METHOD OF MANUFACTURING BILLET FOR PRODUCING STEEL PIPE, AND METHOD OF MANUFACTURING STEEL PIPE FOR LINE PIPE

摘要

PROBLEM TO BE SOLVED: To provide a method of manufacturing a billet for producing a steel pipe of adequate surface quality, which has a low yielding ratio and does not cause performance degradation due to strain aging, and to provide a method of manufacturing a steel pipe for line pipe. SOLUTION: The billet used in manufacturing steel pipe includes, by mass%, 0.10-0.20% C, 0.15-0.40% Si, 0.80-1.60% Mn, 0.01-0.080% Al, 0.0005-0.060% N, the balance Fe with impurities, and the contents of N and Al of which satisfy the expression: log (Al×N)<=-3.6...(a), is manufactured by tapping a molten steel 60-80% against all tapping amounts from a converter, which contains remained oxygen of 30 ppm or more, then adding Al as a reducing agent to the molten steel during tapping, in a steelmaking process, adjusting the composition of the molten steel, and casting the molten steel. The billet is made a pipe by hot piercing and rolling, and then the structure of the steel pipe made to have a bainitic structure or a tempered structure of bainite by heat treatment.