STRETCH-REDUCED STEEL PIPE AND ITS MANUFACTURING METHOD

摘要

PROBLEM TO BE SOLVED: To provide a stretch-reduced steel pipe having excellent workability and its manufacturing method. SOLUTION: The stretch-reduced steel pipe has a structure in which secondary phases having an average major axis to minor axis ratio of <=2 and an average grain size of <=3μm are distributed into ferritic phases in a recrystallized ferrite structure. It is desirable that: the average major axis to minor axis ratio of the recrystallized ferrite is <=3; the average grain size of the recrystallized ferrite structure is 3-60μm; and the area ratio of the secondary-phase structure is >=1%. In the method for manufacturing the stretch-reduced steel pipe, stretch reduction is finished at 500 deg.C to (A1 point-80 deg.C) and reheating is performed in the temperature range between (A1 point-50 deg.C) and the A1 point at >=30 deg.C/s heating rate and cooling is performed down to <=(A1 point-100 deg.C) at >=5 deg.C/s cooling rate.