Effect of Tempcore Processing on Mitigating Problems of Tramp Elements in Low C Steel Produced from Recycled Material

The effect of tramp elements in the steel was intensively studied. It was found that the solubility of tramp elements decreased as the temperature decreased under normal cooling conditions. The tramp elements (Cu, Pb, and Sn) diffused toward the grain boundaries, and intermetallic compounds or rich phases which have low melting points were formed, causing reduction in ductility and failure during the bending test. Rebars with Cu content which were left to air cooling after the last step showed drop in elongation, up to 32%. On contrast, the samples with high percentage of tramp elements (Cu, Pb, and Sn) in the billet, which were rolled and subjected to Tempcore process, did not show drop in elongation or failure in bending test (especially for rebar with diameter less than 32 mm); however, copper must be less than 0. 35 mass% to prevent the precipitation of Cu-rich zones of critical size in 32 mm. When quenching was applied, the tramp elements remained in the interstitial supersaturated solid solution positions inside the grains and would not have the chance to diffuse and form precipitates, hindering the copper precipitates from reaching the critical size necessary for impairing the properties. This would hinder the occurrence of the harmful effect of the tramp elements on the elongation or the hot shortness after rolling.