Method for producing for grain-oriented electromagnetic steel sheet

Abstract

Provided is a method for producing grain-oriented electromagnetic steel sheets having good magnetic properties through a continuous process that performs casting molten steel to manufacturing hot-rolled sheets using a component composition that conforms to an inhibitor-free method, all within a series of equipment. The method for producing grain-oriented electromagnetic steel sheets according to an embodiment of the present invention involves continuously casting molten steel having a prescribed component composition to produce a slab with a thickness of 30 mm to 80 mm, followed by a heating step for continuously heating the slab, a hot rolling step for hot rolling the slab to obtain a hot-rolled sheet, and a cooling step for the hot-rolled sheet. Subsequently, the hot-rolled sheet is annealed (optional), cold-rolled once or twice with an intermediate annealing in between, decarburized, and finish-annealed. The present invention is such that, while optimizing conditions of each step, in particular, in cold rolling, the value obtained by subtracting an entry-side steel plate temperature from an exit-side steel plate temperature in a first pass is 30°C to 150°C, and characterized in that if a rolling speed is λ mpm, and a time taken for the sheet to pass between the first and second passes is t seconds, then λ × t ≥ 90.

Description

Manufacturing method of grain-oriented electrical steel sheets

[0001] This invention relates to a method for manufacturing grain-oriented electrical steel sheets. More particularly, this invention relates to a method for manufacturing grain-oriented electrical steel sheets having stable magnetic properties by cold rolling under specific conditions when using a continuous process in which a series of equipment is used to carry out everything from casting molten steel to manufacturing hot-rolled sheets.

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

[0003] Grain-oriented electrical steel sheets have a crystalline structure in which the <001> orientation, which is the easy magnetization axis of iron, is highly aligned in the rolling direction of the steel sheet. Such a texture is formed through secondary recrystallization during the manufacturing process of grain-oriented electrical steel sheets, particularly during finish annealing, which preferentially causes the growth of crystal grains with the {110}<001> orientation, known as the Goss orientation. Therefore, the crystal orientation of the secondary recrystallized grains has a significant influence on the magnetic properties of the grain-oriented electrical steel sheet.

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

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

[0006] When applying the inhibitor-free method described above, a conventional slab heating furnace (gas furnace) is used for heating the slab at low temperatures. Because the composition does not contain strong inhibitors, the effect of suppressing grain growth by inhibitors is weak. Therefore, in cold rolling, it is necessary to introduce strain into the <100> / / ND grains, which are a factor in the deterioration of magnetic properties, so that the <100> / / ND grains are easily eroded by crystal grains of other orientations during decarburization annealing.

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

[0008] However, in the process of continuously obtaining hot-rolled coils from cast slabs, the proportion of columnar crystal structure in the slab's microstructure is higher than in normal hot rolling. As a result, even after decarburization and annealing, the magnetic properties deteriorate because <100> / / ND, which is a factor that degrades magnetic properties, remains on the surface of the hot-rolled coils.

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

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

[0011] Furthermore, for the reasons mentioned above, the inhibitorless method has a weak effect in suppressing grain growth, resulting in the retention of <100> / / ND, which is a factor in the deterioration of magnetic properties, during the primary crystal stage. Consequently, there is a problem in that the magnetic properties tend to deteriorate.

[0012] On the other hand, when a process of continuously obtaining hot-rolled coils from slabs cast thinner than usual is applied to the manufacturing process of grain-oriented electrical steel sheets, there is a problem in that a large proportion of columnar crystals is present in the slab, and the thickness of the sheet before hot rolling is thinner than when manufacturing hot-rolled sheets in the usual way, which inevitably results in a smaller rolling ratio during hot rolling. As a result, there is a large amount of <100> / / ND before cold rolling, and consequently the magnetic properties deteriorate.

[0013] In view of the above problems, the present invention aims to provide a method for manufacturing grain-oriented electrical steel sheets that has good magnetic properties by using a component composition that conforms to the inhibitorless method and by carrying out a continuous process in which the process from casting molten steel to manufacturing hot-rolled sheets is carried out using a series of equipment.

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

[0015] However, simply subjecting molten steel with a composition conforming to the inhibitor-less method to a continuous casting and hot-rolling process did not yield good magnetic properties, nor did it even allow for secondary recrystallization. Therefore, the inventors diligently pursued further research and arrived at the findings described in (1) and (2) below.

[0016] (1) In order to produce hot-rolled sheets from molten steel with a composition conforming to the inhibitor-free method by a continuous casting and hot-rolling process, and then to induce secondary recrystallization, it was necessary to optimize the conditions of the continuous casting and hot-rolling process. Specifically, it was found that the following conditions (A) to (D) are important. (A) The slab thickness in the casting process should be 30 mm or more and 80 mm or less. (B) After casting, the slab should be heated under predetermined conditions (heating time: 5 minutes or more and 35 minutes or less, maximum temperature reached: 1000°C or more and 1290°C or less). (C) The surface temperature of the slab at the start of hot rolling should be 900°C or more and 1250°C or less, and a hot-rolled sheet with a thickness of 1.0 mm or more and 3.5 mm or less should be obtained by hot rolling in 3 to 7 passes. (D) Within 200 seconds after the end of hot rolling, the hot-rolled sheet should be cooled so that the surface temperature of the hot-rolled sheet is 650°C or less.

[0017] (2) By satisfying all of the above conditions (A) to (D), it was possible to manufacture hot-rolled sheets from molten steel with a composition conforming to the inhibitorless method by a continuous casting and hot-rolling process, and then induce secondary recrystallization. However, the magnetic properties obtained after secondary recrystallization were not necessarily sufficient compared to the case in which a general slab reheating process was adopted.

[0018] An investigation into the causes of these problems revealed that the texture during primary recrystallization was degraded compared to when slabs were cast at normal thicknesses. Differences in texture were mainly observed in the orientation group belonging to <100> / / ND. This was thought to be because, when casting thinly, the proportion of columnar crystals in the slab increased compared to when slabs were cast at normal thicknesses of 200 mm or more. Furthermore, the thickness of the hot-rolled sheet in the manufacturing process of grain-oriented electrical steel sheets was thinner than that of normal hot-rolled sheets, resulting in a smaller reduction ratio during hot rolling. Consequently, sufficient strain could not be introduced during cold rolling, and as a result, recrystallization of recrystallization nuclei of other orientations did not occur. After decarburization and annealing, crystals belonging to the orientation group belonging to <100> / / ND remained, degrading the crystal orientation of the Goss orientation during secondary recrystallization.

[0019] Therefore, in investigating the conditions under which a sufficient amount of strain can be introduced in the cold rolling process, it was found that the value obtained by subtracting the temperature of the incoming steel sheet from the temperature of the exiting steel sheet in the first pass should be between 30°C and 150°C, and that it is important to set λ × t ≥ 90 when the rolling speed is λ (mpm) and the time taken for the sheet to pass between the first and second passes is t seconds. As a result, it becomes possible to introduce sufficient strain to the orientation group belonging to <100> / / ND during cold rolling, and during decarburization annealing, the <100> / / ND grains are eroded by crystal grains of other orientations, reducing the amount of residual grain. Consequently, the Goss orientation grains grow more easily during secondary recrystallization, making it possible to manufacture grain-oriented electrical steel sheets with good magnetic properties.

[0020] Based on the above findings, the gist of the present invention is as follows: [1] A step of continuously casting molten steel having a composition in mass% or mass ppm of C: 0.08% or less, Si: 2.0% or more and 4.5% or less, Mn: 0.50% or less, acid-soluble Al: 20 ppm or more and 120 ppm or less, S: less than 50 ppm, and N: 80 ppm or less, with the content of Se, Te, and O each suppressed to less than 50 ppm, and the remainder consisting of Fe and unavoidable impurities, to produce a slab with a thickness of 30 mm or more and 80 mm or less; and a subsequent heating step of heating the slab using a heating device under conditions of a heating time of 5 minutes or more and 35 minutes or less, and the maximum temperature reached on the surface of the slab being 1000°C or more and 1290°C or less. Subsequently, a hot rolling step to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm, wherein the surface temperature of the slab at the start of hot rolling is 900°C to 1250°C and the reduction schedule is 3 to 7 passes; a step to cool the hot-rolled sheet within 200 seconds after the completion of the hot rolling, under the condition that the surface temperature of the hot-rolled sheet is 650°C or lower; an optional step to perform hot-rolled sheet annealing on the hot-rolled sheet; and a step to obtain a cold-rolled sheet by subjecting the hot-rolled sheet to one or two or more cold-rollings with an intermediate annealing in between, wherein in at least one of the cold-rolling steps, (i) the value obtained by subtracting the entry-side steel sheet temperature from the exit-side steel sheet temperature in the first pass is 30°C to 150°C. (ii) A method for manufacturing grain-oriented electrical steel sheets, comprising: (ii) a step of having a rolling speed of λ mpm and a time of t seconds for the sheet to pass between the first and second passes, such that λ × t ≥ 90; a step of subsequently performing decarburization annealing on the cold-rolled sheet to obtain a decarburized annealed sheet; an optional step of performing nitriding treatment on the cold-rolled sheet during the decarburization annealing or on the decarburized annealed sheet after the decarburization annealing; and a step of subsequently applying an annealing separating agent to the decarburized annealed sheet and performing finish annealing.

[0021] [2] The method for manufacturing grain-oriented electrical steel sheets according to [1] above, wherein the cold rolling is performed two or more times, and all times the conditions (i) and (ii) are satisfied.

[0022] [3] The method for producing a grain-oriented electrical steel sheet according to [1] or [2] above, wherein the heating step is performed in a non-oxidizing atmosphere having an oxygen concentration of 3.0% by volume or less.

[0023] [4] The heating step includes: a first heating step of setting the surface temperature of the slab to 950°C or more and 1150°C or less using a first heating device as the heating device; and thereafter, a second heating step of setting the surface temperature of the slab to the maximum attained temperature using a second heating device different from the first heating device as the heating device, The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [3] above.

[0024] [5] The method for producing a grain-oriented electrical steel sheet according to [4] above, wherein the first heating device is a tunnel furnace and the second heating device is an induction heating device.

[0025] [6] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [5] above, wherein the component composition further contains, in mass%, one or more selected from the group consisting of: Ni: 1.500% or less, Sn: 0.500% or less, Sb: 0.500% or less, Cu: 0.500% or less, P: 0.500% or less, Cr: 1.500% or less, Mo: 0.500% or less, B: 0.0200% or less, Nb: 0.0100% or less, Co: 0.0100% or less, Ti: 0.0200% or less, Zn: 0.0500% or less, Bi: 0.0200% or less, W: 0.0030% or less, Pb: 0.0300% or less, Ga: 0.0100% or less, Ge: 0.0300% or less, As: 0.0300% or less, and Ag: 0.0300% or less.

[0026] According to the present invention, problems caused by residual <100> / / ND, which become apparent when a continuous casting and hot rolling process is adopted using a component composition according to the inhibitor-less method, can be solved.

[0027] That is, according to the method for producing a grain-oriented electrical steel sheet of the present invention, it is possible to produce a grain-oriented electrical steel sheet having good magnetic properties by a continuous process in which everything from casting molten steel to producing a hot-rolled sheet is performed by a series of equipment lines, while using a component composition according to the inhibitor-less method.

[0028] [Method for producing grain-oriented electrical steel sheet] A method for producing a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises: a step of continuously casting molten steel having a predetermined component composition to produce a slab; a heating step of heating the slab under predetermined conditions thereafter; a hot rolling step of subjecting the slab to hot rolling under predetermined conditions thereafter to obtain a hot-rolled sheet; and a step of cooling the hot-rolled sheet under predetermined conditions thereafter. In the method for producing a grain-oriented electrical steel sheet according to the present embodiment, thereafter, hot-rolled sheet annealing is performed as necessary, then cold rolling is performed once or two or more times with intermediate annealing interposed, and further decarburization annealing and finish annealing are performed, thereby obtaining a product steel sheet.

[0029] (Component composition of molten steel and slab) First, the component composition of molten steel and slab will be described. Unless otherwise specified, the "%" indication regarding components means mass%. Similarly, unless otherwise specified, the "ppm" indication means mass ppm.

[0030] C: 0.08% or less When C remains in the final product sheet, it causes magnetic aging and induces magnetic property deterioration. If the C content in the molten steel and the slab is excessive, the load in the decarburization step increases, and the C content in the final product sheet cannot be sufficiently reduced. Therefore, the C content in the molten steel and the slab is set to 0.08% or less. On the other hand, C has a function of suppressing grain coarsening during hot rolling and improving the structure before cold rolling. In addition, in cold rolling, C improves the texture after primary recrystallization through interaction with dislocations. From this viewpoint, the C content is preferably 0.01% or more.

[0031] Si: 2.0% or more and 4.5% or less Si is an element that exhibits the effect of reducing iron loss by increasing electrical resistance. From the viewpoint of obtaining this effect, the Si content is set to 2.0% or more. On the other hand, if the Si content is excessive, cold rolling becomes significantly difficult, so the Si content is set to 4.5% or less.

[0032] Mn: 0.50% or less. If the Mn content is too high, the primary recrystallized texture deteriorates, making it difficult to obtain secondary recrystallized grains that are highly concentrated in the Goss orientation. From this viewpoint, the Mn content should be 0.50% or less. On the other hand, Mn is an element that has the effect of improving hot workability. From the viewpoint of obtaining this effect, it is preferable that the Mn content be 0.01% or more.

[0033] Acid-soluble Al: 20 ppm to 120 ppm. Since this embodiment relates to the inhibitorless method, it is necessary to reduce the content of Al, which is an inhibitor-forming component, as much as possible. From this viewpoint, the Al content is set to 120 ppm or less. When applying the inhibitorless method, if only secondary recrystallization is considered, Al is not necessarily required. However, Al has the effect of reducing O, which is an impurity, during the refining stage of molten steel, and also creates dense Al on the surface during secondary recrystallization annealing. ２ O ３ A film is formed, reducing the effects of nitriding and other processes from the surrounding atmosphere. For this reason, the Al content should be 20 ppm or more.

[0034] S: Less than 50 ppm, N: 80 ppm or less. Since this embodiment relates to an inhibitor-less method, the content of S and N, which are inhibitor-forming components, must be reduced as much as possible. If the S and N content is excessive, precipitation is more likely to occur in the continuous casting and hot rolling process, and the structure becomes non-uniform. Therefore, the S content should be less than 50 ppm and the N content should be 80 ppm or less. The lower limit of the S and N content is preferably 0 ppm. However, it is difficult to completely remove S and N, and extreme reduction of S and N leads to a significant increase in manufacturing costs. From the viewpoint of manufacturing costs, the S content is preferably 10 ppm or more, and the N content is preferably 20 ppm or more.

[0035] Se, Te, and O: Less than 50 ppm each. If the content of Se and Te is excessive, Se and Te oxides are formed, making secondary recrystallization difficult. These elements are known to segregate at the center during casting. In this invention, where hot rolling is performed immediately after casting without reheating the slab, Se and Te may remain as coarse precipitates in the center of the cast slab, reducing ductility during subsequent cold rolling and causing the steel sheet to become brittle. Therefore, the content of Se and Te should be less than 50 ppm each, preferably 30 ppm or less. In addition, O forms oxides and remains as inclusions in the final product, degrading the magnetic properties, so the O content needs to be kept below 50 ppm. The content of Se, Te, and O may be 0 ppm.

[0036] The remainder of the components other than those listed above consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permitted 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 introduced unintentionally. These elements may be inevitably present in amounts of approximately 0.010% or less. Furthermore, for the purpose of further improving performance, the component composition of the molten steel and slab may optionally contain the following elements.

[0037] Ni: 1,500% or less. Ni has the effect of improving magnetic properties by increasing the uniformity of the hot-rolled sheet structure. On the other hand, if the Ni content is excessive, secondary recrystallization becomes unstable, and the magnetic properties deteriorate. Therefore, when Ni is included, the Ni content should be 1,500% or less. From the viewpoint of obtaining the above effect from Ni addition, it is preferable that the Ni content be 0.005% or more.

[0038] Sn: 0.500% or less Sb: 0.500% or less Cu: 0.500% or less Sn, Sb, and Cu are elements that can be considered auxiliary inhibitors through grain boundary segregation, and may be useful in inhibitor-less methods that do not actively utilize inhibitors from precipitates. On the other hand, if the content of these elements is excessive, the possibility of secondary recrystallization failure increases. Therefore, when these elements are included, the content of each element should be 0.500% or less. From the viewpoint of obtaining the effects of adding these elements, it is preferable that the Sn content be 0.001% or more, the Sb content be 0.005% or more, and the Cu content be 0.010% or more. It is more preferable that the Sn content be 0.010% or more.

[0039] P: 0.500% or less Cr: 1.500% or less P and Cr have the effect of improving the forsterite film formation reaction. On the other hand, if the content of these elements is too high, the forsterite film formation may be accelerated too much, and the film may peel off. Therefore, when these elements are included, the P content should be 0.500% or less and the Cr content should be 1.500% or less. From the viewpoint of obtaining the above effects from the addition of these elements, it is preferable that the P content be 0.005% or more and the Cr content be 0.010% or more.

[0040] Mo: 0.500% or less B: 0.0200% or less Nb: 0.0100% or less Mo, B, and Nb all contribute to suppressing grain growth and have the effect of improving texture and stabilizing secondary recrystallization. On the other hand, if the content of these elements is excessive, they precipitate and function as strong inhibitors, which is undesirable in the inhibitor-less method. Therefore, when these elements are included, the Mo content should be 0.500% or less, the B content 0.0200% or less, and the Nb content 0.0100% or less. A B content of 0.0050% or less is more preferable. From the viewpoint of obtaining the above effects from the addition of these elements, it is preferable that the Mo content be 0.005% or more, the B content be 0.0001% or more, and the Nb content be 0.0005% or more. A Mo content of 0.010% or more is more preferable.

[0041] Co: 0.0100% or less Ti: 0.0200% or less Zn: 0.0500% or less Bi: 0.0200% or less W: 0.0030% or less Pb: 0.0300% or less Ga: 0.0100% or less Ge: 0.0300% or less As: 0.0300% or less Ag: 0.0300% or less Co, Ti, Zn, Bi, W, Pb, Ga, Ge, As, and Ag are grain boundary segregation elements that suppress grain growth and grain boundary movement, thereby enhancing magnetic properties. On the other hand, if the content of these elements is excessive, they may precipitate as carbides, nitrides, or oxides in the steel, or they may precipitate as elemental metals without being able to maintain a solid solution state with the steel, which can lead to a deterioration of the final magnetic properties. Therefore, when these elements are included, their content should be below the above upper limit. It is more preferable that the Ti content be 0.0050% or less. From the viewpoint of obtaining the above effects by adding these elements, it is preferable that the Co content be 0.0005% or more, the Ti content be 0.0001% or more, the Zn content be 0.0001% or more, the Bi content be 0.0001% or more, the W content be 0.0001% or more, the Pb content be 0.0001% or more, the Ga content be 0.0001% or more, the Ge content be 0.0001% or more, the As content be 0.0001% or more, and the Ag content be 0.0001% or more. It is more preferable that the Ti content be 0.0010% or more, the Bi content be 0.0050% or more, and the Pb, Ge, As, and Ag content be 0.0010% or more each.

[0042] (Casting Process) In this embodiment, molten steel having the above-mentioned component composition is first continuously cast to produce slabs with a thickness of 30 mm to 80 mm. With a component composition that does not contain inhibitors, there is a possibility of excessive grain growth during hot rolling following casting, during transport, and during waiting after processing. By optimizing the slab thickness, it becomes possible to achieve sufficient microstructure control during subsequent hot rolling. The effect is easier to obtain with a thicker slab, but in this embodiment, when the final thickness of the hot-rolled sheet is determined by a series of hot rolling processes without dividing it into rough rolling and finish rolling, it is extremely difficult to directly roll a slab cast to a thickness exceeding 80 mm in manufacturing. Therefore, the slab thickness is set to 80 mm or less. Also, if the slab thickness is less than 30 mm, it is not possible to set a sufficient reduction ratio during hot rolling, and the effect of suppressing microstructure deterioration cannot be obtained. Therefore, the slab thickness is set to 30 mm or more.

[0043] (Heating Process) Next, the slab obtained in the casting process is subjected to the heating process directly, i.e., in a continuous process. In the heating process, the slab is heated using a heating device under the conditions that the heating time is 5 minutes or more and 35 minutes or less, and the maximum temperature reached on the surface of the slab is 1000°C or more and 1290°C or less. Note that the "temperature of the slab surface" in the heating process refers to the temperature of the entire portion of the slab's upper and lower surfaces, excluding the area within 100 mm from both ends in the width direction of the slab towards the center in the width direction. This is because the ends in the width direction of the slab are cooled and heated not only from the upper and lower surfaces but also from the sides, and therefore often do not represent the typical temperature of the slab surface.

[0044] If this type of heating is not performed, the slab temperature will gradually decrease after casting until hot rolling begins. In this case, the driving force for precipitation will gradually increase, and trace amounts of precipitate-forming elements may form precipitates. To avoid this gradual increase in the driving force for precipitation, the slab is heated from the surrounding area to suppress unwanted precipitation.

[0045] Heating time: 5 minutes to 35 minutes, and maximum surface temperature of the slab: 1000°C to 1290°C Generally, it is difficult to perform casting at high speed, so in processes that involve continuous casting and hot rolling, a certain amount of time is often required before hot rolling. When it takes about 10 minutes or more from the start of casting until hot rolling, precipitate-forming elements that inevitably get mixed in sometimes precipitate in the form of nitrides, sulfides, etc. This problem does not necessarily occur in all slabs, but it tends to occur especially when scrap is used as the iron source. To avoid the precipitation of unavoidable impurity elements, it was effective to heat the slab for 5 minutes to 35 minutes until the surface temperature of the slab reached 1000°C to 1290°C before hot rolling.

[0046] If the heating time is less than 5 minutes, phenomena such as precipitation and solid solution require a certain amount of time to occur, resulting in an inability to obtain a stable effect and ultimately leading to a deterioration of the magnetic properties. Therefore, the heating time should be 5 minutes or more, preferably 8 minutes or more. On the other hand, if the heating time exceeds 35 minutes, it leads to coarsening of the crystal grain size and ultimately leads to a deterioration of the magnetic properties. Therefore, the heating time should be 35 minutes or less, preferably 20 minutes or less. Note that "heating time" refers to the time during which heat is applied to the slab by the heating device. If the heating device is a tunnel furnace, it refers to the time the slab is inside the tunnel furnace. If the heating device is an induction heating device, it refers to the time the slab is under the influence of the magnetic field of the induction heating device.

[0047] If the maximum temperature reached on the slab surface during the heating process is less than 1000°C, the uneven temperature distribution during casting will be reflected, resulting in the unavoidable precipitation of impurity elements in some areas. This will cause uneven grain size during primary recrystallization, making secondary recrystallization impossible. Therefore, the maximum temperature reached should be 1000°C or higher. On the other hand, if the maximum temperature reached exceeds 1290°C, breakout may occur during hot rolling. Furthermore, the homogenization of the precipitation state is excessive, resulting in excessively low deformation resistance and making it difficult to control the shape after rolling. Therefore, the maximum temperature reached should be 1290°C or lower, preferably 1250°C or lower.

[0048] The heating process is preferably carried out in a non-oxidizing atmosphere with an oxygen concentration of 3.0 volume% or less. This is because if the oxygen concentration in the atmosphere is high, carbon in the steel will be decarburized as CO by the oxygen in the atmosphere on the slab surface, promoting the coarsening of the columnar crystal structure near the surface and causing bleed defects. The oxygen concentration may be 0.0 volume%. The non-oxidizing atmosphere is not particularly limited and may include carbon-based combustion gases, ammonia-based combustion gases, and N2, which are by-products of the steelworks. ２ It can be composed of one or more gases selected from the group consisting of inert gases such as Ar. In particular, when using carbon-based combustion gas or ammonia-based combustion gas, which are by-products of steel mills, oxygen or air is introduced to generate the combustion gas, but CO, CO ２ No, no ２ It is preferable that the amount of unreacted oxygen in the total gas volume, including the above, be 3.0% by volume or less.

[0049] In this process, since the slabs are continuously supplied to the next process, it is preferable to use an open furnace such as a tunnel furnace. When an open furnace is used, the atmosphere inside the furnace is greatly affected by the atmosphere, but by using methods such as sealing gas, the oxygen concentration inside the furnace can be reduced to 3.0 volume percent or less, creating a non-oxidizing atmosphere and suppressing decarburization from the slab surface. If a more airtight structure can be applied, atmosphere control will be easier.

[0050] Preferably, the two-stage heating process includes a first heating step in which a first heating device is used to raise the surface temperature of the slab to 950°C or higher and 1150°C or lower, and a second heating step in which a second heating device different from the first heating device is used to raise the surface temperature of the slab to a maximum temperature of 1000°C or higher and 1290°C or lower. This allows the heating rate of the first and second heating steps to be controlled individually. In the case of two-stage heating, a heating time of 5 minutes or more and 35 minutes or less refers to the total heating time of the first and second heating steps.

[0051] In the heating process, when the surface temperature of the slab exceeds 1000°C, the rate of oxidation on the surface increases significantly, and when it exceeds 1150°C, almost the entire surface of the slab becomes covered with liquid phase scale. This liquid phase scale can be a cause of velvet defects. Therefore, in the second heating process, which aims for a maximum temperature of 1000°C to 1290°C, it is preferable to use an induction heating device capable of rapid heating. For example, it is preferable to use a tunnel furnace for the first heating process and an induction heating device for the second heating process. By making the second heating process rapid, grain growth, which tends to progress especially in the high-temperature range, can be suppressed, and texture deterioration caused by coarse grains can be more effectively suppressed, and as a result, velvet defects can also be suppressed.

[0052] (Hot Rolling Process) Next, the slab is hot-rolled under predetermined conditions to obtain a hot-rolled sheet. In the hot-rolling process, which determines the final thickness of the hot-rolled sheet, the temperature is relatively low, and dislocations are introduced into the steel sheet. Since dislocations function as nuclei for precipitation, there is a possibility that the precipitation of precipitate-forming elements that are mixed in will proceed. By setting a specific temperature and reduction schedule, unwanted precipitation can be suppressed and a uniform state can be maintained.

[0053] The surface temperature of the slab at the start of hot rolling (hot rolling start temperature): 900°C or higher and 1250°C or lower. If the hot rolling start temperature is below 900°C, precipitation is likely to occur due to the introduction of strain during hot rolling. Unwanted precipitation functions as a local grain growth inhibitory force, leading to non-uniform grain size during primary recrystallization, and causing deterioration of the texture and poor secondary recrystallization. Therefore, the hot rolling start temperature should be 900°C or higher, preferably 950°C or higher. On the other hand, if the hot rolling start temperature exceeds 1250°C, dislocations are introduced and recovery occurs, resulting in insufficient strain introduction, deterioration of the texture, and failure to obtain good magnetic properties. Therefore, the hot rolling start temperature should be 1250°C or lower.

[0054] Reduction Schedule: 3 to 7 Passes If the number of hot rolling passes is less than 3, the reduction per pass becomes too large, increasing the risk of edge cracking at the edges of the hot-rolled sheet. Therefore, the number of hot rolling passes should be 3 or more. On the other hand, if the number of hot rolling passes exceeds 7, the reduction ratio per pass becomes too low, resulting in insufficient strain being introduced by hot rolling, leading to a decrease in the recrystallization rate, which is a factor in the deterioration of the texture. Therefore, the number of hot rolling passes should be 7 or less, preferably 6 or less.

[0055] Hot-rolled sheet thickness: 1.0 mm or more and 3.5 mm or less. In controlling the structure of grain-oriented electrical steel sheets, the reduction ratio during cold rolling is also an extremely important factor. If hot rolling is attempted to reduce the thickness of the hot-rolled sheet to less than 1.0 mm, it becomes impossible to maintain an appropriate reduction ratio during cold rolling, leading to deterioration of the texture. Therefore, the thickness of the hot-rolled sheet should be 1.0 mm or more. On the other hand, if the thickness of the hot-rolled sheet exceeds 3.5 mm, it also becomes impossible to maintain an appropriate reduction ratio during cold rolling, leading to deterioration of the texture. Therefore, the thickness of the hot-rolled sheet should be 3.5 mm or less.

[0056] (Cooling Process) Next, within 200 seconds after the completion of hot rolling, the hot-rolled sheet is cooled under conditions that the surface temperature of the hot-rolled sheet is 650°C or lower. After hot rolling, the dislocation density increases, making it easier for precipitates to form. If the cooling time from the end of hot rolling until the surface temperature of the hot-rolled sheet reaches 650°C is 200 seconds or less, the uneven precipitation of precipitate-forming elements caused by unavoidable impurities can be suppressed. The lower limit of this cooling time is not particularly limited, and cooling may be started immediately after the completion of hot finish rolling, provided there are no constraints on the equipment configuration.

[0057] (Winding process) Next, the hot-rolled sheet can be wound up to obtain a hot-rolled coil.

[0058] (Hot-rolled sheet annealing process) After that, the hot-rolled sheet is subjected to hot-rolled sheet annealing as necessary. 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 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 850°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.

[0059] (Cold Rolling Process) Next, 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. Normally, by introducing a sufficient amount of strain during cold rolling, the amount of residual grains with the <100> / / ND orientation in the texture is reduced after decarburization annealing because grains with other orientations are eroded by grains with other orientations. However, when casting thin sheets, the structure in the slab contains many columnar crystal structures, and the thickness of the hot-rolled sheet is thinner than usual, making it difficult to introduce sufficient strain during cold rolling. Also, because the reduction amount during cold rolling is small, the rise in material temperature due to processing heat is small, making temperature control by cold rolling more difficult than usual. Therefore, the above problems were overcome by controlling the amount of lubricating oil, the temperature of the lubricating oil, the rolling speed, and the inter-pass time. In other words, in this embodiment, it is important that the following conditions (i) and (ii) are met in at least one of the cold rolling processes.

[0060] (i) Value obtained by subtracting the entry steel plate temperature from the exit steel plate temperature in the first pass: 30°C or more and 150°C or less In order to introduce a sufficient amount of strain for the <100> / / ND oriented grains to be eroded during decarburization annealing, it is necessary to segregate the dislocations with dissolved carbon and fix the dislocations. For this reason, it is important that the value obtained by subtracting the entry steel plate temperature in the first pass from the exit steel plate temperature in the first pass is 30°C or more and 150°C or less. If the above value is less than 30°C, the diffusion distance of dissolved carbon becomes short, and dissolved carbon cannot segregate onto the dislocations between the exit of the first pass and the entry of the second pass, so the dislocations cannot be fixed, and sufficient strain cannot be introduced into the <100> / / ND oriented grains during cold rolling, leading to deterioration of the texture. For this reason, the above value is 30°C or more, preferably 40°C or more. Furthermore, if the above value exceeds 150°C, the dissolved carbon segregates excessively at dislocations, reducing the dispersion of dissolved carbon within the crystal grains. Since dissolved carbon within the crystal grains is one of the factors that causes non-uniform deformation, even if cold rolling is performed in that state, sufficient strain cannot be introduced into the <100> / / ND oriented grains, leading to deterioration of the texture. Therefore, the above value is 150°C or lower, preferably 140°C or lower.

[0061] (ii) Let the rolling speed be λ mpm and the time taken for the sheet to pass between the first and second passes be t seconds, then λ × t ≥ 90. The amount of dissolved carbon that segregates dislocations increases with time. Therefore, the smaller the time t taken for the sheet to pass between the first and second passes, the shorter the diffusion distance of the dissolved carbon, and the less dissolved carbon segregates dislocations between the exit of the first pass and the entry of the second pass, so the dislocations are not fixed. As a result, sufficient strain cannot be introduced into the <100> / / ND oriented grains during cold rolling, which leads to deterioration of the texture. Also, the smaller the rolling speed λ, the slower the strain rate, so the magnetic flux density B ８Recrystallization nuclei with a Goss orientation, which are useful for improving the properties, become less likely to form. However, if the rolling speed λ is too fast, the time t required for sheet metal passage between the first and second passes becomes shorter, so there is a trade-off relationship between λ and t. For this reason, it is important to express it in terms of the parameter λ×t, and to obtain good magnetic properties, λ×t should be 90 or higher, preferably 130 or higher. A larger λ×t is preferable, so there is no particular upper limit, but in this embodiment, λ×t is generally 35000 or less.

[0062] Cold rolling includes not only general cold rolling performed at room temperature, but also warm rolling, in which the temperature of a hot-rolled sheet is raised to a temperature higher than room temperature, for example, between 100°C and 300°C.

[0063] If cold rolling is performed only once, conditions (i) and (ii) must be satisfied in that cold rolling. If cold rolling is performed two or more times with intermediate annealing in between, the effects of the present invention can be obtained if conditions (i) and (ii) are satisfied in at least one of those times. However, in order to obtain the effects of the present invention more fully, it is preferable that conditions (i) and (ii) be satisfied in all of the cold rolling. The preferred number of cold rollings is two, with one intermediate annealing in between. In this case, the reduction ratio of the first cold rolling is preferably 18% or more and 45% or less, and the reduction ratio of the second cold rolling is preferably 75% or more and 93% or less.

[0064] 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.

[0065] (Decarburization Annealing Process) 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 which is optimal for secondary recrystallization. The second purpose of this annealing is to decarburize the carbon contained in the steel by using a wet hydrogen nitrogen or wet hydrogen argon atmosphere with a dew point of 40°C to 70°C, and at the same time form an oxide film on the surface by the above annealing atmosphere. For this reason, it is desirable that the annealing temperature (holding temperature) for decarburization annealing be in the temperature range of approximately 800°C to less than 950°C. The holding time at the annealing temperature is preferably 30 seconds to 300 seconds. Furthermore, to further improve the texture, it is effective to increase the heating rate during the heating process of decarburization annealing. Specifically, improvement can be expected by increasing the heating rate between 500°C and 700°C to 80°C / s or more.

[0066] Nitriding treatment may be applied to the cold-rolled sheet during decarburization annealing, or to the decarburized annealed sheet after decarburization annealing. Nitriding treatment can increase the amount of nitride precipitates, particularly near the surface layer of the steel sheet where nitrogen is supplied from an external source, and can further improve the non-uniformity of the precipitates. The nitriding treatment can be performed using the conventional method used for grain-oriented electrical steel sheets. As a method of performing nitriding treatment during decarburization annealing, for example, the decarburization annealing can be maintained in a humid hydrogen nitrogen atmosphere (e.g., 75% H). ２ +25%N ２ Methods include performing the process under the specified conditions and, after holding, blowing ammonia gas onto the steel plate while maintaining the same atmosphere, or introducing the plate into a mixed gas atmosphere of hydrogen, nitrogen, and ammonia after holding. Another method for performing nitriding after decarburization annealing is to first cool the decarburized annealed plate to room temperature, then raise the temperature again to between 400°C and 900°C, and anneal it with a mixed gas of hydrogen, nitrogen, and ammonia.

[0067] (Annealing separator coating step) Next, an annealing separator is applied to the surface of the decarburized annealed sheet. Magnesia (MgO) can be used as the main component of the annealing separator to form a forsterite coating on the steel sheet surface after finish annealing. In this case, adding an appropriate amount of Ti oxide, Sr compound or the like into the separator can further facilitate the formation of the forsterite coating. In particular, the addition of an auxiliary agent that promotes the uniform formation of the forsterite coating also advantageously works to improve the peeling property of the coating. The method for applying the annealing separator is not particularly limited, and a method of applying a solution obtained by dissolving the annealing separator in a solvent, a method of adhering a pre-formed sheet-shaped annealing separator, and the like can be appropriately adopted.

[0068] (Finish annealing step) Next, finish annealing is performed for secondary recrystallization and forsterite coating formation. The annealing atmosphere is N ２ , Ar, H ２ , or any mixed gas of these is suitable. Since precipitation of trace components in the final product leads to deterioration of magnetic properties, for component purification, the maximum annealing temperature is preferably 1100°C or higher and 1280°C or lower, and the soaking time is preferably 3 hours or more and 50 hours or less. Since the grain-oriented electrical steel sheet obtained by the present invention has little variation in magnetic properties within a coil, it is desirable to perform finish annealing on a coil with a weight of 5 tons or more, more preferably 10 tons or more, in consideration of economic efficiency.

[0069] (Insulating coating forming step) After the above finish annealing, an insulating coating can be further formed on the surface of the steel sheet. The type of such an insulating coating is not particularly limited, and any known insulating coating is suitable. For example, a method described in Japanese Patent Laid-Open No. 50-79442 and Japanese Patent Laid-Open No. 48-39338, in which a coating solution containing phosphate-chromate-colloidal silica is applied to a steel sheet and baked at about 800°C, is preferable.

[0070] Regarding the grain-oriented electrical steel sheet obtained as the final product, the component composition of the steel sheet base, after purification during finish annealing and removal of the insulating and forsterite coatings, is as follows: The component composition contains Si: 2.0% to 4.5%, Mn: 0.01% to 0.5%, acid-soluble Al: 10 ppm to less than 60 ppm, and S: 5 ppm to less than 50 ppm, with the content of Se, Te, and O suppressed to less than 50 ppm each. Furthermore, the content of other elements in the steel may decrease depending on the finish annealing conditions, such as being incorporated into the forsterite coating or released into the gas phase, so the concentration will be lower than that of the slab.

[0071] [Example 1] Molten steel having a composition of C: 0.06%, Si: 3.2%, Mn: 0.05%, acid-soluble Al: 80 ppm, S: 20 ppm, N: 30 ppm, Cr: 0.030%, and P: 0.010%, with the remainder being Fe and unavoidable impurities, was refined. In the laboratory, casting, heating, hot rolling, and cooling processes were carried out in succession under the conditions shown in Table 1 to obtain hot-rolled sheets.

[0072] The heating process was carried out in an atmosphere of blast furnace combustion gas (oxygen concentration: 1.8 vol%), and was a single-stage heating process using only the heating furnace. After the completion of hot rolling, the cooling time until the surface temperature of the hot-rolled sheet reached 650°C was adjusted by controlling the amount of water used.

[0073] The hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 1020°C for a soaking time of 40 seconds, scale was removed by pickling, and then a cold-rolled sheet with a final thickness of 0.27 mm was obtained by one cold-rolling pass. The cold-rolling conditions were that the difference between the exit sheet temperature and the entry sheet temperature in the first pass was 90°C, the rolling speed was λ mpm, and the time taken for the sheet to pass between the first and second passes was t seconds, with λ × t ≥ 90. This cold-rolled sheet was then subjected to annealing at 840°C for 150 seconds at 55% H ２ -45%N ２The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization to obtain decarburized annealed sheets. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1200°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0074] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ Table 1 shows the results. As is clear from Table 1, good magnetic properties were obtained in grain-oriented electrical steel sheets manufactured under the conditions according to the present invention.

[0075]

[0076] [Example 2] Molten steel having a composition of C: 0.04%, Si: 3.4%, Mn: 0.07%, P: 0.060%, acid-soluble Al: 100 ppm, S: 10 ppm, N: 60 ppm, Sb: 0.008%, and Mo: 0.010%, with the remainder being Fe and unavoidable impurities, was refined. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession using the conditions shown in No. 10 and No. 17 of Example 1 (see Table 1) to obtain a hot-rolled sheet.

[0077] Subsequently, the hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 900°C for a soaking time of 50 seconds, scale was removed by pickling, and then a cold-rolled sheet with a final thickness of 0.23 mm was obtained by one cold-rolling pass. The values ​​obtained by subtracting the entry-side steel sheet temperature from the exit-side steel sheet temperature in the first pass of the cold-rolling process, and the value of λ × t are shown in Table 2. In Table 2, Nos. 1-4 are examples of hot-rolled sheets manufactured using the conditions shown in No. 10 of Example 1, and Nos. 5-8 are examples of hot-rolled sheets manufactured using the conditions shown in No. 17 of Example 1.

[0078] This cold-rolled sheet was subjected to a process at 840°C for 150 seconds at 55% H ２ -45%N２ The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization to obtain decarburized annealed sheets. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0079] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ Table 2 shows the results. As is clear from Table 2, good secondary recrystallization occurred in all examples, but in the inventive example where the value obtained by subtracting the entry steel sheet temperature from the exit steel sheet temperature in the first pass of the cold rolling process, and λ×t were optimized, better magnetic properties were obtained.

[0080]

[0081] [Example 3] Molten steel having a component composition containing the elements shown in Table 3, with the remainder being Fe and unavoidable impurities, was refined. In Table 3, the analytical value of Al represents the content of acid-soluble Al. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession to obtain a hot-rolled sheet.

[0082] In the casting process, a slab with a thickness of 70 mm was used. In the heating process, the slab was placed in the heating furnace when its surface temperature reached 1000°C and heated to 1180°C in 15 minutes. The atmosphere inside the heating furnace was that of combustion gas from a blast furnace (oxygen concentration as shown in Table 3). In the hot rolling process, the starting temperature was 1100°C, and a hot-rolled sheet with a thickness of 2.2 mm was produced in four passes. After the completion of hot rolling, the cooling time until the surface temperature of the hot-rolled sheet reached 650°C was 110 seconds.

[0083] A hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 1000°C for 40 seconds, scale was removed by pickling, and then a cold-rolled sheet with a final thickness of 0.25 mm was obtained by one cold-rolling pass. The cold-rolling conditions were that the difference between the exit sheet temperature and the entry sheet temperature in the first pass was 90°C, the rolling speed was λ mpm, and the time taken for the sheet to pass between the first and second passes was t seconds, with λ × t ≥ 90. This cold-rolled sheet was then subjected to annealing at 840°C for 150 seconds at 55% H ２ -45%N ２ The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization to obtain decarburized annealed sheets. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[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 Table 3 shows the results. The surface of the obtained grain-oriented electrical steel sheet was also observed, and the presence or absence of velvet defects was visually determined, and the results are shown in Table 3. As is clear from Table 3, good magnetic properties were obtained in the grain-oriented electrical steel sheet manufactured under the conditions according to the present invention. In addition, velvet defects were suppressed by reducing the oxygen concentration in the atmosphere inside the heating furnace.

[0085]

[0086] [Example 4] Molten steel having a component composition containing the elements shown in Table 4, with the remainder being Fe and unavoidable impurities, was refined. In Table 4, the analytical value of Al represents the content of acid-soluble Al. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession to obtain a hot-rolled sheet.

[0087] In the casting process, a slab with a thickness of 70 mm was used. In the heating process, the slab was placed in the heating furnace when its surface temperature reached 1000°C. Under some conditions (Table 4, Nos. 2, 6, 8), the slab was heated to 1250°C in 15 minutes in the heating furnace. Under the remaining conditions (Table 4, Nos. 1, 3-5, 7, 9), the slab was heated to 1100°C in 10 minutes in the heating furnace, and then rapidly heated to 1250°C using an induction heating device, where it was held for 5 minutes. The atmosphere inside the heating furnace was that of combustion gas from a blast furnace (oxygen concentration of 3.0 vol%). The atmosphere of the induction heating device was that of N with an oxygen concentration of 5.0 vol%. ２ A gaseous atmosphere was created.

[0088] Subsequently, the hot rolling process was carried out with a starting temperature of 1050°C, and a hot-rolled sheet with a thickness of 2.8 mm was produced in three passes. After the completion of hot rolling, the cooling time until the surface temperature of the hot-rolled sheet reached 650°C was 90 seconds.

[0089] 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 a cold-rolled sheet with a final thickness of 0.23 mm was obtained by one cold-rolling pass. The cold-rolling conditions were that the difference between the exit sheet temperature and the entry sheet temperature in the first pass was 90°C, the rolling speed was λ mpm, and the time taken for the sheet to pass between the first and second passes was t seconds, with λ × t ≥ 90. This cold-rolled sheet was then subjected to annealing at 840°C for 150 seconds at 55% H ２ -45%N ２ The sheets were then annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization to obtain decarburized annealed sheets. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 8 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and magnesium phosphate was applied to the surface of the finished annealed sheets and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[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. ８Table 4 shows the results. The surface of the obtained grain-oriented electrical steel sheet was also observed, and the presence or absence of velvet defects was visually determined, and the results are shown in Table 4. As is clear from Table 4, good magnetic properties were obtained in the grain-oriented electrical steel sheet manufactured under the conditions according to the present invention. In addition, velvet defects were suppressed by performing the heating process in two stages.

[0091]

[0092] [Example 5] Molten steel having a composition of C: 0.04%, Si: 3.4%, Mn: 0.07%, P: 0.060%, acid-soluble Al: 100 ppm, S: 10 ppm, N: 60 ppm, Sb: 0.008%, and Mo: 0.010%, with the remainder being Fe and unavoidable impurities, was refined. In the laboratory, the casting process, heating process, hot rolling process, and cooling process were carried out in succession using the conditions shown in No. 10 of Example 1 (see Table 1) to obtain a hot-rolled sheet.

[0093] Subsequently, the hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 900°C for a soaking time of 50 seconds. After removing scale by pickling, it was cold-rolled twice, with an intermediate annealing in between, to obtain a cold-rolled sheet with a final thickness of 0.23 mm. The reduction ratio in the first pass was 25%, and the reduction ratio in the second pass was 90%. The intermediate annealing was performed under conditions of a soaking temperature of 1050°C and a holding time of 120 seconds. The values ​​obtained by subtracting the entry steel sheet temperature from the exit steel sheet temperature in the first pass of the first and second cold-rolling processes, and the value of λ × t, are shown in Table 5.

[0094] This cold-rolled sheet was subjected to a process at 840°C for 150 seconds at 55% H ２ -45%N ２ The sheets were annealed in a humid atmosphere with a dew point of 60°C for decarburization and primary recrystallization to obtain decarburized annealed sheets. Subsequently, an annealing separating agent mainly composed of MgO was applied to the surface of the decarburized annealed sheets, and a finish annealing process including a secondary recrystallization process and a purification process was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 10 hours to obtain finished annealed sheets. An insulating coating consisting of colloidal silica and aluminum phosphate was applied to the surface of the finished annealed sheets and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0095] For the obtained grain-oriented electrical steel sheet, the magnetic flux density (B) at an excitation magnetic field of 800 A / m and excitation frequency of 50 Hz was measured in accordance with JIS C2550-1:2011. ８ The magnetic flux density (B) obtained was measured. ８ Table 5 shows the results. As is clear from Table 5, although good secondary recrystallization occurred in all examples, in the inventive example in which the rolling conditions were optimized in at least one cold rolling step, better magnetic properties were obtained, and in the inventive example in which the rolling conditions were optimized in two cold rolling steps, even better magnetic properties were obtained.

[0096]

[0097] [Example 6] An experiment was conducted to investigate the effect of nitriding treatment using a hot-rolled sheet prepared under the conditions of No. 1 in Table 5. No. 1 in Table 6 is the same as No. 1 in Table 5. In Nos. 2 and Nos. 3 in Table 6, decarburization annealing was performed under the same conditions as No. 1 in Table 5, and then the nitriding treatment described in Table 6 was performed. In No. 2 in Table 6, immediately after holding at 840°C for 150 seconds during decarburization annealing, the temperature was reduced to 800°C and left at 60% H for 30 seconds. ２ +20%N ２ +20%NH ３ The sample was introduced into a mixed gas atmosphere. In No. 3 of Table 6, after decarburization annealing was completed and the sample was cooled to room temperature, it was then annealed again at 650°C for 120 seconds at 60% H. ２ +20%N ２ +20%NH ３ The material was introduced into a mixed gas atmosphere. The nitrogen content increased by 0.012% in No. 2 and 0.011% in No. 3. Subsequently, similar to Example 5, an annealing separation agent mainly composed of MgO was applied to the surface of the decarburized annealed sheet, and finish annealing, including a secondary recrystallization process and a purification process, was carried out under conditions of a maximum temperature of 1150°C and a soaking time of 8 hours to obtain a finish annealed sheet. An insulating coating consisting of colloidal silica and magnesium phosphate was applied to the surface of the finish annealed sheet and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0098] 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. ８The magnetic flux density (B) obtained was measured. ８ Table 6 shows the results. As is clear from Table 6, 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 magnetic properties were obtained by nitriding treatment.

[0099]

[0100] This invention can be applied to the manufacture of grain-oriented electrical steel sheets.

Claims

1. A process to produce a slab with a thickness of 30 mm to 80 mm by continuously casting molten steel having a composition in mass% or mass ppm, containing C: 0.08% or less, Si: 2.0% to 4.5%, Mn: 0.50% or less, acid-soluble Al: 20 ppm to 120 ppm, S: less than 50 ppm, and N: 80 ppm or less, with the content of Se, Te, and O each suppressed to less than 50 ppm, and the remainder consisting of Fe and unavoidable impurities; and a heating process thereafter, in which the slab is heated using a heating device for a heating time of 5 minutes to 35 minutes and the maximum temperature reached on the surface of the slab is 1000°C to 1290°C. Subsequently, a hot rolling step to obtain a hot-rolled sheet with a thickness of 1.0 mm to 3.5 mm, wherein the surface temperature of the slab at the start of hot rolling is 900°C to 1250°C and the reduction schedule is 3 to 7 passes; a step to cool the hot-rolled sheet within 200 seconds after the completion of the hot rolling, under the condition that the surface temperature of the hot-rolled sheet is 650°C or lower; an optional step to perform hot-rolled sheet annealing on the hot-rolled sheet; and a step to obtain a cold-rolled sheet by subjecting the hot-rolled sheet to one or two or more cold-rollings with an intermediate annealing in between, wherein in at least one of the cold-rolling steps, (i) the value obtained by subtracting the entry-side steel sheet temperature from the exit-side steel sheet temperature in the first pass is 30°C to 150°C. (ii) A method for manufacturing grain-oriented electrical steel sheets, comprising: (ii) a step of having a rolling speed of λ mpm and a time of t seconds for the sheet to pass between the first and second passes, such that λ × t ≥ 90; a step of subsequently performing decarburization annealing on the cold-rolled sheet to obtain a decarburized annealed sheet; an optional step of performing nitriding treatment on the cold-rolled sheet during the decarburization annealing or on the decarburized annealed sheet after the decarburization annealing; and a step of subsequently applying an annealing separating agent to the decarburized annealed sheet and performing finish annealing.

2. The method for manufacturing grain-oriented electrical steel sheets according to claim 1, wherein the cold rolling is performed two or more times, and all times the conditions (i) and (ii) are satisfied.

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

4. The method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 3, wherein the heating step comprises: a first heating step in which a first heating device is used as the heating device to raise the surface temperature of the slab to 950°C or more and 1150°C or less; and a second heating step in which a second heating device different from the first heating device is used as the heating device to raise the surface temperature of the slab to the maximum temperature reached.

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

6. The above component composition is further divided by mass percent into: Ni: 1.500% or less, Sn: 0.500% or less, Sb: 0.500% or less, Cu: 0.500% or less, P: 0.500% or less, Cr: 1.500% or less, Mo: 0.500% or less, B: 0.0200% or less, Nb: 0.0100% or less, Co: 0.0100% or less, Ti: 0.0200% or less, Zn A method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 5, comprising one or more selected from the group consisting of: 0.0500% or less, Bi: 0.0200% or less, W: 0.0030% or less, Pb: 0.0300% or less, Ga: 0.0100% or less, Ge: 0.0300% or less, As: 0.0300% or less, and Ag: 0.0300% or less.

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