Effect of Ferrite Status on Mechanical Properties of Hot-Rolled Directly Quenched and Partitioned Steel

Abstract

Hot-rolling direct quenching and partitioning (HDQ&P) processes were applied to both low carbon high silicon vanadium micro-alloyed steel and low carbon low silicon steel with phosphorus addition. Proeutectoid ferrite with an area fraction of about 30% was introduced into some of the sheets. Microstructures were characterized using SEM, TEM and XRD. Mechanical properties were investigated by means of uniaxial tensile test. Austenite stabilization, retained austenite stability, tensile deformation and fracture were comprehensively analyzed by making the comparison between the two steels and between the sheets with and without ferrite considering the different precipitation statuses in ferrite. Experimental results showed that the high silicon vanadium micro-alloyed steel contained more retained austenite with higher stability compared with the low silicon phosphorus added steel. Filled with much more carbides with larger sizes, the martensite in the low silicon phosphorus added steel exhibited a much salient tempered feature. The high silicon vanadium added steel possessed higher strength and ductility than the low silicon phosphorus added steel. The introduction of ferrite can result in an extremely obvious post-necking elongation drop in the low silicon phosphorus added steel. The dispersed V-bearing precipitates and high silicon content in ferrite can increase the yield strength and simultaneously diminish the hardness difference between ferrite and martensite, which can improve their compatible deformation capability and then enhance the work hardening ability and finally raise both the UTS and elongation of the steel.