Iron eventually replaced Bronze due to availability of materials. Due to trade disruptions, tin was unavailable for a period of time, during which use of iron became more widespread. By the time tin became available again, iron was seen as cheaper, stronger and lighter and use of bronze faded. Unlike bronze which can be melted in small kilns, iron has to be worked at high temperatures, and needs a specialized furnace to allow it to reach its melting point.
Mixing iron with carbon created steel- The higher the carbon content, the harder the steel, but harder steel is also more brittle and so amounts of carbon varied in use depending on what the tool/ weapon was needed for.