Steel alloys are primarily iron and between 0.002% and 2% carbon. The later is called high carbon steel and is used for heat treated parts. Sometimes other elements are added for strength or other properties, such as silicon and manganese for spring steel or chromium for harder steels.
Replacing carbon with silicon, seeing as the two elements are in the same periodic group, has been tried. Unfortunately, the practical properties of steel (strength, durability, melting point, machinability, and heat-treat-ability) require the carbon atom.
Aluminum alloys are practical too, and have replaced steel in many applications. Aluminum is also more plentiful in the earth's crust than either iron or carbon and is easier to machine, however it is not as strong or durable as steel.
In steel making, carbon is either added or removed to reach some standard percentage of the element in proportion with the other elements. This is done to conform to SAE product standards. Once cannot make steel without carbon.
The steel industry exhales CO2, CO, H2, CH4, and other gasses as a result of the reactions required to meet various specifications. There are a few interesting and potentially practical ways to improve the sustainability of the steel industry.
- Recycle steel by SAE number so that the process is mostly removal of surface films and particulate matter, followed by melting and reforming into standard stock.
- Reduce consumption by improving engineering and manufacturing processes.
- Find ways to minimize the proportion of carbon in the input ore and recycled materials so that the steel making process consumes (rather than produces) carbon based gases.
- Use catalytic conversion to convert CO to CO2 and then capture all the CO2 and feed it to plants, phytoplankton, or artificial photosynthesis beds.
This last item is quite a ways down the research and development road, but it is perhaps the most sustainable of the four.