JP2008285076A - Rack-and-pinion steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rack-and-pinion steering device hardly damaging a rack even when applied with collision or stress in using besides little damage caused in manufacturing the rack. <P>SOLUTION: The rack 21 for this rack-and-pinion steering device is made of steel having a carbon content of 0.35-0.55 mass%. That is, the rack is manufactured in a manner that it is molded by cold forging after rod-like or tubular material made of steel is softened and annealed, and is further subjected to induction hardening. The material is softened and annealed, thereby the maximum particle diameter of spheroidized cementite is set to 0.5-7 μm and also hardness is set to Hv 200 or below. A surface layer portion cured by induction hardening is formed on the surface of the rack 21. However, since the inside of the surface layer portion is not hardened by the induction hardening, it is formed as an unhardened portion of hardness of Hv 350 or below. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a rack and pinion type steering device used in an automobile or the like.

  In passenger cars, a rack and pinion mechanism is mainly used as a mechanism for converting the rotation of the steering shaft into the motion of the left and right steered wheels because of its high rigidity and light weight. The rack of the rack-and-pinion steering device is made of a medium carbon steel material (for example, a steel material corresponding to S35 to S55C), and is usually manufactured as follows. That is, after quenching and tempering a rod-shaped material obtained by rolling a medium carbon steel material, teeth that mesh with the teeth of the pinion are formed by cutting, and induction hardening is performed on the teeth. As described above, since the rack is formed by cutting, there is a problem in that the manufacturing requires a lot of labor and time and is expensive.

Therefore, there is a method of forming the rack by plastic working. According to the plastic working, the manufacturing cost is reduced because much labor and time are not required for the manufacturing as compared with the cutting. In recent years, it has been proposed to form a rack by plastic working using a tubular material such as a pipe instead of a rod-shaped material (see Patent Documents 1 to 4). By manufacturing the rack from a tubular material, the weight of the automobile can be reduced.
Japanese Patent Laid-Open No. 10-58081 Japanese Patent Laid-Open No. 2001-79639 Japanese Patent No. 3442298 JP 2006-103644 A

However, when the rack is formed by cold forging, intense processing is performed. Therefore, if the structure of the material is not optimized, the material may be cracked during cold forging.
In addition, the rack has the role of a car frame, and if it is damaged, the steering wheel cannot be operated. Therefore, the rack is designated as an important safety part, and high reliability and strength are required. ing. For example, it is required to have an excellent impact resistance property that does not break even when an automobile receives an impact by riding on a curb or the like, and a property that does not break even when a bending stress is applied.

In order to satisfy such a requirement, the rack is usually made of medium carbon steel, and a hardened hardened layer is formed on the surface by induction hardening to enhance wear resistance and strength against bending stress. However, even if the strength against bending stress is increased by induction hardening, if an excessive load is applied and a crack occurs in the hardened hardened layer, the crack propagates to the inside and the rack breaks, making the handle operation impossible. There was a risk of becoming.
Therefore, the present invention solves the above-mentioned problems of the prior art, and in addition to being hard to be damaged during the manufacture of the rack, the rack and rack is not easily damaged even when an impact or stress is applied during use. It is an object to provide a pinion type steering device.

In the case where the rack of a rack-and-pinion type steering device is formed by cold forging, not only the relationship between the hardness of the material and the formability but also the relationship between the structure of the material and the formability has been earnestly studied. The present invention has been completed by obtaining knowledge about the structure state of a material from which good moldability can be obtained.
That is, in order to solve the problem, the present invention has the following configuration. A rack-and-pinion type steering device according to a first aspect of the present invention meshes with the pinion, a steering shaft that is rotated by a driver's steering, a pinion that is connected to the steering shaft and rotates as the steering shaft rotates. A rack and pinion type steering apparatus including a rack coupled to a wheel, wherein the rack satisfies the following three conditions.

Condition A) A rod-like material or tubular material made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less and subjected to soft annealing was obtained by forming by cold forging. Is.
Condition B) By the soft annealing, the maximum particle size of the spheroidized cementite is 0.5 μm or more and 7 μm or less, and the hardness is Hv 200 or less.
Condition C) It has a surface layer portion that has been quenched and hardened by induction hardening, and a non-quenched portion that has not been quenched, and the hardness of the non-quenched portion is Hv 350 or less.

  The rack and pinion type steering device of the present invention is less likely to be damaged during manufacturing of the rack, and is less likely to be damaged even when an impact or stress is applied during use.

An embodiment of a rack and pinion type steering apparatus according to the present invention will be described in detail with reference to FIG.
A steering shaft 11 having a steering wheel 10 fixed to the upper end portion is supported inside a steering shaft housing 12 so as to be rotatable about an axis. Further, the steering shaft housing 12 is fixed at a predetermined position in the vehicle interior with the lower portion inclined toward the front of the vehicle.

  The rack and pinion mechanism that converts the rotation of the steering shaft 11 into the movements of the left and right steered wheels 15 and 15 is supported by the rack 21 that is movable in the axial direction and the rack 21 teeth that are supported obliquely with respect to the axis of the rack 21. And a pinion 22 having teeth meshing with each other, and a rack 21 and a cylindrical rack housing 23 that supports the pinion 22. The rack and pinion mechanism is arranged substantially horizontally in the engine room at the front of the vehicle so that the longitudinal direction thereof is along the width direction of the vehicle.

Further, the upper end portion of the pinion 22 and the lower end portion of the steering shaft 11 are connected by two universal joints 25 and 26. Further, steered wheels 15 and 15 are connected to both ends of the rack 21.
When a steering torque (rotational force) is applied to the steering wheel 10 by the driver, the steering shaft 11 rotates, and the pinion 22 rotates as the steering shaft 11 rotates. Then, the rotation of the pinion 22 is converted into a slide motion in the left-right direction of the rack 21 by the rack and pinion mechanism, and the steered wheels 15 and 15 are driven to steer the automobile.

  The rack and pinion type steering device of the present embodiment may be provided with a so-called power steering mechanism. That is, the steering torque is detected by a torsion bar (not shown) attached to the steering shaft 11, and the output of the electric motor 13 (rotational force assisting steering) is controlled based on the detected steering torque. The output of the electric motor 13 is supplied to an intermediate portion of the steering shaft 11 (may be supplied to the pinion 22), and is combined with the steering torque to drive the steered wheels 15 and 15 by a rack and pinion mechanism. Converted into movement.

  In this rack and pinion type steering apparatus, the rack 21 is made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less. The rack 21 is formed by cold forging after soft annealing on a rod-like material or tubular material (hereinafter also referred to as a material) made of steel as described above, and further by induction hardening. It is manufactured.

  This material has a hardness of Hv200 or less by soft annealing. The material contains ferrite, spheroidized cementite, and acicular cementite (see FIG. 2 which is a structure diagram of S45C). The maximum particle size of spheroidized cementite is 0.5 μm to 7 μm by softening annealing. It is as follows. Furthermore, a surface layer portion that is quenched and hardened by induction hardening is formed on the surface of the rack 21. The inside of the surface layer portion is not quenched by induction hardening and is a non-quenched portion. And the hardness of this non-hardened part is Hv350 or less.

Since such a rack 21 has excellent strength, it is difficult to cause damage even when a large impact or load is applied. In addition, since the material contains ferrite, spheroidized cementite, and acicular cementite and is excellent in plastic workability, damage such as cracking hardly occurs when forming the rack 21 by cold forging.
Carbon is an element necessary for imparting strength and hardenability to steel, and is required for the performance required for the rack 21 (for example, excellent impact resistance characteristics that do not break even when subjected to an impact, or bending stress). However, in order to impart the property of not breaking, the content in the steel needs to be 0.35% by mass or more. However, if the content exceeds 0.55% by mass, the workability of the steel is lowered and the productivity may be deteriorated.

  Moreover, by softening and annealing before forming the material by cold forging, the hardness is reduced and plastic workability is improved, and damage during forming is suppressed. In order to obtain sufficient plastic workability, the hardness of the material subjected to soft annealing needs to be Hv200 or less, but the lower the hardness, the higher the plastic workability, so the hardness is Hv170. The following is preferable.

  In addition, not only the hardness of the material, but also the plastic workability varies depending on the state and size of carbides in the steel (that is, the structure state). That is, as the particle size of the carbide increases when soft annealing is performed, the carbon concentration in the ferrite, which is the base structure, decreases, and the plastic workability improves. Accordingly, the larger the particle size of the spheroidized cementite, the better the plastic workability. If the soft annealing is insufficient, the maximum particle size of the spheroidized cementite will be less than 0.5μm, the deformation resistance will be large and the plastic workability will be insufficient, so cracking will occur when the material is formed by cold forging. Damage may occur.

  However, if the particle size of the spheroidized cementite is too large, damage will occur in the spheroidized cementite itself when forming by cold forging, or the coarse cementite-ferrite interface will be the source of strain concentration and voids. It is easy to form and break. Furthermore, after the molding, induction hardening is applied to the teeth of the rack to form a hardened layer (surface layer part). However, it may be difficult to dissolve into the base tissue, and sufficient hardness may not be obtained. For these reasons, the maximum particle size of the spheroidized cementite is preferably 7 μm or less.

  Furthermore, although the non-quenched portion of the rack 21 is work-hardened by cold forging, when the work hardening is too high and the hardness of the non-quenched portion exceeds Hv 350, the toughness becomes insufficient and an impact load is applied. In addition, the rack 21 may be damaged. Therefore, the hardness of the non-quenched portion needs to be Hv 350 or less. In order to make the rack 21 more difficult to break, the hardness of the non-quenched portion is preferably Hv 250 or less.

  Below, an example of the manufacturing method of the rack 21 is demonstrated. First, soft annealing is performed on a rod-like material or a tubular material made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less. An example of soft annealing conditions is shown. The material is held at 740 to 860 ° C. above the A1 transformation point for 0.1 h or more, then cooled to 680 to 720 ° C. at a cooling rate of 20 to 70 ° C./h, and held for 1 to 5 hours. Then, it cools to 620-680 degreeC with the cooling rate of 10-100 degreeC / h, and also cools to 500-560 degreeC with the cooling rate of 10-150 degreeC / h. By such soft annealing, the steel becomes a structure containing ferrite, spheroidized cementite, and acicular cementite.

Next, the material subjected to soft annealing is subjected to cold forging as shown in FIGS. 3 and 4 and formed into a predetermined shape. First, an example in which a solid bar material having a circular cross section is formed by cold forging will be described with reference to FIG.
First, the solid bar-shaped material 31 is placed on the die 32 in which the groove 32a having an arcuate cross section is formed. The curvature radius of the groove 32a is set larger than the radius of the solid bar-shaped material 31, and the solid bar-shaped material 31 is disposed in the groove 32a (see FIG. 3A). Then, by pressing the punch 33 against the solid rod-shaped material 31 from above and pressing the punch 33 downward, the solid rod-shaped material 31 is brought into close contact with the groove 32a and deformed. Since the contact surface of the punch 33 with the solid rod-shaped material 31 is flat, a flat portion 31a is formed on the solid rod-shaped material 31 (see FIG. 3B). Thereby, the solid rod-shaped material 31 becomes a shape having the flat surface portion 31a and the cylindrical surface portion 31b. In addition, the contact surface with the solid rod-shaped material 31 of the punch 33 is not limited to a planar shape, and may have a taper.

  Next, a die 34 having a groove 34a deeper than the above-described groove 32a is prepared, and the solid bar-shaped material 31 is disposed in the groove 34a with the cylindrical surface portion 31b facing downward (bottom side of the groove 34a). . At this time, since the width of the groove 34a is designed to be slightly smaller than the diameter of the solid rod-shaped material 31, the solid rod-shaped material 31 is not completely accommodated in the groove 34a ((c) of FIG. 3). See). Then, when the punch 35 having the tooth groove 35 a is pressed against the flat portion 31 a of the solid bar-shaped material 31 and the punch 35 is pressed downward, the solid bar-shaped material 31 is pushed into the groove 34 a of the die 34. At that time, both sides of the solid bar-shaped material 31 (the portions contacting the side walls of the grooves 34a of the die 34) are squeezed and deformed to become parallel planes, and the teeth 31c corresponding to the tooth grooves 35a are formed. It is formed on the flat portion 31a of the solid rod-shaped material 31 (see (d) of FIG. 3). And the meat for the squeezed portions on both sides of the solid rod-shaped material 31 is supplied to the teeth 31c, and the shape of the teeth 31c becomes larger.

  Next, a die 36 in which a groove 36a having a substantially rectangular cross section is formed and a punch 37 in which a groove 37a having a circular arc cross section is prepared. On the bottom surface of the groove 36a formed in the die 36, a tooth groove 36b corresponding to the tooth 31c formed in the flat portion 31a of the solid rod-shaped material 31 is formed. On the other hand, the groove 37 a formed in the punch 37 has a shape corresponding to the cylindrical surface portion 31 b of the solid rod-shaped material 31.

  When the solid rod-shaped material 31 is disposed in the groove 36a formed in the die 36, the width of the groove 36a is slightly smaller than the diameter of the solid rod-shaped material 31, so that the solid rod-shaped material 31 is completely in the groove 36a. Is not accommodated (see FIG. 3E), but when the punch 37 is pressed against the solid bar material 31 from above and pressed downward, the solid bar material 31 is pushed into the groove 36a of the die 36, The shape of the actual bar-shaped material 31 is adjusted. Since both sides of the solid bar-shaped material 31 (parts contacting the side wall of the groove 36a of the die 36) are flat, no surplus is produced when the solid bar-shaped material 31 is pushed into the groove 36a of the die 36. (See (f) in FIG. 3).

  Next, an example in which a tubular material 41 having a circular cross section is formed by cold forging will be described with reference to FIG. In FIG. 4, the same reference numerals as those in FIG. 3 are assigned to the same or corresponding parts as in FIG. Since the forming method is exactly the same as that of the solid rod-shaped material 31, detailed description is omitted. However, when the tubular material 41 is formed by cold forging, the diameter of the hollow hole 42 is set in the hollow hole 42. It is preferable to put another member 43 having substantially the same diameter. At the stage of first deformation (see FIG. 4B), the hollow member 42 is filled with the separate member 43, and the tubular material 41 and the separate member 43 are integrated.

  The material formed into the shape of the rack in this way was induction-hardened and tempered to form a hardened surface layer portion, and then subjected to grinding and superfinishing to complete the rack 21. The conditions for induction hardening are not particularly limited, and examples include conditions such as an output current of 220 to 270 A, a frequency of 100 kHz, a heating time of 3 to 5 seconds, and a cooling time of 10 to 15 seconds.

〔Example〕
Hereinafter, the present invention will be described more specifically with reference to examples. First, the relationship between the crack occurrence rate when cold forging the material and the maximum particle size of spheroidized cementite contained in the steel constituting the material was investigated.
Soft annealing of 25mm diameter round bar material or tubular material made of steel equivalent to S45C specified in Japanese Industrial Standards JIS (carbon content is 0.45 mass%) under the same conditions as in the previous example To obtain a structure containing ferrite, spheroidized cementite, and acicular cementite. Furthermore, the softening annealing conditions were slightly changed to produce spheroidized cementite having different maximum particle sizes and hardnesses (see Table 1).

  Such a material was cold forged as shown in FIGS. 3 and 4 and formed into a rack shape. And it was confirmed by microscopic observation whether the microcrack had generate | occur | produced in the front-end | tip of the tooth | gear which is easy to generate | occur | produce a crack. 100 materials each having the same maximum particle size of spheroidized cementite were prepared and subjected to cold forging, and the occurrence rate of microcracks (breakage rate) was calculated. The results are shown in Table 1. Moreover, the graph of FIG. 5 shows the relationship between the maximum particle diameter of spheroidized cementite and the crack generation rate.

In addition, the hardness shown in Table 1 is the hardness of the raw material in the stage after performing softening annealing. Moreover, the measuring method of the maximum particle diameter of spheroidized cementite is as follows. The material that had been softened and annealed was broken, and the cross section was polished and then etched with a nital etchant. The etched cross section was magnified 1000 times with an optical microscope, photographs were taken at five locations, and the particle size of the spheroidized cementite was measured. Of the various shapes of cementite, those having an aspect ratio (major axis / minor axis) of 2.0 or less were defined as spheroidized cementite.
As can be seen from the graphs in Table 1 and FIG. 4, when the maximum particle size of the spheroidized cementite was 0.5 μm or more, microcracks were not generated by cold forging.

  Next, the relationship between the hardness and toughness of the non-quenched portion of the rack was investigated as follows. A steel material made of steel corresponding to S45C defined in Japanese Industrial Standards JIS (carbon content is 0.45 mass%) was softened and annealed under the same conditions as in the above example. Then, cold forging was performed at various processing rates to form Charpy impact test specimens having different hardnesses (see Table 2).

  In addition, the measuring method of hardness is as follows. The rack was broken and the cross section was polished to give a mirror surface. And the hardness of ten places among this mirror surface was measured with the Vickers hardness meter, and the average value was calculated | required. Further, Comparative Example 5 is a Charpy made of a tempered material obtained by subjecting a steel material similar to the above to a tempering treatment (treatment of holding at 850 ° C. for 30 minutes followed by water cooling and tempering at 700 ° C. for 20 minutes). It is a test piece for impact test and is not cold forged. Such a tempered material is used as a material for a gear cutting rack (a rack formed by cutting teeth) that is currently used in many rack and pinion type steering devices.

The Charpy impact strength of the obtained specimen for Charpy impact test was measured using a Charpy impact strength measuring device. The results are shown in Table 2. The Charpy impact strength in Table 2 is shown as a relative value when the Charpy impact strength of Comparative Example 5 is 1.
As can be seen from Table 2, when the hardness exceeded 350, the Charpy impact strength was lower than that of the current tempered material. Since this test piece has not been induction-hardened, it can be seen from this result that the toughness of the material is insufficient when the hardness of the non-quenched portion is more than 350.

It is a figure explaining the composition of the rack and pinion type steering device concerning the present invention. It is a structure chart of ferrite, spheroidized cementite, and acicular cementite contained in the material. It is a figure explaining the process of giving cold forging to a rod-shaped raw material, and shape | molding in the shape of a rack. It is a figure explaining the process of giving cold forging to a tubular raw material and shape | molding in the shape of a rack. It is a graph which shows the relationship between the largest particle size of spheroidization cementite, and a crack generation rate.

Explanation of symbols

11 Steering shaft 21 Rack 22 Pinion 31 Bar material 41 Tubular material

Claims (1)

A rack-and-pinion type steering including: a steering shaft that is rotated by a driver's steering; a pinion that is connected to the steering shaft and that rotates as the steering shaft rotates; and a rack that meshes with the pinion and is connected to a wheel. A rack and pinion type steering device, wherein the rack satisfies the following three conditions:
Condition A) A rod-like material or tubular material made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less and subjected to soft annealing was obtained by forming by cold forging. Is.
Condition B) By the soft annealing, the maximum particle size of the spheroidized cementite is 0.5 μm or more and 7 μm or less, and the hardness is Hv 200 or less.
Condition C) It has a surface layer portion that has been quenched and hardened by induction hardening, and a non-quenched portion that has not been quenched, and the hardness of the non-quenched portion is Hv 350 or less.