Method for manufacturing hot rolled sheet for grain-oriented electrical steel sheet, method for manufacturing grain-oriented electrical steel sheet, line of equipment for manufacturing 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 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

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 manufacturing equipment for hot-rolled sheets for grain-oriented electrical steel sheets, and hot-rolled sheets for grain-oriented electrical steel sheets.

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

[0003] A common manufacturing method for grain-oriented electrical steel sheets involves using precipitates called inhibitors to induce secondary recrystallization of grains with a Goss orientation during secondary recrystallization annealing. Examples include the method using AlN described in Patent Document 1 and the method using MnS or MnSe described in Patent Document 2, both of which have been put into practical industrial use. These inhibitor-based methods require heating the slab at high temperatures of 1300°C or higher for complete solid solution of the inhibitor components, but they are extremely useful for stably developing secondary recrystallized grains.

[0004] On the other hand, a technique for developing Goss-oriented crystal grains through secondary recrystallization in materials that do not contain inhibitor components has been disclosed in Patent Document 3, etc. This technique aims to eliminate impurities such as inhibitor components as much as possible, thereby making the dependence of grain boundary energy on the grain boundary orientation difference angle during primary recrystallization apparent, and allowing for secondary recrystallization of grains with Goss orientation without the use of inhibitors. This effect is called the texture inhibition effect. This method does not require fine dispersion of inhibitors in the steel, and therefore does not require the high-temperature slab heating that was previously essential, offering significant 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.

[0006] Japanese Patent Publication No. 40-15644, Japanese Patent Publication No. 51-13469, Japanese Patent Publication No. 2000-129356, Japanese Patent Publication No. 2013-512332

[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.

[0009] To solve the above problems, the inventors diligently studied hot rolling conditions using slabs with a high columnar crystal structure in a continuous process (hereinafter also referred to as the continuous casting and hot rolling process) in which a series of equipment is used to carry out the process from casting molten steel to manufacturing hot-rolled sheets. As a result, they first found that secondary recrystallization occurs and the sheet can have the properties of a grain-oriented electrical steel sheet if the following conditions (A) to (C) are met: (A) The slab thickness in the casting process is 30 mm or more and 80 mm or less. (B) After casting, heating is performed under predetermined 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) A hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less is obtained by hot rolling in 3 to 7 passes or less.

[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 mass% containing 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%, 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. In this process, six passes were used, and the reduction ratio for each pass was varied. Furthermore, the first and second passes were performed without lubrication, and the coefficient of friction between the rolling rolls and the slab was set to 0.37. The coefficient of friction was evaluated by measuring the critical engagement angle. Details of the evaluation method will be described later. 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.

[0013] A thin section of a 120 mm 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 was annealed at a soaking temperature of 1050°C for 30 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 material was annealed in a humid atmosphere 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] 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. ８was measured. The obtained magnetic flux density (B ８ ) is shown in Fig. 1 in relation to the reduction ratios in the first and second passes. From this result, it can be seen that when the reduction ratio is 30.0% or more in one or both of the first and second passes, the magnetic flux density (B ８ ) is good.

[0016] <Experiment 2> Molten steel having the same component composition as in Experiment 1 was produced in a laboratory vacuum melting furnace, and a slab with a thickness of 60 mm was obtained by pouring it into a mold. While the temperature of the cast slab was high, the slab was heated under the conditions that the heating time was 20 minutes and the maximum temperature reached on the surface of the slab was 1100°C. Then, hot rolling was performed on the slab to obtain a hot-rolled sheet with a thickness of 2.6 mm. At this time, the number of passes was set to 6 passes, and the reduction ratio in the first pass was set to 33.3%. Also, in the first pass, the friction coefficient between the rolling roll and the slab was variously changed by changing the lubrication conditions or the roughness of the rolling roll. The friction coefficient was evaluated by measuring the limiting bite angle. The treatment was continuously performed from after the casting of the slab until the production of the hot-rolled sheet, and the experiment was carried out under the condition that the temperature did not decrease as much as possible.

[0017] Then, the hot-rolled sheet was subjected to hot-rolled sheet annealing with a soaking temperature of 950°C and a soaking time of 10 seconds. After removing the scale by pickling, it was cold-rolled to obtain a cold-rolled sheet with a final thickness of 0.23 mm. This cold-rolled sheet was annealed for decarburization and primary recrystallization in a moist atmosphere of 840°C for 150 seconds, 60% H ２ - 40% N ２ , dew point 45°C to obtain a decarburized annealed sheet. Then, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and finish annealing for secondary recrystallization was performed under the conditions of a maximum temperature of 1200°C and a soaking time of 5 hours to obtain a finish annealed sheet. An insulating coat composed of colloidal silica and aluminum phosphate was applied to the surface of the finish annealed sheet, and baked at 850°C to obtain a final grain-oriented electrical steel sheet.

[0018] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B ８ ) 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 (B ８The results of organizing ) in relation to the friction coefficient in the first pass are shown in Fig. 2. From these results, it can be seen that when the friction coefficient in the first pass with a high reduction ratio is 0.20 or more, the magnetic flux density (B ８ ) is good.

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

[0020] In this experiment, a slab having a columnar crystal structure obtained by thin slab casting was used as the starting material. Columnar crystals have a peculiar deviation in crystal orientation where <100> aligns in the direction of elongation. The direction of elongation is basically the thickness direction of the slab, i.e., the ND direction, and this peculiar crystal orientation is <100> / / ND. This orientation is known to be difficult 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 the grain-oriented electrical steel sheet. That is, it can be said that reducing this orientation as much as possible before secondary recrystallization contributes to the improvement of the magnetic flux density.

[0021] Therefore, regarding the conditions where the magnetic flux density obtained in Experiment 1 is excellent (the reduction ratio is 50.0% in the first pass and 16.7% in the second pass, hereinafter referred to as "Condition A") and the conditions where the magnetic flux density is poor (the reduction ratio is 25.0% in the first pass and 22.2% in the second pass, hereinafter referred to as "Condition B"), the frequency of the {100}<011> orientation, which is frequent after primary recrystallization, among the <100> / / ND groups that inhibit the growth of Goss orientation grains in the grain structure after primary recrystallization was investigated. The investigation method was to measure the incomplete pole figures of the 110, 200, and 211 planes by the X-ray pole method, calculate the orientation distribution function (ODF) by the ADC (Arbitrary Defined Cell) method from the data, and represent the frequency of the {100}<011> orientation by the random intensity ratio at the position of (φ1, Φ, φ2) = (0°, 0°, 45°) in Euler notation.

[0022] The results are shown in Fig. 3. The {100}<011> orientation intensity under condition A with excellent magnetic flux density is clearly low. In this experiment, experiments were conducted to change the hot rolling conditions. First, by increasing the reduction ratio in passes with a large reduction amount, such as the first pass or the second pass, the introduction of strain increased, and it is considered that the <100> / / ND columnar crystal structure was effectively destroyed, successfully reducing <100> / / ND before secondary recrystallization. Similarly, when the friction coefficient was high, the magnetic flux density increased, and it is considered that shear force was applied to the steel sheet due to friction, promoting strain introduction.

[0023] The gist of the present invention completed based on the above findings is as follows. [1] 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: a total of 0.025% or less, and the balance consists of Fe and inevitable impurities. A step of continuously casting molten steel having a component composition to produce a slab with a thickness of 30 mm or more and 80 mm or less; Then, a heating step of heating the slab under the conditions that the heating time is 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; Then, a hot rolling step of hot rolling the slab under the conditions that the rolling schedule is 3 passes or more and 7 passes or less, and in one or both of the first pass and the second pass, the reduction ratio is 30.0% or more and the friction coefficient between the rolling roll of that pass and the slab is 0.20 or more to obtain a hot rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less. A method for producing a hot rolled sheet for a grain-oriented electrical steel sheet.

[0024] [2] The manufacturing method of the hot rolled sheet for a grain-oriented electrical steel sheet 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 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% or mass ppm; [Group B] One or more 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, in mass% or mass ppm. A method for producing a hot-rolled sheet for grain-oriented electrical steel according to [1] or [2] above, comprising one or more groups selected from the group consisting of [Group C], where 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%,

[0026] [4] A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets as described in any one of [1] to [3] 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.

[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 production line of hot-rolled sheets for grain-oriented electrical steel sheets, comprising: 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, controlled so that the heating time is 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; and a hot rolling mill that hot-rolls the slab to obtain a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less, under the conditions that the reduction schedule is 3 passes or more and 7 passes or less, 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 the said pass is 0.20 or more; and these are arranged in order.

[0029] [7] The production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets as described in [6] above, wherein the slab heating device 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 items [1] to [3] above.

[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.

[0032] The reduction ratio of the first and second passes of hot rolling is determined by the magnetic flux density B. ８ This graph shows the effect on the magnetic flux density B. The coefficient of friction between the rolling rolls and the slab during hot rolling affects the magnetic flux density B. ８ This graph shows the effect on magnetic flux density B. ８ Condition A and magnetic flux density B were high. ８ This graph shows the {100}<011> orientation strength in the texture after decarburization annealing under condition B, where the strength was low. This is a schematic diagram showing a production equipment row 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; and then hot rolling the slab under predetermined conditions to obtain a hot-rolled sheet.

[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: 0.10% 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 (C) 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% 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 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 has the effect of improving 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 Se (either or both): 0.025% or less in total. By including acid-soluble Al, N, and either or both S and Se within the above content ranges, precipitates containing these elements can act as inhibitors, improving the final magnetic properties. Exceeding the upper limits of each content raises concerns that the precipitates may become too coarse, preventing them from exhibiting a 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 either or both 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 either or both 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 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.

[0040] [Group A] By including one or more of the following in mass%, the recrystallized texture is improved, and the final magnetic properties can be improved. If the content of each exceeds the upper limit, the effect saturates and becomes a cost-increasing factor. While there are no particular limitations on the lower limit of the content of each element, in order to obtain the above effects, 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%, or in 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 of the above, carbides and nitrides of these elements are formed finely, and the grain size after annealing is refined, improving the bending characteristics and suppressing sheet threading problems. If the content of each exceeds the upper limit, the effect saturates and becomes a factor that increases 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 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] By including one or more elements selected from the above, these elements can be concentrated or compounded 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. While there are no particular limitations on the lower limit of the content of each element, in order to obtain the above effects, 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, first, molten steel having the above-mentioned component composition is 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 the effect is easier to obtain with thicker slabs, 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 hot rough rolling and hot finish rolling, it is extremely difficult to directly roll slabs cast with a thickness exceeding 80 mm during manufacturing. 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. These problems do not necessarily occur in all slabs, but they tended to occur especially 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 in the thickness direction of the plate remains, 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. If the heating device is a tunnel furnace, it means the time during which the slab is inside the tunnel furnace. 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 or higher and 1290°C or lower. If the maximum surface temperature of the slab during the heating process is less than 1000°C, it reflects the uneven temperature distribution during casting, resulting in the unavoidable precipitation of impurity elements in some areas. This leads to uneven grain size in the recrystallized material during primary recrystallization, making secondary recrystallization impossible. Therefore, the maximum surface temperature should be 1000°C or higher, preferably 1100°C or higher. On the other hand, if the maximum surface temperature exceeds 1290°C, there is a possibility of breakout during subsequent hot rolling, and the deformation resistance of the steel becomes too low, making it difficult to control the shape after rolling. Therefore, the maximum surface temperature 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, there is a possibility that the precipitation of precipitate-forming elements that are mixed in will proceed. By setting a specific reduction schedule, unwanted precipitation can be suppressed and a uniform state can be maintained.

[0049] Reduction 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] If the reduction ratio is 30.0% or more in either or both of the first and second passes, and the coefficient of friction between the rolling rolls and the slab in that pass is 0.20 or more, and the reduction ratios in both the first and second passes are less than 30.0%, the introduction of strain is insufficient, and the columnar crystal structure having the <100> / / ND orientation cannot be effectively destroyed, resulting in the inability 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 either or both of the first and second passes. There is no particular upper limit to the reduction ratios 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 ratios in the first and second passes be 55.0% or less.

[0051] In at least one of the first and second passes in which the reduction ratio is 30.0% or more (hereinafter referred to as the "specific pass"), it is important that 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 introduction of strain due to friction is insufficient, and the columnar crystal structure having the <100> / / 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 in the specific pass is preferably 0.25 or higher. The higher the coefficient of friction in the specific pass, the easier it is for strain to be introduced into the columnar crystal and the more effective it is, so there is no particular upper limit. However, in order to prevent the rolling load on the hot rolling mill from becoming too high, the coefficient of friction in the specific pass is preferably 0.50 or less, and more preferably 0.40 or less.

[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 experiment 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 if θ is small, the slab is pulled in the direction of the roll gap and rolled, but if 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 the 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 a constant thickness. Specifically, a rectangular slab sample with a thickness of 50 mm was used, 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 above method was used because this experiment used small laboratory samples. 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 or more and 3.5 mm or less. 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 of 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, in order to retain the strain introduced during hot rolling, it is preferable to rapidly cool the sheet immediately after hot rolling. After that, the hot-rolled sheet can be wound to obtain a hot-rolled coil.

[0055] [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.

[0056] [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.

[0057] First, the hot-rolled sheet is subjected to hot-rolled sheet annealing as needed. As described above, in order to reduce the <100> / / ND orientation caused by the columnar crystal structure as much as possible in order to improve magnetic properties, it is extremely desirable to perform hot-rolled sheet annealing. In that 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), but it is preferable to be 10 seconds or more in order to reduce the <100> / / ND orientation, and 240 seconds or less in order to suppress deterioration of surface quality due to pickup.

[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. As an example of performing nitriding treatment during the decarburization annealing process, the decarburization annealing process may be held 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.

[0062] 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.

[0063] 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. 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 is 1150°C or higher for 10 hours or more. Furthermore, H ２ Because the purification is accelerated by the atmosphere, H ２ It is desirable to create an atmosphere containing 50% or more of [the substance].

[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 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 consists of 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 thereto. 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 examples include a tunnel furnace that can pass through the slab and an induction heating device that can induction heat the slab.

[0068] The hot rolling mill 50 performs a hot rolling process to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm by hot rolling the slab. 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.

[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 in succession 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 was subjected to hot-rolled sheet annealing at a soaking temperature of 1025°C for 30 seconds, scale was removed by pickling, and then cold-rolled to a final sheet thickness of 0.23 mm. This cold-rolled sheet was then subjected to annealing at 840°C for 100 seconds at 50% H. ２ -50%N ２ The sheets were annealed in a humid atmosphere with a dew point of 45°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 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 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 860°C to obtain the final grain-oriented electrical steel sheet.

[0072] 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 following is shown in Table 1. 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.

[0073]

[0074] [Example 2] Molten steel having 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 was annealed at a soaking temperature of 1100°C for 20 seconds, scale was removed by pickling, and then cold-rolled to a final thickness of 0.20 mm. This cold-rolled sheet was then annealed at 820°C for 100 seconds at 50% H. ２ -50%N ２ The sheets were then annealed in a humid atmosphere with a dew point of 35°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 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 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 820°C to obtain the final grain-oriented electrical steel sheet.

[0076] 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 following is shown in Table 2. As is clear from Table 2, good magnetic properties were obtained in grain-oriented electrical steel sheets manufactured from hot-rolled sheets manufactured under the conditions of the present invention.

[0077]

[0078] [Example 3] Molten steel having 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 examples, the slab was immediately heated using an induction heating method after heating, and the maximum surface temperature of the slab was reached at 1150°C in 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 plate with a thickness of 3.3 mm. In this experiment, seven passes were used, with a reduction ratio of 40.0% in the first pass. 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 were subjected to hot-rolled sheet annealing at a soaking temperature of 1150°C for a soaking time of 20 seconds, and the presence or absence of this hot-rolled sheet annealing is recorded 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 at 45% H ２ -55%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 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 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 840°C to obtain the final grain-oriented electrical steel sheet.

[0080] 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 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 high-temperature hot-rolled sheet annealing.

[0081]

[0082] [Example 4] Molten steel having a composition consisting of 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 slab with a thickness of 50 mm. While the temperature of the cast slab was still high, the slab was heated for 10 minutes under conditions that the maximum temperature reached on the surface of the slab was 1200°C. Subsequently, the slab was subjected to hot rolling to obtain a hot-rolled sheet with a thickness of 2.4 mm. In this process, five passes were used, and the reduction ratio of the first pass was set to 40.0%. This pass was performed as an unlubricated rolling, 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.

[0083] Subsequently, the hot-rolled sheet was subjected to hot-rolled sheet annealing 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 subjected to annealing at 830°C for 120 seconds at 45% H. ２ -55%N ２ The sheets were then annealed in a humid atmosphere with a dew point of 55°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 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 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 840°C to obtain the final grain-oriented electrical steel sheet.

[0084] 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 components are listed 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]

[0086] [Example 5] Molten steel having 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 subjected to hot rolling to obtain a hot-rolled sheet with a thickness of 2.4 mm. In this process, six passes were used, and the reduction ratio of the first pass was set to 55.5%. This pass was performed as unlubricated rolling, and the coefficient of friction between the rolling rolls and the slab was set to 0.36. 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.

[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 at 60% H. ２ -40%N ２ Then, annealing for decarburization and primary recrystallization was performed in a humid atmosphere with a dew point of 45°C to obtain a decarburized annealed plate.

[0088] Subsequently, three conditions were implemented to investigate the effects of nitriding. In No. 1 of Table 5, no nitriding treatment was performed. In No. 2 of Table 5, intermediate nitriding was performed. In the intermediate nitriding method, immediately after holding at 830°C for 100 seconds for decarburization annealing, the temperature was reduced to 750°C and left at 60% H for 20 seconds. ２ +20%N ２ +20%NH ３The material was introduced into a mixed gas atmosphere. In No. 3 of Table 5, an additional nitriding method was performed. In the additional nitriding method, after decarburization annealing was completed and the temperature was cooled to room temperature, it was annealed again at 750°C for 20 seconds at 60% H. ２ +20%N ２ +20%NH ３ The mixture was introduced into a mixed gas atmosphere. The increase in nitrogen 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] 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 following is shown in Table 5. 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.

[0091]

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

[0093] 100 Hot-rolled sheet manufacturing equipment for grain-oriented electrical steel sheets 10 Continuous casting machine 20 Slab 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% of C: 0.10% or less, Si: 1.5% to 4.5%, Mn: 0.02% to 0.30%, 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; and a heating process thereafter, in which the slab is heated for a heating time of 5 minutes to 30 minutes and the maximum temperature reached on the surface of the slab is 1000°C to 1290°C. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel sheets, comprising: a hot-rolling step in which the slab is subjected to hot-rolling 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, thereby obtaining a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm 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 includes an induction heating step performed using an induction heating device.

3. The component composition is: [Group A] One or more 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% or mass ppm; [Group B] One or more 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, in mass% or mass ppm. A method for producing a hot-rolled sheet for grain-oriented electrical steel according to claim 1 or 2, comprising one or more selected from the group consisting of [Group C] 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%, 4. A method for manufacturing a hot-rolled sheet for grain-oriented electrical steel according to any one of claims 1 to 3, 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.

5. The method for manufacturing a grain-oriented electrical steel sheet according to claim 4, 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.

6. A production equipment array for hot-rolled sheets for grain-oriented electrical steel sheets, comprising: a continuous casting machine that continuously casts molten steel having the component composition described in claim 1 or 3 to produce slabs with a thickness of 30 mm or more and 80 mm or less; a slab heating device that heats the slab, controlled so that the heating time is 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; and a hot rolling mill that hot-rolls the slab to obtain a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less, under the conditions that the reduction schedule is 3 passes or more and 7 passes or less, 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 the said pass is 0.20 or more; and these are arranged in order.

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

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 claims 1 to 3.

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