M3 Microstructure Control Theory and Technology of the Third-Generation Automotive Steels with HighStrength and High Ductility
WANG Cunyu1,CHANG Ying2(),ZHOU Fengluan1,CAO Wenquan1,DONG Han1,3,WENG Yuqing1
1.Special Steel Institute, Central Iron and Steel Research Institute, Beijing 100081, China 2.School of Automotive Engineering, Dalian University of Technology, Dalian 116024, China 3.School of Material Science and Engineering, Shanghai University, Shanghai 200444, China
Cite this article:
WANG Cunyu,CHANG Ying,ZHOU Fengluan,CAO Wenquan,DONG Han,WENG Yuqing. M3 Microstructure Control Theory and Technology of the Third-Generation Automotive Steels with HighStrength and High Ductility. Acta Metall Sin, 2020, 56(4): 400-410.
An important topic is the achievement of high strength and high plasticity for the development of automotive steels. Present article reviews the M3 (multiphase, metastable and multiscale) microstructure and property control theory and technology of high-strength and high-ductility third-generation automotive steels, as well as new challenges. M3 microstructure and property-microstructure control theory provide theoretical support for the development of steels with high strength and high plasticity. Transformation induced plasticity (TRIP) effect of metastable austenite has a significant influence on properties and microstructure of steels. On the one hand, it can enhance the work-hardening rate and thereby improve strength and plasticity of steels. On the other hand, it causes some new problems, such as the increase of the shear edge crack sensitivity, the decrease of hydrogen induced delayed fracture properties, and more complex transformation behavior of metastable austenite under cyclic loading. At present, the quality consistency and basic research on application are insufficient for the high-strength and high-plasticity steels with metastable austenite. As a widely-applied product, the automotive steels need be evaluated in microstructure evolution and properties from the whole chain including composition design, microstructure control, cutting process, forming process, joining process and service performance. The evaluation results will provide the basis for the improvement of microstructure control theory and technology. Full consideration will be given in the technical applicability and cost of products.
Fund: National Key Research and Development Program of China(2017YFB0304401);National Key Research and Development Program of China(2016YFB0101605);National Natural Science Foundation of China(51971050);National Natural Science Foundation of China(51571048);National Basic Research Program of China(2010CB630803)
Fig.3 M3 (multiphase, metastable, multiscale) microstructure control design, positive/reverse phase transformation process and performance control principle (A3—temperature of polymorphic transformation γ?α in Fe-C phase diagram, A1—temperature of eutectoid reaction in Fe-C phase diagram, Ms—martensite transformation start temperature, TMCP—thermo mechanical control process)
Fig.4 Relation between metastable austenite fraction and strength-ductility balance (Rm—tensile strength, δ—elongation) (a)[41] and effect of austenite fraction on work-hardening behavior of medium-Mn steel (dσ/dε—work-hardening rate) (b)
Fig.5 Typical microstructures of the third-generation automotive steel(a) hot-rolled medium Mn steel(b) cold-rolled medium Mn steel(c) QP980 steel (M/A—martensite/austenite)
Fig.6 Transformation of metastable austenite of 0.13C-5Mn steel under different stress-cycle conditions[71](a) 435 MPa, 1000 cyc (b) 435 MPa, 4000 cyc (c) 600 MPa, 20 cyc (d) 600 MPa, 100 cyc
Fig.7 Effect of metastable austenite fraction on delayed fracture properties[72]
Fig.8 Effects of metastable austenite in the rollover zone (a), burnish zone (b) and fracture zone (c) on work-hardening behavior of sheared edge[74]
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