Influence of Hot Ni Shot Peening on Surface Hot Shortness

Steel scraps are used as a resource of electric steel making. However, reuse of the scrap leads to increase the amount of tramp elements such as Cu and Sn. In hot rolling process, Cu segregates at the scale/steel interface. Furthermore, this Cu segregation layer causes surface cracking. This brittleness is called surface hot shortness. Previous studies have shown that surface hot shortness is suppressed by addition of Ni. However, the addition of Ni increases in the cost of steel making. The authors have reported the influence of shot peening on suppression of surface hot shortness in pervious study. After hot oxidation, the shot peened scale/steel interface was rugged. The rugged scale/steel interface occluded the segregated Cu into the scale. The surface hot shortness was suppressed by the shot peening. However, the influence of shot peening didn't completely suppressed surface hot shortness. Whereat, this paper shows the influences of hot Ni shot peening on surface hot shortness. The shot peening grid was changed from cast iron to Ni. Surface hot shortness was suppressed by the hot Ni shot peening. After the hot Ni shot peening process, Ni diffused phase at the surface of specimens were observed with the Cu segregated layer by EDX analysis. The melting temperature of Cu segregated layer was increased by Cu-Ni alloying. In addition, the refined grains at the surface of specimens were observed by optical microscope. This suggests that the refined grains and increased lattice defects such as dislocations are developed the microscopically heterogeneous oxidation. Consequently, the hot Ni shot peening can suppress the surface hot shortness by Ni enrichment and grain refinement in surface layer.