Continuous atmosphere heat treatment furnace and heat treatment method

Abstract

To provide a continuous atmosphere heat treatment furnace which can execute annealing treatment involving recarburization reaction relative to an object to be heated while suppressing a used amount of an atmosphere gas.SOLUTION: A continuous atmosphere heat treatment furnace 1 comprises: conveyance means 41 which conveys an object W to be heated; an inlet side purge chamber 10 in which a vacuum purge in a chamber is executed; a vacuum heating chamber 14 which is provided at the downstream side in a conveyance direction of the inlet side purge chamber 10, and in which the object W to be heated is heated in a vacuum atmosphere; heat treatment chambers 18A, 18B which are provided at the downstream side in a conveyance direction of the vacuum heating chamber 14 and have in-chamber atmosphere control means for controlling an in-chamber atmosphere upon heat-treating the object W to be heated; and an outlet side purge chamber 20 which is provided at the downstream side in a conveyance direction of the heat treatment chambers, and in which the vacuum purge at the inside of the chambers and the cooling of the object W to be heated are executed. The in-chamber atmosphere control means adjusts a flow rate of the atmosphere gas to be fed into the chambers so that an index value of a carbon potential determined from a CO concentration and a CO2 concentration reaches a prescribed value.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a continuous atmospheric heat treatment furnace for continuously heat-treating an object to be heated, and a heat treatment method using the same. [Background technology]

[0002] Conventionally, annealing has been performed on steel parts (workpieces) for the purpose of improving machinability in subsequent processes, etc. As a continuous atmospheric heat treatment furnace used for annealing, for example, the one described in Patent Document 1 below is known.

[0003] In the annealing process, a reducing and carburizing gas consisting mainly of CO, H2, and N2 is used as the atmospheric gas, and the process is carried out while suppressing decarburization and oxidation. For heated objects whose surfaces have been decarburized in the previous process, a recarburization reaction based on an equilibrium reaction occurs, mitigating the effects of the previous process. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-176217 Summary of the Invention [Problem to be solved by the invention]

[0005] When annealing is performed using a continuous atmospheric heat treatment furnace, it is necessary to continuously supply atmospheric gas into the furnace to create an atmosphere suitable for the treatment, which increases the amount of gas used. In particular, if there is oxide scale on the surface of the object to be heated, the consumption of CO in the atmospheric gas is promoted, which increases the amount of gas used. It is possible to remove the oxide scale before the annealing treatment, but this would require an additional process.

[0006] The present invention has been made in light of the above circumstances and has an object to provide a continuous atmospheric heat treatment furnace and a heat treatment method that are capable of performing an annealing treatment involving a recarburization reaction on an object to be heated while reducing the amount of atmospheric gas used. [Means for solving the problem]

[0007] The continuous atmospheric heat treatment furnace according to the first aspect of the present invention is defined as follows: A continuous atmospheric heat treatment furnace for heat-treating a workpiece, comprising: A transport means for transporting the object to be heated; an inlet side purge chamber in which vacuum purging of the interior of the chamber is performed; a vacuum heating chamber provided downstream of the inlet purge chamber in a conveying direction, in which the object to be heated is heated in a vacuum atmosphere; a heat treatment chamber provided downstream in a conveying direction of the vacuum heating chamber and having an atmosphere control means for controlling an atmosphere in the chamber when the object to be heated is heat-treated; an outlet-side purge chamber provided downstream of the heat treatment chamber in a transport direction, in which vacuum purging of the interior of the chamber and cooling of the object to be heated are performed; Equipped with The indoor atmosphere control means adjusts the flow rate of the atmospheric gas supplied into the room so that the indoor atmosphere contains CO gas, CO2 gas, and 0.01 vol. % or less of hydrocarbon gas, and so that a carbon potential index value determined from the CO concentration and CO2 concentration becomes a predetermined value.

[0008] According to the continuous atmospheric heat treatment furnace of the first aspect thus defined, the heat treatment in the vacuum heating chamber in the first stage is performed in a vacuum atmosphere, so that the use of atmospheric gas during this period can be omitted. Also, the heat treatment in a vacuum atmosphere can remove oxide scale from the surface of the object to be heated. In the continuous atmospheric heat treatment furnace of the first aspect, heat treatment accompanied by a recarburization reaction is performed in the latter heat treatment chamber. Since the oxide scale is removed in the former heat treatment, it is possible to avoid an increase in the amount of gas used in the latter heat treatment due to promotion of consumption of CO in the atmospheric gas caused by a reaction with the oxide scale. Therefore, according to the continuous atmospheric heat treatment furnace of the first aspect, it is possible to perform an annealing treatment accompanied by a recarburization reaction on an object to be heated while suppressing the amount of atmospheric gas used.

[0009] Here, a vacuum pump may be connected to the heat treatment chamber so that the interior of the chamber can be depressurized (second aspect). By carrying out the heat treatment under reduced pressure, the amount of atmospheric gas used can be further reduced. Furthermore, in a conventional atmospheric heat treatment furnace, it takes time for the atmosphere to become suitable for treatment when restarting after shutdown, so it is necessary to maintain the indoor atmosphere even when no treatment is being performed. However, with the heat treatment furnace of the second aspect, the indoor atmosphere can be changed to a state suitable for treatment in a short time by degassing with a vacuum pump.

[0010] Furthermore, in the present invention, a configuration can be adopted in which a plurality of the heat treatment chambers are provided, and each heat treatment chamber has a means for controlling the temperature and / or pressure therein (third aspect). In this way, the temperature and pressure can be changed for each heat treatment chamber.

[0011] In addition, the present invention can be configured to include an inert atmosphere heating chamber between the inlet purge chamber and the vacuum heating chamber, in which the object to be heated is heated in an inert gas atmosphere (fourth aspect). In this way, the object to be heated can be quickly heated to a predetermined temperature.

[0012] In addition, in the present invention, an intermediate cooling chamber in which the object to be heated is cooled can be further provided between the vacuum heating chamber and the heat treatment chamber. In this case, the intermediate cooling chamber can be configured to include a storage treatment chamber formed as a partition therein, a circulation fan for circulating a cooling gas, and a gas cooling means including a gas cooler and a heater provided on a flow path of the cooling gas (fifth aspect). In this way, it is possible to deal with various types of annealing. In addition, since the temperature of the cooling gas is adjusted by the gas cooler and the heater, the object to be heated can be cooled at a desired cooling gradient.

[0013] Here, the intermediate cooling chamber may be configured to include the chamber atmosphere control means (sixth aspect).

[0014] The heat treatment method according to the seventh aspect of the present invention is defined as follows: The objects to be heated are stacked using a jig having a plurality of stages of spaces for accommodating objects to be heated in the vertical direction, and the objects to be heated are heat-treated using the continuous atmospheric heat treatment method according to claim 1. A vacuum furnace capable of creating a vacuum atmosphere inside is, by its nature, a round container, and the effective dimensions for accommodating objects to be heated inside are large in the height direction. According to the heat treatment method of the seventh aspect, the load capacity of objects to be heated in a heat treatment furnace (vacuum furnace) can be increased by stacking objects to be heated using a jig.

[0015] Here, the jig may be provided with an air guide plate that guides the atmospheric gas toward the space accommodating the object to be heated (eighth aspect). In this way, the temperature difference between the upper and lower objects to be heated during cooling can be kept small, allowing for more uniform cooling. [Brief description of the drawings]

[0016] [Figure 1] 1 is a diagram showing a schematic overall configuration of a continuous atmospheric heat treatment furnace according to an embodiment of the present invention. [Diagram 2] 2 is a diagram showing a configuration from an inlet side purge chamber to a vacuum heating chamber in the continuous atmospheric heat treatment furnace of FIG. 1. [Diagram 3] 2 is a diagram showing a configuration from an intermediate cooling chamber to an outlet-side purge chamber in the continuous atmospheric heat treatment furnace of FIG. 1. FIG. [Figure 4] FIG. 4 is a cross-sectional view of the intermediate cooling chamber taken from a different direction than FIG. 3. [Diagram 5] FIG. 2 is an explanatory diagram showing elements related to indoor atmosphere control and indoor pressure control in the embodiment. [Figure 6] FIG. 2 is a diagram showing an example of a heat pattern and a pressure pattern of isothermal annealing performed in the continuous atmospheric heat treatment furnace of the embodiment. [Figure 7] FIG. 2 is a diagram showing an example of a heat pattern and a pressure pattern of spheroidizing annealing performed in the continuous atmospheric heat treatment furnace of the embodiment. [Figure 8] FIG. 13 is a diagram showing a jig for stacking objects to be heated. [Figure 9] 13 is a diagram showing the configuration of an intermediate cooling chamber through which cooling gas flows laterally. FIG. [Figure 10] FIG. 13 is a diagram showing another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Next, embodiments of the present invention will be described in detail below.

[0018] Fig. 1 shows a schematic overall configuration of a continuous atmospheric heat treatment furnace according to one embodiment of the present invention. In the figure, 1 is a continuous atmospheric heat treatment furnace (hereinafter, may be simply referred to as a heat treatment furnace). In this example, a heat target W such as a steel part is continuously heat-treated while being accommodated inside a basket-shaped jig S (heat target accommodation space).

[0019] The heat treatment furnace 1 is equipped with a steel furnace body 4 having a generally cylindrical shape extending long in the left-right direction in Fig. 1. An inlet 6 for charging is formed on the left side of the furnace body 4 in the figure, and an outlet 7 for unloading is formed on the right side of the furnace body 4 in the figure. The inlet 6 and the outlet 7 are provided with doors 8 and 9, respectively, which are opened and closed by an air cylinder type opening and closing device 22.

[0020] The inside of the furnace body 4 is divided into sections, an entrance side purge chamber 10, an inert atmosphere heating chamber 12, a vacuum heating chamber 14, an intermediate cooling chamber 16, heat treatment chambers 18A and 18B, and an exit side purge chamber 20, along the direction in which the object W to be heated is transported. Each chamber is equipped with a pressure-resistant furnace shell, and the atmospheric gas in the chamber is sucked by a vacuum pump, so that each chamber can be made into a vacuum state (reduced pressure state). In addition, between each chamber, specifically between the entrance side purge chamber 10, the inert atmosphere heating chamber 12, the vacuum heating chamber 14, the intermediate cooling chamber 16, the heat treatment chambers 18A and 18B, and the exit side purge chamber 20, an air cylinder type opening and closing device 23 is provided, which drives the openings 24, 25, 26, 27, 28, and 29 provided at the openings of each chamber to open and close. The openings of each chamber can be closed airtight by these doors.

[0021] In addition, a partitioned chamber 31 is formed between each chamber. The partitioned chamber 31 airtightly isolates the area between each chamber (for example, between the inlet side purge chamber 10 and the inert atmosphere heating chamber 12), including the area where two opposing doors (for example, doors 24, 24 shown in FIG. 1) move up and down, from the outside, and prevents outside air from entering the room when the door provided at the opening of each chamber is opened.

[0022] 1, conveying rollers 41 are arranged in parallel along the conveying direction in each chamber constituting the heat treatment furnace 1. Metal rollers made of stainless steel, heat-resistant cast steel, etc. can be used as the rollers 41. In some cases, it is also possible to use rollers made of C / C composite, which has high heat resistance and little loss of strength up to about 2000°C. A plurality of rollers 41 arranged in each of the entrance purge chamber 10, the inert atmosphere heating chamber 12, the vacuum heating chamber 14, the intermediate cooling chamber 16, the heat treatment chambers 18A and 18B, and the exit purge chamber 20 respectively constitute roller groups 43, 44, 45, 46, 47, 48, and 49. Each of these roller groups is independently driven to sequentially transport the object to be heated W loaded into the entrance purge chamber 10 downstream in the transport direction (to the right in the figure).

[0023] FIG. 2 is a diagram showing the configuration of the heat treatment furnace 1 from the inlet side purge chamber 10 to the vacuum heating chamber 14. As shown in FIG. The inlet side purge chamber 10 is a section that prevents the air from entering the inert atmosphere heating chamber 12. The inlet side purge chamber 10 is connected to a degassing pipe 52 extending from a vacuum pump 51 and an N2 gas supply pipe 53 connected to an N2 gas source (not shown). When the object to be heated W is loaded from the inlet 6 and the door 8 is closed, the air in the inlet side purge chamber 10 is exhausted to the outside of the chamber through the vacuum pump 51. When the pressure is restored, N2 gas is supplied into the chamber through the pipe 53 to restore the pressure to atmospheric pressure.

[0024] 2, the inert atmosphere heating chamber 12 has a heat-resistant insulating material therein, which constitutes an insulating wall 55. The inside of the insulating wall 55 is a storage and processing chamber 56 in which the object to be heated W is stored and heat-treated, and a pipe 57 for supplying N2 gas into the storage and processing chamber 56 is connected. The accommodating and processing chamber 56 is provided with a heater 58 as a heating means, and a convection heating fan 59 that stirs and convects the N2 gas supplied into the accommodating and processing chamber 56, accelerating the temperature rise of the object to be heated W. Reference numeral 60 in the figure denotes a motor that rotates the fan 59. Further, a degassing pipe 62 extending from a vacuum pump 61 is connected to the inert atmosphere heating chamber 12, so that the pressure in the accommodation / processing chamber 56 can be reduced to a vacuum pressure (10 Pa or less).

[0025] The vacuum heating chamber 14 is a section in which the object W to be heated is heated at a predetermined temperature in a vacuum atmosphere. The vacuum heating chamber 14 has a heat-resistant insulating material therein, which constitutes an insulating wall 63. The inside of the insulating wall 63 is a storage processing chamber 64 in which the object W to be heated is stored and heat-treated. The storage processing chamber 64 is provided with a heater 67 as a heating means, and its output is controlled by a control unit (not shown). A vacuum pump 65 for reducing the pressure inside the storage processing chamber 64 to a predetermined vacuum pressure (e.g., 10 Pa) is also connected to the vacuum heating chamber 14 via piping 66. One of the advantages of heating the object W in a vacuum atmosphere is that it is possible to remove oxide scale from the surface of the object W. In this example, vacuum heating is performed prior to heat treatment in the subsequent heat treatment chambers 18A and 18B to remove oxide scale from the surface of the object W, thereby preventing the atmospheric gas from reacting with the scale and being consumed in the heat treatment chambers 18A and 18B. To enhance the scale removal effect, it is desirable to heat the object W to 750°C or higher in the vacuum heating chamber 14 in an atmosphere of 10 Pa or less.

[0026] FIG. 3 is a diagram showing a configuration from the intermediate cooling chamber 16 to the outlet side purge chamber 20 in the heat treatment furnace 1. As shown in FIG. The intermediate cooling chamber 16 is a section capable of cooling the object to be heated W to a predetermined temperature. As shown in the figure, the intermediate cooling chamber 16 has a heat-resistant insulating material therein, which constitutes an insulating wall 68, and the inside of the insulating wall 68 is a storage treatment chamber 69 that stores and heat-treats the object to be heated W. Openings 70 and 71 are provided at the top and bottom of the insulating wall 68 that forms the storage treatment chamber 69 to allow a cooling gas to pass therethrough.

[0027] FIG. 4 is a cross-sectional view of the intermediate cooling chamber 16, more specifically, a cross-sectional view in a direction perpendicular to the conveying direction. In the figure, 73 is a circulation fan that circulates the cooling gas inside the intermediate cooling chamber 16, and 74 is a motor that rotates the circulation fan 73. The circulation fan 73 is provided on the left side of the accommodating and processing chamber 69 in the figure, in the region between the furnace shell 5 and the accommodating and processing chamber 69. In addition, on the right side of the accommodating and processing chamber 69 in the figure, a partition wall 75 that closes the region between the furnace shell 5 and the accommodating and processing chamber 69 (the region on the right side in the figure) is provided. In this example, the circulation fan 73 generates a circulation flow 76 of cooling gas that includes an outward path 76a that passes from top to bottom within the accommodation processing chamber 69 and a return path 76b that passes upward between the furnace shell 5 and the accommodation processing chamber 69, as shown by the arrows in Figure 4.

[0028] A gas cooler 78 reduces the temperature of the cooling gas by heat exchange. The gas cooler 78, like the circulation fan 73, is provided on the left side of the accommodation treatment chamber 69 in the figure, between the furnace shell 5 and the accommodation treatment chamber 69, and reduces the temperature of the cooling gas that has become hot after passing through the heated object W.

[0029] Reference numeral 79 denotes a heater for heating the cooling gas, which in this embodiment is disposed directly above the containing and processing chamber 69 at a position facing the upper opening 70 . Reference numeral 80 denotes a temperature sensor that detects the temperature of the cooling gas flowing through the intermediate cooling chamber 16, and is installed at a position between the heater 79 and the object to be heated W in the gas flow path 76 to detect the temperature of the cooling gas on the secondary side of the heater 79. In this example, a control unit (not shown) connected to the temperature sensor 80 controls the output of the heater 79 so that the temperature of the cooling gas detected by the temperature sensor 80 coincides with a preset target gas temperature. The temperature sensor 80 can also be installed at a position between the object to be heated W and the gas cooler 78 in the gas flow path 76, or on the secondary side of the gas cooler 78.

[0030] The gas cooler 78 , the circulation fan 73 , the heater 79 , the temperature sensor 80 and the control unit constitute a gas cooling means in the intermediate cooling chamber 16 . In this gas cooling means, as shown in FIG. 4, the cooling gas flows downward through the opening 70 at the top of the insulating wall 68 by the rotation of the circulation fan 73, hits the high-temperature object W to cool it. At this time, the cooling gas, which has become hot by heat exchange with the object W, flows out of the opening 71 at the bottom of the insulating wall 68, flows upward through the flow path (return path 76b) between the furnace shell 5 and the accommodation treatment chamber 69, passes through the gas cooler 78, and is temporarily lowered in temperature. The cooling gas is then heated by the heater 79 and adjusted to a predetermined temperature. The cooling gas with the adjusted temperature hits the object W again to cool it. That is, in this example, not only the object W accommodated in the accommodation treatment chamber 69 is simply cooled, but also the object W can be cooled at a desired cooling gradient by controlling the temperature of the cooling gas.

[0031] Here, the atmospheric gas supplied to the intermediate cooling chamber 16 is a mixed gas containing CO gas and CO2 gas, and a recarbonization reaction can be caused in the heated object W by PF control, which controls the carbon potential index value (PF) determined by the ratio of CO2% to the square of CO% in the indoor atmospheric gas as shown in the following formula (1) so that it becomes a target value. PF=(CO%) 2 / CO2% … Formula (1)

[0032] FIG. 5 is a diagram showing elements related to indoor atmosphere control and indoor pressure control. As shown in the figure, a gas introduction pipe 82 connected to the intermediate cooling chamber 16 is connected to a CO gas supply pipe 83 and a CO2 gas supply pipe 86, respectively.

[0033] The CO gas supply pipe 83 supplies CO gas from a CO gas source 84 to the gas introduction pipe 82 and supplies the CO gas into the room via the gas introduction pipe 82, and is equipped with a flow rate control valve 85. The CO2 gas supply pipe 86 supplies CO2 gas from a CO2 gas source 87 to the gas introduction pipe 82 and supplies the gas into the room via the gas introduction pipe 82, and is equipped with a flow rate control valve 88. The CO gas supply pipe 83 and the CO2 gas supply pipe 86 are used to supply CO gas and CO2 gas into the room in PF control using a carbon potential index value (PF).

[0034] In FIG. 5, 89 is an analyzer and 90 is an atmosphere control unit. The analyzer 89 measures the CO 2 gas concentration (CO 2 %) and CO gas concentration (CO %) in the room, and sends the measurement signal to the atmosphere control unit 90. The atmosphere control unit 90 is connected to the analyzer 89 as well as the flow rate control valves 85 and 88 on the CO gas supply pipe 83 and the CO2 gas supply pipe 86. The atmosphere control unit 90 receives a measurement signal from the analyzer 89, calculates the carbon potential index value (PF) shown in the above formula (1), and further calculates a control output corresponding to the opening degree of each flow rate control valve so that the calculated actual PF value follows the reference value of the carbon potential index value selected in advance according to the room temperature in the intermediate cooling chamber 16 and the heat treatment content of the workpiece. Then, control signals corresponding to these control outputs are input to each of the flow rate control valves 85 and 88, respectively, and the flow rates of CO gas and CO2 gas introduced into the chamber are adjusted via these flow rate control valves. That is, the analyzer 89, the atmosphere control unit 90, and the flow rate control valves 85 and 88 constitute an indoor atmosphere control means in the intermediate cooling chamber 16.

[0035] Next, the indoor pressure control in the intermediate cooling chamber 16 will be described. In this example, a control for maintaining the indoor pressure at a predetermined pressure is performed separately from the above-mentioned PF control. In this case, a pressure detector 91 shown in Fig. 5, a vacuum pump 93 connected to the intermediate cooling chamber 16 via a pipe 92 (see Fig. 3), and a pressure control unit 94 are used. The pressure detector 91 detects the pressure inside the chamber and transmits a signal corresponding to the detected pressure value (detected value). The pressure control unit 94 is connected to the pressure detector 91 and the vacuum pump 93, and controls the output of the vacuum pump 93 so as to obtain a predetermined pressure inside the chamber.

[0036] The heat treatment chambers 18A and 18B are sections in which the object to be heated W is heat-treated in a reducing and carburizing gas atmosphere, and as shown in Fig. 3, have a heat-resistant insulating material inside, which constitutes an insulating wall 97. The inside of the insulating wall 97 is a storage processing chamber 98 in which the object to be heated W is stored and heat-treated. The storage processing chamber 98 is provided with a heater 99 and a fan 100 as heating means, and the output of the heater 99 is controlled by a control unit (not shown). In addition, the heat treatment chambers 18A and 18B are connected to a degassing pipe 103 extending from a vacuum pump 102, like the other processing chambers, and are capable of reducing the pressure and suctioning.

[0037] The gases supplied to the heat treatment chambers 18A and 18B are CO gas and CO2 gas. A CO gas supply pipe and a CO2 gas supply pipe are connected to the gas introduction pipes 82 connected to the heat treatment chambers 18A and 18B, respectively, so that the carbon potential index value (PF) in the room atmosphere gas can be controlled. Therefore, the amount of C in the surface layer of the heat object W treated in the heat treatment chambers 18A and 18B is adjusted by a recarbonization reaction. Note that the configuration for controlling the room atmosphere and the room pressure in the heat treatment chambers 18A and 18B is the same as that of the intermediate cooling chamber 16, so a detailed explanation is omitted here.

[0038] The outlet side purge chamber 20 is a section that prevents air from entering the heat treatment chambers 18A and 18B. The outlet side purge chamber 20 is connected to a degassing pipe 106 extending from a vacuum pump 105 and a N2 gas supply pipe 107 connected to an N2 gas source (not shown). The room is also equipped with a gas cooler 108 for cooling the ambient gas and a fan (not shown) for circulating the ambient gas, and the object to be heated W can be cooled by circulating the ambient gas (N2 gas).

[0039] Next, a series of heat treatment operations in the heat treatment furnace 1 will be described using an example in which isothermal annealing is performed as shown in Fig. 6. First, the roller group 43 is driven to load the object W to be heated into the inlet side purge chamber 10. When the door 8 is closed, the pressure is reduced using the vacuum pump 51, and the air in the chamber is discharged to the outside.

[0040] After the evacuation in the entrance purge chamber 10 is completed, the exit door 24 of the entrance purge chamber 10 and the entrance door 24 of the inert atmosphere heating chamber 12 are opened, the roller groups 43, 44 are driven to transport the object W to be heated into the inert atmosphere heating chamber 12, and the door 24 is closed. The object W transported into the inert atmosphere heating chamber 12 is rapidly heated by convection heating in an inert atmosphere at a predetermined normal pressure or pressurized state (101 to 280 kPa).

[0041] After heating in the inert atmosphere heating chamber 12 is completed, the interior of the inert atmosphere heating chamber 12 is evacuated, and under the same vacuum pressure as the vacuum heating chamber 14, the door 25 on the exit side of the inert atmosphere heating chamber 12 and the door 25 on the entrance side of the vacuum heating chamber 14 are opened, the roller groups 44, 45 are driven, the object W to be heated is transported into the vacuum heating chamber 14, and the door 25 is closed. The object W to be heated transported into the vacuum heating chamber 14 is heat-treated at a predetermined temperature (910°C) in a vacuum atmosphere (10 Pa or less). At this time, the oxide scale on the surface of the object to be heated can be removed.

[0042] Next, after the heat treatment in the vacuum heating chamber 14 is completed, the door 26 on the exit side of the vacuum heating chamber 14 and the door 26 on the entrance side of the intermediate cooling chamber 16 are opened, the roller groups 45, 46 are driven to transport the object W to be heated into the intermediate cooling chamber 16, and the door 26 is closed. In the intermediate cooling chamber 16, the object W to be heated is cooled to a target temperature (650°C). At this time, an atmospheric gas is supplied into the intermediate cooling chamber 16, and the circulation fan 73 is rotated to circulate the atmospheric gas. At this time, the temperature of the atmospheric gas hitting the object W to be heated is adjusted by the gas cooler 78 and the heater 79, so that the object W to be heated can be cooled at a desired cooling gradient (for example, 0.1 to 10°C / s).

[0043] Next, after the cooling treatment in the intermediate cooling chamber 16 is completed, the exit door 27 of the intermediate cooling chamber 16 and the entrance door 27 of the heat treatment chamber 18A are opened, the roller groups 46, 47 are driven to transport the object W to the heat treatment chamber 18A, and the door 27 is closed. In the heat treatment chamber 18A, the object W is soaked under a predetermined pressure while being maintained at 650° C., which is the isothermal holding temperature in isothermal annealing.

[0044] At this time, the carbon potential index value (PF) in the indoor ambient gas in the heat treatment chamber 18A is controlled to be a target value, so that if the surface of the workpiece W has been decarburized in the previous process, the decarburized portion is recarburized. This soaking process accompanied by the recarburization reaction is continued while moving to the heat treatment chamber 18B.

[0045] Next, after the soaking treatment in the heat treatment chamber 18B is completed, the interior of the heat treatment chamber 18B is evacuated to the same vacuum pressure as the exit side purge chamber 20, the exit side door 29 of the heat treatment chamber 18B and the entry side door 29 of the exit side purge chamber 20 are opened, the roller groups 48, 49 are driven to transport the object W to the exit side purge chamber 20, and the door 29 is closed. After the pressure is restored by the atmospheric gas (N2 gas) in the exit side purge chamber 20, the object W is cooled by circulating the atmospheric gas while being cooled by the gas cooler 108. After cooling, the door 9 is opened and the object W is carried out, completing a series of operations related to the heat treatment of the object W.

[0046] Although a series of heat treatment operations in the heat treatment furnace 1 has been described above using the case of isothermal annealing as an example, it is also possible to perform spheroidizing annealing as shown in Fig. 7. In this case, the object W to be heated is heated to 750°C in the vacuum heating chamber 14, and then slowly cooled to 660°C at a substantially constant cooling rate (15°C / h) through the intermediate cooling chamber 16 and the heat treatment chambers 18A and 18B.

[0047] As described above, in the continuous atmospheric heat treatment furnace 1 of this embodiment, the heat treatment in the vacuum heating chamber 14 in the first stage is performed in a vacuum atmosphere, so that the use of atmospheric gas during this period can be omitted. In addition, since the oxide scale on the surface of the object W can be removed by the heat treatment in a vacuum atmosphere, it is possible to avoid an increase in the amount of gas used in the subsequent heat treatment using atmospheric gas, which would be caused by the reaction with the oxide scale promoting the consumption of CO in the atmospheric gas. Therefore, according to the continuous atmospheric heat treatment furnace 1 of this embodiment, the object can be subjected to an annealing treatment accompanied by a recarburization reaction while suppressing the amount of atmospheric gas used.

[0048] In the continuous atmospheric heat treatment furnace 1 of this embodiment, the heat treatment chambers 18A and 18B are connected to a vacuum pump and are configured to be able to reduce the pressure inside the chambers, so that the amount of gas used can be reduced by performing heat treatment under reduced pressure. Furthermore, when restarting after a shutdown, the atmosphere inside the chambers can be quickly replaced with a state suitable for treatment by degassing the vacuum pump.

[0049] In addition, the continuous atmospheric heat treatment furnace 1 of this embodiment is provided with an inert atmosphere heating chamber 12 between the inlet side purge chamber 10 and the vacuum heating chamber 14, in which the object to be heated W is heated in an inert gas atmosphere, and the object to be heated W can be quickly heated to a predetermined temperature.

[0050] In addition, in the continuous atmospheric heat treatment furnace 1 of this embodiment, the intermediate cooling chamber 16 provided between the vacuum heating chamber 14 and the heat treatment chamber 18A is configured to include a gas cooling means including a circulation fan 73 for circulating the cooling gas, and a gas cooler 78 and a heater 79 provided on the cooling gas flow path, and in the intermediate cooling performed between the vacuum heating chamber 14 and the heat treatment chamber 18A, the heated object W can be cooled at a desired cooling gradient.

[0051] Next, a modification of this embodiment will be described. A vacuum furnace with a vacuum atmosphere inside is, by its characteristics, a round container as shown in Fig. 4, and the effective dimension for accommodating the object W increases in the height direction. Therefore, in order to increase the productivity of the continuous atmospheric heat treatment furnace 1 described above, it is effective to stack the object W in the vertical direction to increase the load capacity of the object W.

[0052] 8 is a diagram showing a jig for stacking objects to be heated vertically. The jig SB in the figure includes a lower container 111, a support 112 extending upward from a corner of the lower container 111, and an upper container 114 supported by the support 112, and is capable of stacking objects to be heated W vertically. The lower vessel 111 and the upper vessel 114 each have a bottom plate 120, a side plate 121 and an upper plate 123, and are generally shaped like a rectangular parallelepiped. An interior of each vessel is defined as a space 125 for accommodating an object to be heated.

[0053] Here, the upper plate 123 of the lower container 111 and the upper container 114 is an air guide plate that guides the horizontal cooling air (cooling gas) indicated by the arrow in the figure toward the heated object accommodation space 125, and is inclined obliquely upward. A gas intake port 126 that opens toward the upstream side of the horizontal cooling air is formed between the side plate 121 and the tip of the air guide plate 123. Meanwhile, a number of through holes are formed in the bottom plate 120.

[0054] According to the jig SB configured in this manner, in each of the lower container 111 and the upper container 114, horizontal cooling air can be taken in through the gas inlet 126. Then, as shown by the arrow in the figure, the direction of the cooling air that has entered the jig can be changed by the air guide plate 123 so that it flows downward through the heated object accommodation space 125.

[0055] FIG. 9 is a diagram showing the configuration of an intermediate cooling chamber through which cooling gas flows laterally. In the intermediate cooling chamber 16B shown in the figure, openings 70, 71 for passing cooling gas are provided on the left and right side walls of the accommodating and processing chamber 69. A heater 79 is disposed at a position facing the opening 70 on the left side of the figure, and a suction fan 130 is disposed so that its suction port overlaps with the opening 71 on the right side of the figure. Gas coolers 78 are disposed above and below the accommodating and processing chamber 69.

[0056] In the intermediate cooling chamber 16B configured in this manner, the suction fan 130 generates a circulating flow 76 of cooling gas passing through the storage and processing chamber 69 from left to right, as shown by the arrows in the figure, so that uniform cooling can be performed on the upper heated object W and the lower heated object W stacked by the jig SB.

[0057] Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be configured in various modified forms without departing from the spirit of the present invention. (1) FIG. 10 is a schematic diagram showing another embodiment of the continuous atmospheric heat treatment furnace of the present invention. As shown in FIG. 10(A), the continuous atmospheric heat treatment furnace of the present invention may have four treatment chambers, and the arrangement and number of the various treatment chambers can be changed as appropriate. For example, by providing a plurality of heat treatment chambers as shown in FIG. 10(B), the temperature and pressure can be changed for each heat treatment chamber in which recarburization is performed. Also, by providing an inert atmosphere heating chamber as shown in FIG. 10(C), the object to be heated can be heated quickly. Also, by providing an intermediate cooling chamber as shown in FIG. 10(D), various types of annealing can be handled. Also, since the higher the temperature, the more the recarburization reaction is promoted, it is also possible to provide a heat treatment chamber upstream of the intermediate cooling chamber in the conveying direction as shown in FIG. 10(E). (2) In addition, as the gas supplied to the heat treatment chamber of the above embodiment, it is also possible to use an RX gas containing CO, H2 and N2 as main components.

[0058] (3) In addition, the atmosphere can be used as the pressure recovery gas for the inlet purge chamber in the above embodiment. In this case, the pressure recovery for the inlet purge chamber can be performed after the object to be heated is transferred to the inert atmosphere heating chamber or the vacuum heating chamber. In addition, the atmosphere can be introduced into the inlet purge chamber through a pipe or by opening a separately provided air release valve. (4) In the intermediate cooling chamber of the above embodiment, the containing treatment chamber may be made of metal. When metal is used, the cooling gas passage can be easily formed into a complex shape and the cooling efficiency can be improved, so that the chamber can be preferably used for isothermal annealing. [Explanation of symbols]

[0059] 1. Continuous atmospheric heat treatment furnace 10 Inlet side purge chamber 12 Inert atmosphere heating chamber 14 Vacuum heating chamber 16,16B Intermediate cooling room 18A, 18B Heat treatment room 20 Outlet side purge chamber 41 Roller (conveying means) 73 Circulation Fan 78 Gas Cooler 79 Heater 123 Upper plate (air guide plate) 125 Heat storage space S,SB jig W Heated object

Claims

1. A continuous atmospheric heat treatment furnace for heat-treating a workpiece, comprising: A transport means for transporting the object to be heated; an inlet side purge chamber in which vacuum purging of the interior of the chamber is performed; a vacuum heating chamber provided downstream of the inlet purge chamber in a conveying direction, in which the object to be heated is heated in a vacuum atmosphere; a heat treatment chamber provided downstream in a conveying direction of the vacuum heating chamber and having an atmosphere control means for controlling an atmosphere in the chamber when the object to be heated is heat-treated; an outlet-side purge chamber provided downstream of the heat treatment chamber in a transport direction, in which vacuum purging of the interior of the chamber and cooling of the object to be heated are performed; Equipped with The indoor atmosphere control means controls the indoor atmosphere to be controlled by CO gas, CO 2 gas, and 0.01% by volume or less of hydrocarbon gas, and 2 A continuous atmospheric heat treatment furnace that adjusts the flow rate of the atmospheric gas supplied into the chamber so that the carbon potential index value, determined from the concentration, becomes a specified value.

2. 2. The continuous atmospheric heat treatment furnace according to claim 1, wherein a vacuum pump is connected to the heat treatment chamber, and the interior of the chamber can be depressurized by the vacuum pump.

3. 2. The continuous atmospheric heat treatment furnace according to claim 1, wherein a plurality of said heat treatment chambers are provided, and each heat treatment chamber has a means for controlling an internal temperature and / or an internal pressure.

4. 2. The continuous atmospheric heat treatment furnace according to claim 1, further comprising an inert atmosphere heating chamber provided between the inlet side purge chamber and the vacuum heating chamber, for heating the object to be heated in an inert gas atmosphere.

5. 2. The continuous atmospheric heat treatment furnace according to claim 1, further comprising an intermediate cooling chamber between the vacuum heating chamber and the heat treatment chamber in which the object to be heated is cooled, the intermediate cooling chamber having a storage treatment chamber partitioned therein, and a gas cooling means including a circulation fan for circulating a cooling gas, and a gas cooler and a heater provided on a flow path of the cooling gas.

6. 6. The continuous atmospheric heat treatment furnace according to claim 5, wherein said intermediate cooling chamber is provided with said chamber atmosphere control means.

7. 2. A heat treatment method comprising stacking the objects to be heated using a jig having a plurality of stages of spaces for accommodating the objects to be heated in the vertical direction, and heat treating the objects to be heated using the continuous atmospheric heat treatment furnace according to claim 1.

8. The heat treatment method according to claim 7 , wherein the jig is provided with an air guide plate for guiding the atmospheric gas toward the space accommodating the object to be heated.

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