Patent Publication Number: US-7722728-B2

Title: Heat treatment method and heat treatment apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heat treatment method and a heat treatment apparatus for steel products. 
     2. Description of the Related Art 
     Atmosphere control is important in heat treatment of a steel product, and such atmosphere control is performed by controlling CP (carbon potential) in a heat treatment atmosphere. Conventionally, there is disclosed a method of stabilizing CP at a constant value by controlling a supply amount of enriched gas (C m H n  gas) based on CP during carburization heat treatment of a steel product (Japanese Patent Publication No. Hei 5-15782). There is also disclosed a method of stabilizing CP by feedback control such as proportional control, PID control or the like (Japanese Patent Application Laid-open No. 2003-013136). 
     However, in a conventional heat treatment furnace, there is a problem such that when an opening of the furnace is opened for carrying a workpiece in or out, air enters inside the furnace and decreases CP largely. Particularly, there is a problem such that when CP is feedback controlled, a control response (CP) overshoots. Furthermore, there are cases such that a control response becomes unstable to cause hunting, or take a long time to reach a target value. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a heat treatment method and a heat treatment apparatus capable of stabilizing CP inside a furnace. 
     In order to solve the above-described problems, according to the present invention, a heat treatment method for supplying transforming gas and enriched gas inside a furnace and heat treating a workpiece inside the furnace is provided, which includes the steps of: performing feedback control of carbon potential by operating a supply flow rate of the enriched gas based on carbon potential inside the furnace; stopping the feedback control at any one of before an opening of the furnace is opened, while the opening of the furnace is open, and after the opening of the furnace is closed and before an atmosphere outside the furnace begins to flow into the furnace and increasing a supply flow rate of the transforming gas from a supply flow rate thereof immediately before the feedback control is stopped; and resuming the feedback control when a furnace pressure reaches a predetermined pressure after the opening of the furnace is closed. According to such a heat treatment method, decrease or disturbance of CP inside the furnace can be prevented even when air enters the furnace due to effects of opening/closing the opening. 
     This heat treatment method may further include the step of returning, when the furnace pressure reaches the predetermined pressure after the opening of the furnace is closed, the supply flow rate of the transforming gas to the supply flow rate thereof immediately before the feedback control is stopped. Furthermore, the heat treatment method may further include the step of increasing, when the feedback control is stopped, the supply flow rate of the enriched gas from a supply flow rate thereof immediately before the feedback control is stopped. In this manner, decrease of CP can be suppressed more effectively. 
     Further, the opening may be a carry-out port for carrying out a workpiece from the furnace, and the method may further include the steps of: opening the carry-out port of the furnace in a state that an exit of an oil tank chamber provided outside the carry-out port of the furnace is closed and carrying a workpiece into the oil tank chamber; and opening the exit of the oil tank chamber after the carry-out port of the furnace is closed and carrying out the workpiece from the oil tank chamber. 
     Further, according to the present invention, a heat treatment apparatus for supplying transforming gas and enriched gas inside a furnace and heat treating a workpiece inside the furnace is provided, which includes: a first regulator for regulating an opening degree of a transforming gas flow regulating valve provided on a supply path of the transforming gas and a second regulator for regulating an opening degree of an enriched gas flow regulating valve provided on a supply path of the enriched gas; and a feedback control system including the second regulator for performing feedback control of carbon potential, in which the first regulator increases the opening degree of the transforming gas flow regulating valve at any one of before an opening of the furnace is opened, while the opening of the furnace is open, and after the opening of the furnace is closed and before an atmosphere outside the furnace begins to flow into the furnace, and decreases the opening degree of the transforming gas flow regulating valve when a furnace pressure reaches a predetermined pressure after the opening of the furnace is closed, and in which the second regulator stops the feedback control at any one of before an opening of the furnace is opened, while the opening of the furnace is open, and after the opening of the furnace is closed and before an atmosphere outside the furnace begins to flow into the furnace, and resumes the feedback control when a furnace pressure reaches a predetermined pressure after the opening of the furnace is closed. 
     Further, according to the present invention, a heat treatment apparatus for supplying transforming gas and enriched gas inside a furnace and heat treating a workpiece inside the furnace is provided, which includes: a first transforming gas supply path and a second transforming gas supply path for supplying the transforming gas inside the furnace; a first regulator for regulating opening/closing of an open/close valve provided on the second transforming gas supply path and a second regulator for regulating an opening degree of an enriched gas flow regulating valve provided on a supply path of the enriched gas; and a feedback control system including the second regulator for feedback controlling carbon potential, in which the first regulator opens the open/close valve at any one of before an opening of the furnace is opened, while the opening of the furnace is open, and after the opening of the furnace is closed and before an atmosphere outside the furnace begins to flow into the furnace, and closes the open/close valve when a furnace pressure reaches a predetermined pressure after the opening of the furnace is closed, and in which the second regulator stops the feedback control at any one of before an opening of the furnace is opened, while the opening of the furnace is open, and after the opening of the furnace is closed and before an atmosphere outside the furnace begins to flow into the furnace, and resumes the feedback control when a furnace pressure reaches a predetermined pressure after the opening of the furnace is closed. 
     In this heat treatment apparatus, the second regulator may increase the opening degree of the enriched gas flow regulating valve when the feedback control is stopped. Further, the heat treatment apparatus may further include: a second enriched gas supply path for supplying the enriched gas inside the furnace, in which the second regulator may close an open/close valve provided on the second enriched gas supply path while the feedback control is performed, and open the open/close valve provided on the second enriched gas supply path when the feedback control is stopped. 
     The opening may be a carry-out port for carrying out a workpiece from the furnace, and an oil tank chamber may be provided outside the carry-out port of the furnace. Also, the heat treatment apparatus may further include a passing port for passing a workpiece provided between a carburizing chamber and a diffusing chamber provided inside the furnace; and a shutter for closing the passing port. Accordingly, atmospheres in the carburizing chamber and the diffusing chamber can be stabilized further. 
     Precisely, the present invention is for preventing disturbance of CP occurring when the opening of the furnace is opened or closed, and preventing a conventional phenomenon such that CP decreases due to sucking in air or the like by a negative pressure generated inside the furnace when the opening is opened or closed, by increasing the supply flow rate of the transforming gas or both the supply flow rate of the transforming gas and the supply flow rate of the enriched gas according to the opening or closing of the opening. By stabilizing CP, efficiency of heat treatment such as carburization for example is improved. Furthermore, in the case of carburization treatment, it is also possible to perform carburization treatment with high efficiency by providing a shutter between the carburizing chamber and the diffusing chamber, and keeping CP appropriately inside the diffusing chamber while maintaining high CP in the carburizing chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view describing the structure of a carburization treatment apparatus; 
         FIG. 2  is a graph describing a change of a pressure inside a heat treatment furnace; 
         FIG. 3  is a graph describing a change of CP inside a carburizing chamber; 
         FIG. 4  is a graph describing a change of a supply flow rate of enriched gas to the carburizing chamber; 
         FIG. 5  is a graph describing a change of a supply flow rate of transforming gas to the carburizing chamber; 
         FIG. 6  is a schematic cross-sectional view describing the structure of a carburization treatment apparatus according to another embodiment; 
         FIG. 7  is a schematic cross-sectional view describing the structure of a carburization treatment apparatus according to another embodiment; 
         FIG. 8  is a graph showing variations of a target value of a furnace temperature and a target value of CP in experiment 1; 
         FIG. 9  is a graph showing variations of a target value of a furnace temperature and a target value of CP in comparative experiment 1; 
         FIG. 10  is a graph showing variations of a target value of a furnace temperature and a target value of CP in comparative experiment 2; 
         FIG. 11  is a graph showing variations of measured values of a furnace temperature and CP obtained in the experiment 1; 
         FIG. 12  is a graph showing variations of measured values of a furnace temperature and CP obtained in the comparative experiment 1; 
         FIG. 13  is a graph showing variations of measured values of a furnace temperature and CP obtained in the comparative experiment 2; 
         FIG. 14  is a graph showing carbon concentration distributions in a workpiece subjected to treatment of the experiment 1, a workpiece subjected to treatment of the comparative experiment 1, and a workpiece subjected to treatment of the comparative experiment 2; and 
         FIG. 15  is a chart showing ECD in a workpiece subjected to treatment of the experiment 1, a workpiece subjected to treatment of the comparative experiment 1, and a workpiece subjected to treatment of the comparative experiment 2. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. As shown in  FIG. 1 , a carburization treatment apparatus  1  as a heat treatment apparatus which implements a carburization treatment method as a heat treatment method according to the present invention has a heat treatment furnace  3  for performing heat treatment on a workpiece  2  that is a steel product. Inside the heat treatment furnace  3 , a degreasing chamber  10  as a carry-in chamber, a preheating chamber  11 , a carburizing chamber  12 , a diffusing chamber  13 , and a quenching chamber  14  are provided in this order from a front side toward a rear side (from the left to the right in  FIG. 1 ). Behind the heat treatment furnace  3 , an oil tank chamber  16  is provided. 
     At a front part of the heat treatment furnace  3 , a carry-in port  21  is provided as an opening for carrying a workpiece  2  into the degreasing chamber  10  inside the heat treatment furnace  3 , and a door  22  for opening/closing the carry-in port  21  is provided. 
     Between the degreasing chamber  10  and the preheating chamber  11 , a passing port  31  for passing a workpiece  2  is formed, and a shutter  32  for shutting the passing port  31  is provided. Between the preheating chamber  11  and the carburizing chamber  12 , a passing port  33  for passing a workpiece  2  is formed, and a shutter  34  for shutting the passing port  33  is provided. Between the carburizing chamber  12  and the diffusing chamber  13 , a passing port  35  for passing a workpiece  2  is formed, and a shutter  36  for shutting the passing port  35  is provided. Between the diffusing chamber  13  and the quenching chamber  14 , a passing port  37  for passing a workpiece  2  is formed, and a shutter  38  for shutting the passing port  37  is provided. When treating a workpiece  2  in the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 , the passing ports  31 ,  33 ,  35 , and  37  can be shut by the shutters  32 ,  34 ,  36 , and  38  respectively. It should be noted that when the passing ports  31 ,  33 ,  35 , and  37  are shut by the shutters  32 ,  34 ,  36 , and  38 , atmospheres in the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  are communicable with each other via gaps between the passing ports  31 ,  33 ,  35 ,  37  and the respective shutters  32 ,  34 ,  36 , and  38 . 
     At a rear part of the heat treatment furnace  3 , a carry-out port  41  as an opening for carrying out a workpiece  2  from the heat treatment furnace  3  and carrying it into the oil tank chamber  16  is formed, and a door  42  for opening/closing the carry-out port  41  is provided. The aforementioned oil tank chamber  16  is provided outside the carry-out port  41  and is communicable with the heat treatment furnace  3  via the carry-out port  41 . In the door  42 , a hole  42   a  is provided. 
     At a lower part of the heat treatment furnace  3 , a roller conveyor  50  for carrying a workpiece  2  from the carry-in port  21  toward the carry-out port  41  side is provided. The workpiece  2  is carried by the roller conveyor  50  to pass through the passing ports  31 ,  33 ,  35 , and  37  sequentially and is carried into and out of the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  sequentially. It should be noted that plural workpieces  2  can be carried in a line in a carrying direction of the roller conveyor  50  into the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 . 
     To the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 , transforming gas supply paths  61 ,  62 ,  63 , and  64  are connected respectively for supplying transforming gas (RX gas). The transforming gas is mainly constituted of CO (carbon monoxide) gas, H 2  (hydrogen) gas, and N 2  (nitrogen) gas, and includes a minute amount of CO 2  (carbon dioxide) and H 2 O (water). On the transforming gas supply paths  61 ,  62 ,  63 , and  64 , transforming gas flow regulating valves  71 ,  72 ,  73 , and  74  are provided respectively. Opening degrees of the transforming gas flow regulating valves  71 ,  72 ,  73 , and  74  are regulated by an output signal from a first regulator  90 . 
     Further, to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 , enriched gas supply paths  82 ,  83 , and  84  for supplying utility gas (city gas) or the like for example as enriched gas (C m H n  gas) are connected respectively. On the enriched gas supply paths  82 ,  83 , and  84 , enriched gas flow regulating valves  92 ,  93 , and  94  are provided respectively. Opening degrees of the enriched gas flow regulating valves  92 ,  93 , and  94  are regulated by an output signal from a second regulator  100 . 
     Furthermore, to the quenching chamber  14 , an air supply path  104  for supplying air is connected. On the air supply path  104 , an air flow regulating valve  105  is provided. At an upper part of the degreasing chamber  10 , an excess  106  for exhausting air is provided. At upper parts of the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 , fans  110  for stirring an atmosphere in each chamber are provided respectively, and moreover, although not being shown, heaters for heating an atmosphere in each chamber are provided respectively. Also, in the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 , oxygen (O 2 ) sensors  112 ,  113 , and  114  for measuring CP in each chamber are provided respectively. It is arranged that detected values of the respective oxygen sensors  112 ,  113 , and  114  are transferred to the second regulator  100 . 
     The regulator  100  has a function to calculate CP in each of the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  based on detected values of the oxygen sensors  112 ,  113 , and  114 , and also has a function of a PID (proportional-integral-differential) regulating meter to regulate an opening degree of each of the enriched gas flow regulating valves  92 ,  93 , and  94  based on CP in each of the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 . Specifically, the regulator  100  compares CP in each of the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  obtained by calculation with a target value thereof, obtains an operation amount for each of the enriched gas flow regulating valves  92 ,  93 , and  94  to make each CP become the target value, and sends operation signals to the enriched gas flow regulating valves  92 ,  93 , and  94 . Then, in response to the operation signals from the regulator  100 , opening degrees of the enriched gas supply flow regulating valves  92 ,  93 , and  94  are regulated respectively, thereby regulating enriched gas supply flow rates from the enriched gas supply paths  82 ,  83 , and  84  respectively. Namely, there are arranged a PID control system  122  as a feed back control system having the oxygen sensor  112 , the regulator  100  and the enriched gas flow regulating valve  92 , a PID control system  123  as a feed back control system having the oxygen sensor  113 , the regulator  100  and the enriched gas flow regulating valve  93 , and a PID control system  124  as a feed back control system having the oxygen sensor  114 , the regulator  100  and the enriched gas flow regulating valve  94 . CP in the carburizing chamber  12  is controlled by the PID control system  122 , CP in the diffusing chamber  13  is controlled by the PID control system  123 , and CP in the quenching chamber  14  is controlled by the PID control system  124 . 
     At a lower part of the oil tank chamber  16 , an oil tank  130  is provided. Also, an exit  131  for carrying out a workpiece  2  from the oil tank chamber  16  is formed, and a door  132  for opening/closing the exit  131  is provided. Further, on an upper part of the oil tank chamber  16 , an excess  133  for exhausting air and a transforming gas supply path  134  for supplying transforming gas to the oil tank chamber  16  are attached. 
     It should be noted that the atmosphere inside the heat treatment furnace  3  is exhausted via the excess  106 , flows into the oil tank chamber  16  via the hole  42   a  of the door  42  and is exhausted via the excess  133 . Further, as described above, when the passing ports  31 ,  33 ,  35  and  37  are shut by the shutters  32 ,  34 ,  36 , and  38  respectively, atmospheres in the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  are communicable with each other, and during heat treatment of a workpiece  2 , the atmosphere in the heat treatment furnace  3  flows generally from the diffusing chamber  13  through the carburizing chamber  12 , the preheating chamber  11 , and the degreasing chamber  10  sequentially to be exhausted via the excess  106 . Also, it flows from the diffusing chamber  13  to the quenching chamber  14 , flows into the oil tank chamber  16  via the hole  42   a  of the door  42 , and is exhausted via the excess  133 . In this way, atmospheres in the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  are regulated preferably. Particularly, when the shutter  36  is provided between the diffusing chamber  13  and the carburizing chamber  12 , it is possible to prevent flow of an atmosphere from the diffusing chamber  13  into the carburizing chamber  12 , thereby preventing increase of CP in the diffusing chamber  13 . Also, a furnace pressure inside the heat treatment furnace  3  can be controlled by regulating opening degrees of the excesses  106  and  133 . 
     Also, in the carburization treatment apparatus  1 , a sequencer  140  for controlling processes in the carburization treatment apparatus  1  is provided. The aforementioned regulator  90  and  100  are connected to the sequencer  140  via a network or the like. 
     Next, carburizing treatment processes of a workpiece  2  using the carburization treatment apparatus  1  constructed as above will be explained. First, the carry-in port  21  of the heat treatment furnace  3  is opened, a workpiece  2  is carried into the degreasing chamber  10 , the carry-in port  21  is closed, and degreasing treatment is performed. In the degreasing chamber  10 , the workpiece  2  is heated to approximately 80° C. Next, the passing port  31  is opened, the workpiece  2  is moved from the degreasing chamber  10  to the preheating chamber  11 , and the passing port  31  is closed. In the preheating chamber  11 , the workpiece  2  is heated to approximately 940° C. After the preheating, the passing port  33  is opened, the workpiece  2  is moved from the preheating chamber  11  to the carburizing chamber  12 , and the passing port  33  is closed. In the carburizing chamber  12 , the workpiece  2  is heated to approximately 950° C. and carburization treatment is performed for a predetermined period of time. Cp in the carburizing chamber  12  is maintained at a relatively high value, approximately 1.1% for example, by PID control. After the carburization treatment, the passing port  35  is opened, the workpiece  2  is moved from the carburizing chamber  12  to the diffusing chamber  13 , and the passing port  35  is closed. In the diffusing chamber  13 , the workpiece  2  is heated to approximately 950° C., and diffusion treatment is performed for a predetermined period of time. Cp in the diffusing chamber  13  is maintained at approximately 0.8% by PID control. After the diffusion, the passing port  37  is opened, the workpiece  2  is moved from the diffusing chamber  13  to the quenching chamber  14 , and the passing port  37  is closed. In the quenching chamber  14 , the workpiece  2  is cooled down to approximately 850° C., and quenching is performed for a predetermined period of time. CP in the quenching chamber  14  is maintained at approximately 0.7% by PID control. After the quenching, the carry-out port  41  of the heat treatment furnace  3  is opened, the workpiece  2  is carried into the oil tank chamber  16 , and the carry-out port  41  is closed. Then, in the oil tank chamber  16 , the workpiece  2  is dipped in the oil tank  130  to perform oil quenching and then pulled out of the oil tank  130 , and thereafter the exit  131  is opened to carry out the workpiece  2 . As described above, a series of treatment in the carburization treatment apparatus  1  is completed. 
     Incidentally, when the carry-in port  21  and the exit  131  of the oil tank chamber  16  are both closed while opening/closing the carry-out port  41  of the heat treatment furnace  3 , continuing the PID control of CP values can cause a problem of inefficiency in control of CP values.  FIG. 2  shows a change of a pressure inside the heat treatment furnace  3  when the carry-out port  41  is opened with the carry-in port  21  and the exit  131  being closed.  FIG. 3  and  FIG. 4  show a change of CP in the carburizing chamber  12  and a change of a supply flow rate of enriched gas from the enriched gas supply path  82  at this time, respectively. With the carry-in port  21  and the exit  131  of the oil tank chamber  16  being closed, when the carry-out port  41  of the heat treatment furnace  3  starts to open (S 1  in  FIG. 2 ), an atmosphere having a low temperature in the oil tank chamber  16  is heated up by radiant heat from the inside of the heat treatment furnace  3  and expands rapidly, thereby increasing the pressure inside the heat treatment furnace  3  as shown in  FIG. 2 . Thereafter, when the carry-out port  41  starts to close (S 2  in  FIG. 2 ), the pressure in the heat treatment furnace  3  drops rapidly. When the carry-out port  41  is closed (S 3  in  FIG. 2 ), the pressure in the heat treatment furnace  3  continues to drop, and thereafter air is sucked in from the outside of the heat treatment furnace  3 . Accordingly, as shown by a chain dashed line in  FIG. 3 , CP in the carburizing chamber  12  drops rapidly. When the PID control of the PID control system  122  is continued as it is while the CP thus drops rapidly, the enriched gas supply flow rate from the enriched gas supply path  82  is controlled to rise rapidly as shown by a chain dashed line in  FIG. 4 , and the CP in the carburizing chamber  12  overshoots as shown by the chain dashed line in  FIG. 3 . Then, a problem occurs such as making the CP unstable to cause hunting, or taking a long time to reach a target value, or the like, and thus the control cannot be done favorably. Accordingly, in this embodiment, the PID control of the PID control system  122  is stopped when the carry-out port  41  is opened/closed so as to prevent the CP from becoming unstable. In this way, as shown by a double chain dashed line in  FIG. 3 , even when the CP in the carburizing chamber  12  decreases, the CP can be made close to the target value stably. Furthermore, in this embodiment, in addition to stopping the PID control, by increasing the transforming gas supply flow rate from the transforming gas supply path  62  and the enriched gas supply flow rate from the enriched gas supply path  82 , decrease in pressure inside the carburizing chamber  12  and decrease in CP inside the carburizing chamber  12  are prevented. For the same reason, in the diffusing chamber  13  and the quenching chamber  14 , the PID control in the PID control systems  123  and  124  are stopped when the carry-out port  41  is opened/closed, and moreover, the transforming gas supply flow rates from the transforming gas supply paths  63  and  64  and the enriched gas supply flow rates from the enriched gas supply paths  83 ,  84  are increased. 
     To describe specifically, first, before the carry-out port  41  is opened, the enriched gas supply flow rates from the enriched gas supply paths  82 ,  83 , and  84  are regulated by the PID control systems  122 ,  123 , and  124  respectively, and the transforming gas supply flow rates from the transforming gas supply paths  62 ,  63 , and  64  are maintained at a constant flow rate respectively by maintaining opening degrees of the transforming gas flow regulating valves  72 ,  73 , and  74  constantly as shown in  FIG. 5 . Then, immediately before the carry-out port  41  is opened, an instruction is given from the sequencer  140  to the regulator  100  to stop the PID control and increase the opening degrees of the enriched gas flow regulating valves  92 ,  93 , and  94 , and then as shown by a solid line in  FIG. 4 , the supply flow rates from the enriched gas supply paths  82 ,  83 , and  84  are increased to a predetermined value. Also, an instruction is given from the sequencer  140  to the regulator  100  to increase the opening degrees of the transforming gas flow regulating valves  72 ,  73 , and  74 , and then as shown by a solid line in  FIG. 5 , the supply flow rates from the transforming gas supply paths  62 ,  63 , and  64  are increased to a predetermined value respectively. After a predetermined time T 1  has passed since the PID control is thus stopped and the supply flow rates of the enriched gas and the transforming gas are increased from the supply flow rates of immediately before the PID control is stopped, an instruction to open the carry-out port  41  is given from the sequencer  140  to a not-shown opening/closing drive mechanism of the door  42 . Thereafter, after a predetermined time T 2  has passed since the PID control is stopped, an instruction to resume the PID control is given from the sequencer  140  to the regulator  100 . Thus, the opening degrees of the enriched gas flow regulating valves  92 ,  93 , and  94  become close to the state before the PID control is stopped, and as shown by the solid line in  FIG. 4 , the enriched gas supply flow rates from the enriched gas supply paths  82 ,  83 , and  84  respectively decrease to be close to the state before the PID control is stopped. Also, an instruction to decrease the opening degrees of the transforming gas flow regulating valves  72 ,  73 , and  74  is given from the sequencer  140  to the regulator  90 , and as shown in  FIG. 5 , the supply flow rates of the transforming gas supply paths  62 ,  63 , and  64  return respectively to the state before the PID control is stopped. By the above method, the CP can be maintained approximately constantly as shown by a solid line in  FIG. 3 . 
     It should be noted that the predetermined time T 2  may be determined in advance so as to assure a sufficient time based on experiments. For example, an average time may be determined from passing of the predetermined time T 1  after the PID control is stopped and the supply flow rates of the enriched gas and the transforming gas are increased, through opening of the carry-out port  41 , carrying out of the workpiece  2 , closing of the carry-out port  41 , until approximating thereafter of the furnace pressure in the heat treatment furnace  3  to a predetermined pressure, for example a furnace pressure before the carry-out port  41  is opened, so as to adopt the required time thereof as the predetermined time T 2 . Specifically, it may be set such that, after the carry-out port  41  is closed and the furnace pressure returns to a predetermined pressure, for example a furnace pressure before the carry-out port  41  is opened, the PID control is resumed and the opening degrees of the transforming gas flow regulating valves  72 ,  73 , and  74  are set back. Thus, after the furnace pressure increases sufficiently and thus there is no more suction of air, the PID control can be resumed and also the supply flow rates of the transforming gas can be set back. Even when the PID control is resumed, the CP can be prevented from becoming unstable, and by increasing the supply flow rate of the transforming gas while air is sucked into the furnace, decrease in CP can be securely prevented. 
     According to such a carburization treatment apparatus  1 , by increasing the supply flow rates of the transforming gas and the enriched gas when opening the carry-out port  41  of the heat treatment furnace  3 , decrease of the furnace pressure inside the heat treatment furnace  3  can be prevented, and moreover, entrance of air into the heat treatment furnace  3  and decrease of CP inside the heat treatment furnace  3  can be prevented. By stopping the feedback control of CP when the carry-out port  41  of the heat treatment furnace  3  is opened, the CP can be prevented from becoming unstable. Stabilization of CP can be achieved easily without performing complex control setting. By stabilizing CP in the heat treatment furnace  3 , carburization treatment can be performed effectively. For example, during treatment of a workpiece  2  in the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13  or the quenching chamber  14 , when the carry-out port  41  is opened and another workpiece  2  is moved from the quenching chamber  14  to the oil tank chamber  16 , variation of CP in each of the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  can be suppressed. Therefore, respective treatment in the degreasing chamber  10 , the preheating chamber  11 , the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  can be performed favorably. Moreover, improvement in reliability of treatment effects and reduction in treating time can be achieved. 
     As above, the preferred embodiment of the present invention has been explained, but the present invention is not limited to such an example. It will be clear for those skilled in the art that various types of variation examples and modification examples may be devised within the range of the technical ideas described in the appended claims, and it will be understood that such examples belong to the technical scope of the present invention as a matter of course. 
     In the above embodiment, the method is explained in which the supply flow rates of the transforming gas and the enriched gas are increased simultaneously and decreased after the same predetermined time T 2 , but the timing to increase or decrease the supply flow rates of the transforming gas and the enriched gas is not limited to this. For example, a time T 3  to increase the enriched gas supply flow rate may be set shorter than the time T 2  to increase the transforming gas supply flow rate. An increase start time of the supply flow rate of the transforming gas and an increase start time of the supply flow rate of the enriched gas may be different from each other. 
     Also, in the above embodiment, operations such as stopping the PID control, starting of increasing the supply flow rate of the transforming gas, starting of increasing the supply flow rate of the enriched gas, and so on are performed immediately before the carry-out port  41  is opened, but these operations may be performed after the carry-out port  41  is opened, instead of before the carry-out port  41  is opened. Specifically, when these operations are performed after the carry-out port  41  is closed and before an atmosphere outside the furnace begins to flow into the heat treatment furnace  3 , it is possible to prevent decrease or disturbance of CP. For example, the above-described operations may be performed while the carry-out port  41  is open. Also, with an average time from closing of the carry-out port  41  to starting of flow of an atmosphere outside the furnace into the heat treatment furnace  3  being determined in advance, the above-described operations may be performed before this time passes. Also, the above-described operations may be performed after the carry-out port  41  is closed and before the furnace pressure inside the heat treatment furnace  3  decreases to a predetermined value. 
     Also, in the above embodiment, the supply flow rates of the transforming gas and the enriched gas are increased together, but only the supply flow rate of the transforming gas may be increased while keeping the supply flow rate of the enriched gas at the supply flow rate of immediately before the PID control is stopped. Specifically, only by stopping the PID control and increasing the supply flow rate of the transforming gas, decrease of CP accompanying opening/closing of the carry-out port  41  can be prevented sufficiently. 
     In the above-described embodiment, the supply flow rates of the transforming gas and the enriched gas are regulated by regulating the opening degrees of the transforming gas flow regulating valves  72 ,  73 , and  74  and the opening degrees of the enriched gas flow regulating valves  92 ,  93 , and  94  respectively, but with second transforming gas supply paths for supplying the transforming gas being provided in the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  respectively for example, the transforming gas supply flow rate may be increased by supplying the transforming gas from the second transforming gas supply paths only when the carry-out port  41  is opened. Similarly, with second enriched gas supply paths for supplying the enriched gas being provided in the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  respectively for example, the enriched gas supply flow rate may be increased by supplying the enriched gas from the second enriched gas supply paths only when the carry-out port  41  is opened. 
     For example, as shown in  FIG. 6 , other than the first transforming gas supply paths  62 ,  63 , and  64 , second transforming gas supply paths  152 ,  153 , and  154  for increasing the transforming gas are connected to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  respectively. In the shown example, the respective second transforming gas supply paths  152 ,  153 , and  154  are bypass circuits provided between a supply source of the transforming gas and a downstream side of the transforming gas flow regulating valves  72 ,  73 , and  74  of the transforming gas supply paths  62 ,  63 , and  64 . On the second transforming gas supply paths  152 ,  153 , and  154 , open/close valves  156 ,  157 , and  158  are provided respectively. Open/close operations of the respective open/close valves  156 ,  157 , and  158  are regulated by an output signal from the first regulator  90 ′. This first regulator  90 ′ performs operations to open the open/close valves  156 ,  157 , and  158  at any one of immediately before the carry-out port  41  is opened, while the carry-out port  41  is open, and after the carry-out port  41  is closed and before an atmosphere outside the furnace begins to flow into the heat treatment furnace  3 , and close the open/close valves  156 ,  157 , and  158  when the furnace pressure reaches a predetermined pressure after the carry-out port  41  is closed. In such an arrangement, in a normal state before the carry-out port  41  is opened, the respective open/close valves  156 ,  157 , and  158  are closed and thus the transforming gas is not supplied from the second transforming gas supply paths  152 ,  153 , and  154 , but a constant flow amount of transforming gas is supplied from the first transforming gas supply paths  62 ,  63 , and  64  respectively. Then, at any one of immediately before the carry-out port  41  is opened, while the carry-out port  41  is open, and after the carry-out port  41  is closed and before an atmosphere outside the furnace begins to flow into the heat treatment furnace  3 , an instruction to open the respective open/close valves  156 ,  157 , and  158  is given from the sequencer  140  to the first regulator  90 ′. Thus, the open/close valves  156 ,  157 , and  158  are opened, and a constant flow amount of transforming gas is supplied from the second transforming gas supply paths  152 ,  153 , and  154  to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  respectively. In other words, it is a state that in addition to the constant flow amount of the transforming gas from the first transforming gas supply paths  62 ,  63 , and  64 , the constant flow amount of the transforming gas is supplied from the second transforming gas supply paths  152 ,  153 , and  154 , which increases the supply flow rate of the transforming gas to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 . Then, after the carry-out port  41  is closed and the furnace pressure inside the heat treatment furnace  3  becomes a predetermined pressure, an instruction to close the open/close valves  156 ,  157 , and  158  is given from the sequencer  140  to the first regulator  90 ′. Thus, the open/close valves  156 ,  157 , and  158  are closed again, thereby returning to the state of supplying the transforming gas only from the first transforming gas supply paths  62 ,  63 , and  64 . In other words, the supply flow rate of the transforming gas to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  decreases and returns to the supply flow rate of immediately before the PID control is stopped. Also in this manner, the supply flow rate of the transforming gas to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  can be controlled preferably, and thus decrease of CP accompanying opening/closing of the carry-out port  41  can be prevented preferably. 
     Also, as shown in  FIG. 7  for example, other than the first enriched gas supply paths  82 ,  83 , and  84 , second enriched gas supply paths  162 ,  163 , and  164  for increasing the enriched gas are connected to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  respectively. In the shown example, the respective second enriched gas supply paths  162 ,  163 , and  164  are bypass circuits provided between a supply source of the enriched gas and a downstream side of the enriched gas flow regulating valves  92 ,  93 , and  94  of the enriched gas supply paths  82 ,  83 , and  84 . On the second enriched gas supply paths  162 ,  163 , and  164 , open/close valves  166 ,  167 , and  168  are provided respectively. Open/close operations of the respective open/close valves  166 ,  167 , and  168  are regulated by an output signal from the second regulator  100 ′. This second regulator  100 ′ performs operations to close the respective open/close valves  166 ,  167 , and  168  when PID control is performed, and open the respective valves  166 ,  167 , and  168  when the PID control is stopped. In such an arrangement, in a normal state before the carry-out port  41  is opened, the respective open/close valves  166 ,  167 , and  168  are closed and thus the enriched gas is not supplied from the second enriched gas supply paths  162 ,  163 , and  164 , but the enriched gas is supplied from the first enriched gas supply paths  82 ,  83 , and  84  respectively while being regulated based on the PID control. Then, at any one of immediately before the carry-out port  41  is opened, while the carry-out port  41  is open, and after the carry-out port  41  is closed and before an atmosphere outside the furnace begins to flow into the heat treatment furnace  3 , an instruction to open the respective open/close valves  166 ,  167 , and  168  is given from the sequencer  140  to the second regulator  100 ′ together with an instruction to stop the PID control. Thus, the open/close valves  166 ,  167 , and  168  are opened, and a constant flow amount of enriched gas is supplied from the second enriched gas supply paths  162 ,  163 , and  164  to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  respectively. In other words, it is a state that the supply flow rates from the first enriched gas supply paths  82 ,  83 , and  84  are maintained at the supply flow rates of immediately before the PID control is stopped, and in addition to this enriched gas from the first enriched gas supply paths  82 ,  83 , and  84 , the constant flow amount of the enriched gas is supplied from the second enriched gas supply paths  162 ,  163 , and  164 , which increases the supply flow rate of the enriched gas to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14 . Then, after the carry-out port  41  is closed and the furnace pressure inside the heat treatment furnace  3  becomes a predetermined pressure, an instruction to close the open/close valves  166 ,  167 , and  168  is given from the sequencer  140  to the first regulator  100 ′ together with the instruction to resume the PID control. Thus, the open/close valves  166 ,  167 , and  168  are closed again. In other words, it returns to a state that the supply flow rate of the enriched gas to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  is decreased, and the enriched gas is supplied only from the first enriched gas supply paths  82 ,  83 , and  84  while being regulated based on the PID control. Also in this manner, the supply flow rate of the enriched gas to the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  can be controlled preferably, and thus decrease of CP accompanying opening/closing of the carry-out port  41  can be prevented preferably. 
     In the above-described embodiment, the PID control is performed by the PID control systems  122 ,  123 , and  124 , but it may be arranged to control CP by any other feedback control. For example, the regulator  100  may be provided with a function of a PI (proportional-integral) regulating meter, where respective CP in the carburizing chamber  12 , the diffusing chamber  13 , and the quenching chamber  14  are each controlled by a PI control system as a feed back control system constituted of the oxygen sensor  112 ,  113 , or  114 , the regulator  100 , and the enriched gas flow regulating valves  92 ,  93 , or  94 . 
     EXAMPLE 
     The inventors of the present invention performed the following experiment 1, comparative experiment 1, and comparative experiment 2 for verifying effects of the present invention. In all of the experiment 1, the comparative experiment 1, and the comparative experiment 2, a heat treatment furnace of batch type is used, a workpiece is inserted into the furnace, and atmospheres similar to those in the carburizing chamber, the diffusing chamber and the quenching chamber in the sequential type heat treatment furnace as shown in this embodiment are realized in order, thereby treating the workpiece. Then, carbon concentration distribution near the surface of the workpiece is measured after the treatment. Note that an SS400 round bar complying with the JIS standard is used as a dummy workpiece. 
     [Experiment 1] 
     As an atmosphere similar to that in the carburizing chamber  12  of the heat treatment furnace  3 , an atmosphere in which the target value of a furnace temperature is 950° C. and the target value of CP (a measured value by electromotive force value method (oxygen sensor method), the same used below) is 1.1% was maintained for approximately 60 minutes (refer to  FIG. 8 , treatment A 1 ). Subsequently, as an atmosphere similar to that in the diffusing chamber  13 , an atmosphere in which the target value of a furnace temperature is 950° C. and the target value of CP is 0.8% was maintained for approximately 45 minutes (treatment A 2 ). Subsequently, as an atmosphere similar to that in the quenching chamber  14 , an atmosphere in which the target value of a furnace temperature is 850° C. and the target value of CP is 0.75% was maintained for approximately 30 minutes (treatment A 3 ). Note that the concentration of CO 2  in the transforming gas is 0.20%. 
     [Comparative Experiment 1] 
     In the comparative experiment 1, variation of the furnace temperature and the target value of CP is set similarly to the experiment 1, and further, as shown in  FIG. 9 , operations to decrease and return the CP intermittently are performed. Specifically, as treatment in a conventional sequential type heat treatment furnace, a phenomenon is recreated such that the CP decreases every time an opening is opened/closed for carrying in or out a workpiece. Decreasing CP is performed three times in the treatment A 1  at predetermined time periods, twice in the treatment A 2  at predetermined time periods, and twice in the treatment A 3  at predetermined time periods. Note that the time between starting of decreasing CP and returning to the original CP is approximately seven minutes for each operation. Also, decreasing CP is realized by stopping supply of the enriched gas and introducing oxygen. The concentration of CO 2  in the transforming gas is 0.20% similarly to the experiment 1. 
     [Comparative Experiment 2] 
     In the comparative experiment 2, the target value of CP in the treatment A 1  in the comparative experiment 1 is changed to 0.9% (refer to  FIG. 10 , treatment A 1 ′). Further, the concentration of CO 2  in the transforming gas is changed to 0.40%. Other conditions are the same as in the comparative experiment 1. 
     [Experimental Results and Examination] 
       FIG. 11  is a graph of measured values of furnace temperatures and CP obtained in the experiment 1.  FIG. 12  is a graph of measured values of furnace temperatures and CP obtained in the comparative experiment 1.  FIG. 13  is a graph of measured values of furnace temperatures and CP obtained in the comparative experiment 2. Note that CP inside the furnace is calculated based on detected values from the oxygen sensors. Due to effects of CH4 (methane) or the like existing inside the furnace, measured CP values in  FIG. 11 ,  FIG. 12 , and  FIG. 13  are higher than actual CP values in the furnace.  FIG. 14  shows measured values (mean values) of respective carbon concentration distributions of a workpiece treated in the experiment 1, a workpiece treated in the comparative experiment 1, and a workpiece treated in the comparative experiment 2. As is clear from  FIG. 14 , in the workpiece treated in the experiment 1, the higher carbon concentration is obtained as compared with the workpiece treated in the comparative experiment 1 and the workpiece treated in the comparative experiment 2. Also, the mean value of ECD (depth of carburization from the surface of a workpiece to a position where carbon concentration is approximately 0.4%) is 0.54 mm in the workpiece treated in the experiment 1, which is 0.49 mm in the workpiece treated in the comparative experiment 1, and thus a difference as large as 0.05 mm is generated (refer to  FIG. 15 ). From the above, it is verified that, by preventing decrease of CP during treatment, the ECD can be improved and the carburization treatment can be performed effectively. Also, in actual sequential type carburization treatment, there may be a case that the frequency of occurrence of CP decrease, namely the frequency of opening/closing the opening for carrying in or out a workpiece, is larger than in the comparative experiments 1 and 2. In such a case, it is conceivable that effects to be obtained by preventing the CP decrease during treatment becomes much larger, and thus it is inferred that the reduction in treatment time or the like can be achieved. Further, from the results of the comparative experiment 1 and the comparative experiment 2, it is also found that, by increasing the target value of CP during carburization (treatment A 1  and A 1 ′) from 0.9% to 1.1%, the carbon concentration distribution can be improved, and the ECD can be increased by approximately 0.12 mm. Therefore, it is verified that increasing CP during carburization is effective for improving treatment efficiency. 
     The present invention can be applied to a carburization treatment apparatus. 
     According to the present invention, by increasing supply flow rate of transforming gas or enriched gas when the opening of the furnace is opened, decrease of CP inside the furnace can be prevented even when air enters the furnace. By stopping feedback control of CP when the opening of the furnace is opened, the CP can be prevented from becoming unstable. Stabilization of CP can be achieved simply without performing complex control setting. By stabilizing CP inside the furnace, carburization treatment can be performed effectively. Furthermore, CP can be stabilized even in an atmosphere with high CP, and thus the carburization treatment can be performed with high efficiency.