Material design concept to improve HIC resistance of linepipe steels based on PHIC parameter, which describes hardenability of center segregation region was developed with theoretical consideration to partitioning of solute elements in the liquid/solid phase. By using PHIC parameter, alloying elements were optimized for reducing the hardness in the center segregation region. Advance controlled cooling process was also introduced in order to achieve homogeneous microstructure and mechanical properties in the plate. Thus, process window that balances higher strength, toughness, HIC resistance and moderate surface hardness has been successfully extended.


Linepipe material for sour gas service primarily needs to have crack resistant properties. However, applications of sour linepipes are expanding toward deep water or cold remote regions, which require higher toughness and/or heavier wall thickness as well as higher strength. In order to improve crack resistance properties, many kinds of treatment in the steel making and the plate rolling process are usually applied for sour linepipe steels. One of the basic treatments is to prevent initiation of HIC by controlling inclusions, such as MnS, in the steels. Therefore, lower level of sulfur content and Ca treatment, which prevents the formation of elongated MnS by forming spherical CaS, need to be applied (Taira, Tsukada, Kobayashi, Tanimura, Inagaki and Seki, 1981; Jones and Gray, 1993). Another basic treatment is reducing center segregation, because the center segregation area shows increased hardness and HIC mainly occurs in this area. For this purpose, contents of C, P, Mn and other alloying elements that causes center segregation need to be limited. Conventional linepipe steels for sour service are produced by applying the above treatments. For the steels of Grade X60 and higher, the accelerated cooling process after controlled rolling is applied for obtaining high strength with lean steel compositions. Plate rolling conditions for sour linepipe steels also need to be controlled (Tamehiro, Takeda, Yamada, Matsuda and Yamamoto, 1984; Endo, Doi, Ume, Kakihara and Nagae, 1997). Higher cooling rates in the accelerated cooling process after rolling gives tremendous benefit for obtaining higher strength and superior toughness even for thicker plates. This paper firstly summarizes the effect of alloying elements, plate rolling and cooling conditions on sour resistant properties and mechanical properties of high strength sour linepipe steels. However, even applying above measures, it becomes difficult to fulfill recent stringent requirement for sour linepipes, such as heavy wall thickness and higher toughness. Secondly, recent alloy design concept to improve HIC resistance based on PHIC, which is a parameter to describe hardenability of center segregation region is introduced. Finally, advanced accelerated cooling apparatus to realize homogeneous surface hardness is referred in the context of sulphide stress corrosion cracking (SSCC) resistance.

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