KR100359242B1 - Low temperature heating method of high magnetic flux density oriented electrical steel sheet - Google Patents

Abstract

The present invention is intended to be able to obtain a fine hot-rolled structure even when the C content of the small steel is extremely low in manufacturing a high magnetic flux density oriented electrical steel sheet having excellent magnetic properties. According to the present invention, there is provided a low-temperature heating method for producing a high magnetic flux density oriented electrical steel sheet, in which the slab is prepared by reducing the amount of C in the molten steel and containing an appropriate amount of Zr, and then producing the slab at a low temperature of 1,250 ° C or lower. Hot rolling is carried out, and the cold rolling is carried out once to obtain the final thickness, followed by decarbonization annealing, annealing to form a new precipitate, which is used as an inhibitor. Wherein the slab is 1.0-4.8 wt% Si, 0.005-0.019 wt% C, 0.05-0.2 wt% Mn, 0.01-0.07 wt% Zr, 0.020 wt% or less Al, 0.007 S or less in weight%, N or less in 0.01 weight%, Fe which remain | survives, and an unavoidable impurity are contained.

Description

Low temperature heating method of high magnetic flux oriented electrical steel sheet

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet used as an iron core of an electrical apparatus such as a transformer, more specifically, the slab heating temperature is lower than 1,250 ℃, to form an inhibitor after rolling is completed to the final product thickness The present invention relates to a method for producing an electrical steel sheet having excellent directionality.

A grain-oriented electrical steel sheet has an aggregate structure of {110} <001> orientation in the rolling direction, and has been newly manufactured by many researchers since the manufacturing method was first presented by US Patent No. 1,965,559 to NP Goss. The invention and improvement of the method have been made. In order to manufacture a grain-oriented electrical steel sheet, an inhibitor that functions to inhibit grain growth is indispensable, and as such inhibitors, fine precipitates or segregation elements are used. Therefore, the grain-oriented electrical steel sheet is suppressed by primary recrystallization grains by effective control of inhibitors, and by secondary recrystallization structure obtained by selectively growing grains of {110} &lt; 001 &gt; It has excellent magnetic properties.

At present, industrially produced grain-oriented electrical steel sheet uses one of MnS, MnS + AlN, and MnS (Se) + Sb as an inhibitor. Among them, the method using only MnS as an inhibitor has lower magnetic flux density than the other two methods. When a product having a high magnetic flux density is used as an iron core, since the size of the electric apparatus can be reduced and the size of the apparatus can be reduced, much efforts have been made to increase the magnetic flux density in the field of manufacturing oriented electrical steel sheets.

Among the above methods, the most excellent magnetic property is a method using MnS + AlN as an inhibitor, and proceeds to the processes of hot slab heating, hot rolling, precipitation annealing, cold rolling, decarbonization annealing, and high temperature annealing. The high temperature annealing refers to a process of developing a collective structure in the {110} <001> direction by causing secondary recrystallization on a coil. High temperature annealing prevents sticking between steel sheets in any method using an inhibitor, and reacts with an oxide layer formed on the surface of the steel sheet during decarbonization annealing to form a glassy film, thereby providing insulation. It is carried out by applying an annealing separator (MgO is the main component). The final product is obtained by performing an insulation coating on a plate having an aggregate structure of the {110} &lt; 001 &gt; orientation obtained by high temperature annealing. The method of using MnS (Se) + Sb as an inhibitor proceeds to 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 seen above, in the manufacturing technique of oriented electrical steel sheet, what kind of precipitates or grain boundary segregation elements will be used, and how to configure each process to obtain a stable {110} <001> azimuth structure becomes an important factor. have.

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 recrystallized grains can be inhibited until just before the secondary recrystallization occurs, and thermally stable at high temperatures immediately before the secondary recrystallization takes place and not easily decomposed. Such a condition is satisfied and currently used industrially is the method using the aforementioned MnS, AlN, MnSe.

There are advantages and disadvantages in the production of each of the grain-oriented electrical steel sheets currently used industrially. First, a method of using MnS as an inhibitor is disclosed in Japanese Patent Publication No. 30-3651 by M. F. Littmann. This method is cold rolled twice, including intermediate annealing, and a stable secondary recrystallization structure can be obtained. However, this method cannot obtain a high magnetic flux density, and there is a problem in that manufacturing costs are high due to two cold rolling.

Representative method for producing a grain-oriented electrical steel sheet using MnS + AlN is shown in Japanese Patent Publication No. 40-15644, in which the cold rolled once with a high reduction ratio of 80% or more to produce a grain-oriented electrical steel sheet do. This method is cold rolled once and has the advantage of obtaining a high magnetic flux density product. However, in industrial production, the manufacturing conditions are very strict and each process condition must be strictly controlled.

A representative 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 achieves a high magnetic flux density but performs two cold rollings, using toxic and expensive elements such as Sb and Se.

In addition, these methods suffer from a fundamental problem that is more serious than the disadvantages mentioned above. 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 because at least MnS or AlN must be heated to the temperature required for solid solution. That is, MnS, AlN, etc. contained in the small steel component is heated by heating for a long time at a high temperature to be hot-rolled, and then cooled to make a precipitate having an appropriate size and distribution. 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. However, in actual 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 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 because there are many calories used.

(2) As the surface part of the slab flows down to the molten state, the repair cost of the heating furnace is high and the life is shortened.

(3) The columnar tablet, which is a solidification structure developed on the surface of the slab, grows coarsely, and deep cracks are generated in the width direction of the plate in the subsequent hot rolling process, thereby significantly reducing the error rate.

For the same reason, manufacturing a grain-oriented electrical steel sheet by lowering the slab reheating temperature may have many effects in terms of manufacturing cost and error rate.

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 depending 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 coating an annealing separator containing a nitriding compound on 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 addition, a representative method for the time point of nitriding is shown in Japanese Patent Publication No. 2-228425, in which nitrogen is introduced into steel by a nitriding process performed on a plate after hot rolling or before final cold rolling. There is a method of making a precipitate, and there is a method of nitriding and decarburizing when decarbonization is performed after the cold rolling is completed, as shown in Japanese Patent Publication No. 2-294428. However, in this method, there is a problem that the secondary recrystallization becomes unstable, and Japanese Patent Publication No. 3-2324 is improved from Japanese Patent Publication No. After growing to a certain extent or more, a method of nitriding with ammonia gas is proposed.

All of the above methods use a method of injecting nitrogen, which is generated by ammonia decomposing at about 500 ° C. or more, by nitriding with ammonia gas, into the steel sheet. This is because nitrogen entered into the steel sheet reacts with Al and Si, which are elements already present in the steel, to form nitride, and the nitride is used as an inhibitor. At this time, among the nitrides formed therein, AlN and (Al, Si) N are mainly Al-based nitrides.

That is, in the conventional method for producing a grain-oriented electrical steel sheet, precipitates are formed inside the steel sheet by heating the slab to a low temperature and nitriding the same with a material or gas having nitriding ability in the steel sheet. However, since all of these methods decarbonize the steel sheet containing about 0.050% of carbon to remove carbon and then add nitrogen, it is inevitable to add a new process to the 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 compound having a nitriding ability to the annealing separator has a serious problem that causes a large amount of defects in the forsterite layer formed on the surface of the steel sheet to deteriorate the product. In addition, since the amount of S or N contained in the steel is relatively high, much unintended MnS or AlN is produced after hot rolling. This decreases the primary recrystallized grain size after decarbonization annealing, and thus requires a very strong inhibitor to smoothly obtain the secondary recrystallized structure. Therefore, the secondary recrystallized structure can be obtained only if the small and fine precipitate can be formed uniformly. This should be strictly controlled to a very narrow range of the particle size after decarbonization annealing, and the amount of nitriding after nitriding must also be strictly controlled, which makes industrial manufacture difficult.

For this method to be applied industrially, the following two problems must be addressed. First, process improvement is needed that does not significantly change existing equipment. This is to reduce the manufacturing cost and enhance the applicability of the new technology. Secondly, stable oriented 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, which ultimately relates to manufacturing costs.

The present invention is to solve the above problems, to produce a fine hot-rolled structure even if the C content of the small steel to extremely low in producing a high magnetic flux density oriented electrical steel sheet having excellent magnetic properties.

Figure 1-Photograph showing a precipitate functioning as a particle growth inhibitor function of the component system according to the present invention

According to this invention for achieving the above object, there is provided a low-temperature heating method for producing a high magnetic flux density oriented electrical steel sheet, in which the slab is prepared by reducing the amount of C in the molten steel and containing an appropriate amount of Zr, The slab is heated at a low temperature of 1,250 ° C. or lower, hot rolled, cold rolled once to a final thickness, decarbonized annealing, nitrided to form a new precipitate, and used as an inhibitor.

Wherein the slab is 1.0-4.8 wt% Si, 0.005-0.019 wt% C, 0.05-0.2 wt% Mn, 0.01-0.07 wt% Zr, 0.020 wt% or less Al, 0.007 Re-heating of the slab before the hot rolling is contained, containing S or less by weight%, N or less by 0.01 weight%, Fe, which forms the remainder, and inevitable impurities, and is performed at a temperature of 1,050-1,250 ° C.

[Description of Preferred Embodiment of the Invention]

Hereinafter, a preferred embodiment of a low temperature heating method for producing a high magnetic flux density oriented electrical steel sheet according to 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.

Si is an element which increases the specific resistance of steel and remarkably improves iron loss characteristics, and is an element that is essential for the production of grain-oriented electrical steel sheet. The amount of addition is determined by various limiting factors, and it is common to contain about 2.95-3.5% in commercial products. The upper limit is set to the extent that industrial cold rolling can be stably performed. In a special and strictly controlled rolling method, strength rolling containing about 4.5% Si is known. If the content of Si is 1.0% or less, the addition effect is insignificant and does not have much significance. Therefore, in this embodiment, the Si content is set at 1.0-4.8%.

In the conventional method, Al finally became nitrides of the AlN and (Al, Si) N forms to act as inhibitors. However, Al is not used in this example with respect to the inhibitor. However, Al, like Si, is an element that increases the specific resistance of steel, so it is advantageous to add Al. When the Al content is more than 0.020%, AlN precipitates are formed in combination with nitrogen to decrease the primary recrystallization grain size, which leads to unstable development of secondary recrystallization. In addition, when a large amount of precipitates of AlN are formed after nitriding, the overall suppression force becomes too large, and thus, the balance of the force of appropriate suppression force according to the grain size is broken, making development of secondary recrystallization difficult. Therefore, in this Example, the content of Al is set at 0.020%.

C is an element added to refine the hot rolled structure, but after functioning during hot rolling, it becomes an impurity and has to be removed because it adversely affects its magnetic properties. When 3% of Si is contained, when about 0.018% of C is contained, α-γ transformation occurs during hot rolling, which may function to refine the hot rolled structure. If the amount of Si increases, the C content must also increase to function. From this point of view, the lower limit of the content of C should be 0.020% or more. On the other hand, C is an element that deteriorates the characteristics of electrical equipment such as transformers by causing magnetic aging when left in the final product, and is strictly controlled at 0.003% or less in the final product. Therefore, the decarburization process is necessarily included in the production of the grain-oriented electrical steel sheet, and in view of ease of decarburization, the amount of C in the small steel is advantageously low. However, when Zr is added as in the present invention, the hot rolled structure becomes fine even when the amount of C is lowered. Therefore, in this example, the content of C is set at 0.005-0.019%.

Mn is a component that increases the electrical resistance and lowers the iron loss. However, too much content causes a decrease in magnetic flux density. Therefore, in this example, the Mn content is set at 0.05-0.2%.

Zr is one of the characteristic elements of this invention, which is maintained in solid solution in the steel, and after slab is cold rolled to the final product thickness, it is combined with nitrogen that enters the steel by decomposition of ammonia gas in the nitriding process. Acts as. If the amount is less than 0.01%, the amount of inhibitor is insufficient to obtain a stable secondary recrystallized tissue, and if it exceeds 0.07%, the nitride is coarse to obtain a stable secondary recrystallized tissue. Therefore, in this example, the content of Zr is set at 0.01-0.07%.

N is reinforced during simultaneous decarburization and nitriding, so that the amount entering into the impurity is sufficient when dissolving. No adverse effects due to the addition of nitrogen have been found, but if it exceeds 0.01%, the primary recrystallized grain size becomes smaller by reacting with Al in the steel to form coarse AlN precipitates. Therefore, in this embodiment, the content of N is set at 0.01% or less.

S is an element with high segregation, and it is preferable that it is not contained as much as possible for hot workability. In order to reduce the content of S to extremely low during steelmaking, additional costs are required due to the desulfurization process. Therefore, S is a kind of element contained as unavoidable and unavoidable impurities. However, when the content of S exceeds 0.007%, the first recrystallized grains are reduced by reacting with Mn included in the steel to synthesize MnS, so the content of S is preferably maintained at 0.007% or less.

Hereinafter, a low temperature heating method for manufacturing a high magnetic flux density 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 1,050-1,250 占 폚. If the heating temperature is less than 1,050 ℃, the work during hot rolling is difficult, and if it is more than 1,250 ℃ does not significantly affect the magnetic properties, it is because the advantages of low-temperature heating of the slab, which is an object of the present invention and characteristics. That is, it is preferable to keep the heating temperature as low as possible but maintain the hot rolling at 1,050 ° C or more, which will not be difficult. In the conventional method for producing a grain-oriented 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 is dissolved by high temperature heating of the slab, and the size and distribution of the slab must be reprecipitated during hot rolling. However, in this embodiment, since the slab is cold rolled to the final product thickness to form an inhibitor, no high temperature heating of the slab to control the precipitate is required.

Hot rolled plates are annealed in the range of 900-1150 ° C. In the related art, partial employment and reprecipitation of precipitates during hot-rolled sheet annealing have been performed to maintain a stable precipitate distribution, and then maintained at 1,100-1,150 ° C. and then quenched when it reached about 900 ° C. However, the present invention does not need to consider the viewpoint of precipitates, so that annealing is performed in the range of 900-1,150 ° C. in order to homogenize the hot rolled structure and improve pickling properties, and the cooling method does not require strict control.

Next, the annealed 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 primary recrystallization. Decarburization is performed in a mixed gas atmosphere of wet hydrogen and nitrogen, and decarburization capacity is determined by the hydrogen partial pressure and the vapor pressure at this time.

The decarburized plate is nitrided by annealing in an atmosphere containing ammonia. Nitriding is a method in which dry ammonia gas is placed in a furnace where decomposed nitrogen enters the steel sheet. The final amount of carbon after decarbonization annealing is lowered to 20 ppm or less, and the amount of nitride after annealing is controlled to a desired amount by changing annealing time and ammonia concentration. Nitrogen annealing temperature is set at 850-950 ° C., and the amount of ammonia is easily changed to 100-1,000 ppm by adjusting the annealing time in the range of about 0.1-5.0% by volume fraction.

When decarburization and nitriding are performed in the same furnace, removal and nitriding of C are simultaneously performed. At this time, the atmosphere in the annealing furnace is formed by introducing a small amount of dry ammonia gas into a mixed atmosphere of wet hydrogen and 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.

After annealing or simultaneous decarburization and annealing, 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 wound state. The high temperature annealing consists of a temperature rising section for developing a 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. As a rule of thumb, 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 rising process, and 100% hydrogen is used because the pure annealing is a process of removing harmful elements in the river by maintaining in a reducing atmosphere. desirable.

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

The formation of the precipitate used as an inhibitor in the present invention will be described. Since the component system of this invention contains Si, Mn, Al, and Zr, after the hot rolling, nitrides or carbides of these alone or in combination may be formed. In fact, there is a precipitate formed during casting, which is mostly coarse Zr (C, N). In comparison with the thermodynamic reaction priorities for the precipitates produced by the above-mentioned elements, nitrides of Zr are formed, followed by Al nitrides. In addition, since Zr is an element that forms carbide, it reacts with C in the steel to form ZrC. Comparing these formation enthalpy, ZrN is -88 Kcal, AlN is -76.1 Kcal, ZrC is -48.27 Kcal. In other words, ZrN is preferentially generated, followed by AlN and ZrC. However, as mentioned above, most of the precipitates in the hot rolled sheet state are Zr (C, N). It is thought that when ZrN is first generated, C gathers around and becomes Zr (C, N). However, at this stage the content of nitrogen is low below 0.006%, so that AlN is hardly formed. Another precipitate observed in the hot rolled sheet state is coarse MnS combined with S contained as impurities. However, since the precipitate present in the hot rolled sheet is not directly used as an inhibitor, it does not mean much. However, these precipitates should be properly controlled because they affect the particle size of primary recrystallized grains during decarbonization annealing. Substantially the precipitate used as an inhibitor is a precipitate formed after nitriding.

The type and size of precipitate after nitriding is different from that of hot rolled sheet. In the decarburization process before nitriding, the content of C in the steel is less than 20 ppm, so that almost no carbides or carbonitrides are observed.

In this invention, when nitrogen is added in a simultaneous decarburization and annealing process, due to the formation enthalpy compared with the above, nitrogen preferentially reacts with Zr in the steel to form ZrN, which 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, ZrN precipitates preferentially.

As a result of observing the precipitate after simultaneous decarburization and nitriding in the method according to the present invention, it was found that a large number of ZrN was formed. 1 is a micrograph of the precipitate at about 900 ° C. during the high temperature annealing process after simultaneous decarburization and nitriding. The precipitates found were many hundreds of microns. 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 annealing the hot rolled sheet. Therefore, the main precipitate in the steel sheet produced according to this invention is ZrN, and it is assumed that this nitride acts as an inhibitor.

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

In this experiment it contains 0.017% C, 3.15% Si, 0.09% Mn, 0.006% S, 0.013% Al, 0.0065% N and the balance Fe and unavoidable elements, As shown in Table 1 below, each of the steel sheet slabs containing Zr was heated to 1,200 ° C., and then hot rolled to obtain a hot rolled sheet having a plate thickness of 2.3 mm. The hot rolled sheet was annealed at 1,120 ° C. for 2 minutes, quenched in water at 100 ° C., pickled, and cold rolled to a thickness of 0.30 mm.

TABLE 1

The cold rolled plate was 155 seconds in an atmosphere containing a mixed gas of 25% H ₂ and 75% N ₂ with a dew point temperature of 48 ° C and a dry NH ₃ in a furnace maintained at 875 ° C. Simultaneous decarburization and nitriding were performed. At this time, NH ₃ gas was 0.3% by volume. 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 consisting of 25% N ₂ and 75% H ₂ , and maintained for 10 hours in an atmosphere consisting of 100% H ₂ after reaching 1,200 ° C.

The magnetic properties of the steel sheets of each experimental example obtained according to the change in the addition amount of Zr as shown in Table 1 are shown in Table 2. Magnetic properties were measured for the magnetic flux density (B ₁₀ ) induced in the specimen under a magnetic field of 1,000A / m.

TABLE 2

As shown in Table 2, in Comparative Example 1, in which Zr was not added, a grain-oriented electrical steel sheet could not be obtained, and in the case of Inventive Examples 1 to 4 in which Zr was added within a range defined in a preferred embodiment of the present invention, a good electrical steel sheet could be obtained. It can be seen that.

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, even if the C content of the steel is extremely low, a fine hot-rolled structure can be obtained, and magnetic properties by forming ZrN by simultaneous decarburization and nitriding and using it as an inhibitor of grain growth This excellent high magnetic flux density oriented electrical steel sheet can be produced.

Claims (1)

The steel sheet slab is heated and hot rolled, the hot rolled sheet is annealed, the final thickness is obtained by one cold rolling, decarburization and nitriding is performed simultaneously to form an inhibitor, and then annealing separator is applied to the surface of the steel sheet. In the method for producing a grain-oriented electrical steel sheet having a high magnetic flux density by high temperature annealing, The slab is 1.0-4.8 wt% Si, 0.005-0.019 wt% C, 0.05-0.2 wt% Mn, 0.01-0.07 wt% Zr, 0.020 wt% or less Al, 0.007 wt% It contains the following S, 0.01 weight% or less N, remainder Fe, and an unavoidable impurity, Decarburization and nitriding were performed simultaneously on the slab rolled to the final thickness to form ZrN precipitates, Reheating of the slab before hot rolling is carried out at a low temperature of 1,250 ℃ or less low-temperature manufacturing method of a high magnetic flux density oriented electrical steel sheet.