JP2008274386A - Surface-treated stainless steel excellent in design characteristics and corrosion resistance and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface-treated stainless steel whose corrosion resistance can be markedly improved by electropolishing its surface without deteriorating its design. <P>SOLUTION: The stainless steel excellent in design characteristics and corrosion resistance is obtained by conducting a surface treatment on the surface of an austenitic stainless steel from which scale has been removed after rolling and solution heat treatment. Its surface achieves a surface undulation having a roughness Ra of ≥1.8 μm formed by design characteristics improvement treatment and acquires a passive film having a markedly high Cr content compared to a base metal by an electropolishing treatment and a passivation treatment. The area of Cu-enriched regions observed in pitting parts after a pitting potential measurement test is 0.1 μm<SP>2</SP>per 1.0 cm<SP>2</SP>measured area. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a surface-treated stainless steel excellent in design and corrosion resistance and a method for producing the same.

Among stainless steels, austenitic stainless steels have excellent corrosion resistance that can almost prevent the occurrence of rust without relying on corrosion prevention by painting unless they are used in an atmosphere environment, especially in environments close to the coast. Have. In addition, it is used in many building materials and building structures because of its cleanliness and excellent design.

And when using such stainless steel for building materials etc., in order to improve the designability of building materials etc., giving unevenness to the surface of stainless steel intentionally is performed. That is, when unevenness is given to the surface, the reflection of light from the surface is affected, so that the brightness changes depending on the viewing angle. Compared to the case of a simple flat surface with few such changes, this change in brightness will make building materials stand out more three-dimensionally, giving viewers a sense of quality, and improving the evaluation of design properties such as building materials It is thought that it has.

However, when unevenness is imparted to the surface in order to improve the designability in this way, another problem occurs. Stainless steel is usually manufactured by rolling and performing shots, pickling, etc. to remove scale on the surface. Although the steel material in this state has irregularities on the surface, since it is white, it has poor metallic luster and poor design. Therefore, when using for building materials etc. which require designability, the process which removes a white surface and gives an unevenness | corrugation again by hairline grinding | polishing process etc. is generally performed. However, since such processing has a significant adverse effect on the corrosion resistance improvement effect of the passive film on the stainless steel surface, even if it does not occur almost completely, rust generation is completely suppressed. There is a problem that it is impossible.

Conventionally, in order to solve this problem of deterioration in corrosion resistance, after machining for imparting irregularities to the surface, pickling again (immersing in a nitric acid solution having a concentration of about 20%) or using a resin for the surface The covering is performed. However, even if the surface is pickled with unevenness, it is difficult to increase the corrosion resistance to the level obtained when pickling on a smooth surface, and only an insufficient effect can be obtained. . In addition, the method of covering with a urethane resin or the like is not a problem as long as it is relatively new, but the coating may be partially destroyed over time. In this case, rust is generated from the location where the coating is destroyed. However, there is a problem that the design is greatly deteriorated due to the influence of the destruction of the coating, and the repair becomes difficult.

In addition to the above-described method for improving corrosion resistance, a surface treatment technique called electropolishing is known as disclosed in Patent Document 1. Electropolishing is well known as a technique that can mirror the surface of a metal, and is a technique that is known to further improve the inherent corrosion resistance of stainless steel, particularly when applied to stainless steel.

That is, as shown in Non-Patent Document 1, when electropolishing is applied to stainless steel, a passive film having an increased Cr concentration compared to the Cr concentration of the base material is generated on the surface, and the corrosion resistance is greatly increased. It is to improve. For example, when electrolytic polishing is applied to the surface of SUS304, it has been confirmed that excellent corrosion resistance that can withstand even in coastal areas can be obtained.

Japanese Examined Patent Publication No. 54-42938 Surface Technology Vol. 41, No. 3, pp. 17-20 Published by Surface Technology Association

However, when electrolytic polishing as shown in Patent Document 1 is performed on a surface provided with unevenness for improving the design of stainless steel, there are the following problems.
In other words, the technique of electropolishing is characterized in that the surface is mirror-finished, and the surface after the treatment becomes a surface having no irregularities with a very small surface roughness, as will be described. In this case, even if unevenness is imparted to the folding surface to improve the design, the effect cannot be obtained at all, and the design cannot be improved even if the corrosion resistance is improved.

For such a problem, it is conceivable as a countermeasure to perform mild electropolishing so that surface irregularities remain to such an extent that the designability does not deteriorate. However, as a result of detailed investigations by the present inventors, it has been found that excellent corrosion resistance peculiar to electropolishing cannot be obtained even if a slight electropolishing treatment is performed after imparting irregularities to the surface.

The present invention was made in order to solve the above-mentioned problems, and is a novel surface-treated stainless steel that can sufficiently obtain the effect of improving the corrosion resistance originally obtained by electropolishing while ensuring excellent designability while leaving irregularities. And it aims at providing the manufacturing method.

The invention according to claim 1 of the present invention is a stainless steel obtained by rolling and performing surface treatment on austenitic stainless steel whose surface has been scale-removed after solution heat treatment. Ra 1.8μm or more by performing a design improvement process to give irregularities to the surface as needed without performing mechanical grinding and polishing finishing to increase the smoothness of the surface after descaling And the maximum Cr concentration is equal to or higher than the average Cr concentration of the substrate + 3σ ₁ (σ ₁ : standard deviation of the Cr concentration of the substrate) by the electrolytic polishing process and the passivation process. It has a passive film on its surface, and changes the potential from natural potential to noble side in 1.0 mol / liter saline solution at 30 ° C. for a surface of 1.0 cm ² until a current of 1000 μA flows. anode In pitting of when the pole, Cu concentrated region (Cu concentration is the average Cu concentration + 3 [sigma] ₂ or more matrix region; sigma _2: the standard deviation of the Cu concentration in the matrix) area is a 0.1 [mu] m ² or less It is a surface-treated stainless steel excellent in design and corrosion resistance.

The point of the present invention is that the surface of the austenitic stainless steel that has been subjected to rolling, solution heat treatment, and surface descaling treatment and the vicinity thereof have a circle-equivalent diameter that clearly has a higher Cu concentration than the substrate. In the presence of a Cu-enriched region having a size of several hundred nm to several tens of μm, for example, even after electrolytic polishing, the Cu-enriched region is still present. The pitting corrosion potential is low even if it is performed, and excellent corrosion resistance cannot be obtained. After removing this Cu-concentrated region and performing a mild electropolishing treatment and a passivation treatment, the surface has irregularities. It is in the point which discovered newly that it was able to ensure very excellent corrosion resistance, even if it was in the state left.

The points that have led to the completion of the present invention will be described below.
As a method of measuring the corrosion resistance of stainless steel, a method of measuring the pitting potential by anodic polarization is a well-known method. This pitting corrosion potential means a potential at which the passivation caused by the passive film existing on the stainless steel surface collapses at any point on the test surface. It can be judged that the higher the pitting potential is, the more passive film having higher resistance to oxidation is formed, and the higher the corrosion resistance is.

And the pit called pitting corrosion is formed in the test piece after a pitting corrosion potential measurement in the place where the passive state film collapsed. Therefore, the present inventors measured the pitting potential for the test pieces prepared by changing the conditions in various ways, and investigated the pits after the measurement in detail. As a result, a specimen with a low pitting potential and poor corrosion resistance must have a Cu-enriched region with a circle-equivalent diameter of about several hundred nm to several tens of μm (a number that does not have a clear boundary with the substrate). The presence of a net of about 10 μm or a particle of about several hundred nm in diameter with a clear boundary with the substrate) was confirmed in the pitting portion, whereas the pitting potential was In the pit of the test piece having a high corrosion resistance, the Cu concentrated region cannot be detected, and this Cu concentrated region is a material in which austenitic stainless steel is shot and pickled after solution heat treatment. It has been found that it exists in a concentration of several tens of μm from the surface. Since the Cu concentration region has a clearly higher Cu concentration than the substrate, the region where the Cu concentration is equal to or higher than the average Cu concentration of the substrate + 3σ ₂ (σ ₂ : standard deviation of the Cu concentration of the substrate). The Cu enrichment region is defined and described in claim 1. The shape of Cu concentrated region, the number may vary, limited in number are likely to obscure the claims, considered appropriate limitation of the area, the observation surface 1.0 cm ² around the area 0 .1 μm ² or less was defined as the claimed range of the surface-treated stainless steel according to the present invention (Claim 1).

Therefore, the present inventors have studied the efficient method for removing this Cu-concentrated region in order to improve the corrosion resistance, and have obtained the following knowledge to complete the present invention.
(1) The Cu-enriched region is present in the surface layer portion having a depth of several tens of μm from the surface. This is because, when the surface is oxidized and scale is generated, Fe is preferentially oxidized compared to Cu, so that a concentrated region of Cu is partially generated and enters the substrate surface by rolling. Estimated.

 (2) Since the Cu enriched region is concentrated on the surface or a position very close to the surface, it can be removed by electropolishing. However, if polishing is performed until the Cu-concentrated region is completely removed by electrolytic polishing, the corrosion resistance can be improved by performing a passivation treatment thereafter, but the surface irregularities are reduced and the design is greatly reduced. To do. Further, after the electrolytic polishing process is performed until the Cu-concentrated region is completely removed, it is possible to perform a surface irregularity imparting process for improving the design property. Therefore, it becomes a very disadvantageous condition in terms of productivity and cost. Therefore, as a result of further investigation, it has been found that when concentrated in an aqueous nitric acid solution that preferentially dissolves Cu and does not dissolve the substrate, the Cu concentrated region can be removed without losing the surface irregularities. .

 (3) The Cu concentrated region exists at a position close to the surface, but a part of the Cu concentrated region exists without being exposed on the surface. Even when immersed in an aqueous nitric acid solution in such a state, the Cu concentrated region cannot be completely removed. However, when the electrode is immersed in an acid such as an aqueous nitric acid solution after performing an electropolishing treatment for a short time so that the Cu-concentrated region is exposed on the surface, there is a problem without corroding stainless steel as a mother phase. In addition, the Cu-enriched region can be removed and the state in which the unevenness remains can be maintained, so that the design is not deteriorated. In addition, even when the unevenness imparting process is performed after removing the Cu-enriched region, the electrolytic polishing process, which is the pretreatment, can be processed in a short time, which is similarly disadvantageous in terms of cost and productivity. Nor.

 (4) When electropolishing and subsequent passivation treatment are performed without removing the Cu-enriched region, the corrosion resistance of the substrate adjacent to the partially existing Cu-enriched region is other than Since only the passive film having a reduced value compared with the region is produced, the measurement of the pitting corrosion potential causes the passive film in the region having such a low corrosion resistance to collapse preferentially, and only a low pitting potential is obtained. And a Cu-enriched region remaining in the pitting portion is observed. In other words, when actually used, it is impossible to prevent the occurrence of rust from such a weak point. On the other hand, when the electropolishing treatment and the passivation treatment are performed after removing the Cu-enriched region, since there is no portion that can be said to be such a weak point, an extremely high pitting corrosion potential is obtained, and unevenness is obtained. In spite of the adverse effects due to the presence of, excellent corrosion resistance can be ensured.

Next, each component of the surface-treated stainless steel according to claim 1 will be described in detail.
First, a steel material to be used is an austenitic stainless steel such as SUS304, and a product having a desired shape and size is manufactured by hot rolling or the like, followed by solution heat treatment. Usually, in the solution heat treatment state, scales are generated on the surface by heating during rolling and heat treatment, and this is removed by shot, pickling or the like. Up to this point, there is no difference from the conventional method.

  In this state, unevenness by shot processing has already been generated on the surface. If you want to change the appearance of irregularities to improve the design, in addition to the steel material obtained in this way, a process to give different designs by intentionally imparting irregularities to the surface (Claim 4).

Here, the case of performing mechanical grinding and polishing to improve the smoothness of the surface is excluded when grinding and polishing to improve the surface smoothness are performed. Since the Cu concentrated region is also removed simultaneously by the processing, there is no possibility that the corrosion resistance of the passive film is lowered due to the presence of the Cu concentrated region. Since it is difficult to apply the present invention to the above, it is excluded from the scope of the invention.

Since the reflection of light changes due to the presence of the unevenness, the brightness changes depending on the viewing angle, thereby improving the design of the product and enhancing the luxury. In addition, as a process which provides an unevenness | corrugation, it can carry out by methods, such as the finishing process by shots, such as a steel shot and a glass bead, the cold rolling using the rolling roll which can provide an unevenness | corrugation intentionally. As a result, the surface is processed so that a surface with a roughness of Ra of 1.8 μm or more remains even after the second electrolytic polishing process or passivation process described later is performed. Further, the surface treatment for providing the unevenness may be either before or after the copper removal treatment. This is because it is possible to achieve both excellent design and corrosion resistance as long as the unevenness is processed before the film is formed by the second electropolishing treatment and passivation treatment described later.

  In addition, the lower limit of the surface roughness Ra is set to 1.8 μm or more, as a matter of course, when the roughness is smaller than this, it is not possible to ensure excellent design properties. This is to distinguish clearly. That is, in the prior art, the presence of the Cu-concentrated region that causes a decrease in corrosion resistance and the means for efficiently removing it are not clarified, so that excellent corrosion resistance cannot be obtained if the unevenness remains. Considering that it was difficult to make a product excellent in design and corrosion resistance at the same time, because the unevenness was almost lost (that is, Ra is less than 1.8 μm) when trying to improve the corrosion resistance by sufficient electropolishing. The present invention is limited in order to clarify that excellent corrosion resistance can be secured with the unevenness remaining. The surface roughness Ra is more preferably 3.0 μm or more in order to ensure high design properties.

However, it is possible to process the unevenness after improving the corrosion resistance by electrolytic polishing, but it is inefficient and extremely costly to reduce the roughness once and reversely apply the unevenness again. In addition to being a high process, the passivation film is destroyed by the unevenness processing and the corrosion resistance is impaired, so that it is not normally selected.

Next, the surface of the produced stainless steel must have a passive film whose maximum Cr concentration is equal to or higher than the Cr concentration of the substrate + 3σ ₁ (σ ₁ : standard deviation of the Cr concentration of the substrate). More specifically, in the stainless steel according to claim 2, the Cr concentration of the substrate is 16.0 to 20.0% by mass%, and about 17 to 22 atom% (σ ₁ is about ₁ % by atomic%). On the other hand, the Cr concentration in the coating is preferably 25 atomic% or more, and Cr / Fe is preferably 0.8 or more in terms of atomic ratio. The O content is about 30 to 60 atomic%, and the thickness is very thin and about 6 nm or less. It is a well-known technique that such a coating film with a high Cr concentration is generated by the electropolishing process. However, in the present invention, as already described, the electropolishing process and the passivation are performed after removing the Cu-enriched region. Since it is generated by the treatment, it has a feature that it is very strong in terms of corrosion resistance as compared with the coating formed without considering the existence of the Cu concentrated region.

Here, “the anodic polarization until the potential is changed from the natural potential to the noble side at 30 ° C. and 1 mol / liter of saline and a current of 1000 μA flows” is a test method for pitting potential measurement that is normally performed. Is. However, if the test piece preparation procedure in the pitting corrosion potential measuring method specified in JISG0577 is carried out, the surface state changes greatly. Therefore, the preparation procedure of the test piece specified in JIS such as polishing is not executed, It is necessary to measure the pitting potential in a state where the treated test piece is masked leaving a test area of 1.0 cm ² . This measurement method is the same for all the examples described later (including conventional examples).

When the pitting corrosion potential is measured, the pitting corrosion occurs at the weakest and weakest corrosion resistance among the passive films formed on the surface. Therefore, if the Cu-enriched region is present in the passive film, the substrate will naturally pitting near the periphery of the Cu-enriched region. A part with a high Cu content can be detected in the part. Since the Cu concentration in this Cu-concentrated region is so high that the value does not overlap with the substrate, the Cu-concentrated region having the above-described definition is observed in the surface-treated stainless steel of the present invention. The area is limited to 0.1 μm ² or less per 1.0 cm ² surface.

Here, the lower limit of 0.1 μm ² is not described in the sense that the Cu concentration region below this is positively allowed. As shown in the examples described later, since the Cu-concentrated region exceeding 0.1 μm ² was observed for all test pieces with insufficient corrosion resistance and poor Cu-concentrated region removal treatment, this is expressed in words. Therefore, it is limited in this way. When the copper removal treatment is sufficiently performed, the Cu concentrated region is not detected as shown in the examples described later, and the feature of the present invention is based on the complete removal of the Cu concentrated region. This is as already explained.

Next, the stainless steel targeted by the present invention, as described in claim 2, in mass%, C: 0.08% or less, Si: 1.00% or less, Mn: 2.50% or less. P: 0.045% or less, S: 0.030% or less, Ni: 7.0-15.0%, Cr: 16.0-20.0%, Mo: 3.0% or less, Austenitic stainless steel comprising the balance Fe and impurity elements, or, if necessary, Cu: 4.0% or less, N: 0.30% or less, Nb: 0.15% or less, one or more A great effect can be obtained by applying the method of the present invention to steel containing.

That is, the level of corrosion resistance obtained by the method according to the present invention is almost the same level as that of a duplex stainless steel such as SUS329J1, which is known to have the highest corrosion resistance among stainless steels (at a pitting potential of 800 mV vs SCE or more). There is little point in applying the present invention to duplex stainless steels that are originally excellent in corrosion resistance. Conversely, even if the present invention is applied to ferritic and martensitic stainless steels, which are cheaper than austenitic stainless steels but have inferior corrosion resistance, the material has a corrosion reaction only in the state of being immersed in the electrolytic solution. Therefore, sufficient corrosion resistance improvement effect cannot be obtained. Therefore, the present invention is suitable to be applied only to austenitic stainless steel, and more preferably applied to steel having the above-described components. To explain specifically by steel type name, it goes without saying that SUS304 is included, but other than SUS304L, SUS304N1, SUS304N2, SUS304LN, SUS304J3, SUSXM7, SUS316, SUS316L, SUS316N and the like are included in the specified component range. It is.

In addition, since an above described component is a well-known component also including the range of JIS steel, it is thought that it is not necessary to explain the reason for a component limitation in particular, The description is abbreviate | omitted.

Next, the manufacturing method of the surface-treated stainless steel according to the present invention described in claim 3 is the surface treatment of austenitic stainless steel that has been rolled, subjected to solution heat treatment, and descaled by shot, pickling, or the like. A first electropolishing treatment step for exposing a Cu-enriched region to the surface; a copper removal treatment step for removing the Cu-enriched region on the surface; a second electropolishing treatment and passivation. It comprises a passive film production step by treatment.

  In the method for producing a surface-treated stainless steel according to the present invention, austenitic stainless steel is used as a target, and after manufacturing into a predetermined shape by hot rolling or the like, a solution heat treatment is performed, and the scale generated on the surface is shot, pickled, etc. The steel material removed by (1) is used.

  The target steel material is exactly the same as that of claim 2 already described, and as described in claim 5, in mass%, C: 0.08% or less, Si: 1.00% or less, Mn: 2. 50% or less, P: 0.045% or less, S: 0.030% or less, Ni: 7.0-15.0%, Cr: 16.0-20.0%, Mo: 3.0% In addition, it contains one or more of Cu: 4.0% or less, N: 0.30% or less, Nb: 0.15% or less as necessary, and the balance from Fe and impurity elements A great effect can be obtained by using the austenitic stainless steel.

  In the present invention, in order to ensure a design property that gives a high-class feeling to a person by the above-described method, the steel material is subjected to a design property improving treatment for imparting irregularities to the surface as necessary (Claim 4). . However, even with shots for removing scales, it is possible to give irregularities so that Ra is 1.8 μm or more depending on the shot conditions. There is no need to process. However, in order to obtain excellent design properties, it is desirable that Ra is 3.0 μm or more, and even when a surface having a roughness of 1.8 μm or more can be secured by a shot for scale removal, Furthermore, it is of course possible to perform a design improvement process step.

And in this invention, the 1st electrolytic polishing process is performed with respect to the prepared steel materials. However, the electropolishing treatment here does not completely remove the Cu concentrated region, but the Cu concentrated region can be completely removed by the copper removal treatment in the subsequent step. The purpose is to expose the surface, and a very short treatment time is sufficient. On the other hand, if the process is continued more than necessary, if unevenness has already been provided in the previous process, the unevenness will be reduced, and the design and luxury will be lost. is required.

  This first electropolishing treatment step allows the Cu enriched region to be exposed on the surface, so that Cu enrichment can be easily performed by performing a copper removal treatment such as immersion in a nitric acid aqueous solution in this state. Regions can be removed. The copper removal treatment may be any solution as long as it can dissolve only the Cu-concentrated region in a short time without corroding the base portion other than the Cu-concentrated portion, for example, a concentration of 20% or more. The treatment is possible by using an aqueous nitric acid solution (claim 6). Here, the reason why the lower limit is set to 20% is that if the concentration is lower than this, it may take time for the copper removal treatment, or sufficient copper removal treatment may not be possible.

  In addition, after the design improvement process which provides an unevenness | corrugation, you may implement a 1st electrolytic polishing process and a copper removal process, and the designability which provides an unevenness | corrugation after a 1st electrolytic polishing process and a copper removal treatment process. Improvement processing may be performed. This is because, as long as the copper removal treatment is completed before the second electrolytic polishing treatment step, which is a subsequent step, excellent corrosion resistance can be secured in the end.

  Then, after the surface irregularity imparting and the copper removal treatment for improving the design properties are completed, a second electrolytic polishing treatment and a passivation treatment are further performed. The purpose of this treatment is different from the first electropolishing treatment, and is a treatment performed to form a passive film having excellent corrosion resistance by forming a film having a high Cr concentration on the surface. At this time, since the Cu-concentrated region that adversely affects the corrosion resistance of the coating has already been removed in the previous step, the coating does not weaken locally, and a very excellent passive coating over the entire surface. Can be formed. In addition, since this step is not intended for polishing the surface and it is necessary to leave the unevenness in order not to deteriorate the designability, the process can be carried out in a short time as in the first polishing process. It needs to be terminated. Here, the passivation treatment can be performed by, for example, a method of immersing in about 5 to 10% nitric acid aqueous solution or about 14% phosphoric acid aqueous solution.

Next, the effect obtained by the present invention will be clarified by examples.
SUS304 (0.05% C-0.46% Si-% by mass) was hot-rolled as a test material used as an example, subjected to solution heat treatment, shot for scale removal, and pickling finish. 0.99% Mn-0.031% P-0.002% S-0.24% Cu-8.08% Ni-18.15% Cr-0.19% Mo-0.08% N) did.

This test material was subjected to surface treatment by changing the processing conditions for imparting surface irregularities, copper removal treatment, electrolytic polishing treatment time before and after copper removal treatment, etc., and maximum Cr in the passive film after the surface treatment. The concentration was measured and the pitting potential was measured after the treatment to evaluate the influence of the treatment conditions on the corrosion resistance. Moreover, about each test piece, while confirming the presence or absence of the Cu concentration area | region in the pitting corrosion part after pitting corrosion potential measurement, and measuring the surface roughness, it implemented also about design evaluation.

Regarding the electrolytic polishing treatment, the copper removal treatment, and the passivation treatment, those subjected to the treatment were evaluated under common conditions. However, the current density in the electropolishing treatment was evaluated by changing the numerical value because the electrolyte solution deteriorates faster if it is too high. The conditions are shown in Tables 1 and 2. Specific conditions of each test piece including the current density are summarized in Table 4 described later. The DS shown in Table 4 means that the steel sheet is hot-rolled, subjected to a solution heat treatment, shot and pickled, and is not subjected to any additional unevenness processing. It means a state.

Regarding the processing of irregularities on the surface, shots, pickling treatments, steel shots and glass bead shot treatments to change the appearance of irregularities, and hairline polishing finishes were evaluated, respectively. . The steel shot used shot particles with a particle size of 0.4 mm, and the glass bead shot used shot particles of low alkali borosilicate glass with a particle size of 0.5 mm.

Next, evaluation conditions will be described.
The corrosion resistance was evaluated by measuring the pitting potential as described above. The pitting corrosion potential is measured by immersing the surface-treated test piece in a 1 mol / liter saline solution at 30 ° C. so that the test area is 1.0 cm ^2, and the potential of the test piece is naturally reduced by a potentiostat. The voltage was gradually changed from the potential to the noble side, and the anodic polarization curve was measured. As described above, the work such as polishing as defined by JIS is not performed when preparing a test specimen for measurement. Then, the current density from the anodic polarization curve is anodically polarized until 1000μA / cm ^2, the pitting potential was measured by determining the potential of when it becomes 100 .mu.A / cm ^2, Table 4, the average value showed that. The potential was shown based on a saturated calomel electrode at 30 ° C. At the same time, three or more tests were performed under the same conditions in order to evaluate the relationship between the presence of the Cu enriched region and the obtained corrosion resistance. Then, in order to accurately investigate whether or not there is a Cu-enriched region in the pitting portion after the pitting corrosion potential measurement, the position of the pitting portion is confirmed by SEM (scanning electron microscope), and the CuMA is detected by EPMA analysis. An investigation was conducted to determine whether or not there is a portion in which the concentration of Cu was increased by simultaneously measuring the Cu concentration of the substrate and its variation (standard deviation). Table 4 shows whether or not a Cu-concentrated region can be confirmed in the pitting portion, and Table 5 shows the average Cu concentration and area of the test piece in which the Cu-concentrated region can be confirmed.

Next, a design property evaluation method will be described.
In the evaluation of the design property, the judgment varies depending on the person, and it is difficult to measure it as some value with a measuring instrument. However, it is a fact that the change in the reflection of light due to the unevenness of the surface affects the design properties, and it has been confirmed that there is a deep correlation with the surface roughness. Therefore, the surface roughness (arithmetic mean roughness Ra) of each test piece is measured under the conditions shown in Table 3, and 10 persons are selected for the evaluation of the design properties, and the five-stage evaluation (the maximum is 5). In Table 4, the average evaluation score of 10 people is 4 points or more, ◎, 3 points or more and less than 4 points, and less than 3 points ×.

  Among the evaluation results shown in Table 4, Test Nos. 1 to 4 are examples that satisfy all the conditions of the present invention described above, and 5 to 12 are those in which the copper removal treatment is untreated, No. 1 is a comparative example that does not satisfy the conditions of the present invention in that the electrolytic polishing treatment is not performed, and 13 to 15 are evaluations of ordinary SUS304 that is not subjected to electrolytic polishing treatment or passivation treatment for improving corrosion resistance The results are shown.

Among the comparative examples, Nos. 7 and 10 have no confirmed Cu enriched region in the pitting portion, and the pitting potential is a high value of about 950 mV, and excellent results are obtained with respect to corrosion resistance. ing. However, in the case of 7, as a result of electropolishing treatment as long as 80 minutes at a current density as high as 20 A / dm ² , the Cu concentrated region is removed and the corrosion resistance is improved. As a result, the unevenness imparted to the surface is considerably removed and the surface becomes close to a smooth surface, and the design properties are greatly reduced. In the case of 10 as well, the Cu-enriched region was removed by # 150 hairline polishing, and the corrosion resistance was excellent, but the surface roughness was extremely smooth at 0.46 μm, and the design was greatly reduced. It is.

  In the comparative examples except 7 and 10, no copper removal treatment was performed except for 8, and the electropolishing treatment was not performed until the Cu enriched region on the surface was removed, so that a high pitting potential could be obtained. It was not possible, and a Cu concentration area | region was confirmed in the pitting corrosion part. The detected average Cu concentration in the Cu-enriched region was much higher than the Cu concentration in the substrate, even when the variation (standard deviation) in the Cu concentration in the substrate was taken into account. In these comparative examples, since the surface roughness is not so lowered, there is no problem with the design property, but the corrosion resistance is significantly inferior.

For test No. 8, although copper removal treatment has been performed, the first electropolishing treatment step necessary as the preceding step has not been carried out, so that a part of the Cu concentrated region is not exposed on the surface. By performing the copper removal treatment in the state, as shown in Tables 4 and 5, the removal of the Cu concentrated region was incomplete, and excellent corrosion resistance was not obtained.

Moreover, since test No. 13-15 which is a prior art example is not electropolishing and passivating, the surface is formed with a passivated film in the normal shot and pickling process for 13. Nos. 14 and 15 have only a passive film formed by natural oxidation, and this film is very fragile, so that the corrosion resistance is greatly inferior.

  In contrast to these comparative examples, in Nos. 1 to 4 which are the examples of the present invention, a light electrolytic polishing treatment is performed in advance to expose the Cu-concentrated region existing near the surface and the surface, and then remove. Since copper treatment is performed, Cu that adversely affects corrosion resistance is reliably removed. Also, the first electropolishing treatment step before the copper removal treatment and the second electropolishing treatment after the copper removal treatment are performed only for the purpose of improving the corrosion resistance, unlike the usual purpose of the electropolishing. Even if the surface unevenness processing performed for improving the design property is completed, the purpose can be sufficiently achieved. Therefore, as is apparent from the results in Table 4, the surface-treated stainless steel of the present invention can easily obtain a stainless steel excellent in design and corrosion resistance, and can satisfy only one of the conventional performances. As compared with the case where no is obtained, a remarkable effect can be obtained.

Moreover, in order to confirm the grounds that the present invention exhibits excellent corrosion resistance, the composition of the surface coating formed after the passivation treatment was analyzed by XPS analysis (X-ray photoelectron spectroscopy). The results of Example 4 are shown in FIG. From this figure, when the average Cr concentration and standard deviation of the substrate are calculated with a distance of 6 nm or more from the surface as the substrate, the average Cr concentration is 20.9 atomic percent and the standard deviation is 0.7 atomic percent, and the maximum Cr of the surface portion The concentration is 29.6 atomic%. Therefore, in the surface portion, the Cr and O concentrations are increased, the Fe concentration is decreased, and the maximum Cr concentration is equal to or higher than the average Cr concentration of the substrate + 3σ ₁ (σ ₁ : standard deviation of the Cr concentration of the substrate). It was confirmed that a passive film was formed. In addition, the same result was obtained for the test piece of the present invention for the test piece not shown here. As described above, it is a well-known fact that such a passivation film can be formed by the electropolishing treatment and the subsequent passivation treatment.

Here, it is described in FIG. 1 that the C concentration is high and the Cr concentration is reduced from the surface to around 1 nm. It is generally detected when XPS analysis is performed on a test piece that has been placed and stored. Since the contamination layer does not indicate the composition state of the passive film on the surface, it can be ignored.

Next, an example of the result of observing the pitting portion after the pitting potential measurement is shown. FIG. 2 is an SEM photograph of the pitting portion of test No. 9 as a comparative example. Then, FIG. 3 shows the result of EPMA analysis at the analysis site A on this photograph (the burr-shaped site at the edge of the pitting portion). In the case of the test piece No. 9 that has not been subjected to the first electrolytic polishing treatment and the subsequent copper removal treatment and has not been subjected to the electrolytic polishing treatment for such a long time that the Cu concentrated region is removed. EPMA analysis detected a Cu-enriched region containing Cu equal to or higher than the average Cu concentration of the substrate + 3σ ₂ (σ ₂ : standard deviation of the Cu concentration of the substrate) (in SUS304, which is a material used as an example) Is only contained as an impurity, and such a peak cannot be confirmed in places other than the Cu-concentrated region in the pitting portion.) On the other hand, although the same figure is not shown about the test piece of this invention, the peak of Cu is not confirmed in EPMA analysis, but the presence or absence of Cu concentration area | region has a big influence on the corrosion resistance. This result could be confirmed.

Furthermore, Test Nos. 3 and 4 were conducted in order to confirm the effect when the current density during the electropolishing treatment was lowered, but it was necessary as compared with the case where the current density was high. Although there is a point that the processing time of the electrolytic polishing process becomes long, it has been confirmed that the same effect as that obtained when the current density is high can be obtained.

As described above, in the surface-treated stainless steel of the present invention, after performing a mild electropolishing treatment, a copper removal treatment is performed, and after removing a Cu-concentrated region that causes a decrease in the corrosion resistance of the passive film, A coating with extremely excellent corrosion resistance is formed by electrolytic polishing treatment and passivation treatment, so that it is possible to obtain corrosion resistance comparable to that of duplex stainless steel despite being an austenitic stainless steel, close to the coast Even if it is used in an environment, it is possible to obtain an excellent effect that the generation of rust can be suppressed for a long period of time.

The figure which shows the XPS analysis result of the surface of test No. 4 which is an Example of this invention Scanning electron micrograph of pitting portion after measurement of pitting corrosion potential of test No. 9, which is a comparative example EPMA analysis result of pitting portion after measurement of pitting corrosion potential of test No. 9 which is a comparative example

Claims (6)

A stainless steel that has been subjected to surface treatment on austenitic stainless steel that has been rolled and subjected to solution heat treatment, and whose surface has been descaled, in order to increase the surface smoothness after descaling. It has surface undulations with a roughness of Ra1.8μm or more by applying a design improvement process that gives irregularities to the surface as needed without mechanical grinding and polishing being performed. In addition, the electrolytic polishing process and the passivation process have a passive film on the surface whose maximum Cr concentration is equal to or higher than the average Cr concentration of the substrate + 3σ ₁ (σ ₁ : standard deviation of the Cr concentration of the substrate). targeting surface of .0Cm ^2, at 30 ° C. the potential from the natural potential saline 1.0 mol / l is changed to noble side, the pitting of when the anodic polarization up to a current flows in 1000Myuei, Cu Region (Cu concentration is the average Cu concentration + 3 [sigma] ₂ or more matrix region; sigma _2: the standard deviation of the Cu concentration in the matrix) design of the area is equal to or is 0.1 [mu] m ² or less, excellent corrosion resistance Surface treated stainless steel. The chemical composition of the stainless steel used is C: 0.08% or less, Si: 1.00% or less, Mn: 2.50% or less, P: 0.045% or less, and S: 0.00% in mass%. 030% or less, Ni: 7.0 to 15.0%, Cr: 16.0 to 20.0%, Mo: 3.0% or less, and if necessary, Cu: 4.0% or less, It contains one or more of N: 0.30% or less, Nb: 0.15% or less, and consists of the balance Fe and impurity elements, and is excellent in design and corrosion resistance. Surface treated stainless steel A surface treatment method for an austenitic stainless steel that has been rolled, solution heat treated, and descaled, and includes a first electropolishing process that exposes a Cu enriched region on the surface, and a Cu concentration on the surface. Of a surface-treated stainless steel with excellent design and corrosion resistance, characterized by comprising a copper removal treatment step for removing the activated region and a second electropolishing treatment and a passivation film generation step by passivation treatment Method. The design improvement process process which provides an unevenness | corrugation so that the roughness Ra of the final product surface may be 1.8 micrometers or more is performed before performing the second electropolishing process. A method for producing surface-treated stainless steel with excellent design and corrosion resistance. The chemical composition of the stainless steel used is C: 0.08% or less, Si: 1.00% or less, Mn: 2.50% or less, P: 0.045% or less, and S: 0.00% in mass%. 030% or less, Ni: 7.0 to 15.0%, Cr: 16.0 to 20.0%, Mo: 3.0% or less, and if necessary, Cu: 4.0% or less, 5. One or more of N: 0.30% or less and Nb: 0.15% or less are contained, and the balance consists of Fe and impurity elements. 5. Of surface-treated stainless steel with excellent design and corrosion resistance. The method for producing a surface-treated stainless steel having excellent design properties and corrosion resistance according to any one of claims 3 to 5, wherein a copper removal treatment step comprising immersing in a 20% or more nitric acid aqueous solution is performed. .