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Hybrid nanostructure stainless steel with super-high strength and toughness
Available online at www.sciencedirect.com
ScienceDirect ScienceDirect Procedia Engineering 00 (2017) 000–000
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 207 (2017) 1791–1796
International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017, Kingdom International Conference on theCambridge, TechnologyUnited of Plasticity, ICTP 2017, 17-22 September 2017, Cambridge, United Kingdom
Hybrid nanostructure stainless steel with super-high strength and Hybrid nanostructure stainless steel with super-high strength and toughness toughness a,b a,b b b Gang Niu , Huibin Wu *, Da Zhang , Na Gong Gang Niua,b, Huibin Wua,b*, Da Zhangb, Na Gongb Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing 100083, China
a
b Beijing Engineering Technology Research Center of Special Steel for Traffic and Energy, Beijing 100083, China Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing 100083, China b Beijing Engineering Technology Research Center of Special Steel for Traffic and Energy, Beijing 100083, China
a
Abstract Abstract The objective of the present study is to obtain the super-high strength and toughness stainless steel through hybrid nanostructure design. The present hybrid nanostructured stainless was fabricated two toughness cold rollingstainless and annealing processes. The The objective of the study is to obtain the steel super-high strengthbyand steel through hybrid second cold-rolled and annealing was carried out on bimodal microstructure which was obtained by the first cold-rolled and nanostructure design. The hybrid nanostructured stainless steel was fabricated by two cold rolling and annealing processes. The annealing on the original microstructure. The bimodal structure consisting of nanometer grains and a small amount of micrometer second cold-rolled and annealing was carried out on bimodal microstructure which was obtained by the first cold-rolled and grains which were distributed in band.The After the second andnanometer annealing, bothand thea small original coarse grain zone annealing on the original microstructure. bimodal structurecold-rolled consisting of grains amount of micrometer (micrometer grains) and fine grain zone (nanometer grains) formed their own finer bimodal microstructure and the bands the grains which were distributed in band. After the second cold-rolled and annealing, both the original coarse grain ofzone grain distribution became narrower. In our investigations, the hybrid nanostructure obtained by two cold rolling and annealing (micrometer grains) and fine grain zone (nanometer grains) formed their own finer bimodal microstructure and the bands of the processes had finerbecame grains narrower. and narrower bands of the grain the distribution. Meanwhile the resultsby of two tensile experiment showed that grain distribution In our investigations, hybrid nanostructure obtained cold rolling and annealing the yield strength (1221 MPa) tensilebands strength (1376 MPa) of the hybrid nanostructure wereofgreatly in the situation processes had finer grains and and narrower of the grain distribution. Meanwhile the results tensileimproved experiment showed that was not significant the bimodal microstructure. It is obvious that the that the decrease the toughness t=45.3%) the yield strengthof (1221 MPa) and(εtensile strength (1376 MPa) ofcompared the hybridwith nanostructure were greatly improved in the situation yieldthe strength of of thethe hybrid nanostructure increased 2.7 times and the with total the elongation remained atItthe level ofthat 45.3% was not by significant compared bimodal still microstructure. is obvious the that decrease toughness (εt=45.3%) compared with the original microstructure. Mainly because we embed the soft micrometer grains into the hard nanometer grains yield strength of the hybrid nanostructure increased by 2.7 times and the total elongation still remained at the level of 45.3% to form thewith lamellar interphase structure. The back-stress hardening, dislocation hardening, TWP andhard TRIPnanometer effect produced compared the original microstructure. Mainly because we embed the soft micrometer grainseffect into the grains by the hybrid nanostructure during tensile process contributed to high strength and toughness. And this contribution more to form the lamellar interphase structure. The back-stress hardening, dislocation hardening, TWP effect and TRIP effect was produced pronounced when the width of lamellar become narrower. by the hybrid nanostructure during tensile process contributed to high strength and toughness. And this contribution was more © 2017 The when Authors. by Elsevier Ltd.narrower. pronounced the Published width of lamellar become the scientific committee of the International Conference on the Technology Peer-review responsibility of Elsevier © 2017 The under Authors. Published by Ltd. © 2017 The Authors. Published by Elsevier Ltd. of Plasticity . responsibility Peer-review under of the scientific committee of the International Conference on the Technology Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. of Plasticity.
* Corresponding author. Tel.: 13910297962; fax: 62332947. E-mail address: [email protected] * Corresponding author. Tel.: 13910297962; fax: 62332947. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Plasticity . Peer-review under responsibility of the scientific committee Plasticity.
of the International Conference on the Technology of of the International Conference on the Technology of
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.940
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Gang Niu et al. / Procedia Engineering 207 (2017) 1791–1796 Gang Niu / Procedia Engineering 00 (2017) 000–000
Keywords: Austenitic stainless steel, Hybrid nanostructure, Reverse transformation, Strength, Ductility
1. Introduction Austenitic stainless steels have been widely used in automotive components and biomedical field due to its special properties of being non-magnetic, corrosion-resistant and easy to shape [1,2]. However, further application of austenitic stainless steels is limited by its relatively low strength. And the biocompatibility of this material is obviously inferior to calcium phosphate and bio-active glass. Recent reviews concerning ultrafine-grained steel were presented by Misra of Louisiana State University [3-6]. It has been observed that nanocrystals with sizes ≤ 500 nm tend to improve the cell viability and promote the formation of bone lipoprotein. The micron grains of 0.5~2 μm in size can help to enhance the cell adhesion and stimulate metabolic activity, and this makes stainless steel with micro/nanometer composite structure possess better human histocompatibility compared to that of the traditional coarse-grained medical stainless steel. Microstructural refinement is one of the most important methods of strengthening metallic materials to yield lightweight components with improved performance [7-9]. In particular, the extremely high yield strength has been achieved in the ultrafine-grained (UFG) or nanograined (NG) stainless steels, wherein the yield strength well resembled the Hall-Petch relationship [10]. However, at the same time, the ductility of stainless steels with the homogeneous UFG/NG microstructures is considerably low compared with their coarsegrained (CG) microstructures [11-13]. And the inferior ductility has severely limited the aforementioned applications of this material. The low ductility is mainly caused by the low strain hardening [14-17], which is the result of their small grain sizes. In recent years, several research efforts were made to achieve a combination of high strength and reasonable ductility through creating heterogeneous microstructures, and it was clearly demonstrated that the bimodal grain size distribution is a simple and effective method to obtain heterogeneous microstructures with high strength and sufficient ductility [18-23]. Thus it can be seen if the stainless steel was prepared into the properly heterogeneous microstructure, it not only have good biocompatibility, but also have a good matches of strength and the ductility. Here, we try to design a particularly hybrid nanostructured low-Ni high-Mn austenitic stainless steel based on the bimodal microstructure, which will realize by two cold rolling and annealing processes. It is mainly fabricated by the reverse transformation of strain-induced martensite and the recrystallization of deformed austenite. 2. Experimental procedure The chemical compositions of commercial low-Ni high-Mn austenitic stainless steel were (in wt. %): C-0.091 Si0.35 Mn-10.1 Cr-13.8 Ni-1.25 Cu-0.90 N-0.11 Nb-0.12 and balanced by Fe. The commercial steel sheets of 7.6 mm thickness were first heated to the 1050 °C and held for 12 min to solution treatment. The initial austenite grain size was in the range of 5~20 μm as shown in Fig. 1. Then, the experiments of the first cold-rolled and annealing were carried out. The cold deformation with 70% total thickness reduction was carried out in the solution-treated samples. The annealing experiments were conducted on CCT-AV-II simulated annealing experiment machine for the samples of cold rolled [24]. The heating rate was 30 °C/s to the holding temperature of 800 °C, the soaking time was 10 s on continuous annealing and the cooling rate was 50 °C/s down to 300 °C followed by free cooling. Next, the second cold-rolled and annealing experiments were carried out on the bimodal microstructure obtained by the first coldrolled and annealing. The cold deformation with 50% total thickness reduction was carried out in the bimodal samples. And the heating rate was 30 °C/s to the holding temperature of 720 °C, the soaking time was 1 s on continuous annealing and then cold to room temperature with the same cooling process. The microstructural evolutions were analyzed using optical microscope (Zeiss Axiovert 40MAT) and the scanning electron microscopy (Zeiss Ultra 55) All specimens were prepared using an electropolishing at a voltage of 15 V for 25 s, and the electrolyte contained 20 vol.% of perchloric acid and 80% of ethanol. The electron backscatter diffraction was applied to determine the microscopic characterization. The tensile tests were carried out at room temperature using the CMT5105 tensile machine. Non-standard tensile tests specimen geometry, the 10 mm gauge length, had to be used. And the strain rate was 10-3s-1.
Gang Niu et al. / Procedia Engineering 207 (2017) 1791–1796 Gang Niu / Procedia Engineering 00 (2017) 000–000
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100μm Fig. 1. Original grain structure of low-Ni high-Mn austenitic stainless steel after solution treatment
3. Results and discussion The first annealed microstructure (800 °C for 10 s) and the grain size distribution of the low-Ni high-Mn austenitic stainless steel with 70% cold deformation were shown in Fig. 2. It can be found that the nanometer grains and the micrometer grains obviously presented heterogeneous lamella structure. And this kind of lamella microstructure was mainly composed of nanometer grains accounted for about 75%. Besides, the peak value of nanometer grains and micrometer grains were about 500 nm and 1.1 μm, respectively. This was the bimodal microstructures obtained by the first cold-rolled and annealing. Fig. 3 shows the second cold-rolled and annealed microstructure. As previous research work [25], the coarse grain zone (micrometer grains) and fine grain zone (nanometer grains) of the bimodal microstructures also produced the strain-induced martensite and deformed austenite after cold-rolling. After the proper annealing process (720 °C for 1 s), these two zones formed their own finer bimodal microstructure and the bands of the grain distribution became narrower due to the bands of the straininduced martensite and deformed austenite in the bimodal microstructure became narrower under the second coldrolling. So, the hybrid nanostructure obtained by two cold rolling and annealing processes had finer grains and narrower bands of the grain distribution compared with the bimodal microstructure.
10μm Fig. 2. The bimodal microstructure and the grain size distribution of experimental steel. The coarse grain zone and fine grain zone as shouwn in black box and red box, respectively. Number of the measured grains (N) was gived in the figure
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3μm Fig. 3. The hybrid nanostructure of the low-Ni high-Mn austenitic stainless steel
Fig. 4. Mechanical properties of the hybrid nanostructure, the bimodal microstructure and the original microstructure (a) Engineering stress-strain curves; (b) Strain hardening rate curves
The mechanical properties of the hybrid nanostructure, the bimodal microstructure and the original microstructure were shown in Fig. 4. The bimodal microstructure had better comprehensive mechanical properties with yield stress (σy) ~738 MPa and total elongation (εt) ~56.4% compared with the original microstructure (σ y=332 MPa, εt=65.6%). And more importantly, the results of tensile experiment showed that the yield strength (1221 MPa) and tensile strength (1376 MPa) of the hybrid nanostructure were dramatically improved in the situation that the decrease of the toughness (εt=45.3%) was not significant compared with the bimodal microstructure. It was obvious that the yield strength of the hybrid nanostructure increased by 2.7 times and the total elongation still remained at the sufficient level of 45.3% compared with the original microstructure was shown in Fig. 4(a). The extraordinarily high yield strength of the hybrid nanostructure was attributed to the back-stress hardening and dislocation hardening [22]. The soft lamellae of recrystallized micrometer grains will start plastic deformation first during tensile process. However, they were constrained by surrounding hard lamellae of reversed nanometer grains. Therefore, the dislocation in such grains was piled up and blocked at lamella interfaces, which were actually grain boundaries. This produced a longrange back stress [22] or [26,27] to make it difficult for dislocation to slip in the lamellae of micrometer grains until
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the surrounding lamellae of nanometer grains started to yield at a larger global strain and to stop the dislocation source from emitting more dislocations. It means that the soft lamellae constrained by hard lamella matrix have appeared much stronger than when they are not constrained. This was one of the primary reasons for the high yield strength of the hybrid nanostructure. Besides, the increase of dislocation density with tensile strain has also caused an increase in stress, i.e., dislocation hardening. And with the decrease of grain size, the effect of dislocation strengthening was more apparent, which was caused by the increase in grain boundaries. The back-stress hardening and dislocation hardening contributed to the high strain hardening which can greatly improve the strength of the hybrid nanostructure at the early strain stage was shown in Fig. 4(b). And the strength of the hybrid nanostructure with finer grains and narrower bands of the grain distribution was much greater than the bimodal microstructure and original microstructure. After the samples yielded, the transformation of strain-induced martensite would occur in further tension, i.e., TRIP [28,29] effect which allowed the materials to undergo uniform plastic deformation with high strain hardening as shown in Fig. 4(b). So the good match of strength and ductility was possessed in austenitic stainless steel. In the same type of microstructure, the rate of the formation of strain-induced martensite in tensile straining mainly depended on the grain size [30]. When the grain size was within the micron range, the increase of grain size favored the formation of strain-induced martensite. However, in nanometer grains, the forming ability of strain-induced martensite was even better than in the coarse-grained microstructure. So, the original microstructure easily transformed to martensite in tensile straining, which maintained the strain hardening of it at a high level. But, the softer lamellae of the hybrid nanostructure were easier to deform than hard lamellae. Therefore, this caused plastic strain partitioning where the soft lamellae carried much higher plastic strain than hard lamellae. In addition, the size of micrometer grains in soft lamellae of the hybrid nanostructure was smaller than that of original microstructure and bimodal microstructure. So, the rate of the formation of strain-induced martensite in the hybrid nanostructure was lower than original microstructure and bimodal microstructure. However, when the tensile strain reached an enough level, the rate of the formation of strain-induced martensite in the hybrid nanostructure increased rapidly due to the deformation of large numbers of the nanometer grains. And the strain hardening rate also increased quickly as shown in Fig. 4(b). Therefore, the TRIP effect caused continuous plastic deformation of the hybrid nanostructure stainless steel. And it combined with back-stress hardening and dislocation hardening to make the hybrid nanostructure stainless steel have excellent comprehensive mechanical properties. 4. Conclusions This research obtained the hybrid nanostructure stainless steel with super-high strength and toughness through the reverse transformation of strain-induced martensite and the recrystallization of deformed austenite design, which was fabricated by two cold rolling and annealing processes. The main characteristic of the hybrid nanostructure was that the soft micrometer grains embed into the hard nanometer grains to form the lamellar interphase structure. The yield strength (1221 MPa) and tensile strength (1376 MPa) of the hybrid nanostructure were dramatically improved, and the yield strength was increased by 2.7 times and the total elongation still remained at the sufficient level of 45.3% compared with the original microstructure. The back-stress hardening, dislocation hardening, TWIP effect and TRIP effect were responsible for the continuously high strain-hardening rate in tension, which has led directly to the high strength and sufficient ductility. With the decrease of the grains size and the narrow of the lamellar, these effects were more pronounced in the hybrid nanostructure compared with the bimodal microstructure and the original microstructure. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grant Nos. 51474031). References [1] H.W. Huang, Z.B. Wang, J. Lu, K. Lu, Fatigue behaviors of AISI 316L stainless steel with a gradient nanostructured surface layer [J]. Acta Mater. 87 (2015) 150–160.
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Karjalainen, Advancing nanograined/ultrafine-grained structures for metal implant technology: Interplay between grooving of nano/ultrafine grains and cellular response [J], Mater. Sci. Eng. C, 30 (2010) 1050–1059. [7] R.Z. Valiev, M.J. Zehetbauer, Y. Estrin, H. W. Höppel, Y. Ivanisenko, H. Hahn, G. Wilde, H. J. Roven, X. Sauvage, T. G. Langdon, The innovation potential of bulk nanostructured materials [J]. Advanced Eng. Mater., 9 (2007) 527–533. [8] Y. Estrin, A. Vinogradov, Extreme grain refinement by severe plastic deformation: a wealth of challenging science [J]. Acta mater., 61 (2013) 782–817. [9] R. Zheng, Y. Sun, K. Ameyama, C. Ma, Optimizing the strength and ductility of spark plasma sintered Al 2024 alloy by conventional thermomechanical treatment [J]. Mater. Sci. Eng. A, 590 (2014) 147–152. [10] X.H. Chen, J. Lu, L. Lu, K. Lu, Tensile properties of a nanocrystalline 316L austenitic stainless steel [J]. Scripta Mater., 52 (2005) 1039– 1044. [11] H. Ueno, K. Kakihata, Y. Kaneko, S. Hashimoto, A. Vinogradov, Nanostructurization assisted by twinning during equal channel angular pressing of metastable 316L stainless steel[J]. J. Mater. Sci, 46 (2011) 4276–4283. [12] Y. Mine, Z. Horita, Y. Murakami, Effect of hydrogen on martensite formation in austenitic stainless steels in high-pressure torsion [J]. Acta Mater., 57 (2009) 2993–3002. [13] M. Eskandari, A. Najafizadeh, A. Kermanpur, Effect of strain-induced martensite on the formation of nanocrystalline 316L stainless steel after cold rolling and annealing [J]. Mater. Sci. Eng. A, 519 (2009) 46–50. [14] X.L. Wu, P. Jiang, L. Chen, F.P. Yuan, Y. T. Zhu, Extraordinary strain hardening by gradient structure [J]. PNAS, 111 (2014) 7197–7201. [15] T.H. Fang, W.L. Li, N.R. Tao, K. Lu, Revealing extraordinary intrinsic tensile plasticity in gradient nano-grained copper [J]. Science, 331 (2011) 1587–1590. [16] X. Wu, F. Yuan, M. Yang, P. Jiang, C. Zhang, L. Chen, Nanodomained nickel unite nanocrystal strength with coarse-grain ductility [J]. Scientific reports, 5 (2015) 11728. [17] Y. Wei, Y. Li, L. Zhu, Y. Liu, X. Lei, G. Wang, Y. Wu, et at. Evading the strength–ductility trade-off dilemma in steel through gradient hierarchical nanotwins[J]. Nature commun., 5 (2014). [18] X. Guo, G. Yang, G.J. Weng, The saturation state of strength and ductility of bimodal nanostructured metals [J]. Mater. Lett., 175 (2016) 131–134. [19] R. Zheng, Y. Sun, W. Xiao, K. Ameyama, C. Ma, Nanostructured Al 87 Ni 8.5 Ce 3 Fe 1 Cu 0.5 alloy prepared by mechanical milling spark plasma sintering and hot extrusion [J]. Mater. Sci. Eng. A, 606 (2014) 426–433. [20] Y. Wang, M. Chen, F. Zhou, E. Ma, High tensile ductility in a nanostructured metal[J]. Nature, 419 (2002) 912–915. [21] A. M.K. Esawi, N. T. Aboulkhair, Bi-modally structured pure aluminum for enhanced strength and ductility[J]. Mater. Des., 83 (2015) 493– 498. [22] X. Wu, M. Yang, F. Yuan, G. Wu, Y. Wei, X.X. Huang, Y.T. Zhu, Heterogeneous lamella structure unites ultrafine-grain strength with coarse-grain ductility [J]. PNAS, 112 (2015) 14501–14505. [23] S.K. Vajpai, M. Ota, Z. Zhang, K. Ameyama, Three-dimensionally gradient harmonic structure design: an integrated approach for high performance structural materials [J]. Mater. Res. Lett., 4 (2016) 191–197. [24] X. Zhang, Z. Wen, R. Dou, G. Zhou, Z. Li, Evolution of microstructure and mechanical properties of cold-rolled SUS430 stainless steel during a continuous annealing process. Mater. Sci. Eng. A 598(2014) 22-27 [25] H. Wu, G. Niu, J. Cao, Annealing of strain-induced martensite to obtain micro/nanometre grains in austenitic stainless. Materials Science and Technology, 33 (2017) 480-486. [26] C.W. Sinclair, G. Saada, J.D. Embury, Role of internal stresses in co-deformed two-phase materials [J]. Philos. Mag., 86 (2006) 4081–4098. [27] H. Mughrabi, Dislocation wall and cell structures and long-range internal stresses in deformed metal crystals [J]. Acta metall., 31 (1983) 1367–1379. [28] S. Zaefferer, J. Ohlert, W. Bleck, A study of microstructure, transformation mechanisms and correlation between microstructure and mechanical properties of a low alloyed TRIP steel[J]. Acta Mater., 52 (2004) 2765-2778. [29] P. Behjati, A. Kermanpur, A. Najafizadeh, H. Samaei Baghbadorani, J.-G. Jung, Y.-K. Lee, Enhanced mechanical properties in a highmanganese austenitic steel through formation of nano grains, nanotwinned austenite grains, nano carbides and TRIP[J]. 610 (2014) 273-278. [30] A. Kisko, R.D.K. Misra, J. Talonen, L.P. Karjalainen, The influence of grain size on the strain-induced martensite formation in tensile straining of an austenitic 15Cr–9Mn–Ni–Cu stainless steel [J]. Mater. Sci. Eng. A, 578 (2013) 408–416.
ScienceDirect ScienceDirect Procedia Engineering 00 (2017) 000–000
Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 207 (2017) 1791–1796
International Conference on the Technology of Plasticity, ICTP 2017, 17-22 September 2017, Kingdom International Conference on theCambridge, TechnologyUnited of Plasticity, ICTP 2017, 17-22 September 2017, Cambridge, United Kingdom
Hybrid nanostructure stainless steel with super-high strength and Hybrid nanostructure stainless steel with super-high strength and toughness toughness a,b a,b b b Gang Niu , Huibin Wu *, Da Zhang , Na Gong Gang Niua,b, Huibin Wua,b*, Da Zhangb, Na Gongb Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing 100083, China
a
b Beijing Engineering Technology Research Center of Special Steel for Traffic and Energy, Beijing 100083, China Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing 100083, China b Beijing Engineering Technology Research Center of Special Steel for Traffic and Energy, Beijing 100083, China
a
Abstract Abstract The objective of the present study is to obtain the super-high strength and toughness stainless steel through hybrid nanostructure design. The present hybrid nanostructured stainless was fabricated two toughness cold rollingstainless and annealing processes. The The objective of the study is to obtain the steel super-high strengthbyand steel through hybrid second cold-rolled and annealing was carried out on bimodal microstructure which was obtained by the first cold-rolled and nanostructure design. The hybrid nanostructured stainless steel was fabricated by two cold rolling and annealing processes. The annealing on the original microstructure. The bimodal structure consisting of nanometer grains and a small amount of micrometer second cold-rolled and annealing was carried out on bimodal microstructure which was obtained by the first cold-rolled and grains which were distributed in band.The After the second andnanometer annealing, bothand thea small original coarse grain zone annealing on the original microstructure. bimodal structurecold-rolled consisting of grains amount of micrometer (micrometer grains) and fine grain zone (nanometer grains) formed their own finer bimodal microstructure and the bands the grains which were distributed in band. After the second cold-rolled and annealing, both the original coarse grain ofzone grain distribution became narrower. In our investigations, the hybrid nanostructure obtained by two cold rolling and annealing (micrometer grains) and fine grain zone (nanometer grains) formed their own finer bimodal microstructure and the bands of the processes had finerbecame grains narrower. and narrower bands of the grain the distribution. Meanwhile the resultsby of two tensile experiment showed that grain distribution In our investigations, hybrid nanostructure obtained cold rolling and annealing the yield strength (1221 MPa) tensilebands strength (1376 MPa) of the hybrid nanostructure wereofgreatly in the situation processes had finer grains and and narrower of the grain distribution. Meanwhile the results tensileimproved experiment showed that was not significant the bimodal microstructure. It is obvious that the that the decrease the toughness t=45.3%) the yield strengthof (1221 MPa) and(εtensile strength (1376 MPa) ofcompared the hybridwith nanostructure were greatly improved in the situation yieldthe strength of of thethe hybrid nanostructure increased 2.7 times and the with total the elongation remained atItthe level ofthat 45.3% was not by significant compared bimodal still microstructure. is obvious the that decrease toughness (εt=45.3%) compared with the original microstructure. Mainly because we embed the soft micrometer grains into the hard nanometer grains yield strength of the hybrid nanostructure increased by 2.7 times and the total elongation still remained at the level of 45.3% to form thewith lamellar interphase structure. The back-stress hardening, dislocation hardening, TWP andhard TRIPnanometer effect produced compared the original microstructure. Mainly because we embed the soft micrometer grainseffect into the grains by the hybrid nanostructure during tensile process contributed to high strength and toughness. And this contribution more to form the lamellar interphase structure. The back-stress hardening, dislocation hardening, TWP effect and TRIP effect was produced pronounced when the width of lamellar become narrower. by the hybrid nanostructure during tensile process contributed to high strength and toughness. And this contribution was more © 2017 The when Authors. by Elsevier Ltd.narrower. pronounced the Published width of lamellar become the scientific committee of the International Conference on the Technology Peer-review responsibility of Elsevier © 2017 The under Authors. Published by Ltd. © 2017 The Authors. Published by Elsevier Ltd. of Plasticity . responsibility Peer-review under of the scientific committee of the International Conference on the Technology Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. of Plasticity.
* Corresponding author. Tel.: 13910297962; fax: 62332947. E-mail address: [email protected] * Corresponding author. Tel.: 13910297962; fax: 62332947. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Plasticity . Peer-review under responsibility of the scientific committee Plasticity.
of the International Conference on the Technology of of the International Conference on the Technology of
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity. 10.1016/j.proeng.2017.10.940
1792 2
Gang Niu et al. / Procedia Engineering 207 (2017) 1791–1796 Gang Niu / Procedia Engineering 00 (2017) 000–000
Keywords: Austenitic stainless steel, Hybrid nanostructure, Reverse transformation, Strength, Ductility
1. Introduction Austenitic stainless steels have been widely used in automotive components and biomedical field due to its special properties of being non-magnetic, corrosion-resistant and easy to shape [1,2]. However, further application of austenitic stainless steels is limited by its relatively low strength. And the biocompatibility of this material is obviously inferior to calcium phosphate and bio-active glass. Recent reviews concerning ultrafine-grained steel were presented by Misra of Louisiana State University [3-6]. It has been observed that nanocrystals with sizes ≤ 500 nm tend to improve the cell viability and promote the formation of bone lipoprotein. The micron grains of 0.5~2 μm in size can help to enhance the cell adhesion and stimulate metabolic activity, and this makes stainless steel with micro/nanometer composite structure possess better human histocompatibility compared to that of the traditional coarse-grained medical stainless steel. Microstructural refinement is one of the most important methods of strengthening metallic materials to yield lightweight components with improved performance [7-9]. In particular, the extremely high yield strength has been achieved in the ultrafine-grained (UFG) or nanograined (NG) stainless steels, wherein the yield strength well resembled the Hall-Petch relationship [10]. However, at the same time, the ductility of stainless steels with the homogeneous UFG/NG microstructures is considerably low compared with their coarsegrained (CG) microstructures [11-13]. And the inferior ductility has severely limited the aforementioned applications of this material. The low ductility is mainly caused by the low strain hardening [14-17], which is the result of their small grain sizes. In recent years, several research efforts were made to achieve a combination of high strength and reasonable ductility through creating heterogeneous microstructures, and it was clearly demonstrated that the bimodal grain size distribution is a simple and effective method to obtain heterogeneous microstructures with high strength and sufficient ductility [18-23]. Thus it can be seen if the stainless steel was prepared into the properly heterogeneous microstructure, it not only have good biocompatibility, but also have a good matches of strength and the ductility. Here, we try to design a particularly hybrid nanostructured low-Ni high-Mn austenitic stainless steel based on the bimodal microstructure, which will realize by two cold rolling and annealing processes. It is mainly fabricated by the reverse transformation of strain-induced martensite and the recrystallization of deformed austenite. 2. Experimental procedure The chemical compositions of commercial low-Ni high-Mn austenitic stainless steel were (in wt. %): C-0.091 Si0.35 Mn-10.1 Cr-13.8 Ni-1.25 Cu-0.90 N-0.11 Nb-0.12 and balanced by Fe. The commercial steel sheets of 7.6 mm thickness were first heated to the 1050 °C and held for 12 min to solution treatment. The initial austenite grain size was in the range of 5~20 μm as shown in Fig. 1. Then, the experiments of the first cold-rolled and annealing were carried out. The cold deformation with 70% total thickness reduction was carried out in the solution-treated samples. The annealing experiments were conducted on CCT-AV-II simulated annealing experiment machine for the samples of cold rolled [24]. The heating rate was 30 °C/s to the holding temperature of 800 °C, the soaking time was 10 s on continuous annealing and the cooling rate was 50 °C/s down to 300 °C followed by free cooling. Next, the second cold-rolled and annealing experiments were carried out on the bimodal microstructure obtained by the first coldrolled and annealing. The cold deformation with 50% total thickness reduction was carried out in the bimodal samples. And the heating rate was 30 °C/s to the holding temperature of 720 °C, the soaking time was 1 s on continuous annealing and then cold to room temperature with the same cooling process. The microstructural evolutions were analyzed using optical microscope (Zeiss Axiovert 40MAT) and the scanning electron microscopy (Zeiss Ultra 55) All specimens were prepared using an electropolishing at a voltage of 15 V for 25 s, and the electrolyte contained 20 vol.% of perchloric acid and 80% of ethanol. The electron backscatter diffraction was applied to determine the microscopic characterization. The tensile tests were carried out at room temperature using the CMT5105 tensile machine. Non-standard tensile tests specimen geometry, the 10 mm gauge length, had to be used. And the strain rate was 10-3s-1.
Gang Niu et al. / Procedia Engineering 207 (2017) 1791–1796 Gang Niu / Procedia Engineering 00 (2017) 000–000
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100μm Fig. 1. Original grain structure of low-Ni high-Mn austenitic stainless steel after solution treatment
3. Results and discussion The first annealed microstructure (800 °C for 10 s) and the grain size distribution of the low-Ni high-Mn austenitic stainless steel with 70% cold deformation were shown in Fig. 2. It can be found that the nanometer grains and the micrometer grains obviously presented heterogeneous lamella structure. And this kind of lamella microstructure was mainly composed of nanometer grains accounted for about 75%. Besides, the peak value of nanometer grains and micrometer grains were about 500 nm and 1.1 μm, respectively. This was the bimodal microstructures obtained by the first cold-rolled and annealing. Fig. 3 shows the second cold-rolled and annealed microstructure. As previous research work [25], the coarse grain zone (micrometer grains) and fine grain zone (nanometer grains) of the bimodal microstructures also produced the strain-induced martensite and deformed austenite after cold-rolling. After the proper annealing process (720 °C for 1 s), these two zones formed their own finer bimodal microstructure and the bands of the grain distribution became narrower due to the bands of the straininduced martensite and deformed austenite in the bimodal microstructure became narrower under the second coldrolling. So, the hybrid nanostructure obtained by two cold rolling and annealing processes had finer grains and narrower bands of the grain distribution compared with the bimodal microstructure.
10μm Fig. 2. The bimodal microstructure and the grain size distribution of experimental steel. The coarse grain zone and fine grain zone as shouwn in black box and red box, respectively. Number of the measured grains (N) was gived in the figure
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3μm Fig. 3. The hybrid nanostructure of the low-Ni high-Mn austenitic stainless steel
Fig. 4. Mechanical properties of the hybrid nanostructure, the bimodal microstructure and the original microstructure (a) Engineering stress-strain curves; (b) Strain hardening rate curves
The mechanical properties of the hybrid nanostructure, the bimodal microstructure and the original microstructure were shown in Fig. 4. The bimodal microstructure had better comprehensive mechanical properties with yield stress (σy) ~738 MPa and total elongation (εt) ~56.4% compared with the original microstructure (σ y=332 MPa, εt=65.6%). And more importantly, the results of tensile experiment showed that the yield strength (1221 MPa) and tensile strength (1376 MPa) of the hybrid nanostructure were dramatically improved in the situation that the decrease of the toughness (εt=45.3%) was not significant compared with the bimodal microstructure. It was obvious that the yield strength of the hybrid nanostructure increased by 2.7 times and the total elongation still remained at the sufficient level of 45.3% compared with the original microstructure was shown in Fig. 4(a). The extraordinarily high yield strength of the hybrid nanostructure was attributed to the back-stress hardening and dislocation hardening [22]. The soft lamellae of recrystallized micrometer grains will start plastic deformation first during tensile process. However, they were constrained by surrounding hard lamellae of reversed nanometer grains. Therefore, the dislocation in such grains was piled up and blocked at lamella interfaces, which were actually grain boundaries. This produced a longrange back stress [22] or [26,27] to make it difficult for dislocation to slip in the lamellae of micrometer grains until
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the surrounding lamellae of nanometer grains started to yield at a larger global strain and to stop the dislocation source from emitting more dislocations. It means that the soft lamellae constrained by hard lamella matrix have appeared much stronger than when they are not constrained. This was one of the primary reasons for the high yield strength of the hybrid nanostructure. Besides, the increase of dislocation density with tensile strain has also caused an increase in stress, i.e., dislocation hardening. And with the decrease of grain size, the effect of dislocation strengthening was more apparent, which was caused by the increase in grain boundaries. The back-stress hardening and dislocation hardening contributed to the high strain hardening which can greatly improve the strength of the hybrid nanostructure at the early strain stage was shown in Fig. 4(b). And the strength of the hybrid nanostructure with finer grains and narrower bands of the grain distribution was much greater than the bimodal microstructure and original microstructure. After the samples yielded, the transformation of strain-induced martensite would occur in further tension, i.e., TRIP [28,29] effect which allowed the materials to undergo uniform plastic deformation with high strain hardening as shown in Fig. 4(b). So the good match of strength and ductility was possessed in austenitic stainless steel. In the same type of microstructure, the rate of the formation of strain-induced martensite in tensile straining mainly depended on the grain size [30]. When the grain size was within the micron range, the increase of grain size favored the formation of strain-induced martensite. However, in nanometer grains, the forming ability of strain-induced martensite was even better than in the coarse-grained microstructure. So, the original microstructure easily transformed to martensite in tensile straining, which maintained the strain hardening of it at a high level. But, the softer lamellae of the hybrid nanostructure were easier to deform than hard lamellae. Therefore, this caused plastic strain partitioning where the soft lamellae carried much higher plastic strain than hard lamellae. In addition, the size of micrometer grains in soft lamellae of the hybrid nanostructure was smaller than that of original microstructure and bimodal microstructure. So, the rate of the formation of strain-induced martensite in the hybrid nanostructure was lower than original microstructure and bimodal microstructure. However, when the tensile strain reached an enough level, the rate of the formation of strain-induced martensite in the hybrid nanostructure increased rapidly due to the deformation of large numbers of the nanometer grains. And the strain hardening rate also increased quickly as shown in Fig. 4(b). Therefore, the TRIP effect caused continuous plastic deformation of the hybrid nanostructure stainless steel. And it combined with back-stress hardening and dislocation hardening to make the hybrid nanostructure stainless steel have excellent comprehensive mechanical properties. 4. Conclusions This research obtained the hybrid nanostructure stainless steel with super-high strength and toughness through the reverse transformation of strain-induced martensite and the recrystallization of deformed austenite design, which was fabricated by two cold rolling and annealing processes. The main characteristic of the hybrid nanostructure was that the soft micrometer grains embed into the hard nanometer grains to form the lamellar interphase structure. The yield strength (1221 MPa) and tensile strength (1376 MPa) of the hybrid nanostructure were dramatically improved, and the yield strength was increased by 2.7 times and the total elongation still remained at the sufficient level of 45.3% compared with the original microstructure. The back-stress hardening, dislocation hardening, TWIP effect and TRIP effect were responsible for the continuously high strain-hardening rate in tension, which has led directly to the high strength and sufficient ductility. With the decrease of the grains size and the narrow of the lamellar, these effects were more pronounced in the hybrid nanostructure compared with the bimodal microstructure and the original microstructure. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grant Nos. 51474031). References [1] H.W. Huang, Z.B. Wang, J. Lu, K. Lu, Fatigue behaviors of AISI 316L stainless steel with a gradient nanostructured surface layer [J]. Acta Mater. 87 (2015) 150–160.
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