Linepipe steels, which transport oil and gas, should have high strength, high toughness, excellent corrosion resistance and superior weldability. [1][2][3] The current demand is towards the larger-diameter and higher-pressure linepipes to improve the efficiency of transmission, resulting in more stringent specifications for the linepipe steels. In addition to the above mentioned properties, the steels should have low yield ratio (yield strength/tensile strength) for the safety concern. The lower yield ratio means the higher resistance to deformation from yielding to plastic instability, 4,5) preventing the sudden decrease in the strength. According to the engineering specifications, API X-70 steels should have the yield strength higher than 480 MPa, impact energy larger than 100 J at Ϫ40°C and the yield ratio smaller than 85 %. It has been shown in the previous study that the achievement of low yield ratio (around 80 %) and high toughness (ductile-brittle transition temperature of around Ϫ100°C) is possible in the 440 MPa grade C-Mn steel by controlling the thermomechanical process.6) However, it becomes more difficult to maintain adequate values of yield ratio and toughness as strength increases to a higher level. The basic difficulty in optimizing these properties comes from the fact they are often inversely correlated, i.e. an increase in the strength is achieved at the expense of the yield ratio and toughness, and vice versa. Therefore to develop the high performance linepipe steels, the individual effect of microstructural features on these properties should be clearly understood.One of the ways of decreasing the yield ratio is the utilization of hard second phase in the microstructure. Numerous studies on so-called dual phase steels showed that these steels have low yield ratio due to the presence of hard martensite or bainite in soft ferrite matrix. 7,8) Shikanai et al.,9) analyzed the relationship between the volume fraction and morphology of second phase and the yield ratio by finite element method (FEM) and reported that the steel should have soft matrix with around 50 % volume fraction of hard second phase to obtain low yield ratio. It has also been suggested that the larger difference in the strength between the two phases is more desirable to obtain low yield ratio.9,10) However these dual phase steels generally have low yield strengths which do not meet the property requirement for X-70 linepipe steels. Moreover the large difference in the strength between the two phases might have a deleterious effect on toughness since cracks can easily nucleate at the hard second phase particles at low temperatures. Recently, more emphasis is placed on developing acicular ferrite or bainite base steels for linepipe applications.11-13) However, the toughness of these steels is often disappointing. Therefore new microstructures are needed to obtain the adequate combinations of strength, toughness and yield ratio. ISIJ International, Vol. 42 (2002) The present study aims at elucidating the effects of microstructu...