| 摘要 |
A low alloy steel comprising, by mass %, C: 0.10 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, Cr: 1.0 to 2.0%, Mo: 0.05 to 2.0%, Al: 0.10 % or less and Ti: 0.002 to 0.05%, and with a Ceq value obtained by the following formula (1) of 0.65 or more, with the balance being Fe and impurities, wherein in the impurities, P is 0.025% or less, S is 0.010% or less, N is 0.007% or less, and B is less than 0.0003%, and the number per unit area of M23C6 type precipitates (M: a metal element) whose grain diameter is 1 mum or more is 0.1/mm2 or less. This invention provides a low alloy steel possessing both hardenability and toughness and improves the resistance to sulfide stress corrosion cracking. <?in-line-formulae description="In-line Formulae" end="lead"?>Ceq=C+(Mn/6)+(Cr+Mo+V)/5 formula (1)<?in-line-formulae description="In-line Formulae" end="tail"?> where C, Mn, Cr, Mo and V in the formula (1) denote the mass % of respective elements.
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