Stainless steel with excellent porosity resistance in natural freshwater and brackish water environments.

Abstract

To provide a stainless steel with superior pitting resistance in natural fresh water and brackish water environments where the natural potential repeatedly fluctuates between a noble potential (high potential) and a negative potential (low potential).SOLUTION: A stainless steel comprises, in mass%, C: over 0.018 and 0.30% or less, Si: 0.01 to 4.00%, Mn: 0.01 to 3.00%, P: 0.001 to 0.05%, S: 0.0001 to 0.01%, Cr: 9 to 23%, Ni: 0.001% or more and less than 2.0%, Cu: 0.001 to 1.5%, Mo: 0.001 to 1.5%, Sn: 0.001 to 0.300%, Al: over 0.03 and 4.00% or less, N: 0.001 to 0.100%, and O: 0.0001 to 0.01%, and further comprises, in mass%, one or two or more selected from Zr: 0.001 to 0.1%, V: 0.001 to 0.2%, W: 0.001 to 0.1%, Ta: 0.001 to 0.1%, and Hf: 0.001 to 0.1%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to stainless steel having excellent pore - resistance in a natural fresh - water / brackish - water environment, which has excellent pore - resistance in an environment directly contacting natural fresh - water / brackish - water, such as products for storing or transporting natural fresh - water / brackish - water, estuary dams, sluice gates, etc.

Background Art

[0002] Conventionally, for members directly contacting natural water, such as storage tanks, pipeline pipes for transportation, waterways, estuary dams, sluice gates, etc., various stainless steels have been used depending on salt concentration and temperature conditions. Particularly, in a seawater environment with a chloride ion concentration of about 20,000 ppm, high - Cr stainless steel with a Cr content exceeding 25% is used. On the other hand, for fresh water (river water / lake water) with a low salt concentration where the general chloride ion concentration is less than 200 ppm, austenitic stainless steels such as SUS304 steel and SUS316L steel are used. Further, in a brackish - water environment where fresh water with a concentration of 200 - 20,000 ppm is mixed with seawater, various steel grades are selected and used depending on the concentration. However, austenitic stainless steels are very expensive because they contain a large amount of expensive Ni.

[0003] In Patent Document 1, ferritic stainless steel is used for a corrugated steel plate - made waterway member as a member of a waterway. The corrosion resistance is evaluated by the area ratio of red rust generation. By using stainless steel instead of the conventional galvanized material, it is stated that the generation of red rust can be reduced and the life - cycle cost can be reduced.

[0004] In stainless steel used in liquids in various environments, it is known that corrosion progresses more easily in the gap part than in the flat part (crevice corrosion). Also, it is known that the more the natural potential of stainless steel immersed in the liquid becomes noble (higher potential), the easier it is for pitting corrosion to occur and grow.

[0005] In natural water, because it contains microorganisms, a biofilm forms on the surface of stainless steel, and it is said that the spontaneous potential of the metal shifts to a noble potential due to the action of the metabolic products of these organisms. Patent Document 2 confirms that the spontaneous potential shifts to a noble potential due to the accumulation of microorganisms, and then discloses that stainless steel with added V is suitable as a material that exhibits crevice corrosion resistance, making it less susceptible to crevice corrosion even at a noble potential.

[0006] As described above, stainless steel components used in environments that come into direct contact with natural freshwater and brackish water are known to experience crevice corrosion if there are gaps. Therefore, they are designed to minimize the formation of gaps. Consequently, structural gaps, such as those at the joints between parts, are minimal.

[0007] As mentioned above, corrosion is less likely to occur in the flat parts (planar surfaces) of stainless steel components compared to the gaps. It is conceivable that crevice corrosion may occur in the gaps between the metal and the biofilm due to the accumulation of microorganisms (Patent Document 2), but in actual use, there is flow, and light substances such as microorganisms are carried away, so it is rare for them to continue to form gaps.

[0008] As described above, while the flat surfaces offer a less corrosive environment than the gaps due to the exchange of water in contact with them, the flat surfaces represent a significantly larger area ratio than the gaps. Therefore, the progression of pitting corrosion, which is unique to stainless steel and occurs on the flat surfaces, largely determines the total amount of water leakage.

[0009] As mentioned above, pitting corrosion occurs and grows more easily as the natural potential becomes more noble, so it is necessary to use materials with higher corrosion resistance than in controlled environments. However, except for critical areas such as lifelines where water leakage is unacceptable, leaks can occur, but the equipment is used as is, and repairs are carried out after a certain period of time.

[0010] Furthermore, conventionally known materials are thought to have their lifecycle (LCC) calculated based on a typical 40-year total cost, thus setting the product life at around 40 years. Therefore, they do not satisfy the recent need for longer lifespans of 50 years or more.

[0011] Furthermore, it is generally known that stainless steel is prone to pitting corrosion near the waterline. In this area, the steel material is positioned perpendicular to the ground surface, and horizontal pitting corrosion is crucial for the formation of holes. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2018-162594 [Patent Document 2] Japanese Patent Publication No. 2001-254149 [Overview of the project] [Problems that the invention aims to solve]

[0013] The authors, through studies using test specimens positioned at various angles, found that pitting corrosion tends to progress more easily in the vertical direction along gravity than in the horizontal direction. Specifically, metal ions and chloride ions produced by corrosion accumulate inside pitting holes of about 0.1 to 1 mm in diameter, forming a low-pH, highly corrosive liquid called anolite. It was hypothesized that this anolite is greatly affected by gravity and tends to accumulate downwards, causing corrosion to progress vertically. Therefore, it was considered that steel materials positioned horizontally are more prone to corrosion in the thickness direction and to developing holes than steel materials positioned perpendicular to the Earth's surface.

[0014] As described later, it was found that the natural potential of the environment constantly fluctuates between noble potential (high potential) and noble potential (low potential). However, none of the publicly available literature has been able to conduct tests that simulate the actual fluctuations of natural potential, and therefore, it cannot be said that the optimal component design has been achieved.

[0015] This invention relates to stainless steel with excellent porosity resistance in natural freshwater and brackish water environments where the natural potential repeatedly fluctuates between noble potential (high potential) and noble potential (low potential), and provides stainless steel (particularly ferritic and martensitic stainless steel) that enables the long-term operation of equipment using steel materials. [Means for solving the problem]

[0016] From the above perspective, the inventors first determined the target area. To ensure a lifespan of over 50 years, the aim is to reduce welds, which cause deterioration of corrosion resistance, and to adopt mechanical fastening structures using fittings and components such as bolts, nuts, and screws. As mentioned above, the area ratio of flat surfaces to gaps is overwhelmingly large, so the progression of pitting corrosion, which is unique to stainless steel and occurs on flat surfaces, can be said to affect the total amount of water leakage. Furthermore, even on flat surfaces, as mentioned above, corrosion progresses more in the thickness direction and holes are more likely to form in steel materials positioned horizontally than in steel materials positioned perpendicular to the ground surface. Therefore, we investigated ways to suppress the occurrence of pitting corrosion when flat surfaces are placed horizontally in liquid.

[0017] First, in a river environment to which the material is exposed, we measured the natural potential, an indicator of the degree of pitting corrosion acceleration, using super stainless steel as the monitoring electrode. Super stainless steel, which has extremely high corrosion resistance, was used to eliminate the possibility of measurement noise due to electrode corrosion. As a result, it was found that the natural potential of the environment is constantly fluctuating. Furthermore, when the corrosion behavior of ferritic stainless steel was monitored simultaneously, it was found that even when pitting corrosion occurred on the test piece when exposed to an environment with a high natural potential, repassivation occurred after the natural potential of the environment decreased, stopping the progression of pitting corrosion, and that pitting corrosion progressed intermittently. From this, we learned that not only the pitting corrosion occurrence behavior, as evaluated in the past, but also the fact that pitting corrosion is less likely to occur after repassivation has a significant impact on porosity. Through laboratory tests simulating the environment, we investigated additive elements that are less likely to occur after repassivation and have the effect of suppressing the progression of pitting corrosion in the horizontal direction.

[0018] Assuming that welding would not be used, the stainless steel was formulated without Nb and Ti. Similarly, aiming for low cost, the basic concept for material development was to create ferritic and martensitic stainless steels with a Cr content of less than 23% and a low amount of expensive Ni added.

[0019] As a result, we discovered that including one or more of Zr, V, W, Ta, and Hf in stainless steel promotes repassivation and suppresses the recurrence of pitting corrosion, and that even with trace amounts of additives, extremely good horizontal porosity can be ensured. This led to the identification of an appropriate amount of additive, and thus the present invention was completed.

[0020] The gist of this invention is as follows: [1] In mass%, It contains C: greater than 0.018% and less than or equal to 0.30%, Si: 0.01 to 4.00%, Mn: 0.01 to 3.00%, P: 0.001 to 0.05%, S: 0.0001 to 0.01%, Cr: 9 to 23%, Ni: greater than 0.001% and less than 2.0%, Cu: 0.001 to 1.5%, Mo: 0.001 to 1.5%, Sn: 0.001 to 0.300%, Al: greater than 0.03% and less than or equal to 4.00%, N: 0.001 to 0.100%, and O: 0.0001 to 0.01%. Furthermore, this stainless steel, characterized by its excellent porosity resistance in natural freshwater and brackish water environments, contains one or more of the following by mass%, with the remainder being Fe and unavoidable impurities: Zr: 0.001-0.1%, V: 0.001-0.6%, W: 0.001-0.1%, Ta: 0.001-0.1%, and Hf: 0.001-0.1%. [2] The stainless steel described in [1], characterized in that the stainless steel is ferritic stainless steel or martensitic stainless steel, and has excellent porosity in natural freshwater and brackish water environments. [Effects of the Invention]

[0021] According to the present invention, it is possible to significantly improve the horizontal pore resistance in natural fresh water and brackish water, and it is possible to provide a ferritic stainless steel for natural fresh water and brackish water environments that can be used without adding excessive alloy components, which has extremely high industrial value.

Embodiments for Carrying Out the Invention

[0022] The present invention provides a ferritic stainless steel applicable to natural fresh water and brackish water environments. Considering the influence of tide levels on the chloride ion concentration, natural fresh water with a maximum chloride ion concentration of 2000 ppm is assumed. Further, the ferritic stainless steel of the present invention is used in the form of a steel plate formed by molding in the above environment.

[0023] First, the inventors measured the natural potential of stainless steel immersed in rivers and lakes across various regions for over one year, and clarified that the natural potential continuously fluctuates and the range of its fluctuation.

[0024] As a result, although there are differences depending on the steel type, it was found that the natural potential of stainless steel generally fluctuates within a range of about 0 to 0.6 V vs. SSE (hereinafter referred to as V).

[0025] Next, the inventors evaluated the progress of pitting corrosion of various stainless steels in a simulated natural water environment. A more severe brackish water environment was simulated. A solution containing about 2000 ppm of chloride ions (low potential solution) and a solution obtained by adding hydrogen peroxide solution to the low potential solution to a concentration of 0.5% (high potential solution) were prepared. Hydrogen peroxide solution has the effect of nobilizing the potential. The high potential solution simulates the situation where the natural potential of the environment is noble, and the low potential solution simulates the situation where the natural potential of the environment is base. Although there are differences depending on the steel type, the low potential solution shows a natural potential of about 0 V and the high potential solution shows a natural potential of about 0.3 to 0.6 V, which is appropriate as a simulation of the natural fresh water and brackish water environment.

[0026] The test pieces were 30 x 50 x 1 mm and finished with #400 wet polishing. As the components of the stainless steel, Nb and Ti were not added, and aiming at low cost, the Cr content was less than 19% and the addition amount of expensive Ni was small.

[0027] First, in step 1, the test specimen was placed in a high-potential solution heated to 30°C for 8 hours without stirring, with the widest surface (evaluation surface) facing horizontally upwards, to promote the occurrence of pitting corrosion. After that, it was washed with unused low-potential solution. Furthermore, in step 2, without drying, the specimen was placed in an unused low-potential solution heated to 30°C for 16 hours, with the evaluation surface facing upwards, as in step 1, to promote the cessation of pitting corrosion that occurred in step 1. After that, it was washed again with unused low-potential solution. Finally, in step 3, the specimen was placed in a high-potential solution heated to 30°C for 96 hours, with the evaluation surface facing upwards, as in step 1, to promote the occurrence of pitting corrosion again. After that, the pitting corrosion depth was measured after washing and rust removal.

[0028] Measurements were performed using the depth of focus method with an optical microscope. The maximum depth from the surface direction of each sample was compiled as a characteristic value from the measured pitting depths, and elements that have a pitting inhibition effect and their addition amounts were determined.

[0029] The results are shown in the examples described below. As is clear from the examples, it was found that by adding one or more of V, W, Zr, Ta, and Hf to stainless steel, the occurrence of pitting corrosion when the natural potential between the immersion solution and the stainless steel repeatedly changes from high potential to low potential to high potential can be suppressed.

[0030] Using the methods described above, we were able to develop stainless steel that can be used in simulated natural freshwater environments.

[0031] The following are the reasons for limiting the constituent elements of the present invention. Regarding the component composition, % refers to mass % unless otherwise specified.

[0032] V, W, Zr, Ta, and Hf are elements that are poorly soluble in anolite, which reacts with water and inhibits passivation, and are thought to suppress the recurrence of pitting corrosion by forming highly barrier oxide or hydroxide films, thus forming the core of this invention. In particular, pitting corrosion occurs when the spontaneous potential becomes nobler, stops when the spontaneous potential becomes less nobler, and then becomes less likely to occur again when the spontaneous potential becomes nobler, so the rate of pitting corrosion progression is greatly reduced. V exhibits its effect when added at concentrations exceeding 0.001%. However, excessive addition increases costs, so the upper limit was set at 0.6%. For better porosity, an addition of 0.004-0.01% is preferable. A more preferable range is 0.05-0.1%. Furthermore, W, Zr, Ta, and Hf exhibit their effects when added at concentrations of 0.001% or higher. More preferable lower limits are 0.003% for W, 0.007% for Zr, 0.002% for Ta, and 0.003% for Hf. On the other hand, since adding large amounts increases costs, the upper limit was set at 0.1%. To obtain better porosity resistance, it is preferable to add 0.01% or more of each.

[0033] Carbon (C) is detrimental to the porosity of stainless steel, and lower levels are preferable. However, considering refining costs, the lower limit was set at over 0.018%. Furthermore, since levels exceeding 0.300% significantly accelerate the occurrence of pitting corrosion, an upper limit of 0.300% was set.

[0034] The amount of Si added was set to 4.00% or less, which is within the typical composition range for hot-rollable stainless steel that does not affect porosity. Furthermore, since manufacturing costs increase when the Si content is less than 0.01%, the lower limit was set to 0.01%.

[0035] Mn is a solid solution strengthening element, and adding large amounts degrades corrosion resistance. For this reason, the amount added was limited to 3.00% or less. Also, since manufacturing costs would be high if the Mn content was less than 0.01%, it was set to 0.01% or more.

[0036] While a low P content is desirable from the standpoint of reducing the frequency of pitting corrosion and improving hot workability, a P content of 0.001% or higher was set because lower levels would result in higher manufacturing costs. On the other hand, since a P content exceeding 0.05% drastically deteriorates hot workability, the upper limit was set at 0.05% or lower.

[0037] When sulfur (S) is added in large quantities, it forms inclusions that reduce corrosion resistance, and this becomes particularly noticeable above 0.01%. Therefore, the amount added was limited to 0.01% or less. However, since amounts below 0.0001% result in high manufacturing costs, the amount was set to 0.0001% or more.

[0038] Cr is a basic component of stainless steel. Since corrosion resistance decreases if the Cr content is less than 9.0%, the addition was set at 9.0% or more. While a higher Cr content improves the frequency of pitting corrosion and the ability to re-passivate, the effect saturates in natural freshwater if it exceeds 23.0%, so the upper limit was set at 23.0%.

[0039] Ni, Cu, and Mo are elements that further improve porosity, and are particularly effective in freshwater applications where chloride ion concentration is high or where maintenance cannot be performed for extended periods. Therefore, in this invention, Ni, Cu, and Mo are all added. The effect appears when each is added at a concentration of 0.001% or more. More preferable lower limits are 0.010% for Ni, 0.060% for Cu, and 0.002% for Mo. Even more preferable lower limits are 0.055% for Ni, 0.115% for Cu, and 0.015% for Mo. However, considering processability, the upper limits are less than 2.0% for Ni and 1.5% for Cu and Mo.

[0040] Sn is an effective element for improving corrosion resistance, and its effect is stably exhibited at an addition level of 0.001% or higher, so the lower limit is set at 0.001%. On the other hand, excessive addition will saturate the effect in natural freshwater, so the upper limit is set at 0.30%. Furthermore, because interaction with Cu causes hardening and adversely affects moldability, the preferable concentration is 0.01-0.15%.

[0041] Since aluminum is used as a deoxidizing agent, it inevitably remains at a concentration of over 0.03%. Therefore, this was set as the lower limit. Adding large amounts would significantly degrade the manufacturability, so the upper limit was set at 4.00%.

[0042] Since a lower nitrogen content is preferable when workability is a priority, the lower limit was set to 0.001% considering refining costs. Adding large amounts of nitrogen to ferritic stainless steel degrades corrosion resistance due to the precipitation of chromium nitride, so the upper limit was set to 0.100%.

[0043] O mainly exists as an oxide, and if a large amount remains, it can cause surface scratches during manufacturing, so the upper limit was set at 0.01%. Lower O levels are preferable, and considering cost reduction, the lower limit was set at 0.0001%.

[0044] The above are the essential elements in this invention, with the remainder being Fe and unavoidable impurities.

[0045] Other applications include storage tanks, transport line pipes, and open channels and pipe-shaped channels made by joining mechanically processed steel plates.

[0046] The stainless steel of the present invention is preferably ferritic stainless steel or martensitic stainless steel. Ferritic stainless steel is suitable for forming complex shapes due to its excellent workability, while martensitic stainless steel has high yield strength and is difficult to process, but it is expected to increase the rigidity of the product. If the heat treatment temperature is set in a range where austenite is not formed, it becomes ferritic stainless steel, and if the heat treatment temperature is set in a range where ferrite is not formed, it becomes martensitic stainless steel. What is formed at the heat treatment temperature is determined by the alloy composition. [Examples]

[0047] The present invention will be further described below based on examples. Tables 1 and 2 show the chemical compositions of the steel of the present invention and comparative steel, which were melted and cast by electric furnace vacuum melting to produce steel ingots. These steel ingots were hot-rolled to a thickness of 5 mm, and then 1.0 mm thick steel plates were produced by a combination of cold rolling and annealing. Test pieces measuring 30 x 50 x 1 mm were taken from the steel plates. Immersion tests were conducted in simulated natural water for a total of 120 hours as described above, and the pitting corrosion resistance was investigated by examining the depth of pitting corrosion after the tests.

[0048] A solution containing approximately 2000 ppm of chloride ions was used as the low-potential solution. A high-potential solution was prepared by adding hydrogen peroxide to the low-potential solution to a concentration of 0.5%. Hydrogen peroxide has the effect of nobility in potential. The high-potential solution simulates a situation where the natural potential of the environment is noble, while the low-potential solution simulates a situation where the natural potential of the environment is noble. Although there are differences depending on the type of steel, the low-potential solution shows a natural potential of about 0V, and the high-potential solution shows a natural potential of about 0.3-0.6V, which is appropriate for simulating natural freshwater and brackish water environments.

[0049] In step 1, the test specimen was placed in a high-potential solution heated to 30°C for 8 hours without stirring, with its widest surface (evaluation surface) facing horizontally upwards. It was then washed with unused low-potential solution. Without drying, in step 2, the specimen was placed in unused low-potential solution heated to 30°C for 16 hours, with the evaluation surface facing upwards, as in step 1. It was then washed again with unused low-potential solution. Finally, in step 3, the specimen was immersed in a high-potential solution heated to 30°C for 96 hours, with the evaluation surface facing upwards, as in step 1. Afterward, the specimen was washed, rust removed, and the pitting corrosion depth was measured.

[0050] Measurements were performed using an optical microscope with the depth of focus method. The maximum depth from the surface direction of each sample among the measured pitting depths was compiled as a characteristic value.

[0051] The porosity was evaluated using a three-level scoring system based on the criteria shown below, and the results are displayed in the "Test Results" section of Tables 1 and 2. 1: Pitting depth less than 0.1 mm 2: Pitting depth 0.1 mm or more and less than 0.5 mm 3: Pitting depth of 0.5 mm or more A rating of 1 indicates very high porosity and allows for maintenance-free use. A rating of 2 indicates good porosity and is at a level that can withstand actual use. A rating of 3 indicates a steel grade that will develop holes during use and will not meet the expected lifespan.

[0052] [Table 1]

[0053] [Table 2]

[0054] The results are shown in Tables 1 and 2. Component compositions that fall outside the scope of the present invention are underlined.

[0055] As shown in Tables 1 and 2, the examples of the present invention No. 1 to 38 received ratings of 1 and 2, indicating that they are steel grades suitable for practical use.

[0056] In contrast, as shown in Table 2, all of the comparative examples No. A to J received a score of 3, indicating that they were not at a level that would satisfy practical needs. Comparative example No. J did not contain any of V, W, Zr, Ta, or Hf, and the test result was poor, with a score of 3. Comparative examples No. A, B, G, and I all had high-than-average values ​​for C, S, N, and Mn, respectively, and their test results were poor, receiving a score of 3. Comparative examples No. C, D, E, F, and H all showed low levels of Ni, Cr, Cu, Mo, and Sn, respectively, and their test results were poor, receiving a score of 3.

[0057] As described above, it can be seen that the steel of the present invention has extremely superior porosity compared to the comparative steel.

Claims

1. In mass percent, It contains C: greater than 0.018% and 0.300% or less, Si: 0.01 to 4.00%, Mn: 0.01 to 3.00%, P: 0.001% to 0.05%, S: 0.0001 to 0.01%, Cr: 9 to 23%, Ni: 0.001% or more and less than 2.0%, Cu: 0.001 to 1.5%, Mo: 0.001 to 1.5%, Sn: 0.001 to 0.300%, Al: greater than 0.03% and 4.00% or less, N: 0.001 to 0.100%, O: 0.0001 to 0.01%, Furthermore, this stainless steel, characterized by containing one or more of the following in mass%, in the following proportions: Zr: 0.001-0.1%, V: 0.001-0.6%, W: 0.001-0.1%, Ta: 0.001-0.1%, and Hf: 0.001-0.1%, with the remainder being Fe and unavoidable impurities, exhibits excellent porosity resistance in natural freshwater and brackish water environments.

2. The stainless steel with excellent porosity resistance in natural freshwater and brackish water environments according to claim 1, characterized in that the stainless steel is a ferritic stainless steel or a martensitic stainless steel.

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