| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength oil well steel suitable for thick-walled products, excellent in toughness and SSC resistance, and having YS of 110 ksi class or above, and inexpensively manufacturing method therefor. SOLUTION: This steel is a low alloy steel which has a composition containing 0.15-0.3% C, 0.2-1.5% Cr, 0.1-1% Mo, 0.05-0.3% V, and 0.003-0.1% Nb and in which the total amount of precipitating carbides is 1.5-4% and an MC carbide comprises 5-45% of the total amount of the carbides, and further, the proportion of an M23C6 carbide is <=(200/t)% when (t) represents product thickness. The steel can be manufactured by subjecting a steel with the above composition to heating up to >=1150 deg.C and to hot working at >=1000 deg.C finishing temperature, applying direct quenching from >=900 deg.C and tempering at a temperature between 550 deg.C and the Ac1 transformation point, and subjecting the steel to reheating up to 850-1000 deg.C and quenching and then to tempering at a temperature between 650 deg.C and the Ac1 transformation point at least once.
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