Study on Thermal Process and Microstructure & Properties of High Strength Automotive Steel

Abstract

This paper studies how the Nb content affected mechanical properties and microstructures of automotive steel processed by quenching and tempering (Q-T) and quenching-partitioning-tempering (Q-P-T) with the help of tensile testers, X-ray diffractometers, scanning electron microscopes, transmission electron microscopes and other equipment. It was demonstrated that when the Nb content remained the same, Agt (total elongation at maximum force), A (elongation after fracture) and R m·A (the product of strength with ductility) of the steel processed by Q-P-T were higher than those of the steel processed by Q-T; and when the steel was treated by the same thermal process, as the Nb content increased, its R p0.2 (yield strength) and R m (tensile strength) would grow, while A gt, A and R m·A would rise first and then decline. No matter if treated by Q-T or Q-P-T, the microstructures of the steel containing 0% and 0.03% of Nb separately were always lath martensite and film-like retained austenite located between laths. When processed by Q-P-T, the steel containing 0.03% Nb saw the largest strength-ductility product. This was because Nb could help refine the sizes of the original austenite grain and the lath martensite and precipitate nano-sized NbC; and the Q-P-T process was more effective in thickening the film-like retained austenite in steel and promoting the dispersed precipitation of the NbC.