Microstructure evolution and mechanical properties of a hot-rolled directly quenched and partitioned steel containing proeutectoid ferrite

A low carbon V microalloyed steel was treated by hot-rolling direct quenching and partitioning (HDQ&P) processes. The microstructures were characterized by polygonal proeutectoid ferrite and lath martensite accompanying with both blocky and film-like retained austenite. This kind of HDQ&P steel possesses a lower yield ratio and similar tensile strength and elongation when compared with the existing HDQ&P steel without ferrite. Partitioning processes with different time were designed to optimize the characteristics of the retained austenite and to control its stability. The microstructure-properties relationship, the stability of the retained austenite, and the transformation-induced plasticity (TRIP) behavior were investigated by comparing the microstructures and mechanical properties of the HDQ&P sheets with those of the TRIP sheets. The results show that the introduction of proeutectoid ferrite can ensure the low yield strengths of the materials and simultaneously intensify the inhomogeneous distributions of carbon and silicon in the untransformed austenite. The particular element distributions result in a considerable amount of large blocky retained austenite locating on the ferrite/martensite boundaries or in some regions surrounded by ferrite. The high tensile strength of the HDQ&P steel can be attributed to the major martensitic structure, the V-bearing precipitates in ferrite and the TRIP effect of the retained austenite. The outstanding combination of strength, yield ratio and ductility, which synthesizes the advantages of dual-phase (DP) steel, TRIP steel and Q&P steel, indicates that the HDQ&P steel has a great potential for practical application.