KR100480002B1 - A method for manufacturing grain oriented electrical steel sheet with superior magnetic property - Google Patents

Abstract

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet, the purpose of which is to identify the relationship between the edge crack of the hot rolled sheet and the cooling conditions until the slab is introduced into the rough mill, and based on this strictly control the cooling conditions of the slab The present invention provides a method of manufacturing a grain-oriented electrical steel sheet which can prevent the occurrence of edge cracks in the hot rolled sheet and further reduce the magnetic deviation of the final product.

In order to achieve the above object, the present invention, in the method for producing a grain-oriented electrical steel sheet including re-heating the silicon steel slab in the heating furnace and then induction into the rough rolling mill, followed by finishing rolling, The technical gist of the present invention relates to a method of manufacturing a grain-oriented electrical steel sheet having excellent magnetic properties, including rough rolling between a center temperature of the silicon steel slab drawn into the rough rolling mill and an edge temperature within 50 ° C.

Description

A method of manufacturing oriented electrical steel sheet having excellent magnetic properties {A METHOD FOR MANUFACTURING GRAIN ORIENTED ELECTRICAL STEEL SHEET WITH SUPERIOR MAGNETIC PROPERTY}

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet applied to the iron core material of electrical equipment such as transformer, more specifically by controlling the cooling conditions after the slab extraction by minimizing hot cracked edge crack while hot magnetic The present invention relates to a method for economically manufacturing high magnetic flux density oriented electrical steel sheets.

A grain-oriented electrical steel is a material having an aggregate structure (goth structure) in which the <001> direction is oriented on the {110} plane in which the magnetization of iron is directed in the rolling direction. It is used. In general, the grain-oriented electrical steel sheet has a final thickness through a rolling process and then selectively grows {110} <001> primary recrystallized grains by secondary recrystallization through an annealing process, thereby securing a good goth structure for magnetic properties. In this manufacturing technique, it is very important that the precipitates (inhibitors) are finely and uniformly dispersed in the cavity, thereby suppressing the coarse growth of the primary recrystallized grains before the start of the second recrystallization, and selectively growing only {110} <001>. Do.

Conventionally, S is mainly added to secure a goth structure with MnS precipitates, whereas recently, Al is newly added to increase the degree of integration of the goth structure. This Al combines with N in the steel sheet to form a dispersed phase with AlN and suppresses the growth of the primary recrystallization and plays a role of making the secondary recrystallization having high density. A grain-oriented electrical steel sheet obtained by using a high-density goth bearing using AlN and MnS is generally referred to as a high magnetic flux density grain-oriented electrical steel sheet. This technology was first disclosed by US Patent Nos. 3287183 and 3636579, and a number of techniques for manufacturing high magnetic flux density oriented electrical steel sheets have been published. There is no addition of Al, such as the addition of Se and the addition of Sb, or the addition of Cu, Sn, etc. in steel, but the practical application in the industry is the coexistence of Al and S worldwide. It is.

The normal action when using Al and S as an inhibitor is as follows. Al and S form AlN or MnS, respectively, to form a precipitate dispersed phase, and these particles cause secondary recrystallization by suppressing the movement of the grain boundary at the grain boundaries of the primary recrystallization. This particle needs to be as small as possible in order to effectively suppress primary recrystallization. Therefore, it is common knowledge that slabs made in steelmaking are usually heated to a very high temperature, for example, around 1400 ° C. for the purpose of solidifying AlN and MnS in a furnace of a hot rolling process. In the heating furnace, the solid solution is used to finish the rolling in a short time at a high temperature so as not to precipitate as much as possible during the rolling, and in the subsequent hot-rolled sheet annealing process, MnS and AlN are precipitated to form an effective precipitation dispersed phase.

On the other hand, in recent years, it is required to reduce the iron loss of electrical steel sheet for the purpose of sex energy by reducing the power loss in the transformer and power transmission, and as a manufacturer of electrical steel sheet, all the techniques are used to develop low iron loss materials. As a result of this research, the magnetic domain control technology and the technology of reducing the iron loss by thinning the thickness of steel sheet have been developed, and actual product thicknesses of 0.3mm, 0.27mm, etc. are produced and sold with 0.23mm, 0.20mm. In order to manufacture such low iron loss materials, it is necessary to increase the degree of integration of the aggregated structure (110) <001>. That is, the fine and uniform presence of the so-called inhibitor in the dispersion phase of the dispersion becomes important, and therefore, the heating conditions, the temperature, and the time in the heating furnace of the hot combustion furnace are managed in a stricter direction. As a result, it takes sufficient time at higher temperatures. Moreover, it is important to prevent precipitation in hot rolling and also to prevent precipitation during rolling in hot finishing mills.

However, when making hot-rolling heating and rolling conditions into a strict direction, various problems arise. One of them is the difference in magnetic properties measured in the longitudinal direction of the coil. Even if the heater is sufficiently heated and the inhibitor is in a solid state, the difference in precipitation dispersion occurs in rolling because the relationship between temperature and time is inevitably changed at the front and rear ends of the slab. That is, compared with the front end of the slab, the phenomenon that the rear end is poor in relation to the temperature drop during rolling and the relationship between temperature and time occurs. For this reason, even if the characteristics of the front end are good, the characteristics of the rear end are bad, and the magnetic characteristic variation in the coil longitudinal direction is large. This not only lowers the product yield rate of the factory, but also raises the cost, and results in a mismatch between production and order.

Another big problem is the phenomenon of edge cracking in hot rolling. This cause is not simple because many factors overlap. However, experience shows that the higher the hot rolling temperature, the more cracks are generated. In other words, cracks occur when the magnetic properties are good, and on the contrary, when the magnetic properties are bad, a good edge state is shown. This crack sometimes appears to be 100mm or more in width. Since this crack must be trimmed after rolling, it is not only a big factor in the drastic reduction of the error rate, but also a big problem in that it can produce a product that cannot be sold in a factory that needs to produce a product of a predetermined width in response to an order. It can be said.

It was a general idea that the cracks mentioned above were due to the formation of coarse grains by high temperature heating in the furnace. To date, several techniques have been proposed to eliminate this crack. The techniques so far have been about making coils by hot rolling without generating any cracks in the slabs with these large grains. As an example, Japanese Patent Laid-Open No. 3-47601 proposes a technique for preventing edge cracking by rolling in rough rolling of hot rolling, which reduces the large crystal by promoting recrystallization of the large crystal in rolling. will be. In addition, Japanese Patent Laid-Open No. 6-122005 and Japanese Patent No. 57165102 disclose techniques for preventing edge cracks by equipment, cooling during rolling, edge heating, and the like. However, these methods and other proposed technologies require investment of enormous new facilities to prevent edge cracking, which is not only economical but also cannot completely solve the edge crack. Therefore, it is extremely common to recognize that edge cracks inevitably occur when trying to improve the characteristics of directional electrical steel, especially high magnetic flux density oriented electrical steel using AlN and MnS as an inhibitor, and this is inevitable. It's coming to think.

However, the present inventors have confirmed through practical operation that there is a limit in the improvement of productivity and the error rate unless the edge crack is eliminated in the manufacture of directional electrical steel sheet, especially high magnetic flux density oriented electrical steel sheet using AlN and MnS as an inhibitor. In addition, numerous studies have been conducted over the years to identify the cause of edge cracks and prevent the occurrence of edge cracks. The specific technology is described in detail in Korean Patent Application Nos. 1996-44545, 1996-69482, 1997-73574, 1999-13215, and the like. An overview of each technology follows.

In 1996-44545, the major cause of cracks was found in glass S, which does not exist as MnS, and prevented edge cracking by minimizing the amount of S in steelmaking. A method of securing magnetism through a method such as S addition in a high temperature annealing atmosphere has been proposed.

In 1996-69482, the edge crack confirmed that glass S of the edge portion of the plate was particularly problematic, and it was excellent in magnetism by applying Mn to the slab edge part before charging the steel slab manufactured in the performance into the heating furnace. A method for producing a high magnetic flux density oriented electrical steel sheet without edge cracks has been proposed. 1997-73574 finds that if S is lowered in the steelmaking stage, coarse MnS produced in the performance stage can be employed even if the heating temperature is lowered, and a method for preventing edge cracking by lowering the heating temperature is proposed. .

In 1999-13215, a technique for preventing hot rolled sheet edge cracking by controlling a slab heating rate during slab heating before hot rolling is disclosed.

Although the edge cracks can be remarkably reduced with the techniques proposed by the present inventors, as the operation continues, the variation in terms of product quality and edge cracks has become a problem, and it is recognized that these methods are not enough. Reached. In order to prevent the edge crack of the hot rolled sheet, the present inventors continue to study and propose the present invention based on the results of the research.

The present invention identifies the relationship between the edge cracks of hot rolled sheets and the cooling conditions until the slab is introduced into the rough mill, and based on this, the cooling conditions of the slabs are strictly controlled to prevent the occurrence of hot cracked edge cracks and furthermore, It is an object of the present invention to provide a method for manufacturing a grain-oriented electrical steel sheet capable of reducing magnetic deviation.

In order to achieve the above object, the present invention, in the method for producing a grain-oriented electrical steel sheet including re-heating the silicon steel slab in the heating furnace and then induction into the rough rolling mill, followed by finishing rolling, And rough rolling with a deviation between the center temperature of the silicon steel slab drawn into the rough rolling mill and the edge temperature within 50 ° C.

Hereinafter, the present invention will be described in detail.

The inventors have investigated the correlation between the cooling conditions and the hot-rolled sheet edges through simulation experiments on the cooling conditions after the slab is extracted from the heating furnace, and based on these results, the hot-rolled sheet edge cracks are controlled by strictly controlling the cooling conditions. The present invention has been completed by the conclusion that it can reduce and also improve the magnetic properties of the final product.

Usually, slabs having a thickness of 200 to 250 mm after heating in a heating furnace at a predetermined temperature at a predetermined temperature are extracted for rolling, and then roughly rolled at 1320 to 1250 ° C. to form a bar having a thickness of 40 mm. Subsequently, it is continuously rolled from finishing rolling to the final thickness and wound with a hot rolled coil. In practice, hot rolled edge cracks are observed in hot rolled coils, but some are observed in the state of bars. The cracks observed in the bar state are caused by rough rolling, which means that it is difficult to completely solve the edge cracks without controlling the working conditions before rough rolling.

The inventors of the present invention have carefully measured the temperature change in the course of cooling the slabs extracted from the furnace, and found that the temperature deviations of the slabs were large before the rough rolling. In other words, the large temperature deviation of each slab region is directly investigated without overlooking the possibility that the phase ratio of each region may be different. Therefore, the lower the temperature in the state of transformation from δ ferrite to austenite, the more austenite ratio becomes. I could confirm that. Rough rolling is necessarily performed under a mixed structure of ferrite and austenite. When the temperature of the edge portion decreases, the edge portion becomes more austenite than the center portion. This leads to the conclusion that the deformation imbalance at the center and the edge of the rough rolling acts as a major cause of cracking in the bar.

On the basis of this point of view, after thorough examination and experimentation of the working conditions, it was found that optimizing the slab cooling conditions before hot rolling did not produce edge cracks in the bar state after rough rolling. In other words, the slab cooling condition to eliminate the edge cracks generated in rough rolling is to make the temperature deviation in the slab width direction small, and it can be deduced that the temperature deviation between the edge part and the center part is within 50 ℃. there was. The temperature deviation varies depending on the size of the slab. In the case of the actual production line conditions, that is, when the slab size is 200 to 250 mm thick and 900 to 1100 mm wide, the temperature deviation between the edge and the center can be within 50 ° C. It is derived that the time to be extracted and transported to rough rolling within 50 seconds can be achieved.

As a method of not being able to be within 50 seconds due to the operating conditions, the effect of the present invention can be obtained by preventing the temperature drop of the edge portion through the installation of a heating furnace or the installation of an edge heater between the heating furnace and the rough rolling machine.

In this way, reducing the temperature deviation of the center and the edge of the pre-rolled slab, the edge crack prevention and the widthwise magnetic deviation in the final product is greatly reduced, thereby improving the overall magnetism. In addition, it is possible to simultaneously achieve the improvement of the error rate and the magnetic improvement through the reduction of edge cracks.

The present invention can be usefully applied to the oriented electrical steel sheet which is reheated at a high temperature of about 1200 ℃ or more frequently occurs edge cracks. As the grain-oriented electrical steel sheet, Si: 2.0% to 4.0%, C: 0.02% to 0.10%, Sol-Al: 0.01% to 0.05%, Mn: 0.03% to 0.10%, N: 0.005% to 0.015%, and S: 0.007% by weight Typical examples include steels containing 0.030% and Cu: 0.01-0.15%.

Below. The present invention will be described in detail by way of examples.

Example 1

As a component of steelmaking, Si: 3.20%, C: 0.070%, Sol-Al: 0.028%, Mn: 0.079%, N: 0.090%, S: 0.015%, Cu: 0.08%, remaining Fe and other inevitably The steel which consists of impurities contained was dissolved. A slab with a thickness of 200 mm and a width of 1000 mm was cast from the steel of this component, and the heating temperature was 1375 ° C. for a total of 290 minutes, followed by extraction, rough rolling, and finishing rolling to prepare a hot rolled plate having a thickness of 2.3 mm. In addition, the hot rolled sheet was heat-treated at 1120 ° C. and then cold rolled to a final thickness of 0.3 mm. The coil was subjected to decarbonization annealing at 845 ° C., and then an annealing separator containing MgO as a main component was applied, followed by final high temperature annealing at 1200 ° C. At this time, by controlling the moving time from the heating furnace to the rough rolling after extraction, the temperature deviation between the center temperature of the slab and the edge part is different, so that the widthwise temperature deviation is the edge crack of the hot rolled sheet, the edge crack of the bar after rough rolling, and the final The effect on the magnetic properties of the product was investigated. The occurrence state of the edge crack of the hot rolled plate and the magnetization of these final hot annealing plate is shown in Table 1 below.

division Travel time (seconds) Temperature deviation of slab center and edge part (℃) Cracks in the bar Hot Rolled Edge Cracking Rate (%) Magnetic flux density (B10, Tesla) Within 5mm Within 10mm Within 15mm Inventive Example 35 10 radish 100 0 0 1.94 40 30 radish 100 0 0 1.94 50 50 radish 95 5 0 1.94 Comparative example 60 60 U 80 20 5 1.93 65 80 U 65 25 10 1.92 80 100 U 55 30 15 1.92 90 120 U 40 40 20 1.92

As shown in Table 1, when the slab movement time from the heating furnace to the rough rolling after the extraction is within the range of the present invention, no edge crack is observed in the bar after the rough rolling, and the coarse edge crack in the hot rolled sheet. It can be seen that the incidence rate of is significantly reduced, and the magnetism of the final product is also good.

As described above, according to the present invention, there is a useful effect of preventing edge cracks in the bar after rough rolling and further reducing the magnetic deviation in the width direction of the hot rolled sheet.

Claims (4)

In the method for producing a grain-oriented electrical steel sheet comprising re-heating a silicon steel slab in a heating furnace, drawing into a rough rolling mill, rough rolling, and then finishing rolling. The method of manufacturing a grain-oriented electrical steel sheet having excellent magnetic characteristics, characterized in that the rough rolling is carried out within 50 ℃ of the deviation between the center temperature and the edge temperature of the silicon steel slab extracted from the heating furnace and introduced into the rough mill. The method of claim 1, wherein the thickness of the slab is 200 ~ 250mm, the width is 900 ~ 1100mm of the directional electrical steel sheet having excellent magnetic properties, characterized in that the movement time of the slab from the heating furnace to the rough rolling mill within 50 seconds Manufacturing method. The method of claim 1, wherein a heating furnace or an edge heater is installed in the heating furnace and the rough rolling section to prevent temperature reduction of the slab. According to any one of claims 1 to 3, wherein the silicon steel slab is Si: 2.0 to 4.0%, C: 0.02 to 0.10%, Sol-Al: 0.01 to 0.05%, Mn: 0.03 to 0.10% by weight , N: 0.005% to 0.015%, S: 0.007% to 0.030%, Cu: 0.01% to 0.15%, remaining Fe and other inevitable impurity containing impurities, characterized in that the manufacturing method of the excellent grain-oriented electrical steel sheet.