KR100490999B1 - Method For Manufacturing Grain-Oriented Silicon Steel Containing High Silicon By Strip Casting Process - Google Patents

Abstract

The present invention relates to a method for producing a grain-oriented electrical steel sheet used in the iron core of a generator or a transformer, etc., by using a vertical twin roll strip casting method to produce a grain-oriented electrical steel sheet under appropriate conditions, Si can be added up to 5.5%, The purpose of the present invention is to provide a method for producing a high silicon grain-oriented electrical steel sheet by strip casting, which does not add C, so that the decarburization process can be omitted.

The present invention relates to a method for producing a high silicon grain-oriented electrical steel sheet using the strip casting method, wherein the molten silicon steel containing wt%, C: 0.003% or less, and Si: 2.0 to 5.5% in a nitrogen atmosphere at 20 to 30 ° C. After casting the strip to the thickness of 2.0mm or less with the superheat of molten metal, the surface of the strip is pickled and brushed, and then cold rolled to the thickness of 0.3mm or less by using the two-stage cold rolling method including intermediate annealing. The main point of the present invention is a method for producing a high silicon grain-oriented electrical steel sheet by a strip casting method, which is produced by heat treatment at 1100-1200 ° C for 10-20 hours at high temperature.

Description

Method for manufacturing high silicon oriented electrical steel sheet by strip casting method {Method For Manufacturing Grain-Oriented Silicon Steel Containing High Silicon By Strip Casting Process}

The present invention relates to a method for producing a grain-oriented electrical steel sheet used in the iron core of a generator or a transformer, and more particularly, to a method for producing a high silicon grain-oriented electrical steel sheet by a strip casting method.

Oriented electrical steel is a representative soft magnetic material widely used in iron cores of generators and transformers, and various methods have been studied for reducing iron loss and increasing magnetic flux density for more than 100 years. The representative method of reducing iron loss is to increase the Si content, and to increase the magnetic flux density, the {110} [001] texture is known as the Goss texture by giving high orientation to the crystal structure of silicon steel sheet. Methods of increasing the density of are known.

However, increasing the Si content for reducing iron loss lowers the cold workability of the material, so that when the Si content is added to 3.5% or more, it is known that cold rolling is impossible in the conventional manufacturing method.

However, it has been reported that ribbons manufactured by rapid solidification method can be cold rolled by more than 70% even at 4.5 wt% Si content (IEEE Transactions on Magnetics, Vol. MAG23, No. 5, September 1987).

However, in this case, since the thickness is cast into a ribbon of 300 μm or less, it is very difficult to cold roll at a reduction ratio of more than 80% to obtain a well-developed recrystallized Goss texture. Special methods are required, such as rolling together.

Conventional grain-oriented electrical steel sheet is manufactured through various complex processes such as manufacturing slab by continuous casting, controlled cooling, hot rolling, annealing, cold rolling (80%), primary recrystallization, decarburization heat treatment, and secondary recrystallization heat treatment. do.

In this manufacturing process, the addition of carbon in the molten metal and subsequent control of the carbon content in the final product by decarburization heat treatment play a very important role in the existing manufacturing method.

In the slab manufactured by the conventional continuous casting method to manufacture oriented electrical steel sheet, there are huge columnar tablets and when reheated and heat treated, large grains of 10cm or more are formed. Recrystallization often occurs.

This problem was solved by increasing the C content and rapidly increasing the volume ratio of the γ phase upon reheating, and transforming the α phase into fine grains by nucleating at the grain boundaries of the γ phase during controlled cooling after hot rolling.

As shown in FIG. 1, the carbon added in the manufacture of the high magnetic flux density oriented electrical steel sheet is increased in the Fe + Si alloy, the α + γ region is expanded, the nitrogen solubility of the γ phase is very large compared to the α phase 1000 If the γ phase is present at a temperature above ℃, in the controlled cooling process after hot rolling into the α phase in the temperature range of 1000 ℃ ~ 800 ℃, the solid solubility of nitrogen is rapidly reduced to precipitate a fine AlN.

However, the added carbon causes magnetic aging in the final product, so it must be decarburized to 30 ppm or less by heat treatment.

In the manufacture of grain-oriented silicon steel sheet, as described above, in order to reduce the coarse grain size in the continuous casting and hot rolling processes, the α↔ γ transformation is used, and secondary recrystallization is induced by suppressing the grain growth during the first recrystallization. As an inhibitor, AlN and MnS should be finely dispersed and dispersed. In this case, the difference in the solubility of N in austenite and ferrite is also used, and the solubility of MnS in reusing the coarsened MnS during annealing before hot rolling is also C. It is closely related to the amount added.

Therefore, in the existing method for producing oriented electrical steel sheet, these precipitates must be subjected to hot rolling, preheating, and controlled cooling to distribute the precipitates in appropriate sizes (about 200 to 300Å) and fractions.

In addition, to obtain a fine AlN distribution, C should be added up to about 0.06 wt%. In addition, the increase in the C content is closely related to the Si content.

It is well known that increasing the Si content in grain-oriented electrical steel increases electrical resistance and reduces iron loss.

However, when 4 wt% or more of Si is added, the ductility of the material is drastically reduced, which makes it difficult to cold roll. Therefore, Si of up to about 3 wt% is usually added.

Takahashi et al. Explained why it is difficult to increase the Si content in connection with the C content. In order to make AlN act as an inhibitor in the manufacture of conventional Hi-B grade oriented electrical steel sheets, the austenite fraction is 40-50 vol. In this case, when Si is 3.25wt%, C should be added 0.07 ~ 0.08wt%.

If Si content is increased, about 0.1wt% of C must be added to add 4wt% Si, and if more than 0.08wt% of C is added, decarburization is insufficient even after decarburization, resulting in magnetic aging in the final product. It could happen.

Therefore, in recent years, the necessity of not only improving the magnetic properties of oriented electrical steel sheets but also reducing manufacturing costs by shortening the manufacturing process is greatly demanded.

The vertical twin roll sheet casting method is a continuous casting process that has been intensively studied in recent years to reduce production costs and develop new materials. It uses a roll or drum type continuous casting apparatus that is water-cooled to have a thickness of 1-10 mm that is almost the same as that of the final product. Because it can be cast, it is possible to greatly reduce or omit the hot rolling process part in the existing manufacturing process, and it is quenched by the water-cooling roll during casting, so that a fine structure distribution can be obtained and the solute of the solute can be increased. to be.

Examples of this kind of technology include those described in JP-A-58-157250, JP-A-60-184449, JP-A-62-130749 and the like.

To date, its applicability in stainless steel has been intensively examined, and in some cases it has been put to practical use and used in commercial production.

However, in the case of electrical steel sheet, the electrical steel sheet was cast by some vertical twin roll casting method, but the Si content was 3.2wt%, similar to the general manufacturing process (EP 0 540 405 A1), or cold rolled steel in the manufacture of high silicon steel of 3.5wt% Si or more. No mention is made of post processes such as heat treatment (Stahl und Eisen 111 (1991) Nr. 2 61-66).

In the case of using the rapid solidification method for the production of grain-oriented electrical steel sheet has been registered by European Patent 0 390 160 A1 in 1990 and US Patent 5,049,204 in 1991 by Mizoguchi et al in Japan, and US Patent 5,417,772 in 1995 by Bavay et al in France. Registered.

EP-A-0 390 160 (or US Patent 5,049,204) regulates the secondary cooling rate of strips solidified from molten metal to induce secondary recrystallization to form goss grains with a {110} <001> orientation. It was.

However, US Pat. No. 5,417,772 attempts to form goss grains on the surface of the strip by adjusting the heat exchange coefficient between the roll surface and the molten metal rather than the secondary cooling rate.

EP-A-0 390 160 (or US Pat. No. 5,049,204) manufactures strips with a thickness of 0.7 to 3.0 mm and maintains a cooling rate of 50 ° C / sec or more, and a secondary cooling rate of 10 ° C / s or more between 1300 ° C and 900 ° C. Fats and oils; C; 0.03 to 0.10%, Si; 2.5 to 4.5%, Mn; 0.2 to 0.15%, S: 0.01 to 0.05%, acid soluble Al: 0.01 to 0.04%, N: 0.003 to 0.015% and A process of cold rolling, decarburization, coating, and secondary recrystallization heat treatment of up to 80% with one or more intermediate annealing with one or more intermediate annealing is proposed. In US Patent 5,417,772, thickness of 5mm or less, C: 0.01 to 0.1%, Si: 2% or more, maintaining the roll average temperature of 400 ° C or less, heat exchange coefficient between the roll and the metal of 0.1cal / cm ² s ° C or more, strip width It is proposed to process cold rolling, decarburization, coating, and secondary recrystallization heat treatment of up to 80% with a rolling force of 50 kgf / mm or less and one or more intermediate annealing.

In the above two inventions, the C content is added at least 0.01% in order to refine the grains and precipitates by using the γ↔α transformation, and the C content of 30 ppm or less is maintained in the final product through the decarburization process in the post-treatment process.

The present inventors have conducted research and experiments to solve the above-mentioned problems of the prior art, and based on the results, the present invention proposes the present invention. The purpose of the present invention is to provide a method for manufacturing a high silicon grain-oriented electrical steel sheet by strip casting, which can add up to 5.5% of Si by producing electrical steel sheet and eliminate the decarburization process because C is not added. .

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention provides a method for manufacturing a high silicon grain electrical steel sheet using a strip casting method,

At a wt%, a molten silicon steel containing C: 0.003% or less and Si: 2.0 to 5.5% was cast to a thickness of 2.0 mm or less with a melt superheat of 20 to 30 ° C. under a nitrogen atmosphere, and the surface of the strip After stripping, brushing, and cold-rolling to a thickness of 0.3 mm or less using a two-stage cold rolling method including an intermediate annealing, a strip casting method manufactured by high temperature heat treatment at 1100-200 ° C for 10-20 hours It relates to a method for producing a high silicon oriented electrical steel sheet by.

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

In the present invention, it is preferable to set the C content to 0.003% (30 ppm) or less, because the magnetic aging can be prevented without the final decarburization process, and the more preferable C content is 0.001% or less.

Even if the strip casting method is used, cold rolling is required to produce more than 85% of the final product. At this time, if the Si content is high, the ductility of the material is reduced, so that cracking occurs during cold rolling, and thus rolling is impossible.

In other words, when the content of Si exceeds 5.5%, cold rolling is impossible, so the content of Si is limited to 5.5% or less.

The lower limit thereof is not particularly limited, but in view of the magnetic properties, the lower limit of the Si content is preferably set to 2.0%.

The C and Si are set to the above-mentioned content, and the content of the remaining Al, Mn, S, and N is preferably to have a composition of a conventional Hi-B grade oriented electrical steel sheet.

More preferred composition of the molten metal in the present invention is C: 0.003% or less, Si: 2.0-5.5%, Al: 0.01-0.02%, Mn: 0.03-0.05%, S: 0.02-0.03%, and N: 0.01-0.02 It is made up of%.

Molten superheat indicates how much higher the casting takes place than the liquidus temperature (T _L ) of the molten metal.

However, in the Fe-Si alloy, T _L decreases as the Si content increases, so even if the melt superheat is 20 ° C., various temperatures may occur depending on the Si content.

In the present invention, if the temperature is less than 20 ° C, the solidification completion point during strip casting is formed so much higher than the nip point of the roll, so that excessive pressure is applied to the roll, which may damage the surface of the roll. Since the completion point is formed below the nip point of the roll, in the case of the Fe-Si alloy having a low temperature strength, there is a fear that the strip is broken during casting.

Therefore, in this invention, it is preferable to set melt superheat degree to 20-30 degreeC.

In the present invention, the initial casting thickness is set in consideration of the final thickness that can be produced only by cold rolling of 85% or more without hot rolling, and is set to 2.0 mm or less, preferably 1.6 mm or less.

Strip casting is preferably carried out in a nitrogen atmosphere.

In the case of strip casting as described above, the grain size of the cast strip can be controlled to 200 μm or less, and fine AlN and MnS of 300 μm or less can be deposited.

Cold rolling is preferably performed by a two-stage cold rolling method including intermediate annealing, and cold rolling is preferably performed at a thickness of 0.1 to 0.3 mm.

The high temperature heat treatment step is preferably performed at 1100-1200 ° C. for about 10-20 hours because the secondary recrystallized goth grains are sufficiently grown when the high temperature heat treatment is performed under these conditions.

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

Example 1

Using a vertical twin roll strip caster, a strip of 1.6 mm thick is made of a hard-working alloy consisting of C: 0.001%, Si: 4.5%, Al: 0.014%, Mn: 0.05%, S: 0.013%, and N: 0.015%. Was cast. The molten metal superheat was set at 20 deg. The cast strips were made into cold rolled specimens of 0.25 mm thickness using intermediate annealing and two stage cold rolling. At this time, the intermediate annealing was performed at 1050 ° C. for 5 minutes in a dry hydrogen atmosphere. The cold rolled strip was subjected to secondary recrystallization heat treatment at 1200 ° C. for 20 hours at 100% H ₂ atmosphere for 20 hours, and the structure was observed and the orientation of the crystal grains was observed as the pole, and the results are shown in FIG. 5.

When the alloy liquid phase contacts the rotating water-cooled mold during strip casting, a cell-shaped solidification layer is formed on the surface of the roll by rapid solidification. At the same time, the liquid layer having a predetermined thickness in front of the solidification cell has a liquidus temperature. Under cooling (T _L ) or less, the isotropic dendritic phases floating in the supercooled liquid layer grow together at the front of the cell.

The thickness of the cell layer which solidifies first on the surface of the roll is about 70 to 80 µm, and no initial solidification structure in dendritic form was observed in the crystal grains in this layer, and initial solidification structure was observed in equiaxed dendritic crystal grains.

As shown in (a) and (b) of FIG. 2, the liquid phase having low melt superheat [FIG. 2 (a)] is the thickness of the liquid layer supercooled than the liquid having high superheat [FIG. 2 (b)]. Since is thickened, the equiaxed crystal dendrite is widely distributed.

The temperature distribution in the thickness direction of the strip casting process has the highest temperature at the center layer and the lower temperature toward the surface portion, so that when the equiaxed crystal grains grow and the coagulation layer leaves the supercooled liquid layer, the temperature of the liquid phase becomes higher than the solid phase. As the heat flow changes from the liquid phase to the solid phase, the dendritic phase of the columnar tablet begins to grow from the solidified liquid interface.

The growth of the columnar dendritic phase stops as it meets the columnar growth on the opposite side of the solidification point.

The change of the melt superheat changes the position of the solidification point and the rate of cross-sectional reduction of the cast strip is determined by the change of position of the solidification point.

Due to the higher solidification completion point at 20 ° C, the cross-sectional reduction rate was high, and as a result of the high temperature deformation and dynamic recrystallization, crystal grains were formed over the entire thickness of the specimen.

The texture of the strip was analyzed for the case where the melt superheat was 20 ° C., and the results are shown in FIG. 3.

As shown in FIG. 3, when the melt superheat is 20 ° C., a goth texture is developed in the subsurface layer (0.8 point of thickness). In the BCC metal, the goth texture is known as a texture developed by shear deformation.

Therefore, in the melt superheat at 20 ° C., a goth texture was formed by shear deformation due to friction with the roll in the surface layer as a condition of high temperature deformation of the strip as described above.

Therefore, at 20 ° C molten superheat, goth grains which could act as nuclei of secondary recrystallization were formed by hot deformation during strip casting.

As shown in Fig. 5 (a), the size of the crystal grains after the final heat treatment has a few cm, indicating that secondary recrystallization has occurred by the present invention.

In FIG. 5 (b), the + position of the pole figure is within an error range of 5 ° at a position indicating a goth orientation ({110} <001>), which is, according to the present invention, the nuclei of the secondary recrystallization. It can be seen that a goth grain can be formed which can act as.

Therefore, in the present invention, it is possible to obtain the final secondary recrystallized goth texture even if the C content is controlled to 30 ppm or less from the initial molten metal to the alloy component.

As mentioned above, the role of C in the manufacture of oriented electrical steel sheets can be replaced by the rapid solidification effect of strip casting, so it is necessary to add C, which is harmful in the final product and undergoes decarburization, as in the conventional manufacturing process. none.

Therefore, in the present invention, the decarburization process performed before the secondary recrystallization heat treatment in the existing manufacturing process may be omitted by controlling the C content to 30 ppm or less from the thin casting process of the molten metal.

Example 2

A molten metal consisting of C: 0.001%, Si: 4.5%, Al: 0.014%, Mn: 0.05%, S: 0.013%, and N: 0.015% is produced by strip casting with molten iron at a temperature of 30 ° C. at a thickness of 1.6 mm. The prepared strips were made into cold rolled specimens of 0.25 mm thickness using intermediate annealing and two stage cold rolling. At this time, the intermediate annealing was performed at 1050 ° C. for 5 minutes in a dry hydrogen atmosphere. The cold rolled strip was subjected to secondary recrystallization heat treatment at 1200 ° C. for 20 hours at 100% H ₂ , followed by high temperature heat treatment. As a result of observing the structure photograph and the pole figure, the same result as in Example 1 was obtained.

In case of strip casting with molten superheat of 30 ℃, since the solidification completion point is located much closer to the nip point of the roll than in case of molten superheat of 20 ℃, since the strip reduction rate is low, the strip deformation is small. .

As shown in FIG. 2B, when the molten metal superheat is high, the thickness of the supercooled liquid layer is narrow. Since the columnar dendritic phase develops because the equiaxed crystal grains do not grow significantly and escape from the supercooled liquid layer, the size of the layer where the equiaxed crystals appear is very small and there is little difference in size of the crystal grains in the isometric dendritic layer. Instead, the columnar dendrite grows very large.

The texture of the strip was analyzed for the case where the molten metal superheat was 30 ° C., and the results are shown in FIG. 4.

As shown in FIG. 4, when the molten metal superheat is 30 ° C., the surface part shows a disordered aggregate form as a result of macroscopic analysis of the aggregate, but in the analysis of the micro aggregate using EBSD (Electron Back Scattered Diffraction) Goth grains were found between disordered equiaxed grains, and they were also found to act as Goss nuclei, secondary recrystallization in secondary recrystallization heat treatment after cold rolling.

In the present invention, by producing a grain-oriented electrical steel sheet under appropriate conditions using the strip casting method, fine precipitates can be precipitated, so that the decarburization process can be omitted without adding C, and the grain size of the cast strip is finely controlled. Since the fraction of low angle grain boundary with low orientation difference between grains is high, crack generation is suppressed during cold working, and thus the Si content can be increased to 5.5%.

1 is α, γ phase Fe-Si state diagram showing relationship with C content

Figure 2 is a schematic diagram showing the change of the solidification layer according to the molten metal superheat when the metal is rapidly solidified, (a) is a melt superheat of 20 ℃ (b) shows a case of melt superheat of 30 ℃

FIG. 3 shows the orientation distribution function (ODF) calculated from the measured values to show the change in the texture in the thickness direction of the strip when the melt superheat is 20 ° C. FIG.

4 is an orientation distribution function (ODF) calculated from measured values to show the change in texture in the thickness direction of the strip when the melt superheat is 30 ° C.

FIG. 5 shows the surface photograph and (100) pole figure of the final secondary recrystallized heat treatment strip, (a) shows a scanning electron micrograph, and (b) shows (100) pole figure.

Claims (2)

In the production of high silicon oriented electrical steel sheet using the strip casting method, At a wt%, a molten silicon steel containing C: 0.003% or less and Si: 2.0 to 5.5% was cast to a thickness of 2.0 mm or less with a melt superheat of 20 to 30 ° C. under a nitrogen atmosphere, After pickling and brushing the surface of the strip, it is cold rolled to a thickness of 0.3 mm or less by using a two-stage cold rolling method including an intermediate annealing, and then subjected to high temperature heat treatment at 1100-1200 ° C. for 10-20 hours. Method for producing a high silicon oriented electrical steel sheet by the strip casting method characterized in that According to claim 1, wherein the composition of the molten silicon steel is C: 0.003% or less, Si: 2.0-5.5%, Al: 0.01-0.02%, Mn: 0.03-0.05%, S: 0.02-0.03%, and N: 0.01-0.02% High silicon directionality by strip casting method characterized in that Manufacturing method of electrical steel