JP2010018068A - Rack and pinion type steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rack and pinion type steering device hardly being damaged in manufacturing. <P>SOLUTION: A rack 21 for the rack and pinion type steering device is made of steel having a carbon content of 0.35 mass% or more and 0.55 mass% or less and a sulfur content of 0.035 mass% or less. That is, the rack is manufactured in such a manner that it is molded into a prescribed shape by cold forging after rod-like or tubular material made of the steel is softened and annealed, and is further subjected to induction hardening. The material (the material after dehardening and annealing and before cold forging) has hardness of HV200 or less by dehardening and annealing, and has a structure containing spheroidal cementite, acicular cementite, and ferrite. Further, the relationship between Vickers hardness H of the material and an amount Cs (mass%) of sulfur contained in the steel satisfies the expression; H+2,000×Cs≤240. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a rack and pinion type steering device used for 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 type 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 6). By manufacturing the rack from a tubular material, the weight of the automobile can be reduced.
JP-A-10-58081 Japanese Patent Laid-Open No. 2001-79639 Japanese Patent No. 3442298 JP 2006-103644 A JP 2007-105751 A JP 2007-144433 A

However, steel with a carbon content of 0.35 mass% or more and 0.55 mass% or less that is generally used as a rack material is a difficult-to-process material. There was a risk of damage such as cracking.
Accordingly, it is an object of the present invention to provide a rack and pinion type steering device that solves the above-described problems of the prior art and is less likely to be damaged during manufacture.

In the case where the rack of the rack-and-pinion type steering device is formed by cold forging, the present inventors have studied the sulfur content in the steel constituting the material, the hardness of the material, and the structure of the material. The present invention has been completed by obtaining knowledge about the relationship between this point and the moldability of the material.
That is, in order to solve the problem, the present invention has the following configuration. The rack and pinion type steering device according to claim 1 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 five conditions.

Condition A) A material made of steel and subjected to soft annealing is formed into a predetermined shape by cold forging and further subjected to induction hardening.
Condition B) The amount of sulfur contained in the steel is 0.035% by mass or less.
Condition C) The material has a structure containing spherical cementite, acicular cementite, and ferrite.
Condition D) The Vickers hardness of the material is HV200 or less.
Condition E) The Vickers hardness H of the material and the amount Cs (mass%) of sulfur contained in the steel satisfy the formula H + 2000 × Cs ≦ 240.

As described above, if the content of sulfur in the steel constituting the material, the hardness of the material, and the structure of the material are controlled, the formability of the material is improved. Therefore, when manufacturing a rack by forming the material by cold forging, there is almost no risk of damage such as cracking in the material.
The rack and pinion type steering apparatus according to claim 2 of the present invention is the rack and pinion type steering apparatus according to claim 1, wherein the amount of carbon contained in the steel is 0.35 mass% or more and 0.55. It is characterized by being not more than mass%.
Furthermore, the rack and pinion type steering apparatus according to claim 3 of the present invention is the rack and pinion type steering apparatus according to claim 2, wherein the steel is S35C to S55C and SAE defined in Japanese Industrial Standard JIS G4051. It is one of 1035 to 1055 defined in the standard.

Here, the critical significance of each numerical value under each condition will be described.
[Sulfur content]
When manganese sulfide, which is a non-metallic inclusion, is present in steel, damage such as cracking is likely to occur in the material during cold forging. Therefore, if the content of sulfur (S) in the steel is reduced, the production of manganese sulfide is suppressed, and the material is less likely to be damaged during cold forging. In order to sufficiently suppress the occurrence of damage to the material when forming the rack by cold forging the material, the sulfur content in the steel needs to be 0.035% by mass or less. .

[Carbon content]
Carbon (C) is an element necessary for ensuring the strength and surface hardness of steel after induction hardening. Since the rack requires static strength, bending fatigue strength, and pitching fatigue strength, the carbon content is preferably 0.35 mass% or more. However, if the content is more than 0.55% by mass, the hardness becomes too high and the moldability may be impaired, so 0.55% by mass or less is preferable.

[About Vickers hardness of material]
The material in the present invention has good formability because it has been subjected to soft annealing, but when soft annealing is insufficient, deformation resistance increases, damage such as cracking May occur. These causes include the occurrence of pearlite in the structure and insufficient spheroidization of carbides. In order for the material to have good moldability even if the soft annealing is insufficient, the Vickers hardness of the material needs to be HV200 or less.

[Relationship between Vickers hardness of material and amount of sulfur contained in steel]
As the sulfur content in steel increases, the amount of manganese sulfide increases, and this manganese sulfide becomes the starting point of damage such as cracking during cold forging, but just by reducing the sulfur content in steel. The occurrence of damage cannot be suppressed sufficiently. Causes of damage during cold forging include inclusions such as manganese sulfide as well as the structure of the material. If both are not suitable, the occurrence of damage cannot be sufficiently suppressed. In order to sufficiently suppress the damage of the raw material during cold forging, it is necessary that the Vickers hardness H of the raw material and the amount Cs of sulfur contained in the steel satisfy the expression H + 2000 × Cs ≦ 240.

  The rack and pinion type steering device of the present invention is less likely to be damaged during manufacture.

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 movement 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 motion.

  In this rack and pinion type steering device, the rack 21 is made of steel having a carbon content of 0.35 mass% to 0.55 mass% and a sulfur content of 0.035 mass%. ing. Then, the rack 21 is softened and annealed to a rod-like material or tubular material (hereinafter also referred to as a material) made of steel as described above, and then formed into a predetermined shape by cold forging, and further induction-hardened. It is manufactured by giving.

  This material (the material after soft annealing and before cold forging) has a hardness of HV200 or less by soft annealing. Further, this material (a material after soft annealing and before cold forging) has a structure containing spherical cementite, acicular cementite, and ferrite. Furthermore, the Vickers hardness H of the material (the material after soft annealing and before cold forging) and the amount Cs (mass%) of sulfur contained in the steel satisfy the formula H + 2000 × Cs ≦ 240. Furthermore, a hardened layer that has been hardened by induction hardening is formed on the surface of the rack 21. The inside of the hardened layer is not quenched by induction hardening and is a non-quenched portion.

Thus, since the content of sulfur in the steel constituting the material, the hardness of the material, and the structure of the material are controlled, the formability of the material is good. Therefore, when the rack 21 is manufactured by forming the material by cold forging, there is almost no possibility that damage such as cracking occurs in the material.
In addition, as steel constituting the material, S35C, S38C, S40C, S43C, S45C, S48C, S50C, S53C, S55C defined in Japanese Industrial Standard JIS G4051, and 1035, 1037, 1038, defined in SAE standard, 1039, 1040, 1042, 1043, 1044, 1045, 1046, 1049, 1050, 1053, 1055 are preferable.

  Below, an example of the manufacturing method of the rack 21 is demonstrated. First, soft annealing is applied to a rod-like material or tubular material made of steel having a carbon content of 0.35% by mass to 0.55% by mass and a sulfur content of 0.035% by mass or less. Apply. 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 structure becomes a structure containing spherical cementite, acicular cementite, and ferrite.

Next, the material subjected to soft annealing is subjected to cold forging as shown in FIGS. 2 and 3 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 rod-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 rod-shaped material 31, and the solid rod-shaped material 31 is disposed in the groove 32a (see FIG. 2A). 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. 2B). 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) in FIG. 2). 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. 2). 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. 2E), 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. 2).

  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. 3, the same or corresponding parts as those in FIG. 2 are denoted by the same reference numerals as those 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. 3B), 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 subjected to induction hardening and tempering to form a hardened layer on the surface, 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. The relationship between the crack occurrence rate when cold forging the material, the Vickers hardness of the material and the sulfur content in the steel was investigated.
Conditions similar to the above-mentioned example are applied to a round bar material or a tubular material having a diameter of 25 mm made of steel (S35C, S45C, or S55C defined in Japanese Industrial Standards JIS) having alloy components as shown in Tables 1-6. To give a structure containing spherical cementite, acicular cementite, and ferrite.
Such a material was cold forged as shown in FIGS. 2 and 3 and formed into a rack shape. And it was confirmed by microscopic observation whether the microcrack has 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 Vickers hardness and the same sulfur content in the steel were prepared and subjected to cold forging, and the occurrence rate of microcracks (breakage rate) was calculated. The results when the material is a round bar material are shown in Tables 1 to 3, and the results when the material is a tubular material are shown in Tables 4 to 6. Further, from the Vickers hardness H of the material and the sulfur content Cs (mass%) in the steel, a value of H + 2000 × Cs (hereinafter referred to as a formula value) is calculated, and the formula value and the crack occurrence rate The relationship is shown in the graphs of FIGS. FIG. 4 is a graph when the material is a round bar material, and FIG. 5 is a graph when the material is a tubular material.

In addition, the Vickers hardness shown to Tables 1-6 is the hardness of the raw material in the stage just before performing cold forging. In addition to the alloy components (carbon, silicon, manganese) shown in Tables 1 to 6 and sulfur which is an impurity, steel contains chromium and inevitable impurities contained in ordinary steel. The impurities are phosphorus, copper, nickel, nitrogen, and oxygen. The contents thereof are 0.03% by mass or less for phosphorus, 0.3% by mass or less for copper, 0.2% by mass or less for nickel, and nitrogen. Is 0.03% by mass or less, and oxygen is 0.003% by mass or less.
As can be seen from the graphs of FIGS. 4 and 5, if the formula value exceeds 240, cracking occurred when cold forging was performed. If the sulfur content in the steel is high, the amount of manganese sulfide increases remarkably, so this manganese sulfide is considered to be the starting point of cracking.

It is a figure explaining the composition of the rack and pinion type steering device concerning the present invention. 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 formula value (H + 2000 * Cs) in case a raw material is a round bar raw material, and a crack generation rate. It is a graph which shows the relationship between a formula value (H + 2000xCs) in case a raw material is a tubular raw material, and a crack generation rate.

Explanation of symbols

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

Claims (3)

A rack-and-pinion type steering comprising: a steering shaft that is rotated by a driver's steering; a pinion that is coupled to the steering shaft and that rotates as the steering shaft rotates; and a rack that meshes with the pinion and is coupled to a wheel. The rack and pinion type steering apparatus, wherein the rack satisfies the following five conditions.
Condition A) A material made of steel and subjected to soft annealing is formed into a predetermined shape by cold forging and further subjected to induction hardening.
Condition B) The amount of sulfur contained in the steel is 0.035% by mass or less.
Condition C) The material has a structure containing spherical cementite, acicular cementite, and ferrite.
Condition D) The Vickers hardness of the material is HV200 or less.
Condition E) The Vickers hardness H of the material and the amount Cs (mass%) of sulfur contained in the steel satisfy the formula H + 2000 × Cs ≦ 240.   The rack and pinion type steering device according to claim 1, wherein the amount of carbon contained in the steel is 0.35 mass% or more and 0.55 mass% or less.   The rack and pinion type steering device according to claim 2, wherein the steel is one of S35C to S55C defined in Japanese Industrial Standard JIS G4051 and 1035 to 1055 defined in SAE Standard. .

Images