Analysis on the development trend of steel technology for auto parts

In 2013, China’s automobile market demand was around 20.8 million, with a growth rate of 7%. It is estimated that the total number of automobiles in China will reach 23.5 million in 2015 and 25 million in 2020. The development of automobiles is moving in the direction of high power output, lightweight, high performance, long life, noise reduction, stable operation, safety and reliability, environmental protection, energy saving, economical and low cost, easy processing, and multiple varieties. Steel for auto parts is a key part of a car, and a manufacturing material for the core components to ensure the running performance of the car. The development direction of automobiles puts forward higher requirements for the development of steel for auto parts.

Product requirements: high toughness, high precision, energy-saving and environmentally-friendly auto parts steels are used in the four major systems of automobiles:

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Steel for engine system: The engine is the source of power for the car. Typical parts include crankshafts, connecting rods, cams, etc., mainly made of non-quenched and tempered steel, fuel injection systems are mainly made of steel for fuel pump nozzles, and valve steels are mainly made of valve steel.
Transmission and transmission system steel: The transmission and transmission system is the soul of a car, in which gear rings, gear hubs, gears, and gear shafts are mainly gear steel. The transmission system is more like the waist and spine of an animal, including transmission shafts, half shafts, front shafts, etc. The transmission shaft is mainly made of steel.
Suspension and steering system steel: The suspension system is the leg 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 joints, steering gears, hubs, bearings, etc. The hubs and bearings are mainly bearing steel, and the universal joints, ball joints, and steering gears are mainly gear steel.
Standard parts system steel: mainly cold heading steel for fasteners.
Automotive industry application system requirements: auto parts have the characteristics of high performance, long life, easy processing, low noise, high surface accuracy, and high strength; to achieve lightweight and compact cars, and reduce CO2 emissions by improving efficiency.
The requirements of the automotive industry on materials are: auto parts steel has the characteristics of high strength and toughness, easy cutting, high dimensional accuracy, low alloying, environmental friendliness, and omission of heat treatment. See the attached table for the development trend of common technologies for the development of steel materials for auto parts.
Technology trends: precise control, uniform composition, microalloying
The technological progress of automobiles has driven the technological development of auto parts, and the technological development of parts has promoted the technological progress and technological innovation of automobiles.
High-quality gear steel, high-quality spring steel, high-quality non-quenched and tempered steel, high-quality fastener steel, high-quality bearing steel, and high-quality valve steel are the steels for automotive parts that require special alloy steels. Representative of key materials. High-quality special steel for auto parts is developing in the direction of lightweight, high performance, long life, stable operation, low noise, safety, energy saving, low cost, easy processing, and multiple 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.
The development trend of high-quality gear steel. High-quality gear steel has been continuously developed in terms of variety, easy cutting, environmental protection, low alloying to reduce costs, low noise, stable operation, good steel uniformity, and small heat treatment deformation.
The technical quality characteristic value of the evaluation gear steel is concentrated in the aspects of hardenability, purity, grain size, band structure and so on.
The level of hardenability and the stability of the hardenability value are important indicators for evaluating the quality of gear steel. The pursuit goal of hardenability zone control is 3HRC.
The impact of gear steel cleanliness on gear fatigue life has attracted more and more attention. The inclusion of oxides and sulfides 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 car carburized gears. The pursuit goal of gear steel oxygen content control is [O]10ppm. The addition of Ti and Ca is prohibited in high-quality gear steel, and Ti0.01% and Ca0.0005% in steel are stipulated. The pursuit goal of non-metallic inclusion control is A2 level. , B2 level, C1 level, D1 level.
The grain size is an important indicator of gear steel. The fine and uniform austenite grains in the gear steel are quenched to obtain a fine martensite structure, which significantly improves the fatigue performance of the gear and reduces the deformation of the gear after heat treatment. The fine and uniform austenite grain size has a special contribution to the strength and toughness of the parts. In particular, it is of great significance to improve the fracture toughness of gears and enhance the brittle fracture resistance of gears. The grain size of gear steel is required to be level 6. At present, in order to obtain high temperature (>960℃) carburized gear steel in the world, the microalloying elements Nb, V, Zr, etc. are added or compounded during smelting to form alloy carbonitrides in the steel, and the grain size of the steel is 8 grade.
The band structure is a structural defect of steel. For gear steel, severe band structure will affect the uniformity of carburizing, increase the degree of quenching deformation, and make carburized gears poor in dimensional accuracy. Therefore, the pursuit goal of band structure control of gear steel is not more than level 2.
The development trend of high-quality non-quenched and tempered steel. Non-quenched and tempered steel is the addition of microalloying elements (V, Ti, Nb, Al, B or N) to medium and low carbon steel or medium and low carbon manganese steel, through controlled rolling (forging) and controlled cooling process, to give full play to the precipitation The effects of strengthening, fine-grain strengthening and phase change strengthening make the steel need not be quenched and tempered after hot rolling (forging), and its strength and hardness can reach the level of quenched and tempered steel, and it is a high-efficiency and energy-saving steel with a certain degree of plasticity and toughness. Non-quenched and 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., presents a trend of gradually replacing quenched and tempered steel with non-quenched and tempered steel. Among them, starting from reducing processing steps and improving productivity, the selection of steel materials for connecting rods is showing a trend of gradually replacing non-breaking materials with breaking materials.
The characteristic values ​​for evaluating the technical quality of non-quenched and tempered steel include carbon equivalent, carbon segregation, oxygen content, grain size, decarburized layer, mechanical properties, ferrite content, inclusion level, etc. For example, the pursuing goal of carbon equivalent range value control is 0.02%. The pursuing goal of carbon segregation control of the full cross-section of steel is 0.03%. The pursuing goal of austenite grain size control is not coarser than 8. The microstructure should be pearlite + ferrite. The decarburization layer requires the depth of decarburization on each side of the steel (ferrite + transition layer). The pursuit goal of control is 0.5% of the steel diameter. The goal of oxygen content control is 15ppm. The goal of band structure control is 1.5.
The development trend of high-quality spring steel. The two main factors affecting spring design stress are fatigue resistance and elasticity reduction performance, which have become the subject of current spring steel research and development. 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 and plastic, that is, the tensile strength is 2000 MPa, and the reduction of area is 50%; high fatigue strength and resistance Corrosion fatigue performance; excellent elasticity resistance.
In addition, high-quality spring steel requires good economy. The research and development of its new steel grades, on the one hand, 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 Under the condition of change, ultra-high strength can be achieved through processes such as thermomechanical heat treatment, induction heat treatment and online heat treatment under the premise of fully ensuring economic efficiency. At present, foreign deformation heat treatment and induction heat treatment processes have been widely used in actual production, so that in-line spring steel wires are subjected to an oil quenching and tempering heat treatment process.
The development trend of high-quality cold heading steel. The automotive industry has an increasing demand for high-precision, high-strength fasteners, steel structure connection pairs and non-standard shaped parts. Therefore, the demand for high-purity, high-performance, and high-quality cold heading steel is more urgent. In the process of transforming fasteners into multi-purpose high-precision auto parts, technological progress has played a key role.
The development direction of cold heading steel is non-quenched and tempered steel, boron steel and ultra-fine grained steel, and non-quenched and tempered cold heading steel without heat treatment has attracted much attention. Non-quenched and tempered cold heading steel adopts microalloying, controlled rolling and controlled cooling and other strengthening and toughening methods. In the process of processing fasteners, the spheroidizing annealing before cold drawing and the quenching and tempering treatment after forming can be omitted, which can also reduce The decarburization tendency of the threaded part improves the yield.
The characteristic values ​​for evaluating the technical quality of high-quality cold-headed steel include cold-heading properties, decarburization layer, surface quality, etc. The pursuing goal of cold heading control is 1/5, and the pursuing goal of decarburization layer control is the diameter of hot-rolled decarburization layer 0.3%D.
The development trend of high-quality bearing steel. Bearing steel is mainly used to manufacture rolling elements and rings of rolling bearings. Bearings should have the characteristics of long life, high precision, low heat generation, high speed, high rigidity, low noise, high wear resistance, etc. Therefore, the bearing steel is required to have high hardness, uniform hardness, high elastic limit, high contact fatigue strength, The necessary toughness, certain hardenability, and corrosion resistance in atmospheric lubricants. In order to meet the above performance requirements, the chemical composition uniformity of the bearing steel, the content and type of non-metallic inclusions, the size and distribution of carbides, and the decarburization are strict. Bearing steel generally develops in the direction of high quality, high performance and multiple varieties.
Evaluation of the quality of bearing materials mainly depends on the purity and uniformity of the materials. The purity of the material means that there are as few inclusions in the material as possible. The uniformity of the material means that the inclusions and carbide particles in the material are small and dispersed. Specifically, bearing steel mainly develops in two directions: high cleanliness and diversified performance. Improving the cleanliness of bearing steel, especially reducing the oxygen content in the steel, can significantly extend the life of the bearing. The goal of oxygen content control in bearing steel is 5ppm, the goal of oxide inclusion control in steel is the inclusion size of 10m, the goal of titanium content control is 5ppm, and the goal of nitrogen content control is 30ppm.