Steel, by definition, is a mixture of iron & carbon with the optional addition of other elements to enhance it's characteristics. Thus "Decarbonised Steel" does not mean that in it's final condition it has "No" carbon. According to "Machinery's Handbook" in the Bessemer process air is blown through the molten iron burning out the carbon & other impurities, a batch of 15 tons containing about 3/4 ton of carbon. After this what is left is virtually a vat of molten wrought iron, to which is added the proper amount of carbon & manganese for the desired grade of steel. The "decarbonized" referes to the process of making the steel from cast "Pig Iron" which contains a large amount of free carbon. Note the 3/4 tons of carbon in 15 tons of cast iron amounts to 5% while Steel having 1¼% (only ¼ as much) is considered a very "High Carbon" steel, with "Mild Steels" generally falling into a range of .2% or less. Thus while 1500lbs of carbon were removed from that 15 ton batch of cast only about 60lbs would be added back to make a 1020 mild steel.
