Steel for steel tube with excellent sulfide stress cracking resistance

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%;
• Mo:
• 0.2 to 1.0%;
• and
• O:
• up to 0.0015%;
• one or more of Nb:
• 0.005 to 0.1%, Zr:
• 0.005 to 0.1%, V:
• 0.005 to 0.5% and B:
• 0.0003 to 0.005%;
• the balance being Fe, Mg and impurities,
• wherein the content of Mg in the steel is not less than 1.0 ppm and not more than 5.0 ppm;
• 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.