JPS644578B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a high-strength spring steel applied to the manufacture of high-strength springs used in automobiles, aircraft, various industrial machines, etc. The present invention relates to a method of manufacturing a high-strength spring. (Prior art) For example, a valve spring used in an internal combustion engine is 150
They are often used at temperatures near ℃ and are subjected to repeated loads due to high-speed compression, making them one of the most severely used springs. Conventionally, oil-tempered wire has been commonly used as spring steel for internal combustion engine valve springs, and the Japanese Industrial Standards (JIS) also specify SWO-V (carbon steel oil-tempered wire for valve springs).
JIS G3561) SWOCV-V (Chromium vanadium steel oil tempered wire for valve springs; JIS G3565) SWOSC-V (Silicon chrome steel oil tempered wire for valve springs; JIS G3566) is specified. Among these materials, the SWOSC-V material has superior fatigue strength and fatigue resistance compared to other oil-tempered wires for valve springs, and is therefore widely used as a material for valve springs in internal combustion engines. When even higher fatigue strength is required, this steel is subjected to nitriding or soft nitriding treatment to increase surface hardness and improve fatigue strength. However, looking at recent development trends in internal combustion engines,
There is a growing demand for higher output and lighter weight than ever before. Therefore, valve springs are also required to have a high stress design and a long service life, and are being subjected to even more severe conditions. Conventional surface treatments are no longer able to adequately meet the demands for higher output and lighter weight, and the development of new materials has become necessary. (Purpose of the Invention) This invention was made in view of the above circumstances, and is used as a high-strength valve spring material that enables high stress and long life, especially in response to high rotation and high output of internal combustion engines. The purpose of the present invention is to provide a high-strength spring steel that can achieve high stress and long life not only in internal combustion engines but also in other fields of use, and further, by using the above-mentioned spring steel. The object is to provide a method for manufacturing high strength springs. (Structure of the Invention) The spring steel according to the present invention has C:
0.40~0.75%, Si: 1.0~3.0%, Mn: 0.5~1.5%,
Ni: 2.0~6.0%, Cr: 0.1~1.5%, Mo: 0.05~1.0
%, V: 0.05 to 0.5%, and the balance is Fe and impurities. In this embodiment, among the impurities, [S]≦0.010%,
[O]≦0.0015%, [N]≦0.010%, and
The amount of residual austenite after quenching should be 10% or more, and for example, for the three elements C, Ni, and Si, the remaining austenite should be regulated to 35・C (%) + 2・Si (%) + Ni (%) ≧ 23%. It is characterized by having an austenite content of 10% or more. Further, the method for manufacturing a high-strength spring according to the present invention uses the above-mentioned high-strength spring steel, and after quenching, the high-strength spring steel is cooled into a predetermined spring shape. It is characterized by being inter-formed and then tempered to give it a predetermined strength. The high-strength spring steel according to the present invention consists of the above-mentioned components, and the strength is adjusted by precipitating fine carbides in a high-tough matrix, refining the crystal grains, and refining. This is how it was done. The high-strength spring steel according to the present invention is capable of achieving a high level of strength, and also has a major feature that it can be cold-formed to form a predetermined spring shape such as a coil spring. It is. In this case, in terms of cold formability, retained austenite in the quenched state is more desirable.
It is effective to set it to 10% or more and to adopt a process of cold forming (for example, coil forming) at this stage. On the other hand, in the case of ordinary oil-tempered wire, a process is adopted in which the wire is tempered by successive quenching and tempering, and then cold-formed (for example, coil-formed). When manufacturing high-strength springs using steel for high-strength springs, it is possible to manufacture high-strength springs by a process that is quite different from conventional ones. Next, the reason for limiting the composition range (weight %) of high-strength spring steel according to this explanation will be explained. C (carbon): C is an effective element for increasing the strength of steel, but if it is less than 0.40%, it will not be possible to obtain the strength necessary for a spring, and if it exceeds 0.75%, reticular cementite will easily form. Since this would impair the fatigue strength of the spring, it was set in the range of 0.40 to 0.75%. Si (silicon): Si is an element that improves the strength of steel by solid solution in ferrite and is effective in improving the fatigue resistance of springs, but if it is less than 1.0%, it is not necessary for springs. Unable to obtain fatigue resistance, 3.0%
If it exceeds 1.0%, the toughness may deteriorate and free carbon may be generated during heat treatment, so the content was set in the range of 1.0 to 3.0%. Mn (manganese): Mn is an element that is effective in deoxidizing steel and improving the hardenability of steel, and for this purpose, it is necessary to contain it in an amount of 0.5% or more. However, if it exceeds 1.5%, the hardenability becomes excessive, deteriorating the toughness, and is likely to cause deformation during hardening, so it is set in the range of 0.5 to 1.5%. Ni (Nickel): In addition to improving the toughness after quenching and tempering in this patented steel, Ni is used to intentionally form a large amount of retained austenite during quenching and utilize this to improve the toughness after quenching and tempering. Another important objective is to enable shaping (for example cold coiling). Therefore, if it is less than 2.0%, sufficient toughness improvement and retained austenite cannot be ensured, and if it exceeds 6%, the toughness improvement effect is saturated and the cost increases, so it was set in the range of 2.0 to 6.0%. Cr (Chromium): Cr is an element effective in preventing decarburization and graphitization of high carbon steel, but if it is less than 0.1%, these effects cannot be fully expected, and if it is less than 1.5%.
Since toughness deteriorates when the content exceeds 0.1% to 1.5%. Mo (molybdenum): Mo is an element effective in improving the fatigue resistance of springs, but if it is less than 0.05%, such an effect will not be sufficiently obtained, and if it exceeds 1.0%, the effect will be saturated. Moreover, composite carbides are formed which are not dissolved in austenite. If the amount of these composite carbides increases and becomes a large lump, they cause the same damage as non-metallic inclusions and may reduce the fatigue strength of the steel. Therefore, Mo is
The range was 0.05 to 1.0%. V (vanadium): V is an element that has a large grain refining effect during low-temperature rolling, can improve spring characteristics and increase reliability, and also contributes to precipitation hardening during quenching and tempering. In order to obtain such an effect, it is necessary to contain 0.05% or more. However, if it exceeds 0.5%, the toughness deteriorates and the spring properties are reduced, so the content was set in the range of 0.05 to 0.5%. In addition, to improve the machinability of steel,
S: 0.4% or less, Te: 0.15% or less, Pb: 0.3% or less, Bi: 0.3% or less, Se: 0.3% or less, Ca: 0.01%
One or more of these can be contained as appropriate in the following ranges, and in order to increase strength and weather resistance through precipitation hardening, Cu can also be contained in a range of 0.3 to 3%. Al: 0.01~0.1%, Ti: 0.01~0.3%,
Nb: 0.01~0.3%, Ta: 0.01~0.3%, Zr: 0.01~
One or more of these can be contained in a range of 0.3%, and in order to increase hardenability, B:
It is also possible to add 0.0005-0.01%. Furthermore, [S] is an element that impairs the fatigue strength of springs, and the lower the [S] content, the more reliable the spring will be, so the upper limit is regulated to 0.010% depending on the purpose of use, etc. It is also desirable that [O] generates oxide-based inclusions, which tend to become the starting point of fatigue failure, so it is also desirable to regulate the upper limit to 0.0015% depending on the purpose of use, etc., and [N] Since TiN-based inclusions are generated and reduce the fatigue strength of steel, it is also desirable to limit the upper limit to 0.010% depending on the purpose of use. Furthermore, the amount of residual austenite after quenching is 10
% or more to improve cold formability and enable cold forming into a predetermined spring shape after quenching. In this case, for example, C, Ni,
Regarding the three elements of Si, 35・C (%) + 2・Si (%) +
It is also desirable to regulate Ni (%)≧23% so that the amount of retained austenite after quenching is 10% or more. (Example) Next, an example of the present invention will be explained along with a comparative example. Conventionally, as a criterion for determining the high strength of spring steel, a spring is actually formed and then the spring is subjected to an appropriate load for a certain amount of time. There is a method of tightening it and determining the amount of sag before and after tightening. This amount of settling is
It shows a good correlation with the hardness of the material, and the higher the hardness of the material, the smaller the amount of sag. on the other hand,
It is also known that there is a good correlation between tensile strength and hardness. Therefore, the degree of increase in strength of the spring can be evaluated by measuring the tensile strength of the material instead of measuring the amount of set.
Therefore, in the following test examples, it was decided to judge whether the strength of the spring was increased based on the fatigue resistance and tensile strength. Therefore, first, steels having chemical compositions No. 1 to No. 39 shown in Table 1 were melted and then ingot-formed, followed by decomposition rolling and wire rod rolling to produce spring steel wires. Next, tensile test pieces, fatigue test pieces, and fatigue test pieces were cut out from each of these wire rods, quenched at 900°C for 30 minutes in oil, tempered to a predetermined temperature, and then finished into the predetermined test piece shape. . Thereafter, the following tests were conducted.

【table】

[Table] Test Example 1 In Table 1, test steels No. 1 to No. 7 were prepared to investigate the influence of C on mechanical properties. The tensile test results are shown in Figures 1 and 2. When C is less than 0.40%, the tensile strength σ _B decreases significantly as shown in Figure 1, and the spring is not strong enough. If it exceeds this value, the aperture decreases significantly as shown in FIG. 2, and the spring is insufficient in toughness. By the way, chrome vanadium steel oil tempered wire for valve springs specified in JIS G3565,
The aperture is 40% when the wire diameter is 3.5mmφ or more, and when the wire diameter is 3.5mmφ
Below, it is specified as 45% or more, and the tensile strength σ _B is specified as 180Kgf/mm ² for a wire diameter of 8mmφ, but the high strength spring steel according to the present invention exceeds the above specification. The tensile strength σ _B of 200 Kgf/mm ² or more was also obtained, and it is clear that test steel Nos. 2 to 6 satisfy this requirement. Next, the same sample was also examined for its resistance to settling. In this case, instead of a test using a solid coil spring, a screw creep test was used for evaluation. The test conditions at this time were to apply shear prestrain to the test piece, apply a load stress of 100 Kgf/ ^mm2 to this test piece for 96 hours at an ambient temperature of 150°C, and measure the amount of shear creep strain at that time. did. The results are also shown in Table 1. As shown in Table 1, the residual shear strain of the steel of the present invention is considerably lower than that of the comparative steel, and is sufficient to support higher strength springs. Test Example 2 Test steels No. 8 to No. 14 in Table 1 were prepared for the purpose of investigating the influence of Si on mechanical properties. Furthermore, FIG. 3 shows the results of evaluating the effect of Si on the yield strength ratio (σ _0.2 /σ _B ), and FIG. 4 shows the results of evaluating the effect of Si on the basis of the aperture. As shown in FIG. 3, when the Si content is lower than 1.0%, the yield strength ratio decreases significantly and the spring cannot function adequately. Furthermore, as shown in Figure 4, when Si exceeds 3.0%, the aperture decreases significantly,
In this case as well, the spring lacks toughness. Furthermore, the results of evaluating the fatigue resistance by a torsional creep test are shown in Table 1, and the residual shear strain of the steel of the present invention is lower than that of the comparative steel, and the Si content is 1.0.
When the content is 3.0%, it is sufficient to increase the strength of the spring. Test Example 3 In Table 1, the test steels shown in No. 15 to No. 29 are Ni
This study was prepared with the purpose of investigating the effect on the mechanical properties of Figures 5 and 6 show the tensile strength σ _B and the aperture, respectively.
This is an evaluation of the effect of Ni. As shown in Table 1 and Figures 5 and 6, when the C content is around 0.50% and the Ni content is less than 2.0% (No. 15), the area of area decreases significantly and the spring lacks toughness. . On the other hand, when Ni exceeds 6.0% (No. 27), the toughness and ductility are sufficient, but the strength tends to decrease, and the effect of Ni is saturated. In addition, No. 28 and 29 contain 0.74% and 0.82% of C, respectively.
Ni is added to improve the toughness and ductility of high C materials.
The Ni content was approximately 7%, but as shown in Figure 6, even if the Ni content was increased, sufficient narrowing could not be obtained.
It is judged that the improvement effect of Ni on toughness and ductility occurs when the C content is up to 0.75%. Furthermore, the results of evaluating the fatigue resistance by a torsional creep test are shown in Table 1, and the residual shear strain of the steel of the present invention is lower than that of the comparative steel.
When the Ni content is within the range of 2.0 to 6.0%, it is sufficient to increase the strength of the spring. Test Example 4 Test steels No. 30 to No. 34 in Table 1 were prepared for the purpose of investigating the influence of S on corrosion fatigue strength. Here, for the corrosion fatigue test, a rotary bending fatigue test piece was prepared from a wire rod, heated at 900℃ for 30 minutes, cooled in oil, and heated to 400℃.
After being heated for 2 hours and air-cooled for quenching and tempering, the specimen was left in salt water spray for 120 hours to form corrosion pits on the surface of the specimen, and then a fatigue test was conducted. Table 1 shows the durability limit values, and it can be seen that the fatigue strength of the steels with S content regulated to 0.010% or less is clearly improved. Test Example 5 The test steels No. 35 to No. 39 in Table 1 are [O], [N]
This sample was prepared for the purpose of investigating the effect of fatigue strength on fatigue strength. For the fatigue test piece, a rotary bending fatigue test piece was cut from the wire rod prepared above, heated at 900℃ for 30 minutes, cooled in oil,
After heating at 400°C for 2 hours and cooling in the air for quenching and tempering, a fatigue test was conducted in the atmosphere. Table 1 shows the durability limit values, where [O] is 0.0015
% or less, and those in which [N] is restricted to 0.010% or less to reduce oxide-based inclusions and TiN-based inclusions, respectively, are found to have improved fatigue strength. Test Example 6 When manufacturing a coil spring by cold forming from the high-strength spring steel wire rod according to the present invention, cold forming is performed using retained austenite produced in the quenched state. The following steps can be considered. (1) Quenching → Cold forming → Tempering (2) Quenching → Cold forming → Subcello treatment → Tempering (3) Quenching → Tempering → Cold forming → Tempering In any case, the process shown here is quenching. What they have in common is that they utilize retained austenite. Therefore, as shown in Table 2, samples with different retained austenite were selected from Table 1 and cold-formed into a coil shape after quenching, and the formability at that time was judged by the occurrence of cracks and the amount of springback. . The results are shown in Table 2.

[Table] As is clear from Table 2, when the residual austenite is less than 10%, the amount of spring back is large when molded under the same conditions, and to eliminate this, a larger molding force is required, leading to cracking. It was confirmed that there are cases where It was also found that if the retained austenite content is 10% or more, the cold formability is better as the retained austenite content increases. In addition, as shown in Table 1, in order to satisfy the residual austenite content of 10% or more that allows cold forming, 35・C (%) + 2・Si for C, Ni, and Si must be
(%) + Ni% value (“Y value” in Table 1)
It is abbreviated as ) is 23% or more. (Effects of the Invention) As explained above, the high-strength spring steel according to the present invention has C: 0.40 to 0.75% by weight,
Si: 1.0~3.0%, Mn: 0.5~1.5%, Ni: 2.0~6.0
%, Cr: 0.1-1.5%, Mo: 0.05-1.0%, V: 0.05
Contains ~0.5%, the balance consists of Fe and impurities,
More preferably, in the impurity, [S]≦
0.010%, [O]≦0.0015%, [N]≦0.010%, and the amount of residual austenite after quenching is set to be 10% or more depending on the purpose of use, so high strength spring steel is Therefore, it is suitable as a material for valve springs of internal combustion engines, for example, and can be made to correspond to the high rotation and high output of internal combustion engines in the future, and has an extremely high stress and long life. High strength springs can be produced by using the high tensile strength spring steel according to the present invention, quenching, cold forming into a predetermined spring shape, and then tempering to give a predetermined strength. It has the remarkable effect that it is possible to obtain the following.

[Brief explanation of drawings]

Figures 1 and 2 are graphs showing the results of investigating the effect of C on the tensile strength and area of area of spring steel, and Figures 3 and 4 are graphs showing the effect of Si on the yield strength ratio and area of area of spring steel. Figures 5 and 6 are graphs showing the results of investigating the effects of Ni on the tensile strength and reduction of area of spring steel.

Claims (1)

[Claims] 1% by weight, C: 0.40-0.75%, Si: 1.0-3.0
%, Mn: 0.5-1.5%, Ni: 2.0-6.0%, Cr: 0.1
1.5%, Mo: 0.05-1.0%, V: 0.05-0.5%, with the balance consisting of Fe and impurities. 2 In impurities, [S]≦0.010%, [O]≦
The high-strength spring steel according to claim 1, which is regulated to 0.0015% and [N]≦0.010%. 3. The high-strength spring steel according to claim 1 or 2, wherein the amount of retained austenite after quenching is 10% or more. 4 Regarding the three elements C, Ni, and Si, 35・C (%)
The high-strength spring steel according to claim 3, in which the amount of retained austenite is set to 10% or more by regulating +2.Si (%) + Ni (%)≧23%. 5 Weight%: C: 0.40-0.75%, Si: 1.0-3.0
%, Mn: 0.5-1.5%, Ni: 2.0-6.0%, Cr: 0.1
~1.5%, Mo: 0.05~1.0%, V: 0.05~0.5%, using high strength spring steel with the balance consisting of Fe and impurities, and after quenching, cold forming into the specified spring shape, and then A method of manufacturing a high-strength spring, which is characterized by applying a tempering treatment to impart a predetermined strength.