JP4487749B2 - Constant velocity joint inner ring and manufacturing method thereof - Google Patents

Description

  The present invention relates to a constant velocity joint inner ring used for a drive axle of a power transmission device of an automobile or an industrial machine, and a manufacturing method thereof, in particular, excellent in fatigue strength, and excellent in machinability and productivity in a manufacturing process. The present invention relates to an inner ring of a constant velocity joint and a manufacturing method thereof.

2. Description of the Related Art Conventionally, in a power transmission device for an automobile, the driving axle on the wheel side is usually via a constant velocity joint 3 to transmit power from the drive shaft 1 to the hub 2 of the wheel as shown in FIG. is there.
The constant velocity joint 3 is a combination of an outer ring 4 and an inner ring 5. That is, the inner ring 5 is slidably fixed to the inner side of the mouse part 4a via a ball 6 fitted in a ball raceway groove formed on the inner surface of the mouse part 4a of the outer ring 4, and the drive shaft 1 is attached to the inner ring 5. On the other hand, the stem portion 4b of the outer ring 4 is splined to the hub 2, for example, to transmit power from the drive shaft 1 to the hub 2 of the wheel.

  The spline portion, which is a fitting portion with the drive shaft of the inner ring of the constant velocity joint, is subjected to rolling fatigue accompanied by a slight slip, that is, slip rolling fatigue when performing repeated power transmission. Therefore, the inner ring of the constant velocity joint is subjected to induction hardening and tempering after hot forging, cold forging, cutting and rolling, etc. are processed into a predetermined shape. In general, a hardened surface layer is formed to ensure a sliding rolling fatigue life, which is an important characteristic for this type of application (see Patent Document 1).

  On the other hand, due to recent environmental problems, there is a strong demand for compactness, weight reduction, and long life of the above parts, as represented by the demand for weight reduction for automobile parts. There is a demand for further improvement in strength.

  In general, the alloying of steel materials and the regulation of cold working conditions are effective for increasing the strength of materials that contribute to improving fatigue strength, but the addition of alloys causes a decrease in workability and machinability. From the viewpoint of improving the efficiency of industrial production and reducing the cost, problems remain. Moreover, in order to provide a large working rate in cold working, it is necessary to appropriately insert an annealing step during cold working, and in this case as well, problems remain in productivity and manufacturing cost.

  As a solution to the above problem, Patent Document 2 discloses a technique for improving machinability while maintaining cold forgeability by graphitizing C in steel. However, in this technology, since the Si content is as high as 1.9 to 3.0 mass%, the deformation resistance during cold forging is large, and the formed graphite is also large and has low deformability, so this technology is used industrially. It ’s difficult.

  In addition, Patent Document 3 discloses a technique for improving machinability by graphitizing C in steel, but in this method, quenching before graphitization is indispensable. There are problems in manufacturing cost and manufacturing efficiency.

  Furthermore, Patent Document 4 discloses a technique for improving the cold workability by precipitating graphite, but it still takes a long time for graphitization, so that there remains a problem for industrial use. is there.

On the other hand, in Patent Document 5, in hot working a steel material in which the contents of C, Si, Mn, B, Al, N, and Cr are hot-worked, by specifying the cooling rate after hot work, graphite is specified. A technique for improving the machinability by fine precipitation is disclosed.
JP 2002-371320 A Japanese Patent Laid-Open No. 51-57621 JP 49-103817 A Japanese Patent Laid-Open No. 3-140411 JP 2002-180185 A

  Here, the technique described in Patent Document 5 has made it possible to improve machinability without incurring strength deterioration. However, as described above, due to recent environmental problems, fatigue strength is further increased. In order to obtain higher fatigue strength, when the solid solution of C is suppressed, the precipitation of graphite is also suppressed and the machinability varies greatly. There remains room for improvement where it is difficult to stably obtain a steel material that balances strength with high order.

  As described above, when trying to form a constant velocity joint inner ring by cold forging using a conventionally developed graphite precipitation steel, it is disadvantageous in terms of productivity, manufacturing cost, and quality control. In particular, when graphite precipitated steel is applied to the constant velocity joint inner ring, it is always difficult to achieve both the fatigue strength of the part and the machinability in the manufacturing process of the constant velocity joint inner ring depending on the amount and state of precipitation of the graphite. It becomes a problem.

  Accordingly, the present invention provides a constant velocity joint inner ring with extremely good machinability (hereinafter simply referred to as machinability) in cutting for finishing into a part shape after hot working, and having excellent fatigue characteristics, It is an object of the present invention to provide a manufacturing method thereof.

  According to the study by the inventors, graphitizing and precipitating carbon in the steel material is effective for enhancing the machinability of the constant velocity joint inner ring. It has been found that the hardness of the parent phase itself decreases, and that the graphite acts as a lubricant during cutting, thereby suppressing an increase in the temperature of the tool. In addition, it is indispensable to finely disperse graphite in order to improve machinability. The reason is that the machinability improvement effect of graphite, along with the lubrication effect of graphite, the material is deformed in the shear region during cutting, and a crack is formed at the interface between the graphite and the parent phase. This is based on the mechanism of becoming. That is, as the graphite is finely dispersed and the average distance between the graphite and the graphite is shorter, the cracks are more easily connected.

  On the other hand, in order to increase the fatigue strength, it is important to increase the hardness of the base metal structure and to minimize the amount of non-metallic inclusions contained in the steel material. In particular, regarding the increase in hardness, the effect of increasing C is remarkable. Furthermore, focusing on the fact that the high C that brings about this hardness increase also raises the C supersaturation degree of the steel to make it easy to precipitate graphite, and the conditions under which the machinability can be improved simultaneously with the high hardness. , Repeated examination.

  By the way, the performance required for the constant velocity joint inner ring is manufacturability (machinability) and sliding rolling fatigue strength, and it is desirable that the rolling fatigue strength is also high. As described above, as described above, the addition of C has a remarkable effect of improving the fatigue strength. Therefore, increasing the amount of C is effective for improving the material properties inexpensively and easily. Here, when C contained in a high proportion is precipitated as graphite, graphitized C improves machinability, but there is a saturation point in the machinability improvement effect related to the amount of precipitation. The decrease in fatigue strength accompanying the precipitation of graphite is related to both the strengthening contribution of C itself, that is, the decrease in the ratio itself contributing to the strengthening of steel as a solid solution and pearlite, and the propagation of fatigue cracks due to graphite. It was revealed that the fatigue strength decreased.

  Here, as for the addition C, the amount contributing to improvement of machinability as graphite, the solid solution content and the strengthening contribution through pearlite precipitation were separated and evaluated by the method described later. It was found that less than 1 mass% of C contributed little. That is, if the amount of C precipitation as graphite increases, machinability improves at the expense of fatigue strength, but in order to combine both characteristics of fatigue strength and machinability, 1 mass% of the amount of added C It has been newly found that the above C needs to be precipitated as graphite.

  Furthermore, as a result of investigation, since the amount of precipitation of C is related to the amount of supersaturated component of the parent phase, in order to precipitate 1 mass% or more of added C as graphite, the heating temperature before hot working must be set. It has also been found that it is effective to set the temperature to 950 ° C. or higher. That is, industrially, by adopting a temperature of 950 ° C. or higher as a heating temperature prior to hot working such as forging, the effect of C solid solution can be obtained, and a predetermined graphite is cooled during cooling after the subsequent working. An amount of precipitation can be obtained.

On the other hand, the inventors paid attention to the following points as characteristics of steel materials related to propagation of fatigue cracks.
That is, despite the increase in strength, the fatigue strength may not always improve. This reduction in fatigue strength is a problem that directly affects the life of the component. The inventors presumed that this was caused by the fact that the graphite precipitates were prone to fatigue crack initiation and propagation sites. However, it was clarified that it does not cause the fatigue strength to decrease. It has also been clarified that the fine precipitation of graphite can be achieved with a remarkable effect by setting the total work rate during hot working to 70% or more and making the entire metal structure fine structure.

As a result of examining this knowledge together with the improvement of the machinability described above, when the processing rate in hot working is 70% or more, the particle size of graphite has reached a fine level of 5 μm or less, and the fatigue strength deteriorated. It has been clarified that not only there is no adverse effect, but the strength is improved, and machinability is easily improved at the same time. That is, from the viewpoint of the balance between machinability and fatigue strength, the amount of precipitation of added C as graphite is controlled, and from the viewpoint of coexistence of machinability and fatigue strength, the precipitation form of graphite is fine. It is important to make it.
Based on the above findings, the present invention has been led.

That is, the gist configuration of the present invention is as follows.
(1) C: 0.2 to 1.5 mass%,
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and
N: 0.001 to 0.015 mass%
The steel structure is composed of ferrite, cementite and graphite, and the graphite is precipitated as graphite grains having an average particle size of 5 μm or less and a particle size of 10 μm or less. A constant velocity joint inner ring characterized by being made of carbon steel having a C amount of 1 mass% to 50 mass% of the total C amount.

(2) C: 0.2 to 1.5 mass%,
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and
N: 0.001 to 0.015 mass%
And having the composition of the balance Fe and inevitable impurities , consisting of ferrite, cementite and graphite, and the graphite has an average particle size of 5 μm or less and a C amount precipitated as graphite particles having a particle size of 10 μm or less. A constant velocity joint inner ring comprising a carbon steel having a structure of 1 mass% or more and 50 mass% or less of the total C amount and a quenching structure by induction hardening applied to a portion where at least fatigue characteristics are required.

(3) In the above (1) or (2), the carbon steel further comprises
Mo: 3.0mass% or less,
W: 3.0mass% or less,
Al: 0.06 mass% or less,
Ti: 0.05 mass% or less,
Ni: 3.0mass% or less,
Co: 3.0mass% or less,
V: 0.1 mass% or less,
Cu: 1.5 mass% or less,
Nb: 0.07 mass% or less and
Ta: 0.20mass% or less
A constant velocity joint inner ring characterized by containing one or more selected from the group consisting of:

(4) In any one of the above (1) to (3) , the carbon steel further includes
Ca: 0.008 mass% or less,
Mg: 0.005 mass% or less,
Zr: 0.10 mass% or less,
Pb: 0.30 mass% or less,
Bi: 0.30 mass% or less,
Te: 0.30 mass% or less,
Se: 0.30 mass% or less and
REM: A constant velocity joint inner ring characterized by containing one or more selected from 0.20 mass% or less .

(5) C: 0.2 to 1.5 mass%,
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and
N: 0.001 to 0.015 mass%
Carbon steel having a composition composed of the balance Fe and inevitable impurities, and heating the material at 950 ° C. or higher, and thereafter processing rate R1 of 1150 ° C. or higher, less than 1150 ° C. to over 980 ° C. A method for producing a constant velocity joint inner ring , wherein hot working is performed with a total machining rate of 70% or more, which is a total value of a machining rate R2 and a machining rate R3 of 980 ° C. or lower and 750 ° C. or higher .

(6) C: 0.2 to 1.5 mass%,
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and N: 0.001 to 0.015 mass%
Carbon steel having a composition comprising the balance Fe and inevitable impurities, and heating the material at a temperature of 950 ° C. or higher, followed by a processing rate R1 of 1150 ° C. or higher and a processing of less than 1150 ° C. to more than 980 ° C. that all working rate is the sum of the working ratio R3 at a rate R2,980 ° C. or less 750 ° C. or higher to facilities for hot working of 70% or more, further performing induction hardening at a site at least fatigue characteristics are required A method of manufacturing a constant velocity joint inner ring, which is characterized.

(7) In the above (5) or (6), the material further includes
Mo: 3.0mass% or less,
W: 3.0mass% or less,
Al: 0.06 mass% or less,
Ti: 0.05 mass% or less,
Ni: 3.0mass% or less,
Co: 3.0mass% or less,
V: 0.1 mass% or less,
Cu: 1.5 mass% or less,
Nb: 0.07 mass% or less and
Ta: 0.20mass% or less
The manufacturing method of the constant velocity joint inner ring | wheel characterized by including 1 type, or 2 or more types chosen from these .

(8) In any one of the above (5) to (7) , the material further includes
Ca: 0.008 mass% or less,
Mg: 0.005 mass% or less,
Zr: 0.10 mass% or less,
Pb: 0.30 mass% or less,
Bi: 0.30 mass% or less,
Te: 0.30 mass% or less,
Se: 0.30 mass% or less and
REM: A method for producing a constant velocity joint inner ring, comprising one or more selected from 0.20 mass% or less .

(9) The method for manufacturing a constant velocity joint inner ring according to any one of the above (5) to (8), wherein the processing rate R2 is 0 to 10%.

  According to the present invention, it is possible to stably obtain a constant velocity joint inner ring having both excellent fatigue characteristics and machinability. As a result, for example, it is excellent for the demand for weight reduction of automobile parts.

The present invention will be specifically described below.
That is, as shown in FIG. 2, the constant velocity joint inner ring 5 has at least a rolling surface 5a of the ball 6 interposed between the constant velocity joint outer ring 4 shown in FIG. The fitting surface 5b is required to be excellent in fatigue characteristics. Furthermore, in the constant velocity joint inner ring 5, it is also possible to form a hardened surface layer 7 by induction hardening on one or both of the rolling surface 5a and the fitting surface 5b, and also on other portions. is there.

In such a constant velocity joint inner ring, it is first necessary to be made of carbon steel having a steel structure made of ferrite, cementite (iron carbide) and graphite. Since pearlite has a layered structure of ferrite and cementite, it is included in the above.
Here, as described above, from the viewpoint of the balance between machinability and fatigue strength, it is possible to control the amount of precipitation of added C as graphite, and also from the viewpoint of coexistence of machinability and fatigue strength, It is important to make the precipitate form fine. Therefore, graphite has an average particle size of 5 μm or less, and the amount of C deposited as graphite particles with a particle size of 10 μm or less is 1 mass% or more of the total C content, so that both machinability and fatigue strength can be achieved at a higher order. Realize.

That is, in the present invention, the steel structure is composed of ferrite, cementite and graphite, and the graphite has an average particle size of 5 μm or less, and the amount of C precipitated as graphite particles having a particle size of 10 μm or less is 1 mass of the total C amount. % Or more.
First, the reason why graphite is necessary in the structure is that machinability is inferior if graphite is not deposited when the steel material is cut. The reason why the balance other than graphite is ferrite and cementite is that when the graphite particles of the amount and size described later are precipitated in the steel material, the steel structure becomes ferrite and cementite and pearlite which is a mixed structure of both.

  Further, the graphite needs to have an average particle size of 5 μm or less. That is, if the average particle diameter of the graphite grains exceeds 5 μm, the graphite grains become fatigue crack initiation / propagation sites, and the fatigue strength is lowered. In particular, since the rolling fatigue life is sensitive to the particle size of graphite, the average particle size of graphite is preferably 2 μm or less. Moreover, in order to fully exhibit the tool lubricating effect of graphite, it is preferable that the average particle diameter of graphite is 1 μm or more.

Furthermore, the amount of C precipitated as graphite needs to be 1 mass% or more of the total amount of C in the steel. If 1mass% or more of C in the steel is not precipitated as graphite, when cutting steel, the machinability of the steel is poor, so the life of the cutting tool is shortened, resulting in poor productivity and manufacturing costs. Will lead to an increase. Preferably, the amount of precipitated C is 5 mass% or more of the total amount of C in steel.
On the other hand, in the present invention, in order to increase the fatigue strength, it is preferable to increase the strength by solute C or carbide (cementite), and therefore the amount of C precipitated as graphite is based on the total amount of C in steel. Must be 50 mass% or less.

  Here, the ratio of the amount of C deposited as graphite to the total amount of C in steel is observed with a scanning electron microscope, the area ratio of the precipitated graphite is measured with an image analyzer, and this is defined as the volume ratio of precipitated graphite. It can be determined by calculating the graphitized C content rate from the specific gravity of graphite and the volume fraction of precipitated graphite. In the present invention, since the finely precipitated graphite needs to be 1 mass% or more of the total C amount, the above-mentioned area ratio is measured for graphite grains having a particle size of 10 μm or less, and the C amount ratio is calculated. Shall. The reason for measuring the area ratio of graphite particles having a particle size of 10 μm or less is that graphite particles having a particle size exceeding 10 μm do not contribute to improvement of machinability even if they are precipitated. In order to further improve the machinability, it is particularly preferable that the amount of C deposited as graphite grains having a particle size of 5 μm or less is 1 mass% or more and less than 50 mass% of the total C amount.

Next, the component composition of the carbon steel constituting the constant velocity joint inner ring of the present invention will be specifically described.
C: 0.2-1.5 mass%
C directly affects the improvement of fatigue strength. When the C content is less than 0.2 mass%, the effect of improving the fatigue strength is not sufficient. On the other hand, when it exceeds 1.5 mass%, the absolute amount of precipitation of the graphite becomes too large even if the structure control is performed, and the fatigue strength is reduced. In order to decrease, the C content is set to 0.2 to 1.5 mass%. A more preferable amount of C is 0.3 to 0.9 mass%.

Si: 0.3 to 2.0 mass%
Si is an important element in controlling the precipitation form of graphite. When the Si content is less than 0.3 mass%, the deposition rate of graphite is slow, and even if hot working under the conditions described later is performed, the graphite cannot be sufficiently precipitated, resulting in poor machinability. On the other hand, if the content exceeds 2.0 mass%, the fatigue strength decreases, and the size of graphite tends to increase and the deformability tends to decrease. Therefore, the content is limited to the range of 0.3 to 2.0 mass%. A more preferable Si amount is 1.3 to 2.0 mass%.

Mn: 1.5 mass% or less
Mn is an element effective in improving strength and fatigue strength, and is preferably contained at 0.1 mass% or more, more preferably 0.35 mass% or more. On the other hand, if the content exceeds 1.5 mass%, the effect of improving the strength is saturated, and the fatigue strength tends to decrease. Therefore, the content is limited to the range of 1.5 mass% or less.

B: 0.0005 to 0.015 mass%
B binds to N in the steel and exists in the steel as BN, thereby increasing the number of graphite precipitation sites and promoting the fine precipitation of graphite. When the B content is less than 0.0005 mass%, the effect is not sufficient, and fine graphite grains cannot be obtained. On the other hand, if it exceeds 0.015 mass%, the graphite tends to be coarsened, and the grain boundary strength is lowered to lower the fatigue strength. Therefore, the addition amount of B is limited to the range of 0.0005 to 0.015 mass%.

N: 0.001 to 0.015 mass%
In order to form the above-mentioned BN, N needs to contain 0.001 mass% or more. On the other hand, when the content exceeds 0.015 mass%, the fatigue strength also decreases, so the content is limited to the range of 0.001 to 0.015 mass%.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Mo: 3.0mass% or less
Mo is an element useful for improving fatigue strength through strength improvement. Preferably, it is added at 0.1 mass% or more, but adding more than 3.0 mass% leads to deterioration of machinability, so that it is 3.0 mass% or less. It is preferable to be in the range.

W: 3.0 mass% or less W is also an element useful for improving fatigue strength through strength improvement, and is preferably added in an amount of 0.1 mass% or more, but adding more than 3.0 mass% leads to deterioration of machinability. The range is 3.0 mass% or less.

Al: 0.06 mass% or less
Al is added as a steel deoxidizer, preferably at 0.005 mass% or more. However, if the content exceeds 0.06 mass%, the machinability and fatigue strength are reduced, and therefore, the content is preferably set to a range of 0.06 mass% or less.

Ti: 0.05 mass% or less
Ti is an element useful for refining crystal grains due to the pinning effect of TiN, and is preferably added at 0.002 mass% or more, but if added over 0.05 mass%, fatigue strength is reduced. A range of 0.05 mass% or less is preferable.

Ni: 3.0mass% or less
Ni is effective in increasing the strength and preventing cracking when Cu is added, and is preferably added at 0.05 mass% or more, but if added over 3.0 mass%, it tends to cause fire cracking, so the range is 3.0 mass% or less. It is preferable that

Co: 3.0mass% or less
Co is also an element effective for increasing the strength. Preferably, it is added at 0.1 mass% or more, but if added over 3.0 mass%, it tends to cause fire cracking, so it is preferably within the range of 3.0 mass% or less.

V: 0.1 mass% or less V is a useful element that produces a microstructure refining effect by pinning by being precipitated as carbide, and is preferably added at 0.005 mass% or more, but even if added over 0.1 mass%, it is effective. Is not only saturated, but also causes an increase in the price of the steel material.

Cu: 1.5 mass% or less
Cu is a useful element that improves the strength by solid solution strengthening and precipitation strengthening, and also contributes effectively to improving hardenability, so 0.05 mass% or more is preferably added. However, if the content exceeds 1.5 mass%, cracking is likely to occur during hot working, and manufacturing becomes difficult. Therefore, the content may be within a range of 1.5 mass% or less.

Nb: 0.07 mass% or less
Nb has an effect of pinning grain growth by precipitation, and is preferably added at 0.005 mass% or more, but even if added over 0.07 mass%, the effect is saturated, so the range is 0.07 mass% or less. It is preferable.

Ta: 0.2 mass% or less
Ta is also a useful element for pinning grain growth by precipitation, and it is preferably added at 0.02 mass% or more, but adding more than 0.2 mass% not only saturates the effect but also tends to reduce hot workability. Therefore, the range is 0.2 mass% or less.

Ca: 0.008 mass% or less
Ca is a useful element that spheroidizes inclusions and improves fatigue properties, and is preferably added at 0.0001 mass% or more, but if added over 0.008 mass%, inclusions tend to become coarse and deteriorate fatigue properties. For this reason, it is preferable that the range be 0.008 mass% or less.

Mg: 0.005 mass% or less
Mg is an element that contributes to improvement of machinability by forming an oxide, and it is preferable to add it at 0.0001 mass% or more, but excessive addition leads to coarsening of the oxide and lowers fatigue properties, so 0.005 mass % Or less is preferable.

Zr: 0.1 mass% or less
Zr is also an element that forms an oxide and contributes to improved machinability, and is preferably added at 0.005 mass% or more. However, excessive addition leads to coarsening of the oxide and decreases fatigue characteristics, so 0.1 mass % Or less is preferable.

Pb, Bi, Te, Se and REM
Pb, Bi, Te, Se, and REM are all elements that contribute to machinability improvement. Pb is 0.003 mass% or more, Bi is 0.003 mass% or more, Te is 0.005 mass% or more, and Se is 0.005 mass% or more. REM is preferably added at 0.001 mass% or more, but excessive addition is harmful to fatigue strength, so Pb is 0.3 mass% or less, Bi is 0.3 mass% or less, Te is 0.3 mass% or less, Se is It is preferable to add 0.3 mass% or less and REM 0.2 mass% or less.

The balance other than the above elements is Fe and inevitable impurities. Inevitable impurities include P, S, O and Cr.
That is, P lowers the fatigue strength by lowering the grain boundary strength, and also has the detrimental effect of promoting cracking, but is acceptable up to 0.05 mass%.
S forms MnS in steel and has the effect of improving the machinability. However, if it is contained in excess of 0.02 mass%, it segregates at the grain boundary and lowers the grain boundary strength. acceptable. Furthermore, it is more preferable to reduce to 0.003 mass% or less.
O exists in steel as an oxide inclusion, but if the O content is large, the fatigue life is reduced. Considering this point, the allowable upper limit is 0.02 mass%.
Since Cr suppresses the precipitation of graphite, it is not preferable to contain Cr. However, 0.1 mass% or less is acceptable.

In order to stably obtain sufficient graphite precipitation for improving machinability, it is particularly preferable to set C: 0.7 mass% or more and Si: 1.3 mass% or more.
The preferred component composition range has been described above, but in the present invention, it is not sufficient to limit the component composition to the above range, and it is important to adjust the steel structure of the constant velocity joint inner ring as described above. .

Next, conditions for manufacturing the constant velocity joint inner ring of the present invention will be described.
First, a steel bar obtained by hot-rolling a steel material adjusted to a predetermined component composition is cut into a predetermined length, and then a constant-velocity joint inner ring shape is roughly formed by hot forging, and further rolled by cutting. The surface is molded and the outer peripheral portion is processed, and if necessary, the surface is normalized, and then a groove for spline coupling of the drive shaft to the mating surface is formed by rolling to obtain a product. Furthermore, induction hardening can be performed as necessary.

  At this time, it is necessary to set the heating temperature for hot working to 950 ° C. or higher and then to a processing rate of 70% or higher. By setting the heating temperature during hot working to 950 ° C. or higher, C in the steel is dissolved, and the processing rate during hot working is set to 70% or higher to refine the structure. And, by refining the structure by strong processing with a processing rate of 70% or more and BN precipitation by containing appropriate amounts of B and N, a large amount of graphite precipitation sites are generated, and in the cooling process after hot working, 1 mass% or more is precipitated as fine graphite grains, and the average particle diameter of the graphite is 5 μm or less.

  That is, if the heating temperature during hot working is less than 950 ° C., the graphitization of C in the subsequent cooling process becomes insufficient even if the processing rate is set to 70% or more. It becomes impossible to make the ratio of C amount 1 mass% or more. If the heating temperature during hot working is higher than 1200 ° C, the crystal grain size becomes coarse, so 1200 ° C or lower is preferable.

On the other hand, when the processing rate is less than 70%, the graphite grains are coarsened, so that the average grain diameter of the graphite grains cannot be made 5 μm or less. Here, in order to graphitize C with the above amount and average particle diameter, it is preferable that the cooling rate to 600 ° C. after hot working is 0.2 ° C./s or less. Moreover, it is preferable that the cooling rate after lowering to 600 ° C. is 10 ° C./s or less.
In the present invention, the processing rate at the time of hot processing means the rate of change in the area of the cross section perpendicular to the processing direction before and after processing, and the cross-sectional area before processing S ₀ and the cross-sectional area after processing S _3. (S ₀ −S ₃ ) / S ₀ × 100 (%).

  Here, in the hot working, the total working rate in the temperature range of 1150 ° C. or higher and / or 980 ° C. or lower and 750 ° C. or higher is set to 70% or higher, and no processing is performed in the temperature range lower than 1150 ° C. and higher than 980 ° C. ( It is particularly preferable that the processing rate is 10% or less even when the processing rate is 0%.

  That is, in the temperature range of 980 ° C. or lower and 750 ° C. or higher, strong processing is performed with a processing rate of 70% or higher to refine the structure, and as described above, BN is precipitated with an appropriate amount of B and N contained. By generating a large amount of graphite precipitation sites, 1 mass% or more of C in the steel is precipitated as fine graphite grains in the cooling process after hot working, and the average particle size of graphite is moderately fine And In addition, when performing hot working, the temperature range of less than 1150 ° C. and over 980 ° C. is a temperature range in which coarse precipitation of BN is remarkable. Processing induced precipitation and alignment result in continuous form precipitation or long precipitation connected in the processing direction. The BN becomes a preferential precipitation site of graphite in a graphite precipitation temperature range at a lower temperature, and has a remarkable tendency to induce long graphite precipitation. In order to avoid such a situation, if the processing rate in the temperature range below 1150 ° C. and over 980 ° C. is 10% or less, preferably 0%, the average particle size of graphite can be made very fine, and the average particle size is 1 ˜2 μm can be achieved.

The total processing rate in the temperature range of 1150 ° C. or higher and / or 980 ° C. or lower and 750 ° C. or higher, and the processing rate in the temperature range lower than 1150 ° C. and higher than 980 ° C. shall be as follows. That is, when the initial cross-sectional area S0, the cross-sectional area S1 after processing at 1150 ° C. or higher, the cross-sectional area S2 after processing below 1150 ° C. and higher than 980 ° C., and the cross-sectional area S3 after processing at 750 ° C. or higher below 750 ° C. Processing rate R1 above ℃, processing rate R2 below 1150 ° C and above 980 ° C, processing rate R3 below 980 ° C and above 750 ° C,
R1 (%) = (S0-S1) / S0 × 100
R2 (%) = (S1-S2) / S0 × 100
R3 (%) = (S2-S3) / S0 × 100
And In the production method of the present invention, it is most preferable that R1 + R3 is 70% or more and R2 is 10% or less.

  If the total processing rate in the temperature range of 1150 ° C or higher and / or 980 ° C or lower and 750 ° C or higher is 70% or higher, and the processing in the temperature range below 1150 ° C and higher than 980 ° C is 10% or lower, the processing strain is BN The effect on the precipitation form is minimal and the alignment is slight. The amount of C deposited as graphite grains having an average particle diameter of 1-2 μm and a particle diameter of 5 μm or less is 1 mass% or more and less than 50 mass% of the total C amount. Thus, a constant velocity joint inner ring excellent in rolling fatigue characteristics can be obtained while sufficiently exhibiting the tool lubricating effect of graphite.

  Thereafter, as described above, the inner ring of the constant velocity joint can be obtained by forming a rolling surface and an outer peripheral portion by surface cutting and further forming a groove on the fitting surface by rolling.

  The constant velocity joint thus obtained is further subjected to induction hardening on a portion where fatigue characteristics are required, for example, a part represented by a rolling surface and / or a mating surface, or the entire surface, thereby providing extremely high strength and fatigue. It is possible to impart strength. That is, the precipitated graphite is re-dissolved in the matrix at the outer peripheral portion of the part and quenched, whereby a hardened layer can be formed. Induction hardening may be performed at a heating temperature of 800 ° C. or higher. If it is less than 800 ° C., a cured layer cannot be obtained sufficiently.

  In addition, when induction hardening is performed, a hardened layer is formed on the surface to form a hardened structure, so that the structure composed of the above-described ferrite, cementite, and graphite is not formed, but portions other than the hardened layer are formed of ferrite, cementite, and graphite. An organization consisting of

  Steel having the chemical composition shown in Table 1 was melted in a converter, cast into a 400 × 540 mm bloom by a continuous casting machine, and then hot rolled into a 150 mm square billet. The billet was hot rolled into a steel bar having a diameter of 27 mm to 57 mm, and then air-cooled.

Next, after cutting this steel bar to a predetermined length, as shown in Table 2, after forming a constant velocity joint inner ring shape (outer diameter: 45 mm and inner diameter: 20 mm) by various heating temperatures and hot forging and air cooling, The rolling surface and the outer peripheral portion were formed by cutting, and a groove for spline coupling was formed on the fitting surface by rolling.
For some samples (No. 75 and 76), for comparison, after air cooling following the hot forging, graphitization (700 ° C. × 6 h) was performed, and the amount of precipitated graphite / total amount of C A product with more than 50% was also produced.

  While observing the metal structure of the cross section of the constant velocity joint inner ring thus obtained, the average area ratio of the precipitated graphite observed in the scanning electron microscope structure was measured by an image analyzer, and the precipitation was determined from the specific gravity and the precipitation amount ratio. The C amount rate was calculated. However, in some samples, undissolved large-sized graphite remained in the structure, but was distinguished from these and treated by treating graphite having a precipitation diameter of 10 μm or less as precipitated graphite.

In addition, the obtained constant velocity joint inner ring is fitted with a drive shaft on its fitting surface, and is mounted on the mouse portion of the constant velocity joint outer ring via a ball (steel ball), while the constant velocity joint outer ring stem By fitting a hub to the part, a constant velocity joint unit was obtained (see FIG. 1). The specifications of the ball, outer ring, drive shaft, and hub are as follows.
Ball: Quenched and tempered steel of high carbon chromium bearing steel SUJ2 Outer ring: Induction-quenched and tempered steel of carbon steel for machine structure Hub: Induction-quenched and tempered steel of carbon steel for machine structure Drive shaft: Induction-hardened and tempered steel of carbon steel for machine structure

  Next, in this power transmission system that uses this constant velocity joint unit to transmit the rotational movement of the drive shaft to the hub via the inner ring and inner ring of the constant velocity joint, the sliding rolling fatigue of the fitting portion of the drive shaft and the ball An endurance test on rolling fatigue of the rolling contact surface was conducted.

  Here, the sliding rolling fatigue test uses a torsional fatigue tester with a maximum torque of 4900 N · m, and the frequency between the hub of the constant velocity joint unit and the drive shaft under the condition of a maximum torque of 700 N · m. The torsional force was repeatedly applied by swinging at 2 Hz, and the time until the spline part (fitting surface with the drive shaft) of the constant velocity joint inner ring was damaged due to slip rolling was evaluated as the sliding rolling fatigue life.

  In the rolling fatigue test, power is transmitted under the conditions of torque: 900 N · m, operating angle (angle formed by the inner ring axis and drive shaft axis): 20 °, and rotation speed: 300 rpm. The time until separation occurred on the rolling surface was evaluated as the rolling fatigue life. Here, the dimensions and shapes of the drive shaft, constant velocity joint outer ring, etc. were set so that the constant velocity joint inner ring would be the weakest part during the durability test.

  As for machinability, the chip length in the cutting process of the outer periphery of the inner ring is compared with the case of S40C, and the case where the average length is shorter than the average chip length in the same condition cutting of S40C is good, equivalent or The long case was evaluated as bad. Peripheral cutting conditions were as follows: peripheral speed 300m / min, feed 1mm / rev., Carbide tool, cutting depth 1mm.

  The machinability is closely related to the presence of graphite in the structure, and as shown in Table 2, good machinability can be obtained with any material in which graphite is precipitated by 1 mass% or more of the total C content. I understand. Further, the fatigue characteristics are good when the average particle diameter of graphite is 5 μm or less.

  Furthermore, the constant velocity joint inner ring produced under the same conditions as above was quenched or tempered using an induction hardening device with a frequency of 50 Hz, and then the rolling surface and the outer peripheral part were finished and subjected to outer peripheral cutting. By comparing the chip lengths, the machinability was compared with that of conventional S40C material. The results are shown in Table 3 as finish workability. Similarly to the above, the sliding rolling fatigue strength of the fitting portion of the drive shaft and the rolling fatigue strength of the rolling surface of the ball were also evaluated. The results are collectively shown in Table 3, and it can be seen that good characteristics exceeding the performance of S40C practical for automobile parts can be obtained.

It is a fragmentary sectional view of a constant velocity joint. It is sectional drawing which shows the surface hardening layer in a constant velocity joint inner ring | wheel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Drive shaft 2 Hub 3 Constant velocity joint 4 Outer ring 4a Mouse | mouth part 4b Stem part 5 Inner ring 5a Rolling surface 5b Fitting surface 6 Ball 7 Surface hardening layer

Claims (9)

C: 0.2-1.5 mass%
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and
N: 0.001 to 0.015 mass%
Having a composition of the balance Fe and inevitable impurities,
The steel structure is composed of ferrite, cementite and graphite, and the graphite has an average particle size of 5 μm or less, and the amount of C precipitated as graphite grains having a particle size of 10 μm or less is 1 mass% to 50 mass% of the total C content. A constant velocity joint inner ring made of carbon steel. C: 0.2-1.5 mass%
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and
N: 0.001 to 0.015 mass%
Having a composition of the balance Fe and inevitable impurities,
A steel structure comprising ferrite, cementite and graphite, wherein the graphite has an average particle size of 5 μm or less, and the amount of C precipitated as graphite particles having a particle size of 10 μm or less is 1 mass% to 50 mass% of the total C content; A constant velocity joint inner ring made of carbon steel having at least a hardened structure by induction hardening applied to a portion requiring fatigue characteristics. The carbon steel according to claim 1 or 2, further comprising:
Mo: 3.0mass% or less,
W: 3.0mass% or less,
Al: 0.06 mass% or less,
Ti: 0.05 mass% or less,
Ni: 3.0mass% or less,
Co: 3.0mass% or less,
V: 0.1 mass% or less,
Cu: 1.5 mass% or less,
Nb: 0.07 mass% or less and
Ta: 0.20mass% or less
A constant velocity joint inner ring characterized by containing one or more selected from the group consisting of: The carbon steel according to any one of claims 1 to 3 , further comprising:
Ca: 0.008 mass% or less,
Mg: 0.005 mass% or less,
Zr: 0.10 mass% or less,
Pb: 0.30 mass% or less,
Bi: 0.30 mass% or less,
Te: 0.30 mass% or less,
Se: 0.30 mass% or less and
REM: A constant velocity joint inner ring characterized by containing one or more selected from 0.20 mass% or less . C: 0.2-1.5 mass%
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and
N: 0.001 to 0.015 mass%
Carbon steel having a composition composed of the balance Fe and inevitable impurities, and heating the material at 950 ° C. or higher, and thereafter processing rate R1 of 1150 ° C. or higher, less than 1150 ° C. to over 980 ° C. A method for producing a constant velocity joint inner ring , wherein hot working is performed with a total machining rate of 70% or more, which is a total value of a machining rate R2 and a machining rate R3 of 980 ° C. or lower and 750 ° C. or higher . C: 0.2-1.5 mass%
Si: 0.3-2.0mass%
Mn: 1.5 mass% or less,
B: 0.0005 to 0.015 mass% and N: 0.001 to 0.015 mass%
Carbon steel having a composition comprising the balance Fe and inevitable impurities, and heating the material at a temperature of 950 ° C. or higher, followed by a processing rate R1 of 1150 ° C. or higher and a processing of less than 1150 ° C. to more than 980 ° C. that all working rate is the sum of the working ratio R3 at a rate R2,980 ° C. or less 750 ° C. or higher to facilities for hot working of 70% or more, further performing induction hardening at a site at least fatigue characteristics are required A method of manufacturing a constant velocity joint inner ring, which is characterized. 7. The material according to claim 5, wherein the material further includes
Mo: 3.0mass% or less,
W: 3.0mass% or less,
Al: 0.06 mass% or less,
Ti: 0.05 mass% or less,
Ni: 3.0mass% or less,
Co: 3.0mass% or less,
V: 0.1 mass% or less,
Cu: 1.5 mass% or less,
Nb: 0.07 mass% or less and
Ta: 0.20mass% or less
The manufacturing method of the constant velocity joint inner ring | wheel characterized by including 1 type, or 2 or more types chosen from these . The material according to any one of claims 5 to 7 , further comprising:
Ca: 0.008 mass% or less,
Mg: 0.005 mass% or less,
Zr: 0.10 mass% or less,
Pb: 0.30 mass% or less,
Bi: 0.30 mass% or less,
Te: 0.30 mass% or less,
Se: 0.30 mass% or less and
REM: A method for producing a constant velocity joint inner ring, comprising one or more selected from 0.20 mass% or less . 9. The method for manufacturing a constant velocity joint inner ring according to claim 5, wherein the processing rate R2 is 0 to 10%.

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