CN111868292B

CN111868292B - Vacuum carburization method and method for manufacturing carburized component

Info

Publication number: CN111868292B
Application number: CN201980020012.1A
Authority: CN
Inventors: 藤堂尚二; 中谷贵行; 今高秀树
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-03-22
Filing date: 2019-03-22
Publication date: 2022-03-29
Anticipated expiration: 2039-03-22
Also published as: WO2019182140A1; JPWO2019182140A1; CN111868292A; JP6583600B1

Abstract

In the vacuum carburizing treatment method according to the present embodiment, when the flow rate of the carburizing gas calculated from the diffusion flux of carbon in the surface layer of the steel material obtained by diffusion simulation using the diffusion equation is defined as the theoretical carburizing gas flow rate (FT), the time at which the actual carburizing gas flow rate is equal to the theoretical carburizing gas flow rate after the start of the carburizing process is defined as the crossover time te, the time from the start to the end of the carburizing process is defined as the carburizing time ta, and the time at 1/5 of the carburizing time is defined as the reference time ta/5, in the preceding carburizing process (S1) from the start to the crossover time te of the carburizing process, the actual carburizing gas Flow Rate (FR) is made to be: a theoretical carburizing gas flow rate (FT (te)) at a reference time ta/5 from the start of the carburizing step or more and a theoretical carburizing gas flow rate (FT (20s)) at a time 20 seconds from the start of the carburizing step or less; in the later carburizing step (S2) from the intersection time te to the carburizing time ta, the actual carburizing gas Flow Rate (FR) is set to be within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate (FT).

Description

Vacuum carburization method and method for manufacturing carburized component

Technical Field

The present application relates to a vacuum carburization method and a method for manufacturing a carburized component.

Background

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

However, the hydrocarbon gas as the carburizing gas is thermodynamically unstable. Therefore, when the carburizing temperature is high, the carburizing gas is easily decomposed into carbon, hydrogen, and the like. When the carburizing temperature is high, the carburizing gas molecules move actively. Due to the active motion, the carburizing gas molecules collide with each other at high speed, and the carburizing gas decomposes. Coal and tar are produced by decomposition of the carburizing gas. In this case, the surface carbon concentration and carburization depth are not uniformly distributed. Therefore, the surface layer of the carburized component cannot be maintained at a constant quality. Therefore, in the vacuum carburizing treatment method, the unevenness in the distribution of the carbon concentration on the surface of the carburized component and the unevenness in the distribution of the carburized depth of the surface layer are suppressed. In the following description, the non-uniform distribution of the carbon concentration in the surface of the carburized component and the non-uniform distribution of the carburized depth in the surface layer of the carburized component are referred to as "carburization non-uniform distribution".

Proposed techniques for suppressing the unevenness of the carburized distribution are Japanese patent laid-open Nos. 8-325701 (patent document 1), 2016-148091 (patent document 2), 2002-173759 (patent document 3), and 2005-350729 (patent document 4).

In the vacuum carburizing method described in patent document 1, a workpiece made of a steel material is vacuum-heated in a heating chamber of a vacuum carburizing furnace, and a carburizing gas is supplied into the heating chamber to perform a carburizing process. In this vacuum carburizing treatment method, gaseous chain unsaturated hydrocarbons are used as the carburizing gas. The inside of the heating chamber is subjected to carburizing treatment in a vacuum state of 1kPa or less. Patent document 1 describes that the generation of coal is suppressed and uniform carburization is possible.

In the vacuum carburizing treatment method described in patent document 2, a carburizing gas is injected into a carburizing chamber in which a reduced-pressure atmosphere is performed, thereby carburizing a workpiece disposed in the carburizing chamber. In this vacuum carburizing treatment method, the gas injection amount of the carburizing gas injected into the carburizing chamber is calculated based on the volume of the workpiece in the loaded state in the carburizing chamber, the volume of the carburizing chamber, the total surface area of the workpiece, and a constant set based on the type of the carburizing gas. And injecting the carburizing gas of the calculated gas injection amount into the carburizing chamber. Patent document 2 describes that the occurrence of spot-like excess carburization can be prevented.

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

In the vacuum carburizing method described in patent document 4, the relationship V ═ f (t) between the theoretical flow rate V of the carburizing gas required for the carburizing treatment and the carburizing time t is calculated based on the internal diffusion of the material from the carburized depth and the surface carbon concentration. In the pre-carburizing period of the carburizing step, the carburizing time flow rate V1 is supplied sufficiently higher than the theoretical flow rate V and is free from sooting. Further, in the late stage of carburization following the early stage of carburization, a diffusion time flow rate V2 smaller than the theoretical flow rate V is supplied. Patent document 4 describes that the generation of coal can be prevented and the remaining of cementite can be reduced.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 8-325701

Patent document 2: japanese laid-open patent publication No. 2016-148091

Patent document 3: japanese laid-open patent publication No. 2002-173759

Patent document 4: japanese patent laid-open publication No. 2005-350729

However, the uneven carburization distribution can be suppressed by another method different from the vacuum carburization methods of patent documents 1 to 4.

Disclosure of Invention

Problems to be solved by the invention

The purpose of the present application is to provide a vacuum carburizing method and a method for manufacturing a carburized component, which can suppress unevenness in carburization distribution.

Means for solving the problems

The vacuum carburization method according to the present application is a vacuum carburization method for performing a vacuum carburization process on a steel material in a vacuum carburizing furnace, and the vacuum carburization method includes:

a heating step of heating the steel material at a carburizing temperature;

a soaking step of soaking the steel material at the carburizing temperature after the heating step;

a carburizing step of maintaining the steel material at the carburizing temperature while supplying a carburizing gas, which is an acetylene gas, into the vacuum carburizing furnace after the soaking step;

a diffusion step of stopping supply of the carburizing gas into the vacuum carburizing furnace after the carburizing step, and holding the steel material at the carburizing temperature; and the number of the first and second groups,

a quenching step of quenching the steel material after the diffusion step,

in the carburizing step, the carburizing step is performed,

defining an actual flow rate of said carburizing gas as an actual carburizing gas flow rate,

defining a flow rate of the carburizing gas required for the vacuum carburizing treatment of the steel material, which is calculated from a diffusion flux of carbon of a surface layer of the steel material obtained by a diffusion simulation using a diffusion equation, as a theoretical carburizing gas flow rate,

defining a time point at which the actual carburizing gas flow rate is equal to the theoretical carburizing gas flow rate after the start of the carburizing process as an intersection time te,

the time from the start to the end of the carburizing step is defined as a carburizing time ta,

the time 1/5 of the carburizing time ta is defined as a reference time ta/5, and at this time,

the carburizing step includes:

a first carburizing step of performing carburizing from the start of the carburizing step to the intersection time te; and the number of the first and second groups,

a late carburizing step from the crossover time te to the carburizing time ta,

in the early carburizing step, the carbon content of the carburized steel is controlled,

and enabling the actual carburizing gas flow to be as follows: the theoretical carburizing gas flow rate at the reference time ta/5 from the start of the carburizing step is equal to or higher than the theoretical carburizing gas flow rate at the 20 second time from the start of the carburizing step,

in the post-carburizing step, the carbon content of the steel is controlled,

and enabling the actual carburizing gas flow to be within the range of 1.00-1.20 times of the theoretical carburizing gas flow.

The method for manufacturing a carburized component according to the present application includes:

and a step of subjecting the steel material to the vacuum carburization method.

ADVANTAGEOUS EFFECTS OF INVENTION

The vacuum carburizing treatment method can inhibit the uneven carburizing distribution. The method for manufacturing a carburized component according to the present application can manufacture a carburized component in which the variation in carburization distribution is suppressed.

Drawings

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

Fig. 2 is a graph showing a temporal change in an actual carburizing gas flow rate and a temporal change in a theoretical carburizing gas flow rate in a conventional carburizing process.

Fig. 3 is a graph showing the relationship between the difference Δ F (NL/min) between the actual carburizing gas flow rate introduced at the start of the preceding carburizing step and the theoretical carburizing gas flow rate at time ta/5 of 1/5 of the carburizing time ta from the start of the carburizing step, and the carbon concentration difference (mass%) on the surface of the carburized component.

Fig. 4 is a diagram showing a change with time in an actual carburizing gas flow rate in the carburizing step by the vacuum carburizing treatment method according to the present embodiment.

Fig. 5 is a schematic diagram for explaining a method of adjusting the actual carburizing gas flow rate FR in the carburizing step at the later stage in the vacuum carburizing treatment method according to the present embodiment.

Fig. 6 is a diagram showing an example of a heating pattern of the vacuum carburizing treatment method according to the present embodiment.

Fig. 7 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process of test No. 1 to test No. 8 in the example.

Fig. 8 is a graph showing the change with time in the actual carburizing gas flow rate in the carburizing process of test No. 9 in the example.

Fig. 9 is a graph showing the change with time in the actual carburizing gas flow rate in the carburizing process of test No. 10 in the example.

Fig. 10 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process of test No. 11 and test No. 12 in the example.

Fig. 11 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process of test No. 13 and test No. 14 in the example.

Fig. 12 is a graph showing the change with time in the actual carburizing gas flow rate in the carburizing process of test No. 15 in the example.

Fig. 13 is a graph showing the change with time in the actual carburizing gas flow rate in the carburizing process of test No. 16 in the example.

Fig. 14 is a graph showing the change with time in the actual carburizing gas flow rate in the carburizing process of test No. 17 in the example.

Detailed Description

The present inventors have studied a method for suppressing the unevenness of the carburized distribution in a carburized part in the vacuum carburization treatment method. The present inventors first focused attention on a carburizing gas flow rate required for a vacuum carburizing process of a steel material in a carburizing step in the vacuum carburizing process. The optimal carburizing gas flow rate in the carburizing step can be calculated as a theoretical carburizing gas flow rate based on diffusion simulation using a diffusion equation.

[ theoretical carburizing gas flow ]

In the vacuum carburizing treatment method of the present embodiment, acetylene is used as the carburizing gas. The decomposition of acetylene is controlled in speed by carbon diffusion at the surface of the steel material to be carburized. That is, the larger the diffusion flux of carbon entering the steel from the surface of the steel, the larger the amount of acetylene decomposed.

In the vacuum carburization, carbon diffuses in the steel, that is, Fick's 1 st Law holds. At this time, the gas flow rate of the carburizing gas (acetylene gas) required to make the carbon concentration at a predetermined depth position from the steel surface by the vacuum carburizing treatment a desired concentration is defined as a theoretical carburizing gas flow rate FT.

The amount of change in diffusion flux J (mm mass%/s) of carbon invading from the steel surface and the carbon concentration per unit time is calculated by a known diffusion simulation using a diffusion equation

So that the theoretical carburizing gas flow rate FT can be calculated. Specifically, the theoretical carburizing gas flow rate can be obtained by the following method.

Diffusion plateIn the case of generation (that is, in the case where Fick's Law 1 is established), the diffusion flux J of carbon entering from the surface of the steel material is defined by the formula (1), and the amount of change in the carbon concentration per unit time is

Is defined by formula (2).

Wherein D is the diffusion coefficient (mm) of carbon in the steel²And C is the mass concentration (% by mass) of carbon, z is the displacement (mm) in the depth direction from the steel surface, and t is the time (sec) from the start of the carburizing step.

Is a partial differential sign.

If the amount of change in the carbon concentration is calculated based on the gradient of the chemical potential, the diffusion driving force of carbon can be strictly controlled. At this time, the diffusion flux J (mm. mol%/s) of carbon is defined by the formula (3), and the temporal change in carbon concentration is defined by the formula (4).

Wherein m is the mobility (mm) of carbon²mol/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), and t is the time (sec).

Is a partial differential sign。

Wherein the driving force for diffusion of carbon is as in formula (3)

And (4) partial. In addition, the austenite (γ) in the vacuum carburization process has a carbon concentration as low as 2% or less, and the molar concentration and the mass concentration are substantially proportional to each other. Therefore, the formula (3) can be expressed in terms of mass concentration (% by mass). When expression (3) is expressed in mass%, the diffusion flux J (mm · mass%/s) of carbon is defined by expression (5), and the temporal change in carbon concentration is defined by expression (2).

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

Diffusion simulations for calculating the theoretical carburizing gas flow rate FT were performed in the following manner using the above-mentioned fick's law 1 (equations (1), (3), and (5)), and fick's law 2 (equations (2) and (4)).

In the vacuum carburizing treatment using acetylene as the carburizing gas, carbon enters from the surface of the steel material due to decomposition of the carburizing gas on the surface of the steel material. It is assumed that the carbon concentration in the steel material increases until the carbon concentration is in equilibrium with graphite on the steel material surface in the carburizing step. Therefore, the boundary condition in the simulation of the diffusion of carbon in the steel surface in the vacuum carburization treatment is defined as "the carbon concentration in the steel surface is balanced with graphite". The diffusion simulation was performed as described below on the premise described above.

[ calculation method in diffusion simulation ]

First, mesh data is created in which the surface layer of a steel material to be subjected to vacuum carburization is divided into a plurality of cells. 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 cells may gradually increase from the surface of the steel material in the depth direction. In this case, the ratio of the sizes of the adjacent cells is 0.80 to 1.25, preferably 0.90 to 1.10. The size of the cell is not limited thereto. The object for diffusion simulation may be one-dimensional. When the shape of the steel material is a round bar or a cylinder, the grid data can be processed as one dimension by setting the grid data to a cylinder coordinate system. Further, if the diameter of the steel material (round bar or cylinder) is 50 times or more the diffusion distance, the treatment can be performed in the same manner as the flat surface. Here, the diffusion distance means √ Dt. The diffusion coefficient D is calculated from the carbon concentration of the steel and the carburization temperature. The time t (sec) is a carburizing time. In the vacuum carburizing treatment, when the carburizing step and the diffusion step are each performed 2 or more times, the time t is a time from the start of the first carburizing step to the end of the last carburizing step (in the case where the carburizing step is performed only 1 time, the time from the start to the end of the carburizing step). For example, using SCM415 specified in JIS G4053 (2008), the diffusion distance √ Dt is 0.20mm when the carburization temperature is 950 ℃ and the carburization time is 51 minutes. In this case, if the diameter of the steel material is 10mm or more, the steel material can be processed in the same manner as the plane. In addition, using SCM420 specified in JIS G4053 (2008), the diffusion distance √ Dt is 0.21mm when the carburizing temperature is 950 ℃ and the carburizing time is 51 minutes. Then, the analysis time (step time) of the diffusion simulation is set. The step time is not particularly limited, and is, for example, 0.001 to 1.0 second.

As described above, the steel material surface is in an equilibrium state with graphite. Therefore, the equilibrium phase and the equilibrium composition in an equilibrium state with graphite at the carburizing temperature are determined by known thermodynamic calculations based on the chemical composition of the steel material to be subjected to the vacuum carburizing treatment. The chemical composition of the steel material to be subjected to the vacuum carburization is subjected to thermodynamic calculation in consideration of dilution due to an increase in the C concentration until the C concentration increases to a point where graphite appears as an equilibrium phase. For example, when the C concentration is increased by 7 mass%, the weight of the steel material itself becomes 1.07 times. Therefore, the concentration of the other elements than C was thermodynamically calculated based on the chemical composition set to 1/1.07 times. The content of C in the steel, the chemical potential of C, and the concentration of solid solution C in austenite can be specified from the equilibrium phase and the equilibrium composition obtained by thermodynamic calculations. Known thermodynamic calculation software can be used for the thermodynamic calculation. Known thermodynamic calculation software is, for example, under the trade name Pandat (trademark).

Similarly, inside the steel material except the surface of the steel material, there are cases of vacuum carburization and cases of cementite (θ) precipitation. At this time, carbon in the steel material is distributed into cementite and austenite. Therefore, the equilibrium phase and the equilibrium composition in the steel material other than the steel material surface at the carburizing temperature are determined by the thermodynamic calculation described above. Similarly to the steel surface, the equilibrium phase, the equilibrium composition, the C content in the steel, the chemical potential of C, and the solid-solution C concentration in austenite can be specified in the steel.

The diffusion coefficient D of carbon in austenite in the steel material may be a value obtained in advance through experiments using a steel material to be subjected to vacuum carburization, or may be data reported as experimental data. For example, as the diffusion coefficient (m) of C in austenite²(s), the following equation can be used with reference to what is proposed by Gray g.

D＝4.7×10^-5×exp(－1.6×C－(37000－6600×C)/1.987/T)

In the formula, "C" represents the solid-solution C concentration (mass%) in austenite, and T represents the carburizing temperature (K).

Mobility m (m) of carbon in austenite in steel material²The/s) can be determined from the diffusion coefficient D and thermodynamic calculations. The formula obtained by formulating this is as follows.

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

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

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

(A) The concentration of solid-solution C in austenite (i.e., the concentration of diffused C) in each cell at the carburizing temperature is determined based on the carbon concentration in each cell and the thermodynamic calculation results. At this time, it is assumed that C in cementite is fixed and only solid solution C in austenite diffuses.

(B) In each cell, based on the specific solid solution C concentration, the diffusion flux J in each cell is obtained by a difference method using formula (1), formula (3), or formula (5). At this time, as described above, the solid-solution carbon concentration on the steel surface is set to the solid-solution carbon concentration (C) at the solid-solution limit in the state of equilibrium with graphite_sat). Based on diffusion flux J from the steel surface₀The acetylene flow rate was determined with the carburizing efficiency set to 100%. The obtained acetylene flow rate was defined as the theoretical carburizing gas flow rate at the step time.

(C) The C concentration of each cell at the time when the step time passes is determined based on the diffusion flux J obtained in each cell.

(D) And judging whether cementite is generated or not as an equilibrium phase based on the thermodynamic calculation result. The time required for the generation of cementite is not considered (that is, (a) at the next step time is determined).

(E) When the carburizing process was performed 2 or more times, the diffusion process was simulated between the carburizing processes, and the subsequent carburizing process was simulated. In the diffusion step, the diffusion flux J from the surface of the steel material is measured₀The calculation of (a) to (D) is performed with zero.

The above calculation is obtained for each step time, and the theoretical carburizing gas flow rate FT in the carburizing step is obtained. The "theoretical carburizing gas flow rate" FT can be represented as a theoretical carburizing gas flow rate curve by plotting the theoretical carburizing gas flow rate FT at each carburizing time in a graph in which the horizontal axis represents the elapsed time (carburizing time) from the start of carburizing and the vertical axis represents the theoretical carburizing gas flow rate FT. Fig. 1 is a view showing an example of a relationship between a theoretical carburizing gas flow rate and time, which is calculated from the diffusion flux of carbon in the surface layer of the steel material obtained by the above-described diffusion simulation. ● in FIG. 1 indicates the theoretical carburizing gas flow rate FT at each time. Curve C in fig. 1_1.00Representing the theoretical carburizing gas flow curve.

It should be noted that the theoretical carburizing gas flow rate curve C_1.00May be represented by formula (6).

F＝A/√t (6)

Wherein A is per 1m defined by the formula (7)²The carburizing gas flow rate (NL/min) of (1), t represents the time (min) from the start of carburizing.

A＝a×T²+b×T+c (7)

In formula (6), a, b, and c are constants determined by the chemical composition of the steel material, and T is the carburization temperature (deg.c). For example, in the case of SCM420 prescribed in JIS G4053 (2008), when determined by the diffusion simulation described above, a is 8.52 × 10^-5B-0.140, c-58.2. In the case of SCM415 prescribed in JIS G4053 (2008), when determined by the diffusion simulation described above, a is 8.64 × 10^-5、b＝－0.141、c＝59.0。

Therefore, based on the equation (6) which is an approximate equation, the theoretical carburizing gas flow rate FT at each carburizing time in the actual carburizing process may be calculated.

[ vacuum carburizing treatment method according to the present embodiment ]

The actual flow rate of the carburizing gas at the time of the vacuum carburizing treatment is defined as "actual carburizing gas flow rate" FR. The present inventors have conducted investigations and studies on the matters of the case where the actual carburizing gas flow rate FR largely deviating from the relationship of the theoretical carburizing gas flow rate FT at the carburizing time is assumed to be used, as shown in fig. 1.

Fig. 2 is a graph showing a temporal change in the actual carburizing gas flow rate FR and a temporal change in the theoretical carburizing gas flow rate FT in the conventional carburizing process. In fig. 2, the vertical axis represents the carburizing gas flow rate (NL/min), and the horizontal axis represents the time (min) from the start of the carburizing step. As described above, the solid line FR in fig. 2 shows the actual carburizing gas flow rate FR in the conventional carburizing process. Dotted line C of FIG. 2_1.00The theoretical carburizing gas flow rate FT is expressed as described above.

Referring to fig. 2, the time from the start of the carburizing step to the end of the carburizing step is defined as a carburizing time ta (that is, the carburizing step is ended at the time of the time ta from the start of the carburizing step). Further, a time at which the actual carburizing gas flow rate FR is initially equal to the theoretical carburizing gas flow rate FT is defined as the crossover time te. The period from the start of the carburizing step to the crossover time te is defined as the previous carburizing step (S1). Further, the time from the intersection time te to the carburizing time ta is defined as a later carburizing step (S2).

In the early carburizing step (S1), 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 step of the conventional vacuum carburizing treatment method, the actual carburizing gas flow rate FR in the early carburizing step (S1) is insufficient, and the carburizing distribution on the steel surface becomes uneven. On the other hand, in the late stage carburizing step (S2), the actual carburizing gas flow rate FR is larger than the theoretical carburizing gas flow rate FT (curve C)_1.00) High. Therefore, in the later carburizing step S2, the actual carburizing gas flow rate FR is excessive and remains in the vacuum carburizing furnace. As a result, in the later carburizing step (S2), coal and tar are generated by the residual carburizing gas, and the carburization distribution on the steel surface becomes uneven.

Based on the above findings, the present inventors considered that the carburizing gas flow rate curve C is theoretically calculated in the carburizing step_1.00To control the actual carburizing gas flow rate FR.

However, as shown in fig. 2, in the early carburizing step (S1), the theoretical carburizing gas flow rate curve C is compared with the late carburizing step (S2)_1.00Is steep. Therefore, in actual operation, this theoretical carburizing gas flow rate curve C is combined_1.00It is very difficult to adjust the actual carburizing gas flow rate FR.

Further, in the early carburizing step (S1), at the start of the carburizing step (t is 0), the theoretical carburizing gas flow rate FT is infinite as understood from the above equation (5). Therefore, it is extremely difficult to introduce the actual carburizing gas flow rate FR equal to the theoretical carburizing gas flow rate FT at the initial stage of the early carburizing step (S1).

Therefore, the present inventors investigated that the carburization maldistribution in the previous carburizing step (S1) is suppressed by a different method in the previous carburizing step (S1), instead of setting the actual carburizing gas flow rate FR in conjunction with the theoretical carburizing gas flow rate FT. As a result, the present inventors have obtained the following findings.

As described above, in the carburizing step (the early carburizing step (S1) + the late carburizing step (S2)), the actual carburizing gas flow rate supplied to the vacuum carburizing furnace is defined as FR, and the theoretical carburizing gas flow rate is defined as FT. Among the time required for the carburizing step (i.e., the carburizing time ta), a time corresponding to 1/5 from the start of the carburizing step is defined as a reference time ta/5. The theoretical carburizing gas flow rate FT at the reference time ta/5 is defined as the theoretical carburizing gas flow rate FT (ta/5). Further, a value obtained by subtracting the theoretical carburizing gas flow rate FT (ta/5) from the actual carburizing gas flow rate FR is defined as a flow rate difference Δ F. The flow rate difference Δ F is expressed by the following equation.

Flow difference Δ F ═ FR-FT (ta/5)

The present inventors considered whether or not the carburization distribution unevenness in the steel material after the carburization time ta passed can be suppressed even if there is a difference in flow rate between the actual carburization gas flow rate FR and the theoretical carburization gas flow rate FT to some extent in the initial stage of the carburization step. Therefore, the relationship between the flow rate difference Δ F and the carbon concentration difference (mass%) on the steel surface at the reference time ta/5 was examined, and fig. 3 was obtained. Fig. 3 is a graph showing the relationship between the difference Δ F (NL/min) between the actual carburizing gas flow rate introduced at the start of the preceding carburizing step and the theoretical carburizing gas flow rate at time 1/5 (reference time) ta/5 of the carburizing time ta from the start of the carburizing step, and the difference in the surface layer carbon concentration (mass%) of the carburized component. Fig. 3 is made based on the results of the examples described later. The difference in the carbon concentration of the surface layer of the steel material is an example of an index indicating the unevenness of the carbon concentration distribution, that is, the carburization distribution, of the surface layer of the steel material.

Referring to fig. 3, when the flow rate difference Δ F at the time of the reference time ta/5 is 0.0 NL/minute or more, the difference in the carbon concentration of the surface layer of the steel material becomes significantly smaller than that in the case where the flow rate difference Δ F is less than 0.0 NL/minute, and the difference in the carbon concentration of the surface layer hardly changes even if Δ F increases. That is, in the graph of fig. 3, an inflection point exists at a flow rate difference Δ F at the reference time ta/5 in the vicinity of 0.0 NL/minute.

Based on the results of fig. 3, the present inventors have found that, in the early carburizing step (S1), that is, in the initial carburizing step in the carburizing step in which the theoretical carburizing gas flow rate FT exceeds the actual carburizing gas flow rate FR, the carburizing distribution unevenness can be sufficiently suppressed by setting the actual carburizing gas flow rate FR to be equal to or higher than the theoretical carburizing gas flow rate FT (ta/5) at the reference time ta/5 instead of matching the actual carburizing gas flow rate FR to the theoretical carburizing gas flow rate FT.

Fig. 4 is a diagram showing a change with time in the actual carburizing gas flow rate FR in the carburizing step by the vacuum carburizing treatment method according to the present embodiment. In fig. 4, the vertical axis represents the carburizing gas flow rate (NL/min), and the horizontal axis represents the carburizing time (min). The broken line FT in the figure indicates the theoretical carburizing gas flow rate FT. The solid line FR indicates the actual carburizing gas flow rate FR.

Referring to fig. 4, in the vacuum carburizing treatment method according to the present embodiment, the actual carburizing gas flow rate FR in the previous carburizing step (S1) is set to be equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) at the reference time ta/5. In this case, as shown in fig. 3, the carburization distribution unevenness of the steel material after the vacuum carburization can be sufficiently suppressed.

On the other hand, if the actual carburizing gas flow rate FR in the early carburizing step (S1) is too large as compared with the theoretical carburizing gas flow rate FT (20S) at the time of 20 seconds from the start of the carburizing step, the actual carburizing gas flow rate FR will be too large as compared with the theoretical carburizing gas flow rate FT at the initial stage of the later carburizing step (S2). In this case, in the late carburizing step (S2), it takes an excessive amount of time to decrease the actual carburizing gas flow rate FR to the theoretical carburizing gas flow rate FT. Therefore, an excessive amount of the carburizing gas remains in the vacuum carburizing furnace. Excess carburizing gas produces coal on the steel surface. As a result, the carburization distribution of the steel material becomes uneven. Therefore, the actual carburizing gas flow rate FR in the previous carburizing step (S1) is set to be equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) at the reference time ta/5, but the upper limit of the actual carburizing gas flow rate FR in the previous carburizing step (S1) must be considered. Therefore, in the present embodiment, the actual carburizing gas flow rate FR in the preceding carburizing step (S1) is set to be equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) at the reference time ta/5 and equal to or less than the theoretical carburizing gas flow rate FT (20S) at the time 20 seconds from the start of the carburizing step. In this case, in the later carburizing step (S2), under the conditions for the adjustment described below, the excess carburizing gas can be prevented from remaining in the vacuum carburizing furnace, and the carburization distribution of the carburized component (steel material) can be prevented from becoming uneven.

In fig. 4, the actual carburizing gas flow rate FR from the start of the carburizing step (t is 0) to the intersection time te at which the actual carburizing gas flow rate FR is initially equal to the theoretical carburizing gas flow rate FT (i.e., at the end of the previous carburizing step (S1)) is kept constant. In this case, the adjustment of the actual carburizing gas flow rate FR in the early carburizing step (S1) is easy. However, the actual carburizing gas flow rate FR in the early carburizing step (S1) (time t is 0 to te) may not be fixed. When the actual carburizing gas flow rate FR in the previous carburizing step (S1) is equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) at the reference time ta/5 and equal to or less than the theoretical carburizing gas flow rate FT (20S) at the time 20 seconds from the start of the carburizing step, the change with time of the actual carburizing gas flow rate FR in the previous carburizing step (S1) is not particularly limited. For example, the actual carburizing gas flow rate FR in the preceding carburizing step (S1) may be increased or decreased as time passes. Further, the actual carburizing gas flow rate FR in the preceding carburizing step (S1) may repeatedly increase and decrease with time.

In the present embodiment, in the early carburizing step (S1), the actual carburizing gas flow rate FR is set to be equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) at the reference time ta/5 and equal to or less than the theoretical carburizing gas flow rate FT (20S) at the time 20 seconds from the start of the carburizing step. On the other hand, in the late stage carburizing step (S2) after the early stage carburizing step (S1), it is not so difficult to control the actual carburizing gas flow rate FR in conjunction with the theoretical carburizing gas flow rate FT. Therefore, in the later carburizing step (S2) (t ═ te to ta), the actual carburizing gas flow rate FR is controlled in accordance with the theoretical carburizing gas flow rate FT. This can suppress the excess carburizing gas from remaining in the vacuum carburizing furnace. As a result, the generation of coal and tar can be reduced, and the carburization distribution unevenness of the steel material can be suppressed.

However, in actual operation, it is difficult to follow the theoretical carburizing gas flow rate curve C without error_1.00The actual carburizing gas flow rate FR is so controlled. Therefore, the present inventors investigated the relationship between the actual carburizing gas flow rate FR and the theoretical carburizing gas flow rate FT and the carburization distribution unevenness in the later carburizing step (S2). As a result, it was found that, in the late carburizing step (S2), the variation in the carburizing distribution can be suppressed by maintaining the actual carburizing gas flow rate FR in the range of 1.00 to 1.20 times the theoretical carburizing gas flow rate FT.

Fig. 5 is a schematic diagram for explaining a method of adjusting the actual carburizing gas flow rate FR in the late carburizing step (S2) in the vacuum carburizing treatment method according to the present embodiment. Curve C in fig. 5_1.20Is a curve showing the change in the carburizing gas flow rate 1.20 times the theoretical carburizing gas flow rate FT. Curve C is shown below_1.20Referred to as "theoretical carburizing gas flow curve" C_1.20. Theoretical carburizing gas flow rate curve C in fig. 5_1.00Is a curve showing the change in the carburizing gas flow rate 1.00 times the theoretical carburizing gas flow rate FT. In the later carburizing step (S2), the actual carburizing gas flow rate FR is shown by the theoretical carburizing gas flow rate curve C in fig. 5_1.00Curve C of theoretical carburizing gas flow_1.20Within the range of (3), that is, within the range of the shaded portion in fig. 5.

In the later carburizing step (S2), if the actual carburizing gas flow rate FR is less than 1.00 times the theoretical carburizing gas flow rate FT, the amount of carbon supply required for the vacuum carburizing process becomes insufficient. In this case, the area near the carburizing gas nozzle is likely to be carburized among the steel surface, and the carburizing gas is not sufficiently supplied to the area far from the carburizing gas nozzle, which makes carburizing difficult. As a result, the carburization distribution becomes uneven.

On the other hand, if the actual carburizing gas flow rate FR exceeds 1.20 times the theoretical carburizing gas flow rate FT in the late carburizing step (S2), the actual carburizing gas flow rate FR becomes too large in the late carburizing step (S2). In this case, coal is generated, and the carburization distribution becomes uneven. More specifically, as described below.

The average velocity of the gas molecules is inversely proportional to the square root of the mass. In the case of using acetylene as the carburizing gas, the rate of hydrogen gas having a molecular weight of 2 is 3.6 times that of acetylene having a molecular weight of 26. In this way, the diffusion rate of hydrogen is higher than that of acetylene, and therefore, the atmosphere in the vacuum carburizing furnace is easily made uniform. As a result, the carburization distribution unevenness is reduced. Therefore, in order to reduce the carburization maldistribution, it is effective to reduce the proportion of acetylene and increase the proportion of hydrogen in the atmosphere in the vacuum carburizing furnace.

When the actual carburizing gas flow rate FR exceeds 1.20 times the theoretical carburizing gas flow rate FT, the proportion of the carburizing gas (acetylene) increases and the proportion of the hydrogen gas decreases in the atmosphere in the vacuum carburizing furnace. In this case, the uniformity of the furnace atmosphere is reduced, and the effect of suppressing the carburization distribution unevenness due to hydrogen gas cannot be sufficiently obtained. When the actual carburizing gas flow rate FR exceeds 1.20 times the theoretical carburizing gas flow rate FT, the frequency at which the acetylene molecules collide with each other increases. The collision of acetylene molecules with each other produces coal. As a result, the carburization distribution of the steel material becomes uneven.

Therefore, in the vacuum carburizing treatment method according to the present embodiment, in the late carburizing step (S2), the actual carburizing gas flow rate FR is adjusted (controlled) to be within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate FT.

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

[1] The vacuum carburization method of (1) is a vacuum carburization method for performing a vacuum carburization process on a steel material in a vacuum carburization furnace, and the vacuum carburization method includes:

a heating step of heating the steel material at a carburizing temperature;

a quenching step of quenching the steel material after the diffusion step,

in the carburizing step, the carburizing step is performed,

defining a time point when the actual carburizing gas flow rate is equal to the theoretical carburizing gas flow rate after the carburizing process is started as an intersection time te,

the carburizing step includes:

a late carburizing step from the crossover time te to the carburizing time ta,

in the post-carburizing step, the carbon content of the steel is controlled,

[2] The vacuum carburization method according to [1], wherein,

when 1/10 times of the carburizing time ta is defined as a time ta/10,

the actual carburizing gas flow rate is set to be equal to or greater than the theoretical carburizing gas flow rate at the time ta/10 from the start of the carburizing step.

[3] The vacuum carburization method according to [2], wherein,

when 1/30 times the carburizing time ta is defined as time ta/30,

the actual carburizing gas flow rate is set to be equal to or greater than the theoretical carburizing gas flow rate at the time ta/30 from the start of the carburizing step.

[4] The vacuum carburization method of (1) to (3), wherein,

and fixing the actual carburizing gas flow.

[5] Method for producing carburized component

The method comprises the step of subjecting a steel material to the vacuum carburization method according to any one of [1] to [4 ].

The vacuum carburizing method and the method for manufacturing a carburized component according to the present embodiment will be described in detail below.

[ vacuum carburization method ]

Fig. 6 is a diagram showing an example of a heating pattern of the vacuum carburizing treatment method according to the present embodiment. Referring to fig. 6, the vacuum carburization method according to this embodiment includes: a heating step (S10); a soaking step (S20); a carburizing step (S30); a diffusion step (S40) and a quenching step (S50). The details of each step are described below.

[ heating Process (S10) ]

In the heating step (S10), the steel material is heated at a carburizing temperature. The steel material to be subjected to the vacuum carburization may be a steel material provided by a third party or a steel material produced by performing the vacuum carburization method. The chemical composition of the steel material is not particularly limited. A known steel material subjected to carburizing treatment may be used. The steel material is, for example, an alloy steel material for machine structural use defined in JIS G4053 (2008). More specifically, SCr415, SCr420, SCM415, and the like defined in JIS G4053 (2008) are exemplified.

The prepared steel material may be a hot-worked steel material or a cold-worked steel material. Examples of the hot working include hot rolling, hot extrusion, and hot forging. The cold working is, for example, cold rolling, cold drawing, cold forging, or the like. The steel material may be a steel material subjected to machining such as cutting after being hot-worked or cold-worked.

In the heating step (S10), the steel material is inserted into a vacuum carburizing furnace and heated to the carburizing temperature Tc. The heating step (S10) is a step known in the vacuum carburizing treatment method. The carburizing temperature Tc may be a known temperature. The carburization temperature Tc is A_c3Above the transformation point. The carburizing temperature Tc is preferably in the range of 900 to 1130 ℃. When the carburizing temperature Tc is 900 ℃ or higher, the heat conduction by radiation increases, and the temperature in the vacuum carburizing furnace is easily uniform. As a result, the carburization distribution unevenness of the steel material is likely to be reduced. When the carburizing temperature is 1130 ℃ or lower, the grain size of the steel material can be prevented from becoming coarse, and the strength of the steel material can be suppressed from decreasing. The lower limit of the carburizing temperature Tc is more preferably 910 ℃, and still more preferably 920 ℃. The upper limit of the carburizing temperature Tc is more preferably 1100 ℃, and still more preferably 1080 ℃.

[ soaking step (S20) ]

In the soaking step (S20), the steel material is held at the carburizing temperature Tc for a predetermined time. Hereinafter, the holding time in the soaking step (S20) is referred to as soaking time. The soaking step (S20) is a step known in the vacuum carburizing treatment method. The soaking time may be appropriately adjusted according to the shape and/or size of the steel material. The soaking time is preferably 10 minutes or more. More specifically, when a cross section perpendicular to the longitudinal direction of the steel material is converted into a circle, the soaking time is preferably 30 minutes or more per 25mm equivalent circle diameter. For example, when the circle-equivalent diameter is 30mm, the soaking time is preferably 36 minutes or more. The 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 100Pa or less. In the heating step (S10) and/or the soaking step (S20), introduction of nitrogen gas and vacuum evacuation by a vacuum pump are performed, and a nitrogen atmosphere of 1000Pa or less may be used.

[ carburizing step (S30) ]

In the carburizing step (S30), the inside of the vacuum carburizing furnace is preliminarily set to a low pressure or vacuum before the start of carburizing. Low pressure or vacuum means, for example, 10Pa or less. If the pressure in the vacuum carburizing furnace is low, the frequency of collision between molecules of the carburizing gas decreases. That is, the frequency of decomposition of the carburizing gas in the atmosphere becomes small. Therefore, the generation of coal and tar can be suppressed by spraying the surface of the steel material at a low pressure as quickly as possible. As a result, the surface carbon concentration of the steel material can be rapidly increased. In carburizing from the start of carburizing to the end of carburizing (time ta), the furnace internal pressure is set to 1 to 1000 Pa.

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

[ carburizing gas ]

In the present embodiment, the carburizing gas used in the carburizing step in the vacuum carburizing treatment method is acetylene gas.

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

In the present embodiment, the purity of acetylene as the carburizing gas may be 98% or more. As the acetylene, for example, acetylene dissolved in acetone, acetylene dissolved in Dimethylformamide (DMF) may be used as the carburizing gas. Acetylene dissolved in DMF is preferably used as the carburizing gas. In this case, the contamination of the solvent into the furnace atmosphere can be suppressed. When the supply source for supplying acetylene to the vacuum carburizing furnace is a gas holder, the primary pressure when acetylene is supplied from the gas holder into the vacuum carburizing furnace is preferably 0.5MPa or more. When the pressure is supplied to the vacuum carburizing furnace, the pressure is preferably reduced to 0.20MPa or less by using a pressure reducing valve.

[ details of carburizing step (S30) ]

The carburizing step (S30) includes, as described above: a first carburizing step (S1) and a second carburizing step (S2). The early carburizing step (S1) is a step during a period from the start of the carburizing step (t ═ 0) to a crossover time t ═ te. The late carburizing step (S2) is a step during a period from the intersection time te to the carburizing time ta.

[ preparation in advance ]

Before the vacuum carburizing treatment method is performed, as a preliminary preparation, the above-described diffusion simulation using the diffusion equation is performed, the theoretical carburizing gas flow rate FT corresponding to the target steel material is calculated, and the temporal change in the theoretical carburizing gas flow rate FT at the carburizing time ta in the carburizing step (S30) as shown in fig. 1 is obtained.

[ Pre-carburizing step (S1) ]

As shown in fig. 5, in the early carburizing step (S1), the actual carburizing gas flow rate FR is set to: the theoretical carburizing gas flow rate FT (ta/5) at the reference time (ta/5) from the start of the carburizing step (t ═ 0) is not less than the theoretical carburizing gas flow rate FT (20s) at the time of 20 seconds from the start of the carburizing step (t ═ 0).

If the actual carburizing gas flow rate FR in the early carburizing step (S1) is smaller than the theoretical carburizing gas flow rate FT (ta/5), the supply of the carburizing gas is excessively insufficient in the early carburizing step (S1). In this case, the steel material (carburized part) subjected to the vacuum carburization method has a large variation in carburization distribution. On the other hand, if the actual carburizing gas flow rate FR in the previous carburizing step (S1) exceeds the theoretical carburizing gas flow rate FT (20S), the actual carburizing gas flow rate FR becomes excessive. In this case, it takes time to adjust the actual carburizing gas flow rate FR to be within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate FT after the crossing time te elapses. Therefore, in the later carburizing step (S2), the carburizing gas remains in the vacuum carburizing furnace in excess, and coal is likely to be produced. As a result, the carburized part (steel material) manufactured by the vacuum carburization method has a large variation in carburization distribution.

In the early carburizing step (S1), if the actual carburizing gas flow rate FR is set to be equal to or higher than the theoretical carburizing gas flow rate FT (ta/5) and equal to or lower than the theoretical carburizing gas flow rate FT (20S) and on the premise that the condition that the actual carburizing gas flow rate FR in the later carburizing step (S2) described later is satisfied, the carburizing distribution unevenness of the carburized component (steel material) after the vacuum carburizing treatment can be sufficiently suppressed. The actual carburizing gas flow rate FR in the early carburizing step (S1) can be adjusted by a known method. For example, the actual carburizing gas flow rate FR may be adjusted by adjusting the flow rate of the carburizing gas supplied to the vacuum carburizing furnace using a supply valve, or may be adjusted by another known method. The adjustment of the actual carburizing gas flow rate may also be performed by a known control device of the vacuum carburizing furnace. The actual carburizing gas flow rate FR is adjusted by adjusting the opening degree of a control device, such as the above-described supply valve.

The time 1/30 of the carburizing time ta from the start to the end of the carburizing step (S30) is defined as ta/30. The theoretical carburizing gas flow rate at time ta/30 from the start (t ═ 0) of the carburizing process (S30) is defined as FT (ta/30). Further, a time of 1/10 of the carburizing time ta is defined as ta/10. The theoretical carburizing gas flow rate at time ta/10 from the start (t ═ 0) of the carburizing process (S30) is defined as FT (ta/10). When the time ta/10 is 20 seconds or more, the lower limit of the actual carburizing gas flow rate FR in the early carburizing step (S1) is preferably the theoretical carburizing gas flow rate FT (ta/10). When the time ta/30 is 20 seconds or more, a more preferable lower limit of the actual carburizing gas flow rate FR in the early carburizing step (S1) is the theoretical carburizing gas flow rate FT (ta/30). In this case, the carburization distribution unevenness is further reduced.

As described above, the theoretical carburizing gas flow rate FT calculated by diffusion simulation based on the diffusion equation is a curve in which the gas flow rate gradually decreases with the passage of time, and is a curve approximated by equation (5). The carburizing time ta is not particularly limited, and is a time longer than 20 seconds, for example, 3 minutes to 120 minutes.

The actual carburizing gas flow rate FR in the early carburizing step (S1) is preferably fixed. The "actual carburizing gas flow rate FR constant" referred to herein includes a range in which the actual carburizing gas flow rate FR varies by ± 5.0%. That is, in the case where the actual carburizing gas flow rate FR is X (NL/min), the "actual carburizing gas flow rate FR constant" in the present specification means that the actual carburizing gas flow rate FR is within a range of X ± 5.0% (NL/min). When the actual carburizing gas flow rate FR is fixed, the control device controls the flow rate of the carburizing gas to be fixed. At this time, the control device adjusts the opening degree of the supply valve and the like. The flow rate may vary within ± 5.0% due to a response speed of the control device, a control error, and the like. Therefore, as described above, the "actual carburizing gas flow rate FR fixed" includes the range in which the actual carburizing gas flow rate FR varies by ± 5.0%. The response speed of the control device for adjusting the flow rate of the carburizing gas is preferably 5 seconds or less at 98% response. The control error is preferably within ± 5.0%.

If the actual carburizing gas flow rate FR in the previous carburizing step (S1) is fixed, the adjustment (control) of the actual carburizing gas flow rate FR becomes easy.

As described above, the actual carburizing gas flow rate FR in the early carburizing step (S1) may be fixed, and may vary from the theoretical carburizing gas flow rate FT (ta/5) or higher to the theoretical carburizing gas flow rate FT (20S) or lower. That is, the actual carburizing gas flow rate FR in the early carburizing step (S1) may increase with time, may decrease with time, or may increase with time within a range of not less than the theoretical carburizing gas flow rate FT (ta/5) and not more than the theoretical carburizing gas flow rate FT (20S).

[ late carburizing step (S2) ]

In the later carburizing step (S2), the actual carburizing gas flow rate FR is set to be within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate FT. As shown in fig. 5, with the actual carburizing gas flow rate FR lying on the theoretical carburizing gas flow rate curve C_1.00Curve C of theoretical carburizing gas flow_1.20In such a way that the actual carburizing gas flow rate FR is adjusted. Therefore, in the later carburizing step (S2), the excess carburizing gas can be prevented from remaining in the vacuum carburizing furnace. As a result, the generation of coal and tar can be reduced, and the occurrence of uneven carburization distribution in the carburized component (steel material) after the vacuum carburization method is performed can be suppressed.

In the later carburizing step (S2), if the actual carburizing gas flow rate FR is less than 1.00 times the theoretical carburizing gas flow rate FT, the gas flow rate is insufficient. Therefore, the distribution of the carburizing gas in the vacuum carburizing furnace becomes nonuniform. For example, the concentration of the carburizing gas is high in the vicinity of the supply nozzle of the carburizing gas, and the concentration of the carburizing gas is low in a region distant from the supply nozzle. As a result, the carburization distribution becomes uneven in the steel material after the vacuum carburization process.

On the other hand, in the late carburizing step (S2), if the actual carburizing gas flow rate FR exceeds 1.20 times the theoretical carburizing gas flow rate FT, the carburizing gas is excessively supplied. In this case, the carburization distribution of the carburized component (steel material) after the vacuum carburization method is performed becomes large.

Therefore, in the later carburizing step (S2), the actual carburizing gas flow rate FR is set to be within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate FT. As described above, the response speed of the control device for adjusting the flow rate of the carburizing gas is preferably 5 seconds or less at 98% response. The control error is preferably within ± 5.0%.

In the later carburizing step (S2), the upper limit of the actual carburizing gas flow rate FR is preferably 1.18 times, and more preferably 1.15 times, the theoretical carburizing gas flow rate FT. In this case, the steel material after the vacuum carburizing treatment step can be further suppressed from having a nonuniform carburized distribution.

[ carburizing pressure in the carburizing step (S30) ]

The pressure of the carburizing gas (carburizing gas pressure) in the carburizing step (S30) is not particularly limited. Preferably, the carburizing gas pressure in the early carburizing step (S1) is higher than the carburizing gas pressure in the later carburizing step (S2). In this case, in the later carburizing step (S2), the generation of coal is further suppressed. It is further preferable that the carburizing gas pressure in the later carburizing step (S2) is decreased with the passage of time. The carburizing pressure in the carburizing step (S30) is preferably 1kPa or less.

[ carburizing time ta in the carburizing step (S30) ]

The carburizing time ta, which is the time from the start (t being 0) to the end of the carburizing step (S30), is appropriately set according to the target carbon concentration of the surface layer of the steel material after the vacuum carburizing treatment step. The carburizing time ta may be determined according to the diffusion simulation described above using the diffusion equation. The carburizing time ta may be determined in advance by performing a vacuum diffusion treatment test and based on experimental data. The longer the carburizing time ta is, the more preferable. Curve C of theoretical carburizing gas flow FT when carburizing time ta is long_1.00The inclination of (2) is gentle. Therefore, the adjustment of the actual carburizing gas flow rate FR becomes easy. As described above, the lower limit of the carburizing time ta is preferably 3 minutes, and more preferably 3.5 minutes. The upper limit of the carburizing time ta is preferably 120 minutes, and more preferably 60 minutes.

[ diffusion step (S40) ]

The diffusion step (S40) is a step known in the vacuum carburization method. In the diffusion step (S40), the supply of the carburizing gas to the vacuum carburizing furnace is stopped, and the steel material is held at the carburizing temperature Tc for a predetermined time. In the diffusion step (S40), carbon that has entered the steel material in the carburizing step is diffused into the steel material. This reduces the carbon concentration in the surface layer that has been increased in the carburizing step, and increases the carbon concentration at a predetermined depth. In the diffusion step (S40), nitrogen gas may be introduced into the vacuum carburizing furnace and vacuum-exhausted by a vacuum pump, and the atmosphere may be a nitrogen gas atmosphere of 1000Pa or less or a vacuum atmosphere. The vacuum is, for example, 10Pa or less. The vacuum carburizing furnace is set to a nitrogen atmosphere or a vacuum state of 1000Pa or less, thereby suppressing the intrusion and detachment of carbon from the steel surface.

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

[ quenching Process (S50) ]

In the quenching step (S50), the steel material after the carburizing step (S30) and the diffusion step (S40) is held at the quenching temperature (Ts) for a predetermined time, and then quenched (quenched). Thereby, the surface layer portion of the steel material having a high C concentration becomes martensite to form a hardened layer. The quenching step (S50) is a step known in the vacuum carburizing treatment method.

As shown in fig. 6, when the quenching temperature Ts is lower than the carburizing temperature Tc, the steel material after the diffusion step (S40) is cooled to the quenching temperature Ts. The cooling rate in this case is not particularly limited. Considering the processing time in the vacuum carburization process, the cooling rate is preferably higher. The cooling speed is preferably 0.02-30.00 ℃/s. The cooling rate here is a value obtained by dividing the temperature difference between the carburizing temperature Tc and the quenching temperature Ts by the cooling time.

The steel material having the quenching temperature Ts lower than the carburizing temperature Tc may be cooled by a known cooling method. For example, the steel material may be cooled by cooling the steel material under vacuum, or may be cooled by gas cooling. When the steel material is left to stand under vacuum, it is preferably left to stand under a pressure of 100Pa or less. When the steel material is cooled by gas cooling in the cooling, it is preferable to use an inert gas as the cooling gas. As inert gas, for example, nitrogen and/or helium are preferably used. As the inert gas, in particular, nitrogen gas which is inexpensive and can be used is preferably used. By using an inert gas as the cooling gas, oxidation of the steel material can be prevented.

The steel is held at a quenching temperature Ts for a predetermined timeThereafter, the steel is quenched. Quenching temperature Ts if A₃Phase transition point (Ar)₃Phase transition point) or above, there is no particular limitation. The lower limit of the quenching temperature Ts is preferably 800 ℃, more preferably 820 ℃, and still more preferably 850 ℃. The upper limit of the quenching temperature Ts is preferably 1130 ℃, more preferably 1100 ℃, still more preferably 950 ℃, still more preferably 900 ℃, and still more preferably 880 ℃.

As the quenching method in the quenching step (S50), a known quenching method is used. Quenching methods are, for example, water cooling, oil cooling.

The above vacuum carburization method is performed to form a carburized part from a steel material. In the vacuum carburizing treatment method according to the present embodiment, diffusion simulation based on a diffusion equation is performed to calculate the theoretical carburizing gas flow rate FT of the steel material to be vacuum carburized. The carburizing step (S30) is divided into a first carburizing step (S1) and a second carburizing step (S2). In the early carburizing step (S1), the actual carburizing gas flow rate FR is set to: the theoretical carburizing gas flow rate FT (ta/5) at the time when the reference time (ta/5) has elapsed since the start (t ═ 0) of the carburizing step (S30) is equal to or greater than the theoretical carburizing gas flow rate FT (20S) at the time when 20 seconds have elapsed since the start (t ═ 0) of the carburizing step (S30). In the later carburizing step (S2), the actual carburizing gas flow rate FR is adjusted to be within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate FT. This can suppress occurrence of uneven carburization distribution in the steel material after the vacuum carburization step.

The vacuum carburization method may further include another step. For example, the vacuum carburizing treatment method may be performed after the quenching step (S50) and then the tempering step. The tempering step may be performed under known conditions. For example, in the tempering process, the alloy is treated with A_c1The steel material is kept at a temperature not higher than the transformation point for a predetermined time and then cooled.

[ method for producing carburized component ]

The method for manufacturing a carburized component according to this embodiment includes a step of manufacturing a carburized component by applying the above-described vacuum carburizing method to a steel material. In the carburized component manufactured through the above steps, the variation in the carburized distribution can be suppressed.

Examples

Hereinafter, the effect of the vacuum carburization method according to this embodiment will be described in more detail with reference to examples. The conditions in the following examples are examples of conditions employed to confirm the feasibility and effects of the vacuum carburization method according to the present embodiment. Therefore, the vacuum carburization method according to the present embodiment is not limited to this example of the conditions.

A steel pipe for machine structural use (hereinafter referred to as steel pipe) having a chemical composition corresponding to SCM415 defined in JIS G4053 (2008) and a round bar corresponding to SCM415 were prepared. The C contents of the steel pipes and round bars of the respective test numbers were 0.15 mass%. The steel tube had a diameter of 34mm, a wall thickness of 4.5mm and a length of 110 mm. The diameter of the round bar is 26mm, and the length is 70 mm. The evaluation of the vacuum carburization was performed using a round bar, and a steel pipe was used as a sample for examining the unevenness in carburization distribution occurring at the arrangement position of the round bar in the vacuum carburizing furnace.

The total surface area (m) of the round bar and the steel pipe obtained by vacuum carburization for each test number²) Defined as the surface area (m) of the steel²). The surface area of the steel material is determined by the following formula.

Steel surface area ═ surface area per 1 steel pipe × number of steel pipes + surface area per 1 round bar × number of round bars

The surface area of the resulting steel material is shown in table 1. In test nos. 1 to 4, 9 to 13, 16 and 17, 248 steel pipes and 3 round rods were used. In test No. 5, 496 steel pipes and 3 round bars were used. In test nos. 6 and 7, 124 steel pipes and 3 round rods were used. In test nos. 8, 14 and 15, 62 steel pipes and 3 round rods were used.

[ Table 1]

First, a diffusion simulation using a diffusion equation is performed to obtain a theoretical carburizing gas flow rate.Specifically, the round bar and the steel pipe are divided into a plurality of cells of 2 μm or more in the thickness direction. In addition, the step time in the diffusion simulation is set to 0.002 to 0.02 seconds. In the chemical composition of the steel pipe and the round bar (SCM415), the equilibrium composition in an equilibrium state with graphite at the surface at the carburization temperature was determined by thermodynamic calculation. Further, the equilibrium composition inside the steel material at the carburizing temperature, the chemical potential of carbon, and the mobility of carbon were determined. The thermodynamic calculation uses the trade name Pandat (trademark). Further, the database uses PanFe. Further, the mobility (m) of carbon²The following equation is used for s).

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

Wherein C in the formula is a solid-solution C concentration (mass%) in austenite, and T is a carburizing temperature (K).

The target carbon concentration on the surface of the steel pipe and the round bar was set to 0.7 mass%, and the target carbon concentration at a depth of 1.0mm from the surface was set to 0.40 mass%. The diffusion simulations of (a) to (D) described above were performed for each step time under the above-mentioned precondition, and the theoretical carburizing gas flow rate FT was obtained for each step time.

As a result of calculating the theoretical carburizing gas flow rate FT, the theoretical carburizing gas flow rate FT can be approximated by the following equation.

FT＝A/√t (6)

A＝a×T²+b×T+c (7)

In the case of this embodiment (SCM415), a is 8.64 × 10^-5、b＝－0.141、c＝59.0。

After the theoretical carburizing gas flow rate FT was calculated, the actual vacuum carburizing treatment was performed by the following method. First, a cage made of a stainless steel material (SUS 316 specified in JIS G4303 (2012)) which has been sufficiently carburized was prepared. The number of steel pipes was uniformly arranged in the cage, and 3 round bars were disposed at the center of the cage, at the front left of the cage, and at the rear right of the cage. As described above, the round bar was used as a test material, and the steel pipe was used as a sample for confirming the occurrence of the uneven carburization distribution due to the arrangement position of the round bar.

The cage with the steel material (steel pipe and round bar) placed therein was inserted into a vacuum carburizing furnace, and vacuum carburization was performed. Then, carburized parts of test nos. 1 to 17 were obtained. The conditions in the vacuum carburization are shown in table 1.

Specifically, in each test number, the vacuum carburization treatment was performed as follows. The vacuum carburizing treatment in each test number maintained the pressure in the furnace at 10Pa or less. In the heating step, the round bars of each test number were heated to the carburizing temperature Tc shown in table 1. After the heating step, a soaking step is performed. In the soaking step, the steel material (round bar) is held at the carburizing temperature Tc for 60 minutes.

After the soaking step, the carburizing step is performed. In the carburizing step, acetylene is supplied as a carburizing gas in a vacuum carburizing furnace. The carburizing pressure in the carburizing step is kept at 1kPa or less. The carburizing time ta (minutes) in the carburizing step is shown in table 1.

The carburizing time in the carburizing step and the diffusion time in the diffusion step were adjusted with the target of setting the carburizing time ta in the carburizing step and the carbon concentration at the depth of 1.0mm of the round bar as described above to 0.40 mass%. The actual carburizing gas flow rate is shown in fig. 7 to 14 below. Hereinafter, the set values of the actual carburizing gas flow rate FR for test nos. 1 to 17 will be described with reference to fig. 7 to 14.

Fig. 7 is a graph showing changes with time in the actual carburizing gas flow rate in the carburizing process of test No. 1 to test No. 8. FR in FIG. 7 represents the actual carburizing gas flow rates of the respective test numbers 1 to 8. C_1.00Curve for theoretical carburizing gas flow FT (theoretical carburizing gas flow curve C)_1.00)。C_1.20Is a curve representing a carburizing gas flow rate 1.20 times the theoretical carburizing gas flow rate FT (theoretical carburizing gas flow rate curve C)_1.20). In test nos. 1 to 8, the actual carburizing gas flow rate FR is equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S) in the early carburizing step (S1). Test No. 1 to testIn experiment No. 8, in the later carburizing step (S2), the actual carburizing gas flow rate FR is the theoretical carburizing gas flow rate curve C_1.00Curve C with carburizing gas flow_1.20Within the range of (a). The actual carburizing gas flow rate was adjusted and measured by using a flow meter (trade name: Mass flow controller D3665, manufactured by coflock co.

Fig. 8 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process for test No. 9. Referring to fig. 8, in test No. 9, in the early carburizing step (S1), the actual carburizing gas flow rate FR is smaller than the theoretical carburizing gas flow rate FT (ta/5). In the late carburizing step (S2), the actual carburizing gas flow rate FR is the theoretical carburizing gas flow rate curve C_1.00Curve C with carburizing gas flow_1.20Within the range of (a).

Fig. 9 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process for test No. 10. Referring to fig. 9, in test No. 10, the actual carburizing gas flow rate FR was equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S) in the early carburizing step (S1). However, in the late carburizing step (S2), there is a case where the actual carburizing gas flow rate FR is smaller than the theoretical carburizing gas flow rate curve C_1.00The area of (a).

Fig. 10 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process for test No. 11 and test No. 12. Referring to fig. 10, in test No. 11 and test No. 12, the actual carburizing gas flow rate FR was equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S) in the early carburizing step (S1). However, in the late carburizing step (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20The area of (a).

Fig. 11 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process for test No. 13 and test No. 14. Referring to fig. 11, in test nos. 13 and 14, in the early carburizing step (S1), the actual carburizing gas flow rate FR is equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and is equal to or greater than the theoretical carburizing gas flow rate FT (ta/5)FT (20s) or less. However, at the initial stage of the late carburizing process (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20。

Fig. 12 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process for test No. 15. Referring to fig. 12, in test No. 15, in the early carburizing step (S1), the actual carburizing gas flow rate FR exceeds the theoretical carburizing gas flow rate FT (20S). Therefore, in the late carburizing step (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20。

Fig. 13 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process for test No. 16. Referring to fig. 13, in test No. 16, the actual carburizing gas flow rate FR was fixed in all the carburizing steps.

Fig. 14 is a graph showing the temporal change in the actual carburizing gas flow rate in the carburizing process of test No. 17. In test No. 17, the actual carburizing gas flow rate FR was smaller than the theoretical carburizing gas flow rate FT (ta/5) at the later stage of the earlier carburizing step (S1). I.e. the actual carburizing gas flow rate FR decreases too fast.

After the carburizing step, the round bar was subjected to a diffusion step for a diffusion time (minutes) shown in table 1, so that carbon that had entered the round bar was diffused into the round bar. The diffusion step is performed at a furnace pressure of 10Pa or less while maintaining the carburizing temperature. The processing conditions in the diffusion step are shown in table 1.

The column "FT (20 s)" in table 1 shows the theoretical carburizing gas flow rate (NL/min) at 20 seconds from the start of the carburizing step. The column "FT (ta/5)" shows the theoretical carburizing gas flow rate (NL/min) at the time when the reference time (ta/5) has elapsed since the start of the carburizing process. The column "crossing time te" describes the time (minutes) at which the actual carburizing gas flow rate FR is initially equal to the theoretical carburizing gas flow rate FT. The column "FR under te" describes the actual carburizing gas flow rate (NL/min) at the crossover time te. The column "initial gas flow rate" describes the actual carburizing gas flow rate (NL/min) at a carburizing process start time t of 0. The column "final gas flow rate" describes the actual carburizing gas flow rate (NL/min) at the carburizing time ta.

Further, the "maximum gas flow rate ratio" in the column of "late stage carburizing step" in table 1 represents the maximum value when the flow rate ratio of the following formula is obtained at each time of the late stage carburizing step.

Flow ratio of actual carburizing gas flow to theoretical carburizing gas flow

The "minimum gas flow rate ratio" in the column of "late stage carburizing step" in table 1 represents the minimum value of the flow rate ratio obtained by the above equation at each time in the late stage carburizing step. In short, a maximum gas flow rate ratio of 1.20 or less and a minimum gas flow rate ratio of 1.00 or more means that the actual carburizing gas flow rate FR is within a range of 1.00 to 1.20 times the theoretical carburizing gas flow rate in the late carburizing step.

After the diffusion step, the round bar was cooled to 860 ℃. Then, the temperature was maintained at the quenching temperature (860 ℃ C.) for 30 minutes. After the holding, the round bar was immersed in oil at 120 ℃ to carry out oil quenching. Tempering is carried out on the round bar after quenching. The tempering temperature was set at 170 ℃ and the holding time at the tempering temperature was set at 2 hours.

Through the above-described manufacturing steps, a carburized part (round bar) is manufactured by performing a vacuum carburization process.

[ evaluation test ]

The carbon concentration and the depth at which the carbon concentration of the surface layer of the carburized part (round bar) of each test number reached 0.40 mass% (hereinafter referred to as carburization depth) were measured, and the carburization maldistribution was evaluated.

[ test for measuring carbon concentration in the surface layer of carburized component ]

The carburized part (round bar) of each test number in the state of being inserted into the vacuum carburizing furnace was cut in a range of 20mm in the longitudinal direction of the carburized part from the upper end face and in a range of 5mm in the longitudinal direction of the carburized part from the lower end face. Hereinafter, the range of 20mm from the upper end surface is referred to as "upper end surface test piece", and the range of 5mm from the lower end surface is referred to as "lower end portion".

The peripheral surfaces of the upper end face test piece and the remaining portion (hereinafter referred to as a main body portion) where the lower end portion was cut off were subjected to turning. In the turning, the surface layer portion from the surface of the round bar to a depth of 0.30mm was ground at every 0.05mm depth pitch. The carbon concentration of the cut powder at each depth position of the taken 0.05mm pitch was measured. Through the above steps, the carbon concentration at the pitch of 0.05mm was determined for the surface layer region from the surface to the depth of 0.30mm of the 3 carburized parts (the center position of the cage, the front left position of the cage, and the rear right position of the cage) of each test number. The 6 carbon concentrations of the carburized components disposed at the center of the cage from the surface to 0.30mm are defined as carbon concentrations a1 to a6 (mass%) in order from the surface. The carbon concentrations of 6 carbons from the surface to 0.30mm of the carburized component disposed at the front left of the cage were defined as carbon concentrations B1 to B6 (mass%) in order from the surface. The 6 carbon concentrations of the carburized components disposed at the right rear of the cage from the surface to 0.30mm are defined as carbon concentrations C1 to C6 (mass%) in order from the surface. Then, the difference between the maximum value and the minimum value of the carbon concentration obtained at the same depth position in the 3 carburized parts was obtained. Specifically, the maximum value and the minimum value among the carbon concentrations a1, B1, and C1 in the region from the surface to the depth position of 0.05mm were selected, and the difference between the carbon concentrations was defined as Δ 1. Similarly, the maximum value and the minimum value among the carbon concentrations a2, B2, and C2 in the region from the 0.05mm depth position to the 0.10mm depth position from the surface were selected, and the difference between the carbon concentrations was defined as Δ 2. Through the above steps, Δ 1 to Δ 6 were obtained, and the arithmetic average value of Δ 1 to Δ 6 was defined as "difference in surface layer carbon concentration" (mass%). The obtained results are shown in the column "difference in surface layer carbon concentration (% by mass)" in table 1.

[ carburization depth measurement test ]

The carbon concentration in the surface layer portion of the circumferential surface was measured using the above-described upper end face test piece. Specifically, the carbon concentration of the cross section (cross section perpendicular to the longitudinal direction of the upper end surface test piece) of the upper end surface test piece at a position 20mm from the upper end surface was measured in the radial direction from a depth position of 2mm from the surface toward the surface. Specifically, line analysis by an 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 a region in which the carbon concentration was 0.40 mass% or more (hereinafter referred to as carburization depth) was determined for each of the 3 upper end surface test pieces. The average of the differences between the maximum value and the minimum value of the carburized depth obtained from each upper end face test piece was defined as "0.40 mass% depth difference" (mm). The obtained results are shown in the column of "0.40 mass% depth difference (mm)" in table 1.

[ evaluation results ]

Referring to table 1, the case where the difference in the carbon concentration of the surface layer was 0.030 mass% or less and the difference in the depth of 0.40 mass% carbon concentration was 0.05mm or less was evaluated as excellent as the vacuum carburization method with less variation in the carburized distribution.

Referring to table 1 and fig. 7 to 14, in test No. 1 to test No. 8 (fig. 7), in the early carburizing step (S1), the actual carburizing gas flow rate FR is equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S). In test nos. 1 to 8 and further in the later carburizing step (S2), the actual carburizing gas flow rate FR is the theoretical carburizing gas flow rate curve C_1.00Curve C with carburizing gas flow_1.20Within the range of (a). As a result, the difference in the carbon concentration of the surface layer was 0.030 mass% or less and the difference in the depth of 0.40 mass% was 0.05mm or less. That is, the carburized part has a small variation in carburization distribution.

In test nos. 1 to 3, 5, 6, and 8, the actual carburizing gas flow rate FR in the early carburizing step (S1) is equal to or greater than the theoretical carburizing gas flow rate FT (ta/10). Therefore, the difference in the surface layer carbon concentration and the difference in the depth of 0.40 mass% are equal to or less than those in test nos. 4 and 7 in which the actual carburizing gas flow rate FR in the previous carburizing step (S1) is smaller than the theoretical carburizing gas flow rate FT (ta/10).

Further, in test nos. 1 and 5, the actual carburizing gas flow rate FR in the previous carburizing step (S1) is equal to or greater than the theoretical carburizing gas flow rate FT (ta/30). Therefore, the difference in the surface layer carbon concentration is small and the difference in the depth of 0.40 mass% is small as compared with test numbers 2 to 4 and 6 to 8 in which the actual carburizing gas flow rate FR in the previous carburizing step (S1) is smaller than the theoretical carburizing gas flow rate FT (ta/30).

On the other hand, in test No. 9 (fig. 8), in the early carburizing step (S1), the actual carburizing gas flow rate FR is smaller than the theoretical carburizing gas flow rate FT (ta/5). Therefore, the difference in the surface carbon concentration exceeds 0.030 mass% and the difference in the 0.40 mass% depth exceeds 0.05mm, resulting in a large variation in the carburized distribution of the carburized component.

In test No. 10 (fig. 9), the actual carburizing gas flow rate FR was equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S) in the early carburizing step (S1). However, in the late carburizing step (S2), there is a case where the actual carburizing gas flow rate FR is smaller than the theoretical carburizing gas flow rate curve C_1.00The area of (a). Therefore, the difference in the surface carbon concentration exceeds 0.030 mass% and the difference in the 0.40 mass% depth exceeds 0.05mm, resulting in a large variation in the carburized distribution of the carburized component.

In test nos. 11 and 12 (fig. 10), the actual carburizing gas flow rate FR was equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S) in the early carburizing step (S1). However, in the late carburizing step (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20The area of (a). Therefore, at least the difference in the surface layer carbon concentration exceeds 0.030 mass%, and the carburized part has a large variation in the carburization distribution.

In test nos. 13 and 14 (fig. 11), in the early carburizing step (S1), the actual carburizing gas flow rate FR is equal to or greater than the theoretical carburizing gas flow rate FT (ta/5) and equal to or less than the theoretical carburizing gas flow rate FT (20S). However, at the initial stage of the late carburizing process (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20. Therefore, at least the difference in the surface layer carbon concentration exceeds 0.030 mass%, and the carburized part has a large variation in the carburization distribution.

In test No. 15 (fig. 12), in the early carburizing step (S1), the actual carburizing gas flow rate FR exceeds the theoretical carburizing gas flow rate FT (20S). Therefore, in the late carburizing step (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20. This is because the actual carburizing gas flow rate is too large in the early carburizing step (S1), and as a result, the initial stage of the later carburizing step (S2) cannot be achievedReducing the actual carburizing gas flow FR to the carburizing gas flow curve C_1.20The following. Therefore, the difference in the surface layer carbon concentration exceeds 0.030 mass%, and the carburized part has a large variation in the carburization distribution.

In test No. 16 (fig. 13), the actual carburizing gas flow rate was fixed in all carburizing steps. Therefore, in the late carburizing step (S2), the actual carburizing gas flow rate FR exceeds the carburizing gas flow rate curve C_1.20. Therefore, the 0.40 mass% depth difference exceeds 0.05mm, and the carburized part has a large variation in carburization distribution.

In test No. 17 (fig. 14), the actual carburizing gas flow rate FR was smaller than the theoretical carburizing gas flow rate FT (ta/5) at the later stage of the earlier carburizing step (S1). Further, in the latter stage of the later carburizing step (S2), the actual carburizing gas flow rate FR is smaller than the carburizing gas flow rate curve C_1.00. Therefore, the difference in the surface carbon concentration exceeds 0.030 mass% and the difference in the 0.40 mass% depth exceeds 0.05mm, resulting in a large variation in the carburized distribution of the carburized component.

The embodiments of the present invention are explained above. However, the above-described embodiments are merely illustrative for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate without departing from the scope of the present invention.

Claims

1. A vacuum carburization method for performing a vacuum carburization process on a steel material in a vacuum carburizing furnace, comprising:

a heating step of heating the steel material at a carburizing temperature;

a quenching step of quenching the steel material after the diffusion step,

in the carburizing step, the carburizing step is performed,

the carburizing step includes:

a late carburizing step from the crossover time te to the carburizing time ta,

and enabling the actual carburizing gas flow to be as follows: the theoretical carburizing gas flow rate at the reference time ta/5 from the start of the carburizing step is equal to or higher than the theoretical carburizing gas flow rate at the 20 second time from the start of the carburizing step;

in the post-carburizing step, the carbon content of the steel is controlled,

2. The vacuum carburization processing method according to claim 1, wherein,

when 1/10 times the carburizing time ta is defined as time ta/10,

3. The vacuum carburization processing method according to claim 2, wherein,

when 1/30 times the carburizing time ta is defined as time ta/30,

4. A vacuum carburization processing method according to any one of claim 1 to claim 3, wherein,

and fixing the actual carburizing gas flow.

5. A method for manufacturing a carburized component is provided,

the method comprises the step of subjecting a steel material to the vacuum carburization method according to any one of claims 1 to 4.