KR20020041021A - Method for manufacturing unidirectional electrical steel sheet with high magnetic flux density - Google Patents

Abstract

PURPOSE: A method is provided to manufacture a unidirectional electrical steel sheet with superior high magnetic flux density more economically by adding Al and N added in steelmaking process in a minimum quantity capable of forming an inhibitor required for causing second recrystallization. CONSTITUTION: The method for manufacturing a unidirectional electrical steel sheet with high magnetic flux density comprises the processes of hot rolling the slab after completely solid solution treating the AlN deposited by reheating a slab comprising 2.0 to 4.0 wt.% of Si, 0.0060 to 0.030 wt.% of acid soluble Al, 0.0030 wt.% or less of N, 0.025 to 0.060 wt.% of C, 0.32 wt.% or less of Mn, 0.02 to 0.1 wt.% of Ni, 0.02 to 0.1 wt.% of Cr, 0.007 wt.% or less of S and a balance of Fe and other inevitable impurities in the temperature range which is between 1000 to 1200 deg.C, and above a theoretic solid solution temperature of AlN formed by reacting Al and N added to steelmaking process; re-solid solution treating AlN deposits produced during hot rolling by heating the hot rolled steel sheet to a temperature of the theoretic solid solution temperature of AlN added in the steelmaking process or more again; annealing the hot rolled steel sheet so that an inhibitor of uniformed AlN is finely deposited by rapidly cooling the resulting steel sheet in the temperature range of 800 to 1000 after slowly cooling the heated steel sheet to a temperature of 800 to 1000 deg.C; cold rolling the resulting steel sheet to a thickness of 0.20 to 0.35 mm using one time of cold rolling process or two times of cold rolling processes including intermediate annealing after pickling the annealed steel sheet; reinforcing the inhibitor by simultaneously performing decarburization and nitriding on the cold rolled steel sheet at a temperature of 800 to 950 deg.C for 180 seconds or more under a mixed gas atmosphere of hydrogen and nitrogen having a dew point of 40 to 70 deg.C and containing nitrogen ions, thereby forming fine AlN; and performing finishing high temperature annealing on the resulting material after coating an annealing separating agent having a principal constituent of MgO on the decarburized and nitrided steel sheet.

Description

Method for Manufacturing Unidirectional Electrical Steel Sheet With High Magnetic Flux Density

The present invention relates to a method of manufacturing a unidirectional electrical steel sheet used as iron core materials of large sized rotary machines such as various transformers, electric motors, generators, and other electronic devices, and more particularly, to manufacture high magnetic flux density oriented electrical steel sheets capable of low-temperature heating of slabs. It is about a method.

In general, a unidirectional electrical steel sheet is composed of grains having a Goss texture, which is a crystal orientation of the steel plate surface is (110) plane and the crystal orientation of the rolling direction is parallel to the [001] axis. It is a soft magnetic material with excellent characteristics. In particular, the high magnetic flux density oriented electrical steel sheet has excellent magnetic properties since the orientation of the (110) [001] grains in the rolling direction is aligned in a range of 3 ° or less. The magnetic properties of oriented electrical steel are mainly expressed by magnetic flux density and core loss. The magnetic flux density is the magnitude of magnetic flux induced in the oriented electrical steel sheet by a certain magnetic field (1000A / m), B10 (Tesla). The soft magnetic material having high magnetic flux density, that is, excellent magnetization property, is advantageous because it can reduce the size of an electric device. Iron loss [W17 / 50 (W / kg)] is a power loss consumed by thermal energy in an iron core with 50 Hz alternating current under a magnetic field of constant intensity (1.7 Tesla). . For that reason, demand for directional electrical steel sheets with low iron loss is increasing in order to save energy.

The grain-oriented electrical steel sheet is composed of grains in the (110) [001] orientation only. In the primary recrystallized structure formed by heat treatment after cold rolling, grains having a specific orientation, that is, the orientation of (110) [001], grow abnormally. This selective and abnormal grain growth is called secondary recrystallization. An excellent high magnetic flux density oriented electrical steel sheet can be obtained when the degree of orientation of the grains in the rolling direction is excellent with complete secondary recrystallization by grains of the (110) [001] orientation.

There are several prerequisites for obtaining a complete secondary recrystallization.

First, the grains of the (110) [001] orientation, which are the nucleus of the secondary recrystallization, must exist in the primary recrystallization tissue.

In general, grains of the (110) [001] orientation are formed in the hot rolled sheet by shear deformation during hot rolling, and are also present in the primary recrystallized structure after cold rolling and heat treatment. The larger the cold rolling rate, the smaller the grains in the (110) [001] orientation are, but only the (110) [001] grains which are extremely excellent in the orientation in the rolling direction remain. When these grains are secondary recrystallized, the orientation in the rolling direction is also reduced. The good magnetic flux density is obtained.

Second, crystal growth of primary recrystallized grains with different orientations around them must be suppressed until grains of the (110) [001] orientation are secondary recrystallized. Generally, those used to suppress crystal growth of grains are solid elements such as B, P, and Sn or precipitates such as MnS (Se), AlN, Cu ₂ S, and BN, which are either dissolved or precipitated at grain boundaries to prevent grain migration. Interfering has the effect of inhibiting grains from growing. As described above, the employment element or precipitate that inhibits the growth of grains is often inhibited. In order for the inhibitor to exhibit a strong deterrent, the distribution of the inhibitor in the primary recrystallization tissue is very important along with the selection of the appropriate inhibitor. Only when a large amount of inhibitor is finely and uniformly distributed can the growth of the primary recrystallized grains be surely suppressed to obtain a complete secondary recrystallization. To this end, an appropriate amount of inhibitor element was added in the steelmaking step, and a slab was heated to a high temperature of 1400 ° C. to sufficiently dissolve the elements that are inhibitors.

Third, grains of different orientations in which crystal growth is suppressed should be well encroached upon secondary recrystallization of (110) [001] grains. Many studies have shown that grains in the {111} <112> orientation correspond to grains in the (110) [001] orientation about 35 °, and when these grains are large around the (110) [001] grain, Secondary recrystallization is said to occur stably. Therefore, in order to obtain grains in the {111} <112> orientation, it is necessary to stabilize the structure by hot-rolled sheet annealing and to make uniform primary recrystallized structure by appropriate cold rolling and heat treatment. Increasing the cold rolling rate improves the orientation of the (110) [001] grains in the rolling direction, but on the contrary, the grains in the {111} <112> azimuth become less and cannot form a stable secondary recrystallization. In addition, the high cold rolling rate increases the driving force of crystal growth, so that crystal growth of recrystallized grains is not suppressed, and secondary recrystallization becomes unstable due to normal crystal growth.

Such a complete secondary recrystallization requires a uniform primary recrystallization structure composed of (110) [001] grains with good orientation, strong inhibitors, and grains mainly in the {111} <112> orientation. Therefore, much research on oriented electrical steel sheet has focused mainly on the selection of inhibitors with excellent grain growth suppression effect and preconditions for enabling such inhibitors to effectively suppress grain growth. So far, precipitates such as MnS, AlN, MnSe, and BN have been found to be effective as inhibitors. Together with these precipitates, the conditions under which the precipitates can inhibit grain growth, such as slab reheating, hot rolling and winding temperatures, and Hot rolled sheet annealing condition, cold rolling rate and finishing hot annealing condition to obtain uniform primary recrystallization structure have been studied.

In recent years, development has been carried out to lower the heating temperature of the slab and to omit some processes in order to increase productivity and reduce manufacturing costs. As described above, a complete secondary recrystallization requires a large amount of inhibitor to be finely and uniformly distributed in the primary recrystallization structure, which requires heating the slab to a high temperature of 1400 ° C. where the inhibitor can be sufficiently employed.

The slab heating at such a high temperature is simply to finely deposit the inhibitor, but the damage thereof is very large.

Coarse microstructure due to high temperature slab heating generates streaks that hinder the magnetic properties of the final product, and edge cracks occur during hot rolling, greatly reducing product productivity. In addition, a high temperature slab heating operation forms an oxide layer called fayalite on the surface of the slab. The oxide layer has a very low melting point and melts at a temperature higher than 1300 ° C. At this time, the molten slag accumulates in the furnace, which requires frequent furnace maintenance and periodic furnace repair. As mentioned above, due to the difficulty of high temperature heating work of slab and the increase in production cost, many studies have been conducted to reduce slab heating temperature at each steel mill.

As an example, Japanese Patent Application Laid-Open No. 1-230721 proposes a method of manufacturing a high magnetic flux density oriented electrical steel sheet by a method of forming an AlN as an inhibitor using ammonia gas after decarbonization annealing during the manufacturing process. .

Since this method does not form AlN as a strong inhibitor in the steelmaking stage, the slab reheating temperature can be drastically reduced, and after decarbonization annealing, an independent and additional immersion annealing with ammonia gas is introduced to introduce the AlN as an inhibitor. It is a method of manufacturing a density oriented electrical steel sheet.

This manufacturing method is characterized in that the slab is heated at a low temperature of 1200 ° C. where the inhibitor is not completely employed, unlike the conventional high flux density oriented electrical steel sheet manufacturing method in which the slab is heated to a high temperature of 1400 ° C. where the inhibitor is completely employed. It is hot-rolled and then nitrided using ammonia gas in the process between the final cold rolling process and the high temperature annealing process to introduce the inhibitor necessary for secondary recrystallization.

That is, the major difference from the existing manufacturing method is that the inhibitor is not completely employed in the slab heating step.

In the steelmaking process, with addition of the main components, the contents of Al and N components, which are the main components of the inhibitor, are adjusted, and then a slab is made. At this time, the AlN, which is an inhibitor, grows coarsely. The method of heating the slab at high temperature in order to solidify such coarsely grown AlN into the steel was a method of manufacturing a conventional high magnetic flux density oriented electrical steel sheet, and the method proposed in Japanese Patent Publication No. 1-230721 was The slab is heated only to a temperature that can be hot rolled without completely coagulating AlN grown coarse, and nitriding with ammonia gas after final cold rolling forms the inhibitor AlN necessary for secondary recrystallization.

This method is a manufacturing method in which AlN is already formed in the slab and the remaining unreacted acid-soluble Al reacts with the N component introduced into the nitriding process in a later step to form AlN, an inhibitor, which is added in the steelmaking process. It is a method of forming an inhibitor using only a part of Al content.

On the other hand, Japanese Patent Laid-Open No. 84-056522 contains about 0.006 to 0.026 wt% of acid-soluble Al and 0.003 to 0.013 wt% of N to heat the slab to a temperature at which the inhibitor is not completely dissolved to form AlN as a coarse precipitate. A method for producing a grain-oriented electrical steel sheet is presented.

In the case of the method described above, considering the fact that the content of N in the steel which has undergone the steelmaking step is 30 ppm or less, a process of additionally adding N is required, which leads to a complicated manufacturing process and disadvantages in terms of productivity.

On the other hand, when the AlN, which is an inhibitor, is heated in a state in which it is not completely dissolved in the slab, and is hot rolled, coarse AlN remains in the hot rolled plate. In this case, fine AlN is precipitated in which the coarse AlN serves as an inhibitor It is very easy to cause unstable secondary recrystallization because it is very likely to grow by eroding fine AlN during high temperature annealing where secondary recrystallization occurs. Therefore, in order to suppress such a phenomenon, the above-described method requires a very difficult manufacturing process to maintain the appropriate grain size after complex hot-rolled sheet annealing and decarbonization annealing.

The present inventors have conducted research and experiments to solve the above-mentioned problems of the prior art, and based on the results, the present invention provides secondary recrystallization of Al and N components added in the steelmaking process. By adding as little as possible the required inhibitor, the slab can be heated at low temperature, and the unidirectional electrical steel sheet having excellent high magnetic flux density characteristics by reinforcing the inhibitor by hot-rolled sheet annealing and decarburization-precipitation treatment after cold rolling. To provide a method to manufacture more economically, the purpose is.

According to the present invention, in the method for producing a high magnetic flux density unidirectional electrical steel sheet, Si: 2.0 to 4.0%, acid-soluble Al: 0.0060 to 0.030%, N: 0.0030% or less, C: 0.025 to 0.060%, Mn: Slabs containing 0.32% or less, Ni: 0.02% to 0.1%, Cr: 0.02% to 0.1%, and S: 0.007% or less, and the balance of Fe and other unavoidable impurities are not lower than the theoretical employment temperature of AlN added in the steelmaking process. After reheating between 1200 ° C and hot rolling, the hot rolled plate is heated above the theoretical employment temperature of AlN, then slowly cooled to a temperature of 800-1000 ° C, and then quenched at a temperature of 800-1000 ° C to obtain an homogeneous inhibitor. After the hot-rolled sheet annealing is carried out so that fine precipitation of AlN occurs, the hot-rolled sheet is cold rolled to 0.20 to 0.35mm in thickness by one or two cold rolling methods including intermediate annealing, and the dew point is 40-70 ° C. In a mixed gas atmosphere of hydrogen and nitrogen containing nitrogen ions, Decarburizing and nitriding at the same time for more than 180 seconds at the same time to form additional fine AlN to reinforce the inhibitor, apply annealing separator mainly containing MgO, and then finish high temperature annealing to perform high magnetic flux density unidirectional electrical steel sheet It relates to a method of manufacturing.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present inventors have conducted extensive research to improve the above-mentioned problems of the prior art, and introduced the minimum acid-soluble Al and N contents required to form the inhibitors required for secondary recrystallization in the steelmaking process to obtain additional ferroalloy input. By suppressing the production cost and lowering the AlN completely and reprecipitating it to make the most of the inhibitor and strengthening the inhibitor by forming additional AlN precipitate by nitrogen ion introduced by decarburization and nitriding treatment performed at the final thickness It is to propose a method that can be more economically produced a high magnetic flux density unidirectional electrical steel sheet by a strong inhibitor.

That is, the present invention is a method of manufacturing a high magnetic flux density unidirectional electrical steel sheet, 0.0060 ~ 0.030wt%, and N component of 0.0060 to 0.030wt%, which is the main component of the inhibitor which plays an important role in the formation of secondary recrystallization in the steelmaking process By adding ˜0.0030 wt%, the AlN precipitates formed by reacting these components are designed to be completely dissolved in the slab heating furnace at least 1000 ℃ and the slab heating is performed by heating the steel sheet slab at a low temperature between 1000∼1200 ℃. After securing the complete employment of the inhibitor AlN in the hot-rolled sheet annealing, which is also a post-process, the AlN was heated to a temperature higher than the complete employment temperature of AlN by addition of Al and N components, and then slowly cooled to a temperature of 800-1000 ° C. By quenching at a temperature of 1000 ℃ to form a fine AlN to improve the crystal growth inhibition of the inhibitor.

In addition, it is a feature of the present invention to reinforce the AlN inhibitor by forming additional AlN precipitates by nitrogen ions introduced by the decarburization and nitriding treatment performed at the final thickness.

In general, a lot of research has been conducted on the full employment of AlN, which is widely used as an inhibitor in oriented electrical steel sheets. In particular, it is well established by Iwayama and the theoretical employment temperature of AlN is defined as follows.

Log [Alwt%] [Nwt%] =-10062 / T + 2.72 (T: Kelvin temperature)

The above relation is importantly used to predict the temperature at which AlN formed during slab production can be completely dissolved in slab heating by using Al and N content added during the production of grain-oriented electrical steel sheet. Therefore, the present invention provides a uniform and fine AlN precipitate distribution to the microstructure before secondary recrystallization by allowing Al and N to be completely dissolved in slab heating while allowing low-temperature slab heating from this relationship. In addition, since an additional nitrogen component can be introduced into the steel sheet through the decarburization and at the same time in the annealing process after cold rolling, it is possible to greatly reduce the amount of nitrogen added in the steelmaking process. As a result of the study, the inventors confirmed that reducing the amount of nitrogen added to 0.0030 wt% or less in the steelmaking process can lower the theoretical solid solution temperature of AlN to 1200 ° C. or below, and also hot roll after slab heating. When the nitrogen content is controlled below 0.0030wt%, the Al content is up to 0.0300wt%, and since the AlN theoretical employment temperature rises above 1200 ℃, the complete solid solution of AlN is not required for low temperature slab heating below 1200 ℃. It will not be possible. By optimizing the content of Al and N added in the steelmaking process, the slab heating temperature required for the employment of the inhibitor is lowered to 1000 ~ 1200 ° C, and the slab low temperature heating and the formation of fine AlN precipitates according to the full employment of the inhibitor make the secondary stable It is possible to manufacture unidirectional electrical steel sheets with recrystallization and excellent high magnetic flux density.

On the other hand, in order to obtain a more stable and excellent high magnetic flux density, it is necessary to distribute the AlN inhibitors with a strong suppression force evenly in the steel sheet. Unstable This is because the AlN is completely employed by the slab heating at 1000-1200 ° C, but the deposition of the precipitate may be uneven due to the partial precipitation of the slab in the hot rolling process of the slab.

Therefore, it is necessary to reheat the hot rolled plate to re-employ the precipitates which may have been formed during hot rolling, and to quench them to make very fine precipitates. Of course, since the hot rolled sheet has a portion that is not recrystallized during hot rolling and has a very uneven microstructure, it is necessary to perform annealing to make the microstructure uniform. Therefore, the present inventors heat the slab by heating the slab to a low temperature of 1000-1200 ° C. with AlN component design, and then hot-roll the slab so that AlN is completely dissolved and then react the Al and N components to which the hot-rolled plate is added again in the process. The AlN precipitates formed were heated above the reusable temperature, and then cooled slowly to a temperature of 800 to 1000 ° C. where fine precipitates were formed, and then quenched at a temperature of 800 to 1000 ° C.

The hot-rolled sheet annealing ensures a uniform microstructure and a very fine and uniform distribution of AlN precipitates, thereby making it possible to manufacture a unidirectional electrical steel sheet having high magnetic flux density magnetic properties. If the hot-rolled sheet annealing temperature is lower than the theoretical employment temperature of AlN, the AlN formed during the hot rolling grows coarsened to form a non-uniform precipitate distribution, resulting in unstable secondary recrystallization and heat flux density characteristics.

On the other hand, after the acid-soluble Al added in the steelmaking process and N sufficiently reacts to form fine AlN, acid-soluble Al that does not react with N remains. Therefore, the nitrogen ions introduced through the decarburization and nitriding at the final thickness react with the remaining phase soluble Al to form AlN, thereby reinforcing the fine AlN inhibitor which is generated in the hot-rolled sheet annealing to suppress grain growth. Vehicle recrystallization and excellent magnetic properties are secured.

Hereinafter, the reason for limitation of the component of this invention and its component range is demonstrated.

Si is a basic composition of electrical steel sheet to increase the specific resistance of the material serves to lower the core loss (core loss). If the Si content is less than 2.0wt%, the resistivity decreases and the iron loss characteristics deteriorate. If the content is over 4.0wt%, the iron content becomes excessively brittle, making the cold rolling extremely difficult and unstable secondary recrystallization. Silver is preferably 2.0 to 4.0 wt%.

The Al component is a major component of the inhibitor that forms grains of AlN and suppresses grain growth with N. The Al content of acid solubility that reacts with N to form AlN is important rather than the total amount of Al. When the acid-soluble Al exceeds 0.030 wt%, the ratio of coarse stretched grains to the hot rolled sheet increases relatively. Therefore, the hot rolled plate heating temperature for recrystallization of the stretched grains is increased to 1200 ° C. or more. Such heat treatment affects the distribution of precipitates in the hot rolled sheet, so that the primary recrystallization structure is not uniform and the {112} <110> aggregate structure is developed. Such aggregates do not contribute to secondary recrystallization of the {110} <001> component. In addition, when AlN is formed by reacting with the N component introduced during the simultaneous decarburization and nitriding treatment after cold rolling, a non-uniform precipitate distribution is formed, resulting in unstable secondary recrystallization. If the acid-soluble Al content is less than 0.0060wt%, not only precipitates sufficient to inhibit the growth of crystal grains are formed in the first place, but secondary recrystallization becomes unstable because it is difficult to reinforce the inhibitor by further nitriding.

N is an essential component in the formation of secondary recrystallization because it is a member of the inhibitor that reacts with acid-soluble Al to form AlN precipitates to inhibit the growth of primary recrystallized grains. One of the characteristics of the present invention is that by lowering this N component to 0.0030 wt% or less, the slab reheating can be performed at a low temperature of 1000 to 1200 ° C., and the AlN precipitate can be formed very strong through the complete re-incorporation of AlN. will be. Therefore, it is not necessary to add more than 0.0030wt% of N component in the steelmaking process. This is because the addition of fine AlN precipitates with strong inhibitory power can be easily performed by adding N component through simultaneous decarburization and nitriding treatment, which is a process after cold rolling. If more than 0.0030wt% is added, it is impossible to achieve full employment of AlN by low-temperature slab heating, and coarse AlN is formed to decrease grain growth inhibition effect. Therefore, it is preferable to add N component below 0.0030wt%, and to add an additional N component in the post-processing simultaneous decarburization and nitriding process.

C is added to 0.02wt% or more to promote the austenite transformation of steel to refine the hot-rolled structure during hot rolling, and to promote the solid solution and precipitation of AlN to enhance the effect as an inhibitor, but when it remains in the final product, Because it forms a magnetic deterioration, it must be managed to 30ppm or less in the final product through decarburization process. In the present invention, because the slab is heated at a low temperature, the role of carbon is not very important, and therefore, even if the carbon content is added only 0.025 wt% or more, the effect of carbon may be sufficiently seen. However, when the carbon content is more than 0.060wt%, the decarburization takes a long time and the nitriding treatment is hindered by excess carbon, and the reinforcement of the inhibitor becomes unstable, so that sufficient decarburization and nitriding treatment is not performed. .

Mn has the effect of reducing the iron loss by increasing the specific resistance similar to Si, and promotes austenite transformation during slab heating to facilitate the solid solution of AlN. However, when 0.32% or more is added, coarse MnS is formed by combining with a small amount of S. Since AlN tends to precipitate around coarse MnS, it prevents even precipitation formation of AlN.

S is an ingredient that combines with Mn to form an emulsion to suppress grain growth, but when added excessively, S is segregated in the center part during slab heating and adversely affects the microstructure. In addition, when coarse precipitates are combined with Mn to prevent even precipitation of AlN, the present invention is preferably managed at 0.007% or less.

Cr delays the recrystallization of the deformed crystal grains during hot rolling, thereby promoting the development of the {110} <001> component, and helps to form a stable forsterite layer during high temperature annealing. If Cr is less than 0.02%, the effect of addition cannot be obtained. If more than 0.20% of Cr is added, a large amount of stretched tissue exists in the hot rolled plate, and cold rolling becomes difficult due to severe work hardening during cold rolling.

Ni is an austenite forming element such as carbon and Mn, and helps to form evenly precipitated and uniform hot rolled structure of the inhibitor AlN. If it is less than 0.02% like Cr, addition effect is not obtained, and if it is added more than 0.10%, cold rolling becomes difficult due to severe work hardening during cold rolling, and it is desirable to add it to 0.10% or less because it is an expensive alloy. Do.

The steel components of the present invention are as described above, and the other components are composed of Fe and inevitable trace impurities. The steel material of the above components can be used as a raw material of the present invention even when manufactured using any conventional dissolution method, ingot method, performance method, or the like.

Hereinafter, manufacturing conditions are demonstrated.

The heating temperature of the slab of the electrical steel sheet composed of the above-described components is 1000 ~ 1200 ℃ such low temperature slab heating temperature is possible by controlling the content of Al and N components added in the steelmaking process. The most important feature of the present invention is that in spite of such low temperature slab heating, by fully employing the inhibitor AlN, fine AlN precipitates having a strong crystal growth inhibition necessary for secondary recrystallization can be obtained. When the slab is heated to 1200 ° C. or higher, the slab surface portion melts and flows down due to high temperature oxidation, which greatly shortens the life of the heating furnace and causes a failure of the slab heating equipment. It is also disadvantageous in terms of production cost because it uses more energy than necessary. The heating temperature of 1000 ° C. or less causes the rolling load to become very large during hot rolling, making the rolling itself difficult, and since the AlN is not well dissolved, the microstructure of the hot rolled sheet and the primary recrystallization sheet becomes very unstable. Therefore, the steel sheet slab heated under the slab reheating conditions as described above is preferably hot rolled to a thickness of 2.6 to 1.5 mm in consideration of the optimum rolling rate and wound at a temperature of 600 ° C. or lower.

The hot rolled sheet is present in a state in which most of the AlN is dissolved, but some AlN is precipitated during the cooling process after hot rolling, these precipitates are likely to continue to grow in the subsequent heat treatment process. In addition, the microstructure after hot rolling has a very nonuniform microstructure in the thickness direction. In particular, there are crystal grains elongated in the rolling direction during hot rolling in the center of the hot rolled sheet. Such stretched grains do not form a secondary recrystallized structure. Therefore, in order to give a uniform microstructure to such a hot rolled sheet and to form fine AlN having a strong crystal growth inhibitory power, hot rolled sheet preannealing should be performed. One of the characteristics of the present invention is that AlN precipitates finely after heating the hot rolled sheet above the theoretical employment temperature of AlN formed by reacting Al and N components added in the steelmaking process to form a fine and uniform AlN precipitate. After cooling slowly to the temperature of 800-1000 degreeC, it rapidly cools at the temperature of 800-1000 degreeC, and forms a strong precipitate distribution. If the hot rolled plate is not heated above the theoretical employment temperature as described above, the AlN formed during hot rolling is not reusable and continues to grow. As a result, the crystal growth inhibitory power of the AlN inhibitor is lowered and the secondary recrystallization becomes unstable. Therefore, the annealing of the hot-rolled sheet is a temperature of 800 ~ 1000 ℃ to re-use the AlN precipitated by heating above the theoretical employment temperature of AlN formed by the reaction of Al and N component added in the steelmaking process and then AlN begins to precipitate fine It is preferable to quench at. If the annealing temperature of such a hot rolled sheet is numerically defined, it is preferable to limit the slab heating temperature of the inventive component system to 1000-1200 ° C, and the annealing temperature must be Al and N added in the steelmaking process as described above. It should be above the theoretical employment temperature of AlN formed by the reaction of the components. Once heated above the theoretical employment temperature, it is slowly cooled to a temperature of 800 to 1000 ° C., preferably 900 to 950 ° C., more preferably 950 ° C., and then quenched at this temperature. Fine AlN precipitation does not occur. This reduces the grains of {110} <001>, the nuclei of secondary recrystallization, among the grains recrystallized in the simultaneous decarburization and nitriding after cold rolling.

On the contrary, when quenched at a temperature of 800 ° C. or lower, AlN is already precipitated and grown, and thus a strong inhibitor cannot be obtained. Therefore, after heating above theoretical employment temperature, it should be quenched at a temperature of 800 ~ 1000 ℃ to precipitate fine AlN. The hot rolled sheet thus annealed is pickled and rolled by one cold rolling method including one time or intermediate annealing up to a final thickness of 0.20 to 0.35 mm. Cold rolling below 0.2mm is inferior in workability due to excessive deformation stress, and the crystal growth driving force becomes very large, making it difficult to control as an inhibitor. Cold rolling to a thickness of 0.35 mm or more is not preferable because the orientation in the (110) [001] direction is lowered, and the iron loss worsens when the thickness is thick.

The cold rolled sheet to the final thickness has a dew point of 40 ~ 70 ℃, heat treatment to perform decarburization and nitriding at the same time by maintaining the temperature of 800 ~ 950 ℃ for more than 180 seconds in a mixed gas atmosphere of hydrogen and nitrogen containing nitrogen ions Nitrogen ions introduced into the steel sheet form additional fine AlN to reinforce the inhibitor. The present inventors have already filed a patent for a simultaneous decarburization and nitriding treatment method. (Korean Patent Application No. 96-63078)

The content of nitrogen ions introduced into the steel sheet by decarburization and nitriding is suitably 50-500 ppm.

If the content of nitrogen ion is less than 50ppm, there is no meaning of reinforcement of inhibitor due to the small amount of precipitated AlN formed, and if it exceeds 500ppm, N-component which is not reacted with acid-soluble Al increases and is formed just before secondary recrystallization. It inhibits the development of the light coating layer and degrades the surface quality.

When the decarburization and nitriding treatment temperature is 800 ° C. or lower, decarburization is difficult. Secondary recrystallization becomes unstable because a temperature of 950 ° C. or higher coarsens the primary recrystallization structure. In addition, if the dew point is lower than 40 ℃ or the holding time is less than 180 seconds decarburization is not made properly.

On the other hand, if the dew point is 70 ° C or higher, the surface oxide layer of the primary recrystallization plate is formed thick, so that the formation of the forsterite layer formed during high temperature annealing becomes unstable. Nitriding treatment uses all kinds of gases capable of producing nitrogen ions, for example, gas such as ammonia, to form nitrides by fixing nitrogen ions on the surface of steel sheet. In this way, if decarburization and nitriding are performed simultaneously in a single annealing furnace, there are great advantages in terms of productivity and equipment efficiency.

As described above, the first recrystallization plate subjected to decarburization and nitriding is coated with MgO, an annealing separator, and subjected to final finishing annealing. In order to obtain high magnetic flux density characteristics, the finish annealing is performed at a temperature rising rate of 10 to 20 ° C./hr to 1100-1250 ° C., preferably to 1200 ° C., to be cracked for 10 hours or more. At this time, in order to form a good forsterite layer and to remove impurities from the steel sheet, it is preferable to use dry water extraction or a mixed gas of hydrogen and nitrogen.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

The following table is based on the component system containing Si: 3.12%, C: 0.05%, Mn: 0.2%, S: 0.005%, Cr: 0.05%, Ni: 0.05% by weight and the balance of Fe and other unavoidable impurities Slab was prepared by changing the content of acid-soluble Al and N as shown in 1 and adding and dissolving the same. The slab heating temperature was calculated by calculating the theoretical employment temperature of Al and N added as shown in Table 1 below to perform slab heating at a temperature higher and lower than the theoretical solid solution temperature and to hot roll to make a 2.3 mm thick hot rolled sheet. The hot-rolled sheet annealing was heated to or above the theoretical solid solution temperature for each hot-rolled sheet as shown in Table 1 and then heated to a temperature lower than that, then cooled slowly to 950 ℃, and then quenched at 950 ℃ to AlN precipitate. The hot rolled sheet thus annealed is cold rolled to 0.30 mm in thickness after pickling, and then 180 seconds at a temperature of 875 ° C. in a mixed gas atmosphere containing nitrogen ions obtained by decomposition of ammonia at a dew point of 52 ° C. Decarburization and nitriding were carried out simultaneously. The high temperature annealing formed a secondary recrystallization by heating up to 1200 degreeC by the temperature rising rate of 15 degree-C / hr in the water atmosphere containing 50% nitrogen, and cracking for 20 hours.

Magnetic flux density was measured for the grain-oriented electrical steel sheet manufactured as described above, and the results are shown in Table 1 below.

Al (wt%) N (wt%) Theoretical employment temperature (℃) Slab heating temperature (℃) Annealing Temperature (℃) Magnetic flux density (Tesla) Remarks 0.035 0.0030 1230 1200 1200 1.77 Comparative Material 1 0.030 0.0040 1242 1200 1200 1.82 Comparative Material 2 0.025 0.0020 1160 1200 1200 1.91 Invention 1 0.025 0.0020 1160 1200 1100 1.85 Comparative Material 3 0.025 0.0010 1100 1150 1100 1.90 Invention 2 0.025 0.0010 1100 1150 1000 1.86 Comparative Material 4 0.020 0.0010 1083 1100 1100 1.92 Invention 3 0.020 0.0010 1083 1100 1000 1.83 Comparative Material 5 0.015 0.0004 993 1000 1000 1.91 Invention 4 0.010 0.0020 1083 1100 1100 1.90 Invention 5 0.010 0.0020 1083 1000 1000 1.75 Comparative Material 6 0.005 0.0010 981 1000 1000 1.72 Comparative Material7

As shown in Table 1, when the content of the acid-soluble Al exceeds 0.03wt%, it can be seen that the magnetic flux density is inferior because AlN is not completely re-used by slab heating at 1200 ° C.

In addition, even if the content of acid-soluble Al and the content of N component hit the management range and reheat the slab above the theoretical employment temperature, and the complete heating of AlN occurs, the preanneal temperature of the hot-rolled sheet is less than the theoretical employment temperature of AlN. It can be seen that the density is somewhat lower.

And even if the slab is heated above the theoretical employment temperature, when the content of acid-soluble Al added in the steelmaking process is extremely low, such as 0.005 wt%, the finely deposited AlN precipitates are very small and thus excellent magnetic flux density is not obtained. It can be seen.

However, when the content of acid-soluble Al and N was well within the control range and the slab heating and hot-rolled sheet annealing were performed above the theoretical employment temperature of AlN (Inventive Materials 1-5), fine AlN precipitate distribution was observed. It can be seen that it is possible to manufacture an excellent high magnetic flux density unidirectional electrical steel sheet.

As described above, the present invention is to design the optimum amount of Al and N components added to the steelmaking process to enable low-temperature slab heating at the time of slab heating and to fully employ AlN, so that the slab heating operation of general carbon steel and low-temperature slab heating Slab heating at low temperature continuously without mutual interference, minimizing the amount of Al and N components, reducing production cost and forming fine AlN precipitates with strong suppression ability by AlN full employment. Since the steel sheet can be manufactured, it is much easier to manufacture than the prior art, and there is an effect of providing a method of manufacturing a high magnetic flux density unidirectional electrical steel sheet having high productivity.

Claims (3)

In the manufacturing method of high magnetic flux density unidirectional electrical steel sheet, Si: 2.0 to 4.0%, acid soluble Al: 0.0060 to 0.030%, N: 0.0030% or less, C: 0.025 to 0.060%, Mn: 0.32% or less Theoretical employment of AlN formed by reacting Al and N added in the steelmaking process, a slab containing Ni: 0.02 to 0.1%, Cr: 0.02 to 0.1%, S: 0.007% or less, and the balance of Fe and other unavoidable impurities AlN precipitated by reheating between 1000 ~ 1200 ℃ and completely heated, then hot rolled, and the hot rolled sheet is heated above the theoretical working temperature of AlN added in the steelmaking process to produce AlN precipitates produced during hot rolling. After re-stocking, it is slowly cooled to a temperature of 800 to 1000 ° C., and then quenched at a temperature of 800 to 1000 ° C., followed by hot-rolled sheet annealing to induce fine precipitation of uniform AlN as an inhibitor. Including cold rolling method twice to thickness 0.20 ~ 0.35mm Cold rolling, denitration and nitriding at the temperature of 800 to 950 ° C for more than 180 seconds in a mixed gas atmosphere of nitrogen and hydrogen containing nitrogen ions at a dew point of 40 to 70 ° C to form additional fine AlN After reinforcing, and then applying an annealing separator containing MgO as a main component, and then finishing high temperature annealing, characterized in that the high magnetic flux density unidirectional electrical steel sheet manufacturing method. The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet according to claim 1, wherein the temperature at which the hot rolled sheet is quenched at annealing is 900 to 950 ° C. The high magnetic flux density unidirectional electrical steel sheet manufacturing method according to claim 1 or 2, wherein the content of nitrogen ions introduced by decarburization and nitriding is 50-500 ppm.