Nanoscale Cementite Precipitates and Comprehensive Strengthening Mechanism of Steel
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STEEL is the fundamental material of a country’s industrialization. Strength is one of the important performances of steel structures. For over half a century, ferrous-metallurgy and materials scientists from different countries have performed much in-depth research into steel’s strengthening mechanisms. For single-phase alloy, Hall–Petch proposed the grain refinement strengthening theory[1,2] based on the dislocation pileup model. For steel in which second-phase precipitation exists, many authors proposed various kinds of precipitation strengthening mechanisms, based on the interaction between secondphase particles and dislocation motion, the most important of which are the shearing mechanism and bypass (looping) mechanism.[3–6] JIE FU and KEMING FANG, Professors, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, China, Wuhan University of Science and Technology, Wuhan 430081, P.R. China. GUANGQIANG LI, Professor, is the Director of the Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, China, Wuhan University of Science and Technology. Contact e-mail: [email protected] XINPING MAO, Chief Engineer, is with Guangzhou Zhujiang Steel Co., Ltd., Guangzhou 510730, P.R. China Manuscript submitted March 29, 2010. Article published online August 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
High-strength low-alloy (HSLA) steel containing microalloying elements such as V, Nb, and Ti is a typical steel with a precipitation strengthening mechanism, in which nanoscale V, Nb, and Ti carbides have a significant precipitation strengthening effect. However, because it is hard to determine the volume fractions of different types and sizes of nanoscale particles, by existing theoretical formulas, the contribution of precipitation strengthening to steel strength can only be analyzed in a qualitative way rather than by quantitative calculation, which prevents further research into steel’s precipitation strengthening mechanisms. Based on chemical phase analysis, X-ray small-angle scattering (XSAS), room temperature organic (RTO) analysis, high-resolution transmission electron microscopy (TEM), and steel tempering rapid-cooling technology, the precipitated nanoscale particles were quantitatively analyzed. The contribution of precipitation strengthening to the yield strength was calculated. Subsequently, the steel’s strengthening mechanisms are discussed based on this calculation, and a comprehensive strengthening mechanism of steel is proposed. II. NANOSCALE IRON-CARBON PRECIPITATES IN HSLC STEEL Thin slab continuous casting and rolling process (referred to hereinafter as TSCR), one of the contemVOLUME 42A, DECEMBER 2011—3797
porary steel production processes, has been growing rapidly in the world in recent years. Over 95 pct of TSCR products are low-carbon steels with 0.06 to 0.25 pct carbon conten
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