JP2002030338A - High concentration carburizing and quenching method of steel, and high concentration carburizing and quenching parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high concentration carburizing and quenching method of steel for obtaining the excellent mechanical properties such as the hardness of a surface part, ware resistance, fatigue resistance, large softening resistance by a simple treatment process at lower carburizing temperatures, and easily suppressing generation of soot, and a high concentration carburizing hardened parts appropriate for, in particular, car parts. SOLUTION: This high concentration carburizing and quenching method includes a carbo-nitriding step for carbo-nitriding steel at the temperature of 800-880 deg.C, and a hardening step for successively hardening the steel at the temperature higher than the carbo-nitriding temperature. In the carbo-nitriding step, network carbide is solid-solved in the austenite by setting, for example, the concentration of carbon (C) of the carburizing atmosphere to be 0.7-1.2%, and adding 3-8% ammonium gas (NH3), and when the austenite is transformed to the martensite by the hardening, the solid-solved carbide is re-precipitated, and granularly distributed in the martensite in a substantially uniform manner.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to general mechanical parts and driving force transmitting parts which require contact fatigue strength and abrasion resistance of gears, bearings, cam components, etc., and in particular, hardness due to frictional heat of rotation and sliding. The present invention relates to a high-concentration carburizing and quenching method suitable for automobile parts and the like accompanied by a decrease, and a high-concentration carburizing and quenching part to which this method is applied.

[0002]

2. Description of the Related Art Generally, in mechanical parts, it is often necessary to increase the surface hardness of the parts to provide wear resistance and to keep the inside of the parts relatively soft to maintain toughness. For this purpose, the steel is often subjected to carburizing quenching or carbonitriding quenching. The steel used has a carbon content of 0.2
% Of carbon steel and alloy steel, such as carbon case hardened steel, alloy case hardened steel, steel for machine structural use, and steel for mechanical mechanical structure. Chromium steel, chromium molybdenum steel, nickel chromium molybdenum steel and the like are typical materials.

[0003] Incidentally, the carburizing and quenching method includes A
There are two methods: a normal carburizing method performed in a carburizing atmosphere with a carbon concentration equal to or less than the cm transformation, and a method in which the carbon concentration in the atmosphere is increased to a high carbon concentration equal to or higher than the intersection point of the Acm line corresponding to the processing temperature to generate carbide. In such a case, the latter high-carbon carburizing method (or high-concentration carburizing method) is often adopted. In the case of the high-concentration carburizing method, carbides are precipitated in a network along grain boundaries. The material on which the carbides are precipitated has higher hardness and higher wear resistance than the ordinary carburizing method, but has a disadvantage in that the material strength is reduced.

[0004] Therefore, it has been generally practiced to granulate the reticulated carbide by utilizing the A ₁ transformation. That is, from the carburizing temperature (normally 900 ° C. or higher), A ₁
Lowered to below the transformation temperature or for a long time kept at a temperature near the A ₁ transformation temperature, and granulated reticulated carbide, then, it is subjected to hardening treatment by heating again.

For example, US Pat.
No. 5,610) describes the following method as a method for producing a surface-hardened part suitable for automobile parts, and particularly excellent in bending fatigue strength. That is, for example, after carburizing at 930 ° C., gradually cooling at a rate of, for example, 20 ° C./min, then heating and maintaining at a temperature of 800 ° C., temporarily lowering to 750 ° C., and then quenching, A production method in which a surface layer has a mixed structure mainly composed of martensite and retained austenite is described.

[0006] Further, US Pat.
No. 3,749) discloses a method of hardening the surface of a rolling bearing component by carburizing at 970 ° C., cooling and holding at about 300 ° C., and quenching at 805 ° C.
A manufacturing method in which annealing is performed at 610 ° C. before quenching at 805 ° C. is described.

Further, Japanese Patent Application Laid-Open No. 11-117059 discloses a steel component for preventing the precipitation of cementite during high-temperature carburization at 980 ° C. or higher, and a carburizing method thereof. In the prior art section of
The following method is described as a recent carburizing method.

That is, in recent years, RX gas carburization using RX gas (endothermic modified gas) as a carrier gas and butane gas or the like as an enriched gas has been carried out in the range of 950 to 1000.
A high-temperature carburizing method performed at a high temperature of ° C. and a vacuum carburizing method of carburizing and diffusing in a reducing atmosphere in which hydrocarbon gas is decomposed under reduced pressure are also performed.

Further, as a special carburizing method, a method has been proposed in which a carburizing cycle is repeated twice or more, and at least one of the cycles is carburized in an atmosphere having a carbon concentration of Acm or more.

[0010] In addition to the above-mentioned carburizing method, there has been proposed a method of performing high-concentration carburizing using steel having a special composition in order to make it difficult to form a network-like carbide.

[0011]

However, the conventional high-concentration carburizing method and the high-concentration carburized quenched part have the following problems. 1) As described above, in the case of the high-concentration carburizing method, it is necessary to suppress the reduction in the strength of the material by eliminating the network-like carbide, but the carburizing temperature is high and the treatment process is complicated. In the case of the high-concentration carburizing method, residual austenite is liable to be generated, so that the hardness tends to be low, and there is a problem that the wear resistance is lowered. 2) In the case of the high-concentration carburizing method, it is necessary to increase the carbon concentration of the atmosphere as described above. In this case, soot is generated in the treatment process, and a so-called sooting problem is likely to occur. When soot adheres to the furnace material of the processing equipment, there is a problem in the life of the furnace material, and when it adheres to the processing member, there is a problem in that the glitter of the component is impaired. 3) In addition, as a high-concentration carburized and quenched part, a high surface partial hardness, abrasion resistance, and the like can be obtained by using a simplified treatment method without using a steel having a special composition as described above.
Those that have obtained fatigue resistance and the like are required. In particular, in automotive parts and the like that involve sliding at high temperatures,
Not only high hardness but also high softening resistance, that is, a component in which the surface hardness of the component at a high temperature does not lower than the surface hardness of the component at a normal temperature is required.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object of the present invention is to obtain a desired mechanical property by a simple carburizing temperature and a simple processing step as compared with the prior art, and It provides a high concentration carburizing and quenching method for steel that does not cause soot generation problems, and has excellent mechanical properties such as surface partial hardness, wear resistance, fatigue resistance, and large softening resistance.
In particular, it is an object of the present invention to provide a high-concentration carburized and quenched part suitable as an automobile part.

[0013]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a high-concentration carburizing and quenching method in which a steel material is carbonitrided at a temperature of 800 to 880 ° C. A quenching step of quenching the steel material at a temperature higher than the carbonitriding temperature (the invention of claim 1).

The invention of claim 1 can be said to be the following high-concentration carburizing and quenching method based on the action on the iron-carbon system equilibrium diagram. That is, in the method according to claim 1, the carbonitriding step is a step of depositing carbides at a temperature and a carbon concentration higher than the Acm transformation, and the quenching step is a network-like precipitation precipitated along grain boundaries. Only carbide is dissolved in austenite, leaving carbide precipitated at boundaries of crystal grains, and when austenite is transformed into martensite by quenching, the solid-dissolved carbide is re-precipitated to be substantially uniform in martensite. (The invention of claim 2).

Further, as an embodiment of the invention of claim 1 or 2, in the method of claim 1 or 2, the carbonitriding step comprises the step of reducing the carbon (C) concentration of the carburizing atmosphere to 0.7 to 1.0. The process is performed by adding 2 to 8% and adding ammonia gas (NH ₃ ) at 3 to 8% (the invention of claim 3).

That is, in the above method, the carburizing temperature is
The temperature and the carbon concentration are set higher than the Acm transformation and lower than the quenching temperature, and carbonitriding is performed at 800 to 880 ° C. lower than the conventional method to precipitate carbides. For example, it is quenched by heating at a temperature higher by 20 to 40 ° C. In the process of heating to the quenching temperature, only the network-like carbides precipitated along the grain boundaries are dissolved in austenite, leaving the carbides precipitated at the boundaries of the crystal grains, and re-precipitating the dissolved carbides. In addition, it can be distributed almost uniformly and granularly in martensite as a quenched structure.

In the above method, by adding ammonia gas to the atmosphere, the A ₁ transformation point and the Acm transformation line move to the lower left in the iron-carbon system equilibrium diagram.
Therefore, carbides can be precipitated by setting the carbon concentration of the atmosphere to the concentration of ordinary carburizing treatment, and the treatment temperature can be lowered.
Therefore, the generation of soot is also easily suppressed.

According to the high concentration carburizing and quenching method for steel according to the first to third aspects of the present invention, the carburizing temperature is lower and the treatment process is simplified as compared with the conventional method. It is possible to provide a high-concentration carburized and quenched part having excellent mechanical properties such as wear resistance, fatigue resistance, and large softening resistance. As the target steel material, the above-described carbon case hardened steel, alloy case hardened steel, steel for machine structural use, steel for alloy machine structural use, and the like can be used.

As an embodiment of the first to third aspects of the present invention, the fourth aspect of the present invention is more preferable. That is, in the method according to any one of claims 1 to 3, after the quenching process, a sub-zero process and a tempering process are included. The sub-zero treatment step is, for example, a step of performing a deep cooling treatment at −60 ° C., and is suitable for transforming retained austenite into martensite and improving hardness and wear resistance, as is well known. Further, the tempering treatment step is suitable for tempering at, for example, about 200 ° C. to partially precipitate carbides, nitrides, and the like, and to partially remove and adjust macroscopic internal stress caused by quenching. is there.

Further, in order to prevent soot from adhering to the surface of the processing component, the invention of claim 4 is preferable. That is,
In the method according to the third aspect, the C concentration and the NH ₃ addition amount are controlled so that soot does not adhere to the surface of the treated steel material.

Next, as the invention relating to the high concentration carburized and quenched parts, the inventions of claims 6 to 8 are preferable. First,
According to the invention of claim 6, in a high-concentration carburized and quenched part such as case hardened steel or steel for machine structural use, the carburized layer has a structure in which granular carbides are substantially uniformly dispersed in martensite, and The member hardness H _V (300 g) from the component surface to a depth of at least 0.5 mm is 550 or more. This makes it possible to provide a component having high surface partial hardness, wear resistance, fatigue resistance, and the like without using steel having a special composition.

According to a seventh aspect of the present invention, in the sixth aspect, the case hardening steel is a chromium molybdenum steel, and has a component surface hardness of at least 200 ° C. and a component surface hardness at room temperature. Shall be equivalent. This makes it possible to provide a chromium molybdenum steel component that requires a large softening resistance and is suitable for an automobile component or the like that slides or rotates with frictional heat.

According to an eighth aspect of the present invention, in the sixth or seventh aspect, the high-concentration carburized and quenched part comprises a pinion shaft, a governor shaft, a fluid bearing shaft, a valve seat, a gear sleeve, Automobile parts such as turbocharger-related parts. In particular, the pinion shaft is required to have no decrease in surface hardness at around 220 ° C. and to be improved in wear resistance and seizure resistance. Conventionally, the surface of a component is subjected to electroless Ni plating. The governor shaft is hard chrome plated. By applying the high-concentration carburized and quenched parts of the present invention, it is possible to omit the implementation of these platings, and the technical and commercial value of the present invention is great.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

(Embodiment 1) FIG. 1 shows Embodiment 1 of the present invention.
The high-concentration carburizing and quenching process related to the above is shown. In the figure, the member to be treated is chromium molybdenum steel SCM415.
(JIS G 4105), the dimensions are 18mm in diameter and 100mm in length
And As shown in FIG. 1, in the first carbonitriding process, the carbonitriding temperature was 830 ° C., the carbon concentration in the atmosphere was 0.9%, the ammonia gas was added at 5%, and the treatment time was 240 minutes. Thereafter, the mixture is heated to 860 ° C., maintained at that temperature for 5 minutes, quenched with quenching oil at 60 ° C., further quenched, subjected to a subzero treatment at −60 ° C., returned to room temperature, and then cooled to 170 ° C. for 120 minutes. Tempering for minutes. In this example, the rate of temperature rise when heating to 860 ° C. was 20 ° C./10 minutes. In order to make only the network-like carbides precipitated along the grain boundaries form a solid solution in austenite, the temperature rising rate is increased to 860.
It is better to extend the holding time at ° C.

The surface hardness of the member processed by the above, in 960 H _V (300 g), very high hardness was obtained. According to the normal carburizing and quenching method, generally, H _V (300 g)
Is about 720 to 780. The surface hardness distribution and the metal structure of the carburized layer will be described later together with different examples.

(Embodiment 2) FIG. 2 shows Embodiment 2 of the present invention.
The high-concentration carburizing and quenching process related to the above is shown. In the figure, the member to be treated is chrome molybdenum steel SCM435.
(JIS G 4105), the dimensions are 20mm in diameter and 100mm in length
And As shown in FIG. 2, in the first carbonitriding process, the carbonitriding temperature was 820 ° C., the carbon concentration of the atmosphere was 0.8%, ammonia gas was added at 5%, and the treatment time was 120 minutes. Then, it heated to 840 degreeC and hold | maintained at that temperature for 1 minute, and performed the same process as Example 1 after that. The temperature rise rate up to 840 ° C. was the same as in Example 1. The surface hardness of the member treated as described above is
In 940 H _V (300g), it is likewise very high hardness as in Example 1 were obtained.

(Embodiment 3) FIG. 3 shows the measurement results of the surface hardness and the hardness distribution in the depth direction of the treated member according to Embodiment 3. 6 and 7 show micrographs of the surface structure and the cross-sectional structure of the metal structure of the carburized layer according to Example 3. The member to be processed in the third embodiment is a chrome molybdenum steel SCM4 as the member for the pinion shaft.
18 (JISG 4105). The high concentration carburizing and quenching process was the same as in Example 1.

As shown in FIG. 3, the surface hardness was extremely high at 980, and even at a depth of 0.5 mm from the surface, a component having a high hardness of 620 was obtained.

According to the micrographs of the surface structure and the cross-sectional structure shown in FIGS. 6 and 7, there was no reticulated carbide.
It can be seen that fine granular carbides (white portions in the photograph) are precipitated in the martensite, and that the granular carbides are distributed from the surface to deep portions. In addition,
In the photograph of FIG. 7, four diamond-shaped black portions are diamond indentations during hardness measurement.

The pinion shaft member according to this embodiment has high abrasion resistance and seizure resistance due to the high hardness and carbide distribution as described above, and the surface hardness decreases as the temperature of the member increases. It did not occur up to 220 ° C., and it was confirmed that it had a large softening resistance.

Example 4 FIG. 4 shows the measurement results of the surface hardness and the hardness distribution in the depth direction of the treated member according to Example 4. 8 and 9 show micrographs of the surface structure and the cross-sectional structure of the metal structure of the carburized layer according to Example 4. The member to be treated in Example 4 was chrome molybdenum steel SCM435 (JIS G 4105). The high concentration carburizing and quenching process was the same as in Example 2.

As shown in FIG. 4, the surface hardness was as high as 940, and even at a depth of 0.5 mm from the surface, a part having a hardness as high as 600 was obtained. further,
The hardness is 580 even at a depth of 1.0 mm from the surface.
And showed high hardness.

According to the micrographs of the surface structure and the sectional structure shown in FIGS. 8 and 9, as in Example 3, no network-like carbide was present, and fine granular carbide was precipitated in martensite. It can also be seen that it is distributed from the surface to a deep position.

Example 5 FIG. 5 shows the measurement results of the coefficient of friction for evaluating the wear resistance and seizure resistance of the treated member according to Example 5. The test piece in this example is a chromium molybdenum steel SC as a member for a pinion shaft.
M415 (JIS G 4105).

In FIG. 5, the vertical axis represents the friction coefficient, and the horizontal axis represents the test time (second). In the figure, the measurement result indicated by A indicates that the same high-concentration carburizing and quenching treatment process as in Example 1 was performed as a surface treatment on a substrate obtained by subjecting SCM415 to conventional ordinary carburizing and tempering. Shows the measurement results, the measurement results shown by B, without the surface treatment,
That is, Table 1 below shows the measurement results of the untreated products.

FIG. 5 shows the results measured by a Falex-type abrasion tester. Table 1 below shows samples such as fixed test pieces and rotating test pieces and test conditions.

[0038]

[Table 1]

As is clear from the results shown in FIG. 5, the rotating test piece of A subjected to the high-concentration carburizing and quenching treatment of the present invention is:
Compared with B, the coefficient of friction is considerably smaller, and it can be seen that the wear resistance and seizure resistance of rotating members such as a pinion shaft member and a fluid bearing shaft are improved by applying the present invention.

The five embodiments have been described above.
Various changes in the processing steps are possible within the scope of this technical idea. In particular, the cooling temperature in quenching, the processing temperature in sub-zero processing and tempering, the processing time, the processing atmosphere, and the like are not limited to the above embodiments.

[0041]

As described above, according to the present invention, in the high-concentration carburizing and quenching method, a carbonitriding step of carbonitriding steel at a temperature of 800 to 880 ° C. And a quenching treatment step of quenching at a high temperature. In the carbonitriding treatment step, the carbon (C) concentration of the carburizing atmosphere is set to 0.7 to 1.2%, and the ammonia gas (NH ₃ ) is set to 3 to 8%. %, The granular carbide can be distributed almost uniformly in the martensite from the surface to the deep part by a simple treatment step with a lower carburizing temperature than in the prior art.

As a result, it has excellent mechanical properties such as surface hardness, abrasion resistance, fatigue resistance, and large softening resistance, and has the above-mentioned general mechanical structural parts and general driving force transmitting parts, especially automobile parts. It is possible to provide a high-concentration carburized and quenched part suitable as the above. Further, the high-concentration carburized and quenched part of the present invention is a high-speed steel (high-speed steel) applied as a tool steel because of its stable hardness at a high temperature of its member hardness.
It can also be expected to be used as an alternative material to

Further, it is easy to adjust the size and distribution of the granular carbide in the high-concentration carburizing and quenching, and furthermore, to adjust to the desired mechanical properties, depending on the processing conditions, and also to suppress the generation of soot on members. Easy to control.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a high-concentration carburizing and quenching process according to the present invention.

FIG. 2 is a diagram showing an example of a process different from FIG. 1 according to the present invention;

FIG. 3 is a diagram showing a measurement result of a surface hardness and a hardness distribution in a depth direction of a processing member according to Example 3.

FIG. 4 is a view showing measurement results of a surface hardness and a hardness distribution in a depth direction of a processing member according to Example 4.

FIG. 5 is a view showing a measurement result of a friction coefficient of a processing member according to the fifth embodiment.

FIG. 6 is a micrograph of the surface structure of the carburized layer metal structure of the treated member according to Example 3.

FIG. 7 is a micrograph of a cross-sectional structure of a metal structure of a carburized layer of a processing member according to Example 3.

FIG. 8 is a micrograph of the surface structure of the metal structure of the carburized layer of the treated member according to Example 4.

FIG. 9 is a micrograph of a cross-sectional structure of the metal structure of the carburized layer of the processing member according to Example 4.

[Explanation of symbols]

 None

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Claims (8)

[Claims] 1. A steel material comprising: a carbonitriding step of carbonitriding a steel material at a temperature of 800 to 880 ° C .; and a quenching step of subsequently quenching the steel material at a temperature higher than the carbonitriding temperature. High concentration carburizing and quenching method. 2. The method according to claim 1, wherein the carbonitriding step is a step of depositing carbide at a temperature and a carbon concentration higher than the Acm transformation, and the quenching step is performed along grain boundaries. Only the network-like carbide is solid-dissolved in austenite, leaving the carbide precipitated at the boundaries of the crystal grains, and when the austenite is transformed into martensite by quenching, the solid-dissolved carbide is re-precipitated to form a precipitate in the martensite. A highly concentrated carburizing and quenching method for a steel material, wherein the method is a step of substantially uniformly distributing the particles in a granular manner. 3. The method according to claim 1, wherein
In the carbonitriding process, the carbon (C) concentration in the carburizing atmosphere is reduced.
0.7-1.2%, ammonia gas (NH _Three) To 3
A steel characterized in that it is a step of treating by adding up to 8%.
High concentration carburizing and quenching of materials. 4. The method according to claim 1, further comprising a sub-zero treatment step and a tempering treatment step after the quenching treatment step. . 5. The method according to claim 3, wherein the C concentration and NH are set so that soot does not adhere to the surface of the treated steel material.
₃ High concentration carburizing and quenching of steel characterized by controlling the amount of addition. 6. In a high-concentration carburized and quenched part such as case-hardened steel or steel for machine structural use, the carburized layer has a structure in which granular carbides are substantially uniformly dispersed in martensite, and the carburized layer is formed from the surface of the part. Member hardness H _V (300
g) is not less than 550, a high-concentration carburized and hardened part made of steel. 7. The steel material according to claim 6, wherein the case hardening steel is a chromium molybdenum steel, and the surface hardness of the component is at least 200 ° C. and is equal to the surface hardness of the component at room temperature. High concentration carburized parts. 8. The high-concentration carburized part according to claim 6, wherein the high-concentration carburized part is an automobile part such as a pinion shaft, a governor shaft, a fluid bearing shaft, a valve seat, a gear sleeve, and a turbocharger-related part. High-concentration carburized and hardened parts of steel, characterized by the following: