| 摘要 |
PURPOSE:To obtain high strength steel excellent in sulfide cracking resistance, by applying two-stage hot processing to steel, wherein the amount of C, Nb and Mn and the amount of P and S in impurities are specified and Cu, V and one of Ca, REM or the like are added, under a specific condition before direct hardening. CONSTITUTION:The composition of steel consists of, on a wt% basis, 0.15-0.45 C, 0.08 or less Si, 0.01-0.30 Mn, 0.20-1.50 Cr, 0.05-0.80 Mo, 0.01-0.10 Nb, 0.01-0.10 Al, according to necessity, 0.0005-0.005 B, 0.05-0.50 Cu, 0.01-0.10 V and one or more of 0.001-0.30 Ca and 0.001-0.050 REM and the remainder of Fe and inevitable impurities. In this case, P in said impurities is set to 0.015 or less and S therein to 0.010 or less. This steel is subjected to primary hot processing and, after transformation is once completed, the processed steel is again heated to Ac3-Ac3+200 deg.C to be subjected to secondary hot processing. Succeedingly, the processed steel is directly hardened from an austenite state and subsequently heated in the above mentioned temp. range to be subjected to hardening treatment one or more times. Subsequently, annealing treatment is applied at a temp. of Ac1 transformation point of less. |
