Thin grain-oriented electrical steel sheet and method for producing same

Abstract

Provided is a method for producing a thin grain-oriented electrical steel sheet. The method for producing a grain-oriented electrical steel sheet according to the present invention comprises the steps of: producing a hot-rolled sheet by hot-rolling a slab; the hot-rolled plate is subjected to hot rolling annealing treatment; cold-rolling the hot-rolled annealed sheet to produce a cold-rolled sheet; carrying out primary recrystallization annealing of decarburization and nitriding on the cold-rolled sheet; and performing secondary recrystallization annealing on the cold-rolled sheet subjected to the primary recrystallization annealing.

Description

Technical Field

[0001] The present invention relates to the manufacture of a thin grain-oriented electrical steel sheet. Specifically, it relates to a grain-oriented electrical steel sheet and a manufacturing method thereof. The grain-oriented electrical steel sheet reduces the magnetic deviation of thin products by controlling the residual Al content in the slab and the annealing conditions of the hot-rolled sheet according to the hot-rolled thickness, and controlling the nitriding amount of the steel sheet according to the cold-rolled thickness. Background Art

[0002] Grain-oriented electrical steel sheets are mainly used as core materials for transformers, motors, generators, and other electronic devices. The final product of the grain-oriented electrical steel sheet has a texture in which the orientation of the grains is oriented along the (100)

[001] direction, and has extremely excellent magnetic properties in the rolling direction. Therefore, it can be used as a core material for transformers, motors, generators, and other electronic devices. In order to reduce energy loss, low iron loss is required, and in order to miniaturize generators, high magnetic flux density is required.

[0003] The iron loss of the grain-oriented electrical steel sheet is divided into hysteresis loss and eddy current loss. In order to reduce the eddy current loss, efforts such as increasing the specific resistivity and reducing the product thickness are required. In the direction of reducing the product thickness, although there is difficulty in rolling the grain-oriented electrical steel sheet of difficult-to-roll products into ultra-thin materials, the biggest difficulty and the problem that needs to be overcome in manufacturing ultra-thin material products with extremely low iron loss characteristics is that it is necessary to very strongly maintain the aggregation degree of the Goss orientation as the secondary recrystallization structure of the grain-oriented electrical steel sheet.

[0004] From the perspective of the problems in rolling when manufacturing ultra-thin products, when manufacturing a grain-oriented electrical steel sheet through a low-temperature heating method and a single strong cold rolling process, the generally known optimal reduction ratio is about 90%. In order to ensure a cold rolling rate of 90%, the hot-rolled sheet thickness needs to be hot-rolled to a thickness of 1.5 - 2.3 mmt or less. As the hot-rolled thickness becomes thinner, a high reduction ratio is required. Due to reasons such as the maintenance of the hot-rolling temperature, the shape of the edge (edge) part of the hot-rolled sheet such as edge scab, or the top and tail of the coiled sheet, the productivity is reduced. In addition, as the length of the hot-rolled coil increases, there will inevitably be a difference in the rolling time and a difference in the hot-rolling temperature between the top and tail of the coiled sheet, which is more unfavorable for forming uniform fine precipitates in the length direction of the coiled sheet. In addition, when heating the slab for hot rolling, when the slab moves in the hot-rolling reheating furnace, a temperature deviation occurs because the temperature of the contact part with the slide rail is lower than that of the non-contact part, and a difference in solid solution precipitates (fine precipitates) will inevitably occur in the length direction of the hot-rolled sheet. This difference causes a problem of magnetic property deviation in the final product.

[0005] More importantly, as the product thickness decreases, during the secondary recrystallization annealing process, especially in the range where secondary recrystallization with Gaussian orientation occurs, the loss of precipitates from the surface accelerates, making it difficult to maintain a strong Gaussian orientation aggregation degree. This is a problem directly related to the magnetic properties of the product, so it is difficult to ensure very low iron loss characteristics by manufacturing ultra-thin products. As the thickness decreases, instead of Gaussian orientation, the grains grow as the loss of precipitates accelerates, forming coarse grains penetrating the thickness direction, so even after long-term high-temperature annealing, it is not easy to eliminate, resulting in uneven iron loss.

[0006] As a method for overcoming the loss of precipitates, a method of preventing the loss of precipitates by increasing the fraction of N2 gas during the secondary recrystallization annealing process has been proposed, but this has the problem of causing surface defects, such as nitrogen outlets on the product plate surface, etc.

[0007] An economical manufacturing method using the simultaneous decarburization and carburization method has also been proposed. During the process of manufacturing a decarburized plate by the simultaneous decarburization and carburization method, it is clearly shown that there are differences in the surface particle size and the center layer particle size, and it is proposed that it needs to be controlled within a certain range.

[0008] A technique for significantly improving magnetism by including segregation elements such as Sb, P, Sn, etc. has also been proposed. When manufacturing ultra-thin products by adding more segregation elements, as an auxiliary inhibitor to compensate for the loss of precipitates, segregation elements are used, but when added in excess, there is a problem that it is difficult to perform ultra-thin rolling, and the oxide layer becomes uneven and thinner, and the substrate coating characteristics are poor, having the side effect of causing further loss of precipitates, so magnetism cannot be stably ensured.

[0009] In the primary recrystallization annealing process when manufacturing ultra-thin products, a method of adjusting the oxidation ability at the front end and nitriding treatment has also been proposed. However, in the manufacturing of ultra-thin products, there is a problem that the influence of the loss of precipitates becomes very sensitive.

[0010] In addition, a method of adding Cr to the slab and adjusting the amount of nitriding gas added at the front end and the rear end during the primary recrystallization annealing process has been proposed. However, this method maintains a uniform nitrogen amount in the thickness direction of the steel plate, but due to the uneven distribution of AlN precipitates, there is still a problem of deviation in magnetic properties. In addition, by adding Cr, as the depth of the oxide layer deepens, the thickness of the base coating becomes thicker, and there is a problem of an increase in the proportion of the coating in the product when manufacturing ultra-thin materials.

[0011] [Prior Art Documents]

[0012] [Patent Documents]

[0013] (Patent Document 1) Korean Patent Publication No. 2001-0060418 Summary of the Invention

[0014] (1) Technical Problem to be Solved

[0015] The object of the present invention is to provide a grain-oriented electrical steel sheet and a method for manufacturing the same. The grain-oriented electrical steel sheet can reduce the magnetic deviation of thin products by controlling the residual Al content in the slab and the annealing conditions of the hot-rolled sheet according to the hot-rolled thickness, and controlling the nitriding amount of the steel sheet according to the cold-rolled thickness.

[0016] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the present invention based on the entire text of the specification of the present invention.

[0017] (2) Technical Solution

[0018] One aspect of the present invention relates to a method for manufacturing a thin grain-oriented electrical steel sheet, the manufacturing method comprising the following steps: hot-rolling a slab to manufacture a hot-rolled sheet, wherein, by weight%, the slab contains: Si: 2.5 - 4.0%, C: 0.03 - 0.09%, Al: 0.0150 - 0.0400%, Mn: 0.04 - 0.15%, S: less than 0.010% (except 0%), N: 0.0020 - 0.0120%, P: 0.010 - 0.050%, one or more of Sn and Sb in total: 0.03 - 0.12%, the balance of Fe and other inevitable impurities; subjecting the hot-rolled sheet to hot-rolled annealing treatment; cold-rolling the hot-rolled annealed sheet to manufacture a cold-rolled sheet; subjecting the cold-rolled sheet to primary recrystallization annealing for decarburization and nitriding; and subjecting the cold-rolled sheet after the primary recrystallization annealing to secondary recrystallization annealing, wherein the contents of Al and N in the slab satisfy the following relational expressions 1 to 2, when the temperature difference between the heating temperature T1 and the soaking annealing temperature T2 in the annealing step of the hot-rolled sheet is △T, △T≥200°C is satisfied, and the cooling rate when cooling from T1 to T2 is controlled to be 30°C / second (sec) or less, and the amount of nitrogen △N added to the cold-rolled steel sheet by nitriding in the step of the primary recrystallization annealing satisfies the following relational expression 3.

[0019] [Relational Expression 1]

[0020] 0.039 - 0.01Ht ≤ [Al] - 27 / 14×[N] ≤ 0.047 - 0.01Ht

[0021] [Relational Expression 2]

[0022] 15.5 - 5Ht ≤ [Al] / [N]

[0023] In Relations 1 to 2, [Al] and [N] respectively represent the contents of Al and N in the slab (wt%), and Ht represents the thickness of the final hot-rolled sheet (mm).

[0024] [Relation 3]

[0025] [Al] - 27 / 14×[N] + (0.23 - Ct) 0.125 ≤ (27 / 14)△N ≤ 0.03

[0026] Wherein, [Al] and [N] respectively represent the contents of Al and N in the slab (wt%), and Ct represents the thickness of the final cold-rolled sheet

[0027] The slab may further contain 0.02 - 0.15% of Cr.

[0028] The slab may further contain 0.01 - 0.2% of Cu and 0.01 - 0.05% of Bi.

[0029] The slab may contain one or more of Ti in the range of 0.002 - 0.01% and V in the range of 0.002 - 0.01% with the sum of the contents being 0.002 - 0.01%.

[0030] The primary recrystallization annealing process may be carried out at a temperature of 800 - 900°C for 60 - 180 seconds.

[0031] In addition, another aspect of the present invention relates to a thin grain-oriented electrical steel sheet which, by weight, comprises: Si: 2.5 - 4.0%, C: 0.03 - 0.09%, Al: 0.0150 - 0.0400%, Mn: 0.04 - 0.15%, S: not more than 0.010% (except 0%), N: 0.0020 - 0.0120%, P: 0.010 - 0.050%, one or more of Sn and Sb in total: 0.03 - 0.12%, the balance being Fe and other inevitable impurities, and the amount of nitrogen △N increased in the steel sheet by nitriding satisfies the following Relation 3.

[0032] [Relation 3]

[0033] [Al] - 27 / 14×[N] + (0.23 - Ct) 0.125 ≤ (27 / 14)△N ≤ 0.03

[0034] Wherein, [Al] and [N] respectively represent the contents of Al and N in the steel sheet (wt%), and Ct represents the thickness of the steel sheet.

[0035] (III) Beneficial effects

[0036] The grain-oriented electrical steel sheet according to an embodiment of the present invention can specifically reduce the magnetic deviation of the thin grain-oriented electrical steel sheet product and improve the magnetic properties by controlling the residual Al content in the slab according to the hot-rolled thickness and the annealing conditions of the hot-rolled sheet, and further by controlling the amount of nitriding of the steel sheet according to the cold-rolled thickness. Embodiment

[0037] Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement the present invention. However, the present invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0038] The method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention includes the following steps: heating a steel slab and then performing hot rolling to manufacture a hot-rolled sheet; annealing the hot-rolled sheet; performing cold rolling on the hot-rolled annealed sheet to manufacture a cold-rolled sheet; performing primary recrystallization annealing on the cold-rolled sheet for decarburization and nitriding; and performing secondary recrystallization annealing on the cold-rolled sheet after completion of the primary recrystallization annealing.

[0039] Hereinafter, the method for manufacturing the electrical steel sheet of the present invention will be described in the process sequence.

[0040] [Manufacture of Hot-Rolled Sheet]

[0041] First, in the present invention, a steel slab is hot-rolled to manufacture a hot-rolled sheet. By weight %, the steel slab contains: Si: 2.5 - 4.0%, C: 0.03 - 0.09%, Al: 0.0150 - 0.0400%, Mn: 0.04 - 0.15%, S: 0.010% or less (except 0%), N: 0.0020 - 0.0120%, P: 0.010 - 0.050%, one or more of Sn and Sb in total: 0.03 - 0.12%, and the balance of Fe and other inevitable impurities.

[0042] At this time, in the present invention, before manufacturing the hot-rolled sheet, a step of heating the steel slab may further be included. Through the heating step, precipitates can be partially solid-soluted. Further, the coarsening growth of the columnar crystal structure of the slab can be prevented, and cracks generated along the width direction of the sheet in the subsequent hot-rolling process can be prevented. However, when the heating temperature of the slab is too high, the surface portion of the slab melts, which may lead to the maintenance of the heating furnace and shorten the life of the heating furnace. Therefore, in the present invention, it is preferable to set the heating temperature in the range of 1130 - 1200°C. When the heating temperature of the slab is too low, the above heating effect cannot be expected. In addition, in the present invention, the steel slab may not be heated, and the continuously cast steel slab may be directly hot-rolled.

[0043] Hereinafter, the reasons for restricting the alloy composition and its content of the slab of the present invention will be described, where the content of each component element is based on weight%.

[0044] Si: 2.5 - 4.0%

[0045] The function of silicon (Si) is to increase the specific resistance of the grain-oriented electrical steel sheet material to reduce the core loss, that is, the iron loss. When the Si content is too low, the specific resistance decreases, and thus the iron loss may deteriorate. When the Si content is too high, the brittleness of the steel increases, the toughness decreases, the incidence of plate breakage during the rolling process increases, and the weldability deteriorates, generating a load during the cold rolling operation and not reaching the plate temperature required for the rolling process during cold rolling, and the formation of secondary recrystallization may become unstable. Therefore, in the present invention, the Si content is preferably controlled in the range of 2.5 - 4.0%, more preferably in the range of 3.0 - 3.5%.

[0046] C: 0.03 - 0.09%

[0047] Carbon (C) is an element that induces the formation of the austenite phase. As the C content increases, the ferrite-austenite phase transformation in the hot rolling process is activated, and the elongated hot-rolled strip structure formed in the hot rolling process increases, thereby suppressing the growth of ferrite grains in the hot-rolled sheet annealing process. In addition, as the C content increases, the texture is improved after cold rolling, especially the Goss fraction increases, due to the increase in the elongated hot-rolled strip structure with a strength higher than that of the ferrite structure and the refinement of the initial particles of the cold rolling starting structure, that is, the hot-rolled sheet annealing structure. This is because, due to the residual C present in the steel sheet after hot-rolled sheet annealing, the rolling effect becomes larger during cold rolling, increasing the Goss fraction in the primary recrystallized grains. Therefore, the higher the C content, the more beneficial it is, but the decarburization annealing time becomes longer during the subsequent decarburization nitriding annealing, damaging the productivity. If the decarburization at the initial stage of heating is insufficient, the primary recrystallized grains will be uneven, resulting in unstable secondary recrystallization. In addition, due to the magnetic aging phenomenon, the magnetic properties may deteriorate. Therefore, in the present invention, the C content is preferably restricted to the range of 0.03 - 0.09%. More preferably, the C content can be in the range of 0.05 - 0.07%. As described above, carbon is removed by decarburization during the primary recrystallization annealing process, and the finally manufactured grain-oriented electrical steel sheet can contain 0.005% or less of C.

[0048] Al: 0.0150 - 0.0400%

[0049] Aluminum (Al) combines with N and precipitates as AlN. However, during decarburization and nitriding annealing, nitrides in the form of (Al, Si, Mn)N and AlN are formed as fine precipitates, thereby playing a strong role in inhibiting grain growth. Thus, a certain amount or more of dissolved Al is required. When the Al content is too low, the number and volume fraction of the formed precipitates decrease, and the effect of inhibiting grain growth may be insufficient. When too much Al is contained, the precipitates grow coarsely, and the effect of inhibiting grain growth decreases. Therefore, in the present invention, Al is preferably controlled in the range of 0.0150 - 0.0400%, and more preferably, in the range of 0.0200 - 0.0380%.

[0050] Mn: 0.04 - 0.15%

[0051] Similar to Si, manganese (Mn) also has the effect of increasing the specific resistance and reducing iron loss. The Mn reacts with Si and the nitrogen introduced by nitriding treatment to form precipitates of (Al, Si, Mn)N, which is an important element for inhibiting the growth of primary recrystallization grains and causing secondary recrystallization. In addition, Mn forms sulfide (Surfide) precipitates together with Cu to improve the uniformity of primary recrystallization grains and plays a role as a partial auxiliary inhibitor in the formation of secondary recrystallization. However, when too much Mn is contained, in order to adjust the (Cu, Mn)S fine precipitates, it is necessary to increase the slab reheating temperature. In this case, the primary recrystallization grains become extremely fine, and it is necessary to raise the temperature of primary recrystallization annealing above the range, and it leads to non-uniform grains. Therefore, the upper limit of the Mn content can be limited to 0.15%.

[0052] In addition, when too much Mn is added, in addition to Fe2SiO4 on the steel plate surface, a large amount of (Fe, Mn) and Mn oxides are formed, which hinder the base coating formed during secondary recrystallization annealing, thereby reducing the surface quality. In the primary recrystallization annealing process, the phase transformation between ferrite and austenite becomes non-uniform, and as a result, the size of the primary recrystallization grains becomes non-uniform, and in turn, the secondary recrystallization becomes unstable.

[0053] N: 0.0020 - 0.0120%

[0054] Nitrogen (N) is an element that reacts with Al etc. to refine grains. When these elements are appropriately distributed, as described above, the structure after cold rolling can be appropriately refined, which helps to ensure an appropriate primary recrystallization grain size. However, when the N content is excessive, the primary recrystallization grains are overly refined. As a result, due to the fine grains, the driving force for grain growth during secondary recrystallization becomes large, and grains with an unfavorable orientation may also grow, so this is not preferred. In addition, when too much N is added, the primary recrystallization grains are overly refined. As a result, due to the fine grains, secondary recrystallization with an unfavorable orientation is formed, and thus the magnetic properties may deteriorate. Therefore, the N content is set to 0.0120% or less. Additionally, when the N content is too low, the effect of suppressing primary recrystallization is too weak, and a stable effect of suppressing grain growth may not be obtained. Therefore, the slab may contain 0.0020 - 0.0120% of N. More specifically, it may also contain N in the range of 0.0025 - 0.0100%. During the secondary recrystallization annealing process, part of the N is removed, so the finally manufactured grain-oriented electrical steel sheet may contain N in the range of 0.005% or less.

[0055] S: 0.010% or less

[0056] Sulfur (S) is an element with a high solution temperature and severe segregation during hot rolling. Therefore, it is preferably avoided as much as possible, but S is one of the inevitable impurities contained during steelmaking. In addition, since S forms (Mn, Cu)S, which affects the uniformity of primary recrystallization grains, the content of S can be limited to 0.010% or less. More specifically, the content of S can be in the range of 0.001 - 0.008%.

[0057] In addition, the slab of the present invention may contain one or more of Sn and Sb and P.

[0058] Total of one or more of Sn and Sb: 0.03 - 0.12%

[0059] Tin (Sn) and antimony (Sb) are grain boundary segregation elements and elements that impede the movement of grain boundaries, and are thus known as crystallization growth inhibitors. In addition, in the primary recrystallization texture, by increasing the grain fraction of the Goss orientation, the Goss orientation nuclei that grow into the secondary recrystallization texture increase, and the size of the secondary recrystallization microstructure decreases. Therefore, the smaller the grain size, the smaller the eddy current loss, and thus the iron loss of the final product is reduced. When the sum of Sn and Sb is too small, there is no addition effect. When the sum of Sn and Sb is too large, the grain growth inhibition force increases excessively. In order to relatively increase the grain growth driving force, it is necessary to reduce the grain size of the primary crystal microstructure. Therefore, it is necessary to perform decarburization annealing at a low temperature, and thus it is impossible to control an appropriate oxide layer, and thus a good surface cannot be ensured. More preferably, the content of one or more of the Sn and Sb is separately in the range of 0.04 - 0.12% or the total amount thereof is in the range of 0.04 - 0.12%.

[0060] P: 0.01 - 0.05%

[0061] Phosphorus (P) is an element that exhibits an effect similar to that of Sn and Sb, and can play an auxiliary role in segregating at the grain boundaries to impede the movement of the grain boundaries while suppressing grain growth. In addition, in terms of the microstructure, P has the effect of improving the {110}<001> texture. When the content of P is too small, there is no addition effect. When too much P is added, the brittleness increases and the rollability may be significantly reduced. Therefore, the content of P is in the range of 0.01 - 0.05%. More preferably, the content of P can be in the range of 0.015 - 0.045%.

[0062] In the present invention, the following component elements can be added as needed.

[0063] Ti: 0.002 - 0.010%

[0064] Titanium (Ti) is a strong nitride-forming element. In the step before hot rolling, TiN is formed, thereby reducing the N content and finely precipitating, thereby suppressing grain growth. When Ti is added within an appropriate range, it shows the effect of suppressing grain growth by forming TiN precipitates and the effect of reducing the deviation of grain diameters in the coil by reducing the fine precipitation of AlN.

[0065] V: 0.002 - 0.010%

[0066] Vanadium (V) is a carbide and nitride-forming element, and suppresses grain growth by fine precipitation. By adding V within an appropriate range, it shows the effect of suppressing grain growth by forming fine precipitates and the effect of reducing the deviation of grain diameters in the coil.

[0067] The sum of one or more of Ti and V: 0.002 - 0.010%

[0068] When adding the above-mentioned Ti and / or V in the present invention, the total of one or more of the Ti and V is preferably in the range of 0.002 - 0.010%, and more preferably, the total of one or more of the Ti and V can be in the range of 0.003 - 0.007%.

[0069] Cu: 0.01 - 0.20%

[0070] Cu combines with S to precipitate as CuS, mainly mixes with MnS to form (Mn, Cu)S, thereby playing a role in inhibiting grain growth. In addition, like Mo, Cu can form a large number of precisely oriented Goss grains in the structure of the hot-rolled surface part, reduce the grain size after secondary recrystallization, reduce the eddy current loss, so reduce the iron loss of the final product, grow a large number of precisely oriented Goss grains, and thus the magnetic flux density will also increase. When adding Cu to the steel plate, when the Cu content is less than 0.01%, the effect is insufficient, and when the Cu content exceeds 0.20%, the precipitates grow coarsely, and the effect of inhibiting grain growth may be reduced.

[0071] Cr: 0.02 - 0.15%

[0072] Chromium (Cr) is an element that promotes oxidation formation. When adding chromium (Cr) in the range of 0.02 - 0.15%, it inhibits the formation of a dense oxide layer on the surface layer and helps to form a fine oxide layer in the depth direction. By adding Sb and Sn while adding an appropriate range of Cr content, it is easier to form primary recrystallization with excellent uniformity. By adding Cr, it is possible to overcome the phenomenon of non-uniform primary recrystallization grains caused by the delay of decarburization and nitriding due to the increase in the content of Sb and Sn, thereby forming primary recrystallization grains with excellent uniformity, and Cr is an element that shows the effect of improving magnetism. When the Cr content does not reach the lower limit value, the effect is weak, and when the Cr content exceeds the upper limit value, an excessive oxide layer is formed, so its effect is reduced, and due to the addition of high-price alloys, it will cause an increase in cost, so it is not preferred.

[0073] Impurity elements

[0074] In addition to the above elements, impurities such as Zr that are inevitably mixed in can be included. Zr and the like are strong carbide and nitride forming elements, so it is preferably not added as much as possible, and its content should be 0.01% or less.

[0075] In addition to the above elements, the balance contains iron (Fe). In one embodiment of the present invention, the addition of elements other than the above alloy components is not excluded, and within the scope of not damaging the technical idea of the present invention, it can be added in various ways. When further containing additional elements, the additional elements can be included in place of the balance of Fe.

[0076] In the present invention, the slab having the above composition can be hot-rolled to produce a hot-rolled sheet with a thickness of 1.5 - 2.3 mm.

[0077] At this time, the present invention is characterized in that the Al and N contents in the slab are composed to satisfy the following relational expressions 1 and 2 according to the thickness of the hot-rolled sheet.

[0078] [Relational expression 1]

[0079] 0.039 - 0.01Ht ≤ [Al] - 27 / 14 × [N] ≤ 0.047 - 0.01Ht

[0080] [Relational expression 2]

[0081] 15.5 - 5Ht ≤ [Al] / [N]

[0082] Wherein, [Al] and [N] respectively represent the contents of Al and N in the slab (wt%), and Ht represents the thickness of the final hot-rolled sheet (mm).

[0083] When [Al] - 27 / 14 × [N] in the relational expression 1 is less than the left side, the amount of precipitates of AlN formed by nitriding before secondary recrystallization annealing is insufficient. When [Al] - 27 / 14 × [N] is greater than the right side, the inhibitory power of AlN as an inhibitor is insufficient, so it may lead to coarsening of the grains in the surface layer and the center layer of the steel sheet.

[0084] As the thickness of the hot-rolled sheet becomes thinner, the hot-rolling time is long, the temperature difference in the length direction increases, the distribution of fine AlN precipitates remaining in the hot-rolling is uneven, and the deviation of magnetic properties increases. Therefore, it is necessary to control the contents of Al and N to satisfy the relational expression 1.

[0085] In addition, when [Al] / [N] in the relational expression 2 is too small, the amount of precipitates of AlN increases, the amount of non-uniform precipitates increases, and it is not conducive to the uniform control of precipitates in the subsequent process.

[0086] Next, in the present invention, the obtained hot-rolled sheet is annealed.

[0087] In the present invention, this hot-rolled sheet annealing process can be carried out by heating to a temperature range T1 of 1000 - 1150 °C, then cooling to a temperature range T2 of 800 - 950 °C and performing soaking annealing, and then performing normal temperature cooling.

[0088] At this time, in the present invention, it is characterized in that the temperature difference (ΔT) between the heating temperature T1 and the soaking annealing temperature T2 is controlled to be 200 °C or more. When the temperature difference (ΔT) is lower than 200 °C, the precipitation driving force is insufficient, and thus problems such as an increase in magnetic deviation caused by non-uniform precipitates may occur.

[0089] In addition, in the present invention, the cooling rate when cooling from T1 to T2 is preferably controlled to be 30 °C / second or less. When the cooling rate exceeds 30 °C / second, precipitation deviations at each position are caused, and thus a problem of an increase in magnetic deviation may occur.

[0090] In addition, in the present invention, the hot-rolled annealed sheet can be cold-rolled to manufacture a cold-rolled sheet.

[0091] In the present invention, the cold rolling can be carried out by one-time heavy cold rolling, or can also be carried out through multiple passes. During the rolling process, a rolling pass effect is provided by warm rolling at a temperature of 200 - 300 °C more than once, and the final thickness can be manufactured to be 0.14 - 0.23 mm. The cold-rolled cold-rolled sheet can be nitrided in a decarburization and recrystallization of the deformed structure and nitriding gas during a primary recrystallization annealing process.

[0092] Next, in the present invention, the cold-rolled sheet is subjected to primary recrystallization annealing.

[0093] In one embodiment of the present invention, the cold-rolled sheet is subjected to decarburization nitriding treatment in the primary recrystallization annealing process, and nitriding is carried out in the soaking step in the primary recrystallization annealing process. That is, the nitriding step can be carried out in a separate soaking zone, or can be carried out in a soaking zone provided with a shielding layer that hinders the flow of nitriding gas to the front end and the rear end.

[0094] In the primary recrystallization process, by appropriately adding a hydrogen + nitrogen atmosphere gas and a nitriding gas with controlled oxidation ability, the surface grains are appropriately grown and inhibited, and nitriding is smoothly carried out inside the steel sheet. Specifically, the nitrogen amount of the steel sheet increased by nitriding satisfies the following relational expression 3.

[0095] [Relational Expression 3]

[0096] [Al] - 27 / 14×[N] + (0.23 - Ct) 0.125 ≤ (27 / 14)ΔN ≤ 0.03

[0097] Wherein, [Al] and [N] respectively represent the contents (weight %) of Al and N in the slab, and Ct represents the final cold-rolled sheet thickness

[0098] In the present invention, when △N is too small, nitrogen cannot penetrate into the interior of the steel plate and only exists on the surface layer, resulting in a decrease in magnetism. On the other hand, when △N is too large, the grain growth in the surface layer part of the steel plate is strongly inhibited, resulting in a decrease in magnetism.

[0099] In the present invention, the nitriding gas can be used without limitation as long as it can decompose nitrogen at the temperature of the primary recrystallization annealing process and penetrate into the interior of the steel plate. Specifically, the nitriding gas may contain one or more of ammonia and amine. The nitriding process can be carried out for 30 - 100 seconds.

[0100] In the present invention, decarburization can also be carried out in the step of the primary recrystallization annealing. Decarburization can be carried out in an atmosphere with an oxidation ability (PH2O / PH2) of 0.3 to 0.7. Through decarburization, the steel plate can contain 0.005% or less of carbon, and more preferably can contain 0.003% or less of carbon.

[0101] In addition, in the present invention, the soaking temperature of the step of the primary recrystallization annealing can be carried out at a temperature of 800 - 900 °C. When the temperature is too low, primary recrystallization may not occur, or nitriding may not proceed smoothly. When the temperature is too high, the primary recrystallization growth is too large, resulting in a decrease in magnetism.

[0102] Subsequently, in the present invention, the cold-rolled plate that has completed the primary recrystallization annealing is subjected to secondary recrystallization annealing. The purpose of the secondary recrystallization annealing is generally to form a {110}<001> texture through secondary recrystallization, to form a glassy film through the reaction of the oxide layer formed during decarburization and MgO to impart insulation, and to remove impurities that damage magnetic properties. As a method of secondary recrystallization annealing, in the temperature rising range before secondary recrystallization occurs, it is maintained as a mixed gas of nitrogen and hydrogen to protect the nitride as a grain growth inhibitor, so that secondary recrystallization develops well, and after secondary recrystallization is completed, it is maintained in a 100% hydrogen atmosphere for a long time to remove impurities.

[0103] During the secondary recrystallization annealing process, the surface oxide layer generated during the primary recrystallization annealing process reacts with the annealing separator to form a base coating. The composition of the base coating is different from that of the base steel plate. For example, when MgO is used as the annealing separator, it contains bohaien forsterite.

[0104] In addition, in the present invention, after the secondary recrystallization annealing, it may further include a step of forming an insulating coating. The method of forming the insulating coating is well-known, so the specific description is omitted.

[0105] The grain-oriented electrical steel sheet of the present invention manufactured by the manufacturing process described above may contain, by weight %: Si: 2.5 - 4.0%, C: 0.03 - 0.09%, Al: 0.015 - 0.040%, Mn: 0.04 - 0.15%, S: less than 0.01% (except 0%), N: 0.002 - 0.012%, P: 0.010 - 0.050%, one or more of Sn and Sb in total: 0.030 - 0.12%, the balance being Fe and other inevitable impurities, and the amount of nitrogen ΔN added to the steel sheet by nitriding may satisfy the following relational expression 3.

[0106] [Relational expression 3]

[0107] [Al] - 27 / 14×[N] + (0.23 - Ct) 0.125 ≤ (27 / 14)ΔN ≤ 0.03

[0108] Wherein, [Al] and [N] respectively represent the contents (weight %) of Al and N in the steel sheet, and Ct represents the steel sheet thickness. Detailed implementation mode

[0109] Hereinafter, the present invention will be described in detail by way of preferred embodiments of the present invention. However, the following embodiments are merely a preferred embodiment of the present invention, and the present invention is not limited to the following embodiments.

[0110] (Example 1)

[0111] The slab having the composition shown in Table 1 below was subjected to vacuum melting, and then an ingot was manufactured. In Table 1 below, Steel grades A to C are inventive steels whose alloy compositions satisfy the scope of the present invention, Steel grades D to E are comparative steels that do not satisfy relational expression 1, and Steel grade F is a comparative steel that does not satisfy relational expression 2.

[0112] Next, the ingot was heated at a temperature of 1150 °C for 210 minutes and then hot-rolled to manufacture a hot-rolled sheet with a thickness of 2.0 mm t. In addition, the hot-rolled sheet was heated to a heating temperature T1 and then cooled to T2 and held for 90 seconds under the hot-rolled sheet annealing conditions shown in Table 2 below, and rapidly cooled in water, and cold-rolled once to a thickness of 0.20 mm after pickling.

[0113] Next, the cold-rolled sheet was held in a humid atmosphere and ammonia mixed gas atmosphere of 50 volume % (v%) hydrogen and 50 volume % nitrogen at a temperature of about 800 - 900 °C, thereby performing decarburization and nitriding annealing heat treatment. At this time, the carbon content in the annealed hot-rolled sheet was 30 ppm or less, and the nitriding amount was as shown in Table 2 below.

[0114] In addition, MgO, which serves as an annealing separator, is coated on the decarburized and annealed steel plate, and final annealing is performed in the form of coiled sheet. The final annealing is carried out in a mixed atmosphere of 25 vol% nitrogen and 75 vol% hydrogen up to 1200 °C, and after reaching 1200 °C, it is held in a 100% hydrogen atmosphere for more than 10 hours and then furnace-cooled. Thereafter, an insulating coating composition containing a mixed solution of metal phosphate and colloidal silica is coated on the steel plate, and heat treatment is performed to form an insulating coating.

[0115] As described above, the maximum and minimum values of the magnetic flux density and iron loss of the obtained electrical steel sheet are measured and shown in Table 2 below. For magnetic properties, the iron loss is measured under the conditions of 1.7 Tesla and 50 Hz using the single sheet measurement method, and the size (Tesla) of the magnetic flux density induced in a magnetic field of 800 A / m is measured. In addition, the magnetic properties of the entire coiled sheet are measured, and the maximum and minimum values thereof are shown in Table 2 below.

[0116] [Table 1]

[0117]

[0118] The residual components in Table 1 are Fe and inevitable impurities. In addition, Alr represents [Al] - 27 / 14×[N] in Relationship 1.

[0119] [Table 2]

[0120]

[0121] As shown in Tables 1 to 2 above, in the case of Invention Examples 1 to 3 where the steel plate alloy composition, Relationships 1 to 2, and manufacturing conditions satisfy the scope of the present invention, it can be confirmed that the deviations of both iron loss and magnetic flux density are small.

[0122] In contrast, in Comparative Example 1 where the manufacturing process conditions do not satisfy Relationship 3, it can be confirmed that the precipitates formed in the DNL (decarburization and nitriding process) are insufficient, resulting in large deviations in iron loss and magnetic flux density.

[0123] In addition, in Comparative Examples 2 to 3 where the hot-rolled plate annealing heat treatment conditions deviate from the scope of the present invention, it can be seen that the deviations of iron loss and magnetic flux density are large.

[0124] In addition, in the case of Comparative Examples 4 to 5 using Comparative Steels D to E where the steel plate alloy composition does not satisfy Relationship 1, the deviations of iron loss and magnetic flux density are large, and in the case of Comparative Example 6 using Steel Grade F where the steel plate alloy composition does not satisfy Relationship 2, it can be seen that the deviations of iron loss and magnetic flux density are large.

[0125] (Example 2)

[0126] A slab having the composition shown in Table 33 below is subjected to vacuum melting to produce an ingot. Among the steel grades in Table 3 below, steel grades G to I are inventive steels whose alloy compositions satisfy the scope of the present invention, steel grades J to K are comparative steels that do not satisfy Relationship 1, and steel grade L is a comparative steel that does not satisfy Relationship 2.

[0127] Next, the ingot is heated at a temperature of 1150 °C for 200 minutes and then hot-rolled to produce a hot-rolled sheet with a thickness of 1.8 mm t. In addition, under the hot-rolled sheet annealing conditions shown in Table 4 below, the hot-rolled sheet is heated to the heating temperature T1, cooled to T2, held for 90 seconds, rapidly cooled in water, pickled, and then cold-rolled once to a thickness of 0.18 mm.

[0128] Next, the cold-rolled sheet is held in a humid atmosphere and ammonia mixed gas atmosphere of 50% by volume of hydrogen and 50% by volume of nitrogen at a temperature of about 800 - 900 °C, and decarburization and nitriding annealing heat treatment are performed. At this time, the carbon content in the annealed hot-rolled sheet is 30 ppm or less, and the nitriding amount is as shown in Table 4 below.

[0129] In addition, MgO as an annealing separator is coated on the steel sheet subjected to decarburization annealing heat treatment, and final annealing is performed in the form of a coiled sheet. The final annealing is carried out to 1200 °C in a mixed atmosphere of 25% by volume of nitrogen and 75% by volume of hydrogen. After reaching 1200 °C, it is held in a 100% hydrogen atmosphere for more than 10 hours and then furnace-cooled. After that, an insulating coating composition containing a mixture of metal phosphate and colloidal silica is coated on the steel sheet, and heat treatment is performed to form an insulating coating.

[0130] As described above, the maximum and minimum values of the magnetic flux density and iron loss of the produced electrical steel sheet are measured and shown in Table 4 below. For magnetic properties, the iron loss is measured under the conditions of 1.7 tesla and 50 Hz using the single-sheet measurement method, and the size (tesla) of the magnetic flux density induced in a magnetic field of 800 A / m is measured. In addition, the magnetic properties of the entire coiled sheet are measured, and their maximum and minimum values are shown in Table 4 below.

[0131] [Table 3]

[0132]

[0133] In Table 3, the remaining components are Fe and unavoidable impurities. In addition, Alr represents [Al] - 27 / 14 × [N] of Relationship 1.

[0134] [Table 4]

[0135]

[0136] As shown in Tables 3 to 4 above, in the case of Invention Examples 1 to 3 where the alloy composition of the steel sheet, Relations 1 to 2, and the manufacturing conditions satisfy the scope of the present invention, it can be confirmed that the deviations of iron loss and magnetic flux density are both small.

[0137] In contrast, in Comparative Examples 7 to 12 where the steel sheet composition components (relations) or the manufacturing process conditions deviate from the scope of the present invention, it can be confirmed that the deviations of iron loss and magnetic flux density are both large. In particular, in Comparative Example 7 where the manufacturing process conditions do not satisfy Relation 3, insufficient precipitation of DNL results in large deviations of iron loss and magnetic flux density.

[0138] From the descriptions of the above embodiments, in the case of Invention Examples 1 to 6 where the content ranges of Al and N in the steel sheet according to the hot-rolled sheet thickness are appropriately ensured within the proposed ranges, the AP annealing conditions are optimized, and the nitriding conditions according to the cold-rolled sheet thickness are optimized during the primary recrystallization annealing, it can be confirmed that the deviations of iron loss and magnetic flux density are both small.

[0139] On the other hand, in the case of Comparative Examples 1 to 12 where the content ranges of Al and N in the steel sheet according to the hot-rolled sheet thickness are not within the ranges proposed by the present invention, or the AP annealing conditions deviate from the ranges proposed by the present invention, or the nitriding conditions according to the cold-rolled sheet thickness during the primary recrystallization annealing deviate from the scope of the present invention, it can be confirmed that the iron loss and magnetic flux density are poor and their deviations are large.

[0140] The present invention is not limited to the above embodiments and can be manufactured in various different forms. Those of ordinary skill in the technical field to which the present invention pertains can understand that it can be implemented by other specific embodiments without changing the technical idea or essential features of the present invention. Therefore, it should be understood that the above-described embodiments are exemplary in all aspects and not restrictive.

Claims

1. A method for manufacturing a thin grain-oriented electrical steel sheet, comprising the following steps: The slab is hot-rolled to produce a hot-rolled sheet. By weight percentage, the slab contains: Si: 2.5 - 4.0%, C: 0.03 - 0.09%, Al: 0.0150 - 0.0400%, Mn: 0.04 - 0.15%, S: less than 0.010% and excluding 0%, N: 0.0020 - 0.0120%, P: 0.010 - 0.050%, one or more of Sn and Sb in total: 0.03 - 0.12%, the balance of Fe and other inevitable impurities; the hot-rolled sheet is subjected to hot-rolled annealing treatment; the hot-rolled annealed sheet is cold-rolled to produce a cold-rolled sheet; the cold-rolled sheet is subjected to primary recrystallization annealing for decarburization and nitriding; and the cold-rolled sheet after the primary recrystallization annealing is subjected to secondary recrystallization annealing, wherein, the contents of Al and N in the slab satisfy the following relational expressions 1 to relational expression 2, When the temperature difference △T between the heating temperature T1 and the soaking annealing temperature T2 in the annealing step of the hot-rolled sheet satisfies △T≥200°C, and the cooling rate when cooling from T1 to T2 is controlled below 30°C / second, and the amount of nitrogen △N added to the cold-rolled steel sheet by nitriding in the step of the primary recrystallization annealing satisfies the following relational expression 3, [Relational expression 1] 0.039 - 0.01Ht ≤ [Al] - 27 / 14×[N] ≤ 0.047 - 0.01Ht [Relational expression 2] 15.5 - 5Ht ≤ [Al] / [N] In relational expressions 1 to 2, [Al] and [N] respectively represent the contents of Al and N in the slab, in weight percentage, and Ht represents the thickness of the final hot-rolled sheet, in mm, [Relationship 3] [Al] - 27 / 14×[N] + (0.23 - Ct) 0.125 ≤ (27 / 14)△N ≤ 0.03 wherein, [Al] and [N] respectively represent the contents of Al and N in the slab, in weight percentage, and Ct represents the thickness of the final cold-rolled sheet.

2. The manufacturing method of the thin grain-oriented electrical steel sheet according to claim 1, wherein, The slab further contains 0.02 - 0.15% of Cr.

3. The manufacturing method of the thin grain-oriented electrical steel sheet according to claim 1, wherein, The slab further contains 0.01 - 0.2% of Cu or 0.01 - 0.05% of Bi.

4. The manufacturing method of the thin grain-oriented electrical steel sheet according to claim 1, wherein, The slab contains one or more of Ti: 0.002 - 0.010% and V: 0.002 - 0.010% with the sum of the contents in the range of 0.002 - 0.010%.

5. The manufacturing method of the thin grain-oriented electrical steel sheet according to claim 1, wherein, The process of the primary recrystallization annealing is carried out at a temperature of 800 - 900°C for 60 - 180 seconds.

6. A thin grain-oriented electrical steel sheet. By weight percentage, the thin grain-oriented electrical steel sheet contains: Si: 2.5 - 4.0%, C: 0.03 - 0.09%, Al: 0.0150 - 0.0400%, Mn: 0.04 - 0.15%, S: less than 0.010% and excluding 0%, N: 0.0020 - 0.0120%, P: 0.010 - 0.050%, one or more of Sn and Sb in total: 0.03 - 0.12%, the balance of Fe and other inevitable impurities, and the amount of nitrogen △N added to the steel sheet by nitriding satisfies the following relational expression 3, [Relationship 3] [Al] - 27 / 14×[N] + (0.23 - Ct) 0.125 ≤ (27 / 14)△N ≤ 0.03 Among them, [Al] and [N] respectively represent the contents of Al and N in the steel sheet, in weight percentage, and Ct represents the thickness of the steel sheet.

7. The thin grain-oriented electrical steel sheet according to claim 6, wherein, The steel sheet further contains 0.02 - 0.15% of Cr.

8. The thin grain-oriented electrical steel sheet according to claim 6, wherein, The steel plate further contains 0.01 - 0.2% of Cu or 0.01 - 0.05% of Bi.

9. The thin grain-oriented electrical steel sheet according to claim 6, wherein, The steel plate contains one or more of Ti in the range of 0.002 - 0.010% and V in the range of 0.002 - 0.010% with the sum of the contents being 0.002 - 0.010%.

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