JP6298881B2 - Ferritic stainless steel and manufacturing method thereof - Google Patents

Description

The present invention relates to ferritic stainless steel excellent in crevice corrosion resistance in normal temperature water and warm water, and a method for producing the same.
This application claims priority based on international patent application PCT / JP2014 / 058541 filed on March 26, 2014, the contents of which are incorporated herein by reference.

  Stainless steel has excellent corrosion resistance and strength characteristics, and is therefore used as a tank material for storing water and hot water. In particular, compared to austenitic stainless steel, ferritic stainless steel has features such as a low thermal expansion coefficient and excellent resistance to stress corrosion cracking. Further, in recent years, the application of ferritic stainless steel has become widespread in order to avoid the price fluctuations of Ni raw materials that are rare metals. In particular, SUS430J1L (18Cr-Cu) and SUS444 (19Cr-2Mo), which are high-purity ferritic stainless steels having excellent corrosion resistance after welding, are typical steel types of ferritic stainless steels.

  This high-purity ferritic stainless steel is used as a material for various water-based equipment and tanks by taking advantage of the above characteristics. In particular, as a material for tanks, high-purity ferritic stainless steel is used for water storage tanks and ice heat storage tanks at room temperature. It is used. Among these stainless steel tanks, tanks installed in household water heaters such as electric water heaters and Ecocute (registered trademark) generally have a cylindrical body so that they can withstand the source pressure of water pipes. It has a capsule-type structure in which a bowl-shaped end plate portion is placed on the top and bottom of each. In an industrial mass production process, since the body part and the end plate part are generally overlapped and welded, a clearance is formed on the structure. In this weld gap, crevice corrosion, which is a cause of local corrosion of stainless steel, is likely to be induced. Even in the case of SUS444 or SUS445J1 (22Cr-1.2Mo) described above, water leakage due to crevice corrosion may occur when used in an area with poor water quality or when the welding condition or gap structure is inappropriate. Some water storage tanks have a structure in which there is no clearance due to welding. In this case, the water storage tank has a structure in which panel-shaped stainless steel plates are joined by bolting with resin packing interposed therebetween. In this case, there is a gap between the stainless steel and the packing, and if the water quality is poor, this site is also a site of concern for corrosion. Many tanks and tanks for storing water at room temperature have the same structure as the tank for storing warm water, but the corrosive environment becomes milder as the water temperature is lower. For this reason, high-purity ferritic stainless steel with lower Cr and less Mo than SUS444 (Cr content is low and Mo content is reduced) may be used. SUS444 is often used. In addition, due to the structure, stress is likely to concentrate in the weld gap in the above-described capsule-shaped tank, and there is a concern that it may be broken due to changes in water pressure during hot water supply or water supply. For this reason, a durability test for changing the water pressure in the tank is performed as a test at the time of manufacturing the tank. If the welded structure or welding conditions are inappropriate, cracks occur in this part (weld gap), so the strength of the weld gap needs to be increased. In order to increase the strength of the material, there is a method of adding a solid solution strengthening element, and some of them reduce the toughness of the material. When toughness falls, problems, such as a crack arising in a steel plate at the time of manufacture, may arise.

  Regarding the corrosion resistance of the weld gap, Patent Document 1 discloses a method of improving corrosion resistance by adding Ti and Al to high CrMo steel (steel having a large amount of Cr and Mo). In order to improve crevice corrosion resistance, it is known to add Cr and Mo, which are main elements of stainless steel, as described above. However, these Cr and Mo are rare metals, and reduction of the amount of use is also desired worldwide. In particular, Mo is subject to speculative price fluctuations, and its reduction is desired.

  From this point of view, a technique for forming a Si-enriched layer on the surface layer is known as a technique for exerting excellent crevice corrosion resistance even when the amount of Cr and Mo, which are main elements of stainless steel, is reduced. For example, Patent Document 4 discloses that a Si enriched layer can be formed on the surface using a production method in a reduction furnace using hydrogen called BA (bright annealing), and the Si enriched layer improves corrosion resistance. Yes.

  Further, as described above, generally, hot water storage / storage tanks require excellent repeated fatigue strength against changes in water pressure. However, it is desirable that the strength of the welded portion is high because a strong impact due to water pressure is applied to the welded portion at the time of water supply and water stoppage. Regarding the method of improving the strength of the welded portion of ferritic stainless steel, Patent Document 2 discloses a method of adding Si to the material itself and precipitating NbC, and further discloses a method of adding Ni to Patent Document 3. Has been.

  However, when the present inventors examined these prior arts, it was found that further improvements were necessary. That is, in the above-described prior art, a large amount of Si is added to steel. However, as a result of the study by the present inventors, it has been found that these additions improve the tensile strength, but reduce the toughness of the hot-rolled sheet, which is likely to cause a problem in the manufacturing process. Therefore, in order to improve toughness in the prior art, it is necessary to suppress components such as Si, which these prior arts need to add in a large amount.

As for the electrolytic method for removing Si on the surface, Patent Document 5 discloses a method for improving descalability by combining sulfuric acid and nitric acid. Further, Patent Document 6 discloses a method of descaling in a solution in which sulfate ions or sodium ions are mixed with nitric acid.
All of these are basically intended to remove oxide scale containing Si from the surface, and are not intended to actively leave only Si as in the present invention.

JP-A-5-70899 JP 2008-291303 A JP 2011-184732 A JP-A-6-279950 JP 11-61500 A JP 2005-232546 A

An object of the present invention is to provide a ferritic stainless steel that can suppress both water leakage due to crevice corrosion in a water storage tank and a hot water storage tank and fracture of a weld due to fluctuations in water pressure during water supply and water stoppage.
In the present invention, while reducing the addition amount of Cr and Mo together with Ni, which is required to be reduced as a rare metal, and by not adding more than the necessary amount of Si and Ti that reduce the toughness of the welded portion, The purpose is to simultaneously improve water leakage due to crevice corrosion and fracture of the weld due to fluctuations in water pressure.

As a result of intensive studies by the present inventors on the method for solving the above-mentioned problems, it is necessary to actively leave Si only on the surface and to add Si to the material (base material) to the minimum necessary. I found it. In addition, as a method of leaving Si only on this surface, it has been clarified that it is important to optimize the electrolytic descale conditions in the annealing pickling process at the time of manufacturing the steel sheet.
That is, in the actual stainless steel sheet manufacturing process, in the electrolytic descale process, current is supplied to the steel sheet by indirect energization. This is carried out by placing a pair of electrodes apart from the steel sheet immersed in the electrolytic solution and passing a current through the electrodes. At this time, the steel plate facing the positive electrode of the electrode becomes a negative electrode (cathode), the steel plate facing the negative electrode of the electrode becomes a positive electrode (anode), and the electrode and the steel plate form a closed circuit through the electrolytic solution. At this time, the oxidation reaction is promoted on the anode side surface of the steel sheet, and mainly the oxide of Cr is dissolved. On the other hand, the Fe oxide is mainly dissolved on the cathode side surface of the steel plate.
Although it is known that Si oxides are difficult to remove by the electrolytic method, it has been found that Si oxides remain on the surface appropriately by controlling the electrolyte and current density as described later. Appropriate descaling is performed by controlling the conditions in the case of electrolysis using this steel plate as an anode and the conditions in the case of electrolysis using a steel plate as a cathode.
Here, the total amount of electricity (= current value × time) supplied from the negative electrode (the steel plate side is the anode) and the positive electrode (the steel plate side is the cathode) is constant. At this time, if the electrode area is the same, the current density of the anode and the cathode is constant, but if one electrode area is increased, the electrolysis time is increased, but the current density is decreased. Thus, by appropriately reducing the electrolysis time and current value (current density per unit area of the steel sheet), oxides mainly composed of Cr and Fe on the surface are dissolved and removed.
Through various experiments, it has been found that by controlling the current density and time of electrolysis, it is possible to leave an appropriate amount of Si oxide even if the amount of Si added to the steel is minimal. did. Furthermore, on the cathode side, it was also clarified that the reduction and dissolution efficiency of the oxide scale mainly composed of Fe oxide can be improved and the metal precipitation due to the reduction can be suppressed by shortening the current with a high current density. Therefore, the anode / cathode electrolysis time ratio during electrolysis is preferably more than 1.0. Thereby, it became possible for Si oxide to cover a steel surface layer moderately, and to ensure the corrosion resistance more than the corrosion resistance expected from content of Cr and Mo contained in a base material. The reason is considered as follows. Of course, the Si oxide of the surface layer itself functions as a protective film and acts to improve the corrosion resistance. Furthermore, with the Si oxide coverage of the present embodiment, Al, Ca, and Si, which are referred to as a calcium component contained in tap water, are likely to be deposited on the surface layer while the steel plate is used as a hot water tank or a water storage tank. For this reason, it discovered that corrosion resistance improved further by forming a more stable oxide film in the surface layer.
This invention is made | formed based on the said knowledge, and has the following requirements.

(1) By mass%, C: 0.020% or less, N: 0.025% or less, Si: 0.08 to 0.50%, Mn: 0.01 to 1.0%, P: 0.035 %: S: 0.01% or less, Cr: 13.0 to 25.0%, Al: 0.005 to 0.300%, and Nb: 0.01 to 0.60%, the balance being Crevice corrosion resistance in water at 45 ° C., comprising Fe and inevitable impurities, satisfying the following formula (1), and having a surface coverage of the surface Si oxide of 10 to 50% Ferritic stainless steel with excellent resistance.
C _s ≧ −0.45Cr + 28 (1)
C _s = 3Si _sur + Cr _sur (A)
However, equation (1) Cr in represents the Cr content of the ferritic stainless steel (mass%), _{C s} is the value calculated from the equation (A), Si in the formula (A) _sur represents the Si concentration ratio (atomic%) in the surface layer, and Cr _sur represents the Cr concentration ratio (atomic%) in the surface layer.
(2) Further, in mass%, including Ti: 0.03 to 0.30%, and satisfying the following formula (3), according to (1), in water at 45 ° C. Ferritic stainless steel with excellent crevice corrosion resistance.
Nb / Ti> 1.0 (3)
However, Nb and Ti in Formula (3) show Nb content (mass%) and Ti content (mass%) in ferritic stainless steel, respectively.
(3) Further, in mass%, Cu: 2.0% or less, Ni: 2.0% or less, Sn: 0.50% or less, Sb: 0.50% or less, V: 0.50% or less, Zr : 0.50% or less, B: 0.0050% or less, Ta: 0.10% or less, Ca: 0.010% or less, Mg: 0.0050% or less, Ga: 0.010% or less, and REM: The ferritic stainless steel having excellent crevice corrosion resistance in water at 45 ° C. according to (1) or (2), comprising at least one selected from 0.100% or less.
(4) By mass%, C: 0.020% or less, N: 0.025% or less, Si: 0.08 to 0.50%, Mn: 0.01 to 1.0%, P: 0.035 %: S: 0.01% or less, Cr: 13.0 to 25.0%, Al: 0.005 to 0.300%, Nb: 0.01 to 0.60%, and Mo: 0.03 -3.0%, the balance consists of Fe and inevitable impurities, satisfies the following formula (1 ') and formula (2), and the surface coverage of the surface Si oxide is 10-50% you wherein there, ferritic stainless steel with excellent crevice corrosion resistance in a 85 ° C. water.
C _s ≧ −0.45 (Cr + 3Mo) +34 (1 ′)
C _s = 3Si _sur + Cr _sur (A)
Cr + 3Mo: 18-30 (2)
However, the formula (1 '), Cr in the formula (2), Mo is, Cr content of the ferritic stainless steel (mass%), shows the Mo content (mass%), respectively, _{C s} has the formula ( It is a value calculated from A), Si _sur in formula (A) indicates the Si concentration ratio (atomic%) in the surface layer, and Cr _sur indicates the Cr concentration ratio (atomic%) in the surface layer.
(5) In addition, by mass% Ti: it comprises from 0.03 to 0.30%, and characterized by satisfying the following equation (3), according to (4), in 85 ° C. water Ferritic stainless steel with excellent crevice corrosion resistance.
Nb / Ti> 1.0 (3)
However, Nb and Ti in Formula (3) show Nb content (mass%) and Ti content (mass%) in ferritic stainless steel, respectively.
(6) Further, in mass%, Cu: 2.0% or less, Ni: 2.0% or less, Sn: 0.50% or less, Sb: 0.50% or less, V: 0.50% or less, Zr : 0.50% or less, B: 0.0050% or less, Ta: 0.10% or less, Ca: 0.010% or less, Mg: 0.0050% or less, Ga: 0.010% or less, and REM: characterized in that it comprises one or more selected 0.100% or less (4) or (5) described in, ferritic stainless steel excellent in crevice corrosion resistance in a 85 ° C. water.
(7) Ferritic stainless steel excellent in crevice corrosion resistance in water at 45 ° C. according to any one of (1) to (3), which is used for a water storage tank.
(8) which is used for the hot water storage tank (4) to according to any one of (6), ferritic stainless steel excellent in crevice corrosion resistance in a 85 ° C. water.
(9) A step of performing a descaling process on the heat treatment material having the chemical composition according to any one of (1) to (6), wherein in the descaling process, a positive electrode and a negative electrode are used. The heat treatment material is indirectly sandwiched, and the heat treatment material and the electrode are immersed in an electrolytic solution to perform alternating electrolysis, and 50 to 300 g / L of sodium sulfate and 50 g / L or more of nitrate ions are used as the electrolytic solution. The ratio of the electrolysis time of the anode / cathode in the alternating electrolysis is set to more than 1.0 and 6.0 or less using the aqueous solution that is contained. A method for producing ferritic stainless steel having excellent crevice corrosion resistance in water at 45 ° C or 85 ° C.

  In the ferritic stainless steel according to one embodiment of the present invention, the content of expensive elements such as Cr and Mo is suppressed, and both excellent crevice corrosion resistance and excellent toughness of the welded portion can be achieved. Therefore, ferritic stainless steel that satisfies the characteristics required for water storage and hot water storage tanks that did not exist conventionally can be provided at low cost.

FIG. 1 is a schematic diagram showing the relationship between the amount of Cr (mass%) of the base material and the value of 3Si _sur + Cr _sur (atomic%) regarding the result of the corrosion test of the crevice in water at 45 ° C. FIG. 2 is a schematic diagram showing the relationship between the value of Cr + 3Mo (mass%) of the base metal and the value of 3Si _sur + Cr _sur (atomic%) regarding the results of the crevice corrosion test in 85 ° C. water.

The knowledge obtained from the experiments conducted by the present inventors and the experimental results are shown below.
Test materials were manufactured as follows. Ferritic stainless steel was melted in a vacuum melting furnace. No. in Table 1 1 and no. The steel components were adjusted to a composition of 18 to obtain a steel material for a corrosion test against normal temperature water. No. in Table 5 21 and no. The steel components were adjusted to a composition of 38, and a steel material for a corrosion test against warm water was obtained.
These steel materials were hot-rolled to produce hot-rolled sheets having a thickness of 5 mm. The hot rolled steel sheet was subjected to scale removal, cold rolling, and heat treatment to produce a cold rolled sheet having a thickness of 0.8 mm. A part of the hot-rolled sheet in the middle of the process was used for toughness evaluation described later. Soaking process (finish annealing) for 1 minute at 980 ° C. in an atmosphere simulating LNG combustion exhaust gas (gas composition: 3% O ₂ -12% CO ₂ -balance N ₂ , dew point 40 ° C.) To obtain a heat-treated material. The heat treatment material was subjected to the following descaling treatment (electrolytic treatment).

In the descaling treatment, pretreatment was performed on some heat-treated materials according to the salt method and the neutral salt electrolysis method.
In the pretreatment by the salt method, a commercial descaling alkaline salt mainly composed of NaOH was maintained at 450 ° C., and the steel sheet was immersed in this alkaline salt for 10 seconds.
In the pretreatment by the neutral salt electrolysis method, an aqueous sodium sulfate solution having a concentration of 150 g / L was prepared using a commercially available sodium sulfate reagent, and this aqueous solution was used as an electrolytic solution. The test piece was used as a sample electrode, and two SUS304 stainless steel plates were placed in parallel at 10 mm intervals on both sides of the test piece as a counter electrode. The current density and electrolysis time were set as follows, simulating alternating electrolysis by indirect energization of the actual machine. The steel plate was energized for 1 second at each of the current densities of −100 mA / cm ² and +100 mA / cm ² , this energization was performed 10 times, and electrolysis was performed for a total of 20 seconds. Since the anode time and the cathode time are each 1 second, the anode / cathode time ratio in this case is 1.0. The amount of electricity is 10 kQ / m ² . The electrolysis conditions were controlled using a function generator and a potentiostat.

Next, the heat treatment material was subjected to electrolytic treatment. Using commercially available reagents containing nitrate ions and Na ₂ SO ₄ , aqueous solutions having concentrations shown in Tables 3 and 7 were prepared, and this aqueous solution was used as an electrolytic solution. As a reagent containing nitrate ions, a special grade nitric acid aqueous solution was used. However, a reagent containing sodium nitrate or other nitrate ions can be used as long as other conditions are satisfied. The electrolysis method was the same as the pretreatment by the neutral salt electrolysis method described above. The electrolysis time was changed so that the ratio of the anode time to the cathode time was 1.0 to 8.0. The total amount of electricity at that time was fixed at 10 kQ / m ² which was the same as the pretreatment by the neutral salt electrolysis method described above. The actual amount of electricity per unit area varies depending on the effect of the plate passing speed and the electrolysis efficiency in the electrolyte (effect of solution conductivity), etc., but if it is in the range of about 1 to 30 kQ / m ² , The purpose of the embodiment is achieved.
Depending on conditions, the heat treatment material was immersed in an aqueous solution of nitric hydrofluoric acid after electrolysis. The acid concentration was adjusted to 60 g / L nitric acid and 15 g / L hydrofluoric acid using each reagent. The temperature was 40 ° C. and the immersion time was 10 to 20 seconds.
The test material was manufactured by the above.

The evaluation of the test material after electrolysis (in some cases, after electrolysis and nitric hydrofluoric acid treatment) was performed as follows.
Descalability was evaluated as follows. The specimen was observed visually and with a 10-fold magnifier. When no scale was observed, or when the number of scales was one per field, it was judged that the descaling was completed and the descaleability was good (B). When two or more scales remained in one field of view, it was judged that the descaling was not completed and the descalability was poor (C).
The Si coverage on the surface was measured as follows. First, in order to reduce the dispersion | variation by a measurement position, five places were selected arbitrarily from the measurement sample. Then, for each location, using a field emission Auger electron spectrometer (FE-AES), the condition of the outermost layer in the field of view observed at 1000 times under the condition that the acceleration voltage is 10 keV and the beam current value is 10 nA. Si distribution map (mapping image) was obtained. The image of the obtained distribution map was binarized for each of the five locations to determine the area ratio of the locations where Si was present, and the average value was taken as the Si coverage (surface coverage of the surface Si oxide).
Surface (surface layer) Cr concentration (Cr concentration ratio, atomic%): Cr _sur and surface (surface layer) Si concentration (Si concentration ratio, atomic%): Si _sur were measured as follows. Using a scanning FE Auger electron spectrometer (FE-SAM), a concentration profile in the depth direction was measured under the condition that the Ar sputtering rate was 15 nm / min for the field of view observed at 1000 times. The elements to be measured were O, Fe, Cr, Si, and Al.
Here, the surface layer is defined as a portion from the position where the oxygen O concentration is ½ of the maximum concentration in the oxygen O concentration profile, that is, the position from the position where the oxygen O concentration is half-value to the outermost surface. Since the oxygen O is concentrated on the surface in order to generate an oxide by heat treatment, the portion where the oxygen O is concentrated is used as a surface layer.
Next, the maximum concentration of Cr and Si in the surface layer was defined as follows. That is, in the element concentration profile of only the cation element excluding O from the measurement element, the maximum concentration of Cr is the Cr concentration of the surface layer (atomic%): Cr _sur , and the maximum concentration of Si is the Si concentration of the surface layer (atomic%) ): Si _sur . In detail, in order to reduce the variation due to the measurement position, the maximum concentration of Cr and the maximum concentration of Si were measured at arbitrary five locations, and the average values thereof were defined as Cr _sur and Si _sur . Here, depending on the components, heat treatment, and descaling conditions, the above elements may not be concentrated in the surface layer, and the concentration in the surface layer may be lower than the concentration in the base material. In this case, the maximum value is the concentration near the boundary between the surface layer and the base material, and this concentration is almost equal to the analysis value of the base material.

  A sample to be subjected to the corrosion test was prepared as follows using the test material. A first test plate having a thickness of 0.8 mm × width 50 mm × length 300 mm, and the same size as the first test plate, and was bent to 10 ° at a position 10 mm from the end of the width 50 mm. A second test plate was prepared. The first and second test plates were stacked such that the bent portion of the second test plate was in contact with the first test piece at an angle of 10 °. In order to make contact at 10 °, it is necessary to keep the distance between the first and second test pieces at about 0.28 mm, so the thickness between the first and second test pieces is 0.27 mm, which is slightly thinner than the interval. Of another stainless steel plate. TIG welding was performed while the two contact portions were stacked to obtain a welding material. TIG welding was performed at a speed of 50 cpm with the current value adjusted in the range of 100 to 150 A so that the back bead was formed. During welding, shielding with Ar gas was carried out on both the back side and the front side to suppress oxide generation due to welding heat as much as possible. The welded material was cut to 25 mm in the longitudinal direction and cut along the center of the weld line in the width direction to obtain a weld clearance test piece having a width of 30 mm × length of 25 mm. All cut end faces were mirror-polished and the test results were not affected by the end face form.

The corrosion test was performed as follows.
First, in a corrosion test for water at room temperature (evaluation of corrosion resistance), a test solution was prepared by adding NaCl and copper sulfate reagents to tap water in Hikari-shi, Yamaguchi Prefecture so that the Cl ⁻ amount was 2000 ppm and the Cu ²⁺ amount was 1 ppm. Was made. Tap water was collected without passing through a filter such as a water purifier. Cu ²⁺ was added in order to bring the potential close to the actual environment and accelerate corrosion. The test solution was filled into a large lidded flask and the temperature was maintained at 45 ° C. The sample was immersed in this test solution, and the sample was held for 6 months in a state where air of 0.1 L / min was blown into the test solution. Every week, the whole amount of the test solution was replaced with a new test solution. After the test, a sample was taken out and the clearance was opened to evaluate the presence or absence of corrosion and the depth of corrosion. The corrosion depth was measured in units of 1 micron by a depth of focus method using an optical microscope, and the deepest corrosion depth in the sample was adopted as the corrosion depth of the sample. The case where the value of the corrosion depth was 100 microns or less was regarded as acceptable. Specifically, a test material having a corrosion depth exceeding 100 microns was evaluated as C (bad). A specimen having a corrosion depth of 40 to 100 microns was evaluated as B (good). A specimen having a corrosion depth of less than 40 microns was evaluated as A (excellent).
In the corrosion test for hot water (corrosion resistance evaluation), the test solution, the experiment method, and the test period were the same as the above-described corrosion test for water at room temperature, and only the liquid temperature was 85 ° C. The evaluation method was the same as the corrosion test for water at room temperature.

First, the results of an evaluation test at normal temperature (evaluation of crevice corrosion resistance against water at normal temperature) will be described.
No. of the example of the present invention. No. 1 steel and the comparative example No. 1. The results using 13 steel materials are shown in Tables 3 and 4.
No. Even when the electrolytic method of this embodiment is applied to the steel material No. 1 and the ratio of the electrolysis time of the anode / cathode is 1.0 or less, the Si coverage on the surface is less than 10%, It showed a corrosion depth of over 100 microns.
The same tendency was obtained when the conventional general salt method, neutral salt electrolysis method, and nitric acid hydrofluoric acid immersion were combined.
On the other hand, when a descaling treatment (electrolytic treatment) is performed under the electrolysis conditions of the present embodiment using the electrolytic solution having the composition of the present embodiment (electrolytic material), Si remains on the surface at a ratio of 10% to 50%. The corrosion depth was 100 microns or less.
Also in the electrolysis method of the present embodiment, when the ratio of the anode / cathode electrolysis time exceeded 6.0, the scale was not sufficiently removed and the descale was not completed. In the corrosion test, the corrosion depth exceeded 100 microns.
No. When bright annealing (BA treatment) was applied to steel No. 1 (BA treatment material), although Si remained on the surface, the Si coverage was large at about 90%, and the corrosion depth in the corrosion test was also high. The result exceeded 100 microns.

After the corrosion test, the welding material was taken out and the appearance was observed. As a result, regarding the flat part without corrosion, the BA treated material maintained the same metallic luster as before the test. On the other hand, in the electrolytic material of the present invention example, the surface was slightly thin and hazy. When the surface of the flat portion was examined by GDS (High Frequency Glow Discharge Spectroscopy), oxygen and Cr were slightly concentrated on the surface of the BA-treated material. On the other hand, in the electrolytic material of the example of the present invention, Al, Si, and Ca were concentrated on the surface in addition to oxygen. The Si coverage was also evaluated by the method described above. As a result, in the BA-treated material, Si was uniformly present on the surface. On the other hand, in the electrolytic material of the present invention example, it was found that Si was present on the steel surface at a ratio of 10 to 50% in terms of area ratio.
From this result, it has been clarified that when Si oxide is present on the steel surface in an amount of 10 to 50%, long-term corrosion resistance to tap water at room temperature is improved. The reason for this is presumed to be that Al, Si, and Ca, which are generally referred to as calcite, are easily deposited on the surface as described above, and this functions as a protective film on the surface. In the case of the BA-treated material, the Si oxide was present smoothly and uniformly on the surface, so that it is considered that the chalk component was difficult to precipitate. In Tables 3 and 4, the comparative example No. In 18 steel materials, even when electrolytic treatment was performed under the conditions of this embodiment, the amount of Si remaining on the surface was small and the Si coverage was low. For this reason, in the corrosion test, the corrosion depth exceeded 100 microns. This indicates that not only the amount of Cr and Mo in the base material (steel material) but also the amount of Si on the surface affects the corrosion resistance. In addition, this time, it evaluated in the clearance part of the base material to which sufficient Ar gas shielding was given at the time of welding. When the gas shield is omitted at the time of welding, an oxide scale due to heat is generated. In this case, if Si is concentrated on the surface before welding, the oxide scale due to heat is also suppressed. Moreover, since the scale has a composition in which Al, Si, Ca, etc. are likely to precipitate, the same effect is exhibited and excellent corrosion resistance is obtained.

Subsequently, the relationship between the amount of Cr in the base material (steel material) and the amount of Cr and Si on the surface was investigated. The aforementioned No. No. 1 steel and No. 1 steel. Along with No. 18 steel, No. 1 in Table 1. No. 2 steel, No. 2 The steel material of No. 3 was subjected to a descaling process (electrolytic process) under the conditions of this embodiment, and a test material was manufactured. Specifically, the anode / cathode electrolysis time ratio was 4.0. The total amount of electricity was constant at 10 kQ / m ² which was the same as the conditions in Table 3.
The shape of the sample to be subjected to the corrosion test and the conditions of the corrosion test were also the same as those in Tables 3 and 4 described above. A specimen having a corrosion depth measured in the corrosion test exceeding 100 microns was evaluated as C (bad). A specimen having a corrosion depth of 100 microns or less and 40 microns or more was evaluated as B (good). A specimen having a corrosion depth of less than 40 microns was evaluated as A (excellent). The result is shown in FIG. In Figure 1, the Cr content is x-axis and y-axis values of C _s to be described later, were plotted evaluation results. Specifically, the data of the test material evaluated as C (bad) was plotted with a cross mark. The data of the test material evaluated as B (good) was plotted with white circles. Data of the specimen evaluated as A (excellent) was plotted with double circles. Further, a line segment represented by y = −0.45x + 28 (C _s = −0.45Cr + 28) is drawn in FIG.

In the present _embodiment, it will be described the reason for using Cr + _3Mo, a _{C s} = 3Si _{sur + Cr} sur as an index.
Generally, Cr and Mo are known as elements that improve corrosion resistance. The present inventors have also found that leaving Si oxide on the surface layer has the same effect as Cr and Mo on the corrosion resistance of the gap, particularly the corrosion resistance of the weld gap. That is, the Cr concentration (mass%) of the base material (steel material) and the value (atomic%) calculated from the concentration of Cr and Si on the surface (surface layer); C _s = 3Si _sur + Cr _sur is the following formula (1) It was confirmed that excellent crevice corrosion resistance can be obtained when the above condition is satisfied.
C _s ≧ −0.45Cr + 28 (1)
It was also confirmed that the higher the value of C _s = 3Si _sur + Cr _sur , the better the corrosion resistance even if the value of the Cr concentration of the base material is low. This coefficient was obtained by regression from other results not shown. Although not shown in the figure, when the Cr concentration of the base material was less than 13%, the corrosion resistance was poor even if the value of C _s = 3Si _sur + Cr _sur was high. This is considered to be because it is necessary to add Cr to the steel material in a minimum amount in order to ensure the corrosion resistance with the surface film. From this result, the lower limit of the Cr concentration of the base material was set to 13.0%. On the other hand, if the Cr concentration exceeds 25.0%, the workability deteriorates and the cost increases, so 25.0% was made the upper limit of the Cr concentration. The Cr concentration is preferably 15.0 to 24.0%, more preferably 17.0 to 23.0%.
In addition, toughness was evaluated using a hot-rolled sheet having a thickness of 5 mm obtained in the above-described production process of the test material. A test piece having a thickness of 3 mm, a length of 55 mm, and a width of 10 mm was cut out from the hot-rolled sheet by cutting and grinding, and a 2 mm V-notch was machined in the center in the longitudinal direction of the test piece and subjected to a Charpy test. . The Charpy test was conducted at 0 ° C., and if the absorbed energy was 50 J / cm ² or more, the Charpy test was accepted. The evaluation results of toughness will be described in detail in Examples, but cracking occurred when the amount of Si, Ti, or Cr was larger than the range of the present embodiment. This is considered to be due to the fact that the crystal grains in the welded portion are coarsened and that the elements such as Si, Ti, Cr, and the like have a large amount of elements that lower the toughness, so that the tendency is prominent in the welded portion.

Next, the corrosion resistance in a warm water environment (crevice corrosion resistance against warm water) was evaluated.
No. of the example of the present invention. No. 21 steel and comparative example No. 21. The results using 38 steel materials are shown in Tables 7 and 8. Similar to the test at room temperature, No. When 21 steel materials were processed within the conditions of this embodiment, Cr and Si were concentrated on the surface, and descaling was completed. Moreover, the corrosion resistance in a warm water environment was also favorable. On the other hand, no. No. 38 steel and No. 38 steel. Even in the case of 21 steel materials, when the conditions of this embodiment were not satisfied, the result was inferior in descaling and inferior in corrosion resistance in a hot water environment.

Next, the relationship between the amount of Cr and Mo in the base material and the amount of Cr and Si on the surface was investigated. The aforementioned No. No. 21 steel and no. Along with No. 38 steel material, No. No. 22 steel, no. 23 steel materials were subjected to a descaling treatment (electrolytic treatment) under the conditions of the present embodiment to produce test materials. Specifically, the anode / cathode electrolysis time ratio was 4.0. The total amount of electricity was constant at 10 kQ / m ² which was the same as the conditions in Table 3. Other conditions and criteria were the same as those at the room temperature test described above.
The obtained results are shown in FIG. In Figure 2, the value of Cr + 3Mo the x-axis, the values of _{C s} as y-axis, were plotted evaluation results. As in FIG. 1, the data of the specimen evaluated as C (bad) was plotted with a cross mark. The data of the test material evaluated as B (good) was plotted with white circles. Data of the specimen evaluated as A (excellent) was plotted with double circles. In FIG. 2, a line segment represented by y = −0.45x + 34 (C _s = −0.45 (Cr + 3Mo) +34) is drawn.
When the value of Cr + 3Mo (mass%) of the base material and the value (atomic%) calculated from the concentration of Cr and Si on the surface (surface layer); C _s = 3Si _sur + Cr _sur satisfies the following formula (1 ′) It was confirmed that excellent crevice corrosion resistance was obtained.
C _s ≧ −0.45 (Cr + 3Mo) +34 (1 ′)
Further, it was confirmed that the higher the value of C _s = 3Si _sur + Cr _sur , the better the corrosion resistance even if the value of Cr + 3Mo of the base material is low. Although not shown in the figure, when the value of Cr + 3Mo of the base material is less than 18%, even if the value of C _s = 3Si _sur + Cr _sur is high, the corrosion resistance is poor. From this result, the lower limit of the value of Cr + 3Mo was set to 18%. On the other hand, when the value of Cr + 3Mo exceeds 30%, the workability is lowered and the cost is also increased. For this reason, 30% was made the upper limit of the value of Cr + 3Mo. The value of Cr + 3Mo is preferably 19 to 27%, more preferably 20 to 26%.
Of course, these materials, which have excellent crevice corrosion resistance against hot water, have sufficient crevice corrosion resistance even at lower temperature room temperature water and can be applied at room temperature. .

  The ferritic stainless steel of the present invention was made based on the knowledge obtained from the above experimental results. The ferritic stainless steel according to the embodiment will be described in detail below.

(First embodiment)
The range of the element content of this embodiment will be described below. The unit “%” indicating the element content means mass%.
Cr is the most important element for securing the corrosion resistance of stainless steel, and stabilizes the ferrite structure. For this reason, at least 13.0% Cr is necessary. Increasing the Cr content also improves the corrosion resistance. However, since Cr is a rare metal, not only suppression of its use is desired, but addition of an excessive amount of Cr also deteriorates workability and toughness that becomes a problem during production. For this reason, the upper limit of the Cr amount is set to 25.0%. The amount of Cr is desirably 16.0 to 24.0%, and more desirably 18.0 to 23.0%.

  Si is an important element as a deoxidizing element, and is also effective for corrosion resistance and oxidation resistance. In the present embodiment, Si is a main element that concentrates on the surface and improves the corrosion resistance. However, the addition of an excessive amount of Si not only decreases the toughness of the base material but also increases the strength. Thereby, workability and manufacturability are lowered. Therefore, the Si amount in the base material is set to 0.08% to 0.50%, and various characteristics are improved by promoting the concentration of Si to the surface layer. The amount of Si is desirably 0.10 to 0.35%, and more desirably 0.12 to 0.29%.

  Since C reduces intergranular corrosion resistance and workability, it is necessary to reduce the content thereof. For this reason, the upper limit of the C amount is set to 0.02% or less. Since excessively reducing the amount of C worsens the refining cost, the amount of C is more preferably 0.002 to 0.015%.

  N, like C, lowers the intergranular corrosion resistance and workability, so it is necessary to reduce the N content. For this reason, the upper limit of the N amount is set to 0.025% or less. However, since excessively reducing the amount of N worsens the refining cost, the amount of N is more preferably 0.002 to 0.015%.

  Mn is an important element as a deoxidizing element. However, when an excessive amount of Mn is added, MnS that becomes a starting point of corrosion is easily generated. An excessive amount of Mn destabilizes the ferrite structure. For this reason, Mn content shall be 0.01-1.00% or less. The amount of Mn is more desirably 0.05 to 0.50%.

  P not only lowers weldability and workability, but also easily causes intergranular corrosion, so the amount of P needs to be kept low. Therefore, the P content is 0.035% or less. The amount of P is more preferably 0.001 to 0.025%.

  Since S produces water-soluble inclusions that are the starting points of corrosion such as CaS and MnS, it is necessary to reduce the amount of S. Therefore, the S amount is set to 0.010% or less. However, since excessive reduction of the amount of S causes cost deterioration, the amount of S is more desirably 0.0002 to 0.0500% or less.

  Al is important as a deoxidizing element, and also has an effect of controlling the composition of nonmetallic inclusions and refining the structure. However, the addition of an excessive amount of Al leads to coarsening of non-metallic inclusions, which may be a starting point for product wrinkles. Therefore, the lower limit value of the Al amount is set to 0.005%, and the upper limit value of the Al amount is set to 0.300%. The amount of Al is desirably 0.007% to 0.15%, and more desirably 0.010 to 0.10%.

  Nb is an extremely important element for fixing C and N and suppressing intergranular corrosion of the base metal and the welded portion in the ferritic stainless steel. However, the addition of an excessive amount of Nb decreases the workability, so the range of the Nb amount is 0.01 to 0.60%. The amount of Nb is desirably 0.05 to 0.50%, and more desirably 0.10 to 0.40%.

Furthermore, you may contain the following elements as needed.
Ti, like Nb, is a very important element for fixing C and N, suppressing intergranular corrosion of the weld, and improving workability, and is added as necessary. However, the addition of an excessive amount of Ti reduces the toughness of the material. Further, a lot of hard Ti-based nonmetallic inclusions are generated, which causes surface defects during production. For this reason, the range of Ti amount is 0.03 to 0.30%. The amount of Ti is desirably 0.05 to 0.27%, and more desirably 0.07 to 0.20%.
In addition, when adding Ti, in order to stabilize C and N and to suppress a toughness fall, the balance of Ti amount and Nb amount becomes important. For this reason, it is necessary to add Ti and Nb so that Nb / Ti exceeds 1.0. Nb / Ti is preferably 1.2 to 6.0, and more preferably 1.4 to 4.0.

  Cu has the effect of reducing the active dissolution rate when corrosion occurs. However, the addition of an excessive amount of Cu reduces workability, and in some cases, the eluted Cu ions may accelerate corrosion. For this reason, when adding Cu, the range of Cu amount shall be 2.0% or less. The amount of Cu is desirably 0.20 to 1.5%, and more desirably 0.25 to 1.1%.

  Ni suppresses the active dissolution rate and has excellent repassivation characteristics due to a small hydrogen overvoltage. However, the addition of an excessive amount of Ni lowers the workability and makes the ferrite structure unstable, so the upper limit of the Ni amount is set to 2.0%. The amount of Ni is desirably 0.10 to 1.2%, and more desirably 0.20 to 1.1%.

  When Sn is added, the active dissolution rate is lowered and the corrosion resistance is improved. In particular, the effect can be obtained by adding a small amount of Sn. However, the addition of an excessive amount of Sn reduces workability. For this reason, when adding Sn, the range of Sn content shall be 0.50% or less. The amount of Sn is desirably 0.01 to 0.40%, and more desirably 0.03 to 0.30%.

  Sb has the same effect as Sn. Therefore, when adding Sn, the range of Sn content shall be 0.50% or less. The amount of Sn is desirably 0.01 to 0.40%, and more desirably 0.03 to 0.30%.

  V and Zr have the effect of improving weather resistance and crevice corrosion resistance. If V is added while suppressing the amount of Cr and Mo, excellent workability can be secured. However, the addition of excessive amounts of V and Zr saturates the effect of reducing workability and improving corrosion resistance. For this reason, the lower limits of the V amount and the Zr amount are each 0.03%, and the upper limits of the V amount and the Zr amount are each 0.50%. Each of the V amount and the Zr amount is more preferably 0.05 to 0.30%.

  B is an effective grain boundary strengthening element for improving secondary work brittleness. However, the addition of an excessive amount of B causes a solid solution strengthening of ferrite and decreases ductility. For this reason, the upper limit of the B amount is set to 0.0050%. The amount of B is more desirably 0.0002 to 0.0020%.

  Ta is an element that improves corrosion resistance, particularly acid resistance. However, the addition of an excessive amount of Ta reduces the workability and also saturates the effect of improving the corrosion resistance. For this reason, when adding Ta, the upper limit of Ta amount is made 0.10%. The amount of Ta is more desirably 0.00010 to 0.0050%.

  Ca is an element used in the production process of stainless steel as a deoxidizing element. However, when an excessive amount of Ca is added, a large amount of Ca-based nonmetallic inclusions may be generated, which may become a starting point of corrosion. Therefore, when adding Ca, the amount of Ca is made 0.010% or less. The amount of Ca is more desirably 0.0001 to 0.0050%.

  Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizer. Mg acts as a crystallization nucleus of TiN. TiN becomes a solidification nucleus of the ferrite phase in the solidification process and promotes crystallization of TiN. Thereby, the ferrite phase can be finely generated during solidification. By miniaturizing the solidified structure, it is possible to prevent the occurrence of surface defects due to coarse solidified structures such as ridging and roping of the product. Furthermore, workability is improved. For this reason, Mg is added as needed. When adding Mg, in order to express these effects, the amount of Mg is made 0.0001% or more. However, if the Mg content exceeds 0.0050%, manufacturability deteriorates, so the upper limit of the Mg content is 0.0050%. Preferably, considering the manufacturability, the Mg amount is set to 0.0003 to 0.0020%.

  REM (rare earth metal element) is an element that improves the hot workability and the cleanliness of steel and is effective in improving the corrosion resistance of the present embodiment, and may be added as necessary. When added, the REM amount (total amount of rare earth metal elements) is set to 0.001% or more in order to exhibit the effect. However, the addition of an excessive amount of REM leads to an increase in alloy cost and a decrease in manufacturability, so the upper limit of the REM amount is set to 0.100%. Preferably, considering the effect, economy and manufacturability, the REM amount is 0.001 to 0.050%. REM refers to a generic name of two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu), and examples thereof include La, Ce, and Nd. .

  Ga is an element having an effect of improving the toughness of stainless steel. However, addition of an excessive amount of Ga deteriorates manufacturability and increases costs. For this reason, when adding Ga, the amount of Ga shall be 0.010% or less. The Ga content is more preferably 0.0001 to 0.005%.

Next, the surface layer of the ferritic stainless steel of this embodiment will be described.
The surface coverage of the surface Si oxide is 10 to 50%, preferably 15 to 45%, and more preferably 20 to 40%. As a result, Al, Si, and Ca, which are generally referred to as “calcium”, contained in tap water are likely to precipitate on the surface. The chlorine component deposited on the surface functions as a protective film on the surface. For this reason, the crevice corrosion resistance excellent with respect to the tap water at normal temperature is obtained for a long time.
The surface coverage of the surface Si oxide is measured by the method described above.

The following formula (1) is satisfied.
C _s ≧ −0.45Cr + 28 (1)
Cr in Formula (1) shows Cr content (mass%) in ferritic stainless steel (base material). C _s is a value calculated from the following equation (A).
C _s = 3Si _sur + Cr _sur (A)
In the formula (A), Si _sur represents the Si concentration ratio (atomic%) in the surface layer, and Cr _sur represents the Cr concentration ratio (atomic%) in the surface layer.
Surface Cr concentration (Cr concentration ratio, atomic%): Cr _sur and surface Si concentration (Si concentration ratio, atomic%): Si _sur are measured as described above.
From the result of FIG. 1, by satisfying the formula (1), excellent crevice corrosion resistance can be obtained for a long period of time with respect to room temperature (45 ° C.) tap water.

(Second Embodiment)
The ferritic stainless steel of the second embodiment has excellent crevice corrosion resistance against water at normal temperature and warm water.
The chemical composition of the ferritic stainless steel of the second embodiment is different from the chemical composition of the ferritic stainless steel of the first embodiment in that it contains Mo: 0.03 to 3.0%. Components other than Mo and the operation and effects thereof are the same as those in the first embodiment. For this reason, description regarding components other than Mo is omitted.

  Mo is effective in repairing the passive film and is an extremely effective element for improving the corrosion resistance. Furthermore, when Mo and Cr are combined, the effect of improving pitting corrosion resistance is obtained. When the amount of Mo is increased, the corrosion resistance is improved, but the workability and toughness are lowered, and the cost is increased. For this reason, the upper limit of the Mo amount is set to 3.0%. In order to exhibit these characteristics, it is necessary to contain Mo in an amount of at least 0.03%. The amount of Mo is desirably 0.30 to 2.0%, and more desirably 0.45 to 2.0%.

The surface layer of the ferritic stainless steel of this embodiment will be described.
The surface coverage of the surface Si oxide is 10 to 50%, preferably 15 to 45%, and more preferably 20 to 40%. Since the method for measuring the surface coverage of the surface Si oxide and the effects thereof are the same as those in the first embodiment, description thereof is omitted.
The following expression (1 ′) is satisfied.
C _s ≧ −0.45 (Cr + 3Mo) +34 (1 ′)
Cr and Mo in the formula (1 ′) indicate the Cr content (mass%) and the Mo content (mass%) in the ferritic stainless steel (base material), respectively. C _s is a value calculated from the following equation (A).
C _s = 3Si _sur + Cr _sur (A)
In the formula (A), Si _sur represents the Si concentration ratio (atomic%) in the surface layer, and Cr _sur represents the Cr concentration ratio (atomic%) in the surface layer. The measurement method of Si _sur and Cr _sur is the same as that of the first embodiment.
Furthermore, the following expression (2) is also satisfied.
Cr + 3Mo: 18-30 (2)
Cr and Mo in the formula (2) indicate the Cr content (mass%) and the Mo content (mass%) in the ferritic stainless steel, respectively.

From the results of FIG. 2, by satisfying the formula (1 ′) and the formula (2), excellent crevice corrosion resistance with respect to 85 ° C. tap water (hot water) can be obtained for a long time. It has sufficient crevice corrosion resistance even to cold water at room temperature.
The value of Cr + 3Mo in the formula (2) is preferably 19 to 27%, more preferably 20 to 25%.

(Third embodiment)
The present embodiment relates to a method for manufacturing the ferritic stainless steel of the first and second embodiments.
Steel components are obtained by adjusting the steel components so as to have the chemical composition described in the first and second embodiments. Next, the steel material is subjected to hot rolling, scale removal, cold rolling, and heat treatment to produce a cold rolled sheet. Finish annealing is applied to the cold-rolled sheet to obtain a heat treatment material.
The conditions of the manufacturing method of well-known ferritic stainless steel are applied to the above process.

The heat treatment material is subjected to the following descaling treatment (electrolytic treatment).
If necessary, a pretreatment is performed according to a salt method or a neutral salt electrolysis method. The pretreatment conditions are not particularly limited, and known conditions are applied.
Next, electrolytic treatment is performed.
An aqueous solution containing 50 to 300 g / L sodium sulfate and 50 g / L or more nitrate ions is used as the electrolytic solution. Examples of nitrate ions include various compounds containing nitrate ions such as nitric acid and sodium nitrate. The amount of nitric acid in the electrolytic solution is preferably 70 to 300 g / L, and more preferably 80 to 150 g / L. The amount of sodium sulfate (Na ₂ SO ₄ ) in the electrolytic solution is 50 to 300 g / L, preferably 70 to 200 g / L, and more preferably 90 to 180 g / L. Further, the pH of the electrolytic solution is not particularly limited, but is preferably lower. Specifically, the pH of the electrolytic solution is preferably −1 to 6, and more preferably more than 0 to less than 3.
The heat treatment material is sandwiched indirectly (without contact with the heat treatment material) between the positive electrode and the negative electrode, and the heat treatment material and the electrode are immersed in the electrolytic solution. In this state, current is supplied to the heat treatment material to perform alternating electrolysis.

When electrolysis is performed between the negative electrode and the heat treatment material (anode electrolysis), the heat treatment material becomes an anode, and the surface of the heat treatment material is oxidized and dissolved. Specifically, the oxide mainly composed of Cr is dissolved and removed from the surface of the heat treatment material.
When electrolysis is performed between the positive electrode and the heat treatment material (cathode electrolysis), the heat treatment material becomes a cathode, and oxide scale mainly composed of Fe oxide is reduced and dissolved.
The time during which electrolysis is performed between the negative electrode and the heat treatment material is referred to as anode electrolysis time, and the time during which electrolysis is performed between the positive electrode and the heat treatment material is referred to as cathode electrolysis time.
The ratio of anode / cathode electrolysis time is more than 1.0 and 6.0 or less. The ratio of anode / cathode electrolysis time is preferably 1.5 to 5.5, more preferably more than 2.0 to 5.0.
Specifically, when electrolysis is performed between the negative electrode and the heat treatment material, the anode electrolysis time is lengthened to lower the anode current value (current density per unit area of the heat treatment material). Thereby, from the surface of the heat treatment material (steel plate), an oxide mainly composed of Cr and Fe is dissolved and removed, and an appropriate amount of Si oxide is left.
When electrolysis is performed between the positive electrode and the heat treatment material, the cathode electrolysis time is shortened to increase the cathode current value (current density per unit area of the heat treatment material). Thereby, the efficiency of reduction and dissolution of oxide scale mainly composed of Fe oxide is improved, and metal precipitation due to reduction can be suppressed.
Thereby, Si oxide can coat | cover a steel surface layer moderately, and can obtain corrosion resistance more than the corrosion resistance expected from the chemical composition of a base material.

Example 1
Steel materials having the chemical composition shown in Table 1 were manufactured by a normal high purity ferritic stainless steel manufacturing method. A method for producing a test material by performing a descaling process on a steel material and a method for producing a sample used for the test are the same as those in the above-described embodiment.
Specifically, the steel material was rolled and heat-treated to produce a cold-rolled plate having a thickness of 0.8 mm. The cold-rolled plate is subjected to a soaking treatment (finish annealing) for 1 minute in an atmosphere simulating LNG combustion exhaust gas (gas composition: 3% O ₂ -12% CO ₂ -balance N ₂ , dew point 40 ° C), A heat-treated material was obtained. The temperature of finish annealing was set in the range of 900 to 980 ° C. based on the recrystallization temperature of each material.

The heat treatment material was subjected to a descaling process under the same conditions as in the previous embodiment. An electrolytic solution having a nitrate ion concentration of 80 to 150 g / L and a Na ₂ SO ₄ concentration of 70 to 200 g / L was prepared using a commercially available nitric acid reagent. The test piece was used as a sample electrode, and two SUS304 stainless steel plates were placed 10 mm apart on both sides to form a counter electrode. The current density and electrolysis time were controlled using a potentiostat to simulate alternating electrolysis by indirect energization of the actual machine. Regarding the electrolysis time, the total of one anode time and cathode time (time for one cycle) was set to 5 seconds, and the ratio of the electrolysis time of the anode / cathode was changed from 1.0 to 8.0. By repeating this cycle four times, the total electrolysis time was kept constant at 20 seconds. In this case, the current density of each of the anode electrolysis and the cathode electrolysis was changed in accordance with the electrolysis time so that the total amount of electricity was constant at 10 kQ / m ² .
The test material was manufactured by the above.

Descaleability after electrolysis, surface Si coverage, and surface (surface layer) Cr concentration (Cr concentration ratio, atomic%): Cr _sur and surface (surface layer) Si concentration (Si concentration ratio, atomic%): Si _sur was measured by the method described in the embodiment.

By the method described in the embodiment, a weld clearance test piece having a clearance angle of 10 ° and a width of 30 mm × 25 mm was produced.
A corrosion test at normal temperature (evaluation of crevice corrosion resistance against water at normal temperature) was performed by the method described in the embodiment. A specimen having a corrosion depth in the gap exceeding 100 microns was evaluated as C (bad). A specimen having a corrosion depth of 40 to 100 microns was evaluated as B (good). A specimen having a corrosion depth of less than 40 microns was evaluated as A (excellent).

Toughness was evaluated by the method described in the embodiment. Specifically, a sample having a thickness of 3 mm, a length of 55 mm, and a width of 10 mm was cut out from a hot-rolled sheet having a thickness of 5 mm obtained in the course of manufacturing the test material. A 2 mm V-notch was machined at the center in the longitudinal direction of the sample and subjected to a Charpy test. The Charpy test was performed at 0 ° C., and the test material having an absorption energy of 50 J / cm ² or more was evaluated as acceptable (B). A specimen having an absorbed energy of less than 50 J / cm ² was evaluated as rejected (C).
The obtained results are shown in Tables 2-4.

Specimen No. 1 to 16 were produced by having a chemical composition within the range of the first embodiment and being subjected to descaling (electrolysis) under the conditions of the third embodiment. Specimen No. In 1-16, Si coverage of the surface layer was 10-50%. Moreover, the corrosion depth measured by the corrosion test at normal temperature was below the standard value (100 microns), and showed excellent crevice corrosion resistance.
On the other hand, the test material No. 17-20 have a chemical composition outside the range of the first embodiment. For this reason, even if the descaling treatment (electrolysis) is performed under the conditions of the third embodiment, the Si coverage of the surface layer is as low as less than 10%, or the Si coverage is large and the descaling has not been completed ( Evaluation of C). For this reason, the corrosion depth measured in the corrosion test at room temperature exceeded the reference value (100 microns). In addition, specimen No. In No. 19, the amount of Si was larger than the range of the first embodiment, and cracking occurred in the toughness test of the hot-rolled sheet.

(Example 2)
Sample materials were manufactured under the same conditions as in Example 1 except that steel materials having chemical compositions shown in Table 5 were used. In the same manner as in Example 1, a weld gap test piece was produced.
A corrosion test in a warm water environment (evaluation of crevice corrosion resistance against warm water) was performed in the same manner as the corrosion test at normal temperature except that the liquid temperature was 85 ° C.
Various characteristics were evaluated in the same manner as in Example 1.
The obtained results are shown in Tables 5-8.

Specimen No. 21 to 36 had a chemical composition within the range of the second embodiment, and were manufactured by being descaled (electrolyzed) under the conditions of the third embodiment. Specimen No. In 21-36, Si coverage of the surface layer was 10-50%. The corrosion depth measured in the hot water corrosion test was below the standard value (100 microns), indicating excellent corrosion resistance.
On the other hand, the test material No. 37 to 40 have a chemical composition outside the range of the second embodiment. For this reason, even if the descaling treatment (electrolysis) is performed under the conditions of the third embodiment, the Si coverage of the surface layer is as low as less than 10%, or the Si coverage is large and the descaling has not been completed ( Evaluation of C). For this reason, the corrosion depth measured in the corrosion test with warm water exceeded the reference value (100 microns). In addition, specimen No. In No. 39, the amount of Si was larger than the range of the second embodiment, and cracking occurred in the toughness test of the hot-rolled sheet.

  From the above results, the test material having a chemical composition within the range of the first and second embodiments and subjected to the descaling treatment under the conditions of the third embodiment is excellent in crevice corrosion resistance. It has become clear that it has excellent properties and manufacturability.

  The ferritic stainless steel of the present embodiment has excellent crevice corrosion resistance against water at normal temperature and warm water. For this reason, if it is various containers and tanks which store water and hot water, the outstanding effect is exhibited irrespective of the structure. In addition to the tank, the same effect is exhibited in facilities that use tap water, such as a bathtub and kitchen equipment. In addition, for example, a drain water collector of a latent heat recovery type gas water heater and a structure that comes into contact with condensed water in a heat exchange device such as the heat exchanger, an automobile exhaust system material or exterior material, various batteries The ferritic stainless steel of the present embodiment can be widely applied even in parts that require corrosion resistance such as materials and welded pipes. In addition, as described above, the ferritic stainless steel of the present embodiment can be preferably applied not only to the base material but also to a structure that is used in a TIG welded state and requires corrosion resistance.

Claims (9)

% By mass
C: 0.020% or less,
N: 0.025% or less,
Si: 0.08 to 0.50%,
Mn: 0.01 to 1.0%
P: 0.035% or less,
S: 0.01% or less,
Cr: 13.0-25.0%,
Al: 0.005 to 0.300%, and Nb: 0.01 to 0.60%,
The balance consists of Fe and inevitable impurities,
A ferritic stainless steel excellent in crevice corrosion resistance in water at 45 ° C., characterized by satisfying the following formula (1) and having a surface coverage of a surface Si oxide of 10 to 50%.
C _s ≧ −0.45Cr + 28 (1)
C _s = 3Si _sur + Cr _sur (A)
However, equation (1) Cr in represents the Cr content of the ferritic stainless steel (mass%), _{C s} is the value calculated from the equation (A), Si in the formula (A) _sur represents the Si concentration ratio (atomic%) in the surface layer, and Cr _sur represents the Cr concentration ratio (atomic%) in the surface layer. The crevice corrosion resistance in water at 45 ° C. according to claim 1, further comprising, by mass%, Ti: 0.03 to 0.30% and satisfying the following formula (3): Ferritic stainless steel with excellent properties.
Nb / Ti> 1.0 (3)
However, Nb and Ti in Formula (3) show Nb content (mass%) and Ti content (mass%) in ferritic stainless steel, respectively. Further, in terms of mass%, Cu: 2.0% or less, Ni: 2.0% or less, Sn: 0.50% or less, Sb: 0.50% or less, V: 0.50% or less, Zr: 0.0. 50% or less, B: 0.0050% or less, Ta: 0.10% or less, Ca: 0.010% or less, Mg: 0.0050% or less, Ga: 0.010% or less, and REM: 0.100 The ferritic stainless steel having excellent crevice corrosion resistance in water at 45 ° C. according to claim 1, wherein the ferritic stainless steel contains at least one selected from the group consisting of 1% or less. % By mass
C: 0.020% or less,
N: 0.025% or less,
Si: 0.08 to 0.50%,
Mn: 0.01 to 1.0%
P: 0.035% or less,
S: 0.01% or less,
Cr: 13.0-25.0%,
Al: 0.005 to 0.300%,
Nb: 0.01 to 0.60%, and Mo: 0.03 to 3.0%,
The balance consists of Fe and inevitable impurities,
Satisfy the following formula (1 ') and formula (2), and you surface coverage of the surface layer of Si oxide, wherein 10 to 50% crevice corrosion resistance in a 85 ° C. water Ferritic stainless steel with excellent resistance.
C _s ≧ −0.45 (Cr + 3Mo) +34 (1 ′)
C _s = 3Si _sur + Cr _sur (A)
Cr + 3Mo: 18-30 (2)
However, the formula (1 '), Cr in the formula (2), Mo is, Cr content of the ferritic stainless steel (mass%), shows the Mo content (mass%), respectively, _{C s} has the formula ( It is a value calculated from A), Si _sur in formula (A) indicates the Si concentration ratio (atomic%) in the surface layer, and Cr _sur indicates the Cr concentration ratio (atomic%) in the surface layer. Furthermore, by mass% Ti: includes 0.03 to 0.30%, and characterized by satisfying the following equation (3), according to claim 4, crevice in a 85 ° C. water Ferritic stainless steel with excellent corrosivity.
Nb / Ti> 1.0 (3)
However, Nb and Ti in Formula (3) show Nb content (mass%) and Ti content (mass%) in ferritic stainless steel, respectively. Further, in terms of mass%, Cu: 2.0% or less, Ni: 2.0% or less, Sn: 0.50% or less, Sb: 0.50% or less, V: 0.50% or less, Zr: 0.0. 50% or less, B: 0.0050% or less, Ta: 0.10% or less, Ca: 0.010% or less, Mg: 0.0050% or less, Ga: 0.010% or less, and REM: 0.100 % of claim 4 or 5, characterized in that it comprises one or more selected from the following, ferritic stainless steel excellent in crevice corrosion resistance in a 85 ° C. water. The ferritic stainless steel excellent in crevice corrosion resistance in water at 45 ° C according to any one of claims 1 to 3, which is used for a water storage tank. According to any one of claims 4-6, which is used for hot water storage tank, a ferritic stainless steel excellent in crevice corrosion resistance in a 85 ° C. water. A step of performing a descaling treatment on the heat treatment material having the chemical composition according to any one of claims 1 to 6,
In the descaling process, the heat treatment material is indirectly sandwiched between positive and negative electrodes, the heat treatment material and the electrode are immersed in an electrolytic solution, and alternating electrolysis is performed.
As the electrolytic solution, an aqueous solution containing 50 to 300 g / L of sodium sulfate and 50 g / L or more of nitrate ions is used.
The ratio of the electrolysis time of the anode / cathode in the alternating electrolysis is more than 1.0 and 6.0 or less, and the water at 45 ° C or 85 ° C according to any one of claims 1 to 6 Of ferritic stainless steel with excellent crevice corrosion resistance.

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