| 摘要 |
PURPOSE:To produce a 80kg/cm<2> high tensile steel plate having excellent resistance to stress relief annealing cracking by subjecting successively a specifically composed steel slab to hot rolling, direct hardening and tempering under specific conditions. CONSTITUTION:The steel slab consisting of the compsn. contg., by weight %, 0.05-0.15% C, 0.10-0.50% Si, 0.7-1.4% Mn, 0.40-0.80% Cr, <=8.0% Ni, 0.30-0.60% Mo, <=0.05% V, 0.010-0.040% Nb, 0.010-0.040% solAl and <=0.008% S is prepd. Such steel slab is subjected to the hot rolling consisting of rolling at 1,050-1,250 deg.C heating temp. and finishing at >=800 deg.C. The water cooling is then started at >=750 deg.C surface temp. of the rolling material and the material is cooled down to <=150 deg.C at an average cooling rate of >=100 deg.C/min down to 400 deg.C in the region right under the surface layer correspond ing to 1/4 thickness to execute direct hardening. The material is then subjected to tempering in a 500-700 deg.C range.
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