JPH09268364A - Method for carburizing treatment for austenitic stainless steel and austenitic stainless steel product obtained thereby - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for carburizing treatment for austenitic stainless steel by which its surface hardness can be improved without deteriorating the strength of a base metal and, moreover, a hard surface layer having corrosion resistance higher than that of the base metal can be formed. SOLUTION: This is a method for carburizing treatment for austenitic stainless steel in which, prior to carburizing treatment, austenitic stainless steel is held in a heating state in an atmosphere of gaseous fluorine, next, the temp. at the time of carburizing treatment is set to 400 to 680 deg.C, and then, the carburizing treatment is executed. The austenitic stainless steel is composed of stable austenitic stainless steel contg. 1 to 6% molybdenum or stable austenitic stainless steel contg. 13 to 25% chromium, and a carburizing hardened layer having corrosion resistance higher than that of the base metal is formed by the carburizing treatment.

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 austenitic stainless steel to harden its surface and improve corrosion resistance, and an austenitic stainless steel product obtained thereby. Is.

[0002]

2. Description of the Related Art Austenitic stainless steel is widely used because of its high corrosion resistance and high decorativeness.
For example, fasteners such as bolts, nuts, screws, washers, and pins are made of an austenitic stainless material in view of the above characteristics. In addition, in machinery and equipment for fields requiring high corrosion resistance such as food machinery, chemical plants, and nuclear power, various machine parts such as various shafts, impellers, dies, springs, chains, valve parts, etc. Austenitic stainless steel products are applied. However, unlike most carbon steel materials, most of the above austenitic stainless steel products have been strengthened by strengthening the material strength of the austenitic stainless steel products in the intermediate processing step until finishing the final shape. For example, cold or warm processing represented by press working, extrusion molding, punching and the like densifies the crystal structure of austenitic stainless steel itself, and so-called work hardening improves the material strength. In strengthening materials in such intermediate processing steps, there are restrictions due to the shape of bolts, nuts, etc., and there are restrictions due to mold costs, etc., during extrusion processing, etc. Naturally, there are limits. Therefore, when fasteners such as bolts, nuts and screws and austenitic stainless steel products such as pump shafts, bearings and springs are required to have a special surface rigidity and seizure prevention function, hard chrome plating, Ni-P, etc. Wet plating or physical vapor deposition (physica)
(1 vapor deposition, PVD) or the like, or a permeation hardening treatment such as nitriding or carburizing.

[0003]

However, in the above-mentioned wet plating or film coating such as PVD, the film formed on the surface of the product is peeled off, which shortens the life of the product. Cause Therefore,
The application of permeation hardening treatment such as carburizing is considered.

Of the above-mentioned penetration hardening treatments, the nitriding treatment is a method of infiltrating nitrogen atoms from the surface of the austenitic stainless material into the inside thereof to form a hard nitride layer on the surface. However, with this method, although the surface hardness of the product is improved, there is a major problem that the essential corrosion resistance is reduced. That is, in the nitriding hardened layer, the chromium atoms contained in the austenitic stainless steel material itself (the improvement of corrosion resistance is realized by the chromium atoms) are consumed as chromium nitrides of CrN and Cr ₂ N. It is considered that the corrosion resistance decreases due to the decrease in the amount of solid solution chromium in the base material. Further, the formation of this nitride causes problems such as swelling of the surface of the product, deterioration of the surface roughness, and magnetism.

As another method of the above penetration hardening treatment, there is a carburizing method. The carburizing method that has been often performed conventionally is to contact the surface of the product with a gas containing carbon,
Carbon atoms are permeated into the surface layer to form a hard carburized layer. In such a carburizing method, the treatment is generally carried out at a temperature of 700 ° C. or higher, which is the A ₁ transformation point of iron, in consideration of the permeation rate of carbon atoms and the solid solution limit. However, in this case, the temperature is much higher than the recrystallization temperature of iron (the recrystallization temperature of iron is approximately 450 ° C.) for a long time, and the base material of austenitic stainless steel reinforced by work hardening is maintained. Has a major drawback that it is softened by recrystallization and the product strength is significantly reduced. Moreover, when austenitic stainless steel is carburized at a high temperature as described above, chromium carbide precipitates in the carburized layer, and the solid solution chromium component of the austenitic stainless steel is consumed for the formation of carbides. The amount decreases,
There is also a problem that the corrosion resistance is remarkably lowered. Therefore, the carburizing process for austenitic stainless steel is
The reality is that nothing has been done so far.

The present invention has been made in view of the above circumstances, and a hard surface layer having a significantly improved surface hardness without lowering the strength of the base material and having a corrosion resistance higher than that of the base material. It is an object of the present invention to provide a method for carburizing austenitic stainless steel capable of forming a steel and austenitic stainless steel products obtained thereby.

[0007]

In order to achieve the above object, the method of carburizing austenitic stainless steel according to the present invention is to hold austenitic stainless steel in a heated state in a fluorine gas atmosphere prior to carburizing. ,
A method of carburizing austenitic stainless steel, in which the temperature during the carburizing treatment is set to a temperature of 400 to 680 ° C., wherein the austenitic stainless steel contains molybdenum in an amount of 1 to 6% by weight. 13-25 stainless steel or chrome
The gist of the present invention is stable austenitic stainless steel containing 100% by weight, and a carburized hardened layer having corrosion resistance higher than that of the base material is formed by the above carburizing treatment.

Further, in the austenitic stainless steel product of the present invention, the base material contains 1 to 6% by weight of stable austenitic stainless steel or chromium.
It is formed from stable austenitic stainless steel containing 3 to 25% by weight, and a surface layer having a depth of 5 to 70 μm from the surface is hardened by the infiltration of carbon atoms to form a carburized hardened layer. The gist is that it is formed to have a micro Vickers hardness of 500 to 1050 (HV), the carburized hardened layer is formed of an austenite phase in which chromium carbide particles are not present, and has corrosion resistance higher than that of the base material.

[0009] The present inventors have found that to increase the surface hardness for the austenitic stainless steel in the process of superimposing a series of studies, when the carburizing treatment, the pre-treatment with a fluorine gas, to the austenitic stainless steels A ₁ We conceived that carburizing at temperatures below the transformation point would be possible, and based on this, we conducted a series of studies. In the process of this research, as in the above idea, if the treatment with the fluorine-based gas is performed before the carburizing treatment or simultaneously with the carburizing treatment,
It is possible to carburize stainless steel, which has been regarded as impossible in the past. In particular, the temperature of the carburizing treatment is not higher than 700 ° C. as in the prior art but 680 ° C. or lower, preferably 500 ° C. It has been found that carburization can be effectively performed even at a temperature lower than ℃. Then, by further studying, by using stable type stainless steel as the austenitic stainless steel, ferrite is not precipitated even in the intermediate processing before carburization, and the austenite single phase state is maintained, so that the hardness of the carburized layer is It has been found that the carburized product is high and uniform, and magnetism does not occur in the carburized product. Moreover, among the above stable stainless steels, if the stable stainless steel containing 1 to 6% by weight of molybdenum or the stable stainless steel containing 13 to 25% by weight of Cr, the carburized layer obtained has a corrosion resistance higher than that of the base material. The present invention has been achieved by discovering that here,
Stable stainless steel means that at room temperature, even if it is processed into a predetermined product shape, ferrite does not form metallically,
A stainless steel that exhibits a complete austenitic phase.

The reason why a carburized layer having a corrosion resistance higher than that of the base material is obtained is not clear at present, but a barrier band derived from the C-enriched layer formed in the surface layer by carburization is not formed. It is presumed that this is partly because the metal ions are formed and the diffusion of metal ions is blocked. By doing so, a surface layer having a depth of 5 to 70 μm from the surface of the austenitic stainless steel product is formed in the carburized hardened layer, and the hardness of the carburized hardened layer is formed in a micro Vickers hardness of 500 to 1050 Hv. To be done. Moreover, the carburized hardened layer is composed of an austenite phase in which chromium carbide does not precipitate, and it has been found that the carburized hardened layer exhibits a higher degree of corrosion resistance than the base metal. In addition, the product obtained by this carburization does not have the problems such as swelling of the surface and deterioration of the surface roughness, which have been problems in the conventional nitriding treatment.

The molybdenum is considered to contribute to the improvement of the corrosion resistance of the carburized hardened layer, but as known as a ferrite stabilizing element, it is an obstacle to the stabilization of the austenitic phase of austenitic stainless steel. Therefore, if a large amount of molybdenum is added, it is necessary to increase the addition amount of the austenite stabilizing element such as Ni or N, which leads to an increase in raw material cost and manufacturing cost. Therefore,
In the present invention, stable austenitic stainless steel containing 1 to 6% by weight of molybdenum is used, but if cost is emphasized, it is about 1.0 to 3% by weight like a material standardized as SUS316 series. It is desirable to adjust the amount to be added.

In the present invention, the austenite phase in which chromium carbide particles are absent means an X-ray diffractometer (X-Ray) commonly used for crystal structure analysis of metal materials.
Diffraction meter)
_{23 C 6, Cr 7 C 3} , Cr 3 crystalline carbide C ₂ and the like refers to the austenite phase can not be confirmed. That is, since the crystal structure of the austenite phase (γ-phase), which is the base phase of austenitic stainless steel, is a face-centered cubic lattice and the lattice constant is a = 3.59Å, a specific diffraction peak is obtained by X-ray diffraction. can get. On the other hand, Cr ₂₃ C ₆ is
Even with the same face-centered cubic lattice, the lattice constant is a = 10.6.
Å, and Cr ₇ C ₃ is a trigonal crystal and has a lattice constant a = 1.
4.0Å, c = 4.53Å, Cr ₃ C ₂ is orthorhombic and has a lattice constant a = 5.53Å, b = 2.821Å, c
= 11.49Å. Therefore, these chromium carbides have different crystal structures and lattice constants from the austenite phase, and generate diffraction peaks different from those obtained in the austenite phase. Therefore, when chromium carbide is present in the carburized hardened layer, a peak of chromium carbide which is not seen in the case of the austenite single phase appears by X-ray diffraction. On the other hand, the carburized hardened layer in the present invention is an austenite phase in which chromium carbide does not exist, and the lattice of the austenite phase of the matrix phase is isotropically strained and expanded by the invasion and solid solution of carbon atoms. Chromium carbide peaks do not appear even by diffraction.

Further, in the present invention, the stable stainless steel refers to stainless steel which exhibits an austenite phase in which ferrite is not generated metallically even when processed into a predetermined product shape at room temperature, as described above. FIG. 4 shows the relationship between the Cr equivalent and the Ni equivalent (Schaeffle).
In the state diagram of r), it means a stainless steel having a composition in which the Cr equivalent and the Ni equivalent are within the range of the region (A) shown in the figure. Here, the Cr equivalent and the Ni equivalent are numerical values represented by the following formulas (1) and (2), respectively.

[0014]

## EQU1 ## Cr equivalent = Cr weight% + Mo weight% + 1.5 × Si weight% + 0.5 × Nb weight% (1)

[0015]

[Equation 2] Ni equivalent = Ni weight% + 30 × C weight% + 0.5 × Mn weight% (2)

In the present invention, the corrosion resistance is determined by
Samples of carburized material and untreated base material are held under the same accelerated corrosion environment under the same conditions, with significant difference in the index indicating the degree of corrosion. Here, the accelerated corrosion environment is
Examples of the environment include, but are not limited to, an environment such as salt spray, immersion in physiological saline, immersion in an acidic solution such as an aqueous HCl solution.

[0017]

Next, an embodiment of the present invention will be described in detail.

According to the present invention, stable austenitic stainless steel containing 1 to 6% by weight of molybdenum or 13% of Cr is used.
A stable austenitic stainless steel containing 25 wt% is subjected to a carburizing treatment after being pretreated with fluorine gas or at the same time as the pretreatment.

Examples of the stable austenitic stainless steel include S containing 1 to 3% by weight of molybdenum.
In addition to US316, SUS316L, and SUS317, molybdenum up to 5 to 6% by weight is further added, and N which is an austenite stabilizing element is 0.1 to 0.4% by weight, Ni.
Stainless steel containing up to 22 to 25% by weight and molybdenum not included, 13 to 25% by weight of Cr and 8 to 2 of Ni
Examples include austenitic stainless steel materials such as SUS304 and SUS310 containing about 2% by weight. In the present invention, these are called base materials.

The amount of molybdenum added to the stable austenitic stainless steel is preferably in the range of 1 to 6% by weight from the viewpoint of improving the corrosion resistance of the carburized hardened layer. A range of 3% by weight is more preferable.

Such stable austenitic stainless steel materials are often used for fasteners such as bolts, nuts, screws, washers, pins and the like. In the present invention, the austenitic stainless steel product includes not only the above fasteners but also a wide range of industrial fields such as chains, watch cases, spinning shuttle tips, fine gears, knives, etc. Includes mechanical parts used in.

Next, the austenitic stainless steel as described above is used.
Prior to the carburizing process,
The fluoridation treatment is performed in the atmosphere. The above fluorination treatment
May be performed simultaneously with the carburizing treatment. For this fluorination treatment
For, a fluorine-based gas is used. As the above-mentioned fluorine-based gas
NF_Three, BF_Three, CF_Four, HF, SF₆, C_TwoF
₆, WF₆, CHF_Three, SiF_Four, ClF_ThreeConsists of etc.
Fluorine compound gas can be used, which may be used alone or
Are used in combination of two or more. Also, except for these gases
In addition, other fluorine-based gas containing fluorine (F) in the molecule
It can be used as the fluorine-based gas. Also this
Fluorine compound gas such as
F made_TwoGas or premade F_TwoGas is also on
It can be used as the fluorine-based gas. like this
Fluorine compound gas and F_TwoGas is a mixed gas
Used. Then, the fluorine compound gas, F_TwoGas, etc.
The fluorine-based gas can be used by itself,
Usually N_TwoUsed after diluted with an inert gas such as gas
You. Fluorine-based gas itself in such diluted gas
The concentration of the body is, for example, 10,000 to 100 on a volume basis.
000 ppm, preferably 20000-7000
0 ppm, more preferably 30,000 to 50,000 p
pm. Most practical as this fluorine-based gas
NF is _ThreeIt is. Above NF_ThreeIs gaseous at room temperature
It has high chemical stability and is easy to handle. this
NF like_ThreeThe gas is usually N_TwoCombined with gas
And used within the above concentration range.

To describe the fluorination treatment in more detail, in the present invention, first, virgin (untreated) austenitic stainless steel is placed in a furnace and kept in a heated state under a fluorine-based gas atmosphere of the above concentration, Fluoride treatment.
In this case, heating and holding the austenitic stainless steel itself is, for example, 250 to 600 ° C., preferably 250 to 600 ° C.
It is carried out by maintaining the temperature at 500 ° C. The holding time of the austenitic stainless steel in the fluorine-based gas atmosphere is usually set to tens of minutes to tens of minutes. By treating the austenitic stainless steel in such a fluorine-based gas atmosphere, the passive film containing Cr ₂ O ₃ formed on the surface of the austenitic stainless steel changes to a fluoride film. This fluorinated film is expected to facilitate the permeation of carbon atoms used for carburization compared to the above passive film, and the surface of austenitic stainless steel becomes a surface state in which carbon atoms easily permeate due to the above fluorination treatment. It is supposed to be.

Next, after carrying out the fluorination treatment as described above, a carburizing treatment is carried out. The carburizing treatment is performed on the austenitic stainless steel itself at a temperature of 400 to 680 ° C., preferably 4
Temperature of 00-600 ° C, more preferably 400-500 ° C
The carburizing gas consisting of CO + H ₂ mixed gas or RX [RX component is CO: 23% by volume +
CO ₂ : 1% by volume + H ₂ : 31% by volume + H ₂ O: 1% by volume + balance: N ₂ ] A carburizing gas or the like made of a gas is used to carry out the carburizing gas atmosphere in the furnace. As described above, in the present invention, the carburizing treatment is performed at a low temperature that does not cause softening and solution treatment of the core of the austenitic stainless steel.
When a CO + H ₂ mixed gas is used as the carburizing gas, the concentration of the CO + H ₂ mixed gas is 2 to CO.
10% by volume, 30 to 40% by volume of H ₂ and the balance N ₂ are preferable.

The above-mentioned fluorination treatment and carburization treatment are carried out, 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 its outer shell, 3 is a heater, 4 is an inner container, 5 is a gas introduction pipe, 6 is an exhaust pipe, 7 is a motor, 8 is a fan, 11 is a cage made of wire mesh, and 13 is a vacuum pump. , 14
Is an exhaust gas treatment device, 15 and 16 are cylinders, 17 is a flow meter, and 18 is a valve. An austenitic stainless steel product 10 is put in the muffle furnace 1, a cylinder 16 is connected to a flow path, and a fluorine-based gas such as NF ₃ is introduced into the muffle furnace 1 to perform a fluorination treatment while heating. The gas is extracted from the exhaust pipe 6 by the action of the vacuum pump 13 and is detoxified in the exhaust gas treatment device 14 and is discharged to the outside. Next, the cylinder 15 is connected to the flow path, the above-mentioned carburizing gas is introduced into the muffle furnace 1 to carry out carburizing treatment, and thereafter,
The gas is discharged to the outside via the exhaust pipe 6 and the exhaust gas treatment device 14. A fluorination treatment and a carburization treatment are performed by this series of operations.

By such treatment, a diffusion and penetration layer (carburized hardening layer) of "carbon" is formed deeply and uniformly on the surface of austenitic stainless steel. This carburized hardened layer is a state in which a large amount of carbon atoms form a solid solution in the austenite phase, which is the matrix phase, to cause lattice distortion. Exhibits corrosion resistance superior to that of wood.

SUS316 plate piece which is a typical austenitic stainless steel was used as a sample, and this is shown in FIG.
It was placed in the muffle furnace 1 shown in FIG. 1 for fluorination and carburization. That is, first, NF ₃ + N ₂ (NF ₃ :
10% by volume, N ₂ : 90% by volume) in a fluorine-based gas atmosphere at 350 ° C. for 20 minutes for fluorination, and then the fluorine-based gas is discharged from the furnace, followed by carburizing gas CO + CO ₂ + H ₂ mixed gas (CO: 38 vol% + CO _2: 2% by volume + H _2: 60 vol%) was introduced into the furnace, and held for 18 hours at 450 ° C., and carburized. As a result, the surface hardness Hv = 85 on the surface of the SUS316 plate material.
0 (core Hv = 220 to 230) and a carburized hardened layer having a depth of 20 μm were formed. When the sample on which this carburized hardened layer was formed was subjected to the salt spray test of JIS 2371, no rust was generated even after 480 hours. Further, the carburized hardened layer was not etched by the Birla reagent (acidic picric acid alcohol solution) used for the corrosion resistance test of the stainless steel structure, but was barely etched by aqua regia. Moreover, the cured sample had almost no deterioration in surface roughness, and neither dimensional change due to swelling nor magnetism occurred.

The inventors of the present invention further investigated various combinations of types of austenitic stainless steel pieces, carburizing temperature, and the like. As a result, when the carburizing temperature exceeded 680 ° C., as described above, It was confirmed that the core portion of 1 was extremely likely to be softened and that the corrosion resistance of the hardened layer was significantly reduced. That is, from the viewpoint of corrosion resistance, the carburizing temperature is preferably 680 ° C. or lower, and more preferably 500 ° C. or lower, the better result is obtained. A more preferable carburizing temperature is 400 to 500 ° C.

In the present invention, the carburizing temperature is high.
Especially when the temperature exceeds 450 ° C, even a slight amount of Cr
_{twenty three}C₆It is said that such carbides precipitate on the surface of the carburized hardened layer.
A phenomenon occurs. However, even in this case,
Carburized products of HF + HNO _ThreeSolution, HCl + HNO_Three
Precipitation by immersion in a strong acid such as a solution and pickling
Carbides have been removed to provide better corrosion resistance than the base metal, and typically
High surface hardness of Curse hardness Hv = 850 or more is secured
You. That is, the austenator that has been carburized as described above.
Ito series stainless steel products have a carburized hard surface formed on their surface.
The surface of the chemical layer becomes black due to carburization, and
The above carburizing and hardening due to the small amount of oxygen atoms present in the atmosphere.
An iron-based internal oxide layer may be formed on the outermost layer of the layer.
You. However, such an internal oxide layer has the above-mentioned HF +
HNO_ThreeSolution, HCl + HNO_ThreeSoak in a strong acid such as a solution
It can be removed by pickling and removing the above-mentioned precipitated carbide.
After removal by pickling, the corrosion resistance is higher than that of the base metal and Hv =
A surface hardness of 850 or more is secured. And the above strength
The internal oxide layer was removed by dipping in acid.
Stenite stainless steel products are the same as before carburizing.
Such a bright state is exhibited. The X-ray diffraction pattern shown in FIG.
Chart C is carburized from SUS316 plate at 480 ° C.
After that, the concentration is 5% by volume HF + 15% by volume HNO_ThreeOf solution
X-ray diffraction char of treated product which was soaked in strong acid for 20 minutes and pickled
That the above carbides are not observed at all.
Recognize.

This will be described in more detail. That is, when the surface of the treated product is carefully observed after the carburizing treatment, there is a dark-colored layer at a depth of 2 to 3 μm from the surface in the outermost layer, which indicates that this is an internal oxide layer of austenitic stainless steel. It was confirmed by line diffraction. This can be considered as follows. That is, in an atmosphere containing CO in the range of 400 to 500 ° C., a region where a carburizing reaction (2CO → CO ₂ + C) and an oxidation reaction of Fe (4CO ₂ + 3Fe → 4CO + Fe ₃ O ₄ ) may coexist simultaneously. Therefore, it is considered that the internal oxide layer was formed by the carburizing treatment for this reason. The internal oxide layer of such austenitic stainless steel is
This was not seen in the conventional carburizing method of treating at a temperature of 700 ° C. or higher.

For example, SUS316L (C content: 0.02% by weight, Cr content: 17.5% by weight, Ni content: 12.0% by weight, Mo, which was carburized at 480 ° C. for 12 hours, was used.
(Content: 2.0% by weight) As an example of a socket bolt and a washer, the hardened layer depth was 30 μm, and the surface hardness was Hv = 910 in micro-Vickers, but the surface was black. Next, these black carburized products are
After being dipped in a 5% by volume HF + 25% by volume HNO ₃ solution heated to 0 ° C. for 20 minutes for pickling, and then soft blasted, almost the appearance of a bright state before carburizing treatment was obtained. When this was subjected to a salt spray test of JIS 2371, it did not rust at all even after 2000 hours,
Furthermore, in organic and inorganic acid resistance tests and elution tests with physiological saline, it was confirmed that they exhibited corrosion resistance superior to that of the base material.

The treatment liquid used for the above-mentioned pickling is not particularly limited, and various types such as hydrofluoric acid, nitric acid, hydrochloric acid and sulfuric acid can be used. These may be used alone, but as described above, they can also be used as a mixed solution of nitric acid + hydrofluoric acid, nitric acid + hydrochloric acid, sulfuric acid + nitric acid and the like. Further, although they can be used at room temperature, they may be used as a high temperature solution. Further, electrolytic treatment using an electrolytic solution such as nitric acid or sulfuric acid may be performed.

Thus, according to the carburizing treatment of the present invention,
The treated product exhibits higher corrosion resistance than the austenitic stainless base material, which is considered to be due to the following two reasons. That is, first of all,
In the present invention, by performing a fluorination treatment prior to carburization, it is possible to carry out the carburizing treatment at a low temperature of 680 ° C. or lower. The solid solution chromium component present in austenitic stainless steel (which exerts corrosion resistance) is less likely to precipitate as carbides such as Cr ₇ C ₃ and Cr ₂₃ C ₆ and the amount of precipitation is reduced, which results in austenitic stainless steel. A large amount of solid solution chromium component remains in stainless steel. This prevents deterioration of the corrosion resistance of the base material. This the SUS316 member NF _3: 10 vol% +
N ₂ : 4 at 300 ° C. in a fluorine-based gas atmosphere of 90% by volume
After 0 minute fluorination treatment, CO: 32% by volume + CO ₂ :
3% by volume + H ₂ : 65% by volume carburizing gas atmosphere at 600 ° C. for 4 hours (X-ray diffraction chart is shown in FIG. 3), 450 ° C. for 16 hours (FIG. 2) This is supported by contrasting the X-ray diffraction results of the cured layer of the X-ray diffraction chart B) shown with that of the untreated product (X-ray diffraction chart A shown in FIG. 2). That is, as is clear from the figure, in the carburized product 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 is precipitated, and the amount of the solid solution chromium component remaining in the austenitic stainless steel is small. On the other hand, in the case of carburizing at 450 ° C. (B shown in FIG. 2), the peak of Cr ₂₃ C ₆ is hardly recognized, so that the precipitation amount of the above-mentioned carbide is remarkably small, and therefore in the austenitic stainless It seems that there is a large amount of solid solution chromium component remaining in, and the corrosion resistance is high. Second,
Since stable stainless steel containing 1 to 6% by weight of molybdenum or stable austenitic stainless steel containing 13 to 25% by weight of chromium was used, a barrier band derived from the C-enriched layer formed in the surface layer was formed. Therefore, it is considered that the carburized hardened layer exhibits corrosion resistance higher than that of the base metal due to the prevention of diffusion of metal ions and the effect of molybdenum addition which can contribute to improvement of acid resistance of austenitic stainless steel.

Further, it is considered that the improvement of the hardness of the carburized product is caused by the generation of austenite lattice strain due to the penetration of carbon atoms. This is the peak position of the austenite phase in the X-ray diffraction of the carburized product at 450 ° C (Fig. 2
B) shown in Fig. 2 and the peak position of the austenite phase of the pickled product after carburizing at 480 ° C (C shown in Fig. 2) are SUS.
The peak position of the austenite phase of the 316 untreated product (Fig. 2
It shifts to the lower angle side (left side) than A) shown in
This is supported by the fact that it is clear that the carburized products (B and C shown in FIG. 2) have austenite lattice strain. The above X-ray diffraction was performed using RINT150.
No. 0 device, 50 kV, 200 mA, Cu target, and conditions.

In the low temperature region of 500 ° C. or lower, the carburized hard layer requires a considerable amount of time to obtain a thick film because the diffusion rate of C under the austenite structure is considerably low. For example, SUS316L with the thickest hardened layer
Even in the case of a system, at a carburizing temperature of 490 ° C, the treatment time is 1
A carburized layer of 37 μm can be formed in 2 hours.
Even if carburized for a long time, the total carburized hardened layer is only 49 μm. Therefore, it takes 70 hours or more to obtain a hardened layer depth of more than 70 μm. Probably
Such long treatments would be less economical. However, even in the case of drill tapping that requires a deepest hardened layer, if a hardened layer of 40 μm is formed, SPCC (cold rolled steel sheet) with a thickness of 2.3 t can be sufficiently drilled, which is economical. A useful carburized hardened layer can be obtained within the range of the treatment time that does not cause the loss.

Also, carburized at 480 ° C. for 16 hours (D), carburized at 450 ° C. and pickled (E),
5 to 7 show the results of EPMA analysis of the carbon concentration in the carburized hardened layer of the carburized (F) at 600 ° C. and 600 ° C., respectively. A typical temperature range in the present invention is 48
Carburized at 0 ° C (D) [Fig. 5] and 450 ° C
(E) [Fig. 6] that was treated with
It has reached 1.8 to 2.0% by weight. On the other hand, 60
In the case of treatment (F) at 0 ° C. (FIG. 7), the maximum carbon concentration is as low as 1.03% by weight. As described above, the present invention has another feature that the carbon concentration of the carburized hardened layer is very high, which is one of the causes for forming the carburized hardened layer having high hardness. In the carburized hardened layer formed in the present invention, the maximum carbon concentration is shown in FIG.
It is the outermost surface as is clear from the EPMA analysis result of FIG. 7. The maximum carbon concentration on this surface varies depending on the carbon potential of the atmospheric gas during the carburizing treatment, but is 400 to 500 which is the temperature range that is preferably implemented in the present invention.
It has been found that the maximum carbon concentration in the carburized hardened layer formed by the treatment at ℃ is in the range of 1.2 to 2.6% by weight. When the maximum carbon concentration in the carburized hardened layer is 1.7 to 2.6% by weight, the effect of improving the surface hardness is further increased.

[0037]

As described above, the present invention holds the austenitic stainless steel in a heated state in a fluorine-based gas atmosphere prior to or at the same time as the carburizing treatment. Can be as low as 400-680 ° C. Therefore, without impairing the high workability of austenitic stainless steel at all,
It is possible to realize high surface hardness and also obtain higher corrosion resistance than that of the base material. And in the present invention, since the surface hardness as described above is improved by the carburizing treatment,
The phenomenon of deterioration of the surface roughness caused by the nitriding treatment, the decrease in dimensional accuracy due to the swelling, and the magnetism are not caused at all.

In the austenitic stainless steel product thus obtained, a surface layer having a depth of 5 to 70 μm from the surface of the austenitic stainless steel product is hardened by the infiltration of carbon atoms to form a carburized hardened layer. The hardness of the carburized hardened layer is 500-11 in micro Vickers hardness.
It is formed to 80 (Hv), preferably 500 to 1050 (Hv). Moreover, since the carburized hardened layer is formed from the austenite phase without depositing chromium carbide, the obtained product exhibits the corrosion resistance higher than that of the base material due to the formation of the C-concentrated band in the outermost surface layer. Therefore, it is useful for fasteners such as bolts, nuts, screws, etc., to various mechanical parts used in general industrial fields, that is, various shafts, impellers, bearings, springs, valve parts, etc. . Further, it is particularly promising as a material for machine parts used in fields such as food machinery, chemical plants, and semiconductor industries.

Next, examples will be described.

[0040]

EXAMPLE As an example, SUS316 (Cr content: 17
% By weight, Ni content: 13.5% by weight, Mo content: 2.5
% By weight, C content: 0.07% by weight, balance: Fe) rolled plate pieces (2.5 t × 15 × 15) and SUS304 (C
r content: 18.5% by weight, Ni content: 8.5% by weight, C
Content: 0.08% by weight, balance: Fe) rolled plate pieces (2.
A plurality of 5t × 15 × 15) were prepared. The core hardness of these materials is SUS316 and Hv = 220-
Hv = 170 to 180 for 230 and SUS304 material. When these materials were heated to 320 ° C. in the furnace shown in FIG. 1, a mixed gas of NF ₃ : 20% by volume and the balance: N ₂ was blown for 15 minutes for fluorination treatment, and then purged with N ₂ gas. The temperature was raised to 480 ° C. Then, H ₂ : 31
Vol% + CO: 21 vol% + CO _2: 1 vol% + remainder
A N ₂ carburizing gas was introduced, and the carburizing treatment was carried out by holding in that atmosphere for 15 hours. Further, subsequently, these treated products were heated at 55 ° C. to obtain 3% by volume HF + 15% by volume HNO.
_It was immersed in ₃ solutions for 30 minutes and cleaned.

As a result of measuring the depth of the carburized hardened layer and the surface hardness, the depth of the carburized hardened layer was 32 μm for the SUS316 material.
m, surface hardness Hv = 980, and in SUS304 material, carburized hardened layer depth 28 μm, surface hardness Hv = 1080
Met.

The carburized products of the SUS316 material and the SUS304 material and the plate pieces of the respective untreated products were used as samples, and the samples were heated to 50 ° C. in 5% by volume HCl.
Corrosion resistance was evaluated by quantifying the metal ion elution concentration in the solution after immersion for 3 hours in the aqueous solution by atomic absorption spectrometry. The results are shown in Table 1 below.

[0043]

[Table 1]

As can be seen from the results in Table 1 above, SUS
The carburized 316 material exhibited significantly better corrosion resistance than the untreated material (that is, the base material). Also, SUS30
Among the four materials, the carburized product exhibited better corrosion resistance than the untreated material.

[Brief description of drawings]

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

FIG. 2 is an X-ray diffraction chart of SUS316 untreated product, SUS316 plate material carburized at 450 ° C., and SUS316 plate material carburized at 480 ° C. followed by strong acid immersion treatment. is there.

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

FIG. 4 is a diagram showing a relationship between Cr equivalent and Ni equivalent.

FIG. 5 is an EPMA analysis result of a treated product in which a SUS316 plate material is carburized at 480 ° C.

FIG. 6 is an EPMA analysis result of a product obtained by carburizing a SUS316 plate material at 450 ° C.

FIG. 7 shows an EPMA analysis result of a treated product in which a SUS316 plate material is carburized at 600 ° C.

Claims (6)

[Claims] 1. An austenitic system in which austenitic stainless steel is maintained in a heated state in a fluorine-based gas atmosphere prior to the carburizing process, and then the carburizing process is performed at a temperature of 400 to 680 ° C. A method of carburizing stainless steel, wherein the austenitic stainless steel contains stable austenitic stainless steel containing 1 to 6% by weight of molybdenum or 13 to 13% of chromium.
A method for carburizing austenitic stainless steel containing 25% by weight of a stable austenitic stainless steel, wherein a carburizing hardened layer having a corrosion resistance higher than that of the base material is formed by the carburizing treatment. 2. The temperature at the time of carburizing treatment is 400-500.
The method for carburizing austenitic stainless steel according to claim 1, wherein the method is set to ℃. 3. The method for carburizing austenitic stainless steel according to claim 1, wherein the heating in a fluorine-based gas atmosphere is carried out at a temperature range of 250 to 500 ° C. for the austenitic stainless steel. 4. The carburizing method for austenitic stainless steel according to claim 1, wherein the pickling treatment is performed after the carburizing treatment. 5. The base material is formed of stable austenitic stainless steel containing 1 to 6% by weight of molybdenum or stable austenitic stainless steel containing 13 to 25% by weight of chromium, and has a depth of 5 to 70 μm from the surface. Is hardened by the infiltration of carbon atoms to form a carburized hardened layer, and the hardness of this carburized hardened layer is formed to a micro Vickers hardness of 500 to 1050 (Hv), and the carburized hardened layer is formed of chromium carbide particles. An austenitic stainless steel product which is formed from a non-existing austenite phase and has corrosion resistance equal to or higher than that of the base material. 6. The maximum carbon concentration in the carburized hardened layer is 1.2 to
The austenitic stainless steel product according to claim 5, which is 2.6% by weight.