| 摘要 |
A steel which is excellent in delayed fracture resistance containing, by mass%, C: 0.10 to 0.55%, Si: 0.01 to 3%, and Mn: 0.1 to 2%, further containing one or more of Cr: 0.05 to 1.5%, V: 0.05 to 0.2%, Mo: 0.05 to 0.4%, Nb: 0.001 to 0.05%, Cu: 0.01 to 4%, Ni: 0.01 to 4%, and B: 0.0001 to 0.005%, and having a balance of Fe and unavoidable impurities, the structure being a mainly tempered martensite structure, the surface of the steel being formed with (a) a nitrided layer having a thickness from the surface of the steel of 200 µm or more and a nitrogen concentration of 12.0 mass% or less and higher than the nitrogen concentration of the steel by 0.02 mass% or more and (b) a low carbon region having a depth from the surface of the steel of 100 µm or more to 1000 µm or less and having a carbon concentration of 0.05 mass% or more and 0.9 time or less the carbon concentration of the steel. |
