Non-directional electromagnetic steel sheet

Abstract

To provide a non-oriented magnetic steel sheet in which nitridation is suppressed, in particular deterioration of iron loss of intermediate frequency is suppressed, in a non-oriented magnetic steel sheet containing much Al and thin in a sheet thickness.SOLUTION: A non-oriented magnetic steel sheet comprising, in mass%, Si: 2.0 to 3.5%, Al: 0.20 to 2.50%, Mn: 0.7% or smaller, and residual Fe and impurities, having a product thickness of 0.35 mm or smaller and a nitrogen content ratio NR that is a ratio of estimated surface layer nitrogen content expressed by a formula (1) relative to an internal nitrogen content satisfying a formula (2). Ns={t×Nt- (t-0.01)×Nm } / 0.01 - formula (1) NR=Ns / Nm≤30. formula (2) [In the formula (1), Nt: a nitrogen content (mass%) in the non-oriented magnetic steel sheet a steel sheet thickness as it is, Nm: a nitrogen content (mass%) contained in a part from a position of 5 μm in a sheet thickness direction from one surface of the steel sheet to a position of 5 μm in the sheet thickness direction from the other surface of the steel sheet, and t: a steel sheet thickness (mm).]SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] This invention relates to non-oriented electrical steel sheets. [Background technology]

[0002] Non-oriented electrical steel sheets are widely used primarily as core materials for rotating machinery. In particular, in response to recent energy conservation demands, there is a need for stable, good magnetic properties, productivity, and cost reduction. Furthermore, the increasing consumption in the transportation equipment sector and the rapid progress of electrification in developing countries have led to a rapid increase in demand for non-oriented electrical steel sheets.

[0003] Since non-oriented electrical steel sheets are used in electrical equipment, their magnetic properties are important. To ensure good iron loss characteristics (low iron loss values) in terms of magnetic properties, high-grade non-oriented electrical steel sheets that have better iron loss characteristics than the steel grades 35A440 and 50A470 specified in JIS C 2552 (2014) have traditionally contained Al (alkaline alloy) in the second highest amount after Si (silicon).

[0004] Furthermore, in recent years, among high-grade non-oriented electrical steel sheets with superior iron loss characteristics, there has been an increase in steel grades containing even more Al. In addition to neutralizing N, Al, like Si, has the effect of improving resistivity, but in recent years, Al content has been adjusted considering cost and the effect of improved characteristics (cost performance). Moreover, in non-oriented electrical steel sheets, there is a strong tendency to reduce the thickness of the steel sheet in order to reduce eddy currents in order to improve iron loss in the higher frequency range. Production technologies for non-oriented electrical steel sheets with high Al content have existed for a long time, and there are many such examples, such as Patent Documents 1 to 6. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 62-054023 [Patent Document 2] Japanese Patent Application Laid-Open No. 03-294426 [Patent Document 3] Japanese Patent Application Laid-Open No. 04-268025 [Patent Document 4] Japanese Patent Application Laid-Open No. 06-248398 [Patent Document 5] Japanese Patent Application Laid-Open No. 2007-262431 [Patent Document 6] Japanese Patent Application Laid-Open No. 2010-174376 [Non-Patent Document]

[0006] [Non-Patent Document 1] Yoshihiko Oda, et al., "Development of Electromagnetic Steel Sheet for High-Efficiency Motor by Ultra-Low S Technology", Transactions of the Institute of Electrical Engineers of Japan, A (Fundamentals, Materials and Common Parts), 2003, Vol. 123, No. 1, pp. 83-88 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] The technologies of Patent Documents 1 to 6 do not focus on the phenomenon of nitridation on the surface of the steel sheet during final annealing.

[0008] Now, the non-oriented electromagnetic steel sheet is manufactured so that its magnetic properties become desired properties by changing the final annealing conditions in addition to the components, the conditions of the intermediate processes during manufacturing, and the cold rolling ratio. The final annealing is basically continuously performed by changing the temperature in a non-oxidizing (dry) atmosphere with a mixed gas of nitrogen and hydrogen. At this time, depending on the conditions, the magnetic properties may be inferior to expectations.

[0009] The present invention has been made in view of the above problems, and the object of the present invention is to provide a non-oriented electromagnetic steel sheet that contains a large amount of Al for improving magnetic properties, particularly for improving eddy current loss, has a thin plate thickness, suppresses nitridation from the surface of the steel sheet during final annealing, and particularly suppresses the deterioration of iron loss at intermediate frequencies. [Means for Solving the Problems]

[0010] The inventors discovered that when producing thin, non-oriented electrical steel sheets, the iron loss at intermediate frequencies deteriorates as the sheet thickness decreases. After diligent investigation, the inventors confirmed that the cause was an increase in the nitrogen content of the steel sheet due to nitriding in the atmosphere, and that this nitrogen deteriorates the magnetic properties, particularly the iron loss at intermediate frequencies.

[0011] Based on the above findings, the gist of the present invention is as follows: [1] In mass percent, Si: 2.0%~3.5% Al: 0.20%~2.50% It contains Mn: 0.7% or less, with the remainder being Fe and impurities. The plate thickness is 0.35 mm or less. This is the ratio of the estimated surface nitrogen content (Ns) to the internal nitrogen content (Nm), as shown by the following formula (1). As shown in equation (2) below Nitrogen content ratio NR 、2.2~30 A non-oriented electrical steel sheet characterized by satisfying the following conditions. Ns={t×Nt-(t-0.01)×Nm} / 0.01 ··· Equation (1) NR = Ns / N m ··· Equation (2) Here, in equation (1) above, Nt: Nitrogen content (mass%) in non-oriented electrical steel sheet with the same thickness. Nm: Nitrogen content (mass%) contained in the portion of the steel plate from a position 5 μm from one side in the thickness direction to a position 5 μm from the other side in the thickness direction. t: Steel plate thickness (mm) That is the case. [2] Furthermore, in mass%, Sn is 0.02 Contains ~0.1% Characterized by possessing Please Non-oriented electrical steel sheet as described in Item 1. [3] Furthermore, the non-oriented electrical steel sheet according to claim 1 or 2 is characterized by containing P: 0.005 to 0.10% by mass.

[0012] Electrical steel sheets are generally used at commercial frequencies of 50 Hz or 60 Hz, but the non-oriented electrical steel sheet according to the present invention is related to the electrical steel strip for intermediate frequencies (400 Hz to 10 kHz) specified in JIS C 2550-3 (2011). The magnetic properties are mainly evaluated at 1.0 T, 400 Hz with an iron loss of W10 / 400. In the following, the magnetic properties at intermediate frequencies may be referred to as intermediate frequency characteristics. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a non-oriented electrical steel sheet with a high Al content and thin thickness that exhibits excellent magnetic properties at intermediate frequencies, and in particular suppresses the deterioration of iron loss at intermediate frequencies. [Brief explanation of the drawing]

[0014] [Figure 1] This figure shows an example of a Scanning Electron Microscope (SEM) image of the thickness cross-section of a non-oriented electrical steel sheet (a), as well as an example of a mapping image showing the distribution of N (b) and an example of a mapping image showing the distribution of Al (c) obtained by Energy Dispersive X-ray Spectroscopy (EDS). [Figure 2] This figure shows an example of a GDS (Grow Discharge Emission Spectroscopy) measurement result illustrating the nitrogen distribution in the thickness direction of the plate. [Figure 3] This figure shows microstructure images (cross-sections in the thickness direction parallel to the rolling direction (L section)) of non-oriented electrical steel sheets with different thicknesses or NRs. [Figure 4] This is a schematic diagram showing the nitrogen content at various points on a steel plate. [Figure 5] This figure shows the relationship between nitrogen content ratio NR and iron loss in W10 / 400. [Figure 6] This figure shows the relationship between the amount of hydrogen in the atmosphere during final annealing, the sulfur content of the steel sheet, and the amount of nitriding of the steel sheet before and after final annealing. [Figure 7]This figure shows the relationship between the amount of hydrogen in the atmosphere during final annealing, the dew point, and the amount of nitriding of the steel sheet before and after final annealing. [Figure 8] This figure shows the relationship between the amount of hydrogen in the atmosphere during final annealing, the partial pressure ratio PH2O / PH2, and the amount of nitriding of the steel sheet before and after final annealing. [Modes for carrying out the invention]

[0015] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

[0016] Non-oriented electrical steel sheets inevitably contain impurities such as N, S, Nb, Ti, and C during their manufacturing process. Elements other than N are not introduced after the steelmaking process. In the final annealing stage, a mixture of nitrogen and hydrogen gas is used in the annealing atmosphere, and it has been suggested that nitrogen may affect the magnetic properties of non-oriented electrical steel sheets.

[0017] Furthermore, in recent years, the electrification of transportation equipment, particularly automobiles, has been actively pursued as a measure against global warming. In this field, since the drive equipment is mounted on the transportation equipment, it is necessary to reduce its weight and improve its efficiency as much as possible. For this reason, the electrical steel sheets that form the iron core of the drive equipment require low iron loss. As is well known, iron loss consists of hysteresis loss and eddy current loss, and technically, efforts are made to reduce both.

[0018] Among the methods for reducing eddy current losses, the main ones are increasing resistivity and reducing plate thickness. To increase resistivity, the content of components, especially Si and Al, is increased. Regarding the reduction of plate thickness, products with a thickness of 0.20 mm, thinner than the JIS standard of 0.35 mm, have been commercialized, and improvements in production technology are being made to include even thinner plates of 0.15 mm. However, the thinner the plate, the greater the effect of the skin effect, so the properties of the steel plate surface affect the magnetic properties.

[0019] On the other hand, hysteresis loss can be improved by reducing impurities, improving the texture, and coarsening the crystal grains.

[0020] These principles and guidelines follow conventional thinking. However, within this framework, we discovered that the "state" of the surface layer of the final product greatly influences the intermediate frequency characteristics of thin materials. In other words, although thin electrical steel sheets have been produced until now, it has been observed that iron loss does not improve as much as expected as the sheet thickness decreases. After diligent investigation, the inventors found that this phenomenon largely depends on the composition of the steel material and the final annealing conditions.

[0021] The inventors of this invention conducted a precise analysis of products with inferior magnetic properties and found the following: Figure 1 shows an example of (a) an SEM image (secondary electron image), (b) an EDS elemental mapping image showing the N distribution state, and (c) an EDS elemental mapping image showing the Al distribution state of a cross-section of a non-oriented electrical steel sheet. The black areas in (a) are AlN. As can be seen from (a) to (c), precipitates mainly composed of AlN were found to be scattered within 5 μm from the surface of the steel sheet. Figure 2 is an example of GDS measurement showing the nitrogen distribution in the thickness direction of the steel sheet before and after nitriding. As shown in Figure 2, it can be seen that there are regions with high nitrogen concentration within 5 μm from both the surface and back surface of the steel sheet after nitriding.

[0022] As can be seen from Figure 2, the inventors discovered that the cause of this is that nitrogen in the annealing atmosphere penetrates the steel sheet, causing it to be nitrided. The inventors then confirmed that the AlN formed on the surface of the steel sheet degrades the magnetic properties, particularly the iron loss at intermediate frequencies. In other words, after diligently investigating and examining this phenomenon, the inventors found that in Al-containing non-oriented electrical steel sheets, nitriding occurs depending on the composition and annealing process conditions, resulting in poor iron loss.

[0023] <Comparison of crystal grain sizes in the thickness direction due to surface nitriding> First, the inventors analyzed steel sheets in detail to understand the actual conditions. Figure 3 shows microstructure photographs (cross-sections in the thickness direction parallel to the rolling direction (L section)) of non-oriented electrical steel sheets with different thicknesses or NRs. (a) shows the case of a steel sheet with a thickness of 0.5 mm and NR=39, (b) shows the case of a steel sheet with a thickness of 0.35 mm and NR=146 (Nt=0.0072%, Nm=0.0012%), and (c) shows the case of a steel sheet with a thickness of 0.35 mm and NR=9.1 (Nt=0.0016%, Nm=0.0016%). Note that NR is the ratio of the estimated surface nitrogen content Ns to the internal nitrogen content Nm, as shown by the following formula (1), although details will be described later.

[0024] Comparing Figures 3(b) and 3(c) regarding the cross-sectional crystalline structure of the steel sheets, no significant difference in the crystalline structure inside the steel sheets due to nitrogen content is observed. However, in the highly nitrided material (b), which has a high nitrogen content on the surface of the steel sheet, the crystal grains on both surfaces are smaller than those in the less nitrided material (c), which has a low nitrogen content on the surface of the steel sheet. (c) is a less nitrided material, and the crystal grains on both surfaces are not smaller than those inside the steel sheet. The steel sheet shown in Figure 3(a) is a polynitrided material with a high nitrogen content on the surface, similar to the steel sheet shown in (b), but it differs in thickness from the steel sheet shown in (b). The final annealing time differs depending on the thickness, and the thicker steel sheet shown in (a) has a longer final annealing time and a higher amount of nitriding. Therefore, the steel sheet shown in (a) has a higher frequency of small crystal grains on the surface compared to the steel sheet shown in (b). However, although the steel sheet shown in (a) has an NR=39, its thickness is 0.50 mm, so the deterioration of intermediate frequency iron loss is small, and it is a reference example not included in the present invention.

[0025] In general, in the field of electrical steel sheets and grain-oriented electrical steel sheets, when evaluating crystal size, the surface grain size is often not considered (see Patent Document 2). This is because the grain size of the surface crystals does not contribute to secondary recrystallization, and the uniformity of the inner layers is slightly more important than that of the surface. However, in non-oriented electrical steel sheets, primary recrystallization is applied instead of secondary recrystallization, so the surface grain state is particularly important for thin materials of 0.35 mm or less.

[0026] Incidentally, while the grain growth of the surface layer of a steel sheet is inferior to that of the inner layer due to the pinning phenomenon of grain boundary movement on the surface itself, the magnetic properties are superior when the grain size is large and uniform throughout the entire thickness of the sheet. Therefore, steel sheets in which the grain size of the surface differs from that of the interior due to nitriding have inferior magnetic properties. In particular, in the case of non-oriented electrical steel sheets used in fields with frequencies higher than commercial frequencies, the effect of surface nitriding is greater because the sheet thickness is thinner than conventional sheets.

[0027] In addition, considering that precipitates affecting the pinning effect on the surface of the steel sheet are also important, we conducted a detailed observation of the steel sheet surface and investigated the relationship between surface precipitates and magnetic properties. We confirmed that surface precipitates also have a significant impact on magnetic properties. As previously mentioned, we found that the cause of this is AlN on the surface, which was formed by nitrogen introduced by nitriding during the final annealing process and Al in the steel sheet.

[0028] As described above, the surface layer is important in thin non-oriented electrical steel sheets for two reasons. One is the suppression of grain growth due to nitriding, and the other is the deterioration of hysteresis loss due to precipitates. It was found that both of these are caused by nitriding during the final annealing process.

[0029] <Nitrogen content ratio NR> Since nitrogen nitrided during the final annealing process is concentrated in the surface layer (up to 5 μm from the surface of the steel sheet), the estimated surface nitrogen content Ns in this region was estimated using the following equation (1). The nitrogen content ratio NR (=Ns / Nm), which is the ratio of the estimated surface nitrogen content Ns to the internal nitrogen content Nm contained in the portion from 5 μm in the thickness direction from one side of the steel sheet to 5 μm in the thickness direction from the other side of the steel sheet, satisfies the following equation (2).

[0030] Ns={t×Nt-(t-0.01)×Nm} / 0.01 ··· Equation (1) NR=Ns / Nm≦30 ··· Formula (2) Here, Nt: Nitrogen content (mass%) of steel plate with its original thickness (total thickness nitrogen content) Nm: Nitrogen content (mass%) (internal nitrogen content) contained in the portion of the steel plate from a position 5 μm in the thickness direction from one side of the steel plate to a position 5 μm in the thickness direction from the other side of the steel plate. t: Steel plate thickness (mm)

[0031] The internal nitrogen content (Nm) can be determined by grinding 5 μm from each side of the steel plate and measuring the N content of the steel plate after grinding. A schematic diagram showing the relationship between the steel plate thickness and nitrogen content is shown in Figure 4.

[0032] Figure 5 shows the relationship between the nitrogen content ratio NR and the iron loss at W10 / 400 for steel sheets manufactured with a nitrogen and hydrogen mixed gas, where the final annealing atmosphere was (a) nitrogen: 100 vol%, hydrogen: 0 vol%, (b) nitrogen: 75 vol%, hydrogen: 25 vol%, (c) nitrogen: 50 vol%, hydrogen: 50 vol%, (d) nitrogen: 25 vol%, hydrogen: 75 vol%, and (e) nitrogen: 0 vol%, hydrogen: 100 vol%,. In detail, Figure 5 shows the relationship between the nitrogen content ratio NR and the iron loss W10 / 400 of steel sheets that were subjected to continuous casting of molten steel with C:0.0014%, Si:3.2%, Al:1.2%, Mn:0.6%, S:0.0009%, and N:0.0018% using a conventional method, hot rolling, hot rolling annealing at 970°C for 70 seconds, pickling, cold rolling to 0.20 mm, 0.25 mm, and 0.30 mm, and final annealing at 1020-1035°C for 35-56 seconds in sequence. The final annealing atmosphere is as described above. In Figure 5, the horizontal axis shows the nitrogen content ratio NR, and the vertical axis shows the iron loss W10 / 400 (W / kg).

[0033] In all cases shown in Figure 5(a) to (d), AlN precipitates were not present at depths greater than 5 μm, suggesting that the effect of AlN is limited to the surface layer of 5 μm.

[0034] The nitrogen content ratio NR is 30 or less. A nitrogen content ratio NR of 30 or less indicates that nitriding of the steel sheet surface is suppressed and AlN deposition is suppressed. In this case, even a steel sheet with a high Al content and thin thickness exhibits excellent magnetic properties at intermediate frequencies. Preferably, the nitrogen content ratio NR is 20 or less.

[0035] <plate thickness> The thickness of the non-oriented electrical steel sheet according to this embodiment is 0.35 mm or less. There is no particular lower limit. However, although it is conventionally known that the properties of the electrical steel sheet itself improve when the sheet thickness is reduced, if it is too thin, it may lead to an increase in production costs, an increase in the cost of electrical equipment, and a decrease in the fill ratio. Therefore, the sheet thickness is preferably 0.15 mm or more.

[0036] <Component composition> Next, the component composition of the non-oriented electrical steel sheet according to this embodiment will be described. In the following description of the component composition, % refers to mass %. The non-oriented electrical steel sheet according to this embodiment contains Si: 2.0% to 3.5%, Al: 0.20% to 2.50%, and Mn: 0.7% or less, with the remainder being Fe and impurities. Furthermore, the non-oriented electrical steel sheet according to this embodiment preferably contains at least one of S: 0.0010% to 0.0030%, Sn: 0.02% to 0.1%, C: 0.0050% or less, and P: 0.005% to 0.10%. In addition, the non-oriented electrical steel sheet according to this embodiment may contain Cu, Cr, and Ni as needed.

[0037] Si: 2.0~3.5% Si is an element that increases resistivity and contributes to improving iron loss characteristics. If the Si content is less than 2.0%, the iron loss improvement effect cannot be sufficiently obtained, so the Si content should be 2.0% or more. Preferably, the Si content is 2.5% or more. On the other hand, if the Si content exceeds 3.5%, the mechanical properties deteriorate significantly, so the Si content should be 3.5% or less.

[0038] Al: 0.20~2.50% Al is an element that detoxifies N and increases its resistivity, thereby contributing to improved iron loss characteristics. If the Al content is less than 0.20%, the resistivity is low and iron loss is poor, and the phenomenon of nitriding during final annealing is hardly observed. For this reason, the Al content should be 0.20% or more. For thin, non-oriented electrical steel sheets (intermediate grade and above) where iron loss is important, it is preferable to contain 0.3% or more Al. On the other hand, if the Al content exceeds 2.50%, Nors clogging may occur during continuous casting, making continuous casting extremely difficult. For this reason, the Al content should be 2.50% or less. In addition, if the Al content is high, coarse oxides may form on the slab surface, reducing mechanical properties and causing surface defects. For this reason, the Al content is preferably 2.0% or less, and more preferably 1.5% or less.

[0039] Mn: 0.7% or less Mn is an element that detoxifies sulfur (S) and increases its resistivity, thereby contributing to improved iron loss characteristics. If the Mn content is higher than 0.7%, MnS is formed, fixing the S in the steel sheet, eliminating the negative catalytic effect of S, activating the surface state, and making nitriding more likely. Furthermore, a high Mn content makes it difficult to achieve extremely low carbon content in steelmaking. For this reason, the Mn content should be 0.7% or less. Considering production costs, a Mn content of 0.5% or less is preferable.

[0040] Sn: 0.02~0.1% Sn is an element that segregates at grain boundaries and suppresses nitriding. If the Sn content is less than 0.02%, the nitriding suppression effect of Sn may not be sufficiently obtained, so a Sn content of 0.02% or more is preferable. A Sn content of 0.05% or more is more preferable. Furthermore, a Sn content of 0.1% or less is preferable. This is because exceeding this level may result in significantly poor cold rolling performance and may not provide the desired nitriding prevention effect. If nitriding is suppressed by optimizing the final annealing conditions, intentional addition is unnecessary.

[0041] Impurities are elements that inevitably become mixed into the molten steel from the steel raw materials and / or during the steelmaking process, and are permissible in amounts that do not impair the properties of the non-directional electrical steel sheet. Examples include S, N, C, P, Ti, Cu, Cr, Ni, etc., which are inevitably mixed in as described later.

[0042] S: 0.0010~0.0030% As described in Non-Patent Document 1, it has long been said that a lower S content is preferable, but it has been found that this is not necessarily the case. Figure 6 shows the relationship between the amount of hydrogen in the atmosphere during final annealing, the S content of the steel sheet, and the amount of nitriding of the steel sheet before and after final annealing for steel sheets manufactured using molten steel with different S content, with the final annealing atmosphere being a mixture of nitrogen and hydrogen gas as follows: (a) nitrogen: 95% by volume, hydrogen: 5% by volume, (b) nitrogen: 80% by volume, hydrogen: 20% by volume, (c) nitrogen: 65% by volume, hydrogen: 35% by volume, and (d) nitrogen: 50% by volume, hydrogen: 50% by volume. In detail, Figure 6 shows the relationship between the amount of hydrogen in the atmosphere during the final annealing, the S content of the steel sheet, and the amount of nitriding before and after the final annealing of a steel sheet obtained by sequentially performing a final annealing at 1025°C for approximately 35 to 40 seconds, using a conventional method to continuously cast molten steel with the following composition: C:0.0012%, Si:3.1%, Al:1.25%, Mn:0.6%, P:0.002%, S:0.0004%, 0.0019%, 0.0036%, 0.0052%, Sn:0.01%, and N:0.0006%. The final annealing atmosphere is as described above. Figure 6 shows the amount of nitriding (ppm) before and after final annealing, with the horizontal axis representing hydrogen concentration (volume %) and the vertical axis representing sulfur content (ppm). Figure 6 shows that nitriding is promoted when sulfur content is low. If nitriding during final annealing can be sufficiently suppressed, the sulfur content may be less than 0.0010%. However, if nitriding is not performed, the grain size will become too large and eddy current loss will increase unless the final annealing conditions are precisely controlled. Also, if the sulfur content is low, nitriding will be promoted and hysteresis loss will increase if the annealing conditions are not sufficiently controlled, which is undesirable. Considering the ease of manufacturing and cost of the entire process, a sulfur content of 0.0010 to 0.0030% is preferable.

[0043] C: 0.005% or less The carbon content (C) is preferably 0.005% or less during the steelmaking stage and 0.005% or less in the product stage (non-oriented electrical steel sheet according to this embodiment). Carbon (C) is an element that inhibits magnetic properties and is therefore removed by decarburization annealing before finish annealing. If the carbon content is 0.005% or more, carbon compounds may precipitate during use of electrical equipment incorporating the non-oriented electrical steel sheet, causing magnetic aging and degrading the equipment's performance. For this reason, the carbon content is preferably 0.005% or less. If the carbon content exceeds 0.010% during the steelmaking stage, the decarburization annealing time in the annealing process becomes longer in order to reduce the carbon content to 0.005% or less in the product stage, reducing productivity. For this reason, the carbon content of the molten steel is preferably 0.010% or less. The carbon content in the product stage is more preferably 0.003% or less. While there is no specific lower limit, reducing the carbon content to below 0.001% during the molten steel stage significantly increases steelmaking costs. Therefore, 0.001% is the practical lower limit for carbon content in the finished product of practical steel sheets. The partial pressure ratio P of the atmosphere during the final annealing, as described later... H2O / P H2 If this value is set to 0.1 or higher, the carbon content of the product can easily be reduced to 0.005% or less.

[0044] P: 0.005~0.10% The P content is preferably 0.005% or more. If the P content is less than 0.005%, the {111} plane orientation increases, which may result in inferior magnetic properties. The P content is more preferably 0.008% or more. On the other hand, if it exceeds 0.10%, the toughness and ductility decrease significantly, so the P content is preferably 0.10% or less. The P content is more preferably 0.05% or less.

[0045] Ti: 0.005% or less Ti is an element that inevitably mixes into molten steel during the steelmaking process. A Ti content of 0.005% or less is preferable, as it does not impair the magnetic properties of non-oriented electrical steel sheets. More preferably, the Ti content is 0.002% or less. There is no particular lower limit.

[0046] N: 0.0030% or less As previously mentioned, nitrogen (N) enters during the final annealing stage, but considering the overall purity, it is preferable to have as little N as possible during the steelmaking stage. If the N content exceeds 0.0030%, it forms AlN, TiN, etc., before the final annealing, hindering grain growth. For this reason, the N content is preferably 0.0030% or less. More preferably, the N content is 0.0020% or less. There is no particular lower limit.

[0047] Cu, Cr, and Ni: 0.05% or less Cu, Cr, and Ni are elements that inevitably become mixed in, but if their respective content is 0.05% or less, they do not significantly affect the magnetic properties. Therefore, the content of Cu, Cr, and Ni is preferably 0.05% or less. There is no particular lower limit.

[0048] The above chemical composition can be measured using general analytical methods for steel.

[0049] Up to this point, the non-oriented electrical steel sheet according to this embodiment has been described. Next, an example of a method for manufacturing the non-oriented electrical steel sheet according to this embodiment will be described. As previously stated, the thickness of the thin electrical steel strip specified in JIS C 2558 (2015) is included in this application. However, in conventional technology, thin electrical steel strip is manufactured by the so-called reroll method. This method involves further cold rolling after final annealing. The thickness of the non-oriented electrical steel sheet according to this embodiment is equivalent to that of the rerolled material, but further cold rolling is omitted to avoid increased costs.

[0050] <Manufacturing method for non-oriented electrical steel sheets> The non-oriented electrical steel sheet according to this embodiment is manufactured through refining, continuous casting, hot rolling, hot-rolled sheet annealing, cold rolling, and final annealing. Refining, continuous casting, and hot rolling may be carried out by known methods. Hot-rolled sheet annealing, cold rolling, and final annealing will be described in detail below.

[0051] <Hot-rolled sheet annealing> The non-oriented electrical steel sheet according to this embodiment is a thin non-oriented electrical steel sheet with better iron loss characteristics than steel grades 35A440 and 50A470 specified in JIS C 2552 (2014). Therefore, hot-rolled sheet annealing is applied to prevent rignification of the surface properties. The sheet thickness at that time is approximately 1.6 mm to 2.3 mm. For example, a hot-rolled sheet with a thickness of approximately 1.8 mm is used for a non-oriented electrical steel sheet with a product thickness of 0.25 mm, and a hot-rolled sheet with a thickness of approximately 2.2 mm is used for a non-oriented electrical steel sheet with a product thickness of 0.35 mm. The temperature for hot-rolled sheet annealing is preferably 850°C to 1000°C, and the annealing time is preferably around 60 seconds.

[0052] <Cold rolling> After hot-rolled sheet annealing, descaling is performed and then cold-rolled. The hot-rolled sheet is subjected to one cold-rolling or two or more cold-rolling processes with an intermediate annealing in between to obtain a sheet of the desired thickness. Cold rolling may be performed at room temperature (10-30°C), or the sheet may be heated to a higher temperature, for example, around 200°C, and then warm-rolled. Since the product thickness is thin, it is preferable to roll it in a reverse cold mill.

[0053] <Final annealing conditions> In the final annealing stage, the conditions to be controlled are the annealing atmosphere (gas ratio), dew point Dp, and partial pressure ratio P. H2O / P H2、 These are the annealing temperature and annealing time. First, referring to Figure 7, we consider the annealing atmosphere, dew point Dp, and partial pressure ratio P. H2O / P H2 This will be explained. Figure 7 is a diagram showing the relationship between the amount of hydrogen in the atmosphere during final annealing (H2%), the dew point (Dp(°C)), and the amount of nitriding of the steel sheet before and after final annealing (ppm). The final annealing conditions are very important for obtaining the non-oriented electrical steel sheet according to this embodiment, and there are two conditions, Condition A and Condition B, as shown in Figure 7. Condition A is the condition for obtaining the non-oriented electrical steel sheet according to this embodiment prioritizing low cost, and the partial pressure ratio (P H2O / P H2): It is 0.10 to 0.06, the amount of hydrogen in the atmosphere (H2%) is 10 to 20% by volume, and the balance is nitrogen and water vapor in the range of the above partial pressure ratio. Condition B is the condition for obtaining the non-oriented electromagnetic steel sheet according to the present embodiment with priority on quality, and the partial pressure ratio (P H2O / P H2 ): It is 0.0010 to 0.0015, the amount of atmospheric hydrogen (H2%) is 35 to 45% by volume, and the balance is nitrogen and water vapor in the range of the above partial pressure ratio. Note that the partial pressure ratio P H2O / P H2 is the ratio of the water vapor partial pressure P H2 to the hydrogen partial pressure P H2O in the annealing atmosphere.

[0054] Subsequently, the annealing temperature and annealing time will be described. The final annealing for manufacturing a non-oriented electromagnetic steel sheet having good iron loss characteristics exceeding 50A470 and 35A360 of the non-oriented electromagnetic steel sheet is annealed at 900°C to 1075°C for about 45 seconds to 75 seconds in order to ensure grain growth properties. Regarding the annealing atmosphere, when heating and soaking are indirect such as radiant tube or element heating, the dew point Dp of the annealing atmosphere in the furnace becomes -30°C or lower due to the introduced gas. This annealing condition corresponds to Condition B shown in FIG. 7. On the other hand, when a direct flame such as a non-oxidizing furnace confronts the steel sheet, even if it is called non-oxidizing, the dew point Dp becomes about 15°C. Such annealing conditions correspond to Condition A.

[0055] Nitridation of the steel sheet occurs during the final annealing, and its form can be judged by the generation state of AlN. The depth of nitridation is usually at most 5 μm per side. This AlN deteriorates the magnetic properties. To prevent the generation of this AlN, the components are important, and the final annealing conditions are also important. The final annealing is basically performed in a mixed gas of nitrogen and hydrogen.

[0056] When the atmosphere is pseudo-wet (the partial pressure ratio (P H2O / P H2 ): 0.10 to 0.06 is called pseudo-wet.) in Condition A, an oxide of Al is formed on the surface of the steel sheet, the intrusion of nitrogen into the steel sheet is hindered, and almost no AlN is formed. Specifically, the partial pressure ratio P H2O / P H2This is the case when the value is greater than 0.10. However, the partial pressure ratio P H2O / P H2 When the value exceeds 0.2, the oxide layer becomes thicker (more numerous), and properties other than iron loss deteriorate. Also, the partial pressure ratio P H2O / P H2 Even if the ratio is 0.10 or less, nitriding can be suppressed if the annealing temperature is 1000°C or less. If the annealing temperature exceeds 1000°C, shorten the annealing time as much as possible or increase the H2 ratio as much as possible (P H2 It is preferable to increase the hydrogen content. However, considering the cost, the hydrogen content is limited to 50% by volume. By doing so, a non-oriented electrical steel sheet with as little AlN as possible on the surface, satisfying the above formula (2), can be obtained. However, the partial pressure ratio P H2O / P H2 If the value is less than 0.06, lowering the dew point will not satisfy equation (2) above. Note that increasing the amount of hydrogen will satisfy equation (2) above, but this will lead to increased costs.

[0057] As in condition B, the partial pressure ratio P H2O / P H2 To lower the dew point, indirect heating is used, and the furnace dew point Dp is -30°C or lower. In this case, if the hydrogen content in the atmosphere is less than 20% by volume, the surface of the steel sheet is activated, and the steel sheet will nitride even if the nitrogen content in the atmosphere is low. Conventionally, it was thought that steel sheets would not nitride under these conditions, but in the case of non-oriented electrical steel sheets containing Al, even if the furnace dew point is low, if the nitrogen content is high, nitriding will easily occur and the magnetic properties will deteriorate.

[0058] Of course, conditions between conditions A and B can be considered as conditions for manufacturing non-oriented electrical steel sheets according to this embodiment, but when heating steel sheets using a non-oxidizing furnace, it is difficult to control the dew point (Dp). On the other hand, the hydrogen ratio can be easily controlled in a non-oxidizing furnace. For this reason, although it is costly, the region of condition B is easy to control and the magnetic properties are good.

[0059] Here, Figure 8 shows the amount of hydrogen in the atmosphere (H2%) and the partial pressure ratio (P H2O / P H2), and the relationship between the amount of nitriding (ppm) before and after the final annealing of the steel sheet are shown. According to Figure 8, the ambient hydrogen content (H2%) is high, and the partial pressure ratio (P H2O / P H2 In the region where the partial pressure ratio (P) is large, it can be seen that the amount of nitriding is small. Considering only the small amount of nitriding, the manufacturing conditions in this region are preferable. However, in steel types with a high Al content, as in the present invention, the partial pressure ratio (P) is large. H2O / P H2 If the dew point (Dp) is high, the surface of the steel plate oxidizes and its magnetic properties deteriorate. Also, maintaining a high dew point (Dp) under annealing conditions requires a humidifier, which is undesirable from the standpoint of quality and cost. In other words, in reality, the conditions in the lower right direction in Figure 8, i.e., a high amount of hydrogen in the atmosphere and a partial pressure ratio (P) are desirable. H2O / P H2 A condition where ) is small is preferable. Up to this point, an example of a method for manufacturing non-oriented electrical steel sheets according to this embodiment has been described. [Examples]

[0060] The following are examples of the present invention. The conditions in these examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to the conditions used in the following examples. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0061] Molten steel having the components shown in Table 1 was continuously cast using a known method to obtain slabs. The obtained slabs were then subjected to hot rolling, hot-rolled plate annealing, pickling, cold rolling, and final annealing under the conditions shown in Table 2 to produce samples.

[0062] [Table 1]

[0063] [Table 2]

[0064] Furthermore, the magnetic properties were evaluated using an iron loss of W10 / 400 at 1.0T and 400Hz, targeting the intermediate frequency (400Hz to 10kHz) specified in JIS C 2550-3 (2011). The characteristic standards for thin non-oriented electrical steel sheets are shown in Table 3. Table 3 shows the typical values ​​and upper limits for W10 / 400. The typical value is equivalent to the average value of the characteristic distribution, and the upper limit is the limit value that the user will be satisfied with. The typical values ​​for W10 / 400 for sheet thicknesses of 0.15mm, 0.20mm, 0.25mm, 0.30mm, and 0.35mm are 9.5W / kg, 10.5W / kg, 12.0W / kg, 13.5W / kg, and 14.0W / kg, respectively. Furthermore, the upper limits for W10 / 400 were set to 10.0 W / kg, 12.0 W / kg, 13.0 W / kg, 14.5 W / kg, and 16.0 W / kg, respectively. In this embodiment, the upper limits were used to evaluate the magnetic properties. That is, for example, when the product plate thickness was 0.25 mm, an example where the iron loss W10 / 400 was 13.0 W / kg or less was evaluated as having good magnetic properties, and an example where it exceeded 13.0 W / kg was evaluated as having poor magnetic properties. When the product plate thickness was 0.30 mm, an example where the iron loss W10 / 400 was 14.5 W / kg or less was evaluated as having good magnetic properties, and an example where it exceeded 14.5 W / kg was evaluated as having poor magnetic properties. Furthermore, for product plate thicknesses of 0.35 mm, examples where the iron loss W10 / 400 is 14.0 W / kg or less are evaluated as having good magnetic properties, and examples where it exceeds 16.0 W / kg are evaluated as having poor magnetic properties. The evaluation results are shown in Table 4.

[0065] [Table 3]

[0066] [Table 4]

[0067] Table 4 shows that C6 has better magnetic properties in terms of iron loss W10 / 400 for C5 and C6. This is because, although the nitriding amount is lower for C5, the partial pressure ratio P at the final annealing stage is lower. H2O / P H2Since C6 < C5, it is presumed that this is due to oxidation of the steel sheet surface. Also, for Steel No. A1 and Steel No. A2, Steel No. A2 had a higher Sn content and overall less nitridation. Production No. B1 has a final annealing atmosphere of 5 vol% hydrogen - 95 vol% nitrogen, which is a condition where nitridation is likely to occur. However, in the examples of C8 and C9, since Steel No. A2 containing Sn was used, nitridation was suppressed and the magnetic properties were good.

Industrial Applicability

[0068] As described above, according to the present invention, a non - oriented electromagnetic steel sheet with excellent magnetic properties at intermediate frequencies can be manufactured and has high applicability.

Claims

1. In mass percent, Si: 2.0% to 3.5%, Al: 0.20% to 2.50%, It contains Mn: 0.7% or less, with the remainder being Fe and impurities. The plate thickness is 0.35 mm or less. A non-oriented electrical steel sheet characterized in that the nitrogen content ratio NR shown in the following formula (2), which is the ratio of the estimated surface nitrogen content Ns shown in the following formula (1) to the internal nitrogen content Nm, satisfies 2.2 to 30. Ns={t×Nt-(t-0.01)×Nm} / 0.01... Formula (1) NR=Ns / Nm... Formula (2) Here, in equation (1) above, Nt: Nitrogen content (mass%) of non-oriented electrical steel sheet with the same thickness as the steel sheet. Nm: Nitrogen content (mass%) contained in the portion of the steel plate from a position 5 μm from one side in the thickness direction to a position 5 μm from the other side in the thickness direction. t: Steel plate thickness (mm) That is the case.

2. Furthermore, the non-oriented electrical steel sheet according to claim 1 is characterized by containing 0.02 to 0.1% of Sn by mass.

3. The non-oriented electrical steel sheet according to claim 1 or 2, further characterized in that it contains P: 0.005 to 0.10% by mass.

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