Process Parameters On The Microstructure Evolution Of A Low Carbon Microalloyed High Strength Dual Phase Steel

With a number of unique mechanical properties such as low yielding point, an increased strain hardening rate at lower strains, high strength, good ductility, dual phase steels attract much attention all over the world in recent years. Dual phase steels have been widely used in automobile parts and other cold worked parts. The microstructural features of dual phase steels are a dispersion of a hard martensite phase in a matrix of ferrite. The strength of dual phase steels is attributed to hard martensite and ductility to soft ferrite. The volume fraction of ferrite and martensite can be changed by the chemical composition of steel design and different thermomechanical process and heat treatment.The process parameters of reheating temperature, deformation, deformation temperature, cooling rate, relaxation time etc were conducted on a Gleeble-3500. The microstructural observation and characterization were performed on a Zeiss optical microscope and a LEO1450 electronic microscope.The austenite maintained a small grain size during the reheating temperature ranging from 1100℃-1250℃. The growth of austenite was strongly retard by Nb and Ti nitride and carbide. There was an orientation relationship between austenite and ferrite when the deformation was small whereas the relationship was not obvious for large deformation. The layer-like martensite/austenite(M/A) was distributed in the matrix of lath-like or plate-like ferrite at higher deformation temperatures, while it was island-like dispersed in the matrix of lath-like or plate-like ferrite at lower deformation temperatures. The length of ferrite packets obviously became smaller with decrease of deformation temperatures whereas the width did not changed greatly. The M/A was island-like dispersed in the matrix of polygonal ferrite at higher cooling rates, whereas the M/A was changed into layer-like in the acicular ferrite matrix at lower cooling rates. The length of ferrite packets obviously became larger at higher end cooling temperatures. The volume fraction of M/A reached a maximum at the relaxation of 60s while it remained nearly constant when the relaxation time was shorter and longer. The optimized effect of relaxation time on the microstructure refinement was 60s in this study. The size of ferrite packets was smaller with shorter relaxation time and became larger with increasing of relaxation time.The optimized grain-refined dual phase with acicular ferrite and M/A can be realized by the following way. The steel is reheated to1150oC with the speed of 15oC/s and held for 600s, then cooled to 850oC at the speed of 1oC/s, followed by reduction of 30% at this temperature and relaxed for 60s, then immediately cooled to 550oC at the speed of 15oC/s and finally cooled to room temperature with the speed of 1oC/s.