Alloy structural steel introduction 


 This kind of steel, as a suitable hardenability, the appropriate metal heat treatment, microstructure is uniform sorbite, bainite or fine pearlite, which has a high tensile strength and yield strength ratio (usually about 0.85), higher toughness and fatigue strength and lower toughness - brittle transition temperature, can be used to make cross-section of a larger size machine parts. 
 Alloying elements in the role of structural steel 
 There are three aspects: ① increasing steel hardenability. Hardenability steel is quenched, quenched to martensite from the surface layer from the depth, is to achieve 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 increase the hardenability of steel, in which Mn, Mo, Cr, B the strongest effect, followed by Ni , Si, Cu. The strong carbide forming elements such as V, Ti, Nb, etc., can only increase when dissolved in austenite hardenability of steel. ② affect the process of tempering steel. As the alloying elements can hinder the tempering of steel in a variety of atomic diffusion, which at the same temperature and the carbon steel compared to generally play the delayed decomposition of martensite and carbides grew up in the role of the aggregation, thereby enhancing the steel tempering stability, is to enhance resistance to temper softening steel, V, W, Ti, Cr, Mo, Si more significant role, Al, Mn, Ni is not obvious. Contain high content of carbide forming elements such as V, W, Mo and other steel, tempering at 500 ~ 600 ℃, the precipitation of fine particles dispersed special carbides such as V4C3, Mo2C, W2C, etc., instead of part of coarse alloy cementite, so that the strength of steel is no longer decreased but increased, that is a secondary sclerosis (see tempering). Mo on the temper brittleness of steel have to stop or weaken the effect. ③ affect the strengthening and toughening steel. Ni solid solution strengthening by way of strengthening the ferrite; Mo, V, Nb and other carbide forming elements, only way to diffuse hardening of the solid solution strengthening Youyi way to improve steel yield strength; carbon reinforcement the most significant. In addition, adding these alloying elements, are generally austenite grain refinement, increased grain boundary strengthening effect. Factors that affect the toughness of the steel complex, Ni improve the toughness of the steel; Mn austenite grain coarsening easy to make, sensitive to the temper brittleness; lower P, S content and increase the purity of steel, to improve the toughness of steel is important role (see metal enhanced). 
 Classification 
 Alloy structural steel is generally divided into the surface of quenched and tempered structural steel and hardened steel. ① quenched and tempered structural steel the carbon content of the general type of steel is about 0.25% to 0.55% for the given section size of the structural parts in the quenching (quenching and tempering), if the cross-section along the hardenability, the mechanical properties good, if not thoroughly quenched, the microstructure occurs in the free ferrite, the toughness decreased. Has a tendency to temper brittleness of steel such as manganese, chromium steel, nickel chromium steel, rapid cooling after tempering. Such critical diameter of steel quenching, with the grain size and content of alloying elements increases, for example, 40Cr and 35SiMn steel is about 30 ~ 40mm, and 40CrNiMo and 30CrNi2MoV steel is about 60 ~ 100mm, commonly used in manufacturing under larger load shaft, connecting rods and other structural parts. ② surface hardened structural steel for the manufacture of hard and wear-resistant surface and heart Ministry of flexible components, such as gears and shafts. Department of toughness to make parts heart high carbon content in steel should be low, generally from 0.12 to 0.25%, along with the right amount of alloying elements to ensure appropriate hardenability. Nitrided steel need to join is easy to form nitrides of alloying elements (such as Al, Cr, Mo, etc.). Carburizing or carbonitriding steel by 850 ~ 950 ℃ after carburizing or carbonitriding, quenching and tempering at low temperatures (about 200 ℃) state to use. Nitrided steel treated by nitrogen (480 ~ 580 ℃), the direct use, is no longer quenched and tempered. Edit this paragraph the production process 
 Steel and steel according to quality requirements, alloy structural steel smelting, can be used BOF, open hearth furnace, electric arc furnace; or in combination with ESR, vacuum degassing. Ingot casting or die casting can be used. Slow cooling or hot ingot should send forging, rolling. Ingot heating, even temperature and should strive to have enough holding time to improve the segregation defects and to avoid forging, rolling deformation when the uneven; forging, rolling steel, small size, especially about 0.2% carbon carburizing steel above 600 ℃ should be rapidly cooled, so as not to increase the band structure; forging greater cross-section, should take measures to eliminate the stress and the white point. Quenched and tempered steel hardened into martensite as possible, and then tempered into sorbite organizations; carburizing steel carburizing process, the diffusion layer the concentration gradient should not be too large to avoid grain boundary diffusion layer in a row on the mesh carbide; nitride steel must be heat treated to obtain the required performance, and then the final finishing order for nitrogen. Nitriding treatment in addition to Cui Bo "white layer" grinding to remove, the longer processing. 
 Other information 
 Alloy structural steel can be divided into general and special purpose alloy steel. The former includes high-strength low-alloy steel, cold steel, high strength steel, hardened steel, quenched and tempered steel and non-quenched and tempered steel; which include spring steel, ball bearing steel, free cutting steel, cold steel stamping. Requires a high yield strength, tensile strength and fatigue strength, as well as adequate ductility and toughness. Generally use the electric arc furnace and BOF smelting, demanding the use of refining, electroslag remelting or vacuum processing, vacuum induction furnace smelting or double vacuum smelting, the appropriate heat treatment. This type of steel alloy element content is quite high, there are corrosion resistant steel, heat-resistant steel, wear-resistant steel, magnet, and has other special physical and chemical properties of special steel. Alloy structural steel used in ships, vehicles, aircraft, missiles, weapons, railways, bridges, pressure vessels, machine tools and other structures. Alloy structural steel than carbon steel has better mechanical properties, especially heat treatment and excellent performance. The grade is usually based on "Digital + element symbol + number" approach to represent. Grades in the beginning of a two-digit average carbon content of steel the million points, after the number of elements and symbols that contain the content of alloying elements and their average percentage. If the alloying element content of less than 1.5%, its content is not marked. High quality grades of steel in the rear to increase the symbol "A" for example, 16Mn, 20Cr, 40Mn2, 30CrMnSi, 38CrMoAlA and so on. Alloy structural steel thin uniform classification and a unified digital code number Code alloy structural steel (including spring steel round gold) fine classification 
 AO × × × × Mn (X), MnMo (X) series steel 
 Al × × × × SiMn (X), SiMnMo (X) series steel 
 A2 × × × × Cr (X), CrSi (X), CrMn (X), CrV (x), CrMnSi (X) series steel 
 A3 × × × × CrMo (X), CrMoV (X) series steel 
 A4 × × × × CrNi (X) series steel 
 A5 × × × × CrNiMo (X), CrNiW (X) series steel 
 A6 × × × × Ni (X), NiMo (X), NiCoMo (X), Mo (X), MoWV (X) series steel 
 A7 × × × × B (X), MnB (X), SiMnB (X) series steel 
 A8 × × × × (temporarily empty) 
 A9 × × × × other alloy steel