Herein, it is shown that the hot‐stamped steel exhibits high hardenability with a critical cooling rate of about 0.7 °C s−1 due to its unique chemical composition. The microstructure of the annealed sheet consists of ferrite, spherical carbides, and intergranular martensite. Thereinto, the generation of intergranular martensite can eliminate the yield point elongation and reduce the ratio of yield strength to ultimate tensile strength. Furthermore, compared with commercial 30MnB5 steel, the experimental steel (40Mn2CrNbV) quenched sheet shows excellent mechanical properties: yield strength (YS) = 1361 MPa, ultimate tensile strength (UTS) = 2422 MPa, and total elongation (TE) = 6.1%. Additionally, it is found that the Cr7C3 with a mean diameter of 240 ± 50 nm in the annealed quenching sheet can hinder dislocation movement to increase the yield strength. The coherent Nb‐rich (Nb, V)C precipitate (a mean diameter is <50 nm) in the cold‐rolled quenched sheet can improve the mechanical properties of the hot‐stamped steel.
In this work, we developed a third-generation automobile steel with high strength and elongation of 0.1C–11Mn medium manganese steel using a simple quenching and low-temperature tempering heat treatment procedure. At different austenitisation temperatures, the microstructure and mechanical properties of manganese steel were examined. The microstructure included tempered martensite (TM), austenite, and ultra-fine-grained ferrite. The steel achieved the highest strength and elongation of 39.1GPa·% when it was quenched at 630°C for 10 min and tempered at 450°C for 5 min. In conclusion, dislocation strengthening was the primary factor to increase yield strength. At 630°C, the steel achieved its best mechanical properties because of a higher austenite stability, work-hardening capability, and synergistic impact of ultra-fine-grained ferrite and martensite.
To improve the production process and produce high-quality annealed drawn and ironed (DI) plate, continuous annealing experiments were carried out at 620 °C, 640 °C, 680 °C, and 720 °C, and the effect of continuous annealing temperature on the microstructure, mechanical characteristics, and texture of annealed DI plate were clarified. The microstructure was tested with a scanning electron microscope (SEM); the mechanical properties and weighted average of the plastic strain ratio (r¯) were measured using a tension test; and the texture characterizations were tested by X-ray powder diffractometer (XRD) and electron backscatter diffraction (EBSD). The results reveal that, with the increase of the annealing temperature, the average grain size grew from 5.14 μm to 6.56 μm, the yield strength and tensile strength decreased, and the elongation increased. The rolling textures drastically reduced after annealing. When annealed at a lower temperature of 620 °C, the texture content of {111} <110> was the highest. When the annealing temperature increased to 640 °C, 680 °C and 720 °C, the texture content of {111} <112> was higher than that of {111} <110>. The mechanical properties of the DI plate that was annealed at 640 °C are the best, with a higher r¯ value and a lower planar anisotropy value.
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