JP4311984B2 - Rack bar, steering device using the same, and electric power steering device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a rack bar for steering an automobile, and a steering device and an electric power steering device using the same.
[0002]
[Prior art]
The most important characteristic required for an automotive steering rack bar is that it does not break brittlely even when an excessive load is applied statically or dynamically, and it bends even if an excessive load is applied. By causing deformation (bending deformability), it is required that the part does not completely break and separate.
The current manufacturing process of the rack bar is applied to the following processes: rolled steel, quenching and tempering, machining, induction hardening and tempering, and finishing. In order to avoid brittle fracture as much as possible, the toughness of the material is improved by performing quenching and tempering treatment instead of direct processing and induction quenching of the rolled steel, and then processing the parts and induction quenching. By applying the treatment, brittle fracture is prevented.
[0003]
Conventionally, an attempt has been made to prevent brittle fracture while omitting the quenching and tempering treatment in order to reduce the manufacturing cost in the steel material for the rack bar (Patent Document 1).
[0004]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-8189
[0005]
[Problems to be solved by the invention]
In recent years, electric power steering devices have become popular mainly for small cars because they can improve fuel efficiency by 3 to 5% compared to hydraulic power steering devices.
However, in the electric power steering device, the stress acting on the meshing portion between the rack bar and the pinion as a steered shaft extending in the left-right direction of the automobile is significantly larger than that of the hydraulic power steering device (for example, 6 10 times higher).
[0006]
On the other hand, in order to reduce the cost due to the mass production effect by using common parts, the rack bar having the same specifications as the rack bar used in the hydraulic power steering apparatus is also used in the electric power steering apparatus. For this reason, it is disadvantageous in terms of wear resistance and strength (including fatigue strength) of the rack bar in medium-sized cars and higher where high stress acts, and as a result, the adoption of the electric power steering device is limited to the center of small cars.
[0007]
The rack bar of Patent Document 1 that omits quenching and tempering has a drawback that the toughness for bending deformability is insufficient.
On the other hand, when performing normal quenching or tempering, there are problems that the tempering time is long and the manufacturing cost is high.
The present invention has been made in view of the above problems, and an object thereof is to provide a rack bar, a steering device, and an electric power steering device that are inexpensive and have excellent bending deformability.
[0008]
[Means for Solving the Problems and Effects of the Invention]
  In order to achieve the above object, the present invention provides a round bar-shaped main body having a diameter D formed using steel that has been quenched and tempered, a flat portion provided on a part of the peripheral surface of the main body, In the rack bar provided with the rack tooth forming part provided on the flat part and formed with a plurality of rack teeth, the carbon content of the steel is 0.50 to 0.60% by mass, and at least the rack tooth forming part A hardened layer subjected to induction hardening and tempering is provided, and the surface hardness of the rack tooth forming portion is 680 to 800 HV in terms of Vickers hardness,As a result of the tempering treatment being performed at an atmospheric temperature of 660 ° C. or more and 700 ° C. or less for a time of 20 minutes or less with respect to the round bar subjected to quenching treatment as an intermediate for manufacturing the rack bar, a product As a book rack bar,In the portion where the depth from the surface of the back portion of the rack tooth forming portion is (3/4) D, the total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%, and the regenerated pearlite structure is The area percentage is 0 to 50%, and the total area percentage of the tempered bainite structure, tempered martensite structure and regenerated pearlite structure is 50 to 100%.It is saidA rack bar is provided.
[0009]
Usually, the rack teeth are formed to a depth of about D / 4. Therefore, the depth from the surface of the back surface of the rack tooth forming portion is (3/4) D (hereinafter simply referred to as “(3/4) D”. Part ") generally corresponds to the boundary between the quenched part and the unquenched part. Even if a rack bar that has received a large load undergoes excessive bending deformation and cracks partially occur, it remains in the (3/4) D portion at an area percentage of at least 30%. Since the tempered bainite structure and the tempered martensite structure prevent the propagation of cracks, it is possible to prevent the rack bar from being broken and broken into two parts. In addition, the reason why the area percentage of the regenerated pearlite structure is set to 50% or less in the (3/4) D part is that the toughness is not lowered.
[0010]
By hardening the steel having a carbon (C) content of 0.50 to 0.60 mass%, the hardness and toughness necessary for a rack bar are imparted. That is, by setting the carbon content to 0.50% by mass or more, the wear resistance of the rack teeth can be increased by induction hardening, whereas when the carbon content exceeds 0.60% by mass, the rack bar resistance Since the impact property is lowered and it becomes easy to cause cracking during induction hardening, the carbon content is set in the range of 0.50 to 0.60% by mass.
[0011]
Moreover, the reason why the surface hardness of the rack tooth forming portion is limited to 680 to 800 HV is as follows. That is, if the surface hardness is less than 680 HV, the surface hardness of the rack tooth forming portion becomes insufficient and the fatigue limit for bending fatigue is lowered. On the other hand, if it exceeds 800 HV, the toughness of the surface layer portion decreases, and static load or quasi-static This is because the bending strength against the mechanical load is insufficient. Therefore, by setting the surface hardness of the rack tooth forming portion to 680 to 800 HV, the fatigue limit against bending fatigue is increased and sufficient bending strength against static or quasi-static load is ensured.
[0012]
Moreover, it is preferable if the boron content of the steel is 5 to 30 ppm. By adding such a small amount of boron, induction hardenability can be improved. That is, the grain boundary of the induction-hardened part can be strengthened, and the toughness can be increased to significantly improve the bending deformability (cracking resistance). As boron content for obtaining such an effect, at least 5 ppm or more is necessary, but even if boron exceeding 30 ppm is contained, the effect is saturated, so it is preferable to set it in the range of 5 to 30 ppm. .
[0013]
Moreover, it is preferable if the effective depth of the hardened layer (corresponding to the effective hardened layer depth) at the bottom of the rack tooth forming portion is 0.1 to 1.5 mm from the surface of the bottom of the tooth. When the effective depth exceeds 1.5 mm, when subjected to a high impact, there is a tendency to bend locally at one place in the longitudinal direction of the rack bar and bend into a "<" shape. As a result, the pinion may not be able to move on the rack bar. On the other hand, if the effective depth is less than 0.1 mm, the bending strength near the base of the rack tooth may be insufficient. Therefore, by setting the effective depth in the range of 0.1 to 1.5 mm, the entire rack bar bends gently when a heavy load is applied while ensuring the tooth root bending strength, thus ensuring the emergency steering performance. It becomes possible. A more preferable effective depth is 0.3 to 1.2 mm.
[0014]
  If it is said rack bar, it can be used conveniently for a steering device, Furthermore, it can be used suitably for an electric power steering device.
  Further, the present invention provides a round bar-shaped main body having a diameter D, a flat portion provided on a part of the peripheral surface of the main body, and a rack tooth forming portion provided on the flat portion and formed with a plurality of rack teeth. In this method, a round bar made of steel having a carbon content of 0.50 to 0.60% by mass is quenched to obtain a primary intermediate, and then an ambient temperature of 660 to 700 ° C. Tempering in a time of 20 minutes or less in a furnace heated to 1 and cooled to room temperature, the depth (from the surface of the steel back corresponding to the back of the rack teeth of the rack tooth forming portion ( 3/4) The reclaimed pearlite structure of the D portion is 0 to 50% in terms of area percentage, and the total of the tempered bainite structure, the tempered martensite structure and the regenerated pearlite structure of the depth (3/4) D portion is 50 to 10 in area percentage After forming a flat portion on the peripheral surface of the tertiary intermediate obtained by drawing out the secondary intermediate and then processing the rack teeth on the formed flat portion The rack tooth forming part is formed, induction hardening is performed on the formed rack tooth forming part, and then the surface hardness of the rack tooth forming part is set to 680 to 800 HV in terms of Vickers hardness. The present invention provides a method for manufacturing a rack bar.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus in which a steering rack according to an embodiment of the present invention is used. Referring to FIG. 1, an electric power steering system (EPS) 1 is connected to a steering shaft 3 connected to a steering member 2 such as a steering wheel, and to the steering shaft 3 via a universal joint 4. An intermediate shaft 5, a pinion shaft 7 connected to the intermediate shaft 5 via a universal joint 6, and rack teeth 8 a meshing with a pinion 7 a provided at the tip of the pinion shaft 7. And a rack bar 8 as a steered shaft extending in the direction.
[0016]
The rack bar 8 is supported in a linearly reciprocable manner through a plurality of bearings (not shown) in a housing 17 fixed to the vehicle body. Both end portions of the rack bar 8 project to both sides of the housing 17, and tie rods 9 are coupled to the respective end portions. Each tie rod 9 is connected to a corresponding steering wheel 10 via a corresponding knuckle arm (not shown).
When the steering member 2 is operated and the steering shaft 3 is rotated, this rotation is converted into a linear motion of the rack bar 8 along the left-right direction of the automobile by the pinion 7a and the rack teeth 8a. Thereby, the steering of the steering wheel 10 is achieved.
[0017]
The steering shaft 3 is divided into an input shaft 3a connected to the steering member 2 and an output shaft 3b connected to the pinion shaft 7. These input and output shafts 3a and 3b are on the same axis line via a torsion bar 11. They are connected to each other so as to be relatively rotatable.
A torque sensor 12 is provided for detecting a steering torque based on the relative rotational displacement between the input and output shafts 3a and 3b via the torsion bar 11, and the torque detection result of the torque sensor 12 is obtained from an ECU (Electric Control Unit: electronic Control unit) 13. The ECU 13 controls the voltage applied to the steering assisting electric motor 15 via the drive circuit 14 based on a torque detection result or a vehicle speed detection result given from a vehicle speed sensor (not shown). The output rotation of the electric motor 15 is decelerated via the speed reduction mechanism 16 and transmitted to the pinion shaft 7 via the output shaft 3b and the intermediate shaft 5, and converted into a linear motion of the rack bar 8, thereby assisting steering. .
[0018]
As the speed reduction mechanism, a small gear 16a such as a worm shaft that is connected to a rotating shaft (not shown) of the electric motor 15 so as to be integrally rotated, and a worm wheel that meshes with the small gear 16a and is connected to the output shaft 16b so as to be integrally rotatable. A gear mechanism including a large gear 16b such as can be exemplified.
2A is a partial cross-sectional side view of the rack bar 8, and FIG. 2B is a cross-sectional view taken along the line bb of FIG. 2A. The rack bar 8 includes a round bar-shaped main body 20 having a diameter D, a flat portion 21 provided on a part of the peripheral surface 20 a of the main body 20, and a rack tooth forming portion 22 provided on the flat portion 21.
[0019]
The flat portion 21 extends in a predetermined length parallel to the axis line 23 of the main body 20 and has a predetermined width. The rack tooth forming portion 22 includes a plurality of rack teeth 8a provided above and a tooth bottom portion 24 provided between adjacent rack teeth 8a.
The main body 20 is formed using a steel bar that has been subjected to induction hardening and short-time tempering described later. The carbon content of the steel forming the main body 20 is 0.50 to 0.60 mass%. The reason why the carbon content is 0.50% by mass is to increase the wear resistance of the rack teeth 8a by subjecting the steel material to induction hardening, which will be described later. However, if the carbon content exceeds 0.60 mass%, the impact resistance characteristics of the rack bar 8 are deteriorated, and it becomes easy to cause a burning crack during the induction heat treatment. Therefore, the carbon content is set to 0.60% by mass or less, preferably 0.56% by mass or less.
[0020]
Further, the steel constituting the main body 20 preferably contains 5 to 30 ppm of boron (B). By adding boron of 5ppm or more, the grain boundary of the induction-hardened part can be strengthened, and the toughness can be increased to significantly improve the bending deformability (cracking resistance), while boron exceeding 30ppm is contained. However, since the effect is saturated, it is preferably set in the range of 5 to 30 ppm.
As other components of steel, Si content 0.05-0.5 mass%, Mn content 0.2-1.5 mass%, S content 0.06 mass% or less, Cr content 1.5 It contains not more than mass% (not including 0 mass%), and the balance is Fe and inevitable impurities.
[0021]
At least the rack tooth forming portion 22 of the rack bar 8 is provided with a hardened layer 25 by induction hardening and tempering performed after the rack teeth 8a are formed. The surface hardness of the rack tooth forming portion 22 is set to 680 to 800 HV in terms of Vickers hardness. If it is less than 680 HV, the surface hardness of the rack tooth forming portion 22 becomes insufficient, and the fatigue limit for bending fatigue is lowered. On the other hand, if it exceeds 800 HV, the toughness of the surface layer portion decreases, and static load or quasi-static This is because the bending strength against the load is insufficient.
[0022]
Moreover, in the rack tooth formation part 22, it is preferable that the effective depth d of the hardened layer 25 in the tooth bottom part 24 between the rack teeth 8a is in the range of 0.1 to 1.5 mm from the surface of the tooth bottom part 24. Here, the effective depth d of the hardened layer 25 is defined by the distance from the surface to the position of the hardness of 450 HV, and corresponds to the effective hardened layer depth.
When the effective depth d of the hardened layer 25 in the tooth bottom part 24 exceeds 1.5 mm, when receiving a high impact, it bends locally at one place in the middle part of the rack bar 8 in the longitudinal direction. The pinion 7a may not be able to move on the rack bar 8 as a result. On the other hand, if the effective depth d of the hardened layer 25 is less than 0.1 mm, the bending strength near the root of the rack tooth 8a may be insufficient.
[0023]
Therefore, by setting the effective depth d of the hardened layer 25 at the tooth bottom portion 24 in the range of 0.1 to 1.5 mm, the rack bar 8 as a whole is loaded when a large load is applied while ensuring the root bending strength of the rack teeth 8a. Is designed to bend gently to ensure emergency steering performance. The effective depth d of the hardened layer 25 at the root portion 24 is more preferably 0.3 to 1.2 mm.
Usually, the rack teeth 8a are formed to a depth of about D / 4. Further, on the peripheral surface 20a of the main body 21, a portion 27 [(3/4) D portion with a depth of (3/4) D from the surface of the back surface portion 26 opposite to the rack tooth forming portion 22 in the diameter direction. 27), the total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%, the regenerated pearlite structure is 0 to 50% in terms of area percentage, and the tempered bainite structure and tempered martensite. The total area percentage of the tissue and the regenerated perlite tissue is set to be 50 to 100%. This can be observed from an electron micrograph of the cut surface of the rack bar 8. In addition, the diameter D of said round bar is about 20-40 mm, Preferably it is 20-38 mm, More preferably, it is about 20-36 mm.
[0024]
Further, it is preferable that no residual ferrite is generated from the surface of the tooth bottom 24 to a depth of 0.1 mm. If residual ferrite is generated, the strength may be locally reduced, so that this is excluded.
Next, a method for manufacturing the rack bar 8 will be described. First, as shown in FIG. 3, a solid (in some cases hollow) round bar made of steel having a carbon content of 0.50 to 0.60 mass% is heated to a heating temperature T1 of 780 ° C. or higher. Quench by controlled cooling to room temperature with water cooling. The reason for setting the heating temperature T1 to 780 ° C. or higher is to suppress residual ferrite. The heating temperature T1 is preferably 800 ° C. or higher. The upper limit of the heating temperature T1 is usually about 860 ° C., preferably about 850 ° C.
[0025]
In the controlled cooling, the total of the bainite structure and martensite structure in the (3/4) D portion 27 is 30% (area percentage) or more, preferably 40% (area percentage) or more, more preferably 50%. It is necessary to carry out so that it may become (area percentage) or more. Such controlled cooling conditions can be appropriately set according to the composition of the steel and the like. For example, in the region where the temperature is about 800 to 300 ° C. (preferably 750 to 350 ° C.), the cooling rate is 30 to 80 °. It is preferable to cool at C / second (preferably 40 to 70 ° C./second).
[0026]
The primary intermediate in which the bainite structure and martensite structure thus obtained are introduced is tempered for a treatment time t of 20 minutes or less including the temperature rising process, and is air-cooled to room temperature. The atmosphere temperature T2 of the furnace used for the tempering process is set to 660 ° C. or higher. If the furnace atmosphere temperature T2 is set to 660 ° C. or higher, the Vickers hardness of the tempered secondary intermediate can be reduced (for example, 320 HV or lower) even in the case of tempering for 20 minutes or less. ), It is possible to improve cutting workability when the flat portion 21 and the rack tooth forming portion 22 are later processed. The furnace atmosphere temperature T2 is preferably 680 to 700 ° C.
[0027]
The reason for setting the tempering treatment time t to 20 minutes or less is to reduce the manufacturing cost. The tempering treatment time t is preferably set to 15 minutes or less.
In the above (3/4) D part 27, the total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%, the regenerated pearlite structure is 0 to 50% in area percentage, and tempered. In order to control the structure after tempering so that the total area percentage of the bainite structure, tempered martensite structure and regenerated pearlite structure is 50 to 100%, the above tempering conditions (660 ° C. or higher, 20 minutes) Within the range of ()), it is preferable that the tempering temperature is not too high and the tempering time is not too long. If the tempering temperature is too high or the tempering time is too long, the area percentage of the bainite structure and martensite structure introduced by controlled cooling is likely to be reduced, and the pearlite structure is likely to be regenerated, resulting in lower bending properties. Because.
[0028]
In this way, the secondary intermediate obtained by the tempering process is drawn, and then a part of the peripheral surface of the secondary intermediate is milled to form a flat portion 21. By performing broaching, a rack tooth forming portion 22 including a plurality of rack teeth 8a is formed. Next, the rack tooth forming part 22 is subjected to induction hardening with a heating time of 5.5 seconds and a cooling time of 10 seconds with water cooling, for example, and then subjected to a tempering treatment at 170 ° C. for 1.5 hours, for example. A Vickers hardness of 680 to 800 HV is achieved on the surface of the tooth forming portion 22 to form the rack bar 8.
[0029]
In the rack bar 8 thus obtained, as described above, it is possible to ensure the necessary wear resistance and the necessary bending strength as the rack teeth 8a. Further, the tempered bainite structure and the tempered martensite structure remaining in the (3/4) D portion 27 prevent the crack bar 8 from being broken into two parts, thereby preventing the rack bar 8 from being broken in two. Can do.
In addition, by setting the effective depth d of the hardened layer 25 of the tooth bottom portion 24 to 0.1 to 1.5 mm from the surface of the tooth bottom portion 24, the rack bar 8 as a whole can be secured when a large load is applied while ensuring the root bending strength. It is possible to ensure gentle steering performance by turning gently.
[0030]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples.
Example
C content 0.53% by mass, Si content 0.23% by mass, Mn content 0.8% by mass, S content 0.018% by mass, Cr content 0.30%, B content 0.015 % Steel was rolled into a steel bar having a diameter of 30 mm, heated to a heating temperature of 780 ° C., and then controlled to room temperature. The cooled steel bar was tempered by allowing it to stay in a furnace heated to an atmospheric temperature of 660 ° C. for 15 minutes. The steel bar after tempering was allowed to cool. More preferably, the heating temperature is 820 ° C, and the ambient temperature during tempering is 690 ° C.
[0031]
The steel bar thus obtained was drawn to a diameter of 27.5 mm, and then the flat portion 21 was formed by cutting, and the rack teeth 8a were formed on the flat portion 21 to form the rack tooth forming portion 22. . Next, the rack tooth forming portion 22 was subjected to induction hardening with a heating time of 5.5 seconds and a cooling time by water cooling of 10 seconds, and then tempered at 170 ° C. for 1.5 hours, A hardened layer 25 was provided on 22 to produce the rack bar of the example.
[0032]
In the embodiment, the surface hardness of the rack tooth forming portion 22 is 710 HV. Depth from the surface of the back surface portion 26 of the rack tooth forming portion 22 (3/4) In the D portion 27, the total area percentage of the tempered bainite structure and the tempered martensite structure is 90%, and the regenerated pearlite structure has an area. The percentage is 0%. The effective depth d of the hardened layer 25 of the bottom portion 24 of the rack tooth forming portion 22 is 0.7 mm from the surface of the bottom portion 24.
Comparative example
C content 0.46% by mass, Si content 0.19% by mass, Mn content 0.86% by mass, S content 0.053% by mass, Cr content 0.13% by mass, The steel material not containing was formed into a steel bar by rolling, heated to a heating temperature of 850 ° C., and then cooled to room temperature. The cooled steel bar was tempered by staying in a furnace heated to an ambient temperature of 610 ° C. for 30 minutes or more. The steel bar after tempering was allowed to cool. Using the bar thus obtained, a rack bar of a comparative example was manufactured in the same manner as in the examples.
[0033]
In the comparative example, normal induction hardening and tempering are performed on the rack tooth forming portion. The surface hardness of the rack teeth is 650 HV. (3/4) In part D, the total area percentage of the tempered bainite structure and the tempered martensite structure is 70%, and the regenerated pearlite structure is 20% in terms of area percentage. The effective depth of the hardened layer at the root is 0.3 mm from the surface of the root.
The following tests were conducted using two each of these examples and comparative examples.
Positive input static fracture test
A test apparatus as shown in FIG. 4 was used. The rack bar 8 of the example or the rack bar of the comparative example was assembled in the housing 17, and both ends of the housing 17 were fixed to the fixed columns 31. The rack bar 8 was fixed at the neutral position, and drive torque was applied to the pinion shaft 7 from the rotary actuator 32 connected to the pinion shaft 7. The drive torque was increased, leading to destruction.
[0034]
The load when cracks occur in the rack bar is 305 J in the example, whereas it is 188 J in the comparative example, and the fracture strength of the example is about 62% higher than the fracture strength of the comparative example. It turned out to be.
Reverse input static fracture test
A test apparatus as shown in FIG. 5 was used. The rack bar 8 of the example or the rack bar of the comparative example was incorporated in the housing 17, and both ends of the housing 17 were fixed to the fixed support 31 via the mount rubber 33. The pinion shaft 7 was fixed at the neutral position via the joint 34, the end of the rack bar 8 was pushed by the load cylinder 35 via the load cell 36, and a load was applied until cracking noise was confirmed. The output of the dynamic strain meter 37 connected to the load cell 36 was recorded on the recorder 38.
[0035]
As a result, the crack initiation load of the example is 92 N · m on average, while the crack initiation load of the comparative example is 51 N · m on average, and the fracture strength of the example is compared with the fracture strength of the comparative example. It was found that the increase was about 80%.
Reverse input impact fracture test
A test apparatus as shown in FIG. 6 was used. The rack bar 8 of the example or the rack bar of the comparative example was assembled in the housing 17, and both ends of the housing 17 were fixed to a pair of fixed arms 40 fixed to the fixed column 39. The housing 17 is arranged upright so that the end near the pinion shaft 7 is on the top. The pinion shaft 7 is fixed to the fixed support 41 at the neutral position. The receiving member 42 was fixed to the end of the rack bar 8 on the side close to the pinion shaft 7.
[0036]
A weight 44 is provided above the receiving member 42 so as to be movable up and down by the guide bar 43. A load cell 45 is fixed to the lower portion of the weight 44. The weight 44 to which the load cell 45 is fixed is 100 kg, the distance between the load cell 45 and the receiving member 42 is set to 20 cm, the weight 44 and the load cell 45 are dropped and collide with the receiving member 42 until the load cell 45 is damaged. The number of drops was examined.
A dynamic strain meter 46 was connected to the load cell 45, and the output of the dynamic strain meter 46 was recorded on an electromagnetic oscilloscope 47.
[0037]
As a result of the test, the comparative example resulted in destruction in 3 times on average, while the example resulted in destruction in 15 times on average. It was proved that the reverse input impact strength of the example was remarkably superior to that of the comparative example.
Bending strength test
A test apparatus as shown in FIG. 7 was used. The rack bar 8 of the example or the rack bar of the comparative example was assembled in the housing 17, and both ends of the housing 17 were fixed to the fixed columns 48. With the rack bar 8 protruding as far as possible from the end of the housing 17 on the side close to the pinion shaft, the receiving member 49 fixed to the tip of the rack bar 8 is pushed by the load cylinder 50 through the load cell 51, and the rack bar 8 The bending load was applied until the maximum load was obtained.
[0038]
The output of the dynamic strain meter 52 connected to the load cell 51 was guided to the load meter 53, and the load was measured. As a result, the maximum load was 8.6 KN in the example and 7.4 KN in the comparative example. Thus, it was confirmed that the example had a bending strength of about 16% increase as a comparative example. Moreover, it was confirmed that both of them “bend” without breaking.
Positive input durability test
A test apparatus as shown in FIG. 8 was used. The rack bar 8 of the example or the rack bar of the comparative example was assembled in the housing 17, and both ends of the housing 17 were fixed to the fixed columns 54. Servo actuators 55 were connected to both ends of the rack bar 8, respectively. A rotary actuator 58 is connected to the pinion shaft 7 via a joint 56 and a torque meter 57, and drive torque is applied to the pinion shaft 7 by the rotary actuator 58. The driving torque was 50 N · m, the frequency was 0.1 to 0.2 Hz, and the number of repetitions was 30,000.
[0039]
After the test was completed, the amount of wear at the portion engaged with the pinion was measured. The average of the examples was 8.7 μm, while the average of the comparative examples was 27.8 μm. % Wear has been demonstrated to decrease.
Reverse input durability test
A test apparatus as shown in FIG. 9 was used. The rack bar 8 of the example or the rack bar of the comparative example was assembled in the housing 17, and both ends of the housing 17 were fixed to the fixing columns 59. The pinion shaft 7 is fixed to the neutral position via the joint 60, and the axial force is applied to the rack bar 8 from the servo actuator 61 via the tie rod 9 connected to the end of the rack bar 8 on the side close to the pinion shaft 7. . The axial force applied to the rack bar 8 was 9.8 kN, and the test was performed at a frequency of 5 Hz until breakage.
[0040]
As a result, the comparative example was damaged at 350,000 times, whereas the example was not damaged at 700,000 times.
Bending fatigue test
A test piece 62 shown in FIG. 10A was made of the same material as in the example. The test piece 62 is a substantially round shaft having a total length L of 90 mm. A constricted portion 65 is formed with a cross-sectional curvature of R5 around a position where the distance N is 40 mm from one end 62a of the test piece 62. The minimum diameter R of the constricted portion 65 is 8 mm. One end 62a side of the constricted portion 65 is a cylindrical portion 63 having a diameter P of 12 mm. Further, a taper portion 64 having a 1/20 taper that gradually increases in diameter as the other end 62b side goes to the other end 62b side with the constricted portion 65 interposed therebetween. The maximum diameter Q of the tapered portion 64 is 14 mm. A similar comparative piece was prepared for the comparative example.
[0041]
The bending fatigue test of the test piece 62 or the comparative piece was performed using the test apparatus shown in FIG. The remaining taper portion 64 was fixed to the tapered support hole 67 of the fixed column 66 in a state where the portion with the distance M from the one end 62 a of the test piece 62 protruding in a cantilever manner was cantilevered. A bending load was repeatedly applied at a frequency of 20 Hz by a load cylinder 69 via a rolling roller 68 in the vicinity of one end 62a of the fixed test piece 62, and the stress and the number of repetitions were measured to obtain an SN curve.
[0042]
As a result of the test, in the smoothed portion of the SN curve (the portion where the stress converges), the stress of the comparison piece is 1270 MPa, whereas the stress of the test piece 62 is 1450 MPa, and the fatigue strength is about 15%. Proven to improve.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering apparatus including a rack bar according to an embodiment of the present invention.
2A is a partially cutaway side view of the rack bar, and FIG. 2B is a cross-sectional view taken along the line bb of FIG. 2A.
FIG. 3 is a graph showing the relationship between time and temperature during heat treatment.
FIG. 4 is a schematic view of a test apparatus for a positive input static destructive test.
FIG. 5 is a schematic diagram of a test apparatus for a reverse input static destructive test.
FIG. 6 is a schematic view of a test apparatus for a reverse input impact test.
FIG. 7 is a schematic view of a test apparatus for a bending strength test.
FIG. 8 is a schematic view of a test apparatus for a positive input durability test.
FIG. 9 is a schematic view of a test apparatus for a reverse input durability test.
FIG. 10 (a) is a schematic side view of a test piece, and FIG. 10 (b) is a schematic diagram of a test apparatus for a bending fatigue test.
[Explanation of symbols]
1 Electric power steering system (EPS)
2 Steering member
3 Steering shaft
3a Input shaft
3b Output shaft
5 Intermediate shaft
7 Pinion shaft
7a pinion
8 rack bars
8a rack teeth
11 Torsion bar
12 Torque sensor
13 ECU
15 Electric motor
16 Reduction mechanism
20 body
20a circumference
21 Flat part
22 Rack tooth forming part
23 axis
24 tooth bottom
25 Hardened layer
26 Back side
27 (3/4) D part [(3/4) D part]
62 Specimen
D diameter
d Effective depth

Claims (6)

A round bar-shaped main body having a diameter D formed using quenched and tempered steel, a flat portion provided on a part of the peripheral surface of the main body, and a plurality of rack teeth provided on the flat portion. In a rack bar comprising a formed rack tooth forming portion,
The carbon content of the steel is 0.50 to 0.60 mass%,
At least a hardened layer subjected to induction hardening and tempering is provided on the rack tooth forming part,
The surface hardness of the rack tooth forming part is 680 to 800 HV in terms of Vickers hardness,
As a result of the tempering treatment being performed at an atmospheric temperature of 660 ° C. or more and 700 ° C. or less for a time of 20 minutes or less with respect to the round bar subjected to quenching treatment as an intermediate for manufacturing the rack bar, a product As for this rack bar, the depth from the surface of the back part of the rack tooth forming part is (3/4) D, and the total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%. there are, reproduction pearlite is 0-50% in area percentage, and rack tempered bainite structure, the total area percentage of tempered martensite structure and regeneration pearlite structure, characterized in that there is a 50-100% bar.   The rack bar according to claim 1, wherein the boron content of the steel is 5 to 30 ppm.   3. The rack bar according to claim 1, wherein an effective depth of the hardened layer at a tooth bottom portion of the rack tooth forming portion is 0.1 to 1.5 mm from a surface of the tooth bottom portion.   A steering apparatus using the rack bar according to claim 1, 2 or 3.   An electric power steering apparatus using the rack bar according to claim 1, 2 or 3. Manufactures a rack bar having a round bar-shaped main body with a diameter D, a flat portion provided on a part of the peripheral surface of the main body, and a rack tooth forming portion provided on the flat portion and formed with a plurality of rack teeth. In the way to
After obtaining a primary intermediate by quenching a round bar made of steel having a carbon content of 0.50 to 0.60 mass%,
In a furnace heated to an ambient temperature of 660 to 700 ° C., tempering is performed in a time of 20 minutes or less, and cooled to room temperature, whereby the steel corresponding to the back surface portion of the rack tooth of the rack tooth forming portion is formed. The reclaimed pearlite structure in the portion of depth (3/4) D from the surface of the back portion is made 0 to 50% in area percentage, and the tempered bainite structure and tempered martensite in the portion of depth (3/4) D. To obtain a secondary intermediate in which the total of the structure and the regenerated perlite structure is 50 to 100% by area percentage
Next, after forming a flat portion on the peripheral surface of the tertiary intermediate obtained by drawing the secondary intermediate,
Processing rack teeth on the formed flat part to form a rack tooth forming part,
Manufacturing the rack bar characterized in that the surface hardness of the rack tooth forming part is set to 680 to 800 HV in terms of Vickers hardness by performing induction hardening after the formed rack tooth forming part is subjected to induction hardening. Method.

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