Scrap recycling is the most powerful way to reduce the carbon footprint in steel making. Precious resources and more than 80% of energy can be saved as compared to the primary steel production by means of the blast furnace (BF). Due to a stagnant use of steel products in Western Europe, the recycling rate should substantially increase during the next years. However, pronounced material integration and multi-material use in technical products lead to a contamination of steel scrap. Some of these contaminations cannot be economically removed during remelting and refining in the electric arc furnace (EAF) and therefore, a gradual accumulation of detrimental impurities, like Cu or Sn, causes a deterioration of quality [1]. Accordingly, the respective recycling steel grades are not sufficient to fulfil the demanding specifications of flat products, e.g. in the packaging or automotive industry. Strong segregations at the prior austenite grain boundaries leads to the problem of hot shortness, i.e., massive embrittlement of hot-formed steel products. In the present study, concepts of increasing the Cu concentration have been reconsidered. Adding between 0.5 and 2 wt.% Cu in low- and medium carbon steel as well as in the quench and tempering steel 42CrMo4 (AISI 4140, produced by vacuum induction melting) led to an improvement of the static and cyclic mechanical properties [2-3]. This improvement is associated with the precipitation of nano-sized, semi coherent Cu precipitates during an ageing treatment. The precipitates were identified by means of atom probe tomography (APT) and were shown to result in a substantial strength increase. The results are discussed using the CALPHAD approach (computational thermodynamics) with respect to a future design of Cu-tolerant recycling steel grades for a circular economy.

References

[1] J. M. Cullen; J. Allwood; M. D. Bambach Enivronmental Science and Technology, 2012, 46, 13048-13055.

[2] A. Gramlich; T. Hinrichs; H. Springer; U. Krupp Steel Research International, 2022, early view, 2200623.

[3] D. Görzen; H. Schwich; B. Blinn; W. Song; U. Krupp; W. Bleck; T. Beck International Journal of Fatigue, 2021, 144, 106042.