Correlation between surface defects induced at different stages during hot rolling and bendability of high-performance steel

During hot rolling of steel, various defects are introduced in the surface and near surface region of the steel strip. These defects will propagate and evolve through the different stages of the hot rolling process and, finally, affect the bendability during subsequent manufacturing processes. Understanding the connection between surface defects and oxide layers and the bendability of high-performance steels is critical for forming process optimisation. This study is to investigate the surface/near surface defects and oxide layers of a high-performance martensitic wear-resistant steel at different stages of the hot rolling process (roughing mill, finishing mill, and after the final processing) and correlate these to the bendability. Pre- and post-bend test analyses were performed using scanning electron microscopy with energy-dispersive spectroscopy, 3D white-light interferometry, X-ray diffraction, and microhardness measurements. Friction free bending tests were performed using an equipment similar as the original VDA 238-100 tool, but upscaled in size managing higher loads. The bendability of samples in levelled condition after finishing process is reduced compared to as-rolled condition. The results show that the formation of red scale between the steel substrate and primary oxides significantly affects the bendability of the martensitic wear-resistant steel in both as-rolled and levelled conditions. Detailed characterisation indicated that crack initiation during bending was primarily concentrated at the red scale region as a result of topographical changes and local deformation of the near surface microstructure. Generation of secondary cracks caused by abrasive wear from mechanical brushing also impact bendability.