| 摘要 |
PROBLEM TO BE SOLVED: To provide a martensitic stainless steel in which no heat treatment for strength regulation is required and which has strength of a grade of 552-689 MPa proof stress in an as-hot-rolled or as-welded state and also superior corrosion resistance and toughness and can be suitably used as a stock for a large- diameter welded steel pipe such as UOE pipe, a manufacturing method thereof, and also provide a method for manufacturing a welded steel pipe. SOLUTION: This steel has a composition which consists of <=0.03% C, 0.05-1% Si, 0.05-2% Mn, 12-20% Cr, 3-6.0% Ni, 0.001-0.1% Al and the balance essentially Fe and in which the contents of P, S, N, O (oxygen) and Ti among impurities are made to <=0.025% P, <=0.01% S, <=0.02% N, <=0.01% O and <=0.05% Ti, respectively, and inequalities, [Cr-Ni-14>=0] and [Cr-2×Ni-12.5<=0] are satisfied. Further, a martensitic phase comprises 55-90 vol.% of the metallic structure of the steel.
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