STEEL FOR STEEL PIPE HAVING EXCELLENT SULFIDE STRESS CRACKING RESISTANCE

摘要

The present invention provides a steel for steel tubes which simultaneously satisfies a plurality of characteristics, specifically, a steel for steel tubes with excellent sulfide stress cracking resistance, including, by mass%: C: 0.2 to 0.7%; Si: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: not more than 0.005%; P: not more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: not more than 0.007%; Cr: 0. 1 to 1.5%; and Mo: 0.2 to 1.0%; the balance being Fe, Mg and impurities, being characterized in that: the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0 ppm; and non-metallic inclusions of not less than 50% of the total number of those in steel each having the maximum bulk size of not less than 1 µm and comprising two or more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N have such a morphology that Mg-Al-O-based oxides exist at the central part of the inclusion, Ca-Al-based oxides and/or Ca-Al-based oxysulfides enclose the Mg-Al-O-based oxides, and Ti-containing-carbonitrides or -carbides further exist on a complete or partial periphery of the Ca-Al-based oxides and/or Ca-Al-based oxysulfides.