Iron Ingot is one of several alloys you can choose from to craft items that call for an alloy. If you are using a blueprint that calls for multiple types of alloys (indicated in parentheses in the ingredient list with the number of types required and the amount needed of each type) you can choose from Cobalt Ingot, Copper Ingot, Iridium Ingot, Iron Ingot, Silver Ingot or Tin Ingot. For extremely high-quality blueprints you may have to travel extensively to acquire all the types required.
Iron Ingot can be used to craft certain items requiring alloy.
Including Cannons and other Artillery.