JPS63213645A - Spring steel excellent in free-cutting property and sag resistance - Google Patents

Abstract

PURPOSE:To improve free-cutting property without deteriorating sag resistance, by adding proper amounts of S, Te, Pb, Ca, etc., to an N-containing high-Si spring steel with a specific composition. CONSTITUTION:The spring steel has a composition consisting of, by weight, 0.40-0.70% C, 1.00-2.50% Si, 0.50-2.00% Mn, 0.012-0.030% N, one or more kinds among 0.03-0.40% S, 0.001-0.15% Te, 0.03-0.20% Pb, and 0.001-0.050% Ca, and the balance Fe with impurity elements and further containing, if necessary, either or both of 0.20-1.0% Cr and/or 0.03-0.50% Nb and 0.03-0.50% V. This steel has superior free-cutting property, sag resistance, and durability.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a coil spring 1 used in a suspension system of an automobile, etc.
This invention relates to a spring steel suitable for laminated leaf springs and torsion bars that has excellent free machinability and resistance to fatigue.

(Prior Art) In recent years, there has been a very strong demand for lightweight transportation equipment such as automobiles. Therefore, there has been a strong demand for reducing the weight of the springs themselves used in these suspension systems.

The most efficient way to reduce weight for suspension springs is to increase the design stress. However, when springs are manufactured using conventional spring steel, a phenomenon called "settling" in which the height of the spring decreases becomes more noticeable over time. Increased ``sag'' leads to a decrease in the height of the bumper, which poses a safety problem, so it was not possible to increase the design stress.

On the other hand, in the spring manufacturing process, two surfaces are decarburized.

For the purpose of removing flaws, processing both end faces of springs, manufacturing Taber coil springs, etc., surface cutting such as beering, centerless, etc., and grinding may be required.

However, since the spring material has a high carbon concentration of around 0.6%, the hardness after rolling is about HRC 30.

Therefore, the current situation is that tool life is usually ensured by annealing to lower the hardness during cutting.

However, the machinability is still not sufficient.

Furthermore, there is a strong desire to improve machinability, and attempts have been made to omit the annealing process.

Traditionally, it has been used as a spring steel with excellent resistance to fatigue.

As it has become known that Si in the spring swell is an effective element for improving the fatigue resistance, SUP 7, which has an even higher Si content than 5UP6, has come to be used more frequently.

In addition, Nb is an element that further increases resistance to fatigue.
It was found that the effects of V, N, etc. were large, and JP-A-57-
No. 32353, JP-A-60-103155, etc. have been filed.

However, such elements that improve fatigue resistance are not desirable elements in terms of machinability, and there has been a strong desire to develop steel for springs that is excellent in both free machinability and fatigue resistance.

(Problems to be Solved) The purpose of the present invention is to meet the demands of the above-mentioned industry and provide a steel for springs that has a resistance to 2-settling that is equal to or better than the recently developed network, and that is also excellent in free machinability. be.

(Means for Solving the Problems) With this background in mind, the inventors of the present invention have conducted extensive research and have found that an appropriate amount of N or N is added to high-Si spring steel.

By adding an appropriate amount of one or more of S=Pb+Cat Te to the spring steel to which N, Nb, and V have been added, free machinability can be improved without impairing the excellent settling resistance. It was a success.

The samples in Table 1 were melted in a 30 kg vacuum melting furnace. 76-2
0,6 C-2 with only N content changed to 50 ppm
,O5i~0.8Mn~0.35V was quenched and tempered, observed with a scanning electron microscope, and the number of precipitated carbonitrides and torsional croup strain after 96 hours were measured. This is what is shown.

The details of the teardrop creep test method will be described later, so they are omitted here.

As shown in Table 1, it can be seen that as Nil increases, the number of precipitates increases. Further, the torsional creep strain after 96 hours decreases with the increase in precipitates, and the setting resistance improves, that is, the first invention steel of the present invention has a weight ratio of C0.4 to 0. 7%,
S i 1, O~2, 5%, Mn 0
．． 5-2.0%, N 0.012-0.03%, S 0.03-0.4%, Te 0.0
01-0.15%, Pb 0.03-0.2%.

The second invention steel contains one or more of ICa 0.001 to 0.50%, and the balance consists of Fe and impurity elements, and the second invention steel is the first invention steel with Cr 0.2 to 1%.
．． It contains 0% and improves hardenability.

3.4 Invention steel has V 0.03 to 0.0 compared to 1st and 2nd invention steel.
5%, and one or two of Nb 0.03 to 0.5%.

The reasons for limiting the composition of the steel of the present invention will be explained below.

The C amount was set to 0.40 to 0.70%, which is 0.40%.
This is because if the content is less than 0.70%, sufficient strength cannot be obtained by quenching and tempering, and if the content exceeds 0.70%, the toughness is significantly reduced and machinability is further reduced.

The reason for setting the Si amount to 1.00 to 2.50% is 1.00%.
This is because if the Si content is less than 2.50%, the effect of increasing the strength of the substrate and improving the resistance to sagging cannot be obtained sufficiently by solid solution of Si in the ferrite. This is because the effect of improving the settling property is saturated and decarburization tends to occur during rolling.

The reason for setting the Mn amount to 0.50 to 2.00% is 0.50
This is because hardenability is insufficient if it is less than 2.0%.
This is because if the content exceeds the above range, the toughness will be impaired.

N has a high affinity with AI and partially becomes AIN in steel, preventing coarsening of crystal grains. On the other hand, solid solution N that does not combine with other elements such as AI has the effect of fixing dislocations introduced by cold working such as setting and improving the resistance to settling. Furthermore, N combined with Nb and V is uniformly distributed as nitrides and carbonitrides in a quenched and tempered state with a diameter of about 0.1 micron. Since such precipitates act as obstacles to the movement of dislocations, they greatly improve the resistance to settling.

The content of N that acts like this is Q, 012~0.0
The reason why it is set at 3% is that if it is less than 0.012%, the above effect low hole may occur.

Like Mn, Cr is an element that improves hardenability and is effective in preventing decarburization, and is an element that is added in an appropriate amount as necessary. In order to obtain these effects, it is necessary to contain at least 0.20% or more, and the lower limit is set to 0.20%. However, if the content exceeds 1.00%, the effect of improving the sagging resistance will be inhibited, so the upper limit was set at 1300%.

The V and Nb amounts were each set to 0.03 to 0.50%.

This is because if the content is less than 0.03%, the effect of improving the fatigue resistance cannot be sufficiently obtained, and if the content exceeds 0.50%, the effect will be saturated and the price of the steel will increase. .

By adding these V and Nb in combination, it is possible to greatly improve the settling resistance compared to when they are added alone.

S, Te, Pb, "A" are elements that improve the machinability of the steel of the present invention, S 0.03-0.46%, Te
0.001-0.15%, Pb 0.03-0.2
0%, and Ca 0.001 to 0.05%, because if it is less than the lower limit, the machinability improvement effect is small, and if it is added beyond the upper limit, it will remain in the steel in the form of large inclusions, which will reduce the durability. This is to reduce fatigue life.

(Example) Below, the characteristics of the steel of the present invention will be clarified by comparing it with conventional steel and comparative steel through examples.

Table 2 shows the chemical composition of the test steel.

In Table 2, the Y steel is the conventional 1i1sUP 7, the Z steel is the comparison steel in which V and Nb are added to the Y steel, the A-X steel is the invention steel, the A-F steel is the first invention steel, G-L steel is the second invention steel, M-R@ is the third invention steel, and S-X steel is the fourth invention steel.

Table 3 shows the results of examining the machinability of the invention steel, comparative steel, and conventional steel. Table 3 shows that test steels A-Z in Table 2 were rolled to 14 mmΦ and then drilled with a high-speed steel drill.Evaluation was made based on the drilling time.As is clear from Table 3, the invention steel Certain A to Xw4 are conventionally (7) SIJP7 (
This steel has superior machinability compared to steel (Y steel) and comparative steel (steel Z) containing Nb and V.

Table 4 shows the results of a torsional creep test for evaluating the fatigue resistance of the test steels A-Z. In the weeping creep test, the test steel was rolled to 2011IIIlΦ, and then a test piece with a parallel portion diameter of 9[ll01] was prepared.

Then, it was quenched and tempered to have a Vickers hardness of 550. After setting, torsion torque was applied so that the surface of the parallel part had a shear stress of 100 kgf/mm, and the creep strain was measured and evaluated after 96 hours.The experiment was conducted in an air-conditioned room at a constant temperature of 20°C. Performed at.

Care was taken to ensure that there would be no increase or decrease in sag due to temperature changes. Coil springs are subject to torsional torque during use, and sagging is considered to be a type of creep phenomenon, so the sagging resistance of coil spring materials can be evaluated based on two results.

As is clear from Table 4, steels A to X, which are the steels of the present invention, have improved fatigue resistance due to the addition of elements that improve free machinability, such as S, Te, Pb, and Ca. No. 1.2 Invention steel showed better resistance to fatigue than Y steel, which is the conventional steel, and No. 3 steel with Nb and V added thereto.
, 4 inventive steel showed extremely superior sag resistance compared to comparative mzm, which is SUP 7 with Nb and V added.

In order to confirm the effectiveness of the present invention steel as a solid spring, coil springs having the specifications shown in Table 4 were prepared for four representative steel types among the test steels, conventional steel, comparative steel, and invention steel. After forming and shot peening, the average window cuff
5kgf/wri”, stress amplitude 40kgf/
The residual shear strain was measured after stress was repeatedly applied 200,000 times in mm''. The results are shown in Table 6.

Koshishaji Table 5 Table 6 As is clear from Table 6, there are no steels according to the invention.

However, the addition of elements that improve free machinability, such as S, Te, Pb, and Ca, sometimes resulted in poor fatigue resistance. Further, even though the test was repeated up to 500,000 times under the same stress conditions, none of the coil springs of the steel of the present invention broke.

By containing appropriate amounts of one or more of S, Te, Pb, and Ca, we succeeded in improving machinability without reducing durability and resistance to fatigue.

It is possible to improve the tool life during cutting during spring manufacturing, and it is also possible to omit annealing during cutting, which has extremely high practicality.

Claims (4)

[Claims] (1) Weight ratio: C0.40-0.70%, Si1
．． 00-2.50%, Mn0.50-2.00%, N0
．． 012~0.030%, S0.03~0.40%,
Te0.001-0.15%, Pb0.03-0.20
%, one or two of Ca0.001-0.050%
A spring steel with excellent free machinability and fatigue resistance, characterized by containing at least 100% Fe and the remainder consisting of Fe and impurity elements. (2) Weight ratio: C0.40-0.70%, Si1
．． 00-2.50%, Mn0.50-2.00%, Cr
0.20-1.0%, N0.012-0.030%,
S0.03~0.40%, Te0.001~0.15%
, Pb0.03-0.20%, Ca0.001-0.0
A spring steel with excellent free machinability and fatigue resistance, characterized by containing one or more of 50% of the elements, and the remainder consisting of Fe and impurity elements. (3) Weight ratio: C0.40-0.70%, Si1
．． 00-2.50%, Mn0.50-2.00%, N0
．． 012~0.030% and Nb0.03~0.50%
, contains one or two of V0.03-0.50%, and further contains S0.03-0.40% and Te0.001-0.001.
0.15%, Pb0.03-0.20%, Ca0.00
Contains one or more of 1 to 0.050%,
A spring steel with excellent free machinability and fatigue resistance, characterized by the balance being Fe and impurity elements. (4) C0.40-0.70%, Si1 in terms of weight ratio
．． 00-2.50%, Mn0.50-2.00%, Cr
0.20-1.0%, N0.012-0.030%,
Contains one or two of Nb0.03-0.50%, V0.03-0.50%, and S0.03-0.03%.
40%, Te0.001~0.15%, Pb0.03~
0.20%, Ca 0.001-0.050%, and one or more of them, with the balance consisting of Fe and impurity elements. Spring steel with excellent free machinability and fatigue resistance. .