Microstructural, mechanical and tribological characterisation of roll materials for the finishing stands of the hot strip mill for steel rolling

The microstructure, mechanical and tribological properties for three different materials, high speed steel, high chromium iron and indefinite chill iron, used for hot strip mill work rolls have been evaluated. Microstructural characterisation was performed using light optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. The mechanical and tribological properties were evaluated using micro-Vickers indentation and scratch testing in combination with post-test microscopy. The microstructures of the investigated materials were found to be rather complex with a number of secondary phases and also materials with similar nominal composition display significant differences with respect to distribution, size and morphology of carbides. Scratch testing, including detection of friction coefficient, acoustic emission and penetration depth, gives valuable information concerning the mechanical and tribological response on a microscopic level of the investigated materials. Type, amount, distribution, size and morphology of the secondary phases in the materials have a strong impact on the surface deformation and wear mechanisms during scratching. Cracking and chipping are frequently observed in connection to the ridges surrounding the scratches. However, cross-sectional analyses of the scratched microstructures reveal that cracking of the brittle carbide phases may extend to significant depths>100 µm, reducing the mechanical strength of the material. Based on the results, it is believed that a more isotropic microstructure, e.g., obtained via a powder metallurgy process, with finer carbides would result in improved properties and performance in a hot rolling application.