Hot-rolled steel sheet for non-oriented electrical steel sheet, method for producing hot-rolled steel sheet for non-oriented electrical steel sheet, and method for producing non-oriented electrical steel sheet

Abstract

The hot-rolled steel sheet for non-oriented electrical steel sheet of the present invention contains Si, Mn, Al, Ti, Nb, V, and Zr as chemical components in predetermined amounts, and when observed in a cross section parallel to the rolling direction and the thickness direction, AlN having an equivalent circle diameter of 10 to 200 nm is present in the grains and grain boundaries of ferrite grains, and the number density of the AlN present in the grains and grain boundaries is 8.0 pieces / μm with respect to the observed area. 2 The number density of the AlN particles present at the grain boundaries is set to 40 particles / μm with respect to the grain boundary area. 2 the following.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet for non-oriented electrical steel sheets capable of improving magnetic properties, a method for producing the hot-rolled steel sheet for non-oriented electrical steel sheets, and a method for producing the non-oriented electrical steel sheets. Background Art

[0002] Non-oriented electrical steel sheets are primarily used as core materials for rotating machinery and other applications. In recent years, demand for higher equipment efficiency has increased, even in areas where low-grade non-oriented electrical steel sheets are used. Consequently, even low-grade non-oriented electrical steel sheets are required to achieve higher magnetic flux density and lower iron loss while maintaining cost.

[0003] Furthermore, with the recent development of inverter control in rotating machines, there is a demand for improvement in iron loss at high frequencies. Therefore, even low-grade non-oriented electromagnetic steel sheets are required to have reduced iron loss at high frequencies.

[0004] Low-grade non-oriented electrical steel sheets generally have a low Si content and a chemical composition that produces an α-γ transformation (ferrite-austenite transformation) during the manufacturing process. Previously, methods have been proposed for improving the magnetic properties of such low-grade non-oriented electrical steel sheets by omitting hot-rolled sheet annealing.

[0005] For example, Patent Document 1 proposes a method of completing hot rolling at or above the Ar3 transformation point and then slowly cooling at a rate of 5°C / s or less in the temperature range from the Ar3 transformation point to the Ar1 transformation point. However, this cooling rate is difficult to achieve in industrial production processes.

[0006] Patent Document 2 proposes a method of adding Sn to steel and controlling the hot rolling finish temperature according to the Sn concentration to achieve a high magnetic flux density. However, this method limits the Si concentration to 0.4% or less, which is insufficient for achieving low iron loss.

[0007] Patent Document 3 proposes a steel sheet that achieves high magnetic flux density and excellent grain growth during stress relief annealing by limiting the heating and finishing temperatures during hot rolling. However, this method lacks a step such as auto-annealing to replace hot-rolled sheet annealing, and therefore cannot achieve high magnetic flux density.

[0008] Patent Document 4 proposes a method for increasing magnetic flux density by controlling the chemical composition of steel and hot-rolling conditions. To address the technical issue of AlN finely precipitating at α grain boundaries during the γ→α phase transformation, which hinders grain growth during auto-annealing of the hot-rolled sheet, Patent Document 4 controls the finishing rolling temperature to 800°C (Ar1 + 20°C) and the coiling temperature to 780°C or higher. However, this method does not address the fundamental technical issue of AlN precipitation during the γ→α phase transformation.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 06-192731

[0012] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-241554

[0013] Patent Document 3: Japanese Patent Application Laid-Open No. 2007-217744

[0014] Patent Document 4: International Publication No. 2013 / 069754 Summary of the Invention

[0015] Technical problem to be solved by the invention

[0016] As mentioned above, low-grade non-oriented electrical steel sheets generally have a chemical composition that causes an α-γ phase transformation during the manufacturing process. Conventional technology has attempted to improve the magnetic properties of such low-grade non-oriented electrical steel sheets by performing autogenous annealing after hot rolling, rather than annealing the hot-rolled steel sheets. However, as mentioned above, these conventional technologies have not yet achieved satisfactory magnetic properties. In particular, improvements in iron loss at high temperatures have been insufficient.

[0017] The present invention has been made in view of the above circumstances and has an object to provide a hot-rolled steel sheet for non-oriented electrical steel sheet having excellent iron loss characteristics at high frequencies in addition to general magnetic characteristics, a method for producing the hot-rolled steel sheet for non-oriented electrical steel sheet, and a method for producing the non-oriented electrical steel sheet.

[0018] Technical means for solving technical problems

[0019] The gist of the present invention is as follows.

[0020] (1) A hot-rolled steel sheet for non-oriented electrical steel sheet according to one embodiment of the present invention,

[0021] As chemical components, in mass%,

[0022] C: 0.005% or less,

[0023] Si: 0.10-1.50%,

[0024] Mn: 0.10~0.60%,

[0025] P: 0.100% or less,

[0026] Al: 0.20-1.00%,

[0027] Ti: 0.0010~0.0030%,

[0028] Nb: 0.0010~0.0030%,

[0029] V: 0.0010~0.0030%,

[0030] Zr: 0.0010~0.0030%,

[0031] N: 0.0030% or less,

[0032] Sn: 0-0.20%,

[0033] Sb: 0~0.20%,

[0034] The rest is composed of Fe and impurities.

[0035] When observed in a cross section parallel to the rolling direction and the plate thickness direction, AlN with an equivalent circle diameter of 10 to 200 nm exists within the ferrite grains and at the grain boundaries.

[0036] The number density of the AlN present in the crystal grains and the grain boundaries was 8.0 pieces / μm with respect to the observation area. 2 Below, and

[0037] The number density of the AlN particles present in the grain boundaries is 40 particles / μm with respect to the grain boundary area. 2 the following.

[0038] (2) In the hot-rolled steel sheet for non-oriented electrical steel sheet as described in (1),

[0039] As chemical components, in mass %,

[0040] Sn: 0.02~0.20%,

[0041] Sb: 0.02~0.20%

[0042] At least one of .

[0043] (3) A method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to one embodiment of the present invention is the method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet as described in (1) or (2) above,

[0044] Heat the slab to a temperature range of 1050°C to 1180°C.

[0045] The heated slab is subjected to rough rolling,

[0046] The rough rolled material after the rough rolling is kept in a temperature range of 850°C or higher and below the Ar1 point,

[0047] The rough rolled material after holding is reheated to a temperature range exceeding the Ar1 point and below the Ac1 point.

[0048] The rough rolled material immediately after heating is subjected to finish rolling under the condition that the finishing temperature after finish rolling is 800° C. or higher and Ar1 point or lower.

[0049] The finished rolled material is coiled at a temperature of 750°C to 850°C.

[0050] The slab contains, as chemical components, in mass %,

[0051] C: 0.005% or less,

[0052] Si: 0.10-1.50%,

[0053] Mn: 0.10~0.60%,

[0054] P: 0.100% or less,

[0055] Al: 0.20-1.00%,

[0056] Ti: 0.0010~0.0030%,

[0057] Nb: 0.0010~0.0030%,

[0058] V: 0.0010~0.0030%,

[0059] Zr: 0.0010~0.0030%,

[0060] N: 0.0030% or less,

[0061] Sn: 0-0.20%,

[0062] Sb: 0~0.20%,

[0063] The remainder is composed of Fe and impurities.

[0064] (4) The method for producing a non-oriented electrical steel sheet of the present invention is a method for producing a non-oriented electrical steel sheet using the hot-rolled steel sheet for non-oriented electrical steel sheet as described in (1) or (2) above.

[0065] The hot-rolled steel sheet for non-oriented electrical steel sheet is cold-rolled without annealing the hot-rolled steel sheet.

[0066] The cold-rolled material after the cold rolling is subjected to final annealing at a temperature of 800° C. or higher and below the Ac1 point.

[0067] Effects of the Invention

[0068] According to the above aspects of the present invention, a hot-rolled steel sheet for non-oriented electrical steel sheet having excellent iron loss characteristics at high frequencies in addition to general magnetic characteristics, a method for producing the hot-rolled steel sheet for non-oriented electrical steel sheet, and a method for producing the non-oriented electrical steel sheet can be provided. DETAILED DESCRIPTION

[0069] Below, preferred embodiments of the present invention are described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the scope of the present invention. In addition, in the following numerical ranges, the lower limit and upper limit are included in the range. The value indicated as "less than" or "exceeds" is not included in the numerical range. In addition, unless otherwise specified, the "%" for the content of each element refers to "mass %".

[0070] In the hot-rolled steel sheet for non-oriented electrical steel sheet of the present embodiment, the chemical composition and the production conditions are controlled in a complex and inseparable manner, and the form of AlN contained in the hot-rolled steel sheet is controlled.

[0071] For example, in a non-oriented electrical steel sheet having a chemical composition that causes an α-γ phase transformation during the manufacturing process and manufactured by autogenous annealing after hot rolling instead of annealing the hot-rolled sheet, it is preferable to allow sufficient grain growth during autogenous annealing and final annealing after hot rolling in order to improve the magnetic properties.

[0072] However, AlN contained in the hot-rolled steel sheet pins the grain boundaries and inhibits grain growth. Therefore, the less AlN contained in the hot-rolled steel sheet, the better.

[0073] For example, Patent Document 4, mentioned above, attempts to reduce the AlN content in steel sheets. While the technology disclosed in Patent Document 4 may indeed be able to reduce the AlN content in steel sheets to some extent, the technology disclosed in Patent Document 4 cannot fundamentally suppress the precipitation of AlN during the γ→α transformation. In particular, a significant amount of AlN precipitates at the grain boundaries of ferrite (α) grains. Consequently, grain growth is not sufficient during autogenous annealing and final annealing after hot rolling.

[0074] In this embodiment, the chemical composition and manufacturing conditions are controlled in a complex and inseparable manner to reduce the amount of AlN present within α-phase grains and at grain boundaries, particularly at α-phase grain boundaries. As a result, the grains grow sufficiently during autogenous annealing and final annealing after hot rolling, resulting in a non-oriented electrical steel sheet that exhibits excellent iron loss characteristics at high frequencies in addition to general magnetic properties.

[0075] Patent Document 4 also mentions the AlN number density in the steel sheet after final annealing. However, since the AlN precipitated during the hot rolling process is presumably Ostwald ripened during the final annealing, reducing the AlN number density, this figure cannot necessarily be compared with the AlN number density in the hot-rolled steel sheet of the present embodiment. Furthermore, the crystal structure of the steel sheet after hot rolling is deformed by processing during the subsequent cold rolling, and recrystallized and grains grow during the final annealing. Therefore, the ferrite grain boundaries after hot rolling and after final annealing do not necessarily match.

[0076] Hot rolled steel sheet for non-oriented electrical steel sheet according to the present embodiment

[0077] As chemical components, in mass%,

[0078] C: 0.005% or less,

[0079] Si: 0.10-1.50%,

[0080] Mn: 0.10~0.60%,

[0081] P: 0.100% or less,

[0082] Al: 0.20-1.00%,

[0083] Ti: 0.0010~0.0030%,

[0084] Nb: 0.0010~0.0030%,

[0085] V: 0.0010~0.0030%,

[0086] Zr: 0.0010~0.0030%,

[0087] N: 0.0030% or less,

[0088] Sn: 0-0.20%,

[0089] Sb: 0~0.20%,

[0090] The rest is composed of Fe and impurities.

[0091] When observed in a cross section parallel to the rolling direction and the plate thickness direction, AlN with an equivalent circle diameter of 10 to 200 nm exists within the ferrite grains and at the grain boundaries.

[0092] The number density of the AlN present in the crystal grains and the grain boundaries was 8.0 pieces / μm with respect to the observation area. 2 Below, and

[0093] The number density of the AlN particles present in the grain boundaries is 40 particles / μm with respect to the grain boundary area. 2 the following.

[0094] <Chemical Composition of Hot-Rolled Steel Sheet>

[0095] First, regarding the hot-rolled steel sheet for non-oriented electrical steel sheet according to the present embodiment, the reasons for limiting the chemical composition of the steel will be described.

[0096] In the present embodiment, the hot-rolled steel sheet contains basic elements as chemical components, contains optional elements as needed, and the balance is composed of Fe and impurities.

[0097] C: 0.005% or less

[0098] C is a harmful element that degrades iron loss and causes magnetic aging. The C content is 0.005% or less. The C content is preferably 0.003% or less. The lower the C content, the better, and the lower limit may be 0%. However, considering industrial productivity, the C content may exceed 0%. It may also be 0.0015% or more, 0.0020% or more, or 0.0025% or more.

[0099] Si: 0.10-1.50%

[0100] Si is an element that increases the inherent resistivity of steel and reduces iron loss. Therefore, the lower limit of the Si content is 0.10%. On the other hand, excessive addition reduces magnetic flux density. Therefore, the upper limit of the Si content is 1.50%. Preferably, the lower limit of the Si content may be 0.50%, and the upper limit of the Si content may be 1.20%.

[0101] Mn: 0.10~0.60%

[0102] Mn increases the inherent resistivity of steel and coarsens sulfides, rendering them harmless. Therefore, the lower limit of the Mn content is 0.10%. On the other hand, excessive addition can embrittle the steel and increase costs. Therefore, the upper limit of the Mn content is 0.60%.

[0103] P: 0.100% or less

[0104] P may also increase the hardness of steel sheets, but it can also cause embrittlement. The P content should be 0.100% or less. The P content is preferably 0.08%. The lower the P content, the better, and the lower limit can be 0%. However, considering industrial productivity, the P content can be 0.001% or more.

[0105] Al: 0.20~1.00%

[0106] Al is a deoxidizing element that also increases resistivity, raises the α-γ transformation point, and forms AlN. Therefore, the lower limit of the Al content is 0.20%. On the other hand, excessive addition can reduce magnetic flux density and workability. Therefore, the upper limit of the Al content is 1.00%. The upper limit of the Al content is preferably 0.80%.

[0107] Ti: 0.0010~0.0030%

[0108] Ti is a nitride-forming element, but unlike AlN, it precipitates abundantly as nitrides even in the γ phase. In this embodiment, Ti is important as a nitride-forming element to suppress the fine precipitation of AlN at the α grain boundaries during the γ→α transformation. Therefore, the lower limit of the Ti content is 0.0010%. On the other hand, excessive addition can form carbides, worsening grain growth during final annealing. Therefore, the upper limit of the Ti content is 0.0030%.

[0109] Nb: 0.0010~0.0030%

[0110] Nb is a nitride-forming element, but unlike AlN, it precipitates abundantly as nitrides even in the γ phase. In this embodiment, Nb is important as a nitride-forming element to suppress the fine precipitation of AlN at the α grain boundaries during the γ→α transformation. Therefore, the lower limit of the Nb content is 0.0010%. On the other hand, excessive addition will form carbides, worsening grain growth during final annealing. Therefore, the upper limit of the Nb content is 0.0030%.

[0111] V: 0.0010~0.0030%

[0112] V is a nitride-forming element, but unlike AlN, it precipitates abundantly as nitrides even in the γ phase. In this embodiment, V is important as a nitride-forming element to suppress the fine precipitation of AlN at the α grain boundaries during the γ→α transformation. Therefore, the lower limit of the V content is 0.0010%. On the other hand, excessive addition will form carbides, worsening grain growth during final annealing. Therefore, the upper limit of the V content is 0.0030%.

[0113] Zr: 0.0010~0.0030%

[0114] Zr is a nitride-forming element, but unlike AlN, it precipitates as nitrides even in the γ phase. Zr is important as a nitride-forming element to suppress the fine precipitation of AlN at the α grain boundaries during the γ→α phase transformation. Therefore, the lower limit of the Zr content is 0.0010%. On the other hand, excessive addition can form carbides, worsening grain growth during final annealing. Therefore, the upper limit of the Zr content is 0.0030%.

[0115] N: 0.0030% or less,

[0116] N is an element that generates AlN and is not favorable for grain growth. In this embodiment, the upper limit of the allowable N content, which allows for harmless N, is 0.0030% or less. The lower the N content, the better, and the lower limit can be 0%. However, considering industrial productivity, the N content can be 0.0001% or more. For example, when the N content is 0.0001% or more, AlN is easily generated, which can easily hinder grain growth.

[0117] Sn: 0~0.20%

[0118] Sb: 0~0.20%

[0119] Sn and Sb improve the texture after cold rolling and recrystallization and increase the magnetic flux density. Therefore, Sn and Sb can be included as needed. For example, the lower limit of the Sn content and Sb content is preferably 0.02%, and more preferably 0.03%. On the other hand, excessive addition will make the steel brittle. Therefore, the upper limit of the Sn content and Sb content is 0.20%. The upper limit of the Sn content and Sb content is preferably 0.10%.

[0120] The above-mentioned effects can be obtained by containing at least one of Sn and Sb. Therefore, as a chemical composition, it is preferable to contain at least one of Sn: 0.02 to 0.20% or Sb: 0.02 to 0.20% in terms of mass%.

[0121] The chemical composition of the hot-rolled steel sheet according to the present embodiment described above corresponds to a chemical composition that causes α-γ transformation during the manufacturing process.

[0122] Furthermore, in this embodiment, impurities may be included as chemical components. "Impurities" refer to elements whose presence does not impair the effects of this embodiment, and are elements that are introduced during industrial steel plate production from raw material ores, scrap, or the manufacturing environment. The upper limit of the total impurity content may be, for example, 5%.

[0123] The above chemical composition can be measured by general analytical methods. For example, the chemical composition can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition is determined by measuring a 35 mm square test piece collected from a steel plate using a Shimadzu Corporation ICPS-8100 (measuring device) under conditions based on a pre-made calibration curve. In addition, C can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0124] Slabs are formed by casting molten steel adjusted to achieve the composition of the hot-rolled steel sheet described above. The slab casting method is not particularly limited. Furthermore, during research and development, even when steel ingots are formed using a vacuum melting furnace or the like, the same effects as those obtained with the composition described above have been confirmed.

[0125] <AlN contained in hot-rolled steel sheets>

[0126] Regarding the hot-rolled steel sheet for non-oriented electrical steel sheet according to the present embodiment, the reasons for limiting the amount of AlN contained in the hot-rolled steel sheet will be described.

[0127] As described above, in this embodiment, the chemical composition and manufacturing conditions are controlled in a complex and inseparable manner to control the morphology of AlN contained in the hot-rolled steel sheet. In particular, in this embodiment, precipitation of AlN at the grain boundaries of α grains is suppressed.

[0128] In the hot-rolled steel sheet for non-oriented electrical steel sheet of the present embodiment, AlN having an equivalent circle diameter of 10 to 200 nm is present in the grains and grain boundaries of ferrite grains (α grains) when observed in a cross section parallel to the rolling direction and the thickness direction.

[0129] The number density of AlN present in the grains and grain boundaries (total number density) is 8.0 pieces / μm with respect to the observation area. 2 Below, and

[0130] The number density of AlN present in the grain boundaries (number density in the grain boundaries) is 40 pieces / μm with respect to the grain boundary area. 2 the following.

[0131] In this embodiment, the size of AlN that has the greatest influence on grain growth is controlled to have an equivalent circle diameter of 10 to 200 nm. In the hot-rolled steel sheet for non-oriented electrical steel sheet of this embodiment, AlN of the above size is contained within the α grains and at the grain boundaries.

[0132] The number density of AlN particles of the above size existing in the grains and grain boundaries of the α grains exceeds 8.0 particles / μm with respect to the observation area. 2 When the grain growth during self-annealing and final annealing becomes insufficient. As a result, the magnetic flux density and iron loss characteristics of the non-oriented electrical steel sheet are reduced. The number density of AlN of the above size existing in the grains and grain boundaries of α grains is 8.0 pieces / μm with respect to the observed area. 2 On the other hand, the smaller the number density of AlN particles of the above size existing in the α-grains and at the grain boundaries, the better, and the lower limit may be 0 particles / μm with respect to the observation area. 2However, it is difficult to actually make the number density 0 / μm 2 In industry, the number density of AlN particles of the above size existing in the grains and grain boundaries of α grains is sometimes 0.1 particles / μm with respect to the observation area. 2 above.

[0133] In addition, in order to improve the iron loss characteristics at high frequencies, it is not sufficient to simply control the number density (total number density) of AlN of the above size existing in the grains and grain boundaries of the α grains. It is preferable to control the number density of AlN of the above size existing in the grain boundaries of the α grains (number density in the grain boundaries).

[0134] The number density of AlN particles of the above size existing in the grain boundaries of α grains exceeds 40 particles / μm with respect to the grain boundary area. 2 When the grain growth during self-annealing and final annealing becomes insufficient. As a result, the iron loss characteristics at high frequencies are degraded as a non-oriented electrical steel sheet. The number density of AlN particles of the above size existing in the grain boundaries of α grains is 40 particles / μm with respect to the grain boundary area. 2 The number density is preferably 35 / μm. 2 On the other hand, the smaller the number density of AlN particles of the above size existing in the grain boundaries of α grains, the better, and the lower limit may be 0 particles / μm relative to the grain boundary area. 2 However, it is difficult to actually make the number density 0 / μm 2 In industry, the number density of AlN particles of the above size existing in the grain boundaries of α grains is sometimes 0.5 particles / μm relative to the grain boundary area. 2 above.

[0135] The AlN contained in the hot-rolled steel sheet can be determined using TEM-EDS (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy). For example, a thin film sample with a cross section parallel to the rolling direction and the thickness direction is collected from the hot-rolled steel sheet as the observation surface. Based on the observation and quantitative analysis results of TEM-EDS, it is sufficient to determine the precipitates with an atomic ratio of Al to N of approximately 1:1 in the observation field. The diameter of the determined AlN area converted into a circle is defined as the equivalent circle diameter. The AlN with an equivalent circle diameter of 10 to 200 nm present in the observation field (observation area) is determined, and the number density of AlN present in the grains and grain boundaries of α grains (total number density) and the number density of AlN present in the grain boundaries of α grains (number density in grain boundaries) are calculated. For example, the observation field can be at least 10μm×10μm. The number of AlN particles present at the grain boundaries can be defined as the number of AlN particles present within a distance of 0.2 μm from the grain boundaries into each grain, and the grain boundary area can be defined as the value obtained by multiplying the total distance of the grain boundaries in the image obtained by TEM-EDS observation by 0.4 μm. Alternatively, to derive the equivalent circle diameter, the image obtained by TEM-EDS observation can be read using a scanner or the like and analyzed using commercially available image analysis software.

[0136] <Method for Manufacturing Hot-Rolled Steel Sheet>

[0137] Next, a method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to the present embodiment will be described.

[0138] The method for producing a hot-rolled steel sheet for a non-oriented electrical steel sheet according to the present embodiment is the method for producing the hot-rolled steel sheet described above, and is specifically as follows:

[0139] Heat the slab to a temperature range of 1050°C to 1180°C.

[0140] The heated slab is subjected to rough rolling.

[0141] The rough rolled material after the rough rolling is kept at a temperature range of 850°C or higher and below the Ar1 point.

[0142] The rough rolled material after the above holding is reheated to a temperature range exceeding the Ar1 point and below the Ac1 point.

[0143] The rough rolled material immediately after heating is subjected to finish rolling under the condition that the finishing temperature after finish rolling is 800°C or higher and Ar1 point or lower.

[0144] The finished rolled material is coiled at a temperature of 750°C to 850°C.

[0145] The above slab contains, as chemical components, in mass %,

[0146] C: 0.005% or less,

[0147] Si: 0.10-1.50%,

[0148] Mn: 0.10~0.60%,

[0149] P: 0.100% or less,

[0150] Al: 0.20-1.00%,

[0151] Ti: 0.0010~0.0030%,

[0152] Nb: 0.0010~0.0030%,

[0153] V: 0.0010~0.0030%,

[0154] Zr: 0.0010~0.0030%,

[0155] N: 0.0030% or less,

[0156] Sn: 0-0.20%,

[0157] Sb: 0~0.20%,

[0158] The remainder is composed of Fe and impurities.

[0159] In this embodiment, the coil is auto-annealed after hot finishing, aiming to improve the magnetic properties of the non-oriented electrical steel sheet. For example, in this embodiment, the slab heating temperature during hot rolling is set to 1050°C to 1180°C, rough rolling is performed, the rough-rolled material is held at 850°C to the Ar1 point, the held rough-rolled material is heated to a temperature above the Ar1 point and below the Ac1 point, finish rolling is performed, and the finished material is coiled at 750°C to 850°C. These manufacturing conditions effectively suppress the precipitation of AlN at the grain boundaries of the α phase. As a result, grains grow preferentially during auto-annealing and final annealing, achieving excellent iron loss and magnetic flux density as a non-oriented electrical steel sheet.

[0160] The chemical composition of the slab is the same as that of the hot-rolled steel sheet described above. In the production of non-oriented electrical steel sheet, the chemical composition barely changes from the slab to the hot-rolled steel sheet. The chemical composition of the slab corresponds to the chemical composition that produces the α-γ phase transformation during the manufacturing process.

[0161] To prevent precipitates from re-solubilizing and finely precipitating, and to prevent degradation of iron loss, the slab heating temperature is set to 1180°C or lower. However, if the slab heating temperature is too low, deformation resistance increases, and the load of hot rolling increases. Therefore, the slab heating temperature is set to 1050°C or higher. The lower limit of the slab heating temperature is preferably 1080°C. The upper limit of the slab heating temperature is preferably 1150°C, and more preferably 1130°C.

[0162] The conditions for rough rolling are not particularly limited, and known rough rolling conditions may be applied.

[0163] The rough-rolled material after rough rolling is kept below the Ar1 point so that it is transformed into the α phase. The Ar1 point refers to the temperature at which the phase transformation to the α phase is completed during cooling. In this embodiment, since Ti, Nb, V and Zr must be contained as chemical components, nitrides of Ti, Nb, V and Zr are generated in the γ phase, the number of AlN in the steel decreases, and the content of dissolved N in the steel decreases. However, a part of the N is still dissolved in the steel. Therefore, the rough-rolled material after rough rolling is kept below the Ar1 point so that the steel structure is transformed into a single-phase structure of the α phase with low N solubility. As a result, the dissolved N in the steel precipitates in large quantities as nitrides (such as AlN). By implementing such a heating cycle and controlling the amount of dissolved N, it is possible to suppress the large-scale precipitation of nitrides after finish rolling.

[0164] The inventors of the present invention conducted research and found that the AlN precipitated after rough rolling and before finish rolling is ultimately difficult to become AlN present in the grain boundaries of the α phase. The detailed reason is not clear at present, but it is believed that even if AlN precipitates at the grain boundaries after rough rolling and before finish rolling, the location of AlN (grain boundaries or within grains) will change according to the dynamic and static structural changes caused by finish rolling. Therefore, it is believed that the number of AlN present in the grain boundaries of the α phase is reduced. That is, in this embodiment, it is important to precipitate a large amount of N dissolved in the steel as nitrides (such as AlN) after rough rolling and before finish rolling, and not to redissolve the nitrides after finish rolling. For example, it is believed that if the nitrides are redissolved after finish rolling, the N redissolved in the steel will preferentially precipitate as AlN at the grain boundaries of the α phase during the cooling process after finish rolling.

[0165] For the reasons mentioned above, the rough rolled material after rough rolling is kept below the Ar1 point. On the other hand, if the holding temperature is too low, nitrides will not precipitate and grow easily. Therefore, the rough rolled material after rough rolling is kept at 850°C or above.

[0166] The cooling rate for cooling the rough-rolled material to a temperature range of 850°C to the Ar1 point after rough rolling is not particularly limited. However, after rough rolling, it is preferably cooled to a temperature range of 850°C to the Ar1 point at an average cooling rate of 0.1 to 2°C / sec. An average cooling rate of less than 0.1°C / sec results in poor production efficiency, while an average cooling rate exceeding 2°C / sec may hinder precipitation or growth of nitrides.

[0167] The rough-rolled material, held in the temperature range of 850°C to the Ar1 point, is then reheated to a temperature range exceeding the Ar1 point and below the Ac1 point. As mentioned above, the Ar1 point is the temperature at which the phase transformation to the α phase ends during cooling. The Ac1 point is the temperature at which the phase transformation to the γ phase begins during heating. While the rough-rolled material held in the temperature range of 850°C to the Ar1 point transforms into a single-phase α phase, at this temperature, the finishing and coiling temperatures of the rough-rolled material become too low. Therefore, to increase the finishing and coiling temperatures and enhance the auto-annealing effect in the coiled state, the rough-rolled material after holding is reheated. When the reheating temperature exceeds the Ac1 point, a phase transformation from the α phase to the γ phase occurs, and nitrogen redissolves in the steel. This redissolved nitrogen precipitates as nitrides (e.g., AlN) during the cooling process after finish rolling. This precipitation, in particular, occurs in large quantities at the grain boundaries of the α phase, hindering grain growth during auto-annealing and final annealing. Therefore, the reheating temperature is set below the Ac1 point. On the other hand, to achieve a sufficient auto-annealing effect by increasing the finishing and coiling temperatures, the reheating temperature should be above the Ar1 point. Furthermore, multiple heating cycles are possible as long as they are within this temperature range. The reheating method and form are not particularly limited, and induction heating, for example, may be used. The Ar1 and Ac1 temperatures can be determined experimentally.

[0168] Finish rolling is performed on the rough rolled material that has been reheated to a temperature range exceeding the Ar1 point and below the Ac1 point. The finishing temperature of the finishing rolling is set to be 800°C or higher and below the Ar1 point. As mentioned above, the Ar1 point refers to the temperature at which the phase transformation to the α phase is completed during cooling. If the finishing temperature of the finishing rolling is lower than 800°C, a sufficient coiling temperature cannot be ensured. Therefore, the finishing temperature of the finishing rolling is 800°C or higher. On the other hand, if the finishing temperature of the finishing rolling exceeds the Ar1 point, the γ phase partially remains as a steel structure in the finished rolled material, and a γ→α phase transformation occurs during coiling after the finishing rolling. The N dissolved in the γ phase precipitates at the grain boundaries of the α phase, which, as a result, hinders the grain growth during self-annealing and final annealing. Therefore, the finishing temperature of the finishing rolling is set to be below the Ar1 point.

[0169] The coiling temperature for the finished rolled material is set to 750°C or higher and 850°C or lower. If the coiling temperature is lower than 750°C, the grains will not grow sufficiently during auto-annealing. Therefore, the coiling temperature should be set to 750°C or higher. On the other hand, if the coiling temperature exceeds 850°C, the surface scale (surface oxide) of the finished rolled material will be excessive, and the detergency during pickling will be reduced. Therefore, the coiling temperature should be set to 850°C or lower.

[0170] Hot-rolled steel sheets produced under the aforementioned manufacturing conditions contain a low amount of AlN within α-phase grains and at grain boundaries, particularly at α-phase grain boundaries. This allows for sufficient grain growth during autogenous annealing and final annealing after hot rolling, resulting in non-oriented electrical steel sheets with excellent iron loss characteristics at high frequencies in addition to typical magnetic properties.

[0171] <Method for Manufacturing Non-Oriented Magnetic Steel Sheet>

[0172] Hereinafter, a method for producing a non-oriented electrical steel sheet according to the present embodiment will be described.

[0173] The method for producing a non-oriented electrical steel sheet according to the present embodiment is a method for producing a non-oriented electrical steel sheet using the above-mentioned hot-rolled steel sheet.

[0174] The hot rolled steel sheets produced under the above-mentioned production conditions are cold rolled without annealing.

[0175] The cold-rolled material after the above-mentioned cold rolling is subjected to final annealing at a temperature of 800° C. or higher and Ac1 point or lower.

[0176] The hot-rolled steel sheet produced under the above-mentioned production conditions is pickled, then cold-rolled, and finally annealed. The cold-rolling conditions are not particularly limited. Known cold-rolling conditions may be applied.

[0177] The final annealing temperature is set to be 800°C or higher and below the Ac1 point. If the final annealing temperature is lower than 800°C, unrecrystallized structures remain, degrading magnetic properties. Therefore, the final annealing temperature is set to be 800°C or higher. On the other hand, if the final annealing temperature exceeds the Ac1 point, an α→γ phase transformation occurs, deteriorating magnetic properties. Therefore, the final annealing temperature is set to be below the Ac1 point.

[0178] Furthermore, the final annealing time is preferably 10 seconds to 600 seconds. If the final annealing time is within the above range, the crystal grains can be sufficiently grown.

[0179] The non-oriented electrical steel sheet produced by satisfying the above-mentioned production conditions is excellent in not only general magnetic properties but also iron loss properties at high frequencies.

[0180] The lower the iron loss of non-oriented electrical steel sheets, the better. For example, the iron loss W15 / 50 is preferably less than 5.2 W / kg, and the iron loss W10 / 200 is preferably less than 18.0 W / kg. Furthermore, the higher the magnetic flux density of non-oriented electrical steel sheets, the better. For example, the magnetic flux density B50 is preferably 1.69 T or higher, and the magnetic flux density B25 is preferably 1.62 T or higher.

[0181] Furthermore, the magnetic properties of an electromagnetic steel sheet, such as magnetic flux density, can be measured using known methods. For example, the magnetic properties of an electromagnetic steel sheet can be measured using a method based on the Epstein test specified in JIS C2550:2011, or the single sheet magnetic properties test method (Single Sheet Tester: SST) specified in JIS C2556:2015. Furthermore, during research and development, when steel ingots are formed using a vacuum melting furnace, etc., it is difficult to obtain test pieces of the same size as those used in actual machine manufacturing. In this case, for example, a test piece measuring 55 mm in width and 55 mm in length can be obtained and measured using the single sheet magnetic properties test method. Furthermore, the obtained result can be multiplied by a correction factor to obtain a measurement value equivalent to that obtained using the Epstein test method. In this embodiment, measurements are performed using a method based on the single sheet magnetic properties test method.

[0182] [Example 1]

[0183] The effects of one embodiment of the present invention are further specifically described through examples. The conditions in the examples are merely examples of conditions employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be employed within the scope of the present invention as long as they do not deviate from the spirit of the present invention and achieve the objectives of the present invention.

[0184] <Example 1>

[0185] Slabs having the chemical compositions listed in Tables 1A and 1B were hot rolled to a thickness of 2.5 mm under the production conditions of the hot rolling marks listed in Tables 2A and 2B, and the hot-rolled steel sheets were coiled.

[0186] [Table 1A]

[0187]

[0188] [Table 1B]

[0189]

[0190] [Table 2A]

[0191]

[0192] [Table 2B]

[0193]

[0194] The chemical composition of the produced hot-rolled steel sheet was equivalent to that of the slab. A test piece was cut from the center of the hot-rolled steel sheet in the width direction, and a transmission electron microscope (TEM) specimen was prepared so that a cross section parallel to the rolling direction and the thickness direction could be observed. The transmission electron microscope (TEM) observed a 10 μm × 10 μm field of view, and the number density of AlN particles with an equivalent circle diameter of 10 to 200 nm was calculated as described above. The results are shown in Tables 3A to 3C.

[0195] [Table 3A]

[0196]

[0197] [Table 3B]

[0198]

[0199] [Table 3C]

[0200]

[0201] Furthermore, the hot-rolled steel sheets were pickled and then cold-rolled to 0.5 mm to obtain cold-rolled steel sheets, which were then subjected to final annealing under the conditions of the final annealing mark described in Table 4 to obtain non-oriented electrical steel sheets.

[0202] [Table 4]

[0203]

[0204] 55 mm square test pieces were cut from the non-oriented electrical steel sheets after final annealing in parallel with the rolling direction and the sheet width direction. Iron loss and magnetic flux density were measured according to the single sheet magnetic property test method (JIS C 2556:2015), and the average values ​​in the L and C directions were determined.

[0205] Regarding iron loss, in addition to W15 / 50, a conventionally common evaluation index, W10 / 200, representing the iron loss when used at high frequencies, was also measured. W15 / 50 refers to the iron loss of a non-oriented electrical steel sheet excited at 50 Hz to 1.5 T, while W10 / 200 refers to the iron loss of a non-oriented electrical steel sheet excited at 200 Hz to 1.0 T.

[0206] The magnetic flux density was measured as B50 and B25. B50 refers to the magnetic flux density when a magnetic field of 5000 A / m is applied to the non-oriented electrical steel sheet at 50 Hz, and B25 refers to the magnetic flux density when a magnetic field of 2500 A / m is applied to the non-oriented electrical steel sheet at 50 Hz.

[0207] The samples were judged as passing if W15 / 50 was less than 5.2 W / kg, W10 / 200 was less than 18.0 W / kg, B50 was 1.69 T or more, and B25 was 1.62 T or more. The results are shown in Tables 3A to 3C.

[0208] As shown in Tables 3A to 3C, the inventive examples satisfy both the chemical composition and AlN number density requirements, resulting in excellent magnetic properties. In contrast, as shown in Tables 3A to 3C, the comparative examples do not satisfy either the chemical composition or AlN number density requirements, resulting in poor manufacturability and poor magnetic properties.

[0209] Furthermore, in Comparative Examples No. d30 and No. d31, the Ti, Nb, V, and Zr contents in the slab composition did not meet the preferred ranges. Furthermore, after rough rolling, the rough-rolled material was not held within the temperature range of 850°C or higher and the Ar1 point or lower, and the rough-rolled material was not heated to a temperature range exceeding the Ar1 point and below the Ac1 point. In Comparative Examples No. d30 and No. d31, care was taken to prevent the steel sheet temperature from dropping during rough and finish rolling. Therefore, even without reheating after rough rolling, the finish rolling end temperature was 800°C or higher. Although the finish rolling end temperature was 800°C or higher in Comparative Examples No. d30 and No. d31, the lack of holding and reheating after rough rolling prevented optimal control of the AlN number density in the hot-rolled steel sheets. Consequently, in Comparative Examples No. d30 and No. d31, the W15 / 50 ratio was met for non-oriented electrical steel sheets, but the W10 / 200 ratio was not excellent.

[0210] <Example 2>

[0211] Slabs having the chemical compositions listed in Tables 1A and 1B were hot rolled to a thickness of 2.5 mm under the production conditions of the hot rolling marks listed in Tables 2A and 2B, and the hot-rolled steel sheets were coiled.

[0212] The chemical composition of the produced hot-rolled steel sheet was equivalent to that of the slab. A test piece was cut from the center of the hot-rolled steel sheet in the width direction, and a transmission electron microscope (TEM) specimen was prepared so that a cross section parallel to the rolling direction and the thickness direction could be observed. The TEM observation field was 10 μm × 10 μm, and the number density of AlN particles with an equivalent circle diameter of 10 to 200 nm was calculated as described above. The results are shown in Table 5.

[0213] [Table 5]

[0214]

[0215] Furthermore, the hot-rolled steel sheets were pickled and then cold-rolled to 0.5 mm to obtain cold-rolled steel sheets, which were then subjected to final annealing under the conditions of the final annealing mark described in Table 4 to obtain non-oriented electrical steel sheets.

[0216] 55 mm square test pieces were cut from the non-oriented electrical steel sheets after final annealing in parallel with the rolling direction and the sheet width direction. Iron loss and magnetic flux density were measured according to the single sheet magnetic property test method (JIS C 2556:2015), and the average values ​​in the L and C directions were determined.

[0217] The iron loss was measured at W10 / 200, which is the iron loss when used at high frequencies, in addition to W15 / 50, a conventional general evaluation index. The magnetic flux density was measured at B50 and B25.

[0218] As in Example 1, the samples were judged as acceptable if W15 / 50 was less than 5.2 W / kg, W10 / 200 was less than 18.0 W / kg, B50 was 1.69 T or more, and B25 was 1.62 T or more. The results are also shown in Table 5.

[0219] As shown in Table 5, the examples of the present invention satisfy the chemical composition and AlN number density, and therefore have excellent magnetic properties.

[0220] Industrial Availability

[0221] According to the above aspects of the present invention, a hot-rolled steel sheet for non-oriented electrical steel sheet, a method for producing a hot-rolled steel sheet for non-oriented electrical steel sheet, and a method for producing a non-oriented electrical steel sheet that exhibit excellent iron loss characteristics at high frequencies in addition to general magnetic characteristics can be provided. Therefore, the present invention has high industrial applicability.

Claims

1. A hot-rolled steel sheet for non-oriented electromagnetic steel sheet, characterized in that: As chemical components, in mass%, C: 0.005% or less, Si: 0.10-1.50%, Mn: 0.10~0.60%, P: 0.100% or less, Al：0.20～1.00％、 Ti: 0.0010~0.0030%, Nb: 0.0010~0.0030%, V：0.0010～0.0030％、 Zr：0.0010～0.0030％、 N: 0.0030% or less, Sn: 0-0.20%, Sb: 0~0.20%, The rest is composed of Fe and impurities. When observed in a cross section parallel to the rolling direction and the plate thickness direction, AlN with an equivalent circle diameter of 10 to 200 nm exists within the ferrite grains and at the grain boundaries. The number density of the AlN present in the crystal grains and the grain boundaries was 8.0 pieces / μm with respect to the observation area. 2 the following, The number density of the AlN particles present in the grain boundaries is 40 particles / μm with respect to the grain boundary area. 2 The following, and When the hot-rolled steel sheet is cold-rolled without annealing and annealed at 800° C. or higher and below the Ac1 point, the iron loss W15 / 50 and the iron loss W10 / 200 are measured. The iron loss W15 / 50 is less than 5.2 W / kg, and the iron loss W10 / 200 is less than 18.0 W / kg.

2. The hot-rolled steel sheet for non-oriented electrical steel sheet according to claim 1, wherein: As chemical components, in mass%, Sn: 0.02~0.20%, Sb: at least one of 0.02% to 0.20%.

3. A method for producing a hot-rolled steel sheet for non-oriented electrical steel sheet, the method for producing a hot-rolled steel sheet for non-oriented electrical steel sheet according to claim 1 or 2, characterized in that: Heat the slab to a temperature range of 1050°C to 1180°C. The heated slab is subjected to rough rolling, The rough rolled material after the rough rolling is kept at a temperature range of 850°C or higher and below the Ar1 point. The rough rolled material after holding is reheated to a temperature range exceeding the Ar1 point and below the Ac1 point. The rough rolled material immediately after heating is subjected to finish rolling under the condition that the finishing temperature after finish rolling is 800° C. or higher and Ar1 point or lower. The finished rolled material is coiled at a temperature of 750°C to 850°C. The slab contains, as chemical components, in mass %, C: 0.005% or less, Si: 0.10-1.50%, Mn: 0.10~0.60%, P: 0.100% or less, Al：0.20～1.00％、 Ti: 0.0010~0.0030%, Nb: 0.0010~0.0030%, V：0.0010～0.0030％、 Zr：0.0010～0.0030％、 N: 0.0030% or less, Sn: 0-0.20%, Sb: 0~0.20%, The remainder is composed of Fe and impurities.

4. A method for producing a non-oriented electrical steel sheet using the hot-rolled steel sheet for non-oriented electrical steel sheet according to claim 1 or 2, characterized in that: The hot-rolled steel sheet for non-oriented electrical steel sheet is cold-rolled without annealing the hot-rolled steel sheet. The cold-rolled material after the cold rolling is subjected to final annealing at a temperature of 800° C. or higher and below the Ac1 point.

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