Exceptional Work-Hardening Behavior of Medium-Carbon High-Silicon Low-Alloy Steels
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An exceptional work-hardening behavior is reported in a set of newly developed medium-carbon high-silicon low-alloy steels consisting of ferrite, bainite, and retained austenite. Each strain-hardening curve is associated with three strain-hardening exponents, which change systematically due to the dynamic change in microstructure during loading. Manufacturing processes, such as deep drawing and bending, require materials with high formability and high work-hardening response. The austenite-based stainless steels and twin-induced plasticity steels exhibiting excellent work-hardening tendency[1,2] incur high cost due to heavy alloy addition and complex processing steps. Thus, new steels with excellent combination of tensile properties at reduced cost are of practical importance. In our previous communications,[3,4] we have demonstrated that newly developed high-strength low-alloy silicon-based transformation-induced plasticity (TRIP) steels have the potential to exhibit good work-hardening response at par with the austenite-based steels. Currently, an interesting finding is that the work-hardening response (Figure 7 in Reference 4) of each of the reported steels is associated with three work-hardening exponents (nH), inherently related to the change in microstructures during tensile loading. We are reporting this new finding with a detailed mechanism for evolution of three nH values and their variation during the course of tensile loading. Moreover, in the present study, a continuously increasing value of nH with strain (characteristic of austenitic high-Mn steels[2]) has been observed in low-alloyed steels during their tensile deformation. The primary motive of this article is to
A. VARSHNEY, Ph.D. Student, and S. SANGAL and K. MONDAL, Professors, are with the Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, UP, 208016, India. Contact email: [email protected] Manuscript submitted May 17, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
understand the variation in nH values with strain for steels developed by different heat-treatment cycles and to report an increase in nH value with strain during tensile deformation of a selected set of previously developed low-alloy steels.[3,4] Tensile specimens as per ASTM E8M were extracted from high-carbon high-silicon steel with composition 0.61C-1.71Si-0.86Mn. The specimens were continuously cooled for 0 to 40 seconds after austenitization at 1313 K (1040 C) followed by austempering at 573 K, 623 K, and 673 K (300 C, 350 C, and 400 C). A schematic of heat-treatment cycles superimposed on a transformation diagram is shown in Figure 1. The red curve in the figure shows a continuous cooling transformation curve corresponding to the formation of 0.1 pct ferrite, while the blue curve corresponds to the formation of 0.1 pct bainite during isothermal transformation. Details of the heat-treatment cycles and processing of the steels are already reported.[4] Scanning electron microscopic (SEM) analysis was carried out using a Zeiss field emission
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