This paper describes the development of high strength heavy wall seamless pipes of X80 to X100 grade for ultra-deep water application. Steel pipes with higher strength generally tend to have low fracture toughness either in pipe body or in weld joint and low weldability. Therefore, improvement of fracture toughness and weldability are particularly important with respect to development of higher strength seamless pipes. Metallurgical research in laboratory test was carried out and the effect of microstructure of quenched and tempered steel on strength and toughness was particularly investigated. As a result uniform lower bainite phase containing no or minimized coarse martensite-austenite (M-A) constituent at a quenched condition is suitable microstructure to perform high strength and high fracture toughness after tempering. The steel having such microstructure showed excellent performance even in the high strength grade of X100. Lowering a transformation temperature from austenite phase to bainite phase during quenching process is effective to obtain suitable microstructure by adding and controlling alloy elements such as Mn, Cr, Mo. In order to suppress an increase in carbon equivalent as Pcm value by addition of alloy element, lowering content of carbon is necessary. As a consequence of the low Pcm value mitigation of hardening in coarse grain heat affected zone (HAZ) and good toughness were confirmed by welding tests. A trial production of the developed steel based on a new metallurgical design mentioned above was conducted by applying inline heat treatment process in medium-size seamless mill. In conjunction with tremendously rapid cooling system of inline heat treatment facility the seamless pipes of the trial production achieve grades X80 of 40 mm wall thickness and X80 to X100 grades of 20 mm WT by changing tempering temperature. A good combination of high strength and good fracture toughness was confirmed.

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