What Is Structural Steel?
Structural steel refers to steel that meets specific strength and formability grades. Formability is expressed as the elongation after the tensile test is interrupted. Structural steel is generally used for bearing and other purposes, where the strength of steel is an important design criterion.
Structural steel
- Structural steel refers to steel that meets specific strength and formability grades. Formability to break after tensile test
- Compared with ordinary carbon structural steel, high-quality carbon structural steel has a lower content of sulfur, phosphorus and other non-metallic inclusions. According to different carbon content and uses, this type of steel is roughly divided into three categories:
Structural steel alloy structural steel
- This type of steel has high tensile strength and yield strength due to its proper hardenability. After suitable metal heat treatment, the microstructure is homogeneous sorbite, bainite, or very fine pearlite. Ratio (generally around 0.85), higher toughness and fatigue strength, and lower toughness-brittleness transition temperature, which can be used to manufacture machine parts with larger cross-section sizes.
- The role of alloying elements in structural steel
- There are three aspects: Increase the hardenability of steel. Hardenability refers to the depth of the martensite layer from the surface layer when the steel is quenched, and is the main parameter to obtain good comprehensive properties. Except for Co, almost all alloy elements such as Mn, Mo, Cr, Ni, Si and C, N, B, etc. can improve the hardenability of steel. Among them, 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 in austenite. Affects the tempering process of steel. Because alloying elements can hinder the diffusion of various atoms in the steel during tempering, compared with carbon steel at the same temperature, it generally plays a role in delaying the decomposition of martensite and the growth of carbides, thereby improving The tempering stability of steel, that is, improving the tempering softening resistance of steel, the effects of V, W, Ti, Cr, Mo, Si are more significant, and the effects of Al, Mn, and Ni are not obvious. Steels containing higher levels of carbide-forming elements such as V, W, Mo, etc., when tempered at 500 to 600 ° C, precipitate small and special carbonized material points such as V4C3, Mo2C, W2C, etc., instead of some larger alloys Cementite makes the strength of the steel no longer decrease but increases, that is, secondary hardening occurs (see tempering). Mo has the effect of preventing or weakening the tempering brittleness of steel. Affects the strengthening and toughening of steel. Ni strengthens ferrite by solid solution strengthening; Carbide-forming elements such as Mo, V, and Nb increase the yield strength of steel both by diffusion hardening and solid solution strengthening; carbon has the most significant strengthening effect. In addition, the addition of these alloying elements generally refines the austenite grains and increases the strengthening effect of the grain boundaries. The factors that affect the toughness of steel are more complicated. Ni improves the toughness of steel. Mn is easy to coarsen austenite grains and is sensitive to tempering brittleness. Reducing P and S content and improving the purity of steel are important to improve the toughness of steel. Role (see metal strengthening).
- Alloy structural steel is generally divided into quenched and tempered structural steel and case hardened structural steel.
- The carbon content of quenched and tempered structural steels is generally about 0.25% to 0.55%. For structural parts of a given cross-sectional size, when quenched and tempered during quenching and tempering, mechanical properties Good. If the hardening is impervious and free ferrite appears in the microstructure, the toughness decreases. For steels with a tendency to temper brittleness, such as manganese steel, chromium steel, nickel-chromium steel, etc., they should be cooled quickly after tempering. The quenching critical diameter of this type of steel increases with the increase in grain size and alloying element content. For example, 40Cr and 35SiMn steels are about 30 to 40mm, while 40CrNiMo and 30CrNi2MoV steels are about 60 to 100mm. Structural components such as shafts and connecting rods with large loads.
- Surface hardened structural steel is used to manufacture hard and wear-resistant surface parts and flexible core parts, such as gears and shafts. In order to make the core toughness of the part high, the carbon content in the steel should be low, generally 0.12 to 0.25%, and there should be a suitable amount of alloying elements to ensure proper hardenability. Nitrided steel also needs to add alloying elements (such as Al, Cr, Mo, etc.) that are easy to form nitrides. After carburizing or carbonitriding steel, after carburizing or carbonitriding at 850 950 , it is quenched and used under low temperature tempering (about 200 ). Nitrided steel is directly used after nitriding treatment (480 580 ), without quenching and tempering treatment.
Carbon steel
- Production Process
- According to the type of steel and the quality requirements of the steel, the smelting of alloy structural steel can use oxygen top-blown converters, open hearth furnaces, electric arc furnaces; or remelting with electroslag and vacuum degassing. Ingots can be cast or die cast. The steel ingot should be slowly cooled or hot-forged and rolled. When the steel ingot is heated, the temperature should be uniform and sufficient holding time to improve segregation defects and avoid uneven deformation during forging and rolling. Forged and rolled steel is small in size, especially carburizing with 0.2% carbon. For steel, it should be cooled quickly above 600 to avoid aggravating the band structure. Forgings with large cross-sections, measures should be taken to eliminate internal stress and white spots. Quenched and tempered steel should be quenched into martensite structure as much as possible, and then tempered into morselite structure. During carburizing steel, the concentration gradient of carburizing layer should not be too large, so as to avoid continuous network on the grain boundary of carburizing steel. Carbide; nitrided steel must first be heat treated to obtain the required properties, and then finally finished for nitriding. After the nitriding treatment, the brittle "white layer" is ground and removed, and no further processing is performed. A type of classified carbon steel. The carbon content is about 0.05% to 0.70%, and some can be as high as 0.90%. Can be divided into two types of ordinary carbon structural steel and high-quality carbon structural steel. The former contains more impurities and is low in price. It is used in places where performance is not high. Most of its carbon content is less than 0.30%, manganese content is less than 0.80%, and the strength is low. it is good. Except for a few cases, it is generally used without heat treatment. Mostly made of bar steel, shaped steel, steel plate, etc. There are many uses and large quantities, mainly used in railways, bridges, various construction projects, manufacturing various metal components that bear static loads, and mechanical parts and general welded parts that do not need heat treatment that are not important. High-quality carbon structural steel The steel is pure, has few impurities and good mechanical properties, and can be used after heat treatment. According to the manganese content, it is divided into two groups: ordinary manganese content (less than 0.80%) and higher manganese content (0.80% to 1.20%). The carbon content is less than 0.25%, and it is used directly without heat treatment, or through carburizing, carbonitriding, etc. to manufacture small and medium gears, shafts, piston pins, etc .; the carbon content is 0.25% to 0.60%, typical steel Nos. 40, 45, 40Mn, 45Mn, etc., are often quenched and tempered to produce various mechanical parts and fasteners; carbon content exceeds 0.60%, such as 65, 70, 85, 65Mn, 70Mn, etc., mostly used as springs Use of steel.
- Mechanical properties
- This type of steel mainly guarantees mechanical properties, so its grade reflects its mechanical properties, which is expressed by Q + numbers, where "Q" is the Chinese pinyin prefix of the word "yield" at the yield point, and the number represents the value of the yield point. For example, Q275 represents the yield point as 275Mpa. If the letters A, B, C, and D are marked after the grade, it means that the quality grade of the steel is different, the quantity containing S and P decreases in order, and the quality of steel increases in order. If the letter "F" is marked at the end of the grade, it is boiling steel, the mark "b" is semi-killed steel, and those without "F" or "b" are calm steel. For example, Q235-A · F represents a grade A boiling steel with a yield point of 235Mpa, and Q235-C represents a grade C killed steel with a yield point of 235Mpa.
- Quality score
- Carbon structural steels are generally not heat treated and are used directly in the supplied state. Generally, Q195, Q215, and Q235 steels have a low carbon mass fraction, good welding performance, good plasticity and toughness, and a certain strength. They are often rolled into thin plates, rebars, welded steel pipes, etc., and are used in bridges, constructions, and other ordinary screws, Nuts and other parts. Q255 and Q275 steel have a slightly higher mass fraction of carbon, higher strength, better plasticity and toughness, and can be welded. Generally, rolled steel, strip and steel plates are used as structural parts, and simple mechanical links, gears, and couplings are manufactured. Knots, pins and other parts.
- display method
- The grade of carbon structural steel consists of four parts: the letter representing the yield point, the yield point value, the quality grade symbol, and the deoxidation method symbol. For example: Q235-A · F.
- chemical composition
- The grade and chemical composition (smelting analysis) of the steel shall meet the requirements of Table 1.
- Table 1
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- Note: The upper limit of manganese content for Q235A and B boiling steel is 0.60%.
- 1.1.1 The silicon content of boiling steel is not more than 0.07%; the silicon content of semi-killed steel is not more than 0.17%; the lower limit of silicon content of calm steel is 0.12%.
- 1.1.2 Grade D steel should contain sufficient elements to form a fine grain structure, for example, the content of acid-soluble aluminum in the steel is not less than 0.015% or the total aluminum content is not less than 0.020%.
- 1.1.3 The residual elements chromium, nickel and copper in the steel should not exceed 0.30% each, and the nitrogen content of the oxygen converter steel should not exceed 0.008%. If the supplier can guarantee, no analysis is required.
- With the consent of the purchaser, the copper content of Grade A steel may not be greater than 0.35%. At this time, the supplier shall analyze the copper content and indicate its content in the quality certificate.
- 1.1.4 The residual content of arsenic in steel should not be greater than 0.080%. The arsenic content of steel smelted from pig iron with arsenic-containing ore is stipulated by the supplier and the buyer through agreement. If the raw material does not contain arsenic, the analysis of the arsenic content in the steel can be omitted.
- 1.1.5 In order to ensure that the mechanical properties of steel products comply with the requirements of this standard, the lower limits of carbon, manganese content of each grade of grade A steel and the carbon and manganese contents of other grades of grade steel may not be used as delivery conditions, but Analysis) should be stated in the quality certificate.
- 1.61.1.6 When supplying commercial steel ingots (including continuous casting slabs) and slabs, the supplier shall ensure that the chemical composition (melting analysis) meets the requirements of Table 1, but in order to ensure that the properties of rolled steel meet the requirements of this standard, each grade The chemical composition of grade B steel can be adjusted appropriately according to the requirements of the demander, and another agreement can be made.
- 1.2 The allowable deviation of the chemical composition of the finished steel and commercial slab shall comply with the requirements of Table 1 in GB222.
- The deviation of chemical composition of the finished steel and commercial slab of the boiling steel is not guaranteed.
- 2 Smelting method
- Steel is smelted by oxygen converter, open hearth or electric furnace. Unless the purchaser has special requirements, and it is stated in the contract, the smelting method is generally determined by the supplier.
- 3 Delivery status
- Steel is generally delivered in hot rolled (including controlled rolling) conditions. According to the requirements of the purchaser, after the agreement between the two parties, it can also be delivered in the normalized state (except for A-grade steel).