Automotive industry

Development status of oil quenched and tempered spring steel wire for automotive use

The automotive industry is the largest user of spring steel, consuming about 60% of spring steel production. Oil-quenched and tempered spring steel wire, which is currently the ideal material for valve springs and suspension springs in automobiles, is also receiving more and more attention. This paper discusses the latest research progress and development of oil-quenched and tempered spring steel wire for automotive applications in terms of chemical composition, metallurgical quality, cross-section shape, surface quality and heat treatment process.

First, the use of oil quenched and tempered spring steel wire, performance requirements and production process

1.1 automotive oil quenched and tempered spring steel wire use car suspension springs: suspension springs are repaired or peeled cold-drawn steel wire and then quenched and tempered spring steel wire. Suspension springs in actual use to be frequently subjected to high stress impact loads, so it requires good fatigue resistance, shock resistance and resistance to elasticity reduction.

Engine valve springs: valve springs belong to the key parts in the internal combustion engine, its role is to control the engine intake and exhaust. Because of its use in high speed, high stress and high temperature, so its bearing capacity and elasticity reduction indicators for the safety of the car and engine life are critical.

1.2 Requirements for automotive oil-quenched and tempered spring steel wire

The development trend of automotive valve springs and suspension springs is to improve the use of stress, reduce their weight, reduce the size and improve the fatigue life. Requirements for spring steel wire is: ① improve the chemical composition of steel and improve the fatigue strength of steel wire, hardenability, temper resistance and adaptability to high-temperature environments; ② improve the cleanliness of the steel and change the type of inclusions, limit the number of inclusions and the size of the spring in order to overcome the early failure of the spring; ③ to improve the fatigue life, general fatigue of the domestic spring is required (2.3 ~ 5.0) x 107 times, while the products of outstanding foreign companies have generally reached 5.5 to 5.0 times, the fatigue life of spring steel wire, the fatigue of spring steel wire is required (2.3~5.0) x 107 times, while foreign companies have generally reached 5.5 to 10 times. Excellent enterprise products have generally reached 5 × 108 times; ④ refine the grain, improve the yield strength and toughness to improve the spring spring performance; ⑤ change the shape of the cross-section to obtain the spring loaded with the ideal stress distribution; ⑥ elimination of surface scratches and cracks, reduce the decarburisation layer in order to prolong the service life in the high-stress conditions.

1.3 oil quenching and tempering spring steel wire production process

1.3.1 Suspension spring oil-quenched and tempered steel wire production process Raw materials → surface treatment → drawing and sizing; specifications of steel wire → heating → oil quenching → tempering treatment → flaw detection → oiling → winding → inspection → packaging → storage.

1.3.2 Valve spring oil quenching and tempering steel wire production process

① The production process of steel wire to be stripped is as follows: raw material → stripping → intermediate annealing → surface treatment → drawing; specifications of steel wire → oil quenching → tempering → flaw detection → oiling → reeling → inspection → packaging → storage.

② not peeled steel wire production process with 1.3.1.

Second, oil quenching and tempering spring steel wire properties of the research status quo

2.1 chemical composition and properties of oil-quenched and tempered spring steel wire has undergone three stages: Si.Mn system → Cr.V system → Cr-Si system. At present, the main Cr-V system, Cr-Si system is mainly.

With the development of the automotive industry, the fatigue strength of the spring and spring reduction performance requirements are increasingly high, from Table I is not difficult to see, as a spring to improve the strength of the element C, the overall trend of its content is to gradually increase. The alloying elements have gradually changed from Si and Mn to Cr, V and Cr, Si, the main reason for which is to increase the fatigue strength, corrosion resistance and spring resistance. 2.1.1 Influence of chemical composition on the fatigue properties of springs

Springs are parts that work under alternating loads, and their damage is mainly in the form of fatigue fracture. Therefore, the fatigue strength of springs is particularly important. Many scholars have carried out a lot of research in this area, and obtained some results.

In order to improve the strength of CrSi steel in the Si content increased to 2.15%, and the addition of ω (Ni) = 2.0%, ω (V) = 0.2%, so that the fatigue strength increased to 1300MPa in 570 ~ 1280MPa stress fatigue life of more than 500,000 times. At the same time the relaxation resistance has also been improved, better than SAE9254. Garphyttan company introduced a higher strength OTEVAN90, tensile strength of 2100MPa, is because of the material in the Si-Cr steel on the basis of the addition of Ni, so that the material toughness significantly improved.

HiromuIzumida et al. in the study of austenite grain size on the fatigue strength of high Si oil tempered steel wire fatigue strength found that: compared with the conventional high-strength alloys oil quenched and tempered steel wire, high Si oil quenched and tempered steel wire fatigue strength is higher. Mainly due to the refinement of austenite grain size smaller martensite. The fatigue strength of the wire is increased. Therefore, by controlling the carbide precipitation refinement can be obtained with higher fatigue limit and better toughness of the wire. The addition of V and Nb to SAE9260 produces precipitation strengthening and effectively prevents the growth of austenite grains, thus increasing the tensile yield strength and torsional yield strength, and increasing the service stress by 98 MPa compared with that of SAE9260. In addition, the addition of Ti generates TiC precipitates in the steel below the micron level. It can absorb the diffusible hydrogen, so that it does not diffuse, thus reducing the effect of hydrogen embrittlement. Corrosion fatigue test results show that: in the S, . = 1200MPa conditions, the fatigue life of UHSl900 steel more than 500,000 times, and SAE9260 fatigue life (S., = 1000MPa) is equivalent to use. Nitriding and shot peening is also an important way to improve the strength of steel wire, through the stress shot of spring steel wire in the ‘90% fatigue life reliability’ of this indicator than conventional shot peening can be increased by a maximum of 6.5 times, while the surface nitriding not only improve the surface hardness of the spring (HV>900), to improve the abrasion resistance of the spring. More importantly, it can introduce a higher residual stress field. This improves the fatigue resistance of the spring. Wan Jiekui et al. SWOSC-V, OTEVA a 75SC spring 450 ℃ nitrogen and carbon co-penetration. So that the spring fatigue performance has been improved. In addition, SWOHS-V high-temperature nitriding and shot peening treatment. Its fatigue performance and only nitriding treatment of SWOSC-VTS compared to improve 25%.

2.1.2 Influence of chemical composition on spring elasticity reduction

In addition to fatigue strength. Elasticity reduction is also one of the important performance parameters of springs. Improvements in spring resistance can be achieved by increasing the strength of the spring blank and by increasing the temper resistance of the steel.

KawakamiH et al. found that Si.Cr-V steel with mass fractions of Si, Cr, and V of 1.5%, 0.5%, and 0.2%, respectively, had good spring resistance. In addition, the addition of 0.01% to 0.025% of N in the steel can form a fine AIN grain, so that the spring relaxation resistance is improved. V-containing steel due to V tempered at 450 ℃ above the second precipitation hardening will occur. Improved tempering resistance, so that the spring resistance to spring reduction improved.

2.2 Metallurgical quality and performance

The quantity and particle size of non-metallic inclusions is one of the most important factors affecting the performance of springs under alternating stress. In recent years. In the steelmaking widely used in secondary metallurgy and other technologies to produce spring steel, so that the mass fraction of oxygen in the steel is less than 15x10-6, to reduce the fatigue performance of the non-metallic inclusions content, change the inclusions morphology and particle size, to improve the fatigue limit of the spring. Usually used smelting process has electric furnace + furnace refining and converter or converter + furnace refining. Electric furnace + furnace refining can improve the purity of steel, in the electric furnace smelting based on the addition of furnace refining process. The purpose of OT refining is to reduce the number of inclusions in the steel, lower the level of inclusions, fine-tune the composition of the steel (C, Mn, Si, Cr, etc.), and remove the harmful elements in the steel (e.g., S, P, etc.). Converter and converter + furnace refining can reduce the impurities and impurity elements brought by the scrap, improve the purity of spring steel raw materials, coupled with the furnace refining measures, can produce high quality spring steel. Comparison of the two, the former can control the oxygen content of 21 rng/m3, but the amount of nitrogen even after vacuum treatment also reaches 75 ~ 150mg/m3, while the content of impurity elements is higher; the use of converter smelting, as a result of the use of blast furnace iron to join the amount of scrap is small, the purity of molten steel to improve the nitrogen content can be controlled in the 38mg/m3 below. At present, our country from Japan and Germany import raw materials are used in converter smelting. Ultra-pure spring steel production is based on the needs of automobile engine valve springs and the development of a smelting technology. After ultra-pure smelting of spring steel inclusions inclusions in the maximum size (or thickness) is less than 15μm, and is diffusely distributed. Thus, the inclusions on the spring fatigue life is reduced to the maximum possible, to achieve the harmless steel wire inclusions, so that the performance of the steel has been improved.

2.3 surface quality and performance

In steel wire, surface defects seriously affect the quality of steel wire, reducing the fatigue life of steel wire. These defects mainly include scratches, cracks, folds and pits. Their presence will cause stress concentration, so that the actual stress on the surface increases and the formation of fatigue failure of the initial point. Therefore, surface defects must be eliminated in order to extend service life under high stress conditions. In order to improve the surface quality of the spring, people have taken a variety of measures, such as billet grinding, low-temperature rolling (to reduce decarburisation), post-rolling control of the cold, etc.; peeling or grinding of the coil can be removed cracks and decarburisation layer and get a smooth surface; non-destructive flaw detection of the specified defects above the depth of the defect due to the drawing or loading and unloading of transport caused by the defects of the scratches to be identified and marked so that in the rolled into a spring can be easily picked out; some springs are also Electrolytic polishing to further improve the surface quality of the spring.

2.4 Wire cross-section shape and properties

In addition to heat treatment, spring manufacturing process and other considerations, the shape of the cross-section of spring steel wire research has been carried out for many years. Round cross-section of steel wire made of compression coil spring inner ring radial bearing the maximum shear stress, along the circumference of the circle quickly decreases, resulting in a very bad stress distribution. Therefore, a circular cross-section is not an ideal cross-section and does not allow the material to be used to its full potential. The elliptical cross-section also did not attract more interest, and then proposed a semi-circular and semi-elliptical cross-section and improved, and now this cross-section of steel wire has been better used in valve springs. This cross-section to avoid the maximum stress peaks in the inner ring, the stress distribution curve tends to flatten, but there are still differences in stress around. Another advantage of the ovoid cross-section is that it is smaller in height than the round cross-section, thus increasing the number of active coils and making the spring more flexible than a spring with a round cross-section at the same stress. Different combinations of short and long axes as well as elliptical edges inwards or outwards can give different benefits in terms of stress or deflection. Therefore the search for the optimum cross-section has to be continued. The domestic demand for valve springs made of steel wires of ovoid cross-section is also increasing from year to year.

2.5 Oil quenching and tempering equipment, production technology and performance

Oil quenching and tempering is the production of oil quenching and tempering spring steel wire the most important process, which determines the final organisation and performance of spring steel wire. In recent years, oil quenching and tempering equipment and production technology has gone through the following stages.

2.5.1 Heating technology Production of spring steel wire, the choice of reasonable heating technology is to obtain a good tempering flexural organisation of the basic guarantee. At present the main use of heating technology has a tube furnace heating technology and induction heating technology.

(1) tube furnace heating technology: tube furnace heating benefits are stable and easy to control the process, energy consumption is small, the raw material of the original organisation adaptability, but covers a large area. At present, Japan's Sugita's suspension spring steel wire and Sweden's valve spring steel wire plus Feiteng are used in the tube furnace heating.

(2) induction heating technology: induction heating production of spring steel wire with a high degree of integration of equipment, covers an area of less, fast heating speed and other advantages, but its application of frequency, feed speed, quenching and heating temperature, insulation, quenching medium, tempering temperature, tempering insulation, tempering cooling medium after the choice is extremely important. At present, in the induction heating technology research has also achieved some results. Li Shuliang and others on the induction heating temperature control of the two ways and system components were studied. It is found that the temperature control system can save raw materials, heat evenly and reduce labour intensity after establishing a reasonable mathematical model and closed-loop control. Li Gui change et al. through the transformation of induction heating equipment, to achieve the refinement of grain, improve the organisation, reduce stress, improve product performance, reduce energy consumption, and greatly improve the mechanical properties of the product. In addition, through the induction heating equipment power supply frequency research. Xiao Jin and others have designed a high power factor; adjust the power paradigm with large; low switching losses; high output efficiency; power supply structure is simple: anti-interference ability of induction heating power supply.

In addition to the above two heating technology, there are a number of heating technology is also used in the production of oil quenching and tempering spring steel wire. The use of electric contact rapid heating technology. But the steel wire after heat treatment to obtain ultra-fine grain, and make the tempered flexural organisation micronisation. Su Deda et al. rescue found with electric contact rapid heating, graded oil quenching and tempering continuous treatment of 4Cr13 steel wire should be selected 1070 ~ 1200 ℃ oil quenching. And with homemade electric contact rapid heating continuous oil quenching and tempering equipment trial production of 4Cr13 oil quenched steel wire, its use of good results. Electron beam or laser heating technology. It has the characteristics of very fast heating speed, high phase transition temperature, very short austenitising time and very fast quenching and cooling time. Through the study of laser heating surface quenching, Liu Li et al. found that the wear resistance of the steel heated and quenched by laser is significantly higher than that of the steel without laser treatment, and the grain on the surface of the treated steel is significantly refined.

2.5.2 Tempering method

At present, in the spring steel wire production process, lead bath tempering is one of the more commonly used tempering methods. In addition, there are also tempering with fluid particle furnace.

Lead bath tempering although lower cost, easy to achieve production, but its pollution is more serious. Fluidised particle furnace is an efficient, energy-saving, non-polluting, flexible heat treatment furnace. Its application and development are getting more and more attention.

III. Conclusion

This paper mainly outlines the development status of oil quenched-tempered spring steel wire for automobiles in terms of chemical composition, metallurgical quality, cross-section shape, surface quality and heat treatment process. In summary, the development and production of spring steel wire from the raw materials, the first is to control the chemical composition of steel, the development of new steel grades, in order to adapt to the growing demand for high-performance spring steel wire quality. Secondly, improve the purity of steel to ensure uniform hardenability of steel. Third, improve the surface quality and dimensions, explore new cross-sectional shapes to ensure that the performance and service life of the spring, and finally, the development and use of advanced steel wire oil quenching - tempering equipment and processes to adapt to the development of high-performance spring production requirements.