Non-oriented electromagnetic steel sheet

Abstract

This non-oriented electromagnetic steel sheet has a chemical composition that contains, in terms of mass%: at most 0.0050% of C; 2.40-5.00% of Si; 0.10-1.50% of Mn; more than 0.010% but less than 0.030% of sol. Al; at most 0.15% of P; at most 0.0050% of S; 0.0010-0.0050% of N; 0.0006-0.0050% of Ti; 0-0.080% of Sn; 0-0.080% of Sb; 0-0.0050% of V; 0-0.0050% of Zr; 0-0.0050% of Nb; 0-0.10% of Cr; 0-0.10% of Cu; 0-0.10% of Ni; 0-0.05% of Mo; 0-0.0050% of B; 0-0.0100% of Mg; 0-0.0100% of Ca; 0-0.0100% of REM; and 0-0.0100% of Bi, with the balance consisting of Fe and impurities.

Description

Non-oriented electrical steel sheet

[0001] The present invention relates to a non-oriented electrical steel sheet. The present application claims priority based on Japanese Patent Application No. 2025-067710 filed in Japan on April 16, 2025, the content of which is incorporated herein by reference.

[0002] In recent years, for the purpose of global warming countermeasures, in the automotive field, vehicles driven by motors, such as hybrid vehicles with excellent fuel efficiency and pure electric vehicles, have been increasingly popularized. In other fields as well, efficiency regulations for motors such as air conditioners and refrigerators, and efficiency regulations for various motors used in industrial applications are becoming increasingly strict. Non-oriented electrical steel sheets are functional materials used as core materials for motors, and are required to have low iron loss and high magnetic flux density in order to improve motor efficiency.

[0003] As a means of reducing the iron loss of non-oriented electrical steel sheets, there is a means of containing alloying elements such as Si, and in the case of high-grade non-oriented electrical steel sheets, Si is generally contained at around 3 mass%. However, when Si is contained excessively, there is a problem that the magnetic flux density decreases (deteriorates). Therefore, in non-oriented electrical steel sheets, the content of alloying elements such as Si is adjusted according to the required balance between iron loss and magnetic flux density.

[0004] For example, Patent Document 1 discloses a non-oriented electrical steel sheet having both excellent high-frequency iron loss and magnetic flux density, and a method for manufacturing the same. Patent Document 1 shows that good magnetic properties can be obtained by adding appropriate amounts of Si, Al, and Mn in combination, reducing the sheet thickness, and appropriately controlling the average crystal grain size.

[0005] Japanese Unexamined Patent Publication No. 11-222653

[0006] On the other hand, with non-oriented electrical steel sheets, increasing the content of Si and other alloying elements leads to an increase in surface defects that occur during the manufacturing process. Since surface defects must be removed before the product is shipped, the formation of surface defects during the manufacturing process leads to a decrease in yield and an increase in manufacturing processes such as finishing. In addition, if the coil becomes smaller after surface defects are removed, the frequency of coil replacement in the motor core (iron core) pressing process increases, which also degrades the productivity of the pressing process. Furthermore, if the coil becomes excessively small after surface defects are removed and does not meet the coil unit weight specifications agreed upon with the customer, it may have to be discarded without being able to be shipped, which can lead to a decrease in yield and thus a decrease in manufacturing costs. In contrast to this, while Patent Document 1 considers improving magnetic properties, it does not mention anything about reducing surface defects.

[0007] Therefore, in view of the above problems, the present invention aims to provide a non-oriented electrical steel sheet with low iron loss and suppressed (low frequency of) surface defect formation.

[0008] To address the above-mentioned problems, the inventors focused on the relationship between the occurrence of surface defects and the chemical composition of non-oriented electrical steel sheets, assuming a sol. Al content of more than 0.010% and less than 0.030%, and conducted extensive research. As a result, they discovered that by including 0.0006 to 0.0050% of Ti, an element generally known to adversely affect magnetic properties, it is possible to suppress the generation of surface defects while keeping the increase in iron loss low.

[0009] The present invention has been made in view of the above findings. The gist of the present invention is as follows: [1] A non-oriented electrical steel sheet according to one aspect of the present invention has the following composition in mass%, C: 0.0050% or less, Si: 2.40 to 5.00%, Mn: 0.10 to 1.50%, sol. Al: greater than 0.010% and less than 0.030%, P: 0.15% or less, S: 0.0050% or less, N: 0.0010 to 0.0050%, Ti: 0.0006 to 0.0050%, Sn: 0 to 0.080%, Sb: 0 to 0.080%, V: 0 to 0.0050%, Zr: 0 to 0.0050%, Nb: 0 to 0.0050% It has a chemical composition consisting of 0-0.10% Cr, 0-0.10% Cu, 0-0.10% Ni, 0-0.05% Mo, 0-0.0050% B, 0-0.0100% Mg, 0-0.0100% Ca, 0-0.0100% REM, 0-0.0100% Bi, and the remainder being Fe and impurities.

[0010] According to the above aspects of the present invention, it is possible to obtain a non-oriented electrical steel sheet with low iron loss and suppressed (low frequency of) surface defect formation.

[0011] This figure shows the relationship between the frequency of surface defects and the Ti content, based on Example 1. This figure shows the relationship between iron loss W10 / 400 (iron loss when excited at magnetic flux density: 1.0T, frequency: 400Hz) and the Ti content, based on Example 1.

[0012] The following describes a preferred embodiment of the present invention: a non-oriented electrical steel sheet (a non-oriented electrical steel sheet according to this embodiment) and a method for manufacturing the same. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, in the numerical limit range indicated by "~" below, the values ​​at both ends are included in the range as the lower and upper limits. On the other hand, numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Unless otherwise specified, "%" for the content of each element means "mass%". The non-oriented electrical steel sheet according to this embodiment has the chemical composition described later, and is excellent in terms of yield and manufacturing cost, and also has low iron loss, making it suitable as a non-oriented electrical steel sheet for motors that require high efficiency.

[0013] (Chemical Composition) The reason for limiting the chemical composition of the non-oriented electrical steel sheet according to this embodiment will be explained.

[0014] C: 0.0050% or less. Carbon (C) is generally present as an impurity and is an element that degrades magnetic properties. Therefore, the C content should be 0.0050% or less. Preferably, the C content is 0.0045% or less, more preferably 0.0040% or less, and even more preferably 0.0036% or less. On the other hand, a lower C content is preferable, so it may be 0%, but since it is not easy to achieve a C content of 0% industrially, it may be greater than 0%. Also, reducing the C content increases the time required for decarburization in the steelmaking process, which increases costs. Therefore, preferably, the C content is 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0012% or more.

[0015] Si: 2.40-5.00% Si is an effective element for increasing the resistivity of steel sheets and reducing iron loss. Therefore, the Si content should be 2.40% or more. Preferably, the Si content is 2.85% or more, and more preferably 3.35% or more. On the other hand, if the Si content is excessive, the magnetic flux density will decrease significantly. Therefore, the Si content should be 5.00% or less. Preferably, the Si content is 4.50% or less, more preferably 4.15% or less, and even more preferably 4.10% or less.

[0016] Mn: 0.10-1.50% Mn is an effective element for increasing the resistivity of steel sheets and reducing iron loss. Therefore, the Mn content should be 0.10% or more. Preferably, the Mn content is 0.20% or more, and more preferably 0.30% or more. On the other hand, Mn has a higher alloy cost compared to Si and Al. Therefore, the Mn content should be 1.50% or less. Preferably, the Mn content is 1.00% or less. Even more preferably, it is 0.75% or less.

[0017] Sol. Al: greater than 0.010% and less than 0.030%. Al is an effective element for increasing the resistivity of steel sheets and reducing iron loss. Therefore, in the non-oriented electrical steel sheet according to this embodiment, the sol. Al (acid-soluble Al) content is greater than 0.010%. On the other hand, from a cost standpoint, the sol. Al content is less than 0.030%.

[0018] P: 0.15% or less. Although phosphorus (P) is generally present as an impurity, it is also a solid solution strengthening element, and if the P content is excessive, the steel sheet hardens, making cold rolling difficult. For this reason, the P content should be 0.15% or less. Preferably, the P content is 0.10% or less, and more preferably 0.07% or less. On the other hand, the P content may be 0%, but considering refining costs, etc., the P content may be 0.005% or more or 0.01% or more.

[0019] S: 0.0050% or less. S is generally present as an impurity and combines with Mn in the steel to form fine MnS, which inhibits the growth of crystal grains during annealing and degrades the magnetic properties of non-oriented electrical steel sheets. For this reason, the S content should be 0.0050% or less. Preferably, the S content is 0.0045% or less, and more preferably 0.0040% or less. On the other hand, a lower S content is preferable, so it may be 0%, but considering refining costs, etc., it may be greater than 0%, such as 0.0001% or more, or 0.0003% or more.

[0020] N: 0.0010 to 0.0050% N is generally known to be an element that degrades magnetic properties, and conventionally, it has been common to control the N content at a low level. However, as a result of the inventors' research, it has become clear that if the N content is reduced too much, the occurrence of surface defects in the steel sheet increases. In other words, it has become clear that by including N to an extent that does not degrade magnetic properties, the occurrence of surface defects can be reduced, and a new finding has been obtained that there is a range of N content that allows for a balance between magnetic properties and manufacturing costs. To obtain this effect, the N content is set to 0.0010% or more. The N content is preferably 0.0015% or more, and more preferably 0.0020% or more. On the other hand, N is also an element that combines with Al to form fine AlN, inhibits the growth of crystal grains during annealing, and degrades magnetic properties. For this reason, the N content is set to 0.0050% or less. The N content is preferably 0.0045% or less, and more preferably 0.0040% or less.

[0021] Ti: 0.0006 to 0.0050% Ti is generally known to be an element that degrades magnetic properties, and conventionally, it has been common to control the Ti content at a low level. However, as a result of the inventors' research, it has become clear that if the Ti content is reduced too much, the occurrence of surface defects in the steel sheet increases. In other words, it has become clear that the occurrence of surface defects can be reduced by including Ti to an extent that does not degrade the magnetic properties, and that there is a range of Ti content that allows for a balance between magnetic properties and manufacturing costs. For this reason, the non-oriented electrical steel sheet according to this embodiment contains a certain amount of Ti. Specifically, the Ti content is 0.0006% or more. Preferably, the Ti content is 0.0010% or more, and more preferably 0.0015% or more. On the other hand, if the Ti content exceeds 0.0050%, the above effect saturates, and the deterioration of magnetic properties becomes significant. For this reason, the Ti content is 0.0050% or less. The Ti content is preferably 0.0045% or less, and more preferably 0.0040% or less.

[0022] In the non-oriented electrical steel sheet according to this embodiment, the chemical composition basically contains Si, Mn, sol, Al, N, and Ti, and optionally one or more of C, P, and S, with the remainder being Fe and impurities. On the other hand, even if one or more of Sn, Sb, V, Zr, Nb, Cr, Cu, Ni, Mo, B, Mg, Ca, REM (rare earth elements), and Bi are included in place of a portion of the remaining Fe, the effect of reducing surface defects will not be impaired. However, since including any of these elements will increase the alloy cost, if they are included, the content of each should be within the following ranges. Furthermore, it is preferable that the total content of these elements be 1.00% or less. The Sn content should be 0.080% or less. Preferably, it should be 0.065% or less. More preferably, it should be 0.045% or less or 0.040% or less. The Sb content should be 0.080% or less. Preferably, it is 0.055% or less. More preferably, it is 0.035% or less. The V content is 0.0050% or less. Preferably, it is 0.0040% or less. More preferably, it is 0.0030% or less. The Zr content is 0.0050% or less. Preferably, it is 0.0040% or less. More preferably, it is 0.0030% or less. The Nb content is 0.0050% or less. Preferably, it is 0.0040% or less. More preferably, it is 0.0030% or less. The Cr content is 0.10% or less. Preferably, it is 0.07% or less. More preferably, it is 0.04% or less. The Cu content is 0.10% or less. Preferably, it is 0.07% or less. More preferably, it is 0.04% or less. The Ni content is 0.10% or less. Preferably, it is 0.07% or less. More preferably, it is 0.04% or less. The Mo content is 0.05% or less. Preferably, it is 0.03% or less. More preferably, it is 0.01% or less. The B content is 0.0050% or less. Preferably, it is 0.0040% or less. More preferably, it is 0.0030% or less. The Mg content is 0.0100% or less. Preferably, it is 0.0050% or less. More preferably, it is 0.0030% or less. The Ca content is 0.0100% or less. Preferably, it is 0.0050% or less.More preferably, the content is 0.0030% or less. The REM content is 0.0100% or less. Preferably, it is 0.0050% or less. More preferably, it is 0.0030% or less. The Bi content is 0.0100% or less. Preferably, it is 0.0050% or less. More preferably, it is 0.0030% or less. These elements do not need to be included, so the lower limit of the content of each element is 0%. Furthermore, within the above ranges, these elements may be added arbitrarily, or they may be introduced from the raw materials or mixed in as impurities during the manufacturing process.

[0023] Remainder: Fe and impurities The remainder of the chemical composition, other than the elements mentioned above, is Fe and impurities. That is, in the non-oriented electrical steel sheet according to this embodiment, the chemical composition may contain Si, Mn, sol. Al, N, and Ti, with the remainder being Fe and impurities, or it may contain Si, Mn, sol. Al, N, and Ti, and further contain one or more selected from the group consisting of C, P, S, Sn, Sb, V, Zr, Nb, Cr, Cu, Ni, Mo, B, Mg, Ca, REM, and Bi, with the remainder being Fe and impurities. The impurities are usually 0.50% or less in total, but preferably 0.10% or less.

[0024] The above chemical composition is that of a non-oriented electrical steel sheet that does not include insulating coatings, etc. That is, if the non-oriented electrical steel sheet has a base steel sheet and a coating formed on the surface of the base steel sheet, the above chemical composition is that of the base steel sheet. If the non-oriented electrical steel sheet has an insulating coating on its surface, the insulating coating is removed from both the front and back surfaces by grinding, and the sample is used for measuring the chemical composition.

[0025] (Grain size) In the non-oriented electrical steel sheet according to this embodiment, the metal structure is not limited, but from the viewpoint of obtaining better magnetic properties, the average grain size is preferably 30 to 200 μm. More preferably 40 to 190 μm, even more preferably 50 to 180 μm, and even more preferably 60 to 170 μm.

[0026] The average grain size is determined by the following method: On a cross section perpendicular to the thickness of a non-oriented electrical steel sheet, parallel to the rolling direction, lines 5 mm long are drawn parallel to the sheet surface at three locations: the center of the sheet thickness and two locations at the 1 / 4 thickness mark. The number of grain boundaries crossing each of these lines is measured. The sum of the number of grain boundaries crossing each line is divided by the total measurement length (15 mm) to obtain L. Using this L, the average grain size D is calculated as D = 1.11 × L.

[0027] (Sheet Thickness) In the non-oriented electrical steel sheet according to this embodiment, the sheet thickness is not limited, but it is known that iron loss decreases as the sheet thickness decreases. For this reason, a sheet thickness of 0.50 mm or less is preferred. The sheet thickness is more preferably 0.35 mm or less, and even more preferably 0.30 mm or less. On the other hand, the thinner the sheet thickness, the lower the productivity of the motor core. For this reason, a sheet thickness of 0.10 mm or more is preferred. The sheet thickness is more preferably 0.15 mm or more.

[0028] Regarding plate thickness, if the nominal plate thickness is clearly defined, the nominal plate thickness shall be considered as the plate thickness of the non-oriented electrical steel sheet. On the other hand, if the nominal plate thickness is not clearly defined, the plate thickness may be measured using a micrometer.

[0029] (Magnetic Properties) The non-oriented electrical steel sheet according to this embodiment has the above configuration, which provides excellent magnetic properties. When considering application to various products, it is preferable that the iron loss W10 / 400 when excited at a magnetic flux density of 1.0 T and a frequency of 400 Hz is within the following ranges for each sheet thickness: When the sheet thickness exceeds 0.30 mm: W10 / 400 is 25.0 W / kg or less When the sheet thickness exceeds 0.27 mm and is 0.30 mm or less: W10 / 400 is 17.0 W / kg or less When the sheet thickness exceeds 0.25 mm and is 0.27 mm or less: W10 / 400 is 16.0 W / kg or less When the sheet thickness exceeds 0.20 mm and is 0.25 mm or less: W10 / 400 is 14.0 W / kg or less When the sheet thickness is 0.20 mm or less: W10 / 400 is 13.0 W / kg or less

[0030] W10 / 400 is measured in accordance with the method for measuring the magnetic properties of electrical steel strips using an Epstein tester as specified in JIS C 2550-1:2011.

[0031] (Insulating Coating) The non-oriented electrical steel sheet according to this embodiment may have an insulating coating on its surface. That is, the non-oriented electrical steel sheet according to this embodiment may have a base steel sheet and an insulating coating formed on the surface of the base steel sheet. The insulating coating may be a known coating. For example, the insulating coating may be a coating consisting only of an organic chemical composition, a coating consisting only of an inorganic chemical composition, or an insulating coating consisting of an organic-inorganic composite. From the viewpoint of reducing environmental impact, it is preferable that the insulating coating does not contain chromium. Furthermore, the insulating coating may be an insulating coating that exhibits adhesive properties when heated and pressurized. Examples of insulating coatings that exhibit adhesive properties include coatings containing acrylic resin, phenolic resin, epoxy resin, or melamine resin.

[0032] <Manufacturing Method> Below, an example of a manufacturing method for non-oriented electrical steel sheets according to this embodiment will be described. As long as the non-oriented electrical steel sheet according to this embodiment has the above-described configuration, the manufacturing method is not particularly limited. The manufacturing method described below is one example for manufacturing non-oriented electrical steel sheets according to this embodiment, and is a preferred example of a manufacturing method for non-oriented electrical steel sheets according to this embodiment.

[0033] The non-oriented electrical steel sheet according to this embodiment can be manufactured by a manufacturing method having the following steps: (I) Hot rolling step (II) Hot rolled sheet annealing step (III) Pickling step (IV) Cold rolling step (V) Finish annealing step Furthermore, when manufacturing a non-oriented electrical steel sheet with an insulating coating on the surface, the following step may also be included: (VI) Insulating coating formation step Preferred conditions for each step will be described below. For conditions and steps not described, known conditions may be applied.

[0034] (Hot Rolling Process) In the hot rolling process, hot-rolled steel sheets are obtained by hot-rolling a steel ingot or steel billet (hereinafter also referred to as "slab") having the above chemical composition. In hot rolling, slabs obtained by general methods such as continuous casting or ingot rolling are charged into a heating furnace, heated, held, and then hot-rolled. In this case, if the temperature of the slab is high, hot rolling may be performed without charging it into a heating furnace. The various conditions for hot rolling are not particularly limited.

[0035] (Hot-rolled sheet annealing process) In the hot-rolled sheet annealing process, annealing (hot-rolled sheet annealing) is performed on the hot-rolled steel sheet as needed. By performing hot-rolled sheet annealing, even better magnetic properties can be obtained in non-oriented electrical steel sheets. Hot-rolled sheet annealing may be performed by either box annealing or continuous annealing. When performing by box annealing, it is preferable to hold the sheet in the temperature range of 700 to 900°C for 1 to 100 hours. When performing by continuous annealing, it is preferable to hold the sheet in the temperature range of 850 to 1100°C for 1 to 180 seconds. Other conditions for hot-rolled sheet annealing are not particularly limited. Furthermore, the hot-rolled sheet annealing process may be omitted.

[0036] (Pickling Process) In the pickling process, scale formed on the surface of hot-rolled steel sheets used in the cold-rolling process is removed by pickling. The pickling process may be performed after the hot-rolling process and before the cold-rolling process. If the hot-rolled steel sheets are subjected to hot-rolled sheet annealing, the pickling may be performed either before or after the hot-rolled sheet annealing. The pickling conditions are not limited and may be those known.

[0037] (Cold Rolling Process) In the cold rolling process, the hot-rolled steel sheet, after the pickling process or the hot-rolled sheet annealing process, is cold-rolled to obtain a steel sheet (cold-rolled steel sheet) with the final finished thickness (the same thickness as the final product). The conditions for cold rolling, such as the steel sheet temperature and rolling roll diameter, are not particularly limited and can be appropriately selected depending on the chemical composition of the hot-rolled steel sheet and the desired thickness of the steel sheet. If the final finished thickness is thin, or if the steel sheet has a high alloying element content and therefore high deformation resistance, the steel sheet may be cold-rolled once to a certain thickness (intermediate thickness), then subjected to intermediate annealing, and then cold-rolled again to the final finished thickness. The intermediate thickness can be appropriately selected depending on the chemical composition of the steel sheet and the final finished thickness. When performing intermediate annealing by box annealing, it is preferable to hold the sheet at a temperature range of 700 to 900°C for 1 to 100 hours. When performing continuous annealing, it is preferable to maintain the temperature in the range of 850 to 1100°C for 1 to 180 seconds.

[0038] (Finish Annealing Process) In the finish annealing process, the steel sheet obtained by the cold rolling process (cold-rolled steel sheet) is heated to a finish annealing temperature within a predetermined temperature range and held therefor to perform finish annealing. When the average grain size is to be 30 to 200 μm, it is preferable to set the finish annealing temperature to 900 to 1200°C. By setting the finish annealing temperature to 900°C or higher, the average grain size can be set to 30 μm or higher. On the other hand, by setting the finish annealing temperature to 1200°C or lower, excessive grain growth can be suppressed, and the average grain size can be set to 200 μm or lower. The time for holding in the temperature range of 900 to 1200°C (finish annealing time) is not particularly limited, but it is preferable to set it to 1 second or more in order to more reliably obtain good magnetic properties. On the other hand, from the viewpoint of productivity, it is preferable to set the finish annealing time to 120 seconds or less. Other conditions for finish annealing are not particularly limited.

[0039] (Insulating Coating Formation Process) In the insulating coating formation process, after the finish annealing process, an insulating coating is formed on the surface of the steel sheet according to a general method. The insulating coating may be a coating consisting only of organic chemical composition, a coating consisting only of inorganic chemical composition, or an insulating coating consisting of an organic-inorganic composite. From the viewpoint of reducing environmental impact, it is preferable that the insulating coating does not contain chromium. Furthermore, the insulating coating may be an insulating coating that exhibits adhesive properties when heated and pressurized. Examples of insulating coatings that exhibit adhesive properties include coatings containing acrylic resin, phenolic resin, epoxy resin, or melamine resin. An insulating coating can be formed by applying a coating liquid that will become an insulating coating to the surface of the steel sheet and drying it.

[0040] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are just one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0041] (Example 1) A slab with a thickness of 260 mm was produced by continuous casting. The obtained slab was heated to 1140°C, held for 6 hours, and then hot-rolled to obtain a hot-rolled steel sheet with a thickness of 1.9 mm. These hot-rolled steel sheets were subjected to hot-rolled sheet annealing by continuous annealing, in which the sheets were held at a temperature of 970°C for 25 seconds, followed by pickling. The pickled hot-rolled steel sheet was cold-rolled to obtain a steel sheet (cold-rolled steel sheet) with a width of 1140 mm and a sheet thickness of 0.25 mm. These steel sheets were subjected to finish annealing in which they were held at a finish annealing temperature of 1000°C for 15 seconds. After the finish annealing, a chromium-free insulating coating was formed on the surface of the steel sheet to obtain a non-oriented electrical steel sheet having the insulating coating on the surface.

[0042] The chemical composition of the obtained non-oriented electrical steel sheet was measured. The results are shown in Table 1. The chemical composition here is an analysis result using a measurement sample from which the insulating coating has been removed by grinding the surface (both the front and back surfaces) of the non-oriented electrical steel sheet having the insulating coating on the surface. In the analysis, ICP-emission spectrometry was used for Si, Mn, sol. Al, and P, ICP-mass spectrometry for Ti, combustion-infrared absorption method for C and S, and inert gas fusion-thermal conductivity method for N.

[0043] Further, the average crystal grain size D of the obtained non-oriented electrical steel sheet was determined by the following method. On a vertical cross-section in the sheet thickness direction parallel to the rolling direction of the non-oriented electrical steel sheet, at a total of three positions: the center in the sheet thickness direction, and positions at 1 / 4 of the sheet thickness (1 / 4 thickness positions) from the front surface and the back surface respectively, a 5 mm line parallel to the sheet surface was drawn at each position, and the number of grain boundaries crossing each line was measured. The value obtained by dividing the total measured length (15 mm) by the total number of grain boundaries crossing each line was defined as L. Using this L, the average crystal grain size D was determined as D = 1.11 × L. As a result, the average crystal grain size was in the range of 0.080 to 0.110 mm (80 to 110 μm).

[0044] Further, for the obtained non-oriented electrical steel sheet, magnetic properties and the occurrence status of surface flaws were evaluated in the following manner.

[0045] (Magnetic Properties) The magnetic properties were evaluated using W10 / 400, which is the iron loss when excited at a magnetic flux density of 1.0 T and a frequency of 400 Hz. Specifically, 16 test pieces each with dimensions of 30 mm in the direction perpendicular to the rolling direction of the coil (C direction) and 280 mm in the longitudinal direction of the coil (L direction), and 16 test pieces each with dimensions of 30 mm in the longitudinal direction of the coil (L direction) and 280 mm in the direction perpendicular to the rolling direction of the coil (C direction) were sampled from the non-oriented electrical steel sheet. For these 32 test pieces, W10 / 400 was measured using a 25 cm Epstein tester specified in JIS C 2550-1:2011. Here, the density used for iron loss measurement was 7.60 g / cm ３ was used. In this example, when W10 / 400 was 14.0 W / kg or less, it was judged that the iron loss was low.

[0046] (Surface Defects) By observing the surfaces of the steel sheet (both the front surface and the back surface), the number of surface defects occurring per 5000 m in the length direction (rolling direction) was investigated. Specifically, by visual inspection, defects with a width of 1 mm or more and a length of 10 mm or more were detected, and the defects that could be felt by touching with a fingertip were determined as surface defects to be counted. It was judged that when the number of surface defects was 20 pieces / 5000 m or less, the occurrence of surface defects was suppressed, and when the number was 10 pieces / 5000 m or less, the occurrence of surface defects was more preferably suppressed.

[0047] The results are shown in Table 1. Fig. 1 shows the relationship between the occurrence frequency of surface defects and the Ti content, and Fig. 2 shows the relationship between iron loss W10 / 400 and Ti content. The circles ○ are the plots of A1 to A5 in Table 1.

[0048]

[0049] As can be seen from Table 1, Fig. 1 and Fig. 2, when the Ti content or N content was low, the number of surface defects generated was high. On the other hand, when the Ti content or N content was high, the iron loss W10 / 400 was inferior.

[0050] (Example 2) A slab with a thickness of 240 mm was produced by continuous casting. The obtained slab was heated to 1160°C and held for 5 hours, then hot-rolled to obtain a hot-rolled steel sheet with a thickness of 2.0 mm. These hot-rolled steel sheets were subjected to continuous annealing, holding at a temperature of 1030°C for 10 seconds, followed by pickling. After pickling, the hot-rolled steel sheets were cold-rolled to obtain steel sheets (cold-rolled steel sheets) with a width of 970 to 1250 mm and a thickness of 0.25 mm, 0.27 mm, or 0.30 mm. These steel sheets were subjected to finish annealing, holding at a finish annealing temperature of 960°C for 50 seconds. After finish annealing, a chromium-free insulating coating was formed on the surface of the steel sheet to obtain a non-oriented electrical steel sheet with an insulating coating on the surface.

[0051] The chemical composition of the obtained non-oriented electrical steel sheets was measured. The chemical composition here is the result of analysis using measurement samples obtained by grinding the surface (both front and back surfaces) of non-oriented electrical steel sheets that had an insulating coating on their surface to remove the insulating coating. For the analysis, ICP-emission spectrometry was used for Si, Mn, sol. Al, P, Sn, Sb, Cr, Cu, and Ni; ICP-mass spectrometry was used for Ti, V, Zr, Nb, and Mo; combustion-infrared absorption spectrometry was used for C and S; and inert gas fusion-thermal conductivity spectrometry was used for N. The results are shown in Table 2.

[0052] Furthermore, the grain size of the obtained non-oriented electrical steel sheet was measured in the same manner as in Example 1. As a result, the average grain size was 85 to 117 μm.

[0053] The surface defects of the obtained non-oriented electrical steel sheets were evaluated in the same manner as in Example 1. Regarding magnetic properties, the non-oriented electrical steel sheets with a thickness of 0.25 mm were evaluated in the same manner as in Example 1. For the non-oriented electrical steel sheets with thicknesses of 0.27 mm and 0.30 mm, 14 test pieces were taken with dimensions of 30 mm in the direction perpendicular to the coil's rolling direction (C direction) and 280 mm in the coil's longitudinal direction (L direction), and another 14 test pieces with dimensions of 30 mm in the coil's longitudinal direction (L direction) and 280 mm in the direction perpendicular to the coil's rolling direction (C direction). For these 28 test pieces, the iron loss was evaluated by measuring W10 / 400 using a 25 cm Epstein tester as specified in JIS C 2550-1:2011. Here, the density used for measuring iron loss was 7.60 g / cm³. ３ The following was used. In this example, low iron loss was determined if the W10 / 400 was 17.0 W / kg or less for a steel plate with a thickness of 0.30 mm, 16.0 W / kg or less for a steel plate with a thickness of 0.27 mm, and 14.0 W / kg or less for a steel plate with a thickness of 0.25 mm. The results are shown in Table 3.

[0054]

[0055]

[0056] As can be seen from Tables 2 and 3, surface defects in the steel sheets were reduced when the Ti and N content were within the specified range. On the other hand, when the Ti or N content was excessively low, as in No. K and No. L, many surface defects occurred in the steel sheets.

[0057] (Example 3) A slab with a thickness of 230 mm was produced by continuous casting. The obtained slab was heated to 1170°C and held for 4 hours, then hot-rolled to obtain a hot-rolled steel sheet with a thickness of 1.7 mm. After pickling these hot-rolled steel sheets, hot-rolled sheet annealing was performed by holding the sheet at a temperature of 820°C for 10 hours using box annealing. These hot-rolled steel sheets were cold-rolled to obtain steel sheets (cold-rolled steel sheets) with a width of 1010 to 1240 mm and a thickness of 0.20 mm or 0.25 mm. Finish annealing was performed on these steel sheets by holding them at a finish annealing temperature of 990°C for 70 seconds. After finish annealing, a chromium-free insulating coating was formed on the surface of the steel sheet to obtain a non-oriented electrical steel sheet with an insulating coating on the surface.

[0058] The chemical composition of the obtained non-oriented electrical steel sheets was measured. The chemical composition here is the result of analysis using measurement samples obtained by grinding the surface (both front and back surfaces) of non-oriented electrical steel sheets that had an insulating coating on their surface to remove the insulating coating. For the analysis, ICP-emission spectrometry was used for Si, Mn, sol. Al, P, Sn, Sb, and B; ICP-mass spectrometry for Ti, Ca, Mg, REM, and Bi; combustion-infrared absorption spectrometry for C and S; and inert gas fusion-thermal conductivity spectrometry for N. The results are shown in Table 4.

[0059] Furthermore, the grain size of the obtained non-oriented electrical steel sheet was measured in the same manner as in Example 1. As a result, the average grain size was 120 to 132 μm.

[0060] The surface defects of the obtained non-oriented electrical steel sheets were evaluated in the same manner as in Example 1. Regarding magnetic properties, the non-oriented electrical steel sheets with a thickness of 0.25 mm were evaluated in the same manner as in Example 1. For the non-oriented electrical steel sheets with a thickness of 0.20 mm, 18 test pieces were taken with dimensions of 30 mm in the direction perpendicular to the coil's rolling direction (C direction) and 280 mm in the coil's longitudinal direction (L direction), and another 18 test pieces with dimensions of 30 mm in the coil's longitudinal direction (L direction) and 280 mm in the direction perpendicular to the coil's rolling direction (C direction). For these 36 test pieces, the iron loss was evaluated by measuring W10 / 400 using a 25 cm Epstein tester as specified in JIS C 2550-1:2011. Here, the density used for measuring iron loss was 7.60 g / cm³.３ In this example, a steel plate with a thickness of 0.25 mm was judged to have low iron loss if its W10 / 400 was 14.0 W / kg or less, and a steel plate with a thickness of 0.20 mm was judged to have low iron loss if its W10 / 400 was 13.0 W / kg or less. The results are shown in Table 5.

[0061]

[0062]

[0063] As can be seen from Tables 4 and 5, surface defects in the steel sheets were reduced when the Ti and N content were within the specified range. On the other hand, as in Nos. S and T, when the Ti or N content was excessively low, many surface defects occurred in the steel sheets.

[0064] According to the present invention, it is possible to obtain a non-oriented electrical steel sheet with low iron loss and suppressed (low frequency of) surface defect formation.

Claims

1. In mass%, C: 0.0050% or less, Si: 2.40-5.00%, Mn: 0.10-1.50%, sol. Al: greater than 0.010%, less than 0.030%, P: 0.15% or less, S: 0.0050% or less, N: 0.0010 to 0.0050%, Ti: 0.0006 to 0.0050%, Sn: 0 to 0.080%, Sb: 0 to 0.080%, V: 0 to 0.0050%, Zr: 0 to 0.0050%, Nb: 0 to 0.0050%, Cr: 0 to 0.10%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Mo: 0 to 0.05%, B: 0 to 0.0050%, Mg: 0 to 0.0100%, Ca: 0 to 0.0100% A non-oriented electrical steel sheet characterized by having a chemical composition consisting of REM: 0 to 0.0100%, Bi: 0 to 0.0100%, and the remainder: Fe and impurities.

Images