WO2019131602A1

WO2019131602A1 - Nitrided steel member, and method and apparatus for producing nitrided steel member

Info

Publication number: WO2019131602A1
Application number: PCT/JP2018/047505
Authority: WO
Inventors: 陽一渡邊; 泰平岡
Original assignee: パーカー熱処理工業株式会社
Priority date: 2017-12-27
Filing date: 2018-12-25
Publication date: 2019-07-04
Also published as: JP2021042398A

Abstract

This nitrided steel member contains a carbon steel or a low alloy steel as a matrix phase, and is characterized by having a hardened layer having a martensite structure containing 0.8% or more of nitrogen at the surface of the member, having a diffusion layer, in which nitrogen diffuses into the matrix phase, below the hardened layer, and in that the hardened layer has a thickness of 2-50 µm from the surface of the nitrided steel member, the diffusion layer extends to a depth of more than 100 µm from the surface of the nitrided steel member, and the hardness of the diffusion layer at a depth of 100 µm from the surface of the nitrided steel member is at least 100 HV more than the hardness at a depth of 2 mm from the surface of the nitrided steel member.

Description

Nitride steel member and method and apparatus for producing nitride steel member

The present invention relates to a nitride steel member and a method and an apparatus for manufacturing the nitride steel member. More specifically, the present invention relates to a nitride steel member having excellent fatigue resistance useful for gears, crankshafts, and the like for automobile transmissions, and a method and apparatus for manufacturing the nitride steel member.

Among surface hardening treatments of steel materials, the need for nitriding treatment, which is low heat treatment distortion treatment, is high, and in recent years, the interest in atmosphere control technology for gas nitriding treatment has been particularly increasing.

In the basic structure configuration obtained by the gas nitriding process, a compound layer which is iron nitride is formed on the surface, and a hardened layer called a diffusion layer is formed inside. The hardened layer is usually made of an alloy nitride such as Si or Cr as a base material component.

In addition to the temperature and time of the gas nitriding process, the atmosphere in the gas nitriding furnace is also used to control the thickness (depth) of each of these two layers and / or the type of iron nitride on the surface, etc. It is controlled appropriately. Specifically, the nitriding potential (K _N ) in the gas nitriding furnace is appropriately controlled.

For example, through this control, the volume fraction (type of iron nitride) of γ 'phase (Fe ₄ N) and ε phase (Fe _2-3 N) in the compound layer formed on the surface of the steel material is controlled It is done. Specifically, it is known that the fatigue resistance is improved by forming the γ ′ phase more than the ε phase (“Heat treatment”, Volume 55, No. 1, pages 1 and 2 (Hiraoka, Yoichi Watanabe, Takeshi Ishida): Non-Patent Document 1). Furthermore, a nitrided steel member in which the bending fatigue strength and the surface fatigue are improved by the formation of the γ ′ phase is also provided (Japanese Patent Application Laid-Open No. 2013-221203: Patent Document 1).

As described above, it is already known that the? 'Phase is formed in the compound layer on the surface of the steel material to improve the fatigue resistance. However, even if the gas nitriding process is performed to form a large amount of γ 'phase, the compound layer contains not only a small amount of ε phase, but in fact, it becomes a two phase state of γ' phase and ε phase (Japanese Patent Application Laid-Open No. 2016-211069: Patent Document 2). That is, there is a limit in forming a compound layer mainly based on the? 'Phase in order to improve the fatigue strength. Further, even in the compound layer in which the γ ′ phase and the ε phase are controlled in a controlled manner, a large number of voids are formed in the vicinity of the surface layer of the compound layer. These voids tend to develop into fatigue cracks.

On the other hand, when nitriding treatment is carried out at a temperature above the eutectoid transformation point (about 590 ° C) of the Fe-N binary alloy, a compound layer is formed on the surface, and if quenching is performed thereafter, the nitrogen-containing martensitic structure is present below Is formed. Nitriding treatment in the temperature range is referred to as carbonitriding treatment in contrast to conventional nitriding treatment.

However, in the carbonitriding treatment, austenite in the structure near the surface (except for the compound layer on the surface) is stabilized, and a large amount of austenite remains even if it is quenched after that. Therefore, the strain after the heat treatment is about the same as the nitriding treatment. In addition, this stabilized austenite is transformed to a hard martensitic structure by being reheated to a temperature of 250 to 300.degree.

For example, STKM13 (a kind of carbon steel) is subjected to nitridation treatment at 640 ° C. for 90 minutes, and further to nitrogen treatment at 660 ° C. for 40 minutes and then reheat treatment at 280 ° C. for 90 minutes. It is hardened until "(New concept and practice of carburization and nitridation", Agne Technical Center, p. 142-147, 2013 (Teruaki Watanabe): Non-Patent Document 2). However, there is a problem that the compound layer on the surface remains.

Furthermore, even if a JIS-SPCC (a type of cold rolled steel sheet) is subjected to nitridation treatment at 700 ° C., a compound layer is formed on the surface, and a hardened layer of nitrogen martensitic structure is formed on the lower part in subsequent quenching. ("European Conference on Heat Treatment and Surface Engineering A3 TS Congress (Nice, France, 2017)", pp. 26-29 (Y. Kawata and T. Kidachi): Non-Patent Document 3). That is, also in this case, the compound layer on the surface remains.

On the other hand, a hardened layer of martensitic structure having a thickness of 0.35 mm or more can be obtained without forming a compound layer by carrying out a nitriding treatment at 800 ° C. and then quenching it, and the fatigue resistance can be improved. Have been reported ("The 5th heat treatment technology seminar text of Japan heat treatment technology", 2012, (5) pages 1 to 8 (Mashiro Okumiya): Non-patent document 4).
Patent Document 1 cited by this specification is JP-A-2013-221203, and Patent Document 2 cited by this specification is JP-A-2016-211069. In addition, Non-Patent Document 1 cited by this specification is “Heat treatment”, Volume 55, No. 1, pages 1-2 (Taira Hiraoka, Yoichi Watanabe, Takeshi Ishida), and non-patents cited by this specification. Reference 2 is "New concept and practice of carburization and nitridation", Agne Technical Center, p. 142-147 (Watanabe Teruaki) 2013, and Non-Patent Document 3 cited in this specification is "European Conference on Heat Treatment and Surface Engineering A3 TS Congress (Nice, France, 2017), pp. 26-29 (Y. Kawata and T. Kidachi), and Non-Patent Document 4 cited in this specification is “Japanese Heat Treatment Technology”. Association 5th Heat Treatment Technology Seminar Text ", 2012, (5) pp. 1-8 (Masahiro Okumiya).

JP, 2013-221203, A JP, 2016-211069, A

The fatigue failure of machine parts results from notches which are subjected to high load stress, for example the root of a gear tooth. In the notched portion, a stress distribution according to the shape and load environment occurs only in the surface layer region (from the surface to the inside of a predetermined depth). For this reason, it is desirable to harden only the surface layer region so as not to impair the toughness and the machinability of the steel material.

However, the conventionally practiced nitrogen treatment does not sufficiently meet such a demand. In the techniques disclosed in Non-Patent Document 2 and Non-Patent Document 3, unnecessary compound layers remain on the surface, so they are not suitable for curing the surface region. On the other hand, in the technique disclosed in Non-Patent Document 4, although the unnecessary compound layer is not formed, the cured layer is too thick and the thermal strain / transformation strain is large, which is also not suitable for curing the surface region.

The inventor of the present invention repeated intensive studies and various experiments to limit the configuration of the processing furnace and control the temperature and the nitriding potential of the nitriding treatment with high precision to thereby make the surface region hardened as desired. It was found that the members could be manufactured.

The present invention has been made based on the above findings. An object of the present invention is to provide a nitrided steel member whose surface region is hardened as desired, and a manufacturing method and apparatus for manufacturing such a nitrided steel member.

The present invention is a nitrided steel member having a carbon steel or low alloy steel as a matrix, and having a hardened layer having a martensitic structure containing 0.8% or more of nitrogen by mass% on the surface, wherein the hardened layer The lower portion is provided with a diffusion layer in which nitrogen is diffused in the matrix, and the hardened layer has a thickness of 2 μm to 50 μm from the surface of the nitrided steel member, and the diffusion layer is the nitrided portion. The hardness of the diffusion layer at a depth of 100 μm from the surface of the nitrided steel member, which extends to a depth of more than 100 μm from the surface of the steel member, and at a depth of 2 μm from the surface of the nitrided steel member Is a nitride steel member characterized in that it is larger than 100 HV.

According to the present invention, since the hardened layer having a martensitic structure containing 0.8% or more of nitrogen is limited to a thickness of 2 μm to 50 μm from the surface of the nitrided steel member, the heat treatment strain / transformation strain is small. . Moreover, the hardness of the diffusion layer at a depth of 100 μm from the surface of the nitride steel member is 100 HV or more than the hardness at a depth of 2 mm from the surface of the nitride steel member, so that the hardened layer is thin. Sufficient strength can be guaranteed.

As carbon steel, carbon steel whose carbon content is 0.1% or more in mass% can be used, for example. Moreover, as low alloy steel, SCr420, SCM415, etc. can be utilized, for example.

Further, the present invention is a method of manufacturing a nitrided steel member having a carbon steel or a low alloy steel as a mother phase using a circulation type processing furnace provided with a guide cylinder and a stirring fan, and at the time of nitriding treatment, The temperature range in the circulating process furnace is controlled to 610 ° C. to 660 ° C., and the nitriding potential in the circulating process furnace is controlled to the range of 0.06 to 0.3 at the time of the nitriding treatment It is a manufacturing method of the nitriding steel member characterized by the above.

According to the method of manufacturing a nitrided steel member of the present invention,
A hardened layer having a martensitic structure containing 0.8% or more of nitrogen is provided on the surface, and a diffusion layer in which nitrogen is diffused in the matrix is provided below the hardened layer, and the hardened layer is It has a thickness of 2 μm to 50 μm from the surface of the nitrided steel member, and the diffusion layer extends to a depth of more than 100 μm from the surface of the nitrided steel member, and 2 mm from the surface of the nitrided steel member It is possible to manufacture a nitrided steel member characterized in that the hardness of the diffusion layer at a depth of 100 μm from the surface of the nitrided steel member is greater than 100 HV or more than the hardness in the depth.

Further, the present invention is provided with a circulation type processing furnace having a guide cylinder and a stirring fan, and during nitriding processing, the temperature range in the circulation type processing furnace is controlled to 610 ° C. to 660 ° C. In the above, the nitriding potential in the circulation type processing furnace is controlled in the range of 0.06 to 0.3.

According to the apparatus for manufacturing a nitrided steel member of the present invention,
A hardened layer having a martensitic structure containing 0.8% or more of nitrogen is provided on the surface, and a diffusion layer in which nitrogen is diffused in the matrix is provided below the hardened layer, and the hardened layer is It has a thickness of 2 μm to 50 μm from the surface of the nitrided steel member, and the diffusion layer extends to a depth of more than 100 μm from the surface of the nitrided steel member, and 2 μm from the surface of the nitrided steel member It is possible to manufacture a nitrided steel member characterized in that the hardness of the diffusion layer at a depth of 100 μm from the surface of the nitrided steel member is greater than 100 HV or more than the hardness in the depth.

In the apparatus for manufacturing a nitride steel member according to the present invention, for example, ammonia gas and ammonia decomposition gas are introduced into the circulation type processing furnace. In this case, in order to control the nitriding potential, the manufacturing apparatus changes a ratio of introduction of the ammonia gas and the amount of introduction of the ammonia decomposition gas while keeping the total flow rate constant; It is preferable that the second control for changing the introduction amount of the ammonia gas can be selectively performed in a state in which the introduction of the ammonia decomposition gas is stopped.

Effect of the present invention

3 is a cross-sectional photomicrograph of a nitrided steel member according to an embodiment of the present invention. It is a cross-sectional microscope picture of the nitrided steel member of FIG. 1 before reheat treatment. It is a cross-sectional microscope picture of a comparative example. It is a graph which shows the example of an experiment about hardness distribution. It is the schematic of the manufacturing apparatus of the nitride steel member by one Embodiment of this invention. It is a schematic sectional drawing of a circulation type processing furnace (horizontal gas nitriding furnace). It is a graph which shows the example of 1st control. It is a graph which shows the example of 1st control. It is a graph which shows the example of 2nd control. It is a graph which shows the example of 2nd control. It is the schematic which shows the example of the jig inserted in a furnace. It is a figure which shows the form of the Ono type rotation bending fatigue test piece.

Hereinafter, preferred embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

(Configuration of one embodiment of a nitrided steel member)
FIG. 1 is a sectional photomicrograph of a nitrided steel member 100 according to an embodiment of the present invention. As shown in FIG. 1, the nitrided steel member 100 of the present embodiment is provided with a hardened layer 101 having a martensitic structure containing 0.8% or more of nitrogen on the surface, and in the lower part of the hardened layer 101, a matrix phase is provided. And a diffusion layer 102 in which nitrogen is diffused. The matrix (base material) of the present embodiment is a carbon steel having a carbon content of 0.45% by mass.

Hardened layer 101 has a thickness of about 15 μm from the surface of nitrided steel member 100, which is in the range of 2 μm to 50 μm. Diffusion layer 102 extends from the surface of nitrided steel member 100 to a depth exceeding 100 μm. Then, the hardness (for example, about 310 HV) of diffusion layer 102 at a depth of 100 μm from the surface of nitrided steel member 100 from the hardness (for example, about 180 HV) at a depth of 2 mm from the surface of nitrided steel member 100 It is larger than 100 HV.
(Manufacturing conditions of one embodiment of a nitrided steel member)

The nitrided steel member 100 of the present embodiment is subjected to the nitronitriding treatment under the treatment conditions of the treatment temperature: 640 ° C., the nitriding potential: 0.16, and the treatment time: 2 hours using a circulation type treatment furnace described later It is quenched and further reheated at 250 ° C. for 2 hours. In the photograph of FIG. 1, the hardened layer 101 is strongly corroded (blackened) by the corrosive liquid for texture observation.

A cross-sectional micrograph before the reheating treatment is shown in FIG. In this state, most of the region corresponding to the hardened layer 101 is an austenite phase and does not have sufficient hardness. By performing the reheat treatment, the martensitic structure in the austenite phase is increased, and accordingly, sufficient hardness can be obtained.
(Configuration of comparative example)

A cross-sectional micrograph of Comparative Example 150 is FIG. The comparative example 150 was subjected to carbonitriding treatment using a circulation type treatment furnace described later under the treatment conditions of treatment temperature: 640 ° C., nitriding potential: 0.32 (> 0.3), treatment time: 2 hours. It is quenched afterward.

As shown in FIG. 3, in Comparative Example 150, a compound layer 153 is formed on the surface, and a hardened layer 151 having a martensitic structure is provided below the compound layer 153, and further, the lower side of the hardened layer 151. , And the diffusion layer 152 in which nitrogen is diffused in the matrix phase. As described above, an unnecessary compound layer is formed under manufacturing conditions where the nitriding potential is high.
(Effect of nitrided steel members)

According to the nitrided steel member 100 of the present embodiment, the hardened layer having a martensitic structure containing 0.8% or more of nitrogen is limited to a thickness of 2 μm to 50 μm from the surface of the nitrided steel member, so heat treatment is performed. Low strain / transformation strain. Also, the hardness of the diffusion layer at a depth of 100 μm from the surface of the nitride steel member is 100 HV or more than the hardness at a depth of 2 μm from the surface of the nitride steel member, so that the hardened layer is thin. Sufficient strength can be guaranteed.
(Range of nitrogen concentration of hardened layer)

The nitrogen concentration of the hardened layer 101 is the result of considering the stability of the austenite phase at room temperature. That is, by containing 0.8% or more of nitrogen (more preferably by 1.0% or more of nitrogen), most of the austenite phase is stabilized at room temperature when quenched, ie, during quenching Martensitic transformation does not occur. As a result, the strain is extremely small as compared to the case where martensitic transformation occurs during quenching. (Martensitic transformation is promoted in subsequent reheat treatment to increase hardness.)
(Range of thickness of hardened layer)

With regard to the thickness of the hardened layer 101, basically, the thicker the thickness, the better the fatigue strength. However, depending on the load environment of the nitrided steel member 100, there is a case that there is no further effect of improving the fatigue strength (the effect is saturated) even if the thickness is further improved. Specifically, depending on the shape of the nitrided steel member 100 and the load environment, for example, the stress distribution in the notch may differ. Therefore, the thickness of the hardened layer 101 can be appropriately selected depending on the shape of the nitrided steel member 100 and the load environment.

However, manufacturing conditions (processing temperature such that the condition that “the hardness of the diffusion layer at the depth of 100 μm from the surface of the nitride steel member is 100 HV or more greater than the hardness at the depth of 2 mm from the surface of nitride steel member”) At a temperature of 610 ° C. to 660 ° C. and a nitriding potential of 0.06 to 0.3, the thickness of the hardened layer 101 is 2 to 50 μm.

Specifically, the alloy component-based carbon steel in which the hardened layer 101 tends to be thick (specifically, S50C steel, 50 μm which is the result when carbonizing at a processing temperature of 660 ° C., nitriding potential: 0.17) Is the upper limit value.

On the other hand, as a condition for forming the hardened layer 101 on the entire surface of the nitrided steel member 100 (there is no case where the hardened layer 101 is not locally formed), 2 μm is set as the lower limit value.
(Condition of hardness of diffusion layer)

The nitrided steel member 100 of this embodiment is characterized in that not only the hardened layer 101 but also the diffusion layer 102 has sufficient hardness. Fig. 4 is a graph showing that JIS-S45C steel (carbon steel) and JIS-SCM415 steel (Cr-Mo steel) are subjected to nitriding treatment for 1.5 hours at various temperatures shown in the drawing and then quenched. Furthermore, the hardness distribution of each test piece reheated at 250 ° C. for 2 hours is shown. As shown in FIG. 4, the surface hardness at a depth position of 100 μm (= 0.1 mm) can be made in the range of about 300 to 500 HV by selecting the nitriding temperature and the steel type. The

The surface hardness after nitriding treatment is generally often obtained at a depth of 50 μm from the surface. However, in the nitrided steel member 100 of the present embodiment, in order to avoid the influence of the hardened layer 101 having a martensitic structure, the hardness at a depth position of 100 μm from the surface is to be evaluated.

As shown in FIG. 4, in order to obtain a hardness distribution equal to or higher than that of the S45C steel (carbon steel) nitrided at 580 ° C. (represented by the line), carbonitriding at a temperature of 660 ° C. It is necessary to process. Furthermore, although not shown, even if JIS-SACM 645 steel, which has a higher alloy content than SCM 415 steel, if it is carbonized at a temperature of 660 ° C. or less, the hardness at a depth of 100 μm should be increased to 500 HV or more Was possible.

On the other hand, the hardness at a depth of 2 mm from the surface is defined as an evaluation object for an internal structure not affected by nitriding.

(Configuration of manufacturing apparatus for nitrided steel members)
First, the basic matter of the gas nitriding treatment will be explained chemically. In the gas nitriding treatment, nitriding represented by the following formula (1) in the processing furnace (gas nitriding furnace) in which the article to be treated is disposed A reaction occurs.
NH ₃ → [N] + 3 / 2H ₂ (1)

At this time, the nitriding potential K _N is defined by the following equation (2).
K _N = P _{NH 3} / P _{H 2} ^3/2 (2)
Here, P _NH3 is the ammonia partial pressure in the furnace, and P _H2 is the hydrogen partial pressure in the furnace. The nitriding potential K _N is known as an index indicating the nitriding ability of the atmosphere in the gas nitriding furnace.

On the other hand, in the furnace during gas nitriding treatment, a part of the ammonia gas introduced into the furnace is thermally decomposed into hydrogen gas and nitrogen gas according to the reaction of the equation (3).
NH ₃ → 1/2 N ₂ + 3/2 H ₂ (3)

In the furnace, the reaction of the formula (3) mainly occurs, and the nitriding reaction of the formula (1) can be almost neglected quantitatively. Therefore, if the in-furnace ammonia concentration consumed in the reaction of the equation (3) or the hydrogen gas concentration generated in the reaction of the equation (3) is known, the nitriding potential can be calculated. That is, since hydrogen and nitrogen to be generated are 1.5 mol and 0.5 mol respectively from 1 mol of ammonia, if the ammonia concentration in the furnace is measured, the hydrogen concentration in the furnace can also be understood, and the nitriding potential should be calculated. Can. Alternatively, if the in-furnace hydrogen concentration is measured, the in-furnace ammonia concentration can be known, and the nitriding potential can be calculated again.

The ammonia gas flowed into the gas nitriding furnace is discharged to the outside of the furnace after circulating in the furnace. That is, in the gas nitriding process, the existing gas is continuously discharged to the outside of the furnace by continuously flowing fresh (new) ammonia gas into the furnace with respect to the existing gas in the furnace (pushed by the supply pressure) .

Here, if the flow rate of ammonia gas introduced into the furnace is small, the gas residence time in the furnace will be long, so the amount of ammonia gas to be decomposed will increase and nitrogen gas generated by the decomposition reaction + The amount of hydrogen gas increases. On the other hand, if the flow rate of ammonia gas introduced into the furnace is high, the amount of ammonia gas discharged out of the furnace without being decomposed increases and the amount of nitrogen gas + hydrogen gas generated in the furnace decreases Do.

Now, FIG. 5 is a schematic view showing a manufacturing apparatus for manufacturing a nitrided steel member according to an embodiment of the present invention. As shown in FIG. 5, the manufacturing apparatus 1 of the present embodiment includes the circulation type processing furnace 2, and only two types of ammonia and ammonia decomposition gas are used as the gas introduced into the circulation type processing furnace 2. ing. The ammonia decomposition gas is a gas also called AX gas, and is a mixed gas consisting of nitrogen and hydrogen in a ratio of 1: 3. However, as introduced gas, (1) only ammonia gas, (2) only two types of ammonia and ammonia decomposition gas, (3) only two types of ammonia and nitrogen gas, or (4) ammonia and ammonia decomposition gas Only three types of nitrogen gas may be selected.

An example of the cross-sectional structure of the circulation type processing furnace 2 is shown in FIG. In FIG. 6, a cylinder 202 called retort is disposed in a furnace wall (also called a bell) 201, and a cylinder 204 called an internal retort is disposed further inside thereof. The introduced gas supplied from the gas introducing pipe 205 passes around the object to be treated and then passes through the space between the two

cylinders

202 and 204 by the action of the stirring fan 203 as shown by the arrows in the figure. It circulates. 206 is a flared gas hood, 207 is a thermocouple, 208 is a lid for the cooling operation, and 209 is a fan for the cooling operation. The circulation type processing furnace 2 is also called a horizontal gas nitriding furnace, and the structure itself is known.

The workpiece S is carbon steel or low alloy steel, and is, for example, a crankshaft or a gear, which is an automobile part.

Further, as shown in FIG. 5, in the processing furnace 2 of the surface hardening treatment apparatus 1 of the present embodiment, a furnace opening / closing lid 7, a stirring fan 8, a stirring fan drive motor 9, and an atmosphere gas concentration detection device 3 , A nitriding potential regulator 4, a programmable logic controller 30, and an in-furnace introduced gas supply unit 20.

The stirring fan 8 is disposed in the processing furnace 2, rotates in the processing furnace 2, and stirs the atmosphere in the processing furnace 2. The stirring fan drive motor 9 is connected to the stirring fan 8 so as to rotate the stirring fan 8 at an arbitrary rotational speed.

The atmosphere gas concentration detection device 3 is configured by a sensor that can detect the hydrogen concentration or the ammonia concentration in the processing furnace 2 as the atmosphere gas concentration in the furnace. The detection main body of the sensor is in communication with the inside of the processing furnace 2 through the atmosphere gas pipe 12. In the present embodiment, the atmosphere gas pipe 12 is formed by a path that directly communicates the sensor main body of the atmosphere gas concentration detector 3 with the processing furnace 2, and the furnace gas waste pipe connected to the exhaust gas combustion decomposition device 41 halfway 40 are connected. Thereby, the atmosphere gas is distributed to the gas to be discarded and the gas supplied to the atmosphere gas concentration detection device 3.

Further, after the atmosphere gas concentration detection device 3 detects the atmosphere gas concentration in the furnace, the atmosphere gas concentration detection device 3 outputs an information signal including the detected concentration to the nitriding potential regulator 4.

The nitriding potential regulator 4 includes an in-furnace nitriding potential calculator 13 and a gas flow rate output adjuster 30. The programmable logic controller 31 also has a gas introduction amount control device 14 and a parameter setting device 15.

The in-furnace nitriding potential calculation unit 13 calculates the nitriding potential in the processing furnace 2 based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection unit 3. Specifically, an arithmetic expression of the nitriding potential programmed according to the actual furnace introduced gas is incorporated, and the nitriding potential is calculated from the value of the atmosphere gas concentration in the furnace.

The parameter setting device 15 includes, for example, a touch panel, and can set and input a total flow rate of gas introduced into the furnace, a gas type, a processing temperature, a target nitriding potential, and the like. Each setting parameter value set and input is transmitted to the gas flow rate output adjusting means 30.

Then, the gas flow rate output adjusting means 30 sets the nitriding potential calculated by the in-furnace nitriding potential calculating device 13 as an output value, sets a target nitriding potential (the set nitriding potential) as a target value, and outputs ammonia gas and ammonia decomposition gas. Control is performed with each introduction amount as an input value. More specifically, the first control for changing the introduction ratio of the ammonia gas and the ammonia decomposition gas while maintaining the total flow rate of the ammonia gas introduction amount and the ammonia decomposition gas constant, and the ammonia gas in a state where the introduction of the ammonia decomposition gas is stopped It is possible to selectively carry out a second control for changing the introduction amount of. The output value of the gas flow rate output adjustment means 30 is transmitted to the gas introduction amount control means 14.

The gas introduction amount control means 14 sends control signals to the first supply amount control device 22 for ammonia gas and the second supply amount control device 26 for ammonia decomposition gas in order to realize the introduction amount of each gas. It has become.

The in-furnace introduced gas supply unit 20 of the present embodiment includes a first in-furnace introduced gas supply unit 21 for ammonia gas, a first supply control device 22, a first supply valve 23, and a first flow meter 24. ,have. Further, the in-furnace introduced gas supply unit 20 of the present embodiment includes a second in-furnace introduced gas supply unit 25 for ammonia decomposition gas (AX gas), a second supply amount control device 26, and a second supply valve 27. , And a second flow meter 28.

In the present embodiment, the ammonia gas and the ammonia decomposition gas are mixed in the in-furnace gas introduction pipe 29 before entering the processing furnace 2.

The first in-furnace introduction gas supply unit 21 is formed of, for example, a tank filled with a first in-furnace introduction gas (in this example, ammonia gas).

The first supply amount control device 22 is formed by a mass flow controller, and is interposed between the first in-furnace introduced gas supply unit 21 and the first supply valve 23. The opening degree of the first supply amount control device 22 changes in accordance with the control signal output from the gas introduction amount control means 14. In addition, the first supply control unit 22 detects the amount of supply from the first in-furnace introduced gas supply unit 21 to the first supply valve 23, and sends an information signal including the detected supply to the gas introduction control means 14. It is designed to output. The control signal may be used for correction of control by the gas introduction amount control means 14 or the like.

The first supply valve 23 is formed by a solenoid valve that switches the open / close state according to the control signal output from the gas introduction amount control means 14, and between the first supply amount control device 22 and the first flow meter 24. It is interspersed.

The second furnace introduction gas supply unit 25 is formed of, for example, a tank filled with a second furnace introduction gas (in this example, an ammonia decomposition gas).

The second supply control device 26 is formed by a mass flow controller, and is interposed between the second in-furnace introduced gas supply unit 25 and the first supply valve 27. The opening degree of the first supply control unit 26 changes in accordance with the control signal output from the gas introduction control unit 14. Further, the third supply control unit 26 detects the amount of supply from the second in-furnace introduced gas supply unit 25 to the second supply valve 27, and sends an information signal including the detected supply to the gas introduction control means 14. It is designed to output. The control signal may be used for correction of control by the gas introduction amount control means 14 or the like.

The second supply valve 27 is formed by an electromagnetic valve that switches the open / close state according to the control signal output from the gas introduction amount control means 14, and between the second supply amount control device 26 and the second flow meter 28. It is interspersed.

(Operation of manufacturing apparatus for nitrided steel members (manufacturing method))
Next, the operation of the manufacturing apparatus 1 of the present embodiment will be described. First, the article S is introduced into the circulation type processing furnace 2 and the circulation type processing furnace 2 is heated to a desired processing temperature. Thereafter, a mixed gas of ammonia gas and ammonia decomposition gas or only ammonia gas is introduced into the processing furnace 2 at a set initial flow rate from the in-furnace introduced gas supply unit 20. The setting initial flow rate can also be set and input in the parameter setting device 15, and is controlled by the first supply amount control device 22 and the second supply amount control device 26 (both mass flow controllers). Further, the stirring fan drive motor 9 is driven to rotate the stirring fan 8 and stir the atmosphere in the processing furnace 2.

The in-furnace nitriding potential calculator 13 of the nitriding potential regulator 4 calculates the in-furnace nitriding potential (at the beginning, it is a very high value (because there is no hydrogen in the furnace) but there is decomposition of ammonia gas (hydrogen generation) Decreases as the process progresses), and it is determined whether or not the sum of the target nitriding potential and the reference deviation value is exceeded. The reference deviation value can also be set and input in the parameter setting device 15.

When it is determined that the calculated value of the nitriding potential in the furnace falls below the sum of the target nitriding potential and the reference deviation value, the nitriding potential adjuster 4 causes the gas introduction amount control means 14 to introduce the introduction amount of gas introduced into the furnace. Start control of the

The furnace nitriding potential calculator 13 of the nitriding potential controller 4 calculates the furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal. Then, the gas flow rate output adjusting means 30 uses the nitriding potential calculated by the in-furnace nitriding potential calculator 13 as an output value, and uses the target nitriding potential (the set nitriding potential) as a target value. Implement PID control with an input value. Specifically, in the PID control, the first control for changing the introduction ratio of the ammonia gas and the introduction amount of the ammonia decomposition gas while keeping the total flow rate of the introduction amount of the ammonia gas constant and stopping the introduction of the ammonia decomposition gas A second control of changing the introduction amount of ammonia gas in the state is selectively performed. In the PID control, each set parameter value set and input by the parameter setting device 15 is used. As the setting parameter value, for example, different values are prepared in accordance with the value of the target nitriding potential.

Then, the gas flow rate output adjusting means 30 controls the introduction amount of each in-furnace introduced gas as a result of the PID control. Specifically, the gas flow rate output adjustment means 30 determines the flow rate of each gas, and the output value is transmitted to the gas introduction amount control means 14.

The gas introduction amount control means 14 sends control signals to the first supply amount control device 22 for ammonia gas and the second supply amount control device 26 for ammonia decomposition gas in order to realize the introduction amount of each gas.

By the control as described above, the nitriding potential in the furnace can be stably controlled in the vicinity of the target nitriding potential. As a result, it is possible to carry out the nitridation treatment of the article S with extremely high quality.

Furthermore, depending on the type and shape of the material of the product S to be processed, it is possible to carry out the cooling step after the nitridation treatment in the manufacturing apparatus 1 concerned. However, if a sufficient hardness can not be obtained after reheating at the cooling rate of the manufacturing apparatus 1, the article S is removed from the furnace while maintaining the heating temperature after the nitriding treatment in the manufacturing apparatus 1. It is necessary to transport it to the quenching device (e.g., an oil tank) and then to quench it. Alternatively, it is necessary to take out the object S after cooling in the manufacturing apparatus 1 from the manufacturing apparatus 1, raise the temperature again to the heating temperature in another heating furnace equipped with a quenching device, and then quench it.

In addition, although the reheating step can also be performed in the manufacturing apparatus 1, it is generally performed in another tempering furnace outside the furnace.

(On the selection of the first control and the second control)
An example in which the first control is adopted is shown in FIGS. 7A and 7B. In the example of FIGS. 7A and 7B, the total flow rate of the introduced amount of ammonia gas and the introduced amount of ammonia decomposition gas is constant at 166 (l / min), and the nitriding potential is as high as 0.16. It is controlled.

An example in which the second control is adopted is shown in FIGS. 8A and 8B. In the example of FIGS. 8A and 8B, the introduction of the ammonia decomposition gas is stopped, and only the introduction amount of the ammonia gas is feedback-controlled in small steps in the vicinity of 220 (l / min), whereby the nitriding potential is 0.16. It is controlled with high precision.

From the viewpoint of control stability and process safety, it is preferable that the first control be performed. However, when there is a large amount of insertion of the article S in the furnace (for example, when the surface area of the article S exceeds 7 m ² ), many decomposition reactions of the formula (3) occur, so the first control has a nitriding potential. It is difficult to control with high accuracy. In such a case, it is preferable to shift to the second control to perform nitriding potential control.

(About the importance of the guide tube (internal retort))
According to the experiment of the present inventor, when the guide cylinder 204 (internal retort) is removed from the manufacturing apparatus 1 and the nitriding treatment is performed (comparative example), the compound layer is formed on the surface of the article S to be treated. That was confirmed. (In the comparative example, in addition to the removal of the guide cylinder 204, the positions of the stirring fan 203 and the gas introduction pipe 205 also moved to the center of the ceiling in the furnace.)

Specifically, in the case of using the manufacturing apparatus 1 and in the case of the comparative example, the carbonizing treatment is performed at a treatment temperature of 640 ° C., a nitriding potential of 0.16, and a treatment time of 2 hours. After the nitrogen treatment, it was transported to an oil tank separately installed outside the furnace while maintaining the temperature, and then cooling was performed (hereinafter, the procedure of carrying to the oil tank after the carbonizing treatment and cooling as described above; Called As the article S, using the jig shown in FIG. 9, as a steel material at the center of the A surface (furnace lid side), the B surface (center in the furnace), and the C surface (deep side in the furnace). A coin-shaped test piece of S45C steel and having a diameter of 20 x 5 mm was used.

When the thickness of the M layer of the cross section at the time of observing the structure in a state similar to FIG. As shown, in the case of the example, a hardened layer of nitrogen martensite having a thickness of 18 to 20 μm was obtained on any surface.

On the other hand, in the case of the comparative example, the compound layer was formed on any surface, and it was recognized that the hardened layer thickness by martensite tends to be larger as the surface was set in the depth direction. It is considered that this is because the in-furnace uniformity of the nitriding potential is not good.

※ CL presence / absence: presence or absence of compound layer, M layer thickness: hardened layer thickness by nitrogen martensitic structure

(Verification of hardness)
The treatments of the example of Table 2 and the comparative example were performed on S45C steel having a shape as shown in FIG.

In the example, after nitriding treatment at a treatment temperature of 640 ° C., a nitriding potential of 0.16, and a treatment time of 2 hours, it was oil cooled and reheating treatment was carried out at 250 ° C. for 2 hours. As a result, a hardened layer of martensitic structure was obtained with a thickness of 20 μm on the surface. The difference (ΔHV) between the hardness of the diffusion layer 102 at a depth of 100 μm from the surface and the hardness at a depth of 2 mm from the surface was 135 HV> 100 HV.

In Comparative Example 1, oil cooling was performed after carbonitriding at a treatment temperature of 570 ° C., a nitriding potential of 0.25 (known as a value for forming a γ ′ phase), and a treatment time of 3.5 hours. As a result, a 10 μm γ ′ phase rich compound layer was obtained on the surface. The difference (ΔHV) between the hardness of the diffusion layer 102 at a depth of 100 μm from the surface and the hardness at a depth of 2 mm from the surface was 140 HV> 100 HV.

In Comparative Example 2, after a nitronization treatment with a treatment temperature of 640 ° C., a nitriding potential of 0.32 (known as a value for forming a γ ′ phase), and a treatment time of 2 hours, oil cooling was carried out, and A reheat treatment of time was performed. As a result, a compound layer with a thickness of 20 μm was obtained on the surface, and a hardened layer with a nitrogen martensitic structure of 15 μm was obtained under the compound layer. The difference (ΔHV) between the hardness of the diffusion layer 102 at a depth of 100 μm from the surface and the hardness at a depth of 2 mm from the surface was 135 HV> 100 HV.

In Comparative Example 3, after carbonitriding at a treatment temperature of 700 ° C., a nitriding potential of 0.1, and a treatment time of 1.5 hours, it was oil-cooled and subjected to reheat treatment at 250 ° C. for 2 hours. As a result, a hardened layer of martensitic structure was obtained with a thickness of 40 μm on the surface. The difference (ΔHV) between the hardness of the diffusion layer 102 at a depth of 100 μm from the surface and the hardness at a depth of 2 mm from the surface was 70 HV <100 HV.

Furthermore, the rotational bending fatigue strength of each of the test pieces was evaluated using the Ono type rotational bending fatigue tester (Shimadzu Corporation, H7 type). The test load was carried out at 56 kgf and 60 kgf, and the rotational speed was made common at 3000 rpm. Evaluation of test results, 10 ⁵ rotates in 56Kgf, and pass those celebrated 10 ⁷ rotating at 60 kgf (in the table ○), otherwise was evaluated as unacceptable (× in the table).

Although the example achieved the target life with any test load, in Comparative Example 2 and Comparative Example 3, the target life could not be achieved with any of the test loads. In addition, in the comparative example 3, internal fracture was carried out by any test load, and the hardness of the diffusion layer was insufficient, which was in agreement with the fact.

DESCRIPTION OF SYMBOLS 1 manufacturing apparatus of nitrided steel member 2 circulation type processing furnace 3 atmosphere gas concentration detection apparatus 4 nitriding potential regulator 5 internal retort 6 retort 7 furnace opening / closing lid 8 stirring fan 9 stirring fan drive motor 12 atmosphere gas piping 13 nitriding potential calculation in furnace Device 14 Gas flow control device 15 Parameter setting device (touch panel)
20 In-furnace gas supply unit 21 first in-furnace introduction gas supply unit 22 first in-furnace gas supply control device 23 first supply valve 25 second in-furnace introduction gas supply unit 26 second in-furnace gas supply control device 27 second Supply valve 29 In-furnace introduced gas introduction piping 30 Gas flow rate output adjustment device 31 Programmable logic controller 40 In-furnace gas disposal piping 41 Exhaust gas combustion decomposition device 100 One embodiment steel nitride member 101 hard layer 102 diffusion layer 150 steel nitride of comparative example Member 153 Compound layer 151 Hardened layer 152 Diffusion layer 201 Furnace wall or bell 202 Retort 203 Agitating fan 204 Guide tube (internal retort)
205 gas inlet tube 206 flared gas exhaust or gas hood 207 thermocouple 208 lid for cooling operation 209 blower for cooling operation

Claims

A nitride steel member having a carbon steel or low alloy steel as a matrix,
The surface is provided with a hardened layer having a martensitic structure containing 0.8% or more of nitrogen by mass%,
A diffusion layer in which nitrogen is diffused in the matrix is provided under the hardened layer.
The hardened layer has a thickness of 2 μm to 50 μm from the surface of the nitrided steel member,
The diffusion layer extends from the surface of the nitrided steel member to a depth of over 100 μm,
A nitrided steel member characterized in that the hardness of the diffusion layer at a depth of 100 μm from the surface of the nitrided steel member is 100 HV or more larger than the hardness at a depth of 2 mm from the surface of the nitrided steel member.
The nitrided steel member according to claim 1, characterized in that a carbon steel having a carbon content of 0.1% or more in mass% is used as a matrix.
A method of manufacturing a nitrided steel member having a carbon steel or a low alloy steel as a matrix by using a circulation type processing furnace provided with a guide tube and a stirring fan,
During nitriding, the temperature range in the circulating processing furnace is controlled to 610 ° C. to 660 ° C.,
A method of manufacturing a nitrided steel member, wherein the nitriding potential in the circulating treatment furnace is controlled to a range of 0.06 to 0.3 during the nitriding treatment.
It has a circulation type processing furnace with a guide tube and a stirring fan,
During nitriding, the temperature range in the circulating processing furnace is controlled to 610 ° C. to 660 ° C.,
At the time of the nitriding treatment, the nitriding potential in the circulation type processing furnace is controlled in the range of 0.06 to 0.3.
Ammonia gas and ammonia decomposition gas are introduced into the circulating treatment furnace,
The manufacturing apparatus is configured to control the nitriding potential
First control for changing the introduction ratio of the ammonia gas and the ammonia decomposition gas while keeping the total flow rate of the introduction amount of the ammonia gas and the introduction amount of the ammonia decomposition gas constant;
A second control that changes the introduction amount of the ammonia gas while stopping the introduction of the ammonia decomposition gas;
The apparatus for manufacturing a nitrided steel member according to claim 4, wherein the apparatus can selectively carry out.