Abstract
The effect of tramp elements Cu and Sn on the hot workability of mild steels was investigated in this paper. A hot rolling condition was simulated by oxidizing followed by tensile-deforming and the hot workability was assessed by measuring the number of surface cracks. The microstructure at the scale/steel interface was closely observed, and the relationship between the surface cracking and the microstructure was established. Sn as low as 0.04 % increased the number of cracks in an 0.3 % Cu bearing steel, while 0.3 % Ni suppressed the cracking in an 0.3%Cu-0.04%Sn bearing steel. An energy dispersive X-ray spectroscopy (EDX) at the subscale layer of the oxidized Cu-Sn bearing steel revealed that an 82%Cu-7%Sn-Fe alloy phase formed at the scale/steel interface. The phase calculation using the Thermo-Calc computer program showed that Sn decreased the solubility limit for Cu in Fe, thus increasing the amount of liquid Cu enriched alloy and enhancing the surface cracking by Cu liquid embrittlement. On the other hand, an addition of 0.3 % Ni to the 0.3%Cu-0.04%Sn bearing steel formed two different Cu-Sn phases at the surface of the steel; a 12%Cu-19%Ni-Fe layer under the steel surface and a liquid 63%Cu-12%Sn-12%Ni-Fe alloy placed above the layer. The amount of the 63%Cu-Sn-Ni-Fe alloy was a little, and the 12%Cu-Ni-Fe layer was in a large amount. In addition, the phase calculation at an oxidation temperature 1 100°C showed that the Cu and Ni enriched layer was solid. Therefore, the addition of 0.3 % Ni suppressed the surface cracking in the Cu-Sn bearing steel. A mechanism of the reduced amount of liquid by increasing Ni content was discussed in terms of local equilibrium and inward diffusion of Cu atoms from the Cu enriched layer into the steel.
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