How chemical compositions affect the mechanical properties of steel pipe ?
Carbon, a basic element, has the greatest influence on the properties of steel pipe. The influence of different carbon content on the properties of steel is different according to the content of impurity elements in the steel and the different cooling conditions after rolling. With the increase of carbon content in the steel, the hardness of carbon steel in the hot rolled state increases linearly, while the plasticity and toughness decrease. In the subeutectoid range, the influence of carbon on tensile strength is that the tensile strength increases with the increase of carbon content. While beyond eutectoid range, it decreases with the increase of carbon content. In addition, when carbon content increases, the corrosion resistance of carbon steel decreases, while carbon also makes carbon steel welding performance and cold processing (stamping, drawing) performance become worse.
The content of silicon in carbon steel is no more than 0.50%. Silicon is also a useful element in steel. In rimmed steel, silicon content is very low. Silicon is added to steel as a deoxidizing element. The silicon content in killed steel is generally 0.12 ~ 0.37%. Silicon increases the fluidity of molten steel. In addition to forming non-metallic inclusions, silicon is dissolved in ferrite. With the increase of silicon content, the tensile strength, yield point, elongation, surface shrinkage and impact toughness of the steel increased significantly.
In carbon steel, manganese is a beneficial element. Manganese is added to steel as an element for deoxidation and sulfur removal. For killed steel, manganese can improve the deoxidation effect of silicon and aluminum, and form manganese sulfide with sulfur, which can reduce the harm of sulfur in steel to a considerable extent. Manganese has a good effect on the mechanical properties of carbon steel. It can improve the hardness and strength of steel after hot rolling. The reason is that manganese melts into ferrite and causes solution strengthening. Therefore, the manganese content of each furnace should be controlled strictly and stably according to the technical requirements in the refining process.
Generally speaking, phosphorus is a harmful element in steel. It comes from raw materials such as ore and pig iron. Phosphorus can improve the strength of steel, but reduce the plasticity and toughness, especially make the brittle transition temperature of steel rise sharply, that is, increase the cold brittleness (low temperature brittleness) of steel. Due to the harmful effects of phosphorus, and the large segregation of phosphorus, the content of phosphorus should be strictly controlled.
However, in the steel with low carbon content, the damage of phosphorus is relatively small. In this case, phosphorus can be used to improve the strength of steel. In addition, other beneficial effects of phosphorus are utilized where appropriate, such as increasing the atmospheric corrosion resistance of steel, such as container steel; Improve magnetism, such as electrical silicon steel; Improve the machinability of steel and reduce the bond of hot rolled sheet.
Generally speaking, sulfur is also a harmful element, which mainly comes from the raw material and combustion product, sulfur dioxide. The greatest harm of sulfur is that it causes steel to crack during hot working, which is called hot embrittlement. Sulfur can improve the machinability of steel, which is the beneficial effect of sulfur.
Nitrogen in steel comes from the furnace charge, and the molten steel absorbs nitrogen from the furnace gas and atmosphere during smelting and casting. Nitrogen causes quenching aging and deformation aging of carbon steel, which has a significant effect on the properties of carbon steel. Due to the aging effect of nitrogen, the hardness and strength of steel are increased, but the plasticity and toughness decrease, especially under the condition of deformation aging. Therefore, for ordinary low alloy steel, the aging phenomenon is harmful, thus nitrogen is a harmful element. But for some fine grain steels and vanadium and niobium steels, nitrogen becomes a beneficial element due to the effect of nitride on grain refinement. In addition, as an alloying element, nitrogen is used in stainless acid-resistant steel. In addition, the nitriding treatment method can make machine parts obtain excellent comprehensive mechanical properties, thus prolonging the service life of the parts.
Hydrogen in steel is absorbed from either corrosive water-bearing furnace charge or a furnace gas containing steam. Hydrogen is very harmful to steel. One is to cause hydrogen embrittlement, that is, under the action of the ultimate stress of steel, after a certain period of time, in the absence of any warning under the circumstances of sudden fracture, often cause disastrous consequences. The second is to cause a large number of fine crack defects inside the steel – white dot, smooth silver dot in the steel vertical end face. After the pickling, the end face show more hair filaments crack. White dot makes the elongation of steel significantly decreased, especially the end shrinkage rate and impact toughness reduce more, sometimes may be close to zero. Therefore, steel with white dot is not available. This defect occurs mainly in alloy steel.
Oxygen（O）and other nonmetal inclusions
The solubility of oxygen in steel is very low, and almost exists in steel in the form of oxide inclusion, such as FeO, AL2O3, MnO, CaO, MgO, etc. In addition, there are FeS, MnS, silicates, nitrides and phosphates in steel. These inclusions break the continuity of the steel matrix and often become the starting point of cracks under both static and dynamic loads. The various states of these non-metallic inclusions affect the properties of steel to varying degrees, especially the plasticity, toughness, fatigue strength and corrosion resistance of steel. Therefore, nonmetallic inclusions should be strictly controlled.