Article ID: TETSU-2024-137
Copper bearing steel is hard to recycle because it shows severe surface cracking in hot working due to the formation of copper liquid phase on the interface between scale and steel, i.e. hot shortness. Suppression of the hot shortness is a critical issue to attain sustainable society. In order to clarify the mechanism of the surface cracking in hot rolling, in this study, 0.30%Cu with/without 0.15%Ni bearing square steel bars with controlled oxide scale were hot rolled in a laboratory. The height was reduced 5%, 25% or 40% at 1273 K, 1373 K or 1473 K. After the hot rolling at 1273 K, few cracks were found on both upper and side surface where no copper liquid phase appear. After the hot rolling at 1373 K or 1473 K, surface cracks shallower than 50 µm were found on upper surface, while surface cracks deeper than 100 µm were found on side surface of the rolled bars. Depth and width of cracks increased with an increase in height reduction. On the upper surface, cracks were suppressed by temperature drop due to heat transfer to the cold roll. The copper liquid phase on the interface between scale and steel was transformed to solid. On the other hand, edge cracks were formed on side surface (free surface).
