| abstract |
The present invention concerns a steel for steel tubes that simultaneously satisfies a variety of characteristics, especially a steel for steel tubes with excellent resistance to fracture by sulfur stress, consisting of, in% of mass: C: 0.2 to 0.7%; Yes: 0.01 to 0.8%; Mn: 0.1 to 1.5%; S: no more than 0.005%; P: no more than 0.03%; Al: 0.0005 to 0.1%; Ti: 0.005 to 0.05%; Ca: 0.0004 to 0.005%; N: no more than 0.007%; Cr: 0.1 to 1.5%; and Mo: 0.2 to 1.0%; where the balance is Fe, Mg and impurities, and that is characterized in that: the content of Mg in the steel is not less than 1.0 ppm and is not more than 5.0 ppm; and nonmetallic inclusions of not less than 50% of the total amount found in steel have a maximum individual size of not more than 1 µm and consist of two or more elements of Ca, Al, Mg, Ti and Nb and two or more elements of O, S and N having a morphology such that there are oxides based on Mg-Al-O in the central part of the inclusion, oxides based on Ca-Al and / or oxysulfides based on Ca-Al encompasses the oxides with Mg-Al-0 base, and the carbonitrides or carbides that contain Ti or also exist in a complete or partial periphery of oxides with Ca-Al base and / or oxysulfides with Ca-Al base. |