The effect of a tramp element Cu on the hot workability of steels was investigated in this paper. The number of surface cracks occurring in the specimens which were tensile-deformed after oxidized was measured to assess the effect of small amounts of Cu (0.3%) and Ni (0.15%) on hot workability. The microstructure of a scale/steel interface was closely observed and the relationship between the surface cracking and the microstructure was established. For an 0.3% Cu bearing steel, the surface hot cracking occurred only at 1100 degrees C oxidation due to a liquid Cu enriched phase, i.e., 87%Cu-Fe, formed at the scale/steel interface. Both at 1000 degrees C and at 1200 degrees C oxidations, solid Cu enriched phases formed at the steel interface, which did not cause surface cracking. A liquid Cu enriched phase in a small amount was occluded into the scale at 1200 degrees C. An addition of 0.15% Ni suppressed the surface cracking of 0.3% Cu bearing steel by eliminating all the Cu enriched liquid phases. At 1100 degrees C oxidation, Ni addition enhanced the occlusion of a solid Cu enriched phase, i.e., 66%Cu-15%Ni-Fe, and left a solid phase, 16%Cu-16%Ni-Fe, at the scale/steel interface. At 1200 degrees C oxidation, Ni addition also enhanced the occlusion of solid phases, 16%Cu-27%Ni-Fe and 10%Cu-10%Ni-Fe, and left a solid phase, 6%Cu-1%Ni-Fe at the interface. A mechanism of formation of various Cu-Ni enriched phases in 0.3%Cu-0.15%Ni bearing steel was discussed in terms of equilibrium diagrams and the difference in diffusion rate of Cu and Ni.
