Non-oriented electrical steel sheet and method for manufacturing the same

Abstract

A non-oriented electromagnetic steel sheet in which: a base material has a chemical composition of, by mass, 0.0050% or less of C, over 3.70% and up to 4.60% of Si, over 0.20% and up to 0.50% of Mn, 0.23-0.75% of Al, 0.030% or less of P, 0.0018% or less of S, 0.0040% or less of N, 0.005-0.040% of Sn, and 0-0.040% of Sb, the balance being Fe and impurities; the relationship [4.2≤Si+Al+0.5×Mn≤4.9] is satisfied; the N content [N]s from the surface of the base material to a position at 20 μm in the depth direction is 0.0060% or less; the average crystal grain size of the base material is 50-120 μm; the saturation magnetic flux density is 1.945 T or greater; the tensile strength is 600 MPa or greater; and the plate thickness is 0.10-0.30 mm.

Description

Technical Field

[0001] The present invention relates to a non-oriented electromagnetic steel sheet and a method for manufacturing the same.

Background Art

[0002] In recent years, global environmental problems have attracted attention, and the demand for efforts towards energy conservation has been increasing further. In particular, the improvement of the efficiency of electrical equipment is strongly desired. For this reason, even in non-oriented electromagnetic steel sheets widely used as core materials for motors or generators, the demand for improving magnetic properties has been further strengthened. This tendency is remarkable in drive motors for electric vehicles and hybrid vehicles, as well as motors for air conditioner compressors.

[0003] The motor cores of various motors as described above are composed of a stator which is a stator and a rotor which is a rotor. The characteristics required for the stator and rotor constituting the motor core are different from each other. For the stator, excellent magnetic properties (low iron loss and high magnetic flux density), particularly low iron loss and high saturation magnetic flux density are required, while for the rotor, excellent mechanical properties (high strength) are required.

[0004] Since the characteristics required for the stator and rotor are different, by separately producing a non-oriented electromagnetic steel sheet for the stator and a non-oriented electromagnetic steel sheet for the rotor, desired characteristics can be realized. However, preparing two types of non-oriented electromagnetic steel sheets leads to a decrease in yield.In order to realize the high strength required for the rotor while realizing the low iron loss and high saturation magnetic flux density required for the stator, non-oriented electromagnetic steel sheets having excellent strength and excellent magnetic properties have been conventionally studied.

[0005] For example, in Patent Documents 1 to 4, attempts have been made to realize excellent magnetic properties and high strength.

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] International Publication No. 2020 / 091043 [Patent Document 4] Japanese Patent Publication No. 2010-90474 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, achieving non-oriented electrical steel sheets that combine high strength and low iron loss requires the inclusion of large amounts of alloying elements, as disclosed in Patent Documents 1 to 4. This presents the problem of reduced toughness and increased susceptibility to fracture during cold rolling. Furthermore, increasing the alloying level can sometimes lead to a decrease in saturation magnetic flux density.

[0008] This invention was made to solve these problems and aims to provide a stable supply of non-oriented electrical steel sheets with high strength and excellent magnetic properties. [Means for solving the problem]

[0009] The present invention is summarized as follows: non-oriented electrical steel sheet and a method for manufacturing the same.

[0010] (1) The chemical composition of the base material is, in mass%, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75% P:0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050% Nb: Less than 0.0050% Less than 0.0050% Zr, Less than 0.0050% V, Less than 0.200% Cu, Less than 0.500% Ni, 0.005 - 0.040% Sn, 0 - 0.040% Sb, The balance is Fe and impurities, satisfies the following formula (i), The N content [N]s from the surface of the base material to a position 20 μm in the depth direction is 0.0060% or less, The average crystal grain size of the base material is 50 - 120 μm, The saturation magnetic flux density is 1.945 T or more, The tensile strength is 600 MPa or more, The plate thickness is 0.10 - 0.30 mm, Non - oriented electrical steel sheet. 4.2 ≦ Si + Al + 0.5×Mn ≦ 4.9 ···(i) However, the element symbols in the above formula are the content (mass%) of each element.

[0011] (2) The base material has an insulating film on its surface, The non - oriented electrical steel sheet according to (1) above.

[0012] (3) A method for manufacturing the non - oriented electrical steel sheet according to (1) or (2) above, by mass%, C: 0.0050% or less, Si: more than 3.70% and 4.60% or less, Mn: more than 0.20% and 0.50% or less, Al: 0.23 - 0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: less than 0.0050%, Nb: less than 0.0050%, Zr: less than 0.0050%, V: less than 0.0050%, Cu: less than 0.200%, Ni: Less than 0.500% Sn: 0.005~0.040%, Sb: 0~0.040%, The remainder consists of Fe and impurities. For a steel ingot having a chemical composition satisfying the following equation (i), The process involves, in order, a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process at a soaking temperature of 650-780°C for 8-36 hours, a cold rolling process to reduce the sheet thickness to 0.10-0.30 mm, and a finish annealing process at a soaking temperature of 880-1020°C for 1 second to 10 minutes. A method for manufacturing non-oriented electrical steel sheets. 4.2≦Si+Al+0.5×Mn≦4.9 (i) However, the element symbols in the above formula represent the content (mass %) of each element. [Effects of the Invention]

[0013] According to the present invention, a non-oriented electrical steel sheet having high strength and excellent magnetic properties can be obtained. [Modes for carrying out the invention]

[0014] As a result of diligent research conducted by the inventors to solve the above problems, we have obtained the following findings.

[0015] To achieve high strength, low iron loss, and high saturation magnetic flux density while ensuring toughness during cold rolling, it is necessary to optimize the content of the main alloying elements, Si, Mn, and Al.

[0016] Specifically, the mixture contains Si in a concentration of over 3.70% to 4.60%, as Si has the highest solid solution strengthening ability and contributes most to the increase in electrical resistance. In addition, to obtain good grain growth properties, it contains 0.23% or more Al. On the other hand, to suppress the deterioration of the saturation magnetic flux density, the Al content is kept to 0.75% or less. Furthermore, although Mn has the lowest solid solution strengthening ability among the three elements, it contributes to the increase in electrical resistance with minimal toughness deterioration, and therefore contains over 0.20% Mn.

[0017] Through repeated investigations, the inventors found that nitriding of the surface layer of the steel sheet degrades iron loss. Although the mechanism is not yet clear, they discovered that Mn influences the nitriding of the surface layer of the steel sheet. To suppress the degradation of iron loss due to nitriding of the surface layer of the steel sheet, the Mn content is set to 0.50% or less. Furthermore, it was found that Sn also has the effect of suppressing nitriding of the surface layer of the steel sheet. Therefore, Sn is included at a concentration of 0.005-0.040%.

[0018] In typical continuous hot-rolled sheet annealing, scale-covered hot-rolled sheets are inserted into the annealing furnace, resulting in the formation of scale that is difficult to remove during pickling after annealing. Therefore, mechanical descaling, such as shot blasting, is necessary before pickling. However, with high-alloy steels as mentioned above, shot blasting causes twinning deformation on the surface of the steel sheet, and this twinning deformation can easily lead to problems such as sheet fracture and edge cracking during cold rolling.

[0019] In this invention, the hot-rolled sheet is pickled before annealing, and then annealed in a batch furnace. Since the scale on the hot-rolled sheet can be easily removed by pickling, shot blasting is unnecessary and twinning deformation does not occur. Therefore, good toughness can be ensured even with high-alloy steel, and the occurrence of problems such as sheet fracture and edge cracking during cold rolling can be suppressed.

[0020] This invention is based on the above findings. The requirements of this invention will be described in detail below.

[0021] 1. Overall Structure The non-oriented electrical steel sheet according to one embodiment of the present invention has high strength and excellent magnetic properties, making it suitable for both stators and rotors. Furthermore, it is preferable that the non-oriented electrical steel sheet according to this embodiment has an insulating coating on the surface of the base material described below.

[0022] 2. Chemical composition of the base material The reasons for the limitations on each element are as follows. Note that in the following explanation, "%" for content refers to "mass%".

[0023] C: 0.0050% or less Carbon (C) is an element that causes iron loss degradation in 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 should be 0.0050% or less. Preferably, the C content is 0.0040% or less, more preferably 0.0035% or less, and even more preferably 0.0030% or less. Furthermore, 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.

[0024] Si: more than 3.70% and less than 4.60% Silicon (Si) is an element that increases the electrical resistance of steel, reduces eddy current loss, and improves high-frequency iron loss in non-oriented electrical steel sheets. Furthermore, because Si has a large solid solution strengthening capacity, it is also an effective element for increasing the strength of non-oriented electrical steel sheets. To obtain these effects, the Si content should be greater than 3.70%. Preferably, the Si content is 3.80% or more, more preferably 3.90% or more, and even more preferably 4.00% or more. On the other hand, if the Si content is excessive, the workability deteriorates significantly, making it difficult to carry out cold rolling. Therefore, the Si content should be 4.60% or less. Preferably, the Si content is 4.50% or less, and more preferably 4.40% or less.

[0025] Mn: More than 0.20% and less than 0.50% Manganese (Mn) is an effective element for increasing the electrical resistance of steel, reducing eddy current losses, and improving high-frequency iron loss in non-oriented electrical steel sheets. However, if the Mn content is too low, the effect of increasing electrical resistance is small, and iron loss deteriorates due to the precipitation of fine sulfides (MnS) in the steel. Therefore, the Mn content should be greater than 0.20%. Preferably, the Mn content is 0.25% or more, and more preferably 0.30% or more. On the other hand, if the Mn content is excessive, nitriding of the surface layer of the steel sheet becomes excessive, and iron loss deteriorates. Therefore, the Mn content should be 0.50% or less. Preferably, the Mn content is 0.45% or less, and more preferably 0.40% or less.

[0026] Al: 0.23~0.75% Aluminum (Al) is an element that reduces eddy current losses by increasing the electrical resistance of steel, thereby improving high-frequency iron loss in non-oriented electrical steel sheets. Al also improves iron loss by improving the texture. Furthermore, although not to the same extent as Si, Al contributes to increasing the strength of non-oriented electrical steel sheets through solid solution strengthening. Additionally, the addition of an appropriate amount of Al suppresses the refinement of AlN, which is formed by AlN bonding with N in the steel, improving grain growth during finish annealing and suppressing iron loss degradation caused by the fine AlN itself.

[0027] To obtain these effects, the Al content should be 0.23% or higher. Preferably, the Al content should be 0.25% or higher, and more preferably 0.27% or higher. Note that the nitriding of the surface layer of the steel sheet, as mentioned above, is more likely to occur as the Al content increases. As a result, iron loss deteriorates. However, in this invention, by controlling the content of Mn and Sn, nitriding of the surface layer of the steel sheet can be suppressed even when the Al content is high. Therefore, the effects of this invention are particularly pronounced when the Al content is high. That is, the effects of this invention are more pronounced when the Al content is, for example, greater than 0.45% or 0.47% or higher. On the other hand, if the Al content is excessive, the toughness deteriorates and the risk of fracture during cold rolling increases. Therefore, the Al content should be 0.75% or less. Preferably, the Al content should be 0.70% or less, and more preferably 0.65% or less.

[0028] In this embodiment, the electrical resistance of the steel is ensured by appropriately controlling the content of Si, Al, and Mn. Furthermore, from the viewpoint of ensuring strength, it is also necessary to appropriately control the content of Si, Al, and Mn. On the other hand, an upper limit is also necessary from the viewpoint of ensuring saturation magnetic flux density and toughness. Therefore, in addition to the content of Si, Al, and Mn being within the above ranges, it is necessary to satisfy the following equation (i). The value of the middle side of the following equation (i) is preferably 4.3 or more, more preferably 4.4 or more, preferably 4.8 or less, and more preferably 4.7 or less.

[0029] 4.2≦Si+Al+0.5×Mn≦4.9 (i) However, the element symbols in the above formula represent the content (mass %) of each element.

[0030] P:0.030% or less Phosphorus (P) is present in steel as an impurity, and if its content is excessive, the toughness of non-oriented electrical steel sheets will be significantly reduced. Therefore, the P content should be 0.030% or less. Preferably, the P content is 0.025% or less, and more preferably 0.020% or less. However, since extreme reduction of the P content may lead to an increase in manufacturing costs, preferably, the P content is 0.003% or more, and more preferably 0.005% or more.

[0031] S: 0.0018% or less S (sulfur) is an element that increases iron loss by forming fine precipitates of MnS, thereby degrading the magnetic properties of non-oriented electrical steel sheets. Therefore, the sulfur content should be 0.0018% or less. Preferably, the sulfur content should be 0.0016% or less, and more preferably 0.0014% or less. However, since extreme reduction of the sulfur content may lead to increased manufacturing costs, preferably, the sulfur content should be 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0032] N: 0.0040% or less Nitrogen (N) is an element that is inevitably mixed into steel, and it forms nitrides, increasing iron loss and degrading the magnetic properties of non-oriented electrical steel sheets. Therefore, the N content should be 0.0040% or less. Preferably, the N content is 0.0030% or less, and more preferably 0.0020% or less. However, since extreme reduction of the N content may lead to an increase in manufacturing costs, it is preferable that the N content be 0.0005% or more.

[0033] Ti: Less than 0.0050% Titanium (Ti) is an element that inevitably contaminates steel and can combine with carbon or nitrogen to form precipitates (carbides, nitrides). When carbides or nitrides are formed, these precipitates themselves degrade the magnetic properties of the non-oriented electrical steel sheet. Furthermore, they inhibit the growth of crystal grains during finish annealing, further degrading the magnetic properties of the non-oriented electrical steel sheet. Therefore, the Ti content should be less than 0.0050%. Preferably, the Ti content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the Ti content may lead to increased manufacturing costs, it is preferable that the Ti content be 0.0005% or more.

[0034] Nb: Less than 0.0050% Niobium (Nb) is an element that contributes to increased strength by bonding with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Therefore, the Nb content should be less than 0.0050%. Preferably, the Nb content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the Nb content may lead to increased manufacturing costs, it is preferable that the Nb content be 0.0001% or more.

[0035] Zr: Less than 0.0050% Zr (zirconium) is an element that contributes to increased strength by bonding with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Therefore, the Zr content should be less than 0.0050%. Preferably, the Zr content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the Zr content may lead to increased manufacturing costs, it is preferable that the Zr content be 0.0001% or more.

[0036] V: Less than 0.0050% Vanadium (V) is an element that contributes to increased strength by bonding with carbon or nitrogen to form precipitates (carbides, nitrides), but these precipitates themselves degrade the magnetic properties of non-oriented electrical steel sheets. Therefore, the V content should be less than 0.0050%. Preferably, the V content is 0.0040% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less. However, since extreme reduction of the V content may lead to increased manufacturing costs, it is preferable that the V content be 0.0001% or more.

[0037] Cu: Less than 0.200% Copper (Cu) is an element that inevitably becomes mixed into steel. Intentionally including Cu increases the manufacturing cost of non-oriented electrical steel sheets. Therefore, in this embodiment, it is not necessary to actively include Cu, and an impurity level is sufficient. The Cu content is set to less than 0.200%, which is the maximum value that can inevitably be mixed in during the manufacturing process. The Cu content is preferably 0.150% or less, and more preferably 0.100% or less. There is no particular lower limit to the Cu content, however, an extreme reduction in Cu content may lead to 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.

[0038] Ni: Less than 0.500% Nickel (Ni) is an element that is inevitably present in steel. However, since Ni is also an element that improves the strength of non-oriented electrical steel sheets, it may be intentionally included. However, because Ni is expensive, the Ni content should be less than 0.500%. Preferably, the Ni content is 0.400% or less, and more preferably 0.300% or less. There is no particular lower limit to the Ni content, but an extreme reduction in Ni content may lead to an increase in manufacturing costs. Therefore, preferably, the Ni content is 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more. Furthermore, if Ni is intentionally included, it is preferable that the Ni content be 0.200% or more.

[0039] Sn: 0.005~0.040% Sn (tin) is a useful element for ensuring low iron loss in non-oriented electrical steel sheets by segregating on the surface of the base material and suppressing oxidation and nitriding during annealing. In addition, Sn also has the effect of improving the texture by segregating at grain boundaries and increasing the magnetic flux density of non-oriented electrical steel sheets. To obtain these effects, the Sn content should be 0.005% or more. Preferably, the Sn content should be 0.010% or more, and more preferably 0.015% or more. On the other hand, if the Sn content is too high, the toughness of the steel decreases, making cold rolling difficult. Therefore, the Sn content should be 0.040% or less. Preferably, the Sn content should be less than 0.040%, more preferably 0.035% or less, and even more preferably 0.030% or less.

[0040] Sb: 0~0.040% Antimony (Sb), like tungsten (Sn), is a useful element for ensuring low iron loss in non-oriented electrical steel sheets by segregating on the surface of the base material and suppressing oxidation and nitriding during annealing. Sb also improves the texture by segregating at grain boundaries, thereby increasing the magnetic flux density of the non-oriented electrical steel sheet. Therefore, Sb may be included as needed. However, excessive Sb content can reduce the toughness of the steel, making cold rolling difficult. For this reason, the Sb content should be 0.040% or less. Preferably, the Sb content is 0.030% or less. To reliably obtain the above effects, the Sb content should be 0.005% or more, and more preferably 0.010% or more.

[0041] In the chemical composition of the base material of the non-oriented electrical steel sheet of the present invention, the remainder is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of steel due to raw materials such as ore and scrap, and various factors in the manufacturing process, and which are acceptable within a range that does not adversely affect the present invention.

[0042] Furthermore, there are no specific restrictions on the content of Cr and Mo as impurity elements. In the non-oriented electrical steel sheet according to this embodiment, the properties of the non-oriented electrical steel sheet according to this embodiment are not particularly affected even if these elements are included in a range of 0.5% or less each. Similarly, the properties of the non-oriented electrical steel sheet according to this embodiment are not particularly affected even if Ca and Mg are included in a range of 0.002% or less each. The properties of the non-oriented electrical steel sheet according to this embodiment are not particularly affected even if rare earth elements (REM) are included in a range of 0.004% or less. In this embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements.

[0043] Although oxygen (O) is also an impurity element, its presence in the range of 0.035% or less does not affect the properties of the non-oriented electrical steel sheet according to this embodiment. Since oxygen can also be mixed into the steel during the annealing process, its presence in the slab stage (i.e., ladle value) in the range of 0.010% or less does not particularly affect the properties of the non-oriented electrical steel sheet according to this embodiment.

[0044] In addition to the elements mentioned above, impurity elements such as Pb, Bi, As, B, and Se may be present, but as long as their content is within the range of 0.0050% or less, it will not impair the properties of the non-oriented electrical steel sheet according to this embodiment.

[0045] The chemical composition of the base material of the non-oriented electrical steel sheet according to this embodiment can be determined using various known measurement methods. For example, it can be measured using ICP emission spectrometry, gravimetric analysis, or spark discharge emission spectrometry. In addition, C and S can be measured using combustion-infrared absorption spectrometry, N can be measured using inert gas combustion-thermal conductivity spectrometry, and O can be measured using inert gas fusion-nondispersive infrared absorption spectrometry.

[0046] Furthermore, in this embodiment, from the viewpoint of suppressing nitriding in the surface layer of the steel sheet, the N content [N]s from the surface of the base material to a depth of 20 μm is set to 0.0060% or less. If [N]s is 0.0060% or less, it is possible to suppress iron loss deterioration. [N]s is preferably 0.0055% or less, and more preferably 0.0050% or less.

[0047] The nitrogen content [N]s from the surface of the base material to a depth of 20 μm is measured by the following procedure. First, the nitrogen content [N]1 of the non-oriented electrical steel sheet, from which the insulating coating has been removed by heated alkaline solution, is measured. Next, 20 μm of each surface of the non-oriented electrical steel sheet is removed by chemical polishing, and the nitrogen content [N]2 of the sample after removal is measured. Then, [N]s is calculated from the measured [N]1, [N]2, and the thickness t (μm) of the non-oriented electrical steel sheet using the following formula. [N]s=(t×[N]1-(t-40)×[N]2) / 40

[0048] 3.Crystal grain size In this embodiment, the average grain size of the base material is set to 50 to 120 μm. By setting the average grain size of the base material to 50 μm or more, it is possible to suppress the deterioration of hysteresis loss and improve magnetic properties. On the other hand, by setting the average grain size to 120 μm or less, it is possible to improve the strength of the steel and suppress the deterioration of iron loss due to increased eddy current loss. The average grain size is preferably 60 μm or more, and more preferably 70 μm or more. Furthermore, the average grain size is preferably 110 μm or less, and more preferably 100 μm or less.

[0049] In this invention, the average grain size of the base material shall be determined in accordance with JIS G 0551:2013 "Microscopic Test Method for Grain Size of Steel".

[0050] 4. Magnetic properties In the non-oriented electrical steel sheet according to this embodiment, excellent magnetic properties refer to iron loss W. 10 / 400 This means that the saturation magnetic flux density (Bs) is low and high.

[0051] Here, iron loss W 10 / 400 This refers to the iron loss that occurs under the conditions of a maximum magnetic flux density of 1.0 T and a frequency of 400 Hz, and shall be measured in accordance with the Epstein method specified in JIS C 2550-1:2011. In this invention, the iron loss W is 10 / 400 A low W / kg value is defined as 14.5 W / kg or less for plate thicknesses of 0.26 mm or more, 12.5 W / kg or less for plate thicknesses of 0.21 to 0.25 mm, and 11.0 W / kg or less for plate thicknesses of 0.20 mm or less.

[0052] Furthermore, the saturation magnetic flux density Bs is measured using a vibrating sample magnetometer (VSM). In the non-oriented electrical steel sheet according to this embodiment, the magnetic flux density Bs is 1.945T or higher regardless of the sheet thickness.

[0053] 5. Mechanical properties The non-oriented electrical steel sheet according to this embodiment has high strength, specifically a tensile strength of 600 MPa or more. A tensile strength of 605 MPa or more is preferred, and 610 MPa or more is more preferred. Here, the tensile strength is measured by performing a tensile test in accordance with JIS Z 2241:2011.

[0054] 6. Plate thickness In the non-oriented electrical steel sheet according to this embodiment, the sheet thickness is set to 0.10 mm or more from the viewpoint of manufacturing costs for cold rolling and finish annealing. On the other hand, from the viewpoint of reducing iron loss, the sheet thickness is set to 0.30 mm or less. Therefore, the sheet thickness of the non-oriented electrical steel sheet according to this embodiment is 0.10 to 0.30 mm.

[0055] Furthermore, the effect of nitriding on the surface layer of the steel sheet becomes more pronounced as the sheet thickness decreases. In other words, if nitriding on the surface layer of the steel sheet is not suppressed, the iron loss deteriorates more significantly as the sheet thickness decreases. However, in the present invention, nitriding on the surface layer of the steel sheet is suppressed, so the deterioration of iron loss can be suppressed even when the sheet thickness is thin. Therefore, the effects of the present invention are more pronounced when the sheet thickness is, for example, less than 0.25 mm or 0.20 mm or less.

[0056] 7. Insulating coating In the non-oriented electrical steel sheet according to this embodiment, it is preferable that the base material surface has an insulating coating. Since the non-oriented electrical steel sheet is used after the core blank has been punched out and then laminated, providing an insulating coating on the surface of the base material can reduce eddy currents between the sheets, thereby reducing eddy current losses as a core.

[0057] The type of insulating coating is not particularly limited, and known insulating coatings used as insulating coatings for non-oriented electrical steel sheets can be used. Examples of such insulating coatings include composite insulating coatings mainly composed of inorganic materials and further containing organic materials. Here, a composite insulating coating is an insulating coating mainly composed of at least one of the following: metal chromate salts, metal phosphate salts, or inorganic materials such as colloidal silica, Zr compounds, or Ti compounds, with fine organic resin particles dispersed in it. In particular, from the viewpoint of reducing the environmental burden during manufacturing, which has been a growing need in recent years, insulating coatings using metal phosphate salts, Zr or Ti coupling agents, or carbonates or ammonium salts thereof as starting materials are preferably used.

[0058] The amount of insulating coating applied is not particularly limited, but for example, 200-1500 mg / m² per side. 2 It is preferable to use a concentration of approximately 300-1200 mg / m² per side. 2 It is more preferable to do so. By forming the insulating film so that the amount of adhesion falls within the above range, it is possible to maintain excellent uniformity. When measuring the amount of insulating film adhesion afterward, various known measurement methods can be used. For example, a method of measuring the mass difference before and after immersion in a sodium hydroxide aqueous solution, or a fluorescence X-ray method using a calibration curve can be used as appropriate.

[0059] 8. Manufacturing method The non-oriented electrical steel sheet according to this embodiment can be manufactured by sequentially performing a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process, a cold rolling process, and a finish annealing process on a steel ingot having the chemical composition described above, under the conditions shown below. Furthermore, if an insulating film is to be formed on the surface of the base material, an insulating film formation process is performed after the finish annealing process. Each process will be described in detail below.

[0060] <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 sheet. There is no particular requirement for the heating temperature of the steel ingot before hot-rolling, but it is preferable to set it to, for example, 1050 to 1250°C. There is also no particular requirement for the thickness of the hot-rolled sheet after hot-rolling, but it is preferable to set it to, for example, about 1.5 to 3.0 mm, taking into account the final thickness of the base material.

[0061] <Acid washing process> The hot-rolled sheet described above is subjected to pickling to remove the scale layer formed on the surface of the base material. Here, the pickling conditions, such as the concentration of the acid used, the concentration of the accelerator used, and the temperature of the pickling solution, are not particularly limited and can be known pickling conditions.

[0062] <Batch-type hot-rolled sheet annealing process> Subsequently, hot-rolled sheet annealing is performed with the aim of reducing iron loss in the steel sheet. Hot-rolled sheet annealing is carried out using a batch annealing furnace, with a soaking temperature of 650-780°C and a soaking time of 8-36 hours. By soaking for 8 hours or more, the metal structure is sufficiently homogenized, the coarsening of precipitates progresses, and a sufficient improvement in iron loss can be obtained. On the other hand, if the soaking temperature exceeds 780°C or the soaking time exceeds 36 hours, the grain size becomes coarser, the toughness decreases, and fracture occurs during cold rolling.

[0063] The batch annealing furnace maintains a non-oxidizing atmosphere, which can be a mixed atmosphere of H2 and N2 with an H2 content of 1 to 100 volume percent (i.e., H2 + N2 = 100 volume percent). Even in an atmosphere containing N2, nitriding of the surface layer of the base material can be suppressed if the Mn and Sn content is appropriate. However, from the viewpoint of more reliably suppressing nitriding, an atmosphere with H2:100 volume percent is preferred.

[0064] <Cold rolling process> Cold rolling is performed on the steel sheet after annealing the hot-rolled sheet described above. In cold rolling, the sheet is rolled at a reduction ratio such that the final thickness of the base material is 0.10 to 0.30 mm.

[0065] <Finishing Annealing Process> Following the cold rolling described above, finish annealing is performed. In the method for manufacturing non-oriented electrical steel sheets according to this embodiment, it is preferable to use a continuous annealing furnace for finish annealing. Finish annealing is performed under conditions of a soaking temperature of 880 to 1020°C and a soaking time of 1 second to 10 minutes. It is preferable to use a mixed atmosphere of H2 and N2 with an H2 proportion of 1 to 100 volume percent (i.e., H2 + N2 = 100 volume percent) and a dew point of the atmosphere of -50 to +10°C.

[0066] If the soaking temperature is below 880°C, the grain size becomes finer and the iron loss deteriorates, which is undesirable. If the soaking temperature exceeds 1020°C, not only is the strength insufficient, but nitriding occurs in the surface layer, which also deteriorates the iron loss, making it undesirable. Furthermore, if the soaking time is less than 1 second, sufficient grain growth cannot occur. On the other hand, if the soaking time exceeds 10 minutes, it leads to an increase in manufacturing costs.

[0067] <Insulating film formation process> Following the above-mentioned finish annealing, an insulating coating formation process is carried out as needed. Here, the method for forming the insulating coating is not particularly limited, and the treatment solution may be applied and dried using a known method using a known insulating coating forming solution as shown below. Examples of known insulating coatings include composite insulating coatings mainly composed of inorganic materials and further containing organic materials.

[0068] A composite insulating coating is an insulating coating that mainly consists of at least one of the following: a metal salt such as a metal chromate salt or a metal phosphate salt, or an inorganic substance such as colloidal silica, a Zr compound, or a Ti compound, with fine organic resin particles dispersed within it. In particular, from the viewpoint of reducing the environmental burden during manufacturing, which has been a growing need in recent years, insulating coatings using a metal phosphate salt, a Zr or Ti coupling agent as a starting material, or insulating coatings using a carbonate or ammonium salt of a metal phosphate salt, a Zr or Ti coupling agent as a starting material are preferably used.

[0069] The surface of the base material on which the insulating coating is formed may be subjected to any pretreatment before applying the treatment solution, such as degreasing with an alkali or pickling with hydrochloric acid, sulfuric acid, or phosphoric acid. Alternatively, the treatment solution may be applied to the surface of the base material directly after finish annealing without any of these pretreatments.

[0070] The non-oriented electrical steel sheet obtained as described above has excellent properties such as high saturation magnetic flux density, low iron loss, and high strength, making it suitable as a material for both rotor cores and stators.

[0071] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. [Examples]

[0072] Slabs having the chemical composition shown in Table 1 were heated to 1150°C, then hot-rolled at a finishing temperature of 850°C and a finishing thickness of 2.0 mm, and wound at 600°C to obtain hot-rolled steel sheets. After removing scale from the obtained hot-rolled steel sheets by pickling, they were annealed in a batch annealing furnace at the soaking temperature and soaking time shown in Table 2. The resulting steel sheets were then cold-rolled to a thickness of 0.20 mm. Furthermore, finish annealing was performed in a mixed atmosphere of H2:20%, N2:80%, and dew point:-30°C at the soaking temperature and soaking time shown in Table 2 for 20 seconds. After finish annealing, an insulating coating consisting of aluminum phosphate and an acrylic-styrene copolymer resin emulsion with a particle size of 0.2 μm was applied to the steel sheets and baked in air at 350°C.

[0073] [Table 1]

[0074] [Table 2]

[0075] For each test material obtained, the average grain size was measured in a cross-section parallel to the rolling direction of the base material, in accordance with JIS G 0551:2013 "Microscopic test method for grain size of steel". In addition, Epstein test specimens were taken from the rolling direction and width direction of each test material, and the iron loss W was measured by Epstein testing in accordance with JIS C 2550-1:2011. 10 / 400 The saturation magnetic flux density was evaluated using a vibrating sample magnetometer (VSM).

[0076] Next, JIS No. 5 tensile test specimens were taken from each test material in accordance with JIS Z 2241:2011, with the longitudinal direction coinciding with the rolling direction of the steel plate. Then, tensile tests were performed using these specimens in accordance with JIS Z 2241:2011, and the tensile strength was measured.

[0077] Furthermore, the nitrogen content [N]s from the surface of the base material to a depth of 20 μm was measured using the following procedure. First, the nitrogen content [N]1 of the non-oriented electrical steel sheet, from which the insulating coating was removed by heated alkaline solution, was measured. Next, 20 μm of each surface of the non-oriented electrical steel sheet was removed by chemical polishing, and the nitrogen content [N]2 of the sample after removal was measured. Then, [N]s was calculated from the measured [N]1, [N]2, and the thickness t (μm) of the non-oriented electrical steel sheet using the following formula. [N]s=(t×[N]1-(t-40)×[N]2) / 40

[0078] The results mentioned above are shown in Table 2.

[0079] Tests No. 2, 3, 6, 7, 9, 11, 15-17, 20, 21, 26, 27, 29, and 30, which satisfy the provisions of the present invention, were found to have low iron loss, high saturation magnetic flux density, and high tensile strength of 600 MPa or more.

[0080] In contrast to these, in comparative studies No. 1, 4, 5, 8, 10, 12-14, 18, 19, 22-25, 28, and 31-33, iron loss W 10 / 400Either the material is inferior, the saturation magnetic flux density is poor, or the toughness deteriorates significantly, making manufacturing difficult.

[0081] Specifically, in Test No. 1, the Mn content was lower than the specified range, resulting in a large amount of fine MnS precipitation and poor iron loss. In Test No. 4, the S content was higher than the specified range, resulting in a large amount of MnS precipitation and poor iron loss. In Test No. 5, the Sn content was lower than the specified range, resulting in a high [N]s content and poor iron loss. In Test No. 8, the Sn content was higher than the specified range, causing toughness to deteriorate and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0082] In Test No. 10, the Mn content was higher than the specified range, resulting in a high [N]s value and inferior iron loss. In Test No. 12, the Si + Al + 0.5 × Mn content was higher than the specified range, resulting in inferior saturation magnetic flux density. In Test No. 13, the Si content and Si + Al + 0.5 × Mn content were higher than the specified range, leading to degraded toughness and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0083] In Test No. 14, the hot-rolled sheet annealing temperature was higher than the specified range, resulting in degraded toughness and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties. In Test No. 18, the hot-rolled sheet annealing temperature was lower than the specified range, resulting in a smaller average grain size after finish annealing and inferior iron loss. In Test No. 19, the finish annealing temperature was lower than the specified range, resulting in a smaller average grain size and inferior iron loss. In Test No. 22, the finish annealing temperature was higher than the specified range, resulting in a larger average grain size and a higher [N]s value, leading to inferior iron loss and inferior strength.

[0084] In Test No. 23, the Si content was below the specified range, resulting in inferior tensile strength. In Test No. 24, the Si + Al + 0.5 × Mn content was below the specified range, resulting in inferior iron loss. Furthermore, in Test No. 25, the Al content was below the specified range, resulting in a smaller average grain size after finish annealing and inferior iron loss. Finally, in Test No. 28, the Al content was above the specified range, leading to degraded toughness and fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0085] In Test No. 31, the soaking time during hot-rolled sheet annealing was longer than specified, resulting in grain coarsening and degraded toughness, causing fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties. In Test No. 32, the soaking temperature during hot-rolled sheet annealing was higher than specified, resulting in grain coarsening and degraded toughness, causing fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties. Furthermore, in Test No. 33, the Al content was lower than specified, resulting in a degraded texture and precipitation of fine AlN, leading to poor iron loss. [Industrial applicability]

[0086] As described above, according to the present invention, non-oriented electrical steel sheets with high strength and excellent magnetic properties can be obtained at low cost.

Claims

1. The chemical composition of the base material is, in mass percent, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: Less than 0.200% Ni: Less than 0.500%, Sn: 0.005-0.040%, Sb: 0 to 0.040%, The remainder consists of Fe and impurities. The following equation (i) is satisfied, The N content [N]s from the surface of the base material to a depth of 20 μm is 0.0060% or less. The average grain size of the base material is 50 to 120 μm. The saturation magnetic flux density is 1.945 T or higher. The tensile strength is 600 MPa or more. The plate thickness is 0.10 to 0.30 mm. Non-oriented electrical steel sheet. 4.2≦Si+Al+0.5×Mn≦4.9...(i) However, the element symbols in the above formula represent the content (mass %) of each element.

2. The surface of the base material has an insulating coating, The non-oriented electrical steel sheet according to claim 1.

3. A method for manufacturing a non-oriented electrical steel sheet according to claim 1 or claim 2, In mass percent, C: 0.0050% or less, Si: more than 3.70% and less than 4.60%, Mn: more than 0.20% and less than 0.50%, Al: 0.23-0.75%, P: 0.030% or less, S: 0.0018% or less, N: 0.0040% or less, Ti: Less than 0.0050%, Nb: Less than 0.0050% Zr: Less than 0.0050%, V: Less than 0.0050%, Cu: Less than 0.200% Ni: Less than 0.500%, Sn: 0.005-0.040%, Sb: 0 to 0.040%, The remainder consists of Fe and impurities. For a steel ingot having a chemical composition satisfying the following equation (i), The process involves, in order, a hot rolling process, a pickling process, a batch-type hot-rolled sheet annealing process at a soaking temperature of 650 to 780°C for 8 to 36 hours, a cold rolling process to reduce the sheet thickness to 0.10 to 0.30 mm, and a finish annealing process at a soaking temperature of 880 to 1020°C for 1 second to 10 minutes. A method for manufacturing non-oriented electrical steel sheets. 4.2≦Si+Al+0.5×Mn≦4.9...(i) However, the element symbols in the above formula represent the content (mass %) of each element.

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