Structure-property relationship in vanadium micro-alloyed TRIP steel subjected to the isothermal bainite transformation process

Abstract

Structure-property relationship in vanadium micro-alloyed TRIP steel is investigated focusing on austenite transformation, precipitation behavior and mechanical performance. Microstructural evolution of the steel during processing was characterized mainly using EBSD, TEM and APT techniques; and its influence on mechanical properties was discussed. The TRIP steel exhibits complex microstructure including intercritical ferrite (IF), bainitic ferrite (BF), retained austenite (RA) and martensite. After identical intercritical annealing (IA), IF fraction keeps constant, BF and RA fractions increase while martensite fraction decreases with decreasing isothermal bainite transformation (IBT) temperature from 470 to 380 °C. The initially precipitated vanadium carbides (VC) in the hot-rolled specimen undergo coarsening during IA process, accompanied with some newly precipitated VC in either ferrite or in austenite, confirmed by the co-existence of small-sized and coarsened VC after IA process, as well as occurrence of K–S orientation relationship between VC and the α-Fe matrix. Ultra-fine average grain size of less than 1 μm in the TRIP heat-treated steel is achieved by the refining effect of these nano-scale VC precipitates, which can inhibit phase boundary (PB) migration during IA process and promote bainite transformation during subsequent IBT process. The higher volume fraction of RA accompanied with higher carbon enrichment intensifies sustainable strain hardening and finally increased ductility of the specimens treated under lower IBT temperature. In the IBT-380 specimen with the highest RA and BF fractions, the occurrence of micro-yielding phenomenon is a result of the Cottrell atmosphere in soft domain BF and stress relaxation due to RA to martensite transformation, during which the soft domain IF/RA/BF and hard domain martensite yields step by step. An excellent mechanical performance is thus achieved in the IBT-380 specimen, meeting the requirements for the 3rd generation advanced auto-steel.