JP2011195947A - Color austenitic stainless steel material having corrosion resistance and high hardness and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a color austenitic stainless steel material having high corrosion resistance and high hardness which is hardly injured, is excellent in wear resistance and corrosion resistance, has beautiful hue and superior decorative property, moreover, can retreat and reuse the used material and parts, permits resource economization and can reduce manufacturing expenses, and to provide a method of manufacturing the color austenitic stainless steel material having corrosion resistance and high hardness.SOLUTION: The color austenitic stainless steel material having corrosion resistance and high hardness to which the superior corrosion resistance, high surface hardness and various colors are imparted by subjecting the material and parts manufactured with the austenitic stainless steel to the nitriding heat treatment and the oxidation treatment after a surface working process is provided.

Description

The present invention relates to an austenitic stainless steel material and a method for producing the same, and more specifically, a material or a part produced with austenitic stainless steel is subjected to a nitriding heat treatment (nitriding) and a surface processing step, followed by an oxidation treatment. The present invention relates to a high-corrosion resistance and high-hardness color austenitic stainless steel material having excellent corrosion resistance, high surface hardness and various colors, and a method for producing the same.

Austenitic stainless steel used in fields where corrosion resistance is important is generally based on iron containing 18 wt% chromium and 8 wt% nickel. Call. An austenitic stainless steel containing 1 to 3% by weight of Mo in this 18-8 stainless steel is also generally used.
These austenitic stainless steels have many types of steel standardized by KS, such as STS304, STS316, STS310, etc., and they have excellent corrosion resistance and are used in daily life, household and office supplies, accessories, beauty instruments, food industry and chemicals. Widely used in industry.
On the other hand, there is a growing desire to create various aesthetics in daily life using hue.
Conventional color forming methods for such stainless steel include, for example, the following.
The hard film of Ti or Zr formed on the surface of stainless steel by CVD or PVD method has high hardness but low adhesion to the base material and can cause peeling. The disadvantage is that only can be implemented. Another problem is that it cannot provide corrosion resistance comparable to that of stainless steel as a base material.
On the other hand, the UK INCO method (UKpat, 275, 781) is formed by immersing stainless steel in an acidic Cr solution to form an oxide layer with a thickness of 1 μm and made of Fe, Ni and Cr oxides. Although various colors are manifested by the interference of light appearing by the layer, the thickness of the oxide layer formed at this time is only a few hundred mm, so the film thickness is thin and the surface hardness is low. Scratches are likely to occur.
When stainless steel is oxidized in a high-temperature oxidizing atmosphere, hues such as gold, brown, and blue can be imparted. However, the oxide film having a thickness of about Å is easily damaged as in the case of the INCO method. .
In addition, since the above methods cannot increase the hardness of the base material, it is difficult to use as a use in which a large load acts.
On the other hand, when the color oxide film layer peels off or changes color during use, the color material produced by the color forming method has a problem in that it cannot be used again after being reprocessed. .

The present invention has been proposed in consideration of all the above-mentioned problems, and the object of the present invention is that scratches do not occur well, wear resistance and corrosion resistance are excellent, hue is beautiful, and decoration is excellent. High-corrosion-resistant and high-hardness color austenitic stainless steel material that can save resources and reduce manufacturing costs because it can be reprocessed and reused after use. Is to provide.

In order to achieve such an object, according to the present invention, (a) a step of performing a nitriding heat treatment on the austenitic stainless steel material; (b) a step of performing a surface processing step on the steel material subjected to the nitriding heat treatment. And (c) a step of performing an oxidation heat treatment for forming a color oxide film layer on the steel material that has been subjected to the surface processing step, wherein the color austenitic stainless steel material having high corrosion resistance and high hardness is provided. A manufacturing method is provided.
In the present invention, the nitriding heat treatment is a gas method, a plasma method or a salt bath method in order to form a nitrogen supersaturated solid solution having a high hardness on the surface of a stainless steel material and a nitrogen diffusion layer in the lower part thereof. The heat treatment can be carried out at a temperature range of ˜450 ° C. for 1 to 30 hours, and the phase composition of the nitrogen supersaturated solid solution formed on the surface of the stainless steel material is preferably S-phase. Features.
The surface processing step is for removing the oxide generated on the surface during nitriding heat treatment to form a surface roughness of 0.1 to 5 μmRa. Sanding, shot peening, buffing, lapping, polishing or vapor shot Etc. can be implemented.
The formation of the color oxide layer is intended to realize various and beautiful colors on the surface of stainless steel materials and parts subjected to surface processing, and it is 10 seconds to 20 hours in an oxidizing gas atmosphere at 150 to 600 ° C. In this case, the oxidizing gas atmosphere is any one selected from oxygen, air, water vapor, or nitrogen dioxide, or a mixed gas of two or more thereof. Moreover, nitrogen gas can also be mixed and used for these gases.

The high corrosion resistance and high hardness color austenitic stainless steel materials and parts of the present invention as described above provide the following effects.
First, it is possible to provide a material and a part that are not easily damaged and have excellent wear resistance and corrosion resistance.
Second, it is possible to provide materials and parts having a beautiful hue and excellent decorativeness.
Third, after use, materials and parts can be reprocessed and reused, so that resources can be saved and manufacturing costs can be reduced.

First, in the present invention, a nitriding heat treatment is performed on a material or a part made of austenitic stainless steel by a gas method, a plasma method or a salt bath method at a temperature range of 300 to 450 ° C. for 1 to 30 hours. The nitrogen supersaturated solid solution formed at this time has a thickness of 1 to 30 μm, a hardness of 1000 to 1800 HV, and a phase composition of S-phase.
When a nitriding heat treatment is performed on a stainless steel material having high corrosion resistance at a temperature exceeding 450 ° C., the corrosion resistance is lowered for the following reason.
The high corrosion resistance of stainless steel is due to the passive film of chromium formed on the surface of the steel. When nitriding heat treatment is performed at a temperature exceeding 450 ° C, chromium nitride (CrN, Cr ₂ N, etc.) is formed on the surface of the steel. In the vicinity of such chromium nitrides, the amount of chromium dissolved in the steel is reduced. Accordingly, a chromium passive film is not formed in the vicinity of the chromium nitride or the amount formed is insufficient, and the chromium nitride causes a so-called sensitization phenomenon to the corrosive environment, thereby reducing the corrosion resistance.
Accordingly, if a nitriding heat treatment is performed at 450 ° C. or lower, such a chromium nitride is not deposited on the surface and an S-phase nitrogen solid solution is formed, but this S-phase nitrogen solid solution is inherent to stainless steel. It has a high surface hardness of 1000 HV or more without deteriorating corrosion resistance.
In particular, an S-phase formed by performing a nitriding heat treatment in a temperature range of 400 ° C. or less is known to have a level of corrosion resistance similar to that in the case of not performing the nitriding heat treatment.
Although there are many studies on the properties of the S-phase, it is a face-centered cube with a lattice constant of 0.378 nm (FCC, Face Centered Cubic), where nitrogen is supersaturated with a maximum of 22 atomic percent of solid solution of nitrogen. It is a solid solution.
On the other hand, the reason why the corrosion resistance is improved by the nitriding heat treatment of the stainless steel material is that (a) the formation of a passive film of nitrogen is promoted on the surface of the stainless steel and (b) the activated stainless steel This is because the concentration of nitrogen was increased on the surface.
By the way, when the nitriding heat treatment is performed at a temperature exceeding 450 ° C., CrN is precipitated instead of the S-phase, and the corrosion resistance is lowered. Therefore, it is desirable that the nitriding heat treatment is performed in the temperature range of 300 to 450 ° C. for 1 to 30 hours so that the S-phase thickness becomes 1 to 30 μm.
When the nitriding heat treatment is performed at a temperature lower than this, since the diffusion rate of nitrogen atoms is remarkably low, a sufficiently thick S-phase and a nitrogen diffusion layer cannot be obtained and a high external load cannot be supported. Furthermore, when the nitriding heat treatment is performed for a long time, an S-phase and a nitrogen diffusion layer having a desired thickness can be obtained, but this is not economical.
The nitriding heat treatment can be performed by a gas method, a plasma method, or a salt bath method. Here, the nitriding heat treatment means a pure nitriding heat treatment (nitriding) performed in a pure nitriding atmosphere and a mixture containing carbon in a pure nitriding atmosphere. Used in a broad sense including nitrocarburizing performed in an atmosphere.
The nitriding heat treatment by the medium gas method in the nitriding heat treatment step can be performed by the following method.
First, prior to the nitriding heat treatment, a fluorination treatment is performed to facilitate the penetration of nitrogen atoms. Fluorine-based gas used for the fluorination treatment includes fluorine compound gas composed of NF ₃ , BF ₃ , CF ₄ , HF, SF ₆ , C ₂ F _6, WF _6, etc. It can also be used by mixing, or nitrogen gas can be diluted here.
The fluorination treatment is carried out by introducing a stainless steel material and parts into a heat treatment furnace having a nitriding heat treatment temperature of 300 to 450 ° C., creating a fluorine-based gas atmosphere, and maintaining this atmosphere. In such a fluorine-based gas atmosphere, the maintenance time of the stainless steel material and parts is usually set within a range of several minutes to several tens of minutes in order to set it in an appropriate time depending on its shape and dimensions.
This fluorination treatment facilitates the penetration of nitrogen atoms into the surface layer of stainless steel. The reason for this is as follows.
The surface of the stainless steel material is formed with a passive film that inhibits the permeation and diffusion of nitrogen atoms that cause nitriding. When the stainless steel material on which such a passive film is formed is heated under the fluorine gas atmosphere as described above, the passive film is converted into a fluoride film. Since this fluoride film allows nitrogen atoms to penetrate more easily than the passive film, stainless steel can be transferred in a surface state that is easy to nitride, thereby forming a deep and uniform nitride layer.
As described above, the nitriding heat treatment is performed by maintaining the stainless steel in which nitrogen atoms are easily penetrated by the fluorination treatment in a gas nitriding atmosphere at a temperature of 300 to 450 ° C. In this case, the gas nitriding atmosphere is pure NH ₃ , NH ₃ and an endothermic gas, or a mixed gas of NH ₃ and CO ₂ , but generally the gas is mixed with nitrogen gas. .
Alternatively, the nitriding heat treatment can be carried out using a mixed gas of ammonia gas and hydrocarbon. In this case, the fluorine-based gas is used before the nitriding heat treatment as described above due to the surface activation effect by the hydrocarbon gas. Thus, the gas nitriding heat treatment can be performed without performing the pretreatment.
Salt bath nitriding is _NaCN, KCN, NaCNO, KCNO, by dipping K 2 CO ₃ and _Na 2 CO ₃ stainless steel constant time in a mixed molten salt such as CN ^- nitride nitride, or by nitriding carburizing reaction ^- or CNO This is a step of performing heat treatment.
In plasma nitriding, stainless steel is charged into a vacuum chamber and the pressure is reduced to a certain level, and then a reaction gas such as nitrogen, hydrogen, and hydrocarbon is mixed at a predetermined ratio and introduced into the vacuum chamber. Thus, when a voltage is applied so that the stainless steel becomes a cathode, glow discharge is generated and the nitriding reaction proceeds.
At this time, it is important to perform a pretreatment for activating the surface of the stainless steel by sputtering before introducing the reaction gas. For example, sputtering is performed by glow heating in a mixed gas atmosphere of Ar and hydrogen of about 0.5 to 5 torr in a temperature range of 300 to 450 ° C., and high-temperature gas ions collide with the metal surface to form an oxide film on the outermost surface. This is a step of removing the adsorbed contamination layer. If this pretreatment operation is not sufficient, the formed nitride layer may be insufficient, or no nitride layer may be formed at all.
The S-phase formed by performing a nitriding heat treatment in the temperature range of 300 to 450 ° C. provides a stainless steel having a high surface hardness of 1000 HV or higher without reducing the inherent corrosion resistance of the stainless steel.
Subsequently, a surface processing step is performed on the material and parts subjected to the nitriding heat treatment.
When the nitriding heat treatment process is performed, for example, after the completion of the nitriding heat treatment, when the heat treatment is taken out at a high temperature and contacted with air, oxides are generated on the surface of the stainless steel due to oxygen in the air. Since the material hinders the formation of a color oxide film layer by oxidation, which will be described later, it must be removed.
Also, the color realization by the formation of the oxide film is due to the interference phenomenon between the light reflected from the surface of the oxide layer and the light reflected through the oxide layer and reflected from the surface of the base material. A desired beautiful hue cannot be obtained by irregular reflection.
Accordingly, in order to adjust the surface roughness by removing oxides on the surface of the stainless steel in the surface processing step, the roughness after the surface processing is preferably in the range of 0.1 to 5 μmRa. The process can be performed by sanding, shot peening, buffing, lapping, polishing or vapor shot.
Subsequently, an oxidation treatment for forming a color oxide film is performed on the stainless steel subjected to the surface processing step.
The color formation by oxidation forms a unique color according to the thickness of the oxide layer, and the oxidation treatment can be performed by oxidizing for 10 seconds to 20 hours in an oxidizing gas atmosphere at 150 to 600 ° C. At this time, the oxidizing gas used may be oxygen, air, water vapor, and nitrogen dioxide alone or in combination of two or more thereof. Alternatively, these gases can contain nitrogen.
When the oxidation treatment temperature is less than 150 ° C. or the oxidation treatment time is less than 10 seconds, the oxidation treatment is not performed, so that a desired color cannot be obtained, and the oxidation treatment temperature is higher than 600 ° C. Alternatively, when the oxidation treatment time exceeds 20 hours, the S-phase formed during the nitriding heat treatment decomposes in the ferrite, CrN and austenite phases, the corrosion resistance is lowered, and the hardness is rapidly lowered.
On the other hand, the same color can be obtained by forming oxide layers having the same level of thickness by low-temperature oxidation treatment at low temperatures or short-time oxidation treatment at high temperatures.
That is, even when heated for a short time at a high temperature of 800 ° C. for several seconds to several tens of seconds as in the high-frequency heat treatment, the same hue as when heated for a long time at a low temperature can be obtained.
During the oxidation treatment, the thickness of the oxide layer formed varies depending on the oxidation temperature, time and oxidizing atmosphere (inside and outside of several tens to several hundreds of liters), and the hue corresponding to the thickness of this oxide layer is can get.
On the other hand, when peeling or discoloration of the color oxide film layer occurs during use, the color oxide film layer may be removed and re-oxidized in the surface processing step to give a color and reused.
High corrosion resistance and high hardness color austenitic stainless steel materials and parts according to the present invention have high surface hardness, can withstand high loads, and are not easily damaged by friction. Is maintained. Moreover, it may be used for washing machine parts, faucets, toilet articles, interior / exterior materials for buildings, and cruise ship decorations that are exposed to a corrosive environment with excellent corrosion resistance due to the S-phase formed during nitriding heat treatment.
In addition, the color material and parts according to the present invention can be reused even if the color oxide film layer is peeled off or discolored during use, so that resources can be saved and manufacturing costs can be reduced. There is.
Hereinafter, the present invention will be described more specifically through examples.
<Example 1>
Pipes having an outer diameter of 30 mmφ, a thickness of 1 mm, and a length of 300 mm were manufactured from STS304, STS316, and STS310 materials. The pipe was placed in a pit type furnace set at 400 ° C., and the air in the furnace was replaced with N 2, and then a fluorine-based gas (a mixed gas of 8% by volume of NF ₃ and 92% by volume of N ₂ ) was injected. The state was maintained for 20 minutes. Next, after replacing the fluorine-based gas with N ₂ , nitriding treatment was performed for 15 hours in an atmosphere of a nitriding gas (NF ₃ 60% by volume, CO ₂ 10% by volume, and N ₂ 30% by volume) at 400 ° C. Air cooled.
When nitriding was performed in this way, the thicknesses of the compound layers of the STS304, STS316, and STS310 materials were 15 μm, 18 μm, and 14 μm, respectively, and the hardnesses of the compound layers were 1010 Hv, 1070 Hv, and 1100 Hv, respectively. In addition, as a result of evaluating the corrosion resistance in a salt spray test (KS D 9502), there was no rusting even after 600 hours.
After buffing the nitriding pipe as described above and setting its surface roughness to 1.2 μmRa, it was maintained in an air atmosphere at 480 ° C. for 5 hours. As a result, a golden hue was obtained and the hue of each material The difference was not great.
<Example 2>
A disk-shaped test piece having a thickness of 5 mm and a diameter of 20 mm was manufactured from STS304 and STS316 materials, and plasma nitriding was performed. The plasma nitriding conditions were carried out for 13 hours in a glow discharge state by introducing a mixed gas of 80% by volume of N ₂ and 20% by volume of H ₂ and depressurizing at 5 torr, applying a test piece of about 500 V as a cathode. Decided to do. At this time, the temperature of the test piece was set to 390 ° C., and the furnace was cooled to room temperature under high vacuum after nitriding.
When plasma nitriding was performed as described above, the thickness of the compound layer was 12 μm and 14 μm, respectively, with respect to STS304 and STS316, and the hardness of the compound layer was 1020 Hv and 1200 Hv, respectively. As a result of evaluating the corrosion resistance in a salt spray test (KS D 9502), there was no rust even after 600 hours.
This specimen was polished to a surface roughness of 0.9 μmRa, and then oxidized for 5 hours in a mixed gas atmosphere of 50% oxygen and 50% air at 450 ° C. As a result, a golden hue was obtained. It was.
<Example 3>
Salt bath nitridation was performed on 11 mm diameter balls made with STS316 and STS310. Salt bath nitriding is a method of immersing for 8 hours in a mixed molten salt composed of 45 wt% NaCN, 45 wt% Na ₂ CO ₃ and 15 wt% (NaK) ₄ Fe (CN) ₆ maintained at 450 ° C. It carried out in. When salt bath nitridation was performed in this way, the thickness of the compound layer was 22 μm and 19 μm, respectively, with respect to STS316 and STS310, and the hardness of the compound layer was about 1000 Hv for both of the two materials.
Then, barrel polishing was performed on the salt bath nitrided balls. The barrel polishing was carried out by centrifuging the barrel for 20 minutes at a rotation speed of 200 prm by immersing it in a mixture of 500 cc water, 800 cc grinding stone with a diameter of 3 mm and 30 cc compound for 100 balls.
The polished ball was oxidized for 5 hours in a mixed gas atmosphere of oxygen of 60% by volume and nitrogen of 40% by volume at 500 ° C. As a result, a golden hue was obtained, and the corrosion resistance was evaluated by a salt spray test. There was no rusting after hours.
As described above, the present invention has been described by way of limited embodiments, but the technical idea of the present invention and the equivalents of the claims described below are equivalent to those having ordinary knowledge in the technical field to which the present invention belongs. Various modifications and variations are possible within the scope.

Claims (7)

(a) performing a nitriding heat treatment on the austenitic stainless steel material;
(b) performing a surface processing step on the steel material subjected to the nitriding heat treatment; and
(c) A method for producing a high-corrosion-resistant and high-hardness color austenitic stainless steel material comprising a step of performing an oxidation heat treatment for forming a color oxide film layer on the steel material subjected to the surface processing step . The nitriding heat treatment is performed by a gas method, a salt bath method or a plasma method in a temperature range of 300 to 450 ° C for 1 to 30 hours to form an S-phase nitride layer on the surface. Of high color corrosion resistance and high hardness color austenitic stainless steel material. 2. The high corrosion resistance and high hardness color according to claim 1, wherein the surface processing step is performed by any one method selected from sanding, shot peening, buffing, lapping, polishing, and vapor shot. Method for producing austenitic stainless steel. The high corrosion resistance and high corrosion resistance according to claim 1, wherein the oxidation heat treatment step for forming the color oxide film layer is performed in an oxidizing gas atmosphere at a temperature range of 150 to 600 ° C for 10 seconds to 20 hours. A method for producing high-hardness color austenitic stainless steel. The oxidizing gas atmosphere is any one or a mixed gas selected from the group consisting of oxygen, air, water vapor and nitrogen dioxide, or a gas further containing nitrogen in these gases. The method for producing a color austenitic stainless steel material having high corrosion resistance and high hardness according to claim 4. The high corrosion resistance according to any one of claims 1 to 5, wherein the color stainless steel material can be reused after being subjected to the surface processing step and color oxidation treatment after use. And a method for producing a high-hardness color austenitic stainless steel material. A color austenitic stainless steel material having high corrosion resistance and high hardness produced by the method according to any one of claims 1 to 5.