Effect of Microstructures on Yield Strength in Hot-Stamped Steel Sheet

For automotive structural parts, hot stamping is being increasingly used because of the need for both higher fuel efficiency and crashworthiness. The yield strength of hot-stamped steel sheet is lower than that of water-quenched steel sheet. The microstructure of hot-stamped low-carbon steel sheet comprises lath martensite and retained austenite. Due to the slow cooling rate below the martensite start temperature, the martensite formed by hot stamping is auto-tempered. To clarify the factor dominating the yield strength of the hot-stamped steel, the authors herein investigated the effect of microstructures on the yield strength with heat-treated specimens at various cooling rates and heating temperatures. Consequently, it was clarified that the yield strength of the auto-tempered low-carbon steel depends on grain, dislocation, solute carbon, carbide, and retained austenite. As far as the present experiment is concerned, the retained austenite is the most effective factor on the yield strength.