JP5335595B2 - Chrome-plated stainless steel plate with excellent post-processing corrosion resistance - Google Patents

Description

  The present invention relates to a corrosion-resistant chromium-plated stainless steel sheet having an anti-corrosion plating layer in which the outer layer is a hydrated chromium oxide layer and the inner layer is a metal chromium layer, and particularly excellent corrosion resistance even after cold working the steel sheet. The present invention relates to a chrome-plated stainless steel sheet that exhibits

  2. Description of the Related Art Conventionally, techniques for improving corrosion resistance by performing various surface treatments such as plating, painting, and chemical conversion treatment on stainless steel materials are known. Among them, the chrome-plated stainless steel material is preferable from the viewpoint of design and is used in harsh environments such as automobile parts. The following technique is disclosed regarding such a highly corrosion-resistant chromium-plated stainless steel material.

In Patent Document 1 below, the surface of a stainless steel product is directly plated with chromium having a thickness of 0.1 to 1.0 μm, and the current density of the anodic polarization curve in 0.25 N hydrochloric acid (50 ° C.) is 100 μA / cm ^2. Potential V100. t. p. Describes a method of applying a treatment such as a heat treatment in the air so that the voltage becomes 0.5 V (Ag / AgCl) or more. Here, since the measurement conditions (0.25 N hydrochloric acid, 50 ° C.) of the anodic polarization curve, which was regarded as a corrosion resistance evaluation method, are too severe, the target V100. t. p. It is not easy to obtain the heat treatment, and heat treatment is required to be left in the atmosphere of 100 to 300 ° C. for 0.5 to 24 hours within 48 hours (desirably immediately after plating) after chromium plating. A product with such special incidental requirements has a problem in that productivity is hindered due to excessive quality depending on applications and complicated processes.

  Further, in Patent Document 2 below, the natural potential of a chromium plating product composed of chromium plating and an oxide film at 5% NaCl, pH: 10 to 11 is −0.3 V (vs. Ag / AgCl) or more. And a method of performing a wet oxidation treatment such as an anodic electrolytic oxidation treatment or a chemical oxidation treatment. Although this method has a difference in the corrosion resistance evaluation method as compared with Patent Document 1, it is the same as Patent Document 1 in that an auxiliary process is required, and there is a problem that waste liquid processing is also necessary because wet processing is required. there were.

  Further, in Patent Document 3 below, the natural potential of a chromium plating product composed of chromium plating and an oxide film at 5% NaCl, pH: 10 to 11 is −0.3 V (vs. Ag / AgCl) or more. A method of performing plasma treatment after chromium plating is described. This method requires an incidental process as in Patent Documents 1 and 2.

  Further, all of the above-mentioned conventional techniques are techniques when plating is performed on a processed and molded product, and is not related to corrosion resistance after processing a product plated with chromium in advance.

  Generally, many pinholes and microcracks are introduced into the chromium plating layer, and there are also disadvantages that it is hard and poor in ductility and is easily damaged by cold working. In order to support a wide range of applications such as various automotive parts, building materials, kitchen appliances, etc., it is important to have sufficient corrosion resistance even when cold-worked after plating, especially for a wide range of applications. It is an indispensable element to provide sufficient post-processing corrosion resistance for the stainless steel plate to be processed. However, this has not been clarified by the prior art.

Japanese Patent No. 2668714 JP-A-2005-232529 JP 2007-56282 A

  This invention makes it a subject to provide the chromium plating stainless steel plate excellent in the corrosion resistance after a process.

  The corrosion resistance of metallic materials depends on the severity of the corrosive environment. In a mild environment, sufficient corrosion resistance can be obtained even with a low-grade material, and in a severe environment, sufficient corrosion resistance cannot be obtained unless it is a high-grade material. Therefore, in the present invention, the corrosive environment to be applied is roughly divided into two. In other words, as a means to reproduce the harsh environment when used outdoors such as automobile parts and outdoor building materials, it will be evaluated by a combined cycle corrosion test (hereinafter abbreviated as CCT) to which repeated dry and wet cycles are applied. As a means for simulating a mild environment when used indoors such as kitchens and indoor building materials, it was decided to evaluate using a salt spray test (hereinafter abbreviated as SST). In the following, explanation will be given in the order of harsh environment and mild environment.

(Technology for severe corrosive environments)
The inventors first applied chromium plating to a ferritic stainless steel plate having a Cr content of 17% by electroplating using a Sargent bath, which is a conventionally known standard chromium plating bath, and electrolyzed it. A steel plate sample with varying plating thickness was produced by changing the time, and a draw bead processing was applied to a test piece collected from this sample, and then subjected to a corrosion resistance test to investigate the relationship between the chromium plating thickness and the post-processing corrosion resistance. The result is shown in FIG. The plating bath composition was chromic acid: 100 g / L, sulfuric acid: 1.0 g / L, the bath temperature was 50 ° C., the current density was 20 A / dm ² , and the electrolysis time was 0.5 to 15 sec. The thickness of the plating layer was determined by measuring the element concentration profile in the thickness direction of chromium and oxygen using a glow discharge optical emission spectrometer. For the draw bead processing, a method of performing a drawing process with a plate thickness reduction rate of 20% while pressing an SKD11 mold having a bead portion R = 4 mm and a bead height of 4 mm with 800 kg was applied. As a corrosion resistance evaluation method, a CCT test according to JASO M610-92 was used, and the degree of rusting was evaluated using RN described in JIS G 0595 “Stainless Steel Surface Rust Generation Evaluation Method” (2004) as an index.

  FIG. 1 shows that the post-processing corrosion resistance is governed by the plating thickness. A thinner plating thickness is advantageous for the corrosion resistance after processing. This is because if the plating thickness is thick, damage to the plating layer due to processing becomes severe. However, it can be seen that the variation in corrosion resistance after processing is large even with the same plating thickness.

  As a result of investigating this cause, the present inventors have found that the variation in corrosion resistance after processing depends on the properties of the chromium plating layer before processing. That is, as shown in FIG. 2, it was found that there is a clear correlation between the cathode current density of the sample before processing and the corrosion resistance after processing.

  The post-processing corrosion resistance of the chromium plating material is governed by the area ratio of the ground iron exposed by the local destruction of the plating layer. The chrome plating layer contains latent defects such as pinholes and microcracks, and the plating layer breaks from these as a result of processing, leading to the exposure of the ground iron. Therefore, if there are few latent defects in the as-plated state, the exposed post-processed iron is less, and as a result, the post-processing corrosion resistance is better.

  The number of latent defects can be expressed by the cathode current density. The outer layer of the plating layer is a hydrated chromium oxide, and the cathode reaction hardly occurs here. If this outer layer is defective and the underlying metallic chromium layer or the underlying ground iron is exposed, a cathode reaction occurs. Therefore, the magnitude of the cathode current density corresponds to the defect area ratio of the outer plating layer, and this correlates with the amount of latent defects in the entire chromium plating layer.

  From the above, in order to obtain good post-processing corrosion resistance, it is necessary that the plating layer thickness is appropriate, and in addition, it is more desirable that the cathode current density in the as-plated state is appropriate. Got.

Next, the influence of the material was examined. Cold-rolled sheets are prepared using laboratory melts with various steel components, and chromium plating is applied by electroplating using a sergeant bath. The plating thickness is 0.02 μm and the cathode current density is 0.3-0. A 6 μA / cm ² chromium plating sample was prepared, and the corrosion resistance after processing was investigated by the same method as described above. An example of the results is shown in FIG.

From this, it can be seen that when the Cr content of the material is insufficient, satisfactory post-processing corrosion resistance cannot be obtained. The reason for this is that galvanic corrosion occurs at the local destruction site of the plating layer during processing. Therefore, the natural potential of the material needs to be more noble than the potential of the plating layer, and in order to satisfy this, the Cr content must be at least 15.0% by mass%.
(Technology for mild corrosive environment)
Using the same technique as in the harsh environment described above, a ferritic stainless steel plate having a Cr content of 13% was subjected to chromium plating, and a steel plate sample was produced by changing the electrolysis time to change the plating thickness. After adding a draw bead processing to the piece, it was subjected to a corrosion resistance test to investigate the relationship between the chromium plating thickness and the post-processing corrosion resistance. The result is shown in FIG. As the corrosion resistance evaluation method, the SST test described in JIS Z 2371 was used, and the degree of rusting was evaluated using the RN described in JIS G 0595 “Stainless steel surface rust generation evaluation method” (2004) as an index.

  As shown in FIG. 5, the corrosion resistance after processing is governed by the plating thickness as in the CCT test result, and as shown in FIG. 6, there is a clear correlation between the cathode current density of the sample before processing and the corrosion resistance after processing. This is the same as the CCT test result.

From the above, as in the harsh environment, even in a mild environment, good corrosion resistance can be obtained if the plating layer thickness is appropriate, and in addition, if the cathode current density in the as-plated state is appropriate, more The conclusion was desirable.
What is different from the harsh environment is the requirement of the substrate. Chromium plating with appropriate plating thickness and cathode current density was applied to cold-rolled sheets made from laboratory melts with various steel components, and the corrosion resistance after processing was investigated by the same method as described above. An example of the result is shown in FIG.

  From this, it can be seen that if the Cr content of the substrate is 10.5% or more, satisfactory post-processing corrosion resistance can be obtained, and the Cr content in the harsh environment need not be less than 15.0%.

The present invention is configured based on the above knowledge, and the gist thereof is the following contents as described in the claims.
(1) On a surface of a stainless steel plate base material containing Cr: 15.0 to 30.0% by mass%, and having an anticorrosion plating layer in which an outer layer is a hydrated chromium oxide layer and an inner layer is a metallic chromium layer The anticorrosion plating layer has a thickness of 0.01 to 0.10 μm, and the anticorrosion plating layer has a cathode current density measured in a 3.5% NaCl solution at 30 ° C. in an open air state. A chromium-plated stainless steel sheet excellent in post-processing corrosion resistance , characterized by being 2.5 μA / cm ² or less under the condition of silver standard electrode reference −0.6 V.
(2) On a surface of a stainless steel plate base material containing Cr: 10.5 to 15.0% by mass%, and having an anticorrosion plating layer in which the outer layer is a hydrated chromium oxide layer and the inner layer is a metallic chromium layer The anticorrosion plating layer has a thickness of 0.01 to 0.10 μm, and the anticorrosion plating layer has a cathode current density measured in a 3.5% NaCl solution at 30 ° C. in an open air state. A chromium-plated stainless steel sheet excellent in post-processing corrosion resistance , characterized by being 2.5 μA / cm ² or less under the condition of silver standard electrode reference −0.6 V.

  As described above, according to the present invention, it is possible to provide a chrome-plated stainless steel sheet having excellent corrosion resistance after processing, and there are significant industrially useful effects.

It is a figure which shows the relationship between a plating layer thickness and the CCT corrosion resistance after a process. It is a figure which shows the relationship between the cathode current density in an as-plated state, and the CCT corrosion resistance after a process. It is a figure which shows the relationship between Cr content of a raw material, and the CCT corrosion resistance after a process. It is a figure which shows the shape of the tool used for draw bead processing. It is a figure which shows the relationship between a plating layer thickness and SST corrosion resistance after a process. It is a figure which shows the relationship between the cathode current density in an as-plated state, and the SST corrosion resistance after a process. It is a figure which shows the relationship between Cr content of a raw material, and SST corrosion resistance after a process.

  Hereinafter, the present invention will be described in detail.

  First, the stainless steel plate material in the present invention will be described.

  The material in the present invention is a stainless steel plate containing Cr: 15.0 to 30.0% by mass% for a severe corrosive environment, and Cr: 10.5 for a mild corrosive environment. A stainless steel plate containing ˜15.0% is used. The metal structure of the steel sheet is not limited as long as the content of Cr, which is a basic element for ensuring corrosion resistance after processing of the chromium-plated stainless steel sheet, is within the above range. Any of ferritic, austenitic, martensitic, or mixed structure steels may be used. Further, alloy elements such as Mo, Ni, Cu, Ti, Nb, C, and N that contribute to corrosion resistance other than Cr may be adjusted as necessary with reference to the prior art.

  The reason for limiting the Cr content of the stainless steel sheet to be applied in the present invention will be described below.

  Cr is a main element that dominates the corrosion resistance of the material and contains an appropriate amount. By processing, the plating layer is locally destroyed and a part where the iron is exposed is formed in part, but in order to suppress the occurrence of corrosion and corrosion of the part as much as possible, the space between the plating layer and the iron It is necessary to make the potential difference as small as possible. For this purpose, the minimum Cr content is 15.0% by mass for harsh environments and 10.5% for mild environments, below which galvanic corrosion occurs. As shown in FIG. 5, satisfactory post-processing corrosion resistance cannot be obtained. The upper limit of Cr content does not need to be specified from the viewpoint of corrosion resistance, but considering the workability and cost of the material itself, 30.0% by mass% for harsh environments, mild environment The upper limit is preferably 15.0%.

  The stainless steel plate having the above composition is manufactured by a normal stainless steel plate manufacturing method in which a steel piece melted and refined in a converter or an electric furnace is subjected to hot rolling, pickling, cold rolling, annealing, finish pickling, and the like. Is done.

  Next, the anticorrosion plating layer in this invention is demonstrated.

  The anticorrosion plating layer in the present invention is such that the lower layer (inner layer) is a metal chromium layer and the upper layer (outer layer) is a hydrated chromium oxide layer and has a thickness of 0.01 to 0.10 μm.

  The reason why the hydrated chromium oxide layer is disposed on the upper layer (outer layer) of the plating layer is that no cathode reaction occurs on this surface. When a plating defect that leads to the exposure of the ground iron occurs due to the processing, the part becomes a corrosion starting point. Here, in order for corrosion to continue and grow, it is necessary in the area where the cathode that supports the dissolution of the anode of the ground iron part exists. However, if the periphery of the defect is covered with a hydrated chromium oxide layer, the cathodic reaction does not occur here, so anodic dissolution cannot be supported, and even if the iron is exposed, it is extremely small and on the exposed surface. If the cathodic reaction at is small, corrosion does not grow easily. Thus, the hydrated chromium oxide layer plays an extremely important role in inhibiting corrosion growth. The presence of the hydrated chromium oxide layer is determined by the bonding of metal-oxygen and metal-oxygen-hydrogen using surface analysis methods such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption edge fine structure analysis (EXAFS). This can be confirmed by identification. In addition, the presence and thickness of the hydrated chromium oxide layer can be ascertained by measuring the element concentration distribution in the thickness direction of the plating layer using Auger electron spectroscopy (AES) or glow discharge electron spectroscopy (GDS). it can.

  The reason for defining the thickness of the plating layer is to ensure good post-processing corrosion resistance. That is, as shown in FIGS. 1 and 5, when the plating layer thickness exceeds 0.10 μm, satisfactory corrosion resistance cannot be obtained. This is because, as the plating layer becomes thicker, strain concentration on latent defects such as microcracks increases, causing large-scale plating layer damage and increasing the exposed area of the ground iron. If the exposed area of the steel is increased, the cathodic reaction proceeds on the surface of the steel, even if the surrounding area is covered with hydrated chromium oxide, so that corrosion grows and the corrosion resistance deteriorates. On the other hand, if the thickness of the plating layer is less than 0.01 μm, the pinhole increases and the exposure of the base iron increases, so that the corrosion resistance becomes insufficient.

  Therefore, the plating layer thickness is not too thick and not too thin, and should be controlled in the range of 0.01 to 0.10 μm. To obtain more preferable corrosion resistance, the plating layer thickness should be kept in the range of 0.01 to 0.05 μm. good. In addition, the plating layer thickness prescribed | regulated by this invention is the sum total of the thickness of a lower layer (inner layer): Metal chromium layer and an upper layer (outer layer): Hydrated chromium oxide layer.

Furthermore, it is desirable that the latent defects contained in the plating layer before processing be as small as possible. This is because the larger the number of latent defects, the more easily the plating layer is damaged by processing. The number of latent defects can be known using the cathode current density as an index. The cathode reaction does not occur on the hydrated chromium oxide layer that covers the upper layer of the plating layer, but only on the lower metal chromium layer or the iron base. As shown in FIG. 2 and FIG. 6, a plating layer with better corrosion resistance can be obtained. The cathode current density referred to here depends mainly on the plating conditions because it corresponds to the latent defect area ratio. However, since the plating thickness to be formed is extremely thin, it also includes scratches introduced by handling after plating. It is an indicator. The desirable condition set in the present invention is that the cathode current density measured in a 3.5% NaCl solution at 30 ° C. in the open atmosphere is 2.5 μA / s under the condition of a silver / silver chloride standard electrode reference of −0.6 V. cm ² or less.

The anticorrosion plating layer satisfying the above requirements can be obtained by a normal electroplating method. The plating bath composition is not particularly limited, and a conventionally known sergeant bath can be used. As the plating bath, a composition of chromic acid: 100 to 400 g / L, sulfuric acid: 1.0 to 4.5 g / L is preferable. As plating conditions, temperature: 45 to 55 ° C., current density: 10 to 80 A / dm. The condition of ² is preferable. When electroplating is performed using a plating bath mainly composed of chromic acid, the plating layer structure automatically becomes a two-layer structure in which the lower layer is a metallic chromium layer and the upper layer is a hydrated chromium oxide layer.

  The thickness of the plating layer is obtained from the concentration distribution data of Cr and O in the thickness direction obtained by glow discharge optical emission spectrometry. That is, the chromium concentration at a depth of 1.0 μm from the surface is defined as the chromium concentration in the base material, and the depth position from the surface where the oxygen concentration is 5.0% higher and the oxygen concentration is less than 10.0% is plated. Defined as thickness. The equipment used in the present invention is JY5000RF-PSS type manufactured by JOBIN YVON, and analysis conditions include Current Method Program: CNBistel-05NNN-0, Mode: Constant Electric Power 40W, Ar Pressure: 775 MPaAsec 980 MPaAsec. Sampling Time: 0.020 (sec / point).

  The cathode current density was measured by a potentiostat method using a potentiostat with a silver / silver chloride standard electrode as a reference electrode in a static solution at 30 ° C. and 3.5% NaCl in an open atmosphere. It is determined from the cathode polarization curve. In the present invention, a potentiostat PS-08 type manufactured by Toho Giken Co., Ltd. was used, and the cathode polarization curve was measured at a sweep rate of 20 mV / min from the time when 1 minute passed after the test piece was immersed in the solution. The current density at a standard electrode reference of −0.6 V was determined as the cathode current density referred to in the present invention.

  Further, as a method for evaluating post-processing corrosion resistance, a strip test piece having a width of 40 mm is subjected to a draw bead process, and after degreasing and end face sealing, a CCT test or an SST test is performed for evaluation. In draw bead processing, a lubricating oil (Castroll No. 122) is applied in advance to a strip test piece, and a pair of tools having the shape shown in FIG. 4 is pressed with a load of 800 kg while pulling speed: 200 mm / min and a plate thickness reduction rate of 20%. The drawing process shall be performed. The CCT test is performed under the conditions specified in JASO M610-92. The degree of eruption after 30 cycles has been evaluated using the RN specified in JIS G0595 as an index, and an RN of 6.5 or higher is accepted. The SST test is an SST test described in JIS Z 2371. The degree of eruption after exposure to 1000 Hr is evaluated using RN of JIS G 0595 as an index, and RN 6.5 or higher is passed.

Example 1
Stainless steel having the composition shown in Table 1 was melted in a 150 kg vacuum melting furnace and cast into a 50 kg steel ingot, and then hot-rolled, hot-rolled sheet annealed, pickled, cold rolled, intermediate annealed, pickled, cold rolled, and finished. A steel plate having a thickness of 0.8 mm was manufactured through an annealing-finish pickling process.

  Using this stainless steel plate as a material, an anticorrosion plating layer was formed by electroplating using a sergeant bath. The thickness of the plating layer and the cathode current density of this sample were measured, and the corrosion resistance after the draw bead processing was evaluated.

The plating bath composition is chromic acid: 100 g / L, sulfuric acid: 1.0 g / L, the bath temperature is 50 ° C., the current density is 20 A / dm ^2, and the electrolysis time is changed in the range of 0.5 to 15 sec. The layer thickness was changed.

  The thickness of the plating layer is obtained from the element profile of chromium and oxygen in the thickness direction using a glow discharge emission spectroscopic analyzer by the above-described method. Lower layer (inner layer): metallic chromium layer and upper layer (outer layer): hydrated chromium oxidation This is the sum of the thicknesses of physical layers.

The draw bead processing is performed by pressing a pair of tools having a shape shown in FIG. 4 on a strip test piece having a width of 40 mm applied in advance with lubricating oil (Castroll No. 122) at 800 kg, and a drawing rate: 200 mm / min. A method of drawing 20% was taken.
As a corrosion resistance evaluation method, the CCT test of JASO M610-92 was used, and the test period was 30 cycles. The degree of rusting was evaluated using RN defined in JIS G0595 as an index, and RN 6.5 points or more were evaluated as passing.

Table 2 shows the test conditions and results. This invention No. Nos. 1 to 7 and Nos. 11 to 13 and 15 showed excellent post-processing corrosion resistance because the Cr content and plating layer thickness of the materials were appropriate and the cathode current density was also in a desirable range. Reference Example No. Nos. 14, 16, and 17 are different from the present invention in that only the cathode current density is out of the desired range. Compared to 13.15, the corrosion resistance is slightly inferior, but is sufficiently satisfactory. On the other hand, Comparative Example No. Nos. 101 to 103 have comparatively low Cr content in the material. Nos. 108 and 109 have an inappropriate plating layer thickness. Since Nos. 104 to 107 have inappropriate plating thickness and cathode current density, satisfactory post-processing corrosion resistance cannot be obtained. Comparative Example No. 201 and 202 show the corrosion resistance of the stainless steel plate itself when not plated. Based on these, the present invention No. 1 made of 17% Cr steel plate was used. No. 3 and 12-17, it turns out that the post-process corrosion resistance exceeding a 17Cr-1.2Mo steel plate (comparative example No. 202) is obtained by the chromium plating of this invention.
(Example 2)
A test having the same contents as in Example 1 was performed except that the stainless steel material shown in Table 3 was used and the SST test (test time 1000 Hr) described in JIS Z 2371 was used as the corrosion resistance evaluation test.

Table 4 shows the test conditions and results. This invention No. Nos. 301 to 309 exhibited excellent post-processing corrosion resistance because the Cr content and plating layer thickness of the material were appropriate and the cathode current density was also in a desirable range. Reference Example No. Nos. 310 to 312 are in the present invention No. 3 because only the cathode current density is out of the desirable range. Compared with 303 and 307 to 309, the corrosion resistance is slightly inferior, but it is at a level that is sufficiently satisfactory. On the other hand, Comparative Example No. Nos. 501 and 502 have comparatively low Cr content in the material. Nos. 507 and 508 have an inappropriate plating layer thickness. Since 503 to 506 are inappropriate in plating thickness and cathode current density, satisfactory post-processing corrosion resistance cannot be obtained. Comparative Example No. Reference numerals 509 and 510 denote the corrosion resistance of the 13 to 14% Cr stainless steel sheet itself when plating is not performed. Corrosion resistance is insufficient because anticorrosion plating is not applied. The effects of the present invention were confirmed by the above examples.

Claims (2)

On the surface of a stainless steel plate base material containing Cr: 15.0 to 30.0% by mass%, the anticorrosion plating layer comprising an outer layer consisting of a hydrated chromium oxide layer and an inner layer consisting of a metal chromium layer, The plating layer has a thickness of 0.01 to 0.10 μm, and the anticorrosion plating layer has a cathode current density measured in a 3.5% NaCl solution at 30 ° C. in an open air state. A chromium-plated stainless steel sheet excellent in post-processing corrosion resistance , characterized by being 2.5 μA / cm ² or less under a condition of reference −0.6V . Mass% Cr: the surface of the stainless steel substrate containing 10.5 to 15.0%, the outer layer has a corrosion plated layer inner layer with hydrated chromium oxide layer is formed of a metal chromium layer, the anticorrosive The plating layer has a thickness of 0.01 to 0.10 μm, and the anticorrosion plating layer has a cathode current density measured in a 3.5% NaCl solution at 30 ° C. in an open air state. A chromium-plated stainless steel sheet excellent in post-processing corrosion resistance , characterized by being 2.5 μA / cm ² or less under a condition of reference −0.6V .

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