| 摘要 |
A high strength steel material 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 both of V: 1.5% or less and Mo: 3.0% or less, the contents of V and Mo satisfying V+1/2Mo>0.4%, further containing one or more of Cr: 0.05 to 1.5%, 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 material being formed with (a) a nitrided layer having a thickness from the surface of the steel material of 200 μm or more and a nitrogen concentration of 12.0 mass % or less and higher than the nitrogen concentration of the steel material by 0.02 mass % or more and (b) a low carbon region having a depth from the surface of the steel material 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 material. |
