KR20000042205A - Method of manufacturing high magnetic flux oriented electric steel sheet excellent in magnetism and economy - Google Patents

Abstract

PURPOSE: A method of manufacturing a high magnetic flux oriented electric steel sheet is provided to be economic and excellent in magnetic characteristic by forming an inhibitor through decarbonization and nitrification and simplifying the final annealing condition. CONSTITUTION: An oriented electric steel sheet consists of, by weight %: not more than 0.1% of C, 1.0-4.8% of Si, 0.010-0.05% of Al, 0.05-2.0% of Mn, not more than 100ppm of N, not more than 0.01% of S, Fe and incidental impurities. The oriented electric steel sheet is heated at a temperature of 1100-1250°C and passes through the processes of hot rolling, annealing, nitrifying and final annealing.

Description

Manufacturing method of high magnetic flux density oriented electrical steel with excellent magnetic properties and economical efficiency

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet used as an iron core such as a transformer, and more particularly, a high magnetic flux density grain-oriented electrical steel sheet having excellent magnetic properties is subjected to slab heating at a low temperature and simultaneously decarburization and nitriding. The present invention relates to a method for producing at low cost by shortening the final annealing time.

A grain-oriented electrical steel sheet has a structure of {110} <1> azimuth in the rolling direction to significantly improve the magnetic flux density, thereby reducing the loss when used as a component of an electrical apparatus. It is possible to obtain the aggregate structure of the {110} <1> direction in the rolling direction by a combination of various manufacturing processes. Generally, components, slab heating, hot rolling, hot rolled sheet annealing, primary recrystallization annealing, final annealing, etc. It is important to be very tightly controlled.

Such oriented electrical steel sheet suppresses the growth of the primary recrystallized grains, and exhibits excellent magnetic properties by the secondary recrystallized structure obtained by selectively growing the grains in the {110} <1> orientation among the grains whose growth has been suppressed. Therefore, the growth inhibitory agent (hereinafter referred to as 'inhibitor') of the primary recrystallized grain is particularly important. In the final high temperature annealing process, it is directional that crystal grains (hereinafter, referred to as 'secondary recrystallization') having an aggregate structure of {110} <1> azimuth stably grow among the grains inhibited by inhibitors are directional. It is the core of electric steel sheet manufacturing technology.

Specifically, as an inhibitor, artificially made fine precipitates or segregation elements are used, and in order for the growth of all primary recrystallized grains to be suppressed until just before secondary recrystallization occurs in the final annealing process, these precipitates may have sufficient amount and appropriate amount. It should be evenly distributed in size, and thermally stable up to the high temperature just before secondary recrystallization occurs and not easily decomposed. Secondary recrystallization starts in the final annealing process because these inhibitors grow or decompose as the temperature increases and they lose their ability to inhibit the growth of the primary recrystallized grains. .

MnS, AlN, MnSe, etc. are inhibitors that are widely used industrially because the above mentioned conditions are satisfied.

Among them, a representative known technique for manufacturing an electrical steel sheet using only MnS as an inhibitor is presented in Japanese Patent Publication No. 40-15644. The manufacturing method is a stable secondary recrystallization structure by performing two cold rolling processes including intermediate annealing. To get. However, this method using only MnS as an inhibitor cannot obtain a high magnetic flux density, and there is a problem in that the manufacturing cost is high because it is manufactured by two cold rolling. In the field of electrical steel sheet, the magnetic flux density is required to be high, because the use of a product having a high magnetic flux density as an iron core makes it possible to miniaturize the electric equipment, and for this reason, much efforts have been made to increase the magnetic flux density.

There is a method for producing a grain-oriented electrical steel sheet using MnS and AlN at the same time as an inhibitor, a typical known technique is that disclosed in Japanese Patent Publication No. 40-15644. In this method, the product is cold-rolled once with a high reduction ratio of 80% or higher to obtain a product having a high magnetic flux density. Specifically, the method consists of a series of processes of hot slab heating, hot rolling, hot rolled sheet annealing, cold rolling, decarbonization annealing, and final annealing. In this case, as mentioned above, the final annealing refers to a process of developing a collective structure in the (110) [001] direction by causing secondary recrystallization in a coiled state. In the final annealing process, an annealing separator mainly composed of MgO before annealing is applied to the surface of the steel sheet to prevent the steel sheets from sticking to each other, and the oxide layer formed on the surface of the steel sheet during decarbonization annealing, in any method using an inhibitor. I react to form a glass film to impart insulation to the steel sheet. In this way, the final annealing is performed on the steel sheet having the aggregate structure in the (110) [001] direction by the final annealing to obtain the final product.

Another method is to prepare a grain-oriented electrical steel sheet using MnSe and Sb as inhibitors, and a representative known technique is disclosed in Japanese Patent Publication No. 51-13469. The manufacturing method consists of a series of processes of high temperature slab heating, hot rolling, hot rolled sheet annealing, primary cold rolling, intermediate annealing, secondary cold rolling, decarbonization annealing, and final annealing. Although this method has an advantage of obtaining a high magnetic flux density, it is subjected to two cold rollings and uses expensive Sb or Se as an inhibitor, resulting in a high manufacturing cost and a problem that these elements are toxic.

In addition, these methods suffer from a more serious underlying problem than the disadvantages mentioned above. In other words, MnS (Se) or AlN contained in the slab of oriented electrical steel sheet should be reheated and employed for a long time at high temperature to be used as an inhibitor by making a precipitate having an appropriate size and distribution during hot rolling and then cooling. For this purpose, the slab must be heated to high temperature.

Specifically, the method using MnS as an inhibitor is 1300 ° C., the method using MnS + AlN as an inhibitor is 1350 ° C., and the method using MnSe + Sb as an inhibitor can be obtained by reheating the slab to 1320 ° C. or higher to obtain high magnetic flux density. Known. In fact, in industrial production, in order to obtain a uniform temperature distribution to the inside in consideration of the size of the slab, it is reheated to a temperature of almost 1400 ℃.

If the slab is heated at a high temperature for a long time as described above, there is a problem in that manufacturing cost is high due to a large amount of heat used, and a surface part of the slab flows down to a molten state, causing a high cost of repairing the furnace, and a problem of shortening the life of the furnace. have. In particular, when the columnar tablet, which is a solidifying structure developed on the surface of the slab, grows coarsely by prolonged high temperature heating, there is a problem of causing a deep crack in the width direction of the plate in the subsequent hot rolling process to significantly reduce the error rate. .

Therefore, it is possible to produce a grain-oriented electrical steel sheet by lowering the reheating temperature of the slab, which may bring many beneficial effects in terms of manufacturing cost and error rate. Therefore, many methods have not been studied in recent years that do not use MnS having high solubility temperature as an inhibitor. This is made possible by techniques that produce nitrides at appropriate points in the manufacturing process by a method known as nitriding, rather than relying entirely on inhibitors from the elements contained in the steel components. This method solves the above problems by lowering the reheating temperature of the slab, and the necessary inhibitor is made by nitriding at the final plate thickness, and is commonly referred to as a technique for producing oriented electrical steel sheet by low temperature heating method.

The nitriding treatment includes nitriding a steel sheet in a gas atmosphere having a nitriding capacity after the decarburization process, applying a nitriding compound to the steel sheet by applying an annealing separator, and applying a gas having a nitriding capability to the atmosphere during a high temperature annealing process. Various methods are known, including the inclusion of gas into the steel sheet.

Of these, nitriding steel sheets in a gas atmosphere having nitriding ability after decarburization is most commonly used. Currently, a method of supplying nitrogen into the steel sheet in a separate nitriding process including ammonia gas after decarburization using Al-based nitride is disclosed in Japanese Patent Laid-Open No. Hei 1-230721 and Japanese Patent Laid-Open No. 1-283324. Presented. Meanwhile, a method of simultaneously performing decarbonization and nitride annealing is shown in Korean Patent Application Publication No. 97-43184, and Korean Patent Application No. 97-28305 discloses a method of simultaneously performing decarburization and nitriding using a component system different from the previous patent. Is being presented.

As for the time point of nitriding treatment, a method of first performing decarbonization annealing, growing the crystal grains to a certain extent or more, and nitriding with ammonia gas is proposed in Japanese Patent Application Laid-Open No. 3-2324. A method for simultaneously and nitriding is proposed in Korean Patent Application No. 97-28305 and Korean Patent Application No. 97-37247.

Nitriding by ammonia gas in the above method utilizes the property that ammonia decomposes into hydrogen and nitrogen at about 500 ° C. or higher, and injects nitrogen generated by decomposition into the steel sheet. This is to form a nitride by reacting with nitrogen, Al, Si, etc. already in the steel to form a nitride and used as an inhibitor. At this time, one of the nitrides formed as an inhibitor is an Al-based nitride of AlN and (Al, Si) N.

The above methods provide a method of manufacturing a grain-oriented electrical steel sheet by heating the slab to a low temperature and nitriding the steel sheet with a material or gas capable of nitriding to form new precipitates in the steel sheet.

As mentioned above, the nitriding gas is represented by ammonia, and the action and problems when nitriding it after decarbonization is completed are as follows. Nitriding by decomposition of ammonia gas can be carried out if it is 500 degreeC or more which is the decomposition temperature of ammonia gas. However, at a relatively low temperature directly above 500 ° C., the diffusion rate of nitrogen in the steel sheet is very slow, so that nitriding time is required for a long time. If it is 800 ° C. or more, nitriding becomes easy, but primary recrystallized grains are easy to grow. Uneven distribution leads to instability in secondary recrystallization. Therefore, the suitable temperature range of nitriding can be seen as 500-800 ° C. However, if the nitriding temperature is low and the nitriding treatment time is too long, there is a problem in productivity, and the nitriding temperature is actually performed in the range of 700 to 800 占 폚. A method of nitriding based on such technical idea is disclosed in Korean Patent Publication No. 95-4710.

At such a temperature, the decomposition reaction of ammonia and the diffusion of nitrogen are active. Therefore, very precise control of the nitriding conditions is required in order to add the desired amount of nitrogen in the river. That is, the amount of nitriding is determined by the concentration of ammonia, the nitriding temperature and the nitriding time, and the appropriate amount of nitriding should be determined by the combination of these conditions. In consideration of productivity, since nitriding should be performed in a short time, the concentration of ammonia and nitriding temperature should be high. In this case, nitriding is performed in a short time, and the nitrogen concentration at the surface portion of the steel sheet is mainly increased. Therefore, the variation of each part of the steel sheet becomes very large. Almost no nitriding occurs in the center of the steel sheet, and unevenness in each of the positions is also severe in the surface portion.

In addition, the amount of nitriding is greatly affected by the state of the steel sheet. Typical examples include surface roughness, grain size, and constituents. Surface roughness refers to the roughness of the steel sheet. If the surface of the steel sheet is rough, the contact area with the atmospheric gas increases, which causes variation in the amount of nitriding. If the grain size is small, the grain boundary per unit area increases, and the diffusion of nitrogen through the grain boundary occurs faster and preferentially than the diffusion in the grain, resulting in a deviation of the amount of nitride. As a constituent, a variation in the amount of nitride can be brought about according to the relative amount of the element in the steel sheet to facilitate the nitride. This variation in the amount of nitrogen ultimately leads to defects in the film, which can be solved by a combination of the atmosphere and heat treatment temperature of the final annealing, as suggested in Korean Patent Application No. 97-65356.

In producing a grain-oriented electrical steel sheet using nitride as an inhibitor, nitrogen concentration in the atmosphere becomes very important in the final annealing process, similarly to the conventional high temperature heating method or the low temperature heating method. Therefore, all methods require a box annealing furnace, which is an annealing furnace capable of converting atmospheric gases. This is because the high temperature annealing process, which is the final annealing of the grain-oriented electrical steel sheet, should be separated into a section causing secondary recrystallization and a section removing impurities. In other words, in the temperature rising section where secondary recrystallization occurs, secondary recrystallization occurs only when nitrogen is present in the atmosphere to prevent the nitrides in the steel sheet from being decomposed and lost, and after the second recrystallization, the nitride must be decomposed and discharged into the atmosphere. to be. Therefore, if the nitrogen atmosphere is maintained as a whole, the secondary recrystallization occurs stably, but many impurities remain in the final plate and the magnetic properties deteriorate. On the contrary, if the whole section is maintained in the hydrogen atmosphere, the inhibitor is lost in the elevated temperature range, and thus the secondary recrystallized tissue cannot be obtained. Therefore, there is a problem that a manufacturer who does not have a box annealing furnace for converting the atmosphere cannot manufacture a high magnetic flux density oriented electrical steel sheet in this manner, which is proposed in the final annealing step as proposed in Korean Patent Application No. 98-53688. The solution was made possible by applying the MgO slurry mixed with a small amount of Si ₃ N ₄ to the steel sheet before entering, and performing the final annealing in a 100% hydrogen atmosphere.

The final annealing process, as mentioned above, is a very important step in obtaining a secondary recrystallized structure with a {110} <1> orientation. In particular, according to the method disclosed in Korean Patent Publication No. 95-4710 for nitrification after decarburization, the method includes transforming the precipitate produced after nitriding annealing in the final annealing process. Specifically, precipitates formed after nitriding are Si ₃ N ₄ or (Si, Mn) N precipitates, which are thermally unstable and easily decomposed. Therefore, these precipitates cannot be used because they do not meet the conditions mentioned above. Therefore, it is necessary to replace them with thermally stable precipitates such as AlN and (Al, Si) N to function as inhibitors. When nitride is formed by denitrification and nitride annealing, at least 4 hours must be maintained at a temperature of 700-800 ° C. in a subsequent annealing process to transform into a precipitate that can be used as an inhibitor. This means that the final annealing process is long and must be very tightly controlled, which is also very disadvantageous in terms of manufacturing cost.

The present invention is to solve the above problems, in the manufacture of oriented electrical steel sheet of low temperature heating method using ammonia gas, simultaneously performing decarburization and nitriding to form an inhibitor, simplifying the annealing conditions during final annealing and economical An object of the present invention is to provide a technique for manufacturing a grain-oriented electrical steel sheet having excellent magnetic properties.

According to this invention for achieving the above object, there is provided a method of manufacturing a high magnetic flux density oriented electrical steel sheet excellent in magnetic properties and economics. In this method, the slab of a grain-oriented electrical steel sheet is reheated and hot rolled at a low temperature of 1,250 ° C. or lower, followed by hot roll annealing, then cold rolling to obtain a final sheet thickness, and simultaneously decarburizing and nitriding to form an inhibitor to form a final annealing. It comprises a step, and to simplify the annealing conditions by eliminating the transformation of the inhibitor in the final annealing step.

Here, the electrical steel slab is 0.1 wt% or less C, 1.0-4.8 wt% Si, 0.010-0.05 wt% Al, 0.05-2.0 wt% Mn, 100 ppm or less N, 0.01 wt% S or less,%, balance Fe, and inevitable impurities.

Since the Cu, Cr, and Ni are contained in the electrical steel sheet slab, the AlN-based nitride may be finally used as an inhibitor.

[Description of Preferred Embodiment of the Invention]

Hereinafter, a method of manufacturing a high magnetic flux density oriented electrical steel sheet excellent in magnetic properties and economic efficiency according to a preferred embodiment of the present invention will be described in detail.

First, the components of the high magnetic flux density oriented electrical steel sheet manufactured according to this embodiment will be described. In the following description,%, which is a unit of content, means weight% unless otherwise stated.

Component system of the electrical steel sheet slab according to Example 1 contains C, Si, Al, Mn, N and S, the role and content of each element is as follows.

C is an element added to refine the hot rolled structure, and after C has a function during hot rolling, it becomes an impurity and has to be removed because it adversely affects its magnetic properties. If the content is too high, coarse carbides precipitate and it becomes difficult to remove carbon upon decarbonization. Therefore, in this embodiment, it is set at 0.1% or less.

Si is a component added to increase the electrical resistance of the steel sheet to lower the iron loss. If the content is 4.8% or more, cold rolling is impossible, and when it is 1.0% or less, the addition effect is hardly obtained. Therefore, in this example, the content of Si is set at 1.0-4.8%.

Al is finally a nitride in the form of AlN, (Al, Si) N and (Al, Si, Mn) N and acts as an inhibitor. When the content is 0.010% or less, sufficient effect as an inhibitor cannot be expected. If too high, Al-based nitride grows coarsely, and the inhibitor's ability is reduced. Therefore, in this example, the content of Al is set at 0.010-0.050%.

Since N is reinforced during simultaneous decarburization and nitride annealing, the amount to be taken into the impurity at the time of dissolution is sufficient. No adverse effects from the addition of nitrogen have been found, but hot rolling becomes difficult when it exceeds 100 ppm. Therefore, in this embodiment, the content of N is set to 100 ppm or less.

Mn is an element that increases the electrical resistance and has an effect of lowering iron loss. When the content is too high, Mn causes a decrease in magnetic flux density. Therefore, in this example, the content of Mn is set at 0.05-2.0%.

S is preferably an element which has a high solubility temperature and high segregation during hot rolling so that it is not contained as much as possible. However, S is an unavoidable impurity contained in steelmaking. Even so, the content of S is preferably limited to 0.01% or less.

Example 2 relates to the case where the electrical steel slab is made of a component system additionally containing Cu, Cr, and Ni in addition to the element of Example 1 above. In this example, the Cu content is set at 0.3-0.7%, and the Ni and Cr contents are set at 0.03-0.07%, respectively.

Hereinafter, a method of manufacturing a grain-oriented electrical steel sheet according to this embodiment will be described in detail for each process.

In this embodiment, the heating temperature of the slab before hot rolling is set to 1,100-1,250 占 폚. When the heating temperature is 1,100 ℃ or less, it is difficult to work during hot rolling, and when the heating temperature is 1,250 ℃ or more does not significantly affect the magnetic properties, but the advantage of low temperature heating of the slab, which is one of the characteristics of the present invention is insignificant. That is, it is preferable to keep the heating temperature as low as possible but not less than 1,050 ° C. so as not to be difficult for hot rolling.

The steel sheet slab heated as above is hot rolled by a conventional method. In the current commonly used method, the final thickness of the hot rolled sheet is usually 2.0-2.5 mm. Hot rolled plate is hot rolled and then cold rolled to make final thickness 0.35, 0.30, 0.27, 0.23mm. Although hot-rolled sheet annealing has various methods, this embodiment takes the most general method of heating after cooling to 900 ° C. or higher, and even such general method can obtain sufficiently good characteristics.

The cold rolled plate simultaneously performs decarbonization and nitride annealing in a mixed gas atmosphere of ammonia + hydrogen + nitrogen. In general, nitriding is used as an inhibitor by adding nitrogen to steel to form a nitride, and thus it is possible in any process after cold rolling. That is, nitrogen may be introduced into the steel by nitriding with ammonia gas during decarbonization annealing or a separate nitriding annealing process after decarburization. However, this embodiment is characterized by simultaneously performing decarburization and nitriding. This is because the annealing temperature greatly affects the formation state and type of nitride, which will be described in detail.

Atmospheric conditions of simultaneous decarburization and annealing of this embodiment use three types of gases: ammonia, hydrogen, and nitrogen, but ammonia is dry, hydrogen and nitrogen are mixed, and wet is introduced into the furnace. . The humidity of the mixed gas of hydrogen and nitrogen depends on the annealing temperature and the composition ratio of the gas, and is set to maximize the decarburization capacity. For example, it is determined at about 63 ° C. when using 25% nitrogen + 75% hydrogen, and about 46 ° C. when using 75% nitrogen + 25% N 2. In addition, the annealing temperature of simultaneous decarburization and nitriding is preferably performed at 810-950 ° C. If the annealing temperature is lower than 800 ° C, decarburization takes a long time, and the size of the primary recrystallization is small, so that it is difficult to develop a stable secondary recrystallization structure at the time of final annealing. If the annealing temperature is higher than 950 ° C, it is difficult to control the rate of the nitriding reaction, and the grains grow excessively or unevenly, making it difficult to develop a stable secondary recrystallized structure during final annealing. The annealing time of simultaneous decarburization and nitriding is determined by the annealing temperature and the concentration of ammonia gas introduced. For example, when 1% ammonia and 75% nitrogen + 25% hydrogen at a dew point of 46 ° C. were added to 875 ° C., 30 ppm-40 ppm carbon and 150-170 ppm nitrogen were obtained after 30 seconds. Therefore, the annealing time requires 30 seconds or more.

In general, in the manufacture of grain-oriented electrical steel sheet, an annealing separator based on MgO is applied to the steel sheet, followed by final annealing using a box annealing furnace, and then {110} <1> texture in the rolling direction by secondary recrystallization. By obtaining the oriented electrical steel sheet having excellent magnetic properties is produced. The purpose of the final annealing is to remove impurities that impair insulation and impair magnetic properties by forming a {110} <1> texture in the secondary recrystallization rolling direction and forming a glassy film formed by the reaction of an oxide layer and MgO formed during decarburization. In the general method of final annealing, the secondary recrystallization is well developed by protecting the nitride, which is a particle growth inhibitor, by maintaining a mixed gas of nitrogen and hydrogen in an elevated temperature range before the secondary recrystallization occurs, and after the second recrystallization is completed. It is kept in 100% hydrogen atmosphere for a long time to remove impurities. In this embodiment also follows the above method in principle. However, the manufacturing cost can be reduced by applying an economical annealing method which is one of the characteristics of this invention.

In the existing method of denitrification and nitriding, the transformation of precipitates occurs in the final annealing process. In this method, the temperature of annealing is 700-800 ° C. After nitriding, Si ₃ N ₄ and (Si, Mn) N are formed in the surface layer of the steel sheet, and they are AlN in the range of 700-800 ° C during the final annealing. It is changed to or (Al, Si) N to act as an inhibitor. Therefore, it should be maintained for a long time at 700-800 ℃ in the final annealing step, or the temperature increase rate of this temperature section should be very slow. Therefore, the final annealing time is lengthened, and it is difficult to control the temperature increase rate. However, the precipitates produced during simultaneous decarburization and annealing of this embodiment are AlN, (Al, Si) N, (Al, Si, Mn) N, which do not require transformation of precipitates at high temperature annealing and are directly used as inhibitors. . Therefore, the long-term maintenance and the limit of the temperature increase rate for the temperature rising section of the final annealing become unnecessary, which means that the annealing time can be greatly shortened. The generation of this kind of precipitate is thought to be due to the difference in annealing temperature. That is, at a temperature above 810 ° C., Si ₃ N ₄ or (Si, Mn) N cannot be stably present, and the diffusion of nitrogen also occurs very rapidly, resulting in Al series nitrides throughout the entire steel sheet.

Originally, in the final annealing, the second recrystallization interval is generally slowed down in temperature so that grains having a {110} <1> orientation can be easily grown. That is, when comparing the grain growth driving force of the grains, the grains having the {110} <1> orientation are larger than the grains of other orientations, so that they grow preferentially. However, the driving force difference between the grains is very small, and if the temperature increase rate is fast, all grains are easily grown, and the grains in the {110} <1> azimuth become poor in the rolling direction. Since the secondary recrystallization start temperature is 900 ° C. or higher in the component system of this embodiment, even if the temperature rises faster, the secondary recrystallization does not affect the development of the secondary recrystallization. Therefore, it is possible to increase the temperature increase rate until at least 900 ℃ can be significantly shortened the final annealing time. For example, a rapid heating up to 900 ° C and a temperature range of 900-1200 ° C are gradually heated to allow stable secondary recrystallization and glassy film formation, and then maintained at a high temperature of about 1200 ° C to remove impurities. It is possible. Specific methods thereof will be described later in the Examples.

Hereinafter, the characteristics of this invention will be described metallurgically.

Nitriding is carried out simultaneously with a separate nitriding process or decarburization after decarburization, and nitrogen generated by decomposition of ammonia gas enters the inside of the steel sheet to form nitride. The method of nitriding after decarburization is performed at 700-800 ° C., and the method of simultaneous decarburization and nitriding at 810-950 ° C. in ammonia + hydrogen + nitrogen atmosphere. However, these two methods are based on metallurgical dissimilar ideologies that go beyond simply nitriding or differences in annealing temperature conditions. This will be described in detail.

In the method of forming a precipitate through a separate nitriding process after decarburization, the annealing temperature is performed at 800 ° C. or lower, and nitrides such as Si ₃ N ₄ , (Si, Mn) N are formed. Such precipitates are thermodynamically low in energy and are easily formed, but are very thermally unstable. Therefore, these precipitates are easily decomposed when the temperature is high, and can not be used as an inhibitor of the grain-oriented electrical steel sheet. In addition, since the annealing temperature is low, the diffusion of nitrogen is not very active, and nitride is concentrated in the surface portion of the steel sheet. Therefore, in the subsequent annealing process, they are decomposed again to be reprecipitated with other elements present in the steel sheet. The precipitate formed here is used as an inhibitor as a stable nitride such as AlN or (Al, Si) N.

Simultaneous decarburization and nitriding to form precipitates require annealing temperatures of 810 ° C or higher. In consideration of decarburization, the annealing temperature is too long at temperatures below 800 ° C., so that it is not industrially useful. Nitrogen can be spread evenly in the entire thickness direction of the steel sheet by enabling the diffusion of nitrogen. Is set in consideration of the temperature. In this temperature range, unstable nitrides such as Si ₃ N ₄ or (Si, Mn) cannot be formed, and thermally very stable precipitates such as AlN, (Al, Si) N, (Al, Si, Mn) N are formed throughout the steel sheet. It is formed evenly distributed. Therefore, it can be used as an inhibitor without the need for reprecipitation in the subsequent final annealing process. The difference in phenomena as described above gives a big difference in the final annealing process, but the simultaneous decarburization and nitriding method can bring about a different cost in terms of manufacturing cost since the final annealing heat treatment pattern can be different.

In the following, an experiment performed on a steel sheet manufactured by the method according to the present invention will be described.

Experiment 1

A grain-oriented electrical steel slab containing 3.11% Si, 0.054% C, 0.10% Mn, 0.029% Al, 0.0030% S, 0.0064% N and the balance Fe and unavoidable impurities A hot rolled sheet having a thickness of 2.3 mm was prepared by heating the wafer at 1,200 ° C. for 210 minutes and hot rolling. The hot rolled sheet was heated to 1050 ° C., held at 900 ° C. for 90 seconds, quenched in water, pickled, and cold rolled to a thickness of 0.30 mm, respectively.

(A) The cold rolled plate was kept at 150 ° C by simultaneously adding 75% H ₂ and 25% N ₂ mixed atmosphere and 1% dry ammonia in a furnace maintained at 875 ° C for 150 seconds. Decarburization, nitriding.

(B) Cold rolled plates were decarbonized in a mixed atmosphere of 75% H ₂ and 25% N ₂ with a dew point temperature of 64 ° C. in a furnace kept at 850 ° C., and then 30 seconds in a stream held at 770 ° C. Ammonia was added for nitriding.

Nitrogen content of the steel plate nitrided by each method (A) and (B) was the same in the range between 190-210 ppm.

MgO, which is an annealing separator, was applied to the steel sheet, and the coils were wound up to form a final annealing. Final annealing was performed using a box annealing furnace at 25% N2 + 75% H2 atmosphere up to 1200 ° C. At this time, the temperature increase rate up to 900 ° C. was increased to 10, 20, 30, 50, 100 ° C./hr., And 900-1200 ° C. was set to 20 ° C./hr.

The magnetic flux density and iron loss characteristics measured for each example in this experiment are shown in Table 1.

Nitriding Method Heating rate up to 900 ℃ at final annealing (℃ / hr) Magnetic flux density (B10. Telsa) division A 10 1.927 Inventive Steel 1 20 1.918 Inventive Steel 2 30 1.920 Invention Steel 3 50 1.912 Inventive Steel 4 100 1.899 Inventive Steel 5 B 10 1.924 Comparative Steel 1 20 1.921 Comparative Steel 2 30 1.853 Comparative Steel 3 50 1.811 Comparative Steel 4 100 1.707 Comparative Steel 5

As shown in Table 1, in the case of nitriding by the B method, only the final annealing at 10, 20 ° C / hr was shown to have excellent magnetic properties. However, in the case of nitriding by the A method, all the experiments were performed. Examples show excellent characteristics. Here, the limit of the temperature increase rate test up to 100 ° C / hr takes into account the practically applicable range when processing a large coil in a large industrial furnace.

Experiment 2

3.20% Si, 0.019% C, 0.24% Mn, 0.018% Al, 0.003% S, 0.0055% N, 0.015% P, 0.5% Cu, 0.05% The oriented electrical steel slab containing Cr, 0.05% Ni, balance Fe, and unavoidable impurities was heated at 1,250 ° C. for 210 minutes, and then hot rolled to prepare a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was heated to 1050 ° C., held at 900 ° C. for 90 seconds, quenched with water, pickled, and cold rolled to a thickness of 0.27 mm, respectively. The cold rolled plate is kept at 150 ° C for 25 seconds with a mixed atmosphere of 25% H ₂ and 75% N ₂ with a dew point temperature of 46 ° C and 1% dry ammonia simultaneously for simultaneous decarburization and nitriding. Treated.

MgO, which is an annealing separator, was applied to the steel sheet, and the coils were wound up to form a final annealing. Final annealing was performed using a box annealing furnace at 25% N2 + 75% H2 atmosphere up to 1200 ° C. At this time, the temperature increase rate up to 900 ° C. was increased to 10, 20, 30, 50, 100 ° C./hr., And 900-1200 ° C. was set to 20 ° C./hr.

The magnetic flux density and iron loss characteristics measured for each example in this experiment are shown in Table 2.

division Temperature raising rate up to 900 ℃ when final annealing (℃ / hr) Magnetic flux density (B10, Tesla) Inventive Steel 6 10 1.933 Inventive Steel 7 20 1,928 Inventive Steel 8 30 1.925 Inventive Steel 9 50 1.922 Inventive Steel 10 100 1.901

As shown in Table 2, it can be seen that even in the component system exemplified above, the magnetic properties are excellent regardless of the temperature increase rate at the time of final annealing.

Experiment 3

A grain-oriented electrical steel slab containing 3.11% Si, 0.054% C, 0.10% Mn, 0.029% Al, 0.0030% S, 0.0064% N and the balance Fe and unavoidable impurities After heating 210 minutes at 1,200 ° C., hot rolling was performed to prepare a hot rolled plate having a thickness of 2.3 mm. The hot rolled sheet was heated to 1050 ° C., held at 900 ° C. for 90 seconds, quenched in water, pickled, and cold rolled to a thickness of 0.30 mm, respectively. The cold rolled plate is kept at a constant temperature by simultaneously adding 75% H ₂ and 25% N ₂ mixed atmosphere and 1% dry ammonia into the furnace maintained at a constant temperature to maintain a certain time. Nitriding treatment gave a final nitrogen of 170-220 ppm. At this time, the simultaneous decarburization, nitriding temperature was changed to 750, 770, 800, 810, 870, 950 ℃, the time was changed to 120-240 seconds to control the amount of nitriding.

MgO, which is an annealing separator, was applied to the steel sheet, and the coils were wound up to form a final annealing. Final annealing was performed using a box annealing furnace at 25% N2 + 75% H2 atmosphere up to 1200 ° C. At this time, the temperature increase rate up to 900 ° C. was 50 ° C./hr., And the 900-1200 ° C. range was 20 ° C./hr.

The magnetic flux density and iron loss characteristics measured for each example in this experiment are shown in Table 3.

division Simultaneous Decarburization, Nitride Annealing Temperature (℃) Simultaneous Decarburization, Nitride Annealing Time Simultaneous Decarburization, Nitride Annealing Time Magnetic flux density (B10, Tesla) Comparative Steel 6 750 240 175 1.725 Comparative Steel 7 770 210 200 1.824 Comparative Steel 8 800 190 204 1.863 Inventive Steel 11 810 170 212 1.904 Inventive Steel 12 870 140 217 1.928 Inventive Steel 13 950 120 220 1.921

As shown in Table 3, even when the annealing temperature and time are adjusted to the appropriate amount of nitriding by controlling the annealing temperature and time during simultaneous decarburization and nitriding, excellent magnetic properties cannot be obtained when the annealing temperature is lower than 800 ° C. If the annealing temperature is higher than 950 ° C, there is a difficulty in controlling the grain size and there is a problem in industrial application. Therefore, the appropriate temperature range for simultaneous decarburization and annealing is limited to 810-950 ° C.

While the invention has been described above based on the preferred embodiments thereof, these embodiments are intended to illustrate rather than limit the invention. It will be apparent to those skilled in the art that various changes, modifications, or adjustments to the above embodiments can be made without departing from the spirit of the invention. Therefore, the protection scope of the present invention will be limited only by the appended claims, and should be construed as including all such changes, modifications or adjustments.

As can be seen from the above, according to the present invention, when manufacturing a grain-oriented electrical steel sheet by low temperature heating method, both decarburization and nitriding are performed simultaneously, and the temperature increase rate of the first half of the final annealing can be increased to obtain an excellent high magnetic flux density grain-oriented electrical steel sheet. The final annealing time can be greatly reduced.

Claims (2)

In the method of manufacturing a grain-oriented electrical steel sheet comprising the step of reheating the hot-rolled slab of a grain-oriented electrical steel sheet at a temperature of 1,250 ℃ or less, hot rolling, then performing annealing, then cold rolling to obtain a final sheet thickness The steel sheet slab may contain 0.1 wt% or less of C, 1.0-4.8 wt% of Si, 0.010-0.05 wt% of Al, 0.05-2.0 wt% of Mn, 100 ppm or less of N, and 0.01 wt% or less. Consisting of S and the remaining Fe and inevitable impurities, The final annealing step is a method for producing a grain-oriented electrical steel sheet, characterized in that the nitriding treatment is carried out simultaneously with the decarburization in the temperature range of 810-950 ℃. The method of claim 1, The steel sheet slab is 0.3-0.7% by weight of Cu, 0.03-0.07% by weight of Ni and 0.03-0.07% by weight of Cr production method for producing a grain-oriented electrical steel sheet further comprises.