According to my personal understanding, this is iron-based-based, adding various elements to meet different needs. Their metal elements can be the same (or different), but in different proportions. In addition, they also add nonmetals (carbon, silicon, etc. ), thus forming a wide variety. By analogy, each aluminum-based copper-based product (various elements) is manufactured according to a certain proportion, and the ultimate goal is to meet various requirements (strength, toughness, rigidity,. . . . )。