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%.
The stability of retained austenite was improved by the dual-stable C-Mn partitioning process. The phase transformation and element diffusion of dual-stable C-Mn partitioning process of tested steel were investigated by means of EPMA, SEM, OM, tensile testing machine, and other analysis methods. The effects of the first and second austenite stabilization time on the microstructure and mechanical properties of low-C-Si-Mn steel were studied, respectively. The enrichment of C and Mn elements is obvious after the dual-stable C-Mn partitioning process, and the microstructure of the tested steel is constituted of martensite, ferrite, and retained austenite. Compared with the conventional Q&P steel, the tensile strength of the steel treated by the dual-stable C-Mn partitioning process is slightly lower, but the plasticity is improved significantly. The tensile strength is 875-910 MPa, the elongation is 20-24%, and the product of strength and elongation can reach 21 GPa·%.
In this paper, the TRIP590 steel was used for C-Mn partitioning. The influence of C-Mn partitioning on the microstructure and mechanical properties of the steel was studied. SEM, EPMA, XRD and tensile tests were used to characterize the microstructure of the tested steel, calculate the content of retained austenite, and analyze the enrichment of C atoms and Mn atoms and mechanical properties. The results show that there was a lot of lath martensite and scattered ferrite in the microstructures of the Q&P steel and C-Mn partitioning steel. After C-Mn partitioning, the content of ferrite was increased. The enrichment of C and Mn in the C-Mn partitioning steel was relatively apparent, and the concentration of the atoms in the center of martensite was significantly higher than at the boundary between martensite and ferrite. Mn-rich areas were also C-rich areas. Compared with the Q&P steel, the C-Mn partitioning steel had a larger amount of retained austenite, higher elongation and PSE.
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