Abstract:
The invention provides a carburization treatment method in which a carburization treatment is conducted simultaneously with an operation of supplying a hydrocarbon gas and an oxidative gas into a furnace kept under a reduced pressure. Preferably, the internal pressure within the furnace is kept at 0.1 to 101 kPa, the hydrocarbon gas is one, two or more than two kinds of gases selected from the group consisting of C 3 H 8 , C 3 H 6 , C 4 H 10 , C 2 H 2 , C 2 H 4 , C 2 H 6  and CH 4 , while the oxidative gas is an air, an O 2  gas, or CO 2  gas.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to carburization treatment methods for carburizing steel material and a carburization treatment apparatus suitable for carrying out the carburization treatment methods.  
         [0003]     2. Description of the Related Art  
         [0004]     Various methods are known for carburizing steel material, such as a gas carburization method, a vacuum carburization method, and a plasma carburization method, with each having both advantages and disadvantages.  
         [0005]     However, one gas carburization method has a disadvantage of the generation of a large amount of CO 2  gas and a possibility of an explosion. A further problem associated with this method is that intergranular oxidation will occur on the surface of the steel material. On the other hand, another gas carburization method using an endothermic gas makes it necessary to employ a metamorphism furnace, hence suffering from a problem of high equipment cost.  
         [0006]     A vacuum carburization method is associated with a problem in that once the carbon concentration on the surface of a steel material is increased to a predetermined solid solubility, a large amount of soot will be undesirably generated. As a result, not only does the carburization equipment need a comparatively long time and a considerably high cost for maintenance, but also such equipment does not have sufficient versatility. Moreover, another problem associated with this method is that it is difficult to perform a carbon potential control in an atmosphere within the furnace, if compared with the above-described gas carburization methods. In addition, a plasma carburization method is said to be low in productivity.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is an object of the present invention to provide improved, new and economical carburization treatment methods which can be effectively used to replace any one of the above-described conventional carburization methods. It is another object of the present invention to provide an improved carburization treatment apparatus which is suitable for carrying out the carburization treatment methods provided according to the present invention.  
         [0008]     In order to achieve the above objects of the present invention, a carburization treatment method according to the present invention comprises performing the carburization treatment while supplying a hydrocarbon gas and an oxidative gas into a furnace kept at a reduced pressure.  
         [0009]     With the use of the present invention, since it is possible to dispense with an exhaust gas burning process (which was needed in the above-described conventional gas carburization method), the CO 2  gas generation amount can be reduced so as to reduce an explosion possibility. Further, since it is not necessary to employ a metamorphism furnace, the amount of gas necessary to be used in the carburization treatment can be reduced, thereby rendering the whole process of carburization treatment more economical. Moreover, different from the above-described vacuum carburization method, since the method of the invention makes it possible to supply not only the hydrocarbon gas but also an oxidative gas, and since it is possible to control the carbon potential of the atmosphere within the furnace, the generation of soot can be prevented, thereby rendering easier the maintenance of the furnace.  
         [0010]     As a preferred embodiment of the present invention, the carburization treatment is conducted while supplying a hydrocarbon gas and an oxidative gas, and an inert gas is further supplied during the carburization treatment. With the use of this method, it is possible to increase the gas amount within the furnace, thereby making it possible to ensure a uniform temperature rise and thus a uniform carburization treatment.  
         [0011]     Further, as another embodiment of the present invention, it is preferable that the internal pressure within the furnace is 0.1 to 101 kPa. In other words, if the internal pressure within the furnace is lower than 0.1 kPa, it is impossible to ensure a desired carburization capability. On the other hand, if the internal pressure within the furnace is larger than 101 kPa, since such an internal pressure is generally close to atmospheric pressure, a problem will be caused which is similar to that associated with the above-described conventional gas carburization method.  
         [0012]     Furthermore, in the above-described method according to the present invention, the hydrocarbon gas may be at least one selected from the group consisting of C 3 H 8 , C 3 H 6 , C 4 H 10 , C 2 H 2 , C 2 H 4 , C 2 H 6  and CH 4 , while the oxidative gas may be air, O 2  gas or CO 2  gas.  
         [0013]     Moreover, in the method according to the present invention, a carbon potential of the atmosphere within the furnace is controlled by controlling the amount of at least one of the hydrocarbon gas and the oxidative gas. At this time, the amount of at least one of the hydrocarbon gas and the oxidative gas is controlled by carrying out at least one of the following measurements which include: measurement of CO gas partial pressure, measurement of CO gas concentration, measurement of CO 2  gas partial pressure, measurement of CO 2  gas concentration, measurement of O 2  gas partial pressure, measurement of O 2  gas concentration, measurement of H 2  gas partial pressure, measurement of H 2  gas concentration, measurement of CH 4  gas partial pressure, measurement of CH 4  gas concentration, measurement of H 2 O partial pressure, measurement of H 2 O concentration, and measurement of a dew point, all within the furnace.  
         [0014]     On the other hand, a carburization treatment apparatus according to the present invention comprises a hydrocarbon gas supply unit for supplying a hydrocarbon gas into a furnace; an oxidative gas supply unit for supplying an oxidative gas into the furnace; and a vacuum pump for reducing the internal pressure within the furnace. With the use of the carburization treatment apparatus according to the present invention, it is possible to carry out the above-described method of the present invention with a high efficiency. In contrast, a conventional gas carburization furnace is not associated with the use of a vacuum pump, and a conventional vacuum carburization furnace does not contain an oxidative gas supply unit since it is not needed.  
         [0015]     The above carburization treatment apparatus further comprises an in-furnace atmosphere analyser for analysing the atmosphere within the furnace, and a pressure gauge to control the internal pressure within the furnace. With the use of such a carburization treatment apparatus, it is possible to correctly control the atmosphere within the furnace, and also to control and thus reduce the internal pressure within the furnace, thereby rendering it possible to more effectively carry out the above-described method of the present invention.  
         [0016]     In addition, the above-described carburization treatment apparatus further comprises a computing device for computing a carbon potential in accordance with an analysis value fed from the in-furnace atmosphere analyzer, a regulation device for regulating the amount of at least one of the hydrocarbon gas and the oxidative gas in accordance with the computed values fed from the computing device, and a thermo-couple for controlling the internal temperature within the furnace. With the use of this carburization treatment apparatus, it is possible to automatically supply the hydrocarbon gas and/or the oxidative gas into the furnace, and it is also possible to control the internal temperature of the furnace.  
         [0017]     Moreover, in the above-described carburization treatment apparatus, the in-furnace atmosphere analyzer is at least one of the following gauges and meters including CO gas partial pressure gauge, CO gas concentration meter, CO 2  gas partial pressure gauge, CO 2  gas concentration meter, O 2  gas partial pressure gauge, O 2  gas concentration meter, H 2  gas partial pressure gauge, H 2  gas concentration meter, CH 4  gas partial pressure gauge, CH 4  gas concentration meter and dew point hygrometer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is an explanatory view showing a carburization furnace suitable for carrying out the carburization treatment method according to the present invention.  
         [0019]      FIG. 2  is a plan view showing the structure of a carburization quenching apparatus suitable for carrying out the carburization treatment method according to the present invention.  
         [0020]      FIG. 3  is a graph showing an average carbon concentration distribution of a steel material treated in Example 1.  
         [0021]      FIG. 4  is a photograph showing the surface organization of the steel material treated in Example 1.  
         [0022]      FIG. 5  is a graph showing an average carbon concentration distribution of a steel material treated in Example 2.  
         [0023]      FIG. 6  is a photograph showing the surface organization of the steel material treated in Example 2.  
         [0024]      FIG. 7  is also a photograph but showing the crystal particles of the steel material treated in Example 2. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Referring to  FIG. 1 , reference numeral  1  represents a furnace casing, reference numeral  2  represents a thermally insulating material, reference numeral  3  represents an atmosphere stirring fan, reference numeral  4  represents a heater, reference numeral  5  represents a thermal couple for measuring an internal temperature within the furnace, reference numeral  6  represents a pressure gauge for use in controlling and reducing an internal pressure within the furnace, reference numeral  7  represents a sampling device for sampling an atmosphere within the furnace, reference numeral  8  represents an analyzer for analyzing an atmosphere within the furnace, such an analyzer may be a CO gas partial pressure gauge or a CO gas concentration meter. Reference numeral  9  represents an analyzer for analyzing an atmosphere within the furnace, but such an analyzer may be a CO 2  gas partial pressure gauge or a CO 2  gas concentration meter. Reference numeral  30  represents a further analyzer for analyzing an atmosphere within the furnace, such an analyzer may be an O 2  gas partial pressure gauge or an O 2  gas concentration meter. Reference numeral  10  represents a mass flow controller provided in connection with a hydrocarbon gas supply unit  10   a  for controlling an amount of hydrocarbon gas to be supplied to the furnace. Reference numeral  11  represents another mass flow controller provided in connection with an oxidative gas supply unit  11   a  for controlling an amount of an oxidative gas to be supplied to the furnace. Reference numeral  12  represents a vacuum pump for reducing an internal pressure within the furnace. Reference numeral  13  represents a carbon potential computing device, reference numeral  14  represents a regulation device for sending regulation signals to the mass flow controllers  10  and  11  in accordance with the computed values fed from the carbon potential computing device  13 . Here, the thermally insulating material  2  is preferably made of a ceramic fiber having a low heat radiation and a low heat accumulation.  
         [0026]     With regard to the aforementioned carburization furnace having the above-described construction, the pressure reduction adjustment within the furnace can be carried out by controlling the discharge of an atmosphere from the furnace, by virtue of the pressure gauge  6  and the vacuum pump  12 . Further, the carbon potential of an atmosphere within the furnace may be controlled in a manner described as follows, so that it is possible to maintain a high carbon potential which is slightly below a carbon solid solubility. At this time, the analysis values fed from the internal atmosphere analyzers  8 ,  9  and  30  are introduced into the carbon potential computing device  13 . Then, the adjustment gauge  14 , in accordance with the computed values provided by the carbon potential computing device  13 , operates to send an adjustment signal to the mass flow controller  10  (for controlling the hydrocarbon gas supply amount) as well as to the mass flow controller  11  (for controlling the oxidative gas supply amount). In this way, it is possible to adjust an amount of at least one of the hydrocarbon gas and the oxidative gas being supplied into the furnace, thereby effectively controlling the carbon potential of an atmosphere within the furnace.  
         [0027]     The control of an amount of the hydrocarbon gas and/or the oxidative gas being supplied into the furnace may be effected by measuring the partial pressure of at least one of various kinds of gases forming an atmosphere within the furnace. However, it is also possible to perform the same control by measuring the concentration of at least one of various kinds of gases forming the atmosphere within the furnace. For example, it is possible to measure the partial pressure or the concentration of at least one of CO gas, CO 2  gas, O 2  gas, H 2  gas and CH 4  gas (together forming an atmosphere within the furnace), by utilizing various partial pressure gauges (CO gas partial pressure gauge, CO 2  gas partial pressure gauge, O 2  gas partial pressure gauge, H 2  gas partial pressure gas and CH 4  gas partial pressure gas) or various concentration meters (CO gas concentration meter, CO 2  gas concentration meter, O 2  gas concentration meter, H 2  gas concentration meter and CH 4  gas concentration meter), thereby effecting correct control of the supply amount of the hydrocarbon gas and/or the oxidative gas when being supplied into the furnace.  
         [0028]     Furthermore, it is possible to control an amount of the hydrocarbon gas and/or the oxidative gas being supplied into the furnace, by measuring the partial pressure of H 2 O or the concentration of H 2 O within the furnace, or by measuring the dew point of an atmosphere gas within the furnace using a dew point hygrometer.  
         [0029]     In this way, with the use of the various methods as described in the above, it is possible to correctly control an amount of the hydrocarbon gas and/or the oxidative gas being supplied into the furnace, thereby making it possible to keep an atmosphere within the furnace at a high carbon potential which is slightly below the carbon solid solubility.  
         [0030]     Referring to  FIG. 2 , reference numeral  15  represents an inlet door, reference number  16  represents a transportation room, reference numeral  17  represents a carburization room, reference numeral  18  represents a gas cooling room, reference numeral  19  represents an oil quenching room, reference numeral  20  represents an outlet door, while reference numerals  21   a ,  21   b  and  21   c  all represent partition doors. Here, the carburization room  17  is identical to the carburization room in the carburization furnace shown in  FIG. 1 .  
         [0031]     An initial state of the carburization quenching apparatus will be described as follows. Namely, the inlet door  15 , the outlet door  20  and the partition doors  21   a ,  21   b  and  21   c  are all closed. The carburization room  17  is heated to a quenching temperature and then kept at this temperature, while the pressure within the carburization room is controlled at 0.1 kPa or lower. Similarly, the pressure within the quenching room  19  is also kept at 0.1 kPa or lower, while the quenching oil within the quenching room  19  is heated to a temperature suitable for steel material quenching treatment. At this time, the transportation room  16  is under atmospheric pressure.  
         [0032]     Starting from the above-described initial state, at first, the inlet door  15  is opened so that steel material is introduced into the transportation room  16 . Then, the inlet door  15  is closed and the pressure within the transportation room  16  is reduced to 0.1 kPa or lower. Subsequently, the partition door  21   a  located between the transportation room  16  and the carburization room  17  is opened so that the steel material is moved to the carburization room  17 . Then, the partition wall  21  is closed. On the other hand, although not shown in the drawings, an apparatus for transporting the steel material may be a chain device (for use in the transportation room  16  as well as in the oil quenching room  19  and driven by a motor, and may also be a roller hearth for use in the carburization room  17 ).  
         [0033]     Then, after the partition door  21   a  is closed, the pressure within the carburization room  17  recovers to a predetermined pressure such as 100 kPa by virtue of N 2  gas, while the temperature within the carburization room is elevated to the carburization temperature. Subsequently, after the carburization room has been kept at the carburization temperature for 30 minutes, N 2  gas is discharged from the carburization room  17 , so that the pressure within the carburization room  17  is reduced to 0.1 kPa or lower.  
         [0034]     Afterwards, a predetermined amount of hydrocarbon gas and a predetermined amount of oxidative gas are supplied to the carburization room  17  by way of a purge line, so that an internal pressure within the carburization room  17  is allowed to be restored to its carburization pressure. Upon pressure restoration and based on the computation result obtained by processing the data representing the measured CO 2  partial pressure or CO 2  concentration, the carburization room  17  is allowed to control, with the use of a control line, the supply amount of at least one of the hydrocarbon gas and the oxidative gas. However, at this time, the carbon potential is set with reference to a carbon solid solubility which depends on a carburization temperature, so that such a carbon potential will be within a predetermined range so as not to produce soot.  
         [0035]     After having performed the carburization treatment for a predetermined time period, the supply of the hydrocarbon gas as well as the oxidative gas to the carburization room  17  is stopped, and the atmosphere within the carburization room  17  is discharged so as to have the steel material kept under a reduced pressure, thereby adjusting the carbon concentration on the surface of the steel material. Then, the temperature within the carburization room  17  is lowered to the quenching temperature, and the partition door  21   a  is opened. Further, the partition door  21   c  located between the transportation room  16  and the quenching room  19  is opened, so that the steel material is transferred, under a reduced pressure, to the quenching room  19  by way of the transportation room  16 , thereby performing an oil quenching treatment. After the quenching treatment, the steel material is taken out of the treatment system by way of the outlet door  20 . At this moment, an adjustment of the carbon concentration on the surface of the steel material is allowed to be performed, and at the same time a control of the quenching temperature is carried out.  
         [0036]     Furthermore, in the case of a high temperature carburization treatment (1050° C.) which requires an adjustment of crystal particles, after an adjustment has been performed on the carbon concentration on the surface of the treated steel material, the steel material is transported to the gas cooling room  18  by way of the transportation room  16  as well as the partition door  21   b . Then, after the pressure has been restored to a predetermined value (for example, 100 kPa) by means of N 2  gas, the steel material is cooled and the N 2  gas is discharged, so that the pressure over the steel material is reduced to 1 kPa or lower. In this way, under a reduced pressure and by way of the transportation room  16 , the steel material is returned to the carburization room  17  so as to be heated again to a temperature suitable for a reheating treatment. Moreover, the carburization room  17  is kept at the reheating temperature for 30 minutes. Then, the N 2  gas is discharged so that the pressure within the carburization room is reduced to 1 kPa or lower. Subsequently, the steel material is transported to the quenching room  19  by way of the transportation room  16 , thereby performing an oil quenching treatment. In this way, after the quenching treatment has been finished, the steel material is taken out of the treatment system by way of the outlet door  20 .  
         [0037]     In fact, the inventors of the present invention have conducted the carburization treatment using the method of the present invention, with an actual process and results thereof being discussed in the following.  
       EXAMPLE 1  
       [0038]     Sections of steel material SCM 420 in the form of test pieces each having a diameter of 20 mm and a length of 40 mm were disposed at nine positions (upper and lower corner portions as well as in the central area) within the carburization room  17  whose internal temperature was controlled at 950° C. and whose internal pressure was controlled at 0.1 kPa or lower. Then, the pressure within the carburization room  17  was restored to 100 kPa by charging the room with N 2  gas, while the internal temperature thereof was kept at 950° C.  
         [0039]     After the carburization room  17  had been kept under the above-described conditions for 30 minutes, its internal pressure was reduced to 0.1 kPa by virtue of gas discharge. Subsequently, C 3 H 8  gas and CO 2  gas were supplied into the carburization room  17 , each at a flow rate of 3.5 L/min so as to increase the internal pressure to 1.7 kPa.  
         [0040]     Next, with the internal pressure of the carburization room  17  kept at 1.7 kPa, the amount of C 3 H 8  gas and/or CO 2  gas being supplied to the carburization room was changed so as to control the carbon potential to 1.25%. Then, the interior of the carburization room  17  was kept at 950° C. for 57 minutes.  
         [0041]     Subsequently, the supply of C 3 H 8  gas and/or CO 2  gas was stopped and the internal pressure within the carburization room  17  was reduced to 0.1 kPa by virtue of gas discharge. Then, this internal pressure was kept for 37 minutes, while the internal temperature of the carburization room  17  was lowered to 870° C. during a subsequent time period of 30 minutes. Then, the steel material was transported to the quenching room  19  by way of the transportation room  16 , thereby starting the oil quenching treatment.  
         [0042]     The average carbon concentration distribution of the steel material treated in this example is shown in  FIG. 3 . In fact, the carbon concentrations shown in this graph represent the average values of the carbon concentrations of the steel material pieces located at the aforementioned nine positions. As a result, an effective carburization depth (0.36% C) could be found to be 0.7 mm, which was an appropriate value. Further, a photograph representing the surface organization of the treated steel material is shown in  FIG. 4 . It is to be noted that there were no abnormal layers formed on the surface of the steel material treated in the above described process.  
         [0043]     When a carburization lead time of the carburization treatment in Example 1 was compared with a carburization lead time of the gas carburization treatment (which is a conventional process) using an endothermic gas, it was found that the conventional gas carburization treatment using an endothermic gas needed 118 minutes as its carburization lead time, while the carburization lead time of the carburization treatment in Example 1 was only 94 minutes, thus making it possible to shorten the carburization lead time by about 20%. In this way, using the carburization treatment method actually carried out in Example 1, it becomes possible to obtain a carburized layer having a desired depth using a shorter time period than required by the above described conventional gas carburization treatment (which requires the use of an endothermic gas). Therefore, the total energy consumption can be reduced and thus the desired economic advantage can be achieved. Moreover, since there is no soot being generated, the pieces of steel material can be placed at any position within the furnace without any limitation. In addition, the use of the present invention makes it possible to obtain carburized layers which are relatively uniform and differ little from each other in their physical and chemical properties.  
       EXAMPLE 2  
       [0044]     Example 2 is used to explain how a high temperature carburization can be carried out. Namely, sections of steel material pieces which were identical to those used in Example 1 were disposed at nine positions within the carburization room  17  whose internal temperature was controlled at 1050° C. and whose internal pressure was controlled at 0.1 kPa or lower. Then, the pressure within the carburization room  17  was restored to 100 kPa by charging the room with N 2  gas, while the internal temperature thereof was kept at 1050° C.  
         [0045]     After the carburization room  17  had been kept under the above-described conditions for 30 minutes, its internal pressure was reduced to 0.1 kPa by virtue of gas discharge. Subsequently, C 3 H 8  gas and CO 2  gas were supplied into the carburization room  17  at a flow rate of 14 L/min so as to increase the internal pressure to 1.7 kPa.  
         [0046]     Next, with the internal pressure of the carburization room  17  kept at 1.7 kPa, the supply amount of CO 2  gas was controlled at a constant flow rate of 10 L/min, while the supply amount of C 3 H 8  gas was changed so as to have the carbon potential controlled at 1.4%. Then, the interior of the carburization room  17  was kept at 1050° C. for 16 minutes.  
         [0047]     Subsequently, the supply of C 3 H 8  gas and CO 2  gas was stopped and the internal pressure within the carburization room  17  was reduced to 0.1 kPa by virtue of gas discharge. This internal pressure was kept for 16 minutes. Afterwards, the steel material was cooled and then heated again so as to adjust the size of the crystal particles.  
         [0048]     In more detail, the steel material was transported from the carburization room  17  to the gas cooling room  18  by way of the transportation room  16 . Then, the interior of the gas cooling room  18  was restored to 100 kPa by charging the room with N 2  gas, followed by cooling the same for 15 minutes. Afterwards, the N 2  gas was discharged and the internal pressure within the gas cooling room  18  was reduced to 0.1 kPa or lower. At this time, the steel material was transported into the carburization room  17  by way of the transportation room  16 . Then, the steel material was heated so as to increase its temperature, with the heating process being conducted under a condition in which the N 2  gas was still present and the internal pressure within the carburization room was 100 kPa. After this condition had been kept for 30 minutes, the internal pressure within the carburization room  17  was reduced to 0.1 kPa by virtue of gas discharge, while the steel material was transported to the quenching room  19  by way of the transportation room  16 , thereby starting the oil quenching treatment.  
         [0049]     The average carbon concentration distribution of the steel material treated in this example is shown in  FIG. 5 . In fact, similar to the above example shown in  FIG. 3 , the carbon concentrations shown in this graph represent the average values of the carbon concentrations of the steel material pieces located at the aforementioned nine positions. As a result, an effective carburization depth (0.36% C) was found to be 0.73 mm, which was an appropriate value. Further, a photograph indicating the surface organization of the treated steel material is shown in  FIG. 6 . It is to be noted that there were no abnormal layers formed on the surface of the steel material treated in the above described process. In addition, one example of a crystal particle photograph is shown in  FIG. 7 . Here, the crystal particle size was #9, which was an appropriate value.  
         [0050]     In this way, since the treatment temperature was set at 1050° C., which is a high temperature, and since the carbon potential was set at 1.4%, the carburization lead time of the carburization treatment in Example 2 could be greatly reduced. In fact, the carburization lead time in this example was reduced by about 73% compared with the aforementioned conventional gas carburization treatment (which uses an endothermic gas). Accordingly, using the carburization treatment method actually carried out in Example 2, it becomes possible to obtain a carburized layer having a desired depth, using a reduced time period than that required by the above described conventional gas carburization treatment (which uses an endothermic gas). Therefore, it is possible to reduce the total energy consumption. Moreover, since there is no soot being generated, the pieces of steel material can be placed at any position within the furnace without any limitation. In this way, the use of the present invention makes it possible to obtain carburized layers which are relatively uniform and differ little from each other in their physical and chemical properties.  
       EXAMPLE 3  
       [0051]     Example 3 was conducted based on Example 1 but using a different carburization pressure from that used in Example 1. Namely, sections of steel material pieces which were identical to those used in Example 1 were disposed at nine positions within the carburization room  17  whose internal temperature was controlled at 950° C. and whose internal pressure was controlled at 0.1 kPa or lower. Then, the pressure within the carburization room  17  was restored to 100 kPa by charging the room with N 2  gas, while the internal temperature thereof was kept at 950° C.  
         [0052]     After the carburization room  17  had been kept under the above described conditions for 30 minutes, its internal pressure was reduced to 0.1 kPa by virtue of gas discharge. Subsequently, C 3 H 8  gas and CO 2  gas were supplied into the carburization room  17 , each at a flow rate of 15 L/min so as to increase the internal pressure to 100 kPa.  
         [0053]     Next, with the internal pressure of the carburization room  17  kept at 100 kPa, the supply amount of CO 2  gas and/or the supply amount of C 3 H 8  gas were changed so as to have the carbon potential controlled at 1.25%. Then, the interior of the carburization room  17  was kept at 950° C. for 57 minutes.  
         [0054]     Subsequently, the supply of C 3 H 8  gas and CO 2  gas was stopped and the internal pressure within the carburization room  17  was reduced to 0.1 kPa by virtue of gas discharge. Then, this internal pressure was kept for 37 minutes, while the internal temperature of the carburization room  17  was lowered to 870° C. during a subsequent time period of 30 minutes. Afterwards, the steel material was transported to the quenching room  19  by way of the transportation room  16 , hence starting the oil quenching treatment.  
         [0055]     As a result, an effective carburization depth (0.36% C) of the treated steel material in this example was found to be 0.72 mm, which was an appropriate value, and no soot was generated.