JP4197459B2 - Steel bar for steering rack - Google Patents

Description

【００４７】
【表２】

【００４８】
表１〜２のうち実験例２０〜２８は成分設計が不適切な例である。すなわち実験例２０ではＣ量が不足しているため、耐摩耗性が不十分である。実験例２１は逆にＣ量が多いため耐衝撃特性が不十分である。実験例２２ではＭｎ量が少なく焼入性に劣る為、焼戻しベイナイト組織及び焼戻しマルテンサイト組織の合計量が不足しており、曲げ変形能が不十分である。実験例２３はＭｎ量が過剰となっているため、高周波焼入れの際に硬化層が深くなってしまい、曲げ変形能が不十分となる。実験例２４〜２７では、Ｂ又はＴｉ量が不適切であるために耐衝撃性が不十分である。実験例２８はＣｒ量が過剰であるため、高周波焼入れの際に硬化層が深くなってしまい、曲げ変形能が不十分となる。
【００４９】
また実験例２９〜３６から明らかなように、成分設計が適切であっても、組織が不適切であると諸特性が不十分となる。すなわち実験例２９〜３１では焼戻しベイナイト組織及び焼戻しマルテンサイト組織の合計量が不足しており、曲げ変形能が不十分となる。実験例３２〜３４では再生パーライト組織が多すぎるため、曲げ変形能が不十分となる。実験例３５〜３６では、焼戻しベイナイト組織、焼戻しマルテンサイト組織、及び再生パーライト組織の合計量が不足しているため、曲げ変形能が不十分となる。
【００５０】
これらに対して実験例１〜１９は、成分設計及び組織の両方が適切であるため、耐衝撃特性及び耐摩耗性の両方に優れており、しかも亀裂の進展・貫通を防止できる。
【００５１】
【発明の効果】
本発明のステアリングラック用棒鋼によれば、成分及び組織の両方が適切に制御されているため、耐摩耗性が改善されながらも耐衝撃特性にも優れており、しかも亀裂の進展・貫通を防止することができる。
【図面の簡単な説明】
【図１】図１は実験例で用いる試験ディスクの形状を示す概略斜視図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel bar useful for manufacturing a rack (steering rack) used for a steering gear of an automobile and a manufacturing method thereof.
[0002]
[Prior art]
Automobile steering is classified into a type that is power-assisted by hydraulic pressure (hydraulic power steering) and a type that is power-assisted by electricity (electric power steering). The hydraulic power steering is the mainstream in the past, and assists the steering force with pressure oil discharged from an oil pump that operates using engine output. However, hydraulic power steering uses a part of the engine output as a power source, so that there is a problem that the fuel consumption of the automobile is lowered. On the other hand, the electric power steering assists the steering force by an electric motor that is operated by electric energy from the battery, and can improve the fuel consumption of the automobile in comparison with the hydraulic power steering. The spread is progressing.
[0003]
Incidentally, the steering rack is one of the main components constituting the steering, and also plays a skeleton role of the automobile. In addition, if the steering rack breaks, the steering wheel operation becomes impossible, so it is designated as an important safety part, and high reliability and strength characteristics are required. In other words, the steering rack has 1) excellent impact resistance that does not cause damage even if the automobile rides on a curb or the like, 2) does not break even when bending stress acts (crack resistance), and 3) Wear resistance of rack teeth is required.
[0004]
Conventionally, S45C steel (see Patent Documents 1 and 2), medium carbon steel (see Patent Documents 3 and 4), and the like are used for such a steering rack, and by forming a hardened surface layer by induction hardening. In addition to enhancing wear resistance, it also increases strength against bending stress (cracking resistance) (see Patent Documents 1 and 2).
[0005]
However, even if the bending strength is increased by induction hardening, once an excessive load is applied and a crack occurs once in the induction hardened layer, the crack propagates and penetrates into the interior, leading to breakage. It becomes impossible. Furthermore, in the electric power steering which has been spreading in recent years, the contact surface pressure between the steering rack and the pinion gear tends to be higher than that of the hydraulic power steering, and the S45C steel (Patent Documents 1 and 2) has wear resistance. Is lacking. Moreover, although the amount of C of medium carbon steel (patent documents 3 and 4) is not clear, even if it uses C excessively and wear resistance is improved, an impact resistance characteristic will fall.
[0006]
Patent Document 5 describes that when B is added and induction-hardened, brittle fracture does not occur even when an excessive load is applied, and fracture is prevented by bending deformation. This steel for steering racks in Patent Document 5 is a steel that omits the quenching and tempering treatment, and its structure is substantially ferrite pearlite.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. Sho 62-178472 (page 3, upper left column, third line, column of conventional technology)
[Patent Document 2]
Japanese Patent Application Laid-Open No. Sho 62-180018 (second page, lower left column, line 15; conventional technology column)
[Patent Document 3]
JP 2000-153336 A (paragraph 0013 second line)
[Patent Document 4]
JP 2001-79639 A (paragraph 0015 second line)
[Patent Document 5]
JP 10-8189 A (paragraph 0006, column of Examples)
[0008]
[Problems to be solved by the invention]
Note that the present inventors have found that even if a bending test is performed after preparing a steel with a B-added quenching and tempering treatment omitted according to the method of Patent Document 5 and performing induction testing, cracks propagate and penetrate to reach fracture. Have confirmed (see Experimental Example 31 described later).
[0009]
The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is a steering wheel that has improved impact resistance while being excellent in impact resistance, and can prevent the progress and penetration of cracks. The object is to provide a rack steel bar and a method for manufacturing the same.
[0010]
[Means for Solving the Problems]
As a result of intensive studies to solve the above problems, the present inventors have been able to prevent the impact resistance from being lowered if B is added even if the amount of C is increased. Furthermore, a tempered bainite structure and a tempered pearlite structure. Further, the present inventors have found that if the regenerated pearlite structure is controlled within a predetermined range, the bending deformability can be enhanced, and even if a crack occurs once, the progress / penetration of the crack can be prevented.
[0011]
That is, the steel bar for a steering rack according to the present invention has C: 0.50 to 0.60% (meaning mass%, hereinafter the same), Si: 0.05 to 0.5%, Mn: 0.2 to It contains 1.5%, B: 0.0005 to 0.003%, and Ti: 0.005 to 0.05%, and 0.005 to 0.1% Al and 0.005% as other elements. Containing both 002-0.02% N and / or 1.5% or less (not including 0%) of Cr, the balance consisting of Fe and inevitable impurities,
The gist is that the quenching and tempering structure of the portion of depth D / 4 (D indicates the diameter of the steel bar) from the surface of the steel bar is adjusted as shown in 1), 2) and 3) below. is there.
[0012]
1) 30-100% in total of tempered bainite structure and tempered martensite structure (area percentage)
2) Regenerated pearlite structure is 0 to 50% (area percentage)
3) A total of 50 to 100% (area percentage) of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure.
The steel bar for steering rack may further contain a free-cutting element (S, Pb, Bi, Te, Mg, Ca, REM, Zr, etc.).
[0013]
The steel bar for a steering rack according to the present invention is obtained by quenching a steel bar obtained by rolling a steel slab from a temperature of 780 ° C. or higher, and a total of the bainite structure and the martensite structure at a depth D / 4 of 30 to 100% (area %)
It can manufacture by putting in the furnace heated to the atmospheric temperature of 660-720 degreeC, performing a short-time tempering process for 20 minutes or less, and cooling to room temperature.
[0014]
In the present specification, the term “quenching” means quenching at the time of quenching / tempering of the steel bar after rolling, and is used separately from the term “induction quenching”.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Steel bars for steering racks of the present invention are: C: 0.50-0.60% (meaning mass%, the same applies hereinafter), Si: 0.05-0.5%, Mn: 0.2-1.5 %, B: 0.0005 to 0.003%, and Ti: 0.005 to 0.05%, and 0.0005 to 0.1% Al and 0.002 to 0 as other elements 0.02% N and / or 1.5% or less (not including 0%) Cr. The balance is Fe and inevitable impurities.
[0016]
The reasons for limiting the above components are as follows.
[0017]
The reason why the C content is 0.50% or more is to sufficiently improve the wear resistance when a steering rack (for example, a steering rack for electric power steering) is used. The preferable C content is 0.52% or more. However, if the content of C is too large, the impact resistance characteristics of the steering rack deteriorate. Therefore, the C content is 0.60% or less, preferably 0.58% or less.
[0018]
The reason why the Si content is 0.05% or more is to perform deoxidation of the steel material. A preferable Si content is 0.10% or more, particularly 0.15% or more. However, when there is too much content of Si, the machinability at the time of forming a rack tooth will fall. Therefore, the Si content is 0.5% or less, preferably 0.35% or less, more preferably 0.30% or less.
[0019]
The reason why the Mn content is 0.2% or more is not only to increase the strength of the steel, but also to increase the hardenability and facilitate the introduction of the bainite structure, thereby bending the steel when the steel is processed into a steering rack. This is to increase the deformability. A preferable Mn content is 0.5% or more, particularly 0.7% or more. However, when there is too much content of Mn, the hardened layer by induction hardening will become deep too much, and a bending deformability will fall. Therefore, the Mn content is 1.5% or less, preferably 1.3% or less, more preferably 1.2% or less.
[0020]
The reason why the content of B is set to 0.0005% or more is to ensure impact resistance even in the steel according to the present invention in which the C content is increased. A preferable B content is 0.0007% or more. However, if the B content is excessively increased, a harmful B-based compound is generated, and the impact resistance is decreased. Therefore, the B content is 0.003% or less, preferably 0.0025% or less, and more preferably 0.0020% or less.
[0021]
Ti binds to N in the steel to form TiN, thereby suppressing the generation of BN, and is effective in securing the above-described effect by B. Accordingly, the Ti content is 0.005% or more, preferably 0.010% or more, and more preferably 0.012% or more. However, if the Ti content is excessively increased, the impact resistance characteristics of the steering rack are deteriorated. Therefore, the Ti content is 0.05% or less, preferably 0.04% or less, and more preferably 0.035% or less.
[0022]
The reason why Al and N are contained is that by forming AlN, the austenite grains during induction hardening can be refined. The Al content is 0.0005% or more, preferably 0.010% or more, and more preferably 0.020% or more. The N content is 0.002% or more, preferably 0.003% or more, and more preferably 0.004% or more. However, if the contents of Al and N are excessively increased, the impact resistance is deteriorated. Therefore, the Al content is 0.1% or less, preferably 0.08% or less, more preferably 0.05% or less. The N content is 0.02% or less, preferably 0.01% or less, more preferably 0.007% or less.
[0023]
The reason for containing Cr is to improve hardenability. The lower limit of the Cr content is not particularly limited, but is, for example, about 0.05%, preferably about 0.08%, and more preferably about 0.10%. However, when there is too much content of Cr, the hardened layer by induction hardening will become deep too much, and a bending deformability will be insufficient. Therefore, the Cr content is, for example, 1.5% or less, preferably 1.0% or less, and more preferably 0.50% or less.
[0024]
The steering rack steel bar of the present invention may contain a free-cutting element (S, Pb, Bi, Te, Mg, Ca, REM, Zr, etc.) if necessary. The amount of these free-cutting elements is, for example, S: 0.06% or less (not including 0%), Pb: 0.3% or less (not including 0%), Bi: 0.2% or less (0 %), Te: 0.1% or less (not including 0%), Mg: 0.01% or less (not including 0%), Ca: 0.01% or less (not including 0%) REM: 0.01% or less (not including 0%), Zr: 0.3% or less (not including 0%). These free-cutting elements can be added alone or in combination of two or more.
[0025]
In the steel bar for a steering rack according to the present invention, the quenching and tempering structure at a depth D / 4 (D indicates the diameter of the steel bar) from the surface of the steel bar is adjusted as shown in 1), 2) and 3) below. ing.
[0026]
1) Total of tempered bainite structure and tempered martensite structure [hereinafter sometimes referred to as “TB + TM”] is 30 to 100% (area percentage)
2) Regenerated pearlite structure is 0 to 50% (area percentage)
3) The total of the tempered bainite structure, tempered martensite structure and regenerated pearlite structure [hereinafter sometimes referred to as TB + TM + RP] is 50 to 100% (area percentage)
Hereinafter, the reason for performing these organization controls will be described.
[0027]
The tempered bainite structure and the tempered martensite structure are structures introduced by quenching and tempering the rolled steel bar, and prevent the progress and penetration of cracks generated in the induction hardened layer when used as a steering rack. It is effective for. In other words, the induction rack is extremely hard at the induction hardened part (surface part), so if a large bend is applied, the induction hardened layer near the root of the steering tooth (usually around the D / 4 part) is prone to cracking, but induction hardening is applied. If a tempered bainite structure or a tempered martensite structure remains at the boundary between the part and the part that has not been induction-hardened, it is possible to prevent the cracks generated in the induction-hardened layer from progressing to the inside, and to prevent cracks in the steering rack itself. Can be prevented. Therefore, the sum (TB + TM) of the bainite structure and tempered martensite structure of the D / 4 portion is set to 30% or more. Preferably it is 40% or more, More preferably, it is 50% or more.
[0028]
The regenerated pearlite structure is a structure introduced in the tempering step, and is distinguished from the pearlite structure of steel as it is rolled. Unlike the tempered bainite structure and tempered martensite structure, the regenerated pearlite structure is not only useful for preventing the progress and penetration of cracks, but when only the regenerated pearlite structure increases, the bending deformability is rather lowered. Therefore, the reproduction perlite structure is 50% or less. Preferably it is 40% or less, More preferably, it is 30% or less. Moreover, when the reclaimed pearlite structure is reduced, the impact resistance tends to be further improved.
[0029]
Even if a tempered bainite structure and a tempered martensite structure are introduced by quenching and tempering, if a ferrite / pearlite structure or a soft ferrite structure remains from the rolled material, it is still impossible to prevent the progress / penetration of cracks. Therefore, it is necessary to reduce the structure of the as-rolled material. In other words, it is necessary to increase the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure. The total (TB + TM + RP) of the three structures introduced by these quenching and tempering steps is 50% or more, preferably 60% or more, and more preferably 70% or more.
[0030]
The diameter of the steel bar for the steering rack of the present invention is not particularly limited, but is usually about 10 to 40 mm, preferably about 15 to 38 mm, and more preferably about 20 to 36 mm in consideration of processing into a steering rack.
[0031]
The steel bar for the steering rack as described above, for example, is to roll a steel slab having the above composition, quench the obtained steel bar and introduce a bainite structure and a martensite structure, and then perform a tempering treatment at a high temperature for a short time. Can be manufactured.
[0032]
In such a production method, the heating temperature for quenching 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. In addition, a soft ferrite layer is generated, and the strength of the steering rack is insufficient. The upper limit of the heating temperature is, for example, about 860 ° C., preferably about 850 ° C. If the heating temperature is too high, the bending of the steel bar tends to increase during quenching.
[0033]
The quenching cooling condition is that the total amount of bainite structure and martensite structure introduced by the quenching (total amount in D / 4 part; area percentage) is at least the total amount of the tempered bainite structure and martensite structure described above. It is necessary to set to be the same. It is convenient to use controlled cooling for such cooling. The conditions for controlled cooling can be set as appropriate according to the composition of the steel. For example, it is desirable to cool the temperature range from 800 to 300 ° C. at a cooling rate of 30 to 80 ° C./second.
[0034]
The tempering temperature (furnace temperature) is about 660 to 720 ° C., preferably about 680 to 700 ° C., and the tempering time (residence time including the temperature rising process) is about 20 minutes or less, preferably about 15 minutes or less. If the tempering temperature is too high or the tempering time is too long, the reclaimed pearlite structure increases too much.
[0035]
The steel bar for a steering rack according to the present invention has excellent impact resistance while improving wear resistance, and also has excellent bending deformability, and therefore, a steering rack (especially a steering rack for electric power steering). Very useful.
[0036]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
[0037]
Experimental Examples 1-36
Steel materials having the components shown in Tables 1 and 2 were melted and rolled into a steel bar having a diameter of 30 mm. Subsequently, after heating to the temperature shown in Tables 1-2, it quenched by controlling cooling to room temperature. In the controlled cooling, the structure of the bar steel was controlled by changing the amount of water and the water cooling time. The cooled steel bar was tempered by being retained in the furnace heated to the atmospheric temperature shown in Tables 1-2 for the time shown in Tables 1-2. The steel bar after tempering was allowed to cool.
[0038]
The structure of D / 4 part of the steel bar after quenching and the structure of D / 4 part of the steel bar after tempering were observed with an electron microscope (magnification 5000 times), martensite structure and bainite structure, and tempered martensite structure, tempered. The area ratios of the bainite structure and the regenerated pearlite structure were determined.
[0039]
In addition, the following tests were conducted in order to investigate crack resistance (bending deformability), impact resistance characteristics, and wear resistance characteristics when the steel bars after tempering were used as a steering rack.
[0040]
[Bending test]
The steel bar after tempering was drawn to a diameter of 27.5 mm and then cut to form rack teeth. The depth of the rack teeth is about D / 4. Next, a steering rack was prepared by induction-quenching the teeth under the following conditions.
[0041]
Induction hardening condition coil: For surface hardening (diameter 40mm, thickness 2mm)
Voltage: 4.0 kV
Current: 4.5A
Frequency: 40kHz
Heating method: moving quenching (moving speed 3.0mm / sec)
Cooling: A mixed solvent of soluble oil and water was used, and a three-point bending test was performed using a steering rack obtained by using a steering rack distance of 400 mm and a pressing point on the opposite side of the teeth of the steering rack.
[0042]
×: A crack generated in the induction hardened layer propagated and penetrated to the inside, and the steering rack was broken into two. ○: The crack stopped halfway and did not break [impact test]
The steel bar after tempering was drawn to a diameter of 27.5 mm, and then a JIS No. 3 U-notch test piece was cut out from D / 4 part, and the surface layer on the notch forming side was induction-hardened. The induction hardening conditions were the same as in the bending test except that the moving speed was 3.5 mm / second. The obtained specimen was subjected to a Charpy impact test (test temperature: room temperature) in accordance with JIS Z2242, and the impact value was determined.
[0043]
[Abrasion test]
A disc having the same characteristics as the tempered steel bar obtained in the experimental example was prepared. That is, a steel material having the same composition as the steel bar of the experimental example is melted, forged to a diameter of 53 mm by hot forging, cut into a 15 mm thick disk, and then quenched and tempered under the same conditions as in each experimental example. It was.
[0044]
Next, it was machined into a two-stage semicircular plate (upper diameter 44 mm, upper thickness 3 mm; lower diameter 50 mm, lower thickness 5 mm) as shown in FIG. 1, and the upper surface layer was induction-hardened. . The induction hardening conditions were the same as in the bending test except that the moving speed was 2.5 mm / sec. The obtained test disk was subjected to a pin-on-disk wear test, and the wear loss of the test piece was measured. The detailed conditions of the wear test are as follows.
[0045]
Lubrication: Dry test piece surface roughness: Ra 0.25 μm
Peripheral speed: 0.05 m / sec Surface pressure: 0.05 GPa
Pin: SUJ2 [diameter 5 mm, hardness (HRC) 64]
The results are shown in Tables 1-2.
[0046]
[Table 1]

[0047]
[Table 2]

[0048]
Of Tables 1 and 2, Experimental Examples 20 to 28 are examples in which the component design is inappropriate. That is, in Experimental Example 20, since the amount of C is insufficient, the wear resistance is insufficient. On the contrary, since Experimental Example 21 has a large amount of C, the impact resistance characteristics are insufficient. In Experimental Example 22, since the amount of Mn is small and the hardenability is poor, the total amount of the tempered bainite structure and the tempered martensite structure is insufficient, and the bending deformability is insufficient. In Experimental Example 23, since the amount of Mn is excessive, the hardened layer becomes deep during induction hardening, and the bending deformability becomes insufficient. In Experimental Examples 24-27, since the B or Ti amount is inappropriate, the impact resistance is insufficient. In Experimental Example 28, since the amount of Cr is excessive, the hardened layer becomes deep during induction hardening, and the bending deformability becomes insufficient.
[0049]
Further, as is apparent from Experimental Examples 29 to 36, even if the component design is appropriate, various characteristics become insufficient if the tissue is inappropriate. That is, in Experimental Examples 29 to 31, the total amount of the tempered bainite structure and the tempered martensite structure is insufficient, and the bending deformability becomes insufficient. In Experimental Examples 32-34, since there are too many reclaimed pearlite structures, the bending deformability becomes 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 is insufficient, the bending deformability becomes insufficient.
[0050]
On the other hand, since Experimental Examples 1 to 19 are suitable for both the component design and the structure, they are excellent in both impact resistance and wear resistance, and can prevent the progress and penetration of cracks.
[0051]
【The invention's effect】
According to the steel bar for a steering rack of the present invention, both the composition and the structure are appropriately controlled, so that the wear resistance is improved and the impact resistance is excellent, and the progress and penetration of cracks are prevented. can do.
[Brief description of the drawings]
FIG. 1 is a schematic perspective view showing the shape of a test disk used in an experimental example.

Claims (3)

C: 0.50-0.60% (meaning mass%, the same applies hereinafter), Si: 0.05-0.5%, Mn: 0.2-1.5%, B: 0.0005-0 0.003%, and Ti: 0.005 to 0.05%,
Further, as other elements, both 0.0005 to 0.1% Al and 0.002 to 0.02% N and / or 1.5% or less (not including 0%) Cr are contained. ,
The balance is a steel bar made of Fe and inevitable impurities,
A steering rack characterized in that the quenching and tempering structure of a portion of a depth D / 4 (D indicates the diameter of the steel bar) from the surface of the steel bar is adjusted as shown in 1), 2) and 3) below. Steel bar.
1) 30-100% in total of tempered bainite structure and tempered martensite structure (area percentage)
2) Regenerated pearlite structure is 0 to 50% (area percentage)
3) A total of 50 to 100% (area percentage) of the tempered bainite structure, the tempered martensite structure, and the regenerated pearlite structure Furthermore, S: 0.06% or less (not including 0%), Pb: 0.3% or less (not including 0%), Bi: 0.2% or less (not including 0%), Te: 0.0. 1% or less (excluding 0%), Mg: 0.01% or less (not including 0%), Ca: 0.01% or less (not including 0%), REM: 0.01% or less (0 %), Zr: 0.3% or less (excluding 0%), at least one selected from the following. A steel slab containing the element according to claim 1 or 2 is rolled,
The obtained steel bar was quenched from a temperature of 780 ° C. or more, and the bainite structure and the martensite structure at a depth of D / 4 were made 30 to 100% (area percentage) in total,
A method for producing a steel bar for a steering rack, wherein the steel plate is placed in a furnace heated to an atmospheric temperature of 660 to 720 ° C, subjected to a short tempering treatment for 20 minutes or less, and cooled to room temperature.

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