Edge Retention/Rolling Test Stand

[Jan]

KG, an oversimplified answer to your question may be:

If the amount of carbon in molten iron is below 2.1% we have steel, but if it is above 2.1% we have cast iron.

Typically we distinguish between:
Low carbon steel with less than 0.25% carbon content,
Medium carbon steel with some 0.3 to 0.5% carbon content,
High carbon steel with some 0.55% to 0.95% carbon content and
Very high carbon steel with some 1 to 2% carbon content.

The highest possible carbon content has a chemical compound called cementite (Fe3C) which has 6.7 weight % of carbon. Often the excess of carbon in high carbon steels is present as separate cementite grains.

Jan