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

Abstract

The non-oriented electromagnetic steel sheet contains prescribed chemical components, has a tensile strength of 580 MPa or more, an average crystal grain size of a recrystallized portion of a steel base of 50 μm or less, and, among inclusions contained in the steel base, the number of inclusions having a Mg content of more than 5 mass% and a Mn content of 5 mass% or more is 5 times or more compared with the number of inclusions having a Mg content of 5 mass% or less and a Mn content of 5 mass% or more.

Description

Technical Field

[0001] This invention relates to non-oriented electromagnetic steel sheets and methods for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2021-61872, filed on March 31, 2021, the contents of which are incorporated herein by reference. Background Technology

[0002] In recent years, the increasing energy efficiency of electrical equipment worldwide has placed higher demands on non-oriented electromagnetic steel sheets used in motors, requiring higher performance characteristics. In particular, the need for reduced iron loss has intensified, leading to increased Si or Al content, improved inherent resistance, and larger crystal grain size, thereby achieving lower iron loss.

[0003] Patent document 1 discloses a non-oriented electromagnetic steel sheet with lower iron loss that eliminates the nitriding problem that is significant in steel sheets that de-toxicize sulfides and promote grain growth.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2005-330527 Summary of the Invention

[0007] The technical problem that the invention aims to solve

[0008] The use of electric motors in hybrid electric vehicles is conducted in environments accompanied by stress changes caused by speed variations corresponding to acceleration and deceleration operations, vehicle body vibration, and magnet vibration in the magnet insertion hole. Therefore, high strength suitable for high-speed rotation or stress concentration areas, as well as fatigue characteristics under repeated stress, i.e., high fatigue strength, are required.

[0009] However, in high-strength non-oriented electromagnetic steel sheets with tensile strengths of 580 MPa or higher, fatigue strength that matches the strength is sometimes not achieved. The objective of this invention is to provide a non-oriented electromagnetic steel sheet possessing a tensile strength of 580 MPa or higher and excellent fatigue strength.

[0010] Technical means for solving technical problems

[0011] The inventors of this invention have conducted in-depth research on the fatigue strength of non-oriented electromagnetic steel sheets. The results show that in electromagnetic steel sheets with low fatigue strength, the deformation of soft MnS becomes the initiation point for cracks. Furthermore, in electromagnetic steel sheets with high fatigue strength, the amount of MnS in inclusions is relatively low.

[0012] Further research revealed that fatigue strength was improved by making the sulfides in the inclusions Mg-containing, further hardened inclusions.

[0013] This invention is based on the above insights and further research, and its purpose is as follows.

[0014] (1) A non-oriented electromagnetic steel sheet according to one aspect of the present invention, characterized in that, by mass%, it contains Si: 2.5-4.5%, sol.Al: 0-2.0%, Mn: 0.1-3.5%, C: 0-0.0030%, P: 0-0.10%, S: 0-0.0030%, N: 0-0.050%, O: 0-0.050%, Mg: 0.0003-0.0050%, Ti: 0-0.0030%, V: 0-0.10%, Sb: 0-0.10%, Nd: 0-0.10%, Bi: 0-0.10%, W: 0-0.10%, Nb: 0-0.10%, Y: 0-0.10%. And selected from one or more of the group consisting of Ni: 0-0.5%, Cr: 0-0.5%, Cu: 0-0.5%, Sn: 0-0.2%, La: 0-0.0050%, and Ce: 0-0.0050%, with the remainder being Fe and impurities, having a tensile strength of 580 MPa or more, having an average crystal grain size of 50 μm or less in the recrystallized portion of the steel matrix, and having an equivalent circle diameter of 1 μm or more and an S content of 5% by mass or more in the steel matrix, having a number of inclusions with a Mg content of 5% or less and a Mn content of 5% or more than 5% by mass, and having a Mg content of 5% or more than 5% by mass, having a Mg content of 5% or more than 5% by mass, having a Mn content of 5% or more than 5% by mass, having a Mg content of 5% or more than 5% by mass, having a Mg content of 5% or more than 5% by mass, having a Mn content of 5% or more than 5% by mass, having a Mg content of 5% or more than 5% by mass, having a Mg content of 5% or more than 5% by mass, having a Mg content of 5% or more than 5% by mass, having an Mn ...

[0015] (2) In the non-oriented electromagnetic steel sheet described in (1) above, the number density of inclusions with an equivalent circle diameter of 5 μm or more may be less than 1.0 inclusions / mm. 2 .

[0016] (3) Alternatively, the non-oriented electromagnetic steel plate described in (1) above may have a thickness of less than 0.30 mm.

[0017] (4) Another embodiment of the present invention is a method for manufacturing a non-oriented electromagnetic steel sheet, which is a method for manufacturing the non-oriented electromagnetic steel sheet described in (1) above, characterized in that it includes: a process of manufacturing a steel billet by casting, a process of heating the steel billet, a process of hot rolling the heated steel billet to form a hot-rolled steel sheet, a process of coiling the hot-rolled steel sheet, a process of cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet, and a process of finally annealing the cold-rolled steel sheet to obtain a non-oriented electromagnetic steel sheet; in the casting, the cooling rate of 1300°C to 1200°C is set to 50°C / s or less; in the heating of the steel billet, the dwell time at a center temperature of 1100°C or higher is set to less than 2 hours (excluding 0); in the coiling process, the coiling temperature of the hot-rolled steel sheet is set to 700°C or higher; and in the final annealing, the maximum temperature reached is set to 700 to 900°C.

[0018] Invention Effects

[0019] According to the present invention, a non-oriented electromagnetic steel sheet with a tensile strength of 580 MPa or more and excellent fatigue strength can be obtained. Detailed Implementation

[0020] The embodiments of the present invention will now be described in detail.

[0021] In a non-oriented electromagnetic steel sheet according to one embodiment of the present invention, fatigue strength is improved by making the sulfides in the inclusions in the steel matrix further hardened inclusions containing Mg. In the present invention, further hardened inclusions containing Mg are defined as follows: In elemental concentration analysis based on Energy Dispersive X-ray Spectrometry (hereinafter referred to as "EDS"), when quantifying all detected elements (excluding C), inclusions in which Mg is detected at more than 5% by mass and S is detected at more than 5% by mass are defined as "further hardened inclusions containing Mg". In the present invention, among the above-mentioned further hardened inclusions containing Mg, inclusions with an equivalent circle diameter of 1 μm or more are considered.

[0022] Furthermore, among inclusions with an equivalent circle diameter of 1 μm or more and an S content of 5% or more by mass, the number of inclusions with a Mg content of less than 5% by mass and an Mn content of 5% or more by mass is set to be more than 5 times the number of inclusions with a Mg content of more than 5% by mass and an Mn content of more than 5% by mass.

[0023] As a result, the hardening of inclusions and the occurrence of inclusion deformation leading to the initiation of kerf cracks are reduced, and fatigue strength is improved. Preferably, among inclusions in the steel matrix with an equivalent circle diameter of 1 μm or more and an S content of 5% by mass or more, the number of inclusions with a Mg content higher than 5% by mass and an Mn content of 5% by mass or more is at least 10 times that of inclusions with a Mg content of 5% or less and an Mn content of 5% or more.

[0024] Inclusion analysis was performed using SEM to observe the entire thickness of the plate. To obtain a suitable observation area, the observation surface could be prepared by tilting the plate during grinding or by stacking multiple steel plates. The steel plate was cut to include both the width (rolling perpendicular direction) and thickness direction as the observation surface, and then resin was applied. Next, the observation surface of the resin-filled steel plate was ground. The observation area can be varied depending on the number of inclusions present, but is typically set at 5 mm. 2 The above describes the determination of inclusion size. Inclusion size is measured using image analysis software, and the measurement is performed based on the diameter converted to an equivalent circle. Inclusions can be analyzed using EDS (Electronic Data Separation), and the composition of each inclusion is defined based on the average value of the entire block.

[0025] In each field of view, the number of inclusions with an equivalent circle diameter of 1.0 μm or more and an S content of 5% by mass or more was determined. Specifically, firstly, inclusions in each field of view were identified based on contrast. Among the identified inclusions, elemental concentration analysis (EDS analysis) was performed on inclusions with an equivalent circle diameter of 1.0 μm or more. Quantification was performed on all detected elements (excluding C) to identify inclusions with an equivalent circle diameter of 1.0 μm or more, a Mg content of less than 5% by mass and a Mn content of 5% by mass or more, and inclusions with an equivalent circle diameter of 1.0 μm or more, a Mg content higher than 5% by mass and a Mn content of 5% by mass or more.

[0026] 5mm 2 Among the inclusions identified within the field of view of the observed area above, those with an equivalent circle diameter of 1.0 μm or more, an S content of 5% or more by mass and an Mn content of 5% or more by mass are identified. The number of inclusions with a Mg content of less than 5% by mass and the number of inclusions with a Mg content of more than 5% by mass are determined, and their ratios are calculated.

[0027] The number density (numbers / mm²) is calculated based on the total area of ​​the observed field of view. 2 When calculating the number density, the third decimal place of the obtained value is rounded. Furthermore, the number density is determined using a device that enables component analysis of a scanning electron microscope (SEM-EDS device).

[0028] Furthermore, the presence of large inclusions can become the initiation point of cracks, sometimes reducing the fatigue limit. Therefore, it is preferable to have fewer large inclusions. Specifically, the number density of inclusions with an equivalent circle diameter of 5 μm or more is preferably less than 1.0 inclusions / mm. 2 .

[0029] There are no particular limitations on the method of setting the number density of inclusions with an equivalent circle diameter of 5 μm or more to be less than 1.0 inclusions / mm2. However, for example, the following methods can be used: during the period when the molten steel is in the tundish, the inclusions are made to float to the surface of the molten steel; in continuous casting, the electromagnetic brake is applied to the molten steel in the mold to increase the time when the molten steel is in the mold, so that the inclusions float to the surface of the molten steel; or a mold that is long in the vertical direction is used to increase the time when the molten steel is in the mold, so that the inclusions float to the surface of the molten steel, etc., and then the floated inclusions are removed.

[0030] Furthermore, by producing inclusions containing Mg as specified in this invention, it is also helpful to set the number density of inclusions with an equivalent circle diameter of 5 μm or more to less than 1.0 inclusions / mm. 2 Large inclusions with an equivalent circumference diameter of 5 μm or more are considered, for example, Al₂O₃ or MnS. Of Al and Mg, Mg has a higher deoxidizing power than Al. Therefore, in this invention, which flexibly utilizes Mg, Al₂O₃ is easily reduced secondaryly during the steelmaking stage to generate MgO. If Al₂O₃ is kept in its original state, it has a high specific gravity and is easily retained in the molten steel, while MgO has a low specific gravity and is easily floated, thus making it easier to remove. Furthermore, regarding MnS, Mg is more likely to form sulfides than Mn, and it also precipitates more readily at high temperatures than MnS. Therefore, the amount of MnS precipitation is reduced. Moreover, the uniformly dispersed MgS becomes the precipitation site for the later-precipitated MnS, thereby suppressing the formation frequency of large MnS that are formed as monomers. Therefore, by controlling Mg inclusions, the formation of large inclusions can be suppressed.

[0031] In the non-oriented electromagnetic steel sheet of this embodiment, the microstructure is approximately 100% ferrite. The remaining portion of the microstructure consists of inclusions, etc. Furthermore, the average grain size of the recrystallized portion is 50 μm or less. When the grains are coarse, the strength decreases. Here, the recrystallized portion refers to grains (recrystallized grains) among the ferrite particles with an aspect ratio (length in the rolling direction / length in the thickness direction) of 3 or less. On the other hand, the aspect ratio of the non-recrystallized grains is greater than 3. The grain size of the non-recrystallized grains and the recrystallized grains can be determined by observing the microstructure and obtaining the grain size of the non-recrystallized grains and the recrystallized grains by the following method. A test piece with an observation surface including the rolling direction and the thickness direction of the steel sheet is prepared, and the center of the thickness of the sheet is observed. After grinding the observation surface of the test piece to a mirror finish, it is immersed in a 3% nitric acid ethanol etching solution for 10 seconds, and the texture is revealed by etching. The etched observation surface is observed at 500x magnification using an optical microscope. Grains with an aspect ratio of 3 or less are determined from the etched observation surface, and the average grain size is calculated from this. The average grain size is determined according to JIS G0551:2013 "Steel - Microscopic Test Method for Grain Size". The lower limit of the average grain size of the recrystallized portion does not need to be particularly limited, but if the grains are excessively fine, the average grain size becomes too small, and the shape of the steel sheet may sometimes deteriorate. Therefore, the average grain size of the recrystallized portion is preferably 10 μm or more, more preferably 12 μm or more, and even more preferably 15 μm or more. Furthermore, the proportion of non-recrystallized grains can also be 100%. In cases where no recrystallized grains are observed, the average grain size specified in this invention is set to 0.

[0032] The non-oriented electromagnetic steel sheet of this embodiment has a tensile strength of 580 MPa or higher. Since chemical compositions used to improve tensile strength are known, they can be adjusted appropriately. The chemical composition of the non-oriented electromagnetic steel sheet of this embodiment can, for example, be the chemical composition described later. Furthermore, in order to achieve a tensile strength of 580 MPa or higher, as described later, the final annealing temperature needs to be adjusted.

[0033] Tensile strength was determined using tensile test piece No. 13B according to JIS Z2241:2011.

[0034] The chemical composition of the non-oriented electromagnetic steel sheet of this embodiment is not particularly limited if it is a composition applicable to non-oriented electromagnetic steel sheets with a tensile strength of 580 MPa or higher. Below, examples of preferred chemical compositions for the non-oriented electromagnetic steel sheet of this embodiment are shown. In the description of the chemical composition, "%" indicates "mass %". The non-oriented electromagnetic steel sheet of this embodiment contains, by mass %, Si: 2.5–4.5%, sol.Al: 0–2.0%, Mn: 0.1–3.5%, C: 0–0.0030%, P: 0–0.10%, S: 0–0.0030%, N: 0–0.050%, O: 0–0.050%, Mg: 0.0003–0.0050%, Ti: 0.0030% or less, V: 0–0.10%. Sb: 0-0.10%, Nd: 0-0.10%, Bi: 0-0.10%, W: 0-0.10%, Nb: 0-0.10%, Y: 0-0.10%, and one or more selected from the group consisting of Ni: 0-0.5%, Cr: 0-0.5%, Cu: 0-0.5%, Sn: 0-0.2%, La: 0-0.0050%, and Ce: 0-0.0050%, with the remainder being Fe and impurities.

[0035] Si is an element that increases the strength of steel plates. Furthermore, Si is an element that increases resistivity and is included to reduce iron loss. From the viewpoint of its effects and preventing a decrease in saturation magnetic flux density or embrittlement of the steel, the Si content is preferably 2.5% to 4.5%. The Si content is preferably 2.8% or more, more preferably 3.0% or more. Furthermore, the Si content is more preferably 4.2% or less, and even more preferably 4.0% or less.

[0036] Like Si, sol.Al is an element that increases resistivity and is contained to reduce iron loss. The effect of reducing iron loss can also be achieved through Si, so it is not necessary to include sol.Al. Therefore, the sol.Al content can also be 0%. The sol.Al content can be 0.3% or more, 0.4% or more, 0.5% or more, or 0.6% or more. On the other hand, from the viewpoint of preventing a decrease in saturation magnetic flux density, the sol.Al content is preferably set to 2.0% or less. The sol.Al content is more preferably 1.8% or less, and more preferably 1.5% or less. The sol.Al content can also be 1.2% or less. Here, sol.Al refers to acid-soluble Al that is soluble in acid and has not become an oxide such as Al₂O₃, and is measured as Al obtained by subtracting the insoluble residue on the filter paper generated during the Al analysis process.

[0037] Like Si and sol.Al, manganese (Mn) increases resistivity and is therefore included to reduce iron loss. Furthermore, Mn is an element that improves the strength of steel sheets. From the viewpoint of this effect and preventing a decrease in saturation magnetic flux density or embrittlement of the steel, the Mn content is preferably set at 0.1% to 3.5%. The Mn content is more preferably 0.4% or more, more preferably 0.6% or more, and even more preferably 0.8% or more. The Mn content can also be 0.9% or more, 1.0% or more, or 1.2% or more. Furthermore, the Mn content is more preferably 3.3% or less, more preferably 3.5% or less. The Mn content can also be 3.0% or less.

[0038] Carbon (C) is present as an impurity. To reduce iron loss, the C content is preferably set to 0.0030% or less. The C content is more preferably 0.0025% or less, and even more preferably 0.0020% or less. The lower limit of the C content is not particularly limited, and it can also be 0%, but from the point of view of manufacturing cost, the C content can also be 0.0010% or more.

[0039] Phosphorus (P) is an element that increases the strength of steel plates. The strength of steel plates can also be increased by silicon (Si) or manganese (Mn), so it is possible to omit P. Therefore, the P content can be 0%. The P content can also be 0.01% or more, 0.02% or more, or 0.04% or more. On the other hand, from the viewpoint of preventing embrittlement of the steel plate, the P content is preferably set to 0.10% or less. The P content is more preferably 0.08% or less, and even more preferably 0.06% or less. The P content can also be 0.04% or less.

[0040] Sulfur (S) is present as an impurity. To reduce iron loss, the S content is preferably set to 0.0030% or less. More preferably, it is 0.0025% or less, and even more preferably 0.0020% or less. Furthermore, to form inclusions with an S content of 5% by mass or more, the S content of the steel plate is higher than 0%. The S content can also be 0.0006% or more, or 0.0007% or more.

[0041] Nitrogen (N) is present as an impurity. To reduce iron loss, the N content is preferably set to 0.050% or less. When the N content is 0.050% or less, the formation of excess inclusions or precipitates is suppressed, which can further suppress the decrease in magnetic properties or fatigue strength. The N content can also be 0.0027% or less, 0.0025% or less, or 0.0020% or less. Furthermore, non-oriented electromagnetic steel sheets may not contain N, so the lower limit of the N content can also be 0%, but to suppress the increase in cost due to excess, the N content is preferably set to 0.0010% or more. The N content can also be 0.0014% or more, 0.0017% or more, or 0.0020% or more.

[0042] O is present as an impurity. To reduce iron loss, the O content is preferably set to 0.050% or less. When the O content is 0.050% or less, the formation of excess inclusions or precipitates is suppressed, which can further suppress the reduction of magnetic properties or fatigue strength. The O content can also be 0.0027% or less, 0.0025% or less, or 0.0020% or less. In addition, non-oriented electromagnetic steel sheets can be made to contain no O, so the lower limit of O content can also be 0%, but in order to suppress the increase in cost due to excess, the O content is preferably 0.0010% or more. The O content can also be 0.0014% or more, 0.0017% or more, or 0.0020% or more.

[0043] Mg is an element that reduces iron loss by promoting grain growth, and it also enhances fatigue strength by making sulfides in inclusions harder. To achieve this effect, considering cost, the Mg content is preferably set at 0.0003–0.0050%. A more preferred Mg content is 0.0005% or more, and even more preferred is 0.0010% or more. A more preferred Mg content is 0.0040% or less, and even more preferred is 0.0030% or less.

[0044] Ti is an element present as an impurity. Ti couples with C, N, O, etc., in the steel matrix to form tiny precipitates such as TiN, TiC, and Ti oxides, which hinder grain growth during annealing and deteriorate magnetic properties. Therefore, the Ti content is preferably set to 0.0030% or less. More preferably, it is 0.0020% or less, and even more preferably 0.0010% or less. Ti is not necessary, therefore the lower limit of its content is 0%. Considering refining costs, the Ti content can also be set to 0.0003% or more, or 0.0005% or more.

[0045] The remaining chemical components are Fe and impurities. Impurities are components present in the raw materials or introduced during manufacturing, not components intentionally included in the steel sheet. Examples of impurities include Zn and B.

[0046] Furthermore, the non-oriented electromagnetic steel sheet of this embodiment may also contain V: 0-0.10%, Zr: 0-0.10%, Sb: 0-0.10%, Nd: 0-0.10%, Bi: 0-0.10%, W: 0-0.10%, Nb: 0-0.10%, Y: 0-0.10%, and Ca: 0-0.0050%.

[0047] V and Nb are elements that contribute to increasing the strength of non-oriented electromagnetic steel sheets. V and Nb may be omitted, and their contents can be 0%, but to achieve the aforementioned effect, the contents of V and Nb are preferably 0.0010% or more. The contents of V and Nb can also be 0.0023% or more. On the other hand, if V and N are excessively present, fine precipitates can hinder grain growth, resulting in a disadvantage in iron loss; therefore, the contents of V and Nb are preferably 0.10% or less. The contents of V and Nb are preferably 0.0050% or less.

[0048] Zr, Nd, Bi, W, and Y are elements that reduce fine precipitates and improve grain growth. As a result, productivity is improved. Since these elements can be omitted, the content of each of Zr, Nd, Bi, W, and Y can be 0%, but to achieve the aforementioned effect, the content of each of Zr, Nd, Bi, W, and Y is preferably 0.0010% or more. The content of each of Zr, Nd, Bi, W, and Y is more preferably 0.0015% or more. On the other hand, even if Zr, Nd, Bi, W, and Y are present in excess, the aforementioned effect will saturate; therefore, the content of each of Zr, Nd, Bi, W, and Y is preferably 0.10% or less. The content of each of Zr, Nd, Bi, W, and Y is preferably 0.0010% or less.

[0049] Sb is an element that improves magnetic properties, such as increasing B50. Sb may be absent, and the Sb content may be 0%, but to achieve the aforementioned effects, the Sb content is preferably 0.0050% or more. The Sb content is more preferably 0.01% or more. On the other hand, even if Sb is present in excess, the aforementioned effects will saturate, so the Sb content is preferably 0.10% or less. The Sb content is preferably 0.05% or less.

[0050] In addition to the elements mentioned above, the steel sheet may contain one or more elements selected from the group consisting of Ni, Cr, Cu, Sn, La and Ce, instead of a portion of Fe.

[0051] Ni is an element that increases the electrical resistance of steel plates and reduces iron loss. Ni is not necessarily present; the lower limit for Ni content is 0%. Even trace amounts of Ni can achieve the desired effect, but to reliably obtain the desired effect, the Ni content is preferably 0.01% or more, and more preferably 0.02% or more. From a product cost perspective, the Ni content is preferably 0.5% or less, and more preferably 0.4% or less.

[0052] Cr is an element that improves corrosion resistance and high-frequency properties. Cr is not necessarily present; the lower limit for Cr content is 0%. Even trace amounts of Cr can achieve the desired effect, but to reliably obtain the desired effect, the Cr content is preferably 0.01% or more, and more preferably 0.02% or more. From a product cost perspective, the Cr content is preferably 0.5% or less, and more preferably 0.4% or less.

[0053] Cu is an element that increases the electrical resistance of steel plates and reduces iron loss. The presence of Cu is not necessary; the lower limit for Cu content is 0%. Even trace amounts of Cu can achieve the desired effect, but to reliably achieve this effect, the Cu content is preferably 0.01% or more, and more preferably 0.02% or more. From the perspective of product cost and preventing steel embrittlement, the Cu content is preferably 0.5% or less, and more preferably 0.4% or less.

[0054] Sn is an element that promotes the development of a crystal orientation that is preferred for magnetic properties. The presence of Sn is not necessary; the lower limit for Sn content is 0%. Even trace amounts of Sn can provide the desired effect, but to reliably achieve this effect, the content is preferably 0.01% or more, more preferably 0.02% or more. From the viewpoint of preventing deterioration of magnetic properties, the Sn content is preferably 0.2% or less, more preferably 0.1% or less.

[0055] La (La) is an element that coarsens sulfides, improves grain growth during heat treatment, and contributes to low iron loss. La is not necessary; the lower limit for La content is 0%. Even trace amounts of La can achieve its effect, but to reliably obtain the desired effect, the La content is preferably 0.005% or more, and more preferably 0.0010% or more. From the viewpoint of preventing deterioration of magnetic properties, the La content is preferably 0.0050% or less, and more preferably 0.0030% or less.

[0056] Ce is an element that coarsens sulfides, improves grain growth during heat treatment, and contributes to low iron loss. Ce is not necessarily present; the lower limit for Cu content is 0%. Even trace amounts of Ce can achieve its effect, but to reliably obtain the desired effect, the Ce content is preferably 0.005% or more, more preferably 0.0010% or more. From the viewpoint of preventing deterioration of magnetic properties, the Ce content is preferably 0.0050% or less, more preferably 0.0030% or less.

[0057] In addition to the elements mentioned above, one or more elements selected from the group consisting of As, Ga, Ge, Se, Co, and Pb may be included in the range of 0 to 0.01%, instead of Fe.

[0058] In this embodiment, the thickness of the non-oriented electromagnetic steel sheet is preferably less than 0.30 mm. When the sheet thickness is less than 0.30 mm, the magnetic properties are reduced.

[0059] Next, a method for manufacturing a non-oriented electromagnetic steel sheet according to an embodiment of the present invention will be described.

[0060] First, steel billets with a specified chemical composition are manufactured. The billets are first smelted in a converter or electric furnace, and then further degassed under vacuum as needed to obtain molten steel. The resulting molten steel is then continuously cast, or cast into ingots and rolled into billets with a thickness of approximately 30–400 mm. At this stage, the cooling rate from 1300°C to 1200°C is set to 50°C / s or less. If this cooling rate is too fast, MgS cannot be preferentially produced over MnS, the number of inclusions with Mg content higher than 5% by mass and Mn content higher than 5% by mass decreases, resulting in a decrease in the fatigue strength of the non-oriented electromagnetic steel sheet. The thickness of the billet can also be set to 150 mm or more. Alternatively, the thickness of the billet can be 350 mm or less.

[0061] After manufacturing the steel billet, it is reheated and hot-rolled to produce hot-rolled steel sheet. During the heating of the billet, the dwell time at a center temperature above 1100°C is set to less than 2 hours (excluding 0). If this dwell time is too long, the amount of sulfides with low Mg content increases, resulting in a decrease in the fatigue strength of the non-oriented electromagnetic steel sheet. The hot rolling conditions are not particularly limited. For example, the final rolling temperature can be set to 700–1050°C.

[0062] After hot rolling, the sheet is coiled, annealed, and then cold rolled. The coiling temperature for hot rolling is set to 700°C or higher. For example, the coiling temperature for hot rolling is set to 700–1000°C. When the coiling temperature is below 700°C, S in the solid solution state at that time forms individual MnS, which easily forms MnS without Mg, and is therefore not preferred. Other conditions are not particularly limited. Annealing of the hot-rolled sheet can also be omitted. Annealing of the hot-rolled sheet can be carried out, for example, in continuous annealing at 950°C to 1050°C for 10 seconds to 3 minutes. Cold rolling can be carried out, for example, in a temperature range of room temperature to 300°C with a rolling yield of 70–90%.

[0063] After cold rolling to produce cold-rolled steel sheets, the sheets undergo final annealing to obtain non-oriented electromagnetic steel sheets. To produce non-oriented electromagnetic steel sheets with high tensile strength and excellent fatigue strength, the final annealing is performed at a low temperature. Specifically, the maximum reaching temperature is set to 700–900°C, and the soaking time is set, for example, to 10–60 seconds. Here, the soaking time refers to the time spent holding the maximum reaching temperature at -10°C. A more suitable maximum reaching temperature can be adjusted appropriately based on the chemical composition. By performing final annealing at a low temperature, grain growth is suppressed, and the average grain size of the recrystallized portion is set to below 50 μm, resulting in non-oriented electromagnetic steel sheets with high tensile strength and excellent fatigue strength.

[0064] The non-oriented electromagnetic steel sheet manufactured as described above has a high tensile strength of over 580 MPa and excellent fatigue strength.

[0065] Fatigue strength can be obtained through a pulsating tensile test based on JIS Z2273:2011. Specifically, fatigue test pieces are collected from non-oriented electromagnetic steel sheets with the rolling direction aligned with the tensile direction. The test piece shape adopts JIS 2-15 test piece. After grinding the end faces of the parallel section and the R section with 600 grit abrasive paper, a pulsating tensile test is performed at room temperature and in atmospheric atmosphere. The maximum stress that does not fracture after a stress ratio of 0.10, a frequency of 20 Hz, and 2 million stress load repetitions is defined as the fatigue strength (MPa). In this invention, if the fatigue strength is 450 MPa or higher, it is considered to have excellent fatigue strength.

[0066] An example of a preferred embodiment of the present invention will be described, but the present invention is not limited to the above. The above is an example; any configuration that has substantially the same structure and performs the same effect as the technical concept described in the technical solution of the present invention is included within the technical scope of the present invention. For example, in the manufacturing method of the non-oriented electromagnetic steel sheet of this embodiment, other known manufacturing processes may also be included.

[0067] Example

[0068] The present invention will now be illustrated with specific examples, but it is not limited thereto. Various modifications and variations are readily apparent to those skilled in the art within the scope of the ideas described in the technical solutions, and it should be understood that these also fall within the technical scope of the present invention.

[0069] (Example 1)

[0070] Electromagnetic steel sheets were manufactured using a steel grade with the following chemical composition: Si: 3.3%, sol.Al: 0.7%, Mn: 1.2%, C: 0.002%, P: 0.02%, S: 0.0010%, Ti: 0.0015%, and the remainder being Fe and impurities (without Mg). A steel grade containing 0.0013% Mg was used to replace the Fe in the above steel grades. The tensile strength and fatigue strength were then determined using the methods described above.

[0071] In the manufacturing of electromagnetic steel sheets, the cooling rate of casting from 1300℃ to 1200℃ is set to 30℃ / s, the dwell time of the billet at a center temperature above 1100℃ before hot rolling is set to 1 hour, the coiling temperature is set to 750℃, and the final annealing temperature is varied between 750℃ and 1000℃.

[0072] Table 1 shows the final annealing temperature and the tensile strength and fatigue strength of various electromagnetic steel sheets with different Mg contents. In this experiment, a tensile strength of 580 MPa or higher and a fatigue strength of 450 MPa or higher were considered good. Good results were obtained when the final annealing temperature was set to 750–800 °C for the Mg-added electromagnetic steel sheets with the above chemical composition.

[0073] [Table 1]

[0074]

[0075] (Example 2)

[0076] Non-oriented electromagnetic steel sheets with the compositions shown in Tables 2A and 2B are manufactured with the thicknesses shown in Table 3A under the following conditions: cooling rate during casting, residence time above 1100°C in the heating furnace before hot rolling, coiling temperature, and maximum reaching temperature of final annealing.

[0077] The aforementioned method was used to determine the percentage of inclusions (Mg content less than 5% by mass and Mn content greater than 5%) per 1 mm of the manufactured non-oriented electromagnetic steel sheet. 2 The number of inclusions per 1 mm of "inclusions with Mg content higher than 5% and Mn content higher than 5%" 2 The number of crystals, the average crystal size of the recrystallized part, tensile strength, and fatigue strength.

[0078] [Microscopic Tissue Observation Experiment]

[0079] The microstructure of steel plates from each test number was observed using the following method to determine the grain size of the recrystallized portion of the ferrite structure. Test pieces were prepared with surfaces encompassing both the rolling and thickness directions of each steel plate. The observation surfaces of the test pieces were mirror-polished and then immersed in a 3% nitric acid-ethanol etching solution for 10 seconds. The texture was then revealed through etching. The etched observation surfaces were observed using an optical microscope at 100x magnification in three fields. Regions where the aspect ratio (length in the rolling direction / length in the thickness direction) of the ferrite grains was determined to be less than 3. The average grain size of the ferrite in these regions was determined according to JIS G0551:2013 "Steel – Microscopic Test Method for Grain Size".

[0080] [Test for Determining the Number and Density of Inclusions]

[0081] Eight 0.25mm thick steel plates were overlapped, and the test specimens were filled with resin with the surface including both the width direction (rolling right angle direction) and the thickness direction as the observation surface. The observation surface of the resin-filled test specimens was then ground. The entire thickness area of ​​the overlapping plates was observed using a SEM with EDS. The size of inclusions was measured using image analysis software, and the diameter was calculated and converted to the equivalent circle diameter. (For 5mm...) 2 The scope of observation included all inclusions with an equivalent circle diameter of 1 μm or larger. Furthermore, EDS analysis was performed on inclusions larger than 1 μm, and the number density of inclusions with Mg content between 5% and 5% were determined among inclusions with both S and Mn contents of 5% or larger. The number density (inclusions / mm²) was calculated based on the total area. 2 When calculating the number density of inclusions with an equivalent circle diameter of 1 μm or more, round the third decimal place. For inclusions with an equivalent circle diameter greater than 5 μm, the number density is determined based on the total area, regardless of the composition, only by size. Since no ratio calculation is needed for these inclusions, round the second decimal place to obtain the number density.

[0082] [Magnetic properties]

[0083] The iron loss W10 / 400 of a non-oriented electromagnetic steel plate at a frequency of 400 Hz and a magnetic flux density of 1.0 T was determined using the method for determining the magnetic properties of an electromagnetic steel strip using the Epstein test apparatus as described in JISC2550-1:2011.

[0084] The results are shown in Table 3B. In Table 3B, "Mg: less than 5%" means "inclusions with a Mg content of less than 5% by mass and a Mn content of more than 5% per 1 mm". 2 The number of inclusions, "Mg: higher than 5%" means "inclusions with Mg content higher than 5% and Mn content higher than 5%" per 1 mm 2The number and ratio represent the ratio of the number of inclusions with Mg content higher than 5% and Mn content higher than 5% to the number of inclusions with Mg content lower than 5% by mass and Mn content higher than 5%. Furthermore, "5 μm or more" in Table 3 indicates the number density (inclusions / mm) of inclusions with an equivalent circle diameter of 5 μm or more. 2 ).

[0085] [Table 2A]

[0086]

[0087] [Table 2B]

[0088]

[0089] [Table 3A]

[0090]

[0091] [Table 3B]

[0092]

[0093] According to the present invention, it can be confirmed that a non-oriented electromagnetic steel sheet with a tensile strength of 580 MPa or more and a fatigue strength of 450 MPa or more is obtained.

Claims

1. A non-oriented electromagnetic steel sheet, characterized in that, The chemical composition, by mass%, contains 2.5–4.5% Si. sol.Al: 0-2.0% Mn: 0.1–3.5% C: Below 0.0030% P: below 0.10% S: below 0.0030% N: below 0.050% O: Below 0.050% Mg: 0.0003~0.0050% Ti: below 0.0030% V：0～0.10％、 Sb: 0~0.10% Nd: 0–0.10% Bi: 0-0.10% W：0～0.10％、 Nb: 0–0.10% Y：0～0.10％、 and from Ni: 0-0.5% Cr：0～0.5％、 Cu: 0-0.5% Sn: 0-0.2% La: 0–0.0050%, and Ce: 0–0.0050%. Choose one or more from the groups formed. Remaining components: Fe and impurities. Tensile strength above 580MPa The average crystal grain size of the recrystallized portion of the steel matrix is ​​less than 50 μm. Among the inclusions in the steel matrix with an equivalent circular diameter of 1 μm or more and an S content of 5% by mass or more, the number of inclusions with a Mg content of less than 5% by mass and an Mn content of 5% by mass or more is more than 5 times that of inclusions with a Mg content of more than 5% by mass and an Mn content of more than 5% by mass.

2. The non-oriented electromagnetic steel sheet according to claim 1, characterized in that, The number density of inclusions with an equivalent circle diameter of 5 μm or more is less than 1.0 inclusions / mm. 2 .

3. The non-oriented electromagnetic steel sheet according to claim 1, characterized in that, The plate thickness is less than 0.30 mm.

4. A method for manufacturing a non-oriented electromagnetic steel sheet, comprising the method for manufacturing the non-oriented electromagnetic steel sheet as described in claim 1, characterized in that, have: The process of manufacturing steel billets through casting. The process of heating the steel billet, The process of hot-rolling the heated steel billet to produce hot-rolled steel sheet. The coiling process for coiling the hot-rolled steel sheet. The process of cold-rolling the hot-rolled steel sheet, and the production of cold-rolled steel sheet. The process of finally annealing the cold-rolled steel sheet to obtain a non-oriented electromagnetic steel sheet; In the casting process, the cooling rate from 1300°C to 1200°C is set to less than 50°C / s. In the heating of the steel billet, the dwell time at a center temperature above 1100°C is set to be less than 2 hours and does not include 0. In the coiling process, the coiling temperature of the hot-rolled steel sheet is set to above 700°C. In the final annealing, the maximum temperature reached is set to 700–900°C.

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