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

Abstract

Provided is a method of hot rolling that obtains a good shape for hot-rolled coils. A method of hot rolling according to the present disclosure includes hot rolling a slab after heating the slab in a heating furnace. A maximum arrival temperature of the slab in the heating furnace is designated as T1 in °C, a temperature of the slab is raised to T1 at least 10 minutes before the slab is extracted from the heating furnace, the temperature of the slab upon extraction from the heating furnace is designated as T2 in °C. T1 ≥ 1200, T2 ≤ T1 - 20, and the slab has a chemical composition such that the γ phase ratio at T1 is 10 mol% or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of hot rolling, a method of producing a hot-rolled coil, and a method of producing a grain-oriented electrical steel sheet.BACKGROUND

[0002] Grain-oriented electrical steel sheets are typically produced by using precipitates called inhibitors to induce secondary recrystallization of Goss orientation ({110}<001>) grains during final annealing. For example, JP S40-15644 B discloses a method using AlN as an inhibitor, and JP S51-13469 B discloses a method using MnS or MnSe as an inhibitor. Both of these methods have been put into industrial use. The use of these inhibitors is a useful method for stably developing secondary recrystallized grains, but because the precipitates must be finely dispersed, slab heating needs to be performed at a high temperature of 1300 °C or higher before hot rolling.

[0003] However, high-temperature heating of slabs not only increases the equipment costs but also increases the amount of scale generated during hot rolling, resulting in a decrease in yield and making equipment maintenance more complicated.

[0004] On the other hand, a production technique that does not use the inhibitors described above (an inhibitor-less method) has also been proposed. For example, a technique has been proposed in which secondary recrystallization is achieved through the control of texture, using a higher-purity steel without adding inhibitor-forming components to the slab (Patent Literature (PTL) 1).CITATION LISTPatent Literature

[0005] PTL 1: JP 2000-129356 ASUMMARY(Technical Problem)

[0006] Materials containing almost no inhibitor-forming components do not require slab heating at high temperatures of 1,300 °C or higher. Therefore, hot rolling can be carried out using slab heating equipment such as gas furnaces that are commonly used in steel manufacturing, without using special furnaces for slab heating. However, in some products, the longitudinal ends of the coil after hot rolling (hot-rolled coil) have in some cases become deformed, which may lead to meandering in the subsequent hot-rolled sheet annealing process or to sheet fracture in the cold rolling process. This has been a factor hindering production, particularly on an industrial scale.

[0007] The present disclosure advantageously solves the aforementioned problems and aims to provide a method of hot rolling that obtains a good shape for a slab before hot rolling by appropriately controlling the slab heating conditions before hot rolling, thereby obtaining a good shape for a hot-rolled coil, and a method of producing a hot-rolled coil and a grain-oriented electrical steel sheet that includes a process of hot rolling a slab by this method of hot rolling.(Solution to Problem)

[0008] To solve the above-mentioned problems, we have investigated in detail the hot rolling conditions of hot-rolled coils in which shape defects actually occurred and have found that shape defects occur at the longitudinal ends of the hot-rolled coils in the following cases. (1) The maximum arrival temperature reached by the slab during slab heating before hot rolling is 1200 °C or higher, and the slab has a chemical composition in which the γ phase ratio at the maximum arrival temperature is 10 mol% or less. (2) When extracted from the heating furnace, the ends of the slab lifted by the extraction apparatus (extractor) are deformed and sink.

[0009] Here, the slab temperature can be determined by separately passing a slab equipped with a thermocouple through a heating furnace and recording the temperature change of the slab at each position in the heating furnace in advance. For example, thermocouples can be attached at a total of six locations, at the center in the slab longitudinal direction and at both longitudinal ends, on the surface and in the center of the thickness direction. The average value of the six measurements at each position in the heating furnace can be used as the temperature of the slab at that position. If the slab temperature is actually measured in the heating furnace, that value may be used.

[0010] The y phase ratio can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) produced by Thermo-Calc Software AB.

[0011] Here, a conventional method of controlling slab heating is as follows.

[0012] Generally, one of the purposes of slab heating in general steel is to lower the deformation resistance of the steel by raising the temperature of the slab, thereby enabling hot rolling under a large reduction to obtain hot-rolled coils. To fulfill this role, it is necessary to control the temperature of the slabs extracted from the heating furnace to a temperature suitable for hot rolling. The object of control is the temperature of the slab at the time of extraction from the heating furnace, and before extraction, the temperature does not need to be higher than the temperature of the slab at the time of extraction. Therefore, it is common to gradually increase the temperature of the slab in the heating furnace and control the temperature to reach the maximum arrival temperature at the time of extraction.

[0013] 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 interior of the slab. Once an element is solute, it may reprecipitate if the steel temperature drops. For this reason as well, the slab temperature at the time of extraction is commonly controlled to be the maximum arrival temperature.

[0014] When the slab is extracted from the heating furnace after being heated to the maximum arrival temperature, the slab is lifted by an extraction apparatus with a plurality of claws, called an extractor. The slab is then placed on conveying rollers and subjected to hot rolling.

[0015] During extraction, the slab is supported at four points if using a four-claw extractor, and at eight points if using an eight-claw extractor, allowing the slab to be lifted. When a high-temperature slab is lifted with few support points, the ends of the slab are deformed and sink downward due to their own weight. FIG. 1 is a schematic diagram illustrating a state in which the edges of a high-temperature slab being lifted by a four-claw extractor are deformed and sink downward due to their own weight.

[0016] Creep deformation, which occurs particularly at high temperatures, is more likely to occur in the α (ferrite) phase, while the deformation rate in the y (austenite) phase is slower. Therefore, creep deformation, in which the slab ends sink during extraction, is more likely to occur when steel with a low y phase ratio is extracted. The amount of sinking deformation of the slab is also related to the placement of the extractor claws 2 and the slab 1. As illustrated in FIG. 1, if the length from the extractor claw 2 (slab support) located furthest at the end of the slab to the end of the slab 1 is taken as the overhang length Li, then in the case of a slab with a width of 0.9 m to 1.2 m, a thickness of 170 mm to 240 mm, and a total length in the longitudinal direction of 5 m to 15 m, for example, it has been found that deformation is likely to occur if the overhang length L 1 exceeds 1.2 m. In FIG. 1, deformation (sinking width L2) occurs at both ends of the slab, causing the ends to sink.

[0017] It is presumed that the above-described mechanism causes a slab with sunken ends to be extracted from the heating furnace and hot rolled, resulting in poor shape at the longitudinal ends of the hot-rolled coil.

[0018] Based on the presumed mechanism, we investigated the possibility of improving the shape of the hot-rolled coil.

[0019] Here, electrical steel sheets usually contain a high concentration of Si in order to improve the final magnetic properties. Si stabilizes the α phase and reduces the y phase ratio when heated at high temperatures. C is also an element that has a large effect on the y phase ratio. As C has the effect of improving the hot-rolled microstructure and the texture during primary recrystallization, an appropriate amount of C is added from the perspective of improving the final magnetic properties. Therefore, it is difficult to adopt a method of increasing the y phase ratio at high temperatures by significantly changing the composition of an electrical steel sheet that has already undergone the production process.

[0020] The maximum arrival temperature of the slab during slab heating is usually set in order to dissolve trace amounts of impurity elements and precipitate-forming elements and to homogenize the slab. When trace elements are intentionally added to improve the magnetic properties, lowering the maximum arrival temperature is not necessarily an option, but it is possible to lower the temperature of the slab when it is extracted from the heating furnace. If the cause of the poor shape of the longitudinal ends of the hot-rolled coil is the poor shape of the slab ends that occurs during extraction, the shape can likely be improved by lowering the temperature at which the slab is extracted from the heating furnace, where deformation occurs.

[0021] Therefore, we utilized the fact that in actual systems, when elements that have been dissolved and homogenized reprecipitate, 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 and the like. We thus devised a method in which, after first raising the temperature of the slab to achieve homogenization, the temperature of the slab is lowered to a range in which no reprecipitation of impurities or precipitate-forming elements occurs and the slab remains homogenous, thereby suppressing slab deformation during lifting and transportation by an extractor at the time of extraction.

[0022] We completed the present disclosure based on the above findings, and the primary features of the present disclosure are as follows. [1] A method of hot rolling, comprising: hot rolling a slab after heating the slab in a heating furnace, wherein a maximum arrival temperature of the slab in the heating furnace is designated as T 1 in °C, a temperature of the slab is raised to Ti at least 10 minutes before the slab is extracted from the heating furnace, the temperature of the slab upon extraction from the heating furnace is designated as T 2 in °C, T 1 ≥ 1200 , T 2 ≤ T 1 − 20 , and the slab has a chemical composition such that a y phase ratio at T 1 is 10 mol% or less. [2] The method of hot rolling according to [1], wherein the hot rolling includes two consecutive passes, each pass is performed in a temperature range of 1030 °C or higher and 1150 °C or lower, with a rolling reduction of 50 % or less and a strain rate of 15 s -1< or more, and a time between passes of 15 s or longer. [3] The method of hot rolling according to [1] or [2], wherein the slab is a steel slab comprising 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 inevitable impurities. [4] The method of hot rolling according to [3], wherein the slab further comprises, in mass %, one or more 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 of producing a hot-rolled coil, comprising hot rolling a slab by the method of hot rolling according to any one of [1] to [4] to obtain a hot-rolled coil. [6] The method of producing a hot-rolled coil according to [5], wherein a widthwise thickness variation in a range of 1 % to 2 % is 1.5 times or less a widthwise thickness variation in a range of 20 % to 21 %, with an entire longitudinal length of the hot-rolled coil being 100 % and one end in a longitudinal direction being 0 %. [7] A method of producing a grain-oriented electrical steel sheet, the method comprising hot rolling a slab by the method of hot rolling according to any one of [1] to [4], performing hot-rolled sheet annealing on a resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, and performing primary recrystallization annealing and final annealing. (Advantageous Effect)

[0023] According to the method of hot rolling of the present disclosure, the shape of the slab ends before hot rolling is improved, thereby obtaining a good shape for the longitudinal ends of the obtained hot-rolled coil, and ultimately making it possible to pass the grain-oriented electrical steel sheet more stably in the production processes after the hot rolling process. As a result, grain-oriented electrical steel sheets can be produced much more easily than before.

[0024] According to the present disclosure, a method of producing a hot-rolled coil and a grain-oriented electrical steel sheet can be provided, where the method includes a process of hot rolling a slab by the above method of hot rolling.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the accompanying drawings: FIG. 1 is a schematic diagram illustrating a state in which the edges of a high-temperature slab being lifted by a four-claw extractor are deformed and sink downward due to their own weight.DETAILED DESCRIPTION

[0026] The method of hot rolling of the present disclosure will now be described in detail.

[0027] A method of hot rolling according to the present disclosure includes hot rolling a slab after heating the slab in a heating furnace, wherein a maximum arrival temperature of the slab in the heating furnace is designated as Ti (units: °C; omitted below), a temperature of the slab is raised to Ti at least 10 minutes before the slab is extracted from the heating furnace, and the temperature of the slab upon extraction from the heating furnace is designated as T 2 (units: °C; omitted below). T 1 and T 2 satisfy the following Expressions (1) and (2). T 1 ≥ 1200 T 2 ≤ T 1 − 20

[0028] Here, the time from when the slab is charged into the heating furnace until it is extracted is usually 120 minutes or more and 300 minutes or less. The conveying speed of the slab in the heating furnace is usually constant.

[0029] In the method of the present disclosure, a slab having a chemical composition such that the γ phase ratio at T 1 is 10 mol% or less is used. When a slab having a chemical composition in which the y phase ratio is 5 mol % or less is used, a greater effect can be obtained. No lower limit is placed on the y phase ratio, and the case of 0 mol % is also applicable.

[0030] The chemical composition of the slab preferably satisfies the following. Hereinafter, "%" indications regarding chemical compositions are "mass%" unless otherwise specified.C: 0.03 % or more and 0.08 % or less

[0031] C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, and therefore, from the perspective of improving the final magnetic properties, the C content is preferably 0.03 % or more. On the other hand, if the C content exceeds 0.08 %, even if decarburization annealing is performed it becomes difficult to reduce the C content to 50 ppm or less, the content at which magnetic aging does not occur. From this perspective, the C content is preferably limited to 0.08 % or less.Si: 2.0 % or more and 8.0 % or less

[0032] Si is an element useful in reducing iron loss by increasing electrical resistance. In order to obtain good magnetic properties, the Si content is preferably 2.0 % or more. The Si content is more preferably 2.8 % or more. On the other hand, Si is also an element that increases the brittleness of steel. From the perspectives of reducing the risk of fracture during sheet passing and suppressing deterioration of cold rolling manufacturability, the Si content is preferably limited to 8.0 % or less. The Si content is more preferably 4.5 % or less.Mn: 0.005 % or more and 3.0 % or less

[0033] Mn is a useful element from the perspectives of improving hot workability and controlling the formation of an oxide film during primary recrystallization. From this perspective, the Mn content is preferably 0.005 % or more. The Mn content is more preferably 0.01 % or more. On the other hand, from the perspective of avoiding deterioration of the magnetic properties due to deterioration of the primary recrystallized texture, the Mn content is preferably limited to 3.0 % or less. The Mn content is more preferably 0.5 % or less.Al: Less than 0.0100 %, N: 0.0060 % or less, O: 0.0060 % or less, S + 0.405 × Se: 0.0060 % or less

[0034] If Al is excessive, it may be difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition. The Al content is therefore preferably limited to less than 0.0100 %. The Al content is more preferably 0.0800 % or less.

[0035] To prevent the formation of silicon nitrides after purification annealing in the production of the steel sheet after final annealing, the N content is preferably limited to 0.0060 % or less. The N content is more preferably 0.0040 % or less.

[0036] O forms oxides and inhibits deterioration of the magnetic properties of the steel sheet after final annealing. The O content is therefore preferably limited to 0.0060 % or less. The O content is more preferably 0.0030 % or less.

[0037] To stably obtain a secondary recrystallized microstructure, the total amount of S and Se multiplied by 0.405 is preferably limited to 0.0060 % or less. This total amount is more preferably 0.0040 % or less.

[0038] Al, N, O, S and Se are inhibitor components.

[0039] The essential components and the inhibitor components have been described above, but one or more optional components selected from the elements described below may also be included as appropriate. 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.

[0040] Ni is a useful element in terms of improving the microstructure of a hot-rolled coil for better magnetic properties thereof. In order to fully obtain this effect, when Ni is included, the Ni content is preferably 0.005 % or more. On the other hand, if Ni is excessive, secondary recrystallization becomes unstable and magnetic properties deteriorate. The Ni content is therefore preferably 1.50 % or less.

[0041] Sn, Sb, Cu, Mo, P, Cr, B, and Bi are grain boundary segregation elements that can improve various properties, but if they are in excess, the development of secondary recrystallized grains can be inhibited. From these perspectives, when these elements are contained, the amounts are set to the above ranges.

[0042] Nb, Ti and Te are precipitate-forming elements that may improve various properties, but excessive amounts may make secondary recrystallization unstable. From these perspectives, when these elements are contained, the amounts are set to the above ranges.

[0043] The slab used in the method of the present disclosure preferably has a chemical composition containing the aforementioned essential components and optional components, with the balance being Fe and inevitable impurities.

[0044] The slabs used in the method of the present disclosure can be produced by refining molten steel adjusted to a desired chemical composition by a known method using a converter, electric furnace, or the like, and then subjecting the steel to vacuum treatment, if necessary, followed by a conventional method of ingot casting or method of continuous casting. Alternatively, a thin slab or thinner cast steel having a thickness of 100 mm or less may be directly produced by direct casting and used as a slab.

[0045] The slab used in the method of the present disclosure has a chemical composition such that the y phase ratio is 10 mol% or less at T 1 , which is the maximum arrival temperature reached by the slab in the heating furnace. The y phase ratio can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) produced by Thermo-Calc Software AB.

[0046] After adjusting the chemical composition of the slab, the aforementioned thermodynamic software Thermo-calc ver. 2019b (database TCFE7) can be used to set the maximum arrival temperature T 1 at which trace elements are uniformly dissolved or do not precipitate coarsely and unnecessarily exert an inhibition effect. At this time, the y phase ratio at T 1 is similarly calculated and confirmed to be 10 mol% or less.

[0047] Alternatively, the maximum arrival temperature T 1 reached by the slab in the heating furnace may be set, and the chemical composition may then be adjusted so that the y phase ratio is 10 mol% or less at this value of T 1 .

[0048] In the method of the present disclosure, the maximum arrival temperature T 1 is also the maximum arrival temperature reached by the slab in the heating furnace and satisfies Expression (1) above. That is, T 1 is a temperature of 1200 °C or higher. At temperatures above 1200 °C, the slab can be easily homogenized. To suppress creep deformation, T 1 can be preferably set to 1300 °C or lower. T 1 is more preferably 1260 °C or lower.

[0049] In the method of the present disclosure, the slab temperature is raised to T 1 at least 10 minutes before extraction from the heating furnace in order to dissolve trace components uniformly and prevent an unnecessary inhibition effect from being exerted. The slab temperature is preferably raised to T 1 between 30 minutes and 15 minutes before extraction.

[0050] In the method of the present disclosure, the slab temperature T 2 at the time of extraction from the heating furnace satisfies the above Expression (2) in order to suppress creep deformation. That is, T 2 is set to a temperature that is 20 °C or more lower than T 1 . To avoid a situation in which the homogenized precipitate-forming elements are reprecipitated due to a lower temperature, thereby impairing the homogenization effect, T 2 is preferably a temperature 20 °C to 80 °C lower than T 1 .

[0051] T 2 is preferably 1200 °C or lower to suppress creep deformation. T 2 is preferably set to 1120 °C or higher to prevent reprecipitation of precipitates.

[0052] By using the above-described heat pattern, creep deformation, which occurs when the slab edge sinks downward when the slab is lifted by the extractor during extraction, can be suppressed. The method of the present disclosure is advantageous when the length (overhang length) from the endmost extractor claw supporting the slab to the end of the slab is relatively large (for example, when the length exceeds 1.2 m for a slab that is 0.9 m to 1.2 m wide, 200 mm to 240 mm thick, and 5 m to 15 m long in the longitudinal direction). For example, in the case of a slab having a width of 0.9 m to 1.2 m, a thickness of 200 mm to 240 mm, and a total length in the longitudinal direction of 5 to 15 m, the method of the present disclosure is more advantageous when the overhang length is 1.5 m or more and 4.0 m or less. However, the slab used in the method of the present disclosure is not limited to the above-described shape (width, thickness, overall length).

[0053] When the slab composition has a y phase ratio exceeding 10 mol% at the maximum arrival temperature T 1 , or when heating in a range such that the maximum arrival 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 disclosure is applied under these conditions, an improvement in the shape defects of the slab ends can be expected, but the degree 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 arrival temperature. Therefore, over a range in which poor slab shape does not represent a major problem, operation that actively uses the heat pattern of the disclosure is not rational from the perspective of energy efficiency.

[0054] To lower the slab temperature of an already heated slab by 20 °C or more, a temperature difference of 50 °C or more is required in the furnace. Since the temperature of the slab itself drops only slowly, it is extremely difficult to lower the temperature sufficiently during extraction so that the temperature during heated extraction is at least 20 °C lower than the maximum arrival temperature, unless the slab reaches the maximum arrival temperature at least 10 minutes before extraction. Here, the temperature inside the furnace is the temperature of the atmospheric gas inside the furnace and can be measured by a sensor such as a thermocouple. When a plurality of sensors is installed above, below, left, and right of the slab inside the furnace, the average value of the measurements from each sensor can be used. Instead of measuring with a sensor, control may be performed based on a calculated value of the slab temperature.

[0055] 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, and the like. Conventional heating furnaces can be improved as appropriate.

[0056] The length (overhang length) from the extractor claw to the end of the slab when extracting the slab does not have to be the same at both ends. For example, there may be cases in which the overhang length at one end is relatively large (e.g., more than 1.2 m for a slab with a width of 0.9 m to 1.2 m, a thickness of 200 mm to 240 mm, and a total length of 5 m to 15 m in the longitudinal direction), and the overhang length at the other end is relatively small (e.g., 1.2 m or less for a slab with a width of 0.9 m to 1.2 m, a thickness of 200 mm to 240 mm, and a total length of 5 m to 15 m in the longitudinal direction). In such a case, the temperature inside the furnace on the side where the overhang length is relatively long may be actively lowered, and the slab temperature at only that end may be made 20 °C or more lower than the maximum arrival temperature.

[0057] The slab extracted after the above-described slab heating is homogenized steel, and the overhanging part by the extractor (slab ends) sinks only slightly during extraction.

[0058] Next, the slab is subjected to hot rolling, which is relatively easy because sinking and defectiveness of shape at the slab ends have been controlled.

[0059] During the hot rolling process in the method of the present disclosure, 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 or higher to 1150 °C or lower.

[0060] To improve the shape of the longitudinal end portions of the hot-rolled coil, the time between two consecutive passes is preferably 15 s or more, the rolling reduction of each pass is preferably 50 % or less, and the strain rate is preferably 15 s -1< or more.

[0061] The method of the present disclosure uses a slab in which the y phase ratio is 10 mol% or less at T 1 , which is the maximum arrival temperature in the heating furnace, and the y phase ratio normally reaches its maximum in the temperature range of 1030 °C or higher to 1150 °C or lower. Generally, austenite has a higher deformation resistance than ferrite and is less likely to deform even when pressed. Therefore, the rolling reduction for each pass is limited to 50 % or less. From the perspective of homogenizing the structure of the hot-rolled coil, the rolling reduction is preferably 15 % or more. The rolling reduction is more preferably 20 % or more.

[0062] The time between passes is preferably set to 15 s or longer, since by doing so, dislocations formed once by deformation are recovered or disappear by recrystallization, and rolling can be performed without excessively increasing deformation resistance. The time between passes is preferably 120 s or shorter in order to suppress the formation of precipitates nucleated by dislocations generated during deformation.

[0063] The strain rate is preferably 15 s -1< or more, since this facilitates improving the shape of the longitudinal ends of the hot-rolled coil. The strain rate is preferably 50 s -1< or less.

[0064] Here, the strain rate ε can be calculated using the following Ekelund expression. ε ≒ υ R R ′ h 1 2 2 − r ⋅ r

[0065] In the expression, v R is the roller peripheral speed (mm / s), R' is the roll radius (mm), h 1 is the roll entry side thickness (mm), and r is the rolling reduction (%).

[0066] To allow the entry speed of the slab 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 peripheral speed is preferably 4000 mm / s or more. For these reasons, the roll peripheral speed is also preferably 8000 mm / s or less.

[0067] To allow the load to be controlled easily within an appropriate range, the roll radius is preferably 700 mm or more. For this reason, the roll radius is also preferably 1300 mm or less.

[0068] Typically, the longitudinal ends of a hot-rolled coil have larger thickness variation (the value obtained by subtracting the minimum thickness from the maximum thickness) than the steady-state portion including the central portion. However, according to the method of the present disclosure, the thickness variation at the longitudinal ends of the hot-rolled coil can be suppressed. For example, the widthwise thickness variation width in a range of 1 % to 2 % can be controlled to be 1.5 times or less the widthwise thickness variation width in a range of 20 % to 21 %, with the entire longitudinal length of the hot-rolled coil being 100 % and one end in the longitudinal direction being 0 % (the other end being 100 %). At both longitudinal ends of the hot-rolled coil, the widthwise thickness variation in a range of 1 % to 2 % is preferably 1.5 times or less the widthwise thickness fluctuation in a range of 20 % to 21 %.

[0069] The present disclosure also relates to a method of producing a grain-oriented electrical steel sheet, the method including hot rolling a slab by the method of hot rolling according to the present disclosure, performing hot-rolled sheet annealing on the resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, next optionally performing decarburization annealing, and then performing final annealing. The hot-rolled sheet has an improved shape at the longitudinal end, which can suppress meandering, fracture during the cold rolling process, and the like.

[0070] It is important for the hot-rolled sheet annealing to be performed at 1150 °C or lower. If the hot-rolled sheet annealing temperature exceeds 1150 °C, the inhibitor-forming elements that are inevitably mixed in will become solute and will reprecipitate unevenly during cooling, making it difficult to achieve a homogenous primary recrystallization microstructure 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 becomes too coarse, which is also disadvantageous in realizing an appropriate primary recrystallization microstructure. The hot-rolled sheet annealing is preferably performed at 900 °C or higher to promote recrystallization.

[0071] After the hot-rolled sheet annealing, cold rolling is performed once, or twice or more with intermediate annealing performed therebetween. In cold rolling, it is effective to perform aging treatment once, or twice or more, at a rolling temperature of 80 °C or higher and 150 °C or lower, with the temperature between rolling passes being increased to 100 °C or higher and 300 °C or lower, in order to develop a Goss texture.

[0072] Next, decarburization annealing is optionally performed to reduce the C content to 50 mass ppm or less, at which no magnetic aging occurs. The C content is preferably reduced to 30 mass ppm or less.

[0073] Next, primary recrystallization annealing is performed. The purpose of this primary recrystallization annealing is to subject the cold rolled sheet with a rolled microstructure to primary recrystallization and to adjust to the optimum primary recrystallized grain size for secondary recrystallization, and also to decarburize the carbon contained in the steel by setting the annealing atmosphere to a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere and simultaneously form an oxide film on the surface by the aforementioned oxidizing atmosphere. Therefore, the primary recrystallization annealing is preferably performed at 750 °C or higher with the dew point induced in an H 2 mixed atmosphere. The primary recrystallization annealing is preferably performed at 900 °C or lower with the dew point induced in an H 2 mixed atmosphere. During the temperature increase in the primary recrystallization annealing, the heating rate is preferably set to 200 °C / s or higher between 550 °C and 680 °C, as doing so can further enhance the effect of texture improvement. A technique of increasing the Si content by siliconizing after decarburization annealing may also be used in combination.

[0074] Thereafter, a final annealing is performed to develop a secondary recrystallized microstructure. In this case, a forsterite film may be formed using an annealing separator containing MgO as the main component. The forsterite film can be better formed by the addition of appropriate amounts of Ti oxides, Sr compounds, and the like to the separating agent. In particular, the addition of additives that promote uniform forsterite film formation is also advantageous for improving separation properties. Any annealing separator, such as Al 2 O 3 , may also be used to suppress film formation.

[0075] The final annealing needs to be performed at 800 °C or higher to induce secondary recrystallization, but the heating rate up to 800 °C does not have a significant effect on the magnetic properties. Hence, any condition may be used. The annealing atmosphere may be any of N 2 , Ar, or H 2 , or a mixed gas containing two or more of these. To promote secondary recrystallization more effectively, isothermal holding near the secondary recrystallization temperature may be employed. However, since the same effect can be obtained by, for example, slowing down the heating rate, isothermal holding is not essential. Precipitation of trace elements in the final product leads to a deterioration in magnetic properties. The maximum annealing temperature is therefore set to 1100 °C or higher for element purification.

[0076] After the final annealing, an insulating coating may be further applied to the steel sheet surface and baked. The type of insulating coating is not particularly limited, and any known insulating coating can be used. For example, preferred methods are described in JP S50-79442 A and JP S48-39338 A, where a coating liquid containing phosphate-chromate-colloidal silica is applied on a steel sheet and then baked at a temperature of around 800 °C.

[0077] Furthermore, the shape of the steel sheet can be adjusted by performing flattening annealing. The flattening annealing may also be combined with baking of the insulating coating.EXAMPLES

[0078] The present disclosure will be described in detail with reference to examples, but the present disclosure is not limited to these examples.[Example 1]

[0079] A steel slab (1 m wide, 180 mm thick, 8 m long) containing Si: 3.2 % to 3.4%, C: 0.035 % to 0.055 %, Mn: 0.07 %, Al: 0.0050 % to 0.0080 %, N, O, and S + 0.405 × Se: less than 0.0060 % each, with the balance being Fe and inevitable impurities, and containing no inhibitor components, was heated using the heating pattern illustrated in Table 1 in a heating furnace with a structure that allows the slab ends to have an overhang length of 1.3 m when the slab is supported by an extractor (extraction apparatus) used during heating furnace extraction. The time required from when the maximum temperature (maximum arrival temperature) in the furnace was reached to when the extraction started was recorded as 0 minutes when the timing of the start of extraction and the timing of arrival at the maximum temperature coincided. After extraction, the third and fourth passes of four-pass rough rolling were carried out under the conditions illustrated in Table 1. The diameter of the rolls of the rolling mill was 800 m, and the peripheral speed of the rolls was controlled so as to obtain the strain rate illustrated in Table 1. Subsequently, finish hot rolling with multiple passes in the temperature range of 850 °C to 950 °C was performed to finish to a thickness of 2.2 mm.

[0080] For the obtained hot-rolled coil (total length 1000 m, width 1 m), 300 mm long strip samples (1 m × 300 mm) were cut out 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. In addition, to evaluate the shape of the steady-state portion, samples were similarly cut out at six positions 2 m apart, starting from a position 200 m away from the end. For each sample, the thickness profile in the coil width direction was measured using a laser profilometer, the difference between the maximum and minimum values was calculated, and the thickness was evaluated as the ratio of the thickness difference between the longitudinal end side and the steady-state portion. As is clear from Table 1, the shapes of the Examples were improved compared to the Comparative Examples.[Table 1]

[0081] Table 1Steel No.Added components (mass%)*Slab temperatureTime from reaching maximum temperature until starting extractionγ phase ratioRough rolling third passRough rolling fourth passTime between third and fourth passesDifference in thickness at longitudinal end / difference in thickness at steady state portionNotesSiCMnAlNS + 0.405 SeMaximum temperature inside furnaceAt the start of extractionMaximum temperatureTemperatureEach rolling reductionStrain rateTemperatureEach rolling reductionStrain rate13.40.0350.070.00600.0040.00121190 °C1150 °C25 min1%1035 °C25 %25 s -1< 1010 °C38 %40 s -1< 28 s1.4Comparative Example1220 °C1220 °C0 min0 %1110 °C30 %25 s -1< 1080 °C30 %25 s -1< 45 s1.7Comparative Example1220 °C1195 °C18 min0 %1080 °C30 %25 s -1< 1050 °C30%25 s -1< 43 s1.1Example1220 °C1195 °C18 min0 %1080 °C55 %45 s -1< 1050 °C30 %28 s -1< 50 s1.3Example1255 °C1230 °C15 min0%1160 °C31 %25 s -1< 1120 °C42%40 s -1< 38 s1.8Comparative Example23.20.040.070.00800.0050.00381210 °C1190 °C11 min7%1090 °C28 %25 s -1< 1070 °C18 %13 s -1< 90 s1.3Example1210 °C1190 °C11 min7%1110 °C32 %30 s -1< 1090 °C44 %42 s -1< 14 s1.3Example1200 °C1175 °C20 min9%1070 °C30%25 s -1< 1050 °C30 %25 s -1< 38 s1.1Example1220 °C1205 °C8 min6 %1150 °C48 %50 s -1< 1100 °C25 %20 s -1< 55 s1.5Comparative Example1245 °C1195 °C29 min2%1100 °C35 %30 s -1< 1060 °C35 %30 s -1< 20 s1.1Example33.40.0550.070.00500.0030.00241120 °C1120 °C0 min18 %1000 °C25 %25 s -1< 970 °C38 %40 s -1< 28 s1.5Comparative Example1230 °C1230 °C0 min11%1160 °C28 %30 s -1< 1110 °C45 %45 s -1< 35 s1.4Comparative Example1250 °C1185 °C35 min9%1100 °C18%22 s -1< 1070 °C55 %46 s -1< 55 s1.2Example1250 °C1185 °C35 min9%1100 °C25 %25 s -1< 1070 °C26 %21 s -1< 50 s1.1Example* The balance is Fe and incidental impurities. [Example 2]

[0082] Steel slabs containing the components illustrated in Table 2, with the balance being Fe and inevitable impurities, and having a calculated y phase ratio of 10 mol% or less over the entire temperature range, were hot rolled under the set of conditions also illustrated in Table 2 using a heating furnace structure that allows the slab ends to have an overhang length of 1.3 m when the slab is supported by an extractor (extraction apparatus) used during extraction from the heating furnace.

[0083] Two coils were produced for the hot-rolled coils under the same set of conditions, and one of the coils was used to evaluate the shape of the longitudinal end portion in the same manner as in Example 1.

[0084] The hot-rolled coil from which no samples were taken was subjected to hot-rolled sheet annealing at an end-point temperature of 1020 °C, and it was confirmed whether the sheet moved transversely and meandered for 20 mm or more during sheet passing. Thereafter, the sheet was subjected to primary cold rolling at 100 °C in a reverse mill to a thickness of 1.7 mm, followed by intermediate annealing at 900 °C for 1 minute, and then to reverse secondary cold rolling again, with coiling aging treatment at 200 °C during the process, to a sheet thickness of 0.22 mm. Subsequently, primary recrystallization annealing was performed with 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 composed of MgO: 95 % and TiO 2 : 5 % was then applied to the steel sheet surface as an aqueous slurry, and the steel sheet was subjected to secondary recrystallization annealing. A coating liquid containing phosphate-chromate-colloidal silica at a 3:1:3 weight ratio was applied to the surface of the final annealed sheet obtained in this way and was baked thereon at 800 °C. The magnetic properties of the widthwise center of the obtained product sheet coil were also confirmed.

[0085] The results are listed in Table 2.

[0086] As is clear from Table 2, the Examples had improved shape and production stability. Furthermore, in the Example using Steel No. 6, good magnetic properties were obtained. Although a reverse mill is used in these Examples, meandering within the line in a continuous line such as a tandem mill may lead to fracture during cold rolling. The Examples are thus highly effective. [Example 3]

[0087] Steel slabs containing the components illustrated in Table 3, with the balance being Fe and inevitable impurities, and having a calculated y phase ratio of 10 mol% or less over the entire temperature range, were hot rolled under the set of conditions also illustrated in Table 3 using a heating furnace structure that allows the slab ends to have an overhang length of 1.3 m when the slab is supported by an extractor (extraction apparatus) used during extraction from the heating furnace.

[0088] Two coils were produced for the hot-rolled coils under the same set of conditions, and one of the coils was used to evaluate the shape of the longitudinal end portion in the same manner as in Example 1.

[0089] The hot-rolled coils from which no samples were taken were checked for meandering in the same manner as in Example 2, and their magnetic properties were also confirmed.

[0090] The results are listed in Table 3.

[0091] As is clear from Table 3, in the Examples, the production stability is improved and good magnetic properties are obtained.INDUSTRIAL APPLICABILITY

[0092] According to the method of hot rolling of the present disclosure, the shape of the slab ends before hot rolling is improved, thereby obtaining a good shape for the longitudinal ends of the obtained hot-rolled coil, and ultimately making it possible to pass the grain-oriented electrical steel sheet more stably in the production processes after the hot rolling process. A grain-oriented electrical steel sheet can therefore be produced much easier than before. According to the present disclosure, a method of producing a grain-oriented electrical steel sheet can be provided, where the method includes a process of hot rolling a slab by the above method of hot rolling. A hot-rolled coil with a good slab shape can also be provided.REFERENCE SIGNS LIST

[0093] 1Slab 2Extractor claw L 1 Overhang length L 2 Sinking width

Claims

1. A method of hot rolling, comprising: hot rolling a slab after heating the slab in a heating furnace, wherein a maximum arrival temperature of the slab in the heating furnace is designated as T1 in °C, a temperature of the slab is raised to T1 at least 10 minutes before the slab is extracted from the heating furnace, the temperature of the slab upon extraction from the heating furnace is designated as T2 in °C, T 1 ≥ 1200 , T 2 ≤ T 1 − 20 , and the slab has a chemical composition such that a γ phase ratio at T1 is 10 mol% or less.

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

3. The method of hot rolling according to claim 1 or 2, wherein the slab is a steel slab comprising 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 inevitable impurities.

4. The method of hot rolling according to claim 3, wherein the slab further comprises, in mass %, one or more 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 of producing a hot-rolled coil, comprising hot rolling a slab by the method of hot rolling according to any one of claims 1 to 4 to obtain a hot-rolled coil.

6. The method of producing a hot-rolled coil according to claim 5, wherein a widthwise thickness variation width in a range of 1 % to 2 % is 1.5 times or less a widthwise thickness variation width in a range of 20 % to 21 %, with an entire longitudinal length of the hot-rolled coil being 100 % and one end in a longitudinal direction being 0 %.

7. A method of producing a grain-oriented electrical steel sheet, the method comprising hot rolling a slab by the method of hot rolling according to any one of claims 1 to 4, performing hot-rolled sheet annealing on a resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, and performing primary recrystallization annealing and final annealing.

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