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

Abstract

Provided is a hot rolling method for producing a hot-rolled steel coil with good shape. The hot rolling method is a method in which a slab is heated using a heating furnace, and then hot-rolled, and in which the temperature of the slab is brought to a maximum temperature T1 (unit: °C) in the heating furnace before the slab is withdrawn from the heating furnace for 10 minutes or more, and the temperature of the slab at the time of withdrawal from the heating furnace is set to T2 (unit: °C), where T1 ≥ 1200, and T2 ≤ T1 - 20, and the slab has a composition in which the γ phase ratio at T1 (unit: °C) is 10 mol% or less.

Description

Technical Field

[0001] This invention relates to a hot rolling method, a method for manufacturing hot-rolled steel coils, and a method for manufacturing directional electromagnetic steel sheets. Background Technology

[0002] The manufacture of directional electromagnetic steel sheets typically utilizes precipitates known as inhibitors, achieved by causing secondary recrystallization of Gaussian-oriented ({110}<001>) grains during the final annealing process. For example, Japanese Patent Publication No. 40-15644 discloses a method using AlN and MnS as inhibitors, and Japanese Patent Publication No. 51-13469 discloses a method using MnS and MnSe as inhibitors; both have been industrially applied. These methods using inhibitors are useful for stably promoting the growth of secondary recrystallized grains, but because the precipitates must be finely dispersed, slab heating before hot rolling is required at temperatures above 1300°C.

[0003] However, in addition to increasing equipment costs, the high-temperature heating of slabs also increases the amount of oxide scale generated during hot rolling, resulting in problems such as reduced yield and complicated equipment maintenance.

[0004] On the other hand, manufacturing techniques that do not use the inhibitors mentioned above (inhibitor-free methods) have also been proposed. For example, a technique has been proposed that utilizes steel with higher purity obtained by eliminating inhibitor-forming components in the slab, and exhibits secondary recrystallization by controlling the texture (aggregate structure) (Patent Document 1).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2000-129356 Summary of the Invention

[0008] Since billets containing almost no inhibitors do not require slab heating at temperatures above 1300°C, hot rolling can be performed using conventional slab heating equipment such as gas furnaces used in steel manufacturing, without the need for special furnaces. However, in some products, shape defects occur at the long-side ends of the hot-rolled coils, which may lead to serpentine behavior during subsequent hot-rolled annealing processes and plate breakage during cold rolling processes, particularly hindering industrial-scale production.

[0009] The present invention advantageously solves the above-mentioned problems, and its object is to provide a hot rolling method that, by appropriately controlling the heating conditions of the slab before hot rolling, results in a good shape of the slab before hot rolling, thereby resulting in a good shape of the hot-rolled steel coil; and a method for manufacturing hot-rolled steel coils and directional electromagnetic steel sheets, including the process of hot rolling the slab using the hot rolling method.

[0010] In order to solve the above-mentioned problems, the inventors conducted a detailed investigation into the hot rolling conditions of hot-rolled steel coils that actually produced shape defects, and found that shape defects occurred at the ends of the long side of the hot-rolled steel coils under the following conditions.

[0011] (1) In the slab heating before hot rolling, the slab reaches a maximum temperature of 1200℃ or above, and the slab has a composition with a γ phase fraction of less than 10 mol% at the maximum temperature.

[0012] (2) When the slab is pulled out of the heating furnace, the end of the slab lifted by the extraction device (extraction machine) sinks and deforms.

[0013] Here, the slab temperature can be determined by separately passing the slab equipped with thermocouples through a heating furnace and pre-recording the temperature changes of the slab at various locations within the furnace. Thermocouples can be installed at, for example, a total of six locations: the center along the long side of the slab, the respective surfaces at the ends of the long sides along the long side, and the center along the thickness direction. The average of the measurements taken at these six locations within the heating furnace can be used as the temperature of the slab at that location. This value can be used when actually measuring the slab temperature inside the heating furnace.

[0014] In addition, the γ phase index can be calculated using Thermo-calcver.2019b (database TCFE7), a thermodynamic software developed by Thermo-Calc Software AB.

[0015] Here, the traditional method for controlling slab heating is as follows.

[0016] One of the purposes of heating slabs in ordinary steel production is to reduce the deformation resistance of the steel by raising the slab to a high temperature, thus enabling hot rolling with a large reduction to obtain hot-rolled steel coils. To achieve this, the temperature of the slab extracted from the furnace needs to be controlled at a suitable temperature for hot rolling. The controlled temperature is the temperature of the slab at the moment of extraction from the furnace, and it does not need to be higher than the temperature of the slab at previous extraction times. Therefore, it is usually controlled by gradually increasing the slab temperature within the furnace and reaching the maximum reachable temperature upon extraction.

[0017] Furthermore, in the manufacturing of directional electromagnetic steel sheets, it is also required to dissolve trace amounts of impurity elements and precipitate-forming elements to homogenize the slab. If the steel temperature decreases, the elements that have been dissolved once may precipitate again. Therefore, from this perspective, the temperature of the slab is usually controlled by ensuring that the temperature reaches its maximum at the moment of extraction.

[0018] The slab, heated to its maximum temperature, is lifted from the furnace by a pull-out device with multiple claws, called a pull-out machine, and placed on conveyor rollers for hot rolling.

[0019] For the slab during extraction, if it is a 4-jaw extractor, it will be supported and lifted at 4 points; if it is an 8-jaw extractor, it will be supported and lifted at 8 points. If the hot slab is lifted with few support points, the ends of the slab will sink and deform downwards due to their own weight. Figure 1 This is a schematic diagram showing the state in which the end of a hot slab lifted by a four-jaw extractor sinks and deforms downward due to its own weight.

[0020] Especially at high temperatures, creep deformation tends to occur in the α (ferrite) phase, while the γ (austenite) phase has a slower strain rate. Therefore, the creep deformation resulting in the slab end sinking during extraction is more likely to occur when steel with a low γ phase ratio is extracted. Furthermore, the amount of slab sinking deformation is also related to the claw 2 of the extractor and the placement position of the slab 1. For example... Figure 1 As shown, if the length from the claw 2 (slab support) of the extractor located at the end closest to the slab end to the end of the slab 1 is defined as the cantilever length L1, for 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 in the long side direction of 5 to 15 m, it is found that deformation is prone to occur when the cantilever length L1 exceeds 1.2 m. Figure 1 In the middle, deformation such as sinking of the slab ends occurs at both ends (sinking amplitude L2).

[0021] It is speculated that due to the mechanism described above, the slab with the end in a sunken shape is drawn out of the heating furnace and hot rolled, thus the poor shape of the long side end of the hot rolled steel coil can be observed.

[0022] Based on the hypothesized mechanism, the inventors investigated the possibility of improving the shape of hot-rolled steel coils.

[0023] Here, to improve the final magnetic properties of electromagnetic steel sheets, a high concentration of Si is typically included in the steel. Si stabilizes the α phase and reduces the γ phase inclination during high-temperature heating. Additionally, C can also be cited as an element that significantly affects the γ phase inclination, but C has the effect of improving the hot-rolled structure and the aggregate structure during primary recrystallization; therefore, from the viewpoint of improving the final magnetic properties, there exists an appropriate amount. Thus, it is difficult to improve the γ phase inclination at high temperatures by drastically altering the composition of the electromagnetic steel sheet already manufactured in the process.

[0024] The maximum temperature reached by the slab during slab heating is usually set based on the goal of homogenizing the slab by solidifying trace amounts of impurity elements and precipitate-forming elements. While it's possible to lower the maximum temperature by including trace elements to improve magnetic properties, it's not always feasible to select a lower maximum temperature, but the temperature at which the slab is removed from the furnace can be reduced. If the poor shape of the long side of the hot-rolled steel coil is due to a poor shape at the slab end during removal, then lowering the temperature at which the deformed slab is removed from the furnace significantly increases the likelihood of improving the shape.

[0025] Therefore, the inventors have effectively utilized the fact that in the actual system, there exists a supersaturated temperature region where precipitation will not occur even if the temperature is lower than the precipitation temperature calculated by thermodynamic equilibrium calculations during element re-precipitation after a single solid solution and homogenization process. They have conceived of a method that, after homogenizing the slab by heating it to a high temperature once, heats the slab to a low temperature within a range that does not cause the re-precipitation of impurities or precipitate-forming elements and maintains the homogeneous state of the slab, thereby suppressing slab deformation during extraction and conveying by the extraction machine.

[0026] Based on the above understanding, the main idea of ​​the present invention completed by the inventors is as follows.

[0027] [1] A hot rolling method, wherein a slab is heated in a heating furnace and then hot rolled.

[0028] More than 10 minutes before the slab is removed from the heating furnace, the slab temperature is brought to the highest temperature reached by the slab in the heating furnace, namely T1 (unit: °C), and the slab temperature at the time of removal from the heating furnace is set as T2 (unit: °C).

[0029] here,

[0030] T1≥1200,

[0031] T2≤T1-20,

[0032] The above-mentioned slab has a composition with a γ phase fraction of less than 10 mol% at T1 (unit: °C).

[0033] [2] According to the hot rolling method of [1], wherein the hot rolling includes two consecutive passes, each pass being in a temperature range of 1030°C to 1150°C with a reduction rate of less than 50% and a strain rate of 15 s. －1 The above procedure is performed, with a time interval of 15 seconds or more between each pass.

[0034] [3] According to the hot rolling method of [1] or [2], wherein the above-mentioned slab is a steel billet having the following composition: containing, by mass %

[0035] C: 0.03%~0.08%

[0036] Si: 2.0%–8.0%

[0037] Mn: 0.005%~3.0%

[0038] Al: less than 0.0100%

[0039] O: Below 0.0060%

[0040] N: below 0.0060%, and

[0041] S + 0.405 × Se: below 0.0060%,

[0042] The remainder consists of Fe and unavoidable impurities.

[0043] [4] According to the hot rolling method of [3], wherein the above-mentioned slab further contains, by mass percent, one or more of the following components:

[0044] Ni: 0.005%~1.50%

[0045] Sn: 0.01%~0.50%

[0046] Sb: 0.005%~0.50%

[0047] Cu: 0.01%–0.50%

[0048] Mo: 0.01%–0.50%

[0049] P: 0.0050%~0.50%

[0050] Cr: 0.01%~1.50%

[0051] Nb: 0.0005%~0.0200%

[0052] Ti: 0.0005%~0.0200%

[0053] B: 0.0005%~0.0200%

[0054] Te: 0.0005%~0.0200%, and

[0055] Bi: 0.0005%~0.0200%.

[0056] [5] A method for manufacturing hot-rolled steel coil, wherein a slab is hot-rolled by any one of the hot rolling methods in [1] to [4] to obtain a hot-rolled steel coil.

[0057] [6] According to the manufacturing method of hot-rolled steel coil of [5], the total length of the long side of the hot-rolled steel coil is set to 100%, the end of one side of the long side is set to 0%, and the thickness variation in the width direction within the range of 1 to 2% is less than 1.5 times the thickness variation in the width direction within the range of 20 to 21%.

[0058] [7] A method for manufacturing a directional electromagnetic steel sheet, wherein a slab is hot rolled by any one of the hot rolling methods in [1] to [4], the hot rolled sheet is annealed, and then cold rolled once or twice or more with intermediate annealing, and then recrystallization annealing and final product annealing are performed.

[0059] According to the hot rolling method of the present invention, by ensuring a good shape at the end of the slab before hot rolling, the long-side end of the resulting hot-rolled steel coil can be well-shaped, thereby making the directional electromagnetic steel sheet more stable in subsequent manufacturing processes after the hot rolling process. As a result, directional electromagnetic steel sheets can be manufactured much more easily than before.

[0060] According to the present invention, a method for manufacturing hot-rolled steel coils and directional electromagnetic steel sheets, including the process of hot rolling a slab using the hot rolling method, can also be provided. Attached Figure Description

[0061] Figure 1 This is a schematic diagram showing the state in which the end of a hot slab lifted by a four-jaw extractor sinks and deforms downward due to its own weight. Detailed Implementation

[0062] The hot rolling method of the present invention will be described in detail below.

[0063] This invention is a hot rolling method that uses a heating furnace to heat a slab before hot rolling. At least 10 minutes before the slab is removed from the heating furnace, the slab temperature is brought to the maximum temperature T1 (unit: °C, hereinafter omitted) within the furnace. The slab temperature at the time of removal from the furnace is set as T2 (unit: °C, hereinafter omitted). T1 and T2 satisfy the following equations (1) and (2).

[0064] T1≥1200 (1)

[0065] T2≤T1-20 (2)

[0066] Here, the time from when the slab is loaded into the heating furnace until it is removed is typically 120 to 300 minutes. The conveying speed of the slab inside the heating furnace is usually constant.

[0067] In the method of the present invention, a slab composed of a component having a γ phase fraction of 10 mol% or less at T1 is used. Greater effects can be achieved when using a slab composed of a component having a γ phase fraction of 5 mol% or less. Furthermore, the lower limit of the γ phase fraction is not particularly limited, and it can also be applied at 0 mol%.

[0068] The composition of the slab preferably meets the following conditions. Unless otherwise specified, "%" in relation to composition refers to "mass %".

[0069] C: 0.03%~0.08%

[0070] Carbon (C) has the effect of improving the microstructure of hot-rolled structures and the aggregate structure during primary recrystallization. Therefore, from the viewpoint of improving the final magnetic properties, a content of 0.03% or more is preferred. On the other hand, if the C content exceeds 0.08%, it is difficult to reduce C to below 50 ppm without inducing magnetic aging, even with decarburization annealing. From this perspective, C is preferably limited to 0.08% or less.

[0071] Si: 2.0%–8.0%

[0072] Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, Si is preferably contained at 2.0% or more, more preferably at 2.8% or more. On the other hand, Si is also an element that increases the brittleness of steel. From the perspective of reducing the risk of fracture during sheet rolling and suppressing the deterioration of cold rolling properties, Si is preferably limited to 8.0% or less, more preferably 4.5% or less.

[0073] Mn: 0.005%~3.0%

[0074] Mn is also a useful element from the viewpoint of improving hot workability and controlling the formation of oxide films during primary recrystallization. From this perspective, Mn is preferably contained at 0.005% or more, more preferably at 0.01% or more. On the other hand, from the viewpoint of avoiding the deterioration of magnetic properties due to the degradation of the aggregate structure during primary recrystallization, Mn is preferably limited to 3.0% or less, more preferably 0.5% or less.

[0075] Al: less than 0.0100%, N: less than 0.0060%, O: less than 0.0060%, S + 0.405 × Se: less than 0.0060%.

[0076] If Al is in excess, it may be difficult to obtain a secondary recrystallization structure due to the effect of texture inhibition. Therefore, Al is preferably limited to less than 0.0100%, and more preferably less than 0.0800%.

[0077] To prevent the formation of Si nitrides after purification annealing during the manufacturing of the final product board, N is preferably limited to 0.0060% or less, more preferably 0.0040% or less.

[0078] In order to suppress the formation of oxides and the deterioration of magnetic properties in the final product board, O is preferably limited to 0.0060% or less, more preferably 0.0030% or less.

[0079] In order to stably obtain a secondary recrystallized structure, the total amount of S and Se obtained by multiplying by 0.405 is preferably limited to 0.0060% or less, more preferably 0.0040% or less.

[0080] Al, N, O, S, and Se are the inhibitory components.

[0081] The essential and inhibitory components have been described above, but it is also appropriate to further include one or more elements selected from the elements described below as any component.

[0082] Ni: 0.005%~1.50%

[0083] Sn: 0.01%~0.50%

[0084] Sb: 0.005%~0.50%

[0085] Cu: 0.01%–0.50%

[0086] Mo: 0.01%–0.50%

[0087] P: 0.0050%~0.50%

[0088] Cr: 0.01%~1.50%

[0089] Nb: 0.0005%~0.0200%

[0090] Ti: 0.0005%~0.0200%

[0091] B: 0.0005%~0.0200%

[0092] Te: 0.0005%~0.0200%, and

[0093] Bi: 0.0005%~0.0200%

[0094] Ni is a useful element for improving the microstructure of hot-rolled steel coils and thus enhancing their magnetic properties. To fully achieve this effect, when Ni is present, the content of Ni is preferably 0.005% or more. On the other hand, if Ni is in excess, secondary recrystallization becomes unstable and the magnetic properties deteriorate; therefore, the content of Ni is preferably 1.50% or less.

[0095] Sn, Sb, Cu, Mo, P, Cr, B, and Bi are grain boundary segregation elements that can improve various properties, but if in excess, they may hinder the growth of secondary recrystallized grains. Based on these considerations, the amounts within the aforementioned range are used when these elements are present.

[0096] Nb, Ti, and Te are precipitate-forming elements that sometimes improve various properties, but if in excess, they may destabilize secondary recrystallization. Therefore, in cases containing these elements, the amounts within the range described above should be used.

[0097] The slab used in the method of the present invention preferably has the following composition: containing the above-mentioned essential components and, depending on the circumstances, any other components, with the remainder consisting of Fe and unavoidable impurities.

[0098] The slabs used in the method of this invention can be manufactured by refining molten steel with the desired composition using known methods such as converters or electric furnaces, performing vacuum treatment as needed, and then using conventional ingot casting or continuous casting methods. Alternatively, thin castings with a thickness of 100 mm or less can be directly manufactured using direct casting methods and used as slabs.

[0099] The slab used in the method of this invention has a composition with a γ phase fraction of less than 10 mol% at the highest temperature reached by the slab in the heating furnace, i.e., T1. The γ phase fraction can be calculated using the thermodynamic software Thermo-calc ver.2019b (database TCFE7) developed by Thermo-Calc Software AB.

[0100] After adjusting the composition of the slab, the thermodynamic software Thermo-calc ver.2019b (database TCFE7) can be used to set the highest reaching temperature T1, where trace amounts of components are uniformly dissolved or do not precipitate coarsely and unnecessarily exhibit an inhibitory effect. At this point, the γ phase fraction at T1 is calculated and confirmed to be below 10 mol%.

[0101] Alternatively, the maximum temperature reached by the slab in the heating furnace, T1, can be set, and the composition can be adjusted so that the γ phase rate at T1 is below 10 mol%.

[0102] In the method of the present invention, the highest reached temperature T1 is the highest reached temperature of the slab in the heating furnace, satisfying the above formula (1). That is, T1 is a temperature of 1200°C or higher. If it is 1200°C or higher, the slab can be easily homogenized. From the perspective of suppressing creep deformation, T1 is preferably 1300°C or lower, and more preferably 1260°C or lower.

[0103] In the method of the present invention, from the perspective of ensuring uniform solid solution of trace components without exhibiting unnecessary inhibitory effects, the slab temperature is reached at least 10 minutes before being withdrawn from the heating furnace. Preferably, the slab temperature is reached at T1 between 15 and 30 minutes before withdrawal.

[0104] In the method of the present invention, from the perspective of suppressing creep deformation, the slab temperature T2 when it is pulled out of the heating furnace satisfies the above formula (2). That is, T2 is a temperature that is 20°C or more lower than T1. In order to avoid the redeposition of homogenized precipitate-forming elements due to low temperature and damage to the homogenization effect, T2 is preferably a temperature that is 20 to 80°C lower than T1.

[0105] From the perspective of suppressing creep deformation, T2 is preferably below 1200°C. From the perspective of suppressing the re-precipitation of precipitates, T2 is preferably above 1120°C.

[0106] By employing the thermal mode described above, creep deformation that causes the slab end to sink downwards can be suppressed when the slab is lifted by the extraction machine during extraction. The method of the present invention is advantageous when the length (cantilever length) from the claw of the extraction machine supporting the slab end to the end of the slab is relatively large (e.g., exceeding 1.2m for slabs with a width of 0.9–1.2m, a thickness of 200–240mm, and a total length in the long side direction of 5–15m). For example, in slabs with a width of 0.9–1.2m, a thickness of 200–240mm, and a total length in the long side direction of 5–15m, the method of the present invention is further advantageous when the cantilever length is 1.5m–4.0m. However, the slab used in the method of the present invention is not limited to slabs with the shapes (width, thickness, total length) described above.

[0107] When the slab composition has a γ phase fraction exceeding 10 mol% at the maximum reaching temperature T1, or when heating is performed within a range where the maximum reaching temperature is less than 1200°C, the deformation when the slab is lifted by the extraction machine during extraction is not as significant. Under these conditions, the application of the thermal mode of the present invention can also be expected to improve the shape defects at the slab ends, but the effect is relatively small. Generally, heating furnaces improve fuel utilization efficiency during heating by gradually heating the slab to the target maximum reaching temperature. Therefore, from an energy efficiency point of view, it is not reasonable to actively use the thermal mode of the invention when the shape defects of the slab are not a major problem.

[0108] To reduce the slab temperature by more than 20°C after a single heating cycle, a temperature difference of at least 50°C is required within the furnace. Since the slab's temperature decreases slowly, ensuring the temperature at extraction is at least 20°C lower than the maximum reached temperature is crucial. If the slab hasn't reached the maximum reached temperature more than 10 minutes before extraction, sufficient cooling during extraction is extremely difficult. Here, the furnace temperature refers to the temperature of the atmosphere within the furnace, which can be measured using sensors such as thermocouples. If multiple sensors are installed around the slab within the furnace, averaging the readings from each sensor is possible. Alternatively, control can be based on calculated slab temperatures instead of sensor measurements.

[0109] In addition, since the slab itself carries heat, it is advantageous to have different systems of burners, furnace wall structures that suppress radiant heat, and convection control mechanisms for atmospheric gases at different locations in the furnace in order to create a large temperature difference inside the furnace. Therefore, it is possible to make appropriate improvements to the existing furnaces.

[0110] The length from the claw of the extractor to the end of the slab (cantilever length) can differ at both ends. For example, one end may have a relatively large cantilever length (e.g., exceeding 1.2m for slabs with a width of 0.9–1.2m, a thickness of 200–240mm, and a total length along the long side of 5–15m), while the other end may have a relatively small cantilever length (e.g., less than 1.2m for slabs with a width of 0.9–1.2m, a thickness of 200–240mm, and a total length along the long side of 5–15m). In such cases, the furnace temperature on the side with the larger cantilever length can be aggressively reduced, ensuring that the slab temperature is at least 20°C lower than the maximum reaching temperature, only at that end.

[0111] The slab that has been heated and extracted as described above has been homogenized in the steel and can achieve a state where the cantilever part (end of the slab) of the extractor sinks little during extraction.

[0112] Next, the slab is hot rolled. Since the poor shape of the slab end is suppressed, the rolling is relatively easy.

[0113] In the hot rolling process of the method of the present invention, at least two consecutive rolling passes from the slab stage to the production of the intermediate slab can be performed in a temperature range of 1030°C to 1150°C.

[0114] From the perspective of improving the shape of the long side end of hot-rolled steel coil, it is preferable that the time between two consecutive passes is more than 15 seconds, the reduction rate of each pass is less than 50%, and the strain rate is 15 seconds. －1 above.

[0115] The method of this invention uses slabs with a γ phase fraction of less than 10 mol% at the highest reached temperature (T1) in the heating furnace, typically with the γ phase fraction being highest in the temperature range of 1030°C to 1150°C. Generally, austenite has higher deformation resistance than ferrite and is not easily deformed even when pressed. Therefore, the reduction rate for each pass is limited to less than 50%. From the perspective of homogenizing the microstructure of the hot-rolled steel coil, the reduction rate is preferably 15% or more, more preferably 20% or more.

[0116] From the perspective of enabling rolling without excessively increasing deformation resistance, the time between passes is preferably 15 seconds or more, so that dislocations formed during a single deformation can be recovered or eliminated due to recrystallization. From the perspective of suppressing the formation of precipitates with dislocations generated during deformation as nuclei, the time between passes is preferably 120 seconds or less.

[0117] From the perspective of easily improving the shape of the long side end of the hot-rolled steel coil, the strain rate is preferably 15s. -1 The above. The preferred strain rate is 50 s. -1 the following.

[0118] Here, the strain rate ε can be calculated using the following Ekelund formula.

[0119]

[0120] In the formula, v R R' is the roller circumferential speed (mm / s), H' is the roller radius (mm), h1 is the thickness of the roller inlet side plate (mm), and r is the reduction rate (%).

[0121] From the perspective of being able to control the entry speed of the slab onto the roll within an appropriate range and to keep the slab in a held state through friction with the roll at an earlier stage, the roll circumferential speed is preferably 4000 mm / s to 8000 mm / s.

[0122] From the perspective of being able to easily control the load within an appropriate range, the roller radius is preferably 700mm to 1300mm.

[0123] Typically, the thickness variation (the value obtained by subtracting the minimum thickness from the maximum thickness) is larger at the long-side ends of a hot-rolled steel coil compared to the constant portion including the central portion. However, according to the method of the present invention, the thickness variation at the long-side ends of the hot-rolled steel coil can be suppressed. For example, by setting the total length of the long side of the hot-rolled steel coil to 100% and setting one end of the long side direction to 0% (the other end to 100%), the thickness variation in the width direction within the range of 1 to 2% can be controlled to be less than 1.5 times that within the range of 20 to 21%. At both ends of the long side direction of the hot-rolled steel coil, the thickness variation in the width direction within the range of 1 to 2% is preferably less than 1.5 times that within the range of 20 to 21%.

[0124] This invention also relates to a method for manufacturing directional electromagnetic steel sheets, which involves hot rolling a slab using the hot rolling method of this invention, followed by hot rolling annealing of the resulting hot-rolled sheet, then cold rolling once or twice or more with intermediate annealing, followed by decarburization annealing as needed, and finally final annealing. Because the shape of the long-side end of the hot-rolled sheet is improved, it can suppress serpentine behavior and breakage during the cold rolling process.

[0125] It is crucial that the annealing of hot-rolled steel sheets be carried out below 1150°C. If the annealing temperature exceeds 1150°C, the inhibitory components inevitably mixed in will dissolve and recrystallize unevenly upon cooling, making it difficult to achieve a uniform primary recrystallization structure and hindering the growth of secondary recrystallization. Furthermore, if the annealing temperature exceeds 1150°C, the resulting grain size will be excessively coarse, which is also detrimental to achieving a suitable primary recrystallization structure. From the perspective of promoting recrystallization, annealing of hot-rolled steel sheets is preferably carried out at 900°C or higher.

[0126] After hot-rolled sheet annealing, cold rolling is performed once or twice or more with intermediate annealing. From the perspective of promoting Gaussian structure growth, it is effective to use a cold rolling temperature of 80℃~150℃ and to perform aging treatment once or twice or more to raise the temperature between rolling passes to 100℃~300℃.

[0127] Next, decarburization annealing is performed as needed to reduce the C content to below 50 ppm by mass without inducing magnetic aging, preferably to below 30 ppm by mass.

[0128] Next, a recrystallization annealing is performed. The purpose of this recrystallization annealing is to adjust the primary recrystallization grain size of the cold-rolled sheet with the rolled structure to the optimal size for secondary recrystallization. By using a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere, the carbon contained in the steel is decarburized, and an oxide film is formed on the surface using the aforementioned oxidizing atmosphere. Therefore, the primary recrystallization annealing is preferably performed at 750°C to 900°C under a mixed H2 atmosphere with the dew point introduced. During the heating of the primary recrystallization annealing, a heating rate of 200°C / s or more between 550°C and 680°C can further improve the improvement effect on the aggregate structure, and is therefore preferred. Alternatively, a technique for increasing the Si content by silicon diffusion after decarburization annealing can also be used.

[0129] Then, secondary recrystallization occurs through final annealing. At this stage, an annealing separating agent primarily composed of MgO can be used to form a forsterite coating. Adding appropriate amounts of Ti oxides, Sr compounds, etc., to the separating agent further promotes the formation of the forsterite coating. In particular, adding aids that promote uniform forsterite coating formation also helps improve exfoliation properties. Alternatively, any annealing separating agent, such as Al2O3, can be used to inhibit coating formation.

[0130] To exhibit secondary recrystallization, the final annealing needs to be carried out at a temperature above 800°C. The heating rate up to 800°C does not significantly affect the magnetic properties, so any conditions are acceptable. For the annealing atmosphere, N2, Ar, or H2, or a mixture containing two or more of these gases, are suitable. To more favorablely induce secondary recrystallization, isothermal holding near the secondary recrystallization temperature can be performed; however, since the same effect can be achieved by slowing the heating rate, isothermal holding is not necessary. If trace components precipitate in the final product, it will lead to deterioration of the magnetic properties; therefore, for component purification, the maximum annealing temperature should be above 1100°C.

[0131] After the final product is annealed, an insulating film can be further coated onto the surface of the steel plate and then sintered. There are no particular limitations on the type of insulating film; any known insulating film can be used. For example, the method described in Japanese Patent Application Publication Nos. 50-79442 and 48-39338, which involves coating a steel plate with a coating solution containing phosphate-chromate-colloidal silica and sintering it at approximately 800°C, is preferred.

[0132] Alternatively, planarization annealing can be performed to adjust the shape of the steel sheet. Planarization annealing can also be performed together with the sintering treatment of the insulating film.

[0133] Example

[0134] The present invention will be described in detail through embodiments, but the present invention is not limited to the embodiments.

[0135] [Example 1]

[0136] A steel billet (1m wide, 180mm thick, 8m total length along the long side) containing Si: 3.2-3.4%, C: 0.035-0.055%, Mn: 0.07%, Al: 0.0050-0.0080%, N: less than 0.0060%, O: less than 0.0060%, S + 0.405 × Se: less than 0.0060%, with the remainder consisting of Fe and unavoidable impurities, and without inhibitory components, was heated in a heating furnace under the heating mode shown in Table 1. The furnace had a structure with a 1.3m cantilever length at the end of the billet when supported by an extraction machine (extraction device) used during furnace extraction. The time required from reaching the highest temperature in the furnace (maximum reached temperature) to the start of extraction was recorded as 0 minutes if the start of extraction coincided with the reaching of the highest temperature. After extraction, the third and fourth passes of a four-pass roughing process were performed under the conditions shown in Table 1. The mill roll diameter was 800 μm, and the roll circumferential speed was controlled in a manner that allowed for the strain rates shown in Table 1. Next, a multi-pass finishing hot rolling process was carried out in a temperature range of 850–950 °C to achieve a thickness of 2.2 mm.

[0137] For the obtained hot-rolled steel coil (total length 1000m, width 1m), to evaluate the shape of the long-side end, short strip samples (1m × 300mm) were cut along the rolling direction at positions 10m, 12m, 14m, 16m, 18m, and 20m away from the long-side end of the steel coil, each 300mm long. Similarly, for the shape evaluation of the constant portion, samples were cut at 6 points at 2m intervals, 200m away from the end. For each sample, the thickness profile in the width direction of the steel coil was measured using a laser profilometer, and the difference between its maximum and minimum values ​​was calculated. The evaluation was based on the ratio of the thickness difference between the long-side end and the constant portion. As shown in Table 1, the shape of the inventive example was improved compared to the comparative example.

[0138] [Table 1]

[0139]

[0140] [Example 2]

[0141] Using the same heating furnace as shown in Table 2, a steel billet containing the composition shown in Table 2, with the remainder consisting of Fe and unavoidable impurities, and with a calculated γ phase fraction of less than 10 mol% over all temperature range is hot rolled. The heating furnace has a structure in which the cantilever length of the slab end is 1.3 m when the slab is supported by an extractor (extraction device) used when it is pulled out of the heating furnace.

[0142] Two hot-rolled steel coils were produced under the same conditions. One coil was used, and the shape of the long side end was evaluated using the same method as in Example 1.

[0143] Hot-rolled steel coils without sample collection were annealed at 1020°C to confirm whether they exhibited lateral movement or serpentine behavior exceeding 20mm during plate passage. Then, using a reversible mill, they were cold-rolled once to 1.7mm at 100°C, followed by intermediate annealing at 900°C for 1 minute, and then reversibly cold-rolled again, with an intermediate winding aging treatment at 200°C to achieve a plate thickness of 0.22mm. Next, a 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 60s. An annealing separating agent of 95% MgO and 5% TiO2 was applied to the steel plate surface in the form of an aqueous slurry for secondary recrystallization annealing. A coating solution containing phosphate-chromate-colloidal silica in a weight ratio of 3:1:3 was applied to the surface of the resulting annealed sheet, and sintering was performed at 800°C. The magnetic properties of the central portion of the width of the resulting steel coil were also confirmed.

[0144] The results are shown in Table 2.

[0145] As shown in Table 2, the shape was improved and manufacturing stability was enhanced in the inventive examples. Furthermore, good magnetic properties were obtained in the inventive examples using steel No. 6. In addition, this embodiment utilizes a reversible rolling mill, which is highly effective because fractures during cold rolling can occur in continuous production lines such as tandem rolling mills due to the serpentine movement within the production line.

[0146] [Table 2]

[0147]

[0148] [Example 3]

[0149] Using the same heating furnace as shown in Table 3, a steel billet containing the composition shown in Table 3, with the remainder consisting of Fe and unavoidable impurities, and with a calculated γ phase fraction of less than 10 mol% over all temperature ranges, is hot rolled. This heating furnace has a structure in which the cantilever length of the slab end is 1.3 m when the slab is supported by an extractor (extraction device) used when it is pulled out of the heating furnace.

[0150] Two hot-rolled steel coils were produced under the same conditions. One coil was used, and the shape of the long side end was evaluated using the same method as in Example 1.

[0151] For hot-rolled steel coils for which no samples were taken, the presence of serpentine behavior was confirmed using the same method as in Example 2, and the magnetic properties were also confirmed.

[0152] The results are shown in Table 3.

[0153] [Table 3]

[0154]

[0155] As shown in Table 3, in the invention example, manufacturing stability is improved, and good magnetic properties are obtained.

[0156] Production availability

[0157] According to the hot rolling method of the present invention, the shape of the slab end before hot rolling is good, thereby resulting in a good shape of the long side end of the obtained hot-rolled steel coil. This further makes the through-plate more stable in subsequent manufacturing processes after the hot rolling process of the directional electromagnetic steel sheet, and compared with the past, it is extremely easy to manufacture directional electromagnetic steel sheets. According to the present invention, it is also possible to provide a method for manufacturing directional electromagnetic steel sheets including the process of hot rolling a slab using this hot rolling method, as well as a hot-rolled steel coil with a good slab shape.

[0158] Symbol Explanation

[0159] 1. Slab

[0160] 2. Claws of the extraction machine

[0161] L1 cantilever length

[0162] L2 Sinking Amplitude L2

Claims

1. A hot rolling method, wherein a slab is heated in a heating furnace and then hot rolled. More than 10 minutes before the slab is removed from the heating furnace, the slab temperature is brought to the highest temperature reached inside the heating furnace, T1, and the slab temperature at the time of removal from the heating furnace is set as T2. The units of T1 and T2 are °C. here, T1≥1200， T2≤T1-20, The slab has a composition with a γ phase fraction of less than 10 mol% at T1.

2. The hot rolling method according to claim 1, wherein, It includes two consecutive hot rolling passes, each passing in a temperature range of 1030℃ to 1150℃ with a reduction rate of less than 50% and a strain rate of 15s. －1 The above procedure is performed, with a time interval of 15 seconds or more between each pass.

3. The hot rolling method according to claim 1 or 2, wherein, The slab is a steel billet having the following composition: containing, by mass % C：0.03％～0.08％、 Si: 2.0%–8.0% Mn: 0.005%~3.0% Al: less than 0.0100% O: Below 0.0060% N: below 0.0060%, and S + 0.405 × Se: less than 0.0060%, the remainder consists of Fe and unavoidable impurities.

4. The hot rolling method according to claim 3, wherein, The slab further contains, by weight percent, one or more of the following components: Ni: 0.005%~1.50% Sn: 0.01%~0.50% Sb: 0.005%~0.50% Cu: 0.01%–0.50% Mo: 0.01%–0.50% P：0.0050％～0.50％、 Cr：0.01％～1.50％、 Nb: 0.0005%~0.0200% Ti: 0.0005%~0.0200% B：0.0005％～0.0200％、 Te: 0.0005%~0.0200%, and Bi: 0.0005%~0.0200%.

5. A method for manufacturing hot-rolled steel coil, wherein a slab is hot-rolled to obtain a hot-rolled steel coil by any one of the hot-rolling methods described in claims 1 to 4.

6. The method for manufacturing hot-rolled steel coils according to claim 5, wherein, The total length of the long side of the hot-rolled steel coil is set to 100%, and one end of the long side is set to 0%. The thickness variation in the width direction within the range of 1 to 2% is less than 1.5 times that within the range of 20 to 21%.

7. A method for manufacturing a directional electromagnetic steel sheet, comprising hot rolling a slab using the hot rolling method described in any one of claims 1 to 4, performing hot rolling annealing on the obtained hot-rolled sheet, performing one or more cold rolling processes including intermediate annealing, performing one recrystallization annealing, and performing final product annealing.

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