The Influence of the Ni Content on Surface Crack Formation in Continuous Casting for Cu- and Sn-Containing Steels

The increased use of low-quality scrap leads to higher concentrations of unwanted tramp elements in steel, such as Cu, Sn, and Ni, which can significantly affect downstream processing by causing surface hot shortness during continuous casting or hot rolling. This study focuses on the role of Ni in mitigating hot shortness caused by Cu and Sn in continuous casting for a medium-carbon construction steel with a specific focus on determining critical tramp element thresholds across different bending temperatures. Investigations were performed using the In Situ material characterization by bending tests (IMC-B) to evaluate crack susceptibility, alongside simultaneous thermal analysis (STA) to analyze oxidation mechanisms in detail. The results show that under oxidizing conditions, the presence of Cu and Sn leads to the formation of low-melting, Cu-rich phases, resulting in a substantial increase in surface defects over a wide temperature range, even for moderate so-called Cu-equivalent (Cueq), a value that reflects the impact of Cu, Sn, and Ni in terms of hot shortness. While the addition of sufficient Ni can suppress the negative effects of Cu and Sn, the required Ni content, and therefore also the critical Cueq, depends heavily on the bending temperature: at 1100 °C, the critical Cueq is relatively high at approximately 0.5 wt pct, whereas it drops within the 950 to 1000 °C range to only 0.05 to 0.10 wt pct.