JP5869072B2 - Method for surface modification of stainless steel sheet - Google Patents

Description

  The present invention relates to a method for rapidly modifying the surface of a stainless steel plate.

  Addition of nitrogen is extremely effective for improving the corrosion resistance of stainless steel. As a method for adding nitrogen, mainly, a pressure melting method in which nitrogen is added in a refining stage, a solid-phase nitrogen absorption method in which stainless steel absorbs nitrogen from an atmospheric gas in a high-temperature solid phase state, and the like (see Patent Document 1) ). In the pressure dissolution method, since nitrogen has a strong solid solution strengthening effect, difficult workability after addition of nitrogen becomes a problem. On the other hand, in the solid-phase nitrogen absorption method, nitrogen is added after the steel material is processed or formed, so that the problem of difficult workability can be avoided. Further, since the solid-phase nitrogen absorption method has a larger nitrogen absorption amount than the pressure dissolution method, it can be expected that the corrosion resistance of the stainless steel will be greatly improved.

  In the solid-phase nitrogen absorption method, nitrogen is absorbed from the atmospheric gas while exposing the stainless steel to a high temperature. For this reason, not only the composition of the steel material and the properties of the material itself such as the surface oxide film, but also environmental factors such as the heat treatment temperature and atmospheric gas, and the heat treatment time, the amount of nitrogen absorbed on the material surface and the diffusion depth of nitrogen into the material. Greatly affects. For example, a surface oxide film already present before heat treatment prevents diffusion of nitrogen into the material. In this case, considering the manufacturing cost, in order to perform the nitrogen absorption treatment of the steel sheet as it is without performing a special pretreatment for removing the oxide film, the oxide film is reduced and removed in the initial stage during the heat treatment. It is possible to do. Further, by performing heat treatment at an appropriate temperature and nitrogen partial pressure, high concentration of nitrogen can be dissolved in the steel sheet without forming nitrides or oxides. When nitrides and oxides are generated, diffusion of nitrogen into the material is hindered, and the corrosion resistance of the steel sheet is impaired.

  In order to absorb a high concentration of nitrogen in a conventional general solid-phase nitrogen absorption method, it takes several tens of minutes or more for the heat treatment. For this reason, stainless steel is exposed to a high temperature for a long time, and there are problems such as coarsening of crystal grains and an increase in manufacturing cost. In addition, if the heat treatment temperature is lowered too much to prevent the crystal grains from becoming coarse, problems such as formation of nitrides and insufficient diffusion depth of nitrogen occur if the heat treatment time is shortened too much.

  On the other hand, when the nitrogen absorption treatment is performed on a continuous production line such as bright annealing (BA) treatment, the heat treatment time is only tens of seconds. Therefore, in order to sufficiently absorb nitrogen in such a short time and diffuse to a sufficient depth inside the material, it is important to heat-treat the target stainless steel at an appropriate temperature and atmospheric gas conditions. . Specifically, ferritic stainless steel, austenitic / ferritic stainless steel, and austenitic stainless steel have different components, so that the appropriate temperature and nitrogen partial pressure in the atmospheric gas are different. For example, in Patent Document 2, austenite-ferritic stainless steel (duplex stainless steel) is targeted, and an austenite-enriched layer in which a predetermined amount of nitrogen is absorbed is formed on the surface by subjecting this steel plate to a bright heat treatment. doing.

JP 2006-316338 A JP 2004-353041 A

  As described above, the conventional solid-phase nitrogen absorption method removes the oxide film in the initial stage of heat treatment, diffuses high-concentration nitrogen to a sufficient depth with a short heat treatment, and suppresses the coarsening of crystal grains. I can't do it. Moreover, in patent document 2, since the austenite rich layer which absorbed nitrogen is thin, the effect of a corrosion resistance improvement is not enough. Accordingly, an object of the present invention is to provide a method for modifying the surface of a stainless steel plate that can provide high corrosion resistance by forming a high concentration of nitrogen in a short time without forming nitrides. .

  In order to achieve the above object, as a result of intensive research, the present inventors have found a method for rapidly dissolving high-concentration nitrogen in various stainless steel plates by controlling the heat treatment temperature and atmospheric gas, and It came to be completed. In other words, the stainless steel plate surface can be improved by allowing the stainless steel plate to pass through a high-temperature atmosphere gas containing nitrogen gas and reducing gas to quickly dissolve solid nitrogen at a high concentration on the stainless steel plate surface, thereby providing excellent corrosion resistance. I found a quality method. The present invention has been made by repeating experiments based on this finding and finding the most desirable conditions.

  In particular, the essence of the method for modifying the surface of a stainless steel plate according to the present invention is to perform the treatment in a temperature range where the stainless steel becomes an austenite single phase and in a region where the stainless steel is in equilibrium with nitrogen gas. The austenite phase has high nitrogen solubility and is advantageous for nitrogen absorption. Furthermore, it is possible to maintain the nitrogen atoms on the surface of the stainless steel in a solid solution state by appropriately setting the nitrogen partial pressure and using a temperature range where no nitride is formed. This region was discovered by the present inventors through extensive research, and by performing heat treatment so as to satisfy this condition, high-concentration nitrogen can be rapidly dissolved, and sufficient from the surface. Can be diffused deeply.

That is, the surface modification method for a stainless steel sheet according to the present invention has a thickness of 1140 to 1210 ° C. in a furnace having an atmosphere in which the partial pressure of nitrogen is 0.7 atm and the balance is made of a reducing gas. 0.5 to 2.0 mm, Cr: 18.0 to 20.0 mass%, Ni: 9.0 to 13.0 mass%, balance: Fe and unavoidable impurities and a temperature at which the steel plate equilibrates with the austenite single phase at 30 Hold for ~ 90 seconds, solidify 0.3 mass% or more of nitrogen while forming an austenite single phase on the surface of the steel sheet, cool the steel sheet in the furnace while maintaining the gas atmosphere, and nitrogen is solidified on the surface of the steel sheet. The molten austenite single phase is maintained .

  According to the present invention, by controlling the atmospheric gas and the heat treatment temperature to form an austenite single phase on the surface of the steel plate, it is possible to efficiently dissolve high-concentration nitrogen to a sufficient depth. The austenite single phase on the surface of the stainless steel plate contains nitrogen at a concentration higher than the center of the plate thickness of the steel plate, contains 0.3 mass% or more of nitrogen, and has a thickness of 10 μm or more (hereinafter referred to as nitrogen). Represented as an enriched layer). In addition, since the treatment is performed in a temperature range where the austenite single phase is obtained and in a region where nitrogen gas is in equilibrium, nitrogen atoms on the surface of the stainless steel can be maintained in a solid solution state, thereby preventing the formation of nitrides. it can. Therefore, the corrosion resistance of the stainless steel plate surface can be effectively improved.

Hereinafter, the reason for the numerical limitation in the surface modification method of the stainless steel plate of the present invention will be described.
Nitrogen partial pressure: 0.25 to 0.7 atm Nitrogen gas is an indispensable gas for dissolving nitrogen atoms on the steel plate surface. If the nitrogen partial pressure is less than 0.25 atm, it is difficult to obtain a nitrogen-enriched layer. That is, the diffusion depth of nitrogen from the steel sheet surface is less than 10 μm, or the nitrogen concentration is less than 0.3 mass%. On the other hand, when the nitrogen partial pressure exceeds 0.7 atm, the solid solution state of nitrogen cannot be maintained, and nitride is easily formed. For this reason, nitrogen partial pressure shall be 0.25-0.7 atmosphere . Further, the remainder of the atmospheric gas needs to be a reducing gas in order to prevent oxidation of the steel sheet. In normal processing, the total pressure may be 1 atm. However, the partial pressure of nitrogen may be 0.25 to 0.7 atm, and the total pressure is not necessarily limited to this.

Heat treatment temperature: 1079-1210 ° C
In the nitrogen atmosphere, an austenite single phase is formed on the stainless steel plate surface. When the heat treatment temperature is less than 1079 ° C., the solid solution and diffusion of nitrogen are insufficient, and it is difficult to obtain a nitrogen-enriched layer, or depending on the steel type, the austenite single phase is not maintained and the solid solution state of nitrogen atoms is maintained. Therefore, nitride is easily formed. On the other hand, when the heat treatment temperature exceeds 1210 ° C., the crystal grains become coarse and the toughness decreases, or depending on the steel type, a ferrite phase is formed and the solubility of nitrogen is reduced, so that it is difficult to obtain a high concentration nitride enriched layer. Therefore, the heat treatment temperature is set to 1,079-1210 ° C..

Plate thickness: 0.5-2.0mm
In order to allow the steel sheet to quickly absorb nitrogen during the heat treatment time of the manufacturing process, it is necessary to quickly reach the target temperature, that is, 1079 to 1210 ° C. If the plate thickness is too thick, it takes time to raise the temperature of the steel sheet, and the nitrogen absorption time of the steel sheet is substantially shortened within the limited heat treatment time during the manufacturing process, making it difficult to obtain a nitrogen-enriched layer. On the other hand, if the plate thickness is too thin, the steel plate is likely to be deformed during processing at a high temperature, causing a problem in quality. For this reason, plate thickness shall be 0.5-2.0 mm.

Holding time: 30-90s
The amount of nitrogen that can be dissolved in each steel type is determined by the heat treatment temperature and the nitrogen partial pressure. In an environment of appropriate temperature and nitrogen partial pressure, nitrogen is absorbed to the surface of the steel plate to the equilibrium concentration and diffuses into the steel plate. For example, if the holding time is sufficiently long, it is possible to diffuse nitrogen to the central part of the steel sheet and distribute it uniformly in the thickness direction. However, when considering the continuous atmosphere heat treatment such as BA line, the heat treatment time is only tens of seconds. In the present invention, in consideration of the manufacturing cost, nitrogen is diffused from the steel sheet surface to a predetermined depth within such a short time. For this reason, the holding time at the above temperature is set to 30 to 90 s.

Nitrogen concentration: 0.3 mass% or more
In order to obtain sufficient corrosion resistance, the concentration of nitrogen dissolved in the surface of the steel sheet is set to 0.3 mass% or more. By performing the heat treatment under the conditions as described above, a nitrogen-enriched layer having such a high nitrogen concentration and 10 μm or more from the surface can be obtained.

  In the present invention, in order to provide sufficient corrosion resistance and withstand mechanical damage such as scratches, the thickness of the nitrogen-enriched layer is set to 10 μm or more, but it is only necessary to obtain necessary corrosion resistance. The thickness may be set according to the purpose, and may be less than 10 μm.

  In the present invention, specifically, for example, ferritic stainless steel, austenitic / ferritic stainless steel, and austenitic stainless steel can be used. By appropriately changing the temperature range of the heat treatment depending on the steel type, high-concentration nitrogen can be efficiently diffused to a sufficient depth without forming nitrides.

  Hereinafter, a preferable composition component range and heat treatment temperature range in the case of using ferritic stainless steel, austenitic / ferritic stainless steel, and austenitic stainless steel will be described.

1 Ferritic stainless steel
1-1 Composition component range
(Cr: 24.0-26.0 mass%)
Cr is an essential element because it is an element that improves corrosion resistance and is also a ferrite-forming element. In order to sufficiently secure the corrosion resistance of the base material, 24 mass% or more is necessary. However, if it exceeds 26 mass%, an intermetallic compound such as a σ phase is formed, or Cr nitride is formed during nitrogen absorption treatment. Since it is easily formed, the corrosion resistance of the steel material is lowered. For this reason, Cr content shall be 24.0-26.0 mass%.

1-2 heat treatment temperature range (1079 ~ 1140 ℃)
The temperature range in which an austenite single phase is obtained is 1079 to 1140 ° C., and nitride (Cr nitride) is not generated in this temperature range, so that nitrogen can be dissolved efficiently. When the heat treatment temperature is less than 1079 ° C., Cr nitride is formed. On the other hand, when the heat treatment temperature exceeds 1140 ° C., a ferrite phase is formed, so that it is difficult to dissolve nitrogen at a high concentration and it is difficult to obtain a nitride-enriched layer. Therefore, the heat treatment temperature range is set to 1079 to 1140 ° C.

2 Austenitic ferritic stainless steel
2-1 Composition component range
(Cr: 24.0-26.0 mass%)
Cr is added for the same reason as described above. In order to ensure the corrosion resistance of the base material, 24 mass% or more is necessary. However, if it exceeds 26 mass%, an intermetallic compound such as a σ phase is formed, or Cr nitride is formed during nitrogen absorption treatment. Because it is easy, it reduces the corrosion resistance of steel. On the other hand, if the Cr content is less than 24 mass%, a sufficient amount of nitrogen cannot be dissolved during the nitrogen absorption treatment, so that the effect of improving the corrosion resistance is small. For this reason, Cr content shall be 24.0-26.0 mass%.

(Ni: 5.0-8.0 mass%)
Ni is an austenite forming element, and in order to maintain the austenite-ferrite two-phase structure of stainless steel containing a large amount of Cr and Mo, addition of 5.0 mass% or more is necessary. On the other hand, if added over 8.0 mass%, the ferrite phase decreases and it becomes difficult to maintain the austenite-ferrite two-phase structure, so the Ni content is set to 5.0 to 8.0 mass%.

(Mo: 3.0-4.0 mass%)
Mo is an element that improves corrosion resistance while being a ferrite-forming element. When the Mo content is less than 3.0 mass%, it is difficult to ensure the corrosion resistance of the base material. On the other hand, if it exceeds 4.0 mass%, it reacts with Cr during absorption of intermetallic compounds such as the σ phase and nitrogen, and forms nitrides, thus reducing the corrosion resistance. For this reason, Mo content shall be 3.0-4.0 mass%.

2-2 Heat treatment temperature (1193 to 1208 ° C)
In austenitic ferritic stainless steel, in order to change the two-phase structure to an austenite single-phase structure, it is necessary to perform heat treatment in a temperature range of 1193 to 1208 ° C. In this steel type, when the heat treatment temperature is lower than 1193 ° C., the dissolution rate of nitrogen is extremely slow, and it is difficult to obtain a nitrogen-enriched layer of 10 μm or more. Moreover, since the ferrite phase is formed when the heat treatment temperature exceeds 1208 ° C., the solubility of nitrogen decreases.

3 Austenitic stainless steel (1)
3-1 Composition component range
(Cr: 18.0 to 20.0 mass%)
As described above, Cr is an element that improves the corrosion resistance. In order to ensure the corrosion resistance of the base material, 18.0 mass% or more of Cr is required. However, if it exceeds 20.0 mass%, an intermetallic compound such as a σ phase is formed, so that the corrosion resistance is lowered. For this reason, Cr content shall be 18.0-20.0 mass%.

(Ni: 9.0 to 13.0 mass%)
Ni is an austenite forming element, and in order to maintain the austenite single phase structure of stainless steel, 9.0 mass% or more is necessary. However, if it exceeds 13.0 mass%, not only the cost increases, but also the solid solubility limit of nitrogen is lowered, so that the amount of nitrogen solid solution is lowered. For this reason, Ni content shall be 9.0-13.0 mass%.

3-2 Heat treatment temperature (1140-1210 ° C)
In the austenitic stainless steel (1), nitrogen absorption treatment is performed in a temperature range of 1140 to 1210 ° C. When the heat treatment temperature is less than 1140 ° C., the dissolution rate of nitrogen is extremely slow, and it is difficult to obtain a nitrogen-enriched layer of 10 μm or more. On the other hand, when it exceeds 1210 ° C., the crystal grains are likely to be coarsened.

4 Austenitic stainless steel (2)
4-1 Composition component range
(Cr: 16.0 to 18.0 mass%)
As described above, Cr is an element that improves the corrosion resistance. In order to ensure the corrosion resistance of the base material, in the case of simultaneous addition of Mo, the Cr content needs to be 16 mass% or more. However, if it exceeds 18.0 mass%, an intermetallic compound such as a σ phase is formed, so that the corrosion resistance decreases. For this reason, Cr content shall be 16.0-18.0 mass%.

(Ni: 12.0 to 15.0 mass%)
Ni is an austenite forming element, and in a stainless steel containing a large amount of Mo and Cr, which are ferrite forming elements, it is necessary to increase the content in order to maintain an austenite single phase structure. When the Ni content is less than 12.0 mass%, it is difficult to maintain the austenite single phase structure. On the other hand, when it exceeds 15.0 mass%, not only the cost increases, but also the solid solubility limit of nitrogen is lowered, so that the amount of nitrogen solid solution is lowered. For this reason, Ni content was 12.0-15.0 mass%.

(Mo: 2.0-3.0 mass%)
Mo is an element that improves corrosion resistance while being a ferrite-forming element. In order to obtain the effect, the Mo content is set to 2.0 to 3.0 mass%. If the Mo content is less than 2.0 mass%, it is difficult to ensure the corrosion resistance of the base material. On the other hand, if it exceeds 3.0 mass%, it becomes impossible to maintain an austenite single phase structure, and furthermore, since it reacts with Cr at the time of nitrogen absorption and intermetallic compounds such as σ phase, a nitride is formed, so that the corrosion resistance is lowered.

4-2 Heat treatment temperature (1139-1210 ° C)
In the austenitic stainless steel (2), the heat treatment temperature is set to 1139 to 1210 ° C. In this temperature range, since an austenite single-phase structure can be formed without forming nitrides, nitrogen can be effectively dissolved. When the heat treatment temperature is lower than 1139 ° C., Cr nitride is formed. On the other hand, if the heat treatment temperature exceeds 1210 ° C., a ferrite phase is formed, so that it is difficult to dissolve nitrogen in a high concentration and it is difficult to obtain a nitride-enriched layer.

  According to the present invention, high corrosion resistance can be imparted to a stainless steel plate by forming a high concentration of nitrogen in a short time without forming nitrides.

In Fe-25Cr ferritic stainless steel, it is a figure which shows the relationship between the temperature range in which the austenite single phase in a nitrogen partial pressure of 0.25 atmosphere is formed, and the nitrogen concentration which dissolves in steel. It is a figure which shows the cross-sectional structure | tissue of Fe-25Cr ferritic stainless steel processed for 60 s at 1130 degreeC in nitrogen partial pressure 0.25 atmosphere. It is a graph which shows the anodic polarization curve of Fe-25Cr ferritic stainless steel before and behind a nitrogen absorption process.

Next, the present invention will be described in more detail. First, ferritic stainless steel will be described with reference to FIG. Ferritic stainless steel is composed of Cr: 24.0 to 26.0 mass%, the balance Fe and inevitable impurities, and examples thereof include Fe-25Cr steel. FIG. 1 shows the temperature range in which an austenite single phase is formed and the concentration of nitrogen dissolved in the steel (that is, the solid solubility limit) when the nitrogen partial pressure is 0.25 atm in Fe-25Cr ferritic stainless steel. It is a figure which shows the relationship. The temperature range in which an austenite single phase is obtained is 1079 to 1140 ° C., and the concentration of nitrogen dissolved in the austenite phase when treated in a nitrogen atmosphere of 0.25 atm is expressed by formula (1). In this temperature region, since Cr nitride is not generated, nitrogen can be dissolved efficiently. At temperatures below 1079 ° C., Cr nitride is formed. On the other hand, when the temperature exceeds 1140 ° C., a ferrite phase is formed, so that the solubility of nitrogen is lowered and it is difficult to obtain a nitrogen-enriched layer.
mass% N = −0.000049 × T (° C.) + 6.672 (1)

The austenitic ferritic stainless steel consists of Cr: 24.0-26.0 mass%, Ni: 5.0-8.0 mass%, Mo: 3.0-4.0 mass%, the balance Fe and inevitable impurities, Examples thereof include SUS329J4L duplex stainless steel. In SUS329J4L, in order to change the two-phase structure to an austenite single-phase structure, it is necessary to perform heat treatment in an atmosphere gas containing nitrogen gas of 0.25 to 0.7 atm in a temperature range of 1193 to 1208 ° C. For example, the concentration of solute nitrogen when stainless steel consisting of Cr: 25 mass%, Ni: 6.0 mass%, Mo: 3.25 mass%, the balance Fe and unavoidable impurities is treated in a nitrogen atmosphere of 0.5 atm. It is represented by Formula (2). In this steel type, when the heat treatment temperature is lower than 1193 ° C., the dissolution rate of nitrogen is extremely slow, and it is difficult to obtain a nitrogen-enriched layer of 10 μm or more. Moreover, when it exceeds 1208 degreeC, a ferrite phase will be formed and the solubility of nitrogen will fall.
mass% N = −0.000036 × T (° C.) + 5.277 (2)

The austenitic stainless steel is composed of Cr: 18.0 to 20.0 mass%, Ni: 9.0 to 13.0 mass%, the balance Fe and inevitable impurities, and examples thereof include SUS304L. Specifically, in SUS304L composed of Cr: 19.0 mass%, Ni: 11.0 mass%, the balance Fe and unavoidable impurities, the nitrogen dissolved in the austenite phase when the nitrogen partial pressure is 0.5 atm. The concentration is expressed by equation (3). In this steel type, when the heat treatment temperature is less than 1140 ° C., the dissolution rate of nitrogen is extremely slow, and it is difficult to obtain a nitrogen-enriched layer of 10 μm or more. On the other hand, when it exceeds 1210 ° C., the crystal grains become coarse. For this reason, heat processing temperature shall be 1140-1210 degreeC.
mass% N = −0.00110 × T (° C.) + 1.715 (3)

Further, as other austenitic stainless steels, Cr: 16.0 to 18.0 mass%, Ni: 12.0 to 15.0 mass%, Mo: 2.0 to 3.0 mass%, the balance Fe and inevitable impurities For example, SUS316L. Specifically, in SUS316L austenitic stainless steel consisting of Cr: 17.0 mass%, Ni: 13.0 mass%, Mo: 2.5 mass%, the balance Fe and unavoidable impurities, the nitrogen partial pressure is 0.5 atm. In this case, the austenite single phase structure is maintained without forming nitrides in the temperature range of 1139 to 1210 ° C. The concentration of nitrogen dissolved in the austenite phase when treated in a nitrogen atmosphere of 0.5 atm is expressed by the formula (4).
mass% N = −0.0087 × T (° C.) + 1.4005 (4)

  As described above, the amount of nitrogen that can be dissolved in each steel type is determined by the temperature of the heat treatment and the nitrogen partial pressure. In an environment with appropriate temperature and partial pressure of nitrogen, nitrogen is absorbed to the surface of the steel plate to the equilibrium concentration and diffuses into the steel plate. If the treatment time is sufficiently long, nitrogen reaches the center of the steel plate, and therefore it can be uniformly distributed in the thickness direction. However, in terms of manufacturing cost, the heat treatment time is only tens of seconds. For this reason, in the present invention, the thickness of the nitrogen-enriched layer is set to 10 μm or more in order to provide a heat treatment holding time of 30 to 90 s and to provide sufficient corrosion resistance and to withstand mechanical damage such as scratches. . However, since it is only necessary to obtain necessary corrosion resistance, the thickness may be set according to the purpose, and may be less than 10 μm.

  Note that raising the heat treatment temperature is the most effective method for rapidly diffusing nitrogen. For this reason, even within the above temperature range, it is preferable to perform the treatment at a temperature as high as possible while suppressing the coarsening of crystal grains. Another method for rapidly diffusing nitrogen is to remove an oxide film such as a passive film on the surface of the steel sheet. This is because the surface oxide film prevents absorption of nitrogen and delays diffusion. For example, it is possible to promote diffusion of nitrogen into the steel sheet by mixing reducing hydrogen gas into the atmospheric gas and absorbing nitrogen while reducing the oxide film with hydrogen during heat treatment. In this case, the partial pressure of hydrogen gas is preferably 0.30 atm or more and 0.75 atm or less. Moreover, it is necessary to make a dew point low, and -30 degrees C or less is preferable. If it exceeds −30 ° C., the tendency to oxidize becomes strong and the ability to reduce hydrogen decreases.

  Hereinafter, an example of the present invention will be described. Stainless steel cold-rolled steel sheets of Fe-25Cr (ferrite type), SUS329J4L (austenite / ferrite two-phase type), and SUS304L and SUS316L (austenite type) having the composition components shown in Table 1 were prepared by a normal manufacturing method. A test piece of 20 mm × 30 mm × thickness (0.5 to 2.0) mm was taken from this cold rolled steel sheet, wet-polished to Emily No. 2000, degreased, and then subjected to nitrogen absorption under the conditions shown in Table 2. Treatment (heat treatment) was performed.

The conditions for the nitrogen absorption treatment are as shown in Table 2. First, an atmosphere furnace having a heating chamber and a cooling chamber was evacuated to 10 ⁻³ Pa, and the atmosphere gases shown in Table 2 were introduced into the atmosphere furnace. The flow rate of the atmospheric gas was kept constant at 1 liter / min, and the dew point was controlled to −45 ° C. or lower. Then, after reaching the temperature shown in Table 2, the test piece was put in and held for the time shown in Table 2, and then the test piece was moved from the heating chamber to the cooling chamber having the same atmospheric gas, and the austenite single phase was Rapid cooling was performed to maintain.

  The structure of each test piece obtained as described above was observed. Tissue observation was performed on the surface and cross-sectional structures using an optical microscope, a scanning electron microscope (SEM), and an electron beam backscatter diffraction method (EBSD). A part of the sample for observing the cross-sectional structure was produced by a cross-sectional ion processing apparatus (CP). The austenite phase formation was confirmed by this structure observation. An example is shown in FIG. FIG. 2 is a cross-sectional structure of Fe-25Cr ferritic stainless steel treated at 1130 ° C. for 60 s when the nitrogen gas in the atmosphere gas is 0.25 atm. From FIG. 2, it was confirmed that the Fe-25Cr steel whose surface before the heat treatment had a ferrite single phase structure had an austenite single phase structure formed on the surface after the heat treatment. Similarly, SUS329J4L steel, which had an austenite-ferrite dual phase structure, was also confirmed to have formed an austenite single phase structure after heat treatment.

  In addition, the crystal grain size and the presence or absence of nitride formation were evaluated by structure observation. In the present invention, the crystal grain size was judged as ○ when less than 50 μm, and as x when 50 μm or more. In addition, since nitride deteriorates the corrosion resistance, the case where the nitride was not generated is indicated as ◯, and the case where the nitride is generated is indicated as x. Further, the nitrogen concentration distribution in the depth direction from the surface of the test piece was measured by a high-frequency glow emission analyzer (GD-OES) to identify the nitrogen-enriched layer. Here, the nitride-enriched layer is a layer composed of an austenite single phase and having a nitrogen concentration of 0.3 mass% or more, and the thickness from the surface is 10 μm or more, and “less than 10 μm” is x. These results are shown in Table 3.

  Table 3 shows the effect of the heat treatment temperature on the evaluation items of each steel type. In each steel plate, when the heat treatment temperature was too low (1060 ° C.), the thickness of the nitrogen-enriched layer became insufficient or nitride was generated. On the other hand, if the heat treatment temperature is too high (1230 ° C. or 1220 ° C.), an austenite single phase is not formed and a ferrite phase is formed, so that nitrogen cannot be dissolved efficiently and nitrogen having a thickness of 10 μm or more. An enriched layer was not obtained. Moreover, since the heat treatment temperature was high, the crystal grains became coarse. From this, it was found that it is necessary to perform the heat treatment in a temperature range where an austenite single phase is obtained. That is, 1079 to 1140 ° C. for ferritic stainless steel (Fe-25Cr), 1193 to 1208 ° C. for austenitic ferritic stainless steel (SUS329J4L), 1140 to 1210 ° C. for austenitic stainless steel (SUS304L), and other austenitic stainless steels For steel (SUS316L), a temperature range of 1139-1210 ° C is suitable.

  In addition, Table 3 shows the effect of the heat treatment time on the evaluation items of each steel type. In each steel plate, when the heat treatment temperature was a suitable temperature (1130 ° C., 1150 ° C. or 1200 ° C.), if the heat treatment time was less than 30 s, a sufficiently thick nitrogen-enriched layer could not be obtained. On the other hand, when the heat treatment time exceeded 90 s, the crystal grains became coarse. From this, it was found that the heat treatment time is suitably between 30 and 90 seconds.

  Table 4 shows the thickness of the nitrogen-enriched layer of each test piece and its nitrogen concentration as an example. The sample No. 1 sample is Fe-25Cr (ferrite type) which has been subjected to nitrogen absorption treatment for 60 s at a nitrogen partial pressure of 0.25 atm and 1130 ° C., and the sample No. 2 sample is at a nitrogen partial pressure of 0.5 atm and 1200 ° C. SUS329J4L (two-phase system) subjected to nitrogen absorption treatment for 60 s. Sample No. 3 is SUS304L that has been subjected to nitrogen absorption treatment for 60 s at a partial pressure of nitrogen of 0.7 atm and 1150 ° C., and Sample No. 4 is nitrogen absorption for 60 s at a partial pressure of nitrogen of 0.5 atm and 1150 ° C. It is SUS316L (austenite type) which processed. Table 4 shows that in all steel types, the nitrogen-enriched layer is very thick, and the nitrogen-enriched layer contains more nitrogen than the base material.

  Next, the influence of the thickness of the steel sheet was examined. A stainless cold-rolled steel plate similar to the above was prepared, a 10 mm × 120 mm × thickness (0.2-3.0) mm test piece was collected, wet-polished to Emily No. 2000, and degreased, Nitrogen absorption treatment (heat treatment) was performed. The nitrogen absorption treatment was performed by placing a test piece horizontally on a table with a fulcrum interval of 100 mm, the nitrogen partial pressure in the atmosphere gas was 0.50 atm, and the heat treatment time was 60 s. After heat treatment, the deformation of each steel plate was observed and the nitrogen-enriched layer was measured. At this time, those without distortion were marked with ◯, and those with distortion were marked with X. Further, the thickness of the nitride-enriched layer was 10 μm or more, and the case of less than 10 μm was made x. These results are shown in Table 5.

  Table 5 shows the influence of the plate thickness. In each steel plate, when the plate thickness was less than 0.5 mm, distortion of the plate due to heat treatment was observed. On the other hand, when the plate thickness exceeded 2.0 mm, the thickness of the formed nitrogen-enriched layer was insufficient. This is considered to be because when the plate thickness is large, the rate of temperature increase becomes slow, and the soaking time at a predetermined temperature is shortened. From this, it was found that 0.3 to 2.0 mm is suitable for the thickness of each steel plate.

  Furthermore, in order to evaluate the corrosion resistance of each steel plate, a corrosion test was conducted. In the corrosion test, the critical pitting temperature (CPT) is obtained by performing anodic polarization in a 20 wt% NaCl aqueous solution and measuring the pitting corrosion potential of each test piece using the samples Nos. 1 to 4 described above. It was. The results are shown in Table 6. As an example, FIG. 3 shows anode polarization curves of Fe-25Cr ferritic stainless steel before and after nitrogen absorption treatment.

  Table 6 and FIG. 3 show the influence of nitrogen absorption treatment on the corrosion resistance. From Table 6 and FIG. 3, in Fe-25Cr steel, the critical pitting corrosion temperature (CPT) before nitrogen absorption was 5 ° C., and pitting corrosion occurred at this temperature, but after nitrogen absorption treatment, CPT was 85 ° C. Improved. In addition, CPT was improved in other types of test pieces. In Table 6, in particular, Fe-25Cr steel and SUS329J4L steel absorbed nitrogen more than SUS304L and SUS316L, so the improvement in corrosion resistance was most remarkable. From these things, it has confirmed that the corrosion resistance of a steel plate could be improved by nitrogen absorption treatment.

Claims (1)

In a furnace having an atmosphere in which the partial pressure of nitrogen is 0.7 atm and the balance is made of a reducing gas, in a temperature range of 1140 to 1210 ° C., the plate thickness is 0.5 to 2.0 mm, and Cr: 18.0 ~ 20.0 mass%, Ni: 9.0 to 13.0 mass%, the steel sheet consisting of the balance Fe and inevitable impurities is held for 30 to 90 seconds at a temperature equilibrated with the austenite single phase,
While forming an austenite single phase on the surface of the steel plate, 0.3 mass% or more of nitrogen was dissolved,
Cooling the steel sheet in a furnace while maintaining the gas atmosphere,
A method for modifying a surface of a stainless steel sheet, wherein the surface of the steel sheet is maintained in an austenite single phase in which nitrogen is dissolved.

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