Weather-resistant ferritic stainless steel

Abstract

To provide ferritic stainless steel excellent in corrosion resistance, in particular, weather resistance in high purity ferritic stainless steel in which a Si content in the steel is set to 1.0 mass% or more, specific components contained in the steel are appropriated, and specific S-based inclusions in the steel are controlled.SOLUTION: A ferritic stainless steel having a predetermined chemical composition, in which S-based inclusions, which are inorganic compounds containing S, are present in the stainless steel, and among the S-based inclusions, specific S-based inclusions having an equivalent diameter size of 1 μm or more and with an S content of 1 mass% or more are present on the surface of stainless steel at a number ratio of 15 or less / mm2, and when each content of Cr, Si, Mo, Cu, Ni, Al, Ca, Mg in stainless steel is expressed by {Cr}, {Si}, {Mo}, {Cu}, {Ni}, {Al}, {Ca}, {Mg}, respectively, these provide ferritic stainless steels excellent in weather resistance satisfying predetermined relationship of formulas (1) and (2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to ferritic stainless steel with excellent weather resistance. [Background technology]

[0002] Traditionally, ferritic stainless steel has been widely used in building materials, kitchen materials, and electrical components because it is inexpensive due to its lack of nickel content or its low nickel content. However, conventional ferritic stainless steel has inferior corrosion resistance compared to austenitic stainless steel, such as SUS304. Therefore, recently, high-purity ferritic stainless steel has been developed with improved corrosion resistance by adding 1.0 mass% of silicon to ferritic stainless steel, which has reduced carbon and nitrogen impurities through improved manufacturing technology.

[0003] However, ferritic stainless steel is sometimes used in outdoor environments, particularly corrosive environments such as salt-damaged environments, for example, in building panels and piping. Until now, weather resistance has been improved in ferritic stainless steel by adding alloying elements such as Cr and Mo to suppress surface rusting. However, adding large amounts of Cr and Mo goes against the goal of making ferritic stainless steel inexpensive. Furthermore, ferritic stainless steel has the property that pitting corrosion progresses faster and is more difficult to stop once it occurs, compared to austenitic stainless steel containing Ni. Ferritic stainless steel with reduced C and N impurity elements and 1.0 mass% Si added is effective in suppressing pitting corrosion in aqueous solutions, but it has the problem of not having sufficient weather resistance in outdoor corrosive environments.

[0004] For example, Patent Document 1 discloses a stainless steel welded structure and a stainless steel plate for welding with excellent toughness in the welded part, achieved by ensuring strength through Si addition, adding Ni, and reducing the size of inclusions in the weld metal to 1 μm or less in order to improve the strength of the base material and the welded part. Patent Document 2 discloses a ferritic stainless steel and welded structure that improves weldability while ensuring corrosion resistance and strength by suppressing burn-through by setting Ni, Mn, and Mo within appropriate ranges, and suppressing insufficient penetration by setting Si and Cu within appropriate ranges. Patent Document 3 discloses a ferritic stainless steel that suppresses high-temperature salt damage by setting Ni, Mo, and Si within appropriate ranges, and improves pitting corrosion resistance by setting Cr, Si, Mo, and Ni within appropriate ranges. Patent Document 4 discloses a ferritic stainless steel that improves high strength and crevice corrosion resistance by specifying the amounts of Si, Cr, and Mo, and specifying the number of metal layers on the surface to reduce the starting point of crevice corrosion.

[0005] However, none of the patent documents 1 to 4 take into account weather resistance, which is a problem when used outdoors. In particular, high-purity ferritic stainless steel, which has improved pitting corrosion resistance and tensile strength due to the addition of Si, has the problem of not taking into account inclusions containing S (sulfur), which are surface defects that become corrosion initiation points in outdoor salt-damaged environments. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2010-229470 [Patent Document 2] Japanese Patent Publication No. 2018-165384 [Patent Document 3] Japanese Patent Publication No. 2019-112709 [Patent Document 4] Japanese Patent Publication No. 2020-164937 [Overview of the project] [Problems that the invention aims to solve]

[0007] Therefore, the present invention has been made in view of the above problems, and aims to provide a high-purity ferritic stainless steel in which the Si content in the steel exceeds 1.0 mass%, the specific components contained in the steel are optimized, and specific S-type inclusions present in the steel are controlled, thereby providing a ferritic stainless steel with excellent corrosion resistance, in particular, excellent weather resistance. [Means for solving the problem]

[0008] Therefore, the features of the present invention are listed below. (1) In mass%, C: 0.10% or less, Si: more than 1.0% and less than 3.0%, Mn: 1.0% or less, Ni: 2.0% or less, P: 0.10% or less, S: 0.0020% or less, Cr: 13.0% or more and 32.0% or less, N: 0.10% or less, At least one of Nb and Ti: 6(C+N)% or more and 0.7% or less. Cu: 2.0% or less, Mo: 3.0% or less, Al: 0.01% or more and 0.20% or less, Ca: 0.010% or less, Mg: 0.010% or less, A ferritic stainless steel having a chemical composition containing O: 0.0040% or less, with the remainder being Fe and unavoidable impurities, In the aforementioned stainless steel, there are S-based inclusions, which are inorganic compounds containing S. Of the aforementioned S-type inclusions, specific S-type inclusions with a diameter equivalent size of 1 μm or more and an S content of 1% by mass or more have a number ratio of 15 inclusions / mm² on the surface of the stainless steel. 2 The following: When the respective contents of Cr, Si, Mo, Cu, Ni, Al, Ca, and Mg in the stainless steel are represented by {Cr}, {Si}, {Mo}, {Cu}, {Ni}, {Al}, {Ca}, and {Mg}, respectively, {Al}, {Ca}, and {Mg} satisfy the relationship of the following formula (1), and {Cr}, {Si}, {Mo}, {Cu}, and {Ni} satisfy the relationship of the following formula (2), a ferritic stainless steel excellent in weather resistance. Formula (1): {Al} + 10{Ca} + {Mg} ≤ 0.20 Formula (2): {Cr} + 1.7{Si} + 1.2{Mo} + 0.4({Cu} + {Ni}) > 19.0 (2) The chemical composition is At least one of V, W, Co, and Zr: 1.0% by mass or less, REM: 0.10% by mass or less, Sn: 0.10% by mass or less, and B: further containing not more than 0.01% by mass, the ferritic stainless steel excellent in weather resistance according to (1).

Effect of the Invention

[0009] [[ID=s24]] The present invention can provide a ferritic stainless steel excellent in weather resistance by making the Si content in the steel exceed 1.0% by mass, optimizing specific components contained in the steel, and controlling specific S-based inclusions present in the steel.

Mode for Carrying Out the Invention

[0010] [[ID=s31]] Hereinafter, embodiments of the present invention will be described. The following description is an example of an embodiment of the present invention and does not limit the scope of the claims.

[0011] In order to solve the problems, the present inventors have intensively studied the action effects of additive elements and inclusions that improve corrosion resistance, particularly weather resistance, in ferritic stainless steel, and have obtained the following new findings to complete the present invention. [[ID=s37]]

[0012] [[ID=3s9]] The ferritic stainless steel of the present invention (hereinafter simply referred to as "stainless steel") has Fe as its main component and contains Cr, and its metal structure is mainly composed of the ferrite phase. In order to possess mechanical properties such as tensile strength and elongation, as well as processing characteristics during manufacturing, it has the chemical composition shown below. Furthermore, the stainless steel of the present invention is used in many corrosion-resistant applications. For example, it is often required to have weather resistance, such as in building materials where rust suppression is important in outdoor corrosive environments, and in piping where suppression of perforation due to corrosion is important. For this reason, in order to improve weather resistance, the number of inclusions that serve as corrosion initiation points has been reduced. These inclusions exist in the steel of the stainless steel, and those exposed on the surface dissolve in corrosive environments, creating gaps between the base material of the stainless steel and the inclusions. A potential difference is generated inside the gap with the outside, which becomes a driving force for corrosion. In addition, metal ions dissolved by corrosion, especially Cr ions, form hydrates, which lowers the pH of the surrounding environment. This tendency is strong in the gap between the base material and the inclusions because ion diffusion is difficult, making repassivation difficult and allowing pitting corrosion to progress. The larger this gap becomes, the more likely the pH inside the gap is to decrease.

[0013] Here, the inclusions are inorganic compounds containing sulfur (S) (hereinafter referred to as "S-system inclusions"). These S-system inclusions contain sulfides such as MnS and CaS. In addition to sulfides that precipitate on their own, Al, Ca, and Mg oxides also aggregate and precipitate together with the precipitated sulfides to form S-system inclusions.

[0014] These S-type inclusions are soluble in moisture in corrosive environments and act as corrosion initiation points; therefore, it is necessary to reduce their proportion. The methods for reduction are as follows (1) and (2). (1) Reduce the number of sulfur-based inclusions by reducing the sulfur content in stainless steel. (2) Control the content of Al, Ca, and Mg, which form oxides and aggregate with S or sulfides to form S-based compounds. In particular, control the content of Ca, which has a strong tendency to form sulfides.

[0015] Furthermore, sulfur-based inclusions dissolve in corrosive environments, creating gaps between them and the stainless steel base material. A potential difference is generated within these gaps, which acts as a driving force for corrosion. In addition, when corrosion occurs within these gaps, metal ions from the stainless steel dissolve. The dissolved chromium ions form hydrates, lowering the pH of the surrounding environment. This tendency is particularly strong in gaps beneath inclusions, where ion diffusion is less likely to occur, making repassivation difficult and allowing pitting corrosion to progress. Stainless steel is used in many corrosion-resistant applications. Weather resistance is often required, such as in building materials where rust suppression is important in outdoor corrosive environments, and in piping where preventing perforation due to corrosion is crucial. To improve weather resistance, it is common practice to add alloying elements such as chromium and molybdenum to conventional ferritic stainless steel. However, since stainless steel contains a certain number of inclusions that act as corrosion initiation sites, it was necessary to add more alloying elements than necessary to satisfy the weather resistance requirement. Furthermore, ferritic stainless steel tends to experience faster corrosion (pitting) progression and less resistance to re-passivation (re-stopping) compared to austenitic stainless steel containing nickel. Furthermore, silicon (Si) is cited as a solid solution element that improves the corrosion resistance of the base material. However, while the addition of Si to steel improves corrosion resistance, it does not improve weather resistance. This is because the corrosion-improving effect of Si is effective in neutral environments, and no effect of suppressing corrosion progression has been observed in low pH environments such as the inside of pitting corrosion. Therefore, weather resistance is ensured while adding small amounts of Cu and Ni, which effectively act against pitting corrosion in the low pH environment caused by crevice corrosion. The ferritic steel of the present invention will be described in detail below.

[0016] The stainless steel of the present invention has a chemical composition, in mass%, of C: 0.10% or less, Si: greater than 1.0% and 3.0% or less, Mn: 1.0% or less, Ni: 2.0% or less, P: 0.1% or less, S: 0.0020% or less, Cr: 13.0% to 32.0%, N: 0.1% or less, at least one of Nb and Ti: 6(C+N)% to 0.7%, Cu: 2.0% or less, Mo: 3.0% or less, Al: 0.01% to 0.20%, Ca: 0.010% or less, Mg: 0.010% or less, and O: 0.0040% or less, with the remainder being Fe and unavoidable impurities.

[0017] (chemical composition) The reasons for the limitations on each essential additive element are explained below. Note that in the following descriptions of each component of the chemical composition, "mass%" is simply represented as "%". (C: 0.10% or less) Carbon (C) is inevitably present in stainless steel. However, since its content reduces workability and corrosion resistance, it is preferable to have a lower content, so the upper limit should be 0.10% or less. In particular, it has a significant effect of being the starting point for corrosion and reducing weather resistance, and it is even preferable to reduce it to 0.05% or less to further improve corrosion resistance, especially pitting corrosion resistance and weather resistance. On the other hand, C is an interstitial solid solution element and has a strong tendency to segregate at grain boundaries, thus contributing to the strengthening of grain boundaries. Therefore, C is also effective in suppressing grain boundary segregation of phosphorus (P). To obtain these effects of C, it is preferable to set the lower limit to 0.0005% or more.

[0018] (Si: more than 1.0% and less than 3.0%) Silicon (Si) is effective as a deoxidizing element, improving oxidation resistance. It also increases tensile strength and hardness, contributing to improved mechanical strength. Furthermore, by increasing the Si content to over 1.0%, corrosion resistance, especially pitting corrosion resistance in neutral environments such as saltwater, can be improved. On the other hand, it acts as a solid solution strengthening element, leading to a decrease in workability and weldability toughness, so the upper limit should be 3.0% or less. To ensure deoxidation, oxidation resistance, mechanical strength, and pitting corrosion resistance, considering the respective effects and manufacturability, the Si content is preferably between 1.0% and 2.5%. Furthermore, considering the respective effects and manufacturability, the preferred range is between 1.1% and 2.0%.

[0019] (Mn: 1.0% or less) Manganese (Mn) is an effective element for deoxidation and fixation of sulfur (S). However, it can form MnS sulfides, which can be the starting point for corrosion, and it can also destabilize the ferrite phase, leading to a decrease in oxidation resistance. Therefore, the upper limit should be 1.0% or less. On the other hand, to ensure the desulfurization effect of deoxidizing oxygen (O) in the matrix phase and fixing sulfur (S) by converting it into sulfides, it is preferable to set the lower limit to 0.01% or more. The preferred range is 0.02 to 0.8%, taking into account the respective effects and manufacturing costs.

[0020] (Ni:2.0% or less) Nickel (Ni) is an effective element for improving corrosion resistance, and in particular, it is effective in suppressing the progression of pitting corrosion in low pH environments and in suppressing crevice corrosion between the base material and inclusions. To obtain resistance to crevice corrosion, it is preferable that the Ni content be greater than 0.03%. On the other hand, if the Ni content exceeds 2.0%, it destabilizes the ferrite phase, leading to an increase in alloy costs and a decrease in workability due to increased material strength, so the upper limit for Ni content should be 2.0%. The preferred range for Ni content is 1.8% or less, taking into consideration performance and alloy costs.

[0021] (P:0.10% or less) Phosphorus (P) is an element that inhibits processability and weldability, and a lower content is better; therefore, the upper limit should be 0.10% or less. However, excessive reduction leads to increased refining costs, so it is preferable to set the lower limit at 0.005% or more. A more preferable range, considering manufacturing costs, is 0.008 to 0.050%.

[0022] (S:0.0020% or less) Sulfur (S) is inevitably present in stainless steel. It segregates at grain boundaries, reducing the toughness, hot workability, and corrosion resistance of welds. Furthermore, it forms inorganic compounds as sulfides with other elements, reducing corrosion resistance, especially weather resistance. Therefore, a lower S content is preferable, and the upper limit should be 0.0020% or less. However, excessive reduction leads to increased raw material and refining costs, so it is preferable to set the lower limit at 0.0001% or more. A more preferable range is 0.0002 to 0.0018%, taking into account the suppression of weather resistance degradation and manufacturing costs.

[0023] (Cr: 13.0~32.0%) Cr (chromium) is a fundamental element of the ferritic stainless steel of the present invention and is an essential element for ensuring oxidation resistance, corrosion resistance, and especially weather resistance. Considering the salty environment outdoors, the lower limit of Cr content is set at 13.0% or more to ensure weather resistance. The upper limit of Cr content is set at 32.0%, preferably 25.0% or less, from the viewpoint of raw material costs and manufacturability such as reduction of hot-rolled sheet toughness.

[0024] (N: 0.10% or less) Since nitrogen (N), like carbon (C), reduces processability and corrosion resistance, a lower N content is preferable, and the upper limit should be 0.10% or less. However, excessive reduction leads to increased refining costs, so it is preferable to set the lower limit at 0.001%. Although N, like carbon, is an interstitial solid solution element, it has a small tendency to segregate at grain boundaries and contributes little to strengthening the grain boundaries. There are concerns about reduced corrosion resistance due to sensitization, so the preferred range is 0.005 to 0.050%, considering performance and manufacturing costs.

[0025] (At least one of Nb and Ti: 6(C+N)% or more and 0.7% or less) Niobium (Nb) and titanium (Ti) are effective elements that improve workability and resistance to intergranular corrosion, as well as improving the metal structure, through their action as stabilizing elements that fix carbon (C) and nitrogen (N). When adding them, their respective effects will be realized, so the amount should be 6(C+N) or more relative to the amount of C and N added. However, excessive addition will lead to an increase in alloy cost and a decrease in manufacturability due to an increase in recrystallization temperature, so it should be kept below 0.7%. Furthermore, excessive addition will lead to a decrease in surface quality during manufacturing. The preferred range, considering the effects, alloy cost, and manufacturability, is 0.05-0.5% for at least one of Nb and Ti. A more preferred range is 0.08-0.5%.

[0026] (Cu:2.0% or less) Copper (Cu) is a suitable element for obtaining workability and weather resistance. In particular, it has the effect of suppressing the progression of pitting corrosion in low pH environments. To obtain this effect, it is preferable that the Cu content be greater than 0.03%. On the other hand, if the Cu content exceeds 2.0%, it destabilizes the ferrite phase, leading to an increase in alloy cost and a decrease in workability due to increased material strength, so the upper limit of the Cu content should be 2.0%. The preferred range for Cu content is 0.04 to 1.5%, taking into consideration performance and alloy cost.

[0027] (Mo: 3.0% or less) Mo (molybdenum) is an effective element for obtaining oxidation resistance, pitting corrosion resistance, and weather resistance, in addition to oxidation resistance, similar to Ni and Cu. In particular, it is effective in suppressing the progression of pitting corrosion in low pH environments. For these effects to be achieved, the Mo content is preferably 0.10% or more. However, excessive content will increase alloy costs and hinder the manufacturability of hot and cold working, so the upper limit of the Mo content should be 3.0% or less.

[0028] (Al: 0.01% or more and 0.20% or less) Aluminum (Al) is an extremely effective deoxidizing element. However, it can lead to a decrease in the toughness and weldability of steel, so its upper limit should be 0.20% or less. Al, in particular, forms oxides with Si, reducing the amount of O (oxygen) in the matrix. However, if the Al content increases, the amount of oxides in the steel will increase, and these will combine with sulfides to form soluble inclusions, which can become the starting point for corrosion and reduce weather resistance. Therefore, an upper limit of 0.20% or less is preferable. On the other hand, the lower limit should be 0.01% or more, taking the deoxidizing effect into consideration. A more preferable range is 0.01 to 0.17%, taking manufacturability and performance into consideration.

[0029] (Ca:0.010% or less) Calcium (Ca) is an extremely effective deoxidizing element. In particular, it forms oxides with Si, reducing the amount of oxygen (O) in the matrix phase. Furthermore, it is an element that improves hot workability and the cleanliness of stainless steel, and is added as needed. The Ca content is preferably 0.0003% or more to exhibit these effects. However, if the Ca content exceeds 0.010%, oxides and sulfides are more likely to form, and in particular, sulfides (CaS) are soluble in water, which can become the starting point for corrosion and reduce weather resistance, so the upper limit of the Ca content is set at 0.010%. Preferably, it is 0.009% or less, taking into consideration manufacturability and oxidation resistance.

[0030] (Mg:0.010% or less) Magnesium (Mg) is an extremely effective deoxidizing element. In particular, it forms oxides with Si, reducing the amount of oxygen (O) in the matrix. Furthermore, it is an element that improves hot workability and the cleanliness of stainless steel, and is added as needed. The Mg content is preferably 0.0003% or more to exhibit these effects. However, a high Mg content reduces manufacturability, and an increase in oxides in the steel leads to the formation of soluble inclusions by combining with sulfides, which can become a starting point for corrosion and reduce weather resistance. Therefore, the upper limit of the Mg content is set at 0.010%. Preferably, it is 0.009% or less, taking into consideration manufacturability and oxidation resistance.

[0031] (O: Less than 0.0040%) Oxygen (O) is inevitably present in stainless steel sheets. It forms oxides with other additive elements. When sulfides aggregate on these oxides, they act as soluble inclusions that become corrosion initiation points, leading to a decrease in weather resistance. Therefore, the O content should be kept below 0.0040%. More preferably, it should be kept below 0.0035%. However, excessive reduction leads to increased raw material and refining costs, so it is preferable to set the lower limit at 0.0001% or higher.

[0032] (Percentage of S-type inclusions) Furthermore, the stainless steel of the present invention contains S-based inclusions, which are inorganic compounds containing S. Among these S-based inclusions, specific S-based inclusions with a diameter equivalent size of 1 μm or more and an S content of 1% by mass or more are present on the surface of the stainless steel at a number ratio of 15 pieces / mm². 2 The following applies:

[0033] S-type inclusions dissolve in water in corrosive environments, creating gaps between them and the stainless steel base material. A potential difference is generated within these gaps, driving corrosion. Furthermore, when pitting corrosion occurs within these gaps, metal ions from the stainless steel leach out. The leached Cr ions form hydrates, lowering the pH of the surrounding environment. This tendency is particularly strong in gaps beneath the inclusions, where ion diffusion is less likely to occur, making repassivation difficult and allowing pitting corrosion to progress. Ferritic stainless steels with low Ni content corrode faster than austenitic stainless steels, making S-type inclusions more likely to act as corrosion initiation points. Therefore, weather resistance is improved by reducing the size or number of S-type inclusions that act as corrosion initiation points.

[0034] Among S-type inclusions, specific S-type inclusions with a diameter equivalent size of 1 μm or more and an S content of 1% by mass or more have a number ratio of 15 pieces / mm on the surface of stainless steel. 2 The following measures will suppress rust formation on the surface of this stainless steel. The number proportion of specific S-type inclusions was observed by mirror-finishing the rolled surface and measured as follows: First, specific S-type inclusions were detected using a FE-SEM (Field Emission Scanning Electron Microscope: Hitachi High-Tech Corporation SU-5000) instrument, and then the composition and number proportion of those inclusions were measured by point analysis using an EDX (Energy Dispersive X-ray Spectrometer).

[0035] Furthermore, among the detected inclusions, the diameter equivalent size ((long side × short side) 1 / 2 The number of specific sulfurous inclusions with a diameter of 1 μm or more and an EDX analysis component sulfur content of 1% by mass or more was determined. The number of specific sulfurous inclusions was divided by the observation area to calculate the number ratio.

[0036] Specific S-type inclusions are S-type inclusions with an equivalent diameter size of 1 μm or more. Here, "equivalent diameter size" refers to the value obtained by raising the product of the long side dimension and the short side dimension of the shape to the power of 1 / 2. This allows the size to be uniquely determined even if the shape is irregular. S-type inclusions with an equivalent diameter size of less than 1 μm are not likely to become the starting point of corrosion because their small size creates a potential difference between the inside and outside of the gap that acts as a driving force for corrosion, and diffusion is easily carried out when pitting corrosion occurs within the gap, resulting in only a slight decrease in pH. Furthermore, if the sulfur content of the inclusions is less than 1%, the inclusions do not dissolve and form gaps between them and the base material, making them less likely to become corrosion initiation points. Therefore, even if inclusions are present on the surface of stainless steel, the number ratio of specific sulfur-based inclusions with a diameter equivalent size of 1 μm or more and a sulfur content of 1% or more should be 15 inclusions / mm². 2 By following the steps below, rust formation can be suppressed and the deterioration of weather resistance can be prevented.

[0037] Furthermore, in the stainless steel of the present invention, when the content of Cr, Si, Mo, Cu, Ni, Al, Ca, and Mg is expressed as {Cr}, {Si}, {Mo}, {Cu}, {Ni}, {Al}, {Ca}, and {Mg} respectively, {Al}, {Ca}, and {Mg} satisfy the relationship shown in formula (1) below, while {Cr}, {Si}, {Mo}, {Cu}, and {Ni} satisfy the relationship shown in formula (2) below. Formula (1): {Al}+10{Ca}+{Mg}≦0.20 Formula (2): {Cr}+1.7{Si}+1.2{Mo}+0.4({Cu}+{Ni})>19.0

[0038] (Regarding equation (1)) The stainless steel of the present invention satisfies formula (1), which is expressed as {Al}, {Ca}, and {Mg}, respectively. The stainless steel of the present invention employs a method of reducing the proportion of specific sulfur-based inclusions to improve weather resistance. By adding more than 1.0% and up to 3.0% Si to the stainless steel of the present invention, high corrosion resistance can be obtained even in ferritic stainless steels. Furthermore, by adding more than 1.0% Si, it is possible to ensure pitting corrosion resistance while reducing the content of costly Cr and Mo. However, the improvement in pitting corrosion resistance by adding Si is mainly effective in neutral environments, and therefore does not effectively work in gaps between specific sulfur-based inclusions and the base material that form a low pH environment, so improvement in weather resistance by adding Si could not be expected. However, since the stainless steel of the present invention reduces specific sulfur-based inclusions, which are corrosion initiation points, weather resistance can be further improved by adding Si.

[0039] The stainless steel of the present invention satisfies the relationship given by formula (1) in terms of Al, Ca, and Mg content {Al}, {Ca}, and {Mg}. This makes it possible to reduce the number proportion of specific sulfur-based inclusions. The stainless steel of this invention specifies the content of Al, Ca, and Mg, which form sulfur-based inclusions through the aggregation of S or sulfides. Al and Mg form oxides and aggregate with S, etc., to form sulfur-based inclusions. On the other hand, the addition of Al and Mg is essential because it is possible to drastically reduce the amount of O (oxygen) in the stainless steel by adding them in combination with Si, and therefore the amount of addition needs to be controlled. Furthermore, Ca has a stronger tendency to form oxides compared to Al and Mg, and also tends to combine with S to form sulfides. For this reason, we found that Ca has a 10 times greater tendency to form sulfur-based inclusions compared to Al and Mg, and derived equation (1).

[0040] Furthermore, the number of specific sulfur-based inclusions present on the surface of stainless steel that have a diameter equivalent size of 1 μm or more and a sulfur content of 1% by mass or more is 15 inclusions / mm². 2 To control the process as described below, the value of equation (1) should be set to 0.20 or less. This will control specific S-type inclusions while ensuring the weather resistance of the stainless steel. If the value on the left side of equation (1) exceeds 0.20, it falls outside the above limiting range. In particular, the number ratio of specific S-type inclusions is 15 pieces / mm 2 It is difficult to control the following. Also, the number ratio is 10 pieces / mm 2 To achieve the following, it is preferable to set the value of equation (1) to 0.15 or less.

[0041] (Regarding equation (2)) Furthermore, the ferrite steel of the present invention satisfies the relationship shown in formula (2) below, when the content of Cr, Si, Mo, Cu, and Ni is expressed as {Cr}, {Si}, {Mo}, {Cu}, and {Ni} respectively. Formula (2): {Cr}+1.7{Si}+1.2{Mo}+0.4({Cu}+{Ni})>19.0

[0042] Stainless steel undergoes pitting corrosion when its passive film is locally destroyed by halogen ions such as chloride ions in corrosive environments. The occurrence and progression of this pitting corrosion vary greatly depending on the elements contained in the stainless steel. Generally, the pitting corrosion index (PI = Cr + 3Mo), based on Cr, has been known as an indicator of the pitting corrosion resistance of stainless steel. The stainless steel of the present invention satisfies equation (2), which was newly obtained by discovering that Mo, Cu, and Ni are effective in controlling the progression of pitting corrosion in crevice environments such as gaps and inside pitted areas, which are low pH environments.

[0043] The stainless steel of the present invention employs pitting potential measurement as a method for simply evaluating the pitting corrosion resistance ranking. Among the additive elements it contains, many additive elements were measured, and attention was paid to Si, Cu, and Ni, for which particularly effective results were found, in addition to Cr and Mo. The effects of their respective contents {Cr}, {Si}, {Mo}, {Cu}, and {Ni} were investigated. The pitting potential measurement was carried out in accordance with JIS G 0577, and the potential at which the current value in a 3.5 mass% NaCl aqueous solution at 30°C exceeds 100 μA / cm 2 was defined as the pitting potential V‘c100. In the stainless steel of the present invention, when it contains more than 1.0% of {Si}, it was found that the pitting potential is improved not only by an increase in {Cr} but also by an increase in {Si}, and the effect is 1.7 times that of {Cr}. Although this effect is not necessarily clear, from the analysis of the passive film, it is presumed that the oxides of Si generated in the inner layer of the film and at the steel interface exhibit the effect of suppressing the destruction of the passive film by halogen ions, particularly in a neutral solution environment such as salt water. Furthermore, when Si is added in excess of 1.0%, when it contains {Mo}, it was found that the effect is 1.2 times that of {Cr}. When it contains {Cu} and {Ni}, it was found that the effect is 0.4 times that of {Cr}. Therefore, the stainless steel of the present invention is expressed as {Cr}+1.7{Si}+1.2{Mo}+0.4({Cu}+{Ni}) in formula (2).

[0044] Furthermore, the pitting potential V'c100 was compared with SUS430J1L (19Cr ferritic stainless steel), SUS443J1 (21Cr ferritic stainless steel), and SUS304 (18Cr-8Ni austenitic stainless steel). By setting formula (2) of the present invention to 19.0, it was possible to achieve a pitting potential of 0.20V, equivalent to that of SUS430J1L (19Cr ferritic stainless steel). Also, by setting formula (2) of the present invention to 21.0, it was possible to achieve a pitting potential of 0.30V, equivalent to that of SUS443J1 (21Cr ferritic stainless steel) and SUS304 (18Cr-8Ni austenitic stainless steel). Therefore, formula (2) of the present invention was set to 19.0 or higher. In addition, if formula (2) in the present invention exceeds 25.0, the Si content {Si} in the steel tends to become too high, resulting in excessively high tensile strength and hardness, reduced workability, and higher manufacturing costs. Similarly, a higher content of Mo, Cu, and Ni in the steel results in the disadvantage of higher raw material costs. For this reason, the upper limit of equation (2) is preferably set to 25.0.

[0045] Furthermore, the stainless steel of the present invention may be further enriched with the following optional additive elements as needed.

[0046] (V, W, Co, Zr: 1.0% or less) V (vanadium), W (tungsten), Co (cobalt), and Zr (zirconium) are elements effective in improving corrosion resistance. In particular, by reducing the amount of solid-solution carbon and nitrogen through the formation of carbonitrides, they contribute to improving the corrosion resistance and workability of high-purity ferritic stainless steel, and are therefore added as needed. The content of V, W, Co, and Zr should preferably be 0.01% or more for each to exhibit its effect. If the content of each exceeds 1.0%, it leads to an increase in alloy cost and a decrease in manufacturability, and causes hardening and a decrease in toughness due to solid-solution strengthening and precipitation strengthening, so the upper limit of the content of each should be 1.00% or less. The preferred range for the content of each is 0.02 to 0.50%, taking into consideration workability, manufacturability, and alloy cost.

[0047] (Sn:0.10% or less) Sn (tin) is an element that is effective in improving corrosion resistance and secondary workability by suppressing grain boundary segregation of P, and is added as needed. For each of these effects to manifest, the lower limit of the Sn content is preferably 0.01% or more. However, if the Sn content exceeds 0.10%, the alloy cost increases and the manufacturability of hot and cold working is hindered, so the upper limit of the Sn content should be 0.10% or less.

[0048] (REM: 0.10% or less) Rare earth elements (REMs) can be added as needed because they improve hot workability and steel purity, and significantly enhance oxidation resistance and hot workability. Their content should be 0.001% or more, which is the amount at which their effects manifest. However, excessive addition of REMs only leads to increased alloy costs and decreased manufacturability; therefore, the upper limit for each REM content should be 0.10% or less. Preferably, considering effectiveness, economy, and manufacturability, at least one type should be present in a concentration of 0.001% to 0.080%. REM refers to rare earth metals of the lanthanide series, actinide series, and composite metals thereof, such as Ce, Pr, and Sm.

[0049] (B: 0.010% or less) Boron (B) is an element that improves hot workability and resistance to secondary work brittleness, and its addition to ferritic stainless steel is effective. The B content is preferably 0.0003% or higher to achieve these effects. However, if the B content exceeds 0.010%, it leads to a decrease in elongation and fatigue strength, so the upper limit is set at 0.010%. Preferably, considering material cost and workability, the B content is set to 0.0005-0.0080%.

[0050] (The remainder is Fe and unavoidable impurities) The remainder consists of Fe and unavoidable impurities, such as As and Sb. Here, unavoidable impurities refer to components that are mixed in during the industrial production of stainless steel due to various factors in the raw materials such as ore and scrap, and the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0051] (Manufacturing method) A preferred method for manufacturing the stainless steel of the present invention having the chemical composition described above will now be explained. In conventional methods for manufacturing stainless steel, the sulfur (S) content is reduced by increasing the refining time and reducing the scrap ratio. In conventional methods, it is common to reduce S by extending the refining time in the converter. On the other hand, in order to control the content of trace amounts of S, simply extending the refining time significantly hinders manufacturability, resulting in high manufacturing costs and therefore not being practically implemented. The reduction of S content in the ferritic steel of the present invention is achieved not only by extending the refining time, but also by reducing the scrap ratio, which is the main cause of the inclusion of S and other impurities. That is, it is desirable to have a scrap ratio of 40% or less, and more preferably 35% or less. This makes it possible to ensure the weather resistance targeted by the present invention. [Examples]

[0052] The present invention will be described in detail based on the following embodiments. However, the present invention is not limited to the embodiments shown below.

[0053] Table 1 shows the content of essential additive elements and, in some cases, optional additive elements in Examples 1-6 and Comparative Examples 1-7. Ferritic stainless steel having the chemical composition (mass%) shown in the Examples and Comparative Examples was melted down and hot-rolled at a heating temperature of 1150-1250°C to produce hot-rolled steel sheets with a thickness of 4 mm. Subsequently, the sheets were annealed at 1000°C, pickled, and then cold-rolled to a thickness of 0.8 mm, followed by finish annealing and pickling at 870-1000°C. The following evaluations were performed using these sheets.

[0054] [Table 1]

[0055] The characteristics and performance of Examples 1-6 and Comparative Examples 1-7 were evaluated below.

[0056] Table 2 evaluates the properties of the stainless steel of the present invention, including the number ratio of specific S-type inclusions and the calculated values ​​of equations (1) and (2).

[0057] The proportion of specific S-type inclusions was evaluated as follows: A 10mm x 10mm test specimen was cut from the manufactured steel plate by machining. At this time, the specimen was taken at a point at least 20mm away from the widthwise edge of the manufactured annealed and pickled plate. Next, the specimen was embedded in resin so that the rolled surface would be the observation surface. Then, the resin-embedded specimen was polished to a mirror finish by wet polishing. Afterward, specific S-type inclusions were detected and their composition was measured using FE-SEM (Hitachi High-Tech Corporation SU-5000) and EDX point analysis. The measurement was performed using the automated inclusion analysis function, under the following conditions. Measurement range: Measurement area 5mm 2 It measures (2.0mm x 2.5mm). Observation magnification: 200x (1 field of view 0.48 mm × 0.64 mm), with 5% overlap, 18 fields of view were measured to obtain the above measurement range. The minimum particle size detected was 0.47 μm. The EDX analysis beam diameter is 0.05 μm. Of the detected inclusions, the diameter ((long side × short side)) 1 / 2 The number of specific sulfurous inclusions with a diameter of 1 μm or more and an EDX analysis component sulfur concentration of 1% by mass or more was determined. The number of specific sulfurous inclusions was divided by the observation area to obtain the number ratio (inclusions / mm²). 2 ) was calculated.

[0058] [Table 2]

[0059] Table 3 evaluates the weather resistance and pitting corrosion resistance of the stainless steel surface according to the present invention. Here, weather resistance is expressed as the rust area ratio after a combined salt-wet-dry cycle test (CCT). A smaller rust area ratio indicates better weather resistance of stainless steel in a salt-damaged environment. In particular, it can be evaluated using both the number of specific S-type inclusions shown in Table 2 and the value on the left side of equation (1). Furthermore, pitting resistance was evaluated by measuring the pitting potential. Since a higher pitting potential suppresses the occurrence of pitting, the value on the left side of equation (2) can be used to represent pitting resistance.

[0060] The method for evaluating weather resistance is described in detail below. After cutting out a test specimen measuring 50 mm in width and 100 mm in the rolling direction, one side surface of the test specimen was wet-polished with #600 grit. Next, three sides of the test specimen (excluding one side in the width direction) were coated with resin (one-component condensation type RTV rubber KE44 manufactured by Shin-Etsu Silicone Co., Ltd.). Then, two polyethylene tubes measuring 20 mmφ × 10 mm were bonded onto a 70 mm × 150 mm bakelite plate, and the unpolished surface of the test specimen was placed on top and bonded. A combined salt-dry-wet cycle test (CCT) was performed on the sample obtained in this way. The sample was placed in the CCT apparatus with the surface of the test specimen at a 75° angle to the horizontal plane and the side of the test specimen not coated with resin facing downwards. Three cycles were performed, each consisting of 5% salt spray (35°C, 2 hours), drying (60°C, 25% RH, 4 hours), and wetting (50°C, 95% RH, 2 hours). After that, the sample was washed with water and dried, and the rust area ratio on the surface of the test specimen was evaluated. Furthermore, in this evaluation, if the rating number (RN) is less than 8 (corresponding to a rust area ratio exceeding 0.25%), it is marked as "×" indicating poor weather resistance; if the RN is 8 or higher (corresponding to a rust area ratio of 0.25% or less), it is marked as "〇" indicating excellent weather resistance; and if the RN is 9.5 or higher (corresponding to a rust area ratio of 0.05% or less), it is marked as "◎" indicating extremely excellent weather resistance. Furthermore, the evaluation of this rust-affected area ratio conforms to the "Annex JC (Normative) Rating Number Method" of JIS Z 2371: Salt spray test method.

[0061] Furthermore, we will explain the evaluation method for pitting corrosion resistance. A test specimen measuring 20 mm x 15 mm was prepared by shearing the test material. A wire was spot-welded to one end of the specimen, and the area other than the 10 mm x 10 mm test surface was coated with silicone resin. A 3.5% NaCl aqueous solution was used as the test solution, and the test was performed at 30°C under Ar degassing. After completely immersing the test surface in the above NaCl aqueous solution and leaving it for 10 minutes, the anode current density was measured from the natural electrode potential to 500 μA / cm² using the potentiokinetic method with a potential sweep rate of 20 mV / min. 2 The potential was measured until it reached a certain value, and the anodic polarization curve was obtained. The pitting potential was 100 μA / cm² on the anodic polarization curve. 2 Among the potentials corresponding to each, the most noble value was defined as the pitting potential (V). In this evaluation, if the pitting potential was less than 0.20V, it was marked as "× (unacceptable)" because sufficient pitting resistance could not be obtained; if the pitting potential was between 0.20V and 0.30V, it was marked as "〇 (good)" because pitting resistance equivalent to that of 19Cr-containing ferritic stainless steel (SUS304J1L) was obtained; and if the pitting potential was 0.30V or higher, it was marked as "◎ (excellent)" because pitting resistance equivalent to that of 21Cr-containing ferritic stainless steel and 18-8 austenitic stainless steel (SUS443J1, SUS304) was obtained.

[0062] [Table 3]

[0063] As shown in Tables 1 and 2, the stainless steels of Examples 1 to 6 satisfy the appropriate range of the present invention in terms of the number ratio of S-type inclusions, the value of formula (1), and the value of formula (2). As a result of the findings shown in Table 3, it can be seen that the stainless steels of Examples 1 to 6 all received a rating of "○" or higher in terms of weather resistance and pitting corrosion resistance.

[0064] In contrast, Comparative Example 1 had a high sulfur content of 0.0085% by mass in the stainless steel, and consequently, the number ratio of sulfur-based inclusions was 30.3 (pieces / mm³). 2 ) was high and outside the scope of the present invention, resulting in poor weather resistance ("×").

[0065] Comparative Example 2 had a high content of Ca and Mg in the stainless steel, and consequently, the number ratio of specific S-type inclusions was 22.7 (pieces / mm³). 2 The value of equation (1) was high at 0.222, which is outside the scope of the present invention, and therefore the weather resistance was poor, marked with "×".

[0066] Comparative Example 3 has an Al content of 0.130% in stainless steel, which is within the range, but the number percentage of specific S-type inclusions is 20.3 (pieces / mm³). 2 The value of (1) was high, and the value of (1) was also high at 0.221, which is outside the scope of the present invention, so the weather resistance was poor, marked with "×".

[0067] Comparative Example 4 had a high Al content of 3.198% in the stainless steel, and the number of specific S-type inclusions was 16.7 (pieces / mm³). 2 The value of equation (1) was high at 3.240, which is outside the scope of the present invention, and therefore the weather resistance was poor, marked with "×".

[0068] Comparative Example 5 has a low Al content of 0.003% in stainless steel, which is outside the range, and a high O content of 0.007%, so the value of formula (1) is within the range, but the number percentage of specific S-type inclusions is 17.5 (pieces / mm³). 2 ) was high and outside the scope of the present invention, resulting in poor weather resistance ("×").

[0069] Comparative Example 6 had a low Si content of 0.23% in the stainless steel, and the value of equation (2) was outside the range, resulting in poor weather resistance and pitting corrosion resistance, both of which were marked "×".

[0070] Comparative Example 7 had a low Cr content of 11.03% in the stainless steel, and the value of equation (2) was outside the range, resulting in poor weather resistance and pitting corrosion resistance, both of which were marked "×".

[0071] From the results of Examples 1-6 and Comparative Examples 1-7, it can be seen that in order to obtain the weather resistance and pitting corrosion resistance targeted by the present invention, it is important that the chemical composition range specified in the present invention, the number ratio of specific S-type inclusions, and the values ​​of formulas (1) and (2) are within the range specified in the present invention. Furthermore, the addition of trace elements such as V, W, Co, Zr, REM, Sn, and B, and the preferred manufacturing method specified in the present invention are effective in improving weather resistance and pitting corrosion resistance.

Claims

1. In mass percent, C: 0.10% or less, Si: more than 1.0% and less than 3.0%, Mn: 1.0% or less, Ni: 2.0% or less, P: 0.10% or less, S: 0.0020% or less, Cr: 13.0% or more and 32.0% or less, N: 0.10% or less, At least one of Nb and Ti: 6(C+N)% to 0.7% Cu: 0.10% or less, Mo: 3.0% or less Al: 0.01% or more and 0.20% or less, Ca: 0.0003% or more and 0.010% or less, Mg: 0.0003% or more and 0.010% or less, A ferritic stainless steel having a chemical composition containing 0.0040% or less of O, with the remainder being Fe and unavoidable impurities, In the aforementioned ferritic stainless steel, there are S-based inclusions, which are inorganic compounds containing S. Of the aforementioned S-type inclusions, specific S-type inclusions with a diameter equivalent size of 1 μm or more and an S content of 1% by mass or more have a number ratio of 15 inclusions / mm² on the surface of the ferritic stainless steel. ２ The following: When the respective contents of Cr, Si, Mo, Cu, Ni, Al, Ca, and Mg in the ferritic stainless steel are expressed as {Cr}, {Si}, {Mo}, {Cu}, {Ni}, {Al}, {Ca}, and {Mg}, {Al}, {Ca}, and {Mg} satisfy the relationship shown in equation (1) below, A weather-resistant ferritic stainless steel in which {Cr}, {Si}, {Mo}, {Cu}, and {Ni} satisfy the relationship shown in formula (2) below. Formula (1): {Al}+10{Ca}+{Mg}≦0.20 Formula (2): {Cr}+1.7{Si}+1.2{Mo}+0.4({Cu}+{Ni})>19.0

2. The aforementioned chemical composition is At least one of V, W, Co, and Zr: 1.0% by mass or less. REM: 0.10% by mass or less, Sn: 0.10% by mass or less, B: The weather-resistant ferritic stainless steel according to claim 1, further containing 0.01% by mass or less of B.

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