JP2003041339A - Steel material for constant velocity joint outer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel material for a constant velocity joint outer of an automobile, superior in cold forgeability. SOLUTION: This steel material includes 0.4-0.6% C, 0.05% or less Si, 0.10-0.4% Mn, 0.10% or less Cr, Ti, B, and Al, or further Mo, and controlled S as an impurity of 0.004% or less, and further controlled O, N, and P of a reasonable value or less. The manufacturing method comprises sequentially hot rolling and softening annealing the above steel material, preferably to make the spheroidizing rate of cementite to be 30% or more. Thereby, components having complex shapes can be formed by cold forging, without performing intermediate annealing between many stages of cold forging.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material for a structural member of an automobile, and more particularly to a steel material having excellent cold forgeability, which is suitable for a constant velocity joint outer which is important for a structural member.

[0002]

2. Description of the Related Art In cold forging, processing is performed without heating the material, so that the equipment is simple, the finishing dimensional accuracy is excellent, the material yield is high, and the die (tool) life is long. Has advantages such as a small amount of cutting after molding, and is applied to the manufacture of automobile parts such as constant velocity joint outers.

However, in the cold forging temperature range, the steel used has a high deformation resistance and a low deformability. In order to carry out cold forging industrially, it is important to suppress the hardness of the material (steel material) used and realize low deformation resistance,
If the content of the hardenability-improving element is reduced in order to achieve low deformation resistance, there is a problem that the induction hardenability deteriorates. In the manufacture of automobile parts and the like, after being processed into a predetermined size and shape by cold forging, the surface is often hardened by a treatment such as induction hardening, and deterioration of hardenability becomes a problem. Therefore, there has been a demand for a steel material having both excellent cold forgeability and induction hardenability.

In response to such a demand, for example, Japanese Patent Laid-Open No. 2-
No. 129341 discloses C: 0.40 to 0.60%, Si: 0.05% or less, Mn: 0.20 to 0.65%, Al: 0.01 to 0.05%, Cr: 0.30%.
As the following, a carbon steel for machine structural use has been proposed, which further contains Ti and B and is excellent in cold forgeability and induction hardenability in which S, O and N as impurities are reduced to a predetermined amount or less.
Further, in Japanese Patent Laid-Open No. 2-145745, C: 0.25 to 0.65%,
Si: 0.15% or less, Mn: 0.60% or less, B: 0.0005 to 0.0050
%, Ti: 0.005 to 0.05%, Mo, V, and S for 0.015% or less have been proposed for cold forging steel.

In Japanese Patent Laid-Open No. 9-268344, C: 0.
45 to 0.60%, Si: 0.01 to 0.15%, Mn: 0.10 to 1.00%, C
r: 0.3% or less, Al: 0.015 to 0.050%, further containing Ti and B, and carbides having an average particle size of 5 μm or less dispersed in an average particle interval of 20 μm or less and excellent in cold forgeability. Input steel has been proposed. In addition, JP-A-10-960
No. 47 discloses C: 0.25 to 0.65%, Si: 0.15% or less, M
n: 0.60% or less, B: 0.0005 to 0.0050%, Ti: 0.005 to
There has been proposed a cold forging steel containing 0.05%, further containing Mo, V, and Cr, or further containing Nb, Ta, and Zr, and having S reduced to 0.015% or less.

[0006]

SUMMARY OF THE INVENTION
JP-A-2-129341, JP-A-2-145745, JP-A-9-2683
Even if the steel materials described in Japanese Patent Publication No. 44 and Japanese Patent Application Laid-Open No. 10-96047 are used, the workability is high and the complicated shape is similar to that of the constant velocity joint outer of an automobile, a typical example of which is shown in FIG. In order to prevent cracking of the steel material and increase of forming load, cold forging is performed in multiple steps and after performing intermediate annealing between each forging step to recover the deformability of the steel material. I was molding.

Particularly, in the production of constant velocity joint outers for automobiles, performing the intermediate annealing between the cold forging steps of the respective stages requires not only the cost required for the heat treatment itself but also the cost required for the heat treatment itself.
There has been a problem that the cost is increased due to the generation of intermediate inventory due to the complicated process, and the manufacturing cost of the constant velocity joint outer is increased. For these reasons, there has been a strong demand for a steel material for constant velocity joint outers, which is capable of manufacturing a product having a high degree of workability and a complicated shape without intermediate annealing.

An object of the present invention is to solve the above-mentioned problems of the prior art advantageously, and to propose a steel material for an automobile constant velocity joint outer which is inexpensive and excellent in cold forgeability.

[0009]

Means for Solving the Problems The present inventors first conducted various studies on factors that enable the omission of intermediate annealing in the cold forging process of the constant velocity joint outer for automobiles. First, the present inventors, without performing intermediate annealing, in order to be able to form a component having a complicated shape by cold forging, based on the idea that it is essential to improve the deformability of the steel material, Various experiments and studies were conducted on the occurrence of cracks during cold forging.

As a result, it was found that cracks during cold forging mainly originated from fine cracks caused by delamination of the interface between the ferrite matrix phase of steel and MnS. Moreover, the present inventors significantly reduced the S content to 0.004 mass% or less, whereby the amount of MnS in the steel was remarkably reduced, and the size was refined, and the ferrite matrix phase and Mn
It was found that the generation of fine cracks caused by the peeling of the interface with S can be suppressed. Furthermore, it was also found that the anisotropy of the deformability of steel material was significantly reduced with the miniaturization of the MnS size.

The present inventors have also found that fine cracks, which are the starting points of cracks during cold forging, may occur due to fracture of rod-shaped carbide (cementite).
From this, the present inventors have found that the carbide (cementite)
It was also found that setting the spheroidization ratio of the above to a predetermined value (30%) or more is also effective in preventing cracking during cold forging. By using these methods, the present inventors require intermediate annealing by cold forging of a constant velocity joint outerwear for automobiles having a high workability and a complicated shape, which has been conventionally impossible. I found that it can be molded without.

The present invention has been completed by further studies based on the above findings. That is, in the present invention, in mass%, C: 0.4 to 0.6%, Si: 0.05% or less, M
n: 0.10 to 0.4%, Cr: 0.10% or less, Ti: 0.005 to 0.05
%, B: 0.0003 to 0.0030%, Al: 0.01 to 0.05%, and S, O, N, and P as impurities, S: 0.004% or less, O: 0.0020% or less, N: 0.007% or less, P: 0.010
% Or less, excellent cold forgeability, characterized by having a composition consisting of balance Fe and unavoidable impurities, is a steel material for automobile constant velocity joint outer, in the present invention, the composition In addition, in further mass%, Mo: 0.
In the present invention, it is preferable that the content is 05 to 0.2%. Further, in the present invention, the following formula (1) (cementite spheroidization rate) = (the number of cementite particles having an aspect ratio of less than 2) / (the total number of cementite particles) ... It is preferable that the cementite spheroidization rate A defined in (1) has a structure of 30% or more.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the composition of the steel material of the present invention will be described in detail. Hereinafter, mass% is simply referred to as%. C: 0.4 to 0.6% C is an element effective in ensuring the surface hardness and effective hardening depth during induction hardening, and is actively utilized.
However, if the C content is less than 0.4%, it becomes difficult to secure the strength required for machine parts, while if the C content exceeds 0.6%, the deformability and the deformation resistance increase, and the cold forgeability deteriorates. . Therefore, C is limited to the range of 0.4 to 0.6%.

Si: 0.05% or less Si dissolves in the ferrite matrix during spheroidizing annealing to increase the deformation resistance after cold forging, so it is preferable to reduce it as much as possible, but up to 0.05% is acceptable. Mn: 0.10-0.4% Mn is an element that is effective in ensuring hardenability, but at the same time is an element that increases the deformation resistance during cold forging,
In the present invention, it is 0.10 to 0.4%. When Mn is less than 0.10%,
The induction hardenability is insufficient, while if it exceeds 0.4%, the deformation resistance increases and the cold forgeability deteriorates.

Cr: 0.10% or less Cr is an element that forms a solid solution in carbides during annealing and makes the carbides insoluble, thereby deteriorating the induction hardenability and increasing the deformation resistance during cold forging. Then, it is preferable to reduce as much as possible, but 0.10% is acceptable. Ti: 0.005 to 0.05% Ti has a strong affinity with C and N, and has the action of forming precipitates such as carbides, nitrides or carbonitrides to reduce the solid solution C and N in ferrite. . This allows
Strain aging is suppressed, and the deformation resistance during cold forging is reduced.
Further, Ti is an element useful for effectively exerting the hardenability improving effect of B, but if it is less than 0.005%, these effects are not sufficiently recognized. On the other hand, if the content exceeds 0.05%, coarse nitrides are formed, the deformability during cold forging is reduced, and the rolling fatigue life is significantly reduced.
Therefore, Ti is limited to the range of 0.005 to 0.05%.

B: 0.0003 to 0.0030% B is an effective element for improving hardenability, but 0.00
If it is less than 03%, its effect is small. On the other hand, if it exceeds 0.0030%, its effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, B is 0.
It was limited to the range of 0003 to 0.0030%.

Al: 0.005 to 0.05% Al is an element which acts as a deoxidizing agent and also has an effect of effectively exerting the hardenability improving effect of B by forming AlN by combining with N. , Less than 0.005%, the effect is insufficient. On the other hand, even if the content exceeds 0.05%, the effect is saturated and the effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, Al is 0.005-0.05.
It was limited to the range of%.

In the present invention, Mo may be added, if necessary, in addition to the above-mentioned components for the purpose of improving the hardenability. Mo: 0.05-0.2% Mo is an element that improves hardenability, but less than 0.05% is less effective, while more than 0.20% contains:
Work hardening becomes large, and deformation resistance during cold forging increases. Therefore, Mo is preferably limited to 0.05 to 0.2%.

Further, in the present invention, S as an impurity,
O, N, P are reduced below a predetermined value. S: 0.004% or less S is an element that forms MnS in steel and improves machinability, but MnS becomes a starting point of cracking during cold forging and deteriorates cold forgeability. When the molding material is required to have extremely high deformability, such as the constant velocity joint outer of the present invention, it is necessary to reduce S as much as possible. By reducing S to 0.004% or less,
The amount of MnS in steel is significantly reduced, and the size of MnS present is also reduced. Therefore, by reducing the S content to 0.004% or less, the deformability during cold forging is significantly improved, including anisotropy, and it is difficult to perform cold forging without intermediate annealing. The quick joint outer can be formed by cold forging without intermediate annealing.

O: 0.0020% or less O forms an oxide type non-metallic inclusion with Al, Mn, Si, etc. in steel, and deteriorates both cold forgeability and rolling contact fatigue property. Therefore, it is necessary to reduce O as much as possible, and in the present invention, it is limited to 0.0020% or less. N: 0.007% or less N forms a solid solution in ferrite to cause strain aging and increases deformation resistance, and at the same time, forms a BN by combining with B to reduce the effective B content and improve the hardenability of B. Reduce. From this, it is necessary to reduce N as much as possible, but 0.007
Up to% is acceptable.

P: 0.010% or less P acts effectively for improving the machinability, but on the other hand, it embrittles the ferrite phase and deteriorates the cold forgeability. Further, P segregates at the grain boundaries during quenching and tempering to lower the grain boundary strength, lowering the resistance to the propagation of fatigue cracks and lowering the fatigue strength. From this, it is necessary to reduce P as much as possible, but up to 0.010% is allowed.

The balance other than the above components is Fe and inevitable impurities. The steel material of the present invention, in addition to the above composition,
If necessary, the cementite spheroidization rate A defined by the following formula (1) A = (the number of cementite particles having an aspect ratio of less than 2 / (the total number of cementite particles) ..... (1) is 30%. It is preferable to have the above structure .. The spheroidization rate of cementite is 30.
%, The deformability at the time of cold forging is improved and the deformation resistance is significantly reduced as compared with those of less than 30%.

Next, a preferred method for producing the steel material of the present invention will be described. First, it is preferable to melt the molten steel having the above-mentioned composition by a commonly known melting method such as a converter, and then make it into a steel material having a predetermined size and shape by a commonly known casting method such as a continuous casting method. Then, these steel materials are sequentially subjected to a hot rolling step and a softening annealing treatment step to obtain a steel material having a predetermined size and shape. In the hot rolling step, the steel material having a predetermined size and shape may be used, and the hot rolling conditions are not particularly limited. From the viewpoint of microstructure refinement after hot rolling, the heating temperature of the steel material is preferably 1100 ° C or lower.

The softening / annealing step is preferably a hot rolling step, and is preferably performed on a steel material rolled into a predetermined size and shape.
It is preferable to heat and hold at 00 to 760 ° C. for 3 to 20 hours to reduce the hardness and to make the carbide spherical. By this softening / annealing step, the cementite spheroidization rate A defined by the following formula (1): (the number of cementite particles having an aspect ratio of less than 2 / (the total number of cementite particles)) (1) Of 30% or more is preferable.By setting the spheroidization rate of cementite to 30% or more, the deformability at the time of cold forging is improved and the deformation resistance is significantly increased as compared with those of less than 30%. descend.

Next, using the steel material of the present invention as a forming material,
A preferred process for manufacturing a constant velocity joint outer for an automobile will be described. The steel material of the present invention is used as a forming material, and the forming material is usually made into a component having a predetermined size by a cold forging step and a cutting step. If the steel material is used in the present invention, the component can be manufactured without the intermediate annealing step.

The cold forging may be carried out by an ordinary cold forging machine, and it is preferable to perform forging a plurality of times by using a die having a predetermined shape according to the deformation resistance. When using the steel material of the present invention, intermediate annealing is not performed in the cold forging step. In the cold forging process, the time between each forging process is 12
It is preferably 0 s or less. After the cold forging step, a cutting step is performed so as to obtain a component having a predetermined size.

Then, these parts having predetermined dimensions are heated in whole or in part to the Ac ₃ transformation point or higher, quenched, and then subjected to a heat treatment step of tempering to obtain products.
Needless to say, as the quenching and tempering conditions in the heat treatment step, the temperature and time can be appropriately selected according to the desired performance, except that the quenching temperature is set to the Ac ₃ transformation point or higher.

[0028]

Example A steel material having the composition shown in Table 1 was used, and the steel material was
By a hot rolling process of heating at 850 to 1100 ° C and rolling, a straight bar having a cross section of 52 mmφ was formed. Then, these straight rods are subjected to a softening annealing process of 700 to 760 ° C. for 3 to 7 hours,
Steel material. Among the steel materials, No. A, No. B and No. C are those whose compositions are within the scope of the present invention, No.
D and No. E are comparative examples in which the amount of S is outside the scope of the present invention, and No. F is
It is equivalent to JIS standard S48C.

First, for these steel materials, (1) MnS distribution state investigation, (2) cementite spheroidization rate investigation, (3) cold forgeability test, (4) cold forging test into solid constant velocity joint outer shape (5) An induction hardenability test was conducted. Each test method is shown below. (1) MnS distribution state investigation The distribution of MnS in steel was investigated based on the ASTM E45 method. Measurement area is 0.5mm ² × 320 field of view (total field of view area 160mm ² )
The worst field of view (ASTM-A method evaluation) and the number of fields at Thin 0.5 points (ASTM-D 0.5T field number) were measured.

(2) Investigation of cementite spheroidization rate A test piece was taken from a 1 / 4d part of a steel material (straight rod), and after polishing, it was corroded by a Picral solution, and 5 sections were observed using a scanning electron microscope. , 10 fields were imaged at a magnification of 5000 times at each location. Based on this image, the aspect ratio of each cementite particle was measured using an image analyzer, and the cementite spheroidization rate A in each molding material was calculated as follows (1) Cementite spheroidization rate A = (cementite with an aspect ratio of less than 2 Number of particles / (total number of cementite particles) ... (1) was used for calculation.

(3) Cold forgeability test A cylindrical test piece of 15 mmφ × 22.5 mmH was sampled by machining from a material of 20 mmφ obtained by drawing each softened and annealed steel material with a total area reduction ratio of 60%. did. At this time, the test pieces were sampled in two directions, a parallel direction and a vertical direction with respect to the drawing process direction.

A cold compression test was carried out using these test pieces, and 10 test pieces each at various compression rates. The deformation resistance of the material was calculated from the load during the test. After the test, check for the occurrence of cracks on the side of each test piece,
Crack generation rate at each compression rate (number of cracked test pieces / 10
Number) was calculated. From the test results, the compression rate at which the crack generation rate was 50% was determined as the limit compression rate. Furthermore, the ratio of the critical compression ratios in the two types of test piece sampling directions was determined as the critical ratio.

(4) Cold forging test for outer shape of constant velocity joint outer joint Each of the softened and annealed steel materials is subjected to a cold forging step consisting of three stages of cold forging to obtain the shape schematically shown in FIG. Size: 100
It was molded into a solid constant velocity joint outer shape with a size of mmφ × 150 mml. In the cold forging of each stage, the material as cold forged in the previous stage was used as the material of the next stage, and the intermediate annealing was not performed.
The occurrence of cracks at each forging stage was measured using n = 50 test pieces. The crack occurrence rate (%) was calculated by (number of test pieces with cracks) / 50 × 100.

(5) Induction hardenability test In the induction hardenability test, 30 mmφ × 100 mml test pieces were taken from a steel material, and these test pieces had a frequency of 15 kHz and an output of 114 k.
After induction hardening under the moving quenching conditions of W and a test piece moving speed of 10 mm / s, tempering was performed at 150 ° C for 1 hour. The surface hardness (HRC) and the hardening depth (effective hardening depth) at which Hv: 400 or more were measured for the test pieces after the heat treatment.

The results are shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

In each of the examples of the present invention, MnS was miniaturized,
The amount of MnS is also significantly reduced. Compared to this, S
In steel materials No. 6 to No. 8 whose content is out of the range of the present invention,
Miniaturization and reduction of the amount of MnS have not been realized. Moreover, all of the examples of the present invention show a high limit compression rate of 69% or more in the direction parallel to the drawing direction. The limit ratio, which is the ratio of the limit compression rate in the direction parallel to the drawing direction and the limit compression rate in the direction perpendicular to the drawing direction, shows a high value of 0.86 or more.
It is considered that these effects are due to the decrease in the amount of MnS and the miniaturization accompanying the decrease in the amount of S. On the other hand, in the comparative examples (steel materials No. 6 to No. 8) out of the range of the present invention, the limit compression ratio in the direction parallel to the drawing direction is 52 to 65%, and the limit ratio is 0.
It is as low as 58 or less, has low cold forgeability and is anisotropic.

In the example of the present invention, the solid constant velocity joint outer having a complicated shape can be formed by cold forging without intermediate annealing. In the example of the present invention, no cracks were generated until the final cold forging in the third step. On the other hand, in Comparative Examples outside the scope of the present invention, most of the cracks were generated by the final cold forging in the third stage. Example of the present invention in which the spheroidizing rate of cementite is within a suitable range (steel materials No. 2 and N
In o.5), cracking was not observed in the final cold forging in the third step.

Further, it can be seen that each of the examples of the present invention has sufficiently high induction hardenability. The effective hardening depth after induction hardening and tempering showed a value equal to or higher than that of the comparative example having substantially the same C content. Steel No. 1 using the Mo-containing steel material shows higher induction hardenability than Steel No. 3 containing no Mo.

[0041]

As described above, according to the present invention, it is possible to form a constant velocity joint outer having a high workability and a complicated shape by cold forging without performing intermediate annealing. It is possible to obtain a constant velocity joint outer with excellent dimensional accuracy as compared with other molding methods such as, for example, at a low cost, and it is possible to achieve a marked effect in industry.

[Brief description of drawings]

FIG. 1 is an explanatory view schematically showing an example of the shape of a constant velocity joint outer.

[Explanation of symbols]

1 joint outer 2 inner 3 ball steel

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshiyuki Hoshino             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Shinichi Amano             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works (72) Inventor Masayoshi Saga             19 Matsuyama-cho, Moka City, Tochigi Prefecture Honda Motor Co., Ltd.             Tochigi Co., Ltd. (72) Inventor Yu Terauchi             19 Matsuyama-cho, Moka City, Tochigi Prefecture Honda Motor Co., Ltd.             Tochigi Co., Ltd. (72) Inventor Naohiro Ogura             19 Matsuyama-cho, Moka City, Tochigi Prefecture Honda Motor Co., Ltd.             Tochigi Co., Ltd. (72) Inventor Yutaka Sato             19 Matsuyama-cho, Moka City, Tochigi Prefecture Honda Motor Co., Ltd.             Tochigi Co., Ltd.

Claims (3)

[Claims] 1. In mass%, C: 0.4 to 0.6%, Si: 0.05% or less, Mn: 0.10 to 0.4%, Cr: 0.10% or less, Ti: 0.005 to 0.05%, B: 0.0003 to 0.0030%, Al : 0.005-0.05%, S, O, N, P as impurities S: 0.004% or less, O: 0.0020% or less, N: 0.007% or less, P: 0.010% or less, the balance Fe and A steel material for a constant velocity joint outer of an automobile, which is excellent in cold forgeability and has a composition of inevitable impurities. 2. In addition to the above composition, M% by mass is further included.
O: 0.05-0.2% is contained, Claim 1 characterized by the above-mentioned.
Steel material for constant velocity joint outer described in. 3. The steel material for a constant velocity joint outer according to claim 1, having a structure having a cementite spheroidization rate A defined by the following formula (1) of 30% or more. Note (cementite spheroidization rate) = (number of cementite particles having an aspect ratio of less than 2) / (total number of cementite particles) ......... (1)