Effect of hot stamping and quenching & partitioning process on microstructure and mechanical properties of ultra-high strength steel

Abstract

In order to better achieve the purpose of automobile lightweight, hot stamping and quenching & partitioning (HS-Q&P) process has become a hot research focus. This process includes a hot stamping followed by a controlled quenching and partitioning process. In this paper, the effect of different austenitizing temperatures and time, as well as different holding pressure and partitioning temperatures and time on the microstructure evolution and mechanical properties of the tested steel was systematically studied. The resulting microstructure was characterized by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD), and the volume fraction of retained austenite and carbon concentration in austenite were measured by x-ray diffraction (XRD). The results show that, the microstructure of HS-Q&P steel was finer and smoother than that of Q&P steel. During the pressure holding and partitioning process, the carbon atoms diffuse from martensite to austenite, thereby improving the stability of the retained austenite. Because both retained austenite and ferrite can improve the product of strength and elongation (PSE), the PSE of two phase zone austenitized HS-Q&P steel is better than that of completely austenitized HS-Q&P steel. When the austenitizing temperature was 780 °C and the pressure holding time was 90 s, the HS-Q&P steel reached the best mechanical properties. Its tensile strength was 1350 MPa, elongation was 14.24%, and PSE was 19.22 GPa%.

Keywords

• carbon partitioning
• HS-Q&P
• PSE
• EBSD
• retained austenite