JP2016128591A - Ferritic stainless steel for hot water storing and water storing vessel excellent in weld zone toughness and water leak resistance and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve resistance for water leak due to crevice corrosion of a water storing and hot water storing tank of ferritic stainless steel and breakage inhibition of a weld zone due to water pressure variation during water supply and stop by inhibiting reduction of toughness of the weld zone while minimizing addition of Cr and Mo.SOLUTION: There is provided ferritic stainless steel containing, by mass%, C:0.020% or less, N:0.025% or less, Si:0.08 to 0.3%, Mn:0.01 to 1.0%, P:0.035% or less, S:0.01% or less, Cr:16 t 24%, Mo:0.5 to 2.5%, Al:0.005 to 0.2%, and Nb:0.10 to 0.60%, satisfying following formulae (1) and (2) and having surface coating percentage of Si oxide of a surface layer of 10 to 50%. (1) Cs≥-5.0(Cr+3Mo)+155, Cs=3Si+Cr, (2) Cr+3Mo:18 to 27, where Siand Crmean surface layer cation concentration ratio (atom%).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a ferritic stainless steel for water storage and hot water storage containers (tanks) having excellent weld toughness and water leakage resistance, and a method for producing the same.

  Stainless steel has been used as a tank material for storing water and hot water because of its excellent corrosion resistance, strength, and earthquake resistance. In particular, ferritic stainless steel has features such as a smaller coefficient of thermal expansion than austenitic stainless steel and excellent stress corrosion cracking resistance. In recent years, as an avoidance from the high price of Ni raw materials, Its application is widespread, and SUS444 (19Cr-2Mo) is the representative steel type.

  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 is a capsule type with a bowl-shaped end plate part on top and bottom. In an industrial mass production process, the body and the end plate are generally welded one on top of the other, so a clearance is formed on the structure. This weld crevice easily induces crevice corrosion, which is the cause of local corrosion of stainless steel. Even in the case of SUS444, water leakage due to crevice corrosion may occur if the water quality is poor, the welding conditions or the gap structure is inappropriate. May occur. Some water storage tanks do not have a gap due to welding. In this case, however, they have a structure in which panel-shaped stainless steel plates are joined together with resin packing and bolted together. In this case, there is a gap between the stainless steel and the packing, and if this is the case, the water may be corroded if the water quality is poor.

  The above-mentioned capsule-shaped weld gap tends to concentrate stress due to its structure, and there is a concern that it will break due to changes in water pressure during hot and cold water supply. For this reason, a durability test that changes the water pressure in the tank was conducted as a test at the time of manufacturing the tank. If the welded structure and welding conditions are inappropriate, cracks will occur in this part, so that the toughness reduction of the weld gap is suppressed. It becomes important.

  Patent Document 1 discloses a method of improving the corrosion resistance of the weld gap by using high CrMo steel and adding Ti and Al. 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 the reduction of the amount of use thereof is desired worldwide. In particular, since the speculative price fluctuation occurs in Mo, the 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 Cr and Mo, which are main elements of stainless steel, are reduced. For example, Patent Document 5 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.

  In addition, as described above, hot water storage / storage tanks generally require repeated fatigue strength due to changes in water pressure. However, since a strong impact due to water pressure is applied to the welded part at the time of water stoppage, it is desirable that the toughness indicating the strength against the impact at the welded part is also high. Regarding the method for improving the weld strength of ferritic stainless steel, Patent Document 2 discloses a method of adding Si to the material itself and depositing NbC, and Patent Document 3 discloses a method of adding Ni. .

  However, when the present inventors examined these prior arts, it was found that further improvements were necessary. In other words, in the above-described prior art, a large amount of Si is added to steel, but as a result of the study by the present inventors, toughness is obtained by adding Si or Ti that improves the tensile strength of the material. It turns out that it falls. 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, a method of improving descalability by combining sulfuric acid and nitric acid is disclosed in Patent Document 6, and a method of descaling in a solution in which sulfuric acid ions or sodium ions are mixed with nitric acid. Is disclosed in Patent Document 7. 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 2009-41041 A JP-A-6-279950 JP 11-61500 A JP 2005-232546 A

  The present invention minimizes the addition of Cr and Mo, which are required to be reduced as rare metals, and does not add more than necessary Si or Ti, which lowers the toughness of the welded portion. The purpose is to simultaneously improve the suppression of water leakage due to crevice corrosion and the prevention of fracture of welds due to fluctuations in water pressure at the time of water stoppage, which are problems in water storage and hot water storage tanks.

  As a result of intensive studies on the method for solving the above-mentioned problems, the present inventors have found that Si should be actively left only on the surface and the addition to the material should be kept to a minimum. In addition, as a method of leaving Si only on this surface, it was clarified that it is important to optimize the electrolytic descale conditions in the annealing pickling process at the time of steel plate production. In other words, in the actual production electrolysis process, the supply of current to the steel sheet is performed by sandwiching the positive and negative electrodes indirectly through the steel sheet and through the electrolyte, so the negative electrode (the steel sheet side becomes the anode) The total amount of electricity (= current value × time) supplied from the positive electrode (the steel plate is the cathode) is constant. Here, by lengthening the anode time for oxidizing and dissolving the surface, that is, by appropriately reducing the anode current value (current density per unit area of the steel plate), the surface Cr and Fe-based oxides are dissolved and removed. It has been found that even if the amount of Si added to the steel is the minimum necessary, it is possible to leave an appropriate Si oxide. At the same time, 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 if the cathode side has a high current density and a short time. Therefore, it is desirable that the anode / cathode electrolysis time ratio during electrolysis is more than 1.0. As a result, the Si oxide appropriately coated the steel surface layer, and it became possible to ensure the corrosion resistance higher than the Cr and Mo contents of the base material. The reason for this is that the surface Si oxide itself acts as a protective coating to improve corrosion resistance, and the Si oxide coverage of this embodiment is also used as a tap water component during use as a hot water tank or water storage tank. The present inventors have found that the corrosion resistance is further improved by forming a stable oxide film on the surface layer because Al, Ca, and Si, which are contained in the calcium oxide, are easily deposited on the surface layer.

This invention is made | formed based on the said knowledge, and makes the following structures a summary.
That is, the present invention
(1) By mass%, C: 0.020% or less, N: 0.025% or less, Si: 0.08-0.3%, Mn: 0.01-1.0%, P: 0.035 % Or less, S: 0.01% or less, Cr: 16 to 24%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.2%, Nb: 0.10 to 0.60% The balance consists of Fe and inevitable impurities, satisfies the following formulas (1) and (2), and the surface coverage of the surface Si oxide is 10 to 50%. Ferritic stainless steel for hot water storage and storage containers with excellent toughness and water leakage resistance.
(1) Cs ≧ −5.0 (Cr + 3Mo) +155
Cs = 3Si _sur + Cr _sur
(2) Cr + 3Mo: 18-27
Here, Cr and Mo mean the base material concentration (mass%) of each element, and Si _sur and Cr _sur mean the surface cation concentration ratio (at%) of Si and Cr, respectively.
(2) Further, in terms of% by mass, Ti: 0.03 to 0.25% is satisfied and the following formula (3) is satisfied. Excellent ferritic stainless steel for hot water storage and storage containers.
(3) Nb / Ti> 1
Here, Nb and Ti mean the base material concentration (mass%) of each element.
(3) In addition, (1) or (2), characterized by containing one or more of Cu: 2.0% or less, Ni: 2.0% or less, and Sn: 0.5% by mass% Ferritic stainless steel for hot water storage / water storage containers as described in (2), which has excellent weld toughness and water leakage resistance.
(4) Further, in mass%, V: 0.5% or less, Zr: 0.5% or less, B: 0.005% or less, including one or more, (1) The ferritic stainless steel for hot water storage / storage containers excellent in welded portion toughness and water leakage resistance according to any one of (3).
(5) In the final finish annealing pickling step of stainless steel, electrolytic treatment is performed with an aqueous solution containing 50 g / L or more of nitric acid, and the anode / cathode electrolysis time ratio in indirect energization at the time of electrolytic treatment exceeds 1.0, 6 The method for producing a ferritic stainless steel for hot water storage / storage containers having excellent weld toughness and water leakage resistance according to any one of the above (1) to (4), wherein the ferritic stainless steel is excellent in weld toughness and water leakage resistance.

  The ferritic stainless steel of the present invention and the manufacturing method thereof can achieve both the crevice corrosion resistance and the toughness of the welded portion while suppressing the content of expensive elements such as Cr and Mo. 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.

It is a figure which shows the influence which Cr + 3Mo and 3Si _sur + Cr _sur exert on corrosion depth.

The present invention has been found as follows.
The test material was manufactured as follows. The developed steel composition and the comparative steel composition are ferritic stainless steels shown in Nos. 1 to 3 and 13 of Table 1 melted by a vacuum melting furnace, and a cold rolled sheet having a thickness of 0.8 mm was manufactured by rolling and heat treatment. . This is a gas composition simulating LNG combustion exhaust gas: 3% O ₂ -12% CO ₂ —N ₂ bal. Finish annealing was performed at a dew point of 40 ° C. and a soaking heat treatment of 980 ° C. for 1 minute, and this heat treated material was subjected to the following descaling treatment.

In the descaling process, the salt method and the Rusner electrolysis method were implemented as part of the pretreatment. The salt method was carried out by immersing the steel sheet for 10 seconds in a commercial descaling alkali salt mainly composed of NaOH held at 450 ° C. The electrolytic solution of the Rusner electrolysis method was a 150 g / L aqueous solution using a commercially available sodium sulfate reagent. The electrolysis conditions were as follows: a test piece was used as a sample electrode, and two SUS304 stainless steel plates were placed in parallel on both sides of the test piece by 10 mm. The current density and electrolysis time were simulated by alternating electrolysis using indirect energization of the actual machine. In the Rusner electrolysis method in this test, the current density was ± 100 mA / cm ² , and 10 seconds each was carried out for 20 seconds in total. The anode / cathode time ratio at this time is 1.0 because it is 1 second, and the amount of electricity is 10 kQ / m ² . The electrolysis conditions were controlled using a function generator and a potentiostat.

The electrolytic solution of the developed electrolytic method was an aqueous solution having a concentration shown in Table 2 using a commercially available nitric acid reagent. The electrolysis method was the same as the Lusner method described above, and the electrolysis time was changed from 1.0 to 8.0 in the ratio of anode time to cathode time. The total amount of electricity at that time was fixed at 10 kQ / m ² as in the previous Lusner electrolysis method. The actual amount of electricity per unit area may vary depending on the speed of plate passing, the efficiency of electrolysis in the electrolyte (the effect of solution conductivity), etc., but this range is approximately 1 to 30 kQ / m ^2. The object of the invention is achieved.

  Depending on the conditions, immersion in a nitric hydrofluoric acid solution was performed after electrolysis. The 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 evaluation after electrolysis (in some cases, after the treatment with nitric hydrofluoric acid after electrolysis) was performed as follows.

  The scale is not observed visually or with a magnifier of 10 times, or even if there is one in one field of view, it is judged that the descaling has been completed. It was judged.

  The surface coverage of the Si oxide on the surface layer is obtained by mapping the amount of Si on the outermost layer and binarizing the image using a scanning FE Auger electron spectrometer under the conditions of an acceleration voltage of 10 keV and a beam current value of 10 nA. The rate was taken as the coverage.

Surface Cr: Cr _sur and Si concentration: Si _sur are C, O, and the like when the concentration profile in the depth direction is measured at an Ar sputtering rate of 15 nm / min using the same scanning FE Auger electron spectrometer. The maximum concentration of Cr and Si in the concentration profile of only the removed cation element was used.

  Samples for the corrosion test were prepared as follows. Prepare a test plate of 0.8 mm thickness × 50 mm width × 300 length and a similar test plate that is the same size and bent at 10 ° at the end 10 mm out of the width 50 mm. The test pieces were stacked so as to be in contact with each other at an angle of 10 °. In order to make contact at 10 °, it is necessary to keep the distance between the two test pieces at about 0.28 mm, so another stainless steel with a thickness of 0.27 mm that is slightly thinner than the gap is sandwiched between the two pieces. It is. The two contact portions were overlapped and TIG welded to obtain a weld gap test piece. TIG welding was carried out at a speed of 50 cm / min, adjusted at a current value of 100 to 150 A so that a back bead was formed. Shielding with Ar gas was performed on both sides during welding to suppress oxide generation due to welding heat as much as possible. This 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 gap test piece having a width of 30 mm × 25 mm. The cut end faces were all mirror-polished and the influence of the end face form was removed.

The corrosion test was performed as follows. The test solution was prepared by adding a NaCl reagent equivalent to 2000 ppm in terms of Cl ⁻ to tap water in Hikari, Yamaguchi Prefecture. Tap water was collected without using a filter such as a water purifier. The sample was immersed in a state where it was filled in a large flask with a lid and kept at 85 ° C., and kept for 6 months in a state where air was blown at 0.1 L / min. The whole amount of the test solution was replaced every week. After the test, the sample was taken out and the clearance was opened to evaluate the presence of corrosion and the depth of corrosion. The corrosion depth was measured in units of 1 μm by the depth of focus method using an optical microscope, and the case where the value was 50 μm or less was regarded as acceptable.

The relationship between the Cr and Mo contents of the base metal and the Cr and Si contents on the surface was investigated. Surface Cr: Cr _sur and Si concentration: Si _sur are C, when the concentration profile in the depth direction was measured at an Ar sputtering rate of 10 nm / min using a scanning FE Auger electron spectrometer as described above. This is the maximum concentration (at%) of Cr and Si in the concentration profile of only the cation element excluding O and the like.

First, steel plates were produced under the production conditions shown in Table 2 using the steel No1 material and steel No13 material shown in Table 1. Furthermore, using steel Nos. 2 and 3 in Table 1, in the final finish annealing pickling step, electrolytic treatment was performed with an aqueous solution containing nitric acid of 50 g / L or more, and the anode / cathode electrolysis time ratio in indirect energization during the electrolytic treatment Was set to 3.0. The total amount of electricity was fixed at 10 kQ / m ² which was the same as in Table 2. The shape of the corrosion test sample and its conditions were also the same as those in Table 2 above. Based on the corrosion depth of the corrosion test, the case where the corrosion depth exceeded 50 μm was evaluated as “x”, the case where 50 μm or less was 20 μm or more was evaluated as “◯”, and the case where less than 20 μm was evaluated as “◎”. The result is shown in FIG. FIG. 1 is a plot of corrosion test results with the horizontal axis representing Cr + 3Mo and the vertical axis representing Cs = 3Si _sur + Cr _sur .

Here, the reason why Cr + 3Mo and Cs = 3Si _sur + Cr _sur are used as indices in the present invention will be described. In general, Cr and Mo are known as elements for improving corrosion resistance, but it has been found that leaving Si oxide on the surface layer has the same effect as the corrosion resistance of the gap, especially the corrosion resistance of the weld gap. It depends. That is, the correlation between the Cr + 3Mo of the base material and the Cr and Si concentrations on the surface; Cs = 3Si _sur + Cr _sur is recognized, and the higher the value of Cs = 3Si _sur + Cr _sur , the lower the value of the base material Cr + 3Mo. It was also confirmed that the corrosion resistance was improved. This coefficient is obtained by regression from other results not shown.

And it was recognized that there is little corrosion in the area | region above the straight line described in FIG. When the region above this straight line is expressed in the form of a function, the following equation (1) is obtained.
(1) Cs ≧ −5.0 (Cr + 3Mo) +155
Cs = 3Si _sur + Cr _sur

Although not shown in the figure, when the base material Cr + 3Mo is less than 18, even if the value of Cs = 3Si _sur + Cr _sur is high, the corrosion resistance is poor. This is considered to be because it is necessary to add the minimum materials Cr and Mo in order to ensure the corrosion resistance with the surface coating. From this result, the lower limit of Cr + 3Mo was determined to be 18. On the other hand, if Cr + 3Mo exceeds 27, the workability deteriorates and the cost increases, so this was made the upper limit. Preferably, it is 20-26. Based on the above results, the above equation (2) was determined.

  Next, the effect of the surface coverage of the surface Si oxide will be described. When the surface coverage of the surface Si oxide was compared with the corrosion depth evaluation result in the corrosion test, the surface depth of the surface Si oxide was less than 10% or more than 50%, and the corrosion depth was It became clear that it exceeded 50 micrometers. In contrast, it was clarified that the corrosion resistance in the long-term corrosion test using tap water is improved when 10 to 50% of Si oxide is present on the steel surface. The reason for this is that, in addition to the surface Si oxide itself exhibiting excellent corrosion resistance in the neutral chloride aqueous solution as in this test, Al, Si, and Ca, which are generally referred to as chlorinated components, are contained in tap water. It is presumed that it tends to deposit on the surface, which functions as a protective film on the surface. Therefore, in the present invention, the surface coverage of the surface Si oxide is set in the range of 10 to 50%. On the other hand, when the Si oxide on the steel surface exceeds 50%, it means that the oxide scale generated by the heat treatment remains thick on the surface, and in this case, a portion with inferior corrosion resistance remains immediately below it, so Becomes bigger. Therefore, the upper limit of Si oxide is set to 50%.

  As for the toughness of the welded part, a TIG welded test piece subjected to bead-on-plate welding was produced and an adhesion bending test was performed. The welding conditions were 70 A, 50 cm / min, and Ar gas shielding was performed. Although this result will be described in detail in the description of the examples, cracks occurred in the adhesion bending test when Si, Ti, and Cr were larger than the range of the present invention. This is considered to be due to the fact that the tendency became noticeable in the welded part because it contained a large amount of elements such as Si, Ti, Cr, etc. that lower the toughness in addition to the coarsening of the welded part.

  The range of the element content of the present invention will be described below. Unless otherwise specified,% means mass%.

  Cr is the most important element for securing the corrosion resistance of stainless steel, and at least 16% is necessary to stabilize the ferrite structure. Increasing Cr improves corrosion resistance, but not only is the suppression of its use as a rare metal, but excessive addition reduces workability and manufacturability as well as the above-mentioned weld toughness, so the upper limit is 24%. It was. It is preferably 16.5 to 23%, more preferably 16.5 to 22.0%.

  Si is an important element as a deoxidizing element and is also effective in corrosion resistance and oxidation resistance. In the present invention, Si is a main element that enhances corrosion resistance by concentration on the surface. However, excessive addition not only reduces the toughness of both the base metal and the welded part, but also increases the strength excessively, thereby reducing workability and manufacturability. Therefore, the content of the base material was set to 0.08% to 0.3%, and various characteristics were improved by promoting Si concentration on the surface layer. Desirably, it is 0.10 to 0.29%, and more desirably 0.15 to 0.28%.

  Mo is an element that is effective in repairing a passive film and is very effective in improving corrosion resistance. Furthermore, there exists an effect which improves pitting corrosion resistance in combination with Cr. Increasing Mo improves corrosion resistance, but lowers workability and toughness and increases costs, so the upper limit is set to 2.5%. In order to exhibit these characteristics, it is necessary to contain Mo at least 0.50%. Desirably, it is 0.70 to 2.0%, and more desirably 0.9 to 2.0.

  C lowers the intergranular corrosion resistance and workability, so it is necessary to reduce the content thereof, so the upper limit was made 0.02% or less. Since excessive reduction deteriorates the refining cost, it is more preferably 0.002 to 0.015%.

  N, like C, reduces intergranular corrosion resistance and workability, so its content must be reduced, so its upper limit was made 0.025% or less. However, excessive reduction deteriorates the refining cost, so 0.002 to 0.015% is more desirable.

  Mn is an important element as a deoxidizing element. However, if added excessively, MnS, which becomes a starting point of corrosion, is easily generated, and the ferrite structure is destabilized. 0% or less. More desirably, it is 0.05 to 0.5%.

  P not only deteriorates weldability and workability, but also easily causes intergranular corrosion. Therefore, P needs to be kept low. Therefore, the content is set to 0.035% or less. More desirably, it is 0.001 to 0.02%.

  S needs to be reduced because it generates water-soluble inclusions that cause corrosion such as CaS and MnS. Therefore, the content is made 0.01% or less. However, excessive reduction causes cost deterioration, so 0.0002 to 0.005% is more desirable.

  Al is important as a deoxidizing element, and also has the effect of controlling the composition of non-metallic inclusions and refining the structure. However, excessive addition leads to coarsening of non-metallic inclusions, which may be the starting point for product wrinkling. Therefore, the lower limit is set to 0.005% and the upper limit is set to 0.2%. More desirably, it is 0.007% to 0.10%.

  Nb is an extremely important element for fixing C and N and suppressing intergranular corrosion of the base metal and welds in ferritic stainless steel. However, excessive addition reduces workability, so the range was made 0.6% or less. Desirably, it is 0.1 to 0.5%, and more desirably 0.15 to 0.4%.

  Furthermore, you may contain the following element as needed.

  Ti is a very important element for fixing C and N like Nb, suppressing intergranular corrosion of the welded portion and improving workability, and is added as necessary. However, excessive addition causes a reduction in the toughness of the raw material and causes surface flaws during production due to the large formation of hard Ti-based nonmetallic inclusions, so the range is 0.03 to 0. .25%. More desirably, the content is 0.07 to 0.20%.

  In addition, when adding Ti, in order to stabilize C and N and to suppress toughness fall, balance with Nb becomes important. For this purpose, it is necessary to add Nb so that Nb / Ti exceeds 1.0, and more desirably, Nb / Ti should be added so as to exceed 1.1. Therefore, the formula (3) is determined.

  Cu has the effect of reducing the active dissolution rate when corrosion occurs. However, excessive addition deteriorates workability, and in some cases, the eluted Cu ions may accelerate corrosion. Therefore, when added, the range was made 2% or less. More preferably, it is 0.2 to 1.5%, and still more preferably 0.25 to 1.1%.

  Ni suppresses the active dissolution rate and has excellent repassivation characteristics due to a small hydrogen overvoltage. However, excessive addition reduces workability and makes the ferrite structure unstable, so the upper limit was made 2%. Desirably, it is 0.1 to 1.2%, and more desirably 0.2 to 1.1%.

  Sn also improves the corrosion resistance by reducing the active dissolution rate due to its addition, and its effect can be obtained especially by adding a small amount. However, excessive addition reduces the workability, so when added, the range was made 0.5% or less. More preferably, it is 0.01 to 0.4%, and still more preferably 0.04 to 0.3%.

  V and Zr improve the weather resistance and crevice corrosion resistance, and if V is added while suppressing the use of Cr and Mo, excellent workability can be secured. However, excessive addition of V and Zr lowers workability and also saturates the effect of improving corrosion resistance, so the lower limit of V and Zr is 0.03% and the upper limit is 0.50%. More desirably, it is 0.05 to 0.30%.

  B is an effective grain boundary strengthening element for improving secondary work embrittlement, but excessive addition causes solid solution strengthening of ferrite and causes a drop in ductility. For this reason, the upper limit is made 0.005%. More desirably, it is 0.0002 to 0.0020%.

  The manufacturing method of the ferritic stainless steel for hot water storage / storage containers of this invention is demonstrated.

  Using the material having the above-mentioned components of the present invention, it is manufactured by a normal high-purity ferritic stainless steel manufacturing method, and in the final finish annealing pickling step, electrolytic treatment is carried out with an aqueous solution containing nitric acid of 50 g / L or more, By setting the anode / cathode electrolysis time ratio in indirect energization at the time of electrolytic treatment to be more than 1.0 and 6.0 or less, the ferritic stainless steel for hot water storage / storage container of the present invention can be manufactured.

  In the final finish annealing pickling process, if nitric acid is less than 50 g / L, descaling becomes insufficient and results in not satisfying the formula (1), but electrolytic treatment is performed with an aqueous solution containing nitric acid of 50 g / L or more. Thus, the steel of the present invention can be obtained without such a problem.

  In addition, when the anode / cathode electrolysis time ratio (anode electrolysis time / cathode electrolysis time) in indirect energization at the time of electrolytic treatment is 1.0, the formula (1) is not satisfied and the surface coverage of the surface Si oxide is 10 If it exceeds 6.0 and the scale is not removed sufficiently, it will be judged that the scale cannot be scaled, and the surface coverage of the Si oxide will exceed 50%. If the electrolysis time ratio exceeds 1.0 and is 6.0 or less, the ferritic stainless steel for hot water storage / storage container of the present invention can be produced without causing such problems.

  In the case where the Si content is close to the upper limit within the range of the present invention and the surface coverage of the surface Si oxide is too high than the target, the anode / cathode electrolysis time ratio is 1. By increasing the value within the range of more than 0 and not more than 6.0 or / and increasing the concentration of nitric acid within the range, the surface coverage of the surface Si oxide can be produced as intended.

  Steels having the chemical composition shown in Table 1 were produced as follows. That is, a cold-rolled sheet having a thickness of 0.8 mm was produced by rolling and heat treatment. This was annealed in the atmosphere for 1 minute soaking. The temperature of finish annealing was set between 900 and 980 ° C. based on the recrystallization temperature of the amount of square bars. This heat-treated material was subjected to the following descaling treatment.

Descaling was also performed as described above. The electrolyte was 150 g / L using a commercially available nitric acid reagent. Electrolysis conditions were as follows: the test piece was placed on the sample electrode, and two SUS304 stainless steel plates were placed 10 mm apart on both sides. The current density and electrolysis time were controlled using a potentiostat to simulate alternating electrolysis by indirect energization of the actual machine. As for the electrolysis time, a cycle of one anode time and cathode time was set to 5 seconds, and the time ratio was set to 4.0. By repeating this cycle four times, the total electrolysis time was kept constant for 20 seconds. In this case, the current density was changed according to the electrolysis time so that the total amount of electricity was constant at 10 kQ / m ² . As described above, the cases shown in Table 1 are manufactured by applying the electrolytic conditions of the present invention.

As for the descaleability after electrolysis, the scale is not observed visually or with a magnifier of 10 times, or even if there is one in one field of view, it is judged that the descaling has been completed, and if more scale remains Therefore, it was judged that descaling was impossible. As described above, the surface coverage of the surface Si oxide was also derived from the Si mapping image using a scanning FE Auger electron spectrometer. Similarly, Cr: Cr _{sur on the} surface and Si _sur : Si _sur were set to the maximum concentrations of Cr and Si in the cation concentration profile in the depth direction using a scanning FE Auger electron spectrometer.

  Samples to be subjected to the corrosion test were also produced as 30 mm wide × 25 mm long weld gap test pieces having a gap angle of 10 °, manufactured under the same conditions as described above. The corrosion test was a 6-month immersion test using tap water as described above, and the corrosion depth in the crevice was evaluated as x when the value exceeded 50 μm, ◯ when 20 μm or more and 50 μm or less, and ◎ when less than 20 μm.

  As for the toughness of the welded part, a TIG welded test piece subjected to bead-on-plate welding was produced, and an adhesion bending test was performed. The welding conditions were 70 A, 50 cpm, and an argon shield was implemented.

  This result is the present invention component range as shown in Table 1 right: Steel Nos. 1 to 11 satisfy the formulas (1) and (2), and the Si coverage of the surface layer is 10 to 50%. The corrosion depth of the corrosion test was below the standard value, indicating excellent corrosion resistance. On the other hand, the steel Nos. 12 to 15 which are components outside the scope of the present invention have a surface depth of Si coverage of less than 10%, or a large amount of Si and insufficient descaling. became. Moreover, in steel No14 where Si is higher than the present invention, cracks occurred in the toughness test of the weld.

  Next, Table 2 shows the results of manufacturing under the manufacturing conditions shown in Table 2 using the steel No1 material of the present invention and the steel No13 material of the comparative example. Conditions other than the manufacturing conditions shown in Table 2 were the same as those shown in Table 1 above. No. No. A material and an anode / cathode electrolysis time ratio of 1.0. In the case of B and J, equation (1) was not satisfied, the surface coverage of the surface Si oxide was less than 10%, and the corrosion test showed a corrosion depth exceeding 50 μm. Further, as an electrolyte, No. having a nitric acid concentration of 30 g / L. In the case of I, the descaling was insufficient, the formula (1) was not satisfied, and the corrosion depth was worse. This showed the same tendency even when the conventional salt method, Rusner electrolysis method, and nitric acid hydrofluoric acid immersion were combined. The anode / cathode electrolysis time ratio exceeded 6.0. In the case of G, it was judged that the scale could not be removed sufficiently and de-scaling was impossible, the surface coverage of the Si oxide exceeded 50%, and the corrosion depth exceeded 50 μm in the corrosion test.

  Bright annealing of this material: No. In the case of K, although Si remained on the surface, the coverage was as high as 90%, and the corrosion depth in the corrosion test exceeded 50 μm. In the appearance observation taken out after the corrosion test, the BA-treated material maintained the same metallic luster as before the test in the flat part without corrosion, whereas the electrolytic material of the present invention was transformed into a slightly cloudy surface. It was. When this flat surface is examined by GDS, oxygen and Cr are slightly concentrated on the surface of the BA-treated material, whereas in the condition material of the present invention, Al, Si, and Ca are concentrated on the surface in addition to oxygen. It was. Moreover, as a result of taking a Si mapping image of the surface before the test by FE-AES, and evaluating this by image processing, the surface Si is uniformly present in the BA material. It was found that the area ratio was 10 to 50%. From this, it was clarified that the corrosion resistance in the long-term corrosion test using tap water is improved when 10 to 50% of Si oxide is present on the steel surface. The reason for this is presumably because Al, Si, and Ca, which are generally called “calky”, contained in tap water tend to precipitate 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 it seems that it was difficult to precipitate the chalk content.

  Further, in Table 2, No. 1 using steel No. 13 as a comparative example. In L and M, even when the electrolytic treatment was performed under the conditions of the present invention, the equation (1) was not satisfied and Si remained on the surface and the coverage was low. Therefore, even in the corrosion test, the corrosion depth exceeded 50 μm. It was. This indicates that not only the amount of Cr and Mo in the base material but also the amount of Si on the surface affects the corrosion resistance. By the way, this time, we evaluated the base metal part with sufficient Ar gas shield during welding. However, if the gas shield is omitted during welding, the same effect is exhibited even if oxide scale is generated by heat.

  On the other hand, when implemented with the electrolytic solution composition and electrolysis conditions of the present invention, the formula (1) was satisfied, Si remained on the surface at a rate of 10% to 50%, and the corrosion depth was 50 μm or less.

  The ferritic stainless steel of the present invention exhibits its excellent effect regardless of its structure as long as it is various containers and tanks for storing water and hot water. Even if water is not stored, the same effect can be obtained in facilities that use tap water, such as bathtubs and kitchen equipment. Further, the present invention can be widely applied to a portion requiring corrosion resistance, such as a drain water recovery device of a latent heat recovery type gas water heater, its heat exchanger, and various welding pipes. Further, as described above, not only the base material but also the structure used as it is with TIG welding is suitable for applications having corrosion resistance.

Claims (5)

% By mass, C: 0.020% or less, N: 0.025% or less, Si: 0.08 to 0.3%, Mn: 0.01 to 1.0%, P: 0.035% or less, S: 0.01% or less, Cr: 16-24%, Mo: 0.5-2.5%, Al: 0.005-0.2%, Nb: 0.10-0.60% The balance is composed of Fe and inevitable impurities, satisfies the following formulas (1) and (2), and has a surface coverage of the surface Si oxide of 10 to 50%. Ferritic stainless steel for hot water storage containers with excellent water leakage resistance.
(1) Cs ≧ −5.0 (Cr + 3Mo) +155
Cs = 3Si _sur + Cr _sur
(2) Cr + 3Mo: 18-27
Here, Cr and Mo mean the base material concentration (mass%) of each element, and Si _sur and Cr _sur mean the surface cation concentration ratio (at%) of Si and Cr, respectively. Furthermore, the hot water storage / excellence in weld part toughness and water leakage resistance according to claim 1, characterized in that, in mass%, Ti: 0.03 to 0.25% is satisfied and the following formula (3) is satisfied. Ferritic stainless steel for water storage containers.
(3) Nb / Ti> 1
Here, Nb and Ti mean the base material concentration (mass%) of each element.   Furthermore, by mass%, Cu: 2.0% or less, Ni: 2.0% or less, Sn: 0.5% 1 type or 2 types or more are included, The feature of Claim 1 or 2 characterized by the above-mentioned. , Ferritic stainless steel for hot water storage containers with excellent weld toughness and water leakage resistance.   Furthermore, by mass%, V: 0.5% or less, Zr: 0.5% or less, B: 0.005% or less, one or two or more types are included. Ferritic stainless steel for hot water storage and storage containers with excellent weld toughness and water leakage resistance.   In the final finish annealing pickling process for stainless steel, electrolytic treatment is performed with an aqueous solution containing nitric acid at 50 g / L or more, and the anode / cathode electrolysis time ratio in indirect energization during electrolytic treatment is more than 1.0, 6.0 or less The manufacturing method of the ferritic stainless steel for hot water storage / storage containers excellent in weld part toughness and water leakage resistance according to any one of claims 1 to 4.

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