| 摘要 |
PROBLEM TO BE SOLVED: To improve tensile strength and toughness and to remove weld crack by specifying the composition of a steel, particularly the relation of B content with Cu, Ni, and Mo, to reduce weld cracking sensitivity and also specifying rolling and heat treatment conditions. SOLUTION: A steel stock has a composition which consists of, by weight, 0.05-0.14% C, 0.01-0.5% Si, 0.5-1.5% Mn, <=0.015% P, <=0.005% S, 0.30-0.50% Cu, 0.15-0.50% Ni, 0.30-0.60% Cr, 0.05-0.4% Mo, 0.015-0.045% Nb, <=0.030% Ti, 0.005-0.10% Al, 0.001-0.007% N, B in an amount in the range shown by inequality I, and the balance Fe and in which the value of P cm defined by equation II is regulated to <=0.20. The steel stock is heated to 1050 to 1250 deg.C, rolled, and finished to prescribed plate thickness at 800 to 900 deg.C. The resultant steel plate is directly hardened and then tempered at 350 to 550 deg.C. By this method, the high tensile strength steel plate of >75 mm plate thickness, having >=570 MPa tensile strength and superior toughness and excellent in weldability in a cold district, can be obtained.
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