Metallurgical industry is used to calling iron, chromium, manganese and their alloys (mainly alloy steel and steel) ferrous metals. In fact, pure iron and chromium are silvery white, and manganese is silvery gray. They are called ferrous metals because the surface of steel is often covered with a layer of black ferroferric oxide, while manganese and chromium are mainly used for smelting alloy steel, so people call iron, chromium, manganese and their alloys ferrous metals. In addition, people specially classify these three metals and their alloys as one category, while all other metals and alloys are classified as non-ferrous metals, because steel occupies an extremely important position in the national economy and is one of the important symbols to measure a country's national strength; Its output accounts for about 95% of the world's total metal output. Of all metals, chromium is the hardest and the most difficult to corrode. People often mix chromium into steel to make hard and corrosion-resistant chromium steel. Chrome steel is a good material for construction machinery, gun barrels, tanks and armored vehicles. Adding more than 12% chromium and a certain amount of nickel in steelmaking can smelt stainless steel. Chromium is also an essential metal for electroplating (commonly known as cromet). Hard and tough manganese steel can be produced by adding about 13% manganese in steelmaking. People use manganese steel to make buckets of steel mills, ball bearings, bulldozers and excavators and other parts that are easy to wear. High manganese steel is also used to make helmets, tank armor and armor-piercing projectiles.