I visited the Cat factory in Peoria a couple of years ago on one of the ACMOC-organized tours. One place we visited was the foundry, and our tour host was a guy that used to manage the foundry full-time - he's still there but in a very light capacity now. Anyway, he explained that a large proportion of the steel they used in the foundry is scrap. I should have thought to ask him how much sorting they did of the steel so as to know "exactly" what was in it, but he went on to say that after they do an initial melt, the engineers "assess" the steel in the lab and then send out paper bags of additives for the operators to throw (literally) into the cauldron (can't remember what he called that big pot). It changed every time they cooked up some steel because the make-up of the recycled input varied from batch to batch. Quite interesting and yet they produced the same engine blocks every time with the same finished material.