Formation mechanism of surface cracks and subsurface cracks in SAE1548: Medium carbon sulfur-containing V-N micro-alloyed bloom

Abstract

The three-dimensional (3D) morphology, spatial distribution and two-dimensional (2D) distribution of each cross-section of the surface cracks were investigated in detail with the help of industrial computed tomography (CT) equipment. Micro-structure and inclusions analysis around the cracks shows that the surface cracks originated from the mold, are induced by large-size inclusions, and propagates along the austenite grain boundary. The third brittle temperature range of SAE1548 is 650–900 °C due to the precipitation of the precipitates (AlN, V(C,N)) along the austenite grain boundary. However, the surface temperature of the bloom in the pulling and straightening section is just in this range, which makes the surface crack easy to expand and the subsurface crack easy to initiate. The macrostructure study suggested that both the mold cooling and secondary cooling processes are not optimal. As expected, the formation mechanism of cracks in SAE1548 is clarified.