Ultrahigh Ductility, High-Carbon Martensitic Steel
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wo decades, the development of advanced high-strength steels (AHSSs) has been the driving force of the automotive industry, including dual-phase (DP) steels,[1] transformation-induced plasticity (TRIP) steels,[2] quenching and partitioning (Q&P) and quenching–partitioning–tempering steels,[3] (Q–P–T) steels,[4] in an effort to raise the strength and keep enough ductility. However, the increase in strength results in the loss of ductility, as shown in Figure 1 (Figure 1 is modified from Reference 5 by adding the data of Q–P–T steel in this work.). Since the properties of strength and ductility are mutually exclusive,[6] the product of strength and elongation (PSE) as a comprehensive property is used to evaluate the good from the bad properties. Based on the concept of PSE, AHSSs are classified into three generations:[7–12] the ‘‘first generation’’ of AHSSs (PSE is less than 30 GPa pct), the ‘‘second generation’’ (larger than 50 GPa pct), and the third (next) generation (between them). In AHSSs, Figure 1 shows that martensitic steels possess the highest strength, but their ductility is the worst. Their strength can be greater than 1600 MPa, but the elongation is only about 10 pct. As is well known, the martensitic matrix has higher strength than the ferrite SHENGWEI QIN, YU LIU, and QINGGUO HAO, Ph.D. Students, XUNWEI ZUO, Lecturer, YONGHUA RONG and NAILU CHEN, Professors, are with School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Rd, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted March 21, 2016. Article published online July 28, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
matrix in DP steels or TRIP steels; it also has higher strength than bainite in TRIP steels since carbon content in martensite is higher than that in ferrite or bainite. We have noted that high-carbon martensitic steels are hardly paid close attention to in the study of AHSSs. There may be two reasons: (1) High-carbon martensite exhibits worse ductility than does medium-carbon or low-carbon martensite since high-carbon martensite has higher dislocation density and, in turn, demonstrates worse deformation ability; and (2) in high-carbon martensitic steels, there is some brittle twin-type martensite, which is thermal-induced martensite from the transformation of prior austenite during quenching or strain-induced martensite from the transformation of retained austenite during deformation; in particular, the strain-induced, twin-type martensite without tempering will be more detrimental to ductility and will exhibit the stronger sensitivity to the notch.[13] Based on this analysis, there may be two approaches to enhance the ductility of high-carbon martensitic steels: (1) Raise the deformation ability (cracking resistance) of the martensitic matrix by properly reducing the carbon content in the martensitic matrix; and (2) raise the mechanical stability of retained austenite by increasing the carbon content in retained austenite and dispersing retained austenite so that the str
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