| 摘要 |
PROBLEM TO BE SOLVED: To obtain a steel increased in tensile strength and hardness even in the case of extremely large thickness and having excellent weldability and sufficient machinability by specifying the chemical composition of a structure- hardened-type steel. SOLUTION: This steel has a composition consisting of 0.03-0.25% C, 0-0.2% Si, 0-0.9% Mn, 1.5-5% Ni, 0-18% Cr, 0.05-1% of (Mo+W/2), 0-0.3% S, 0.5-3% of either or both of Al and Cu, and the balance Fe with impurities resulting from refining, particularly N. If necessary, this steel can contain 0.0005-0.015% B, further 0-0.3% of one or more elements among V, Nb, Zr, Ta, and Ti, and further 0-0.3% of one or more elements among Pb, Se, Te, and Bi. Moreover, this steel satisfies inequalities I, II. At this time, in the inequality I,α=1.4 is satisfied in the case of Cr<8%, whileα=0 is satisfied in the case of Cr>=8%; in the inequality II, kB=0.8 is satisfied in the case of B=0.0005 to 0.015%, while kB=0 is satisfied in the case where B content takes a value other than the above. Further, Kth/Tr<=3 is satisfied in the case of Cr<=5%.
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