You are absolutely correct sir...carbon is not an impurity in steel, it is however in "iron" and it is that particular impurity that produces steel. A case in point though...too little carbon makes for a soft blade, none at all is simply iron, too much carbon resultes in to rigid a metal and is called "cast iron".Gokul43201 said:Carbon is definitely NOT considered an impurity in steel - in fact, it is a primary alloying element (at least in low allow steels) and is what makes steel harder than iron. And, the carbon content of a steel (or any other metal, for that matter) can not be controlled by heat-treatment. What the heat-treatment does is determine the microstructure of the steel, and this is what primarily determines the hardness (yes, the total carbon content is also important, but that is not affected by heating/cooling). The hardness is usually a nealy linear function of the amount of a phase known as martensite, that is stabilized by the heat-treatment. It is also a function of the particle size, which too is dependent on the cooling rate.
Slow cooling, or annealing, generally results in a tougher but softer steel, while rapid quenching results in a harder but more brittle steel. There are more factors that go into determining the properties of the steel, and they would take up a good chapter to describe.