To provide a high strength steel pipe stock which has a low yield ratio after pipe making without deteriorating its toughness properties, and to provide a production method thereof.
A steel slab having a composition 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. After that, the steel slab is subjected to hot rolling so as to satisfy a cumulative draft of ≥75% at ≤850°C and a rolling finishing temperature of 700-800°C. Subsequently, water cooling is performed from (the rolling finishing temperature to the rolling finishing temperature -50°C) 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.% of the whole structure is formed.
