KR20020045214A - A method for manufacturing high permeability grain oriented electrical steel sheet - Google Patents

Abstract

PURPOSE: A method for manufacturing a high permeability grain oriented electrical steel sheet with high magnetic density is provided, in which edge cracks of hot rolled steel sheet can be prevented by regulating cooling rate properly over the temperature range from a dual phase of ferrite and austenite to ferrite monophase. CONSTITUTION: In a manufacturing method of a grain oriented electrical steel sheet where a steel slab comprising Si 2.0-4.0 wt.%, C 0.03-0.10 wt.%, Sol-Al 0.01-0.05 wt.%, Mn 0.03-0.10 wt.%, N 0.005-0.015 wt.%, S 0.007-0.030 wt.%, Cu 0.01-0.15 wt.%, a balance of Fe and other inevitable impurities undergoes rough rolling, finish rolling, coiling, preliminary annealing, cold rolling, decarburization annealing, applying of annealing separator, and hot annealing in order, the present invention is characterized in that cooling rate is regulated at 20 to 30°C/s over a temperature range of from end point of finish rolling to 810-860°C.

Description

A method for manufacturing high magnetic flux density oriented electrical steel sheet {A METHOD FOR MANUFACTURING HIGH PERMEABILITY GRAIN ORIENTED ELECTRICAL STEEL SHEET}

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet used as a core material such as a transformer, and more particularly, by optimizing the hot rolling conditions during the manufacturing process, the grain-oriented electrical steel that can minimize the hot-rolled sheet edge crack It relates to a method for producing a steel sheet.

Recently, in order to reduce power loss in transformers and power transmission, development of low iron loss materials capable of lowering iron losses in electrical steel sheets has been promoted. As a result of this research, the magnetic domain control technology and the technology of lowering the iron loss by thinning the thickness of steel sheet were developed. Actually, the product thickness of 0.3mm, 0.27mm, etc. is 0.23mm, 0.20mm. In the production of such low iron loss materials, it is necessary to increase the degree of integration of the aggregate structure (110) <001>, so that the heating conditions in the heating furnace of the hot-burning furnace are so as to distribute the dispersed dispersion phase, the so-called inhibitor finely and uniformly. , Temperature, and time should be tightly controlled. That is, a sufficient time at a higher temperature, to prevent precipitation in hot rolling, and to prevent precipitation during rolling in a high temperature finishing mill.

By the way, in the case of strictly controlling the hot-rolled heating and rolling conditions, various problems arise, one of which is that the magnetic properties are different in the longitudinal direction of the coil. The reason for this is as follows. Even when the heater heated sufficiently in the heating furnace is in the solid state, the difference in precipitation dispersion occurs because the relationship between temperature and time inevitably varies at the front end and the rear end of the slab during rolling. That is, compared with the front end of the slab, the rear end of the slab occurs in the relationship between the temperature drop during the rolling, the temperature and the time, the phenomenon occurs. As a result, the magnetic characteristic variation in the coil length direction becomes large. This not only lowers the product yield rate of the factory, but also raises the cost, making the balance between production and order unbalanced.

Another big problem is the phenomenon of edge cracking during hot rolling. The crack has a size of 100 mm or more in the width direction, and since it must be trimmed after rolling, not only does it cause a huge drop in the error rate, but also a factory that needs to produce a product having a predetermined width in response to an order, It becomes a big problem to manufacture a product that cannot be sold.

There are many causes of such cracks, and it is a general idea that crystal grains become coarse due to high temperature heating in a heating furnace. That is, in general, when the reheating temperature and the hot rolling temperature are increased to improve the magnetic properties, a lot of edge cracks are generated, and conversely, when the reheating temperature and the hot rolling temperature are lowered to prevent the edge cracks, the magnetic properties are deteriorated.

Many techniques have been proposed for the purpose of eliminating these cracks, and so far, techniques have been directed to hot rolling coils with minimal cracking in slabs with these large grains. For 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 the rolling. In addition, Japanese Patent Laid-Open No. 6-122005 and Japanese Patent Laid-Open No. 57-165102 disclose techniques for preventing edge cracks by equipment, cooling during rolling, edge heating, and the like.

However, this method and other proposed techniques have not only been uneconomical, but also could not completely solve the edge cracks because of the huge investment of new equipment to prevent edge cracks.

Therefore, edge cracks inevitably occur in directional electrical steel sheets, particularly high magnetic flux density oriented electrical steel sheets using AlN and MnS as inhibitors, and it is until now considered to be an unavoidable phenomenon.

However, the inventors of the present invention, in the manufacture of high magnetic flux density oriented electrical steel sheet using AlN and MnS as an inhibitor, confirms through the practical operation that there is a limit to the improvement in productivity and the error rate unless the edge crack is eliminated, We have applied for techniques for determining the cause of edge cracks and preventing the occurrence of edge cracks. Such techniques include Korean Patent Application Nos. 96-44545, 96-69482, 97-73574, 98-49081, and 99-13215, and in the above-mentioned 96-44545, Identifying the main cause is in glass S that does not exist as MnS and preventing edge cracks by reducing the amount of S in the steelmaking stage as much as possible, and adding S to MgO additive or S in final high temperature annealing atmosphere. We proposed a method of securing magnetism through the method of. In the above-mentioned 96-69482, it is confirmed that the generation of edge cracks is caused by the glass S of the edge portion of the plate, and by applying Mn to the slab edge portion in the state before charging the steel slab manufactured in the performance into the heating furnace, A method of manufacturing a high magnetic flux density oriented electrical steel sheet which is excellent and has no hot-rolled sheet edge crack is proposed. 97-73574 found that lowering the S in the steelmaking step can cause the coarse MnS produced in the playing step to be dissolved even if the heating temperature is lowered, and proposed a method for preventing edge cracks by lowering the heating temperature. . In 98-49081, it was found that the slab cooling method has a significant effect on edge crack generation during hot rolling, and proposed a method capable of preventing edge cracks by limiting the high pressure water injection method and the number of injection times. Further, in the above-mentioned 99-13215, the effect of the slab heating rate in the heating furnace on the edge crack of the hot rolled sheet is investigated, and the preheating temperature (Tp) of the preheating zone and the heating temperature (Th) of the heating zone are 145≥Th-Tp. A method that can prevent edge cracking by satisfying a condition of ≥ 55 has been proposed.

However, according to the above methods, the edge cracks were remarkably reduced, but when the operation was unstable, it was recognized that the variation in the edge cracks was a problem, such as a large number of edge cracks, and these methods were not enough. Reached.

Accordingly, the present inventors have conducted research and experiments to improve the problems of the conventional methods described above, and have proposed the present invention based on the results. The present invention has been made in the two-phase region of ferrite and austenite from the end of finishing rolling. It is an object of the present invention to provide a method of manufacturing a high magnetic flux density oriented electrical steel sheet which can minimize the occurrence of hot-rolled sheet edge cracks by appropriately controlling a cooling rate in a temperature range up to a temperature transforming into a ferrite single phase region.

The present invention for achieving the above object,

By weight%, Si: 2.0 to 4.0%, C: 0.03 to 0.10%, Sol-Al: 0.01 to 0.05%, Mn: 0.03 to 0.10%, N: 0.005 to 0.015%, S: 0.007 to 0.030%, Cu: Preparation of oriented electrical steel sheet which is wound by rough rolling and finishing rolling of steel slab composed of 0.01 ~ 0.15%, balance Fe and other unavoidable impurities, followed by preannealing, cold rolling, decarbonization annealing, and hot annealing. In the method,

It relates to a method of manufacturing a high magnetic flux density oriented electrical steel sheet characterized in that the cooling rate of the temperature section from the end of the finishing rolling to 810 ~ 860 ℃ controlled to 20 ~ 30 ℃ / s.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The inventors of the present invention can reduce the edge cracks generated in the hot rolled sheet to a remarkable level by controlling the heating furnace operating conditions and the rough rolling operating conditions. Found. In particular, as a result of a long time scrutiny, even after the coil has been observed, the edge cracks have not been observed immediately after being transported to the post process. It was concluded that it is impossible to manufacture a hot rolled coil without edge cracking. As described above, it was observed that a few days after the winding was observed after a few days because the microcracks were advanced before the winding and the microcracks were observed immediately after the winding. As a result, the microcracks are formed depending on the coil. And, it can be seen that these microcracks develop into coarse cracks over time. Accordingly, the present inventors have come to the conclusion that in order to make a sound coil without edge cracks of a hot rolled sheet, it is indispensable to fundamentally block the cause of the microcracks.

The inventors of the present invention carefully examined the possible time periods of microcracks by dividing them before finishing rolling and after finishing rolling. In particular, the inventors of the present invention prevent the microcracks from growing during the process of cooling and winding the material after finishing rolling. De focused.

That is, the hot-rolled sheet after finishing finishing is cooled through the running table to reach the winding machine and wound up, and it is found that the microcracks generated before the finishing rolling progress during the cooling process, and the progress of the microcracks during cooling is observed. Research and experiment were conducted on how to prevent this. In addition, since the microcracks generated after finishing rolling are considered to be related to brittleness of the material, the present invention has been completed by focusing on the fact that the cooling rate after winding is involved in brittleness of the material.

Hereinafter, the steel component and manufacturing conditions of this invention are demonstrated.

In the present invention, in general, by weight% used in grain-oriented electrical steel sheet, Si: 2.0 to 4.0%, C: 0.03 to 0.10%, Sol-Al: 0.01 to 0.05%, Mn: 0.03 to 0.10%, N: 0.005 to 0.015% , S: 0.007% to 0.030%, and Cu: 0.01% to 0.15%. The steel slab having such a composition is reheated and then hot rolled. In the present invention, the heating of the hot rolled slab can be performed not only at a high temperature but also at a low temperature. For example, even in the case of low temperature heating of 1300 ° C. or less, in the case of steel grades in which the finishing rolling end temperature is 1000 ° C. or more, the winding temperature is terminated at around 550 ° C., and edge cracks occur, it is applicable in any case.

On the other hand, in the hot rolling operation of a grain-oriented electrical steel sheet, the slab having a thickness of about 200 ~ 250mm is heated to 1300 ~ 1400 ℃ in a heating furnace, and then roughly rolled into a bar having a thickness of about 40mm, Roll to the final thickness through rolling. Normal finishing rolling is performed using a finishing mill comprised of 6 to 7 stands, starting rolling of the first stand at 1200 ° C or higher and ending rolling at the final stand at 1000 ° C or higher.

As described above, the hot rolled sheet after finishing finishing reaches the winding machine via a running table and is wound. In the usual case, after finishing finishing finishing at around 1000 ° C, at least 50 ° C / s or 60 ° C / s, in particular, The higher the temperature range, the greater the cooling rate, while cooling to the winder at a speed of 70 ° C./s or more up to 900 ° C., while in the present invention, the temperature range from 810 to 860 ° C. after the finishing rolling is performed at a speed of 20 to 30 ° C./s. It is characterized by cooling, for the following reasons.

At the end of finishing rolling, the microstructure in the steel sheet becomes two phases of austena art and ferrite. In the process of cooling to winding, the austenite is transformed into ferrite, and then becomes a ferrite single phase region. The inventors of the present invention, the cooling rate for the temperature range from the mixed phase (two phase) region of the austenite and ferrite to 810 ~ 860 ℃ to become a single phase region of the ferrite only affects the brittleness of the material In view of the fact that the quenching becomes fine when the transformation from austenite to ferrite becomes fine, the experiments were repeated by differentiating the cooling rate before and after the transformation zone. As a result, it was found that lowering the cooling rate from the end of rolling to the temperature range of 810 to 860 ° C. to 30 ° C./s or less has a great effect on the prevention of fine cracks. It has also been found that adjusting the cooling rate in this way can be realized by adjusting the water pressure of the cooling water. That is, in order to obtain the above-mentioned effect, the cooling rate in the temperature section should be set at 30 ° C / s or less. However, if the cooling rate is less than 20 ° C./s, the cooling rate after 860 ° C. is too large to make the winding temperature match with the length of a normal running table. Therefore, the lower limit of the cooling rate is set to 20 ° C./s. .

On the other hand, there is no particular limitation on the cooling rate in the temperature range from 810 to 860 ° C. to the winder, which becomes the single phase region of the ferrite only in the mixed phase (two phase) region of the austenite and ferrite. On the other hand, the winding temperature is preferably set in a range that can ensure a stable magnetic properties in the final product, it is usually set to a temperature range of 500 ~ 550 ℃.

Then, the hot rolled steel sheet wound in the coil state is transferred to the subsequent process.

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

(Example)

By weight, Si: 3.12%, C: 0.060%, Sol-Al: 0.025%, Mn: 0.085%, N: 0.0087%, S: 0.019%, Cu: 0.08%, remaining Fe and other unavoidable impurities The steel slab made of ash was reloaded in a furnace of 1375 ° C. for a total of 285 minutes and then extracted. This was rough-rolled to prepare a bar having a thickness of 40 mm, followed by finishing rolling and hot rolling to a thickness of 2.3 mm. At this time, the finishing rolling temperature is set to 1150 ℃, the cooling rate from the end of finishing rolling to 850 ℃ changed as shown in Table 1 to investigate the edge crack of the hot rolled sheet, the results are shown in Table 1 below .

In addition, each hot rolled sheet thus prepared was heat-treated at 1120 ° C., and then cold rolled to a final thickness of 0.29 mm in five passes. The coil was decarbonized at 845 占 폚, and thereafter, an annealing separator containing MgO as a main component was applied, followed by final high temperature annealing at 1200 占 폚. Thereafter, the magnetic properties of the final high temperature annealing plate thus obtained were measured, and the results are shown in Table 1 below.

division Cooling speed from finishing finishing to 850 ° C (° C / sec) Edge crack incidence (%) Magnetic flux density B ₁₀ (Tesla) 5mm or less 5 ~ 10mm 10-15mm Inventive Example 1 20 100 0 0 1.95 Inventive Example 2 25 100 0 0 1.95 Inventive Example 3 30 100 0 0 1.95 Comparative Example 1 35 95 5 0 1.94 Comparative Example 2 40 90 10 0 1.94 Comparative Example 3 50 90 10 0 1.94 Comparative Example 4 70 85 10 5 1.94

As shown in Table 1, in the case of Inventive Examples (1) to (3) in which the cooling rate from finishing finishing to 850 ° C is in the range of 20 to 30 ° C / s, the edge crack of 5 mm or more does not occur. Therefore, the edge state of the hot rolled sheet is very good, it can be seen that the magnetic properties of the final product is also excellent.

According to the present invention as described above, it is possible to secure an excellent magnetic properties while significantly reducing the occurrence of edge cracks in the hot rolled sheet, there is an effect that can be applied to products requiring high magnetic flux density and low iron loss.

Claims (1)

By weight%, Si: 2.0 to 4.0%, C: 0.03 to 0.10%, Sol-Al: 0.01 to 0.05%, Mn: 0.03 to 0.10%, N: 0.005 to 0.015%, S: 0.007 to 0.030%, Cu: Preparation of oriented electrical steel sheet which is wound by rough rolling and finishing rolling of steel slab composed of 0.01 ~ 0.15%, balance Fe and other unavoidable impurities, followed by preannealing, cold rolling, decarbonization annealing, and hot annealing. In the method, Method of manufacturing a high magnetic flux density oriented electrical steel sheet characterized in that the cooling rate of the temperature section from the end of the finishing rolling to 810 ~ 860 ℃ controlled to 20 ~ 30 ℃ / s