There is no unified definition and concept for special steel. It is generally believed that special steel refers to a type of steel that has special chemical composition, is produced using special processes, has special structure and properties, and can meet special needs. Compared with ordinary steel, special steel has higher strength and toughness, physical properties, chemical properties, biocompatibility, and process performance. China, Japan, and the European Union have similar definitions of special steel, dividing it into three categories: high-quality carbon steel, alloy steel, and high alloy steel (with alloy elements greater than 10%). Among them, alloy steel and high alloy steel account for 70% of the production of special steel. The main types of steel include special carbon structural steel, carbon tool steel, carbon spring steel, alloy spring steel, alloy structural steel, ball bearing steel, alloy tool steel, high alloy tool steel, high-speed tool steel, stainless steel, heat-resistant steel, as well as high-temperature alloys, precision alloys, and electrothermal alloys. Currently, there are nearly 2000 special steel grades and approximately 50000 varieties and specifications in the world. In addition to a wide variety of types, special steel also exhibits characteristics different from ordinary steel in terms of specifications. In addition to plates, pipes, wires, strips, rods, and profiles, there are also composite materials, surface alloying materials, surface treatment materials, precision forging materials, precision castings, powder metallurgy products, etc.