Ferritic stainless steel sheet

Abstract

To provide a stainless steel sheet that enables coexistence of excellent soft magnetic properties and corrosion resistance while lowering material cost, in cases where the Si content is more than 1.0 mass%.SOLUTION: A stainless steel sheet with a specified chemical composition is characterized in that coarse inclusions are inclusions having an equivalent circle diameter of 2.0 μm or more, the number density of the coarse inclusions is 100 / mm2 or more and less than 400 / mm2, and composite inclusions are inclusions among the coarse inclusions satisfying [0.5≤([O]+[S]) / ([Mg]+[Ca])≤10] and [2.0≤[Ti]+[Nb]], and the proportion of the composite inclusions to the coarse inclusions is 30% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a ferritic stainless steel sheet. [Background technology]

[0002] Conventionally, in the trend toward miniaturization and high efficiency of electric vehicles and electrical appliances, electromagnetic materials such as permalloy and electromagnetic steel sheets have been used as electromagnetic materials with excellent soft magnetic properties. For this reason, stainless steels with improved magnetic properties have been developed, as shown in Patent Documents 1 to 6.

[0003] Patent Document 1 describes a martensitic stainless steel containing 0.08 to 0.20% C and 11.5 to 18.0% Cr, in which the carbide density in the structure is 5 × 10 5 pieces / mm 2 As described above, magnetic stainless steel is disclosed that has a ferrite grain size of 7 μm or more, a residual magnetic flux density Br of 0.9 T or less, and a squareness ratio of 0.65 or less.

[0004] In addition, Patent Document 2 discloses that in martensitic stainless steel containing C: 0.08 to 0.20% and Cr: 11.5 to 18.0%, the maximum magnetic permeability μ m 1500 or more, 0.2% proof stress 30kgf / mm 2 The above magnetic stainless steel is disclosed.

[0005] Furthermore, Patent Document 3 discloses that, in steel containing, by weight, C: less than 0.005%, Si: 0.1 to 1.5%, and Cr: 9.0 to 17.0%, by performing magnetic annealing at a temperature range of 800 to 850°C and a soaking time of 0 to 10 minutes, the maximum magnetic permeability μ m A soft magnetic stainless steel with a modulus of 10,000 or more has been disclosed.

[0006] Patent Document 4 also describes a magnet that has a composition containing, by weight, C: 0.02% or less and Cr: 9.0 to 17.0%, and has a crystal plane intensity ratio K expressed by a predetermined formula of 10 or more and a maximum magnetic permeability μ maxA soft magnetic stainless steel sheet having a thickness of 2.0 mm or less and a magnetic strength of 2000 emu or more is disclosed.

[0007] Patent Document 5 discloses a free-cutting soft magnetic stainless steel containing, by mass, 0.02 to 0.15% C, 0.5 to 3.0% Si, and 10 to 22% Cr. Patent Document 6 discloses a hysteresis motor in which a stator yoke that constitutes the stator of the hysteresis motor together with an exciting coil has a composition containing 0.05 mass% or less C, 3.0 mass% or less Si, and 5.0 to 20.0 mass% Cr. [Prior art documents] [Patent documents]

[0008] [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 Application Laid-Open No. 2000-064000 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-152354 [Patent Document 6] Japanese Patent Application Laid-Open No. 2009-038907 Summary of the Invention [Problem to be solved by the invention]

[0009] However, in recent years, it is expected that electromagnetic materials will be used in highly corrosive environments such as automatic water heaters and automatic sprinklers, etc. For this reason, there is a demand for the development of materials that not only have soft magnetic properties but also excellent corrosion resistance.

[0010] One method for improving the corrosion resistance of electromagnetic materials such as electrical steel sheets is to form a nickel plating coating on the surface of the electromagnetic material. However, nickel is relatively expensive, which increases material costs. Another method for improving corrosion resistance is to add molybdenum (Mo). However, molybdenum is an element equally or more expensive than nickel, which also increases material costs. Therefore, soft magnetic stainless steel sheets that have excellent corrosion resistance without the need for surface treatments such as nickel plating and without the addition of molybdenum (Mo) have been attracting attention.

[0011] Recently, it has been found that adding more than 1.0 mass% of Si to ferritic stainless steel sheets is effective in improving corrosion resistance. However, when ferritic stainless steel sheets contain a large amount of Si, that is, more than 1.0%, C and N are activated, promoting the precipitation of coarse carbonitrides and the like, which reduces soft magnetic properties.

[0012] Although Patent Documents 1 and 2 both describe carbonitrides, the stainless steels disclosed in these documents are martensitic stainless steels with a high C content, leaving room for improvement in terms of soft magnetic properties. Furthermore, these documents make no mention of corrosion resistance, leaving room for improvement in terms of corrosion resistance as well.

[0013] Although Patent Document 3 describes corrosion resistance, it does not specifically describe inclusions. Therefore, the stainless steel disclosed in this document has room for further improvement in terms of both soft magnetic properties and corrosion resistance. Furthermore, Patent Documents 4 to 6 do not fully consider corrosion resistance, and the stainless steels disclosed in these documents have room for further improvement in terms of corrosion resistance.

[0014] Therefore, in a stainless steel sheet containing more than 1.0 mass % of Si, it is difficult to achieve both soft magnetic properties and corrosion resistance while reducing material costs.

[0015] In view of the above, an object of the present invention is to provide a stainless steel sheet that, when the Si content exceeds 1.0 mass %, can achieve both excellent soft magnetic properties and corrosion resistance while reducing material costs. [Means for solving the problem]

[0016] The present invention has been made to solve the above problems, and the gist of the present invention is the following stainless steel sheet.

[0017] (1) Chemical composition, in mass%, C: 0.001 to 0.010%, Si: more than 1.0% and less than 2.5%, Mn: 1.00% or less, P:0.030% or less, S: 0.0050% or less, O: 0.0050% or less, Ni: 0.001 to 0.030%, Cr: 15-25%, Mo: 0.01 to 0.50%, Al: 0.01 to 0.20%, Ti: 0.01 to 0.50% Nb: 0.01 to 0.50%, N: 0.001 to 0.030%, Mg: Contains 0.0001 to 0.0030% The balance is Fe and impurities. The following formula (i) is satisfied: When inclusions having a circle equivalent diameter of 2.0 μm or more are defined as coarse inclusions, the number density of the coarse inclusions is 100 pieces / mm 2 More than 400 pieces / mm 2 is less than A ferritic stainless steel sheet in which, when coarse inclusions that satisfy the following formulas (ii) and (iii) are considered to be composite inclusions, the number ratio of the composite inclusions to the coarse inclusions is 30% or more: 0.0002≦Mg / (Ti+Nb)<0.1500 (i) 0.5≦([O]+[S]) / ([Mg]+[Ca])≦10 (ii) 2.0≦[Ti]+[Nb] (iii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the stainless steel plate, and is set to zero if not contained, and each element symbol in the above formulas (ii) and (iii) represents the content (atomic%) of each element in the coarse inclusions, and is set to zero if not contained.

[0018] (2) The chemical composition further contains, in place of a portion of the Fe, B: 0.0050% or less, Cu: 1.0% or less, V: 0.50% or less, W: 0.50% or less, Ca: 0.01% or less, Zr: 0.50% or less, Co: 0.50% or less, Ga: 0.10% or less, REM: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, and Sn: 0.01% or less, The ferritic stainless steel sheet according to (1) above, containing one or more selected from the group consisting of:

[0019] (3) A ferritic stainless steel sheet according to (1) or (2) above, in which the area ratio of the complex inclusions is 1.0% or less. [Effects of the Invention]

[0020] According to the present invention, when the Si content exceeds 1.0 mass %, it is possible to obtain a stainless steel sheet that can achieve both excellent soft magnetic properties and corrosion resistance while reducing material costs. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram showing the relationship between the number density of coarse inclusions and soft magnetic properties. [Figure 2] Figure 2 is a structural photograph showing the shape of the complex inclusions. DETAILED DESCRIPTION OF THE INVENTION

[0022] The present inventors have conducted various studies to achieve corrosion resistance and soft magnetic properties, particularly low residual magnetic flux density and low coercive force, and have come to the following findings.

[0023] (a) It has been known that adding Si is effective in reducing iron loss. For example, electrical steel sheets containing 3% or more by mass have low remanence and low coercivity, and are widely used in the iron cores of solenoid valves, where responsiveness is required. Thus, adding Si to steel sheets is generally effective in improving soft magnetic properties. On the other hand, in stainless steel sheets containing a certain amount of Cr or more, adding Si in excess of 1.0% by mass tends to increase the activity of C and N, promoting the formation of fine carbonitrides during the manufacturing process. Carbonitrides containing C and N inhibit domain wall movement, increasing remanence and coercivity, and ultimately degrading soft magnetic properties. Thus, adding Si alone to stainless steel sheets is not sufficient to improve soft magnetic properties.

[0024] (b) Therefore, the present inventors investigated reducing C and N, suppressing the expensive Mo content, and then increasing the Si content to more than 1.0 mass%. For example, a certain amount of Ti and Nb is added, and then more than 1.0 mass% Si is added. However, when Mo is also reduced, it is difficult to improve both corrosion resistance and soft magnetic properties. The present inventors discovered that this is because fine precipitates such as carbonitrides precipitate during manufacturing after hot rolling, and a certain amount or more of coarse precipitates are formed.

[0025] (c) We found that the fine precipitation of carbonitrides can be suppressed by adding 0.0002% or more Mg. Compared to Fe, Al, and Si, Mg has a stronger tendency to combine with O to form oxides. It also combines with S to form sulfides. This leads to the formation of minute oxides, MgO and MgS, during the steelmaking process. Furthermore, the formation of coarse carbonitrides, such as TiN, NbN, TiC, and NbC, is promoted around the oxides MgO and MgS in the solid-liquid coexistence region and during slab heating. The formation of coarse carbonitrides reduces the amount of solute C and N in the steel, improving the purity of the stainless steel and suppressing the formation of fine carbonitrides during heat treatment after hot rolling. However, the formation of excessive coarse precipitates can adversely affect soft magnetic properties.

[0026] (d) Therefore, it was revealed that it is effective to control not only the chemical composition but also the manufacturing conditions from hot rolling to magnetic annealing within a predetermined range in order to suppress the formation of fine precipitates and keep the amount of coarse precipitates within a certain range.

[0027] An embodiment of the present invention has been made based on the above findings. Each requirement of the stainless steel sheet of this embodiment will be described in detail below.

[0028] 1.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" for the content of each element means "% by mass" unless otherwise specified.

[0029] C: 0.001 to 0.010% Carbon (C) is an impurity element contained in steel. Furthermore, when carbon is contained in the matrix, workability and corrosion resistance are reduced. It also forms carbides with other elements, reducing the number density of coarse inclusions and degrading soft magnetic properties. Therefore, the C content is 0.010% or less. The C content is preferably 0.008% or less. While it is preferable to reduce the C content as much as possible, excessive reduction of C increases refining costs. Therefore, the C content is 0.001% or more, and more preferably 0.002% or more.

[0030] Si: More than 1.0% and less than 2.5% Silicon (Si) is a deoxidizing element that purifies the matrix. It also increases hardness and contributes to improved mechanical strength. It also improves corrosion resistance, particularly pitting corrosion resistance in neutral environments such as saltwater. It also stabilizes the ferrite phase, increases electrical resistance, reduces iron loss, and improves soft magnetic properties. Therefore, the Si content is greater than 1.0%. However, excessive Si content reduces workability and weldability. Furthermore, it may actually reduce soft magnetic properties and corrosion resistance. For this reason, the Si content is set to 2.5% or less. Taking into account these effects and manufacturability, the Si content is preferably greater than 1.0% and less than 2.0%.

[0031] Mn: 1.00% or less Manganese (Mn) is a deoxidizing element that purifies the matrix. It also forms sulfides with sulfur (S), purifying the matrix and improving soft magnetic properties. However, excessive Mn content forms sulfides (MnS), increasing corrosion initiation sites and reducing corrosion resistance. It also destabilizes the ferrite phase, forming a martensite phase and potentially reducing soft magnetic properties. To avoid a reduction in soft magnetic properties, the Mn content is 1.00% or less. To achieve the above effects and ensure the high-purity effect of deoxidizing oxygen (O) in the matrix and forming sulfur (S) sulfides, the Mn content is preferably 0.01% or more. Considering the above effects and manufacturability, the Mn content is preferably 0.05% or more and 0.40% or less.

[0032] P:0.030% or less P (phosphorus) is an impurity element contained in steel. P forms phosphides that reduce soft magnetic properties and also reduce workability and weldability. For this reason, the P content is 0.030% or less. From the viewpoint of manufacturability, the P content is preferably 0.025% or less. It is preferable to reduce the P content as much as possible, but excessive reduction of P increases refining costs. For this reason, the P content is preferably 0.001% or more.

[0033] S: 0.0050% or less S (sulfur) is an impurity element contained in steel. S forms sulfides, which increase the number of corrosion initiation sites and reduce corrosion resistance, especially pitting corrosion resistance. For this reason, the S content is 0.0050% or less. The S content is preferably 0.0020% or less. On the other hand, sulfides act as nuclei for the precipitation of carbonitrides of other elements, promoting the formation and coarsening of carbonitrides during refining and hot rolling. As a result, the formation of fine carbonitrides, which are harmful to soft magnetic properties, is suppressed. For this reason, the S content is preferably 0.0002% or more.

[0034] O: 0.0050% or less O (oxygen) is an impurity element contained in steel. O forms oxides with metal elements such as Fe, Al, Si, Mg, and Ca, and reduces soft magnetic properties. For this reason, the O content is 0.0050% or less. On the other hand, trace amounts of MgO and CaO act as nucleation starting points for carbonitrides and promote their formation and growth during refining and hot rolling. Furthermore, excessive reduction in O content leads to increased refining costs, so the O content is preferably 0.0001% or more. Taking into account each effect and manufacturability, the O content is preferably 0.0010% or more and 0.0040% or less.

[0035] Ni: 0.001 to 0.030% Ni (nickel) is an element effective in improving corrosion resistance. In the stainless steel sheet of this embodiment, the Ni content is 0.001% or more to obtain sufficient corrosion resistance. However, if Ni is contained in excess, the ferrite phase becomes unstable and the soft magnetic properties deteriorate. For this reason, the Ni content is 0.030% or less. In consideration of corrosion resistance and soft magnetic properties, the Ni content is preferably 0.010% or more and 0.025% or less.

[0036] Cr: 15~25% Cr (chromium) is a basic element of stainless steel and is an essential element for ensuring corrosion resistance, especially weather resistance and oxidation resistance. Cr is also a ferrite-forming element, and has the effect of suppressing the formation of martensite and improving soft magnetic properties. For this reason, the Cr content is 15% or more. However, if Cr is contained in excess, the saturation magnetic flux density decreases and the Cr 23 The precipitation of C6 and other elements increases the residual magnetic flux density and coercive force. As a result, the soft magnetic properties deteriorate. For this reason, the Cr content is 25% or less. The Cr content is preferably 16 to 19%, and more preferably 17 to 18%.

[0037] Mo: 0.01 to 0.50% Mo (molybdenum), like Ni and Cu, is an element that improves oxidation resistance as well as corrosion resistance. It is particularly effective in improving pitting corrosion resistance, for example, in suppressing the progression of pitting corrosion in a low pH environment. Therefore, the Mo content is 0.01% or more. However, excessive Mo content increases alloy costs. Furthermore, it reduces manufacturability during hot working and cold working. Therefore, the Mo content is 0.50% or less. In consideration of manufacturability, the Mo content is preferably 0.05% or more and less than 0.30%.

[0038] Al: 0.01 to 0.20% Aluminum (Al) is an extremely effective deoxidizing element. For this reason, the Al content is 0.01% or more. However, excessive Al content reduces the toughness and weldability of the steel. It also reduces the soft magnetic properties. For this reason, the Al content is 0.20% or less. Al, particularly with Si, forms oxides, reducing O (oxygen) in the matrix. However, an excessive amount of Al oxide hinders domain wall motion and reduces the soft magnetic properties. However, if the Al content is 0.20% or less, there is no effect on the soft magnetic properties. Taking into account the above effects and manufacturability, the Al content is 0.02% or more and 0.15% or less.

[0039] Ti: 0.01 to 0.50% Ti (titanium) forms coarse carbonitrides, TiN and TiC, using MgO and CaO as nuclei, contributing to the high purity of the matrix and improving workability and soft magnetic properties. Therefore, the Ti content is 0.01% or more. The Ti content is 0.05% or more. However, excessive Ti content promotes the precipitation of Ti phosphides and sulfides, resulting in an excessive number of coarse inclusions and degrading soft magnetic properties. Therefore, the Ti content is 0.50% or less. The Ti content is preferably 0.20% or less.

[0040] Nb: 0.01 to 0.50% Like Ti, Nb (niobium) forms coarse carbonitrides, NbN and NbC, using MgO and CaO as nuclei, improving workability and soft magnetic properties. Therefore, the Nb content is 0.01% or more. However, excessive Nb content increases alloy costs and promotes the precipitation of Fe and Laves phases, degrading soft magnetic properties. Therefore, the Nb content is 0.50% or less. Taking the above effects into consideration, the Nb content is preferably 0.01% or more and 0.50% or less, and more preferably 0.05% or more and 0.20% or less.

[0041] N: 0.001 to 0.030% N (nitrogen) is an impurity element contained in steel. Like C, N reduces workability, so the N content is 0.030% or less. Furthermore, N forms nitrides with other additive elements, which may cause excessive coarse inclusions and reduce soft magnetic properties. Therefore, the N content is preferably 0.020% or less. Furthermore, in consideration of manufacturability, the N content is preferably 0.015% or less. It is preferable to reduce the N content as much as possible, but excessive reduction of N increases refining costs. For this reason, the N content is 0.001% or more.

[0042] Mg: 0.0001 to 0.0030% Magnesium (Mg) is an extremely effective deoxidizing element. In particular, it forms oxides with Si and Ca, reducing the O (oxygen) content in the matrix. The MgO oxide formed during refining, casting, and hot rolling serves as the nucleation site for TiN, TiC, NbN, and NbC, thereby enhancing the purity of stainless steel and promoting the coarsening of these precipitates. This suppresses the precipitation of large amounts of fine carbonitrides during manufacturing processes after hot-rolled sheet annealing, leading to improved soft magnetic properties. For this reason, the Mg content is 0.0001% or more. To achieve the full effect, the Mg content is preferably 0.0002% or more. However, excessive Mg content increases the number density of MgO, reducing the size of individual carbonitrides and hindering domain wall motion. It also increases the number of coarse inclusions, resulting in reduced soft magnetic properties. For this reason, the Mg content is 0.0030% or less. In consideration of manufacturability and soft magnetic properties, the Mg content is preferably 0.0015% or less.

[0043] In addition to the above elements, one or more elements selected from the group consisting of B, Cu, V, W, Ca, Zr, Co, Ga, REM, Y, Hf, and Sn may be contained within the ranges shown below. In other words, the lower limit of the above elements is 0%. The reasons for limiting each element will be explained below.

[0044] B: 0.0050% or less B (boron) has the effect of improving hot workability and resistance to secondary work embrittlement. Furthermore, when added to stainless steel, it has the effect of improving workability. Therefore, boron may be added as needed. However, excessive B content reduces elongation. In addition, nitrides are formed, degrading soft magnetic properties. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0030% or less, and more preferably 0.0015% or less. On the other hand, to obtain the above effects, the B content is preferably 0.0005% or more.

[0045] Cu: 1.0% or less Cu (copper) has the effect of improving corrosion resistance. In particular, it is effective in pitting corrosion resistance, which inhibits the progression of pitting corrosion in a low pH environment. It also has the effect of improving workability. Therefore, it may be added as needed. However, excessive Cu content destabilizes the ferrite phase and reduces soft magnetic properties. Furthermore, it increases alloy costs, increases material strength, and reduces workability. For this reason, the Cu content is 1.0% or less. The Cu content is preferably 0.5% or less, and more preferably 0.1% or less. On the other hand, to obtain the above effects, the Cu content is preferably 0.03% or more.

[0046] V: 0.50% or less V (vanadium) has the same effect as Ti and Nb, and reduces solute C and N by forming carbonitrides, thereby improving corrosion resistance and soft magnetic properties. Therefore, it may be added as needed. However, excessive V content increases alloy costs and reduces manufacturability. Furthermore, excessive precipitates are formed, resulting in poor soft magnetic properties. For this reason, the V content is 0.50% or less. The V content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, to obtain the above effects, the V content is preferably 0.01% or more.

[0047] W: 0.50% or less W (tungsten) has the effect of improving corrosion resistance by dissolving in steel. Therefore, it may be contained as needed. However, excessive W content increases alloy costs and reduces manufacturability. Furthermore, solid solution strengthening and precipitation strengthening harden the steel, reducing elongation. Therefore, the W content is 0.50% or less. The W content is preferably 0.45% or less. On the other hand, to obtain the above effect, the W content is more preferably 0.01% or more.

[0048] Ca: 0.01% or less Ca (calcium) is an extremely effective deoxidizing element, with the same effect as Mg. In particular, it forms oxides with Si and has the effect of reducing O (oxygen) in the matrix. Therefore, it may be added as needed. However, excessive Ca content forms oxides and sulfides, which hinder domain wall motion and reduce soft magnetic properties. Furthermore, since the sulfides formed by Ca are water-soluble, pitting corrosion resistance is reduced. For this reason, the Ca content is 0.01% or less. The Ca content is preferably 0.009% or less, and more preferably 0.008% or less. On the other hand, to obtain the above effects, the Ca content is preferably 0.0003% or more.

[0049] Zr: 0.50% or less Zr (zirconium) has the effect of improving the cleanliness of steel and improving soft magnetic properties and resistance to secondary work embrittlement. Therefore, it may be added as needed. However, excessive Zr content increases alloy costs and reduces manufacturability. For this reason, the Zr content is 0.50% or less. The Zr content is preferably 0.47% or less, and more preferably 0.45% or less. On the other hand, to obtain the above effects, the Zr content is preferably 0.01% or more.

[0050] Co:0.50% or less Co (cobalt) has the effect of improving the cleanliness of steel and improving soft magnetic properties and resistance to secondary work embrittlement. Therefore, it may be added as needed. However, excessive Co content increases alloy costs and reduces manufacturability. For this reason, the Co content is 0.50% or less. The Co content is preferably 0.45% or less, and more preferably 0.40% or less. On the other hand, to obtain the above effects, the Co content is preferably 0.01% or more.

[0051] Ga: 0.10% or less Ga (gallium) has the effect of improving the cleanliness of steel and significantly improving oxidation resistance and hot workability. Therefore, it may be added as needed. However, if Ga is added in excess, oxides are easily formed, hindering domain wall motion and degrading soft magnetic properties. Furthermore, alloy costs increase and manufacturability decreases. For this reason, the Ga content is 0.10% or less. The Ga content is preferably 0.09% or less, and more preferably 0.05% or less. On the other hand, to obtain the above effects, the Ga content is preferably 0.001% or more.

[0052] REM: 0.10% or less REM (rare earth metals) have the effect of improving the cleanliness of steel and significantly improving its oxidation resistance and hot workability. Therefore, they may be added as needed. However, excessive REM content easily forms oxides, hindering domain wall motion and degrading soft magnetic properties. It also increases alloy costs and reduces manufacturability. For this reason, the REM content is 0.10% or less. The REM content is preferably 0.09% or less, and more preferably 0.05% or less. On the other hand, to achieve the above effects, the REM content is preferably 0.001% or more.

[0053] REM refers to a total of 16 elements, including Sc and lanthanides, and the REM content above refers to the total content of these elements.Industrially, REM is often added in the form of misch metal.

[0054] Y: 0.10% or less Y (yttrium) has the effect of improving the cleanliness of steel and significantly improving oxidation resistance and hot workability. Therefore, it may be added as needed. However, if Y is added in excess, it easily forms oxides, hindering domain wall motion and degrading soft magnetic properties. In addition, it increases alloy costs and reduces manufacturability. For this reason, the Y content is 0.10% or less. The Y content is preferably 0.09% or less, and more preferably 0.06% or less. On the other hand, to obtain the above effects, the Y content is preferably 0.001% or more.

[0055] Hf: 0.10% or less Hf (hafnium) has the effect of improving the cleanliness of steel and significantly improving oxidation resistance and hot workability. Therefore, it may be added as needed. However, if Hf is added in excess, it easily forms oxides, hindering domain wall motion and degrading soft magnetic properties. It also increases alloy costs and reduces manufacturability. For this reason, the Hf content is 0.10% or less. The Hf content is preferably 0.09% or less, and more preferably 0.05% or less. On the other hand, to obtain the above effects, the Hf content is preferably 0.001% or more.

[0056] Sn: 0.01% or less Like B, Sn (tin) has the effect of improving secondary workability. Therefore, it may be contained as needed. However, if Sn is contained in excess, the material strength increases significantly and workability decreases. Therefore, the Sn content is 0.01% or less. The Sn content is preferably 0.005% or less, and more preferably 0.002% or less. On the other hand, in order to obtain the above effect, the Sn content is preferably 0.0005% or more.

[0057] In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during the industrial production of stainless steel sheet due to various factors in raw materials such as ore and scrap, and in the manufacturing process, and are acceptable within a range that does not adversely affect this embodiment.

[0058] (i) 0.0002≦Mg / (Ti+Nb)<0.1500 (i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the stainless steel sheet, and if the element is not contained, it is set to zero.

[0059] The value in equation (i), Mg / (Ti + Nb), is an index showing the contribution of Mg to the formation of M(C,N), which will be described later. When Mg / (Ti + Nb) is 0.0002 or greater, the Mg content is sufficient relative to Ti and Nb. This results in an increase in the number density of MgO and MgS, which in turn increases the precipitation of M(C,N) with these as nuclei. Therefore, C and N are sufficiently immobilized in the solid-liquid coexistence region and during slab heating in the steelmaking process. Therefore, in downstream processes with low annealing temperatures, such as cold-rolled sheet annealing, the precipitation of fine carbonitrides is suppressed, improving soft magnetic properties. Therefore, Mg / (Ti + Nb) should be 0.0002 or greater, preferably 0.0003 or greater, and more preferably 0.0005 or greater.

[0060] If the Mg / (Ti+Nb) ratio is less than 0.1500, the number density of MgO and MgS increases, increasing the number of nucleation starting points for M(C,N). This promotes the formation of coarse M(C,N) particles without increasing the number density, improving soft magnetic properties. On the other hand, if the Mg / (Ti+Nb) ratio is 0.1500 or more, the number of nucleation starting points for the above-mentioned M(C,N) particles becomes excessive, resulting in the individual M(C,N) particles being refined to less than 2.0 μm and an excessive number density, which degrades the magnetic properties. Therefore, it is preferable that the Mg / (Ti+Nb) ratio be less than 0.1500 and not more than 0.1000.

[0061] 2.Inclusions 2-1. Coarse inclusions The stainless steel sheet of this embodiment contains inclusions. In this application, inclusions include not only compounds formed during the steelmaking and casting processes, but also compounds precipitated during subsequent manufacturing processes. In other words, inclusions refer to all compounds observed separately from the matrix.

[0062] In the stainless steel sheet of this embodiment, from the viewpoint of soft magnetic properties, fine inclusions are suppressed and the amount of coarse inclusions is kept within a certain range. That is, when inclusions having an equivalent circle diameter of 2.0 μm or more are defined as coarse inclusions, the number density of the coarse inclusions is 100 pieces / mm 2 More than 400 pieces / mm 2 The density of coarse inclusions is less than 100 pieces / mm 2 If the density is less than 100 pieces / mm, fine inclusions will inevitably precipitate in the subsequent process, increasing the residual magnetic flux density and coercive force, resulting in a decrease in soft magnetic properties. 2 More than 150 pieces / mm 2 More preferably, it is equal to or greater than this.

[0063] Furthermore, if the circle equivalent diameter is 2.0 μm or more, the fine precipitation of Ti and Nb-based precipitates in the post-processing can be suppressed, but the number density is 400 particles / mm 2 In the above cases, the distance between the inclusions becomes shorter, which in turn pins the domain wall motion, resulting in a sharp increase in the residual magnetic flux density Br×coercivity Hc, and a decrease in the soft magnetic properties.

[0064] Therefore, the density of coarse inclusions is 400 pieces / mm 2 The density of coarse inclusions is less than 390 pieces / mm 2 Preferably, it is:

[0065] 2-2.Composite inclusions 2-2-1. Number ratio of composite inclusions In the stainless steel sheet of this embodiment, among the above-mentioned coarse inclusions having an equivalent circle diameter of 2.0 μm or more, coarse inclusions that satisfy the following formulas (ii) and (iii) are considered to be composite inclusions. 0.5≦([O]+[S]) / ([Mg]+[Ca])≦10 (ii) 2.0≦[Ti]+[Nb] (iii) The element symbols in the formulas (ii) and (iii) above represent the content (atomic %) of each element in the coarse inclusions, and if the element is not contained, it is set to 0. In the present application, for ease of understanding, the values ​​in brackets are expressed in atomic %.

[0066] As shown in FIG. 2, composite inclusions generally have a shape in which an oxysulfide is present near the center and is surrounded by carbonitrides. Here, the term "oxysulfide" refers to oxides and sulfides, which may be bonded singly or in a composite form. That is, the term refers to inclusions containing at least one of O and S. Furthermore, M(C,N) represents a carbonitride of element M, where M contains one or more elements selected from Ti and Nb. Furthermore, M may contain elements other than Ti and Nb as long as the content is less than 1 mass%. That is, M(C,N) includes at least one of Ti and Nb, and further includes at least one of C and N. In the stainless steel sheet of this embodiment, in consideration of measurement accuracy and the like, coarse inclusions that satisfy the above formulas (ii) and (iii) are recognized as composite inclusions.

[0067] In the stainless steel sheet of this embodiment, the ratio of the number of composite inclusions to the number of coarse inclusions is 30% or more. If the ratio of the number of composite inclusions to the number of coarse inclusions is less than 30%, the number density of oxysulfides, which serve as the nuclei of M(C,N), will be insufficient, and C and N that are not solidified in the solid-liquid coexistence region and during slab heating in the steelmaking process will finely precipitate in subsequent processes, increasing the remanence and coercivity. As a result, the soft magnetic properties will deteriorate. For this reason, the ratio of the number of composite inclusions to the number of coarse inclusions is 30% or more, preferably 35% or more, and more preferably 40% or more.

[0068] Although there is no particular upper limit to the number ratio, if the upper limit exceeds 80%, the oxysulfides that act as nucleation starting points for M(C,N) will be in an excessive state, resulting in the precipitation of a large amount of fine M(C,N) and an increase in the number density of coarse inclusions, which may lead to increases in the residual magnetic flux density and coercive force. For this reason, the number ratio is preferably 80% or less.

[0069] 2-2-2. Area ratio of composite inclusions The area ratio of the above-mentioned composite inclusions is preferably 1.0% or less of the observation field. When the area ratio of the composite inclusions is 1.0% or less, domain wall movement is less likely to be hindered, and the residual magnetic flux density and coercive force are more likely to decrease. As a result, the soft magnetic properties are improved. In order to further promote domain wall movement and further decrease the residual magnetic flux density and coercive force, the area ratio of the composite inclusions is preferably 0.9% or less. The lower limit of the area ratio of the composite inclusions is not particularly limited, but is usually 0.3%.

[0070] 2-3.Measuring method for inclusions The measurement of coarse inclusions is carried out using the following procedure. The TD surface of the steel plate (the surface perpendicular to the plate thickness direction and rolling direction) is buffed, and images observed at 500x magnification using a JEOL SEM (IT-510A) are binarized and analyzed. Nikon NIS-Elements is used for binarization. After binarization, dimensional measurements are automatically performed by the analysis software, and the number of inclusions with a circle equivalent diameter of 2 μm or more is counted and converted into a number density per unit area. The binarization of the SEM image is continued until a total of 50 inclusions of 2 μm or more are measured.

[0071] The number proportion of composite inclusions is determined using the following procedure. The chemical composition of 50 inclusions determined as coarse inclusions using the above method is analyzed using SEM-EDX to examine the composition. The EDX analysis conditions are observation magnification of 500x or more, acceleration voltage of 15 kV, WD of 10 mm, and Cps x measurement time (seconds) = 300,000. If a portion of the interior or interface of a coarse inclusion is confirmed to have a chemical composition that satisfies formulas (ii) and (iii), it is considered to be a composite inclusion. The number proportion of composite inclusions is calculated by dividing the number of composite inclusions by the number of coarse inclusions (50).

[0072] The area ratio of composite inclusions is calculated using the following procedure: The sum of the areas of the individual composite inclusions is calculated by allocating it to the area of ​​the observation field. The area of ​​each composite inclusion is automatically calculated using binarization software (NIS-Elements).

[0073] 3. Target characteristics In this application, the product of residual magnetic flux density Br and coercive force Hc is used as an index for comprehensively assessing soft magnetic properties. Since the smaller Br [T] and Hc [A / m], the better, the smaller this product is, the better the soft magnetic properties. If the product of Br [T] and Hc [A / m] is 14.0 or less, the material has good soft magnetic properties and can be used in practical components. Furthermore, if it is 10.0 or less, the material will easily respond to changes in external magnetic fields, allowing for efficient component operation.

[0074] Furthermore, the residual magnetic flux density Br is preferably 0.5 T or less, and more preferably 0.3 T or less. Furthermore, the coercive force Hc is preferably 100 A / m or less, and more preferably 50 A / m or less. By keeping the residual magnetic flux density Br and coercive force Hc within these ranges, it is possible to improve responsiveness while suppressing malfunctions of switches, relays, etc.

[0075] The remanence Br and coercivity Hc were measured using a single sheet magnetic test (SST test) as specified in JIS C 2556, using 55mm square test pieces cut by electrical discharge machining according to the following procedure. In this test, the sample was positioned so that a magnetizing force was applied in the rolling direction. After demagnetization, the magnetizing force was applied up to 8000 A / m, then swept down to -8000 A / m and returned to 8000 A / m to obtain a magnetization curve (BH curve). With the vertical axis representing B (magnetic flux density) and the horizontal axis representing H (magnetizing force), the intercepts on the Y and X axes represent the remanence Br and coercivity Hc, respectively. These measured values ​​indicate the responsiveness to an external magnetic field; the smaller the value, the easier it is to return to its initial state and the better the soft magnetic properties.

[0076] In this application, corrosion resistance is evaluated by pitting potential. Here, a pitting potential of 0.20 V or higher is judged to be good corrosion resistance. The pitting potential is measured by the following procedure. Based on JIS G 0577, the current value in a 3.5 mass % NaCl aqueous solution at 30°C is 100 μA / cm 2 The potential exceeding this value is defined as the pitting potential V'c100, and this value is measured.

[0077] 4. Steel plate type and thickness The stainless steel sheet of this embodiment has the above-mentioned chemical composition and is therefore a ferritic stainless steel sheet. The thickness of the ferritic stainless steel sheet of this embodiment is preferably 0.3 to 5.0 mm depending on the application.

[0078] 5.Applications The stainless steel sheet of this embodiment has both pitting corrosion resistance and soft magnetic properties, making it suitable for use in relays, motors, and shielding cases for electric vehicles. Furthermore, because it also has corrosion resistance, it can also be used in automatic sprinklers, automatic water heaters, household electrical appliances, and other appliances used in wet environments.

[0079] 6. Manufacturing method The stainless steel sheet of this embodiment can be stably produced, for example, by the following production method.

[0080] The stainless steel sheet of this embodiment is manufactured, for example, by the steps of melting, hot rolling, (annealing the hot-rolled sheet as necessary), cold rolling, annealing the cold-rolled sheet, and magnetic annealing.

[0081] 6-1.Hot rolling Steel having the above chemical composition is melted and a material for hot rolling, such as a slab, is produced. The obtained material for hot rolling is heated at a temperature of 1150 to 1300°C and hot rolled to produce a hot-rolled sheet. Hot rolling must be carried out at a higher temperature than usual to promote coarsening of carbonitrides, and the heating temperature of the slab is set within the above-mentioned temperature range.

[0082] Here, if the heating temperature of the material for hot rolling is 1150°C or higher, the solid solution of precipitates other than carbonitrides will proceed, and the number density of coarse inclusions will be 100 pieces / mm 2 More than 400 pieces / mm 2 Furthermore, it is possible to promote the precipitation and coarsening of M(C,N) using MgO and MgS precipitated (crystallized) during refining as nuclei, and since the size of each coarse inclusion can be increased, the number density can be reduced.

[0083] On the other hand, materials for hot rolling become easily deformed at temperatures above 1300°C, significantly reducing manufacturability. For this reason, the heating temperature of materials for hot rolling is 1300°C or lower. In order to suppress abnormal growth of oxide scale and improve descaling properties, the heating temperature of materials for hot rolling is preferably 1250°C or lower.

[0084] Furthermore, the heating time of the material for hot rolling is preferably 1 hour or more. When heated at 1150°C or higher for 1 hour or more, carbonitride-forming elements such as Ti are sufficiently diffused, facilitating the formation and growth of carbonitrides around oxysulfides, thereby making it possible to increase the number ratio of complex inclusions to 30% or more. Furthermore, the heating time of the material for hot rolling is preferably 12 hours or less. This is because if the heating time of the material for hot rolling exceeds 12 hours, the growth of complex inclusions is promoted, and the area ratio of complex inclusions is likely to exceed 1.0%.

[0085] Conditions for rough hot rolling, finish hot rolling, and subsequent cooling may be selected appropriately within typical ranges. After hot rolling, coiling is preferably performed at 550°C or less to avoid the formation of unnecessary precipitates. Furthermore, in order to suppress 475 embrittlement, the coiling temperature is more preferably 450°C or less. The lower limit of the coiling temperature is preferably 50°C or more to ensure the toughness of the hot-rolled sheet. In particular, a coiling temperature of 100°C or more increases the toughness of the hot-rolled sheet, improving manufacturability. Therefore, the coiling temperature is preferably 100°C or more.

[0086] 6-2.Hot-rolled sheet annealing The obtained hot-rolled sheet is annealed. This annealing is called hot-rolled sheet annealing. Hot-rolled sheet annealing is performed to promote recrystallization. The annealing temperature during hot-rolled sheet annealing is preferably 800 to 1050°C. This is because an annealing temperature of 800°C or higher can sufficiently promote recrystallization. Furthermore, if the annealing temperature is 1050°C or lower, the formation of unnecessary precipitates during cooling can be suppressed. In particular, in order to suppress the formation of unnecessary precipitates such as FeTiP, the annealing temperature is more preferably 900 to 1050°C. Furthermore, hot-rolled sheet annealing may be omitted to reduce manufacturing costs.

[0087] 6-3.Cold rolling The hot rolled sheet is then cold rolled to produce a cold rolled sheet. The rolling mill may be either a Sendzimir mill or a tandem mill. In cold rolling, the roll roughness, roll diameter, rolling oil, number of rolling passes, rolling speed, rolling temperature, etc. may be appropriately selected within a general range. The cold rolling rate may be within a normal range, but in particular, ND / / which deteriorates the soft magnetic properties may be selected. <111> In order to suppress the growth of defects, the rolling reduction is preferably less than 90%. Intermediate annealing may be performed during cold rolling, and the number of cold rolling steps is not limited to one.

[0088] 6-4.Cold-rolled sheet annealing The obtained cold-rolled sheet is annealed. This annealing is called cold-rolled sheet annealing. Cold-rolled sheet annealing is performed for the purpose of promoting recrystallization and homogenizing the structure using the strain introduced by cold rolling as a driving force. The annealing temperature T1 during cold-rolled sheet annealing is preferably 800 to 1050°C. If the annealing temperature T1 is 800°C or higher, recrystallization is promoted and a uniform structure can be obtained.

[0089] Furthermore, if the annealing temperature T1 is 1050°C or less, the formation of unnecessary precipitates that occur during cooling is suppressed, and the deterioration of soft magnetic properties can be suppressed. In particular, in order to promote recrystallization and suppress the formation of unnecessary precipitates, the annealing temperature T1 is more preferably 900 to 1050°C. Precipitates include, for example, M(C,N), Fe2Nb, FeTiP, Ti4C2S2, (Fe,Cr) 23 C6 is an example.

[0090] When the above-mentioned hot-rolled sheet annealing and cold-rolled sheet annealing are performed, either batch annealing or continuous annealing may be used. Furthermore, the atmosphere for each annealing may be air or, if necessary, bright annealing using an inert gas atmosphere such as hydrogen gas or nitrogen gas. Alternatively, annealing in a vacuum may be used. After the hot-rolled sheet annealing and the cold-rolled sheet annealing, shot blasting, salt treatment, pickling, electrolytic pickling, etc. may be performed. Furthermore, for example, a tension leveler process for shape correction may be performed after the cold-rolled sheet annealing, as long as the effects of the present invention are not impaired.

[0091] 6-5.Magnetic annealing Furthermore, in the annealing of this embodiment, the cold-rolled sheet that has undergone final annealing is subjected to magnetic annealing. Magnetic annealing is annealing to correct the magnetization of the cold-rolled annealed sheet. The annealing temperature T2 of the magnetic annealing is preferably 950°C or higher and 1250°C or lower. If the annealing temperature T2 in the magnetic annealing is 950°C or higher, it is possible to correct the deviation in magnetization that has occurred. Furthermore, if the annealing temperature T2 in the magnetic annealing is 1250°C or lower, it is easy to suppress the formation of precipitates that occur during cooling.

[0092] The annealing temperature T2 of magnetic annealing must be at least 50°C higher than the annealing temperature T1 of cold-rolled sheet annealing. That is, the relationship between the annealing temperature T2 of magnetic annealing and the annealing temperature T1 of cold-rolled sheet annealing satisfies the following formula (a). This is because grain growth during magnetic annealing can be promoted by having the relationship between the annealing temperature T2 of magnetic annealing and the annealing temperature T1 of cold-rolled sheet annealing satisfy the following formula (a).

[0093] T2-T1≧50 (a) In the above formula, each symbol is defined as follows: T1: Annealing temperature for cold-rolled sheet annealing (℃) T2: Annealing temperature of magnetic annealing (℃)

[0094] The temperature rise rate when raising the temperature to the annealing temperature T2 of the magnetic annealing is 15 to 100°C / min. If the temperature rise rate to T2 is 15°C / min or more, the coarsening of crystal grains is promoted. Similarly, if the temperature rise rate to T2 is 100°C / min or less, the deterioration of soft magnetic properties can be suppressed. The temperature rise rate to T2 is preferably 50°C / min or less.

[0095] The annealing atmosphere is preferably a vacuum environment or an inert gas atmosphere. In the case of vacuum annealing, the degree of vacuum is 1.0×10 -2 torr, preferably less than 1.0 x 10 -3 More preferably, it is less than torr.

[0096] The annealing time is preferably 1 to 10 hours, more preferably 1 to 3 hours, taking productivity into consideration. After holding at the annealing temperature, the material may be cooled as needed. The cooling method may be cooling with Ar gas or the like, or water cooling, air cooling, or furnace cooling. Other magnetic annealing conditions may be determined as appropriate depending on the product to be applied, the intended use, etc.

[0097] EXAMPLES The stainless steel sheet according to the present invention will be described in more detail below with reference to examples, but the present embodiment is not limited to these examples. [Example]

[0098] Steels having the chemical compositions shown in Tables 1 and 2 were melted to produce slabs, which were used as materials for hot rolling. The resulting slabs were hot rolled under the conditions shown in Table 3 to produce hot-rolled sheets. The resulting hot-rolled sheets were subjected to hot-rolled sheet annealing, cold rolling, cold-rolled sheet annealing, and magnetic annealing under the conditions shown in Table 2 to produce ferritic stainless steel sheets. After the hot-rolled sheet annealing and cold-rolled sheet annealing, shot blasting and pickling were performed. Other conditions not listed in the tables were set to satisfy the preferred ranges in the specification.

[0099] [Table 1]

[0100] [Table 2]

[0101] [Table 3]

[0102] The obtained stainless steel sheets were observed and measured for inclusions by the methods described below. In addition, the remanence, coercivity, and pitting potential were measured, and the soft magnetic properties and pitting corrosion resistance were evaluated by the evaluation methods described below.

[0103] (Inclusion measurement) The TD surface of the steel sheet (the surface perpendicular to the thickness direction and rolling direction) was buffed, and the 500x magnification images observed using a JEOL SEM (IT-510A) were binarized. The binarization process was performed using Nikon's NIS-Elements software. The dimensions of the inclusions were automatically measured using binarization software, and the number of coarse inclusions with a circle equivalent diameter of 2 μm or more was counted and converted to a number density per unit area. Binarization was continued until a total of 50 coarse inclusions were counted. The chemical composition of the coarse inclusions was then analyzed using SEM-EDX analysis. The EDX analysis conditions were a magnification of 10,000x, an accelerating voltage of 15 kV, a working distance of 10 mm, and Cps × measurement time (seconds) = 300,000. Sites within or at the interface of coarse inclusions where the chemical composition satisfied equations (ii) and (iii) were confirmed were defined as composite inclusions. The proportion of the number of composite inclusions was determined by dividing the number of composite inclusions by the number of coarse inclusions (50).

[0104] The area ratio of composite inclusions was calculated by dividing the sum of the areas of the individual composite inclusions by the total area of ​​the observed field. The area of ​​each composite inclusion was calculated automatically by binarization software (NIS-Elements).

[0105] (soft magnetic properties) Test pieces measuring 55 mm square were cut from the magnetically annealed material using electrical discharge machining. Soft magnetic properties were evaluated using a single sheet magnetic test (SST test) in accordance with JIS C 2556. Single sheet magnetic measurement was performed using the H-coil method. The test piece, which had a primary and secondary winding, was sandwiched between an excitation yoke on the top and bottom to form a closed circuit. The test piece was positioned so that the magnetizing force was applied in the rolling direction. The measurement conditions were as follows: after demagnetizing the test piece, a magnetizing force of 8000 A / m was applied and then swept down to -8000 A / m. A DC hysteresis curve (BH curve) was then obtained by sweeping the force down to 8000 A / m. With the magnetizing force H on the horizontal axis and the magnetic flux density B on the vertical axis, the coercive force Hc was measured at the intersection with the horizontal axis, and the residual magnetic flux density Br was measured at the intercept with the vertical axis.

[0106] Since a small residual magnetic flux density (Br) and a small coercive force (Hc) are preferable, soft magnetic properties are evaluated by the product of Br and Hc. If the value of Br x Hc is 14.0 or less, the material is evaluated as "Good" (○), indicating practical use, and if it is 10.0 or less, the material is evaluated as "Excellent" (◎), indicating a higher performance. If the value exceeds 14.0, the material is evaluated as "Poor" (×), indicating inferior soft magnetic properties.

[0107] Pitting potential measurements were performed in accordance with JIS G 0577. The test material was sheared to prepare corrosion resistance test pieces measuring 20 mm x 15 mm. A conducting wire was spot-welded to one end of the test piece, and the entire surface except for the 10 mm x 10 mm test surface was covered with silicone resin. A 3.5% aqueous NaCl solution was used as the test solution, and the test was performed at 30°C in an Ar deaerated atmosphere. The test surface was completely immersed in the above NaCl aqueous solution and left for 10 minutes. After that, the anodic current density was measured from the natural electrode potential using a potentiostat at a potential sweep rate of 20 mV / min until the anodic current density reached 500 μA / cm. 2 The potential was measured until it reached 100 μA / cm on the anodic polarization curve. 2 The most noble potential among the potentials corresponding to the above was taken as the pitting potential (V).

[0108] If the pitting potential was less than 0.20 V, the pitting resistance was deemed insufficient and rated as "× (unacceptable)," whereas if the pitting potential was 0.20 V or more and less than 0.30 V, the pitting resistance was deemed good, equivalent to that of 19Cr-containing ferritic stainless steel (SUS304J1L), and rated as "◯ (good)." The results are summarized in Table 4.

[0109] [Table 4]

[0110] From the results shown in Table 4, all of the invention examples C1 to C26 satisfied the range of chemical composition, the number density of coarse inclusions, and the proportion of complex inclusions. As a result, the soft magnetic properties were all rated "Good" or better, and the pitting corrosion resistance was also rated "Good." Furthermore, it can be seen that invention example C3 achieved excellent soft magnetic properties due to the iron loss reduction effect of Si. All of invention examples C20 to C23 satisfied the preferred chemical composition, and achieved excellent soft magnetic properties. Furthermore, although C26 is an invention example, it did not satisfy the requirements for the area ratio of complex inclusions, so its Br × Hc value was slightly lower than the other invention examples.

[0111] In contrast, in comparative examples c1 to c24, at least one of the chemical composition, the number density of coarse inclusions, and the proportion of complex inclusions is outside the range of this embodiment, and therefore at least one of the soft magnetic properties and corrosion resistance is reduced.

[0112] In Comparative Example c1, the C content exceeds the upper limit, and Cr 23 It can be seen that an increase in carbides such as C6 reduces the number density of coarse inclusions, resulting in an "x" for soft magnetic properties. In comparative example c2, the Si content is below the lower limit, and the iron loss reduction effect is not sufficiently achieved, resulting in an "x" for soft magnetic properties. It can also be seen that the effect of Si on improving the pitting potential is not sufficiently achieved, resulting in an "x" for corrosion resistance.

[0113] It can be seen that in Comparative Example c3, the Si content exceeds the upper limit, and the increase in precipitates due to C and N activation leads to an excess of coarse inclusions, resulting in soft magnetic properties of "×". It can be seen that in Comparative Example c4, the Mn and S content exceeds the upper limit, and the increase in water-soluble MnS results in corrosion resistance of "×". It can be seen that in Comparative Example c5, the P content exceeds the upper limit, and the increase in phosphides (such as FeTiP) results in soft magnetic properties of "×".

[0114] It can be seen that Comparative Example c6 contains O and Al exceeding the upper limit, resulting in an increase in oxides (Al2O3, etc.), and therefore the soft magnetic properties are "x". Comparative Example c7 does not contain Ni and Mo, and therefore the corrosion resistance is "x". It can be seen that Comparative Example c8 contains Ni and Mo exceeding the upper limit, and therefore the ferrite phase becomes unstable, and therefore the soft magnetic properties are "x".

[0115] It can be seen that in Comparative Example c9, the Cr content is less than the lower limit, so the corrosion resistance is marked "x". In Comparative Example c10, the Cr content exceeds the upper limit, so the Cr content is 23 It can be seen that the amount of Cr-based precipitates such as C6 increases, and the soft magnetic properties are marked as "x." It can be seen that the comparative example c11 does not contain Ti, and the immobilization of C and N is insufficient, so the soft magnetic properties are marked as "x."

[0116] It can be seen that in Comparative Example c12, the Ti content exceeds the upper limit, and the increase in Ti-based precipitates such as FeTiP results in an excess of coarse inclusions, resulting in an "x" soft magnetic property. It can be seen that in Comparative Example c13, the Nb content is absent, and the immobilization of C and N is insufficient, resulting in insufficient formation of coarse inclusions, resulting in an "x" soft magnetic property. It can be seen that in Comparative Example c14, the Nb content exceeds the upper limit, resulting in an increase in Nb-based precipitates such as Fe2Nb, resulting in an "x" soft magnetic property.

[0117] It can be seen that in Comparative Example c15, the N content exceeded the upper limit, increasing nitrides such as CrN and resulting in an excess of coarse inclusions, resulting in an "x" soft magnetic property. It can be seen that in Comparative Example c16, the Mg content was below the lower limit, resulting in insufficient formation of coarse carbonitrides of 2.0 μm or more, resulting in fine precipitation in a subsequent process, resulting in an "x" soft magnetic property. It can be seen that in Comparative Example c17, the Mg content exceeded the upper limit, resulting in an excess of coarse inclusions, resulting in an "x" soft magnetic property.

[0118] In Comparative Example c18, the Mg / ([Ti] + [Nb]) ratio was below the lower limit, resulting in a low proportion of composite inclusions and fine precipitates formed in subsequent processes, resulting in a soft magnetic property rating of "x." In Comparative Example c19, the Mg / ([Ti] + [Nb]) ratio exceeded the upper limit, resulting in an increased number of M(C,N) nucleation sites. This resulted in a high proportion of composite inclusions, but the individual coarse inclusions were refined to less than 2.0 μm and precipitated in large quantities. This resulted in M(C,N) inhibiting domain wall motion, resulting in a soft magnetic property rating of "x." In Comparative Example c20, the excessive O content increased the amount of oxides, and the optional additional elements exceeded the upper limit, resulting in a soft magnetic property rating of "x."

[0119] In Comparative Example c21, the slab heating temperature was low at 1000°C, which resulted in insufficient formation of M(C,N) during heating, resulting in a decrease in the number density of coarse inclusions and a result of the soft magnetic properties being "×". In Comparative Example c22, the magnetic annealing temperature was low at 850°C, resulting in a decrease in the number ratio of complex inclusions, insufficient correction of magnetization, and a result of the soft magnetic properties being "×". In Comparative Example c23, the heating rate for magnetic annealing was fast at 200°C / s, resulting in a decrease in the number density of coarse inclusions, which prevented uniform correction of magnetization, resulting in a result of the soft magnetic properties being "×". In Comparative Example c24, formula (a) was not satisfied, resulting in a decrease in the number ratio of complex inclusions, which resulted in insufficient correction of magnetization during magnetic annealing, resulting in a result of the soft magnetic properties being "×".

Claims

1. The chemical composition, in mass%, is C: 0.001 to 0.010%, Si: more than 1.0% and less than 2.5%, Mn: 1.00% or less, P: 0.030% or less, S: 0.0050% or less, O: 0.0050% or less, Ni: 0.001 to 0.030%, Cr: 15-25%, Mo: 0.01-0.50%, Al: 0.01-0.20%, Ti: 0.01 to 0.50%, Nb: 0.01 to 0.50%, N: 0.001-0.030%, Mg: 0.0001 to 0.0030%; The balance is Fe and impurities. The following formula (i) is satisfied: When inclusions having a circle equivalent diameter of 2.0 μm or more are defined as coarse inclusions, the number density of the coarse inclusions is 100 pieces / mm ２ More than 400 pieces / mm ２ is less than A ferritic stainless steel sheet in which, when coarse inclusions that satisfy the following formulas (ii) and (iii) are considered to be composite inclusions, the number ratio of the composite inclusions to the coarse inclusions is 30% or more: 0.0002≦Mg / (Ti+Nb)<0.1500...(i) 0.5≦([O]+[S]) / ([Mg]+[Ca])≦10...(ii) 2.0≦[Ti]+[Nb]...(iii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the stainless steel plate, and is set to zero if the element is not contained, and each element symbol in the above formulas (ii) and (iii) represents the content (atomic %) of each element in the coarse inclusions, and is set to zero if the element is not contained.

2. The chemical composition further contains, in place of a portion of the Fe, B: 0.0050% or less, Cu: 1.0% or less, V: 0.50% or less, W: 0.50% or less, Ca: 0.01% or less, Zr: 0.50% or less, Co: 0.50% or less, Ga: 0.10% or less, REM: 0.10% or less, Y: 0.10% or less, Hf: 0.10% or less, and Sn: 0.01% or less, The ferritic stainless steel sheet according to claim 1, comprising one or more selected from the group consisting of:

3. 3. The ferritic stainless steel sheet according to claim 1, wherein the area ratio of the complex inclusions is 1.0% or less.

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