WO2021039911A1

WO2021039911A1 - Vacuum carburization treatment method and production method for carburized component

Info

Publication number: WO2021039911A1
Application number: PCT/JP2020/032388
Authority: WO
Inventors: 尚二藤堂; 秀樹今高
Original assignee: 日本製鉄株式会社
Priority date: 2019-08-29
Filing date: 2020-08-27
Publication date: 2021-03-04
Also published as: CN114341392A; CN114341392B; JPWO2021039911A1; JP7201092B2

Abstract

Provided is a vacuum carburization treatment method in which variations in carburization can be inhibited. In a preceding carburization step of the vacuum carburization treatment method according to an embodiment of the present invention, an actual carburization gas flow rate is not less than a theoretical carburization gas flow rate at time ta/10, where ta represents time at which the carburization step has been completed, and is not more than a theoretical carburization gas flow rate four seconds after the start of the preceding carburization step. In a subsequent carburization step, an actual carburization gas flow rate during a period of time from time t0 to time 4t0 is FA√(t0/t) to FA, and an actual carburization gas flow rate during a period of time from time 4t0 to time ta is FA√(t0/t) to 2FA√(t0/t), where t0 represents time at which the partial pressure of acetylene after the start of the carburization step first becomes not less than 0.8 times of the partial pressure of hydrogen, and FA represents an actual carburization gas flow rate at the start of the preceding carburization step.

Description

Vacuum carburizing method and manufacturing method of carburized parts

The present invention relates to a vacuum carburizing treatment method and a method for manufacturing carburized parts. In this specification, the carburized steel parts are referred to as "carburized parts".

Steel parts that require high surface fatigue strength are manufactured by subjecting steel materials to surface hardening treatment. One of the surface hardening treatment methods is a vacuum carburizing treatment method. The vacuum carburizing treatment method includes a carburizing step and a diffusion step. In the carburizing step, a carburizing gas, which is a hydrocarbon gas, is introduced to increase the carbon concentration on the surface of the steel material heated to the carburizing temperature. The hydrocarbon gas is, for example, acetylene, propane, or the like. In the diffusion step, after the carburizing step, the introduction of the carburized gas is stopped to diffuse carbon in the depth direction of the surface layer of the steel material. The carbon concentration in the surface layer of the steel material is controlled by adjusting the time of the carburizing step and the diffusion step.

However, the hydrocarbon gas, which is a carburized gas, is thermodynamically unstable. Therefore, when the carburizing temperature is high, the carburized gas is easily decomposed into carbon, hydrogen, and the like. When the carburizing temperature is high, the carburized gas molecules move actively. Due to the vigorous movement, the carburized gas molecules collide with each other at high speed, and the carburized gas decomposes. Soot and tar are generated by the decomposition of carburized gas. In this case, the surface carbon concentration and carburizing depth vary. Therefore, the surface layer of the carburized parts cannot be maintained at a constant quality. Therefore, the vacuum carburizing treatment method is required to suppress variations in the carbon concentration on the surface of the carburized parts and variations in the carburizing depth of the surface layer. In the following description, the variation in the carbon concentration on the surface of the carburized part and the variation in the carburizing depth of the surface layer of the carburized part are referred to as "carburizing variation".

Techniques for suppressing variation in carburization include JP-A-8-325701 (Patent Document 1), JP-A-2016-148091 (Patent Document 2), JP-A-2002-173759 (Patent Document 3), and JP-A-2005. It is proposed in Japanese Patent Application Laid-Open No. 350729 (Patent Document 4) and Japanese Patent Application Laid-Open No. 2012-7240 (Patent Document 5).

In the vacuum carburizing treatment method described in Patent Document 1, a work made of a steel material is vacuum-heated in a heating chamber of a vacuum carburizing furnace, and a carburizing gas is supplied to the heating chamber to perform the carburizing treatment. In this vacuum carburizing method, a gaseous chain unsaturated hydrocarbon is used as the carburizing gas. Then, the carburizing treatment is carried out in a vacuum state of 1 kPa or less in the heating chamber. It is described in Patent Document 1 that this makes it possible to uniformly carburize while suppressing the generation of soot.

In the vacuum carburizing treatment method described in Patent Document 2, the object to be treated placed in the carburizing chamber is carburized by injecting carburizing gas into the carburizing chamber in a depressurized atmosphere. In this vacuum carburizing method, the gas injection amount of the carburized gas to be injected into the carburizing chamber is the volume of the object to be carburized in the packed state, the volume of the carburizing chamber, the total surface area of the object to be carburized, and the carburizing. Calculated based on the constant set based on the type of gas. Then, the carburized gas of the calculated gas injection amount is injected into the carburizing chamber. It is described in Patent Document 2 that this can prevent the occurrence of spot-like excessive carburizing.

In the vacuum carburized atmosphere gas control system described in Patent Document 3, propane gas is used as the carburized gas. In this control system, the carburized gas is supplied into the vacuum carburizing furnace in which the carburized material is set. Then, the carbon generated by the thermal decomposition reaction of the carburized gas is solid-solved and diffused into the carburized material to perform the carburizing treatment of the carburized material. In this control system, the partial pressure of hydrogen gas generated by this pyrolysis reaction is constantly measured during the carburizing process. Then, the amount of carburized gas supplied into the furnace is adjusted and controlled in real time based on the measured value. Patent Document 3 describes that this makes it possible to stably produce high-quality carburized steel.

In the vacuum carburizing treatment method described in Patent Document 4, the relationship V = f (t) between the theoretical flow rate V of the carburized gas required for the carburizing treatment and the carburizing time t is determined by the carburizing depth and the surface carbon concentration of the material. Calculated based on internal diffusion. Then, in the early stage of carburizing in the carburizing step, the flow rate V1 at the time of carburizing, which is sufficiently larger than the theoretical flow rate V and does not cause sooting, is supplied. Further, in the latter stage of carburizing following the first stage of carburizing, a diffusion flow rate V2 smaller than the theoretical flow rate V is supplied. It is described in Patent Document 4 that this makes it possible to reduce the residual cementite while preventing the generation of soot.

In the vacuum carburizing method described in Patent Document 5, the time change of the theoretical flow rate of the carburized gas required for the carburizing treatment is obtained based on the diffusion of carbon into the product to be treated. Then, based on the time change of the theoretical flow rate, the partial pressure ratio of hydrogen generated by the carburizing reaction at the theoretical flow rate to the total pressure in the treatment chamber is defined as the theoretical hydrogen partial pressure ratio. The time change of the theoretical hydrogen partial pressure ratio is obtained, and the time change of the theoretical hydrogen partial pressure ratio is compared with the time change of the hydrogen partial pressure ratio with respect to the total pressure in the treatment chamber during the actual carburizing treatment. Based on these approximation degrees, the degree of variation in carburizing quality within the same operation batch is determined. It is described in Patent Document 5 that this can improve the reproducibility of the quality of the carburized parts and reduce the quality variation of the carburized parts.

Japanese Unexamined Patent Publication No. 8-325701 Japanese Unexamined Patent Publication No. 2016-148091 JP-A-2002-173759 Japanese Unexamined Patent Publication No. 2005-350729 Japanese Unexamined Patent Publication No. 2012-7240

However, the variation in carburizing may be suppressed by another method different from the vacuum carburizing treatment methods of Patent Documents 1 to 5.

An object of the present disclosure is to provide a vacuum carburizing treatment method and a method for manufacturing carburized parts capable of suppressing carburizing variation.

The vacuum carburizing method according to the present disclosure
This is a vacuum carburizing method that performs vacuum carburizing treatment on steel materials in a vacuum carburizing furnace.
A heating process that heats the steel material to the carburizing temperature,
After the heating step, a heat equalizing step of soaking the steel material at the carburizing temperature and
After the heat soaking step, a carburizing step of holding the steel material at the carburizing temperature while supplying a carburizing gas which is an acetylene gas into the vacuum carburizing furnace.
After the carburizing step, a diffusion step of stopping the supply of the carburized gas into the vacuum carburizing furnace and holding the steel material at the carburizing temperature,
A quenching step of performing quenching on the steel material after the diffusion step, and
With
In the carburizing step
The flow rate of the carburized gas supplied into the vacuum carburizing furnace is defined as the actual carburized gas flow rate.
The flow rate of the carburized gas required for the vacuum carburizing treatment of the steel material is defined as the theoretical carburized gas flow rate.
The completion time of the carburizing process is defined as ta.
When the first time after the start of the carburizing step that the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is defined as t0,
The carburizing step is
A partial pressure measuring step of continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace to specify the time t0.
The early carburizing process from the start of the carburizing process to the time t0, and
The late carburizing step from time t0 to time ta,
Including
In the early carburizing process,
The actual carburized gas flow rate is set to be equal to or more than the theoretical carburized gas flow rate at time ta/10 and equal to or less than the theoretical carburized gas flow rate 4 seconds after the start of the carburizing step.
In the late carburizing step,
When the actual carburized gas flow rate of the early carburizing step is defined as FA and the time from the start of the carburizing step is defined as time t.
The actual carburized gas flow rate during the period from time t0 to time 4t0 is set to FA√ (t0 / t) or more and FA or less.
The actual carburized gas flow rate from the time 4t0 to the time ta is set to FA√ (t0 / t) or more and 2FA√ (t0 / t) or less.

The method for manufacturing carburized parts according to the present disclosure is as follows.
The steel material is provided with a step of carrying out the above-mentioned vacuum carburizing treatment method.

The vacuum carburizing treatment method of the present disclosure can suppress variations in carburizing. The method for manufacturing carburized parts of the present disclosure can manufacture carburized parts in which carburizing variations are suppressed.

FIG. 1 is a diagram showing an example of the relationship between the theoretical carburized gas flow rate and time calculated by the diffusion flux of carbon on the surface layer of the steel material obtained by the diffusion simulation using the diffusion equation. FIG. 2 is a diagram showing the time course of the actual carburized gas flow rate in the conventional carburizing step and the time course of the theoretical carburized gas flow rate. FIG. 3 shows the time course of the actual carburized gas flow rate in the carburizing step of the vacuum carburizing treatment method according to the present embodiment (see the figure below) and the time course of the acetylene partial pressure and the hydrogen partial pressure in the atmosphere of the vacuum carburizing furnace in the carburizing step (see the figure below). It is a figure which shows (the above figure). FIG. 4 is a diagram showing an example of a heat pattern of the vacuum carburizing treatment method of the present embodiment. FIG. 5 is a diagram showing an example of a gas flow rate set value in the early carburizing step of the vacuum carburizing treatment method of the present embodiment. FIG. 6 is a diagram showing an example of a gas flow rate set value of the vacuum carburizing treatment method of the present embodiment. FIG. 7 is a diagram showing an example of a gas flow rate set value of the vacuum carburizing treatment method of the present embodiment, which is different from FIG. FIG. 8 is a diagram showing an example of a gas flow rate set value of the vacuum carburizing treatment method of the present embodiment, which is different from FIGS. 6 and 7. FIG. 9 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing process of test numbers 1, test number 5, and test numbers 7 to 12. FIG. 10 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing process of test numbers 2 to 4 and test number 6. FIG. 11 is a schematic diagram of the gas flow rate set value and the gas analysis value in the carburizing step of test numbers 13 and 14. FIG. 12 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing process of test numbers 15 to 17. FIG. 13 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing step of test number 18. FIG. 14 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing step of test number 19. FIG. 15 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing step of test number 20. FIG. 16 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing step of test number 21.

The present inventors investigated a method for suppressing carburizing variation in carburized parts in the vacuum carburizing treatment method. The present inventors first focused on the existence of carburized gas that was supplied into the vacuum carburizing furnace but was exhausted without causing a carburizing reaction. A part of the carburized gas that did not cause the carburizing reaction becomes soot and adheres to the steel material to be vacuum carburized. Soot is a source of carbon. Therefore, carbon is excessively supplied to the portion of the steel material to which soot is attached. Therefore, the adhesion of soot tends to cause variations in carburizing. On the other hand, if the carburizing gas flow rate is excessively reduced in order to suppress the adhesion of soot, the carburizing reaction becomes insufficient. In this case as well, carburizing variation is likely to occur.

Based on the above findings, the present inventors have come up with the idea of theoretically defining the flow rate of carburized gas that invades the surface of the steel material from the atmosphere inside the vacuum carburizing furnace in the carburizing process. In the present specification, the "theoretical carburized gas flow rate" is the carburized gas flow rate required to bring the carbon concentration at a predetermined depth position from the surface of the steel material to a desired concentration, and is all carburized gas. Means the carburized gas flow rate on the assumption that is used for the carburizing reaction. The present inventors adjust the flow rate of carburizing gas supplied to the vacuum carburizing furnace in the actual vacuum carburizing treatment (hereinafter referred to as the actual carburizing gas flow rate) based on the theoretical carburizing gas flow rate specified in advance. It was considered that the amount of carburized gas that does not contribute to the carburizing reaction can be suppressed and the carburizing reaction can be prevented from being insufficient, and as a result, the variation in carburizing can be suppressed.

As the vacuum carburizing process progresses, the gradient of carbon concentration becomes gentle, so the diffusion flux of carbon that invades the inside of the steel material from the surface of the steel material decreases. The flow rate of carburized gas entering the steel material from the atmosphere inside the furnace decreases with the passage of time. Therefore, the theoretical carburized gas flow rate is a function that fluctuates with the passage of time from the start of carburizing gas supply (start of carburizing process). The theoretical carburized gas flow rate can be obtained based on diffusion simulation or experimentally. Hereinafter, the determination of the theoretical carburized gas flow rate based on the diffusion simulation will be described as an example of the method of determining the theoretical carburized gas flow rate. However, as described above, the method for determining the theoretical carburized gas flow rate is not limited to the diffusion simulation.

[Theoretical carburized gas flow rate]
In the vacuum carburizing treatment method of the present embodiment, acetylene is used as the carburizing gas. The decomposition of acetylene is rate-determined by the diffusion of carbon in the surface layer of the steel material to be carburized. That is, the larger the diffusion flux of carbon that invades the inside of the steel material from the surface of the steel material, the larger the amount of decomposition of acetylene. When carburizing with a carburizing gas other than acetylene, a chemical reaction other than the carburizing reaction is assumed as described later. Therefore, it is difficult to apply it to the vacuum carburizing method of the present embodiment.

In the vacuum carburizing process, carbon diffuses in the steel material, that is, the first law of Fick is established. The flow rate of carburized gas (acetylene gas) required to bring the carbon concentration at a predetermined depth from the surface of the steel material to the desired concentration by vacuum carburizing, and all the carburized gas is used for the carburizing reaction. The carburized gas flow rate on the premise of this is defined as the theoretical carburized gas flow rate FT (t). Here, t is the time from the start of the carburizing process. The start time of the carburizing process means the time when the carburizing gas is started to be supplied into the furnace as described later. FT (t) corresponds to a value obtained by converting the flow rate of carbon invading the surface of the steel material into the flow rate of acetylene gas. In the following description, the theoretical gas flow rate is also simply referred to as "FT".

The theoretical carburized gas flow rate FT diffuses, for example, the diffusion flux J (mm · mass% / s) of carbon entering from the surface of the steel material and the amount of change in carbon concentration per unit time (∂C / ∂t). It can be calculated by calculating based on a well-known diffusion simulation using an equation. Specifically, the theoretical carburized gas flow rate can be obtained by the following method.

When diffusion occurs (that is, when the first law of Fick is established), the diffusion flux J of carbon invading from the surface of the steel material is defined by the equation (1), and the amount of change in carbon concentration per unit time (that is, when the first law of Fick is established) ∂C / ∂t) is defined by equation (2).
J = -D (∂C / ∂z) (1)
∂C / ∂t ＝－∂J / ∂z (2)
Here, D is the diffusion coefficient (mm ² / s) of carbon in the steel material. C is the mass concentration (mass%) of carbon. z is the displacement (mm) in the depth direction from the surface of the steel material. t is the time (seconds) from the start of the carburizing process. ∂ is a partial derivative symbol.

If the amount of change in carbon concentration is calculated based on the gradient of the chemical potential, the diffusion driving force of carbon will be treated strictly. In this case, the diffusion flux J (mm · mol% / s) of carbon is defined by the formula (3), and the time change of the carbon concentration is defined by the formula (4).
J = -mx (∂μ / ∂z) (3)
∂x / ∂t ＝－∂J / ∂z (4)
Here, m is the mobility of carbon (mm ² · mol / J · s). x is the molar concentration of carbon (mol%). μ is the chemical potential (J / mol) of carbon. z is the displacement (mm) in the depth direction. T in the formula (4) is the time (s) from the start of the carburizing step. ∂ is a partial derivative symbol.

Here, the driving force for carbon diffusion is the part of (∂μ / ∂z) in the equation (3). Further, the carbon concentration in austenite (γ) in the vacuum carburizing treatment is as small as 2% or less, and the molar concentration and the mass concentration are almost proportional to each other. Therefore, the formula (3) may be expressed in terms of mass concentration (mass%). When the formula (3) is expressed in% by mass, the diffusion flux J (mm · mass% / s) of carbon is defined by the formula (5), and the time change of the carbon concentration is defined by the formula (2).
J = -mC (∂μ / ∂z) (5)
C in the formula (5) is a carbon concentration (mass%).

Theoretical carburized gas flow rate FT using the first law of Fick (formulas (1), (3) and (5)) and the second law of Fick (formulas (2) and (4)). The diffusion simulation for calculating is performed by the following method.

In the vacuum carburizing treatment using acetylene as the carburizing gas, carbon invades the steel material from the surface of the steel material due to the decomposition of the carburizing gas. On the surface of the steel during the carburizing process, it is assumed that the carbon concentration in the steel increases until it is in equilibrium with graphite. Therefore, the boundary condition in the carbon diffusion simulation on the steel surface in the vacuum carburizing treatment is defined as "the carbon concentration on the steel surface is in equilibrium with graphite". Based on the above assumptions, the diffusion simulation is carried out as follows.

[Calculation method in diffusion simulation]
First, mesh data is created in which the surface layer of the steel material to be vacuum carburized is divided by a plurality of cells. A well-known size is sufficient for the size of each cell. The cell size is, for example, 1 to 500 μm. The size of the cell may be gradually increased from the surface of the steel material in the depth direction. In that case, the size ratio of adjacent cells is 0.80 to 1.25, preferably 0.90 to 1.10. However, the cell size is not limited to this. The object for which the diffusion simulation is performed may be one-dimensional. When the shape of the steel material is a round bar or a cylinder, it can be treated as one dimension by using the mesh data as a cylindrical coordinate system. Further, if the diameter of the steel material (round bar or cylinder) is 50 times or more the diffusion distance of carbon in the steel, it may be treated in the same way as a flat surface. The diffusion distance here is √Dt. The diffusion coefficient D is calculated from the carbon concentration of the steel material and the carburizing temperature. The time t (seconds) is the carburizing time (carburizing step implementation time). For example, when SCM415 specified in JIS G 4053 (2008) is used as a steel material and the carburizing temperature is 950 ° C. and the carburizing time is 51 minutes, the diffusion distance √Dt is 0.20 mm. In this case, if the diameter of the steel material is 10 mm or more, it may be handled in the same way as a flat surface. When SCM420 specified in JIS G 4053 (2008) is used as a steel material and the carburizing temperature is 950 ° C. and the carburizing time is 51 minutes, the diffusion distance √Dt is 0.21 mm. In addition, the analysis time (step time) of the diffusion simulation is set. The step time is not particularly limited, but is, for example, 0.001 to 1.0 second.

In the vacuum carburizing process, a carburizing step is carried out, and then a diffusion step is carried out. The carburizing step and the diffusion step may be set multiple times. For example, when the carburizing step and the diffusion step are set twice, the first carburizing step is carried out, and the first diffusion step is carried out after the first carburizing step. Further, the second carburizing step is carried out after the first diffusion step, and the second diffusion step is carried out after the second carburizing step. When the carburizing step and the diffusion step are carried out a plurality of times in this way, the theoretical carburized gas flow rate in the previous carburizing step is reset and the theoretical carburized gas flow rate in the next carburizing step is newly set for each carburizing step. ..

When the nth carburizing step (n is a natural number of 1 or more) is carried out and then the n + 1th carburizing step is carried out with a diffusion step of less than 1/10 of the nth carburizing step time, the nth time is carried out. The carburizing step and the n + 1th carburizing step are considered to be one carburizing step. That is, in this case, the theoretical gas flow rate set in the nth carburizing step is used as it is in the n + 1th carburizing step without being reset. In other words, if the diffusion process time between the nth carburizing step and the n + 1th carburizing step is 1/10 or more of the nth carburizing step time, the n + 1th carburizing step is the nth carburizing step. Reset the theoretical carburized gas flow rate of and set a new theoretical carburized gas flow rate.

As mentioned above, it is assumed that the carbon concentration on the surface of the steel material is in equilibrium with graphite. Therefore, based on the chemical composition of the steel material to be vacuum carburized, the equilibrium phase and equilibrium composition in an equilibrium state with graphite at the carburizing temperature are obtained by a well-known thermodynamic calculation. The chemical composition of the steel material to be vacuum carburized is thermodynamically calculated by increasing the C concentration until graphite appears as an equilibrium phase, taking into consideration that it is diluted by an increase in the C concentration. For example, when the C concentration increases by 7% by mass, the mass of the steel material itself increases 1.07 times. Therefore, the thermodynamic calculation is performed based on the chemical composition in which the concentrations of the elements other than C are 1 / 1.07 times. From the equilibrium phase and the equilibrium composition obtained by thermodynamic calculation, the C content in the steel material, the chemical potential of C, and the solid solution C concentration dissolved in austenite can be specified. Well-known thermodynamic calculation software can be used for thermodynamic calculation. The well-known thermodynamic calculation software is, for example, the trade name Pandat ™.

Similarly, cementite (θ) may precipitate inside the steel material other than the surface of the steel material in the case of vacuum carburizing. In this case, carbon (C) in the steel material is distributed to cementite and austenite. Therefore, the equilibrium phase and the equilibrium composition inside the steel material other than the steel material surface at the carburizing temperature are obtained by the above-mentioned thermodynamic calculation. Similar to the surface of the steel material, the equilibrium phase, the equilibrium composition, the C content in the steel material, the chemical potential of C, and the solid solution C concentration dissolved in austenite can be specified inside the steel material.

For the diffusion coefficient D of carbon in austenite in the steel material, a numerical value obtained in advance by an experiment using the steel material to be vacuum carburized may be used, or data reported as experimental data may be used. Good. For example, as the diffusion coefficient D (m ² / s) of C in austenite, Gray G. et al. The following equation may be used with reference to the one proposed by Tibbetts et al.
D = 4.7 × 10 ^-5 × exp (-1.6 × C- (37000-6600 × C) /1.987 / T)
Here, "C" in the formula is the solid solution C concentration (mass%) in austenite, and T is the carburizing temperature (K).

^{The mobility m (m 2} / s) of carbon in austenite in steel can be obtained from the diffusion coefficient D and thermodynamic calculation. The following formula is a formulation of the mobility m.
m = 1.54 × 10 ^-15 exp (-1.61 × C- (17300-2920 × C) / T)
Here, "C" in the formula is the solid solution C concentration (mass%) in austenite, and T is the carburizing temperature (K).

Next, the C concentration of the surface layer obtained by the vacuum carburizing treatment is set. Specifically, the target carbon concentration in the outermost cell and the target carbon concentration at a predetermined depth are set. Further, as an initial value, the solid solution C concentration in all cells = the C concentration (C ₀ ) of the chemical composition of the steel material (core), and the cementite precipitation amount is 0 in all cells.

Based on the above preconditions, the following calculation is performed for each step time.
(A) Based on the carbon concentration in each cell and the thermodynamic calculation result, the solid solution C concentration (that is, the concentration of diffused C) in austenite in each cell at the carburizing temperature is specified. At this time, it is assumed that C in cementite is fixed and only solid solution C in austenite diffuses.
(B) In each cell, the diffusion flux J in each cell is obtained by the difference method using the formula (1), the formula (3) or the formula (5) based on the specified solid solution C concentration. At this time, as described above, the solid solution carbon concentration on the surface of the steel material is set to the solid solution carbon concentration (C _sat ) at the solid solution limit in an equilibrium state with graphite. _{Based on the diffusion flux J 0} from the surface of the steel material, the acetylene flow rate is determined with the carburizing efficiency as 100%. The obtained acetylene flow rate is defined as the theoretical carburized gas flow rate at that step time.
(C) Based on the obtained diffusion flux J in each cell, the C concentration of each cell at the time when the step time elapses is determined.
(D) Based on the thermodynamic calculation result, it is determined whether cementite is generated as an equilibrium phase. The time required for cementite formation is ignored (that is, (A) is determined in the next step time).
(E) When the carburizing step is performed twice or more, the diffusion step between the carburizing steps is simulated, and then the carburizing step is simulated. In the diffusion step, calculations (A) to (D) are performed with _{the diffusion flux J 0} from the surface of the steel material as zero.

_{The above calculation is obtained for each step time, and the diffusion flux J 0} (t) of carbon from the surface of the steel material per unit surface area of the steel material during the carburizing step is obtained. _{Then, the diffusion flux J 0} (t) per unit surface surface of the steel material is converted into the acetylene gas flow rate, and further ^{multiplied by the surface surface S (m 2} ) of the steel material to be vacuum carburized, the theory at time t. The carburized gas flow rate FT (t) is obtained. In the figure in which the horizontal axis is the elapsed time from the start of carburizing (carburizing time) and the vertical axis is the theoretical carburized gas flow rate FT, the theoretical carburized gas flow rate FT is plotted by plotting the theoretical carburized gas flow rate FT at each carburizing time. It can be expressed as a theoretical carburized gas flow rate curve. FIG. 1 is a diagram showing an example of the relationship between the theoretical carburized gas flow rate and time calculated by the diffusion flux of carbon on the surface layer of the steel material obtained by the above diffusion simulation. ● in FIG. 1 indicates the theoretical carburized gas flow rate FT at each time. _{Curve C 1.00 in} FIG. 1 shows a theoretical carburized gas flow rate curve.

The approximate expression of the theoretical carburized gas flow rate curve C _1.00 can be expressed by the equation (6).
FT = S × A / √t (6)
Here, FT is the theoretical carburized gas flow rate (NL / min). A in the formula (6) can be expressed by the formula (7). T in the formula (6) is the time (minutes) from the start of the carburizing process.
A = a × T ² + b × T + c (7)
In the formula (7), a, b and c are constants determined by the chemical composition of the steel material, and T is the carburizing temperature (° C.). For example, when the steel material is SCM420 specified in JIS G 4053 (2008), a = 8.52 × ^10-5 , b = −0.140, and c = as determined by the above diffusion simulation. It is 58.2. When the steel material is SCM415 specified in JIS G 4053 (2008), a = 8.64 × ^10-5 , b = −0.141, and c = 59.0 as determined by the above diffusion simulation. is there.

Equation (6), which is an approximate expression of the theoretical carburized gas flow rate FT, is also regarded as the theoretical carburized gas flow rate FT in this specification. That is, based on the equation (6), the theoretical carburized gas flow rate FT at each carburizing time may be obtained in the actual carburizing step.

In the above explanation, the theoretical carburized gas flow rate was obtained based on a well-known diffusion simulation using the diffusion equation as an example of the method for determining the theoretical carburized gas flow rate. However, the theoretical carburized gas flow rate may be determined by other methods. For example, the theoretical carburized gas flow rate can be determined experimentally.

For example, the method of obtaining the theoretical gas flow rate by experiment is as follows. The vacuum carburizing treatment is carried out on the steel material having the same chemical composition as the steel material to be actually vacuum carburized. The flow rate of the carburized gas supplied to the vacuum carburizing furnace is kept constant, and the partial pressure of acetylene and the partial pressure of hydrogen in the vacuum carburizing furnace are continuously measured during the carburizing process. Then, the first time t0 at which the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is 1/10 or less of the time ta (that is, the total carburizing process time) which is the completion time of the carburizing process. Find the minimum value FAmin. Based on the obtained carburized gas flow rate FAmin, the theoretical carburized gas flow rate FT = FAmin√ (t0 / t).

As described above, the theoretical carburized gas flow rate is equal to the carburized gas flow rate used for the carburizing reaction in contact with the surface of the steel material. Therefore, the theoretical carburized gas flow rate is not affected by the size and shape of the heat treatment furnace.

[Vacuum carburizing method of this embodiment]
The flow rate of the carburized gas actually supplied to the vacuum carburizing furnace during the vacuum carburizing process is defined as the "actual carburized gas flow rate" FR. The present inventors investigated and examined the events assumed when the actual carburized gas flow rate FR, which greatly deviates from the relationship of the theoretical carburized gas flow rate FT at the carburizing time, as shown in FIG. 1 is used.

FIG. 2 is a diagram showing a time-dependent change in the actual carburized gas flow rate FR in the conventional carburizing step and a time-dependent change in the theoretical carburized gas flow rate FT. The vertical axis of FIG. 2 shows the carburized gas flow rate (NL / min), and the horizontal axis shows the time (minutes) from the start of the carburizing process. As described above, the solid line FR in FIG. 2 shows the actual carburized gas flow rate FR in the conventional carburizing step. The broken line C _{1.00 in} FIG. 2 indicates the theoretical carburized gas flow rate FT as described above.

With reference to FIG. 2, the start time of the carburizing process is defined as "0", and the completion time of the carburizing process is defined as "ta". That is, the carburizing step is performed from time 0 to time ta. The completion time ta is set in advance according to the set value of the carbon concentration at the predetermined depth position of the steel material after the carburizing treatment. Further, the time during which the actual carburized gas flow rate FR first becomes equal to the theoretical carburized gas flow rate FT is defined as "te".

The period from the start of the carburizing process to the time te is defined as the period S100. The period from time te to time ta is defined as the period S200. In period S100, the actual carburized gas flow rate FR is lower than the _{theoretical carburized gas flow rate FT (curve C 1.00).} Therefore, in the carburizing step of the conventional vacuum carburizing treatment method, the actual carburized gas flow rate FR in the period S100 is insufficient. In this case, on the surface of the steel material, a portion where the carburizing reaction is sufficient and a portion where the carburizing reaction is insufficient occur. Therefore, the carburizing variation on the surface of the steel material becomes large. In addition, the desired carbon concentration may not be obtained on the surface layer of the steel material. On the other hand, in the period S200, the actual carburized gas flow rate FR is higher than the _{theoretical carburized gas flow rate FT (curve C 1.00).} Therefore, in the period S200, the actual carburized gas flow rate FR becomes excessive and remains in the vacuum carburizing furnace. As a result, in period S200, soot and tar are generated by the residual carburized gas. In this case, the carburizing variation on the surface of the steel material becomes large.

Based on the above investigation results, the present inventors considered to control the actual carburized gas flow rate FR in accordance with _{the theoretical carburized gas flow rate curve C 1.00 during the carburizing step.}

However, as shown in FIG. 2, in the initial period S100 of the carburizing process, the slope of the _{theoretical carburized gas flow rate curve C 1.00 is steeper than that in the subsequent period S200.} Therefore, it was found that it is very difficult to adjust the actual carburized gas flow rate FR according to the slope of the theoretical carburized gas flow rate curve C _{1.00 during the actual operation period S100.}

Further, in the initial period S100 of the carburizing process, at the start of the carburizing process (t = 0), when the above equation (6) is adopted, the theoretical carburized gas flow rate FT becomes infinite. Therefore, in actual operation, it is extremely difficult to introduce an actual carburized gas flow rate FR equal to the theoretical carburized gas flow rate FT at the beginning of the period S100.

Therefore, the present inventors considered not only considering the theoretical carburized gas flow rate FT as an element for actually controlling the carburized gas flow rate, but also considering other factors. Actually, the gas component in the atmosphere in the vacuum carburizing furnace changes according to the carburized gas flow rate FR. This change in gas composition causes variation in carburizing and generation of soot. Therefore, the present inventors paid attention not only to the theoretical carburized gas flow rate FT but also to the gas component in the atmosphere of the vacuum carburizing furnace as an element for actually controlling the carburized gas flow rate.

The present inventors paid attention to the partial pressure of hydrogen and the partial pressure of acetylene in the atmosphere in the vacuum carburizing furnace. The hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace can be measured by a well-known analyzer. The analyzer is, for example, a quadrupole mass spectrometer.

The analyzed hydrogen partial pressure was generated in the vacuum carburizing furnace by the reaction based on the following formula.
C ₂ H ₂ → 2 C + H ₂

Hydrogen partial pressure is an index of the amount of carburizing reaction in the carburizing process. That is, the hydrogen partial pressure is an index of the degree of suppression of carburizing variation. On the other hand, the partial pressure of acetylene means the amount of surplus gas that did not cause a carburizing reaction, and is an index of the amount of soot and tar generated.

In the vacuum carburizing treatment using acetylene, the chemical reaction is extremely fast immediately after the start of the carburizing process, that is, immediately after the start of supply of acetylene into the furnace. That is, the rate of carbon entering the surface of the steel material immediately after the start of the carburizing process is extremely high. Therefore, if the flow rate of acetylene (carburized gas flow rate) supplied into the furnace is small, most of the atmosphere in the furnace becomes hydrogen gas. As a result, the hydrogen partial pressure in the furnace becomes high and the acetylene partial pressure becomes low. On the other hand, if the flow rate of acetylene gas supplied into the furnace (vacuum carburizing gas flow rate) is large, acetylene gas that does not cause a carburizing reaction remains in the furnace. In this case, the hydrogen partial pressure in the furnace becomes low, and the acetylene partial pressure becomes high. Therefore, the amount of carburizing reaction on the surface of the steel material can be estimated by monitoring the partial pressure of hydrogen and the partial pressure of acetylene in the furnace.

If the actual carburized gas flow rate FR can be controlled based on the theoretical carburized gas flow rate FT and the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, the present inventors can reduce the carburizing variation in the vacuum carburizing treatment. It was thought that it could be suppressed and the generation of soot could be suppressed. Therefore, the present inventors further investigated and obtained the following findings.

(A) If the carburizing gas flow rate is small at the initial stage of the carburizing process (around period S100), the carburizing reaction amount is small. Therefore, the rate at which the acetylene partial pressure rises is slow. As a result, the variation in carburizing becomes large, and the carbon concentration in the surface layer of the carburized parts also becomes low.

(B) The completion time of the carburizing process is defined as ta. As described above, the completion time ta is set in advance according to the set values of the surface carbon concentration and the carburizing depth of the steel material after the carburizing treatment. Then, the time from the start time of the carburizing process to 1/10 of the completion time ta is defined as ta / 10. The theoretical carburized gas flow rate at time _{ta / 10} is defined as FT ta / 10. If the actual carburized gas flow rate FR at the initial stage of the carburizing process is set to the theoretical carburized gas flow rate FT _{ta / 10} or more at time ta / 10, the hydrogen partial pressure in the atmosphere in the vacuum carburizing furnace rises rapidly, but it is early. The rate of increase in acetylene partial pressure increases. As a result, the shortage of the carburizing reaction amount at the initial stage of the carburizing process can be suppressed, and the carburizing variation can be reduced.

(C) On the other hand, if the actual carburized gas flow rate FR at the initial stage of the carburizing process is too large, the partial pressure of acetylene in the furnace rises excessively quickly. In this case, excess acetylene gas remains in the furnace. As a result, soot or tar is generated, and carburizing variation occurs. The theoretical carburized gas flow rate 4 seconds after the start of the carburizing process is defined _{as FT 4.} If the actual carburized gas flow rate FR is FT ₄ or less at the initial stage of the carburizing process, it is possible to prevent the actual carburized gas flow rate FR in the furnace from becoming excessively large. Therefore, the variation in carburizing can be suppressed.

(D) If the actual carburized gas flow rate FR is maintained at a high level, the acetylene partial pressure gradually increases. Therefore, the partial pressure of acetylene greatly exceeds the partial pressure of hydrogen at any time. In this case, in the atmosphere inside the vacuum carburizing furnace, excess gas that does not cause a carburizing reaction is excessively present. Therefore, soot generated by the surplus gas is generated and adheres to the surface of the carburized parts. As a result, the carburizing variation becomes large.

(E) In the carburizing step, when the partial pressure of acetylene becomes 0.8 times or more the partial pressure of hydrogen, if the actual carburized gas flow rate FR is maintained or reduced, excess gas is suppressed in the atmosphere in the vacuum carburizing furnace. can do. Therefore, the variation in carburizing can be suppressed.

Based on the above findings, the present inventors can secure a sufficient amount of carburizing reaction at the initial stage of the carburizing process by adjusting the actual carburized gas flow rate FR in the carburizing process as shown in (I) to (III) below. It was thought that it could be done, and after that, excess gas could be suppressed to suppress the generation of soot and tar, and the variation in carburizing could be reduced.

Here, each term is defined as follows.
Time ta: Completion time of carburizing process Time t0: First time when the partial pressure of acetylene becomes 0.8 times or more of the partial pressure of hydrogen after the start of the carburizing process Time ta / 10: From the start time of the carburizing process to the completion time ta 1/10 time Time 4t0: Time when four times the period from the start of the carburizing process to the time t0 elapses after the start of the carburizing process Early carburizing step S1: Period from the start of the carburizing process to time t0 Late carburizing step S2 : Period from time t0 to time ta Actual carburizing gas flow rate FR: Carburizing gas (acetylene) flow rate actually supplied to the vacuum carburizing furnace Theoretical carburizing gas flow rate FT _{ta / 10} : Theoretical carburizing gas flow rate theory at time ta / 10 Carburizing gas flow rate FT ₄ : Theoretical carburizing gas flow rate 4 seconds after the start of the carburizing process

When the above terms are defined, the actual carburized gas flow rate FR is adjusted as shown in (I) to (III) below, as shown in FIG.
(I) In the first stage carburizing step S1, the actual carburized gas flow rate FR is set to FT _{ta / 10} or more and FT ₄ or less. When the actual carburized gas flow rate FR is constant in the first carburizing step S1, that value is taken as the actual carburized gas flow rate FA.
(II) In the late carburizing step S2, the actual carburized gas flow rate FR is set to FA × √ (t0 / t) or more and FA or less in the period of time t0 to 4t0.
(III) In the late carburizing step S2, the actual carburized gas flow rate FR is set to FA × √ (t0 / t) or more and 2FA × √ (t0 / t) or less in the period from time 4t0 to time ta.
Here, t is the time from the start of carburizing.

FIG. 3 shows the time course of the actual carburized gas flow rate in the carburizing step of the vacuum carburizing treatment method according to the present embodiment (see the figure below) and the time course of the acetylene partial pressure and the hydrogen partial pressure in the atmosphere of the vacuum carburizing furnace in the carburizing step (see the figure below). It is a figure which shows (the above figure). With reference to FIG. 3, in the present embodiment, the actual carburized gas flow rate FR is adjusted within the range of the hatched region in FIG. 3 during the period from time t0 to time ta. The curve that is the lower limit of the hatching region is a curve of carburized gas flow rate = FA × √ (t0 / t). The upper limit of the hatching region is a curve of carburized gas flow rate = 2FA × √ (t0 / t). Both FA × √ (t0 / t) and 2FA × √ (t0 / t) are equations proportional to the equation (6) of the theoretical carburized gas flow rate FT.

As described above, the time t0 is the first time after the start of the carburizing step, when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure. As shown in FIG. 3, in the initial stage of the early carburizing step S1, the hydrogen partial pressure rises more rapidly than the acetylene partial pressure. This is because the carburizing reaction occurs actively. The hydrogen partial pressure rises rapidly and then begins to fall before the acetylene partial pressure. Then, as a result of the decrease in the hydrogen partial pressure, the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure. At this point, that is, after the start of the carburizing step, the first time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is defined as the time t0. The "0.8" times referred to here is a value obtained by rounding down the second decimal place of the calculated value of the acetylene partial pressure / hydrogen partial pressure ratio.

The vacuum carburizing treatment method according to the present embodiment completed based on the above knowledge has the following configuration.

[1]
This is a vacuum carburizing method that performs vacuum carburizing treatment on steel materials in a vacuum carburizing furnace.
A heating process that heats the steel material to the carburizing temperature,
After the heating step, a heat equalizing step of soaking the steel material at the carburizing temperature and
After the heat soaking step, a carburizing step of holding the steel material at the carburizing temperature while supplying a carburizing gas which is an acetylene gas into the vacuum carburizing furnace.
After the carburizing step, a diffusion step of stopping the supply of the carburized gas into the vacuum carburizing furnace and holding the steel material at the carburizing temperature,
A quenching step of performing quenching on the steel material after the diffusion step, and
With
In the carburizing step
The flow rate of the carburized gas supplied into the vacuum carburizing furnace is defined as the actual carburized gas flow rate.
The flow rate of the carburized gas required for the vacuum carburizing treatment of the steel material is defined as the theoretical carburized gas flow rate.
The completion time of the carburizing process is defined as ta.
When the first time after the start of the carburizing step that the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is defined as t0,
The carburizing step is
A partial pressure measuring step of continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace to specify the time t0.
The early carburizing process from the start of the carburizing process to the time t0, and
The late carburizing step from time t0 to time ta,
Including
In the early carburizing process,
The actual carburized gas flow rate is set to be equal to or more than the theoretical carburized gas flow rate at time ta/10 and equal to or less than the theoretical carburized gas flow rate 4 seconds after the start of the carburizing step.
In the late carburizing step,
When the actual carburized gas flow rate of the early carburizing step is defined as FA and the time from the start of the carburizing step is defined as time t.
The actual carburized gas flow rate during the period from time t0 to time 4t0 is set to FA√ (t0 / t) or more and FA or less.
The actual carburized gas flow rate from the time 4t0 to the time ta is set to FA√ (t0 / t) or more and 2FA√ (t0 / t) or less.
Vacuum carburizing method.

[2]
The vacuum carburizing method according to [1].
In the late carburizing step,
In the period of time 4t0 to time ta, the actual gas carburizing flow rate is reduced by the method (A) or (B) with the passage of time.
Vacuum carburizing method.
(A) The actual carburized gas flow rate is gradually reduced by repeating the maintenance and reduction of the actual carburized gas flow rate.
(B) The actual carburized gas flow rate is gradually reduced with the passage of time.

[3]
The vacuum carburizing treatment method according to [1] or [2].
The theoretical carburized gas flow rate is determined based on a diffusion simulation using a diffusion equation.
Vacuum carburizing method.

[4]
It is a method of manufacturing carburized parts.
A step of carrying out the vacuum carburizing treatment method according to any one of [1] to [3] is provided on the steel material.
Manufacturing method of carburized parts.

Hereinafter, the vacuum carburizing treatment method and the manufacturing method of carburized parts according to the present embodiment will be described in detail.

[Vacuum carburizing method]
FIG. 4 is a diagram showing an example of a heat pattern of the vacuum carburizing treatment method of the present embodiment. With reference to FIG. 4, the vacuum carburizing treatment method of the present embodiment includes a heating step (S10), a soaking step (S20), a carburizing step (S30), a diffusion step (S40), and a quenching step (S50). ) And. The details of each step will be described below.

[Heating step (S10)]
In the heating step (S10), the steel material is heated to the carburizing temperature. The steel material to be subjected to the vacuum carburizing treatment may be provided by a third party or may be manufactured by a person who implements the vacuum carburizing treatment method. The chemical composition of the steel material is not particularly limited. It is sufficient to use a well-known steel material to be carburized. The steel material is, for example, an alloy steel material for machine structure specified in JIS G 4053 (2008). More specifically, the steel material is, for example, SCr415, SCr420, SCM415, etc. specified in JIS G 4053 (2008).

The steel material to be prepared may be a hot-worked steel material or a cold-worked steel material. Hot working is, for example, hot rolling, hot extrusion, hot forging and the like. Cold working includes, for example, cold rolling, cold drawing, cold forging and the like. The steel material may be one that has been subjected to machining typified by cutting after being hot-worked or cold-worked.

In the heating step (S10), the steel material is charged into the vacuum carburizing furnace and the steel material is heated to the carburizing temperature Tc. The heating step (S10) is a well-known step in the vacuum carburizing treatment method. A well-known temperature is sufficient for the carburizing temperature Tc. The carburizing temperature Tc is _{equal to} or higher than the Ac3 transformation point. The preferred range of carburizing temperature Tc is 900 to 1130 ° C. When the carburizing temperature Tc is 900 ° C. or higher, heat transfer due to radiation becomes high, and the temperature in the vacuum carburizing furnace tends to be uniform. As a result, the carburizing variation of the steel material tends to be small. When the carburizing temperature is 1130 ° C. or lower, it is possible to prevent the crystal grain size of the steel material from becoming coarse, and it is possible to suppress a decrease in the strength of the steel material. A more preferable lower limit of the carburizing temperature Tc is 910 ° C, and even more preferably 920 ° C. A more preferable upper limit of the carburizing temperature Tc is 1100 ° C., and even more preferably 1080 ° C.

[Heat soaking step (S20)]
In the heat equalizing step (S20), the steel material is held for a predetermined time at the carburizing temperature Tc. Hereinafter, the holding time in the heat equalizing step (S20) is also referred to as a heat equalizing time. The heat soaking step (S20) is a well-known step in the vacuum carburizing treatment method. The soaking time can be appropriately adjusted depending on the shape and / or size of the steel material. Preferably, the soaking time is 10 minutes or more. More specifically, when the cross section perpendicular to the longitudinal direction of the steel material is converted into a circle, the preferable heat equalization time is 30 minutes or more per 25 mm of the equivalent diameter of the circle. For example, when the equivalent circle diameter is 30 mm, the soaking time is preferably 36 minutes or more. The preferable upper limit of the soaking time is preferably 120 minutes, more preferably 60 minutes.

The pressure in the furnace in the heating step (S10) and the soaking step (S20) is not particularly limited. The pressure in the furnace in the heating step (S10) and the soaking step (S20) may be, for example, 100 Pa or less. In the heating step (S10) and / or the soaking step (S20), nitrogen gas may be introduced and vacuum exhausted by a vacuum pump to create a nitrogen atmosphere of 1000 Pa or less. In the soaking step (S20), the inside of the vacuum carburizing furnace is made low pressure or vacuum by at least the start of the carburizing step (S30). For example, in the soaking step (S20), the inside of the vacuum carburizing furnace is set to 10 Pa or less by the start of the carburizing step (S30).

[Carburizing step (S30)]
In the present specification, the carburizing step (S30) means a step of supplying carburizing gas in a furnace under reduced pressure or vacuum. That is, the time when the carburizing gas is started to be supplied into the furnace under reduced pressure or vacuum after the soaking step (S20) is the start of the carburizing step (S30). In the carburizing step (S30), the carburized gas is supplied into the furnace while maintaining the inside of the furnace at a low pressure. Since the pressure inside the furnace is low, the frequency of collision between carburized gas molecules is reduced. That is, the frequency of decomposition of the carburized gas in the atmosphere inside the furnace is reduced. Therefore, by supplying the carburized gas to the surface of the steel material under low pressure, the generation of soot and tar can be suppressed. As a result, the surface carbon concentration of the steel material can be rapidly increased. In the carburizing step (S30) from the start of carburizing to the end of carburizing (time ta), for example, the inside of the furnace is set to 1 to 1000 Pa. However, the pressure inside the furnace in the carburizing step (S30) is not limited to the above range.

In the carburizing step (S30), a carburized gas is introduced into the vacuum carburizing furnace, and the steel material is held at the carburizing temperature Tc for a predetermined time.

[Carburized gas]
In the present embodiment, the carburizing gas used in the carburizing step (S30) of the vacuum carburizing treatment method is acetylene gas.

Propane gas is often used in the conventional vacuum carburizing treatment. However, in addition to the carburizing reaction, propane gas also causes a decomposition reaction into methane, ethylene, acetylene, hydrogen and the like. Most of the methane and ethylene produced by the decomposition reaction do not contribute to the carburizing reaction and are exhausted from the vacuum carburizing furnace. Therefore, when propane gas is used, the theoretical carburized gas flow rate FT cannot be calculated by diffusion simulation using the diffusion flux of carbon obtained by the diffusion equation. On the other hand, acetylene is unlikely to cause reactions other than carburizing. Therefore, the theoretical carburized gas flow rate FT can be calculated by a diffusion simulation using the diffusion flux of carbon obtained by the diffusion equation.

In this embodiment, the purity of acetylene, which is a carburized gas, may be 98% or more. As the acetylene, for example, acetylene dissolved in acetone or acetylene dissolved in dimethylformamide (DMF) may be used as the carburizing gas. Preferably, acetylene dissolved in DMF is used as the carburizing gas. In this case, it is possible to suppress the mixing of the solvent into the atmosphere inside the furnace. When the supply source of acetylene to the vacuum carburizing furnace is a cylinder, the primary pressure when supplying acetylene from the cylinder into the vacuum carburizing furnace is preferably 0.5 MPa or more. When supplying to a vacuum carburizing furnace, preferably, the pressure is reduced to 0.20 MPa or less by using a pressure reducing valve.

[Details of carburizing process (S30)]
The carburizing step (S30) includes a partial pressure measuring step S0, an early carburizing step S1, and a late carburizing step S2. The details of each step will be described below.

[Advance preparation]
Before implementing the vacuum carburizing treatment method, the theoretical carburized gas flow rate FT according to the target steel material is determined as a preliminary preparation, and the carburizing step (S30) is completed up to the completion time ta as shown in FIG. The time course of the theoretical carburized gas flow rate FT is obtained. The theoretical carburized gas flow rate FT may be determined based on a diffusion simulation or an experiment.

[Partial pressure measurement step S0]
In the partial pressure measurement step S0, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are measured during the carburizing step (S30). Specifically, the partial pressure of hydrogen and the partial pressure of acetylene in the atmosphere in the vacuum carburizing furnace are continuously measured. Here, "continuously" means measuring the hydrogen partial pressure and the acetylene partial pressure a plurality of times over time. The hydrogen partial pressure and the acetylene partial pressure may be measured continuously, or may be measured at predetermined time intervals. The measurement is performed using a well-known partial pressure measuring device. The partial pressure measuring instrument is, for example, a quadrupole mass spectrometer. However, as the partial pressure measuring instrument, a partial pressure measuring instrument other than the quadrupole mass spectrometer may be used.

In the partial pressure measurement step S0, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are measured over time. That is, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are monitored. The time t0 (the first time after the start of the carburizing step that the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure) is determined based on the hydrogen partial pressure and the acetylene partial pressure measured over time.

When using a quadrupole mass analyzer as a partial pressure measuring instrument, the quadrupole mass analyzer measures each component gas (hydrogen, acetylene) in order. Therefore, the measurement time of hydrogen partial pressure and the measurement time of acetylene are different. The analysis time of each component (hydrogen, acetylene) of the quadrupole mass analyzer is preferably 0.2 seconds or more and 2.0 seconds or less, and the analysis interval is preferably 4.0 seconds or less.

For example, in the case of using a quadrupole mass analyzer as a voltage division measuring instrument, hydrogen is analyzed in 0.5 seconds, acetylene is analyzed in 0.5 seconds, and 2.0 from the start of hydrogen analysis. After a second, assume that hydrogen is analyzed again in 0.5 seconds and then acetylene is analyzed in 0.5 seconds. In this case, the analysis time of each component (hydrogen, acetylene) is 0.5 seconds, and the analysis interval is 2.0 seconds. In the following description, the analysis period of each component is defined as "analysis step". Further, the period from the start time of the measurement step to the start time of the next measurement step is defined as "analysis interval". In the case of the above example, the analysis step is 1.0 second (hydrogen analysis time 0.5 seconds + acetylene analysis time 0.5 seconds), and the analysis interval is 2.0 seconds.

When a quadrupole mass analyzer is used as the partial pressure measuring instrument, the time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure, that is, the time t0 in FIG. 3 is determined by the following method. Let t1 be the start time of an analysis step and t2 be the completion time of that analysis step. During the analysis step, the hydrogen partial pressure may be measured first, or the acetylene partial pressure may be measured first. Further, the start time of the next analysis step is defined as t3, and the completion time of the analysis step is defined as t4. At this time, the analysis period is a time between time t1 and time t3.

In this case, the acetylene partial pressure obtained in the analysis step at time t1 to time t2 is 0.8 times or more the hydrogen partial pressure obtained in the same analysis step (that is, the analysis step at time t1 to time t2). And the hydrogen partial pressure obtained in the analysis step at the next time t3 to time t4 after the lapse of the analysis interval is 1.25 of the acetylene partial pressure obtained in the analysis step at time t1 to time t2. When it is double or less, the completion time t2 of the analysis step in which the acetylene partial pressure is measured is defined as the time t0.

Not only is the partial pressure of acetylene more than 0.8 times the partial pressure of hydrogen obtained in the same analysis step, but the partial pressure of hydrogen obtained in the next analysis step is the acetylene obtained in the previous analysis step. The reason for the condition that the partial pressure is 1.25 times or less is as follows. If the carburized gas starts to flow into the furnace after the hydrogen partial pressure measurement is completed and before the acetylene partial pressure measurement in the analysis steps of time t1 to time t2, this is the case. The hydrogen partial pressure obtained in the analysis step is zero. Therefore, the acetylene partial pressure obtained in this analysis step is always 0.8 times or more the hydrogen partial pressure. When the completion time of this analysis step is determined to be time t0, it does not mean that the acetylene gas is actually sufficiently introduced into the furnace. Therefore, it is necessary not to recognize such a case as time t0. In the above case, the hydrogen partial pressure measured in the next analysis step (time t3 to time t4) after the lapse of the analysis interval is 1.25 times larger than the acetylene partial pressure obtained in the previous analysis step. Exceed. This is because the partial pressure of hydrogen rises sharply due to the introduction of acetylene gas.

On the other hand, if the acetylene partial pressure obtained as a result of sufficient introduction of the carbonized gas into the furnace is 0.8 times or more the hydrogen partial pressure obtained in the same analysis step, after the analysis interval has elapsed. The partial pressure of hydrogen obtained in the next analysis step is 1.25 times or less the partial pressure of acetylene obtained in the previous analysis step. This is because, as shown in FIG. 3, when the carburized gas is sufficiently introduced into the furnace, the hydrogen partial pressure does not increase with the passage of time, but rather decreases.

Therefore, when a quadrupole mass analyzer is used as the partial pressure measuring instrument, the obtained acetylene partial pressure is 0.8 times or more the hydrogen partial pressure obtained in the same analysis step, and the analysis interval elapses. If the hydrogen partial pressure obtained in the subsequent next analysis step is 1.25 times or less the acetylene partial pressure obtained in the previous analysis step, the completion time of the analysis step in which the acetylene partial pressure is measured is measured. t2 is defined as time t0.

The gas in the furnace (hydrogen, acetylene) may be analyzed in the furnace, or may be extracted and analyzed outside the furnace. When analyzing the gas in the furnace, a partial pressure measuring device installed in the furnace is used. The partial pressure measuring device may be a measuring device other than the above-mentioned quadrupole mass spectrometer. Further, the partial pressure measuring device may be used properly for each component gas. For example, the acetylene partial pressure may be analyzed by a quadrupole mass analyzer, and the hydrogen partial pressure may be analyzed by another pressure dividing measuring instrument.

In the carburizing step (S30), the carburizing gas is supplied under the above-mentioned reduced pressure. Therefore, the carburized gas rapidly undergoes a carburizing reaction in the entire furnace. Therefore, the measurement result of the partial pressure of the gas in the furnace is unlikely to vary in the furnace. That is, the analysis result of the gas in the furnace can be regarded as almost uniform in the furnace.

[First term carburizing process S1]
As shown in FIG. 3, the period from the start of the carburizing step (S30) to the first time t0 when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is defined as the early carburizing step S1. In the early carburizing step S1, the actual carburized gas flow rate FR is adjusted so as to satisfy the following condition I.
(I) In the first stage carburizing step S1, the actual carburizing gas flow rate FR is set to be the theoretical carburizing gas flow rate FT _{ta / 10} or more and the theoretical carburizing gas flow rate FT ₄ or less.

FIG. 5 is a diagram showing an example of a gas flow rate set value in the early carburizing step S1 of the vacuum carburizing treatment method of the present embodiment. In the early carburizing step S1, the actual carburized gas flow rate FR is set within the range of the hatching region in FIG. 5 (FT _{ta / 10} or more and FT ₄ or less).

If the actual carburized gas flow rate FR in _{the early carburizing step S1 is less than the theoretical carburized gas flow rate FT ta / 10} at time ta / 10, the supply of carburized gas is too insufficient in the early carburizing step S1. In this case, the carburizing variation becomes large in the steel material (carburized parts) subjected to the vacuum carburizing treatment method. On the other hand, if the actual carburized gas flow rate FR in the first carburizing step S1 _{exceeds the theoretical carburized gas flow rate FT 4} at 4 seconds from the start of the carburizing process, the actual carburized gas flow rate FR is too large. In this case, it takes time to adjust the actual carburized gas flow rate FR to FA × √ (t0 / t) or more and 2FA√ (t0 / t) or less by time 4t0 after the lapse of time t0. Therefore, excess gas (acetylene gas) remains excessively in the vacuum carburizing furnace, and soot is likely to be generated. As a result, the carburizing variation becomes large in the carburized parts (steel materials) manufactured by carrying out the vacuum carburizing treatment method.

If the actual carburizing gas flow rate FR in the first carburizing step S1 is the theoretical carburizing gas flow rate FT _{ta / 10} _{or more at time ta / 10 and the theoretical carburizing gas flow rate FT 4} or less at 4 seconds from the start of the carburizing process, On the premise that the conditions II and III of the actual carburizing gas flow rate FR in the late carburizing step S2 described later are satisfied, the carburizing variation of the carburized parts (steel material) after the vacuum carburizing treatment can be sufficiently suppressed. The actual carburized gas flow rate FR in the early carburizing step S1 can be adjusted by a well-known method. For example, the flow rate of the carburized gas supplied to the vacuum carburizing furnace may be adjusted by the supply valve to adjust the actual carburized gas flow rate FR, or the actual carburized gas flow rate FR may be adjusted by another well-known method. May be good. In fact, the adjustment of the carburized gas flow rate FR may be carried out by a well-known control device of the vacuum carburizing furnace. The control device actually adjusts the carburized gas flow rate FR by adjusting the opening degree of the supply valve described above, for example.

It is preferable that the actual carburized gas flow rate FR in the first carburizing step S1 is constant. If the actual carburized gas flow rate FR is constant, the fluctuations of the hydrogen partial pressure and the acetylene partial pressure in the furnace can be measured with high accuracy. If the actual carburized gas flow rate FR in the early carburizing step S1 fluctuates, the fluctuation of the hydrogen partial pressure in the furnace and the fluctuation of the acetylene partial pressure are affected by the fluctuation of the actual carburized gas flow rate FR. If the actual carburized gas flow rate FR in the first carburizing step S1 is constant, the fluctuations of the hydrogen partial pressure and the acetylene partial pressure in the furnace can be measured with high accuracy. Therefore, it is preferable that the actual carburized gas flow rate FR in the early carburizing step S1 is constant. For example, as shown in FIG. 3, the actual carburized gas flow rate FR in the early carburizing step S1 is preferably constant. In this case, the value of the actual carburized gas flow rate FR that was constant throughout the previous carburizing step S1 becomes the actual carburized gas flow rate FA in the first carburizing step S1. However, in actual operation, it is a well-known technical common sense to those skilled in the art that the actual carburized gas flow rate does not become completely constant according to the set value and fluctuates within a certain range from the set value. Therefore, when the actual carburized gas flow rate FR in the previous carburizing step S1 is constant, the actual carburized gas flow rate FR allows a margin of ± 10% of the set value. That is, when the actual carburized gas flow rate FR changes within ± 10% of the specific set value through the previous carburizing step S1, the set value is set as the value of the actual carburized gas flow rate FA in the previous carburizing step. That is, in the present specification, FA means the carburized gas flow rate within the range of ± 10% of the set value in the early carburizing step S1. Preferably, FA is within the range of ± 5% of the set value in the early carburizing step S1.

[Late carburizing process S2]
As shown in FIG. 3, the period from the time t0 to the completion time ta of the carburizing step is defined as the late carburizing step S2. In the late carburizing step S2, the actual carburized gas flow rate FR is adjusted so as to satisfy the following conditions II and III.
(II) In the late carburizing step S2, the actual carburized gas flow rate FR is set to FA × √ (t0 / t) or more and FA or less in the period of time t0 to 4t0.
(III) In the late carburizing step S2, the actual carburized gas flow rate FR is set to FA × √ (t0 / t) or more and 2FA × √ (t0 / t) or less in the period from time 4t0 to time ta.
Here, t is the time from the start of carburizing.

In short, in the late carburizing step S2, the actual carburized gas flow rate FR is adjusted so as to be within the hatching range in FIG. As a result, it is possible to prevent excess carburizing gas from remaining in the vacuum carburizing furnace in the late carburizing step S2. As a result, the generation of soot and tar can be reduced, and the carburizing variation of the carburized parts (steel material) after the vacuum carburizing treatment method can be suppressed.

[Condition II]
If the actual carburized gas flow rate is less than FA × √ (t0 / t) in the period t0 to 4t0 of the late carburizing step S2, the gas flow rate is insufficient. In this case, the distribution of the carburized gas varies in the vacuum carburizing furnace. For example, the concentration of carburized gas is high in the vicinity of the carburized gas supply nozzle, and the concentration of carburized gas is low in the region away from the supply nozzle. As a result, the carburizing variation becomes large in the steel material after the vacuum carburizing treatment step.

On the other hand, if the actual carburized gas flow rate exceeds FA in the period of time t0 to 4t0 in the late carburizing step S2, the carburized gas is excessively supplied. In this case, soot and tar are generated by this surplus gas. As a result, the carburizing variation of the carburized parts (steel material) after the vacuum carburizing treatment becomes large.

Therefore, the actual carburized gas flow rate FR is set to FA × √ (t0 / t) or more and FA or less in the period of time t0 to 4t0 in the late carburizing step S2. In this case, the carburized gas flow rate required for the carburizing reaction can be sufficiently secured and the generation of soot and tar can be suppressed, provided that the conditions I and III are satisfied. As a result, it is possible to suppress the occurrence of carburizing variation of carburized parts. As described above, the actual carburized gas flow rate FR allows a margin of ± 10% of the set value. Therefore, as described above, there is a similar margin for the actual carburized gas flow rate FA in the early carburizing step S1. That is, in the present specification, the actual carburized gas flow rate FA in the early carburizing step means the carburized gas flow rate within the range of ± 10% of the set value of the actual carburized gas FR in the early carburizing step S1. Further, until the middle of the time t0 to 4t0 of the late carburizing step S2, the actual carburized gas flow rate FR is maintained by FA following the early carburizing step S1, and then the actual carburized gas flow rate is FA to FA × √ (t0 /). It may be adjusted within the range of t).

[About Condition III]
If the actual carburized gas flow rate is less than FA × √ (t0 / t) in the period of time 4t0 to ta in the late carburizing step S2, the gas flow rate is insufficient. In this case, the distribution of the carburized gas varies in the vacuum carburizing furnace. For example, the concentration of carburized gas is high in the vicinity of the carburized gas supply nozzle, and the concentration of carburized gas is low in the region away from the supply nozzle. As a result, the carburizing variation becomes large in the steel material after the vacuum carburizing treatment step.

On the other hand, if the actual carburized gas flow rate exceeds 2FA × √ (t0 / t) in the period of 4t0 to ta in the late carburizing step S2, the carburized gas is excessively supplied. In this case, soot and tar are generated by this surplus gas. As a result, the carburizing variation of the carburized parts (steel material) after the vacuum carburizing treatment becomes large.

Therefore, the actual carburized gas flow rate FR is set to FA × √ (t0 / t) or more and 2FA × √ (t0 / t) or less in the period of time 4t0 to ta in the late carburizing step S2. In this case, the carburized gas flow rate required for the carburizing reaction can be sufficiently secured and the generation of soot and tar can be suppressed, provided that the conditions I and II are satisfied. As a result, it is possible to suppress the occurrence of carburizing variation of carburized parts.

In the late carburizing step S2, if the actual carburized gas flow rate FR satisfies the conditions II and III, the change with time of the actual carburized gas flow rate FR is not particularly limited. For example, as shown in FIG. 6, the reduction of the actual carburized gas flow rate FR may be started within the period of time 4t0 to ta in the late carburizing step S2.

In the late carburizing step S2, as shown in FIG. 6, the actual carburized gas flow rate FR may be maintained and reduced repeatedly with the passage of time to gradually reduce the actual carburized gas flow rate FR. Further, as shown in FIG. 7, in the late carburizing step S2, the actual carburized gas flow rate FR may be gradually reduced with the passage of time. Further, as shown in FIG. 8, the actual carburized gas flow rate FR may be gradually decreased and then increased with the passage of time. In short, if the conditions II and III are satisfied in the late carburizing step S2, the time-dependent fluctuation of the actual carburized gas flow rate FR is not particularly limited.

[Carburizing gas pressure in carburizing step (S30)]
The carburizing gas pressure (carburizing gas pressure) in the carburizing step (S30) is not particularly limited. Preferably, the carburized gas pressure in the early carburizing step S1 is made higher than the carburized gas pressure in the late carburizing step S2. In this case, the generation of soot is further suppressed in the late carburizing step S2. More preferably, the carburized gas pressure in the late carburizing step S2 is lowered with the passage of time. The preferable carburizing gas pressure in the carburizing step (S30) is 1 kPa or less.

[Time ta of carburizing step (S30)]
The time ta, which is the time from the start (t = 0) of the carburizing step (S30) to the completion, is before the start of the vacuum carburizing treatment according to the target carbon concentration of the surface layer of the steel material after the vacuum carburizing treatment step. It is set as appropriate. The time ta may be determined by the above-mentioned diffusion simulation using the diffusion equation. The time ta may be determined from experimental data by conducting a vacuum diffusion treatment test in advance. The longer the time ta, the better. The longer the time ta, the easier it is to actually adjust the carburized gas flow rate FR. The preferred lower limit of the time ta is 50 seconds, more preferably 1 minute (60 seconds), and even more preferably 3 minutes (180 seconds). The preferred upper limit of the time ta is 120 minutes, more preferably 60 minutes.

[Diffusion step (S40)]
The diffusion step (S40) is a well-known step in the vacuum carburizing treatment method. In the diffusion step (S40), the supply of the carburized gas to the vacuum carburizing furnace is stopped, and the steel material is held at the carburizing temperature Tc for a predetermined time. In the diffusion step (S40), the carbon that has entered the steel material in the carburizing step (S30) is diffused inside the steel material. As a result, the carbon concentration of the surface layer increased in the carburizing step (S30) decreases, and the carbon concentration of a predetermined depth increases. In the diffusion step (S40), nitrogen gas is introduced into the vacuum carburizing furnace and vacuum exhaust is performed by a vacuum pump to create a nitrogen atmosphere of 1000 Pa or less, or to create a vacuum. The vacuum is, for example, 10 Pa or less. By setting the inside of the vacuum carburizing furnace to a nitrogen atmosphere of 1000 Pa or less or a vacuum state, the invasion and desorption of carbon from the surface of the steel material is suppressed.

The holding time in the diffusion step (S40) is appropriately set according to the target carbon concentration of the surface layer of the steel material after the vacuum carburizing treatment step. Therefore, the holding time in the diffusion step (S40) is not particularly limited.

[Quenching process (S50)]
In the quenching step (S50), the steel material for which the carburizing step (S30) and the diffusion step (S40) have been completed is held at the quenching temperature (Ts) for a predetermined time, and then rapidly cooled (quenched). As a result, the surface layer portion of the steel material having an increased C concentration is transformed into martensite to form a hardened layer. The quenching step (S50) is a well-known step in the vacuum carburizing treatment method.

As shown in FIG. 4, when the quenching temperature Ts is lower than the carburizing temperature Tc, the steel material after the diffusion step (S40) is cooled to the quenching temperature Ts. The cooling rate in this case is not particularly limited. Considering the processing time of the vacuum carburizing process, it is preferable that the cooling rate is high. The preferred cooling rate is 0.02 to 30.00 ° C./sec. The cooling rate referred to here is the temperature difference between the carburizing temperature Tc and the quenching temperature Ts divided by the cooling time.

When the quenching temperature Ts is less than the carburizing temperature Tc, a known cooling method may be used as the cooling method for the steel material. For example, the steel material may be cooled by allowing it to cool under vacuum, or the steel material may be cooled by gas cooling. When the steel material is allowed to cool under vacuum, it is preferably allowed to cool at a pressure of 100 Pa or less. When the steel material is cooled by using gas cooling in cooling, it is preferable to use an inert gas as the cooling gas. As the inert gas, for example, nitrogen gas and / or helium gas is preferably used. As the inert gas, it is particularly preferable to use inexpensively available nitrogen gas. By using an inert gas as the cooling gas, oxidation of the steel material can be suppressed.

After holding the steel material at the quenching temperature Ts for a predetermined time, the steel material is rapidly cooled. Quenching temperature Ts is not particularly limited as long as _{A 3} transformation point _{(A r3} transformation point) or more. The preferred lower limit of the quenching temperature Ts is 800 ° C., more preferably 820 ° C., and even more preferably 850 ° C. The preferred upper limit of the quenching temperature Ts is 1130 ° C., more preferably 1100 ° C., still more preferably 950 ° C., still more preferably 900 ° C., still more preferably 880 ° C.

A known quenching method is used as the quenching method in the quenching step (S50). The quenching method is, for example, gas cooling, water cooling, or oil cooling.

Implement the above vacuum carburizing method and use steel as a carburized part. In the vacuum carburizing treatment method of the present embodiment, the theoretical carburized gas flow rate FT for the steel material to be vacuum carburized is used. Then, the carburizing step (S30) is divided into an early carburizing step S1 and a late carburizing step S2 at the first time after the start of the carburizing step when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure. Then, the actual carburized gas flow rate FR is adjusted so that the condition I is satisfied in the early carburizing step S1 and the conditions II and III are satisfied in the late carburizing step S2. As a result, it is possible to suppress the occurrence of carburizing variation in the steel material after the vacuum carburizing treatment.

In addition, the vacuum carburizing treatment method of this embodiment may further include other steps. For example, the vacuum carburizing treatment method may include a tempering step after the quenching step (S50). It suffices to carry out the tempering process under well-known conditions. For example, in the tempering step, the steel material is held at a temperature equal to or lower than the _{Ac1 transformation point for a predetermined time, and then cooled.}

Further, in the vacuum carburizing treatment method of the present embodiment, the carburizing step (S30) and the diffusion step (S40) may be repeated a plurality of times. In this case, as described above, the time ta and the theoretical carburized gas flow rate FT are determined for each carburizing step (S30).

[Manufacturing method of carburized parts]
The method for manufacturing a carburized part of the present embodiment includes a step of manufacturing a carburized part by carrying out the above-mentioned vacuum carburizing treatment method on a steel material. In the carburized parts manufactured by the above steps, variations in carburizing can be suppressed.

Hereinafter, the effect of the vacuum carburizing treatment method of the present embodiment will be described more specifically by way of examples. The conditions in the following examples are one condition example adopted for confirming the feasibility and effect of the vacuum carburizing treatment method of the present embodiment. Therefore, the vacuum carburizing treatment method of the present embodiment is not limited to this one-condition example.

A steel pipe for mechanical structure (hereinafter referred to as a steel pipe) having a chemical composition corresponding to SCM415 specified in JIS G 4053 (2008) and a round bar corresponding to SCM415 were prepared. The C content of the steel pipe and the round bar of each test number was 0.15% by mass. The diameter of the steel pipe was 34 mm, the wall thickness was 4.5 mm, and the length was 110 mm. The diameter of the round bar was 26 mm and the length was 70 mm. The evaluation of the vacuum carburizing treatment was performed with a round bar, and the steel pipe was used as a dummy material for investigating the variation in carburizing depending on the position of the round bar in the vacuum carburizing furnace.

^{The total surface area (m 2} ) of the round bar and steel pipe vacuum carburized in each test number was defined as the surface area of the steel material (m ^2). The surface area of the steel material was calculated by the following formula.
Surface area of steel material = surface area per steel pipe x number of steel pipes + surface area per round bar x number of round bars Table 1 shows the surface area of the obtained steel material. In test numbers 1 to 5, 10 to 13, 15 and 16, 18 to 21, 248 steel pipes and 3 round bars were used. In test number 6, 496 steel pipes and 3 round bars were used. In test numbers 7-9, 14 and 17, 124 steel pipes and 3 round bars were used.

First, a diffusion simulation using the diffusion equation was performed to obtain the theoretical carburized gas flow rate. Specifically, it was divided into a plurality of cells having a thickness of 2 μm or more in the thickness direction of the round bar and the steel pipe. The step time in the diffusion simulation was set to 0.002 to 0.02 seconds. In the chemical composition of the steel pipe and the round bar (SCM415), the equilibrium composition in the equilibrium state with graphite on the surface at the carburizing temperature was obtained by thermodynamic calculation. Furthermore, the equilibrium composition inside the steel material at the carburizing temperature, the chemical potential of carbon, and the mobility of carbon were determined. The trade name Pandat ™ was used for the thermodynamic calculation. Further, the database used the trade name PanFe ™. The following formula was used for the mobility of carbon (m ^{2 / s).}
m = 1.54 × 10 ^-15 exp (-1.61 × C- (17300-2920 × C) / T)
Here, C in the formula is the solid solution C concentration (mass%) in austenite, and T is the carburizing temperature (K).

The target value of carbon concentration on the surface of the steel pipe and round bar was set to 0.70% by mass, and the target value of carbon concentration at a depth of 1.0 mm from the surface was set to 0.40% by mass. With the above as a precondition, the diffusion simulations (A) to (D) described above were carried out for each step time, and the theoretical carburized gas flow rate FT for each step time was obtained.

As a result of calculating the theoretical carburized gas flow rate FT, the theoretical carburized gas flow rate FT could be approximated to the following equation.
FT = S × A / √t (6)
^{Here, A is the carburized gas flow rate (NL / min) per 1 m 2} defined by the formula (7), and t is the time (minute) from the start of carburizing. Further, S indicates the surface area of the steel material (m ² ).
A = a × T ² + b × T + c (7)
In the case of this example (SCM415), a = 8.64 × ^10-5 , b = −0.141, and c = 59.0.

After calculating the theoretical carburizing gas flow rate FT, the actual vacuum carburizing treatment was carried out by the following method. First, a basket made of a fully carburized stainless steel material (SUS316 specified in JIS G 4303 (2012)) was prepared. The above-mentioned number of steel pipes were arranged evenly in an upright position in the car, and three round bars were placed in an upright state in the center of the car, in front of the left side of the car, and in the back right of the car. As described above, the round bar was used as a test material, and the steel pipe was used as a dummy material for confirming the occurrence of carburizing variation due to the arrangement location of the round bar.

A car with steel materials (steel pipes and round bars) was inserted into a vacuum carburizing furnace, and vacuum carburizing was performed. Then, carburized parts of test numbers 1 to 21 were obtained. The conditions for the vacuum carburizing treatment were as shown in Table 1.

Specifically, in each test number, vacuum carburizing treatment was carried out as follows. In the vacuum carburizing treatment at each test number, the pressure in the furnace was kept below 10 Pa. In the heating step, the round bars of each test number were heated to the carburizing temperature Tc shown in Table 1. After the heating step, a soaking step was carried out. In the soaking step, the steel material (round bar) was held for 60 minutes at the carburizing temperature Tc.

After the heat soaking process, a carburizing process was carried out. In the carburizing step, acetylene was supplied as a carburizing gas into the vacuum carburizing furnace. The carburized gas pressure in the carburizing step was kept below 1 kPa. The completion time ta (minutes) of the carburizing step was as shown in Table 1.

As described above, the carburizing time in the carburizing step and the diffusion time in the diffusion step were adjusted with the goal of setting the carbon concentration of the round bar to 0.40% by mass at a depth of 1.0 mm.

In the carburizing step, the gas in the atmosphere in the vacuum carburizing furnace was analyzed with a quadrupole mass analyzer to continuously measure the hydrogen partial pressure and the acetylene partial pressure. The mass-to-charge ratio (m / z) of hydrogen was set to 2, and the mass-to-charge ratio of acetylene was set to 26. The analysis time was 0.5 seconds and the analysis interval was 4 seconds. Based on the obtained hydrogen partial pressure and acetylene partial pressure, the time t0 (the first time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure) was determined.

The changes over time in the actual carburized gas flow rate of each test number were as shown in FIGS. 9 to 16. Hereinafter, the set values of the actual carburized gas flow rate FR of test numbers 1 to 21 will be described with reference to FIGS. 9 to 16.

FIG. 9 is a diagram showing changes over time in the actual carburized gas flow rate FR in the carburizing steps of

test numbers

1, 5, 7 to 12. With reference to FIG. 9, in

test numbers

1, 5, 7 to 12, the actual carburized gas flow rate FR at the carburizing process start time (t = 0) was defined as FA. FA was FT _{ta / 10} or more and FT ₄ or less. The actual carburized gas flow rate FR was kept constant as FA until the time ts before the time t0 was exceeded and the time 4t0 was reached. After time ts, the actual carburized gas flow rate was gradually reduced along the curve C2 (= FA × √ (ts / t)). As a result, the actual carburized gas flow rate FR was FT _{ta / 10} or more and FT ₄ or less in the early carburizing step S1. Further, in the late carburizing step S2, the actual carburized gas flow rate FR in the period from time t0 to time 4t0 was FA√ (t0 / t) or more and FA or less. Further, the actual carburized gas flow rate FR from time 4t0 to time ta was FA√ (t0 / t) or more and 2FA√ (t0 / t) or less.

FIG. 10 is a diagram showing changes over time in the actual carburized gas flow rate FR in the carburizing steps of test numbers 2 to 4 and 6. With reference to FIG. 10, in test numbers 2 to 4 and 6, the actual carburized gas flow rate FR at the carburizing process start time (t = 0) was defined as FA. FA was FT _{ta / 10} or more and FT ₄ or less. The actual carburized gas flow rate FR was kept constant as FA until the time ts before the time t0 was exceeded and the time 4t0 was reached. The time ts in the period from time t0 to time 4t0 in FIG. 10 was later than the time ts in the period from time t0 to time 4t0 in FIG. After time ts, the actual carburized gas flow rate FR was gradually reduced along the curve C2 (= FA × √ (ts / t)). As a result, the actual carburized gas flow rate FR was FT _{ta / 10} or more and FT ₄ or less in the early carburizing step S1. Further, in the late carburizing step S2, the actual carburized gas flow rate FR in the period from time t0 to time 4t0 was FA√ (t0 / t) or more and FA or less. Further, the actual carburized gas flow rate FR from time 4t0 to time ta was FA√ (t0 / t) or more and 2FA√ (t0 / t) or less.

FIG. 11 is a diagram showing changes over time in the actual carburized gas flow rate FR in the carburizing steps of test numbers 13 and 14. With reference to FIGS. 11, in test numbers 13 and 14, the FA, which is the actual carburized gas flow rate FR at the carburizing process start time (t = 0), was less than _{FT ta / 10.} Then, at a time ts later than the time ta/10, the actual carburized gas flow rate FR was gradually reduced in the same manner as the theoretical carburized gas flow rate FT. During the carburizing step, the partial pressure of acetylene in the vacuum carburizing furnace was not more than 0.8 times the partial pressure of hydrogen. Therefore, t0 was not specified during the vacuum carburizing treatment.

FIG. 12 is a diagram showing changes over time in the actual carburized gas flow rate FR in the carburizing steps of test numbers 15 to 17. With reference to FIG. 12, in test numbers 15 to 17, the FA, which is the actual carburized gas flow rate FR at the carburizing process start time (t = 0), is FT _{ta / 10} or more and FT ₄ or less. It was. However, at the time ts before the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure, the actual carburized gas flow rate FR starts to gradually decrease, and the actual carburized gas flow rate FR becomes FA × √ (ts / t). It was adjusted to be. Therefore, during the carburizing step, the partial pressure of acetylene in the vacuum carburizing furnace did not become 0.8 times or more the partial pressure of hydrogen. Therefore, t0 was not specified during the vacuum carburizing treatment.

FIG. 13 is a diagram showing the time course of the actual carburized gas flow rate FR in the carburizing step of test number 18. With reference to FIG. 13, in test number 18, the FA, which is the actual carburized gas flow rate FR at the carburizing process start time (t = 0), was FT _{ta / 10} or more and FT ₄ or less. Then, the actual carburized gas flow rate FR was kept constant as FA until the time ts that exceeded the time t0 and exceeded the time 4t0. After time ts, the actual carburized gas flow rate FR was gradually reduced along the curve C2 (= FA × √ (ts / t)). As a result, the actual carburizing gas flow rate FR is FT _{ta / 10} or more and FT ₄ or less in the early carburizing step S1, and the actual carburizing in the period from time t0 to time 4t0 in the late carburizing step S2. The gas flow rate FR was FA√ (t0 / t) or more and FA or less. However, the actual carburized gas flow rate FR from time 4t0 to time ta exceeded 2FA√ (t0 / t).

FIG. 14 is a diagram showing the time course of the actual carburized gas flow rate FR in the carburizing step of test number 19. With reference to FIG. 14, in test number 19, FA, which is the actual carburized gas flow rate FR at the carburizing process start time (t = 0), was less than _{FT ta / 10.} Further, the actual carburized gas flow rate FR after that was made constant by FA. In Test No. 19, the partial pressure of acetylene in the vacuum carburizing furnace did not become 0.8 times or more the partial pressure of hydrogen during the carburizing step. Therefore, t0 was not specified during the vacuum carburizing treatment.

FIG. 15 is a diagram showing the time course of the actual carburized gas flow rate FR in the carburizing step of test number 20. With reference to FIG. 15, in test number 20, FA, which is the actual carburized gas flow rate FR at the carburizing process start time (t = 0), was FT _{ta / 10} or more and FT ₄ or less. Then, the actual carburized gas flow rate FR was kept constant as FA until the time ts exceeding the time t0 and less than the time 4ta. After time ts, the actual carburized gas flow rate FR was gradually reduced along a curve C2 (= FB × √ (ts / t)) using an FB (see FIG. 15) lower than the theoretical carburized gas flow rate FT at time ts. As a result, the actual carburized gas flow rate FR was less than FA√ (t0 / t) in the late carburizing step S2.

FIG. 16 is a diagram showing the time course of the actual carburized gas flow rate FR in the carburizing step of test number 21. With reference to FIG. 16, in test number 21, the FA, which is the actual carburized gas flow rate FR at the carburizing process start time (t = 0), was FT _{ta / 10} or more and FT ₄ or less. Then, the actual carburized gas flow rate FR was kept constant as FA until the time ts exceeding the time t0 and less than the time 4ta. After time ts, the actual carburized gas flow rate FR was reduced. However, between the time 4ta and the time ta, there was a period in which the actual carburized gas flow rate FR exceeded 2FA√ (t0 / t).

The actual carburized gas flow rate was adjusted and measured using a flow meter (manufactured by Cofflock Co., Ltd., trade name: mass flow controller D3665).

After the carburizing step, the diffusion step was carried out on the round bar at the diffusion time (minutes) shown in Table 1, and the carbon that had penetrated into the round bar was diffused into the round bar. The diffusion step was carried out at a pressure in the furnace of 10 Pa or less while maintaining the carburizing temperature. The diffusion time (minutes) was as shown in Table 1.

In the "FT _{ta / 10} " column in Table 1, the theoretical carburized gas flow rate (NL / min) at the time ta / 10 is described. In the "FT ₄ " column, the theoretical carburized gas flow rate (NL / min) at 4 seconds from the start of the carburizing process is described. In the "hour t0 (minute)" column, the hour t0 (minute) is described. In the "hour 4t0 (minutes)" column, the time 4t0 (minutes) is described. In the "hours ts (minutes)" column, the time ts (minutes) at which the carburizing gas flow rate FR actually started to gradually decrease is described. In the "FR ≧ FA × √ (t0 / t)?" Column, it is described whether or not the actual carburized gas flow rate was FA × √ (t0 / t) or more in the time 4t0 to the time ta. When "YES", it means that the actual carburized gas flow rate FR was FA × √ (t0 / t) or more. When it is "NO", it means that the actual carburized gas flow rate FR was less than FA × √ (t0 / t). In the "FR ≦ 2FA × √ (t0 / t)?" Column, it is described whether or not the actual carburized gas flow rate FR was 2FA × √ (t0 / t) or less in the time 4t0 to the time ta. When "YES", it means that the actual carburized gas flow rate FR was 2FA × √ (t0 / t) or less. When it is "NO", it means that the actual carburized gas flow rate FR exceeds 2FA × √ (t0 / t). "Diffusion time (minutes)" indicates the diffusion time (minutes) in the diffusion step.

After the diffusion step, the round bar was cooled to 860 ° C. Then, it was held at the quenching temperature (860 ° C.) for 30 minutes. After holding, the round bar was immersed in oil at 120 ° C. and oil-quenched. The round bar after quenching was tempered. The tempering temperature was 170 ° C., and the holding time at the tempering temperature was 2 hours.

By the above manufacturing process, vacuum carburizing treatment was carried out to manufacture carburized parts (round bars).

[Evaluation test]
The carbon concentration of the surface layer of the carburized part (round bar) of each test number and the depth at which the carbon concentration was 0.40% by mass (hereinafter referred to as the carburized depth) were measured to evaluate the carburizing variation.

[Carburized parts surface carbon concentration measurement test]
In the carburized parts (round bars) of each test number in the state of being inserted into the vacuum carburizing furnace, a range of 20 mm in the longitudinal direction of the carburized parts from the upper end surface and a range of 5 mm in the longitudinal direction of the carburized parts from the lower end surface were cut. .. Hereinafter, the range of 20 mm from the upper end surface is referred to as an "upper end surface test piece", and the portion within a range of 5 mm from the lower end surface is referred to as a "lower end portion".

Turning was performed on the circumferential surface of the upper end surface test piece and the remaining part where the lower end portion was cut (hereinafter referred to as the main body portion). In the turning process, chips from the surface layer portion from the surface of the round bar to a depth of 0.30 mm were collected at every 0.05 mm depth pitch. The carbon concentration of the chips at each depth position at a pitch of 0.05 mm was measured. Through the above steps, the three carburized parts of each test number (center position of the car, left front position of the car, and right back position of the car) have a 0.05 mm pitch in the surface layer region from the surface to a depth of 0.30 mm. The carbon concentration in was determined. The six carbon concentrations from the surface of the carburized part arranged at the center of the car to 0.30 mm were defined as carbon concentrations A1 to A6 (mass%) in order from the surface. The six carbon concentrations up to 0.30 mm from the surface of the carburized parts arranged at the front left side of the car were defined as carbon concentrations B1 to B6 (mass%) in order from the surface. The six carbon concentrations from the surface of the carburized parts arranged at the back right of the car to 0.30 mm were defined as carbon concentrations C1 to C6 (mass%) in order from the surface. Then, in the three carburized parts, the difference between the maximum value and the minimum value of the carbon concentration obtained at the same depth position was obtained. Specifically, the maximum value and the minimum value were selected from the carbon concentrations A1, B1, and C1 in the region from the surface to a depth of 0.05 mm, and the difference value of the carbon concentration was defined as Δ1. Similarly, the maximum value and the minimum value were selected from the carbon concentrations A2, B2, and C2 in the region from the surface to the depth position of 0.05 mm to 0.10 mm, and the difference value of the carbon concentration was defined as Δ2. By the above steps, Δ1 to Δ6 were obtained, and the arithmetic mean value of Δ1 to Δ6 was defined as “surface carbon concentration difference” (mass%). The obtained results are shown in the "Surface carbon concentration difference (mass%)" column of Table 1.

Furthermore, all arithmetic average values of carbon concentrations A1 to A6, B1 to B6, and C1 to C6 were defined as surface average carbon concentrations (mass%). The obtained results are shown in the "Surface average carbon concentration (mass%)" column of Table 1.

[Carburizing depth measurement test]
Using the above-mentioned upper end surface test piece, the carbon concentration of the surface layer portion of the circumferential surface was measured. Specifically, the carbon concentration of the cross section (cross section perpendicular to the longitudinal direction of the upper end surface test piece) at a position 20 mm from the upper end surface of the upper end surface test piece is measured in the radial direction from a depth position 2 mm from the surface toward the surface. It was measured. Specifically, line analysis was performed by EPMA (electron probe mouth analyzer) to measure the carbon concentration in the radial direction (depth direction). Based on the measurement results, the depth of the region where the carbon concentration is 0.40% by mass or more (hereinafter referred to as carburizing depth) was determined for each of the three upper end surface test pieces. The average of the difference between the maximum value and the minimum value of the carburizing depth obtained from each upper end surface test piece was defined as "0.40 mass% depth difference" (mm). The obtained results are shown in the "0.40 mass% depth difference (mm)" column of Table 1.

[Evaluation results]
With reference to Table 1, a method having a surface carbon concentration difference of 0.030% by mass or less and a depth difference of 0.40% by mass of 0.05 mm or less is excellent as a vacuum carburizing treatment method having a small carburizing variation. I evaluated that it was.

With reference to Table 1, in test numbers 1 to 12, the actual carburized gas flow rate FR was FT _{ta / 10} or more and FT ₄ or less in the early carburizing step S1. Further, in the late carburizing step S2, the actual carburized gas flow rate FR was FA × √ (t0 / t) or more and FA or less in the period of time t0 to 4t0. Further, in the late carburizing step S2, the actual carburized gas flow rate FR was FA × √ (t0 / t) or more and 2FA × √ (t0 / t) or less in the period from time 4t0 to time ta. Therefore, the average carbon concentration of the surface layer was 0.680 mass% or more, the surface carbon concentration difference was 0.030 mass% or less, and the depth difference of 0.40 mass% was 0.05 mm or less. That is, the carburizing variation of the carburized parts was small.

On the other hand, in test numbers 13 and 14, as shown in FIGS. 11 and 1, the actual carburized gas flow rate (FA) in the early carburizing step was less than _{FT ta / 10.} Therefore, the average carbon concentration in the surface layer was less than 0.680% by mass, and carburizing was not sufficiently performed.

In test numbers 15 to 17, as shown in FIGS. 12 and 1, the actual carburized gas flow rate (FA) at the start of carburizing was FT _{ta / 10} or more and FT ₄ or less, but the acetylene partial pressure was 0 of the hydrogen partial pressure. The actual carburized gas flow rate FR was gradually reduced before it became more than 0.8 times. Therefore, the average carbon concentration in the surface layer was less than 0.680% by mass, and carburizing was not sufficiently performed.

In test number 18, as shown in FIG. 13 and Table 1, the time ts for gradually reducing the actual carburized gas flow rate FR was later than the time 4t0. As a result, the actual carburized gas flow rate FR after the gradual decrease exceeded 2FA × √ (t0 / t). As a result, the difference in carbon concentration in the surface layer exceeded 0.030% by mass, and the carburizing variation of the carburized parts was large.

In test number 19, as shown in FIG. 14 and Table 1, the actual carburized gas flow rate FR was constant at a value FA of less than _{FT ta / 10.} Therefore, the 0.40 mass% depth difference exceeded 0.05 mm, and the carburizing variation of the carburized parts was large.

In test number 20, as shown in FIG. 15 and Table 1, the value FA of the actual carburizing gas flow rate at the start of carburizing was FT _{ta / 10} or more and FT ₄ or less, but the actual carburizing was performed between time 4t0 and time ta. There was a period during which the gas flow rate FR was less than FA × √ (t0 / t). Therefore, the average carbon concentration in the surface layer was less than 0.680% by mass, and carburizing was not sufficiently performed. Further, the difference in surface carbon concentration exceeded 0.030% by mass, the difference in depth of 0.40% by mass exceeded 0.05 mm, and the carburizing variation of the carburized parts was large.

In test number 21, as shown in FIG. 16 and Table 1, the value FA of the actual carburizing gas flow rate at the start of carburizing was FT _{ta / 10} or more and FT ₄ or less, but the actual carburizing was performed between time 4t0 and time ta. There was a period when the gas flow rate FR exceeded 2FA × √ (t0 / t). Therefore, the difference in carbon concentration in the surface layer exceeded 0.030% by mass, and the carburizing variation of the carburized parts was large.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

Claims

This is a vacuum carburizing method that performs vacuum carburizing treatment on steel materials in a vacuum carburizing furnace.
A heating process that heats the steel material to the carburizing temperature,
After the heating step, a heat equalizing step of soaking the steel material at the carburizing temperature and
After the heat soaking step, a carburizing step of holding the steel material at the carburizing temperature while supplying a carburizing gas which is an acetylene gas into the vacuum carburizing furnace.
After the carburizing step, a diffusion step of stopping the supply of the carburized gas into the vacuum carburizing furnace and holding the steel material at the carburizing temperature,
A quenching step of performing quenching on the steel material after the diffusion step, and
With
In the carburizing step
The flow rate of the carburized gas supplied into the vacuum carburizing furnace is defined as the actual carburized gas flow rate.
The flow rate of the carburized gas required for the vacuum carburizing treatment of the steel material is defined as the theoretical carburized gas flow rate.
The completion time of the carburizing process is defined as ta.
When the first time after the start of the carburizing step that the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is defined as t0,
The carburizing step is
A partial pressure measuring step of continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace to specify the time t0.
The early carburizing process from the start of the carburizing process to the time t0, and
The late carburizing step from time t0 to time ta,
Including
In the early carburizing process,
The actual carburized gas flow rate is set to be equal to or more than the theoretical carburized gas flow rate at time ta/10 and equal to or less than the theoretical carburized gas flow rate 4 seconds after the start of the carburizing step.
In the late carburizing step,
When the actual carburized gas flow rate of the early carburizing step is defined as FA and the time from the start of the carburizing step is defined as time t.
The actual carburized gas flow rate during the period from time t0 to time 4t0 is set to FA√ (t0 / t) or more and FA or less.
The actual carburized gas flow rate from the time 4t0 to the time ta is set to FA√ (t0 / t) or more and 2FA√ (t0 / t) or less.
Vacuum carburizing method.
The vacuum carburizing treatment method according to claim 1.
In the late carburizing step,
In the period of time 4t0 to time ta, the actual gas carburizing flow rate is reduced by the method (A) or (B) with the passage of time.
Vacuum carburizing method.
(A) The actual carburized gas flow rate is gradually reduced by repeating the maintenance and reduction of the actual carburized gas flow rate.
(B) The actual carburized gas flow rate is gradually reduced with the passage of time.
The vacuum carburizing treatment method according to claim 1 or 2.
The theoretical carburized gas flow rate is determined based on a diffusion simulation using a diffusion equation.
Vacuum carburizing method.
It is a method of manufacturing carburized parts.
A step of carrying out the vacuum carburizing treatment method according to any one of claims 1 to 3 is provided on the steel material.
Manufacturing method of carburized parts.