High strength steel and high strength bolt excellent in delayed fracture resistance and methods of production of same

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 certain thickness range and a nitrogen concentration higher than the nitrogen concentration of the steel by 0.02 mass % or more and (b) a low carbon region having a certain depth range from the surface of the steel and having a carbon concentration of 0.9 time or less the carbon concentration of the steel.

主权项

1. A high strength steel 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%, andhaving a balance of Fe and unavoidable impurities, the structure being a tempered martensite structure having an area ratio of 85% or more,
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, wherein a hardness of the surface of the steel is less than or equal to a hardness of the remaining portion of the steel.