JP2007224353A - Rack-and-pinion electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve wear resistance at an engaging surface between a rack and a pinion in a rack-and-pinion electric power steering device of column-assisted or pinion-assisted type. <P>SOLUTION: At least either of a rack 11 and a pinion 21 is produced by subjecting a stock composed of steel having a composition consisting of, by mass, 0.20 to 0.6% C, 2.5 to 4.5% Cr, 0.5 to 1.5% Mn, 0.10 to 1.5% Si, 0.5 to 1.5% Mo, ≤12 ppm O, ≤50 ppm Ti, 0.020 to 0.06% Al, 100 to 300 ppm N and the balance Fe with inevitable impurities to working into prescribed shape and then applying heat treatments including carburizing or carbonitriding treatment, quenching treatment and tempering treatment. Moreover, hardness in the surface layer part between the tooth surface (engaging surface) and a position at a depth of 0.10 mm from the tooth surface is made to Hv 600 or above. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a rack and pinion type electric power steering apparatus.

The rack-and-pinion type steering mechanism meshes the pinion of the pinion shaft connected to the steering shaft with the rack of the rack shaft connected to the wheel, and the rotational force and amount of rotation of the pinion and the axial thrust of the rack and By converting to a stroke, the rotation of the steering shaft is converted to the motion of the wheel.
This rack-and-pinion type steering mechanism is a manual steering device that converts only the rotational force applied to the steering shaft by the driver's steering to the movement of the wheel, or the hydraulic power steering that generates the rotational force that assists the steering by a hydraulic pump. It is used in an apparatus, an electric power steering apparatus that generates a rotational force for assisting steering by an electric motor, and the like. In addition, as an electric power steering device using a rack and pinion type steering mechanism, a column assist type that transmits the rotational force generated by the electric motor to the steering shaft, a pinion assist type that transmits the rotational force to the pinion shaft, And a rack assist type that transmits the rotational force to the rack shaft.

  Of the devices using such a rack and pinion type steering mechanism, in the manual steering device, only the rotational force applied by the driver's steering is converted into wheel motion via the rack and pinion type steering mechanism. The torque applied to the rack and pinion is small. Further, in the hydraulic power steering device, the rotational force for assisting the steering is converted into the motion of the wheel by the piston of the hydraulic cylinder integrally provided on the rack shaft without using the rack and pinion type steering mechanism. The torque applied to the andpinion is small. Further, in the rack assist type electric power steering apparatus, the rotational force for assisting the steering is converted into the wheel motion by a ball screw or the like without using the rack and pinion type steering mechanism, so that the load is applied to the rack and the pinion. The torque is about the same as that of the hydraulic power steering device.

  On the other hand, in the column assist type and pinion assist type electric power steering devices, the torque that combines the rotational force applied to the steering shaft by the driver's steering and the rotational force generated by the electric motor is applied to the rack and Loaded on the pinion. Therefore, in these electric power steering devices, the torque applied to the rack and pinion is more than 10 times larger than that of the manual steering device, hydraulic power steering device, and rack assist type electric power steering device. There is a problem that the tooth surfaces (meshing surfaces) of the rack and the pinion that are meshed with each other easily wear.

  In the rack and pinion type steering mechanism, a rack guide is disposed on the back side of the tooth surface of the rack, and the rack guide is supported in a state of being pressed against the pinion. At this time, the rack guide has a relatively small pressing force so that the rack and the pinion can be smoothly engaged while being slightly stroked so as to allow the variation in the inter-axis distance between the rack and the pinion due to the gear accuracy. (For example, about 200N). However, at the time of power transmission, a separation force mainly generated by the gear pressure angle (for example, 10000 N, which is at most about the same as the rack thrust force) is applied, so the rack guide is pushed back and retracted to the full stroke. When the rotational force is reversed, the transmission tooth surface is switched and the separation force is once reduced to zero, so that the separated rack collides with the pinion vigorously and a striking sound (rattle sound) is generated.

Here, the separation force varies not only during power transmission but also due to input vibration from the wheel side such as road surface irregularities and shimmies, so rattle noise is generated even when only the inertial force is acting on the steering shaft. . For this reason, in the electric power steering device, since the rotational force by the electric motor is also applied compared to the manual steering device in which only the rotational force by the steering of the driver is applied to the steering shaft, the inertial force acting on the steering shaft is more than doubled. Sound is easily generated.
At this time, if wear occurs on the meshing surfaces of the rack and the pinion, the stroke retreat amount of the rack guide increases and the collision speed between the rack and the pinion increases, so that the rattle noise further increases and the steering feeling is increased. Gets worse.

  Therefore, in order to make the rack and pinion meshing surfaces less likely to wear, the rack and pinion are heated against a material made of alloy steel for mechanical structure (SMnC, SCr, SCM, SNCM, etc.) defined in JIS. After forming into a predetermined shape by performing mechanical processing such as cold forging, warm forging, cold forging, etc., surface hardening treatment (carburizing quenching, induction hardening, soft nitriding treatment, etc.) is performed on those meshing surfaces, In general, the surface hardness is set to HRC55 to 63, and the core hardness at the base of the meshing surface is made about 10 to 20% smaller than the surface hardness.

Moreover, in patent document 1-patent document 3, in order to make it hard to produce abrasion on the meshing surface of a rack and a pinion, the technique shown below is proposed.
In Patent Document 1, it is proposed to improve wear resistance by providing a plurality of recesses by shot peening on the meshing surfaces of the rack and the pinion to reduce the sliding resistance of the meshing surfaces.

  In Patent Document 2, a power transmission component (pinion and rack) having a meshing surface for transmitting power is made of carburized steel (SCr420H, SCM420H, SNCM220H, SNCM420H, SNCM815, SAE4320, SAE5120, etc.), and at least a meshing surface is formed. In the surface layer portion (portion from the surface to about 50 μm), it has been proposed that the hardness is HRC 63 to 68 and the amount of retained austenite is 13 to 30% by volume.

In Patent Document 3, a first hardened layer having a hardness of HRC68 (Hv940) or higher is formed by quenching on the meshing surface of the rack and pinion and the back surface of the rack, and then the first hardened layer is further formed on the first hardened layer. It has been proposed to form a second hardened layer that is 20% or more harder than the layer, has a retained austenite amount of 0% by volume to less than 10% by volume, and has a hardened layer depth of 9 μm or more by shot peening.
JP 2001-278074 A Japanese Patent Laid-Open No. 7-54049 JP 2005-125972 A

However, in recent years, with further increase in output of the rack and pinion type electric power steering device, it has been required to further improve the wear resistance on the meshing surfaces of the rack and pinion. This request may not be met. Here, in order to make it difficult to cause wear on the meshing surfaces of the rack and the pinion, a method of reducing the surface pressure applied to the meshing surface by increasing the tooth width while maintaining the number of meshing teeth can be considered. When it is adopted, it is necessary to increase the outer diameter of the rack, which causes a new problem that the weight increases.
Therefore, an object of the present invention is to improve the wear resistance of the meshing surfaces of the rack and the pinion in the column and pinion assist type rack and pinion type electric power steering apparatus.

  In order to solve such a problem, a steering shaft that is rotated by a driver's steering, a pinion shaft that is connected to the steering shaft and has a pinion, a rack shaft that is connected to a wheel and has a rack that meshes with the pinion, A rack-and-pinion type electric power steering apparatus, wherein the rack and the pinion type electric power steering apparatus are configured to transmit the rotational force to the steering shaft or the pinion shaft. And at least one of the pinions in terms of mass ratio, the C content is 0.20% to 0.6%, the Cr content is 2.5% to 4.5%, and the Mn content is 0.5. % 1.5% or less, Si content 0.10% or more 1.5% or less, Mo content 0.5% or more 1.5% or less, O content 12ppm or less After processing a material made of steel having a Ti content of 50 ppm or less, an Al content of 0.020% or more and 0.06% or less, an N content of 100 ppm or more and 300 ppm or less, and the balance being Fe and inevitable impurities into a predetermined shape Obtained by heat treatment including carburizing or carbonitriding treatment, quenching treatment, and tempering treatment, and the hardness of the surface layer portion from the meshing surface to a depth of 0.10 mm is Hv600 or more. Provided is a rack and pinion type electric power steering apparatus.

According to this, at least one of the rack and the pinion is subjected to carburizing or carbonitriding treatment, quenching treatment and tempering treatment using a material made of surface hardened steel containing a lot of Cr and Mo having an effect of improving wear resistance. By performing surface hardening heat treatment including, by curing the surface layer part forming the meshing surface (rack tooth surface and pinion tooth surface) to a deeper part than the conventional (part from the surface to 0.10 mm), The wear resistance on the meshing surfaces of the rack and the pinion can be improved.
That is, in the rack and pinion type electric power steering device of the present invention, after processing a material made of the following specific steel into a predetermined shape at least one of the rack and the pinion, carburizing or carbonitriding, quenching, And a heat treatment including a tempering treatment, and the surface layer portion forming the meshing surface was as follows.

[About steel]
<C content: 0.20 to 0.6 mass%>
C (carbon) is dissolved in the base to improve the hardness after quenching and tempering, and bonds with carbide forming elements such as Fe (iron), Cr (chromium), Mo (molybdenum), V (vanadium), etc. Thus, it has the effect of improving wear resistance by forming carbides. If the C content of the steel constituting the material is too small, these effects cannot be obtained, and the time required for carburizing or carbonitriding to obtain the required hardened layer depth is increased, leading to an increase in cost, In some cases, δ ferrite is generated and the toughness is lowered. On the other hand, if the C content is too high, coarse eutectic carbides are likely to be produced during steelmaking, and the necessary rolling fatigue life and strength cannot be obtained, and forgeability, cold workability, and machinability are reduced. The cost may increase. Therefore, the C content of the steel constituting the material is 0.20 mass% or more and 0.6 mass% or less, and preferably 0.20 mass% or more and 0.5 mass% or less.

<Cr content: 2.5 to 4.5 mass%>
Cr (chromium) has a function of improving the hardenability, temper softening resistance, corrosion resistance, and rolling fatigue life by solid solution in the base. In addition, the interstitial solid solution elements such as C and N (nitrogen) are made difficult to move and the base structure is stabilized, so that it has an effect of greatly suppressing a decrease in life when hydrogen enters. Furthermore, since the carbide finely distributed in the steel becomes a carbide such as (Fe, Cr) ₃ C or (Fe, Cr) ₇ C ₃ having high hardness, it also has an effect of improving wear resistance. In order to obtain these effects, the Cr content of the material steel is 2.5 mass% or more.
On the other hand, if the Cr content of the steel constituting the material is too large, cold workability, machinability, and carburization processability are reduced, leading to an increase in cost, or coarse eutectic carbide is generated, The required rolling fatigue life and strength may not be obtained. Therefore, the Cr content of the steel constituting the material is 4.5% by mass or less, and preferably 3.5% by mass or less.

<Mn content: 0.5% by mass or more and 1.5% by mass or less>
Mn (manganese) acts as a deoxidizing agent during steelmaking, and in the same way as Cr, it dissolves in the matrix and lowers the Ms point to ensure the necessary amount of retained austenite and improve hardenability. Have the effect of In order to obtain these effects, the Mn content of the steel constituting the material is 0.5% by mass or more, preferably 0.8% by mass or more.
On the other hand, if the Mn content is too high, not only cold workability and machinability are lowered, but also the Ms point is significantly lowered, so that a large amount of retained austenite remains and the necessary hardness is obtained. It may disappear. Therefore, the Mn content of the steel constituting the material is 1.5% by mass or less, preferably 1.2% by mass or less.

<Si content: 0.10% by mass or more and 1.5% by mass or less>
Si (Silicon), like Mn, acts as a deoxidizer during steelmaking, and, like Cr and Mn, strengthens the base martensite, thereby improving hardenability, rolling fatigue life, and tempering. Has the effect of improving softening resistance. In order to obtain these effects, the Si content of the material steel is set to 0.10% by mass or more.
On the other hand, if the Si content is too high, the machinability, forgeability, cold workability, and carburization processability may be reduced. Therefore, the Si content of the steel constituting the material is 1.5% by mass or less, preferably 0.7% by mass or less, and more preferably 0.5% by mass or less.

<Mo content: 0.5 mass% or more and 1.5 mass% or less>
Similar to Cr, Mo (molybdenum) has the effect of improving the hardenability, temper softening resistance, corrosion resistance, and rolling fatigue life by solid solution in the base, and interstitial solid solution elements such as C and N. It has the effect of stabilizing the tissue by making it difficult to move, and greatly suppressing the decrease in life when hydrogen enters. Mo also has the effect of improving wear resistance by forming fine carbides such as Mo ₂ C. In order to obtain these effects, the Mo of the material steel is 0.5 mass% or more.
On the other hand, if the Mo content is too high, the cold workability and machinability will decrease, leading to an increase in cost, and coarse eutectic carbides will be generated, making it impossible to obtain the necessary rolling fatigue life and strength. There is a case. Therefore, the Mo content of the steel constituting the material is 1.5% by mass or less.

<O content: 12 ppm or less>
O (oxygen) produces oxide-based non-metallic inclusions (Al ₂ O ₃ ) and significantly reduces the rolling fatigue life. For this reason, although it is preferable to make O content rate of the steel which makes a raw material as small as possible, if it is 12 ppm or less, it is permissible.
<Ti content: 50 ppm or less>
Ti (titanium) produces TiN-based nonmetallic inclusions with high hardness and small plastic deformability. This TiN-based non-metallic inclusion becomes a stress concentration source and reduces the rolling fatigue life. For this reason, although it is preferable to make Ti content rate of steel which makes a raw material as small as possible, if it is 50 ppm or less, it is permissible.

<Al content: 0.020 mass% or more and 0.06 mass% or less>
Al (aluminum) is finely dispersed in the steel as AlN, and has an action of preventing austenite crystal grains from coarsening during heat treatment (particularly carburizing or carbonitriding). In order to obtain this effect, the Al content of the material steel is set to 0.020% by mass or more.
On the other hand, if the Al content is too high, a large amount of oxide inclusions (Al ₂ O ₃ ) are generated, and the rolling fatigue life is reduced. Therefore, the Al content of the steel constituting the material is 0.06% by mass or less.

<N content: 100 ppm or more and 300 ppm or less>
N has an effect of preventing austenite crystal grains from coarsening by forming Al and nitride. In order to obtain this effect, the N content of the steel constituting the material is set to 100 ppm or more.
On the other hand, when there is too much N content rate, the effect will be saturated and steel manufacture will become difficult. Therefore, the N content of the steel constituting the material is set to 300 ppm or less.

[About the surface layer forming the meshing surface]
In order to improve the wear resistance on the meshing surfaces of the rack and pinion, the hardness of the surface layer portion from the meshing surface to a depth of 0.10 mm is set to Hv 600 or more. Further, in order to further improve the wear resistance on the meshing surfaces of the rack and the pinion, the surface layer portion from the meshing surface to a depth of 0.05 mm is subjected to shot peening treatment on the meshing surface after the heat treatment. The hardness is preferably Hv 800 or more. Furthermore, in order to further improve the wear resistance on the meshing surfaces of the rack and the pinion, the amount of retained austenite in the surface layer portion from the meshing surface to a depth of 0.05 mm is preferably 20% by volume or less.

  According to the rack and pinion type electric power steering apparatus of the present invention, after processing a material made of a specific steel into a predetermined shape, at least one of the rack and the pinion is subjected to carburizing or carbonitriding, quenching, and tempering. When the hardness of the surface layer portion from the meshing surface to a depth of 0.10 mm is set to Hv 600 or more, the wear resistance on the meshing surfaces of the rack and pinion can be improved. Therefore, even when the column assist type and pinion assist type rack-and-pinion type electric power steering device is increased in output, it is possible to prevent wear on the meshing surfaces of the rack and pinion, and to prevent rattling noise. .

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a longitudinal sectional view showing a column assist type rack and pinion type electric power steering apparatus as an example of the rack and pinion type electric power steering apparatus of the present invention.
As shown in FIG. 1, the rack and pinion type electric power steering device is connected to a wheel and supported in an oblique direction with respect to an axial center of the rack shaft 1 and a rack shaft 1 that is movable in the axial direction. A pinion shaft 2 connected to the steering shaft, and a cylindrical housing 3 that supports the rack shaft 1 and the pinion shaft 2 are provided.

The rack shaft 1 is provided with a rack 11 having a tooth surface (meshing surface) on one end side in the axial direction, and a wheel is connected to the other end side in the axial direction via a tie rod. The rack shaft 1 is supported by the housing 3 in a state where the tooth surface of the rack 11 is pressed against the pinion 2 shaft by the rack guide 4.
The rack guide 4 includes a roller 43 rotatably supported on a pin 42 by a needle bearing 41, a cylindrical holder 44 that holds the pin 42 in the rack guide 4, and a disc spring 45. And an adjustment cover 46 that presses against the back surface of the lens.

The pinion shaft 2 is provided with a pinion 21 having a tooth surface (meshing surface) that meshes with the tooth surface of the rack 11 on one end side in the axial direction (left side in FIG. 1). The pinion shaft 2 is supported by the housing 3 by a needle bearing 5 and a ball bearing 6 provided so as to sandwich both end surfaces of the pinion 21 in the axial direction.
The needle bearing 5 includes an inner peripheral surface of a bearing hole 31 formed in the housing 3, an outer peripheral surface 22 of the pinion shaft 2, and a plurality of needles 51 disposed so as to be able to roll between them. The ball bearing 6 includes an inner ring 61, an outer ring 62, and a ball 63. The inner ring 61 of the ball bearing 6 is fitted to the outer peripheral surface 23 of the pinion shaft 2 and is fixed between the ring 7 attached to the outer peripheral surface 23 and caulked and one end surface in the axial direction of the pinion 21. . The outer ring 62 of the ball bearing 6 is fixed in a state where it is pressed by a ring nut 33 screwed into a male screw on the opening side of a bearing hole 32 formed in the housing 3.

On the other hand, a connecting hole 24 is formed on the other end side in the axial direction of the pinion shaft 2 (on the right side in FIG. 1) to be connected to the steering shaft connected to the steering wheel via a universal joint. An electric motor (not shown) is connected to the other axial end of the pinion shaft 2.
In the rack-and-pinion type electric power steering apparatus of this configuration, when the driver steers the steering wheel and applies a rotational force to the steering shaft, the applied rotational force is applied to the pinion shaft 2 via a torsion bar (not shown). Communicated. At this time, since the torsion bar is twisted due to the resistance on the wheel side, the output of the electric motor that detects the twist is controlled. Therefore, when the driver steers the steering wheel, the rotational force applied to the steering shaft is transmitted to the rack 11 through the pinion shaft 2 and the pinion 21 together with the rotational force generated by the electric motor, thereby changing the direction of the wheels. Can be changed.

In the present embodiment, the rack 11 and the pinion 21 are produced by the following procedure.
First, after processing the raw material which consists of steel of each composition shown in Table 1 into a predetermined shape, after performing the quenching process by the method shown in Table 1, the tempering process by heat-maintaining at 160-270 degreeC for 1-2 hours Then, a hardened surface layer X was formed on the tooth surface of the rack 11 and the tooth surface of the pinion 21.
In “carburization quenching” shown in Table 1, oil quenching was performed after heating and holding at a predetermined temperature shown in Table 1 for a predetermined time in a furnace in which a mixed gas (RX gas + enriched gas) was introduced. Further, in the “carbonitriding quenching” shown in Table 1, oil quenching is performed after heating and holding at a predetermined temperature shown in Table 1 for a predetermined time in a furnace in which a mixed gas (RX gas + enrich gas + ammonia gas) is introduced. It was. Furthermore, in the “quenching” shown in Table 1, oil quenching was performed after heating and holding at the predetermined temperature shown in Table 1 for a predetermined time.

Next, the tooth surface of the rack 11 and the tooth surface of the pinion 21 were polished.
Next, in the examples of “with shot peening” shown in Table 1, shot peening was applied to the tooth surfaces of the rack 11 and the pinion 21 to form the hardened layer Y on the upper surface of the hardened layer X. This shot peening process is performed by injecting a steel ball (shot material) having an average particle diameter of 75 μm at an injection pressure of 0.4 MPa and then injecting SiC (shot material) having an average particle diameter of 5 μm at the same injection pressure. It was.

That is, when both the hardening heat treatment consisting of carburizing or carbonitriding treatment, quenching treatment, and tempering treatment and shot peening treatment are performed on the tooth surface of the rack 11 and the tooth surface of the pinion 21, as shown in FIG. The hardened layer X by heat treatment and the hardened layer Y by shot peening are formed on the tooth surface of the rack 11 and the tooth surface of the pinion 21.
Using the sample for destructive inspection of the rack 11 thus obtained, the hardness of the surface layer portion A from the tooth surface to a depth of 0.10 mm and the hardness of the surface layer portion B from the tooth surface to a depth of 0.05 mm. The thickness was measured by the Vickers hardness test method defined in JIS Z 2241. The results are also shown in Table 1.

Further, the amount of retained austenite present in the surface layer portion B from the tooth surface to a depth of 0.05 mm was measured with a known X-ray diffractometer using the obtained rack 11 sample for destructive inspection. The results are also shown in Table 1.
Further, among the obtained racks 11, the residual compressive stress at a position of 0.05 mm from the tooth surface is measured with an X-ray diffractometer using a sample for destructive inspection of the rack 11 in which the tooth surface is shot peened. did. As a result, all were 1000 MPa or more.
In addition, in the obtained sample for destructive inspection of the pinion 21, the same result as the case where the same hardening heat treatment and shot peening processing were performed using the same steel as the sample for destructive inspection of the rack 11 was obtained.

In addition, after the obtained rack 11 life sample and pinion 21 life test sample were incorporated into the rack-and-pinion electric power steering apparatus shown in FIG. 1, the life test was performed under the following conditions. . In this life test, the amount of iron powder in the lubricant (grease) is measured every 20 to 30 hours with an upper limit of 2000 hours, and the amount of iron powder is 0.3 mass% or more with respect to the entire grease. The rotation time of the pinion 21 was calculated as the lifetime. In addition, this life test was performed 10 times for each sample. And this result was combined with Table 1 by L10 lifetime based on the Weibull distribution curve.
<Life test conditions>
Pinion rotation speed: 300 min ^-1
Surface pressure: 2.3 MPa
Test temperature: Room temperature (about 28 ° C)

As shown in Table 1, the rack 11 and the pinion 21 are manufactured using the steel of the present invention range, and the hardness of the surface layer portion A from the tooth surface of the rack 11 and the tooth surface of the pinion 21 to a depth of 0.10 mm is set. , Hv600 or higher. 1-No. In No. 14, other No. 15-No. Compared with 34, it had a long life.
No. 1-No. 14, the surface layer portion B from the tooth surface of the rack 11 and the tooth surface of the pinion 21 to a depth of 0.05 mm, an example in which the hardness is Hv 800 or more, or the amount of residual austenite is 20% by volume or less In the example, the service life was longer.
Furthermore, no. 1 and No. No. 13 result, No. 13 12 and No. From the result of 14, it can be seen that the carbonitriding has a longer life than the carburizing treatment.

  From the above results, the rack 11 and the pinion 21 are manufactured using the steel of the present invention, and the hardness of the portion from the tooth surface to the depth of 0.10 mm is set to Hv 600 or more, whereby the rack 11 and the pinion 21 are obtained. As a result, it was confirmed that the rack and pinion type electric power steering apparatus can have a long life.

It is a longitudinal cross-sectional view which shows an example of the rack and pinion type electric power steering device of the present invention. It is explanatory drawing which shows the structure of the rack and pinion of the rack and pinion type electric power steering device of the present invention.

Explanation of symbols

1 rack shaft 11 rack 2 pinion shaft 21 pinion 3 housing 4 rack guide

Claims (3)

A steering shaft that is rotated by a driver's steering, a pinion shaft that is connected to the steering shaft and has a pinion, a rack shaft that is connected to a wheel and meshes with the pinion, and a rotational force that assists the steering is generated. A rack-and-pinion type electric power steering apparatus, wherein the rotational force is transmitted to the steering shaft or the pinion shaft.
At least one of the rack and the pinion is a mass ratio, the C content is 0.20% to 0.6%, the Cr content is 2.5% to 4.5%, and the Mn content is 0. 0.5% or more and 1.5% or less, Si content is 0.10% or more and 1.5% or less, Mo content is 0.5% or more and 1.5% or less, O content is 12 ppm or less, Ti content is 50ppm or less, Al content is 0.020% or more and 0.06% or less, N content is 100ppm or more and 300ppm or less, the material made of steel with the balance being Fe and inevitable impurities is processed into a predetermined shape, then carburized or carburized A rack-and-pinion obtained by performing heat treatment including nitriding treatment, quenching treatment, and tempering treatment, and the hardness of the surface layer portion from the meshing surface to a depth of 0.10 mm is Hv600 or more Electric power Tearing device.   By performing shot peening on the meshing surface after the heat treatment, the hardness of the surface layer portion from the meshing surface to a depth of 0.05 mm is Hv 800 or more. The rack and pinion type electric power steering apparatus according to claim 1.   The rack-and-pinion electric power steering apparatus according to claim 1 or 2, wherein the amount of retained austenite in the surface layer portion from the meshing surface to a depth of 0.05 mm is 20% by volume or less.

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