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Influence of cooling paths on microstructural characteristics and precipitation behaviors in a low carbon V–Ti microalloyed steel
Materials Science & Engineering A 594 (2014) 389–393
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Influence of cooling paths on microstructural characteristics and precipitation behaviors in a low carbon V–Ti microalloyed steel Jun Chen, Meng-yang Lv, Shuai Tang, Zhen-yu Liu n, Guo-dong Wang The State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box 105, No. 11, Lane 3, Wenhua Road, HePing District, Shenyang 110819, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 25 September 2013 Accepted 29 September 2013 Available online 8 October 2013
Based on ultra fast cooling, the microstructural characteristics, precipitation behaviors and mechanical properties of a low-carbon V–Ti microalloyed steel were investigated in details using optical microscope, electron back-scattered diffraction and transmission electron microscope. The results show that the ferrite grains can be slightly refined, the sheet spacings of interphase precipitation can be also slightly reduced and the number fraction of ferrite grains with higher precipitation hardening can be significantly enhanced by increasing cooling rate (by comparisons of air cooling and furnace cooling), and a ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to produce hot rolled high strength steel with the tensile strength of 810 MPa, elongation of 24% and yield ratio of 0.82. While for furnace cooling after ultra fast cooling, its tensile strength, elongation and yield ratio is only 750 MPa, 22% and 0.84, respectively. The interphase precipitation in V–Ti microalloyed steel was observed, and these nanometer-sized carbides were detected as (V, Ti)C using energy dispersive X-ray spectroscopy spectra. In addition, the precipitation hardening was estimated as 313 MPa and 293 MPa for air cooling and furnace cooling after ultra fast cooling, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: V–Ti microalloyed steel Ultra fast cooling Interphase precipitation Mechanical properties
1. Introduction Microalloyed steels utilize chemical composition design of low carbon content and are microalloyed with niobium, vanadium and titanium, or other additions, such as molybdenum, boron, etc. The increase of strength is attributed to grain refinement, solid-solution strengthening, dislocation strengthening and precipitation hardening. Moreover, the precipitation hardening attracts more and more researchers' attentions. In 2004, Funakawa et al. developed Ti–Mo bearing high strength steels with tensile strengths of 780 MPa and excellent formability. The microstructure of these steels consists of ferritic matrix with nanometer-sized carbides, and the precipitation hardening due to these nanometer-sized carbides has been estimated as 300 MPa [1]. Since then, the mechanism of interphase precipitation and precipitation hardening have been further understood [2–8]. Moreover, The (Ti, Mo)C [1–5], VC [6], (Nb, Ti)C, (Nb, Ti, Mo)C [7], NbC, V(C, N), (V, Nb)C and (V, Cr)C [8] interphase precipitation has been observed. However, the interphase precipitation, the chemical composition of precipitates and precipitation hardening in V–Ti microalloyed steel have not been reported in details. Besides interphase precipitation behaviors in microalloyed steel, effects of chemical composition and thermomechanical parameters on transformation in microalloyed steels were also investigated [9–12]. For hot rolling practices, strain-
n
Corresponding author. Tel.: þ 86 24 8368 0571; fax: þ86 24 2390 6472. E-mail address: [email protected] (Z.-y. Liu).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.09.086
induced precipitation can hardly be avoided due to relatively longer rolling time, which significantly lowers precipitation hardening. But using ultra fast cooling can greatly suppress the amount of precipitation in austenite and increase supersaturation ratio in ferrite [13], and the precipitation hardening can be enhanced. So, at the condition of ultra fast cooling, it is of significance to understand effects of cooling paths on microstructure, precipitation behaviors and mechanical properties in V–Ti microalloyed steel. In the current study, the effects of cooling paths on microstructure, interphase precipitation behaviors and mechanical properties were investigated using optical microscope (OM), electron back-scattered diffraction (EBSD), transmission electron microscope (TEM), tensile testing and Vickers-microhardness testing. Microstructural characteristics and precipitation behaviors for two different cooling paths were clarified. The chemical composition of nanometer-sized carbides was detected using qualitative energy dispersive X-ray spectroscopy (EDXS). The amount of precipitation hardening was also estimated.
2. Experimental procedure The chemical composition of the tested steel is shown in Table 1. The alloy was prepared by vacuum melting and then cast into ingots, which were forged into 70 mm 70 mm 100 mm square billets. The square billets were hot rolled using two-high 450 mm experimental hot rolling mill followed by ultra fast cooling (UFC) system. The schematic representation of thermomechanical control
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J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
process (TMCP) is shown in Fig. 1. The billets were reheated to 1250 1C and held for 2 h to dissolve all of the carbides. After that, the recrystallization controlled rolling with the initial rolling temperature (Tin) of 1150 1C, the finishing temperature (Tout) of 1050 1C and the total reduction ratio (R) of 85% was performed. And the reduction schedule is 70 mm-52 mm-38 mm-28 mm22 mm-17 mm-13 mm-10 mm. After hot rolling, the hot rolled plates were cooled to room temperature using cooling path A or cooling path B. Specimens were cut from hot rolled plates and their surfaces along thickness direction and rolling direction were mechanically polished and then etched in 4% nital solution for the observation of OM (LEICA DMIRM). The specimens were also electropolished in a mixture of 12.5% perchloric acid and 87.5% absolute ethyl alcohol at 25 1C using voltage of 20 V for 20 s. The thin foils were prepared by mechanical abrasion at first and then twin-jet electropolished in a mixture of 9% perchloric acid and 91% absolute ethyl alcohol at 40 1C using voltage of 30 V. The thin foils were examined using FEG-TEM (FEI Tecnai G2 F20). Mechanical properties in rolling direction were tested using CMT-5105 electron universal testing machine controlled by computer. Standard round tensile samples with diameter of 8 mm, original gauge length of 40 mm and parallel length of 60 mm were tested at room temperature with a cross beam speed of 3 mm/min. Table 1 Chemical composition of the tested steel (wt%). Si
Mn
P
S
V
Ti
Al
N
0.06
0.31
1.31
0.005
0.003
0.06
0.11
0.06
0.0042
Temperature
C
Time Fig. 1. Schematic representation of thermomechanical control process.
The yield strength, tensile strength and elongation were all determined. In order to investigate precipitation hardening, The Vickers-microhardness within ferrite grains was tested using Vickers-durometer with a load of 25 g. The testing standard followed the guidelines of ISO 6507-1: 2005(E) [14].
3. Results and discussions 3.1. Microstructural characteristics and precipitation behaviors with two different cooling paths 3.1.1. Optical metallography The optical metallographs of tested steel with two different cooling paths are shown in Fig. 2. For cooling path A, the microstructure composed of ferrite and perlite can be observed. The white phase is allotriomorphic ferrite, and the dark phase is perlite, marked as white arrows in Fig. 2(a). Some fine ferrite grains in local zones can be observed, marked as white circles in Fig. 2(a). While for the cooling path B, only ferrite phase can be observed. Some fine ferrite grains in local zones can be also observed in Fig. 2(b), also marked as white circles in Fig. 2(b). However, the number of these fine grains for cooling path A is pronouncedly higher than that for cooling path B. 3.1.2. Electron back-scattered diffraction orientation maps The results of electron back-scattered diffraction (EBSD) analysis are presented in Fig. 3, showing the low-angle grain boundaries with misorientation ranging from 21 to 151 in white lines and high-angle grain boundaries with misorientation higher than 151 in black lines. The ferrite grain size distributions of tested steel are depicted in Fig. 4. It can be clearly seen that all ferrite grains show irregular shape and a great number of small ferrite grains can be observed, marked as black arrows in Fig. 3(a) and (b), but the number of fine ferrite grains (d ¼3–5 μm) for cooling path A is obviously higher than that for cooling path B, as shown in Fig. 4. This result is in better agreement with that of the observation of optical metallographs, indicating that using air cooling after ultra fast cooling can remain larger number of fine ferrite grains, however, the microstructural homogeneity for cooling path A is worse than that for cooling path B. In addition, the average ferrite grain size for cooling paths A and B was estimated as 9.5 μm and 10.9 μm, respectively, showing that the ferrite grains can be slightly refined by increasing cooling rate (by comparisons of air cooling and furnace cooling after ultra fast cooling). 3.1.3. Transmission electron microscopy It is well known that the parallel row-like characteristic of interphase precipitation can only be observed when the electron beam direction is paralleled to plans of row precipitates [3]. So we tilted thin foils to make the zone axis of plans of row precipitates
Perlite
20μm Fig. 2. Optical microstructure of tested steel with (a) cooling path A and (b) cooling path B.
20μm
J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
391
RD
RD 10μm
ND
10μm
ND
25
25
20
20
Rel. Frequency, %
Rel. Frequency, %
Fig. 3. Orientation maps of tested steel with (a) cooling path A and (b) cooling path B.
15 10 5 0
5
10
15
20
25
30
35
40
Ferrite grain size, µm
15 10 5 0
5
10
15
20
25
30
35
40
Ferrite grain size, µm
Fig. 4. Ferrite grain size distributions of tested steel with (a) cooling path A and (b) cooling path B.
along the electron beam direction, and the transmission electron micrographs and the representative energy dispersive X-ray spectroscopy (EDXS) spectra of fine carbides are presented in Fig. 5, showing that the interphase precipitation and random precipitation are both observed for two different cooling paths. However, the sheet spacings for cooling path A range from 22 nm to 25 nm, while for cooling path B, range from 28 nm to 35 nm, indicating that the sheet spacings of interphase precipitation can be refined by increasing cooling rate. The ferrite transformation for higher cooling rate takes place at lower temperature, as a result, the sheet spacings of interphase precipitation can be reduced. Moreover, Okamoto et al. [8] and Yen et al. [3] also indicated that lowering temperature can reduce sheet spacings of interphase precipitation. However, the interphase precipitation within some ferrite grains cannot be observed, as shown in Fig. 5(b). Furthermore, both interphase precipitation (Zone A in Fig. 5(d)) and random precipitation (Zone B in Fig. 5(d)) within a given ferrite grain can be also observed. In addition, Fig. 5(e) shows that these nanometer-sized particles are mainly (V, Ti)C.
paths. For cooling path A, The single-peak can be observed in Fig. 6 (a), while for cooling path B, The double-peak can be observed in Fig. 6(b). Moreover, the number fraction of ferrite grains with Vickers-microhardness higher than 260 HV for cooling path A is 77%, while for cooling path B, that is only 31%. Additionally, Campos et al. [15] indicated that the double-peak behavior represents two different grains, i.e., some grains with interphase precipitation showing a higher Vickers-microhardness peak and others without interphase precipitation showing a lower Vickersmicrohardness peak. However, based on aforementioned TEM observation, it can be seen that the interphase precipitation is present in some ferrite grains, and it occupies parts of a particular ferrite grain. Apart from interphase precipitation, fine random precipitates are observed. So the higher Vickers-microhardness within ferrite grains should be owed to precipitation hardening of fine interphase or random precipitation particles. From Fig. 6, it can be deduced that the number fraction of ferrite grains with a higher precipitation hardening can be significantly enhanced using a higher cooling rate (by comparisons of air cooling and furnace cooling after ultra fast cooling).
3.2. Mechanical properties with two different cooling paths 3.2.1. Tensile properties The mechanical properties including yield strength (YS), tensile strength (TS), elongation (A), yield ratio (YR) and strain hardening exponent (n) are given in Table 2, and the value, as shown in Table 2, is the average value of three parallel standard round tensile samples. Although the tested steel is ferrite microstructure with the average ferrite grain size of 10 μm, the higher strength can be gained owing to higher precipitation hardening. 3.2.2. Vickers-microhardness within ferrite grains Fig. 6 represents the Vickers-microhardness distributions from 100 ferrite grains of tested steel with two different cooling
3.2.3. Strengthening mechanism For ferritic steels, if the dislocations strengthening and precipitation hardening are not considered, their yield strength can be expressed as follows [3,16]: sy ¼ Δs0 þ ΔsSS þΔsGB ¼ 53:9 þ ð32:34½Mn þ 83:16½Si þ360:36½C þ 354:2½NÞ þ 17:402d
1=2
ð1Þ
where sy is yield strength in MPa, Δs0 is friction stress of pure iron in MPa, ΔsSS is solid-solution strengthening in MPa, ΔsGB is grain boundaries strengthening in MPa, [Mn], [Si], [C] and [N] are the average mass fraction of manganese, silicon, carbon and nitrogen in solution, respectively in %, d is ferrite grain diameter in mm. [Mn] and [Si] in ferrite matrix is 1.31 wt% and 0.31 wt%,
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~22.2nm ~23.0nm
~22.2nm
~25.1nm 100nm
~2 4.7nm
200nm
~31.2nm
~33.0nm ~34.2nm
Zone B
~28.1nm
Zone A
100nm
~31.2nm
200nm
Fig. 5. Transmission electron micrographs of thin foils with (a) and (b) cooling path A and (c) and (d) cooling path B and (e) representative EDXS spectra of fine carbides.
In addition, the dislocations strengthening can be estimated using as follows:
Table 2 Mechanical properties of tested steel processed by TMCP. Process
YS (MPa)
TS (MPa)
A (%)
YR
n
Cooling path A Cooling path B
664.3 635.3
813.7 753.3
24.0 22.4
0.82 0.84
0.134 0.133
ΔsDIS ¼ Mαμbρ1=2
ð4Þ
log f½Ti½Cgα ¼ 4:40 9575=T
ð2Þ
where M is Taylor factor of 2.75, α is constant of 0.435, μ is shear modulus of 80.3 GPa, b is Burgers vector of 0.248 nm, ρ is dislocation density using a value of 5 1013 m 2. The total yield strength can be expressed as Eq. (5), so the increment of precipitation hardening by nanometer-sized particles of (V, Ti)C can be calculated by subtracting friction stress of pure iron, solid-solution strengthening, grain boundaries strengthening and dislocation strengthening from total yield strength which was tested by tensile testing. The components of yield strength, i.e., Δs0, ΔsSS, ΔsGB, ΔsDIS and ΔsOROWAN, and total yield strength are presented in Table 3.
log f½V½Cgα ¼ 8:05 12265=T
ð3Þ
sy ¼ Δs0 þ ΔsSS þΔsGB þ ðΔsDIS 2 þ ΔsOROWAN 2 Þ1=2
respectively. Based on solubility product Eqs. (2) and (3) [17], free carbon content is estimated as 0.022 wt%. Furthermore, free nitrogen content should be 0 owing to nitrogen fixed by the formation of stable TiN. The ferrite grain diameter is 9.5 μm and 10.9 μm for cooling path A and cooling path B, respectively.
ð5Þ
J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
25
25
Higher than 260HV Rel. Frequency, %
Rel. Frequency, %
Higher than 260HV
20
20 15 10 5 0 230
393
240
250
260
270
280
290
300
15 10 5 0 200 210 220 230 240 250 260 270 280 290
Hv (25g)
Hv (25g)
Fig. 6. Vickers-microhardness distributions from 100 ferrite grains of tested steel with (a) cooling path A and (b) cooling path B.
Table 3 Components of yield strength for cooling path A and cooling path B. Process
Cooling path A
Cooling path B
Δs0 (MPa) ΔsSS (MPa) ΔsGB (MPa) ΔsDIS (MPa) ΔsOROWAN (MPa) sy (MPa)
53.9 76.1 178.5 168.5 313.4 664.3
53.9 76.1 166.7 168.5 293.7 635.3
4. Conclusions At the condition of ultra fast cooling, effects of cooling paths on microstructure, precipitation behaviors and mechanical properties of V–Ti microalloyed steel were investigated and the following conclusions were drawn: (1) The microstructure consisting of ferrite and a little perlite is observed at cooling path A, while for cooling path B, only ferrite microstructure can be observed. Moreover, the ferrite grains can be slightly refined, the sheet spacings of interphase precipitation can be also slightly reduced and the number fraction of ferrite grains with higher precipitation hardening can be significantly enhanced by increasing cooling rate (by comparisons of air cooling and furnace cooling after ultra fast cooling). (2) The higher tensile strength of 810 MPa for cooling path A is attained, while for cooling path B, its tensile strength is only 750 MPa. But they both show excellent plasticity and a lower yield ratio. (3) The precipitation hardening was estimated as 313 MPa and 293 MPa for cooling path A and cooling path B, respectively. (4) The nanometer-sized carbides were detected as (V, Ti)C particles based on analysis of qualitative energy dispersive X-ray spectroscopy (EDXS) spectra.
Acknowledgments This work is supported by the Fundamental Research Funds for the Central Universities (N110607003), the National Natural Science Foundation of China (51204049) and the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of PR China. References [1] Y. Funakawa, T. Shiozaki, K. Tomita, T. Yamamoto, E. Maeda, ISIJ International 44 (2004) 1945–1951. [2] H.W. Yen, C.Y. Huang, J.R. Yang, Scripta Materialia 61 (2009) 616–619. [3] H.W. Yen, P.Y. Chen, C.Y. Huang, J.R. Yang, Acta Materialia 59 (2011) 6264–6274. [4] S. Mukherjee, I.B. Timokhina, C. Zhu, S.P. Ringer, P.D. Hodgson, Acta Materialia 61 (2013) 2521–2530. [5] I.B. Timokhina, P.D. Hodgson, S.P. Ringer, R.K. Zheng, E.V. Pereloma, Scripta Materialia 56 (2007) 601–604. [6] Y.J. Zhang, G. Miyamoto, K. Shinbo, T. Furuhara, Scripta Materialia 69 (2013) 17–20. [7] J.H. Jang, Y.U. Heo, C.H. Lee, H.K.D.H. Bhadeshia, D.W. Suh, Materials Science & Technology 29 (2013) 309–313. [8] R. Okamoto, A. Borgenstam, J. Ågren, Acta Materialia 58 (2010) 4783–4790. [9] P. Cizek, B.P. Wynne, C.H.J. Davies, B.C. Muddle, P.D. Hodgson, Metallurgical and Materials Transactions A 33 (2002) 1331–1349. [10] P.A. Manohar, T. Chandra, C.R. Killmore, ISIJ International 38 (1996) 1486–1493. [11] B. Eghbali, A. Abdollah-Zadeh, Journal of Materials Processing Technology 180 (2006) 44–48. [12] X. Chun, Q. Sun, X. Chen, Materials & Design 28 (2007) 2523–2527. [13] J. Chen, S. Tang, Z.Y. Liu, G.D. Wang, Y.L. Zhou, Journal of Materials Science 47 (2012) 4640–4648. [14] ISO International Standard, Standard test method for Vickers-hardness test of metallic materials, ISO 6507-1, 2005(E). [15] S.S. Campos, E.V. Morales, H.J. Kestenbach, Materials Characterization 52 (2004) 379–384. [16] F.B. Pickering, Physical Metallurgy and the Design of Steels [M], Applied Science Publishing Ltd., London (1978) 1978; 63. [17] K.A. Taylor, Scripta Metallurgica et Materialia 32 (1995) 7–12.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Influence of cooling paths on microstructural characteristics and precipitation behaviors in a low carbon V–Ti microalloyed steel Jun Chen, Meng-yang Lv, Shuai Tang, Zhen-yu Liu n, Guo-dong Wang The State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box 105, No. 11, Lane 3, Wenhua Road, HePing District, Shenyang 110819, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 25 September 2013 Accepted 29 September 2013 Available online 8 October 2013
Based on ultra fast cooling, the microstructural characteristics, precipitation behaviors and mechanical properties of a low-carbon V–Ti microalloyed steel were investigated in details using optical microscope, electron back-scattered diffraction and transmission electron microscope. The results show that the ferrite grains can be slightly refined, the sheet spacings of interphase precipitation can be also slightly reduced and the number fraction of ferrite grains with higher precipitation hardening can be significantly enhanced by increasing cooling rate (by comparisons of air cooling and furnace cooling), and a ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to produce hot rolled high strength steel with the tensile strength of 810 MPa, elongation of 24% and yield ratio of 0.82. While for furnace cooling after ultra fast cooling, its tensile strength, elongation and yield ratio is only 750 MPa, 22% and 0.84, respectively. The interphase precipitation in V–Ti microalloyed steel was observed, and these nanometer-sized carbides were detected as (V, Ti)C using energy dispersive X-ray spectroscopy spectra. In addition, the precipitation hardening was estimated as 313 MPa and 293 MPa for air cooling and furnace cooling after ultra fast cooling, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: V–Ti microalloyed steel Ultra fast cooling Interphase precipitation Mechanical properties
1. Introduction Microalloyed steels utilize chemical composition design of low carbon content and are microalloyed with niobium, vanadium and titanium, or other additions, such as molybdenum, boron, etc. The increase of strength is attributed to grain refinement, solid-solution strengthening, dislocation strengthening and precipitation hardening. Moreover, the precipitation hardening attracts more and more researchers' attentions. In 2004, Funakawa et al. developed Ti–Mo bearing high strength steels with tensile strengths of 780 MPa and excellent formability. The microstructure of these steels consists of ferritic matrix with nanometer-sized carbides, and the precipitation hardening due to these nanometer-sized carbides has been estimated as 300 MPa [1]. Since then, the mechanism of interphase precipitation and precipitation hardening have been further understood [2–8]. Moreover, The (Ti, Mo)C [1–5], VC [6], (Nb, Ti)C, (Nb, Ti, Mo)C [7], NbC, V(C, N), (V, Nb)C and (V, Cr)C [8] interphase precipitation has been observed. However, the interphase precipitation, the chemical composition of precipitates and precipitation hardening in V–Ti microalloyed steel have not been reported in details. Besides interphase precipitation behaviors in microalloyed steel, effects of chemical composition and thermomechanical parameters on transformation in microalloyed steels were also investigated [9–12]. For hot rolling practices, strain-
n
Corresponding author. Tel.: þ 86 24 8368 0571; fax: þ86 24 2390 6472. E-mail address: [email protected] (Z.-y. Liu).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.09.086
induced precipitation can hardly be avoided due to relatively longer rolling time, which significantly lowers precipitation hardening. But using ultra fast cooling can greatly suppress the amount of precipitation in austenite and increase supersaturation ratio in ferrite [13], and the precipitation hardening can be enhanced. So, at the condition of ultra fast cooling, it is of significance to understand effects of cooling paths on microstructure, precipitation behaviors and mechanical properties in V–Ti microalloyed steel. In the current study, the effects of cooling paths on microstructure, interphase precipitation behaviors and mechanical properties were investigated using optical microscope (OM), electron back-scattered diffraction (EBSD), transmission electron microscope (TEM), tensile testing and Vickers-microhardness testing. Microstructural characteristics and precipitation behaviors for two different cooling paths were clarified. The chemical composition of nanometer-sized carbides was detected using qualitative energy dispersive X-ray spectroscopy (EDXS). The amount of precipitation hardening was also estimated.
2. Experimental procedure The chemical composition of the tested steel is shown in Table 1. The alloy was prepared by vacuum melting and then cast into ingots, which were forged into 70 mm 70 mm 100 mm square billets. The square billets were hot rolled using two-high 450 mm experimental hot rolling mill followed by ultra fast cooling (UFC) system. The schematic representation of thermomechanical control
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J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
process (TMCP) is shown in Fig. 1. The billets were reheated to 1250 1C and held for 2 h to dissolve all of the carbides. After that, the recrystallization controlled rolling with the initial rolling temperature (Tin) of 1150 1C, the finishing temperature (Tout) of 1050 1C and the total reduction ratio (R) of 85% was performed. And the reduction schedule is 70 mm-52 mm-38 mm-28 mm22 mm-17 mm-13 mm-10 mm. After hot rolling, the hot rolled plates were cooled to room temperature using cooling path A or cooling path B. Specimens were cut from hot rolled plates and their surfaces along thickness direction and rolling direction were mechanically polished and then etched in 4% nital solution for the observation of OM (LEICA DMIRM). The specimens were also electropolished in a mixture of 12.5% perchloric acid and 87.5% absolute ethyl alcohol at 25 1C using voltage of 20 V for 20 s. The thin foils were prepared by mechanical abrasion at first and then twin-jet electropolished in a mixture of 9% perchloric acid and 91% absolute ethyl alcohol at 40 1C using voltage of 30 V. The thin foils were examined using FEG-TEM (FEI Tecnai G2 F20). Mechanical properties in rolling direction were tested using CMT-5105 electron universal testing machine controlled by computer. Standard round tensile samples with diameter of 8 mm, original gauge length of 40 mm and parallel length of 60 mm were tested at room temperature with a cross beam speed of 3 mm/min. Table 1 Chemical composition of the tested steel (wt%). Si
Mn
P
S
V
Ti
Al
N
0.06
0.31
1.31
0.005
0.003
0.06
0.11
0.06
0.0042
Temperature
C
Time Fig. 1. Schematic representation of thermomechanical control process.
The yield strength, tensile strength and elongation were all determined. In order to investigate precipitation hardening, The Vickers-microhardness within ferrite grains was tested using Vickers-durometer with a load of 25 g. The testing standard followed the guidelines of ISO 6507-1: 2005(E) [14].
3. Results and discussions 3.1. Microstructural characteristics and precipitation behaviors with two different cooling paths 3.1.1. Optical metallography The optical metallographs of tested steel with two different cooling paths are shown in Fig. 2. For cooling path A, the microstructure composed of ferrite and perlite can be observed. The white phase is allotriomorphic ferrite, and the dark phase is perlite, marked as white arrows in Fig. 2(a). Some fine ferrite grains in local zones can be observed, marked as white circles in Fig. 2(a). While for the cooling path B, only ferrite phase can be observed. Some fine ferrite grains in local zones can be also observed in Fig. 2(b), also marked as white circles in Fig. 2(b). However, the number of these fine grains for cooling path A is pronouncedly higher than that for cooling path B. 3.1.2. Electron back-scattered diffraction orientation maps The results of electron back-scattered diffraction (EBSD) analysis are presented in Fig. 3, showing the low-angle grain boundaries with misorientation ranging from 21 to 151 in white lines and high-angle grain boundaries with misorientation higher than 151 in black lines. The ferrite grain size distributions of tested steel are depicted in Fig. 4. It can be clearly seen that all ferrite grains show irregular shape and a great number of small ferrite grains can be observed, marked as black arrows in Fig. 3(a) and (b), but the number of fine ferrite grains (d ¼3–5 μm) for cooling path A is obviously higher than that for cooling path B, as shown in Fig. 4. This result is in better agreement with that of the observation of optical metallographs, indicating that using air cooling after ultra fast cooling can remain larger number of fine ferrite grains, however, the microstructural homogeneity for cooling path A is worse than that for cooling path B. In addition, the average ferrite grain size for cooling paths A and B was estimated as 9.5 μm and 10.9 μm, respectively, showing that the ferrite grains can be slightly refined by increasing cooling rate (by comparisons of air cooling and furnace cooling after ultra fast cooling). 3.1.3. Transmission electron microscopy It is well known that the parallel row-like characteristic of interphase precipitation can only be observed when the electron beam direction is paralleled to plans of row precipitates [3]. So we tilted thin foils to make the zone axis of plans of row precipitates
Perlite
20μm Fig. 2. Optical microstructure of tested steel with (a) cooling path A and (b) cooling path B.
20μm
J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
391
RD
RD 10μm
ND
10μm
ND
25
25
20
20
Rel. Frequency, %
Rel. Frequency, %
Fig. 3. Orientation maps of tested steel with (a) cooling path A and (b) cooling path B.
15 10 5 0
5
10
15
20
25
30
35
40
Ferrite grain size, µm
15 10 5 0
5
10
15
20
25
30
35
40
Ferrite grain size, µm
Fig. 4. Ferrite grain size distributions of tested steel with (a) cooling path A and (b) cooling path B.
along the electron beam direction, and the transmission electron micrographs and the representative energy dispersive X-ray spectroscopy (EDXS) spectra of fine carbides are presented in Fig. 5, showing that the interphase precipitation and random precipitation are both observed for two different cooling paths. However, the sheet spacings for cooling path A range from 22 nm to 25 nm, while for cooling path B, range from 28 nm to 35 nm, indicating that the sheet spacings of interphase precipitation can be refined by increasing cooling rate. The ferrite transformation for higher cooling rate takes place at lower temperature, as a result, the sheet spacings of interphase precipitation can be reduced. Moreover, Okamoto et al. [8] and Yen et al. [3] also indicated that lowering temperature can reduce sheet spacings of interphase precipitation. However, the interphase precipitation within some ferrite grains cannot be observed, as shown in Fig. 5(b). Furthermore, both interphase precipitation (Zone A in Fig. 5(d)) and random precipitation (Zone B in Fig. 5(d)) within a given ferrite grain can be also observed. In addition, Fig. 5(e) shows that these nanometer-sized particles are mainly (V, Ti)C.
paths. For cooling path A, The single-peak can be observed in Fig. 6 (a), while for cooling path B, The double-peak can be observed in Fig. 6(b). Moreover, the number fraction of ferrite grains with Vickers-microhardness higher than 260 HV for cooling path A is 77%, while for cooling path B, that is only 31%. Additionally, Campos et al. [15] indicated that the double-peak behavior represents two different grains, i.e., some grains with interphase precipitation showing a higher Vickers-microhardness peak and others without interphase precipitation showing a lower Vickersmicrohardness peak. However, based on aforementioned TEM observation, it can be seen that the interphase precipitation is present in some ferrite grains, and it occupies parts of a particular ferrite grain. Apart from interphase precipitation, fine random precipitates are observed. So the higher Vickers-microhardness within ferrite grains should be owed to precipitation hardening of fine interphase or random precipitation particles. From Fig. 6, it can be deduced that the number fraction of ferrite grains with a higher precipitation hardening can be significantly enhanced using a higher cooling rate (by comparisons of air cooling and furnace cooling after ultra fast cooling).
3.2. Mechanical properties with two different cooling paths 3.2.1. Tensile properties The mechanical properties including yield strength (YS), tensile strength (TS), elongation (A), yield ratio (YR) and strain hardening exponent (n) are given in Table 2, and the value, as shown in Table 2, is the average value of three parallel standard round tensile samples. Although the tested steel is ferrite microstructure with the average ferrite grain size of 10 μm, the higher strength can be gained owing to higher precipitation hardening. 3.2.2. Vickers-microhardness within ferrite grains Fig. 6 represents the Vickers-microhardness distributions from 100 ferrite grains of tested steel with two different cooling
3.2.3. Strengthening mechanism For ferritic steels, if the dislocations strengthening and precipitation hardening are not considered, their yield strength can be expressed as follows [3,16]: sy ¼ Δs0 þ ΔsSS þΔsGB ¼ 53:9 þ ð32:34½Mn þ 83:16½Si þ360:36½C þ 354:2½NÞ þ 17:402d
1=2
ð1Þ
where sy is yield strength in MPa, Δs0 is friction stress of pure iron in MPa, ΔsSS is solid-solution strengthening in MPa, ΔsGB is grain boundaries strengthening in MPa, [Mn], [Si], [C] and [N] are the average mass fraction of manganese, silicon, carbon and nitrogen in solution, respectively in %, d is ferrite grain diameter in mm. [Mn] and [Si] in ferrite matrix is 1.31 wt% and 0.31 wt%,
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J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
~22.2nm ~23.0nm
~22.2nm
~25.1nm 100nm
~2 4.7nm
200nm
~31.2nm
~33.0nm ~34.2nm
Zone B
~28.1nm
Zone A
100nm
~31.2nm
200nm
Fig. 5. Transmission electron micrographs of thin foils with (a) and (b) cooling path A and (c) and (d) cooling path B and (e) representative EDXS spectra of fine carbides.
In addition, the dislocations strengthening can be estimated using as follows:
Table 2 Mechanical properties of tested steel processed by TMCP. Process
YS (MPa)
TS (MPa)
A (%)
YR
n
Cooling path A Cooling path B
664.3 635.3
813.7 753.3
24.0 22.4
0.82 0.84
0.134 0.133
ΔsDIS ¼ Mαμbρ1=2
ð4Þ
log f½Ti½Cgα ¼ 4:40 9575=T
ð2Þ
where M is Taylor factor of 2.75, α is constant of 0.435, μ is shear modulus of 80.3 GPa, b is Burgers vector of 0.248 nm, ρ is dislocation density using a value of 5 1013 m 2. The total yield strength can be expressed as Eq. (5), so the increment of precipitation hardening by nanometer-sized particles of (V, Ti)C can be calculated by subtracting friction stress of pure iron, solid-solution strengthening, grain boundaries strengthening and dislocation strengthening from total yield strength which was tested by tensile testing. The components of yield strength, i.e., Δs0, ΔsSS, ΔsGB, ΔsDIS and ΔsOROWAN, and total yield strength are presented in Table 3.
log f½V½Cgα ¼ 8:05 12265=T
ð3Þ
sy ¼ Δs0 þ ΔsSS þΔsGB þ ðΔsDIS 2 þ ΔsOROWAN 2 Þ1=2
respectively. Based on solubility product Eqs. (2) and (3) [17], free carbon content is estimated as 0.022 wt%. Furthermore, free nitrogen content should be 0 owing to nitrogen fixed by the formation of stable TiN. The ferrite grain diameter is 9.5 μm and 10.9 μm for cooling path A and cooling path B, respectively.
ð5Þ
J. Chen et al. / Materials Science & Engineering A 594 (2014) 389–393
25
25
Higher than 260HV Rel. Frequency, %
Rel. Frequency, %
Higher than 260HV
20
20 15 10 5 0 230
393
240
250
260
270
280
290
300
15 10 5 0 200 210 220 230 240 250 260 270 280 290
Hv (25g)
Hv (25g)
Fig. 6. Vickers-microhardness distributions from 100 ferrite grains of tested steel with (a) cooling path A and (b) cooling path B.
Table 3 Components of yield strength for cooling path A and cooling path B. Process
Cooling path A
Cooling path B
Δs0 (MPa) ΔsSS (MPa) ΔsGB (MPa) ΔsDIS (MPa) ΔsOROWAN (MPa) sy (MPa)
53.9 76.1 178.5 168.5 313.4 664.3
53.9 76.1 166.7 168.5 293.7 635.3
4. Conclusions At the condition of ultra fast cooling, effects of cooling paths on microstructure, precipitation behaviors and mechanical properties of V–Ti microalloyed steel were investigated and the following conclusions were drawn: (1) The microstructure consisting of ferrite and a little perlite is observed at cooling path A, while for cooling path B, only ferrite microstructure can be observed. Moreover, the ferrite grains can be slightly refined, the sheet spacings of interphase precipitation can be also slightly reduced and the number fraction of ferrite grains with higher precipitation hardening can be significantly enhanced by increasing cooling rate (by comparisons of air cooling and furnace cooling after ultra fast cooling). (2) The higher tensile strength of 810 MPa for cooling path A is attained, while for cooling path B, its tensile strength is only 750 MPa. But they both show excellent plasticity and a lower yield ratio. (3) The precipitation hardening was estimated as 313 MPa and 293 MPa for cooling path A and cooling path B, respectively. (4) The nanometer-sized carbides were detected as (V, Ti)C particles based on analysis of qualitative energy dispersive X-ray spectroscopy (EDXS) spectra.
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