KR20040064589A - Nitrification method of metal material - Google Patents

Abstract

PURPOSE: To provide a method for performing nitriding at high nitriding efficiency so that a high N material is manufactured, wherein the nitriding is advantageous in terms of productivity in separately manufacturing steel materials and in manufacturing high functional steel. CONSTITUTION: The method comprises a process of nitriding a metallic material having temperature that is 20 deg.C or more than that of the nitriding atmosphere as maintaining 600 deg.C or more of temperature at a nitriding atmosphere comprising gases in which one of the constituting elements of a gas molecule is N (except N2 gas) and having a temperature range from a room temperature to 800 deg.C or less, wherein the gases in which one of the constituting elements of a gas molecule is N are 0.5 to 100 % (volume ratio) of the nitriding atmosphere, wherein at least one gas out of the gases in which one of the constituting elements of a gas molecule is N is ammonia gas, wherein the nitriding atmosphere contains 1.0 vol.% or more of hydrogen gas, wherein the nitriding atmosphere is a mixed gas comprising ammonia gas and nitrogen gas, the nitrogen gas is 10 vol.% or more of the nitriding atmosphere, and amount of nitrogen gas/(amount of nitrogen gas+amount of hydrogen gas) is 0.60 or more, and the atmospheric gas has a dew point of -10 deg.C or more.

Description

Nitriding method of metal materials {NITRIFICATION METHOD OF METAL MATERIAL}

The present invention provides a material containing nitriding gas for the purpose of imparting desirable properties with respect to strength, fatigue properties, abrasion resistance, toughness, and the like in the manufacturing process of steel sheets and steel materials used in containers, automobiles, home appliances, building materials, and the like. The present invention relates to a method of nitriding in a nitriding furnace that maintains a nitriding atmosphere, and more particularly, to a method of efficiently nitriding a material while suppressing deterioration to nitriding due to handling of the nitriding atmosphere.

In steel products used in various aspects, various characteristics (for example, strength as a structural member or workability for forming a member, welding property with other members and weldability as strength of a joint, toughness in use, and other members during long-term use) Wear resistance at the site of contact with the body, corrosion resistance in various environments, magnetic properties as an electric device member, or corrosion resistance or designability).

MEANS TO SOLVE THE PROBLEM This inventor controls material | concentration which considers the improvement of strength, workability, toughness, abrasion resistance, magnetic property, corrosion resistance, etc. with respect to high N steel containing a large amount of N so that it cannot think conventionally, and concentrates N instead of conventional C. In this way, steels with structure control, and electronic steel sheets with controlled nitride forms in steels were developed. These are disclosed in, for example, Japanese Patent Laid-Open No. 2002-020834, Japanese Patent Laid-Open No. 2002-012948, and the like. The great feature of these steels is that they do not necessarily contain a large amount of Cr and Ni, and contain a large amount of N (nitrogen) that cannot be reached by a manufacturing method in which component adjustment is made in conventional molten steel. For this purpose, the method of component adjustment with respect to the steel of a solid state is needed, and it is thought that it is effective to employ | adopt the method of nitriding a material in gas atmosphere as a monopoly method. However, in order to contain a large amount of N whose N content exceeds 0.1%, since the production efficiency is low by the conventional method and it becomes the obstacle of practical use, the present inventor has proposed a very efficient gas nitriding method in Japanese Patent Application 2002-90647. Proposed in the issue.

In addition, if the components can be changed in the following steps of the manufacturing process by nitriding about 0.01% even if not a large amount up to 0.1%, materials having various materials can be separated and manufactured using the same material, thereby reducing the inventory of semi-finished products. In addition, we expect significant improvements in the steel manufacturing process. Even in such a case, if efficient nitriding is possible, the nitriding time can be shortened and the nitriding facility can be simplified, and the practical use of the nitriding process is promoted.

However, also in the technique described in Japanese Patent Application No. 2002-90647 filed by the present inventor, another improvement in nitriding efficiency is also required.

In order to nitride the steel, it is necessary to maintain the steel in a high temperature nitriding atmosphere, but there is a possibility that the nitriding itself, which maintains the nitriding atmosphere, may also be nitrided and deteriorated. In particular, for a long time in a high temperature and high concentration ammonia atmosphere having high nitriding efficiency, a guide roll, a furnace member, a burner, etc., which are provided for passing steel materials through the furnace, may be nitrided and fall off. Since nitriding occurs on hot metal surfaces, it is a solution to cover these members with a material other than metal, or with a material other than metal, or to cool the furnace members with a water-transmitted tube or the like. Considering this is not practical.

In addition, the nitriding of the furnace itself means that the gas component introduced into the furnace due to nitriding is not efficiently consumed for nitriding steel materials.

In addition, when the temperature of the nitriding atmosphere in the nitriding furnace is set to be higher than the temperature required for nitriding the material in order to increase the nitriding efficiency, not only the nitriding of the furnace member is promoted, but also the gas itself does not exist in the nitriding material. Since it decomposes and the nitriding capacity falls, the nitriding efficiency does not improve as expected.

As described above, in the prior art, there has been a strong demand for improvement of the nitriding method for the efficient production of high N materials.

[Patent Document 1]

Japanese Patent Laid-Open No. 2002-020834

[Patent Document 2]

Japanese Patent Publication No. 2002-012948

The present invention solves the problems of the prior art as described above, and provides a method of nitriding with high nitriding efficiency when applying a nitriding method that is advantageous from the viewpoint of productivity in terms of the production of the steel material and the production of the high-performance steel. An object of this invention is to enable the production of an efficient high N material.

1 shows a nitriding method A of the present invention.

2 shows a nitriding method B of the present invention.

3 is a view showing a nitriding method C of the present invention.

4 shows a nitriding method D of the present invention;

Fig. 5 shows the nitriding method E of the present invention.

MEANS TO SOLVE THE PROBLEM In order to solve the subject mentioned above, the present inventors earnestly examined the nitriding conditions of the material in a nitriding atmosphere, and the structure of the equipment which perform nitriding, and acquired the following knowledge. In other words, the nitriding atmosphere itself is capable of nitriding the metal material by maintaining the temperature of the metal material itself, which can be placed in the nitriding atmosphere even at a relatively low temperature, above the temperature of the nitriding atmosphere. According to this method, since the temperature of the furnace and the furnace member which maintains an atmosphere can be suppressed below the temperature required for nitriding material, it becomes possible to suppress nitriding of the furnace itself, and it is possible to use a higher concentration of nitriding atmosphere. Become.

In addition, since the nitriding gas itself can be suppressed from being decomposed by keeping the nitriding atmosphere itself at a relatively low temperature, the nitriding efficiency can be realized by setting the metal material temperature higher than conventionally.

As a result, the decomposition of the nitriding gas on the surface of the metal material is promoted to increase the nitriding efficiency, and also to increase the diffusion rate toward the center of the metal material of the N atoms intruding from the surface of the metal material. It is easy to obtain a component homogeneously steel material, and can also suppress formation of the Fe nitride film on the surface of the metal material which may be a problem by nitriding at low temperature. In addition, since the metal material temperature can be set higher than conventionally, it is possible to use various kinds of thermally more stable gases other than ammonia gas as the nitride gas decomposed on the surface of the metal material. The present invention has been made in consideration of the most suitable nitriding conditions and nitriding equipments in view of the conventional metal material manufacturing process and facility capability in the past based on the above findings, and the gist thereof is as follows. (1) As a heating method of the metal material, a method other than the heat conduction from the nitriding atmosphere is matched with the heat conduction or is employed without heating by the heat conduction. ② Set the temperature of the nitriding atmosphere lower than the temperature of the metal material. ③ The nitriding atmosphere is controlled to obtain the best nitriding efficiency in the above state.

Specifically, the gist of the present invention is as described below in the claims.

(1) Nitriding a metallic material at least 600 ° C and at least 20 ° C higher than the temperature of the nitriding atmosphere in a nitriding atmosphere at or above 800 ° C, including a gas whose N constituent element is N (excluding N2 gas). Nitriding method of a metal material including the process of making.

(2) The nitriding method of the metal material as described in (1) characterized by the gas whose one of the constituent elements of a gas molecule is N is 0.5-100% (volume rate) of an atmosphere.

(3) The method of nitriding metal materials according to (1) or (2), wherein at least one kind of gas in which one of the constituent elements of the gas molecules is N is ammonia gas.

(4) The nitriding method of any one of (1) to (3), wherein the nitriding atmosphere contains hydrogen gas having a volume ratio of 1.0% or more.

(5) The nitriding atmosphere is a mixed gas containing ammonia and nitrogen, the nitrogen gas is 10% or more (volume ratio) of the nitriding atmosphere, and the nitrogen gas amount / (nitrogen gas amount + hydrogen gas amount) is 0.60 or more ( The nitriding method of the metal material in any one of 1)-(3).

(6) The nitriding method of the metal material in any one of (1)-(5) characterized by the dew point of a nitriding atmosphere gas being -10 degreeC or more.

(7) The metal material is heated by a method other than thermal conduction from the nitriding atmosphere, and subsequently, the metal material is charged into the nitriding atmosphere, and the metal material is nitrided until the temperature of the metal material becomes 50 ° C or more lower than the temperature at charging. The nitriding method of the metal material in any one of (1)-(6) characterized by holding in atmosphere.

(8) The method of nitriding a metal material according to any one of (1) to (7), wherein the metal material is heated in a nitriding atmosphere by a means different from the heat conduction from the nitriding atmosphere.

(9) The nitriding method of the metal material as described in any one of (1)-(8) characterized by heating a metal material using an electricity supply heating or an induction heating.

(10) The method of nitriding a metal material according to any one of (1) to (9), wherein the N content is increased by 0.0002% or more by mass% by the step of nitriding the metal material.

(11) The method for nitriding a metal material according to any one of (1) to (10), wherein the metal material is a steel material.

The present invention will be described in detail below.

Although the present invention can be applied to a metal material having all components, the following mainly explains the application to steel materials.

First, the reason for limitation of the atmosphere component in this invention is demonstrated in detail below. It is an essential condition of the present invention that a gas having N as one of the constituent elements of the gas molecules in the atmosphere is present. Examples of the gas include ammonia gas and nitrogen dioxide gas. Hereinafter, in this specification, these gases are described as "nitriding gas." Nitriding in the present invention is a main mechanism in which the atomic N generated by the decomposition of the nitriding gas on the hot metal surface penetrates the steel, and the present invention controls the decomposition of the nitriding gas and the reaction with the steel surface. It is. In the present invention, however, nitrogen is not included in the nitriding gas. This is because nitrogen is difficult to decompose relatively stably and therefore is not suitable for the purpose of the present invention, which increases the nitriding efficiency.

The nitriding gas is preferably ammonia gas because of the nitriding efficiency, that is, the decomposition reaction rate on the surface of the hot metal, the ease of use in the working environment, and the like. Of course, mixing and using various types of nitriding gases does not impair the effects of the present invention. The atmosphere containing nitriding gas to which nitriding is performed is described below as "nitriding atmosphere" in the present specification. The concentration of nitriding gas in the nitriding atmosphere has a great influence on the nitriding efficiency. If any nitriding gas is present, there is a small possibility that nitriding of steel materials may occur, but the concentration for causing industrially efficient nitriding is made 0.5% or more by volume ratio. The upper limit of this concentration is not necessary, and it is also possible to set it as 100% in complete nitrogen gas atmosphere.

It does not specifically limit about gas components other than nitriding gas in nitriding atmosphere. It is possible to include inert gas such as argon and hydrogen or nitrogen used in heat treatment of ordinary steel materials.

In order to improve nitriding efficiency, the control of the concentration of gas components other than these nitriding gases and the amount of water inevitably contained, that is, the atmospheric dew point may also be effective. In particular, high nitriding efficiency may be obtained by setting hydrogen to 1% or more or the dew point to -10 ° C or more. Although the reason is not clear, there is a possibility that the decomposition of water vapor or the state of oxygen on the surface of the steel sheet affects the penetration of N into the steel caused by the decomposition of the nitride gas on the steel surface and its decomposition. In addition, there is a possibility that hydrogen supply is caused by decomposition of steam, which may cause any problem, but in addition to the hydrogen supply source, introduction of hydrogen gas from the outside, decomposition of nitriding gas such as ammonia gas, etc. are overlapped. I think it's very complicated.

It shows below about these density | concentrations when nitriding atmosphere contains ammonia gas, nitrogen, and hydrogen. In this case, nitriding of the steel is mainly caused by decomposition of the ammonia gas, but with this, the abundance ratio of nitrogen gas and hydrogen gas in the nitriding atmosphere also changes to produce atomic nitrogen and atomic hydrogen. In this case, when the volume ratio nitrogen gas is 10% or more, the hydrogen gas 1% or more, and the nitrogen gas amount / (nitrogen gas amount + hydrogen gas amount) is 0.60 or more, very high efficiency nitriding becomes possible. The nitriding atmosphere concentration is macroscopically in a nitriding furnace, in the microscopic sense, in the vicinity of the surface of the steel where the nitriding reaction is reliably occurring, in the vicinity of the surface of the furnace member, or in the case of gas decomposition or synthesis, or in which the nitriding proceeds continuously. Although it is not always possible to be constant both spatially and temporally in the actual industry, for example, if it is difficult to determine the average concentration of the nitriding atmosphere, which is considered to be valid as a guideline for controlling the nitriding efficiency of steel materials, it is introduced into the nitriding furnace. It is also possible to determine based on the flow rate of the gas.

Temperature conditions during nitriding are the most important factor of the present invention. A feature of the present invention is that the temperature of the steel is made higher than the temperature of the nitriding atmosphere. In the conventional nitriding method, the nitriding atmosphere is heated to a temperature necessary for nitriding a material, and the nitriding is carried out at the same time as heating and maintaining the steel by keeping the steel therein. For this reason, the reduction of the metal material nitriding efficiency by nitriding itself by nitriding and the decomposition of the nitriding gas itself other than the metal material surface becomes an obstacle, and the upper limit of the temperature of nitriding atmosphere and the upper limit of the temperature which nitrides a material are barely. It was 800 degreeC.

In the present invention, the steel is heated by a method that does not conform to the heat conduction from the nitriding atmosphere, and the nitriding atmosphere itself is at a lower temperature than the heated steel, so that the nitriding furnace itself or the nitriding gas other than the surface of the metal material itself is used. Problems such as deterioration of the metal material nitriding efficiency due to decomposition can be avoided. In order to nitrate steel materials efficiently, it is necessary to make the temperature of steel materials 600 degreeC or more. Preferably it is 700 degreeC or more, More preferably, it is 800 degreeC or more, 900 degreeC or more, and very quick nitriding becomes possible. At the same time, the diffusion of N in the steel into the steel from the steel surface also occurs quickly, so that it becomes possible to suppress the uniformity of the N concentration in the steel and the formation of Fe nitride which may be generated on the surface and impair the characteristics. However, for example, in a material that controls the texture of the steel aggregates in the cold rolling process, when the temperature becomes very high, the texture of the aggregates controlled by the transformation may be lost. Also, by forming a concentration difference consciously in the thickness direction of the steel, In order to improve a characteristic, since the effect of a characteristic improvement may become small, it needs attention. In particular, considering only nitriding efficiency, heating to 1000 degreeC or more is also possible.

On the other hand, the temperature of the nitriding atmosphere is preferably 800 ° C. As described above, if the high temperature nitriding atmosphere is maintained in the heat treatment furnace, the nitriding efficiency is lowered by nitriding the furnace itself, and the high temperature nitriding gas becomes unstable and decomposes in the absence of a nitriding object. This is because the lowering of the nitriding efficiency. The temperature of the nitriding atmosphere is preferably 750 ° C. or less, more preferably 650 ° C. or less, and when it is 550 ° C. or less, nitriding of the furnace itself hardly occurs. Of course, even at room temperature without any heating does not impair the effects of the present invention.

It is an important feature of the present invention to raise the temperature of the steel at least 20 ° C or higher than the temperature of the nitriding atmosphere at least during the nitriding period. Without a difference of 20 ° C. or more, it is impossible to achieve the effect of the present invention of improving the nitriding efficiency of the metal material while suppressing the nitriding of the furnace or decomposition of the nitriding gas itself. However, this temperature difference does not need to exist throughout the nitriding process. That is, for example, when a steel material having a temperature higher than 20 ° C. or higher is inserted into the nitriding atmosphere, and the nitriding is carried out, when the heating means of the steel in the nitriding atmosphere is not described, The temperature decreases and soon approaches the ambient temperature. Even in such a case, since the advantages of nitriding with high efficiency in the previous stage of nitriding can be obtained, the present invention shall be included.

The temporary period is preferably 1 second or more, more preferably 3 seconds or more. In this case, if the temperature difference is too small, the advantages of the present invention are also reduced. Therefore, it is preferable to set the temperature difference, that is, the temperature drop of the steel material accompanying nitriding, to 50 ° C or more. However, when the nitriding atmosphere is lower than 550 ° C., the nitriding efficiency becomes very small and approaches zero. In order to obtain the effect of the present invention, the temperature of the steel is maintained as long as possible and the temperature difference is maintained as much as possible. It is a matter of course that it is preferable to maintain the temperature higher than the nitriding atmosphere. To this end, it is preferable to apply a facility that can heat the steel in the nitriding atmosphere. Since the use of heat conduction from the atmosphere as a heating means of steel impairs the spirit of the present invention, it is preferable to use means such as energization heating or induction heating as this means.

By using such a means, the temperature rising only of steel materials becomes possible in the state which avoided the temperature rise of the thing in which temperature rise is unnecessary, such as a nitriding atmosphere and a furnace member. Thereby, it becomes possible to hold | maintain 900 degreeC or more steel materials for a long time in the nitriding atmosphere about room temperature, for example. In this case, since the temperature of the nitriding atmosphere or the temperature of the furnace member is increased by the heated steel, it may be considered that nitriding or decomposition of the nitriding gas itself occurs at an undesired place. In order to avoid this, it is preferable to consider the flow rate of the nitriding atmosphere or the like in contact with the steel. In other words, it is desirable to control the nitriding by blowing a large amount of gas to the surface of the steel material that is heating the high-speed gas, and to remove the heat from the furnace. In addition, laser irradiation, infrared irradiation, etc. can also be considered as a heating method of steel materials, but these high-energy rays collide with nitride gas molecules in a nitriding atmosphere before they reach the steel materials, and thus decompose gas molecules. It is necessary to consider the fall of efficiency.

In various fields, a technique is employed in which a high concentration of N is contained in the material, particularly in the surface layer, thereby making it highly functional by surface hardening or the like. When the amount of nitriding is very small, the effect of the invention is also small. Therefore, it is preferable to apply when the increase in N content by nitriding is 0.0002% or more in mass%. In particular, when applied to nitriding only the surface, the case where the increase of nitrogen content in nitriding site | part is considerably large is considered, and said 0.0002% is the average value of the nitrided material.

This invention is effective especially when a large amount of N addition is required, and the amount of increase of N content by nitriding becomes like this. Preferably it is 0.01% or more, More preferably, it is 0.1% or more, More preferably, it is 0.3% or more. In the case of steel, unless the so-called stainless steel containing a large amount of Cr, Ni and the like is used, the upper limit of N that can be obtained by component adjustment by molten steel, which is a conventional manufacturing method, is only about 0.03%. In particular, when applied to the techniques disclosed in Japanese Patent Laid-Open No. 2002-020834 and Japanese Patent Laid-Open No. 2002-012948 filed by the inventor, the industrial effect is absolute.

In addition, the holding time in nitriding atmosphere in nitriding is decided by the balance with the target characteristic and N amount, and is not specifically limited. In some cases, the object can be achieved by holding a few seconds, and when it is necessary to contain a high concentration of N up to the center of the steel plate or the large steel plate with a thick plate thickness, a long time maintenance is required. In the case of continuous annealing, the limit is only 30 minutes. However, by using box annealing or the like, treatment of several hours or more can be performed. Considering operability and productivity, 2 seconds to 20 days are practical ranges. Although the timing of nitriding can be performed at any time between the cast and the product, the nitriding utilizes the diffusion of N from the surface into the steel, and the thinner and smaller the thickness and size of the material, the easier the nitriding of high concentration. In general, nitriding is advantageous after being processed into a shape close to the final product.

In the case of the steel sheet manufactured in a general process, it is preferable to perform it in the process after hot finishing rolling, and in manufacture of a normal cold rolled steel sheet, nitriding according to this invention as a part of the cooling process after recrystallization annealing is in a state of production. good.

The present invention is employed in the first half of the steel manufacturing process to contain a high concentration of N in the steel, and then adds a step of performing material control or structure control by high temperature treatment or correction at a suitable temperature, or by adding a nitride layer. It is also possible to add a step of imparting good thermal history to diffusion, or to adopt the nitriding step according to the present invention after imparting recrystallization and appropriate properties by reaching the maximum temperature of the annealing step.

The use of the present invention is not limited to the shape or the like, but can be applied not only to application to metal materials and steel materials as materials but also to processed members used in automobiles, containers, buildings and the like.

(Example)

Evaluation of nitriding efficiency was performed using the experimental equipment shown in FIG. 1 thru | or FIG. The used material is a commercially available steel sheet and rail steel. Fig. 1 heats a steel sheet in an induction heating furnace in a plate of a coil of a cold rolled steel sheet, and then immediately blows a gas containing nitriding gas into the nitriding furnace to perform nitriding. 2, the roll was used as an electrode, and the steel plate between electrode rolls was energized and heated, and at the same time, between the rolls, the inside of the nitriding furnace filled with nitriding atmosphere was filled. 3, the rail steel was placed in a batch nitriding furnace, and the rail was heated by an energization heating furnace. 4 is a conventional batch heating furnace. Fig. 5 controls the atmosphere and the atmosphere temperature in the furnace in the continuous annealing line of the ordinary steel sheet. In addition, although not shown, since surface oxidation of the steel sheet at the position where nitriding is not performed is prevented and the surface forming is equivalent to that obtained in the production of ordinary steel, parts other than the nitriding furnace are also suitable as in the conventional manufacturing method. It is very desirable to be controlled by the atmosphere.

Table 1 shows the evaluation results of the conditions of nitriding furnace and nitriding efficiency. Nitriding atmosphere is made into the mixed gas of ammonia gas, nitrogen gas, and hydrogen gas, and is shown in Table 1 by composition volume%. In addition, the atmosphere of the heating furnace in nitriding method D is 80% of nitrogen and 20% of hydrogen gas.

Nitriding efficiency was evaluated by N content by the chemical analysis of the steel materials before and after nitriding. In the invention example that satisfies the conditions of the present invention, even in the same nitriding atmosphere and the same or short nitriding time, the N increase in the steel is high, indicating that the nitriding efficiency of the present invention is high. Is smaller than the comparative material.

On the other hand, in the comparative example which does not satisfy the conditions of the present invention, even though the same nitriding atmosphere and the same or short nitriding time, the N increase in the steel value was low, indicating that the nitriding efficiency was low. Since it is necessary to apply the high temperature nitriding atmosphere deviating from the conditions of this invention which make temperature higher 20 degreeC or more than atmospheric temperature, furnace material nitriding becomes "a lot".

As described above, according to the present invention, the metal material can be nitrided for the purpose of imparting desirable properties with respect to strength, fatigue properties, abrasion resistance, toughness, and the like in the manufacturing process of the metal material including the steel sheet or the steel. In the production of material classification of materials, nitriding can be carried out with high nitriding efficiency by applying a nitriding method that is advantageous from the viewpoint of productivity, and it is possible to produce a high-productivity material separation and exhibit industrially useful and remarkable effects.

Claims (11)

A process of nitriding a metal material that is at least 600 ° C. and at least 20 ° C. higher than the temperature of the nitriding atmosphere in a nitriding atmosphere at or above 800 ° C. including a gas having one of the constituent elements of the gas molecule (excluding N 2 gas). Nitriding method of a metal material comprising the. The method for nitriding a metal material according to claim 1, wherein a gas in which one of the constituent elements of the gas molecules is N is 0.5 to 100% (volume ratio) of the nitriding atmosphere. The method for nitriding a metal material according to claim 1 or 2, wherein at least one kind of gas in which one of the constituent elements of the gas molecule is N is ammonia gas. The nitriding method according to any one of claims 1 to 3, wherein the nitriding atmosphere contains hydrogen gas having a volume ratio of 1.0% or more. The nitriding atmosphere is a mixed gas containing ammonia and nitrogen, the nitrogen gas is 10% or more (volume ratio) of the atmosphere, and the amount of nitrogen gas / (nitrogen gas amount + hydrogen). The amount of gas) is 0.60 or more. The method for nitriding a metal material according to any one of claims 1 to 5, wherein the dew point of the atmospheric gas is -10 ° C or higher. The metal material according to any one of claims 1 to 6, wherein the metal material is heated by a method other than heat conduction from the nitriding atmosphere, the metal material is subsequently charged into the nitriding atmosphere, and the temperature of the metal material is charged. A method of nitriding a metal material, characterized in that it is held in a nitriding atmosphere until it is 50 ° C. or more lower than the temperature. The method of nitriding a metal material according to any one of claims 1 to 7, wherein the metal material is heated in a nitriding atmosphere by a means different from heat conduction from the nitriding atmosphere. The method of nitriding a metal material according to any one of claims 1 to 8, wherein the metal material is heated using energized heating or induction heating. The method for nitriding a metal material according to any one of claims 1 to 9, wherein the N content is increased by 0.0002% or more by mass% by the step of nitriding the metal material. The method for nitriding a metal material according to any one of claims 1 to 10, wherein the metal material is a steel material.