TECHNICAL FIELD

The present invention relates to a continuous furnace where to apply a nitriding treatment to a steel member.

BACKGROUND ART

Steel members such as gears used in automobile transmissions are required to be high in pitting resistance and bending fatigue strength. As a method to meet such requirements, there has been known a method that applies a nitriding treatment to a steel member to form, on a surface of the steel member, an iron nitride compound layer including a γ′ phase.

Patent Document 1 discloses a method that applies a 529 to 650° C. heat-treatment to a steel member under a NH3gas atmosphere to form an iron nitride compound layer on a surface of the steel member, then after once discharging the atmosphere gas in a process chamber, newly supplies an inert gas or a reducing gas, and performs a denitrification treatment by exposing the steel member having 500 to 650° C. to an atmosphere of the inert gas or the reducing gas for a predetermined time. In Patent Document 1, the iron nitride compound layer composed of an ε phase and a γ′ phase is formed on the surface of the steel member by this method.

PRIOR ART DOCUMENT

Patent Document

Patent Document 1: Japanese Laid-open Patent Publication No. 2016-65263

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

The nitriding treatment of Patent Document 1 is performed in a batch-type furnace, but the batch-type furnace is low in productivity and its processable number per lot is limited, leading to an increase in treatment cost. Therefore, a series of the nitriding treatment is desirably performed continuously using a continuous furnace. However, in order to execute a nitriding treatment like that of Patent Document 1 in a continuous furnace including continuous process chambers, it is necessary to control the atmosphere gas independently in each of the process chambers, resulting in a complicated furnace structure.

The present invention was made in consideration of the above circumstances, and has an object to improve productivity of a nitrided steel member by performing a nitriding treatment in a continuous furnace, the nitriding treatment being a process that forms an iron nitride compound layer composed of an ε phase or of the ε phase and a γ′ phase and thereafter precipitates the γ′ phase in the iron nitride compound layer.

Means for Solving the Problems

The present inventors have found out that it is possible to perform the aforesaid nitriding treatment in the continuous furnace by controlling the temperature of an atmosphere gas in the furnace instead of controlling a nitriding potential KN of the atmosphere gas in the furnace. Specifically, the present invention to solve the aforesaid problem is a continuous nitriding treatment furnace where to apply a nitriding treatment to a steel member, the continuous nitriding treatment furnace including: a nitriding chamber into which the steel member is carried; a heater that heats an atmosphere gas in the nitriding chamber; and a control part that is configured to control an atmosphere gas temperature in the nitriding chamber, by adjusting a heat value of the heater so as to cause the nitriding chamber to have a first nitriding zone and a second nitriding zone whose atmosphere gas temperatures are different, the second nitriding zone being located downstream of the first nitriding zone in terms of a conveyor line and being lower in the atmosphere gas temperature than the first nitriding zone by 25° C. to 150° C., and that is configured to perform a control of executing the nitriding treatment that forms an iron nitride compound layer on a surface of the steel member in the first nitriding zone, by adjusting flow rates of gases included in a process gas for the nitriding treatment so as to attain a nitriding potential KN with which the iron nitride compound layer composed of an ε phase or of the ε phase and a γ′ phase is formed on the surface of the steel member, and precipitates the γ′ phase in the iron nitride compound layer in the second nitriding zone, wherein, when an atmosphere gas in the first nitriding zone flows into the second nitriding zone, a value calculated by subtracting the nitriding potential KN of the first nitriding zone from the nitriding potential KN of the second nitriding zone becomes −0.1 to 0. The nitriding potential KN is calculated by the following expression.
KN=P(NH3)/P(H2)3/2

P(NH3): partial pressure of NH3gas, P(H2): partial pressure of H2gas

The present invention according to another aspect s a continuous nitriding treatment method of applying a nitriding treatment to a steel member in a continuous furnace, the method including: controlling an atmosphere gas temperature in a nitriding chamber into which the steel member is carried, so as to cause the nitriding chamber to have a first nitriding zone and a second nitriding zone whose atmosphere gas temperatures are different, the second nitriding zone being located downstream of the first nitriding zone in terms of a conveyor line and being lower in the atmosphere gas temperature than the first nitriding zone by 25° C. to 150° C.; and performing the nitriding treatment that: forms an iron nitride compound layer on a surface of the steel member in the first nitriding zone to which a process gas for the nitriding treatment has been supplied, with gases included in the process gas being adjusted in flow rates so as to attain a nitriding potential KN with which the iron nitride compound layer composed of an ε phase or of the ε phase and a γ′ phase is formed on the surface of the steel member; and precipitates the γ′ phase in the iron nitride compound layer in the second nitriding zone configured such that a value calculated by subtracting the nitriding potential KN of the first nitriding zone from the nitriding potential KN of the second nitriding zone becomes −0.1 to 0 when an atmosphere gas in the first nitriding zone flows into the second nitriding zone.

Effect of the Invention

According to the present invention, the nitriding treatment that forms the iron nitride compound layer composed of the ε phase or of the ε phase and the γ′ phase and thereafter precipitates the γ′ phase in the iron nitrided compound layer can be performed in the continuous furnace. This makes it possible to improve productivity of a nitrided steel member.

