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Intercritical annealing temperature dependence of hydrogen embrittlement behavior of cold-rolled Al-containing medium-Mn steel
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Intercritical annealing temperature dependence of hydrogen embrittlement behavior of cold-rolled Alcontaining medium-Mn steel Yongjian Zhang a,*, Chengwei Shao b, Jiaojiao Wang a, Xiaoli Zhao a, Weijun Hui a a
Materials Science and Engineering Research Center, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China b China Aero-Polytechnology Establishment, Beijing 100028, PR China
highlights
graphical abstract
Hydrogen embrittlement (HE) of Al-containing medium-Mn steel was evaluated. HE susceptibility increases with increasing intercritical annealing temperature. Both the stability and amount of austenite have important influence on the HE behavior.
article info
abstract
Article history:
The present investigation attempts to evaluate the influence of intercritical annealing
Received 5 May 2019
temperature (TIA) on the hydrogen embrittlement (HE) of a cold-rolled Al-containing
Received in revised form
medium-Mn steel (Fe-0.2C-4.88Mn-3.11Al-0.62Si) by using electrochemical hydrogen-
23 June 2019
charging, slow strain rate tensile test and scanning electron microscope. The results
Accepted 29 June 2019
show that an excellent combination of strength and ductility (the product of ultimate
Available online xxx
tensile strength and total elongation) up to ~53 GPa$% was obtained for the specimen intercritically annealed at an intermediate temperature of 730 C, whereas the HE index
Keywords:
increases significantly with an increase in TIA up to 850 C. Being different from the typical
Hydrogen embrittlement
dimple ductile fracture for the uncharged specimen, the hydrogen-charged specimen ex-
Medium-Mn steel
hibits a mixed brittle interface decohesion and ductile intragranular fracture mode in the
Intercritical annealing temperature
crack initiation region and the brittle fracture fraction increases with increasing TIA. Both
Austenite stability
the stability and amount of austenite play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel. Thus, it is suggested that suitable TIA should be explored
* Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.ijhydene.2019.06.204 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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to guarantee the safety service of automotive parts made of this type of steel in addition to acquiring excellent mechanical properties. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, environment protection, energy saving and security concerns have greatly stimulated the research and development of advanced high strength steels (AHSSs) [1]. Among these achievements, medium-Mn steel (typically containing 3e10 wt% Mn), which is featured by an excellent combination of strength and ductility as well as reasonable materials and manufacturing costs, has been regarded as one potential candidate for the third generation AHSSs for automotive car body applications [2e4]. Medium-Mn steel usually exhibits a martensitic microstructure after hot- and/or coldrolling because of its high Mn content and thus improved hardenability, and an ultrafine-grained (UFG) microstructure consisting of retained austenite (RA) with fraction up to ~60 vol% at room temperature after heat treatment in the aþg two phase region (intercritical annealing (IA)) [5e7]. It is known that the substantially enhanced tensile properties of medium-Mn steels are mainly derived from the transformation-induced plasticity (TRIP) effect of RA during tensile deformation, and the TRIP effect relys on the fraction and stability degree of RA [4,8]. Therefore, previous investigations on medium-Mn steels were mainly focused on methods to optimize the overall tensile properties by controlling chemical compositions and IA conditions [2,3]. Among IA parameters such as heating and cooling rate, annealing temperature (TIA) and annealing time, the selection of an optimum TIA range has attracted particular interests for obtaining a good balance between the phase stability and fraction of RA [2]. More recently, Al has been frequently added to medium-Mn steels mainly for the purpose of modulating the austenite stability and reducing the probability of cementite formation [2,3,7e9], although there are production difficulties in steelmaking such as nozzle clogging [3]. Hydrogen embrittlement (HE) is a widely known phenomenon in high strength materials [10]. The ever-increasing efforts towards developing a hydrogen-based energy supply and consumption chain has driven great attention to HE problem in hydrogen-rich environments with an increase in the strength level of AHSSs as well as the development of hydrogen-resistant steels [11e28]. The study of the HE behavior of a quenching & portioning (Q&P) steel revealed that the TRIP effect could increase its susceptibility to HE [15]. Therefore, there is concern that TRIP-assisted medium-Mn steels are possibly more susceptible to HE than conventional AHSSs due to their larger faction of RA which would fully or partially transforms into martensite during plastic deformation [2,3]. In fact, a HE index (HEI) defined as the relative total elongation (TEL) loss as high as 74e87% was found in a medium-Mn steel (0.1Ce7Mn-0.5Si, wt.%) [25]. Moreover, it has been found that the HE susceptibility of high-Mn steels is
particular sensitive to their compositions which affect austenite phase stability, hydrogen uptake behavior and the material's stacking fault energy et al. [18]. Therefore, it is worthwhile to systematically investigate the HE behavior of medium-Mn steels; however, only a few studies were conducted till now [25e29]. Our previous studies concerning the effect of IA time on the HE susceptibility of both cold- and warm-rolled medium-Mn steels have demonstrated that an increase in the IA time led to an increase of the HE susceptibility [27,28]. TIA is the most important processing parameter for medium-Mn steels, however, as far as we known, no reports have attempted to evaluate the effect of TIA on the HE susceptibility of medium-Mn steel. Therefore, in the present investigation, the influence of TIA on the HE behavior of a coldrolled Al-containing medium-Mn steel was explored by using slow strain rate tensile (SSRT) test in order to be able to understand and predict the potential hydrogen damage. The results are also beneficial for promoting the industrial application of medium-Mn steel as well as ensuring safety performance of components made of this type of steel under hydrogen-related service environments.
Material and experimental procedure The chemical composition of the tested steel is Fe-0.20C0.62Si- 4.88Mn-3.11Al-0.004P-0.005S (wt.%). The steel was melted in a vacuum induction furnace and cast into 110 kg ingot. The ingot was solution treated at 1220 C for 2 h and then hot-forged into plates of 30 mm thickness. The forged plates were reheated to 1200 C and held at that temperature for 2 h and then hot-rolled to sheets of 5.5 mm thickness followed by air cooling. After annealing at 750 C for 60 min and air cooled to room temperature, the hot-rolled sheets were cold-rolled with a thickness reduction of ~67% to a final thickness of 1.8 mm. The cold-rolled steel sheets were intercritically annealed at 700 C, 730 C, 770 C and 850 C for 10 min, respectively, and cooled in air to room temperature (henceforth referred as the CR700, CR730, CR770 and CR850 specimens, respectively). Standard smooth tensile specimens (1.2 mm thickness, 5 mm width and 25 mm gauge length) were cut longitudinally along the rolling direction. The volume fraction of RA was determined via X-rays diffraction (XRD, Rigaku D/MAX 2500) using a Cu-Ka radiation source. The tests were operated at 150 mA and 40 kV. The integrated intensities of the (200)a, (211)a, (200)g, (220)g and (311)g diffraction peaks were used for the calculations. Microstructures were examined using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD). The SEM specimens were etched in a 2 vol% nital
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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solution after standard grinding and polishing procedure. Specimens for both EBSD and XRD were electro-polished in an ethanol solution with 20% perchloric acid at 18 V to remove the residual stress. Hydrogen was introduced into the specimens by electrochemical charging in a 0.25 mol/L H2SO4 solution with 0.5 g/L thiourea. The charging current density was 0.2 mA/cm2 and the charging time was 60 min. Hydrogen analysis were conducted at a constant heating rate of 100 C/h to a maximum temperature of 800 C using a thermal desorption spectrometry (TDS, HTDS-002) equipped with a quadrupolar mass spectrometer. SSRT tests were carried out on a SUNS UTE5305 tensile testing machine operated at a nominal strain rate of 5.1 105 s1 at room temperature [30]. The relative TEL loss was adopted to express the HE susceptibility index (HEI): HEI ð%Þ ¼ ð1 TELH =TEL0 Þ 100%
(1)
where TEL0 and TELH are the TEL for the uncharged and hydrogen-charged specimens, respectively. The fracture surfaces of the fractured SSRT specimens were observed by SEM.
Results Microstructural characterization As shown in Fig. 1, all the specimens except the CR850 one exhibit an mixed microstructure of coarse elongated d-ferrite (EF) and an UFG constituent consisting of intercritical a-ferrite (IF) and rod- and globular-shaped RA, which is similar to that of other cold-rolled medium-Mn steels with suitable alloying addition of Al [8,9]. The high Al content resulted in an expanded two-phase region of d-ferrite and austenite at the
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soaking temperature (~1200 C); during subsequent hot- and cold-rolling, the d-ferrite was remarkably elongated and refined [8,9,26]. The fine size IF and RA phases were formed during the IA treatment, and the formation of this kind of microstructure is because of the rather high stored energy in the heavily cold deformed structure which provides a high driving force for recrystallization [31]. An increase in TIA accelerated the formation of equiaxed microstructure. There appeared a great amount of martensite for the CR850 specimen (Fig. 1(d)), this is because the TIA as high as 850 C resulted in a significantly decreased stability of austenite. Fig. 2 presents the phase distribution of the CR730, CR770 and CR 850 specimens characterized by EBSD. It is clear that the rod- and globular-shaped RA is primarily located at the boundary regions of ferrite. Moreover, the fractions of highangle grain boundaries with misorientation angle larger than 15 were 73.8%, 55.9% and 48.1% for the CR730, CR770 and CR 850 specimens, respectively. It is clear from Figs. 1 and 2 that there is a texture in the cold-rolled steel sheets after IA at different temperatures for 10 min. Therefore, an orientation distribution function (ODF) analysis was carried out to measure the texture more accurately. Fig. 3 presents the 42 ¼ 45 section obtained from the EBSD analysis of the CR730, CR770 and CR 850 specimens. As for all the specimens, one can see that the orientations of the RA are primarily the Brass texture {110}<112> as shown in Fig. 3(a), (c) and (e), while it is characterized as the RotatedCube {001}<110> and/or g-fiber {111} components for the ferrite. Similar texture characteristics were also found in a hot-rolled and intercritically annealed 0.3Ce6Mne3Al-1.5Si medium-Mn steel [32]. The changes of the average size of RA with TIA are presented in Fig. 4, which exhibited a gradual increase with an
Fig. 1 e SEM micrographs of the tested specimens intercritically annealed at (a) 700 C, (b) 730 C, (c) 770 C and (d) 850 C (EF: elongated d-ferrite; IF: intercritical a-ferrite; RA: retained austenite; M: martensite.). Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 2 e EBSD micrographs of specimens intercritically annealed at (a) 730 C, (b) 770 C and (c) 800 C. Green and red colors denote bcc and fcc phase regions, respectively, the blue lines depict the low angle boundaries with misorientation lying in between 2 and 15 and the black lines present the high angle boundaries with misorientation larger than 15 . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) increase in TIA and the increase was more significant when TIA was higher than 730 C. This phenomenon is primarily due to faster diffusion of Mn at higher TIA [33]. The results of XRD revealed that the RA fraction exhibits an increase-decrease pattern with TIA and a maximum RA fraction of ~19 vol% was obtained for the CR770 specimen, while only ~5 vol% of RA was obtained for the CR850 specimen as shown in Fig. 5.
was intercritically annealed at ever higher TIA of 850 C. The CHi is not considered in the present investigation for such hydrogen is usually regarded to do not lead to a deterioration of mechanical properties [34]. By the way, the lowtemperature peak temperature shifts slightly to a higher temperature for the HR850 specimen, indicating the change of main hydrogen trapping sites, that is, the existence of many martensite phases.
Mechanical properties HE susceptibility As shown in Table 1, the ultimate tensile strength (UTS) increases while the yield strength (YS) decreases with an increase in TIA, and the TEL shows an increase-decrease pattern with TIA like that of the RA fraction and a maximum value of 46.5% was obtained when TIA was 770 C. As a result, an excellent combination of strength and ductility (the product of UTS and TEL, UTS TEL) as high as 52.7 GPa$% was obtained for the CR730 specimen. These trends are similar to those reported in the literature [8,32,33]. Notably, there were a sharp increase of the UTS and a remarkable decrease of the TEL when TIA is 850 C. This is primarily because of the significant decrease of the RA fraction and correspondingly the sharp increase of the martensite fraction after air cooling to room temperature due to the significantly decreased stability of austenite, which will be discussed later.
Hydrogen desorption behavior The hydrogen desorption curves of tested specimens intercritically annealed at different TIA are presented in Fig. 6(a). There are double desorption peaks, i.e., low- and hightemperature peaks, for all the hydrogen-charged specimens with their peak temperatures being ~100 C and ~370 C, respectively. Similar results were also found in other mediumMn steels [26e28]. It is generally regarded that hydrogen corresponding to low-temperature peak represents weak trapped or reversible hydrogen, whereas hydrogen corresponding to high-temperature peak represents strong trapped or irreversible hydrogen [34]. Fig. 6(b) reveals that the concentration corresponding to reversible hydrogen (CHr) slightly decreased with increasing TIA up to 770 C, while the concentration corresponding to irreversible hydrogen (CHi) remained almost unchanged with increasing TIA up to 770 C. There was a slight increase of CHr and a slight decrease of CHi when the specimen
Fig. 7 shows the SSRT engineering stress-strain curves of the specimens with different TIA before and after hydrogencharging. Regarding the uncharged specimens, the CR730 and CR770 specimens exhibit discontinuous yielding behavior, as was also found in other cold-rolled and intercritically annealed medium-Mn steels [2,9,25,28]. This phenomenon is mainly related to the heterogeneous deformation of martensitic transformation or the size of RA [35]. There was a significant change in the tensile deformation behavior after hydrogen charging. Both the UTS and TEL decreased after hydrogen-charging, whereas the YS and the initial strain hardening rate of the hydrogen-charged specimens except the CR850 were almost identical to those of the uncharged specimens, confirming the good reproducibility of the test. For the CR850 specimen, fracture occurred within the elastic regime after hydrogen-charging. As shown in Table 1, HEI increases gradually with increasing TIA, i.e., the CR700 specimen demonstrates the lowest HE susceptibility, while the CR850 specimen exhibits the highest HE susceptibility. This means that the tested steel experience a significant degradation of mechanical properties with increasing TIA.
SSRT fracture surface Fig. 8 presents the fracture appearances of the tested specimens. At both sides a brittle zone, i.e., crack initiation region, as indicated by dotted lines, can be detected for all the hydrogen-charged specimens (Fig. 8(c), (d) and (f)) as compared with the uncharged specimen (Fig. 8(a)) except for the CR850 specimen. The fraction of brittle zone increases with an increase in TIA. The central part of the fracture surface of the hydrogen-charged specimens as well as the uncharged specimen is still ductile with delamination fracture, which is
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 3 e 42 ¼ 45 section of the orientation distribution function (ODF) obtained from EBSD analysis of specimens intercritically annealed at (a,b) 730 C, (c,d) 770 C and (e,f) 800 C, (g) 42 ¼ 45 section of ODF representing typical texture components. ODF for (a,c,e) austenite and (b,d,f) for ferrite in the specimens. related to the texture of the microstructure as mentioned above [32], and was often found in ultrafine elongated grain structure and laminated composites [36,37] and heavy warm rolled Al-containing medium-Mn steel [29]. The CR850 specimen demonstrates a totally brittle fracture appearance both before and after hydrogen-charging (Fig. 8(b) and (e)). When having a closer look at the crack initiation region, fine dimple fractures can be observed for both the uncharged and hydrogen-charged specimens except the CR850 as shown in Fig. 9. However, further careful observation of the fracture surfaces revealed that there are still differences between the uncharged and hydrogen-charged specimens.
All the uncharged specimens exhibit only empty dimples (Fig. 9(a), (c) and (e)), indicating their relatively high ductility as shown in Fig. 7. Both empty dimples and dimples filled with grains (white arrows in Fig. 9(b), (d) and (f) and 10) were found for all the hydrogen-charged specimens except the CR850. Similar fracture characteristic was also found by others in medium-Mn steels with primarily equiaxed microstructure [25,27,28]. Both the uncharged and hydrogencharged CR850 specimens exhibit a mixed mode fracture of quasi-cleavage and intergranular, and the fraction of intergranular fracture increased after hydrogen-charging (Fig. 9(g) and (h) see Fig. 10).
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 4 e The distribution and dependence of the RA size on TIA of the tested steel. (a) CR700; (b) CR730; (c) CR770; (d) the average RA size. Note that the size is the average of both the RA and martensite for the TIA of 850 C.
Fig. 5 e XRD patterns and the measured RA fractions of the tested specimens. (a) XRD patterns before tensile tests; (b) XRD patterns after tensile tests; (c) the measured RA fractions; (d) transformation ratio of RA.
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Table 1 e Tensile properties of the tested cold-rolled steel intercritically annealed at different TIA. Specimen
UTS (MPa)
YS (MPa)
TEL (%)
UTS TEL (GPa$%)
HEI (%)
831 882 1060 1369
754 707 616 675
39.1 59.8 37.1 12.8
32.5 52.7 39.3 17.5
15.3 64.9 78.6 83.4
CR700 CR730 CR770 CR850
Fig. 6 e (a) Hydrogen desorption rate curves (TDS curves) and (b) hydrogen contents of the tested samples intercritically annealed at different temperatures.
Fig. 7 e Engineering stress-stain curves of SSRT for specimens intercritically annealed at (a) 700 C, (b) 730 C, (c) 770 C and (d) 850 C.
Discussion Mechanical stability of RA As can been seen from Table 1, the mechanical properties of the tested steel exhibit an increase-decrease pattern with TIA, and a maximum value of UTS TEL as high as 52.7 GPa$% was obtained when the tested steel was intercritically annealed at
730 C (Table 1). This result reveals that the mechanical properties of the tested steel are sensitive to TIA, as has been found by many other researchers [7,8,32,33,36e41]. This phenomenon is primarily due to the dependence of the amount and mechanical stability of RA on TIA [2,39]. The RA fraction also shows an increase-decrease pattern with TIA (Fig. 3(c)), which is closely related to the reverse transformation from martensite to austenite, the partitioning of elements between a and g and the grain growth, which occur during the IA
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 8 e Low magnification SEM fractographs of the (a,b) uncharged and (c,d,e,f) hydrogen-charged SSRT specimens intercritically annealed at (a,d) 770 C, (b,e) 850 C, (c) 700 C and (f) 730 C. Dotted lines indicate brittle zones. process [2]. The mechanical stability of RA, which refers to its resistance to transform into martensite upon increasing in stress and strain, decreases with increasing TIA (Fig. 4(d)). Many factors, such as the concentration of strong stabilizing elements (for example, Mn and C) in RA and the size and morphology of RA, affect the mechanical stability of RA [42,43]. Table 2 shows the variations of Mn and C concentrations in different phases with TIA. The Mn concentrations in different phases were based on the EDS analysis of 40 randomly selected phases. Both the Mn and C concentrations decreased with increasing TIA, which is one of the main reasons for the decreasing stability of RA with increasing TIA, while both the Al and Si concentrations changed slightly with TIA. Moreover, the Mn concentration in different phases increased in the following order: EF, IF and RA. These results suggest the partitioning of Mn among different phases during the IA process, as was also confirmed by others [7,8,33,42,43]. The size of RA also strongly affects its mechanical stability and a reduction in its size is well known to enhance its stability by suppressing the martensite transformation [8,44,45]. The strengthening of austenite through grain refining and alloying increases the mechanical free energy required to transform austenite into martensite, which lowers the ability to nucleate martensite [8,42]. The RA size increases with an increase in TIA as shown in Fig. 3. Therefore, both the decreasing of the Mn and C concentrations in RA and the increasing of RA size are primarily attributed to the decreased RA stability with increasing TIA of the tested steel. Moreover, these trends are more remarkable for the CR850 specimen, which cause a remarkable decreasing of the RA fraction and the presence of a large amount of martensite as mentioned above.
HE behavior The excellent combination of strength and ductility of medium-Mn steels primarily stems from the TRIP effect during deformation [2e4,8]. However, the deformation-induced fresh martensite (DIM) is regarded to be most vulnerable to HE [3,15,26], which can be explained as follows. The DIM
possesses high carbon content because of the high carbon content of the original RA (Table 2). This kind of high carbon DIM does not experience any annealing or tempering treatment, and thus it is inherently brittle and has rather high susceptibility to HE, which can be further confirmed by the tensile properties and HE behavior of the CR850 specimen. The CR850 specimen, which has ~46.39 vol% martensite besides EF, IF and RA (~4.75 vol%), has much lower ductility and significantly higher HE susceptibility (Table 1). To verify this, the hydrogen-charged CR770 and CR850 specimens were pre-strained 5% and were then crosssectioned longitudinally and carefully observed under a SEM. It is obvious that almost no sub-cracks could be observed for the CR770 specimen (Fig. 11(a)), while the CR850 specimen was already fractured and a considerable number of subcracks in the uniform deformation region were found (Fig. 11(b)). These sub-cracks were formed either through cracking of hard martensite (white arrows) or by interface decohesion between martensite and ferrite phases (yellow arrows) for the hydrogen-charged CR850 specimen, as was also usually found in DP steels [17,22]. The sub-crack initiation might result from stress concentration caused by the different strain hardening rates of martensite and ferrite [17,46]. These sub-cracks prefer to propagate along the interface boundaries of martensite and ferrite (Fig. 11(c)), and thus formed intergranular fracture as shown in Fig. 9(g). These results strongly demonstrate the detrimental influence of hard martensite on promoting the early formation of sub-cracks. The presence of hydrogen would lower the cohesive bonding energy between lattice atoms [47], and significantly promote the formation of intergranular cracking as shown in Fig. 9(h), and thus exhibits much higher HE susceptibility (Table 1). Although the microstructures of the CR700, CR730 and CR770 specimens do not consist of any original martensite (Fig. 1(a), (b) and (c)), DIM would be transformed from the RA gradually with increasing tensile strain depending on the mechanical stability of RA, i.e., the higher the RA stability is, the later the DIM would be transformed, and thus its detrimental influence would be delayed. As the stability of RA decreases while its fraction increases with increasing TIA up to
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 9 e SEM fractographs in crack initiation region of (a,c,e) uncharged and (b,d,f) hydrogen-charged SSRT specimens: (a,b) CR700; (c,d) CR730; (e,f) CR770; (g,h) CR850. 770 C, the amount of DIM increases (Fig. 4), and thus causes an increasing trend of the HE susceptibility with an increase in TIA (Table 1 and Fig. 7). Notably, it has been confirmed that austenite is a sink for hydrogen primarily because of its high hydrogen solubility and low hydrogen diffusivity [15,48e50], and a linear relationship between CHr and the RA fraction was found in high carbon martensitic steels [48]. It was also found that a notable increase of CH from ~0.35 ppm for a full martensitic
microstructure to ~1.87 ppm for a reversion treated martensitic microstructure containing ~35 vol% RA in a TRIPassisted maraging steel (0.01Ce9Mne3Ni-1.4Al, wt%) [49]. Therefore, it was at first expected that there would be a slight increase of CHr with increasing TIA up to 770 C. However, the present study found that there was a slight decrease of CHr with increasing TIA up to 770 C, i.e., with increasing the RA fraction (Figs. 4 and 5). Thus, it is reasonable to regard that most of the hydrogen was not trapped within the RA because
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Fig. 10 e Highly magnified SEM fractographs in crack initiation region of hydrogen-charged SSRT specimens: (a) CR770; (b) CR850.
Table 2 e Concentrations of Mn and C in different phases (experimental error was given in the parenthesis). Specimen
Annealing temperature ( C)
Mn (wt.%) add.
CR700 CR730 CR770 CR850
700 730 770 850
4.88 4.88 4.88 4.88
RA 6.82 6.61 6.45 5.75
± 0.52 ± 0.71 ± 0.57 ± 0.49
IF 5.15 4.46 4.44 4.32
± 0.53 ± 0.21 ± 0.22 ± 0.23
C (wt.%) EF 4.38 ± 4.34 ± 4.29 ± 4.22 ±
0.14 0.19 0.15 0.17
add.
RA
0.20 0.20 0.20 0.20
0.665 0.596 0.592 0.503
Note: EF refers to elongated d-ferrite; IF refers to intercritical a-ferrite; RA refers to retained austenite.
Fig. 11 e SEM fractographs of the cross-sectional area of the 5% pre-strained (a) CR770 and (b,c) CR850 specimens.
of the extremely low hydrogen diffusivity in it [51] and the short charging time (60 min) applied, while it is most likely that most of the hydrogen was trapped by the interfaces between g and a phases, as was also pointed out by others [48,52], this is because the g/a interface is considered to be a
strong hydrogen trap [50]. Therefore, the area of phase interfaces decreases with an increase in the RA size i.e., TIA, and correspondingly a decrease of CHr (Table 3). Moreover, the hydrogen trapped by the g/a interface would be inherited into the interface of the DIM and ferrite [25,26], and thus
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Table 3 e Determination of hydrogen concentration per the phase interfaces area per unit volume (106 g). Specimen CR700 CR730 CR770 CR850
CHr (ppmw)
DRA size (mm)
VRA (vol%)
SVRA ( 1017, mm2)
CHr/SVRA ( 1017, g/mm2)
1.36 1.20 1.16 1.29
0.42 0.51 0.65 1.53
15.26 16.37 19.26 4.75
2.19 1.93 1.78 e
0.63 0.62 0.65 e
Note: DRAeRA size (diameter); VRAeVolume fraction of RA; SVRAethe area of phase interfaces per unit volume.
facilitated the brittle fracture occurred along the phase interfaces (Fig. 9). By assuming the RA as globular shape, the amount of phase interfaces area per unit volume (SVRA) could be expressed as [27]: SVRA ¼ 6 VRA =DRA
(2)
where DRA and VRA are the average diameter and volume fraction of RA, respectively. As shown in Table 3, CH/SVRA remains almost unchanged, although both SVRA and CH decrease with increasing TIA, which reveals that almost identical amount of hydrogen was trapped at the RA/ferrite interfaces. This result further confirms that the stability and amount of RA plays a critical role in determining the HE susceptibility of the tested steel. By the way, the existence of a large amount of martensite causes a slight increase of CHr for the CR850 specimen.
SSRT fracture characteristics One significant finding of the fracture surfaces of hydrogencharged SSRT specimens is the presence of dimples filled grains besides empty dimples (dimples without grain) as shown in Fig. 9(b), (d) and (f). This phenomenon, which is different from those of other AHSSs, has also been reported in cold-rolled [25,28] and warm-rolled [27] medium-Mn steels. It was proposed that this kind of fracture was generated by intergranular cracking occurring along the interface boundaries of RA grains by the hydrogen-enhanced decohesion (HEDE) mechanism that dissolved hydrogen reduce the cohesive bonding energy [25]. As shown in Fig. 12, the average Mn content in the dimples with grain is 6.52 ± 0.37 wt%, while the average Mn content in the empty dimples is 4.78 ± 0.34 wt% for the CR770 specimen;
which is in accordance with the results in Table 2. The average size of the traces of detached grains is ~0.61 mm, which is similar to the size of RA (~0.65 mm) for the CR770 specimen. Moreover, further statistical analysis of randomly selected 200 dimples of the hydrogen-charged CR700, CR730 and CR770 specimens demonstrates that the ratio of dimples filled with grains has a nearly linear relationship with the fraction of transformed RA (DIM) after SSRT as shown in Fig. 13, indicating an increase of the fraction of brittle interface decohesion fracture with DIM, i.e., the facture surfaces became increasingly brittle with increasing TIA. These results strongly suggest that these “grains” are DIM transformed from the RA and brittle interface decohesion occurred along the interface of DIM and ferrite. This kind of brittle interface decohesion fracture is an indication of less ductility for the hydrogencharged specimens (Fig. 7). Therefore, the characteristic fracture of the tested TRIPassisted medium-Mn steel can be explained as follows. For the specimens with no initial martensite after the IA treatment, a certain amount of deformation, which depends on the mechanical stability of RA, is needed for the transformation of RA into martensite (DIM); once the DIM was formed, brittle interface decohesion would occur along the boundaries of DIM and deformed ferrite through the HEDE mechanism [25], and thus caused the characteristic dimples filled with DIM. Notably, the presence of hydrogen especially the enrichment of hydrogen at g/a interface would significantly promote the formation of brittle interface decohesion [12,17,25]. For the region absent of RA, i.e., the IF and EF grains, it must have suffered considerable deformation before the formation of DIM; the HE of ferrite would deform and fracture by the hydrogen-enhanced local plasticity (HELP) mechanism [25], and thus formed the ductile fracture characterized by empty dimple morphology. For the specimen like CR850 having
Fig. 12 e (a) SEM fractographs in crack initiation region of hydrogen-charged CR770 specimen and (b) the distribution of Mn content in dimples with grain and without grain. Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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increasing TIA. Therefore, both the stability and amount of RA play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2017RC024).
references
Fig. 13 e Variations of the ratio of fracture dimples filled with grains with the transformed RA fraction after SSRT tests of hydrogen-charged SSRT specimens.
considerable amount of initial martensite before tensile deformation, martensite cracking and/or brittle interface decohesion along the boundaries of martensite and ferrite would occur at the start of deformation (Fig. 11(b)), and thus formed a mixed mode fracture of quasi-cleavage and intergranular.
Conclusions (1) The tested Al-containing medium-Mn steel exhibit a mixed microstructure of coarse elongated d-ferrite and an UFG constituent consisting of and rod- and globularshaped RA when TIA is lower than 850 C. The average size of RA exhibits a gradual increase with an increase in TIA, while the RA fraction exhibits an increasedecrease pattern with TIA. (2) The UTS increases while the YS decreases with an increase in TIA, whereas both the TEL and UTS TEL show an increase-decrease pattern with TIA. An excellent strength-ductility balance as high as ~53 GPa$% was obtained for the specimen annealed at 730 C. (3) Unlike that of UTS TEL, the HEI increases monotonically with an increase in TIA, the CR700 specimen demonstrates the lowest HE susceptibility, while the CR850 specimen exhibits the highest HE susceptibility. (4) Unlike that of dimple ductile fracture for the uncharged specimens, the hydrogen-charged specimens except the CR850 exhibit a mixed fracture of dimple ductile and brittle interface decohesion fracture mode. The CR850 specimen exhibits a mixed mode fracture of quasicleavage and intergranular. The facture surfaces became increasingly brittle with increasing TIA. (5) Both the decreasing of the C and Mn concentrations in the RA and the increasing of the RA size are primarily attributed to the decreased mechanical stability of RA with increasing TIA. Therefore, the amount of deformation-induced martensite increases, and then causes an increasing tendency of HE susceptibility with
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Intercritical annealing temperature dependence of hydrogen embrittlement behavior of cold-rolled Alcontaining medium-Mn steel Yongjian Zhang a,*, Chengwei Shao b, Jiaojiao Wang a, Xiaoli Zhao a, Weijun Hui a a
Materials Science and Engineering Research Center, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China b China Aero-Polytechnology Establishment, Beijing 100028, PR China
highlights
graphical abstract
Hydrogen embrittlement (HE) of Al-containing medium-Mn steel was evaluated. HE susceptibility increases with increasing intercritical annealing temperature. Both the stability and amount of austenite have important influence on the HE behavior.
article info
abstract
Article history:
The present investigation attempts to evaluate the influence of intercritical annealing
Received 5 May 2019
temperature (TIA) on the hydrogen embrittlement (HE) of a cold-rolled Al-containing
Received in revised form
medium-Mn steel (Fe-0.2C-4.88Mn-3.11Al-0.62Si) by using electrochemical hydrogen-
23 June 2019
charging, slow strain rate tensile test and scanning electron microscope. The results
Accepted 29 June 2019
show that an excellent combination of strength and ductility (the product of ultimate
Available online xxx
tensile strength and total elongation) up to ~53 GPa$% was obtained for the specimen intercritically annealed at an intermediate temperature of 730 C, whereas the HE index
Keywords:
increases significantly with an increase in TIA up to 850 C. Being different from the typical
Hydrogen embrittlement
dimple ductile fracture for the uncharged specimen, the hydrogen-charged specimen ex-
Medium-Mn steel
hibits a mixed brittle interface decohesion and ductile intragranular fracture mode in the
Intercritical annealing temperature
crack initiation region and the brittle fracture fraction increases with increasing TIA. Both
Austenite stability
the stability and amount of austenite play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel. Thus, it is suggested that suitable TIA should be explored
* Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.ijhydene.2019.06.204 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
2
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to guarantee the safety service of automotive parts made of this type of steel in addition to acquiring excellent mechanical properties. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In recent years, environment protection, energy saving and security concerns have greatly stimulated the research and development of advanced high strength steels (AHSSs) [1]. Among these achievements, medium-Mn steel (typically containing 3e10 wt% Mn), which is featured by an excellent combination of strength and ductility as well as reasonable materials and manufacturing costs, has been regarded as one potential candidate for the third generation AHSSs for automotive car body applications [2e4]. Medium-Mn steel usually exhibits a martensitic microstructure after hot- and/or coldrolling because of its high Mn content and thus improved hardenability, and an ultrafine-grained (UFG) microstructure consisting of retained austenite (RA) with fraction up to ~60 vol% at room temperature after heat treatment in the aþg two phase region (intercritical annealing (IA)) [5e7]. It is known that the substantially enhanced tensile properties of medium-Mn steels are mainly derived from the transformation-induced plasticity (TRIP) effect of RA during tensile deformation, and the TRIP effect relys on the fraction and stability degree of RA [4,8]. Therefore, previous investigations on medium-Mn steels were mainly focused on methods to optimize the overall tensile properties by controlling chemical compositions and IA conditions [2,3]. Among IA parameters such as heating and cooling rate, annealing temperature (TIA) and annealing time, the selection of an optimum TIA range has attracted particular interests for obtaining a good balance between the phase stability and fraction of RA [2]. More recently, Al has been frequently added to medium-Mn steels mainly for the purpose of modulating the austenite stability and reducing the probability of cementite formation [2,3,7e9], although there are production difficulties in steelmaking such as nozzle clogging [3]. Hydrogen embrittlement (HE) is a widely known phenomenon in high strength materials [10]. The ever-increasing efforts towards developing a hydrogen-based energy supply and consumption chain has driven great attention to HE problem in hydrogen-rich environments with an increase in the strength level of AHSSs as well as the development of hydrogen-resistant steels [11e28]. The study of the HE behavior of a quenching & portioning (Q&P) steel revealed that the TRIP effect could increase its susceptibility to HE [15]. Therefore, there is concern that TRIP-assisted medium-Mn steels are possibly more susceptible to HE than conventional AHSSs due to their larger faction of RA which would fully or partially transforms into martensite during plastic deformation [2,3]. In fact, a HE index (HEI) defined as the relative total elongation (TEL) loss as high as 74e87% was found in a medium-Mn steel (0.1Ce7Mn-0.5Si, wt.%) [25]. Moreover, it has been found that the HE susceptibility of high-Mn steels is
particular sensitive to their compositions which affect austenite phase stability, hydrogen uptake behavior and the material's stacking fault energy et al. [18]. Therefore, it is worthwhile to systematically investigate the HE behavior of medium-Mn steels; however, only a few studies were conducted till now [25e29]. Our previous studies concerning the effect of IA time on the HE susceptibility of both cold- and warm-rolled medium-Mn steels have demonstrated that an increase in the IA time led to an increase of the HE susceptibility [27,28]. TIA is the most important processing parameter for medium-Mn steels, however, as far as we known, no reports have attempted to evaluate the effect of TIA on the HE susceptibility of medium-Mn steel. Therefore, in the present investigation, the influence of TIA on the HE behavior of a coldrolled Al-containing medium-Mn steel was explored by using slow strain rate tensile (SSRT) test in order to be able to understand and predict the potential hydrogen damage. The results are also beneficial for promoting the industrial application of medium-Mn steel as well as ensuring safety performance of components made of this type of steel under hydrogen-related service environments.
Material and experimental procedure The chemical composition of the tested steel is Fe-0.20C0.62Si- 4.88Mn-3.11Al-0.004P-0.005S (wt.%). The steel was melted in a vacuum induction furnace and cast into 110 kg ingot. The ingot was solution treated at 1220 C for 2 h and then hot-forged into plates of 30 mm thickness. The forged plates were reheated to 1200 C and held at that temperature for 2 h and then hot-rolled to sheets of 5.5 mm thickness followed by air cooling. After annealing at 750 C for 60 min and air cooled to room temperature, the hot-rolled sheets were cold-rolled with a thickness reduction of ~67% to a final thickness of 1.8 mm. The cold-rolled steel sheets were intercritically annealed at 700 C, 730 C, 770 C and 850 C for 10 min, respectively, and cooled in air to room temperature (henceforth referred as the CR700, CR730, CR770 and CR850 specimens, respectively). Standard smooth tensile specimens (1.2 mm thickness, 5 mm width and 25 mm gauge length) were cut longitudinally along the rolling direction. The volume fraction of RA was determined via X-rays diffraction (XRD, Rigaku D/MAX 2500) using a Cu-Ka radiation source. The tests were operated at 150 mA and 40 kV. The integrated intensities of the (200)a, (211)a, (200)g, (220)g and (311)g diffraction peaks were used for the calculations. Microstructures were examined using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectrometer (EDS) and electron backscatter diffraction (EBSD). The SEM specimens were etched in a 2 vol% nital
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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solution after standard grinding and polishing procedure. Specimens for both EBSD and XRD were electro-polished in an ethanol solution with 20% perchloric acid at 18 V to remove the residual stress. Hydrogen was introduced into the specimens by electrochemical charging in a 0.25 mol/L H2SO4 solution with 0.5 g/L thiourea. The charging current density was 0.2 mA/cm2 and the charging time was 60 min. Hydrogen analysis were conducted at a constant heating rate of 100 C/h to a maximum temperature of 800 C using a thermal desorption spectrometry (TDS, HTDS-002) equipped with a quadrupolar mass spectrometer. SSRT tests were carried out on a SUNS UTE5305 tensile testing machine operated at a nominal strain rate of 5.1 105 s1 at room temperature [30]. The relative TEL loss was adopted to express the HE susceptibility index (HEI): HEI ð%Þ ¼ ð1 TELH =TEL0 Þ 100%
(1)
where TEL0 and TELH are the TEL for the uncharged and hydrogen-charged specimens, respectively. The fracture surfaces of the fractured SSRT specimens were observed by SEM.
Results Microstructural characterization As shown in Fig. 1, all the specimens except the CR850 one exhibit an mixed microstructure of coarse elongated d-ferrite (EF) and an UFG constituent consisting of intercritical a-ferrite (IF) and rod- and globular-shaped RA, which is similar to that of other cold-rolled medium-Mn steels with suitable alloying addition of Al [8,9]. The high Al content resulted in an expanded two-phase region of d-ferrite and austenite at the
3
soaking temperature (~1200 C); during subsequent hot- and cold-rolling, the d-ferrite was remarkably elongated and refined [8,9,26]. The fine size IF and RA phases were formed during the IA treatment, and the formation of this kind of microstructure is because of the rather high stored energy in the heavily cold deformed structure which provides a high driving force for recrystallization [31]. An increase in TIA accelerated the formation of equiaxed microstructure. There appeared a great amount of martensite for the CR850 specimen (Fig. 1(d)), this is because the TIA as high as 850 C resulted in a significantly decreased stability of austenite. Fig. 2 presents the phase distribution of the CR730, CR770 and CR 850 specimens characterized by EBSD. It is clear that the rod- and globular-shaped RA is primarily located at the boundary regions of ferrite. Moreover, the fractions of highangle grain boundaries with misorientation angle larger than 15 were 73.8%, 55.9% and 48.1% for the CR730, CR770 and CR 850 specimens, respectively. It is clear from Figs. 1 and 2 that there is a texture in the cold-rolled steel sheets after IA at different temperatures for 10 min. Therefore, an orientation distribution function (ODF) analysis was carried out to measure the texture more accurately. Fig. 3 presents the 42 ¼ 45 section obtained from the EBSD analysis of the CR730, CR770 and CR 850 specimens. As for all the specimens, one can see that the orientations of the RA are primarily the Brass texture {110}<112> as shown in Fig. 3(a), (c) and (e), while it is characterized as the RotatedCube {001}<110> and/or g-fiber {111} components for the ferrite. Similar texture characteristics were also found in a hot-rolled and intercritically annealed 0.3Ce6Mne3Al-1.5Si medium-Mn steel [32]. The changes of the average size of RA with TIA are presented in Fig. 4, which exhibited a gradual increase with an
Fig. 1 e SEM micrographs of the tested specimens intercritically annealed at (a) 700 C, (b) 730 C, (c) 770 C and (d) 850 C (EF: elongated d-ferrite; IF: intercritical a-ferrite; RA: retained austenite; M: martensite.). Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 2 e EBSD micrographs of specimens intercritically annealed at (a) 730 C, (b) 770 C and (c) 800 C. Green and red colors denote bcc and fcc phase regions, respectively, the blue lines depict the low angle boundaries with misorientation lying in between 2 and 15 and the black lines present the high angle boundaries with misorientation larger than 15 . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) increase in TIA and the increase was more significant when TIA was higher than 730 C. This phenomenon is primarily due to faster diffusion of Mn at higher TIA [33]. The results of XRD revealed that the RA fraction exhibits an increase-decrease pattern with TIA and a maximum RA fraction of ~19 vol% was obtained for the CR770 specimen, while only ~5 vol% of RA was obtained for the CR850 specimen as shown in Fig. 5.
was intercritically annealed at ever higher TIA of 850 C. The CHi is not considered in the present investigation for such hydrogen is usually regarded to do not lead to a deterioration of mechanical properties [34]. By the way, the lowtemperature peak temperature shifts slightly to a higher temperature for the HR850 specimen, indicating the change of main hydrogen trapping sites, that is, the existence of many martensite phases.
Mechanical properties HE susceptibility As shown in Table 1, the ultimate tensile strength (UTS) increases while the yield strength (YS) decreases with an increase in TIA, and the TEL shows an increase-decrease pattern with TIA like that of the RA fraction and a maximum value of 46.5% was obtained when TIA was 770 C. As a result, an excellent combination of strength and ductility (the product of UTS and TEL, UTS TEL) as high as 52.7 GPa$% was obtained for the CR730 specimen. These trends are similar to those reported in the literature [8,32,33]. Notably, there were a sharp increase of the UTS and a remarkable decrease of the TEL when TIA is 850 C. This is primarily because of the significant decrease of the RA fraction and correspondingly the sharp increase of the martensite fraction after air cooling to room temperature due to the significantly decreased stability of austenite, which will be discussed later.
Hydrogen desorption behavior The hydrogen desorption curves of tested specimens intercritically annealed at different TIA are presented in Fig. 6(a). There are double desorption peaks, i.e., low- and hightemperature peaks, for all the hydrogen-charged specimens with their peak temperatures being ~100 C and ~370 C, respectively. Similar results were also found in other mediumMn steels [26e28]. It is generally regarded that hydrogen corresponding to low-temperature peak represents weak trapped or reversible hydrogen, whereas hydrogen corresponding to high-temperature peak represents strong trapped or irreversible hydrogen [34]. Fig. 6(b) reveals that the concentration corresponding to reversible hydrogen (CHr) slightly decreased with increasing TIA up to 770 C, while the concentration corresponding to irreversible hydrogen (CHi) remained almost unchanged with increasing TIA up to 770 C. There was a slight increase of CHr and a slight decrease of CHi when the specimen
Fig. 7 shows the SSRT engineering stress-strain curves of the specimens with different TIA before and after hydrogencharging. Regarding the uncharged specimens, the CR730 and CR770 specimens exhibit discontinuous yielding behavior, as was also found in other cold-rolled and intercritically annealed medium-Mn steels [2,9,25,28]. This phenomenon is mainly related to the heterogeneous deformation of martensitic transformation or the size of RA [35]. There was a significant change in the tensile deformation behavior after hydrogen charging. Both the UTS and TEL decreased after hydrogen-charging, whereas the YS and the initial strain hardening rate of the hydrogen-charged specimens except the CR850 were almost identical to those of the uncharged specimens, confirming the good reproducibility of the test. For the CR850 specimen, fracture occurred within the elastic regime after hydrogen-charging. As shown in Table 1, HEI increases gradually with increasing TIA, i.e., the CR700 specimen demonstrates the lowest HE susceptibility, while the CR850 specimen exhibits the highest HE susceptibility. This means that the tested steel experience a significant degradation of mechanical properties with increasing TIA.
SSRT fracture surface Fig. 8 presents the fracture appearances of the tested specimens. At both sides a brittle zone, i.e., crack initiation region, as indicated by dotted lines, can be detected for all the hydrogen-charged specimens (Fig. 8(c), (d) and (f)) as compared with the uncharged specimen (Fig. 8(a)) except for the CR850 specimen. The fraction of brittle zone increases with an increase in TIA. The central part of the fracture surface of the hydrogen-charged specimens as well as the uncharged specimen is still ductile with delamination fracture, which is
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 3 e 42 ¼ 45 section of the orientation distribution function (ODF) obtained from EBSD analysis of specimens intercritically annealed at (a,b) 730 C, (c,d) 770 C and (e,f) 800 C, (g) 42 ¼ 45 section of ODF representing typical texture components. ODF for (a,c,e) austenite and (b,d,f) for ferrite in the specimens. related to the texture of the microstructure as mentioned above [32], and was often found in ultrafine elongated grain structure and laminated composites [36,37] and heavy warm rolled Al-containing medium-Mn steel [29]. The CR850 specimen demonstrates a totally brittle fracture appearance both before and after hydrogen-charging (Fig. 8(b) and (e)). When having a closer look at the crack initiation region, fine dimple fractures can be observed for both the uncharged and hydrogen-charged specimens except the CR850 as shown in Fig. 9. However, further careful observation of the fracture surfaces revealed that there are still differences between the uncharged and hydrogen-charged specimens.
All the uncharged specimens exhibit only empty dimples (Fig. 9(a), (c) and (e)), indicating their relatively high ductility as shown in Fig. 7. Both empty dimples and dimples filled with grains (white arrows in Fig. 9(b), (d) and (f) and 10) were found for all the hydrogen-charged specimens except the CR850. Similar fracture characteristic was also found by others in medium-Mn steels with primarily equiaxed microstructure [25,27,28]. Both the uncharged and hydrogencharged CR850 specimens exhibit a mixed mode fracture of quasi-cleavage and intergranular, and the fraction of intergranular fracture increased after hydrogen-charging (Fig. 9(g) and (h) see Fig. 10).
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 4 e The distribution and dependence of the RA size on TIA of the tested steel. (a) CR700; (b) CR730; (c) CR770; (d) the average RA size. Note that the size is the average of both the RA and martensite for the TIA of 850 C.
Fig. 5 e XRD patterns and the measured RA fractions of the tested specimens. (a) XRD patterns before tensile tests; (b) XRD patterns after tensile tests; (c) the measured RA fractions; (d) transformation ratio of RA.
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Table 1 e Tensile properties of the tested cold-rolled steel intercritically annealed at different TIA. Specimen
UTS (MPa)
YS (MPa)
TEL (%)
UTS TEL (GPa$%)
HEI (%)
831 882 1060 1369
754 707 616 675
39.1 59.8 37.1 12.8
32.5 52.7 39.3 17.5
15.3 64.9 78.6 83.4
CR700 CR730 CR770 CR850
Fig. 6 e (a) Hydrogen desorption rate curves (TDS curves) and (b) hydrogen contents of the tested samples intercritically annealed at different temperatures.
Fig. 7 e Engineering stress-stain curves of SSRT for specimens intercritically annealed at (a) 700 C, (b) 730 C, (c) 770 C and (d) 850 C.
Discussion Mechanical stability of RA As can been seen from Table 1, the mechanical properties of the tested steel exhibit an increase-decrease pattern with TIA, and a maximum value of UTS TEL as high as 52.7 GPa$% was obtained when the tested steel was intercritically annealed at
730 C (Table 1). This result reveals that the mechanical properties of the tested steel are sensitive to TIA, as has been found by many other researchers [7,8,32,33,36e41]. This phenomenon is primarily due to the dependence of the amount and mechanical stability of RA on TIA [2,39]. The RA fraction also shows an increase-decrease pattern with TIA (Fig. 3(c)), which is closely related to the reverse transformation from martensite to austenite, the partitioning of elements between a and g and the grain growth, which occur during the IA
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 8 e Low magnification SEM fractographs of the (a,b) uncharged and (c,d,e,f) hydrogen-charged SSRT specimens intercritically annealed at (a,d) 770 C, (b,e) 850 C, (c) 700 C and (f) 730 C. Dotted lines indicate brittle zones. process [2]. The mechanical stability of RA, which refers to its resistance to transform into martensite upon increasing in stress and strain, decreases with increasing TIA (Fig. 4(d)). Many factors, such as the concentration of strong stabilizing elements (for example, Mn and C) in RA and the size and morphology of RA, affect the mechanical stability of RA [42,43]. Table 2 shows the variations of Mn and C concentrations in different phases with TIA. The Mn concentrations in different phases were based on the EDS analysis of 40 randomly selected phases. Both the Mn and C concentrations decreased with increasing TIA, which is one of the main reasons for the decreasing stability of RA with increasing TIA, while both the Al and Si concentrations changed slightly with TIA. Moreover, the Mn concentration in different phases increased in the following order: EF, IF and RA. These results suggest the partitioning of Mn among different phases during the IA process, as was also confirmed by others [7,8,33,42,43]. The size of RA also strongly affects its mechanical stability and a reduction in its size is well known to enhance its stability by suppressing the martensite transformation [8,44,45]. The strengthening of austenite through grain refining and alloying increases the mechanical free energy required to transform austenite into martensite, which lowers the ability to nucleate martensite [8,42]. The RA size increases with an increase in TIA as shown in Fig. 3. Therefore, both the decreasing of the Mn and C concentrations in RA and the increasing of RA size are primarily attributed to the decreased RA stability with increasing TIA of the tested steel. Moreover, these trends are more remarkable for the CR850 specimen, which cause a remarkable decreasing of the RA fraction and the presence of a large amount of martensite as mentioned above.
HE behavior The excellent combination of strength and ductility of medium-Mn steels primarily stems from the TRIP effect during deformation [2e4,8]. However, the deformation-induced fresh martensite (DIM) is regarded to be most vulnerable to HE [3,15,26], which can be explained as follows. The DIM
possesses high carbon content because of the high carbon content of the original RA (Table 2). This kind of high carbon DIM does not experience any annealing or tempering treatment, and thus it is inherently brittle and has rather high susceptibility to HE, which can be further confirmed by the tensile properties and HE behavior of the CR850 specimen. The CR850 specimen, which has ~46.39 vol% martensite besides EF, IF and RA (~4.75 vol%), has much lower ductility and significantly higher HE susceptibility (Table 1). To verify this, the hydrogen-charged CR770 and CR850 specimens were pre-strained 5% and were then crosssectioned longitudinally and carefully observed under a SEM. It is obvious that almost no sub-cracks could be observed for the CR770 specimen (Fig. 11(a)), while the CR850 specimen was already fractured and a considerable number of subcracks in the uniform deformation region were found (Fig. 11(b)). These sub-cracks were formed either through cracking of hard martensite (white arrows) or by interface decohesion between martensite and ferrite phases (yellow arrows) for the hydrogen-charged CR850 specimen, as was also usually found in DP steels [17,22]. The sub-crack initiation might result from stress concentration caused by the different strain hardening rates of martensite and ferrite [17,46]. These sub-cracks prefer to propagate along the interface boundaries of martensite and ferrite (Fig. 11(c)), and thus formed intergranular fracture as shown in Fig. 9(g). These results strongly demonstrate the detrimental influence of hard martensite on promoting the early formation of sub-cracks. The presence of hydrogen would lower the cohesive bonding energy between lattice atoms [47], and significantly promote the formation of intergranular cracking as shown in Fig. 9(h), and thus exhibits much higher HE susceptibility (Table 1). Although the microstructures of the CR700, CR730 and CR770 specimens do not consist of any original martensite (Fig. 1(a), (b) and (c)), DIM would be transformed from the RA gradually with increasing tensile strain depending on the mechanical stability of RA, i.e., the higher the RA stability is, the later the DIM would be transformed, and thus its detrimental influence would be delayed. As the stability of RA decreases while its fraction increases with increasing TIA up to
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 9 e SEM fractographs in crack initiation region of (a,c,e) uncharged and (b,d,f) hydrogen-charged SSRT specimens: (a,b) CR700; (c,d) CR730; (e,f) CR770; (g,h) CR850. 770 C, the amount of DIM increases (Fig. 4), and thus causes an increasing trend of the HE susceptibility with an increase in TIA (Table 1 and Fig. 7). Notably, it has been confirmed that austenite is a sink for hydrogen primarily because of its high hydrogen solubility and low hydrogen diffusivity [15,48e50], and a linear relationship between CHr and the RA fraction was found in high carbon martensitic steels [48]. It was also found that a notable increase of CH from ~0.35 ppm for a full martensitic
microstructure to ~1.87 ppm for a reversion treated martensitic microstructure containing ~35 vol% RA in a TRIPassisted maraging steel (0.01Ce9Mne3Ni-1.4Al, wt%) [49]. Therefore, it was at first expected that there would be a slight increase of CHr with increasing TIA up to 770 C. However, the present study found that there was a slight decrease of CHr with increasing TIA up to 770 C, i.e., with increasing the RA fraction (Figs. 4 and 5). Thus, it is reasonable to regard that most of the hydrogen was not trapped within the RA because
Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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Fig. 10 e Highly magnified SEM fractographs in crack initiation region of hydrogen-charged SSRT specimens: (a) CR770; (b) CR850.
Table 2 e Concentrations of Mn and C in different phases (experimental error was given in the parenthesis). Specimen
Annealing temperature ( C)
Mn (wt.%) add.
CR700 CR730 CR770 CR850
700 730 770 850
4.88 4.88 4.88 4.88
RA 6.82 6.61 6.45 5.75
± 0.52 ± 0.71 ± 0.57 ± 0.49
IF 5.15 4.46 4.44 4.32
± 0.53 ± 0.21 ± 0.22 ± 0.23
C (wt.%) EF 4.38 ± 4.34 ± 4.29 ± 4.22 ±
0.14 0.19 0.15 0.17
add.
RA
0.20 0.20 0.20 0.20
0.665 0.596 0.592 0.503
Note: EF refers to elongated d-ferrite; IF refers to intercritical a-ferrite; RA refers to retained austenite.
Fig. 11 e SEM fractographs of the cross-sectional area of the 5% pre-strained (a) CR770 and (b,c) CR850 specimens.
of the extremely low hydrogen diffusivity in it [51] and the short charging time (60 min) applied, while it is most likely that most of the hydrogen was trapped by the interfaces between g and a phases, as was also pointed out by others [48,52], this is because the g/a interface is considered to be a
strong hydrogen trap [50]. Therefore, the area of phase interfaces decreases with an increase in the RA size i.e., TIA, and correspondingly a decrease of CHr (Table 3). Moreover, the hydrogen trapped by the g/a interface would be inherited into the interface of the DIM and ferrite [25,26], and thus
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Table 3 e Determination of hydrogen concentration per the phase interfaces area per unit volume (106 g). Specimen CR700 CR730 CR770 CR850
CHr (ppmw)
DRA size (mm)
VRA (vol%)
SVRA ( 1017, mm2)
CHr/SVRA ( 1017, g/mm2)
1.36 1.20 1.16 1.29
0.42 0.51 0.65 1.53
15.26 16.37 19.26 4.75
2.19 1.93 1.78 e
0.63 0.62 0.65 e
Note: DRAeRA size (diameter); VRAeVolume fraction of RA; SVRAethe area of phase interfaces per unit volume.
facilitated the brittle fracture occurred along the phase interfaces (Fig. 9). By assuming the RA as globular shape, the amount of phase interfaces area per unit volume (SVRA) could be expressed as [27]: SVRA ¼ 6 VRA =DRA
(2)
where DRA and VRA are the average diameter and volume fraction of RA, respectively. As shown in Table 3, CH/SVRA remains almost unchanged, although both SVRA and CH decrease with increasing TIA, which reveals that almost identical amount of hydrogen was trapped at the RA/ferrite interfaces. This result further confirms that the stability and amount of RA plays a critical role in determining the HE susceptibility of the tested steel. By the way, the existence of a large amount of martensite causes a slight increase of CHr for the CR850 specimen.
SSRT fracture characteristics One significant finding of the fracture surfaces of hydrogencharged SSRT specimens is the presence of dimples filled grains besides empty dimples (dimples without grain) as shown in Fig. 9(b), (d) and (f). This phenomenon, which is different from those of other AHSSs, has also been reported in cold-rolled [25,28] and warm-rolled [27] medium-Mn steels. It was proposed that this kind of fracture was generated by intergranular cracking occurring along the interface boundaries of RA grains by the hydrogen-enhanced decohesion (HEDE) mechanism that dissolved hydrogen reduce the cohesive bonding energy [25]. As shown in Fig. 12, the average Mn content in the dimples with grain is 6.52 ± 0.37 wt%, while the average Mn content in the empty dimples is 4.78 ± 0.34 wt% for the CR770 specimen;
which is in accordance with the results in Table 2. The average size of the traces of detached grains is ~0.61 mm, which is similar to the size of RA (~0.65 mm) for the CR770 specimen. Moreover, further statistical analysis of randomly selected 200 dimples of the hydrogen-charged CR700, CR730 and CR770 specimens demonstrates that the ratio of dimples filled with grains has a nearly linear relationship with the fraction of transformed RA (DIM) after SSRT as shown in Fig. 13, indicating an increase of the fraction of brittle interface decohesion fracture with DIM, i.e., the facture surfaces became increasingly brittle with increasing TIA. These results strongly suggest that these “grains” are DIM transformed from the RA and brittle interface decohesion occurred along the interface of DIM and ferrite. This kind of brittle interface decohesion fracture is an indication of less ductility for the hydrogencharged specimens (Fig. 7). Therefore, the characteristic fracture of the tested TRIPassisted medium-Mn steel can be explained as follows. For the specimens with no initial martensite after the IA treatment, a certain amount of deformation, which depends on the mechanical stability of RA, is needed for the transformation of RA into martensite (DIM); once the DIM was formed, brittle interface decohesion would occur along the boundaries of DIM and deformed ferrite through the HEDE mechanism [25], and thus caused the characteristic dimples filled with DIM. Notably, the presence of hydrogen especially the enrichment of hydrogen at g/a interface would significantly promote the formation of brittle interface decohesion [12,17,25]. For the region absent of RA, i.e., the IF and EF grains, it must have suffered considerable deformation before the formation of DIM; the HE of ferrite would deform and fracture by the hydrogen-enhanced local plasticity (HELP) mechanism [25], and thus formed the ductile fracture characterized by empty dimple morphology. For the specimen like CR850 having
Fig. 12 e (a) SEM fractographs in crack initiation region of hydrogen-charged CR770 specimen and (b) the distribution of Mn content in dimples with grain and without grain. Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204
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increasing TIA. Therefore, both the stability and amount of RA play a critical role in governing the HE behavior of TRIP-assisted medium-Mn steel.
Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (Grant No. 2017RC024).
references
Fig. 13 e Variations of the ratio of fracture dimples filled with grains with the transformed RA fraction after SSRT tests of hydrogen-charged SSRT specimens.
considerable amount of initial martensite before tensile deformation, martensite cracking and/or brittle interface decohesion along the boundaries of martensite and ferrite would occur at the start of deformation (Fig. 11(b)), and thus formed a mixed mode fracture of quasi-cleavage and intergranular.
Conclusions (1) The tested Al-containing medium-Mn steel exhibit a mixed microstructure of coarse elongated d-ferrite and an UFG constituent consisting of and rod- and globularshaped RA when TIA is lower than 850 C. The average size of RA exhibits a gradual increase with an increase in TIA, while the RA fraction exhibits an increasedecrease pattern with TIA. (2) The UTS increases while the YS decreases with an increase in TIA, whereas both the TEL and UTS TEL show an increase-decrease pattern with TIA. An excellent strength-ductility balance as high as ~53 GPa$% was obtained for the specimen annealed at 730 C. (3) Unlike that of UTS TEL, the HEI increases monotonically with an increase in TIA, the CR700 specimen demonstrates the lowest HE susceptibility, while the CR850 specimen exhibits the highest HE susceptibility. (4) Unlike that of dimple ductile fracture for the uncharged specimens, the hydrogen-charged specimens except the CR850 exhibit a mixed fracture of dimple ductile and brittle interface decohesion fracture mode. The CR850 specimen exhibits a mixed mode fracture of quasicleavage and intergranular. The facture surfaces became increasingly brittle with increasing TIA. (5) Both the decreasing of the C and Mn concentrations in the RA and the increasing of the RA size are primarily attributed to the decreased mechanical stability of RA with increasing TIA. Therefore, the amount of deformation-induced martensite increases, and then causes an increasing tendency of HE susceptibility with
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Please cite this article as: Zhang Y et al., Intercritical annealing temperature dependence of hydrogen embrittlement behavior of coldrolled Al-containing medium-Mn steel, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.06.204