Non-oriented electrical steel sheet and its manufacturing method

Abstract

The chemical composition of a base material of this non-oriented electromagnetic steel sheet is, by mass%, 0–0.0050% C, 3.8–4.9% Si, 0.05–1.20% Mn, more than 0.02% and 0.50% or less sol. Al, 0–0.030% P, 0–0.0030% S, 0–0.0030% N, 0% or more and less than 0.0050% Ti, 0% or more and less than 0.0050% Nb, 0% or more and less than 0.0050% Zr, 0% or more and less than 0.0050% V, 0% or more and less than 0.200% Cu, 0% or more and less than 0.500% Ni, 0–0.100% Sn, and 0–0.100% Sb, with the remainder being Fe and impurities. This non-oriented electromagnetic steel sheet satisfies the expressions [4.3≤Si+sol. Al+0.5×Mn≤5.0], [B50(0°)-B50(45°)≤0.16], and [(B50(0°)+2×B50(45°)+B50(90°)) / 4≥1.57], and the tensile strength thereof is 580 MPa or greater.

Description

[Technical field]

[0001] The present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof. This application claims priority based on Japanese Patent Application No. 2021-023510, filed on February 17, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, global environmental issues have been attracting attention, and the demand for energy conservation efforts has been increasing. Among the demands for energy conservation efforts, there is a strong demand for high efficiency in electrical equipment. For this reason, there is an increasing demand for improved magnetic properties in non-oriented electrical steel sheets, which are widely used as iron core materials for motors, generators, etc. This tendency is particularly noticeable in drive motors for electric vehicles and hybrid vehicles, and motors for air conditioner compressors.

[0003] The motor cores of the various motors described above are composed of a stator, which is a stationary part, and a rotor, which is a rotating part. The characteristics required for the stator and rotor that make up the motor core are different from each other. The stator is required to have excellent magnetic characteristics (low iron loss and high magnetic flux density), especially low iron loss, while the rotor is required to have excellent mechanical characteristics (high strength).

[0004] Since the characteristics required for the stator and rotor are different, the desired characteristics can be achieved by producing a non-oriented electrical steel sheet for the stator and a non-oriented electrical steel sheet for the rotor separately. However, preparing two types of non-oriented electrical steel sheets causes a decrease in yield. Therefore, in order to achieve the high strength required for the rotor and the low iron loss required for the stator without stress relief annealing, non-oriented electrical steel sheets with excellent strength and magnetic properties have been studied.

[0005] For example, Patent Documents 1 and 2 attempt to achieve high strength and excellent magnetic properties. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2019 / 017426 [Patent Document 2] International Publication No. 2020 / 091039 [Patent Document 3] Japanese Patent Application Publication No. 2013-91837 [Patent Document 4] Japanese Patent Publication No. 2002-14691 [Patent Document 5] Japanese Patent Publication No. 2001-295003 Summary of the Invention [Problem to be solved by the invention]

[0007] However, to realize a non-oriented electrical steel sheet that combines high strength and low iron loss, it is necessary to contain a large amount of alloying elements as disclosed in Patent Documents 1 and 2, which causes a problem that toughness is reduced and breakage is likely to occur during cold rolling. In addition, in Patent Document 1, when sufficiently low iron loss is required for a stator, further stress relief annealing is required.

[0008] The present invention has been made to solve these problems, and has an object to stably provide a non-oriented electrical steel sheet having high strength and excellent magnetic properties. [Means for solving the problem]

[0009] The present invention relates to the following non-oriented electrical steel sheet and a method for producing the same.

[0010] (1) The non-oriented electrical steel sheet according to one embodiment of the present invention has The chemical composition of the base material is, in mass%, C: 0 to 0.0050%, Si: 3.8-4.9%, Mn: 0.05-1.20%, sol.Al: more than 0.02%, less than 0.50%, P: 0~0.030%, S: 0~0.0030%, N: 0~0.0030%, Ti: 0% or more, less than 0.0050% Nb: 0% or more, less than 0.0050% Zr: 0% or more, less than 0.0050% V: 0% or more, less than 0.0050%, Cu: 0% or more, less than 0.200% Ni: 0% or more, less than 0.500% Sn: 0~0.100%, Sb: 0 to 0.100%, and The balance is Fe and impurities. The following formulas (i) to (iii) are satisfied: Tensile strength is 580MPa or more. the law of nature, The plate thickness is 0.10 to 0.35 mm. The average iron loss W around the circumference, which is the average of the iron losses in the rolling direction, the 45° direction from the rolling direction, and the 90° direction from the rolling direction 10 / 400 but, For plate thicknesses over 0.30mm and 0.35mm or less, the limit is 16.0W / kg or less. For plate thicknesses over 0.25mm and 0.30mm or less, 15.0W / kg or less. For plate thicknesses over 0.20mm and 0.25mm or less, 13.0W / kg or less. If the plate thickness is 0.20 mm or less, it is 12.0 W / kg or less. do. 4.3≦Si+sol.Al+0.5×Mn≦5.0 (i) In the above formula (i), the element symbols indicate the content (mass %) of each element. B 50 (0°)-B 50 (45°)≦0.16 (ii) (B 50 (0°) + 2×B 50 (45°)+B 50 (90°)) / 4≧1.57 (iii) However, B in the above formulas (ii) and (iii) 50 (0°) is the magnetic flux density (T) at a magnetizing force of 5000A / m in the rolling direction, B 50(45°) is the magnetic flux density (T) at a magnetizing force of 5000A / m in the direction 45° from the rolling direction, B 50 (90°) is the magnetic flux density (T) at a magnetizing force of 5000 A / m in a direction 90° from the rolling direction.

[0011] (2) The non-oriented electrical steel sheet according to (1) above, wherein the chemical composition is, in mass%, Sn: 0.005 to 0.100%, and Sb: 0.005 to 0.100%, The ink may contain one or two selected from the following:

[0012] (3) The non-oriented electrical steel sheet according to (1) or (2) above may have an insulating coating on the surface of the base material.

[0013] (4) A method for producing a non-oriented electrical steel sheet according to another embodiment of the present invention is a method for producing the non-oriented electrical steel sheet according to any one of (1) to (3) above, In mass%, C: 0 to 0.0050%, Si: 3.8-4.9%, Mn: 0.05-1.20%, sol.Al: more than 0.02%, less than 0.50%, P: 0~0.030%, S: 0~0.0030%, N: 0~0.0030%, Ti: 0% or more, less than 0.0050% Nb: 0% or more, less than 0.0050% Zr: 0% or more, less than 0.0050% V: 0% or more, less than 0.0050%, Cu: 0% or more, less than 0.200% Ni: 0% or more, less than 0.500% Sn: 0~0.100%, Sb: 0 to 0.100%, and The balance is Fe and impurities. For a steel ingot having a chemical composition that satisfies the following formula (i), Hot rolling process, Without hot-rolled sheet annealing, The first cold rolling process reduces the plate thickness to 1.0 mm or less. Intermediate annealing process with soaking temperature of 800 to 1050℃ and soaking time of 1 to 300 seconds. A secondary cold rolling process with a reduction ratio of 65% to 85%, and Annealing temperature is 850~1050℃ The heat soaking time is 1 to 300 seconds. The final annealing process is then carried out. 4.3≦Si+sol.Al+0.5×Mn≦5.0 (i) In the above formula (i), the element symbols indicate the content (mass %) of each element. Effect of the Invention

[0014] According to the above-described embodiment of the present invention, a non-oriented electrical steel sheet having high strength and excellent magnetic properties can be stably obtained. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present inventors have conducted extensive research to solve the above problems and have come to the following findings.

[0016] Silicon, manganese and aluminum are elements that increase the electrical resistance of steel and reduce eddy current loss. These elements also contribute to increasing the strength of steel.

[0017] Among Si, Mn and sol.Al, Si is the element that contributes most efficiently to increasing electrical resistance and strength. sol.Al has the second highest effect of increasing electrical resistance and strength after Si. On the other hand, Mn has a slightly lower effect of increasing electrical resistance and strength compared to Si and sol.Al.

[0018] For these reasons, in this embodiment, the contents of Si, sol. Al and Mn are adjusted within appropriate ranges to achieve high strength and improved magnetic properties.

[0019] Next, the improvement of toughness during cold rolling of the above-mentioned steel plate containing large amounts of Si, sol. Al and Mn was examined. In the past, when alloy elements such as Si, sol.Al, and Mn were added in large amounts in steel to increase the strength of the steel, the toughness was reduced, and as a result, there was a problem that breakage was likely to occur during cold rolling. Therefore, the present inventors have intensively studied how to improve the toughness of steel sheets (high alloy steels) containing a large amount of alloy elements during cold rolling, and have found that by omitting hot-rolled sheet annealing, breakage during cold rolling can be suppressed even for high alloy steels. Specifically, they have found that the toughness of high alloy steels during two cold rolling steps can be ensured by pickling a hot-rolled sheet that has not been annealed, performing primary cold rolling to a thickness of 1 mm or less, performing intermediate annealing, and then performing secondary cold rolling.

[0020] The double cold rolling method has been investigated in the past. For example, Patent Documents 3 to 5 attempt to achieve excellent magnetic properties and high strength.

[0021] However, in the method disclosed in Patent Document 3, the Goss orientation {110} <001> The magnetic flux density in the rolling direction B 50 is good, but B 50 The anisotropy of becomes excessively large. B 50 When an electromagnetic steel sheet with a large anisotropy of the magnetic properties is used for a motor core, the smooth rotation of the motor is hindered. The means disclosed in Patent Document 4 is not satisfactory in terms of strength because the contents of Si, Mn and Al are low. The means disclosed in Patent Document 5 uses B 50 However, the reduction ratio in the final cold rolling must be increased to 85% or more, which means that the plate thickness at the start of the final cold rolling must be increased, resulting in insufficient toughness and a risk of fracture during rolling if the Si content is high.

[0022] As a result of further investigation, the present inventors have found that a steel sheet having excellent toughness during cold rolling, high strength and good magnetic properties, and further having a B 50They found that in order to realize non-oriented electrical steel sheet with small anisotropy in axial direction, it is important to appropriately control the sol. Al content, the sheet thickness at the start of the secondary cold rolling, and the reduction ratio of the secondary cold rolling.

[0023] The present invention has been made based on the above findings. Preferred embodiments of the present invention will be described in detail below. However, the present invention is not limited to the configurations disclosed in the present embodiments, and various modifications are possible without departing from the spirit of the present invention.

[0024] 1. Overall structure The non-oriented electrical steel sheet according to the present embodiment has high strength and excellent magnetic properties, and is therefore suitable for both stators and rotors. In the manufacture of the non-oriented electrical steel sheet according to the present embodiment, the sheet has excellent toughness during cold rolling, and breakage during rolling is suppressed, allowing stable manufacture. In addition, the non-oriented electrical steel sheet according to the present embodiment preferably has an insulating coating on the surface of the base material (silicon steel sheet) described below.

[0025] 2. Chemical composition of the base material In the chemical composition of the base material of the non-oriented electrical steel sheet according to this embodiment, the reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%." A numerical range described with "~" includes the lower limit and the upper limit.

[0026] C: 0 to 0.0050% C (carbon) is an element that causes iron loss degradation of non-oriented electrical steel sheets. If the C content exceeds 0.0050%, the iron loss of the non-oriented electrical steel sheet deteriorates, and good magnetic properties cannot be obtained. Therefore, the C content is set to 0.0050% or less. The C content is preferably 0.0040% or less, more preferably 0.0035% or less, and even more preferably 0.0030% or less. The C content may be 0%. However, since it is difficult to make the C content 0% in practical steel sheets in terms of production, the C content may be more than 0%. Since C contributes to increasing the strength of non-oriented electrical steel sheets, if this effect is to be obtained, the C content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0027] Si: 3.8~4.9% Silicon (Si) is an element that increases the electrical resistance of steel, reduces eddy current loss, and improves the high-frequency iron loss of non-oriented electrical steel sheets. In addition, Si has a large solid solution strengthening ability, so it is also an effective element for increasing the strength of non-oriented electrical steel sheets. In order to obtain these effects, the Si content is set to 3.8% or more. The Si content is preferably 3.9% or more, more preferably more than 4.0%, and even more preferably 4.1% or more. On the other hand, if the Si content is excessive, the workability is significantly deteriorated, making it difficult to perform cold rolling. Therefore, the Si content is set to 4.9% or less. The Si content is preferably 4.8% or less, and more preferably 4.7% or less.

[0028] Mn: 0.05 to 1.20% Mn (manganese) is an element that is effective in increasing the electrical resistance of steel, reducing eddy current loss, and improving the high-frequency iron loss of non-oriented electrical steel sheets. If the Mn content is too low, the effect of increasing the electrical resistance is small, and fine sulfides (MnS) are precipitated in the steel, which may cause insufficient grain growth during finish annealing. Therefore, the Mn content is set to 0.05% or more. The Mn content is preferably 0.20% or more, more preferably 0.23% or more, and even more preferably 0.40% or more. On the other hand, if the Mn content is excessive, the magnetic flux density of the non-oriented electrical steel sheet decreases significantly. Therefore, the Mn content is set to 1.20% or less. The Mn content is preferably 1.10% or less, and more preferably 1.00% or less.

[0029] sol.Al: more than 0.02%, less than 0.50% Sol.Al (aluminum) is an element that has the effect of reducing eddy current loss by increasing the electrical resistance of steel and improving the high frequency iron loss of non-oriented electrical steel sheets. Sol.Al is also an element that contributes to increasing the strength of non-oriented electrical steel sheets by solid solution strengthening, although not as much as Si. In order to obtain these effects, the sol.Al content is made to be more than 0.02%. The sol.Al content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and more preferably 0.20% or more. On the other hand, if the sol.Al content is excessive, the anisotropy of the magnetic flux density of the non-oriented electrical steel sheet increases. Therefore, the sol.Al content is made to be 0.50% or less. The sol.Al content is preferably 0.45% or less, more preferably 0.40% or less, and even more preferably 0.35% or less. In the present embodiment, sol. Al means acid-soluble Al, and indicates solute Al that is present in the steel in a solid solution state.

[0030] In this embodiment, the electrical resistance of the steel is ensured by appropriately controlling the contents of Si, sol.Al, and Mn. Also, from the viewpoint of ensuring strength, it is necessary to appropriately control the contents of Si, sol.Al, and Mn. On the other hand, from the viewpoint of ensuring magnetic flux density and toughness, an upper limit of the total content of Si, sol.Al, and Mn is also required. Therefore, in addition to the contents of Si, sol.Al, and Mn being within the above ranges, it is necessary to satisfy the following formula (i). From the viewpoint of ensuring the electrical resistance and strength of the steel, the value of the middle part of the following formula (i) is preferably 4.4 or more, and more preferably 4.5 or more. On the other hand, from the viewpoint of ensuring the magnetic flux density and toughness of the steel, the value of the middle part of the following formula (i) is preferably 4.9 or less, and more preferably 4.8 or less.

[0031] 4.3≦Si+sol.Al+0.5×Mn≦5.0 (i) In the above formula, the element symbols indicate the content (mass %) of each element.

[0032] P: 0~0.030% P (phosphorus) is contained in steel as an impurity, and if its content is excessive, the toughness of the non-oriented electrical steel sheet is significantly reduced. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.025% or less, and more preferably 0.020% or less. The P content may be 0%. However, since an extreme reduction in the P content may cause an increase in manufacturing costs, the P content is preferably 0.003% or more, and more preferably 0.005% or more.

[0033] S: 0 to 0.0030% S (sulfur) is an element that increases iron loss by forming fine precipitates of MnS, and deteriorates the magnetic properties of non-oriented electrical steel sheets. Therefore, the S content is set to 0.0030% or less. The S content is preferably 0.0020% or less, more preferably 0.0018% or less, and even more preferably 0.0015% or less. The S content may be 0%. Note that an extreme reduction in the S content may cause an increase in manufacturing costs, so the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0034] N: 0~0.0030% N (nitrogen) is an element that is inevitably mixed into steel, and forms nitrides that increase core loss and deteriorate the magnetic properties of non-oriented electrical steel sheets. Therefore, the N content is set to 0.0030% or less. The N content is preferably 0.0025% or less, and more preferably 0.0020% or less. The N content may be 0%. However, since an extreme reduction in the N content may cause an increase in manufacturing costs, the N content is preferably 0.0005% or more.

[0035] Ti: 0% or more, less than 0.0050% Ti (titanium) is an element that is inevitably mixed into steel, and can combine with carbon or nitrogen to form precipitates (carbides, nitrides). When carbides or nitrides are formed, these precipitates themselves deteriorate the magnetic properties of the non-oriented electrical steel sheet. Furthermore, the growth of crystal grains during finish annealing is inhibited by the carbides or nitrides, which deteriorates the magnetic properties of the non-oriented electrical steel sheet. Therefore, the Ti content is less than 0.0050%. The Ti content is preferably 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. The Ti content may be 0%. Note that an extreme reduction in the Ti content may cause an increase in manufacturing costs, so the Ti content is preferably 0.0005% or more.

[0036] Nb: 0% or more, less than 0.0050% Nb (niobium) is an element that contributes to high strength by combining with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves deteriorate the magnetic properties of non-oriented electrical steel sheets. Therefore, the Nb content is less than 0.0050%. The Nb content is preferably 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. In addition, the Nb content is more preferably below the measurement limit, and more preferably less than 0.0001%. Since the lower the Nb content, the better, the Nb content may be 0%.

[0037] Zr: 0% or more, less than 0.0050% Zr (zirconium) is an element that contributes to high strength by combining with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves deteriorate the magnetic properties of non-oriented electrical steel sheets. Therefore, the Zr content is less than 0.0050%. The Zr content is preferably 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. In addition, the Zr content is more preferably below the measurement limit, and more preferably 0.0001% or less. The lower the Zr content, the better, so the Zr content may be 0%.

[0038] V: 0% or more, less than 0.0050% V (vanadium) is an element that contributes to high strength by combining with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves deteriorate the magnetic properties of non-oriented electrical steel sheets. Therefore, the V content is set to less than 0.0050%. The V content is preferably 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. The V content is more preferably below the measurement limit, and more preferably 0.0001% or less. The lower the V content, the better, so the V content may be set to 0%.

[0039] Cu: 0% or more, less than 0.200% Cu (copper) is an element that is inevitably mixed into steel. Intentionally including Cu increases the manufacturing cost of the non-oriented electrical steel sheet. Therefore, in this embodiment, it is not necessary to actively include Cu, and it may be included at an impurity level. The Cu content is less than 0.200%, which is the maximum value that can be unavoidably included in the manufacturing process. The Cu content is preferably 0.150% or less, and more preferably 0.100% or less. The Cu content may be 0%. The lower limit of the Cu content is not particularly limited, but an extreme reduction in the Cu content may cause an increase in manufacturing costs. Therefore, the Cu content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0040] Ni: 0% or more, less than 0.500% Ni (nickel) is an element that is inevitably mixed into steel. However, Ni is also an element that improves the strength of non-oriented electrical steel sheets, so it may be intentionally contained. However, since Ni is expensive, the Ni content is less than 0.500%. The Ni content is preferably 0.400% or less, and more preferably 0.300% or less. The Ni content may be 0%. The lower limit of the Ni content is not particularly limited, but an extreme reduction in the Ni content may cause an increase in manufacturing costs. Therefore, the Ni content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0041] Sn: 0 to 0.100% Sb: 0 to 0.100% Sn (tin) and Sb (antimony) are useful elements that contribute to ensuring low iron loss in non-oriented electrical steel sheets by segregating on the surface of the base material and suppressing oxidation and nitridation during annealing. In addition, Sn and Sb also have the effect of improving the texture by segregating at the grain boundaries and increasing the magnetic flux density of the non-oriented electrical steel sheet. Therefore, at least one of Sn and Sb may be contained as necessary. However, if the content of these elements is excessive, the toughness of the steel may decrease and cold rolling may become difficult. Therefore, the content of Sn and Sb is 0.100% or less, respectively. The content of Sn and Sb is preferably 0.060% or less, respectively. The content of Sn and Sb may be 0%, respectively. If it is desired to reliably obtain the above effects, the content of at least one of Sn and Sb is preferably 0.005% or more, more preferably 0.010% or more.

[0042] In the chemical composition of the base material (silicon steel sheet) of the non-oriented electrical steel sheet according to this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in due to various factors in raw materials such as ores and scraps and in the manufacturing process when industrially producing steel, and are permissible within a range that does not adversely affect the properties of the non-oriented electrical steel sheet according to this embodiment.

[0043] In this embodiment, the contents of Cr and Mo as impurity elements are not particularly specified. In the non-oriented electrical steel sheet according to this embodiment, even if these elements are contained in the range of 0.5% or less, there is no particular effect on the properties of the non-oriented electrical steel sheet according to this embodiment. In addition, even if Ca and Mg are contained in the range of 0.002% or less, there is no particular effect on the properties of the non-oriented electrical steel sheet according to this embodiment. Even if rare earth elements (REM) are contained in the range of 0.004% or less, there is no particular effect on the properties of the non-oriented electrical steel sheet according to this embodiment. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the above REM content refers to the total content of these elements.

[0044] Although O is also an impurity element, even if it is contained in a range of 0.05% or less, it does not affect the properties of the non-oriented electrical steel sheet according to this embodiment. Since O may be mixed into steel during the annealing process, even if it is contained in a range of 0.01% or less in the slab stage (i.e., ladle value), there is no particular effect on the properties of the non-oriented electrical steel sheet according to this embodiment.

[0045] In addition to the above elements, elements such as Pb, Bi, As, B, and Se may be contained as impurity elements. As long as the content of each of these elements is in the range of 0.0050% or less, the properties of the non-oriented electrical steel sheet according to this embodiment are not impaired.

[0046] The chemical composition of the base material of the non-oriented electrical steel sheet according to this embodiment may be measured using ICP emission spectrometry or spark discharge emission spectrometry. In addition, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas combustion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. When the steel sheet to be measured has an insulating coating or the like, this is removed before measuring the chemical composition.

[0047] 3.Magnetic properties In the non-oriented electrical steel sheet according to the present embodiment, excellent magnetic properties are obtained by measuring the average core loss W 10 / 400 is low, and the magnetic flux density B 50 is high and B 50 This means that the anisotropy of the iron loss W 10 / 400 means the iron loss that occurs under the condition of a maximum magnetic flux density of 1.0 T and a frequency of 400 Hz, and the magnetic flux density B 50 means the magnetic flux density in a magnetic field of 5000 A / m.

[0048] The characteristics averaged all around refer to the average values ​​of the characteristics in the rolling direction, the characteristics in a direction at 45° from the rolling direction, and the characteristics in a direction at 90° from the rolling direction, as described below. Note that the direction at 90° from the rolling direction refers to the direction perpendicular to the rolling direction within the sheet surface (i.e., the direction perpendicular to the rolling direction and the sheet thickness direction). All-around average W10 / 400 =(W 10 / 400 (0°)+2×W 10 / 400 (45°)+W 10 / 400 (90°)) / 4 All round average B 50 =(B 50 (0°) + 2×B 50 (45°)+B 50 (90°)) / 4

[0049] Also, B 50 The anisotropy of is herein denoted as ΔB 50 and is as follows: ΔB 50 =B 50 (0°)-B 50 (45°)

[0050] Specifically, when the thickness of the non-oriented electrical steel sheet is more than 0.30 mm and is less than 0.35 mm, the average core loss is 10 / 400 16.0W / kg or less, over 0.25mm, 0.30mm or less, the average W 10 / 400 15.0W / kg or less, over 0.20mm, 0.25mm or less: Average W 10 / 400 is 13.0W / kg or less, and the average W 10 / 400 is 12.0W / kg or less, and ΔB 50 is 0.16T or less, and the average B 50 In this embodiment, the above magnetic properties (iron loss W 10 / 400 and magnetic flux density B 50 ) is measured using magnetic test pieces in each direction according to the Epstein test specified in JIS C 2550-1 (2011).

[0051] 4. Mechanical properties In the non-oriented electrical steel sheet according to this embodiment, having high strength means that the tensile (maximum) strength in the rolling direction is 580 MPa or more. The non-oriented electrical steel sheet according to this embodiment has a tensile strength of 580 MPa or more. The tensile strength is preferably 590 MPa or more. Here, the tensile strength is measured by carrying out a tensile test in accordance with JIS Z 2241 (2011).

[0052] The non-oriented electrical steel sheet according to this embodiment has high strength and excellent magnetic properties (especially B 50 Anisotropy of (ΔB 50 )) can be achieved at the same time. This has not been possible by simply increasing the alloying strength as in the past. In this embodiment, by optimizing the total content of alloying elements that contribute to strength (formula (i)) and controlling the conditions of the manufacturing method (particularly the secondary cold rolling step and the finish annealing step) described later, it is possible to achieve a non-oriented electrical steel sheet that achieves both high strength and excellent magnetic properties.

[0053] Therefore, the non-oriented electrical steel sheet of this embodiment can be suitably used as an iron core material for rotating machines such as drive motors and generators for electric vehicles and hybrid vehicles, and compressor motors for air conditioners and large air conditioners.

[0054] 5. Insulation coating In the non-oriented electrical steel sheet according to the present embodiment, it is preferable that an insulating coating is provided on the surface of the base material. Since the non-oriented electrical steel sheet is used after being punched into a core blank and then laminated, by providing an insulating coating on the surface of the base material, it is possible to reduce eddy currents between the sheets and reduce eddy current loss in the core.

[0055] In the present embodiment, the type of the insulating coating is not particularly limited, and a known insulating coating used as an insulating coating for non-oriented electrical steel sheets can be used. Examples of such insulating coatings include composite insulating coatings that are mainly made of inorganic materials and further contain organic materials.

[0056] Here, the composite insulating coating refers to an insulating coating that is mainly composed of at least one of a metal salt such as a metal chromate or a metal phosphate, or an inorganic substance such as colloidal silica, a Zr compound, or a Ti compound, and in which fine organic resin particles are dispersed. In particular, from the viewpoint of reducing the environmental load during production, which has become increasingly necessary in recent years, an insulating coating that uses a metal phosphate, a Zr or Ti coupling agent as a starting material, or an insulating coating that uses a carbonate or ammonium salt of a metal phosphate, a Zr or Ti coupling agent as a starting material is preferably used.

[0057] The amount of the insulating coating is not particularly limited, but may be, for example, 200 to 1500 mg / m per side. 2 The coating thickness is preferably about 300 to 1200 mg / m per side. 2 It is more preferable that the insulating coating is formed so that the coating weight falls within the above range, thereby making it possible to maintain excellent uniformity. When the coating weight of the insulating coating is measured after the fact, various known measuring methods can be used, and for example, a method of measuring the mass difference before and after immersion in an aqueous sodium hydroxide solution, or a fluorescent X-ray method using a calibration curve method, etc. may be appropriately used.

[0058] The non-oriented electrical steel sheet according to the present embodiment has been described above, but the average crystal grain size of the non-oriented electrical steel sheet according to the present embodiment is not particularly limited. However, if the average crystal grain size becomes too small without the crystal grains becoming coarse, there is a concern that the iron loss may worsen. On the other hand, if the crystal grains become excessively coarse and the average crystal grain size becomes too large, not only the strength may decrease but also the eddy current loss may worsen. Therefore, the average crystal grain size of the non-oriented electrical steel sheet is preferably 50 μm to 120 μm. The average crystal grain size may be 60 μm or more, and further 70 μm or more. The average crystal grain size may be 100 μm or less. The average crystal grain size can be measured, for example, by the cutting method of JIS G0551 (2020) in a cross section parallel to the rolling direction and the plate thickness direction.

[0059] In addition, the sheet thickness of the non-oriented electrical steel sheet according to this embodiment is not particularly limited. Usually, the thinner the sheet thickness, the lower the iron loss, but the higher the manufacturing cost. In view of this, if the sheet thickness is 0.10 mm or more, the iron loss can be kept lower and the cost increase can be suppressed. In addition, if the sheet thickness is 0.35 mm or less, the low iron loss can be maintained. Therefore, the sheet thickness of the non-oriented electrical steel sheet according to this embodiment is preferably 0.10 to 0.35 mm. More preferably, it is 0.15 to 0.30 mm.

[0060] 6. Manufacturing method The non-oriented electrical steel sheet according to this embodiment is not particularly limited in the manufacturing method, but can be manufactured by, for example, sequentially carrying out a hot rolling step, a pickling step, a first cold rolling step, an intermediate annealing step, a second cold rolling step, and a finish annealing step under the conditions shown below on a steel ingot having the above-mentioned chemical composition. In addition, when an insulating coating is formed on the surface of the base material (silicon steel sheet), an insulating coating forming step is carried out after the above-mentioned finish annealing step. Each step will be described in detail below.

[0061] <Hot rolling process> A steel ingot (slab) having the above chemical composition is heated and the heated steel ingot is hot rolled to obtain a hot rolled steel sheet. The heating temperature of the steel ingot when subjected to hot rolling is not particularly specified, but is preferably, for example, 1050 to 1250° C. The thickness of the hot rolled steel sheet after hot rolling is also not particularly specified, but is preferably, for example, about 1.5 to 3.0 mm, taking into consideration the efficiency of hot rolling and subsequent processes.

[0062] <Acid washing process> After the hot rolling process, pickling is performed without annealing the hot-rolled sheet. In general, after the hot rolling process, pickling is often performed after annealing the hot-rolled sheet. However, in the case of steel containing a large amount of alloy elements as in this embodiment, if hot-rolled sheet annealing is performed, toughness may deteriorate and break during cold rolling. Therefore, in this embodiment, hot-rolled sheet annealing is omitted. Specifically, the hot-rolled sheet is pickled without annealing, and the scale layer formed on the surface of the base material is removed. Here, pickling conditions such as the concentration of the acid used in pickling, the concentration of the accelerator used in pickling, and the temperature of the pickling solution are not particularly limited, and may be known pickling conditions.

[0063] <First cold rolling process> After pickling, the plate thickness is reduced to 1.0 mm or less. If the plate thickness after reduction exceeds 1.0 mm, there is a high risk of fracture during secondary cold rolling. The plate thickness after reduction is more preferably 0.9 mm or less, and even more preferably 0.8 mm or less.

[0064] <Intermediate annealing process> After the first cold rolling, intermediate annealing is performed to improve the magnetic properties of the non-oriented electrical steel sheet. Regarding the heat treatment conditions in intermediate annealing, the first cold rolled sheet is annealed at 800 to 1050°C for 1 to 300 seconds. If the soaking temperature in intermediate annealing is too low, the magnetic flux density B 50 Therefore, the soaking temperature in the intermediate annealing is set to 800°C or higher, preferably 850°C or higher, and more preferably 900°C or higher. On the other hand, if the soaking temperature in the intermediate annealing is too high, there is a risk of fracture during the secondary cold rolling. Therefore, the soaking temperature in the intermediate annealing is set to 1050°C or lower, preferably 1040°C or lower, and more preferably 1030°C or lower. In addition, if the soaking time in the intermediate annealing is too short, the magnetic flux density B 50The soaking time in the intermediate annealing may decrease. Therefore, the soaking time in the intermediate annealing is set to 1 sec or more, preferably 5 sec or more, and more preferably 10 sec or more. On the other hand, if the soaking time in the intermediate annealing is too long, the manufacturing cost may increase. Therefore, the soaking time in the intermediate annealing is set to 300 sec or less, preferably 200 sec or less, and more preferably 100 sec or less. Since the rolling oil from the first cold rolling is attached to the steel sheet before the intermediate annealing, it is preferable to perform a degreasing treatment.

[0065] <Secondary cold rolling process> After the intermediate annealing, secondary cold rolling is performed. In the secondary cold rolling, the base material is rolled at a reduction rate of 65% or more and less than 85% so that the final plate thickness is 0.10 to 0.35 mm. If the reduction rate in the secondary cold rolling is too low, the magnetic flux density B 50 Anisotropy of (ΔB 50 ) may become large. In addition, if the reduction rate in the secondary cold rolling is excessively reduced, iron loss may deteriorate. Therefore, the reduction rate in the secondary cold rolling is preferably 67% or more. On the other hand, if the reduction rate in the secondary cold rolling is too high, the plate thickness at the start of rolling may become too thick and may break. Therefore, the reduction rate in the secondary cold rolling is preferably 83% or less. In addition, when the intermediate annealing is performed in an oxidizing atmosphere, it is preferable to remove the scale before performing the secondary cold rolling.

[0066] <Finishing annealing process> After the secondary cold rolling, finish annealing is performed. In the method for producing a non-oriented electrical steel sheet according to the present embodiment, it is preferable to use a continuous annealing furnace for the finish annealing.

[0067] Regarding the finish annealing conditions, the soaking temperature (annealing temperature) is set to 850 to 1050° C. Other conditions for the finish annealing step are preferably a soaking time of 1 to 300 sec, a mixed atmosphere of H2 and N2 (i.e., H2+N2=100% by volume) with a H2 ratio of 5 to 100% by volume, and an atmospheric dew point of 30° C. or lower.

[0068] If the soaking temperature is less than 850°C, the grain size becomes fine and the core loss of the non-oriented electrical steel sheet deteriorates, which is not preferable. If the soaking temperature exceeds 1050°C, the strength of the non-oriented electrical steel sheet becomes insufficient, which is not preferable, as it increases the manufacturing cost. The soaking temperature is more preferably 875 to 1025°C, and even more preferably 900 to 1000°C. If the soaking time is less than 1 sec, the grains cannot be sufficiently coarsened. If the soaking time exceeds 300 sec, it increases the manufacturing cost. The proportion of H2 in the atmosphere is more preferably 10 to 90% by volume. The dew point of the atmosphere is preferably low from the viewpoint of increasing the magnetic flux density. The dew point of the atmosphere is more preferably 10°C or less, even more preferably 0°C or less, and even more preferably -10°C or less.

[0069] <Insulating film formation process> After the above-mentioned finish annealing, an insulating film forming step is carried out as necessary. The method for forming the insulating film is not particularly limited, and a known insulating film forming treatment liquid as described below may be used, and the treatment liquid may be applied and dried by a known method. An example of a known insulating film is a composite insulating film mainly made of an inorganic material and further containing an organic material.

[0070] The composite insulating coating is, for example, an insulating coating that is mainly composed of at least one of a metal salt such as a metal chromate or a metal phosphate, or an inorganic substance such as colloidal silica, a Zr compound, or a Ti compound, and in which fine organic resin particles are dispersed. In particular, from the viewpoint of reducing the environmental load during production, which has become increasingly necessary in recent years, an insulating coating that uses a metal phosphate, a Zr or Ti coupling agent as a starting material, or an insulating coating that uses a carbonate or ammonium salt of a metal phosphate, a Zr or Ti coupling agent as a starting material is preferably used.

[0071] Before applying the treatment liquid to the surface of the base material on which the insulating coating is formed, any pretreatment may be performed, such as a degreasing treatment with an alkali or an acid pickling treatment with hydrochloric acid, sulfuric acid, phosphoric acid, etc. The treatment liquid may be applied to the surface of the base material as it is after finish annealing without performing such pretreatment.

[0072] The present invention will be described in more detail below with reference to examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these example conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved. EXAMPLES

[0073] A slab having the composition shown in Table 1 was heated to 1150°C, and then hot-rolled at a finishing temperature of 850°C and a finishing thickness of 2.0 mm, and then coiled at 650°C to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was subjected to pickling to remove surface scale without hot-rolled sheet annealing, and then to primary cold rolling to a thickness of 0.7 mm. Next, the steel sheet after the primary cold rolling was degreased, and then intermediate annealing was performed at 970°C for 40 seconds to obtain an intermediate annealed sheet. The intermediate annealed sheet was subjected to secondary cold rolling to a thickness of 0.20 mm to obtain a cold-rolled steel sheet. Furthermore, a finish annealing was performed at 1000°C for 20 seconds in a mixed atmosphere of H2: 15%, N2: 85%, and a dew point of -30°C. After that, an insulating coating was applied to manufacture a non-oriented electrical steel sheet and use it as a test material.

[0074] The insulating coating is made of aluminum phosphate and an acrylic-styrene copolymer resin emulsion with a particle size of 0.2 μm, and the coating weight is 1000 mg / m 2 The coating was applied so that the thickness was 100 μm and then baked at 350°C in air to form the coating.

[0075] [Table 1]

[0076] [Table 2]

[0077] For each test material obtained, Epstein test pieces were taken from the rolling direction, the 45° direction from the rolling direction, and the 90° direction from the rolling direction, and the magnetic properties in each direction (iron loss W 10 / 400 and magnetic flux density B 50 The average iron loss around the circumference W 10 / 400 is 12.0W / kg or less, the magnetic flux density B 50 is 1.57T or more, and ΔB 50 When the magnetic field strength was 0.16 T or less, the test materials were judged to have excellent magnetic properties and passed the test. When this condition was not met, the test materials were judged to have poor magnetic properties and failed the test. This pass condition was set because the final thickness of the cold-rolled material for each test material was 0.20 mm or less.

[0078] Furthermore, JIS No. 5 tensile test pieces were taken from each test material in accordance with JIS Z 2241 (2011) so that the longitudinal direction coincided with the rolling direction of the steel plate. Then, using the above test pieces, a tensile test was performed in accordance with JIS Z 2241 (2011) to measure the tensile strength. A tensile strength of 580 MPa or more was judged to have high strength and pass. A tensile strength of less than 580 MPa was judged to have poor strength and fail.

[0079] The results of the Epstein test and the tensile test are shown in Table 2. Note that the underlines in Tables 1 and 2 indicate compositions outside the scope of the present invention. Also, "-" in the chemical composition table shown in Table 1 means that the content of the corresponding element is 0% in significant figures (numbers up to the least significant digit) defined in this embodiment.

[0080] It was found that in Test Nos. 2 to 4, 6, 7, 9, 12 to 14, and 16 to 18, in which the chemical composition of the steel sheet satisfied the provisions of the present invention, the average iron loss around the circumference was low, the average magnetic flux density around the circumference was high, the anisotropy of the magnetic flux density was small, and the tensile strength was high, being 580 MPa or more.

[0081] In contrast, in Test Nos. 1, 5, 8, 10, 11, 15 and 19 to 23, which are comparative examples, at least one of the magnetic properties and the tensile strength was poor, or the toughness was significantly deteriorated, making production difficult.

[0082] Specifically, in test No. 1, the Si content was lower than the specified range, resulting in poor tensile strength, and in test No. 8, formula (i) was not satisfied, resulting in poor tensile strength.

[0083] In test No. 5, formula (i) was not satisfied, and in test No. 15, the P content exceeded the specified range, so that the toughness deteriorated and fractured during cold rolling, and it was not possible to measure the tensile strength and magnetic properties. Similarly, in test No. 20, the Si content and formula (i) were not satisfied, and in test No. 22, the Sn content exceeded the specified range, and in test No. 23, the Sb content exceeded the specified range, so that the toughness deteriorated and fractured during cold rolling, and it was not possible to measure the tensile strength and magnetic properties.

[0084] In test No. 10, the sol.Al content exceeded the specified range, resulting in poor anisotropy of magnetic flux density. In test No. 11, the S content exceeded the specified range, resulting in poor iron loss. In test No. 19, the C content exceeded the specified range, resulting in poor iron loss. In test No. 21, the sol.Al content was lower than the specified range, resulting in poor iron loss. EXAMPLES

[0085] A slab of steel type I in Table 1 was heated to 1150°C, and then hot-rolled at a finishing temperature of 850°C and a finishing thickness of 2.0 mm, and then coiled at 650°C to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was not annealed, but the surface scale was removed by pickling, and the sheet was rolled down to the thickness shown in Table 3 to obtain a primary cold-rolled sheet. The primary cold-rolled sheet of each thickness was degreased, and then intermediate annealed for 30 seconds at the soaking temperature shown in Table 3 to obtain an intermediate annealed sheet. The intermediate annealed sheet was subjected to secondary cold rolling to a thickness of 0.20 mm to obtain a cold-rolled steel sheet. Furthermore, in a mixed atmosphere of H2: 15%, N2: 85%, and a dew point shown in Table 3, final annealing was performed for 20 seconds at the soaking temperature shown in Table 3. An insulating coating was then applied to produce a non-oriented electrical steel sheet, which was used as a test material. As shown in Test No. 40, a comparative example was also carried out in which hot-rolled sheet annealing was performed under soaking conditions of 950° C.×60 seconds.

[0086] [Table 3]

[0087] The insulating coating is made of aluminum phosphate and an acrylic-styrene copolymer resin emulsion with a particle size of 0.2 μm, and the coating weight is 900 mg / m 2 The coating was applied so that the thickness was 100 μm and then baked at 350°C in air to form the coating.

[0088] For each test material obtained, Epstein test pieces were taken from the rolling direction, the 45° direction from the rolling direction, and the 90° direction from the rolling direction, and the magnetic properties in each direction (iron loss W 10 / 400 and magnetic flux density B 50 The average iron loss around the circumference W 10 / 400 is 12.0W / kg or less, the magnetic flux density B 50 is 1.57T or more, and ΔB 50 When the magnetic field strength was 0.16 T or less, the test material was judged to have excellent magnetic properties and passed the test. When this condition was not met, the test material was judged to have poor magnetic properties and failed the test. This pass condition was set because the plate thickness of each test material was 0.20 mm or less.

[0089] Furthermore, JIS No. 5 tensile test pieces were taken from each test material in accordance with JIS Z 2241 (2011) so that the longitudinal direction coincided with the rolling direction of the steel plate. Then, using the above test pieces, a tensile test was performed in accordance with JIS Z 2241 (2011) to measure the tensile strength. A tensile strength of 580 MPa or more was judged to have high strength and pass. A tensile strength of less than 580 MPa was judged to have poor strength and fail.

[0090] The results of the Epstein test and the tensile test are shown in Table 3.

[0091] It was found that in Test Nos. 25 to 27, 30 to 32, and 36 to 38, in which the sheet thickness after the first cold rolling, the intermediate annealing temperature, and the reduction ratio of the second cold rolling satisfied the provisions of the present invention, the average iron loss around the circumference was low, the average magnetic flux density around the circumference was high, the anisotropy of the magnetic flux density was small, and the tensile strength was high, at 580 MPa or more.

[0092] In contrast, in Test Nos. 24, 28, 29, 33 to 35, and 39 to 41, which are comparative examples, the magnetic properties were poor, the tensile strength was poor, or the toughness was significantly deteriorated, making production difficult.

[0093] Specifically, in Test Nos. 24 and 41, the sheet thickness after the first cold rolling was greater than the specified range, so the toughness deteriorated and the sheet broke during the second cold rolling, making it impossible to measure the tensile strength and magnetic properties. In Test No. 33, the intermediate annealing temperature was higher than the specified range, so the toughness deteriorated and the sheet broke during the second cold rolling, making it impossible to measure the tensile strength and magnetic properties. In Test No. 40, hot-rolled sheet annealing was performed, so the toughness deteriorated and the sheet broke during the first cold rolling, making it impossible to measure the tensile strength and magnetic properties.

[0094] Furthermore, in Test No. 28, the reduction ratio of the secondary cold rolling was lower than the specified range, resulting in poor anisotropy of the magnetic flux density, and in Test No. 29, the intermediate annealing temperature was lower than the specified range, resulting in poor average magnetic flux density around the circumference.

[0095] In addition, in test No. 34, the final annealing temperature was lower than the specified temperature, resulting in poor average core loss around the circumference, while in test No. 35, the final annealing temperature was higher than the specified temperature, resulting in poor tensile strength.

[0096] In Test No. 39, the reduction ratio in the secondary cold rolling was lower than the specified range, and the finish annealing temperature was also lower than the specified range, resulting in poor iron loss. [Industrial Applicability]

[0097] As described above, according to the present invention, a non-oriented electrical steel sheet having high strength and excellent magnetic properties can be obtained.

Claims

1. The chemical composition of the base material is, in mass%, C: 0 to 0.0050%, Si: 3.8-4.9%, Mn: 0.05-1.20%, Sol. Al: more than 0.02%, less than 0.50%, P: 0 to 0.030%, S: 0-0.0030%, N: 0 to 0.0030%, Ti: 0% or more and less than 0.0050%; Nb: 0% or more, less than 0.0050%; Zr: 0% or more and less than 0.0050%; V: 0% or more and less than 0.0050%; Cu: 0% or more and less than 0.200%; Ni: 0% or more and less than 0.500%; Sn: 0-0.100%, Sb: 0 to 0.100%, and The balance is Fe and impurities. The following formulas (i) to (iii) are satisfied: The tensile strength is 580 MPa or more, The plate thickness is 0.10 to 0.35 mm, The average iron loss W 10 / 400 around the circumference, which is the average of the iron losses in the rolling direction, the direction at 45° from the rolling direction, and the direction at 90° from the rolling direction, is If the plate thickness is more than 0.30 mm and is 0.35 mm or less, the load is 16.0 W / kg or less. For plate thicknesses over 0.25 mm and 0.30 mm or less, 15.0 W / kg or less; If the plate thickness is more than 0.20 mm and is 0.25 mm or less, the load is 13.0 W / kg or less. When the plate thickness is 0.20 mm or less, it is 12.0 W / kg or less. Non-oriented electrical steel sheet. 4.3≦Si+sol. Al+0.5×Mn≦5.0...(i) In the above formula (i), the element symbols indicate the content (mass %) of each element. B ５０ (0°) ５０ (45°)≦0.16 ・・・(ii) (B ５０ (0°)+2×B ５０ (45°)+B ５０ (90°)) / 4≧1.57 ・・・(iii) However, B in the above formula (ii) and formula (iii) ５０ (0°) is the magnetic flux density (T) at a magnetizing force of 5000 A / m in the rolling direction, B ５０ (45°) is the magnetic flux density (T) at a magnetizing force of 5000 A / m in the direction 45° from the rolling direction, B ５０ (90°) is the magnetic flux density (T) at a magnetizing force of 5000 A / m in a direction 90° from the rolling direction.

2. The chemical composition, in mass%, Sn: 0.005 to 0.100%, and Sb: 0.005-0.100%, Contains one or two selected from The non-oriented electrical steel sheet according to claim 1.

3. The base material has an insulating coating on its surface. The non-oriented electrical steel sheet according to claim 1 or 2.

4. A method for producing the non-oriented electrical steel sheet according to any one of claims 1 to 3, comprising the steps of: In mass percent, C: 0 to 0.0050%, Si: 3.8-4.9%, Mn: 0.05-1.20%, Sol. Al: more than 0.02%, less than 0.50%, P: 0 to 0.030%, S: 0-0.0030%, N: 0 to 0.0030%, Ti: 0% or more and less than 0.0050%; Nb: 0% or more, less than 0.0050%; Zr: 0% or more and less than 0.0050%; V: 0% or more and less than 0.0050%; Cu: 0% or more and less than 0.200%; Ni: 0% or more and less than 0.500%; Sn: 0-0.100%, Sb: 0 to 0.100%, and The balance is Fe and impurities. For a steel ingot having a chemical composition that satisfies the following formula (i), Hot rolling process, A primary cold rolling process in which the sheet thickness is reduced to 1.0 mm or less without performing hot-rolled sheet annealing; An intermediate annealing process in which the soaking temperature is 800 to 1050°C and the soaking time is 1 to 300 seconds. A secondary cold rolling step having a rolling reduction of 65% or more and less than 85%; and A final annealing process is then carried out in which the annealing temperature is 850 to 1050°C and the soaking time is 1 to 300 seconds. Manufacturing method of non-oriented electrical steel sheet. 4.3≦Si+sol. Al+0.5×Mn≦5.0...(i) In the above formula (i), the element symbols indicate the content (mass %) of each element.

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