- Email: [email protected]
Hot ductility of high alloy Fe–Mn–C austenite TWIP steel
Author’s Accepted Manuscript Hot ductility of high alloy Fe-Mn-C austenite TWIP steel Peng Lan, Haiyan Tang, Jiaquan Zhang
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(16)30204-0 http://dx.doi.org/10.1016/j.msea.2016.02.086 MSA33391
To appear in: Materials Science & Engineering A Received date: 5 January 2016 Revised date: 26 February 2016 Accepted date: 28 February 2016 Cite this article as: Peng Lan, Haiyan Tang and Jiaquan Zhang, Hot ductility of high alloy Fe-Mn-C austenite TWIP steel, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.02.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot ductility of high alloy Fe-Mn-C austenite TWIP steel Peng Lan*, Haiyan Tang, Jiaquan Zhang School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 10083, P.R. China *Corresponding author:. Peng Lan([email protected]) Present/permanent address:
Room 417, Yejinshengtai Building, NO.30 Xueyuan Road, Haidian District, Beijing, P.R. China.
Abstract The high temperature tensile behavior of Fe-22Mn-0.7C Twinning induced plasticity (TWIP) steel has been experimentally investigated from 700 ˚C to 1250 ˚C. It is shown that TWIP steel presents higher tensile strengths and lower reduction area(RA) than low carbon(LC) steel under as cast condition. There is just one peak in each stress-strain curve of TWIP steel, while multi-peak has been observed in that of LC steel above 900 ˚C. The hot ductility of as cast Fe-22Mn-0.7C TWIP steel is not appreciable with most of RAs lower than 40% in the temperature range of 700~1250 ˚C. Interdendritic brittle cracks are observed in the fracture of as cast samples without any dimple. The hot ductility of Fe-22Mn-0.7C TWIP steel increases effectively with RAs higher than 60% in the range of 900~1200 ˚C when the Mn and C microsegregation ratio decrease to 0.9~1.1 and 0.75~1.3 respectively, together with the grain size reducing to 50 μm in diameter. Increasing the strain rate results in the increase of RAs of as cast TWIP steel, in which more uniform deformation and less microcrack in the matrix has been observed, along with deformation bands along the stretching direction. The inevitable solute microsegregation Mn and C, along with the microporosity in as cast structure result in the matrix homogeneity weakening of TWIP steel, leading to the deterioration of deformation continuity in the interdendritic zone. Dynamic recrystallization(DRX) retardation by high Mn alloying with extremely large grains is also a factor influencing the hot ductility of TWIP steel, although it improves the RA of as cast matrix in a very limited level. According to the stacking fault energy calculation, the low hot ductility of as cast TWIP steel with noteless necking behavior in the hot tensile test is not related to mechanical twining.
Keywords: Fe-Mn-C alloy; hot ductility; TWIP steel; matrix homogeneity; dynamic
recrystallization 1. Introduction Twinning induced plasticity(TWIP) steel has been regarded as one of the most potential structural material for automobile with the demand of safety and lightweight, attributed to its excellent combination of high strength and high 1
ductility[1,2]. It has been more than fifteen years since high Mn TWIP steel was first reported, however, the proportion of TWIP steel components in body-in-white is still very low. Not only the high alloying cost but also the insufficient understanding on smelting, casting and processing influence its application [3,4]. It is still of great value to develop a suitable technique to cast high Mn TWIP steel by conventional equipment with the advantage of investment saving. However, the sensitivity to surface cracking of TWIP steel is extremely high during casting and hot rolling [5, 6], leading to the increase of cost and decrease of yield. The high temperature tensile behavior of TWIP steel under as cast condition is the necessity to evaluate the crack sensitivity during continuous casting and hot processing [7]. In our previous work [8, 9], coarse solidification grains and serious solute microsegregation were observed in Fe-22Mn-0.7C TWIP steel, which deteriorated its processability and the final mechanical property. Besides, low solidification temperature, wide mushy zone spacing and high tendency to form interdendritic porosity have been also revealed by present authors [10, 11]. According to the experimental results by Bleck et al.[12], Kang et al. [13], and Salas-Reyes et al. [14], the reduction area (RA) of TWIP steel in static tensile test was lower than 40% within the temperature range between 700 ˚C and 1200 ˚C. However, in Hamada and Karjalainen’s[15] research, the hot ductility of Fe-Mn-C TWIP steel was appreciable with RA higher than 60%, which was better than that of 304 stainless steel within the temperature range of 1000~1200 ˚C. Also, the mechanism on the hot ductility of TWIP steel has not been widely agreed, for which grain boundary sliding, impure particles and matrix homogeneity are respectively employed in different literatures[12-15]. Consequently, more effort is needed to spend on the research of high temperature deformation behavior of TWIP steel. The aim of the present work is to get a clear understanding of the speciality of Fe-22Mn-0.7C TWIP steel during hot tensile process, and to investigate the hot ductility mechanism by analyzing the related factors including solute concentration, matrix homogeneity, grain size, facture morphology, phase transformation and strain rate. The relationship between hot ductility and dynamic recrystallization (DRX) of as cast TWIP steel has been discussed accordingly. 2. Experiment and methodology The Fe-22Mn-0.7C TWIP steel was produced by a 25 Kg vacuum medium frequency induction melting furnace, as well as the low carbon(LC) steel. The specimen of high Mn austenite stainless steel(SS) was cut from a continuous casting slab. The measured chemical compositions of these steels are shown in Table 1. The concentration of C and S was measured through infrared absorptiometric method after combustion in current oxygen, and Mn content was determined by perchloric acid oxidation trivalent manganese titrimetric method. In addition, inert gas carrier melting thermo-conductimetric method was employed for the measurement N concentration, while inert gas carrier melting infrared absorptiometric method was introduced for measuring O. The contents of other alloying elements (Si, Cr, Ni and P) were measured by ICP-OES (Inductively coupled plasma optical emission spectrometric) methodology. The specimen for tensile test with a cylindrical shape was cut from dendrite grains 2
zone, which was 120 mm in length and 10 mm in diameter with a thread at each end about 10 mm long. The sketch of specimen and sampling location are shown in Fig. 1. Gleeble 1500 thermomechanical simulator was employed for the measurement of high temperature deformation behavior. The heating and tensile-testing profile was established according to the matrix phase transformation behavior of the samples. For TWIP steel with single austenite matrix, specimens were heated to the temperature 50 ˚C higher than the pretested with the rate of 10 ˚C·s-1 and soaked for 2min, followed by a cooling operation to the test temperature by 1.67 ˚C·s-1 , then soaked for another 2min and stretched. For other steels, specimens were heated to 1350 ˚C by 10 ˚C·s-1, followed by a soaking process about 5min, then cooled to test temperature with the rate of 1.67˚C·s-1 and tensile-tested. The tensile strain rate was set as a constant rate of 10-3s-1. High strain rate (10-2s-1, 10-1s-1, 1 s-1) tests were conducted to investigate the hot deformation mechanism of TWIP steel. The detail of the tensile-testing profile can be found in our previous study [8, 9].
TWIP LC SS
Table 1 Chemical composition of investigated steels, wt.% C Si Mn P S Cr Ni T.O T.N 0.73 0.17 22.03 0.0056 0.0060 15 60 0.15 0.20 0.71 0.0038 0.0052 15 52 0.045 0.40 1.03 0.036 0.004 18.35 8.02 0.06
Fe Bal. Bal. Bal.
Fig.1 (a) Sketch of static tensile specimen, (b) Sampling location of TWIP steel and LC steel from ingot, and (c) Sampling location of stainless steel from slab. The fracture morphology of TWIP steel was observed by JSM-6480LV scanning electron microscopy (SEM), while the matrix microstructure was analyzed by Leica DMR optical microscopy (OM) after grinding and Nital etching. The microsegregation of as cast TWIP steel specimens was examined by JEOL JXA-8230 electron probe microanalyzer(EPMA), and the phase transformation of the tensile specimen before and after tensile test was determined by MAC-21 ultra-high-power X-ray diffractometer(XRD) with CuKα radiation at a rate of 5 degree·min-1. 3
In order to further investigate the hot ductility mechanism, hot forging operation was conducted on Fe-22Mn-0.7C steel to get finer grain size and better matrix homogeneity. The as cast ingots of TWIP steel with 120 mm in diameter and 250 mm (excluding hot-top) in height were heated to 1200 ˚C and soaked for 2 hours, then forged into a rectangular bar sized 30mm×80mm×1000mm. The finishing temperature was higher than 900 ˚C, and the hot forged sample was cooled to room temperature by the air. The grain morphology of hot forged sample at elevated temperatures was observed by VL2000DX high temperature confocal laser scanning microscope (HT-CLSM). The static tensile test and matrix homogeneity examination was carried out with the same method as mentioned above. 3. Results and discussion 3.1 Stress-strain behavior of TWIP steel True stress-strain curves of as cast TWIP steel, LC steel and as forged TWIP steel are shown in Fig. 2(a), (b) and (c). It can be seen that Fe-Mn-C TWIP steel exhibits more obvious strain hardening performance than LC steel, especially when the test temperature is below 1000 °C. The occurrence of DRX is observed with the fluctuation and multi-peak characteristic in the true stress-strain curves of as cast LC steel when the tensile temperature is above 900 °C, while it also happens in as forged TWIP samples when the temperature is higher than 1000°C. Nevertheless, it is absent in as cast TWIP steel throughout the whole temperature range. When the tensile temperature is no more than 1200°C, the yield strength and tensile strength of as cast TWIP steel are higher than that of LC steel. Taking Fig. 2(a) and (c) into comparison, the as forged TWIP shows higher yield strength and tensile strength than the as cast samples in the temperature range of 700~900°C. However, at higher temperatures the tensile strengths of as forged TWIP steel are closed to that of as cast ones, but with much longer tensile strains. 350
True stress, MPa
250
200
(b)
300
250
True stress, MPa
300
700℃ 800℃ 900℃ 1000℃ 1100℃ 1200℃ 1300℃
350
700℃ 800℃ 900℃ 1000℃ 1100℃ 1200℃ 1300℃
(a)
Peak point 150
100
50
200
Peak point 150
100
50
0
0
0.0
0.2
0.4
0.6
0.8
True strain
0.0
0.2
0.4
True strain
4
0.6
0.8
350
700 ℃ 800 ℃ 900 ℃ 1000 ℃ 1100 ℃ 1200 ℃ 1300 ℃
(c)
300
True stress, MPa
250 200
Peak point 150 100 50 0 0.0
0.2
0.4
0.6
0.8
Ture strain
Fig. 2 Ture stress-strain curves of different specimens (a) As cast TWIP steel (b) As cast LC steel, and (c) As forged TWIP steel 3.2 Hot ductility of TWIP steel Hot ductility curves of as cast TWIP steel, LC steel and SS steel are shown in Fig.3. The RAs of TWIP steel are very low with all measured values less than 40%. Although SS steel is in the same matrix phase with TWIP steel, the former presents much better hot ductility than the latter. LC steel shows the best hot ductility with highest RAs excluding the temperature range of 750~900°C. Mintz and Arrowsmith[16] experimentally investigated the hot ductility from numerous steels and proposed the critical value to estimate matrix crack sensitivity was about 40%, below which the cracking possibility would increase obviously. It is shown in Fig.3 that at high temperature above 1200°C, the RA of TWIP steel decreases as temperature increases, which is the first brittle zone for TWIP steel. RAs within the second brittle zone from 1200°C to 950°C are all below 40%, and it nearly levels off. The third brittle zone for TWIP steel is around 950~800°C, in which the weakest hot ductility is obtained. Temperature ranges for different brittle zones of the investigated steels are shown in Table 2. From 700°C to 1300°C high Mn TWIP steel shows the most serious brittle performance, while austenite stainless steel exhibits low RAs but acceptable hot ductility. Fig.4 shows the hot ductility curves of as forged and as cast TWIP steel, along with the experiment results from previous research. It is worthy to notice that RAs from 900°C to 1200 °C after hot forging treatment are nearly twice higher than that of as cast specimens. Bleck et al.[12] and Kang et al.[13] carried out hot tensile experiment of as cast TWIP steels with the composition of Fe-23Mn-0.6C and Fe-18Mn-0.6C respectively, and the strain rates were set as 2×10-3s-1 and 3×10-3s-1. It was revealed by them that RAs of TWIP steel from 700 °C to 1350 °C were rarely low, which were unexpectedly close to the values in the present work. Hamada and Karjalainen [15] made a research on the hot ductility of Fe-22Mn-0.6C TWIP steel within the temperature range of 700~1300°C, in which all RAs were higher than 60%. Since the specimen is under as rolled condition, the hot ductility measured by Hamada is comparable to the present result of as forged TWIP steel. Besides, the high RA of TWIP steel in Hamada’s experiment can also be explained by the high tensile strain rate (1s-1). It is already reported by us that the RA increases with strain 5
rate increasing, and they are in a logarithmic linear relationship when the strain rate are between 1×10-3s-1~1s-1[9]. A recent work on the hot ductility of as cast Fe-18Mn-0.6C TWIP steel by Han et al. [17] has been reported. Comparing to the hot ductility of the present TWIP from dendrite zone with a strain rate of 10-3s-1, RAs measured by Han et al. within the temperature range from 800°C to 1100°C are also much higher [17]. It is found that the specimens in Han’s experiment were sampled from equiaxed zone with the strain rate of 1s-1. The combination effect of these two factors leads to the high ductility of TWIP steel, even better than that of as forged and as rolled specimen[15]. It can be inferred that matrix homogeneity improvement and grain size refinement in the as forged Fe-22Mn-0.7C TWIP steel should be the explanation to the improvement hot ductility. In other words, the weak matrix homogeneity and large grains should be the factors lowering the hot ductility of the present as cast TWIP steel. Table 2 Temperature ranges of different brittle zone for various steels 1st brittle zone 2nd brittle zone 3rd brittle zone TWIP 1200~solidus 1200~950°C 950~800°C LC Above 1300°C 850~800°C SS Above 1250°C TWIP LC SS
100 90
Reduction ratio of area
80 70 60 50 40 30 20 10 0 700
800
900
1000
1100
1200
1300
Temperature, C
Fig.3 Hot ductility curves of investigated steels 90
-3 -1
TWIP as forged, 10 s -3 -1 TWIP as cast, 10 s -2 -1 TWIP as cast, 10 s -1 -1 TWIP as cast, 10 s -1 TWIP as cast, 1s -2 -1 TWIP as cast, 10 s -1 -1 TWIP as cast, 10 s -1 TWIP as cast, 1s -3 -1 23Mn-0.6C as cast, 210 s [12] -3 -1 18Mn-0.6C as cast, 310 s [13] -1 22Mn-0.6C as rolled, 1s [15] -1 18Mn-0.6C as cast, 5s [17]
80
Reduction ration of area,%
70 60 50 40 30 20 10 0 700
800
900
1000
1100
1200
1300
1400
Temperature,C
Fig. 4 Hot ductility curves of TWIP steel under different conditions, together with the results from references[12],[13],[15] and [17] 3.3 Matrix phase of TWIP steel 6
Fig. 5 shows the XRD result of TWIP steel sample before and after tensile test at 900°C. It is seen that the matrix phase of Fe-22Mn-0.7C TWIP steel is always austenite independent of processing and testing condition. No obvious difference in the diffraction pattern has been observed from samples before and after tensile-testing. It should be paid more attention that the characteristic of diffraction peak is quite different for the samples from different preparing condition. By comparing Fig.5 (a), (b) and (c), (d) it can be found that the width of each peak reduces remarkably after hot forging operation. At the same time, the height of γ(220) peak increases significantly while that of γ(311) peak descends considerably. The decrease of peak width represents the grains refinement during the hot working process, while the height change indicates the rotating of grain crystal orientation in the matrix. It is obvious that a part of as cast grains with (311) crystal orientation rotates to (220) orientation during forging. It is well known that the onset of ferrite films along austenite grain boundaries is the widely accepted mechanism to explain the hot ductility loss within the temperature range of austenite decomposition. The austenite stability may decrease when the solute microsegregation is quite serious. In a recent report it is found the matrix of as cast Fe-22Mn-3Si-3Al TWIP steel is a mixture of austenite and ferrite due to the uneven solute distribution[18]. However, in the present research the high carbon alloying in TWIP steel stabilizes the austenite quite well with no ferrite or martensite detected. It can be drawn that phase transformation should not be the explanation for the low hot ductility of Fe-22Mn-0.7C TWIP steel in this temperature range. (a)
(111)
(111)
(b)
Intensity /a.u.
Intensity/a.u.
(311)
(100) (311)
(100)
(220) (220)
40
50
60
70
80
40
90
50
60
(c)
80
90
(100)
(d)
(111)
Intensity/a.u.
(111)
Intensity/a.u.
70
2/()
2/()
(220)
(220)
(100)
(311)
(311)
40
50
60
70
80
40
90
50
60
70
80
90
2
2
Fig. 5 X-ray diffraction results of TWIP steel. (a)as cast sample before test, (b)as cast sample after test, (c)as forged sample before test, and (d)as forged sample after test. 7
3.4 Matrix homogeneity and grain size of TWIP steel Fig. 6 shows the solutes distribution of TWIP steel samples from as cast and as forged conditions. The area-scanning results of Mn distribution in the matrix by EPMA are presented in Fig. 6(a) and (b), along with the corresponding line-scanning results of Mn and C shown in Fig. 6(c)~(f). It is shown that the matrix of as cast TWIP steel is inhomogeneous with obvious Mn segregation in the interdendritic zone, and it becomes much more uniform in the as forged TWIP steel sample, although the dendrite structure can be observed illegibly. Besides, in the interdendritic zone in as cast sample where solute microsegregation is serious, the microporosity can also be observed, as marked by white circles in Fig.6(a). However, such a corresponding relationship between microsegregation and microporosity is not very obvious in as forged TWIP steel. According to quantitative results in Fig. 6(c)~(f), it is clear that the C solute distributes in accordance with Mn in both as cast and as forged TWIP steel samples. The microsegregation ratio in Fig. 6 is defined as the local solute concentration to the average solute concentration. For the present Fe-22Mn-0.7C TWIP steel, Mn segregation ratio is between 0.8 and 1.3 in as cast matrix, while it reduces to 0.9~1.1 after hot forging. At the same time, C segregation ratio decreases from 0.6~1.5 under as cast condition to 0.75~1.3 after hot working treatment. The standard deviation(StDev) is employed to describe the dispersion level of solute concentration for the line-scanning results. The StDev of Mn concentration in the as forged sample is less than half of that from as cast sample, indicating the increase of solute distribution homogeneity. However, the StDev of C concentration from the as forged TWIP steel reduces just by one third compared to that of the as cast sample, implying the ineffective improvement of C diffusion. Actually, the thermal diffusion ability of C atoms should be higher than that of Mn with the consideration of atom size. It seems that the experimental results in the present research contradict with the previous theory. In fact, it is the interaction effect of solute atoms that explains the above inconsistency. There are two factors influencing the thermal diffusion behavior of C atoms from Mn. On one hand, Mn addition increases the diffusion activated energy due to the increase of lattice distortion[19]; on the other hand, Mn alloying decreases the activity of C atoms by forming the Mn-C atoms pair[20]. Fig. 7 shows the ten possible octahedral structures in Fe-Mn-C austenite matrix. According to the statistical analysis, when Mn concentration is lower than 33at%, the number of Mn atoms around C atom in the octahedron should be less than two, as shown in Fig.7(a)~(d). However, the stability of FeMnC cluster increases as the number of Mn in the octahedron decreases, according to the first principle calculation results by Von Appen and Dronskowski[21]. Accordingly, Mn atoms are readily to occupy most of the positions around C atoms to form the stable octahedral structure, as shown in Fig.7(h)~(j). That is also in good agreement with Kirkaldy and Brown’s[20] conclusion that Mn lowered the activity of C in austenite and stimulated C diffusion to high-Mn areas. This leads to an interactive effect of Mn and C on the microsegregation stability in the interdendritic zone, as shown in Fig. 6(e) and (f). 8
(a) m
(b) n
StDev=0.1322
2.0
(c)
Carbon microsegregation ratio
Manganese microsegregation ratio
2.0
1.5
1.0
0.5
0.0
StDev=0.1504
(d) 1.5
1.0
0.5
0.0
0
100
200
300
400
500
0
100
300
2.0
StDev=0.0616
(e)
400
500
StDev=0.1151
(f) Carbon microsegregation ratio
Manganese microsegregation ratio
2.0
200
Distance, um
Distance, um
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0 0
100
200
300
400
500
0
Distance, um
100
200
300
400
500
Distance, um
Fig. 6 Solutes distribution of TWIP steel. (a)Mn distribution in as cast matrix, (b)Mn distribution in as forged matrix, (c)Mn microsegregation ratio along line m in (a), (d)C microsegregation ratio along line m in (a), (e)Mn microsegregation ratio along line n 9
in (b), (f)C microsegregation ratio along line n in (b) The relationship between hot ductility and solute alloying concentration of TWIP steel has been barely reported. In our previous paper[9], it is inferred that high Mn alloying should take a promoting effect on the development of dendrite structure in steels, which has been proved by the experimental results from Yang et al.[22]. It is widely known that the coarser the steel matrix, the weaker the hot ductility. Consequently, the inevitable solute microsegregation, together with the microporosity in the TWIP steel should be the most predominant factor influencing hot ductility. (a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Fig. 7 Octahedral strcture in Fe-Mn-C austenite matrix.(a)Fe6C, (b)Fe5MnC, (c)Fe4Mn2C-cis, (d)Fe4Mn2C-trans, (e)Fe3Mn3C-fac, (f)Fe3Mn3C-mer, (g)Fe2Mn4C-cis, (h)Fe2Mn4C-trans, (i)FeMn5C, (j)Mn6C (a)
(b)
Fig. 8 Grain morphology of TWIP steel. (a) As cast(by OM), (b) as forged(by HT-CLSM) Fig. 8 shows the grain morphology of TWIP steel under as cast and as forged conditions, which is traced manually with drawing software. OM and HT-CLSM are employed to observe the as cast and as forged samples under room temperature and 900 ˚C respectively. It can be seen that the as cast grains are in elongated shape while the as forged grains are almost equiaxed. The grain size of TWIP steel from as 10
cast matrix is larger than 500μm in minor axis direction, while that of as forged samples is around 50μm in diameter. According to the opinion of Sah et al.[23], the kinetics of DRX is accelerated by decrease in grain size for nickel alloys. Wray[24], Crowther and Mintz[25] carried out series of experiments to investigate the effect of initial grain size on DRX of steel austenite grains. It is proposed by them that earlier onset and faster rate of DRX can be promoted by a finer grain structure. Fernandez et al.[26] and Wang et al.[27] experimentally studied the DRX behavior of Nb/Nb-Ti microalloyed steels and Mn-Cr gear steel respectively, in which they interpreted that DRX was more apt to take place in fine austenite grains structure. The grain refinement should also take some positive effect on the hot ductility improvement of as forged TWIP steel. 3.5 Fracture morphology of TWIP steel Fracture morphologies of TWIP steel under different tensile conditions are shown in Fig. 9. It is seen that at each temperature TWIP steel samples present brittle fracture with interdendritic cracks at the strain rate of 10-3 s-1. The facture morphology of samples tensile-tested at 900 ˚C is more brittle than that from 1100 ˚C, which can be confirmed by the presence of flat dendrite boundaries and lamellar interdendritic layers, as marked by the white trace in Fig.9(a). As tensile temperature increases to 1100 ˚C, the lamellar layers in the interdendritic zone disappear, in place by unsmooth interdendritic grain boundaries, as marked in Fig.9(c). The appearance of lamellar layers at 900 ˚C should be in relation to the solute microsegregation, which dissolves into the adjacent dendrites at 1100 ˚C. Hot ductility improvement induced by matrix homogeneity increase has already been proved by the RA comparison of as cast and as forged samples, in which grain refinement impact should be also taken into consideration. As the tensile strain rate increases, the fracture morphology becomes ductile with high proportion of dimples, although the interdendritic cracks are still observed, as shown in Fig.9(e) and (f). Under this situation the RA at 950 ˚C increases from 31.9% at 10-3 s-1 to 55.2% at 10-1 s-1, which is well agreed with the fracture characteristic. The tensile facture of as forged TWIP steel is full of dimples with no interdendritic crack, as shown in Fig.(g) and (h). Almost no inclusion has been found in the origin of the dimple, suggesting the steel purity control is acceptable. (a)
(b) R.A.=22.0%
11
(c)
(d) R.A.=32.4%
(e)
(f) R.A.=55.2%
(g)
(h) R.A.=67.4%
Fig. 9 Fracture morphology of TWIP steel at different conditions. (a),(b)Temperature: 900 ˚C, as cast, 10-3 s-1; (c),(d) Temperature: 1100 ˚C, as cast, 10-3 s-1; (e),(f) Temperature: 950 ˚C, as cast, 10-1 s-1; (g),(h)Temperature: 1200 ˚C, as forged, 10-3 s-1 (a) (b)
Stretching (c)
(d)
Fig. 10 Cracks initiation and propagation in as cast TWIP steel.(a) Temperature: 850 ˚C, 12
strain rate: 10-3s-1,(b) Temperature: 1150 ˚C, strain rate: 10-3s-1, (c) Temperature: 1200 ˚C, strain rate: 10-2s-1,(d)Temperature: 1200 ˚C, strain rate: 1s-1 Fig. 10 shows cracks initiation and propagation in as cast TWIP steel during tensile test. In most of the cases, crack initiates at dendrite boundaries, and then propagates along dendrite growth direction, as shown in Fig. 10(a) and (b). This leads to the interdendritic brittle fracture of as cast TWIP steel, as observed in Fig. 9(a)~(d). There are also some cracks initiating from the sample surface, and then spreading into the matrix inside. It is noteworthy that plenty of microcrack and microporosity are observed in the dendrite boundaries area near the fracture, as marked by the white circles in Fig.10. That means crack can easily initiate and propagate along dendrite boundary, where solidification defects locate and the matrix homogeneity is the weakest. When the interdendritic solute microsegregation and microporosity is serious enough, crack can link each other quickly. As tensile strain rate increases, the nearby microcrack becomes blunted, as shown in Fig.10(c) and (d). The sample matrix from the strain rate of 1s-1 seems more uniform with obvious deformation bands along the stretching direction. Actually, the ductile fracture at higher strain rate results from the prevention of crack growth along dendrite boundaries within shorter deformation time, leading to the accordant deformation of dendrite and interdendritic zone. DRX is a matrix softening behavior which plays a beneficial role in the improvement of hot ductility. However, no dynamically recrystallized grain is observed in Fig.10, even under the low strain rate high temperature conditions. 4. Mechanism of Hot ductility loss in TWIP steel The hot ductility of as cast TWIP steel is not quite appreciable according to the present research, which is also coincident with experimental results from Bleck et al.[12], Kang et al.[13] and Salas-Reyes et al.[14]. However, the related mechanism has not been discussed clearly yet. There are three temperature ranges for the brittle behavior of TWIP steel, as presented in Table 2. The first brittle zone between 1200 ˚C and solidus is due to the weakening of interdendritic area by the prior melting of solute microsegregation layers. The second brittle zone of TWIP steel is from 950 ˚C to 1200 ˚C, in which RAs are nearly a constant. The hot ductility increases with the increase of strain rate in this range, suggesting the low RA is not induced by the formation of (Fe,Mn)(O,S) as in other steels. Poor matrix homogeneity in as cast TWIP steel with serious microsegregation and microporosity is the predominant explanation, while DRX retarding by high Mn high C alloying with extremely large grains should be the following reason. This will be discussed in the next paragraphs in detail. The third brittle zone for the present TWIP steel is within the temperature of 800~950 ˚C, where the lowest RAs exist. According to XRD results, only austenite has been detected in the matrix. The widely accepted mechanism for the third brittle zone of common steels by the formation of ferrite film and carbonitride along grain boundary cannot be employed here. By comparing the facture morphology from 900 ˚C and 1100˚C, it is found that the undissolved segregated layers in the interdendritic zone weakens the deformation continuity of the matrix, leading to the early fracture with low RA. 13
In order to elucidate the deformation mechanism of Fe-22Mn-0.7C TWIP steel in the second brittle zone, factors influencing steel hot ductility, such as solute concentration, matrix homogeneity, grain size, phase transformation and mechanical twinning, will be discussed further here. TWIP steel is the one of the newly developed advanced high strength steels with extremely high manganese addition, and the relationship between the low hot ductility and high solute alloying has not been widely discussed. Fig. 11 shows the correlation of RA and Mn concentration from as cast steels with a strain rate in the order of 10-3 s-1. Table 3 presents the chemical composition of each sample from references. It can be seen that RA decreases linearly with Mn concentration increasing in the temperature range of 1000~1200 ˚C. The reasons for the loss of hot ductility of high Mn TWIP steels are quite diverse. Kang et al.[13] tentatively proposed that grain boundary sliding at high temperature result in the hot ductility deterioration of TWIP steel, but he did not explain it further. Hamada and Karjalainen[15], Salas-Reyes et al.[14] made similar experiments on as rolled and as cast TWIP steels respectively and they attributed the low hot ductility to grain boundary sliding based on EBSD graphs and fracture morphology. However, the sliding behavior in Hamada’s study occurred along austenite grains boundaries, while that in Mejia’s work happened in interdendritic area. Besides, Han et al.[17] studied the influence of Ni on the hot ductility of high Mn TWIP steel. Low melting point eutectic particles, randomly distributed MnS inclusions and DRX delay by high stacking fault energy are found to be the reasons for the RA drop of TWIP steel. Brune et al.[30] measured the RAs of low Mn and high Mn steel. It is demonstrated that the formation of complex precipitation is the main cause for the low hot ductility of Fe-18Mn-0.3C-0.8Al TWIP steel. Yang et al.[22] made an investigation on the hot ductility of steel samples with different Mn addition. He pointed out that the inadequate Al addition and excessive S alloying was the factors resulting in the low RA of TWIP steel. It can be drawn that more attention has been paid to the effect of Al/S/P/Ti/Nb/V alloying on the hot ductility of TWIP steel in previous analysis[14,30-36] other than that of Mn. It can be seen from Fig. 11 the RA of steel decreases as Mn concentration increases, which is nearly independent of other solute concentration. According to the comparison of hot tensile behavior from the three steel grades in the present work, the RA of high Mn TWIP steel is naturally lower than that of LC steel and SS steel. The inevitable solute microsegregation Mn and C(Fig. 6), along with the microporosity in as cast matrix(Fig.6 and Fig. 10) result in the homogeneity decrease of TWIP steel, leading to the ductility loss of the matrix, especially the dendrite boundaries zone. This conclusion also can be proved by Bleck et al.[12], in which the Fe-23Mn-0.6C steel presented lower RAs with a less uniform solidification structure compared to medium Mn and low Mn steel. In addition, interdendritic fracture in the literatures by Cagala et al.[37], Salas-Reyes et al. [14], Yang et al.[22] and Han et al.[17] is also the evidence for the ductility weakening of dendrite boundaries. Carbon is another solute element in the present TWIP steel, which shows limited effect on matrix hot ductility. There are two factors impacting the RA the austenite TWIP steel by C. On one hand, the higher the C in TWIP steel, the wider the mushy 14
zone during solidification. As a consequence, the microsegregation and microporosity in the interdendritic zone is more liable to form[38], resulting in the unsound structure with low cracking resistance. On the other hand, carbon is more apt to segregate along the dendrite boundary due to the low solute partition coefficient during solidification. There will be an interactive effect of Mn and C on the segregation stability when we take the formation of stable Mn6C octahedron into consideration[21]. It is obvious that high carbon alloying takes a negative effect on as cast hot ductility of TWIP steel, even though it is not dominant. Sulfur and phosphorus are regarded as detrimental elements on steel hot ductility. According to the investigation by Kang et al.[32] and Yang et al.[22], the formation of sulfide and phosphide compound with low melting point leads to the weakness of hot ductility. In a recent work by Lv et al.[39], the RA of high Mn steel started to decrease when the concentration of S and P is higher than 0.006 wt.% respectively. Considering the S and P level in the present TWIP steel, their detrimental effect can be neglected. Present, 1000C Kang et al., 1000C [13] Salas-Reyes et al., 1000C [14] Zeng et al., 1000C [28] Bleck et al., 1000C [12] Fan et al., 1000C [29] Yang et al., 1000C [22] Present, 1100C Salas-Reyes et al., 1100C [14] Zeng et al., 1100C [28] Bleck et al., 1100C [12] Fan et al., 1100C [29] Yang et al., 1100C [22] Present, 1200C Zeng et al., 1200C [28] Bleck et al., 1200C [12] Fan et al., 1200C [29] Yang et al., 1200C [22]
100
80
RA, %
60
40
20
0 0
5
10
15
20
25
Mn concentration, wt%
Fig. 11 Correlation of RA and Mn concentration from as cast steels Table 3 Chemical composition of as cast steels from references, wt.% C Mn Si P S N Al Reference 1 0.61 18.07 0.009 0.006 0.013 0.047 Kang et al.[13] 2 0.61 21.91 0.01 0.0066 0.016 Kang et al.[13] 3 0.56 21.0 1.3 0.012 1.6 Salas-Reyes et al.[14] 4 0.111 4.87 0.03 0.011 0.002 0.0045 0.007 Zeng et al.[28] 5 0.55 23.1 0.26 0.005 0.0003 0.0084 Bleck et al.[12] 6 0.84 15.7 0.27 0.007 0.003 0.0095 Bleck et al.[12] 7 0.86 9.3 0.27 0.012 0.0059 0.0061 Bleck et al.[12] 8 0.17 1.42 0.36 0.033 0.012 Fan et al.[29] 9 0.57 17.07 0.007 0.0005 0.0043 2.1 Yang et al.[22] 10 0.59 8.68 0.007 0.0005 0.0038 2.1 Yang et al.[22] 11 0.67 0.028 0.007 0.0005 0.0029 2.1 Yang et al.[22] DRX is another vital factor that affects the hot ductility of steel. The fluctuation and 15
multi-peak in stress-strain curves(Fig. 2) of as cast LC steel and as forged TWIP steel is the characteristic of DRX. However, DRX is absent in the as cast TWIP steel, for which no evidence in the stress-strain behavior or matrix microstructure is found. Both steel composition and grain size show important effects on the DRX. Wary experimentally examined DRX in steels alloyed with 0~9.5 wt.% Mn and found high Mn addition retarded DRX in tensile test[24]. Cabanas et al. investigated the hot deformation behavior of Fe-Mn binary alloys containing 1~20 wt.% Mn[40]. They pointed out that the activation energy for hot working increased with Mn content increasing, and high Mn alloying delayed DRX. Hamada et al.[41] and Han et al.[17] studied the hot deformation behavior of TWIP steel respectively, and both of them regarded Mn as a DRX delay element. On the opposite, in the experiment by Wietbrock et al.[42] Fe-28Mn-0.3C presented the earlier onset of DRX than Fe-23Mn-0.3C and Fe-28Mn-0.6C TWIP steel. They proposed that increased Mn content promoted DRX of TWIP steel. According to the result in the present work, DRX is retarded by high Mn alloying without the consideration of grain size difference, which can be seen clearly in Fig. 2 and Fig. 10. Besides, carbon addition in TWIP steel also influences the hot ductility. Wary[24] made a comparison between the onset strain of DRX of the steels with 0.051~0.71 C alloying. The weakest retardation effect on DRX of C was found compared to Ni, Mn, Si, and P in steels. Crowther and Mintz[43] measured the hot ductility of steels containing 0.04~0.65 wt.% C. He suggested the activation energy of DRX increased with C content increasing. As a result, DRX in the present Fe-22Mn-0.7C TWIP steel is thoroughly suppressed by the high alloying of Mn and C. Taking the microsegregation of Mn and C into account, the possibility of DRX becomes even lower. For TWIP steel with no phase transformation, the dendrite interface is coherent with austenite grains boundary[44]. It is well accepted that the most probable location for the occurrence of DRX is grain boundary, however, under as cast condition the segregated Mn and C in interdendritic boundaries retard DRX more strongly. Grain size is also an important parameter in analyzing DRX behavior in steels. It is illustrated by Wary[24] that a fine-grain structure can promote the early onset and fast rate of DRX in steel. Sah et al.[23], Crowther and Mintz[25], and Fernandez et al.[26] also obtained the similar conclusion based on their own experiments. The grain refinement of TWIP steel by hot forging gives rise to higher RA than that from as cast TWIP steel with coarser grains. Nevertheless, grain size is not the most influential factors for hot ductility of TWIP steel. DRX occurred at 900 ˚C in the tensile test of LC steel under as cast condition, in which the grain size should be in the same order of as cast TWIP steel. Consequently, the composition speciality of TWIP steel is more influential on retardation of DRX. It is noteworthy that DRX just partly occurs in as cast TWIP steel during hot deformation in the reported literatures. Moreover, it improves the hot ductility of the as cast matrix in a very limited level. In the present and Kang et al.’s[13] study, no dynamically recrystallized grain has been observed in the as cast matrix. Han et al.[17] examined the grain morphology of TWIP steel after hot tensile test, they found DRX only occurred just in a few grains near the fracture. DRX fraction was measured in 16
the experiment by Salas-Reyes et al.[14]. For metal mold cast non-microalloy TWIP steel, Nb-TWIP steel and Mo–TWIP steel, DRX fraction was 5%, 0 and 2.5%, and the RA was 60%, 60% and 65% respectively. At the meanwhile, the RA from sand mold cast nan-microalloy TWIP steel, Nb-TWIP steel and Mo-TWIP steel was 40%, 70% and 60%, and the corresponding DRX faction was 1%, 0 and 0.5% respectively. It can be seen that the sample with a lower DRX fraction can also give a higher RA, and vice versa. As a result, the hot ductility of as cast TWIP steel is not mainly dependent on DRX. 200
Fe-20Mn-0.4C Present Fe-22mn-0.7C Present Fe-30Mn-1.0C Present Fe-30Mn-0.2C-2.5Al-0.6Si Baradaran[46] Fe-22Mn-0.6C Dumay[47] Fe-22Mn-0.6C Allain[48]
SFE, mJm
-2
150
100
50
Dislocation slip Twining -martensite Solute concentration range
0 0
100
200
300
400
500
600
Temperature, C
Fig. 12 SFE variation against temperature in TWIP steels Twinning behavior is the most distinctive characteristic of TWIP steel, which makes sample with more uniform deformation and more delayed necking. Stacking fault energy(SFE) has been regarded as the most widely used parameter to determine the occurrence of mechanical twinning. A two sublattice model has been employed to calculate the SFE in TWIP steel in previous work[10]. Fig. 12 shows the SFE variation against temperature in different TWIP steels, together with the results from Baradaran et al.[45], Dumay et al.[46] and Allain et al.[47]. The SFE predicted by present model increases monotonously with temperature increasing, which agrees well with that from the previous research. When the temperature is above 300˚C, the thermodynamic database from SGTE is not reliable. Even so, it can be estimated that the higher the temperature, the larger the SFE. According to the investigation by Allain et al.[47] and Hamada et al.[41], phase transformation will be the toughening mechanism when SFE is lower than 18 mJ·mol-1 in high Mn steel, while twinning and partial dislocation slipping will occur when SFE is within the range of 18~60 mJ·mol-1. For the system with higher SFE perfect dislocation slipping will be the main role for the plasticity. The SFE for present TWIP steel with 20~30 wt.% Mn and 0.4~1.7 wt.% C(Concentration fluctuation is induced by microsegregation) in the as cast matrix will exceed the SFE range for twinning when the temperature is above 300 ˚C. Therefore, the noteless necking behavior of as cast TWIP steel in the tensile test is not related to mechanical twining. Jung et al.[48] measured the high temperature twinning behavior of Fe-18Mn-0.6C TWIP steel by XRD methodology. They found the twinning fraction was higher than 30% at room temperature and lower than 15% at 300 ˚C. Asghari et al.[49] observed the grain morphology of Fe-18Mn-2Si-2Al TWIP steel after hot compression. It was reported the twinning fraction decreased dramatically 17
when the temperature was higher than 300 ˚C. Koyama et al.[50] examined the twinning density of tensile-tested Fe-18Mn-1.15C TWIP steel by EBSD technology, which was nearly zero at 200 ˚C. It can be concluded that the deformation mechanism of the present TWIP steel at the concerned temperature range should be not significantly affected by twinning behavior. 5. Conclusions The high temperature tensile behavior of Fe-22Mn-0.7C TWIP steel has been experimentally investigated, and the related factors influencing the deformation mechanism, such as solute concentration, matrix homogeneity, grain size, facture morphology, phase transformation and strain rate, have been discussed. The conclusions can be drawn as follows: 1) TWIP steel presents higher tensile strengths and lower RAs than LC steel from 700 ˚C to 1250 ˚C under as cast condition. There is just one peak in each stress-strain curve of TWIP steel, while multi-peak has been observed in that of LC steel above 900 ˚C. 2) The hot ductility of as cast Fe-22Mn-0.7C TWIP steel is not appreciable with most of RAs lower than 40% in the temperature range of 700~1250 ˚C. Interdendritic brittle cracks are observed in the fracture of as cast samples without any dimple. 3) The hot ductility of Fe-22Mn-0.7C TWIP steel increases effectively with RAs higher than 60% in the range of 900~1200 ˚C when the Mn and C microsegregation ratio decrease to 0.9~1.1 and 0.75~1.3 respectively, together with the grain size reducing to 50μm in diameter. 4) Increasing the strain rate results in the increase of RAs of as cast TWIP steel, in which more uniform deformation and less microcrack in the matrix has been observed, along with deformation bands along the stretching direction. 5) The inevitable solute microsegregation Mn and C, along with the microporosity in as cast structure result in the matrix homogeneity weakening of TWIP steel, leading to the deterioration of deformation continuity in the interdendritic zone. 6) DRX retardation by high Mn alloying with extremely large grains is also a factor influencing the hot ductility of TWIP steel, although it improves the RA of as cast matrix in a very limited level. 7) According to the SFE calculation, the low hot ductility of as cast TWIP steel with noteless necking behavior in the hot tensile test is not related to mechanical twining. Acknowledgement
The research is supported by Fundamental Research Funds for the Central Universities NO. FRF-TP-15-066A1 in P.R. China. The authors are sincerely grateful to the financial funding. Reference
[1] S.H. Kim, H. Kim, N.J. Kim, Brittle intermetallic compound makes ultrastrong low-density steel with large ductility, Nature 518(2015).77-79. [2] O. Grässel, G. Frommeyer, Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels, Mater. Sci. 18
Technol. 14(1998) 1213-1217. [3] O. Bouaziz, S. Allain, C. P. Scott, P. Cugy, D. Barbier, High manganese austenitic twinning induced plasticity steels: a review of the microstructure properties relationships, Curr. Opin. Solid St. M. 15(2011) 141-168. [4] B.C. De Cooman, O. Kwon, K.G. Chin. State-of-the-knowledge on TWIP steel, Mater. Sci. Technol. 28(2012) 513-527. [5] K.H. Moon, M.S. Park, S. Yoo, J.K. Park, J.W. Cho, G. Shin, Molten mold flux technology for continuous casting of the ULC and TWIP steel, in: F. Marquis, PRICM: 8 Pacific Rim International Congress on Advanced Materials and Processing. John Wiley & Sons Inc., 2013, pp. 735-745. [6] S.H. Wang, Z.Y. Liu, W.N. Zhang, G.D. Wang, J.L. Liu, G.F. Liang, Microstructure and mechanical property of strip in Fe-23Mn-3Si-3Al TWIP steel by twin roll casting, ISIJ Int. 49(2009) 1340-1346. [7] B. Mintz, D.N. Crowther, Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting, Int. Mater. Rev. 55(2010)168-196. [8] Y.C. Wang, P. Lan, Y. Li, J.Q. Zhang, Effect of Alloying Elements on Mechanical Behavior of Fe-Mn-C TWIP Steel, J. Mater. Eng. 43(2015) 30-38. [9] P. Lan, L. Song, C. Du, J.Q. Zhang, Analysis of solidification microstructure and hot ductility of Fe-22Mn-0.7C TWIP steel, Mater. Sci. Technol. 30(2014) 1297-1304. [10] P. Lan, Analysis of solidification characteristics and structure performance on TWIP steels for automotive, Doctoral dissertation, University of Science and Technology Beijing, 2015. [11] P. Lan, J.Q. Zhang, Thermophysical properties and solidification defects of Fe–22Mn–0.7C TWIP steel, Steel Res. Int., 87(2016)250-261. [12] W. Bleck, K. Phiu-on, C. Heering, G. Hirt, Hot workability of as-cast high manganese-high carbon steels, Steel Res. Int. 78(2007) 536-545. [13] S.E. Kang, A. Tuling, J.R. Banerjee, W.D. Gunawardana, B. Mintz, Hot ductility of TWIP steels, Mater. Sci. Technol. 27(2011) 95-100. [14] A.E. Salas-Reyes, I. Mejía, A. Bedolla-Jacuinde, A. Boulaajaj, J. Calvo, J.M. Cabrera, Hot ductility behavior of high-Mn austenitic Fe–22Mn–1.5 Al–1.5 Si–0.45 C TWIP steels microalloyed with Ti and V, Mater. Sci. Eng. 611A (2014) 77-89. [15] A.S. Hamada, L.P. Karjalainen, Hot ductility behaviour of high-Mn TWIP steels, Mater. Sci. Eng. 528A(2011)1819-1827. [16] B. Mintz, J.M. Arrowsmith, Hot-ductility behaviour of C–Mn–Nb–Al steels and its relationship to crack propagation during the straightening of continuously cast strand, Metals Technol. 6(1979) 24-32. [17] K. Han, J. Yoo, B. Lee, I. Han, C. Lee, Effect of Ni on the hot ductility and hot cracking susceptibility of high Mn austenitic cast steel, Mater. Sci. Eng. 618A(2014) 295-304. [18] A. Ferraiuolo, A. Smith, J.G. Sevillano, F. de las Cuevas, P. Karjalainen, H. Gouveia, M. M. Rodrigues, Metallurgical Design of High-Strength Austenitic Fe-C-Mn Steels with Excellent Formability (Metaldesign), Publications Office of the European Union. [19] R. Zhu, Y. Lu, S. Li, Z. Chao, S. Wang, Effect of C-Mn atomic couples on austenitic high manganese steel, J. Iron Steel Res. 9(1997) 57-60. 19
[20] J.S. Kirkaldy, L.C. Brown, Diffusion behaviour in ternary, multiphase systems, Can. Metall. Quart. 2(1963)89-115. [21] J.V. Appen, R. Dronskowski, Carbon-Induced ordering in manganese-rich austenite-A density-functional total-energy and chemical-bonding study[J]. Steel Res. Int. 82(2011) 101–107. [22] J. Yang, Y.N. Wang, X.M. Ruan, R.Z. Wang, K. Zhu, Z.J. Fan, Y.C. Wang, C.B. Li, X.F. Wang, Effects of manganese content on solidification structures, thermal properties, and phase transformation characteristics in Fe-Mn-Al-C steels, Metall. Mater. Trans. 63B(2015) 1365-1375. [23] J.P. Sah, G.J. Richardson, C.M. Sellars, Grain-size effects during dynamic recrystallization of nickel, Metal Sci. 8(1974) 325-331. [24] P.J. Wray. Effect of composition and initial grain size on the dynamic recrystallization of austenite in plain carbon steels, Metall. Trans. 15A(1984) 2009-2019. [25] D.N. Crowther, B. Mintz, Influence of grain size on hot ductility of plain C–Mn steels, Mater. Sci. Technol. 2(1986) 951-955. [26] A. I. Fernández, P. Uranga, B. López, J.M. Rodriguez-Ibabe, Dynamic recrystallization behavior covering a wide austenite grain size range in Nb and Nb-Ti microalloyed steels, Mater. Sci. Eng. 361A(2003) 367-376. [27] B.X. Wang, X.H. Liu, G.D. Wang. Dynamic recrystallization behavior and microstructural evolution in a Mn–Cr gear steel, Mater. Sci. Eng. 393A(2005) 102-108. [28]Q. Zeng, B. Zhang, F. Wang, High temperature ductility of Q345 B and AH32 continuous casting slabs at different positions, J. Unv. Sci. Technol. Beijing 36(2014)183-188. [29] Y. Fan, M. Wang, H. Zhang, H. Tao, P. Zhao, S. Li, Hot plasticity and fracture mechanism of the third generation of automobile steel, J. Unv. Sci. Technol. Beijing 35(2013) 607-612. [30] T. Brune, D. Senk, R. Walpot, B. Steenken, Hot ductility behavior of boron containing microalloyed steels with varying manganese contents, Metall. Mater. Trans. 46B(2015) 1400-1408. [31] S.E. Kang, A. Tuling, I. Lau, J.R. Banerjee, B. Mintz, The hot ductility of Nb/V containing high Al, TWIP steels, Mater. Sci. Technol. 27(2011) 909-915. [32] S.E. Kang, J.R. Banerjee, B. Mintz, Influence of S and AlN on hot ductility of high Al, TWIP steels, Mater. Sci. Technol. 28(2012) 589-596. [33] S.E. Kang, J.R. Banerjee, E.M. Maina, B. Mintz, Influence of B and Ti on hot ductility of high Al and high Al, Nb containing TWIP steels, Mater. Sci. Technol. 29(2013) 1225-1232. [34] S.E. Kang, J.R. Banerjee, A. Tuling, B. Mintz, Influence of P and N on hot ductility of high Al, boron containing TWIP steels, Mater. Sci. Technol. 30(2014)1328-1335. [35] I. Mejía, A.E. Salas-Reyes, J. Calvo, J.M. Cabrera, Effect of Ti and B microadditions on the hot ductility behavior of a High-Mn austenitic Fe-23Mn-1.5 Al-1.3 Si-0.5 C TWIP steel, Mater. Sci. Eng. 648A(2015) 311-329.
20
[36] A.E. Salas-Reyes, I. Mejía, A. Bedolla-Jacuinde, A. Boulaajaj, J. Calvo, J.M. Cabrera, Hot ductility behavior of high-Mn austenitic Fe–22Mn–1.5 Al–1.5 Si–0.45 C TWIP steels microalloyed with Ti and V, Mater. Sci. Eng. 611A(2014) 77-89. [37] M. Cagala, E. Mazancová, P. Bábková, P. Lichý, P. Kozelský, Hot ductility of two high manganese steels, in: METAL 2011, Proc. 20th Int. Metall. Mater. Conf., 2011, pp. 486–491. [38] P. Lan, J.Q. Zhang, Numerical analysis of macrosegregation and shrinkage porosity in large steel ingot, Ironmak. Steelmak. 41(2014) 598-606. [39] B. Lv, F.C. Zhang, M. Li, L.H. Qian, T.S. Wang, Effects of phosphorus and sulfur on the thermoplasticity of high manganese austenitic steel, Mater. Sci. Eng. 527A(2010) 5648-5653. [40] N. Cabanas, J. Penning, N. Akdut, B.C. De Cooman, High-temperature deformation properties of austenitic Fe-Mn alloys, Metall. Mater. Trans. 37A(2006) 3305-3315. [41] A.S. Hamada, M.C. Somani, L.P. Karjalainen, High temperature flow stress and recrystallization behavior of high-Mn TWIP steels, ISIJ Int. 47(2007) 907-912. [42] B. Wietbrock, W. Xiong, M. Bambach, G. Hirt, Effect of temperature, strain rate, manganese and carbon content on flow behavior of three ternary Fe-Mn-C (Fe-Mn23-C0. 3, Fe-Mn23-C0. 6, Fe-Mn28-C0. 3) High-manganese steels, Steel Res. Int. 82(2011) 63-69. [43] D.N. Crowther, B. Mintz, Influence of carbon on hot ductility of steels, Mater. Sci. Technol. 2(1986) 671-676. [44] S. Tsuchiya, M. Ohno, K. Matsuura, Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels, Acta Mater. 60(2012) 2927-2938. [45] A.H. Baradaran, A. Zarei-Hanzaki, H.R. Abedi, S.M. Fatemi-Varzaneh, A. Imandoust, The ductility behavior of a high-Mn twinning induced plasticity steel during cold-to-hot deformation, Mater. Sci. Eng. 561A(2013) 411-418. [46] A. Dumay, J.P. Chateau, S. Allain, S. Migot, O. Bouaziz, Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe-Mn-C steel, Mater. Sci. Eng. 483A(2008) 184-187. [47] S. Allain, J.P. Chateau, O. Bouaziz, S. Migot, N. Guelton, Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe-Mn-C alloys, Mater. Sci. Eng. 387A(2004) 158-162. [48] J.E. Jung, J. Park, J.S. Kim, J.B. Jeon, S.K. Kim, Y.W. Chang, Temperature effect on twin formation kinetics and deformation behavior of Fe-18Mn-0.6C TWIP steel, Met. Mater. Inter. 20(2014) 27-34. [49] A. Asghari, A. Zarei-Hanzaki, M. Eskandari, Temperature dependence of plastic deformation mechanisms in a modified transformation-twinning induced plasticity steel, Mater. Sci. Eng. 579A(2013) 150-156. [50] M. Koyama, T. Sawaguchi, K. Tsuzaki, TWIP effect and plastic instability condition in an Fe–Mn–C austenitic steel, ISIJ Int. 53(2013) 323-329. Figure captions: 21
Fig.1 (a) Sketch of static tensile specimen, (b) Sampling location of TWIP steel and LC steel from ingot, and (c) Sampling location of stainless steel from slab. Fig. 2 Ture stress-strain curves of different specimens (a) As cast TWIP steel (b) As cast LC steel, and (c) As forged TWIP steel Fig.3 Hot ductility curves of investigated steels Fig. 4 Hot ductility curves of TWIP steel under different conditions, together with the results from references[12],[13],[15] and [17] Fig. 5 X-ray diffraction results of TWIP steel. (a)as cast sample before test, (b)as cast sample after test, (c)as forged sample before test, and (d)as forged sample after test. Fig. 6 Solutes distribution of TWIP steel. (a)Mn distribution in as cast matrix, (b)Mn distribution in as forged matrix, (c)Mn microsegregation ratio along line m in (a), (d)C microsegregation ratio along line m in (a), (e)Mn microsegregation ratio along line n in (b), (f)C microsegregation ratio along line n in (b) Fig. 7 Octahedral strcture in Fe-Mn-C austenite matrix.(a)Fe6C, (b)Fe5MnC, (c)Fe4Mn2C-cis, (d)Fe4Mn2C-trans, (e)Fe3Mn3C-fac, (f)Fe3Mn3C-mer, (g)Fe2Mn4C-cis, (h)Fe2Mn4C-trans, (i)FeMn5C, (j)Mn6C Fig. 8 Grain morphology of TWIP steel. (a) As cast(by OM), (b) as forged(by HT-CLSM) Fig. 9 Fracture morphology of TWIP steel at different conditions. (a),(b)Temperature: 900 ˚C, as cast, 10-3 s-1; (c),(d) Temperature: 1100 ˚C, as cast, 10-3 s-1; (e),(f) Temperature: 950 ˚C, as cast, 10-1 s-1; (g),(h)Temperature: 1200 ˚C, as forged, 10-3 s-1 Fig. 10 Cracks initiation and propagation in as cast TWIP steel.(a) Temperature: 850 ˚C, strain rate: 10-3s-1,(b) Temperature: 1150 ˚C, strain rate: 10-3s-1, (c) Temperature: 1200 ˚C, strain rate: 10-2s-1,(d)Temperature: 1200 ˚C, strain rate: 1s-1 Fig. 11 Correlation of RA and Mn concentration from as cast steels Fig. 12 SFE variation against temperature in TWIP steels
22
www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(16)30204-0 http://dx.doi.org/10.1016/j.msea.2016.02.086 MSA33391
To appear in: Materials Science & Engineering A Received date: 5 January 2016 Revised date: 26 February 2016 Accepted date: 28 February 2016 Cite this article as: Peng Lan, Haiyan Tang and Jiaquan Zhang, Hot ductility of high alloy Fe-Mn-C austenite TWIP steel, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2016.02.086 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hot ductility of high alloy Fe-Mn-C austenite TWIP steel Peng Lan*, Haiyan Tang, Jiaquan Zhang School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 10083, P.R. China *Corresponding author:. Peng Lan([email protected]) Present/permanent address:
Room 417, Yejinshengtai Building, NO.30 Xueyuan Road, Haidian District, Beijing, P.R. China.
Abstract The high temperature tensile behavior of Fe-22Mn-0.7C Twinning induced plasticity (TWIP) steel has been experimentally investigated from 700 ˚C to 1250 ˚C. It is shown that TWIP steel presents higher tensile strengths and lower reduction area(RA) than low carbon(LC) steel under as cast condition. There is just one peak in each stress-strain curve of TWIP steel, while multi-peak has been observed in that of LC steel above 900 ˚C. The hot ductility of as cast Fe-22Mn-0.7C TWIP steel is not appreciable with most of RAs lower than 40% in the temperature range of 700~1250 ˚C. Interdendritic brittle cracks are observed in the fracture of as cast samples without any dimple. The hot ductility of Fe-22Mn-0.7C TWIP steel increases effectively with RAs higher than 60% in the range of 900~1200 ˚C when the Mn and C microsegregation ratio decrease to 0.9~1.1 and 0.75~1.3 respectively, together with the grain size reducing to 50 μm in diameter. Increasing the strain rate results in the increase of RAs of as cast TWIP steel, in which more uniform deformation and less microcrack in the matrix has been observed, along with deformation bands along the stretching direction. The inevitable solute microsegregation Mn and C, along with the microporosity in as cast structure result in the matrix homogeneity weakening of TWIP steel, leading to the deterioration of deformation continuity in the interdendritic zone. Dynamic recrystallization(DRX) retardation by high Mn alloying with extremely large grains is also a factor influencing the hot ductility of TWIP steel, although it improves the RA of as cast matrix in a very limited level. According to the stacking fault energy calculation, the low hot ductility of as cast TWIP steel with noteless necking behavior in the hot tensile test is not related to mechanical twining.
Keywords: Fe-Mn-C alloy; hot ductility; TWIP steel; matrix homogeneity; dynamic
recrystallization 1. Introduction Twinning induced plasticity(TWIP) steel has been regarded as one of the most potential structural material for automobile with the demand of safety and lightweight, attributed to its excellent combination of high strength and high 1
ductility[1,2]. It has been more than fifteen years since high Mn TWIP steel was first reported, however, the proportion of TWIP steel components in body-in-white is still very low. Not only the high alloying cost but also the insufficient understanding on smelting, casting and processing influence its application [3,4]. It is still of great value to develop a suitable technique to cast high Mn TWIP steel by conventional equipment with the advantage of investment saving. However, the sensitivity to surface cracking of TWIP steel is extremely high during casting and hot rolling [5, 6], leading to the increase of cost and decrease of yield. The high temperature tensile behavior of TWIP steel under as cast condition is the necessity to evaluate the crack sensitivity during continuous casting and hot processing [7]. In our previous work [8, 9], coarse solidification grains and serious solute microsegregation were observed in Fe-22Mn-0.7C TWIP steel, which deteriorated its processability and the final mechanical property. Besides, low solidification temperature, wide mushy zone spacing and high tendency to form interdendritic porosity have been also revealed by present authors [10, 11]. According to the experimental results by Bleck et al.[12], Kang et al. [13], and Salas-Reyes et al. [14], the reduction area (RA) of TWIP steel in static tensile test was lower than 40% within the temperature range between 700 ˚C and 1200 ˚C. However, in Hamada and Karjalainen’s[15] research, the hot ductility of Fe-Mn-C TWIP steel was appreciable with RA higher than 60%, which was better than that of 304 stainless steel within the temperature range of 1000~1200 ˚C. Also, the mechanism on the hot ductility of TWIP steel has not been widely agreed, for which grain boundary sliding, impure particles and matrix homogeneity are respectively employed in different literatures[12-15]. Consequently, more effort is needed to spend on the research of high temperature deformation behavior of TWIP steel. The aim of the present work is to get a clear understanding of the speciality of Fe-22Mn-0.7C TWIP steel during hot tensile process, and to investigate the hot ductility mechanism by analyzing the related factors including solute concentration, matrix homogeneity, grain size, facture morphology, phase transformation and strain rate. The relationship between hot ductility and dynamic recrystallization (DRX) of as cast TWIP steel has been discussed accordingly. 2. Experiment and methodology The Fe-22Mn-0.7C TWIP steel was produced by a 25 Kg vacuum medium frequency induction melting furnace, as well as the low carbon(LC) steel. The specimen of high Mn austenite stainless steel(SS) was cut from a continuous casting slab. The measured chemical compositions of these steels are shown in Table 1. The concentration of C and S was measured through infrared absorptiometric method after combustion in current oxygen, and Mn content was determined by perchloric acid oxidation trivalent manganese titrimetric method. In addition, inert gas carrier melting thermo-conductimetric method was employed for the measurement N concentration, while inert gas carrier melting infrared absorptiometric method was introduced for measuring O. The contents of other alloying elements (Si, Cr, Ni and P) were measured by ICP-OES (Inductively coupled plasma optical emission spectrometric) methodology. The specimen for tensile test with a cylindrical shape was cut from dendrite grains 2
zone, which was 120 mm in length and 10 mm in diameter with a thread at each end about 10 mm long. The sketch of specimen and sampling location are shown in Fig. 1. Gleeble 1500 thermomechanical simulator was employed for the measurement of high temperature deformation behavior. The heating and tensile-testing profile was established according to the matrix phase transformation behavior of the samples. For TWIP steel with single austenite matrix, specimens were heated to the temperature 50 ˚C higher than the pretested with the rate of 10 ˚C·s-1 and soaked for 2min, followed by a cooling operation to the test temperature by 1.67 ˚C·s-1 , then soaked for another 2min and stretched. For other steels, specimens were heated to 1350 ˚C by 10 ˚C·s-1, followed by a soaking process about 5min, then cooled to test temperature with the rate of 1.67˚C·s-1 and tensile-tested. The tensile strain rate was set as a constant rate of 10-3s-1. High strain rate (10-2s-1, 10-1s-1, 1 s-1) tests were conducted to investigate the hot deformation mechanism of TWIP steel. The detail of the tensile-testing profile can be found in our previous study [8, 9].
TWIP LC SS
Table 1 Chemical composition of investigated steels, wt.% C Si Mn P S Cr Ni T.O T.N 0.73 0.17 22.03 0.0056 0.0060 15 60 0.15 0.20 0.71 0.0038 0.0052 15 52 0.045 0.40 1.03 0.036 0.004 18.35 8.02 0.06
Fe Bal. Bal. Bal.
Fig.1 (a) Sketch of static tensile specimen, (b) Sampling location of TWIP steel and LC steel from ingot, and (c) Sampling location of stainless steel from slab. The fracture morphology of TWIP steel was observed by JSM-6480LV scanning electron microscopy (SEM), while the matrix microstructure was analyzed by Leica DMR optical microscopy (OM) after grinding and Nital etching. The microsegregation of as cast TWIP steel specimens was examined by JEOL JXA-8230 electron probe microanalyzer(EPMA), and the phase transformation of the tensile specimen before and after tensile test was determined by MAC-21 ultra-high-power X-ray diffractometer(XRD) with CuKα radiation at a rate of 5 degree·min-1. 3
In order to further investigate the hot ductility mechanism, hot forging operation was conducted on Fe-22Mn-0.7C steel to get finer grain size and better matrix homogeneity. The as cast ingots of TWIP steel with 120 mm in diameter and 250 mm (excluding hot-top) in height were heated to 1200 ˚C and soaked for 2 hours, then forged into a rectangular bar sized 30mm×80mm×1000mm. The finishing temperature was higher than 900 ˚C, and the hot forged sample was cooled to room temperature by the air. The grain morphology of hot forged sample at elevated temperatures was observed by VL2000DX high temperature confocal laser scanning microscope (HT-CLSM). The static tensile test and matrix homogeneity examination was carried out with the same method as mentioned above. 3. Results and discussion 3.1 Stress-strain behavior of TWIP steel True stress-strain curves of as cast TWIP steel, LC steel and as forged TWIP steel are shown in Fig. 2(a), (b) and (c). It can be seen that Fe-Mn-C TWIP steel exhibits more obvious strain hardening performance than LC steel, especially when the test temperature is below 1000 °C. The occurrence of DRX is observed with the fluctuation and multi-peak characteristic in the true stress-strain curves of as cast LC steel when the tensile temperature is above 900 °C, while it also happens in as forged TWIP samples when the temperature is higher than 1000°C. Nevertheless, it is absent in as cast TWIP steel throughout the whole temperature range. When the tensile temperature is no more than 1200°C, the yield strength and tensile strength of as cast TWIP steel are higher than that of LC steel. Taking Fig. 2(a) and (c) into comparison, the as forged TWIP shows higher yield strength and tensile strength than the as cast samples in the temperature range of 700~900°C. However, at higher temperatures the tensile strengths of as forged TWIP steel are closed to that of as cast ones, but with much longer tensile strains. 350
True stress, MPa
250
200
(b)
300
250
True stress, MPa
300
700℃ 800℃ 900℃ 1000℃ 1100℃ 1200℃ 1300℃
350
700℃ 800℃ 900℃ 1000℃ 1100℃ 1200℃ 1300℃
(a)
Peak point 150
100
50
200
Peak point 150
100
50
0
0
0.0
0.2
0.4
0.6
0.8
True strain
0.0
0.2
0.4
True strain
4
0.6
0.8
350
700 ℃ 800 ℃ 900 ℃ 1000 ℃ 1100 ℃ 1200 ℃ 1300 ℃
(c)
300
True stress, MPa
250 200
Peak point 150 100 50 0 0.0
0.2
0.4
0.6
0.8
Ture strain
Fig. 2 Ture stress-strain curves of different specimens (a) As cast TWIP steel (b) As cast LC steel, and (c) As forged TWIP steel 3.2 Hot ductility of TWIP steel Hot ductility curves of as cast TWIP steel, LC steel and SS steel are shown in Fig.3. The RAs of TWIP steel are very low with all measured values less than 40%. Although SS steel is in the same matrix phase with TWIP steel, the former presents much better hot ductility than the latter. LC steel shows the best hot ductility with highest RAs excluding the temperature range of 750~900°C. Mintz and Arrowsmith[16] experimentally investigated the hot ductility from numerous steels and proposed the critical value to estimate matrix crack sensitivity was about 40%, below which the cracking possibility would increase obviously. It is shown in Fig.3 that at high temperature above 1200°C, the RA of TWIP steel decreases as temperature increases, which is the first brittle zone for TWIP steel. RAs within the second brittle zone from 1200°C to 950°C are all below 40%, and it nearly levels off. The third brittle zone for TWIP steel is around 950~800°C, in which the weakest hot ductility is obtained. Temperature ranges for different brittle zones of the investigated steels are shown in Table 2. From 700°C to 1300°C high Mn TWIP steel shows the most serious brittle performance, while austenite stainless steel exhibits low RAs but acceptable hot ductility. Fig.4 shows the hot ductility curves of as forged and as cast TWIP steel, along with the experiment results from previous research. It is worthy to notice that RAs from 900°C to 1200 °C after hot forging treatment are nearly twice higher than that of as cast specimens. Bleck et al.[12] and Kang et al.[13] carried out hot tensile experiment of as cast TWIP steels with the composition of Fe-23Mn-0.6C and Fe-18Mn-0.6C respectively, and the strain rates were set as 2×10-3s-1 and 3×10-3s-1. It was revealed by them that RAs of TWIP steel from 700 °C to 1350 °C were rarely low, which were unexpectedly close to the values in the present work. Hamada and Karjalainen [15] made a research on the hot ductility of Fe-22Mn-0.6C TWIP steel within the temperature range of 700~1300°C, in which all RAs were higher than 60%. Since the specimen is under as rolled condition, the hot ductility measured by Hamada is comparable to the present result of as forged TWIP steel. Besides, the high RA of TWIP steel in Hamada’s experiment can also be explained by the high tensile strain rate (1s-1). It is already reported by us that the RA increases with strain 5
rate increasing, and they are in a logarithmic linear relationship when the strain rate are between 1×10-3s-1~1s-1[9]. A recent work on the hot ductility of as cast Fe-18Mn-0.6C TWIP steel by Han et al. [17] has been reported. Comparing to the hot ductility of the present TWIP from dendrite zone with a strain rate of 10-3s-1, RAs measured by Han et al. within the temperature range from 800°C to 1100°C are also much higher [17]. It is found that the specimens in Han’s experiment were sampled from equiaxed zone with the strain rate of 1s-1. The combination effect of these two factors leads to the high ductility of TWIP steel, even better than that of as forged and as rolled specimen[15]. It can be inferred that matrix homogeneity improvement and grain size refinement in the as forged Fe-22Mn-0.7C TWIP steel should be the explanation to the improvement hot ductility. In other words, the weak matrix homogeneity and large grains should be the factors lowering the hot ductility of the present as cast TWIP steel. Table 2 Temperature ranges of different brittle zone for various steels 1st brittle zone 2nd brittle zone 3rd brittle zone TWIP 1200~solidus 1200~950°C 950~800°C LC Above 1300°C 850~800°C SS Above 1250°C TWIP LC SS
100 90
Reduction ratio of area
80 70 60 50 40 30 20 10 0 700
800
900
1000
1100
1200
1300
Temperature, C
Fig.3 Hot ductility curves of investigated steels 90
-3 -1
TWIP as forged, 10 s -3 -1 TWIP as cast, 10 s -2 -1 TWIP as cast, 10 s -1 -1 TWIP as cast, 10 s -1 TWIP as cast, 1s -2 -1 TWIP as cast, 10 s -1 -1 TWIP as cast, 10 s -1 TWIP as cast, 1s -3 -1 23Mn-0.6C as cast, 210 s [12] -3 -1 18Mn-0.6C as cast, 310 s [13] -1 22Mn-0.6C as rolled, 1s [15] -1 18Mn-0.6C as cast, 5s [17]
80
Reduction ration of area,%
70 60 50 40 30 20 10 0 700
800
900
1000
1100
1200
1300
1400
Temperature,C
Fig. 4 Hot ductility curves of TWIP steel under different conditions, together with the results from references[12],[13],[15] and [17] 3.3 Matrix phase of TWIP steel 6
Fig. 5 shows the XRD result of TWIP steel sample before and after tensile test at 900°C. It is seen that the matrix phase of Fe-22Mn-0.7C TWIP steel is always austenite independent of processing and testing condition. No obvious difference in the diffraction pattern has been observed from samples before and after tensile-testing. It should be paid more attention that the characteristic of diffraction peak is quite different for the samples from different preparing condition. By comparing Fig.5 (a), (b) and (c), (d) it can be found that the width of each peak reduces remarkably after hot forging operation. At the same time, the height of γ(220) peak increases significantly while that of γ(311) peak descends considerably. The decrease of peak width represents the grains refinement during the hot working process, while the height change indicates the rotating of grain crystal orientation in the matrix. It is obvious that a part of as cast grains with (311) crystal orientation rotates to (220) orientation during forging. It is well known that the onset of ferrite films along austenite grain boundaries is the widely accepted mechanism to explain the hot ductility loss within the temperature range of austenite decomposition. The austenite stability may decrease when the solute microsegregation is quite serious. In a recent report it is found the matrix of as cast Fe-22Mn-3Si-3Al TWIP steel is a mixture of austenite and ferrite due to the uneven solute distribution[18]. However, in the present research the high carbon alloying in TWIP steel stabilizes the austenite quite well with no ferrite or martensite detected. It can be drawn that phase transformation should not be the explanation for the low hot ductility of Fe-22Mn-0.7C TWIP steel in this temperature range. (a)
(111)
(111)
(b)
Intensity /a.u.
Intensity/a.u.
(311)
(100) (311)
(100)
(220) (220)
40
50
60
70
80
40
90
50
60
(c)
80
90
(100)
(d)
(111)
Intensity/a.u.
(111)
Intensity/a.u.
70
2/()
2/()
(220)
(220)
(100)
(311)
(311)
40
50
60
70
80
40
90
50
60
70
80
90
2
2
Fig. 5 X-ray diffraction results of TWIP steel. (a)as cast sample before test, (b)as cast sample after test, (c)as forged sample before test, and (d)as forged sample after test. 7
3.4 Matrix homogeneity and grain size of TWIP steel Fig. 6 shows the solutes distribution of TWIP steel samples from as cast and as forged conditions. The area-scanning results of Mn distribution in the matrix by EPMA are presented in Fig. 6(a) and (b), along with the corresponding line-scanning results of Mn and C shown in Fig. 6(c)~(f). It is shown that the matrix of as cast TWIP steel is inhomogeneous with obvious Mn segregation in the interdendritic zone, and it becomes much more uniform in the as forged TWIP steel sample, although the dendrite structure can be observed illegibly. Besides, in the interdendritic zone in as cast sample where solute microsegregation is serious, the microporosity can also be observed, as marked by white circles in Fig.6(a). However, such a corresponding relationship between microsegregation and microporosity is not very obvious in as forged TWIP steel. According to quantitative results in Fig. 6(c)~(f), it is clear that the C solute distributes in accordance with Mn in both as cast and as forged TWIP steel samples. The microsegregation ratio in Fig. 6 is defined as the local solute concentration to the average solute concentration. For the present Fe-22Mn-0.7C TWIP steel, Mn segregation ratio is between 0.8 and 1.3 in as cast matrix, while it reduces to 0.9~1.1 after hot forging. At the same time, C segregation ratio decreases from 0.6~1.5 under as cast condition to 0.75~1.3 after hot working treatment. The standard deviation(StDev) is employed to describe the dispersion level of solute concentration for the line-scanning results. The StDev of Mn concentration in the as forged sample is less than half of that from as cast sample, indicating the increase of solute distribution homogeneity. However, the StDev of C concentration from the as forged TWIP steel reduces just by one third compared to that of the as cast sample, implying the ineffective improvement of C diffusion. Actually, the thermal diffusion ability of C atoms should be higher than that of Mn with the consideration of atom size. It seems that the experimental results in the present research contradict with the previous theory. In fact, it is the interaction effect of solute atoms that explains the above inconsistency. There are two factors influencing the thermal diffusion behavior of C atoms from Mn. On one hand, Mn addition increases the diffusion activated energy due to the increase of lattice distortion[19]; on the other hand, Mn alloying decreases the activity of C atoms by forming the Mn-C atoms pair[20]. Fig. 7 shows the ten possible octahedral structures in Fe-Mn-C austenite matrix. According to the statistical analysis, when Mn concentration is lower than 33at%, the number of Mn atoms around C atom in the octahedron should be less than two, as shown in Fig.7(a)~(d). However, the stability of FeMnC cluster increases as the number of Mn in the octahedron decreases, according to the first principle calculation results by Von Appen and Dronskowski[21]. Accordingly, Mn atoms are readily to occupy most of the positions around C atoms to form the stable octahedral structure, as shown in Fig.7(h)~(j). That is also in good agreement with Kirkaldy and Brown’s[20] conclusion that Mn lowered the activity of C in austenite and stimulated C diffusion to high-Mn areas. This leads to an interactive effect of Mn and C on the microsegregation stability in the interdendritic zone, as shown in Fig. 6(e) and (f). 8
(a) m
(b) n
StDev=0.1322
2.0
(c)
Carbon microsegregation ratio
Manganese microsegregation ratio
2.0
1.5
1.0
0.5
0.0
StDev=0.1504
(d) 1.5
1.0
0.5
0.0
0
100
200
300
400
500
0
100
300
2.0
StDev=0.0616
(e)
400
500
StDev=0.1151
(f) Carbon microsegregation ratio
Manganese microsegregation ratio
2.0
200
Distance, um
Distance, um
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0 0
100
200
300
400
500
0
Distance, um
100
200
300
400
500
Distance, um
Fig. 6 Solutes distribution of TWIP steel. (a)Mn distribution in as cast matrix, (b)Mn distribution in as forged matrix, (c)Mn microsegregation ratio along line m in (a), (d)C microsegregation ratio along line m in (a), (e)Mn microsegregation ratio along line n 9
in (b), (f)C microsegregation ratio along line n in (b) The relationship between hot ductility and solute alloying concentration of TWIP steel has been barely reported. In our previous paper[9], it is inferred that high Mn alloying should take a promoting effect on the development of dendrite structure in steels, which has been proved by the experimental results from Yang et al.[22]. It is widely known that the coarser the steel matrix, the weaker the hot ductility. Consequently, the inevitable solute microsegregation, together with the microporosity in the TWIP steel should be the most predominant factor influencing hot ductility. (a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Fig. 7 Octahedral strcture in Fe-Mn-C austenite matrix.(a)Fe6C, (b)Fe5MnC, (c)Fe4Mn2C-cis, (d)Fe4Mn2C-trans, (e)Fe3Mn3C-fac, (f)Fe3Mn3C-mer, (g)Fe2Mn4C-cis, (h)Fe2Mn4C-trans, (i)FeMn5C, (j)Mn6C (a)
(b)
Fig. 8 Grain morphology of TWIP steel. (a) As cast(by OM), (b) as forged(by HT-CLSM) Fig. 8 shows the grain morphology of TWIP steel under as cast and as forged conditions, which is traced manually with drawing software. OM and HT-CLSM are employed to observe the as cast and as forged samples under room temperature and 900 ˚C respectively. It can be seen that the as cast grains are in elongated shape while the as forged grains are almost equiaxed. The grain size of TWIP steel from as 10
cast matrix is larger than 500μm in minor axis direction, while that of as forged samples is around 50μm in diameter. According to the opinion of Sah et al.[23], the kinetics of DRX is accelerated by decrease in grain size for nickel alloys. Wray[24], Crowther and Mintz[25] carried out series of experiments to investigate the effect of initial grain size on DRX of steel austenite grains. It is proposed by them that earlier onset and faster rate of DRX can be promoted by a finer grain structure. Fernandez et al.[26] and Wang et al.[27] experimentally studied the DRX behavior of Nb/Nb-Ti microalloyed steels and Mn-Cr gear steel respectively, in which they interpreted that DRX was more apt to take place in fine austenite grains structure. The grain refinement should also take some positive effect on the hot ductility improvement of as forged TWIP steel. 3.5 Fracture morphology of TWIP steel Fracture morphologies of TWIP steel under different tensile conditions are shown in Fig. 9. It is seen that at each temperature TWIP steel samples present brittle fracture with interdendritic cracks at the strain rate of 10-3 s-1. The facture morphology of samples tensile-tested at 900 ˚C is more brittle than that from 1100 ˚C, which can be confirmed by the presence of flat dendrite boundaries and lamellar interdendritic layers, as marked by the white trace in Fig.9(a). As tensile temperature increases to 1100 ˚C, the lamellar layers in the interdendritic zone disappear, in place by unsmooth interdendritic grain boundaries, as marked in Fig.9(c). The appearance of lamellar layers at 900 ˚C should be in relation to the solute microsegregation, which dissolves into the adjacent dendrites at 1100 ˚C. Hot ductility improvement induced by matrix homogeneity increase has already been proved by the RA comparison of as cast and as forged samples, in which grain refinement impact should be also taken into consideration. As the tensile strain rate increases, the fracture morphology becomes ductile with high proportion of dimples, although the interdendritic cracks are still observed, as shown in Fig.9(e) and (f). Under this situation the RA at 950 ˚C increases from 31.9% at 10-3 s-1 to 55.2% at 10-1 s-1, which is well agreed with the fracture characteristic. The tensile facture of as forged TWIP steel is full of dimples with no interdendritic crack, as shown in Fig.(g) and (h). Almost no inclusion has been found in the origin of the dimple, suggesting the steel purity control is acceptable. (a)
(b) R.A.=22.0%
11
(c)
(d) R.A.=32.4%
(e)
(f) R.A.=55.2%
(g)
(h) R.A.=67.4%
Fig. 9 Fracture morphology of TWIP steel at different conditions. (a),(b)Temperature: 900 ˚C, as cast, 10-3 s-1; (c),(d) Temperature: 1100 ˚C, as cast, 10-3 s-1; (e),(f) Temperature: 950 ˚C, as cast, 10-1 s-1; (g),(h)Temperature: 1200 ˚C, as forged, 10-3 s-1 (a) (b)
Stretching (c)
(d)
Fig. 10 Cracks initiation and propagation in as cast TWIP steel.(a) Temperature: 850 ˚C, 12
strain rate: 10-3s-1,(b) Temperature: 1150 ˚C, strain rate: 10-3s-1, (c) Temperature: 1200 ˚C, strain rate: 10-2s-1,(d)Temperature: 1200 ˚C, strain rate: 1s-1 Fig. 10 shows cracks initiation and propagation in as cast TWIP steel during tensile test. In most of the cases, crack initiates at dendrite boundaries, and then propagates along dendrite growth direction, as shown in Fig. 10(a) and (b). This leads to the interdendritic brittle fracture of as cast TWIP steel, as observed in Fig. 9(a)~(d). There are also some cracks initiating from the sample surface, and then spreading into the matrix inside. It is noteworthy that plenty of microcrack and microporosity are observed in the dendrite boundaries area near the fracture, as marked by the white circles in Fig.10. That means crack can easily initiate and propagate along dendrite boundary, where solidification defects locate and the matrix homogeneity is the weakest. When the interdendritic solute microsegregation and microporosity is serious enough, crack can link each other quickly. As tensile strain rate increases, the nearby microcrack becomes blunted, as shown in Fig.10(c) and (d). The sample matrix from the strain rate of 1s-1 seems more uniform with obvious deformation bands along the stretching direction. Actually, the ductile fracture at higher strain rate results from the prevention of crack growth along dendrite boundaries within shorter deformation time, leading to the accordant deformation of dendrite and interdendritic zone. DRX is a matrix softening behavior which plays a beneficial role in the improvement of hot ductility. However, no dynamically recrystallized grain is observed in Fig.10, even under the low strain rate high temperature conditions. 4. Mechanism of Hot ductility loss in TWIP steel The hot ductility of as cast TWIP steel is not quite appreciable according to the present research, which is also coincident with experimental results from Bleck et al.[12], Kang et al.[13] and Salas-Reyes et al.[14]. However, the related mechanism has not been discussed clearly yet. There are three temperature ranges for the brittle behavior of TWIP steel, as presented in Table 2. The first brittle zone between 1200 ˚C and solidus is due to the weakening of interdendritic area by the prior melting of solute microsegregation layers. The second brittle zone of TWIP steel is from 950 ˚C to 1200 ˚C, in which RAs are nearly a constant. The hot ductility increases with the increase of strain rate in this range, suggesting the low RA is not induced by the formation of (Fe,Mn)(O,S) as in other steels. Poor matrix homogeneity in as cast TWIP steel with serious microsegregation and microporosity is the predominant explanation, while DRX retarding by high Mn high C alloying with extremely large grains should be the following reason. This will be discussed in the next paragraphs in detail. The third brittle zone for the present TWIP steel is within the temperature of 800~950 ˚C, where the lowest RAs exist. According to XRD results, only austenite has been detected in the matrix. The widely accepted mechanism for the third brittle zone of common steels by the formation of ferrite film and carbonitride along grain boundary cannot be employed here. By comparing the facture morphology from 900 ˚C and 1100˚C, it is found that the undissolved segregated layers in the interdendritic zone weakens the deformation continuity of the matrix, leading to the early fracture with low RA. 13
In order to elucidate the deformation mechanism of Fe-22Mn-0.7C TWIP steel in the second brittle zone, factors influencing steel hot ductility, such as solute concentration, matrix homogeneity, grain size, phase transformation and mechanical twinning, will be discussed further here. TWIP steel is the one of the newly developed advanced high strength steels with extremely high manganese addition, and the relationship between the low hot ductility and high solute alloying has not been widely discussed. Fig. 11 shows the correlation of RA and Mn concentration from as cast steels with a strain rate in the order of 10-3 s-1. Table 3 presents the chemical composition of each sample from references. It can be seen that RA decreases linearly with Mn concentration increasing in the temperature range of 1000~1200 ˚C. The reasons for the loss of hot ductility of high Mn TWIP steels are quite diverse. Kang et al.[13] tentatively proposed that grain boundary sliding at high temperature result in the hot ductility deterioration of TWIP steel, but he did not explain it further. Hamada and Karjalainen[15], Salas-Reyes et al.[14] made similar experiments on as rolled and as cast TWIP steels respectively and they attributed the low hot ductility to grain boundary sliding based on EBSD graphs and fracture morphology. However, the sliding behavior in Hamada’s study occurred along austenite grains boundaries, while that in Mejia’s work happened in interdendritic area. Besides, Han et al.[17] studied the influence of Ni on the hot ductility of high Mn TWIP steel. Low melting point eutectic particles, randomly distributed MnS inclusions and DRX delay by high stacking fault energy are found to be the reasons for the RA drop of TWIP steel. Brune et al.[30] measured the RAs of low Mn and high Mn steel. It is demonstrated that the formation of complex precipitation is the main cause for the low hot ductility of Fe-18Mn-0.3C-0.8Al TWIP steel. Yang et al.[22] made an investigation on the hot ductility of steel samples with different Mn addition. He pointed out that the inadequate Al addition and excessive S alloying was the factors resulting in the low RA of TWIP steel. It can be drawn that more attention has been paid to the effect of Al/S/P/Ti/Nb/V alloying on the hot ductility of TWIP steel in previous analysis[14,30-36] other than that of Mn. It can be seen from Fig. 11 the RA of steel decreases as Mn concentration increases, which is nearly independent of other solute concentration. According to the comparison of hot tensile behavior from the three steel grades in the present work, the RA of high Mn TWIP steel is naturally lower than that of LC steel and SS steel. The inevitable solute microsegregation Mn and C(Fig. 6), along with the microporosity in as cast matrix(Fig.6 and Fig. 10) result in the homogeneity decrease of TWIP steel, leading to the ductility loss of the matrix, especially the dendrite boundaries zone. This conclusion also can be proved by Bleck et al.[12], in which the Fe-23Mn-0.6C steel presented lower RAs with a less uniform solidification structure compared to medium Mn and low Mn steel. In addition, interdendritic fracture in the literatures by Cagala et al.[37], Salas-Reyes et al. [14], Yang et al.[22] and Han et al.[17] is also the evidence for the ductility weakening of dendrite boundaries. Carbon is another solute element in the present TWIP steel, which shows limited effect on matrix hot ductility. There are two factors impacting the RA the austenite TWIP steel by C. On one hand, the higher the C in TWIP steel, the wider the mushy 14
zone during solidification. As a consequence, the microsegregation and microporosity in the interdendritic zone is more liable to form[38], resulting in the unsound structure with low cracking resistance. On the other hand, carbon is more apt to segregate along the dendrite boundary due to the low solute partition coefficient during solidification. There will be an interactive effect of Mn and C on the segregation stability when we take the formation of stable Mn6C octahedron into consideration[21]. It is obvious that high carbon alloying takes a negative effect on as cast hot ductility of TWIP steel, even though it is not dominant. Sulfur and phosphorus are regarded as detrimental elements on steel hot ductility. According to the investigation by Kang et al.[32] and Yang et al.[22], the formation of sulfide and phosphide compound with low melting point leads to the weakness of hot ductility. In a recent work by Lv et al.[39], the RA of high Mn steel started to decrease when the concentration of S and P is higher than 0.006 wt.% respectively. Considering the S and P level in the present TWIP steel, their detrimental effect can be neglected. Present, 1000C Kang et al., 1000C [13] Salas-Reyes et al., 1000C [14] Zeng et al., 1000C [28] Bleck et al., 1000C [12] Fan et al., 1000C [29] Yang et al., 1000C [22] Present, 1100C Salas-Reyes et al., 1100C [14] Zeng et al., 1100C [28] Bleck et al., 1100C [12] Fan et al., 1100C [29] Yang et al., 1100C [22] Present, 1200C Zeng et al., 1200C [28] Bleck et al., 1200C [12] Fan et al., 1200C [29] Yang et al., 1200C [22]
100
80
RA, %
60
40
20
0 0
5
10
15
20
25
Mn concentration, wt%
Fig. 11 Correlation of RA and Mn concentration from as cast steels Table 3 Chemical composition of as cast steels from references, wt.% C Mn Si P S N Al Reference 1 0.61 18.07 0.009 0.006 0.013 0.047 Kang et al.[13] 2 0.61 21.91 0.01 0.0066 0.016 Kang et al.[13] 3 0.56 21.0 1.3 0.012 1.6 Salas-Reyes et al.[14] 4 0.111 4.87 0.03 0.011 0.002 0.0045 0.007 Zeng et al.[28] 5 0.55 23.1 0.26 0.005 0.0003 0.0084 Bleck et al.[12] 6 0.84 15.7 0.27 0.007 0.003 0.0095 Bleck et al.[12] 7 0.86 9.3 0.27 0.012 0.0059 0.0061 Bleck et al.[12] 8 0.17 1.42 0.36 0.033 0.012 Fan et al.[29] 9 0.57 17.07 0.007 0.0005 0.0043 2.1 Yang et al.[22] 10 0.59 8.68 0.007 0.0005 0.0038 2.1 Yang et al.[22] 11 0.67 0.028 0.007 0.0005 0.0029 2.1 Yang et al.[22] DRX is another vital factor that affects the hot ductility of steel. The fluctuation and 15
multi-peak in stress-strain curves(Fig. 2) of as cast LC steel and as forged TWIP steel is the characteristic of DRX. However, DRX is absent in the as cast TWIP steel, for which no evidence in the stress-strain behavior or matrix microstructure is found. Both steel composition and grain size show important effects on the DRX. Wary experimentally examined DRX in steels alloyed with 0~9.5 wt.% Mn and found high Mn addition retarded DRX in tensile test[24]. Cabanas et al. investigated the hot deformation behavior of Fe-Mn binary alloys containing 1~20 wt.% Mn[40]. They pointed out that the activation energy for hot working increased with Mn content increasing, and high Mn alloying delayed DRX. Hamada et al.[41] and Han et al.[17] studied the hot deformation behavior of TWIP steel respectively, and both of them regarded Mn as a DRX delay element. On the opposite, in the experiment by Wietbrock et al.[42] Fe-28Mn-0.3C presented the earlier onset of DRX than Fe-23Mn-0.3C and Fe-28Mn-0.6C TWIP steel. They proposed that increased Mn content promoted DRX of TWIP steel. According to the result in the present work, DRX is retarded by high Mn alloying without the consideration of grain size difference, which can be seen clearly in Fig. 2 and Fig. 10. Besides, carbon addition in TWIP steel also influences the hot ductility. Wary[24] made a comparison between the onset strain of DRX of the steels with 0.051~0.71 C alloying. The weakest retardation effect on DRX of C was found compared to Ni, Mn, Si, and P in steels. Crowther and Mintz[43] measured the hot ductility of steels containing 0.04~0.65 wt.% C. He suggested the activation energy of DRX increased with C content increasing. As a result, DRX in the present Fe-22Mn-0.7C TWIP steel is thoroughly suppressed by the high alloying of Mn and C. Taking the microsegregation of Mn and C into account, the possibility of DRX becomes even lower. For TWIP steel with no phase transformation, the dendrite interface is coherent with austenite grains boundary[44]. It is well accepted that the most probable location for the occurrence of DRX is grain boundary, however, under as cast condition the segregated Mn and C in interdendritic boundaries retard DRX more strongly. Grain size is also an important parameter in analyzing DRX behavior in steels. It is illustrated by Wary[24] that a fine-grain structure can promote the early onset and fast rate of DRX in steel. Sah et al.[23], Crowther and Mintz[25], and Fernandez et al.[26] also obtained the similar conclusion based on their own experiments. The grain refinement of TWIP steel by hot forging gives rise to higher RA than that from as cast TWIP steel with coarser grains. Nevertheless, grain size is not the most influential factors for hot ductility of TWIP steel. DRX occurred at 900 ˚C in the tensile test of LC steel under as cast condition, in which the grain size should be in the same order of as cast TWIP steel. Consequently, the composition speciality of TWIP steel is more influential on retardation of DRX. It is noteworthy that DRX just partly occurs in as cast TWIP steel during hot deformation in the reported literatures. Moreover, it improves the hot ductility of the as cast matrix in a very limited level. In the present and Kang et al.’s[13] study, no dynamically recrystallized grain has been observed in the as cast matrix. Han et al.[17] examined the grain morphology of TWIP steel after hot tensile test, they found DRX only occurred just in a few grains near the fracture. DRX fraction was measured in 16
the experiment by Salas-Reyes et al.[14]. For metal mold cast non-microalloy TWIP steel, Nb-TWIP steel and Mo–TWIP steel, DRX fraction was 5%, 0 and 2.5%, and the RA was 60%, 60% and 65% respectively. At the meanwhile, the RA from sand mold cast nan-microalloy TWIP steel, Nb-TWIP steel and Mo-TWIP steel was 40%, 70% and 60%, and the corresponding DRX faction was 1%, 0 and 0.5% respectively. It can be seen that the sample with a lower DRX fraction can also give a higher RA, and vice versa. As a result, the hot ductility of as cast TWIP steel is not mainly dependent on DRX. 200
Fe-20Mn-0.4C Present Fe-22mn-0.7C Present Fe-30Mn-1.0C Present Fe-30Mn-0.2C-2.5Al-0.6Si Baradaran[46] Fe-22Mn-0.6C Dumay[47] Fe-22Mn-0.6C Allain[48]
SFE, mJm
-2
150
100
50
Dislocation slip Twining -martensite Solute concentration range
0 0
100
200
300
400
500
600
Temperature, C
Fig. 12 SFE variation against temperature in TWIP steels Twinning behavior is the most distinctive characteristic of TWIP steel, which makes sample with more uniform deformation and more delayed necking. Stacking fault energy(SFE) has been regarded as the most widely used parameter to determine the occurrence of mechanical twinning. A two sublattice model has been employed to calculate the SFE in TWIP steel in previous work[10]. Fig. 12 shows the SFE variation against temperature in different TWIP steels, together with the results from Baradaran et al.[45], Dumay et al.[46] and Allain et al.[47]. The SFE predicted by present model increases monotonously with temperature increasing, which agrees well with that from the previous research. When the temperature is above 300˚C, the thermodynamic database from SGTE is not reliable. Even so, it can be estimated that the higher the temperature, the larger the SFE. According to the investigation by Allain et al.[47] and Hamada et al.[41], phase transformation will be the toughening mechanism when SFE is lower than 18 mJ·mol-1 in high Mn steel, while twinning and partial dislocation slipping will occur when SFE is within the range of 18~60 mJ·mol-1. For the system with higher SFE perfect dislocation slipping will be the main role for the plasticity. The SFE for present TWIP steel with 20~30 wt.% Mn and 0.4~1.7 wt.% C(Concentration fluctuation is induced by microsegregation) in the as cast matrix will exceed the SFE range for twinning when the temperature is above 300 ˚C. Therefore, the noteless necking behavior of as cast TWIP steel in the tensile test is not related to mechanical twining. Jung et al.[48] measured the high temperature twinning behavior of Fe-18Mn-0.6C TWIP steel by XRD methodology. They found the twinning fraction was higher than 30% at room temperature and lower than 15% at 300 ˚C. Asghari et al.[49] observed the grain morphology of Fe-18Mn-2Si-2Al TWIP steel after hot compression. It was reported the twinning fraction decreased dramatically 17
when the temperature was higher than 300 ˚C. Koyama et al.[50] examined the twinning density of tensile-tested Fe-18Mn-1.15C TWIP steel by EBSD technology, which was nearly zero at 200 ˚C. It can be concluded that the deformation mechanism of the present TWIP steel at the concerned temperature range should be not significantly affected by twinning behavior. 5. Conclusions The high temperature tensile behavior of Fe-22Mn-0.7C TWIP steel has been experimentally investigated, and the related factors influencing the deformation mechanism, such as solute concentration, matrix homogeneity, grain size, facture morphology, phase transformation and strain rate, have been discussed. The conclusions can be drawn as follows: 1) TWIP steel presents higher tensile strengths and lower RAs than LC steel from 700 ˚C to 1250 ˚C under as cast condition. There is just one peak in each stress-strain curve of TWIP steel, while multi-peak has been observed in that of LC steel above 900 ˚C. 2) The hot ductility of as cast Fe-22Mn-0.7C TWIP steel is not appreciable with most of RAs lower than 40% in the temperature range of 700~1250 ˚C. Interdendritic brittle cracks are observed in the fracture of as cast samples without any dimple. 3) The hot ductility of Fe-22Mn-0.7C TWIP steel increases effectively with RAs higher than 60% in the range of 900~1200 ˚C when the Mn and C microsegregation ratio decrease to 0.9~1.1 and 0.75~1.3 respectively, together with the grain size reducing to 50μm in diameter. 4) Increasing the strain rate results in the increase of RAs of as cast TWIP steel, in which more uniform deformation and less microcrack in the matrix has been observed, along with deformation bands along the stretching direction. 5) The inevitable solute microsegregation Mn and C, along with the microporosity in as cast structure result in the matrix homogeneity weakening of TWIP steel, leading to the deterioration of deformation continuity in the interdendritic zone. 6) DRX retardation by high Mn alloying with extremely large grains is also a factor influencing the hot ductility of TWIP steel, although it improves the RA of as cast matrix in a very limited level. 7) According to the SFE calculation, the low hot ductility of as cast TWIP steel with noteless necking behavior in the hot tensile test is not related to mechanical twining. Acknowledgement
The research is supported by Fundamental Research Funds for the Central Universities NO. FRF-TP-15-066A1 in P.R. China. The authors are sincerely grateful to the financial funding. Reference
[1] S.H. Kim, H. Kim, N.J. Kim, Brittle intermetallic compound makes ultrastrong low-density steel with large ductility, Nature 518(2015).77-79. [2] O. Grässel, G. Frommeyer, Effect of martensitic phase transformation and deformation twinning on mechanical properties of Fe–Mn–Si–AI steels, Mater. Sci. 18
Technol. 14(1998) 1213-1217. [3] O. Bouaziz, S. Allain, C. P. Scott, P. Cugy, D. Barbier, High manganese austenitic twinning induced plasticity steels: a review of the microstructure properties relationships, Curr. Opin. Solid St. M. 15(2011) 141-168. [4] B.C. De Cooman, O. Kwon, K.G. Chin. State-of-the-knowledge on TWIP steel, Mater. Sci. Technol. 28(2012) 513-527. [5] K.H. Moon, M.S. Park, S. Yoo, J.K. Park, J.W. Cho, G. Shin, Molten mold flux technology for continuous casting of the ULC and TWIP steel, in: F. Marquis, PRICM: 8 Pacific Rim International Congress on Advanced Materials and Processing. John Wiley & Sons Inc., 2013, pp. 735-745. [6] S.H. Wang, Z.Y. Liu, W.N. Zhang, G.D. Wang, J.L. Liu, G.F. Liang, Microstructure and mechanical property of strip in Fe-23Mn-3Si-3Al TWIP steel by twin roll casting, ISIJ Int. 49(2009) 1340-1346. [7] B. Mintz, D.N. Crowther, Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting, Int. Mater. Rev. 55(2010)168-196. [8] Y.C. Wang, P. Lan, Y. Li, J.Q. Zhang, Effect of Alloying Elements on Mechanical Behavior of Fe-Mn-C TWIP Steel, J. Mater. Eng. 43(2015) 30-38. [9] P. Lan, L. Song, C. Du, J.Q. Zhang, Analysis of solidification microstructure and hot ductility of Fe-22Mn-0.7C TWIP steel, Mater. Sci. Technol. 30(2014) 1297-1304. [10] P. Lan, Analysis of solidification characteristics and structure performance on TWIP steels for automotive, Doctoral dissertation, University of Science and Technology Beijing, 2015. [11] P. Lan, J.Q. Zhang, Thermophysical properties and solidification defects of Fe–22Mn–0.7C TWIP steel, Steel Res. Int., 87(2016)250-261. [12] W. Bleck, K. Phiu-on, C. Heering, G. Hirt, Hot workability of as-cast high manganese-high carbon steels, Steel Res. Int. 78(2007) 536-545. [13] S.E. Kang, A. Tuling, J.R. Banerjee, W.D. Gunawardana, B. Mintz, Hot ductility of TWIP steels, Mater. Sci. Technol. 27(2011) 95-100. [14] A.E. Salas-Reyes, I. Mejía, A. Bedolla-Jacuinde, A. Boulaajaj, J. Calvo, J.M. Cabrera, Hot ductility behavior of high-Mn austenitic Fe–22Mn–1.5 Al–1.5 Si–0.45 C TWIP steels microalloyed with Ti and V, Mater. Sci. Eng. 611A (2014) 77-89. [15] A.S. Hamada, L.P. Karjalainen, Hot ductility behaviour of high-Mn TWIP steels, Mater. Sci. Eng. 528A(2011)1819-1827. [16] B. Mintz, J.M. Arrowsmith, Hot-ductility behaviour of C–Mn–Nb–Al steels and its relationship to crack propagation during the straightening of continuously cast strand, Metals Technol. 6(1979) 24-32. [17] K. Han, J. Yoo, B. Lee, I. Han, C. Lee, Effect of Ni on the hot ductility and hot cracking susceptibility of high Mn austenitic cast steel, Mater. Sci. Eng. 618A(2014) 295-304. [18] A. Ferraiuolo, A. Smith, J.G. Sevillano, F. de las Cuevas, P. Karjalainen, H. Gouveia, M. M. Rodrigues, Metallurgical Design of High-Strength Austenitic Fe-C-Mn Steels with Excellent Formability (Metaldesign), Publications Office of the European Union. [19] R. Zhu, Y. Lu, S. Li, Z. Chao, S. Wang, Effect of C-Mn atomic couples on austenitic high manganese steel, J. Iron Steel Res. 9(1997) 57-60. 19
[20] J.S. Kirkaldy, L.C. Brown, Diffusion behaviour in ternary, multiphase systems, Can. Metall. Quart. 2(1963)89-115. [21] J.V. Appen, R. Dronskowski, Carbon-Induced ordering in manganese-rich austenite-A density-functional total-energy and chemical-bonding study[J]. Steel Res. Int. 82(2011) 101–107. [22] J. Yang, Y.N. Wang, X.M. Ruan, R.Z. Wang, K. Zhu, Z.J. Fan, Y.C. Wang, C.B. Li, X.F. Wang, Effects of manganese content on solidification structures, thermal properties, and phase transformation characteristics in Fe-Mn-Al-C steels, Metall. Mater. Trans. 63B(2015) 1365-1375. [23] J.P. Sah, G.J. Richardson, C.M. Sellars, Grain-size effects during dynamic recrystallization of nickel, Metal Sci. 8(1974) 325-331. [24] P.J. Wray. Effect of composition and initial grain size on the dynamic recrystallization of austenite in plain carbon steels, Metall. Trans. 15A(1984) 2009-2019. [25] D.N. Crowther, B. Mintz, Influence of grain size on hot ductility of plain C–Mn steels, Mater. Sci. Technol. 2(1986) 951-955. [26] A. I. Fernández, P. Uranga, B. López, J.M. Rodriguez-Ibabe, Dynamic recrystallization behavior covering a wide austenite grain size range in Nb and Nb-Ti microalloyed steels, Mater. Sci. Eng. 361A(2003) 367-376. [27] B.X. Wang, X.H. Liu, G.D. Wang. Dynamic recrystallization behavior and microstructural evolution in a Mn–Cr gear steel, Mater. Sci. Eng. 393A(2005) 102-108. [28]Q. Zeng, B. Zhang, F. Wang, High temperature ductility of Q345 B and AH32 continuous casting slabs at different positions, J. Unv. Sci. Technol. Beijing 36(2014)183-188. [29] Y. Fan, M. Wang, H. Zhang, H. Tao, P. Zhao, S. Li, Hot plasticity and fracture mechanism of the third generation of automobile steel, J. Unv. Sci. Technol. Beijing 35(2013) 607-612. [30] T. Brune, D. Senk, R. Walpot, B. Steenken, Hot ductility behavior of boron containing microalloyed steels with varying manganese contents, Metall. Mater. Trans. 46B(2015) 1400-1408. [31] S.E. Kang, A. Tuling, I. Lau, J.R. Banerjee, B. Mintz, The hot ductility of Nb/V containing high Al, TWIP steels, Mater. Sci. Technol. 27(2011) 909-915. [32] S.E. Kang, J.R. Banerjee, B. Mintz, Influence of S and AlN on hot ductility of high Al, TWIP steels, Mater. Sci. Technol. 28(2012) 589-596. [33] S.E. Kang, J.R. Banerjee, E.M. Maina, B. Mintz, Influence of B and Ti on hot ductility of high Al and high Al, Nb containing TWIP steels, Mater. Sci. Technol. 29(2013) 1225-1232. [34] S.E. Kang, J.R. Banerjee, A. Tuling, B. Mintz, Influence of P and N on hot ductility of high Al, boron containing TWIP steels, Mater. Sci. Technol. 30(2014)1328-1335. [35] I. Mejía, A.E. Salas-Reyes, J. Calvo, J.M. Cabrera, Effect of Ti and B microadditions on the hot ductility behavior of a High-Mn austenitic Fe-23Mn-1.5 Al-1.3 Si-0.5 C TWIP steel, Mater. Sci. Eng. 648A(2015) 311-329.
20
[36] A.E. Salas-Reyes, I. Mejía, A. Bedolla-Jacuinde, A. Boulaajaj, J. Calvo, J.M. Cabrera, Hot ductility behavior of high-Mn austenitic Fe–22Mn–1.5 Al–1.5 Si–0.45 C TWIP steels microalloyed with Ti and V, Mater. Sci. Eng. 611A(2014) 77-89. [37] M. Cagala, E. Mazancová, P. Bábková, P. Lichý, P. Kozelský, Hot ductility of two high manganese steels, in: METAL 2011, Proc. 20th Int. Metall. Mater. Conf., 2011, pp. 486–491. [38] P. Lan, J.Q. Zhang, Numerical analysis of macrosegregation and shrinkage porosity in large steel ingot, Ironmak. Steelmak. 41(2014) 598-606. [39] B. Lv, F.C. Zhang, M. Li, L.H. Qian, T.S. Wang, Effects of phosphorus and sulfur on the thermoplasticity of high manganese austenitic steel, Mater. Sci. Eng. 527A(2010) 5648-5653. [40] N. Cabanas, J. Penning, N. Akdut, B.C. De Cooman, High-temperature deformation properties of austenitic Fe-Mn alloys, Metall. Mater. Trans. 37A(2006) 3305-3315. [41] A.S. Hamada, M.C. Somani, L.P. Karjalainen, High temperature flow stress and recrystallization behavior of high-Mn TWIP steels, ISIJ Int. 47(2007) 907-912. [42] B. Wietbrock, W. Xiong, M. Bambach, G. Hirt, Effect of temperature, strain rate, manganese and carbon content on flow behavior of three ternary Fe-Mn-C (Fe-Mn23-C0. 3, Fe-Mn23-C0. 6, Fe-Mn28-C0. 3) High-manganese steels, Steel Res. Int. 82(2011) 63-69. [43] D.N. Crowther, B. Mintz, Influence of carbon on hot ductility of steels, Mater. Sci. Technol. 2(1986) 671-676. [44] S. Tsuchiya, M. Ohno, K. Matsuura, Transition of solidification mode and the as-cast γ grain structure in hyperperitectic carbon steels, Acta Mater. 60(2012) 2927-2938. [45] A.H. Baradaran, A. Zarei-Hanzaki, H.R. Abedi, S.M. Fatemi-Varzaneh, A. Imandoust, The ductility behavior of a high-Mn twinning induced plasticity steel during cold-to-hot deformation, Mater. Sci. Eng. 561A(2013) 411-418. [46] A. Dumay, J.P. Chateau, S. Allain, S. Migot, O. Bouaziz, Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe-Mn-C steel, Mater. Sci. Eng. 483A(2008) 184-187. [47] S. Allain, J.P. Chateau, O. Bouaziz, S. Migot, N. Guelton, Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe-Mn-C alloys, Mater. Sci. Eng. 387A(2004) 158-162. [48] J.E. Jung, J. Park, J.S. Kim, J.B. Jeon, S.K. Kim, Y.W. Chang, Temperature effect on twin formation kinetics and deformation behavior of Fe-18Mn-0.6C TWIP steel, Met. Mater. Inter. 20(2014) 27-34. [49] A. Asghari, A. Zarei-Hanzaki, M. Eskandari, Temperature dependence of plastic deformation mechanisms in a modified transformation-twinning induced plasticity steel, Mater. Sci. Eng. 579A(2013) 150-156. [50] M. Koyama, T. Sawaguchi, K. Tsuzaki, TWIP effect and plastic instability condition in an Fe–Mn–C austenitic steel, ISIJ Int. 53(2013) 323-329. Figure captions: 21
Fig.1 (a) Sketch of static tensile specimen, (b) Sampling location of TWIP steel and LC steel from ingot, and (c) Sampling location of stainless steel from slab. Fig. 2 Ture stress-strain curves of different specimens (a) As cast TWIP steel (b) As cast LC steel, and (c) As forged TWIP steel Fig.3 Hot ductility curves of investigated steels Fig. 4 Hot ductility curves of TWIP steel under different conditions, together with the results from references[12],[13],[15] and [17] Fig. 5 X-ray diffraction results of TWIP steel. (a)as cast sample before test, (b)as cast sample after test, (c)as forged sample before test, and (d)as forged sample after test. Fig. 6 Solutes distribution of TWIP steel. (a)Mn distribution in as cast matrix, (b)Mn distribution in as forged matrix, (c)Mn microsegregation ratio along line m in (a), (d)C microsegregation ratio along line m in (a), (e)Mn microsegregation ratio along line n in (b), (f)C microsegregation ratio along line n in (b) Fig. 7 Octahedral strcture in Fe-Mn-C austenite matrix.(a)Fe6C, (b)Fe5MnC, (c)Fe4Mn2C-cis, (d)Fe4Mn2C-trans, (e)Fe3Mn3C-fac, (f)Fe3Mn3C-mer, (g)Fe2Mn4C-cis, (h)Fe2Mn4C-trans, (i)FeMn5C, (j)Mn6C Fig. 8 Grain morphology of TWIP steel. (a) As cast(by OM), (b) as forged(by HT-CLSM) Fig. 9 Fracture morphology of TWIP steel at different conditions. (a),(b)Temperature: 900 ˚C, as cast, 10-3 s-1; (c),(d) Temperature: 1100 ˚C, as cast, 10-3 s-1; (e),(f) Temperature: 950 ˚C, as cast, 10-1 s-1; (g),(h)Temperature: 1200 ˚C, as forged, 10-3 s-1 Fig. 10 Cracks initiation and propagation in as cast TWIP steel.(a) Temperature: 850 ˚C, strain rate: 10-3s-1,(b) Temperature: 1150 ˚C, strain rate: 10-3s-1, (c) Temperature: 1200 ˚C, strain rate: 10-2s-1,(d)Temperature: 1200 ˚C, strain rate: 1s-1 Fig. 11 Correlation of RA and Mn concentration from as cast steels Fig. 12 SFE variation against temperature in TWIP steels
22