Note that generally speaking the term steel as mentioned means it is an alloy containing carbon. The silicon, sulfer & phoserphorus are more or less from the process of smelting I believe. If one sees the term "Alloy Steel" this can normally be taken to mean some other elemrnt than carbon has been added to the mix. Chromium, nickel, molynedum, vanadium etc are common alloying elements.
It would seem that about 1/16% of carbon is the minimum to be classified as steel rather than iron. This puts it into the class known as 1006, the 10 designating a plain carbon steel & the 06 the percentage. Note that under this original SAE 4-digit designation no provision was made for alloys containing more than 1% average carbon content. I believe the steels having more than this were all listed under a special tool steel designation.
Ordinary Grey Cast iron as I recall was in about a 5% carbon range, but don't hold me to this. I suspect the 2% given by Rocketman is correct, but this would I think be found only in very high grade castings, & in what is also referred to as "Semi-Steel" castings. Another fine grained cast iron is known as "Meehanite". In my early machine shop days (pre CNC Lathes) we used to get this in plate form to use for making tracer templates. It was considerably less dirty to machine than ordinary grey iron.
