An Investigation Of Quchening And Partitioning Process For A Of V-microalloyed Low Carbon High Strength Steel

The application of advanced high-strength steels are of great significance for improving the safety and energy saving of automobiles.As the representation of the third generation high-strength steels,quenching and partitioning(Q&P)steel has higher strength and better comprehensive mechanical properties.Vanadium(V)has a significant effect on improving the mechanical properties of steel,which is a rich mineral resource in the west of Panzhihua,China.Presently,the effect of V-microalloyed on microstructure and properties of Q&P steel has not been reported.Through the observing the microstructure and testing the mechanical properties,this paper systematically studies the process-microstructure-property relationship of V-microalloyed Q&P steel,and analyzes the effect of V on bainite and retained austenite,strengthening mechanism and the precipitation rule of V carbides.The microstructure and properties of V-microalloyed steel and V-free steel were systematically compared.As researched objects,0.24C-1.50Si-1.90Mn(0V steel)and0.24C-1.50Si-1.90Mn-0.16V(0.16 V steel)were treat by one-step Q&P heat treatment and measured tensile mechanical properties,OM and SEM were used to observe the microstructure,EBSD for analyzing retained austenite,TEM for V-carbide precipitation.The microstructures of the two steels consist of blocky ferrite,lath tempered martensite,M/A-island,blocky fresh martensite,film or blocky retained austenite.The tensile strength of V-free Q&P steel is 937 ? 1102 MPa,while V-microalloyed Q&P steel is 1007? 1249 MPa.The influence of V on microstructure evolution in one-step Q&P process was analyzed.V in Q&P steel will inhibit the bainite transformation,speed up C homogenization time of retained austenite and reduce the "optimal partitioning(quenching)temperature".From quantitatively contrast of the low-angle grain boundaries of the two steels,it is inferred that V was mainly solid-solution in the matrix,solidsolution strengthening and fine-grain strengthening occurred,and the stacking fault energy would also be reduced.The relationship between microstructure and properties of steels with different Vcontents under air cooling one-step Q&P process was explored.It can improve plasticity by increasing air cooling process and increasing ferrite content.SEM results showed that the microstructure consisted of ferrite,lath martensite,blocky martensite,M/A-island,and bainite.As the air cooling time increasing,ferrite content increases and the martensite content decreases.The higher the V content,the more M/A islands,and the less bainite.The tensile strength is 863 ? 1053 MPa,and the elongation is 15.5 ? 25.5%,higher plasticity and Product Strength and Plasticity than traditional one-step Q&P process.The relationship between the microstructure and properties of different V-content steels in the austenite reverse one-step Q&P process was investigated.Before the Q&P process,high temperature austenitization and quenching processes were used to make V more fully solid-dissolved in the original austenite to achieve the effect of grain refinement.SEM showed that the mirostructure was composed of ferrite,tempered martensite,fresh martensite,M/A-island,and bainite.Fresh martensite,M/A-island,bainite were increased with partitioning tempetaure.Ferrite was increased,bainite was decreased,and fresh martensite was increased with increasing of the V content.The tensile strength is 822 ? 1024 MPa and the elongation is 18.0 ? 27.5%.