| 摘要 |
PROBLEM TO BE SOLVED: To produce an ultrahigh tensile strength steel excellent in stress corrosion cracking resistance and low temp. toughness by forming its structure into a one essentially consisting of a mixed tempered one of acicular and massive martensite and incorporating the carbides of Cr or the like therein. SOLUTION: The components of a steel are composed of, by weight, 0.06 to 0.12% C, 0.02 to 0.25% Si, 0.05 to 0.50% Mn, >11.0 to 13% Ni, and the balance iron. Furthermore, one or more kinds among 0.8 to 2.5% Mo, >1.0 to 3% Cr, 0.05 to 0.20% V and 0.01 to 0.10% Al are incorporated therein. Moreover, one or more kinds among Cu, Nb, Ti, Ca and rare earth metals may be incorporated therein. The slab thereof is heated at 1000 to 1250 deg.C and is subjected to hot working at >=50% reduction of area, theΔworking is finished at >=700 deg.C, and it is cooled to <=150 deg.C. After that, it is heated at the Ac3 -40 deg.C to less than the Ac3 , is thereafter quenched and is tempered at less than the Ac1 . Thus, its structure is formed into a mixed tempered one of acicular and massive martensite. In this way, the steel applicable even in a severe using environment can be provided.
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