The advantages of high strength, lightweight, good processing performance and welding performance, and good reusability make the application scope of steel structure continue to expand. In order to ensure structural quality and safety, these steels should have high strength and toughness, as well as good processing properties. Therefore, it is particularly important to understand the influence of various elements on the properties of steel. This article will share the influence of common elements in steel structures on steel properties.
The main chemical components in steel
Carbon is the main element of steel. When the carbon content in the steel is below 0.8%, as the carbon content increases, the strength, and hardness of the steel increase, while the elongation decreases, and the ductility and toughness decrease. But when the carbon content is above 1.0%, as the carbon content increases, the increase in tensile strength slows down. As the carbon content increases, the welding performance of steel becomes worse (steel with a carbon content greater than 0.3% has a significant decrease in weldability), the corrosion resistance decreases, and welding performance and cold working (stamping, drawing) performance deteriorate.
Manganese is used to deoxidize and desulfurize and exists in steel during steel-making. It is a beneficial element in steel. The content of manganese in carbon steel is generally 0.25-0.80%. Manganese has strong deoxidation and desulfurization ability, it can eliminate or reduce the hot brittleness caused by oxygen and sulfur, and can also be combined with sulfur to form MnS, thereby eliminating the harmful effects of sulfur to a considerable extent and greatly improving the hot working character of steel. It can improve the strength and hardness of steel at the same time.
Silicon is a kind of deoxidizer, its deoxidation effect is stronger than Mn, and its content in steel is ≤0.50%. Silicon can increase the fluidity of molten steel. Each increase of 0.1% Silicon in carbon steel can increase the tensile strength of hot-rolled steel by about 7.8-8.8 MN/m2 and the yield point by about 3.9-4.9 MN/m2. The elongation decreases by about 0.5% and the reduction of area shrinkage and impact toughness is not obvious, but when the Si content exceeds 0.8-1.0%, the area shrinkage decreases, especially the impact toughness is significantly reduced. Silicon exists in steel as silicic acid, which is harmful to wire drawing.
Generally speaking, sulfur is a harmful element, and the biggest hazard of sulfur is to cause steel to crack during hot processing, that is, to produce hot shortness. The cause of hot shortness is the severe segregation of sulfur. By adding Mn to avoid the formation of FeS in the steel to prevent hot shortness. Mn has a stronger affinity for S than Fe, so S and Mn in molten steel will form MnS preferentially. Sulfur has an adverse effect on the welding performance of steel, and it is easy to cause hot cracking of the weld. Therefore, when manufacturing parts that require finer surface roughness and less stringent strength requirements, free-cutting steel with high S content can be used.
Generally speaking, phosphorus is a harmful impurity element. As the phosphorus content increases, the strength, yield ratio, and hardness of steel increase, while the plasticity and toughness decrease significantly. In particular, the lower the temperature, the greater the impact on plasticity and toughness. These are the main harmful effects of phosphorus. Phosphorus can improve the cutting performance and corrosion resistance, so the content of phosphorus can be appropriately increased in the free-cutting steel. Phosphorus also reduces the weldability of steel significantly. But phosphorus can improve the wear resistance and corrosion resistance of steel, so other elements can be used as alloy elements in low alloy steel.
Oxygen is a harmful element in steel. The solubility of oxygen in steel is very small. In steel, almost all oxygen exists in the form of oxides. As the oxygen content in the steel increases, the strength of the steel increases, but the performance and toughness of the steel decrease, and oxides are included. The presence of oxygen will cause the hot brittleness of steel.
N causes quenching aging and deformation aging of carbon steel, thereby affecting the properties of carbon steel. Due to the aging effect of N, the hardness and strength of the steel increase, the performance and toughness are reduced, and the weldability and the cold brittleness become worse. For ordinary low alloy steel, the aging phenomenon is harmful, so nitrogen is a harmful element. Nitrogen can reduce the adverse effects of aluminum, silver, vanadium, and other elements, improve the performance of steel, and can be used as an alloying element of low-alloy steel.
It is added to steel as a deoxidizer. The content of aluminum in carbon steel is generally less than 0.10%. The aluminum part added to the steel will form A12O3 with oxygen, and partly dissolve in solid iron. AlN is formed with different heating or cooling conditions, which can prevent the growth of bulk crystal grains.
Cold working hardening and age hardening
As time goes by, the residual carbon and oxygen solid solution substances in the pure ferrite precipitate out gradually, forming free carbides or oxide particles to restricting the plastic deformation of the pure ferrite, which is aging. Aging will increase the strength of steel and reduce plasticity and toughness. The aging process can range from a few days to several decades. The plastic deformation of steel during cold working (normal temperature processing) increases the yield point of the steel while reducing plasticity and toughness is called cold work hardening.
The effect of heat treatment on the properties of steel
Heat treatment is a process in which the metal part is heated to a suitable temperature in a certain medium, and after it is kept at this temperature for a certain period of time, it is cooled at different speeds. The overall heat treatment of steel generally has four basic processes: annealing, normalizing, and quenching.
The purpose of annealing is to reduce the hardness of metal materials and improve plasticity to facilitate cutting, reduce residual stress, and improve the uniformity of structure and composition. There are many types of annealing processes, such as complete annealing, diffusion annealing, incomplete annealing, isothermal annealing, spheroidizing annealing, stress relief annealing, and crystallization annealing.
Normalizing is mainly to improve the mechanical properties of low-carbon steel, improve the cutting type, refine the grains, eliminate structural defects, and prepare for the subsequent heat treatment. For large castings, forgings, and steel, normalizing can refine grains. After normalizing low carbon steel, fine flake pearlite can be obtained, which can increase the hardness and improve the workability.
Quenching is a heat treatment process in which the steel is austenitized and cooled at an appropriate cooling rate so that the component is dissolved into solid solution in all or a certain range in the cross-section, and then the unstable structure such as martensite is rapidly grown. Quenching can greatly improve the strength and hardness of steel.
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