JP5083857B2 - Covering member and manufacturing method thereof - Google Patents

Description

  The present invention relates to a covering member having a coating on the surface of a base material made of stainless steel and a method for producing the same, and more particularly to surface treatment of a base material made of austenitic stainless steel.

  Stainless steel is a low-carbon iron-based alloy containing about 12 mass% or more of Cr. Stainless steel exhibits excellent corrosion resistance because it stabilizes by forming a stable passive film on the surface. The alloy element deeply involved in the formation of the passive film is chromium (Cr), and when the Cr concentration exceeds 12 mass%, the corrosion resistance is rapidly improved, and it hardly corrodes depending on the environment. Among stainless steels, austenitic stainless steels are used in a very wide range of materials such as interior and exterior materials for buildings, vehicles, and various chemical plants because of their stable chemical and mechanical properties at high and low temperatures. However, even austenitic stainless steel corrodes depending on the environment in which it is used, and therefore further improvement in corrosion resistance is required. One example of a method for improving the corrosion resistance of stainless steel is to form a coating such as an amorphous carbon (diamond-like carbon: DLC) film having high corrosion resistance on the surface of stainless steel.

  When forming a film on the surface of a base material made of stainless steel, the base material is subjected to nitriding treatment in order to improve the adhesion between the base material and the film. For example, Patent Document 1 discloses a coating rod in which a base material made of austenitic stainless steel is rolled and then subjected to nitriding to form a DLC film on the surface thereof. However, when this rod is used in an environment where corrosion is likely to occur, the DLC film peels even in an environment where untreated austenitic stainless steel does not corrode. This is because corrosion occurred in the nitrided base material, and rust volume-expanded at the interface between the base material and the DLC film to lift the DLC film. That is, depending on the conditions for nitriding the substrate, the corrosion resistance of the austenitic stainless steel decreases.

Therefore, in Patent Document 2, in order to improve corrosion resistance, after austenitic stainless steel is subjected to solution heat treatment, the surface is ground, and carburizing treatment and nitriding treatment are performed in a state where the austenite structure appears on the surface to be treated. The surface structure of the austenitic stainless steel obtained in this way is disclosed. Further, Non-Patent Document 1 describes that as a condition for nitriding austenitic stainless steel to generate an “S phase” having excellent corrosion resistance on the surface, 450 ° C. or lower so that a Cr compound does not precipitate in the S phase. It is described that nitriding is performed at a low temperature.
JP 2004-344759 A JP 2005-272978 A Ichii, et al., “Low-temperature nitriding of austenitic stainless steel”, Surface Technology, 2003, Vol. 54, No. 3, p. 28-31

  The present inventors have paid attention to the processing-induced martensite generated when processing austenitic stainless steel. In Patent Document 1 and Patent Document 2, work-induced martensite is generated by rolling or grinding austenitic stainless steel. As will be described in detail later, the present inventors thought that the corrosion resistance of the stainless steel after nitriding treatment is reduced due to the presence of the martensite structure. Therefore, as described in Non-Patent Document 1, even if nitriding is performed at a low temperature of 450 ° C. or lower, depending on the state of the austenitic stainless steel to be nitrided, there is a problem that the corrosion resistance after nitriding treatment is lowered.

  In view of the above problems, the present invention provides a covering member having a base material that exhibits the same degree of corrosion resistance as a base material even after being subjected to nitriding after processing a base material made of stainless steel having an austenitic structure, and a method for manufacturing the covering member. The purpose is to provide.

The covering member of the present invention is formed by nitriding a surface layer portion of a processed material obtained by processing a base material made of stainless steel having an austenitic structure, and a reference electrode made of silver chloride in a 5 mass% sodium chloride aqueous solution kept at 40 ° C. a substrate showing the natural immersion potential of nobler than -200mV the natural immersion potential measurement measured using, e Bei and a coating formed on at least a portion of the surface of said surface layer portion, wherein the coating is amorphous It is characterized by being one or more of a carbon film, a chromium nitride film, a titanium nitride film, and a metal film .

The method for producing a coated member of the present invention includes a pretreatment step for reducing work-induced martensite from a work material generated by work-induced martensite by processing a base material made of stainless steel having an austenite structure, and the work material. and the nitriding treatment step of forming a nitride layer performs a nitriding treatment, and a coating formation step of forming a film on the surface of the nitride layer, Ri more name the film formation step, an amorphous carbon film, a chromium nitride film, It is a step of forming one or more of a titanium nitride film and a metal coating .

  In the manufacturing method of the covering member of the present invention, it is desirable that the pretreatment step is a step of removing a surface portion of the processed material generated by concentration of processing-induced martensite on the surface side. Alternatively, the pretreatment step is preferably a step of austenitizing the work-induced martensite by performing a solution heat treatment on the processed material.

Moreover, the manufacturing method of the coating | coated member of this invention is a processing process which makes the base material which consists of stainless steel which has an austenitic structure warm-working or hot-working at 30 degreeC or more higher temperature than stainless steel Md30, and making it into a processed material When the nitriding treatment step of forming a nitride layer performs a nitriding treatment on the workpiece, and the film formation step of forming a film on the surface of the nitride layer, Ri more name the film forming step, the amorphous carbon film And a step of forming one or more of a chromium nitride film, a titanium nitride film, and a metal film .

The covering member of the present invention is a highly corrosion-resistant covering member in which a coating is coated on the surface of a base material having a natural immersion potential measured no less than −200 mV measured under the above conditions. That is, even when the base material is nitrided after the base material is processed, the corrosion resistance is kept high. Since the covering member of the present invention has high corrosion resistance of the base material, peeling of the coating film accompanying corrosion of the base material is also reduced, and as a result, high corrosion resistance is maintained.

  The corrosion resistance of the base material is mainly influenced by the amount of martensite present in the surface layer portion (that is, the portion to be nitrided) of the processed material, as will be described in detail later. In the covering member of the present invention, the natural immersion potential of the substrate is used as an index indicating the amount of martensite present in the surface layer portion of the processed material.

  Moreover, according to the manufacturing method of the covering member of the present invention, before the nitriding treatment step, the processing-induced martensite is reduced from the processed material generated by the processing-induced martensite (pretreatment step) or has an austenite structure. A base material made of stainless steel is warm-worked or hot-worked at a temperature 30 ° C. higher than the stainless steel Md30 (machining process). By reducing the processing-induced martensite existing at the site where the nitrided layer is formed by the pretreatment process or the processing process, a decrease in the corrosion resistance of the base material after the nitriding process is suppressed.

  Hereinafter, the covering member of the present invention and the manufacturing method thereof will be described in detail.

  FIG. 1 is a cross-sectional view schematically showing a conventional base material obtained by nitriding a processed material obtained by processing a base material made of austenitic stainless steel. By processing a base material made of austenitic stainless steel, when stress that helps shear deformation acts on the base material, a part of the base material made of austenitic structure transforms into a martensite structure. (It is also known as “strain-induced transformation” or “machining-induced transformation”). When the workpiece is subjected to nitriding treatment, the surface layer portion of the workpiece is nitrided and the nitride layer 10 is formed, but the surface layer portion of the workpiece has martensite (work-induced martensite) induced by stress-induced transformation. . In the martensite structure, nitrogen atoms easily enter and the nitriding progresses quickly. Therefore, the nitrogen diffused in the surface layer portion is easily combined with chromium, which is an alloy element of stainless steel, and easily forms a composite compound 15 of chromium, nitrogen, and carbon. . Therefore, the periphery of the composite compound 15 becomes a low chromium layer 16 in which chromium is reduced. In the low chromium layer 16, the chromium concentration is lower than that of the stainless steel of the base material, and the portion where the chromium concentration is less than 12 mass% is no longer stainless steel, so that a stable passive film is not formed and corrosion proceeds. The starting point. As a result, the corrosion resistance of the base material 1 is lower than the corrosion resistance of the base material and the processed material.

  On the other hand, the covering member of the present invention includes a base material formed by nitriding a surface layer portion of a processed material obtained by processing a base material made of stainless steel having an austenitic structure, and the base material is 5 mass% chloride held at 40 ° C. A natural immersion potential measured from a reference electrode made of silver chloride in an aqueous sodium solution and a noble natural immersion potential from -200 mV is shown. That is, the base material constituting the covering member of the present invention exhibits excellent corrosion resistance. The reason why the base material exhibits a noble natural immersion potential of −200 mV is because there are few sites with low chromium concentration (low chromium layer) present in the surface layer portion of the base material. A portion with a low chromium concentration is less likely to be formed as the amount of martensite present in the surface layer portion of the workpiece to be nitrided is reduced. A substrate with a reduced amount of martensite present in the surface layer of the processed material exhibits a natural immersion potential nobler than -200 mV, preferably nobler than -150 mV, more preferably noble natural dip potential more than -100 mV. Indicates. In addition, the preparation method of a base material and the measuring method of natural immersion potential are mentioned later.

  The base material is made of stainless steel having an austenite structure. The stainless steel that forms the base material may be stainless steel that generates work-induced martensite by cold working, but because it wants to suppress the amount of work-induced martensite generated as much as possible, the phase stability of the austenite structure is low. High stainless steel is preferred. The phase stability of the austenite structure is expressed by, for example, Ms point or Md30. The Ms point is the temperature at which martensitic transformation starts, Md30 is an index for estimating the ease with which work-induced martensite is generated, and 50% of work-induced martensite is 0.3% of the true tensile strain 0.3. The resulting temperature. The Ms point and Md30 can be obtained from the composition of stainless steel using the following mathematical formulas.

  In the above formula, for example, “% C” is the ratio of carbon contained in the stainless steel when the stainless steel is 100 mass% (unit: mass%), and other elements are the same. It is. Here, three types of austenitic stainless steel plates (thickness 0.15 mm) A, B, and C having different compositions were subjected to solution heat treatment, and then processing induction of a workpiece subjected to rolling with a reduction in sheet thickness of 20% at room temperature. When the amount of martensite is measured, the results shown in Table 1 are obtained.

For the measurement of the amount of processing-induced martensite, the saturation magnetization of 100% processing-induced martensite of SUS304 is 150 emu / g, and the magnetization of the object to be measured (working material) with an applied magnetic field of 18.8 kOe (15 MA / m) is used. A method of estimating from a ratio with a value can be used. The saturation magnetization σs [emu / g] of the object to be measured is a vibration sample type magnetometer (VSM-3S-15 manufactured by Toei Kogyo Co., Ltd. in this measurement), and the applied magnetic field is 18.8 kOe (15 MA / m) at maximum. You can sweep and measure. The amount of martensite is calculated from the following equation.
Martensite amount [%] = (σs / 150) × 100
For example, if σs of the object to be measured is 30 emu / g, the martensite amount is 20%.

  Table 1 also shows the Ms point and Md30 values calculated using the above formula. As the values of Ms point and Md30 are smaller, the amount of processing-induced martensite generated is suppressed. That is, the base material is preferably austenitic stainless steel having an Ms point of 0 ° C. or lower, further −100 ° C. or lower, and an Md30 of 90 ° C. or lower, further 50 ° C. or lower. Specifically, austenitic stainless steel having a basic composition of 18% Cr-8% Ni standardized by JIS is preferable, and examples thereof include SUS301, SUS304, and SUS316.

  Cr is an alloy element related to the corrosion resistance of the substrate. The amount of Cr in the stainless steel constituting the base material depends on the amount of martensite structure produced in the processed material, but when the stainless steel is 100 mass%, it is 12 mass% or more, and further 15 mass% or more, 17 mass% or more. Is preferred. When the amount of Cr is in the above range, a substrate excellent in corrosion resistance showing a noble natural immersion potential from -200 mV can be obtained.

Covering member of the present invention is provided with the substrate, a coating formed on at least a portion of the surface of the surface layer portion of the substrate, the. The coating is applied to at least a part of the surface of the substrate, that is, the surface of the nitride layer formed by nitriding. The coating is preferably one or more of an amorphous carbon film, a chromium nitride film, a titanium nitride film, and a metal coating. The film may be an inorganic film or a metal film such as Cr or Ni, but is preferably a film having at least corrosion resistance. Particularly preferred are an amorphous carbon film (DLC film), a chromium nitride film, a titanium nitride film, and the like. With these coatings, the corrosion resistance of the substrate is maintained high. In particular, the DLC film is excellent in mechanical properties such as wear resistance and solid lubricity, and in addition to corrosion resistance, it also has insulation, visible / infrared light transmittance, oxygen barrier properties, etc., so the DLC film is coated. In addition to corrosion resistance, other characteristics are also imparted to the covering member of the present invention.

Although there is no particular limitation on the film thickness of the coating , for example, in the case of a DLC film, it is preferably 0.05 to 50 μm. If the film thickness of the DLC film is within this range, it exhibits excellent corrosion resistance and excellent adhesion to the substrate.

  The manufacturing method of the covering member of the present invention includes a pretreatment process or a processing process, a nitriding process, and a film forming process. The manufacturing method of the covering member of the present invention suppresses a decrease in corrosion resistance due to nitriding treatment by setting the processed material before nitriding the surface layer to a state in which there is little or no processing-induced martensite. Therefore, a pretreatment step for reducing the processing-induced martensite from the processing material generated by processing-induced martensite by processing a base material made of stainless steel having an austenitic structure, or a base material consisting of stainless steel having an austenitic structure Is processed at a temperature higher by 30 ° C. or higher than Md30 of stainless steel to obtain a processed material by hot working or hot working.

  The pretreatment step is a step of reducing the processing induced martensite from the processed material generated by the processing induced martensite by processing the base material. The base material may be processed by general processing methods such as plastic processing such as rolling and press processing, cutting processing, grinding processing, and shear processing, but the amount of processing-induced martensite generated depends on processing conditions. Also changes. For this reason, it is desirable to perform processing under conditions in which processing-induced martensite is difficult to be generated. In the obtained processed material, processing-induced martensite is generated, but is generally concentrated on the surface side of the processed material.

  As a method of reducing the processing-induced martensite from the processed material, there is a method of removing the surface portion of the processed material. For example, in the case of surface processing such as cutting, work-induced martensite is concentrated and generated on the surface side of the work material. Therefore, the surface portion of the work material is removed, so that nitriding is performed in a subsequent nitriding process. It is possible to reduce the processing-induced martensite in the surface layer portion. When removing the surface portion of the processed material, it is only necessary to remove several μm from the surface of the processed material, so that a large dimensional change does not occur in the base material. Further, only the surface portion needs to be removed, and since it is not necessary to process the entire processed material, deformation of the processed material is also suppressed.

  In order to remove the surface portion of the workpiece, the surface portion may be dissolved using an acid. Specifically, a method of dissolving and removing the surface portion by electrolytic dissolution or chemical dissolution can be mentioned. In particular, electrolytic polishing or chemical polishing is desirable because stress does not act on the workpiece due to polishing and processing-induced martensite is not generated. Further, the surface roughness can be kept small (10-point average roughness Rzjis is 0.1 to 3 μm). Electropolishing and chemical polishing may be performed under the conditions normally used for surface treatment of stainless steel.

  It is desirable that the surface portion removed in the pretreatment step is removed with a thickness of 0.1 to 20 μm, or further 1 to 20 μm. Further, as a method of reducing the work-induced martensite from the work material, a method of converting the work-induced martensite to austenite by subjecting the work material to a solution heat treatment may be used. In the solution heat treatment, for example, after being held at 950 to 1150 ° C. for 1 minute or longer, it may be rapidly cooled to a low temperature.

  In the manufacturing method of the covering member of the present invention, the pretreatment step is a processing step in which the base material is warm-worked or hot-worked at a temperature higher by 30 ° C. than Md30 of stainless steel to obtain a work material. You can also. In the processing step, the base material is processed under a temperature condition in which processing-induced martensite is difficult to be generated. Although the temperature of warm working or hot working depends on the alloy composition of the base material to be worked, it is desirable to work at a temperature higher by 30 ° C. or more by 60 ° C. than Md30. The processing method is not particularly limited, and examples thereof include plastic processing such as rolling processing and press processing, cutting processing, grinding processing, and shearing processing.

  The base material used in the pretreatment process and the processing process is as described above.

  The nitriding step is a step of forming a nitrided layer by nitriding the workpiece. In the processed material that has undergone the pretreatment step or the processing step, the austenite structure occupies most of the surface layer portion to be nitrided. Nitrogen atoms are dissolved in the austenite structure by nitriding, but the above complex compound is hardly formed. Therefore, the base material obtained by nitriding the processed material that has undergone the pretreatment process or the processing process is difficult to form a portion with a low Cr concentration and exhibits high corrosion resistance. Specifically, the base material has a natural soaking potential nobler than -200 mV as measured by a natural soaking potential measured using a reference electrode made of silver chloride in a 5 mass% sodium chloride aqueous solution kept at 40 ° C., It is desirable to show a noble natural immersion potential more than 150 mV and -100 mV.

The nitriding treatment step is preferably a nitriding treatment by an ion nitriding method, a gas nitriding method or a molten salt nitriding method, and any method can be used as long as it is performed under the conditions normally used for the surface treatment of stainless steel. Can also produce a desired substrate. For example, in the case of a gas nitriding method, nitrogen is dissolved on the surface of the workpiece even at a low processing temperature by introducing a corrosive gas that destroys the passive film together with ammonia gas. In the molten salt nitriding method, a molten salt containing nitride ions (N ³⁻ ) is used. At this time, an electrochemical nitriding treatment is performed on the surface of the stainless steel by using the molten salt as an electrolyte. Even if there is a dynamic film, a nitride layer can be easily formed on the surface layer of the non-treated material.

The nitriding treatment temperature is not particularly limited, but is preferably 350 to 600 ° C., more preferably 370 to 470 ° C. The nitriding depth is not particularly limited, but is preferably 0.1 to 200 μm, more preferably 1 to 50 μm. When the nitriding temperature and the nitriding depth are in the above ranges, the surface of the substrate can be made sufficiently hard, and it is desirable in terms of adhesion between the substrate and the coating .

The film forming step is a step of forming a film on the surface of the nitride layer. The coating can be formed by a known CVD method or PVD method such as a plasma CVD method, an ion plating method, or a sputtering method. However, as represented by the sputtering method, the PVD method is a directional film forming method. Therefore, in order to form a film uniformly by the PVD method, it is necessary to dispose a plurality of targets in the apparatus or to rotate a substrate on which the film is formed. As a result, the structure of the film forming apparatus becomes complicated and expensive. In addition, in the PVD method, it is not easy to form a film on an inner peripheral surface of a base material having a complicated shape, for example, a cylindrical base material.

  On the other hand, since the plasma CVD method forms a film with a reactive gas, it can be easily formed even in a complicated shape. Further, the structure of the film forming apparatus is simple and inexpensive. Examples of the plasma CVD method include a high-frequency plasma CVD method using high-frequency discharge and a direct-current plasma CVD method using direct-current discharge. In particular, the direct-current plasma CVD method is preferable because it can be performed by a film forming apparatus having a simple configuration including a vacuum furnace and a direct-current power source. In addition, if the nitriding process is a nitriding process using a plasma nitriding process, the nitriding process and the film forming process are performed using the same apparatus only by changing the type of gas introduced into the vacuum furnace. Can do.

For example, when an amorphous carbon film is formed by a direct-current plasma CVD method, first, a base material is placed in a vacuum vessel, and a reaction gas and a carrier gas are introduced. Then, plasma ions in the reaction gas may be attached to the substrate by discharge. If an amorphous carbon film (DLC-Si film) containing hydrocarbon gas such as methane (CH ₄ ) or acetylene (C ₂ H ₂ ) or silicon is formed as the reaction gas, TMS [Si ( A silicon compound gas such as CH ₃ ) ₄ ], SiH ₄ , SiCl ₄ , or SiH ₂ F ₄ may be used, and hydrogen gas, argon gas, or the like may be used as the carrier gas.

The film forming temperature is not particularly limited, but is preferably 600 ° C. or lower. The film formation time may be appropriately selected according to the desired film thickness.

  As mentioned above, although embodiment of the coating | coated member of this invention and its manufacturing method was described, the coating | coated member of this invention and its manufacturing method are not limited to the said embodiment. The covering member and the manufacturing method thereof according to the present invention can be implemented in various forms with modifications, improvements, etc. that can be made by those skilled in the art without departing from the gist of the present invention.

  Next, the present invention will be described more specifically with reference to examples.

[Production of substrate]
As a base material, a round bar base material (diameter 6.5 mm, length 60 mm, surface roughness Rzjis 1.5 μm) made of austenitic stainless steel SUS304 was prepared. The Ms point of SUS304 calculated from the above formula was −150 ° C., and Md30 was 30 ° C. On the surface of the round bar-shaped base material, cutting was performed under the condition that the shear deformation generated in the base material was suppressed as much as possible (processing step), and a # A0 base material was produced.

  Further, after performing the same processing as # A0, nitriding treatment was performed by a plasma nitriding treatment method described later (nitriding treatment step), and # A0-1 and # A0-2 substrates were produced. Note that nitriding was performed at 500 ° C. for # A0-1 and 400 ° C. for # A0-2.

  Further, after the same processing as # A0, a commercially available chemical polishing solution was used under predetermined conditions, and the surface portion from the outermost surface to 5 μm was removed (pretreatment step). Thereafter, nitriding treatment was performed by a plasma nitriding treatment method described later (nitriding treatment step) to obtain a base material of # A1-2. In addition, # A1-2 was nitrided at 400 ° C.

[Example 1]
A DLC-Si film was formed on the surface of the base material of # A1-2 by the plasma CVD method described later (film formation process), and the covering member of Example 1 was produced.

[Comparative Example 1]
A DLC-Si film was formed on the surface of the base material of # A0-1 by the plasma CVD method described later, and a covering member of Comparative Example 1 was produced.

[Comparative Example 2]
A DLC-Si film was formed on the surface of the base material of # A0-2 by the plasma CVD method described later, and a covering member of Comparative Example 2 was produced.

<Nitriding process and film formation process>
The nitriding treatment and the formation of the DLC-Si film were performed using a DC plasma CVD apparatus shown in FIG. As shown in FIG. 2, the direct-current plasma CVD apparatus 2 includes a stainless steel container 20, a base 21, a gas introduction pipe 22, and a gas outlet pipe 23. The gas introduction pipe 22 is connected to various gas cylinders (not shown) through valves (not shown). The gas outlet pipe 23 is connected to a rotary pump (not shown) and a diffusion pump (not shown) via a valve (not shown).

<Plasma nitriding treatment>
The workpiece 3 was placed on the base 21 installed in the container 20. Next, the container 20 was sealed, and the gas in the container 20 was exhausted by a rotary pump and a diffusion pump connected to the gas outlet pipe 23. Hydrogen gas was introduced into the container 20 through the gas introduction pipe 22 at 14 sccm (standard cc / min), and the gas pressure was set to about 133 Pa. Thereafter, a DC voltage of 200 V was applied between the stainless steel anode plate 24 and the base 21 provided inside the container 20 to start discharging. Then, the temperature was raised by ion bombardment until the surface temperature of the workpiece 3 reached the nitriding temperature. Next, nitrogen gas 500 sccm and hydrogen gas 40 sccm were introduced from the gas introduction tube 22, and plasma nitriding was performed at a pressure of about 425 Pa, a voltage of 300 V (current 1 to 2 A), 400 ° C. or 500 ° C. for 1 hour. When the cross-sectional structure of the obtained base material was observed, the nitriding depth was about 10 μm.

  After the plasma nitriding treatment, hydrogen gas and argon gas are introduced through the gas introduction tube 22 by 30 sccm at a pressure of about 500 Pa and a voltage of 250 V (current 1 to 2 A), and sputtering is performed at the same temperature as the plasma nitriding treatment. Fine irregularities were formed on the surface.

<DLC-Si film deposition>
When the DLC-Si film is formed on the surface of the base material following the plasma nitriding treatment, methane and TMS, which are raw material gases, are supplied from the gas introduction pipe 22, and each gas is supplied with methane: 50 sccm, TMS: 1 sccm, Hydrogen gas: 30 sccm, argon gas: introduced at a flow rate of 30 sccm, a pressure of about 400 Pa, a voltage of 300 V (current 1 to 2 A), a film formed for 40 minutes at the same temperature as the plasma nitriding treatment, and a DLC film thickness of about 3 μm A Si film was obtained.

[Evaluation of substrate]
The natural immersion potential and the corrosion rate were measured for the four types of substrates obtained by the above procedure. The measurement method will be described with reference to FIGS.

  The natural immersion potential measurement includes a sample electrode E1 produced using any of the above four types of base materials, a reference electrode E2 made of silver chloride, an electrometer (not shown), and a container C. A measuring device was used (FIG. 3). The container C was filled with a 5 mass% sodium chloride aqueous solution as the test liquid L, and the temperature of the test liquid L was set to 40 ° C., and then the sample electrode E1 and the reference electrode E2 were inserted. In this state, the potential difference ΔE (natural immersion potential) between the sample electrode E1 and the reference electrode E2 was measured with an electrometer. The results are shown in Table 2.

  The corrosion rate was measured by the polarization resistance method. For the measurement of the corrosion rate, a sample electrode E1 produced using any of the above four types of base materials, a reference electrode E2 made of silver chloride, a platinum electrode (counter electrode) E3, and an electrometer (not shown) And a measuring device comprising the container C (FIG. 4). The container C was filled with a 5 mass% sodium chloride aqueous solution as the test liquid L, and the temperature of the test liquid L was set to 40 ° C., and then the electrodes E1, E2, and E3 were inserted. A potential of the electric potential when polarized by flowing a minute current ΔI (adjusted so that ΔE does not exceed 10 mV. In this measurement, an arbitrary value of 0.1 to 10 μA) is passed between the sample electrode E1 and the platinum electrode E3. The amount of change was determined by measuring the potential difference ΔE between the sample electrode E1 and the reference electrode E2 with an electrometer. Since the corrosion rate is proportional to ΔI / ΔE, the relative corrosion rate was obtained by obtaining the ΔI / ΔE value for each substrate. The results are shown in Table 2. In Table 2, the corrosion rate is a relative value when the corrosion rate of the # A0 substrate is 1. In this measurement, a corrosion rate meter HK-103 manufactured by Hokuto Denko Corporation was used as the above measuring device.

  The base material of # A0, which is a state of a processed material that has been subjected only to cutting, showed a noble natural immersion potential at +30 mV. That is, the # A0 base material is excellent in corrosion resistance.

  Next, in the # A0-1 and # A0-2 base materials that were subjected to nitriding after cutting, the natural immersion potentials were -400 mV and -210 mV, respectively, and the corrosion rates were 70 times that of the # A0 base material. It was more than twice. That is, since the processing-induced martensite generated by the cutting process was present in the # A0 base material, the corrosion resistance was lowered by nitriding the base material. In addition, the # A0-2 base material having a low nitriding temperature of 400 ° C. showed a higher natural immersion potential than the # A0-1 base material nitrided at 500 ° C., but the processing-induced martensite Even if the base material containing slag was subjected to a low temperature treatment, it was not possible to suppress a decrease in corrosion resistance.

  The natural immersion potential of the # A1-2 base material whose surface was chemically polished before the nitriding treatment was +30 mV, and the corrosion rate was 0.6 (when the base material of # 0 was 1). That is, the # A1-2 base material showed no reduction in corrosion resistance after nitriding. This is because the portion where the work-induced martensite produced by cutting is concentrated and present is removed by chemically polishing the surface portion.

[Evaluation of coated member]
A salt spray test was performed on the covering members of Example 1 and Comparative Examples 1 and 2. The salt spray test was performed by placing a covering member in a test tank maintained at 40 ° C., and spraying a 5 mass% sodium chloride aqueous solution as a test liquid in the form of a mist into the test tank. The surface of the covering member 48 hours after the start of the test was visually observed to check for the occurrence of rust. The results are shown in Table 3. In Table 3, “◯” indicates that rust did not occur, and “x” indicates that rust occurred.

  The covering member of Example 1 was excellent in corrosion resistance without peeling of the DLC-Si film. On the other hand, in the covering members of Comparative Examples 1 and 2, rust was generated. This is because the test solution entered from a defect penetrating in the thickness direction of the DLC-Si film and corroded the surface of the substrate.

It is sectional drawing which shows typically the conventional base material obtained by nitriding the processing material which has a process induction martensite. It is the schematic of a direct-current plasma CVD apparatus. It is the schematic of the apparatus used for the measurement of the natural immersion potential of a base material. It is the schematic of the apparatus used for the measurement of the corrosion rate by a polarization resistance method.

Claims (13)

Natural immersion potential measured using a reference electrode made of silver chloride in a 5 mass% sodium chloride aqueous solution formed by nitriding the surface layer portion of a processed material obtained by processing a base material made of stainless steel having an austenitic structure. A substrate showing a natural immersion potential nobler than -200 mV by measurement,
A coating coated on at least a part of the surface of the surface layer portion;
Bei example, said coating is an amorphous carbon film, a chromium nitride film, the covering member, characterized in that at least one of titanium nitride film and a metal film.   The covering member according to claim 1 whose Ms point of said stainless steel is 0 ° C or less and Md30 is 90 ° C or less.   The covering member according to claim 1, wherein the base material exhibits a natural immersion potential nobler than −150 mV. A pretreatment step for reducing the processing-induced martensite from the processing material generated by the processing-induced martensite by processing a base material made of stainless steel having an austenitic structure;
Nitriding treatment step of nitriding the work material to form a nitride layer;
A film forming step of forming a film on the surface of the nitride layer;
Ri More Na, the film forming step, manufacture of the covering member, characterized in that a step of forming an amorphous carbon film, a chromium nitride film, one or more of titanium nitride film and a metal film Method. The said pre-processing process is a manufacturing method of the coating | coated member of Claim 4 which is a process of removing the surface part of the said processed material which the process induction martensite concentrated on the surface side and produced | generated. The method for manufacturing a covering member according to claim 5 , wherein the pretreatment step is a step of dissolving and removing the surface portion by electrolytic dissolution or chemical dissolution. The said pre-processing process is a manufacturing method of the coating | coated member of Claim 4 which is a process of austenitizing a process induction martensite by carrying out the solution heat treatment of the said processed material. A processing step in which a base material made of stainless steel having an austenite structure is warm-worked or hot-worked at a temperature 30 ° C. or higher than Md30 of stainless steel to obtain a work material;
Nitriding treatment step of nitriding the work material to form a nitride layer;
A film forming step of forming a film on the surface of the nitride layer;
Ri More Na, the film forming step, manufacture of the covering member, characterized in that a step of forming an amorphous carbon film, a chromium nitride film, one or more of titanium nitride film and a metal film Method. 5. The processed material after nitriding treatment exhibits a natural immersion potential nobler than −200 mV as measured by a natural immersion potential measured using a reference electrode made of silver chloride in a 5 mass% sodium chloride aqueous solution maintained at 40 ° C. 5. The manufacturing method of the coating | coated member in any one of -8 . The method for manufacturing a covering member according to claim 9 , wherein the natural immersion potential is nobler than −150 mV. The method for producing a covering member according to any one of claims 4 to 8 , wherein the stainless steel has an Ms point of 0 ° C or lower and an Md30 of 90 ° C or lower. The method for manufacturing a covering member according to any one of claims 4 to 8 , wherein the nitriding treatment step is a nitriding treatment by an ion nitriding method, a gas nitriding method, or a molten salt nitriding method. The said nitriding process process is a process of nitriding at 350-600 degreeC, The manufacturing method of the coating | coated member in any one of Claims 4-8 .

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