Effects of the Finish Rolling Temperature on mechanical properties and microstructure evolution of line pipe steel

Mechanical properties of line pipe steels used in the deep sea or in the severe cold climate depend on alloying elements and manufacturing processes and many efforts have been made to enhance the properties of the line pipe steels. In this study, for systematic approaches to the process design of line pipe steels, its phase diagram was calculated using a Thermo-Calc™ program. The calculations indicated that A₃ is around 840°C. Setting the FRT(Finish Rolling Temperature) above A₃ appears to be critical to the increase of strength and toughness of line pipe steels by increasing the amounts of acicular ferrite and at the same time by decreasing the precipitation of proeutectoid ferrite. In the case of the FRT below A₃, relatively large amounts of proeutectoid ferrite are precipitated from the matrix with the carbon contents of the austenite phase around ferrite being enriched.