Transformation and Precipitation Behavior in Heat Treatment On-Line Process

by Toyohisa Shinmiya, Nobuyuki Ishikawa , Haruo Nakamichi, Shigeru Endo, Nobuo Shikanai, Koichi Ihara, Shinichi Kakihara

Publisher - JFE Steel Corporation, Fukuyama, Japan

Category - General Novel

There is a strong need for higher strength linepipe even for sour gas service because of total cost reduction in pipeline system. However, at present, commercial use of sour resistant linepipe is limited up to Grade X65. Fine low carbon bainite microstructure, which is obtained by controlled rolling and Accelerated cooling process, has an excellent balance of high strength and high toughness. However, there is a limitation for balancing higher strength and crack resistant property, because harder second phase, such as martensite-austenite constituent (MA), is formed and cracks propagate along the second phase easily. In order to develop Grade X70 linepipe steels, extensive investigations for improving resistance to hydrogen induced cracking (HIC) in NACE sour environment were conducted using advanced plate manufacturing technology. Heat treatment on-line process was applied subsequently after Accelerated cooling for precipitation hardening by fine carbides. Homogeneous microstructure without MA was also obtained through thickness of the plate by this new process. It was proved that precipitation hardened ferrite-bainite steels have extremely high resistance against HIC for Grade X70. Mechanical properties and microstructural characteristics of this newly developed steel were introduced in this paper.

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