Method of producing steel pipes, and welded pipes
View Patent ↗The high yield ratio problem intrinsic in high-strength steel pipes is solved and a method of producing a steel pipe, while securing the pipe roundness is provided. A steel pipe produced from a steel plate by forming and welding is subjected to 0.3 to 1.2% of expansion and then to 0.1 to 1.0% of reduction. The steel pipe produced by this method has a structure in which martensite and/or bainite accounts for more than 80% as expressed in terms of area percentage, and has a yield strength of not lower than 551 MPa and a yield ratio of not higher than 93%. The pipe reduction is desirably carried out at a lower work ratio as compared with the pipe expansion.
1. A method of producing a steel pipe having a microstructure at least 80%, as expressed in terms of area percentage, of which comprises martensite and/or bainite and having a yield strength of not lower than 551 MPa; comprising a step of forming and welding a steel plate into a steel pipe, expanding, by 0.3 to 1.2%, the steel pipe, and then reducing the expanded steel pipe by 0.1 to 1.0%.
2. A method of producing a steel pipe according to claim 1 , wherein the percentage of pipe reduction is smaller than the percentage of pipe expansion.
3. A method of producing a steel pipe according to claim 1 , wherein the steel pipe after expansion and reduction has a yield strength of not lower than 689 MPa.
4. A method of producing a steel pipe according to claim 2 , wherein the steel pipe after expansion and reduction has a yield strength of not lower than 689 MPa.
5. A welded steel pipe produced by any of the methods according to claims 1 to 4 .
6. A welded steel pipe produced by the method according to claim 2 .
7. A welded steel pipe produced by the method according to claim 3 .
8. A welded steel pipe produced by the method according to claim 4 .