By increasing the carbon content, steel can be made hot at temperatures above 1200oF. Normally, high carbon steel is further heat treated, such as quenched and tempered, to increase the yield and tensile strengths. Because of the high carbon content, when you quench steel in water, as an example, the steel becomes very hard, (i.e. the Rockwell can increase substantially). In fact, if it becomes too hard, the material is quenched in oil or salt instead of water, which allows it to be very tough and still allows it to be reduced because the yield strength is less than the tensile strength. Expensive, high carbon steels are found in agricultural products such as plows, cultivator disks, lawnmower blades, cutting blades, etc. Of course, high carbon steels can be annealed to bring down to a very low yield strength or heat treated to a tough product of 150,000 PSI yield strength.