Steel for steel tube with excellent sulfide stress cracking resistance
View Patent ↗The present invention provides a steel which simultaneously satisfies a plurality of characteristics, specifically, a steel for tubes with excellent sulfide stress cracking resistance, including, 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 is not less than 1.0 ppm and not more than 5.0 ppm; and inclusions of not less than 50% of the total number of those in steel have such a morphology that Mg—Al—O-based oxides exist at the central part of the inclusion, Ca—Al-based oxides enclose the Mg—Al—O-based oxides, and Ti-containing-carbonitrides further exist on a periphery of the Ca—Al-based oxides.
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,
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.
2. 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.