Microstructures and Mechanical Properties of a New As-Hot-Rolled High-Strength DP Steel Subjected to Different Cooling S
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TRODUCTION
HIGH-STRENGTH low-alloy (HSLA) steels are widely used for automotive applications, primarily from the viewpoint of fuel efficiency and weight saving. The high strength is mainly attributed to microalloying additions (e.g., Nb, V, and Ti) and thermo-mechanical controlled processing (TMCP). Recently, HSLA steels with tensile strength of 780 MPa have been used for vehicle components.[1–7] It is realized as an important branch of HSLA steels that advanced high-strength steels (AHSS) such as ferrite/martensite dual-phase (FMDP) steel and ferrite/bainite dual-phase (FBDP) steel can be cost effective with a good combination of strength and ductility.[8,9] The dual-phase (DP) steels are JUN HU, Ph.D. Candidate, LIN-XIU DU, Professor, CAI-RU GAO, Associate Professor, and TONG-ZI YANG, Ph.D. Candidate, are with The State Key Laboratory of Rolling and Automation, Northeastern University, P.O. Box 105, No. 11, Lane 3, Wenhua Road, HePing District, Shenyang 110819, P.R. China. Contact e-mail: [email protected] JIANJUN WANG, Associate Professor, is with the Institute of Materials Research, School of Material and Metallurgy, Northeastern University. AN-YANG WANG, Researcher, is with the AVIC Shenyang Aircraft Corporation, Shenyang 110034, P.R. China. R.D.K. MISRA, Distinguished Professor and Stuller Endowed Chair in Metallurgy, is with the Laboratory for Excellence in Advanced Steel Research, Center for Structural and Functional Materials, University of Louisiana at Lafayette, P.O. Box 44130, Lafayette, LA 70504-4130. Manuscript submitted June 27, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
ductile because of the presence of a soft ferrite phase and the strength is attributed to hard martensite or bainite constituent. The conventional ferrite/martensite dualphase steels are characterized by low yield ratio (yield strength/tensile strength) because of the significant difference in hardness of the two phases. The low yield ratio means high resistance to deformation from yielding to plastic instability. However, newer engineering specifications require high yield strength.[10] Thus, it is important to obtain both high yield strength and low yield ratio.[11] The application area of soft ferrite and hard martensite DP steels is limited because the interface of the two phases is identified as the site for nucleation of voids during ductile fracture. The martensite–ferrite interface strength can approach 2.4 to 2.5 GPa.[12,13] Nanoscale TiC particles play a significant role in precipitation hardening and consequently influence yield strength of DP steels.[14] Moreover, high Si addition is also considered to increase the yield strength by substitutional solid solution strengthening and promoting TiC precipitation.[9,15] From the perspective of obtaining a different yield ratio, it is meaningful to develop a new low carbon high-strength DP steel and study the variation of yield ratio in steels subjected to different cooling schedules. This aspect has not been studied to the best of our understanding. Currently, there is strong interes
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