Material Design of Hot-rolled Steel Coils for Heavy Wall X70 Spiral Linepipes

摘要

Higher strength and toughness has been required for linepipe steelsusing for long-distance pipelines of natural gas transportation. In orderto achieve high strength and DWTT properties for heavy wall X70spiral linepipes, the microstructure refinement of bainitic ferrite and thesuppression of hard second phase such as MA (martensite-austeniteconstituent) are essential in hot rolled steel coils. Niobium (Nb) is veryeffective to have precipitation strengthening and bainitic ferrite grainrefinement through the increase of controlled rolling (CR) ratio inaustenite non-recrystallization region. In this study, the effect of Nbcontent on the toughness of simulated coarse grain heat affected zone(CG-HAZ) and actual welded joint were investigated. As a result,higher Nb steels had good toughness in heavy wall high strength steels.Titanium (Ti) addition was also useful to suppress austenite graingrowth by austenite grain pinning effect. It is concluded that it is veryimportant to control the amount of precipitates including NbC, TiN andTiC for obtaining target mechanical properties in both of base metaland welded joint. Details of the microstructure changes will bediscussed in this paper.