JP2549039B2 - Carbonitriding heat treatment method for high strength gears with small strain - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to a carbonitriding heat treatment method,
In particular, the present invention relates to a carbonitriding heat treatment method for case-hardening steel capable of producing a gear for an automobile transmission with less distortion during quenching and less noise.

[0002]

2. Description of the Related Art Generally, case hardening steel is used for automobile gears in order to improve fatigue resistance and surface wear resistance. In this case, the case-hardening steel is machined into a predetermined shape such as a gear and then carbonitrided for surface hardening. That is, carbon and nitrogen are infiltrated into the surface of a low alloy steel having a relatively low carbon content, and then quenching and tempering are performed to harden the tooth surface by making it martensite and to secure the toughness of the gear. From the point of view, the core of the gear has a ferrite-pearlite structure that partially contains martensite or bainite. However, the gear core is shown in Fig. 1 (B).
It is the central part of the gear shown by the mesh. Utilizing the difference in texture between the tooth surface and the gear core is a basic principle for obtaining fatigue strength and toughness, and in the carbonitriding and quenching method, it is sufficient to utilize the mass effect of the cooling rate. What has been achieved is a well-known fact.

However, although the gear core part has a ferrite-pearlite structure containing a part of martensite or bainite due to the mass effect, the inside of the tooth (see FIG. 1).
(A portion inside the carburized layer of the tooth shown by the hatched portion in (A))
Has a faster cooling rate than the core, and therefore has a mixed structure of martensite, which is a quenched structure, and part of bainite. At this time, transformation stress due to martensite formation (stress due to volume expansion that occurs when changing from austenite to martensite structure) occurs, so heat treatment deformation cannot be avoided and gear precision cannot be maintained. . In particular,
Despite the strictest restrictions on noise, automotive transmission gears are small and thin, resulting in martensite and bainite microstructures inside the teeth, which is the largest contributor to gear noise. It has become.

Therefore, conventionally, a method of improving accuracy by adding gear grinding to quenching distortion has been used. In addition, nitriding treatments performed at low temperatures such as ion nitriding, gas soft nitriding, and tuftride are being studied. Further, Japanese Patent Application Laid-Open No. 55-6456 discloses a method of forming a ferrite-pearlite structure by an isothermal annealing process and then hardening only the surface by a soft nitriding process.
In addition, a method in which after the ferrite / pearlite structure is once formed, the A ₁ transformation point of the surface layer is lowered by nitriding treatment to promote austenitization and a martensite structure is obtained only in the surface layer by quenching (Metal Temporary Special Issue, 1990, Vo1
8, P25) is disclosed.

[0005]

However, in the mechanical grinding of gears, the carbonitriding layer is partially shaved off, which increases the number of manufacturing steps and makes it difficult to apply to parts having complicated shapes.
Further, the surface hardness and the residual stress are uneven, which is a quality problem. On the other hand, in the methods of ion nitriding, gas soft nitriding, tuftride and JP-A-55-6456, since the quench hardened layer is thin and the internal hardness is low,
Sufficient fatigue strength cannot be obtained. Further, in the method of lowering the A ₁ transformation point of the surface layer by nitriding treatment to promote austenitization and obtaining the martensite structure only in the surface layer by rapid cooling, hardness is reduced due to formation of an abnormal compound layer in the carbonitrided portion, and There was a problem that sufficient fatigue strength could not be obtained due to the low internal hardness.

That is, the conventional machining method sacrifices economy and the quality of the gear itself, and even when the material is used, distortion can be reduced by preventing martensite inside the tooth, but the original characteristics of the gear are obtained. This is a sacrifice of high fatigue strength, and an effective method for forming a complete ferrite and pearlite structure inside the tooth while maintaining high strength is desired.

[0007]

Means for Solving the Problems The inventors of the present invention have dealt with such a situation, and have developed a material for carbonitrided gears in which distortion due to carbonitriding and quenching treatment is small and excellent hardening characteristics are obtained. Much research has been done to develop it. That is, the carbonitriding treatment is an extremely effective method for reducing the A ₁ transformation point only in the surface layer and thus making only the surface layer austenite while forming the ferrite / pearlite structure inside the tooth. The internal strength of the tooth mainly depends on the steel composition. Therefore, for the purpose of developing an effective method for forming a complete ferrite and pearlite structure inside the tooth while maintaining high strength, as a result of conducting a detailed study from both aspects of carbonitriding conditions and steel composition, the following findings were obtained. Obtained.

In the carbonitriding treatment, the surface compound layer,
The formation of the internal oxide layer and the incompletely hardened layer causes a decrease in the hardness of the surface layer. To prevent the formation of these, it is effective to increase the hardenability so that the surface layer is sufficiently hardened while reducing Cr and Si. In order to increase the depth of the hardened layer, it is necessary to deepen the austenite from the surface layer by carbonitriding, and it is effective to suppress the formation of the surface compound layer.

The decrease in hardness due to the ferrite / pearlite structure inside the tooth can be suppressed by the precipitation hardening and miniaturization effects of carbonitride. Further, addition of a specific amount of V is effective from the viewpoint of suppressing the formation of the compound layer. Therefore, based on these findings, in order to clarify the concrete means of achievement, further research and studies were carried out, and immediately after the carbonitriding treatment was applied to the gear manufactured from case hardening steel of a specific chemical composition, During the quenching of the tooth, which is a source of noise, the structure inside the tooth is made to be a ferrite and fine pearlite structure without lowering the surface hardness by holding in the temperature range for a specified time and then quenching and tempering. We have completed a new carbonitriding heat treatment method for gears that can significantly reduce the strain and significantly improve quietness.

That is, the gist of the present invention is as follows. (1)% by weight, C: 0.1% or more and less than 0.3%, S
i: less than 0.15%, Mn: 0.30% or more and 1.50%
Less than, Cr: 0.30% or more and less than 1.00%, Mo:
Gears made of steel containing 0.10% or more and less than 1.00% and V: 0.05% or more and less than 1.00% with the balance Fe and impurities are carbonitrided, and immediately thereafter, Or once A of the tooth surface part (carbonitriding part)
r ₁ is cooled to a temperature range below the transformation point, after heating to above Ac ₃ transformation point of the tooth surface portion again (carbonitriding unit), the teeth inside the above Ar ₃ transformation point of the tooth surface portion (carbonitriding unit) ( Hold the temperature in the temperature range below the Ar ₁ transformation point of the (non-carbonitriding part) for 5 minutes or more and less than 2 hours to form a fine ferrite pearlite inside the tooth while keeping the tooth surface in the austenite state, and then quench and temper. Thus, a carbonitriding heat treatment method for a high-strength gear with small strain, characterized in that the inside of the tooth has a fine ferrite-pearlite structure.

However, the inside of the tooth is the inside of the carburized layer of the tooth shown by the shaded portion in FIG. 1 (A). (2) In% by weight, C: 0.1% or more and less than 0.3%, S
i: less than 0.15%, Mn: 0.30% or more and 1.50%
Less than, Cr: 0.30% or more and less than 1.00%, Mo:
0.10% or more and less than 1.00%, V: 0.05% or more and less than 1.00%, and Nb: 0.02% or more and 0.20% or less, Ti: 0.01% or more and 0.20. %, One or more of the steel containing the balance Fe and impurities is carbonitrided and then immediately or once Ar _{1 of the} tooth surface (carbonitrided part)
After cooling to a temperature range below the transformation point and heating again to the Ac ₃ transformation point of the tooth surface portion (carbonitriding portion) or higher, inside the tooth (non-carburized) above the Ar ₃ transformation point of the tooth surface portion (carbonitriding portion) By maintaining the temperature range below the Ar ₁ transformation point of the nitrided part) for 5 minutes or more but less than 2 hours to make the tooth surface fine ferrite pearlite while keeping the tooth surface in an austenite state, and then quenching and tempering , A method for carbonitriding heat treatment of a high-strength gear with small strain, characterized in that the inside of the tooth is made of fine ferrite pearlite.

However, the inside of the tooth is the inside of the carburized layer of the tooth shown by the shaded portion in FIG. 1 (A). Each constituent element of the present invention will be described below. First, the chemical composition of the alloy steel to which the carbonitriding heat treatment method of the present invention can be applied will be described.

The alloy steel of the present invention can be made into a ferrite / pearlite structure inside the tooth by cooling in a furnace for carbonitriding, and has an internal hardness sufficient to maintain rigidity and toughness as a gear. Must. On the other hand, since this alloy steel is carbonitrided, it must have a chemical composition that is not adversely affected by carbonitriding. C is an element added to secure the strength required for automobile gears, especially the core strength, but if it is less than 0.1%, such an effect cannot be sufficiently obtained, and 0.3% or more. In that case, the toughness is reduced and the steel becomes brittle, which makes it difficult to use as a carburizing steel, so the content thereof is set to 0.1% or more and less than 0.3%.

Si is an element that has a significant adverse effect on the grain boundary oxidation of carburizing steel, and when the content is 0.15% or more, remarkable grain boundary oxidation is formed in the carburized layer, and the material properties of the carburizing steel deteriorate. Therefore, the content is set to less than 0.15%. M
n is an element necessary for imparting strength, toughness, and hardenability to steel, but when it is 1.50% or more, it becomes a hard structure of bainite or martensite in cooling after hot rolling, and after cutting etc. 1.5 because it is not suitable for secondary processing
Less than 0%. However, if the addition amount of Mn is less than 0.30%, the effect of improving the hardenability is not sufficient, and the content thereof is 0.30% or more.

Cr forms a compound layer in the carbonitrided layer of steel, lowers the surface hardness and, at the same time, inhibits the solid solution of N in iron, and raises the austenitizing temperature of the carbonitrided layer to austenite. Inhibit. At 1.00% or more, a compound layer is formed remarkably and nitriding treatment is hindered, so 1.00%
Less than However, if the added amount of Cr is less than 0.30%, the effect of improving the hardenability is not sufficient, and the content of Cr is 0.1.
30% or more.

Mo is an element which improves the wear resistance, hardenability and mechanical properties of steel without forming an intergranular oxide layer, and is particularly useful for increasing the hardness inside the tooth. The effect is not sufficient, and if it is 1.00% or more, the toughness is deteriorated, so the content is made 0.10% or more and less than 1.00%. V
Is an element that increases the hardness of ferrite / pearlite inside the tooth due to the precipitation hardening of carbonitride.
%, The effect is not sufficient, and even if 1.00% or more is contained, the effect is saturated, so 0.05% or more and 1.00
Less than%.

Ti and Nb are elements that generate carbonitrides and are effective in refining carburized crystal grains. To obtain the effect, at least one element of Nb is 0.02% or more and T is T.
It is necessary that the content of i is 0.01% or more. However, Nb
If the content of Ti exceeds 0.20% and the content of Ti exceeds 0.20%, the effect is saturated, so the upper limit is 0.20% for Nb,
Ti is set to 0.20%.

The characteristics can be further improved by adding the following elements in addition to the above components.
Ca is an element necessary for suppressing the stretching of MnS and reduces anisotropy in fatigue of gears. To give the effect of suppressing the stretching of MnS, Ca is contained in an amount of 0.0010% or more. However, even if the content exceeds 0.010%, the effect is saturated and the economy is impaired, so the upper limit is 0.010%.
And

Next, the carbonitriding method is gas carbonitriding.
Method or liquid carbonitriding method is preferable, and carbonitriding depth
Is 0.5 mm or more from the viewpoint of surface hardness and residual stress.
Preferably. Surface carbon content is more than eutectoid and less than Acm
Immerse at a temperature of 850-980 ° C with carbon potential
By carbonitriding, carbonitriding compared to non-carbonitriding parts
Ar of the chemical department ₃The transformation point drops by about 50 ° C (carbonitriding temperature:
930 ° C, carbon potential: 0.9%, N potene
Shall: 2%, the present inventors at JIS SCM420
Experimental example).

After carbonitriding, the temperature is 660 to 700 ° C. by cooling the furnace.
After cooling to a temperature above the Ar ₃ transformation point on the tooth surface and below the Ar ₁ transformation point inside the tooth, and holding for a certain period of time, the tooth inside the non-carbonitrided portion is transformed into ferrite and fine pearlite, The carbonitrided portion of the surface is maintained in an austenite state. The holding time is preferably 5 minutes or more and less than 2 hours from the viewpoint of temperature uniformity and heat treatment cost (longer holding time does not have an advantage in quality and is uneconomical).

More preferable conditions for refining the crystal grains in the carbonitrided portion are as follows: after carbonitriding, the tooth surface is gradually cooled to the Ar ₁ transformation point or lower, and then the tooth surface portion is Ac ₃ transformation point or higher. Reheat to. At this time, the austenite grains in the carbonitrided portion are refined. Next, after the Ar ₃ transformation point of the tooth surface portion is set to be equal to or lower than the Ar ₁ transformation point of the inside of the tooth and held for a certain period of time, the inside of the tooth of the non-carbonitrided portion remains as fine ferrite / pearlite and is Only the carbonitrided part transforms into fine-grained austenite. The holding time is preferably 5 minutes or more and less than 2 hours in view of temperature uniformity and heat treatment cost.

As described above, by austenitizing the surface, making ferrite and fine pearlite inside the tooth, and then quenching in an oil or salt bath, all the carbonitrided parts of the tooth surface can be made into martensite and already transformed. The inside of the finished tooth can be maintained with unhardened ferrite and fine pearlite. After that, in order to remove internal stress of the carbonitriding part and increase strength and tenacity, 150
A tempering process at ~ 200 ° C is performed.

The present invention will be further described below with reference to examples.

[0024]

Example: Steel having the chemical composition shown in Table 1 was melted to 162 mm.
After slab-rolling into a square billet, 50 mmφ steel bar was finished by hot rolling. Next, the steel bar thus obtained is hot forged and normalized, and then machined to produce a gear having an outer diameter of 87 mm, a module 3, 27 teeth and a tooth width of 20 mm, and carbonitriding these samples. gave. First, the sample is carbon potential: 0.9%, N potential: 2%,
After carbonitriding at a temperature of 930 ° C, No. 1 is 840 ° C above the Ar ₃ transformation point on the tooth surface and inside the tooth, and No. 2 is above the Ar ₁ transformation point inside the tooth and above the Ar ₃ transformation point on the surface. More than 75
0 ℃, No. 3 is 630 ℃, which is lower than the Ar ₃ transformation point of the tooth surface.
After furnace cooling to 30 minutes and holding for 30 minutes, each was cooled in an oil bath at 130 ° C. No. 4 to No. 11 are shown in Table 1 at temperatures above the Ar ₃ transformation point of the tooth surface (carbonitriding portion) and below the Ar ₁ transformation point of the inside of the tooth (non-carbonitriding portion), and No. 12 to No. .1
_No. 6 was gradually cooled to 600 ° C. below the Ar ₁ transformation point of the tooth surface, then reheated to 850 ° C. above the Ac ₃ transformation point of the tooth surface, and then Ar ₁ inside the tooth (non-carbonitrided portion). The tooth surface portion (carbonitrided portion) was kept at a temperature not lower than the Ar ₃ transformation point below the transformation point for 30 minutes, and then cooled in an oil bath at 130 ° C. Then, all samples were tempered at 180 ° C. for 1 hour. For the amount of strain, the tooth profile error was measured according to JIS B1702, and is also shown in Table 1 and Table 2.

[0025] No. hardened above the Ar ₃ transformation point inside the tooth
In No. 1, the inside of the tooth becomes martensite and the amount of tooth profile distortion is large,
Further, in No. 2 which is hardened above the Ar ₁ transformation point inside the tooth and above the Ar ₃ transformation point in the surface, the tooth interior has a ferrite-pearlite structure that contains some martensite, so the reduction of the tooth profile distortion is sufficient. is not. Further, in No. 3 hardened with less than Ar _{3 of the} tooth surface portion, the surface portion also has a bainite and pearlite structure, and the surface hardness required as a gear cannot be obtained.

Further, above the Ar ₃ transformation point of the tooth surface,
Moreover, although quenching is performed at a temperature below the Ar ₁ transformation point inside the tooth, No. 4 having a higher Mn than the range of the present invention has a low tooth profile distortion amount, but the tooth internal hardness is too high and the toughness is low. , Not suitable for practical use. Further, No. 5 in which V is too small compared to the range of the present invention has a small amount of tooth profile distortion, but the tooth internal hardness is insufficient and is not suitable for practical use.

On the other hand, No. 6 within the scope of the present invention
In all of ~ 16, the inside of the tooth has a ferrite-pearlite structure, the hardness inside the tooth is sufficient, the tooth profile distortion amount can be significantly reduced, and the fatigue strength is high due to the suppression of the internal oxide layer and the incompletely hardened layer. Is clear.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

As described above, according to the present invention, the heat treatment strain is reduced by forming the ferrite / pearlite structure inside the tooth of the automobile gear manufactured by carbonitriding and quenching, and at the same time, by controlling the structure of the surface layer. The present invention provides a high-strength automotive transmission gear with less noise and improved fatigue strength, and has an extremely remarkable industrial effect.

[Brief description of drawings]

FIG. 1A is an explanatory diagram of a tooth inside of a gear and FIG. 1B is an explanatory diagram of a gear core portion.

Claims (2)

(57) [Claims] 1. By weight%, C: 0.1% or more and less than 0.3%, Si: less than 0.15%, Mn: 0.30% or more 1.
Less than 50%, Cr: 0.30% or more and less than 1.00%, M
O: 0.10% or more and less than 1.00%, V: 0.05% or more and less than 1.00%, gears made of steel with the balance Fe and impurities were carbonitrided, and then Immediately or after cooling to a temperature range below the Ar ₁ transformation point of the tooth surface portion (carbonitriding portion) and heating again to above the Ac ₃ transformation point of the tooth surface portion (carbonitriding portion), the tooth surface portion (carburizing portion) The temperature inside the tooth above the Ar ₃ transformation point of the nitrided portion) (below the non-carbonitriding portion) below the Ar ₁ transformation point is maintained for 5 minutes or more and less than 2 hours to keep the tooth surface in an austenitic state and A carbonitriding heat treatment method for high-strength gears with small strains, characterized in that a fine ferrite-pearlite structure is formed inside the tooth by making a proper ferrite-pearlite, and then quenching and tempering. However, the inside of the tooth is the inside of the carburized layer of the tooth shown by the hatched portion in FIG. 2. In% by weight, C: 0.1% or more and less than 0.3%, Si: less than 0.15%, Mn: 0.30% or more 1.
Less than 50%, Cr: 0.30% or more and less than 1.00%, M
O: 0.10% or more and less than 1.00%, V: 0.05% or more and less than 1.00%, Nb: 0.02% or more and 0.20% or less, Ti: 0.01% or more and 0 Gears made of steel containing 20% or less of one or more and the balance of Fe and impurities are carbonitrided, and then immediately or once after the tooth surface (carbonitrided) Ar
_After cooling to a temperature range of ₁ transformation point or lower and heating again to the Ac ₃ transformation point of the tooth surface portion (carbonitriding portion) or higher, after the Ar ₃ transformation point of the tooth surface portion (carbonitriding portion) or higher, the inside of the tooth (non- (Carbonitriding part) Hold the temperature in the temperature range below the Ar ₁ transformation point for 5 minutes or more and less than 2 hours to form a fine ferrite pearlite inside the tooth while keeping the tooth surface in an austenite state, and then quench and temper. By this, a carbonitriding heat treatment method for a high-strength gear with small strain, characterized in that the inside of the tooth is made of fine ferrite pearlite. However, the inside of the tooth is the inside of the carburized layer of the tooth shown by the hatched portion in FIG.