This refers to the engineering and alloy steel used for building structure, including the weldability of high-strength alloy steel structural steel, alloy steel, railway with alloy steel, geological drilling with alloy steel, pressure vessel with alloy steel, high manganese steel, etc. This kind of steel is used as the engineering and construction structure, in alloy steel, such as total steel alloy content is low, but the production, the heavy usage & have spent . < BR > 9. The mechanical structure with steel & have spent &nbsp; < BR > & have spent &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; This kind of steel is suitable for manufacturing machinery and mechanical parts of alloy steel. It is on the basis of quality carbon steel, appropriately add one or several kinds of alloy elements, to improve the strength, toughness and hardenability of steel. This kind of steel after heat treatment are usually used. Mainly includes the commonly used two kinds of alloy structural steel and alloy spring steel, including conditioning treatment, surface hardening treatment of the alloy steel, cold plastic molding with alloy steel. According to the chemical composition of basic system can be divided into Mn steel, SiMn steel, Cr steel, CrMo steel, CrNiMo steel, Ni, B department of steel & steel have spent . < BR >