Method for producing hot-rolled sheet for grain-oriented electrical steel sheet, method for producing grain-oriented electrical steel sheet, production equipment line for hot-rolled sheet for grain-oriented electrical steel sheet, and hot-rolled sheet for grain-oriented electrical steel sheet

Abstract

The present invention provides a method for producing a hot-rolled sheet that is suitable for production of a grain-oriented electrical steel sheet having stable magnetic properties, by a continuous process which uses a component composition in accordance with an inhibitorless method and in which steps from casting of molten steel to production of the hot-rolled sheet are performed with a sequential equipment line. A method for producing a hot-rolled sheet for a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises: a step for continuously casting molten steel having a prescribed component composition to produce a slab having a thickness of 30-80 mm; a heating step for subsequently heating the slab before the temperature of a surface of the slab becomes lower than a prescribed value; a hot rolling step for subsequently hot rolling the slab to obtain a hot-rolled sheet; and a step for subsequently cooling the hot-rolled sheet. The method is characterized in that conditions in each step are optimized and, in particular, in the final pass of hot rolling, the rolling reduction ratio is 5.0-50.0% and the strain rate is not less than 50.0 s-1.

Description

Method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets, method for manufacturing grain-oriented electrical steel sheets, equipment array for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets, and hot-rolled sheets for grain-oriented electrical steel sheets

[0001] The present invention relates to a method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets, a method for manufacturing grain-oriented electrical steel sheets, a set of equipment for manufacturing hot-rolled sheets 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 hot-rolled sheets suitable for manufacturing grain-oriented electrical steel sheets having stable magnetic properties by a continuous process in which the process from casting molten steel to manufacturing the hot-rolled sheets is carried out using a series of equipment.

[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 have a crystalline structure in which the <001> orientation, which is the easy magnetization axis of iron, is highly aligned in the rolling direction of the steel sheet. Such a texture is formed through secondary recrystallization during the manufacturing process of grain-oriented electrical steel sheets, particularly during finish annealing, which preferentially causes the growth of crystal grains with the {110}<001> orientation, known as the Goss orientation. Therefore, the crystal orientation of the secondary recrystallized grains has a significant influence on the magnetic properties of the grain-oriented electrical steel sheet.

[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 of grain-oriented electrical steel sheets requires high-temperature slab heating to solidify inhibitor-forming elements, resulting in extremely high manufacturing costs. To address this problem, a method has been developed that can induce secondary recrystallization without containing 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 utilize 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 increases the purity to reduce resistance to grain boundary movement, thereby making the inherent difference in grain boundary movement speed, which depends on the grain boundary characteristics, 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-free method described above, a conventional slab heating furnace (gas furnace) is used for heating the slab at low temperatures. Because the composition does not contain strong inhibitors, if trace elements with grain growth inhibiting properties are mixed into the steel, it has been observed that the quality within the slab becomes uneven due to temperature inconsistencies in the heating furnace.

[0007] Furthermore, as a means of avoiding high-temperature slab heating, a process for continuously obtaining hot-rolled coils from cast slabs is also being investigated. For example, in the technology described in Patent Document 5, after casting a slab with a maximum thickness of 70 mm, hot rolling is started at a temperature exceeding 1200°C to produce hot-rolled coils, thereby successfully obtaining hot-rolled coils suitable for the manufacturing process of grain-oriented electrical steel sheets using conventional inhibitors.

[0008] However, 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.

[0009] U.S. Patent No. 1,965,559, Japanese Patent Publication No. 40-15644, Japanese Patent Publication No. 51-13469, Japanese Unexamined Patent Publication No. 2000-129356, Japanese Unexamined Patent Publication No. 2008-69391

[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 view of the above problems, the present invention aims to provide a method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets and a set of manufacturing equipment that enables the production of hot-rolled sheets suitable for the manufacture of grain-oriented electrical steel sheets having stable magnetic properties by a continuous process in which the process from casting of molten steel to the manufacture of hot-rolled sheets is carried out using a component composition conforming to the inhibitorless method and a series of equipment.

[0013] A continuous process (hereinafter also referred to as the continuous casting and hot-rolling process) that carries out the process from molten steel casting to the manufacture of hot-rolled sheets using a series of equipment eliminates the step of slab reheating, which has the potential to suppress the factors causing quality variations due to grain growth during slab reheating, which are unique to the inhibitor-less method. On the other hand, since the component composition according to 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.

[0014] However, simply subjecting molten steel with a composition conforming to the inhibitor-less 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) and (2) below.

[0015] (1) In order to produce hot-rolled sheets from molten steel with a composition conforming to the inhibitor-free method by a continuous casting and hot-rolling process, and then to induce secondary recrystallization, it was necessary to optimize the conditions of the continuous casting and hot-rolling process. Specifically, it was found that the following conditions (A) to (D) are important. (A) The slab thickness in the casting process should be 30 mm or more and 80 mm or less. (B) After casting, heating should be performed under predetermined conditions (heating time: 5 minutes or more and 25 minutes or less, maximum temperature reached: 1000°C or more and 1290°C or less) before the surface temperature of the slab falls below 850°C. (C) The surface temperature of the slab at the start of hot rolling should be 900°C or more and 1250°C or less, and a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less should be obtained by hot rolling in 3 to 7 passes. (D) Within 200 seconds after the end of hot rolling, the hot-rolled sheet should be cooled so that the surface temperature of the hot-rolled sheet is 650°C or less.

[0016] (2) By satisfying all of the above conditions (A) to (D), it was possible to manufacture hot-rolled sheets from molten steel with a composition conforming to the inhibitorless method by a continuous casting and hot-rolling process, and then induce secondary recrystallization. However, the magnetic properties obtained after secondary recrystallization were not necessarily sufficient compared to the case in which a general slab reheating process was adopted.

[0017] An investigation into these problems revealed that the texture during primary recrystallization was degraded compared to the common process of reheating the slab after it had cooled to below 500°C following casting. The differences in texture were mainly observed in the orientation group belonging to <100> / / ND. This was thought to be due to the fact that when casting thin slabs compared to casting slabs with a normal thickness of 200 mm or more, the proportion of columnar crystals in the slab increases, and as a result, the grain size of the hot-rolled sheet becomes coarser due to the influence of the texture of the columnar crystals.

[0018] Therefore, we investigated the conditions for controlling the crystal grain size to an appropriate level by recrystallizing the microstructure after casting during hot rolling. We found that in the final pass of hot rolling, the reduction ratio should be between 5.0% and 50.0%, and the strain rate should be 50.0 s. －１ We found that the above conditions are important. By applying these conditions, it becomes possible to introduce appropriate dislocations during hot rolling and increase the number of recrystallization nuclei. As a result, it became possible to control the average grain size of recrystallized grains to 30 μm or more and 300 μm or less in the surface layer from the surface to 1 / 5 of the thickness of the hot-rolled sheet. By performing hot-rolled sheet annealing (optional process), cold rolling, decarburization annealing, and finish annealing (secondary recrystallization annealing) on ​​the hot-rolled sheet obtained in this way, it becomes possible to manufacture grain-oriented electrical steel sheets with good magnetic properties.

[0019] Based on the above findings, the gist of the present invention is as follows: [1] A step of continuously casting molten steel having a composition in mass% or mass ppm of C: 0.08% or less, Si: 2.0% or more and 4.5% or less, Mn: 0.50% or less, acid-soluble Al: 20 ppm or more and 120 ppm or less, S: less than 50 ppm, and N: 80 ppm or less, with the content of Se, Te, and O each suppressed to less than 50 ppm, and the remainder consisting of Fe and unavoidable impurities, to produce a slab with a thickness of 30 mm or more and 80 mm or less; and thereafter a heating step of heating the slab using a heating device for a heating time of 5 minutes or more and 25 minutes or less, and the maximum temperature reached on the surface of the slab being 1000°C or more and 1290°C or less, before the surface temperature of the slab falls below 850°C. Subsequently, the surface temperature of the slab at the start of hot rolling is 900°C to 1250°C, the reduction schedule is 3 to 7 passes, and in the final pass, the reduction ratio is 5.0% to 50.0%, and the strain rate is 50.0 s. －１ A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets, comprising: a hot-rolling step of hot-rolling the slab under the above conditions to obtain a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less; and a step of cooling the hot-rolled sheet within 200 seconds after the completion of the hot-rolling, under the condition that the surface temperature of the hot-rolled sheet becomes 650°C or less.

[0020] [2] The heating step in the method for manufacturing a hot-rolled sheet for a grain-oriented electrical steel sheet according to [1] above is performed in a non-oxidizing atmosphere with an oxygen concentration of 3.0% by volume or less.

[0021] [3] The heating step includes: a first heating step of using a first heating device as the heating device to set the temperature of the surface of the slab to 950°C or higher and 1150°C or lower; and then, a second heating step of using a second heating device different from the first heating device as the heating device to set the temperature of the surface of the slab to the maximum temperature reached. This is the method for manufacturing a hot-rolled sheet for a grain-oriented electrical steel sheet according to [1] or [2] above.

[0022] [4] In the method for manufacturing a hot-rolled sheet for a grain-oriented electrical steel sheet according to [3] above, the first heating device is a tunnel furnace, and the second heating device is an induction heating device.

[0023] [5] In the method for manufacturing a hot-rolled sheet for a grain-oriented electrical steel sheet according to any one of [1] to [4] above, the component composition further contains, in mass%, one or more selected from the group consisting of Ni: 1.500% or less, Sn: 0.500% or less, Sb: 0.500% or less, Cu: 0.500% or less, P: 0.500% or less, Cr: 1.500% or less, Mo: 0.500% or less, B: 0.0200% or less, Nb: 0.0100% or less, Co: 0.0100% or less, Ti: 0.0200% or less, Zn: 0.0500% or less, Bi: 0.0200% or less, W: 0.0030% or less, Pb: 0.0300% or less, Ga: 0.0100% or less, Ge: 0.0300% or less, As: 0.0300% or less, and Ag: 0.0300% or less.

[0024] [6] A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets as described in any one of [1] to [5] above, comprising: an optional step of hot-rolled sheet annealing; a step of cold-rolling the hot-rolled sheet once or two or more times with an intermediate annealing in between to obtain a cold-rolled sheet; a step of decarburizing the cold-rolled sheet to obtain a decarburized annealed sheet; an optional step of nitriding the cold-rolled sheet during the decarburizing annealing or the decarburized annealed sheet after the decarburizing annealing; and a step of applying an annealing separating agent to the decarburized annealed sheet and performing a finish annealing.

[0025] [7] A continuous casting machine that continuously casts molten steel having the component composition described in [1] or [5] above to produce a slab with a thickness of 30 mm or more and 80 mm or less; a heating device that heats the slab, controlled so that the heating time is 5 minutes or more and 25 minutes or less, and the maximum temperature reached on the surface of the slab is 1000°C or more and 1290°C or less, before the surface temperature of the slab falls below 850°C; and a heating device that heats the slab, controlled so that the surface temperature of the slab at the start of hot rolling is 900°C or more and 1250°C or less, the reduction schedule is 3 passes or more and 7 passes or less, and in the final pass the reduction ratio is 5.0% or more and 50.0% or less, and the strain rate is 50.0 s －１ A production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets, comprising: a hot-rolling mill that performs hot-rolling on the slab under the above conditions to obtain a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less; and a cooling device that cools the hot-rolled sheet within 200 seconds after the completion of the hot-rolling, under the condition that the surface temperature of the hot-rolled sheet is 650°C or less; all of these are arranged in order.

[0026] [8] The heating device is a production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets as described in [7] above, which heats the slab in a non-oxidizing atmosphere with an oxygen concentration of 3.0 volume% or less.

[0027] [9] The heating apparatus comprises a first heating apparatus that performs a first heating step of raising the surface temperature of the slab to 950°C or higher and 1150°C or lower, and a second heating apparatus different from the first heating apparatus that then performs a second heating step of raising the surface temperature of the slab to the maximum temperature reached, the assembly of hot-rolled sheets for grain-oriented electrical steel sheets as described in [7] or [8] above.

[0028]

[10] A production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets as described in [9] above, wherein the first heating device is a tunnel furnace and the second heating device is an induction heating device.

[0029]

[11] A hot-rolled sheet for grain-oriented electrical steel manufactured by the method for manufacturing a hot-rolled sheet for grain-oriented electrical steel described in any one of [1] to [5] above, wherein the average particle size of the recrystallized grains in the surface layer from the surface to 1 / 5 of the thickness is 30 μm or more and 300 μm or less.

[0030] The present invention can 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 of having to perform hot rolling at extremely high temperatures in the continuous casting and hot rolling process.

[0031] In other words, according to the method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets and the manufacturing equipment array of the present invention, it is possible to manufacture hot-rolled sheets suitable for the production of grain-oriented electrical steel sheets with stable magnetic properties by using a component composition that conforms to the inhibitorless method and carrying out the process from casting molten steel to manufacturing hot-rolled sheets using a series of equipment arrays in a continuous process.

[0032] This is a schematic diagram showing a production equipment array 100 for hot-rolled sheets for grain-oriented electrical steel sheets according to one embodiment of the present invention.

[0033] [Method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets] A method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets according to one embodiment of the present invention comprises the steps of: continuously casting molten steel having a predetermined component composition to produce a slab; then, heating the slab under predetermined conditions before the surface temperature of the slab falls below a predetermined value; then, hot rolling the slab under predetermined conditions to obtain a hot-rolled sheet; and then, cooling the hot-rolled sheet under predetermined conditions.

[0034] (Composition of Molten Steel and Slabs) First, the composition of molten steel and slabs will be explained. Unless otherwise specified, "%" in relation to the components means mass percent. Similarly, unless otherwise specified, "ppm" refers to mass ppm.

[0035] C: 0.08% or less. If carbon (C) remains in the final product plate, it causes magnetic aging and leads to magnetic degradation. If the carbon content in the molten steel and slab is excessive, the load in the decarburization process becomes high, and the carbon content in the final product plate cannot be sufficiently reduced. Therefore, the carbon content in the molten steel and 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. In addition, during cold rolling, carbon improves the texture after primary recrystallization through interaction with dislocations. From this viewpoint, it is preferable that the carbon content be 0.01% or more.

[0036] Si: 2.0% to 4.5% Si is an element that reduces iron loss by increasing electrical resistance. From the viewpoint 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.

[0037] Mn: 0.50% or less. If the Mn content is too high, the primary recrystallized texture deteriorates, making it difficult to obtain secondary recrystallized grains that are highly concentrated in the Goss orientation. From this viewpoint, the Mn content should be 0.50% 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.

[0038] Acid-soluble Al: 20 ppm to 120 ppm. Since this embodiment relates to the inhibitorless method, it is necessary to reduce the content of Al, which is an inhibitor-forming component, as much as possible. From this viewpoint, the Al content is set to 120 ppm or less. When applying the inhibitorless 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 also creates dense Al on the surface during secondary recrystallization annealing. ２ O ３ A film is formed, reducing the effects of nitriding and other processes from the surrounding atmosphere. For this reason, the Al content should be 20 ppm or more.

[0039] S: Less than 50 ppm, N: 80 ppm or less. Since this embodiment relates to an inhibitor-less method, the content of S and N, which are inhibitor-forming components, 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, and the structure becomes non-uniform. Therefore, the S content should be less than 50 ppm and the N content should be 80 ppm or less. The lower limit of the S and N content is preferably 0 ppm. However, it is difficult to completely remove S and N, and extreme reduction of S and N leads 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.

[0040] Se, Te, and O: Less than 50 ppm each. If the content of Se and Te is excessive, Se and Te oxides 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, O forms oxides and remains as inclusions in the final product, degrading the magnetic properties, so the O content needs to be kept below 50 ppm. The content of Se, Te, and O may be 0 ppm.

[0041] The remainder of the components other than those listed above consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the purpose of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of unavoidable impurities include elements that are mixed in unintentionally. These elements may be inevitably present in amounts of about 0.010% or less. Furthermore, for the purpose of further improving performance, the component composition of the molten steel and slab may optionally contain the following elements.

[0042] Ni: 1,500% or less. Ni has the effect of improving magnetic properties by increasing the uniformity of the hot-rolled sheet structure. On the other hand, if the Ni content is excessive, secondary recrystallization becomes unstable, and the magnetic properties deteriorate. Therefore, when Ni is included, the Ni content should be 1,500% or less. From the viewpoint of obtaining the above effect from Ni addition, it is preferable that the Ni content be 0.005% or more.

[0043] Sn: 0.500% or less Sb: 0.500% or less Cu: 0.500% or less Sn, Sb, and Cu are elements that can be considered auxiliary inhibitors through grain boundary segregation, and may be useful in inhibitor-less methods that do not actively utilize inhibitors from precipitates. 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, the content of each element should be 0.500% or less. From the viewpoint of obtaining the effects of adding these elements, 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 0.010% or more. It is more preferable that the Sn content be 0.010% or more.

[0044] P: 0.500% or less Cr: 1.500% or less P and Cr have the effect of improving the forsterite film formation reaction. 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 0.500% or less and the Cr content should be 1.500% or less. From the viewpoint of obtaining the above effects from the addition of these elements, it is preferable that the P content be 0.005% or more and the Cr content be 0.010% or more.

[0045] Mo: 0.500% or less B: 0.0200% or less Nb: 0.0100% or less Mo, B, and Nb all contribute to suppressing grain growth and have the effect of improving texture and stabilizing secondary recrystallization. On the other hand, if the content of these elements is excessive, they precipitate and function as strong inhibitors, which is undesirable in the inhibitor-less method. Therefore, when these elements are included, the Mo content should be 0.500% or less, the B content 0.0200% or less, and the Nb content 0.0100% or less. A B content of 0.0050% or less is more preferable. From the viewpoint of obtaining the above effects from the addition of these elements, it is preferable that the Mo content be 0.005% or more, the B content be 0.0001% or more, and the Nb content be 0.0005% or more. A Mo content of 0.010% or more is more preferable.

[0046] Co: 0.0100% or less Ti: 0.0200% or less Zn: 0.0500% or less Bi: 0.0200% or less W: 0.0030% or less Pb: 0.0300% or less Ga: 0.0100% or less Ge: 0.0300% or less As: 0.0300% or less Ag: 0.0300% or less Co, Ti, Zn, Bi, W, Pb, Ga, Ge, As, and Ag are grain boundary segregation elements that suppress grain growth and grain boundary movement, thereby enhancing magnetic properties. On the other hand, if the content of these elements is excessive, they may precipitate as carbides, nitrides, or oxides in the steel, or they may precipitate as elemental metals without being able to maintain a solid solution state with the steel, which can lead to a deterioration of the final magnetic properties. Therefore, when these elements are included, their content should be below the above-mentioned upper limit. It is more preferable that the Ti content be 0.0050% or less. From the viewpoint of obtaining the above-mentioned effects from the addition of these elements, it is preferable that the content of these elements be above the above-mentioned lower limit. It is more preferable that the Ti content be 0.0010% or more, the Bi content be 0.0050% or more, and the Pb, Ge, As, and Ag content be 0.0010% or more each.

[0047] (Casting Process) In this embodiment, molten steel having the above-mentioned component composition is first continuously cast to produce slabs with a thickness of 30 mm to 80 mm. With a component composition that does not contain inhibitors, there is a possibility of excessive grain growth during hot rolling following casting, during transport, and during waiting after processing. By optimizing the slab thickness, it becomes possible to achieve sufficient microstructure control during subsequent hot rolling. The effect is easier to obtain with a thicker slab, but in this embodiment, when the final thickness of the hot-rolled sheet is determined by a series of hot rolling processes without dividing it into rough rolling and finish rolling, it is extremely difficult to directly roll a slab cast to a thickness exceeding 80 mm in manufacturing. Therefore, the slab thickness is set to 80 mm or less. Also, if the slab thickness is less than 30 mm, it is not possible to set a sufficient reduction ratio during hot rolling, and the effect of suppressing microstructure deterioration cannot be obtained. Therefore, the slab thickness is set to 30 mm or more.

[0048] (Heating Process) Next, the slab obtained in the casting process is subjected to the heating process directly, i.e., in a continuous process. In the heating process, the slab is heated using a heating device for a heating time of 5 minutes or more and 25 minutes or less, and the maximum temperature reached on the slab surface is between 1000°C and 1290°C, before the temperature of the slab surface falls below 850°C. Note that "temperature of the slab surface" in the heating process refers to the temperature of the entire portion of the slab's upper and lower surfaces, excluding the area within 100 mm from both ends in the width direction of the slab towards the center in the width direction. This is because the ends in the width direction of the slab are cooled and heated not only from the upper and lower surfaces but also from the sides, and therefore often do not represent the typical temperature of the slab surface.

[0049] If this type of heating is not performed, the slab temperature will gradually decrease after casting until hot rolling begins. In this case, the driving force for precipitation will gradually increase, and trace amounts of precipitate-forming elements may form precipitates. To avoid this gradual increase in the driving force for precipitation, the slab is heated from the surrounding area to suppress unwanted precipitation.

[0050] Surface temperature of the slab at the start of the heating process: 850°C or higher. In this embodiment, since a component composition following the inhibitorless method is adopted, if the surface temperature of the slab is low, below 1200°C, specifically above 850°C, it is possible to maintain a state of complete solid solution or a state in which precipitation does not progress for a certain period of time. In the continuous casting and hot rolling process, the ability to cool the slab to a low temperature is a major advantage. If the surface temperature of the slab at the start of the heating process exceeds 1200°C, cooling from the slab surface layer may not be sufficient, and breakout may occur where the unsolidified molten steel in the center of the slab breaks through the surface layer (solidified layer) and flows out. Therefore, it is preferable to keep the surface temperature of the slab at the start of the heating process below 1200°C. On the other hand, if the surface temperature of the slab at the start of the heating process is below 850°C, it becomes extremely difficult to maintain a state of complete solid solution of trace amounts of precipitated elements. As a result, this can affect the grain growth suppression force and become a cause of poor secondary recrystallization. Furthermore, excessive energy is required to perform the subsequent hot rolling at the appropriate temperature, negating the benefits of a continuous process. Therefore, the surface temperature of the slab at the start of the heating process should be 850°C or higher.

[0051] Heating time: 5 minutes to 25 minutes, and maximum surface temperature of the slab: 1000°C to 1290°C Generally, it is difficult to perform casting at high speed, so in processes that involve continuous casting and hot rolling, a certain amount of time is often required before hot rolling. When it takes about 10 minutes or more from the start of casting until hot rolling, precipitate-forming elements that inevitably get mixed in sometimes precipitate in the form of nitrides, sulfides, etc. This problem does not necessarily occur in all slabs, but it tends to occur especially when scrap is used as the iron source. To avoid the precipitation of unavoidable impurity elements, it was effective to heat the slab for 5 minutes to 25 minutes until the surface temperature of the slab reached 1000°C to 1290°C before hot rolling.

[0052] If the heating time is less than 5 minutes, phenomena such as precipitation and solid solution require a certain amount of time to occur, resulting in an inability to obtain a stable effect and ultimately leading to a deterioration of the magnetic properties. Therefore, the heating time should be 5 minutes or more, preferably 8 minutes or more. On the other hand, if the heating time exceeds 25 minutes, it leads to coarsening of the crystal grain size and ultimately leads to a deterioration of the magnetic properties. Therefore, the heating time should be 25 minutes or less, preferably 20 minutes or less. Note that "heating time" refers to the time during which heat is applied to the slab by the heating device. If the heating device is a tunnel furnace, it refers to the time the slab is inside the tunnel furnace. If the heating device is an induction heating device, it refers to the time the slab is under the influence of the magnetic field of the induction heating device.

[0053] If the maximum temperature reached on the slab surface during the heating process is less than 1000°C, the uneven temperature distribution during casting will be reflected, resulting in the unavoidable precipitation of impurity elements in some areas. This will cause uneven grain size during primary recrystallization, making secondary recrystallization impossible. Therefore, the maximum temperature reached should be 1000°C or higher. On the other hand, if the maximum temperature reached exceeds 1290°C, breakout may occur during hot rolling. Furthermore, the homogenization of the precipitation state is excessive, resulting in excessively low deformation resistance and making it difficult to control the shape after rolling. Therefore, the maximum temperature reached should be 1290°C or lower, preferably 1250°C or lower.

[0054] The heating process is preferably carried out in a non-oxidizing atmosphere with an oxygen concentration of 3.0 volume% or less. This is because if the oxygen concentration in the atmosphere is high, carbon in the steel will be decarburized as CO by the oxygen in the atmosphere on the slab surface, promoting the coarsening of the columnar crystal structure near the surface and causing bleed defects. The oxygen concentration may be 0.0 volume%. The non-oxidizing atmosphere is not particularly limited and may include carbon-based combustion gases, ammonia-based combustion gases, and N2, which are by-products of the steelworks. ２ It can be composed of one or more gases selected from the group consisting of inert gases such as Ar. In particular, when using carbon-based combustion gas or ammonia-based combustion gas, which are by-products of steel mills, oxygen or air is introduced to generate the combustion gas, but CO, CO ２ No, no ２It is preferable that the amount of unreacted oxygen in the total gas volume, including the above, be 3.0% by volume or less.

[0055] In this process, since the slabs are continuously supplied to the next process, it is preferable to use an open furnace such as a tunnel furnace. When an open furnace is used, the atmosphere inside the furnace is greatly affected by the atmosphere, but by using methods such as sealing gas, the oxygen concentration inside the furnace can be reduced to 3.0 volume percent or less, creating a non-oxidizing atmosphere and suppressing decarburization from the slab surface. If a more airtight structure can be applied, atmosphere control will be easier.

[0056] Preferably, the two-stage heating process includes a first heating step in which a first heating device is used to raise the surface temperature of the slab to 950°C or higher and 1150°C or lower, and a second heating step in which a second heating device different from the first heating device is used to raise the surface temperature of the slab to a maximum temperature of 1000°C or higher and 1290°C or lower. This allows the heating rate of the first and second heating steps to be controlled individually. In the case of two-stage heating, a heating time of 5 minutes or more and 25 minutes or less refers to the total heating time of the first and second heating steps.

[0057] In the heating process, when the surface temperature of the slab exceeds 1000°C, the rate of oxidation on the surface increases significantly, and when it exceeds 1150°C, almost the entire surface of the slab becomes covered with liquid phase scale. This liquid phase scale can be a cause of velvet defects. Therefore, in the second heating process, which aims for a maximum temperature of 1000°C to 1290°C, it is preferable to use an induction heating device capable of rapid heating. For example, it is preferable to use a tunnel furnace for the first heating process and an induction heating device for the second heating process. By making the second heating process rapid, grain growth, which tends to progress especially in the high-temperature range, can be suppressed, and texture deterioration caused by coarse grains can be more effectively suppressed, and as a result, velvet defects can also be suppressed.

[0058] (Hot Rolling Process) Next, the slab is hot-rolled under predetermined conditions to obtain a hot-rolled sheet. In the hot-rolling process, which determines the final thickness of the hot-rolled sheet, the temperature is relatively low, and dislocations are introduced into the steel sheet. Since dislocations function as nuclei for precipitation, there is a possibility that the precipitation of precipitate-forming elements that are mixed in will proceed. By setting a specific temperature and reduction schedule, unwanted precipitation can be suppressed and a uniform state can be maintained.

[0059] The surface temperature of the slab at the start of hot rolling (hot rolling start temperature): 900°C or higher and 1250°C or lower. If the hot rolling start temperature is below 900°C, precipitation is likely to occur due to the introduction of strain during hot rolling. Unwanted precipitation functions as a local grain growth inhibitory force, leading to non-uniform grain size during primary recrystallization, and causing deterioration of the texture and poor secondary recrystallization. Therefore, the hot rolling start temperature should be 900°C or higher, preferably 950°C or higher. On the other hand, if the hot rolling start temperature exceeds 1250°C, dislocations are introduced and recovery occurs, resulting in insufficient strain introduction, deterioration of the texture, and failure to obtain good magnetic properties. Therefore, the hot rolling start temperature should be 1250°C or lower.

[0060] Reduction Schedule: 3 to 7 Passes If the number of hot rolling passes is less than 3, the reduction per pass becomes too large, increasing the risk of edge cracking at the edges of the hot-rolled sheet. Therefore, the number of hot rolling passes should be 3 or more. On the other hand, if the number of hot rolling passes exceeds 7, the reduction ratio per pass becomes too low, resulting in insufficient strain being introduced by hot finish rolling, leading to a decrease in the recrystallization rate, which is a factor in the deterioration of the texture. Therefore, the number of hot rolling passes should be 7 or less, preferably 6 or less.

[0061] Hot-rolled sheet thickness: 1.0 mm or more and 3.5 mm or less. In controlling the structure of grain-oriented electrical steel sheets, the reduction ratio during cold rolling is also an extremely important factor. If hot rolling is attempted to reduce the thickness of the hot-rolled sheet to less than 1.0 mm, it becomes impossible to maintain an appropriate reduction ratio during cold rolling, leading to deterioration of the texture. Therefore, the thickness of the hot-rolled sheet should be 1.0 mm or more. On the other hand, if the thickness of the hot-rolled sheet exceeds 3.5 mm, it also becomes impossible to maintain an appropriate reduction ratio during cold rolling, leading to deterioration of the texture. Therefore, the thickness of the hot-rolled sheet should be 3.5 mm or less.

[0062] Final pass: The reduction ratio is 5.0% or more and 50.0% or less, and the strain rate is 50.0 s －１ In the present embodiment where the slab thickness in the casting process is extremely thinner than the general slab thickness, it is difficult to obtain a sufficient reduction ratio for microstructure control in the continuous hot rolling process, and microstructure control by hot rolling has an important meaning. Therefore, in the present embodiment, in the final pass of hot rolling, the reduction ratio is 5.0% or more and 50.0% or less, and the strain rate is 50.0 s －１ or more. This enables the production of a grain-oriented electrical steel sheet having good magnetic properties. When the reduction ratio in the final pass is less than 5.0%, dislocations cannot be sufficiently retained in the steel, and the recrystallization rate cannot be increased during annealing after the subsequent process. As a result, the grain structure during primary recrystallization deteriorates. Therefore, the reduction ratio in the final pass is 5.0% or more, preferably 20.0% or more. On the other hand, when the reduction ratio in the final pass exceeds 50.0%, defects such as ear cracking may occur during hot rolling. Therefore, the reduction ratio in the final pass is 50.0% or less, preferably 45.0% or less. Also, when the strain rate in the final pass is less than 50.0 s －１ , the time for the introduced dislocations to recover becomes long, and the effect of retaining dislocations in the subsequent process becomes insufficient. Therefore, the strain rate in the final pass is 50.0 s －１ or more. The upper limit of the strain rate in the final pass is not particularly limited. However, from the perspective of equipment specifications, the strain rate in the final pass is generally 500 s －１ or less.

[0063] Note that the strain rate ε is calculated using the following Ekelund's formula. Here, v Ｒ is the roll peripheral speed (mm / s), R' is the roll radius (mm), h １ is the roll inlet side plate thickness (mm), and r is the reduction ratio (%).

[0064] (Cooling Process) Next, within 200 seconds after the completion of hot rolling, the hot-rolled sheet is cooled under conditions that the surface temperature of the hot-rolled sheet is 650°C or lower. After hot rolling, the dislocation density increases, making it easier for precipitates to form. If the cooling time from the end of hot rolling until the surface temperature of the hot-rolled sheet reaches 650°C is 200 seconds or less, the uneven precipitation of precipitate-forming elements caused by unavoidable impurities can be suppressed. The lower limit of this cooling time is not particularly limited, and cooling may be started immediately after the completion of hot finish rolling, provided there are no constraints on the equipment configuration.

[0065] (Winding process) Next, the hot-rolled sheet can be wound up to obtain a hot-rolled coil.

[0066] [Hot-rolled sheet for grain-oriented electrical steel] The hot-rolled sheet for grain-oriented electrical steel according to one embodiment of the present invention is a hot-rolled sheet manufactured by 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 by reference.

[0067] In this embodiment, the hot-rolled sheet has an average grain size of recrystallized grains of 30 μm or more and 300 μm or less in the surface layer up to 1 / 5 of its thickness. By performing hot-rolled sheet annealing (optional process), cold rolling, decarburization annealing, and finish annealing (secondary recrystallization annealing) on ​​such a hot-rolled sheet, it is possible to manufacture grain-oriented electrical steel sheets with good magnetic properties.

[0068] The "average grain size of recrystallized grains" in a hot-rolled sheet shall be determined by the following method. First, the surface layer from the surface to 1 / 5 of the thickness is observed using an optical microscope in a cross-section perpendicular to the rolling direction of the hot-rolled sheet. The observation field of view shall be 1 mm for each of the two surface layers on the front and back. ２ Then, the average equivalent diameter of the recrystallized grains is determined from the total area and total number of recrystallized grains present within the observation field, and this is defined as the "average grain size of the recrystallized grains".

[0069] Furthermore, 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. In other words, by employing the manufacturing method of this embodiment, the value obtained by dividing the precipitated Al content by the acid-soluble Al content 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 will be quantified by removing the surface layer from the hot-rolled sheet up to one-quarter of its thickness, using the remaining central portion 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 resulting residue. The content of acid-soluble aluminum will 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] [Method for Manufacturing Grain-Grain Electrical Steel Sheets] In a method for manufacturing grain-grain electrical steel sheets according to one embodiment of the present invention, a hot-rolled sheet for grain-grain electrical steel sheets obtained by the above method is used, hot-rolled sheet annealing is performed as needed, followed by one or two or more cold-rolling processes with an intermediate annealing in between, and then decarburization annealing and finish annealing to produce a finished steel sheet.

[0073] First, the hot-rolled sheet is annealed as needed. From the viewpoint of improving magnetic properties, it is desirable to perform hot-rolled sheet annealing. In this case, it is preferable that the soaking temperature during hot-rolled sheet annealing be 850°C or higher so that recrystallization occurs. There is no particular upper limit to the soaking temperature. However, in order to suppress deterioration of surface quality due to pickup, it is preferable that the soaking temperature be 1200°C or lower. There is no particular limit to the holding time at the soaking temperature (soaking time), but from the viewpoint of improving magnetic properties, it is preferable to be 10 seconds or more, and in order to suppress deterioration of surface quality due to pickup, it is preferable to be 240 seconds or less.

[0074] Subsequently, the hot-rolled sheet is subjected to cold rolling once or two or more times with an intermediate annealing in between to obtain a cold-rolled sheet. This cold rolling includes not only general cold rolling performed at room temperature, but also warm rolling in which the temperature of the hot-rolled sheet is raised to a temperature higher than room temperature, for example, between 100°C and 300°C.

[0075] From the viewpoint of improving the microstructure, the soaking temperature in intermediate annealing is preferably 900°C or higher, and more preferably 1000°C or higher. Furthermore, to suppress deterioration of surface quality due to picking, the soaking temperature is preferably 1200°C or lower. From the viewpoint of improving the microstructure, the holding time at the soaking temperature in intermediate annealing is preferably 30 seconds or higher, and more preferably 60 seconds or higher. Furthermore, to suppress deterioration of surface quality due to picking, the holding time at the soaking temperature is preferably 240 seconds or lower.

[0076] 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 wet hydrogen nitrogen or wet hydrogen argon atmosphere with a dew point of 40°C to 70°C, 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 approximately 800°C to less than 950°C. The holding time at the annealing temperature is preferably 30 seconds to 300 seconds. 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 setting the heating rate between 500°C and 700°C to 80°C / s or more.

[0077] Nitriding treatment may be applied to the cold-rolled sheet during decarburization annealing, or to the decarburized annealed sheet after decarburization annealing. Nitriding treatment can increase the amount of nitride precipitates, particularly near the surface layer of the steel sheet where nitrogen is supplied from an external source, and can further improve the non-uniformity of the precipitates. The nitriding treatment can be performed using the conventional method used for grain-oriented electrical steel sheets. As a method of performing nitriding treatment during decarburization annealing, for example, the decarburization annealing can be maintained in a humid hydrogen nitrogen atmosphere (e.g., 75% H). ２ +25%N ２ Methods include performing the process under the specified conditions and, after holding, blowing ammonia gas onto the steel plate while maintaining the same atmosphere, or introducing the plate into a mixed gas atmosphere of hydrogen, nitrogen, and ammonia after holding. Another method for performing nitriding after decarburization annealing is to first cool the decarburized annealed plate to room temperature, then raise the temperature again to between 400°C and 900°C, and anneal it with a mixed gas of hydrogen, nitrogen, and ammonia.

[0078] Next, an annealing separation agent is applied to the surface of the decarburized annealed sheet. Magnesia (MgO) can be used as the main component of the annealing separation agent to form a forsterite film on the surface of the steel sheet after finish annealing. At this time, adding an appropriate amount of Ti oxide or Sr compound to the separation agent can further improve the formation of the forsterite film. In particular, the addition of an auxiliary agent that promotes uniform formation of the forsterite film is also advantageous in improving the peeling characteristics of the film. The method of applying the annealing separation agent is not particularly limited, and methods such as applying a solution in which the annealing separation agent is dissolved in a solvent, or attaching a sheet of the annealing separation agent that has been prepared in advance, can be used as appropriate.

[0079] Next, finish annealing is performed for secondary recrystallization and forsterite film formation. The annealing atmosphere is N ２ Ar, H ２, or any of these mixed gases are suitable. Since the precipitation of trace components in the final product can lead to a deterioration of magnetic properties, it is preferable that the maximum annealing temperature be between 1100°C and 1280°C, and the soaking time be between 3 hours and 50 hours, in order to purify the components. The grain-oriented electrical steel sheet obtained by the present invention has little variation in magnetic properties within the coil, so considering economics, it is desirable to finish anneal the coil with a weight of 5 tons or more, more preferably 10 tons or more.

[0080] After the above-mentioned finish annealing, an insulating coating can be further formed on the surface of the steel sheet. 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 sheet and bake it at about 800°C.

[0081] 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 contains Si: 2.0% to 4.5%, Mn: 0.01% to 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.

[0082] [Manufacturing Equipment Arrangement for Hot-Rolled Sheets for Grain-Grain Electrical Steel] Referring to Figure 1, the manufacturing equipment arrangement 100 for hot-rolled sheets for grain-grain electric steel according to one embodiment of the present invention comprises a continuous casting machine 10, a heating device 20, a hot rolling mill 50, a cooling device 60, and a coiler 70 arranged in order, enabling a continuous casting and hot-rolling process. In the example of Figure 1, the heating device 20 consists of a first heating device 20A, which is a tunnel furnace, and a second heating device 20B, which is an induction heating device located downstream thereof, but the present invention is not limited thereto.

[0083] The continuous casting machine 10 continuously casts molten steel having the aforementioned component composition to produce slabs with a thickness of 30 mm to 80 mm.

[0084] The heating device 20 heats the slab, and during this process, the heating time is controlled to be between 5 and 25 minutes, and the maximum temperature reached on the slab surface is between 1000 and 1290°C, before the temperature of the slab surface falls below 850°C. The type of heating device 20 is not particularly limited and examples include a tunnel furnace through which the slab can pass, and an induction heating device capable of induction heating the slab.

[0085] The heating device 20 preferably heats the slab in a non-oxidizing atmosphere with an oxygen concentration of 3.0 volume% or less.

[0086] The heating device 20 preferably includes a first heating device 20A that performs a first heating step to raise the surface temperature of the slab to 950°C or higher and 1150°C or lower, and a second heating device 20B that then performs a second heating step to raise the surface temperature of the slab to a maximum temperature of 1000°C or higher and 1290°C or lower. The second heating device 20B is a different piece of equipment from the first heating device 20A. As shown in Figure 1, the first heating device 20A is preferably a tunnel furnace. Since the second heating step is preferably performed by rapid heating, as shown in Figure 1, the second heating device 20B is preferably an induction heating device.

[0087] The hot rolling mill 50 performs a hot rolling process on the slab to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm. At that time, the hot rolling mill 50 is positioned so that the surface temperature of the slab at the start of hot rolling is between 900°C and 1250°C.

[0088] The reduction schedule for the hot rolling mill 50 is 3 to 7 passes. Furthermore, the final pass of the hot rolling mill 50 has a reduction ratio of 5.0% to 50.0% and a strain rate of 50.0 s. －１ It is important that the rolling capacity is as described above.

[0089] The cooling device 60 cools the hot-rolled sheet under the condition that the surface temperature of the hot-rolled sheet becomes 650°C or lower within 200 seconds after the completion of hot rolling. To achieve such rapid cooling, it is preferable that the cooling device 60 has a water cooling mechanism.

[0090] The coiler 70 winds up the hot-rolled sheet discharged from the cooling device 60 to form a hot-rolled coil.

[0091] [Example 1] Molten steel having a composition of C: 0.06%, Si: 3.2%, Mn: 0.05%, acid-soluble Al: 80 ppm, S: 20 ppm, N: 30 ppm, Cr: 0.030%, and P: 0.010%, with the remainder being Fe and unavoidable impurities, was refined. In the laboratory, casting, heating, hot rolling, and cooling processes were carried out in succession under the conditions shown in Table 1 to obtain hot-rolled sheets.

[0092] The heating process was carried out in an atmosphere of blast furnace combustion gas (oxygen concentration: 1.8 vol%), and was a single-stage heating process using only the heating furnace. In the final pass of hot rolling, the reduction ratio was 40.0% and the strain rate was 52.8 s. －１ The cooling time after the completion of hot rolling, until the surface temperature of the hot-rolled sheet reached 650°C, was adjusted by controlling the amount of water used.

[0093] The 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 then annealed at 840°C for 150 seconds at 55% H. ２ -45%N ２ The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization 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 a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1200°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0094] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ Table 1 shows the results. As is clear from Table 1, good magnetic properties were obtained in grain-oriented electrical steel sheets manufactured from hot-rolled sheets manufactured under the conditions of the present invention.

[0095]

[0096] [Example 2] Molten steel having a composition of C: 0.04%, Si: 3.4%, Mn: 0.07%, P: 0.060%, acid-soluble Al: 100 ppm, S: 10 ppm, N: 60 ppm, Sb: 0.008%, and Mo: 0.010%, with the remainder being Fe and unavoidable impurities, was refined. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession using the conditions shown in No. 11 and No. 15 of Example 1 (see Table 1) to obtain a hot-rolled sheet. However, the reduction ratio and strain rate for each pass in the hot rolling process were as shown in Table 2. In Table 2, Nos. 1-3 are examples using the conditions shown in No. 11 of Example 1, and Nos. 4-7 are examples using the conditions shown in No. 15 of Example 1.

[0097] A test specimen was taken from a portion of the obtained hot-rolled sheet, and the average grain size of the recrystallized grains was determined using the method described above, which is shown in Table 2.

[0098] A hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 900°C for 50 seconds, scale was removed by pickling, and then two cold-rolling processes, including intermediate annealing, were performed to produce a cold-rolled sheet with a final thickness of 0.23 mm. The intermediate annealing was performed under conditions that resulted in a maximum temperature of 1050°C. This cold-rolled sheet was then annealed at 840°C for 150 seconds at 55% H ２ -45%N ２The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization 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 a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0099] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ The results are shown in Table 2. As is clear from Table 2, good secondary recrystallization occurred in all examples, but in particular, in the inventive example in which the reduction ratio and strain rate in the final pass of hot rolling were optimized, the average grain size of the recrystallized grains was between 30 μm and 300 μm, resulting in better magnetic properties.

[0100]

[0101] [Example 3] Molten steel having a component composition containing the elements shown in Table 3, with the remainder being Fe and unavoidable impurities, was refined. In Table 3, the analytical value of Al represents the content of acid-soluble Al. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession to obtain a hot-rolled sheet.

[0102] In the casting process, a slab with a thickness of 70 mm was used. In the heating process, when the surface temperature of the slab reached 1000°C, it was placed in the heating furnace and heated to 1180°C in 15 minutes. The atmosphere inside the heating furnace was that of combustion gas from a blast furnace (oxygen concentration as shown in Table 3). In the hot rolling process, the starting temperature was 1100°C, and a hot-rolled sheet with a thickness of 2.2 mm was produced in four passes. In the final pass, the reduction ratio was 36.8%, and the strain rate was 51.0 s. －１ The cooling time after the completion of hot rolling until the surface temperature of the hot-rolled sheet reached 650°C was set to 110 seconds.

[0103] A test specimen was 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). Also, the 1 mm cross-section parallel to the rolling direction of this test specimen is shown. ２ SEM observations were performed on the field of view, and the average particle size was determined by arithmetic mean calculation of the individual particle sizes of the precipitated MnS, which are shown in Table 3.

[0104] The hot-rolled sheet was annealed at a soaking temperature of 1000°C for 40 seconds, scale was removed by pickling, and then cold-rolled to a final thickness of 0.25 mm. This cold-rolled sheet was then annealed at 840°C for 150 seconds at 55% H. ２ -45%N ２ The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization 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 a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0105] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ Table 3 shows the results. The surface of the obtained grain-oriented electrical steel sheet was also observed, and the presence or absence of velvet defects was visually determined, and the results are shown in Table 3. As is clear from Table 3, good magnetic properties were obtained in the grain-oriented electrical steel sheet manufactured from hot-rolled sheet manufactured under the conditions of the present invention. In addition, velvet defects were suppressed by reducing the oxygen concentration in the atmosphere inside the heating furnace.

[0106]

[0107] [Example 4] Molten steel having a component composition containing the elements shown in Table 4, with the remainder being Fe and unavoidable impurities, was refined. In Table 4, the analytical value of Al represents the content of acid-soluble Al. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession to obtain a hot-rolled sheet.

[0108] In the casting process, a slab with a thickness of 70 mm was used. In the heating process, the slab was placed in the heating furnace when its surface temperature reached 1000°C. Under some conditions (Table 4, Nos. 2, 6, 8), the slab was heated to 1250°C in 15 minutes in the heating furnace. Under the remaining conditions (Table 4, Nos. 1, 3-5, 7, 9), the slab was heated to 1100°C in 10 minutes in the heating furnace, and then rapidly heated to 1250°C using an induction heating device, where it was held for 5 minutes. The atmosphere inside the heating furnace was that of combustion gas from a blast furnace (oxygen concentration of 3.0 vol%). The atmosphere of the induction heating device was that of N with an oxygen concentration of 5.0 vol%. ２ A gaseous atmosphere was created.

[0109] Subsequently, the hot rolling process began at a starting temperature of 1050°C, and a hot-rolled sheet with a thickness of 2.8 mm was produced in three passes. In the final pass, the reduction ratio was 49.6% and the strain rate was 58.2 s. －１ The cooling time after the completion of hot rolling until the surface temperature of the hot-rolled sheet reached 650°C was set to 90 seconds.

[0110] The 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 then treated at 840°C for 150 seconds at 55% H ２ -45%N ２ The sheets were then annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization 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 a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 8 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and magnesium phosphate was applied to the surface of the finished annealed sheets and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0111] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ Table 4 shows the results. The surface of the obtained grain-oriented electrical steel sheet was also observed, and the presence or absence of velvet defects was visually determined, and the results are shown in Table 4. As is clear from Table 4, good magnetic properties were obtained in the grain-oriented electrical steel sheet manufactured from hot-rolled sheet manufactured under the conditions of the present invention. In addition, velvet defects were suppressed by performing the heating process in two stages.

[0112]

[0113] [Example 5] An experiment was conducted to investigate the effect of nitriding treatment using a hot-rolled sheet prepared under the conditions of No. 1 in Table 4. No. 1 in Table 5 is the same as No. 1 in Table 4. In Nos. 2 and 3 in Table 5, decarburization annealing was performed under the same conditions as No. 1 in Table 4, and then the nitriding treatment described in Table 5 was performed. In No. 2 in Table 5, immediately after holding at 840°C for 150 seconds during decarburization annealing, the temperature was reduced to 800°C and left at 60% H for 30 seconds. ２ +20%N ２ +20%NH ３ The material was introduced into a mixed gas atmosphere. In No. 3 of Table 5, after decarburization annealing was completed and the material was cooled to room temperature, it was then annealed again at 650°C for 120 seconds at 60% H. ２ +20%N ２ +20%NH ３ The material was introduced into a mixed gas atmosphere. The nitrogen content increased by 0.012% in No. 2 and 0.011% in No. 3. Subsequently, similar to Example 4, an annealing separation agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and finish annealing, including a secondary recrystallization process and a purification process, was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 8 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and magnesium phosphate was applied to the surface of the finish annealed sheet and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0114] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011.８ The magnetic flux density (B) obtained was measured. ８ Table 5 shows the results. As is clear from Table 5, grain-oriented electrical steel sheets manufactured from hot-rolled sheets manufactured under the conditions of the present invention exhibited good magnetic properties, and even better magnetism was obtained by nitriding treatment.

[0115]

[0116] This invention can be applied to the manufacture of hot-rolled sheets and grain-oriented electrical steel sheets.

[0117] 100 Hot-rolled sheet manufacturing equipment for grain-oriented electrical steel sheets 10 Continuous casting machine 20 Heating equipment 20A First heating equipment (tunnel furnace) 20B Second heating equipment (induction heating equipment) 50 Hot rolling mill 60 Cooling equipment 70 Coiler

Claims

1. A process to produce a slab with a thickness of 30 mm to 80 mm by continuously casting molten steel having a composition in mass% or mass ppm, containing C: 0.08% or less, Si: 2.0% to 4.5%, Mn: 0.50% or less, acid-soluble Al: 20 ppm to 120 ppm, S: less than 50 ppm, and N: 80 ppm or less, with the content of Se, Te, and O each suppressed to less than 50 ppm, and the remainder consisting of Fe and unavoidable impurities; and a heating process thereafter, before the surface temperature of the slab falls below 850°C, the slab is heated using a heating device for a heating time of 5 minutes to 25 minutes, and the maximum temperature reached on the surface of the slab is 1000°C to 1290°C. Subsequently, the surface temperature of the slab at the start of hot rolling is 900°C to 1250°C, the reduction schedule is 3 to 7 passes, and in the final pass, the reduction ratio is 5.0% to 50.0%, and the strain rate is 50.0 s. －１ A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets, comprising: a hot-rolling step of hot-rolling the slab under the above conditions to obtain a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less; and a step of cooling the hot-rolled sheet within 200 seconds after the completion of the hot-rolling, under the condition that the surface temperature of the hot-rolled sheet becomes 650°C or less.

2. The method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets according to claim 1, wherein the heating step is carried out in a non-oxidizing atmosphere with an oxygen concentration of 3.0 volume% or less.

3. The method for manufacturing a hot-rolled sheet for grain-oriented electrical steel according to claim 1 or 2, wherein the heating step comprises: a first heating step in which a first heating device is used as the heating device to raise the surface temperature of the slab to 950°C or higher and 1150°C or lower; and a second heating step in which a second heating device different from the first heating device is used as the heating device to raise the surface temperature of the slab to the maximum temperature reached.

4. The method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets according to claim 3, wherein the first heating device is a tunnel furnace and the second heating device is an induction heating device.

5. The above component composition is further divided by mass percent into: Ni: 1.500% or less, Sn: 0.500% or less, Sb: 0.500% or less, Cu: 0.500% or less, P: 0.500% or less, Cr: 1.500% or less, Mo: 0.500% or less, B: 0.0200% or less, Nb: 0.0100% or less, Co: 0.0100% or less, Ti: 0.0200% or less, Zn: 0 A method for producing a hot-rolled sheet for grain-oriented electrical steel according to any one of claims 1 to 4, comprising one or more selected from the group consisting of 0.500% or less, Bi: 0.0200% or less, W: 0.0030% or less, Pb: 0.0300% or less, Ga: 0.0100% or less, Ge: 0.0300% or less, As: 0.0300% or less, and Ag: 0.0300% or less.

6. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel according to any one of claims 1 to 5, comprising: an optional step of hot-rolled sheet annealing; a step of cold-rolling the hot-rolled sheet once or two or more times with an intermediate annealing in between to obtain a cold-rolled sheet; a step of decarburizing the cold-rolled sheet to obtain a decarburized annealed sheet; an optional step of nitriding the cold-rolled sheet during the decarburizing annealing or the decarburized annealed sheet after the decarburizing annealing; and a step of applying an annealing separating agent to the decarburized annealed sheet and performing finish annealing.

7. A continuous casting machine for continuously casting molten steel having the component composition described in claim 1 or 5 to produce a slab with a thickness of 30 mm or more and 80 mm or less; a heating device for heating the slab, controlled so that the heating time is 5 minutes or more and 25 minutes or less, and the maximum temperature reached on the surface of the slab is 1000°C or more and 1290°C or less, before the surface temperature of the slab falls below 850°C; and a method for hot rolling in which the surface temperature of the slab is 900°C or more and 1250°C or less at the start of hot rolling, the reduction schedule is 3 passes or more and 7 passes or less, the reduction ratio is 5.0% or more and 50.0% or less in the final pass, and the strain rate is 50.0 s －１ A production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets, comprising: a hot-rolling mill that performs hot-rolling on the slab under the above conditions to obtain a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less; and a cooling device that cools the hot-rolled sheet within 200 seconds after the completion of the hot-rolling, under the condition that the surface temperature of the hot-rolled sheet is 650°C or less; all of these are arranged in order.

8. The heating device heats the slab in a non-oxidizing atmosphere with an oxygen concentration of 3.0 volume% or less, in the production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets according to claim 7.

9. The heating apparatus comprises a first heating apparatus that performs a first heating step of raising the surface temperature of the slab to 950°C or higher and 1150°C or lower, and a second heating apparatus different from the first heating apparatus that then performs a second heating step of raising the surface temperature of the slab to the maximum temperature reached, the apparatus for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets according to claim 7 or 8.

10. The production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets according to claim 9, wherein the first heating device is a tunnel furnace and the second heating device is an induction heating device.

11. A hot-rolled sheet for grain-oriented electrical steel manufactured by the method for manufacturing a hot-rolled sheet for grain-oriented electrical steel described in any one of claims 1 to 5, wherein the average particle size of recrystallized grains in the surface layer from the surface to 1 / 5 of the thickness is 30 μm or more and 300 μm or less.

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