JPH09111408A - Low strain type carburized and quenched steel stock for gear - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine the chemical composition of a steel stock for gear, minimal in strain. SOLUTION: A steel stock, having a composition which consists of, by weight, 0.10-0.35% C, 0.50-2.5% Si, 0.20-2.50% Mn, 0.01-2.50% Cr, 0.01-2.0% Ni, and the balance iron with inevitable impurities and in which Ac3 , represented by equation, Ac3 =920-203√C+44.7Si-30Mn-11Cr+40Al-15.2Ni+13.1W+104V+40 Ti, is regurated to 850-960 deg.C and also DI, represented by equation, DI=7.95√C (1+0.70Si)(1+3.3Mn)(1+2.16Cr)(1+0.36Ni)(1+5.0V), is regulated to 30-250mm, is used. This steel stock is carburized at 850-1,000 deg.C, hardened at 800-950 deg.C, and tempered, by which the noncarburized zone of the steel stock is provided with dual-phase structure consisting of martensite containing 10-70area% ferrite. Further, one or more kinds selected from the group consisting of 0.01-0.70% W and 0.01-1.0% V and/or the group consisting of 0.005-2.0% Al, 0.005-1.0% Ti, 0.005-0.50% Nb, and 0.005-0.50% Zr are added. It is desirable to regulate DI to 30-150mm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for use as a steel material for gears of automobiles, construction machines, industrial machines and the like.
The present invention relates to a low distortion type steel material for a carburized and quenched gear having a very small amount of distortion during carburizing and quenching.

[0002]

2. Description of the Related Art For example, in recent automobiles, quietness during driving is remarkably improved, but noise is generated during driving. This is mainly due to gear noise generated from gears. The gear noise is generated due to a gear meshing defect. Such gear meshing defect is caused by carburizing and quenching or nitrocarburizing to harden the surface of a semi-finished gear formed into a predetermined shape. (Hereinafter collectively referred to as carburizing and quenching).

That is, during carburizing and quenching of a steel material for a gear, a transformation stress due to the formation of martensite, that is, a stress due to volume expansion generated when the austenite structure is transformed into a martensite structure, is generated, so that the steel material is distorted. This cannot be avoided, and as a result, gear noise occurs because the dimensional accuracy of the gear cannot be maintained high. In particular, in a transmission gear for an automobile, despite its extremely strict noise limits, its structure is small and its wall thickness is thin, so that the structure inside the gear is mainly composed of martensite including bainite. , Distortion is likely to occur during carburizing and quenching, which is the largest cause of gear noise.

[0004] Therefore, in order to improve the dimensional accuracy of the gear, the carburized and hardened gear semi-finished product is machine-cut,
There is a method of partially removing the carburized layer and performing a tooth shape correction process for reducing the amount of quenching distortion. However, such correction of the tooth profile by mechanical grinding not only significantly reduces the productivity due to an increase in the number of manufacturing processes, but also significantly increases the manufacturing cost due to the mechanical grinding process, and causes unevenness in surface hardness and residual stress. Therefore, there is a problem in quality.

[0005] In view of the above, gear steel is often used after carburizing and quenching without performing a tooth shape correction process.
Therefore, in order to improve the dimensional accuracy of a carburized and hardened gear semi-finished product, it is necessary to reduce hardening distortion. Such carburizing and quenching distortion is greatly affected by the hardenability of the steel material. Furthermore, carburizing and quenching is usually about
Since the heat treatment is performed at a high temperature of 920 ° C., coarsening of austenite crystal grains during carburization is also considered to be one of the causes of distortion. Furthermore, recently, in order to shorten the carburizing time and improve the productivity, a method of increasing the carburizing temperature and thereby increasing the quenching temperature has been tried.

Various studies have been made on methods for reducing the amount of quenching distortion of steel materials for gears. For example, for example, the chemical composition of steel materials is limited to a specific narrow range so that the hardenability becomes the lower limit of the Jominy band. A method is known in which the hardenability is controlled to a low level by controlling the temperature within the steel.
No. 247848 and Japanese Unexamined Patent Publication No. 59-123743 disclose that, in order to suppress grain coarsening during carburization and heat retention, an appropriate amount of grain refining elements such as Al, Ti, and Nb are contained in steel. It discloses a method of finely adjusting crystal grains by adding (hereinafter referred to as Prior Art 1).

Japanese Patent Application Laid-Open No. Hei 5-70925 discloses Si, M
After performing a carbonitriding process on a semi-finished gear made of steel having a chemical composition limited to a specific range such as n, Cr, Mo, and V, it is subjected to A of a tooth surface portion, that is, a carbonitrided portion (hereinafter the same).
r ₁ is cooled to a temperature range of below the transformation point. Next, the tooth surface is maintained in an austenite state by maintaining it in a temperature range that is equal to or higher than the Ar ₃ transformation point of the tooth surface portion and equal to or lower than the Ar ₁ transformation point of the tooth interior, that is, the non-carburized portion (hereinafter, the same). While making the inside of the tooth fine ferrite pearlite, then quenching and tempering, the carbonitrided part of the tooth surface becomes martensite, and the inside of the tooth that has already undergone transformation is quenched. A method of maintaining ferrite and fine pearlite which are not present (hereinafter referred to as Prior Art 2) is disclosed. FIG. 4 is a schematic perspective view illustrating the inside of the gear teeth, the tooth surface, and the gear core.

[0008] For example, Japanese Patent Application Laid-Open No. 3-260048 discloses that
A method of reducing heat treatment distortion by nitriding at a low temperature such as tuftride, gas nitriding, or gas nitrocarburizing (hereinafter, referred to as
Prior art 3).

[0009]

However, the above-described prior arts have the following problems. In the prior art 1, since the crystal grains are finely adjusted, the coarsening of the crystal grains during carburization and heat retention can be suppressed, so that the variation of the quenching distortion inside the teeth can be reduced, and the quenching distortion can be reduced. This has the advantage that it can be made uniform. However, the prior art 1 has a problem in that it is not possible to sufficiently reduce the strain because there is a limit in suppressing the occurrence of strain associated with the martensitic transformation.

The prior art 2 has an advantage that quenching distortion due to volume expansion caused by martensite generation can be reduced by forming a ferrite-pearlite structure inside the tooth. However, in prior art 2, since the inside of the tooth, that is, the non-carburized portion has a ferrite-pearlite structure, it is difficult to secure sufficient toughness, and the heat treatment temperature must be strictly controlled. There is a problem that the operation becomes complicated, not only hindering the productivity but also increasing the cost.

In the prior art 3, since a hard nitrogen compound layer can be formed on the surface, a surface hardened layer having good abrasion resistance can be obtained.
Since the processing is performed in the low temperature range, there is an advantage that the deformation of the processing component is small. However, the prior art 3 has a shallow hardened layer depth and requires a long nitriding treatment of 50 to 100 hours to obtain a sufficient hardened layer. It has the disadvantage of being high.

[0012] Accordingly, an object of the present invention is to solve the above-mentioned problems, to reduce the amount of distortion after ordinary and efficient carburizing and quenching and tempering, and therefore to reduce the dimensional accuracy. Low distortion type that can easily and efficiently heat-treat gears of automobiles, construction machines, industrial machines, and the like that do not generate gear noise during use. It is an object of the present invention to provide a carburized and hardened gear steel.

[0013]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and have obtained the following findings.

The main factor affecting the amount of carburizing and quenching strain of steel for gears is the strain caused by volume expansion that occurs when the austenite structure is transformed into martensite structure. It has been found that the ferrite / martensite two-phase structure after carburizing and quenching is used to mix the ferrite into the austenitic structure at the time of heating and the amount of quenching strain is dramatically reduced.

[0015] In the present invention, it is also an important object to provide a steel material capable of producing gears under heat treatment conditions of easy and economical carburizing and quenching. Moreover, it is an essential requirement that the steel material of the present invention has a structure in which ferrite is mixed in a martensite structure by carburizing and quenching. Therefore, the steel material of the present invention needs to have an Ac ₃ transformation temperature higher than a normal carburizing and quenching temperature range.

Therefore, as a result of detailed study on the influence of elements such as Si, Mn, Cr, Al, and V in steel on the Ac ₃ transformation temperature, it was found that by appropriately limiting the contents of these elements, The ferrite / martensite two-phase structure can be easily obtained even under the carburizing condition of No. 1, and by adding an appropriate amount of ferrite strengthening element, the inside of the tooth, that is, the non-carburized part is strengthened and the fatigue strength of the tooth surface part is improved. Therefore, it was found that the quenching strain amount can be dramatically reduced without lowering the fatigue strength of the tooth root.

The present invention has been made on the basis of the above findings, and the steel material for a low distortion type carburized and quenched gear according to the first aspect of the present invention is characterized in that: C: 0.10 to 0.35 wt.%, Si: 0.50 to 2.5
wt.%, Mn: 0.20 to 2.50 wt.%, Cr: 0.01 to 2.50 wt.%, and Ni: 0.01 to 2.0 wt.%, and the balance: chemical composition consisting of iron and inevitable impurities. And, below
(1) Formula: Ac ₃ = 920-203 √C + 44.7Si-30Mn-11Cr + 40Al-15.2Ni + 13.1W + 104V + 40Ti ------------ Calculated by (1) Ac ₃ point parameter is between 850 and 960
Within the range of ℃, the following formula (2): D _I = 7.95√C (1 + 0.70Si) (1 + 3.3Mn) (1 + 2.16Cr) (1 + 0.36Ni) (1 + 5.0V)- ----------- (2) ideal critical diameter (D _I) calculated by the 30~250mm
A steel material having a chemical composition within the range of, wherein the steel material is subjected to a carburizing treatment within a temperature range of 850 to 1000 ° C., and then subjected to a quenching treatment within a temperature range of 800 to 950 ° C. Then, it is characterized in that the structure of the non-carburized part of the steel material obtained as described above is subjected to a tempering treatment and is a two-phase structure composed of martensite containing 10 to 70 area% of ferrite. Is. In the present invention, the Ac ₃ point parameter and the ideal critical diameter are calculated by the above equations (1) and (2).
When calculating (D _I ), there are terms related to a predetermined component element on the right side of the formulas (1) and (2), but the chemical component composition is not limited, but Al, W, V The content of Ti and Ti shall be calculated as 0 (zero). Hereinafter, the same applies to the second to fifth aspects of the present invention. The same applies to comparative steels and conventional steels in examples described later.

The steel material for a low distortion type carburized and quenched gear according to the second aspect of the present invention further includes, in addition to the steel material for a gear according to the first aspect, a group consisting of the following chemical components: tungsten (W): 0.01 to 0.70 wt.% And Vanadium (V): It contains at least one element selected from 0.01 to 1.0 wt.%.

The steel material for a low-strain type carburized and quenched gear according to the third aspect of the present invention further includes, in addition to the steel material for a gear according to the first aspect, a group consisting of the following chemical components: aluminum (Al): 0.005 to 2.0 wt.%, Titanium (Ti): 0.005 to 1.0 wt.%, Niobium (Nb): 0.005 to 0.50 wt.%, And zirconium (Zr): at least one selected from 0.005 to 0.50 wt.% It contains an element.

The steel material for a low distortion type carburized and quenched gear according to the fourth aspect of the present invention is a steel material for a gear according to the first aspect of the present invention, and further includes a group consisting of the following chemical components: tungsten (W): 0.01 to 0.70 wt.% And vanadium (V): at least one element selected from 0.01 to 1.0 wt.%, And a group consisting of the following chemical composition: aluminum (Al): 0.005 to 2.0 wt.%, Containing at least one element selected from titanium (Ti): 0.005 to 1.0 wt.%, Niobium (Nb): 0.005 to 0.50 wt.%, And zirconium (Zr): 0.005 to 0.50 wt.% It is.

According to a fifth aspect of the present invention, there is provided a steel material for a low distortion type carburized and quenched gear according to any one of the first to fourth aspects of the present invention, wherein the ideal critical diameter (D _I ) is in the range of 30 to 150 mm.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, Si, which is an element for increasing the Ac ₃ transformation temperature and improving hardenability,
By increasing the contents of V, and Al, Ti, and W, which raise the Ac ₃ transformation temperature, it is possible to easily form a ferrite-martensite two-phase structure by carburizing and quenching treatment, and the ferrite has an expansion strain of martensite. By absorbing the above, the amount of quenching strain is significantly reduced, and furthermore, the hardness of the core portion of the gear during quenching (hereinafter referred to as "gear core portion" (see Fig. 4)) can be sufficiently secured, so Fatigue strength comparable to that obtained.

In addition, in the case of automobile gears, shot peening is often performed for the purpose of improving the root fatigue strength. However, according to the steel material of the present invention, the formation of a grain boundary oxide layer on the surface is suppressed. In addition, since no quenching defective structure is generated, when the shot peening treatment is performed, the tooth root fatigue strength increases without deteriorating the surface roughness. Further, Si, W, and V increase the temper softening resistance and improve the surface fatigue strength.

As described above, in the present invention, each element in the steel material exerts various functions and effects, and the chemical component elements to be contained in the steel material include essential components and selected components. Then, the selected components were divided into two groups. Among the functions and effects of W and V as selective components, they are common in improving hardenability, so they are classified into the first group.
Since b and Zr are common in suppressing quenching distortion by refining crystal grains, they are included in the second group.

Next, the reason why the chemical composition of the steel material for carburized and quenched gears of the present invention is limited to the above range will be described. (1) Carbon (C) Carbon is a basic element necessary for ensuring the strength of the gear core by carburizing and quenching, and it must be contained at least 0.10 wt.% In order to exert its effect. Is required,
If the content is less than 0.10 wt.%, It takes a long time to obtain an effective carburized hardened layer depth, so that it is industrially impossible. However, when the carbon content exceeds 0.35 wt.%, Toughness is deteriorated and machinability is lowered. Therefore, the carbon content, 0.
It should be limited to the range of 10-0.35%.

(2) Silicon (Si) Silicon is an element that plays an important role in the present invention as described below. That is, silicon is effective in preventing grain boundary oxidation of the surface layer, is a ferrite forming element,
c _{3 It} is effective for raising the transformation point and is a relatively inexpensive element. Further, the tempering softening resistance is increased to improve the surface fatigue strength. However, the silicon content
If the content is less than 0.50 wt.%, The concentration of silicon in the surface layer which is inevitably present in the carburizing gas during the carburizing process is too low. As a result, the fatigue strength is reduced. On the other hand, when the silicon content is excessively beyond 2.5 wt.%, Too much ferrite amount, not only the strength and toughness is decreased, Si0 ₂ system results nonmetallic inclusions increases, and conversely This leads to a decrease in fatigue strength. Therefore, the silicon content should be limited to the range of 0.50-2.5 wt.%.

(3) Manganese (Mn) Manganese is an element effective for improving the hardenability and ensuring the strength of the gear core. In order to exert its effect, manganese is required to be 0.20 wt.% Or more. It is necessary to contain it. However, since manganese has an effect of greatly lowering the Ac ₃ transformation point, if its content exceeds 2.50 wt.%, Not only does it become impossible to obtain a two-phase structure of martensite and ferrite, but also it becomes harder. Becomes too high, leading to deterioration of machinability. Therefore, the manganese content should be limited to the range of 0.20-2.50 wt.%.

(4) Chromium (Cr) Chromium, like manganese, is an element effective in improving the hardenability, and 0.01% is necessary to exert its action.
It is necessary to contain at least wt.%. However, chromium has the effect of lowering the Ac ₃ transformation point like manganese. Therefore, if its content exceeds 2.50 wt.%, The two-phase structure of martensite and ferrite cannot be obtained any more. No, the hardness is too high,
This leads to deterioration of machinability. Therefore, the chromium content, 0.01 ~
It should be limited to the range of 2.50 wt.%.

(5) Nickel (Ni) Nickel is an element effective for enhancing hardenability and toughness, and 0.01 wt.% Is required to exert its action.
It is necessary to contain the above. However, when the nickel content exceeds 2.0 wt.%, The hardness becomes too high, the machinability is deteriorated, and nickel is an expensive element, which causes an economic disadvantage. Therefore, the nickel content should be limited to the range of 0.01 to 2.0 wt.%.

(6) Tungsten (W) Tungsten, like molybdenum, raises the Ac ₃ transformation point and is effective for ferrite formation. Further, it increases temper softening resistance and improves surface fatigue strength. It is an element effective in improving toughness and root fatigue strength, and in order to exert its action, it is necessary to contain 0.01 wt.% Or more. However, tungsten is also an expensive element, and even if its content exceeds 0.70 wt.%, There is an economic disadvantage for its effect. Therefore,
Tungsten content should be limited to the range of 0.01 to 0.70 wt.%. When tungsten and molybdenum are added in combination, the total amount is desirably 0.70 wt.% Or less. If it exceeds 0.70 wt.%, The carburizing and quenching strain becomes large, which is not preferable.

(7) Vanadium (V) Vanadium has a large effect of increasing the transformation point of Ac ₃ , and also enhances hardenability, improves root fatigue strength, increases temper softening resistance, and improves surface fatigue strength. In addition, it has the effect of forming carbonitrides, making crystal grains finer, and suppressing quenching distortion. To exert the effect, the content is 0.01 wt.% Or more. It is necessary. However, vanadium content is 1.
When the content exceeds 0 wt.%, The effect is saturated and not only economical disadvantages are caused, but also the amount of carbonitrides is increased and the toughness is reduced. Therefore, the vanadium content is reduced to 0.01 to 1.0 wt.
Should be limited within the range.

(8) Aluminum (Al) Aluminum combines with nitrogen to form AlN, and by making crystal grains finer, reduces distortion during quenching and is effective in improving toughness and fatigue strength. Element. For this purpose, it must be contained at 0.005 wt.% Or more. Aluminum, like silicon, is a ferrite-forming element and can significantly increase the Ac ₃ transformation point economically. However, when the aluminum content exceeds 2.0 wt.% And becomes large, the amount of alumina-based inclusions increases, and the toughness and fatigue strength decrease. Therefore,
The aluminum content should be limited to the range of 0.005 to 2.0 wt.%. When silicon and aluminum are used together, the total amount is desirably regulated to 2.6 wt.% Or less in order to ensure the cleanliness and toughness of the steel.

(9) Titanium (Ti) Titanium is also a ferrite-forming element, has a large effect of increasing the transformation point of Ac ₃ , is an element effective in refining austenite crystal grains, and has a carburized part and a tooth. It has the effect of increasing the internal yield strength and contributing to the improvement of fatigue strength. To achieve this effect, 0.005 wt.%
It is necessary to contain the above. However, when the titanium content exceeds 1.0 wt.%, The effect is saturated and not only economical disadvantages are caused, but also the amount of carbonitride becomes excessively large, and the toughness is reduced. Therefore, the titanium content, 0.005
It should be limited to the range of ~ 1.0 wt.%.

(10) Niobium (Nb) Niobium is also an effective element for refining austenite crystal grains, and in order to exhibit its effect, 0.005 wt.
It is necessary to contain the above. However, when the niobium content exceeds 0.50 wt.%, The effect is saturated and not only economical disadvantages are caused, but also the amount of carbonitrides is increased and the toughness is reduced. Therefore, the niobium content is 0.005
It should be limited to the range of ~ 0.50 wt.%.

(11) Zirconium (Zr) Zirconium, like titanium and niobium, is also an effective element for refining austenite crystal grains. In order to exhibit its effect, it is necessary to contain at least 0.005 wt.%. is necessary. However, the zirconium content is 0.50w
If the content exceeds t.%, the effect is saturated and economic disadvantages are caused. In addition, the amount of carbonitride is increased and the toughness is reduced. Therefore, the zirconium content is reduced from 0.005 to 0.50 wt.%
Should be limited within the range.

The contents of P, Cu and O as unavoidable impurities in the steel of the present invention are desirably as low as possible. N is added for the purpose of refining crystal grains, if necessary, up to 0.20 wt.%. In order to improve machinability, S, Pb, Ca
And a free-cutting element such as Se.

(12) Ac _3- point parameter: FIG. 5 shows an example of a heat treatment pattern in a conventional carburizing process by a conventional method.
After carburizing the gear steel at 920 ° C. and diffusing the carbon into the steel, 85% lower than the carburizing temperature to reduce distortion.
It is kept at 0 ° C. and then quenched by quenching with oil or the like. Therefore, if the Ac ₃ point parameter calculated by the following equation (1) for steel for gears is less than 850 ℃, 85 after carburization
Even at 0 ° C., ferrite cannot be secured in austenite. On the other hand, when the above Ac ₃ point parameter exceeds 960 ° C, the amount of ferrite in austenite becomes excessive and the strength of the gear core becomes insufficient. Therefore, the following formula (1) of the steel of the present invention: Ac ₃ = 920-203 √C + 44.7Si-30Mn-11Cr + 40Al-15.2Ni + 13.1W + 104V + 40Ti ----------- --- Set the Ac ₃ point parameter calculated by (1) to 850 to 960
It should be limited to within the range of ° C.

(13) Ideal Critical Diameter (D _I ): Ideal Critical Diameter
(D _I ) is a value representing the hardenability of steel. Typically,
The austenitic grain size number of the steel product required when the steel product is used as the steel product is No. 8, which is the same for carburized and quenched gears. In order to secure the desired fatigue strength, the following formula (2) is used to calculate the ideal critical diameter (D _I ) of the steel when the austenite grain size number is 8.
Formula: D _I = 7.95√C (1 + 0.70Si) (1 + 3.3Mn) (1 + 2.16Cr) (1 + 0.36Ni) (1 + 5.0V) ------------ --- It is necessary that the ideal critical diameter (D _I ) value calculated by (2) is 30 mm or more. On the other hand, the above ideal critical diameter
If the (D _I ) value exceeds 250 mm, the effect of absorbing the transformation strain of martensite due to the ferrite mixed in the austenite structure is lost, and the quenching strain increases.
Therefore, the austenite particle size number was set to 8
The ideal critical diameter (D _I ) value calculated by the equation (2) is 30
The chemical composition of the gear should be limited to within the range of ~ 250 mm. In order to further reduce the quenching distortion, it is desirable to limit the value to a range of 30 to 150 mm. Note that when the austenite grain size number is other than No. 8, since the coefficient of the right side of equation (2) is determined in accordance with the grain size number, calculated value using the calculation formula for D _I corresponding to the austenite grain size number is, above The chemical composition of the gear should be limited so as to fall within the above range.

Regarding Carburizing and Quenching Temperature Next, the carburizing temperature for the steel material should be a temperature at which the carburizing treatment can be performed easily and efficiently.
When the carburizing temperature is lower than 850 ° C., the diffusion rate of C is low, and it takes a long time to obtain a desired carburizing depth. On the other hand, if the carburizing temperature exceeds 1000 ° C., the crystal grains are likely to become coarse and the oxidation of the surface of the steel material becomes remarkable, so that the surface fatigue characteristics are reduced. Therefore, the carburizing temperature should be limited to the range of 850-1000 ° C.

The quenching temperature after carburizing is 800
If the temperature is lower than 0 ° C, it takes a long time to lower the furnace temperature of the carburizing furnace to that temperature. On the other hand, when the quenching temperature exceeds 950 ° C., it becomes difficult to secure the ferrite area% in the martensite structure obtained after quenching to a desired value, and the amount of quenching distortion increases. Therefore, the quenching temperature should be limited to the range of 800 to 950C.

About the amount of ferrite in the structure inside the tooth (structure in the non-carburized portion) When the amount of ferrite in the structure inside the tooth, which is the non-carburized portion after carburizing and tempering, is less than 10%, the transformation strain of martensite is sufficiently reduced. It cannot be absorbed and the amount of quenching distortion cannot be suppressed to a small value. On the other hand,
If it exceeds 70%, it becomes difficult to secure desired strength and toughness inside the tooth. Therefore, the amount of ferrite in the tissue inside the tooth should be limited to the range of 10-70%. At this time, martensite may partially contain retained austenite and / or bainite.

[0042]

Next, the present invention will be described in more detail with reference to examples and comparative examples. Range of conditions of the present invention shown in Table 1 (chemical composition, Ac ₃ point parameter, ideal critical diameter (D _I ), carburizing temperature, quenching temperature, and ferrite area% of non-carburizing part after carburizing and tempering) Inventive steel Nos. 1 to 13 and comparative steel Nos. 1 to 7 and conventional steel Nos. 1 to 4 which are out of the conditions of the present invention shown in Tables 2 and 3 An ingot was prepared.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

Comparative Steel No. 1 is a steel having a C content lower than the range of the present invention, a Si content exceeding the range of the present invention, and a high Ac ₃ point parameter of 962 ° C., Comparative Steel No. 2 Has a Mn content higher than the range of the present invention, an Ac ₃ point parameter lower than the range of the present invention, and an ideal critical diameter (D _I ) smaller than that of the present invention. Comparative steel No. 3 has a Cr content of the present invention. Steel having a higher Ac ₃ point parameter than the range of the present invention, comparative steel No. 4 is a steel having a high C content exceeding the range of the present invention, and comparative steel No. 5 is a steel containing Si and Mn. Steels whose amount is below the range of the present invention, Ac ₃ point parameter is below the range of the present invention, and ideal critical diameter (D _I ) is also below the range of the present invention, Comparative Steel No. 6 has Ni, W and Ti contents. However, the comparative steel No. 7 has a Ni and Ti content within the range of the present invention. Many Beyond, Ac ₃ point parameter is lower than the range of the present invention steel.

Conventional steel Nos. 1 to 4 are steels specified by the conventional JIS, conventional steel No. 1 is JIS SMnC420, conventional steel No. 2 is JIS SCM420, conventional steel No. 3 is
JIS SNCM420, and conventional steel No. 4 is JIS SCM4
35, all of which are steels having a low Si content and an Ac _three- point parameter outside the scope of the present invention.

The above invented steels, comparative steels and conventional steel ingots were hot-rolled to prepare round steel bars having a diameter of 20 to 90 mm, and the obtained round steel bars were subjected to normalizing treatment. From the round bar after the normalizing treatment, a quenched strain test piece and a fatigue test piece were collected. After subjecting each specimen to carburizing and quenching / tempering, the amount of carburizing and quenching, the rotational bending fatigue property, and the gear fatigue property were tested. Furthermore, after the normalizing round bar steel of 20 mm was subjected to carburizing and quenching and tempering, a tensile test piece and an impact test piece were sampled and tested for strength and toughness. Each test method is as follows.

(1) Carburizing and quenching strain amount: A Navy C test piece was prepared from a round steel bar having a diameter of 65 mm. FIG. 1 shows a front view of a navy C test piece, and FIG. 2 shows a side view thereof. As shown in both figures, the navy C test piece 1 has an opening 2 and a circular space 3 in a disk-shaped body, and the dimensions of each part of the test piece are as follows. Specimen diameter (a): 60mm, thickness (b): 12mm, diameter of circular space
(c): 34.8 mm, opening distance (d): 6 mm, distance (p) between the center of the test piece and the center of the opening circle: 10.2 mm.

The amount of strain after carburizing and quenching / tempering is measured by
The change rate of the opening interval of the navy C test piece before and after carburizing and quenching was measured. When a gear steel is used to form a gear using a steel material having a large distortion such that the amount of strain after carburizing and quenching / tempering with a navy C test piece exceeds 1.0%, and this is carburized and quenched / tempered, Deformation occurs, and the tooth profile must be corrected by mechanical grinding, and mechanical grinding cannot be omitted. In order to be able to use as a gear with carburized quenching and tempering without performing tooth profile correction grinding, the distortion amount after carburizing quenching and tempering in the Navy C test piece must be 1.0% or less, Further, in order to be able to use the gear without performing the tooth profile correction grinding irrespective of the shape and size of the gear, the content is more preferably 0.5% or less.

Ten Navy C test pieces 1 having the above-mentioned shape were produced for each test steel, and after carburizing / quenching and then tempering the test piece 1, the opening interval (d) of this test piece Measure the rate of change before and after carburizing and tempering,
This value was defined as the strain amount of carburizing and quenching. The test result was represented by the average value of the number of test repetitions n = 10 and its variation. The above results are shown in Tables 4 to 6. The other test results below are also shown in the above table.

[0052]

[Table 4]

[0053]

[Table 5]

[0054]

[Table 6]

(2) Ferrite area% of non-carburized part:
Using the test pieces that had been subjected to carburizing and quenching strain measurement, the ferrite area% of the ferrite-martensite two-phase structure in the non-carburized part of each sample steel after carburizing and quenching was measured by a microscopic test, and It is defined as the ferrite area%, and this ferrite area% is shown in the above table.

(3) Rotating Bending Fatigue Properties: A test piece with a diameter of 10 mm in the parallel portion was taken from a round bar steel with a diameter of 20 mm, and a notch having a depth of 1 mm in the direction perpendicular to this was formed in the parallel portion (stress concentration coefficient α =
1.8) was applied over the entire circumference to prepare a rotating bending fatigue test piece, and the test piece was carburized and quenched and tempered under the same conditions as those applied to the navy C test piece, and then shot. Peening treatment (Arc height: 0.6mmA,
(Coverage: 300%), and the test piece subjected to such treatment was subjected to 10 ⁷ using an Ono-type rotary bending fatigue tester.
The rotating bending fatigue test was performed twice, and the rotating bending fatigue strength was measured. The above table shows the measurement results of rotating bending fatigue strength.

(4) Gear fatigue characteristics, grain boundary oxide layer depth, quenching failure layer depth and effective hardened layer depth: diameter 90 mm
The outer diameter of 75mm, tooth width 10mm,
A test gear with module 2.5 and 28 teeth was prepared and subjected to carburizing and quenching / tempering and shot peening treatment under the same conditions as the above-mentioned rotational bending fatigue characteristics. Using a fatigue tester, a gear fatigue test was performed at a rotation speed of 3000 rpm, and the torque value that did not break at the number of repetitions of 10 ⁷ was determined as the tooth root strength of the gear. The table above shows the gear fatigue durability torque test results, and
The presence or absence of chipping was indicated. Furthermore, the tooth portion was cut out from the gear subjected to the gear fatigue test to prepare a predetermined test piece, and the grain boundary oxide layer depth associated with carburizing and quenching, the quenching failure layer depth, and the effective hardened layer depth were measured. , These are shown in the table above.

(5) Strength and impact value: JIS No. 4 tensile test piece (parallel part diameter: 14 mm φ) and JIS No. 3 Charpy test piece were prepared from a 25 mmφ round bar after carburizing and tempering, and tensile test and impact test Then, the strength of the gear core and the toughness of the gear core were evaluated. The above table shows the test results of strength and toughness.

The following matters are clear from Tables 1 to 3 and Tables 4 to 6. Comparative Steel No. 1 had a C content lower than the range of the present invention, a Si content exceeding the range of the present invention, and a high Ac ₃ point parameter of 962 ° C., and therefore had a high ferrite area ratio. The core strength, rotary bending fatigue strength and gear fatigue endurance torque are low. Comparative steel No. 2 is
A steel having a Mn content higher than the range of the present invention, an Ac ₃ point parameter lower than the range of the present invention, and an ideal critical diameter (D _I ) lower than that of the present invention, and thus the quenching strain exceeded 1%. . Comparative Steel No. 3 had a large Cr content exceeding the range of the present invention and an Ac ₃ point parameter lower than the range of the present invention, so that the quenching strain exceeded 1%. Comparative steel
Since No. 4 has a high C content exceeding the range of the present invention,
The core has low toughness. Comparative Steel No. 5 has Si and Mn contents lower than the range of the present invention, an Ac ₃ point parameter lower than the range of the present invention, and an ideal critical diameter (D _I ) also lower than the range of the present invention. Is greater than 1%,
Low core strength, rotary bending fatigue strength, and gear fatigue endurance torque. Comparative Steel No. 6 has a high Ni, W, and Ti contents exceeding the range of the present invention, so the toughness of the core is low. Comparative Steel No. 7 has a high Ni and Ti content exceeding the range of the present invention, and therefore has low toughness in the core.

Further, in the conventional steel Nos. 1 to 4, the ferrite area ratio is 5 to 8%, which is outside the range of the present invention and is small.
The grain boundary oxide layer depth and the quenching failure layer depth were large, and the quenching strain was large.

On the other hand, the steel Nos. 1 to 13 of the present invention are
Compared with conventional steel, the grain boundary oxidation layer is significantly reduced, no quenching failure layer is observed at all, and the carburizing and quenching characteristics are the effective hardened layer depth of carburization, the strength of the gear core, and the impact of the gear core. The value is equal to or higher than that of conventional steel, and the area ratio of ferrite is 12 to 68 within the range of the present invention.
% Of ferrite-martensite two-phase structure, the quenching strain amount was as small as in the range of 0 to 1%, and there was little variation among lots. FIG. 3 shows the relationship between the ideal critical diameter (D _I ) of the steel of the present invention and the conventional steel and the strain of carburizing and quenching. As is apparent from the figure, the heat treatment distortion of the gear is significantly reduced by the present invention, and the distortion is reduced from 0 to about 40% of the conventional steel.

As is further clear, in the comparative steels No. 2 to 7 and the conventional steels No. 1 to 4, chipping occurred on the tooth surface in the low torque region. On the other hand, the invention steel Nos. 1 to 13
Has better fatigue strength and tooth root strength than conventional steel, and has no quenching failure layer. Increased Si content increases tempering softening resistance, chipping does not occur, and surface pressure Strength was also strengthened.

[0063]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, the strain amount due to the carburizing and quenching treatment is 2.4 to 10% of that of the conventional steel.
It is possible to suppress the value to a small value of 0 to 1.0% compared to about 3.3%, and it is possible to obtain a steel material for gears having excellent tooth root strength by a normal carburizing and quenching treatment. , A construction machine which is suitable as a gear for an automobile without tooth profile modification and which requires tooth profile modification after carburizing and quenching,
In the case of gears for industrial machines, etc., the amount of distortion due to carburizing and quenching can be reduced, so that it is not necessary to modify the tooth profile, and therefore, a steel material for a low distortion type carburizing and quenching gear capable of reducing machining costs and improving productivity. And many industrially advantageous effects are brought about.

[Brief description of the drawings]

FIG. 1 is a front view showing an example of a test piece (a navy C test piece) for measuring the amount of strain of carburizing and quenching.

FIG. 2 is a side view of FIG. 2;

FIG. 3 is an ideal critical diameter (D _I ) of the steel of the present invention and the conventional steel.
4 is a graph showing a relationship between the amount of carburizing and quenching distortion.

FIG. 4 is a schematic perspective view illustrating the inside and the surface of the gear teeth.

FIG. 5 is a graph showing an example of a heat treatment pattern of carburizing and quenching by a conventional method.

[Explanation of symbols]

 1 Navy C test piece 2 Opening 3 Circular space 4 Tooth inside (non-carburized part) 5 Tooth surface part (carburized part) 6 Gear core

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C22C 38/58 C22C 38/58 C23C 8/22 C23C 8/22 F16H 55/06 F16H 55/06

Claims (5)

[Claims] 1. Carbon (C): 0.10 to 0.35 wt.%, Silicon (Si): 0.50 to 2.5 wt.%, Manganese (Mn): 0.20 to 2.50 wt.%, Chromium (Cr): 0.01 to 2.50 wt. .% And nickel (Ni): 0.01 to 2.0 wt.%, Balance: chemical composition consisting of iron and inevitable impurities, and the following formula (1): Ac ₃ = 920-203 √ C + 44.7Si-30Mn-11Cr + 40Al-15.2Ni + 13.1W + 104V + 40Ti ------------ The Ac ₃ point parameter calculated by (1) is 850 to 960.
Within the range of ℃, the following formula (2): D _I = 7.95√C (1 + 0.70Si) (1 + 3.3Mn) (1 + 2.16Cr) (1 + 0.36Ni) (1 + 5.0V)- ----------- (2) ideal critical diameter (D _I) calculated by the 30~250mm
A steel material having a chemical composition within the range of, wherein the steel material is subjected to a carburizing treatment within a temperature range of 850 to 1000 ° C., and then subjected to a quenching treatment within a temperature range of 800 to 950 ° C. Then, a tempering treatment is performed, and the structure of the non-carburized portion of the steel material thus obtained is a two-phase structure composed of martensite containing 10 to 70 area% of ferrite. Steel material for low distortion type carburized and quenched gears. 2. At least one element selected from the group consisting of the following chemical components: tungsten (W): 0.01 to 0.70 wt.%, And vanadium (V): 0.01 to 1.0 wt.%. The low distortion type carburized and quenched gear steel material according to claim 1, wherein 3. A group consisting of the following chemical components: aluminum (Al): 0.005 to 2.0 wt.%, Titanium (Ti): 0.005 to 1.0 wt.%, Niobium (Nb): 0.005 to 0.50 wt.%, And The low distortion type carburized and hardened gear steel material according to claim 1, further comprising at least one element selected from the group consisting of: zirconium (Zr): 0.005 to 0.50 wt.%. 4. A group consisting of the following chemical components: at least one element selected from the group consisting of tungsten (W): 0.01 to 0.70 wt.% And vanadium (V): 0.01 to 1.0 wt.%; Group consisting of component composition: aluminum (Al): 0.005 to 2.0 wt.%, Titanium (Ti): 0.005 to 1.0 wt.%, Niobium (Nb): 0.005 to 0.50 wt.%, And zirconium (Zr): 0.005 2. The steel material for a low-strain carburized and quenched gear according to claim 1, further comprising at least one element selected from .about.0.50 wt.%. 5. The ideal critical diameter (D _I ) is 30 to 150 mm.
Any one of claims 1 to 4 within the range of
4. A low distortion type steel material for a carburized and quenched gear as described in (1).