Ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties.

Abstract

To provide a ferritic stainless steel excellent in pitting-corrosion resistance and soft magnetic properties, with a Si content over 1.0 mass% to duly regulate a specific component contained in a steel as well as to control an inclusion present in the steel and to control a crystal orientation of a crystal grain in the steel.SOLUTION: A ferritic stainless steel sheet contains C, Si, Mn, P, S, O, Cr, Mo, N, Al, Nb, Ti, etc. The contents of the Cr, Si and Mo in the steel sheet and the contents of the Nb and Ti respectively satisfy prescribed conditional expressions, and a precipitate present in the steel sheet has an average size of 5.0 μm or less in an equivalent circle diameter. An area ratio A of the precipitate is 2.0% or less in a base material of the stainless steel sheet. The ferritic stainless steel sheet is excellent in pitting-corrosion resistance and soft magnetic properties.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties. [Background technology]

[0002] Traditionally, permalloy and electrical steel sheets have been used as metallic materials with excellent soft magnetic properties. However, permalloy is expensive due to the large amount of nickel (Ni) added. Electrical steel sheets also have high manufacturing costs due to the nickel plating treatment to compensate for corrosion resistance. Ferritic stainless steel, which contains no Ni or only a small amount of Ni, is gaining recognition as an alternative metallic material. Until now, ferritic stainless steel, which possesses both soft magnetic properties and corrosion resistance, has ensured corrosion resistance by adding molybdenum (Mo). However, because alloying elements such as Mo are expensive, it has not been possible to provide inexpensive metallic materials.

[0003] Recently, as a result of the inventors' diligent research, it has been found that adding more than 1.0 mass% of Si to ferritic stainless steel sheets is effective in improving pitting corrosion resistance. However, adding a large amount of Si to ferritic stainless steel sheets has the problem that C and N are activated, promoting the deposition of carbonitrides and other materials, and reducing soft magnetic properties. Furthermore, adding a large amount of Mo to the steel or applying Ni plating to the surface of the steel sheet would be contrary to the objective of manufacturing ferritic stainless steel sheets at low cost. Therefore, there is a problem in that ferritic stainless steel sheets with more than 1.0 mass% of Si added to improve pitting corrosion resistance are difficult to manufacture at low cost and possess both pitting corrosion resistance and soft magnetic properties.

[0004] For example, Patent Document 1 describes a martensitic stainless steel containing C: 0.08-0.20% and Cr: 11.5-18.0%, wherein the carbide density in the microstructure is 5 × 10⁻⁶. 5 pieces / mm 2The above discloses a stainless steel for magnetic applications having a ferrite particle size of 7 μm or more, a residual magnetic flux density Br: 0.9 T or less, and a square aspect ratio of 0.65 or less. Furthermore, Patent Document 2 discloses a martensitic stainless steel containing one or two of the following: C: 0.08~0.20%, Cr: 11.5~18.0%, Mo: 0.05~1.30%, or W: 0.05~0.80%, with a maximum magnetic permeability μ m 1500 or more, 0.2% proof stress 30kgf / mm 2 The above describes a stainless steel for magnetic applications. Furthermore, Patent Document 3 discloses a steel containing, by weight %, less than 0.005% C, 0.1-1.5% Si, 1.0% or less Mn, 0.04% or less P, 0.01% or less S, 9.0-17.0% Cr, 0.02% or less N, 1.0% or less Ni, 1.0% or less Al, and 1.0% or less Ti, with the remainder being Fe and unavoidable impurities, and by subjecting it to magnetic annealing in the temperature range of 800-850°C and soaking time of 0-10 min, the maximum magnetic permeability μ m The document discloses a soft magnetic stainless steel with a magnetic coefficient of 10,000 or higher.

[0005] Furthermore, Patent Document 4 describes a material having a composition in weight percent of C: 0.02% or less, N: 0.02% or less, Mn: 1.0% or less, Cr: 9.0-17.0%, with the remainder being Fe and unavoidable impurities, a crystal plane intensity ratio K expressed by a predetermined formula of 10 or more, and a maximum magnetic permeability μ maxPatent document 5 discloses a soft magnetic stainless steel sheet with a thickness of 2.0 mm or less and having a magnetic force of 2000 emu or more. Patent document 5 also discloses a free-cutting soft magnetic stainless steel containing, by mass%, C: 0.02~0.15%, Si: 0.5~3.0%, Mn: 2.0% or less, S: 0.1~0.5%, Cr: 10~22%, Al: 0.01~4.0%, Ti: 0.5~1.5%, with the remainder being Fe and unavoidable impurities. Furthermore, Patent Document 6 describes a hysteresis motor comprising a rotor formed of a semi-hard magnetic material and a stator that generates a rotating magnetic field applied to the rotor, wherein the stator yoke, which constitutes the stator of the hysteresis motor together with the excitation coil, contains C: 0.05 mass% or less, N: 0.05 mass% or less, Si: 3.0 mass% or less, Mn: 1.0 mass% or less, Ni: 1.0 mass% or less, P: 0.04 mass% or less, S: 0.01 mass% or less, Cr: 5.0 to 20.0 mass%, Ti: 0.5 mass% or less, with the remainder being Fe and unavoidable impurities. 4.3×%Cr+19.1×%Si>40.2···Formula (1) 64×%Si+35×%Cr+480×%Ti ≥221 × %C + 247 × %N + 40 × %Mn + 80 × %Ni + 460 ... Equation (2) t≧0.23÷f 1 / 2 ·Formula (3) The present invention discloses a hysteresis motor that has a composition satisfying equations (1) and (2), and is formed from an Fe-Cr soft magnetic stainless steel plate with a thickness t that satisfies equation (3) when the operating frequency is f (kHz).

[0006] However, while Patent Documents 1 to 3 describe carbonitrides, they do not describe other precipitates, nor do they describe pitting corrosion resistance. Similarly, while Patent Documents 4 to 6 describe carbonitrides and other precipitates, they do not describe pitting corrosion resistance. Therefore, there is a problem in that these documents do not consider how to create a ferritic stainless steel sheet containing more than 1.0 mass% Si that is inexpensive and possesses both excellent pitting corrosion resistance and soft magnetic properties. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 05-171369 [Patent Document 2] Japanese Patent Application Publication No. 06-013220 [Patent Document 3] Japanese Patent Application Publication No. 10-176250 [Patent Document 4] Japanese Patent Publication No. 2000-064000 [Patent Document 5] Japanese Patent Publication No. 2006-152354 [Patent Document 6] Japanese Patent Publication No. 2009-038907 [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, the present invention has been made in view of the above problems, and aims to provide an inexpensive ferritic stainless steel with excellent pitting corrosion resistance and soft magnetic properties by setting the Si content to more than 1.0 mass%, optimizing the specific components contained in the steel, and controlling the inclusions present in the steel and the crystal orientation of the crystal grains in the steel. [Means for solving the problem]

[0009] The features of the present invention are listed below. (1) In mass%, C: 0.020% or less, Si: more than 1.0% and less than 2.5%, Mn: 1.0% or less, P: 0.040% or less, S: Less than 0.0030% O: Less than 0.0040% Cr:15.5% or more and 23.0% or less Mo: 0.5% or less, N: 0.020% or less, Al: 0.20% or less, Nb: 0.30% or less, and a ferritic stainless steel sheet having a chemical composition containing Ti: 0.30% or less, with the balance being Fe and inevitable impurities, when the respective contents of Cr, Si, Mo, Nb and Ti in the stainless steel sheet are represented by {Cr}, {Si}, {Mo}, {Nb} and {Ti}, respectively, {Cr}, {Si} and {Mo} satisfy the relationship of the following (Formula 1), and {Nb} and {Ti} satisfy the relationship of the following (Formula 2), the precipitates present in the stainless steel sheet have an average size of 5.0 μm or less in terms of the equivalent circle diameter, and the area ratio A of the precipitates to the base metal of the stainless steel sheet satisfies the relationship of the following (Formula 3), a ferritic stainless steel sheet excellent in pitting corrosion resistance and soft magnetic properties. (Formula 1): PI = {Cr} + 2{Si} + 3{Mo} ≥ 19.0 (Formula 2): 0.10 ≤ ({Nb} + {Ti}) ≤ 0.30 (Formula 3): A ≤ 2.0% (2) When measuring the crystal orientation on a plane parallel to the rolling plane of the stainless steel sheet, the total area ratio S1 of crystal grains with an RD plane orientation of <110> and crystal grains with an RD plane orientation of <112> is 80% or less, and the area ratio S2 of crystal grains with an RD plane orientation of <111> is 10% or less, the ferritic stainless steel sheet excellent in pitting corrosion resistance and soft magnetic properties according to (1). (3) The chemical composition is, in mass%, B: 0.0050% or less, Ni: 1.0% or less, Cu: 1.0% or less, V: 0.50% or less, W: 0.50% or less, Ca: 0.0100% or less, Mg: 0.010% or less, Zr: 0.50% or less, Co: 0.50% or less, Ga: 0.10% or less, [[ID=4l]] La: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, and, A ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties as described in (1) or (2), further containing one or more selected from the group REM:0.10% or less. [Effects of the Invention]

[0010] This invention provides an inexpensive ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties by setting the Si content in the steel to more than 1.0 mass%, optimizing the specific components contained in the steel, and controlling the inclusions present in the steel and the crystal orientation of the crystal grains within the steel. [Modes for carrying out the invention]

[0011] Embodiments of the present invention are described below. Note that the following description is an example of embodiments of this invention and does not limit the scope of these claims.

[0012] In order to solve the problem, the inventors of this invention diligently investigated the effects of additive elements that improve pitting corrosion resistance and soft magnetic properties in ferritic stainless steel sheets, and obtained the following new findings, leading to the present invention.

[0013] The ferritic stainless steel sheet of the present invention (hereinafter simply referred to as "stainless steel sheet") contains Cr as its main component, has a structure mainly consisting of a ferrite phase, and has the following chemical composition. Furthermore, the stainless steel sheet of the present invention can be used in many applications requiring pitting corrosion resistance. For example, because it is ferritic and has excellent soft magnetic properties, it is used in relays, motors, and shield cases for electric vehicles. In addition, its corrosion resistance makes it suitable for environments that use water, such as automatic watering systems, automatic sprinklers, and household electrical appliances.

[0014] Conventionally, it has been known that adding Si is effective in improving soft magnetic properties. Si-added electrical steel sheets generally provide high magnetic flux density even at low external magnetic fields, and have therefore been widely used in the cores of solenoid valves and other applications. Thus, adding Si to ordinary steel sheets has been effective in improving soft magnetic properties. However, adding more than 1% Si to ordinary steel sheets can increase the activity of N and C, promoting the precipitation of carbonitrides. Such carbonitrides act as pitting corrosion initiation points, leading to a decrease in pitting corrosion resistance. Furthermore, carbonitrides pinn magnetic domain wall movement, also leading to a decrease in soft magnetic properties. In addition, carbonitrides reduce the grain size of the steel sheet before cold rolling, and make it easier to generate grains with crystal orientations that have low soft magnetic properties. Thus, simply adding Si has limited effectiveness in improving soft magnetic properties. Therefore, when Si is added to a high-purity ferritic stainless steel sheet with reduced C and N content, with the intention of reducing costs compared to existing materials such as Mo-added steel, it is possible to obtain a stainless steel sheet with excellent corrosion resistance, particularly in both pitting corrosion resistance and soft magnetic properties, by controlling the manufacturing conditions from hot rolling to the final steel sheet, thereby controlling the state of precipitates and the crystal orientation of the generated crystal grains. The stainless steel sheet of the present invention will be described in detail below.

[0015] The stainless steel sheet of the present invention has a chemical composition in mass%, C: 0.020% or less, Si: more than 1.0% and less than 2.5%, Mn: 1.0% or less, P: 0.040% or less, S: Less than 0.0030% O: Less than 0.0040% Cr:15.5% or more and 23.0% or less Mo: 0.5% or less, N: 0.020% or less, Al: 0.20% or less, Nb: 0.30% or less, It contains less than 0.30% Ti, with the remainder consisting of Fe and unavoidable impurities.

[0016] (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: 0.020% or less) Carbon (C) is an element that is inevitably present in stainless steel. However, because its presence in the matrix reduces workability and corrosion resistance, a lower C content is preferable. Furthermore, it forms nonmagnetic carbides with other added metals, hindering domain wall movement and thus reducing soft magnetic properties, so the upper limit should be 0.020% or less. In addition, by preferably reducing the C content to 0.05% or less, corrosion resistance, particularly pitting corrosion resistance, can be further improved. On the other hand, C is an interstitial solid solution element and has a strong tendency to segregate at grain boundaries, so it is preferable to set the lower limit to 0.001% or more in order to contribute to strengthening the grain boundaries.

[0017] (Si: more than 1.0% and less than 2.5%) Silicon (Si) is an effective element for deoxidation and improves oxidation resistance. It also increases hardness and contributes to improved mechanical strength. Furthermore, it can improve corrosion resistance, especially pitting corrosion resistance in neutral environments such as saltwater. It also increases electrical resistivity, thereby reducing iron loss and improving soft magnetic properties. Therefore, the lower limit of Si addition should be at least 1.0%. On the other hand, Si acts as a solid solution strengthening element, leading to a decrease in workability and weldability, and excessive addition promotes the formation of precipitates, leading to a decrease in soft magnetic properties, so the upper limit should be 2.5% or less. Considering the respective effects and manufacturability, the Si content is preferably between 1.0% and 2.0%.

[0018] (Mn: 1.0% or less) Manganese (Mn) is a deoxidizing element and is effective in fixing oxygen (O). However, it can form MnS sulfides, which can be the starting point for corrosion. Furthermore, Mn destabilizes the ferrite phase, forming a martensite phase and leading to a decrease in soft magnetic properties and oxidation resistance. Therefore, the upper limit of the Mn content should be 1.0% or less. On the other hand, in order to ensure the deoxidizing effect of Mn on oxygen (O) in the matrix phase and the fixation of sulfur (S) by forming sulfides of sulfur (S), it is preferable that the lower limit of the Mn content be 0.01% or more. Considering the above effects and manufacturing costs, it is more preferable that the Mn content be 0.05 to 0.5%.

[0019] (P:0.040% or less) Phosphorus (P) is an element that is inevitably present in stainless steel. Since the phosphides that P forms reduce soft magnetic properties and also hinder workability and weldability, a lower P content is preferable, and therefore the upper limit of the P content should be 0.040% or less. However, excessive reduction of P leads to increased refining costs, so it is preferable to set the lower limit of the P content at 0.005% or more. A more preferable range for the P content, taking manufacturing costs into consideration, is 0.010 to 0.030%.

[0020] (S:0.0030% or less) S (sulfur) is an element that is inevitably present in stainless steel. S reduces the soft magnetic properties of stainless steel due to the sulfides it forms, and it also tends to segregate at grain boundaries, reducing hot workability, corrosion resistance, and especially weather resistance. Therefore, a lower S content is preferable, and the upper limit of the S content should be 0.0030% or less. However, excessive reduction of S leads to increased raw material and refining costs, so it is preferable to set the lower limit of the S content at 0.0001% or more. A more preferable range for the S content is 0.0010 to 0.0020%, taking into consideration the suppression of embrittlement and manufacturing costs.

[0021] (O: Less than 0.0040%) Oxygen (O) is an element that is inevitably present in stainless steel sheets. It forms oxides with other additive elements and acts as a site that hinders magnetic domain wall movement, thus degrading soft magnetic properties, making it an undesirable element. Furthermore, the formed oxides reduce mechanical properties such as workability and toughness, so the O content should be less than 0.0040%, preferably 0.0035% or less. An even more preferable O content is 0.0010% or less. However, excessive reduction of O leads to increased raw material and refining costs, so it is preferable to set the lower limit at 0.0001% or more.

[0022] (Cr: 15.5% or more and 23.0% or less) Cr (chromium) is a fundamental element of the ferritic stainless steel of this invention and is an essential element for ensuring corrosion resistance, particularly weather resistance and oxidation resistance. Like Si, Cr is an effective component that suppresses the formation of martensite and improves soft magnetic properties. This effect can be achieved by including 15.5% or more of Cr. However, adding more than 23.0% of Cr reduces the saturation magnetic flux density and increases hardness, degrading workability. Therefore, the Cr content should be between 15.5% and 23.0%. To further enhance these effects, it is preferable to have a Cr content of 16.0-20.0%.

[0023] (Mo: 0.5% or less) Mo (molybdenum) is an effective element for obtaining weather resistance in addition to corrosion resistance and oxidation resistance, similar to Ni and Cu. In particular, it is effective in suppressing the progression of pitting corrosion in low pH environments. For each of these effects to be exhibited, the Mo content is preferably 0.10% or more. However, excessive content will increase the alloy cost and hinder the manufacturability of hot and cold working, so the upper limit of the Mo content should be 0.5% or less.

[0024] (N:0.020% or less) Nitrogen (N) is an element that is inevitably present in stainless steel sheets. Like carbon (C), N reduces workability and corrosion resistance, so a lower N content is preferable. Furthermore, since N forms nitrides with other additive elements and reduces soft magnetic properties, the upper limit of the N content should be 0.020% or less. However, excessive reduction of N leads to increased refining costs, so it is preferable to set the lower limit of the N content at 0.001%. In addition, although N is an interstitial solid solution element like carbon, it has a small tendency to segregate at grain boundaries and contributes little to strengthening the grain boundaries. Since it sensitizes the corrosion targeted by this invention, the preferred range for the N content is 0.005 to 0.015%, considering performance and manufacturing costs.

[0025] (Al: 0.20% or less) Aluminum (Al) is an extremely effective deoxidizing element. However, because it can lead to a decrease in the toughness and weldability of steel, the upper limit of the Al content should be 0.20% or less. Al, in particular, forms oxides with Si, reducing the amount of O (oxygen) in the matrix phase. However, if the amount of oxides increases, it can hinder magnetic domain wall movement, reducing soft magnetic properties and becoming a starting point for corrosion, thus reducing pitting corrosion resistance. However, if the Al content is 0.20% or less, there will be no decrease in soft magnetic properties or pitting corrosion resistance. Furthermore, the lower limit of the Al content should preferably be 0.01% or more, taking into account the deoxidizing effect, and considering manufacturability and performance, the Al content should preferably be between 0.01% and 0.10%.

[0026] (Nb:0.30% or less) Niobium (Nb) is an effective element for processability and corrosion resistance due to its ability to fix carbon and nitrogen. However, because it forms carbonitrides and hinders domain wall movement, thereby reducing soft magnetic properties, the Nb content should be 0.30% or less. Preferably, the Nb content should be 0.01% or more, which is the amount at which the respective effects manifest. The preferred range for the Nb content is 0.01 to 0.25%, taking into account the respective effects, alloy cost, and manufacturability.

[0027] (Ti: 0.30% or less) Titanium (Ti), like Nb, is an effective element for processability and corrosion resistance due to its ability to fix C and N. However, because it forms carbonitrides and hinders magnetic domain wall movement, thereby reducing soft magnetic properties, the Ti content should be 0.30% or less. Preferably, the Ti content should be 0.01% or more, which is the amount at which the respective effects manifest. The preferred range for Ti content is 0.01 to 0.25%, taking into account the respective effects, alloy cost, and manufacturability.

[0028] Furthermore, the stainless steel sheet of the present invention may contain the following optional additive elements as needed.

[0029] (B:0.0050% or less) Boron (B) is an element that improves hot workability and resistance to secondary work brittleness, and its addition to stainless steel is effective. The B content is preferably 0.00050% or more to achieve these effects. However, since a high B content leads to a decrease in elongation and fatigue strength, the upper limit is set at 0.0050%. Preferably, considering material cost and workability, the content is set at 0.0005 to 0.0020%.

[0030] (Ni: 1.0% or less) Nickel (Ni) is an effective element for corrosion resistance and is effective against weathering in crevice corrosion. In order to obtain pitting corrosion resistance in the stainless steel of the present invention, it is preferable that the Ni content be greater than 0.03%. On the other hand, if the Ni content exceeds 1.0%, it destabilizes the ferrite phase, leading to a decrease in soft magnetic properties, an increase in alloy costs, and a decrease in workability due to increased material strength. Therefore, the upper limit of the Ni content is set at 1.0%. The preferred range for the Ni content is 0.8% or less, taking into consideration performance and alloy costs.

[0031] (Cu:1.0% or less) Copper (Cu) is an effective element for corrosion resistance and is suitable for obtaining workability and pitting corrosion resistance. In particular, it is effective in suppressing the progression of pitting corrosion in low pH environments. In order to obtain workability by delaying P segregation in the stainless steel of the present invention, it is preferable that the Cu content be greater than 0.03%. On the other hand, if the Cu content exceeds 1.0%, it destabilizes the ferrite phase, leading to a decrease in soft magnetic properties, an increase in alloy costs, and a decrease in workability due to increased material strength, so the upper limit of the Cu content is set at 1.0%. The preferred range for the Cu content is 0.05 to 0.5%, taking into consideration performance and alloy costs.

[0032] (V:0.50% or less) Vanadium (V) is an effective element for improving corrosion resistance. In particular, by reducing solid-solution carbon and nitrogen through the formation of carbonitrides, it contributes to improving the corrosion resistance and soft magnetic properties of high-purity ferritic stainless steel, and is therefore added as needed. The V content is preferably 0.01% or more, which is necessary for the effects to manifest. If the content exceeds 0.50%, it leads to an increase in alloy costs and a decrease in manufacturability, as well as a decrease in soft magnetic properties due to excessive precipitates, so the upper limit for the content of each element should be 0.50% or less. The preferred range for the content of each element is 0.02 to 0.30%, taking into account workability, manufacturability, and alloy costs.

[0033] (W: 0.50% or less) W is an effective element for improving corrosion resistance, and since it contributes to improved corrosion resistance by solid-solution in steel, it should be added as needed. When added, the amount should be 0.01% or more, which is the amount at which the effects of each element become apparent. Excessive addition leads to increased alloy costs and decreased manufacturability, and in particular, additions exceeding 0.5% cause hardening and decreased elongation due to solid-solution strengthening and precipitation strengthening, so the upper limit should be 0.50% or less. The preferred range is 0.02-0.3%, considering performance, manufacturability, and alloy cost.

[0034] (Co, Zr: both 0.50% or less) Co (cobalt) and Zr (zirconium) are effective elements for improving the purity of steel and obtaining soft magnetic properties and resistance to secondary processing brittleness, and should be added as needed. When added, the amount should be 0.01% or more, which is the amount at which their effects manifest. Excessive addition leads to increased alloy costs and decreased manufacturability, so the upper limit should be 0.50%. The preferred range is 0.02-0.3%, considering performance, manufacturability, and alloy cost.

[0035] (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%, it forms oxides and sulfides, hindering magnetic domain wall movement and reducing soft magnetic properties. Moreover, since the formed sulfides are water-soluble, this leads to a decrease in pitting corrosion resistance. Therefore, the upper limit of the Ca content is set at 0.010%. Preferably, it is 0.009% or less, considering manufacturability and oxidation resistance.

[0036] (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 phase. 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, Mg reduces manufacturability and leads to a decrease in corrosion resistance and pitting corrosion resistance due to the formation of many oxides and sulfides. Also, because it forms oxides and sulfides that hinder magnetic domain wall movement and reduce soft magnetic properties, 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.

[0037] (Ga:0.10% or less) Gallium (Ga) is added as needed because it improves hot workability and the cleanliness of steel, and significantly enhances oxidation resistance and hot workability. The content of each is set to 0.001% or more, which is the amount at which their respective effects manifest. However, since Ga readily forms oxides, hindering magnetic domain wall movement and reducing soft magnetic properties, and also leading to increased alloy costs and decreased manufacturability, the upper limit is set to 0.10% or less. Preferably, considering the effects, economy, and manufacturability, at least one of them is added in an amount of 0.001 to 0.050%.

[0038] (Y, Hf, La, REM: all less than 0.10%) Yttrium (Y), hafnium (Hf), lanthanum (La), and rare earth elements (REM) are 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, since La, Y, Hf, and REM readily form oxides, hindering magnetic domain wall movement and reducing soft magnetic properties, and also leading to increased alloy costs and decreased manufacturability, the upper limit for the content of each of Y, Hf, La, and REM should be 0.10% or less. Preferably, considering the effects, economy, and manufacturability, at least one of them should be present in a quantity of 0.001-0.050%. REM refers to rare earth metals of the lanthanide series, actinide series, and composite metals thereof, such as Ce, Pr, and Sm.

[0039] (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.

[0040] Furthermore, in the stainless steel sheet of the present invention, when the content of Cr, Si, Mo, Nb, and Ti in the stainless steel sheet is expressed as {Cr}, {Si}, {Mo}, {Nb}, and {Ti} respectively, {Cr}, {Si}, and {Mo} satisfy the following relationship (Equation 1), and {Nb} and {Ti} satisfy the following relationship (Equation 2). The precipitates present in the stainless steel sheet have an average size of 5.0 μm or less in terms of equivalent circle diameter, and the area ratio A of the precipitates in the base material of the stainless steel sheet satisfies the following relationship (Equation 3). (Formula 1): PI={Cr}+2{Si}+3{Mo}≧19.0 (Formula 2): 0.10≦({Nb}+{Ti})≦0.30 (Formula 3): A≦2.0%

[0041] (Regarding Equation 1) Furthermore, the ferritic steel sheet of the present invention satisfies the relationship (Equation 1) when the content of Cr, Si, and Mo is expressed as {Cr}, {Si}, and {Mo}, respectively. (Formula 1):{Cr}+2{Si}+3{Mo}≧19.0

[0042] Stainless steel sheets containing Cr are susceptible to corrosion in environments containing halogen ions such as chloride ions. The passive film is locally destroyed by the action of chloride ions, and pitting corrosion progresses as these areas preferentially fail. The progression of this pitting corrosion varies greatly depending on the elements contained in the stainless steel sheet. Generally, the pitting index (PI = Cr + 3Mo) is known as an indicator of the pitting corrosion resistance of stainless steel sheets. The stainless steel sheet of the present invention satisfies (Equation 1), which was obtained by newly discovering the effect of Si as the pitting index (PI).

[0043] The stainless steel sheet of this invention employs pitting potential measurement as a method for evaluating pitting corrosion resistance. Many of the additive elements were measured, and in addition to Cr and Mo, Si, which showed a particularly significant effect, was focused on. The influence of each of the {Cr}, {Si}, and {Mo} content was investigated. Pitting potential measurement was performed in accordance with JIS G 0577, with a current value of 100 μA / cm² in a 3.5 mass% NaCl aqueous solution at 30°C. 2 A potential exceeding a certain value was defined as the pitting potential V'c100. In the stainless steel sheet of the present invention, when the content exceeds 1.0% {Si}, it was found that pitting corrosion resistance is improved not only by increasing {Cr} but also by increasing {Si}, and the effect is twice that of {Cr}. The mechanism by which this effect occurs is not entirely clear, but from the analysis of the passivation film, it is inferred that the Si oxides formed in the inner layer of the film and at the steel interface exert an effect of suppressing the destruction of the passivation film by halogen ions. Furthermore, it was found that even when more than 1.0% of Si is added, the effect of containing {Mo} is three times that of {Cr}. Therefore, the pitting index (PI) of the stainless steel sheet of the present invention is expressed as PI = {Cr} + 2{Si} + 3{Mo}.

[0044] Furthermore, the pitting potential V'c100 was compared with that of SUS430J1L (19Cr ferritic stainless steel sheet), SUS443J1 (21Cr ferritic stainless steel sheet), and SUS304 (18Cr-8Ni austenitic stainless steel sheet). By setting the PI (Equation 1) of the stainless steel sheet of the present invention to ≥ 19.0, a pitting potential of 0.20V, equivalent to that of SUS430J1L (19Cr ferritic stainless steel sheet), was achieved. Also, by setting the PI of the stainless steel sheet of the present invention to ≥ 21.0, a pitting potential of 0.30V, equivalent to that of SUS443J1 (21Cr ferritic stainless steel sheet) and SUS304 (18Cr-8Ni austenitic stainless steel sheet), was achieved. Therefore, the pitting index (PI) of the stainless steel sheet of the present invention was set to 19.0 or higher. Furthermore, if the pitting index (PI) of the stainless steel sheet of the present invention exceeds 25.0, the Si content in the steel tends to become too high, resulting in excessively high tensile strength and hardness, reduced workability, and the formation of excessive precipitates, which degrades the soft magnetic properties. Similarly, if the Mo content in the steel increases, it results in higher raw material costs. For these reasons, it is preferable to set the upper limit of the pitting index (PI) to 25.0.

[0045] (Regarding Equation 2) Furthermore, the ferritic steel sheet of the present invention satisfies the relationship (Equation 2) when the Nb and Ti content in the stainless steel sheet are expressed as {Nb} and {Ti}, respectively. (Formula 2): 0.10≦({Nb}+{Ti})≦0.30

[0046] Niobium (Nb) and titanium (Ti) are both stabilizing elements that fix carbon (C) and nitrogen (N), thereby improving workability and resistance to intergranular corrosion, and are also effective in improving the metal structure. When added, each element exhibits its own effect, so the total content of Nb and Ti in stainless steel sheets should be 0.10% or more. Furthermore, Nb and Ti can suppress the decrease in the soft magnetic properties of stainless steel by forming oxides and nitrides of carbon and nitrogen. However, if there is an excess of Nb and Ti, the large amount of oxides and nitrides generated will become pinning sites for magnetic domain wall movement, reducing the soft magnetic properties of coercivity and magnetic flux density. In addition, excessive addition of Nb and Ti leads to an increase in alloy costs and a decrease in manufacturability due to the increase in recrystallization temperature, so the upper limit of the total content of Nb and Ti should be 0.30% or less. Moreover, excessive addition of Nb and Ti may lead to a decrease in surface quality during manufacturing. Therefore, considering the effects, alloy cost, and manufacturability, the preferred range for the total content of Nb and Ti is preferably 0.15 to 0.25%.

[0047] (Regarding Equation 3) Furthermore, in the ferrite steel sheet of the present invention, the precipitates present in the steel sheet have an average size of 5.0 μm or less in terms of the equivalent circle diameter, and the area ratio A of the precipitates occupying the base material of the stainless steel sheet satisfies the relationship of (Equation 3). (Equation 3): A ≤ 2.0%

[0048] Furthermore, in the stainless steel sheet of the present invention, precipitates are formed from oxides, sulfides (e.g., MnS), carbonitrides (e.g., (Nb, Ti)C, M 23 C6-type carbides, carbonitrides such as Ti(C,N), (Nb, Ti)(C,N), etc.), phosphides (e.g., FeTiP, etc.), and alloy phases containing Nb, Ti, Fe, etc. (e.g., Fe2Nb, etc.). These precipitates become pinning sites that hinder domain wall movement and deteriorate the soft magnetic properties. Furthermore, the effect of this pinning site is affected by the size and area ratio of the precipitates. Therefore, in the stainless steel sheet of the present invention, the precipitates present in the steel sheet have an average size of 5.0 μm or less in terms of the equivalent circle diameter, and the area ratio A of the precipitates occupying the base material of the stainless steel sheet satisfies the relationship of (Equation 3). Thereby, the deterioration of the soft magnetic properties due to the precipitates can be suppressed.

[0049] The precipitates were measured for the equivalent circle diameter and area ratio as follows. After chemically etching the rolling surface, precipitates of 0.1 μm or more were extracted using an optical microscope. The extracted precipitates were classified into the following three categories according to JIS G 0555 (Microscopic test method for non-metallic inclusions in steel), and the average size (observation number n = 10) was determined as the equivalent circle diameter. Thereafter, the area ratio of each category is expressed as "(the number of measured precipitates in each category) × (the area obtained from the equivalent circle diameter) / (the observation field area) × 100".

[0050] Furthermore, the precipitates were classified into the following three types based on the shape of the precipitates observed with an optical microscope and the results of the composition analysis of the precipitates. (1) Type A precipitates (sulfide type): Precipitates with rounded or sharp tips and an aspect ratio of 3 or greater. These correspond to groups A and C in JIS G 0555 (Annex A (Normative) Standard diagrams of groups A, B, C, D and DS inclusions). (2) B-type precipitates (oxide-based, carbonitride-based): Precipitates that do not deform, are angular, and have an aspect ratio of less than 3. These correspond to groups B and D of the JIS standard. (3) C-type precipitates (phosphide-based, Nb, and Ti-containing alloy phases): Circular or nearly circular precipitates with an aspect ratio of less than 3. These correspond to Group DS (individual inclusions) in the JIS standard. Here, the area ratio A of the precipitates represents the sum of the area ratio A1 of the A-type precipitates, the area ratio A2 of the B-type precipitates, and the area ratio A3 of the C-type precipitates.

[0051] The average size of the precipitates is determined as the equivalent diameter of a circle. Here, the "equivalent diameter of a circle" refers to the diameter of a perfect circle that corresponds to the area of ​​a figure drawn in an image or the like. By setting the equivalent diameter of a circle to 5.0 μm or less, which is used as a convenient numerical value when comparing multiple complex figures, the obstruction to magnetic domain wall movement is reduced. Similarly, by setting the equivalent diameter of the precipitates to 5.0 μm or less, it is possible to suppress the precipitates from becoming the starting point of pitting corrosion. Furthermore, the lower limit of the average size of the precipitates is set to 0.01 μm in terms of the equivalent diameter of a circle, due to reasons such as the detection limit of the measuring device.

[0052] Furthermore, the area fraction A of the precipitates should be kept below 2.0%, as shown in (Equation 3). If the area fraction exceeds 2.0%, the number of pinning sites that hinder domain wall movement increases, and the soft magnetic properties deteriorate. Therefore, by keeping the area fraction A below 2.0%, domain wall movement is not hindered, and the deterioration of soft magnetic properties can be suppressed.

[0053] At this time, the precipitates were classified by shape. However, none of the precipitates classified by shape—Type A precipitates (sulfide type), Type B precipitates (oxide type, carbonitride type), and Type C precipitates (phosphide type, alloy phase containing Nb and Ti)—possessed magnetic properties significant enough to affect the steel sheet, and their function as pinning sites was equivalent. Therefore, it was found that excellent soft magnetic properties could be obtained by setting the equivalent circle diameter of the precipitates to 5.0 μm or less and the area ratio A to 2.0% or less.

[0054] (Surface crystal orientation) Furthermore, when measuring the crystal orientation of the stainless steel sheet of the present invention on a plane parallel to the rolling surface, the RD plane orientation is <110> The crystal grains are such that the RD plane orientation is <112> The total area fraction S1 of the crystal grains is 80% or less, and the RD plane orientation is <111> It is preferable that the area fraction S2 of the crystal grains is 10% or less. <100> The area fraction S3 containing the crystal grains, and all other RD plane orientations are <210> The sum of the area fraction S4 of the crystal grains, which are of higher orientations, and other factors is set to 100%.

[0055] The crystal orientation is measured as follows: The sample was cut from the center of a steel sheet, measuring 10 mm L × 10 mm W, and then thinned from the rolled surface until the thickness was halved. The rolled surface was then finished with colloidal silica, and the thinned surface (parallel to the rolled surface) was measured using electron backscatter diffraction (EBSD) in combination with a scanning electron microscope (SEM). Furthermore, the area fraction of crystal grains with a specific crystal orientation is measured using OIM software (manufactured by TSL Solutions Co., Ltd.) as follows. The orientation of each measurement point measured using the above apparatus is color-coded according to its position and illustrated to obtain an IPF (Inverse Pole Figure) map. At this time, the proportion of the area of ​​the measurement field in which crystal grains with an angular difference from each orientation that is within the tolerance angle (Tolerance Angle) of 15° or less occupy is calculated as the area fraction of crystal grains with a single crystal orientation.

[0056] Furthermore, the soft magnetic properties are measured as follows. A test specimen with an outer diameter of φ45 mm, an inner diameter of Φ33 mm, and a thickness of 0.8 mm was cut from the center of the width of a steel plate using electrical discharge machining. Then, a primary winding was wound around the test specimen 100 times, and a secondary winding was wound around it 100 times, and it was subjected to a ring test using a BH tracer. During this test, an external magnetic field was applied up to a maximum of 1000 oorsted (Oe), and the magnetic flux density (B1: Gauss (G)) at 1000 oorsted (Oe) was measured from the initial magnetization curve, and the coercivity (Hc (oorsted: Oe)) was measured from the BH curve.

[0057] Ferritic stainless steel is <111> The orientation is the hard magnetization axis, <100> The orientation is the easy magnetization axis. Other crystal orientations have magnetization eases between these two. Therefore, depending on the actual direction of use, when measuring the crystal orientation on a plane parallel to the rolling plane, it is the hard magnetization axis. <111> Stainless steel that can suppress orientation, reduce coercivity, and increase magnetic flux density is desirable. However, when ferritic stainless steel containing Si is made into a steel sheet under normal manufacturing conditions, the rolling stable orientation is... <110> Crystal grains having orientation, <111> It is prone to forming crystal grains with orientation, and the effect of improving soft magnetic properties cannot be fully obtained.

[0058] The stainless steel of the present invention, by controlling the chemical composition and manufacturing conditions, has an RD plane orientation. <110> The crystal grains and RD plane orientation are <112> The total area fraction S1 of the crystal grains is 80% or less, and the RD plane orientation is <111> The area fraction S2 of the crystal grains is reduced to 10% or less. This allows the coercivity to be reduced to 1.00 oorsted (Oe) or less, making it possible to follow changes in the external magnetic field. To follow changes in the external magnetic field, it is preferable to reduce it to 0.85 oorsted (Oe) or less. Furthermore, by setting the area ratio S1 to 80% or less and the area ratio S2 to 10% or less, the magnetic flux density (B1) at an external magnetic field of 1000 oorsted (Oe) during measurement can be increased to 1800 gauss (G) or more, preferably 2000 gauss (G) or more.

[0059] A method for manufacturing a stainless steel sheet having the chemical composition described above will now be explained. The method for manufacturing a stainless steel sheet of this embodiment includes, for example, the steps of steelmaking - hot rolling - hot-rolled sheet annealing - pickling - cold rolling - cold-rolled sheet annealing. In this manufacturing process, a stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties is obtained by coiling after hot rolling under the conditions shown below, then hot-rolled sheet annealing, followed by cold rolling and cold-rolled sheet annealing, and then magnetic annealing.

[0060] Hot-rolled sheet annealing, cold-rolled sheet annealing, and magnetic annealing can all be batch-type or continuous-type annealing. Furthermore, the atmosphere for each annealing process may be bright annealing in a non-oxidizing atmosphere such as hydrogen gas or nitrogen gas, or annealing in air, if necessary. After cold-rolled sheet annealing, salt treatment, pickling, electrolytic pickling, etc., may be performed. In addition to these, to the extent that it does not impair the effects of the present invention, for example, a tension leveling process for shape correction may be performed after cold-rolled sheet annealing. The stainless steel sheet of the present invention is particularly suitable for applications requiring magnetic properties. In such cases, products with excellent magnetic properties can be obtained by performing magnetic annealing after processing into parts. The stainless steel of the present invention is suitable for use in motor parts such as motor cores, relays and solenoid valves, and their cores, yokes, connectors and housings. For this purpose, after cold-rolled sheet annealing, magnetic annealing is performed after forming processes such as cutting or electrical discharge machining, depending on the application.

[0061] In the stainless steel sheet of the present invention, it is possible to ensure the pitting corrosion resistance and soft magnetic properties targeted by the present invention even when manufactured under normal process conditions, provided that the above chemical composition is satisfied. However, in order to satisfy the requirements for a suitable metallic structure, it is preferable to manufacture it as follows. The finishing temperature in the hot rolling process may be within a general range, but it should be 900°C or higher. The coiling temperature can be set in the range of 300°C to 900°C as needed, but in the case of the stainless steel of the present invention, it should be 500°C or lower, and the cooling rate between finishing and coiling should be 20°C / second or higher. This suppresses excessive precipitation of precipitates. Next, after hot working, the hot-rolled sheet is annealed at 900°C or higher, preferably 920°C to 1000°C. The heating time is not particularly limited, but from the viewpoint of completing recrystallization, it is preferably 10 to 120 seconds.

[0062] Next, the cold rolling process may be carried out using either a zenzimir mill or a tandem mill. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, and rolling temperature should be appropriately selected within a general range. Intermediate annealing may be performed during the cold rolling process. Cold rolling is followed by annealing of the cold-rolled sheet. The annealing temperature should be 900°C or higher to remove processing strain, and 950°C or lower to suppress grain growth. In particular, the soaking temperature should be between 900°C and 950°C. If the annealing temperature of the cold-rolled sheet is below 900°C, recrystallization may be insufficient. Conversely, if the annealing temperature of the cold-rolled sheet exceeds 950°C, grain growth may be insufficiently suppressed. There is no particular limit to the heating time in the cold-rolled sheet annealing, but from the viewpoint of promoting recrystallization, it is preferably between 10 and 120 seconds.

[0063] Furthermore, the conditions for magnetic annealing can be appropriately determined according to the applicable product, application, etc., but the heating temperature for magnetic annealing should be 900°C or higher, preferably 950°C or lower. It is desirable to heat for 1 to 10 hours, and more preferably for 1 to 3 hours. By performing magnetic annealing in this manner, processing strain is removed and the crystal grains are coarsened, resulting in improved magnetic properties, and the magnetic flux density B1 and coercivity can be enhanced. As explained above, hot-rolled sheet annealing, cold-rolled sheet annealing, and magnetic annealing performed after these processes are preferably carried out at temperatures of 900°C or higher.

[0064] At this time, in order to make the equivalent circular diameter of the precipitates 5.0 μm or less, hot rolling should be performed to a plate thickness of 8.0 mm or less. This will simultaneously allow the area ratio A of the precipitates to be 2.0% or less. Furthermore, by performing hot rolling to a plate thickness of 5.0 mm or less, the RD plane orientation will be <111> The area ratio S2 of the crystal grains can be kept to 10% or less. Furthermore, the manufacturing method of the stainless steel sheet of this embodiment makes it possible to obtain a stainless steel sheet with a coercivity of 0.85 oorsted or less, a magnetic flux density (B1) of 1800 gauss (G) or more, and a pitting potential of 0.20 V or more. [Examples]

[0065] 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.

[0066] Table 1 shows the content of essential additive elements and, in some cases, optional additive elements in steel grades A to K used in Examples 1 to 13 and Comparative Examples 1 to 7.

[0067] [Table 1]

[0068] Table 2 shows the manufacturing conditions for ferritic stainless steel having the chemical composition (mass%) shown in Table 1. The process involves melting the stainless steel, hot rolling it at a heating temperature of 800°C to 1050°C, winding it at a cooling rate of 15 to 60°C / s at a winding temperature of 300°C to 600°C, and producing hot-rolled steel sheets with thicknesses ranging from 3.5 mm to 8.0 mm. These sheets were then annealed and pickled at 900°C or higher. Following this, cold rolling was performed, cold-rolled sheet annealing was carried out at 950°C or lower, and then pickling was performed. Next, magnetic annealing was performed at 950°C or 1150°C. The manufacturing conditions for Examples 1 to 13 and Comparative Examples 1 to 7 are shown below.

[0069] [Table 2]

[0070] The characteristics of Examples 1-13 and Comparative Examples 1-7 were evaluated below. Table 3 evaluates the properties of the stainless steel of the present invention, including the value on the left side of (Equation 1) relating to pitting corrosion resistance, the middle value of (Equation 2), the average equivalent circular diameter of the precipitates, the area ratio A of the precipitates, and the area ratios A1, A2, and A3 of the A, B, and C system precipitates, as well as the area ratios S1, S2, and S3 for each crystal orientation parallel to the rolling direction.

[0071] The left-hand value of Equation 1 relating to pitting corrosion resistance and the middle value of Equation 2 relating to the evaluation of precipitates were calculated based on the chemical composition for each steel type shown in Table 1.

[0072] The precipitates were measured as follows. A 10mmL × 10mmW plate was cut from the center of the width of the stainless steel sheet used as the sample, and the thickness was reduced from the rolled surface to the center of the plate thickness. After chemical etching of the rolled surface, precipitates of 0.1 μm or larger were observed using an optical microscope. The precipitates were separated into the following three categories in accordance with JIS G 0555 (Microscopic examination method for nonmetallic inclusions in steel), and the average size (number of observations: 10) was determined as the equivalent diameter of a circle. The size of the precipitate is determined as the equivalent diameter of a circle. The equivalent diameter of a circle is the diameter of a perfect circle that corresponds to the area of ​​a figure drawn in an image or other image. The area ratio for each category is expressed as "(Number of measurements in each category) × (Area calculated from the equivalent circle diameter) / (Observation field area) × 100".

[0073] Based on their shape, the precipitates were classified into three types: Type A precipitates (sulfide-based), Type B precipitates (oxide-based, carbonitride-based), and Type C precipitates (phosphide-based, alloy phases containing Nb and Ti) using an optical microscope. Compositional analysis of the precipitates was also performed. Inclusions were detected using a FE-SEM (Field Emission Scanning Electron Microscope: JEOL Ltd.), and the composition of these areas was further measured by point analysis using an EDX (Energy Dispersive X-ray Spectrometer). Here, the precipitate area ratio A represents the sum of the area ratios A1 (Type A precipitates), A2 (Type B precipitates), and A3 (Type C precipitates).

[0074] Furthermore, the crystal orientation and the area ratio of crystal grains having the assumed crystal orientation were measured as follows. A 10mmL × 10mmW plate was cut from the center of the width of a steel sheet, and the thickness was reduced from the rolled surface so that the plate thickness was halved. The rolled surface was then finished with colloidal silica, and the reduced-thickness surface (rolled surface) was measured using electron backscatter diffraction (EBSD) in combination with a scanning electron microscope (SEM). Next, the orientation of each measurement point obtained in this way was illustrated with different colors according to its position to obtain an IPF (Inverse Pole Figure) map. At this time, the area ratio was calculated as the proportion of the area of ​​the measurement field in which crystal grains with an angular difference from each orientation (tolerance angle of 15° or less) occupied by a single crystal grain was defined as the area ratio of the area of ​​the measurement field in which this crystal grain occupied a single crystal orientation.

[0075] [Table 3]

[0076] Next, Table 4 shows the evaluation of the pitting corrosion resistance and soft magnetic properties of the stainless steel surface of the present invention using coercivity and magnetic flux density (B1).

[0077] Furthermore, we will explain the evaluation method for pitting corrosion resistance. A corrosion resistance 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. 2Among 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.

[0078] Next, we will explain the method for evaluating soft magnetic properties. A test specimen with an outer diameter of φ45 mm, an inner diameter of Φ33 mm, and a thickness of 0.8 mm was cut from the center of the width of a steel plate using electrical discharge machining. Then, a primary winding was wound around the test specimen 100 times, followed by a secondary winding 100 times, and the specimen was subjected to a ring test using a BH tracer. During this test, an external magnetic field was applied up to a maximum of 1000 oorsted (Oe), and the magnetic flux density (B1) at 1000 oorsted (Oe) was measured from the initial magnetization curve, and the coercivity (Hc) was measured from the BH curve. Here, we evaluated that if the coercivity is 0.85 oorsted (Oe) or less, it can respond to high-frequency changes in magnetization and is therefore suitable for practical application; if it is between 0.85 oorsted (Oe) and 1.0 oorsted (Oe), it can follow changes in magnetization and is therefore suitable for practical application; and if it exceeds 1.0 oorsted (Oe), it is deemed unsuitable for practical application because it is slow to respond to changes in magnetization and has poor soft magnetic properties. Furthermore, the study evaluated that a magnetic flux density (B1) of 2000G or higher is practically preferable because sufficient magnetic flux density can be obtained even with a low external magnetic field, a magnetic flux density between 1800G and 2000G is suitable for practical use because sufficient magnetic flux density can be obtained even with a low external magnetic field, and a magnetic flux density below 1800G is not applicable to practical use because sufficient magnetic flux density cannot be obtained even with a high external magnetic field.

[0079] [Table 4]

[0080] As shown in Tables 3 and 4, the stainless steels of Examples 1 to 13 satisfy the appropriate range of the present invention in terms of the value on the left side of Equation 1, the middle value of Equation 2, the equivalent circular diameter of the precipitate, the area fraction A, and the area fractions S1 and S2 related to the crystal orientation. As a result, as shown in Table 4, all of the stainless steels in Examples 1 to 13 exhibit excellent pitting corrosion resistance, rated "○" or higher. Furthermore, all of the stainless steels in Examples 1 to 13 have a magnetic flux density (B1) of 1800G or higher and a coercivity of 1.0 oorsted (Oe) or less, indicating that they are all suitable for practical use.

[0081] In contrast, Comparative Example 1 has low manufacturing conditions for hot rolling temperature, coiling temperature, and cooling rate, resulting in a large area ratio A of precipitates in the stainless steel, and high area ratios S1 and S2 related to crystal orientation. This falls outside the scope of the present invention, and it can be seen that the magnetic flux density (B1) is less than 1800G, and the coercivity exceeds 1.0 oorsted (Oe), making it unsuitable for practical use.

[0082] Comparative Example 2 had a value of 19.0 or higher on the left side of (Equation 1), indicating a "○" rating for pitting corrosion resistance. On the other hand, C and O exceeded the upper limit, resulting in a large area fraction A of precipitates in the stainless steel, and a high area fraction S2 related to crystal orientation, which falls outside the scope of the present invention. As a result, the magnetic flux density (B1) is less than 1800G, and the coercivity exceeds 1.0 oorsted (Oe), indicating that it is not applicable to practical use.

[0083] Comparative Example 3 had a value of 19.0 or higher on the left side of (Equation 1), indicating a "○" rating for pitting corrosion resistance. On the other hand, because the Si content in the stainless steel was low at 0.9%, the magnetic flux density (B1) was less than 1800G, and the coercivity exceeded 1.0 oorsted (Oe), indicating that it is not suitable for practical use.

[0084] Comparative Example 4 had a value of 19.0 or higher on the left side of (Equation 1), indicating a "○" rating for pitting corrosion resistance. On the other hand, the Si content in the stainless steel was high at 2.6%, and the area fraction A of precipitates and the area fraction S2 related to crystal orientation were high, which is outside the scope of the present invention. As a result, the magnetic flux density (B1) was less than 1800G, and the coercivity exceeded 1.0 oorsted (Oe), indicating that it is not applicable to practical use.

[0085] Comparative Example 5 has a magnetic flux density (B1) of 1800G or more and a coercivity of 1.0 oorsted (Oe) or less, but the value of (Equation 1) is low, indicating poor pitting corrosion resistance ("×").

[0086] Comparative Example 6 has a magnetic flux density (B1) of 1800G or more and a coercivity of 1.0 oorsted (Oe) or less, but the value of (Equation 1) is low, indicating poor pitting corrosion resistance ("×").

[0087] Comparative Example 7 had a value of 19.0 or higher on the left side of (Equation 1), indicating a "○" rating for pitting corrosion resistance. However, the value in the middle of (Equation 2) was outside the scope of the invention, the equivalent circular diameter of the precipitate exceeded 5.0 μm, the magnetic flux density (B1) was less than 1800 G, and the coercivity exceeded 1.0 Ørsted (Oe), indicating that it is not suitable for practical use.

[0088] From the results of Examples 1-13 and Comparative Examples 1-7, it can be seen that in order to obtain the target pitting corrosion resistance and soft magnetic properties of the present invention, it is important that the range of chemical composition specified in the present invention, the value of the left side of (Equation 1) relating to pitting corrosion, the middle value of (Equation 2), the average equivalent circular diameter of the precipitate, the area fraction A of the precipitate, and the area fractions S1 and S2 relating to the crystal orientation are within the range of the present invention. Furthermore, the addition of trace elements such as B, Ni, Cu, V, W, Ca, Mg, Zr, Co, Ga, La, Hf, Y, and REM, and the preferred manufacturing method specified in the present invention are effective in improving pitting corrosion resistance and soft magnetic properties.

Claims

1. In mass percent, C: 0.020% or less, Si: more than 1.0% and less than 2.5%, Mn: 1.0% or less, P: 0.040% or less, S: Less than 0.0030% O: Less than 0.0040%, Cr: 15.5% or more and 23.0% or less, Mo: 0.5% or less N: 0.020% or less, Al: 0.20% or less, Nb: 0.30% or less, A ferritic stainless steel sheet having a chemical composition containing 0.30% or less of Ti, with the remainder being Fe and unavoidable impurities, When the respective contents of Cr, Si, Mo, Nb, and Ti in the aforementioned stainless steel sheet are represented as {Cr}, {Si}, {Mo}, {Nb}, and {Ti}, {Cr}, {Si}, and {Mo} satisfy the following relationship (Equation 1), {Nb} and {Ti} satisfy the following relationship (Equation 2): The precipitates present in the stainless steel sheet have an average size of 5.0 μm or less in terms of equivalent diameter, and the area ratio A of the precipitates in the base material of the stainless steel sheet satisfies the following relationship (Equation 3), thereby providing a ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties. (Formula 1): PI={Cr}+2{Si}+3{Mo}≧19.0 (Formula 2): 0.10≦({Nb}+{Ti})≦0.30 (Formula 3): A≦2.0%

2. The ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties according to claim 1, wherein when the crystal orientation is measured on a plane parallel to the rolling surface of the stainless steel sheet, the area ratio S1 of the total of crystal grains with an RD plane orientation of <110> and crystal grains with an RD plane orientation of <112> is 80% or less, and the area ratio S2 of crystal grains with an RD plane orientation of <111> is 10% or less.

3. The aforementioned chemical composition, in mass%, B: 0.0050% or less, Ni: 1.0% or less, Cu: 1.0% or less, V: 0.50% or less, W: 0.50% or less, Ca: 0.0100% or less, Mg: 0.010% or less, Zr: 0.50% or less, Co: 0.50% or less, Ga: 0.10% or less, La: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, A ferritic stainless steel sheet with excellent pitting corrosion resistance and soft magnetic properties according to claim 1 or 2, further containing one or more selected from the group REM: 0.10% or less.

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