Carburization treatment method

1. A carburization treatment method comprising:

introducing steel material into a transportation room, wherein the transportation room is configured to transport the steel material between a carburization room, an oil quenching room, and a gas cooling room;

reducing the pressure in the transportation room to 0.1 kPa or lower;

transporting the steel material after the pressure of the transportation room has been reduced from the transportation room to the carburization room by way of opening a first partition door located between the transportation room and the carburization room;

elevating a pressure in the carburization room by introducing N 2 gas therein and elevating a temperature in the carburization room after the first partition door is closed;

reducing the pressure in the carburization room to 0.1 kPa or lower by discharging the N 2 gas;

supplying a predetermined amount of hydrocarbon gas and a predetermined amount of oxidative gas to the carburization room so that the internal pressure of the carburization room is 0.1 to 101 kPa, the hydrocarbon gas being at least one 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 ;

controlling the amount of hydrocarbon gas and the amount of oxidative gas in the carburization room with the internal pressure of the carburization room kept at 0.1 to 101 kPa such that an atmosphere within the carburization room has a high carbon potential which is slightly below a carbon solid solubility limit;

stopping the supply of hydrocarbon gas and oxidative gas and reducing the pressure in the carburization room; and

after a predetermined amount of time, lowering the temperature in the carburization room to an oil quenching temperature and opening the first partition door;

transporting the steel material to the oil quenching room from the carburization room, via the transportation room by opening a second partition door located between the transportation room and the oil quenching room; and

performing an oil quenching treatment on the steel material in the oil quenching room.

2. The carburization treatment method according to claim 1 , wherein the atmosphere having a high carbon potential slightly below the carbon solid solubility is maintained by directly supplying at least one of the hydrocarbon gas and the oxidative gas into the carburization room.

3. The carburization treatment method according to claim 2 , wherein the oxidative gas is O 2 gas or CO 2 gas.

4. The carburization treatment method according to claim 2 , wherein the amount of at least one of the hydrocarbon gas and the oxidative gas being supplied to the carburization room for maintaining the atmosphere having a high carbon potential slightly below the carbon solid solubility is controlled by carrying out at least one of the following measurements: 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 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 dew point, all within the carburization room.

5. The carburization treatment method according to claim 1 , comprising, after the oil quenching, removing the steel material through an outlet door.

6. The carburization treatment method according to claim 5 , comprising transporting the steel material to the gas cooling room from the carburization room, via the transportation room, by opening a third partition door located between the gas cooling room and the transportation room.