QUENCHED AND TEMPERED STEEL PIPE WITH HIGH FATIGUE LIFE, AND ITS MANUFACTURING METHOD
Provided is a steel pipe suitable for machine structure members that are required to be lightweight, of which the strength is increased and the fatigue life is prolonged with preventing the increase in the material cost and the production cost. The steel pipe is a high fatigue life quenched/tempered steel pipe having a composition comprising, % by mass, C: 0.1 to 0.4%, Si: 0.5 to 1.5%, Mn: 0.3 to 2%, P: at most 0.02%, S: at most 0.01%, Cr: 0.1 to 2%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, Al: at most 0.1%, B: 0.0005 to 0.01%, N: at most 0.01%, and optionally at least one of Ni: at most 0.5%, Ca: at most 0.02%, Mo: at most 0.5% and V: at most 0.5%, with a balance of Fe and inevitable impurities, in which the mean grain size of the precipitated carbides is at most 0.5 μm and of which the hardness in the center part of the wall thickness in the cross section perpendicular to the longitudinal direction of the steel pipe is at least 400 HV.
1 . A high fatigue life quenched/tempered steel pipe having a composition comprising, % by mass, C: 0.1 to 0.4%, Si: 0.5 to 1.5%, Mn: 0.3 to 2%, P: at most 0.02%, S: at most 0.01%, Cr: 0.1 to 2%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, Al: at most 0.1%, B: 0.0005 to 0.01%, and N: at most 0.01%, with a balance of Fe and inevitable impurities, in which the mean grain size of the precipitated carbides is at most 0.5 μm and of which the hardness in the center part of the wall thickness in the cross section perpendicular to the longitudinal direction of the steel pipe is at least 400 HV.
2 . The high fatigue life quenched/tempered steel pipe as claimed in claim 1 , wherein the composition further contains at least one of Ni: at most 0.5%, Ca: at most 0.02%, Mo: at most 0.5% and V: at most 0.5%.
3 . The high fatigue life quenched/tempered steel pipe as claimed in claim 1 , which has a wall thickness t of from 1 to 7 mm and an outer diameter D of from 10 to 45 mm and satisfies D/t≧4.
4 . A method for producing a high fatigue life quenched/tempered steel pipe, comprising quenching treatment of keeping an unquenched starting steel pipe having a composition that comprises, % by mass, C: 0.1 to 0.4%, Si: 0.5 to 1.5%, Mn: 0.3 to 2%, P: at most 0.02%, S: at most 0.01%, Cr: 0.1 to 2%, Ti: 0.01 to 0.1%, Nb: 0.01 to 0.1%, Al: at most 0.1%, B: 0.0005 to 0.01%, and N: at most 0.01%, with a balance of Fe and inevitable impurities, at 900 to 1100° C. for 10 to 60 seconds and then rapidly cooling it, followed by tempering treatment of keeping it at 280 to 380° C. for 10 to 60 minutes.
5 . The method for producing a high fatigue life quenched/tempered steel pipe as claimed in claim 4 , wherein the starting steel pipe further contains at least one of Ni: at most 0.5%, Ca: at most 0.02%, Mo: at most 0.5% and V: at most 0.5%.
6 . The method for producing a high fatigue life quenched/tempered steel pipe as claimed in claim 4 , wherein the starting steel pipe is formed by welding a steel plate having a wall thickness t of from 1 to 7 mm into a pipe having an outer diameter D of from 10 to 45 mm and satisfying D/t≧4.
7 . The high fatigue life quenched/tempered steel pipe as claimed in claim 2 , which has a wall thickness t of from 1 to 7 mm and an outer diameter D of from 10 to 45 mm and satisfies D/t≧4.
8 . The method for producing a high fatigue life quenched/tempered steel pipe as claimed in claim 5 , wherein the starting steel pipe is formed by welding a steel plate having a wall thickness t of from 1 to 7 mm into a pipe having an outer diameter D of from 10 to 45 mm and satisfying D/t≧4.