What is the range of carbon content in plain carbon steels?

What happens if carbon content exceeds the higher range and where is it used for? Differentiate the cast iron from steel as regards their microstructure.

What should be the conditions of cooling in the heat treatment of 0.45% carbon steel for the development of: (1) finer pearlitic structure, and (2) 100% martensitic structure?

With reference to iron-carbon phase diagram, what would be the final microstructure of steel with (a) 0.80% carbon, (b) 0.40% carbon and (c) 0.90% carbon under equilibrium cooling?