JPH07179936A - Steel for thin leaf springs with excellent heat sag - Google Patents

Abstract

(57) [Summary] [Composition] C: 0.40 to 0.70% by weight, Si: 0.15 to 0.
35%, Mn: 0.70 to 1.50%, Cr: 0.80 to 1.50%, Al: 0.010
~ 0.050%, V: 0.05 ~ 0.50%, N: 0.01 ~ 0.03%,
After the steel consisting of the balance Fe and impurity elements is heated to 820 to 950 ° C to austenite, it is austempered at a temperature of 320 to 400 ° C, then 330 to 450 ° C and 10 ° C above the austempering temperature. A steel for thin leaf springs, which is a steel that has been tempered at a high temperature as described above, and has an excellent heat sag property, which is characterized by having a tempered bainite structure. [Effect] It becomes possible to manufacture a thin leaf spring having excellent heat settling property while suppressing heat treatment distortion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for leaf springs which is excellent in heat sag and is capable of significantly reducing heat treatment distortion.

[0002]

2. Description of the Related Art Thin leaf springs are used in various applications such as automobiles and various machines. Conventional materials for thin leaf springs include:
SK5, which is one of the carbon tool steels, is widely used,
This SK5 is processed into a predetermined spring shape, heated and austenitized, and then poured into a salt solution, 300 to 400
It is manufactured by subjecting it to austempering or quenching and tempering by quenching to ℃. Then, the target mechanical characteristics are adjusted by changing the austempering temperature and the tempering temperature.

Recently, the demands for thin leaf springs, as well as other components, have become extremely strict in various aspects. In particular, in terms of usage environment, thin leaf springs are often applied to parts where the temperature rises considerably during use such as automobile engine parts. In this case, the spring shape is constantly changed by the force constantly applied to the spring. Since the settling phenomenon that changes in temperature becomes more pronounced than that at room temperature, it has been desired to develop a spring steel having excellent thermal settling property.

Materials have been actively developed since the 1950s in order to suppress the settling phenomenon that occurs during the use of springs, and various new materials such as coil springs for suspensions and valve springs for automobile engines have been developed. Has been developed and a patent application has been filed. However, since the materials shown up to now emphasize the sag phenomenon, the measure for improvement is to optimize the components, and the heat treatment is the same quenching and tempering treatment as the conventional one. It was a thing.

However, with respect to the thin leaf spring, the dimensional accuracy requirements are very strict, so that the quenching and tempering treatment causes the heat treatment strain to be very large, and the strict dimensional accuracy requirements cannot be satisfied. It is dealt with by restraining and heat treating. However, it takes a lot of man-hours to heat-treat with restraint, which has been a serious problem in manufacturing.

Further, when SK5 described above is used, the quenching and tempering material is slightly inferior in heat settling property, and there is a problem in heat treatment distortion. Austempered material has large heat settling property. It had the problem of being inferior.

With respect to the above-mentioned problems, the applicant of the present invention has disclosed in Japanese Patent Laid-Open No.
The inventions described in JP-A-9398 and JP-A-5-179399 have already been applied for. The invention described in this publication is characterized by subjecting a steel, which is hardened by adding an appropriate amount of Mn and Cr to ensure hardenability, and refined crystal grains by adding V and N, to austempering. Thus, the heat settling property is superior to the conventional one and the heat treatment distortion can be remarkably improved.

[0008]

The steel for thin leaf springs described in the above publication has significantly improved thermal sag compared to SK5 which is a conventional steel, and heat treatment strain can be suppressed to a small level.
However, recently, the demand for improving the heat settling property of a thin leaf spring has become more strict, and it has a heat settling property more excellent than that of the invention described in the above-mentioned publication, and at the time of heat treatment like the invention of the above-mentioned publication. There has been a strong demand for the development of new steel that does not require the members to be constrained. An object of the present invention is to solve the above-mentioned problems, and to provide a steel for thin leaf springs, which is more excellent in heat settling property than the steel of the above-mentioned publication and can suppress heat treatment strain to be equal to or less than that.

[0009]

In view of the above-mentioned drawbacks of conventional steels, the present inventors have ensured excellent heat sag,
In addition, as a result of intensive studies to find a component system and a heat treatment method capable of reducing heat treatment strain, the present invention has been completed, and is based on the following new findings.

The inventors of the present invention have considered that it is difficult to perform quenching and tempering treatment in order to improve the heat treatment strain, and therefore, similar to the invention described in the above publication, an austempering treatment is adopted,
In addition to conducting a detailed investigation of the optimum component system for obtaining excellent heat sag by austempering, the austempering conditions were also reviewed.

As a result, it has been found that the heat treatment strain can be reduced to the same or less magnitude while securing a more excellent heat sinking property as compared with the invention described in the above-mentioned publication due to the effect of the combination of the following constitutions. . By adding an appropriate amount of Mn and Cr to give hardenability,
A uniform bainite structure is obtained without ferrite-pearlite transformation during austempering. Not only mere austempering treatment, but then tempering treatment is performed at a temperature higher by 10 ° C. or more than the austempering treatment temperature. V, Al, and N are added in appropriate amounts, and fine carbonitrides are precipitated by austempering and tempering to improve the thermal sag.

A feature of the present invention is to find out a component system which can obtain excellent mechanical properties after austempering treatment. Not only simple austempering treatment but also subsequent tempering treatment is performed at a temperature higher than the austempering treatment temperature. It has been found that by carrying out the heat treatment, the heat settling property is remarkably improved and the heat treatment strain can be suppressed to a small level as compared with the case where no tempering is performed.

That is, the thin leaf spring steel of the present invention, which is excellent in heat sag, has a weight ratio of C: 0.40 to 0.70% and Si:
0.15 to 0.35%, Mn: 0.70 to 1.50%, Cr: 0.80 to 1.50%
, Al: 0.010 to 0.050%, V: 0.05 to 0.50%, N: 0.010 to 0.0
820 steel containing 30% and the balance Fe and impurity elements
After heating to ~ 950 ° C for austenite, 320 ~ 4
Austempered at a temperature of 00 ° C and then 330
~ 450 ℃ and above the austempering temperature
It is a steel that has been tempered at a temperature higher than 10 ° C. and is characterized in that its structure is a tempered bainite structure.

The reasons for limiting the components of the thin leaf spring steel of the present invention having excellent heat sag will be described below. C: 0.40 to 0.70% C is an essential element for securing the strength required as spring steel, and it is necessary to contain 0.40% or more. However, if contained in a large amount, austenite remains after the heat treatment and the thermal sag property is deteriorated, so the upper limit was made 0.70%.

Si: 0.15 to 0.35% Si is an element necessary for deoxidation during steelmaking, and 0.15% or more is required to obtain the effect. However, when contained in a large amount, the hardness cannot be sufficiently lowered by annealing performed to facilitate processing into a predetermined shape such as punching and bending, and the processing after annealing becomes difficult. Further, Si is known as an element that improves the settling resistance when performing quenching and tempering, but when performing austempering as in the present invention, austenite remains after the heat treatment and becomes heat. Therefore, the upper limit was set to 0.35%.

Mn: 0.70 to 1.50% Mn is an element for ensuring the necessary hardenability, and during austempering, ferrite / pearlite transformation and bainite transformation do not occur during cooling to the austempering temperature. Thus, it is an element necessary for completing all the transformations during the holding at the austempering temperature to obtain a uniform bainite structure. Therefore, it is necessary to contain at least 0.70%. However, if a large amount is contained, austenite remains after heat treatment, and the thermal settling property deteriorates, so the upper limit was made 1.50%.

Cr: 0.80 to 1.50% Cr is an indispensable element for securing necessary hardenability like Mn and for obtaining a uniform bainite structure. Therefore, it is necessary to contain at least 0.80%. However, if contained in a large amount, the hardness due to annealing before working into a predetermined shape increases, so the upper limit was made 1.50%.

Al: 0.010 to 0.050% Al is an element necessary for deoxidation, and is also an element necessary for refining crystal grains by precipitating AlN to improve thermal sag. Therefore, the content of 0.010% or more is required. However, even if contained in a large amount, the effect saturates, and the fatigue properties deteriorate due to the precipitation of huge inclusions, so the upper limit was made 0.050%.

V: 0.05 to 0.50% V has the effect of refining the crystal grains, and has the effect of finely forming carbonitrides after heat treatment and improving the thermal sag property by precipitation strengthening. Therefore, at least 0.05%
The above contents are required. However, even if it is contained in a large amount, the above effect is saturated, and a huge carbonitride is generated to deteriorate the fatigue properties, so the upper limit was made 0.50%.

N: 0.010 to 0.030% N is an element effective in improving thermal fatigue resistance by forming carbonitrides in combination with Al and V, and refining crystal grains and strengthening precipitation by the carbonitrides. . Even if N is not added, it is mixed in from the air when dissolved and contained as an impurity.
In the present invention, it is necessary to positively add it during steel making, and the lower limit value was made 0.010%. However, even if contained in a large amount, the effect will be saturated, and N ₂ gas may be generated when cooling the molten steel to cause defects inside the material,
The upper limit was 0.030%.

Next, the reasons for limiting the heat treatment conditions for the steel of the present invention will be described below. Austenitizing temperature 820 ~ 95
The reason for setting the temperature to 0 ° C. is that the austenite single phase state may not be obtained below 820 ° C., and if the temperature exceeds 950 ° C., the crystal grains become coarse and the heat settling property deteriorates.

The reason why the austempering temperature is 320 to 400 ° C. is that if the temperature is lower than 320 ° C., the time required for the completion of bainite transformation is long and the heat treatment time is long and there is a problem in manufacturing. This is because the hardness after heat treatment becomes low, and the required strength cannot be obtained.

The tempering treatment is an essential step in the present invention for improving the heat settling property. The lower limit of the processing temperature was set to 330 ° C because the effect of improving the heat settling effect cannot be obtained unless tempering is performed at a temperature 10 ° C or higher higher than the austempering processing temperature. It is set at 10 ℃ higher, and the upper limit is 450.
The reason why the temperature is set to ℃ is that when the temperature exceeds 450 ℃, the hardness decreases and the required strength cannot be obtained.

Since the present invention is applied to a thin leaf spring, it is necessary to process it into a predetermined spring shape before performing the heat treatment shown in the present invention. This processing is carried out by hot-rolling, cold-rolling or the like after melting to a predetermined thickness, annealing if necessary, and then cold-working such as punching and bending using a press or the like. However, according to the present invention, regardless of the contents of the steps, excellent heat sag can be secured by subjecting the steel having the above-mentioned components to the heat treatment.

[0025]

The steel for thin leaf springs excellent in heat sag of the present invention is
The required hardenability is secured by adding Mn and Cr, and V, Al, N
Of steel with precipitation strengthening and grain refinement by precipitation of carbonitrides by austempering and tempering at a temperature higher than the austempering temperature It was possible to suppress the strain after heat treatment to be small while obtaining the above.

[0026]

EXAMPLES Next, the characteristics of the steels of the present invention will be clarified by examples in comparison with comparative steels and conventional steels. Tables 1 and 2 show the chemical composition of the test materials and the test results. Table 1 is an example in which the components satisfy the conditions of the present invention, and Table 2 is an example in which the components do not satisfy the conditions of the present invention. Of Table 1, No.1-10
Is an example of the present invention, and Nos. 11 to 18 are comparative examples in which the heat treatment conditions do not satisfy the conditions of the present invention. In Table 2, Nos. 19 to 28 are examples in which the heat treatment conditions are within the scope of the present invention, but the components do not satisfy the conditions of the present invention. Of these, No. 28 is JIS steel SK5. Is an example using. N
o.29 and 30 are examples of conventional heat treatment of JIS steel SK5, 29 is quench-tempered and 30 is austempered.

30 kg of test steel having the components shown in Tables 1 and 2
It was melted in a vacuum induction electric furnace, a torsion spring was produced by forging and machining, heat treatment was performed under the conditions shown in the above table, and strain and heat sag after heat treatment were evaluated. The strain after heat treatment was evaluated by defining the difference in height of the abutment when the torsion spring after heat treatment was placed on a flat plate as flatness, and determining the flatness. In the state before heat treatment, the difference in height of the abutment is almost 0 because it is processed by machining, but the strain increases due to heat treatment, and the greater the degree of strain, the greater the difference and the flatness. Also grows.

Regarding the thermal sag property, the same one as the torsion spring used for the evaluation of the heat treatment strain was used, and the load necessary for expanding the abutment to a predetermined interval (L ₀ ) was measured before the test. After that, fix the abutment with the width (L ₁ ) expanded and leave it at 150 ° C for 96 hours, measure the load again in the same way as before the test, and measure the load reduction rate against the load before the test. It was evaluated by determining.

[0029]

[Table 1]

[0030]

[Table 2]

As is apparent from Tables 1 and 2, comparing No. 11 to 18, which are comparative examples whose heat treatment conditions are outside the scope of the present invention, with No. 1 to 10, which are the examples of the present invention, 12 is an austenitizing temperature, 13 and 14 are austempering temperatures do not satisfy the conditions of the present invention, so the heat settling property is greatly inferior, and 15 is the tempering temperature is high and the strength decreases and heat settling It is inferior in sex. No. 16 is an example in which the temper tempering effect was insufficient due to the low tempering temperature, and no temper tempering treatment was performed at all, so no temper tempering effect was obtained at all. It is a thing. 18
Is an example in which quenching and tempering was performed without performing austempering. In this case, the thermal settling property is not significantly different from that of the present invention, but is significantly inferior in terms of heat treatment distortion.

Next, among the comparative examples in which the components are outside the scope of the present invention,
Comparing No. 19 to 27 with the present invention, except for 24, since some components are out of the range, the heat settling property is inferior to that of the present invention, and 24 is the Cr content. Therefore, the results shown in Table 1 are equivalent to those of the present invention,
The annealing hardness increased, making punching difficult. No. 28, which is a comparative example obtained by subjecting JIS steel SK5 to heat treatment by the method of the present invention, also has a poor thermal sag property because it is not a component suitable for obtaining an excellent thermal sag property.

Further, the conventional examples Nos. 29 and 30 are significantly inferior to those of the present invention in either flatness or thermal fatigue resistance. On the other hand, the present invention, as shown in No. 1 ~ 10 Examples, while suppressing the heat treatment strain to a low value that does not cause a problem in production, it was confirmed that excellent thermal sag property can be obtained. .

[0034]

As described above, the thin leaf spring steel of the present invention, which is excellent in heat sag, has its components adjusted to a range suitable for austempering treatment, and after the austempering treatment, the temperature is set to a temperature higher than the austempering treatment temperature. By carrying out tempering at a temperature higher than 10 ° C, it was possible to secure excellent thermal sag while suppressing heat treatment distortion to a small level.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Sugimoto 1 Wanowari, Arao-cho, Tokai-shi, Aichi Aichi Steel Co., Ltd. Address No. 1 in the Central Technology Center Co., Ltd.

Claims (1)

[Claims] 1. A weight ratio of C: 0.40 to 0.70%, Si: 0.15
~ 0.35%, Mn: 0.70 ~ 1.50%, Cr: 0.80 ~ 1.50%, Al: 0.
010 to 0.050%, V: 0.05 to 0.50%, N: 0.010 to 0.030%, the balance 820 to 950 steel containing Fe and impurity elements
After austenitizing by heating to ℃, austempering is performed at a temperature of 320 to 400 ℃, and then 330 to 450 ℃.
A steel for thin leaf springs having an excellent heat sag property, which is a steel that has been tempered at a temperature higher than the austempering temperature by 10 ° C. or more, and has a tempered bainite structure.