Manufacturing method and manufacturing facility for oriented electromagnetic steel sheet

Abstract

Provided are: a method for manufacturing an oriented electromagnetic steel sheet, the method being capable of significantly improving manufacturability; and a facility capable of realizing the same. This method for manufacturing an oriented electromagnetic steel sheet involves a series of steps of: hot-rolling a steel slab having a predetermined component composition; then annealing the hot-rolled sheet; performing cold rolling once or at least twice so that the total rolling reduction from the thickness of the obtained hot-rolled sheet to the thickness of a product after the cold rolling is at least 80%; performing primary recrystallization annealing; subsequently, applying an annealing separation agent onto the surface of the steel sheet; and then performing final finish annealing and flattening annealing for flattening, wherein the occupancy rate of carbides at crystal grain boundaries of recrystallized grains in the hot-rolled sheet before the cold rolling and after the annealing of the hot-rolled sheet is set to at least 80%, and then the first reduction in the cold rolling is performed under the conditions in which the strain rate is at most 200 / sec, the rolling reduction is at most 30%, and the steel sheet temperature when biting into a roll is at most 90ºC.

Description

Manufacturing method and equipment for grain-oriented electrical steel sheet

[0001] The present invention relates to a method and equipment for manufacturing a grain-oriented electrical steel sheet.

[0002] Grain-oriented electrical steel sheets are steel sheets with excellent magnetic properties that have a crystalline structure (Goss orientation) in which the <001> orientation, the axis of easy magnetization of iron, is highly concentrated in the rolling direction of the steel sheet. One method proposed to improve the magnetic properties of grain-oriented electrical steel sheets is to control the morphology of C in the steel by controlling the cooling process after annealing before final cold rolling.

[0003] For example, Patent Document 1 proposes a technique for precipitating fine carbides with a particle size of 100 Å to 500 Å by subjecting an annealed steel sheet to rapid cooling and aging treatment under specific conditions. Also, Patent Document 2 proposes a technique for increasing solute C by cooling an annealed steel sheet at a cooling rate of 150°C / min or more in a temperature range of 600 to 300°C.

[0004] The techniques proposed in Patent Documents 1 and 2 control carbon in steel as very fine carbides or dissolved C, and when dislocations are introduced during cold rolling, the dissolved C or the like adheres to the dislocations to form a Cottrell atmosphere, thereby promoting non-uniform deformation during cold rolling, modifying the cold-rolled texture, and improving the texture after primary recrystallization.

[0005] This effect is also known in general steel as a technique for increasing the {110} strength in the texture after recrystallization during annealing after cold rolling. In grain-oriented electrical steel sheets, the {110}<001> orientation is ultimately accumulated using a metallurgical phenomenon called secondary recrystallization, and the {110} texture can act as a good nucleus for secondary recrystallization. Therefore, in grain-oriented electrical steel sheets, the technique of forming carbides within the crystal grains is extremely common.

[0006] JP-A-58-157917 JP-A-52-094825

[0007] In recent years, the need for energy conservation has led to a demand for electrical steel sheets with lower iron loss, and products that meet this need are being manufactured by thinning the steel sheets and refining the magnetic domains.

[0008] It is known that silicon added to steel increases the electrical resistance of the steel, reduces Joule heat generated during use, and significantly contributes to improving iron loss. As a result, electrical steel sheets containing a large amount of silicon can achieve good iron loss. However, silicon is also known to embrittle steel, and it is generally very difficult to process steel with a silicon content of more than 4.0% by mass by rolling.

[0009] Generally, steel materials containing a large amount of alloying elements have high strength and are often difficult to roll. Furthermore, as mentioned above, Si promotes embrittlement of the material, so even a content of a few percent by mass can cause breakage problems. Therefore, various methods are often used to manufacture electrical steel sheets, such as using a rolling mill with high rigidity, a reverse mill instead of a tandem mill, or warm rolling at a high temperature to soften the material.

[0010] On the other hand, in order to provide flexibility in production, it is desirable to be able to manufacture using not only rolling mills suitable for manufacturing electrical steel sheets, but also various other rolling mills.

[0011] Therefore, the inventors of the present invention have investigated reducing the reduction rate per pass from the viewpoint of reducing the load on the rolling mill and the rolling rolls during rolling, and as a result have encountered an unexpected problem in that fractures tend to occur in a range where the rolling speed is relatively slow.

[0012] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing grain-oriented electrical steel sheets that can significantly improve manufacturability, and a manufacturing facility for grain-oriented electrical steel sheets that can realize this method.

[0013] In order to solve the above problem, the inventors first investigated materials that had fractured after the first pass of hot rolling. As a result, they confirmed that multiple deformation twins had formed in the fractured materials, and that the formed twins had an effect on adjacent crystals by propagating through grain boundaries.

[0014] It is known that twins can cause material embrittlement by cutting other twins or interacting with dislocations, so the inventors believed that suppressing the formation of such twins would be particularly effective in suppressing fracture.

[0015] Typically, twins are formed so that the atoms constituting a material are mirror-symmetrical with respect to the twin plane. Therefore, they are unlikely to form when dislocations are introduced into the crystal and a large amount of distortion is present in the crystal lattice. Under the conditions under which fracture occurred, a low rolling reduction was used to reduce the rolling load, and at the same time, the rolling speed was relatively slow to improve stability during sheet threading through the rolling mill. The inventors suspected that rolling under these conditions may have resulted in the formation of numerous deformation twins before plastic deformation due to dislocations occurred. However, these rolling conditions were set to allow rolling not only on rolling mills designed for electrical steel sheet production, but also on many general-purpose rolling mills. Therefore, methods such as increasing the reduction rate or using a high rolling speed, which increase the burden on the rolling mill, are not desirable countermeasures.

[0016] Therefore, the inventors realized that it is difficult to completely suppress the formation of twins, and decided to establish a policy to reduce the amount of twins generated by preventing twins from propagating to other adjacent crystals, even if twins do occur, and to conduct research on this. As a result, the inventors came up with the idea of ​​precipitating carbides at grain boundaries as a way to impart a function like a barrier to the propagation of twins to grain boundaries, since twins propagate across grain boundaries. The details of the experiments that led to the above findings are described below.

[0017] <Experiment 1> A steel slab for grain-oriented electrical steel sheet (hereinafter also simply referred to as "steel slab") was prepared, having a composition containing, by mass%, 0.03% C, 4.2% Si, 0.1% Mn, 0.02% sol. Al, 50 ppm S, 100 ppm Se, and 60 ppm N, with the remainder being Fe and other elements each reduced to less than 60 ppm. The prepared steel slab was heated at 1380°C and then hot-rolled to obtain a hot-rolled coil (hereinafter also referred to as "hot-rolled coil") having a thickness of 2.5 mm. Test specimens were cut from the obtained hot-rolled coil, and the test specimens were subjected to hot-rolled sheet annealing in a laboratory experimental furnace at a temperature of 980°C. An experiment was conducted in which cooling after hot-rolled sheet annealing was controlled.

[0018] First, referring to Non-Patent Document 1 (New Edition of Steel Materials and Alloy Elements, p. 395), carbides (Fe ３ The conditions under which precipitation of C) is considered possible are a cooling rate of 1.5°C / sec in the temperature range of 700°C or lower and 600°C or higher, and a residence time in the above temperature range of 1 minute or longer.

[0019] A section was cut from the resulting hot-rolled annealed sheet so that a cross section perpendicular to the rolling direction could be observed. After etching with nital, the central part of the sheet was continuously observed using a scanning electron microscope (SEM) covering 500 μm in the thickness direction and 1 mm in the width direction perpendicular to the rolling direction (sheet width direction). The results showed that carbides precipitated at 85% of the grain boundaries within the observed field of view. When etched with nital, the steel portion was etched, while the carbides remained unetched. Therefore, if carbides formed on the grain boundaries, the film-like carbides were observed with a contrast different from that of the steel matrix. Furthermore, by analyzing the same area using a high-resolution electron probe microanalyzer (EPMA), it was possible to determine the state of C enrichment on the grain boundaries and quantify the extent to which the entire grain boundary length within the observed field of view was covered with carbides. Furthermore, during cooling after the hot-rolled sheet annealing, almost no carbides precipitated in the temperature range from the hot-rolled sheet annealing temperature to 700°C or higher. Therefore, after cooling at an arbitrary cooling rate (for example, 20°C / sec) in the temperature range from the hot-rolled sheet annealing temperature to 700°C, the cooling conditions were changed so that a constant cooling rate was maintained in the temperature range of 700°C to 600°C, and the residence time in the temperature range of 700°C to 600°C was changed. The cooling rate in the temperature range of 600°C or less was set to 50°C / sec. The steel sheet thus obtained was subjected to a cooling treatment to remove Fe carbides precipitated on the grain boundaries. ３ C was evaluated to obtain the samples shown in Table 1. Table 1 shows the relationship between the residence time in the temperature range of 700°C or lower and 600°C or higher and the grain boundary occupancy rate of carbides.

[0020]

[0021] <Experiment 2> A steel slab was prepared having a composition, by mass%, of 0.02% C, 4.8% Si, 0.3% Mn, and 0.005% sol. Al, with the balance being Fe and other elements, with impurities such as S, N, Se, and O each reduced to 50 ppm or less. The prepared steel slab was heated to 1150°C and then hot-rolled to obtain a hot-rolled coil with a thickness of 2.5 mm. Test specimens were cut from the obtained hot-rolled coil and subjected to hot-rolled sheet annealing in a laboratory experimental furnace, with the test specimens reaching a temperature of 990°C. The test specimens were rapidly cooled at a rate of 50°C / s in the temperature range from 700°C to 600°C. The cooling rate in the temperature range from 600°C to 500°C after hot-rolled sheet annealing was changed, and the residence time in the temperature range from 600°C to 500°C was controlled to change. After annealing the hot-rolled sheet, since carbides do not precipitate at grain boundaries in the temperature range of 700°C or higher, any cooling pattern was acceptable. Cooling was performed at 30°C / sec, and after the residence time in the temperature range of 600°C or lower and 500°C or higher, rapid cooling was performed at 60°C / sec. The grain boundary occupancy of carbides after annealing was quantitatively evaluated using the SEM observation described above. Table 2 shows the relationship between the residence time in the temperature range of 600°C or lower and 500°C or higher and the grain boundary occupancy of carbides. Comparing the results of Tables 1 and 2 reveals that, as described in Non-Patent Document 1, precipitation of grain boundary carbides is more likely to proceed when the steel is held in the temperature range of 600°C or lower and 500°C or higher. Although carbide precipitation also proceeds at temperatures above 600°C, considering that cooling for more than one minute is not practical during actual manufacturing, controlling the residence time, especially in the temperature range of 600°C or lower and 500°C or higher, is extremely important.

[0022]

[0023] The obtained samples were subjected to cold rolling with a first pass reduction of 20% and a strain rate of 150 / sec, followed by multiple passes of rolling to a thickness of 1.0 mm. While the sheets did not completely break, some cracks were observed, resulting in crack defects. The number of samples actually rolled under each experimental condition was used as the parameter, and the number of samples with crack defects was used as the crack occurrence rate, resulting in the results shown in Figure 1.

[0024] As shown in Figure 1, the inventors have found that, to prevent fracture problems that occur during cold rolling under specific rolling conditions of low reduction and low strain rate, it is effective to set the carbide occupancy rate to 80% or more of the grain boundaries of recrystallized grains in the steel sheet before cold rolling, regardless of the cooling pattern. Furthermore, when materials in which cracks occurred under relatively low reduction were observed, it was confirmed that many deformation twins were formed. However, once the reduction had progressed to a certain extent, the processed structure became complex and intricate due to dislocations, and it was not easy to interpret this as a change in twin density.

[0025] The results obtained are considered to be extremely useful knowledge from the perspective of improving manufacturability. Meanwhile, in the production of grain-oriented electrical steel sheets, carbide control for texture control is also important. When the material is held at high temperatures with a high diffusion rate for a long time, promoting grain boundary precipitation, the carbon concentration within the crystal grains naturally decreases significantly.

[0026] Therefore, the inventors have studied extensively how to increase the carbide occupancy rate at the grain boundaries while retaining as much carbon as possible within the crystal grains, by maintaining the temperature at which carbides are formed at the grain boundaries for only the time required for nucleation, and performing the stage in which the precipitates grow at a low temperature at which the diffusion rate is as slow as possible, and have completed the present invention.

[0027] That is, the present invention that solves the above problems is as follows. [1] A method for producing a grain-oriented electrical steel sheet, which comprises a series of steps including hot rolling a steel slab containing, by mass%, C: 0.01% to 0.10%, Si: 2.0% to 6.5%, and Mn: 0.01% to 0.5%, followed by hot-rolling annealing, and then cold rolling one or more times from the thickness of the obtained hot-rolled sheet to the thickness of the product after cold rolling with a total reduction of 80% or more, followed by primary recrystallization annealing, applying an annealing separator to the surface of the steel sheet, followed by final annealing and flattening annealing for flattening, characterized in that, after the hot-rolled sheet annealing, the occupancy rate of carbides relative to the grain boundaries of recrystallized grains in the hot-rolled sheet before the cold rolling is set to 80% or more, and the initial cold-rolling reduction is set at a strain rate of 200 / s or less, a reduction of 30% or less, and the steel sheet temperature when being engaged in the rolls is set to 90°C or less.

[0028] [2] The method for producing a grain-oriented electrical steel sheet according to [1], wherein one or more intermediate annealing steps are performed between the two or more cold rolling steps.

[0029] [3] The method for producing a grain-oriented electrical steel sheet according to [1] or [2], wherein during cooling after annealing the hot-rolled sheet, the residence time in a temperature range of 600°C or lower and 500°C or higher is 10 seconds or longer.

[0030] [4] The method for producing a grain-oriented electrical steel sheet according to [1] or [2], wherein, during cooling after the hot-rolled sheet annealing, the residence time in the temperature range of 600°C or less and 500°C or more is 3 seconds or more and less than 10 seconds, the average cooling rate in the temperature range of 500°C or less and 200°C or more is 10°C / second or less, and cooling is performed at a cooling rate of 15°C / second or more until coil winding.

[0031] [5] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [4], wherein the heating rate in the primary recrystallization annealing is 200°C / sec or more in a temperature range of 550°C or more and 680°C or less.

[0032] [6] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [5], wherein the steel slab further contains, in addition to the chemical composition, sol. Al: 0.010% to 0.050%, N: 0.004% to 0.015%, and S+0.4Se: 0.010% to 0.050%, in mass%.

[0033] [7] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [5], wherein the steel slab further contains, in addition to the above-mentioned chemical composition, sol. Al: less than 0.010%, and each of the elements S, N, and O: 60 ppm or less, by mass%.

[0034] [8] In addition to the above-mentioned chemical composition, the steel slab further contains, in mass%, Ni: 0.005% or more and 1.50% or less, Sn: 0.01% or more and 0.50% or less, Sb: 0.005% or more and 0.50% or less, Cu: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, P: 0.0050% or more and 0.50% or less, Cr: 0.01% or more and 1.50% or less, Nb: 0.0005% or more and 0.0200% or less, B: 0.0005% or more and 0.0200% or less, Te: 0.0005% or more and 0.020 10. The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [7], wherein the steel sheet contains one or more selected from the group consisting of: Co: 0.0001% or more and 0.0100% or less, Ga: 0.0001% or more and 0.0100% or less, Zn: 0.0001% or more and 0.500% or less, Bi: 0.0005% or more and 0.0200% or less, Pb: 0.001% or more and 0.3% or less, Ge: 0.001% or more and 0.3% or less, As: 0.001% or more and 0.3% or less, and Ag: 0.001% or more and 0.3% or less.

[0035] [9] Equipment used in manufacturing grain-oriented electrical steel sheet, the equipment comprising: a plurality of cooling sections for cooling a steel strip that has been annealed to a temperature of 700°C or less in multiple stages; a thermometer provided at least halfway through each cooling section or on the outlet side thereof for measuring the temperature of the steel strip; a control unit for controlling the cooling rate in each cooling section by using the temperatures measured by the thermometers for feedback control; and at least one cooling water removal unit provided between the cooling sections for removing cooling water from the steel strip, wherein the equipment is capable of setting the coil winding temperature to 100°C or less.

[0036]

[10] The manufacturing equipment for grain-oriented electrical steel sheet according to [9], wherein the plurality of cooling sections include a first cooling section that controls the residence time of the steel strip in a temperature range of 600°C or lower and 500°C or higher to 3 seconds or longer and less than 10 seconds, and a second cooling section that controls the residence time of the steel strip in a temperature range of 500°C or lower and 200°C or higher to 30 seconds or longer.

[0037] According to the present invention, it is possible to provide a method for manufacturing grain-oriented electrical steel sheets that can significantly improve manufacturability, and equipment that can realize this method.

[0038] 1 is a diagram showing the relationship between the grain boundary occupancy rate of carbides and the probability of crack occurrence. FIG. 2 is a schematic diagram showing an example of manufacturing equipment for the grain-oriented electrical steel sheet according to the present invention.

[0039] (Method for manufacturing grain-oriented electrical steel sheet) Hereinafter, an embodiment of the present invention will be described. The method for manufacturing grain-oriented electrical steel sheet according to the present invention is a method for manufacturing grain-oriented electrical steel sheet through a series of steps, including hot rolling a steel slab containing, by mass%, C: 0.01% to 0.10%, Si: 2.0% to 6.5%, and Mn: 0.01% to 0.5%, followed by hot-rolling annealing, cold rolling the resulting hot-rolled sheet to the thickness of the resulting hot-rolled sheet, followed by one or more cold rolling steps with a total rolling reduction of 80% or more to the thickness of the product, primary recrystallization annealing, applying an annealing separator to the surface of the steel sheet, final annealing, and flattening annealing for flattening. Here, after annealing the hot-rolled sheet and before cold rolling, the occupancy rate of carbides relative to the grain boundaries of recrystallized grains in the hot-rolled sheet is set to 80% or more, and the initial reduction in cold rolling is set to a strain rate of 200 / sec or less, a reduction ratio of 30% or less, and the steel sheet temperature when being bitten into the rolls is set to 90°C or less.

[0040] [Steel Slab] In the present invention, a steel slab for grain-oriented electrical steel sheet is used as a starting material. First, the chemical composition of the steel slab will be described. In the following description of the chemical composition, "%" represents "mass %" and "ppm" represents "mass ppm" unless otherwise specified.

[0041] The present invention is advantageous in improving manufacturability when low reduction and low speed rolling are required due to the use of a rolling mill that does not have the mill rigidity, load capacity, or equipment for performing warm rolling, or when low reduction and low speed rolling are required due to process requirements. Therefore, many other components and manufacturing processes can be adopted that are similar to those used in the manufacture of general grain-oriented electrical steel sheets. However, the contents of C, Si, and Mn are limited for the following reasons.

[0042] C: 0.01% or more and 0.10% or less C is an essential element for precipitating carbides on grain boundaries and improving texture. However, if the C content exceeds 0.10%, it is difficult to decarburize during the final decarburization annealing, and the carbon remains in the product, causing iron loss degradation known as magnetic aging. Therefore, the C content is set to 0.10% or less. Furthermore, if the C content is less than 0.01%, various precipitation controls cannot be adequately performed. Therefore, the C content is set to 0.01% or more. From the viewpoints of manufacturability and magnetic properties, the C content is preferably set to 0.02% or more and 0.06% or less.

[0043] Si: 2.0% or more and 6.5% or less Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, the Si content must be 2.0% or more. On the other hand, Si also increases the brittleness of steel. If the Si content exceeds 4.5%, the risk of fracture during equipment threading increases and cold rolling properties also deteriorate significantly. However, in the present invention, Si can provide the effect of suppressing cracking, so the Si content can be higher than usual. However, since the magnetostriction properties required along with iron loss saturate at 6.5%, adding more Si does not significantly improve the magnetic properties. Therefore, the Si content is set to 6.5% or less. Since the risk reduction effect during equipment threading is not zero, the Si content is preferably set to 2.8% or more and 4.5% or less.

[0044] Mn: 0.01% or more and 0.5% or less Mn is a useful element from the viewpoint of controlling oxide film formation during primary recrystallization, but if it is less than 0.01%, it is not effective in terms of controlling oxide film formation. Therefore, the Mn content is set to 0.01% or more. On the other hand, Mn also has the effect of improving hot workability during manufacturing, but if the Mn content exceeds 0.5%, the primary recrystallization texture deteriorates, leading to deterioration of magnetic properties. Therefore, the Mn content is set to 0.5% or less.

[0045] Other typical compositions are as follows. In the present invention, it is sufficient that the steel slab has a composition that allows a grain-oriented electrical steel sheet to be obtained by sequentially performing known processes, namely, hot rolling, hot-rolled sheet annealing, cold rolling to the final sheet thickness in one pass, decarburization annealing (which also serves as primary recrystallization annealing), and final finish annealing (which also serves as secondary recrystallization annealing and purification annealing). Therefore, it is possible to use a composition that utilizes an inhibitor component to develop secondary recrystallized grains, or, as shown in Patent Document 3, etc., it is also possible to develop secondary recrystallized grains without using a precipitation-type inhibitor (AlN, MnS, MnSe, etc.). The preferred contents of the inhibitor components are as follows, depending on the type:

[0046] <When an inhibitor component is used> Sol. Al: 0.010% or more and 0.050% or less N: 0.004% or more and 0.015% or less S + 0.4Se: 0.010% or more and 0.050% or less If the sol. Al content is less than 0.010%, the magnetic flux density of the produced grain-oriented electrical steel sheet decreases. On the other hand, if the sol. Al content exceeds 0.050%, secondary recrystallization becomes unstable. Therefore, the sol. Al content is preferably 0.010% or more and 0.050% or less.

[0047] If the N content is less than 0.004%, AlN does not precipitate properly during the manufacturing process, making it difficult to control the grain size. Furthermore, if the N content exceeds 0.015%, it can cause frequent surface defects called blisters. Therefore, the N content is preferably 0.004% or more and 0.015% or less. The N content can be changed as needed by applying a nitriding process during the manufacturing process, and in many cases, sufficient precipitates can be formed with an N content of 0.010% or less.

[0048] S + 0.4Se: 0.010% or more and 0.050% or less. The absolute amounts of Se and S as inhibitor components are insufficient when the S content + 0.4 × Se content is less than 0.010%. On the other hand, if S + 0.4Se exceeds 0.050%, purification during final annealing becomes difficult. Therefore, it is preferable that S + 0.4Se be 0.010% or more and 0.050% or less. S and Se can be used as inhibitors in the form of MnSe and MnS, respectively, or as a composite of these, Mn(S,Se). Furthermore, AlN-based inhibitors and MnSe and / or MnS-based inhibitors can coexist, resulting in a synergistic effect.

[0049] <When no precipitation inhibitor components are contained> Sol. Al: less than 0.010% S: 60 ppm or less N: 60 ppm or less O: 60 ppm or less When no precipitation inhibitor components are contained, the contents of sol. Al, S, and O, which are precipitation inhibitor-forming elements, are limited to extremely low levels. Specifically, they are limited to sol. Al: less than 0.010%, S: 60 ppm or less, and O: 60 ppm or less. If these amounts are exceeded, it becomes difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition.

[0050] It is desirable to keep N at 60 ppm or less to prevent the formation of Si nitrides after purification annealing. It is also desirable to reduce the contents of nitride-forming elements Ti, Nb, B, Ta, and V to 0.050% or less, respectively. This is to prevent deterioration of iron loss by not interfering with the texture inhibition effect.

[0051] The contents of the inhibitor components are as described above, but by adding grain boundary segregation elements to these, magnetic properties can be improved. The elements include Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Mo: 0.01% to 0.50%, P: 0.0050% to 0.50%, Cr: 0.01% to 1.50%, and Nb: 0.0005% to 0.02%. 00% or less, B: 0.0005% or more and 0.0200% or less, Te: 0.0005% or more and 0.0200% or less, Co: 0.0001% or more and 0.0100% or less, Ga: 0.0001% or more and 0.0100% or less, Zn: 0.0001% or more and 0.500% or less, and Bi: 0.0005% or more and 0.0200% or less.

[0052] Furthermore, Pb, Ge, As, Ag, etc. may be contained in the range of 0.001% or more and 0.3% or less, respectively. These elements may be used alone or in combination, thereby improving the iron loss.

[0053] A steel slab, which is a starting material having the above-mentioned composition, is heated at an appropriate temperature according to the composition system, and then subjected to hot rolling, including rough rolling and finish rolling, to produce a hot-rolled sheet. When the steel slab contains precipitation-type inhibitor components, it is heated to a temperature range of 1350°C to 1450°C inclusive to completely dissolve Al, Se, S, and the like. On the other hand, when the steel slab does not contain precipitation-type inhibitor components, if the heating temperature is too high, the inhibitor-forming components dissolved during heating will precipitate non-uniformly and finely during hot rolling, which will locally suppress grain boundary migration, resulting in a highly non-uniform grain size distribution and inhibiting the development of secondary recrystallized grains in the Goss orientation. Therefore, it is preferable to use a relatively low heating temperature, for example, 1250°C or less. The hot-rolling conditions are not particularly limited, and may be the same as those typically used for producing grain-oriented electrical steel sheets.

[0054] The hot-rolled sheet obtained as described above is subjected to hot-rolled sheet annealing. In hot-rolled sheet annealing, it is preferable to perform soaking treatment at a temperature of 800°C or higher and 1150°C or lower for 20 seconds or longer in order to homogenize the hot-rolled structure. The effect of the present invention was greater in terms of crack suppression when the maximum annealing temperature was less than 1000°C. Although the mechanism is not clear, when hot-rolled sheet annealing is performed at a low temperature of less than 1000°C, recrystallization nuclei are not formed, and a relatively coarse structure caused by hot rolling remains in the center of the sheet thickness, which may make it more likely to develop into cracks, and it is believed that this is why the crack suppression effect of the present invention is exerted.

[0055] Next, in the cooling performed after annealing the hot-rolled sheet, one of the following controls is required from the viewpoint of carbide control.

[0056] 1) The residence time in the temperature range of 600°C to 500°C is set to 10 seconds or more. Holding at a higher temperature reduces the driving force for carbide precipitation, so a longer holding time may be necessary. Holding at a temperature in this temperature range for 10 seconds or more allows carbides to form on the grain boundaries. However, because the holding temperature is high, much of the carbon present within the grains diffuses to the grain boundaries, lowering the carbon concentration within the grains. As described above, under certain rolling conditions, increasing the grain boundary occupancy rate by carbides can reduce the cracking rate and improve manufacturability. On the other hand, because the utilization of carbides within the grains is limited, when using these conditions, the texture obtained after primary recrystallization annealing is inferior, and the magnetic properties of the product may be slightly inferior. However, because these conditions allow for relatively easy improvement in productivity, they can be applied when higher levels of magnetic properties are not required. While there is no specific upper limit to the residence time, the effect of crack suppression saturates even with a longer residence time, and a shorter time is preferable from the perspective of magnetic properties. Therefore, a residence time of 15 seconds or less is desirable.

[0057] 2) The residence time in the temperature range of 600°C to 500°C is set to 3 seconds or more but less than 10 seconds, and the average cooling rate in the temperature range of 500°C to 200°C is set to 10°C / second or less (i.e., residence time 30 seconds or more). Cooling is performed at 15°C / second or more before coiling. By setting the residence time in the temperature range of 600°C to 500°C or more to 3 seconds or more, carbide nuclei can be formed on the grain boundaries. If the residence time in this temperature range is less than 3 seconds, grain boundary precipitation does not proceed sufficiently. On the other hand, if the residence time in this temperature range is 10 seconds or more, precipitation at the grain boundaries may proceed too much, and even if the cooling rate below 500°C is controlled, an appropriate carbide state may not be achieved. In other words, the necessary amount of carbon in the grains may be reduced, which may prevent the texture improvement effect from being achieved. Therefore, it is preferable to set the residence time in the temperature range to 3 seconds or more but less than 10 seconds.

[0058] The cooling rate is reduced in the temperature range of 500°C to 200°C, where the carbon diffusion rate is extremely slow compared to the temperature range of 600°C to 500°C, and the residence time is set to 30 seconds or more. This heat pattern allows for some degree of intragranular carbon diffusion to the grain boundaries while simultaneously retaining a certain amount of carbon within the grains. After appropriate grain boundary precipitation, the cooling rate is increased again to suppress intragranular carbon diffusion and ensure appropriate intragranular carbide precipitation. After at least the residence treatment for grain boundary precipitation, the cooling rate must be increased again to a cooling rate of 15°C / second or more. This minimizes the impact on the texture while improving manufacturability. As a result, the cooled steel strip has carbides formed on the grain boundaries, and the thickness of the carbide depleted zone (depleted zone) formed near the grain boundaries is reduced to 20% or less of the grain size.

[0059] In particular, to realize the heat pattern shown in 2), the cooling section following the continuous annealing furnace in the grain-oriented electrical steel sheet manufacturing facility needs the following mechanism as shown in the schematic diagram in Figure 2: 1. It has multiple cooling sections (two cooling sections 1 and 2 in the example of Figure 2) that cool the steel strip in multiple stages in the temperature range where the steel strip temperature is 600°C or less. In the temperature range from the hot-rolled sheet annealing temperature to 700°C or more, Fe carbide is cooled. ３C hardly precipitates, and although precipitation does progress in the temperature range of 600°C or higher, the precipitation rate is very slow. Therefore, cooling to 600°C after annealing is not particularly limited from the perspective of carbide control, but unnecessarily slowing the cooling rate increases the required line length. Therefore, a general cooling rate, for example, 5°C / sec to 40°C / sec, is sufficient. 2. The cooling section (cooling section 1 in the example of Figure 2) that cools the steel strip in the temperature range of 600°C to 500°C or higher has a thermometer (thermometer 1 in the example of Figure 2) that can measure the temperature of the steel strip midway through the cooling section or at the exit side. 3. A mechanism (controller) is provided that measures the steel strip temperature using the thermometer described in 2 above, feedback-controls the steel strip temperature, controls the cooling rate in the temperature range of 600°C to 500°C, and controls the residence time in that temperature range to 3 seconds or more and less than 10 seconds. 4. In the temperature range of 500°C or lower and 200°C or higher, a gradual temperature drop occurs even with natural cooling. Therefore, at the outlet of the cooling section responsible for cooling in the temperature range of 600°C or lower and 500°C or higher, cooling water remains on the steel strip. To prevent excessive cooling, at least one mechanism (cooling water removal section) is provided between the cooling sections to easily wipe and remove the cooling water. It is preferable to provide the cooling water removal section before the cooling section that significantly reduces the cooling rate. Cooling water removal sections can also be provided between all cooling sections. 5. A slow cooling section with a temperature maintenance function that ensures a residence time of 30 seconds or more in the temperature range of 500°C or lower and 200°C or higher is provided, or a natural cooling section without an active cooling function. 6. After slow cooling or natural cooling, a cooling section (cooling section 2 in the example of Figure 2) is provided between the exit coil winding mechanism and the steel strip, which is responsible for cooling the steel strip in a temperature range of 200°C or less, and a thermometer (thermometer 2 in the example of Figure 2) capable of measuring the steel strip temperature is provided midway through the cooling section or on the exit side. 7. A mechanism is provided that measures the steel strip temperature using the thermometer described in 6 above, performs feedback control of the steel strip temperature, and can achieve a stable cooling rate of 15°C / second or more in the target cooling section. Because the steel strip is also at a relatively low temperature in the target cooling section, multiple functions can be provided, such as pickling following cooling.8. When the coil is finally wound at the exit of the facility, it has the function of being cooled to below 100°C.

[0060] Next, the obtained hot-rolled and annealed sheet is subjected to cold rolling. At this time, the total reduction ratio from the thickness of the hot-rolled sheet to the thickness of the final product sheet is 80% or more. Because such high reduction puts a heavy burden on the rolling mill, a strain rate of 200 / sec or less and a reduction ratio of 30% or less are adopted as the reduction conditions for the first pass by the rolling mill. The above rolling conditions are applied due to manufacturing and equipment constraints, from the viewpoint of texture control, etc., and are not necessarily recommended as the optimal rolling conditions for the production of grain-oriented electrical steel sheets. For example, equipment constraints include a case where, although it is normally desired to achieve the final plate thickness in one rolling pass without intermediate annealing, there is a limit to the reduction rate per pass due to issues with the rolling load; or a case where, when manufacturing using a manufacturing method in which rolling is performed twice or more times with intermediate annealing once or more, the reduction rate per rolling pass is low, but there is no rolling mill that can adjust the number of passes like a reverse rolling mill, so a tandem rolling mill has to be used, and the number of stands is determined by the specifications of the rolling mill, so the rolling speed in the first pass is inevitably slow.

[0061] If rolling is not performed under such special conditions, it is not necessary to occupy the grain boundaries with carbides as specified in the present invention. Similarly, when the steel sheet temperature exceeds 90°C when it is engaged with the work rolls during the first pass reduction, the present invention does not need to be applied because, if the steel sheet temperature is high, deformation due to dislocations is likely to occur, and twinning deformation is suppressed. There are no particular restrictions on cold rolling as long as the final sheet thickness is obtained. If possible, as in many conventional techniques, the texture can be improved by performing warm rolling, which uses processing heat to increase the steel sheet temperature after the first pass.

[0062] Next, the final cold-rolled sheet is subjected to primary recrystallization annealing. The purpose of this primary recrystallization annealing is to primarily recrystallize the cold-rolled sheet having a rolled texture, adjust the primary recrystallized grain size to an optimum size for secondary recrystallization, and decarburize the carbon contained in the steel by using a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere as the annealing atmosphere, while simultaneously forming an oxide film on the surface in the oxidizing atmosphere. For this reason, the primary recrystallization annealing is performed in a H ２ The dew point is introduced in a mixed atmosphere, and the primary recrystallization annealing is performed at 750° C. to 900° C. During the temperature increase in the primary recrystallization annealing, the texture improving effect can be further enhanced by setting the temperature increase rate to 200° C. / sec or more in the temperature range of 550° C. to 680° C.

[0063] An annealing separator is applied to the surface of the above-mentioned primary recrystallization annealed steel sheet. Magnesia (MgO) is used as the main component of the annealing separator to form a forsterite film on the surface of the steel sheet after secondary recrystallization annealing. In this case, adding an appropriate amount of Ti oxide, Sr compound, etc. to the separator can further enhance the formation of the forsterite film. In particular, adding an auxiliary agent that promotes uniform forsterite film formation is also advantageous for improving release properties.

[0064] This is followed by final annealing for secondary recrystallization and forsterite film formation. ２ , Ar, H ２ Alternatively, any of these mixed gases is suitable. To more effectively perform secondary recrystallization, the material can be isothermally maintained near the secondary recrystallization temperature. However, this does not necessarily require isothermal maintenance, as slowing the rate of temperature increase can also be effective. If trace elements precipitate in the final product, this can lead to deterioration of the magnetic properties, so the maximum annealing temperature is preferably 1100°C or higher to purify the elements.

[0065] After the above-mentioned final annealing, an insulating coating can be further applied to the surface of the steel sheet and baked. The type of such insulating coating is not particularly limited, and any conventionally known insulating coating is suitable. For example, a method in which a coating liquid containing phosphate, chromate, and colloidal silica, as described in JP-A-50-79442 and JP-A-48-39338, is applied to the steel sheet and baked at a temperature of about 800°C, is suitable.

[0066] Furthermore, the shape of the steel sheet can be adjusted by flattening annealing, and further, this flattening annealing can also be used as a baking treatment for the insulating coating.

[0067] Example 1 A steel slab was prepared having a composition, by mass%, of 0.05% C, 3.2% Si, 0.04% Mn, 0.0200% sol. Al, 100 ppm Se, 100 ppm N, 60 ppm S, and less than 50 ppm O, with the remainder being Fe and unavoidable impurities. The prepared steel slab was heated to 1350°C and hot-rolled to produce a 2.0 mm-thick hot-rolled sheet. The hot-rolled sheet was then annealed at 990°C for 30 seconds using the equipment according to the present invention, and then cooled under the cooling conditions listed in Table 3. A sample was cut from the longitudinal end and width center of the obtained coil so that the cross section perpendicular to the rolling direction could be observed. Next, the cut samples were etched with nital, and then SEM observations were performed continuously at the center of the plate thickness, covering 500 μm in the plate thickness direction and 1 mm in the direction perpendicular to the rolling direction (plate width direction). The obtained SEM images were analyzed to determine the carbide occupancy rate relative to the total grain boundary length within the field of view. Subsequently, cold rolling was performed using either one or two rolling processes until the final plate thickness was achieved, and some conditions included intermediate annealing at 1030°C for 20 seconds. The final plate thickness was 0.22 mm to 0.35 mm. Subsequently, primary recrystallization annealing was performed, with a heating rate of 250°C / sec in the temperature range of 550°C to 680°C, a soaking temperature of 800°C, and a soaking time of 30 seconds. If an in-line fracture occurred during the process, it was counted as one fracture, and the in-line fracture rate was calculated using the number of coils threaded over a one-week period as the modulus. The steel sheet after primary recrystallization was composed of 95% MgO and 95% TiO. ２A water slurry of an annealing separator containing 5% phosphate, chromate, and colloidal silica was applied to the surface of the steel sheet, and the steel sheet was subjected to final finish annealing. A coating liquid containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:3 was applied to the surface of the finish annealed sheet obtained in this way, and the sheet was baked at 800°C. In this way, a product sheet coil was obtained as a grain-oriented electrical steel sheet.

[0068]

[0069] The magnetic properties of the product sheet coil obtained as described above were investigated at the width center. After stress relief annealing at 800°C for 3 hours, test pieces of 30 mm x 280 mm with a total mass of 500 g or more were cut out from the position corresponding to the outer winding of the coil during final annealing, and the magnetic properties were measured by the Epstein test specified in JIS C2550. ８ The magnetic flux density (T) at a magnetizing force of 800 A / m was measured. The results are shown in Table 4.

[0070]

[0071] As is clear from Table 4, it was confirmed that the present invention can suppress the incidence of in-line fractures even when rolling conditions that are prone to cracking are applied, and that the magnetic properties of the grain-oriented electrical steel sheet are also maintained at a good level.

[0072] Example 2 A steel slab containing, by mass, 0.04% C, 3.3% Si, and 0.05% Mn, as well as the other components shown in Table 5, was prepared. The prepared steel slab was heated to 1200°C and then hot-rolled to form a hot-rolled sheet. The hot-rolled sheet was then annealed in an inventive annealing furnace at 980°C for 60 seconds, cooled at 30°C / s in a temperature range of 950°C to 400°C, held for 3 to 150 seconds in a temperature range of 400°C to 250°C, and then cooled at 30°C / s in a temperature range of 250°C to 100°C. The holding (retention) times in the temperature ranges of 400°C to 250°C and 500°C to 200°C are shown in Table 5. Samples were cut from the resulting coil at the longitudinal end and width center so that the direction perpendicular to the rolling direction could be observed. Next, the cut samples were etched with nital, and then SEM observations were performed continuously at the center of the sheet thickness, covering 500 μm in the sheet thickness direction and 1 mm in the direction perpendicular to the rolling direction (sheet width direction). The obtained SEM images were analyzed to determine the carbide occupancy rate relative to the total grain boundary length within the field of view. Next, rolling was performed using a 6-std tandem rolling mill not normally used for electrical steel sheet production, with the rolling mill inlet temperature at 40°C. The reduction ratio and strain rate of the first pass, as well as the final sheet thickness after rolling, are shown in Table 5. Twenty coils were rolled under the same conditions, and evaluation was performed with a rating of × if two or more coils (breakage rate during rolling 10%) broke, and a rating of ◯ if one or fewer coils broke. In addition, for coils that broke in the first half of rolling, cross sections were cut from the broken portion along the rolling direction, and the length per unit area of ​​grain boundaries with a twin orientation relationship (60 degrees around the <111> axis with respect to the matrix orientation: tolerance 15 degrees) was also evaluated using EBSD. Coils that broke in the later stages of rolling were not evaluated because it was difficult to determine whether twins were present or absent. Subsequently, primary recrystallization annealing was performed in a temperature range of 400°C to 700°C, with a heating rate of 200°C / s, a soaking temperature of 850°C, and a soaking time of 40 seconds. Next, an annealing separator containing MgO as the main component was applied to the steel sheet and subjected to final finish annealing. A coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the finish annealed sheet obtained as described above, and flattening annealing was performed at 850°C for 30 seconds. In this way, a product sheet coil was obtained as a grain-oriented electrical steel sheet.

[0073] The magnetic properties of the width center of the product sheet coil obtained as described above were investigated. After final annealing, a test piece of 30 mm x 280 mm was cut out from the position corresponding to the outer winding of the coil so that the total mass was 500 g or more. ８ The relationship between the obtained magnetic flux density and each experimental condition is shown in Table 5.

[0074]

[0075] As is clear from Table 5, the inventive examples were able to maintain good magnetic properties while suppressing the in-line breakage rate.

[0076] According to the present invention, it is possible to provide a method for manufacturing grain-oriented electrical steel sheets that can significantly improve manufacturability, and equipment that can realize this method.

Claims

1. A method for producing a grain-oriented electrical steel sheet comprising a series of steps of hot rolling a steel slab containing, by mass%, C: 0.01% to 0.10%, Si: 2.0% to 6.5%, and Mn: 0.01% to 0.5%, followed by hot-rolling annealing, cold rolling the resulting hot-rolled sheet to a thickness of a product, followed by one or more cold rolling steps with a total reduction of 80% or more, followed by primary recrystallization annealing, applying an annealing separator to the surface of the steel sheet, followed by final annealing and flattening annealing for flattening, a method for producing a grain-oriented electrical steel sheet, characterized in that after the hot-rolled sheet annealing and before the cold rolling, an occupation ratio of carbides to the grain boundaries of recrystallized grains in the hot-rolled sheet is set to 80% or more, and the initial reduction in the cold rolling is set to a strain rate of 200 / sec or less, a reduction ratio of 30% or less, and the temperature of the steel sheet when it is bitten into the rolls is 90°C or less.

2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein one or more intermediate annealing steps are performed between the two or more cold rolling steps.

3. The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein during cooling after the hot-rolled sheet annealing, the retention time in a temperature range of 600°C to 500°C is 10 seconds or more.

4. 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein, during cooling after the hot-rolled sheet annealing, a residence time in a temperature range of 600°C or lower and 500°C or higher is set to 3 seconds or longer and shorter than 10 seconds, an average cooling rate in a temperature range of 500°C or lower and 200°C or higher is set to 10°C / sec or shorter, and cooling is performed at a cooling rate of 15°C / sec or faster before coiling.

5. The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the heating rate in the primary recrystallization annealing in a temperature range of 550°C or higher and 680°C or lower is 200°C / second or higher.

6. 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel slab further contains, in mass%, sol. Al: 0.010% to 0.050%, N: 0.004% to 0.015%, and S+0.4Se: 0.010% to 0.050%, in addition to the above-mentioned chemical composition.

7. 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel slab further contains, in addition to the above-mentioned chemical composition, sol. Al: less than 0.010%, and each of the elements S, N, and O: 60 ppm or less, by mass%.

8. In addition to the above-mentioned chemical composition, the steel slab further contains, in mass %, Ni: 0.005% or more and 1.50% or less, Sn: 0.01% or more and 0.50% or less, Sb: 0.005% or more and 0.50% or less, Cu: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, P: 0.0050% or more and 0.50% or less, Cr: 0.01% or more and 1.50% or less, Nb: 0.0005% or more and 0.0200% or less, B: 0.0005% or more and 0.0200% or less, Te: 0.0005% or more and 0.0 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel sheet contains at least one element selected from the group consisting of Cr: 0.0001% or more and 0.0100% or less, Co: 0.0001% or more and 0.0100% or less, Ga: 0.0001% or more and 0.0100% or less, Zn: 0.0001% or more and 0.500% or less, Bi: 0.0005% or more and 0.0200% or less, Pb: 0.001% or more and 0.3% or less, Ge: 0.001% or more and 0.3% or less, As: 0.001% or more and 0.3% or less, and Ag: 0.001% or more and 0.3% or less.

9. This equipment is used when manufacturing grain-oriented electrical steel sheet, and includes: a plurality of cooling sections that cool a steel strip that has been annealed to a temperature of 700°C or less in multiple stages; a thermometer that measures the temperature of the steel strip, provided at least halfway through each cooling section or on the outlet side; a control unit that controls the cooling rate in each cooling section by using the temperatures measured by the thermometers for feedback control; and at least one cooling water removal unit that removes cooling water from the steel strip, provided between the cooling sections, and wherein the coil winding temperature can be set to 100°C or less.

10. 10. The manufacturing equipment for grain-oriented electrical steel sheet according to claim 9, wherein the plurality of cooling sections include a first cooling section that controls the residence time of the steel strip in a temperature range of 600°C or lower and 500°C or higher to 3 seconds or longer and less than 10 seconds, and a second cooling section that controls the residence time of the steel strip in a temperature range of 500°C or lower and 200°C or higher to 30 seconds or longer.