Ultrafine-Grained Multiphase Steels with Different Microstructural Constitutions Fabricated Through Annealing of Tempere
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is an increasing demand within the steel industry to develop new methods of manufacturing high-strength steels with enhanced properties at low cost. Ultrafine-grained (UFG) steels, which have shown great potential for enhancing both strength and toughness, have drawn much attention in recent years.[1] With strong grain-refinement strengthening, lean compositions can be used to produce high-strength steels. Up to now, various methods have been proposed to obtain a UFG structure. For instance, the serve plastic deformation (SPD) methods, including high-pressure torsion (HPT),[2] equal-channel angular pressing (ECAP),[3] and accumulative roll bonding (ARB),[4] are promising approaches for the production of UFG metals; however, they demand specific processes and facilities. Thus, the SPD method is restricted from being applied to mass manufacturing. Another well-established route for
XU WANG and RENDONG LIU are with the Technology Center of Ansteel Corporation, Anshan, 114009, China. RAN DING and JIANGUO HE are with the School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China. Contact e-mail: [email protected] AIMIN ZHAO is with the Engineering Research Institute, University of Science and Technology Beijing, Beijing, 100083, China. Manuscript submitted August 29, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
producing UFG steels depends on thermo-mechanical processes, including warm rolling/ausforming,[5,6] dynamic strain-induced transformation (DSIT) of ferrite,[7] and cold rolling of as-quenched martensite.[8] Compared with SPD, this kind of method is more capable of mass industrial production.[1] Particularly, cold rolling of as-quenched martensite is a simple method for producing UFG steels, which is characterized by quick grain subdivision without large deformation.[8] However, it is difficult to deform the as-quenched martensite at room temperature due to the large deformation load and prevalent transverse cracking,[9] especially for a sufficiently alloyed steel. Another issue associated with UFG materials with single-phase microstructures is that their ductility at room temperature is not guaranteed by grain refinement compared with their dramatically enhanced strength.[1] Several methods have been proposed to deal with this shortcoming in low-alloyed UFG steels, such as developing bimodal structures and introducing fine carbide or martensite islands as the second phase.[5,10] Accordingly, the work hardening is improved by the dislocation multiplication and slipping inside the relatively large grains or by geometrically necessary dislocations generated near the second phases. Thus, the uniform elongation of UFG steels could be improved without much loss of their strength. Recently, the current authors have reported the utilization of tempered martensite as the pre-microstructure before subsequent deformation and heat treatment to produce UFG quenching and partitioning (Q&P) steels.[11] Encouragingly, the rolling load and the risk of transverse cracking were reduced, and the
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