JPH08176791A - Method for carburizing steel - Google Patents

Abstract

PURPOSE: To establish a method for carburizing steel capable of obtaining a high-quality carburized material free from the occurrence of abnormally carburized layers, abnormally grown and coarsened austenite grains, treatment strains, etc., by a short-time treatment on an industrial scale with a good workability. CONSTITUTION: The steel to be treated is tempered prior to the carburization treatment to form the structure mainly composed of one or >=2 kinds of 'martensite structure', 'bainite structure', 'spheroidal carbide structure' or 'ferrite structure finely dispersed with carbonitride'. The steel is thereafter carburized at a temp. below the Ae3 point (more preferably right under the Ae1 point) by a plasma carburization method. The simultaneous execution of nitriding and carburizing is possible as well by simultaneously penetrating and diffusion nitrogen at this plasma carburization period. The steel may be subjected to a nitriding treatment, reheating hardening and tempering treatment, high-frequency hardening, etc., after the plasma carburization.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-concentration carburizing method for steel, which allows a carburizing material of good quality to be obtained in a short time with good workability.

[0002]

2. Description of the Related Art At present, the surface hardening method by carburization is widely used as a means for improving the performance (various fatigue strength, wear resistance, etc.) and reliability of machine structural steel. Most of the commonly used carburizing means are “gas carburizing”, and in special cases, a salt bath may be used.

In any case, since carburization is based on a mechanism in which carbon atoms generated by the reduction reaction of CO or CN on the steel surface permeate into the steel, the carburization reaction is necessarily accompanied by the reduction reaction. . That is, CO or C by steel
Carburization proceeds by the reduction of N, in other words, the oxidation of steel. Therefore, the carburizing treatment of steel inevitably causes oxidation of the steel, and this oxidation causes the grain boundary oxidation on the steel surface and the accompanying incomplete quenching structure near the grain boundaries, that is, an "abnormal carburizing layer". Since these abnormal carburizing layers cause deterioration of the characteristics of the carburizing layer, methods for preventing this have been developed from both aspects of steel type components and carburizing process. In addition,
Typical of new methods related to carburizing process are "vacuum carburizing method" and "plasma carburizing method" (see Japanese Patent Laid-Open No. 2-145759, which can directly infiltrate carbon atoms without reduction reaction). ) ”, But these methods are suddenly attracting attention because they do not generate an abnormal carburizing layer.

On the other hand, in the current carburizing process (gas carburizing, etc.), the condition of "3 to 5 hours at 920 to 930 ° C" is generally adopted. Is being considered as a serious problem.
However, since the conventional carburizing process is based on "a mechanism for generating carbon atoms by a reduction reaction and diffusing and permeating the carbon atoms from the surface of the steel material to the inside by thermal diffusion" as described above,
By increasing the carburizing temperature, the processing time can be greatly shortened. However, the increase in carburizing temperature is
Abnormal growth and coarsening of Stenite grains "," Reducing the life of carburizing furnace "
Or, it is difficult to carry out because it introduces a new problem such as “distortion during carburization”, and it is the current situation that it is eager to achieve a temperature rise of 20 to 30 ° C. up to about 950 ° C. at most. .

That is, even in the present carburizing treatment at a treating temperature of 920 to 930 ° C., austenite crystal grains grow abnormally during the carburizing treatment depending on the material of the steel material and the previous history of the parts to be treated. The fact is that the problem of coarsening and significantly deteriorating the characteristics of parts has not been completely overcome. Therefore, the above-mentioned "increasing the carburizing temperature for shortening the carburizing time" remarkably promotes this abnormal grain growth, so that normal carburizing, particularly in a temperature range of 950 ° C or higher, is even desperate. Moreover, when the carburizing temperature is raised to about 950 ° C., the life of the furnace is remarkably shortened, and it is difficult to cause an excessive burden of equipment costs.

Further, it is usual that the steel material after the carburizing treatment is quenched as it is, but the carburizing temperature is 93
If the temperature is raised to over 0 ° C, the thermal strain during quenching after that,
Deformation distortion is large, which has a fatal adverse effect on the dimensional accuracy of parts. Of course, much effort has been paid in terms of heat treatment technology in order to minimize the dimensional deviation due to this heat treatment strain, but since the problem of this strain can be greatly improved without difficulty when the heat treatment temperature decreases, recently The trend is toward lowering the heat treatment temperature and improving quality.

However, in the gas carburizing method, the treatment temperature is 7
If the temperature is lowered to around 50 ° C, the balance between the generation of C due to the decomposition of gas on the steel surface and the permeation and diffusion of C will be disrupted, and the permeation and diffusion of carbon atoms into the steel will be delayed. It will remain inside. This soot causes an explosive combustion when air is mixed into the furnace, leading to a risk of causing an explosion in the carburized portion of the furnace. Moreover, at such a low temperature, the decomposition of gas and the permeation and diffusion of C are significantly delayed, which causes a problem that the time for carburizing treatment is significantly increased. Is.

The same applies to the case of the vacuum carburizing method, and although it can be said that this is an extremely excellent technique in that an abnormal carburizing layer is not generated, the raw material gas is thermally decomposed on the surface of the steel material to generate carbon atoms (carbon atoms of the generator). A high temperature is required for this purpose, and at 900 ° C. or lower, the gas decomposition reaction cannot catch up, so carburizing is impossible. Further, even in the case of the plasma carburizing method, in the technology known so far, the above-mentioned Japanese Patent Laid-Open No. 2-1
As described in the Example column of 45759, it was difficult to obtain a desired effect unless the treatment was actually performed at a high temperature of about 900 ° C. or higher.

As described above, in the conventional carburizing method, the treatment temperature should be lowered as much as possible in terms of quality.
It was desirable to raise the temperature from the viewpoint of processing time, and it was considered difficult to break the technical limit due to restrictions on the processing temperature from the viewpoint of equipment.

In such a technically closed state,
Recently, a technique for carburizing in the ferrite region of steel has been proposed as a low temperature carburizing method (Japanese Patent Laid-Open No. 3-188256). This method intends to perform carburization in the ferrite region of steel, which has been recognized as impossible in the past, and is a means based on the new investigation that "carburization proceeds even in the ferrite region where the temperature is low". That is, "the diffusion of C is remarkably faster in the low temperature ferrite phase than in the high temperature austenite phase, and the carburizing depth greatly depends on the diffusion of C". It is the essence.

However, although the above-mentioned low temperature carburizing method based on the gas carburizing method gives good results in the treatment in the laboratory equipment, when it is carried out on an industrial scale, Sufficiently satisfied because there is no sufficient and large-scale supply means of carbon atoms to penetrate into the interior, and large-scale equipment technology capable of processing at ferrite region temperature below A ₁ point. There was a problem that it was difficult to stably obtain the results that could be achieved.

In view of the above, the object of the present invention is to provide a high-quality carburized material on an industrial scale that does not cause abnormal carburization layer, abnormal growth coarsening of austenite grains, processing strain, etc. It is to establish a steel carburizing method that can be obtained with good workability by time treatment.

[0013]

The inventors of the present invention have made extensive studies to achieve the above object, and as a result, have been able to obtain the following findings. That is, comparing the diffusion coefficient of C between the ferrite phase at the same temperature and that in the austenite phase, the diffusion rate in the ferrite phase is an order of magnitude larger. Therefore, at the same temperature, the diffusion rate of carbon atoms in the ferrite structure is larger. It is considered to be faster than the austenite structure by one order, and from the viewpoint of the diffusion rate, it is considered that a sufficiently practical carburizing rate can be obtained even in the ferrite region. However, since the solid solution limit amount of C in ferrite is extremely small, it can be said that it is industrially impossible to increase the C concentration by means of carburizing the ferrite surface from the steel surface.

However, when a carbide phase such as a cementite phase is present in the ferrite, there is a phenomenon in which C that has rapidly diffused in the ferrite precipitates as a carbide around this carbide phase. At this time, the "phase equilibrium" does not mean "solid solution / precipitation equilibrium of C in the ferrite single phase", but locally "phase equilibrium between the [microstructure of ferrite + carbide] and the carbide". Therefore, it is theoretically possible to carburize up to the C concentration in the carbide around the carbide. Therefore, fine carbide is uniformly deposited in the ferrite matrix.
If it is dispersed, the above-mentioned phase equilibrium is uniformly and extensively induced in the ferrite matrix, and the increase in the amount of C solid solution in ferrite and the increase in the total C concentration by precipitating it as carbides are rapid. It was thought to be possible.

However, the effect of carburizing on the steel cannot be ensured only by precipitating and dispersing fine carbides in the ferrite matrix and diffusing C from there, and in order to obtain a sufficient carburizing effect, In addition to the "precipitation / dispersion of fine carbide in the ferrite matrix", it is indispensable to supply / infiltrate (carburize) carbon atoms from the surface of the steel in the temperature range where ferrite exists. However, as described above, it is extremely difficult to carburize in a low temperature region where ferrite is present by an ordinary gas carburizing method.

After various studies, the present inventors have found that the "plasma carburizing method" solves this problem. In other words, the plasma carburizing method is extremely effective as a method of sending (carburizing) carbon atoms into the ferrite phase with a low solid solubility limit. According to this method, low temperature "steel in which fine carbides are dispersed in the ferrite matrix" is obtained. The infiltration of carbon atoms will be carried out very smoothly. Then, the carbon atoms that have penetrated into the steel rapidly diffuse by the mechanism described above and contribute to the increase of the C concentration.

The present invention has been made on the basis of the above-mentioned findings and the like. "The steel to be treated is tempered prior to the carburizing" martensite structure "," bainite structure "," spherical carbide structure "or" charcoal structure ". A high-quality carburizing is achieved by forming a structure mainly composed of one or more of "ferrite structure in which nitrides are finely dispersed", and then carburizing at a temperature of less than Ae ₃ point of the steel to be treated by plasma carburizing method. It has a major feature in that it enables stable manufacturing of steel materials in a short time. "

The steel to which the method of the present invention is applied is not particularly limited as long as it exhibits a ferrite phase, and is usually carbon steel, low alloy steel, case hardening steel, spring steel, bearing steel, tool steel, wear resistance. Applicable to all ferritic steels for mechanical structures such as steel and heat resistant steel.

As described above, the present invention prepares a "structure in which a carbide-containing substance is finely dispersed in ferrite" in the steel to be treated, to which the plasma carburizing method is applied particularly to the A of the steel.
e Carburizing at ₃ points or less is the essence, and by advancing the carburizing process while precipitating and growing carbide in the ferrite structure in the low temperature region where ferrite exists, a large amount of The present invention relates to a treatment method for producing a steel having a carbide excess carburized structure in which carbides are uniformly and finely dispersed. The reason why the treatment conditions are limited as described above in the present invention will be explained below together with its action.

[0020]

The present inventors have found that when the plasma carburizing method is applied, carburizing can be performed on a ferritic steel even in a low temperature range of just below Ae ₁ point. Is set to "finely dispersed carbide-containing substance in ferrite", the C concentration in the carburized portion can be sufficiently increased. In other words, this pre-structure consists of "ferrite + carbide particles or carbonitride particles" in which fine carbide-containing substances are uniformly precipitated in ferrite grains. Specifically, a) tempered martensite structure, b) bainite Structure, c) spheroidized annealing structure (spheroidal carbide structure), d) ferrite containing fine precipitates in ferrite grains.
A pearlite structure (for example, a ferrite structure in which carbonitrides such as V-containing non-heat treated steel are finely dispersed), and the like are mainly composed of one or more kinds. As is well known, these structures are characterized by continuous cooling, isothermal transformation,
Since it is realized with good reproducibility by repeated treatments, the history of steel is not particularly limited.

The reason why the steel to be carburized is made to have such a pre-structure is to carburize C at a high concentration, as described above. That is, although the diffusion rate of carbon atoms is significantly faster in ferrite than in austenite, the solid solution limit amount of C in ferrite is extremely small, and therefore C is supplied from the surface of the steel as it is. However, the C concentration cannot be sufficiently increased. However, if the structure of steel is the above-mentioned microstructure in which a carbide phase such as cementite phase is present in ferrite, the "phase equilibrium" becomes "phase equilibrium between [composite microstructure of [ferrite + carbide] and carbide]". Carburization can be performed up to the C concentration in the carbide, and carburization to the desired C concentration can be performed quickly.

It should be noted, however, that the pearlite structure should be noted, and it is difficult to obtain a sufficient effect in carburizing the pearlite structure by the means according to the present invention. Therefore, in the pre-structure containing pearlite, the pearlite ratio should preferably be 50% or less.

Further, in the present invention, the carburizing treatment is carried out in a temperature range below the Ae ₃ point of the steel to be treated. This is Ae ₃
In order to prevent problems such as "abnormal growth and coarsening of austenite grains", "life shortening of carburizing furnace" or "distortion during carburizing treatment", which may occur when carburizing in a high temperature range above the point, This is because a rapid and stable carburizing means for the low temperature ferrite as described above was found. Here, the carburization of the ferrite phase can be performed in the α / γ two-phase region where Ae is ₁ point or more and less than Ae ₃ points, but from the viewpoint of the uniformity of the structure after carburization, the treatment just below Ae ₁ point Preferably, therefore, in order to obtain a higher quality product, the carburizing treatment should be carried out in the temperature range just below Ae ₁ .

By the way, the carburizing treatment according to the present invention is
In particular, the plasma carburizing method must be used. this is,
This is because, as already described, it is impossible or extremely difficult to perform sufficient carburization in a low temperature region where ferrite exists by known carburizing methods other than the plasma carburizing method. However, according to the plasma carburizing method, carbon atoms can be smoothly infiltrated into the ferrite phase having a low solid solubility limit, and the carburized layer having a high C concentration can be quickly and stably formed in combination with the preparation of the predetermined pre-structure. You can

In the method of the present invention, nitriding and carburizing can be simultaneously performed by simultaneously permeating and diffusing nitrogen during plasma carburizing. In this case, unlike the conventional soft nitriding treatment, a surface layer consisting of a nitride layer was formed on the "high carbon concentration ferrite structure" in which carbides were dispersed and precipitated in ferrite deep inside the steel material. A surface-hardened steel material is obtained.

Further, according to the method of the present invention, after carrying out the carburization dispersion precipitation carburizing in the ferrite region by the plasma carburizing method, the existing surface hardening treatment such as nitriding treatment, reheating quenching and tempering treatment, induction hardening, etc. is further combined. You can also

Next, the present invention will be described with reference to examples.

[Examples] Five kinds of steel materials A to E having the chemical compositions shown in Table 1 were prepared, and these were used as a structure before carburizing a) a tempered martensite structure by quenching and tempering, b) a bainite structure by forced air cooling, c) Spheroidal carbide structure by spheroidizing annealing and d) Ferrite-pearlite structure by normalization were prepared. The normalization structure (ferrite-pearlite structure) of the steel material A was such that many fine V compounds were uniformly dispersed and precipitated in the ferrite grains.

[0028]

[Table 1]

Next, these steel materials were subjected to low temperature plasma carburization as the method of the present invention, and gas carburization as the comparative method and the conventional method. Here, in plasma carburizing, the inside of the carburizing furnace is evacuated to 1 Torr and then heated to a predetermined temperature, and Ar, H ₂ gas containing several% of CH ₄ , C _{3 is used} as carburizing gas.
A gas in which a hydrocarbon gas such as H ₈ was mixed was introduced to maintain the furnace pressure at 1 to 10 Torr, and the carburization was performed for a predetermined time. Further, the gas carburizing in the comparative method and the conventional method was carried out experimentally in a small experimental furnace using a mixed gas of CO ₂ and CO as carburizing gas.

The "carburizing depth" and the "carburizing concentration" of each of the carburized materials thus obtained were investigated, and the results are shown in Table 2 together with the carburizing conditions. In addition, "Carburizing depth" is
After the carburizing and quenching, the "depth from the surface" of "the position indicating the hardness which is the average value of the maximum hardness of the carburized layer and the average hardness of the core portion" was defined as the value obtained from the cross-sectional hardness distribution profile. The "carburizing concentration" is a value obtained by shaving the surface layer of the sample after carburizing with a precision lathe at a thickness of 0.1 mm, collecting it, and performing a chemical analysis on it. This is a value corresponding to the average carbon concentration in the area up to 0.1 mm). Further, test Nos. 21 and 22 in Table 2 are those in which the carburizing potential of carburizing gas in the gas carburizing furnace was increased as much as possible, and carburizing was performed in a carbon potential atmosphere of about 1.5.

[0031]

[Table 2]

From the results shown in Table 2, the following can be confirmed. That is, in comparison with the conventional examples of test numbers 19 to 22, all of the test numbers 1 to 14 according to the present invention are low-temperature carburizing at 720 ° C. or less, but have a deeper carburizing depth and carburizing concentration in a shorter carburizing time than the conventional method. Has been obtained.

Further, in the comparative method of test Nos. 15 to 18, an attempt was made for carburizing and precipitating carburizing in the ferrite region by gas carburizing with the same front structure as in the method of the present invention. poor. It should be noted that the steel materials B and C having the prior structure of ferrite pearlite were subjected to plasma carburization under the same conditions as those of the examples of the present invention in Table 2, but almost no carburization was performed, and it was confirmed that they were not practical. Has been done.

[0034]

[Summary of Effects] As described above, according to the present invention, the abnormal carburizing layer, the abnormal growth of austenite grains and the coarsening of
Industrially useful effects such as high-quality carburized steel material can be obtained with good workability by a low-temperature and short-time treatment that does not cause treatment distortion and the like.

Claims (1)

[Claims] 1. Prior to the carburizing treatment, the steel to be treated has a tempered martensite structure, bainite structure, spherical carbide structure, or a structure mainly composed of one or more kinds of ferrite structures in which carbonitride is finely dispersed, After that, the carburizing method of steel is characterized by performing carburizing at a temperature lower than Ae ₃ point of the steel to be treated by plasma carburizing method.