KR100431608B1 - Manufacturing of high magnetic density grain oriented silicon steel - Google Patents

Abstract

The present invention relates to a method of manufacturing a high magnetic flux density unidirectional electrical steel sheet used as an iron core of an electrical apparatus such as a transformer,

The present invention is Si: 2.0-4.8%, Al: 0.005-0.045%, C: 0.030-0.065%, Mn: 0.05-0.2%, S: 0.007% or less, N2: 0.008% or less, balance Fe and inevitable by weight. Hot rolling the electric steel slab made of impurities or the electric steel slab containing B in the composition at 0.001-0.012% at a low temperature of 1,050-1,250 ° C. or lower and hot rolling, and the hot rolled plate in the range of 900 to 1150 ° C. Pre-annealing, pickling the pre-annealed plate and cold rolling to a final thickness, nitriding the cold-rolled plate, and applying an annealing separator to the surface of the annealed plate Characterized in that the manufacturing method of a high magnetic flux density unidirectional electrical steel sheet, characterized in that the step consisting of high temperature annealing on a daily basis, the pre-annealing step is T _MAX (℃) when the carbon content is 0.030 ~ 0.055% by weight ) =-1600 × [wt% C] + 1188 ± 10, the carbon content is 0.055 For 0.065% by weight, _{T MAX (℃) = - 4000} × [wt% C] + 1320 ± 10 which temperature obtained using the equation of rapidly heating up (T _MAX), and by a crack 5-90 seconds and then gradually cooled 890~ It characterized by a method of manufacturing a high magnetic flux density unidirectional electrical steel sheet, characterized in that it is quenched after holding at 950 ℃ for 30-60 seconds.

The present invention configured as described above enables the production of electrical steel sheet having stable magnetic properties by slab low temperature heating.

Description

MANUFACTURING OF HIGH MAGNETIC DENSITY GRAIN ORIENTED SILICON STEEL}

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, more specifically, slab heating temperature is lower than 1,250 ℃, to form an inhibitor after the rolling is completed to the final product thickness The present invention relates to a method for producing an electrical steel sheet having excellent directionality.

The unidirectional electrical steel sheet has an aggregate structure of {110} <001> direction in the rolling direction, and many researchers have invented a new manufacturing method since the manufacturing method was first presented by NP Goss in US Patent 1,965,559. The improvement of properties has been achieved. Inhibitors essential for the production of unidirectional electrical steel sheet have a function of inhibiting the growth of grains and utilize fine precipitates or segregation elements. Therefore, the unidirectional electrical steel sheet is suppressed by the growth of the primary recrystallized grains by effective control of the inhibitor, and by the secondary recrystallized tissue obtained by selectively growing the grains of the {110} <001> orientation among the grains whose growth is suppressed. It has excellent magnetic properties.

Currently, industrially produced unidirectional electrical steel sheet uses one of MnS, MnS + AlN, MnS (Se) + Sb as an inhibitor. Among the above-mentioned methods, the most excellent magnetic property is MnS + AlN as an inhibitor, and is manufactured through hot slab heating, hot rolling, preannealing (precipitation annealing), cold rolling, decarbonization annealing, and high temperature annealing. The method of using MnS (Se) + Sb as an inhibitor is also prepared through the processes of high temperature slab heating, hot rolling, precipitation annealing, primary cold rolling, intermediate annealing, secondary cold rolling, decarbonization annealing, and high temperature annealing.

As shown above, the manufacturing technique of unidirectional electrical steel sheet is completed by what kind of precipitates or grain boundary segregation elements are used and how each process should be configured to obtain a stable {110} <001> azimuth structure. .

The important conditions for the precipitates to function as inhibitors are as follows. Precipitates should be evenly distributed in sufficient quantity and in an appropriate size so that growth of the primary recrystallization grains can be inhibited until just before the secondary recrystallization occurs, and thermally stable to a high temperature immediately before the secondary recrystallization occurs and not easily decomposed. Such conditions are satisfied and currently used industrially are inhibitors of MnS, AlN, MnSe and the like mentioned above.

The production method of each unidirectional electrical steel sheet currently used industrially has advantages and disadvantages. First, a method of using MnS as an inhibitor is disclosed in Japanese Patent Publication No. 30-3651 by M.F.Littmann. In this method, the slab is heated at high temperature and hot rolled, followed by two cold rolling including intermediate annealing, and finally hot annealing to obtain a stable secondary recrystallized structure. However, this method cannot obtain a high magnetic flux density, and there is a problem in that manufacturing cost is high by two cold rolling.

Representative method of manufacturing unidirectional electrical steel sheet using MnS + AlN is shown in Japanese Patent Publication No. 40-15644. In this method, after slab is heated at high temperature and hot rolled, it is higher than 80%. Cold rolling is performed once at a rolling reduction rate to produce a unidirectional electrical steel sheet. This method is cold rolled once and has the advantage of obtaining a high magnetic flux density product. In industrial production, however, the manufacturing conditions are so strict that each process condition must be strictly controlled.

A typical method for producing a grain-oriented electrical steel sheet using MnS (or MnSe) + Sb is shown in Japanese Patent Publication No. 51-13469. This method can obtain a high magnetic flux density but performs cold rolling twice. For example, it is necessary to use toxic and expensive elements such as Sb and Se.

However, these methods suffer from a fundamental problem that is more serious than the above mentioned disadvantages. In other words, in order to obtain a precipitate having a desired size and distribution in each method, all the slabs must be reheated to a high temperature. This is based on the technical idea that MnS or AlN must be heated above the temperature required for solid solution. In other words, MnS or AlN contained in the steel component is heated by heating for a long time at a high temperature to be hot-rolled, and then cooled into a precipitate having an appropriate size and distribution during cooling. It is known that high magnetic flux density can be obtained when the method using MnS is 1,300 ° C., the method using MnS + AlN is 1,350 ° C., and the method using MnS (or MnSe) + Sb is 1,320 ° C. or higher. In actual industrial production, however, in consideration of the size of the slab, in order to obtain a uniform temperature distribution to the inside it is reheated to a temperature close to 1,400 ℃.

When the slab is heated at a high temperature for a long time as described above, the following problems occur.

(1) The production cost is high due to the large amount of calories used,

(2) The surface part of the slab flows down to the molten state, which increases the cost of repairing the heating furnace and shortens the lifespan.

(3) The columnar tablet, which is a solidification structure developed on the surface of the slab, grows coarsely, causing deep cracks in the width direction of the plate in the subsequent hot rolling process, thereby significantly lowering the yield.

For the same reason, manufacturing the unidirectional electrical steel sheet by lowering the slab reheating temperature may have many effects in terms of manufacturing cost and yield.

In order to solve the above problems, methods that do not use MnS having high employment temperature as an inhibitor have been studied in recent years. This can be achieved by making the precipitate at a suitable place in the manufacturing process, without relying entirely on the inhibitor from the elements contained in the steel composition. This method is performed by nitriding treatment as shown in Japanese Patent Laid-Open No. Hei 1-230721 and Japanese Patent Laid-Open No. Hei 1-283324. The nitriding treatment includes applying an annealing separator containing a nitriding compound to a steel sheet, incorporating a nitriding gas into the atmosphere gas during the elevated temperature of the high temperature annealing process, or nitriding gas after cracking in the decarburization process. There is a nitriding steel sheet in the atmosphere.

In this process, carbon of the steel sheet is removed, and nitrogen generated by decomposition of ammonia gas enters the steel sheet. The amount of nitrogen that enters the inside of the steel sheet is influenced by the annealing temperature, the annealing time, and the ammonia fraction in the atmosphere, and is controlled to an appropriate amount of nitrogen according to the steel composition. In addition, the particle size of the primary recrystallization is controlled along with the control of the amount of nitrogen in this process.

After decarburization, after nitriding or simultaneous decarburization annealing, an annealing separator containing MgO as a main component is applied to the surface of the steel sheet and subjected to high temperature annealing in a coil form. The high temperature annealing consists of a temperature rising section for developing the secondary recrystallization structure and a pure annealing section for removing impurities. The temperature increase rate of the temperature increase zone is important because the rearrangement of the precipitate occurs. Empirically, if the rate of temperature rise is too fast, the secondary recrystallization becomes unstable. On the other hand, 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. In order to prevent the loss of nitride used as an inhibitor during the temperature increase process, the atmosphere containing nitrogen is maintained, and the purified annealing is a process of removing harmful elements in the steel by maintaining in a reducing atmosphere, so 100% hydrogen is used.

As described above, the hot rolled sheet is annealed in the range of 900-1150 ° C. in the manufacture of oriented electrical steel sheet of low temperature slab heating method which is nitrided. This is called hot-rolled sheet annealing or pre-annealed, and conditions should be set in consideration of improvement of hot-rolled structure, control of precipitates, and ease of pickling, which is a subsequent process. Existing methods for producing oriented electrical steel sheets by high temperature heating of slabs require very tight control because partial employment and re-precipitation of precipitates occur during hot-rolled sheet annealing to obtain stable precipitate distribution. Specifically, after maintaining at 1,100-1,150 ° C., the precipitates were made to have a desired size and distribution while cooling, and when the temperature reached about 900 ° C., rapid cooling was used to suppress further growth of precipitates. However, in the method of forming the precipitate by nitriding after cold rolling to the final plate thickness, the precipitate is not controlled in this process, so it is performed in the range of 900-1,150 ° C. for the uniformity of the hot rolled structure and the improvement of pickling. To control the uniformity of hot rolled sheet structure and precipitates and to improve final magnetic properties, as in the case of Korean Application No. 1998-58974, after cracking at 1,050-1,150 ° C for 5 to 90 seconds, it is 30 to 890 to 950 ° C. A method of quenching was used after holding for 60 seconds.

However, this method was unable to obtain stable magnetic properties in the final product due to the formation of unwanted precipitates or different textures depending on the component variation of the hot rolled sheet.

Therefore, the present invention is to provide a hot-rolled sheet annealing method of unidirectional electrical steel sheet by slab low temperature heating to obtain the desired hot-rolled sheet structure during pre-annealing and to suppress the formation of unnecessary precipitates in order to obtain excellent and stable magnetic properties even if the components change. Its purpose is to.

As a result of repeating the research to overcome the drawbacks of the prior art and achieve the above object, the following facts were found. That is, the unidirectional electrical steel sheet having a high magnetic flux density can be stably manufactured by performing hot-rolled sheet annealing at 1,050-1,150 ° C. according to carbon content, and maintaining a constant time at 890-950 ° C. to suppress the formation of unnecessary precipitates.

Therefore, in the present invention, Si: 2.0-4.8%, Al: 0.005-0.019%, C: 0.030-0.065%, Mn: 0.05-0.2%, B: 0.001-0.012%, S: 0.007% or less, N ₂ : Less than 0.01%, the rest is composed of Fe and unavoidable impurities, reheating the slab to a temperature of 1,050-1,250 ℃, hot rolling, heating hot rolled plate to 1,050-1,150 ℃ according to carbon content, final plate thickness It is composed of the process of cold rolling once, the process of simultaneously performing decarburization and nitriding, the application of annealing separator to the surface of steel sheet, and the process of high temperature annealing for secondary recrystallization and purifying steel.

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

First, the reason for limitation of the component will be described.

Si is an element that significantly improves iron loss characteristics by increasing the specific resistance of steel, and is an element necessary for manufacturing a unidirectional electrical steel sheet. 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 set by industrially being able to stably perform cold rolling. In a special and strictly controlled rolling method, strength rolling containing about 4.5% Si is known. In the case of 2.0% or less, since the addition effect is insignificant and there is no big meaning, it is preferable to set said Si content to 2.0-4.8%.

In the present invention, Al is finally made of nitrides of the AlN and (Al, Si) N form, and unlike the conventional component system which acts as an inhibitor, there is no significant meaning from the viewpoint of the inhibitor. However, Al, like Si, is an element that increases resistivity. In view of the increase in resistivity, since the effect of addition is less than 0.005%, the Al content is preferably set to 0.005-0.045%.

C is an element added to refine the hot rolled structure. After C is functioned during hot rolling, it becomes an impurity and has to be removed because it adversely affects its magnetic properties. When 3% of Si contains about 0.018% of C, hot rolling Metamorphosis can occur and can function to refine the hot rolled tissue. Therefore, when the amount of Si increases, the amount of C higher than this is required, so it requires more than 0.030%. C is an element that deteriorates the characteristics of electric equipment such as transformers by causing magnetic aging when it remains in the final product, so it is strictly controlled at 0.003% or less in the final product. Therefore, the decarburization process is necessarily included in the manufacture of the unidirectional electrical steel sheet. However, in the present method of simultaneously performing decarburization and nitriding, the amount of C is advantageously low. Therefore, it is preferable that the amount of C is high in terms of miniaturization of the hot rolled structure, but when the coarse carbide is precipitated because the content is too high, it is difficult to remove carbon during simultaneous decarbonation annealing, so it is preferable to select it as 0.02-0.065% or less.

Mn is a component having an effect of increasing the electrical resistance and lowering the iron loss. When the content is too large, Mn causes a decrease in magnetic flux density, so the Mn content is preferably 0.05-0.2%.

B is used as an inhibitor in combination with nitrogen entered into the river by decomposition of ammonia gas in the nitriding process. If the amount is less than 0.001%, the amount of inhibitor is insufficient to obtain a stable secondary recrystallized tissue. If the amount exceeds 0.012%, the secondary recrystallized tissue can be obtained, but the magnetic flux density is confirmed to decrease. Therefore, the content of B is preferably set to 0.001-0.012%.

Since N ₂ is used by reinforcement in the simultaneous decarbonation process, an amount sufficient to enter impurities during dissolution is sufficient. However, nitrogen may be added deliberately, but when it exceeds 0.01%, it reacts with Al contained in the steel to form coarse AlN precipitates, thereby decreasing the primary recrystallized grain size and adversely affecting magnetism. Therefore, N ₂ is preferably made 0.01% or less.

S is a element with severe segregation, so it is desirable to not contain it as much as possible for hot workability, but to make it extremely low through de-S process during steelmaking, there is an additional cost to the process. Therefore, if it exceeds 0.007%, MnS reacts with Mn contained in the steel to make the primary recrystallized grain small, so that S is preferably 0.007% or less.

The following describes the conditions of the manufacturing process of the present invention.

The heating temperature of the slab for electrical steel is preferably selected to be 1,050-1,250 ℃, the reason is that when the heating temperature is less than 1,050 ℃ difficult to work during hot rolling, when the temperature is more than 1,250 ℃ significantly affect the magnetic properties None, but the benefits from low-temperature heating of slabs are greatly reduced. In the conventional method of manufacturing unidirectional electrical steel sheet using AlN or MnS as an inhibitor, the hot rolling process influenced the characteristics of the final product. This is because AlN or MnS must be solid-dissolved by slab high temperature heating and re-precipitated during hot rolling to control grain size and distribution. However, the present invention adopts a method of forming an inhibitor after being cold rolled 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.

Hot rolled steel sheets are annealed in the range of 900-1,150 ° C. In the conventional method using high-temperature slab heating, partial employment and reprecipitation of precipitates occur during hot-rolled sheet annealing, so that they are maintained at 1,100-1,150 ℃ and then slowly cooled to about 900 ℃ in order to obtain a stable distribution of precipitates. It was. However, the method using low-temperature slab heating does not have to consider the viewpoint of precipitates, so it is performed in the range of 900-1,150 ° C for uniformity of the hot rolled structure and pickling improvement, and some excellent magnetic properties can be obtained without strictly controlling the cooling method. . However, the uniformity of the hot rolled sheet tissue has a sensitive influence on the final magnetic properties. This seems to be related to the improvement of the collective structure of hot rolled sheet, which will be explained later. As a result, it was confirmed that the preferred hot-rolled sheet annealing temperature was 1,050-1,150 ° C. according to the carbon content of the hot-rolled sheet. In more detail, depending on the carbon content of the hot-rolled sheet is rapidly heated to the temperature (T _MAX ) obtained by the following equation, and then cracked within 5-90 seconds after the slow cooling, and then quenched after maintaining for 30 to 60 seconds at 890 ~ 950 ℃.

If 0.030~0.055 wt% carbon content _{T MAX (℃) = -1600 [} wt% c] + 1188 ± 10

T _max (℃) = -4000 [wt% c] + 1320 ± 10 for carbon content 0.055 ~ 0.065% by weight

At this time, if the temperature of the hot-rolled sheet after rapid heating does not reach the temperature calculated by the above formula or the crack time is less than 5 seconds, the improvement effect of the hot-rolled sheet structure is insignificant.

Improvement of the hot rolled sheet structure means growing grains having an orientation of {110} <001> to a certain depth in the surface portion. These grains affect the degree of integration of the {110} <001> grains in the rolling direction after the secondary recrystallization. At this temperature, the crack time becomes too long, but if it is higher than the temperature calculated by the above equation, grains grow up to the center of the hot rolled sheet, and when the first recrystallization after cold rolling, very long stretched structures are generated in the rolling direction. These elongated tissues do not easily encroach on the grains of {110} <001> secondary recrystallized during the final high temperature annealing, so they exist as fine grains.

Therefore, annealing higher than the temperature calculated by the above equation or cracking for more than 90 seconds should be avoided. In addition, cracking at 1,050-1,150 ° C. can lead to precipitation of precipitates, which, if not properly controlled, results in worse than the improvement of hot-rolled sheet structure.

That is, in the cooling process after cracking, a large amount of precipitates are precipitated, and when the first recrystallization is performed in a subsequent process, the grain size is reduced, and ultimately, the secondary recrystallization structure cannot be obtained. Therefore, it must be quenched after being cracked for 30-90 seconds in the temperature range of 890-950 ° C. This is because precipitates such as AlN and MnS are most actively precipitated below about 900 ° C. Therefore, they should not be allowed to pass through 900 ° C at high speed after precipitation at a higher temperature. The cracking time of 30-90 seconds is to coarsen some of the precipitates so as not to affect the grain refinement during the formation of primary recrystallized grains. That is, it is not possible to completely suppress the generation of precipitates in the cooling process after cracking at the calculated temperature. Therefore, the precipitates that have already been precipitated in the temperature range of 890-950 ° C are sufficiently grown to prevent the grains from being refined during the production of primary recrystallized grains. If the cracking time is less than 30 seconds, the coarsening effect of the precipitates is insignificant, and if it is maintained for 90 seconds, it grows sufficiently so that it is not necessary to hold it longer than 90 seconds.

Next, the preannealed plate is pickled and cold rolled. At this time, it is preferable to roll to the final thickness by one rolling without intermediate annealing.

Cold rolled plates to final product thickness are annealed for decarburization and nitriding. At this time, the atmosphere in the annealing furnace is performed by putting a small amount of dry ammonia gas into a mixed atmosphere of wet hydrogen + nitrogen.

In this process, carbon in the steel sheet is removed, and nitrogen generated by decomposition of the ammonia gas enters the steel sheet. The amount of nitrogen that enters the inside of the steel sheet is influenced by the annealing temperature, the annealing time, and the ammonia fraction in the atmosphere, and is controlled to an appropriate amount of nitrogen according to the steel composition. In addition, the particle size of the primary recrystallization is controlled along with the control of the nitrogen content. At this time, decarburization may be performed first and nitriding may be adopted.

After annealing, an annealing separator containing MgO as a main component is applied to the surface of the steel sheet and subjected to high temperature annealing on a coil. The high temperature annealing consists of a temperature rising section for developing the secondary recrystallization structure and a pure annealing section for removing impurities. The temperature rise section is important because the rate of temperature rise is caused by rearrangement of precipitates. Empirically, if the rate of temperature rise is too fast, the secondary recrystallization becomes unstable. On the other hand, 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. It is preferable to maintain the atmosphere containing nitrogen in order to prevent the loss of nitride used as an inhibitor during the temperature increase process, and 100% hydrogen is used because the pure annealing is a process of removing harmful elements in the steel by reducing the atmosphere. desirable.

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

Example 1

Electrical steel slabs having the main components shown in Table 1 and consisting of the balance Fe and the unavoidable element were heated to 1,200 ° C., and then hot rolled to obtain hot rolled sheets having a plate thickness of 2.3 mm. The hot rolled sheet was changed from 1,000 ° C to 1,175 ° C at 25 to 50 ° C with a maximum attained temperature for 10 seconds, then cooled slowly and maintained at 910 ° C for 60 seconds, then quenched and rinsed and cold rolled to a thickness of 0.30 mm. It was.

Table 1

Lecture number Component (Wt.%) C Si Mn S Sol-Al N B ① 0.035 2.85 0.080 0.006 0.024 0.0060 0.005 ② 0.043 2.93 0.090 0.005 0.023 0.0065 0.004 ③ 0.049 3.08 0.095 0.005 0.027 0.0068 0.004 ④ 0.056 3.10 0.087 0.005 0.025 0.0075 0.005 ⑤ 0.063 3.10 0.103 0.004 0.029 0.0067 0.009

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 having} a dew point of 47 ° C. and a dry NH ₃ in a furnace maintained at 875 ° C. NH ₃ gas was set at 1.5% by volume fraction. MgO which is an annealing separator was applied to this steel sheet, and final high temperature annealing was performed. The high temperature annealing was heated to 1,200 ° C. at a temperature rising rate of 15 ° C./hr in an atmosphere of 25% N ₂ + 75% H ₂ , and maintained at 100% H ₂ for 20 hours after reaching 1,200 ° C.

The magnetic properties of the steel sheet obtained according to the change in the maximum temperature of the hot rolled sheet annealing are shown in Table 2 below. Magnetic properties were measured by the magnetic flux density (B10) induced in the specimen under a magnetic field of 1,000A / m.

[Table 2]

Lecture number Maximum Reach Temperature (T _MAX ) B10 (Tesla) Remarks A ① 10501075110011251150 1.871.871.881.931.89 Comparative Steel Comparative Steel Development ② 10751100112511501175 1.871.881.931.891.78 Comparative Steels Comparative Steels ③ 10251050107511001125 1.751.871.891.941.88 Comparative Steel Comparative Steel Development B ④ 10501075110011251150 1.891.881.931.881.87 Comparative Steels Comparative Steels ⑤ 10251050107511001150 1.811.891.941.871.88 Comparative Steels Comparative Steels

As shown in Table 2, the magnetic properties of the steel sheet according to the ultra-high temperature change of the hot-rolled sheet annealing under the same conditions as those of the steel number shown in Table 1 showed higher magnetic flux density of 1.93 or more than that of the comparative steel. .

According to the present invention configured as described above, it is possible to suppress the generation of unnecessary precipitates by slab low temperature heating and obtain a desired hot rolled sheet structure, so that stable magnetic properties can be obtained even if there are variations in steel components, and electrical steel sheets having high magnetic flux density. It can be manufactured easily.

Claims (4)

delete By weight% Si: 2.0-4.8%, Al: 0.005-0.045%, C: 0.030-0.065%, Mn: 0.05-0.2%, S: 0.007% or less, N2: 0.008% or less, balance Fe and inevitable impurities Hot-rolling an electrical steel slab or an electrical steel slab containing B in the composition at 0.001-0.012% at a low temperature of 1,050-1,250 ° C., When the hot rolled sheet has a carbon content of 0.030 to 0.055 wt%, the carbon content is 0.055 to 0.065 wt% according to the formula T _MAX (° C.) =-1600 × [wt% C] + 1188 ± 10 Is rapidly heated to a temperature (T _MAX ) obtained by the formula T _MAX (° C.) =-4000 × [wt% C] + 1320 ± 10, cracked for 5 to 90 seconds, then cooled slowly and then cooled to 30- at 890-950 ° C. Pre-annealing and quenching after holding for 60 seconds; Pickling the preannealed plate and cold rolling to a final thickness; Nitriding the cold rolled plate; The method of manufacturing a high magnetic flux density unidirectional electrical steel sheet comprising the step of applying an annealing separator to the surface of the nitride annealing plate to a high temperature annealing. 3. The method of claim 2, wherein the annealing step is performed by adding a small amount of dry ammonia gas to a mixed atmosphere of wet hydrogen + nitrogen. 4. The method of claim 2, The high temperature annealing step is a temperature rising section of 10 ~ 40 ℃ / hr to increase the secondary recrystallization structure, and Method for producing a high magnetic flux density unidirectional electrical steel sheet, characterized in that consisting of a pure annealing section to maintain a reducing atmosphere using 100% hydrogen to remove harmful elements in the steel.