| 摘要 |
PROBLEM TO BE SOLVED: To manufacture the steel product with high energy efficiency by subjecting a steel stock which has a specific composition containing C, Si, Mn, P, S, Mo, Al, Ti, Nb, N, and B and in which yield strength and Mo content satisfy a specific relation to hot working, hardening treatment, and tempering under respectively specified conditions. SOLUTION: The steel stock contains, by weight, 0.10-0.25% C, <=0.5% Si, <=0.5% Mn, <=0.015% P, <=0.005% S, 0.8-2.5% Mo, 0.005-0.1% Al, Ti in an amount in the range between 0.005 and 0.1%; and 3.4 times the content of N or more, 0.01-0.1% Nb, <=0.01%; N, and 0.0005-0.0050% B, and further, a relation of Mo-0.15YS>=-18.8 (where YS is yield strength shown by ksi). The steel stock is heated to >=1,150 deg.C and hot worked. Finish working is completed at a temperature not lower than (Ar3 point +50 deg.C). Quenching is done without delay to perform hardening, and a metallic structure in which as-hardened hardness in the position at the longest distance from the cooling surface becomes >=95% of surface hardness is formed, followed by tempering at 660 to 720 deg.C.
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