Abstract
Abstract The effect of the cooling rate on the hardness and microstructure of the hot-stamped boron steel containing 0.2 mass% carbon was investigated. Sheets with thicknesses of 1.6 and 1.2 mm were heated to 900 °C for 4 min. They were then press formed and simultaneously quench hardened with dies or water quenched. Simulated hot-stamping tests were also performed at various cooling rates. The Vickers hardnesses of the quenched specimens were measured and their cross-sections were observed by optical microscopy and transmission electron microscopy. The hot-stamped specimens had an auto-tempered martensite microstructure and they were softer than the water-quenched specimens, which consisted of lath-martensite. Tempered martensite was distinguished from bainite by observing cementite precipitation. Below the M s temperature, reducing the cooling rate significantly reduces the hardness, even when the cooling rate is higher than the upper critical cooling rate.
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