Development direction of steel for automotive fasteners
Abstract: Introduce the performance requirements of steel for automotive fasteners, as well as the characteristics of smelting and rolling; the current status of related clean bolt steel, microalloyed non-tempered steel, annealing-free steel, boron steel, and high-strength steel with delayed fracture Explain the direction of development; point out the efforts to implement localization of steel for automotive fasteners.
Keywords: automotive fasteners, microalloyed, non-tempered steel, annealed steel, steel, delayed fracture, high strength steel
According to the statistics of China Association of Automobile Manufacturers in January 2010, China's automobile production and sales in 2009 were 13.79 million and 13.64.48 million, respectively, up 48% and 46% respectively. The rapid development of the automotive industry has increased the demand for steel materials. Although China has been the world's largest steel producer for many years, the steel for engine and key component fasteners is mainly based on imported products. The development trend of high performance and light weight of automobiles puts higher demands on the design stress and light weight of automotive fasteners. The high strength of steel for fasteners is the most effective way to solve this requirement.
1. Performance requirements for steel for automotive fasteners
Steel requirements for automotive fasteners:
(1) Good surface quality, high level of dimensional accuracy and low grade non-metallic inclusions and segregation;
(2) It has high tensile strength and good cold heading performance;
(3) It has high fatigue resistance and multiple impact tensile resistance;
(4) It has sufficiently low delayed fracture sensitivity and a low tough-brittle transition temperature.
2. Characteristics of smelting and rolling of steel for automotive fasteners
2.1, smelting technology
It adopts refining outside the furnace and continuous casting process with electromagnetic mixing. Mainly to control the main elemental components such as C, Si, Mn, Cr, Mo in steel to a small range, and to reduce the content of impurities such as P, S, 0, N in steel to achieve high purity of steel. The purpose of low non-metallic inclusions and low segregation level is to improve the cold heading performance of steel and improve the surface quality of steel.
2.2, rolling technology
A high-speed, high-precision rolling process with controlled milk and controlled cooling is used to obtain a rolled heat treated wire (non-tempered or annealed) with good dimensional accuracy (no need for drawing) and as few as possible Surface defects (no need to peel).
3. Current status and development direction of steel for automotive fasteners
3.1, clean bolt steel
In terms of improving the coldness of steel and improving steel, it is necessary to reduce the content of impurity elements in steel as much as possible. Reducing the S content can improve the deformability of the steel, reduce the non-metallic inclusions in the steel, and improve the ductile plasticity; lowering the P content can reduce the deformation resistance of the steel, while reducing the P, S content can also reduce the segregation at the grain boundary. It can reduce the grain boundary embrittlement and improve the delayed fracture resistance of steel. Reducing the 0 content in steel can effectively reduce oxide inclusions and improve the cold working deformation ability of steel.
The Research Institute of Iron and Steel Research conducted a study on the stress corrosion threshold factor KISCC of ML42CrMo steel with high cleanliness. The KISCC of clean steel has a larger margin than that of commercial steel. Therefore, in order to ensure the steel after high strength With high comprehensive performance, the P and S content of high strength bolt steel should be further reduced.
3.2, economical high-strength bolt steel
3.2.1, microalloyed non-tempered steel
The bolts are made of microalloyed non-tempered steel, which can omit the spheroidizing annealing before bolt cold drawing and the quenching and tempering after bolt forming. It can also reduce the decarburization tendency of thread thread and improve the yield of bolts. The economic benefits are very obvious. .
The microalloying elements commonly used in microalloyed non-tempered steels are: Ti, Nb, V, B, and the like. In the heating process before the milk, the microalloying elements should be fully dissolved, and then the fine microalloyed carbonitrides are precipitated during the control of the milk and cooling process, and the austenite grain growth is suppressed to obtain fineness. The grain can improve the toughness and plasticity while improving the strength of the steel. It is worth mentioning that B, trace B can significantly improve the hardenability of steel, and has a significant retardation effect on pearlite transformation, combined with low-temperature rolling, is conducive to the increase of ferrite content, thereby improving the cold heading of steel. performance. The main effects of microalloying elements in steel are shown in Table 1.
The microstructure of microalloyed non-quenched and tempered steel is low carbon manganese-containing ferrite + pearlite type and bainite type, which is due to the role of microalloying elements in grain refinement and the precipitation of carbon and nitrogen in ferrite and pearlite. The strengthening phase of the compound enables the steel to obtain the mechanical properties of the medium Carbon Steel after quenching and tempering without being quenched and tempered. Microalloyed non-tempered steel is mainly used to process 8.8 high-strength bolts. The steel grades that have been applied at home and abroad are: Kobe KNCH8S in Japan, Sumitomo Metal SUC80 in Japan, LF10MnSiTi in China, LF20Mn2, LF18Mn2V and MFT8 produced by Maanshan Iron and Steel Co., Ltd. There are also a small number of 10.9 studs made of microalloyed non-tempered steel. For higher strength grades, such as the 12.9 grade 髙 strength bolts, there are no examples of microalloyed non-tempered steels.
By achieving temperature rolling in the two-phase zone, cold-strengthening non-quenched and tempered steel is moving from microalloying to deformation-induced ultra-fine microstructure, with steel without or with the addition of microalloying elements. It can be used for cold-rolling and processing of high-strength fasteners. It not only saves resources, energy, but also protects the environment. Therefore, the development of energy-saving and environment-friendly steel for the production of high-strength fasteners is a wire rod and rod in the near future. The development direction of material production technology.
3.2.2, free of annealed medium carbon steel or medium carbon alloy steel
Steels that are usually hot-rolled are often subjected to spheroidizing annealing to meet the requirements of cold-fill forming. The purpose is to spheroidal carbides (cementite) in the microstructure of the steel by spheroidizing annealing. A material having low deformation resistance and excellent plasticity is obtained. In the case of annealed medium carbon steel or medium carbon alloy steel, the spherical cementite is directly precipitated from the supercooled austenite under certain conditions. It is proved that the precipitation of cementite from the parent phase in the form of flakes during the transformation of austenite to pearlite is a metastable state. Theoretically, there is a tendency to spontaneously transform the spherical cementite which is more stable to thermodynamics. This Wu Fan et al. also determined the activation energy required for the conversion of flaky cementite into spherical cementite.
Steady-state spherical cementite cannot be precipitated from steady-state austenite. According to research, low carbon steel undergoes deformation slightly higher than Ar3 at high strain rate and large strain, and ultra-fine ferrite of 2 μm or less can be obtained, thereby affecting the decomposition mode of unaltered austenite around it. The austenite accelerates the transformation to the degraded pearlite during subsequent cooling or annealing and slowly coarsens. For medium carbon steel with high pearlite content, the austenite is unbalanced by controlled rolling, and then it is possible to obtain spherical cementite instead of sheet cementite by subsequent controlled cooling. Online softening treatment.
Conventional high-speed wire rod mills are difficult to achieve in-line softening treatment because of the inability to perform low temperature and large deformation controlled rolling and to control the cooling line to be too short.
Japan's Kobe Steel Corporation's 7th Wire Plant remodeled the equipment in 1999, adding a heavy-duty capacity reduction sizing unit and increasing the Steyr air-cooled transportation line from the original 48 m to 100 m. The modified wire rod mill successfully achieved online softening treatment, and the strength of SCM435 cold-rolled steel wire rod produced was below 800 MPa, which was lower than the traditional 900 MPa.
Domestic Maanshan Iron and Steel Co., Ltd. has studied the on-line softening treatment process of high-strength cold-rolled steel earlier. The SWRCH35K-M-free annealing steel produced by the user can directly pull the cold heading production. Anyang Steel's high-speed wire production line adopts Morgan's 6th generation rolling technology and Steyr's air-cooled transportation line with a length of 105 m. After years of exploration, the SCM420-440 series of high-strength cold-rolled steel has also reached the requirements of online softening treatment. The effect, taking SCM435 as an example, the total mass fraction of electrolytic inclusions is 0.005 8%, the center looseness and the central shrinkage level are 0.5, and the hot rolled microstructure is mainly ferrite + pearlite, and the total decarburization depth is less than 0.15 mm, φ6.5~16mm, the grain size of each specification is above 8 and the hardness HRB is about 90. The user can implement a 9 h simplified annealing process, which is more than 2/3 less than the spheroidizing annealing process with more than 30 h.
3.2.3, boron steel
Characteristics of boron steel:
(1) It has good cold deformation ability and can eliminate the annealing treatment before cold deformation;
(2) low quenching and brittleness tendency, which can be quenched with water;
(3) Addition of trace boron may partially replace the addition of expensive alloying elements;
(4) The delayed fracture sensitivity of low carbon steel is relatively low.
The basic principle of boron steel composition design is to reduce the carbon content and improve the cold deformation ability of steel; add a trace amount of boron with a mass fraction of 0.0005% ~ 0.003 5% to compensate for the loss of strength and hardenability due to carbon reduction. Further, alloying elements such as Cr, Mn, and Ti may be added as needed to further improve hardenability.
Usually 8.8 bolts are made of low carbon manganese-containing boron steel equivalent to SAE10B23, and grades 9.8 and 10.9 bolts are made of medium carbon manganese-containing boron steel equivalent to SAE10B35, or ML35MnB, ML370B boron equivalent to ML40Cr and ML35CrMo. Made of steel. However, due to the low tempering softening resistance of boron steel, the tempering temperature is 60-80 lower than that of SCM435 and ML40Cr. Therefore, the 10.9 grade high-strength bolt made of boron steel has high sensitivity to delayed fracture.
The domestic solution to this problem is to improve the comprehensive mechanical properties of boron steel by increasing the V content in boron steel while reducing the P, S and C contents. In China, ML15MnVB and ML20MnVB steel were successfully developed in 1992 and listed in GB/T 6478-2001 "Cold for cold heading and cold extrusion" for the manufacture of engine connecting rod bolts, cylinder head bolts and high-strength bolts for motorcycles. The quality is excellent, and the delayed fracture resistance in the tensile strength range of 1000 ~ 1200MPa is equivalent to or better than SCM435 steel.
Japan's Datong Special Steel has recently developed a 10.9 grade steel for bolts with delayed fracture. Its composition is: w(C) = 0.25%, w(Si) = 0.03%, w(Mn) = 1%, w( P)=0.01%, w(S)=0.002%, w(Cr)=0.3%, w(Ti)=0.05%, w(Nb)=0.025%, w(B)=0.002%, the rolled structure is Low-hardness ferrite + pearlite, due to the reduction of the impurity element P, S content, and the addition of Ti, Nb to refine the grain, the steel has a delayed fracture resistance comparable to or better than 1000-1300 MPa. SCM435 steel has been used to manufacture 10.9 grade automotive bolts.
Domestic Anyang Steel Plant has also successfully developed and produced 10B21 series boron steel according to American Standard, its w(N) is 0.0022% -0.0054%, the average value is 0.0038%; w(0) is 0.0016% -0.0036%, the average value is 0.0028%. The hot-rolled microstructure is ferrite + pearlite, the total decarburization depth is less than 0.15 mm, the grain size of the wire rod is above 8 and the pass rate of 1/2 cold forging is 100%. However, this steel type sometimes has dense corners and small cross cracks during continuous casting, which produces a large amount of crusting on the surface of the wire rod, and the yield is not high. Therefore, the large-scale production of 10B21 series boron steel in Anyang Steel Plant remains to be seen. day.
3.2.4 High-strength steel resistant to delayed fracture
Delayed fracture refers to the sudden brittle failure of a material under static stress after a certain period of time. It is an environmental embrittlement caused by material-environment-stress interaction and is a form of hydrogen-induced deterioration of the material. When the strength exceeds 1200 MPa, the delayed fracture of the high-strength bolt steel becomes very prominent, and the resulting accidents occur frequently. Therefore, the development of high-strength bolt steel with delayed fracture resistance is one of the hotspots of research work at home and abroad in recent years.
Most of the strength bolts are made of tempered martensite, which is more sensitive to delayed fracture. Considering the influence of organizational factors on the delayed fracture resistance of high-strength bolts, an appropriate isothermal treatment process can be used to obtain the complex structure of lower bainite and appropriate martensite and retained austenite, using martensite structure. High strength and good delayed fracture resistance of bainite and austenite structure to achieve good delayed fracture resistance at the strength of the crucible. Or obtaining a fine martensite structure having no grain boundary or less grain boundary carbide by a suitable heat treatment process, and also having good delayed fracture resistance.
At home and abroad, the following measures are often taken in the development of delayed fracture resistant steels.
(1) Reduce grain boundary segregation. The P and S contents of the impurity elements are reduced as much as possible, and the Mn content is lowered to prevent grain boundary embrittlement (P reduces the grain boundary bond strength, S promotes hydrogen absorption in a corrosive environment, and Mn easily causes P and S co-segregation).
(2) Refine the crystal grains. Adding elements such as Ti, Nb, V, and Al to form a dispersed carbonitride to refine austenite grains, improve strength, and improve pavability.
(3) Increase the tempering temperature. Adding elements Mo, V, etc., which are highly resistant to tempering and softening, raise the tempering temperature to avoid the tempering temperature region which easily causes grain boundary embrittlement, and make the carbide fine and uniform.
(4) Adjust the alloying elements. Such as adding Ni, reducing Mn, to obtain higher notch toughness.
(5) Minimize the amount of hydrogen invading the steel surface, that is, add an alloying element that inhibits the formation of corrosion pits, such as Mo.
(6) Harmless intrusion of hydrogen. An appropriate amount of microalloying elements Ti, Nb, and V are added to form fine carbonitrides, thereby uniformly distributing hydrogen and suppressing diffusion of hydrogen.
In recent years, a series of high-strength bolt steels with excellent delayed fracture resistance have been successfully developed at home and abroad. For example, the ADS series of Sumitomo Metals of Japan and the KNDS series of Kobe Steel are all added by microalloying elements and adjusting their contents. And the ratio to obtain the strength of the crucible and the excellent delayed fracture resistance, the use effect is significantly better than the SCM440 steel. Under the guidance of the Iron and Steel Research Institute of China, Dalian Steel Works successfully developed the 1300 MPa grade 髙 strength bolt steel 42CrMoVNb (ADF1) on the basis of 42CrMo steel by reducing the S, P, Si, Mn content. The mechanical properties have been greatly improved, and the 13.9 ~ 14.9 high-strength bolts have been used on Iveco and Cummins engines.
4, the conclusion
The rapid development of China's auto industry has brought huge market opportunities for domestic automotive fastener steel, and the high strength of automotive fastener steel has become an inevitable trend in the market development. Chinese steel enterprises with conditions should accelerate their cars. The research and development and production of steel for fasteners, drawing on the advanced production technology at home and abroad and the achievements already made, improve the localization rate of steel for automotive fasteners.
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