Grain-oriented electrical steel sheet and its manufacturing method

Abstract

The present invention provides a grain-oriented electrical steel sheet in which the residual Al amount in the slab and the amount of nitriding inside the steel sheet are controlled to improve the uniformity of magnetic properties, and a manufacturing method thereof. [Solution] A method for manufacturing grain-oriented electrical steel sheet according to one embodiment of the present invention contains, by weight, 2.5 to 4.0% Si, 0.03 to 0.09% C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (excluding 0%) S, and 0.002 to 0.012% N, with the remainder being Fe and other unavoidably mixed impurities, and includes the steps of hot rolling a slab to produce a hot-rolled sheet, cold rolling the hot-rolled sheet to produce a cold-rolled sheet, performing a primary recrystallization annealing on the cold-rolled sheet, and performing a secondary recrystallization annealing on the cold-rolled sheet after the primary recrystallization annealing is completed.

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 in which the amount of residual Al in the slab and the amount of nitriding inside the steel sheet are controlled to improve the uniformity of magnetic properties, and a manufacturing method thereof. [Background technology]

[0002] Grain-oriented electrical steel sheets are used as iron core materials for stationary equipment such as transformers, electric motors, generators, and other electronic devices. The final product of grain-oriented electrical steel sheets has a texture in which the crystal grains are oriented in the 110

[0001] direction, giving it extremely excellent magnetic properties in the rolling direction, so it is used as iron core materials for transformers, electric motors, generators, and other electronic devices, and low iron loss is required to reduce energy loss, while high magnetic flux density is required to reduce the size of power generating equipment.

[0003] The iron loss of grain-oriented electrical steel sheets can be divided into hysteresis loss and eddy current loss, and reducing eddy current loss requires efforts such as increasing the specific resistivity and reducing the product sheet thickness. While reducing the product thickness presents the challenge of rolling grain-oriented electrical steel sheets, which are difficult to roll, into extremely thin sheets, the biggest challenge and problem that must be overcome in producing extremely thin products with extremely low iron loss characteristics is maintaining a very strong concentration of the Goss orientation, which is the secondary recrystallization structure of grain-oriented electrical steel sheets.

[0004] Considering the issues with rolling when producing ultra-thin products, the optimal reduction ratio for grain-oriented electrical steel, which involves a low-temperature heating process and a single intensive cold rolling process, is generally known to be around 90%. To achieve a 90% cold rolling reduction ratio, the hot-rolled sheet must be hot-rolled to a thickness of 2.0 mm or less. As the hot-rolled thickness becomes thinner, a higher reduction ratio is required, leading to reduced productivity due to issues such as maintaining the hot-rolling temperature, edge scabs on the edges of the hot-rolled sheet, and the shape of the top and tail of the coil. Furthermore, as the length of the hot-rolled coil increases, differences in rolling time and hot-rolling temperature between the top and tail of the coil inevitably occur, making it more difficult to form uniform fine precipitates along the length of the coil. In addition, when a slab is heated for hot rolling, the temperature of the skid contact area is lower than the temperature of the non-contact area when the slab moves in the hot rolling reheating furnace. This causes a temperature deviation, which inevitably results in a difference in solid solution precipitates (fine precipitates) along the length of the hot rolled sheet. This difference can cause problems such as deviations in the magnetic properties of the final product.

[0005] An even more important issue is that as product thickness decreases, the precipitates wash away from the surface more quickly during the secondary recrystallization annealing process, especially in the section where secondary recrystallization of the Goss orientation occurs, making it difficult to maintain a strong concentration of the Goss orientation. This is directly related to the magnetic properties of the product, making it difficult to ensure very low iron loss characteristics when making ultra-thin products.

[0006] One method proposed to overcome this problem is to increase the N2 gas fraction during the secondary recrystallization annealing process to prevent the precipitates from washing away. However, this method has the problem of inducing surface defects such as nitrogen release holes on the surface of the product sheet. An economical manufacturing method using simultaneous decarburization and nitriding has also been proposed. It has been clearly shown that when manufacturing decarburized steel plates using this method, there is a difference between the surface grain size and the center grain size, and it has been suggested that this difference must be controlled within a certain range.

[0007] A technology has also been proposed that dramatically improves magnetic properties by including segregation elements such as Sb, P, and Sn. The segregation elements have been used as auxiliary inhibitors to prevent precipitate washout during the production of ultra-thin products by adding more segregation elements, but excessive addition makes ultra-thin rolling difficult, and excessive addition of segregation elements makes the oxide layer uneven and thin, which deteriorates the properties of the base coating and causes further precipitate washout, making it difficult to ensure stable magnetic properties.

[0008] A method has also been proposed for adjusting the oxidation capacity and nitriding treatment of the front part in the primary recrystallization annealing process when manufacturing ultra-thin products. However, there is a problem in that the manufacturing of ultra-thin products is very sensitive to the effect of precipitate washout. Another proposed method involves adding Cr to the slab and adjusting the amount of nitriding gas input in the front and back stages of the primary recrystallization annealing process. However, while this method maintains a uniform nitrogen content across the steel sheet thickness, it has the problem of uneven distribution of AlN precipitates, which still results in deviations in magnetic properties. Furthermore, adding Cr deepens the oxide layer, which increases the thickness of the base coating, creating problems in the production of ultra-thin products where the coating layer accounts for a large proportion of the product. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a grain-oriented electrical steel sheet and a manufacturing method thereof, specifically, a grain-oriented electrical steel sheet in which the amount of residual Al in the slab and the amount of nitriding inside the steel sheet are controlled to improve the uniformity of magnetic properties, and a manufacturing method thereof. [Means for solving the problem]

[0010] A method for producing a grain-oriented electrical steel sheet according to the present invention includes the steps of: hot rolling a slab containing, by weight%, 2.5 to 4.0% Si, 0.03 to 0.09% C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (excluding 0%) S, and 0.002 to 0.012% N, with the balance being Fe and other unavoidably mixed impurities, and satisfying the following formulas 1 and 2 to produce a hot-rolled sheet; cold rolling the hot-rolled sheet to produce a cold-rolled sheet; performing primary recrystallization annealing on the cold-rolled sheet; and performing secondary recrystallization annealing on the cold-rolled sheet after the primary recrystallization annealing. After the step of primary recrystallization annealing, the following formula 3 is satisfied: [Formula 1] [Al]-27 / 14×[N]≧0.0240 [Formula 2] [Al] / [N]≦14 (In Equations 1 and 2, [Al] and [N] represent the Al and N contents (wt%) in the slab, respectively.) [Formula 3] [N tot ]-[N 1 / 4t~3 / 4t ]≦60×(10×[t]-1) (In Equation 3, [N tot ] means the nitrogen content (ppm) in the entire steel sheet, and [N 1 / 4t~3 / 4t ] means the nitrogen content (ppm) at 1 / 4 to 3 / 4 of the total thickness of the steel sheet, and [t] indicates the thickness of the cold-rolled sheet (mm).

[0011] The slab may further contain one or more of Ti and V, either alone or in total, in an amount of 0.002 to 0.01 wt %.

[0012] The slab may further contain Sn and Sb in a total amount of 0.03 to 0.15 wt %, and P: 0.01 to 0.05 wt %.

[0013] The slab may further contain one or more of Cr: 0.01 wt % or less and Ni: 0.01 wt % or less.

[0014] The primary recrystallization annealing step includes a former process and a latter process, and the amount of nitriding gas input in the former process (A) relative to the total amount of nitriding gas input in the primary recrystallization annealing step (B) satisfies the following formula 4. [Formula 4] 0.05≦[A] / [B]≦[t] (In Equation 4, the unit of the input amount of nitriding gas is Nm 3 / hr, and [t] indicates the thickness of the cold-rolled sheet (mm).

[0015] The execution time of the first step may be 10 to 80 seconds, and the execution time of the second step may be 30 to 100 seconds.

[0016] The former step and the latter step may be carried out at a temperature of 800 to 900°C.

[0017] The former step and the latter step may be carried out in an atmosphere with an oxidizing power (PH2O / PH2) of 0.5 to 0.7.

[0018] The steel sheet after the primary recrystallization annealing can satisfy the following formula 5. [Formula 5] 1≦[G 1 / 4t ]-[G 1 / 2t ]≦3 (In Equation 5, [G 1 / 4t ] means the average grain size (μm) measured at 1 / 4 of the total thickness of the steel sheet, and [G 1 / 2t ] means the average grain size (μm) measured at half the thickness of the steel sheet.)

[0019] The steel sheet after the secondary recrystallization annealing can satisfy the following formula 6. [Formula 6] [D S ] / [D L ]≦0.1 (In Equation 6, [D S ] indicates the number of crystal grains with a grain size of 5 mm or less, and [D L ] indicates the number of crystal grains with a grain size of more than 5 mm.)

[0020] The ratio of maximum Al luminous intensity to maximum Mg luminous intensity of the base coating layer after the secondary recrystallization annealing may be 0.05 to 0.10.

[0021] The grain-oriented electrical steel sheet according to the present invention comprises an electrical steel sheet substrate containing, by weight, 2.5 to 4.0% Si, 0.005% or less (excluding 0%) C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (excluding 0%) S, and 0.0100% or less (excluding 0%) N, with the remainder being Fe and other unavoidably mixed impurities, and a base coating layer located on the electrical steel sheet substrate, wherein the ratio of maximum Al luminescence intensity to maximum Mg luminescence intensity in the base coating layer is 0.05 to 0.10. [Effects of the Invention]

[0022] The grain-oriented electrical steel sheet of the present invention can improve its magnetic properties by adjusting the Al and N contents in the slab and controlling the amount of nitriding depending on the thickness. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 2 is a schematic diagram showing the results of glow discharge optical emission spectroscopy (GDS) of the surface of the grain-oriented electrical steel sheet according to one example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited to these. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Therefore, a 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 for the purpose of referring to particular embodiments only and is not intended to be limiting of the invention.

[0025] As used herein, the singular forms "a," "an," "the," and the like include the plural forms "a," "the," and the like, unless the phrase or the like clearly indicates otherwise. As used in the specification, the meaning of "comprising" embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other properties, regions, integers, steps, operations, elements, and / or components.

[0026] When a part is referred to as being "on" or "above" another part, this may mean that it is exactly on top of the other part or that there may be other parts between them. In contrast, when a part is referred to as being "directly on top of" another part, there are no other parts between them.

[0027] 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 pertains. 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 otherwise defined.

[0028] Unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight. In one embodiment of the present invention, the inclusion of an additional element means that the additional amount of the additional element is included to replace the remaining iron (Fe). Although the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein, the present invention will be described in detail below so that those skilled in the art can easily practice the present invention.

[0029] A method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention includes the steps of hot-rolling a slab to manufacture a hot-rolled sheet, cold-rolling the hot-rolled sheet to manufacture a cold-rolled sheet, performing primary recrystallization annealing on the cold-rolled sheet, and performing secondary recrystallization annealing on the cold-rolled sheet after the primary recrystallization annealing.

[0030] Each step will be explained in detail below. First, a slab is hot rolled to produce a hot-rolled sheet. The slab alloy ingredients are described below. The slab contains, by weight, 2.5 to 4.0% Si, 0.03 to 0.09% C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (excluding 0%) S, and 0.002 to 0.012% N, with the remainder being Fe and other unavoidable impurities.

[0031] Si: 2.50 to 4.00 wt% Silicon (Si) increases the resistivity of grain-oriented electrical steel sheet materials and reduces core loss. If the Si content is too low, the resistivity decreases, resulting in poor core loss. If the Si content is too high, the steel becomes more brittle and its toughness decreases, increasing the incidence of sheet fractures during rolling, reducing weldability, imposing stress on cold rolling operations, preventing the sheet from reaching the temperature required for pass aging during cold rolling, and potentially causing unstable secondary recrystallization. Therefore, the Si content can be 2.5 to 4.0 wt. %. More specifically, it can be 3.0 to 3.5 wt. %.

[0032] C: 0.030 to 0.090% by weight Carbon (C) is an element that induces the formation of the austenite phase. Increased C content activates the ferrite-austenite phase transformation during hot rolling, increases the elongated hot-rolled band structure formed during hot rolling, and suppresses ferrite grain size during the hot-rolled sheet annealing process. Furthermore, increased C content increases the elongated hot-rolled band structure, which has higher strength than ferrite, and refines the initial grain size of the hot-rolled sheet annealed structure, which is the starting structure for cold rolling. This improves the texture after cold rolling, particularly the Goss fraction. This is believed to be because residual C present in the steel sheet after hot-rolled sheet annealing enhances the pass aging effect during cold rolling, increasing the Goss fraction within the primary recrystallized grains. Therefore, while a higher C content is advantageous, it can also reduce productivity by lengthening the decarburization annealing time during subsequent decarburization-nitriding annealing. Insufficient decarburization at the initial stage of heating can lead to nonuniform primary recrystallized grains and unstable secondary recrystallization. In addition, since magnetic properties may be deteriorated due to magnetic aging, the C content may be limited to a range of 0.03 to 0.09 wt%. More specifically, C may be contained in a range of 0.050 to 0.070 wt%. As mentioned above, carbon is removed by decarburization during the primary recrystallization annealing, and the final grain-oriented electrical steel sheet may contain C at 0.005 wt% or less.

[0033] Al: 0.015 to 0.040 wt% Aluminum (Al) combines with N to precipitate AlN. During decarburization and nitriding annealing, it forms fine precipitates, such as (Al, Si, Mn)N and AlN nitrides, which effectively inhibit grain growth. A certain amount of dissolved Al is required. If the content is too low, the number and volume fraction of precipitates formed are low, resulting in an insufficient grain growth inhibition effect. If the Al content is too high, the precipitates grow coarsely, reducing the grain growth inhibition effect. Therefore, the Al content may be 0.015 to 0.040 wt. %. More specifically, it may be 0.0200 to 0.0380 wt. %.

[0034] Mn: 0.040 to 0.150 wt% Manganese (Mn), like Si, increases resistivity and reduces iron loss, and is an important element in suppressing the growth of primary recrystallization grains and inducing secondary recrystallization by reacting with nitrogen introduced by nitriding together with Si to form precipitates of (Al, Si, Mn)N. Mn also forms surfide precipitates together with Cu, improving the uniformity of primary recrystallization grains and partially acting as an auxiliary inhibitor in the formation of secondary recrystallization.

[0035] However, if Mn is contained in an excessive amount, the reheating temperature of the slab must be increased to adjust the fine precipitates of (Cu, Mn)S. In this case, the primary recrystallized grains become extremely fine, and the temperature of the primary recrystallization annealing must be increased beyond the range, which causes non-uniformity of the crystal grains. Therefore, the upper limit of Mn may be limited to 0.15 wt%.

[0036] In addition, excessive addition of Mn causes the formation of large amounts of (Fe, Mn) and Mn oxides in addition to Fe2SiO4 on the surface of the steel sheet, which impedes the formation of the base coating formed during secondary recrystallization annealing and reduces surface quality. It also induces uneven phase transformation between ferrite and austenite during the primary recrystallization annealing process, which causes uneven size of primary recrystallized grains and results in unstable secondary recrystallization.

[0037] N: 0.0020 to 0.0120% by weight Nitrogen (N) is an element that reacts with aluminum and other elements to refine crystal grains. When these elements are appropriately distributed, they help to refine the structure after cold rolling, ensuring an appropriate primary recrystallization grain size, as mentioned above. However, excessive N content is undesirable because it excessively refines the primary recrystallization grains, which in turn increases the driving force for grain growth during secondary recrystallization, potentially leading to the growth of grains with undesirable orientations. Furthermore, excessive N addition can excessively refine the primary recrystallization grains, which can result in the formation of secondary recrystallization with undesirable orientations, resulting in degraded magnetic properties.

[0038] Therefore, the N content is set to 0.0120 wt% or less. On the other hand, if the N content is too low, the primary recrystallization suppression effect is too weak, and a stable grain growth suppression effect may not be obtained. Therefore, the N content in the slab may be 0.0020 to 0.0120 wt%. More specifically, the N content may be 0.0025 to 0.0100 wt%. Since some N is removed during the secondary recrystallization annealing process, the final grain-oriented electrical steel sheet may contain 0.0100 wt% or less of N.

[0039] The Al and N contents in the slab can satisfy the following formulas 1 and 2. [Formula 1] [Al]-27 / 14×[N]≧0.0240 [Formula 2] [Al] / [N]≦14 (In Equations 1 and 2, [Al] and [N] represent the Al and N contents (wt%) in the slab, respectively.) If the left side of Equation 1 is less than 0.0240%, the amount of AlN precipitates formed by nitriding before secondary recrystallization annealing will be insufficient, and the fine AlN precipitates remaining in the extra-thin hot-rolled steel will be unevenly distributed, resulting in increased deviation in magnetic properties. More specifically, the left side of Equation 1 may be 0.0240 to 0.3000%. If the left side of formula 2 is too large, the inhibitory power of AlN is insufficient, which may result in coarsening of the crystal grains in the surface and central layers of the steel sheet. More specifically, the value of the left side of formula 2 may be 5.0 to 13.0.

[0040] S: 0.0100% by weight or less Sulfur (S) is an element that has a high solution temperature during hot rolling and is prone to severe segregation. It is preferable to minimize its content, but it is an unavoidable impurity that is contained during steelmaking. Furthermore, since S forms (Mn, Cu)S and affects the uniformity of primary recrystallized grains, the S content may be limited to 0.0100 wt% or less. More specifically, it may contain 0.0010 to 0.0080 wt%.

[0041] The slab may further contain one or more of Ti and V, either alone or in a total amount of 0.002 to 0.01 wt%. When Ti and V are contained alone, each may be contained in an amount of 0.002 to 0.01 wt%, and when Ti and V are contained simultaneously, the amount of Ti + V may be 0.002 to 0.01 wt%. More specifically, the slab may further contain one or more of Ti and V, either alone or in a total amount of 0.0030 to 0.0070 wt%.

[0042] Ti: 0.002 to 0.01 wt% Titanium (Ti) is a strong nitride forming element, and it becomes TiN before hot rolling, lowering the N content and suppressing grain growth through fine precipitation. When added within the appropriate range, it has the effect of suppressing grain growth through the formation of TiN precipitates and reducing deviations in grain size within the coil by reducing fine AlN precipitates.

[0043] V: 0.002 to 0.01% by weight Vanadium (V) is an element that forms carbides and nitrides, and it suppresses grain growth by forming fine precipitates. When added within the appropriate range, it suppresses grain growth by forming fine precipitates, thereby reducing the deviation in grain size within the coil.

[0044] The slab may further contain Sn and Sb in a total amount of 0.03 to 0.15 wt %, and P: 0.01 to 0.05 wt %.

[0045] Sn and Sb: 0.030 to 0.080 wt% Tin (Sn) and antimony (Sb) are known as grain growth inhibitors because they are grain segregation elements that inhibit grain migration. Increasing the fraction of Goss-oriented grains in the primary recrystallization texture increases the number of Goss-oriented nuclei that grow in the secondary recrystallization texture, thereby reducing the size of the secondary recrystallization microstructure. As the grain size decreases, eddy current loss decreases, resulting in reduced iron loss in the final product. If the total amount of Sn and Sb is too low, the additive effect is ineffective. If the total amount is too high, the grain growth inhibitory effect increases excessively. This increases the driving force for grain growth, which requires a reduction in the grain size of the primary recrystallization microstructure. This requires a low decarburization annealing temperature, which hinders proper control of the oxide layer and results in a poor surface quality. More specifically, one or more of Sn and Sb may be included, either alone or in a combined amount of 0.040 to 0.070 wt.%.

[0046] P: 0.010 to 0.050% by weight Phosphorus (P) is an element that has an effect similar to that of Sn and Sb, and it can segregate in the grain system, hinder the movement of the grain system, and play a supplementary role in suppressing grain growth. <001> It has the effect of improving the texture. If the P content is too low, the addition effect is ineffective, but if added in an excessive amount, brittleness increases and rollability may be significantly impaired. More specifically, P may be contained in an amount of 0.015 to 0.045 wt%.

[0047] The slab may further contain one or more of Cr: 0.01 wt % or less and Ni: 0.01 wt % or less.

[0048] Cr: 0.01% by weight or less Ni: 0.01% by weight or less Chromium (Cr) and nickel (Ni) are disadvantageous in obtaining stable magnetism when manufacturing ultra-thin products, in which the oxide layer becomes deeper, the base coating becomes thicker, and the ratio of the coating layer to the thickness becomes larger. Therefore, the upper limit of each of these elements is set to 0.01% by weight.

[0049] impurity elements In addition to the above elements, unavoidable impurities such as Zr and V may be included. Because Zr, V, and the like are strong carbide-forming elements, it is preferable to avoid adding them as much as possible, and each of them should be contained in an amount of 0.01% by weight or less.

[0050] In addition to the above elements, the remainder is iron (Fe). In one embodiment of the present invention, additional elements in addition to the above alloy components are not excluded, and various elements may be included within a range that does not harm the technical spirit of the present invention. When an additional element is further included, it is included to replace the remainder Fe.

[0051] Prior to the step of producing a hot-rolled sheet, a step of heating the slab to 1230°C or less may be further included. This step may partially solutionize precipitates. Furthermore, this prevents the columnar crystal structure of the slab from growing coarsely, thereby preventing cracks from occurring in the width direction of the sheet during the subsequent hot rolling process, thereby improving yield. If the slab heating temperature is too high, the surface of the slab may melt, requiring repairs to the heating furnace and shortening the furnace's lifespan. More specifically, the slab may be heated at 1130 to 1200°C. It is also possible to hot-roll the continuously cast slab directly without heating it. In the step of producing a hot-rolled sheet, a hot-rolled sheet having a thickness of 1.8 to 2.3 mm can be produced by hot rolling.

[0052] After producing the hot-rolled sheet, the method may further include a step of annealing the hot-rolled sheet, which may be performed by heating the hot-rolled sheet to a temperature of 950 to 1,100°C, cracking at a temperature of 850 to 1,000°C, and then cooling.

[0053] Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet. Cold rolling can be performed through a single strong cold rolling pass or multiple passes. One or more passes of the rolling pass can be warm rolled at a temperature of 200 to 300°C to impart a pass aging effect, resulting in a final thickness of 0.14 to 0.25 mm. The cold-rolled sheet undergoes decarburization and recrystallization of the deformed structure in the primary recrystallization annealing process, and nitriding treatment using nitriding gas.

[0054] Next, the cold-rolled sheet is subjected to primary recrystallization annealing. In one embodiment of the present invention, the step of primary recrystallization annealing is divided into a front-stage process and a rear-stage process, and the amount of nitriding gas introduced in the front-stage process and the rear-stage process are made different. At this time, the former and latter steps are carried out within the crack step of the temperature rising step and the crack step within the primary recrystallization annealing step.

[0055] The former and latter steps may be carried out on separate cracking tables, or may be carried out on a cracking table provided with a shielding film that prevents the flow of nitriding gas to the former and latter steps. By appropriately administering the nitriding gas in the former and latter steps, the surface crystal grains are grown appropriately, and nitriding is smoothly carried out inside the steel sheet, ultimately improving the magnetic properties.

[0056] Specifically, the amount (A) of the nitriding gas introduced in the previous step relative to the total amount (B) of the nitriding gas introduced satisfies the following formula 1. [Formula 1] 0.05≦[A] / [B]≦[t] (In Equation 1, the unit of the amount of nitriding gas input is Nm 3 / hr, and [t] indicates the thickness of the cold-rolled sheet (mm). If the amount of nitriding gas introduced in the pre-stage process is too small, the nitrogen cannot penetrate into the steel sheet and remains only in the surface layer, resulting in poor magnetic properties.On the other hand, if the amount of nitriding gas introduced in the pre-stage process is too large, the grain growth in the surface layer of the steel sheet is significantly suppressed, resulting in poor magnetic properties.

[0057] More specifically, the amount of nitriding gas introduced in the previous step is 0.05 to 3 Nm 3 / hr, and the amount of nitriding gas input in the subsequent process is 1 to 10 Nm 3 It can also be / hr. The nitriding gas may be any gas that can decompose nitrogen at the temperature in the primary recrystallization annealing step and penetrate into the steel sheet. Specifically, the nitriding gas may contain one or more of ammonia and amines. The execution time of the first step may be 10 to 80 seconds, and the execution time of the second step may be 30 to 100 seconds.

[0058] The crack temperature of the primary recrystallization annealing step, i.e., the pre- and post-stage processes, may be 800 to 900°C. If the temperature is too low, the primary recrystallization may not occur or the nitriding may not proceed smoothly. If the temperature is too high, the primary recrystallization may grow too large, resulting in poor magnetic properties. Decarburization may also be performed in the primary recrystallization annealing step. Decarburization may be performed before, after, or simultaneously with the pre- and post-steps. When decarburization is performed simultaneously with the pre- and post-steps, the pre- and post-steps may be performed in an atmosphere with an oxidation potential (PH2O / PH2) of 0.5 to 0.7. Due to decarburization, the steel sheet may contain carbon of 0.005 wt% or less, more specifically, 0.003 wt% or less.

[0059] After the first recrystallization annealing step, the steel sheet may contain 0.0130 wt % or more of nitrogen. As will be described later, the nitrogen content varies depending on the thickness of the steel sheet, and the range refers to the average nitrogen content for the entire thickness.

[0060] The steel sheet after the primary recrystallization annealing can satisfy the following formula 5. [Formula 5] 1≦[G 1 / 4t ]-[G 1 / 2t ]≦3 (In Equation 5, [G 1 / 4t ] means the average grain size (μm) measured at 1 / 4 of the total thickness of the steel sheet, and [G 1 / 2t ] means the average grain size (μm) measured at half the thickness of the steel sheet.)

[0061] The surface grains (G 1 / 4t ) grows large, the secondary recrystallization exceeding 5 mm is not formed, and a very non-uniform secondary recrystallization structure is formed, which can deteriorate the magnetic properties. 1 / 4t When the grain size (N) grows too small, a large amount of fine secondary recrystallization of 5 mm or less is formed, and multiple secondary recrystallization grains with poor orientation are formed, which can deteriorate the magnetic properties. More specifically, the value of Equation 2 may be 1.2 to 2.7. Here, the grain size refers to the grain size measured in a plane parallel to the rolling surface (ND plane).

[0062] The steel sheet after the primary recrystallization annealing can satisfy the following formula 3. [Formula 3] [N tot ]-[N 1 / 4t~3 / 4t ]≦60×(10×[t]-1) (In Equation 3, [N tot ] means the nitrogen content (ppm) in the entire steel sheet, and [N 1 / 4t~3 / 4t ] means the nitrogen content (ppm) at 1 / 4 to 3 / 4 of the total thickness of the steel sheet, and [t] indicates the thickness of the cold-rolled sheet (mm). If the nitrogen content inside the steel sheet is too low, i.e., if the value of the left side of Equation 3 is too large, the ability to inhibit internal grain growth is insufficient, resulting in the formation of numerous defects such as nitrogen release holes in the surface layer and the formation of numerous fine secondary recrystallizations of 5 mm or less, which can result in a deterioration of magnetic properties. More specifically, the value of the left side of Equation 3 may be 0.0030 to 0.0060%.

[0063] Next, the cold-rolled sheet that has undergone the primary recrystallization annealing is subjected to secondary recrystallization annealing. The purpose of secondary recrystallization annealing is roughly divided into the following: <001> The primary functions of secondary recrystallization annealing are to form texture, to provide insulation through the formation of a glassy film caused by the reaction of MgO with the oxide layer formed during decarburization, and to remove impurities that impair magnetic properties. The secondary recrystallization annealing method involves maintaining a nitrogen and hydrogen gas mixture in the temperature-raising section before secondary recrystallization occurs to protect the nitrides, which act as grain growth inhibitors, and to promote the secondary recrystallization. After secondary recrystallization is complete, the material is maintained in a 100% hydrogen atmosphere for a long period of time to remove impurities.

[0064] The steel sheet after secondary recrystallization annealing can satisfy the following formula 6. [Formula 6] [DS] / [DL]≦0.1 (In Equation 6, [DS] represents the number of crystal grains having a grain size of 5 mm or less, and [DL] represents the number of crystal grains having a grain size of more than 5 mm.) During the secondary recrystallization annealing process, the surface oxide layer formed during the primary recrystallization annealing process reacts with the annealing separator to form a base coating layer. The base coating layer is distinguished from the base steel sheet in terms of its composition. For example, if MgO is used as the annealing separator, it will contain hosterite.

[0065] The ratio of the maximum Al luminous intensity to the maximum Mg luminous intensity of the base coating layer may be 0.05 to 0.10. The luminous intensity may be analyzed by glow discharge optical emission spectroscopy (GDS), which is widely known and therefore will not be described in detail. More specifically, the ratio may be 0.06 to 0.10. The method may further include forming an insulating coating layer after the secondary recrystallization annealing. Since the method for forming the insulating coating layer is well known, detailed description thereof will be omitted.

[0066] In one embodiment of the present invention, the deviation of the nitrogen content in the thickness direction of the steel sheet is small, so that the base coating layer is formed uniformly and thinly, and even if the insulating coating layer is formed thinly, appropriate insulation properties can be ensured. In one embodiment of the present invention, by reducing the deviation of the nitrogen content in the thickness direction of the steel sheet, it is possible to form a thin base coating layer after secondary recrystallization, and an additional process of removing the base coating layer may not be required.

[0067] A grain-oriented electrical steel sheet according to one embodiment of the present invention comprises an electrical steel sheet substrate containing, by weight, 2.5 to 4.0% Si, 0.005% or less (excluding 0%) C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (excluding 0%) S, and 0.0100% or less (excluding 0%) N, with the balance being Fe and other unavoidable impurities. The alloying elements of the grain-oriented electrical steel sheet have been described above in relation to the alloying elements of the slab, and therefore a duplicated description will be omitted.

[0068] The grain-oriented electrical steel sheet according to an embodiment of the present invention may include a base coating layer on an electrical steel sheet substrate. The ratio of the maximum Al luminous intensity to the maximum Mg luminous intensity of the base coating layer may be 0.05 to 0.10. This has been explained in the manufacturing method, so a duplicate explanation will be omitted.

[0069] The iron loss (W17 / 50) of the grain-oriented electrical steel sheet may be 0.830 W / kg or less under the conditions of 1.7 Tesla and 50 Hz. More specifically, the iron loss (W17 / 500) may be 0.750 to 0.830 W / kg. More specifically, the difference between the maximum and minimum values ​​of the iron loss (W17 / 50) may be 0.050 W / kg or less. The difference between the maximum and minimum values ​​refers to the difference measured within the entire coil. In this case, the thickness standard may be 0.19 mm.

[0070] The magnetic flux density (B8) induced in the grain-oriented electrical steel sheet under a magnetic field of 800 A / m may be 1.91 T or more. More specifically, it may be 1.91 to 1.95 T. Even more specifically, the difference between the maximum and minimum values ​​of the magnetic flux density (B8) may be 0.025 T or less. The difference between the maximum and minimum values ​​refers to the difference measured within the entire coil.

[0071] Preferred examples and comparative examples of the present invention will be described below. However, the following examples are merely preferred examples of the present invention, and the present invention is not limited to the following examples. [Example]

[0072] Slabs A to F, which have the composition shown in Table 1, were vacuum melted to produce steel containing the remainder Fe and other unavoidable impurities, and then heated at 1150°C for 210 minutes before being hot-rolled to produce hot-rolled sheets with a thickness of 2.0 mm. After pickling, the steel was subjected to a single intensive cold rolling process to a thickness of 0.19 mm or 0.14 mm.

[0073] The cold-rolled sheet was subjected to decarburization and nitriding annealing heat treatment at a temperature of approximately 800 to 900°C in a humid atmosphere of 50% hydrogen and 50% nitrogen and an ammonia mixed gas atmosphere to increase the carbon content to 30 ppm or less and the total nitrogen content to 130 ppm or more. The nitriding gas input rates in the first and second stages were adjusted as shown in Table 2 below, with the first stage lasting 50 seconds and the second stage lasting 70 seconds. The steel sheet thickness, total nitrogen content, and nitrogen content in the central 1 / 4 to 3 / 4 of the steel sheet thickness direction after annealing are summarized in Table 2.

[0074] The steel sheet was coated with MgO as an annealing separator and then coiled for final annealing. The final annealing was performed in a mixed atmosphere of 25v% nitrogen and 75v% hydrogen up to 1200°C, and after reaching 1200°C, the atmosphere was maintained at 100% hydrogen for 10 hours or more, followed by furnace cooling.

[0075] Thereafter, an insulating coating layer-forming composition containing a mixed liquid of metal phosphate and colloidal silica was applied and heat-treated to form an insulating coating layer with the thickness shown in Table 3 below. Table 3 shows the maximum and minimum values ​​of magnetic flux density and iron loss measured under each condition. The magnetic properties were measured using the single sheet measurement method under the conditions of 1.7 Tesla and 50 Hz, and the magnitude of the magnetic flux density (Tesla) induced in a magnetic field of 800 A / m was measured. The magnetic properties of the entire coil were also measured, and the maximum and minimum values ​​are summarized in Table 3 below.

[0076] [Table 1]

[0077] [Table 2]

[0078] [Table 3]

[0079] As can be seen in Table 1, the inventive material, which has an appropriate amount of residual Al and appropriate control of the process conditions during the primary recrystallization annealing, has a uniform nitrogen content throughout the steel sheet thickness, low Al strength in the base coating layer, good coating adhesion, and small deviations in iron loss and magnetic flux density.

[0080] On the other hand, if the residual Al is not adequately secured, if the Al content is excessive compared to the N content, or if the nitrogen content is uneven across the steel sheet thickness, the Al strength of the base coating layer will be relatively high, resulting in poor coating adhesion, inferior iron loss and magnetic flux density, and large deviations.

[0081] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and a person skilled in the art will 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, the above-described embodiments should be understood to be illustrative in all respects and not limiting.

Claims

1. hot rolling a slab containing, by weight %, 2.5 to 4.0% Si, 0.03 to 0.09% C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (excluding 0%) S, and 0.002 to 0.012% N, with the balance being Fe and other unavoidably mixed impurities, and satisfying the following formulas 1 and 2 to produce a hot-rolled sheet; cold-rolling the hot-rolled sheet to produce a cold-rolled sheet; A step of subjecting the cold-rolled sheet to primary recrystallization annealing; and subjecting the cold-rolled sheet that has undergone the first recrystallization annealing to a second recrystallization annealing. After the first recrystallization annealing step, the following formula 3 is satisfied: The method for manufacturing a grain-oriented electrical steel sheet, wherein the ratio of maximum Al luminous intensity to maximum Mg luminous intensity of the base coating layer after the secondary recrystallization annealing is 0.05 to 0.

10. [Formula 1] [Al]-27 / 14×[N]≧0.0240 [Formula 2] [Al] / [N]≦14 (In formulas 1 and 2, [Al] and [N] represent the Al and N contents (wt%) in the slab, respectively.) [Formula 3] [N ｔｏｔ ]-[N １／４ｔ～３／４ｔ ]≦60×(10×[t]-1) (In formula 3, [N ｔｏｔ ] means the nitrogen content (ppm) in the entire steel sheet, and [N １／４ｔ～３／４ｔ ] means the nitrogen content (ppm) at a point between 1 / 4 and 3 / 4 of the total thickness of the steel sheet, and [t] indicates the thickness of the cold-rolled sheet (mm).

2. 2. The method of claim 1, wherein the slab further contains at least one of Ti and V in an amount of 0.002 to 0.01 wt %, either alone or in total.

3. 3. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein the slab further contains Sn and Sb in a total amount of 0.03 to 0.15 wt %, and P: 0.01 to 0.05 wt %.

4. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the slab further contains at least one of Cr: 0.01 wt% or less and Ni: 0.01 wt% or less.

5. The step of performing primary recrystallization annealing includes a front-stage process and a rear-stage process, 5. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the amount (A) of nitriding gas introduced in the preceding step relative to the total amount (B) of nitriding gas introduced in the step of primary recrystallization annealing satisfies the following formula 4: [Formula 4] 0.05≦[A] / [B]≦[t] (In Equation 4, the unit of the amount of nitriding gas fed is Nm ３ / hr, and [t] indicates the thickness of the cold-rolled sheet (mm).

6. 6. The method of claim 5, wherein the first step is performed for 10 to 80 seconds, and the second step is performed for 30 to 100 seconds.

7. The method for producing a grain-oriented electrical steel sheet according to claim 5 or 6, wherein the pre-stage process and the post-stage process are carried out at a temperature of 800 to 900°C.

8. The former step and the latter step are performed by adjusting the oxidation capacity (PH ２ O / PH ２ 8. The method for producing a grain-oriented electrical steel sheet according to claim 5, wherein the heating is carried out in an atmosphere where the temperature of the heating is 0.5 to 0.

7.

9. The invention comprises an electrical steel sheet substrate and a base coating layer located on the electrical steel sheet substrate, the base coating layer containing, by weight, 2.5 to 4.0% Si, 0.005% or less (except 0%) C, 0.015 to 0.040% Al, 0.04 to 0.15% Mn, 0.01% or less (except 0%) S, and 0.0100% or less (except 0%) N, with the balance being Fe and other unavoidably mixed impurities, A grain-oriented electrical steel sheet, characterized in that the ratio of maximum Al luminous intensity to maximum Mg luminous intensity in the base coating layer is 0.05 to 0.10.

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