WO2012176551A1

WO2012176551A1 - Steel product and manufacturing method therefor

Info

Publication number: WO2012176551A1
Application number: PCT/JP2012/061166
Authority: WO
Inventors: 晃司西川; 尚男冨士川; 渡辺　崇則; 岩村　英明; 守弘藤田
Original assignee: エア・ウォーター株式会社
Priority date: 2011-06-24
Filing date: 2012-04-19
Publication date: 2012-12-27
Also published as: JP5044712B1; US20140102592A1; CN103620081B; JP2013007094A; CN103620081A

Abstract

A steel product that combines excellent corrosion resistance and adhesiveness is provided. The steel product is obtained by forming a nitride compound layer and a nitrogen diffusion layer on the surface layer of a steel base material. The nitride compound layer contains a first compound layer that is formed on the nitrogen diffusion layer side, and a second compound layer that is formed on the surface side of the first compound layer. The first compound layer has an ε-structure composed mainly of Fe3N, and the second compound layer has a higher nitrogen concentration than the first compound layer, and has protrusions and recesses formed on the surface. This configuration enables a nitride compound layer with excellent adhesiveness, extremely low carbon content and high corrosion resistance to be formed.

Description

Steel product and manufacturing method thereof

The present invention improves the corrosion resistance and adhesion by performing a surface treatment with nitriding treatment on a steel material, and improves its durability by applying to a back metal such as a brake pad, brake shoe, clutch plate, etc. The present invention relates to a steel product that can be manufactured and a manufacturing method thereof.

Friction members such as brake pads, brake shoes, and clutch plates used in automobiles and the like are generally frequently used in which a friction material is bonded to a steel material as a back metal. The adhesive strength between the friction material and the back metal is related to the performance and quality of the brake and is an extremely important factor from the safety aspect.
In order to obtain a strong adhesive force to the friction material, a method of roughening the surface by a technique such as shot peening has been adopted for the back metal for a long time. In addition, since the adhesive strength is greatly reduced when rust is generated at the adhesive interface, a treatment for forming a chemical conversion coating such as zinc phosphate and a primer layer is performed in consideration of corrosion resistance (for example, Patent Document 1 below). ).
However, the above chemical conversion film has a problem with respect to heat resistance, and has a problem that corrosion resistance and adhesion are lowered by heat. In brakes and clutches, the characteristic that the corrosion resistance and adhesion deteriorate when large frictional heat is generated is extremely undesirable.
On the other hand, nitriding has been applied in a wide range of fields in order to improve the wear resistance and durability of mechanical parts and structural members made of various steel materials. Among them, it is also known that steel materials other than stainless steel can improve corrosion resistance by forming a nitride compound layer on the surface thereof. One of the uses is, for example, application to a back pad of a brake pad. A backing metal obtained by nitriding a steel material excluding stainless steel is more advantageous in terms of corrosion resistance than those obtained by forming the above-described chemical conversion film. Therefore, it has been studied to apply gas soft nitriding to the back metal of the brake pad (for example, Patent Document 2 below).
However, when the back metal is formed by nitriding, it is not sufficient to simply improve the corrosion resistance by forming a nitride compound layer on the surface. In order to improve adhesion when a pad material, which is a friction material, is adhered, it is necessary to form micron-order irregularities on the surface.
Here, since salt bath nitriding treatment is easy to form a relatively uneven surface, it is applied to a back metal (for example, Patent Document 3). In the salt bath nitriding treatment, an Fe—C—N-based compound is formed in the same manner as a nitride formed by a normal gas soft nitriding treatment. For this reason, like patent document 2, it cannot be said that corrosion resistance is enough.

JP-A-5-346129 Shoko 53-47218 JP 62-261726

In Patent Document 2, there is a description that irregularities such as humps and mesh-like protrusions are formed in order to improve adhesion to the friction material. However, it is extremely difficult to form irregularities that sufficiently improve the adhesiveness only by general gas soft nitriding treatment, and there is no mention of a specific method thereof. In addition, the Fe—C—N-based compound formed by gas soft nitriding is not sufficient for its use as a brake pad back metal in terms of its own corrosion resistance. Furthermore, in a nitrided compound layer formed by a general gas soft nitriding treatment, defects that reach from the surface to the diffusion layer are normally formed, which adversely affects corrosion resistance. Because of these problems, the actual situation is that the application of gas nitriding to the back metal has not progressed as a real problem.
On the other hand, in Patent Document 3, a salt bath nitriding treatment is used as a method of forming an Fe—C—N compound. In the salt bath nitriding treatment, unlike the nitriding treatment using a normal gas, it is easy to form irregularities that ensure adhesion performance on the surface. On the other hand, since it is a solid-liquid reaction, a reaction in which a base material component such as Fe dissolves into the salt bath also proceeds simultaneously. For this reason, penetrating defects that reach the diffusion layer are much easier to form than nitriding using gas. Therefore, in order to increase the unevenness of the surface and improve the corrosion resistance compared with the usual salt bath nitriding treatment, it is necessary to form a triiron tetroxide layer on the surface, which also requires a considerable thickness. I can easily imagine what would happen.
That is, when applying a salt bath nitriding treatment, it is desirable to form a thick oxide layer to seal defects, particularly in terms of corrosion resistance, and this method can be expected to improve corrosion resistance to some extent. However, the strength of triiron tetroxide itself is extremely low, and if the oxide layer is formed thick, there is a high possibility that cracks propagate in the triiron tetroxide layer and the friction material peels off. Further, since the number of penetrating defects is large and the size is large as compared with the gas nitriding treatment, sufficient sealing is not easy. Therefore, even if an oxide layer is formed as described in Patent Document 3, it cannot be said that it is a practical method because sufficient corrosion resistance cannot always be obtained.
On the other hand, it is also possible to consider using stainless steel with good corrosion resistance to improve its adhesion or to form a plating layer with some strength. However, these methods are still expensive and difficult to apply. Therefore, there is a strong demand for the development of a technique that achieves both corrosion resistance and adhesion by a relatively inexpensive method.
This invention is made | formed in view of such a situation, and it aims at providing the steel product which has the outstanding corrosion resistance and adhesiveness, and its manufacturing method.

Various investigations were repeated in order to form a surface layer having higher corrosion resistance and higher adhesion on steel while taking advantage of the relatively inexpensive and high corrosion resistance of nitriding using gas. As a result, a nitride compound layer formed by diffusing a halogen element to a depth of 1 μm or more from the surface and then performing nitriding in a gas atmosphere not containing a carbon source is, as shown in FIG. A first compound layer having an ε structure mainly composed of Fe ₃ N, and a heterogeneous material formed on the surface side of the first compound layer, having a higher nitrogen concentration than the first compound layer and having surface irregularities It was ascertained that it was configured to include two layers of the second compound layer. And it turned out that the nitride compound layer containing the said 2 layer has high adhesiveness and corrosion resistance, and came to completion of this invention.
In addition, when an oxide layer mainly composed of iron oxide is formed on the nitride compound layer, unnecessary nitriding, oxide, oxynitride, etc. on the surface are added by adding a peening treatment with fine grains. It was also found that when removed, the adhesion and corrosion resistance were further improved.
That is, the steel product of the present invention is a steel product in which a nitride compound layer and a nitrogen diffusion layer are formed on the surface layer of a base material that is steel,
The nitride compound layer includes a first compound layer formed on the nitrogen diffusion layer side, and a second compound layer formed on the surface side of the first compound layer,
The first compound layer has an ε structure mainly composed of Fe ₃ N,
The second compound layer has a higher nitrogen concentration than the first compound layer and has an uneven surface.
The steel product manufacturing method of the present invention includes a nitride compound layer on the surface of the base material by performing a gas nitriding treatment for diffusing nitrogen after performing a fluorination treatment for diffusing fluorine on the surface of the steel material. A method for producing a steel product for forming a nitrogen diffusion layer,
The fluorination treatment includes a fluorination process in which fluorine reacts with the steel material while introducing the fluorine source gas into the treatment furnace, and fluorine that has been heated and held in a state where the supply of the fluorine source gas is stopped to penetrate the steel material surface. A diffusion step of diffusing to a depth of at least 1 μm,
The gist of the gas nitriding treatment is to reduce and vaporize the diffused fluorine component in a gas atmosphere not containing a carbon source, and to diffuse and infiltrate nitrogen to form a nitride compound layer and a nitrogen diffusion layer. .

In the steel product of the present invention, the nitride compound layer includes a first compound layer formed on the nitrogen diffusion layer side and a second compound layer formed on the surface side of the first compound layer. The first compound layer has an ε structure mainly composed of Fe ₃ N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has irregularities formed on the surface.
The second compound layer has surface irregularities having many intermittent grooves with a predetermined depth. Further, it is formed by growing an Fe—N-based nitride compound outward. Furthermore, it is made of nitride having a higher nitrogen concentration than the first compound layer. For this reason, for example, even when a friction material is bonded and a large shear stress is applied to the friction material, the uneven shape on the surface is not easily destroyed.
In addition, the second compound layer that forms the Fe—N-based surface irregularities, and particularly the first compound layer formed on the nitrogen diffusion layer side, has better corrosion resistance than the Fe—C—N-based compound. Furthermore, due to the effects of the fluorination treatment and the gas nitriding treatment, a denser nitride compound layer is formed, and there are very few defects that penetrate from the surface to the nitrogen diffusion layer. Therefore, it becomes a nitride compound layer with extremely excellent corrosion resistance.
In the steel product of the present invention, when the surface unevenness of the second compound layer has a recess having a depth of 0.5 μm or more at a high density,
A complex uneven shape having a depth of 0.5 μm or more is formed on the surface layer portion, and this becomes a surface layer excellent in adhesiveness to a friction material or the like.
In the steel product of the present invention, the second compound layer has a surface nitrogen concentration of 12% by mass or more.
Increasing the amount of N intrusion into the surface layer so that the surface nitrogen concentration of the second compound layer is 12% by mass or more, and forming a nitride compound with a high N concentration can promote the outward growth of the nitride compound. It becomes possible. Therefore, complex surface irregularities having a predetermined thickness are formed on the surface layer portion, which becomes a surface layer excellent in adhesiveness with a friction material or the like.
In the steel product of the present invention, when the thickness of the second compound layer is 0.7 μm or more,
By increasing the amount of N penetration into the surface layer so that the thickness of the second compound layer is 0.7 μm or more and forming a high N concentration nitride compound, it is possible to promote the outward growth of the nitride compound Become. Therefore, complex surface irregularities having a predetermined thickness are formed on the surface layer portion, which becomes a surface layer excellent in adhesiveness with a friction material or the like.
In the steel product of the present invention, when the thickness of the first compound layer is 5 μm or more,
The entire nitride compound layer becomes denser and the presence of defects penetrating to the nitrogen diffusion layer is extremely reduced, so that the corrosion resistance is remarkably improved.
In the steel product of the present invention, when the actual surface area ratio per unit area of the surface exceeds 1.8,
The true surface area with respect to the apparent surface area is sufficiently large, and the degree of unevenness of the surface unevenness is accordingly high, so that reliable and stable adhesion performance can be ensured.
In the steel product of the present invention, when an oxide layer mainly composed of iron oxide having a thickness of 3 μm or less is formed as the outermost layer on the second compound layer,
Furthermore, corrosion resistance can be improved. That is, the nitride compound layer has a high concentration of invading N. In particular, the nitride compound layer closer to the nitrogen diffusion layer becomes a denser compound layer. However, it is extremely difficult to completely eliminate the penetration defect, and there is a concern about the generation of rust through the route. For this reason, the corrosion resistance is further improved by covering it with an oxide layer. The nitride compound layer according to the present invention has a) a higher corrosion resistance of the nitride compound layer itself than other nitriding methods, b) a relatively dense compound layer can be formed on the nitrogen diffusion layer, and c) a source of rust. Since there are favorable conditions for corrosion resistance, such as the number of through defects that are likely to be small, the volume of through defects in the nitride compound layer can be increased simply by forming an oxide layer with a thickness of 3 μm or less that does not impair adhesion. The portion is sealed to form a surface layer with extremely high corrosion resistance. Therefore, for example, when applied to the back metal of a brake shoe, peeling of the friction material due to the occurrence of corrosion is effectively prevented.
In the steel product of the present invention, when the penetrating defects present in the second compound layer are sealed with an oxide mainly composed of iron oxide,
Furthermore, corrosion resistance can be improved. That is, the nitride compound layer has a high concentration of invading N. In particular, the nitride compound layer closer to the nitrogen diffusion layer becomes a denser compound layer. However, it is extremely difficult to completely eliminate the penetration defect, and there is a concern about the generation of rust through the route. For this reason, the corrosion resistance is further improved by sealing it with an oxide. The nitride compound layer according to the present invention has a) a higher corrosion resistance of the nitride compound layer itself than other nitriding methods, b) a relatively dense compound layer can be formed on the nitrogen diffusion layer, and c) a source of rust. Since there are favorable conditions for corrosion resistance, such as the number of through defects that are likely to be small, the volume of through defects in the nitride compound layer can be increased simply by forming an oxide layer with a thickness of 3 μm or less that does not impair adhesion. The portion is sealed to form a surface layer with extremely high corrosion resistance. Therefore, for example, when applied to the back metal of a brake shoe, peeling of the friction material due to the occurrence of corrosion is effectively prevented.
In the method for producing a steel product of the present invention, a halogenation treatment is performed in which a halogen element is diffused and permeated to a depth of 1 μm or more prior to the gas nitriding treatment. Thereafter, the halogen element is reacted with active hydrogen generated by the decomposition of NH ₃ gas to cause a reduction reaction and vaporize. Subsequently, nitriding is performed in an atmosphere mainly composed of NH ₃ gas containing no carbon source. By doing in this way, the nitrided compound layer which has the surface unevenness | corrugation which has many intermittent grooves of the depth of 0.5 micrometer or more is formed.
A nitrided compound layer formed by a general nitriding treatment has a property that it is very easy to take in carbon present in the surroundings and carbon in the substrate. In contrast, the nitride compound layer formed in the present invention has an extremely low carbon content. That is, the Fe-N-based nitride compound layer is formed instead of the Fe-CN-based. The present invention in which the Fe—N-based nitride compound layer is formed is much more excellent in corrosion resistance than the formation of the Fe—C—N-based nitride compound layer.
Since it is easy to form irregularities on the surface, salt bath nitriding is often used as a surface treatment method for the back metal at present. However, in the salt bath nitriding treatment, since a nitriding treatment is performed using a cyan compound, carbon is inevitably diffused together with nitrogen. For this reason, it has been found that the surface nitride compound layer inevitably contains a high concentration of carbon, which adversely affects the corrosion resistance.
Therefore, in the present invention, nitriding is performed using a nitriding gas that does not contain a carbon source. This makes it possible to form a nitride compound layer having a very low carbon content and high corrosion resistance even if the material contains carbon at a certain high concentration. That is, in a general nitriding treatment, the carbon component contained in the base material is inevitably contained in the nitride compound layer. In the present invention, the surface is activated by the diffusion of fluorine in the fluorination treatment and the subsequent reduction of the fluorine component to facilitate the invasion of N. On the activated surface, the reaction between active hydrogen generated by the decomposition of ammonia gas and carbon existing in the nitride compound layer is promoted. By this reaction, carbon is removed from the nitride compound layer as hydrocarbons or the like.
That is, by controlling the fluorination treatment and the reduction of the fluorine component thereof, a surface irregularity having a large number of intermittent grooves having a depth of 0.5 μm or more, a complex shape and high adhesiveness is formed. In the present invention, it is possible to form the surface irregularities as described above by a surface treatment based on a gas nitriding treatment at a temperature of about 600 ° C. or less. As a result, the surface on which the fluorine component has been reduced becomes an activated state, and the carbon concentration in the nitride compound layer can be reduced, and an Fe—N nitride compound layer with high corrosion resistance can be formed.
In the method for producing a steel product of the present invention, the fluorination treatment is repeated when the fluorination step and the diffusion step are repeated twice or more.
Fluorine can be diffused and penetrated deeper into the surface layer of steel, and the effect of subsequent reduction and vaporization of the fluorine component can be deeply formed to form a thick nitride compound layer with low carbon concentration and high corrosion resistance. Can do.
In the method for producing a steel product of the present invention, after performing the gas nitriding treatment,
In the case of sealing penetration defects existing in the nitride compound layer by forming an oxide layer mainly composed of iron oxide having a thickness of 3 μm or less on the surface,
Furthermore, corrosion resistance can be improved. That is, the nitride compound layer has a high concentration of invading N. In particular, the nitride compound layer closer to the nitrogen diffusion layer becomes a denser compound layer. However, it is extremely difficult to completely eliminate the penetration defect, and there is a concern about the generation of rust through the route. For this reason, the corrosion resistance is further improved by sealing it by oxidation treatment. The nitride compound layer according to the present invention has a) a higher corrosion resistance of the nitride compound layer itself than other nitriding methods, b) a relatively dense compound layer can be formed on the nitrogen diffusion layer, and c) a source of rust. Since there are favorable conditions for corrosion resistance, such as the number of through defects that are likely to be small, the volume of through defects in the nitride compound layer can be increased simply by forming an oxide layer with a thickness of 3 μm or less that does not impair adhesion. The portion is sealed to form a surface layer with extremely high corrosion resistance. Therefore, for example, when applied to the back metal of a brake shoe, peeling of the friction material due to the occurrence of corrosion is effectively prevented.
In the method for producing a steel product of the present invention, after performing the gas nitriding treatment,
By carrying out fine particle peening treatment with an average particle size of 100 μm or less and removing unnecessary nitrides, oxides, oxynitrides, etc. covering the outermost surface, surface irregularities with a depth of 0.5 μm or more are exposed. in case of,
The adhesive strength of the surface irregularities can be further improved. That is, when the nitride particles formed by nitriding are not sufficiently bonded to the nitride compound layer, or when the surface irregularities are covered with nitride, oxide, or oxynitride, the adhesion performance of that portion is reduced. It can be damaged. Therefore, by performing the peening process under weak conditions, the surface irregularities can be exposed and the adhesive force can be further stabilized.
The peening treatment is for adjusting the surface state in order to further improve the adhesion, and does not remove the portion where the through defects are sealed by the oxidation treatment. Thereby, adhesiveness can be improved without impairing the corrosion resistance improved by performing the oxidation treatment.
As described above, the steel product of the present invention and the manufacturing method thereof have high adhesion when a friction material is adhered, for example, by using a surface treatment method mainly including a relatively inexpensive gas nitriding treatment. In addition, it is possible to form a surface layer that is extremely difficult to generate rust. For this reason, for example, when applied to a brake shoe back metal, the friction material does not peel off over a long period of time, resulting in a product with excellent durability.

It is a schematic diagram which shows the layer structure of the nitride compound layer and nitrogen diffusion layer of the steel product of this invention. It is a figure which shows the result of having measured N density | concentration of the surface layer part of an Example and the comparative example 1. FIG. It is a figure which shows the SEM observation result of the surface of an Example and Comparative Examples 1 and 2, and a cross section. It is a measurement result which measured a surface shape about an example and comparative example 2. It is a figure which shows the SEM observation result of the surface of C2-2 which is an Example, and C2-3.

Next, an embodiment for carrying out the present invention will be described.
The steel product of this embodiment is a steel product in which a nitride compound layer and a nitrogen diffusion layer are formed on the surface layer of a base material that is steel.
The nitride compound layer includes a first compound layer formed on the nitrogen diffusion layer side and a second compound layer formed on the surface side of the first compound layer.
The first compound layer has an ε structure mainly composed of Fe ₃ N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has irregularities formed on the surface.
The steel product manufacturing method according to the present embodiment performs a nitriding treatment for diffusing fluorine on the surface of the steel material, and then performing a gas nitriding treatment for diffusing nitrogen, thereby forming a nitride compound layer and nitrogen on the surface layer of the base material. This is a method for producing a steel product in which a diffusion layer is formed.
The fluorination treatment includes a fluorination process in which fluorine reacts with the steel material while introducing the fluorine source gas into the treatment furnace, and fluorine that has been heated and held in a state where the supply of the fluorine source gas is stopped to penetrate the steel material surface. A diffusion step of diffusing to a depth of at least 1 μm,
In the gas nitriding treatment, the diffused fluorine component is reduced and vaporized in a gas atmosphere containing no carbon source, and nitrogen is diffused and penetrated to form a nitride compound layer and a nitrogen diffusion layer.
In this way, a fluorination treatment that performs a fluorination step and a fluorine diffusion step is applied to the steel material surface to form a deep fluorinated layer without excessively increasing the surface fluorine concentration. Then, a nitride compound layer is formed by gas nitriding treatment mainly containing NH ₃ not containing a carbon source. By doing so, an Fe—N-based first compound layer with few penetration defects is formed on the nitrogen diffusion layer side, and a second compound layer having surface irregularities formed on the surface side is formed. Thereby, it can be set as the steel material which has the surface layer excellent in both adhesiveness and corrosion resistance. And, for example, it can be suitably used as a back pad for brake pads, brake shoes, clutch plates and the like having excellent durability.
This embodiment is carbon steel, low alloy steel, high alloy steel, rolled steel for structure, high tensile steel, steel for machine structure, carbon tool steel, alloy tool steel, high speed tool steel, bearing steel, spring steel, skin The present invention can be applied to various steel materials that can form a nitride compound layer, such as burned steel, nitrided steel, stainless steel, heat-resistant steel, and cast forged steel. Among these, as the back metal for brake pads, brake shoes, clutch plates, etc., structural rolled steel, high-tensile steel, and the like can be suitably used.
First, halogenation treatment is performed on these steel materials.
By this halogenation treatment, the oxide film on the surface of the workpiece is removed and a halogenated layer is formed. Further, by performing a dehalogenation element treatment for reducing the halogen component in the halogenated layer and a nitriding treatment, nitrogen is diffused and penetrated from the surface of the object to be treated, thereby forming a nitride compound layer and a nitrogen diffusion layer.
Examples of the halogenation treatment include a fluorination treatment, a chlorination treatment, a bromination treatment, an iodination treatment, and the like, but a fluorination treatment that is easy to handle and easily used industrially can be suitably performed. .
The fluorination treatment is performed by heating and holding at a temperature of 200 to 600 ° C. for a predetermined time in a fluorine source gas atmosphere containing fluorine and / or a fluorine compound such as NF ₃ gas to remove the oxide film on the surface of the steel material and contain fluoride. A fluoride layer is formed.
At this time, if it is only for nitriding treatment, a fluoride layer having a thickness slightly exceeding the thickness of the oxide film formed on the steel surface may be formed. That is, it is sufficient to replace the oxide film with fluoride, specifically, a fluoride layer less than 1 μm, and in many cases, a thinner fluoride layer may be formed.
On the other hand, in this embodiment, a fluoride and fluorine permeation layer having a thickness of 1 μm or more, more preferably about 1.5 to 4.5 μm is formed. By infiltrating fluorine to such deep, then for example when heated in an atmosphere containing NH ₃ gas, and vaporized by the reaction active hydrogen and fluorine produced by decomposition of NH ₃ gas, intermittent Surface irregularities having a large number of grooves are formed.
At this time, the nitriding process also proceeds in parallel with the NH ₃ gas, and the formation of surface irregularities and the nitriding process proceed simultaneously, resulting in a more efficient process. By such treatment, a nitriding treatment layer composed of a nitride compound layer and a nitrogen diffusion layer is formed. The nitride compound layer includes the first compound layer and the second compound layer described above.
The surface roughness of the second compound layer is preferably such that recesses having a depth of 0.5 μm or more have a high density, and more preferably 1 μm or more and 5 μm or less. This is because if the depth of the surface irregularities is too shallow, desired adhesive performance cannot be obtained. On the other hand, if the depth of the surface irregularities is too deep, the bonding force with the first compound layer becomes weak, and there is a risk of causing peeling and the like, resulting in a decrease in adhesion performance. In addition, about the said recessed part 0.5 micrometer or more in depth, it is desirable to exist in two or more places per 50 micrometers length (nominal length) of a surface, More preferably, it is three places or more.
Here, the concentration of the fluorine source gas used for the fluorination treatment is adjusted to an appropriate concentration according to the material of the object to be treated. Here, if the fluorine concentration on the surface of the object to be processed becomes too high, there are too many portions to be vaporized during the nitriding treatment, and the surface portion of the nitride compound layer is pulverized and the adhesiveness tends to deteriorate. Therefore, the fluorine concentration on the surface of the object to be processed before the nitriding treatment step is preferably 60% by mass or less, and more preferably about 50% by mass or less.
Therefore, in the surface treatment method of the present invention, a fluorine diffusion step is provided in order to deeply penetrate the fluorine without extremely increasing the fluorine concentration on the surface of the object to be treated. That is, during the fluorination treatment, a fluorine source gas is allowed to flow in the treatment furnace for a certain period of time to react the object to be treated with fluorine, and the supply of the fluorine source gas is stopped and the fluorine is maintained by heating for a certain period of time. A diffusion step for promoting diffusion and adjusting the fluorine concentration on the surface portion is provided. Thereby, a relatively deep fluorine permeation layer can be formed without excessively increasing the fluorine concentration on the surface of the object to be processed.
Here, the fluorination treatment preferably repeats the fluorination step and the diffusion step twice or more. This is because the diffusion rate of fluorine in the steel material is very slow. Therefore, if the fluorine concentration on the surface is excessively increased, it takes a long time to decrease the concentration by diffusion, resulting in poor efficiency.
Specifically, it is more preferable to use a method in which the fluorine source gas is supplied in several divided portions at a temperature of 200 ° C. or higher, preferably 300 ° C. or higher, more preferably 400 ° C. or higher. As described above, by repeating the fluorination step and the diffusion step, it is possible to form a relatively deep fluorine permeation layer without excessively increasing the fluorine concentration on the surface of the object to be processed.
When the temperature of the fluorination treatment including the fluorination step and the diffusion step is less than 200 ° C., the diffusion rate of fluorine is slow, and it takes a long time to form a fluoride layer of 1 μm or more, resulting in poor productivity. Become. On the other hand, when the temperature of the fluorination treatment exceeds 600 ° C., the diffusion rate of fluorine increases, while the reaction rate between fluorine and steel material increases excessively, making it difficult to control the fluorine concentration on the material surface. Therefore, the temperature of the fluorination treatment is preferably 200 ° C. or more and 600 ° C. or less.
In the fluorination treatment, if the depth of diffusion of fluorine is less than 1 μm, the height of the unevenness formed on the surface becomes insufficient, and the adhesive force with the friction material or the like cannot be sufficiently improved. On the other hand, if the penetration depth of the fluorine exceeds 5 μm, the amount of outgassing on the surface becomes too large and the surface becomes porous, and the irregular shape is easily broken, which tends to cause powdering. Therefore, it is preferable that the depth of diffusion of fluorine in the fluorination treatment is 1 μm or more and 5 μm or less.
What is necessary is just to set suitably about the time which performs a fluorination process and a diffusion process according to a material, the processing state of the surface, etc. If the treatment time is less than 1 minute, it is difficult to sufficiently permeate fluorine. On the other hand, if it exceeds 120 minutes, the penetration depth of fluorine and the fluorine concentration on the surface portion are likely to be excessive, and the productivity is also deteriorated. Therefore, the treatment time for the fluorination treatment is preferably 1 minute or more and 120 minutes or less.
The fluorination treatment can be performed in a furnace that performs nitriding treatment, or the fluorination treatment and the nitriding treatment can be performed in separate furnaces. In addition, the fluorination treatment chamber and the nitridation treatment chamber can be provided separately in the same furnace. At this time, the fluorination treatment and the nitriding treatment are preferably performed separately in different furnaces or rooms. By doing so, it is easy to control the fluorine concentration on the surface of the material and the penetration depth of fluorine.
After performing the above-described fluorination treatment, nitriding treatment is performed.
The nitriding treatment is performed by heating and holding the fluorinated steel product in an atmosphere gas containing a nitriding source gas such as NH ₃ .
By the nitriding treatment, a nitride compound layer including a first compound layer formed on the nitrogen diffusion layer side and a second compound layer formed on the surface side of the first compound layer is formed. The first compound layer has an ε structure mainly composed of Fe ₃ N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has irregularities formed on the surface.
The second compound layer preferably has a high N concentration with a surface nitrogen concentration of 12% by mass or more. As a result, the second compound layer has surface irregularities and exhibits adhesive performance. Such a second compound layer with surface irregularities is formed by forming the irregularities by defluorinating the fluorinated layer, and growing nitrides outwardly on the convex portions formed thereby. The second compound layer formed at this time is considered to be a compound layer containing a considerable amount of a nitride compound having a smaller mass than the first compound layer, and as a result, a compound layer having a complicated uneven shape with higher adhesion. It is.
It is preferable that the surface unevenness of the second compound layer has a high density of recesses having a depth of 0.5 μm or more. This is because if the depth of surface irregularities is less than 0.5 μm, sufficient adhesion performance may not be obtained.
The nitriding conditions are preferably set so that the surface nitrogen concentration is 12% by mass or more. This is because the active N generated by the decomposition of NH ₃ is supplied at a high concentration to the vicinity of the surface, thereby facilitating the diffusion of Fe from the inside to the surface portion in a form attracted to the N. . On the other hand, if the surface nitrogen concentration is excessively increased, the toughness of the second compound layer surface is liable to decrease. Therefore, the surface nitrogen concentration is preferably 20% by mass or less.
The thickness of the second compound layer is preferably 0.7 μm or more, and more preferably 1 μm or more. If the thickness of the second compound layer is less than 0.7 μm, it is difficult to significantly improve the adhesive strength. On the other hand, if the thickness of the second compound layer exceeds 5 μm, the layer having a high nitrogen concentration becomes thick and becomes brittle, so that the adhesive strength may be lowered. Therefore, the thickness of the second compound layer is desirably 0.7 μm or more and 5 μm or less.
The actual surface area ratio per unit area of the surface is preferably set to a value exceeding 1.8. This is because forming the surface irregularities increases the actual surface area per unit area, thereby increasing the adhesion area, thereby improving the adhesion. The actual surface area ratio per unit area is a value obtained by dividing the actual surface area value by the apparent surface area value.
The actual surface area and the actual surface area ratio can be measured by, for example, an apparatus that analyzes a three-dimensional surface shape. Further, the actual surface area ratio may be calculated from the ratio to the linear distance by measuring the length of the uneven portion in a certain cross section. Alternatively, a 3D image may be captured and obtained from the captured image.
If the actual surface area ratio is 1.8 or less, a significant improvement in surface adhesion cannot be expected. On the other hand, when the actual surface area ratio exceeds 5, there is a risk that the strength of bonding the second compound layer having the above-described surface irregularities with the first compound layer as the underlying layer is lowered, and the adhesiveness is lowered. For this reason, the actual surface area ratio is preferably more than 1.8 and not more than 5. Further, considering that the adhesive is adhered and penetrated over the entire surface of the uneven surface, the value is more preferably 2 or more and 4.5 or less.
At this time, if the nitriding temperature is less than 450 ° C., the reaction between the permeated fluorine and the hydrogen entering from the steel material surface becomes slow, and the formation rate of the second compound layer is also slowed, resulting in poor productivity. On the other hand, when the temperature exceeds 650 ° C., the growth of the second compound layer is too fast, and particularly the surface layer portion of the second compound layer becomes extremely porous, and the strength of the formed nitride compound is lowered and can be practically used. There may be no case. Therefore, the nitriding temperature is set to 450 ° C. or higher and 650 ° C. or lower. A more preferable nitriding temperature is 500 ° C. or higher and 600 ° C. or lower.
Depending on the time of the nitriding treatment, surface irregularities which are surface shapes peculiar to the present invention appear. That is, the infiltrated fluorine and hydrogen are reacted to vaporize, and the second compound layer is grown inward and outward. In consideration of the corrosion resistance, it is preferable to set a time sufficient for growing the nitride compound layer including the first compound layer and the second compound layer to have a thickness of 7 μm or more. Specifically, although it depends on the material of the base material to be applied, for example, carbon steel, low alloy steel, high tensile steel, etc., it is desirable that the time is 10 minutes or more and 3 hours or less. When the treatment time is less than 10 minutes, it is difficult to form a desired nitride compound layer and surface shape. In other words, the carbon contained in the material is taken into the compound layer, and it is not sufficient time to decarburize it. On the other hand, even if a treatment exceeding 3 hours is performed, the performance is saturated and the productivity is lowered.
The thickness of the first compound layer is preferably about 5 μm. This is because a relatively dense compound layer is formed on the base material side of the first compound layer formed on the surface.
The nitride compound layer including the first compound layer and the second compound layer preferably has a thickness of 7 μm or more, and more preferably 8 μm or more in consideration of corrosion resistance. On the other hand, when the thickness exceeds 25 μm, the performance including corrosion resistance is saturated and a coarse porous layer is easily formed on the surface. Therefore, the thickness of the nitride compound layer is preferably 7 μm or more and 25 μm or less. In consideration of productivity within a range where performance is hardly deteriorated, the upper limit of the thickness is more preferably 20 μm or less.
The nitriding gas used in the nitriding process does not include a carburizing gas. This is because a nitride compound layer having higher corrosion resistance is formed. Specifically, for example, a method of supplying only NH ₃ gas, NH ₃ gas and nitrogen gas, NH ₃ gas and hydrogen gas, NH ₃ gas, nitrogen gas and hydrogen gas can be exemplified.
Following the nitriding treatment step, a surface oxidation step can be performed as necessary.
By the surface oxidation step, an oxide layer mainly composed of iron oxide having a thickness of 3 μm or less is formed as the outermost layer. The purpose of this oxidation treatment step is to seal the penetrating defect portion leading to the nitrogen diffusion layer existing under the nitride compound layer with the oxide. The method is not particularly limited, and for example, a method of oxidizing in an atmosphere containing an oxidation source gas such as oxygen or water vapor while being heated and held following the nitriding treatment can be employed. Moreover, the method of oxidizing during a cooling process may be sufficient, and the method of oxidizing by carrying out heating maintenance again once after cooling can also be performed.
The steel material nitrided by the method of the present invention has a complex shape with irregularities on the order of microns on the outermost surface, but the first compound layer formed on the nitrogen diffusion layer side has relatively few penetration defects. Therefore, the sealing can be easily performed without performing a strong oxidation treatment. Therefore, the corrosion resistance can be sufficiently improved even by a short-time oxidation treatment using a cooling process.
More specifically, while the temperature of the article to be processed is lowered from about 600 ° C. to about 200 ° C., it is easy to perform cooling in an atmosphere in which an oxidation source gas such as oxygen and / or water vapor exists. Corrosion resistance can be improved. Although it is possible to carry out the oxidation treatment in a relatively low temperature region, in that case, a certain amount of time is required. Therefore, the oxide layer mainly composed of Fe ₃ O ₄ of ₃ μm or less is preferably exposed to an atmosphere containing 3% by volume or less, more preferably 1% by volume or less of the oxidation source gas at a temperature of 450 ° C. or higher. It is preferable to form.
When the oxide layer exceeds 3 μm, for example, when a friction material is adhered and a shear stress is applied, the friction material is easily peeled off due to breakage in the low-strength oxide layer. Therefore, the thickness of the oxide layer to be formed is preferably 3 μm or less, and more preferably 1.5 μm or less. This method is excellent in productivity because a desired oxide layer can be formed within the time for cooling the object to be processed. Moreover, when forming the said oxide layer, in order to exhibit the effect, it is desirable that the thickness shall be 0.2 micrometer or more.
In this embodiment, after the gas nitriding treatment, a fine particle peening treatment can be performed as necessary.
The fine particle peening treatment is a peening treatment with fine particles having an average particle diameter of 100 μm or less, and may be performed following the gas nitriding treatment or may be performed following the oxidizing treatment after the gas nitriding treatment. This removes unnecessary nitrides, oxides, oxynitrides and the like covering the outermost surface, and exposes surface irregularities having a depth of 0.5 μm or more.
That is, since the purpose of the oxidation treatment is to seal through defects as described above, after the oxidation treatment, a fine uneven shape is further given to the surface, and unnecessary particles, nitrides, oxidations are added. There may be a portion where an object, oxynitride or the like is formed so as to cover the uneven surface. In such a case, for example, a peening treatment at a pressure of about 0.1 to 0.4 MPa using fine particles such as glass beads and ceramic particles having an average particle diameter of 100 μm or less, preferably 70 μm or less is added. Thereby, by removing these particles, nitrides, oxides, oxynitrides and the like, a surface state with higher adhesion can be obtained.
As described above, the second compound layer having surface irregularities and excellent adhesion to the first compound layer is formed on the surface of the steel product, and a surface layer having high adhesiveness is formed. In addition, an Fe—N-based first compound layer with few penetration defects and high corrosion resistance is formed on the base material side, and the corrosion resistance is also improved. Furthermore, when an oxidation treatment process that forms a thin oxide layer that does not impair adhesion to friction materials, etc., or a peening treatment process to adjust the surface condition, penetration that exists slightly in the nitride compound layer Defects are sealed, and a surface treatment layer with higher corrosion resistance can be formed.
Therefore, the formed surface treatment layer is a treatment layer which has high adhesion in terms of surface shape and excellent corrosion resistance in which deterioration of the adhesion is suppressed. The steel product surface-treated by the treatment method of the present invention has excellent durability by being applied to a back metal or the like that adheres friction members such as brake pads, brake shoes, and clutch plates used in automobiles, for example. It becomes a structural member.
Next, examples will be described.

As steel materials used for the test, SS400, which is a general structural rolled steel plate, SAPH440, which is a hot rolled steel plate for automobile structures, and SPFH590, which is a workable hot rolled high tensile steel plate for automobiles, were prepared.
Example: After the test pieces of these three steel types were heated to 400 ° C. in a furnace in a nitrogen atmosphere, a fluorination step was carried out for 5 minutes while supplying 10% by volume of NF ₃ gas diluted with nitrogen gas into the furnace. did. Then, the supply of NF ₃ gas was stopped, and a fluorination treatment was performed in which a diffusion process was performed for 15 minutes. Thereafter, the mixture was heated to 500 ° C., and the fluorination treatment layer was reduced (that is, defluorination treatment) for 20 minutes while supplying 70 volume% NH ₃ gas and 30 volume% H ₂ gas into the furnace. Thereafter, the temperature was raised to 550 ° C., 60 minutes, 70 volume% NH ₃ gas and 30 volume% N ₂ gas were supplied into the furnace to perform nitriding treatment, and cooled to room temperature in a nitrogen atmosphere. Note that the thickness of the fluorinated layer was investigated for those cooled in a nitrogen atmosphere simply by performing the above fluorination step and diffusion step, and a 1.5 to 2.0 μm fluorination layer was formed on any specimen. Confirmed that.
Comparative Example 1 (it is a nitriding product using a conventional fluorination treatment): The above three steel type test pieces were heated to 400 ° C. in a furnace in a nitrogen atmosphere and then diluted with nitrogen gas to 2.5% by volume. A fluorination treatment was carried out for 20 minutes while supplying NF ₃ gas in the furnace. Thereafter, the temperature was raised to 550 ° C., 60 minutes, 70 volume% NH ₃ gas and 30 volume% N ₂ gas were supplied into the furnace to perform nitriding treatment, and cooled to room temperature in a nitrogen atmosphere. In addition, about what was cooled in nitrogen atmosphere after implementing the said fluorination process, the thickness of a fluoride layer was investigated, and the fluoride layer of any test piece is 0.4-0.6 micrometer in thickness. It was confirmed that it was thinner than
Comparative Example 2 (Salt bath nitriding product): SAPH440 was prepared by salt bath nitriding treatment at 570 ° C. for 40 minutes.
Table 1 below shows the results of analyzing the surface of these test pieces and the rusting area of the surface when a humidity test is performed in a constant temperature and humidity chamber at a temperature of 60 ° C., a humidity of 80%, and 100 hours. . The specific surface area in the table means that the actual surface area increased due to the formation of surface irregularities with respect to the measurement area, but the actual surface area was measured three-dimensionally (for example, omnifocal plus manufactured by Zeta Instruments) The value of the ratio of the actual surface area / measured area when an optical 3D fine surface shape measuring device can be used) is calculated.

From Table 1, in the Examples, a nitride compound layer having a low carbon concentration is formed on the surface of any steel type. Moreover, it turns out that it has a very high surface nitrogen concentration with respect to Comparative Examples 1 and 2.
In FIG. 2, the result of having measured the density | concentration distribution of the depth direction of N of the outermost surface part of the Example of SAPH440 material and the comparative example 1 is shown. It can be seen that in the example, a portion having a nitrogen concentration exceeding 10% by mass is formed with about 2 μm, whereas in Comparative Example 1, such a portion is hardly formed. Looking at the transition of the nitrogen concentration from the surface of the embodiment, the first compound layer nitrogen concentration to be considered 8% before and after ε (Fe 3 _N) phase, on the surface side, a 11 wt% A second compound layer is formed that appears to contain a ζ (Fe ₂ N) phase with a higher surface nitrogen concentration. The N concentration in each layer has a so-called gradient composition in which the N concentration gradually decreases from the surface, and the second compound layer has a high bonding force with the first compound layer under it even though it is a surface uneven layer, and is loaded. It is thought that the layer is strong against stress.
FIG. 3 shows the results of SEM observation of the surfaces and cross sections of the example, comparative example 1, and comparative example 2. From this surface SEM photograph, it can be seen that many groove-like surface irregularities are formed on the order of microns in the example. In the conventional fluorination treatment method, fluorine was simply concentrated on the surface, but after the appropriate diffusion treatment was applied, the fluoride was reduced and the nitridation treatment was performed. Is formed. That is, by performing a fluorination treatment including a diffusion step, the fluorine is deeply penetrated, and the penetrated fluorine component is reduced and vaporized to form a recess. N penetrates from the surroundings into the remaining convex portion and increases the N concentration, thereby promoting the growth of nitride on the convex portion. As can be seen from the cross-sectional SEM photographs of the examples, the formed second compound layer has a clear color difference from the first compound layer, and since the color is dark, it contains a considerable amount of light elements. You can see that it is a layer. This indicates that N has penetrated from the surrounding area due to the increase in surface area, resulting in a high N concentration, and a nitride having a different composition from that of the nitride in the first compound layer is formed and grown. Thereby, the surface unevenness becomes a more complicated shape. Thereby, in an Example, it is thought that the surface unevenness | corrugation in which the value of a specific surface area is significantly large compared with the comparative examples 1 and 2 is formed.
It can be said that the surface layer structure of the embodiment in which the actual surface area is greatly increased has a surface shape with extremely excellent adhesiveness due to the effect of increasing the bonding area. In addition, the surface layer structure of this Example does not show a large difference among the three steel types used in the test, and other steel types are similar by forming, reducing, and vaporizing a deep fluoride layer by the same method. It means that a surface shape can be formed.
In the outermost surface portion of the cross-sectional SEM photograph of the example of FIG. 3, a second compound layer having surface irregularities constituted by the high N concentration nitride is confirmed. The presence of the second compound layer greatly affects the adhesion.
Further, regarding the corrosion resistance, the examples are clearly superior to the comparative examples 1 and 2. The Fe-N nitride layer having a low carbon concentration is not simply formed, but the first compound layer formed on the nitrogen diffusion layer side of the second compound layer is quite dense with few penetration defects. It shows that it has become.
On the other hand, in the salt bath nitrided product of Comparative Example 2, convex nitride is formed on the surface from the surface SEM photograph, and the surface roughness and specific surface area are increased to increase the adhesive strength. However, as can be seen from the cross-sectional SEM photograph, it cannot be said that the convex nitride has a good bonding state with the underlying nitride. Although relatively large holes are confirmed, it is considered that the adhesion strength when shear stress or the like is applied is far inferior to that in which many surface irregularities are formed as in the example. In addition, as described above, relatively large holes are observed, and it is considered that there are many penetration defects leading to the nitrogen diffusion layer. In addition to being a Fe—C—N-based compound layer, it can also be seen that the corrosion resistance is considerably inferior to the examples.
FIG. 4 shows a surface shape over a length of about 100 μm (nominal length) using a non-contact type measuring device (all-focus plus optical 3D fine surface shape measuring device manufactured by Zeta Instruments) for Example and Comparative Example 2. The measurement result which measured was shown. The figure is shown as a profile viewed from the cross-sectional side. Comparative Example 2 is a salt bath nitriding product that has a large specific surface area and is generally used as a nitriding treatment for brake shoes and the like. Whereas Comparative Example 2 has a comparatively smooth surface uneven shape, the Example has a groove portion (concave portion) having a depth of 0.5 μm or more at a high density of at least 2 to 3 places per nominal length of 50 μm. Existing. Moreover, it turns out that it is a complicated uneven | corrugated shape which has a recessed part and a convex part with a big height difference with a short space | interval. From the above results, in the examples, the value of the specific surface area is increased, and the adhesion area is increased. In addition, the anchor effect due to having a large number of grooves of 0.5 μm or more in a short period is also added. As a result, the surface shape is extremely excellent in adhesion and adhesion.

An adhesion evaluation test was performed using SPFH590 material. As a test piece nitriding method, the test piece was subjected to fluorination treatment under the conditions shown in Table 2. That is, after performing the specified time fluorination step with 10% by volume of NF ₃ gas diluted with nitrogen gas, the supply of NF ₃ gas was stopped and the specified time diffusion step was performed.
In Example B2, the fluorination step for 2.5 minutes and the diffusion step for 5 minutes were performed by the above method, and then the fluorination step for 2.5 minutes and the diffusion step for 10 minutes were further performed.
Example C2 is a 1 minute fluorination step and a 2 minute diffusion step followed by a 1 minute fluorination step and a 3 minute diffusion step, followed by a 1 minute fluorination step and a 5 minute diffusion step. Carried out.
Thereafter, the test pieces were once soaked to 400 ° C., and increased at 3 ° C./min while reducing the fluoride layer to 580 ° C. in an atmosphere of 50 vol% NH ₃ gas and 50 vol% N ₂ gas. Warm up. Thereafter, 100 volume% NH ₃ gas was supplied into the furnace at 580 ° C. for 30 minutes to perform nitriding treatment and cooling. Table 2 also shows the fluorinated layer thickness and surface fluorine concentration of each test piece after fluorination treatment.

From the results of B2 and C2 in Table 2, it can be seen that a thicker fluorinated layer can be formed while the increase in the surface fluorine concentration is suppressed when the fluorination treatment is carried out in several steps.
These test pieces were nitrided by the method described above. At this time, as Example C2-2, after completion of nitridation of C2, cooling was performed in a nitrogen atmosphere containing 0.1% by volume of oxygen to form an oxide layer having an average thickness of about 1.0 μm. did. In addition, as Example C2-3, a sample obtained by projecting the surface of the test piece after cooling C2-2 at a pressure of 0.2 to 0.3 MPa using ceramic fine particles having an average particle diameter of about 60 μm was also produced.
About the test piece which processed on the conditions of an Example and a comparative example, after sticking the surfaces of two test pieces with an adhesive agent, and carrying out the salt spray test using 5% NaCl aqueous solution for 1000 hours, they The adhesive strength test was conducted. The results are shown in Table 3.
In addition, the adhesive strength test was similarly implemented about the test piece (salt bath nitriding goods) of the comparative example 2 in Example 1, and it showed in Table 3 collectively.

From Table 3, it can be seen that the thickness of the second compound layer formed by the method of the present invention has a great influence on the adhesive strength. And it turns out that high adhesive strength is obtained by optimizing the thickness. That is, when the thickness is much less than 1 μm, the improvement of the adhesive strength cannot be expected, and conversely, when the thickness is too thick, the adhesive strength is reduced.
As can be seen from Table 2, the thickness of the second compound layer is greatly influenced by the thickness of the fluoride layer and the fluorine concentration of the fluoride layer. In Comparative Example 1a in which the thickness of the second compound layer is too small, the adhesiveness with the adhesive is low, so that rust easily develops, and as a result, the adhesive strength is low. On the other hand, b of Comparative Example 1 in which the thickness of the second compound layer is excessive seems to be particularly deteriorated in adhesion between the convex portion and the underlying nitride compound layer. This is because the ratio of vaporization when the fluorinated layer was reduced was large, the strength of the concavo-convex part was lowered, and the adhesiveness with the adhesive was also lowered, and the rusting area was increased. It is thought that the adhesive strength was lowered.
On the other hand, it can be seen that the rusting area and the adhesive strength of the example are superior to those of Comparative Example 2. In particular, not only the fluorination step and the diffusion step but also the one in which the fluorination treatment is performed by a method of alternately repeating them has a better result. It can be determined that a surface layer with better corrosion resistance and adhesion can be formed by applying a method of permeating fluorine without excessively increasing the fluorine concentration on the surface.
In addition, as shown in the results of Examples C2-2 and C2-3, the corrosion resistance is particularly improved in the case where the oxidation process is added compared to C2. For this reason, it is considered that the reduction of the adhesive strength due to the generation of rust is extremely difficult, and both of them are considered to exhibit high adhesive strength values.
FIG. 5 shows surface SEM photographs of Examples C2-2 and C2-3. By simply applying a relatively weak oxidation treatment that does not significantly destroy the surface shape, the penetration defects present in the nitride compound layer are sealed, and thus high corrosion resistance is exhibited. In addition, by performing fine particle peening treatment, the surface unevenness shape is greatly destroyed by forming unnecessary particles, nitrides, oxides, oxynitrides, etc., attached to the surface so as to cover the uneven surface. Removed without. This is a result showing that there is a high possibility that the adhesive strength can be increased while maintaining the corrosion resistance. Thus, the addition of the treatment process and the addition of the fine particle peening treatment process are results showing that it is a more preferable embodiment.
From the above results, the steel material surface treatment method of the present invention is a surface treatment method mainly based on a relatively inexpensive gas nitriding treatment, but can form a highly adherent uneven layer on the steel surface and a nitrided compound having high corrosion resistance. A layer can be formed. Accordingly, the steel material surface-treated by the surface treatment method of the present invention has a high adhesiveness when, for example, a friction material is adhered, and a surface layer that is extremely difficult to generate rust is formed. It is difficult to cause problems such as material peeling, that is, it can be used as a member having excellent durability.

The steel product obtained by the production method of the present invention has a surface layer excellent in both surface adhesion and corrosion resistance, and is suitably used as a backing metal for brake pads, brake shoes, clutch plates, etc., which are excellent in durability, for example. can do. The scope of application of the present invention is not limited to this. If it is used in a place where the adhesion performance is required in an environment where corrosion resistance is required, it exhibits excellent performance in various applications.

DESCRIPTION OF SYMBOLS 1 2nd compound layer 2 1st compound layer 3 Nitrogen diffusion layer 4 Base material

Claims

A steel product in which a nitride compound layer and a nitrogen diffusion layer are formed on the surface layer of a base material that is steel,
The nitride compound layer includes a first compound layer formed on the nitrogen diffusion layer side, and a second compound layer formed on the surface side of the first compound layer,
The first compound layer has an ε structure mainly composed of Fe 3 N,
The second compound layer is a layer having a ζ structure containing Fe 2 N higher than the first compound layer until the nitrogen concentration in mass% exceeds 11 mass%,
A steel product, wherein irregularities are formed on the surface of the layer having the ζ structure.
The steel product according to claim 1, wherein the surface roughness of the second compound layer has at least three concave portions with a depth of 0.5 µm or more at a high density of at least three locations per nominal length of 50 µm.
The steel product according to claim 1 or 2, wherein the thickness of the second compound layer is 0.7 µm or more.
The steel product according to any one of claims 1 to 3, wherein the first compound layer has a thickness of 5 µm or more.
The steel product according to any one of claims 1 to 4, wherein the actual surface area ratio per unit area of the surface exceeds 1.8.
6. The steel product according to claim 1, wherein an oxide layer mainly composed of iron oxide having a thickness of 3 μm or less is formed on the second compound layer as an outermost layer.
The steel product according to any one of claims 1 to 6, wherein a penetration defect existing in the second compound layer is sealed with an oxide mainly composed of iron oxide.
A steel product manufacturing method in which a nitride compound layer and a nitrogen diffusion layer are formed on a surface layer of a base material by performing a gas nitriding treatment for diffusing nitrogen after performing a fluorination treatment for diffusing fluorine on the surface of the steel material. There,
The fluorination treatment includes a fluorination process in which fluorine reacts with the steel material while introducing the fluorine source gas into the treatment furnace, and fluorine that has been heated and held in a state where the supply of the fluorine source gas is stopped to penetrate the steel material surface. A diffusion step of diffusing to a depth of at least 1 μm,
In the gas nitriding treatment, the diffused fluorine component is reduced and vaporized in a gas atmosphere containing no carbon source, and nitrogen is diffused and infiltrated to form a nitride compound layer and a nitrogen diffusion layer. Steel product manufacturing method.
The said fluorination process is a manufacturing method of the steel product of Claim 8 which repeats a fluorination process and a diffusion process twice or more.
After performing the gas nitriding process,
The method for producing a steel product according to claim 8 or 9, wherein a penetration defect existing in the nitride compound layer is sealed by forming an oxide layer mainly composed of iron oxide having a thickness of 3 µm or less on the surface.
After performing the gas nitriding process,
By carrying out fine particle peening treatment with an average particle size of 100 μm or less and removing unnecessary nitrides, oxides, oxynitrides, etc. covering the outermost surface, surface irregularities with a depth of 0.5 μm or more are exposed. The method for producing a steel product according to any one of claims 8 to 10.