Hot rolling method, production method for hot-rolled coil, and production method for grain-oriented electromagnetic steel sheet

Abstract

Provided is a hot rolling method that makes it possible to give a hot-rolled coil a favorable shape. The present invention is a hot rolling method that involves heating a slab at a heating furnace before hot rolling the slab. The temperature of the slab is raised to the highest temperature T1 (°C) to be reached by the slab inside the heating furnace at least 10 minutes before the slab is extracted from the heating furnace, and the temperature of the slab at extraction from the heating furnace is set to T2 (°C), where T1≥1200, and T2≤T1-20. The slab has a component composition that makes the γ phase percentage at T1 (°C) no more than 10 mol%.

Description

Hot rolling method, hot rolled coil manufacturing method, and grain-oriented electrical steel sheet manufacturing method

[0001] The present invention relates to a hot rolling method, a method for manufacturing a hot rolled coil, and a method for manufacturing a grain-oriented electrical steel sheet.

[0002] Grain-oriented electrical steel sheets are typically produced by using a precipitate called an inhibitor to induce secondary recrystallization of Goss-oriented ({110}<001>) grains during final annealing. For example, Japanese Patent Publication No. 40-15644 discloses a method using AlN or MnS as an inhibitor, and Japanese Patent Publication No. 51-13469 discloses a method using MnS or MnSe as an inhibitor, both of which have been industrially put to practical use. These inhibitor-based methods are useful for stably developing secondary recrystallized grains, but require finely dispersed precipitates, which necessitates slab heating at a high temperature of 1300°C or higher before hot rolling. However, high-temperature slab heating not only increases equipment costs but also increases the amount of scale formed during hot rolling, resulting in reduced yields and cumbersome equipment maintenance.

[0003] On the other hand, manufacturing techniques that do not use inhibitors (inhibitor-less methods) have also been proposed. For example, a technique has been proposed in which secondary recrystallization is induced by controlling the texture (texture) of a highly purified steel without adding inhibitor-forming components to the slab (Patent Document 1).

[0004] Japanese Patent Application Laid-Open No. 2000-129356

[0005] Since materials containing almost no inhibitor-forming components do not require slab heating at high temperatures of 1,300°C or higher, hot rolling can be carried out using slab heating equipment such as gas furnaces used in general steel manufacturing, without using special furnaces for slab heating. However, in some products, there have been cases where the longitudinal ends of the coil (hot-rolled coil) after hot rolling have become deformed, which can lead to meandering in the subsequent hot-rolled sheet annealing process or sheet breakage in the cold-rolling process, and has been a factor hindering production, particularly on an industrial scale.

[0006] The present invention advantageously solves the above-mentioned problems, and aims to provide a hot rolling method that appropriately controls the slab heating conditions before hot rolling to improve the shape of the slab before hot rolling, thereby improving the shape of the hot-rolled coil, and a method for manufacturing a hot-rolled coil and a grain-oriented electrical steel sheet, which includes a step of hot-rolling a slab using this hot rolling method.

[0007] In order to solve the above-mentioned problems, the inventors conducted a detailed investigation into the hot rolling conditions of hot-rolled coils that actually experienced shape defects, and found that shape defects occurred at the longitudinal ends of the hot-rolled coils in the following cases: (1) The maximum temperature reached by the slab during slab heating before hot rolling is 1200°C or higher, and the slab has a chemical composition such that the γ-phase fraction at the maximum temperature is 10 mol% or less; and (2) When extracted from the heating furnace, the ends of the slab lifted by the extractor are deformed so as to sink.

[0008] The slab temperature can be determined by passing a slab equipped with a thermocouple through a heating furnace and recording the temperature change of the slab at each position in the furnace. Thermocouples can be attached to six locations, for example, at the center of the slab's longitudinal direction and both longitudinal ends, on the surface and at the center of the thickness direction, and the average of the six measurements at each position in the heating furnace can be used as the slab temperature at that location. If the slab temperature is actually measured in the heating furnace, that value can be used. The γ-phase fraction can also be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) manufactured by Thermo-Calc Software AB.

[0009] Conventional slab heating control methods are as follows. One of the purposes of slab heating in general steel is to increase the slab's temperature to reduce its deformation resistance and enable high-reduction hot rolling to obtain hot-rolled coils. To achieve this, the temperature of the slab extracted from the heating furnace must be controlled to a temperature appropriate for hot rolling. The target of control is the temperature of the slab at the time of extraction from the heating furnace; it does not need to be higher than the temperature of the slab at the time of extraction. Therefore, the slab temperature is typically gradually increased in the heating furnace and controlled to reach a maximum temperature at the time of extraction. Furthermore, in the production of grain-oriented electrical steel sheets, it is also necessary to dissolve trace amounts of impurity elements and precipitate-forming elements to homogenize the slab. Once dissolved, elements may re-precipitate when the steel temperature drops. Therefore, the slab temperature at the time of extraction is typically controlled to reach a maximum temperature.

[0010] When the slab is extracted from the heating furnace after being heated to its maximum temperature, it is lifted by a device with multiple jaws called an extractor, placed on a conveying roller, and sent for hot rolling. During extraction, the slab is supported at four points in a four-jaw extractor, and at eight points in an eight-jaw extractor. When a high-temperature slab is lifted with few support points, the edge of the slab deforms and sinks downward due to its own weight. Figure 1 is a schematic diagram showing the edge of a high-temperature slab being lifted by a four-jaw extractor, deforming and sinking downward due to its own weight.

[0011] Creep deformation, which occurs particularly at high temperatures, is likely to occur in the α (ferrite) phase, while the deformation rate of the γ (austenite) phase is slow. Therefore, creep deformation, in which the slab edge sinks during extraction, is likely to occur when steel with a low γ phase ratio is extracted. The amount of slab sinking deformation is also related to the placement of the extractor claws 2 and the slab 1. As shown in Figure 1, the length from the extractor claw 2 (slab support part) located closest to the slab edge to the edge of the slab 1 is called the overhang length L. １For example, in the case of a slab with a width of 0.9 to 1.2 m, a thickness of 170 to 240 mm, and a total length of 5 to 15 m in the longitudinal direction, the overhang length L １ However, it was found that deformation is likely to occur when the depth exceeds 1.2 m. In Figure 1, deformation in which the edge of the slab sinks (sinking width L ２ ) occurs at both ends.

[0012] It is presumed that the slab with sunken ends is extracted from the heating furnace and hot rolled due to the mechanism described above, which is why the poor shape of the longitudinal ends of the hot-rolled coil is observed.

[0013] Based on this speculated mechanism, the inventors investigated the possibility of improving the shape of hot-rolled coils. Here, electrical steel sheets typically contain a high concentration of Si to improve the final magnetic properties. Si stabilizes the α phase and reduces the γ-phase ratio during high-temperature heating. Another element that significantly affects the γ-phase ratio is C. However, C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, so there is an optimum amount of C from the perspective of improving the final magnetic properties. Therefore, it is difficult to adopt a method of significantly changing the composition of electrical steel sheets that have already been manufactured in a process to increase the γ-phase ratio at high temperatures.

[0014] The maximum temperature of the slab during slab heating is usually set to dissolve trace amounts of impurity elements and precipitate-forming elements and homogenize the slab. When trace elements are intentionally added to improve magnetic properties, lowering the maximum temperature is not necessarily an option, but it is possible to lower the temperature of the slab when it is removed from the heating furnace. If the cause of the shape defects at the longitudinal ends of the hot-rolled coil is the shape defects that occur at the slab ends during removal, there is a high possibility that the shape can be improved by lowering the temperature at which the slab is removed from the heating furnace, where deformation occurs.

[0015] Therefore, the inventors have come up with a method of utilizing the fact that in actual systems, when elements that have once been dissolved and homogenized re-precipitate, there is a supersaturated temperature range in which precipitation does not proceed even if the temperature is lower than the precipitation temperature calculated by thermodynamic equilibrium calculations, etc., and after first raising the temperature of the slab to achieve homogenization, the temperature is lowered to a range in which no re-precipitation of impurities or precipitate-forming elements occurs and the slab remains homogenous, thereby suppressing slab deformation when lifted and transported by an extractor during extraction.

[0016] The gist of the present invention, which has been completed by the inventors based on the above findings, is as follows: [1] A hot rolling method in which a slab is heated in a heating furnace and then hot rolled, wherein the slab temperature is made to reach T1 (unit: °C), which is the highest temperature the slab will reach in the heating furnace, at least 10 minutes before the slab is removed from the heating furnace, and the slab temperature at the time of removal from the heating furnace is T2 (unit: °C), where T1 ≥ 1200 and T2 ≤ T1-20, and the slab has a chemical composition such that the γ-phase fraction at T1 (unit: °C) is 10 mol% or less. [2] A hot rolling method comprising two consecutive passes of hot rolling, each pass being performed in a temperature range of 1030°C to 1150°C, with a reduction ratio of 50% or less and a strain rate of 15 s －1[3] The hot rolling method of [1], wherein the slab is a steel slab having a chemical composition containing, in mass%, C: 0.03% or more and 0.08% or less, Si: 2.0% or more and 8.0% or less, Mn: 0.005% or more and 3.0% or less, Al: less than 0.0100%, O: 0.0060% or less, N: 0.0060% or less, and S + 0.405 × Se: 0.0060% or less, with the balance being Fe and unavoidable impurities. [4] The hot rolling method according to [3], wherein the slab further contains, in mass%, one or more elements selected from the group consisting of Ni: 0.005% or more and 1.50% or less, Sn: 0.01% or more and 0.50% or less, Sb: 0.005% or more and 0.50% or less, Cu: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, P: 0.0050% or more and 0.50% or less, Cr: 0.01% or more and 1.50% or less, Nb: 0.0005% or more and 0.0200% or less, Ti: 0.0005% or more and 0.0200% or less, B: 0.0005% or more and 0.0200% or less, Te: 0.0005% or more and 0.0200% or less, and Bi: 0.0005% or more and 0.0200% or less. [5] A method for producing a hot-rolled coil, comprising hot-rolling a slab by any one of the hot rolling methods [1] to [4] to obtain a hot-rolled coil. [6] A method for producing a hot-rolled coil according to [5], wherein the thickness variation in the width direction within a range of 1 to 2%, where the entire longitudinal length of the hot-rolled coil is 100% and one end of the hot-rolled coil is 0%, is 1.5 times or less than the thickness variation in the width direction within a range of 20 to 21%. [7] A method for producing a grain-oriented electrical steel sheet, comprising hot-rolling a slab by any one of the hot-rolling methods [1] to [4], annealing the obtained hot-rolled sheet, followed by cold rolling once or at least two times with intermediate annealing in between, and then performing primary recrystallization annealing and final finish annealing.

[0017] According to the hot rolling method of the present invention, by improving the shape of the slab end portions before hot rolling, the shape of the longitudinal end portions of the obtained hot-rolled coil can be improved, and thus the threading of the grain-oriented electrical steel sheet in the manufacturing process after the hot rolling step can be made more stable. As a result, it becomes possible to manufacture grain-oriented electrical steel sheets much more easily than before. The present invention can also provide a method for manufacturing a hot-rolled coil and a grain-oriented electrical steel sheet, which includes a step of hot-rolling a slab using this hot rolling method.

[0018] This is a schematic diagram showing the state in which the edge of a high-temperature slab being lifted by a four-jaw extractor is deformed and sinking downward due to its own weight.

[0019] The hot rolling method of the present invention will now be described in detail.

[0020] The present invention is a hot rolling method in which a slab is heated in a heating furnace and then hot rolled, in which the slab temperature is allowed to reach T1 (unit: °C, hereinafter omitted), which is the highest temperature the slab will reach in the heating furnace, at least 10 minutes before being removed from the heating furnace, and the slab temperature at the time of removal from the heating furnace is designated as T2 (unit: °C, hereinafter omitted). T1 and T2 satisfy the following formulas (1) and (2): T1≧1200 (1) T2≦T1−20 (2) Here, the time from when the slab is charged into the heating furnace until it is removed is typically 120 minutes or more and 300 minutes or less. The slab transport speed in the heating furnace is typically constant.

[0021] In the method of the present invention, a slab having a composition in which the γ-phase fraction at T1 is 10 mol% or less is used. A greater effect can be achieved when a slab having a composition in which the γ-phase fraction is 5 mol% or less is used. Furthermore, the lower limit of the γ-phase fraction is not particularly limited, and the method can be applied even when the γ-phase fraction is 0 mol%.

[0022] The composition of the slab preferably satisfies the following: Hereinafter, the "%" designation regarding the composition of the component is "mass %" unless otherwise specified.

[0023] C: 0.03% or more and 0.08% or less C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, so from the viewpoint of improving the final magnetic properties, it is preferable to set the C content to 0.03% or more. On the other hand, if the C content exceeds 0.08%, it becomes difficult to reduce the C content to 50 ppm or less, at which magnetic aging does not occur, even when decarburization annealing is performed. From this point of view, it is preferable to limit C to 0.08% or less.

[0024] Si: 2.0% or more and 8.0% or less Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, the Si content is preferably 2.0% or more, more preferably 2.8% or more. On the other hand, Si also increases the brittleness of steel. In order to reduce the risk of breakage during threading and suppress deterioration of cold rolling properties, the Si content is preferably limited to 8.0% or less, more preferably 4.5% or less.

[0025] Mn: 0.005% or more and 3.0% or less Mn is a useful element from the viewpoints of improving hot workability and controlling the formation of oxide films during primary recrystallization. From this viewpoint, the Mn content is preferably 0.005% or more, and more preferably 0.01% or more. On the other hand, to avoid deterioration of the primary recrystallization texture and resulting degradation of magnetic properties, the Mn content is preferably limited to 3.0% or less, and more preferably 0.5% or less.

[0026] Al: less than 0.0100% N: 0.0060% or less O: 0.0060% or less S + 0.405 × Se: 0.0060% or less Excessive Al can make it difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition, so Al is preferably limited to less than 0.0100%, more preferably 0.0800% or less. N is preferably limited to 0.0060% or less, more preferably 0.0040% or less, to prevent the formation of Si nitrides after purification annealing in the production of final product sheets. O forms oxides that inhibit deterioration of the magnetic properties of the final product sheets, so it is preferably limited to 0.0060% or less, more preferably 0.0030% or less. The sum of the amounts of S and Se multiplied by 0.405 is preferably limited to 0.0060% or less, more preferably 0.0040% or less, to stably obtain a secondary recrystallized structure. Al, N, O, S and Se are inhibitor components.

[0027] The essential components and suppressing components have been described above, but one or more of the following elements may be optionally contained: 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 0.0200%, Ti: 0.0005% to 0.0200%, B: 0.0005% to 0.0200%, Te: 0.0005% to 0.0200%, and Bi: 0.0005% to 0.0200%.

[0028] Ni is a useful element that improves the structure of hot-rolled coils and enhances their magnetic properties. To fully obtain this effect, when Ni is added, the Ni content is preferably 0.005% or more. However, excessive Ni makes secondary recrystallization unstable, resulting in deterioration of magnetic properties, so the Ni content is preferably 1.50% or less.

[0029] Sn, Sb, Cu, Mo, P, Cr, B, and Bi are grain boundary segregation elements that can improve various properties, but if they are present in excess, they can inhibit the development of secondary recrystallized grains. For these reasons, when these elements are contained, the amounts should be within the above ranges.

[0030] Nb, Ti, and Te are precipitate-forming elements that can improve various properties, but excessive amounts can make secondary recrystallization unstable. For these reasons, when these elements are added, the amounts should be within the above ranges.

[0031] The slab used in the method of the present invention preferably has a composition containing the above-mentioned essential components and optional components, with the balance being Fe and unavoidable impurities.

[0032] The slabs used in the method of the present invention can be produced by refining molten steel adjusted to a desired composition by a known method using a converter, electric furnace, or the like, and then subjecting it to vacuum treatment, if necessary, followed by a conventional ingot-making method or continuous casting method. Alternatively, thin cast pieces having a thickness of 100 mm or less can be directly produced by a direct casting method and used as slabs.

[0033] The slab used in the method of the present invention has a chemical composition such that the gamma phase fraction is 10 mol% or less at T1, the maximum temperature reached by the slab in the heating furnace. The gamma phase fraction can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) manufactured by Thermo-Calc Software AB. After adjusting the chemical composition of the slab, the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) can be used to set the maximum temperature reached by T1 so that trace elements are uniformly dissolved or do not precipitate coarsely, thereby unnecessarily exerting an inhibition effect. At this time, the gamma phase fraction at T1 is similarly calculated and confirmed to be 10 mol% or less. Alternatively, the maximum temperature reached by the slab in the heating furnace, T1, can be set and the chemical composition adjusted so that the gamma phase fraction is 10 mol% or less at this T1.

[0034] In the method of the present invention, the maximum temperature T1 is the maximum temperature that the slab reaches in the heating furnace and satisfies the above formula (1). That is, T1 is a temperature of 1200°C or higher. If the temperature is 1200°C or higher, the slab can be easily homogenized. In order to suppress creep deformation, T1 can be preferably set to 1300°C or lower, and more preferably 1260°C or lower.

[0035] In the method of the present invention, in order to dissolve trace components uniformly and prevent unnecessary inhibition effects, the slab temperature is allowed to reach T1 at least 10 minutes before extraction from the heating furnace, preferably between 30 minutes and 15 minutes before extraction.

[0036] In the method of the present invention, the slab temperature T2 during extraction from the heating furnace satisfies the above formula (2) in order to suppress creep deformation. That is, T2 is set to a temperature that is at least 20°C lower than T1. To avoid a situation in which homogenized precipitate-forming elements are reprecipitated due to a lower temperature, thereby impairing the homogenization effect, T2 is preferably set to a temperature that is 20 to 80°C lower than T1. In order to suppress creep deformation, T2 is preferably 1200°C or lower. In order to suppress precipitate reprecipitation, T2 is preferably 1120°C or higher.

[0037] The above-described heat pattern can suppress creep deformation, which occurs when the slab edge sinks downward when the extractor lifts the slab during extraction. The method of the present invention is advantageous when the length from the claw of the extractor supporting the slab to the end of the slab (overhang length) is relatively long (e.g., for slabs with a width of 0.9 to 1.2 m, a thickness of 200 to 240 mm, and a total length of 5 to 15 m, the overhang length exceeds 1.2 m). For example, for slabs with a width of 0.9 to 1.2 m, a thickness of 200 to 240 mm, and a total length of 5 to 15 m, the method of the present invention is even more advantageous when the overhang length is 1.5 m or more and 4.0 m or less. However, the slabs used in the method of the present invention are not limited to the above-described shapes (width, thickness, and total length).

[0038] When the slab composition has a γ-phase fraction exceeding 10 mol% at the maximum temperature T1, or when the maximum temperature is below 1200°C, the amount of deformation when the slab is lifted by the extractor during extraction is not very large. Even when the heat pattern of the present invention is applied under these conditions, it is possible to expect an improvement in the shape defects of the slab edge, but the extent of the effect is relatively small. Generally, heating furnaces increase fuel utilization efficiency during heating by gradually heating the slab up to the desired maximum temperature. Therefore, in the range where slab shape defects are not a major problem, it is not rational from the perspective of energy efficiency to actively use the heat pattern of the present invention.

[0039] To lower the temperature of a heated slab by 20°C or more, a temperature difference of 50°C or more is required in the furnace. Because the temperature of the slab itself decreases only slowly, it is extremely difficult to lower the temperature sufficiently during extraction so that the temperature during heating and extraction is 20°C or more lower than the maximum temperature unless the slab reaches its maximum temperature at least 10 minutes before extraction. Here, the furnace temperature is the temperature of the atmospheric gas within the furnace and can be measured using a sensor such as a thermocouple. If multiple sensors are installed in the furnace, such as above, below, left, and right of the slab, the average value of the measurements from each sensor can be used. Instead of sensor measurement, control can also be based on a calculated slab temperature.

[0040] Furthermore, since the slab itself carries heat, in order to create a large temperature difference within the furnace, it is advantageous for the heating furnace to have different systems of burners at different locations within the furnace, a furnace wall structure that suppresses radiant heat, a mechanism for controlling atmospheric gas convection, etc., and conventional heating furnaces can be improved as appropriate.

[0041] The length from the extractor jaws to the end of the slab when extracting the slab (overhang length) does not have to be the same at both ends. For example, the overhang length at one end may be relatively large (e.g., over 1.2 m for a slab with a width of 0.9-1.2 m, a thickness of 200-240 mm, and a total length of 5-15 m), while the overhang length at the other end may be relatively small (e.g., 1.2 m or less for a slab with a width of 0.9-1.2 m, a thickness of 200-240 mm, and a total length of 5-15 m). In such cases, the furnace temperature on the side with the larger overhang length may be actively lowered, and the slab temperature at that end may be at least 20°C lower than the maximum temperature.

[0042] The slab extracted through the above-described slab heating has homogenized steel, and the overhanging part of the extractor (the end of the slab) sinks only slightly during extraction.

[0043] Next, the slab is subjected to hot rolling, which is relatively easy because the slab end portions are prevented from sinking and becoming defective in shape.

[0044] In the hot rolling process of the method of the present invention, at least two consecutive passes of rolling from the slab stage to the sheet bar stage can be carried out in a temperature range of 1030°C to 1150°C. The consecutive two passes should have an inter-pass time of 15 seconds or more, a rolling reduction of 50% or less in each pass, and a strain rate of 15 seconds or less. －1 The above is preferable from the viewpoint of improving the shape of the longitudinal end portions of the hot rolled coil.

[0045] The method of the present invention uses a slab in which the γ-phase fraction is 10 mol% or less at T1, the maximum temperature reached in the heating furnace. The γ-phase fraction is typically greatest in the temperature range of 1030°C to 1150°C. Austenite generally has higher deformation resistance than ferrite and is less likely to deform when rolled. Therefore, the reduction ratio in each pass is limited to 50% or less. The reduction ratio is preferably 15% or more, more preferably 20% or more, from the viewpoint of homogenizing the structure of the hot-rolled coil. The inter-pass time is preferably 15 seconds or more, since dislocations formed by deformation are recovered or eliminated by recrystallization, allowing rolling without excessively increasing deformation resistance. The inter-pass time is preferably 120 seconds or less, since it suppresses the formation of precipitates nucleated by dislocations generated during deformation. The strain rate is preferably 15 seconds or less, since it is easy to improve the shape of the longitudinal end portions of the hot-rolled coil. -1 The strain rate is preferably 50 s -1 It is preferable that:

[0046] Here, the strain rate ε can be calculated using the following Ekelund equation. In the formula, v Ｒ is the roll peripheral speed (mm / s), R' is the roll radius (mm), h １ is the roll entry thickness (mm), and r is the reduction ratio (%).

[0047] The roll peripheral speed is preferably 4000 mm / s or more and 8000 mm / s or less, since this allows the slab's entry speed into the roll to be controlled within an appropriate range and the slab to be held in place by friction with the roll at an earlier stage. The roll radius is preferably 700 mm or more and 1300 mm or less, since this allows the load to be easily controlled within an appropriate range.

[0048] Typically, the longitudinal ends of a hot-rolled coil have larger thickness fluctuations (maximum thickness minus minimum thickness) than the steady-state portion, including the central portion. However, the method of the present invention can suppress thickness fluctuations at the longitudinal ends of the hot-rolled coil. For example, assuming the entire longitudinal length of the hot-rolled coil to be 100%, and one longitudinal end to be 0% (the other end to be 100%), the widthwise thickness fluctuation range of 1 to 2% can be controlled to 1.5 times or less the widthwise thickness fluctuation range of 20 to 21%. At both longitudinal ends of the hot-rolled coil, the widthwise thickness fluctuation range of 1 to 2% is preferably 1.5 times or less the widthwise thickness fluctuation range of 20 to 21%.

[0049] The present invention also relates to a method for producing a grain-oriented electrical steel sheet, which comprises hot-rolling a slab by the hot-rolling method of the present invention, annealing the resulting hot-rolled sheet, cold-rolling it once or twice or more times with intermediate annealing in between, and optionally decarburizing annealing before final annealing. Since the hot-rolled sheet has an improved shape at its longitudinal ends, it is possible to suppress meandering and breakage during the cold-rolling process.

[0050] It is important that the hot-rolled sheet annealing be performed at 1150°C or lower. If the hot-rolled sheet annealing temperature exceeds 1150°C, the inhibitor-forming components inevitably mixed in will dissolve and reprecipitate unevenly during cooling, making it difficult to achieve a uniformly sized primary recrystallized structure and inhibiting the development of secondary recrystallization. Furthermore, if the hot-rolled sheet annealing temperature exceeds 1150°C, the grain size after hot-rolled sheet annealing will become too coarse, which is also disadvantageous in achieving an appropriate primary recrystallized structure. From the viewpoint of promoting recrystallization, the hot-rolled sheet annealing is preferably performed at 900°C or higher.

[0051] After hot-rolled sheet annealing, the steel is cold-rolled once or twice or more times with intermediate annealing in between. In cold rolling, it is effective to carry out aging treatment once or twice or more times at a rolling temperature of 80°C to 150°C and at an inter-rolling temperature of 100°C to 300°C, which is performed at an inter-rolling temperature of 100°C to 300°C, in order to develop a Goss structure.

[0052] Next, decarburization annealing is optionally performed to reduce the C content to 50 mass ppm or less, preferably 30 mass ppm or less, at which magnetic aging does not occur.

[0053] Next, primary recrystallization annealing is performed. The purpose of this primary recrystallization annealing is to perform primary recrystallization on the cold-rolled sheet with a rolled texture, adjusting the primary recrystallized grain size to an optimal size for secondary recrystallization, and decarburizing the carbon contained in the steel by using a wet hydrogen-nitrogen or wet hydrogen-argon 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 in a H2-mixed atmosphere at a dew point of 750°C to 900°C. During primary recrystallization annealing, a heating rate of 200°C / s or higher between 550°C and 680°C is preferred, as this further enhances the texture improvement effect. Furthermore, a technique for increasing the Si content by siliconizing may be used after decarburization annealing.

[0054] The steel is then subjected to final annealing to develop a secondary recrystallized structure. A forsterite film may be formed using an annealing separator primarily composed of MgO. Adding an appropriate amount of Ti oxide or Sr compound to the separator can further enhance the formation of the forsterite film. Adding an auxiliary agent that promotes uniform forsterite film formation is particularly beneficial for improving release properties. Alternatively, an optional annealing separator such as Al2O3 may be used to suppress film formation.

[0055] Final annealing must be performed at 800°C or higher to induce secondary recrystallization, but the heating rate up to 800°C can be any condition since it does not significantly affect the magnetic properties. The annealing atmosphere can be N2, Ar, or H2, or a mixture of two or more of these. To more effectively induce secondary recrystallization, the material can be isothermally maintained near the secondary recrystallization temperature. However, isothermal maintenance is not essential, as a similar effect can be achieved by slowing the heating rate. Because the precipitation of trace elements in the final product leads to a deterioration of the magnetic properties, the maximum annealing temperature is set to 1100°C or higher to purify the elements.

[0056] After the final annealing, an insulating coating may be further applied to the surface of the steel sheet and baked. The type of insulating coating is not particularly limited, and any known insulating coating may be used. For example, a preferred method is to apply a coating liquid containing phosphate, chromate, and colloidal silica to the steel sheet, as described in JP-A-50-79442 and JP-A-48-39338, and bake the coating at about 800°C.

[0057] Furthermore, the steel sheet can be shaped by planarizing annealing, which can also be combined with baking of the insulating coating.

[0058] The present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.

[0059] Example 1: A steel slab (1 m wide, 180 mm thick, and 8 m long) containing 3.2-3.4% Si, 0.035-0.055% C, 0.07% Mn, 0.0050-0.0080% Al, less than 0.0060% N, O, and S + 0.405 × Se, with the remainder consisting of Fe and unavoidable impurities, and containing no inhibitor components, was heated using a heating furnace designed to provide a 1.3 m overhang at the slab edge when supported by an extractor (extraction device) used during furnace extraction, according to the heating pattern shown in Table 1. The time required from reaching the maximum temperature (maximum reached temperature) in the furnace to the start of extraction is recorded as 0 minutes if the timing of the start of extraction and the time of reaching the maximum temperature coincide. After extraction, the third and fourth passes of a four-pass rough rolling process were performed under the conditions shown in Table 1. The diameter of the rolls in the rolling mill was 800 mm, and the roll peripheral speed was controlled to obtain the strain rate shown in Table 1. Subsequently, finish hot rolling was performed in a temperature range of 850 to 950°C, with multiple passes, to finish the material to a thickness of 2.2 mm.

[0060] For the resulting hot-rolled coil (total length 1000 m, width 1 m), 300 mm long strip samples (1 m × 300 mm) were cut in the rolling direction at positions 10 m, 12 m, 14 m, 16 m, 18 m, and 20 m from the longitudinal end of the coil to evaluate the shape of the longitudinal end. Similarly, for evaluation of the shape of the steady-state portion, six samples were similarly cut at 2 m intervals, starting 200 m from the end. For each sample, the thickness profile across the coil was measured using a laser profilometer. The difference between the maximum and minimum values ​​was calculated, and the thickness difference between the longitudinal end and the steady-state portion was used as the ratio of the thickness difference between the longitudinal end and the steady-state portion. As is clear from Table 1, the inventive examples exhibited improved shape compared to the comparative examples.

[0061]

[0062] [Example 2] A steel slab containing the components shown in Table 2, with the balance being Fe and unavoidable impurities, and having a calculated γ phase fraction of 10 mol% or less over the entire temperature range, was hot rolled under the conditions also shown in Table 2 using a heating furnace having a structure in which the overhang length of the slab end was 1.3 m when the slab was supported by an extractor (extraction device) used for extracting from the heating furnace.

[0063] Two hot-rolled coils were produced under the same conditions, and one of them was used to evaluate the shape of the longitudinal end in the same manner as in Example 1. The hot-rolled coil from which no samples were taken was annealed at a temperature of 1020 ° C. to check whether it meandered transversely for 20 mm or more during threading. Subsequently, it was subjected to primary cold rolling at 100 ° C. in a reverse mill to a thickness of 1.7 mm, intermediate annealing at 900 ° C. for 1 minute, and then reverse secondary cold rolling again, with coiling aging treatment at 200 ° C. to reduce the thickness to 0.22 mm. Subsequently, primary recrystallization annealing was performed at a heating rate of 300 ° C. / s between 550 ° C. and 680 ° C., a soaking temperature of 840 ° C., and a soaking time of 60 s. An annealing separator containing 95% MgO and 5% TiO was applied to the steel sheet surface as a water slurry and subjected to secondary recrystallization annealing. The surface of the thus-obtained finish-annealed sheet was coated with a coating solution containing phosphate, chromate, and colloidal silica in a weight ratio of 3:1:3, and baked at 800°C. The magnetic properties of the resulting product sheet coil at the center of its width were also measured. The results are shown in Table 2.

[0064] As is clear from Table 2, the inventive examples showed improved shape and improved manufacturing stability. Furthermore, the inventive examples using Steel No. 6 achieved good magnetic properties. Furthermore, while this example utilized a reverse mill, in a continuous line such as a tandem mill, breakage during cold rolling can occur due to meandering within the line, making the inventive examples highly effective.

[0065]

[0066] Example 3 A steel slab containing the components shown in Table 3, with the balance being Fe and unavoidable impurities, and calculated to have a γ-phase fraction of 10 mol % or less over the entire temperature range, was hot-rolled under the conditions also shown in Table 3 using a heating furnace having a structure in which the overhang length of the slab end was 1.3 m when the slab was supported by an extractor (extraction device) used for extraction from the heating furnace. Two hot-rolled coils were produced under the same conditions, and one of the coils was used to evaluate the shape of the longitudinal end in the same manner as in Example 1. The hot-rolled coil from which no sample was taken was checked for meandering in the same manner as in Example 2, and its magnetic properties were also confirmed. The results are shown in Table 3.

[0067]

[0068] As is clear from Table 3, the inventive examples have improved manufacturing stability and good magnetic properties.

[0069] According to the hot rolling method of the present invention, the shape of the slab ends before hot rolling is improved, thereby improving the shape of the longitudinal ends of the obtained hot-rolled coil, which in turn makes the threading of grain-oriented electrical steel sheet more stable in the manufacturing process after the hot rolling step, making it possible to manufacture grain-oriented electrical steel sheet much easier than before. The present invention can also provide a method for manufacturing grain-oriented electrical steel sheet which includes a step of hot-rolling a slab using this hot rolling method, and a hot-rolled coil having a good slab shape.

[0070] 1 Slab 2 Extractor Claw L １ Overhang length L ２ Sinking width L ２

Claims

1. A hot rolling method in which a slab is heated in a heating furnace and then hot rolled, the slab temperature is caused to reach T1 (unit: °C), which is the highest temperature that the slab will reach in the heating furnace, at least 10 minutes before the slab is removed from the heating furnace, and the slab temperature at the time of removal from the heating furnace is T2 (unit: °C), wherein T1≧1200 and T2≦T1-20, and the slab has a chemical composition such that the gamma phase ratio at T1 (unit: °C) is 10 mol% or less.

2. Includes two consecutive hot rolling passes, with each pass in the temperature range of 1030°C to 1150°C, with a reduction rate of 50% or less and a strain rate of 15 s －1 2. The hot rolling method according to claim 1, wherein the rolling is performed at 0.500° C. or more and the interpass time is 15 s or more.

3. The hot rolling method according to claim 1 or 2, wherein the slab is a steel slab having a composition containing, by mass%, C: 0.03% or more and 0.08% or less, Si: 2.0% or more and 8.0% or less, Mn: 0.005% or more and 3.0% or less, Al: less than 0.0100%, 0: 0.0060% or less, N: 0.0060% or less, and S + 0.405 × Se: 0.0060% or less, with the balance being Fe and unavoidable impurities.

4. The hot rolling method according to claim 3, wherein the slab further contains, in mass%, one or more elements selected from the group consisting of Ni: 0.005% or more and 1.50% or less, Sn: 0.01% or more and 0.50% or less, Sb: 0.005% or more and 0.50% or less, Cu: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, P: 0.0050% or more and 0.50% or less, Cr: 0.01% or more and 1.50% or less, Nb: 0.0005% or more and 0.0200% or less, Ti: 0.0005% or more and 0.0200% or less, B: 0.0005% or more and 0.0200% or less, Te: 0.0005% or more and 0.0200% or less, and Bi: 0.0005% or more and 0.0200% or less.

5. A method for producing a hot-rolled coil, comprising hot-rolling a slab by the hot-rolling method according to any one of claims 1 to 4 to obtain a hot-rolled coil.

6. A method for manufacturing a hot-rolled coil as set forth in claim 5, wherein the widthwise thickness fluctuation range of the hot-rolled coil in the range of 1 to 2%, where the entire longitudinal length of the hot-rolled coil is 100% and one end in the longitudinal direction is 0%, is 1.5 times or less than the widthwise thickness fluctuation range of the hot-rolled coil in the range of 20 to 21%.

7. A method for producing grain-oriented electrical steel sheet, comprising hot rolling a slab by the hot rolling method according to any one of claims 1 to 4, annealing the resulting hot-rolled sheet, cold rolling it once or at least twice with intermediate annealing therebetween, and then subjecting it to primary recrystallization annealing and final finish annealing.

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