What is carbon steel
What is carbon steel?
Carbon steel is an iron carbon alloy with carbon content of 0.0218% ~ 2.11%. Generally, it also contains a small amount of silicon, manganese, sulfur and phosphorus. Generally, the higher the carbon content in carbon steel, the greater the hardness, the higher the strength, but the lower the plasticity.
Classification of carbon steel
- (1) According to the application, carbon steel can be divided into three types: carbon structural steel, carbon tool steel and free cutting structural steel;
- (2) According to smelting method, it can be divided into open hearth steel and converter steel;
- (3) According to deoxidation method, it can be divided into rimmed steel (f), killed steel (z), semi killed steel (b) and special killed steel (TZ);
- (4) According to the carbon content, carbon steel can be divided into low carbon steel (WC ≤ 0.25%), medium carbon steel (WC 0.25% – 0.6%) and high carbon steel (WC > 0.6%);
- (5) According to the quality of steel, carbon steel can be divided into ordinary carbon steel (with higher phosphorus and sulfur content), high quality carbon steel (with lower phosphorus and sulfur content), high quality steel (with lower phosphorus and sulfur content) and super quality steel.
Types of carbon steel
By mass percent of carbon, carbon steel can be divided into low carbon steel, medium carbon steel and high carbon steel. With the increase of carbon content, the hardness of carbon steel increases and the toughness decreases.
1) Low carbon steel – carbon content ≤ 0.25%.
What is low carbon steel?
Mild steel is a kind of carbon steel with carbon content less than 0.25%. Because of its low strength, low hardness and soft, it is also called mild steel. It includes most common carbon structural steel and some high-quality carbon structural steel. Most of them are used for engineering structural parts without heat treatment. Some of them are used for mechanical parts requiring wear resistance after carburizing and other heat treatment.
The annealed microstructure of low carbon steel is ferrite and a small amount of pearlite, with low strength and hardness, good plasticity and toughness. Therefore, it has good cold formability and can be cold formed by crimping, bending, stamping and other methods. The steel also has good weldability. Low carbon steel generally refers to the steel with carbon content between 0.10% and 0.25%. This kind of steel has low hardness and good plasticity. It is easy to adopt cold plastic deformation forming process, welding and cutting. It is commonly used in manufacturing chains, rivets, bolts, shafts, etc.
Characteristics of low carbon steel
The annealed microstructure of low carbon steel is ferrite and a small amount of pearlite, with low strength and hardness, good plasticity and toughness. Therefore, it has good cold formability and can be cold formed by crimping, bending, stamping and other methods. This steel has good weldability. Low carbon steel with low carbon content has low hardness and poor machinability. Normalizing can improve its machinability.
Low carbon steel has great aging tendency, including quenching aging tendency and deformation aging tendency. When the steel is cooled rapidly from high temperature, the carbon and nitrogen in ferrite are in supersaturated state, and it can slowly form the carbon and nitrogen of iron at room temperature. As a result, the strength and hardness of the steel increase, while the plasticity and toughness decrease. This phenomenon is called quenching aging. Low carbon steel will age even if it is air-cooled without quenching. A large number of dislocations are produced in low carbon steel after deformation. The carbon and nitrogen atoms in ferrite interact with the dislocations elastically, and the carbon and nitrogen atoms gather around the dislocations. The combination of carbon and nitrogen atoms with dislocation lines is called the year old Coriolis air mass. This phenomenon is called deformation aging. Deformation aging is more harmful to the plasticity and toughness of low carbon steel than quenching aging. There are obvious upper and lower yield points on the tensile curve of low carbon steel. From the appearance of the upper yield point to the end of the yield extension, the surface wrinkling band formed by the uneven deformation appears on the surface of the specimen, which is called Luders band. Many stamping parts are often scrapped. There are two ways to prevent it. It is a kind of high pre deformation method. The pre deformed steel will also produce Luders band when stamping after a period of time, so the pre deformed steel should not be placed too long before stamping. The other is adding aluminum or titanium into the steel to form a stable compound with nitrogen to prevent the deformation aging caused by the formation of Coriolis air mass.
Types of low carbon steel
Low carbon steel is generally rolled into angle bar, channel bar, I-beam bar, steel pipe, steel strip or steel plate, which is used to make various building components, containers, boxes, furnace bodies and agricultural machinery. High quality low carbon steel rolled into thin plate, making deep drawing products such as car cab and engine hood; It is also rolled into bars for the manufacture of mechanical parts with low strength requirements. Low carbon steel is generally treated without heat treatment before use, and the carbon content is more than 0.15% after carburizing or cyanidation treatment, which is used for shaft, sleeve, sprocket and other parts with high surface temperature and good wear resistance.
Low carbon steel is limited in use due to its low strength. The strength of steel can be greatly improved by increasing the manganese content in carbon steel and adding the alloy elements such as vanadium, titanium and niobium. If the carbon content in steel is reduced and a small amount of aluminum, a small amount of boron and carbide are added to form elements, the ultra-low carbon bainite group can be obtained with high strength and good plasticity and toughness.
2) Medium carbon steel – carbon content > 0.25% ≤ 0.60%.
What is medium carbon steel?
Medium carbon steel is a kind of carbon steel with carbon content of 0.25% ～ 0.60%. It includes most high quality carbon structural steel and some common carbon structural steel. This kind of steel is mostly used to make various mechanical parts, and some are used to make engineering structural parts.
Heat treatment of medium carbon steel
The final heat treatment methods of medium carbon steel include quenching and tempering, low temperature tempering after quenching, low temperature tempering after high frequency quenching, isothermal quenching and medium temperature tempering after quenching.
- (1) Conditioning. The microstructure is tempered sorbite. This structure has good comprehensive mechanical properties, high strength, good plasticity and toughness. The quenched and tempered steel should have good hardenability to ensure uniform microstructure and properties on the whole section of the quenched and tempered parts. Compared with alloy steel, carbon steel has poor hardenability, so it is only suitable for quenching and tempering of medium carbon steel parts with small section size.
- (2) Low temperature tempering after quenching. The microstructure is tempered martensite, which has high strength and proper plasticity and toughness.
- (3) High frequency quenching followed by low temperature tempering. The microstructure of high frequency quenching layer is very fine hidden needle martensite, and tempered martensite can be obtained after low temperature tempering. The effect of this treatment is similar to that of carburizing treatment. Quenching and tempering or normalizing are usually carried out before high frequency quenching. Therefore, after high-frequency quenching and tempering, the core part has high strength, good plasticity and toughness, and the surface layer has high hardness and good wear resistance. In addition, the surface layer of high frequency quenching parts produces compressive stress, which has high fatigue limit and long service life.
- (4) Austempering. The microstructure is bainite, which has high strength and good plasticity and toughness.
- (5) After quenching, it is tempered at medium temperature. The microstructure is tempered sorbite.
Main uses of medium carbon steel
High strength medium carbon quenched and tempered steel has certain plasticity, toughness and strength, good machinability, good comprehensive mechanical properties after quenching and tempering treatment, poor hardenability, easy to crack, low welding performance, good preheating before welding and heat treatment after welding.
Medium carbon steel is mainly used to manufacture high strength moving parts, such as air compressor, piston of pump, impeller of steam turbine, shaft, worm, gear of heavy machinery, wear-resistant surface parts, crankshaft, machine tool spindle, roller, fitter tools, etc.
3) High carbon steel – carbon content > 0.60%.
What is high carbon steel?
High carbon steel is often called tool steel. Its carbon content ranges from 0.60% to 1.70%. It can be quenched and tempered. Hammers and crowbars are made of steel with carbon content of 0.75%; Cutting tools such as drills, taps, reamers, etc. are made of steel with a carbon content of 0.90% to 1.00%.
After proper heat treatment or cold drawing hardening, high carbon steel has high strength and hardness, high elastic limit and fatigue limit. Its cutting performance is acceptable, but its welding performance and cold plastic deformation ability are poor. Due to the high carbon content, cracks are easy to occur during water quenching, so double liquid quenching is often used, and oil quenching is often used for small section parts. This kind of steel is usually used after quenching, tempering or normalizing at medium temperature or surface quenching. It is mainly used for manufacturing springs and wear-resistant parts. Carbon tool steel is a kind of high carbon steel without alloying elements. It is also a kind of tool steel with low cost, good cold and hot workability and wide application range. Its carbon content ranges from 0.65% to 1.35%. It is a special steel for making tools. The density of high carbon steel is 7.81g/cm ³. It can be used in the production of fishing gear.
Properties of high carbon steel
- 1. High hardness (hrc60-65) and good wear resistance can be obtained after heat treatment.
- 2. The hardness is moderate and the machinability is good.
- 3. The raw materials are easy to get and the production cost is low.
- 1. When the working temperature of the cutting tool is higher than 200 ℃, its hardness and wear resistance decrease sharply.
- 2. Low hardenability. When water quenched, the diameter of complete hardening is only 15-18mm; In oil quenching, the maximum diameter or thickness of complete hardening is only about 6 mm, and it is easy to deform and crack.
The hardness and strength of high carbon steel mainly depend on the amount of solid solution carbon in the steel, and increase with the increase of the amount of solid solution carbon. When the amount of solid solution carbon exceeds 0.6%, the hardness does not increase after quenching, but the amount of excess carbide increases, the wear resistance of the steel increases slightly, while the plasticity, toughness and elasticity decrease. Therefore, different steel grades are often selected according to the service conditions and the matching of strength and toughness of steel. For example, 65 steel with lower carbon content can be selected to manufacture spring or spring type parts with less stress. General high carbon steel can be produced by electric furnace, open hearth furnace and oxygen converter. When high quality or special quality is required, electric furnace smelting plus vacuum consumable or electroslag remelting can be used. During smelting, the chemical composition, especially the content of sulfur and phosphorus, should be strictly controlled. In order to reduce segregation and improve isotropic properties, ingots can be annealed at high temperature (especially for tool steels). During hot working, the stop forging (rolling) temperature of hypereutectoid steel should be low (about 800 ℃), the precipitation of coarse network carbide should be avoided after forging and rolling, and slow cooling should be paid attention below 700 ℃ to prevent cracks caused by thermal stress. Surface decarburization should be prevented during heat treatment or hot working (especially for spring steel). There should be enough compression ratio during hot working to ensure the quality and performance of steel.
Welding technology of high carbon steel
1. When the carbon content of high carbon steel is more than 0.60%, the hardening and crack sensitivity after welding are greater, so the weldability is very poor, and it can not be used to manufacture welded structure. It is often used to manufacture parts and components that need more hardness or wear resistance, and its welding work is mainly repair welding.
2. As the tensile strength of high carbon steel is more than 675mpa, the commonly used welding rods are e7015 and e6015. E5016 and E5015 welding rods can be selected when the structural requirements are not high. In addition, chromium nickel austenitic steel electrode can also be used for welding.
3. Welding process
- (1) In order to obtain high hardness and wear resistance, high carbon steel parts need heat treatment, so annealing should be carried out before welding.
- (2) The weldment should be preheated before welding, and the preheating temperature is generally above 250 ~ 350 ℃. The interlayer temperature must not be lower than the preheating temperature during welding.
- (3) After welding, the weldment must be kept warm and cooled slowly, and immediately sent to the furnace for stress relief heat treatment at 650 ℃.
4. High carbon steel has high carbon content, so its weldability is poor. It needs preheating during welding, slow cooling after welding or 350 ℃ low temperature tempering treatment. The specific heat treatment time is determined by the thickness of the workpiece. If preheating is not possible, welding materials with good weldability and crack resistance must be used, but the welding speed must be reduced.
5. High carbon steel belongs to the category of poor welding performance. If it is to be welded, it should be welded under preheating condition, and stress relief heat treatment should be carried out after welding.
Ultra high carbon steel
Ultra high carbon steel is a kind of iron-based alloy with carbon content of 1.0-2.1%. The earliest industrial case of ultra-high carbon copper is Damascus. Its carbon content is 1.5%. In the mid-1970s, Steiner Institute of the United States first carried out the research on Superplasticity of ultra-high carbon steel, and then the National Laboratory of the United States also carried out the research on ultra-high carbon steel, and obtained a series of patents. In addition, Japan and other countries have also carried out research on ultra-high carbon steel. In the 21st century, research on ultra-high carbon steel has been carried out in China. Ultrafine grained ultra-high carbon steel without network carbide was obtained by proper preparation process. It not only has the Superplasticity of high deformation rate at medium and high temperature. And it has good comprehensive mechanical properties at room temperature. The ultra-fine grained ultra-high carbon steel is not only expected to replace part of medium and high carbon steel to make tools, dies, steel wires and structural parts, so as to significantly improve its service life, but also can be connected with itself or other metal matrix materials to prepare new high-performance layered composites by using its good solid-state connection characteristics at medium and high temperature. It is a kind of new material with great industrial application prospect.
According to the quality of the steel (mainly the content of impurities such as sulfur and phosphorus):
Ordinary carbon steel (S ≤0.055％，P ≤0.045％);
What is ordinary carbon steel?
Common carbon steel is the abbreviation of common carbon structural steel. The carbon content is less than 0.38%, and less than 0.25% is the most commonly used. It belongs to low carbon steel, and each metal grade represents the lowest yield point of the steel when its thickness is less than 16mm. Compared with high-quality carbon steel, the limit of carbon content, performance range, phosphorus, sulfur and other residual elements is wider. According to the guarantee conditions of delivery in China and some countries, ordinary carbon steels are divided into three categories: class a steel (class a steel), which only guarantees mechanical properties, but does not guarantee chemical composition; class B steel (class B steel), which only guarantees chemical composition, but does not guarantee mechanical properties; Special steel (C steel), not only to ensure the chemical composition, but also to ensure the mechanical properties. Special steel is often used to manufacture important structural parts.
Application of common carbon steel
This kind of steel can be smelted by oxygen converter, open hearth furnace or electric furnace, and generally hot rolled into steel plate, strip, profile and bar. Steel plates are generally delivered in hot rolling (including controlled rolling) or normalizing condition. The chemical composition, tensile property, impact energy and cold bending property of the steel shall comply with relevant regulations. According to the Chinese national standard GB700-88, this kind of steel is divided into five grades according to the yield point value and four grades according to the quality. The grade is composed of the letter Q representing the yield point, the value of yield point, the symbol of quality grade and the symbol of deoxidation method. This kind of steel has a wide range of applications, most of which are used for welding, riveting or bolting steel structural parts, and a few are used for making various machine parts. Q195 and Q215 steel with lower strength are used to make low carbon steel wire, steel mesh, roof panel, welded steel pipe, anchor bolt and rivet, etc. Q235 steel has medium strength, good plasticity and toughness, and is easy to form and weld. This kind of steel is mostly used as steel bars and steel structural parts, in addition, it is also used as rivets, railway spikes and various mechanical parts, such as bolts, tie rods, connecting rods, etc. Q255 and Q275 steel with high strength are used to make all kinds of agricultural machinery, as well as steel bars and railway fishplates. According to the special performance requirements of some industrial steel, the composition of ordinary carbon structural steel is slightly adjusted to form a series of professional steel, such as riveting steel, bridge steel, pressure vessel steel, hull steel and boiler steel. In addition to strict control of chemical composition and guarantee of conventional properties, some special inspection items, such as low temperature impact toughness, aging sensitivity, gas in steel, inclusion and fracture, are also specified.
Classification of ordinary carbon steel
According to the yield strength grade, it can be divided into five grades. Q235 steel is commonly used in steel structure. The steel grade is composed of four parts: the letter representing yield point, yield point value, quality grade symbol and deoxidation method symbol. For example, q235af, in which “Q” is the first letter of the Chinese phonetic alphabet of steel yield point “Qu”“ 235 “is the yield point value of the steel, indicating that the yield strength of the steel is” 235n / mm ““ A ‘is the quality grade symbol of steel, which is divided into four grades A, B, C and D“ Level a is the lowest level and level D is the highest level F “is the first letter of the Chinese phonetic alphabet of boiling steel, indicating that the steel is boiling steel. If the letter “B” is marked at the end of the steel grade, it indicates that the steel is semi killed steel, and “B” is the first letter of the Chinese phonetic alphabet of the word “semi”“ The letter “Z” is the first letter of the Chinese phonetic alphabet of the word “Zhen” in sedative steel, representing sedative steel“ The letter “TZ” is the first letter of the Chinese phonetic alphabet of “Tezhen” for special killed steel, representing special killed steel. In the expression method of steel grade composition, “Z” and “TZ” symbols are omitted, “F”, “B”, “Z” and “TZ” symbols indicate the deoxidation degree of ingot during casting. Killed steel is fully deoxidized by aluminum, silicon and so on. It emits less gas during casting and has good quality but high price. Rimmed steel is deoxidized by ferromanganese. Due to insufficient deoxidation, there is boiling phenomenon in ingot during casting, and the quality is not uniform enough, but the productivity is high. Between killed steel and rimmed steel is semi killed steel.
High quality carbon steel (S ≤0.040％，P ≤0.040％);
What is high quality carbon steel?
The harmful impurity elements s and P in high-quality carbon steel are lower than those in ordinary carbon steel. Moreover, their smelting methods are different. Ordinary carbon steel is mostly smelted in converter with the lowest cost, while high-quality carbon steel is smelted in open hearth or pure oxygen top blown converter, with better deoxidation and lower impurity content, so its comprehensive mechanical properties and corrosion resistance are better than ordinary carbon steel. Compared with high quality carbon steel, the price of high quality carbon steel is not high, and it is the most widely used carbon steel in engineering.
The indication method and code of high quality carbon steel are in accordance with gb221 standard. 08 F: F represents boiling steel, 08 represents carbon content, expressed in tens of thousands. Another example is 20g, G for special purpose mark, R for pressure and volume steel; G boiler steel; D low temperature steel, 20 represents the average carbon content, expressed in tens of thousands. If it is cast steel, “ZG” should be added before carbon content to indicate cast steel, but not for production methods other than cast steel. If it is rimmed steel or semi killed steel, “F” or “B” should be added.
The chemical composition and mechanical properties of high quality carbon steel are given in GB / t699. The standard lists 31 kinds of materials, such as 08F, 10F, 15F, 08, 10, 20, 25,… 70MN; The commonly used grades of pressure pipes are 08, 10 and 20. The most commonly used grades are 20 steel and 20g plate (the new standard is Q245R). The maximum operating temperature is 425 ℃
Properties and application of high quality carbon steel
08 steel and 10 steel have low carbon content, low strength and good plasticity, good weldability and calenderability, and are usually rolled into sheet or strip. It is mainly used for manufacturing cold stamping parts, such as various instrument panels, containers and gaskets.
15 steel, 20 steel and 25 steel also have good weldability and calenderability. They are often used to manufacture structural parts and parts with low stress and high toughness, such as welding vessels, nuts and screws, and carburized parts with low strength requirements, such as cams and gears. Generally, the heat treatment of carburized parts is quenching (840 ~ 920 ℃) and low temperature tempering after carburizing.
35 steel, 40 steel, 45 steel, 50 steel and 55 steel belong to quenched and tempered steel, which can be used to manufacture parts with high performance requirements, such as gears, connecting rods and shafts. Generally, Quenched and tempered steel should be quenched and tempered to obtain comprehensive mechanical properties with good combination of strength and toughness. Normalizing can be used to replace quenching and tempering for workpieces with low comprehensive mechanical properties, large cross-section size and poor quenching effect.
60 steel, 65 steel, 70 steel, 75 steel, 80 steel and 85 steel belong to spring steel. After proper heat treatment, they can be used to manufacture parts requiring good elasticity and high strength, such as springs and spring washers, and can also be used to manufacture wear-resistant parts. Cold formed springs are usually only subjected to low temperature stress relief. The hot formed spring should be quenched (~ 850 ℃) and tempered (350 ~ 500 ℃). The wear-resistant parts are quenched (~ 850 ℃) and tempered at low temperature (200 ~ 250 ℃).
High-grade fine carbon steel (S≤0.030％，P ≤0.035％).
Carbon structural steel: mainly used for bridges, ships, building components, machine parts, etc;
Carbon structural steel, a kind of carbon steel. The carbon content is about 0.05% ～ 0.70%, and it can be as high as 0.90% in some cases. It can be divided into ordinary carbon structural steel and high quality carbon structural steel. It is mainly used in railway, bridge, all kinds of construction engineering, manufacturing various metal components under static load, unimportant mechanical parts without heat treatment and general welding parts.
Ordinary carbon structural steel: According to the yield strength of steel, carbon structural steel can be divided into five grades: Q195, Q215, Q235, q255 and Q275. Each brand is divided into a, B, C and D grades due to different quality Q215、 Q235 has good plasticity and can be rolled into steel plate, steel bar, steel pipe, etc; Q255、 Q275 can be rolled into forming steel, steel plate, etc.
High quality carbon structural steel: the steel grade is expressed by the average mass fraction of carbon. Such as 20, 45, etc. 20 means C: 0.20% (20 / 10000). Usage: mainly used for manufacturing various machine parts.
Carbon tool steel: mainly used for cutting tools, molds, measuring tools, etc.
What is carbon tool steel?
Carbon tool steel, referred to as carbon steel, refers to high quality high carbon steel with carbon content of 0.7 ~ 1.3%. It is mainly used for manufacturing various cutting tools, measuring tools and moulds. The properties of carbon tool steel depend on the carbon content and the forging process. The hardness of carbon tool steel increases with the increase of carbon content. Carbon tool steel does not contain alloying elements, the price is low.
The steel grade is expressed by the average mass fraction of carbon and preceded by T. Such as T9, T12, etc. T9 means C: 0.9% (9/1000). Application: mainly used for manufacturing all kinds of cutting tools, measuring tools, molds, etc.
Application of carbon tool steel
Carbon steel for cutting tools, moulds and measuring tools. Compared with alloy tool steel, it has good machinability, low price and wide application range, so it is widely used in tool production. Carbon tool steel is divided into carbon cutting tool steel, carbon die steel and carbon gauge steel. Carbon tool steel refers to the carbon tool steel used for making cutting tools, carbon die steel refers to the carbon tool steel used for making cold and hot working dies, and carbon gauge steel refers to the carbon tool steel used for making measuring tools.
This kind of steel is generally delivered in annealed state, and it can also be delivered in non annealed state according to the requirements of the demander. The hardness, fracture structure, network carbide, pearlite structure, quenching hardness, hardenability depth and decarburization layer depth of annealed steel shall meet the requirements of Chinese national standard gb1298. When there are network carbide and lamellar pearlite in this kind of steel, it is easy to produce quenching deformation, cracking and uneven hardness, and reduce the wear resistance of the cutting tool, which is easy to cause the cutting edge collapse and reduce the service life of the cutting tool. In order to prevent the formation of network carbide, the steel should be forged repeatedly and cooled rapidly after forging. The cementite in lamellar pearlite can be spheroidized by spheroidizing annealing. This kind of steel is usually heated by salt bath furnace, which can prevent or reduce decarburization of tool surface. During quenching and cooling, attention should be paid to prevent deformation and cracking. For this reason, graded quenching or isothermal quenching is generally adopted, and some high-frequency quenching is adopted. After quenching, it should be tempered in time to prevent deformation or cracking during parking.
The production cost of carbon tool steel is low and the source of raw materials is convenient; It is easy to be cold and hot processed, and can obtain quite high hardness after heat treatment; Under the condition of low heating, the wear resistance is also good, so it is widely used. Among them, the high quality carbon tool steel has high toughness, which can obtain high finish during grinding, and is suitable for manufacturing tools with complex shape and high precision. However, the red hardness of this kind of steel is poor. When the working temperature exceeds 250 ℃, the hardness and wear resistance decrease rapidly. This kind of steel has low hardenability. When the cross-section size of the tool is larger than 15mm, only the surface layer gets high hardness after water quenching. This kind of steel has narrow quenching temperature range, easy to overheat, large distortion and cracking tendency during quenching, and easy to produce soft spots.
Compared with the alloy tool steel, the hardenability of this kind of steel is low. The hardenability in water is 15 mm, while that in oil is only 5 mm. In addition, the hot hardness of this kind of steel is low. When the working temperature is higher than 250 ℃, the hardness and wear resistance of the steel decrease sharply, so the cutting ability of the steel decreases significantly. Therefore, this kind of steel is only suitable for making tools with small size, simple shape, low cutting speed, small feed and low working temperature.
The carbon content of this kind of steel ranges from 0.65% to 1.35%. There are eight grades of carbon tool steel in Chinese national standard GB 1298-77. T7 steel with low carbon content has good toughness, but low wear resistance. It is suitable for cutting tools of soft materials and tools bearing impact load, such as woodworking tools, sickles, chisels, hammers, etc. T8 steel has better toughness and higher hardness, which is suitable for making punch, scissors and woodworking tools. T8mn steel with high manganese content has good hardenability, which is suitable for making woodworking tools with large fracture, coal mine chisels, stonecutters, hand saw blades and cross grain files with small deformation. T10 steel has good wear resistance and wide application range. It is suitable for making metal cutting tools with poor cutting conditions and high wear resistance requirements, as well as cold stamping dies and measuring tools, such as turning tools, planers, milling cutters, thread rolling plates, wire drawing dies, chisels, calipers and plug gauges. T12 steel has high hardness and good wear resistance, but low toughness. It can be used to make cutting tools and measuring tools that are not impacted and require high hardness and wear resistance, such as scraper, drill, reamer, reamer, tap, die and micrometer. T13 steel is the steel with the highest carbon content in carbon tool steel. Its hardness is very high, but its toughness is low, and it can not bear impact load. It is only suitable for making cutting tools for cutting high hardness materials and tools for processing hard rocks, such as files, cutting knives, drawing dies, carving tools, etc.
Characteristics of carbon tool steel
Carbon tool steel is cheap and easy to get, easy to forge, good machinability. The main disadvantages of carbon tool steel are poor hardenability, need to be quenched with water, salt water or alkali water, high distortion and cracking tendency, low wear resistance and thermal strength. Therefore, carbon tool steel can only be used to make some small hand tools or woodworking tools, as well as small cold working dies with low precision, simple shape, small size and light load, such as small punch, scissors, cold stamping die, cold heading die, etc. In particular, it should be pointed out that carbon tool steel is suitable for cold heading die. According to the working conditions of the cold heading die, the die material should have enough strength and hardness and hardening layer on the working surface and cavity of the die, as well as enough toughness. Therefore, after heat treatment, this kind of die requires a certain hardening layer on the inner hole but not on the outside, So it can keep high toughness. If the hardened layer is too deep, it will crack rapidly due to the heavy impact in the work. However, the cold upsetting die with large size and heavy load will also collapse due to thin hardened layer and too soft matrix.
Carbon tool steels used for cold working die mainly include t7a, T10A and t11a, among which T10A Steel is the most widely used. There are two reasons why T8 steel is seldom used in cold working die: one is that T8 steel has high sensitivity to overheating during quenching and heating, and even under the condition of relatively low heating temperature (780 ~ 790 ℃), the grains of T8 steel are easy to grow up and the toughness is poor; Second, there is no excess carbide in the structure of T8 steel after quenching, so the wear resistance is poor. However, hypereutectoid steels T10A and t11a can obtain relatively fine grains when heated, and are less sensitive to overheating after quenching. After proper heat treatment, they can obtain higher strength and certain toughness. In addition, some carbide remains in the structure of T10A and t11a steel after quenching, which can improve the wear resistance of the die. This is the reason why T10A Steel is widely used.
The hypereutectoid steel T12A with more than 1.1% carbon content has more excess carbides and coarser particles, and the distribution of carbides in the structure is not uniform, so it is easy to form a network or intermittent network, which will deteriorate the mechanical properties of the steel. However, T12A steel can be used as long as the trimming die and scissors with high hardness and wear resistance are required. The wear resistance of hypoeutectoid steel T7 and t7a is lower than that of T10A, but T7 and t7a have better toughness. Therefore, T7 or t7a can be used when making dies with higher toughness requirements.
If carbon tool steel is quenched with water (or salt water or alkali water), it will cause great distortion. The step quenching in alkali bath is an effective measure to solve the quenching distortion of the cold working die, and the cracking phenomenon can be basically eliminated. Adopting 160 ~ 180 ℃ alkali bath step quenching is especially suitable for forming dies that do not need grinding edge, or plastic die, cold heading die, etc. that only need surface hardness and have certain strength and high toughness in the center, or small edge dies (such as punch, blanking die, etc.).
When a deep hardening layer is required, in order to make up for the lack of hardenability, the quenching temperature is generally increased by 20 ~ 40 ℃, and the holding time is appropriately increased. Another method is to first step quench in 150 ~ 170 ℃ alkali bath, and then isothermal stay in 200 ~ 220 ℃ nitrate for a period of time, or first select salt water to cool for a period of time, and then put it into nitrate for isothermal treatment. The purpose is to increase the depth of hardened layer.
Heat treatment of T10
T10 is the most common carbon tool steel with moderate toughness and low production cost. After heat treatment, the hardness of T10 can reach above 60HRC. However, this steel has low hardenability and poor heat resistance (250 ℃), and is mostly used for manufacturing simple tools such as hand tools. Here is a brief introduction to the heat treatment process of T10
Thermoelectric thermometer, medium temperature box resistance furnace.
The heating temperature is 760 ~ 780 ℃, and the furnace is cold. After annealing, the hardness of the steel decreases greatly.
Heating temperature 820 ~ 840 ℃, air cooling.
The heating temperature is 650 ~ 700 ℃.
Quenching medium: oil, heating temperature: 790 ℃, quenching method: for small straight blade, generally vertical slow down.
Temperature 150 ℃, hardness 62Hrc, but easy to crack; Generally, the temperature is 200 ~ 250 ℃ and the hardness is 55 ~ 60HRC.
The above is the general factory for T10 heat treatment process. Because the security waist knife is only used in the family workshop, there is basically no instrument and equipment, and it mainly depends on the visual inspection and experience of the knife maker.
Cast steel: the number of cast steel is preceded by ZG, and the number represents the average mass fraction of steel (expressed in 10000). For example, ZG25 means C: 0.25%. Purpose: it is mainly used for manufacturing parts with complex shape and requiring certain strength, plasticity and toughness, such as gears, couplings, etc.
About 0.25% ~ 0.80%. Solid solution strengthening; Remove FeO and reduce the brittleness of steel; The synthesis of MNS with sulfur can reduce the harmful effect of sulfur. It’s good.
About 0.10% ~ 0.40%, solid solution strengthening; In addition to the bad effect of FeO on the quality of steel, it is beneficial.
FES and Fe form low melting point eutectic (melting point is 985 ℃), which leads to steel embrittlement and cracking during hot working at 1000 ~ 1250 ℃. Harmful.
The strength and hardness increase, but the plasticity and toughness decrease. Harmful.
Manufacturing process of carbon steel
The smelting of carbon steel is usually carried out in converter and open hearth. Converter usually smelts ordinary carbon steel, while open hearth can smelt all kinds of high quality steel. Oxygen top blown converter steelmaking technology is developing rapidly, and it is a trend to replace open hearth steelmaking. All kinds of ingots can be obtained by injecting molten steel into ingot mould. After forging or rolling, the ingot is processed into steel plate, steel strip, steel bar and various section shapes. Carbon steel is generally used directly in hot rolling. When it is used to manufacture tools and various machine parts, it needs heat treatment according to the use requirements; As for steel castings, most of them need heat treatment.
Application of carbon steel
Carbon steel, carbon tool steel, spring steel and their scraps with carbon content of 0.1% – 1.2% can be used for carbon blending and carburizing in metallurgical steel. The following points should be paid attention to when selecting carbon steel return material.
- 1. The contents of phosphorus and sulfur are higher in ingot riser and hot-rolled forging and cutting materials. When smelting low phosphorus and sulfur alloy steel, the consumption should be controlled.
- 2. The components of scrap, scrap strip and scrap secondary steel are not accurate, so the selection should be careful, and it is better to use them after remelting (furnace washing).
- 3. When using carbon steel scrap, spark identification should be carried out and classified.
Use of carbon steel
- Q195 is used to manufacture parts, iron wire, iron ring, pad iron, split pin, pull rod, stamping parts and welding parts with small load.
- Q215a is used to manufacture pull rod, ferrule, washer, carburized ring, carburized parts and weldments.
- Q235aa and grade B are used for manufacturing metal structural parts, carburized parts or carbonitridized parts with low requirements for core strength, pull rods, connecting rods, hooks, couplers, bolts, nuts, sleeves, shafts and connectors; C. Class D is used to manufacture important welded structural parts.
- Q255a is used for manufacturing parts with low strength requirements such as rotating shaft, spindle, hook, pull rod, rocker and wedge. The negative solderability is acceptable.
- Q275 is used to manufacture shaft, sprocket, gear, hook and other parts with high strength requirements.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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