JPH06264992A - Steering rack shaft - Google Patents

Abstract

PURPOSE:To improve the manufacturing efficiency by way of reducing distortion and improving treatment by heat treating a part of a rack tooth formed portion of a shaft main body in the axial direction at least as refining. CONSTITUTION:On a proper part close to an end part of a shaft main body 1, a flat part 3 by plastic working or cutting work of drawing or press working and others is formed. Both sides of this shaft main body 1 are made as a long shaft part 1c and a short shaft part 1c, and between them, a rack tooth formed portion 1a having the flat part 3 and a back part 1b are formed. On the rack tooth formed portion 1a, a plural number of rack teeth 1al are formed. Hereby, at least one part of the rack tooth formed portion 1a in the axial direction is refined 4, and at least one part of the rack tooth formed portion 1a and the back part 1b in the diametrical direction of it are heat treated so that their hardness differs from each other. Consequently, it is possible to extensively reduce distortion after refining 4 of a steering rack shaft A, to easily remove distortion and to improve manufacturing efficiency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces strain by partial tempering, improves machining of rack teeth and the like, improves manufacturing efficiency, lowers manufacturing cost, and has rigidity, toughness and strength. It is related to the steering rack axis which raised.

[0002]

2. Description of the Related Art Conventionally, there has been a steering rack shaft in which the entire shaft is heat-treated in a furnace to improve its strength and toughness. A steering rack shaft that has been entirely tempered is one that forms the steering rack shaft by tempering the entire material, and is used later in areas where abrasion resistance is required, such as rack teeth. Quenching heat treatment is performed, and the tempering is performed as it is, except for the quenching point.

Here, tempering refers to an operation of quenching and then tempering to a relatively high temperature (about 400 ° C. or higher) to obtain a troostite structure or a sorbite structure. The quenching treatment in the tempering is performed for the purpose of uniformly hardening or strengthening the core of the steel material. After quenching, it is preferable to perform high temperature tempering at an appropriate temperature of about 400 ° C or higher. It is normal.

[0004]

When the entire tempering is performed as in the conventional steering rack shaft, as shown by the conventional dotted line in the strain line comparison graph of the heat treatment of FIG. Distortion (bending) had the drawback of becoming large in the shape of a bow. As a result, the strain-removing work takes time, which lowers the manufacturing efficiency and increases the manufacturing cost. Furthermore, even if the distortion caused by the overall tempering is corrected, due to the residual stress in the steel material (steering rack shaft material),
There is a drawback that a large strain is generated each time subsequent heat treatment (rack tooth portion, shaft portion, etc.).

Further, in order to improve the workability of the rack teeth and the like at the rack tooth forming portion to improve the working efficiency and the tool life, it is necessary to make the temper hardness relatively low. Therefore, it has been difficult to improve the strength and toughness of the steering rack shaft and increase the rigidity. That is,
As shown in FIG. 22 (relationship between shaft internal hardness and bending load), if the internal hardness is reduced in order to improve workability, then the bending load with respect to the load becomes weaker. Also,
Workpiece (steering rack shaft) that is strong against bending load
The harder the tool (the higher the internal hardness), the faster the tool damage, the poorer the machining accuracy, the higher the cutting resistance, the higher the cutting temperature, and the faster the progress of wear.

As described above, it is desired to manufacture a steering rack shaft which has good workability in view of manufacturing efficiency and cost, and which has high shaft strength and toughness.

[0007]

Therefore, as a result of earnestly researching in order to solve the above-mentioned problems, the present invention has found that at least a part of an axial direction of a rack tooth forming portion of a shaft main body is adjusted. By using a steering rack shaft etc. that has been heat treated as a quality, distortion is reduced by partial tempering, processing of rack teeth etc. is improved, manufacturing efficiency is improved, manufacturing cost can be reduced, and rigidity It is possible to provide a high steering rack shaft and solve the above problems.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 and FIG. 2 show the entire steering rack shaft A according to this embodiment. FIG. 1 shows a normal steering device, and FIG. 2 shows a power steering device. A through hole 2 is formed in the shaft body 1 of the steering rack shaft A as needed. Therefore, the steering rack shaft A has a solid case having a substantially circular cross section and a hollow case having a substantially circular cross section.

Further, a flat portion 3 formed by plastic working such as drawing or pressing or cutting is formed at an appropriate position near the end of the shaft body 1. A shaft portion 1c having a long length and a shaft portion 1c having a short length are formed on both sides of the shaft body 1, and a rack tooth forming portion 1a having a flat portion 3 and a back surface portion 1b are formed between them. A plurality of rack teeth 1a1, 1a1, ... Are formed in the rack tooth forming portion 1a. The back surface portion 1b indicates a semicircular portion on the diametrically opposite side of the rack tooth forming portion 1a, and the axial length thereof is the flat portion 3a.
Is equivalent to the length of.

At least a portion of the rack tooth forming portion 1a in the axial direction is tempered 4, and at least a portion of the rack tooth forming portion 1a and its diametrical back surface portion 1b are heat treated to have different hardness. ing.

As shown in FIG. 3, the part of the temper 4 is between the end portion 10a of the gearbox 10 and the rack support portion 11 when the steering rack shaft A projects from the gearbox 10 to the maximum extent. Refers to the corresponding location. The range of the corresponding place is represented as S. FIG. 4 is also a partial range S of FIG. 3 and temper 4.
Are the same.

Further, when the steering rack shaft A projects from the gear box 10 to the maximum, there is also a case where the corresponding portion between the end portion 10a of the gear box 10 and the rack supporting elastic device 12 is pointed out. It is almost the same. In other words, when the steering rack shaft A projects from the gear box 10 to the maximum extent, the end portion 1 of the gear box 10 is
0a to the rack support member (rack guide) 11a.

Further, as shown in FIG. 5, when the steering rack shaft A projects from the gear box 10 to the maximum extent, it indicates the corresponding portion between the end portion 10a of the gear box 10 and the rack support portion 11. In this case, there are two corresponding locations, which are represented as ranges S1 and S2. In other words, when the steering rack shaft A is projected from the gear box 10 to the maximum, the end portion 10a of the gear box 10 and the rack support member (rack guide) 11a correspond to each other. Also in FIG. 6, the partial ranges S1 and S2 of the temper 4 are the same as those in FIG.

Further, FIG. 7 shows that when the steering rack shaft A protrudes from the gear box 10 to the maximum extent, the supporting portion near the rack supporting member (rack guide) 11a is used as a corresponding portion and a part of the temper 4 is used. Has been formed. The range of the corresponding portion is represented as S. Also in FIG. 8, a partial range S of the temper 4 is the same as that of FIG. 7.

Further, as shown in FIG. 9, the temper 4 is formed over the entire rack tooth forming portion 1a (a part of the length in the longitudinal direction of the rack shaft). This range is represented as S. In FIG. 10 as well, the partial range S of the temper 4 is the same as that of FIG. 9.

In FIGS. 1 and 2, 13 is a pinion, 14 is a joint, 15 is a tie lot, 16 is a bellows, 1
7 is a piston and 18 is a port.

When the heat treatment of tempering 4 is performed, 40
It is necessary to perform tempering treatment at a high temperature of 0 ° C or higher.

As an experimental example of the temper 4, the material is S4.
Hollow steering rack processing material A of 5C material (JIS)
0 (Outer diameter approx. 25 mm, total length approx. 800 mm, through hole diameter approx. 10 m
m) in the induction coil 19 for induction hardening of the induction hardening device,
At the heating temperature at the time of quenching, that is, at a quenching temperature of about 800 ° C. to 900 ° C., heating is performed for several seconds to several tens of seconds, cooling is performed, and quenching is performed. Thereafter, the rack tooth forming portion (processing portion) is tempered by the induction coil 20 for refining with high frequency at a tempering temperature of about 500 ° C. to about 700 ° C. and a heating time of several seconds to several tens of seconds. As a result, temper 4 is formed, and the metal structure is made into a temper structure (troustite, sorbite structure).

The tempering 4 of the rack tooth forming portion 1a and the back surface portion 1b in the diametrical direction of the rack tooth forming portion 1a having different hardness is performed in the case of the tempering step as shown in FIGS. By heating the steering rack processed material A0 in a fixed state with the induction coil 20 for heat treatment, which heats only the semicircular section of the steering rack processed material A0, by heating only the rack tooth forming portion 1a side for a predetermined time, Back part 1b
The side has a tempering temperature lower than that of the rack tooth forming portion 1a side due to the tempering effect due to the heat effect. This results in different hardness. This is because temper 4 is tempered at a high temperature of 400 ° C. or higher, and the higher the tempering temperature, the lower the hardness.

In the embodiment, the rack tooth forming portion 1
Cross sections having different hardnesses of a (low hardness) and the back surface portion 1b (high hardness) are shown in FIGS. 3, 5, 7 and 9, and in each case, the tissue on the tooth surface side is the back surface side. The density is higher than that, and the higher the density, the lower the hardness. The figure 3,
5, (b) in FIG. 7, FIG. 7, and FIG. 9 are cross-sections in a state where the temper 4 is applied, and FIGS. 3 (c) and 5 (c) show rack teeth while maintaining the state of (b). 1a1 is a cross section finally formed by induction hardening to form a tooth portion quenching layer 5.

As described above, since the rack tooth forming portion 1a and the back surface portion 1b are tempered 4 so as to have different hardness,
The back surface portion 1b can maintain strength during use, the other surface,
The workability can be remarkably improved when the rack tooth 1a1 is formed on the rack tooth forming portion 1a.

In the tempering 4 in the above experimental example, for example, the tempering hardness of the rack tooth forming portion 1a is about 15 to about 28, and the tempering hardness of the back surface portion 1b is about 30 to about 5 HRC.
It was as high as 0. As an experimental example in this case, when the entire steering rack shaft A is fixed and one end on the side having the rack teeth 1a1 is pushed and bent, the rack tooth forming portion 1
Partial refining 4 is provided in a part of a, and HRC is applied on the tooth side of refining hardness.
23, HRC35 on the rear side, the conventional steering rack shaft is the entire temper, and under the condition that this temper hardness is HRC21, the bending load increases by about 50% in the case of partial tempering compared to the conventional whole temper. did.

As another embodiment of the steering rack shaft A, a part of the rack tooth forming portion 1a in the axial direction is heat treated as a temper 4. That is, in this heat treatment, the tempering temperature of the temper 4 of the rack tooth forming portion 1a and the back surface portion 1b is 400 ° C. or higher, and the temperature near this temperature (400 ° C.
It is about 500 ° C.) and satisfies both hardness and workability. Rack tooth forming portion 1a in this case
And the back surface 1b have the same hardness. To obtain the same hardness as described above, as shown in FIG. 16 (b), the steering rack processed material A0 is rotated by the induction coil 19 for hardening which heats only the semicircular section of the steering rack processed material A0. As a state, the entire shaft is heated and the tempering 4 process is performed.

In the embodiment, the rack tooth forming portion 1
A cross section in which the hardness a is the same as that of the back surface portion 1b is shown in FIGS. 4, 6, 8, and 10, and in each case, the density of the tissue in the entire cross section is made uniform. The figure 4,
6, 8 and 10 are cross-sections in a state where the temper 4 is applied, and FIGS. 4C and 6C show the rack teeth while maintaining the state of FIG. 4B. 1a1 is a cross section finally formed by induction hardening to form a tooth portion quenching layer 5.

Further, as another embodiment of the steering rack shaft A, at least a part of the rack tooth forming portion 1a in the axial direction is hardened and tempered to form the rack tooth forming portion 1a.
Only a is tempered 4, and at least a part of the racked tooth forming portion 1a that has been tempered 4 and the diametrical back surface 1b (only tempering) are heat-treated so as to have different hardnesses. This heat treatment is performed in the tempering induction coil 20 that heats only the semicircular section of the steering rack shaft A in the tempering step, as in the embodiment shown in FIGS. 3, 5, 7, and 9. The steering rack shaft A is fixed and only the rack tooth forming portion 1a side is heated for a predetermined time. At this time, the tempering temperature of the rack tooth forming portion 1a side is 400 ° C. or higher, and the temperature is close to this temperature. (400 ° C. to about 500 ° C.), the rack tooth forming portion 1a is tempered 4, and the heating temperature of the back surface portion 1b is lower than 400 ° C. For example, when tempered at a low temperature of 300 ° C., the martensite structure Moreover, the structure can be stabilized without significantly lowering the hardness. As a result, the back surface portion 1b is simply tempered.

Further, FIGS. 11A to 11F are sectional views of still another embodiment of the hollow steering rack shaft A. FIG. 11A shows the rack tooth forming portion 1a in an unadjusted state. The rear surface 1b of the quality portion (raw material state, etc.) is tempered 4. (B) shows that the rack tooth 1a1 is finally induction hardened while maintaining the state of (a), and the tooth portion hardening layer 5 is formed.
Shows a cross section formed as. Further, (c) shows the entire rack tooth forming portion 1a and the back surface portion 1b, that is, the shaft body 1
Only the outer periphery of the cross section of 4 is tempered, and the rack tooth forming portion 1a (low hardness) and the back surface portion 1b (high hardness) are formed so as to have different hardnesses. The central part is a non-heat treated part (raw material state, etc.). (D) shows a cross-section in which the rack tooth 1a1 is finally induction-hardened to form the tooth portion hardening layer 5 while maintaining the state of (c). Also,
(E) shows that the entire rack tooth forming portion 1a and the back surface portion 1b, that is, only the outer periphery of the cross section of the shaft body 1 is tempered 4, and the rack tooth forming portion 1a and the back surface portion 1b have the same hardness. It was formed. The central part is a non-heat treated part (raw material state, etc.). (F) shows a cross-section in which the rack tooth 1a1 is finally induction-hardened to form the tooth portion hardening layer 5 while maintaining the state of (e).

FIGS. 12 (a) to 12 (f) are sectional views of still another embodiment of the solid steering rack shaft A, the concrete mode of which is shown in FIG. ) ~
It is the same as (f).

As shown in FIGS. 18 and 20, partial tempering 4 may be performed at an appropriate position on the shaft portion 1c of the shaft body 1. That is, when the steering rack shaft A projects from the gear box 10 to the maximum, the end portion 1 of the gear box 10
It is a support corresponding point of 0a, and the range of the corresponding point is S
Express as. FIG. 19 shows the temper 4 in FIG. 18 in which the partial ranges S1 and S2 are divided into two.

Regarding the temper 4 in the shaft portion 1c, when the hardness is different by half in sectional view [FIG.
(See FIG. 19B)] and the case where the whole has the same hardness [FIG.
And FIG. 20 (b)].

In the case of the power steering apparatus of FIG. 2, there are the rack tooth 1a1 side and the shaft portion 1c side, and the temper 4 is provided on both sides.

The manufacturing method of the steering rack shaft A will be briefly described. As shown in FIG.
The steering rack processed material A0 [see FIG. 13 (a)] is pressed with the flat portion 3 at an appropriate position [see FIG. 13 (b)], and then the steering processed material A0 is obtained.
Is heated by the induction coil 19 for quenching of the induction hardening device, cooled and quenched, and then the rack tooth forming portion 1a (processing portion) is heated by the induction coil 20 for tempering by high frequency. Temper at 500 ° C to about 700 ° C. Thereby, the temper 4 is formed [see FIG. 13 (c)]. After that,
The rack tooth 1a1 is processed [see FIG. 13 (d)], and finally,
The steering rack shaft A is manufactured by quenching only one of the rack teeth 1a with an induction hardening device [see FIG. 13 (e)].

Another method of manufacturing the steering rack shaft A shown in FIG. 14 is to process the rack teeth 1a1 [FIG.
(See (c)), the portion having the rack teeth 1a1 in the rack tooth forming portion 1a (processing portion) is tempered by the tempering induction coil 20 at a heating temperature of 500 ° C. to about 700 ° C. Are formed [see FIG. 14 (d)]. That is, the order is partially opposite to that of FIG.
The steering rack shaft A is manufactured by the same process as described above.

[0033]

According to the first aspect of the present invention, first, the distortion (bending) of the steering rack shaft A after the refining 4 can be greatly reduced. As a result, if the hardness of the temper 4 is increased (the heating temperature is brought close to 400 ° C.), the strain generally increases on the axis, but in the present invention, the partial temper 4
Therefore, even if the hardness of temper 4 is increased, shaft distortion (bending)
21 [Distortion line comparison graph of heat treatment in FIG. 21 (as a conventional product, the temper hardness is HRC2
0, as one embodiment of the present invention, partial refining and refining hardness: tooth side HRC23, back side HRC35) solid line]. That is, the tempering hardness can be made higher than the conventional tempering of the entire rack shaft, and the desired position can be made extremely strong.

Further, since it is a part of the temper 4, unlike the conventional whole temper, distortion can be easily removed, the manufacturing efficiency can be improved, and the manufacturing cost can be reduced.

According to the second aspect of the invention, at least a portion of the rack tooth forming portion 1a in the axial direction is tempered 4.
When at least a part of the rack tooth forming portion 1a and the diametrical back surface portion 1b are heat-treated so as to have different hardness, when the rack tooth 1a1 is processed after tempering 4, the rack tooth forming portion 1a1 is processed. 1a has relatively low hardness and is easy to process, and the other surface and back surface 1b have high hardness, and must have rigidity, toughness and strength that can sufficiently withstand a load such as bending load during use. There is an advantage that can be.

Also, in the case of claim 3, the same effect as that of claim 2 can be obtained.

Next, in the case of claim 4, when the through hole 2 is provided in the steering rack shaft A to make it hollow,
Generally, when the shaft is hollow, the cross-sectional area is small and the shaft rigidity is inferior to that of solid shaft. However, depending on the hardness of the temper 4, the rack tooth forming portion 1a can be made strong and the shaft diameter can be minimized.

Further, in the case of claim 5, since the temper hardness of the back surface portion 1b can be made higher than that of the rack tooth forming portion 1a even if it is hollow, it is more effective without increasing the shaft diameter. It is possible to provide a lightweight steering rack shaft A.

In the case of claim 6, the same effect as that of claim 5 is obtained.

Further, in claims 7 to 9, since the final product is induction hardened at one position of the rack tooth 1a, it can be made to have excellent abrasion resistance as a tooth, and the above claims 1 to 9 It also has the effect of 3.

Next, in the tenth aspect, except for the rack tooth forming portion 1a, since the heat treatment as the temper 4 is a part of the axial portion 1c in the axial direction, the hardness of the temper 4 is Even if the temperature is increased, the shaft distortion can be made slight, which allows the temper hardness to be higher than that of the conventional rack shaft as a whole, and the desired position can be strengthened, and furthermore, strain relief is easy. Therefore, the manufacturing cost can be reduced.

[0042]

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of a normal steering device provided with the present invention.

FIG. 2 is a cross-sectional view of a main part of a power steering device provided with the present invention.

FIG. 3A is a side view of a main part of the present invention, FIG. 3B is an enlarged sectional view taken along the arrow P3 of FIG. 3A, and FIG. Cross section

FIG. 4A is a side view of a main part of the present invention, FIG. 4B is an enlarged cross-sectional view taken along arrow P4 of FIG. 4A, and FIG. Cross section

FIG. 5 (a) is a side view of a main part of the present invention. (B) is an enlarged sectional view taken along the arrow P5 of (a). Cross section

FIG. 6A is a side view of a main part of the present invention, FIG. 6B is an enlarged cross-sectional view taken along arrow P6 of FIG. 6A, and FIG. Cross section

FIG. 7A is a side view of a main part of the present invention, and FIG. 7B is an enlarged sectional view taken along the arrow P7 of FIG.

FIG. 8A is a side view of a main part of the present invention, and FIG. 8B is an enlarged sectional view taken along the arrow P8 of FIG.

FIG. 9A is a side view of a main part of the present invention, and FIG. 9B is an enlarged sectional view taken along the arrow P9 of FIG.

FIG. 10A is a side view of a main part of the present invention, and FIG. 10B is an enlarged sectional view taken along arrow P10 of FIG.

FIG. 11A is an enlarged cross-sectional view of another embodiment of a hollow steering rack shaft. FIG. 11B is an enlarged cross-sectional view of induction hardening of rack teeth in FIG. 11A, and FIG. 11C is a hollow steering rack shaft. Enlarged sectional view of another embodiment (d) is an enlarged sectional view of induction hardening of rack teeth in (c) (e) is an enlarged sectional view of another embodiment of a hollow steering rack shaft (f) is (e) ) Enlarged sectional view of induction hardening rack teeth

12A is an enlarged sectional view of another embodiment of a solid steering rack shaft. FIG. 12B is an enlarged sectional view in which rack teeth are induction hardened in FIG. 12A. FIG. 12C is a solid steering rack. Enlarged sectional view of another embodiment of the shaft (d) is an enlarged sectional view of induction hardening of rack teeth in (c) (e) is an enlarged sectional view of another embodiment of a solid steering rack shaft (f) Is an enlarged sectional view of induction hardening of rack teeth in (e).

13 (a) to (e) are schematic process diagrams for producing the product of the present invention.

14 (a) to (e) are schematic process diagrams of another embodiment for producing the product of the present invention.

FIG. 15 (a) is a perspective view of a quenching induction coil, and FIG. 15 (b) is a state diagram in which quenching is performed by the quenching induction coil.

FIG. 16A is a schematic diagram of a heat treatment for a non-rotating work. FIG. 16B is a schematic diagram of a heat treatment for a rotating work.

FIG. 17 (a) is a perspective view of a refining induction coil, and (b) is a state diagram in which refining processing is performed by the refining induction coil.

18 (a) is a side view of a main part of a steering rack shaft, and FIG. 18 (b) is an enlarged cross-sectional view taken along the arrow P18 of FIG. 18 (a).

19 (a) is a side view of a main part of a steering rack shaft, and FIG. 19 (b) is an enlarged sectional view taken along the arrow P19 of FIG. 19 (a).

FIG. 20 (a) is a sectional view of a main part of a power steering apparatus of another embodiment provided with a steering rack shaft. (B) is an enlarged sectional view taken along the arrow P20 of (a).

FIG. 21 is a strain line comparison graph by heat treatment.

FIG. 22 is a graph showing the relationship between the internal hardness of the shaft and the bending load.

[Explanation of symbols]

 A ... Steering rack shaft 1 ... Shaft main body 1a ... Rack tooth forming part 1a1 ... Rack tooth 1b ... Back surface part 2 ... Through hole 4 ... Tempering 5 ... Tooth part hardening layer

[Procedure amendment]

[Submission date] January 26, 1994

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 2

[Correction method] Change

[Correction content]

[Fig. 2]

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Fig. 20

[Correction method] Change

[Correction content]

FIG. 20

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kenji Hoshino 1-2757 Hirosawa-cho, Kiryu-shi, Gunma Yamada Manufacturing Co., Ltd.

Claims (10)

[Claims] 1. A steering rack shaft characterized in that at least a part of an axial direction of a rack tooth forming part of a shaft body is heat treated as a refining. 2. At least a portion of the rack tooth forming portion of the shaft body in the axial direction is tempered, and at least a portion of the rack tooth forming portion and a diametrical back surface thereof are heat treated so as to have different hardness. A steering rack shaft that is characterized by 3. A rack tooth forming portion of a shaft main body is quenched and tempered in at least a part in an axial direction, only the rack tooth forming portion is tempered, and at least a part of the rack tooth formed is tempered. A steering rack shaft characterized in that a portion and a diametrical back surface thereof are heat-treated so as to have different hardness. 4. The steering rack shaft according to claim 1, wherein the shaft body is provided with a through hole. 5. The steering rack shaft according to claim 2, wherein the shaft body is provided with a through hole. 6. The steering rack shaft according to claim 3, wherein the shaft body is provided with a through hole. 7. A rack tooth forming portion of a shaft main body is heat treated as a refining in at least a part of an axial direction portion, and the rack tooth forming portion is induction hardened to have a tooth portion hardening layer on the rack tooth. Steering rack shaft characterized by. 8. A rack tooth forming portion of a shaft main body is tempered in at least a part thereof in an axial direction, and at least a part of the rack tooth forming portion and a diametrical back surface thereof are heat treated so as to have different hardness. The steering rack shaft is characterized in that the rack tooth is formed by induction hardening and the rack tooth has a tooth portion hardening layer. 9. A rack tooth forming portion of a shaft main body is hardened and tempered in at least a part in an axial direction, only the rack tooth forming portion is tempered, and the tempered at least part of rack tooth formation is formed. A steering rack shaft characterized in that a portion and a diametral back surface thereof are heat-treated so as to have different hardness, and the rack tooth forming portion is induction hardened to have a tooth portion hardened layer on the rack tooth. . 10. A steering rack shaft, characterized in that a part of the shaft portion other than the rack tooth forming portion of the shaft body in the axial direction is heat treated as a heat treatment.