Microstructure, Precipitation, and Mechanical Properties of V-N-Alloyed Steel After Different Cooling Processes
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INTRODUCTION
CURRENTLY, steel products used in architectural construction are required to have enhanced mechanical properties such as high strength,[1,2] good ductility,[3] and toughness[4] due to the development of civil engineering. For V-N-alloyed 600 MPa grade high-strength steel used in construction industry, these requirements are yield strength, tensile strength, total elongation, and uniform elongation higher than 600 MPa, 730 MPa, 15 pct, and 7.5 pct, respectively. Moreover, the microstructure is required to be mainly composed of refined ferrite (F) and pearlite (P) to obtain a good combination of strength, ductility, and toughness. In low-carbon steels containing V, Olasolo[5] and Baker[6] reported that lowering the transformation temperature tends to refine ferrite grain size and increase dislocation density. In addition to type and proportion of microconstituents, the distribution and particle size of precipitates can also largely determine mechanical properties. JING ZHANG, Ph.D. Candidate, FU-MING WANG, Professor, and ZHAN-BING YANG, Associate Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] CHANG-RONG LI, Professor, is with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China. Manuscript submitted February 16, 2016. Article published online September 16, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
According to the research of Medina[7] and He et al.,[8,9] VN precipitates can promote the nucleation of intragranular ferrite (IGF), consequently, refining grain size from 29 to 16 lm and improving the homogeneity of microstructure, which increases toughness as well as the contribution of grain refinement strengthening to yield strength. Chen et al.[10,11] estimated that the precipitation hardening of (V,Ti)C can reach 300 MPa because of fine random or interphase precipitates. It is an inspiration that refining V(C,N) particle size and sheet spacing through thermo-mechanical controlled process could probably be used in V-N-alloyed steel to greatly increase strength. Moreover, interphase precipitation of nano-scaled carbide particles has been recently applied in low-carbon steels to improve strength and ductility.[12–14] Based on Ashby-Orowan mechanism, finer precipitated particles lead to higher strength and hardness in steels. Through systematically investigating the precipitation behavior of vanadium carbides, several researchers revealed that finer sheet spacing of interphase precipitation and smaller size of particles precipitated in the ferrite matrix can be obtained by decreasing transformation temperature, which also directly results in higher strength.[15–17] It is evident that cooling rate and finish cooling temperature have a strong effect on the above-men
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