Toward Scrap-Tolerant Steels: Investigating the Role of Cu and Sn Micro-Segregations on Solidification Microstructure and Cracking

Abstract A crucial component of lowering the current 8% greenhouse gas emissions (GHG) from the steel industry is increasing circularity. The major obstacle toward enhanced recycling of steel is the impurities that accumulate when a larger fraction of recycled, low-quality steel needs to be used to achieve circularity. In particular, the role of tramp elements (impurities, such as Cu and Sn), which get concentrated during the recycling processes, must be better understood. In this work, we present a study on the model Fe–Cu–Sn system with binary Fe–Cu (up to 5-wt% Cu) and ternary Fe–Cu–Sn (up to 5-wt% Cu, 0.7-wt% Sn) variants. We studied the micro-segregation behavior of Cu and Sn using both experimental and computational thermodynamics and kinetics tools. We saw that up to 4–6 times segregation of Cu and Sn could be confirmed. Moreover, the high alloyed variants (greater than Fe–3Cu and Fe–1Cu–0.15Sn) showed solidification cracking which could be confirmed via microscopy and supported by Scheil solidification calculations. At the same time, alloying with Cu and Sn was seen to refine the ferrite grains in the as-cast material by more than an order of magnitude (from 108 µm → 4.9 µm average diameter). It is proposed that Cu could be successfully used in larger amounts than the current industry practice as an alloying element in steels in cases when hot shortness can be avoided. This would provide an enhanced opportunity for upcycling of lower-quality grades by providing a pathway toward increased strength due to precipitation hardening—making the process of increasing recycling of steel much more lucrative. Graphical Abstract