In 2013, China's auto market demand was around 20.8 million, with a growth rate of 7%. It is estimated that the total number of cars in China will reach 23.5 million in 2015 and 25 million in 2020. The development of automobiles is developing towards high power output, light weight, high performance, long life, noise reduction, stable operation, safety and reliability, environmental protection, energy saving, low cost, easy processing and variety. Steel for automotive parts is a key component of automobiles, and is a manufacturing material for core components that guarantee the running performance of automobiles. The development direction of the automobile puts forward higher requirements for the development of steel for auto parts.
Product requirements: high toughness, high precision, energy saving and environmental protection
Steel for automotive parts is used in the four major systems in the car:
Steel for engine systems: The engine is the source of power for the car. Typical parts include crankshafts, connecting rods, cams, etc., which are mainly non-tempered steel. The fuel injection system is mainly steel for oil pump nozzles, and the valve steel is mainly steel valve steel.
Steel for shifting and transmission systems: The shifting and transmission system is the soul of a car. The gear ring, the hub, the gear and the gear shaft are mainly gear steel. The transmission system is more like the waist and spine of the animal, including the drive shaft, the half shaft, the front axle, etc., and the transmission shaft is mainly made of steel.
Steel for suspension and steering systems: The suspension system is the legs of the car, including suspension springs, stabilizer bars, torsion bars, shock absorbers, etc., mainly spring steel. The steering system is the driving direction of the car, including universal joints, ball heads, steering gears, wheels, bearings, etc., in which the hubs and bearings are mainly bearing steel, and the universal joints, ball heads, steering gears, etc. are mainly gear steel.
Steel for standard parts system: mainly cold-rolled steel for fasteners.
Automotive industry application system requirements: Automotive components have high performance, long life, easy processing, low noise, high surface precision, high strength; to achieve lightweight and compact automotive, reduce CO2 emissions by improving efficiency.
The requirements of the automotive industry for materials are: steel for automotive parts has the characteristics of high strength and toughness, easy cutting, high dimensional accuracy, low alloying, environmental friendliness, and omission of heat treatment. The development trend of common technology for the development of steel materials for automotive parts is shown in the attached table.
Technology trends: precise control, uniform composition, microalloying
The technological advancement of automobiles has driven the technological development of auto parts, and the technological development of parts and components has promoted the technological advancement and technological innovation of automobiles.
High-quality gear steel, high-quality spring steel, high-quality non-tempered steel, high-quality fastener steel, high-quality bearing steel, high-quality valve steel are high requirements for the use of special alloy steel in steel for automotive parts. Representative of key materials. High-quality special steel for auto parts is developing in the direction of light weight, high performance, long life, stable operation, low noise, safety, energy saving, low cost, easy processing and many varieties. The technical quality development direction of high-quality special steel for auto parts is high strength and toughness, high purity, high uniformity, ultra-fine grain size, high surface quality and long fatigue life of steel.
The development trend of high quality gear steel. High-quality gear steels are constantly developing in terms of variety, cutting, environmental protection, low alloying, cost reduction, low noise, smooth operation, good steel uniformity, and small heat treatment deformation.
The technical quality characteristics of gear steels are evaluated in terms of hardenability, purity, grain size, and band structure.
The high degree of hardenability and the stability of the hardenability value are important indicators for evaluating the quality of gear steel. The goal of hardenability belt control is 3HRC.
The influence of the cleanliness of gear steel on the fatigue life of gears has attracted more and more attention. Oxides and sulfides present in the steel, as well as harmful elements such as N, H, O, P, S, etc., will reduce the mechanical properties of the steel and deteriorate the process performance of the steel, thereby affecting the service life of the carburized gear of the automobile. The goal of oxygen content control of gear steel is [O]10ppm. Ti and Ca are not allowed in high-quality gear steel, and Ti 0.01% and Ca 0.0005% are specified in steel. The pursuit of non-metallic inclusion control is A2, B2, C1, and D1.
Grain size is an important indicator of gear steel. After quenching the fine and uniform austenite grains in the gear steel, a fine martensite structure is obtained, which significantly improves the fatigue performance of the gear and reduces the deformation of the gear after heat treatment. Fine uniform austenite grain size has a special contribution to the strength and toughness of the part. In particular, it is of great significance to improve the fracture toughness of the gear and enhance the brittle fracture resistance of the gear. Gear steel grain size requires 6 grades. At present, in order to obtain high temperature (>960 °C) carburized gear steel, the microalloying elements Nb, V, Zr, etc. are added or compounded during smelting, and alloy carbonitride is formed in the steel, and the grain size of the steel is 8 grades.
Banded tissue is a structural defect in steel. For gear steel, severe band structure will affect the uniformity of carburization, increase the degree of quenching deformation, and make the size accuracy of carburizing gears poor. Therefore, the pursuit of the ribbon structure control of gear steel is no more than two.
The development trend of high quality non-tempered steel. Non-quenched and tempered steel is a microalloying element (V, Ti, Nb, Al, B or N) added to medium or low carbon steel or medium and low carbon manganese steel. It is fully controlled by controlled rolling (forging) controlled cooling process. Strengthening, fine grain strengthening and phase transformation strengthening, so that the steel does not need to be quenched and tempered after hot rolling (forging), its strength and hardness can reach the level of quenched and tempered steel, and has high flexibility and energy-saving steel with certain plasticity and toughness. Non-tempered steel is an environmentally friendly steel that meets both high performance and low cost requirements.
The selection of steel materials for automobile crankshafts, connecting rods, front axles, half shafts, I-beams, steering knuckles, knuckle arms, camshafts, etc., shows the trend of replacing quenched and tempered steel with non-tempered steel. Among them, from the viewpoint of reducing the number of processing steps and improving productivity, the selection of the steel material for the connecting rod is showing a tendency to gradually replace the non-broken material with the breaking material.
The characteristic values ​​for evaluating the technical quality of non-quenched and tempered steel include carbon equivalent, carbon segregation, oxygen content, grain size, decarburization layer, mechanical properties, ferrite content, inclusion level and the like. For example, the goal of carbon equivalent range value control is 0.02%. The goal of carbon segregation control for steel full section is 0.03%. The goal of austenite grain size control is not to be coarser than grade 8. The microstructure should be pearlite + ferrite. The decarburization layer requires that the decarburization depth (ferrite + transition layer) control on each side of the steel is aimed at 0.5% of the diameter of the steel. The goal of oxygen content control is 15ppm. The goal of ribbon tissue control is 1.5.
The development trend of high quality spring steel. The two most important factors affecting spring design stress are fatigue resistance and ballistic reduction performance, which have become the subject of research and development of spring steel. At present, spring steel grades are developing in the direction of economy and high performance. The new generation of ultra-high strength spring steel has the following characteristics: ultra high strength - ductile plasticity, that is, tensile strength 2000MPa, section shrinkage 50%; high fatigue strength and Corrosion-resistant fatigue performance; excellent resistance to bullet reduction.
In addition, high quality spring steel requires good economy. The research and development of its new steel grades is to reduce the carbon content and add V and Nb by optimizing the alloying element content of the existing spring steel and adding microalloying elements. On the other hand, the existing steel grades are basically not In the case of change, the process of deformation heat treatment, induction heat treatment, and in-line heat treatment achieves ultra-high strength under the premise of ensuring economical efficiency. At present, foreign deformation heat treatment and induction heat treatment processes have been widely used in actual production, and the online spring steel wire has undergone oil quenching-tempering heat treatment process.
The development trend of high quality cold heading steel. The demand for high-precision, high-strength fasteners, steel joints and non-standard shaped parts in the automotive industry is increasing, so the demand for high-purity, high-performance, high-quality cold heading steel is also more urgent. Technological advances play a key role in transforming fasteners into versatile high-precision automotive parts.
The development direction of cold heading steel is non-tempered steel, boron steel and ultra-fine grain steel, while heat-treated non-tempered cold-rolled steel has attracted much attention. Non-tempered cold-rolled steel can eliminate the spheroidizing annealing before cold drawing and the quenching and tempering after forming by using the method of strengthening and toughening such as microalloying, controlled rolling and controlled cooling, etc. The decarburization tendency of the threaded portion improves the yield.
The characteristic values ​​for evaluating the technical quality of high quality cold heading steel include cold heading, decarburization layer, surface quality and the like. The goal of cold heading control is 1/5, and the goal of decarburization control is to reduce the diameter of the hot-rolled decarburized layer by 0.3% D.
The development trend of high quality bearing steel. Bearing steel is mainly used to manufacture rolling elements and ferrules for rolling bearings. Bearings should have long life, high precision, low heat, high speed, high rigidity, low noise, high wear resistance, etc. Therefore, bearing steels should have high hardness, uniform hardness, high elastic limit, high contact fatigue strength, Requires toughness, certain hardenability, and corrosion resistance in atmospheric lubricants. In order to achieve the above performance requirements, the chemical composition uniformity of bearing steel, the content and type of non-metallic inclusions, the particle size and distribution of carbides, and decarburization are strictly required. Bearing steels generally develop in the direction of high quality, high performance and many varieties.
The quality of the bearing material is evaluated primarily by the purity and uniformity of the material. The purity of a material means that there are as few inclusions as possible in the material. The uniformity of the material means that the inclusions and carbide particles in the material are fine and diffuse. Specifically, bearing steels are mainly developed in two directions: high cleanliness and performance diversification. Increasing the cleanliness of the bearing steel, especially reducing the oxygen content in the steel, can significantly extend the life of the bearing. The pursuit goal of bearing steel oxygen content control is 5ppm. The goal of oxide inclusion control in steel is 10m for inclusions, 5ppm for titanium content control, and 30ppm for nitrogen content control.
Product requirements: high toughness, high precision, energy saving and environmental protection
Steel for automotive parts is used in the four major systems in the car:
Steel for engine systems: The engine is the source of power for the car. Typical parts include crankshafts, connecting rods, cams, etc., which are mainly non-tempered steel. The fuel injection system is mainly steel for oil pump nozzles, and the valve steel is mainly steel valve steel.
Steel for shifting and transmission systems: The shifting and transmission system is the soul of a car. The gear ring, the hub, the gear and the gear shaft are mainly gear steel. The transmission system is more like the waist and spine of the animal, including the drive shaft, the half shaft, the front axle, etc., and the transmission shaft is mainly made of steel.
Steel for suspension and steering systems: The suspension system is the legs of the car, including suspension springs, stabilizer bars, torsion bars, shock absorbers, etc., mainly spring steel. The steering system is the driving direction of the car, including universal joints, ball heads, steering gears, wheels, bearings, etc., in which the hubs and bearings are mainly bearing steel, and the universal joints, ball heads, steering gears, etc. are mainly gear steel.
Steel for standard parts system: mainly cold-rolled steel for fasteners.
Automotive industry application system requirements: Automotive components have high performance, long life, easy processing, low noise, high surface precision, high strength; to achieve lightweight and compact automotive, reduce CO2 emissions by improving efficiency.
The requirements of the automotive industry for materials are: steel for automotive parts has the characteristics of high strength and toughness, easy cutting, high dimensional accuracy, low alloying, environmental friendliness, and omission of heat treatment. The development trend of common technology for the development of steel materials for automotive parts is shown in the attached table.
Technology trends: precise control, uniform composition, microalloying
The technological advancement of automobiles has driven the technological development of auto parts, and the technological development of parts and components has promoted the technological advancement and technological innovation of automobiles.
High-quality gear steel, high-quality spring steel, high-quality non-tempered steel, high-quality fastener steel, high-quality bearing steel, high-quality valve steel are high requirements for the use of special alloy steel in steel for automotive parts. Representative of key materials. High-quality special steel for auto parts is developing in the direction of light weight, high performance, long life, stable operation, low noise, safety, energy saving, low cost, easy processing and many varieties. The technical quality development direction of high-quality special steel for auto parts is high strength and toughness, high purity, high uniformity, ultra-fine grain size, high surface quality and long fatigue life of steel.
The development trend of high quality gear steel. High-quality gear steels are constantly developing in terms of variety, cutting, environmental protection, low alloying, cost reduction, low noise, smooth operation, good steel uniformity, and small heat treatment deformation.
The technical quality characteristics of gear steels are evaluated in terms of hardenability, purity, grain size, and band structure.
The high degree of hardenability and the stability of the hardenability value are important indicators for evaluating the quality of gear steel. The goal of hardenability belt control is 3HRC.
The influence of the cleanliness of gear steel on the fatigue life of gears has attracted more and more attention. Oxides and sulfides present in the steel, as well as harmful elements such as N, H, O, P, S, etc., will reduce the mechanical properties of the steel and deteriorate the process performance of the steel, thereby affecting the service life of the carburized gear of the automobile. The goal of oxygen content control of gear steel is [O]10ppm. Ti and Ca are not allowed in high-quality gear steel, and Ti 0.01% and Ca 0.0005% are specified in steel. The pursuit of non-metallic inclusion control is A2, B2, C1, and D1.
Grain size is an important indicator of gear steel. After quenching the fine and uniform austenite grains in the gear steel, a fine martensite structure is obtained, which significantly improves the fatigue performance of the gear and reduces the deformation of the gear after heat treatment. Fine uniform austenite grain size has a special contribution to the strength and toughness of the part. In particular, it is of great significance to improve the fracture toughness of the gear and enhance the brittle fracture resistance of the gear. Gear steel grain size requires 6 grades. At present, in order to obtain high temperature (>960 °C) carburized gear steel, the microalloying elements Nb, V, Zr, etc. are added or compounded during smelting, and alloy carbonitride is formed in the steel, and the grain size of the steel is 8 grades.
Banded tissue is a structural defect in steel. For gear steel, severe band structure will affect the uniformity of carburization, increase the degree of quenching deformation, and make the size accuracy of carburizing gears poor. Therefore, the pursuit of the ribbon structure control of gear steel is no more than two.
The development trend of high quality non-tempered steel. Non-quenched and tempered steel is a microalloying element (V, Ti, Nb, Al, B or N) added to medium or low carbon steel or medium and low carbon manganese steel. It is fully controlled by controlled rolling (forging) controlled cooling process. Strengthening, fine grain strengthening and phase transformation strengthening, so that the steel does not need to be quenched and tempered after hot rolling (forging), its strength and hardness can reach the level of quenched and tempered steel, and has high flexibility and energy-saving steel with certain plasticity and toughness. Non-tempered steel is an environmentally friendly steel that meets both high performance and low cost requirements.
The selection of steel materials for automobile crankshafts, connecting rods, front axles, half shafts, I-beams, steering knuckles, knuckle arms, camshafts, etc., shows the trend of replacing quenched and tempered steel with non-tempered steel. Among them, from the viewpoint of reducing the number of processing steps and improving productivity, the selection of the steel material for the connecting rod is showing a tendency to gradually replace the non-broken material with the breaking material.
The characteristic values ​​for evaluating the technical quality of non-quenched and tempered steel include carbon equivalent, carbon segregation, oxygen content, grain size, decarburization layer, mechanical properties, ferrite content, inclusion level and the like. For example, the goal of carbon equivalent range value control is 0.02%. The goal of carbon segregation control for steel full section is 0.03%. The goal of austenite grain size control is not to be coarser than grade 8. The microstructure should be pearlite + ferrite. The decarburization layer requires that the decarburization depth (ferrite + transition layer) control on each side of the steel is aimed at 0.5% of the diameter of the steel. The goal of oxygen content control is 15ppm. The goal of ribbon tissue control is 1.5.
The development trend of high quality spring steel. The two most important factors affecting spring design stress are fatigue resistance and ballistic reduction performance, which have become the subject of research and development of spring steel. At present, spring steel grades are developing in the direction of economy and high performance. The new generation of ultra-high strength spring steel has the following characteristics: ultra high strength - ductile plasticity, that is, tensile strength 2000MPa, section shrinkage 50%; high fatigue strength and Corrosion-resistant fatigue performance; excellent resistance to bullet reduction.
In addition, high quality spring steel requires good economy. The research and development of its new steel grades is to reduce the carbon content and add V and Nb by optimizing the alloying element content of the existing spring steel and adding microalloying elements. On the other hand, the existing steel grades are basically not In the case of change, the process of deformation heat treatment, induction heat treatment, and in-line heat treatment achieves ultra-high strength under the premise of ensuring economical efficiency. At present, foreign deformation heat treatment and induction heat treatment processes have been widely used in actual production, and the online spring steel wire has undergone oil quenching-tempering heat treatment process.
The development trend of high quality cold heading steel. The demand for high-precision, high-strength fasteners, steel joints and non-standard shaped parts in the automotive industry is increasing, so the demand for high-purity, high-performance, high-quality cold heading steel is also more urgent. Technological advances play a key role in transforming fasteners into versatile high-precision automotive parts.
The development direction of cold heading steel is non-tempered steel, boron steel and ultra-fine grain steel, while heat-treated non-tempered cold-rolled steel has attracted much attention. Non-tempered cold-rolled steel can eliminate the spheroidizing annealing before cold drawing and the quenching and tempering after forming by using the method of strengthening and toughening such as microalloying, controlled rolling and controlled cooling, etc. The decarburization tendency of the threaded portion improves the yield.
The characteristic values ​​for evaluating the technical quality of high quality cold heading steel include cold heading, decarburization layer, surface quality and the like. The goal of cold heading control is 1/5, and the goal of decarburization control is to reduce the diameter of the hot-rolled decarburized layer by 0.3% D.
The development trend of high quality bearing steel. Bearing steel is mainly used to manufacture rolling elements and ferrules for rolling bearings. Bearings should have long life, high precision, low heat, high speed, high rigidity, low noise, high wear resistance, etc. Therefore, bearing steels should have high hardness, uniform hardness, high elastic limit, high contact fatigue strength, Requires toughness, certain hardenability, and corrosion resistance in atmospheric lubricants. In order to achieve the above performance requirements, the chemical composition uniformity of bearing steel, the content and type of non-metallic inclusions, the particle size and distribution of carbides, and decarburization are strictly required. Bearing steels generally develop in the direction of high quality, high performance and many varieties.
The quality of the bearing material is evaluated primarily by the purity and uniformity of the material. The purity of a material means that there are as few inclusions as possible in the material. The uniformity of the material means that the inclusions and carbide particles in the material are fine and diffuse. Specifically, bearing steels are mainly developed in two directions: high cleanliness and performance diversification. Increasing the cleanliness of the bearing steel, especially reducing the oxygen content in the steel, can significantly extend the life of the bearing. The pursuit goal of bearing steel oxygen content control is 5ppm. The goal of oxide inclusion control in steel is 10m for inclusions, 5ppm for titanium content control, and 30ppm for nitrogen content control.
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