It's not just about heat. Steel is an alloy of iron and carbon. The traditional way to infuse carbon into the metal was to place wrought iron ingots (that is, o

It's not just about heat. Steel is an alloy of iron and carbon. The traditional way to infuse carbon into the metal was to place wrought iron ingots (that is, ones with almost no carbon, too soft for use) into a charcoal fire that was being starved of oxygen, for days. You had to regularly add more charcoal, which both provided the necessary heat and slowly seeped into the metal. As mobility of carbon is very low at low heat, most of the charcoal is wasted as fuel. Around the same time, the Chinese had already figured out the much more efficient process of starting with cast iron (iron saturated with carbon, easy to make but too brittle to use), and heating and working it in a very oxygenated environment, burning out the excess carbon.