Effect of direct quenching and partitioning treatment on mechanical properties of a hot rolled strip steel

Three different online heat treatment processes were designed to study the effects on the mechanical properties of a 0.19C-1.6Si-1.6Mn (wt%) hot rolled strip steel. The microstructures were characterized by means of SEM, TEM, EPMA, and XRD. The mechanical properties were estimated by tensile tests. Results showed that a satisfying combination of strength and ductility could be obtained through the ferrite relaxation and direct quenching and partitioning process. Analysis was also focused on this process. The microstructure contained proeutectoid ferrite grains, martensite packets and blocky or interlath retained austenite, and also contained carbide-free bainite in the case of relatively high quench temperatures. The retained austenite fraction was increased through proeutectoid ferrite and partial bainite transformation, while the tensile strength was also consequently decreased. The most of retained austenite transformed to ferrite under deformation and the elongation was obviously improved.