KR100340495B1 - Method for manufacturing grain oriented electric steel sheet with high magnetic density - Google Patents

Abstract

PURPOSE: A method for manufacturing a grain oriented electric steel sheet with high magnetic density is provided, which can stabilize second recrystallization by simultaneously conducting decarburization and nitrogen enrichment. CONSTITUTION: The method includes the steps of heating a steel slab(150 to 350 mm in thickness) comprising C 0.01 to 0.04 wt.%, Si 2.90 to 3.30 wt.%, Mn 0 15 to 0.30 wt.%, sol.Al 0.010 to 0.040 wt.%, N 0.003 to 0.010 wt.%, 0.015 wt.% or less of P, 0.006 wt.% or less of S, Cu 0.30 to 0.70 wt.%, Ni 0.03 to 0.07 wt.%, Cr 0.03 to 0.07 wt.%, a balance of Fe and incidental impurities at 1130 to 1320 deg.C for 1 to 10 hours; hot rolling the steel slab to the thickness of 1.5 to 2.6 mm; annealing the hot rolled steel sheet at 850 to 1150 deg.C for 30 to 600 sec in nitrogen gas; cold rolling the steel sheet to the thickness of 0.23 to 0.35 mm; decarburizing/nitriding the cold rolled steel sheet at 700 to 950 deg.C for 30 to 600 sec in an atmosphere condition containing nitrogen gas of which dew point is 30 to 70 deg.C; and applying an annealing separator mainly comprised of MgO on the steel sheet, followed by final hot annealing. In the method, decarburization is performed until the amount of residual carbon is less than 30 ppm; the atmosphere condition containing nitrogen gas is a mixed gas of ammonia, hydrogen and nitrogen; the nitriding is performed that the total amount of nitrogen is 130-82.9x(1+£Cu wt.%+10x(Ni wt.%+Crwt.%)|2) ppm; and the final hot annealing is characterized in that it is performed in a mixed gas of hydrogen and nitrogen or dry hydrogen at a temperature elevation rate of 10 to 20 deg.C/hr to the temperature range of 1150 to 1250 deg.C, followed by soaking for 1 to 30 hrs.

Description

Manufacturing method of high magnetic flux density oriented electrical steel sheet by low temperature slab heating method

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet used as iron core materials for transformers, electric motors, generators and other electronic devices, and more particularly, by simultaneously performing decarburization and nitrogen enrichment, the manufacturing process is innovatively shortened. A method of manufacturing a high magnetic flux density oriented electrical steel sheet of low temperature slab heating method in which secondary recrystallization is stabilized.

In general, oriented electrical steel sheets have an aggregate structure (also called 'goth structure') in which the grain orientation is aligned in the (110) [001] direction, and has excellent magnetic properties in the cold rolling direction. It is used as an iron core.

The magnetic properties of oriented electrical steel are mainly represented by magnetic flux density and iron loss. The dual magnetic flux density generally represents the magnetic flux density (B ₁₀ ) induced in the iron core by the magnetic field of 1000 A / m. If the magnetic flux density is high, less iron core material is used. Even if the same performance can be exhibited there is an advantage that can be miniaturized electrical equipment. In addition, the iron loss means the energy loss wasted inferior in the iron core when the magnetic flux density of 1.7 Tesla is obtained by the alternating frequency of 50Hz, the lower the iron loss (W _17/50 ) value of the energy of the electric equipment The efficiency is increased. Therefore, in order to reduce the size and energy saving of electrical equipment, there is an increasing need for a oriented electrical steel sheet having high magnetic flux density and low iron loss.

On the other hand, the (110) [001] texture of the grain-oriented electrical steel sheet is obtained by using the secondary recrystallization phenomenon. Among the fine grains produced by the normal primary recrystallization, the grain of a specific orientation, called the Goss orientation The grain with the orientation of (110) [001] (commonly referred to as the 'nucleus of secondary recrystallization') is an abnormal growth of the entire steel sheet. It is known that a high magnetic flux density is obtained when such secondary recrystallization takes place completely and its orientation is excellent.

In order to stabilize the secondary recrystallization, not only the primary recrystallized grains are uniform in size, but also the orientation of the primary recrystallized grains (hereinafter referred to as 'primary recrystallized grain structure') is eroded into the core of the secondary recrystallization. In the process of growth of secondary recrystallization, the secondary recrystallization should grow in adherence to the ideal [001] direction. That is, it is known that it should be advantageous to develop secondary recrystallized grains having excellent directionality.

For this purpose, it is necessary to suppress the growth of primary recrystallized grains before proper alloy design and consequent process control, especially secondary recrystallization. In other words, the amount and size of precipitates, such as MnS, MnSe, AlN, and Cu ₂ S, which are used as growth inhibitors of the primary recrystallized grains (hereinafter referred to as 'grain growth inhibitors'), and the distribution thereof are well controlled, and are several hundred to 2000 microns in size. It is necessary to ensure the grain growth inhibiting force by ensuring that the fine precipitates and the like are uniformly distributed as much as possible.

In order to control the precipitate distribution, an appropriate amount of precipitate forming element is added in the steelmaking step, and the coarse precipitate formed in the slab after continuous casting is completely dissolved by high temperature slab heating, and the precipitates are fine and uniform in the subsequent hot rolling process. BACKGROUND OF THE INVENTION A method for manufacturing a conventional grain-oriented electrical steel sheet that is controlled so as to be distributed is known.

In the manufacturing process of the conventional grain-oriented electrical steel sheet, as described above, for the solid solution of the precipitate, slab heating for about 5 hours is required at a high temperature of about 1400 ° C. before hot rolling. In other words, during the high temperature slab heating process, an oxide reaction with air is formed on the surface of the slab, and an oxide called fayalite, which is a combination of Si and Fe, is formed, and the oxide has a low melting point. Melting from slag occurs. At this time, the molten water is designed to flow outward, but some of it accumulates in the support in the heating furnace and solidifies at the end of work, which requires internal repair for removing scales. , Cost reduction, productivity reduction, and cost increase. Therefore, a very large benefit can be expected if it is possible to heat the slab at a temperature below 1320 ° C., at which the slab does not melt.

Efforts to lower the slab heating temperature have been vigorously racing around advanced manufacturers, and the main methods are largely divided into two. That is, the method of adjusting the steel component so that the precipitate, which is a grain growth inhibitor, can be employed even in low temperature slab heating, and the low temperature slab heating method through additional precipitate management in the subsequent process without removing the precipitate from the slab heating. Are known.

As such a conventional technique, there is a technique proposed by the inventors of Nobu Yugi, et al. In Korean Patent Publication No. 89-8334. The above proposal is to produce high magnetic flux density oriented electrical steel sheet capable of low temperature slab heating by loading nitrogen into the steel sheet during the manufacturing process to reinforce precipitates such as AlN. However, according to this proposal, additional facilities are needed because nitrogen enrichment (or sedimentation) annealing must be additionally performed after decarbonization, and if the existing equipment is used, the manufacturing process is complicated, such as two annealing. There is a problem of rising manufacturing costs.

Accordingly, the present inventors have studied the present invention at various angles in order to shorten the manufacturing process and prevent the increase in manufacturing cost, and thus, the present invention has been proposed on the basis that the above-mentioned conventional problems can be solved by performing decarbonization annealing and annealing at once. .

That is, the present invention provides a method capable of stably manufacturing high magnetic flux density oriented electrical steel sheet in a shorter manufacturing process while enabling reheating at low temperature.

In the present invention for achieving the above object, in the method of manufacturing a high magnetic flux density oriented electrical steel sheet,

In mid-range, C: 0.01-0.04%, Si: 2.90-3.30%, Mn: 0.15-0.30%, Acid-soluble Al: 0.010-0.040%, N: 0.003-0.010%, P: 0.015% or less, S: 0.006 Steel slab consisting of% or less, Cu: 0.30-0.70%, Ni: 0.03-0.07%, Cr: 0.03-0.07% and the balance Fe and other unavoidable impurities are heated at a temperature of 1130-1320 ° C. for 1-10 hours. Then, hot rolling, pre-annealed hot rolled, cold rolled once to make a cold rolled plate; Dew point is 30-70 ° C .; Decarbonization and nitriding at a temperature of 700-950 ° C. for 30 seconds to 10 minutes in a nitrogen-containing gas atmosphere; Subsequently, after applying an anti-deposition agent containing MgO as a main component, it is comprised including final finishing annealing.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is characterized in that decarburization and nitriding are performed simultaneously (hereinafter referred to as 'simultaneous decarburization and annealing') to ensure stable secondary recrystallization.

According to the present invention, when simultaneous decarbonation annealing of an existing AlN-based high magnetic flux density oriented electrical steel sheet containing about 0.045-0.065% of carbon is possible, an appropriate amount of nitrogen enrichment is possible, but sufficient decarburization is not achieved within a short time. Control is required.

However, if the amount of carbon is added less than usual, the microstructure of the hot rolled plate becomes nonuniform, and as a result, the primary recrystallized microstructure is also non-uniform after co-denitrification annealing. Even if it is secured, the secondary recrystallization development becomes unstable, and high flux density characteristics are not obtained.

The present inventors conducted numerous experiments to prevent the non-uniform distribution of the microstructure of the primary recrystallized grains according to the carbon reduction, and when the appropriate amount of nitrogen is hatched according to the appropriate addition amount of Cu, Ni, Cr, uniform primary recrystallization It was found that tissue was obtained.

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

First, when C is less than 0.01% of steel slabs, grains grow coarsely during slab heating, and the development of secondary recrystallization becomes unstable at the time of the final high temperature annealing. It is not preferable because it takes a long time.

Si is a basic component of electrical steel sheet, which lowers the iron loss by increasing the resistivity of the material, but it is preferable because the iron loss property is bad at an addition content of less than 2.9%, and the cold rolling property is deteriorated when the addition content is more than 3.3%. .

Mn is an element that forms austenite on the slab to facilitate the solid solution of AlN. If it is added less than 0.15%, Mn is not good because the amount of austenite is too small. If the content exceeds 0.30%, the rolling load is too high. It is not good because the plate shape becomes uneven.

Acid-soluble Al is an element necessary for the formation of AlN precipitates. When acid-soluble Al is less than 0.010%, the direction of secondary recrystallization deteriorates and the magnetic flux density is lowered. When it exceeds 0.04%, the development of secondary recrystallization becomes unstable. not.

If N is less than 0.003%, the amount of AlN is insufficient, and if it exceeds 0.010%, it is not preferable because defects in the form of blisters tend to occur in the product.

P is limited to less than 0.015%, which can be controlled without causing an increase in cost in steelmaking, as it may cause plate breakage during cold rolling in more cases than usual.

When S is excessively added, the S segregation in the center of the slab becomes severe, and in order to homogenize it, it is preferable to add the slab at 0.006% or less since the slab needs to be heated to a temperature of the present invention.

Cu, Ni, and Cr are important components added not only to homogenize the microstructure of the hot rolled sheet due to carbon reduction but also to make the primary recrystallized microstructure of the simultaneous decarbonation annealing plate uniform. The addition amounts are 0.3-0.7%, 0.03-0.07%, and 0.03-0.07%, respectively. If any element is added below the lower limit of the addition amount, the first recrystallization microstructure uniformity effect after co-denitrification annealing is insignificant. As a result, secondary recrystallization becomes unstable, leading to deterioration of magnetic properties. In addition, when the upper limit of each component range is exceeded, the effect is not large and rather it is not preferable because it is difficult to decarburize (Cu, Cr), or cost increase due to the addition of expensive alloy (Ni).

The steel component of this invention is as above-mentioned, and consists of the remainder Fe. In addition, inevitable elements (B, Ti, Nb, V) mixed from raw materials during steelmaking may be contained in a trace amount (less than 80ppm). In addition, the silicon steel material as described above can be used as the material of the present invention even when manufactured using any conventional melting method, ingot method, performance method, or the like.

If the thickness of the silicon steel slab made of such a steel component is too thin, hot rolling productivity is lowered, and if the thickness is too thick, the slab heat time should be long, so it is more preferable to control it to 150-350 mm.

When the silicon steel slab is heated before hot rolling, when the heating temperature is less than 1130 ° C., the roll force becomes excessive during hot rolling, which makes it difficult to control the plate shape, and when the temperature exceeds 1320 ° C. Since the temperature is too high to increase the amount of oxidative scale, the slag may be melted and thus excluded from the slab heating temperature range of the present invention. At this time, it is better to heat for 1-10 hours in consideration of cracks and economics to the inside of the slab.

After the hot rolling in a conventional manner, the thickness of the hot rolled sheet at this time is preferably set to 1.5-2.6mm in consideration of the final cold rolling thickness.

After hot rolling as described above, pre-annealed the hot rolled sheet, which is then followed by simultaneous decarbonization annealing for 30 seconds at 850-1150 ° C. in consideration of the formation of primary recrystallized structures of appropriate particle size and the prevention of coarsening of the AlN nitride precipitates. For 10 minutes, it is preferable to use a nitrogen atmosphere to suppress the loss of the precipitate. If the pre-annealed at a temperature and time less than the pre-annealed range of the hot rolled sheet of the present invention, the primary recrystallized grain size becomes fine, and as a result, the secondary recrystallization cannot be developed at a high temperature, and thus excellent magnetic flux density cannot be obtained. Pre-annealing at excess temperatures and times is undesirable because secondary recrystallization becomes unstable due to coarsening of AlN.

The preannealed plate is cold rolled once, and the thickness of the cold rolled plate is preferably 0.23-0.35mm. This is because secondary recrystallization is not well developed when the thickness of the final cold rolled plate is less than 0.23 mm, and it is not preferable because the vortex iron loss property becomes worse when it exceeds 0.35 mm.

The cold rolled cold rolled plate as described above is preferably subjected to simultaneous decarbonation annealing using a mixed gas atmosphere of nitrogen containing a dew point of 30 to 70 ° C. for 30 seconds to 10 minutes at a temperature of 700 to 950 ° C. The reason is that when the annealing temperature is less than 700 ° C or less than 30 seconds, decarburization and nitrogen enrichment are insufficient. If the annealing temperature is higher than 950 ° C, the primary recrystallization structure is too coarse, which makes the secondary recrystallization unstable and thus excellent magnetic flux density. Because you can't get it. In addition, when the annealing time exceeds 10 minutes, the annealing time is long, which is not good because it is uneconomical.

The atmospheric gas may be a nitrogen-containing gas atmosphere for nitrogen enrichment. More preferably, a mixed gas atmosphere of ammonia + hydrogen + nitrogen, which is industrially easy to control decarburization amount and nitrogen enrichment amount, may be used.

At this time, if the dew point of the atmosphere gas is too low, it is not good to increase the annealing time due to the decarburization. Therefore, it is limited to the range of 30-70 ℃.

When performing simultaneous decarbonation annealing as described above, it is necessary to lower the residual carbon amount to 30 ppm or less. That is, if the residual carbon exceeds 30ppm, the secondary recrystallization formed during subsequent high temperature annealing deteriorates, so that excellent magnetic flux density cannot be obtained, and magnetic aging occurs as a product of transformers, resulting in deterioration of iron loss characteristics.

As described above, nitrogen enriched by co-denitrification annealing reacts with excess acid-soluble Al, Cu, Mn, Si, etc. in the steel in the low temperature region during subsequent high temperature annealing to form additional precipitates. The grain growth inhibitory power obtained varies depending on the distribution state such as size.

Therefore, in order to secure an appropriate grain growth inhibiting force, the total nitrogen content in the steel sheet is preferably in the range of 130 to 82.9 x (1 + [ Cu % + 10 x ( Ni % + Cr %) ² ) ppm. That is, less than 130ppm is not preferable because the required minimum precipitates are not formed and the grain growth inhibition is insufficient and the secondary recrystallization becomes unstable. In addition, when the total nitrogen content exceeds 82.9 × (1 + [ Cu % + 10 × ( Ni % + Cr %)] ² ) ppm, not only the primary recrystallization structure is formed unevenly but also coarsening of precipitates during the high temperature annealing It is not preferable that the growth of secondary recrystallization becomes unstable because the growth of the growth rate is not maintained up to a high temperature, so that excellent magnetic flux density is not obtained. At this time, the upper limit of the total nitrogen amount is determined in relation to the content of Cu, Ni, Cr, because Cu, Ni, Cr acts to uniformly distribute the primary recrystallized structure.

After the simultaneous decarbonation annealing as described above, the final finishing annealing, the temperature rise rate is raised to 1150-1250 ℃ by controlling in the range of 10-20 ℃ / hr to fully improve the secondary recrystallization, the orientation It is necessary to crack at this temperature for 1-30 hours. At this time, when the temperature of finishing high temperature annealing is less than 1150 ° C. or the cracking time is less than 1 hour, it is difficult to form a good glass film and remove impurities, and it is uneconomical when the temperature exceeds 1250 ° C. or the time exceeds 30 hours. Since it was excluded from the scope of the present invention.

As the atmospheric gas for high temperature annealing, it is preferable to use dry hydrogen or a mixed gas of hydrogen and nitrogen to remove residual impurities such as N and S after the formation of the glass coating and the completion of the secondary recrystallization.

On the surface of the steel sheet on which the inorganic glass coating is formed by the high temperature annealing, a normal tension coating may be applied after the high temperature annealing for the purpose of improving the insulation property and improving the iron loss by the micronization.

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

EXAMPLE

C: 0.019%, Si: 3.20%, Mn: 0.24%, Acid Soluble Al: 0.018%, N: 0.0055%, S: 0.005%, P: 0.015% By weight ratio Cu, Ni, Cr are as shown in Table 1 below. The addition amount was varied, and a slab of 250 mm thickness composed of the remaining Fe was prepared. The slab was heated at a temperature of 1150 ° C. for 4 hours 30 minutes and then hot rolled to form a 2.0 mm thick hot rolled sheet. Subsequently, the hot rolled sheet was preannealed in a nitrogen gas atmosphere at 950 ° C. for 3 minutes, followed by pickling. A final cold rolled sheet having a thickness of 0.285 mm was obtained by one rolling. Thereafter, decarburization and nitrogen enrichment were carried out using a wet ammonia + hydrogen + nitrogen mixed gas atmosphere having a dew point of 45 ° C. for 3 minutes at 900 ° C., and co-denitrification annealing was carried out to form a primary recrystallized structure.

At this time, in order to change the total nitrogen amount of the steel sheet as shown in Table 1 as the atmosphere gas, ammonia (NH ₃ ) having a different concentration in the range of 0.005-10% by volume and hydrogen having a different concentration in the range of 5-80% A mixed gas consisting of (H ₂ ) and the remaining nitrogen (N ₂ ) was used. Subsequently, a fusion inhibitor containing MgO as a main component was applied to the surface of the steel sheet, followed by finishing high temperature annealing. At this time, the finishing high temperature annealing was performed by a heat treatment cycle of heating up to 1200 ° C. at a temperature rising rate of 20 ° C./hr at 15 ° C. to raise secondary recrystallization, and cooling after cracking for 15 hours. 25% N ₂ + 75 H ₂ was used as an atmospheric gas during temperature rising. It was. After the temperature was raised to 1200 ° C., pure hydrogen gas was used.

Residual carbon content, total mass, uniformity of primary recrystallized microstructure after co-denitrification annealing, and secondary recrystallization rate, , And the magnetic flux density is shown in Table 1 below.

Here, the uniformity of the primary recrystallized microstructure was determined by observing the cross section of the specimen subjected to simultaneous decarbonation annealing with an optical microscope by etching with 3% nital after polishing. The determination method at this time compared the area of grains grown in the thickness direction with the grains of isotropic crystals per 1 mm 2 of the specimen and made uniform if the area of the grains of the isotropic crystals was large, or nonuniform. The secondary recrystallization rate was measured by observing the macrostructure exposed by corrosion of the surface of the finished hot-annealed steel sheet with 20% hydrochloric acid solution warmed to about 80 ° C. In addition, the magnetic flux density is a value obtained by measuring B ₁₀ (magnetic flux density induced at an excitation force of 1000 A / m) with a single-plate magnetometer.

As shown in Table 1, the addition amount of Cu, Ni, and Cr is within the scope of the present invention, and the total nitrogen after simultaneous decarbonation annealing is 130 to 82.9 x (1 + [ Cu % + 10 x ( Ni % + Cr %). In the case of the invention material (1-8) controlled in the range of ² ), a uniform primary recrystallization structure and precipitates such as AlN distributed in appropriate sizes and amounts are obtained, and not only secondary recrystallization occurs completely. The directionality was also improved, showing an excellent magnetic flux density value.

On the other hand, in the case of the comparative material (1,3,5) having a total nitrogen content of less than 130 ppm after co-denitrification annealing, the secondary recrystallization was unstable because an appropriate amount of grain growth inhibition was not obtained.

Further, even when the total nitrogen amount was controlled within the scope of the present invention, the comparative material (7-9) added with any one of the three elements of Cu, Ni, and Cr below the scope of the present invention had uneven primary recrystallization structure. As the result of the in-vehicle crystallization unstable, the low magnetic flux density was shown.

In addition, although the secondary material crystallization completely occurs in the comparative material (10-11) in which the addition amount of Cu and Cr exceeds the scope of the present invention, decarburization deteriorates (more than 30 ppm of residual carbon), and its orientation is thermally decomposed, thus excellent magnetic flux density Couldn't get it.

As described above, according to the present invention, low temperature slab heating is possible, excellent magnetic flux density can be obtained, and in particular, there is a useful effect of shortening the manufacturing process compared with the prior art.

Claims (9)

In the manufacturing method of high magnetic density oriented electrical steel sheet, Weight%, C: 0.01-0.04%, Si: 2.90-3.30%, Mn: 0 15-0.30%, acid soluble Al: 0.010-0.040%, N: 0.003-0.010%, P: 0.015% or less, S: 0.006 Steel slab consisting of% or less, Cu: 0.30-0.70%, Ni: 0.03-0.07%, Cr: 0.03-0.07% and the balance Fe and other unavoidable impurities are heated at a temperature of 1130-1320 ° C. for 1-10 hours. And then, hot rolled, hot rolled plate hot rolled annealing, and then cold rolled once to make a cold rolled plate; Decarbonization annealing at 30-70 ° C. for 30 seconds to 10 minutes at a temperature of 700-950 ° C. in a nitrogen-containing gas atmosphere; Subsequently, after applying an anti-deposition agent containing MgO as a main component, a high temperature-density oriented electrical steel sheet of a low-temperature slab heating method comprising a high temperature finish annealing. The method of manufacturing a high magnetic flux density oriented electrical steel sheet according to claim 1, wherein the steel slab has a thickness of 150-350 mm. The method of manufacturing a high magnetic flux density oriented electrical steel sheet according to claim 1, wherein the hot rolling is performed such that the thickness of the hot rolled sheet is 1: 5-2.6 mm. The method of claim 1, wherein the pre-annealing of the hot rolled sheet is performed in a nitrogen gas atmosphere at a temperature of 850-1150 ° C. for 30 seconds to 10 minutes. The method of manufacturing a high magnetic flux density oriented electrical steel sheet according to claim 1, wherein the cold rolled sheet has a thickness of 0.23-0.35 mm. The method of manufacturing a high magnetic flux density oriented electrical steel sheet according to claim 1, wherein the decarbonization annealing is performed even if the residual carbon content is 30 ppm or less. The method of manufacturing a high magnetic flux density oriented electrical steel sheet according to claim 1, wherein the nitrogen-containing atmosphere gas is a mixed gas of ammonia + hydrogen + nitrogen. The low temperature slab heating method according to claim 7, wherein the nitriding treatment is performed so that the total nitrogen amount is 130 to 82.9 × (1+ [Cu% + 10 × ( Ni % + Cr %)] ² ) ppm. Method for producing fast density oriented electrical steel sheet. The method of claim 1, wherein the finishing high temperature annealing is heated to 1150-1250 ℃ at a rate of 10-20 ℃ / hr in dry hydrogen or a mixed gas atmosphere of hydrogen and nitrogen, and the low temperature characterized in that cracking for 1-30 hours Method for manufacturing high magnetic flux density oriented electrical steel sheet of slab heating method.