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.

The surface and near surface damage features in a martensitic wear resistant steel extracted from three stages (after roughing mill, finishing mill, and levelling) in a hot rolling process has been characterized. Friction free bend tests VDA-238-100 has been performed to assess the deformation behaviour and its correlation to surface properties. The salient conclusions from this study are as follow:⁃After roughing mill, oxide layers of varying coverage and thickness up to 50 μm were observed with a silicon and chromium enriched spinel oxide layer located between the bulk oxide and steel substrate where presence of fayalite (Fe2SiO4) was shown. Localized fracture and removal of the oxides was seen as well as indentation of the oxides into the steel substrate causing local microstructure deformation.⁃After finishing mill the oxide layer thickness was reduced (around 10 μm) with the same silicon and chromium enriched oxide layer closest to the steel substrate. Presence of material transfer from work rolls to steel strip was observed.⁃The levelling introduced abrasive grooves caused by microploughing resulting from interaction with hard particles during mechanical brushing. Deeper valleys were seen in the surface topography compared to as-rolled condition resulting from removal of indented oxides during mechanical brushing. A deformed subsurface microstructure was also observed.⁃The silicon and chromium rich oxide layer improves adhesion of the thicker oxide layer making it difficult to remove during descaling and mechanical brushing.⁃Bend test results showed that the samples in finished condition experience increased plasticity before failure compared to as-rolled condition.⁃The crack initiation was primarily located at regions with presence of silicon rich oxides or defects such as abrasive grooves, surface indents, or local plastically deformed sub-surface areas with reduced ductility.