KR100352594B1 - Hot rolling method of oriented electrical steel sheets with excellent magnetic properties and few hot-rolled sheet edge cracks - Google Patents

Abstract

The present invention relates to a method for producing a grain-oriented electrical steel sheet; The purpose of this study is to strictly control the cooling conditions in the rough rolling process according to the results of research on the correlation between the cooling conditions and the hot-rolled sheet edge cracks in the hot rolling process, and to drastically reduce the hot-rolled sheet edge cracks in the oriented electrical steel sheet. It is to provide a hot rolling method that can improve the magnetic properties of the product.

The present invention having the above object is a method for producing a grain-oriented electrical steel sheet including a hot rolling step of rough rolling and finishing rolling while spraying high pressure water after reheating and extracting a silicon steel slab containing S at a high temperature in a heating furnace. In the technical gist of the present invention, the rough rolling is performed by directly spraying high pressure water on the upper and lower surfaces of the silicon steel slab except for the edge portion. do.

Description

Hot rolling method of oriented electrical steel sheets with excellent magnetic properties and few hot-rolled sheet edge cracks

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet, and more particularly, to a method for economically manufacturing grain-oriented electrical steel sheet by minimizing hot-rolled sheet edge cracks.

A grain-oriented electrical steel sheet is a material having an aggregate structure in which the <1> direction is oriented on the {110} plane in which the magnetization of iron is in the rolling direction. The grain-oriented electrical steel sheet has excellent magnetic properties and is mainly used for iron core materials such as transformers. The properties required for oriented electrical steel sheets are low iron loss and high magnetic flux density. The lower the iron loss, the smaller the energy loss of the electric machine, and the higher the magnetic flux density, the higher the efficiency of the electric machine, which can be miniaturized.

A grain-oriented electrical steel sheet is divided into a conventional grain-oriented electrical steel sheet using one type of primary grain growth inhibitor (for example, MnS) and a high magnetic flux density grain-oriented electrical sheet using two kinds (for example, MnS + AlN). In the conventional manufacturing technique of the conventional grain-oriented electrical steel sheet, the high magnetic flux-density electrical steel sheet is newly added Al in order to increase the degree of integration. Al combines with N in the steel sheet to form a dispersed phase with AlN and suppresses the growth of the primary recrystallization, thereby making secondary recrystallization with high integration. The technology of high magnetic flux density oriented electrical steel sheet which obtains high density goth bearing by using AlN and MnS was first disclosed by US Patent Publication Nos. 3287183 and 3636579, and numerous manufacturing techniques of high magnetic flux density oriented electrical steel sheet This was announced. Al is not added like the technique of adding Se and addition of Sb. However, in the real industry, the coexistence of Al and S is the mainstream in the world. There is also a technique in which Cu, Sn, etc. are contained in steel other than Al and S.

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 this furnace, the solid solution precipitates to be cooled by finishing the rolling in a short time at high temperature so as not to precipitate as much as possible during the rolling, and precipitates MnS and AlN in the subsequent hot-rolled sheet annealing process to form an effective precipitation dispersed phase.

Recently, the low iron loss of electrical steel sheet is required for the purpose of sex energy by reducing the power loss in transformer and power transmission. As a manufacturer of electrical steel sheet, all the technologies are used to develop low iron loss materials. As part of the development of this technology, the magnetic domain control technology and the product of 0.3mm, 0.27mm, etc., the thickness of the product 0.23mm, 0.20mm is also produced and sold. In order to manufacture such a thin material and low iron loss material, it is necessary to increase the degree of integration of the aggregate (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 it is also necessary to enter the finishing mill at a high temperature to prevent precipitation during rolling.

By the way, a problem arises when making hot-rolling heating and rolling conditions into a strict direction. One of them is a phenomenon in which magnetic properties differ when 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 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 cracks occur when the hot rolling temperature is sufficient. 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. This crack needs to be trimmed and removed after rolling, and it is not only a big factor in the drastic reduction of the error rate, but also a product that cannot be sold in a factory that needs to produce a product of a predetermined width in response to an order. This is a big problem.

It was a general idea that this crack was caused by the formation of coarse crystal grains by high temperature heating in a heating furnace. To date, several techniques have been proposed to eliminate this crack. The techniques so far are related to the technique of making the large crystal coil without hot cracking. 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 techniques up to now require a huge investment of new equipment to prevent edge cracks, and are not only economical but also cannot completely solve the edge cracks.

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. Nevertheless, the inventors of the present invention judge that there is a limit in improving productivity and error rate without removing edge cracks in the manufacture of directional electrical steel sheets, particularly high magnetic flux density oriented electrical steel sheets using AlN and MnS as inhibitors. Countless approaches have been pursued over the years to identify the causes and prevent edge cracks. As a result, we succeeded in deciding the cause of the edge crack and the specific method of control. The specific technology is described in detail in Korean Patent Application Nos. 1996-44545, 1996-69482, 1997-73574, and the like. An overview of each technology follows.

1996-44545 identified the major cause of cracks in glass S, which does not exist as MnS, and prevents edge cracking by minimizing the amount of S in the steelmaking stage and adding S to the MgO additive or the final high temperature. A method of securing magnetism through a method such as S addition in an annealing atmosphere has been proposed.

In 1996-69482, the edge crack confirmed that glass S of the edge part of the plate was particularly problematic, and Mn was applied to the slab edge part before the steel slab manufactured in the performance was charged into the heating furnace, thereby having excellent magnetic properties and hot rolling. A method of manufacturing high magnetic flux density oriented electrical steel sheets without plate edge cracks is proposed.

1997-73574 found that lowering S in the steelmaking stage can cause the coarse MnS produced in the playing stage to be dissolved even if the heating temperature is lowered, and proposed a method to prevent edge cracking by lowering the heating temperature.

Although the edge cracks can be remarkably reduced by the technique proposed by the present inventors, as the operation continues, the deviations in terms of quality and edge cracks of products have been highlighted as problems, and they have come to realize that these methods are not enough. . Accordingly, the present inventors continue to study to prevent the edge cracks of the hot rolled sheet, it is possible to significantly reduce the hot cracked edge cracks and improve the magnetic properties of the final product by strictly controlling the cooling conditions in the rough rolling It has been confirmed that the present invention has been proposed.

The present invention is based on the results of a study on the correlation between the cooling conditions and the hot rolled sheet edge cracks in the hot rolling process. The hot rolled sheet edge cracks of the grain-oriented electrical steel sheet can be drastically controlled by strictly controlling the cooling conditions in the rough rolling process. It is an object of the present invention to provide a hot rolling method which can reduce and further improve the magnetic properties of the final product.

In order to achieve the above object, the present invention provides a method for producing a grain-oriented electrical steel sheet including a hot rolling step of rough rolling and finishing rolling while spraying high-pressure water after reheating and extracting silicon steel slab containing S at a high temperature in a heating furnace. In the above rough rolling, the high pressure water is directly sprayed on the upper and lower surfaces of the silicon steel slab except for the edge portion.

Hereinafter, the present invention will be described in detail.

First, the steel slab applied to the present invention is a conventional silicon steel slab containing S and hot rerolled at high temperature. This is because, as mentioned above, the edge crack generated in the hot rolled steel sheet of the grain-oriented electrical steel sheet is caused by segregation of S or the like in coarse grains heated to a high temperature in the reheating process. When reheated at about 1200 ° C or more, grain-oriented electrical steel sheet has a high possibility of segregation such as S due to coarse grains, and the reheating temperature is generally controlled at about 1300-1400 ° C. Such silicon steel slabs are Si: 2.0-4.0%, C: 0.03-0.10%, Sol-Al: 0.01-0.05%, Mn: 0.03-0.10%, N: 0.005-0.015%, S: 0.007- Examples thereof include high magnetic flux density oriented electrical steel sheets including 0.030% and Cu: 0.01-0.15%.

The present inventors conducted a simulation experiment on the cooling conditions of the hot-rolled silicon steel slab containing S as described above, and as a result, the correlation between the cooling conditions and the hot-rolled sheet edge was identified. According to the present invention, it has been concluded that strictly controlling the cooling conditions in rough rolling can greatly reduce the edge crack of the hot rolled sheet and improve the magnetic properties of the final product. Cooling conditions in the rough rolling process derived from the present invention is the first high-pressure water spray method, the second high-pressure water spray frequency, which will be described in detail as follows.

In the conventional hot rolled rolling method of oriented electrical steel sheet, the slab is extracted from the heating furnace, and then the bar is formed in the rough rolling process, and then rolled to the final thickness in the subsequent rolling process. Rough rolling is usually performed by 5 to 7 passes of rolling to make the bar of thickness suitable for finishing rolling, and to remove the surface oxide scale generated before rough rolling to ensure good surface quality of the final rolling plate. do. That is, at the same time as the rolling, the scale generated on the surface of the slab is removed by high pressure water. At this time, the injection of high pressure water directly injects the high pressure water to the entire surface of the slab, such as the upper and lower surfaces and the side and edge of the slab.

In this rough rolling, the slab temperature is generally about 1320-1250 ° C. As the rough rolling progresses and the slab is cooled, the slab structure is transformed from δ ferrite to γ austenite phase, and as the temperature decreases, the austenite The proportion of the phases increases. As a result, the rough rolling is made of a slab in the mixed phase of the ferrite and austenite, it is difficult to uniformly roll due to the difference in deformation resistance of the ferrite and austenite. Direct injection of high-pressure water into the edge of the slab in this state causes rapid supercooling of the edge and increases the temperature deviation between the edge and the center. Accordingly, the edge portion has a larger distribution ratio of austenite than the inside, and the slab center portion and the edge portion have a large difference in structure, resulting in a difference in rollability. It was confirmed through experiments that the rolling property is a direct cause of the edge crack if the difference in the center and the edge portion.

Based on the results of analyzing the cause of the edge crack, the present invention is to solve the difference in the rolling properties in the center and the edge by injecting high-pressure water to the upper and lower surfaces of the slab except the edge. At this time, the edge part can be about 10mm or more in the center direction from the edge, but it is good to remove the scale within 10mm. The scale at the edge part where high pressure water is not directly sprayed is removed by the high pressure water flowing down from the center, so if the area of the edge part where the high pressure water is not sprayed is too large, the scale removal is not smooth.

When controlling the high pressure water injection method according to the present invention, the edge crack is reduced and the magnetic properties are improved, but limiting the number of high pressure water injection edge edge is further reduced.

In addition to the high pressure water spraying method described above, the present inventors have confirmed that the high pressure water spray and itself are harmful to the edge cracks, and the edge crack of the hot rolled sheet can be suppressed while achieving the goal of removing oxide scale through several experiments. It is derived that there are high pressure water jets. That is, in order to remove the scale, high-pressure water is sprayed every pass in the rough rolling to remove the scale, but the present invention is limited to one to three times. If no high-pressure spraying is performed, the remaining scale is pressed into the inside of the steel plate during finishing rolling, and it is difficult to remove the scale in the pickling process, which is a post-process, which adversely affects the characteristics of the final product, making it difficult to use as a product. If it is less than 3 times, the quality characteristics of the surface are not affected at all and a good edge can be obtained. When the number of times of high pressure water injection according to the present invention is limited, high pressure water spraying is preferably performed in the first half of the rough rolling process.

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

Example 1

By weight% Si: 3.22%, C: 0.068%, Sol-Al: 0.029%, Mn: 0.078%, N: 0.091%, S: 0.016%, Cu: 0.09%, remaining Fe and other unavoidable impurities The steel made up was dissolved. The slab is cast from steel of this component and re-heated for a total of 290 minutes in a heating furnace with a maximum heating temperature of 1375 ℃. Then, a high-pressure water is injected in every pass with 7 passes of rough rolling to make a 40mm thick bar. Hot rolled to a thickness of 2.3 mm. In order to investigate the effect of hot water contact on the edge of hot-rolled sheet edge cracks and magnetism when removing the scale from rough rolling, high-pressure water is directly applied to the edge part. The method of the present invention without spraying was compared. In addition, the hot rolled sheet was subjected to heat treatment at 1120 ° C., and then cold rolled to 0.3 mm until the final thickness in 7 passes. The coil was decarbonized 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. The occurrence state of the edge crack of the hot rolled plate and the magnetization of the final high temperature annealing plate is shown in Table 1 below.

division Use of high pressure water at edge Hot Rolled Edge Crack Rate Magnetic flux density (B10, Tesla) Within 5mm Within 10mm Within 15mm Comparative example use 0 30 70 1.92 Inventive Example unused 25 75 0 1.93

As shown in the above table, when high-pressure water is not used at the edge portion when the scale is removed from the rough rolling, the occurrence rate of the coarse edge crack is significantly reduced and it does not affect the magnetism of the final product.

Example 2

By weight% Si: 3.19%, C: 0.076%, Sol-Al: 0.029%, Mn: 0.077%, N: 0.088%, S: 0.016%, Cu: 0.08%, remaining Fe and other unavoidable impurities The steel made up was dissolved. The slab was cast from steel of this component and re-milled for 300 minutes in a heating furnace with a maximum heating temperature of 1375 ° C. After extraction, a 40mm thick bar was made by roughing of 7 passes and hot-rolled to a thickness of 2.3mm by finishing rolling. . In order to investigate the effect of high pressure water injection frequency on the hot rolled sheet edge crack and magnetism in the rough rolling, the high pressure water injection frequency was changed. And did not spray in the second half. In addition, the hot rolled sheet was subjected to heat treatment at 1120 ° C., and then cold rolled to 0.3 mm until the final thickness in 7 passes. The coil was decarbonized 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. The occurrence state of the edge crack of the hot rolled plate and the magnetization of the final high temperature annealing plate is shown in Table 2 below.

High Pressure Water Injection Hot Rolled Edge Crack Rate Magnetic flux density (B10, Tesla) Within 5mm Within 10mm Within 15mm 0 95 5 0 1.94 One 90 10 0 1.93 2 90 10 0 1.93 3 90 10 0 1.93 4 50 40 10 1.93 5 30 45 25 1.92 6 10 45 45 1.92

As shown in Table 2, when the number of injections in the rough rolling is within 1 to 3, it can be seen that the incidence of coarse edge cracks is significantly reduced, and the magnetic properties of the final product are also good.

As described above, the present invention is a simpler method than the prior art, it is possible to significantly reduce the occurrence rate of coarse edge cracks, and further improve the magnetic properties of the final product.

Claims (4)

In the method for producing a grain-oriented electrical steel sheet including a hot rolling step of roughly rolling and finishing rolling while re-extracting silicon steel slab containing S by reheating to a high temperature in a heating furnace and spraying high pressure water, The rough rolling is hot rolling method of a grain-oriented electrical steel sheet having excellent magnetic properties, characterized in that the high-pressure water is directly sprayed on the upper and lower surfaces except for the edge portion of the silicon steel slab. The hot rolling method according to claim 1, wherein the high pressure water spray is performed 1-3 times. The method of claim 1, wherein the edge portion is hot rolling method, characterized in that less than 10mm in the center direction from the edge. The method of claim 1, wherein the steel slab is Si: 2.0-4.0%, C: 0.03-0.10%, Sol-Al: 0.01-0.05%, Mn: 0.03-0.10%, N: 0.005-0.015%, S: 0.007-0.030%, Cu: 0.01-0.15%, the remaining hot rolling method of the grain-oriented electrical steel sheet characterized by consisting of the remaining Fe and other inevitable impurities.