Patent ID: 9175371
Filing Date: 2015-11-03
Classification: C21D,C22C

Abstract:
1. A steel for steel tubes with excellent sulfide stress cracking resistance which comprises, 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%; O: up to 0.0015%; optionally Nb: 0.005 to 0.1%; the balance being Fe, Mg and impurities, and wherein 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.