Structural steel refers to steel to meet specific strength and formability level. Can be formed after the tensile elongation at break, said the test. Structural steel usually used for carrying purposes, the strength of steel in these applications is an important design criteria. Classification structural steel can be broken down as follows: alloy steel, carbon steel, low alloy steel, heat-resistant steel and so on. Low levels of high-quality carbon structural steel and ordinary carbon steel compared to sulfur ﹑ phosphorus and other non-metallic inclusions. Depending on the carbon content and applications, such steel roughly divided into three categories: 1, less than 0.25% C steel is low, especially in the carbon content of less than 0.10% of the 08F, 08Al, etc., because with good drawability and weldability and is widely used as deep drawn parts such as automobile, etc ...... canning, 20G is the main material for the manufacture of ordinary boilers. In addition, low-carbon steel is also widely used as hardened steel, used in machinery manufacturing. 2,0.25 ~ 0.60% C for carbon steel, much used in the quenched state, production machinery parts manufacturing industry. Quenched number 22 ~ 34HRC, mechanical properties can be obtained, but also easy to cut. 3, more than 0.6% C for high carbon steel, used for manufacturing gear ﹑ ﹑ spring rolls, etc., depending on the amount of manganese, manganese content can be divided into ordinary (0.25 to 0.8%) and high manganese content (0.7 to 1.0% and 0.9 to 1.2%), two steel group. Manganese can improve the hardenability of steel, strengthened ferritic improve ﹑ tensile yield strength steel and wear resistance. Usually after a high grade manganese steel additional markings "Mn", such as 15Mn 20Mn ﹑ to distinguish it from the normal amount of carbon manganese steel. Carbon tool steel carbon content of between 0.65 to 1.35%, after heat treatment to obtain high hardness and wear resistance, mainly used in the manufacture of various tools ﹑ ﹑ die cutting tools and measuring tools (see tool steel). Two cases of such steel alloy structural steel, due to having a suitable hardenability, after appropriate heat treatment of metals, Soxhlet uniform microstructure, bainite or fine pearlite, which has a high resistance to tensile strength and yield ratio (typically 0.85), high toughness and fatigue strength, and low ductile - brittle transition temperature, can be used to manufacture a larger cross-sectional dimensions of machine parts. Alloy structural steel elements in the role has three aspects: ① increasing steel hardenability. Hardenability refers steel quenching, starting from the surface to a depth of quenched martensite layer is to obtain good overall performance of the main parameters. In addition to Co, almost all alloying elements such as Mn, Mo, Cr, Ni, Si and C, N, B, etc. can improve the steel hardenability, including the role of Mn, Mo, Cr, B of the strongest, followed by Ni , Si, Cu. The strong carbide forming elements such as V, Ti, Nb, etc., only when dissolved in austenite can increase steel hardenability. ② steel tempering process. Due to the alloying elements in steel tempering can hinder the diffusion of various atoms, which at the same temperature and carbon steel compared to generally play a gathering delayed martensite and carbides grow decomposition, thereby enhancing the stability of steel tempering, i.e. improve temper softening resistance of steel, V, W, Ti, Cr, Mo, Si more significant role, Al, Mn, Ni is not obvious. Contains a high content of carbide forming elements such as V, W, Mo or the like steel, at 500 ~ 600 ℃ tempering, the precipitation of special carbides, such as fine dispersed particles V4C3, Mo2C, W2C, etc., instead of part of coarse alloy cementite, the strength of the steel is no longer declining but increased that occurs secondary hardening (see tempering). Mo steel temper brittleness have prevented or diminished role. ③ affect steel reinforcement and toughening. Ni solid solution strengthening way to strengthen ferrite; Mo, V, Nb and other carbide forming elements, both in the way again dispersion hardening solution strengthening ways to improve the yield strength of steel; strengthening the role of carbon in the most significant. Furthermore, addition of these alloying elements, generally austenite grain refinement, grain boundary strengthening effect increases. The toughness of the steel complex factors, Ni improve toughness; Mn easy to make the austenite grain coarsening, sensitive to temper brittleness; lower P, S content, improve the purity of steel, to improve the toughness of the steel is important action (see the metal reinforcement). Alloy structural steel is generally divided quenched steel and case hardened steel. ① carbon content of quenched and tempered steel type of steel is generally about 0.25% to 0.55% for a given cross-sectional dimensions of the structure, at the time of quenching (quenching and tempering), if the cross section along the hardenability, the mechanical properties well, if not through quenching, microstructure appears freedom ferrite, the toughness decreases. Has a tendency to temper brittleness of steel, such as manganese, chromium steel, nickel-chromium steel, tempering should be fast-cooling. Hardening steels such critical diameter, with the increase of grain size and content of alloying elements increases, for example, 40Cr and 35SiMn steel is about 30 ~ 40mm, while 40CrNiMo and 30CrNi2MoV steel is about 60 ~ 100mm, commonly used in the manufacture of bear larger load shaft, connecting rods and other structural parts. ② surface hardening of structural steel for the manufacture of hard wear-resistant surface and core part of flexible components, such as gears, shafts, etc. To make parts center high toughness steel carbon content should be low, generally in the 0.12 to 0.25%, as well as the right amount of alloying elements, in order to ensure proper hardenability. Nitride need to join easily form nitrides of alloying elements (e.g. Al, Cr, Mo, etc.). Carburizing or carbonitriding steel, 850 ~ 950 ℃ after carburizing or carbonitriding, quenching and tempering at low temperatures (about 200 ℃) condition. Nitriding steel after nitrogen treatment (480 ~ 580 ℃), direct use, no longer deal with quenching and tempering. Carbon structural steel and steel production process according to the requirements of quality steel, smelting alloy structural steel, to be used BOF, open-hearth furnace, electric arc furnace; or plus remelting, vacuum degassing. Ingot casting or molding can be used. Ingot should slow cooling or hot delivery forging, rolling. When the ingot heating temperature uniformity and should seek adequate holding time, in order to improve segregation defects and avoid forging inhomogeneous deformation during rolling; forging, steel rolling after the small size, especially carburizing about 0.2% carbon content steel, at above 600 ℃ rapid cooling should not increase the banded structure; section larger forgings, measures should be taken to eliminate the stress and white spots. Quenched and tempered steel should be as hardened into martensite, and tempered into patenting organizations; carburizing steel carburizing process, the diffusion layer concentration gradient is not too large, in order to avoid a continuous network in the grain boundary diffusion layer carbides; nitriding steel must first heat treated to give the desired performance, then by a final finishing before nitriding. In addition to the grind after nitriding Cuibo "white layer" is removed, the longer processing. A classification of carbon steel. Carbon content of about 0.05% to 0.70% up to 0.90% individual. Can be divided into ordinary carbon steel and high-quality carbon steel structure types. The former contains impurities, low price, performance requirements for where it's most of the carbon content of 0.30%, the manganese content of not more than 0.80%, lower intensity, but the ductility, toughness, cold deformation properties Okay. With few exceptions, generally do not make heat, directly. Made more bars, shaped steel, steel and so on. Many uses, the amount of large, mainly for railways, bridges, all kinds of construction, manufacturing under static load of various metal components and mechanical parts, and generally does not require heat treatment weldments unimportant. Pure steel quality carbon structural steel, low impurity, good mechanical properties, can use after heat treatment. According manganese content is divided into ordinary manganese content (less than 0.80%) and high manganese content (0.80% to 1.20%) groups. Carbon content of 0.25% or less, and more without heat treatment used directly, or through carburizing, carbonitriding and other processing, manufacture of small gears, shafts, piston pins; carbon content of 0.25% to 0.60%, the typical steel No. There 40,45,40Mn, 45Mn, and many more after quenching, the manufacture of mechanical parts and fasteners; carbon content of more than 0.60%, such 65,70,85,65Mn, 70Mn, and many more as the spring steel. The main mechanical properties to ensure that the mechanical properties of these steels, so the grades reflect its mechanical properties, with Q + digital, where "Q" is the yield point "bend" Pinyin word prefix numbers indicate the yield point value, for example, said the yield Q275 point 275Mpa. If the label behind the letter grades A, B, C, D, it means different quality grades of steel, including S, P amount in descending order, in order to improve the quality of steel. If the label behind the grade letter "F" was boiling steel, marked "b" for the semi-killed steel, marked "F" or "b" who was killed. For example Q235-A · F that yield point 235Mpa Class A boiling steel, Q235-C that yield point 235Mpa grade C-killed steel. The mass fraction of carbon steel under normal circumstances is not heat-treated, and direct use in the supply state. Typically Q195, Q215, Q235 low carbon content steel, welding performance, good ductility, toughness, a certain intensity, often rolled into sheets, welded steel pipes, etc., for bridges, buildings and other structures and manufacturing ordinary screws, nuts and other parts. Q255 and Q275 carbon steel mass fraction higher, higher strength, ductility, toughness, can be welded, usually rolled into steel, strip steel and steel for structural parts and manufacturing simple mechanical linkage, gears, couplings Day, pins and other parts. Representation of carbon structural steel grades by letter on behalf of the yield point, yield point value, quality and grade four parts symbols, symbols, etc. deoxy method in order components. For example: Q235-A · F. Grades and chemical composition chemical composition of steel (melting analysis) should be consistent with Table 1. Table 1 Grade Grade Chemical composition C Mn Si SP deoxidation method is not greater than Q195 - 0.06 ~ 0.12 0.25 ~ 0.50 0.30 0.050 0.045 F, b, Z Q215 A 0.09 ~ 0.15 0.25 ~ 0.55 0.30 0.050 0.045 F, b, ZB 0.045 F, b , Z Q235 A 0.14 ~ 0.22 0.30 ~ 0.65 0.30 0.050 0.045 F, b, ZB 0.12 ~ 0.20 0.30 ~ 0.70 0.045 C ≤0.18 0.35 ~ 0.18 0.040 0.040 ZD ≤0.17 0.035 0.035 TZ Q255 A 0.18 ~ 0.28 0.40 ~ 0.70 0.30 0.050 0.045 F, b, ZB 0.045 Q275 - 0.28 ~ 0.38 0.50 ~ 0.80 0.35 0.050 0.045 b, Z NOTE: Q235A, class B limit boiling steel manganese content of 0.60%. ⒌1.1.1 boiling steel silicon content of not more than 0.07%; semi-killed steel silicon content of not more than 0.17%; killed steel lower limit is 0.12% silicon content. ⒌1.1.2 D grade steel should contain enough elements to form a fine grain structure, such as steel, acid-soluble aluminum content of not less than 0.015% or aluminum content of not less than 0.020%. ⒌1.1.3 residual element chromium steel, nickel, copper content should be not more than 0.30% each, the nitrogen content of oxygen converter steel should not exceed 0.008%. If the supplier can guarantee, can not do the analysis. By the need to agree, the copper content of A-grade steel, of not more than 0.35%. In this case, the supplier shall make analysis of copper content, and indicate its content in the quality certificate. ⒌1.1.4 residual content of arsenic in steel should not exceed 0.080%. Steel, with arsenic arsenic content by smelting iron ore smelting provision agreement by both parties. No arsenic as the raw materials for arsenic content in the steel can not do analysis. ⒌1.1.5 mechanical properties of steel in ensuring compliance with the provisions of this standard case, various grades of A grade carbon steel, silicon, manganese and other grades of steel of various grades of carbon and manganese content may not be used as the lower limit of delivery, but its content (melting analysis) should be indicated in the quality certificate. ⒌1.1.6 ingot in the supply of goods (including slab), billets, the supplier shall ensure that the chemical composition (melting analysis) comply with the provisions in Table 1, but in order to ensure the performance of rolled steel conforming to this standard requires that all grades of A, the chemical composition of the steel grade B can be adjusted according to the demand side, and the other set protocol. ⒌1.2 finished steel, steel billets commodity chemical composition tolerances shall comply with the provisions of GB222 Table 1. Boiling steel and finished steel billets commodity chemical composition deviation is not guaranteed. ⒌2 steel smelting by oxygen converter, open hearth or electric furnace smelting, unless the demand side, there are special requirements, and indicate in the contract, smelting method is generally determined by the supplier themselves. Delivery status ⒌3 generally hot-rolled steel (including controlled rolling) delivery. According to the requirements, by mutual agreement, the state can also be normalized delivery (except A grade steel).