Method for manufacturing grain-oriented electrical steel sheets and assembly of equipment for manufacturing grain-oriented electrical steel sheets

Abstract

The present invention provides a method for producing a grain-oriented electrical steel sheet that has excellent magnetic characteristics over the entire length of a coil. A method for producing a grain-oriented electrical steel sheet according to the present invention comprises: a step for casting a molten steel having a specific component composition so as to obtain a slab having a thickness of 30 mm to 180 mm (inclusive); a step for subjecting the slab to rough rolling and finish rolling to obtain a hot rolled sheet, wherein (I) the rough rolling is started within 5 minutes from the time when the surface temperature of the slab falls to 1200°C, and during the time where the surface temperature of the slab is within the range from 900°C to 1150°C (inclusive), and (II) the finish rolling is started within 5 minutes from the end of the rough rolling, while maintaining the surface temperature of the slab within the range from 850°C to 1200°C (inclusive) after the end of the rough rolling; a step for cooling the hot-rolled sheet under the conditions where the cooling time until the surface temperature of the hot-rolled sheet falls to 650°C after the end of the rough rolling is 200 seconds or less; and a step for subsequentially performing cold rolling, decarburization annealing and finish annealing.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing grain-oriented electrical steel sheets, a grid of manufacturing equipment for grain-oriented electrical steel sheets, and hot-rolled sheets for grain-oriented electrical steel sheets. In particular, the present invention relates to a method for manufacturing grain-oriented electrical steel sheets having stable magnetic properties by utilizing a process for continuously manufacturing hot-rolled coils from casting. [Background technology]

[0002] Grain-oriented electrical steel sheets are primarily used as core materials for transformers and other electrical equipment. In recent years, there has been a growing demand for energy-saving core materials. Accordingly, grain-oriented electrical steel sheets, which are the core material, are required to have superior magnetic properties, namely low iron loss and high magnetic flux density.

[0003] Grain-oriented electrical steel sheets are iron with an easy magnetization axis. <001> The texture has a crystal structure in which the orientation is highly aligned with the rolling direction of the steel sheet. Such a texture is called the Goss orientation during the manufacturing process of grain-oriented electrical steel sheets, especially during finish annealing. <001> It is formed through secondary recrystallization, which preferentially promotes the growth of grains with a specific orientation. Therefore, the crystal orientation of the secondary recrystallized grains has a significant impact on the magnetic properties of grain-oriented electrical steel sheets.

[0004] Conventionally, grain-oriented electrical steel sheets are manufactured by the following process: A steel slab containing 4.5% by mass or less of Si and further containing inhibitor-forming elements such as MnS, MnSe, AlN, and BN is heated to 1300°C or higher and then hot-rolled to obtain a hot-rolled sheet. Next, the hot-rolled sheet is annealed as needed, and then cold-rolled once or twice or more with an intermediate annealing in between to obtain a cold-rolled sheet. Next, the cold-rolled sheet is decarburized annealed in a humid hydrogen atmosphere to perform primary recrystallization and decarburization to obtain a decarburized annealed sheet. After applying an annealing separating agent mainly composed of MgO to the decarburized annealed sheet, a finish annealing is performed at 1200°C for about 5 hours to perform secondary recrystallization and purify the inhibitor-forming elements (see, for example, Patent Documents 1 to 3).

[0005] Generally, the manufacturing process for grain-oriented electrical steel sheets is extremely costly because high-temperature slab heating is essential to solidify inhibitor-forming elements. To address this problem, a method has been developed that can induce secondary recrystallization without including inhibitor-forming elements, the so-called inhibitor-less method (see, for example, Patent Document 4). This method is based on a completely different technical concept from conventional methods for manufacturing grain-oriented electrical steel sheets. In other words, conventional methods used precipitates (inhibitors) such as MnS, MnSe, and AlN to induce secondary recrystallization. On the other hand, the inhibitor-less method does not use these inhibitors, but rather reduces resistance to grain boundary movement by increasing the purity of the material, thereby making the inherent difference in grain boundary movement speed, which depends on the characteristics of the grain boundaries, apparent and successfully inducing secondary recrystallization. Therefore, compared to the case where conventional inhibitors are used, high-temperature slab heating to solidify inhibitor-forming elements can be avoided.

[0006] When applying the inhibitor-less method described above, a conventional slab heating furnace (gas furnace) is used for heating the slab at low temperatures. Because it does not have strong inhibitors, if trace elements that inhibit grain growth are mixed into the steel, it has been observed that the quality of the slab becomes uneven due to temperature inconsistencies within the heating furnace.

[0007] Furthermore, as a means of avoiding high-temperature slab heating, processes are being investigated to continuously obtain hot-rolled coils from cast slabs. For example, the technology described in Patent Document 5 successfully obtains hot-rolled coils suitable for the grain-oriented electrical steel sheet process using conventional inhibitors by casting slabs up to 70 mm thick and then starting hot rolling at temperatures exceeding 1200°C to produce hot-rolled coils.

[0008] Furthermore, in order to obtain hot-rolled coils continuously from cast slabs, methods such as starting hot rolling while the slab is still at a temperature exceeding 1200°C after casting, as shown in Patent Document 5, have been considered in order to reproduce the "complete solid solution state of the inhibitor after slab reheating" that occurs in normal hot rolling, but these methods have been difficult to manufacture. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] U.S. Patent No. 1965559 [Patent Document 2] Special Publication No. 40-15644 [Patent Document 3] Special Publication No. 51-13469 [Patent Document 4] Japanese Patent Publication No. 2000-129356 [Patent Document 5] Japanese Patent Publication No. 2008-69391 [Overview of the project] [Problems that the invention aims to solve]

[0010] As mentioned above, the inhibitor-less method, because it does not contain strong inhibitor components, had problems such as uneven heating due to skids, for example, during slab heating in hot rolling, which could lead to differences in grain growth and a non-uniform structure, or the inclusion of trace components which would be amplified.

[0011] On the other hand, applying the process of continuously obtaining hot-rolled coils from slabs cast thinner than usual to the manufacturing process of grain-oriented electrical steel sheets requires maintaining extremely high temperatures during hot rolling to achieve a complete solid solution state of precipitates before the start of hot rolling, which presented challenges for stable production.

[0012] In other words, there is a need for a method to manufacture grain-oriented electrical steel sheets that have stable and excellent magnetic properties throughout the entire length of the coil, by advantageously solving the above-mentioned problems in the process of continuously obtaining hot-rolled coils from slabs while using a component composition that conforms to the inhibitor-less method.

[0013] In view of the above problems, the present invention aims to provide a method for manufacturing grain-oriented electrical steel sheets and a set of manufacturing equipment for grain-oriented electrical steel sheets that can produce grain-oriented electrical steel sheets having excellent magnetic properties along the entire length of the coil. Furthermore, the present invention aims to provide a hot-rolled sheet for grain-oriented electrical steel sheets that can be used as a material for grain-oriented electrical steel sheets having excellent magnetic properties. [Means for solving the problem]

[0014] The process of continuously obtaining hot-rolled coils from a slab (hereinafter also referred to as the continuous casting and hot-rolling process) can eliminate the step of slab reheating, which may suppress the factors causing quality variations due to grain growth during slab reheating, a characteristic of the inhibitor-less method. On the other hand, since the component system following the inhibitor-less method contains almost no inhibitor-forming elements, it is expected that a state of complete solid solution can be maintained even when the slab temperature is lowered to a relatively low temperature after casting. Therefore, the inventors have come to believe that by combining the continuous casting and hot-rolling process with the inhibitor-less method, it may be possible to manufacture grain-oriented electrical steel sheets with better magnetic properties more stably.

[0015] However, simply subjecting molten steel with a composition conforming to the inhibitor-free method to a continuous casting and hot-rolling process did not yield good magnetic properties, nor did it even allow for secondary recrystallization. Therefore, the inventors diligently pursued further research and arrived at the findings described in (1) to (6) below.

[0016] (1) When using a component composition that conforms to the inhibitor-free method, excessive grain growth may occur depending on the waiting time after processing and the transport time during hot rolling. By maintaining an appropriate reduction ratio, excessive coarsening of the microstructure can be suppressed, and texture deterioration before secondary recrystallization can be prevented. The appropriate slab thickness is not uniquely determined as it depends on the capacity of the hot rolling mill, but when the entire hot rolling process is performed at a high temperature, grain growth is more likely to occur, requiring high pressure, while when it is performed at a relatively low temperature, the desired microstructure can be controlled at a lower reduction, and there was a tendency to be able to reduce the slab thickness during casting.

[0017] (2) By adopting a component composition according to the inhibitorless method, even when the slab temperature becomes as low as 1200 °C or less, a state of complete solid solution or a state where precipitation of components contained in trace amounts does not progress can be maintained for a certain period of time. The inventors confirmed by experiments that precipitation does not progress for about 5 minutes from the time when the surface temperature of the slab after casting drops to 1200 °C. Therefore, it is recommended to start rough rolling within 5 minutes from the time when the surface temperature of the slab drops to 1200 °C after casting, while the surface temperature of the slab is within a predetermined range (900 °C or higher and 1150 °C or lower). Preferably, it is within 3 minutes.

[0018] However, generally, the speed of casting steel is very slow, so depending on the line length, it may be difficult to start rough rolling within the recommended time above. In this case, the situation where the temperature of the slab gradually decreases after casting is not desirable, and it is preferable to install a heating device (furnace) having a temperature higher than the surface temperature of the slab to avoid the state where the slab temperature gradually decreases. By suppressing the temperature drop of the slab by the heating device and performing soaking heat treatment, it is allowed to exceed the time until the start of the above-mentioned rough rolling. However, if the time until the start of rough rolling exceeds 20 minutes, precipitation of the inhibitor component contained in a small amount partially progresses, which rather promotes the non-uniformity of the structure and destabilizes the secondary recrystallization.

[0019] (3) It is desirable to perform rough rolling in two or more passes. The reason for this is that when the slab is cast relatively thick, there may be molten steel that has not yet solidified remaining in the center of the slab, so it is necessary to force the solidified parts to contact each other in the first pass of rough rolling. By doing so, a hot-rolled structure can be formed in the second pass and subsequent passes. By introducing dislocations into the structure through strong processing and introducing new recrystallization nuclei, excessive coarsening of the structure can be suppressed and the crystal grain size can be kept uniform.

[0020] (4) When the rough rolling in (3) above is carried out, it is particularly important to perform the following steps. Now, dislocations function as nuclei for recrystallization and also as nuclei when precipitates are formed. For this reason, even though the inhibitor-forming elements are in trace amounts, they are not completely at a content of "0", and a certain amount will precipitate after rough rolling. However, if the reduction in the final stand of rough rolling is severe plastic deformation, dislocations are introduced relatively uniformly into the structure, and a non-uniform precipitation state can also be avoided. If the slab surface temperature at the start of finish rolling is outside the predetermined temperature range (850°C or higher and 1200°C or lower), or if the time from the end of rough rolling to the start of finish rolling exceeds 5 minutes, the dispersion of precipitates becomes non-uniform, affecting even secondary recrystallization. In a continuous casting and hot rolling process, when the plate passing speed is slow and the line length is long, the time from the end of rough rolling to the start of finish rolling may exceed 5 minutes. In this case, there is also a method of increasing the plate passing speed by increasing the reduction ratio in rough rolling and stretching the slab in the rolling direction. However, when the thickness of the slab becomes thinner due to rough rolling, the temperature drop of the slab becomes significant, making it difficult to maintain the target temperature, so it is necessary to provide heating equipment as needed. In particular, it can be said that the time from the end of rough rolling to the start of finish rolling is a condition that should be strongly reflected in the line configuration.

[0021] (5) After finish rolling, the dislocation density becomes higher than that after rough rolling, and precipitates are more likely to form. By cooling to a lower temperature in a shorter time, non-uniform precipitation of inhibitor-forming elements can be suppressed.

[0022] (6) By optimizing the manufacturing conditions described above, it has become clear that it is possible to manufacture a hot-rolled sheet in which the content of precipitated Al targeted at Al nitride that generally functions as an inhibitor is only 1 / 4 or less of the content of acid-soluble Al originally contained in the steel. This hot-rolled sheet avoids a non-uniform precipitation state of trace components that cause secondary recrystallization defects in a continuous casting and hot rolling process, and has sufficient characteristics as a base material for an inhibitor-free component system grain-oriented electrical steel sheet.

[0023] The gist configuration of the present invention completed based on the above findings is as follows. [1] A process to obtain a slab with a thickness of 30 mm to 180 mm by casting molten steel having a composition containing C: 0.08 mass% or less, Si: 2.0 mass% to 4.5 mass%, Mn: 0.5 mass% or less, acid-soluble Al: 20 mass ppm to 120 mass ppm, S: less than 50 mass ppm, and N: less than 80 mass ppm, with the content of Se, Te, and O each suppressed to less than 50 mass ppm, and the remainder consisting of Fe and unavoidable impurities, The process involves subjecting the slab to rough rolling and finish rolling to obtain a hot-rolled sheet, (I) Within 5 minutes from the point when the surface temperature of the slab has dropped to 1200°C, and while the surface temperature of the slab is in the range of 900°C to 1150°C, the rough rolling shall be initiated. (II) After the rough rolling is completed, the process of starting the finish rolling within 5 minutes from the completion of the rough rolling, while maintaining the surface temperature of the slab in the range of 850°C to 1200°C. After the completion of the finish rolling, the process involves cooling the hot-rolled sheet under conditions such that the cooling time until the surface temperature of the hot-rolled sheet drops to 650°C is 200 seconds or less. Subsequently, the process involves cold rolling, decarburization annealing, and finish annealing. A method for manufacturing grain-oriented electrical steel sheets.

[0024] [2] A process to obtain a slab with a thickness of 30 mm to 180 mm by casting molten steel having a component composition containing C: 0.08 mass% or less, Si: 2.0 mass% to 4.5 mass%, Mn: 0.5 mass% or less, acid-soluble Al: 20 mass ppm to 120 mass ppm, S: less than 50 mass ppm, and N: less than 80 mass ppm, with the content of Se, Te, and O each suppressed to less than 50 mass ppm, and the remainder consisting of Fe and unavoidable impurities, After the surface temperature of the slab has dropped to 1200°C or below, the slab is held in an atmosphere of 1000°C to 1250°C for 5 minutes to 20 minutes. Subsequently, the slab is subjected to rough rolling and finish rolling to obtain a hot-rolled sheet, (I) While the surface temperature of the slab is in the range of 900°C to 1150°C, the rough rolling is started. (II) After the rough rolling is completed, the process of starting the finish rolling within 5 minutes from the completion of the rough rolling, while maintaining the surface temperature of the slab in the range of 850°C to 1200°C. After the completion of the finish rolling, the process involves cooling the hot-rolled sheet under conditions such that the cooling time until the surface temperature of the hot-rolled sheet drops to 650°C is 200 seconds or less. Subsequently, the process involves cold rolling, decarburization annealing, and finish annealing. A method for manufacturing grain-oriented electrical steel sheets.

[0025] [3] In the rough rolling, at the final stand, the reduction ratio is 20% or more and 50% or less, and the strain rate is 1.0 s. -1 The above 50s -1 A method for manufacturing grain-oriented electrical steel sheets as described in [1] or [2] above, carried out under the following conditions:

[0026] [4] A method for manufacturing grain-oriented electrical steel sheets according to any one of the above [1] to [3], wherein after the rough rolling is completed and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C to 1200°C.

[0027] [5] A method for manufacturing grain-oriented electrical steel sheets according to any one of the above [1] to [4], wherein the component composition further includes one or more selected from the group consisting of Ni: 0.005 to 1.50%, Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%, P: 0.0050 to 0.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200% by mass%.

[0028] [6] A continuous casting machine that continuously casts molten steel to produce slabs, A rough rolling mill for rough rolling the aforementioned slab, A finishing rolling mill that performs finish rolling on the slab after the rough rolling process to produce a hot-rolled sheet, A coiler for winding the hot-rolled sheet, A series of manufacturing equipment for grain-oriented electrical steel sheets arranged in order, A first heat treatment apparatus is located between the continuous casting machine and the roughing mill and adjusts the surface temperature of the slab to 900°C or more and 1200°C or less. A second heat treatment apparatus is located between the rough rolling mill and the finish rolling mill and adjusts the surface temperature of the slab after rough rolling to 850°C or higher and 1200°C or lower. A third heat treatment apparatus is located between the finish rolling mill and the coiler, and within 200 seconds after the completion of the finish rolling, adjusts the surface temperature of the hot-rolled sheet to 650°C or lower. It has, A series of manufacturing equipment for grain-oriented electrical steel sheets having a feed capacity that allows the slab to move from the roughing mill to the finish rolling mill within 5 minutes.

[0029] [7] The roughing mill, at the final stand, has a reduction ratio of 20% to 50% and a strain rate of 1.0 s -1 The above 50s -1 A series of manufacturing equipment for grain-oriented electrical steel sheets as described in [6] above, having the following rolling capacities:

[0030] [8] A hot-rolled sheet for grain-oriented electrical steel, containing C: 0.08 mass% or less, Si: 2.0 mass% or more and 4.5 mass% or less, Mn: 0.5 mass% or less, acid-soluble Al: 20 mass ppm or more and 120 mass ppm or less, S: less than 50 mass ppm, and N: less than 80 mass ppm, with the content of Se, Te, and O each suppressed to less than 50 mass ppm, and the remainder consisting of Fe and unavoidable impurities, wherein the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al is 0.25 or less.

[0031] [9] The hot-rolled sheet for grain-oriented electrical steel sheets according to [8] above, wherein the component composition further includes one or more selected from the group consisting of Ni: 0.005~1.50%, Sn: 0.001~0.50%, Sb: 0.005~0.50%, Cu: 0.01~0.50%, P: 0.0050~0.50%, Cr: 0.01~1.50%, Mo: 0.01~0.50%, B: 0.0001~0.0200%, Nb: 0.0005~0.0100%, Bi: 0.0001~0.0200%, and Ti: 0.0001~0.0200% by mass%. [Effects of the Invention]

[0032] According to the manufacturing method and production equipment array for grain-oriented electrical steel sheets according to the present invention, grain-oriented electrical steel sheets having excellent magnetic properties throughout the entire length of the coil can be manufactured. Furthermore, grain-oriented electrical steel sheets having excellent magnetic properties can be obtained using hot-rolled sheets for grain-oriented electrical steel sheets according to the present invention as a material.

[0033] According to the present invention, it is possible to solve both the problem caused by the non-uniformity of the microstructure during slab reheating, which is likely to occur in the inhibitorless method, and the problem that hot rolling must be performed at extremely high temperatures in the continuous casting and hot rolling process. Therefore, stable production of grain-oriented electrical steel sheets can be achieved using the continuous casting and hot rolling process. [Brief explanation of the drawing]

[0034] [Figure 1] This is a schematic diagram showing a series of manufacturing equipment 100 for grain-oriented electrical steel sheets according to the present invention. [Figure 2] This graph shows the change in surface temperature of slabs obtained by continuous casting under conditions 1 to 3 of Example 3. [Modes for carrying out the invention]

[0035] [Manufacturing method for grain-oriented electrical steel sheets] A method for manufacturing grain-oriented electrical steel sheets according to one embodiment of the present invention comprises the steps of: casting molten steel having a predetermined component composition to obtain a slab; subjecting the slab to rough rolling and finish rolling to obtain a hot-rolled sheet; cooling the hot-rolled sheet; and thereafter performing cold rolling, decarburization annealing, and finish annealing.

[0036] (Slab composition) First, we will explain the composition of the slab. Unless otherwise specified, "%" in relation to the components means mass percent. Similarly, unless otherwise specified, "ppm" refers to mass ppm.

[0037] C: 0.08% or less If carbon (C) remains in the final product sheet, it can cause magnetic aging and lead to magnetic degradation. If the C content of the slab is excessive, the load during the decarburization process becomes high, and the C content of the final product sheet cannot be sufficiently reduced. Therefore, the C content of the slab should be 0.08% or less. On the other hand, carbon has the function of suppressing grain coarsening during hot rolling and improving the microstructure before cold rolling. Furthermore, in cold rolling, carbon improves the texture after primary recrystallization through interaction with dislocations. From this viewpoint, a C content of 0.01% or more is preferable.

[0038] Si: 2.0% or more and 4.5% or less Si is an element that reduces iron loss by increasing electrical resistance. From the perspective of obtaining this effect, the Si content should be 2.0% or more. On the other hand, if the Si content is too high, cold rolling becomes extremely difficult, so the Si content should be 4.5% or less.

[0039] Mn: 0.5% or less If the Mn content is excessive, the primary recrystallization texture deteriorates, and secondary recrystallized grains highly concentrated in the Goss orientation cannot be obtained. From this viewpoint, the Mn content should be 0.5% or less. On the other hand, Mn is an element that has the effect of improving hot workability. From the viewpoint of obtaining this effect, it is preferable that the Mn content be 0.01% or more.

[0040] Acid-soluble Al: 20ppm or more and 120ppm or less Since this invention relates to an inhibitor-less method, the content of Al, which is an inhibitor-forming component, must be reduced as much as possible. From this viewpoint, the Al content should be 120 ppm or less. When applying the inhibitor-less method, if only secondary recrystallization is considered, Al is not necessarily required. However, Al has the effect of reducing O, which is an impurity, during the refining stage of molten steel, and during secondary recrystallization annealing, it can form a dense Al2O3 film on the surface, reducing the effects of nitriding from the atmosphere. For this reason, the Al content should be 20 ppm or more.

[0041] S: less than 50 ppm, N: less than 80 ppm Since this invention relates to an inhibitor-free method, the content of inhibitor-forming components, S and N, must be reduced as much as possible. If the S and N content is excessive, precipitation is more likely to occur in the continuous casting and hot rolling process, resulting in a non-uniform microstructure. Therefore, the S content should be less than 50 ppm and the N content less than 80 ppm. The lower limit for the S and N content is preferably 0 ppm. However, it is difficult to completely remove S and N, and extreme reductions in S and N lead to a significant increase in manufacturing costs. From the viewpoint of manufacturing costs, the S content is preferably 10 ppm or more and the N content is preferably 20 ppm or more.

[0042] Se, Te, and O: less than 50 ppm each If the content of Se and Te is excessive, sediments and tetraids are formed, making secondary recrystallization difficult. These elements are known to segregate at the center during casting. In this invention, where hot rolling is performed immediately after casting without reheating the slab, Se and Te may remain as coarse precipitates in the center of the cast slab, reducing ductility during subsequent cold rolling and causing the steel sheet to become brittle. Therefore, the content of Se and Te should be less than 50 ppm each, preferably 30 ppm or less. In addition, since O forms oxides and remains as inclusions in the final product, degrading magnetic properties, the O content needs to be kept below 50 ppm. The content of Se, Te, and O may be 0 ppm.

[0043] The remainder of the components other than those listed above consists of Fe and unavoidable impurities, but may optionally contain the following elements.

[0044] Ni: 0.005~1.50% Ni improves magnetic properties by increasing the uniformity of the hot-rolled sheet structure. From the viewpoint of obtaining this effect, it is preferable that the Ni content be 0.005% or more. On the other hand, if the Ni content is too high, secondary recrystallization becomes unstable, and the magnetic properties deteriorate. Therefore, when Ni is included, the Ni content should be 1.50% or less.

[0045] Sn:0.001~0.50%, Sb:0.005~0.50%, Cu:0.01~0.50% Sn, Sb, and Cu are elements that can sometimes be considered auxiliary inhibitors through grain boundary segregation, and may be useful in inhibitor-less processes that do not actively utilize inhibitors from precipitates. From this viewpoint, it is preferable that the Sn content be 0.001% or more, the Sb content be 0.005% or more, and the Cu content be 0.01% or more. On the other hand, if the content of these elements is excessive, the possibility of secondary recrystallization failure increases. Therefore, when these elements are included, their content should be 0.50% or less each.

[0046] P:0.0050~0.50%, Cr:0.01~1.50% P and Cr have the effect of improving the forsterite film formation reaction. From the viewpoint of obtaining this effect, it is preferable that the P content be 0.0050% or more and the Cr content be 0.01% or more. On the other hand, if the content of these elements is too high, the forsterite film formation may be accelerated too much, and the film may peel off. Therefore, when these elements are included, the P content should be 0.50% or less and the Cr content should be 1.50% or less.

[0047] Mo:0.01~0.50%, B:0.0001~0.0200%, Nb:0.0005~0.0100%, Bi:0.0001~0.0200%, and Ti:0.0001~0.0200% All of these elements contribute to suppressing grain growth, improving texture, and stabilizing secondary recrystallization. From the viewpoint of obtaining these effects, it is preferable that the Mo content be 0.01% or more, the B content be 0.0001% or more, the Nb content be 0.0005% or more, the Bi content be 0.0001% or more, and the Ti content be 0.0001% or more. On the other hand, if the content of these elements is excessive, they function as inhibitors, which is undesirable in the inhibitor-less method. Therefore, when including these elements, the Mo content should be 0.50% or less, the B content 0.0200% or less, the Nb content 0.0100% or less, the Bi content 0.0200% or less, and the Ti content 0.0200% or less.

[0048] (Casting process) In this embodiment, first, molten steel having the above-described component composition is cast to obtain a slab with a thickness of 30 mm to 180 mm. By appropriately adjusting the thickness of the slab after casting, it becomes possible to achieve sufficient microstructure control during the subsequent hot rolling. If the slab thickness is less than 30 mm, it is not possible to increase the reduction ratio in rough rolling and finish rolling. If the reduction ratio in rough rolling is insufficient, the uniformity of the microstructure is impaired. If the reduction ratio in finish rolling is insufficient, the dislocation density will not accumulate sufficiently during finish rolling, resulting in insufficient recrystallization in the next process of hot-rolled sheet annealing. If the slab thickness exceeds 180 mm, the engagement angle with the rolling rolls becomes large, making it difficult to perform strong processing. As a result, the reduction ratio per pass decreases, and as with the case of thicknesses less than 30 mm described above, the introduction of dislocations becomes insufficient, resulting in a lack of recrystallization nuclei and impaired microstructure uniformity.

[0049] (Hot rolling process) Following continuous casting, the slab is subjected to rough rolling and finish rolling to obtain a hot-rolled sheet.

[0050] In the first embodiment, (I) rough rolling is started within 5 minutes from the point when the slab surface temperature drops to 1200°C, and while the slab surface temperature is in the range of 900°C to 1150°C. Then, (II) after the rough rolling is completed, finish rolling is started within 5 minutes from the completion of rough rolling, while maintaining the slab surface temperature in the range of 850°C to 1200°C.

[0051] In the second embodiment, after the slab surface temperature has dropped to 1200°C or below, the slab is held in an atmosphere of 1000°C to 1250°C for 5 to 20 minutes. Then, (I) rough rolling is started while the slab surface temperature is in the range of 900°C to 1150°C. Then, (II) after the rough rolling is completed, finish rolling is started within 5 minutes of the completion of rough rolling, while maintaining the slab surface temperature in the range of 850°C to 1200°C.

[0052] Slab surface temperature at the start of rough rolling: 900°C to 1150°C Next, the slab obtained by casting is directly subjected to rough rolling. Rough rolling is started when the slab surface temperature is in the range of 900°C to 1150°C. If the slab surface temperature at the start of rough rolling is below 900°C, the content of precipitated Al in the hot-rolled sheet will be excessive, and the magnetic properties of the grain-oriented electrical steel sheet will deteriorate. If the slab surface temperature at the start of rough rolling exceeds 1150°C, breakout may occur, where the unsolidified molten steel in the center of the slab breaks through the slab surface layer (solidified layer) and flows out. In this invention, by using inhibitor-free components, it is possible to cool the slab to below 1200°C while suppressing precipitation in the steel, thus enabling more stable manufacturing.

[0053] Time from when the slab surface temperature drops to 1200°C until the start of rough rolling: within 5 minutes (First Embodiment) If heating is not applied from outside the manufacturing line, the slab temperature gradually decreases from casting to the start of rough rolling. In this case, the driving force for precipitation gradually increases. If the time from when the slab surface temperature drops to 1200°C until the start of rough rolling is long, the content of precipitated Al in the hot-rolled sheet becomes excessive, and the magnetic properties of the grain-oriented electrical steel sheet deteriorate. Therefore, in the first embodiment, this time is set to within 5 minutes, preferably within 3 minutes. If this time is excessively short, the slab surface temperature at the start of rough rolling becomes high, and breakout may occur. Therefore, it is preferable that this time be 1.0 minute or longer.

[0054] Slab insulation after the slab surface temperature has dropped to 1200°C or below (Second Embodiment) Excessively increasing the casting speed can lead to breakout. Therefore, if the line length to the roughing mill is long, roughing cannot be started within the recommended time in the first embodiment. In this case, to avoid a gradual increase in the precipitation driving force, the slab can be heated from the surroundings to maintain its temperature, extending the time until roughing can be started to 20 minutes. Specifically, the following steps can be added prior to roughing.

[0055] Specifically, after the slab surface temperature drops to 1200°C or below, the slab is held in an atmosphere of 1000°C to 1250°C for 5 to 20 minutes. More precisely, after the slab surface temperature drops to 1200°C or below, the slab is charged into the furnace and heated. From the viewpoint of suppressing the decrease in slab surface temperature at the start of rough rolling and staying within the recommended range, it is preferable that the atmosphere temperature inside the furnace be between 1000°C and 1250°C, and about 100°C (50 to 150°C) higher than the slab surface temperature when it is charged into the furnace. If the holding time is less than 5 minutes, the effect of slab heat retention cannot be sufficiently obtained, and if the holding time exceeds 20 minutes, the slightly contained inhibitor components partially precipitate, leading to non-uniformity of the structure. In this embodiment, since the slab is continuously supplied to the next process, it inevitably becomes an open furnace such as a tunnel furnace. However, if a more airtight structure can be applied, the furnace temperature can be set to the target temperature. Even in cases where isothermal preservation is required, a shorter preservation time is preferable.

[0056] Preferred conditions for rough rolling: At the final stand, the reduction ratio is 20% to 50%, and the strain rate is 1.0 s. -1 The above 50s -1 below The number of passes in rough rolling is not particularly limited, but it is preferable to have between 2 and 7 passes. When the slab is relatively thick, there may be residual unsolidified molten steel in the center of the slab. For this reason, it is preferable to force the solidified parts into contact in the first pass of rough rolling. After that, the hot-rolled structure can be developed in the second and subsequent passes. In rough rolling, the structure is strongly processed to introduce dislocations and new recrystallization nuclei, thereby suppressing excessive coarsening of the structure and maintaining a uniform grain size. For this reason, in the final stand, the reduction ratio is 20% or more, and the strain rate is 1.0 s. -1 The above conditions are preferable. While there are no particular upper limits on the reduction ratio and strain rate at the final stand, excessive processing will saturate the effect of maintaining uniform grain size, requiring equipment with high load-bearing capacity. Therefore, from the viewpoint of using less expensive equipment, the reduction ratio at the final stand should be 50% or less, and the strain rate 50s. -1 The following conditions are preferable.

[0057] The strain rate ε is calculated using the following Ekelund's formula. [Number] Here, v R is the roll peripheral speed (mm / s), R' is the roll radius (mm), h1 is the slab thickness on the roll inlet side (mm), and r is the reduction ratio (%).

[0058] Slab surface temperature at the start of finish rolling: 850°C or higher and 1200°C or lower Time from the end of rough rolling to the start of finish rolling: within 5 minutes The dislocations introduced in rough rolling function as nuclei for recrystallization and also as nuclei when precipitates are formed. Therefore, even if the inhibitor-forming elements are present in trace amounts and their total content is not completely "0", a certain amount of them precipitates after rough rolling. Precipitation progresses from rough rolling to finish rolling. Therefore, after the end of rough rolling, while maintaining the slab surface temperature within the range of 850°C or higher and 1200°C or lower, finish rolling must be started within 5 minutes from the end of rough rolling. If this temperature range is exceeded or the time until the start of finish rolling becomes long, the precipitation of inhibitor-forming elements progresses and the dispersion of inhibitor-forming elements becomes non-uniform, thus affecting secondary recrystallization. When the sheet passing speed is slow and the line length is long, the time from the end of rough rolling to the start of finish rolling may exceed 5 minutes. In this case, there is also a method of increasing the sheet passing speed by increasing the reduction ratio in rough rolling and stretching the slab in the rolling direction. However, when the slab thickness is reduced by rough rolling, the temperature drop of the slab becomes significant and it becomes difficult to maintain the slab temperature at the start of finish rolling within the predetermined range, so it is necessary to provide heating equipment as needed. However, the time from the end of rough rolling to the start of finish rolling cannot be extended even if heating equipment is installed. Therefore, it is necessary to adopt a line configuration considering this time. The lower limit of the time from the end of rough rolling to the start of finish rolling is not particularly limited, and this time can be 0 seconds or more. Incidentally, the thickness of the hot-rolled sheet after finish rolling is preferably 1.3 to 3.5 mm.

[0059] The surface temperature of the hot-rolled sheet at the end of finish rolling (final finishing temperature) is over 650°C, and from the viewpoint of rolling stability, it is preferable that it be between 700°C and 1100°C.

[0060] (cooling process) After the finish rolling is completed, the hot-rolled sheet is cooled under conditions such that the cooling time until the surface temperature of the hot-rolled sheet drops to 650°C is 200 seconds or less. After finish rolling, the dislocation density becomes even higher than after rough rolling, making it easier for precipitates to form. By cooling to a lower temperature in a shorter time, the non-uniform precipitation of inhibitor-forming elements can be suppressed. For this reason, the above cooling time is set to 200 seconds or less. The lower limit of the above cooling time is not particularly limited.

[0061] (Subsequent processes) Subsequently, the steel is subjected to cold rolling, decarburization annealing, and finish annealing to produce finished steel sheets. These subsequent processes are described below.

[0062] First, the hot-rolled sheet is annealed as needed. Then, the hot-rolled sheet is cold-rolled once or twice or more, with an intermediate annealing in between, to obtain a cold-rolled sheet. This cold rolling may be carried out at room temperature, or it may be warm-rolled, where the temperature of the hot-rolled sheet is raised to a higher temperature than room temperature, for example, around 250°C.

[0063] Subsequently, the cold-rolled sheet is subjected to decarburization annealing to obtain a decarburized annealed sheet. This decarburization annealing is also called primary recrystallization annealing. That is, the primary purpose of this annealing is to primary recrystallize the cold-rolled sheet having a rolled structure and adjust it to the primary recrystallized grain size that is optimal for secondary recrystallization. The second purpose of this annealing is to decarburize the carbon contained in the steel by using a humid hydrogen nitrogen or humid hydrogen argon atmosphere, and at the same time form an oxide film on the surface by the above annealing atmosphere. For this reason, it is desirable that the annealing temperature (holding temperature) for decarburization annealing be in the temperature range of 800°C to less than 950°C. Furthermore, to further improve the texture, it is effective to increase the heating rate during the heating process of decarburization annealing. Specifically, improvement can be expected by increasing the heating rate between 500°C and 700°C to 80°C / s or more.

[0064] Next, an annealing release agent is applied to the surface of the decarburized annealed sheet. To form a forsterite film on the surface of the steel sheet after finish annealing, magnesia (MgO) is used as the main component of the annealing release agent. At this time, adding appropriate amounts of Ti oxide or Sr compounds to the release agent can further facilitate the formation of the forsterite film. In particular, the addition of an auxiliary agent that promotes uniform forsterite film formation is also advantageous in improving the peelability of the film.

[0065] Next, finish annealing is performed for secondary recrystallization and forsterite film formation. The annealing atmosphere can be N2, Ar, H2, or a mixture of these gases. Since the deposition of trace components in the final product can lead to deterioration of magnetic properties, it is preferable to set the maximum annealing temperature to 1100°C or higher to purify the components. Since the grain-oriented electrical steel sheet obtained by this invention exhibits little variation in magnetic properties within the coil, it is desirable to perform finish annealing on coils weighing 5 tons or more, more preferably 10 tons or more, considering economic efficiency.

[0066] After the above-mentioned finish annealing, an insulating coating can be applied to the surface of the steel plate and baked. The type of insulating coating is not particularly limited, and any known insulating coating is suitable. For example, a preferred method is to apply a coating solution containing phosphate-chromate-colloidal silica, as described in Japanese Patent Publication No. 50-79442 and Japanese Patent Publication No. 48-39338, to the steel plate and bake it at about 800°C.

[0067] Regarding the grain-oriented electrical steel sheet obtained as the final product, the component composition of the steel sheet base, after purification during finish annealing and removal of the insulating and forsterite coatings, is as follows: The component composition, in mass%, contains Si: 2.0-4.5%, Mn: 0.01-0.5%, acid-soluble Al: 10 ppm to less than 60 ppm, and S: 5 ppm to less than 50 ppm, with the content of Se, Te, and O suppressed to less than 50 ppm each. Furthermore, the content of other elements in the steel may decrease depending on the finish annealing conditions, such as being incorporated into the forsterite coating or released into the gas phase, so the concentration will be lower than that of the slab.

[0068] [Hot-rolled sheet for grain-oriented electrical steel sheet] A hot-rolled sheet for grain-oriented electrical steel according to one embodiment of the present invention is a hot-rolled sheet obtained after the cooling step in the above manufacturing method. The component composition of the hot-rolled sheet of this embodiment is the same as the component composition of the slab described above, so that description shall be applied accordingly.

[0069] The hot-rolled sheet of this embodiment avoids the uneven precipitation of trace components that cause secondary recrystallization defects during the continuous process from casting to hot rolling, thus possessing sufficient properties as a base material for inhibitor-free grain-oriented electrical steel sheets. Specifically, the hot-rolled sheet of this embodiment is characterized by a value of 0.25 or less obtained by dividing the precipitated Al content by the acid-soluble Al content. In the inhibitor-free method, precipitates are not necessarily required for secondary recrystallization. However, if the precipitates (Al nitrides) precipitate unevenly, grain growth will also be uneven, making it difficult for the texture inhibition effect unique to the inhibitor-free method to manifest, resulting in secondary recrystallization defects. In hot-rolled sheets, if the temperature is held at high temperatures for a long time after precipitation, the precipitates (Al nitrides) undergo Oswald growth and become localized. By suppressing the above-mentioned precipitation rate in the hot-rolled sheet, this non-uniformity due to localization can be mitigated to a certain extent. By employing the manufacturing method of this embodiment, the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al can be set to 0.25 or less. A lower value is preferable, and it may be 0.00 or higher.

[0070] The content of precipitated aluminum shall be quantified by removing the surface layer from the top 1 / 4 of the thickness of the hot-rolled sheet (hereinafter also referred to as the surface 1 / 4), using the remaining central part of the sheet as a test specimen, electrolyzing the specimen with a 10% AA-based electrolyte (acetylacetone), filtering, and extracting the residue, and then analyzing the aluminum content of the residue. The content of acid-soluble aluminum shall be the value obtained by conventional wet analysis.

[0071] Furthermore, while the content of other inhibitor-forming elements such as S, Se, and Te is preferably 0 ppm, even if they are not completely removed and remain, a uniform precipitation state can be achieved in which the average precipitate particle size of these elements (MnS, MnSe, MnTe) is 80 nm or less.

[0072] [Manufacturing equipment for grain-oriented electrical steel sheets] Referring to Figure 1, a production equipment array 100 for grain-oriented electrical steel sheets according to one embodiment of the present invention comprises a continuous casting machine 1, a roughing mill 2, a finishing mill 3, and a coiler 4 arranged in order, enabling a continuous casting and hot-rolling process. The continuous casting machine 1 continuously casts molten steel to produce slabs. The roughing mill 2 rough-rolls the slabs obtained by the continuous casting machine 1. The finishing mill 3 finish-rolls the slabs after rough-rolling to produce hot-rolled sheets. The coiler 4 winds the obtained hot-rolled sheets to form hot-rolled coils.

[0073] The grain-oriented electrical steel sheet manufacturing equipment array 100 includes a first heat treatment device 5, a second heat treatment device 6, and a third heat treatment device 7.

[0074] The first heat treatment device 5 is located between the continuous casting machine 1 and the roughing mill 2, and adjusts the surface temperature of the slab to between 900°C and 1200°C. The first heat treatment device 5 measures the surface temperature of the slab just before it is loaded into the roughing mill 2 using a temperature sensing unit 8, and controls the slab temperature so that this measured value reaches the target temperature (between 900°C and 1200°C). Here, if the slab thickness is thin, the temperature will drop more easily, and conversely, if the slab thickness is thick, the temperature will not drop as easily. The holding time in the first heat treatment device 5 is determined by the distance from the continuous casting machine 1 to the roughing mill 2 and the casting speed. For this reason, it is necessary to cool the slab after continuous casting at an appropriate cooling rate to control it to the target temperature. For this purpose, it is desirable that the first heat treatment device 5 has the functions of water cooling, air cooling, heat retention, and heating. In particular, when casting is performed with a fixed slab thickness and casting speed, it is necessary to have one or more of the above functions in order to achieve the target temperature.

[0075] The second heat treatment device 6 is located between the roughing mill 2 and the finishing mill 3 and adjusts the surface temperature of the slab after roughing to between 850°C and 1200°C. Since the transport time from the roughing mill 2 to the finishing mill 3 must be kept short, the distance between the roughing and finishing mills where the second heat treatment device 6 is located must be determined so that the surface temperature of the slab after roughing reaches the target temperature of 850°C to 1200°C. Therefore, it is desirable that the second heat treatment device 6 has the functions of cooling, heat retention, and heating in order to achieve the target temperature at the entry side of the finishing mill 3. Note that, as with the first heat treatment device 5, if the slab thickness, temperature, and feed speed are fixed to some extent, it is sufficient to have one or more of the above functions. In many cases, rough rolling thins the slab and lowers its temperature, necessitating a heating function. This heating requires a mechanism to control the temperature at a fixed distance, making induction heating an effective solution.

[0076] The third heat treatment device 7 is located between the finishing rolling mill 3 and the coiler 4, and adjusts the surface temperature of the hot-rolled sheet to 650°C or below within 200 seconds after the completion of finishing rolling. The third heat treatment device 7 mainly controls cooling in order to adjust the surface temperature of the hot-rolled sheet to 650°C or below after finishing rolling, and therefore needs to have a water cooling mechanism in order to lower the surface temperature of the hot-rolled sheet to 650°C or below within a limited time.

[0077] The grain-oriented electrical steel sheet manufacturing equipment array 100 must have a feed capacity that allows the slab to move from the roughing mill 2 to the finishing mill 3 within 5 minutes.

[0078] In the roughing mill 2, at the final stand, the reduction ratio is 20% to 50%, and the strain rate is 1.0 s. -1 The above 50s -1 It is preferable that the rolling capacity be as follows. [Examples]

[0079] [Example 1] Molten steel was refined, having a composition containing C:0.05%, Si:3.2%, Mn:0.05%, acid-soluble Al:80ppm, S:20ppm, N:30ppm, Se:5ppm or less, Te:5ppm or less, O:20ppm or less, P:0.01%, and Cr:0.03%, with the remainder being Fe and unavoidable impurities. The molten steel was then continuously cast to obtain slabs of the thickness shown in Table 1. Subsequently, hot rolling consisting of rough rolling (4 passes in total) and finish rolling was performed under the conditions shown in Table 1, immediately following casting, to obtain hot-rolled sheets of the thickness shown in Table 1. The hot-rolled sheets were then continuously cooled under the conditions shown in Table 1. After the completion of finish rolling, the cooling time until the surface temperature of the hot-rolled sheets reached 650°C was adjusted by controlling the amount of water.

[0080] Test specimens were taken from a portion of the obtained hot-rolled sheet using the method described above, and the content of precipitated Al was determined. Table 1 shows the content of precipitated Al and the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al (precipitated Al / Sol.Al).

[0081] A hot-rolled sheet was annealed at a soaking temperature of 1020°C for 40 seconds, scale was removed by pickling, and then cold-rolled to a final thickness of 0.27 mm. This cold-rolled sheet was subjected to annealing for decarburization and primary recrystallization to obtain a decarburized annealed sheet. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and a finish annealing process including a secondary recrystallization process and a purification process was performed at a maximum temperature of 1200°C for 10 hours to obtain a finish annealed steel sheet. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish annealed steel sheet and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0082] Thirty-two Epstein test specimens were taken from the obtained grain-oriented electrical steel sheets, and the magnetic flux density (B8) was measured by the Epstein test. The average values ​​of the magnetic flux density (B8) are shown in Table 1. As is clear from Table 1, good magnetic properties were obtained in the grain-oriented electrical steel sheets manufactured under the conditions according to the present invention.

[0083] [Table 1]

[0084] [Example 2] Molten steel having the component composition shown in Table 2 (the remainder being Fe and unavoidable impurities) was refined. Then, hot rolling consisting of casting, rough rolling and finish rolling, and cooling were performed under the conditions of No. 3 in Example 1 to obtain rolled plates.

[0085] Test specimens were taken from a portion of the obtained hot-rolled sheet using the method described above, and the content of precipitated Al was determined. Table 2 shows the content of precipitated Al and the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al (precipitated Al / Sol. Al). In addition, for test specimens taken using the same method, the cross-section in the direction parallel to rolling was 1 mm. 2 SEM observation was performed on the target, and the arithmetic mean of the individual particle sizes of the precipitated MnS was calculated to obtain the average particle size. The results are shown in Table 2.

[0086] Hot-rolled sheets were annealed at a soaking temperature of 900°C for 60 seconds, and scale was removed by pickling. Then, a first cold-rolling was performed to a thickness of 1.5 mm, followed by intermediate annealing at a maximum temperature of 1050°C, and a second cold-rolling to a final thickness of 0.23 mm. This cold-rolled sheet was subjected to annealing for decarburization and primary recrystallization to obtain a decarburized annealed sheet. Subsequently, an annealing separation agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and a finish annealing process including a secondary recrystallization and purification process was performed at a maximum temperature of 1150°C for 10 hours to obtain a finish annealed steel sheet. An insulating coating consisting of colloidal silica and magnesium phosphate was applied to the surface of the finish annealed steel sheet and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0087] Thirty-two Epstein test specimens were taken from the obtained grain-oriented electrical steel sheets, and the magnetic flux density (B8) was measured by the Epstein test. The average values ​​of the magnetic flux density (B8) are shown in Table 2. As is clear from Table 2, good magnetic properties were obtained in the grain-oriented electrical steel sheets manufactured under the conditions according to the present invention.

[0088] [Table 2]

[0089] [Example 3] Molten steel was refined, containing C:0.08%, Si:3.4%, Mn:0.07%, acid-soluble Al:100ppm, S:10ppm, N:60ppm, Se:5ppm or less, Te:5ppm or less, and O:20ppm or less, with the remainder being Fe and unavoidable impurities. The molten steel was then continuously cast to obtain slabs with a thickness of 40 mm. After casting, experiments were conducted under three conditions: Condition 1, where the slab was allowed to cool naturally; Condition 2, where the slab was placed in a furnace with an ambient temperature of 1000°C 4 minutes after the start of casting and held there; and Condition 3, where the slab was placed in a furnace with an ambient temperature of 1100°C 4 minutes after the start of casting, and then changed to a furnace with an ambient temperature of 1000°C 2 minutes later and held there. Figure 2 shows the change in the surface temperature of the slab with respect to time from the start of casting under each condition. Under Condition 1, a temperature drop is constant, and 5 minutes after the slab surface temperature reaches 1200°C (6.5 minutes after the start of casting), the slab surface temperature is approximately 900°C. Under Conditions 2 and 3, the temperature drop is suppressed by charging into the furnace, and the slab is uniformly heated at approximately 1000°C after being charged into the furnace. Under Condition 3, it can be seen that the slab transitions to uniform heating with less temperature drop by passing through a furnace with a temperature 100°C higher than the slab surface temperature at the time of charging. Table 3 shows the slab heating / rough rolling conditions for Conditions 1 to 3.

[0090] Next, rough rolling consisting of two passes was performed under the conditions shown in Table 3, with a reduction ratio of 25% and a strain rate of 3s in the second pass. -1 The material was rolled down under the specified conditions to form a sheet bar with a thickness of 20 mm. Subsequently, it was held or heated with an external heater until finish rolling, and finish rolling was started under the conditions shown in Table 3. Final finishing temperature: After finishing rolling at 800-1050°C to form a hot-rolled sheet with a thickness of 2.0 mm, it was cooled to below 650°C in 100 seconds.

[0091] Test specimens were taken from a portion of the obtained hot-rolled sheet using the method described above, and the content of precipitated Al was determined. Table 3 shows the content of precipitated Al and the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al (precipitated Al / Sol.Al).

[0092] A hot-rolled sheet was annealed at a soaking temperature of 1000°C for 20 seconds, scale was removed by pickling, and then cold-rolled to a final thickness of 0.23 mm. This cold-rolled sheet was subjected to annealing for decarburization and primary recrystallization to obtain a decarburized annealed sheet. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and a finish annealing process including a secondary recrystallization process and a purification process was performed at a maximum temperature of 1150°C for 8 hours to obtain a finish annealed steel sheet. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish annealed steel sheet and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0093] Thirty-two Epstein test specimens were taken from the obtained grain-oriented electrical steel sheets, and the magnetic flux density (B8) was measured by the Epstein test. The average values ​​of the magnetic flux density (B8) are shown in Table 3. As is clear from Table 3, good magnetic properties were obtained in the grain-oriented electrical steel sheets manufactured under the conditions according to the present invention.

[0094] [Table 3]

[0095] [Example 4] Hot-rolled sheets were manufactured from molten steel having a composition of C:0.04%, Si:3.2%, Mn:0.06%, acid-soluble Al:50ppm, S:30ppm, N:50ppm, Se:1ppm, Te:0.3ppm, and O:10ppm, with the remainder being Fe and unavoidable impurities, using a series of equipment with the following specifications and under the following conditions. 1) [Continuous casting machine] Casting thickness: 80mm 2) [Continuous casting machine] Casting speed: 5m / min 3) [First heat treatment apparatus] Post-casting cooling zone length: 15m (passage time required: 3 minutes) 4) [First heat treatment apparatus] Cooling section after the cooling zone until rough rolling: 20m (passage time required: 4 minutes) 5) [Roughing mill] 3 stands, WR diameter: 750 mm (3-stand reduction ratio 35%) 6) [Roughing mill] Exit bar thickness: 25 mm (Exit speed: 16 m / min) 7) [Second heat treatment equipment] Cooling section until finish rolling: 60m (Required passage time: 3.8 minutes) 8) [Finishing Rolling Mill] 7 stands, WR diameter: 700mm 9) [Finishing Rolling Mill] Exit plate thickness: 2.5 mm (Exit speed: 160 m / min) 10) [Third heat treatment apparatus] Cooling section after finish rolling: 50m (passage time required: 0.3 minutes) 11) [Third heat treatment apparatus] Cooling zone length: 200m (transmission time required: 1.3 minutes) Temperature control was performed in steps 4), 7), and 10) above, and feedback to the preceding process was used to appropriately control the casting speed in step 2) and the cooling water control in step 3) of the cooling zone. The coil temperature after step 11) was kept below 650°C. This embodiment 4 is a hot-rolled sheet manufactured under the manufacturing conditions corresponding to claim 1.

[0096] The obtained hot-rolled coils were divided into 10t sections, and the hot-rolled sheets were subjected to hot-rolled sheet annealing at a soaking temperature of 1030°C for 50 seconds. After removing scale by pickling, cold-rolled sheets with a final thickness of 0.27 mm were obtained. These cold-rolled sheets were heated to 800°C at a heating rate of 150°C / s, and then decarburized by soaking with an N2-H2 mixed wet gas to obtain decarburized annealed sheets. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and finish annealing including a secondary recrystallization process and a purification process was performed at a maximum temperature of 1200°C for 10 hours to obtain finish annealed steel sheets. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish annealed steel sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0097] From the obtained grain-oriented electrical steel sheet coil, 32 Epstein test specimens were taken every 200 m along the length of the coil, and the magnetic flux density (B8) was measured by the Epstein test. The magnetic flux density (B8) was 1.903 ± 0.003 T, showing extremely low variability. By applying the present invention method, unlike the conventional slab reheating method, the entire length of the coil is processed continuously, making it possible to manufacture a coil (hot-rolled sheet) in which the influence of transient areas such as slab edges and non-uniformity during slab heating is almost negligible throughout the entire length.

[0098] [Example 5] Hot-rolled sheets were manufactured from molten steel having the same chemical composition as in Example 4, using a set of equipment with the following specifications and under the following conditions. 1) [Continuous casting machine] Casting thickness: 80mm 2) [Continuous casting machine] Casting speed: 3m / min 3) [First heat treatment apparatus] Post-casting cooling zone length: 15m (passage time required: 5 minutes) 4) [First heat treatment apparatus] Cooling section after the cooling zone until rough rolling: 20m (requires passage time of 6.7 minutes) 5) [Roughing mill] 3 stands, WR diameter: 750 mm (3-stand reduction ratio 38%) 6) [Roughing mill] Exit bar thickness: 20 mm (Exit speed: 12 m / min) 7) [Second heat treatment equipment] Cooling section before finish rolling: 60m (passage time required: 5 minutes) 8) [Finishing Rolling Mill] 7 stands, WR diameter: 700mm 9) [Finishing Rolling Mill] Exit plate thickness: 2.0 mm (Exit speed: 120 m / min) 10) [Third heat treatment apparatus] Cooling section after finish rolling: 50m (Required passage time: 0.4 minutes) 11) [Third heat treatment apparatus] Cooling zone length: 200m (transmission time required: 1.7 minutes)

[0099] In the above process, since the time spent in the cooling zone from the cooling zone to rough rolling exceeds 5 minutes, an enclosure was provided around the slab, and heating was performed in a nitrogen gas environment to maintain the slab surface at a constant 1100°C. In addition, to shorten the time spent from rough rolling to finish rolling, the bar thickness after rough rolling was reduced. To compensate for the decrease in surface temperature of the thinner bar, a bar heater was installed between rough rolling and finish rolling, and the temperature at the entrance to finish rolling was set to 1100°C. 11) The coil temperature after this was kept below 650°C. This embodiment 5 is a hot-rolled sheet manufactured under the manufacturing conditions corresponding to claim 2.

[0100] The obtained hot-rolled coils were divided into 10t sections, and the hot-rolled sheets were subjected to hot-rolled sheet annealing at a soaking temperature of 1030°C for 50 seconds. After removing scale by pickling, cold-rolled sheets with a final thickness of 0.23 mm were obtained. These cold-rolled sheets were heated to 800°C at a heating rate of 150°C / s, and then decarburized by soaking with an N2-H2 mixed wet gas to obtain decarburized annealed sheets. Subsequently, an annealing separation agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and finish annealing including a secondary recrystallization process and a purification process was performed at a maximum temperature of 1200°C for 10 hours to obtain finish annealed steel sheets. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish annealed steel sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0101] From the obtained grain-oriented electrical steel sheet coil, 32 Epstein test specimens were taken every 200 m along the length of the coil, and the magnetic flux density (B8) was measured by the Epstein test. The magnetic flux density (B8) was 1.901 ± 0.002 T, showing extremely low variability. By applying the present invention method, unlike the conventional slab reheating method, the entire length of the coil is processed continuously, making it possible to manufacture a coil (hot-rolled sheet) in which the influence of transient areas such as slab edges and non-uniformity during slab heating is almost negligible throughout the entire length. [Industrial applicability]

[0102] This invention can be applied to the manufacture of grain-oriented electrical steel sheets. [Explanation of symbols]

[0103] 100-piece grid of manufacturing equipment for grain-oriented electrical steel sheets 1. Continuous casting machine 2 Roughing mill 3. Finishing Rolling Mill 4 coilers 5. First Heat Treatment Apparatus 6. Second Heat Treatment Apparatus 7. Third Heat Treatment Apparatus 8. Temperature sensing section 20 Final Stand

Claims

1. A process to obtain a slab with a thickness of 30 mm to 180 mm by casting molten steel having a component composition containing C: 0.08 mass% or less, Si: 2.0 mass% to 4.5 mass%, Mn: 0.5 mass% or less, acid-soluble Al: 20 mass ppm to 120 mass ppm, S: less than 50 mass ppm, and N: less than 80 mass ppm, with the content of Se, Te, and O each suppressed to less than 50 mass ppm, and the remainder consisting of Fe and unavoidable impurities, The process involves subjecting the slab to rough rolling and finish rolling to obtain a hot-rolled sheet, (I) Within 5 minutes from the point when the surface temperature of the slab has dropped to 1200°C, and while the surface temperature of the slab is in the range of 900°C to 1150°C, the rough rolling is started. (II) After the rough rolling is completed, the step of starting the finish rolling within 5 minutes from the completion of the rough rolling, while maintaining the surface temperature of the slab in the range of 850°C to 1200°C, After the completion of the finish rolling, the process involves cooling the hot-rolled sheet under conditions such that the cooling time until the surface temperature of the hot-rolled sheet drops to 650°C is 200 seconds or less. Subsequently, the process involves cold rolling, decarburization annealing, and finish annealing. A method for manufacturing grain-oriented electrical steel sheets.

2. A process to obtain a slab with a thickness of 30 mm to 180 mm by casting molten steel having a component composition containing C: 0.08 mass% or less, Si: 2.0 mass% to 4.5 mass%, Mn: 0.5 mass% or less, acid-soluble Al: 20 mass ppm to 120 mass ppm, S: less than 50 mass ppm, and N: less than 80 mass ppm, with the content of Se, Te, and O each suppressed to less than 50 mass ppm, and the remainder consisting of Fe and unavoidable impurities, After the surface temperature of the slab has dropped to 1200°C or below, the slab is held in an atmosphere of 1000°C to 1250°C for 5 minutes to 20 minutes. Subsequently, the slab is subjected to rough rolling and finish rolling to obtain a hot-rolled sheet, (I) While the surface temperature of the slab is in the range of 900°C to 1150°C, the rough rolling is started. (II) After the rough rolling is completed, the step of starting the finish rolling within 5 minutes from the completion of the rough rolling, while maintaining the surface temperature of the slab in the range of 850°C to 1200°C, After the completion of the finish rolling, the process involves cooling the hot-rolled sheet under conditions such that the cooling time until the surface temperature of the hot-rolled sheet drops to 650°C is 200 seconds or less. Subsequently, the process involves cold rolling, decarburization annealing, and finish annealing. A method for manufacturing grain-oriented electrical steel sheets.

3. In the aforementioned rough rolling process, at the final stand, the reduction ratio is 20% to 50%, and the strain rate is 1.0 s. －１ The above 50s －１ A method for manufacturing grain-oriented electrical steel sheets according to claim 1, carried out under the following conditions.

4. In the aforementioned rough rolling process, at the final stand, the reduction ratio is 20% to 50%, and the strain rate is 1.0 s. －１ The above 50s －１ A method for manufacturing grain-oriented electrical steel sheets according to claim 2, carried out under the following conditions.

5. A method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein, after the completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C to 1200°C.

6. A method for manufacturing grain-oriented electrical steel sheets according to claim 2, wherein after the completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C to 1200°C.

7. A method for manufacturing grain-oriented electrical steel sheets according to claim 3, wherein, after the completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C to 1200°C.

8. A method for manufacturing grain-oriented electrical steel sheets according to claim 4, wherein, after the completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C to 1200°C.

9. A method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 8, wherein the aforementioned component composition further includes one or more selected from the group consisting of Ni: 0.005 to 1.50%, Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%, P: 0.0050 to 0.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200%, in mass%,

10. A continuous casting machine that continuously casts molten steel to produce slabs, A rough rolling mill for rough rolling the aforementioned slab, A finishing rolling mill that performs finish rolling on the slab after the rough rolling process to produce a hot-rolled sheet, A coiler for winding the hot-rolled sheet, A series of manufacturing equipment for grain-oriented electrical steel sheets arranged in order, A first heat treatment apparatus is located between the continuous casting machine and the roughing mill and adjusts the surface temperature of the slab to 900°C or more and 1200°C or less. A second heat treatment apparatus is located between the rough rolling mill and the finish rolling mill and adjusts the surface temperature of the slab after rough rolling to 850°C or higher and 1200°C or lower. A third heat treatment apparatus is located between the finish rolling mill and the coiler, and within 200 seconds after the completion of the finish rolling, adjusts the surface temperature of the hot-rolled sheet to 650°C or lower. It has, A grid of manufacturing equipment for grain-oriented electrical steel sheets, having a feed capacity that enables the movement of the slab from the roughing mill to the finish rolling mill within 5 minutes, for carrying out the manufacturing method of grain-oriented electrical steel sheets according to any one of claims 1 to 8.

11. The aforementioned roughing mill, at the final stand, has a reduction ratio of 20% to 50% and a strain rate of 1.0 s. －１ The above 50s －１ A row of manufacturing equipment for grain-oriented electrical steel sheets according to claim 10, having the following rolling capacity.

12. A continuous casting machine for continuously casting molten steel to produce a slab, A rough rolling mill for rough rolling the aforementioned slab, A finishing rolling mill that performs finish rolling on the slab after the rough rolling process to produce a hot-rolled sheet, A coiler for winding the hot-rolled sheet, A series of manufacturing equipment for grain-oriented electrical steel sheets arranged in order, A first heat treatment apparatus is located between the continuous casting machine and the roughing mill and adjusts the surface temperature of the slab to 900°C or more and 1200°C or less. A second heat treatment apparatus is located between the rough rolling mill and the finish rolling mill and adjusts the surface temperature of the slab after rough rolling to 850°C or higher and 1200°C or lower. A third heat treatment apparatus is located between the finish rolling mill and the coiler, and within 200 seconds after the completion of the finish rolling, adjusts the surface temperature of the hot-rolled sheet to 650°C or lower. It has, A grid of manufacturing equipment for grain-oriented electrical steel sheets, having a feed capacity that enables the movement of the slab from the roughing mill to the finish rolling mill within 5 minutes, for carrying out the manufacturing method of grain-oriented electrical steel sheets according to claim 9.

13. The roughing mill has a rolling capacity such that, at the final stand, the reduction ratio is 20% or more and 50% or less, and the strain rate is 1.0 s⁻¹ or more and 50 s⁻¹ or less, in the assembly of equipment for manufacturing grain-oriented electrical steel sheets according to claim 12.

Images