HIGH-STRENGTH SEAMLESS THICK-WALLED STEEL PIPE AND PROCESS FOR PRODUCING SAME

摘要

Provided are: a high-strength seamless thick-walled steel pipe which is excellent in terms of the yield strength and low-temperature toughness of the wall-thickness center; and a process for producing the high-strength seamless thick-walled steel pipe. The high-strength seamless thick-walled steel pipe has excellent low-temperature toughness, and is characterized by having a composition which contains 15.5-18.0% Cr and having a steel structure comprising a ferrite phase and a martensite phase. The steel pipe is further characterized in that in the circumferential-direction cross-section and L-direction (rolling-direction) cross-section of the steel pipe, when any two adjoining ferrite grains in which the difference in crystal orientation between one of the ferrite grains and the other ferrite grain is 15º or greater are regarded as separate grains, then the steel structure has a maximum ferrite grain area of 3,000 µm2 or less and a content of ferrite grains each having an area of 800 µm2 or less of 50% or higher in terms of areal proportion.