Molten (Photo credit: scottwills)
For Iron to be extracted from the raw ore
Molten (Photo credit: scottwills)(basically Iron Oxide), it has to be heated to the point where the iron basically melts off the oxygen, where it can be gathered by the smelters at the bottom of the smelting pot. As it goes through the process of smelting, the Iron picks up a bunch of carbon instead of oxygen. So once the iron cools down and the smelters remove it, it is about 2.5% or more carbon by weight.
That carbon forms microscopic crystals inside the iron; the crystals are hooked onto the iron. So if the crystals break the iron breaks as well: a brittle structure, which is a no-no in useful steel. The carbon crystals, though, are very hard, and they keep the iron from denting and bending so much.
So, basically, the carbon makes the steel harder, but at the same time makes it brittle. Let’s say we need the steel to make a sword. The Japanese would use hard, brittle iron for the edge of the sword, and strong, but dentable (less carbon, as the level of carbon content can vary throughout the smelting furnace, in this case less than .2%) iron for inside and back of the sword. This is so the edge of the blade does not dent and lose its razor-sharpness, and yet the whole thing does not shatter on impact. A different way is to have the whole sword made of metal half way in between. Once the iron has been extracted from the ore, most of it has a large amount of carbon, and so it goes through the bessemer process. In the bessemer process the iron is melted in a huge crucible, then air is blown in from the bottom, and the bubbles filter up through the molten metal. When the oxygen comes up, the carbon hooks onto the oxygen, and the float on up to the top of the molten mass, and then dissipate into the atmosphere, leaving the Iron with a perfect amount of carbon (the workers can decide the amount of carbon left by how long they keep the bubbles going through) in the steel, which is then poured into ingots and sent off to any number of steel working industries, or to a bladesmith who uses the steel to make a beautiful knife, which is bought by a young enthusiast who takes the knife home to use and to admire.