A Steel ingot is produced by mixing coal and iron ingots and then smelting it in a furnace. It is used for multitude of recipes including parts for the spaceship , machines and tools. It can also be compressed into Compressed Steel .
Steel ingots are made by combining 4 coal and 1 iron ingot in a cross shape with the iron ingot in the middle, resulting in steel dust.
Then the Steel dust needs to be smelted in a furnace creating 1 steel ingot.
Detailed Table of how to craft Steel dust. (not in latest version)