Grain-oriented electrical steel sheet and its manufacturing method

Abstract

To provide a grain-oriented electromagnetic steel sheet having improved magnetism by comprising plural cold rolling and decarbonization annealing steps, and a method for producing the same.SOLUTION: A method for producing a grain-oriented electromagnetic steel sheet has the steps of: heating a slab; hot-rolling the slab to produce a hot-rolled steel sheet; subjecting the hot-rolled steel sheet to hot-rolled sheet annealing; subjecting the hot-rolled steel sheet subjected to the hot-rolled sheet annealing to primary cold rolling; subjecting the steel sheet subjected to the primary cold rolling to decarburization annealing; subjecting the steel sheet completed with the decarburization annealing to secondary cold rolling; subjecting the steel sheet completed with the secondary cold rolling to continuous annealing; and subjecting the steel sheet subjected to the continuous annealing to batch annealing.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof, and more particularly to a grain-oriented electrical steel sheet having improved magnetic properties by including multiple cold rolling and decarburization annealing steps, and a manufacturing method thereof. [Background technology]

[0002] Grain-oriented electrical steel sheets are steel sheets with a crystal orientation of {110} <001> It is a soft magnetic material with excellent magnetic properties in the rolling direction, consisting of crystal grains with an alternative name of Goss orientation. Such grain-oriented electrical steel sheets are manufactured by rolling the slab to the final thickness through hot rolling, hot-rolling annealing, and cold rolling after slab heating, and then undergoing primary recrystallization annealing and high-temperature annealing for secondary recrystallization formation. Generally, the secondary recrystallization annealing process of grain-oriented electrical steel requires a low heating rate and long-term purification annealing at high temperature, which is a process that consumes a lot of energy. In order to manufacture grain-oriented electrical steel with excellent magnetic properties by forming secondary recrystallization through such an extreme process, the following process difficulties arise.

[0003] First, there is a temperature difference between the outer and inner coil parts due to heat treatment in the coil state, and it is not possible to apply the same heat treatment pattern to each part, which results in a magnetic difference between the outer and inner coil parts. Second, after decarburization annealing, MgO is coated on the surface, and in the process of forming a base coating during high-temperature annealing, various surface defects occur, reducing the yield. Third, the decarburized sheet that has undergone decarburization annealing is wound into a coil, and then annealed at high temperature, then annealed again for flattening, and then coated with insulation, resulting in a three-step production process that reduces the yield. In order to overcome these process limitations, a technology has been proposed that utilizes normal crystal growth without secondary recrystallization by adjusting the decarburization annealing and cold rolling reduction rate. However, in continuous annealing, the grain growth of the final grains is limited due to the short heat treatment time of moisture, and grains cannot grow to have an optimal grain size, so there is a limit to the improvement of iron loss. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a grain-oriented electrical steel sheet having improved magnetic properties by including multiple cold rolling and decarburization annealing steps, and a manufacturing method thereof. [Means for solving the problem]

[0005] The method for manufacturing a grain-oriented electrical steel sheet according to the present invention includes the steps of heating a slab, hot rolling the slab to manufacture a hot-rolled steel sheet, hot-rolling the hot-rolled steel sheet, first cold rolling the hot-rolled steel sheet that has been annealed, decarburization annealing the first cold-rolled steel sheet, second cold rolling the steel sheet that has been subjected to decarburization annealing, continuous annealing the steel sheet that has been subjected to the second cold rolling, and batch annealing the continuously annealed steel sheet.

[0006] The slab contains, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the remainder being Fe and unavoidable impurities; It is particularly characterized by further containing Mn: 0.1% by weight or less and S: 0.005% by weight or less.

[0007] The step of hot-rolling the slab includes a decarburization process, The hot-rolled sheet annealing step is performed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C. The step of decarburizing and annealing the first cold-rolled steel sheet is characterized by annealing at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.

[0008] The step of decarburizing the first cold-rolled steel sheet is to anneal the steel sheet in an austenite single phase region or in a region where a composite phase of ferrite and austenite exists, After the first cold-rolled steel sheet is decarburized and annealed, the average grain diameter is 150 to 250 μm. The step of subjecting the first cold-rolled steel sheet to decarburization annealing and the step of subjecting the steel sheet to the second cold-rolling after the decarburization annealing are repeated two or more times.

[0009] The continuous annealing step is performed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C for 1 to 5 minutes. The batch annealing step is performed at a temperature of 1000°C to 1200°C and a dew point temperature of -20°C or less for 1 to 8 hours. After the batch annealing step, the {110} <001> The volume fraction of crystal grains forming an angle of 15° or less from the The crystal grains have a diameter of 1000 μm to 5000 μm, and the surface area ratio of the crystal grains is 20 to 70%.

[0010] The grain-oriented electrical steel sheet according to the present invention is characterized in that the area fraction of crystal grains having diameters of 1000 μm to 5000 μm in all crystal grains is 20 to 70%.

[0011] The electrical steel sheet contains, by weight, 1.0% to 4.0% Si, 0.005% or less (excluding 0%) C, and the remainder being Fe and unavoidable impurities; Further containing Mn: 0.1 wt% or less and S: 0.005 wt% or less, {110} <001> The volume fraction of crystal grains forming an angle of 15° or less from the The Goss crystal grains having a ratio (D2 / D1) of the circumscribing circle diameter (D1) to the inscribing circle diameter (D2) of 0.5 or more account for 95% or more by area of ​​all the Goss crystal grains. Effect of the Invention

[0012] The grain-oriented electrical steel sheet according to the present invention has excellent magnetic properties by stably forming large-diameter Goss grains while utilizing normal crystal growth, In addition, since AlN and MnS are not used as grain growth inhibitors, there is no need to heat the slab at high temperatures of 1300°C or higher. In addition, it is no longer necessary to remove precipitates such as N and S, and the purification annealing time can be relatively short, improving productivity. It is also possible to provide a grain-oriented electrical steel sheet having magnetic properties in which cracks are formed in the width direction. [Brief description of the drawings]

[0013] [Figure 1] 1 is a photograph of the surface of a grain-oriented electrical steel sheet manufactured using invention material 2, observed with a scanning electron microscope. [Diagram 2] 1 is a photograph of the surface of the grain-oriented electrical steel sheet manufactured using comparative material 2, observed with a scanning electron microscope. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Terms such as first, second and third are used to describe various parts, components, regions, layers and / or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, the first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention. The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular form includes the plural form unless the text clearly indicates otherwise. As used in the specification, the meaning of "comprising" embodies certain features, regions, integers, steps, operations, elements and / or components and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements and / or components. When a part is referred to as being "on" another part, it may be "directly on" the other part, or there may be other parts interposed between them. In contrast, when a part is referred to as being "directly on" another part, there are no other parts interposed between them. Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless defined. Moreover, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight. In an embodiment of the present invention, the term "additionally contain an additional element" means that the additional element is included in place of the remaining iron (Fe) by an amount corresponding to the additional element.

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to exemplary embodiments thereof, so that those skilled in the art will be able to easily practice the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein. A method for manufacturing a grain-oriented electrical steel sheet according to the present invention includes the steps of heating a slab, hot rolling the slab to manufacture a hot-rolled steel sheet, hot-rolling the hot-rolled steel sheet, first cold rolling the hot-rolled steel sheet that has been annealed, decarburization annealing the first cold-rolled steel sheet, second cold rolling the steel sheet that has been decarburization annealed, continuous annealing the steel sheet that has been second cold rolling, and batch annealing the continuously annealed steel sheet.

[0016] Each step will be explained in detail below. First, the slab is heated. The slab contains, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the remainder being Fe and unavoidable impurities. The reasons for limiting the composition are as follows. Silicon (Si) reduces the magnetic anisotropy of electrical steel sheet and increases resistivity to improve core loss. If the Si content is less than 1.0 wt%, the core loss is inferior, and if it exceeds 4.0 wt%, the brittleness increases. Therefore, the Si content in the grain-oriented electrical steel sheet after the slab and final annealing stages may be 1.0 to 4.0 wt%. More specifically, the Si content is 1.5 to 3.5 wt%.

[0017] Regarding carbon (C), since the Goss crystal grains in the surface layer diffuse to the center during intermediate decarburization annealing and final decarburization annealing, a process is required in which C in the center escapes to the surface layer. More specifically, the C content in the slab is 0.1-0.4 wt%. Furthermore, after the final annealing stage in which decarburization is completed, the carbon content in the final grain-oriented electrical steel sheet is 0.0050 wt% or less, more specifically 0.002 wt% or less. The slab further contains Mn: 0.1 wt % or less and S: 0.005 wt % or less.

[0018] Mn and S form MnS precipitates and impede the growth of Goss grains that diffuse to the center during the decarburization process. Therefore, it is preferable not to add Mn and S. However, taking into consideration the amount that is inevitably mixed in during the steelmaking process, the Mn and S contents in the grain-oriented electrical steel sheet after the slab and final annealing stages are controlled to Mn: 0.1 wt% or less and S: 0.005 wt% or less, respectively. The balance is composed of Fe and unavoidable impurities. The unavoidable impurities are impurities that are mixed in during the steelmaking stage and the manufacturing process of the grain-oriented electrical steel sheet, and are widely known in the art, so a detailed description will be omitted. In particular, components such as Al, N, Ti, Mg, and Ca react with oxygen in the steel to form oxides and need to be strongly suppressed, so each component can be controlled to 0.005 wt% or less. In one embodiment of the present invention, the addition of elements other than the alloy components described above is not excluded, and various elements can be included within a range that does not harm the technical idea of ​​the present invention. When an additional element is further included, it is included in place of the balance Fe.

[0019] More specifically, the slab contains, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the remainder being Fe and unavoidable impurities. The slab heating temperature is 1100°C to 1350°C, which is higher than the usual heating temperature. If the temperature is high during slab heating, there is a problem that the hot-rolled texture becomes coarse and has a negative effect on magnetic properties. However, in the manufacturing method of grain-oriented electrical steel sheet according to the present invention, since the carbon content of the slab is relatively high, the hot-rolled texture does not become coarse even if the slab reheating temperature is high, and since the slab is reheated at a temperature higher than the usual case, it is more advantageous during hot rolling.

[0020] The slab is then hot rolled to produce a hot rolled steel sheet. Hot rolling is carried out to produce a hot-rolled sheet having a thickness of 1.5 to 4.0 mm by applying an appropriate rolling ratio in the final cold rolling stage so that the final product thickness can be produced. The hot rolling temperature and the cooling temperature are not particularly limited, but an example of a wire that has excellent magnetic properties is one in which the hot rolling end temperature is 950°C or less, and the wire is rapidly cooled with water and coiled at 600°C or less. Next, the hot-rolled steel sheet is annealed. At this time, the hot-rolled steel sheet annealing includes a decarburization process. Specifically, the hot-rolled steel sheet is annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C. After the above-mentioned annealing, additional annealing is performed at a temperature of 1000 to 1200° C. and at a dew point temperature of 0° C. or less. After the hot-rolled sheet annealing is performed, pickling is performed.

[0021] Next, a first cold rolling is carried out to produce a cold-rolled steel sheet. In the manufacturing process of conventional grain-oriented electrical steel sheets, it is effective to perform cold rolling once with a reduction rate of nearly 90%. This is because it creates an environment that is favorable for the grain growth of only the Goss grains in the primary recrystallized grains. However, the manufacturing method of grain-oriented electrical steel sheet according to the present invention does not utilize the abnormal grain growth of Goss-oriented grains, but rather causes the Goss grains in the surface layer generated by decarburization annealing and cold rolling to diffuse inward, so it is advantageous to form a large number of Goss-oriented grains distributed in the surface layer. Therefore, when cold rolling is performed at a rolling reduction of 50% to 70%, more specifically, 55% to 65%, a large amount of Goss texture is formed in the surface layer. Next, the cold rolled steel sheet is decarburized and annealed. At this time, the decarburization and annealing step is performed in an austenite single phase region or a region where a composite phase of ferrite and austenite exists. Specifically, annealing is performed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C. The atmosphere is a mixed gas atmosphere of hydrogen and nitrogen. The amount of decarburization during decarburization and annealing is 0.0300% by weight to 0.0600% by weight. After the above-mentioned annealing, additional annealing is performed at a temperature of 1000°C to 1200°C and a dew point temperature of 0°C or less. During this decarburization annealing process, the crystal grains on the surface of the electrical steel sheet grow coarse, but the crystal grains inside the electrical steel sheet remain as fine structures. After this decarburization annealing, the average diameter of the crystal grains is 150 μm to 250 μm. At this time, the crystal grains are surface ferrite crystal grains. The diameter of the crystal grains refers to the diameter of an imaginary circle having the same area as the crystal grain.

[0022] Next, the steel sheet that has been subjected to the decarburization annealing is subjected to a second cold rolling. The second cold rolling is the same as the first cold rolling, so a detailed description thereof will be omitted. The steps of decarburization annealing the cold-rolled steel sheet and second cold rolling the steel sheet after the decarburization annealing may be repeated two or more times. By repeating the steps two or more times, a Goss texture is formed in a large amount in the surface layer. Next, the steel sheet that has completed the secondary cold rolling is subjected to continuous annealing. In the continuous annealing stage, annealing is performed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C. Before continuous annealing, the cold-rolled sheet is subjected to decarburization annealing, and the carbon content is 40% to 60% of the carbon weight of the slab. Therefore, in the continuous annealing stage, the crystal grains formed in the surface layer diffuse inward as the carbon is released. In the continuous annealing stage, decarburization can be performed so that the carbon content in the steel sheet is 0.005% by weight or less.

[0023] The continuous annealing step is performed for 1 to 5 minutes. The purpose of the continuous annealing step is to decarburize the carbon in the steel and grow the grains to a certain size or more. This is because the fraction of Goss grains increases continuously through the process of decarburization and the grain growth immediately thereafter. This is because the Goss grains grow while eating away at the surrounding non-Goss grains. However, considering the productivity of the continuous annealing, the annealing time is limited to within a few minutes, so the grain growth is restricted. In the present invention, it is claimed that the additional batch annealing induces grain growth, which is effective in reducing iron loss. At this time, the Goss fraction does not increase, but the iron loss is reduced due to the effect of the increase in the grain size. After the continuous annealing step, an annealing separator is applied. Annealing separators are widely known in the art, so a detailed description will be omitted. For example, an annealing separator mainly composed of MgO can be used.

[0024] Next, the continuously annealed steel sheet is subjected to batch annealing. Batch annealing means that the steel sheet is wound in a coil and annealed. In the batch annealing step, a texture having the Goss orientation diffused in the continuous annealing step grows. In the method for producing a grain-oriented electrical steel sheet according to the present invention, the Goss texture is different from the case where grains grow by the conventional abnormal grain growth, and the grain diameter is 5 mm or less. Specifically, the grain fraction having a diameter of 1000 um to 5000 um increases. Therefore, compared to the grain-oriented electrical steel sheet produced by the conventional abnormal grain growth, there are many Goss grains having a small grain size, but the grain size is appropriately controlled to minimize iron loss. More specifically, the area fraction of grains having a diameter of 1000 μm to 5000 μm in the total grains is 20 to 70%. At this time, the area fraction of the grains is measured on a plane parallel to the rolling surface (ND surface) of the steel sheet. More specifically, the area fraction of grains having a diameter of 1000 μm to 5000 μm in the total grains is 20 to 60%. More specifically, the area fraction of crystal grains having a diameter of 1000 μm to 5000 μm in the entire crystal grains is 20 to 50%. The batch annealing step can be performed at a temperature of 1000°C to 1200°C and a dew point temperature of -20°C or lower. In addition, the batch annealing is performed for 1 to 8 hours, more specifically, for 2 to 5 hours. In addition, the manufacturing method of the grain-oriented electrical steel sheet according to the present invention has a high Goss fraction, which improves the magnetic properties. <001> The volume fraction of crystal grains making an angle of 15° or less from the surface is 40% or more, more specifically 40% to 75%, and even more specifically 45% to 60%.

[0025] In the grain-oriented electrical steel sheet according to the present invention, the area fraction of crystal grains having a diameter of 1000 μm to 5000 μm among all crystal grains is 20 to 70%. The area distribution of crystal grains has been described in detail in relation to the manufacturing method of grain-oriented electrical steel sheet, so a duplicated description will be omitted. The electrical steel sheet contains, by weight, 1.0% to 4.0% Si, 0.005% or less (except 0%) C, and the balance being Fe and unavoidable impurities. The electrical steel sheet further contains Mn: 0.1% by weight or less and S: 0.005% by weight or less. Except for C, the compositional limitations are the same as those for slabs, so duplicate explanations will be omitted. {110} <001> The volume fraction of crystal grains making an angle of 15° or less from the surface is 40% or more. The ratio (D2 / D1) of the circumscribing circle diameter (D1) to the inscribing circle diameter (D2) of 0.5 or more accounts for 95% or more of the total area of ​​the Goss crystal grains. The crystal grains of the above-mentioned shape are formed by the manufacturing process unique to the present invention.

[0026] The grain-oriented electrical steel sheet according to the present invention has a high Goss fraction, which improves the magnetic properties. 17 / 50 ) is 1.3W / kg or less, and more specifically, iron loss (W 17 / 50 ) is 1 to 1.3 W / kg, more specifically, 1.1 to 1.25 W / kg. 17 / 50 is the magnitude of iron loss (W / kg) induced under 1.7 Tesla and 50 Hz conditions. Hereinafter, specific examples of the present invention will be described. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples.

[0027] Example 1 The slab, containing, by weight, 2.32% Si, 0.195% C, with the remainder being Fe and unavoidable impurities, was heated at a temperature of 1250°C and then hot-rolled, and then hot-rolled at an annealing temperature of 950°C and a dew point temperature of 60°C. The steel sheet was then cooled and pickled, and cold-rolled at a reduction rate of 65% to produce a cold-rolled sheet with a thickness of 0.8 mm. The cold-rolled sheet was again decarburized and annealed at 950°C in a wet mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60°C) for 80 seconds, and then cold-rolled again at a reduction rate of 65% to produce a cold-rolled sheet with a thickness of 0.28 mm. Thereafter, in the final annealing, decarburization annealing was performed for 2 minutes in a wet mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60°C) at a temperature of 950°C, and then heat treatment was continuously performed in a mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60°C) at 1100°C as shown in Table 1, or heat treatment was performed in a coiled state in a mixed gas atmosphere of hydrogen and nitrogen at 1200°C for the time shown in Table 1 below. Table 1 shows the Goss fraction of grains, the area fraction of grains having a size of 1 mm or more and 5 mm or less, and the core loss of grains of the grain-oriented electrical steel sheets after high-temperature annealing according to the examples. <001> The volume fraction of crystal grains that make an angle of 15° or less from the surface was measured. The surface of the finally obtained steel sheet was washed, and the iron loss was measured at 1.7 Tesla and 50 Hz using a single sheet magnetic measurement method.

[0028] [Table 1] As shown in Table 1, it can be confirmed that the surface fraction of crystal grains having diameters of 1 to 5 mm is high in the inventive materials 1 to 4, which were batch annealed for an appropriate time. It can be confirmed that the iron loss is rather superior even though the Goss fraction is relatively low compared to the comparative materials. 1 and 2 show photographs of the surfaces of grain-oriented electrical steel sheets manufactured using Inventive Material 2 and Comparative Material 2, observed with a scanning electron microscope. As can be seen from Figures 1 and 2, it can be seen that the grains of the grain-oriented electrical steel sheet manufactured using Inventive Material 2 were formed relatively large.

[0029] The present invention is not limited to the above-described embodiment and / or examples, but can be manufactured in various different forms, and a person having ordinary skill in the art to which the present invention pertains should understand that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, it should be understood that the above-described embodiment and / or examples are illustrative in all respects and not limiting.

Claims

1. Heating a slab containing, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the balance being Fe and unavoidable impurities; hot rolling the slab to produce a hot rolled steel sheet; annealing the hot-rolled steel sheet; performing a first cold rolling on the hot-rolled steel sheet that has been annealed; decarburization annealing the first cold rolled steel sheet; a step of subjecting the steel sheet having been subjected to the decarburization annealing to a second cold rolling; A step of continuously annealing the steel sheet after the second cold rolling is completed; and The method includes the step of batch annealing the continuously annealed steel sheet, The hot rolling step is performed after heating the slab at a temperature of 1100° C. to 1350° C., The step of manufacturing the hot-rolled steel sheet includes setting the hot-rolling end temperature to 950° C. or less and coiling at 600° C. or less, The thickness of the hot-rolled steel plate is 1.5 to 4.0 mm, The hot-rolled sheet annealing step is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C., The reduction ratios of the first cold rolling step and the second cold rolling step are each 50% to 70%; The step of decarburizing the first cold-rolled steel sheet is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C. The continuous annealing step is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C., The batch annealing step comprises annealing at a temperature of 1000° C. to 1200° C. and a dew point temperature of −20° C. or less for 1 to 8 hours.

2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the step of annealing the hot-rolled sheet includes a decarburization process.

3. 3. The method of claim 1, wherein the step of decarburizing the first cold-rolled steel sheet comprises annealing the steel sheet in an austenite single phase region or in a region where a composite phase of ferrite and austenite exists.

4. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the average diameter of crystal grains after the step of decarburization annealing the primarily cold-rolled steel sheet is 150 to 250 μm.

5. 5. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the step of decarburization annealing the first cold-rolled steel sheet and the step of second cold-rolling the steel sheet after the decarburization annealing are repeated two or more times.

6. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 5, wherein the continuous annealing step comprises annealing for 1 to 5 minutes.

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