JP3005952B2 - Method for carburizing austenitic metal and austenitic metal product obtained by the method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carburizing method for carburizing an austenitic metal to harden its surface, and to an austenitic metal product obtained by the method.

[0002]

Austenitic metals, particularly austenitic stainless steels, are most widely used because of their high corrosion resistance and high decorativeness. In particular, fasteners such as bolts, nuts, screws, washers, pins, etc. are made of austenitic stainless materials in view of the above characteristics. However, the austenitic stainless products as described above have different material strengths from ordinary carbon steel materials, and most of them are enhanced by strengthening in an intermediate processing step until finishing to a final shape. For example, the crystal structure of the austenitic stainless steel itself is densified by press working, extrusion forming, punching, or the like, thereby improving the material strength. The degree of the enhancement of the material strength in such an intermediate processing step is naturally limited due to restrictions on the shapes of bolts and nuts, and restrictions on the die cost in extrusion molding and the like. Therefore, when austenitic stainless steel products such as bolts, nuts, screws, etc. are required to have a material strength at the time of tightening, to prevent seizure, and to be capable of tapping a steel plate, a wet chrome plating or a wet plating such as Ni-P is applied. And physical vapor deposition (physical vapor deposition)
(n, PVD) or the like, or permeation hardening such as nitriding.

[0003]

However, with respect to the above-mentioned wet plating and film coating such as PVD, the film formed on the surface of the austenitic stainless steel product is peeled off and the product life is shortened. The problem of scratching occurs.

[0004] Also, among the penetration hardening treatments such as nitriding,
Nitriding is a method in which nitrogen atoms are permeated from the surface of an austenitic stainless steel material into the inside to form a hard nitrided layer of the surface layer. In this method, although the surface hardness of the austenitic stainless steel product is improved, there is a serious problem that the essential corrosion resistance is lowered. In addition, there are disadvantages that the surface roughness of the stainless steel product is deteriorated, the surface is swollen, and the stainless steel product is magnetized. As described above, the decrease in corrosion resistance due to nitriding is caused by the chromium atoms contained in the austenitic stainless steel material itself (the corrosion resistance is improved by the chromium atoms) in the nitrided hardened layer. ,
The nitride, CrN, is consumed in a chromium nitride of Cr ₂ N, the content thereof is considered to be due to a decrease. In addition, problems such as the surface swelling and the deterioration of the surface roughness are caused by the generation of the nitride as described above.

[0005] As another method of the penetration hardening treatment, there is a carburizing method. However, in the conventional carburizing method, the surface of an austenitic stainless steel product is brought into contact with a gas containing carbon to permeate carbon atoms into a surface layer to form a hard carburized layer. In such a carburizing method, carburizing is generally performed at a temperature of 700 ° C. or more, which is the A ₁ transformation point of iron, in consideration of the permeability of carbon atoms and the solid solution limit. The temperature is kept for a long time at a temperature far exceeding (the recrystallization temperature of iron is approximately 450 ° C.), and there is a major drawback that the strength of the austenitic stainless material itself is significantly reduced. As described above, such a carburizing method has a disadvantage that the strength of the material itself is considerably reduced due to carburization, and therefore, it is applicable to an austenitic stainless steel product which does not originally have such a large material hardness. Is not taken into account. Also, as mentioned earlier, bolts, nuts,
The strength of fasteners such as screws is improved by pressing, extruding, punching, etc. as described above, and is performed by realizing the strength improvement of the entire material. The fact is that the application of the technology of improving the quality is not considered.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and significantly improves the surface hardness without lowering the strength of the austenitic metal itself. It is an object of the present invention to provide a method for carburizing an austenitic metal having an intact hard surface layer and an austenitic metal product obtained by the method.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention is to provide a method for holding an austenitic metal in a heated state in a fluorine-based gas atmosphere prior to carburizing, The first gist is a method of carburizing an austenitic metal which is to be carburized by setting the temperature to 680 ° C. or less, and the surface layer having a depth of 10 to 70 μm from the surface obtained by the carburizing method is made of carbon atoms. To form a carburized hardened layer, and the hardness of the carburized hardened layer is 700 to 700 micro Vickers hardness.
An austenitic metal product formed at 1050 (HV) and having no coarse chromium carbide particles present in the carburized hardened layer is a second aspect.

[0008]

The present inventors have conducted a series of studies to improve the surface hardness of austenitic metals. In the course of carburization, pretreatment with a fluorine-based gas during a carburizing process has resulted in austenitic stainless steels and other austenitic metals. , conceived that it would be allowing carburizing at a ₁ transformation point of the temperature of the steel was repeated series of studies based on this. In the course of this research, if the treatment with a fluorine-based gas is performed prior to or simultaneously with the carburization treatment as in the above-mentioned idea, the carburization treatment that has been considered impossible conventionally becomes possible. It has been found that more effective carburizing can be realized by setting the temperature of the carburizing treatment at a temperature of 680 ° C. or less, preferably at a temperature lower than 500 ° C. instead of the conventional temperature of 700 ° C. or more. By doing so, the surface of the austenitic metal product such as the austenitic stainless product can be reduced by 10 to 70%.
A surface layer having a depth of μm is formed on the carburized layer, and the hardness of the carburized layer is 520 to 1180 H in terms of micro Vickers hardness.
v, preferably from 700 to 1050 Hv, and found that coarse chromium carbide particles did not precipitate in the carburized layer, and reached the present invention. The carburized product obtained in this way has a hard surface layer,
Moreover, the corrosion resistance of the austenitic metal itself is hardly impaired. Also, there are no difficulties such as swelling of the surface and deterioration of the surface roughness.

[0009] The coarse chromium carbide particles generally have a particle size of 0.1.
Although it is 1 to 5 μm, even if finer chromium carbide is contained in the carburized layer, there is no problem in realizing effects such as improvement of surface hardness. When the upper limit of the carbon concentration in the carburized layer is 2.0% by weight or more, the effect of improving the surface hardness is further enhanced. Further, the material of the austenitic metal such as the austenitic stainless steel constituting the austenitic metal product is an austenitic metal such as a stable austenitic stainless steel containing at least 32% by weight of nickel or at least 1.5% by weight of molybdenum. When
The effect that the reduction in corrosion resistance due to carburization is further reduced can be obtained.

Next, the present invention will be described in detail.

According to the present invention, the austenite metal is subjected to a pre-treatment using a fluorine gas, or simultaneously with the pre-treatment, to a carburizing treatment.

As the austenitic metal, austenitic stainless steel, for example, containing at least 50% by weight of iron (hereinafter abbreviated as “wt%”) and 10% by weight of chromium
% Or more austenitic stainless steel. Specifically, one such as SUS316, SUS304, etc.
8-8 stainless steel, SUS310 and 309, which are austenitic stainless steels containing 23 wt% of chromium and 13 wt% of nickel, and a chromium content of 2
Austenite containing 3 wt% and 2 wt% molybdenum-
Ferrite two-phase stainless steel materials and the like can be mentioned. Furthermore, Incoloy (Ni 30-45 wt%, heat resistant steel,
Cr 10 wt% or more, residual Fe, etc.). The austenitic metal also includes a nickel-based alloy containing 45 wt% or more of nickel, 20 wt% of chromium, 30 wt% of iron, and molybdenum. As described above, the austenitic metal in the present invention means all metals that substantially exhibit an austenite phase at room temperature (substantially means that 60 wt% or more has an austenite phase), Fe-Cr-, such as replacing nickel with austenitic stabilizing element manganese
Mn-based metals are also included. In the present invention, these are called base materials.

Among the austenitic metals formed from such austenitic metallic materials, in particular, austenitic stainless materials are widely used for fasteners such as bolts, nuts, screws, washers, pins and the like. . In the present invention, the austenitic metal products such as austenitic stainless products include the above fasteners, chains, watches, cases, spinning shuttle (spindle) tips, fine gears, knives and the like. Various stainless steel products.

Next, the austenitic metal as described above
The genus prior to or the same as carburizing
Occasionally, a fluorination treatment is performed in a fluorine-based gas atmosphere.
For this fluorination treatment, a fluorine-based gas is used. the above
As a fluorine-based gas, NF _Three, BF_Three, CF_Four, H
F, SF₆, C_TwoF₆, WF₆, CHF_Three, SiF_Four ,
ClF_ThreeAnd the like.
Are used alone or in combination of two or more. Ma
In addition, in addition to these gases, contains fluorine (F) in the molecule
Use other fluorine gas as the above fluorine gas
Can be. In addition, such fluorine compound gas is thermally decomposed
F generated by thermal decomposition_TwoGas or in advance
F made_TwoGas should also be used as the above fluorine-based gas
Can be. Such a fluorine compound gas and F_TwoWith gas
Are sometimes mixed and used. And the above fluorine
Compound gas, F_TwoFluorine gas such as gas
Can be used, but usually N_TwoInert gas such as gas
Used after dilution. For such diluted gas
The concentration of the fluorine-based gas itself is
If it is 10,000 to 100000 ppm, preferably
Is 20000 to 70000 ppm, more preferably 3
50,000 to 50,000 ppm. This fluorine gas
NF is the most practical_ThreeIt is. the above
NF_ThreeIs gaseous at room temperature, has high chemical stability,
Easy to handle. Such NF_ThreeThe gas is usually
N above_TwoUsed within the above concentration range in combination with gas
Can be

More specifically, in the present invention, first, a virgin (untreated) austenitic metal is placed in a furnace and heated in a fluorine-based gas atmosphere having the above-mentioned concentration. Fluoride treatment. In this case, the heating and holding are performed by holding the austenitic metal itself at a temperature of, for example, 250 to 600C, preferably 280 to 450C. The holding time of the austenitic metal in the fluorine-based gas atmosphere is usually set to ten minutes to several tens of minutes. By treating the austenitic metal in such a fluorine-based gas atmosphere, the passive film containing Cr ₂ O ₃ formed on the surface of the austenitic metal changes to a fluoride film. This fluoride film is expected to facilitate the penetration of carbon atoms used for carburizing compared to the passive film, and the surface of the austenitic metal is brought into a surface state in which the penetration of “C” atoms is easy by the fluorination treatment. It is presumed that it becomes.

Next, after the above-described fluorine treatment, a carburizing treatment is performed. In the carburizing treatment, the austenitic metal itself is heated to a temperature of 680 ° C or lower, preferably 600 ° C or lower, more preferably 400 to 500 ° C, and a carburizing gas composed of CO ₂ + H ₂ or RX
[Components of RX, 23 vol% of CO (hereinafter abbreviated as “vol%”), 1 vol% of CO ₂ , 31 vol% of H ₂ , H ₂ O 1
vol%, balance N ₂ ] + CO _2, etc., and the inside of the furnace is made into a carburizing gas atmosphere. As described above, in the present invention, the carburizing treatment is performed at a low temperature that does not cause the softening and solutionizing of the core portion of the austenitic metal, which is the greatest feature. In this case, the above CO ₂ +
The ratio of H ₂ is CO _{2 2 to} 10 vol%, H ₂ 30 to 4%.
Preferably 0vol%, RX + CO ₂ is, RX is 80
A ratio of 90 vol% and CO2 of ₃ to 7 vol% is preferable. As a gas used for carburizing, CO + CO ₂ + H ₂ is also used. In this case, each ratio is from CO32 to
_{43vol%, CO 2 2~3vol%,} H 2 55~65
A vol% ratio is preferred.

By performing such treatment, a diffusion and permeation layer of "carbon" is formed deeply and uniformly on the surface of the austenitic metal. This permeation layer is formed in a γ-phase which is a base phase.
It has a form in which a large amount of C is dissolved to cause lattice distortion, significantly improving the hardness as compared with the base material, and maintaining the same corrosion resistance as the base material. For example, a sample of SUS316 plate piece, which is a typical austenitic stainless steel, is placed in a furnace, and NF ₃ + N ₂ (NF
_In a fluorine-based gas atmosphere of 10 vol% and N ₂ 90 vol%), fluorination treatment is performed at 300 ° C. for 40 minutes. Then, the fluorine-based gas is discharged from a furnace, and then CO + CO ₂ + H ₂ (carburizing gas) is discharged. CO32vol%, C
O ₂ 3 vol%, H ₂ 65 vol%) was introduced into the furnace,
It was kept at 450 ° C. for 16 hours and carburized. as a result,
A hardened layer having a surface hardness Hv = 880 (cores 230 to 240) and a depth of 20 μm was formed on the surface of the SUS316 plate material. When this cured sample was subjected to a salt spray test according to JIS 2371, it did not rust at all over 480 hours. In addition, the cured layer is a virular reagent (acidic picric acid alcohol solution) used for a corrosion resistance test of the cured layer.
However, it was not etched and was barely etched by aqua regia. In addition, the cured sample had almost no deterioration in surface roughness, no dimensional change due to swelling, and no magnetism. The present inventors have conducted further studies by variously combining the types of austenitic metal pieces, carburizing temperature, etc., and as a result, when the carburizing temperature exceeds 600 ° C., the softening of the core portion of the austenitic metal has occurred. It has been found that the corrosion resistance of the cured layer is greatly reduced. From the standpoint of corrosion resistance, good results are obtained particularly when the carburizing temperature is set to 600 ° C. or lower, preferably 500 ° C. or lower. More preferred carburizing temperature is
As mentioned above, the temperature is 400 to 500 ° C. In addition, as a result of the study of the present inventors, among austenitic metals, stable austenitic stainless steel containing a large amount of nickel, and also stable austenitic stainless steel containing molybdenum, have good corrosion resistance even after curing. It became clear.

The above-mentioned fluorination treatment and carburization treatment are performed, for example, in a metal muffle furnace as shown in FIG. That is, in this muffle furnace, first, fluorination treatment is performed, and then carburization treatment is performed. In FIG.
1 is a muffle furnace, 2 is an outer shell, 3 is a heater, 4 is an inner vessel, 5 is a gas introduction pipe, 6 is an exhaust pipe, 7 is a motor, 8 is a fan, 11 is a wire mesh basket, and 13 is a vacuum pump. , 14
Denotes an exhaust gas treatment device, 15 and 16 denote cylinders, 17 denotes a flow meter, and 18 denotes a valve. For example, an austenitic stainless steel product 10 is put into the furnace 1, a cylinder 16 is connected to a flow path, a fluorinated gas such as NF ₃ is introduced into the furnace 1, a fluorination treatment is performed while heating, and then an exhaust pipe is provided. From 6, the gas is drawn out by the action of the vacuum pump 13 and detoxified in the exhaust gas treatment device 14 and discharged to the outside. Next, cylinder 1
5 is connected to the flow path, and the above-mentioned carburizing gas is introduced into the furnace 1 to perform carburizing treatment. Thereafter, the gas is discharged to the outside via the exhaust pipe 6 and the exhaust gas treatment device 14. By this series of operations, fluorination treatment and carburization treatment are performed.

As described above, according to the carburizing treatment of the present invention, the treated product retains the high corrosion resistance of the austenitic metal itself, which is considered to be due to the following reasons. That is, in the present invention, by performing the fluorination treatment prior to the carburization, the carburization treatment can be performed at a low temperature of 680 ° C. or less. By
The chromium component present in the austenitic metal (which is considered to exhibit corrosion resistance) is Cr ₇ C ₂ or Cr ₂₃ C
It is considered that it is difficult to precipitate and fix as carbides such as _{6 and the} like, and the amount of the deposit and fixation becomes low, whereby the chromium component remaining in the austenitic metal increases.
This is because the carburized SUS316 products shown in FIGS. 2B to 2C are fluorinated at 300 ° C. for 40 minutes in a fluorine gas atmosphere of NF ₃ 10 vol% + N ₂ 90 vol%, and then CO 2
_{32vol% + CO 2 3vol% +} H 2 65vol% of carburizing 600 ° C. in a gas atmosphere, for 4 hours carburizing (Figure 3), 450 ° C. 16 hours carburizing treatment [Figure 2 (b)] X-ray diffraction results of the cured layer of Is untreated (FIG. 2 (a)). That is, in the carburization at 600 ° C. shown in FIG. 3, the peak of Cr ₂₃ C ₆ is sharp and high. This means that a relatively large amount of the chromium carbide precipitates and the chromium component remaining in the austenitic metal is small. 4
In FIG. 2B carburized at 50 ° C., the Cr ₂₃ C
_Since the peak of ₆ is hardly observed, the precipitation amount of the carbide is remarkably small. Therefore, it is considered that the chromium component remaining in the austenitic metal is large and the corrosion resistance is high.

It is considered that the improvement of the hardness of the carburized product is caused by the generation of γ-lattice strain due to the penetration of carbon atoms. This is a carburized product at 450 ° C [Fig.
(B)], the peak position of the γ-phase in the X-ray diffraction and the peak position of the γ-phase of the pickled product after carburizing at 480 ° C. (FIG. 2 (c)) correspond to the γ-phase of the SUS316 untreated product. Is shifted to the lower angle side (left side) from the peak position of, it is clear that the γ-lattice strain is generated in the carburized products (FIGS. 2B and 2C). The X-ray diffraction was performed using a RINT 1500 apparatus at 50 kV and 200 mV.
A and Cu targets were performed under the conditions.

In the present invention, the temperature of the carburizing treatment is increased, and particularly when the temperature exceeds 450 ° C.
A phenomenon occurs in which carbides such as r ₂₃ C ₆ precipitate on the surface of the hardened layer. However, even in such a case, when the carburized product is immersed in a strong acid such as HF-HNO ₃ or HCl-HNO ₃ , the precipitates are removed, and the corrosion resistance is as good as the base material, and the Vickers hardness Hv is 850 or more. High surface hardness can be maintained. FIG. 2 (c) shows that after the SUS316 plate shown in FIG. 2 (a) is carburized at 480 ° C.
Strong acid with a concentration of 5 vol% HF-15 vol% HNO ₃
FIG. 3 is an X-ray diffraction chart of a treated product immersed for 0 minutes, in which the carbide is not observed at all. Thus, the surface of the carburized hardened layer formed on the surface of the carburized austenitic metal, for example, an austenitic stainless product, is blackened by carburization,
In some cases, the outermost surface layer of the carburized hardened layer is an iron-based internal oxide layer. That is, such an internal oxide layer on the surface has some oxygen atoms in the atmosphere during carburization, depending on the case, and is formed by the presence of the oxygen atoms. As described above, the removal of the internal oxide layer can be performed by immersing in a strong acid such as HF-HNO ₃ or HCl-HNO ₃ to remove the precipitates. Corrosion resistance comparable to that of the base material and high surface hardness of Vickers hardness Hv = 850 or more can be maintained. And the austenitic stainless steel product from which the internal oxide layer has been removed by the above treatment is
It shows a bright state before the carburizing treatment. In more detail, when the surface of the treated product is closely observed after the carburizing treatment, a dark color layer exists at the outermost surface at a depth of 2 to 3 μm from the surface, and this is an internal oxide layer of iron. Was confirmed by X-ray diffraction.
This can be considered as follows. That is, 400-
In an atmosphere containing CO in the range of 500 ° C., carburizing (C
O → CO ₂ + C) and oxidation of Fe simultaneously (4CO ₂ + 3F)
e → 4CO + Fe ₃ O ₄ ) coexist, and it is considered that the internal oxide layer was formed for this reason.
Such an internal oxide layer of iron cannot be seen in the conventional carburizing method in which the treatment is performed at a temperature of 700 ° C. or more. 4
SUS316L (C which has been carburized at 80 ° C for 12 hours
= 0.02 wt%, Cr = 17.5 wt%, Ni = 1
2.0 wt%, Mo = 2.0 wt%) To further describe the examples of socket bolts and washers, the depth of the hardened layer is 30 μm, the surface hardness is Hv = micro Vickers.
910. Next, these black carburized products were immersed in a 5 wt% HF-25 wt% HNO ₃ solution heated to 50 ° C. for 20 minutes, and then subjected to soft blast. Socket bolts and washers with This is JIS
When subjected to a salt spray test of 2371, no rust was formed even after 2,000 hours. In addition, JIS 05
The results of the pitting corrosion test using ferric chloride of No. 78 were almost the same as those of the untreated SUS316 material.

Regarding the carburized hardened layer,
In a low-temperature region of 500 ° C. or less, the diffusion rate of C under the austenite structure is considerably slow. Therefore, even in the case of the SUS316L type having the thickest hardened layer, the processing temperature is 490.
A carburized hardened layer of 37 μm is formed in 12 ° C. for 12 hours, but the total carburized hardened layer is only 49 μm even if the carburizing time is further added for 12 hours. It takes 70 hours or more to obtain a cured layer depth of 70 μm. Presumably, such a long-term treatment would lose economics. Even in the case of drill tapping that requires a hardened layer as deep as possible, 2.3t SPCC with a 40 µm hardened layer
(Cold rolled steel sheet) can be sufficiently drilled, and a useful carburized hardened layer can be obtained within a processing time within which the economic efficiency is not lost.

[0023]

As described above, according to the present invention, the austenitic metal is kept in a heated state in a fluorine-based gas atmosphere prior to or simultaneously with the carburizing treatment. Can be lowered to 680 ° C. or lower. Therefore, high surface hardness can be realized without impairing the corrosion resistance, high workability, and the like of the austenitic metal itself. And, in the present invention, since the surface height as described above is improved by carburizing treatment, a deterioration phenomenon of the surface roughness of the austenitic metal surface, and a decrease in dimensional accuracy based on blistering,
There is no inconvenience such as the austenitic metal itself being magnetic.

In the austenitic metal product such as the austenitic stainless product obtained as described above, the surface layer having a depth of 10 to 70 μm from the surface of the austenitic metal product is hardened by the intrusion of carbon atoms and carburized. And the hardness of the carburized hardened layer is 520 to 1180 (Hv), preferably 7
It is formed between 00 and 1050 (Hv). In addition, since the coarse chromium carbide particles do not precipitate in the carburized hardened layer, the resulting product has the corrosion resistance of the austenitic metal itself and has a high surface hardness. For this reason, among the austenitic metal products, fasteners such as bolts, nuts, and screws made of austenitic stainless steel have excellent performance such as strength at the time of fastening, seizure prevention, and tapping ability for steel plates. In particular, it is useful for applications that require durability as well as decorativeness, for example, fasteners used indoors and outdoors for automobiles.

Next, examples will be described together with comparative examples.

[0026]

Example 1 and Comparative Example 1 As an example, SUS31
6 (Cr content 18 wt%, Ni content 12 wt%, Mo content 2.5 wt%, balance Fe) and SUS304 (C
r content 18 wt%, Ni content 8.5 wt%, balance Fe)
2.5 mm thick plate pieces were prepared. Also, NCF601 (Ni content 60 wt%, C
A 1 mm thick plate piece having an r content of 23 wt% and Fe of 14 wt%) was prepared. As a comparative example, SUS430 which is a ferritic stainless steel (C content 0.06 wt%, Cr content 1
7.5 wt%, balance Fe) and SUS420J ₂ which is a martensitic stainless steel (C content 0.32 wt%)
%, A Cr content of 13 wt%, and a balance of 2.5 mm thick plate pieces of Fe) were prepared.

Next, each of these materials was put into the muffle furnace 1 shown in FIG. 1, and the inside of the furnace 1 was sufficiently vacuum-purged, and then heated to 300 ° C. Then, fluorine gas in that state _{(NF 3 10vol% + N 2} 90vol%)
And the inside of the furnace 1 was made the same pressure as the atmospheric pressure, and fluorination treatment was performed for 10 minutes in that state. Next, after the fluorine-based gas was discharged from the furnace 1, the inside of the furnace 1 was heated to 450 ° C., and while maintaining the temperature, the carburizing gas (CO 10 vol%, C
O _{2 2} % by volume, H ₂ 10% by volume, balance N ₂ ) were introduced, and the mixture was kept for 16 hours to carry out carburizing treatment.

The obtained sample is an example product (SUS3
16, SUS304, NCF601) had a black surface. Such a black surface did not occur in the comparative example product. Next, the above-mentioned black surface layer was rubbed off and the surface hardness and the depth of the carburized hardened layer were determined. In addition, the same measurement was performed for the comparative example product as a control. Table 1 shows the results.

[0029]

[Table 1]

As is evident from the results in Table 1 above, in each of the examples, the surface hardness was significantly improved by carburizing treatment, and a carburized hardened layer was formed. It turns out that such a phenomenon is not seen at all. In addition, SUS316 and SUS30 of these Examples
4, 4, and 5 show micrographs of cross sections of NCF601. These were taken at a magnification of 600 times using an optical microscope. In these figures, a fabric layer, a carburized hardened layer, and a resin layer (black portions) are shown from the lower side of the figures. The resin layer is a layer made of an embedded resin in a state where the sample is embedded in the resin.

Next, each of the above-mentioned examples was polished and polished with emery paper, and JIS 2
371 salt spray test (SST) and 15 wt% HN
It was immersed in a 50 ° C. solution of O ₃ to perform another kind of corrosion resistance test again, and the magnetic permeability was measured. SUS3
Table 2, together with untreated products of 16, SUS304 and NCF601, and nitrided products obtained by nitriding those materials, are shown in Table 2.

[0032]

[Table 2]

The above SUS316, SUS304,
In NCF601, the manufacture of a nitride product as a comparative example is as follows.
This was performed as follows. That is, using the same furnace as that used in the above example, fluorination treatment was performed for 40 minutes using the same fluorine-based gas under the same conditions. Next, after the fluorine-based gas is discharged from the furnace, a nitriding gas (NH ₃ 50
(vol% + N ₂ 25 vol% + H ₂ 25 vol%), the inside of the furnace was heated to 580 ° C., and kept in this state for 3 hours for nitriding treatment.

From the results shown in Table 2 above, the product of the example has a longer time to rust SST than the nitrided product and has a 15 wt% H
It can be seen that there is no change even when immersed in NO ₃ , and the corrosion resistance is remarkably superior to the nitrided product. In addition, it can be seen that the nitrided article has magnetism, whereas the example article has no magnetism. Further, it can be seen that the product of the example has almost no blistering as compared with the nitrided product and has high dimensional accuracy.

[0035]

Embodiment 2 SUS316 (Cr content 17 wt%, Ni
M6 bolts formed by forging from 13 wt%, 3 wt% Mo, balance Fe wire and non-magnetic stainless steel (Cr content: 17.8 wt%, Ni content: 11.5 wt%, Mn content: 1.4 wt%, N content: 0.5 wt%) , The remaining Fe) tapping screw (diameter 4 mm) formed by forging from a wire rod, and the SUS316 sheet material used in Example 1 and SUS30
A sample of four plates was placed in the furnace shown in FIG.
C. and fluorinated in the same manner as in Example 1. Next, a mixed gas with a carburizing gas (CO 50 vol% + H ₂
10 vol% + the balance N ₂ ) was introduced into the furnace, and kept in that state for 32 hours for processing. In this case, the fluorination treatment and the carburization treatment are performed almost simultaneously. Then, the obtained sample was subjected to air blast, and the black layer (1 to
(2 μm thickness) was removed and the surface height was measured.
The hardness of the M6 bolt made of S316 is Hv = 820, the hardness of the non-magnetic tapping screw is Hv = 860, SUS31
The hardness of the six plate materials is Hv = 780, the hardness of the SUS304 plate material is Hv = 830, and the depth of the hardened layer is 18 μm each.
m, 19 μm, 20 μm, and 21 μm.

Next, the example product obtained as described above was immersed in a 60% solution of 15 wt% HNO ₃ for 30 minutes.
After the adhered iron was completely removed, the sample was subjected to the SST test to test the corrosion resistance. As a result, the SUS316 bolt, the non-magnetic stainless steel screw, and the SUS316 plate did not rust at all even after more than 480 hours. Also, 30
The four plate materials slightly rusted after 71 hours. From these results, it is understood that excellent results can be obtained in almost the same manner as in the above-described embodiment.

[0037]

[Embodiment 3] The SUS316 plate used in Embodiment 1 and SU
The S304 plate material and the NCF601 plate material were placed in the same furnace as in Example 1, heated to 400 ° C., and introduced with the same fluorine-based gas as used in Example 1, and subjected to fluorination treatment as in Example 1. The temperature of each material was raised to 480 ° C., and in that state, a carburizing gas (endothermic gas: RX 30 vol%, C
A mixed gas consisting of O ₂ 2.5 vol% and N ₂ 65 vol%) was introduced. Then, after holding in this state for 12 hours, it was taken out. As in Example 1, a black scale was adhered to the surface of the obtained Example product. Therefore, in order to remove the black scale, a strong acid immersion treatment was performed.
That is, the strong acid (HNO ₃ 15 vol%, HF ₃ v
ol% mixed solution) for 10 minutes and then lightly blasted with air blast. As a result, the black scale was removed, and the surface appearance was the same as that of the untreated product (not subjected to fluoridation and carburization). On the other hand, the surface hardness and the depth of the hardened layer were compared between the example product which was not subjected to the strong acid immersion treatment as described above but was simply fluorinated and then subjected to the carburizing treatment and the one subjected to the above strong acid immersion treatment. , And SST rust test. Table 3 shows the results.

[0038]

[Table 3]

As described above, it can be seen that when the strong acid immersion treatment is performed, the corrosion resistance is greatly improved as compared with the case where the strong acid immersion treatment is not performed.

FIG. 2 (c) shows the result of X-ray diffraction treatment of the 316 plate material subjected to the strong acid immersion treatment. In FIG. 2 (c), no Cr carbide was identified. In addition, due to the occurrence of lattice distortion due to the inclusion of a large amount of carbon in the dough γ-phase lattice, the peak position of the γ-phase is shifted to a much lower angle side than that of the untreated material, which is significant. Lattice strain is generated, which proves that it is the origin of hardening.

[0041]

Example 4 The SUS316 plate used in Example 1 was fluorinated in the same manner as in Example 1, and then heated to 600 ° C. Next, a carburizing gas (N ₂ 50 vol% +
RX50 vol%) was introduced, held for 4 hours, and then taken out.

The product of this embodiment has a surface hardness Hv = 900,
The depth of the cured layer was 35 μm. Next, after the surface was polished well, it was subjected to an SST rust test. The time until rusting is 4 hours. Although the result of the SST rusting test is better than that of nitrided products, it is considered that the corrosion resistance of stainless steel is insufficient. The X-ray diffraction result of the product of this example is shown in FIG.
And many sharp diffraction lines of Cr carbide and Mo carbide were observed.

[0043]

Fifth Embodiment Using a bolt made of SUS316 plate and a tapping screw made of non-magnetic stainless steel used in the second embodiment, and using a mixed gas of a fluorine-based gas and a carburizing gas in the same manner as in the third embodiment, Carburizing treatment was performed simultaneously. In this case, the temperature is 510 ° C. and the holding time is 8
Changed to time. The surface hardness of the heads of the screws thus obtained was Hv = 920 and Hv = 980, and the depths of the hardened layers were 26 μm and 28 μm, respectively.

Next, after performing a strong acid immersion treatment in the same manner as in Example 3, the surface hardness was measured.
v = 580 and Hv = 520 were greatly reduced.

The reason for this is that the carburizing temperature was 30 ° C. higher than in the case of Example 3, so that a large amount of chromium carbide was precipitated on the surface layer side. Was removed by erosion,
It is estimated that the surface hardness was reduced.

[0046]

Embodiment 6 A plurality of SUS316 (Cr content: 17.5 wt%, N) having the same core hardness (Hv = 150 to 160) as those subjected to solution treatment (solid solution treatment)
i content 11 wt%, NO content 2 wt%)
S304 (C content 0.06 wt%, Cr content 17.5 w
t%, Ni8wt%, balance Fe) sheet piece and SUS3
A plurality of M6 bolts formed by forging from 16 wires were prepared. Of the plurality prepared, several plates and bolts were placed in the experimental furnace shown in FIG.
° C, and in that state, a fluorine-based gas (NF ₃ 10 vol
1% + N ₂ 90 vol%), and fluorinated.
The sample was taken out of the furnace and used as a fluorinated sample.

Next, the remaining non-fluorinated sample and the fluorinated sample subjected to the fluorination treatment as described above are put together in the experimental furnace shown in FIG. 1 and the temperature is raised to 460 ° C. Carburizing gas (CO20v
ol%, 75 vol% of H _{2, 1} vol% of CO ₂ ) and kept for 12 hours to carry out carburizing treatment.

Of the samples processed as described above,
The surface of each of the fluorinated samples (Example products) was black. On the other hand, the non-fluorinated sample not subjected to the fluoridation treatment (comparative sample) had almost the same appearance and metallic luster before the treatment. Next, when the surface hardness of the above-mentioned examples was examined, Hv = 920 for each.
A surface hardness of 〜１０1050 was indicated. The depth of the hardened layer of each of the examples was 20 μm to 25 μm. On the other hand, no improvement in the surface hardness was observed for the comparative example product not subjected to the fluorination treatment.

[0049]

COMPARATIVE EXAMPLE 2 The SUS316 wire rod used in Example 6 was subjected to forging M6 bolts. The head and the thread portion of this bolt each have a hardness of Hv = 350 to 390 due to the forging. The bolt was put into a normal all-case carburizing furnace at a job shop (a heat treatment contract processing manufacturer) and carburized at 920 ° C. for 60 minutes.

As a result, the surface hardness of the carburized bolt becomes Hv = 580-620 ° C. and the depth of the hardened layer becomes 25
It was 0 μm. However, the hardness of the head and the thread portion of the bolt was remarkably reduced to Hv = 230 to 250. When the carburized bolt was subjected to a salt spray test, red rust was generated on the entire surface in 6 hours.

[0051]

Embodiment 7 SUS316L material and SUS310 material (C = 0.06 wt%, Cr = 25 wt%, Ni = 2
0.5wt%) and XM7 material (C = 0.01wt)
%, Cr = 18.5 wt%, Ni = 9.0 wt%, Cu
= 2.5wt%) and M6 bolts formed from SUS304 material and
In each case, the surface hardness of the head was measured. SUS316L bolt; Hv = 340, SUS310 bolt; Hv =
350, XM7 bolt; Hv = 320, SUS304
Hv = 400. They are then at the time of heating to 350 ° C. at the furnace shown in Figure 1, N ₂ + 5 v
After introducing ol% NF ₃ for 15 minutes, it was switched to N ₂ only and the temperature was raised to 480 ° C. Next, H ₂ 20 vol% +
A carburizing gas of 10 vol% of CO + 1 vol% of CO ₂ + the balance of N ₂ was introduced, and the mixture was taken out in the atmosphere for 15 hours. All the samples were black, but after washing, the surface hardness and the carbonized hardened layer depth were measured.
6; Hv = 880, depth 38 μm, SUS310; Hv
= 920, depth 30 μm, XM7; Hv = 890, depth 33 μm, SUS304; Hv = 1080, depth 20 μ
m. Then, when the cross section of the carbonized hardened layer was corroded with aqua regia and observed under a microscope, the SUS304 bolts showed black in both the hardened layer and the unhardened portion, whereas the SUS304 bolts showed black.
The bolt made of 316 and SUS310 had a carbonized hardened layer that was white and bright, and the bolt made of XM7 had a slightly darker color than the two.

Next, all of these samples were used for 50
It was immersed in a 5 wt% HF-20 wt% HNO ₃ solution at 10 ° C. for 10 minutes and taken out. The state of the carbon hardened layer after immersion in the strong acid was as follows. SUS316; Hv = 86
0, depth = 35 μm, SUS310; Hv = 880, depth = 28 μm, XM7; Hv = 650, depth = 25 μ
m, SUS304; Hv = 450, depth = 5 μm. In addition, SUS316, SUS310, and XM7 acid-immersed products were subjected to a salt spray test according to JIS 2371.
It did not rust over time.

[0053]

[Embodiment 8] After subjecting the SUS316 socket bolt used in Embodiment 1 to the same fluorination treatment as shown in Embodiment 1, at 510 ° C, 20 vol% of H ₂ +10 V of CO2.
ol% + ₁ vol% of CO ₂ + balance N ₂ for 12 hours. The surface hardness of the head was Hv = 1020, and the depth of the carburized hardened layer was 45 μm. Then, at 50 ℃
wt% HF-28 wt% HNO ₃ solution was immersed for 10 hours and taken out, and the state of the hardened layer was examined.
At = 650, the cured layer was 20 μm, which was smaller than before immersion, indicating that the cured layer was etched by the HF-HNO ₃ solution.

[0054]

[Embodiment 9] A 4 mm diameter drill tapping screw (length under the neck 25) was formed from a SUS316L wire containing 2 wt% of Cu.
mm) was forged. This was subjected to carburizing treatment in the same manner as in Example 1 except that only the carburizing treatment conditions were 490 ° C. and the treatment time was 16 hours. After that, the treated product is kept at 55 ° C for 3 hours.
It was immersed in a 15 wt% HNO ₃ solution for 15 hours and shot blasted. When the carburized hardened layer after shot blasting was measured, the surface hardness H
v = 890, and the depth of the hardened layer was 42 μm. Next, 2
A 13t SPCC was prepared, and a drilling test was performed with a hand driver. As a result, drilling performance almost equivalent to that of an iron-based carburized product was obtained.

[0055]

Example 10 The 316L socket bolt and 310 volt used in Example 1 were fluorinated in the same manner as in Example 1, then raised to 430 ° C., and carburized exactly as in Example 1. It was kept for 24 hours using a neutral gas and was taken out. The surface hardness at this time was Hv = 720 for 316 material,
310 material, Hv = 780, the thickness of the cured layer is 21 μm, 1
It was 6 μm.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a furnace used for carburizing treatment of the present invention.

FIGS. 2A and 2B are X-ray diffraction diagrams, wherein FIG. 2A is an X-ray diffraction diagram of a SUS316 unprocessed product, and FIG. 2B is an X-ray diffraction diagram of a processed product obtained by subjecting a SUS316 plate to carburizing at 450 ° C. (C) is an X-ray diffraction diagram of a treated product obtained by subjecting a SUS316 plate material to a carburizing treatment at 480 ° C. and then performing a strong acid immersion treatment.

FIG. 3 is an X-ray diffraction diagram of a treated product obtained by subjecting a SUS316 plate to carburizing at 600 ° C.

FIG. 4 is a photomicrograph of a cross section of a treated product obtained by carburizing a SUS316 plate at 450 ° C.

FIG. 5 is a photomicrograph of a cross section of a treated product obtained by carburizing a SUS304 plate at 450 ° C.

FIG. 6 is a micrograph of a cross section of a processed product obtained by carburizing an NCF601 plate at 450 ° C.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-143263 (JP, A) JP-A-3-1933861 (JP, A) JP-A-2-115344 (JP, A) JP-A-1- 212748 (JP, A) JP-A-63-161170 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 8/22 C23C 8/02

Claims (8)

(57) [Claims] 1. An austenitic metal is kept in a heated state in a fluorine-based gas atmosphere prior to carburizing, and then carburizing is performed by setting the temperature for carburizing to 680 ° C. or lower. Carburizing method for austenitic metal. 2. The temperature at the time of carburizing treatment is 400-500.
The method for carburizing an austenitic metal according to claim 1, wherein the temperature is set to ° C. 3. The carburizing method for an austenitic metal according to claim 1, wherein the heating in the fluorine-based gas atmosphere is performed by setting the temperature of the austenitic metal to 250 to 450 ° C. 4. The method for carburizing austenitic metal according to claim 1, wherein the austenitic metal is austenitic stainless steel. 5. An austenitic metal comprising nickel
The method for carburizing an austenitic metal according to any one of claims 1 to 4, wherein the method is a Ni-based alloy containing 2% by weight or more. 6. A surface layer having a depth of 10 to 70 μm from the surface is hardened by infiltration of carbon atoms to form a carburized hardened layer, and the hardness of the carburized hardened layer becomes 700 to 1050 (HV) in terms of micro Vickers hardness. An austenitic metal product formed, wherein coarse chromium carbide particles are not present in the carburized hardened layer. 7. The austenitic metal product according to claim 6, wherein the austenitic metal is an austenitic stainless steel. 8. The austenitic metal product having a hard surface layer according to claim 6, wherein the material of the austenitic stainless product having a hard surface layer is a stable austenitic stainless steel containing 1.5% by weight or more of molybdenum.