JPH09302456A - High corrosion resistant metallic product and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a product having high corrosion resistance to acid, seawater, chemicals or the like by subjecting an austenitic stainless steel product to fluoriding treatment while being heated and infiltrating carbon atoms therein to form a concentrated layer of carbon to a specified depth. SOLUTION: An austenitic stainless steel product is charged to a muffle furnace 1, gaseous fluorine such as NF3 is introduced therein, and it is subjected to fluoriding treatment while being heated. Next, the gas is drawn out of an exhaust tube 6 by the operation of a vacuum pump, which is made nonpoisonous in an exhaust gas treating device 14 and is released to the outside. Then, a carburizing gas is introduced into the muffle furnace 1, and infiltrating treatment of carbon is executed at 400 to 500 deg.C to form a concentrated layer of carbon on the surface layer of a depth of about 5 to 50μm from the surface of the stainless steel. After that, the gas is exhausted. The maximum carbon concn. of the concentrated layer of carbon is preferably regulated to 1.2 to 2.6wt.%. The austenitic stainless steel preferably contains molybdenum. Moreover, the topmost surface layer is preferably removed after the infiltrating treatment.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a highly corrosion-resistant metal product having mechanical properties and a high degree of corrosion resistance, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, austenitic stainless steel used in fields where corrosion resistance is important has a basic composition containing 18% by weight of chromium and 8% by weight of nickel with iron as a base material. Generally "18-8 stainless steel"
It is what is called. Further, austenitic stainless steel in which 1 to 3% by weight of molybdenum is added to the 18-8 stainless steel is also widely used as a general-purpose steel type.

[0003] These general-purpose austenitic stainless steels include SUS304 and SUS in JIS.
Many steel grades such as 316 have been standardized according to the use and characteristics. Among these, SUS304 is the most commonly used typical general-purpose steel type, but its corrosion resistance may be insufficient depending on the environment in which it is used. For example, under a strong corrosive environment with organic or inorganic acids or salts, chemicals, seawater, halogen gas, SO _2, etc.
Metal products made of 8-8 stainless steel are corroded and cannot be used stably. Therefore, under the corrosive environment as described above, a highly corrosion-resistant stainless steel in which a large amount of molybdenum (3 to 7% by weight) is added to 18-8 stainless steel having the above basic composition may be used. In other words, it is considered that the addition of molybdenum to these high-corrosion-resistant stainless steels strengthens the passive film that is the source of the corrosion-resistant function of stainless steel, and molybdenum is used for corrosive environments such as seawater and sulfuric acid. Steel grades containing up to 5-7% by weight have also been developed. However, molybdenum is a ferrite stabilizing element, and when added in a large amount, a large amount of austenite stabilizing elements such as nickel, copper and nitrogen (N) are added in order to stabilize the austenitic phase of austenitic stainless steel. There must be. Furthermore, in the strong corrosive environment as described above, as a metal material other than austenitic stainless steel, 26C
A high chromium content ferritic stainless steel such as r-4Mo, 29Cr-4Mo-2Ni, a nickel-based alloy material such as Hastelloy or Monel or a titanium alloy material is used,
Alternatively, non-metal materials such as plastic materials and ceramic materials are used.

[0004]

However, in the above-mentioned stainless steel having high corrosion resistance, not only the raw material cost and the steelmaking cost due to the increase of the additional elements are increased, but also the marketability is low, so that the material itself is It will be expensive. Moreover, since it is difficult to work and poor in weldability, it is inferior in mass productivity, and the processing costs such as forming and welding are high. Metal products made from the above stainless steel with high corrosion resistance are general-purpose steel grades such as SUS304. Much more expensive than using. Further, in the case of a nickel-based alloy material or a titanium alloy material, the material cost and the processing cost are further higher than the above-mentioned stainless steel for high corrosion resistance. Also, resin material,
When a non-metal material such as a ceramic material is used, it is inferior to a metal material in terms of reliability such as mechanical strength and the application range is limited at present.

The present invention has been made in view of the above circumstances. An inexpensive general-purpose type stainless steel having a basic composition is used, and a carbon concentrated layer is formed on the surface of the stainless steel to form an acid, seawater, It is an object of the present invention to provide a highly corrosion-resistant metal product which has excellent corrosion resistance against chemicals and the like and is inexpensive, and a method for producing the same.

[0006]

In order to achieve the above object, in the highly corrosion resistant metal product of the present invention, the base material is austenitic stainless steel exhibiting an austenite phase,
The first gist is that the surface layer having a depth of 5 to 50 μm immediately below the passivation film on the surface was formed as a carbon concentrated layer by permeation of carbon atoms.

Further, in the method for producing a highly corrosion-resistant metal product of the present invention, an austenitic stainless steel product whose base material exhibits an austenite phase is held in a heated state in a fluorine-based gas atmosphere, and then at 400 ° C to 500 ° C. A second gist is to perform a carbon atom permeation treatment at a temperature to form a carbon concentrated layer in a surface layer having a depth of 5 to 50 μm from the surface.

[0008] The inventors of the present invention have conducted a series of studies to obtain an inexpensive metal product that can withstand use in a strongly corrosive environment. By performing the processing,
Recalling that the corrosion resistance could be further improved, various experiments were repeated on the surface treatment of austenitic stainless steel. As a result, a metal product made of austenitic stainless steel as a base material
Carbon is diffused and permeated from the outside in a state of being heated to ~ 500 ° C to form a concentrated layer of carbon on the surface, and this concentrated layer has much better corrosion resistance than the austenitic stainless steel which is the base material. The inventors have found that the metal product obtained thereby can be stably used even in a strong corrosive environment, and arrived at the present invention.

That is, according to the present invention, an inexpensive general-purpose stainless steel having a basic composition containing nickel, chromium, or molybdenum in a minimum amount for exhibiting an austenitic phase is used, and the general-purpose stainless steel is processed. Carbon is infiltrated into the surface of the obtained metal product, which shows good corrosion resistance even in a severe corrosive environment, and an inexpensive metal product can be obtained by using general-purpose steel grades. Of.

[0010]

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

The present invention utilizes general-purpose austenitic stainless steel, which is processed into a predetermined product shape, and a metal product whose base material exhibits an austenitic phase in that state is subjected to carbon treatment by surface treatment. And a carbon-enriched layer is formed on the surface layer immediately below the passive film on the surface.

The austenitic stainless steel used in the present invention is not particularly limited as long as it is a stainless steel exhibiting an austenitic phase. For example, JIS
SUS302, SUS302E, SUS2 specified in
02, SUS301, SUS201, SUS301J
1, SUS631, SUS632, SUS303, SU
S303Se, SUS304, SUS304L, SUS
304LN, SUS321, SUS347, SUS30
4N1, SUS304N2, SUS316, SUS31
6N, SUS316L, SUS316LN, SUS31
7, SUS317L, SUS317J1, SUS316
J1, SUS316J1L, SUS305, SUS30
5J1, SUS384, SUS385, SUS309,
SUS309S, SUS310, SUS310S, SU
S330, SUS302B, XM15J1, SUS30
8, various types such as SUS308L are used.

Among these, SUS301 series and SU
S304 exhibits an austenite phase in the solid solution state, but as cold working progresses, ferrite is likely to be generated and corrosion resistance may deteriorate. In particular, SUS3
In the 01 series, this tendency is remarkable. Therefore, SUS304
SUS304N1 and SUS304 in the case of system materials
A steel type added with elements such as nitrogen (N) such as N2, manganese (Mn), copper (Cu) is more suitable. Where S
Even if US304 has a low degree of processing, it does not generate ferrite, and is therefore suitably used in the present invention. In addition, SUS316 containing molybdenum is originally intended to improve the corrosion resistance, the base material itself has better corrosion resistance than SUS304, and the austenite phase is relatively stable. Therefore, SUS316 is suitable for the present invention. Used.

That is, the minimum austenitic stainless steel used in the present invention is nickel, chromium,
It suffices if it contains molybdenum and does not form ferrite at room temperature and exhibits a complete austenite phase. Further, it is more preferable if it is a stable austenitic stainless steel in which ferrite is not formed even when processed at room temperature. Further, even austenitic stainless steel in which ferrite is precipitated by processing can be used as the base material of the present invention if it is made to completely exhibit an austenite phase by solution treatment or the like.

In the present invention, the austenitic stainless steel exhibiting the austenite phase is used as a base material, processed into a desired product shape, and then carbon is permeated to form a carbon concentrated layer.

The carbon enriched layer is formed on the surface of the product by 5
It is preferable to set the thickness to -50 μm, and the thickness is 20 to 30.
It is even more preferable if it is μm. That is, if it is less than 5 μm, it is insufficient to obtain good corrosion resistance, and if it exceeds 50 μm, the processing time becomes longer and the cost becomes disadvantageous.

In the concentrated layer, the infiltrated carbon atoms form an interstitial solid solution in the interstices of the face-centered cubic lattice in the structure of the base material, austenitic stainless steel (which is the face-centered cubic lattice). There is. That is, in the present invention, the carbon atom does not combine with chromium or iron in the base material to form an iron carbide or a chromium carbide. Therefore, the concentrated layer maintains the austenite phase in which carbon atoms infiltrate and form a solid solution. It is considered that the thickened layer significantly improves the corrosion resistance of the austenitic stainless steel.

The austenite phase in which the chromium carbide particles do not exist is the X-ray diffractometer (X-Ray Diffrometer) generally used for the crystal structure analysis of metal materials.
by the action meter), Cr ₂₃ C ₆ , C
r ₇ C ₃ , Cr ₃ C _{2 It} means the austenite phase in which crystalline carbides cannot be confirmed. That is, the austenite phase (γ-phase), which is the base phase of austenitic stainless steel, is
Its crystal structure is a face-centered cubic lattice and the lattice constant is a = 3.59.
Since it is Å, a specific diffraction peak is obtained by X-ray diffraction. On the other hand, Cr ₂₃ C ₆ has a lattice constant of a = 10.6Å even if it has the same face-centered cubic lattice,
₇ C ₃ is a trigonal crystal with a lattice constant of a = 14.0Å, c =
4.53Å, Cr ₃ C ₂ is orthorhombic and has a lattice constant of a = 5.53Å, b = 2.821Å, c = 11.49Å
It is. 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 permeation layer, a peak of chromium carbide which is not seen in the case of the austenite phase single phase appears by X-ray diffraction. On the other hand, the carbon-enriched layer in the present invention has no chromium carbide, and the lattice of the austenite phase of the parent phase is a strained and expanded austenite phase in which the carbon atoms are invaded as a solid solution, The peak of chromium carbide does not appear even by X-ray diffraction.

Conventionally, in the case where the above-mentioned carbon enriched layer is formed on the metal surface, carbon steel or case-hardening steel is carburized at a temperature not lower than the A ₁ transformation point (727 ° C.) of the steel. Keeping in a gas atmosphere to diffuse and infiltrate carbon, and about 0.5 from the surface
So-called carburizing treatment is performed to form a carburized hardened layer having a depth of ˜3 mm. However, the austenitic stainless steel, which places importance on the corrosion resistance, has not been subjected to the carburizing treatment at a high temperature as described above, because the corrosion resistance remarkably decreases and the material strength also decreases. As described above, the reason why the corrosion resistance of the base material is deteriorated by performing the carburizing treatment on the austenitic stainless steel is that the carbon in the base material reacts with the infiltrated carbon by diffusing and infiltrating carbon atoms at a high temperature. And Cr ₂₃ C
Chromium carbides such as ₆ , Cr ₇ C ₃ and Cr ₃ C ₂ are deposited on the defect parts such as grain boundaries and dislocations, so that the solid solution chromium in the base material is consumed and the solid solution chromium contributes to the improvement of corrosion resistance. It is believed that the concentration is significantly reduced.

On the other hand, in the present invention, the temperature at which carbon is permeated is 400 to 500 ° C., which is far lower than the general carburizing temperature range, so that the carbon-concentrated layer contains crystalline chromium carbide. Since it is not formed and is formed from the austenite phase in which chromium carbide does not exist, solid solution chromium is not consumed for the formation of carbide and corrosion resistance does not deteriorate. Not only that, the carbon-enriched layer of the highly corrosion-resistant metal product of the present invention exhibits significantly higher corrosion resistance than the austenitic stainless steel as the base material.

As described above, the reason why the carbon-enriched layer of the highly corrosion-resistant metal product of the present invention exhibits superior corrosion resistance to the austenitic stainless steel as the base material is not clear at present. It is speculated that there are two reasons. That is, first of all, the solid solution of carbon in a high concentration promotes the equalization of the metal structure immediately below the passivation film formed on the outermost surface and strengthens the passivation function of the passivation film. Presumed to be. Secondly, since a high concentration C band is formed in the surface layer portion, it is presumed that a diffusion barrier for metal ions is formed.

Here, in the present invention, the passive film means
Chromium oxide (Cr) formed on the outermost surface of stainless steel
₂ O ₃ ) A thin surface layer mainly composed of a metal oxide.

The carbon concentration in the carbon enriched layer is E
It can be measured by line analysis and area analysis by PMA. 9 to 11 shows SUS316 material at 480 ° C.
Treated with carbon atoms for 16 hours at (a), 450
(B) which has been subjected to pickling treatment after permeation treatment at ℃, and 600
E of carbon concentration in the concentrated layer of (c) carburized at ℃
The PMA analysis result is shown. The maximum carbon concentration is 1.8 to 10 in those (a) [FIG. 9] and [b] [FIG. 10] that are permeated at 480 ° C., which is a typical temperature range in the present invention. It has reached 2.0% by weight. On the other hand, carburized at 600 ° C (c)
In FIG. 11, the maximum carbon concentration is 1.03% by weight.
Moderately low (peaks on the outermost surface are those that detect deposits). As described above, the present invention is characterized in that the carbon concentration of the concentrated layer is extremely high, which is one of the causes for forming the concentrated layer having high hardness. In the present invention, in the formed concentrated layer, the place where the carbon concentration is maximum is
As is clear from the EPMA analysis results of FIGS. 9 to 11, it is the outermost surface.

The maximum carbon concentration on this surface changes depending on the carbon potential of the atmospheric gas during the permeation treatment, but is 400 to 400 which is the temperature range that is preferably implemented in the present invention.
It has been found that the concentrated layer formed by the treatment at 500 ° C. reaches a maximum of 1.2 to 2.6% by weight.
This value is higher than the solid solution limit of 1.7% by weight of carbon in the austenite phase of pure iron at 1100 ° C. In general, the carbon concentration in the carburized layer formed when carburizing stainless steel containing a large amount of chromium at 600 ° C. or higher may be up to 1.5% or higher. It is known in the literature that all are precipitated as the above-mentioned carbides such as Cr ₂₃ C _{6 and} hardly exist in the state of solid solution in the lattice of the austenite phase as in the case of the present invention. As an example, FIG. 8 shows X-ray diffraction data when the present inventors carburized SUS316 stainless steel at 600 ° C. and the lattice constant at this time.
Shown in As is clear from the figure, the peak of Cr ₂₃ C ₆ appears in the stainless steel material that has been carburized at 600 ° C.

Next, a method for producing the highly corrosion-resistant metal product of the present invention by forming the above-mentioned carbon concentrated layer on the surface of the metal product made of austenitic stainless steel will be described.

The highly corrosion-resistant metal product of the present invention is impregnated with carbon atoms in a muffle furnace shown in FIG. 1, for example. In FIG. 1, 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,
13 is a vacuum pump, 14 is an exhaust gas treatment device, 15 and 16
Is a cylinder, 17 is a flow meter, and 18 is a valve.

In the muffle furnace 1, when a carbon enriched layer is formed on a metal product made of austenitic stainless steel, first, a fluorination treatment is performed, and then a carbon atom infiltration treatment is performed. That is, the austenitic stainless steel product 1 is placed in the muffle furnace 1.
0 is put in, the cylinder 16 is connected to the flow path, and a fluorine-based gas such as NF ₃ is introduced into the muffle furnace 1 to perform fluorination while heating. Then, the gas is extracted from the exhaust pipe 6 by the action of the vacuum pump 13, detoxified in the exhaust gas treatment device 14 and discharged to the outside. Next, the cylinder 15 is connected to the flow path, a carburizing gas is introduced into the muffle furnace 1 and heated, and carbon infiltration treatment is performed. After that, the gas is discharged to the outside via the exhaust pipe 6 and the exhaust gas treatment device 14. By this series of operations, carbon atoms permeate the austenite phase of the base material and a carbon enriched layer is formed.

More specifically, first, austenitic stainless steel is formed into a desired product shape, and the austenitic stainless steel product is charged into the muffle furnace 1 and heated to 300 to 400 ° C. in a nitrogen atmosphere. After that, a fluorine-based gas such as NF ₃ is introduced into the furnace, and the state is maintained for 10 to 30 minutes to fluorinate the surface of the austenitic stainless steel product, and the passive film on the surface is once destroyed to clean the product surface. Activate (above, fluorination treatment). Thus, by treating the austenitic stainless steel product in a fluorine-based gas atmosphere, the passive film containing Cr ₂ O ₃ formed on the surface of the austenitic stainless steel product is changed to a fluoride film. It is expected that this fluorinated film will facilitate the permeation of carbon atoms compared to the above-mentioned passive film, and it is speculated that the surface of austenitic stainless steel becomes a surface state in which carbon atom permeation is facilitated by the above fluorination treatment. To be done.

The fluorine-based gas used in the fluorination treatment is not limited to the above NF ₃ , but may be F ₂ , HF, BF.
₃ , CF ₄ , SF ₆ , ClF ₃ , C ₂ F ₆ , WF ₆ , C
Fluorine compound gas composed of HF ₃ , SiF _4, etc. may be mentioned, and these may be used alone or in combination of two or more kinds. In addition to these gases, fluorine (F) in the molecule
Other fluorine-based gases including can also be used as the above-mentioned fluorine-based gas. Further, it is possible to use such a fluorine compound gas F ₂ gas and that generated by thermal decomposition at a thermal decomposition apparatus, as F ₂ gas is also the fluorine-based gas premade. Such fluorine compound gas and F ₂ gas may be mixed and used depending on the case. The fluorine-based gas such as the fluorine compound gas or F ₂ gas may be used alone, but it is usually diluted with an inert gas such as N ₂ gas before use. The concentration of the fluorine-based gas itself in such a diluted gas is, for example, 10,000 to 100,000 ppm, preferably 20,000 to 70,000 ppm, and more preferably, on a volume basis.
It is 30,000 to 50,000 ppm. The above-mentioned NF ₃ has the most practicality as this fluorine-based gas. This is because this NF ₃ is gaseous at room temperature, has high chemical stability and is easy to handle, and is advantageous in terms of safety, operability, use efficiency and the like.

The optimum fluorination temperature is 300 to 400 ° C., but if the temperature of 400 to 500 ° C. is not taken into consideration, taking into consideration that the use efficiency of the fluorine-based gas is slightly deteriorated.

Then, after the above-mentioned fluorination treatment, the fluorine-based gas is stopped, the temperature inside the furnace is further raised to 450 to 480 ° C., and a carburizing gas (for example, CO: 20% by volume, H ₂ : 3).
0% by volume, CO ₂ : 1% by volume, the balance: N ₂ mixed gas) is blown in, and after holding for 3 to 20 hours, it is taken out from the furnace (above, carbon permeation treatment). This infiltration treatment temperature is 50
When it exceeds 0 ° C, the corrosion resistance of the carbon-enriched layer formed by the infiltration treatment becomes lower than that of the austenitic stainless steel which is the base material, and when the temperature further rises, the corrosion resistance rapidly deteriorates. Therefore, it is necessary to perform treatment at a temperature of 500 ° C or lower, preferably 400 to 480 ° C. If the temperature is lower than 400 ° C., the diffusion rate of carbon becomes slow, so that the processing time becomes very long, which is not preferable in terms of cost.

The carburizing gas used in the infiltration treatment is not particularly limited as long as it is a carburizing gas containing CO and H _2, and a CO + H ₂ + CO ₂ mixed gas or a hydrocarbon gas mixed with air. The so-called RX gas (the components of RX gas are, for example, CO: 23% by volume + C)
O ₂ : 1% by volume + H ₂ : 31% by volume + H ₂ O: 1% by volume
+ Remainder: N ₂ ]. Also, as appropriate, C ₃ H ₈ , C
₃ H _6, hydrocarbon gas such as C ₂ H ₄ may be used.

Next, after completion of the permeation treatment, 50 to 7
It is soaked in a HF + HNO ₃ solution heated to 0 ° C. for 30 minutes for pickling treatment (finishing treatment). By this pickling treatment, the surface of the austenitic stainless steel product, which turned black in the above-mentioned permeation treatment, has an appearance almost as bright as the base material. Moreover, even if this pickling treatment is carried out, there is almost no effect on the thickness of the concentrated layer. here,
The concentration of the HF + HNO ₃ solution is HF: 1 to 7% by weight + H
NO _3: 10 to 20% by weight. The pickling treatment after the permeation treatment is not limited to the immersion cleaning treatment using the HF + HNO ₃ solution, and may be the immersion cleaning treatment using HCl + HNO ₃ solution, HNO ₃ solution, H ₂ SO ₄ + HNO ₃ solution or the like. . Where HCl + HNO ₃
The concentration of the solution is HCl: 5 to 20% by weight + HNO ₃ : 1
It is preferably about 5 to 40% by weight, and the concentration of the HNO ₃ solution is preferably about 10 to 30% by weight. The liquid temperature of the solution used for the pickling treatment is not limited to 50 to 70 ° C, and may be 70 ° C or higher, or 50 or lower. Also,
The finishing treatment may be mechanical polishing, chemical polishing, or electrolytic polishing instead of pickling. After this finishing treatment, when taken out into the atmosphere, a passive film is formed again on the surface of the metal product.

By the series of treatments described above, an austenitic stainless steel product having a carbon enriched layer having a depth of 5 to 50 μm from the surface is obtained.

In the present invention, the infiltration treatment temperature for infiltrating carbon is 400 ° C. to 500 ° C., but general carbon steel and alloy steel are transformed into austenite at a low temperature of 600 ° C. or lower (A ₁ transformation point or lower). However, since most of them exhibit ferrite and pearlite phases, carbon hardly dissolves into the steel and carbon does not permeate. On the other hand, austenitic stainless steel exhibits an austenite phase even at the low temperature as described above, but at a low temperature of 500 ° C. or lower, a strong passive film is formed, and nitrogen (N) or carbon (C) in the steel is formed.
The diffusion and permeation of is blocked. In addition, carbon has a larger atomic radius than nitrogen and a smaller affinity with chromium, so that it is more difficult for carbon to diffuse and penetrate into the base material as compared with nitrogen. Therefore, even with austenitic stainless steel, carburization is not normally performed at a low temperature of 500 ° C. or lower. Further, at the above low temperature, a CO + H ₂ + CO ₂ mixed gas or CO + used as a carburizing gas is used.
CO contained in the CO ₂ mixed gas is (2CO → CO ₂
There is a problem that the so-called Boudoor reaction of + C) occurs, carbon is deposited in the furnace and the furnace wall and the like are contaminated. For these reasons, the carbon permeation treatment at the low temperature as described above has not been performed at all. In the present invention, by breaking such technical common sense, the surface of an austenitic stainless steel product is activated by performing a pretreatment using a fluorine-based gas as described above, and a metal product in which a carbon-enriched layer is formed. Was successfully manufactured.

The depth of the carbon-enriched layer is determined by the treatment time and the treatment temperature during the permeation treatment. The maximum carbon concentration at the outermost surface is not significantly affected if the depth of the concentrated layer (that is, the depth of penetration of carbon atoms) is above a certain level. From the viewpoint of corrosion resistance, It is considered that a depth of 5 to 15 μm is sufficient.
The thickened layer is hardened by causing lattice distortion due to solid solution of carbon atoms. If the thickened layer has a depth of 15 to 50 μm, the surface hardness is H by micro Vickers.
It reaches about v850-950. Therefore, in the case of metal products with low core hardness and weak strength, such as those that have undergone solid solution treatment before carbon infiltration treatment, deepening the above-mentioned concentrated layer will prevent product damage, etc. It is preferable in meaning.

The carbon-enriched layer can be easily observed by an ordinary metallographic microscope by etching it with a strong etching solution such as aqua regia. FIG. 2 shows that SUS316 material is subjected to a fluorination treatment at 350 ° C. for 15 minutes in an NF ₃ + N ₂ gas atmosphere, and CO: 21% by volume + H ₂ : 31% by volume.
+ CO ₂ : 5% by volume + remaining N ₂ gas atmosphere, a sample that has been subjected to a carbon infiltration treatment at 480 ° C. for 12 hours is cut, polished, and then etched, and a cross-sectional micrograph magnified 600 times. From the bottom of the figure, the base material, the concentrated layer, and the embedding resin (black portion) are shown. As you can see from the figure,
A carbon concentrated layer is formed on the surface layer of 28 μm.

The structural state of the carbon enriched layer is X
It can be grasped by a line diffraction method (X-ray Diffraction). FIG. 3 shows the result of comparison of the example of the diffraction result with an untreated austenitic stainless steel material as a base material. That is, in FIG. 3, (A) is an X-ray diffraction chart of an untreated SUS316 material,
(B) is an X-ray diffraction chart of SUS316 material that has been subjected to pickling treatment after permeation treatment at 480 ° C. These X
As can be seen from the comparison results of the line diffraction charts, comparing the peak positions of the austenite phase of each chart (peaks indicated by circles in the figure), the peak of the austenite phase of the product (B) after the permeation treatment and the finishing treatment at 480 ° C. Is shifted to a lower angle side (left side) than the peak of the untreated material (A). That is, it is clear from this that the infiltration-treated product (B) has lattice distortion in the austenite phase. It is considered that this lattice strain is that the lattice expands isotropically due to the high concentration and solid solution of carbon, and the strain hardens the concentrated layer. The X-ray diffraction was performed using a RINT 1500 apparatus under the conditions of 50 kV, 200 mA and Cu target.

The thickened layer has completely different characteristics from the carburized hardened layer obtained by the usual carburizing treatment at a high temperature of 700 ° C. or higher. Generally, the solubility of carbon in the austenite phase, which is the parent phase of austenitic stainless steel, is low, and when carburizing is performed at a high temperature of 700 ° C. or higher, excess carbon that has penetrated but cannot be completely dissolved becomes grain boundaries and dislocations. Is deposited on the defective portion such as. In such a case, Cr ₂₃ C ₆ , Cr _{7 can be obtained} by X-ray diffraction.
It can be confirmed that stable carbides such as C ₃ and Fe ₃ C are deposited. That is, as shown in the X-ray diffraction chart (B) of FIG. 3, the carbon-enriched layer of the highly corrosion-resistant metal product of the present invention contains a crystalline chromium carbide such as Cr ₂₃ C ₆ , Cr ₇ C ₃ or the like. It does not precipitate and has an austenite phase similar to that of the base material, and the lattice constant is increased by about 2% from that of the base material. On the other hand, as described above, in the carburized hardened layer treated at high temperature,
Since the infiltrated carbon reacts with chromium or the like to form a carbide, the lattice constant is almost the same as that of the base material, or even if it is increased, it remains 0.02% (see FIG. 8).

Further, in the case of treatment at a high temperature of 700 ° C. or higher, the carburized layer dissolves and disappears in a dozen minutes when immersed in the HF + HNO ₃ solution heated to 50-60 ° C.
On the other hand, in the highly corrosion resistant metal product of the present invention, the above HF +
Even when immersed in the HNO ₃ solution, the thickness of the concentrated layer hardly changes. This indicates that the carbon-enriched layer of the highly corrosion-resistant metal product of the present invention has high corrosion resistance.

Next, the corrosion resistance of the highly corrosive metal product of the present invention will be described. It is considered that the corrosion resistance of austenitic stainless steel depends on the characteristics of the passive film formed on the surface, in particular, the concentration and uniformity of Cr ₂ O ₃ content in the film. It is considered that these are affected not only by the alloy composition of the base material but also by the nonuniformity of the structure immediately below the passive film based on the processing history and the like. The highly corrosion-resistant metal product of the present invention has a base material as an austenite phase, and due to the permeation of carbon atoms, a carbon concentrated layer is formed in the surface layer. It is thought that, when formed, as a result, it exhibits significantly better corrosion resistance than the base material, austenitic stainless steel.

For example, FIG. 4 and FIG.
The measurement result of the anodic polarization curve of the untreated material of S316 and the sample obtained by permeating the SUS316 at 480 ° C. for 12 hours is shown. As you can see from the figure,
A clear peak of the active state region (the active state region is a region where the electric current sharply increases as the potential rises when anodic polarized, and refers to a region where steel melts in the active state: E in FIG. 4) On the other hand, in the sample in which the carbon-enriched layer was formed by the permeation treatment, the peak in the active region was hardly seen. Moreover, regarding the natural electrode potential (the natural corrosion potential of steel: D in the figure), the untreated material is -2.
Compared to 80 mV, the treated product is much more noble with 10 mV. From these facts, it was found that the sample in which the concentrated layer was formed was much more likely to form a passive film than the untreated material, and that the carbon concentrated layer had a metal structure more noble than the base material. . The anodic polarization curve was measured under the following conditions.

FIG. 6 shows the measurement results of the pitting potential of the untreated SUS304 material and the sample in which the SUS304 material was infiltrated at 460 ° C. for 12 hours to form a carbon-enriched layer. As can be seen from the figure, when the potential is gradually increased, the current density of the untreated material rapidly increases at a potential of about 240 mV. In contrast, the permeation-treated products are
There is no sudden increase in current density up to about 920 mV, which is much higher than in the case of the untreated material. That is, since it is considered that pitting corrosion occurs at the time when the current density rapidly increases in this measurement, the highly corrosion-resistant metal product of the present invention is about the pitting corrosion resistance which is a weak point in the austenitic stainless steel material. , Shows that it has been significantly improved. The pitting potential is measured according to JIS 057.
It was performed under the conditions specified in 7.

In the present invention, the basic procedure is to immediately shift to carbon permeation treatment after the fluorination treatment, but in some cases, as an intermediate treatment, after the fluorination treatment, NH ₃ gas is added to the atmosphere gas. Can be added and the mixture can be heated and held for 10 to 30 minutes. It has been confirmed that this intermediate treatment stabilizes at least the subsequent carbon infiltration treatment, and the like. As the intermediate treatment temperature, it is desirable to perform the treatment at a low temperature so as to eliminate the formation of crystalline CrN, and it is preferable to perform the treatment in the range of 400 to 480 ° C. When this intermediate treatment was analyzed by EPMA, the permeated nitrogen (N) was found to overlap with carbon in the very surface layer after the completion of the carbon permeation treatment, and its concentration was about As low as 0.5-1%, in many cases
Since it is removed by the pickling treatment with HF-HNO ₃ solution or the like (finishing process), it does not adversely affect the corrosion resistance of the present invention.

[0045]

As described above, according to the present invention, inexpensive general-purpose stainless steel having a basic composition is used, and a carbon-enriched layer is formed on the surface of the stainless steel. A metal product having excellent chemical resistance and high corrosion resistance can be obtained at low cost. 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 for machine parts used in fields such as food machinery, chemical plants, and semiconductor industries.

The steel types targeted in the present invention are
It is not limited to inexpensive general-purpose stainless steel such as SUS304 and SUS316. That is, the highly corrosion-resistant metal product of the present invention exhibits excellent corrosion resistance by forming a carbon-enriched layer having a corrosion resistance higher than that of the austenitic stainless steel as the base material. Therefore, SUS3
By using austenitic stainless steel, which has better corrosion resistance than the above general-purpose stainless steels such as 17, SUS310, as a base material, a concentrated layer with better corrosion resistance than the base material is formed, and more excellent corrosion resistance is exhibited. It can be done.

Next, examples will be described.

[0048]

[Example 1] Rolled to a plate thickness of 2.5 mm, 15 mm x 1
SUS304 (Cr content: 18.1% by weight, Ni content: 7.9% by weight, C content: 0.08% by weight, balance: Fe) plate piece cut into 5 mm, and SUS316 (Cr
Content: 18.3% by weight, Ni content: 12.4% by weight, M
o content: 2.5% by weight, balance: Fe) A plurality of plate pieces were prepared. In addition, a part of the SUS304 plate piece is
After heating to 1020 ° C. in a vacuum furnace, it was rapidly cooled to carry out a solid solution treatment so as to exhibit an austenite phase. In addition,
Ferrite is deposited on the SUS304 plate pieces that have not been subjected to solution treatment by rolling. Further, the SUS316 plate piece exhibits the austenite phase even after the rolling process. When the hardness of these plate pieces was measured, SUS304
Hv250 for rolled products and Hv2 for SUS316 rolled products
90, the solution-treated product of SUS304 was Hv140.

Next, these plate pieces were charged into the furnace shown in FIG. 1 and heated to 350 ° C. in an N ₂ atmosphere, and then 10
A fluorination treatment was performed by blowing a mixed gas of volume% NF ₃ +90 volume% N ₂ for 15 minutes. After that, the temperature was raised to 480 ° C. in an N ₂ atmosphere, the temperature was maintained for 12 hours to perform a carbon permeation treatment, and then the carbon was taken out. After that, it was immersed in a 3.5 wt% HF + 15 wt% HNO ₃ solution heated to 65 ° C. for 40 minutes for finishing treatment. This finishing treatment (pickling)
As a result, the permeation-treated product, which had a black color, now has a metal appearance equivalent to that of the original untreated product. Table 1 below shows the results of measuring the surface hardness (micro Vickers: Hv) and the carbon concentrated layer depth after the pickling after the permeation treatment and after the pickling.

[0050]

[Table 1]

As is clear from the results in Table 1 above, SU
In the rolled product of S304, since the ferrite is precipitated in the base material by the rolling process, the corrosion resistance is poor even if it is subjected to the rolling process. Is reduced to less than about one third. On the other hand, the SUS304 solution treated product and the SUS316 rolled product that had been subjected to the permeation treatment had no precipitation of ferrite in the base material and exhibited an austenite phase, so the corrosion resistance of the thickened layer was good, and the acid after the permeation treatment was performed. It can be seen that even after washing, the depth of the concentrated layer has hardly decreased.

Next, each plate piece of the rolled SUS304 product was immersed in 20% sulfuric acid heated at 50 ° C., and the weight reduction of the plate piece was measured with the lapse of immersion time. FIG. 7 shows the result. As is clear from the results of FIG. 7, the untreated SUS304 solution treated product had a weight loss of 500 g / m ² or more when immersed for 20 hours or more, but the SUS304 solution treated product was subjected to the permeation treatment. The result is that the weight loss after soaking for 24 hours is about 220 g / m.
² and less than half of the untreated product. Also, SUS
The untreated 316 rolled product had a reduced weight of about 420 g / m ² after being immersed for 24 hours, whereas the SUS316 rolled product subjected to the permeation treatment showed almost no weight reduction. Thus, it can be seen that the SUS304 solution treated product and the SUS316 rolled product subjected to the permeation treatment show significantly better corrosion resistance than the untreated plate piece not subjected to the permeation treatment.

[0053]

Example 2 Generally commercially available SUS304 (Cr
Content: 18.5% by weight, Ni content: 8.5% by weight, C content: 0.08% by weight, balance: Fe) Soft plate material (base material hardness Hv 170 to 180) and SUS316 (Cr content: 17) % By weight, Ni content: 13.5% by weight, Mo content: 2.5% by weight, C content: 0.06% by weight, balance: F
e) A plurality of plate pieces each having a thickness of 2 mm × 10 mm × 10 mm were prepared from the plate material (base material hardness Hv 210 to 230). A portion of these is 440 in the furnace shown in FIG.
The temperature was raised to 0 ° C., and a mixed gas of NF ₃ : 10% by volume + N ₂ : 90% by volume was blown for 25 minutes to perform a fluorination treatment. Then, H ₂ : 32% by volume + CO: 20% by volume + CO ₂ : 1
Volume% + balance: N ₂ carburizing gas was introduced, held for 8 hours for carbon permeation treatment, and then taken out. Next, 5
After immersing in a 4 wt% HF + 15 wt% HNO ₃ solution heated to 5 ° C. for 20 minutes, the surface condition was investigated.
The SUS304 plate piece had a concentrated layer depth of 8 μm and a surface hardness of Hv380. The SUS316 plate piece had a concentrated layer depth of 12 μm and a surface hardness of Hv550. These infiltration-treated products and untreated products were polished with diamond paste and then heated to 65 ° C to obtain 15% by weight.
It was subjected the test sample is subjected to a passivation treatment with HNO ₃ solution. Next, physiological saline (0.5 wt% NaCl solution heated to 37 ° C.) was prepared, each sample was immersed for 1 week, and the elution state of metal ions was investigated by atomic absorption spectrometry. The results are shown in Table 2 below.

[0054]

[Table 2]

As is clear from the results in Table 2 above, the untreated product has more Fe and Ni ions eluted than the product of the present invention. That is, it can be seen that the metal product of the present invention has improved corrosion resistance as compared with the untreated product (that is, the base material) due to the permeation treatment.

[Brief description of drawings]

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

FIG. 2 is an enlarged cross-sectional micrograph of a surface layer portion of the highly corrosion-resistant metal product of the present invention.

FIG. 3 is an X-ray diffraction chart of an untreated SUS316 product and a treated product in which a SUS316 plate material was subjected to a permeation treatment at 480 ° C. and then subjected to a strong acid immersion treatment.

FIG. 4 is a measurement result of an anode polarization curve of an untreated material of SUS316.

FIG. 5 is a measurement result of an anodic polarization curve of a sample obtained by permeating SUS316 at 480 ° C.

[FIG. 6] Untreated material of SUS316 and this SUS316
2 is a measurement result of pitting potential of a sample subjected to permeation treatment at 480 ° C.

FIG. 7 shows the results of a sulfuric acid immersion test of SUS304 solution treated products, SUS316 rolled products, and samples obtained by permeating these at 480 ° C.

FIG. 8 is an X-ray diffraction chart of a treated product obtained by carburizing SUS316 material at 600 ° C.

FIG. 9 is an EPMA analysis result of a treated product obtained by permeating SUS316 material at 480 ° C.

FIG. 10 is an EPMA analysis result of a treated product obtained by permeating SUS316 material at 450 ° C.

FIG. 11 is an EPMA analysis result of a treated product obtained by carburizing SUS316 material at 600 ° C.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Tadashi Hayashida 1-8 Nakahama-cho, Amagasaki-shi, Hyogo Daido Hokusan Co., Ltd. Amagasaki Plant (72) Masaaki Tahara 1-8 Nakahama-cho, Amagasaki-shi, Hyogo Daido Hokusan Co., Ltd. Amagasaki Plant

Claims (9)

[Claims] 1. A base material is made of austenitic stainless steel exhibiting an austenite phase, and a surface layer having a depth of 5 to 50 μm immediately below the passivation film on the surface is formed as a carbon concentrated layer by permeation of carbon atoms. Highly corrosion resistant metal product characterized by being made. 2. The highly corrosion resistant metal product according to claim 1, wherein the austenitic stainless steel is an austenitic stainless steel containing molybdenum. 3. The highly corrosion resistant metal product according to claim 1, wherein the concentrated layer is formed of an austenite phase in which chromium carbide is absent. 4. The maximum carbon concentration of the concentrated layer is 1.2 to 2.6.
The highly corrosion-resistant metal product according to claim 3, wherein the metal product has a weight percentage. 5. An austenitic stainless steel product in which a base material exhibits an austenite phase is held in a heated state in a fluorine-based gas atmosphere, and then carbon atom permeation treatment is performed at a temperature of 400 ° C. to 500 ° C. A method for producing a highly corrosion-resistant metal product, which comprises forming a carbon concentrated layer on a surface layer having a depth of 5 to 50 μm. 6. Heating under a fluorine-based gas atmosphere is 3
The method for producing a highly corrosion-resistant metal product according to claim 5, wherein the temperature is from 00 to 500 ° C. 7. The method for producing a highly corrosion-resistant metal product according to claim 5, wherein a finish treatment for removing the outermost surface layer is performed after the carbon atom infiltration treatment. 8. The finishing treatment is a pickling treatment.
A method for producing the highly corrosion-resistant metal product described. 9. The method according to claim 5, wherein after the heating and holding in a fluorine-based gas atmosphere, NH ₃ gas is added to the atmosphere and the heating and holding is performed, and then the carbon atom permeation treatment is performed. Manufacturing method of highly corrosion resistant metal products.