CN1791696A - Steel bar for steering rack, manufacturing method thereof, and steering rack using same - Google Patents

Abstract

The invention provides a bar steel for a steering rack, which contains C: 0.50 to 0.60 mass%, Si: 0.05 to 0.5 mass%, Mn: 0.2 to 1.5 mass%, B: 0.0005 to 0.003 mass%, Ti: 0.005-0.05 mass%, Al: 0.0005 to 0.1 mass%, and N: 0.002 to 0.02 mass%. The steel bar has a diameter D, and the quenched and tempered structure at a portion of the steel bar having a depth D/4 of the surface thereof satisfies the following conditions I), II) and III). I) The total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%. II) the area percentage of the regenerated pearlite structure is 0-50%. III) the total area percentage of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure is 50 to 100%.

Description

Steel bar for steering rack, manufacturing method thereof, and steering rack using same

technical field

The present invention relates to a steel bar for a steering rack used in a steering device of an automobile, a manufacturing method thereof, and a steering rack using the same.

Background technique

Steering of automobiles is classified into a method of assisting by oil pressure (hydraulic steering assist) and an assisting method by electricity (electric steering assist).

The hydraulic power steering is the current mainstream, which utilizes the engine output to boost the operating force by the pressure oil sprayed from the operating oil pump. However, in the hydraulic power steering, since the output of the engine is used as the power source, there will be a problem of reducing the fuel efficiency of the vehicle.

In contrast, electric power steering uses an electric motor that operates on electrical energy from the battery to assist the steering force. Compared with the above-mentioned hydraulic power steering, electric power steering devices have become more and more popular in recent years because they improve the fuel efficiency of automobiles.

In automobiles, the steering rack extending to the left and right has the following requirements:

1) Excellent impact resistance to the extent that it will not be damaged even if the car is driven on the curb, etc.

2) The property of not breaking even when bending stress is applied (crack resistance), and

3) Wear resistance of rack teeth

In such steering racks, S45C steel (for example, refer to JP-A-62-178472 and JP-A-62-180018), medium carbon steel (for example, JP-A 2000-153336, etc.) have been used in the past. Gazette and JP-A-2001-79639 Gazette), etc. In addition, the wear resistance is improved by forming a surface hardened layer on the surface by induction hardening, and the strength against bending stress (crack resistance) is also improved.

For example, even if the bending strength of the steering rack is increased by induction hardening, if an excessive load acts on the induction hardened layer and temporarily cracks, the cracks may expand inward and break.

In addition, in the electric power steering that has become popular in recent years, the contact surface pressure between the steering rack and the pinion gear (Pinion Gear) tends to be higher than that of the hydraulic power steering, and the wear resistance of S45C steel is insufficient. Also, in medium carbon steel, if the wear resistance is improved by increasing the C content, the impact resistance will decrease.

For example, JP-A-10-8189 describes a steel for steering racks that does not undergo bending deformation due to brittle fracture even when an excessive load is applied when B is added and induction hardened. The steel for the steering rack is steel without quenching and tempering treatment, and its structure is substantially ferrite/pearlite.

Contents of the invention

An object of the present invention is to provide a steel bar for a steering rack that has improved wear resistance and excellent impact resistance, and can prevent cracks from expanding, a method for manufacturing the same, and a steering rack using the same.

The present inventors completed the present invention based on the following findings: even if the amount of C is increased, the drop of impact resistance can be prevented by adding B, and furthermore, if the tempered bainite structure, tempered martensite If the body structure is controlled within a predetermined range, the bending deformation performance can be improved, and even if a crack occurs even a little, the expansion and penetration of the crack can be prevented.

That is, the steel bar for steering racks of the present invention contains C: 0.50 to 0.60% by mass, Si: 0.05 to 0.5% by mass, Mn: 0.2 to 1.5% by mass, B: 0.0005 to 0.003% by mass, Ti: 0.005 to 0.05% by mass % by mass, Al: 0.0005 to 0.1% by mass, and N: 0.002 to 0.02% by mass. Assuming that the diameter of the steel bar for the steering rack is D, the quenched and tempered structure of the steel bar at a depth of D/4 from the surface is adjusted so as to satisfy the following conditions of I), II) and III)

I) The total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%.

II) The area percentage of the regenerated pearlite structure is 0-50%.

III) The total area percentage of tempered bainite structure, tempered martensite structure and regenerated pearlite structure is 50-100%.

In addition, the steel bar for the transmission rack may further contain Cr. In addition, free-machining elements (S, Pb, Bi, Te, Mg, Ca, rare earth elements, Zr, etc.) may also be contained.

The steel bar for the steering rack of the present invention can be produced by quenching the steel bar obtained by rolling the steel sheet from a temperature of 780° C. After the body tissue is 30-100% (area percentage) in total, it is placed in a furnace heated to an ambient temperature of 660-720° C., subjected to a short-time tempering treatment of less than 20 minutes, and cooled to room temperature.

Description of drawings

Fig. 1 is a graph showing the relationship between time and temperature during heat treatment for manufacturing a steel bar for steering racks according to the present invention.

Fig. 2 is a schematic perspective view showing the shape of a test disk used in an experiment example.

3 is a schematic diagram showing a schematic configuration of an electric power steering apparatus including a rotating rack according to an embodiment of the present invention.

FIG. 4A is a partially cutaway side view of the steering rack 8, and FIG. 4B is a cross-sectional view along line 4B-4B of FIG. 4A.

Fig. 5 is a schematic diagram of a test device being subjected to a static destruction test.

Fig. 6 is a schematic diagram of a test device for a reverse input static destruction test.

Fig. 7 is a schematic diagram of a test device for a reverse input impact test.

Fig. 8 is a schematic diagram of a test device for a bending strength test.

Fig. 9 is a schematic diagram of a test device that is undergoing an endurance test.

Fig. 10 is a schematic diagram of a test device for a reverse input durability test.

FIG. 11A is a schematic front view of a test piece, and FIG. 11B is a schematic view of a testing device for a bending fatigue test.

Detailed ways

The steel bar for steering racks of the present invention contains C: 0.50-0.60% by mass, Si: 0.05-0.5% by mass, Mn: 0.2-1.5% by mass, B: 0.0005-0.003% by mass, and Ti: 0.005-0.05% by mass , Al: 0.0005 to 0.1% by mass, N: 0.002 to 0.02% by mass. Moreover, it is preferable to contain 1.5 mass % or less (excluding 0 mass %) of Cr. The remainder is Fe and unavoidable impurities.

The reason for limitation of the said component is as follows.

The reason for setting the C content to 0.50% by mass or more is that the wear resistance can be sufficiently improved when used as a steering rack (for example, a steering rack for electric power steering). The C content is preferably 0.52% by mass or more. However, if the C content is too large, the impact resistance of the steering rack will be reduced. Therefore, the C content is set to be 0.60% by mass or less, preferably 0.58% by mass or less, more preferably 0.56% by mass or less.

The reason for setting the Si content to 0.05% by mass or more is to deoxidize the steel material. The content of Si is preferably 0.10% by mass or more, particularly preferably 0.15% by mass or more. However, if the content of Si is too large, the machinability when forming rack teeth will be reduced. Therefore, the Si content is set to 0.5% by mass or less, preferably 0.35% by mass or less, more preferably 0.30% by mass or less.

The reason for setting the Mn content to 0.2% by mass or more is not only to increase the strength of the steel, but also to increase the bending deformability when the steel is processed into a steering rack by improving the hardenability to facilitate the formation of a bainite structure. The content of Mn is preferably 0.5% by mass or more, particularly preferably 0.7% by mass or more. However, if the content of Mn is too large, the hardened layer by induction hardening will be too deep, and the bending deformation performance will be reduced. Therefore, the content of Mn is 1.5 mass % or less, Preferably it is 1.3 mass % or less, More preferably, it is 1.2 mass % or less.

The reason for setting the B content to 0.0005% by mass or more is to ensure impact resistance in the steel of the present invention in which the C content is increased. The content of B is preferably 0.0007% by mass or more. However, if the content of B is too high, harmful B-based compounds will be produced, and the impact resistance will be reduced instead. Therefore, the content of B is 0.003 mass % or less, Preferably it is 0.0025 mass % or less, More preferably, it is 0.0020 mass % or less.

Ti inhibits the formation of BN by combining with N in the steel to form TiN, and is effective for ensuring the above-mentioned effect of B. Therefore, the content of Ti is 0.005% by mass or more, preferably 0.010% by mass or more, more preferably 0.012% by mass or more. However, if the content of Ti is too large, the impact resistance of the steering rack will conversely be reduced. Therefore, the content of Ti is set to be 0.05% by mass or less, preferably 0.04% by mass or less, more preferably 0.035% by mass or less.

The reason why Al and N are contained is that austenite grains during induction hardening can be refined by forming AlN. The Al content is 0.0005% by mass or more, preferably 0.010% by mass or more, more preferably 0.020% by mass or more. Also, the content of N is 0.002% by mass or more, preferably 0.003% by mass or more, and more preferably 0.004% by mass or more. However, if the content of Al and N is too large, the impact resistance property will be reduced. Therefore, the content of Al is set to be 0.1% by mass or less, preferably 0.08% by mass or less, more preferably 0.05% by mass or less. The content of N is 0.02% by mass or less, preferably 0.01% by mass or less, more preferably 0.007% by mass or less.

The reason for adding Cr is to improve hardenability. Although the lower limit of the Cr content is not particularly limited, it is, for example, about 0.05% by mass, preferably about 0.08% by mass, and more preferably about 0.10% by mass. However, if the content of Cr is too high, the hardened layer formed by induction hardening will be too deep, and the bending deformability will be insufficient. Therefore, the content of Cr is, for example, 1.5% by mass or less, preferably 1.0% by mass or less, more preferably 0.50% by mass or less.

In addition, the steel bar for a steering rack according to the present invention may further contain a machinable element (S, Pb, Bi, Te, Mg, Ca, rare earth element, Zr, etc.) as necessary. The amount of these machinable elements is as follows, for example, S: 0.06 mass % or less (excluding 0 mass %), Pb: 0.3 mass % or less (excluding 0 mass %), Bi: 0.2 mass % or less (excluding 0 mass%), Te: 0.1 mass% or less (excluding 0 mass%), Mg: 0.01 mass% or less (excluding 0 mass%), Ca: 0.01 mass% or less (excluding 0 mass%), rare earth elements ( REM): 0.01% by mass or less (excluding 0% by mass), Zr: 0.3% by mass or less (excluding 0% by mass). These machinability elements can be added alone or in combination of two or more.

Examples of rare earth elements include Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

Therefore, in the steel bar for steering racks of the present invention, the quenched and tempered structure of the steel bar at a depth D/4 (D represents the diameter of the steel bar) from the surface can be adjusted as follows 1), 2) and 3).

1) The total of the tempered bainite structure and the tempered martensite structure (hereinafter, sometimes expressed as "TB+TM") is 30 to 100% (area percentage).

2) The regenerated pearlite structure is 0-50% (area percentage).

3) The total of the tempered bainite structure, tempered martensite structure, and regenerated pearlite structure (hereinafter, sometimes expressed as "TB+TM+RP") is 50 to 100% (area percentage).

Hereinafter, the portion of the steel bar described above at a depth of D/4 from the surface is simply referred to as a D/4 portion.

The reasons for these organizational controls will be described below.

The tempered bainite structure and tempered martensite structure are obtained by quenching and tempering the rolled bar steel, and are useful for preventing cracks in the induction hardened layer when used as a steering rack. Expansion and penetration are effective.

That is, since the induction hardened part (surface part) of the steering rack is extremely hard, if a large bending is applied, the induction hardened layer near the root of the steering rack (usually, near the D/4 part) is likely to be cracked. If the tempered bainite structure and tempered martensite structure remain at the boundary between the induction hardened part and the non-induction hardened part, it can prevent the cracks generated in the induction hardened layer from expanding inward, and can prevent the steering rack own rupture. Therefore, the total (TB+TM) of the fire bainite structure and the tempered martensite structure in the D/4 portion is 30% or more. Preferably it is 40% or more, More preferably, it is 50% or more.

The regenerated pearlite structure is a structure produced in the tempering process, and is a structure different from the pearlite structure of rolled steel. Unlike the tempered bainite structure and the tempered martensite structure, the regenerated pearlite structure does not play a role in preventing the expansion and penetration of cracks, and the too much regenerated pearlite structure reduces the bending deformation performance. Therefore, the regenerated pearlite structure is set at 50% or less. Preferably it is 40% or less, More preferably, it is 30% or less. Also, the regenerated pearlite structure decreases, and the impact resistance tends to be further improved.

Also, even if the tempered bainite structure and tempered martensite structure are obtained by quenching and tempering, if the ferrite-pearlite structure or soft iron If the body structure remains, the expansion and penetration of cracks cannot be prevented.

Therefore, it is necessary to reduce these structures derived from the rolled material, in other words, it is necessary to increase the tempered bainite structure and the tempered martensite structure. The total (TB+TM+RP) of the three structures formed by these quenching and tempering steps is 50% or more, preferably 60% or more, more preferably 70% or more.

The diameter of the steel bar for steering racks of the present invention is not particularly limited, but is usually about 10 to 40 mm in consideration of processing into a steering rack. It is preferably about 15 to 38 mm, more preferably about 20 to 36 mm.

The steel bar for steering racks as described above can be produced by, for example, rolling a steel sheet with the above composition and quenching the obtained steel bar to obtain a bainite structure and a martensite structure. After that, high temperature and short time tempering treatment is carried out.

The heating temperature for quenching in this production method is 780°C or higher, preferably 800°C or higher. If the heating temperature for quenching is too low, TB+TM+RP after tempering tends to be small. Also, a soft ferrite layer is formed, making the steering rack insufficient in strength. The upper limit of the heating temperature is, for example, about 860°C, preferably about 850°C. When the heating temperature is too high, the bending of the bar steel tends to increase during quenching.

The cooling conditions for quenching need to make the total of the bainite structure and martensite structure of the above-mentioned D/4 portion formed by quenching be 30% (area percentage) or more, preferably 40% (area percentage) or more, More preferably, it is 50% (area percentage) or more. Such conditions for controlled cooling can be appropriately set according to the composition of the steel, etc. For example, it is desirable to use a cooling rate of 30 to 80° C./second ( Cooling is preferably performed at 40 to 70° C./sec).

The thus-obtained intermediate into which a bainite structure and a martensite structure have been introduced, with reference to FIG. 1 , is tempered for a treatment time t of less than 20 minutes, preferably less than 15 minutes, including the heating process, and air-cooled to room temperature. . The atmosphere temperature T2 of the furnace used for the tempering treatment is about 660 to 720°C, preferably about 680 to 700°C.

If the atmosphere temperature T2 of the furnace is set above 660°C, even if the tempering is performed for a short time of less than 20 minutes, the Vickers hardness after tempering can be reduced (for example, it can be obtained below 320HV), and the subsequent processing can be improved. Machinability of racks. In addition, the atmosphere temperature T2 of the furnace is preferably 680 to 700°C.

In order to control the structure after tempering, in the above-mentioned D/4 part 27, the total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%, and the area percentage of the regenerated pearlite structure is 0% to 50%, and the total area percentage of tempered bainite structure, tempered martensite structure and regenerated pearlite structure is 50% to 100%, preferably under the above tempering conditions (above 660°C, within 20 minutes) Within the range, do not make the tempering temperature too high and the tempering time too long.

This is because if the tempering temperature is too high and the tempering time is too long, the area percentage of the bainite structure and martensite structure formed by controlled cooling will easily decrease, and pearlite will easily regenerate, resulting in a decrease in bending properties.

The bar steel for steering racks of the present invention obtained in this way has improved wear resistance, excellent impact resistance, and excellent bending deformation performance, so it is extremely useful for steering racks (especially steering racks for electric power steering). it works.

Experimental example 1-36

Steel materials having the compositions shown in Tables 1 to 2 below were melted and rolled into steel bars with a diameter of 30 mm. Then, after heating to the temperature shown in Tables 1-2, quenching was completed by controlling cooling to room temperature. In addition, in this controlled cooling, the structure of the bar steel is controlled by changing the amount of water and the water cooling time. After cooling, the steel bar was tempered by staying in the furnace heated to the atmospheric temperature shown in Tables 1 and 2 for the time shown in Tables 1 and 2. The steel bars after tempering are allowed to cool.

The structure of the D/4 part of the quenched steel bar and the structure of the D/4 part of the tempered steel bar were observed with an electron microscope (5000 times the magnification), and the martensite structure and the bainite structure were obtained. , and the area ratios of tempered martensite, tempered bainite, and regenerated pearlite.

Both the components and structures of Experimental Examples 1 to 19 are properly included in the technical scope of the present invention, and at least one of the components and structures of Experimental Examples 20 to 36 is not suitable, and are outside the scope of the present invention.

In addition, the following experiments were conducted in order to investigate the fracture resistance (bending deformation ability), impact resistance, and wear resistance of tempered bar steel as a steering rack.

Bending test

The tempered bar steel is drawn to form a diameter of 27.5 mm, and then cut to form rack teeth. The depth of rack teeth is about D/4. Next, the tooth portion was subjected to induction hardening under the following conditions to produce a steering rack.

High-frequency quenching conditions

Coil used: for surface quenching (diameter 40mm, thickness 2mm)

Voltage: 4.0kV

Current: 4.5A

Frequency: 40kHz

Heating method: moving quenching (moving speed 3.0mm/s)

Cooling: Mixed solvent of water-soluble oil and water

Using the obtained steering rack, a 3-point bending test was performed with a fulcrum pitch of 400 mm and a pressing position on the opposite side of the steering rack teeth, and evaluation was performed according to the following criteria.

×: The cracks that occurred in the induction hardened layer expanded and penetrated to the inside, and the steering rack was broken in two

○: Cracking stops halfway, no breakage

Impact test

After the tempered steel bar was drawn to have a diameter of 27.5 mm, a JIS No. 3 U-notch test piece was cut out from the D/4 section, and the surface on the side where the notch was formed was subjected to induction hardening. The induction hardening conditions were the same as those of the bending test described above except that the moving speed was 3.5 mm/sec. The pendulum impact test (test temperature: room temperature) was performed on the obtained test piece according to JIS-Z2242, and the impact value was calculated|required.

wear test

A circular plate having the same characteristics as the tempered bar steel obtained in the experimental example was prepared. That is, steel with the same composition as the bar steel of the experimental example was smelted, and after being hot forged into a diameter of 53 mm, and cut into a disc with a thickness of 15 mm, quenching, tempering,

Next, it was machined into a two-layer semicircular plate as shown in Figure 2 (the diameter of the upper layer is 44 mm, the thickness of the upper layer is 3 mm; the diameter of the lower layer is 50 mm, and the thickness of the lower layer is 5 mm), and the upper layer is subjected to induction hardening. The conditions of the induction hardening were the same as those of the bending test described above except that the moving speed was 2.5 mm/sec. The obtained test disk was subjected to a pin on disk wear test to measure the wear reduction amount of the test piece. There are also detailed conditions of the wear test, as shown below.

Lubrication: dry

Test piece surface roughness: Ra0.25μm

Peripheral speed: 0.05m/s

Surface pressure: 0.05Gpa

Pin: SUJ2 (diameter 5mm, Rockwell hardness (HRC) 64)

The results are shown in Tables 1-2.

Table 1 Example Steel composition (unit = mass %; the remainder is Fe and unavoidable impurities) Quenching process Tempering process Bending test Impact value (J/cm ² ) Wear loss (mg) C Si mn B Ti Cr al N Heating temperature (℃) Tissue [B+M] (area%) Furnace temperature (℃) Residence time (minutes) Tissue [TB+TM] (area%) Tissue [RP] (area%) Tissue [TB+TM+RP] (area%) 1 0.52 0.20 0.83 0.0016 0.021 - 0.041 0.0040 810 90 690 10 90 0 90 ○ 56 10 2 0.56 0.18 0.89 0.0014 0.022 - 0.040 0.0042 810 90 690 10 90 0 90 ○ 51 6 3 0.53 0.19 1.06 0.0012 0.023 - 0.048 0.0049 820 90 690 10 90 0 90 ○ 52 8 4 0.54 0.22 0.72 0.0017 0.020 - 0.040 0.0044 820 90 690 10 90 0 90 ○ 54 7 5 0.54 0.24 0.88 0.0008 0.022 - 0.041 0.0042 820 90 690 10 90 0 90 ○ 51 7 6 0.53 0.24 0.76 0.0018 0.022 - 0.045 0.0044 830 90 690 10 90 0 90 ○ 52 6 7 0.54 0.20 0.75 0.0010 0.015 - 0.048 0.0047 820 90 690 10 90 0 90 ○ 53 7 8 0.53 0.22 0.82 0.0014 0.023 - 0.040 0.0041 820 90 690 10 90 0 90 ○ 53 6 9 0.54 0.23 0.83 0.0011 0.022 0.13 0.050 0.0041 830 90 690 10 90 0 90 ○ 53 7 10 0.54 0.24 0.82 0.0016 0.020 0.21 0.048 0.0046 820 90 690 10 90 0 90 ○ 54 8 11 0.53 0.24 0.86 0.0013 0.021 - 0.049 0.0041 820 90 690 10 90 0 90 ○ 52 9 12 0.54 0.21 0.75 0.0014 0.022 - 0.042 0.0049 830 80 690 10 80 0 80 ○ 51 8 13 0.53 0.23 0.73 0.0012 0.022 - 0.048 0.0043 830 70 690 10 70 0 70 ○ 51 8 14 0.53 0.24 0.74 0.0013 0.021 - 0.046 0.0045 820 60 690 10 60 0 60 ○ 52 8 15 0.53 0.20 0.83 0.0011 0.021 - 0.047 0.0044 810 90 690 10 90 0 90 ○ 53 7 16 0.54 0.23 0.84 0.0012 0.021 - 0.040 0.0041 820 90 690 5 90 0 90 ○ 52 8 17 0.55 0.24 0.86 0.0013 0.022 - 0.048 0.0047 820 90 720 10 70 20 90 ○ 50 7 18 0.54 0.21 0.86 0.0010 0.022 - 0.047 0.0049 840 90 690 10 90 0 90 ○ 53 7 19 0.53 0.22 0.77 0.0013 0.022 - 0.040 0.0044 820 90 690 10 90 0 90 ○ 51 8

[B+M] represents the total of bainite structure and martensite structure; [TB+TM] represents the total of tempered bainite structure and tempered martensite structure; [RP] represents regenerated pearlite structure.

[TB+TM+RP] represents the total of tempered bainite structure, tempered martensite structure and regenerated pearlite structure.

Table 2 Example Steel composition (unit = mass %; the remainder is Fe and unavoidable impurities) Quenching process Tempering process Bending test Impact value (J/cm ² ) Wear loss (mg) C Si mn B Ti Cr al N Heating temperature (℃) Tissue [B+M] (area%) Furnace temperature (℃) Residence time (minutes) Tissue [TB+TM] (area%) Tissue [RP] (area%) Tissue [TB+TM+RP] (area%) 20 0.46 0.23 0.82 0.0017 0.022 - 0.042 0.0043 820 90 690 10 90 0 90 ○ 61 62 twenty one 0.64 0.22 0.76 0.0011 0.021 - 0.044 0.0041 830 90 690 10 90 0 90 ○ 16 5 twenty two 0.55 0.21 0.12 0.0015 0.022 - 0.045 0.0043 810 10 690 10 10 0 10 x 51 7 twenty three 0.53 0.24 1.60 0.0017 0.020 - 0.050 0.0045 830 90 690 10 90 0 90 x 53 9 twenty four 0.53 0.21 0.82 0.0002 0.023 - 0.046 0.0048 830 90 690 10 90 0 90 ○ twenty three 8 25 0.54 0.21 0.82 0.0054 0.021 - 0.050 0.0048 830 90 690 10 90 0 90 ○ 14 7 26 0.54 0.23 0.79 0.0014 0.002 - 0.049 0.0047 840 90 690 10 90 0 90 ○ 26 7 27 0.55 0.22 0.88 0.0013 0.113 - 0.041 0.0045 810 90 690 10 90 0 90 ○ twenty four 5 28 0.54 0.22 0.85 0.0014 0.022 1.64 0.044 0.0042 830 90 690 10 90 0 90 x 52 8 29 0.54 0.21 0.88 0.0013 0.021 - 0.050 0.0045 020 20 690 10 20 0 20 x 51 0 30 0.53 0.22 0.83 0.0014 0.024 - 0.043 0.0041 810 20 730 10 10 40 50 x 18 9 31 0.55 0.23 0.80 0.0012 0.023 - 0.043 0.0048 - 0 - - 0 0 0 x twenty one 6 32 0.53 0.22 0.79 0.0014 0.020 - 0.044 0.0041 840 90 750 5 30 60 90 x 10 9 33 0.54 0.22 0.78 0.0014 0.023 - 0.041 0.0042 830 90 750 10 10 70 80 x 15 8 34 0.53 0.23 0.79 0.0012 0.022 - 0.042 0.0043 830 90 740 30 10 60 70 x 17 7 35 0.53 0.21 0.74 0.0013 0.022 - 0.042 0.0049 760 40 690 10 40 0 40 x 52 8 36 0.54 0.20 0.74 0.0014 0.022 - 0.045 0.0045 750 30 690 10 30 0 30 x 51 8

[B+M] represents the total of bainite structure and martensite structure; [TB+TM] represents the total of tempered bainite structure and tempered martensite structure; [RP] represents regenerated pearlite structure.

[TB+TM+RP] represents the total of tempered bainite structure, tempered martensite structure and regenerated pearlite structure.

Experimental examples 20 to 28 in Table 2 are examples in which the design components are not suitable. That is, in Experimental Example 20, the wear resistance was not sufficient because the amount of C was insufficient. On the contrary, in Experimental Example 21, the impact resistance property was insufficient because the amount of C was too large.

In Experimental Example 22, since the amount of Mn was small, the hardenability was poor, so the total amount of the tempered bainite structure and the tempered martensite structure was insufficient, and the bending deformability was insufficient.

In Experimental Example 23, since the amount of Mn was excessive, the hardened layer at the time of induction hardening became deep, and the bending deformability was insufficient.

In Experimental Examples 24 to 27, the impact resistance was insufficient because the amount of B or Ti was not appropriate.

In Experimental Example 28, since the amount of Cr was excessive, the hardened layer at the time of induction hardening became deep, and the bending deformability was insufficient.

Also, from Experimental Examples 29 to 36, it can be seen that even if the components are properly designed, the properties will be insufficient if the structure is not suitable. That is, in Experimental Examples 29 to 31, the total amount of the tempered bainite structure and the tempered martensite structure was insufficient, so the bending deformation was insufficient.

In Experimental Examples 32 to 34, since the regenerated pearlite structure was too much, the bending deformability was insufficient. In Experimental Examples 35 to 36, since the total amount of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure was insufficient, the bending deformability was insufficient.

On the other hand, in Experimental Examples 1 to 19, since both the component design and the structure were suitable, both the impact resistance and the abrasion resistance were excellent, and the expansion and penetration of cracks could be prevented.

According to the steel bar for a steering rack of the present invention, since both the composition and the structure are appropriately controlled, the wear resistance is improved, the impact resistance is also excellent, and the expansion and penetration of cracks can be prevented.

Next, FIG. 3 is a schematic diagram showing a schematic configuration of an electric power steering apparatus including a steering rack using the steel bar for a steering rack of the present invention described above.

Referring to Fig. 3, an electric power steering system (EPS: Electric Power Steering System) 1 has: a steering shaft 3 connected to the steering wheel, that is, a steering member 2; an intermediate shaft 5 connected to the steering shaft 3 through a universal joint 4; The universal joint 6 is connected to the pinion shaft 7 of the intermediate shaft 5; the steering gear has a rack tooth 8a meshing with the pinion 7a provided at the front end of the pinion shaft 7 and serves as a steering shaft extending in the left-right direction of the vehicle Article 8.

The steering rack 8 is supported in a housing 17 fixed to the vehicle body via a plurality of bearings not shown, and can freely reciprocate linearly. A pair of end portions of the steering rack 8 protrude toward both sides of the housing 17 , and are coupled to tie rods 9 at each end portion. Each tie rod 9 is connected to a corresponding steering wheel 10 via a corresponding steering arm (not shown).

When the steering member 2 is operated to rotate the steering shaft 3, the rotation is converted into linear motion of the steering rack 8 in the left-right direction of the vehicle through the pinion 7a and the rack teeth 8a. Thereby, steering of the steering wheel 10 is realized.

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, 3b are connected on the same axis through a torsion bar 11 and can be connected to each other. relative rotation.

A torque sensor 12 is provided to detect torque through the relative rotational displacement between the input and output shafts 3a, 3b via the torsion bar 11, and the torque detection result of the torque sensor 12 is input to an ECU (Electric Control Unit: Electronic Control Unit) 13. In the EUC 13 , the driving of the steering assist electric motor 15 is controlled by the drive circuit 14 based on the torque detection result and the vehicle speed detection result obtained from a vehicle speed sensor not shown.

The output rotation of the electric motor 15 is decelerated by the reduction mechanism 16, transmitted to the pinion shaft 7 through the output shaft 3b and the intermediate shaft 5, and converted into linear motion of the steering rack 8 to assist steering.

As a deceleration mechanism, an exemplary gear mechanism has a pinion 16a, such as a worm shaft, which is connected to an unillustrated rotating shaft of the electric motor 15 and can rotate together; meshes with this pinion 16a and is connected to the output The shaft 16b can rotate together with a large gear 16b such as a worm wheel.

FIG. 4A is a partial sectional front view of the steering rack 8, and FIG. 4B is a sectional view along line 4B-4B of FIG. 4A. The steering rack 8 has a round bar-shaped main body 20 with 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 .

The flat portion 21 has a predetermined width extending for a predetermined length parallel to the axis 23 of the main body 20 . The rack tooth forming portion 22 includes a plurality of the rack teeth 8 a described above, and a tooth bottom portion 24 provided between the adjacent rack teeth 8 a.

The steering rack 8 is formed using the steel bar for the steering rack of the present invention described above. That is, the steel bar for the steering rack used for the steering rack 8 contains C: 0.50 to 0.60% by mass, Si: 0.05 to 0.5% by mass, Mn: 0.2 to 1.5% by mass, B: 0.0005 to 0.003% by mass, Ti: 0.005 to 0.05% by mass, Al: 0.0005 to 0.1% by mass, and N: 0.002 to 0.02% by mass.

In addition, the quenched and tempered structure of the portion of the bar steel at a depth D/4 from the surface (the diameter of the bar steel is D) satisfies the following conditions of I), II) and III).

I) The total area percentage of the tempered bainite structure and the tempered martensite structure is 30 to 100%.

II) The area percentage of the regenerated pearlite structure is 0-50%.

III) The total area percentage of tempered bainite structure, tempered martensite structure and regenerated pearlite structure is 50-100%.

In the steel bar for the steering rack used to form the steering rack 8, Cr is preferably contained in an amount of 1.5% by mass or less (excluding 0% by mass).

Contained in the above-mentioned bar steel for steering rack used to form the steering rack 8, at least one of the following optional ones, S: 0.06 mass % or less (excluding 0 mass %), Pb: 0.3 mass % or less (excluding 0% by mass), Bi: 0.2% by mass or less (0% by mass is not included), Te: 0.1% by mass or less (0% by mass is not included), Mg: 0.01% by mass or less (0% by mass is not included), Ca: 0.01 mass % or less (excluding 0 mass %), rare earth elements: 0.01 mass % or less (excluding 0 mass %), Zr: 0.3 mass % or less (excluding 0 mass %).

As described above, the carbon content of the steel forming the steering rack 8 is 0.50 to 0.60% by mass. The reason for setting the carbon content to 0.50% by mass is to improve the wear resistance of the rack teeth 8 a by subjecting the steel material to induction hardening described later. However, if the carbon content exceeds 0.60% by mass, the impact resistance of the steering rack 8 is reduced, and hot cracks are likely to occur during high-frequency heat treatment. Therefore, the carbon content is set to be 0.60% by mass or less, preferably 0.58% by mass or less, more preferably 0.58% by mass or less.

In addition, in the steel forming the steering rack 8, B is contained in an amount of 5 to 30 ppm. By adding more than 5ppm of B, the grain boundaries of the induction hardened part can be strengthened, especially the toughness can be increased and the bending deformation performance (crack resistance) can be increased. On the other hand, even if the content of B exceeds 30ppm, the effect is saturated, so it is preferable. Set within the range of 5 to 30 ppm.

At least the rack tooth forming portion 22 of the steering rack 8 is provided with a hardened layer 25 by induction hardening and tempering performed after the rack teeth 8 a are formed. The surface hardness of the rack tooth forming portion 22 is set to 680 to 800 HV in Vickers hardness.

This is because if it is less than 680HV, the surface hardness of the rack tooth forming part 22 is insufficient, and the fatigue limit against bending fatigue becomes low. The bending strength of the 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 can be increased, and sufficient bending strength against static load or pseudo-static load can be ensured.

In the rack tooth forming portion 22, the effective hardened layer depth d of the hardened layer 25 of the tooth bottom 24 between the rack teeth 8a is preferably in the range of 0.1 to 1.5 mm from the surface of the tooth bottom 24. Here, the effective hardened layer depth d of the hardened layer 25 is defined as the distance from the surface to the hardness position of 450 HV, and corresponds to the effective hardened layer depth.

When the effective hardened layer depth d of the hardened layer 25 of the tooth bottom portion 24 exceeds 1.5 mm, when a high impact is applied, a part of the middle portion in the longitudinal direction of the steering rack 8 tends to bend into a mountain shape. As a result, there is a possibility that the pinion 7 a cannot move on the steering rack 8 . On the other hand, when the effective hardened layer depth d of the hardened layer 25 is less than 0.1 mm, there is a possibility that the bending strength near the tooth roots of the rack teeth 8a is insufficient.

Therefore, by setting the effective hardened layer depth d of the hardened layer 25 of the tooth bottom portion 24 in the range of 0.1 to 1.5 mm, the root bending strength of the rack tooth 8a can be ensured, and the steering rack 8 as a whole can relax when a large load acts. Curved for exceptional steering performance. The effective hardened layer depth d of the hardened layer 25 of the tooth bottom 24 is more preferably 0.3 to 1.2 mm.

For example, the rack tooth 8 a is formed with the tooth bottom positioned at a depth of about D/4. Therefore, on the peripheral surface 20a of the main body 21, a portion 27 (also referred to as a (3/4)D portion) having a depth of (3/4)D from the surface of the portion 26 facing the rack tooth forming portion 22 in the radial direction 27), which becomes the approximate boundary between the quenched part and the unquenched part.

In the above (3/4) D section 27, the total area percentage of the tempered bainite structure and the tempered martensite structure is set to be 30 to 100%, and the area percentage of the regenerated pearlite structure is set to be 0 to 50%. , and the total area percentage of tempered bainite structure, tempered martensite structure and regenerated pearlite structure is 50-100%. This can be observed from an electron micrograph of a cut surface of the steering rack 8 .

Even if the rack subjected to a heavy load undergoes excessive bending deformation and cracks are formed in a part of it, the tempered bainite structure and tempered martensitic structure with an area percentage of at least 30% remaining in the above (3/4)D portion The body tissue prevents the propagation of cracks and also prevents the rack from breaking into two pieces. In addition, in the above-mentioned (3/4)D portion, the area percentage of the regenerated pearlite structure is set to be 50% or less, because the toughness is not reduced.

In addition, it is preferable not to form residual ferrite to a depth of 0.1 mm from the surface of the tooth bottom portion 24 . The formation of residual ferrite should be excluded because it may reduce the local strength.

Next, a method of manufacturing the steering rack 8 will be described. The flat part 21 is formed by performing milling on a part of the peripheral surface of the bar steel for steering racks of the present invention having the above composition and structure (for example, the composition and structure of Experimental Examples 1 to 19). The flat portion 21 is broached to form a rack tooth forming portion 22 including a plurality of rack teeth 8a. Next, in the rack tooth forming part 22, for example, after induction hardening with a heating time of 5.5 seconds and a water cooling cooling time of 10 seconds, a tempering treatment is performed at, for example, 170° C. for 1.5 hours, thereby forming the rack teeth. The surface of the portion 22 realizes a Vickers hardness of 680-800HV, and forms the steering rack 8 .

In the steering rack 8 thus obtained, as described above, the required wear resistance and the required bending strength can be ensured as the rack teeth 8a. In addition, the tempered bainite structure and tempered martensite structure remaining in the (3/4)D portion prevent the propagation of cracks to the inside, thereby preventing the steering rack 8 from being broken into two stages.

Furthermore, by setting the effective hardened layer depth of the hardened layer 25 of the tooth bottom 24 to 0.1 to 1.5 mm from the surface of the tooth bottom 24, the tooth root bending strength of the rack tooth 8a can be ensured, and the rack can be steered when a large load acts. 8. The whole body is gently curved to ensure extraordinary steering performance. The effective hardened layer depth d is preferably 0.3 to 1.2 mm from the surface of the tooth bottom 24 .

Hereinafter, examples will be given to describe the steering rack of the present invention more specifically.

Example

Using a steel material with a C content of 0.53 mass%, a Si content of 0.23 mass%, a Mn content of 0.8 mass%, a S content of 0.018 mass%, a Cr content of 0.30 mass%, and a B content of 0.015 mass%, the diameter is formed by rolling. After the 30mm steel bar is heated to 780°C, it is then controlled to cool to room temperature. After cooling, the steel bar is tempered by staying in a furnace heated to an ambient temperature of 660° C. for 15 minutes. The steel bars after tempering are allowed to cool. More preferably, the heating temperature is 820°C, and the atmosphere temperature during tempering is 690°C.

The bar steel obtained in this way was drawn to have a diameter of 27.5 mm, and then cut to form a flat portion 21 , and rack teeth 8 a were formed on this flat portion 21 as the rack tooth forming portion 22 . Next, heat the rack tooth forming part 22 for 5.5 seconds, and then perform induction quenching with a cooling time of 10 seconds by water cooling, and then perform tempering treatment at 170° C. for 1.5 hours, and harden the rack tooth forming part 22. Layer 25, fabricated as the rack of the embodiment.

In the embodiment, the surface hardness of the rack tooth forming portion 22 is 710HV. In the depth (3/4) D portion 27 of the rack tooth formation portion 22 from the surface of the back surface portion 26, the total area percentage of the tempered bainite structure and the tempered martensite structure is 90%, and the pearlite is regenerated. Tissue was 0% in area percentage. The effective depth d of the hardened layer 25 of the tooth bottom 24 of the rack tooth forming portion 22 is 0.7 mm from the surface of the tooth bottom 24 .

comparative example

Using a steel material containing 0.46% by mass of C, 0.19% by mass of Si, 0.86% by mass of Mn, 0.053% by mass of S, and 0.13% by mass of Cr, and not containing B, after forming a steel bar by rolling, Heat to 850°C, then cool to room temperature. After cooling, the steel bar is tempered by staying in a furnace heated to an ambient temperature of 610° C. for more than 30 minutes. The steel bars after tempering are allowed to cool. Using the steel bar thus obtained, a steering rack of a comparative example was produced thereafter in the same manner as in the examples.

In the comparative example, normal quenching and tempering were performed on the rack tooth forming portion. The surface hardness of the rack teeth is 650HV. In the (3/4)D section, the total area percentage of the tempered bainite structure and the tempered martensite structure was 70%, and the area percentage of the regenerated pearlite structure was 20%. The effective depth of the hardened layer at the bottom of the tooth is 0.3 mm from the surface of the bottom of the tooth.

Using two of these examples and comparative examples, the following tests were performed.

Positive input static destruction test

Use the test setup shown in Figure 5. The steering rack 8 of the embodiment to the steering rack of the comparative example were inserted into the housing 17, and both ends of the housing 17 were respectively fixed to the fixing struts 31. The steering rack 8 is fixed at a neutral position, and a driving torque is applied to the pinion shaft 7 from a rotary actuator connected to the pinion shaft 7 . The driving torque is continuously increased until it is destroyed.

The load when a crack occurs in the steering rack is 305J in the Example and 188J in the Comparative Example. Comparing the breaking strength of the Example with the breaking strength of the Comparison, it was found that the load increased by about 62%.

Reverse Input Static Destruction Test

Use the test setup shown in Figure 6. The steering rack 8 of the embodiment to the steering rack of the comparative example were inserted into the case 17, and both ends of the case 17 were respectively fixed to the fixed struts 31 via the support rods 33. Fix the pinion shaft 7 at the neutral position through the universal joint (joint) 34, and press the end of the steering rack 8 into the load cylinder 35 through the load cell until it is confirmed that the sound of cracking occurs Apply load. The output of a dynamic strain gauge 37 connected to a load cell 36 is stored in a recorder 38 .

As a result, the average cracking load of the comparative example was 51 N m compared to the average cracking load of the example, which was 92 N m. Comparing the breaking strength of the working example with the breaking strength of the comparative example, it was found that it increased by about 80 %.

Reverse Input Dynamic Destruction Test

Use the test setup shown in Figure 7. The steering rack 8 of the embodiment and the steering rack of the comparative example are inserted into the housing 17 , and both ends of the housing 17 are fixed to a pair of fixing arms 40 fixed to the fixing strut 39 . The housing 17 is disposed with its end near the pinion shaft 7 raised upward. The pinion shaft 7 is fixed to the fixed post 41 at the neutral position. A receiving member 42 is fixed to an end portion of the steering rack 8 near the pinion shaft 7 .

Above the receiving member 42 , a weight 44 supporting free movement up and down is provided through a guide rod 43 , and a force measuring instrument 45 is fixed on the lower part of the weight 44 . The weight of the weight 44 fixing the load cell 45 is 100Kg, 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, collide with the receiving member 42, and the drop is investigated until it is damaged. frequency.

The dynamic strain gauge 46 is connected to the load cell 45 , and the output of the dynamic strain gauge 46 is recorded in the oscilloscope 47 .

As a result of the test, compared to the average of 3 times until the failure of the comparative example, the average of 15 times until the failure of the example. Thus, it was confirmed that the examples were more excellent in the reverse input impact strength than the comparative examples.

Bending strength test

Use the test setup shown in Figure 8. The steering rack 8 of the embodiment to the steering rack of the comparative example were inserted into the housing 17, and both ends of the housing 17 were respectively fixed to the fixing struts 48. FIG. Make the steering rack 8 protrude as far as possible from the end of the housing 17 close to the pinion shaft 7. In this state, the load cylinder 50 presses and fixes the front end of the steering rack 8 by the load cell 51. The member 49 is loaded up to the maximum load obtainable on the steering rack 8 .

The output of the dynamic strain gauge 52 connected to the load cell 51 is introduced into the load cell 53 to measure the load. As a result, the maximum borne load was 8.6KN in the embodiment and 7.4KN in the comparative example. From this, it was confirmed that the Example had an approximately 16% increase in bending strength compared to the Comparative Example. Also, it was confirmed that both sides are "bends" that are not broken.

Entering durability test

Use the test setup shown in Figure 9. The steering rack 8 of the embodiment to the steering rack of the comparative example were inserted into the case 17, and both ends of the case 17 were respectively fixed to the fixing struts 54. FIG. Both ends of the steering rack 8 are respectively connected with servo actuators (servo actuator) 55 . The rotary driver 58 is connected to the pinion shaft 7 through the universal joint 56 and the torque meter 57 , whereby the rotary driver 58 applies driving torque to the pinion shaft 7 . The driving torque is 50N·m, the frequency is 0.1-0.2Hz, and repeated 30,000 times.

After the test was completed, the amount of wear to the meshing portion of the pinion was measured. Compared with the average of 8.7 μm in the example, the average of the comparative example was 27.8 μm, and it was confirmed that the wear amount of the example was reduced by about 70% compared with the comparative example.

Reverse input durability test

Use the test setup shown in Figure 10. The steering rack 8 of the embodiment to the steering rack of the comparative example are inserted into the housing 17, and both ends of the housing 17 are respectively fixed to the fixing struts 59. The pinion shaft 7 is fixed at the neutral position through the universal joint 60, and the steering rack 8 of the servo actuator 61 loads the shaft through the tie rod 9 connected to the end of the steering gear 8 near the pinion shaft 7. force. The axial force loaded on the steering rack 8 was 9.8 kN, and the frequency was 5 Hz until it was damaged. As a result, the Example was not damaged until 700,000 times, compared to 350,000 times when the comparative example was broken.

Bending Fatigue Test

The test piece 62 shown in FIG. 11A was produced from the same raw material as in the example. The test piece 62 is a substantially circular shaft with a total length of 90 mm. A constricted portion 65 having a cross-sectional curvature of R5 was formed centering on a position at a distance N of 40 mm from one end 62 a of the test piece 62 . The minimum diameter R of the constricted portion 65 is 8 mm. The one end 62a side of the constricted portion 65 is a cylindrical portion 62 having a diameter P of 12 mm. In addition, the tapered portion 64 is formed in a 1/20 tapered shape by clamping the other end 62b side of the constricted portion 65 so that the diameter of the other end 62b side is enlarged. The maximum diameter Q of the tapered portion 64 is 14 mm. In the comparative example, the same comparison test piece was produced.

A bending fatigue test was performed from the test piece 62 to the comparative test piece using the test apparatus shown in FIG. 11B . The remaining tapered portion 64 was fixed to the tapered support hole 67 of the fixing post 66 in a state where a portion at a distance M of 50 mm from one end 62 a of the test piece 62 protruded in a cantilever shape. The load was repeatedly applied by the load cylinder 69 at a frequency of 20 Hz by the rotating roller 68 near one end 62a of the fixed test piece 62, the stress and the number of repetitions were measured, and the S-N curve was obtained.

As a result of the test, in the smooth part (stress shrinkage part) of the S-N curve, the stress of the test piece 62 was 1450 MPa compared to the stress of the comparative example test piece 1270 MPa, thereby confirming that the fatigue strength was improved by about 15%.

Claims (12)

1. steel bar for steering rack, it is characterized in that, contain the Al of Ti, 0.0005～0.1 quality % of B, 0.005～0.05 quality % of Mn, 0.0005～0.003 quality % of Si, 0.2～1.5 quality % of C, 0.05～0.5 quality % of 0.50～0.60 quality % and the N of 0.002～0.02 quality %

When the diameter of establishing bar steel was D, the quenching/tempered structure apart from the part of case depth D/4 of bar steel was adjusted to and satisfies following I), II) and condition III),

I) area percentage of the total of tempering bainite tissue and tempered martensite is 30～100%;

II) area percentage of regeneration pearlitic structure is 0～50%;

III) area percentage of the total of tempering bainite tissue, tempered martensite and regeneration pearlitic structure is 50～100%.

2. according to the described steel bar for steering rack of claim 1, it is characterized in that it is following and do not comprise the Cr of 0 quality % to contain 1.5 quality %.

3. according to the described steel bar for steering rack of claim 1, it is characterized in that, also contain from following optional at least a,

S:0.06 quality % is following and not contain 0 quality %, Pb:0.3 quality % following and not contain 0 quality %, Bi:0.2 quality % following and not contain 0 quality %, Te:0.1 quality % following and not contain 0 quality %, Mg:0.01 quality % following and not contain 0 quality %, Ca:0.01 quality % following and do not contain 0 quality %, rare earth element: 0.01 quality % is following and not contain 0 quality %, Zr:0.3 quality % following and do not contain 0 quality %.

4. the manufacture method of a steel bar for steering rack, it is characterized in that, the rolling steel disc that contains following composition, C:0.50～0.60 quality %, Si:0.05～0.5 quality %, Mn:0.2～1.5 quality %, B:0.0005～0.003 quality %, Ti:0.005～0.05 quality %, Al:0.0005～0.1 quality % and N:0.002～0.02 quality %

Bar steel by rolling gained is quenched more than 780 ℃ from temperature, make the area percentage apart from the total of the bainite structure of the part of case depth D/4 and martensitic stucture of bar steel be 30～100% after,

Put into the stove of the atmosphere temperature that is heated to 660～720 ℃ of temperature, carry out the short period of time temper below 20 minutes, be cooled to room temperature, thus, making the area percentage apart from the regeneration pearlitic structure of the part of case depth D/4 of above-mentioned bar steel is 0～50%, and making the area percentage of total of the tempering bainite tissue apart from the part of case depth D/4, tempered martensite and the regeneration pearlitic structure of above-mentioned bar steel is 50～100%

Wherein, D represents the diameter of bar steel.

5. the manufacture method of steel bar for steering rack according to claim 4 is characterized in that, above-mentioned steel disc contains below the 1.5 quality % and do not contain the Cr of 0 quality %.

6. the manufacture method of steel bar for steering rack according to claim 4 is characterized in that, above-mentioned steel disc also contains following choose wantonly out at least a,

S:0.06 quality % is following and not contain 0 quality %, Pb:0.3 quality % following and not contain 0 quality %, Bi:0.2 quality % following and not contain 0 quality %, Te:0.1 quality % following and not contain 0 quality %, Mg:0.01 quality % following and not contain 0 quality %, Ca:0.01 quality % following and do not contain 0 quality %, rare earth element: 0.01 quality % is following and not contain 0 quality %, Zr:0.3 quality % following and do not contain 0 quality %.

7. a steering rack is characterized in that, uses claim 1,2 or 3 described steel bar for steering rack and forms.

8. steering rack according to claim 7 is characterized in that,

Have: main body and be formed at side face local of aforementioned body and contain the rack tooth formation portion of a plurality of rack tooths,

At least be provided with the hardened layer that has been carried out high-frequency quenching processing and temper and has obtained in rack tooth formation portion;

The surface hardness of above-mentioned rack tooth formation portion is counted 680～800HV with Vickers' hardness.

9. steering rack according to claim 8 is characterized in that,

Aforementioned body contains radially with the part of rack tooth formation portion subtend with apart from being the part of 3/4D with the degree of depth of the part surface of above-mentioned rack tooth formation portion subtend radially;

Quenching/the tempered structure of the part of above-mentioned degree of depth 3/4D is adjusted into and satisfies following I), II) and condition III),

I) area percentage of the total of tempering bainite tissue and tempered martensite is 30～100%;

II) area percentage of regeneration pearlitic structure is 0～50%;

III) area percentage of the total of tempering bainite tissue, tempered martensite and regeneration pearlitic structure is 50～100%,

Wherein, D represents the diameter of bar steel.

10. steering rack according to claim 8 is characterized in that,

The tooth bottom is contained in above-mentioned rack tooth formation portion;

The effective case depth of above-mentioned tooth bottom is the surface 0.1～1.5mm apart from the tooth bottom.

11. steering rack according to claim 8 is characterized in that,

The tooth bottom is contained in above-mentioned rack tooth formation portion;

The effective case depth of above-mentioned tooth bottom is the surface 0.3～1.2mm apart from the tooth bottom.

12. steering rack according to claim 8 is characterized in that,

The tooth bottom is contained in above-mentioned rack tooth formation portion;

Residual ferrite is not contained in zone till the degree of depth of the surperficial 0.1mm bottom above-mentioned tooth.

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