Effect of Accelerated Cooling Rate and Finish Rolling Temperature on the Occurrence of Arrowhead Markings in Drop-Weight Tear Test of API 5LX70 Linepipe Nb–V–Ti Steel Plate

Abstract

Four Nb–V–Ti steel plates of API 5LX70 linepipe were produced with changing accelerated cooling rate and finish rolling temperature using thermomechanical controlled process. The changes in ductile fracture surface, brittle fracture surfaces, including initial cleavage fracture and inverse fracture, arrowhead marking surfaces, and delamination were investigated using drop-weight tear test at temperatures of 0 °C and − 10 °C. Based on the drop-weight tear test results, an increase in the accelerated cooling rate, resulting in an increase in the volume fraction of quasi polygonal ferrite, fine polygonal ferrite, and granular ferrite microstructures, increased the percentage of ductile fracture surface and decreased the percentage of brittle fracture surfaces. Moreover, a reduction in the finish rolling temperature led to similar changes in the ductile fracture surface and brittle fracture surfaces due to an increase in the volume fraction of aciculae ferrite. Arrowhead markings surfaces, caused by the presence of banded structures, their percentage was reduced by increasing the accelerated cooling rate and decreasing the finish rolling temperature at the temperature of 0 °C. Furthermore, as the accelerated cooling rate increased, the number, length, thickness, and depth of delaminations were reduced at temperatures of 0 °C and − 10 °C.