JPH04308060A - Steel material for spring - Google Patents

Abstract

PURPOSE:To minimize the amount of change in the dimension of a spring due to low temp. annealing after spring forming by regulating the volume percentage of the body-centered cubic phase, to extend the range of practical application of the volume percentage of the body-centered cubic phase capable of minimizing the above amount of dimensional change by regulating Mo content in a steel material, and to remarkably improve practical utility. CONSTITUTION:The steel material is a steel material for spring prepared by using a solution-treated steel material consisting of a multiphase mixed structure containing face-centered cubic lattice phase and body-centered cubic lattice phase as a starting material and forming it into the prescribed shape and dimensions. Further, the above steel material contains 0.5-4wt.% Mo and the volume percentage of the body-centered cubic lattice phase contained in the solution-treated steel material is regulated to 20-80%.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to steel materials for springs, and more particularly to steel materials for springs such as steel wires for springs and steel plates for leaf springs, in particular dimensional changes during low-temperature annealing after spring forming. Regarding steel materials for springs with small sizes.

0002

[Prior Art] Spring steel is a material (steel wire, steel plate, etc.) for springs such as coil springs and leaf springs, and is made by forming solution-treated steel into a predetermined shape and size using raw material. be. Generally, the forming process is performed repeatedly by cold working such as wire drawing and annealing. At the end of this repetition, a cold working step is performed to ensure spring properties, and therefore the spring steel material is in a so-called cold worked state. For example, steel wire for coil springs is produced by heating the wire to 1150° C., cooling it with water, annealing it, pickling it, and then drawing it to produce a steel wire for springs.

[0003] After the steel material for springs is formed into a spring shape, it is annealed at a low temperature for the purpose of removing residual strain and improving the elastic limit, and then becomes a spring. At this time, it has been well known that dimensional changes occur due to annealing.

For example, so-called spring steel wires such as piano wire, hard steel wire, and stainless steel wire are annealed at 250 to 400° C. after being formed into coil springs. It is known that the outer diameter of stainless steel wire becomes smaller when it is annealed at a temperature of 300 to 400°C (page 314 of "Spring" edited by Japan Spring Association (published by Maruzen Co., Ltd.)). reference).

For products such as torsion springs and tension springs that have straight parts or hook parts at both ends of the coil, the intersection angle changes due to annealing after spring forming, and for products made of piano wire or hard steel wire, the intersection angle at both ends changes. increases and opens, and those made of stainless steel wire close. Although it depends on the shape of the forming material, the dimensions of the forming material change considerably due to annealing after forming. Cold rolled steel plate for springs, SUS301CS
In the case of stainless steel plates for springs such as P, SUS304CSP, etc., and steel plates for leaf springs such as thin plates made of piano wire and stainless steel wire, the dimensions change considerably due to annealing after bending and forming.

[0006]

As mentioned above, dimensional changes occur in spring steel materials due to annealing after spring forming, so workers form the springs with these changes in mind. For example, in the case of coil springs, piano wire and hard steel wire are formed into a spring with an outer diameter larger than the finished dimensions according to specifications, and stainless steel wire is formed into a smaller spring, and dimensional changes occur due to low-temperature annealing. We have worked hard to create springs with the specified dimensions.

However, since the amount of dimensional change mentioned above has subtle differences, it is generally difficult to rely on many years of experience. Therefore, in actual work, it is difficult to The reality is that springs are preformed in advance, annealed at low temperatures, various preliminary tests are conducted to determine the amount of dimensional change, and actual production is based on the results. There is a problem that manufacturing efficiency is reduced, and improvement of this problem is desired. In addition, even if actual production is carried out based on the preliminary tests as described above, the spring dimensions after annealing are not constant and often vary, so there is a problem that it is difficult to improve the precision of the spring dimensions. is also desired.

[0008] For these reasons, the above-mentioned problems can be solved if a steel material for springs that does not undergo dimensional changes during low-temperature annealing treatment after spring forming can be obtained, and the realization of such a steel material for springs is highly desired in the industry. This is beyond words.

[0009] The present invention has been made in view of the above circumstances, and its purpose is to provide a steel material for springs that does not easily undergo dimensional changes during low-temperature annealing treatment after spring forming, thereby improving the productivity of springs. This is intended to improve the precision of spring dimensions.

0010

[Means for Solving the Problems] In order to achieve the above object, a spring steel material according to the present invention has the following structure. That is, the spring steel material according to claim 1 is made of a multiphase mixed structure including a face-centered cubic lattice phase and a body-centered cubic lattice phase, and is formed by forming a solution-treated steel material into a predetermined shape and size as a raw material. In the spring steel material, the steel material is Mo
0.5 to 4 wt%, and the volume fraction of the body-centered cubic lattice phase contained in the solution-treated steel material is 20 to 8
This is a steel material for springs characterized by 0%.

A second aspect of the present invention provides a spring steel material according to the first aspect, wherein the spring steel material is a spring steel wire for a coil spring, a torsion spring, or the like. Further, the spring steel material according to claim 3 is the spring steel material according to claim 1, wherein the spring steel material is a plate spring steel plate.

0012

[Function] As mentioned above, in conventional coil springs, due to low-temperature annealing after coil spring forming, those made of piano wire or hard steel wire contract in outer diameter, while those made of stainless steel wire expand. . In terms of metallographic structure and crystal structure, the former consists mostly of body-centered cubic lattice phases (hereinafter referred to as BCC phases) such as ferrite and martensite.
On the other hand, the latter stainless steel consists mostly of a face-centered cubic lattice phase called austenite (hereinafter referred to as FCC phase). Therefore, the dimensional changes due to low-temperature annealing are in an opposite relationship between those made of the FCC phase and those made of the BCC phase. Therefore, it is thought that in a material consisting of a multiphase mixed structure including an FCC phase and a BCC phase, the two phases cancel each other out, making it difficult for dimensional changes to occur during low-temperature annealing.

[0013] The present invention is based on this idea, and the FC
This was done based on the following knowledge obtained by forming various springs with a multiphase mixed structure including C phase and BCC phase, annealing them at low temperature, and thoroughly investigating the amount of change in spring dimensions before and after annealing. be.

That is, a solution-treated steel material (raw material) is repeatedly cold-worked and annealed to form a cold-worked spring steel material, and various springs are made from the spring steel material. The dimensional change of the spring before and after annealing was measured.
The percentage of the volume of the BCC phase relative to the total volume of the C phase was determined, and the relationship between these volume ratios and the amount of spring dimensional change was investigated.

[0015] As a result, the volume fraction of the BCC phase in the solution-treated steel material (raw material) is in the range of around 50%, that is, A
In the range of (=50-α)% to B(=50+β)%, the spring dimensional change before and after annealing is extremely small, and this range (A to B%) can be achieved by containing 0.5wt% or more of Mo in the steel material. It has been found that the change in spring dimensions before and after annealing can be reduced reliably and stably (that is, with high practicality) by increasing the variation to a range of 20 to 80%. Furthermore, it has been found that the same tendency as described above is exhibited when the spring steel material or the formed spring does not contain deformation-induced martensite (hereinafter referred to as DM phase), which is a type of BCC phase. Therefore, the necessary and sufficient condition for reducing the spring dimensional change before and after annealing as described above is to add 0.5wt% Mo to the steel material.
BC containing the above and not containing DM phase
The aim is to set the C phase volume fraction to 20 to 80%, and to achieve this volume percentage, the BCC phase volume fraction in a solution-treated steel material that does not contain the DM phase must be set to 20 to 80%. .

Therefore, the spring steel material according to the present invention has a face-centered cubic lattice phase (FCC phase) and a body-centered cubic lattice phase (BCC phase).
A spring steel material formed by forming a solution-treated steel material into a predetermined shape and size as a raw material, the steel material having a multiphase mixed structure containing a phase) containing Mo of 0.5 to 4.
wt%, and the volume fraction of the body-centered cubic lattice phase (volume fraction of the BCC phase) contained in the solution-treated steel material is 2.
It is set at 0 to 80%. Therefore, according to the steel material for springs according to the present invention, the above-mentioned necessary and sufficient conditions are satisfied, and the spring dimensional change due to low-temperature annealing after spring forming can be reliably stabilized (highly practical) and extremely small. Become. Therefore, there is no need for a preliminary test, which improves the productivity of the spring, and there is less variation in the dimensions of the spring after annealing, making it possible to achieve high precision in the dimensions of the spring with high practicality.

[0017] The reasons for limiting the above-mentioned Mo content and BCC phase volume fraction will be explained below. If the Mo content is less than 0.5 wt%, the above spring dimensional change will be minimized.The practical range of the BCC phase volume ratio is narrow and practical, and as a result, the spring dimensions may vary, and the dimensions On the other hand, Mo is an expensive element, and if it exceeds 4wt%, it becomes extremely uneconomical, and depending on the steel composition, the steel may become brittle. Therefore, the Mo content is 0.5.
It is necessary to make it ~4wt%.

BCC phase volume fraction: less than 20%, or 80%
%, spring dimensional changes occur due to low-temperature annealing, making it difficult to minimize the amount of spring dimensional change and making it difficult to achieve high precision in spring dimensions. Therefore, the BCC phase volume fraction should be 20 to 80%. There is a need.

For example, FIGS. 1 to 4 are diagrams that prove the above, and FIG. 1 shows a raw wire when solution-treated wire (raw wire) is used as a raw material for spring steel material. The volume fraction of the BCC phase in , and the low temperature annealing after coil spring forming (
The relationship between the outer diameter change rate (Db) at around 300° C. for 10 minutes is shown using the Mo content in the steel material (equivalent to the Mo content in the raw wire) as a parameter. Furthermore, Db
was determined by the following formula (■).

[0020] Db (%) = 100 × (D1-D2)/
D1 ---■ However, in the formula, D1 is the outer diameter (mm) after coil spring forming, and D2 is the outer diameter (mm) after low-temperature annealing.
It is.

As can be seen from FIG. 1, when the volume fraction of the BCC phase in the original wire is around 50%, Db is almost zero and extremely small, but the range of the volume fraction of the BCC phase ( A~B%) Mo content: 0.5wt
narrow when it is less than %, whereas Mo content: 0.5w
When it is t% or more, it is extremely wide, ranging from 20 to 80%, and within this range, Db is approximately zero and no change in spring dimensions occurs. Therefore, by making the steel material contain 0.5 wt% or more of Mo,
And BCC on raw wire (i.e. solution treated steel)
When the phase volume ratio is set to 20 to 80%, Db can be reduced to approximately zero,
In addition, it can be seen that the practical range of the BCC phase volume fraction for reducing Db to approximately zero is wide, and the practicality can be greatly improved. At this time, spring steel material (steel wire) obtained by processing the raw wire
In this case, the BCC phase volume fraction may be even 100% by containing a large amount of DM phase, and Db becomes small. On the other hand, when the BCC phase volume fraction in the raw wire is smaller than 20%, the DM phase is generated by processing and the BCC phase in the spring steel material is reduced.
Although the volume fraction of the CC phase can be made 20 to 80%, even if this is done, Db cannot be made smaller, but rather becomes larger.

FIG. 2 shows the results for the case of a tension spring, and shows that when the Mo content of the steel material is 0.5 wt % or more, the BCC of the raw wire (solution-treated steel material) is
When the phase volume ratio is 20 to 80%, the amount of change in the intersection angle of the hooks at both ends before and after the low-temperature annealing (300°C x 10 minutes) after spring forming is small. Similar results were also obtained for torsion springs.

[0023] Figure 3 shows the results for a formed product, and shows that when the Mo content of the steel material is 0.5 wt% or more, the BCC phase volume fraction in the raw wire (solution-treated steel material) is 2.
At 0% to 80%, the dimensional change of L shown in the figure before and after low temperature annealing (300°C x 10 minutes) is small. Also, as shown in Fig. 4, in the case of a leaf spring, the Mo content of the steel material is 0.5wt.
% or more, BCC phase volume fraction at raw material stage: 2
In the range of 0 to 80%, the amount of change in the V angle before and after low temperature annealing (300°C x 10 minutes) is small.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Example] Various components of Cr-Ni-Si system and Cr
- Manufacture spring steel materials (wire rods and plate materials) made of Ni-Si-Mo steel, manufacture various springs from these materials, B
The CC phase volume fraction and the amount of dimensional change due to low-temperature annealing were measured. The details will be explained below.

(Example 1) 11 wire rods with a diameter of 5.5 mm
After solution treatment at 00℃, pickling and coating treatment, Φ3
．． The wire was drawn to a thickness of 0 mm and then solution-treated at 1100°C. This was used as a raw material (raw wire), and after pickling and coating, it was drawn at a speed of 120 m/min to a diameter of 1.0 m.
I made a wire rod for a spring. At this time, BC at the original line
The volume fraction of the C phase, the nominal amount of Mo, and the chemical composition were varied as shown in Table 1.

Next, using the above spring wire material, the number of turns: 1
It was formed into a coil spring with 5 turns and a spring index D/d:10. Here, D is the outer diameter of the coil spring, and d is the wire diameter. Thereafter, low-temperature annealing was performed at 300° C. for 10 minutes, and the rate of change Db in the spring outer diameter before and after annealing was determined. Table 2 shows the results.
Shown below. A negative Db value means that the outer diameter has shrunk.

[0029] Also, using the above spring wire material, the number of turns: 45
It was formed into a tension spring with 6 turns, D/d: 10, annealed in the same manner as above, the amount of change in the hook intersection angle before and after annealing was determined, and a torsion spring with 6 turns, D/d: 10 was formed. death,
Similarly, the amount of diagonal change before and after annealing was determined. Furthermore, a forming material similar to that shown in FIG. 3 was molded, and the dimensional change in L before and after annealing was similarly determined. These results are shown in Table 2. A negative value means that the angle or L has decreased.

(Example 2) Φ5.5m similar to Example 1
After rolling a wire rod of 2.0 mm into a ribbon-like plate with a thickness of 2.0 mm, it was solution-treated at 1100°C. This was used as a raw material (original plate), and after pickling, cold rolling was performed at a plate thickness reduction rate of 75% to produce a plate material for a spring with a plate thickness of 0.5 mm. The volume fraction of the BCC phase in the original plate is the same as that in the original wire (shown in Table 1).

Next, using the above spring plate material, a plate spring similar to that shown in FIG. 4 (tip bending radius: 8 times the plate thickness)
was molded and annealed in the same manner as in Example 1 (300°C x 10 minutes), and the amount of diagonal change before and after annealing was determined. The results are shown in Table 2. A negative value means that the diagonal has decreased.

As can be seen from Tables 2 and 1, steel materials (raw wire,
Mo content of spring wire rod or original plate) is 0.5 to 4 wt%.
In addition, when the BCC phase volume fraction in the raw material (original wire or original plate) is set to 20 to 80%, the amount of dimensional change (Db, amount of diagonal change) before and after low-temperature annealing is higher than in other cases. etc.) is extremely small.

[0033] In the above example, the solution treatment was carried out at 1100
℃, but is not limited to this temperature, and may be any temperature at which the alloying elements become a solid solution, for example, 1000°C. In addition, if the alloying elements have already been dissolved into a solid solution before such treatment and there is distortion such as processing distortion, it is sufficient to simply heat and anneal to a temperature that can remove the distortion.

[0034]

[Effects of the Invention] The steel material for springs according to the present invention has the structure and function described above, and by adjusting the volume fraction of the body-centered cubic lattice phase contained in the solution-treated steel material, the spring steel material has the structure and function described above. The volume of the body-centered cubic lattice phase makes it possible to reliably and stably reduce the amount of spring dimensional change due to low-temperature annealing after forming, and to minimize the spring dimensional change by adjusting the Mo content in the steel material. The practicality has been greatly improved by expanding the practical range of the rate, and therefore there is no need for preliminary tests, which improves the productivity of springs, and there is less variation in spring dimensions after annealing. This has the effect of increasing the precision of spring dimensions with high practicality.

[Brief explanation of drawings]

[Figure 1] Volume fraction of body-centered cubic lattice phase (BCC phase) and Mo content in raw material (raw wire) of spring steel wire, Mo content, and outer diameter change rate (Db) of coil spring due to low-temperature annealing. FIG.

FIG. 2 is a diagram showing the relationship between the volume fraction of the BCC phase in the original wire, the Mo content, and the amount of change in the hook intersection angle of the tension spring due to low-temperature annealing.

FIG. 3 is a diagram showing the relationship between the volume fraction of the BCC phase in the original wire, the Mo content, and the amount of change in L dimension of the formed product due to low-temperature annealing.

FIG. 4 is a diagram showing the relationship between the volume fraction of the BCC phase and the Mo content in the raw material (original plate) and the amount of change in the V angle of the leaf spring due to low-temperature annealing.

Claims (3)

[Claims] 1. A spring steel material formed from a solution-treated steel material having a multiphase mixed structure including a face-centered cubic lattice phase and a body-centered cubic lattice phase and formed into a predetermined shape and size using the steel material as a raw material. A steel material for a spring, characterized in that the steel material contains 0.5 to 4 wt% of Mo, and the volume fraction of the body-centered cubic lattice phase contained in the solution-treated steel material is 20 to 80%. 2. The spring steel material according to claim 1, wherein the spring steel material is a spring steel wire for a coil spring, a torsion spring, or the like. 3. The spring steel material according to claim 1, wherein the spring steel material is a plate spring steel plate.