Effect of copper and nickel on hot workability of hot-rolled mild steel

The effect of a tramp element Cu on the hot workability of steels was investigated. 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 a 0.3% Cu bearing steel, the surface hot cracking occurred only at 1100 deg C oxidation due to a liquid Cu enriched phase, i.e. 87% Cu-Fe, formed at the scale/steel interface. Both at 1000 deg C and at 1200 deg 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 deg 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 deg 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 deg 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.