PURPOSE: To remarkably improve the resistance of the titled steel to cracking due to hydrogen embrittlement and to blister due to hydrogen, by restricting the amount of Mn in a steel having a specified composition as well as the amount of Ca and REM, restricting the amount of S at the same time, and specifying the quantitative relation among Ca, REM and S.
CONSTITUTION: This steel consists of, by weight, 0.05W0.25% C, 0.010W0.50% Si, 0.05W1.15% Mn, 0.01W0.10% Al, ≤0.030% P, ≤0.0010% S, 0.0005W0.0030% Ca and/or 0.0010W0.010% REM satisfying the equation, and the balance Fe or further contains one or more among ≤0.6% Cu, ≤0.6% Ni and ≤3.0% Cr and/or one or more among ≤1.0% Mo, ≤0.10% Nb, ≤0.15% V and ≤0.10% Zr and/or ≤0.0050% B or ≤0.0050% B and ≤0.10% Ti. The steel contains coarse nonmetallic inclusion having ≤1 coarse grain of ≥50μm grain size per 1kg slab, and the steel is rolled with a hot strip mill and coiled.
NAKAI YOUICHI
KIMURA SHIGEO
NISHIKAWA HIROSHI
KOMAMURA KOUICHI
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