Simply put, without getting into too much detail, the only difference between cast iron and steel is the carbon content. When cast iron is made with coke in a blast furnace it absorbs a huge amount of carbon, which makes it very stable, and very hard in compression, but brittle. Mild steel is cast iron with all but a small percentage of the carbon removed, and is fairly soft. By removing some of the carbon, or diluting it by adding scrap steel to the melt, you get a very tough metal, hard, but much less brittle.