METHOD FOR PRODUCING HIGH STRENGTH STEEL SHEET FOR SOUR RESISTANT LINE PIPES, HIGH STRENGTH STEEL SHEET FOR SOUR RESISTANT LINE PIPES, AND HIGH STRENGTH STEEL PIPE MADE OF HIGH STRENGTH STEEL SHEET FOR SOUR RESISTANT LINE PIPES

摘要

To provide a method for manufacturing a high strength steel sheet for sour resistant line pipes, in which increase in hardness at 0.25 mm below a surface of the steel sheet is suppressed while sufficient strength is secured.SOLUTION: The method for manufacturing a high strength steel for sour resistant line pipes includes a process of heating a billet having a predetermined composition and then hot rolling it to form a steel sheet, a first water-cooling process, a reheating process, a subsequent air-cooling process, and a following second water-cooling process. In the first water-cooling step, the hot rolled steel sheet is water-cooled under a predetermined condition. Then in the reheating process, the steel sheet surface is reheated to a reheating temperature of not less than (Ar+10°C) and not more than (Ar+100°C). In the air-cooling process, the air-cooling start temperature at the surface of the steel sheet is set to be equal to the reheating temperature, and the air-cooling finish temperature at a surface of the steel sheet is set to the Arpoint or more and (Ar+40°C) or lower as well as (the reheating temperature-10°C) or less, and the steel sheet is air-cooled. In the second water-cooling process, the steel sheet is water-cooled with the water-cooling start temperature at the surface of the steel sheet being set to be equal to the air-cooling finish temperature and the water-cooling finish temperature being set to a predetermined condition.SELECTED DRAWING: Figure 1A