Annealing equipment and method for manufacturing grain-oriented electrical steel sheet

Abstract

Provided is an annealing facility which more actively controls carbon in steel, thereby contributing to further improvement of magnetic properties. This annealing facility has a heating zone, a soaking zone, and a cooling zone on a steel strip conveyance line, wherein: the conveyance line can pass a steel strip having a thickness of 2.8 mm or greater; the soaking zone has a means for maintaining the ambient temperature at 900°C or greater; the cooling zone has a means for supplying a refrigerant to the steel strip to set the average cooling rate in a temperature range of 750°C to 120°C to at least 50°C / s; and a means for removing the refrigerant is provided on the outlet side of the cooling zone.

Description

[Technical Field]

[0001] The present invention relates to annealing equipment, particularly to annealing equipment that enables the production of grain-oriented electrical steel sheets with excellent magnetic properties, and to a method for producing grain-oriented electrical steel sheets using the annealing equipment. [Background technology]

[0002] Grain-oriented electrical steel sheets are made by applying a magnetic field to the iron. <001> This steel sheet has excellent magnetic properties and a crystalline structure (Goss orientation) in which the orientation is highly concentrated in the rolling direction of the steel sheet. One method proposed to improve the magnetic properties of grain-oriented electrical steel sheets is to control the form of C in the steel by controlling the cooling process after annealing before final cold rolling.

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

[0004] The technologies proposed in Patent Documents 1 and 2 control carbon in steel as ultrafine carbides or solute C, so that when dislocations are introduced during cold rolling, the solute C and other particles adhere to the dislocations, forming a Cottrell atmosphere that promotes non-uniform deformation during cold rolling, modifies the cold rolling texture, and improves the texture after primary recrystallization. This effect is also known in general steel as a method for increasing the {110} intensity in the texture after recrystallization during annealing after cold rolling. In grain-oriented electrical steel sheets, the {110} <001> In this case, the {110} texture can act as a good nucleus for secondary recrystallization.

[0005] Since the texture after primary recrystallization is strongly affected by the cold rolling conditions, the cold rolling reduction is a very important factor. For example, when producing grain-oriented electrical steel sheets by a single cold rolling without intermediate annealing, it is known that good magnetic properties can be obtained by controlling the texture with a cold rolling reduction of 85% or more. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 58-157917 [Patent Document 2] Japanese Patent Application Publication No. 52-094825 Summary of the Invention [Problem to be solved by the invention]

[0007] The techniques disclosed in the above publications require increasing the cooling rate in annealing before cold rolling in order to convert the carbon in the steel before cold rolling into ultrafine carbides or to convert it into a state of solid solution C. Furthermore, manufacturers of grain-oriented electrical steel sheets are seeking to develop a manufacturing method in which the final thickness is achieved in a single cold rolling pass without intermediate annealing, in order to reduce production costs. However, when the final thickness is achieved in a single cold rolling pass, a high reduction of more than 85% is required in cold rolling to obtain a good primary recrystallization texture.

[0008] Grain-oriented electrical steel sheets are available in thicknesses as thick as 0.35 mm. To achieve this thickness in a single cold rolling pass, especially under high pressures suitable for texture control, the sheet thickness before rolling must be very thick. Grain-oriented electrical steel sheets typically contain 2.0% or more silicon to achieve good magnetic properties. However, silicon is known to be a brittle element, making it difficult to thread thick steel strips stably through a production line without breakage. Furthermore, when cooling thicker sheets, higher cooling performance is expected to be required.

[0009] The present invention advantageously solves the above problems and proposes annealing equipment that contributes to further improving magnetic properties by more actively controlling carbon in steel, and a method for manufacturing grain-oriented electrical steel sheets using this annealing equipment. [Means for solving the problem]

[0010] It is known that the magnetic properties of grain-oriented electrical steel sheets can be improved by controlling precipitates known as inhibitors and by controlling the texture before secondary recrystallization. One of the two key technologies for improving magnetic properties is a technology that minimizes the inhibitor element, known as the inhibitor-less method, which induces secondary recrystallization without the inclusion of inhibitor-forming components, as disclosed in JP 2000-129356 A. Because this technology minimizes the inhibitor element, which is one of the two key technologies for improving magnetic properties, texture control is particularly important for this steel type.

[0011] Therefore, the inventors conducted experiments using grain-oriented electrical steel sheet that did not contain inhibitor components (containing 0.045% C, 3.25% Si, 0.06% Mn, with S, Se, and O each at less than 50 ppm, N at less than 25 ppm, and the remainder being Fe and unavoidable impurities), and clarified the following factors that are extremely important for improving magnetic properties.

[0012] [About the cooling stop temperature and cold rolling reduction ratio for hot-rolled sheet annealing] When producing grain-oriented electrical steel sheets by a single cold rolling pass, the optimum reduction ratio in cold rolling is relatively high, for example, 88%. Generally, the higher the reduction ratio in cold rolling, the sharper the texture becomes, and the more pronounced the improvement effect becomes. However, if the reduction ratio is too high, the {110} <001> The orientation of grains decreases, and secondary recrystallization begins to be unfavorable. From this perspective, the reduction ratio in cold rolling often has the extreme values mentioned above.

[0013] The inventors, in particular, <001> We considered the possibility that the coarse carbides may have contributed to the decrease in the oriented grains. This is because the carbides are harder than the base material and do not deform during cold rolling, which forces the crystal structure around the carbides to deform. <001> It is thought that part of the decrease in oriented grains is due to these coarse carbides.

[0014] As described in Patent Documents 1 and 2, {110} <001> Solute C and ultrafine carbides are used to form oriented grains. Therefore, if thorough cooling management can be implemented to suppress the formation of coarse carbides, it is thought that texture improvement effects can be achieved even at higher rolling reduction rates, resulting in good magnetic properties. Similar results were obtained in newly conducted experiments.

[0015] Typically, gas or water is used for cooling after annealing. If rapid cooling is required, using low-temperature water is efficient. When water is used for cooling, if the water used for cooling remains on the steel strip and is carried into the manufacturing equipment, it may cause corrosion of the equipment. Therefore, by setting the cooling stop temperature of the steel strip to approximately 150°C or higher, the water remaining on the steel sheet after cooling can be evaporated by the heat of the steel sheet, allowing for efficient treatment of the remaining water. As seen in Patent Document 2, the lower limit of the cooling temperature range is set to 300°C or higher, aiming for such efficient manufacturing. Alternatively, when using a process that leaves residual water on the steel sheet, heating with hot air from a heater or dryer may be used to remove the water. However, in either case, maintaining the steel strip at a temperature above 150°C or performing external heat treatment can promote carbide formation, albeit to a certain extent.

[0016] From the above considerations and studies, it was found that, in the cooling during annealing before cold rolling, the steel sheet is cooled in one go to 120°C or less, which is less than 150°C, and then the steel sheet is led to the cold rolling step without incurring a temperature rise of preferably 100°C or more from the cooling stop temperature, thereby making it possible to apply a reduction ratio of, for example, 89% or more in the cold rolling step and obtaining better magnetic properties.

[0017] To achieve the cooling described above, the removal of residual water from the steel sheet is a challenge. However, because the steel sheet needs to be cooled to 120°C or below as described above, the heat from the steel sheet itself or from an external source cannot be used. Therefore, it was discovered that a different residual water removal mechanism than conventional methods is required, such as removing most of the water using equipment that comes into direct contact with the steel sheet, such as a wringer roll or wiper blade, and then blowing it away with dry gas that does not exceed 150°C.

[0018] [Base material thickness measures to achieve high reduction ratios in cold rolling] Increasing the reduction ratio in cold rolling means that the thickness of the base material before cold rolling will be thicker than before. Therefore, annealing equipment requires a transfer line that can transport thicker steel plates. In other words, it is important that the transfer line can pass steel plates with a thickness of 2.8 mm or more. We also considered specific measures to realize such a conveying line and found that the following two methods were effective.

[0019] In other words, in annealing equipment, steel strips are passed through the conveying rolls while being bent multiple times. During this bending process, compressive stress is applied to the inside of the bend and tensile stress is applied to the outside. This tendency becomes more pronounced as the plate thickness increases, so thicker steel plates inevitably have a higher risk of breaking within the equipment. The stress generated during bending is affected by the bending radius, so the larger the diameter of the conveying rolls, the lower the generated stress. From the results of bending tests conducted along the roll shape, it was found that a roll diameter of 950 mm or more is advantageous. A diameter of 1100 mm or more is more desirable, and even more desirable is 1350 mm or more.

[0020] Furthermore, because steel exhibits greater ductility at higher temperatures, maintaining a higher temperature reduces the risk of fracture. In particular, since hot-rolled coils usually enter annealing equipment at the entrance, where they have cooled to room temperature, it is advantageous to provide a preheating facility or a heating facility between the coil discharge and the looper, thereby increasing the steel strip temperature. When conducting the bending test, we also investigated the steel sheet temperature and found that maintaining a steel strip temperature of 70°C or higher significantly reduces the risk of fracture.

[0021] On the other hand, the temperature control after annealing and cooling does not allow for unlimitedly high temperatures. This is because it is necessary to suppress carbide formation after cooling, so excessively high temperatures are undesirable. We have found that a temperature range of 50°C to 120°C is advantageous for suppressing carbide formation while reducing the risk of fracture. In particular, we have also found that when temperatures below 70°C are applied to steel strips, it is desirable for the diameter of the above-mentioned conveying rolls to be 1100 mm or more.

[0022] The present invention was made based on the above findings, and the gist of the present invention is as follows. 1. An annealing facility having a heating zone, a soaking zone, and a cooling zone on a steel strip transport line, wherein the transport line is capable of passing steel strips having a thickness of 2.8 mm or more, the soaking zone has a means for maintaining the ambient temperature at 900°C or higher, the cooling zone has a means for supplying a coolant to the steel strip to achieve an average cooling rate of 50°C / s or higher in the temperature range of 750°C or lower and 120°C or higher, and the annealing facility has a means for removing the coolant on the outlet side of the cooling zone.

[0023] 2. The annealing equipment described in 1 above further comprises a payoff reel at the start end of the conveying line for unwinding the steel strip onto the conveying line and an entry looper for applying tension to the steel strip on the conveying line, and a trimmer between the payoff reel and the entry looper for trimming the edge of the steel strip.

[0024] 3. The annealing equipment according to 1 or 2 above, further comprising a conveying roll for changing the direction of travel of the steel strip passing through the conveying line, the conveying roll having a diameter of 950 mm or more.

[0025] 4. The annealing facility according to 2 or 3 above, further comprising a means for heating the steel strip to 70°C or higher on the entry side of the entry looper.

[0026] 5. An annealing facility as described in any one of 1 to 4 above, wherein the coolant is cooling water at 80°C or less, and the removal means comprises two or more means with different mechanisms for removing the cooling water remaining on the steel strip.

[0027] 6. The annealing facility according to item 5 above, wherein the removing means is both a means for directly removing and a means for indirectly removing the cooling water remaining on the steel strip.

[0028] 7. The annealing facility according to any one of 1 to 6 above, further comprising a means for measuring the temperature of the steel strip and maintaining it at a predetermined temperature, at the outlet side of the removing means.

[0029] 8. A method for producing a grain-oriented electrical steel sheet, comprising hot rolling a steel material containing 0.01% by mass or more and 0.10% by mass or less of C, 2.0% by mass or more and 4.5% by mass or less of Si, and 0.01% by mass or more and 0.5% by mass or less of Mn, and then using the annealing equipment described in any one of 1 to 7 above, hot-rolling the hot-rolled steel strip at a soaking temperature of 900°C or more and at an average cooling rate of 50°C / s or more in a temperature range of 750°C or less and 120°C or more during the cooling process, followed by a single cold rolling with a reduction of 89% or more to obtain a final sheet thickness, followed by decarburization annealing, and then applying an annealing separator to the surface of the steel strip before final annealing.

[0030] 9. The method for producing a grain-oriented electrical steel sheet according to 8 above, wherein the steel strip after hot rolling has a thickness of 2.8 mm or more.

[0031] 10. The method for producing a grain-oriented electrical steel sheet according to 8 or 9, wherein the time during which the steel strip temperature exceeds 120°C after the hot-rolled sheet annealing and before the cold rolling is 0 seconds or more and 120 seconds or less.

[0032] 11. The method for producing a grain-oriented electrical steel sheet according to any one of 8 to 10 above, wherein the heating rate in the primary recrystallization heating step of the decarburization annealing is 200°C / s or more in the temperature range of 550°C or more and 680°C or less. [Effects of the Invention]

[0033] According to the present invention, it is possible to stably produce grain-oriented electrical steel sheets having good magnetic properties. [Brief explanation of the drawings]

[0034] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of an annealing furnace facility. [Figure 2] FIG. 1 is a diagram showing the relationship between the random intensity ratio of the texture and the magnetic flux density and the reduction rate in cold rolling. [Figure 3] FIG. 1 is a diagram showing the relationship between magnetic flux density and reduction rate in cold rolling. DETAILED DESCRIPTION OF THE INVENTION

[0035] The method for producing grain-oriented electrical steel sheet according to the present invention will be specifically described below in the order of steps. In particular, for the hot-rolled sheet annealing step, the configuration of the annealing equipment according to the present invention that is applied to this step will also be described. Note that since the present invention is advantageously suited to the production of ordinary grain-oriented electrical steel sheet that utilizes solute C, other than the chemical composition of the steel material and production conditions shown below, those that conform to the production of ordinary grain-oriented electrical steel sheet can be adopted.

[0036] [Steel material composition] Regarding the composition of steel materials, the contents of the main components C, Si, and Mn are limited for the following reasons. Note that "%" in the composition means "% by mass" unless otherwise specified, and "ppm" means "ppm by mass." C: 0.01% or more and 0.10% or less In the present invention, C is an essential element for improving the primary recrystallization texture using solute C. However, if the C content exceeds 0.10%, it becomes extremely difficult to suppress the formation of coarse carbides, resulting in deterioration of the primary recrystallization texture. For this reason, the C content is limited to 0.10% or less. On the other hand, if the C content is less than 0.01%, the texture improvement effect of solute C cannot be expected. From the viewpoint of magnetic properties, the desirable C content is 0.02% or more and 0.06% or less.

[0037] Si: 2.0% or more and 4.5% or less Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, a Si content of at least 2.0% is necessary. On the other hand, Si also increases the brittleness of steel. If the Si content exceeds 4.5%, the risk of breakage during sheet threading through equipment increases, and cold rolling properties also deteriorate significantly. Therefore, the Si content is limited to 4.5% or less. Since the application of the present invention can reduce the risk during sheet threading through equipment, the desirable Si content is 2.8% or more and 4.5% or less.

[0038] Mn: 0.01% or more and 0.50% or less Mn has the effect of improving hot workability during manufacturing, but if the Mn content exceeds 0.50%, the primary recrystallization texture deteriorates, leading to deterioration of magnetic properties. Therefore, the Mn content is limited to 0.50% or less. Preferably, the Mn content is 0.10% or less. Mn is also a useful element from the perspective of controlling oxide film formation during primary recrystallization, so the content is set to 0.01% or more. If the Mn content is less than 0.01%, no effect can be obtained from the perspective of improving hot workability or controlling oxide film formation.

[0039] The remainder of the above basic components is Fe and unavoidable impurities. In addition to the above basic components, inhibitor-forming components may also be contained. Other typical compositions are as follows: The present invention is applicable to any compositional system that can produce grain-oriented electrical steel sheets by sequentially performing the well-known steps of manufacturing electrical steel sheets, namely, hot rolling, hot-rolled sheet annealing, cold rolling to the final sheet thickness in one pass, decarburization annealing (which also serves as primary recrystallization annealing), and final finish annealing (which also serves as secondary recrystallization annealing and purification annealing). Therefore, a composition that utilizes an inhibitor element to develop secondary recrystallized grains can be adopted. Alternatively, it is also possible to develop secondary recrystallized grains without using a precipitation-type inhibitor (e.g., AlN, MnS, MnSe). Below, the preferred contents of the inhibitor elements are explained according to their types.

[0040] [When using inhibitor components] Sol.Al: 0.01% or more and 0.05% or less, N: 0.004% or more and 0.012% or less If the Sol.Al content is less than 0.01%, the magnetic flux density decreases, while if it exceeds 0.05%, the secondary recrystallization becomes unstable. Therefore, the Sol.Al content is preferably in the range of 0.01% to 0.05%.

[0041] N: 0.004% or more and 0.012% or less If the N content is less than 0.004%, AlN does not precipitate properly during the manufacturing process, making it difficult to control the grain size. Furthermore, if the N content exceeds 0.012%, it can cause frequent surface defects known as blisters. Therefore, the N content is preferably in the range of 0.004% to 0.012%. The N content can be adjusted as needed by applying a nitriding process during manufacturing, so in many cases, sufficient precipitates can be formed with a N content of 0.010% or less.

[0042] Se and / or S (total): 0.01% to 0.05% If the total content of Se and S is less than 0.01%, the absolute amount will be insufficient as an inhibitor component, while if it exceeds 0.05%, it will make purification during final annealing difficult. Therefore, the total content of these elements is preferably in the range of 0.01% to 0.05%. S and Se can be used as inhibitors for MnS and MnSe, respectively, or as a composite of these, Mn(S,Se). An AlN-based inhibitor and an MnSe and / or MnS-based inhibitor can coexist, thereby obtaining a synergistic effect.

[0043] [When no precipitate-type inhibitor components are included] In this case, the contents of Al, N, Se, and S, which are precipitation inhibitor-forming elements, are limited to extremely low values. Specifically, Al is limited to less than 100 ppm, S is limited to 50 ppm or less, and Se is limited to 50 ppm or less. If these amounts are exceeded, it becomes difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition. It is desirable to keep the N content at 50 ppm or less to prevent the formation of Si nitrides after purification annealing. It is also desirable to reduce the contents of the nitride-forming elements Ti, Nb, B, Ta, and V to 50 ppm or less each. This is to prevent deterioration of iron loss without interfering with the effect of texture inhibition.

[0044] The contents and limitations of the inhibitor components are as described above, but the use of grain boundary segregating elements in addition to these can improve properties. Elements such as Cr, Cu, Sn, Sb, Mo, Te, Bi, Pb, Zn, Ge, As, P, In, and Ag can be added in the range of 0.001% to 0.300% each. These elements can be used alone or in combination, which can improve core loss. Ni can also be added in the range of 0.005% to 1.50% to improve the hot-rolled sheet structure and magnetic properties.

[0045] In particular, in steel types that do not use inhibitors, the effect of texture on properties becomes significant, so it is preferable to apply the present invention to steel types that do not contain inhibitor components.

[0046] The starting material slab having the above composition is slab-heated at an appropriate temperature according to the above-mentioned composition system, and then hot-rolled, including rough rolling and finish rolling, to produce a hot-rolled sheet. Regarding the slab heating temperature, if the slab contains precipitation-type inhibitor components, it is preferable to heat it to a temperature in the range of 1350°C to 1450°C inclusive in order to completely dissolve Al, Se, S, etc. On the other hand, if the composition system does not contain precipitation-type inhibitor components, if the slab heating temperature is too high, the inhibitor-forming components dissolved during heating will precipitate non-uniformly and finely during hot rolling. As a result, grain boundary migration is locally suppressed, resulting in a highly non-uniform grain size distribution and the inhibition of the development of secondary recrystallized grains in the Goss orientation. Therefore, it is preferable to use a relatively low heating temperature, for example, 1250°C or less.

[0047] Furthermore, there is no need to impose any particular restrictions on the hot rolling conditions; the hot rolling can be carried out under the usual conditions used for the production of grain-oriented electrical steel sheets. The resulting hot-rolled steel strip must have a thickness appropriate to achieve a reduction ratio of 89% or more in the subsequent cold rolling. Grain-oriented electrical steel sheets are available in thicknesses of 0.30 mm and 0.35 mm, and the steel strip is rolled to a thickness (e.g., 0.29 mm) that takes into account the thickness of the forsterite film and insulating coating formed on the surface. Therefore, considering the thicker thickness specifications, the thickness after hot rolling must be 2.8 mm or more. Note that a thicker thickness requires a higher cooling capacity and increases the rolling load significantly, so the upper limit is preferably 4.0 mm.

[0048] The hot-rolled steel strip thus obtained is subjected to hot-rolled sheet annealing. That is, it is important to perform hot-rolled sheet annealing at a soaking temperature of 900°C or higher and at an average cooling rate of 50°C / s or higher in the temperature range of 750°C or lower and 120°C or higher during the cooling process.

[0049] The reason why the soaking temperature is set to 900°C or higher is that it is necessary to recrystallize and homogenize the worked structure formed by hot rolling. The soaking time does not need to be particularly limited, but from the viewpoint of causing the recrystallization to occur to a certain extent, it is preferably set to 30 seconds or more and 180 seconds or less.

[0050] Next, the steel strip after soaking is cooled. In this cooling process, the average cooling rate in the temperature range of 750°C or less and 120°C or more is set to 50°C / s or more. If this average cooling rate is less than 50°C / s, the {110} <001> This is because coarse carbides precipitate, which is thought to affect grain reduction.

[0051] For the above-mentioned hot-rolled sheet annealing, it is essential to achieve a high reduction rate in cold rolling by using annealing equipment with the following specifications. That is, the annealing equipment to be applied is an annealing equipment having, in order from the upstream side of a conveying line for a hot-rolled steel strip, a heating zone, a soaking zone, and a cooling zone, the conveying line being capable of passing steel strips having a thickness of 2.8 mm or more, the soaking zone having a means for maintaining an atmospheric temperature of 900°C or more, the cooling zone having a means for supplying a medium (coolant) to the steel strip to achieve an average cooling rate of 50°C / s or more in the temperature range of 750°C or less and 120°C or more, and having a means for removing the medium (coolant) on the outlet side of the cooling zone.

[0052] Specifically, as shown in Fig. 1, the system comprises a payoff reel 2 around which a hot-rolled steel strip (hereinafter simply referred to as steel strip) 1 is wound, a conveying line 3 along which the steel strip 1 unwound from the payoff reel 2 passes, a trimmer 4 that trims the edge of the steel strip 1, an entry looper 5 that applies tension to the steel strip 1 on the conveying line 3, an annealing furnace 6 having a heating zone and a soaking zone, a cooling zone 7 that cools the steel strip 1 after it leaves the annealing furnace 6, a means 8 for removing the coolant supplied to the steel strip 1 in the cooling zone 7, and a take-up reel 9. Each of the above components will be described in detail below.

[0053] [Conveyor line] As described above, in the present invention, it is necessary to accept thick steel strips of 2.8 mm or more into the annealing equipment, and therefore it is essential that the conveying line 3 has the performance to be able to pass thick steel strips of 2.8 mm or more without any problems. Specific means to enable this passing include providing a mechanism that can increase the line tension during passing, since the rigidity of steel strips increases as the steel strip becomes thicker, or welding the steel strips to increase the strength of the welds when passing them continuously. Among these, the following passing means A to C are appropriate.

[0054] (Threading method A) A trimmer for trimming the edge of the steel strip 1 is provided between the payoff reel 2 and the entry looper 5. In other words, by trimming the edge of the steel strip 1, stress concentration points such as edge cracks formed during hot rolling can be removed, making it possible to thread steel strips with a thickness of 2.8 mm or more.

[0055] (Threading method B) Steel strip 1 passing through conveyor line 3 undergoes various processes, including looper 5, annealing furnace 6, and cooling zone 7, where its direction of travel is changed between vertical and horizontal directions. This change in direction is typically achieved by changing the winding angle around the conveyor rolls. During this process, the steel strip is repeatedly bent by the conveyor rolls. This bending process applies compressive stress on the inside of the bend and tensile stress on the outside. This tendency becomes more pronounced as the plate thickness increases, inevitably increasing the risk of breakage within the equipment for thicker steel plates. Because the stress generated during bending is affected by the bending radius, larger conveyor roll diameters can reduce the stress generated. To avoid breakage of steel strip 1 during threading, a conveyor roll diameter of 950 mm or greater is advantageous. Therefore, by using conveyor rolls with a diameter of 950 mm or greater, conveyor line 3 can be equipped to handle steel strips with thicknesses of 2.8 mm or greater. The diameter of the transport roll is more preferably 1100 mmΦ or more, and even more preferably 1350 mmΦ or more.

[0056] (Threading means C) A device for heating the steel strip 1 to 70°C or higher is provided on the entry side of the entry looper 5. Since the higher the temperature of a steel strip, the more ductile it becomes, so maintaining a higher temperature reduces the risk of breakage. In particular, since the steel strip 1 typically enters the entry side of an annealing facility after its temperature has been lowered to room temperature, it is advantageous to provide a preheating device or a heating device between the discharge and the looper 5 to increase the steel strip temperature. Maintaining this steel strip temperature at 70°C or higher significantly reduces the risk of breakage. The heating device is not particularly limited as long as it can efficiently heat the steel strip. For example, a coiled steel strip can be charged into a gas furnace, or the steel strip discharged from the coil can be heated using an induction heating (IH) device or the like. If it is difficult to heat the entire width of the steel strip, the above-mentioned effect can be achieved by simply heating the widthwise edge of the steel strip.

[0057] [Heating Zone] The heating zone constituting the front half region of the annealing furnace performs heating so as to maintain the atmospheric temperature of the soaking zone constituting the rear half region of the annealing furnace at 900°C or higher.

[0058] [Solid temperature] The soaking zone has a means for maintaining the atmospheric temperature of the soaking zone at 900°C or higher. Specifically, it is preferable to have a heating means such as an electric heater or a gas burner. Furthermore, since a large amount of energy is required to heat a thick steel strip, it is also effective to use a heating means such as IH (electromagnetic induction heating).

[0059] [Cooling Zone] The cooling zone 7 has a means for supplying a coolant to the steel strip 1 after it has left the soaking zone of the annealing furnace 6, thereby achieving an average cooling rate of 50°C / s or more in the temperature range of 120°C or more and 750°C or less. Specifically, it is preferable to have a means for spraying the coolant toward the steel strip. Furthermore, it is preferable to use cooling water maintained at a constant temperature of 80°C or less as the coolant. This is because, although cooling water is generally recycled in cooling water supply facilities, the temperature of cooling water increases once it has been used. Here, in order to achieve an average cooling rate of 50°C / s or more and to maintain a substantially constant cooling rate within the steel strip coil, it is necessary to maintain a constant temperature of the cooling water.

[0060] [Removal means] The cooling zone 7 has a removal means 8 for reliably removing the refrigerant remaining on the surface of the steel strip 1. To reliably remove the refrigerant, it is preferable to use two or more means with different removal mechanisms for the refrigerant (e.g., cooling water) remaining on the steel strip 1. Specifically, it is preferable to use both a direct removal means and an indirect removal means. Here, direct removal means include a wringer roll and a wiper blade. In addition, indirect removal means includes spraying dry gas at a temperature not exceeding 150°C.

[0061] Furthermore, it is preferable to have a means for measuring the temperature of the steel strip and maintaining it at a predetermined temperature on the outlet side of the removal means 8. That is, it is preferable that the steel strip is not maintained at a temperature exceeding 120°C until cold rolling is performed after the cooling, in order to prevent the formation of carbides from progressing. Therefore, it is recommended to provide a means for measuring the temperature on the outlet side of the removal means 8 and maintaining the temperature in a range of 120°C or less depending on the result of the temperature measurement.

[0062] Incidentally, if cold rolling at a temperature above 120°C is required to improve cold rolling properties, it is recommended to perform cold rolling within 120 seconds after removing the refrigerant and then heating the steel strip held at a low temperature. This takes advantage of the fact that when the steel strip temperature is lowered to near room temperature and then raised again, there is generally an incubation period before nucleation occurs, and therefore the steel does not immediately transition to carbide formation. In other words, warm rolling can also be used to improve the texture.

[0063] In the present invention, the cold rolling step after the above-mentioned hot-rolled sheet annealing is carried out by a single cold rolling step with a rolling reduction of 89% or more, rather than by two cold rolling steps with an intermediate annealing step at a temperature exceeding 700°C. The cold rolling conditions are not particularly limited and can be carried out under any conditions. Incidentally, if the rolling reduction in cold rolling is less than 89%, the above-mentioned hot-rolled sheet annealing before cold rolling is not necessary. When the hot-rolled sheet annealing before cold rolling is carried out appropriately as described above according to the present invention, the {110} <001> However, if the C content in the slab exceeds 0.05%, the {110} orientation will be reduced if the rolling reduction exceeds 95%. <001> Poor secondary recrystallization occurs due to a decrease in grain orientation.

[0064] The present invention is not limited to thick strips, but can also be applied to thin grain-oriented electrical steel sheets with a final thickness of 0.23 mm, etc. In this case, the thickness of the hot-rolled steel sheet used as the mother sheet does not need to be 2.8 mm or more, but since the cooling stop temperature is set to 120°C or less, it is essential to treat the refrigerant remaining on the steel strip after cooling and to manage the temperature of the steel strip in order to ensure stable production.

[0065] The final cold-rolled sheet is then subjected to primary recrystallization annealing. The purpose of this primary recrystallization annealing is to primarily recrystallize the cold-rolled sheet having a rolled texture, adjust the primary recrystallized grain size to an optimal size for secondary recrystallization, and decarburize the carbon contained in the steel by using a wet hydrogen-nitrogen or wet hydrogen-argon annealing atmosphere. At the same time, the oxidizing atmosphere is used to form an oxide film on the surface. For this reason, primary recrystallization annealing is preferably performed at a temperature of 750°C to 900°C in a H2-mixed atmosphere with a dew point. During the primary recrystallization annealing, it is preferable to set the heating rate in the temperature range of 550°C to 680°C to 200°C / s or more, as this further enhances the texture improvement effect.

[0066] An annealing separator is applied to the surface of the above-mentioned primary recrystallization annealed steel sheet. Magnesia (MgO) is used as the main component of the annealing separator to form a forsterite film on the steel sheet surface after secondary recrystallization annealing. In this case, adding an appropriate amount of Ti oxide or Sr compound to the separator can further enhance the formation of the forsterite film. In particular, adding an auxiliary agent that promotes uniform forsterite film formation is also advantageous for improving release properties.

[0067] Following the above process, final annealing is performed to allow for secondary recrystallization and the formation of a forsterite film. N2, Ar, H2, or a mixture of these gases is suitable for the annealing atmosphere. To facilitate secondary recrystallization, the material can be isothermally maintained near the secondary recrystallization temperature. However, isothermal maintenance is not always necessary, as slower heating rates can also be effective. Because the precipitation of trace elements in the final product can lead to a deterioration of magnetic properties, it is preferable to set the maximum annealing temperature to 1100°C or higher to purify the elements.

[0068] 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 insulating coating is not particularly limited, and any conventionally known insulating coating is suitable. For example, a method described in Japanese Patent Laid-Open Nos. 50-79442 and 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. Furthermore, the flattening annealing can also adjust the shape of the steel sheet, and further, this flattening annealing can also serve as a baking treatment for the insulating coating. [Example]

[0069] The inhibitor-free steel slabs, consisting of 0.03% C, 3.4% Si, 0.07% Mn, less than 50 ppm S, Se, and O, and less than 60 ppm N, with the remainder consisting of unavoidable impurities, were heated to 1220°C and hot-rolled to a thickness of 1.8 to 3.5 mm. The annealing equipment according to the present invention was then applied. After soaking at 1030°C for 30 seconds, one slab (the conventional example) was cooled to a cooling stop temperature of 200°C, as in the conventional example, and residual cooling water remaining on the steel sheet was removed by the heat of the steel sheet. The other slab (the inventive example) was cooled to a cooling stop temperature of 120°C, and residual cooling water remaining on the steel sheet was removed by a cooling water removal means (a wringer roll). The cooling rate from 800°C to the cooling stop temperature was adjusted to a constant 80°C / s. The slabs were then pickled at 80°C to remove scale.

[0070] The steel strips, which had been pickled after the hot rolling and annealing, were then cold-rolled (reduction: 83.6-91.6%) to a final thickness of 0.295 mm. Subsequently, primary recrystallization annealing was performed in the temperature range of 550°C to 680°C, with a heating rate of 250°C / s, a soaking temperature of 800°C, and a soaking time of 30 seconds. Test specimens were cut from the width and length centers of the resulting steel strip coil after primary recrystallization, and the texture was measured by X-ray diffraction. The texture was measured by measuring three pole figures, creating ODFs using the ADC method, and then extracting the {110} <001> The random intensity ratio of the orientation was measured. An annealing separator consisting of 95% MgO and 5% TiO2 was then applied to the steel sheet surface as a water slurry, and the steel sheet was subjected to secondary recrystallization annealing. A coating solution containing phosphate, chromate, and colloidal silica in a weight ratio of 3:1:3 was applied to the surface of the final annealed sheet, and the sheet was baked at 800°C.

[0071] The magnetic properties were investigated at the width center of the product sheet coil obtained in this way. After stress relief annealing at 800°C for 3 hours, a 30 mm x 280 mm test piece with a total weight of 500 g or more was cut out from a position corresponding to the outer winding of the coil during secondary recrystallization annealing, and B8 was measured using the Epstein test specified in JIS C2550. The results of the measurements of the random intensity ratio of the texture and the magnetic flux density are shown in Fig. 2 in relation to the reduction rate in cold rolling.

[0072] As shown in FIG. 2, in the manufacturing method according to the present invention, even at a high pressure reduction rate of 89% or more, the {110} <001> It was confirmed that the reduction in orientation was suppressed and the magnetic properties of the grain-oriented electrical steel sheet were improved. On the other hand, in the conventional example where the cooling stop temperature after soaking was set to 200°C, a significant decrease in magnetic flux density occurred when the high pressure reduction rate was 89% or more. It was. [Example]

[0073] A steel slab containing 0.04% C, 3.3% Si, and 0.05% Mn, as well as the other components shown in Table 1, was heated to 1200°C and hot-rolled to form a hot-rolled steel strip. The slab was then subjected to soaking treatment at 1000°C for 60 seconds in an annealing facility according to the present invention, followed by hot-rolled annealing under the cooling conditions and thermal history shown in Table 1, followed by warm rolling using a tandem rolling mill. The initial thickness of the hot-rolled steel strip, as well as the final thickness and rolling reduction, were as shown in Table 1. The slab was then subjected to primary recrystallization annealing at 850°C for 40 seconds, with a heating rate of 200°C / s in the temperature range of 400°C to 700°C. Next, an annealing separator mainly composed of MgO was applied to the steel sheet, followed by secondary recrystallization annealing.

[0074] The resulting secondary recrystallization-annealed sheet was coated with a coating solution containing phosphate, chromate, and colloidal silica in a weight ratio of 3:1:2, and then subjected to flattening annealing at 850°C for 30 seconds. After that, 30 mm x 280 mm test pieces were cut from the position corresponding to the outer coil winding during secondary recrystallization annealing, so that the total weight was 500 g or more. The B8 of these test pieces was measured using the Epstein test specified in JIS C2550. The relationship between the magnetic flux density obtained and each experimental condition is shown in Table 1.

[0075] As is clear from Table 1, under the conditions according to the present invention, better magnetic properties are obtained when the reduction ratio is 89% or more than when it is less than 89%. In particular, when a high reduction ratio is applied, good magnetic properties are obtained by satisfying the conditions of the present invention. For comparison, when a steel sheet was produced under the same conditions as No. 3 in Table 1 except for the soaking temperature of 890°C, the desired texture was not obtained in the secondary recrystallization annealed steel sheet, and the magnetic flux density B8 was confirmed to be less than 1.85 T.

[0076] [Table 1] [Example]

[0077] A steel slab containing 0.05% C, 3.4% Si, 0.08% Mn, 0.015% Al, 0.006% N, 0.003% S, and 0.012% Se was heated to 1400°C and hot-rolled to a 2.6 mm thick hot-rolled steel strip. The annealing equipment of the present invention was then applied, and the steel strip was subjected to a soaking treatment at 1080°C for 60 seconds. The cooling stop temperature was then 80°C, and the remaining cooling water on the steel strip was removed using a cooling water removal device (a wringer roll). The cooling rate from 850°C to the cooling stop temperature during cooling was adjusted to a constant 60°C / s. After the above hot-rolled sheet annealing, pickling was performed at 80°C to remove scale. The pickled steel strip was then cold-rolled. The final thickness and rolling reduction were varied. The obtained cold-rolled sheet was subjected to decarburization annealing in wet hydrogen at 840°C for 2 minutes, after which MgO containing 5% TiO2 by mass was applied as an annealing separator, and then final annealing was performed at 1200°C for 10 hours.

[0078] Several 30mm x 280mm test pieces were cut out from different positions in the width direction from the outermost winding of the obtained coil, and the magnetic flux density was evaluated using a single sheet magnetic measuring device. Figure 3 shows the relationship between the average magnetic flux density obtained and the rolling reduction. As is clear from Figure 3, under the conditions of the invention, good magnetic properties are maintained even at high reduction rates. [Industrial Applicability]

[0079] The annealing equipment of the present invention can be advantageously applied to texture control using solute C and to manufacturing processes that require high pressures for manufacturing purposes. [Explanation of symbols]

[0080] 1 Hot rolled steel strip 2 Payoff Reel 3. Conveyor line 4. Trimmer 5 Inlet looper 6 Annealing furnace 7 Cooling Zone 8 Removal means 9 Take-up reel

Claims

1. An annealing facility used in a method for manufacturing grain-oriented electrical steel sheets, comprising: The method for producing the grain-oriented electrical steel sheet includes hot rolling a steel material containing C: 0.01% by mass or more and 0.10% by mass or less, Si: 2.0% by mass or more and 4.5% by mass or less, and Mn: 0.01% by mass or more and 0.50% by mass or less, and hot-rolling the steel strip after the hot rolling at a soaking temperature of 900°C or more and at an average cooling rate of 50°C / s or more in a temperature range of 750°C or less and 120°C or more during a cooling process, followed by a single cold rolling at a reduction of 89% or more to obtain a final sheet thickness, followed by decarburization annealing, and then applying an annealing separator to the surface of the steel strip, followed by final annealing, The annealing equipment has a heating zone, a soaking zone, and a cooling zone on a conveying line for the steel strip, the conveying line is capable of passing the steel strip having a thickness of 2.8 mm or more, the soaking zone has a means for maintaining an ambient temperature of 900°C or more, the cooling zone has a means for supplying a coolant to the steel strip to achieve an average cooling rate of 50°C / s or more in a temperature range of 750°C or less and 120°C or more, and has a means for removing the coolant on the outlet side of the cooling zone.

2. 2. The annealing equipment according to claim 1, further comprising a payoff reel at the start end of the conveying line for unwinding the steel strip onto the conveying line and an entry looper for applying tension to the steel strip on the conveying line, and a trimmer between the payoff reel and the entry looper for trimming the edge portion of the steel strip.

3. 3. The annealing facility according to claim 1, further comprising a transport roll for changing the direction of travel of the steel strip passing through the transport line, the transport roll having a diameter of 950 mm or more.

4. 3. The annealing facility according to claim 2, further comprising a means for heating the steel strip to 70°C or higher on the entry side of the entry looper.

5. 3. The annealing equipment according to claim 1, wherein the coolant is cooling water at 80°C or less, and the removal means comprises two or more means having different mechanisms for removing the cooling water remaining on the steel strip.

6. 6. The annealing facility according to claim 5, wherein the removing means is both a means for directly removing and a means for indirectly removing cooling water remaining on the steel strip.

7. 3. The annealing facility according to claim 1, further comprising a means for measuring the temperature of said steel strip and maintaining said steel strip at a predetermined temperature, at the outlet side of said removing means.

8. 1. A method for producing a grain-oriented electrical steel sheet, comprising: hot rolling a steel material containing C: 0.01% by mass or more and 0.10% by mass or less, Si: 2.0% by mass or more and 4.5% by mass or less, and Mn: 0.01% by mass or more and 0.50% by mass or less; and hot-rolling the hot-rolled steel strip using the annealing equipment according to claim 1, wherein the hot-rolled steel strip is annealed at a soaking temperature of 900°C or more and at an average cooling rate of 50°C / s or more in a temperature range of 750°C or less and 120°C or more during a cooling process; and then cold rolling the steel strip once at a reduction rate of 89% or more to a final thickness, followed by decarburization annealing, and then applying an annealing separator to the surface of the steel strip, followed by final annealing.

9. The method for producing a grain-oriented electrical steel sheet according to claim 8, wherein the steel strip after hot rolling has a thickness of 2.8 mm or more.

10. 10. The method for producing a grain-oriented electrical steel sheet according to claim 8, wherein the time during which the steel strip temperature exceeds 120°C after the hot-rolled sheet annealing and before the cold rolling is 0 seconds or more and 120 seconds or less.

11. 10. The method for producing a grain-oriented electrical steel sheet according to claim 8, wherein the heating rate in the primary recrystallization heating step of the decarburization annealing is 200°C / s or more in a temperature range of 550°C or more and 680°C or less.

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