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

Abstract

The present invention provides a method for manufacturing a hot rolled sheet that is suitable for manufacturing of a grain-oriented electrical steel sheet having stable magnetic characteristics by a continuous process in which the processes from the casting of molten steel to the production of a hot rolled sheet are executed by a series of equipment. A method for manufacturing a hot rolled sheet for a grain-oriented electrical steel sheet according to one embodiment of the present invention includes: a step for continuously casting molten steel that has a specific component composition so as to produce a slab that has a thickness of 30 mm to 80 mm inclusive; a heating step for subsequently heating the slab under specific conditions; and a hot rolling step for subsequently subjecting the slab to hot rolling so as to obtain a hot rolled sheet that has a thickness of 1.0 mm to 3.5 mm inclusive. In particular, the method is characterized in that: the rolling reduction schedule of the hot rolling step has 3 to 7 passes; in one or both of the first pass and the second pass, the rolling reduction rate is 30.0% or more; and the friction coefficient between the rolling roll and the slab in the first pass and / or the second pass is 0.20 or more.

Description

Technical Field

[0001] The present invention relates to a method for producing a hot-rolled sheet for grain-oriented electrical steel sheets, a method for producing a grain-oriented electrical steel sheet, a production equipment line for a hot-rolled sheet for grain-oriented electrical steel sheets, and a hot-rolled sheet for grain-oriented electrical steel sheets. Background Art

[0002] Grain-oriented electrical steel sheet is a soft magnetic material used as an iron core material for transformers and large generators, and has a texture in which the <001> axis of crystal orientation, which is the easy magnetization axis of iron, is highly aligned with the rolling direction of the steel sheet. Such a texture is achieved by preferentially growing grains of the {110}<001> orientation, called Goss orientation, through secondary recrystallization.

[0003] As a general production method for grain-oriented electrical steel sheets, a method is known in which precipitates called inhibitors are used to cause secondary recrystallization of grains having Goss orientation during secondary recrystallization annealing. For example, there are a method using AlN described in Patent Document 1, a method using MnS or MnSe described in Patent Document 2, all of which have been industrially put into practical use. In these methods using inhibitors, it is necessary to heat the slab at a high temperature of 1300°C or higher for complete solid dissolution of inhibitor components, but this method is extremely useful for stably developing secondary recrystallized grains.

[0004] On the other hand, Patent Document 3 and other documents disclose a technique for developing Goss-oriented grains through secondary recrystallization in a material that does not contain inhibitor components. This technique eliminates impurities such as inhibitor components as much as possible to reveal the dependence of grain boundary energy of grain boundaries during primary recrystallization on the grain boundary orientation difference angle, and allows secondary recrystallization of grains having Goss orientation without using inhibitors. The effect thereof is called the texture inhibition effect. Since this method does not require fine dispersion of inhibitors in steel, and does not require the essential slab heating at high temperature, it has great advantages in terms of both cost and maintenance.

[0005] The manufacturing method of grain-oriented electrical steel sheets using secondary recrystallization as described above has the drawback of requiring many steps and having a relatively high manufacturing cost compared to other steel products. For this reason, in recent years, technologies have been developed to reduce costs by making the slab thickness thinner and performing direct hot rolling. For example, Patent Document 4 discloses a method for obtaining uniform magnetic properties by producing thin slabs with a thickness of about 50 to 100 mm and strictly controlling the hot rolling conditions. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 40-15644 [Patent Document 2] Special Publication No. 51-13469 [Patent Document 3] Japanese Patent Publication No. 2000-129356 [Patent Document 4] Special Publication No. 2013-512332 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, when the inventors conducted manufacturing studies on a lab scale, which is smaller than the actual machine, they found that when hot-rolled coils were continuously obtained from slabs cast thinner than usual using a continuous process that carried out from molten steel casting to hot-rolled sheet production using a series of equipment, the expected magnetic properties could not be obtained. The inventors diligently investigated the cause of this problem and found that this method resulted in a high columnar crystal ratio in the slab, and therefore an appropriate texture was not obtained at the hot-rolled sheet stage, that is, the texture of the steel sheet deteriorated before secondary recrystallization.

[0008] Therefore, 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 manufacturing grain-oriented electrical steel sheets having stable magnetic properties through a continuous process in which the process from casting of molten steel to manufacturing of hot-rolled sheets is carried out by a series of equipment. [Means for solving the problem]

[0009] To solve the above problems, the inventors diligently studied hot rolling conditions using slabs with a high proportion of columnar crystal structure in a continuous process (hereinafter also referred to as the continuous casting and hot rolling process) in which the process from molten steel casting to the manufacture of hot-rolled sheets is carried out using a series of equipment. As a result, they first found that secondary recrystallization occurs and the sheet can acquire the properties of grain-oriented electrical steel by satisfying the following conditions (A) to (C). (A) The slab thickness in the casting process shall be between 30 mm and 80 mm. (B) After casting, heat the product under the specified conditions (heating time: 5 minutes or more and 30 minutes or less, maximum temperature reached: 1000°C or more and 1290°C or less). (C) Obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm by hot rolling with 3 to 7 passes.

[0010] By satisfying all of the above conditions (A) to (C), it was possible to manufacture a hot-rolled sheet by a continuous casting and hot-rolling process, and subsequently induce secondary recrystallization. However, the magnetic properties obtained after secondary recrystallization were not necessarily sufficient compared to the case where a general slab reheating process was adopted. Therefore, the inventors conducted further investigations and found that by optimizing the reduction ratio in the first or second pass of hot rolling and the coefficient of friction between the rolling rolls and the slab in that pass, an appropriate texture could be obtained in the hot-rolled sheet, and ultimately a grain-oriented electrical steel sheet with excellent magnetic properties could be obtained.

[0011] The experiments that led to the completion of this invention will be described below.

[0012] <Experiment 1> Molten steel with a composition of C: 0.03-0.04%, Si: 3.3-3.4%, Mn: 0.07-0.10%, sol.Al: 0.005-0.009%, N: 0.002-0.005%, and S: 0.002-0.005% by mass, with the remainder being Fe and unavoidable impurities, was prepared in a laboratory vacuum melting furnace and poured into a mold to obtain a 60 mm thick slab. While the cast slab was still hot, it was heated for 10 minutes under conditions that the maximum surface temperature reached was 1200°C. Subsequently, the slab was hot-rolled to obtain a 2.6 mm thick hot-rolled sheet. During this process, six passes were used, and the reduction ratio for each pass was varied. Furthermore, the first and second passes were performed using unlubricated rolling, resulting in a friction coefficient of 0.37 between the rolling rolls and the slab. The friction coefficient was evaluated by measuring the critical engagement angle. Details of the evaluation method will be described later. The process was carried out continuously from slab casting to the production of hot-rolled sheets, and the experiment was conducted under conditions that minimized temperature drop.

[0013] A thin section of a 120mm thick slab of the same material, prepared separately for investigation, was cut and immersed in a hot hydrochloric acid solution at 80°C. Microstructural observation revealed that the structure was almost entirely columnar.

[0014] Subsequently, the hot-rolled sheet underwent hot-rolled sheet annealing at a soaking temperature of 1050°C for 30 seconds. After removing scale by pickling, it was cold-rolled to a final thickness of 0.27 mm. This cold-rolled sheet was then annealed at 840°C for 150 seconds in a humid atmosphere of 55% H2-45% N2 with a dew point of 60°C 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 for secondary recrystallization was performed under conditions of a maximum temperature of 1200°C and a soaking time of 5 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 820°C to obtain the final grain-oriented electrical steel sheet.

[0015] The magnetic flux density (B8) of the obtained grain-oriented electrical steel sheets was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz. Figure 1 shows the results of organizing the obtained magnetic flux density (B8) in relation to the reduction ratio of the first and second passes. From these results, it can be seen that the magnetic flux density (B8) is good when the reduction ratio is 30.0% or more in either or both of the first and second passes.

[0016] <Experiment 2> Molten steel with the same chemical composition as in Experiment 1 was prepared in a laboratory vacuum melting furnace and poured into a mold to obtain a 60 mm thick slab. While the cast slab was still hot, it was heated for 20 minutes under conditions that the maximum surface temperature reached was 1100°C. Subsequently, the slab was hot-rolled to obtain a 2.6 mm thick hot-rolled sheet. In this process, six passes were used, with a reduction ratio of 33.3% in the first pass. In the first pass, the friction coefficient between the rolling rolls and the slab was varied by changing the lubrication conditions and the roughness of the rolling rolls. The friction coefficient was evaluated by measuring the critical engagement angle. The process from casting the slab to producing the hot-rolled sheet was carried out continuously, and the experiment was conducted under conditions that minimized temperature drop.

[0017] Subsequently, the hot-rolled sheet underwent hot-rolled sheet annealing at a soaking temperature of 950°C for 10 seconds. After removing scale by pickling, it was cold-rolled to a final sheet thickness of 0.23 mm. This cold-rolled sheet was then annealed at 840°C for 150 seconds in a humid atmosphere of 60% H2-40% N2 with a dew point of 45°C to decarburize and undergo primary recrystallization, resulting in a decarburized annealed sheet. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and a finish annealing for secondary recrystallization was performed under conditions of a maximum temperature of 1200°C and a soaking time of 5 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0018] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B8) at an exciting magnetic field of 800 A / m and an exciting frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. FIG. 2 shows the results of arranging the obtained magnetic flux density (B8) in relation to the friction coefficient in the first pass. From these results, it can be seen that the magnetic flux density (B8) is favorable when the friction coefficient in the first pass with a high rolling reduction is 0.20 or more.

[0019] <Discussion> Summarizing the above two experimental results, it can be seen that favorable magnetic properties can be obtained by increasing the rolling reduction in the early passes of hot rolling and performing rolling with a high friction coefficient. Although the reason for this is not necessarily clear, the inventors consider as follows.

[0020] In the present experiment, a slab having a columnar crystal structure such as that obtained by thin slab casting was used as the starting material. Columnar crystals have a unique crystal orientation deviation in which <100> is aligned along the elongation direction. The elongation direction is basically the thickness direction of the slab, that is, the ND direction, and this unique crystal orientation is <100> / / ND. It is known that this orientation is less likely to introduce strain during rolling, and is a crystal orientation that tends to remain even after recrystallization annealing or the like. On the other hand, it is also known that this orientation significantly inhibits the sharpening of the Goss orientation during secondary recrystallization that characterizes grain-oriented electrical steel sheets. That is, it can be said that reducing this orientation as much as possible before secondary recrystallization contributes to the improvement of magnetic flux density.

[0021] Therefore, for the condition providing excellent magnetic flux density obtained in Experiment 1 (reduction ratios: 50.0% in the first pass, 16.7% in the second pass; hereinafter referred to as "Condition A") and the condition providing poor magnetic flux density (reduction ratios: 25.0% in the first pass, 22.2% in the second pass; hereinafter referred to as "Condition B"), the existing frequency of the {100}<011> orientation, which has high frequency after primary recrystallization among the <100> / / ND groups that inhibit the growth of Goss-oriented grains in the texture after primary recrystallization, was investigated. The investigation method was as follows: incomplete pole figures of the 110, 200 and 211 planes were measured by the X-ray pole method, the orientation distribution function (ODF) was calculated from the obtained data by the ADC (Arbitrary Defined Cell) method, and the existing frequency of the {100}<011> orientation was expressed as the random intensity ratio at the position of (φ1, Φ, φ2)=(0°, 0°, 45°) in Euler notation.

[0022] The results are shown in Figure 3. The {100}<011> orientation intensity of Condition A, which provides excellent magnetic flux density, is obviously lower. In the present experiment, experiments were carried out by changing hot rolling conditions. It is considered that increasing the reduction ratio in passes with large reduction such as the first pass and the second pass increases strain introduction, and effectively destroys the columnar crystal structure having <100> / / ND orientation, thereby succeeding in reducing <100> / / ND before secondary recrystallization. In addition, the reason why the magnetic flux density increased in the case of a high friction coefficient is also substantially the same, and it is considered that friction imparts shearing force to the steel sheet to promote strain introduction.

[0023] The gist configuration of the present invention, which has been completed based on the above findings, is as follows. [1] A step of continuously casting molten steel having a chemical composition comprising, in mass%: C: 0.10% or less, Si: 1.5% or more and 4.5% or less, Mn: 0.02% or more and 0.30% or less, acid-soluble Al: 0.040% or less, N: 0.015% or less, and one or both of S and Se: 0.025% or less in total, with the balance being Fe and unavoidable impurities, to produce a slab having a thickness of 30 mm or more and 80 mm or less; thereafter, a heating step of heating the slab under conditions that the heating time is 5 minutes or more and 30 minutes or less, and the maximum reached temperature of the surface of the slab is 1000°C or more and 1290°C or less; Subsequently, a hot rolling process is performed on the slab to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm, under the conditions that the reduction schedule is 3 to 7 passes, the reduction ratio is 30.0% or more in the first pass and / or the second pass, and the coefficient of friction between the rolling rolls and the slab in that pass is 0.20 or more. A method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets.

[0024] [2] The method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets according to [1] above, wherein the heating step includes an induction heating step performed using an induction heating device.

[0025] [3] The component composition is [Group A] One or more elements selected from the group consisting of Sb: 0.500% or less, Sn: 0.500% or less, Cr: 0.500% or less, Cu: 0.50% or less, Ni: 0.50% or less, Bi: 0.500% or less, P: 0.200% or less, Mo: 0.500% or less, and Co: 0.500% or less, in mass%. [Group B] One or more elements selected from the group consisting of B: 25.0 ppm or less, Nb: 0.020% or less, Ti: 0.0400% or less, V: 0.020% or less, and W: 0.020% or less, expressed in mass percent or mass ppm. [Group C] One or more elements selected from the group consisting of Zn: 0.0200% or less, Zr: 0.020% or less, Pb: 0.0100% or less, As: 0.020% or less, Ag: 0.050% or less, Au: 0.050% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200% or less, and Hf: 0.020% or less, in mass%. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets according to [1] or [2] above, comprising one or more groups selected from the above.

[0026] [4] A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets as described in any one of the above items [1] to [3], An optional step of annealing the hot-rolled sheet, 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. The process of decarburizing and annealing the cold-rolled sheet to obtain a decarburized annealed sheet, An optional step of performing a nitriding treatment on the cold-rolled sheet during the decarburization annealing process, or on the decarburized annealed sheet after the decarburization annealing process, Subsequently, the decarburized annealed plate is treated with an annealing separating agent and then subjected to finish annealing. A method for manufacturing grain-oriented electrical steel sheets.

[0027] [5] A method for manufacturing grain-oriented electrical steel sheets according to [4] above, wherein the hot-rolled sheet annealing is an essential step, and the maximum temperature reached by the hot-rolled sheet during the step is 900°C or higher.

[0028] [6] A continuous casting machine that continuously casts molten steel having the component composition described in [1] or [3] above to produce slabs with a thickness of 30 mm or more and 80 mm or less, A slab heating device that heats the slab, with the heating time controlled to be between 5 minutes and 30 minutes, and the maximum temperature reached on the surface of the slab being between 1000°C and 1290°C, A hot rolling mill that performs hot rolling on a slab to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm, under the conditions that the reduction schedule is 3 to 7 passes, the reduction ratio is 30.0% or more in the first pass and / or the second pass, and the coefficient of friction between the rolling rolls and the slab in that pass is 0.20 or more, A row of manufacturing equipment for hot-rolled sheets of grain-oriented electrical steel, arranged in order.

[0029] [7] The production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets according to [6] above, wherein the slab heating equipment includes an induction heating device.

[0030] [8] 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 the above items [1] to [3]. [Effects of the Invention]

[0031] According to the method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets and the equipment set for manufacturing according to the present invention, it is possible to manufacture hot-rolled sheets suitable for the production of grain-oriented electrical steel sheets having stable magnetic properties through a continuous process in which the process from casting of molten steel to manufacturing of hot-rolled sheets is carried out by a series of equipment sets. [Brief explanation of the drawing]

[0032] [Figure 1] This graph shows the effect of the reduction ratios in the first and second passes of hot rolling on the magnetic flux density B8. [Figure 2] This graph shows the effect of the friction coefficient between the rolling rolls and the slab during hot rolling on the magnetic flux density B8. [Figure 3] {100} in the texture after decarburization annealing under condition A, where magnetic flux density B8 was high, and condition B, where magnetic flux density B8 was low. <011> This is a graph showing directional intensity. [Figure 4] 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. [Modes for carrying out the invention]

[0033] [Manufacturing method for hot-rolled sheets for grain-oriented electrical steel sheets] A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel 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 a heating step of heating the slab under predetermined conditions; and then a hot-rolling step of hot-rolling the slab under predetermined conditions to obtain a hot-rolled sheet.

[0034] (Composition of molten steel and slabs) First, we will explain the component composition of molten steel and slabs. Unless otherwise specified, "%" in the component notation means mass percent. Similarly, unless otherwise specified, "ppm" notation means mass ppm.

[0035] C: 0.10% or less If carbon (C) remains in the final product plate, it causes magnetic aging and leads to magnetic degradation. If the C content of the molten steel and slab exceeds 0.10%, it becomes difficult to reduce the C content to 0.005% or less, which prevents magnetic aging, through decarburization annealing. Therefore, the C content of the molten steel and slab should be 0.10% or less, preferably 0.08% or less. On the other hand, carbon has the effect of improving the texture after decarburization annealing and improving the final magnetism. From this viewpoint, a C content of 0.025% or more is preferable.

[0036] Si: 1.5% or more and 4.5% or less Si is an element that reduces iron loss by increasing electrical resistance. From the viewpoint of obtaining this effect, the Si content should be 1.5% or more, preferably 2.5% 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, preferably 4.0% or less.

[0037] Mn: 0.02% or more and 0.30% or less Mn is an element that improves the hot workability of steel. From the viewpoint of obtaining this effect to the fullest extent, the Mn content should be 0.02% or more, preferably 0.05% or more. On the other hand, if the Mn content is too high, the magnetic flux density of the product plate will decrease. Therefore, the Mn content should be 0.30% or less, preferably 0.15% or less.

[0038] Acid-soluble Al: 0.040% or less, N: 0.015% or less, S and / or Se: 0.025% or less in total. By including acid-soluble Al, N, and one or both of S and Se within the above content ranges, precipitates containing these elements can act as inhibitors, improving the final magnetic properties. Exceeding the upper limit of each content raises concerns that the precipitates may become too coarse, preventing them from exhibiting sufficient pinning effect. The content of acid-soluble Al is preferably 0.004% or more. The N content is preferably 0.002% or more. The content of one or both of S and Se is preferably 0.002% or more in total. The content of acid-soluble Al is preferably 0.030% or less. The N content is preferably 0.009% or less. The content of one or both of S and Se is preferably 0.015% or less in total.

[0039] 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 approximately 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.

[0040] [Group A] In mass%, Sb: 0.500% or less, Sn: 0.500% or less, Cr: 0.500% or less, Cu: 0.50% or less, Ni: 0.50% or less, Bi: 0.500% or less, P: 0.200% or less, Mo: 0.500% or less, and Co: 0.500% or less. By including one or more elements selected from the above, the recrystallized texture is improved, and the final magnetic properties can be enhanced. If the content of each element exceeds the upper limit, the effect saturates, leading to increased costs. There is no particular lower limit for the content of each element, but in order to obtain the above effect, the preferred content of each element is as follows: Sb: 0.005% or more, Sn: 0.005% or more, Cr: 0.005% or more, Cu: 0.01% or more, Ni: 0.01% or more, Bi: 0.005% or more, P: 0.005% or more, Mo: 0.005% or more, and Co: 0.001% or more.

[0041] [Group B] In mass percent or mass ppm, B: 25.0 ppm or less, Nb: 0.020% or less, Ti: 0.0400% or less, V: 0.020% or less, and W: 0.020% or less. By including one or more elements selected from the above, fine carbides and nitrides of these elements are formed, and the grain size after annealing is refined, improving bending characteristics and suppressing sheet threading problems. If the content of each element exceeds the upper limit, the effect saturates and becomes a cost-increasing factor. There is no particular lower limit for the content of each element, but in order to obtain the above effect, the preferred content of each element is B: 0.1 ppm or more, Nb: 0.001% or more, Ti: 0.0005% or more, V: 0.001% or more, and W: 0.001% or more.

[0042] [Group C] In mass%, Zn: 0.0200% or less, Zr: 0.020% or less, Pb: 0.0100% or less, As: 0.020% or less, Ag: 0.050% or less, Au: 0.050% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200% or less, and Hf: 0.020% or less. By including one or more elements selected from the above, these elements are concentrated or form compounded regions at the grain boundaries, thereby strengthening the grain boundaries and suppressing defects caused by grain boundary fracture. If the content of each element exceeds the upper limit, the effect saturates, leading to increased costs. There is no particular lower limit for the content of each element, but in order to obtain the above effect, the preferred content of each element is as follows: Zn: 0.0005% or more, Zr: 0.001% or more, Pb: 0.0001% or more, As: 0.001% or more, Ag: 0.001% or more, Au: 0.001% or more, Ga: 0.0001% or more, Ge: 0.0001% or more, Ca: 0.0005% or more, Mg: 0.0005% or more, REM: 0.0005% or more, and Hf: 0.001% or more.

[0043] (Casting process) In this embodiment, molten steel having the above-described component composition is first continuously cast to produce slabs with a thickness of 30 mm to 80 mm. Optimizing the slab thickness makes it possible to achieve sufficient microstructure control during subsequent hot rolling. While thicker slabs generally yield better results, in this embodiment, where the final thickness of the hot-rolled sheet is determined by a single hot rolling process without separating it into rough and finish rolling, it is extremely difficult to directly roll slabs cast to a thickness exceeding 80 mm. Therefore, the slab thickness is set to 80 mm or less. Furthermore, 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.

[0044] (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 for a heating time of 5 minutes or more and 30 minutes or less, and the maximum temperature reached on the surface of the slab is between 1000°C and 1290°C. In the slab state, the surface temperature is low and the temperature in the center of the plate thickness is high, so if hot rolling is performed afterward, many surface defects due to temperature unevenness occur. To reduce this temperature unevenness, a heat treatment is performed before hot rolling. Furthermore, since it is generally difficult to perform casting at high speed, in processes that perform casting and hot rolling in a continuous manner, 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 are inevitably mixed in sometimes precipitate in the form of nitrides, sulfides, etc. Although this problem does not necessarily occur in all slabs, it tended to occur particularly when scrap was used as the iron source. To avoid the inevitable precipitation of impurity elements, it was effective to heat the slab for 5 to 30 minutes before hot rolling, until the surface temperature of the slab reached between 1000°C and 1290°C.

[0045] Heating time: 5 minutes or more and 30 minutes or less If the heating time is less than 5 minutes, temperature unevenness remains in the thickness direction of the plate, and the occurrence of surface defects cannot be suppressed. Therefore, the heating time should be 5 minutes or more, preferably 8 minutes or more. On the other hand, if the heating time exceeds 30 minutes, the line length must be increased in the continuous casting and hot rolling process, which is inefficient, and scale formation is likely to progress in the temperature range of the heating process, which is undesirable from the viewpoint of yield. Therefore, the heating time should be 30 minutes or less, preferably 20 minutes or less. Note that "heating time" means the time during which heat is applied to the slab by the heating device, and if the heating device is a tunnel furnace, it means the time during which the slab is inside the tunnel furnace, and if the heating device is an induction heating device, it means the time during which the slab is under the influence of the magnetic field of the induction heating device.

[0046] Maximum surface temperature of the slab: 1000°C to 1290°C 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 in the recrystallized material during primary recrystallization, making secondary recrystallization impossible. Therefore, the maximum temperature reached should be 1000°C or higher, preferably 1100°C or higher. On the other hand, if the maximum temperature reached exceeds 1290°C, problems may arise such as breakout during subsequent hot rolling, or the deformation resistance of the steel becoming too low, 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.

[0047] Since the slabs are continuously supplied to the next process, it is preferable to use an open furnace such as a tunnel furnace in the heating process. Furthermore, from the viewpoint of further improving temperature uniformity, it is also preferable that the heating process includes an induction heating process using an induction heating device capable of rapid heating.

[0048] (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, the precipitation of precipitate-forming elements may progress. By setting a specific reduction schedule, unwanted precipitation can be suppressed and a uniform state can be maintained.

[0049] Depression schedule: 3 to 7 passes If the number of hot rolling passes is less than 3, the absolute value of the reduction amount 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, preferably 4 or more. On the other hand, if the number of hot rolling passes exceeds 7, the temperature of the steel sheet drops significantly, making it impossible to properly control precipitates and degrading the final magnetic properties. Therefore, the number of hot rolling passes should be 7 or less, preferably 6 or less.

[0050] In the first pass and / or the second pass, the reduction ratio is 30.0% or more, and the coefficient of friction between the rolling rolls and the slab in that pass is 0.20 or more. If the reduction ratio in both the first and second passes is less than 30.0%, the strain introduction is insufficient. <100> / / If the columnar crystal structure with ND orientation is not effectively destroyed, it will not be possible to obtain a grain-oriented electrical steel sheet with excellent magnetic properties. Therefore, it is important to set the reduction ratio to 30.0% or more in one or both of the first and second passes. There is no particular upper limit to the reduction ratio in the first and second passes. However, if the reduction ratio is set too high in these passes where the absolute value of the reduction is large, edge cracking may occur at the edges of the hot-rolled sheet. Therefore, it is preferable that the reduction ratio in the first and second passes be 55.0% or less.

[0051] It is important that in at least one of the first and second passes where the reduction ratio is 30.0% or more (hereinafter referred to as the "specific pass"), the coefficient of friction between the rolling roll and the slab is 0.20 or higher. If the coefficient of friction in the specific pass is less than 0.20, the strain introduction due to friction is insufficient. <100> / / The columnar crystal structure having an ND orientation cannot be effectively destroyed, and as a result, a grain-oriented electrical steel sheet with excellent magnetic properties cannot be obtained. The coefficient of friction of a specific pass is preferably 0.25 or higher. The upper limit of the coefficient of friction of a specific pass is not particularly limited, as a higher coefficient of friction makes it easier for strain to enter the columnar crystals and is therefore more effective. However, in order to prevent the rolling load on the hot rolling mill from becoming too high, the coefficient of friction of a specific pass is preferably 0.50 or lower, and more preferably 0.40 or lower.

[0052] Method for measuring the coefficient of friction Various methods have been proposed to measure the coefficient of friction between the rolling rolls and the slab, but in the experiments described herein, the coefficient of friction was calculated from the critical engagement angle, which can be easily evaluated. If θ is the angle that the line segment between the point where the slab contacts the rolling rolls during rolling and the center of the rolls makes with the perpendicular direction, then when θ is small, the slab is pulled in the direction of the roll gap and rolled, but when θ is large, it is not pulled in and cannot be rolled. Here, the maximum θ at which the slab is pulled in and rolled is called the "critical engagement angle θ". lim This is called the limiting engagement angle θ. lim The coefficient of friction μ has the following relationship with (1). μ = tanθ lim ...(1) Therefore, θ lim If this is known, the coefficient of friction μ can be derived. θ is calculated geometrically using only the rolling roll diameter and reduction amount. In the experiments described herein, the coefficient of friction was calculated by determining the limiting engagement angle while varying the reduction amount on a slab of constant thickness. Specifically, a rectangular slab sample with a thickness of 50 mm was preheated to 900°C, and the roll gap of a hot rolling mill with a roll diameter of 500 mm was varied. Hot rolling was attempted when the sample surface cooled to 800°C, and the feasibility of engagement was evaluated. At this time, θ was calculated from the smallest roll gap at which engagement did not occur. lim The coefficient was calculated, and μ was derived according to equation (1). This experiment used small laboratory samples, so the above method was adopted. However, when hot rolling is performed continuously as in an actual machine, it is possible to use existing friction coefficient evaluation methods that are obtained from actual data such as rolling torque and rolling load.

[0053] Thickness of hot-rolled sheet: 1.0 mm to 3.5 mm If hot-rolling is attempted to reduce the thickness of the hot-rolled sheet to less than 1.0 mm, it becomes difficult to ensure a uniform thickness throughout the entire length and width. 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, the reduction ratio during cold rolling becomes relatively high, and the magnetic properties deteriorate. Therefore, the thickness of the hot-rolled sheet should be 3.5 mm or less.

[0054] (Cooling process, winding process) After hot rolling, it is preferable to control the temperature of the hot-rolled sheet by water cooling in order to maintain a constant winding temperature. Furthermore, it is preferable to rapidly cool the sheet immediately after hot rolling in order to retain the strain introduced during hot rolling. After that, the hot-rolled sheet can be wound up to obtain a hot-rolled coil.

[0055] [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 manufactured by the above-described manufacturing method.

[0056] [Manufacturing method for grain-oriented electrical steel sheets] In a method for manufacturing grain-oriented electrical steel sheets according to one embodiment of the present invention, a hot-rolled sheet for grain-oriented 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.

[0057] First, the hot-rolled sheet is annealed as needed. As described above, the columnar crystal structure is used to improve the magnetic properties. <100> / / To minimize the ND orientation, it is highly desirable to perform hot-rolled sheet annealing. In this case, it is preferable to set the soaking temperature (the highest temperature reached by the hot-rolled sheet) in the hot-rolled sheet annealing to 900°C or higher so that recrystallization occurs. There is no particular upper limit to the soaking temperature in the hot-rolled sheet annealing. However, in order to suppress deterioration of surface quality due to pickup, it is preferable that the soaking temperature in the hot-rolled sheet annealing is 1200°C or lower. There is no particular limit to the holding time at the soaking temperature (soaking time), <100> / / To reduce ND orientation, a duration of 10 seconds or more is preferable, and to suppress deterioration of surface quality due to pickup, a duration of 240 seconds or less is preferable.

[0058] 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 higher temperature than room temperature, for example, between 100°C and 300°C. Furthermore, performing aging treatment once or multiple times in the range of 100°C to 300°C during cold rolling is effective in changing the recrystallized texture and improving the magnetic properties.

[0059] 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.

[0060] 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 20°C to 80°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 800°C to 900°C. The holding time at the annealing temperature is preferably 30 seconds to 240 seconds. Furthermore, it is preferable that the heating rate to reach the holding temperature in decarburization annealing be 50°C / s to 1000°C / s in order to obtain good magnetic properties.

[0061] Nitriding treatment may be applied to the cold-rolled sheet during the decarburization annealing process, or to the decarburized annealed sheet after the decarburization annealing process. Nitriding treatment can further improve the magnetic properties. The nitriding treatment can be performed using the conventional methods used for grain-oriented electrical steel sheets. Methods for performing nitriding treatment during the decarburization annealing process include, for example, holding the decarburized annealed sheet in a humid hydrogen-nitrogen atmosphere (e.g., 75% H2 + 25% N2), and then, while maintaining that atmosphere, blowing ammonia gas onto the steel sheet, or, after holding, introducing it into a mixed gas atmosphere of hydrogen, nitrogen, and ammonia. Methods for performing nitriding treatment after decarburization annealing include cooling the sheet to room temperature, then raising the temperature again to 400°C to 900°C and annealing it with a mixed gas of hydrogen, nitrogen, and ammonia.

[0062] Next, an annealing separator is applied to the surface of the decarburized annealed sheet. Magnesia (MgO) can be used as the main component of the annealing separator 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 separator 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 for improving the peeling characteristics of the film. The method of applying the annealing separator is not particularly limited, and methods such as applying a solution in which the annealing separator is dissolved in a solvent, or attaching a sheet of the annealing separator that has been prepared in advance, can be used as appropriate.

[0063] Next, finish annealing is performed to induce secondary recrystallization and forsterite film formation. The annealing atmosphere can be N2, Ar, H2, or a mixture of these gases. In finish annealing, it is desirable to hold the steel sheet at 1100°C to 1280°C for 3 to 50 hours in order to remove (purify) the inhibitor components from the steel sheet. More preferably, it should be 1150°C or higher for 10 hours or more. Furthermore, since purification is promoted by an H2 atmosphere, it is desirable to use an atmosphere containing 50% or more H2 in the high-temperature range.

[0064] After the secondary recrystallization annealing described above, it is useful to perform either water washing, brushing, or pickling to remove any adhering annealing separator. Subsequently, performing planar annealing to correct the shape is effective in reducing iron loss. Furthermore, to improve iron loss, it is effective to apply an insulating coating to the surface of the steel sheet before or after planar annealing. The coating may also be baked on during this planar annealing. In this case, it is desirable from the viewpoint of reducing iron loss to use a coating that can impart tension to the steel sheet. It is desirable to deposit inorganic materials onto the surface of the steel sheet using physical vapor deposition or chemical vapor deposition, as this provides excellent coating adhesion and a significant effect in reducing iron loss.

[0065] [Manufacturing equipment for hot-rolled sheets for grain-oriented electrical steel sheets] Referring to Figure 1, a production equipment array 100 for hot-rolled sheets for grain-oriented electrical steel according to one embodiment of the present invention comprises a continuous casting machine 10, a slab heating device 20, a hot rolling mill 50, 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 to this. Also, in the example of Figure 1, a cooling device 60 for cooling the hot-rolled sheet is arranged between the hot rolling mill 50 and the coiler 70, but this is also an optional piece of equipment.

[0066] 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.

[0067] The slab heating device 20 heats the slab, and the heating time is controlled to be between 5 minutes and 30 minutes, and the maximum temperature reached on the surface of the slab is between 1000°C and 1290°C. The type of slab heating device 20 is not particularly limited and includes, for example, a tunnel furnace that can pass through the slab and an induction heating device that can induce heating of the slab.

[0068] The hot rolling mill 50 performs a hot rolling process on a slab to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm. The reduction schedule is 3 to 7 passes. It is important that the hot rolling mill 50 is designed so that in the first pass and / or the second pass, the reduction ratio is 30.0% or more, and the coefficient of friction between the rolling rolls and the slab in those passes is 0.20 or more.

[0069] The cooling device 60 cools the hot-rolled sheet. The coiler 70 winds up the hot-rolled sheet discharged from the cooling device 60 to form a hot-rolled coil. [Examples]

[0070] [Example 1] Molten steel having a composition of C:0.04%, Si:3.2%, Mn:0.10%, sol.Al:0.008%, N:0.003%, and Se:0.004% by mass, with the remainder being Fe and unavoidable impurities, was prepared in a laboratory vacuum melting furnace. A casting process, a heating process, and a hot rolling process were carried out consecutively under the conditions shown in Table 1 to obtain a hot-rolled sheet. In the hot rolling process, the number of passes was set to 4 to 5, and the reduction ratio of the first and second passes was set to 30.0% or more. In addition, the first and second passes were performed without lubrication, and the coefficient of friction between the rolling rolls and the slab was set to 0.36.

[0071] Subsequently, the hot-rolled sheet underwent hot-rolled sheet annealing at a soaking temperature of 1025°C for 30 seconds. After removing scale by pickling, it was cold-rolled to a final sheet thickness of 0.23 mm. This cold-rolled sheet was then annealed at 840°C for 100 seconds in a humid atmosphere of 50% H2-50% N2 with a dew point of 45°C to decarburize and undergo primary recrystallization, resulting in a decarburized annealed sheet. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and a finish annealing for secondary recrystallization was performed under conditions of a maximum temperature of 1200°C and a soaking time of 5 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 860°C to obtain the final grain-oriented electrical steel sheet.

[0072] The magnetic flux density (B8) of the obtained grain-oriented electrical steel sheets was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz. The obtained magnetic flux density (B8) is shown in Table 1. As is clear from Table 1, good magnetic properties were obtained in the grain-oriented electrical steel sheets manufactured from hot-rolled sheets manufactured under the conditions of the present invention.

[0073] [Table 1]

[0074] [Example 2] Molten steel with a composition of C:0.01%, Si:3.1%, Mn:0.08%, sol.Al:0.004%, N:0.002%, and Se:0.003% by mass, with the remainder being Fe and unavoidable impurities, was prepared in a laboratory vacuum melting furnace and poured into a mold to obtain a 30 mm thick slab. While the cast slab was still hot, it was heated for 10 minutes under conditions that the maximum surface temperature reached was 1200°C. Subsequently, the slab was hot-rolled to obtain a 1.7 mm thick hot-rolled sheet. In this process, five passes were used, and the reduction ratios for the first and second passes were set to the values ​​shown in Table 2. Furthermore, the friction coefficient between the rolling rolls and the slab was set to the values ​​shown in Table 2 by changing the lubrication conditions and the roughness of the rolling rolls in both passes. The process from slab casting to hot-rolled sheet fabrication was carried out continuously, and the experiment was conducted under conditions that minimized temperature drop.

[0075] Subsequently, the hot-rolled sheet underwent hot-rolled sheet annealing at a soaking temperature of 1100°C for 20 seconds. After removing scale by pickling, it was cold-rolled to a final sheet thickness of 0.20 mm. This cold-rolled sheet was then annealed at 820°C for 100 seconds in a humid atmosphere of 50% H2-50% N2 with a dew point of 35°C 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 for secondary recrystallization was performed under conditions of a maximum temperature of 1150°C and a soaking time of 5 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 820°C to obtain the final grain-oriented electrical steel sheet.

[0076] The magnetic flux density (B8) of the obtained grain-oriented electrical steel sheets was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz. The obtained magnetic flux density (B8) is shown in Table 2. As is clear from Table 2, good magnetic properties were obtained in the grain-oriented electrical steel sheets manufactured from hot-rolled sheets manufactured under the conditions of the present invention.

[0077] [Table 2]

[0078] [Example 3] Molten steel with a composition of C:0.01%, Si:3.7%, Mn:0.02%, sol.Al:0.005%, N:0.008%, S:0.006%, and Se:0.004% by mass, with the remainder being Fe and unavoidable impurities, was prepared in a laboratory vacuum melting furnace and poured into a mold to obtain a slab with a thickness of 80 mm. While the cast slab was still hot, it was heated for 15 minutes under conditions that the surface temperature of the slab reached 1050°C. In some cases, the slab was immediately heated using induction heating after heating, and the maximum surface temperature reached was 1150°C after a heating time of 15 seconds. The presence or absence of induction heating is shown in Table 3. Subsequently, the slab was hot-rolled to obtain a hot-rolled sheet with a thickness of 3.3 mm. In this process, 7 passes were used, and the reduction ratio of the first pass was 40.0%. This pass was performed as an unlubricated rolling process, and the coefficient of friction between the rolling rolls and the slab was set to 0.36. The process was carried out continuously from the time the slab was cast until the hot-rolled sheet was produced, and the experiment was conducted under conditions that minimized temperature drop.

[0079] Subsequently, in some cases, the hot-rolled sheets underwent hot-rolled sheet annealing at a soaking temperature of 1150°C for 20 seconds, and the presence or absence of this hot-rolled sheet annealing is shown in Table 3. After removing scale by pickling, the sheets were cold-rolled by two cold-rolling processes with an intermediate annealing in between. The first cold-rolling resulted in a sheet thickness of 2.0 mm, and the second cold-rolling resulted in a cold-rolled sheet with a final thickness of 0.23 mm. The intermediate annealing was performed under conditions of a soaking temperature of 1000°C for 30 seconds. These cold-rolled sheets were then annealed at 850°C for 150 seconds in a humid atmosphere of 45%H2-55%N2 with a dew point of 60°C for decarburization and primary recrystallization to obtain decarburized annealed sheets. Subsequently, an annealing separation agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and finish annealing for secondary recrystallization was performed under conditions of a maximum temperature of 1220°C and a soaking time of 10 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 840°C to obtain the final grain-oriented electrical steel sheet.

[0080] The magnetic flux density (B8) of the obtained grain-oriented electrical steel sheets was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz. The obtained magnetic flux density (B8) is shown in Table 3. As is clear from Table 3, even better magnetic properties were obtained by including induction heating in part of the heating process and by performing hot-rolled sheet annealing at high temperatures.

[0081] [Table 3]

[0082] [Example 4] Molten steel containing the elements shown in Table 4, with the remainder being Fe and unavoidable impurities, was prepared in a laboratory vacuum melting furnace and poured into a mold to obtain a 50 mm thick slab. While the cast slab was still hot, it was heated for 10 minutes under conditions that the maximum surface temperature reached was 1200°C. Subsequently, the slab was hot-rolled to obtain a 2.4 mm thick hot-rolled sheet. In this process, five passes were used, with a reduction ratio of 40.0% in the first pass. This pass was performed as an unlubricated roll, and the friction coefficient between the rolling rolls and the slab was set to 0.36. The process from casting the slab to producing the hot-rolled sheet was carried out continuously, and the experiment was conducted under conditions that minimized temperature drop.

[0083] Subsequently, the hot-rolled sheet underwent hot-rolled sheet annealing at a soaking temperature of 1000°C for 20 seconds. After removing scale by pickling, it was cold-rolled to a final sheet thickness of 0.23 mm. This cold-rolled sheet was then annealed at 830°C for 120 seconds in a humid atmosphere of 45% H2-55% N2 with a dew point of 55°C to decarburize and recrystallize, resulting in a decarburized annealed sheet. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and a finish annealing for secondary recrystallization was performed under conditions of a maximum temperature of 1180°C and a soaking time of 10 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 840°C to obtain the final grain-oriented electrical steel sheet.

[0084] The magnetic flux density (B8) of the obtained grain-oriented electrical steel sheet was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz. The obtained magnetic flux density (B8) is shown in Table 4. As is clear from Table 4, good magnetic properties were obtained by using a slab having a component composition according to the present invention.

[0085] [Table 4]

[0086] [Example 5] Molten steel with a composition of C:0.04%, Si:3.2%, Mn:0.06%, sol.Al:0.006%, N:0.002%, and S:0.002% by mass, with the remainder being Fe and unavoidable impurities, was prepared in a laboratory vacuum melting furnace and poured into a mold to obtain a slab with a thickness of 45 mm. While the cast slab was still hot, it was heated for 20 minutes under conditions that the maximum surface temperature reached was 1050°C. Subsequently, the slab was hot-rolled to obtain a hot-rolled sheet with a thickness of 2.4 mm. In this process, six passes were used, with a reduction ratio of 55.5% in the first pass. This pass was performed as an unlubricated roll, and the friction coefficient between the rolling rolls and the slab was set to 0.36. The process from casting the slab to producing the hot-rolled sheet was carried out continuously, and the experiment was conducted under conditions that minimized temperature drop.

[0087] Subsequently, the hot-rolled sheet was annealed at a soaking temperature of 1100°C for 60 seconds, scale was removed by pickling, and then cold-rolled to a final thickness of 0.23 mm. This cold-rolled sheet was then annealed at 830°C for 100 seconds in a humid atmosphere of 60% H2-40% N2 with a dew point of 45°C to decarburize and recrystallize, resulting in a decarburized annealed sheet.

[0088] Subsequently, three conditions were implemented to investigate the effects of nitriding. In Table 5, No. 1, no nitriding treatment was performed. In Table 5, No. 2, intermediate nitriding was performed. In the intermediate nitriding method, immediately after the 100-second holding period at 830°C for decarburization annealing was completed, the temperature was reduced to 750°C, and the sample was then placed in a 60%H2+20%N2+20%NH3 mixed gas atmosphere for 20 seconds. In Table 5, No. 3, additional nitriding was performed. In the additional nitriding method, after the decarburization annealing was completed and the temperature was reduced to room temperature, the sample was again placed in a 60%H2+20%N2+20%NH3 mixed gas atmosphere at 750°C for 20 seconds. The increase in nitrogen content at this time was 0.014% for No. 2 and 0.013% for No. 3.

[0089] Subsequently, an annealing separation agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and finish annealing for secondary recrystallization was performed under conditions of a maximum temperature of 1220°C and a soaking time of 10 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0090] The magnetic flux density (B8) of the obtained grain-oriented electrical steel sheets was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz. The obtained magnetic flux density (B8) is shown in Table 5. As is clear from Table 5, grain-oriented electrical steel sheets manufactured from hot-rolled sheets produced under the conditions of the present invention exhibited good magnetic properties, and even better magnetism was obtained by nitriding treatment.

[0091] [Table 5] [Industrial applicability]

[0092] This invention can be applied to the manufacture of hot-rolled sheets and grain-oriented electrical steel sheets. [Explanation of Symbols]

[0093] 100 Production equipment row for hot-rolled sheets for grain-oriented electrical steel sheets 10 Continuous casting machines 20 Slab heating device 20A First heating device (tunnel furnace) 20B 2nd heating device (induction heating device) 50 Hot Rolling Mill 60 Cooling device 70 Coiler

Claims

1. A process of continuously casting molten steel having a composition in mass%, containing C: 0.10% or less, Si: 2.5% to 4.5%, Mn: 0.02% to 0.17%, acid-soluble Al: 0.040% or less, N: 0.015% or less, and one or both of S and Se: 0.025% or less in total, with the remainder being Fe and unavoidable impurities, to produce a slab with a thickness of 30 mm to 80 mm, Subsequently, a heating step is performed in which the slab is heated for a heating time of 5 minutes or more and 30 minutes or less, and the maximum temperature reached on the surface of the slab is 1000°C or more and 1290°C or less. Subsequently, a hot rolling process is performed on the slab to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm, under the conditions that the reduction schedule is 3 to 7 passes, the reduction ratio is 30.0% or more in the first and second passes or both, and the coefficient of friction between the rolling rolls and the slab in those passes is 0.20 or more. A method for manufacturing hot-rolled sheets for grain-oriented electrical steel sheets.

2. The method for manufacturing a hot-rolled sheet for grain-oriented electrical steel according to claim 1, wherein the heating step includes an induction heating step performed using an induction heating device.

3. The aforementioned component composition is [Group A] One or more elements selected from the group consisting of Sb: 0.500% or less, Sn: 0.500% or less, Cr: 0.500% or less, Cu: 0.50% or less, Ni: 0.50% or less, Bi: 0.500% or less, P: 0.200% or less, Mo: 0.500% or less, and Co: 0.500% or less, in mass percent. [Group B] One or more substances selected from the group consisting of B: 25.0 ppm or less, Nb: 0.020% or less, Ti: 0.0400% or less, V: 0.020% or less, and W: 0.020% or less, expressed in mass percent or mass ppm. [Group C] One or more elements selected from the group consisting of Zn: 0.0200% or less, Zr: 0.020% or less, Pb: 0.0100% or less, As: 0.020% or less, Ag: 0.050% or less, Au: 0.050% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200% or less, and Hf: 0.020% or less, in mass%. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets according to claim 1, comprising one or more groups selected from the above.

4. The aforementioned component composition is [Group A] One or more elements selected from the group consisting of Sb: 0.500% or less, Sn: 0.500% or less, Cr: 0.500% or less, Cu: 0.50% or less, Ni: 0.50% or less, Bi: 0.500% or less, P: 0.200% or less, Mo: 0.500% or less, and Co: 0.500% or less, in mass percent. [Group B] One or more substances selected from the group consisting of B: 25.0 ppm or less, Nb: 0.020% or less, Ti: 0.0400% or less, V: 0.020% or less, and W: 0.020% or less, expressed in mass percent or mass ppm. [Group C] One or more elements selected from the group consisting of Zn: 0.0200% or less, Zr: 0.020% or less, Pb: 0.0100% or less, As: 0.020% or less, Ag: 0.050% or less, Au: 0.050% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, REM: 0.0200% or less, and Hf: 0.020% or less, in mass%. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets according to claim 2, comprising one or more selected from the above.

5. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel according to any one of claims 1 to 4, An optional step of annealing the hot-rolled sheet, 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. The process of decarburizing and annealing the cold-rolled sheet to obtain a decarburized annealed sheet, An optional step of performing a nitriding treatment on the cold-rolled sheet during the decarburization annealing process, or on the decarburized annealed sheet after the decarburization annealing process, Subsequently, the decarburized annealed plate is treated with an annealing separating agent and then subjected to finish annealing. A method for manufacturing grain-oriented electrical steel sheets.

6. A method for manufacturing grain-oriented electrical steel sheets according to claim 5, wherein the hot-rolled sheet annealing is an essential step, and the maximum temperature reached by the hot-rolled sheet during the step is 900°C or higher.

7. A continuous casting machine for continuously casting molten steel having the component composition described in claim 1 or 3 to produce a slab with a thickness of 30 mm or more and 80 mm or less, A slab heating device that heats the slab, with the heating time controlled to be between 5 minutes and 30 minutes, and the maximum temperature reached on the surface of the slab being between 1000°C and 1290°C, A hot rolling mill that performs hot rolling on a slab to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm, under the conditions that the reduction schedule is 3 to 7 passes, the reduction ratio is 30.0% or more in the first pass and / or the second pass, and the coefficient of friction between the rolling rolls and the slab in that pass is 0.20 or more. A row of manufacturing equipment for hot-rolled sheets of grain-oriented electrical steel, arranged in order.

8. The production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets according to claim 7, wherein the slab heating device includes an induction heating device.

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