| 摘要 |
PROBLEM TO BE SOLVED: To provide a low yield ratio high strength electric resistance welded steel pipe for a steel pipe pile and a production method thereof.SOLUTION: The electric resistance welded steel pipe is produced by using steel raw material containing, by mass%, C:0.11 to 0.20%, Si:0.05 to 0.50%, Mn:1.00 to 2.00%, P:0.030% or less, S:0.010%% or less, Al:0.01 to 0.08% or further one or two kind of Nb:0.12% or less and V:0.10% or less satisfying a condition of C-(Nb×12/93+V×12/51)≥0.10, heating the steel raw material to 1150 to 1300°C, subjecting the heated steel raw material to hot rolling, cooling the resultant hot-rolled steel sheet from a finish rolling completion temperature to 550 to 700°C at 10 to 100 s, then winding the hot-rolled steel sheet, and cold-forming the hot-rolled steel sheet to a pipe shape and subjecting the steel sheet formed into pipe shape to electric resistance welding. Thereby a low yield ratio high strength electric resistance welded steel pipe for steel pipe pile is obtained which has: a ferrite phase as a main phase and pearlite/pseudo pearlite having an area ratio of 8 to 30% as a second phase; a yield strength YS:450 MPa or more; a tensile strength TS:590 MPa or more; and a yield ratio:90% or less. |
