Hot rolling method, method for producing hot-rolled coil, method for producing grain-oriented electromagnetic steel sheet, and slab heating equipment

Abstract

Provided is a hot rolling method that improves the shape of a hot-rolled coil. The present invention provides a hot rolling method for hot rolling a slab after the same has been heated in a furnace, wherein: a skid which supports and carries the slab shifts at last once within the furnace, and the highest reached temperature Ta (°C) that the slab reaches from being loaded into the furnace until prior to undergoing the shift most on a slab extraction side of the furnace and the highest reached temperature Tb (°C) that the slab reaches from after undergoing the shift most on the slab extraction side of the furnace until being extracted from the furnace satisfy Tb+80°C>Ta>Tb+10°C; Ta (°C) is the highest reached temperature that the slab reaches within the furnace, and is 1150°C or higher; and the slab has a composition such that the γ phase rate at Ta (°C) is 25 mol% or less.

Description

Hot rolling method, hot rolled coil manufacturing method, grain oriented electrical steel sheet manufacturing method, and slab heating equipment

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

[0002] Grain-oriented electrical steel sheets are typically produced by using a precipitate called an inhibitor to induce secondary recrystallization of Goss-oriented ({110}<001>) grains during final annealing. For example, Patent Document 1 discloses a method using AlN as an inhibitor, and Patent Document 2 discloses a method using MnS or MnSe as an inhibitor, both of which have been industrially implemented. These inhibitor-based methods are useful for stably developing secondary recrystallized grains, but require finely dispersed precipitates, which necessitates slab heating at a high temperature of 1300°C or higher before hot rolling. However, high-temperature slab heating not only increases equipment costs but also increases the amount of scale formed during hot rolling, resulting in reduced yields and cumbersome equipment maintenance.

[0003] On the other hand, manufacturing techniques that do not use inhibitors (inhibitor-less methods) have also been proposed. For example, a technique has been proposed in which secondary recrystallization is induced by controlling the texture (texture) of a highly purified steel without adding inhibitor-forming components to the slab (Patent Document 3).

[0004] Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent Laid-Open No. 2000-129356

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

[0006] The present invention advantageously solves the above-mentioned problems, and aims to provide a hot rolling method that improves the shape of a slab before hot rolling by appropriately controlling the slab heating conditions before hot rolling, thereby improving the shape of a hot-rolled coil; a method for producing a hot-rolled coil and a grain-oriented electrical steel sheet that includes a step of hot-rolling a slab by this hot rolling method; and slab heating equipment that can be used in the hot rolling method.

[0007] In order to solve the above-mentioned problems, the inventors of the present invention have conducted a detailed investigation into the hot rolling conditions of hot rolled coils that have actually suffered from shape defects. The hot rolled coils in question were subjected to slab heating in a walking beam slab heating furnace before hot rolling.

[0008] Figure 1 is a schematic diagram of an example of a walking beam slab heating furnace. In a walking beam slab heating furnace 1, the slab S is generally supported and transported by skids. In Figure 1, the slab S is supported on both the charging side and the unloading side, but this support method is not limited to this. Typically, the skids are arranged alternately, with fixed skids 2a and movable skids 2b. The movable skids 2b move up and down to lift the slab S and transport it little by little from the charging side to the unloading side. If the position of the skids used to support and transport the slab S is always the same relative to the slab S, it would be difficult to heat the underside of the slab S directly above the skids. Therefore, one or more mechanisms called shift skids 3 are often installed within the furnace, and the position of the skids supporting the slab S changes before and after the shift skids. Figure 2 shows a state in which the slab S is supported by three fixed skids 2a or movable skids 2b. When the three movable skids 2b are moving downward, the slab S is supported by the three fixed skids 2a, and when the three movable skids 2b are moving upward, the slab S is supported by the movable skids 2b. Figure 2 shows the latter case. In this case, if the length from the skid 2 located closest to the longitudinal end of the slab S (hereinafter also referred to as the "slab end") among the supporting skids 2 to the longitudinal end of the slab is defined as the overhang length, in the skid arrangement of the heating furnace 1 shown in Figure 1, the overhang length after the shift skid is shifted by the shift skid mechanism 3 is 0.05 mm, compared to the overhang length 0.00 mm before the shift. １ becomes longer.

[0009] The present inventors have found that shape defects frequently occur at the longitudinal end portions of hot-rolled coils in the following cases: (1) The maximum temperature of the slab in the heating furnace before hot rolling is 1150°C or higher, and the slab has a chemical composition in which the γ-phase ratio at the maximum temperature is 25 mol% or less; (2) The overhang length of the slab after shifting at the slab extraction side of the heating furnace is 0.05; １ is the overhang length O before the shift ０ It is more than 10% longer than the previous model.

[0010] The maximum temperature of the slab can be determined by passing a slab with a thermocouple attached through a heating furnace and recording the temperature change of the slab at each position in the furnace. Thermocouples can be attached, for example, to six locations: the center of the slab's longitudinal direction and both longitudinal ends, on the surface and at the center of the thickness direction. The average of the six measurements at each position in the heating furnace can be used as the slab temperature at that location. If the slab temperature is actually measured in the heating furnace, that value can also be used. The γ-phase fraction can also be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) manufactured by Thermo-Calc Software AB.

[0011] It is generally known that creep deformation at high temperatures occurs more easily in the α (ferrite) phase than in the γ (austenite) phase, with a slower rate of deformation. The discovery that a low γ-phase fraction is a characteristic of hot-rolled coils with poor shape suggests that creep deformation in the heating furnace may be the cause of the poor shape. It is presumed that the slab ends creep deformed under their own weight in the heating furnace, resulting in a sinking shape. Because the slab ends are supported by the skid in a cantilevered manner, the creep deformation rate increases as the overhang length increases. It is presumed that this mechanism causes the slabs 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 coils.

[0012] Based on this speculated mechanism, the inventors investigated the possibility of improving the shape of hot-rolled coils. Here, electrical steel sheets typically contain a high concentration of Si to improve the final magnetic properties. Si stabilizes the α phase and reduces the γ-phase ratio during high-temperature heating. Another element that significantly affects the γ-phase ratio is C. However, C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, so there is an optimum amount of C from the perspective of improving the final magnetic properties. Therefore, it is difficult to adopt a method of significantly changing the composition of electrical steel sheets that have already been manufactured in a process to increase the γ-phase ratio at high temperatures.

[0013] The maximum temperature of the slab during slab heating is usually set to dissolve trace amounts of impurity elements and precipitate-forming elements and homogenize the slab. If impurity elements can be sufficiently reduced, it is possible to use a slab heating temperature below 1150°C. In this case, the occurrence of shape defects such as those described above is suppressed. However, in some cases, trace elements are intentionally added to improve magnetic properties, so lowering the maximum temperature is not always an option.

[0014] On the other hand, if the cause of the shape defects at the longitudinal ends of hot-rolled coils is creep deformation at the slab ends, not only the temperature but also the degree of overhang length will have an effect. Therefore, we found that even if the slab reaches the same maximum temperature in the heating furnace, the amount of creep deformation can be suppressed if the overhang length can be controlled.

[0015] The gist of the present invention, which has been completed by the inventors based on the above findings, is as follows: [1] A hot rolling method in which a slab is heated in a heating furnace and then hot-rolled, wherein a skid that supports and transports the slab shifts at least once within the heating furnace, and the maximum temperature Ta (°C) that the slab reaches from when it is charged into the heating furnace until it shifts to the furthest slab removal side of the heating furnace and the maximum temperature Tb (°C) that the slab reaches after it shifts to the furthest slab removal side of the heating furnace and until it is removed from the heating furnace satisfy the relationship Tb + 80°C > Ta > Tb + 10°C, Ta (°C) is the maximum temperature that the slab reaches in the heating furnace and is 1150°C or higher, and the slab has a chemical composition such that the gamma phase fraction at Ta (°C) is 25 mol% or less. [2] Includes two consecutive hot rolling passes, each pass in the temperature range of 1030 ° C or more and 1150 ° C or less, with a reduction rate of 50% or less and a strain rate of 15 s －1[3] The hot rolling method of [1], wherein the slab is a steel slab having a chemical composition, in mass%, of C: 0.02% or more and 0.08% or less, Si: 2.0% or more and 8.0% or less, Mn: 0.005% or more and 3.0% or less, Al: less than 0.0100%, O: 0.0060% or less, N: 0.0060% or less, and S + 0.405 × Se: 0.0060% or less, with the balance being Fe and unavoidable impurities. [4] The hot rolling method according to [3], wherein the slab further contains, in mass%, one or more elements selected from the group consisting of Ni: 0.005% or more and 1.50% or less, Sn: 0.01% or more and 0.50% or less, Sb: 0.005% or more and 0.50% or less, Cu: 0.01% or more and 0.50% or less, Mo: 0.01% or more and 0.50% or less, P: 0.0050% or more and 0.50% or less, Cr: 0.01% or more and 1.50% or less, Nb: 0.0005% or more and 0.0200% or less, Ti: 0.0005% or more and 0.0200% or less, B: 0.0005% or more and 0.0200% or less, Te: 0.0005% or more and 0.0200% or less, and Bi: 0.0005% or more and 0.0200% or less. [5] A method for producing a hot-rolled coil, comprising hot-rolling a slab by any one of the hot rolling methods [1] to [4] to obtain a hot-rolled coil. [6] A method for producing a hot-rolled coil according to [5], wherein the widthwise thickness variation at a position 10 to 20 m from an end of the hot-rolled coil in the longitudinal direction is 1.5 times or less than the widthwise thickness variation at the longitudinal center. [7] A method for producing a grain-oriented electrical steel sheet, comprising hot-rolling a slab by any one of the hot-rolling methods [1] to [4], and subjecting the resulting hot-rolled sheet to hot-rolled sheet annealing, followed by cold rolling once or two or more times with intermediate annealing in between, and then subjecting the hot-rolled sheet to primary recrystallization annealing and final finish annealing.[8] Slab heating equipment for heating slabs before hot rolling, the slab heating equipment comprising: a walking beam type heating furnace having at least one skid for supporting and transporting a slab within the heating furnace and one shift skid mechanism for shifting the position where the skid supports the slab; heat treatment mechanisms that can independently control the temperature before and after the shift skid mechanism on the slab extraction side of the heating furnace; an atmosphere control mechanism that controls the convection of atmospheric gas within the heating furnace; and a furnace temperature control mechanism that controls the heat treatment mechanism and the atmosphere control mechanism on either side of the shift skid mechanism on the slab extraction side of the heating furnace so that the temperature inside the extraction side of the heating furnace is 50°C or more lower than the temperature inside the charging side of the heating furnace.

[0016] According to the hot rolling method of the present invention, by improving the shape of the slab end portions before hot rolling, the shape of the longitudinal end portions of the obtained hot-rolled coil can be improved, and thus the threading of the grain-oriented electrical steel sheet in the manufacturing process after the hot rolling step can be made more stable. As a result, grain-oriented electrical steel sheets can be manufactured much more easily than before. The present invention can also provide a method for manufacturing a hot-rolled coil and a grain-oriented electrical steel sheet, which includes a step of hot-rolling a slab using this hot rolling method, and slab heating equipment that can be used in the hot rolling method.

[0017] Fig. 1 is a schematic diagram of an example of a walking beam type slab heating furnace. Fig. 2 is a schematic diagram showing the state of slab support by skids in the slab heating furnace of Fig. 1. Fig. 3 is a diagram showing the correspondence between slab transportation in a walking beam type heating furnace and an example of a slab temperature profile in the method of the present invention (solid line) and a typical conventional slab temperature profile (dashed line). Fig. 4 is a schematic diagram showing the arrangement of skids before and after shifting in a slab heating furnace.

[0018] The hot rolling method of the present invention will now be described in detail.

[0019] The present invention relates to a hot rolling method for hot rolling a slab after heating it in a heating furnace, in which the heating furnace is equipped with a skid for supporting and transporting the slab, and the skid is shifted at least once within the heating furnace. The skid shift can be performed by a shift skid mechanism. The number of shift skid mechanisms may be one or more. In the method of the present invention, the overhang length O after the shift by the shift skid mechanism closest to the slab extraction side of the heating furnace (hereinafter also referred to as the "final shift skid mechanism") is calculated. １ is the overhang length O before the shift ０ This is useful when using a heating furnace with a skid arrangement where the overhang length is 110% or more of the final shift skid mechanism. １ is the overhang length O before the shift ０ It is particularly advantageous when the overhang length O is 110% or more. １ is, for example, the overhang length O ０ "Before a shift" and "after a shift" refer to the period before starting to change the position of the skid supporting the slab and after completing the change. If the heating furnace has only one shift skid mechanism, this shift skid mechanism corresponds to the final shift skid mechanism. If the heating furnace has multiple shift skid mechanisms, "before a shift by the final shift skid mechanism" refers to the period after a shift by the second-to-final shift skid mechanism has been completed and before a shift by the final shift skid mechanism has been completed.

[0020] In the method of the present invention, the highest temperature that the slab reaches from when it is charged into the heating furnace until it shifts to the slab extraction side of the heating furnace is defined as Ta (unit: °C, hereinafter omitted), and the highest temperature that the slab reaches after it shifts to the slab extraction side of the heating furnace until it is extracted from the heating furnace is defined as Tb (unit: °C, hereinafter omitted), where Ta and Tb satisfy the following relationship: Tb + 80 °C > Ta > Tb + 10 °C. Tb is the highest temperature that the slab reaches from when it shifts by the final shift skid mechanism until it is extracted from the heating furnace.

[0021] As described above, Ta and Tb can be determined by passing a slab equipped with a thermocouple through the heating furnace and recording the temperature change of the slab at each position in the heating furnace in advance. When there is only one shift skid mechanism, the slab reaches its maximum temperature Ta before being shifted by that shift skid mechanism. When there are multiple shift skid mechanisms, the timing at which the slab reaches its maximum temperature Ta is not particularly limited as long as it is between the time the slab is loaded into the heating furnace and the time it is shifted by the final shift skid mechanism. For example, it can be between the time it is shifted by the second-to-final shift skid mechanism and the time it is shifted by the final shift skid mechanism. In this case, the timing at which the slab reaches its maximum temperature Ta may be immediately after being shifted by the second-to-final shift skid mechanism, immediately before it reaches the final shift skid mechanism, or any intermediate time therebetween. The slab may also reach its maximum temperature Ta between the time it is loaded into the heating furnace and the time it reaches the second-to-final shift skid mechanism.

[0022] In the method of the present invention, the maximum temperature Ta is the maximum temperature that the slab reaches in the heating furnace, and is a temperature of 1150°C or higher. The maximum temperature Ta can be set to 1300°C or lower. If Ta is within this range, the slab can be sufficiently homogenized.

[0023] In the method of the present invention, the slab is heated so as to satisfy the condition Tb+80°C > Ta > Tb+10°C.

[0024] When a high-temperature slab is cooled, there is a possibility that elements that have been homogenized by the high temperature may be re-precipitated. Excessively low temperatures reduce the homogenization effect within the slab and may cause deterioration of properties. Therefore, a temperature lower than 80°C above Ta, which is the maximum temperature that the slab can reach in a heating furnace, is not desirable. On the other hand, a temperature lower than Ta by 10°C or more is desirable in order to suppress creep deformation and thereby improve the shape defects at the longitudinal end of the hot-rolled coil.

[0025] Conventional slab heating control methods are as follows. One of the purposes of slab heating in general steel is to increase the slab's temperature to reduce its deformation resistance and enable high-reduction hot rolling to obtain hot-rolled coils. To achieve this, the temperature of the slab extracted from the heating furnace must be controlled to a temperature appropriate for hot rolling. The target of control is the temperature of the slab at the time of extraction from the heating furnace; the temperature of the slab in the heating furnace does not need to be higher than the temperature of the slab at the time of extraction. Therefore, the slab temperature is typically gradually increased in the heating furnace and controlled to reach a maximum temperature at the time of extraction. Furthermore, in the production of grain-oriented electrical steel sheets, it is also necessary to dissolve trace amounts of impurity elements and precipitate-forming elements and to homogenize the slab. Once dissolved, elements may re-precipitate when the steel temperature decreases. Therefore, the slab temperature at the time of extraction is typically controlled to reach a maximum temperature. In addition, since heating furnaces increase fuel utilization efficiency during heating by gradually heating the slabs to the desired maximum temperature, operations that bring large slabs to their maximum temperature at an early stage in the heating furnace are not usually carried out.

[0026] The method of the present invention uses a slab with a composition in which the gamma phase fraction in Ta is 25 mol% or less. Even if the temperature of the slab after the shift is lower than the maximum temperature Ta reached by the slab before the shift at the most extraction side of the heating furnace, the maximum temperature Tb reached by the slab after the shift is higher than (Ta - 80°C). Therefore, elements that have been homogenized through solid solution remain supersaturated, and the progression of precipitation can be easily suppressed. On the other hand, because the maximum temperature Tb is lower than (Ta - 10°C), creep deformation at the slab edge can be easily suppressed.

[0027] FIG. 3 shows the correspondence between the transportation of the slab in the walking beam heating furnace and an example of the slab temperature profile in the method of the present invention (solid line) and a typical conventional slab temperature profile (dashed line).

[0028] Preferably, the method of the present invention satisfies Tb+50°C > Ta > Tb+15°C.

[0029] In the method of the present invention, a slab having a composition in which the γ-phase ratio of Ta is 25 mol% or less is used. By using such a slab, it is possible to prevent excessive formation of pearlite phase derived from the γ-phase in the hot-rolled sheet structure. Because the pearlite phase has a higher strength than the ferrite phase, cold rolling processing can sometimes be difficult, but the method of the present invention can easily avoid such a situation. A slab having a composition in which the γ-phase ratio is 20 mol% or less is preferred. Furthermore, the lower limit of the γ-phase ratio is not particularly limited and may be 0 mol%.

[0030] The composition of the slab preferably satisfies the following: Hereinafter, the "%" designation regarding the composition of the component is "mass %" unless otherwise specified.

[0031] C: 0.02% or more and 0.08% or less C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, so from the viewpoint of enhancing the final magnetic properties, it is preferable to set the C content to 0.02% or more, more preferably 0.03% or more. On the other hand, if the C content exceeds 0.08%, it becomes difficult to reduce the C content to 50 ppm or less, at which magnetic aging does not occur, even when decarburization annealing is performed. From this point of view, it is preferable to limit C to 0.08% or less, more preferably 0.06% or less.

[0032] Si: 2.0% or more and 8.0% or less Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, the Si content is preferably 2.0% or more, more preferably 2.8% or more. On the other hand, Si also increases the brittleness of steel. In order to reduce the risk of breakage during threading and suppress deterioration of cold rolling properties, the Si content is preferably limited to 8.0% or less, more preferably 4.5% or less.

[0033] Mn: 0.005% or more and 3.0% or less Mn is a useful element from the viewpoints of improving hot workability and controlling the formation of oxide films during primary recrystallization. From this viewpoint, the Mn content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, from the viewpoint of avoiding deterioration of the primary recrystallization texture and resulting in deterioration of magnetic properties, the Mn content is preferably limited to 3.0% or less, and more preferably 0.5% or less.

[0034] Al: less than 0.0100% N: 0.0060% or less O: 0.0060% or less S + 0.405 × Se: 0.0060% or less Excessive Al can make it difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition, so Al is preferably limited to less than 0.0100%, more preferably 0.0080% or less. N is preferably limited to 0.0060% or less, more preferably 0.0030% or less, to prevent the formation of Si nitrides after purification annealing in the production of final product sheets. O forms oxides and suppresses deterioration of the magnetic properties of the final product sheets, so it is preferably limited to 0.0060% or less, more preferably 0.0030% or less. The sum of the S content and Se content multiplied by 0.405 is preferably limited to 0.0060% or less, more preferably 0.0040% or less, to stably obtain a secondary recrystallized structure. Al, N, O, S and Se are inhibitor components.

[0035] The essential components and suppressing components have been described above, but one or more of the following elements may be optionally contained: Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Mo: 0.01% to 0.50%, P: 0.0050% to 0.50%, Cr: 0.01% to 1.50%, Nb: 0.0005% to 0.0200%, Ti: 0.0005% to 0.0200%, B: 0.0005% to 0.0200%, Te: 0.0005% to 0.0200%, and Bi: 0.0005% to 0.0200%.

[0036] Ni is a useful element that improves the structure of hot-rolled coils and enhances their magnetic properties. To fully obtain this effect, when Ni is added, the Ni content is preferably 0.005% or more. However, excessive Ni makes secondary recrystallization unstable, resulting in deterioration of magnetic properties, so the Ni content is preferably 1.50% or less.

[0037] Sn, Sb, Cu, Mo, P, Cr, B, and Bi are grain boundary segregation elements that can improve various properties, but if they are present in excess, they can inhibit the development of secondary recrystallized grains. For these reasons, when these elements are contained, the amounts should be within the above ranges.

[0038] Nb, Ti, and Te are precipitate-forming elements that can improve various properties, but excessive amounts can make secondary recrystallization unstable. For these reasons, when these elements are added, the amounts should be within the above ranges.

[0039] The slab used in the method of the present invention preferably has a composition containing the above-mentioned essential components and, optionally, optional components, with the balance being Fe and unavoidable impurities. In this composition, the precipitate components are sufficiently reduced, and the elements that have been homogenized through solid solution can maintain a supersaturated state and suppress the progress of precipitation even if the maximum temperature Tb reached by the slab after shifting by the final shift mechanism is lower than the maximum temperature Ta reached by the slab before shifting by the final shift mechanism.

[0040] The slabs used in the method of the present invention can be produced by refining molten steel adjusted to a desired composition by a known method using a converter, electric furnace, or the like, and then subjecting it to vacuum treatment, if necessary, followed by a conventional ingot-making method or continuous casting method. Alternatively, thin cast pieces having a thickness of 100 mm or less can be directly produced by a direct casting method and used as slabs.

[0041] The slab used in the method of the present invention has a chemical composition such that the γ-phase fraction is 25 mol% or less at Ta, the maximum temperature the slab will reach in the heating furnace. The γ-phase fraction can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) manufactured by Thermo-Calc Software AB. After adjusting the chemical composition of the slab, the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) can be used to calculate the temperature range in which the γ-phase fraction is 25 mol% or less, and the maximum temperature Ta of the slab in the heating furnace can be set based on the calculated temperature range. Alternatively, the maximum temperature Ta of the slab in the heating furnace can be set, and the chemical composition can be adjusted so that the γ-phase fraction is 25 mol% or less at this Ta.

[0042] Slab heating in the method of the present invention can be carried out using slab heating equipment equipped with a walking beam type heating furnace having at least one skid for supporting and transporting the slab within the heating furnace and a shift skid mechanism for shifting the position at which the skid supports the slab.

[0043] The heating furnace is configured such that, on either side of the two slab end portions of the final shift skid mechanism, the distance between the skid located closest to the slab end portion after the shift by the final shift skid mechanism and the skid located one step closer to the center of the slab than that skid is L. １ , before the shift by the final shift skid mechanism, the distance between the skid located closest to the slab edge and the skid one step closer to the center of the slab is L0, the distance L １ is greater than the distance L0, the overhang length O after the shift by the final shift skid mechanism １is the overhang length before the shift. ０ This is because the difference in the width of the slab is likely to be 110% or more, which can result in shape defects, and therefore the benefits of the hot rolling method of the present invention can be fully enjoyed. When a heating furnace is equipped with multiple shift skid mechanisms, the skid arrangement "before shifting by the final shift skid mechanism" corresponds to the skid arrangement of the shift skid mechanism one step before the final one. A schematic diagram is shown in Figure 4. In Figure 4, at the slab end indicated by L0 and L1, the skid located closest to the slab end is the fixed skid 2a, and the skid located one step closer to the center of the slab than that skid is the movable skid 2b.

[0044] As revealed for the first time in this study, slab deformation, which becomes an operational problem when the overhang length increases, is limited to steel grades with a low gamma phase fraction at the target temperature. Typically, the gamma phase appears in iron at high temperatures. In this case, similar issues are likely to occur when steel contains high amounts of gamma phase-suppressing elements such as Si and Al and low amounts of gamma phase-forming elements such as C, Ni, and Cr. Generally, electrical steel sheets contain 2.0% or more Si to reduce iron loss. Furthermore, because impurities affect performance degradation, many elements have strict upper limits on their content, which can lead to slab deformation problems. Meanwhile, most slab heating furnaces in operation are designed and manufactured for general steel materials. Furnaces are designed to gradually heat the slab from the entry side to the exit side, reaching the target temperature (typically the maximum temperature) at the exit side. Naturally, the higher the temperature, the greater the risk of slab deformation. Therefore, the outermost skid of the slab is often shifted outward, as shown in Figure 2, to support the slab over a wider area. The second outermost skid may shift in the same direction, or it may shift inward to achieve overall balance. When the outermost skid shifts outward and the second outermost skid shifts inward, the overhang length increases. If the shift direction (inward or outward) results in uneven support of the slab, a new skid may be installed during the shift. Furthermore, the length and width of steel required by manufacturers are not constant. For example, even when using a heating furnace with the skid arrangement shown in Figure 2, if the required length of the product steel is short, the slab length may also be short, and the leftmost five of the six skids may be used to support the slab. Therefore, to accommodate various product and slab length requirements, multiple heating furnaces with various skid arrangements and shift mechanisms are used in parallel. Applying this invention to slabs that meet the compositional requirements identified during the development of this invention can produce particularly significant improvements when the slab length required for the product does not match the skid arrangement characteristics of the heating furnace.

[0045] The present invention also relates to a slab heating system for use in the method of the present invention, which includes a heat treatment mechanism capable of independently controlling temperatures before and after a shift skid mechanism (final shift skid mechanism) located closest to the slab extraction side of the heating furnace, an atmosphere control mechanism for controlling the convection of atmospheric gas within the heating furnace, and a furnace temperature control mechanism for controlling the heat treatment mechanism and the atmosphere control mechanism so that the furnace temperature on the extraction side of the heating furnace (also referred to as "after the final shift skid mechanism") across the final shift skid mechanism is at least 50°C lower than the furnace temperature on the charging side of the heating furnace (also referred to as "before the final shift skid mechanism"). On the other hand, in order to prevent the slab from becoming excessively cold, it is preferable to avoid the furnace temperature after the final shift skid mechanism being at least 130°C lower than the furnace temperature before the final shift skid mechanism.

[0046] The heat treatment mechanism can be a heating device, such as a burner. For example, the heating furnace can be divided into multiple heating zones (e.g., between shift skid mechanisms), with different burners installed in each heating zone. The atmosphere control mechanism controls the convection of atmospheric gas within the heating furnace. For example, it can be a mechanism with separate intake and exhaust systems for each section of the heating furnace (e.g., each heating zone). The atmosphere control mechanism can also supply inert gas to specific sections of the heating furnace to keep the furnace temperature low in those sections and their surroundings. The atmosphere control mechanism also contributes to furnace temperature control. Note that heating and cooling by controlling the convection of atmospheric gas is performed by the atmosphere control mechanism. The furnace temperature control mechanism controls the heat treatment mechanism and the atmosphere control mechanism so that the furnace temperature after the final shift skid mechanism is at least 50°C lower than the furnace temperature before the final shift skid mechanism. To reduce the slab temperature of a heated slab by 10°C or more, a temperature difference of at least 50°C is required. Here, the furnace temperature can be measured by installing a sensor such as a thermocouple in the heating furnace. For example, if the heating furnace is divided into multiple heating zones, a sensor can be installed in each heating zone. The furnace temperatures before and after the final shift skid mechanism are measured by the sensor installed closest to the final shift skid mechanism. If the heating furnace is divided into multiple heating zones, the furnace temperatures before and after the final shift skid mechanism are measured by the sensor installed in the heating zone adjacent to the final shift skid mechanism. Instead of measuring with a sensor, control may be based on a calculated value of the slab temperature. While changing the skid itself is equivalent to installing a new furnace and involves significant constraints, installing a furnace temperature control mechanism, heat treatment mechanism, and atmosphere control mechanism is easier to achieve.

[0047] The heating furnace may include walls between adjacent heating sections to reduce the effects of radiant heat from the adjacent heating sections. The heating furnace may include an auxiliary skid for supporting the slab.

[0048] Because the slab itself carries heat, control of the heat treatment mechanism and atmosphere control mechanism is necessary to create a large temperature difference inside the furnace. The heat treatment mechanism and atmosphere control mechanism are controlled through the furnace temperature control mechanism so that the desired temperature difference inside the furnace is achieved before and after the final shift skid mechanism. This makes it possible to ensure that the maximum temperature Ta (°C) of the slab before it reaches the final shift skid mechanism and the maximum temperature Tb (°C) of the slab after it has been shifted satisfy the following relationship: Tb + 80°C > Ta > Tb + 10°C

[0049] Depending on the arrangement of the skids, the overhang length of one slab end after the final shift skid mechanism is shifted may be 110% or more, while the other end may be less than 110% of the overhang length of the slab end before the shift. In such cases, the temperature of only one end where the overhang is 110% or more may be lowered to Tb, which allows for more efficient operation.

[0050] The slab heating equipment of the present invention is not limited to the manufacture of grain-oriented electrical steel sheets, but can also be used when heating slabs of steel with a low γ-phase ratio.

[0051] The slab extracted through the above-described slab heating has homogenized steel and also has little sinking at the overhanging positions at the ends of the slab.

[0052] Next, the slab is subjected to hot rolling, which is relatively easy because the slab end portions are prevented from sinking and becoming defective in shape.

[0053] In the hot rolling process of the method of the present invention, at least two consecutive passes of rolling from the slab stage to the sheet bar stage can be carried out in a temperature range of 1030°C to 1150°C. The consecutive two passes should have an inter-pass time of 15 seconds or more, a rolling reduction of 50% or less in each pass, and a strain rate of 15 seconds or less. －1The above-mentioned method is preferable from the viewpoint of improving the shape of the longitudinal end portions of the hot-rolled coil. In particular, even if the inhibitory elements S and Se are present in the slab and form sulfides or selenides in the central layer of the slab, performing two-pass rolling under appropriate conditions in a specific temperature range (1030°C or higher and 1150°C or lower) is advantageous in that it can reduce the size of these sulfides or selenides to a level that would cause problems during cold rolling. In particular, when using Si steel, the temperature range of normal hot rolling forms austenite in addition to ferrite, albeit at a small volume fraction, and therefore separation and destruction due to sulfides and selenides may occur, so the above-mentioned hot rolling process is preferable.

[0054] The method of the present invention uses a slab in which the γ-phase fraction is 25 mol% or less at Ta, the maximum temperature reached in the heating furnace. The γ-phase fraction reaches its maximum near the temperature range of 1030°C to 1150°C. Generally, austenite has higher deformation resistance than ferrite and is less likely to deform when rolled. Therefore, the reduction ratio in each pass is limited to 50% or less. From the viewpoint of homogenizing the structure of the hot-rolled coil, the reduction ratio is preferably 15% or more, more preferably 20% or more. Furthermore, by setting the inter-pass time to 15 s or more, dislocations formed by deformation are recovered or disappear by recrystallization, allowing rolling without excessively increasing deformation resistance. The inter-pass time is preferably 120 s or less to suppress the formation of precipitates nucleated by dislocations generated during deformation. The strain rate is 15 s or less. -1 By setting the strain rate at 50 s or more, it becomes easier to improve the shape of the longitudinal end portion of the hot rolled coil. -1 It can be as follows:

[0055] Here, the strain rate ε can be calculated using the following Ekelund equation.

[0056] In the formula, v Ｒ is the roll 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 (%).

[0057] Typically, the longitudinal ends of a hot-rolled coil exhibit greater thickness variation (maximum thickness minus minimum thickness) than the steady-state portion, including the central portion. However, the method of the present invention can suppress thickness variation at the longitudinal ends of the hot-rolled coil. For example, the widthwise thickness variation at the longitudinal ends of the hot-rolled coil can be controlled to 1.5 times or less the widthwise thickness variation at the steady-state portion of the hot-rolled coil. In this evaluation, the longitudinal ends are defined as positions within 3% of each longitudinal end (i.e., positions between 0 and 3% and between 97 and 100%), assuming the entire longitudinal length of the hot-rolled coil to be 100%. The steady-state portion is preferably evaluated at a position between 10 and 90% from one longitudinal end.

[0058] The present invention also relates to a method for producing a grain-oriented electrical steel sheet, which comprises hot-rolling a slab by the hot-rolling method of the present invention, annealing the resulting hot-rolled sheet, cold-rolling it once or twice or more times with intermediate annealing in between, and then optionally performing decarburization annealing, followed by primary recrystallization annealing and final finish annealing. Since the hot-rolled sheet has an improved shape at its longitudinal ends, it is possible to suppress meandering and breakage during the cold-rolling process.

[0059] It is important that the hot-rolled sheet annealing be performed at 1150°C or lower. If the hot-rolled sheet annealing temperature exceeds 1150°C, the inhibitor-forming components inevitably mixed in will dissolve and reprecipitate unevenly during cooling, making it difficult to achieve a uniformly sized primary recrystallized structure and inhibiting the development of secondary recrystallization. Furthermore, if the hot-rolled sheet annealing temperature exceeds 1150°C, the grain size after hot-rolled sheet annealing will become too coarse, which is also disadvantageous in achieving an appropriate primary recrystallized structure. From the viewpoint of promoting recrystallization, the hot-rolled sheet annealing is preferably performed at 900°C or higher.

[0060] After hot-rolled sheet annealing, the steel is cold-rolled once or twice or more times with intermediate annealing in between. In cold rolling, it is effective to carry out aging treatment once or twice or more times at a rolling temperature of 80°C to 150°C and at an inter-rolling temperature of 100°C to 300°C, which is performed at an inter-rolling temperature of 100°C to 300°C, in order to develop a Goss structure.

[0061] Next, primary recrystallization annealing is performed. The purpose of this primary recrystallization annealing is to perform primary recrystallization on the cold-rolled sheet with a rolled texture, adjusting the primary recrystallized grain size to an optimal size for secondary recrystallization, and decarburizing the carbon contained in the steel by using a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere. At the same time, the oxidizing atmosphere is used to form an oxide film on the surface. Therefore, primary recrystallization annealing is preferably performed in a H2-mixed atmosphere at a temperature between 750°C and 900°C, with a dew point of 100°C. During primary recrystallization annealing, a heating rate of 200°C / s or higher between 550°C and 680°C is preferred, as this further enhances the texture improvement effect. Simultaneously, decarburization annealing is performed to reduce the C content to 50 mass ppm or less, preferably 30 mass ppm or less, at which magnetic aging does not occur. Siliconizing may be used after primary recrystallization annealing to increase the Si content.

[0062] The steel is then subjected to final annealing to develop a secondary recrystallized structure. A forsterite film may be formed using an annealing separator primarily composed of MgO. Adding an appropriate amount of Ti oxide or Sr compound to the separator can further enhance the formation of the forsterite film. Adding an auxiliary agent that promotes uniform forsterite film formation is particularly beneficial for improving release properties. Alternatively, an optional annealing separator such as Al2O3 may be used to suppress film formation.

[0063] Final annealing must be performed at 800°C or higher to induce secondary recrystallization, but the heating rate up to 800°C can be any condition since it does not significantly affect the magnetic properties. The annealing atmosphere can be N2, Ar, or H2, or a mixture of two or more of these. To more effectively induce secondary recrystallization, isothermal holding can be performed near the secondary recrystallization temperature, but a slower heating rate can also achieve the same effect, so isothermal holding is not essential. Because the precipitation of trace elements in the final product leads to a deterioration of the magnetic properties, the maximum annealing temperature is set to 1100°C or higher to purify the elements.

[0064] After final annealing, an insulating coating can be further formed on the surface of the steel sheet. The type of insulating coating is not particularly limited, and any known insulating coating can be used. For example, a preferred method is to apply a coating liquid containing phosphate, chromate, and colloidal silica to the steel sheet, which is described in JP-A-50-79442 and JP-A-48-39338, and bake the coating at about 800°C.

[0065] Furthermore, the steel sheet can be shaped by planarizing annealing, which can also be combined with baking of the insulating coating.

[0066] The present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.

[0067] Example 1: A steel slab (total length 12.2 m, width 1 m, thickness 180 mm) containing 3.2-3.4% Si, 0.035-0.070% C, 0.07% Mn, 0.0050-0.0090% Al, less than 0.0060% N, O, and S + 0.405 × Se, with the remainder consisting of Fe and unavoidable impurities, and containing no inhibitors, was heated using a walking beam furnace according to the heating pattern shown in Table 1. The third and fourth passes of a four-pass rough rolling process were then performed under the conditions shown in Table 1. This was followed by multiple passes of finish hot rolling in the 850-950°C temperature range to a thickness of 2.2 mm. The slab of the invention exhibited suppressed edge subsidence and good shape before hot rolling.

[0068] The walking beam heating furnace is equipped with one shift skid mechanism, which corresponds to the shift skid mechanism closest to the slab extraction side of the heating furnace (final shift skid mechanism). This shift skid mechanism consists of alternating movable and fixed skids, and when the above-mentioned steel slab is placed, the skid arrangement at one end is such that the slab overhang length before and after the shift is 112%. The heating zones before and after the final shift skid mechanism of the heating furnace are separate, and each zone is equipped with a different burner system. The heating zone after the final shift skid mechanism is connected to an inert gas supply system, and the burner and inert gas supply system are connected to a control system. Each heating zone is equipped with a sensor to measure the furnace temperature.

[0069] To evaluate the shape of the longitudinal ends of the obtained hot-rolled coil (total length 1000 m, width 1 m, thickness 2.2 mm), samples were cut from the coil longitudinally at positions 10 m, 12 m, 14 m, 16 m, 18 m, and 20 m from the ends. Similarly, to evaluate the shape of the steady-state portion, six samples were cut at 2 m intervals from a position more than 200 m from the ends. The thickness profile in the width direction was measured, and the difference between the maximum and minimum values ​​was calculated. Evaluation was made based on the ratio of the thickness difference between the longitudinal ends and the steady-state portion. As is clear from the table, it is clear that the shape was improved under the conditions of the invention.

[0070]

[0071] Example 2 A steel slab (total length 8.5 m, width 1 m, thickness 170 mm) containing the components shown in Table 2, with the balance being Fe and unavoidable impurities, and having a calculated γ-phase fraction of 25 mol% or less at the maximum temperature reached during slab heating, was hot rolled using a walking beam heating furnace under the conditions also shown in Table 2. The resulting hot-rolled coil had a width of 1 m and a thickness of 1.6 mm. In the slab of the invention before hot rolling, sinking of the ends was suppressed and the shape was good. The walking beam heating furnace used was the same as in Example 1.

[0072] Two hot-rolled coils were produced under the same conditions, and one of them was used to evaluate the shape of the longitudinal end in the same manner as in Example 1. The hot-rolled coils for which no samples were taken were annealed at a temperature of 1020°C to check whether they would meander or move transversely for 20 mm or more during threading. Subsequently, the coils were subjected to primary cold rolling at 100°C in a reverse mill to a thickness of 1.7 mm, intermediate annealing at 900°C for 1 minute, and then secondary reverse cold rolling, with coiling aging at 200°C during the process, resulting in a thickness of 0.22 mm. Subsequently, primary recrystallization annealing was performed at a heating rate of 300°C / s between 550°C and 680°C, a soaking temperature of 840°C, and a soaking time of 60 seconds. An annealing separator containing 95% MgO and 5% TiO2 was applied to the steel sheet surface as a water slurry, and the steel sheet was then subjected to secondary recrystallization annealing. A coating solution containing phosphate, chromate and colloidal silica in a weight ratio of 3:1:3 was applied to the surface of the finish annealed sheet obtained in this way, and baked at 800°C. The magnetic properties of the center of the width of the product sheet coil of the obtained grain-oriented electrical steel sheet were also confirmed. The magnetic properties were measured in accordance with JIS C2550-1:2011, in which the magnetic flux density (B ８ ) was measured.

[0073] As is clear from Table 2, under the conditions of the present invention, production stability is improved and good magnetic properties are obtained. Furthermore, although a reverse mill was used in this example, in a continuous line such as a tandem mill, meandering within the line can affect breakage during cold rolling.

[0074]

[0075] According to the hot rolling method of the present invention, the shape of the slab ends before hot rolling is improved, thereby improving the shape of the longitudinal ends of the obtained hot-rolled coil, which in turn makes it possible to more stably thread the grain-oriented electrical steel sheet in the manufacturing process after the hot rolling step, and makes it possible to manufacture the grain-oriented electrical steel sheet much easier than before. The present invention can also provide a method for manufacturing a hot-rolled coil and a grain-oriented electrical steel sheet, which includes a step of hot-rolling a slab using this hot rolling method, and slab heating equipment that can be used in the hot rolling method.

[0076] 1 Slab heating furnace 2 Skid 2a Fixed skid 2b Moving skid 3 Shift skid S Slab O ０ Overhang length before shifting at the most slab extraction side of the furnace O １ Overhang length L after shifting at the slab extraction side of the heating furnace ０ Distance L between the skid located closest to the slab edge and the skid one position closer to the center of the slab than the skid before the shift by the final shift skid mechanism １ The distance between the skid closest to the slab edge and the skid one position closer to the center of the slab after the final shift skid mechanism has shifted.

Claims

1. A hot rolling method in which a slab is heated in a heating furnace and then hot rolled, wherein a skid that supports and transports the slab shifts at least once within the heating furnace, and the maximum temperature Ta (°C) that the slab reaches from when it is loaded into the heating furnace and before it shifts to the slab extraction side of the heating furnace and the maximum temperature Tb (°C) that the slab reaches after it shifts to the slab extraction side of the heating furnace and before it is extracted from the heating furnace satisfy Tb + 80°C > Ta > Tb + 10°C, Ta (°C) is the maximum temperature that the slab reaches in the heating furnace and is 1150°C or higher, and the slab has a chemical composition such that the gamma phase ratio at Ta (°C) is 25 mol% or less.

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

3. A hot rolling method according to claim 1 or 2, wherein the slab is a steel slab having a composition containing, by mass%, C: 0.02% or more and 0.08% or less, Si: 2.0% or more and 8.0% or less, Mn: 0.005% or more and 3.0% or less, Al: less than 0.0100%, 0: 0.0060% or less, N: 0.0060% or less, and S + 0.405 × Se: 0.0060% or less, with the balance being Fe and unavoidable impurities.

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

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

6. A method for manufacturing a hot-rolled coil according to claim 5, wherein the widthwise thickness fluctuation range at a position 10 to 20 m from the longitudinal end of the hot-rolled coil is 1.5 times or less than the widthwise thickness fluctuation range at the longitudinal center.

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

8. A slab heating equipment for heating slabs before hot rolling, comprising: a walking beam type heating furnace having at least one skid for supporting and transporting a slab within the heating furnace and one shift skid mechanism for shifting the position at which the skid supports the slab; heat treatment mechanisms capable of independently controlling the temperature before and after the shift skid mechanism on the nearest slab extraction side of the heating furnace; an atmosphere control mechanism for controlling the convection of atmospheric gas within the heating furnace; and a furnace temperature control mechanism for controlling the heat treatment mechanism and the atmosphere control mechanism on either side of the shift skid mechanism on the nearest slab extraction side of the heating furnace so that the temperature inside the extraction side of the heating furnace is 50°C or more lower than the temperature inside the charging side of the heating furnace.

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