JP2010222636A - Surface treatment method of steel product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the surface treatment method of steel product for efficient surface modification of a member to be treated comprising steel product. <P>SOLUTION: The surface treatment method of steel product comprises: a process of performing vacuum carburizing treatment of reducing a pressure at a primary heating temperature T1 and, at the same time, supplying a carburizing gas to the member to be treated; and then, a process of performing nitriding treatment of replacing the carburizing gas with nitriding gas, lowering the temperature to a secondary heating temperature T2 and retaining the temperature at the secondary heating temperature T2. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a surface treatment method for a steel material for efficiently modifying the surface of a member to be treated made of steel.

Conventionally, steel gears are used as various machine parts such as automobiles.
However, if the strength of the steel material is insufficient, the tooth root may be destroyed when a bending stress is applied to the tooth root of the gear. In addition, a sliding force may act on the gear surface portion of the gear to cause a parting of the gear surface portion to cause pitching. In order to prevent such tooth root destruction and suppress pitching, carburizing treatment is often performed on the gear surface portion. Carburizing treatment is performed on the gear surface portion to precipitate carbide, thereby improving the mechanical characteristics of the gear surface portion and securing the tooth root strength, thereby obtaining a highly reliable gear.

  However, the carbide to be precipitated in the carburizing process needs to have a predetermined size and shape. For example, if coarse carbides precipitate in a network form along the crystal grain boundaries, not only the fatigue strength of the gear surface portion decreases, but also cracks may occur during quenching. Therefore, in order to improve the fatigue strength of the gear surface portion, it is necessary to suppress the coarsening of carbides.

Conventionally, various methods have been proposed as a method for dispersing and precipitating fine carbides. For example, Patent Documents 1 and 2, with respect to the gear, and quenched after performing the primary carburization process of supplying the carburizing gas in A _cm line more primary heating temperature, then, A _cm line by A _1-wire or It describes that a secondary carburizing process for supplying a carburizing gas at the following secondary heating temperature is performed, followed by rapid cooling.

In the primary carburizing process, carburizing gas is supplied around the gear member at a predetermined primary heating temperature. Thereby, carbon penetrate | invades from the surface part of a gearwheel. When the primary carburizing treatment is performed for a predetermined time and the gear material is rapidly cooled, precipitation of carbide is suppressed, and an intermediate treatment material in which carbon is supersaturated in the gear surface portion is obtained. The reason for rapid cooling after the primary carburizing treatment is that if the temperature is lowered continuously after the carburizing treatment, coarse carbides are deposited until the secondary heating temperature is reached.
Thereafter, the gear member is heated to a secondary heating temperature slightly lower than the primary heating temperature. As a result, a large amount of carbide nuclei are generated in the carburized portion of the gear material containing carbon in supersaturation, and when carburizing gas is supplied to supply the carburizing gas at the secondary heating temperature, a gear surface layer is formed. Fine carbides are dispersed and deposited.

  For example, Patent Document 3 describes that carburizing and nitriding treatment is repeatedly performed by intermittently supplying a carburizing gas and a nitriding gas into a vacuum furnace. Thus, the coarsening of the carbide can be suppressed by repeatedly performing the carburizing and nitriding treatment. In addition, a large amount of nitrogen enters the gear surface portion, and the hardenability can be improved.

JP 2004-285384 A JP 2007-308792 A JP 2006-28541 A

  In patent documents 1 and 2, it is necessary to perform a carburizing process twice, and a temperature cycle is repeated twice in that case. For this reason, the processing man-hour increases and the manufacturing cost of the gear increases. In addition, the amount of thermal deformation increases. Further, in Patent Document 3, since the carburizing gas and the nitriding gas are intermittently supplied, it is necessary to control the supply ratio and the supply timing of the carburizing gas and the nitriding gas, and the control of the processing conditions is complicated. become. In addition, if a large amount of nitrogen enters the gear surface portion, the hardness of the gear surface portion decreases, so surface processing such as shot peening may be necessary to compensate for this.

  An object of the present invention is to provide a surface treatment method for steel that can efficiently modify the surface of a member to be treated made of steel.

  The first characteristic means of the steel material surface treatment method according to the present invention is a method in which a carburizing gas is supplied to a member to be treated while supplying a carburizing gas while reducing the pressure at the primary heating temperature, and then the carburizing gas is nitrogenated. The temperature is lowered to the secondary heating temperature while substituting with the reactive gas, and the nitriding treatment is performed to maintain the secondary heating temperature.

When performing the carburizing and nitriding treatment, it is necessary to increase the temperature of the treatment atmosphere so that the carbon element can enter from the surface of the member to be treated. At this time, a carburizing gas containing carbon is used as the atmospheric gas. However, since the atmosphere is high in temperature, the excess carburizing gas easily becomes sooted. This soot is a factor that reduces the production efficiency of carburized products, such as the quality of carburized products that adhere to the surface of the member to be processed and each part of the device, and the need for frequent maintenance and cleaning of the device. Become.
In the present invention, in order to improve this point, carburizing and nitriding treatment is performed under a reduced pressure environment. In the vacuum carburizing process, surplus carburizing gas that does not contribute to carburizing is sequentially discharged. For this reason, it can prevent carburizing gas from thermally decomposing and producing soot. Therefore, the quality of the processed product is improved, the frequency of cleaning the apparatus is reduced, and the processing efficiency can be increased.
Furthermore, in the present invention, since the member to be treated is heated only once and the nitriding treatment is performed in the cooling process, the heat treatment efficiency is very good. Further, in this means, after the vacuum carburizing process is performed, the carburizing gas is replaced with the nitriding gas and the nitriding process is performed, so the supply ratio of the carburizing gas and the nitrocarburizing gas can be controlled, It is not necessary to frequently control the supply timing of the oxidizing gas and the nitriding gas, and the processing conditions can be easily controlled. Thus, although this means needs to form a reduced pressure environment during the carburizing and nitriding treatment, the treatment efficiency can be greatly improved as compared with the conventional method in which the heat treatment is performed twice.

  The second characteristic means of the present invention uses a metal mainly containing iron as the member to be treated, and the surface carbon concentration of the member to be treated after the vacuum carburizing treatment and the nitriding treatment is 0.9 mass% or more. There is a point to set.

  With this means, the surface carbon concentration of the member to be treated can be set to a concentration equal to or higher than the eutectoid point, and carbides are likely to precipitate during the carburizing and nitriding treatment.

  The 3rd characteristic means of this invention exists in the point which sets the surface nitrogen concentration of the said to-be-processed member after the said vacuum carburizing process and the said nitriding process to 0.39 mass% or more.

  In order to obtain fine carbon deposits, an appropriate amount of nitrogen element is present in the matrix of the base material in accordance with the amount of carbon element, and the carbon element that precipitates during cooling of the workpiece is coarsened. It is necessary to prevent the nitrogen element from doing so. With the nitrogen concentration of the present invention, the diffusion of carbon element during precipitation can be effectively prevented with respect to the carbon element having the above concentration. As a result, the carbon deposits on the surface portion of the member to be processed become extremely fine, and the mechanical characteristics of the member to be processed can be improved.

  The 4th characteristic means of this invention exists in the point which sets Cr of the said to-be-processed member to 1.00 mass% or less.

  Cr in the steel material combines with carbon to form a carbide. This carbide suppresses solidification in martensite during quenching and softening of the steel material. However, if an amount of Cr exceeding 1.00% by mass is added, it reacts with nitrogen in the nitriding gas during the nitriding treatment to form CrN, and the penetration of nitrogen into the member to be treated is insufficient. Become. Therefore, in this invention, Cr of a to-be-processed member is set to 1.00 mass% or less. Thereby, the formation of CrN during the nitriding treatment is suppressed, and the nitriding treatment can be performed reliably.

  The fifth characteristic means of the present invention is that the rate of temperature decrease from the primary heating temperature to the secondary heating temperature is 2.7 ° C./min or less.

In the nitriding process, the higher the environmental temperature, the higher the nitrogen penetration rate into the member to be treated. However, if the member to be treated is kept at a high temperature for a long time, carbon elements that are not prevented from being diffused by nitrogen, that is, carbon elements existing to some extent deep from the surface of the article to be treated diffuse into deeper portions of the member to be treated. Resulting in. As a result, the carbon concentration in the surface portion of the member to be processed is reduced. On the other hand, when the member to be treated is rapidly cooled, the nitrogen element cannot be infiltrated to a predetermined depth.
Therefore, in this means, the temperature lowering rate at the secondary heating temperature is set to 2.7 ° C./min or less. Thereby, both the carbon concentration and the nitrogen concentration in the vicinity of the surface of the member to be processed can be optimized, and the member to be processed having fine carbon deposits can be obtained.

  The sixth characteristic means of the present invention is that the primary heating temperature is in the range of 900 ° C. to 1050 ° C., and the secondary heating temperature is in the range of 800 ° C. to 880 ° C.

With this means, the material to be treated is maintained in the γ phase in the carburizing process by primary heating, and the temperature of the carburized layer is maintained immediately below the A _cm line in the nitriding process by secondary heating. You can get things. In particular, when the nitriding treatment is held below the A _cm line, carbon deposits are uniformly formed in the entire region of the member to be treated. In other words, when the temperature of the base material is lowered, carbide is first precipitated at the portion reaching the A _cm line. As a result, the carbon concentration decreases in the base material in the vicinity. Since the A _cm line with respect to the lowered carbon concentration is a little lower temperature, no more carbides are precipitated at this site, and the carbides are not further coarsened. With this means, such a phenomenon is caused over the entire base material, and as a result, the carbide of the base material is refined and a member to be processed having excellent mechanical characteristics can be obtained.

It is a figure which shows the temperature pattern of a carburizing nitriding process. It is a phase transformation diagram of a member to be processed. It is a figure which shows the temperature pattern of a carburizing nitriding process (processing condition 1). It is a figure which shows the temperature pattern of a carburizing nitriding process (processing condition 2). It is a figure which shows the temperature pattern of a carburizing nitriding process (processing condition 3). It is a figure which shows the temperature pattern of a carburizing nitriding process (processing condition 4). It is a figure which shows the temperature pattern of a carburizing nitriding process (process condition 5). It is a figure which shows the temperature pattern of a carburizing nitriding process (processing condition 6). It is a figure which shows the temperature pattern of a carburizing nitriding process (processing condition 7).

As a member to be processed according to the present invention, for example, a member that requires particularly excellent mechanical characteristics, such as an automobile gear, a toroidal CVT disk, or a power roller, is used.
The present invention relates to a carburizing and nitriding method for efficiently performing the surface modification of such a member. Hereinafter, the method of the present invention will be described with reference to the drawings and tables.

[Chemical composition of treated material]
The mechanical strength of the member to be treated is affected by the concentration of contained carbon. When the carbon concentration of the base material of the member to be processed is less than 0.10% by mass, the strength of the base material of the member to be processed decreases. On the other hand, if the carbon concentration of the base material of the member to be processed exceeds 0.40% by mass, the hardness of the base material of the member to be processed increases and the workability decreases. Therefore, the member to be treated of the present invention preferably contains carbon in the range of 0.10% by mass to 0.40% by mass.

  FeSi may be added to a steel material used as a machine element as a deoxidizer and a slag viscosity modifier. Thereby, the amount of oxygen of a member to be processed can be reduced, and moderate viscosity can be given to slag. Thus, Si is inevitably mixed during the addition of FeSi as a deoxidizer and viscosity modifier, but in order to maintain the carburizing property, workability, and softening resistance of the member to be treated. It is an element that is also added. When the Si of the member to be processed is less than 0.05% by mass, the member to be processed is easily softened during tempering. On the other hand, when Si of the member to be processed exceeds 1.10% by mass, the carburizing property of the member to be processed is lowered, the hardness of the member to be processed is increased, and the workability is lowered. Therefore, the member to be treated of the present invention preferably contains Si in the range of 0.05% by mass to 1.10% by mass.

Mn, which acts as a deoxidizer, may be added to the steel material during the refining process. Mn is also used to stabilize the austenite by lowering the A ₃ transformation point.
However, when the Mn of the member to be processed is less than 0.30% by mass, the strength of the member to be processed is lowered and the hardenability of the member to be processed is lowered. On the other hand, if the Mn of the member to be processed exceeds 1.20% by mass, the hardness of the member to be processed increases and the workability of the member to be processed decreases. Therefore, in order to maintain the strength, hardenability and workability of the member to be treated, the member to be treated of the present invention preferably contains Mn in the range of 0.30% by mass to 1.20% by mass. .

By adding Cr to the steel material, carbide can be formed, or the carbide can be dissolved in martensite during quenching to improve softening resistance.
On the other hand, if the Cr of the member to be treated exceeds 1.00% by mass, the nitrogen in the nitriding gas easily reacts with the Cr of the member to be treated during the nitriding treatment to form CrN. Nitrogen is less likely to enter the member to be treated. Therefore, in the present invention, the Cr content is 0.50 mass so that the formation of carbides and the softening resistance of the member to be processed can be maintained while the nitrogen in the nitriding gas can easily enter the member to be processed. % To 1.00% by mass, more preferably 0.50% to 0.90% by mass, and still more preferably 0.50% to 0.80% by mass.

  By adding Mo to the steel material, the generation of grain boundary oxides can be suppressed during quenching, and the hardenability of the member to be treated can be improved. However, Mo is an expensive element, and when the amount of Mo added increases, the cost increases. Therefore, the member to be treated of the present invention is preferably in the range of 0.00 mass% to 1.00 mass%, more preferably in the range of 0.10 mass% to 1.00 mass% from the viewpoint of hardenability. It is preferable to contain Mo.

  In steel materials, Ni may be used in order to suppress the low temperature brittleness of steel materials. Ni is preferably added from the viewpoint of hardenability and low-temperature brittleness control, but from the viewpoint of cost and workability, the member to be treated of the present invention has a Ni content in the range of 0.00 mass% to 3.00. It is preferable to contain. In addition, although about 0.10 mass% Ni may mix as an impurity on manufacture, it does not become a problem in particular.

  In the present embodiment, in order to perform the carburizing and nitriding treatment, the member to be treated having C, Si, Mn, Cr, Mo, and Ni as constituent elements is exemplified. However, there is no problem even if other elements such as Ti, Nb, Al, V, and B are added to the constituent elements in accordance with various uses.

[Surface carbon concentration and surface nitrogen concentration of treated material]
In order to efficiently carburize, the surface carbon concentration of the member to be treated must be equal to or higher than the eutectoid point (carbon concentration of 0.80% by mass for iron-carbon steel) so that carbides are likely to precipitate. is there. Therefore, the surface carbon concentration of the member to be treated is set to 0.90% by mass or more.

  Nitrogen is an interstitial element along with carbon. For this reason, it becomes difficult for carbon to diffuse when nitrogen penetrates the member to be treated, and formation of coarse carbides can be suppressed. This nitrogen concentration needs to correspond to the carbon concentration in the base material. Therefore, in this embodiment, the surface nitrogen concentration of the member to be treated is set to 0.39% by mass or more as an adaptation to the carbon concentration. Thereby, nitrogen is easy to occupy the site which can penetrate | invade in the crystal | crystallization of a to-be-processed member, carbon becomes difficult to spread | diffuse, and coarsening of a carbide | carbonized_material can be suppressed.

  Hereinafter, the carburizing and nitriding treatment of the member to be treated according to the present invention will be described.

(Carburizing and nitriding treatment)
FIG. 1 shows a temperature pattern of the carburizing and nitriding treatment. FIG. 2 is a phase transformation diagram of a carbon steel material. Thick lines with arrows in the figure indicate changes in temperature and carbon concentration in the surface portion (carburized layer) of the member to be treated. (A)-(e) of FIG. 1 and FIG. 2 has shown each process in a carburizing nitriding process.

(Temperature raising process)
The vacuum carburizing furnace (not shown) is depressurized, and the vacuum carburizing furnace is heated to the primary heating temperature T1 ° C. in a depressurized state (portion (a) in FIGS. 1 and 2). At this time, X1 point in FIG. 2 is located above the A _3-wire, which is to be processed member γ phase (austenite phase).

(Carburization process)
Subsequently, the carburizing gas is supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace is maintained at the primary heating temperature T1 ° C. for a predetermined time (portion (b) in FIGS. 1 and 2). Processing is performed so that the carbon concentration of the outermost surface of the member to be processed becomes a predetermined value. Thus, by performing the carburizing process under a reduced pressure environment, it is possible to suppress the surplus carburizing gas from becoming sooted. Thus, the carburizing apparatus can be prevented from becoming dirty, and the trouble of cleaning the apparatus is reduced. As a result, the member to be processed can be obtained with high production efficiency. Carbon in the carburizing gas enters the surface portion of the member to be treated, and the surface carbon concentration of the member to be treated increases from C1 to C2% by mass. As a result, a carbon-rich carburized layer is formed on the surface portion of the member to be processed. At this time, the point X2 in FIG. 2 is located above the A _cm line, and the carburized layer of the member to be processed maintains the γ phase.

(Cooling process)
Subsequently, the supply of the carburizing gas is stopped, and when the amount of the carburizing gas becomes sufficiently small, the nitrocarburizing gas is supplied to the inside of the vacuum carburizing furnace while releasing the decompression of the vacuum carburizing furnace. At the same time, the internal temperature of the vacuum carburizing furnace is lowered to the secondary heating temperature T2 at a predetermined cooling rate (part (c) in FIGS. 1 and 2). At this time, nitrogen in the nitriding gas begins to enter the carburized layer of the member to be treated, and the diffusion of carbon is delayed by the intruding nitrogen.

  Both carbon and nitrogen are interstitial elements, and the sites that can enter in the crystal of the member to be processed are determined. Carbon and nitrogen diffuse through the site. When nitrogen enters the member to be processed and occupies the site, carbon becomes difficult to diffuse. For this reason, it is possible to suppress the diffusion of carbon and the precipitation of coarse carbides.

As the primary heating temperature T1 is higher, nitrogen is more likely to enter the member to be processed. In addition, the diffusion coefficient of nitrogen increases. However, the higher the primary heating temperature T1, the larger the carbon diffusion coefficient. Therefore, it is desirable to increase the diffusion coefficient of nitrogen while increasing the concentration of the nitriding gas in the internal atmosphere. For this reason, it is preferable that the cooling rate of the internal temperature of a vacuum carburizing furnace is slow to some extent.
On the other hand, if the cooling rate of the internal temperature of the vacuum carburizing furnace is made too fast, nitrogen will not easily enter the member to be treated, and the effect of delaying the diffusion of carbon by nitrogen may not be fully exhibited. Therefore, it is preferable that the cooling rate of the internal temperature of the vacuum carburizing furnace is not more than a predetermined rate.

(Nitrogenization process)
The internal temperature of the vacuum carburizing furnace is maintained for a predetermined time at the secondary heating temperature T2, that is, the temperature immediately below the A _cm line at the carbon concentration C2 (part (d) in FIGS. 1 and 2). As described above, since the diffusion of carbon is delayed by nitrogen, fine carbides gradually precipitate. Further, in order to promote the precipitation of fine carbides, the point X3 in FIG. 2 is positioned directly below the A _cm line. By doing so, it is possible to deposit carbide at the site of the carbonitrided region of the member to be treated. Thus, the strength of the member to be treated can be improved by dispersing and precipitating fine carbides on the member to be treated.

(Quenching process)
After the carbide is precipitated in a predetermined state, the member to be treated is rapidly cooled (portion (e) in FIGS. 1 and 2). Thereby, a to-be-processed member mainly changes into a martensite phase. In addition to dispersion strengthening with fine carbides, the hardness of the member to be treated increases due to a phase change to a metastable martensite phase. In addition, you may perform a tempering process after a hardening process as needed.

  Examples of the present invention will be described below.

[Used steel]
The following steel materials A to C were used as members to be processed.

  These steel materials A to C were subjected to carburizing and nitriding treatment under the following conditions.

(Processing condition 1)
As shown in FIG. 3, the internal temperature of the gas carburizing furnace was raised to 910 ° C. (primary heating temperature) in a mixed gas (carburizing gas) atmosphere of CO, CO ₂ , N ₂ , and H ₂ . After the temperature increase, the carbon concentration on the outermost surface of the member to be processed was treated to 0.8%, and the internal temperature of the gas carburizing furnace was maintained at 910 ° C. for 120 minutes. After the internal temperature of the gas carburizing furnace was lowered to 850 ° C. (secondary heating temperature) at a cooling rate of 0.7 ° C./min, the internal temperature of the gas carburizing furnace was maintained at 850 ° C. for 30 minutes. Thereafter, the member to be treated was quenched. The proportions of CO, CO ₂ , N ₂ and H ₂ are 20.9%, 0.38% (850 ° C.) / 0.22% (910 ° C.), 40.0% and 38.0%, respectively. . Note that a small amount of CH ₄ is added to keep CO ₂ constant.

(Processing condition 2)
As shown in FIG. 4, the vacuum carburizing furnace was depressurized, and the vacuum carburizing furnace was heated to 930 ° C. (primary heating temperature) in a reduced pressure state. While reducing the vacuum carburizing furnace, acetylene gas (carburizing gas) was supplied into the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. (primary heating temperature) for 40 minutes. It processed so that the carbon concentration of the outermost surface of a to-be-processed member might be 0.8%. The supply of acetylene gas was stopped, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. for 40 minutes. After the internal temperature of the vacuum carburizing furnace was lowered to 850 ° C. (secondary heating temperature) at a cooling rate of 2.7 ° C./min, the internal temperature of the vacuum carburizing furnace was maintained at 850 ° C. for 30 minutes. Thereafter, the member to be treated was quenched.

(Processing condition 3)
As shown in FIG. 5, the vacuum carburizing furnace was depressurized, and the vacuum carburizing furnace was heated to 930 ° C. (primary heating temperature) in a reduced pressure state. While reducing the vacuum carburizing furnace, acetylene gas (carburizing gas) was supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. for 80 minutes. It processed so that the carbon concentration of the outermost surface of a to-be-processed member might be 1.0%. The supply of acetylene gas was stopped while depressurizing the vacuum carburizing furnace. When the amount of acetylene gas was sufficiently reduced, the internal temperature of the vacuum carburizing furnace was lowered to 870 ° C. (secondary heating temperature) at a cooling rate of 2.0 ° C./min. The decompression of the vacuum carburizing furnace was released and ammonia gas (nitrocarburizing gas) was supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 870 ° C. for 180 minutes while maintaining the atmospheric pressure. Thereafter, the member to be treated was quenched.

(Processing condition 4)
As shown in FIG. 6, the vacuum carburizing furnace was depressurized, and the vacuum carburizing furnace was heated to 930 ° C. (primary heating temperature) in a reduced pressure state. While reducing the pressure of the vacuum carburizing furnace, acetylene gas (carburizing gas) was supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. (primary heating temperature) for 80 minutes. It processed so that the carbon concentration of the outermost surface of a to-be-processed member might be 1.0%. The supply of acetylene gas was stopped while depressurizing the vacuum carburizing furnace. When the amount of acetylene gas is sufficiently low, the vacuum carburizing furnace is depressurized and ammonia gas (nitriding gas) is supplied to the inside of the vacuum carburizing furnace. After the internal temperature was lowered to 870 ° C. (secondary heating temperature) at 1.8 ° C./min, the internal temperature of the vacuum carburizing furnace was maintained at 870 ° C. for 30 minutes. Thereafter, the member to be treated was quenched.

(Processing condition 5)
As shown in FIG. 7, the vacuum carburizing furnace was depressurized, and the vacuum carburizing furnace was heated to 930 ° C. (primary heating temperature) in a reduced pressure state. While reducing the pressure of the vacuum carburizing furnace, acetylene gas (carburizing gas) was supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. (primary heating temperature) for 80 minutes. It processed so that the carbon concentration of the outermost surface of a to-be-processed member might be 1.0%. The supply of acetylene gas was stopped while depressurizing the vacuum carburizing furnace. When the amount of acetylene gas is sufficiently low, the vacuum carburizing furnace is depressurized and ammonia gas (nitriding gas) is supplied to the inside of the vacuum carburizing furnace. After lowering the internal temperature to 850 ° C. (secondary heating temperature) at 2.4 ° C./min, the internal temperature of the vacuum carburizing furnace was maintained at 850 ° C. for 60 minutes. Thereafter, the member to be treated was quenched.

(Processing condition 6)
As shown in FIG. 8, the vacuum carburizing furnace was depressurized, and the vacuum carburizing furnace was heated to 930 ° C. (primary heating temperature) in a reduced pressure state. While reducing the pressure of the vacuum carburizing furnace, acetylene gas (carburizing gas) was supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. (primary heating temperature) for 80 minutes. It processed so that the carbon concentration of the outermost surface of a to-be-processed member might be 1.0%. The supply of acetylene gas was stopped while depressurizing the vacuum carburizing furnace. When the amount of acetylene gas is sufficiently low, the vacuum carburizing furnace is depressurized and ammonia gas (nitriding gas) is supplied to the inside of the vacuum carburizing furnace. After the internal temperature was lowered to 870 ° C. (secondary heating temperature) at 2.0 ° C./min, the internal temperature of the vacuum carburizing furnace was maintained at 870 ° C. for 30 minutes. Thereafter, the member to be treated was quenched.

(Processing condition 7)
As shown in FIG. 9, the vacuum carburizing furnace was depressurized, and the vacuum carburizing furnace was heated to 930 ° C. (primary heating temperature) in a reduced pressure state. While reducing the pressure of the vacuum carburizing furnace, acetylene gas (carburizing gas) was supplied to the inside of the vacuum carburizing furnace, and the internal temperature of the vacuum carburizing furnace was maintained at 930 ° C. (primary heating temperature) for 80 minutes. It processed so that the carbon concentration of the outermost surface of a to-be-processed member might be 1.0%. The supply of acetylene gas was stopped while depressurizing the vacuum carburizing furnace. When the amount of acetylene gas is sufficiently low, the vacuum carburizing furnace is depressurized and ammonia gas (nitriding gas) is supplied to the inside of the vacuum carburizing furnace. After the internal temperature was lowered to 850 ° C. (secondary heating temperature) at 2.7 ° C./min, the internal temperature of the vacuum carburizing furnace was maintained at 850 ° C. for 30 minutes. Thereafter, the member to be treated was quenched.

  In this embodiment, ammonia gas is supplied when the temperature lowering of the vacuum carburizing furnace is completed (processing condition 3) or when the temperature lowering of the vacuum carburizing furnace is started (processing conditions 4 to 7). However, nitrogen gas may be supplied in addition to ammonia gas. In this case, nitrogen gas is mainly used to increase the internal pressure of the vacuum carburizing furnace. By supplying ammonia gas and nitrogen gas, the internal pressure of the vacuum carburizing furnace can be quickly increased as compared with the case of supplying ammonia gas alone. As a result, it is possible to suppress the thermal decomposition of ammonia gas into nitrogen and hydrogen. The ratio of ammonia gas and nitrogen gas is appropriately determined according to various conditions such as the material of the member to be processed and the internal temperature of the vacuum carburizing furnace.

  In the present embodiment, an unsaturated hydrocarbon gas such as acetylene or ethylene is used as the carburizing gas. However, it may be a saturated hydrocarbon gas such as propane, or a mixture of the unsaturated hydrocarbon gas and the saturated hydrocarbon gas.

  Various tests and measurements were performed on the steel materials A to C subjected to the carburizing and nitriding treatment under the above processing conditions 1 to 7. SEM observation was performed to examine the presence or absence of precipitates and the size of the precipitates. The surface carbon concentration and the surface nitrogen concentration of the steel materials A to C were measured by EPMA. The surface hardness of the steel materials A to C was measured with a Vickers hardness meter. The contact pressure is 3.3 GPa, the rotation speed is 1400 rpm, and the slip ratio is -40%. A roller pitching test was performed under the condition of an oil temperature of 80 ° C., and the pitching life (number of repetitions) until the pitching was measured.

〔Experimental result〕
The results are shown in Table 2.

(Relationship between surface carbon concentration and precipitates)
The surface carbon concentrations of Comparative Examples 1 and 2 were 0.80 and 0.78% by mass, respectively, and no carbide was precipitated. On the other hand, the surface carbon density | concentration of Comparative Examples 3-5 and Examples 1-3 was in the range of 0.93-1.08 mass%, and the carbide | carbonized_material precipitated. Thereby, when the surface carbon concentration of the carburized layer of the member to be treated is 0.90% by mass or more, it can be seen that the carbide is precipitated.

(Relationship between surface nitrogen concentration and precipitates)
The surface nitrogen concentrations of Comparative Examples 3 to 5 and Examples 1 to 3 were in the range of 0.39 to 1.00% by mass, and coarse carbides did not precipitate. If the surface nitrogen concentration of the carburized layer is low, nitrogen cannot sufficiently occupy the invading sites present in the crystal of the member to be treated. For this reason, carbon diffuses through the site and a coarse carbide precipitates. However, if the surface nitrogen concentration of the carburized layer of the member to be treated is 0.39% by mass or more, coarse carbides do not precipitate.

(Relationship between Cr concentration and precipitates)
Cr density | concentration of Comparative Examples 3-6 and Examples 1-5 was in the range of 0.50-1.00 mass%, and the coarse carbide did not precipitate. When the Cr concentration of the member to be treated is high, nitrogen in the ammonia gas reacts with Cr of the member to be treated during the nitriding treatment, and it is easy to make CrN. For this reason, it becomes difficult for nitrogen in ammonia gas to enter the member to be treated. Thus, if the Cr concentration of the member to be treated is 1.00% by mass or less, nitrogen in the ammonia gas can easily enter the member to be treated, and precipitation of coarse carbides can be suppressed.

(Relationship between cooling rate and pitching life)
In Examples 2, 1, 4, and 5, the primary heating temperature T1, the secondary heating temperature T2, the holding time held at the secondary heating temperature T2, and the temperature lowering time when the temperature is lowered from the primary heating temperature T1 to the secondary heating temperature T2. Table 3 shows the cooling rate, the presence or absence of precipitates, and the pitching life.

実施例２，１，４，５のいずれにおいても炭化物が析出した。降温速度は、１．８〜２．７℃／分の範囲内である。また、実施例２と実施例１とを比較すると、降温速度が小さいほどピッチング寿命が長いことがわかる。これにより、降温速度が、上記範囲の上限値である２．７℃／分以下であれば、疲労強度が向上する。

In all of Examples 2, 1, 4 and 5, carbides were precipitated. The cooling rate is in the range of 1.8 to 2.7 ° C./min. Moreover, when Example 2 and Example 1 are compared, it turns out that a pitching lifetime is long, so that a temperature fall rate is small. Thereby, if the temperature decreasing rate is 2.7 ° C./min or less which is the upper limit of the above range, the fatigue strength is improved.

  The hardness of the member to be processed does not change between the second embodiment and the first embodiment. However, the pitching life of the member to be treated was about twice that of Example 1 compared to that of Example 1. This is presumably because the temperature decrease rate of Example 2 was smaller than the temperature decrease rate of Example 1, and more nitrogen entered the member to be processed, and the softening resistance of the member to be processed was improved. Furthermore, it has also been confirmed that the amount of carbide in the member to be treated is larger in Example 2 than in Example 1, which is considered to contribute to further extending the pitching life.

(Primary heating temperature and secondary heating temperature)
The primary heating temperature is in the range of 900 ° C. to 1050 ° C. so that the member to be treated can maintain the γ phase in the carburizing process. Further, in order to deposit fine carbides in the nitriding step, it is necessary to position the point X3 directly below the A _cm line. For this reason, the secondary heating temperature is set within a range of 800 ° C to 880 ° C.

(Carbide particle size)
Workpiece member result of performing SEM observation for the Example _{1, (Fe, Cr) 3} C fine carbides of about 0.2μm~0.5μm whose main component was confirmed number.

  The present invention can be applied to carburizing and nitriding various members to be processed.

T1 Primary heating temperature T2 Secondary heating temperature

Claims (6)

  After subjecting the workpiece to vacuum carburizing to supply carburizing gas while reducing the pressure at the primary heating temperature, the carburizing gas is replaced with nitriding gas and the temperature is lowered to the secondary heating temperature. A method for surface treatment of a steel material that performs a nitriding treatment that is maintained at a next heating temperature.   The steel member according to claim 1, wherein the member to be treated is a metal mainly containing iron, and the surface carbon concentration of the member to be treated after the vacuum carburizing treatment and the nitriding treatment is set to 0.9 mass% or more. Surface treatment method.   The steel material surface treatment method according to claim 1, wherein a surface nitrogen concentration of the member to be treated after the vacuum carburizing treatment and the nitriding treatment is set to 0.39 mass% or more.   The steel material surface treatment method according to claim 1, wherein Cr of the member to be treated is set to 1.00 mass% or less.   The method for surface treatment of a steel material according to any one of claims 1 to 4, wherein a temperature lowering rate at which the temperature is lowered from the primary heating temperature to the secondary heating temperature is set to 2.7 ° C / min or less.   The steel material surface treatment method according to any one of claims 1 to 5, wherein the primary heating temperature is in a range of 900 ° C to 1050 ° C, and the secondary heating temperature is in a range of 800 ° C to 880 ° C.

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