Patent ID: 10550962

Abstract:
The steel material according to this invention contains, in mass %, C: 0.15 to 0.45%, Si: 0.10 to 1.0%, Mn: 0.10 to less than 0.90%, P: 0.05% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.01% or less, Cr: 0.1 to 2.5%, Mo: 0.35 to 3.0%, and Co: 0.50 to 3.0%, and satisfies expressions (1) and (2), and contains 90% or more of tempered martensite by volume ratio: C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15−Co/6+α≥0.50  (1 )(3C+Mo+3Co)/(3Mn+Cr)≥1.0  (2 )Effective B=B−11(N−Ti/3.4)/14  (3 )where, α in expression (1) is 0.250 when effective B (mass %) defined by expression (3) is 0.0003% or more, and is 0 when the effective B is less than 0.0003.