강관은 질량비로 탄소 0.25%~0.60%, 실리콘 0.01%~2.0%, 망간 0.2%~3.0%, 알루미늄 0.001%~0.1%, 인 0.001%~0.05%, 인 0.02% 또는 더 적은 황, 0.0010% ~ 0.0100% 질소, 0.0003% ~ 0.0050% 붕소 및 0.0001% ~ 0.0050% 칼슘, 나머지는 철 및 부수적 불순물입니다. 강관 원료를 Ac3 변태점 이상으로 가열하여 침지시킨 후, 마무리 온도 900℃에서 (Ac1 변태점) 누적 감소 직경 30℃까지 압연하는 신축 압연 900℃ 이하의 온도 범위 내에서 % 내지 70%.
A raw steel tube has a composition containing, by mass, 0.25% to 0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0% manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05% phosphorus, 0.02% or less sulfur, 0.0010% to 0.0100% nitrogen, 0.0003% to 0.0050% boron, and 0.0001% to 0.0050% calcium, the balance being iron and incidental impurities. The raw steel tube is heated to and soaked at an Ac3 transformation point or higher and is then subjected to stretch-reducing rolling at a finishing temperature of rolling of 900° C. to (Ac1 transformation point) with a cumulative reduction in diameter of 30% to 70% within the temperature range of 900° C. or lower.