| 摘要 |
A steel is subjected to [a step of heating to a temperature higher than point Ac1 and lower than point Ac3 and then cooling], [a step of heating again to a temperature equal to or higher than point Ac3 and then quenching] and [a step of tempering at a temperature equal to or lower than point Ac1] in this order, wherein the steel has been worked into a desired shape by a hot process and has a chemical composition containing 0.15 to 0.65% of C, 0.05 to 0.5% of Si, 0.1 to 1.5% of Mn, 0.2 to 1.5% of Cr, 0.1 to 2.5% of Mo, 0.005 to 0.50% of Ti, 0.001 to 0.50% of Al, and optionally containing at least one element selected from 0.4% or less of Nb, 0.5% or less of V, 0.01% or less of B, 0.005% or less of Ca, 0.005% or less of Mg and 0.005% or less of REM, with the remainder made up by Fe and impurities, wherein Ni, P, S, N and O are contained in the impurities in amounts of 0.1% or less, 0.04% or less, 0.01% or less, 0.01% or less and 0.01% or less, respectively. |
