KR20000041670A - Method of manufacturing oriented electrical steel sheet of low-temperature slab heating style excellent in coating - Google Patents

Abstract

PURPOSE: A method of manufacturing an oriented electrical steel sheet of low-temperature slab heating style is provided to have excellent coating and magnetic characteristic at the same time. CONSTITUTION: A silicon steel slab consists of, by weight %: 0.02-0.08% of C, 2.90-3.30% of Si, 0.15-0.30% of Mn, not more than 0.006% of S, 0.010-0.040% of acid soluble Al, not more than 0.006% of N, Fe and incidental impurities. The silicon steel slab is heated at a temperature of 1100-1250°C and hot-rolled. After that, the silicon steel slab is pre-annealed and cold-rolled once to manufacture a cold-rolled sheet. The cold-rolled sheet is decarbonization annealed to have 50-120ppm of nitrogen mass and coated by an annealing separator. The cold-rolled sheet passes through final annealing of process the sheet at a temperature of 550-700°C for 5-20 hours and then at a temperature of 1030-1060°C for 5-15 hours.

Description

Method for manufacturing oriented electrical steel sheet of low temperature slab heating method with excellent film characteristics

The present invention relates to a method for producing a grain-oriented electrical steel sheet by low temperature slab heating, and in particular, a good glass film is obtained by appropriately controlling the amount of formation of the primary recrystallization growth inhibitor and the finishing high temperature annealing method under nitrogen loading. The present invention relates to a method for producing a grain-oriented electrical steel sheet capable of stably obtaining characteristics and excellent magnetic properties.

A grain-oriented electrical steel sheet has an aggregate structure in which the grain orientations are aligned in the (110) [001] direction, which has excellent magnetic properties in the cold rolling direction. Magnetic properties of oriented electrical steel are mainly represented by magnetic flux density and iron loss. The magnetic flux density is magnetic flux density B ₁₀ induced in the core by the magnetic field of 1000A / m, B10, and iron loss is 1.7Tesla by alternating frequency of 50Hz. The energy loss, W _17/50 , wasted inferiorly within the iron core when the magnetic flux density of is obtained. The use of materials with high magnetic flux density makes it possible to manufacture small and high-performance electrical devices. The less iron loss, the greater the loss of electrical energy.

Since the invention of the method for producing a grain-oriented electrical steel sheet by cold rolling by N.P.Goss, many improvements have been made in improving magnetic properties. It is no exaggeration to say that the history of research on oriented electrical steel sheet is the history of iron loss reduction efforts. Major improvements include thinning the product, or irradiating a laser to the product to finer the magnetic domain. However, all these methods are methods to improve the iron loss characteristics of the material, and the iron loss characteristics after making several sheets of iron cores are greatly influenced by the insulation between the steel sheets and the steel sheets. In order to improve insulation, it depends on how well the glassy film is formed on the surface of the steel sheet during finishing high temperature annealing during the production of grain-oriented electrical steel sheet. That is, a good glassy film having good adhesion and uniformity and a small surface roughness may be combined with a tension coating layer additionally applied in a subsequent process to exhibit excellent insulation.

On the other hand, the (110) [001] texture is obtained by using a secondary recrystallization phenomenon. Secondary recrystallization is a grain of a specific orientation, which is called a Goss orientation, among the fine grains produced by ordinary primary recrystallization. Abnormal growth of grains (nuclei of secondary recrystallization) with an orientation of (110) [001] is called, and the magnetic properties are improved when such secondary recrystallization occurs completely and its orientation is excellent. It is known.

In order to stabilize the secondary recrystallization, the primary recrystallized grains are uniform in size, and the orientation of the primary recrystallized grains (hereinafter referred to as 'primary recrystallized aggregate') is not only encroached on the nucleus of the secondary recrystallization but also the secondary. It is known that in the process of recrystallization, secondary recrystallization adheres to the ideal [001] direction, i.e., it should be advantageous to develop secondary recrystallized grains with good directionality. To achieve this goal, proper alloy design and appropriate process control are therefore required.

Equally important, it is necessary to suppress the growth of primary recrystallized grains before secondary recrystallization occurs. As a grain growth inhibitor for this, precipitates such as MnS, MnSe, AlN, Cu ₂ S, and the like are known. In general, the stronger the grain growth inhibitory effect of the precipitates, the more stable the secondary recrystallization. However, in order to obtain strong grain growth inhibition, the amount, size, and distribution of the precipitates should be well controlled, and it is known that the grain growth inhibitory force increases accordingly if the fine precipitates of several hundred to 2000 microns are uniformly distributed as much as possible. have.

In the case of conventional grain-oriented electrical steel sheet manufacturing for proper control of 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 slab heating, followed by a hot rolling process. The emphasis is on controlling the precipitates to be finely and uniformly distributed. To this end, in the conventional process, the slab heating should be performed at about 1400 ° C. for about 5 hours. At this time, the surface of the hot slab is formed of an oxide called Payalite, in which Si and Fe are mixed by oxidation with air. The oxide has a low melting point, so that even when the slab surface temperature is about 1330 ° C, the molten metal melts from the surface. At this time, the molten slag is designed to flow outward, but part of it accumulates in the support in the furnace and requires internal repair to remove the solidification scale at the end of the work. There are significant cost burdens such as poor performance, reduced productivity, and higher costs. Therefore, if it is possible to heat the slab at a temperature of 1320 ° C. or less, which is a temperature at which the slab does not melt, very large profits can be expected.

Efforts to lower the slab heating temperature have been intensively focused on advanced manufacturers, and slab heating and hot rolling are mainly carried out by adjusting the basic component system, that is, selecting grain growth inhibitors that enable the employment of precipitates even during low temperature slab heating. Unlike the existing high temperature slab heating method that controls the precipitate in the process, the low temperature heating method is known to perform additional precipitate management in the subsequent process is known.

In other words, there are known techniques for forming a precipitate at a suitable place in the manufacturing process, rather than depending entirely on the elements included in the steel composition in forming the precipitate. As such a method, the nitriding treatment method disclosed in Japanese Patent Laid-Open No. 1-230721 and Japanese Patent Laid-Open No. 1-2-283324 is known.

The nitriding treatment method includes applying an annealing separator containing a compound having nitriding ability to a steel sheet, using a nitriding gas as an atmospheric gas during an elevated temperature of a high temperature annealing process, and a nitriding gas after cracking in a decarburization process. Nitriding a steel plate in an atmosphere, etc. are mentioned.

Among these methods, the technology applied to commercialization is appropriate grain growth by incubating an appropriate amount of nitrogen in the steel sheet (hereinafter referred to as 'precipitation') and allowing the nitride precipitates such as AlN formed by the deposition during subsequent high temperature annealing to be uniformly distributed in the steel sheet. By securing the restraining force, the secondary recrystallization is stabilized. Therefore, this is based on the premise of strictly controlling the total nitrogen amount formed in the steel sheet after sedimentation, and the appropriate total nitrogen amount at this time is known to be about 150-350 ppm.

However, in this case, there is a problem that it is difficult to obtain an excellent glassy film after finishing high temperature annealing due to the deposition. That is, the nitrogen constituting the nitride precipitate is decomposed to form a N ₂ gas after reaching the temperature at which the secondary recrystallization is completed at the time of finishing high temperature annealing, and the steel sheet in the process of high temperature annealing after purifying high temperature cracking Will come out. At this time, when the oxide layer or the initial vitreous coating formed before is not uniform, the release of N ₂ is usually unevenly released at a weak portion of the teratogenic coating, and the N ₂ discharge defect (diameter of which can be visually identified on the surface of the steel sheet) A large amount of circular base metal exposed parts of about 0.1-1 mm) are generated. This defect eventually causes very poor characteristics of the final glass coating formed after finishing high temperature annealing such as insulation and adhesion.

In practice, it is known that it is difficult to obtain a good glassy film in the case of the manufacturing method according to the above-mentioned known technology because it is difficult to uniformly make the oxide layer formed upon decarbonization or the initial glassy film formed at the beginning of high temperature annealing.

In order to solve the above problems, the present inventors have continually studied the correlation between the total nitrogen amount deposited in the decarbonization annealing process and the defect of the N ₂ outlet in the finishing annealing process. It was found that it is necessary to reduce, but if the total nitrogen is reduced, the experiment shows that the amount of grain growth inhibition is insufficient and the second recrystallization becomes unstable. Therefore, the present inventors have devised a way to complement the grain growth inhibition while reducing the total nitrogen in the process of sedimentation through continuous research and experiments and came up with the present invention based on the experimental results.

The present invention is a method of manufacturing a grain-oriented electrical steel sheet that enables low-temperature slab heating by the method of sedimentation in the decarburization process, by controlling the total amount of nitrogen loaded after sedimentation to an appropriate amount, and excellent coating by cracking at an appropriate temperature during the elevated temperature To provide a method for producing a grain-oriented electrical steel sheet of low-temperature slab heating method having a magnetic property comparable to the existing method at the same time, the object is.

Method for producing a grain-oriented electrical steel sheet of the present invention for achieving the above object, by weight, C: 0.02 to 0.08%, Si: 2.90 to 3.30%, Mn: 0.15-0.30%, S: 0.006% or less, acid solubility Heating a silicon steel slab composed of Al: 0.010 to 0.040%, N: 0.006% or less, and remaining Fe and other unavoidable impurities to a temperature of 1100 to 1250 ° C;

Rolling the heated slabs annually and then pre-annealing to form a cold rolled sheet by one cold rolling;

Immersing and decarbonizing annealing so that the total nitrogen of the cold rolled sheet is 50 to 120 ppm;

Applying an annealing separator; And

And a primary annealing at 5 to 20 hours at a temperature of 550 to 700 ° C. and a secondary crack at 5 to 15 hours at a temperature of 1030 to 1060 ° C .;

Hereinafter, the present invention will be described in more detail.

In the present invention, by controlling the total nitrogen amount by sedimentation in the decarbonization annealing process to about 120ppm or less, by applying the optimum heat treatment that can contribute to the stabilization of the secondary recrystallization by paying attention to the effect of the heat treatment cycle during the high temperature annealing, insufficient grain growth inhibition In addition, the two objectives of improving the properties of the glassy coating and stabilizing the secondary recrystallization are simultaneously achieved. The present invention is achieved by organic bonding of the components of the silicon steel and the manufacturing process, which will be described in detail below.

[Silicon Steel Slab Components]

C is not good at less than 0.02% because the grains grow coarsely upon heating the slab and the development of secondary recrystallization is unstable at the time of the final high temperature annealing.

If Si is less than 2.90%, excellent iron loss characteristics are not obtained, and if it is more than 3.30%, cold rolling property is deteriorated, which is not preferable.

Mn is an element that forms austenite in the slab to facilitate the solid solution of AlN. When Mn is added below 0.15%, Mn is not good because the amount of austenite is too small, and when it exceeds 0.30%, the roll load during rolling This is not good because it increases too much and the plate shape becomes uneven.

When S is excessively added, the S segregation in the center of the slab becomes severe and homogenizes it, so it is preferable to contain S by 0.006% or less since the slab must be heated to a temperature above the present invention.

Acid-soluble Al is an element necessary for the formation of AlN precipitates. If the acid-soluble Al is less than 0.010%, the direction of the secondary recrystallization deteriorates and the magnetic flux density is lowered. If the acid-soluble Al exceeds 0.040%, the development of the secondary recrystallization becomes unstable.

Since N is used in reinforcing process during the decarbonization annealing, an amount sufficient to enter impurities during dissolution is sufficient. However, if it exceeds 0.006%, the secondary recrystallization becomes unstable because it reacts with Al to form coarse AlN, which is not preferable.

The steel component of the present invention is as described above and is composed of the remaining Fe, even in the case of unavoidable elements (P, Cu, Cr, Ni, B, Ti, Nb, V, etc.) mixed from raw materials such as scrap steel during steelmaking It's okay.

[Manufacture process]

The heating temperature of the silicon steel slab comprised of the above-mentioned steel component is low-temperature reheating, Preferably it is performed at 1100-1250 degreeC. This is not preferable because the roll force during hot rolling is less than 1100 ° C., which makes it difficult to control the plate shape. It is not preferable because the amount of scale on the steel surface increases at temperatures exceeding 1250 ° C. not. If the thickness of the slab is too thin, hot rolling productivity is reduced, if the slab is too thick, the slab heating time should be long, so it is preferable to control the slab to 150 ~ 350mm.

After that, the hot rolled sheet having a thickness of 1.5 to 2.6 mm is usually made in consideration of the subsequent final cold rolled thickness by ordinary hot rolling.

The pre-annealing of the hot rolled sheet is preferably carried out for 30 seconds to 10 minutes at a temperature of 850 ~ 1150 ℃ to improve pickling properties and to prevent coarsening of AlN. At the heating temperature and time less than the present invention, it is not preferable because the film formation that is easy to pickling is difficult and the time required for the pickling process is increased. Not good because it becomes unstable.

The preannealed hot rolled sheet is pickled and adjusted to the final thickness by one cold rolling. At this time, the thickness of the final cold-rolled steel sheet is less than 0.23mm secondary recrystallization is not well developed, if it exceeds 0.35mm it is not preferable because excellent iron loss characteristics are not obtained.

The decarburization annealing process of cold rolled plate obtained by cold rolling is carried out for formation of uniform primary recrystallization structure, decarburization to control residual carbon content below 30ppm and sedimentation to control total nitrogen amount to 50-120ppm. It is preferable to set it as 700-950 degreeC. It is difficult to reduce the residual carbon to below the allowable value when the annealing temperature is less than 700 ° C or the annealing time is less than 30 seconds, and the grains of the surface layer of the steel sheet are coarsened at the annealing time exceeding 950 ° C or more than 10 minutes. This is undesirable because secondary recrystallization becomes unstable. Further, if the dew point of the atmospheric gas during decarbonization annealing is less than 30 ° C, decarburization is insufficient, and if it is above 70 ° C, a nonuniform oxide layer is formed on the surface of the steel sheet, which is not preferable because a good glassy film is not obtained. In the case of decarbonization annealing, if decarburization and sedimentation mass are controlled, any method of decarburization after completion of decarburization to an appropriate level and method of deburring at the same time as annealing can be applied. At this time, the atmosphere gas may be any mixed atmosphere that enables the decarburization of the appropriate level and the sedimentation of the present invention at the same time, but preferably a mixed gas of wet ammonia + hydrogen + nitrogen for industrial control of the sediment mass easily. It is good to use the atmosphere.

One of the characteristics of the present invention, the control of the total nitrogen in the steel sheet by immersion is not preferable when the total nitrogen is less than 50ppm because the amount of nitride precipitates such as AlN is insufficient and the second recrystallization becomes unstable, and exceeds 120ppm Is not preferable because the glass coating is poor due to the occurrence of nitrogen outlet defects during high temperature annealing.

The decarbonized annealing plate may be a conventional annealing separator in order to prevent interplate fusion at high temperature annealing, for example, by weight MgO slurry containing 5% or less of TiO ₂ on the surface of the steel sheet after finishing high temperature Anneal.

The temperature rise rate during finishing hot annealing should be controlled at 10-50 ° C./hr to form an appropriate primary recrystallization texture and to completely induce secondary recrystallization. As an atmospheric gas for high temperature annealing, it is preferable to use dry hydrogen or a mixed gas of hydrogen and nitrogen in order to form a glassy film and to remove residual impurities such as N and S. If the high temperature cracking is performed at a temperature of less than 1150 ° C. or cracked for less than 5 hours, it is difficult to form a good glass film and smoothly remove impurities. If the temperature is higher than 1250 ° C. or cracked for more than 30 hours, it is uneconomical. Excluded from the scope of the present invention.

If the primary cracking temperature is below 550 ° C. or the primary cracking time is less than 5 hours during the high temperature annealing, the nitride formed during the nitriding annealing during the primary cracking process is not uniformly distributed in the steel sheet to obtain appropriate grain growth inhibition. It is not preferable because it becomes impossible and secondary recrystallization becomes unstable. In addition, when the temperature exceeds 700 ° C, the amount of the oxide layer formed by the residual moisture during the high temperature annealing in the coil state becomes excessive, and the oxide layer becomes uneven, which is not good because the glassy film is poorly formed, and it is not necessary for the primary to exceed 20 hours. The crack is not impaired in the magnetic properties, but is excluded from the scope of the invention because it is uneconomic.

The secondary crack treatment during annealing at a high temperature, which is one of the characteristics of the present invention, must be performed when sedimenting in a small amount compared to the known method in the decarbonization annealing process.

As a result of numerous tests, the present inventors have found that secondary cracks are very important from the viewpoint of nucleation of secondary recrystallized tissue having excellent directionality. As well as the stabilization of the secondary recrystallization is improved as a result, it can be confirmed that the magnetic properties comparable to the conventional method is obtained.

If the secondary crack temperature is less than 1030 ℃, the core of the secondary recrystallization with excellent orientation is not generated, excellent magnetic properties can not be obtained, and in the case of the secondary crack exceeding 1060 ℃, decomposition of the nitride precipitate starts to occur rapidly This is undesirable because secondary recrystallization becomes unstable.

If the secondary cracking time is less than 5 hours, the directional improvement effect of the secondary recrystallization is insignificant due to the temperature deviation between the inner coil and outer coil of the coil during annealing in coil state, so that excellent magnetic flux density cannot be stably obtained. Excessive secondary cracks are not desirable as the effect of improving the magnetic properties is not so great, but the productivity decrease is severe.

The coil surface on which the glassy film is formed by the high temperature annealing may be subjected to a tension-coating coating after the high temperature annealing for the purpose of improving the insulation and improving the iron loss due to the finer microstructure.

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

Example 1

By weight%, a 210 mm thick slab composed of C: 0.05%, Si: 3.15%, Mn: 0.23%, S: 0.006%, acid soluble Al: 0.027%, N: 0.003% and the remaining Fe was prepared. This was heated for 5 hours at 1200 ℃ slab and hot rolled to make a hot rolled coil of 2.3mm thickness. Then preannealed and pickled for 2 minutes at 930 ° C, followed by cold rolling once to adjust to a final thickness of 0.285 mm, followed by decarburization at 850 ° C for 3 minutes at 25% H ₂ + 75% N ₂ atmosphere with a dew point of 50 ° C. Annealing was performed, followed by immersion at the same temperature in a dry 1% NH ₃ + 25% H ₂ + 74% N ₂ atmosphere. At this time, the soaking time was changed in the range of 30 seconds to 3 minutes to change the total nitrogen in the steel sheet as shown in Table 1 below. Thereafter, by weight percent, an annealing separator composed of 5% TiO ₂ and the remaining MgO was applied to the surface of the steel sheet, followed by finishing hot annealing. In this case, the high temperature annealing may be performed to stabilize the secondary recrystallization by promoting primary cracking to uniformly distribute the nitride formed during the nitriding annealing in the steel sheet and secondary recrystallization having excellent directionality, as shown in Table 1 below. Different crack conditions were performed. Thereafter, the temperature was raised to 1200 ° C. at a temperature increase rate of 15 ° C./hr to cause secondary recrystallization, and cooled after cracking at the temperature for 10 hours to remove impurities. 25% N ₂ + 75% H ₂ was added as an atmosphere gas during the temperature increase. Pure hydrogen gas was used in the crack section of 1200 degreeC. As described above, the secondary recrystallization rate and magnetic properties were measured for the specimens of which the total nitrogen content in the steel sheet was changed and the cracking temperature and time were changed during the first and second cracking during finishing annealing, and the glass film was formed on the surface of the steel sheet. The results of visual observation of the state are shown in Table 1 below. The secondary recrystallization rate is the result of observing the macrostructure exposed by corrosion of the steel plate surface with 20% hydrochloric acid solution of about 80 ℃ after the end of high temperature annealing, and the magnetic properties of B ₁₀ and W _{17 / 50} was measured.

division Total nitrogen (ppm) Primary crack condition during finish annealing Second crack condition during finish annealing Secondary recrystallization rate Vitreous coating appearance Magnetic flux density Iron loss Temperature (℃) Hours (hr) Temperature (℃) Hours (hr) B ₁₀ (Tesla) W _17/50 (W / kg) Comparative Material 1 40 650 15 1040 20 90% Good 1.78 1.71 Invention 1 50 650 15 1040 20 100% Good 1.92 1.02 Invention 2 80 650 15 1040 20 100% Good 1.92 1.02 Invention 3 120 650 15 1040 20 100% Good 1.94 1.00 Comparative Material 2 150 650 15 1040 20 100% Glassy film defect (defect of nitrogen release outlet) 1.93 1.05 Comparative Material 3 250 500 15 1040 20 95% Good 1.82 1.48 Comparative Material 4 250 750 15 1040 20 100% Glassy film defects 1.92 1.91 Comparative Material 5 250 650 4 1040 18 95% Good 1.81 1.55 Comparative Material 6 250 650 15 Not carried 70% Good 1.65 2.12 Comparative Material7 250 650 15 1020 20 100% Good 1.87 1.29 Comparative Material 8 250 650 15 1070 20 90% Good 1.79 1.69 Comparative Material 9 250 650 15 1060 4 100% Good 1.86 1.31

As shown in Table 1, when the total nitrogen is less than the scope of the present invention after sedimentation in the decarbonization annealing process (Comparative Material 1), the amount of nitride precipitates is insufficient, resulting in unstable development of secondary recrystallization. Inferior. On the other hand, in the case of controlling the total nitrogen amount to an appropriate amount and also controlling the conditions of the primary crack and the primary crack during the high temperature annealing temperature (invention material 1-3), it is possible to obtain a good glassy film and excellent magnetic properties at the same time. Could.

When the total nitrogen amount exceeded the scope of the present invention (Comparative Material 2), the magnetic properties were good, but a high quality glassy film was not formed due to the occurrence of nitrogen release defects. In addition, when the primary cracking temperature is less than 550 ° C. (Comparative Material 3) or when the primary cracking time is less than 5 hours (Comparative Material 5), the nitride formed during the immersion annealing during the primary cracking treatment is uniformly formed in the steel sheet. As it is not distributed, it is not possible to obtain adequate grain growth inhibition, and the second recrystallization becomes unstable. As a result, excellent magnetic properties were not obtained. As a result, not only the oxide layer formed is excessive but also uneven, the magnetic properties are relatively excellent, but the glassy film is poorly formed.

In addition, when the secondary cracking temperature was lower than 1030 ° C. during the high temperature annealing (Comparative Material 7), the core of the secondary recrystallization having excellent directionality was not formed and excellent magnetic properties could not be obtained, and secondary cracking was not performed. In the case of the non-comparative material (Comparative Material 6) or the secondary crack exceeding 1060 DEG C (Comparative Material 8), the appearance of the glassy film was good, but the secondary recrystallization was not stabilized, resulting in inferior magnetic properties.

On the other hand, if the secondary cracking time is less than 5 hours (Comparative Material 9), due to the temperature deviation between the inner coil part and the outer coil part during annealing in the coil state, the secondary recrystallization and the improvement of the direction are insignificant, resulting in excellent magnetic properties. Could not get

As described above, the present invention has the effect of providing a method for producing a grain-oriented electrical steel sheet in which the magnetic properties are also improved while the properties of the glassy coating are also improved.

Claims (4)

After the silicon steel slab is heated, a hot rolled, preannealed, cold rolled sheet is made by one cold rolling, the cold rolled sheet is decarbonized, coated with annealing separator, and finished. In the manufacturing method of electrical steel sheet, The silicon steel slab is by weight, C: 0.02 to 0.08%, Si: 2.90 to 3.30%, Mn: 0.15 to 0.30%, S: 0.006% or less, acid soluble Al: 0.010 to 0.040%, N: 0.006% or less It contains; In the decarbonization annealing, the total nitrogen is 50 to 120 ppm; The finish annealing is a oriented electrical steel sheet of low temperature slab heating method with excellent coating properties, including primary cracking at a temperature of 550 to 700 ° C. for 5 to 20 hours and secondary cracking at a temperature of 1030 to 1060 ° C. for 5 to 15 hours. Manufacturing method. The method according to claim 1, wherein the silicon steel slab is heated at a temperature of 1100 to 1250 ° C. The method of claim 1, wherein the decarbonization annealing is performed at a temperature of 700-950 ° C. The method of claim 1, wherein the immersion is carried out in an atmosphere of ammonia + hydrogen + nitrogen.