JP2009235443A - Method of manufacturing steel with adjusted surface carbon concentration - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing steel with adjusted surface carbon concentration by which the surface carbon concentration of the steel can be easily uniformized. <P>SOLUTION: Carbon concentration adjusting gas containing the predetermined carbon compound gas component is supplied to steel, the equilibrium reaction is established between the carbon in the steel and the carbon concentration adjusting gas. By the establishment of the equilibrium reaction, in a portion where the surface carbon concentration of the steel is saturated, the decarburization reaction is generated in which the carbon compound gas component is generated from the carbon in the steel, and in a non-saturated portion, the carburization reaction is generated in which carbon is supplied into the steel from the carbon compound gas component. By the decarburization reaction and the carburization reaction reversible to each other, the surface carbon concentration of the steel is adjusted. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a steel material, and more particularly, to a method for manufacturing a steel material whose surface carbon concentration is adjusted, and more particularly to a method for manufacturing a steel material whose surface carbon concentration is adjusted by carbon concentration index (carbon potential) control.

  Regarding vacuum carburizing treatment of steel, Patent Document 1 states that “a vacuum carburizing apparatus that performs carburizing while measuring the amount of oxygen in the atmosphere gas and the thermal conductivity of the atmosphere gas in an atmosphere gas containing butane gas under reduced pressure. Is proposed.

  Similarly, Patent Document 2 states that “a steel material that is to be carburized is heated in a carburizing atmosphere in which carburizing gas is supplied under reduced pressure, and city gas or natural gas is used as the carburizing gas, and this is reduced in pressure. A vacuum carburizing method is proposed in which the introduction process, the diffusion holding process, and the exhaust process are performed as one cycle at a temperature of 850 to 1150 ° C., and this is introduced by pulsed means that repeats a plurality of cycles.

  Similarly, Patent Document 3 proposes a vacuum carburizing method that “can perform an unequal interval pulse method while maintaining the same pressure between adjacent carburizing diffusion chambers”.

Further, Patent Document 4 discloses that “a carburizing gas is introduced into a furnace maintained at a high temperature equal to or lower than a reduced pressure atmosphere and a carburizable temperature, and a diffusion process is performed on a workpiece in the furnace. Then, a diffusion treatment is performed on the workpiece in the furnace by heating in a reduced pressure atmosphere.At least in the initial stage of the diffusion treatment, a decarburizing gas is introduced into the furnace to form a surface layer of the workpiece in the furnace. A vacuum carburizing method has been proposed in which a cycle of “decarburizing treatment to reduce or remove cementite in the surface layer of the workpiece” is repeated “successively repeatedly”. Further, in Patent Document 4, as the vacuum carburizing method, carbon dioxide (CO ₂ ) as a decarburizing gas and propane gas (C ₃ H ₈ ) as a carburizing gas are placed in a furnace in which a workpiece is charged. There has been proposed a method of adjusting the surface carbon concentration by introducing a mixed gas obtained by mixing in a decarburization period.

JP 2004-59959 A JP 2004-332074 A JP 2002-115042 A JP 2004-115893 A

  As in the invention of Patent Document 1 or 2, by adjusting the surface carbon concentration of a steel product that requires corners, for example, gears, by carburizing, the surface carbon concentration of the protruding corners is the surface carbon of the flat part. It becomes higher than the concentration, and it is difficult to make the carbon concentration between the corner portion and the flat portion uniform. In addition, even if it performs pulse type carburizing process like invention of patent document 2, although the carbon concentration between a corner | angular part and a flat part can be made small, it is difficult to make uniform.

  According to the unequal interval pulse method of Patent Document 3, it is possible to adjust the carburization and diffusion process time according to the progress of carburization, but it is difficult to make the difference in carbon concentration between the corner and the flat part uniform. .

  According to the invention of Patent Document 4, since propane gas used as a carburizing gas is easily decomposed and is not easily generated, there is actually a problem that it does not reversibly react with carbon in the workpiece. That is, the problem that propane gas is decomposed and only a carburization reaction in which carbon in propane gas enters the workpiece is generated, and a decarburization reaction in which propane gas is generated from carbon in the workpiece is not generated. There is a problem that a carbon exchange reaction is not generated with carbon dioxide used as a decarburizing gas. Therefore, according to the present invention, the surface carbon concentration of the obtained steel is proportional to the processing time and the steel surface area, and it is very difficult to determine the control conditions. It is very difficult to make the surface carbon concentration uniform. In other words, according to the present invention, it is a method of low robustness that requires the control conditions to be changed every time a disturbance occurs.

  The objective of this invention is providing the manufacturing method of the steel materials by which the surface carbon concentration was adjusted which can make the surface carbon concentration of steel materials uniform easily.

  According to the knowledge of the present inventors, conventionally, when a steel workpiece having a corner portion and a flat portion is vacuum carburized, the surface carbon concentration between the corner portion and the flat portion cannot be made uniform. This is because the uniformity of the surface carbon concentration depends on the workpiece shape. Moreover, the difference of the surface carbon concentration obtained by the position which sets a workpiece | work in a furnace is also large. That is, according to the conventional vacuum carburizing method, it is not possible to control “carbon potential” (hereinafter referred to as “CP”) using a carbon exchange reaction between hydrocarbon gas and steel without depending on the workpiece shape. (CP control impossible).

  When hydrocarbon gas, typically acetylene, flows into a heating space, for example, a vacuum carburizing furnace, the hydrocarbon gas becomes carbon and hydrogen when pyrolyzed at the furnace wall other than the surface of the workpiece, for example. . When the amount of hydrocarbon gas introduced into the furnace exceeds the amount that can penetrate and diffuse into the workpiece, the pyrolyzed carbon is adsorbed on the furnace wall and becomes soot. If a soot, that is, a disturbance occurs, the uniformity of the surface carbon concentration cannot be obtained unless the control conditions are changed. In addition, if soot is generated, the maintenance work of the furnace is necessary and the life of the decompression device is shortened. Such changes in equipment over time greatly affect product quality control. Therefore, prevention of soot generation is an important issue in order to continue stable operation.

  Therefore, the present inventors sought a CP control method that can make the surface carbon concentration of steel uniform and prevent the occurrence of flaws based on the following consideration.

  According to this CP control method, the amount of carburization or decarburization per unit time is determined based on the following formula 1.

Carburization or decarburization amount per unit time (mol · cm ² · s) Difference between carbon potential of soot gas and surface carbon concentration of steel (Equation 1)

  That is, the amount of carburization or decarburization per unit time is determined only by the carbon concentration difference at the gas / steel interface, and the carbon diffusion rate in the steel depends on the physical property value of the steel. In the carburizing / diffusion process using gas, in order to adjust the surface carbon concentration of steel, it is essential to control the carbon concentration index (carbon potential (CP)) in the gas. For example, the decarburization process is unsteady. In the same phase, if there is a high or low CP, a CP gradient occurs and carbon mass transfer occurs.

  Also, assuming that the carburizing step and the decarburizing step are performed in the same furnace, the carburizing gas component such as acetylene supplied in excess of the reaction amount with the workpiece in the carburizing step is thermally decomposed, so that Adheres. The CP of the gas depends on the gas partial pressure (composition) and temperature unless soot is generated. However, when soot is generated, the carbon concentration or CP reaches the equilibrium value at the interface between the gas and the furnace inner wall. Since the CP value balanced at the gas-furnace inner wall interface is high, on the contrary, the difference between the CP of the gas and the surface carbon concentration of the steel becomes small, and the amount of carburization or decarburization per unit time becomes small. Conventionally, since there is a problem due to the occurrence of such soot, if carburization and decarburization are carried out in the same furnace, CP control becomes difficult, and eventually it becomes difficult to make the surface carbon concentration uniform. Yes.

  Therefore, the present inventors considered a method of controlling the surface carbon concentration of the carburized steel by the following CP control process.

[Carburization process]
Before describing the CP control process, the carburization of steel will be described. When carburizing steel, for example, when hydrocarbon gas, typically acetylene, is used, it is performed based on the following carburizing reaction.
C ₂ H ₂ (g) ⇔2C (inFe) + H ₂ (g)... Reaction 1

  When acetylene is used as the carburizing gas component, the speed of the carburizing reaction, that is, the normal reaction of reaction 1 on the steel surface is increased, while the decarburization reaction, that is, the reverse reaction of reaction 1 on the steel surface is unlikely to occur. This is because the thermal decomposition of acetylene easily proceeds at a temperature higher than the temperature at which carburization is performed, for example, 850 ° C. or higher.

[CP control process]
In the CP control step, a carbon concentration adjusting gas containing CO / CO + CO ₂ gas as a carbon compound gas component or a carbon concentration adjusting gas containing H ₂ / H ₂ O gas is used, and the following Boudou reaction (reaction 2a) is performed. ), Or an aqueous reaction (reaction 2b), the surface carbon concentration of the steel can be controlled.
2CO⇔C (inFe) + CO ₂ (g) (reaction 2a)
CO ₂ (g) + H ₂ (g) ⇔CO (g) + H ₂ O (g) (reaction 2b)

Reaction 2a is an equilibrium reaction between carbon in the steel and CO / CO + CO ₂ gas, and CO gas is generated from the carbon in the steel when the surface carbon concentration of the steel is saturated due to the establishment of this equilibrium reaction. The carburization reaction in which carbon is supplied from the CO into the steel is generated in the part that is not saturated, and the surface carbon concentration of the steel is adjusted by the reversible decarburization reaction and carburization reaction. . In addition, when the reaction 2a is established, it can be said that carbon exchange occurs between the CO gas and CO ₂ via carbon in the steel. The surface carbon concentration can be set by the CO / CO + CO ₂ gas partial pressure.

Reaction 2b is also an equilibrium reaction between carbon in steel, CO / CO + CO ₂ gas, and H ₂ / H ₂ O gas. By the establishment of this equilibrium reaction, the surface carbon of the steel is similar to that in reaction 2a. The density is adjusted.

  Based on the above findings, the present inventors have intensively studied and as a result, have completed the present invention.

  In the first aspect, the present invention supplies a carbon concentration adjusting gas containing a predetermined carbon compound gas component to a steel material, and establishes an equilibrium reaction between the carbon in the steel material and the carbon concentration adjusting gas, Due to the establishment of the equilibrium reaction, a decarburization reaction in which the carbon compound gas component is generated from the carbon in the steel material is generated in a portion where the surface carbon concentration of the steel material is saturated, and in the steel material in a portion where the carbon compound gas component is not saturated. There is provided a method for producing a steel material in which a carburization reaction in which carbon is supplied from the carbon compound gas component is generated, and a surface carbon concentration of the steel material is adjusted by the reversible decarburization reaction and the carburization reaction.

  According to the present invention, the adjustment of the surface carbon concentration of a steel material is performed using a reversible equilibrium reaction that is resistant to disturbances, in particular, utilizing a mutually reversible decarburization reaction and carburization reaction. Variation in surface carbon concentration or variation in surface carbon concentration among a plurality of steel material lots can be easily uniformized.

According to a preferred embodiment of the present invention, as the carbon compound gas component, a multi-component carbon compound gas, preferably a CO / CO + CO ₂ mixed gas that is carbon exchanged via the steel material is used when the equilibrium reaction is established.

According to the present invention, an inexpensive RX gas (trademark) containing a CO / CO + CO ₂ mixed gas can be practically used as the carbon concentration adjusting gas.

According to a preferred embodiment of the present invention, the surface carbon concentration of the steel material is adjusted under vacuum (vacuum or reduced pressure). According to this embodiment, even when a CO / CO + CO ₂ mixed gas containing oxygen is used as the carbon compound gas component, the grain boundary oxidation of the steel material is prevented as much as possible, and the generation of the grain boundary oxidation stops at the surface layer. The strength of the steel material is maintained at a predetermined level.

  According to a preferred embodiment of the present invention, the surface carbon concentration of the steel material is adjusted in a temperature range of 800 to 1000 ° C.

  According to a preferred embodiment of the present invention, the step of adjusting the surface carbon concentration using the equilibrium reaction is performed only once.

  In a preferred embodiment of the present invention, the carburizing gas component in the case of carburizing steel includes one or more gas components selected from acetylene, propane, ethylene, and butane.

In the step of adjusting the surface carbon concentration using the equilibrium reaction, a predetermined hydrocarbon, a halogenated carbon or the like that causes the following reaction may be added as the carbon compound gas component.
CX (g) ⇔C (inFe) + X (g)
However, X is a halogen or hydrogen.

According to a preferred embodiment of the present invention, an inert gas such as N ₂ or argon may be added in the step of adjusting the surface carbon concentration using the equilibrium reaction.

  According to a preferred embodiment of the present invention, soot generated by the carburization is removed by an oxidizing gas and / or hydrogen gas between the carburizing step and the surface carbon concentration adjusting step using the equilibrium reaction. Includes a scraping process.

  According to a preferred embodiment of the present invention, the steel material has a corner portion and a flat portion, and the carburization step causes the surface carbon concentration of the corner portion to be higher than that of the flat portion, and the equilibrium reaction is performed. By adjusting the surface carbon concentration used, the surface carbon concentration of the protrusion is adjusted within the target range.

  According to a preferred embodiment of the present invention, the steel material is carburized by a carburizing gas component before the carbon concentration adjusting gas is supplied, and the carburizing of the steel material and the supply of the carbon concentration adjusting gas are performed by: It is performed in different furnace chambers.

  According to a preferred embodiment of the present invention, the walls surrounding the carburizing chamber and the carbon concentration adjusting chamber are formed of materials having low reactivity with the carburizing gas component and the carbon concentration adjusting gas component, respectively.

  According to a preferred embodiment of the present invention, the carburizing chamber and the carbon concentration adjusting chamber are configured as a batch type or a continuous type.

  According to a preferred embodiment of the present invention, in the surface carbon concentration adjusting step using the equilibrium reaction, the partial pressure control of the carbon compound gas is performed in response to the sequential CP change of the atmosphere. Feedback control is performed on the operation of the gas flow rate controller for supplying a predetermined gas to the concentration adjusting chamber, the decompression pump, and the stirring fan in the furnace.

  According to a preferred embodiment of the present invention, the surface carbon concentration adjustment according to the present invention is performed after performing the carburizing process under a very high CP at a high temperature.

  According to a preferred embodiment of the present invention, it is possible to reduce the amount of the carbon compound gas component introduced into the atmosphere by adding an inert gas. In addition, a high CP atmosphere can be obtained by adding an inert gas. This is advantageous in the carburizing process.

  According to a preferred embodiment of the present invention, the surface carbon concentration is adjusted under a wide range of conditions from pressurization, normal pressure, reduced pressure to vacuum.

  According to a preferred embodiment of the present invention, when the carburizing step and the surface carbon concentration adjusting step are carried out in the same furnace, in order to remove the soot adhering to the furnace wall due to carburizing, the furnace wall is oxidized into the furnace. It has a structure capable of supplying a reactive gas such as gas or hydrogen gas, and the reactive gas reacts with soot by heating to remove soot.

  According to a preferred embodiment of the present invention, in order to shorten the processing time, it is preferable to use a steel material manufacturing apparatus having a furnace as described below.

  FIG. 1 is an explanatory view of a steel material manufacturing apparatus when a carburizing chamber and a surface carbon concentration adjusting chamber are the same according to a preferred embodiment of the present invention. FIG. 2 is an explanatory diagram of a steel material manufacturing apparatus when separating a carburizing chamber and a surface carbon concentration adjusting chamber according to a preferred embodiment of the present invention.

  Referring to FIG. 1, the furnace of this manufacturing apparatus has a heat insulation multiple structure, and heats the furnace inlet door 1a, the outer heat insulating material 2 of the furnace wall, the reaction gas inlet 3 into the furnace, and the inside of the furnace. Heating tube 4, inner heat insulating material 5 on the furnace wall, vacuum pump 6 for adjusting the pressure in the furnace, fan 7 for stirring the atmosphere in the furnace, pressure control in the furnace connected to the vacuum pump 6 and the fan 7 A device 8 and a heater 9 are included.

  This furnace is used batchwise in the carburizing step and the surface carbon concentration adjusting step. During carburizing, the interior of the furnace (inside the room) is maintained at a high CP in a reduced pressure atmosphere, thereby reducing the carburizing time. When adjusting the surface carbon concentration, the interior of the furnace (inside the room) is maintained in an atmosphere of low CP, for example, CP≈0, and the decarburization time is determined by the carbon diffusion time into the steel, thereby reducing the decarburization time. In this way, the processing time is greatly shortened.

The furnace wall is preferably formed from an inorganic material. For example, the furnace wall is made of SiO ₂ , Al ₂ O ₃ , ZrO ₂ , SiC, C, MgO, Cr ₂ O ₃ , CaO, Ti ₂ O, and complex oxides thereof. Preferably, it is configured. The hydrogen, carbon halide gas is substantially non-reactive with the material, for example, SiC, may be constituted by Si ₃ N _4.

  Referring to FIG. 2, the furnace of this manufacturing apparatus has the same structure as the furnace shown in FIG. 1 except that it has a furnace outlet door 1b. A plurality of the furnaces are connected and used via a work conveyance path, and one furnace is used for a carburizing process and the other furnace is used for a surface carbon concentration adjusting process. This furnace is used continuously in the carburizing step and the surface carbon concentration adjusting step.

  Regarding the furnace atmosphere (indoor atmosphere), it is preferable to supply an inert gas or a reactive gas that reacts with soot, except for the carburizing step and the surface carbon concentration adjusting step. As a combination of a carburizing furnace (carburizing chamber) and a surface carbon concentration adjusting furnace, it is preferable to select a combination that can shorten a processing time for obtaining a physical property value necessary for a workpiece. An example of the combination will be described next.

  3 (A) and 3 (B) are diagrams showing a connection example of a carburizing chamber and a surface carbon concentration adjusting chamber according to a preferred embodiment of the present invention.

  Referring to FIG. 3A, a plurality of surface carbon concentration adjusting chambers D are connected to one carburizing chamber C, and one carburizing chamber C is connected to one surface carbon concentration adjusting chamber D. Has been. Referring to FIG. 3B, a plurality of surface carbon concentration adjusting chambers D are connected to a plurality of carburizing chambers C, respectively. The conveyance path between the carburizing chamber C and the surface carbon concentration adjusting chamber D is maintained in an inert atmosphere.

  In this way, by separating the carburizing chamber C and the surface carbon concentration adjusting chamber D, even if the CP is supersaturated in the carburizing chamber C, the surface carbon concentration adjusting chamber D does not depend on the shape of the workpiece, and the surface carbon of the steel. The concentration can be controlled.

  The reaction interface existing in the furnace as described above will be described. 4 (A) to 4 (C) are explanatory diagrams of a reaction interface according to a preferred embodiment of the present invention. 4A to 4C, the reaction interface existing in the furnace includes a first interface existing between the soot / gas of the furnace wall and a gas / steel (workpiece). There is a second interface present.

  Referring to FIG. 4 (A), at the initial stage of carburizing, the more the gradient from the CP of the gas to the carbon concentration in the steel (CP gradient, concentration gradient) becomes steeper (the steel side is lower), the amount of carbon inflow into the steel increases. To do.

  Referring to FIG. 4C, on the other hand, during the decarburization period, the gradient from the steel surface carbon concentration to the gas (gas phase) CP increases due to carburization, and the gas side has a low gradient, that is, a downward slope. Otherwise, the amount of decarburization will be reduced. Also, the surface carbon concentration of steel decreases due to diffusion into the steel. Even if this diffusion occurs, in order to maintain the amount of decarburization, the CP of the gas may be lowered.

  In actual operation, when soot is deposited on the furnace wall, referring to FIG. 4B, the CP equilibrium at the first interface (between the soot / gas on the furnace wall) is in a very high state, that is, soot precipitation. Located in the area. At this time, the CP gradient of the gas becomes a downward slope from the first interface to the second interface, and the carburization amount increases, but the decarburization amount decreases. In order to eliminate this state, a reactive gas with soot may be introduced into the furnace to make the soot disappear.

  In addition, as long as the carburization process and the surface carbon concentration adjustment process are performed in the same batch furnace and carburization and diffusion are repeated in the same furnace, an increase in CP due to the reaction between the furnace wall components and the gas cannot be avoided. Invite changes over time. Therefore, preferably, the carburizing step and the surface carbon concentration adjusting step are performed independently, and the surface carbon concentration adjusting chamber is always kept in an inert atmosphere outside the processing time, and the atmosphere of the surface carbon concentration adjusting chamber is set to a low CP. maintain.

  For example, after the carburizing process in the carburizing chamber, the workpiece is transferred to the surface carbon concentration adjusting chamber in which the atmosphere is previously maintained at a low CP, so that the CP difference between gas / steel is large from the beginning of the surface carbon concentration adjustment. Is achieved, and the amount of decarburization or the decarburization rate of the excess carbon part increases. Further, by adjusting the CP by changing the gas composition and temperature, the amount of decarburization is further increased and the processing time is shortened. In this way, it becomes easy to adjust the steel material having the corners and the flat part to a uniform carbon concentration while suppressing precipitation of cementite and grain boundary oxidation.

  An embodiment of the present invention will be described below. FIG. 5 is a configuration diagram of an experimental apparatus according to an embodiment of the present invention.

Referring to FIG. 5, this experimental apparatus has a reactor core tube 11a into which a workpiece (steel) W is inserted, a pressure-controlled heating furnace 11 provided with the reactor core tube 11a, and a flow rate connected to the upstream side of the reactor core tube 11a. The control valve 12, the CO gas tank 15, the CO ₂ gas tank 16 and the inert gas tank 17 connected to the upstream side of the flow rate adjustment valve 12 through the flow meter 13 and the pressure reducing valve 14, respectively, and the reactor core tube 11 a are inserted. The thermocouple 18, the pump 20 connected to the downstream side of the core tube 11a via the pressure gauge 19, the gas analyzer 21 connected to the downstream side of the core tube 11a, the pressure gauge 19 and the gas analyzer 21 A personal computer (PC) 22 is connected.

The carbon concentration adjusting gas obtained by mixing CO, CO ₂ and inert gas can flow into the core tube 11a set in the pressure-controlled heating furnace 11 at a flow rate of 50 to 2000 cc / min. The heating temperature of the core tube 11a can be controlled between 500-1500 degreeC. The pressure inside the core tube 11a can be controlled between about 1 kPa (decompression) and about 100 kPa (normal pressure) even while the mixed gas is flowing into the tube.

  The temperature of the workpiece W during processing was measured in real time by the thermocouple 18. The product gas flowing out of the core tube 11a was qualitatively and quantitatively analyzed by a gas analyzer 21, more specifically, gas chromatograph G4000 (manufactured by GL Sciences Inc.), and changes in the gas components and temperature of the product gas with time were recorded.

Gas chromatography analysis conditions are as follows:
Detector: FID
Column length: 3m
Column temperature: 70 ° C
Carrier gas: N2, 120 kPa
Purge gas: N2, 50 ml / min
Injection volume: 3 ml.

[Experiment 1]
Observe the structure of the workpiece W1 that has been subjected only to the vacuum carburizing treatment, the workpiece W2 that has been subjected to the surface carbon concentration treatment according to the present invention after the vacuum carburizing treatment, and the workpiece W3 that has been subjected to the normal gas carburizing treatment after the vacuum carburizing treatment, and their fatigue strength. Was measured. Experimental conditions other than those described above are shown below.

・ Work W1 that has been vacuum carburized only
.. Steel material: Low carbon steel, initial carbon concentration 0.05 to 0.2%
..Carburizing gas: C ₂ H ₂ , C ₃ H ₈ , C ₄ H ₁₀
..Pipe temperature: 800-1050 ° C
..Pipe pressure: 1 kPa

-Work W2 subjected to surface carbon concentration treatment according to the present invention after vacuum carburizing treatment
.. Steel material: Low carbon steel, initial carbon concentration 0.05 to 0.2%
-As described above for carburizing-Carbon concentration adjustment gas: CO gas, CO ₂ and inert gas, CO / CO + CO ₂ gas ratio is, for example, a CP control region of 0.85 or less at normal pressure (Fig. 8 (see (A)) ・ ・ In-pipe temperature: 800-1050 ° C.
..In-pipe pressure: 1 to 100 kPa

・ Work W3 with normal gas carburizing
..Low carbon steel, initial carbon concentration 0.05 to 0.2%
・ ・ Carburization as described above ・ ・ Carburizing gas: CO 3 and CO ₂ mixed gas produced using C ₃ H8 and C ₄ H ₁₀ as raw materials ・ In-pipe temperature: 800 to 1050 ° C.
..In-pipe pressure: 100 kPa

FIG. 6 is a graph showing the results of Experiment 1 according to an example of the present invention. Referring to FIG. 6, W1 subjected only to carburization had the highest fatigue strength with no grain boundary oxidation. In W2, which was subjected to surface carbon concentration adjustment according to the present invention, that is, CP control using CO / CO + CO ₂ gas, grain boundary oxidation occurred, but the fatigue strength was at a level that had no problem in practice. In W3 subjected to normal gas carburization, grain boundary oxidation occurred to a relatively deep layer, and the fatigue strength was reduced. From the above, it was found that sufficient strength can be obtained even if the surface carbon concentration of the steel material is adjusted using a carbon compound gas component containing oxygen, that is, CO / CO + CO ₂ gas.

[Experiment 2]
For SK3 (steel material) and pureFe (pure iron), the surface carbon concentration was adjusted according to the present invention, and the transition of the surface carbon concentration was measured. Experimental conditions other than those described above are shown below.

.. Carbon concentration adjusting gas: CO gas, CO ₂ and inert gas, CO / CO ₂ gas ratio =
..Pipe temperature:
..Pipe pressure:

FIG. 7 is a graph showing the results of Test 2 according to an example of the present invention. Referring to FIG. 7, regardless of the difference in the initial carbon concentration between SK3 (steel material) and pureFe, the surface carbon concentration of both reaches the equilibrium and becomes the same after a predetermined time. This result shows that the surface carbon concentration adjustment according to the present invention is robust against disturbance, and the target uniform surface carbon concentration can be obtained by controlling the CP control, for example, the CO / CO + CO ₂ gas ratio. Is shown.

[Experiment 3]
Next, under a wide range of conditions from reduced pressure (vacuum) to normal pressure (pressurization), the CO gas partial pressure, that is, the CO / CO + CO ₂ gas ratio and the temperature in the pipe are changed to change the surface carbon according to the present invention. It was verified whether the concentration could be adjusted, and in particular, a CO gas partial pressure-atmosphere temperature region in which precipitation of cementite was prevented was examined.

  8 (A) to 8 (C) are graphs showing the results of Experiment 3 according to an example of the present invention. FIG. 8 (A) shows the pressure in the reactor core tube at normal pressure (100 kpa), and FIG. (B) is a graph showing experimental results at 10 kpa, and FIG. 8 (C) is a graph showing experimental results at 1 kpa.

In FIGS. 8A to 8C, cementite did not precipitate in the CP control region indicated by hatching. However, when the CO gas partial pressure and the atmospheric temperature were controlled in the region above the boundary line, cementite precipitated. Therefore, a preferable CO / CO + CO ₂ gas ratio is, for example, 0.75 or less at 800 ° C. and 0.85 or less at 1000 ° C. at normal pressure (100 kPa).

  As described above, the surface carbon concentration control according to the present invention can suppress the formation of cementite by controlling the CO gas partial pressure under a wide range of atmospheric conditions, for example, reduced pressure, vacuum, normal pressure, or increased pressure. I found it possible.

  The present invention is suitably applied to control of the surface carbon concentration of steel products having corner portions, for example, steel parts or steel materials having gear teeth.

It is explanatory drawing of the steel material manufacturing apparatus in case the carburizing chamber and the surface carbon concentration adjustment chamber are the same based on preferable embodiment of this invention. It is explanatory drawing of the steel material manufacturing apparatus in the case of isolate | separating the carburizing chamber and the surface carbon concentration adjustment chamber based on preferable embodiment of this invention. (A) And (B) is a figure which shows the connection example of the carburizing chamber and surface carbon concentration adjustment chamber which concern on preferable embodiment of this invention. (A)-(C) are explanatory drawings of the reaction interface in the carburizing or decarburizing which concerns on preferable embodiment of this invention. It is a block diagram of the experimental apparatus which concerns on one Example of this invention. It is a graph which shows the result of the experiment 1 which concerns on one Example of this invention. It is a graph which shows the result of the experiment 2 which concerns on one Example of this invention. (A)-(C) are graphs showing the results of Experiment 3 according to one embodiment of the present invention, where (A) is the pressure in the furnace core tube at normal pressure (100 kPa), (B) is 10 kPa, (C ) Are graphs showing experimental results at 1 kPa, respectively.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a Furnace entrance door 1b Furnace exit door 2 Outer heat insulating material 3 Reaction gas inflow port 4 Heating pipe 5 Inner heat insulating material 6 Vacuum pump 7 Fan 8 In-furnace pressure control apparatus 9 Heater 11a Furnace core tube 11 Pressure control type heating furnace 12 Flow control valve 13 Flow meter 14 Pressure reducing valve 15 CO gas tank 16 CO ₂ gas tank 17 Inert gas tank 18 Thermocouple 19 Pressure gauge 20 Pump 21 Gas analyzer 22 Personal computer (PC)
C Carburizing chamber D Surface carbon concentration adjustment chamber W, W1, W2, W3 Workpiece

Claims (6)

Supply a carbon concentration adjusting gas containing a predetermined carbon compound gas component to the steel material,
Establishing an equilibrium reaction between the carbon in the steel and the carbon concentration adjusting gas,
Due to the establishment of the equilibrium reaction, a decarburization reaction in which the carbon compound gas component is generated from the carbon in the steel material is generated in a portion where the surface carbon concentration of the steel material is saturated, and in the steel material in a portion where the carbon compound gas component is not saturated. A carburization reaction in which carbon is supplied from the carbon compound gas component is generated,
The surface carbon concentration of the steel material is adjusted by the reversible decarburization reaction and the carburization reaction,
A method for producing a steel material.   The method for producing a steel material according to claim 1, wherein the carbon compound gas component includes a multi-component carbon compound gas capable of carbon exchange via the steel material when the equilibrium reaction is established. The carbon compound gas component is CO / CO + CO ₂ gas,
The establishment of the equilibrium reaction produces the following reversible reaction,
2CO⇔C (inFe) + CO ₂ (g),
However, “inFe” is carbon in the steel material, and “g” is gas.
The manufacturing method of the steel materials of Claim 2 characterized by the above-mentioned. The carbon concentration adjusting gas includes CO / CO + CO ₂ gas, which is the carbon compound gas component, and H ₂ / H ₂ O gas,
The establishment of the equilibrium reaction produces the following reversible reaction,
2CO⇔C (inFe) + CO ₂ (g),
CO ₂ (g) + H ₂ (g) ⇔CO (g) + H ₂ O (g),
However, “inFe” is carbon on the surface of the steel material, and “g” is gas.
The manufacturing method of the steel materials of Claim 2 characterized by the above-mentioned.   The method for producing a steel material according to any one of claims 1 to 4, wherein the surface carbon concentration of the steel material is adjusted under vacuum or reduced pressure. The steel material is carburized by a carburizing gas component before the carbon concentration adjusting gas is supplied,
The method for manufacturing a steel material according to any one of claims 1 to 5, wherein the carburization of the steel material and the supply of the carbon concentration adjusting gas are performed in different furnace chambers.

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