To improve the toughness of line pipes, it is important to control austenite recrystallization and grain growth by adding alloy elements to steels. It is well known that alloy elements influence recrystallization and grain growth behavior through the solute drag effect and pinning effect. Although a good formulation considering these effects has been proposed for recrystallization, a model of grain growth has not been proposed. This paper describes a model of grain growth that considers the solute drag effect, pinning effect, and driving force of grain growth, where the latter takes into account the grain shape. The high performance line pipe with the excellent balance of strength and toughness will be developed by using the model in which the optimum chemical compositions and hot rolling condition for austenite grain refinement are indicated.
High strength and excellent toughness at low temperatures have been required for line pipe steels to achieve safety and reliability in pipelines for oil and natural gas transportation. Control of austenite grains during hot rolling is effective for improving the toughness of steels by grain refinement. In particular, it is important to suppress grain growth after rolling in the recrystallization region for austenite grain refinement, and it is also important to retard recrystallization during rolling in the nonrecrystallization region for increasing the austenite grain boundary area and the formation of a deformation band.
To obtain austenite grains with these desirable characteristics, alloy elements such as Nb, Ti, and Mo are added to steels. These steels is called microalloyed steels. It is well known that grain growth and recrystallization are affected by these alloy elements because of the solute drag effect and the pinning effect of particles. A model of recrystallization that considers the solute drag and pinning effects was proposed (Zurob et al., 2001). This model considered only the solute drag effect of Nb atoms on the mobility and the Zener formula was used to quantify the pinning force in this model (Smith, S.C, 1948).