This paper shows microstructure factors and TMCP conditions for deformability of X100 plates after ageing. Ferrite-bainite microstructure with MA is preferable to bainite microstructure with MA because of low Y/T ratios and S-S curves of the roundhouse type after ageing. In order to obtain this favorable microstructure, lowering reheating and rolling temperature is effective; this process condition increases ferrite nucleation rate by austenite grain refinement, and ferrite phase formation promotes carbon concentration into untransformed austenite and enhances strengthening of bainite microstructure and formation of MA. In addition, control of accelerated cooling is also important to form MA efficiently.
The demand for natural gas is expected to increase rapidly alongside of the world economy growth with the benefit of less carbon dioxide emission than oil and coal in combustion. There have also been needs for high strength linepipes for cost reduction in construction of longdistance pipelines. Furthermore, pipeline projects tend to be planned in severe environments such as in seismic or discontinuous permafrost areas, and deformability of linepipe has lately attracted considerable attention to prevent buckling and fracture in case of large ground movement (Fig. 1). Strain Based Design (SBD) thus begins to be adopted in pipelines in such areas (Dorey, 1999; Glover, 2004). Effective factors have been determined for buckling resistance; S-S curves of the round-house type, high strain hardening coefficients (nvalues), low yield strength to tensile strength and low diameter to thickness (D/t) ratios (Endo, 2000; Ishikawa, 2009). In case of S-S curves with Luders elongation, a small stress increase beyond the yield stress can lead to buckling. In case of high n-values, local strains are diffused by work hardening and buckling can be restrained. Although the most effective is low D/t, pipe thicknesses tend to become smaller with higher strength material and the D/t increases consequently.