Microstructure development and mechanical properties of a C-Mn-Si-Al-Cr cold rolled steel subjected to quenching and partitioning treatment

Abstract

The microstructural evolution and mechanical behavior of Fe-0.2C-2.7Mn-1.2Si-0.8Al-0.3Cr (wt.%) quenching and partitioning steel were studied. Partitioning was performed at 400, 440, and 480 °C for various durations from 30 to 300 s to obtain different fractions of retained austenite. Scanning electron microscopy, dilatometry, and X-ray diffraction analysis as well as uniaxial tensile testing were employed to characterize the microstructural evolutions and mechanical properties. The resulting microstructures were primarily composed of tempered lath martensite, bainitic ferrite, retained austenite, and blocky fresh martensite. The lower partitioning temperature of 400 °C was appropriate for efficient carbon partitioning and stabilization of retained austenite at room temperature. However, the retained austenite fraction decreased with increasing partitioning temperature, due to accelerated bainite formation and carbide precipitation at higher partitioning temperatures. The tensile results revealed that all specimens had tensile strength values of more than 1128 MPa and yield strength values of over 830 MPa. Additionally, prolonging the isothermal holding time had a negative effect on the yield strength due to bainite formation, while it enhanced the elongation. Fractography analysis showed that specimens treated at the partitioning temperature of 400 °C were fractured in a relatively ductile mode, but the fracture nature altered to an intergranular mode after partitioning at higher temperatures.