Structural steel refers to steel that meets specific strength and formability grades. This formability is expressed by the elongation after the break of the tensile test. Structural steel is often used for applications such as load bearing, where the strength of the steel is an important design criterion.

Structural steel can be subdivided into: carbon structural steel, high-quality carbon structural steel, low alloy high strength structural steel, alloy structural steel, spring steel, weather resistant structural steel, easy to chip structural steel, non-tempering mechanical structural steel.

In the classification of steel structure, the content of sulfur, phosphorus and other nonmetallic inclusions is lower in high-quality carbon structural steel than in ordinary carbon structural steel. According to the different carbon content and use, this type of steel is roughly divided into three categories:

● less than 0.25%C is low carbon steel, especially with carbon content of less than 0.10% of 08F, 08Al, etc., because of its good deep drawing and weldability and is widely used as deep drawing parts such as automobiles, cans...... 20G is the main material for the manufacture of ordinary boilers. In addition, low carbon steel is also widely used as carburizing steel for machinery manufacturing.

● 0.25 ~ 0.60%C is medium carbon steel, mostly used in the state of tempering, to make parts of the machinery manufacturing industry. Tempering how many 22~34HRC, can get comprehensive mechanical properties, but also easy to cut.  

● more than 0.6%C is high carbon steel, mostly used for the manufacture of springs, gears, rolls, etc., according to the difference in manganese content, it can be divided into ordinary manganese content (0.25 ~ 0.8%) and higher manganese content (0.7 ~ 1.0% and 0.9 ~ 1.2%) two steel groups. Manganese can improve the hardenability of steel, strengthen ferrite, and improve the yield strength, tensile strength and wear resistance of steel. It is usually marked "Mn" after the grade of steel with high manganese content, such as 15Mn, 20Mn to distinguish it from carbon steel with normal manganese content.

The carbon content of carbon tool steel is between 0.65% and 1.35%, and after heat treatment, high hardness and high wear resistance can be obtained, mainly used in the manufacture of various tools, cutting tools, molds and measuring tools (see tool steel).

Alloy structural steel This kind of steel, due to the appropriate hardenability, after the appropriate metal heat treatment, the microstructure is uniform sortenite, bainite or very fine pearlite, so it has a high tensile strength and yield ratio (generally about 0.85), higher toughness and fatigue strength, and lower toughness - brittleness transition temperature. Can be used to manufacture large section size machine parts.

The role of alloying elements in structural steel has three aspects:

● increase the hardenability of steel. Hardenability refers to the depth of hardening from the surface into the martensite layer when the steel is quenched, and is the main parameter for obtaining good comprehensive properties. In addition to Co, almost all alloying elements such as Mn, Mo, Cr, Ni, Si, C and N, B can improve the hardenability of steel, of which Mn, Mo, Cr, B has the strongest effect, followed by Ni, Si, Cu. The strong carbide forming elements such as V, Ti, Nb, etc., can increase the hardenability of steel only when dissolved into austenite.

● affect the tempering process of steel. Because alloying elements can hinder the diffusion of various atoms in steel during tempering, compared with carbon steel, they generally delay the decomposition of martensite and the accumulation and growth of carbide at the same temperature, thus improving the tempering stability of steel, that is, improving the tempering softening resistance of steel, V, W, Cr, Ti, Mo, Si have a more significant role. The effects of Al, Mn and Ni were not obvious. Steel containing higher content of carbide forming elements such as V, W, Mo, etc., at 500 ~ 600℃ tempering, precipitation of small dispersed special carbide particles such as V4C3, Mo2C, W2C, etc., instead of some of the larger alloy cementing, so that the strength of the steel no longer decreases but increases, secondary hardening (see tempering). Mo can prevent or weaken the tempering brittleness of steel.

● affect the strengthening and toughening of steel. Ni strengthened ferrite by solid solution strengthening. Mo, V, Nb and other carbide forming elements can improve the yield strength of steel by both diffusion hardening and solid solution strengthening. Carbon is the most powerful reinforcer. In addition, the addition of these alloying elements generally refines the austenite grains and increases the strengthening effect of grain boundaries. The factors affecting the toughness of steel are complex,Ni improves the toughness of steel; Mn is easy to coarsene austenite grains and is sensitive to temper brittleness. Reducing the content of P and S and improving the purity of steel play an important role in improving the toughness of steel.

Alloy structural steel is generally divided into tempered structural steel and surface hardened structural steel.

① Tempered structural steel The carbon content of this type of steel is generally about 0.25% ~ 0.55%, for the structural parts of the given cross-section size, in the tempered treatment (quenching and tempering), if the quenching along the cross-section, then the mechanical properties are good, if the quenching is not permeable, there is free ferrite in the microstructure, then the toughness is reduced. For steels with tempering brittleness tendency such as manganese steel, chromium steel, nickel-chromium steel, etc., they should be cooled quickly after tempering. The quenched critical diameter of this kind of steel increases with the increase of grain size and alloying element content, for example, 40Cr and 35SiMn steel pipes are about 30 ~ 40mm, and 40CrNiMo and 30CrNi2MoV steel pipes are about 60 ~ 100mm, which is often used to manufacture shafts, connecting rods and other structural parts that bear large loads.

② Surface hardened structural steel is used to manufacture parts with hard, wear-resistant surface and flexible heart, such as gears, shafts, etc. In order to make the core toughness of the parts high, the carbon content in the steel should be low, generally 0.12% to 0.25%, and there is an appropriate amount of alloying elements to ensure appropriate hardenability. Nitriding steel also needs to add alloying elements that are easy to form nitrides (such as Al, Cr, Mo, etc.). Carburized or carbonitriding steel, after carburizing or carbonitriding at 850 ~ 950 ° C, quenching and tempering at low temperature (about 200 ° C). Nitriding steel after nitriding treatment (480 ~ 580℃), direct use, no longer by quenching and tempering treatment.