JP2002302756A - Surface treated steel member and gas soft nitriding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treated steel member which exhibits stable, excellent performance in the corrosion resistance as and as wear resistance and can shorten its surface treatment time. SOLUTION: The steel member is obtained by performing gas soft-nitriding treatment at 570 to 580 deg.C for 60 min to form a compound layer mainly consisting of epsilon iron nitride (ε-Fe2-3 N) on the surface. On the above gas soft-nitriding treatment, the mixing ratio of gaseous ammonia, nitrogen and carbon dioxide is suitably set, and particularly, the concentration of carbon dioxide is controlled to 20.0 to 80.0 g/h per square meter of the surface area of the material to be treated, so that a stabilization layer in which the concentration of carbon is 0.2 to 1.0 wt.%, and the concentration of nitrogen is 5.0 to 8.0 wt.% is provided in a compound layer having a thickness of 10 to 15 μm. Thus, the reduction of its corrosion resistance is suppressed by the stabilization layer. Further, in a degree at which the compound layer is thinned, the number of production per unit time is increased to improve the productivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel member subjected to a nitrocarburizing treatment and a gas nitrocarburizing method suitable for producing the surface-treated steel member.

[0002]

2. Description of the Related Art A compound layer mainly formed of epsilon iron nitride (ε-Fe _2-3 N) formed by nitrocarburizing treatment is
It has been conventionally known that it greatly contributes to improvement in wear resistance and corrosion resistance of steel members. However, when actually using the steel member subjected to the soft nitriding treatment, some steel members do not rust at all for a long period of time, while those that rust at the initial stage of use or rust after a certain period of use have been used. However, there is a problem that there is considerable variation in the corrosion resistance, and it is difficult to use it stably. It is to be noted that an oxide layer mainly composed of iron sesquioxide (Fe ₃ O ₄ ) is formed on the compound layer formed by the nitrocarburizing process in a part by oxidation process. Even in this case, when the oxide layer on the outermost surface is worn out, the above-mentioned problem of the variation in corrosion resistance occurs, and the fundamental solution cannot be achieved.

Therefore, conventionally, for example, Japanese Patent Application Laid-Open No. 11-269
In the surface-treated steel member described in Japanese Patent No. 630, measures are taken to suppress the variation in corrosion resistance by keeping the carbon concentration distribution in the thickness direction of the compound layer formed by the gas nitrocarburizing treatment within a certain range. It has been confirmed that the compound layer has a thickness of 18 to 20 μm and does not rust (has corrosion resistance) even when its surface layer is ground by 15 μm.

[0004]

However, according to the surface-treated steel member described in the above-mentioned publication, the compound layer has a thickness of 18 to 20.
Since it is formed to have a thickness of μm, it takes a long time to form the compound layer (about 2 hours is required to make the thickness of the compound layer 20 μm), and the number of products per unit time is small. was there. Therefore, in order to increase the number of products per unit time, it is conceivable to manufacture a thin compound layer (5 to 15 μm) under the processing conditions described in the above publication.
In this case, the time required for forming the compound layer can be simply reduced (30 minutes to 1.5 hours). However, as shown in FIG. 3 , the carbon concentration in the compound layer increases while the nitrogen concentration decreases. And poor corrosion resistance. Regarding the above-mentioned problems, the present inventors have conducted intensive studies and found that, in order to increase the number of production per unit time by reducing the thickness of the compound layer, in addition to the carbon concentration distribution in the thickness direction of the compound layer, It was confirmed that the nitrogen concentration distribution greatly affected, and rusting occurred when the surface where the carbon concentration and the nitrogen concentration exceeded a certain range was exposed. In addition, as a result of intensive studies on the gas nitrocarburizing treatment, the gas nitrocarburizing was performed after Fe ₃ C was first formed by carburization, and then Fe ₃ C → Fe ₃ C (N) → Fe ₃ N (C). Although the compound changes, if the amount of carbon dioxide per unit surface area of the material to be treated is too large, the amount of carburizing (carburizing rate) becomes larger than the amount of nitriding (nitrifying rate), and Fe ₃ N (C ) Was delayed, and it was concluded that this would cause a reduction in corrosion resistance. And, focusing on this point, gas nitrocarburizing treatment was performed at various carbon dioxide concentrations.
It has been found that when the concentration of carbon dioxide per unit surface area of the material to be treated exceeds a certain range, rust easily occurs.

The present invention has been made on the basis of the above findings, and it is an object of the present invention to keep the carbon concentration and the nitrogen concentration in the thickness direction of the compound layer formed by the nitrocarburizing treatment within a certain range. By providing a surface-treated steel member capable of exhibiting excellent performance stably in terms of corrosion resistance as well as abrasion resistance even if the thickness of the compound layer is reduced, such a surface-treated steel member is also provided. An object of the present invention is to provide a gas nitrocarburizing method that can be manufactured stably.

[0006]

In order to solve the above-mentioned problems, a surface-treated steel member according to the present invention has a surface-treated steel member having a compound layer mainly composed of epsilon iron nitride formed on the surface by nitrocarburizing treatment. In the compound layer, the carbon concentration is in the range of 0.2 to 1.0% by weight and the nitrogen concentration is in the range of 5.0 to 8.0.
% Of a stable layer, and the thickness of the compound layer is adjusted to 5.0%.
115.0 μm. In the surface-treated steel member configured as described above, the stable layer provided in the compound layer contributes to maintaining the corrosion resistance, and the thickness of the compound layer can be reduced. In the surface-treated steel member, the stable layer may be provided over the entire layer (total thickness) of the compound layer or may be provided partially within the compound layer. The present surface-treated steel member may be formed by laminating an oxide layer mainly composed of triiron tetroxide on the compound layer by oxidation treatment, whereby the wear resistance and corrosion resistance are further improved. I do.

In the gas nitrocarburizing method according to the present invention, a heat treatment is performed in a carbonitriding gas atmosphere containing ammonia, nitrogen and carbon dioxide to form a compound layer mainly composed of epsilon iron nitride on the surface of the material to be treated. In the gas nitrocarburizing method to be formed, the concentration of the carbon dioxide is controlled to 20.0 to 80.0 g / h per 1.0 m ² of surface area of the material to be treated. In the gas nitrocarburizing treatment performed in this manner, a compound layer having excellent corrosion resistance is efficiently formed by suppressing the concentration of carbon dioxide within a predetermined range. In this method, the surface area of the material to be treated can be specified by measuring the weight of the material to be treated or by measuring the volume of the material to be treated. According to the former identification method, the total surface area can be easily grasped when small parts are processed in large quantities at a time. According to the latter identification method, the surface area of the workpiece having a complicated shape can be accurately determined. Can be grasped.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION To manufacture a steel member according to the present invention, a steel member is charged into a nitriding furnace provided with heating means and gas replacement means, and the inside of the nitriding furnace is first evacuated to a vacuum. While introducing nitrogen gas inside, the standard soft nitriding temperature (570
~ 580 ℃). Then, when the temperature is raised to the nitrocarburizing temperature, ammonia gas, nitrogen and carbon dioxide are supplied into the nitriding furnace at a predetermined ratio, and the inside of the nitriding furnace is maintained as a carbonitriding gas atmosphere for a predetermined time to perform a gas nitrocarburizing treatment. Do
After this treatment, for example, the steel member is immersed in oil in an oil tank provided adjacent to the nitriding furnace and rapidly cooled. By this gas nitrocarburizing treatment, a compound layer mainly composed of epsilon iron nitride (ε-Fe _2-3 N) is formed on the surface of the steel member to a thickness of 5 to 15 μm, and a layer under the compound layer is formed. A nitrogen diffusion layer is formed on the steel member, and a steel member having excellent wear resistance and corrosion resistance can be obtained.

In the above gas nitrocarburizing treatment, by appropriately setting the mixing ratio of ammonia gas, nitrogen and carbon dioxide, the carbon concentration in the compound layer becomes 0.2 to 1.0.
A stable layer having a weight percentage of 5.0 to 8.0 weight% is provided. At this time, carbon dioxide is supplied so as to be 20.0 to 80.0 g / h per 1.0 m ² of surface area of the material to be treated. Thereby, the stable layer in the compound layer contributes to the maintenance of stable corrosion resistance, and it becomes possible to provide a surface-treated steel member with little variation in corrosion resistance. The stable layer is desirably provided over the entire thickness of the compound layer. However, when the member to be applied has high abrasion resistance as in the present embodiment, a part of the total thickness of the compound layer is used. May be provided.

Here, in the case where an oxidation treatment is added if necessary, an oxidation furnace provided with heating means and gas replacement means is separately prepared in the same manner as in the above-mentioned nitriding furnace, and the steel which has been subjected to the gas nitrocarburizing treatment is provided therein. First, the inside of the oxidation furnace is evacuated and nitrogen gas is introduced therein, and the temperature is raised to a standard steam oxidation temperature (400 to 500 ° C.). Then, when the temperature is raised to the steam oxidation temperature, steam is blown into the oxidation furnace for a predetermined time, and then taken out of the oxidation furnace and cooled to the atmosphere. By this oxidation treatment, an oxide layer mainly composed of triiron tetroxide (Fe ₃ O ₄ ) is formed on the compound layer by 1 to ₃ μm.
m, and a steel member having more excellent wear resistance and corrosion resistance can be obtained.

[0011]

[Example] Example 1 JIS S35C piston rod material for hydraulic shock absorber (length approx.
(150 mm), and the surface was further ground to finish to a predetermined rod size (diameter 10 mm) and surface roughness (center line average roughness Ra 0.08 μm or less). Next, the rods (piston rods) were set in a dedicated jig with 1,000 rods as one unit. After the grinding oil attached to the rod surface was removed by washing, the rod was charged into a nitriding furnace, and ammonia gas and ammonia gas were removed. , Nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio, and particularly for carbon dioxide, 77.0 g per 1.0 m ² based on the surface area of all rods previously calculated from dimensions.
/ h, in a carbonitriding gas atmosphere, in a carbonitriding gas atmosphere mixed in proportion,
A gas soft nitriding treatment was performed at 60 ° C. for 60 minutes, followed by rapid cooling.

After the above series of treatments, each rod is removed from the jig, and a plurality of the rods are subjected to an analytical test of the compound layer on the surface to determine the carbon concentration distribution and the nitrogen concentration in the thickness direction. Distribution and, for other multiple tubes, conduct JIS Z2371 salt spray test,
The rating number (JIS H8502) was determined from the corrosion area ratio. In addition, as a result of performing a microscope test on a plurality of rods and measuring the thickness of the compound layer, the thickness of the compound layer is
It was 10 to 15 μm.

Example 2 A piston rod material having the same dimensions as in Example 1 was subjected to a gas nitrocarburizing treatment at 570 ° C. for 45 minutes in the same gas atmosphere as in Example 1, and thereafter the same as in Example 1. In the procedure,
An analysis test and a salt spray test of the compound layer on the surface were performed. In addition, as a result of performing microscopic observation on a plurality of rods and measuring the thickness of the compound layer, the thickness of the compound layer was 5 to 5.
It was 8 μm.

Embodiment 3 A piston rod material having the same dimensional conditions as in Embodiment 1 is subjected to a gas nitrocarburizing treatment at 580 ° C. for 60 minutes in the same gas atmosphere as in Embodiment 1, and then the same as in Embodiment 1. In the procedure,
An analysis test and a salt spray test of the compound layer on the surface were performed. In addition, as a result of measuring the thickness of the compound layer by performing microscopic observation on a plurality of rods, the thickness of the compound layer is 12 to
It was 15 μm.

Example 4 The same piston rod material and rod diameter as in Example 1 were used, and the lengths thereof were made different from each other. These were finished to the same surface roughness as in Example 1, and used for the first treatment. Set 2500 rods (piston rods) as a unit in a special jig, and use the rods for the second processing.
1250 pieces were set as a unit in a special jig. next,
Measure the weight of all rods (2500) for the first treatment,
The weight is input to the control device of the gas nitrocarburizing equipment,
The surface area of all rods was calculated. After washing and removing the grinding oil adhering to the surface of the rod for the first treatment, it is charged into a nitriding furnace, and ammonia gas, nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio. In particular, carbon dioxide is supplied so as to be 44.0 g / h per 1.0 m ² based on the surface area of the entire rod determined from the weight, and is maintained at 570 ° C. for 60 minutes in a carbonitriding gas atmosphere. Gas nitrocarburizing treatment, followed by rapid cooling. Next, the rod for the second treatment was measured in the same manner as in the first treatment to determine the surface area of all the rods, the grinding oil attached to the surface was removed by washing, and the rod was charged into a nitriding furnace. Ammonia gas, nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio. At this time, the weight of the second processing rod was found to be half that of the first processing rod, and all the rods (12
The surface area of 50 rods is half (1/2) of the surface area of all rods for the first treatment. Therefore, the supply amount of carbon dioxide was also adjusted to 44.0 g / h per 1.0 m ² based on this surface area, and the gas soft nitriding treatment was performed under the same conditions as the first time. After the above series of treatments, the rods subjected to the first and second treatments are removed from the jig, and finished to a predetermined surface roughness (a center line surface roughness Ra of 0.08 μm or less), and the first treatment is completed. JIS Z2371 salt spray test was conducted on 80 rods of the above and 40 rods which had been subjected to the second treatment to observe the rusting state. Note that the rod 20
The thickness of the compound layer was determined to be 10 to 13 μm by performing a microscopic test on the book and the ten rods that had been subjected to the second treatment and measuring the thickness of the compound layer.

In determining the surface area of all the rods, the rod is not limited to the weight as in the fourth embodiment, but the rod is immersed in a liquid, the volume is measured, and the surface area is determined from the volume and the rod diameter. May be obtained.

Example 5 The same piston rod material as in Example 4 was used to finish it to the same rod dimensions and surface roughness as in Example 4, and 2,500 of them were set in a special jig and placed on the rod surface. After washing and removing the attached grinding oil, it is charged into a nitriding furnace, and ammonia gas, nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio. 3 per 1.0m ² based on the surface area of all rods found
The gas was supplied at a rate of 1.0 g / h, subjected to a gas nitrocarburizing treatment in a carbonitriding gas atmosphere at 570 ° C. for 60 minutes, and then rapidly cooled. After the above series of treatments, each rod is removed from the jig and finished to a predetermined surface roughness (center line surface roughness Ra 0.08 μm or less), and 80 of them are subjected to JIS Z2371 salt spray test. The rusting condition was observed. The thickness of the compound layer was measured by performing a microscopic test on 20 rods, and as a result, the thickness of the compound layer was 10
1313 μm.

Example 6 Using the same piston rod material as in Example 4, this was finished to the same rod dimensions and surface roughness as in Example 4, 2500 of which were set in a special jig and placed on the rod surface. After washing and removing the attached grinding oil, it is charged into a nitriding furnace, and ammonia gas, nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio. 5 per 1.0m ² based on the determined surface area of all rods
The gas was supplied at 8.8 g / h, gas nitrocarburizing treatment was performed at 570 ° C. for 60 minutes in a carbonitriding gas atmosphere, and then quenched. After the above series of treatments, each rod is removed from the jig and finished to a predetermined surface roughness (center line surface roughness Ra 0.08 μm or less). Forty of the rods are subjected to a JIS Z2371 salt spray test. The rusting condition was observed. The thickness of the compound layer was measured by performing a microscopic test on 20 rods, and as a result, the thickness of the compound layer was 10
1313 μm.

Comparative Example 1 The same piston rod material as in Example 1 was mixed with ammonia gas, nitrogen and carbon dioxide at a different ratio from that in Example 1 in a carbonitriding gas atmosphere at 570 ° C. for 60 minutes. The retained gas nitrocarburizing treatment was performed, and then an analysis test and a salt spray test of the compound layer on the surface were performed in the same procedure as in Example 1. In addition, as a result of performing a microscope test on a plurality of rods and measuring the thickness of the compound layer, the thickness of the compound layer was 10 to 15 μm.

Comparative Example 2 Using the same piston rod material as in Example 4, this was finished to the same rod dimensions and surface roughness as in Example 4, and 1250 of them were set in a special jig and placed on the rod surface. After washing and removing the attached grinding oil, it is charged into a nitriding furnace, and ammonia gas, nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio. 8 per 1.0m ² based on the determined surface area of all rods
The mixture was supplied at a rate of 8.0 g / h, subjected to a gas nitrocarburizing treatment at 570 ° C. for 60 minutes in a carbonitriding gas atmosphere, and then rapidly cooled. After the above series of treatments, each rod is removed from the jig and finished to a predetermined surface roughness (center line surface roughness Ra 0.08 μm or less), and 40 of them are subjected to a JIS Z2371 salt water spray test. The rusting condition was observed. The thickness of the compound layer was measured as a result of performing a microscope test on the ten rods and measuring the thickness of the compound layer.
1313 μm.

Comparative Example 3 The same piston rod material as in Example 4 was used and finished to the same rod dimensions and surface roughness as in Example 1, and 1250 of them were set in a special jig and placed on the rod surface. After washing and removing the attached grinding oil, it is charged into a nitriding furnace, and ammonia gas, nitrogen and carbon dioxide are supplied into the furnace at a predetermined ratio. 1 per 1.0m ² based on the surface area of all rods found
The mixture was supplied at a rate of 1.4 g / h and subjected to a gas nitrocarburizing treatment in a carbonitriding gas atmosphere at 570 ° C. for 60 minutes, followed by rapid cooling. After the above series of treatments, each rod is removed from the jig and finished to a predetermined surface roughness (center line surface roughness Ra 0.08 μm or less). Forty of the rods are subjected to a JIS Z2371 salt spray test. The rusting condition was observed. The thickness of the compound layer was measured by performing a microscopic test on ten rods.
55 μm.

Test Results Table 1 and FIGS. 1 and 2 show the distributions of carbon concentration and nitrogen concentration in the compound layer on the surface for Examples 1 to 3 and Comparative Example 1. Note that each plot in the figure shows an average value of a plurality of analysis results. Than this,
The carbon concentration in the thickness direction of the compound layer was 1.0% by weight or less (0.2% by weight or more) in each of the surface-treated piston rods (treated materials) of Example 1, Example 2, Example 3 and Comparative Example 1. It fits in. On the other hand, the nitrogen concentration in the thickness direction of the compound layer was determined in Example 1, Example 2, and Example 3.
In both cases, a region (stable layer) of 5.0% by weight or more was observed on the surface side of the treatment material, whereas the treatment material of Comparative Example 1 contained:
Such a region having a content of 5.0% by weight or more is not recognized.

[0023]

[Table 1]

FIG. 3 shows the results of the salt spray test (test time: 168 hours) for each of the five piston rods (treated materials) subjected to the surface treatment in Examples 1 to 3 and Comparative Example 1. From this, it was confirmed that the treated materials subjected to the surface treatment in Examples 1, 2 and 3 had better corrosion resistance than the treated materials of Comparative Example 1. The reason why the treated materials of Examples 1, 2 and 3 are excellent in corrosion resistance is that the carbon concentration in the compound layer formed by the nitrocarburizing treatment is 0.2 to 1.0% by weight and nitrogen 5.
This is presumed to be due to the presence of a stable layer of 0 to 8.0% by weight.

As described above, the thickness of the compound layer is from 18 to 20 μm in the prior art, but from 5 to 15 μm in the present invention.
m, while in the compound layer,
The carbon concentration is in the range of 0.2-1.0% by weight and the nitrogen concentration is 5.
Since a stable layer in the range of 0 to 8.0% by weight was provided, the formation time of the compound layer was greatly reduced (conventionally, it took about 2 hours.
60 minutes) and increase the number of production per unit time. In addition, stable and excellent performance can be achieved in terms of wear resistance and corrosion resistance.

Table 2 shows Examples 4 to 6 and Comparative Examples 2 and 3.
3 shows the results of a salt spray test on the sample. As a result, no rust was observed in any of the treated products of Examples 4 to 6 until after the test for 72 hours. In addition, from the comparison between the first processed product and the second processed product of Example 4, no difference due to the number of processes performed at one time appeared, and it was found that large-scale processing was possible.
In contrast, the treated products of Comparative Example 2 in which the carbon dioxide concentration was set to be higher than 80.0 g / h · m ² and Comparative Example 3 in which the carbon dioxide concentration was set to be lower than 20.0 g / hm ² were Rust was generated relatively early (within 48 hours), and it was found that controlling the concentration of carbon dioxide within a predetermined range greatly contributed to the improvement of corrosion resistance.

[0027]

[Table 2]

[0028]

As is apparent from the above description, according to the surface-treated steel member of the present invention, the stable layer in which the carbon concentration and the nitrogen concentration are within a predetermined range in the compound layer formed by the nitrocarburizing treatment. And the thickness of the compound layer is reduced, so that not only abrasion resistance but also corrosion resistance can be achieved stably and excellent performance.Moreover, as the compound layer is thinner, the number of production per unit time is reduced. Increase,
Productivity is improved. Further, according to the gas nitrocarburizing method of the present invention, by controlling the concentration of carbon dioxide within a predetermined range in relation to the surface area of the material to be treated, not only the wear resistance, but also the corrosion resistance is stable. A compound layer exhibiting excellent performance can be formed.

[Brief description of the drawings]

FIG. 1 is a graph showing a surface carbon concentration distribution of an example of the present invention in comparison with a comparative example.

FIG. 2 is a graph showing a surface nitrogen concentration distribution of an example of the present invention in comparison with a comparative example.

FIG. 3 is a graph showing the correlation between the thickness of a compound layer and the carbon (C) concentration, nitrogen (N) concentration, and corrosion resistance in the prior art.

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Kaname Ueno 1-6-3 Fujimi, Kawasaki-ku, Kawasaki-shi, Kanagawa F-term in Tokiko Corporation 4K028 AA03 AC07 AC08

Claims (3)

[Claims] 1. A surface-treated steel member having a compound layer mainly composed of epsilon iron nitride formed on its surface by nitrocarburizing treatment, wherein the compound layer has a carbon concentration in the range of 0.2 to 1.0% by weight and a nitrogen concentration in the compound layer. Is provided with a stable layer in the range of 5.0 to 8.0% by weight, and the thickness of the compound layer is set in the range of 5.0 to 15.0 μm. 2. The surface-treated steel member according to claim 1, wherein an oxide layer mainly composed of triiron tetroxide is further formed on the compound layer by oxidation treatment. 3. A gas nitrocarburizing method for performing a heat treatment in a carbonitriding gas atmosphere containing ammonia, nitrogen and carbon dioxide to form a compound layer mainly composed of epsilon iron nitride on the surface of a material to be treated. A gas nitrocarburizing method characterized in that the concentration of the carbon dioxide is controlled to 20.0 to 80.0 g / h per 1.0 m ² of surface area of the material to be treated.