CN114341392B - Vacuum carburization method and carburized component manufacturing method - Google Patents
Vacuum carburization method and carburized component manufacturing method Download PDFInfo
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Classifications
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/06—Surface hardening
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
- C21D1/773—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material under reduced pressure or vacuum
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/20—Carburising
- C23C8/22—Carburising of ferrous surfaces
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
Abstract
Providing: a vacuum carburization method capable of suppressing uneven carburization distribution. In the early stage carburizing step of the vacuum carburizing treatment method of the present embodiment, when the end time of the carburizing step is defined as ta, the actual carburizing gas flow rate is equal to or higher than the theoretical carburizing gas flow rate at the time ta/10 and equal to or lower than the theoretical carburizing gas flow rate at the time 4 seconds from the start of the early stage carburizing step, in the later stage carburizing step, the initial time at which the acetylene partial pressure becomes equal to or higher than 0.8 times the hydrogen partial pressure after the start of the carburizing step is defined as t0, and when the actual carburizing gas flow rate at the start of the early stage carburizing step is defined as FA, the actual carburizing gas flow rate in the period of time t0 to time 4t0 is equal to or higher than FA (t 0/t) and equal to or lower than FA (t 0/t), and the actual carburizing gas flow rate in the period of time 4t0 to time ta is equal to or higher than FA (t 0/t) and equal to or lower than FA (t 0/t).
Description
Technical Field
The present invention relates to a vacuum carburization method and a carburized component manufacturing method. In the present specification, the carburized steel part is referred to as a "carburized part".
Background
Steel parts which are intended to have high surface fatigue strength are manufactured by subjecting steel materials to a surface hardening treatment. One of the surface hardening treatment methods is a vacuum carburization treatment method. The vacuum carburization method includes a carburization step and a diffusion step. In the carburizing step, a carburizing gas is introduced as a hydrocarbon gas, and the carbon concentration of the surface of the steel material heated to the carburizing temperature is increased. 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, hydrocarbon gas as 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, molecules of the carburizing gas move actively. Due to the active movement, the molecules of carburizing gas collide with each other at a high speed, and the carburizing gas is decomposed. Coal and tar are produced by the decomposition of the carburizing gas. At this time, the surface carbon concentration and carburized depth are unevenly distributed. Therefore, the surface layer of the carburized component cannot be maintained at a constant quality. Therefore, in the vacuum carburization method, it is desired to suppress uneven distribution of carbon concentration on the surface of the carburized component and uneven distribution of carburized depth on the surface layer. In the following description, the uneven distribution of the carbon concentration on the surface of the carburized component and the uneven distribution of the carburized depth of the surface layer of the carburized component are referred to as "uneven carburized distribution".
Japanese patent application laid-open No. 8-325701 (patent document 1), japanese patent application laid-open No. 2016-148091 (patent document 2), japanese patent application laid-open No. 2002-173759 (patent document 3), japanese patent application laid-open No. 2005-350729 (patent document 4) and Japanese patent application laid-open No. 2012-7240 (patent document 5) propose techniques for suppressing carburization maldistribution.
The vacuum carburization method described in patent document 1 is to vacuum-heat a workpiece made of steel in a heating chamber of a vacuum carburizing furnace, and to supply a carburizing gas into the heating chamber to perform carburization. In this vacuum carburization method, a gaseous chain unsaturated hydrocarbon is used as a carburizing gas. The inside of the heating chamber is set to a vacuum state of 1kPa or less to perform carburizing treatment. Patent document 1 describes that the occurrence of coal is thereby 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 applied, so that a workpiece placed in the carburizing chamber is carburized. 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 in the loading state of the object to be treated in the carburizing chamber, the volume of the carburizing chamber, the total surface area of the object to be treated, and a constant set based on the type of the carburizing gas. And, the carburizing gas of the calculated gas injection amount is injected into the carburizing chamber. Patent document 2 describes that excessive carburization in the form of spots can be prevented.
In the vacuum carburizing atmosphere gas control system described in patent document 3, propane gas is used as a carburizing gas. In this control system, a carburizing gas is supplied into a vacuum carburizing furnace in which a carburized material is provided. Then, carbon generated by the thermal decomposition reaction of the carburizing gas is dissolved and diffused in the carburized material, and carburization of the carburized material is performed. In the control system, the partial pressure of hydrogen generated by the thermal decomposition reaction is measured in real time in the carburizing process. Based on the measured value, the amount of carburizing gas supplied into the furnace is adjusted and controlled in real time. Patent document 3 describes that high-quality carburizing steel can be stably produced thereby.
In the vacuum carburizing method described in patent document 4, the relation v=f (t) between the theoretical flow rate V of the carburizing gas required for the carburizing process and the carburizing time t is calculated based on the internal diffusion of the material by the carburization depth and the surface carbon concentration. In the carburizing stage of the carburizing step, a sufficient carburizing flow rate V1 is supplied that is larger than the theoretical flow rate V and does not cause black smoking. Further, in the late carburization stage following the early carburization stage, a diffusion flow rate V2 smaller than the theoretical flow rate V is supplied. Patent document 4 describes that the occurrence of coal can be prevented and the residue of cementite can be reduced.
In the vacuum carburization method described in patent document 5, the time change of the theoretical flow rate of the carburizing gas required for the carburization treatment is obtained based on the diffusion of carbon into the inside of the article to be treated. Then, based on the time change of the theoretical flow rate, the partial pressure ratio of hydrogen generated by the carburizing reaction at the theoretical flow rate with respect to the total pressure in the processing chamber is defined as a theoretical hydrogen partial pressure ratio. The time change of the theoretical hydrogen partial pressure ratio is obtained, and the time change of the theoretical hydrogen partial pressure ratio is compared with the time change of the hydrogen partial pressure ratio with respect to the total pressure in the processing chamber at the time of the actual carburizing process. Based on these approximations, the degree of uneven distribution of carburized quality in the same operation lot is determined. Patent document 5 describes that the reproducibility of the quality of a carburized component can be improved, and the quality distribution unevenness of the carburized component can be reduced.
Prior art literature
Patent literature
Patent document 1: japanese patent laid-open No. 8-325701
Patent document 2: japanese patent laid-open publication 2016-148091
Patent document 3: japanese patent laid-open No. 2002-173759
Patent document 4: japanese patent laid-open No. 2005-350729
Patent document 5: japanese patent application laid-open No. 2012-7240
However, by a method different from the vacuum carburization treatment method of patent documents 1 to 5, carburization maldistribution can be suppressed.
Disclosure of Invention
Problems to be solved by the invention
The purpose of the present disclosure is to provide: a vacuum carburization method capable of suppressing uneven carburization distribution and a method for manufacturing a carburized part.
Solution for solving the problem
The vacuum carburization method according to the present disclosure is a vacuum carburization method for performing vacuum carburization on a steel material in a vacuum carburization furnace, the vacuum carburization method including:
a heating step of heating the steel material to a carburizing temperature;
a soaking step of soaking the steel material at the carburizing temperature after the heating step;
a carburizing step of supplying an acetylene gas, that is, a carburizing gas, into the vacuum carburizing furnace after the soaking step, and holding the steel material at the carburizing temperature;
a diffusion step of stopping the supply of the carburizing gas into the vacuum carburizing furnace after the carburizing step, and holding the steel material at the carburizing temperature; and, a step of, in the first embodiment,
a quenching step of quenching the steel material after the diffusion step,
in the foregoing carburization step, the carburization step,
the flow rate of the carburizing gas supplied into the vacuum carburizing furnace is defined as an actual carburizing gas flow rate,
the flow rate of the carburizing gas required for the vacuum carburizing treatment of the steel material is defined as a theoretical carburizing gas flow rate,
The end time of the foregoing carburizing process is defined as ta,
when the initial time period in which the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing step is defined as t0,
the carburizing step includes:
a partial pressure measurement step of continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace to determine the time t 0;
a preliminary carburizing step of starting from the carburizing step to a time t 0; and, a step of, in the first embodiment,
a post carburization step of starting from time t0 to time ta,
in the foregoing preliminary carburization step,
the actual carburizing gas flow rate is set to be equal to or higher than the theoretical carburizing gas flow rate at the time ta/10 and equal to or lower than the theoretical carburizing gas flow rate at the time of 4 seconds from the start of the carburizing process,
in the foregoing post-carburizing process,
the actual carburizing gas flow rate in the preliminary carburizing step is defined as FA, and the time from the start of the carburizing step is defined as time t,
the actual carburizing gas flow rate in the period from the time t0 to the time 4t0 is made to be equal to or higher than FA (t 0/t) and equal to or lower than FA,
the actual carburizing gas flow rate at the time period 4t0 to the time period ta is set to be equal to or greater than FA v (t 0/t) and equal to or less than 2FA v (t 0/t).
The method for producing a carburized component according to the present disclosure includes a step of applying the vacuum carburization method to the steel material.
ADVANTAGEOUS EFFECTS OF INVENTION
The vacuum carburization treatment method can inhibit carburization maldistribution. The method for manufacturing a carburized component of the present disclosure can manufacture a carburized component in which carburization unevenness is suppressed.
Drawings
Fig. 1 is a graph showing an example of a relationship between a theoretical carburizing gas flow rate and time 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 the time-dependent change of the actual carburizing gas flow rate and the time-dependent change of the theoretical carburizing gas flow rate in the conventional carburizing process.
Fig. 3 is a graph showing the change with time of the actual carburizing gas flow rate in the carburizing step (lower graph) and the change with time of the acetylene partial pressure and the hydrogen partial pressure in the atmosphere of the vacuum carburizing furnace in the carburizing step (upper graph) of the vacuum carburizing treatment method of the present embodiment.
Fig. 4 is a diagram showing an example of a thermal mode of the vacuum carburization processing method according to the present embodiment.
Fig. 5 is a diagram showing an example of the gas flow rate set value in the early carburization step of the vacuum carburization processing method of the present embodiment.
Fig. 6 is a diagram showing an example of the gas flow rate set values in the vacuum carburization processing method according to the present embodiment.
Fig. 7 is a view showing an example of the gas flow rate set value of the vacuum carburization processing method according to the present embodiment, which is different from fig. 6.
Fig. 8 is a view showing an example of the gas flow rate set value in the vacuum carburization processing method according to the present embodiment, which is different from fig. 6 and 7.
FIG. 9 is a schematic diagram of the gas flow rate set point and the gas analysis value in the carburizing process of test No. 1, test No. 5, and test No. 7 to test No. 12.
Fig. 10 is a schematic diagram of the gas flow rate set values and the gas analysis values in the carburizing process of test numbers 2 to 4 and 6.
FIG. 11 is a schematic diagram of the gas flow rate set point and the gas analysis value in the carburizing process of test No. 13 and test No. 14.
FIG. 12 is a schematic diagram of the gas flow rate set point and the gas analysis value in the carburizing process of test numbers 15 to 17.
Fig. 13 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing process of test No. 18.
FIG. 14 is a schematic diagram of the gas flow rate set point and the gas analysis value in the carburizing process of test No. 19.
Fig. 15 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing process of test No. 20.
Fig. 16 is a schematic diagram of a gas flow rate set value and a gas analysis value in the carburizing process of test No. 21.
Detailed Description
The present inventors have studied a method of suppressing carburization maldistribution in a carburized part in a vacuum carburization treatment method. The present inventors have focused on the presence of carburizing gas which is supplied into a vacuum carburizing furnace but is exhausted without causing a carburizing reaction. Part of the carburizing gas that does not cause the carburizing reaction becomes coal, and adheres to the steel material that is the subject of the vacuum carburization treatment. Coal serves as a supply source of carbon. Therefore, carbon is excessively supplied to the portion of the steel material where the coal adheres. Therefore, the carburization distribution becomes easily uneven due to the adhesion of the coal. On the other hand, if the carburizing gas flow rate is excessively reduced in order to suppress the adhesion of coal, the carburizing reaction becomes insufficient. In this case, too, carburization maldistribution tends to occur.
Based on the above findings, the present inventors focused on theoretically limiting the flow rate of carburizing gas that intrudes from the atmosphere in the vacuum carburizing furnace to the surface of the steel material in the carburizing step. In the present specification, the "theoretical carburizing gas flow rate" refers to a carburizing gas flow rate required to set the carbon concentration at a predetermined depth position from the surface of the steel material to a desired concentration, and is a carburizing gas flow rate on the premise that all the carburizing gas is used for the carburizing reaction. The present inventors considered that the amount of carburizing gas unfavorable for the carburizing reaction can be suppressed by adjusting the flow rate of the carburizing gas supplied to the vacuum carburizing furnace (hereinafter referred to as actual carburizing gas flow rate) in the actual vacuum carburizing process based on the theoretical carburizing gas flow rate defined in advance, and that the carburizing reaction can be made to be free from defects, as a result, the maldistribution of carburization can be suppressed.
As the vacuum carburization treatment proceeds, the gradient of the carbon concentration becomes gentle, and therefore the diffusion flux of carbon from the steel surface into the steel is reduced. The flow rate of carburizing gas which intrudes into the steel material from the furnace atmosphere decreases with the lapse of time. Therefore, the theoretical carburizing gas flow rate becomes a function that varies with the passage of time from the start of the supply of the carburizing gas (the start of the carburizing process). The theoretical carburizing gas flow rate may be obtained based on diffusion simulation or may be obtained by experiment. Hereinafter, as an example of a method for determining the theoretical carburizing gas flow rate, a description will be given of determining the theoretical carburizing gas flow rate based on diffusion simulation. However, the method for determining the theoretical carburizing gas flow rate is not limited to the diffusion simulation as described above.
[ regarding theoretical carburizing gas flow rate ]
Acetylene is used as the carburizing gas in the vacuum carburizing method of the present embodiment. The decomposition of acetylene is controlled in speed by carbon diffusion at the surface layer of the steel material as a carburization target. That is, the larger the diffusion flux of carbon from the steel surface into the steel, the larger the amount of acetylene decomposition. In the case of carburizing with a carburizing gas other than acetylene, chemical reactions other than carburization are assumed as described later. Therefore, it is difficult to apply the vacuum carburization method of the present embodiment.
In the vacuum carburization treatment, carbon diffuses in the steel material, that is, the 1 st law of fick holds. The flow rate of the carburizing gas (acetylene gas) required to set the carbon concentration at a predetermined depth position from the steel surface by the vacuum carburization treatment to a desired concentration, and the flow rate of the carburizing gas on the premise that all the carburizing gas is used for the carburization reaction, is defined as a theoretical carburizing gas flow rate FT (t). Here, t is a time from the start of the carburizing process. The beginning of the carburizing step is when the carburizing gas starts to be supplied into the furnace, as will be described later. FT (t) corresponds to a value obtained by converting the flow rate of carbon penetrating into the surface of the steel material into the flow rate of acetylene gas. In the following description, the theoretical gas flow rate will also be simply referred to as "FT".
For example, the diffusion flux J (mm. Mass%/s) of carbon intruded from the steel surface and the amount of change in the carbon concentration per unit time are calculated by using a known diffusion simulation based on the diffusion equationThereby, the theoretical carburizing gas flow rate FT can be calculated. Specifically, the theoretical carburizing gas flow rate can be obtained by the following method.
In the case where diffusion occurs (that is, in the case where the 1 st law of fick holds), the diffusion flux J of carbon intruded from the steel surface is defined by formula (1), the amount of change in the carbon concentration per unit time Defined by formula (2).
Here, D is the diffusion coefficient (mm) of carbon in the steel material 2 /s). C is the mass concentration of carbon (mass%). z is a displacement (mm) in the depth direction from the steel surface. t is the time (seconds) from the start of the carburizing process.Is a partial differential symbol.
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. In this case, the diffusion flux J (mm·mol%/s) of carbon is defined by formula (3), and the temporal change in carbon concentration is defined by formula (4).
Where m is the mobility of carbon (mm 2 mol/J.s). x is the molar concentration of carbon (mol%). Mu is the chemical potential of carbon (J/mol). z is the displacement (mm) in the depth direction. T in the formula (4) is a time(s) from the start of the carburizing process.Is a partial differential symbol.
Here, the diffusion driving force of carbon is in formula (3)Part(s). Further, the carbon concentration in austenite (γ) in the vacuum carburization treatment is as small as 2% or less, and the molar concentration is approximately proportional to the mass concentration. Therefore, expression (3) can be expressed in mass concentration (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 the formula (5) is a carbon concentration (mass%).
Using the 1 st law of fick (formulae (1), (3) and (5)), and the 2 nd law of fick (formulae (2) and (4)), a diffusion simulation for calculating the theoretical carburizing gas flow rate FT was performed in the following manner.
In the vacuum carburization treatment using acetylene as a carburizing gas, carbon intrudes into the steel material from the surface of the steel material due to decomposition of the carburizing gas. The surface of the steel material at the carburizing step is assumed to be increased in carbon concentration until the carbon concentration in the steel material is in equilibrium with graphite. Therefore, boundary conditions in the simulation of carbon diffusion on the steel surface in the vacuum carburization treatment are defined as "carbon concentration on the steel surface and graphite balance". Diffusion simulation was performed as follows on the premise described above.
[ calculation method in diffusion simulation ]
First, mesh data for dividing the surface layer of a steel material to be subjected to vacuum carburization processing into a plurality of cells is created. The dimensions of the individual cells may be known. The size of the units is, for example, 1 to 500. Mu.m. The size of the cells may be gradually increased from the surface of the steel material in the depth direction. In this case, the ratio of the sizes of adjacent units is 0.80 to 1.25, preferably 0.90 to 1.10. Wherein the size of the cells is not limited thereto. The object for which the diffusion simulation is performed may be one-dimensional. When the steel material is round bar or cylindrical, the mesh data can be handled as one dimension by setting the cylindrical coordinate system. Further, if the diameter of the steel material (round bar or cylinder) is 50 times or more the diffusion distance of carbon in the steel, the steel material can be treated in the same manner as the plane. Here, the diffusion distance is ∈dt. The diffusion coefficient D is calculated from the carbon concentration and carburization temperature of the steel material. The time t (seconds) is a carburizing time (an implementation time of the carburizing process). For example, when SCM415 specified in JIS G4053 (2008) is used as a steel material, and the carburizing temperature is 950 ℃ and the carburizing time is 51 minutes, the diffusion distance ∈dt is 0.20mm. In this case, if the diameter of the steel material is 10mm or more, the steel material may be treated in the same manner as the plane. When SCM420 defined in JIS G4053 (2008) was used as a steel material, and the carburizing temperature was 950 ℃ and the carburizing time was 51 minutes, the diffusion distance ∈dt was 0.21mm. Then, the analysis time (step time) of the diffusion simulation is set. The step time is not particularly limited, and is set to, for example, 0.001 to 1.0 seconds.
In the vacuum carburization, a carburization step is performed, and thereafter, a diffusion step is performed. The carburizing step and the diffusion step may be performed a plurality of times. For example, in the case where the carburizing process and the diffusion process are performed 2 times, the 1 st carburizing process is performed, and the 1 st diffusion process is performed after the 1 st carburizing process. Further, the diffusion step 1 is followed by the carburization step 2, and the diffusion step 2 is followed by the carburization step 2. When the carburizing process and the diffusion process are performed a plurality of times in this way, the theoretical carburizing gas flow rate in the previous carburizing process is reset for each of the carburizing processes, and the theoretical carburizing gas flow rate in the next carburizing process is newly set.
When the n+1th carburizing step is performed with a diffusion step of 1/10 of the time of the n-th carburizing step interposed therebetween after the n-th carburizing step (n is a natural number of 1 or more), the n-th carburizing step and the n+1th carburizing step are regarded as 1-time carburizing steps. That is, in this case, the theoretical gas flow rate set in the nth carburization step is not newly set, and the process is directly used in the n+1th carburization step. In other words, if the diffusion process time between the nth carburization process and the (n+1) th carburization process is 1/10 or more of the nth carburization process time, the theoretical carburization gas flow rate of the nth carburization process is reset in the (n+1) th carburization process, and a new theoretical carburization gas flow rate is set.
As described above, the carbon concentration on the steel surface is set to be in an equilibrium state with graphite. Therefore, the equilibrium phase and equilibrium composition at the carburizing temperature with the graphite equilibrium state are obtained by a known thermodynamic calculation based on the chemical composition of the steel material to be subjected to the vacuum carburization treatment. The chemical composition of the steel material to be subjected to the vacuum carburization treatment is calculated thermodynamically by increasing the C concentration until the graphite appears as an equilibrium phase in consideration of dilution due to the increase of the C concentration. For example, when the C concentration is increased by 7 mass%, the mass of the steel itself becomes 1.07 times. Therefore, the concentrations of other elements than C were calculated thermodynamically based on the chemical composition set to 1/1.07 times. From the equilibrium phase and the equilibrium composition obtained by thermodynamic calculation, the C content in the steel, the chemical potential of C, and the solid solution C concentration in the austenite can be determined. Well known thermodynamic calculation software may be used in thermodynamic calculation. The well-known thermodynamic calculation software is referred to, for example, under the trade name Pandat (trademark).
Similarly, in the steel material other than the steel material surface, there are cases where vacuum carburization and cementite (θ) precipitation occur. At this time, carbon (C) in the steel material is distributed to 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 were obtained by the thermodynamic calculation described above. In the same manner as 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 the austenite can be determined in the steel.
The diffusion coefficient D of carbon in austenite in the steel material may be a value obtained by experiments in advance using the steel material to be subjected to the vacuum carburization treatment, or may be data reported as experimental data. For example, the diffusion coefficient D (m 2 S) may be referred to as proposed by Gray G.Tibbetts et al, the following formula may be used.
D＝4.7×10 -5 ×exp(-1.6×C-(37000-6600×C)/1.987/T)
Here, "C" in the formula is the solid solution C concentration (mass%) in austenite, and T is the carburizing temperature (K).
Mobility m (m) of carbon in austenite in steel 2 S) can be determined from the diffusion coefficient D and thermodynamic calculations. The formula in which the mobility m is formulated is as follows.
m＝1.54×10 -15 exp(-1.61×C-(17300-2920×C)/T)
Here, "C" in the formula is the solid solution C concentration (mass%) in austenite, and T is the carburizing temperature (K).
Next, the C concentration of the surface layer obtained by the vacuum carburization treatment was set. SpecificallyThe target carbon concentration of the unit at the outermost surface and the target carbon concentration at a predetermined depth are set. Further, as the initial value, a solid solution C concentration in all units=c concentration (C 0 ) The cementite precipitation amount was set to 0 in all the cells.
Based on the above preconditions, the following calculation is performed at each step time.
(A) Based on the carbon concentration in each cell and the thermodynamic calculation result, the solid solution C concentration (i.e., the concentration of diffused C) in austenite in each cell at the carburizing temperature is determined. At this time, it is assumed that C in cementite is fixed, and only solid solution C in austenite diffuses.
(B) Based on the determined solid solution C concentration, the diffusion flux J in each cell is determined by the difference method using the formula (1), the formula (3), or the formula (5). At this time, as described above, the solid solution carbon concentration on the steel surface is set to be the solid solution carbon concentration (C sat ). Based on diffusion flux J from the steel surface 0 The acetylene flow rate was determined by setting the carburizing efficiency to 100%. The acetylene flow rate thus obtained was defined as the theoretical carburizing gas flow rate at this step time.
(C) Based on the obtained diffusion flux J in each cell, the C concentration of each cell at the time when the step time passes is determined.
(D) Based on the result of the thermodynamic calculation, it is judged whether cementite is generated as an equilibrium phase. Note that, the time required for the generation of cementite is disregarded (i.e., the next step time is determined (a)).
(E) When the carburizing process is performed 2 times or more, the diffusion process between the carburizing processes is simulated, and the subsequent carburizing process is simulated. In the diffusion step, a diffusion flux J from the steel surface 0 The values (A) to (D) were calculated with zero.
The above calculation was performed at each step time to determine the diffusion flux J of carbon from the steel surface per unit surface area of the steel during the carburizing step 0 (t). Then, the diffusion flux J per unit surface area of the steel material 0 (t) Converted into acetylene gas flow rate, and multiplied by the surface area S (m) 2 ) The theoretical carburizing gas flow rate FT (t) at time t is obtained. In the 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, the theoretical carburizing gas flow rate FT in each carburizing time is plotted, and the theoretical carburizing gas flow rate FT can be represented in the form of a theoretical carburizing gas flow rate curve. Fig. 1 is a graph showing an example of a relationship between a theoretical carburizing gas flow rate and time calculated from a diffusion flux of carbon in a surface layer of a steel material obtained by the diffusion simulation. The ∈point in fig. 1 shows the theoretical carburizing gas flow rate FT at each time. Curve C in fig. 1 1.00 A theoretical carburizing gas flow curve is shown.
Theoretical carburizing gas flow curve C 1.00 The approximation of (2) can be represented by formula (6).
FT＝S×A/√t (6)
Here, FT is the theoretical carburizing gas flow rate (NL/min). A in the formula (6) can be represented by the formula (7). T in the formula (6) is a time (minutes) from the start of the carburizing process.
A＝a×T 2 +b×T+c (7)
A, b and c in the formula (7) are constants determined by chemical compositions of steel materials, and T is a carburization temperature (°c). For example, when the steel material is SCM420 defined in JIS G4053 (2008), a=8.52×10 when determined by the diffusion simulation described above -5 B= -0.140, c=58.2. When the steel material is SCM415 defined in JIS G4053 (2008), a=8.64×10 when determined by the above diffusion simulation -5 、b＝-0.141、c＝59.0。
Equation (6), which is an approximation of the theoretical carburizing gas flow rate FT, is also referred to as the theoretical carburizing gas flow rate FT in the present specification. That is, based on the equation (6), the theoretical carburizing gas flow rate FT at each carburizing time in the actual carburizing process can be obtained.
In the above description, as an example of a method for determining the theoretical carburizing gas flow rate, the theoretical carburizing gas flow rate is determined based on a well-known diffusion simulation using a diffusion equation. However, the theoretical carburizing gas flow rate may also be determined according to other methods. For example, the theoretical carburizing gas flow rate may also be determined experimentally.
The method for obtaining the theoretical gas flow rate from the experiment is as follows, for example. A steel material having a chemical composition equivalent to that of a steel material actually subjected to vacuum carburization is subjected to vacuum carburization. The flow rate of the carburizing gas supplied to the vacuum carburizing furnace was set to be constant, and the partial pressure of acetylene and the partial pressure of hydrogen in the vacuum carburizing furnace were continuously measured in the carburizing step. Then, a minimum value FAmin of the carburizing gas flow rate at which the first time t0, at which the acetylene partial pressure is 0.8 times or more the hydrogen partial pressure, is 1/10 or less of the time ta (i.e., the total carburization process time) which is the carburization process end time, is obtained. Based on the obtained carburizing gas flow rate FAmin, a theoretical carburizing gas flow rate ft=famin v (t 0/t) is formed.
As described above, the theoretical carburizing gas flow rate is equal to the carburizing gas flow rate for the carburizing reaction by contacting the steel surface. Therefore, the theoretical carburizing gas flow rate is not affected by the size and shape of the heat treatment furnace.
[ vacuum carburization method according to the present embodiment ]
The flow rate of the carburizing gas actually supplied to the vacuum carburizing furnace at the time of the vacuum carburizing treatment is defined as "actual carburizing gas flow rate" FR. The present inventors have studied and studied the matters of the case where the actual carburizing gas flow rate FR greatly deviating from the theoretical carburizing gas flow rate FT relationship under the carburizing time is assumed to be used as shown in fig. 1.
Fig. 2 is a graph showing the time-dependent change of the actual carburizing gas flow rate FR and the time-dependent change of the theoretical carburizing gas flow rate FT in the conventional carburizing process. 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 process. The solid line FR in fig. 2 shows the actual carburizing gas flow rate FR in the conventional carburizing process as described above. Dashed line C of FIG. 2 1.00 The theoretical carburizing gas flow rate FT is indicated as described above.
Referring to fig. 2, the start time of the carburization process is defined as "0", and the end time of the carburization process is defined as "ta". That is, the carburizing process is performed from time 0 to time ta. The end time ta is preset according to a set value of the carbon concentration at a predetermined depth position of the carburized steel material. In addition, the time when the actual carburizing gas flow rate FR initially becomes equal to the theoretical carburizing gas flow rate FT is defined as "te".
The period from the start of the carburizing process to the time te is defined as a period S100. The period from the time te to the time ta is defined as a period S200. In the period S100, the actual carburizing gas flow rate FR is lower than the theoretical carburizing gas flow rate FT (curve C 1.00 ). Therefore, in the carburizing step of the conventional vacuum carburizing method, the actual carburizing gas flow rate FR in the period S100 is insufficient. In this case, a portion where carburization reaction is sufficient and a portion where carburization reaction is insufficient are generated on the surface of the steel material. Therefore, carburization distribution on the steel surface becomes uneven. In addition, a desired carbon concentration may not be obtained in the steel surface layer. On the other hand, in the period S200, the actual carburizing gas flow rate FR is higher than the theoretical carburizing gas flow rate FT (curve C 1.00 ). Therefore, in the period S200, the actual carburizing gas flow rate FR becomes excessive and remains in the vacuum carburizing furnace. As a result, in the period S200, coal and tar are generated due to the residual carburizing gas. In this case, carburization distribution on the steel surface becomes uneven.
Based on the above investigation results, the present inventors considered to combine the theoretical carburizing gas flow rate curve C in the carburizing process 1.00 To control the actual carburizing gas flow rate FR.
However, as shown in fig. 2, in the period S100 at the initial stage of the carburizing process, the theoretical carburizing gas flow rate curve C is compared with the subsequent period S200 1.00 Is steep. Therefore, it is understood that the theoretical carburizing gas flow rate curve C is combined in the period S100 of the actual operation 1.00 It is very difficult to adjust the actual carburizing gas flow rate FR.
Further, in the period S100 at the initial stage of the carburizing process, when the above formula (6) is adopted at the beginning of the carburizing process (t=0), the theoretical carburizing gas flow rate FT becomes infinite. Therefore, in actual operation, 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 period S100.
Therefore, the inventors consider that not only the theoretical carburizing gas flow rate FT but also other factors are considered as factors for controlling the actual carburizing gas flow rate. The gas composition in the atmosphere in the vacuum carburizing furnace varies according to the actual carburizing gas flow rate FR. This change in gas composition causes uneven carburization distribution and generation of coal. Accordingly, the present inventors focused on not only the theoretical carburizing gas flow rate FT but also the gas composition in the atmosphere of the vacuum carburizing furnace as a factor for controlling the actual carburizing gas flow rate.
The present inventors focused on the partial pressure of hydrogen and the partial pressure of acetylene in the atmosphere in the vacuum carburizing furnace. The partial pressure of hydrogen and the partial pressure of acetylene in the atmosphere in the vacuum carburizing furnace can be measured by a known analyzer. The analyzer is, for example, a quadrupole mass spectrometer.
The analyzed hydrogen partial pressure was generated in the vacuum carburizing furnace according to the reaction based on the following formula.
C 2 H 2 →2C+H 2
The hydrogen partial pressure is an index of the carburization reaction amount in the carburization step. That is, the hydrogen partial pressure becomes an index of the degree of suppression of carburization distribution unevenness. On the other hand, the acetylene partial pressure is an index of the amount of residual gas that does not cause carburization reaction, and is an index of the amount of coal and tar produced.
In the vacuum carburization treatment using acetylene, the chemical reaction is extremely rapid immediately after the carburization step, that is, immediately after the supply of acetylene into the furnace is started. That is, the carbon entering speed to the steel surface immediately after the start of the carburization step is extremely fast. Therefore, if the flow rate of acetylene (carburizing gas flow rate) supplied into the furnace is small, the furnace atmosphere becomes substantially hydrogen gas. As a result, the hydrogen partial pressure in the furnace becomes high and the acetylene partial pressure becomes low. On the other hand, if the flow rate of acetylene gas supplied into the furnace (vacuum carburizing gas flow rate) is large, acetylene gas that does not cause a carburizing reaction remains in the furnace. In this case, the hydrogen partial pressure in the furnace becomes low and the acetylene partial pressure becomes high. Therefore, by monitoring the hydrogen partial pressure and the acetylene partial pressure in the furnace, the carburization reaction amount at the steel surface can be estimated.
The present inventors considered that if the actual carburizing gas flow rate FR can be controlled based on the theoretical carburizing gas flow rate FT and the hydrogen partial pressure and acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, the carburization maldistribution can be suppressed and the generation of coal can also be suppressed in the vacuum carburizing treatment. Accordingly, the present inventors have further studied and have obtained the following findings.
(a) In the initial stage of the carburizing process (near the period S100), if the carburizing gas flow rate is small, the amount of carburization reaction is small. Therefore, the acetylene partial pressure rises slowly. As a result, carburized distribution becomes large, and the carbon concentration of the surface layer of the carburized component becomes low.
(b) The end time of the carburizing process is defined as ta. As described above, the end time ta is set in advance according to the set values of the surface carbon concentration and carburization depth of the carburized steel material. Then, a time of 1/10 of the end time ta from the start time of the carburization process is defined as ta/10. The theoretical carburizing gas flow rate at time ta/10 is defined as FT ta/10 . If the actual carburizing gas flow rate FR at the initial stage of the carburizing process is set to the theoretical carburizing gas flow rate FT at a time ta/10 ta/10 As described above, the hydrogen partial pressure in the atmosphere in the vacuum carburizing furnace increases rapidly, but the pressure of acetylene increases at a high rate when the pressure is reduced in the early stage. As a result, the shortage of the carburization reaction amount at the initial stage of the carburization step can be suppressed, and the carburization maldistribution can be reduced.
(c) On the other hand, if the actual carburizing gas flow rate FR at the initial stage of the carburizing process is too high, the partial pressure of acetylene in the furnace excessively increases rapidly. In this case, acetylene gas remains excessively in the furnace. As a result, coal or tar is produced, and carburization distribution becomes uneven. The theoretical carburizing gas flow rate at 4 seconds from the start of the carburizing process was defined as FT 4 . At the initial stage of the carburizing process, if the actual carburizing gas flow rate FR is FT 4 Hereinafter, it is possible to suppress an excessive increase in the actual carburizing gas flow rate FR in the furnace. Therefore, carburization maldistribution can be suppressed.
(d) If the actual carburizing gas flow rate FR is maintained at a large level, the acetylene partial pressure is slowly increased. Therefore, the partial pressure of acetylene at any time greatly exceeds the partial pressure of hydrogen. In this case, excessive gas that does not cause carburization reaction is excessively present in the atmosphere in the vacuum carburizing furnace. Thus, coal derived from surplus gas is generated, and adheres to the surface of the carburized component. As a result, carburization distribution becomes uneven.
(e) In the carburizing step, if the actual carburizing gas flow rate FR is maintained or reduced when the acetylene partial pressure is 0.8 times or more the hydrogen partial pressure, the surplus gas can be suppressed in the atmosphere in the vacuum carburizing furnace. Therefore, carburization maldistribution can be suppressed.
Based on the above findings, the present inventors have found that if the actual carburizing gas flow rate FR in the carburizing process is adjusted as in (I) to (III) below, a sufficient amount of carburization reaction can be ensured at the initial stage of the carburizing process, and thereafter, the generation of coal and tar can be suppressed by suppressing the surplus gas, and thus, the carburization maldistribution can be reduced.
Here, with respect to each term, it is defined as follows.
Time ta: end time of carburization process
Time t0: an initial time period after the start of the carburizing step when the acetylene partial pressure is 0.8 times or more the hydrogen partial pressure
Time ta/10: a time of 1/10 of the end time ta from the start time of the carburizing process
Time 4t0: a time elapsed 4 times the period from the start of the carburizing process to the time t0 after the start of the carburizing process
Early carburization step S1: a period from the start of the carburizing process to a time t0
Post carburization step S2: time period from time t0 to time ta
Actual carburizing gas flow rate FR: carburizing gas (acetylene) flow rate actually supplied to the vacuum carburizing furnace
Theoretical carburizing gas flow rate FT ta/10 : theoretical carburizing gas flow at time ta/10
Theoretical carburizing gas flow rate FT 4 : from the beginning of the carburization processTheoretical carburizing gas flow at 4 seconds
When the above terms are defined, as shown in fig. 3, the actual carburizing gas flow rate FR is adjusted as shown in the following (I) to (III).
(I) In the early carburization step S1, the actual carburized gas flow rate FR is set to FT ta/10 Above and FT 4 The following is given. When the actual carburizing gas flow rate FR is made constant in the early carburizing step S1, this value is taken as the actual carburizing gas flow rate FA.
(II) in the period of time t0 to 4t0 in the post-carburizing step S2, the actual carburizing gas flow rate FR is made to be equal to or greater than FA x (t 0/t) and equal to or less than FA.
(III) in the period of time 4t0 to time ta in the post-carburizing step S2, the actual carburizing gas flow rate FR is set to be not less than FA x (t 0/t) and not more than 2FA x (t 0/t).
Here, t is a time from the start of carburization.
Fig. 3 is a graph showing the change with time of the actual carburizing gas flow rate in the carburizing step (lower graph) and the change with time of the acetylene partial pressure and the hydrogen partial pressure in the atmosphere of the vacuum carburizing furnace in the carburizing step (upper graph) of the vacuum carburizing treatment method of the present embodiment. Referring to fig. 3, in the present embodiment, the actual carburizing gas flow rate FR is adjusted to be within the range of the hatched area in fig. 3 during the period from time t0 to time ta. The curve that becomes the lower limit of the hatched area is a curve of carburizing gas flow rate=fa× (t 0/t). The upper limit of the hatched area is a curve of carburizing gas flow=2fa× (t 0/t). Fa× (t 0/t) and 2fa× (t 0/t) are both formulae proportional to formula (6) of the theoretical carburizing gas flow rate FT.
As described above, the time t0 is set to be the first time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing process. As shown in fig. 3, in the early stage of the early carburization step S1, the hydrogen partial pressure is rapidly increased from the acetylene partial pressure. This is because the carburization reaction actively occurs. After the hydrogen partial pressure rises rapidly, the hydrogen partial pressure begins to fall earlier than the acetylene partial pressure. Then, the hydrogen partial pressure is decreased, and as a result, the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure. The first time at this time, that is, the time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing process is defined as time t0. Here, "0.8" is a value at the 2 nd decimal point excluding the calculated value of the ratio of acetylene partial pressure to hydrogen partial pressure.
The vacuum carburization method according to the present embodiment, which has been completed based on the above findings, has the following configuration.
[1]
A vacuum carburization method for performing vacuum carburization on a steel material in a vacuum carburization furnace, comprising:
a heating step of heating the steel material to a carburizing temperature;
a soaking step of soaking the steel material at the carburizing temperature after the heating step;
A carburizing step of supplying an acetylene gas, that is, a carburizing gas, into the vacuum carburizing furnace after the soaking step, and holding the steel material at the carburizing temperature;
a diffusion step of stopping the supply of the carburizing gas into the vacuum carburizing furnace after the carburizing step, and holding the steel material at the carburizing temperature; and, a step of, in the first embodiment,
a quenching step of quenching the steel material after the diffusion step,
in the foregoing carburization step, the carburization step,
the flow rate of the carburizing gas supplied into the vacuum carburizing furnace is defined as an actual carburizing gas flow rate,
the flow rate of the carburizing gas required for the vacuum carburizing treatment of the steel material is defined as a theoretical carburizing gas flow rate,
the end time of the foregoing carburizing process is defined as ta,
when the initial time period in which the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing step is defined as t0,
the carburizing step includes:
a partial pressure measurement step of continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace to determine the time t 0;
a preliminary carburizing step of starting from the carburizing step to a time t 0; and, a step of, in the first embodiment,
A post carburization step of starting from time t0 to time ta,
in the foregoing preliminary carburization step,
the actual carburizing gas flow rate is set to be equal to or higher than the theoretical carburizing gas flow rate at the time ta/10 and equal to or lower than the theoretical carburizing gas flow rate at the time of 4 seconds from the start of the carburizing process,
in the foregoing post-carburizing process,
the actual carburizing gas flow rate in the preliminary carburizing step is defined as FA, and the time from the start of the carburizing step is defined as time t,
the actual carburizing gas flow rate in the period from the time t0 to the time 4t0 is made to be equal to or higher than FA (t 0/t) and equal to or lower than FA,
the actual carburizing gas flow rate at the time period 4t0 to the time period ta is set to be equal to or greater than FA v (t 0/t) and equal to or less than 2FA v (t 0/t).
[2]
The vacuum carburization processing method according to [1], wherein,
in the foregoing post-carburizing process,
the actual carburizing gas flow rate is reduced by the method (a) or (B) with the lapse of time in the period of time 4t0 to time ta.
(A) Repeating the maintaining and decreasing of the actual carburizing gas flow rate, decreasing the actual carburizing gas flow rate stepwise,
(B) The actual carburizing gas flow rate is gradually reduced with the lapse of time.
[3]
The vacuum carburization treatment method according to [1] or [2], wherein,
the aforementioned theoretical carburizing gas flow rate is determined based on diffusion simulation using a diffusion equation.
[4]
A method for producing a carburized component,
the method of vacuum carburization of steel according to any one of [1] to [3 ].
Hereinafter, a vacuum carburization method and a carburized component manufacturing method according to the present embodiment will be described in detail.
[ vacuum carburization method ]
Fig. 4 is a diagram showing an example of a heating pattern of the vacuum carburization processing method according to the present embodiment. Referring to fig. 4, the vacuum carburization method of the present embodiment includes: a heating step (S10); a soaking step (S20); a carburizing step (S30); a diffusion step (S40) and a quenching step (S50). Details of each step are described below.
[ heating step (S10) ]
In the heating step (S10), the steel material is heated to a carburizing temperature. The steel material to be subjected to the vacuum carburization may be a steel material supplied from a third party or may be a steel material produced by performing a vacuum carburization method. The chemical composition of the steel material is not particularly limited. The known steel material subjected to carburization may be used. The steel material is, for example, an alloy steel material for machine structural use specified in JIS G4053 (2008). More specifically, the steel material is, for example, SCr415, SCr420, SCM415, and the like specified in JIS G4053 (2008).
The steel to be prepared may be a hot-worked steel or a cold-worked steel. The hot working is, for example, hot rolling, hot extrusion, hot forging, or the like. Cold working is, for example, cold rolling, cold drawing, cold forging, or the like. The steel material may be machined, typically by cutting, after being hot-worked or cold-worked.
In the heating step (S10), the steel material is charged into a vacuum carburizing furnace, and the steel material is heated to a carburizing temperature Tc. The heating step (S10) is a well-known step in the vacuum carburization method. The carburizing temperature Tc may be a known temperature. Carburizing temperature Tc is A c3 Above the phase transition point. The preferred range of carburizing temperature Tc is 900-1130 ℃. When the carburizing temperature Tc is 900 ℃ or higher, heat conduction by radiation becomes high, and the temperature in the vacuum carburizing furnace tends to be uniform. As a result, carburized distribution of the steel material is unevenIs easy to be smaller. 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 further preferably 910 ℃, and further 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 a soaking time. The soaking step (S20) is a well-known step in the vacuum carburization 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 the cross section perpendicular to the longitudinal direction of the steel material is converted into a circle, the soaking time is preferably at least 30 minutes per 25mm equivalent diameter. For example, when the equivalent diameter is 30mm, the soaking time is preferably 36 minutes or longer. 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, for example, 100Pa or less. In the heating step (S10) and/or the soaking step (S20), nitrogen gas is introduced and vacuum-exhausted by a vacuum pump, and a nitrogen atmosphere may be 1000Pa or less. In the soaking step (S20), the vacuum carburizing furnace is set to a low pressure or vacuum at least until the carburizing step (S30) is started. For example, in the soaking step (S20), the vacuum carburizing furnace is set to 10Pa or less until the carburizing step (S30) is started.
[ carburizing step (S30) ]
In the present specification, the carburizing step (S30) is a step of supplying a carburizing gas into the furnace under reduced pressure or vacuum. That is, after the soaking step (S20), the carburizing step (S30) is started when the supply of the carburizing gas into the furnace under reduced pressure or vacuum is started. In the carburizing step (S30), a carburizing gas is supplied into the furnace while maintaining the pressure in the furnace at a low level. Since the pressure in the furnace is low, the frequency of collision of molecules of carburizing gas with each other becomes small. That is, the frequency of decomposition of the carburizing gas in the atmosphere in the furnace becomes low. Therefore, the carburizing gas is supplied to the surface of the steel material at a low pressure, and the generation of coal and tar can be suppressed. As a result, the surface carbon concentration of the steel material can be rapidly increased. In the carburizing step (S30) from the start of carburization to the end of carburization (time ta), the furnace pressure is set to, for example, 1 to 1000Pa. However, the furnace internal pressure in the carburizing step (S30) is not limited to the above range.
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 (S30) of the vacuum carburizing treatment method is acetylene gas.
In the conventional vacuum carburization process, a propane gas is often used. However, in addition to the carburizing reaction, a decomposition reaction of propane gas into methane, ethylene, acetylene, hydrogen, or the like occurs. If the amount of methane and ethylene generated by the decomposition reaction is large, the carburizing reaction is not assisted, and the furnace is evacuated from the vacuum carburizing furnace. Therefore, in the case of using propane gas, the theoretical carburizing gas flow rate FT cannot be calculated by using diffusion simulation of the diffusion flux of carbon obtained by the diffusion equation. On the other hand, acetylene is less likely to react other than carburization. Therefore, by using diffusion simulation of the diffusion flux of carbon obtained by the diffusion equation, the theoretical carburizing gas flow rate FT can be calculated.
In the present embodiment, the purity of acetylene as a carburizing gas may be 98% or more. For 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 carburizing gas. In this case, the mixing of the solvent into the furnace atmosphere can be suppressed. When the supply source for supplying acetylene into the vacuum carburizing furnace is a gas tank, the primary pressure when acetylene is supplied from the gas tank into the vacuum carburizing furnace is preferably 0.5MPa or more. When the molten steel is supplied to the vacuum carburizing furnace, the pressure is preferably reduced to 0.20MPa or less by using a pressure reducing valve.
[ detail of carburizing step (S30) ]
The carburizing step (S30) includes: a partial pressure measurement step S0, a pre-carburizing step S1, and a post-carburizing step S2. The following describes each step in detail.
[ preparation in advance ]
Before the vacuum carburization method is performed, the theoretical carburization gas flow FT conforming to the target steel is determined in advance, and the change with time of the theoretical carburization gas flow FT up to the end time ta of the carburization step (S30) shown in fig. 1 is obtained in advance. The theoretical carburizing gas flow rate FT may be determined based on diffusion simulation or may be determined based on experiments.
[ partial pressure measurement step S0]
In the partial pressure measurement step S0, in the carburizing step (S30), the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are measured. Specifically, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace were continuously measured. Here, "continuously" means that the hydrogen partial pressure and the acetylene partial pressure are measured plural times with time. The hydrogen partial pressure and the acetylene partial pressure may be measured continuously, or may be measured at predetermined time intervals. The measurement is performed by a known partial pressure measuring device. The partial pressure detector is, for example, a quadrupole mass spectrometer. However, other partial pressure detectors than a quadrupole mass spectrometer may be used as the partial pressure detector.
In the partial pressure measurement step S0, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace are measured over time. That is, the hydrogen partial pressure and the acetylene partial pressure in the atmosphere inside the vacuum carburizing furnace were monitored. Based on the measured hydrogen partial pressure and acetylene partial pressure over time, a time t0 (the first time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing step) is determined.
When a quadrupole mass spectrometer is used as the partial pressure measuring device, the quadrupole mass spectrometer sequentially measures each component gas (hydrogen and acetylene). Therefore, the measurement time of the hydrogen partial pressure deviates from the measurement time of acetylene. The analysis time of each component (hydrogen and acetylene) in the quadrupole mass spectrometer is preferably 0.2 seconds to 2.0 seconds, and the analysis interval is preferably 4.0 seconds.
For example, in the case of using a quadrupole mass spectrometer as a partial pressure analyzer, it is assumed that acetylene is analyzed at 0.5 seconds after hydrogen is analyzed at 0.5 seconds, hydrogen is again analyzed at 0.5 seconds after 2.0 seconds from the start of hydrogen analysis, and then acetylene is analyzed at 0.5 seconds. In this case, the analysis time of each component (hydrogen and acetylene) was 0.5 seconds, and the analysis interval was 2.0 seconds. In the following description, the analysis period of each component is defined as "analysis step". The period from the start time of the measurement step to the start time of the next measurement step is defined as an "analysis interval". In the case of the above example, the analysis step was 1.0 seconds (analysis time of hydrogen 0.5 seconds+analysis time of acetylene 0.5 seconds), and the analysis interval was 2.0 seconds.
When a quadrupole mass spectrometer is used as the partial pressure measuring device, the determination of the time when the acetylene partial pressure is 0.8 times or more the hydrogen partial pressure, that is, the time t0 in fig. 3, is performed as follows. The start time of an analysis step is set to t1 and the end time of the analysis step is set to t2. In the analysis step, the hydrogen partial pressure may be measured first, or the acetylene partial pressure may be measured first. Further, the start time of the next analysis step is defined as t3, and the end time of the analysis step is defined as t4. At this time, the analysis period is a period between time t1 and time t 3.
In this case, when the partial pressure of acetylene obtained in the analysis step at time t1 to time t2 is 0.8 times or more the partial pressure of hydrogen obtained in the same analysis step (i.e., the analysis step at time t1 to time t 2), and the partial pressure of hydrogen obtained in the analysis step at the next time t3 to time t4 after the analysis interval is 1.25 times or less the partial pressure of acetylene obtained in the analysis step at time t1 to time t2, the end time t2 of the analysis step for measuring the partial pressure of acetylene is defined as time t0.
The reason why the acetylene partial pressure is not only 0.8 times or more of the hydrogen partial pressure obtained in the same analysis step but also 1.25 times or less of the hydrogen partial pressure obtained in the previous 1 analysis step is also the condition is as follows. Assuming that the carburizing gas starts flowing into the furnace at a time t1 to a time t2, the hydrogen partial pressure obtained in the analysis step becomes 0 after the measurement of the hydrogen partial pressure is completed and before the measurement of the acetylene partial pressure. Therefore, the acetylene partial pressure obtained in this analysis step must be 0.8 times or more the hydrogen partial pressure. When the end time of the analysis step is regarded as time t0, acetylene gas is not sufficiently introduced into the furnace. Therefore, this case must not be considered as time t0. In the above case, the hydrogen partial pressure measured at the next analysis step (time t3 to time t 4) after the analysis interval greatly exceeds 1.25 times the acetylene partial pressure obtained at the previous 1 analysis step. This is because the hydrogen partial pressure is abruptly increased by introducing acetylene gas.
On the other hand, when the carbon-impregnated gas is sufficiently introduced into the furnace, the partial pressure of acetylene obtained as a result becomes 0.8 times or more the partial pressure of hydrogen obtained in the same analysis step, and the partial pressure of hydrogen obtained in the next analysis step after the analysis interval becomes 1.25 times or less the partial pressure of acetylene obtained in the previous analysis step. This is because, as shown in fig. 3, when the carburizing gas is sufficiently introduced into the furnace, the hydrogen partial pressure does not increase but decreases with the passage of time.
Therefore, when a quadrupole mass spectrometer is used as the partial pressure measuring device, the end time t2 of the analysis step for measuring the acetylene partial pressure is defined as time t0 when the obtained acetylene partial pressure is 0.8 times or more the hydrogen partial pressure obtained in the same analysis step and the hydrogen partial pressure obtained in the next analysis step after the analysis interval is 1.25 times or less the acetylene partial pressure obtained in the previous 1 analysis step.
The gas in the furnace (hydrogen and acetylene) may be analyzed in the furnace or may be extracted from the outside of the furnace. In the case of analyzing a furnace gas in a furnace, a partial pressure measuring device provided in the furnace is used. The partial pressure measuring instrument may be an instrument other than the four-stage rod mass spectrometer. In addition, a partial pressure measuring device may be used separately for each component gas. For example, acetylene partial pressure can be analyzed using a quadrupole mass spectrometer and hydrogen partial pressure can be analyzed using other partial pressure detectors.
The carburizing step (S30) supplies a carburizing gas under the reduced pressure. Therefore, the carburizing gas rapidly undergoes a carburizing reaction in the whole furnace. Therefore, the partial pressure measurement result of the furnace gas is less likely to be unevenly distributed in the furnace. That is, the analysis result of the furnace gas can be regarded as substantially uniform in the furnace.
[ early carburization step S1]
As shown in fig. 3, a period of time t0 from the start of the carburizing step (S30) to the first time when the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure is defined as a preliminary carburizing step S1. In the early carburizing step S1, the actual carburizing gas flow rate FR is adjusted so as to satisfy the following condition I.
(I) In the early carburizing step S1, the actual carburizing gas flow rate FR is set to the theoretical carburizing gas flow rate FT ta/10 The above theoretical carburizing gas flow rate FT 4 The following is given.
Fig. 5 is a diagram showing an example of the gas flow rate set value in the early carburization step S1 of the vacuum carburization processing method of the present embodiment. In the early carburizing step S1, the actual carburizing gas flow rate FR is set within a range of the hatched area in fig. 5 (FT ta/10 Above and FT 4 The following are described below).
If the actual carburizing gas flow rate FR in the preliminary carburizing step S1 is lower than the theoretical carburizing gas flow rate FT at a time ta/10 ta/10 In the early carburizing step S1, the supply of the carburizing gas is excessively insufficient. In this case, in the steel material (carburized member) subjected to the vacuum carburization method, carburization distribution unevenness becomes large. On the other hand, if the actual carburizing gas flow rate FR in the early carburizing step S1 exceeds the theoretical carburizing gas flow rate FT at the time of 4 seconds from the start of the carburizing step 4 The actual carburizing gas flow rate FR is excessive. In this case, it takes time to adjust the actual carburizing gas flow rate FR to be fa× (t 0/t) or more and 2FA (t 0/t) or less after the time t0 has elapsed until the time 4t 0. Therefore, excessive residual gas (acetylene gas) remains in the vacuum carburizing furnace, and coal is easily generated. As a result, carburized component (steel material) produced by the vacuum carburization method has large uneven carburization distribution.
If the actual carburizing gas flow rate FR in the preliminary carburizing step S1 is set to be the theoretical carburizing gas flow rate FT at a time ta/10 ta/10 Theoretical carburizing gas flow rate FT at 4 seconds from the start of the carburizing process 4 Hereinafter, on the premise that conditions II and III of the actual carburizing gas flow rate FR in the post-carburizing step S2 described later are satisfied, the carburization distribution unevenness of the carburized component (steel material) after the vacuum carburization treatment can be sufficiently suppressed. The actual carburizing gas flow rate FR in the preliminary carburizing step S1 may be adjusted by a known method. For example, the flow rate of the carburizing gas supplied to the vacuum carburizing furnace may be adjusted by a supply valve, the actual carburizing gas flow rate FR may be adjusted, or the actual carburizing gas flow rate FR may be adjusted by another known method. The actual carburizing gas flow rate FR may be adjusted by a known control device of the vacuum carburizing furnace. The control device adjusts the actual carburizing gas flow rate FR by adjusting the opening degree of the supply valve, for example.
The actual carburizing gas flow rate FR in the preliminary carburizing step S1 is preferably constant. If the actual carburizing gas flow rate FR is constant, the variation in the hydrogen partial pressure and acetylene partial pressure in the furnace can be measured with high accuracy. If the actual carburizing gas flow rate FR in the preliminary carburizing step S1 varies, the variation in the hydrogen partial pressure and the variation in the acetylene partial pressure in the furnace are affected by the variation in the actual carburizing gas flow rate FR. If the actual carburizing gas flow rate FR in the preliminary carburizing step S1 is constant, the variation in the hydrogen partial pressure and the acetylene partial pressure in the furnace can be measured with high accuracy. Therefore, the actual carburizing gas flow rate FR in the early carburizing step S1 is preferably constant. For example, as shown in fig. 3, the actual carburizing gas flow rate FR in the early carburizing step S1 is preferably constant. In this case, the value of the actual carburizing gas flow rate FR that is constant in the early carburizing step S1 becomes the actual carburizing gas flow rate FA in the early carburizing step S1. However, in actual operation, the actual carburizing gas flow rate is not completely constant as the set value, and the technical common knowledge known to those skilled in the art is: the self-set value fluctuates to some extent. Therefore, when the actual carburizing gas flow rate FR in the early carburizing step S1 is made constant, the actual carburizing gas flow rate FR allows the remaining amount of ±10% of the set value. That is, when the actual carburizing gas flow rate FR is shifted within ±10% of the specified set value in the early carburizing step S1, the set value is set to the value of the actual carburizing gas flow rate FA in the early carburizing step. That is, in the present specification, FA means a carburizing gas flow rate within a range of ±10% of the set value in the early carburizing step S1. FA is preferably within a range of ±5% of the set value in the early carburization step S1.
[ post carburization step S2]
As shown in fig. 3, a period from time t0 to the carburization process end time ta is defined as a post carburization process S2. In the post-carburizing step S2, the actual carburizing gas flow rate FR is adjusted so as to satisfy the following conditions II and III.
(II) in the period of time t0 to 4t0 in the post-carburizing step S2, the actual carburizing gas flow rate FR is made to be equal to or greater than FA x (t 0/t) and equal to or less than FA.
(III) in the period of time 4t0 to time ta in the post-carburizing step S2, the actual carburizing gas flow rate FR is set to be not less than FA x (t 0/t) and not more than 2FA x (t 0/t).
Here, t is a time from the start of carburization.
In short, in the post-carburizing step S2, the actual carburizing gas flow rate FR is adjusted so as to fall within the range shaded in fig. 3. In this way, in the post-carburizing step S2, excessive carburizing gas can be prevented from remaining in the vacuum carburizing furnace. As a result, the occurrence of coal and tar can be reduced, and uneven carburization distribution of the carburized component (steel material) after the vacuum carburization treatment method is suppressed.
[ concerning condition II ]
In the period of time t0 to 4t0 in the post-carburizing step S2, if the actual carburizing gas flow rate is lower than fa× (t 0/t), the gas flow rate is insufficient. In this case, uneven distribution of carburizing gas occurs in the vacuum carburizing furnace. For example, in the vicinity of the supply nozzle of the carburizing gas, the concentration of the carburizing gas is high, and in the region deviated from the supply nozzle, the concentration of the carburizing gas is low. As a result, carburization distribution unevenness becomes large in the steel material after the vacuum carburization treatment step.
On the other hand, if the actual carburizing gas flow rate exceeds FA during the period of time t0 to 4t0 in the post-carburizing step S2, the carburizing gas is excessively supplied. In this case, coal and tar are produced by the surplus gas. As a result, carburization distribution of the carburized component (steel material) after the vacuum carburization treatment becomes uneven.
Therefore, the actual carburizing gas flow rate FR is set to be fa× (t 0/t) or more and FA or less during the time t0 to 4t0 in the post-carburizing step S2. In this case, the conditions I and III are satisfied, so that the carburizing gas flow rate required for the carburizing reaction can be sufficiently ensured, and the generation of coal and tar can be suppressed. As a result, occurrence of uneven carburization distribution of the carburized component can be suppressed. As described above, the actual carburizing gas flow rate FR allows the remaining amount of ±10% of the set value. Therefore, as described above, the same margin exists for the actual carburizing gas flow rate FA in the early carburizing step S1. That is, in the present specification, the actual carburizing gas flow rate FA in the early carburizing step refers to a carburizing gas flow rate within a range of ±10% of the set value of the actual carburizing gas FR in the early carburizing step S1. Further, the actual carburizing gas flow rate FR may be maintained at FA until the middle of the period of time t0 to 4t0 in the post-carburizing step S2, and then the actual carburizing gas flow rate may be adjusted to be in the range of FA to fa× (t 0/t) in the post-carburizing step S1.
[ concerning condition III ]
In the period of time 4t0 to ta in the post-carburizing step S2, if the actual carburizing gas flow rate is lower than fa× (t 0/t), the gas flow rate is insufficient. In this case, uneven distribution of carburizing gas occurs in the vacuum carburizing furnace. For example, in the vicinity of the supply nozzle of the carburizing gas, the concentration of the carburizing gas is high, and in the region deviated from the supply nozzle, the concentration of the carburizing gas is low. As a result, carburization distribution unevenness becomes large in the steel material after the vacuum carburization treatment step.
On the other hand, if the actual carburizing gas flow rate exceeds 2fa× (t 0/t) during the period of time 4t0 to ta in the post-carburizing step S2, the carburizing gas is excessively supplied. In this case, coal and tar are produced by the surplus gas. As a result, carburization distribution of the carburized component (steel material) after the vacuum carburization treatment becomes uneven.
Therefore, in the period of time 4t0 to ta in the post-carburizing step S2, the actual carburizing gas flow rate FR is set to be fa× (t 0/t) or more and 2fa× (t 0/t) or less. In this case, the conditions I and II are satisfied, so that the carburizing gas flow rate required for the carburizing reaction can be sufficiently ensured, and the occurrence of coal and tar can be suppressed. As a result, occurrence of uneven carburization distribution of the carburized component can be suppressed.
In the post-carburizing step S2, the actual carburizing gas flow rate FR is not particularly limited as long as the conditions II and III are satisfied. For example, as shown in fig. 6, the actual carburizing gas flow rate FR may start to decrease during the period of time 4t0 to ta in the post-carburizing step S2.
In the post-carburizing step S2, as shown in fig. 6, the actual carburizing gas flow rate FR may be gradually decreased by repeating the maintenance and decrease of the actual carburizing gas flow rate FR with the lapse of time. As shown in fig. 7, in the post-carburizing step S2, the actual carburizing gas flow rate FR may be gradually decreased with the lapse of time. Further, as shown in fig. 8, the actual carburizing gas flow rate FR may be gradually decreased with the lapse of time and then increased. In short, in the post-carburizing step S2, the time-lapse change of the actual carburizing gas flow rate FR is not particularly limited as long as the conditions II and III are satisfied.
[ 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. It is preferable that 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 post carburization step S2, the generation of coal can be further suppressed. It is further preferable that the carburizing gas pressure in the post-carburizing step S2 is decreased with the lapse of time. The preferable carburizing gas pressure in the carburizing step (S30) is 1kPa or less.
[ time ta of carburizing step (S30) ]
The time ta from the start (t=0) to the end of the carburizing process (S30) is appropriately set before the start of the vacuum carburizing process according to the target carbon concentration of the surface layer of the steel material after the vacuum carburizing process. The time ta may be determined from the diffusion simulation described above using the diffusion equation. The time ta may be determined from experimental data by performing a vacuum diffusion treatment test in advance. The time ta is preferably long. When the time ta is long, the actual carburizing gas flow rate FR can be easily adjusted. The lower limit of the time ta is preferably 50 seconds, more preferably 1 minute (60 seconds), and still more preferably 3 minutes (180 seconds). The upper limit of the time ta is preferably 120 minutes, more preferably 60 minutes.
[ diffusion step (S40) ]
The diffusion step (S40) is a well-known step 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 intruded into the steel material due to the carburizing step (S30) is diffused into the steel material. This reduces the carbon concentration of the surface layer that has been increased in the carburizing step (S30), and increases the carbon concentration to a predetermined depth. In the diffusion step (S40), nitrogen gas is introduced into the vacuum carburizing furnace and vacuum is exhausted by a vacuum pump, and a nitrogen gas atmosphere of 1000Pa or less or vacuum is set. The vacuum is, for example, 10Pa or less. The inside of the vacuum carburizing furnace is set to be under a nitrogen atmosphere or vacuum state of 1000Pa or less, thereby inhibiting invasion and separation 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 treatment step. Therefore, the holding time in the diffusion step (S40) is not particularly limited.
[ quenching step (S50) ]
In the quenching step (S50), the steel material after the carburizing step (S30) and the diffusion step (S40) are 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 is transformed into martensite to form a hardened layer. The quenching step (S50) is a well-known step in the vacuum carburization method.
As shown in fig. 4, when the quenching temperature Ts is lower than the carburizing temperature Tc, the steel material after the diffusion step (S40) is cooled to the quenching temperature Ts. The cooling rate at this time is not particularly limited. Considering the treatment time in the vacuum carburization treatment step, it is preferable that the cooling rate is increased. Preferably, the cooling rate is 0.02 to 30.00 ℃/sec. The cooling rate is a value obtained by dividing the temperature difference between the carburizing temperature Tc and the quenching temperature Ts by the cooling time.
The cooling method for the steel material when the quenching temperature Ts is lower than the carburizing temperature Tc may be a known cooling method. For example, the steel may be cooled by cooling the steel under vacuum, or the steel may be cooled by gas cooling. When the steel material under vacuum is left to cool, it is preferable to cool the steel material at a pressure of 100Pa or less. In the case of cooling the steel material by gas cooling, it is preferable to use an inert gas as the cooling gas. As the inert gas, for example, nitrogen and/or helium is preferably used. As the inert gas, nitrogen which can be obtained at low cost is particularly preferably used. The use of an inert gas as the cooling gas can suppress oxidation of the steel material.
The steel material is quenched after being kept at the quenching temperature Ts for a predetermined time. If the quenching temperature Ts is A 3 Phase transition point (A) r3 The phase transition point) is not particularly limited. 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 ℃, more preferably 950 ℃, more preferably 900 ℃, more preferably 880 ℃.
As the quenching method in the quenching step (S50), a known quenching method is used. Quenching methods are for example air cooling, water cooling, oil cooling.
The above vacuum carburization method was performed to form a carburized part from a steel material. In the vacuum carburization method of the present embodiment, a theoretical carburizing gas flow rate FT for a steel material to be subjected to vacuum carburization is used. The first time period in which the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing step is divided into a pre-carburizing step S1 and a post-carburizing step S2 in the carburizing step (S30). The actual carburizing gas flow rate FR is adjusted so that the condition I is satisfied in the early carburizing step S1 and the conditions II and III are satisfied in the later carburizing step S2. Thus, occurrence of carburization maldistribution in the steel material after vacuum carburization can be suppressed.
The vacuum carburization method according to the present embodiment may further include other steps. For example, the vacuum carburization method may include a tempering step after the quenching step (S50). The tempering step may be performed under known conditions. For example, in the tempering step, A is used as c1 The steel is kept at a temperature equal to or lower than the transformation point for a predetermined time and then cooled.
In the vacuum carburization method of the present embodiment, the carburization step (S30) and the diffusion step (S40) may be repeated a plurality of times. In this case, as described above, in each carburizing step (S30), the time ta and the theoretical carburizing gas flow rate FT are determined.
[ method for producing carburized component ]
The method for producing a carburized component according to the present embodiment includes a step of producing a carburized component by applying the vacuum carburization method described above to a steel material. In the carburized component produced by the above steps, carburization maldistribution can be suppressed.
Examples
The effect of the vacuum carburization method according to the present embodiment will be described more specifically below by using examples. The conditions in the following examples are examples of conditions used to confirm the possibility and effect of the vacuum carburization processing method according to the present embodiment. Therefore, the vacuum carburization processing method of the present embodiment is not limited to this one example of conditions.
A steel pipe for machine construction (hereinafter referred to as a steel pipe) having a chemical composition corresponding to SCM415 specified in JIS G4053 (2008) and a round bar corresponding to SCM415 were prepared. The C content of each test number steel pipe and round bar was 0.15 mass%. The steel tube had a diameter of 34mm, a wall thickness of 4.5mm and a length of 110mm. The diameter of the round bar is 26mm and the length is 70mm. The evaluation of the vacuum carburization treatment was performed on round bars, and steel pipes were used as samples for investigating carburization distribution irregularities caused by the arrangement positions of the round bars in the vacuum carburization furnace.
Round bars each having been vacuum carburized and each test number was subjected to vacuum carburization, and the total surface area (m 2 ) Defined as the surface area (m) 2 ). The steel surface area was determined by the following equation.
Steel surface area = surface area per 1 steel tube x number of steel tubes + surface area per 1 round bar x number of round bars
The surface areas of the steel materials obtained are shown in Table 1. In test numbers 1 to 5, 10 to 13, 15 and 16, 18 to 21, 248 steel pipes and 3 round bars were used. In test No. 6, 496 steel pipes and 3 round bars were used. In test numbers 7 to 9, 14 and 17, 124 steel pipes and 3 round bars were used.
TABLE 1
First, a diffusion simulation using a diffusion equation was performed to determine a theoretical carburizing gas flow rate. Specifically, the round bar and the steel pipe are divided into a plurality of units of 2 μm or more in the thickness direction. The step time in the diffusion simulation was set to 0.002 to 0.02 seconds. In the chemical composition of the steel pipe and the round bar (SCM 415), the equilibrium composition with graphite at the equilibrium state at the surface at the carburizing temperature was obtained by thermodynamic calculation. Further, the equilibrium composition of the inside of the steel material at the carburizing temperature, the chemical potential of carbon, and the mobility of carbon were obtained. Thermodynamic calculations use the trade name Pandat (trade mark). Further, the database uses a trade name PanFe (trademark). Further, the mobility of carbon (m 2 S) the following formula is used.
m＝1.54×10 -15 exp(-1.61×C-(17300-2920×C)/T)
Here, C in the formula is a solid solution C concentration (mass%) in austenite, and T is a carburizing temperature (K).
The target value of the carbon concentration on the surface of the steel pipe and the round bar was 0.70 mass%, and the target value of the carbon concentration at a depth of 1.0mm from the surface was 0.40 mass%. With the above as a precondition, the diffusion simulations of (a) to (D) were performed for each step time, 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 to the following equation.
FT＝S×A/√t (6)
Here, A is every 1m defined by the formula (7) 2 And t represents a time (minutes) from the start of carburizing. In addition, S represents the surface area (m 2 )。
A＝a×T 2 +b×T+c (7)
In the case of the present embodiment (SCM 415), a=8.64×10 -5 、b＝-0.141、c＝59.0。
After the theoretical carburizing gas flow rate FT was calculated, actual vacuum carburization was performed by the following method. First, a cage formed of a stainless steel material (SUS 316 specified in JIS G4303 (2012)) that is sufficiently carburized was prepared. The steel pipes of the number are uniformly arranged in the cage in a standing state, and 3 round bars are arranged in the center of the cage, the left front side of the cage and the right rear side of the cage in a standing state. As described above, the round bar was used as a test material, and the steel pipe was used as a sample material for confirming occurrence of carburization maldistribution due to the arrangement position of the round bar.
The cage in which the steel materials (steel pipe and round bar) were placed was inserted into a vacuum carburizing furnace, and vacuum carburization was performed. Then, carburized parts of test numbers 1 to 21 were obtained. The conditions in the vacuum carburization treatment are shown in table 1.
Specifically, in each test number, the vacuum carburization treatment was performed as follows. The vacuum carburization 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 carburization temperatures Tc shown in table 1. After the heating step, a soaking step is performed. In the soaking step, the steel (round bar) was held at the carburizing temperature Tc for 60 minutes.
After the soaking step, a carburizing step is performed. In the carburizing step, acetylene is supplied as a carburizing gas in a vacuum carburizing furnace. The carburizing gas pressure in the carburizing step is kept below 1 kPa. The end time ta (minutes) of the carburizing process is described in table 1.
As described above, the carburizing time in the carburizing step and the diffusion time in the diffusion step were adjusted with the carbon concentration of 0.40 mass% at the depth of 1.0mm of the round bar as a target.
In the carburizing step, the atmosphere in the vacuum carburizing furnace was analyzed by a four-stage rod mass spectrometer, and the hydrogen partial pressure and the acetylene partial pressure were continuously measured. The mass-to-charge ratio (m/z) of hydrogen was set to 2, and the mass-to-charge ratio of acetylene was set to 26. The analysis time was 0.5 seconds and the analysis interval was 4 seconds. Based on the obtained hydrogen partial pressure and acetylene partial pressure, a time t0 (the first time when the acetylene partial pressure is 0.8 times or more the hydrogen partial pressure) is obtained.
The actual carburizing gas flow rate of each test number was changed with time as shown in fig. 9 to 16. The actual carburizing gas flow rate FR set values of test numbers 1 to 21 will be described below with reference to fig. 9 to 16.
Fig. 9 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing process of test numbers 1, 5, 7 to 12. Referring to fig. 9, in test numbers 1, 5, and 7 to 12, the actual carburizing gas flow rate FR at the time of starting the carburizing process (t=0) is FA. FA is FT ta/10 Above and FT 4 The following is given. The actual carburizing gas flow rate FR is made constant to FA over time t0 and until time ts before time 4t0 is reached. After time ts, the actual carburizing gas flow rate is gradually reduced by curve C2 (=fa× (ts/t)). As a result, the actual carburizing gas flow rate FR becomes FT in the early carburizing step S1 ta/10 Above and FT 4 The following is given. In the post-carburizing step S2, the actual carburizing gas flow rate FR in the period from time t0 to time 4t0 is not less than FA v (t 0/t) and not more than FA. Further, the actual carburizing gas flow rate FR for the time 4t0 to the time ta is not less than FA (t 0/t) and not more than 2FA (t 0/t).
Fig. 10 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing process of test numbers 2 to 4 and 6. Referring to FIG. 1 0, test numbers 2 to 4, and 6 set the actual carburizing gas flow rate FR at the time of starting the carburizing process (t=0) to FA. FA is FT ta/10 Above and FT 4 The following is given. The actual carburizing gas flow rate FR is made constant to FA over time t0 and until time ts before time 4t0 is reached. The time ts in the period from time t0 to time 4t0 in fig. 10 is a time later than the time ts in the period from time t0 to time 4t0 in fig. 9. After the time ts, the actual carburizing gas flow rate FR gradually decreases in a curve C2 (=fa× (ts/t)). As a result, the actual carburizing gas flow rate FR becomes FT in the early carburizing step S1 ta/10 Above and FT 4 The following is given. In the post-carburizing step S2, the actual carburizing gas flow rate FR in the period from time t0 to time 4t0 is not less than FA v (t 0/t) and not more than FA. Further, the actual carburizing gas flow rate FR for the time 4t0 to the time ta is not less than FA (t 0/t) and not more than 2FA (t 0/t).
Fig. 11 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing process of test numbers 13 and 14. Referring to fig. 11, in test numbers 13 and 14, the actual carburizing gas flow rate FR, i.e., FA at the beginning time of the carburizing process (t=0) is lower than FT ta/10 . Further, at a time ts later than the time ta/10, the actual carburizing gas flow rate FR gradually decreases as in the case of the theoretical carburizing gas flow rate FT. In the carburizing step, the partial pressure of acetylene in the vacuum carburizing furnace is not 0.8 times or more the partial pressure of hydrogen. Therefore, in the vacuum carburization process, t0 is not determined.
Fig. 12 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing process of test numbers 15 to 17. Referring to fig. 12, in test numbers 15 to 17, FA, which is the actual carburizing gas flow rate FR at the beginning time of the carburizing process (t=0), is FT ta/10 Above and FT 4 The following is given. However, when the time ts before the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure, the gradual decrease of the actual carburizing gas flow rate FR starts, and the actual carburizing gas flow rate FR is adjusted so as to be fa× (ts/t). Therefore, in the carburizing step, the partial pressure of acetylene in the vacuum carburizing furnace is not 0.8 times or more the partial pressure of hydrogen. Therefore, in the vacuum carburization process, t0 is not determined.
Fig. 13 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing process of test No. 18. Referring to fig. 13, in test No. 18, FA, which is the actual carburizing gas flow rate FR at the time of starting the carburizing process (t=0), is FT ta/10 Above and FT 4 The following is given. Then, the actual carburizing gas flow rate FR is made constant to FA from the time t0 to the time ts exceeding the time 4t 0. After the time ts, the actual carburizing gas flow rate FR is gradually reduced by a curve C2 (=fa× (ts/t)). As a result, the actual carburizing gas flow rate FR becomes FT in the early carburizing step S1 ta/10 Above and FT 4 In the following, the actual carburizing gas flow rate FR in the period from time t0 to time 4t0 in the post-carburizing step S2 is FA v (t 0/t) or more and FA or less. However, the actual carburizing gas flow rate FR at time 4t0 to time ta exceeds 2FA v (t 0/t).
Fig. 14 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing step of test No. 19. Referring to fig. 14, in test No. 19, the actual carburizing gas flow rate FR, i.e., FA at the beginning time of the carburizing process (t=0) is lower than FT ta/10 . Further, the actual carburizing gas flow rate FR thereafter is made constant to FA. In test No. 19, the acetylene partial pressure in the vacuum carburizing furnace was not 0.8 times or more the hydrogen partial pressure in the carburizing step. Therefore, in the vacuum carburization process, t0 is not determined.
Fig. 15 is a graph showing the change with time of the actual carburizing gas flow rate FR in the carburizing process of test No. 20. Referring to fig. 15, in test No. 20, FA, which is the actual carburizing gas flow rate FR at the time of starting the carburizing process (t=0), is FT ta/10 Above and FT 4 The following is given. Then, the actual carburizing gas flow rate FR is made constant to FA from time ts exceeding time t0 and lower than time 4t 0. After the time ts, the actual carburizing gas flow rate FR is gradually reduced by FB (see fig. 15) lower than the theoretical carburizing gas flow rate FT at the time ts, with a curve C2 (=fb× (ts/t)). As a result, the actual carburizing gas flow rate FR in the post-carburizing step S2 is lower than FA v (t 0/t).
FIG. 16 shows actual carburizing gas in the carburizing process of test No. 21A graph of the time-dependent change in the flow rate FR. Referring to fig. 16, in test No. 21, FA, which is the actual carburizing gas flow rate FR at the time of starting the carburizing process (t=0), is FT ta/10 Above and FT 4 The following is given. Then, the actual carburizing gas flow rate FR is made constant to FA from time ts exceeding time t0 and lower than time 4t 0. After time ts, the actual carburizing gas flow rate FR is reduced. However, there is a period from time 4t0 to time ta in which the actual carburizing gas flow rate FR exceeds 2FA v (t 0/t).
The actual carburizing gas flow rate was adjusted and measured using a flowmeter (manufactured by cofhlock co., ltd., trade name: mass flow controller D3665,3665).
After the carburizing step, a diffusion step was performed on the round bar at the diffusion time (minutes) shown in table 1, so that carbon that 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 diffusion times (minutes) are shown in table 1.
"FT" in Table 1 ta/10 The column indicates the theoretical carburizing gas flow rate (NL/min) at time ta/10. "FT 4 The column indicates the theoretical carburizing gas flow rate (NL/min) at the time point 4 seconds from the start of the carburizing process. The column "time t0 (minutes)" indicates time t0 (minutes). The column "time 4t0 (minutes)" indicates time 4t0 (minutes). The column "time ts (minutes)" indicates the time ts (minutes) at which the gradual decrease of the actual carburizing gas flow rate FR starts. "FR.gtoreq.FA x? The column indicates whether or not the actual carburizing gas flow rate is fa× (t 0/t) or more from time 4t0 to time ta. If "yes", the actual carburizing gas flow rate FR is fa× (t 0/t) or more. In the case of "no", the actual carburizing gas flow rate FR is lower than fa× (t 0/t). "FR.ltoreq.2FA× (t 0/t)? The column indicates whether or not the actual carburizing gas flow rate FR is 2fa× (t 0/t) or less at time 4t0 to time ta. If "yes", the actual carburizing gas flow rate FR is 2fa× (t 0/t) or less. In the case of "no", it means that the actual carburizing gas flow rate FR exceeds 2fa×V (t 0/t). "diffusion time (minutes)" means the diffusion time (minutes) in the diffusion process.
After the diffusion step, the round bar was cooled to 860 ℃. Then, the mixture was kept at the quenching temperature (860 ℃) for 30 minutes. After holding, the round bar was immersed in oil at 120 ℃ and oil quenching was performed. Tempering the quenched round bar. The tempering temperature was set at 170℃and the holding time at the tempering temperature was set at 2 hours.
Through the above-described production steps, a vacuum carburization treatment was performed to produce a carburized part (round bar).
[ evaluation test ]
The carbon concentration and the depth (hereinafter referred to as carburized depth) at which the carbon concentration of the surface layer of each test-numbered carburized part (round bar) reached 0.40 mass% were measured, and carburized distribution unevenness was evaluated.
[ test for measuring carbon concentration in surface layer of carburized component ]
For each test number of carburized parts (round bars) in the state of being inserted into the vacuum carburized furnace, a range of 20mm was cut in the longitudinal direction of the carburized part from the upper end face, and a range of 5mm was cut in the longitudinal direction of the carburized part from the lower end face. Hereinafter, a range of 20mm from the upper end surface is referred to as an "upper end surface test piece", and a portion of 5mm from the lower end surface is referred to as a "lower end portion".
The circumferential surfaces of the upper end surface test piece and the remaining portion (hereinafter referred to as the body portion) of which the lower end portion is cut off are subjected to turning. In the turning, the powder cutting of the surface layer portion from the surface of the round bar to a depth of 0.30mm was performed every 0.05mm depth interval. The carbon concentration of the cut powder at each depth position of the 0.05mm pitch adopted was measured. Through the above steps, the carbon concentration at the pitch of 0.05mm was obtained in the surface layer region from the surface to the depth of 0.30mm of the 3 carburized components (center position of the cage, front left position of the cage, and rear right position of the cage) of each test number. The carbon concentrations of 6 carbon concentrations from the surface to 0.30mm of the carburized component disposed in the central position of the cage were defined as carbon concentrations A1 to A6 (mass%) in this order from the surface. The carbon concentrations of 6 carbon concentrations from the surface to 0.30mm of the carburized component disposed at the left front position of the cage were defined as carbon concentrations B1 to B6 (mass%) in this order from the surface. The carbon concentrations of 6 carbon concentrations from the surface to 0.30mm of the carburized component disposed at the right rear position of the cage were defined as carbon concentrations C1 to C6 (mass%) in this 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 components was obtained. Specifically, among the carbon concentrations A1, B1, and C1 in the region from the surface to the 0.05mm depth position, the maximum value and the minimum value are selected, and the difference value between the carbon concentrations is defined as Δ1. Similarly, among the carbon concentrations A2, B2, and C2 in the region from the depth position of 0.05mm to the depth position of 0.10mm from the surface, the maximum value and the minimum value were selected, and the difference value of the carbon concentrations was defined as Δ2. Through the above steps, Δ1 to Δ6 were obtained, and the arithmetic average value of Δ1 to Δ6 was defined as "surface carbon concentration difference" (mass%). The results obtained are shown in the column "difference in surface carbon concentration (mass%)" in table 1.
Further, the total arithmetic average of the carbon concentrations A1 to A6, B1 to B6, and C1 to C6 is defined as the surface layer average carbon concentration (mass%). The results obtained are shown in the column "average surface carbon concentration (mass%)" in table 1.
[ carburization depth measurement test ]
The carbon concentration of the surface layer portion of the circumferential surface was measured using the above-mentioned upper end surface test piece. Specifically, the carbon concentration of the cross section of the upper end surface test piece at a position 20mm from the upper end surface (a cross section perpendicular to the longitudinal direction of the upper end surface test piece) was measured in the radial direction from a depth position 2mm from the surface toward the surface. Specifically, line analysis using EPMA (electron beam microanalyzer) was performed to measure the radial (depth direction) carbon concentration. Based on the measurement results, the depth of the region (hereinafter referred to as carburized depth) in which the carbon concentration was 0.40 mass% or more was obtained 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 surface test piece was defined as "0.40 mass% depth difference" (mm). The results obtained are shown in the column "0.40 mass% depth difference (mm)" of Table 1.
[ evaluation results ]
Referring to table 1, the vacuum carburization treatment method in which the difference in surface carbon concentration was 0.030 mass% or less and the difference in depth was 0.40 mass% or less was evaluated as excellent as having small carburization distribution unevenness.
Referring to table 1, in test numbers 1 to 12, in the early carburization step S1, the actual carburized gas flow rate FR is FT ta/10 Above and FT 4 The following is given. Further, in the post-carburizing step S2, the actual carburizing gas flow rate FR is fa× (t 0/t) or more and FA or less during the period of time t0 to 4t 0. Further, in the post-carburizing step S2, the actual carburizing gas flow rate FR is fa× (t 0/t) or more and 2fa× (t 0/t) or less during the period of time 4t0 to time ta. Therefore, the average carbon concentration of the surface layer is 0.680 mass% or more, the surface layer carbon concentration difference is 0.030 mass% or less, and the 0.40 mass% depth difference is 0.05mm or less. That is, carburized component has small carburization distribution unevenness.
On the other hand, in test numbers 13 and 14, as shown in FIG. 11 and Table 1, the actual carburizing gas flow rate (FA) in the early carburizing step was lower than FT ta/10 . Therefore, the average carbon concentration in the surface layer is less than 0.680 mass%, and carburization is not sufficiently performed.
In test numbers 15 to 17, as shown in FIG. 12 and Table 1, the actual carburizing gas flow rate (FA) at the beginning of carburization was FT ta/10 Above and FT 4 Hereinafter, however, the actual carburizing gas flow rate FR is gradually reduced until the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure. Therefore, the average carbon concentration in the surface layer is less than 0.680 mass%, and carburization is not sufficiently performed.
In test No. 18, as shown in fig. 13 and table 1, the time ts for gradually decreasing the actual carburizing gas flow rate FR is later than the time 4t 0. As a result, the actual carburizing gas flow rate FR after gradual decrease exceeds 2fa× (t 0/t). As a result, the difference in surface carbon concentration exceeded 0.030 mass%, and carburized distribution was large in the carburized component.
In test No. 19, as shown in fig. 14 and table 1, the actual carburizing gas flow rate FR was constant to be lower than FT ta/10 Is a value FA of (f). Therefore, the depth difference of 0.40 mass% exceeds 0.05mm, and carburization distribution of the carburized part is unevenLarge.
In test number 20, as shown in fig. 15 and table 1, the value FA of the actual carburizing gas flow rate at the start of carburization was FT ta/10 Above and FT 4 Hereinafter, however, there is a period in which the actual carburizing gas flow rate FR becomes lower than fa× (t 0/t) in the period from time 4t0 to time ta. Therefore, the average carbon concentration in the surface layer is less than 0.680 mass%, and carburization is not sufficiently performed. Further, the difference in surface carbon concentration was more than 0.030 mass%, and the difference in depth of 0.40 mass% was more than 0.05mm, so that carburization distribution was large in the carburized part.
In test No. 21, as shown in fig. 16 and table 1, the value FA of the actual carburizing gas flow rate at the start of carburization was FT ta/10 Above and FT 4 Hereinafter, however, there is a period in which the actual carburizing gas flow rate FR exceeds 2fa× (t 0/t) in the period from time 4t0 to time ta. Therefore, the difference in surface carbon concentration exceeds 0.030 mass%, and carburization distribution of the carburized component is large.
The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be modified and implemented as appropriate without departing from the scope of the present invention.
Claims (4)
1. A vacuum carburization method for performing vacuum carburization on a steel material in a vacuum carburization furnace, comprising:
a heating step of heating the steel material to a carburizing temperature;
a soaking step of soaking the steel material at the carburizing temperature after the heating step;
a carburizing step of supplying an acetylene gas, that is, a carburizing gas, into the vacuum carburizing furnace after the soaking step, and holding the steel material at the carburizing temperature;
a diffusion step of stopping the supply of the carburizing gas into the vacuum carburizing furnace after the carburizing step, and holding the steel material at the carburizing temperature; and, a step of, in the first embodiment,
A quenching step of quenching the steel material after the diffusion step,
in the above-mentioned carburizing process, a carburization step,
defining the flow rate of the carburizing gas supplied into the vacuum carburizing furnace as an actual carburizing gas flow rate,
the following flows were defined as theoretical carburizing gas flows: which is a flow rate of the carburizing gas required for adjusting a carbon concentration at a predetermined depth position from the surface of the steel material to a desired concentration, and on the premise that all the carburizing gas is used for a carburizing reaction,
the end time of the carburizing process is defined as ta,
when the initial time period in which the acetylene partial pressure becomes 0.8 times or more the hydrogen partial pressure after the start of the carburizing step is defined as t0,
the carburizing process comprises the following steps:
a partial pressure measurement step of continuously measuring the hydrogen partial pressure and the acetylene partial pressure in the atmosphere in the vacuum carburizing furnace, and determining the time t 0;
a preliminary carburizing step of starting from the carburizing step to a time t 0; and, a step of, in the first embodiment,
a post-carburizing step for starting from the time t0 to a time ta,
in the above-mentioned preliminary carburization step,
the actual carburizing gas flow rate is set to be equal to or higher than the theoretical carburizing gas flow rate at the time ta/10 and equal to or lower than the theoretical carburizing gas flow rate at the time of 4 seconds from the start of the carburizing process,
In the post-carburizing process,
the actual carburizing gas flow rate in the preliminary carburizing step is defined as FA, the FA is within + -10% of a predetermined set value, and when the time from the start of the carburizing step is defined as time t,
the actual carburizing gas flow rate in the period from the time t0 to the time 4t0 is set to beAbove and below the FA, the first antenna is,
the actual carburizing gas flow rate from the time 4t0 to the time ta is set to beAbove and->The following is given.
2. The vacuum carburization processing method according to claim 1, wherein,
in the post-carburizing process,
during the period of time 4t0 to time ta, the actual carburizing gas flow rate is reduced by the method of (A) or (B) with the lapse of time,
(A) Repeating the maintaining and reducing of the actual carburizing gas flow rate, and reducing the actual carburizing gas flow rate stepwise;
(B) The actual carburizing gas flow rate is gradually reduced with the lapse of time.
3. The vacuum carburization processing method according to claim 1 or claim 2, wherein,
the theoretical carburizing gas flow rate is determined based on diffusion simulation using a diffusion equation.
4. A method for producing a carburized component,
The method according to any one of claims 1 to 3, comprising the step of applying the vacuum carburization method to the steel material.
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| JP2005350729A (en) * | 2004-06-10 | 2005-12-22 | Ishikawajima Harima Heavy Ind Co Ltd | Vacuum carburization method |
| JP2007113045A (en) * | 2005-10-19 | 2007-05-10 | Ishikawajima Harima Heavy Ind Co Ltd | Quality control method in vacuum carburizing and vacuum carburizing furnace |
| JP2018095963A (en) * | 2017-12-07 | 2018-06-21 | 國友熱工株式会社 | Vacuum hardening method for metal work and manufacturing method for metal work using the same |
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| JP2005350729A (en) * | 2004-06-10 | 2005-12-22 | Ishikawajima Harima Heavy Ind Co Ltd | Vacuum carburization method |
| JP2007113045A (en) * | 2005-10-19 | 2007-05-10 | Ishikawajima Harima Heavy Ind Co Ltd | Quality control method in vacuum carburizing and vacuum carburizing furnace |
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