CN114341392B - Vacuum carburizing treatment method and manufacturing method of carburized parts - Google Patents

Abstract

Providing: a vacuum carburization method capable of suppressing uneven carburization distribution. In the early stage carburizing step of the vacuum carburizing treatment method of the present embodiment, when the end time of the carburizing step is defined as ta, the actual carburizing gas flow rate is equal to or higher than the theoretical carburizing gas flow rate at the time ta/10 and equal to or lower than the theoretical carburizing gas flow rate at the time 4 seconds from the start of the early stage carburizing step, in the later stage carburizing step, the initial time at which the acetylene partial pressure becomes equal to or higher than 0.8 times the hydrogen partial pressure after the start of the carburizing step is defined as t0, and when the actual carburizing gas flow rate at the start of the early stage carburizing step is defined as FA, the actual carburizing gas flow rate in the period of time t0 to time 4t0 is equal to or higher than FA (t 0/t) and equal to or lower than FA (t 0/t), and the actual carburizing gas flow rate in the period of time 4t0 to time ta is equal to or higher than FA (t 0/t) and equal to or lower than FA (t 0/t).

Description

Vacuum carburizing treatment method and manufacturing method of carburized parts

Technical field

The present invention relates to a vacuum carburizing treatment method and a manufacturing method of carburized parts. It should be noted that in this specification, a carburized steel part is called a "carburized part."

Background technique

Steel parts requiring 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 process and a diffusion process. In the carburizing process, 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. Examples of hydrocarbon gases include acetylene, propane, and the like. In the diffusion process, after the carburizing process, the introduction of the carburizing gas is stopped, and carbon is diffused in the depth direction of the surface layer of the steel material. By adjusting the time of the carburizing process and the diffusion process, etc., the carbon concentration in the surface layer of the steel material can be controlled.

However, hydrocarbon gas as carburizing gas is thermodynamically unstable. Therefore, when the carburizing temperature is high, the carburizing gas is easily decomposed into carbon, hydrogen, etc. When the carburizing temperature is high, the carburizing gas molecules move actively. Due to the active movement, carburizing gas molecules collide with each other at high speed, and the carburizing gas decomposes. Due to the decomposition of carburizing gas, coal and tar are produced. At this time, the surface carbon concentration and carburization depth are unevenly distributed. Therefore, the surface layer of the carburized component cannot be maintained at a constant quality. Therefore, in the vacuum carburizing treatment method, it is desired to suppress the uneven distribution of the carbon concentration on the surface of the carburized component and the uneven distribution of the carburization depth in the surface layer. In the following description, the uneven distribution of carbon concentration on the surface of the carburized component and the uneven distribution of the carburization depth in the surface layer of the carburized component will be referred to as "carburizing uneven distribution."

Japanese Patent Application Laid-Open No. 8-325701 (Patent Document 1), Japanese Patent Application Laid-Open No. 2016-148091 (Patent Document 2), Japanese Patent Application Laid-Open No. 2002-173759 (Patent Document 3), Japanese Patent Application Laid-Open No. 2005-350729 (Patent Document 4) and Japanese Patent Application Publication No. 2012-7240 (Patent Document 5) propose techniques for suppressing uneven distribution of carburization.

The vacuum carburizing treatment method described in Patent Document 1 vacuum-heats a workpiece made of steel in a heating chamber of a vacuum carburizing furnace, and supplies carburizing gas into the heating chamber to perform carburizing. In this vacuum carburizing treatment method, gaseous chain unsaturated hydrocarbons are used as the carburizing gas. Furthermore, the carburizing process is performed by setting the heating chamber to a vacuum state of 1 kPa or less. Patent Document 1 describes that this suppresses the generation of coal and enables uniform carburization.

In the vacuum carburizing treatment method described in Patent Document 2, a carburizing gas is injected into a carburizing chamber in a reduced-pressure atmosphere to carburize the object to be processed placed in the carburizing chamber. In this vacuum carburizing treatment method, a constant is set based on the volume of the carburizing chamber in the loaded state of the object to be processed, the volume of the carburizing chamber, the total surface area of the object to be processed, and the type of carburizing gas. Thereby, the gas injection amount of the carburizing gas injected into the carburizing chamber is calculated. Then, the calculated gas injection amount of carburizing gas is injected into the carburizing chamber. Patent Document 2 describes that this can prevent point-like excessive carburization from occurring.

In the vacuum carburizing atmosphere gas control system described in Patent Document 3, propane gas is used as the carburizing gas. In this control system, carburizing gas is supplied into the vacuum carburizing furnace in which the material to be carburized is installed. In addition, the carbon generated by the thermal decomposition reaction of the carburizing gas is dissolved and diffused into the material to be carburized, and the carburizing treatment of the material to be carburized is performed. In this control system, the partial pressure of hydrogen generated due to the thermal decomposition reaction is measured in real time during the carburizing process. Furthermore, based on the measured value, the amount of carburizing gas supplied to the furnace is adjusted and controlled in real time. Patent Document 3 describes that high-quality carburized steel can be stably produced by this.

In the vacuum carburizing method described in Patent Document 4, the theoretical flow rate V of the carburizing gas required for the carburizing process and the carburizing time t are calculated based on the carburizing depth and surface carbon concentration based on the internal diffusion of the material. The relationship V=f(t). Furthermore, in the early stage of carburization in the carburizing process, the carburizing flow rate V1 is supplied which is sufficiently larger than the theoretical flow rate V and does not cause blackening. Furthermore, in the later stage of carburization that follows the early stage of carburization, a diffusion-time flow rate V2 that is smaller than the theoretical flow rate V is supplied. Patent Document 4 describes that this can prevent the generation of coal and reduce the remaining cementite.

In the vacuum carburizing method described in Patent Document 5, the temporal change in the theoretical flow rate of the carburizing gas required for the carburizing treatment is determined based on the diffusion of carbon into the interior of the workpiece. Then, based on the time change of the theoretical flow rate, the partial pressure ratio of the hydrogen produced by the carburizing reaction under the theoretical flow rate relative 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 determined, and the time change of the theoretical hydrogen partial pressure ratio is compared with the time change of the hydrogen partial pressure ratio relative to the total pressure in the treatment chamber during the actual carburizing treatment. Based on these approximations, the degree of uneven distribution of carburizing quality within the same operation batch is determined. Patent Document 5 describes that this can improve the reproducibility of the quality of carburized parts and reduce uneven quality distribution of carburized parts.

existing technical documents

patent documents

Patent Document 1: Japanese Patent Application Publication No. 8-325701

Patent Document 2: Japanese Patent Application Publication No. 2016-148091

Patent Document 3: Japanese Patent Application Publication No. 2002-173759

Patent Document 4: Japanese Patent Application Publication No. 2005-350729

Patent Document 5: Japanese Patent Application Publication No. 2012-7240

However, uneven carburization distribution can also be suppressed by a method different from the vacuum carburizing treatment methods of Patent Documents 1 to 5.

Contents of the invention

Invent the problem to be solved

An object of the present disclosure is to provide a vacuum carburizing treatment method and a method for manufacturing carburized parts that can suppress uneven carburization distribution.

solutions to problems

The vacuum carburizing treatment method based on the present disclosure is a vacuum carburizing treatment method that performs vacuum carburizing treatment on steel in a vacuum carburizing furnace. The vacuum carburizing treatment method includes:

The heating process is to heat the aforementioned steel to the carburizing temperature;

Soaking process: after the aforementioned heating process, the aforementioned steel material is soaked at the aforementioned carburizing temperature;

In the carburizing process, after the aforementioned soaking process, acetylene gas, that is, carburizing gas, is supplied into the aforementioned vacuum carburizing furnace while maintaining the aforementioned steel material at the aforementioned carburizing temperature;

In the diffusion process, after the aforementioned carburizing process, the supply of the aforementioned carburizing gas to the aforementioned vacuum carburizing furnace is stopped, and the aforementioned steel material is maintained at the aforementioned carburizing temperature; and,

The quenching process is to quench the aforementioned steel material after the aforementioned diffusion process,

In the aforementioned carburizing process,

The flow rate of the carburizing gas supplied into the aforementioned vacuum carburizing furnace is defined as the actual carburizing gas flow rate,

The flow rate of the aforementioned carburizing gas required for the aforementioned vacuum carburizing treatment of the aforementioned steel material is defined as the theoretical carburizing gas flow rate,

Define the end time of the aforementioned carburizing process as ta,

Define the first time after the start of the carburizing process when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure as t0,

The aforementioned carburizing process includes:

Continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, and determining the partial pressure measurement process at the time t0;

A preliminary carburizing step from the start of the aforementioned carburizing step to time t0; and,

The post-carburizing process from the aforementioned time t0 to time ta,

In the aforementioned early carburizing process,

The actual carburizing gas flow rate is set to be greater than the theoretical carburizing gas flow rate at time ta/10 and less than the theoretical carburizing gas flow rate at 4 seconds from the start of the carburizing process,

In the aforementioned late carburizing process,

When the actual carburizing gas flow rate in the early carburizing process is defined as FA, and the time from the start of the carburizing process is defined as time t,

The actual carburizing gas flow rate during the time t0 to time 4t0 is set to be greater than FA√(t0/t) and less than FA,

The actual carburizing gas flow rate from the time 4t0 to the time ta is set to be FA√(t0/t) or more and 2FA√(t0/t) or less.

The method for manufacturing a carburized component based on the present disclosure includes a step of subjecting the steel material to the vacuum carburizing treatment method.

Effect of invention

The vacuum carburizing treatment method of the present disclosure can suppress uneven carburizing distribution. The method of manufacturing a carburized component of the present disclosure can manufacture a carburized component in which uneven carburization distribution is suppressed.

Description of the drawings

FIG. 1 is a diagram showing an example of the relationship between the theoretical carburizing gas flow rate and time calculated from the diffusion flux of carbon in the surface layer of the steel material obtained by diffusion simulation using the diffusion equation.

FIG. 2 is a graph showing the time-dependent change of the actual carburizing gas flow rate and the time-dependent change of the theoretical carburizing gas flow rate in the conventional carburizing process.

3 is a diagram illustrating changes over time in the actual carburizing gas flow rate in the carburizing step of the vacuum carburizing treatment method of the present embodiment (lower graph), and acetylene in the atmosphere of the vacuum carburizing furnace in the carburizing step. A graph showing the time-dependent changes in partial pressure and hydrogen partial pressure (above).

FIG. 4 is a diagram showing an example of a thermal mode of the vacuum carburizing treatment method according to this embodiment.

FIG. 5 is a diagram showing an example of a gas flow rate setting value in the preliminary carburizing step of the vacuum carburizing treatment method according to this embodiment.

FIG. 6 is a diagram showing an example of a gas flow rate setting value in the vacuum carburizing treatment method of this embodiment.

FIG. 7 is a diagram different from FIG. 6 , showing an example of the gas flow rate setting value of the vacuum carburizing treatment method of this embodiment.

FIG. 8 is a diagram different from FIGS. 6 and 7 , showing an example of a gas flow rate setting value in the vacuum carburizing treatment method of this embodiment.

FIG. 9 is a schematic diagram showing gas flow setting values and gas analysis values in the carburizing processes of test numbers 1, 5, and 7 to 12.

FIG. 10 is a schematic diagram of the gas flow rate setting value and the gas analysis value in the carburizing process of Test No. 2 to Test No. 4 and Test No. 6. FIG.

FIG. 11 is a schematic diagram of the gas flow rate setting value and the gas analysis value in the carburizing process of Test No. 13 and Test No. 14.

FIG. 12 is a schematic diagram of the gas flow rate setting value and the gas analysis value in the carburizing process of test numbers 15 to 17.

FIG. 13 is a schematic diagram showing the gas flow rate setting value and the gas analysis value in the carburizing process of test number 18.

FIG. 14 is a schematic diagram of the gas flow rate setting value and gas analysis value in the carburizing process of test number 19.

FIG. 15 is a schematic diagram of the gas flow rate setting value and gas analysis value in the carburizing process of test number 20.

FIG. 16 is a schematic diagram showing the gas flow rate setting value and the gas analysis value in the carburizing process of test number 21.

Detailed ways

The present inventors studied a method of suppressing uneven carburization distribution in carburized parts in a vacuum carburizing treatment method. The present inventors initially focused on the existence of a carburizing gas that is exhausted without causing a carburizing reaction even though it is supplied into a vacuum carburizing furnace. A part of the carburizing gas that does not cause carburizing reaction becomes coal and adheres to the steel material to be subjected to vacuum carburizing treatment. Coal becomes a supply source of carbon. Therefore, carbon is excessively supplied to the portion where coal adheres to the steel material. Therefore, uneven carburization distribution becomes likely to occur due to coal adhesion. On the other hand, if the carburizing gas flow rate is excessively reduced in order to suppress coal adhesion, the carburizing reaction becomes insufficient. In this case, uneven distribution of carburization is also likely to occur.

Based on the above findings, the present inventors focused on theoretically limiting the flow rate of the carburizing gas that invades from the atmosphere in the vacuum carburizing furnace to the surface of the steel material in the carburizing process. In this specification, the "theoretical carburizing gas flow rate" refers to the carburizing gas flow rate required to make the carbon concentration at a predetermined depth position from the surface of the steel material a desired concentration, and refers to the carburizing gas flow rate required to use all the carburizing gas for the carburizing reaction. Carburizing gas flow rate as a prerequisite. The present inventors believe that by adjusting the flow rate of the carburizing gas supplied to the vacuum carburizing furnace during the actual vacuum carburizing process (hereinafter, referred to as the actual carburizing gas flow rate) based on the theoretical carburizing gas flow rate defined in advance, it is possible to suppress inaccuracies. The amount of carburizing gas is favorable for the carburizing reaction, and the carburizing reaction is not insufficient. As a result, uneven distribution of carburizing can be suppressed.

As the vacuum carburizing process proceeds, the gradient of the carbon concentration becomes gentler, and therefore the diffusion flux of carbon that penetrates from the surface of the steel material into the interior of the steel material decreases. The flow rate of the carburizing gas that invades the steel material from the furnace atmosphere decreases with the passage of time. Therefore, the theoretical carburizing gas flow rate becomes a function that changes with the passage of time from the start of the supply of the carburizing gas (the start of the carburizing process). The theoretical carburizing gas flow rate can be determined based on diffusion simulation or experimentally. Hereinafter, as an example of a method of determining the theoretical carburizing gas flow rate, determination of the theoretical carburizing gas flow rate based on diffusion simulation will be described. However, the method for determining the theoretical carburizing gas flow rate is not limited to diffusion simulation as described above.

[About theoretical carburizing gas flow rate]

In the vacuum carburizing treatment method of this embodiment, acetylene is used as the carburizing gas. The decomposition of acetylene is controlled by carbon diffusion in the surface layer of the steel material to be carburized. That is, the greater the diffusion flux of carbon that penetrates from the surface of the steel material into the interior of the steel material, the greater the amount of acetylene decomposed. It should be noted that when carburizing is performed using a carburizing gas other than acetylene, a chemical reaction other than the carburizing reaction is assumed as will be described later. Therefore, it is difficult to apply the vacuum carburizing treatment method of this embodiment.

In the vacuum carburizing process, carbon diffuses in the steel, that is, Fick's first law holds true. The flow rate of the carburizing gas (acetylene gas) required to make the carbon concentration at a predetermined depth from the surface of the steel material through vacuum carburizing treatment reach the desired concentration, and the carburizing process is based on the premise that all the carburizing gas is used for the carburizing reaction. The gas flow rate is defined as the theoretical carburizing gas flow rate FT(t). Here, t is the time from the start of the carburizing process. As will be described later, the start of the carburizing process refers to the time when the carburizing gas starts to be supplied into the furnace. FT(t) corresponds to a value converted into an acetylene gas flow rate from the carbon flow rate that invades the steel surface. In addition, in the following description, the theoretical gas flow rate will also be abbreviated as "FT".

For example, the diffusion flux J (mm·mass %/s) of carbon that invades from the steel material surface and the change amount of carbon concentration per unit time are calculated by a well-known diffusion simulation using a diffusion equation. Thus, the theoretical carburizing gas flow rate FT can be calculated. Specifically, the theoretical carburizing gas flow rate can be obtained by the following method.

When diffusion occurs (that is, when Fick's first law holds true), the diffusion flux J of carbon that invades from the steel surface is defined by equation (1), and the change amount of carbon concentration per unit time It is defined by formula (2).

Here, D is the diffusion coefficient of carbon in the steel material (mm ² /s). C is the mass concentration of carbon (mass %). z is the displacement from the steel surface to the depth direction (mm). t is the time (seconds) from the start of the carburizing process. is the partial differential symbol.

If the change in carbon concentration is calculated based on the gradient of the chemical potential, the diffusion driving force of carbon can be tightly manipulated. In this case, the carbon diffusion flux J (mm·mol%/s) is defined by equation (3), and the time change of the carbon concentration is defined by equation (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 of carbon (J/mol). z is the displacement in the depth direction (mm). t in the formula (4) is the time (s) from the start of the carburizing process. is the partial differential symbol.

Here, the diffusion driving force of carbon is in equation (3) part. In addition, the carbon concentration in the austenite (γ) in the vacuum carburizing process is as small as 2% or less, and the molar concentration and the mass concentration are approximately proportional. Therefore, equation (3) can be expressed in terms of mass concentration (mass %). When formula (3) is expressed in terms of mass %, the carbon diffusion flux J (mm·mass %/s) is defined by formula (5), and the time change of the carbon concentration is defined by formula (2).

C in formula (5) is carbon concentration (mass %).

Using Fick's first law (Formula (1), (3) and Formula (5)) and Fick's second law (Formula (2) and Formula (4)) mentioned above, the following method is used for Diffusion simulation to calculate the theoretical carburizing gas flow rate FT.

In the vacuum carburizing process using acetylene as the carburizing gas, carbon penetrates into the steel material from the surface of the steel material due to the decomposition of the carburizing gas. It is assumed that on the surface of the steel material during the carburizing process, the carbon concentration in the steel material rises until it is in equilibrium with the graphite. Therefore, the boundary condition in the diffusion simulation of carbon on the steel surface during vacuum carburization is defined as "the carbon concentration on the steel surface balances with graphite." Based on the above premise, diffusion simulation is performed as follows.

[Calculation methods in diffusion simulation]

First, mesh data is created in which the surface layer of the steel material to be subjected to vacuum carburizing treatment is divided into a plurality of units. The size of each unit may be a known size. The size of the unit is, for example, 1 to 500 μm. The size of the unit can also be slowly expanded from the surface of the steel to the depth direction. At this time, the size ratio of adjacent units is 0.80 to 1.25, preferably 0.90 to 1.10. However, the size of the unit is not limited to this. Objects for diffusion simulation can be one-dimensional. When the shape of the steel material is a round bar or a cylinder, the grid data can be treated as one-dimensional by setting the cylindrical coordinate system. Furthermore, if the diameter of the steel material (round rod or cylinder) is 50 times or more the diffusion distance of carbon in the steel, it can be treated in the same manner as a flat surface. Here, the so-called diffusion distance refers to √Dt. The diffusion coefficient D is calculated based on the carbon concentration of the steel and the carburizing temperature. The time t (second) is the carburizing time (implementation time of the carburizing process). For example, when SCM415 specified in JIS G 4053 (2008) is used as the steel material, when the carburizing temperature is 950°C and the carburizing time is 51 minutes, the diffusion distance √Dt is 0.20 mm. At this time, if the diameter of the steel material is 10 mm or more, it can be treated in the same manner as a flat surface. It should be noted that when SCM420 specified in JIS G 4053 (2008) is used as the steel material, when the carburizing temperature is 950°C and the carburizing time is 51 minutes, the diffusion distance √Dt is 0.21 mm. Furthermore, the analysis time (step time) of the diffusion simulation is set. The step time is not particularly limited, but is set to 0.001 to 1.0 seconds, for example.

In the vacuum carburizing process, a carburizing process is performed, and then a diffusion process is performed. Sometimes the carburizing process and the diffusion process are performed multiple times. For example, when the carburizing process and the diffusion process are performed twice, the first carburizing process is performed, and the first diffusion process is performed after the first carburizing process. Furthermore, a second carburizing process is performed after the first diffusion process, and a second diffusion process is performed after the second carburizing process. When performing multiple carburizing processes and diffusion processes in this way, in each carburizing process, the theoretical carburizing gas flow rate in the previous carburizing process is reset, and the theoretical carburizing gas flow rate in the next carburizing process is newly set. Carbon gas flow.

It should be noted that after the nth carburizing process (n is a natural number of 1 or more) is performed, the n+1th carburizing process is performed while sandwiching a diffusion process that is less than 1/10 of the time of the nth carburizing process. In the case of the carbonization process, the nth carburizing process and the (n+1)th carburizing process are considered to be the primary carburizing process. That is, in this case, without resetting the theoretical gas flow rate set in the n-th carburizing process, it is directly used in the n+1-th carburizing process. In other words, if the diffusion process time between the nth carburizing process and the n+1th carburizing process is more than 1/10 of the nth carburizing process time, then the n+1th carburizing process will In the process, the theoretical carburizing gas flow rate of the nth carburizing process is reset, and a new theoretical carburizing gas flow rate is set.

As described above, 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 subjected to vacuum carburizing treatment, the equilibrium phase and equilibrium composition in the equilibrium state with graphite at the carburizing temperature are determined by well-known thermodynamic calculations. The chemical composition of the steel material to be subjected to the vacuum carburizing treatment is thermodynamically calculated by increasing the C concentration until graphite appears as an equilibrium phase, taking into account dilution due to an increase in C concentration. For example, when the C concentration increases by 7% by mass, the mass of the steel itself reaches 1.07 times. Therefore, the thermodynamic calculation is performed based on the chemical composition set to 1/1.07 times for the concentration of elements other than C. Based on the equilibrium phase and equilibrium composition obtained through thermodynamic calculations, the C content in the steel material, the chemical potential of C, and the solid solution C concentration in austenite can be determined. Well-known thermodynamic calculation software can be used for thermodynamic calculations. Well-known thermodynamic calculation software is, for example, the trade name Pandat (trademark).

Similarly, vacuum carburization may occur and cementite (θ) may precipitate inside the steel material other than the steel surface. At this time, the carbon (C) in the steel is distributed into cementite and austenite. Therefore, the equilibrium phase and equilibrium composition inside the steel material except the surface of the steel material at the carburizing temperature are obtained through the above-mentioned thermodynamic calculation. Like the steel surface, the equilibrium phase, equilibrium composition, C content in the steel, chemical potential of C, and solid solution C concentration in austenite can also be determined inside the steel.

The diffusion coefficient D of carbon in austenite in the steel material may be a numerical value determined in advance through experiments using the steel material to be subjected to vacuum carburizing treatment, or data reported as experimental data may be used. For example, as the diffusion coefficient D (m ² /s) of C in austenite, the following formula can be used with reference to the content advocated by Gray G. 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 concentration of solid solution C in austenite (mass %), and T is the carburizing temperature (K).

The mobility m (m ² /s) of carbon in the austenite in the steel material can be determined based on the diffusion coefficient D and thermodynamic calculation. The formula formulating the mobility m is the following formula.

m＝1.54×10 ^-15 exp(-1.61×C-(17300-2920×C)/T)

Here, "C" in the formula is the concentration of solid solution C in austenite (mass %), 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 most surface unit and the target carbon concentration in the predetermined depth are set. Furthermore, as the initial value, it is assumed that the solid solution C concentration in all units = the C concentration (C ₀ ) of the chemical composition of the steel material (core part), and the cementite precipitation amount in all units is set to 0.

Based on the above prerequisites, the following calculations are performed at each step time.

(A) Based on the carbon concentration in each unit and the thermodynamic calculation results, determine the concentration of solid solution C in the austenite (that is, the concentration of diffused C) in each unit at the carburizing temperature. At this time, it is assumed that C in cementite is fixed and only solid solution C in austenite diffuses.

(B) In each unit, based on the determined solid solution C concentration, use equation (1), equation (3), or equation (5) to calculate the diffusion flux J in each unit by the difference method. At this time, as mentioned 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 equilibrium with graphite. Based on the diffusion flux J ₀ from the steel surface, the acetylene flow rate was obtained by setting the carburization efficiency to 100%. The calculated acetylene flow rate is defined as the theoretical carburizing gas flow rate under this step time.

(C) Based on the calculated diffusion flux J in each unit, the C concentration of each unit at the time when the step time elapses is determined.

(D) Based on the thermodynamic calculation results, as an equilibrium phase, determine whether cementite is generated. It should be noted that the time required for the generation of cementite is ignored (that is, (A) at the next step time is determined).

(E) When the carburizing process is performed two or more times, a simulation of the diffusion process between the carburizing processes is performed, and a simulation of the subsequent carburizing process is performed. In the diffusion step, the diffusion flux J ₀ from the steel material surface is set to zero, and calculations (A) to (D) are performed.

The above calculation is performed at each step time to obtain 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 process. Then, the diffusion flux J ₀ (t) per unit surface area of the steel material is converted into an acetylene gas flow rate, and further multiplied by the surface area S (m ² ) of the steel material to be subjected to the vacuum carburizing treatment to obtain the diffusion flux J 0 (t) at time t. Theoretical carburizing gas flow FT(t). In a figure in which the horizontal axis represents the elapsed time (carburizing time) from the start of carburization and the vertical axis represents the theoretical carburizing gas flow rate FT, the theoretical carburizing gas flow rate FT at each carburizing time is plotted. Therefore, the theoretical carburizing gas flow rate FT can be expressed in the form of a theoretical carburizing gas flow rate curve. FIG. 1 is a diagram showing an example of the relationship between the theoretical carburizing gas flow rate and time calculated from the diffusion flux of carbon in the surface layer of the steel material obtained by the above-mentioned diffusion simulation. ● in Figure 1 represents the theoretical carburizing gas flow rate FT at each time. Curve C _1.00 in Figure 1 represents the theoretical carburizing gas flow curve.

The approximate formula of the theoretical carburizing gas flow curve C _1.00 can be expressed by formula (6).

FT＝S×A/√t (6)

Here, FT is the theoretical carburizing gas flow rate (NL/min). A in formula (6) can be expressed by 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)

a, b and c in formula (7) are constants determined by the chemical composition of the steel, 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=58.2 are obtained by the above diffusion simulation. When the steel material is SCM415 specified in JIS G 4053 (2008), when calculated from the above diffusion simulation, a=8.64×10 ^-5 , b=-0.141, and c=59.0.

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

In the above description, as an example of a method of determining the theoretical carburizing gas flow rate, the theoretical carburizing gas flow rate is obtained based on a known diffusion simulation using the diffusion equation. However, the theoretical carburizing gas flow rate can also be determined based on other methods. For example, the theoretical carburizing gas flow rate can also be determined based on experiments.

A method for determining the theoretical gas flow rate from experiments is as follows, for example. Vacuum carburizing is performed on steel materials with the same chemical composition as the steel materials that are actually vacuum carburized. The flow rate of the carburizing gas supplied to the vacuum carburizing furnace is kept constant, and the acetylene partial pressure and hydrogen partial pressure in the vacuum carburizing furnace are continuously measured during the carburizing process. Then, the carburizing gas flow rate is determined so that the first time t0 when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure becomes 1/10 or less of the time ta which is the end time of the carburizing process (that is, the full carburizing process time). Minimum value FAmin. Based on the calculated carburizing gas flow rate FAmin, the theoretical carburizing gas flow rate FT=FAmin√(t0/t) is formed.

It should be noted that, as mentioned above, the theoretical carburizing gas flow rate is equal to the carburizing gas flow rate used for the carburizing reaction in contact with the steel surface. Therefore, the theoretical carburizing gas flow rate is not affected by the size and shape of the heat treatment furnace.

[About the vacuum carburizing treatment method of this embodiment]

The flow rate of the carburizing gas actually supplied to the vacuum carburizing furnace during the vacuum carburizing process is defined as the "actual carburizing gas flow rate" FR. The inventors of the present invention investigated and studied a matter as shown in FIG. 1 assuming the use of an actual carburizing gas flow rate FR that greatly deviates from the theoretical carburizing gas flow rate FT relationship at carburizing time.

FIG. 2 is a graph showing the time-dependent change of the actual carburizing gas flow rate FR and the time-dependent change of the theoretical carburizing gas flow rate FT in the conventional carburizing process. The vertical axis of Fig. 2 represents the carburizing gas flow rate (NL/minute), and the horizontal axis represents the time (minutes) from the start of the carburizing process. The solid line FR in FIG. 2 shows the actual carburizing gas flow rate FR in the conventional carburizing process as described above. The dotted line C _1.00 in Fig. 2 represents the theoretical carburizing gas flow rate FT as described above.

Referring to FIG. 2 , the start time of the carburizing process is defined as “0” and the end time of the carburizing process is defined as “ta”. That is, the carburizing process is performed from time 0 to time ta. The end time ta is set in advance based on the set value of the carbon concentration at a predetermined depth position of the carburized steel material. In addition, the time when the actual carburizing gas flow rate FR first becomes equal to the theoretical carburizing gas flow rate FT is defined as "te".

The period from the start of the carburizing process to time te is defined as period S100. The period from time te to time ta is defined as period S200. In the period S100, the actual carburizing gas flow rate FR is lower than the theoretical carburizing gas flow rate FT (curve C _1.00 ). Therefore, in the carburizing process of the conventional vacuum carburizing treatment method, the actual carburizing gas flow rate FR in period S100 is insufficient. In this case, a part where the carburizing reaction is sufficient and a part where the carburizing reaction is insufficient occur on the surface of the steel material. Therefore, the carburization distribution on the steel surface becomes uneven. In addition, the desired carbon concentration may not be obtained in the surface layer of the steel material. On the other hand, in the period S200, the actual carburizing gas flow rate FR is higher than the theoretical carburizing gas flow rate FT (curve C _1.00 ). Therefore, in period S200, the actual carburizing gas flow rate FR becomes excessive and remains in the vacuum carburizing furnace. As a result, coal and tar are generated due to the remaining carburizing gas during period S200. In this case, the uneven distribution of carburization on the surface of the steel material becomes large.

Based on the above investigation results, the present inventors considered controlling the actual carburizing gas flow rate FR in combination with the theoretical carburizing gas flow rate curve C _1.00 in the carburizing process.

However, as shown in FIG. 2 , in the period S100 at the initial stage of the carburizing process, the slope of the theoretical carburizing gas flow rate curve C _1.00 is steeper than in the subsequent period S200 . Therefore, it can be seen that during the actual operation period S100, it is very difficult to adjust the actual carburizing gas flow rate FR based on the slope of the theoretical carburizing gas flow rate curve C _1.00 .

Furthermore, in the period S100 at the initial stage of the carburizing process, when the carburizing process starts (t=0), when the above-mentioned equation (6) is used, the theoretical carburizing gas flow rate FT becomes infinite. Therefore, in actual operation, it is extremely difficult to introduce an actual carburizing gas flow rate FR equal to the theoretical carburizing gas flow rate FT in the early stage of period S100.

Therefore, the present inventors considered that not only the theoretical carburizing gas flow rate FT but also other factors were considered as factors for controlling the actual carburizing gas flow rate. The gas composition in the atmosphere in the vacuum carburizing furnace changes depending on the actual carburizing gas flow rate FR. The change in gas composition causes uneven distribution of carburization and the generation of coal. Therefore, the present inventors focused not only on the theoretical carburizing gas flow rate FT but also on the gas components in the atmosphere of the vacuum carburizing furnace as factors for controlling the actual carburizing gas flow rate.

The present inventors focused on the hydrogen partial pressure and the acetylene partial pressure 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 using a known analyzer. The analyzer is, for example, a quadrupole mass spectrometer.

The hydrogen partial pressure analyzed is generated in the vacuum carburizing furnace according to the reaction based on the following formula.

C ₂ H ₂ →2C+H ₂

The hydrogen partial pressure becomes an index of the carburizing reaction amount in the carburizing process. That is, the hydrogen partial pressure becomes an index of the degree of suppression of uneven carburization distribution. On the other hand, the acetylene partial pressure refers to the amount of remaining gas that does not cause carburization reaction, and is an indicator of the amount of coal and tar produced.

In the vacuum carburizing process using acetylene, the chemical reaction is extremely rapid just after the carburizing process is started, that is, just after the supply of acetylene into the furnace is started. That is, the carbon entering the surface of the steel material immediately after the carburizing process is extremely rapid. Therefore, if the acetylene flow rate (carburizing gas flow rate) supplied to the furnace is small, the atmosphere in the furnace will basically become 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 acetylene gas flow rate (vacuum carburizing gas flow rate) supplied to the furnace is large, acetylene gas that does not cause 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, by monitoring the hydrogen partial pressure and acetylene partial pressure in the furnace, the amount of carburizing reaction on the steel surface can be estimated.

The present inventors believe that if the actual carburizing gas flow rate FR can be controlled based on the theoretical carburizing gas flow rate FT and the hydrogen partial pressure and acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, then the vacuum carburizing process can be suppressed. Carburization is unevenly distributed and can also inhibit coal production. Therefore, the present inventors conducted further studies and obtained the following findings.

(a) In the early stage of the carburizing process (near the period S100), if the carburizing gas flow rate is small, the carburizing reaction amount will be small. Therefore, the partial pressure of acetylene rises slowly. As a result, the carburization distribution becomes uneven and the carbon concentration in the surface layer of the carburized component becomes low.

(b) Define the end time of the carburizing process as ta. As described above, the end time ta is set in advance based on the set values of the surface carbon concentration and the carburization depth of the steel material after carburization. Then, a time that is 1/10 of the end time ta from the start time of the carburizing process is defined as ta/10. The theoretical carburizing gas flow rate at time ta/10 is defined as FT _ta/10 . If the actual carburizing gas flow rate FR at the initial stage of the carburizing process is equal to or greater than the theoretical carburizing gas flow rate FT _{ta/10 at time ta/10} , the hydrogen partial pressure in the atmosphere in the vacuum carburizing furnace will rise rapidly, but in the early stage decreases, the acetylene partial pressure rises faster. As a result, insufficient carburizing reaction amount in the initial stage of the carburizing process can be suppressed, and uneven carburizing distribution can be reduced.

(c) On the other hand, if the actual carburizing gas flow rate FR in the initial stage of the carburizing process is too high, the acetylene partial pressure in the furnace will rise excessively and rapidly. In this case, excess acetylene gas remains in the furnace. As a result, coal or tar is generated, and carburization becomes unevenly distributed. The theoretical carburizing gas flow rate at 4 seconds from the start of the carburizing process is defined as FT ₄ . In the early stage of the carburizing process, if the actual carburizing gas flow rate FR is FT ₄ or less, it can be suppressed that the actual carburizing gas flow rate FR in the furnace becomes excessive. Therefore, uneven distribution of carburization can be suppressed.

(d) If the actual carburizing gas flow rate FR is maintained at a high level, the acetylene partial pressure gradually increases. Therefore, the partial pressure of acetylene will greatly exceed the partial pressure of hydrogen at any time. In this case, excess gas that does not cause the carburizing reaction exists excessively in the atmosphere in the vacuum carburizing furnace. Therefore, coal derived from the remaining gas is generated and adheres to the surface of the carburized component. As a result, the carburization distribution becomes uneven.

(e) In the carburizing process, when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure, if the actual carburizing gas flow rate FR is maintained or reduced, the remaining gas can be suppressed in the atmosphere in the vacuum carburizing furnace. Therefore, uneven distribution of carburization can be suppressed.

Based on the above findings, the present inventors believe that if the actual carburizing gas flow rate FR in the carburizing process is adjusted as follows (I) to (III), a sufficient carburizing reaction amount can be ensured in the early stage of the carburizing process, and Afterwards, residual gas is suppressed, the production of coal and tar is suppressed, and the uneven distribution of carburization can be reduced.

Here, each term is defined as follows.

Time ta: the end time of the carburizing process

Time t0: The first time after the carburizing process starts that the acetylene partial pressure becomes more than 0.8 times the hydrogen partial pressure.

Time ta/10: 1/10 of the end time ta from the start time of the carburizing process

Time 4t0: The elapsed time that is four times the period from the start of the carburizing process to time t0 after the carburizing process is started.

Preliminary carburizing process S1: the period from the start of the carburizing process to time t0

Post-carburizing process S2: period from time t0 to time ta

Actual carburizing gas flow rate FR: Actual carburizing gas (acetylene) flow rate supplied to the vacuum carburizing furnace

Theoretical carburizing gas flow FT _ta/10 : Theoretical carburizing gas flow at time ta/10

Theoretical carburizing gas flow rate FT ₄ : Theoretical carburizing gas flow rate at 4 seconds from the start of the carburizing process

When the above terms are defined, as shown in Figure 3, the actual carburizing gas flow rate FR is adjusted as follows (I) to (III).

(I) In the preliminary carburizing step S1, the actual carburizing gas flow rate FR is set to FT _ta/10 or more and FT ₄ or less. When the actual carburizing gas flow rate FR is made constant in the early carburizing step S1, this value is regarded as the actual carburizing gas flow rate FA.

(II) During the period from time t0 to 4t0 in the late carburizing step S2, the actual carburizing gas flow rate FR is set to FA×√(t0/t) or more and FA or less.

(III) During the period from time 4t0 to time ta in the late carburizing step S2, the actual carburizing gas flow rate FR is set to FA×√(t0/t) or more and 2FA×√(t0/t) or less.

Here, t is the time from the start of carburization.

3 is a diagram illustrating changes over time in the actual carburizing gas flow rate in the carburizing step of the vacuum carburizing treatment method of the present embodiment (lower graph), and acetylene in the atmosphere of the vacuum carburizing furnace in the carburizing step. A graph showing the time-dependent changes in partial pressure and hydrogen partial pressure (above). Referring to FIG. 3 , in the present embodiment, the actual carburizing gas flow rate FR is adjusted to be within the range of the hatched area in FIG. 3 during the period from time t0 to time ta. The curve that becomes the lower limit of the hatched area is a curve of carburizing gas flow rate=FA×√(t0/t). The upper limit of the shaded area is the curve of carburizing gas flow rate=2FA×√(t0/t). FA×√(t0/t) and 2FA×√(t0/t) are both equations proportional to equation (6) of the theoretical carburizing gas flow rate FT.

It should be noted that, as described above, time t0 is the first time after the carburization process is started when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure. As shown in FIG. 3 , in the early stage of the preliminary carburizing step S1 , the hydrogen partial pressure rises rapidly compared to the acetylene partial pressure. This is because the carburizing reaction actively occurs. After the hydrogen partial pressure rises rapidly, it begins to decrease before the acetylene partial pressure. Then, the hydrogen partial pressure decreases, so that the acetylene partial pressure becomes more than 0.8 times the hydrogen partial pressure. This time, that is, the first time after the carburizing process is started when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure, is defined as time t0. It should be noted that the “0.8” times referred to here is the value obtained by discarding the second decimal place of the calculated value of the ratio of acetylene partial pressure/hydrogen partial pressure.

The vacuum carburizing method of this embodiment completed based on the above findings has the following configuration.

[1]

A vacuum carburizing treatment method, which is a vacuum carburizing treatment method for steel materials in a vacuum carburizing furnace. The vacuum carburizing treatment method has:

The heating process is to heat the aforementioned steel to the carburizing temperature;

Soaking process: after the aforementioned heating process, the aforementioned steel material is soaked at the aforementioned carburizing temperature;

In the carburizing process, after the aforementioned soaking process, acetylene gas, that is, carburizing gas, is supplied into the aforementioned vacuum carburizing furnace while maintaining the aforementioned steel material at the aforementioned carburizing temperature;

In the diffusion process, after the aforementioned carburizing process, the supply of the aforementioned carburizing gas to the aforementioned vacuum carburizing furnace is stopped, and the aforementioned steel material is maintained at the aforementioned carburizing temperature; and,

The quenching process is to quench the aforementioned steel material after the aforementioned diffusion process,

In the aforementioned carburizing process,

The flow rate of the carburizing gas supplied into the aforementioned vacuum carburizing furnace is defined as the actual carburizing gas flow rate,

The flow rate of the aforementioned carburizing gas required for the aforementioned vacuum carburizing treatment of the aforementioned steel material is defined as the theoretical carburizing gas flow rate,

Define the end time of the aforementioned carburizing process as ta,

The first time when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure after the carburizing step is started is defined as t0.

The aforementioned carburizing process includes:

Continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, and determining the partial pressure measurement process at the time t0;

The early carburizing process from the start of the aforementioned carburizing process to time t0; and,

The post-carburizing process from the aforementioned time t0 to time ta,

In the aforementioned early carburizing process,

The actual carburizing gas flow rate is set to be greater than the theoretical carburizing gas flow rate at time ta/10 and less than the theoretical carburizing gas flow rate at 4 seconds from the start of the carburizing process,

In the aforementioned late carburizing process,

When the actual carburizing gas flow rate in the early carburizing process is defined as FA, and the time from the start of the carburizing process is defined as time t,

The actual carburizing 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 carburizing gas flow rate from the time 4t0 to the time ta is set to be FA√(t0/t) or more and 2FA√(t0/t) or less.

[2]

The vacuum carburizing method according to [1], wherein:

In the aforementioned late carburizing process,

During the period from the aforementioned time 4t0 to the time ta, as time passes, the aforementioned actual carburizing gas flow rate is reduced by the method (A) or (B).

(A) Repeat the aforementioned maintenance and reduction of the actual carburizing gas flow rate, and gradually reduce the aforementioned actual carburizing gas flow rate,

(B) The actual carburizing gas flow rate is gradually reduced as time passes.

[3]

The vacuum carburizing treatment method according to [1] or [2], wherein,

The aforementioned theoretical carburizing gas flow rate was determined based on diffusion simulation using the diffusion equation.

[4]

A method of manufacturing carburized parts,

It includes the step of subjecting the steel material to the vacuum carburizing treatment method described in any one of [1] to [3].

Hereinafter, the vacuum carburizing treatment method and the manufacturing method of carburized parts based on this embodiment will be described in detail.

[Vacuum carburizing treatment method]

FIG. 4 is a diagram illustrating an example of the heating pattern of the vacuum carburizing method according to this embodiment. Referring to FIG. 4 , the vacuum carburizing treatment method of this embodiment includes a heating process (S10), a soaking process (S20), a carburizing process (S30), a diffusion process (S40), and a quenching process (S50). The details of each process will be described below.

[Heating process (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 a steel material provided by a third party, or may be a steel material manufactured by performing a vacuum carburizing treatment method. The chemical composition of the steel material is not particularly limited. A known steel material that has been carburized may be used. The steel material is, for example, alloy steel for mechanical structures specified in JIS G 4053 (2008). More specifically, steel materials include SCr415, SCr420, SCM415, etc. specified in JIS G 4053 (2008), for example.

The prepared steel material may be hot-processed steel material or cold-processed steel material. Hot working includes, for example, hot rolling, hot extrusion, hot forging, etc. Examples of cold working include cold rolling, cold drawing, cold forging, etc. The steel material may be hot-worked or cold-worked and then subjected to mechanical processing such as cutting.

In the heating step (S10), the steel material is loaded 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. The carburizing temperature Tc may be a known temperature. The carburizing temperature Tc is above the _Ac3 transformation point. The preferred range of carburizing temperature Tc is 900 to 1130°C. If the carburizing temperature Tc is 900° C. or higher, heat conduction due to radiation becomes high, and the temperature in the vacuum carburizing furnace is likely to be uniform. As a result, the uneven carburization distribution of the steel material tends to become smaller. If the carburizing temperature is 1130°C or lower, the grain diameter of the steel material can be prevented from becoming coarse, and a decrease in the strength of the steel material can be suppressed. The lower limit of the carburization temperature Tc is more preferably 910°C, and further preferably 920°C. The upper limit of the carburization temperature Tc is more preferably 1100°C, and further preferably 1080°C.

[Soaking process (S20)]

In the soaking process (S20), the steel material is maintained at the carburizing temperature Tc for a predetermined time. Hereinafter, the holding time in the soaking step (S20) is called soaking time. The soaking step (S20) is a well-known step in the vacuum carburizing treatment method. The soaking time can be adjusted appropriately according to the shape and/or size of the steel. The preferred soaking time is 10 minutes or more. More specifically, when converting a cross section perpendicular to the longitudinal direction of the steel material into a circle, the soaking time is preferably 30 minutes or more per circle equivalent diameter of 25 mm. 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, and 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 is introduced and vacuum exhaust is performed using a vacuum pump, and the nitrogen atmosphere may be 1000 Pa or less. In the soaking process (S20), at least until the carburizing process (S30) starts, the inside of the vacuum carburizing furnace is maintained at low pressure or vacuum. For example, in the soaking process (S20) until the carburizing process (S30) is started, the temperature inside the vacuum carburizing furnace is set to 10 Pa or less.

[Carburizing process (S30)]

In this specification, the carburizing step (S30) refers to a step of supplying carburizing gas in a furnace under reduced pressure or vacuum. That is, after the soaking process (S20), the carburizing process (S30) starts when the carburizing gas is started to be supplied into the furnace under reduced pressure or vacuum. In the carburizing step (S30), the carburizing gas is supplied into the furnace while maintaining the low pressure in the furnace. The pressure inside the furnace is low, so the frequency with which molecules of the carburizing gas collide with each other becomes less frequent. That is, the frequency of decomposition of carburizing gas in the atmosphere in the furnace becomes smaller. Therefore, carburizing gas is supplied to the steel surface at low pressure, thereby suppressing the generation of coal and tar. As a result, the surface carbon concentration of the steel material can be rapidly increased. In the carburizing process (S30) from the start of carburization to the end of carburization (time ta), the temperature inside the furnace is set to 1 to 1000 Pa, for example. However, the furnace pressure in the carburizing step (S30) is not limited to the above range.

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

[Carburizing gas]

In this embodiment, the carburizing gas used in the carburizing step (S30) of the vacuum carburizing treatment method is acetylene gas.

In conventional vacuum carburizing treatments, propane gas was mostly used. However, in addition to the carburization reaction, propane gas also undergoes decomposition reactions into methane, ethylene, acetylene, hydrogen, etc. When a large amount of methane and ethylene are generated due to the decomposition reaction, they do not contribute to the carburizing reaction and are exhausted from the vacuum carburizing furnace. Therefore, when propane gas is used, the theoretical carburizing gas flow rate FT cannot be calculated using diffusion simulation of the diffusion flux of carbon obtained from the diffusion equation. On the other hand, acetylene is not prone to reactions other than carburization. Therefore, the theoretical carburizing gas flow rate FT can be calculated using diffusion simulation of the diffusion flux of carbon obtained from the diffusion equation.

In this embodiment, the purity of acetylene as the carburizing gas may be 98% or more. For acetylene, for example, acetylene dissolved in acetone or acetylene dissolved in dimethylformamide (DMF) can be used as the carburizing gas. Acetylene dissolved in DMF is preferably used as the carburizing gas. In this case, mixing of the solvent into the furnace atmosphere can be suppressed. When the supply source of acetylene to the vacuum carburizing furnace is a gas tank, the primary pressure when supplying acetylene from the gas tank into the vacuum carburizing furnace is preferably 0.5 MPa or more. When supplying to a vacuum carburizing furnace, it is preferable to use a pressure reducing valve to reduce the pressure to 0.20 MPa or less before supplying.

[Details of the carburizing process (S30)]

The carburizing process (S30) includes: a partial pressure measurement process S0, an early carburizing process S1, and a late carburizing process S2. The details of each process will be described below.

[Prepare in advance]

Before implementing the vacuum carburizing treatment method, as a preliminary preparation, the theoretical carburizing gas flow rate FT corresponding to the target steel material is determined in advance, and the theoretical carburizing gas flow rate FT shown in Figure 1 is determined in advance until the end time ta of the carburizing process (S30). Time-dependent changes in the carburizing gas flow rate FT. The theoretical carburizing gas flow rate FT can be determined based on diffusion simulation or based on experiments.

[Partial pressure measurement step S0]

In the partial pressure measurement step S0, in the carburizing step (S30), the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are measured. Specifically, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are continuously measured. Here, "continuously" means that the hydrogen partial pressure and the acetylene partial pressure are measured multiple times over time. The partial pressure of hydrogen and partial pressure of acetylene can be measured continuously or at specified time intervals. The measurement is performed using a known partial pressure measuring device. The partial pressure measuring device is, for example, a quadrupole mass spectrometer. However, as the partial pressure measuring device, a partial pressure measuring device other than a 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. Based on the hydrogen partial pressure and the acetylene partial pressure measured over time, time t0 (the first time after the carburizing process is started when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure) is determined.

When a quadrupole mass spectrometer is used as a partial pressure measuring device, the quadrupole mass spectrometer sequentially measures each component gas (hydrogen, acetylene). Therefore, the measurement time of hydrogen partial pressure deviates from the measurement time of acetylene. The analysis time of each component (hydrogen, acetylene) of the quadrupole mass spectrometer 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, when a quadrupole mass spectrometer is used as a partial pressure measuring device, it is assumed that after hydrogen is analyzed for 0.5 seconds, acetylene is analyzed for 0.5 seconds, and 2.0 seconds after the start of hydrogen analysis, hydrogen is analyzed again for 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 an "analysis step." In addition, the period from the start time of the measurement step to the start time of the next measurement step is defined as the "analysis interval". In the case of the above example, the analysis step is 1.0 seconds (hydrogen analysis time 0.5 seconds + acetylene analysis time 0.5 seconds), and the analysis interval is 2.0 seconds.

When a quadrupole mass spectrometer is used as the partial pressure measuring device, the time required for the acetylene partial pressure to become 0.8 times or more of the hydrogen partial pressure, that is, the time t0 in FIG. 3 is determined as follows. Let the start time of an analysis step be t1 and the end time of the analysis step be t2. In the analysis step, the hydrogen partial pressure can be measured first, or the acetylene partial pressure can be measured first. Furthermore, the start time of the next analysis step is defined as t3, and the end time of the analysis step is defined as t4. At this time, the analysis period is the time between time t1 and time t3.

In this case, the acetylene partial pressure obtained in the analysis step from time t1 to time t2 is 0.8 times the hydrogen partial pressure obtained in the same analysis step (that is, the analysis step from time t1 to time t2). above, and the hydrogen partial pressure obtained in the next analysis step from time t3 to time t4 after the analysis interval is 1.25 times or less the acetylene partial pressure obtained in the analysis step from time t1 to time t2 , the end time t2 of the analysis step for measuring the acetylene partial pressure is defined as time t0.

Not only is the acetylene partial pressure more than 0.8 times the hydrogen partial pressure obtained in the same analysis step, but the hydrogen partial pressure obtained in the next analysis step is less than 1.25 times the acetylene partial pressure obtained in the previous analysis step. The reasons for the conditions are as follows. Assume that when the carburizing gas starts flowing into the furnace, it is in the analysis step from time t1 to time t2, after the measurement of the hydrogen partial pressure is completed, and before the acetylene partial pressure is measured, the hydrogen partial pressure obtained in this analysis step becomes 0. Therefore, the acetylene partial pressure obtained in this analysis step must be at least 0.8 times the hydrogen partial pressure. When the end time of this analysis step is determined as time t0, the acetylene gas is actually not fully introduced into the furnace. Therefore, this situation must not be recognized as time t0. In the above case, the hydrogen partial pressure measured at the next analysis step after the analysis interval (time t3 to time t4) greatly exceeds 1.25 times the acetylene partial pressure obtained in the previous analysis step. This is because the introduction of acetylene gas sharply increases the hydrogen partial pressure.

On the other hand, when carburizing gas is fully introduced into the furnace and the resulting acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure obtained in the same analysis step, the next analysis step after the analysis interval The hydrogen partial pressure obtained in the previous analysis step is less than 1.25 times the acetylene partial pressure obtained in the previous analysis step. This is because, as shown in Fig. 3, when the carburizing gas is fully introduced into the furnace, the hydrogen partial pressure does not increase but decreases with the passage of time.

Therefore, when a quadrupole mass spectrometer is used as the partial pressure measuring device, the obtained acetylene partial pressure is more than 0.8 times the hydrogen partial pressure obtained in the same analysis step, and the next analysis step after the analysis interval has elapsed When the hydrogen partial pressure obtained in is 1.25 times or less the acetylene partial pressure obtained in the previous analysis step, the end time t2 of the analysis step for measuring the acetylene partial pressure is defined as time t0.

It should be noted that the gas in the furnace (hydrogen, acetylene) can be analyzed inside the furnace, or it can be extracted outside the furnace for analysis. 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. Alternatively, separate partial pressure measuring devices can be used for each gas component. For example, a quadrupole mass spectrometer can be used to analyze the acetylene partial pressure and another partial pressure measurer can be used to analyze the hydrogen partial pressure.

In the carburizing step (S30), carburizing gas is supplied under the above-mentioned reduced pressure. Therefore, the carburizing gas rapidly causes carburizing reaction throughout the furnace. Therefore, the partial pressure measurement results of the gas in the furnace are less likely to be unevenly distributed in the furnace. That is, the analysis results of the gas in the furnace can be considered to be substantially uniform within the furnace.

[Preliminary carburizing process S1]

As shown in FIG. 3 , the period from the start of the carburizing process (S30) to the first time t0 when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure is defined as the early carburizing process S1. In the preliminary carburizing step S1, the actual carburizing gas flow rate FR is adjusted so that the following condition I is satisfied.

(I) In the preliminary carburizing step S1, the actual carburizing gas flow rate FR is set to be not less than the theoretical carburizing gas flow rate FT _ta/10 and not more than the theoretical carburizing gas flow rate FT ₄ .

FIG. 5 is a diagram showing an example of the gas flow rate setting value in the preliminary carburizing step S1 of the vacuum carburizing treatment method according to this embodiment. In the preliminary carburizing step S1, the actual carburizing gas flow rate FR is set within the range of the hatched area in FIG. 5 (FT _ta/10 or more and FT ₄ or less).

If the actual carburizing gas flow rate FR in the early carburizing step S1 is lower than the theoretical carburizing gas flow rate FT _{ta/10 at time ta/10} , the supply of the carburizing gas in the early carburizing step S1 is excessively insufficient. In this case, in the steel material (carburized component) subjected to the vacuum carburizing treatment method, the carburization distribution becomes uneven. On the other hand, if the actual carburizing gas flow rate FR in the early carburizing step S1 exceeds the theoretical carburizing gas flow rate FT ₄ at 4 seconds from the start of the carburizing step, the actual carburizing gas flow rate FR is too much. In this case, it takes time to adjust the actual carburizing gas flow rate FR to FA×√(t0/t) or more and 2FA√(t0/t) or less until time 4t0 after time t0 has elapsed. Therefore, excess gas (acetylene gas) remains excessively in the vacuum carburizing furnace, and coal is easily generated. As a result, in the carburized parts (steel materials) manufactured by performing the vacuum carburizing treatment method, the carburization distribution becomes uneven.

If the actual carburizing gas flow rate FR in the early carburizing process S1 is the theoretical carburizing gas flow rate FT at time ta/10, it is not less than _ta/10 and the theoretical carburizing gas flow rate FT ₄ is 4 seconds from the start of the carburizing process. In the following, on the premise that the conditions II and III of the actual carburizing gas flow rate FR in the later-described post-carburizing step S2 are satisfied, uneven carburization distribution of the carburized parts (steel materials) after the vacuum carburizing treatment can be sufficiently suppressed. . The actual carburizing gas flow rate FR in the preliminary carburizing step S1 can be adjusted by a known method. For example, the supply valve can be used to adjust the flow rate of the carburizing gas supplied to the vacuum carburizing furnace to adjust the actual carburizing gas flow rate FR, or other known methods can be used to adjust the actual carburizing gas flow rate FR. The adjustment of the actual carburizing gas flow rate FR can be implemented by a known control device of the vacuum carburizing furnace. The control device adjusts the actual carburizing gas flow rate FR, for example, by adjusting the opening of the supply valve.

It is preferable that the actual carburizing gas flow rate FR in the preliminary carburizing step S1 is constant. If the actual carburizing gas flow rate FR is constant, the changes in the hydrogen partial pressure and the acetylene partial pressure in the furnace can be measured with high accuracy. If the actual carburizing gas flow rate FR in the early carburizing step S1 fluctuates, the fluctuation of the hydrogen partial pressure and the acetylene partial pressure in the furnace will be affected by the fluctuation of the actual carburizing gas flow rate FR. If the actual carburizing gas flow rate FR in the preliminary carburizing step S1 is constant, the changes in 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 carburizing gas flow rate FR in the preliminary carburizing step S1 is constant. For example, as shown in FIG. 3 , the actual carburizing gas flow rate FR in the preliminary carburizing step S1 is preferably constant. In this case, the value of the actual carburizing gas flow rate FR that is constant through the early carburizing step S1 becomes the actual carburizing gas flow rate FA in the early carburizing step S1. However, in actual operation, the actual carburizing gas flow rate is not completely constant as the set value. It is common technical knowledge for those skilled in the art that the set value fluctuates within a certain range. Therefore, when the actual carburizing gas flow rate FR in the early carburizing step S1 is made constant, a margin of ±10% of the set value is allowed for the actual carburizing gas flow rate FR. That is, when the actual carburizing gas flow rate FR changes within ±10% of the determined set value through the early carburizing step S1, the set value is set to the actual carburizing gas flow rate FA in the early carburizing step. value. That is, in this specification, FA refers to the carburizing gas flow rate within the range of ±10% of the setting value in the preliminary carburizing step S1. FA is preferably within the range of ±5% of the setting value in the early carburizing step S1.

[Later carburizing process S2]

As shown in FIG. 3 , the period from time t0 to the carburizing process end time ta is defined as the late carburizing process S2. In the post-carburizing step S2, the actual carburizing gas flow rate FR is adjusted so that the following conditions II and III are satisfied.

(II) During the period from time t0 to 4t0 in the late carburizing step S2, the actual carburizing gas flow rate FR is set to FA×√(t0/t) or more and FA or less.

(III) During the period from time 4t0 to time ta in the late carburizing step S2, the actual carburizing gas flow rate FR is set to FA×√(t0/t) or more and 2FA×√(t0/t) or less.

Here, t is the time since carburizing started.

In short, in the post-carburizing step S2, the actual carburizing gas flow rate FR is adjusted so that it falls within the hatched range in FIG. 3 . Thereby, in the post-carburizing step S2, excess carburizing gas can be suppressed from remaining in the vacuum carburizing furnace. As a result, the generation of coal and tar can be reduced, and uneven carburization distribution of the carburized parts (steel materials) after the vacuum carburizing treatment method is performed can be suppressed.

[About condition II]

During the period from time t0 to 4t0 in the late carburizing step S2, if the actual carburizing gas flow rate is lower than FA×√(t0/t), the gas flow rate is insufficient. In this case, the distribution of the carburizing gas in the vacuum carburizing furnace becomes uneven. For example, the concentration of the carburizing gas is high near the supply nozzle of the carburizing gas, and the concentration of the carburizing gas is low in the area deviated from the supply nozzle. As a result, the carburization distribution becomes uneven in the steel material after the vacuum carburizing process.

On the other hand, if the actual carburizing gas flow rate exceeds FA during the period from time t0 to 4t0 in the late carburizing step S2, the carburizing gas will be supplied excessively. In this case, coal and tar are generated due to the remaining gas. As a result, the carburization distribution unevenness of the carburized component (steel material) after the vacuum carburization process becomes large.

Therefore, during the period from time t0 to 4t0 in the late carburizing step S2, the actual carburizing gas flow rate FR is set to be FA×√(t0/t) or more and FA or less. In this case, by satisfying Condition I and Condition III, the carburizing gas flow rate required for the carburizing reaction can be sufficiently ensured, and the generation of coal and tar can be suppressed. As a result, the occurrence of uneven carburization distribution in the carburized component can be suppressed. It should be noted that, as mentioned above, the actual carburizing gas flow rate FR allows a margin of ±10% of the set value. Therefore, as described above, the same margin exists for the actual carburizing gas flow rate FA in the early carburizing step S1. That is, in this specification, the actual carburizing gas flow rate FA in the early carburizing process refers to the carburizing gas flow rate within the range of ±10% of the set value of the actual carburizing gas FR in the early carburizing process S1. In addition, in the middle of the period from time t0 to 4t0 in the late carburizing step S2, the actual carburizing gas flow rate FR may be maintained at FA, following the early carburizing step S1, and then the actual carburizing gas flow rate may be adjusted to FA～FA×√ within the range of (t0/t).

[About condition III]

During the period from time 4t0 to ta in the late carburizing step S2, if the actual carburizing gas flow rate is lower than FA×√(t0/t), the gas flow rate is insufficient. In this case, the distribution of the carburizing gas in the vacuum carburizing furnace becomes uneven. For example, the concentration of the carburizing gas is high near the supply nozzle of the carburizing gas, and the concentration of the carburizing gas is low in the area deviated from the supply nozzle. As a result, the carburization distribution becomes uneven in the steel material after the vacuum carburizing process.

On the other hand, if the actual carburizing gas flow rate exceeds 2FA×√(t0/t) during the period from time 4t0 to ta in the late carburizing step S2, the carburizing gas will be supplied excessively. In this case, coal and tar are generated due to the remaining gas. As a result, the carburization distribution unevenness of the carburized component (steel material) after the vacuum carburization process becomes large.

Therefore, during the period from time 4t0 to ta in the late carburizing step S2, the actual carburizing gas flow rate FR is set to FA×√(t0/t) or more and 2FA×√(t0/t) or less. In this case, by satisfying Condition I and Condition II, the carburizing gas flow rate required for the carburizing reaction can be sufficiently ensured, and the generation of coal and tar can be suppressed. As a result, the occurrence of uneven carburization distribution in the carburized component can be suppressed.

In the post-carburizing step S2, as long as the actual carburizing gas flow rate FR satisfies Condition II and Condition III, there is no particular limitation on the time-dependent change of the actual carburizing gas flow rate FR. For example, as shown in FIG. 6 , the reduction of the actual carburizing gas flow rate FR may be started during the period from time 4t0 to ta in the post-carburizing step S2.

In the post-carburizing step S2, as shown in Fig. 6, the actual carburizing gas flow rate FR can be maintained and reduced stepwise by repeating the maintenance and reduction of the actual carburizing gas flow rate FR as time passes. In addition, as shown in FIG. 7 , in the post-carburizing step S2 , the actual carburizing gas flow rate FR can be gradually reduced as time passes. Furthermore, as shown in FIG. 8 , the actual carburizing gas flow rate FR can be gradually reduced as time passes and then increased. In short, in the post-carburizing step S2, as long as the conditions II and III are satisfied, there is no particular limitation on the temporal variation of the actual carburizing gas flow rate FR.

[Carburizing air pressure in the carburizing process (S30)]

The pressure of the carburizing gas (carburizing gas pressure) in the carburizing step (S30) is not particularly limited. It is preferable that the carburizing gas pressure in the early carburizing step S1 is higher than the carburizing gas pressure in the late carburizing step S2. In this case, in the post-carburizing step S2, the generation of coal can be further suppressed. It is further preferable to reduce the carburizing gas pressure with the passage of time in the post-carburizing step S2. The preferable carburizing gas pressure in the carburizing step (S30) is 1 kPa or less.

[Time ta of the carburizing process (S30)]

The time ta, which is the time from the start (t=0) to the end of the carburizing process (S30), is appropriately set before starting the vacuum carburizing process based on the target carbon concentration of the surface layer of the steel material after the vacuum carburizing process. The time ta can be determined based on the above diffusion simulation using the diffusion equation. The time ta can be determined by performing a vacuum diffusion treatment test in advance and based on experimental data. The time ta is preferably long. When the time ta is long, the actual carburizing gas flow rate FR becomes easy to adjust. The preferable lower limit of the time ta is 50 seconds, more preferably 1 minute (60 seconds), and still more preferably 3 minutes (180 seconds). The preferable upper limit of time ta is 120 minutes, and more preferably, it is 60 minutes.

[Diffusion process (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 carburizing gas to the vacuum carburizing furnace is stopped, and the steel material is maintained at the carburizing temperature Tc for a predetermined time. In the diffusion step (S40), the carbon that has invaded the steel material in the carburizing step (S30) is diffused into the steel material. As a result, the carbon concentration in the surface layer that became high in the carburizing step (S30) decreases, and the carbon concentration in the predetermined depth increases. In the diffusion step (S40), nitrogen gas is introduced and vacuum exhaust is performed using a vacuum pump in the vacuum carburizing furnace, and the atmosphere is set to a nitrogen atmosphere of 1000 Pa or less or to a vacuum. The vacuum is, for example, 10 Pa or less. By setting the inside of the vacuum carburizing furnace to a nitrogen atmosphere or vacuum state of 1000 Pa or less, the intrusion and detachment of carbon from the surface of the steel material is suppressed.

In addition, the holding time in the diffusion process (S40) is set appropriately based on the target carbon concentration of the surface layer of the steel material after the vacuum carburizing process. Therefore, the holding time in the diffusion step (S40) is not particularly limited.

[Quenching process (S50)]

In the quenching process (S50), the steel material that has completed the carburizing process (S30) and the diffusion process (S40) 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 with a high C concentration transforms 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 at this time is not particularly limited. Considering the processing time of the vacuum carburizing process, a faster cooling rate is more preferable. The cooling rate is preferably 0.02 to 30.00°C/second. The cooling rate here refers to a value obtained by dividing the temperature difference between the carburizing temperature Tc and the quenching temperature Ts by the cooling time.

When the quenching temperature Ts is lower than the carburizing temperature Tc, a known cooling method may be used as the cooling method of the steel material. For example, the steel can be cooled by leaving it to cool under vacuum, or the steel can be cooled by gas cooling. When the steel material is left to cool under vacuum, it is preferably left to cool at a pressure of 100 Pa or less. When the steel material is cooled using gas cooling during cooling, it is preferable to use an inert gas as the cooling gas. As an inert gas, for example, nitrogen and/or helium are preferably used. As the inert gas, it is particularly preferable to use nitrogen which is available at low cost. Using an inert gas as a cooling gas suppresses oxidation of steel.

After maintaining the steel material at the quenching temperature Ts for a prescribed time, the steel material is quenched. The quenching temperature Ts is not particularly limited as long as it is A ₃ transformation point (A _r3 transformation point) or higher. The preferable lower limit of the quenching temperature Ts is 800°C, more preferably 820°C, and still more preferably 850°C. The preferable 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.

As the quenching method in the quenching step (S50), a known quenching method is used. The quenching method is, for example, air cooling, water cooling, or oil cooling.

Implement the above vacuum carburizing treatment method to make steel into carburized parts. In the vacuum carburizing treatment method of this embodiment, the theoretical carburizing gas flow rate FT for the steel material to be subjected to the vacuum carburizing treatment is used. Furthermore, the carburizing process (S30) is divided into an early carburizing process S1 and a late carburizing process S2 at the first time after the carburizing process is started when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure. Then, the actual carburizing gas flow rate FR is adjusted so that condition I is satisfied in the early carburizing step S1 and condition II and condition III are satisfied in the late carburizing step S2. This can suppress the occurrence of uneven carburization distribution in the steel material after vacuum carburization.

It should be noted that the vacuum carburizing treatment method of this embodiment may further include other steps. For example, the vacuum carburizing treatment method may include a tempering process after the quenching process (S50). The tempering process may be carried out under known conditions. For example, in the tempering process, the steel material is held at a temperature below the _Ac1 transformation point for a predetermined time and then cooled.

In addition, in the vacuum carburizing treatment method of this embodiment, the carburizing process (S30) and the diffusion process (S40) can be repeated multiple times. In this case, as described above, the time ta and the theoretical carburizing gas flow rate FT are determined for each carburizing step (S30).

[Manufacturing method of carburized parts]

The method of manufacturing a carburized component according to this embodiment includes a step of subjecting a steel material to the above-mentioned vacuum carburizing treatment method to manufacture a carburized component. In the carburized parts manufactured through the above steps, uneven carburization distribution can be suppressed.

Example

Hereinafter, the effect of the vacuum carburizing treatment method of this embodiment will be demonstrated more specifically using an Example. The conditions in the following examples are an example of conditions adopted in order to confirm the implementation possibility and effect of the vacuum carburizing treatment method of this embodiment. Therefore, the vacuum carburizing treatment method of this embodiment is not limited to this one condition example.

A mechanical structural steel pipe (hereinafter referred to as a steel pipe) having a chemical composition equivalent to SCM415 specified in JIS G 4053 (2008) and a round bar equivalent to SCM415 were prepared. The C content of the steel pipes and round bars of each test number was 0.15% by mass. The diameter of the steel pipe is 34mm, the wall thickness is 4.5mm, and the length is 110mm. The diameter of the round rod is 26mm and the length is 70mm. The evaluation of the vacuum carburizing treatment was performed using round bars, and the steel pipe was used as a sample for investigating the uneven carburization distribution caused by the arrangement position of the round bar in the vacuum carburizing furnace.

The total surface area (m ² ) of the round bar and steel pipe subjected to vacuum carburizing treatment for each test number is defined as the steel surface area (m ² ). The surface area of the steel material is determined using the following formula.

Steel surface area = surface area of each steel pipe × number of steel pipes + surface area of each round bar × number of round bars

The obtained steel material surface area is shown in Table 1. 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 to 9, 14 and 17, 124 steel pipes and 3 round bars were used.

[Table 1]

First, diffusion simulation using the diffusion equation is performed to obtain the theoretical carburizing gas flow rate. Specifically, the round rod and steel pipe are divided into a plurality of units of 2 μm or more in the thickness direction. In addition, the step time in the diffusion simulation was set to 0.002 to 0.02 seconds. In the chemical composition of steel pipes and round rods (SCM415), thermodynamic calculations are used to determine the equilibrium composition in equilibrium with graphite at the surface at the carburizing temperature. 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 (trademark) was used for thermodynamic calculations. Furthermore, the database uses the trade name PanFe (trademark). In addition, the following formula is 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 concentration of solid solution C in austenite (mass %), and T is the carburizing temperature (K).

The target value of the carbon concentration on the surface of the steel pipe and the round rod is 0.70 mass%, and the target value of the carbon concentration at a depth of 1.0 mm from the surface is 0.40 mass%. Taking the above as a prerequisite, the above-mentioned diffusion simulations (A) to (D) are performed at each step time to obtain the theoretical carburizing gas flow rate FT at each step time.

The result of calculating the theoretical carburizing gas flow rate FT is that the theoretical carburizing gas flow rate FT can be approximated by the following formula.

FT＝S×A/√t (6)

Here, A is the carburizing gas flow rate per 1 m ² (NL/minute) defined by equation (7), and t represents the time (minutes) from the start of carburization. In addition, S represents the steel material surface area (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 process is performed using the following method. First, a cage formed of a fully carburized stainless steel material (SUS316 specified in JIS G 4303 (2012)) is prepared. Arrange the above number of steel pipes evenly in the cage in an upright state, and then arrange three round rods in an upright state in the center of the cage, the left front of the cage, and the right rear of the cage. As described above, the round bar was used as the test material and the steel pipe was used as the sample material for confirming the occurrence of uneven carburization distribution due to the arrangement position of the round bar.

A cage in which steel materials (steel pipes and round bars) are arranged is inserted into a vacuum carburizing furnace, and vacuum carburizing treatment is performed. Then, carburized parts of test numbers 1 to 21 were obtained. The conditions in the vacuum carburizing treatment are shown in Table 1.

Specifically, in each test number, vacuum carburizing treatment was performed as follows. The vacuum carburizing treatment in each test number kept the pressure in the furnace below 10Pa. In the heating process, the round bar of each test number was heated to the carburizing temperature Tc shown in Table 1. After the heating process, a soaking process is performed. In the soaking process, the steel material (round bar) is maintained at the carburizing temperature Tc for 60 minutes.

After the soaking process, a carburizing process is performed. In the carburizing process, acetylene is supplied as carburizing gas in the vacuum carburizing furnace. The carburizing gas pressure in the carburizing process is kept below 1kPa. The end time ta (minutes) of the carburizing process is as described in Table 1.

As described above, the carburizing time in the carburizing process and the diffusion time in the diffusion process are adjusted with the carbon concentration at a depth of 1.0 mm of the round rod being 0.40% by mass.

In addition, in the carburizing process, the gas in the atmosphere in the vacuum carburizing furnace is analyzed with a quadrupole mass spectrometer, and the hydrogen partial pressure and the acetylene partial pressure are continuously measured. Let the mass-to-charge ratio (m/z) of hydrogen be 2 and the mass-charge ratio of acetylene be 26. The analysis time is 0.5 seconds and the analysis interval is 4 seconds. Based on the obtained hydrogen partial pressure and acetylene partial pressure, time t0 (the first time when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure) is obtained.

The time-dependent changes in the actual carburizing gas flow rate for each test number are shown in Figures 9 to 16. Hereinafter, the set values of the actual carburizing gas flow rates FR for test numbers 1 to 21 will be explained using FIGS. 9 to 16 .

FIG. 9 is a graph showing changes over time in the actual carburizing gas flow rate FR in the carburizing steps of test numbers 1, 5, and 7 to 12. Referring to FIG. 9 , in test numbers 1, 5, and 7 to 12, the actual carburizing gas flow rate FR at the start time of the carburizing process (t=0) is set to FA. FA is FT _ta/10 or more and FT ₄ or less. After time t0 is exceeded and until time ts before time 4t0 is reached, the actual carburizing gas flow rate FR is kept constant at FA. After time ts, the actual carburizing gas flow rate is gradually reduced according to 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. In addition, in the post-carburizing step S2, the actual carburizing gas flow rate FR during the period from time t0 to time 4t0 is FA√(t0/t) or more and FA or less. Furthermore, the actual carburizing gas flow rate FR from time 4t0 to time ta is FA√(t0/t) or more and 2FA√(t0/t) or less.

FIG. 10 is a graph showing changes over time in the actual carburizing gas flow rate FR in the carburizing steps of test numbers 2 to 4 and 6. FIG. Referring to FIG. 10 , in test numbers 2 to 4 and 6, the actual carburizing gas flow rate FR at the start time of the carburizing process (t=0) is set to FA. FA is FT _ta/10 or more and FT ₄ or less. After time t0 is exceeded and until time ts before time 4t0 is reached, the actual carburizing gas flow rate FR is kept constant at FA. It should be noted that the time ts in the period from time t0 to time 4t0 in FIG. 10 is later than the time ts in the period from time t0 to time 4t0 in FIG. 9 . After time ts, the actual carburizing gas flow rate FR gradually decreases with 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. In addition, the actual carburizing gas flow rate FR during the period from time t0 to time 4t0 in the post-carburizing step S2 is FA√(t0/t) or more and FA or less. Furthermore, the actual carburizing gas flow rate FR from time 4t0 to time ta is FA√(t0/t) or more and 2FA√(t0/t) or less.

FIG. 11 is a graph showing changes over time in the actual carburizing gas flow rate FR in the carburizing steps of test numbers 13 and 14. Referring to Fig. 11, in test numbers 13 and 14, the actual carburizing gas flow rate FR at the start time of the carburizing process (t=0), that is, FA, is lower than FT _ta/10 . Then, at time ts later than time ta/10, the actual carburizing gas flow rate FR is gradually reduced in the same manner as the theoretical carburizing gas flow rate FT. It should be noted that in the carburizing process, the acetylene partial pressure in the vacuum carburizing furnace does not become more than 0.8 times the hydrogen partial pressure. Therefore, in vacuum carburizing treatment, t0 is uncertain.

FIG. 12 is a graph showing changes over time in the actual carburizing gas flow rate FR in the carburizing steps of test numbers 15 to 17. Referring to Fig. 12, in test numbers 15 to 17, the actual carburizing gas flow rate FR, that is, FA, at the carburizing process start time (t=0) is FT _ta/10 or more and FT ₄ or less. However, the actual carburizing gas flow rate FR begins to gradually decrease at the time ts before the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure, and the adjustment is made so that the actual carburizing gas flow rate FR becomes FA×√(ts/t). . Therefore, in the carburizing process, the acetylene partial pressure in the vacuum carburizing furnace does not become more than 0.8 times the hydrogen partial pressure. Therefore, in vacuum carburizing treatment, t0 is uncertain.

FIG. 13 is a graph showing the time-dependent change of the actual carburizing gas flow rate FR in the carburizing process of test number 18. FIG. Referring to FIG. 13 , in test number 18, the actual carburizing gas flow rate FR, that is, FA, at the time when the carburizing process starts (t=0) is FT _ta/10 or more and FT ₄ or less. Then, the actual carburizing gas flow rate FR is kept constant at FA until the time t0 exceeds the time t0 and the time ts exceeds the time 4t0. After time ts, the actual carburizing gas flow rate FR is gradually reduced according to 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 process S1, and the actual carburizing gas flow rate FR during the period from time t0 to time 4t0 in the late carburizing process S2 is FA. Above √(t0/t) and below FA. However, the actual carburizing gas flow rate FR from time 4t0 to time ta exceeds 2FA√(t0/t).

FIG. 14 is a graph showing the time-dependent change of the actual carburizing gas flow rate FR in the carburizing process of test number 19. FIG. Referring to Fig. 14 , in test number 19, the actual carburizing gas flow rate FR, that is, FA, at the time when the carburizing process starts (t=0) is lower than FT _ta/10 . Furthermore, the subsequent actual carburizing gas flow rate FR is made constant to FA. In test number 19, in the carburizing process, the acetylene partial pressure in the vacuum carburizing furnace did not become more than 0.8 times the hydrogen partial pressure. Therefore, in vacuum carburizing treatment, t0 is uncertain.

FIG. 15 is a graph showing the time-dependent change of the actual carburizing gas flow rate FR in the carburizing process of test number 20. Referring to FIG. 15 , in test number 20, the actual carburizing gas flow rate FR, that is, FA, at the time when the carburizing process starts (t=0) is FT _ta/10 or more and FT ₄ or less. Then, the actual carburizing gas flow rate FR is kept constant at FA until time ts exceeds time t0 and is lower than time 4t0. After time ts, the actual carburizing gas flow rate FR is gradually reduced according to the curve C2 (=FB×√(ts/t)) using FB (refer to Figure 15) which is lower than the theoretical carburizing gas flow rate FT at time ts. As a result, the actual carburizing gas flow rate FR in the post-carburizing step S2 is lower than FA√(t0/t).

FIG. 16 is a graph showing the time-dependent change of the actual carburizing gas flow rate FR in the carburizing process of test number 21. Referring to Fig. 16 , in test number 21, the actual carburizing gas flow rate FR, that is, FA, at the time when the carburizing process starts (t=0) is FT _ta/10 or more and FT ₄ or less. Then, the actual carburizing gas flow rate FR is kept constant at FA until time ts exceeds time t0 and is lower than time 4t0. After time ts, the actual carburizing gas flow rate FR is reduced. However, from time 4t0 to time ta, there is a period in which the actual carburizing gas flow rate FR exceeds 2FA√(t0/t).

It should be noted that the actual carburizing gas flow rate is adjusted and measured using a flow meter (manufactured by Cofflock Co., Ltd., trade name: Mass flow controller D3665).

After the carburizing process, the round bar was subjected to a diffusion process for the diffusion time (minutes) shown in Table 1, so that the carbon that had invaded the round bar was diffused into the round bar. While maintaining the carburizing temperature, the diffusion process is performed at a furnace pressure of 10 Pa or less. Diffusion times (minutes) are shown in Table 1.

It should be noted that what is described in the "FT _ta/10 " column in Table 1 is the theoretical carburizing gas flow rate (NL/min) at time ta/10. What is written in the "FT ₄ " column is the theoretical carburizing gas flow rate (NL/min) at the time 4 seconds after the start of the carburizing process. What is written in the “time t0 (minute)” column is time t0 (minute). The "time 4t0 (minutes)" column describes the time 4t0 (minutes). The "time ts (minutes)" column describes the time ts (minutes) when the actual carburizing gas flow rate FR starts to gradually decrease. What is recorded in the "FR≥FA×√(t0/t)?" column is whether the actual carburizing gas flow rate is FA×√(t0/t) or more between time 4t0 and time ta. If "Yes", it means that the actual carburizing gas flow rate FR is FA×√(t0/t) or more. If it is "No", it means that the actual carburizing gas flow rate FR is lower than FA×√(t0/t). What is recorded in the "FR≤2FA×√(t0/t)?" column is whether the actual carburizing gas flow rate FR is 2FA×√(t0/t) or less between time 4t0 and time ta. If "Yes", it means that the actual carburizing gas flow rate FR is 2FA×√(t0/t) or less. If it is "No", it means that the actual carburizing gas flow rate FR exceeds 2FA×√(t0/t). "Diffusion time (minutes)" indicates the diffusion time (minutes) in the diffusion process.

After the diffusion process, the round bar is 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 quenched round bar is tempered. The tempering temperature was set to 170°C, and the holding time at the tempering temperature was set to 2 hours.

Through the above manufacturing process, a vacuum carburizing process is performed to manufacture a carburized component (round rod).

[Evaluation test]

The carbon concentration and the depth at which the carbon concentration reaches 0.40 mass% (hereinafter referred to as the carburization depth) of the carburized parts (round rods) of each test number were measured to evaluate uneven carburization distribution.

[Carbon concentration measurement test on the surface layer of carburized parts]

For the carburized parts (round bars) of each test number that are inserted into the vacuum carburizing furnace, a range of 20 mm is cut from the upper end surface in the length direction of the carburized part, and a range of 20 mm is cut from the lower end surface of the carburized part. Cut off a range of 5mm in the length direction. Hereinafter, the range of 20 mm from the upper end surface will be referred to as the "upper end surface test piece", and the portion within the range of 5 mm from the lower end surface will be referred to as the "lower end portion".

Turning processing was performed on the circumferential surface of the upper end surface of the test piece and the remaining portion of the cut lower end portion (hereinafter referred to as the main body portion). In turning processing, the cutting powder is taken from the surface of the round bar to a depth of 0.30mm at every 0.05mm depth interval. The carbon concentration of the cut powder was measured at each depth position at a distance of 0.05 mm. Through the above process, the 0.05mm pitch was obtained in the surface area from the surface to the depth of 0.30mm for the three carburized parts of each test number (the center position of the cage, the left front position of the cage, and the cage right rear position). carbon concentration below. The six carbon concentrations from the surface to 0.30 mm of the carburized component arranged at the center of the cage are defined as carbon concentrations A1 to A6 (mass %) in order from the surface. The six carbon concentrations from the surface to 0.30 mm of the carburized component arranged at the left front position of the cage are defined as carbon concentrations B1 to B6 (mass %) in order from the surface. The six carbon concentrations from the surface to 0.30 mm of the carburized component arranged at the right rear position of the cage are defined as carbon concentrations C1 to C6 (mass %) in order from the surface. Furthermore, the difference between the maximum value and the minimum value of the carbon concentration obtained at the same depth position among the three carburized parts was determined. Specifically, the maximum value and the minimum value are selected among the carbon concentrations A1, B1, and C1 in the area from the surface to the 0.05 mm depth position, and the difference between these carbon concentrations is defined as Δ1. Similarly, among the carbon concentrations A2, B2, and C2 in the area from a depth position of 0.05 mm to a depth of 0.10 mm from the surface, the maximum value and the minimum value are selected, and the difference between these carbon concentrations is defined as Δ2. Through the above steps, Δ1 to Δ6 are obtained, and the arithmetic mean value of Δ1 to Δ6 is defined as the “surface layer carbon concentration difference” (mass %). The obtained results are described in the "Surface layer carbon concentration difference (mass %)" column of Table 1.

Furthermore, the arithmetic mean value of all the carbon concentrations A1 to A6, B1 to B6, and C1 to C6 is defined as the surface layer average carbon concentration (mass %). The obtained results are described in the "surface layer average carbon concentration (mass %)" column of Table 1.

[Carburization depth measurement test]

Using the above-mentioned upper end surface test piece, the carbon concentration in the surface layer portion of the circumferential surface was measured. Specifically, the carbon concentration of the cross section of the upper end surface test piece at a position 20 mm from the upper end surface (a cross section perpendicular to the longitudinal direction of the upper end surface test piece) was measured in the radial direction toward the surface from a depth position of 2 mm from the surface. Specifically, line analysis using EPMA (electron beam microanalyzer) was performed 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 reaches 0.40 mass% or more (hereinafter referred to as the carburization depth) was determined for each of the three upper end surface test pieces. The average difference between the maximum value and the minimum value of the carburization depth obtained from each upper end surface test piece is defined as "0.40 mass% depth difference" (mm). The obtained results are described in the "0.40 mass% depth difference (mm)" column of Table 1.

[Evaluation results]

Referring to Table 1, the case where the surface layer carbon concentration difference is 0.030 mass% or less and the 0.40 mass% depth difference is 0.05 mm or less is evaluated as excellent as a vacuum carburizing treatment method with small carburizing distribution unevenness.

Referring to Table 1, in test numbers 1 to 12, in the preliminary carburizing step S1, the actual carburizing gas flow rate FR is FT _ta/10 or more and FT ₄ or less. Furthermore, in the post-carburizing step S2, during the period from time t0 to 4t0, the actual carburizing gas flow rate FR is FA×√(t0/t) or more and FA or less. Furthermore, in the post-carburizing step S2, during the period from time 4t0 to time ta, the actual carburizing gas flow rate FR is FA×√(t0/t) or more and 2FA×√(t0/t) or less. Therefore, the average carbon concentration of the surface layer is 0.680 mass% or more, the surface carbon concentration difference is 0.030 mass% or less, and the 0.40 mass% depth difference is 0.05 mm or less. That is, the uneven carburization distribution of the carburized parts is small.

On the other hand, in test numbers 13 and 14, as shown in Figure 11 and Table 1, the actual carburizing gas flow rate (FA) in the early carburizing process was lower than FT _ta/10 . Therefore, the average carbon concentration in the surface layer is less than 0.680% by mass, and carburization is not sufficiently performed.

In test numbers 15 to 17, as shown in Figure 12 and Table 1, the actual carburizing gas flow rate (FA) at the start of carburization is FT _ta/10 or more and FT ₄ or less, but the acetylene partial pressure becomes the hydrogen partial pressure. Before it exceeds 0.8 times, gradually reduce the actual carburizing gas flow rate FR. Therefore, the average carbon concentration in the surface layer is less than 0.680% by mass, and carburization is not sufficiently performed.

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

In test number 19, as shown in Fig. 14 and Table 1, the actual carburizing gas flow rate FR was constant to a value FA lower than FT _ta/10 . Therefore, if the depth difference of 0.40% by mass exceeds 0.05 mm, the carburization distribution of the carburized parts will be large and uneven.

In test number 20, as shown in Fig. 15 and Table 1, the actual carburizing gas flow rate FA at the start of carburization is FT _ta/10 or more and FT ₄ or less. However, during the period from time 4t0 to time ta, there is The period during which the actual carburizing gas flow rate FR becomes lower than FA×√(t0/t). Therefore, the average carbon concentration in the surface layer is less than 0.680% by mass, and carburization is not sufficiently performed. Furthermore, the difference in carbon concentration in the surface layer exceeds 0.030% by mass, and the difference in depth of 0.40% by mass exceeds 0.05 mm, resulting in large uneven carburization distribution of the carburized parts.

In test number 21, as shown in Fig. 16 and Table 1, the actual carburizing gas flow rate FA at the start of carburization is FT _ta/10 or more and FT ₄ or less. However, during the period from time 4t0 to time ta, there is The period during which the actual carburizing gas flow rate FR exceeds 2FA×√(t0/t). Therefore, the difference in carbon concentration in the surface layer exceeds 0.030% by mass, and the carburization distribution of the carburized parts is greatly uneven.

The embodiments of the present invention have been described above. However, the above-described embodiments are only examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented within the scope that does not deviate from the gist of the present invention.

Claims (4)

1. A vacuum carburizing treatment method, which is a vacuum carburizing treatment method for carrying out vacuum carburizing treatment on steel in a vacuum carburizing furnace. The vacuum carburizing treatment method has: The heating process is to heat the steel to carburizing temperature; Soaking process: after the heating process, the steel material is soaked at the carburizing temperature; A carburizing process, after the soaking process, while supplying acetylene gas, that is, carburizing gas, into the vacuum carburizing furnace, the steel material is maintained at the carburizing temperature; A diffusion process, after the carburizing process, stopping the supply of the carburizing gas into the vacuum carburizing furnace and maintaining the steel material at the carburizing temperature; and, a quenching step of quenching the steel material after the diffusion step; In the carburizing process, The flow rate of the carburizing gas supplied into the vacuum carburizing furnace is defined as the actual carburizing gas flow rate, The theoretical carburizing gas flow rate is defined as the theoretical carburizing gas flow rate: the flow rate of the carburizing gas required to adjust the carbon concentration at a predetermined depth position from the surface of the steel material to a desired concentration, and the total amount of the carburizing gas flow rate is defined as the theoretical carburizing gas flow rate. The carburizing gas is used as a prerequisite for the carburizing reaction, The end time of the carburizing process is defined as ta, Define the first time after the carburizing process starts when the acetylene partial pressure becomes 0.8 times or more of the hydrogen partial pressure as t0, The carburizing process includes: Continuously measure the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, and determine the partial pressure measurement process at the time t0; The early carburizing process from the start of the carburizing process to time t0; and, The post-carburizing process from the time t0 to the time ta, In the preliminary carburizing process, The actual carburizing gas flow rate is greater than the theoretical carburizing gas flow rate at time ta/10 and less than the theoretical carburizing gas flow rate at 4 seconds from the start of the carburizing process, In the late carburizing process, The actual carburizing gas flow rate of the early carburizing process is defined as FA, the FA is within the range of ±10% of a certain set value, and the time from the start of the carburizing process is defined as time When t, The actual carburizing gas flow rate during the period from time t0 to time 4t0 is Above and below FA, The actual carburizing gas flow rate from the time 4t0 to the time ta is above and/> the following. 2. The vacuum carburizing treatment method according to claim 1, wherein, In the late carburizing process, During the period from time 4t0 to time ta, as time passes, the actual carburizing gas flow rate is reduced by method (A) or (B), (A) Repeat the maintenance and reduction of the actual carburizing gas flow rate, and reduce the actual carburizing gas flow rate in stages; (B) Gradually reduce the actual carburizing gas flow rate as time passes. 3. The vacuum carburizing method according to claim 1 or claim 2, wherein, The theoretical carburizing gas flow rate is determined based on diffusion simulation using the diffusion equation. 4. A method of manufacturing carburized parts, It includes the step of subjecting the steel material to the vacuum carburizing treatment method according to any one of claims 1 to 3.