KR100345696B1 - A method for manufacturing grain oriented electrical steel sheets by heating its slab at low tempreatures - Google Patents

Abstract

PURPOSE: A method for manufacturing grain oriented electrical steel sheets is provided, which is characterized in that a BN precipitate serving as inhibitor for recrystallization grain growth is formed after a steel sheet is rolled to target thickness. CONSTITUTION: In a method for manufacturing grain oriented electrical steel sheets by heat treatment of slab at low temperature where silicon steel slab is subject to reheating at 1050 to 1250 deg.C, hot rolling, annealing, cold rolling, decarburization annealing, applying an annealing separator and final hot annealing, the method is characterized in that the silicon slab comprises Si 1.0 to 4.8 wt.%, Al 0.005 to 0.019 wt.%, C 0.02 to 0.045 wt.%, Mn 0.05 to 0.2 wt.%, B 0.001 to 0.012 wt.%, 0.008 wt.% or less of N, 0.007 wt.% or less of S, a balance of Fe and incidental impurities; an inhibitor mainly comprised of BN precipitates is formed by conducting simultaneously decarburization and nitriding at an annealing temperature of 850 to 950 deg.C in a mixed gas atmosphere including ammonia, hydrogen and nitrogen during final decarburization annealing, wherein ammonia concentration of the mixed gas is 0.1 to 1.0 vol.%; the hot annealing is conducted at 900 to 1150 deg.C followed by air cooling; the reduction ratio of cold rolling is 84 to 90 %; and the final hot annealing is conducted in such a manner than the steel sheet is soaked for 1 to 30 hrs after it is heated to higher than 1150 deg.C at a temperature elevation rate of 10 to 40 deg.C/hr in a mixed gas atmosphere of dry hydrogen and nitrogen or in a dry hydrogen atmosphere.

Description

A method for manufacturing grain oriented electrical steel sheets by heating its slab at low tempreatures}

The present invention relates to a method for manufacturing a unidirectional electrical steel sheet used as an iron core of an electric device such as a transformer, and more specifically, to low temperature reheating by forming a primary recrystallized grain growth inhibitor after rolling to a final product thickness. This is possible and relates to the method of manufacturing the electrical steel sheet excellent in the orientation.

The unidirectional electrical steel sheet has an aggregate structure in the direction of (110) [001] in the rolling direction, and a new manufacturing method has been developed by many researchers since NP Goss first presented the manufacturing method in US Patent No. 1,965,559. The invention and improvement of properties have been made. Such unidirectional electrical steel sheet suppresses the growth of primary recrystallized grains and exhibits excellent magnetic properties by the secondary recrystallized structure obtained by selectively growing grains in the (110) [001] orientation among the grains whose growth is suppressed. Since the main technology is to inhibit the growth of the primary grains (hereinafter referred to as 'inhibitor') is important. In addition, it is also important to configure each process so that a stable (110) [001] azimuth aggregate can be obtained among grains whose growth is inhibited by an inhibitor.

Specifically, the fine precipitates or segregation elements are mainly used as inhibitors, and these precipitates should be evenly distributed in a sufficient amount and appropriate size so that the growth of the primary grains can be suppressed until just before the secondary recrystallization occurs. It is thermally stable up to the high temperature just before it occurs and should not be easily decomposed. MnS, MnS + AlN, MnS (Se) + Sb are widely known as inhibitors currently used industrially because these conditions are satisfied.

Among them, a representative known technique for manufacturing an electrical steel sheet using only MnS as an inhibitor is shown in Japanese Patent Publication No. 40-15644, and the manufacturing method is stable by performing two cold rollings including intermediate annealing. Recrystallization is being obtained. 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. As such, high magnetic flux density characteristics are required in the field of electrical steel sheet. This is because the use of a product having a high magnetic flux density with the iron core can reduce the size of the electric device, which makes it possible to miniaturize the device. For this reason, a lot of efforts have been made to increase the magnetic flux density.

Representative known technique for producing unidirectional electrical steel sheet using another inhibitor, MnS + AlN, is disclosed in Japanese Patent Publication No. 40-15644. In this method, cold rolling is performed once at a high reduction ratio of 80% or more. To obtain a high magnetic flux density product. However, when this method is applied to industrial production, the manufacturing conditions are very strict and each process condition must be strictly controlled. Specifically, this method consists of a series of processes of hot slab heating, hot rolling, precipitation annealing, cold rolling, decarbonization annealing, and high temperature annealing.

In this case, the high temperature annealing refers to a process of developing a recrystallized structure in the (110) [001] direction by causing secondary recrystallization on the cold rolled coil. In this high temperature annealing process, the annealing separator is applied to the steel sheets before the high temperature annealing in any method using an inhibitor to prevent sticking between the steel sheets, and the oxide layer formed on the surface of the steel sheet during the decarbonization annealing reaction reacts with the annealing separator. In order to form a glassy film, insulation is provided to the steel sheet. In this way, the hot-rolled annealing is applied to the steel sheet having an aggregate structure in the (110) [001] direction to form a final product.

As a representative known technique for manufacturing unidirectional electrical steel sheet using another inhibitor, MnS (Se) + Sb, it is presented in Japanese Patent Publication No. 51-13469. The manufacturing method is hot slab heating and hot rolling. , Precipitation annealing, primary cold rolling, intermediate annealing, secondary cold rolling, decarbonization annealing, high temperature annealing. While this method has the advantage of obtaining a high magnetic flux density, two cold rolling is performed and expensive Sb or Se is used as an inhibitor, resulting in an increase in manufacturing cost and toxicity.

The above methods suffer from a fundamental problem that is more serious than the above mentioned disadvantages. In other words, MnS or AlN contained in the small steel component is heated and heated at a high temperature for a long time to be hot rolled, and then cooled to form a precipitate having an appropriate size and distribution, and used as an inhibitor. It must be reheated.

Specifically, the method using MnS as an inhibitor is 1300 ° C., the method using MnS or AlN as an inhibitor is 1350 ° C., and the method using MnS (Se) + Sb as an inhibitor is more than 1320 ° C. to obtain high magnetic flux density only by reheating the slab. It is known that it can. In fact, in industrial production, it is necessary to reheat to a temperature of almost 1400 ° C. in order to obtain a uniform temperature distribution to the inside in consideration of the size of the slab and the like.

When the slab is heated at a high temperature as described above, there is a problem that the manufacturing cost is high due to the large amount of heat used, and the surface part of the slab flows down to the molten state, which causes a high maintenance cost of the furnace and shortens 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, there are various problems such as causing a deep crack in the width direction of the plate in the subsequent hot rolling process to significantly reduce the error rate.

In order to solve such a problem, when the slab reheating temperature is lowered to manufacture a unidirectional electrical steel sheet, it 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 inhibitors such as MnS having high solidus temperature. This is made possible by techniques that produce precipitates at appropriate places in the manufacturing process, rather than relying entirely on inhibitors from the elements contained in the steel components. As such a method, the nitriding treatment method disclosed in Japanese Patent Application Laid-Open No. Hei 1-230721 and Japanese Patent Application Laid-open No. Hei 1-283324 is known.

In the nitriding treatment method, an annealing separator containing a nitriding compound is applied to a steel sheet, a nitriding gas is applied to the steel sheet into an atmosphere gas during the elevated temperature of the high temperature annealing process, and the nitriding ability after the cracking treatment in the decarburization process is improved. There is a case of nitriding the steel sheet in a gas atmosphere.

In addition, a representative method for the nitriding process is described in Japanese Patent Publication No. 2-228425, which is a method of nitriding a precipitate by hot-rolled plate or nitriding process before final cold rolling. It is made. Alternatively, as disclosed in Japanese Patent Application Laid-Open No. 2-294428, there is a method of simultaneously performing nitriding and decarburization during decarbonization annealing performed after cold rolling is completed, and this method has a problem in that secondary recrystallization becomes unstable. This method has been recently proposed in Japanese Patent Application Laid-Open No. 3-2324, whereby decarbonization annealing is performed first, crystal grains grow to a certain extent or more, and nitriding with ammonia gas.

Nitriding with the ammonia gas above uses a method of injecting nitrogen, which is generated when ammonia is decomposed at about 500 ° C. or higher, into the steel sheet. This is to form a nitride by reacting with nitrogen, Al, Si, etc., which are already present in the steel, which enters the steel sheet, and uses it as an inhibitor. At this time, among the nitrides formed are AlN and (Al, Si) N, which are mainly Al-based nitrides.

All the above methods for reheating the slab at low temperature provide a method of manufacturing unidirectional electrical steel sheet by nitriding the steel sheet with a material or gas having nitriding ability to form precipitates in the steel sheet.

However, all of these methods involve adding nitrogen after removing carbon from a steel sheet containing about 0.050% carbon by a decarbonization annealing process, and thus it is inevitable to add a new process to an existing manufacturing process. In particular, the method of nitriding with gas can be achieved by introducing new equipment or by drastic improvement of existing equipment. In addition, the method of adding a nitriding compound to the annealing separator has a problem of causing a large amount of defects in the forsterite layer formed on the surface of the steel sheet.

In addition, since the amount of S or N contained in the steel is relatively high, much unintentional MnS or AlN is produced after hot rolling. This decreases the primary recrystallization grain size after decarbonization annealing, so a very strong inhibitor is required to obtain the secondary recrystallized structure smoothly. That is, the secondary recrystallized structure can be obtained only if the small and fine precipitates can be formed uniformly. This should be strictly controlled in a very small range after the decarbonization annealing, and the amount of nitriding after nitriding should also be strictly controlled, which makes industrial manufacture difficult.

In order for such a nitriding treatment method to be industrially applied, the following two problems must be decided.

First, there is a need for process improvement that does not significantly change existing equipment. This is because it is necessary to determine whether there is economic value to apply this method even with introduction of new technology along with the problem of manufacturing cost.

Secondly, stable unidirectional electrical steel sheet should be available even if the process is controlled in a relatively wide range. This has a big impact on the error rate of the product and is ultimately related to manufacturing costs.

The present inventors have overcome the drawbacks of the prior art and at the same time study a nitriding method that satisfies the above-mentioned nitriding requirements, reducing the amount of C and making the final thickness of the silicon steel slab containing an appropriate amount of B. By nitridating to form BN precipitates, it was recognized that low temperature reheating was possible and a unidirectional electrical steel sheet having a high magnetic flux density could be produced, and thus the present invention was proposed.

The present invention is basically to provide a low-temperature reheating, and does not change the existing equipment as well as to provide a method for producing a unidirectional electrical steel sheet excellent in magnetic flux density even if the nitriding treatment in a relatively wide range, an object thereof.

1 is a photograph showing the precipitate after the simultaneous decarbonation annealing of the present invention steel

Figure 2 is an electron diffraction photo of BN precipitates by transmission electron microscope of the present invention steel

3 is an analysis diagram of electron diffraction photographs showing the crystal structure of BN;

The present invention for achieving the above object is to reheat the silicon steel slab, and then hot-rolled, hot-rolled sheet annealing, and then cold rolled once to make the final plate thickness, then decarbonized annealing, after applying the annealing separator finish high temperature In the manufacturing method of high magnetic flux density unidirectional electrical steel sheet including annealing step,

The silicon steel slab is in weight%, Si: 1.0-4.8%, Al: 0.005-0.019%, C: 0.02-0.045%, Mn: 0.05-0.2%, B: 0.001-0.012%, N: 0.008% or less, S: 0.007% or less, consisting of residual Fe and other unavoidable impurities, comprising nitriding the final plate to form BN precipitates.

Hereinafter, the steel component system of the present invention will be described in detail.

Si is an element that increases the resistivity of steel and significantly improves iron loss characteristics. The amount of addition is determined by various limiting factors, and in practice, it is generally contained about 2.95-3.5%. The upper limit is determined by the ability to stably cold-roll industrially, but it is known that strength rolling containing about 4.8% of Si is possible in a special and strictly controlled rolling method. Therefore, in the present invention, the addition amount of Si is limited to 1.0-4.8%, but if the content of Si is less than 1.0%, the addition effect is insignificant and does not have much significance.

Al is a nitride of the AlN and (Al, Si) N forms, and unlike the conventional component system which acts as an inhibitor, in the present invention, Al has little meaning from the viewpoint of the inhibitor. However, Al, like Si, is an element that increases resistivity, so it is advantageous to add up to 0.019%, which does not harm hot rolling properties. If more than this, hot rolling workability is reduced. In the conventional manufacturing process, even if this is taken, since it should be used as an inhibitor, up to 0.05% is added, but it is not necessary in the present invention. The lower limit of the amount of Al added may be 0.005% or more because the effect of addition is insignificant at less than 0.005% from the viewpoint of increasing the resistivity. Therefore, Al may be added in 0.005-0.019%.

C is an element added to refine the hot rolled structure. It must be removed because it becomes an impurity after the hot rolling function and becomes an impurity. That is, when 3% of Si is contained, when about 0.018% of C is contained, ferrite-austenite transformation may occur during hot rolling, thereby minimizing the hot rolling structure. Therefore, when the amount of Si is increased, a slightly higher amount of C is required, and therefore, in the present invention, 0.020% or more is required. In addition, in the present invention, since decarburization and nitriding are performed at the same time, the amount of C is advantageously low, so it is preferable to contain it at 0.045% or less. If it contains more than this, it is advantageous in terms of miniaturization of the hot rolled structure, but coarse carbides are precipitated, which makes it difficult to remove carbon during simultaneous decarbonation annealing. As a result, C is added in the range of 0.020-0.045% in the steelmaking stage. If the added amount remains in the final product, it causes self-aging and deteriorates the characteristics of electrical equipment such as transformers. It needs to be strictly controlled. For this purpose, the decarburization process is necessarily included in the manufacture of the unidirectional electrical steel sheet.

Mn is a component having an effect of lowering the iron loss by increasing the electrical resistance. If the content is too large, the magnetic flux density is lowered, so the Mn content is preferably 0.05-0.2%.

B is an element characteristic of the composition of the present invention, which is maintained in solid solution in steel, cold rolled to final product thickness, and decarbonized and nitrified at the same time in combination with nitrogen in the steel by decomposition of ammonia gas as an inhibitor. Is used. At this time, if the amount is less than 0.001%, the amount of the inhibitor is insufficient to obtain a stable secondary recrystallized tissue, if it exceeds 0.012%, it is confirmed that the magnetic flux density decreases. Therefore, the content of B is preferably selected to 0.001-0.012%.

Since N is reinforced by nitriding, an amount sufficient to enter impurities during dissolution is sufficient. However, even if nitrogen is added deliberately, there is no other effect, but if it exceeds 0.008%, it reacts with Al contained in the steel to form coarse AlN precipitates, thereby decreasing the primary recrystallization grain size. In order to obtain the recrystallization structure, the temperature of co-denitrification annealing must be raised. In other words, when the annealing temperature is increased, the non-uniformity of the primary recrystallized grains, which ultimately adversely affects the magnetic properties, it is preferable to select N below 0.008%.

It is preferable that S is not contained as much as possible for hot workability because of high segregation, but there is an additional cost in the process to make extremely low S through the de-S process during steelmaking. Therefore, it may have the amount of S contained in an impurity. However, when the content of S exceeds 0.007%, it reacts with Mn contained in the steel to form MnS, thereby reducing the primary recrystallized grains. Therefore, S is preferably 0.007% or less.

Hereinafter, the process of manufacturing a unidirectional electrical steel sheet using the silicon steel slab comprised as mentioned above is demonstrated.

The heating temperature of the electrical steel slab is preferably selected to 1.050-1250 ℃, the reason is that when the heating temperature is less than 1.050 ℃ difficult to work during hot rolling, and above 1.250 ℃ does not significantly affect the magnetic properties. This is because the benefits from low-temperature heating of slabs are greatly reduced.

In the conventional manufacturing method of unidirectional electrical steel sheet using AlN or MnS as an inhibitor, high temperature slab reheating was inevitable because AlN or MnS was employed by solid slab high temperature heating and then re-precipitated during hot rolling to control the size and distribution. However, the present invention adopts a method of forming an inhibitor after cold rolling to the final product thickness, so that no hot slab heating is required to control the precipitate. Therefore, the heating temperature of the slab is performed in the range of 1.050-1.250 ° C. in consideration of hot rolling workability as described above.

After reheating as above, hot rolling and then annealing, the annealing temperature at this time is preferably in the range of 900-1150 ℃. In the conventional method, in order to obtain a stable distribution of precipitates by causing partial employment and reprecipitation of precipitates during hot-rolled sheet annealing, a method of rapidly cooling after reaching about 900 ° C is used. However, the present invention does not have to consider the viewpoint of precipitates, so that it can be annealed at a temperature of 900-1150 ° C for the uniformity of the hot rolled structure and the improvement of pickling properties. The cooling method at this time does not require strict control, and more preferably air cooling has the effect of slightly improving the magnetic characteristics.

The hot rolled sheet subjected to the annealing as described above is pickled and cold rolled. At this time, the cold rolled is rolled to the final thickness by one rolling without intermediate annealing. And even if the rolling rate at this time changes in the range of 84-90%, a high magnetic flux density can be obtained. This is explained from the result of obtaining a high magnetic flux density even when the final thickness is changed to 0.23 mm, 0.27 mm, 0.30 mm, 0.35 mm using a 2.3 mm hot rolled sheet as shown in the examples.

As described above, the cold-rolled plate to the final product thickness is subjected to nitriding treatment. In this case, any nitriding treatment can be carried out as long as nitrogen is added to the steel sheet. Specifically, for example, a method of nitriding after decarbonization of a cold rolled sheet of final thickness, a method of simultaneously performing decarbonization and nitriding treatment, and a method of applying an annealing separator containing a compound having a nitriding capability to a steel sheet before high temperature annealing Etc. More preferably, a method of simultaneously performing decarbonization and nitriding treatment capable of nitriding without adding a separate process or equipment for nitriding treatment (hereinafter, referred to as 'simultaneous decarbonation annealing') is preferable.

Simultaneous decarbonation annealing of the present invention is more effectively achieved by a method of raising the annealing temperature and lowering the concentration of ammonia than the conventional method. In this way, the carbon concentration in the steel is lowered and the nitrogen concentration is increased.

Specifically, the simultaneous decarbonation annealing temperature of the present invention is preferably set to 850-950 ° C as shown in the examples of the present invention. At this time, the atmosphere in the annealing furnace may be nitrogen gas, but more preferably, a small amount of dry ammonia gas is introduced into a mixed atmosphere of hydrogen and nitrogen. In this process, nitrogen generated by the decomposition of ammonia gas enters the inside of the steel sheet. At this time, the amount of nitrogen that enters the inside of the steel sheet is affected by the annealing temperature, the annealing time, the ammonia fraction in the atmosphere and controlled by the appropriate amount of nitrogen according to the steel composition, the amount of ammonia is 0.1-1.0% It is preferable to adjust the range, and when the amount of ammonia is adjusted in this way, the amount of strong nitrogen is easily changed to 100-1000 ppm.

In the decarburization of the present invention, the carbon of the steel sheet is removed under the annealing conditions, wherein the decarburization capacity is determined by the hydrogen partial pressure and the vapor pressure, and the conditions for the decarburization are finally lowered to 30 ppm or less, more preferably 20 ppm or less. You just need to lose.

On the other hand, the particle size of the primary recrystallized grain is determined by the size and distribution of the precipitate formed after nitriding. The grain size suitable for the suppressing power of the precipitate produced in the component system of the present invention is about 20-30 µm.

After simultaneous decarbonation annealing as described above, an annealing separator containing MgO as a main component is applied to the surface of the steel sheet to finish high temperature annealing in coil form. Specifically, the high temperature annealing consists of a temperature rising section for developing a secondary recrystallization structure and a pure annealing section for removing impurities. At this time, the temperature increase rate is important because the rearrangement of the precipitate occurs, empirically, if the temperature increase rate is too fast, the secondary recrystallization becomes unstable, while if the temperature increase rate is too slow, the annealing time is long and uneconomical. Therefore, the preferable temperature increase rate is 10-40 degreeC / hr. After the temperature is raised to a temperature of 1150 ° C. or more at the same temperature rate, the crack may be cracked for 1-30 hours. At this time, it is preferable to maintain the atmosphere containing nitrogen in order to prevent the loss of the nitride used as an inhibitor during the temperature rising process, and purifying annealing is a process of removing harmful elements in the steel by maintaining in a reducing atmosphere and thus 100% hydrogen. Preference is given to using.

Hereinafter, the metallurgical characteristics of the present invention will be described.

Referring to the formation of precipitates that are used as inhibitors in the simultaneous decarbonation process is as follows.

Since the component system of the present invention contains Si, Mn, B, and Al, nitrides of these alone or in combination may be formed after nitriding. Compared to the above mentioned thermodynamic reaction priorities, the nitrides of Al are formed first, followed by the nitrides of B. This is primarily because AlN is thermodynamically easy to react when nitride is formed at high temperatures during casting. This coarse AlN remains intact after hot rolling.

Since the steel component system of the present invention is lower than 0.008% of the amount of N, almost no other nitride appears to be formed. Other precipitates observed in the hot rolled sheet state are occasionally observed coarse MnS combined with S contained as impurities. At this time, the formed AlN may be subjected to a partial solid solution by performing hot-rolled sheet annealing at a relatively high temperature of 1.120 ° C. or higher, and rapidly cooling after re-precipitation to make a relatively small AlN. In this case, AlN may be used as an inhibitor. However, the present invention can produce a unidirectional electrical steel sheet having excellent magnetic properties by securing a sufficient inhibitor even without this process.

That is, in the present invention, BN formed by adding nitrogen by simultaneous decarbonation annealing to react with B in steel is used as an inhibitor. At this time, if the Al content of the steel is high, even if Al which does not react with AlN remains, BN preferentially precipitates.

This can be clearly seen by thermodynamic considerations.

Specifically, when examining the thermodynamic data of BN and AlN (Metallurgical Thermochemistry, 5th edition, kubaschewski, 1979), the enthalpy of formation of BN is higher than that of AlN, and the free energy value considering entropy is also low. This means that the production of AlN is thermodynamically advantageous over the production of BN. Nevertheless, the formation of BN occurs in practice first, which is explained as follows.

AlN is produced first when pure B and pure Al react with nitrogen to form nitride. However, when B and Al are dissolved together in Fe, nitrogen is added to form nitride to change the aspect. That is, when B and Al present in α (ferrite) -Fe react with N in α-Fe, BN is preferentially formed.

This can be explained by the thermodynamic kinetics, which is due to the difference in diffusion coefficients. This phenomenon is described by Yamanaki et al. Iron. Steel. Inst. There are a number of similar studies, including those reported in Jpn. (1978. 1.8 p404-411).

According to a report by Yamanaka et al., The diffusion rate of B in Fe is about the same as the diffusion rate of N, so that BN is generated even when quenched or wound at a very low temperature. In comparison, the diffusion rate of Al in Fe is very slow compared to B. Therefore, the reaction of nitrogen with any element dissolved in Fe is determined by the diffusion rate of the solid solution.

Also in the present invention it can be seen that a number of BN was formed in Figure 1-3 showing the results of the observation after the simultaneous decarbonation. That is, Figure 1 shows the precipitate after co-denitrification, Figure 2 shows the electron diffraction photo of the precipitate, as shown in Figure 1 BN size of several hundred Å size and its shape forms a square with different sides length There were many things close to the triangle. In addition, FIG. 3 is an analysis of electron diffraction photographs showing the crystal structure of BN. The observed BN is a cubic crystal structure, which is consistent with JCPDS25-1033, which is known to have a distance of (220) plane of 1.2875 mm 3. Other precipitates include coarse MnS existing from the hot rolled sheet, (Si, Mn) N presumed to be formed after nitriding, and AlN alone refined after hot rolled sheet annealing. Therefore, the main precipitate of this component system is BN, and it is assumed that this nitride acts as an inhibitor.

Until now, it has been speculated that the addition of B may have an adjuvant role of inhibitors such as AlN and MnS, but the use of BN as an inhibitor has not been reported. As a result, it has been found that the production of unidirectional electrical steel sheets with high magnetic flux density is possible using BN, which is not known until now, as a main inhibitor.

Hereinafter, the present invention will be described in detail through examples.

Example 1

By weight, Si: 3.15%, Al: 0.013%, C: 0.031%, Mn: 0.09%, N: 0.0065%, S: 0.006% and B as shown in Table 1 below, with the balance Fe and other unavoidable impurities The resulting silicon steel slab was heated to 1200 ° C. and hot rolled to make a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was annealed at 1120 ° C. for 2 minutes, quenched in water at 100 ° C., pickled, and cold rolled to a thickness of 0.30 mm. The cold rolled plate was made into a volume fraction of 0.3% in a mixed gas of 25% H ₂ + 75N ₂ % and a dry NH ₃ gas having a dew point of 48 ° C in a furnace maintained at 875 ° C.

MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the annealing was performed at 25% N ₂ + 75H ₂ % in an atmosphere of 15 ° C./hr at a heating rate of 15 ° C./hr, and reached 100% after reaching 1200 ° C. It was kept for 10 hours in an H ₂ atmosphere.

Then, after measuring the magnetic properties of the steel sheet obtained according to the change in the addition amount of B is shown in Table 1 below. In the embodiment of the present invention, the magnetic properties are measured by the magnetic flux density (B ₁₀ ) induced in the specimen under a magnetic field of 1.000 A / m.

division Amount of B added (% by weight) B ₁₀ (Tesla) Comparative Steel 1 0 1.61 Inventive Steel 1 0.0011 1.91 Inventive Steel 2 0.0033 1.92 Inventive Steel 3 0.0041 1.94 Inventive Steel 4 0.0080 1.92 Inventive Steel 5 0.0110 1.91 Comparative Steel 2 0.0130 1.86

As shown in Table 1 above, it was found that when B was not added (comparative steel (1)), a unidirectional electrical steel sheet could not be obtained, and an appropriate amount of B was required.

Example 2

By weight, Si: 3.10%, Al: 0.014%, Mn: 0.10%, B: 0.0041%, N: 0.0028%, S: 0.0044% and C as shown in Table 1 below, with the balance Fe and other unavoidable impurities The made silicon steel slab was heated to 1150 ° C. and hot rolled to make a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was annealed at 1120 ° C. for 2 minutes, quenched in water at 100 ° C., pickled, and cold rolled to a thickness of 0.30 mm.

The cold rolled plate was subjected to simultaneous decarbonation for 155 seconds in an atmosphere containing a mixed gas of 25% H ₂ + 75% N _{2 with} a dew point of 50 ° C. in a furnace maintained at 875 ° C. and dry NH ₃ . NH ₃ gas was 0.3% by volume fraction. MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the annealing was heated to 1200 ° C. at a temperature rising rate of 15 ° C./hr in an atmosphere of 25% N ₂ + 75% H ₂ , and reached 100 ° C. after 100 ° C. It was kept for 10 hours in a% H ₂ atmosphere.

Thereafter, the amount of residual carbon after simultaneous decarbonation, the amount of nitrogen, and the magnetic flux density of the obtained steel sheet were measured according to the amount of C addition, and the results are shown in Table 2 below.

division Amount of C added (% by weight) Residual carbon (ppm) Nitrogen amount (ppm) B ₁₀ (Tesla) Comparative Steel 3 0.015 11 210 1.83 Inventive Steel 7 0.020 14 200 1.91 Inventive Steel 8 0.045 19 190 1.94 Comparative Steel 4 0.050 31 190 1.90 Comparative Steel 5 0.059 33 210 1.91

As shown in Table 2, it can be seen that when the amount of C is 0.020% or more, high magnetic flux density can be obtained (inventive steel (7-8), comparative steel (4-5)). However, in the case of comparative steel (4-5) containing more than 0.050% of C, if the residual carbon content exceeds 30ppm after simultaneous decarbonation, it may cause self aging and deteriorate the properties because it is used as an iron core for transformers. The amount was found to be preferably managed at 0.020-0.045%.

Example 3

By weight, Si: 3.10%, C: 0.034%, Mn: 0.14%, B: 0.0033%, N: 0.0060%, S: 0.0052% and Al, as shown in Table 1 below, with the balance Fe and other unavoidable impurities The resulting silicon steel slab was heated to 1200 ° C. and hot rolled to make a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was annealed at 1120 ° C. for 2 minutes, air cooled, pickled, and cold rolled to a thickness of 0.27 mm.

The cold rolled plate was subjected to simultaneous decarbonation for 120 seconds in an atmosphere containing a mixed gas of 25% H ₂ + 75% N _{2 with} a dew point of 50 ° C. and dry NH ₃ . At this time, the simultaneous decarbonation annealing temperature was changed to 875 ° C. and 925 ° C. for each, and NH ₃ gas was 0.3% by volume fraction.

MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the annealing was performed at a temperature of 20 ° C / hr at a temperature rising rate of 20 ° C / hr in an atmosphere of 25% N ₂ + 75% H ₂ . It was kept for 10 hours in a% H ₂ atmosphere.

Then, the magnetic properties of the steel sheet obtained according to the change in the addition amount of Al and the simultaneous decarbonation annealing temperature change are shown in Table 3 below. The iron loss at this time was measured by inducing a magnetic flux density of 1.7 Tesla at 50 Hz.

division Amount of Al Added (wt%) Simultaneous decarbonation annealing temperature (℃) W _17/50 (w / kg) B ₁₀ (Tesla) Inventive Steel 9 0.011 875 0.94 1.93 Inventive Steel 10 0.014 0.97 1.94 Inventive Steel 11 0.019 0.99 1.93 Comparative Steel 6 0.022 1.33 1.87 Inventive Steel 12 0.011 925 0.96 1.92 Inventive Steel 13 0.014 1.01 1.93 Inventive Steel 14 0.019 1.01 1.93 Comparative Steel 7 0.022 1.29 1.90

As shown in Table 3, in the case of the comparative steel (6,7) in which the amount of Al is 0.022%, the magnetic flux density is slightly improved when the annealing temperature of simultaneous decarbonation is increased. However, it was found that the primary recrystallization structure was not uniform and secondary recrystallization structure was not fully developed and fine grains existed. As a result, iron loss was higher than that of other steels.

Example 4

By weight, silicon containing Si: 3.15%, C: 0.031%, Al: 0.013%, Mn: 0.09%, B: 0.0033%, N: 0.0065%, S: 0.006% and the balance Fe and other unavoidable impurities The steel slab was heated to 1250 ° C. and hot rolled to make a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was annealed at 1120 ° C. for 2 minutes, cooled under the conditions shown in Table 4, pickled, and cold rolled to a thickness of 0.30 mm.

The cold rolled plate was subjected to simultaneous decarbonation for 155 seconds in an atmosphere containing a mixed gas of 25% H ₂ + 75% N _{2 with} a dew point of 63 ° C. and a dry NH ₃ in a furnace maintained at 875 ° C. NH ₃ gas was 0.3% by volume fraction.

MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the annealing was heated to 1200 ° C. at a temperature rising rate of 15 ° C./hr in a 25% N ₂ + 75% H ₂ atmosphere and reached 1200 ° C. After maintaining for 10 hours at 100% H ₂ atmosphere, the magnetic properties were measured and the results are shown in Table 4 below.

division Cooling condition W _17/50 (w / kg) B ₁₀ (Tesla) Embodiment 1 Quench in water at 100 ℃ 1.04 1.93 Embodiment 2 Air cooling 1.03 1.94

As shown in Table 4, the magnetic properties of the steel sheet obtained after the hot finish annealing according to the cooling conditions after hot-rolled sheet annealing did not have a big difference, it can be seen that the magnetic properties are slightly superior when air-cooled.

Example 5

Silicon steels containing, by weight, Si: 3.15%, C: 0.031%, Al: 0.013%, Mn: 0.09%, B: 0.0033%, N: 0.0065%, S: 0.006%, balance Fe and other unavoidable impurities The slab was heated to 1200 ℃ and hot rolled to make a hot rolled sheet having a plate thickness of 2.3mm. The hot rolled sheet was annealed at 1120 ° C. for 2 minutes, quenched in 100 ° C. water, pickled, and cold rolled to a thickness of 0.23 mm, 0.27 mm, 0.30 mm, and 0.35 mm, respectively.

The cold rolled plate was subjected to simultaneous decarbonation for 155 seconds in an atmosphere containing a mixed gas of 25% H ₂ + 75% N _{2 with} a dew point of 63 ° C. and a dry NH ₃ in a furnace maintained at 875 ° C. NH ₃ gas was 0.3% by volume fraction.

MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the high temperature annealing was heated to 1200 ° C at a temperature rising rate of 15 ° C / hr in an atmosphere of 25% N ₂ + 75% H ₂ and reached 1200 ° C. After maintaining for 10 hours in 100% H ₂ atmosphere after measuring the magnetic properties according to the cold rolling rate, the results are shown in Table 5 below.

division Final thickness (mm) Cold rolling rate (%) W _17/50 (w / kg) B ₁₀ (Tesla) Invention material a 0.35 84.8 1.93 1.09 Invention material b 0.30 87 1.93 1.04 Invention c 0.27 88.3 1.94 0.92 Invention material d 0.23 90 1.94 0.83

As shown in Table 5, when the cold rolling rate is cold rolled in the range of 84-90%, excellent magnetic properties could be obtained.

Example 6

By weight, silicon containing Si: 3.10%, C: 0.036%, Al: 0.014%, Mn: 0.10%, B: 0.0033%, N: 0.0026%, S: 0.052% and the balance Fe and other unavoidable impurities The steel slab was heated to 1200 ° C. and hot rolled to make a hot rolled sheet having a plate thickness of 2.3 mm. The hot rolled sheet was annealed at 900 ° C. for 2 minutes, air cooled, pickled, and cold rolled to a thickness of 0.30 mm.

The cold rolled plate was subjected to simultaneous decarbonation for 120 seconds in an atmosphere containing a mixed gas of 25% H ₂ + 75% N _{2 with} a dew point of 48 ° C. and dry NH ₃ at a volume fraction of 0.3%. At this time, the annealing temperature was changed to 825-975 ℃ as shown in Table 6.

MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the annealing was heated to 1200 ° C. at a temperature rising rate of 15 ° C./hr in an atmosphere of 25% N ₂ + 75% H ₂ , and reached 100 ° C. after 100 ° C. After maintaining for 10 hours at% H ₂ atmosphere, the amount of nitrogen according to the change of annealing temperature and the magnetic flux density after finishing high temperature annealing were measured, and the results are shown in Table 6 below.

division Simultaneous decarbonation annealing temperature (℃) Nitrogen amount (ppm) B ₁₀ (Tesla) Comparative material a 825 110 1.80 Invention material e 850 170 1.93 Invention material f 875 210 1.92 Invention g 900 240 1.95 Invention material h 950 290 1.93 Comparative material b 975 340 1.89

As shown in Table 6, the comparative material (a) having a simultaneous decarbonation annealing temperature of less than 850 ℃ and the comparative material (b) exceeding 950 ℃ was poor magnetic properties. This is considered to be because when the annealing temperature is lower than 850 ° C., the amount of nitrogen in the steel is low and sufficient inhibitors necessary for secondary recrystallization cannot be obtained. In addition, if the annealing temperature is too high, it is considered that the primary recrystallized grains become nonuniform, so that the magnetic properties deteriorate.

Example 7

A silicon steel slab of the same composition used in Example 6 was heated to 1250 ° C. and then hot rolled to form a hot rolled plate having a thickness of 2.3 mm. The hot rolled sheet was annealed at 900 ° C. for 2 minutes, air cooled, pickled, and cold rolled to a thickness of 0.30 mm.

The cold rolled plate was subjected to simultaneous decarbonation for 120 seconds in a mixed gas atmosphere of 25% H ₂ + 75% N _{2 with} a dew point of 48 ° C. in a furnace maintained at 850 ° C. At this time, the volume fraction of NH ₃ was changed to 0.05-1.5% as shown in Table 7 below.

MgO, an annealing separator, was applied to the steel sheet to finish high temperature annealing. At this time, the annealing was heated to 1200 ° C. at a temperature rising rate of 15 ° C./hr in an atmosphere of 25% N ₂ + 75% H ₂ , and reached 100 ° C. after 100 ° C. After maintaining for 10 hours in an atmosphere of% H ₂ , the amount of nitrogen according to the change of the amount of NH ₃ and the magnetic flux density after finishing high temperature annealing were measured, and the results are shown in Table 7 below.

division NH ₃ volume fraction (%) Nitrogen amount (ppm) B ₁₀ (Tesla) Comparative material c 0.05 90 1.81 Invention i 0.1 170 1.92 Invention material j 0.5 220 1.95 Invention k 1.0 290 1.94 Comparative material d 1.5 380 1.89

As shown in Table 7, when the NH ₃ volume fraction was too low (Comparative Material (c)), sufficient nitride could not be obtained and the magnetic properties were low. In addition, when the NH ₃ volume fraction was too high (Comparative Material (d)), the amount of nitrogen in the steel was high and the deterioration of magnetic properties appeared.

As described above, according to the present invention, the low temperature reheating is possible and the nitriding treatment can be performed without changing the existing equipment, and the BN is used as an inhibitor. There is an effect that can provide a method for producing an electrical steel sheet.

Claims (7)

High magnetic flux including the process of reheating the silicon steel slab, followed by hot rolling and hot annealing, then cold rolling once to make the final plate thickness, then decarbonizing the final plate, applying annealing separator and finishing high temperature annealing In the manufacturing method of the density unidirectional electrical steel sheet, The silicon steel slab is in weight%, Si: 1.0-4.8%, Al: 0.005-0.019%, C: 0.02-0.045%, Mn: 0.05-0.2%, B: 0.001-0.012%, N: 0.008% or less, S: 0.007% or less, remainder Fe and other unavoidable impurities, the ammonia concentration of 0.1-1.0% by volume fraction in the decarbonization annealing of the final plate, a mixed gas atmosphere of ammonia + hydrogen + nitrogen and annealing at 850-950 ℃ A method of manufacturing a high magnetic flux density unidirectional electrical steel sheet by slab low temperature heating, characterized by forming an inhibitor which casts BN precipitates by casting at the same time under temperature condition. The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet according to claim 1, wherein the reheating of the silicon steel slab is performed at a temperature of 1050-1250 ° C. The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet according to claim 1 or 2, wherein the hot-rolled sheet annealing is performed after cooling at a temperature of 900-1150 ° C. The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet according to claim 3, wherein the cooling is air cooling. The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet according to claim 1, 2 or 4, wherein the cold rolling is performed with a reduction ratio of 84-90%. The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet according to claim 3, wherein the cold rolling is performed at a reduction ratio of 84-90%. The finishing high temperature annealing according to claim 1, 2 or 4, wherein the finishing high temperature annealing is performed at a temperature of 1150 ° C. or higher at a rate of 10-40 ° C./hr in a dry hydrogen atmosphere or a mixed gas atmosphere of hydrogen and nitrogen for 1-30 hours. A method of manufacturing a high magnetic flux density oriented electrical steel sheet by slab low temperature heating, characterized by cracking.

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