A novel hot stamping steel with superior mechanical properties and antioxidant properties

Abstract

A novel 2000HS hot stamping steel was fabricated for automotive applications. Compared to the traditional 1500HS hot stamping steel, it showed an excellent combination of strength and plasticity, and the thickness of the oxide layer was dramatically reduced during the hot stamping process. The tensile experiment, scanning electron microscope (SEM), electron backscattered diffraction (EBSD), transmission electron microscope (TEM) and x-ray diffraction (XRD) characterization methods were employed, to clarify the best comprehensive mechanical properties of 2000HS steel. Experimental results indicated that the microstructure of Z740-A950-T170 consisted of lath martensite, large M23C6 precipitates, small (Nb, V) C precipitates and retained austenite (4∼6vol.%). The obtained best comprehensive mechanical properties of Z740-A950-T170 sample of the tensile strength was 2160 MPa, the yield strength was up to 1318 MPa, and the total elongation reached at 12%. The addition of Mo element inhibited the coarsening and lengthening of the precipitation. Large M23C6 precipitates (101.2 μm) and small (Nb, V) C precipitates (17.5 μm) played the role of pinning the dislocation and increased the yield strength. Regardless of any process, retained austenite was kept at room temperature (4–6 vol.%), which improving the strength and maintaining the total elongation by transformation induced plasticity (TRIP) effect. In addition, the oxidization layer thickness of the invented steel (12.4 μm) was thinner than that of the 1500HS (23.6 μm) after hot forming at 950 °C for 5 min. The significant reduction of oxidation was due to the high percent of Cr and Si contents within the designed steel, the formation of dense Cr and Si at the bottom of the oxide layer, which improving the antioxidant properties for studied steel.