| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel pipe stock which has a low yield ratio and a low surface hardness after pipe making without deteriorating its toughness properties, and to provide a production method thereof. SOLUTION: A steel slab containing, by mass, 0.04-0.08% C, and further containing one or more kinds of metals selected from Cu, Ni, Cr and Mo so as to satisfy the relation in the inequality of Mn/20+Cu/20+Ni/60+Cr/32+Mo/7≥0.11 is heated to 1,000-1,250°C, and is thereafter subjected to hot rolling so as to satisfy a cumulative draft of≥75% at≤850°C and a rolling finishing temperature of 750-800°C. After that, slow cooling is performed from (the rolling finishing temperature to the rolling finishing temperature -50°C) to 650-750°C at a cooling rate of 1 to 10°C/sec, and, successively, intensive cooling is performed to≤200°C at a cooling rate of 10-30°C/sec, so that a bainite structure in which the ratio of a martensite-austenite coexistent structure having an aspect ratio of≤4.0 is 5-20 vol.% is formed, and its surface hardness Hv is also controlled to≤300. COPYRIGHT: (C)2004,JPO
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