JP3400934B2 - Nitriding steel and nitriding method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitriding steel and a nitriding method therefor. More specifically, when a nitriding treatment is performed, a thick hardened layer is formed on the surface and at the same time, the core is carburized. The present invention relates to a nitriding steel that has hardness almost equal to that of a material and is useful as a material for gears, and a method for nitriding the steel.

[0002]

2. Description of the Related Art Conventionally, as a steel material for gears, Cr steel,
Alloy steels for machine structures, such as Cr-Mo steel and Ni-Cr-Mo steel, are generally used, and this steel material is cut to produce a gear having a predetermined size and shape. And, for the gear that has been cut, the temperature is usually 850 to 900 ° C.
Carburizing and quenching is performed for 8 to 10 hours, and then tempering is performed. Due to such carburizing and quenching, a carburized layer is formed on the surface of the gear to have high hardness, and the core also has a Vickers hardness (HV) of 280 or more, which improves the strength of the gear as a whole. is there.

However, in the case of this carburizing and quenching,
Since the steel is rapidly cooled from the austenite region to undergo the martensitic transformation, volume expansion occurs. Since this volume expansion generally occurs non-uniformly, the accuracy of the tooth profile of the gear is likely to decrease, often causing a problem of gear meshing noise. On the other hand, as a surface hardening method for steel products after machining, in addition to the above-mentioned carburizing and quenching, a nitriding treatment has been conventionally known, and is actually applied to gears.

For this nitriding treatment, NH is applied to nitriding steel such as JIS G4202 which has a large nitriding depth.
Gas nitriding treatment utilizing decomposition of ₃ and salt bath nitriding treatment (tufftride treatment) using cyanide are generally performed. In any of these methods, N in the nascent stage is diffused and permeated from the surface of the steel material toward the core, and a nitride layer having a desired thickness is formed on the surface of the steel material to realize surface hardening.

When this nitriding treatment is applied to, for example, a gear, the temperature at which the nitriding treatment is performed is usually around 570 ° C., which is a temperature below the A ₁ transformation point. Since the phase transformation of steel does not occur, the tooth profile accuracy of gears does not decrease before and after the treatment, and it can be said that it is useful for eliminating meshing noise.

However, when nitriding is applied to gears, especially gears for automobile transmissions,
There are the following problems. The first problem is related to the nitride layer to be formed. For example, in the case of gas nitriding treatment, a nitride layer is formed as follows. First, 5
Place the steel in a nitriding furnace whose temperature is controlled around 70 ° C,
NH ₃ is introduced here. In the initial stage of nitriding, the introduced NH ₃ is on the surface of the steel material and has the following formula: NH ₃ → 3H +
The catalytic cracking reaction represented by N is caused to generate N at the nascent stage, and the N at the nascent stage generates a nucleus of a nitride on the surface of the steel material.

At this time, the atmosphere in the furnace is such that undecomposed residual N
It becomes a mixed atmosphere of H ₃ and H ₂ produced by the decomposition reaction. After that, the concentration of residual NH ₃ in the furnace was reduced to 2
The gas nitriding treatment is advanced while maintaining the content at 0% by volume or more, usually 30 to 40% by volume. On the other hand, the nucleus of the above-mentioned nitride absorbs N to become γ′-Fe ₄ N, and further ε-Fe _x N
(However, x = 2 to 3), and a porous compound layer having a high nitrogen concentration is formed on the surface of the steel material. Therefore, in this compound layer, N is in a state of being accumulated in a supersaturated state. The thickness of this compound layer gradually increases as the nitriding treatment progresses.

Then, N permeates through the above-mentioned porous compound layer and diffuses and permeates toward the core of the steel material to form a nitride-forming element such as Cr, V or Al contained in the steel material. Stable nitrides are generated between and to form a hardened layer. The rate of formation of the hardened layer is generally controlled by the rate of diffusion of N into the steel body, and between the thickness (d) and the treatment time (t), the following equation: d ² = kt (k is a constant It is known that the square law shown in () is established.

From the above, at the end of the nitriding treatment, the nitride layer formed on the surface of the steel material is the above-mentioned porous high nitrogen concentration compound layer, and the steel material It has a two-layer structure in which a hardened layer of high hardness made of a nitride formed between the element and the nitride forming element is located.
In that case, N is in a supersaturated state in the portion of the hardened layer located immediately below the compound layer.

However, in this nitrided layer, the compound layer has a low toughness, and has a property that it is damaged even when subjected to a slight stress and easily peels from the hardened layer. Therefore, when the processed gear is incorporated into the transmission, it may become a foreign substance during the operation of the transmission and adversely affect the function of the transmission, or the peeling point may be the starting point and the gear may crack. .

Further, since this compound layer is a porous layer, its specific surface area is remarkably larger than that of the steel body, and the compound layer grows toward the core while absorbing a large amount of supplied N. To go. Therefore, the N supplied during the nitriding process is consumed in the compound layer without being effectively used for forming the high hardness hardened layer to be originally formed. Therefore, the effective diffusion / permeation amount of N into the steel body is reduced, and the thickness of the formed hardened layer is reduced. That is, there is a tendency that the nitriding depth becomes shallow.

As described above, the nitriding treatment on the gear does not cause the deformation of the gear before and after the treatment, but the compound layer is formed on the surface of the nitride layer, so that the generation of foreign matter and the formation of the hardened layer are prevented. There is a problem that the thickness becomes thin.
Therefore, regarding the nitriding treatment of gears, it is required to suppress the formation of this compound layer as much as possible. For example, in the case of gears for automobile transmission, nitriding treatment capable of suppressing the thickness of the compound layer to 10 μm or less is strongly desired. ing.

The thickness of the above compound layer is reduced.
In order to do so, the following method has been conventionally performed. sand
That is, residual NH in the nitriding process₃20 volume
% From the beginning of nitriding
After a certain time, residual NH ₃Concentration below 10% by volume
Reduce its low concentration NH₃Continue nitriding treatment in the atmosphere
Is a two-step nitriding method.

According to this method, it is possible to suppress the growth of the compound layer over time. However, the residual N
Since the H ₃ concentration is low, there is a problem that the amount of N produced during the nascent stage is small and the thickness of the target hardened layer becomes thin. The second problem is that, in the case of ordinary nitrided steel, the thickness of the hardened layer becomes thin as described above,
The hardness of the core of steel does not become high, and the hardness of the core is HV28.
That is, the strength characteristics are inferior to the carburized material having a value of 0 or more. For example, the bending fatigue strength and pitting fatigue strength of the nitriding material are about 50% lower than those of the carburizing material. Therefore, it is difficult to use the nitrided gear as a substitute for the carburized gear.

Such a problem can be solved by using steel having an HV of about 280 as a raw material. However, in the case of nitriding the gear, considering that cutting of the material is performed before the nitriding, selecting a material with high hardness makes the cutting difficult,
After all, the manufacturing cost of the gear is increased. From this, for example, as a material for nitriding gears,
Before nitriding, there is a material that has a hardness that facilitates cutting, but after nitriding, a material whose core hardness is almost the same as the hardness of the carburized material. Will be required.

The third problem is that, for example, in the case of nitriding a transmission gear, the processing time required for the nitriding processing is limited to about 4 hours at the maximum in view of overall productivity and processing cost. However, it is not always possible to form a sufficiently hardened layer with such a treatment time. Of course, the thickness of the hardened layer can be increased by increasing the treatment time.

Therefore, in consideration of the nitriding treatment of the transmission gear, for example, the development of a steel material capable of forming a hardened layer as thick as possible within a limited treatment time of 4 hours at maximum and a nitriding treatment method thereof. Is required.

[0018]

The present invention solves the above-mentioned problems relating to the nitriding treatment of gears. First, it has good machinability before the nitriding treatment, for example, within a constraint time of 4 hours. Even if the nitriding treatment is performed, a thick hardened layer can be formed and at the same time the compound layer becomes much thinner than before and the hardness of the core is almost the same as that of the carburized material. An object of the present invention is to provide steel and a nitriding treatment method thereof.

[0019]

In order to achieve the above-mentioned object, the present inventors proceeded with the following consideration. First,
Regarding the suppression of the formation of the compound layer, focusing on the effectiveness of the two-step nitriding method described above, the following consideration was added. This 2
In the stepwise nitriding process, the residual N in the atmosphere during the nitriding
When the H ₃ concentration is lowered, the N potential is lowered and the diffusion and penetration of N on the surface of the steel material is suppressed. As a result, the rate of formation of the cured layer, the 2 remaining from law NH ₃ Until that was established on the premise atmosphere high residual NH ₃ concentration
When the concentration is low, the square law starts to be biased, and the formation rate of the hardened layer is significantly reduced. Then, there is a certain time difference between the reduction of the residual NH ₃ concentration and the occurrence of the square law eccentricity, and this time difference is high if the N concentration diffused in the steel at the initial stage of nitriding is high. The bigger it gets.

Therefore, when the residual NH ₃ concentration in the atmosphere is increased in the initial stage of nitriding to increase the amount of N permeating to the surface of the steel material, and then the state is switched to a state in which the residual NH ₃ concentration is low, the compound layer grows. Is suppressed,
The N accumulated at a high concentration on the surface of the compound layer and the hardened layer is thermally diffused toward the steel body, and contributes to the formation of the hardened layer. From the above, it is considered that the compound layer is thin in the case of the two-step nitriding method.

In this case, if the N production amount or the residual NH ₃ concentration is constant, the N permeation amount in the steel material direction is
The more active the surface of the steel material and the larger the contact area between the steel material and N or NH ₃ , the larger the increase. Based on the above consideration regarding the two-step nitriding treatment, the inventors of the present invention have studied the N on the surface of the steel at the initial stage of nitriding.
By increasing the concentration, the hardened layer has been made thicker, and the gas nitriding treatment method described later has been developed which can suppress the formation of the compound layer. Further, the present inventors have developed a precipitation hardening steel having the composition described below, whereby the precipitation hardening proceeds at the same time as the above-mentioned gas nitriding treatment, and as a result, the hardness of the core is almost equal to the hardness of the carburized material. We have also developed nitriding steel.

That is, the nitriding steel of the present invention has a C:
0.05 to 0.15% by weight, Si: 0.50% by weight or less,
Mn: 1.00 wt% or less, Cr: 1.00 to 2.00 wt%, Mo: 0.90 to 1.50 wt%, Al: 0.01
0 to 0.100% by weight, N: 0.0070 to 0.0200
% By weight, Ni: 1.00% by weight or less, V: 0.10
~ 0.30% by weight, Ti: 0.10% by weight or less, Nb:
Contains one or more than 0.030% by weight,
The balance is precipitation hardening type steel consisting of Fe and unavoidable impurities, and the Vickers hardness (HV) of the core becomes 30 or more higher than that before the gas nitriding treatment by the gas nitriding treatment at a temperature of 500 to 600 ° C. Characteristically, it is preferable that S: 0.005 to 0.100% by weight, Pb: 0.03 to 0.35% by weight, Ca: 0.001% by weight.
0-0.0100% by weight, Te: 0.001-0.100
%, Zr: 0.01 to 0.20% by weight of 1 or 2
This steel is designed to improve the machinability by containing at least one kind.

Further, according to the present invention, a steel having the above composition is hot-rolled or hot-forged at a finishing temperature of 900 to 1000 ° C. and then air-cooled to form a microstructure of ferrite and bainite, and at least a core portion is formed. HV hardness of 2
By setting the hardness to 50 or less, and then performing gas nitriding treatment at a temperature of 500 to 600 ° C., a hardened layer made of a nitride is formed on the surface of the nitriding steel, and at the same time, the HV hardness of the core is measured before the gas nitriding treatment. The method for nitriding steel for nitriding treatment, wherein the gas nitriding treatment is surface activation of the steel for nitriding treatment in a fluorine-containing atmosphere at a temperature of 300 to 450 ° C. Then, the atmosphere is switched to an ammonia-containing atmosphere, the temperature of the residual ammonia is maintained at 50 to 70% by volume, and the temperature is raised to 500 to 600 ° C. There is provided a nitriding treatment method for steel for nitriding treatment, which is a treatment for holding the nitriding ratio in% to advance nitriding.

[0024]

First, the nitriding steel of the present invention will be described. This steel is a precipitation hardening steel having the composition described above. In the above composition, C is a basic component necessary for ensuring the strength of the steel, and at the same time, carbides such as Cr, Mo, and V are generated during the nitriding treatment to increase the hardness of the core portion after the nitriding treatment. Is a component of. The higher the C content, the higher the strength of the steel, but if the content exceeds 0.15% by weight, the steel becomes too hard and the machinability deteriorates. When the content is less than 0.05% by weight,
The precipitation amount of carbides during the nitriding treatment is reduced, resulting in insufficient hardness of the core. Therefore, the content of C is set to 0.05 to 0.15% by weight.

Si functions as a deoxidizing agent, and an addition amount of 0.15% by weight or more is sufficient, but if it is more than 0.5% by weight, the toughness and workability of steel are impaired.
The content is set to 0.5% by weight or less. Mn contributes to the deoxidizing action during the melting of steel, is necessary for increasing the hardness of the steel and ensuring the strength, and is a component necessary for generating the bainite structure. However, if the content exceeds 1.00% by weight, the steel becomes too hard and the workability and the toughness decrease, so the content is 1.0.
It is set to 0% by weight or less.

Cr is a component for forming a hardened layer of high hardness nitride on the surface of steel at the time of nitriding treatment and at the same time forming C and carbide in the core to increase the hardness of the core. , Its content is set to 1.00 to 2.00% by weight. If the amount is less than 1.00% by weight, the amount of carbides generated is small and the hardness of the core cannot be increased.
If it exceeds 0% by weight, the cured layer tends to be thin, which is economically disadvantageous.

Mo is a component for forming carbides with C during nitriding to directly increase the hardness of the core, and its content is set to 0.90 to 1.50% by weight. If the amount is less than 0.90% by weight, the above effect cannot be obtained, and if the amount is more than 1.5% by weight, the above effect reaches saturation and particularly expensive Mo is added. Al is a component that functions as a deoxidizing agent and at the same time forms a nitride during the nitriding treatment and contributes to improving the hardness of the steel material surface, and the content thereof is set to 0.010 to 0.100% by weight. 0.
If it is less than 010% by weight, a sufficient deoxidizing effect cannot be obtained,
Further, if it exceeds 0.100% by weight, workability and toughness are deteriorated.

N combines with Al when the molten steel is solidified and forms Al.
It is a component that contributes to the improvement of toughness by refining the crystal grains by forming N, and the content is 0.0070 to 0.020.
It is set to 0% by weight. If it is less than 0.0070% by weight, the above-mentioned effects cannot be obtained, and it is 0.0200% by weight.
If it exceeds, blow holes and pin holes will be generated during solidification.

Ni is a component that contributes to improving the toughness of steel, but it is expensive and economically disadvantageous if it is added in a too large amount, so its content is set to 1.00% by weight or less. V is a component for increasing the hardness of the core by forming carbides during the nitriding treatment as in the case of Mo, while at the same time ensuring the depth of the hardened layer and ensuring the hardness of the steel material surface,
Its content is set to 0.10 to 0.30% by weight.
If it is less than 0.10% by weight, the above effect cannot be obtained, and if it exceeds 0.30% by weight, the toughness of the steel is lowered and the expensive V is wasted.

Ti is a component that exerts the effect of refining the crystal grains in the forged product to enhance the toughness when hot forging this steel to produce an intermediate product. However, if it is blended in a too large amount, large and hard non-metallic inclusions are generated and the workability and toughness of the steel are degraded, so the content is set to 0.10% by weight or less. Nb is also a component that exhibits the same effect as Ti. However, the content is 0.030
Even if it exceeds the weight%, the effect is saturated and the expensive Nb is added, so the content is set to 0.030 weight% or less.

The above-mentioned Ni, V, Ti, and Nb may be contained individually, or two or more kinds selected appropriately may be contained. The nitriding steel of the present invention requires the above composition, but further contains S, Pb, Ca, T.
One or more of e and Zr may be blended to enhance the machinability of steel.

However, if these components are blended in a too large amount, the toughness of the steel will be deteriorated.
0.005 to 0.100% by weight, Pb 0.03 to 0.35% by weight, and Ca 0.000 to 0.10% by weight.
0.0100% by weight, 0.001 to 0.10 for Te
Each content is set to 0% by weight, and in the case of Zr, each content is set to 0.01 to 0.20% by weight.

The nitriding steel of the present invention has a temperature of 1580 to
It can be manufactured by melting the alloy steel having the above composition at 1700 ° C. and casting the molten metal. And this steel is
The hardness of the core is HV2 by performing the hot working described later.
Since it is 50 or less, smooth cutting can be performed, and after that, a nitriding treatment described later forms a good hardened layer on the surface, and at the same time, the hardness of the core portion before nitriding treatment. Compared with this, it has a property that it becomes higher than HV by 30 or more and exhibits the same characteristics as the carburized material.

Next, of the above-mentioned nitriding steels, a steel having a C content of 0.05 to 0.10% by weight is selected and subjected to hot rolling or hot forging at a finishing temperature of 900 to 1000 ° C. After that, air cooling is performed under the condition of 10 to 30 ° C./min. As a result, the structure of the obtained steel becomes a structure mainly composed of soft ferrite and bainite, and the hardness thereof is HV250 or less, and the steel has good machinability.

Here, the finishing temperature during the hot working is 9
If it is lower than 00 ° C, hot workability deteriorates and cracks and defects start to occur, and if the finishing temperature is higher than 1000 ° C, austenite crystal grains become large and the dimensions and mechanical properties of parts after nitriding treatment are increased. In particular, since problems such as non-uniformity of impact properties increase, it is necessary to set the finishing temperature in the range of 900 to 1000 ° C.

The condition of air cooling is not particularly limited, and the steel hot worked at the finishing temperature may be naturally cooled in the atmosphere. The cooling rate at that time is
Usually, it is about 10 to 30 ° C./minute. After cutting the tempered steel into a desired shape,
Gas nitriding is performed.

That is, a gas nitriding treatment at a temperature of 500 to 600 ° C. is performed. Specifically, steel is set in a nitriding furnace, NH ₃ is introduced therein to raise the furnace temperature to 500 to 600 ° C., and then the temperature is maintained. At this time, the residual NH ₃ concentration may be set to 5 to 20% by volume. If the temperature at this time becomes lower than 500 ° C, NH
_Since the catalytic decomposition reaction of ₃ does not proceed sufficiently, the amount of N produced decreases and the formation rate of the hardened layer becomes very slow, and since the carbide does not precipitate, the hardness of the core does not increase, and 6
When the temperature is higher than 00 ° C, it becomes difficult to reduce the thickness of the compound layer, and the generated carbides form a solid solution, so that the hardness of the core portion cannot be improved based on the precipitation hardening of the carbides.

As a result, a hard nitriding layer is formed on the surface of the steel, and at the same time, the above-mentioned contents of Cr and M are contained in the above temperature range.
The carbides of o and V are precipitated and the hardening of the steel proceeds, and the hardness of the core becomes higher by 30 or more in HV than before the gas nitriding treatment. In that case, as described above, a compound layer having a high nitrogen concentration is formed on the surface layer of the nitride layer. Therefore, in order to reduce the thickness of this compound layer, it is preferable to perform the following treatment in the present invention. .

That is, after setting the steel in the nitriding furnace, first, a mixed gas of N ₂ and NF ₃ is introduced into the furnace to make the inside of the furnace a fluorine-containing atmosphere and maintain the temperature at 300 to 450 ° C. . At this time, the active fluorine removes the oxide film and various adsorption films coating the surface of the steel, and the S, P, Si, etc. adhering to the surface during the cutting process are removed by fluorine, The steel surface is activated. That is, since the inactive point for the permeation of N is removed,
The surface of the steel is in a state where it easily forms nitride nuclei.

At this time, if the treatment temperature is lower than 300 ° C., the above-mentioned activation of the steel surface does not proceed sufficiently,
On the other hand, if the temperature is higher than 450 ° C, the activation of the steel material surface proceeds well, but on the other hand, there is a problem that the furnace body and jigs used are severely damaged.
The temperature is set to 450 ° C, preferably 300 to 400 ° C.

Then, the atmosphere in the furnace is switched to the NH ₃ atmosphere, and at the same time, the temperature in the furnace is raised to 500 to 600 ° C. However, since the surface temperature of steel is low in the initial stage of temperature increase, the catalytic decomposition reaction of NH ₃ therein is slow and the amount of N produced is small. Therefore, the residual NH ₃ concentration is set to 5
By increasing the temperature while maintaining a very high concentration of 0 to 95% by volume, a treatment for increasing the N concentration in the nascent stage is taken.

Since the surface of the steel is activated by the fluorine activation treatment, the amount of N permeated into the surface is increased and a thick hardened layer is formed in a short time. Of course,
Also at this time, a compound layer having a high nitrogen concentration grows on the surface layer. Then, when the temperature in the furnace is raised to 500 to 600 ° C., the residual NH ₃ concentration is lowered to 5 to 20% by volume, and nitriding is advanced in that state.

Due to the drastic decrease of the residual NH ₃ concentration, the rate of formation of the hardened layer deviates from the square law up to that point and becomes significantly slower, and at the same time, N accumulated in the compound layer in a supersaturated state is the steel body. It is thermally diffused and participates in the formation of the hardened layer, and at the same time, further growth of the compound layer is suppressed and becomes relatively thin. In this way, a thick hardened layer was formed in the initial stage of nitriding due to the large amount of N permeation, and the supersaturated N of the compound layer was thermally diffused toward the core of the steel, and the growth of the surface layer was virtually stopped. It becomes a thin compound layer.

As described above, according to the nitriding method of the present invention, a nitride layer having a thin brittle compound layer and a thick hardened layer can be formed on the surface of steel. Since the precipitation hardening type steel that is age hardened at the temperature during the nitriding treatment is used, the hardness of the core portion after the nitriding treatment also becomes high.

[0045]

【Example】

1. Change in hardness of core portion Alloy steels having various compositions shown in Tables 1 and 2 were melted and then cast to produce an ingot having a diameter of 130 mm and a length of 230 mm.

[0046]

[Table 1]

[0047]

[Table 2]

Then, each ingot was hot forged at a temperature of 1250 ° C. or higher to a diameter of 30 mm, which was 10 to 20 mm.
After cooling at a cooling rate of ° C / min, a test piece having a diameter of 25 mm was prepared. Then, the test piece was set in a nitriding furnace and the temperature was set to 5
Maintaining the condition of 70 ° C and residual NH ₃ concentration of 40% by volume, 4
A gas nitriding process was performed for an hour. The HV hardness of the center of the obtained nitrided product was measured, and the results are shown in Table 3.

[0049]

[Table 3]

As is clear from Table 3, in each of the steels having the composition of the present invention, the hardness of the core is higher by 30 or more in the HV hardness by the nitriding treatment than before the nitriding treatment. 2. Effect of hot working Alloy steels having the compositions shown in Table 4 were melted and then cast to manufacture an ingot having a diameter of 130 mm and a length of 230 mm.

These ingots were heated to 1250 ° C., hot forged at a forging ratio of 18 and a finishing temperature of 1000 ° C., and then air-cooled to room temperature. The HV hardness of the center of the obtained forged product was measured, and the results are shown in Table 4. Then, each forged product was set in a nitriding furnace, and gas nitriding treatment was performed for 4 hours while maintaining the conditions of a temperature of 570 ° C. and a residual NH ₃ concentration of 40 vol%. The HV hardness of the center of the obtained nitrided product was measured, and the results are shown in Table 4.

[0052]

[Table 4]

From Table 4, the following is clear. (1) The steel of the present invention can be made to have a hardness of HV250 or less by performing hot forging under the exemplified conditions,
Allows cutting. (2) The HV of the core can be increased by 30 or more by performing the gas nitriding treatment under the conditions exemplified for the forged product.

3. Effect of Fluoride Activation Treatment Hot forging under the above-mentioned conditions was performed, and the obtained forged product was subjected to the following gas nitriding treatment. Process 1: The forged product is set in the nitriding furnace, and 10% N
Introducing a mixed gas of F ₃ and 90% N ₂ to increase the furnace temperature to 34
Hold at 0 ° C. for 0.5 hours.

[0055] Then, by switching the furnace atmosphere in NH ₃ atmosphere, then the temperature was raised to the furnace temperature while maintaining the residual NH ₃ concentration 85% by volume to 1.0 hours over 570 ° C., the residual NH ₃ concentration Was reduced to 8% by volume and the state was maintained for 4 hours. Treatment 2 (two-stage nitriding treatment): The forged product was set in a nitriding furnace, NH ₃ was introduced, and nitriding treatment was performed at a furnace temperature of 520 ° C. for 1 hour at a residual NH ₃ concentration of 30% by volume.
Nitriding treatment was performed at 70 ° C. for 3 hours at a residual NH ₃ concentration of 8% by volume.

Process 3: The forged product is set in a nitriding furnace and NH
₃ was introduced, and nitriding treatment was performed at a furnace temperature of 570 ° C. with a residual NH ₃ concentration of 40% by volume for 4 hours. Treatment 4 (conventional gas nitriding treatment): The forged product is set in a nitriding furnace, NH ₃ is introduced, and residual NH _{3 is} maintained at a furnace temperature of 570 ° C.
Nitriding was performed for 4 hours at a concentration of 20% by volume.

For each material obtained by each nitriding treatment,
The thickness (μm) of the compound layer and the thickness of the cured layer were measured according to the following specifications. Measurement of compound layer thickness: Each material was embedded in Bakelite resin, buffed with emery paper # 100 and 0.3 μm aluminum powder, then corroded with picric acid alcohol, and then photographed with an optical microscope (600 times magnification). Take a photograph and measure the thickness of the compound layer.

Measurement of the thickness of the hardened layer: The hardness from the surface to the cross section was measured with a micro Vickers hardness meter by applying a load of 200 g, and the depth to the position where the hardness was 10% higher than the hardness of the core was measured. The above results are shown in Table 5.

[0059]

[Table 5]

As is clear from Table 5, when the steel surface is heat-treated in a fluorine-containing atmosphere prior to the gas nitriding treatment,
The brittle compound layer can be significantly thinned while the thickness of the hardened layer is ensured.

[0061]

As is apparent from the above description, claim 1
Steel is a precipitation hardening type steel that age-hardens at a temperature of 500 to 600 ° C., so that during gas nitriding treatment, phase transformation does not occur and brittleness located on the surface layer of the nitrided layer facilitates exfoliation.
The compound layer is made much thinner than before and the surface is hardened, and at the same time, the HV of the core is increased by 30 or more. Since the steel according to claim 2 further contains a free-cutting element, it has excellent machinability.

According to the third aspect, the hot workability prior to the gas nitriding treatment can make the hardness of the steel HV250 or less and impart the machinability. Then, after the gas nitriding treatment, the hardness of the core can be increased by 30 or more by HV. According to the gas nitriding treatment of the fourth aspect, it is possible to significantly reduce the formation of the brittle and easily peelable compound layer located on the surface layer of the nitrided layer.

As described above, the nitriding steel and the nitriding method thereof according to the present invention can provide a nitrided gear having a high hardness in the core portion and a high strength as a whole, and particularly for manufacturing a gear for an automobile transmission. Its industrial value is extremely large.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tatsuo Fukuzumi 12 Nakamachi, Muroran-shi, Hokkaido Mitsubishi Steel Muroran Special Steel Co., Ltd. Muroran Works (72) Inventor Kenzo Kitano 1-8 Nakahama-cho, Amagasaki-shi Daigohoku San Amagasaki Plant (72) Inventor Naohisa Shimada 1-8 Nakahama-cho, Amagasaki City, Hyogo Daido Hokusan Amagasaki Plant (56) Reference JP-A-55-6456 (JP, A) Hei 7-157842 (JP, A) JP 8-193256 (JP, A) JP 3-1933861 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38 / 00-38/60 C23C 8/26

Claims (5)

(57) [Claims] 1. C: 0.05 to 0.15% by weight, Si:
0.50 wt% or less, Mn: 1.00 wt% or less, Cr:
1.00 to 2.00% by weight, Mo: 0.90 to 1.50% by weight, Al: 0.010 to 0.100% by weight, N: 0.00
70 to 0.0200% by weight, and further Ni: 1.00% by weight
Hereinafter, one or two of V: 0.10 to 0.30% by weight, Ti: 0.10% by weight or less, and Nb: 0.030% by weight or less.
It is a precipitation hardening type steel containing at least one kind and the balance consisting of Fe and unavoidable impurities, and the Vickers hardness (HV) of the core is made to be the hardness before the gas nitriding treatment by the gas nitriding treatment at a temperature of 500 to 600 ° C. comprising a higher becomes properties 30 more than, or
The compound layer formed on the hardened layer by nitriding is
Steel for nitriding treatment, which is 10 μm or less . 2. Further, S: 0.005 to 0.100% by weight, Pb: 0.03 to 0.35% by weight, Ca: 0.001
0-0.0100% by weight, Te: 0.001-0.100
%, Zr: 0.01 to 0.20% by weight of 1 or 2
The nitriding steel according to claim 1, containing at least one kind. 3. C: 0.05 to 0.15% by weight, Si:
0.50 wt% or less, Mn: 1.00 wt% or less, Cr:
1.00 to 2.00% by weight, Mo: 0.90 to 1.50% by weight, Al: 0.010 to 0.100% by weight, N: 0.00
70 to 0.0200% by weight, and further Ni: 1.00% by weight
Hereinafter, one or two of V: 0.10 to 0.30% by weight, Ti: 0.10% by weight or less, and Nb: 0.030% by weight or less.
A steel containing at least one species and the balance consisting of Fe and unavoidable impurities is hot-rolled or hot-forged at a finishing temperature of 900 to 1000 ° C. and then air-cooled to form a microstructure of ferrite and bainite. The Vickers hardness of the core is set to 250 or less, and then the temperature is set to 500 to 6
By performing a gas nitriding treatment at 00 ° C., a hardened layer made of a nitride is formed on the surface of the nitriding steel, and at the same time, the Vickers hardness (HV) of the core is 30 or more than the hardness before the gas nitriding treatment. Higher and hard by gas nitriding
This <br/> a nitriding method nitriding steel according to claim to a compound layer 10μm or less formed on the layer. 4. The gas nitriding treatment is performed at a temperature of 300 to 45.
The surface activation treatment of the nitriding steel is performed in a fluorine-containing atmosphere at 0 ° C., and then the atmosphere is switched to an ammonia-containing atmosphere to maintain the residual ammonia concentration at 50 to 95% by volume at a temperature of 500 to 600. ℃ temperature was raised to a, then by advancing the nitride retains the residual ammonia concentration in 5-20% by volume, on the cured layer by nitriding
4. The compound layer formed on the substrate is 10 μm or less.
For nitriding steel for nitriding. 5. The steel further comprises S: 0.005 to 0.005.
100% by weight, Pb: 0.03 to 0.35% by weight, Ca:
0.0010-0.0100% by weight, Te: 0.001-
0.10 wt%, Zr: 0.01 to 0.20 wt% 1
The method for nitriding steel for nitriding according to claim 4, wherein the nitriding steel contains two or more kinds.