Manufacturing method and equipment for oriented electromagnetic steel sheets

Abstract

This invention provides a method for manufacturing oriented electromagnetic steel sheets that significantly improves manufacturability, and an apparatus for implementing the method. The method for manufacturing oriented electromagnetic steel sheets comprises the following series of steps: hot rolling a steel billet having a specified composition; hot-rolled sheet annealing; cold rolling once or twice with a total reduction rate of 80% or more from the thickness of the obtained hot-rolled sheet to the thickness of the cold-rolled product; recrystallization annealing; coating the steel sheet surface with an annealing separating agent; final product annealing; and planarization annealing for planarization. In this method for manufacturing oriented electromagnetic steel sheets, the grain boundary occupancy of recrystallized carbides in the hot-rolled sheet after annealing and before cold rolling is 80% or more, and the initial reduction during cold rolling is set to a strain rate of 200 / s or less, a reduction rate of 30% or less, and a steel sheet temperature of 90°C or less when biting into the rolls.

Description

Technical Field

[0001] This invention relates to a method and equipment for manufacturing oriented electromagnetic steel sheets. Background Technology

[0002] Oriented magnetic steel sheets are steel sheets with excellent magnetic properties, characterized by a highly concentrated crystalline structure (Gaussian orientation) of the iron's easy magnetization axis, i.e., the <001> orientation, along the rolling direction of the steel sheet. As one method to improve the magnetic properties of oriented magnetic steel sheets, a method has been proposed to control the morphology of carbon (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 annealed steel sheets to rapid cooling and aging treatment under specific conditions. Additionally, Patent Document 2 proposes a technique for increasing the amount of C in solid solution by cooling annealed steel sheets at a cooling rate of 150 °C / min or higher in a temperature range of 600–300 °C.

[0004] The technology proposed in Patent Documents 1 and 2 controls the carbon in steel into extremely fine carbides or solid solution C, and introduces it into dislocations during cold rolling. This causes the solid solution C to be fixed to the dislocations and form a Cottrell atmosphere, thereby promoting uneven deformation during cold rolling, modifying the cold rolling texture, and improving the texture after primary recrystallization.

[0005] This effect is well-known as a method to increase the {110} strength in the texture of steel after recrystallization following cold rolling and annealing. In grain-oriented electromagnetic steel sheets, the final aggregates are concentrated in the {110}<001> orientation using a metallurgical effect known as secondary recrystallization, but at this point, the {110} structure can function as a good nucleus for secondary recrystallization. Therefore, the technique of forming carbides within the grains is very common in grain-oriented electromagnetic steel sheets.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 58-157917

[0009] Patent Document 2: Japanese Patent Application Publication No. 52-094825 Summary of the Invention

[0010] However, in recent years, due to the necessity of energy conservation, there has been a demand for electromagnetic steel sheets with lower iron loss. In fact, products that meet this demand can be manufactured by thinning the steel sheet or refining the magnetic domains.

[0011] It is known that silicon (Si) added to steel increases its electrical resistance, significantly improving iron loss by reducing Joule heating during the use of electromagnetic steel sheets. As a result, electromagnetic steel sheets containing a large amount of Si can achieve good iron loss. However, Si is also known to be an element that embrittles steel, and it is generally very difficult to roll steel containing more than 4.0% by mass.

[0012] Generally speaking, steels with higher alloy content tend to have higher strength, but are more difficult to roll. Furthermore, as mentioned above, because silicon contributes to the embrittlement of the billet, even a content of only a few percent by mass can sometimes cause fracture problems. Therefore, various designs are typically employed in the manufacture of electromagnetic steel sheets: utilizing high-rigidity rolling mills, using reversible rolling mills instead of continuous rolling mills, or employing warm rolling at high temperatures that soften the material.

[0013] On the other hand, in order to make manufacturing more flexible, it is desirable to be able to manufacture not only using rolling mills suitable for electromagnetic steel sheets, but also using various rolling mills.

[0014] Therefore, from the viewpoint of reducing the load on the rolling mill or rolls during rolling, the inventors conducted research on reducing the reduction rate per pass. Unexpectedly, they encountered a problem: fractures were prone to occur at slower rolling speeds.

[0015] The present invention was made to solve the above-mentioned problems, and its object is to provide a method for manufacturing oriented electromagnetic steel sheets that can significantly improve manufacturability and a manufacturing apparatus for oriented electromagnetic steel sheets that can implement the method.

[0016] To address the aforementioned problem, the inventors first conducted a study based on the analysis of materials that fractured after the initial hot rolling pass. The results confirmed that multiple deformation twins formed in the fractured material, and that these twins influenced adjacent crystals by propagating along grain boundaries.

[0017] It is known that twins, through their intersection with other twins or their interaction with dislocations, are a cause of material embrittlement. Therefore, the inventors believe that suppressing the formation of such twins is particularly effective in suppressing fracture.

[0018] Typically, twins form when the atoms constituting the material become mirror objects with the twin plane as their reference point. Therefore, introducing dislocations into the crystal makes them difficult to form, especially under conditions of significant lattice strain. Under fracture conditions, low rolling pressure is set to reduce rolling load, and a slower rolling speed is adopted to improve mill stability during plate handling. Because of these rolling conditions, the inventors deduced that many deformation twins may have already formed before plastic deformation caused by dislocations occurs. However, these rolling conditions are designed to be suitable not only for mills designed for manufacturing electromagnetic steel sheets but also for many commonly used mills. Therefore, methods that increase the reduction rate or set high rolling speeds, which would increase the burden on the mill, are not ideal countermeasures.

[0019] Therefore, the inventors realized that it was difficult to completely suppress twin formation, and thus established a policy to reduce the amount of twins generated, for example, by preventing twins from propagating to adjacent crystals even if they were formed. As a result, the inventors discovered that since twin propagation occurs across grain boundaries, they conceived of a method to precipitate carbides at grain boundaries as a barrier to twin propagation. The experimental details leading to this understanding are described below.

[0020] <Experiment 1>

[0021] The following experiment was conducted: A steel billet for oriented electromagnetic steel sheet (hereinafter referred to as "the billet") was prepared with the following composition: C: 0.03%, Si: 4.2%, Mn: 0.1%, sol.Al: 0.02% by mass, S: 50 ppm, Se: 100 ppm, N: 60 ppm, with the remainder being Fe and other elements reduced to less than 60 ppm. The prepared billet was heated at 1380°C and then hot-rolled to obtain a hot-rolled coil with a thickness of 2.5 mm (hereinafter also referred to as "hot-rolled coil"). Test pieces were cut from the obtained hot-rolled coil and annealed at a temperature of 980°C using a laboratory furnace, with controlled cooling after annealing.

[0022] First, referring to Non-Patent Literature 1 (New Edition of Steel Materials and Alloy Elements p.395), the conditions for the precipitation of carbides (Fe3C) at grain boundaries are as follows: the cooling rate in the temperature range of 600°C to 700°C is 1.5°C / second, and the residence time in the above temperature range is 1 minute or more.

[0023] A portion of the annealed hot-rolled sheet was cut to allow observation of a cross-section perpendicular to the rolling direction. After etching with a nitric acid-ethanol solution, the center of the sheet was continuously observed using a scanning electron microscope (SEM) at a depth of 500 μm along the thickness direction and 1 mm perpendicular to the rolling direction (slab width direction). The results confirmed that 85% of the grain boundaries in the observed field of view were precipitated with carbides. During etching with the nitric acid-ethanol solution, the steel portion was etched, but the carbides remained unetched. Therefore, in the case of carbides forming at the grain boundaries, the carbides extended into a film-like structure were observed with a contrast different from the steel substrate. Furthermore, it was found that a high-resolution electron probe microanalysis (EPMA) instrument could be used to analyze the same extended region, determine the enrichment state of carbon at the grain boundaries, and quantify the degree to which the entire grain boundary length in the observed field of view was covered by carbides. Additionally, during the cooling process after annealing the hot-rolled sheet, almost no carbides precipitated in the temperature range from the annealing temperature to above 700°C. Therefore, the cooling conditions were changed by altering the residence time in the temperature region below 700°C and above 600°C after cooling at an arbitrary cooling rate (e.g., 20°C / second) in the annealing temperature range of hot-rolled steel up to 700°C, and by maintaining a constant cooling rate in the temperature region below 700°C and above 600°C. Then, the cooling rate in the temperature region below 600°C was set to 50°C / second. The resulting steel sheet was evaluated for the carbides Fe3C precipitated at the grain boundaries, and the samples shown in Table 1 were obtained. Table 1 shows the relationship between the residence time in the temperature region below 700°C and above 600°C and the grain boundary occupancy of the carbides.

[0024]

[0025] <Experiment 2>

[0026] A steel billet with the following composition (by mass%) was prepared: C: 0.02%, Si: 4.8%, Mn: 0.3%, sol.Al: 0.005%, with the remainder being Fe and other elements, with S, N, Se, and O impurities reduced to below 50 ppm. The prepared steel billet was heated to 1150°C and then hot-rolled to obtain a hot-rolled coil with a thickness of 2.5 mm. Test pieces were cut from the obtained hot-rolled coil and subjected to hot-rolled plate annealing at 990°C using a laboratory furnace. Rapid cooling at 50°C / second was applied to the temperature range between 700°C and 600°C. Then, the cooling rate in the temperature range between 600°C and 500°C after annealing was varied, and the hot-rolled plate annealing was controlled by changing the residence time in the temperature range between 600°C and 500°C. It should be noted that after annealing the hot-rolled plate, since carbides do not precipitate at grain boundaries in the temperature range above 700°C, any cooling mode can be used, but cooling is performed at 30°C / second. After the dwell time in the temperature range between 600°C and 500°C, rapid cooling is performed at 60°C / second. The grain boundary occupancy of the annealed carbides is quantitatively evaluated through the above-mentioned SEM observation. Table 2 shows the relationship between the dwell time in the temperature range between 600°C and 500°C and the grain boundary occupancy of the carbides. Comparing the results of Table 1 and Table 2, it can be seen that, as described in Non-Patent Literature 1, grain boundary carbides are more likely to precipitate when the temperature range between 600°C and 500°C is maintained. Carbide precipitation also occurs at temperatures above 600°C, but considering that cooling for more than 1 minute is impractical in actual machine manufacturing, it can be said that controlling the dwell time, especially in the temperature range between 600°C and 500°C, is very important.

[0027]

[0028] The obtained samples were subjected to cold rolling in the first pass at a reduction rate of 20% and a strain rate of 150 m / s, followed by multiple rolling passes to produce a sheet thickness of 1.0 mm. While the sheet did not completely fracture, some samples developed cracks, confirming the existence of a certain probability of crack defects. Here, for the experimental conditions, the actual number of samples rolled is used as the overall parameter, and the crack initiation rate is calculated based on the number of samples exhibiting crack defects. Figure 1 The results are shown.

[0029] like Figure 1As shown, the inventors have obtained the following insights: Regarding the fracture problem arising during cold rolling under specific rolling conditions of low pressure and low strain rate, it is effective to maintain a carbide content of over 80% relative to the recrystallized grain boundaries within the steel sheet before cold rolling, regardless of the cooling mode. Furthermore, microstructure observation of billets that developed cracks under lower pressure also confirmed the formation of numerous deformation twins. However, if the reduction reaches a certain stage, it becomes difficult to capture changes in twin density because dislocations transform into a complex, interwoven processing structure.

[0030] The results are considered highly effective from a manufacturability perspective. On the other hand, carbide control for texture control is also crucial in the manufacture of grain-oriented electromagnetic steel sheets. If the grains are held at high temperatures with rapid diffusion for an extended period, grain boundary precipitation occurs, resulting in a significant decrease in the carbon concentration within the grains.

[0031] Therefore, the inventors conducted in-depth research on a method to retain carbon within the grains as much as possible and increase the carbide occupancy rate at the grain boundary by maintaining only the necessary time for nucleation at the temperature at which carbides form at the grain boundary, and by carrying out the precipitate growth stage at a low temperature with the slowest possible diffusion rate, thereby increasing the carbide occupancy rate at the grain boundary. This led to the completion of the present invention.

[0032] That is, the present invention that solves the above-mentioned problems is as follows.

[0033] [1] A method for manufacturing an oriented electromagnetic steel sheet, characterized by the following series of steps: hot rolling a steel billet containing C: 0.01%–0.10%, Si: 2.0%–6.5%, and Mn: 0.01%–0.5% by mass; hot rolling the billet; performing hot-rolled sheet annealing; performing one or more cold rolling cycles with a total reduction rate of 80% or more from the thickness of the obtained hot-rolled sheet to the thickness of the cold-rolled product; performing one recrystallization annealing; then coating the steel sheet surface with an annealing separating agent; and finally performing final product annealing and planarization annealing for planarization.

[0034] In the manufacturing method of this orientation-oriented electromagnetic steel sheet, the carbide content in the hot-rolled sheet after annealing and before cold rolling is 80% or more at the grain boundaries of the recrystallized grains, and the initial reduction of the cold rolling is set to a strain rate of 200 / second or less, a reduction rate of 30% or less, and a steel sheet temperature of 90°C or less when biting into the roll.

[0035] [2] In the method for manufacturing oriented electromagnetic steel sheet according to [1] above, one or more intermediate annealings are performed between the two or more cold rolling processes.

[0036] [3] In the method for manufacturing oriented electromagnetic steel sheet according to [1] or [2] above, during the cooling of the hot-rolled sheet after annealing, the dwell time in the temperature range of 500°C to 600°C is 10 seconds or more.

[0037] [4] According to the manufacturing method of the oriented electromagnetic steel sheet described in [1] or [2] above, during the cooling of the hot-rolled sheet after annealing, the dwell time in the temperature region between 600°C and 500°C is 3 seconds or more and less than 10 seconds, and the average cooling rate in the temperature region between 500°C and 200°C is 10°C / second or less, and the cooling is carried out at a cooling rate of 15°C / second or more until the coil is wound.

[0038] [5] The method for manufacturing an orientation-oriented electromagnetic steel sheet according to any one of [1] to [4] above, wherein the heating rate in the temperature range of 550°C to 680°C during the first recrystallization annealing is 200°C / second or more.

[0039] [6] The method for manufacturing an oriented electromagnetic steel sheet according to any one of [1] to [5] above, wherein the steel billet, in addition to the above composition, further contains sol.Al: 0.010% to 0.050%, N: 0.004% to 0.015%, and S+0.4Se: 0.010% to 0.050% by mass%.

[0040] [7] The method for manufacturing an orientation-oriented electromagnetic steel sheet according to any one of [1] to [5] above, wherein the steel billet, in addition to the above composition, further contains sol.Al: less than 0.010% by mass, and each of the elements S, N and O: less than 60 ppm.

[0041] [8] A method for manufacturing an orientation-oriented electromagnetic steel sheet according to any one of [1] to [7] above, wherein the steel billet, in addition to the above-mentioned composition, further contains, by mass %, an element selected from Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Mo: 0.01% to 0.50%, P: 0.0050% to 0.50%, Cr: 0.01% to 1.50%, Nb: 0.0005% to 0.00%. One or more of the following: 200%, B: 0.0005%–0.0200%, Te: 0.0005%–0.0200%, Co: 0.0001%–0.0100%, Ga: 0.0001%–0.0100%, Zn: 0.0001%–0.500%, Bi: 0.0005%–0.0200%, Pb: 0.001%–0.3%, Ge: 0.001%–0.3%, As: 0.001%–0.3%, and Ag: 0.001%–0.3%.

[0042] [9] A manufacturing apparatus for oriented electromagnetic steel sheet, which is used in manufacturing oriented electromagnetic steel sheet, comprises: multiple cooling zones for multi-stage cooling of steel strip that has reached a temperature of 700°C or below after annealing; a thermometer provided at least at one of the middle or outlet sides of each cooling zone for measuring the temperature of the steel strip; a control unit for controlling the cooling rate of each cooling zone by using the temperature measured by the thermometer for feedback control; and at least one cooling water removal unit provided between the cooling zones for removing the cooling water from the steel strip.

[0043] The equipment for manufacturing this oriented electromagnetic steel sheet is capable of winding the coil at temperatures below 100°C.

[0044]

[10] The manufacturing equipment for the oriented electromagnetic steel sheet according to [9] above, wherein the plurality of cooling zones include: a first cooling zone in which the residence time of the steel strip in a temperature region of 600°C to 500°C is controlled to be 3 seconds or more and less than 10 seconds, and a second cooling zone in which the residence time of the steel strip in a temperature region of 500°C to 200°C is controlled to be 30 seconds or more.

[0045] According to the present invention, a method for manufacturing oriented electromagnetic steel sheets that can significantly improve manufacturability and an apparatus for implementing the method can be provided. Attached Figure Description

[0046] Figure 1 This is a graph showing the relationship between the grain boundary occupancy of carbides and the probability of crack initiation.

[0047] Figure 2 This is a schematic diagram of an example of the manufacturing equipment for the oriented electromagnetic steel sheet according to the present invention. Detailed Implementation

[0048] (Manufacturing method of oriented electromagnetic steel sheet)

[0049] The embodiments of the present invention will be described below. The method for manufacturing the oriented electromagnetic steel sheet according to the present invention is characterized by the following series of steps: hot rolling a steel billet containing C: 0.01%–0.10%, Si: 2.0%–6.5%, and Mn: 0.01%–0.5% by mass; hot rolling annealing; cold rolling once or twice with a total reduction rate of 80% or more from the thickness of the obtained hot-rolled sheet to the thickness of the cold-rolled product; recrystallization annealing; coating the steel sheet surface with an annealing separating agent; and then performing final product annealing and planarization annealing for planarization. Specifically, the grain boundary occupancy rate of recrystallized grains of carbides in the hot-rolled sheet after annealing and before cold rolling is 80% or more, and the initial reduction during cold rolling is set to a strain rate of 200 / s or less, a reduction rate of 30% or less, and a steel sheet temperature of 90°C or less when biting into the roll.

[0050] [Steel billet]

[0051] In this invention, a steel billet for oriented electromagnetic steel sheet is used as the starting material. First, the composition of the steel billet will be described. In the following description of the composition, unless otherwise specified, "%" means "mass %" and "ppm" means "mass ppm".

[0052] This invention is advantageous in improving manufacturability in situations where it is necessary to roll at low pressure and low speed using a rolling mill that lacks the rigidity, load-bearing capacity, and equipment for performing warm rolling, or where rolling at low pressure and low speed is required due to process necessity. Therefore, in particular, other components or manufacturing processes can be adopted using the same components or processes as those used in the manufacture of general-oriented electromagnetic steel sheets. C, Si, and Mn are limited for the following reasons.

[0053] C: 0.01%~0.10%

[0054] Carbon (C) is an essential element for the precipitation of carbides at grain boundaries and for improving texture. However, if the C content exceeds 0.10%, decarburization becomes difficult through the final decarburization annealing process, leaving residues in the product and causing iron loss degradation known as magnetic aging. Therefore, the C content is kept below 0.10%. Furthermore, when the C content is below 0.01%, various precipitation control measures cannot be adequately implemented. Therefore, the C content is kept above 0.01%. From the viewpoint of manufacturability and magnetic properties, the C content is preferably 0.02% to 0.06%.

[0055] Si: 2.0%~6.5%

[0056] Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, the Si content needs to be 2.0% or higher. On the other hand, Si also increases the brittleness of steel. If the Si content exceeds 4.5%, the risk of fracture during plate handling increases, and cold rollability also deteriorates significantly. However, in this invention, since Si can provide a crack-suppressing effect, the Si content can be higher than usual. However, since the magnetostrictive properties required along with iron loss saturate at 6.5%, adding more does not result in a greater improvement in magnetic properties. Therefore, the Si content is kept below 6.5%. Even considering the risk reduction effect during plate handling, the risk will not become zero; therefore, the Si content is preferably 2.8% to 4.5%.

[0057] Mn: 0.01%~0.5%

[0058] From the viewpoint of controlling the formation of the oxide film during primary recrystallization, Mn is a useful element, but below 0.01%, it has no effect from the perspective of controlling oxide film formation. Therefore, the Mn content is kept at 0.01% or more. On the other hand, Mn also has the effect of improving the hot workability during manufacturing, but when the Mn content exceeds 0.5%, the texture of primary recrystallization deteriorates, leading to the degradation of magnetic properties. Therefore, the Mn content is kept at 0.5% or less.

[0059] Other typical compositions are shown below. In this invention, the billet can be subjected to known processes in sequence, namely hot rolling, hot-rolled plate annealing, cold rolling to achieve the final plate thickness in one step, decarburization annealing (also serving as primary recrystallization annealing), and final product annealing (also serving as secondary recrystallization annealing and purification annealing), thereby obtaining a compositional composition suitable for unidirectional electromagnetic steel sheets. Therefore, secondary recrystallization grains can also be grown using inhibitor components, or, as shown in Patent Document 3, secondary recrystallization grains can be grown without using precipitation-type inhibitors (AlN, MnS, MnSe, etc.). The preferred content of the inhibitor component, depending on its type, is as follows.

[0060] <Situations involving the use of inhibitory ingredients>

[0061] Sol.Al: 0.010%~0.050%

[0062] N: 0.004%~0.015%

[0063] S + 0.4Se: 0.010%~0.050%

[0064] When the Sol.Al content is below 0.010%, the magnetic flux density of the manufactured oriented electromagnetic steel sheet decreases. On the other hand, if the Sol.Al content exceeds 0.050%, secondary recrystallization becomes unstable. Therefore, the Sol.Al content is preferably 0.010% to 0.050%.

[0065] If the nitrogen (N) content is below 0.004%, AlN cannot precipitate properly during intermediate processes, making it difficult to control the particle size. Furthermore, if the N content exceeds 0.015%, it leads to frequent surface defects known as blistering. Therefore, the preferred N content is 0.004% to 0.015%. It should be noted that the N content can be changed as needed by employing a nitriding process during manufacturing; in most cases, a N content of 0.010% or less is sufficient to form adequate precipitates.

[0066] S + 0.4Se: 0.010%~0.050%

[0067] If the S content + 0.4 × Se content is less than 0.010%, the absolute amounts of Se and S as inhibitory components are insufficient. On the other hand, if S + 0.4 Se exceeds 0.050%, purification during the final annealing process becomes difficult. Therefore, S + 0.4 Se is preferably 0.010% to 0.050%. It should be noted that S and Se can be used as inhibitors in the forms of MnSe and MnS, respectively, or in the form of their complex, Mn(S, Se). Furthermore, AlN-based inhibitors can coexist with MnSe and / or MnS-based inhibitors, resulting in a synergistic effect.

[0068] <Case without precipitating inhibitor components>

[0069] Sol.Al: Less than 0.010%

[0070] S: below 60ppm

[0071] N: below 60ppm

[0072] O: below 60ppm

[0073] When no precipitation inhibitors are present, the contents of Sol.Al, S, and O, which are precipitation inhibitor forming elements, are limited to extremely low levels. Specifically, the limits are: Sol.Al: below 0.010%, S: below 60 ppm, and O: below 60 ppm. If these amounts are exceeded, it becomes difficult to obtain a secondary recrystallization structure using texture inhibition.

[0074] It should be noted that, for N, to prevent the formation of Si nitrides after purification annealing, the N content is preferably 60 ppm or less. Furthermore, the content of Ti, Nb, B, Ta, and V, which are nitride-forming elements, is also reduced to 0.050% or less. This is to prevent the deterioration of iron loss without hindering the texture suppression effect.

[0075] The content of the inhibitor component is as described above. In addition, the magnetic properties can be improved by using grain boundary segregation elements. As elements, they can be contained in the range of Ni: 0.005%–1.50%, Sn: 0.01%–0.50%, Sb: 0.005%–0.50%, Cu: 0.01%–0.50%, Mo: 0.01%–0.50%, P: 0.0050%–0.50%, Cr: 0.01%–1.50%, Nb: 0.0005%–0.0200%, B: 0.0005%–0.0200%, Te: 0.0005%–0.0200%, Co: 0.0001%–0.0100%, Ga: 0.0001%–0.0100%, Zn: 0.0001%–0.500%, and Bi: 0.0005%–0.0200%.

[0076] In addition, Pb, Ge, As, Ag, etc., can be contained in amounts ranging from 0.001% to 0.3%. These elements can be used alone or in combination, thereby improving iron loss.

[0077] A steel billet, serving as the starting material with the aforementioned composition, is heated to a suitable temperature according to the composition system, and then hot-rolled into a sheet through roughing and finishing rolling. In the case of a composition system containing precipitation inhibitors, the billet is heated to a temperature range of 1350°C to 1450°C to ensure complete solid solution of Al, Se, S, etc. On the other hand, in the case of a composition system without precipitation inhibitors, if the billet is heated to an excessively high temperature, the inhibitor-forming components will precipitate unevenly and finely during hot rolling, thereby locally inhibiting grain boundary movement and resulting in an extremely uneven grain size distribution, hindering the growth of secondary recrystallized grains towards Gaussian orientation. Therefore, a lower heating temperature, such as below 1250°C, is preferred. There are no particular limitations on the hot rolling conditions, as long as they are the conditions commonly used for manufacturing oriented electromagnetic steel sheets.

[0078] The hot-rolled sheet obtained as described above is then subjected to hot-rolled annealing. During hot-rolled annealing, to homogenize the hot-rolled microstructure, it is preferable to perform a homogenization treatment for at least 20 seconds at a temperature between 800°C and 1150°C. The effect of this invention is greater in suppressing cracks when the maximum annealing temperature is below 1000°C. The mechanism is not yet clear, but it is believed that when hot-rolled sheet is annealed at low temperatures below 1000°C, no recrystallization nuclei form, and the coarser microstructure caused by hot rolling remains in the center of the sheet thickness, which may be more prone to crack development. Therefore, this invention can effectively suppress cracks.

[0079] Next, from the perspective of controlling carbides, the cooling performed after annealing of hot-rolled plates requires any of the following controls.

[0080] 1) Ensure that the dwell time in the temperature range between 600℃ and 500℃ is more than 10 seconds.

[0081] The higher the temperature, the lower the driving force for carbide precipitation, thus requiring a longer holding time. By holding the temperature in the aforementioned temperature range for more than 10 seconds, carbides can be formed at the grain boundaries. However, since the holding time is at a high temperature, most of the carbon present within the grains diffuses to the grain boundaries, resulting in a lower carbon concentration within the grains. As mentioned above, under specific rolling conditions, by increasing the grain boundary occupancy of carbides, the crack initiation rate can be reduced, thus improving manufacturability. On the other hand, the utilization of carbides within the grains is limited, and under these conditions, sometimes poor texture is obtained after a single recrystallization annealing, and the magnetic properties of the product deteriorate slightly. However, since these conditions can relatively easily improve productivity, they are applicable when a higher level of magnetic properties is not required. In particular, although no upper limit is set for the dwell time, even with a longer time, the crack suppression effect will saturate, and from the viewpoint of magnetic properties, the shorter the dwell time, the better. Therefore, a dwell time of 15 seconds or less is preferred.

[0082] 2) The dwell time in the temperature range of 600°C to 500°C is 3 seconds or more and less than 10 seconds, and the average cooling rate in the temperature range of 500°C to 200°C is 10°C / second or less (i.e., the dwell time is 30 seconds or more), and cooling is performed at 15°C / second or more until the roll is wound.

[0083] By maintaining a residence time of 3 seconds or more in a temperature region between 500°C and 600°C, carbide nuclei can be formed at grain boundaries. When the residence time in this temperature region is less than 3 seconds, sufficient grain boundary precipitation cannot occur. On the other hand, if the residence time in this temperature region is 10 seconds or more, excessive precipitation towards grain boundaries occurs, and even if the cooling rate is controlled below 500°C, a suitable carbide state may not be achieved. That is, due to the reduction in the necessary amount of carbon within the grains, the texture improvement effect may not be obtained. Therefore, the residence time in the above temperature region is preferably 3 seconds or more and less than 10 seconds.

[0084] Compared to the temperature range of 500°C to 600°C, the cooling rate is reduced in the temperature range of 200°C to 500°C, where carbon diffusion is extremely slow, resulting in a dwell time of 30 seconds or more. This heating mode promotes carbon diffusion from the grains to the grain boundaries to some extent, while retaining a certain amount of carbon within the grains. After appropriate grain boundary precipitation, the cooling rate is increased again to suppress carbon diffusion within the grains, thus making the precipitation state of carbides within the grains appropriate. At least after the dwell treatment for grain boundary precipitation, the cooling rate needs to be increased again, cooling at a rate of 15°C / second or higher. This minimizes the impact on texture while improving manufacturability. As a result, the cooled steel strip becomes a state where carbides form at the grain boundaries, while simultaneously reducing the thickness of the depleted zone (lacking layer) of carbides formed near the grain boundaries to less than 20% of the grain size.

[0085] In particular, to achieve the heating mode shown in 2), the cooling zone after the continuous annealing furnace in the oriented electromagnetic steel sheet manufacturing equipment is as follows: Figure 2 The diagram shows that the following mechanisms are required.

[0086] 1. The system has multiple cooling zones for multi-stage cooling of the steel strip in the temperature range below 600℃. Figure 2 In the example, there are two cooling zones, 1 and 2. However, in the temperature range above 700°C after hot-rolled plate annealing, Fe3C carbides hardly precipitate. Furthermore, although precipitation occurs above 600°C, the precipitation rate is very slow. Therefore, from the viewpoint of controlling carbides, there is no particular limitation on cooling to below 600°C after annealing. However, unnecessarily reducing the cooling rate will increase the necessary production line length. Therefore, a typical cooling rate can be, for example, 5°C / second to 40°C / second.

[0087] 2. Cooling zone for cooling steel strips in a temperature range between 500℃ and 600℃. Figure 2In the example of cooling zone 1, there is a thermometer capable of measuring the temperature of the steel strip located in the middle or at the outlet of the cooling zone. Figure 2 In the example, thermometer 1).

[0088] 3. It has the following mechanism (control unit): it measures the temperature of the steel strip using the thermometer described in section 2 above, performs feedback control on the temperature of the steel strip, controls the cooling rate in the temperature range of 500°C to 600°C, and controls the residence time in the temperature range to be 3 seconds or more and less than 10 seconds.

[0089] 4. In the temperature range of 200°C to 500°C, even with cooling, the temperature will decrease slowly. Therefore, cooling water may remain on the steel strip at the outlet side of the cooling zone that handles the cooling in the temperature range of 500°C to 600°C. To prevent over-cooling, at least one mechanism (cooling water removal section) is provided between the cooling zones for easy wiping to remove the cooling water. The cooling water removal section is preferably located before the cooling zone where the cooling rate is significantly reduced. Alternatively, the cooling water removal section may be located between all the cooling zones.

[0090] 5. A slow cooling zone or a placement cooling zone that does not have an active cooling function, which has the function of ensuring that the residence time in a temperature range of 200°C to 500°C is more than 30 seconds.

[0091] 6. After slow cooling or placement cooling, a cooling zone is provided between the coil winding mechanism on the exit side to cool the steel strip in a temperature range below 200°C. Figure 2 In the example, cooling zone 2), a thermometer for measuring the temperature of the steel strip is located in the middle or at the outlet of the cooling zone. Figure 2 In the example, it is thermometer 2).

[0092] 7. It has a mechanism that measures the steel strip temperature using the thermometer described in section 6 above, performs feedback control on the steel strip temperature, and can stably achieve a cooling rate of 15°C / second or higher in the cooling zone of the object. Because the temperature of the steel strip in the cooling zone of the object is low, it can perform multiple functions such as pickling after cooling.

[0093] 8. When finally winding the roll material at the equipment outlet side, it has the function of cooling to below 100°C.

[0094] Next, the obtained hot-rolled annealed sheet is cold-rolled. At this stage, the total reduction from the thickness of the hot-rolled sheet to the thickness of the final product sheet is 80% or more. Because such high pressure places a heavy burden on the rolling mill, the reduction conditions for the first pass are a strain rate of 200 m / s or less and a reduction rate of 30% or less. These rolling conditions are applied based on manufacturing or equipment constraints, texture control considerations, etc., and are not necessarily recommended as the optimal rolling conditions for the manufacture of oriented electromagnetic steel sheets. For example, as a constraint on equipment, the following situations can be cited: Although the final plate thickness can usually be achieved in one rolling without intermediate annealing, the reduction rate per pass is limited due to rolling load issues; or when manufacturing is carried out by a manufacturing method involving two or more rolling passes with one or more intermediate annealings, the reduction rate per rolling pass becomes lower, but there is no rolling mill like a reversible rolling mill that can adjust the number of passes, so a continuous rolling mill must be used, and the number of stands is determined by the specifications of the rolling mill, so the rolling speed of the first pass must be slower.

[0095] If rolling is not performed under these special conditions, the state of carbide-occupied grain boundaries as specified in this invention is not required. Similarly, during the first pass, even if the steel sheet temperature exceeds 90°C when biting into the work rolls, dislocation deformation easily occurs at high steel sheet temperatures, creating conditions that suppress twinning deformation; therefore, this invention is unnecessary. For cold rolling, no special restrictions are required as long as the final sheet thickness can be obtained. If possible, as in many existing technologies, warm rolling can be performed after the first pass to increase the steel sheet temperature and improve the texture, utilizing processing heat.

[0096] Next, the final cold-rolled sheet undergoes a primary recrystallization annealing. The purpose of this primary recrystallization annealing is to recrystallize the cold-rolled sheet with its rolled structure, adjusting the primary recrystallization grain size to be optimal for secondary recrystallization. Furthermore, it decarburizes the carbon in the steel by using a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere, while simultaneously forming an oxide film on the surface using the aforementioned oxidizing atmosphere. Therefore, the primary recrystallization annealing is performed at a dew point in a mixed H2 atmosphere, between 750°C and 900°C. During the heating phase of the primary recrystallization annealing, maintaining a heating rate of 200°C / second or higher in the temperature range between 550°C and 680°C further enhances the texture improvement effect.

[0097] An annealing release agent is applied to the surface of the steel sheet after the first recrystallization annealing. Magnesium oxide (MgO) is used as the main agent in the annealing release agent to form a forsterite film on the surface of the steel sheet after the second recrystallization annealing. Adding appropriate amounts of Ti oxide or Sr compounds to the release agent further promotes the formation of the forsterite film. In particular, the addition of additives that promote the uniform formation of the forsterite film also improves the peeling properties.

[0098] Next, secondary recrystallization and final annealing are performed to form the forsterite coating. N2, Ar, H2, or mixtures thereof are suitable annealing atmospheres. To facilitate secondary recrystallization, isothermal holding near the secondary recrystallization temperature is recommended. However, slow heating rates are also effective, so isothermal holding is not always necessary. If trace amounts of the component are precipitated in the final product, it will lead to deterioration of the magnetic properties; therefore, to purify the composition, the maximum annealing temperature is preferably 1100°C or higher.

[0099] After the final annealing described above, an insulating film can be further coated onto the surface of the steel plate and then fired. There is no particular limitation on the type of insulating film; any known insulating film is suitable. For example, the method described in Japanese Patent Application Publication No. 50-79442 or Japanese Patent Application Publication No. 48-39338, which involves coating a steel plate with a coating solution containing phosphate-chromate-colloidal silica and firing it at a temperature of approximately 800°C, is preferred.

[0100] In addition, the shape of the steel plate can be adjusted by planarization annealing, and the planarization annealing can also be used as a firing process for insulating film.

[0101] Example

[0102] (Example 1)

[0103] A steel billet with the following composition was prepared: by mass % C: 0.05%, Si: 3.2%, Mn: 0.04%, sol.Al: 0.0200%, Se: 100ppm, N: 100ppm, S: 60ppm, O: less than 50ppm, with the remainder consisting of Fe and unavoidable impurities. The prepared steel billet was heated to 1350°C and then hot-rolled to produce a 2.0 mm thick hot-rolled sheet. Next, using the equipment of this invention, the hot-rolled sheet was annealed at 990°C for 30 seconds and then cooled under the cooling conditions described in Table 3. Samples were cut from the long side end and the center of the width of the obtained coil so that a cross-section perpendicular to the rolling direction could be observed. The cut samples were then etched with a nitric acid-ethanol solution, and SEM observations were performed continuously on the center of the sheet thickness, at a distance of 500 μm in the thickness direction and 1 mm perpendicular to the rolling direction (sheet width direction). The obtained SEM images were analyzed to determine the occupancy of the total grain boundary length of carbides within the field of view. Next, cold rolling was performed under either one or two rolling conditions until the final plate thickness was achieved, with an additional intermediate annealing at 1030°C for 20 seconds. The final plate thickness was 0.22 mm to 0.35 mm. Subsequently, a recrystallization annealing was performed, with a heating rate of 250°C / second in the temperature range of 550°C to 680°C, a soaking temperature of 800°C, and a soaking time of 30 seconds. In the event of in-production breakage during the process, the number of breaks was accumulated, and the number of coils processed within one week was used as the overall parameter to calculate the in-production breakage rate. For the steel plate after the first recrystallization, an annealing separating agent of 95% MgO and 5% TiO2 was applied to the steel plate surface in the form of an aqueous slurry for final annealing. The surface of the thus obtained annealed sheet is coated with a coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3, and then fired at 800°C. This yields a product sheet / coil of oriented electromagnetic steel.

[0104]

[0105] The magnetic properties of the central portion of the width of the product sheet / coil obtained as described above were analyzed. For the magnetic properties, after stress-relief annealing at 800°C for 3 hours, a 30mm × 280mm test piece was cut from the position corresponding to the outer roll of the final annealed sheet, ensuring a total mass of 500g or more. The Epstein test (B8, magnetic flux density at a magnetization of 800 A / m) as specified in JIS C2550 was used to determine the magnetic properties. The results are shown in Table 4.

[0106]

[0107] As shown in Table 4, it is confirmed that according to the present invention, even under rolling conditions that are prone to cracking, the incidence of breakage in the production line can be suppressed, and the magnetic properties of the orientation-oriented electromagnetic steel sheet can be well maintained.

[0108] (Example 2)

[0109] A steel billet containing 0.04% C, 3.3% Si, and 0.05% Mn by mass, and with the composition shown in Table 5, was prepared. The prepared steel billet was heated to 1200°C and then hot-rolled into a hot-rolled sheet. Next, the hot-rolled sheet was annealed at 980°C for 60 seconds in an inventive annealing furnace. It was then cooled at 30°C / second in a temperature range of 950°C to 400°C, held for 3 to 150 seconds in a temperature range of 400°C to 250°C, and cooled at 30°C / second in a temperature range of 250°C to 100°C. The holding (dwelling) times in the temperature ranges of 400°C to 250°C and 500°C to 200°C are shown in Table 5. A sample was cut from the long side end and the center of the width of the obtained coil in a manner perpendicular to the rolling direction. Next, after etching the cut samples with a nitric acid-ethanol solution, SEM observation was performed on the center of the plate thickness, with a thickness of 500 μm and a length of 1 mm perpendicular to the rolling direction (plate width direction). Image analysis was performed on the obtained SEM images to determine the occupancy of the total grain boundary length of carbides within the field of view. Next, rolling was performed using a 6-std (6-stand) continuous rolling mill, which is not typically used in electromagnetic steel sheet manufacturing, at an inlet temperature of 40°C. The reduction rate and strain rate of the first pass, and the final plate thickness after rolling are shown in Table 5. Regarding the entire plate, under the same conditions, rolling 20 coils resulted in a × rating for cases where more than 2 coils broke (10% breakage rate during rolling), and a ○ rating for cases where less than 1 coil broke. Furthermore, for coils that break during the first half of rolling, a cross-section along the rolling direction is cut from the break, and the length per unit area of ​​grain boundaries with twinning orientation relationships (60 degrees relative to the basic orientation around the <111> axis: tolerance 15 degrees) is evaluated using EBSD. For coils that break during the later stages of rolling, it is difficult to determine whether twinning exists, so evaluation is not possible. Subsequently, a single recrystallization annealing is performed in a temperature range of 400°C to 700°C at a heating rate of 200°C / second, a soaking temperature of 850°C, and a soaking time of 40 seconds. Next, an annealing separating agent with MgO as the main agent is coated on the steel sheet for final annealing. The annealed sheet obtained as described above is coated with a coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2, and planarization annealing is performed at 850°C for 30 seconds. Thus, a product sheet / coil of oriented electromagnetic steel sheet is obtained.

[0110] The magnetic properties of the central portion of the width of the product sheet / coil obtained as described above were analyzed. A 30mm × 280mm test piece was cut from the position corresponding to the outer roll of the coil during the final annealing process, ensuring a total mass of 500g or more. The Epstein test (B8(T)) was performed according to JIS C 2550. The relationship between the obtained magnetic flux density and various experimental conditions is shown in Table 5.

[0111]

[0112] As shown in Table 5, in the invention example, the breakage rate in the production line can be suppressed while maintaining good magnetic properties.

[0113] Industrial availability

[0114] According to the present invention, a method for manufacturing oriented electromagnetic steel sheets that can significantly improve manufacturability and an apparatus for implementing the method can be provided.

Claims

1. A method for manufacturing an orientation-oriented electromagnetic steel sheet, characterized in that, The process involves the following steps: hot rolling of a steel billet containing 0.01%–0.10% C, 2.0%–6.5% Si, and 0.01%–0.5% Mn by mass; hot rolling followed by hot-rolled sheet annealing; cold rolling once or twice with a total reduction of 80% or more from the thickness of the obtained hot-rolled sheet to the thickness of the cold-rolled product; recrystallization annealing; coating the steel sheet surface with an annealing separating agent; and finally, final annealing and planarization annealing for planarization. In the manufacturing method of this orientation-oriented electromagnetic steel sheet, the carbide content in the hot-rolled sheet after annealing and before cold rolling is 80% or more at the grain boundaries of the recrystallized grains, and the initial reduction of the cold rolling is set to a strain rate of 200 / second or less, a reduction rate of 30% or less, and a steel sheet temperature of 90°C or less when biting into the roll.

2. The method for manufacturing the oriented electromagnetic steel sheet according to claim 1, wherein, One or more intermediate annealing processes are performed between the two or more cold rolling processes.

3. The method for manufacturing the oriented electromagnetic steel sheet according to claim 1 or 2, wherein, During the cooling process after annealing of the hot-rolled plate, the dwell time in the temperature range of 500°C to 600°C is 10 seconds or more.

4. The method for manufacturing an orientation-oriented electromagnetic steel sheet according to claim 1 or 2, wherein, During the cooling of the hot-rolled sheet after annealing, the dwell time in the temperature region between 600°C and 500°C is 3 seconds or more and less than 10 seconds, and the average cooling rate in the temperature region between 500°C and 200°C is 10°C / second or less, and the cooling is carried out at a cooling rate of 15°C / second or more until the coil is wound.

5. The method for manufacturing the oriented electromagnetic steel sheet according to any one of claims 1 to 4, wherein, During the primary recrystallization annealing, the heating rate in the temperature range between 550°C and 680°C is 200°C / second or higher.

6. A method for manufacturing an orientation-oriented electromagnetic steel sheet according to any one of claims 1 to 5, wherein, In addition to the aforementioned composition, the steel billet further contains, by mass%, sol.Al: 0.010%–0.050%, N: 0.004%–0.015%, and S+0.4Se: 0.010%–0.050%.

7. The method for manufacturing the oriented electromagnetic steel sheet according to any one of claims 1 to 5, wherein, In addition to the aforementioned composition, the steel billet further contains sol.Al (less than 0.010% by mass) and S, N, and O (less than 60 ppm each).

8. A method for manufacturing an orientation-oriented electromagnetic steel sheet according to any one of claims 1 to 7, wherein, In addition to the aforementioned composition, the steel billet further contains, by mass percent, an element selected from Ni: 0.005%–1.50%, Sn: 0.01%–0.50%, Sb: 0.005%–0.50%, Cu: 0.01%–0.50%, Mo: 0.01%–0.50%, P: 0.0050%–0.50%, Cr: 0.01%–1.50%, Nb: 0.0005%–0.0200%, and B: 0.0005%–0.0%. One or more of the following: 200%, Te: 0.0005%–0.0200%, Co: 0.0001%–0.0100%, Ga: 0.0001%–0.0100%, Zn: 0.0001%–0.500%, Bi: 0.0005%–0.0200%, Pb: 0.001%–0.3%, Ge: 0.001%–0.3%, As: 0.001%–0.3%, and Ag: 0.001%–0.3%.

9. A manufacturing equipment for oriented electromagnetic steel sheets, which is used in the manufacturing of oriented electromagnetic steel sheets, and has the following features: Multiple cooling zones are used to cool the steel strip to a temperature below 700°C after annealing. A thermometer is installed at least at any point in the middle or outlet of each cooling zone to measure the temperature of the steel strip; The control unit uses the temperature measured by the thermometer for feedback control to control the cooling rate in each cooling zone; as well as At least one cooling water removal unit is provided between the cooling zones to remove the cooling water from the steel strip; The equipment for manufacturing this oriented electromagnetic steel sheet is capable of winding the coil at temperatures below 100°C.

10. The manufacturing equipment for oriented electromagnetic steel sheets according to claim 9, wherein, The plurality of cooling zones include: a first cooling zone in which the dwell time of the steel strip in a temperature range between 600°C and 500°C is controlled to be more than 3 seconds and less than 10 seconds, and a second cooling zone in which the dwell time of the steel strip in a temperature range between 500°C and 200°C is controlled to be more than 30 seconds.

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