JP2008223126A - Gear made of ductile cast iron - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gear using a cast iron stock having a high strength, a prominently high fatigue strength in the black skin part, and a high hardness equal to that of a steel and a forging, which has excellent machinability to dispense with a grinding which is impossible in a steel and a forging, and having excellent wear resistance and damping capacity, and in which the reduction of working cost can be attained. <P>SOLUTION: A gear stock is composed as cast using ductile cast iron at least comprising 2.4 to 3.3% Cu and 0.01 to 0.05% Sn. It is formed through a gear cutting stage without performing heat treatment. A gear having high facial pressure strength can be obtained as it is. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a ductile cast iron gear, for example, various gears for automobiles, and more particularly, to a gear capable of dealing with a gear having high tensile strength and a high load and low rotation speed, and a high speed rotation and high noise.

  Diesel engine timing gears are used under severe conditions, such as high load and low rotation, and easy contact with the gears. Gears subjected to surface treatment such as nitriding or carburizing and quenching are used.

  Also, newspaper offset double-sided printing presses are used at high speeds, and are used under severe conditions, such as easy contact with the gears when meshing, so normally the surfaces of carbon steel and alloy steel are nitrided and soft-nitrided. Alternatively, a gear subjected to surface treatment such as carburizing and quenching is used. In addition, the machining requires a grinding process because the tooth profile accuracy is required to be high from JIS0 to 1st grade.

  In recent years, it has been required to reduce the noise of the engine. As a timing gear corresponding thereto, a cast iron gear having a large vibration and noise damping characteristic, that is, a small Young's modulus (Young's modulus E = 21000 kgf / steel) In contrast to mm2, cast iron has been proposed with E = 17,000 kgf / mm2).

  As such cast iron gears, spheroidal graphite cast iron (so-called ductile cast iron, hereinafter referred to as such) is suitable because of its strength and hardness. However, as one of means for obtaining strength comparable to that of a steel gear surface-hardened with this ductile cast iron material, the ductile cast iron material is subjected to heat treatment (hereinafter referred to as ADI treatment). Means are provided (for example, see Patent Document 1).

  In particular, due to the emergence of toughened austempered ductile cast iron due to the bainite of the base structure based on austempering, it has been proposed and put into practical use as various gears.

For example, Patent Document 1 describes an austempered ductile cast iron gear employed in a timing gear of a vehicle engine.
There, as a conventional manufacturing method, ductile cast iron of about FCD500 is manufactured by gearing the gear material and hobbing or other gear cutting methods on the same material, and then the tooth surface is shaved and rounded up. There is shown a method comprising a step and a step of subjecting the machined gear to a heat treatment (hereinafter ADI treatment).

  Other conventional methods include the steps of manufacturing gear materials with cast iron, applying ADI processing, and cutting gears such as hobbing on the heat-treated gear materials, then shaving them and precision finishing the tooth surfaces. It is a method of performing.

This method has a drawback that the gear material after ADI processing has a large machining allowance for hobbing such as hobbing, and there is a deterioration in machinability due to work hardening, which is too expensive in practice.
Therefore, as a new method, a gear material is austenitized at a predetermined temperature for a certain period of time, and further maintained at a second predetermined temperature for upper bainite, and then gradually cooled, and then the gear is made by shaving it. A method of forming is proposed.

JP 7-136863 A

According to the manufacturing method described in the above-mentioned Patent Document 1, after gear cutting such as hobbing is performed on a gear material of ductile cast iron in the first step to form a gear coarse material, the gear coarse material is formed as a second step. ADI treatment is performed, and in the final third step, the tooth surface is shaved.
However, this manufacturing method has not been put into practical use because it requires a shaving cost after the ADI treatment.

  In addition, the method of Patent Document 1 has a drawback in that after gear cutting, a gear that is precisely finished by shaving is subjected to heat treatment (ADI treatment), so that deformation due to heat treatment is large and accuracy is deteriorated.

  However, in order to austenite the gear material described above, a step of holding at a second predetermined temperature for a certain time at a predetermined temperature and further to an upper bainite, followed by a slow cooling process, which is shaved to form a gear. In the method, it is the same that the shaving finish is performed on the tooth surface of the gear material after the ADI processing, and it is also true that the ADI processing is troublesome.

  As described above, the austempered ductile cast iron is subjected to a heat treatment process to obtain high strength, so there are problems such as an increase in cost, a decrease in machinability derived from a bainite structure, and a complicated processing process. Therefore, it has not been used as a gear.

  In particular, in various gears, austempered ductile cast iron has extremely high strength and good damping performance and wear resistance, but on the other hand, it has extremely poor machinability. Therefore, conventional austempered ductile cast iron is used as a gear. In doing so, it is necessary to improve the machinability which is the most problematic.

Conventionally, forged products have been variously centered on gears of automobiles and the like.
However, forged products are produced as a raw material by a forging manufacturer, processed by a user, and then heat treated to produce a product.

  As a result, grinding is not performed in order to suppress an increase in processing cost, and naturally the tooth profile accuracy is poor, and noise and vibration are large. Further, since the gear accuracy is poor, there is a problem that the friction is large and the influence on fuel consumption is large.

  Therefore, an object of the present invention is to provide a high load and low load such as a timing gear that can be manufactured at a low cost while ensuring high accuracy and a high vibration and noise damping function without performing heat treatment in a gear made of ductile cast iron. The object is to provide a gear suitable as a rotating gear.

Up to now, it has been difficult to obtain FCD800 (tensile strength of 800 MPa, elongation of 2% or more in ductile cast iron as cast. Cu in the base structure is a pearlite stable component, and its amount increases. Accordingly, the tensile strength increases.
However, Cu is also an element for inhibiting spheroidization of ductile cast iron, and so far, addition of 2% or more has not been tried.

In recent years, the present inventors have developed a ductile cast iron having high strength while keeping as-cast with 2% or more of Cu added (Patent Document 2).
The present invention proposes a gear having a wide range of uses based on the present invention.

  Therefore, in the present invention, the fatigue strength of the black skin portion is high with high strength, and by using a cast iron material, it is excellent in machinability and wear resistance so that a grinding process is unnecessary. The present invention proposes a gear that is excellent in damping capacity, can improve the tooth profile accuracy that can be used for various gears, reduce processing costs, and can reduce weight and noise.

  As a result, the present invention proposes a gear that has strength and hardness comparable to those of steel and forged gears, good machinability, damping and wear resistance, and low manufacturing costs.

  Specifically, it is characterized in that it is formed by ductile cast iron containing Cu: 2.4 to 3.3% by weight, obtained by spheroidizing treatment, and cast by gear cutting without heat treatment. This is a ductile cast iron gear with high surface pressure strength.

  In addition, it is a surface without heat treatment formed by gear cutting after being composed of cast iron using ductile cast iron containing at least Cu: 2.4-3.3% and Sn: 0.01-0.05%. Ductile cast iron gear with high pressure strength.

  Further, it is a ductile cast iron gear that has an as-cast surface pressure strength of 1000 MPa or more.

According to the present invention, since a cast-cast material with high strength and high fatigue strength does not require a heat treatment process such as ADI, vibration and noise with easy tooth profile accuracy, which is easy to process and greatly eliminates the processing process. Gears with large damping function and low noise can be provided at low cost.
Therefore, it can perform a sufficient function as a gear that has conventionally been difficult to prevent noise and vibration such as various gears for automobiles and reduction gears for printing presses, and its application range is wide.

  In addition, according to the first aspect of the present invention, ductile cast iron containing Cu: 2.4 to 3.3% by weight obtained by spheroidizing treatment is casted without gearing without heat treatment. It is possible to provide a ductile cast iron gear having a large surface pressure strength characterized by being formed.

  Furthermore, according to claim 2, after adding at least Cu: 2.4 to 3.3%, using ductile cast iron containing 0.01 to 0.05% Sn, and configured as cast, It is possible to provide a ductile cast iron gear having a high surface pressure strength without the heat treatment formed through the gear cutting process.

  Further, according to claim 3, it is possible to provide the ductile cast iron gear according to claim 1 or 2, which has an as-cast surface pressure strength of 1000 MPa or more.

  As described above, the ductile cast iron gear of the present invention has a high vibration and noise damping function without heat treatment, and a high load, low rotation gear such as a timing gear that can be manufactured at low cost while ensuring high accuracy. A suitable gear can be obtained.

  This inventor used the high intensity | strength ductile cast iron of the chemical component shown in the table | surface of FIG. 1, and implemented the following tests.

  The surface strength of the high-strength ductile cast iron was examined using a two-cylinder tester with simulated gear contact, and compared with conventional ductile cast iron and steel of the same degree of hardness. The possibility of application as a gear material was examined.

  As a result, it has been found that high-strength ductile cast iron has a surface strength superior to that of FCD700 and SCM435, and can be sufficiently applied as a gear material in a medium hardness region.

  The feature of the high-strength ductile cast iron shown in FIG. 1 is the Cu content, which contains about 2.4% Cu. The mechanical properties are shown in the table of FIG.

The average graphite particle size is about 35 μm, the number of graphite particles is about 120 / mm ² , and the matrix has a finer structure than pearlite. The hardness is about HB280. The roller test piece was a cylinder having a diameter of 70 mm and a width of 25 mm, a 3 ′ chamfer was attached to the high-speed roller, and the contact width was 10 mm. The surface roughness was finished to a maximum height of R ₂ = 2 μm by grinding, assuming finishing hobbing with a cermet hob.

The tester used a backup roll type roller tester, and as shown in the table of FIG. 3, a negative slip rate of about −20% was given to the low-speed roller at a high-speed roller rotation speed of 3190 rpm and a low-speed roller rotation speed of 2635 rpm. . Lubricating oil, ISO VG150 equivalent base (specific gravity 0.885 (15 ° C.), viscosity ^{157.1 × 10 6 m 2 /S(40℃),15.42×10 6} m 2 / S (100 ℃)) and Used at an oil amount of 2 L / min and supplied from directly above the roller contact portion.

The test was conducted until the roller surface was damaged or when it did not occur, the high-speed roller repetition number N ₁ = 1.2 × 10 ⁷ (the low-speed roller repetition number N ₂ = 1.0 × 10 ⁷ constant load) Additional operation was performed at.

FIG. 4 shows an outline of test conditions and results. The test was performed under three loads, and the vibration and noise increased and damage occurred at the maximum Hertz stress O _H = 1372MP ₃ and the number of repetitions N ₁ = 4.5 × 10 ⁶ . The damage was a sudden occurrence of a large pit only at one point on the low-speed roller, and was different from the pitching in which the pit gradually increased like a year.

FIG. 5 shows the damage. A large pit of about 2 mm × 1 mm is assumed to have been damaged from the right end to the left.
In addition, many small pits of about 0.1 mm are observed in the contact area, but the operation was continued without increasing vibration and noise due to the influence of these small pits, so the small pits were not considered damaged. The life of the test piece was defined as the occurrence of large pits.

FIG. 6 shows the allowable load with respect to the number of repetitions. For comparison, experimental results of ductile cast iron (FCD700) and medium hardness alloy steel (SCM435) having the same hardness and surface roughness are also shown.
The vertical axis generally uses the maximum Hertz stress and the hardness ratio P _max / HB, but the Young's modulus and Boisson ratio differ between ductile cast iron and steel. The ratio of √F and hardness compared with Hertz stress, ie, √F / HB was used.

The tooth surface strength (fatigue strength against tooth surface damage such as pitching and spalling) is usually designed so that the maximum contact stress acting on the tooth surface is less than the allowable contact stress of the material.
As the maximum contact stress, the maximum Hertz stress (σ _H ) is generally used.
Maximum contact stress acting on tooth surface (σ _H ) ≦ Allowable contact stress of material (σ _Hllm )
Actually, σ _H ≦ σ _Hllm · K ₁ · K ₂ ... K _{n
(K 1 · K 2 · K} n: load distribution coefficient, lubricants coefficients, roughness coefficient ..., etc.)
There are several formulas for what factors are considered and how. (JSME, JGMA, AGMA, ISO, BS, DIN, etc. ("Gear Strength Design Sample": published by the Japan Society of Mechanical Engineers))

σ _Hllm is determined by the material. For example, in the alloy steel with tempering to about HB320, σ _Hllm = 840MPa about, than _{.SIGMA.H-N 1} line diagram in Figure 7, it is possible to read the sigma _Hllm of the present invention product H-FCD.

That is, in general, the value of σ _{H at} the fatigue limit of 10 ⁷ times is σ _Hllm , so σ _Hllm = 1000 MPa. In practical use, it is considered safe to use the value of SCM435 refined to about HB320.
Note that the F / b-N ₁ diagram of FIG. 8 has F / b (vertical load per unit width) corresponding to the value of σ _H as the vertical axis, and is a measure of the transmission load.

  Also, the points in the figure indicate pit occurrence points of φ0.4 mm or more, and the arrows indicate that no damage has occurred.

  The FCD700 is almost the same as or slightly superior to the SCM435, but it can be seen that the product H-FCD of the present invention is remarkably superior to both. Therefore, it is considered that the product H-FCD of the present invention is sufficiently applicable in the medium hardness region. One of the reasons why the surface pressure strength is improved as compared with the conventional FCD is considered to be the influence of copper contained in the base organization.

  The ductile cast iron used in the present invention is characterized by the action of Cu, Sn, and Al, and will be described in detail.

Cu stabilizes austenite, the austenite-perlite transformation occurs at a low temperature, and has the effect of increasing the base pearlite area ratio, reducing the mass effect.
However, the critical amount varies depending on the researcher and is said to be 1.5%, 1.9%, and 2.2%.

  And use beyond this is said to segregate at the austenite grain boundaries and crystallize irregular graphite to inhibit spheroidization. In addition, addition of 2% or more is said to increase the tendency to chill.

  The following experiment was conducted to confirm the pointed out matters. Using a low-frequency induction furnace with a capacity of 2000 kg and using steel scrap and return material to make a molten metal equivalent to FCD450, Cu was added in a change from 1.3% to 2.8%, and then commercially available Fe- The spheroidizing process by sandwich method with Si-Mg-Ca-REM alloy addition was carried out, inoculated at the time of transferring the pouring ladle, and cast into JIS G5502 Y type B (25t × 215l) as a CO2 mold.

The experiment was performed twice for each component. The average value is shown in FIG. As a result, irregular graphite was not confirmed at Cu 1.5% and 2.0%, and some aggregated graphite was formed around spherical graphite at 2.3%, 2.6%, and 2.8%. It was.
Inhibition of spheroidization is expected to start at 2.0 to 2.3% of Cu, but it should be noted that the range of Cu 2.0 to 2.6% is slightly improved in tensile strength. The decrease in elongation is not abrupt. The tensile strength increases because the base is strengthened more densely and becomes superior to the spherical inhibition, and the elongation decreases because the spherical inhibition increases slightly. However, when Cu exceeds 2.6%, spherical inhibition becomes dominant. Moreover, there was no generation of cementite within the content range.

  When a small amount of Sn 0.02% was added to this Cu 2.8% test molten metal, it was found that the graphite shape and matrix structure were improved, and the tensile strength and elongation were greatly improved. The tensile strength obtained by adding Sn 0.02% was (A) and the elongation was (B), which is shown in FIG. Sn has the same effect as Cu, but with the synergistic effect with Cu, it moves to the low temperature side by austenite-perlite transformation, makes the graphite grains finer, and prevents the crystallization of irregular graphite by adding an appropriate amount. Thus, the graphite shape is improved and the elongation is increased. Moreover, it becomes high strength in order to make the base organization more precise.

  Furthermore, it came to discover that Al had a great influence on a base structure in the process which tests Cu and Sn in the cast iron of this invention. That is, it was found that by controlling the Al content of the spheroidizing molten metal, pearlite becomes epoch-making as sorbite, strengthening the base structure and increasing the strength.

  In contrast to Cu and Sn, Al moves the austenite-perlite transformation to the high temperature side, but its influence on the characteristics after spheroidization is very small. It should be noted that a very small amount (0.05%) of Al promotes the oxidation of Cu particles that are liberated beyond the solubility contained in the molten metal before spheroidization. That is, by restricting the Al content before spheroidizing treatment to 0.005% or less and preventing the behavior of this Al during melting, the base of cast iron after spheroidizing treatment is strengthened in reverse. It has a high strength.

  Al forms a compound with oxygen and nitrogen, and most of them are removed as impurities. Therefore, to control the Al content of the molten metal before spheroidizing treatment, the Al content is low, for example, 1.0% or less. It is only necessary to use Fe-Si and avoid the use of steel scraps in which Al is excessively mixed, and it is not necessary to consider the contents of the spheroidizing agent and the inoculum.

  Here, in addition to the above, C, Si, and Mg can use 3% or less, or 4% or more, for example, 2.7% and 4.2%, for example. Further, Si can be 2% or less, 2.7% or more, for example, 1.7% or 2.9%. Mn can be used if it is 1% or less.

Machined (turning, gear cutting, shaving) to a JIS FCD800 standard ductile cast iron gear having the following points made using the material of the present invention, and applied to a cylinder driving device in a double-sided printing machine The following experiments (1) and (2) were performed.
(1) Outline of experimental ductile cast iron helical gears Tooth profile: High-tooth module: 2.5
Number of teeth: 88
Pressure angle: 14.5 °
Twist angle: 10.0 °
(2) Noise measurement test FIG. 10 shows the results of measuring noise in the test apparatus. The FCD800 gear, which is an implementation product of the present invention, was able to reduce visible noise compared to conventional SCM carburized and tempered gears.

  The machinability of the ductile cast iron prepared according to Table 1 was tested in comparison with a comparable steel name (SCM435QT). The results are shown in FIG.

The result is displayed as shown in FIGS. 12-1 and 12-2.
As shown in this table, the cutting resistance at the time of cutting is 75.2%, and the cutting resistance at the time of drilling is 69.4%. As a result, it was found that the machinability is very good.

FIG. 14 shows a table comparing the gears formed according to the present invention and the manufacturing methods of gears formed by other methods.
According to this table, in the case of a crankshaft, the two steps of ADI and grinding are omitted as compared with the conventional heat treatment ADI treatment method. In S45C (H), forging, cutting, induction heat treatment, and grinding are completed in two steps of forging and cutting with FCD800.
SCM requires forging, heat treatment, cutting, heat treatment, and grinding, but the same degree of strength, hardness, and accuracy can be obtained only in the forging, heat treatment, and cutting processes, and its manufacture. The process could be greatly omitted, and the manufacturing cost could be greatly reduced.

The same procedure as in the above test, that is, using a low-frequency induction furnace with a capacity of 2000 kg, using steel scrap and return material to make a molten metal equivalent to FCD450, after adding a desired amount of Cu as shown in FIG. As shown in FIG. 1, a desired amount of Sn is added at the time of carrying out the spheroidizing treatment by the sandwich method of adding Fe—Si—Mg—Ca—REM alloy, and inoculated at the time of transferring the pouring ladle, JIS G5502 Y of CO2 mold Cast into type B (25t × 215l). FIG. 13 shows the mechanical properties and chemical compositions of the inventive products 1 to 4 in which the chemical composition was changed in the same procedure.
Moreover, the microscope picture of the metal structure of this invention product 3 is shown in FIG.

  It can be seen that the tensile strength and elongation are greatly improved by adding Sn as compared to when Cu alone is contained in FIG. 1, and further, even if Cu exceeds 2.6%, it does not decrease. As shown in the micrograph of the metal composition in FIG. 15, there is no cementite or bainite, the sphere has a good shape, fine graphite, and a very dense structure. The ductile cast iron of the present invention sufficiently satisfies the JIS G5502 FCD800-2 standard tensile strength of 800 MPa and elongation of 2% or more when cast.

  As a result of using Fe-Si containing 0.7% Al, the Al content in the molten metal before spheroidizing treatment was 0.005% by weight or less. FIG. 13 shows mechanical properties and chemical compositions of the inventive products 5 to 8 in which the chemical composition was changed in the same procedure as in Example 1 except for the above.

FIG. 16 shows a micrograph of another metal structure of the product of the present invention.
Compared to the inventive products 1 to 4 of FIG. 13, the inventive products 5 to 8 are further improved in tensile strength, and the base is dense like sorbite from the micrograph of the metal composition of FIG. I understand. The ductile cast iron of the present invention sufficiently satisfies the tensile strength of 900 MPa or more and the elongation of 2% or more, which are the standards of ISO 1083 FCD900-2, as cast.

After inoculation at the time of transferring the pouring ladle in the same procedure as in the above experiment and Example 1, it was cast into a mold having an outer diameter of Φ227, an inner diameter of Φ93, and a thickness of 30 mm formed in a green mold. The chemical component analysis results obtained are Cu: 2.9%, Sn: 0.04%, C: 3.7%, Si: 2.5%, Mn: 0.4%, P: 0.00% by weight. 02%, Mg: 0.04%, and the balance was Fe and other inevitable impurities.
The black skin part of the casting was removed by lathe processing, and induction hardening (quenching method: stationary one shot, heating time: 75 seconds) was performed on the inner diameter part, which is harder to quench than the outer diameter. FIG. 17 shows the curing depth, the solid line represents the material of the present invention, and the dotted line represents the comparative material.

In addition, the graph of the comparative material referred to “Cast Iron Production Technology [Modified]” (p112, FIG. 2107) issued by the Shape Materials Center.
As can be seen from the hardening depth shown in FIG. 17, the ductile cast iron of the present invention has a Si value of 2.5%, which is a disadvantageous component for quenching, but also includes typical quenching promoting elements such as Cr, Mo, Ni and the like. It does not contain and has strong hardenability because segregated Cu increases the thermal conductivity and speeds up the heating and cooling, has good fine graphite and a dense structure, and does not generate cementite. is there.

Example of chemical composition (wt%) table of high strength ductile cast iron of the present invention Example of high strength ductile iron Test table of two-cylinder tester of the present invention product Summary table of test conditions and results Photomicrograph showing damage Table showing allowable load for number of cycles H-N ₁ diagram on FIG. 6 of one embodiment of the present invention F / b-N ₁ diagram on FIG. 6 of the same embodiment as above. Graph showing the relationship between tensile strength and elongation of Sn-containing cast iron Noise measurement result table Machining test comparison table Same graph comparison table Same graph comparison table Mechanical properties and chemical composition diagram of the product of the present invention Comparison of manufacturing process with various products Micrograph of an example of the present invention product Photomicrograph of other examples of the present invention The figure which shows the hardening depth of an example of this invention product

Claims (3)

  Ductile with high surface pressure strength obtained by ductile cast iron containing Cu 2.4 to 3.3% by weight and obtained by spheroidizing treatment, and cast by gear cutting without heat treatment Cast iron gear.   At least the surface pressure strength without heat treatment formed through gear cutting process after being composed of cast iron using ductile cast iron containing at least Cu: 2.4-3.3% and Sn: 0.01-0.05% Large ductile cast iron gear.   The ductile cast iron gear according to claim 1 or 2, which has an as-cast surface pressure strength of 1000 MPa or more.

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