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Synopsis :The effect of cooling rate on hardness and microstructure of the hot-stamped boron steel with 0.2 mass% carbon was investigated. After sheet specimen with a thickness of 1.6 mm or 1.2 mm was heated up to 900°C for 4 min, it was press formed and simultaneously quench hardened with dies, or water-quenched. Simulated hot-stamping test was also carried out at various cooling rates. The Vickers hardness of quenched specimens was measured. The microstructure on the cross-section of quenched specimens was observed with optical microscope and transmission electron microscope. The microstrucure of hot-stamped specimen was composed of auto-tempered-martensite and was softer than water-quenched specimen which consisted of lath-martensite. Tempered martensite was distinguished from bainite by observation of cementite precipitation morphology. Cooling rate below the M s point affects hardness significantly, even if cooling rate is higher than the upper critical cooling rate. Decrease in hardness caused by auto-tempering was formulated with the tempering parameter in which was taken account of integration of tempering effect.

The pinning effect is useful for restraining austenite grain growth in low alloy steel and improving heat affected zone toughness in welded joints. We propose a new calculation model for predicting austenite grain growth behavior. The model is mainly comprised of two theories: the solute-drag effect and the pinning effect of TiN precipitates. The calculation of the solute-drag effect is based on the hypothesis that the width of each austenite grain boundary is constant and that the element content maintains equilibrium segregation at the austenite grain boundaries. We used Hillert’s law under the assumption that the austenite grain boundary phase is a liquid so that we could estimate the equilibrium solute concentration at the austenite grain boundaries. The equilibrium solute concentration was calculated using the Thermo-Calc software. Pinning effect was estimated by Nishizawa’s equation. The calculated austenite grain growth at 1473–1673 K showed excellent correspondence with the experimental results.

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The corrosion‐resistive intermetallic compound Fe3Si could non‐electrolytically coat pure iron and mild steel. The disproportional reaction between Si and Si4+ ions deposited the homogeneous Fe3Si layer on the Fe substrates, by using solid silicon and the molten salt composed of NaCl‐KCl‐NaF‐Na2SiF6‐SiO2. SiF4 gas evaporation prevented from continuous siliconization, when only Na2SiF6 was used as Si4+ ion source. By adding SiO2 into the salt as another source, the molten salt became endurable for repeated usage in air. The layer of Fe3Si single phase with silicon mole fractions of 24 ± 1 % grew thicker than 200 μm by immersing for a few hours at 973 – 1173K.