Effect of partitioning procedure on microstructure and mechanical properties of a hot-rolled directly quenched and partitioned steel

Hot-rolling direct quenching and partitioning (HDQP) processes distinguished by the dynamical partitioning procedures and the isothermal partitioning procedures were applied to a low-carbon steel to investigate the differences in the microstructure and the mechanical properties. Microstructures were characterized by means of EPMA, EBSD, TEM and XRD. Mechanical properties were measured by uniaxial tensile tests. Results show that the microstructures of the HDQP sheets are characterized by lath martensite and film-like inter-lath retained austenite. The dynamically partitioned sheets possess narrower martensite laths with higher dislocation densities, compared with the isothermally partitioned sheets. The martensite lath broadening, the dislocation density reduction and the carbide coarsening exist with decreased cooling rate or with prolonged partitioning time. The coarse carbides appearing in the sheet partitioned longer than 5 min promote the decomposition of austenite. X-ray diffraction (XRD) detection results of the specimens with different plastic strains indicate that the retained austenite with the carbon concentration below 1.5 wt% can perform a better transformation behavior with the plastic strain under 5%. The isothermal partitioning processes can improve the average concentration and homogeneity of carbon in the retained austenite but make up part of the retained austenite too stable. Mechanical property results show that the dynamically partitioned sheets possess higher strengths about 1500-1600 MPa and similar elongations about 14-16% with excellent products of strength and elongation (PSE) about 22,000-25,000 MPa%. It is concluded that a dynamical partitioning procedure is preferable for obtaining a HDQP steel with excellent mechanical properties.