Yes, residual Cu is the copper content of steel as detected by chemical analysis. In some cases Cu is added by purpose to increase atmospheric corrosion resistance (e.g., CorTen steels); in other cases, specially in the case of electrical steels, the copper presence is result from the use of contaminated scrap with high Cu contents. Cu is not oxidised in liquid steel, even in a basic oxygen furnace.
On the other hand, a severe surface copper contamination can occur in continuously cast steels due to the wear of the copper mold of the continuous cast machine. The solution here is to plate such mold with nickel.
All the best,
Antonio A. Gorni www.gorni.eng.br