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

Abstract

The present invention provides a method for producing a grain-oriented electrical steel sheet that has excellent magnetic characteristics over the entire length of a coil. A method for producing a grain-oriented electrical steel sheet according to the present invention comprises: a step for casting a molten steel having a specific component composition so as to obtain a slab having a thickness of 30 mm to 180 mm (inclusive); a step for subjecting the slab to rough rolling and finish rolling to obtain a hot rolled sheet, wherein (I) the rough rolling is started within 5 minutes from the time when the surface temperature of the slab falls to 1200°C, and during the time where the surface temperature of the slab is within the range from 900°C to 1150°C (inclusive), and (II) the finish rolling is started within 5 minutes from the end of the rough rolling, while maintaining the surface temperature of the slab within the range from 850°C to 1200°C (inclusive) after the end of the rough rolling; a step for cooling the hot-rolled sheet under the conditions where the cooling time until the surface temperature of the hot-rolled sheet falls to 650°C after the end of the rough rolling is 200 seconds or less; and a step for subsequentially performing cold rolling, decarburization annealing and finish annealing.

Description

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

[0001] The present invention relates to a method for manufacturing grain-oriented electrical steel sheets, a line of manufacturing equipment for grain-oriented electrical steel sheets, and hot-rolled sheets for grain-oriented electrical steel sheets. In particular, the present invention relates to a method for manufacturing grain-oriented electrical steel sheets having stable magnetic properties by utilizing a process for continuously manufacturing hot-rolled coils from casting.

[0002] Grain-oriented electrical steel sheets are materials primarily used in the iron cores of transformers and other devices. In recent years, there has been an increasing demand for more energy-efficient iron cores. Accordingly, grain-oriented electrical steel sheets, which are used as iron core materials, are also required to have better magnetic properties, i.e., low core 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 axis of magnetization of iron, is highly aligned in the rolling direction of the steel sheet. This texture is formed during the manufacturing process of grain-oriented electrical steel sheets, particularly during finish annealing, through secondary recrystallization, which preferentially grows grains with the {110}<001> orientation, known as the Goss orientation. Therefore, the crystal orientation of the secondary recrystallized grains has a significant effect on the magnetic properties of grain-oriented electrical steel sheets.

[0004] Conventionally, grain-oriented electrical steel sheets are manufactured by the following process. A steel slab containing 4.5 mass% or less Si and further containing inhibitor-forming elements such as MnS, MnSe, AlN, and BN is heated to 1300°C or higher and hot-rolled to obtain a hot-rolled sheet. The hot-rolled sheet is then subjected to hot-rolled sheet annealing as necessary, and then cold-rolled once or twice or more times with intermediate annealing in between to obtain a cold-rolled sheet. The cold-rolled sheet is then subjected to decarburization annealing in a wet hydrogen atmosphere to perform primary recrystallization and decarburization, resulting in a decarburized annealed sheet. An annealing separator mainly composed of MgO is applied to the decarburized annealed sheet, and then finish-annealed at 1200°C for about 5 hours to perform secondary recrystallization and purify 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 dissolve inhibitor-forming elements, resulting in extremely high production costs. To address this issue, a method known as the inhibitorless process has been developed that can induce secondary recrystallization without the addition of inhibitor-forming elements (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. Conventional methods induce secondary recrystallization by using precipitates (inhibitors) such as MnS, MnSe, and AlN. On the other hand, the inhibitorless process does not use these inhibitors. Instead, it purifies the steel to reduce resistance to grain boundary migration, thereby revealing the inherent difference in grain boundary migration rate depending on the grain boundary characteristics and successfully inducing secondary recrystallization. Therefore, compared to the conventional method using inhibitors, high-temperature slab heating to dissolve inhibitor-forming elements can be avoided.

[0006] When the inhibitor-less process is applied, a conventional slab heating furnace (gas furnace) is used to heat the slab at low temperatures. Because the process does not have a strong inhibitor, if trace elements that inhibit grain growth are mixed into the steel, the temperature unevenness in the heating furnace can cause uneven quality within the slab.

[0007] Furthermore, as one of the means for avoiding high-temperature slab heating, a process for continuously obtaining hot-rolled coils from cast slabs has also been 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 above 1200°C to obtain a hot-rolled coil, thereby successfully obtaining a hot-rolled coil suitable for a grain-oriented electrical steel sheet manufacturing process that uses a conventional inhibitor.

[0008] Furthermore, as a process for continuously obtaining hot-rolled coils from cast slabs, in order to reproduce the "complete solid solution state of the inhibitor after reheating the slab" that is achieved in ordinary hot rolling, a method has been considered in which hot rolling is started at a temperature exceeding 1200°C after casting, as shown in Patent Document 5, but this process has been difficult to produce.

[0009] US Patent No. 1965559 Specification Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent Laid-Open No. 2000-129356 Japanese Patent Laid-Open No. 2008-69391

[0010] As described above, the inhibitor-less method does not contain strong inhibitor components, and therefore, if uneven heating occurs, for example, due to a skid or the like, during slab heating in hot rolling, there are problems such as differences in grain growth properties resulting in a non-uniform structure, and if trace components are mixed in, these components are accentuated.

[0011] However, in order to apply the process of continuously obtaining hot-rolled coils from slabs that are cast thinner than usual to the manufacturing process of grain-oriented electrical steel sheets, it is necessary to maintain an extremely high temperature during hot rolling in order to achieve a complete solid solution state of the precipitates before the start of hot rolling, which has left challenges in stable production.

[0012] In other words, there is a need for a method for producing grain-oriented electrical steel sheet that has stable and excellent magnetic properties throughout the entire length of the coil by advantageously solving the above-mentioned problems in the process of continuously obtaining hot-rolled coils from slabs while using a component composition that conforms to the inhibitor-less process.

[0013] In view of the above-mentioned problems, the present invention aims to provide a method for manufacturing grain-oriented electrical steel sheets and a line of manufacturing equipment for grain-oriented electrical steel sheets that can produce grain-oriented electrical steel sheets that have excellent magnetic properties throughout the entire coil length. Another object of the present invention is to provide a hot-rolled sheet for grain-oriented electrical steel sheets, which is a raw material for grain-oriented electrical steel sheets with excellent magnetic properties.

[0014] The process of continuously obtaining hot-rolled coils from slabs (hereinafter also referred to as the continuous casting and hot-rolling process) eliminates the need for slab reheating, potentially reducing the quality variation caused by grain growth during slab reheating, which is unique to the inhibitor-less process. On the other hand, the composition system used in the inhibitor-less process contains almost no inhibitor-forming elements, so it is expected that a complete solid solution state 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 combining the continuous casting and hot-rolling process with the inhibitor-less process may enable the more stable production of grain-oriented electrical steel sheets with excellent magnetic properties.

[0015] However, simply subjecting molten steel having a chemical composition according to the inhibitor-less process to a continuous casting and hot rolling process not only fails to provide good magnetic properties, but also fails to produce secondary recrystallization. As a result of intensive research, the inventors have arrived at the findings shown in (1) to (6) below.

[0016] (1) When using a chemical composition according to the inhibitorless process, excessive grain growth may occur depending on the waiting time and transport time after processing during hot rolling. Maintaining an appropriate reduction ratio can suppress excessive coarsening of the structure and prevent deterioration of the texture before secondary recrystallization. The appropriate slab thickness is not uniquely determined because it depends on the capacity of the hot rolling mill. However, when hot rolling is performed entirely at a high temperature, grain growth is more likely to occur, requiring a high reduction pressure. On the other hand, when hot rolling is performed at a relatively low temperature, the desired structure can be controlled at a low pressure, which tends to allow for thinner slab thickness during casting.

[0017] (2) By using a composition that conforms to the inhibitor-less method, it is possible to maintain a state of complete solid solution or a state in which precipitation of trace amounts of components does not proceed for a certain period of time, even when the slab temperature is low, such as 1200°C or below. The inventors have confirmed through experiments that precipitation does not proceed for approximately five minutes after the surface temperature of the slab after casting has dropped to 1200°C. Therefore, it is recommended that rough rolling be started within five minutes after the surface temperature of the slab after casting has dropped to 1200°C, while the surface temperature of the slab is within the specified range (900°C or higher and 1150°C or lower). Preferably, this should be done within three minutes.

[0018] However, because the casting speed of steel is generally very slow, depending on the line length, it may be difficult to start rough rolling within the recommended time. In this case, a gradual decrease in the slab temperature after casting is undesirable, and it is preferable to install a heating device (furnace) with a temperature higher than the surface temperature of the slab to avoid this gradual decrease in the slab temperature. By suppressing the temperature decrease of the slab using a heating device and performing soaking treatment, it is possible to exceed the time until the start of rough rolling. However, if the time until the start of rough rolling exceeds 20 minutes, precipitation of the inhibitor components present in small amounts will progress partially, which will instead promote heterogeneity in the structure and destabilize secondary recrystallization.

[0019] (3) It is desirable to perform rough rolling in two or more passes. The reason for this is that when a slab is cast to a relatively thick thickness, unsolidified molten steel may remain in the center of the slab, so it is necessary to force the solidified portions to come into contact with each other in the first pass of rough rolling. In this way, the hot-rolled structure can be created in the second pass and beyond. By heavily working, dislocations are introduced into the structure, and by introducing new recrystallization nuclei, excessive coarsening of the structure can be suppressed and the crystal grain size can be maintained uniform.

[0020] (4) When the rough rolling (3) above is performed, it is particularly important to perform the following process. Dislocations function as nuclei for recrystallization and also as nuclei for the formation of precipitates. Therefore, even if the amount of inhibitor-forming elements is small, they are not completely "zero." A certain amount of inhibitor-forming elements will precipitate after rough rolling. However, if the reduction in the final stand of rough rolling is strong, dislocations are introduced relatively uniformly into the structure, preventing uneven precipitation. If the slab surface temperature at the start of finish rolling is outside the specified temperature range (850°C or higher and 1200°C or lower) or if the time from the end of rough rolling to the start of finish rolling exceeds 5 minutes, the dispersion of precipitates will be uneven, potentially affecting secondary recrystallization. In a continuous casting and hot rolling process, if the sheet threading speed is slow and the line length is long, the time from the end of rough rolling to the start of finish rolling may exceed 5 minutes. In this case, one method is to increase the reduction rate in rough rolling and elongate the slab in the rolling direction to increase the sheet threading speed. However, when the thickness of the slab is reduced by rough rolling, the temperature of the slab drops significantly, making it difficult to maintain the target temperature, and therefore, it becomes necessary to install heating equipment as needed. In particular, the time from the end of rough rolling to the start of finish rolling is a condition that should be strongly reflected in the line configuration.

[0021] (5) After finish rolling, the dislocation density becomes even higher than after rough rolling, making it easier for precipitates to form. By cooling to a low temperature in a shorter time, it is possible to suppress non-uniform precipitation of inhibitor-forming elements.

[0022] (6) It has become clear that by optimizing the manufacturing conditions described above, it is possible to manufacture a hot-rolled steel sheet in which the content of precipitated Al, which is targeted at Al nitrides that generally function as inhibitors, is only ¼ or less of the content of acid-soluble Al originally contained in the steel. This hot-rolled steel sheet avoids the uneven precipitation of trace elements that causes secondary recrystallization defects in the continuous casting and hot-rolling process, and has sufficient properties to be used as a base material for grain-oriented electrical steel sheets with inhibitor-free components.

[0023] The gist of the present invention, which was completed based on the above findings, is as follows: [1] A process of casting molten steel containing 0.08% by mass or less of C, 2.0 to 4.5% by mass of Si, 0.5% by mass or less of Mn, 20 to 120 ppm by mass of acid-soluble Al, less than 50 ppm by mass of S, and less than 80 ppm by mass of N, with the contents of Se, Te, and O each being reduced to less than 50 ppm by mass, and the balance consisting of Fe and unavoidable impurities, to obtain a slab having a thickness of 30 mm to 180 mm, and a process of rough rolling and finish rolling the slab to obtain a hot-rolled sheet, wherein: (I) the rough rolling is started within 5 minutes after the surface temperature of the slab has dropped to 1200°C and while the surface temperature of the slab is in the range of 900°C to 1150°C, (II) A method for producing a grain-oriented electrical steel sheet, comprising: a step of starting the finish rolling within 5 minutes after the completion of the rough rolling while maintaining the surface temperature of the slab in a range of 850°C to 1200°C; a step of cooling the hot-rolled sheet after the completion of the finish rolling under conditions such that the cooling time until the surface temperature of the hot-rolled sheet decreases to 650°C is 200 seconds or less; and a step of subsequently performing cold rolling, decarburization annealing, and finish annealing.

[0024] [2] A process of casting molten steel having a component composition containing C: 0.08% by mass or less, Si: 2.0% by mass or more and 4.5% by mass or less, Mn: 0.5% by mass or less, acid-soluble Al: 20 ppm by mass or more and 120 ppm by mass or less, S: less than 50 ppm by mass, and N: less than 80 ppm by mass, with the contents of Se, Te, and O each being suppressed to less than 50 ppm by mass, and the balance consisting of Fe and unavoidable impurities, to obtain a slab having a thickness of 30 mm or more and 180 mm or less; a process of holding the slab in an atmosphere of 1000°C or more and 1250°C or less for 5 minutes or less and 20 minutes or less after the surface temperature of the slab has decreased to 1200°C or less; and thereafter, a process of subjecting the slab to rough rolling and finish rolling to obtain a hot-rolled sheet, the process comprising: (I) starting the rough rolling while the surface temperature of the slab is in the range of 900°C or more and 1150°C or less, (II) A method for producing a grain-oriented electrical steel sheet, comprising: a step of starting the finish rolling within 5 minutes after the completion of the rough rolling while maintaining the surface temperature of the slab in a range of 850°C to 1200°C; a step of cooling the hot-rolled sheet after the completion of the finish rolling under conditions such that the cooling time until the surface temperature of the hot-rolled sheet decreases to 650°C is 200 seconds or less; and a step of subsequently performing cold rolling, decarburization annealing, and finish annealing.

[0025] [3] In the rough rolling, in the final stand, the reduction ratio is 20% or more and 50% or less, and the strain rate is 1.0 s －１ Over 50s －１ The method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], which is carried out under the following conditions:

[0026] [4] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [3] above, wherein after completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain a surface temperature of the slab in a range of 850°C or higher and 1200°C or lower.

[0027] [5] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [4] above, wherein the chemical composition further includes, in mass%, one or more selected from the group consisting of Ni: 0.005 to 1.50%, Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%, P: 0.0050 to 0.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200%.

[0028] [6] A line of manufacturing equipment for grain-oriented electrical steel sheets, arranged in this order: a continuous casting machine that continuously casts molten steel to produce slabs; a roughing rolling mill that rough rolls the slabs; a finishing rolling mill that finish rolls the slabs after the rough rolling to produce a hot-rolled sheet; and a coiler that winds up the hot-rolled sheet, the line comprising: a first heat treatment device located between the continuous casting machine and the roughing rolling mill, that adjusts the surface temperature of the slab to 900°C or more and 1200°C or less; a second heat treatment device located between the roughing rolling mill and the finishing rolling mill, that adjusts the surface temperature of the slab after the rough rolling to 850°C or more and 1200°C or less; and a third heat treatment device located between the finishing rolling mill and the coiler, that adjusts the surface temperature of the hot-rolled sheet to 650°C or less within 200 seconds after the end of the finish rolling. A line of manufacturing equipment for grain-oriented electrical steel sheets has a threading capacity that allows the slabs to be moved from the roughing mill to the finishing mill within 5 minutes.

[0029] [7] The roughing mill has a final stand with a rolling reduction of 20% or more and 50% or less and a strain rate of 1.0 s －１ Over 50s －１ [7] A line of manufacturing equipment for grain-oriented electrical steel sheets according to the above [6], having the following rolling capacity:

[0030] [8] A hot-rolled sheet for grain-oriented electrical steel sheet, comprising C: 0.08% by mass or less, Si: 2.0% by mass or more and 4.5% by mass or less, Mn: 0.5% by mass or less, acid-soluble Al: 20 ppm by mass or more and 120 ppm by mass or less, S: less than 50 ppm by mass, and N: less than 80 ppm by mass, wherein the contents of Se, Te, and O are each suppressed to less than 50 ppm by mass, with the balance consisting of Fe and unavoidable impurities, and wherein the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al is 0.25 or less.

[0031] [9] The hot-rolled sheet for grain-oriented electrical steel sheet according to [8] above, wherein the component composition further includes, in mass%, one or more selected from the group consisting of Ni: 0.005 to 1.50%, Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%, P: 0.0050 to 0.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200%.

[0032] The method for manufacturing grain-oriented electrical steel sheets and the line of equipment for manufacturing grain-oriented electrical steel sheets according to the present invention make it possible to manufacture grain-oriented electrical steel sheets having excellent magnetic properties throughout the entire coil length. Furthermore, grain-oriented electrical steel sheets having excellent magnetic properties can be obtained using the hot-rolled sheet for grain-oriented electrical steel sheets according to the present invention as a raw material.

[0033] The present invention can solve both the problem of non-uniform structure during slab reheating, which is likely to occur in the inhibitor-less process, and the problem of having to perform hot rolling at an ultra-high temperature in the continuous casting and hot rolling process, thereby realizing stable production of grain-oriented electrical steel sheets using the continuous casting and hot rolling process.

[0034] 1 is a schematic diagram showing a row 100 of manufacturing equipment for grain-oriented electrical steel sheets according to the present invention. 2 is a graph showing changes in the surface temperature of slabs obtained by continuous casting under conditions 1 to 3 of Example 3.

[0035] [Method for manufacturing grain-oriented electrical steel sheet] A method for manufacturing grain-oriented electrical steel sheet according to one embodiment of the present invention includes the steps of: casting molten steel having a predetermined chemical composition to obtain a slab; rough rolling and finish rolling the slab to obtain a hot-rolled sheet; cooling the hot-rolled sheet; and then cold rolling, decarburization annealing, and finish annealing.

[0036] (Composition of slab) First, the composition of the slab will be described. Note that "%" regarding components means mass % unless otherwise specified. Similarly, "ppm" means mass ppm unless otherwise specified.

[0037] C: 0.08% or less If C remains in the final product sheet, it causes magnetic aging and magnetic deterioration. If the C content of the slab is excessive, the load in the decarburization process becomes high, and the C content of the final product sheet cannot be sufficiently reduced. Therefore, the C content of the slab is set to 0.08% or less. On the other hand, C has the function of suppressing grain coarsening during hot rolling and improving the structure before cold rolling. Furthermore, during cold rolling, C improves the texture after primary recrystallization by interacting with dislocations. From this perspective, the C content is preferably 0.01% or more.

[0038] Si: 2.0% or more and 4.5% or less Si is an element that increases electrical resistance and thereby reduces iron loss. 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.

[0039] Mn: 0.5% or less If the Mn content is excessive, the primary recrystallization texture deteriorates, and secondary recrystallization grains highly concentrated in the Goss orientation cannot be obtained. From this viewpoint, the Mn content is set to 0.5% or less. On the other hand, Mn is an element that exerts the effect of improving hot workability. From the viewpoint of obtaining this effect, the Mn content is preferably 0.01% or more.

[0040] Acid-soluble Al: 20 ppm or more and 120 ppm or less Since the present invention relates to an inhibitor-less process, 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 inhibitor-less process, if only secondary recrystallization is considered, Al is not necessarily required. However, Al has the effect of reducing O, which becomes an impurity, in the refining stage of molten steel, and also forms dense Al on the surface during secondary recrystallization annealing. ２ O ３ The Al content is set to 20 ppm or more, and the Al film formed can reduce the influence of nitriding and other factors from the atmosphere.

[0041] S: less than 50 ppm, N: less than 80 ppm Since the present invention relates to an inhibitor-less process, the contents of S and N, which are inhibitor-forming components, must be reduced as much as possible. If the S content and N content are excessive, precipitation tends to proceed in the continuous casting and hot rolling process, resulting in a non-uniform structure. Therefore, the S content is less than 50 ppm, and the N content is less than 80 ppm. The lower limits of the S content and N content are 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 production costs. From the viewpoint of production costs, the S content is preferably 10 ppm or more, and the N content is preferably 20 ppm or more.

[0042] Se, Te, and O: Less than 50 ppm each. Excessive Se and Te contents result in the formation of Se and Te compounds, making secondary recrystallization difficult. These elements are known to segregate centrally during casting. In the present invention, in which hot rolling is performed directly after casting without slab reheating, Se and Te may remain as coarse precipitates in the center of the slab, reducing ductility during subsequent cold rolling and embrittlement of the steel sheet. Therefore, the Se and Te contents are each less than 50 ppm, preferably 30 ppm or less. Furthermore, O forms oxides that remain as inclusions in the final product, degrading magnetic properties. Therefore, the O content must be kept below 50 ppm. The Se, Te, and O contents may be 0 ppm.

[0043] The balance other than the above components consists of Fe and unavoidable impurities, but may optionally contain the following elements.

[0044] Ni: 0.005 to 1.50% Ni improves the magnetic properties by increasing the uniformity of the hot-rolled sheet structure. From the viewpoint of obtaining this effect, the Ni content is preferably 0.005% or more. On the other hand, if the Ni content is excessive, secondary recrystallization becomes unstable and the magnetic properties deteriorate. Therefore, if Ni is contained, the Ni content is set to 1.50% or less.

[0045] Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%. Sn, Sb, and Cu are elements that are sometimes considered auxiliary inhibitors through grain boundary segregation, and may be useful in inhibitor-less processes that do not actively utilize inhibitors due to precipitates. From this perspective, 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 0.01% or more. On the other hand, excessive contents of these elements increase the possibility of secondary recrystallization defects. Therefore, when these elements are contained, the contents of each of these elements are set to 0.50% or less.

[0046] P: 0.0050 to 0.50%, Cr: 0.01 to 1.50% P and Cr have the effect of improving the reaction for forming a forsterite film. From the viewpoint of obtaining this effect, it is preferable that the P content be 0.0050% or more and the Cr content be 0.01% or more. On the other hand, if the contents of these elements are excessive, the formation of the forsterite film may be excessively promoted, which may result in peeling of the film. Therefore, when these elements are contained, the P content is set to 0.50% or less and the Cr content is set to 1.50% or less.

[0047] Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200%. All of these elements contribute to suppressing grain growth, improving texture, and stabilizing secondary recrystallization. From the viewpoint of achieving these effects, it is preferable that the Mo content be 0.01% or more, the B content be 0.0001% or more, the Nb content be 0.0005% or more, the Bi content be 0.0001% or more, and the Ti content be 0.0001% or more. On the other hand, excessive contents of these elements function as inhibitors and are therefore undesirable in the inhibitorless process. Therefore, when these elements are contained, the Mo content is set to 0.50% or less, the B content to 0.0200% or less, the Nb content to 0.0100% or less, the Bi content to 0.0200% or less, and the Ti content to 0.0200% or less.

[0048] (Casting Process) In this embodiment, first, molten steel having the above-described composition is cast to obtain a slab having a thickness of 30 mm or more and 180 mm or less. By appropriately adjusting the thickness of the cast slab, sufficient microstructural control can be achieved during the subsequent hot rolling. If the slab thickness is less than 30 mm, the reduction ratio in rough rolling and finish rolling cannot be increased. If the reduction ratio in rough rolling is insufficient, the microstructural uniformity is impaired. If the reduction ratio in finish rolling is insufficient, dislocation density is not sufficiently accumulated during finish rolling, resulting in insufficient recrystallization in the subsequent hot-rolled sheet annealing process. If the slab thickness exceeds 180 mm, the engagement angle with the rolling rolls becomes large, making it difficult to perform heavy deformation. As a result, the reduction ratio per single pass decreases, and, as with the case of a thickness less than 30 mm described above, dislocations are not sufficiently introduced, resulting in a lack of recrystallization nuclei and a loss of microstructural uniformity.

[0049] (Hot Rolling Step) Following continuous casting, the slab is subjected to rough rolling and finish rolling to obtain a hot rolled sheet.

[0050] In the first embodiment, (I) the rough rolling is started within 5 minutes from the time when the surface temperature of the slab drops to 1200° C., and while the surface temperature of the slab is in the range of 900° C. to 1150° C. Then, (II) after the rough rolling is completed, finish rolling is started within 5 minutes from the completion of the rough rolling, while the surface temperature of the slab is maintained in the range of 850° C. to 1200° C.

[0051] In a second embodiment, after the surface temperature of the slab drops to 1200°C or less, the slab is held in an atmosphere of 1000°C to 1250°C for 5 to 20 minutes. Then, (I) rough rolling is started while the surface temperature of the slab is in the range of 900°C to 1150°C. Then, (II) after the rough rolling is completed, finish rolling is started within 5 minutes after the completion of the rough rolling while the surface temperature of the slab is maintained in the range of 850°C to 1200°C.

[0052] Slab surface temperature at the start of rough rolling: 900°C or higher and 1150°C or lower Next, the slab obtained by casting is directly subjected to rough rolling. Rough rolling is started when the slab surface temperature is in the range of 900°C or higher and 1150°C or lower. If the slab surface temperature at the start of rough rolling is less than 900°C, the content of precipitated Al in the hot-rolled sheet becomes excessive, and the magnetic properties of the grain-oriented electrical steel sheet deteriorate. If the slab surface temperature at the start of rough rolling exceeds 1150°C, breakout may occur, in which unsolidified molten steel in the center of the slab breaks through the slab surface layer (solidified layer) and flows out. In the present invention, by using an inhibitor-less component, it is possible to cool the slab to 1200°C or lower while suppressing precipitation in the steel, thereby enabling more stable production.

[0053] Time from when the slab surface temperature drops to 1200°C to when rough rolling starts: 5 minutes or less (first embodiment) When heating is not performed from outside the production line, the slab temperature gradually drops from casting to when rough rolling starts. In this case, the driving force for precipitation gradually increases. If the time from when the slab surface temperature drops to 1200°C to when rough rolling starts is long, the content of precipitated Al in the hot-rolled sheet becomes excessive, and the magnetic properties of the grain-oriented electrical steel sheet deteriorate. Therefore, in the first embodiment, this time is set to 5 minutes or less, preferably 3 minutes or less. If this time is too short, the slab surface temperature at the start of rough rolling will be high, which may cause breakouts. Therefore, this time is preferably set to 1.0 minute or more.

[0054] Slab Heat Insulation after the Slab Surface Temperature Drops to 1200°C or Less (Second Embodiment) If the casting speed is excessively high, breakouts may occur. Therefore, if the line length to the roughing mill is long, rough rolling cannot be started within the recommended time period of the first embodiment. In this case, to prevent the driving force of precipitation from gradually increasing, the slab is heated from the outside to maintain the temperature, thereby extending the time until rough rolling begins to 20 minutes. Specifically, the following process can be added prior to rough rolling.

[0055] That is, after the surface temperature of the slab drops to 1200°C or less, the slab is held in an atmosphere of 1000°C to 1250°C for 5 to 20 minutes. Specifically, after the surface temperature of the slab drops to 1200°C or less, the slab is loaded into a furnace and heated. To prevent the slab surface temperature from dropping at the start of rough rolling and maintain the recommended temperature range, the ambient temperature in the furnace is preferably set to 1000°C to 1250°C, and approximately 100°C (50 to 150°C) higher than the surface temperature of the slab when it is loaded into the furnace. Holding times of less than 5 minutes fail to fully achieve the slab heat retention effect. Holding times of more than 20 minutes result in partial precipitation of small amounts of inhibitor components, leading to a non-uniform structure. In this embodiment, the slab is continuously fed to the next process, which inevitably requires an open furnace such as a tunnel furnace. However, if a more airtight structure can be applied, the furnace temperature can be set to the target temperature. Even in such isothermal holding, it is desirable that the holding time be as short as possible.

[0056] Suitable conditions for rough rolling: In the final stand, the reduction ratio is 20% or more and 50% or less, and the strain rate is 1.0 s －１ Over 50s －１ The number of passes in rough rolling is not particularly limited, but is preferably 2 to 7 passes. When the slab is relatively thick, unsolidified molten steel may remain in the center of the slab. For this reason, it is preferable to force the solidified portions to come into contact with each other in the first pass of rough rolling. After that, the hot rolled structure can be created in the second pass and onward. In rough rolling, the structure is heavily processed to introduce dislocations, and new recrystallization nuclei are introduced, thereby suppressing excessive coarsening of the structure and maintaining a uniform grain size. For this reason, in the final stand, the reduction rate is 20% or more and the strain rate is 1.0 s －１ The above conditions are preferable. There are no particular limitations on the upper limits of the reduction rate and strain rate in the final stand, but even if the rolling is performed too strongly, the effect of maintaining uniform grain size will saturate and equipment with a high load capacity will be required. Therefore, from the viewpoint of using cheaper equipment, the reduction rate in the final stand is set to 50% or less and the strain rate is set to 50 s －１ The following conditions are preferred:

[0057] The strain rate ε is calculated using the following Ekeland formula. Here, v Ｒ is the roll peripheral speed (mm / s), R' is the roll radius (mm), h １ is the roll entry side thickness (mm), and r is the rolling reduction (%).

[0058] Slab surface temperature at the start of finish rolling: 850°C to 1200°C Time from the end of rough rolling to the start of finish rolling: within 5 minutes Dislocations introduced during rough rolling function as nuclei for recrystallization and also as nuclei for the formation of precipitates. Therefore, inhibitor-forming elements, even if present in trace amounts, whose content is not completely "0," precipitate to a certain extent after rough rolling. Precipitation progresses between rough rolling and finish rolling. Therefore, after the end of rough rolling, finish rolling must be started within 5 minutes of the end of rough rolling while maintaining the slab surface temperature in the range of 850°C to 1200°C. If the temperature is outside this range or the time until the start of finish rolling is prolonged, the precipitation of inhibitor-forming elements progresses, resulting in uneven dispersion of the inhibitor-forming elements, which can even affect secondary recrystallization. If the sheet threading speed is slow and the line length is long, the time from the end of rough rolling to the start of finish rolling may exceed 5 minutes. In this case, there is also a method of increasing the rolling reduction rate in rough rolling and elongating the slab in the rolling direction to increase the sheet threading speed. However, when the thickness of the slab is reduced by rough rolling, the temperature of the slab drops significantly, making it difficult to maintain the slab temperature within a predetermined range at the start of finish rolling, and therefore, it becomes necessary to install heating equipment as necessary. However, the time from the end of rough rolling to the start of finish rolling cannot be extended even if heating equipment is installed. For this reason, the line configuration must take this time into consideration. The lower limit of the time from the end of rough rolling to the start of finish rolling is not particularly limited, and this time can be 0 seconds or more. The thickness of the hot-rolled sheet after finish rolling is preferably 1.3 to 3.5 mm.

[0059] The surface temperature of the hot-rolled sheet at the end of finish rolling (final finish temperature) is above 650°C, and from the viewpoint of rolling stability, it is preferably 700°C or higher and 1100°C or lower.

[0060] (Cooling step) After the completion of finish rolling, the hot-rolled sheet is cooled under the condition that the cooling time until the surface temperature of the hot-rolled sheet drops to 650°C is 200 seconds or less. After finish rolling, the dislocation density becomes higher than after rough rolling, and precipitates are more likely to form. By cooling to a low temperature in a shorter time, non-uniform precipitation of inhibitor-forming elements can be suppressed. Therefore, the cooling time is set to 200 seconds or less. The lower limit of the cooling time is not particularly limited.

[0061] (Subsequent Processes) The steel sheet is then subjected to cold rolling, decarburization annealing, and finish annealing to produce a steel sheet product. These subsequent processes will be described below.

[0062] First, the hot-rolled sheet is annealed as needed. Then, the hot-rolled sheet is cold-rolled once or twice or more times with intermediate annealing to obtain a cold-rolled sheet. This cold rolling may be performed at room temperature, or may be warm-rolled by raising the temperature of the hot-rolled sheet to a temperature higher than room temperature, for example, about 250°C.

[0063] The cold-rolled sheet is then subjected to decarburization annealing to obtain a decarburized annealed sheet. This decarburization annealing is also called primary recrystallization annealing. That is, the first purpose of this annealing is to cause primary recrystallization of the cold-rolled sheet having a rolled texture and adjust the primary recrystallized grain size to an optimal size 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 as the annealing atmosphere, and simultaneously form an oxide film on the surface. Therefore, the annealing temperature (holding temperature) of the decarburization annealing is preferably in the range of approximately 800°C or higher and lower than 950°C. Furthermore, to further improve the texture, it is effective to increase the heating rate during the heating process of the decarburization annealing. Specifically, improvement can be expected by setting the heating rate between 500°C and 700°C to 80°C / s or higher.

[0064] Next, an annealing separator is applied to the surface of the decarburized 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 finish 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 formation of the forsterite film is also advantageous for improving the release properties of the film.

[0065] Next, final annealing is performed for secondary recrystallization and forsterite film formation. ２ , Ar, H ２ or a mixture of these gases is suitable. If trace elements are precipitated in the final product, this leads to deterioration of the magnetic properties, so the maximum annealing temperature is preferably 1100°C or higher to purify the elements. Since the grain-oriented electrical steel sheet obtained by the present invention has little variation in magnetic properties within the coil, it is desirable to perform finish annealing with a coil weighing 5 tons or more, more preferably 10 tons or more, from an economical standpoint.

[0066] After the above-mentioned finish 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 known insulating coating is suitable. For example, a method described in JP-A-50-79442 and JP-A-48-39338 in which a coating liquid containing phosphate, chromate, and colloidal silica is applied to the steel sheet and baked at about 800°C is suitable.

[0067] As a result of the purification during finish annealing of the grain-oriented electrical steel sheet obtained as a final product, the steel sheet base steel from which the insulating coating and forsterite coating have been removed has the following chemical composition: In other words, the chemical composition contains, in mass %, 2.0 to 4.5% Si, 0.01 to 0.5% Mn, 10 ppm to less than 60 ppm by mass of acid-soluble Al, and 5 ppm to less than 50 ppm by mass of S, with the contents of Se, Te, and O each suppressed to less than 50 ppm by mass. Depending on the finish annealing conditions, other elements may be incorporated into the forsterite coating or released into the gas phase, reducing the content in the steel, and therefore the concentration is lower than that in the slab.

[0068] [Hot-rolled sheet for grain-oriented electrical steel sheet] A hot-rolled sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention is a hot-rolled sheet obtained after the cooling step in the above-described manufacturing method. The chemical composition of the hot-rolled sheet of this embodiment is the same as the chemical composition of the slab described above, and therefore the description therein is incorporated by reference.

[0069] The hot-rolled sheet of this embodiment avoids the non-uniform precipitation of trace elements, which can cause secondary recrystallization defects, during the continuous process from casting to hot rolling, and therefore has sufficient properties as a base material for inhibitor-free grain-oriented electrical steel sheets. Specifically, the hot-rolled sheet of this embodiment is characterized in that the content of precipitated Al divided by the content of acid-soluble Al is 0.25 or less. In the inhibitorless process, precipitates are not necessarily required for secondary recrystallization. However, if precipitates (Al nitrides) are non-uniformly precipitated, grain growth also becomes non-uniform, making it difficult to achieve the texture inhibition effect unique to the inhibitorless process, resulting in secondary recrystallization defects. In hot-rolled sheets, if the sheet is held at high temperatures for a long time after precipitation, the precipitates (Al nitrides) undergo Ostwald ripening and become localized. By suppressing the precipitation rate in the hot-rolled sheet, the non-uniformity caused by localization can be alleviated to a certain extent. By employing the manufacturing method of this embodiment, the value obtained by dividing the content of precipitated Al by the content of acid-soluble Al can be set to 0.25 or less. The lower this value is, the more preferable it is, and it can be 0.00 or more.

[0070] The precipitated Al content is determined by removing the surface layer from the surface to ¼ of the thickness of the hot-rolled sheet (hereinafter also referred to as the surface ¼), using the remaining central portion of the sheet as a test piece, electrolyzing the test piece using a 10% AA-based electrolyte (acetylacetone), filtering, extracting, and analyzing the Al content of the residue. The acid-soluble Al content is determined by a value obtained by ordinary wet analysis.

[0071] The contents of other inhibitor-forming elements such as S, Se, and Te are preferably 0 ppm. However, even if these elements are not completely removed and remain, the precipitates of these elements (MnS, MnSe, MnTe) can have an average precipitate particle size of 80 nm or less, resulting in a uniform precipitate state.

[0072] [Grain-oriented electrical steel sheet manufacturing equipment train] Referring to Figure 1 , a grain-oriented electrical steel sheet manufacturing equipment train 100 according to one embodiment of the present invention comprises a continuous casting machine 1, a roughing mill 2, a finishing rolling mill 3, and a coiler 4 arranged in this order, and is capable of carrying out a continuous casting and hot rolling process. The continuous casting machine 1 continuously casts molten steel to produce slabs. The roughing mill 2 rough-rolls the slabs obtained by the continuous casting machine 1. The finishing rolling mill 3 finish-rolls the slabs after rough rolling to produce a hot-rolled sheet. The coiler 4 coils the obtained hot-rolled sheet into a hot-rolled coil.

[0073] The line of manufacturing equipment 100 for grain-oriented electrical steel sheets includes a first heat treatment device 5 , a second heat treatment device 6 , and a third heat treatment device 7 .

[0074] The first heat treatment device 5 is located between the continuous casting machine 1 and the roughing mill 2 and adjusts the surface temperature of the slab to between 900°C and 1200°C. In the first heat treatment device 5, the surface temperature of the slab immediately before being charged into the roughing mill 2 is measured by the temperature measuring unit 8, and the slab temperature is controlled so that this measured value is the target temperature (between 900°C and 1200°C). Here, when the slab thickness is thin, the temperature tends to decrease, whereas when the slab thickness is thick, the temperature does not decrease easily. The retention time in the first heat treatment device 5 is determined by the distance from the continuous casting machine 1 to the roughing mill 2 and the casting speed. Therefore, it is necessary to subject the slab after continuous casting to a cooling treatment at an appropriate cooling rate and control it to the target temperature. To this end, it is desirable for the first heat treatment device 5 to have the functions of water cooling, natural cooling, heat retention, and heating. In particular, when casting with a fixed slab thickness and casting speed, it is necessary to have one or more of the above functions to achieve the target temperature.

[0075] The second heat treatment device 6 is located between the roughing mill 2 and the finishing mill 3 and adjusts the surface temperature of the slab after rough rolling to between 850°C and 1200°C. Because the transport time from the roughing mill 2 to the finishing mill 3 must be short, the distance between the roughing mill and the finishing mill where the second heat treatment device 6 is located must be determined so that the surface temperature of the slab after rough rolling is the target temperature of between 850°C and 1200°C. Therefore, the second heat treatment device 6 desirably has the functions of cooling, heat retention, and heating to achieve the target temperature at the inlet side of the finishing mill 3. As with the first heat treatment device 5, if the thickness, temperature, and threading speed of the slab are predetermined to a certain extent, it is sufficient for the device to have one or more of the above functions. In many cases, a heating function is required because the slab tends to become thinner and cooler during rough rolling. This heating requires a mechanism to control the temperature at a fixed distance, so induction heating is effective.

[0076] The third heat treatment device 7 is located between the finish rolling mill 3 and the coiler 4, and adjusts the surface temperature of the hot-rolled sheet to 650°C or less within 200 seconds after the end of finish rolling. The third heat treatment device 7 mainly controls cooling in order to adjust the surface temperature of the hot-rolled sheet to 650°C or less after finish rolling, and needs to have a water-cooling mechanism to lower the surface temperature of the hot-rolled sheet to 650°C or less within a limited time.

[0077] The line of manufacturing equipment 100 for grain-oriented electrical steel sheets must have a threading capacity that allows the slabs to be transported from the roughing mill 2 to the finishing mill 3 within five minutes.

[0078] The roughing mill 2 has a rolling reduction of 20% or more and 50% or less in the final stand, and a strain rate of 1.0 s －１ Over 50s －１ It is preferable that the rolling capacity is as follows:

[0079] Example 1 Molten steel was refined having a chemical composition containing 0.05% C, 3.2% Si, 0.05% Mn, 80 ppm acid-soluble Al, 20 ppm S, 30 ppm N, 5 ppm or less Se, 5 ppm or less Te, 20 ppm or less O, 0.01% P, and 0.03% Cr, with the balance being Fe and unavoidable impurities. The molten steel was then continuously cast to obtain a slab having the thickness shown in Table 1. Subsequently, hot rolling consisting of rough rolling (a total of four passes) and finish rolling was performed under the conditions shown in Table 1, consecutively from casting, to obtain a hot-rolled sheet having the thickness shown in Table 1. The hot-rolled sheet was then continuously cooled under the conditions shown in Table 1. After the completion of finish rolling, the cooling time until the surface temperature of the hot-rolled sheet reached 650°C was adjusted by controlling the amount of water.

[0080] Test specimens were taken from parts of the obtained hot-rolled sheets by the method described above, and the precipitated Al content was determined. Table 1 shows the precipitated Al content and the value obtained by dividing the precipitated Al content by the acid-soluble Al content (precipitated Al / Sol. Al).

[0081] 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, followed by pickling to remove scale, and then cold-rolled to a cold-rolled sheet with a final thickness of 0.27 mm. This cold-rolled sheet was subjected to annealing for decarburization and primary recrystallization to obtain a decarburized annealed sheet. An annealing separator mainly composed of MgO was then applied to the surface of the decarburized annealed sheet, and finish annealing, including a secondary recrystallization process and a purification process, was performed at a maximum temperature of 1200°C for a soaking time of 10 hours to obtain a finish-annealed steel sheet. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish-annealed steel sheet and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0082] Thirty-two Epstein test pieces were taken from the obtained grain-oriented electrical steel sheets, and the magnetic flux density (B ８ The magnetic flux density (B ８ ) are shown in Table 1. As is clear from Table 1, the grain-oriented electrical steel sheets manufactured under the conditions according to the present invention have good magnetic properties.

[0083]

[0084] Example 2 A molten steel having the chemical composition shown in Table 2 (the balance being Fe and inevitable impurities) was refined. Then, casting, hot rolling consisting of rough rolling and finish rolling, and cooling were carried out under the conditions of No. 3 in Example 1, to obtain a rolled sheet.

[0085] Test pieces were taken from a part of the obtained hot-rolled sheet by the method described above, and the precipitated Al content was determined. Table 2 shows the precipitated Al content and the value obtained by dividing the precipitated Al content by the acid-soluble Al content (precipitated Al / Sol. Al). In addition, for test pieces taken in the same manner, a 1 mm cross section in the rolling direction was measured. ２ The particles of the precipitated MnS were observed under an SEM, and the particle sizes of the precipitated MnS were arithmetically averaged to obtain the average particle size. The results are shown in Table 2.

[0086] The hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 900°C for 60 seconds, and scale was removed by pickling. Subsequently, the sheet was cold-rolled to a thickness of 1.5 mm by a first cold rolling, intermediate annealing at a maximum temperature of 1050°C, and a second cold rolling to a final thickness of 0.23 mm. This cold-rolled sheet was annealed for decarburization and primary recrystallization to obtain a decarburized annealed sheet. An annealing separator mainly composed of MgO was then applied to the surface of the decarburized annealed sheet, and finish annealing, including a secondary recrystallization process and a purification process, was performed at a maximum temperature of 1150°C for a soaking time of 10 hours to obtain a finish-annealed steel sheet. An insulating coating composed of colloidal silica and magnesium phosphate was applied to the surface of the finish-annealed steel sheet and baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0087] Thirty-two Epstein test pieces were taken from the obtained grain-oriented electrical steel sheets, and the magnetic flux density (B ８ The magnetic flux density (B ８ ) are shown in Table 2. As is clear from Table 2, the grain-oriented electrical steel sheets manufactured under the conditions according to the present invention have good magnetic properties.

[0088]

[0089] Example 3: Molten steel was refined to have a chemical composition containing 0.08% C, 3.4% Si, 0.07% Mn, 100 ppm acid-soluble Al, 10 ppm S, 60 ppm N, 5 ppm or less Se, 5 ppm or less Te, and 20 ppm or less O, with the balance consisting of Fe and unavoidable impurities. The molten steel was then continuously cast to obtain a 40 mm thick slab. Experiments were conducted under the following conditions: Condition 1: the slab was allowed to cool after casting; Condition 2: the slab was charged into a furnace with an ambient temperature of 1000°C four minutes after the start of casting and maintained there; and Condition 3: the slab was charged into a furnace with an ambient temperature of 1100°C four minutes after the start of casting, and then changed to a furnace with an ambient temperature of 1000°C two minutes later. Figure 2 shows the change in the surface temperature of the slab over time from the start of casting under each condition. Under condition 1, the temperature constantly drops, and five minutes after the slab surface temperature reached 1,200°C (six and a half minutes after the start of casting), the slab surface temperature was approximately 900°C. Under conditions 2 and 3, the temperature drop was suppressed by charging the slab into the furnace, and the slab was able to soak at approximately 1,000°C after charging into the furnace. Under condition 3, the slab passed through a furnace whose surface temperature was 100°C higher than when it was charged into the furnace, allowing for a smoother transition to soaking without any temperature drop. Table 3 shows the slab warming / rough rolling conditions for conditions 1 to 3.

[0090] Next, rough rolling consisting of a total of two passes was carried out under the conditions shown in Table 3, with the reduction rate of the second pass being 25% and the strain rate being 3 s －１ The sheet was then rolled under the conditions shown in Table 3. The sheet was then held or heated by an external heater until finish rolling, and finish rolling was started under the conditions shown in Table 3. A hot-rolled sheet having a thickness of 2.0 mm was obtained by finish rolling at a final finish temperature of 800 to 1050°C, and then cooled to 650°C or less in 100 seconds.

[0091] Test specimens were taken from parts of the obtained hot-rolled sheets by the method described above, and the precipitated Al content was determined. Table 3 shows the precipitated Al content and the value obtained by dividing the precipitated Al content by the acid-soluble Al content (precipitated Al / Sol. Al).

[0092] The hot-rolled sheet was subjected to hot-rolled sheet annealing at a soaking temperature of 1000°C for 20 seconds, followed by pickling to remove scale, and then cold-rolled to a final thickness of 0.23 mm. This cold-rolled sheet was subjected to annealing for decarburization and primary recrystallization to obtain a decarburized annealed sheet. An annealing separator mainly composed of MgO was then applied to the surface of the decarburized annealed sheet, and finish annealing, including a secondary recrystallization process and a purification process, was performed at a maximum temperature of 1150°C for a soaking time of 8 hours to obtain a finish-annealed steel sheet. An insulating coating composed of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish-annealed steel sheet and baked at 800°C to obtain the final grain-oriented electrical steel sheet.

[0093] Thirty-two Epstein test pieces were taken from the obtained grain-oriented electrical steel sheets, and the magnetic flux density (B ８ The magnetic flux density (B ８ ) are shown in Table 3. As is clear from Table 3, the grain-oriented electrical steel sheets manufactured under the conditions according to the present invention have good magnetic properties.

[0094]

[0095] Example 4 A hot-rolled sheet was produced from molten steel having a chemical composition containing 0.04% C, 3.2% Si, 0.06% Mn, 50 ppm acid-soluble Al, 30 ppm S, 50 ppm N, 1 ppm Se, 0.3 ppm Te, and 10 ppm O, with the balance being Fe and unavoidable impurities, using equipment trains with the following specifications and conditions: 1) [Continuous casting machine] Casting thickness: 80 mm 2) [Continuous casting machine] Casting speed: 5 m / min 3) [First heat treatment device] Cooling zone length after casting: 15 m (required passage time of 3 minutes) 4) [First heat treatment device] Cooling zone after cooling zone until rough rolling: 20 m (required passage time of 4 minutes) 5) [Roughing rolling mill] 3 stands, WR diameter: 750 mm (3-stand reduction rate of 35%) 6) [Roughing rolling mill] Bar thickness on delivery side: 25 mm (delivery speed 16 m / min) 7) [Second heat treatment device] Cooling zone until finish rolling: 60 m (required passage time of 3.8 minutes) 8) [Finishing rolling mill] 7 stands, WR diameter: 700 mm 9) [Finishing rolling mill] Plate thickness on delivery side: 2.5 mm (delivery speed 160 m / min) 10) [Third heat treatment device] Cooling zone after finish rolling: 50 m (required passage time: 0.3 minutes) 11) [Third heat treatment device] Cooling zone length: 200 m (required passage time: 1.3 minutes) Temperature control was performed in the above 4), 7), and 10), and feedback to the previous process was used to appropriately control the casting speed in 2) and the cooling water control in the cooling zone 3). The coil temperature after 11) was set to 650°C or less. In this Example 4, a hot-rolled sheet was produced under the production conditions corresponding to claim 1.

[0096] The obtained hot-rolled coil was divided into 10 ton sections, and the hot-rolled sections were annealed at a soaking temperature of 1030°C for 50 seconds. After removing scale by pickling, the sections were cold-rolled to a final thickness of 0.27 mm. The cold-rolled sections were heated to 800°C at a heating rate of 150°C / s, and then subjected to N ２ -H ２The steel sheet was decarburized by soaking in a mixed wet gas to obtain a decarburized annealed steel sheet. An annealing separator mainly composed of MgO was then applied to the surface of the decarburized annealed steel sheet, and finish annealing, including a secondary recrystallization process and a purification process, was performed at a maximum temperature of 1200°C for a soaking time of 10 hours to obtain a finish annealed steel sheet. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish annealed steel sheet, and the steel sheet was baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0097] From the obtained coil of grain-oriented electrical steel sheet, 32 Epstein test pieces were taken every 200 m in the longitudinal direction of the coil, and the magnetic flux density (B ８ The magnetic flux density (B ８ ) was 1.903±0.003 T, showing properties with extremely little variation. By applying the method of the present invention, unlike the usual slab reheating method, the entire length of the coil is treated continuously, and therefore it was possible to produce a coil (hot-rolled sheet) that is almost free from the influence of non-steady-state parts such as the slab ends and non-uniformity during slab heating over the entire length.

[0098] Example 5 A hot-rolled steel sheet was produced from molten steel having the same chemical composition as in Example 4 using equipment having the following specifications and conditions. 1) [Continuous casting machine] Casting thickness: 80 mm 2) [Continuous casting machine] Casting speed: 3 m / min 3) [First heat treatment device] Cooling zone length after casting: 15 m (required passage time of 5 minutes) 4) [First heat treatment device] Cooling zone after cooling zone until rough rolling: 20 m (required passage time of 6.7 minutes) 5) [Roughing rolling mill] 3 stands, WR diameter: 750 mm (3-stand reduction rate of 38%) 6) [Roughing rolling mill] Bar thickness on delivery side: 20 mm (delivery speed 12 m / min) 7) [Second heat treatment device] Cooling zone until finish rolling: 60 m (required passage time of 5 minutes) 8) [Finishing rolling mill] 7 stands, WR diameter: 700 mm 9) [Finishing rolling mill] Plate thickness on delivery side: 2.0 mm (delivery speed 120 m / min) 10) [Third heat treatment device] Cooling zone after finish rolling: 50 m (required passage time: 0.4 minutes) 11) [Third heat treatment device] Cooling zone length: 200 m (required passage time: 1.7 minutes)

[0099] In the above process, since the passage time through the natural cooling zone after the cooling zone until rough rolling exceeds 5 minutes, an enclosure was provided around the slab, heating was performed in a nitrogen gas environment, and the slab surface was kept constant at 1100°C. Furthermore, to shorten the passage time from rough rolling to finish rolling, the bar thickness after rough rolling was reduced. To deal with the decrease in the surface temperature of the thinner bar, a bar heater was installed between rough rolling and finish rolling, and the temperature at the inlet side of finish rolling was set to 1100°C. The coil temperature after 11) was set to 650°C or less. In this Example 5, a hot-rolled sheet was produced under the production conditions corresponding to claim 2.

[0100] The obtained hot-rolled coil was divided into 10 ton sections, and the hot-rolled sections were annealed at a soaking temperature of 1030°C for 50 seconds. After removing scale by pickling, the sections were cold-rolled to a final thickness of 0.23 mm. This cold-rolled section was heated to 800°C at a heating rate of 150°C / s, and then subjected to N ２ -H ２ The steel sheet was decarburized by soaking in a mixed wet gas to obtain a decarburized annealed steel sheet. An annealing separator mainly composed of MgO was then applied to the surface of the decarburized annealed steel sheet, and finish annealing, including a secondary recrystallization process and a purification process, was performed at a maximum temperature of 1200°C for a soaking time of 10 hours to obtain a finish annealed steel sheet. An insulating coating consisting of 60% colloidal silica and aluminum phosphate was applied to the surface of the finish annealed steel sheet, and the steel sheet was baked at 850°C to obtain the final grain-oriented electrical steel sheet.

[0101] From the obtained coil of grain-oriented electrical steel sheet, 32 Epstein test pieces were taken every 200 m in the longitudinal direction of the coil, and the magnetic flux density (B ８ The magnetic flux density (B ８ ) was 1.901±0.002 T, showing properties with extremely little variation. By applying the method of the present invention, unlike the usual slab reheating method, the entire length of the coil is treated continuously, and therefore it was possible to produce a coil (hot-rolled sheet) that is almost free from the influence of non-steady-state parts such as the slab ends and non-uniformity during slab heating over the entire length.

[0102] The present invention can be applied to the production of grain-oriented electrical steel sheets.

[0103] 100 Manufacturing equipment row for grain-oriented electrical steel sheet 1 Continuous casting machine 2 Roughing rolling mill 3 Finishing rolling mill 4 Coiler 5 First heat treatment device 6 Second heat treatment device 7 Third heat treatment device 8 Temperature measurement section 20 Final stand

Claims

1. a step of casting a molten steel having a component composition containing C: 0.08% by mass or less, Si: 2.0% by mass or more and 4.5% by mass or less, Mn: 0.5% by mass or less, acid-soluble Al: 20 ppm by mass or more and 120 ppm by mass or less, S: less than 50 ppm by mass, and N: less than 80 ppm by mass, with the contents of Se, Te and O each being suppressed to less than 50 ppm by mass, and the balance consisting of Fe and unavoidable impurities, to obtain a slab having a thickness of 30 mm or more and 180 mm or less; A process of subjecting the slab to rough rolling and finish rolling to obtain a hot-rolled sheet, (I) The rough rolling is started within 5 minutes from the time when the surface temperature of the slab drops to 1200 ° C. and while the surface temperature of the slab is in the range of 900 ° C. or more and 1150 ° C. or less, (II) after completion of the rough rolling, while maintaining the surface temperature of the slab in the range of 850 ° C. or more and 1200 ° C. or less, starting the finish rolling within 5 minutes from the completion of the rough rolling; After the finish rolling is completed, the hot-rolled sheet is cooled under conditions in which the surface temperature of the hot-rolled sheet is lowered to 650° C. in a cooling time of 200 seconds or less. Thereafter, the process includes cold rolling, decarburization annealing, and finish annealing. The method for producing a grain-oriented electrical steel sheet comprising the steps of:

2. a step of casting a molten steel having a component composition containing C: 0.08% by mass or less, Si: 2.0% by mass or more and 4.5% by mass or less, Mn: 0.5% by mass or less, acid-soluble Al: 20 ppm by mass or more and 120 ppm by mass or less, S: less than 50 ppm by mass, and N: less than 80 ppm by mass, with the contents of Se, Te and O each being suppressed to less than 50 ppm by mass, and the balance consisting of Fe and unavoidable impurities, to obtain a slab having a thickness of 30 mm or more and 180 mm or less; After the surface temperature of the slab is lowered to 1200° C. or less, the slab is held in an atmosphere of 1000° C. or more and 1250° C. or less for 5 minutes to 20 minutes; Then, the slab is subjected to rough rolling and finish rolling to obtain a hot-rolled sheet, (I) starting the rough rolling while the surface temperature of the slab is in the range of 900° C. or more and 1150° C. or less; (II) after completion of the rough rolling, while maintaining the surface temperature of the slab in the range of 850 ° C. or more and 1200 ° C. or less, starting the finish rolling within 5 minutes from the completion of the rough rolling; After the finish rolling is completed, the hot-rolled sheet is cooled under conditions in which the surface temperature of the hot-rolled sheet is lowered to 650° C. in a cooling time of 200 seconds or less. Thereafter, the process includes cold rolling, decarburization annealing, and finish annealing. The method for producing a grain-oriented electrical steel sheet comprising the steps of:

3. The rough rolling is performed in a final stand with a rolling reduction of 20% to 50% and a strain rate of 1.0 s －１ More than 50s －１ The method for producing a grain-oriented electrical steel sheet according to claim 1 , which is carried out under the following conditions:

4. The method for producing a grain-oriented electrical steel sheet according to claim 2, wherein the rough rolling is performed in a final stand under conditions where the reduction rate is 20% or more and 50% or less and the strain rate is 1.0 s -1 or more and 50 s -1 or less.

5. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein after completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain a surface temperature of the slab in a range of 850°C or more and 1200°C or less.

6. A method for manufacturing a grain-oriented electromagnetic steel sheet as described in claim 2, wherein after completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C or higher and 1200°C or lower.

7. A method for manufacturing a directional electromagnetic steel sheet as described in claim 3, wherein after completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C or higher and 1200°C or lower.

8. A method for manufacturing a directional electromagnetic steel sheet as described in claim 4, wherein after completion of the rough rolling and before the finish rolling, the slab is subjected to a heat treatment to maintain the surface temperature of the slab in the range of 850°C or higher and 1200°C or lower.

9. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 8, wherein the component composition further comprises, in mass%, one or more selected from the group consisting of Ni: 0.005 to 1.50%, Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%, P: 0.0050 to 0.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200%.

10. a continuous casting machine for continuously casting molten steel to produce slabs; A rough rolling mill that performs rough rolling on the slab; a finishing rolling mill that performs finish rolling on the slab after the rough rolling to produce a hot-rolled sheet; A coiler that winds the hot-rolled sheet; A row of manufacturing equipment for grain-oriented electrical steel sheets, in which the following are arranged in order: A first heat treatment device located between the continuous casting machine and the rough rolling machine and configured to adjust the surface temperature of the slab to 900° C. or more and 1200° C. or less; A second heat treatment device is located between the rough rolling mill and the finish rolling mill and adjusts the surface temperature of the slab after the rough rolling to 850 ° C. or more and 1200 ° C. or less; A third heat treatment device is located between the finishing rolling mill and the coiler, and adjusts the surface temperature of the hot-rolled sheet to 650° C. or less within 200 seconds after the end of the finish rolling. having A line of equipment for manufacturing grain-oriented electrical steel sheets, the line having a threading capacity for moving the slabs from the roughing mill to the finishing mill within 5 minutes.

11. The roughing mill has a final stand with a rolling reduction of 20% to 50% and a strain rate of 1.0 s －１ More than 50s －１ 11. The production line of grain-oriented electrical steel sheet according to claim 10, having a rolling capacity of:

12. A hot-rolled sheet for grain-oriented electrical steel sheet, comprising: C: 0.08% by mass or less; Si: 2.0% by mass or more and 4.5% by mass or less; Mn: 0.5% by mass or less; acid-soluble Al: 20 ppm by mass or more and 120 ppm by mass or less; S: less than 50 ppm by mass; and N: less than 80 ppm by mass; the contents of Se, Te, and O are each suppressed to less than 50 ppm by mass, with the balance consisting of Fe and unavoidable impurities; and a value obtained by dividing the content of precipitated Al by the content of acid-soluble Al is 0.25 or less.

13. The composition further includes, in mass%, one or more selected from the group consisting of Ni: 0.005 to 1.50%, Sn: 0.001 to 0.50%, Sb: 0.005 to 0.50%, Cu: 0.01 to 0.50%, P: 0.0050 to 0.50%, Cr: 0.01 to 1.50%, Mo: 0.01 to 0.50%, B: 0.0001 to 0.0200%, Nb: 0.0005 to 0.0100%, Bi: 0.0001 to 0.0200%, and Ti: 0.0001 to 0.0200%. The hot-rolled sheet for grain-oriented electrical steel sheet according to claim 12.