| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet to be hot-pressed for manufacturing a hot-pressed member having superior toughness and a TS of 1.8 GPa or higher. SOLUTION: The steel sheet has a metallographic structure after having been hot-pressed mainly formed of a self-tempered martensite composed of former austenite grains with sizes of 10μm or smaller. Therefore, the steel sheet comprises 0.25-0.45% C, 0.5-3.0% Mn+Cr, further one or more elements of 0.5% or less Si, 2% or less Ni, 1% or less Cu, 1% or less V and 1% or less Al, and occasionally one or more elements of 0.01% or less B, 1.0% or less Nb, 1.0% or less Mo, 3.42N+0.001≤Ti≤3.42N+0.5, and 0.001-0.005% Ca. The manufacturing method comprises steps of: keeping the steel sheet at a temperature between an Ac<SB>3</SB>point and (Ac<SB>3</SB>point + 100°C) for five minutes or shorter; hot-pressing it; and subsequently cooling it to the Ms point at a cooling rate of the upper critical cooling rate or higher, and then to 150°C from the Ms point at an average cooling rate of 10 to 500°C/s. COPYRIGHT: (C)2006,JPO&NCIPI
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