Effects of Thermo-mechanical Parameters on Microstructural and Mechanical Properties of API X70 Steel

The relationship between the microstructure and the tensile properties of X70 pipeline steel was investigated with the purpose of optimizing the microstructures by a proper selection of preheating and finish rolling temperatures. Four reheat temperatures (900°C, 1000°C, 1100°C, and 1200°C) were selected to study the effects of prior austenite grain size on the resulting microstructures. Two finish rolling temperatures (900°C and 950°C) were chosen with two reduction rates (20% and 40%). The microstructure observations with optical microscopy and SEM show that the increase in the reheating temperature caused an increase in the ferrite grain size as well as the change in shape from polygonal to acicular. The results of mechanical testing showed that decreasing of the finishing rolling temperature causes an increase in the values of yield stress and tensile strength.