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 method of hot rolling that obtains a good shape for hot-rolled coils. The method of hot rolling of the present disclosure includes hot rolling a slab after heating the slab in a heating furnace. Skids that support and transport the slab shift at least once in the heating furnace. A maximum arrival temperature Ta in °C reached by the slab from when the slab is charged into the heating furnace until prior to undergoing a shift furthest on a slab extraction side of the heating furnace and a maximum arrival temperature Tb in °C reached by the slab after undergoing the shift furthest on the slab extraction side of the heating furnace and until being extracted from the heating furnace satisfy Tb + 80 °C > Ta > Tb + 10 °C. Ta is the maximum arrival temperature reached by the slab in the heating furnace and is 1150 °C or higher. The slab has a chemical composition such that a y phase ratio at Ta is 25 mol% or less.

Description

TECHNICAL FIELD

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

[0002] Grain-oriented electrical steel sheets are typically produced by using precipitates called inhibitors to induce secondary recrystallization of Goss orientation ({110}<001>) grains during final annealing. For example, Patent Literature (PTL) 1 discloses a method using AlN as an inhibitor, and Patent Literature (PTL) 2 discloses a method using MnS or MnSe as an inhibitor. Both of these methods have been put into industrial use. The use of these inhibitors is a useful method for stably developing secondary recrystallized grains, but because the precipitates must be finely dispersed, slab heating needs to be performed at a high temperature of 1300 °C or higher before hot rolling.

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

[0004] On the other hand, a production technique that does not use the inhibitors described above (an inhibitor-less method) has also been proposed. For example, a technique has been proposed in which secondary recrystallization is achieved through the control of texture, using a higher-purity steel without adding inhibitor-forming components to the slab (PTL 3).CITATION LISTPatent Literature

[0005] PTL 1: JP S40-15644 B PTL 2: JP S51-13469 B PTL 3: JP 2000-129356 A SUMMARY(Technical Problem)

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

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

[0008] To solve the above problems, we conducted a detailed investigation into the hot rolling conditions of hot-rolled coils that actually suffered shape defects. The targeted hot-rolled coils were subjected to slab heating in a walking beam type slab heating furnace before being hot rolled.

[0009] FIG. 1 is a schematic diagram of an example of a walking beam type slab heating furnace. In a walking beam type slab heating furnace 1, a slab S is generally supported and transported by parts called skids. In FIG. 1, the slab S is supported on both the charging side and the extraction side, but the present disclosure is not limited to such a supporting method. Typically, the skids are arranged alternately with fixed skids 2a and movable skids 2b, and the movable skids 2b move up and down to lift the slab S and transport the slab S little by little from the charging side to the extraction side. If the positions of the skids used to support and transport the slab S are always the same relative to the slab S, it will 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 inside the heating furnace, and the positions of the skids supporting the slab S change before and after the shift skid 3.

[0010] FIG. 2 illustrates a state in which the slab S is supported by three fixed skids 2a or movable skids 2b. When the three movable skids 2b have moved downwards, the slab S is supported by the three fixed skids 2a, and when the three movable skids 2b have moved upwards, the slab S is supported by the movable skids 2b. FIG. 2 illustrates the latter case. In this case, if the overhang length is defined as the length from the skid, among the supporting skids 2, that is located closest to the longitudinal end of the slab S (hereinafter also referred to as the "slab end") to the longitudinal end of the slab, then in the skid arrangement of the heating furnace 1 in FIG. 1, the shift skid mechanism 3 makes an overhang length O 1 after the skid is shifted longer than an overhang length O 0 before the skid is shifted.

[0011] We discovered that shape defects often occur at the longitudinal end of a hot-rolled coil in the following cases. (1) The maximum arrival temperature of the slab in the heating furnace before hot rolling is 1150 °C or higher, and the slab has a chemical composition such that the γ phase ratio at the maximum arrival temperature is 25 mol% or less. (2) The overhang length O 1 of the slab after undergoing the shift furthest on the slab extraction side of the heating furnace is 10 % or more longer than the overhang length O 0 before the shift.

[0012] Here, the maximum arrival temperature of the slab can be determined by separately passing a slab equipped with a thermocouple through a heating furnace and recording the temperature change of the slab at each position in the heating furnace in advance. For example, thermocouples can be attached at a total of six locations, at the center in the slab longitudinal direction and at both longitudinal ends, on the surface and in the center of the thickness direction. The average value of the six measurements at each position in the heating furnace can be used as the temperature of the slab at that position. If the slab temperature is actually measured in the heating furnace, that value may be used.

[0013] The y phase ratio can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) produced by Thermo-Calc Software AB.

[0014] It is generally known that creep deformation occurring at high temperatures is more likely to occur in the α (ferrite) phase, whereas the rate of deformation is slower in the y (austenite) phase. The fact that a characteristic of hot-rolled coils with poor shape is that they have a chemical composition with a low γ-phase ratio suggests that the poor shape may be caused by creep deformation in the heating furnace. In other words, it is estimated that the ends of the slabs have undergone creep deformation due to their own weight inside the heating furnace, causing the ends to sink. The slab end is supported by the skid in a cantilever-like manner. Hence, as the overhang length increases, the creep deformation rate increases. It is presumed that this mechanism causes a slab with sunken ends to be extracted from the heating furnace and hot rolled, resulting in poor shape at the longitudinal ends of the hot-rolled coil.

[0015] Based on the presumed mechanism, we investigated the possibility of improving the shape of the hot-rolled coil.

[0016] Here, electrical steel sheets usually contain a high concentration of Si in order to improve the final magnetic properties. Si stabilizes the α phase and reduces the γ phase ratio when heated at high temperatures. C is also an element that has a large effect on the y phase ratio. As C has the effect of improving the hot-rolled microstructure and the texture during primary recrystallization, an appropriate amount of C is added from the perspective of improving the final magnetic properties. Therefore, it is difficult to adopt a method of increasing the y phase ratio at high temperatures by significantly changing the composition of an electrical steel sheet that has already undergone the production process.

[0017] The maximum arrival temperature of the slab during slab heating is usually set in order to dissolve trace amounts of impurity elements and precipitate-forming elements and to homogenize the slab. If impurity elements and the like can be reduced sufficiently, a slab heating temperature lower than 1150 °C can be adopted. In this case, the occurrence of the aforementioned shape defects is suppressed. However, in some cases, trace elements are intentionally added to improve magnetic properties, and lowering the maximum arrival temperature is not always an option.

[0018] On the other hand, if the cause of the defective shape of the longitudinal end of the hot-rolled coil is creep deformation of the slab end, 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 arrival temperature in the heating furnace, it may be possible to suppress the amount of creep deformation if the overhang length can be controlled.

[0019] We completed the present disclosure based on the above findings, and the primary features of the present disclosure are as follows. [1] A method of hot rolling, comprising: hot rolling a slab after heating the slab in a heating furnace, wherein skids that support and transport the slab shift at least once in the heating furnace, and a maximum arrival temperature Ta in °C reached by the slab from when the slab is charged into the heating furnace until prior to undergoing a shift furthest on a slab extraction side of the heating furnace and a maximum arrival temperature Tb in °C reached by the slab after undergoing the shift furthest on the slab extraction side of the heating furnace and before being extracted from the heating furnace satisfy Tb + 80 ° C > Ta > Tb + 10 ° C , Ta is a maximum arrival temperature reached by the slab in the heating furnace and is 1150 °C or higher, and the slab has a chemical composition such that a y phase ratio at Ta is 25 mol% or less. [2] The method of hot rolling according to [1], wherein the hot rolling includes two consecutive passes, each pass is performed in a temperature range of 1030 °C or higher and 1150 °C or lower, with a rolling reduction of 50 % or less and a strain rate of 15 s -1< or more, and a time between passes of 15 s or longer. [3] The method of hot rolling according to [1] or [2], wherein the slab is a steel slab comprising a chemical composition containing, in mass%, C: 0.02 % 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 inevitable impurities. [4] The method of hot rolling according to [1] or [2], wherein the slab further comprises, in mass%, one or more selected from the group consisting of Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, Ti: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, and Bi: 0.0005 % or more and 0.0200 % or less. [5] A method of producing a hot-rolled coil, comprising hot rolling a slab by the method of hot rolling according to any one of [1] to [4] to obtain a hot-rolled coil. [6] The method of producing a hot-rolled coil according to [5], wherein a widthwise thickness variation width at a position 10 m to 20 m from a longitudinal end of the hot-rolled coil is 1.5 times or less a widthwise thickness variation width at a longitudinal center. [7] A method of producing a grain-oriented electrical steel sheet, the method comprising hot rolling a slab by the method of hot rolling according to any one of [1] to [4], performing hot-rolled sheet annealing on a resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, and performing primary recrystallization annealing and final annealing. [8] Slab heating equipment for performing slab heating before hot rolling, the slab heating equipment comprising: a walking beam type heating furnace comprising at least one each of a skid configured to support and transport a slab in a heating furnace and a shift skid mechanism configured to shift a position at which the skid supports the slab; a heat treatment mechanism capable of independently controlling temperature on both sides of a shift skid mechanism furthest on a slab extraction side of the heating furnace; an atmosphere control mechanism configured to control convection of atmospheric gas in the heating furnace; and a furnace temperature control mechanism configured to control the heat treatment mechanism and the atmosphere control mechanism so that, with respect to the shift skid mechanism furthest on the slab extraction side of the heating furnace, a furnace inner temperature on an extraction side of the heating furnace is 50 °C or more lower than a furnace inner temperature on a charging side of the heating furnace. (Advantageous Effect)

[0020] According to the method of hot rolling of the present disclosure, the shape of the slab ends before hot rolling is improved, thereby obtaining a good shape for the longitudinal ends of the obtained hot-rolled coil, and ultimately making it possible to pass the grain-oriented electrical steel sheet more stably in the production processes after the hot rolling process. As a result, grain-oriented electrical steel sheets can be produced much more easily than before.

[0021] According to the present disclosure, a method of producing a hot-rolled coil and a grain-oriented electrical steel sheet can be provided, where the method includes a process of hot rolling a slab by the above method of hot rolling. Slab heating equipment that can be used in the method of hot rolling can also be provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the accompanying drawings: FIG. 1 is a schematic diagram of an example of a walking beam type slab heating furnace; FIG. 2 is a schematic diagram illustrating the state in which a slab is supported by a skid in the slab heating furnace of FIG. 1; FIG. 3 is a diagram illustrating the correspondence between transportation of a slab in a walking beam type heating furnace on the one hand and an example of a profile of slab temperature in the method of the present disclosure (solid line) and a typical profile of a conventional slab temperature (dashed line) on the other; and FIG. 4 is a schematic diagram illustrating the arrangement of skids before and after a shift in a slab heating furnace. DETAILED DESCRIPTION

[0023] The method of hot rolling of the present disclosure will now be described in detail.

[0024] The present disclosure is a method of hot rolling including hot rolling a slab after heating the slab in a heating furnace. The heating furnace includes skids for supporting and transporting the slab, and the skids shift at least once within the heating furnace. Shifting of the skids may be accomplished by a shift skid mechanism. The number of shift skid mechanisms may be one or more.

[0025] The method of the present disclosure is useful when using a heating furnace having a skid arrangement in which an overhang length O 1 after a shift by the shift skid mechanism furthest on the slab extraction side of the heating furnace (hereinafter also referred to as the "final shift skid mechanism") is 110 % or more of an overhang length O 0 before the shift. It is particularly advantageous if, at both ends of the slab, the overhang length O 1 after the shift by the final shift skid mechanism is 110 % or more of the overhang length O 0 before the shift. The overhang length O 1 can be set to, for example, 130 % or less of the overhang length O 0 . "Before the shift" and "after the shift" refer to before initiating and after completing a change in the position of the skids supporting the slab.

[0026] If the furnace has only one shift skid mechanism, this shift skid mechanism corresponds to the final shift skid mechanism. If the heating furnace has a plurality of shift skid mechanisms, "before undergoing a shift by the final shift skid mechanism" means after undergoing a shift by the second-to-final shift skid mechanism and before undergoing a shift by the final shift skid mechanism.

[0027] In the method of the present disclosure, the maximum arrival temperature reached by the slab from when the slab is charged into the heating furnace until prior to undergoing a shift furthest on the slab extraction side of the heating furnace is designated Ta (units: °C; hereinafter omitted), the maximum arrival temperature reached by the slab after undergoing the shift furthest on the slab extraction side of the heating furnace and before being extracted from the heating furnace is designated Tb (units: °C; hereinafter omitted), and Ta and Tb satisfy Tb + 80 °C > Ta > Tb + 10 °C.

[0028] Tb is the highest temperature the slab reaches after being shifted by the final shift skid mechanism and before being extracted from the heating furnace.

[0029] As described above, Ta and Tb can be determined by separately passing a slab equipped with a thermocouple through a heating furnace and recording the temperature change of the slab at each position in the heating furnace in advance.

[0030] When there is only one shift skid mechanism, the slab reaches the maximum arrival temperature Ta before being shifted by the shift skid mechanism. When there is a plurality of shift skid mechanisms, the timing at which the slab reaches its maximum arrival temperature Ta is not particularly limited as long as it is between the time the slab is charged into the heating furnace and before it is shifted by the final shift skid mechanism. For example, this timing may be between the time when the slab is shifted by the second-to-final shift skid mechanism and before the slab is shifted by the final shift skid mechanism. In this case, the timing at which the slab reaches the maximum arrival temperature Ta may be immediately after the shift by the second-to-final shift skid mechanism, immediately before reaching the final shift skid mechanism, or any time therebetween. After the slab is charged into the heating furnace, the slab may reach the maximum arrival temperature Ta before reaching the second-to-final shift skid mechanism.

[0031] In the method of the present disclosure, the maximum arrival temperature Ta is also the maximum arrival temperature reached by the slab in the heating furnace and is a temperature of 1150 °C or higher. The maximum arrival temperature Ta can be set to 1300 °C or less. If Ta is within this range, the slab can be sufficiently homogenized.

[0032] In the method of the present disclosure, the slab is heated so as to satisfy Tb + 80 ° C > Ta > Tb + 10 ° C .

[0033] When a high-temperature slab is cooled, there is a possibility that elements that have been homogenized as a solute due to the high temperature may be reprecipitated. An excessively low temperature reduces the homogenization effect in the slab and can cause deterioration of characteristics. The temperature is therefore preferably not lowered by more than 80 °C relative to Ta, which is the maximum arrival temperature reached by the slab in the heating furnace. On the other hand, in order to obtain the effect of reducing the shape defects at the longitudinal end portions of the hot-rolled coil by suppressing creep deformation, the temperature is preferably lowered by 10 °C or more relative to Ta.

[0034] Here, a conventional method of controlling slab heating is as follows.

[0035] Generally, one of the purposes of slab heating in general steel is to lower the deformation resistance of the steel by raising the temperature of the slab, thereby enabling hot rolling under a large reduction to obtain hot-rolled coils. To fulfill this role, it is necessary to control the temperature of the slabs extracted from the heating furnace to a temperature suitable for hot rolling. The object of control is the temperature of the slab at the time of extraction from the heating furnace, and 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, it is common to gradually increase the temperature of the slab in the heating furnace and control the temperature to reach the maximum arrival temperature at the time of extraction.

[0036] Furthermore, in the production of grain-oriented electrical steel sheets, it is also necessary to dissolve trace amounts of impurity elements and precipitate-forming elements to homogenize the interior of the slab. Once an element is solute, it may reprecipitate if the steel temperature drops. For this reason as well, the slab temperature at the time of extraction is commonly controlled to be the maximum arrival temperature.

[0037] In addition, since heating furnaces increase fuel utilization efficiency during heating by gradually heating the slab to the desired maximum arrival temperature, operations that bring large-volume slabs to their maximum arrival temperature at an early stage in the heating furnace are not usually carried out.

[0038] In the method of the present disclosure, a slab having a chemical composition such that the y phase ratio in Ta is 25 mol% or less is used. Even if the temperature of the slab after the shift furthest on the extraction side of the heating furnace is lower than the maximum arrival temperature Ta reached by the slab prior to undergoing the shift, the maximum arrival temperature Tb reached by the slab after the shift is higher than (Ta - 80 °C). Therefore, elements that have once become solute and homogenized remain in a supersaturated state, making it easy to suppress the progression of precipitation. On the other hand, since the maximum arrival temperature Tb is lower than (Ta - 10 °C), creep deformation at the slab ends can be easily suppressed.

[0039] FIG. 3 is a diagram illustrating the correspondence between transportation of a slab in a walking beam type heating furnace on the one hand and an example of a profile of slab temperature in the method of the present disclosure (solid line) and a typical profile of a conventional slab temperature (dashed line) on the other.

[0040] In the method of the present disclosure, Tb + 50 ° C > Ta > Tb + 15 ° C is preferably satisfied.

[0041] In the method of the present disclosure, a slab having a chemical composition such that the γ phase ratio at Ta is 25 mol% or less is used. By using such a slab, it is possible to prevent excessive formation of a pearlite phase derived from the γ phase in the hot-rolled sheet microstructure. Since the pearlite phase has a higher strength than the ferrite phase, cold rolling may be difficult in some cases, but the method of the present disclosure can easily avoid such a situation. A slab having a chemical composition such that the γ phase ratio is 20 mol% or less is preferred. No lower limit is placed on the γ phase ratio, which may be 0 mol%.

[0042] The chemical composition of the slab preferably satisfies the following. Hereinafter, "%" indications regarding chemical compositions are "mass%" unless otherwise specified.C: 0.02 % or more and 0.08 % or less

[0043] C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, and therefore, from the perspective of improving the final magnetic properties, the C content is preferably 0.02 % or more. The C content is more preferably 0.03 % or more. On the other hand, if the C content exceeds 0.08 %, even if decarburization annealing is performed it becomes difficult to reduce the C content to 50 ppm or less, the content at which magnetic aging does not occur. From this perspective, the C content is preferably limited to 0.08 % or less. The C content is more preferably 0.06 % or less.Si: 2.0 % or more and 8.0 % or less

[0044] Si is an element useful in reducing iron loss by increasing electrical resistance. In order to obtain good magnetic properties, the Si content is preferably 2.0 % or more. The Si content is more preferably 2.8 % or more. On the other hand, Si is also an element that increases the brittleness of steel. From the perspectives of reducing the risk of fracture during sheet passing and suppressing deterioration of cold rolling manufacturability, the Si content is preferably limited to 8.0 % or less. The Si content is more preferably 4.5 % or less.Mn: 0.005 % or more and 3.0 % or less

[0045] Mn is a useful element from the perspectives of improving hot workability and controlling the formation of an oxide film during primary recrystallization. From this perspective, the Mn content is preferably 0.005 % or more. The Mn content is more preferably 0.010 % or more. On the other hand, from the perspective of avoiding deterioration of the magnetic properties due to deterioration of the primary recrystallized texture, the Mn content is preferably limited to 3.0 % or less. The Mn content is more preferably 0.5 % or less.Al: Less than 0.0100 %, N: 0.0060 % or less, O: 0.0060 % or less, S + 0.405 × Se: 0.0060 % or less

[0046] If Al is excessive, it may be difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition. The Al content is therefore preferably limited to less than 0.0100 %. The Al content is more preferably 0.0080 % or less.

[0047] To prevent the formation of silicon nitrides after purification annealing in the production of the steel sheet after final annealing, the N content is preferably limited to 0.0060 % or less. The N content is more preferably 0.0030 % or less.

[0048] O forms oxides and inhibits deterioration of the magnetic properties of the steel sheet after final annealing. The O content is therefore preferably limited to 0.0060 % or less. The O content is more preferably 0.0030 % or less.

[0049] To stably obtain a secondary recrystallized microstructure, the total amount of S and Se multiplied by 0.405 is preferably limited to 0.0060 % or less. This total amount is more preferably 0.0040 % or less.

[0050] Al, N, O, S and Se are inhibitor components.

[0051] The essential components and the inhibitor components have been described above, but one or more optional components selected from the elements described below may also be included as appropriate. Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, Ti: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, and Bi: 0.0005 % or more and 0.0200 % or less.

[0052] Ni is a useful element in terms of improving the microstructure of a hot-rolled coil for better magnetic properties thereof. In order to fully obtain this effect, when Ni is included, the Ni content is preferably 0.005 % or more. On the other hand, if Ni is excessive, secondary recrystallization becomes unstable and magnetic properties deteriorate. The Ni content is therefore preferably 1.50 % or less.

[0053] Sn, Sb, Cu, Mo, P, Cr, B, and Bi are grain boundary segregation elements that can improve various properties, but if they are in excess, the development of secondary recrystallized grains can be inhibited. From these perspectives, when these elements are contained, the amounts are set to the above ranges.

[0054] Nb, Ti and Te are precipitate-forming elements that may improve various properties, but excessive amounts may make secondary recrystallization unstable. From these perspectives, when these elements are contained, the amounts are set to the above ranges.

[0055] The slab used in the method of the present disclosure preferably has a chemical composition containing the aforementioned essential components and optional components, with the balance being Fe and inevitable impurities. In this chemical composition, the precipitate components are sufficiently reduced, and the elements that have become solute and homogenized can maintain a supersaturated state, with the progression of precipitation being suppressed, even if the maximum arrival temperature Tb reached by the slab after undergoing the shift by the final shift mechanism is lower than the maximum arrival temperature Ta reached by the slab before undergoing the shift by the final shift mechanism.

[0056] The slabs used in the method of the present disclosure can be produced by refining molten steel adjusted to a desired chemical composition by a known method using a converter, electric furnace, or the like, and then subjecting the steel to vacuum treatment, if necessary, followed by a conventional method of ingot casting or method of continuous casting. Alternatively, a thin slab or thinner cast steel having a thickness of 100 mm or less may be directly produced by direct casting and used as a slab.

[0057] The slab used in the method of the present disclosure has a chemical composition such that the γ phase ratio is 25 mol% or less at Ta, which is the maximum arrival temperature reached by the slab in the heating furnace. The γ phase ratio can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) produced by Thermo-Calc Software AB.

[0058] After adjustment of the chemical composition of the slab, the temperature range in which the y phase fraction is 25 mol% or less can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7), and Ta, which is the maximum arrival temperature reached by the slab in the heating furnace, can be set based on the calculated temperature range.

[0059] Alternatively, the maximum arrival temperature Ta reached by the slab in the heating furnace may be set, and the chemical composition may then be adjusted so that the y phase ratio is 25 mol% or less at this value of Ta.

[0060] The slab heating in the method of the present disclosure can be performed using slab heating equipment provided with a walking beam type heating furnace including skids configured to support and transport a slab in a heating furnace and at least one shift skid mechanism configured to shift a position at which the skids support the slab.

[0061] In the heating furnace, on one of the two slab end sides of the final shift skid mechanism, the distance between the skid located furthest on the slab end side and the skid located one skid closer to the center of the slab after the shift by the final shift skid mechanism is L 1 , and the distance between the skid located furthest on the slab end side and the skid one skid closer to the center of the slab before the shift by the final shift skid mechanism is L 0 . In this case, if the distance L 1 is greater than the distance L 0 , the overhang length O 1 after the shift by the final shift skid mechanism easily becomes 110 % or more of the overhang length O 0 before the shift, which could result in poor shape. The benefits of the hot rolling method of the present disclosure can thus be fully enjoyed. In a case in which the furnace has a plurality of shift skid mechanisms, the position of the skid "before the shift by the final shift skid mechanism" corresponds to the position of the skid of the second-to-final shift skid mechanism. FIG. 4 is a schematic diagram. In FIG. 4, at the slab end where L 0 and L 1 are indicated, the skid located furthest on the slab end side is a fixed skid 2a, and the skid located one skid closer to the center of the slab than that skid is a movable skid 2b.

[0062] As has been revealed for the first time by the present disclosure, the deformation of the slab when the overhang length is increased is an operational problem limited to steel types with a low y phase ratio at the target temperature. Normally, the y phase develops in iron at high temperatures. In this case, if the steel contains a large amount of y phase suppressing elements such as Si and Al and a small amount of y phase forming elements such as C, Ni and Cr, a similar problem is likely to occur. Generally, electrical steel sheets contain 2.0 % or more Si to reduce iron loss. Furthermore, since impurities can affect performance degradation, the upper limits of many elements are strictly regulated, and these elements may therefore become components that can lead to the problem of slab deformation.

[0063] On the other hand, most slab heating furnaces currently in operation are designed and produced for general steel materials. The furnace is designed to gradually heat the slab from the entry side to the exit side to a high temperature, so that the target temperature (generally the maximum temperature) is reached at the exit side. Naturally, the higher the temperature, the greater the concern about slab deformation. Hence, with the idea of supporting the slab over a wider area, the skid on the outermost side of the slab is often shifted outward, as illustrated in FIG. 2. The second outermost skid may shift in the same direction or may shift inward for overall balance. When the outermost skid shifts outward and the second outermost skid shifts inward in this way, the overhang length increases. In cases in which the shift direction (inward or outward) causes an imbalance in the locations supporting the slab, a new skid may be installed when the shift occurs.

[0064] Furthermore, the length and width of the steel material required by manufacturers are not fixed. For example, even when using a heating furnace with a skid arrangement as illustrated in FIG. 2, if the required length of the product steel material is short, for example, the length of the slab to be charged will also be short, and the slab may be supported by the five skids on the left out of the six skids. Therefore, to accommodate various required product lengths and slab lengths, a plurality of heating furnaces and heating furnaces with various patterns of skid arrangements and shift mechanisms are used in parallel.

[0065] When a slab that satisfies the set of component conditions identified in the process of completing the present disclosure is used, and there is a mismatch between the slab length required for the product and the skid arrangement characteristics of the heating furnace, application of the present disclosure can yield particularly significant improvements.

[0066] The present disclosure also relates to slab heating equipment used in the method of the present disclosure. The slab heating equipment includes a heat treatment mechanism capable of independently controlling temperature on both sides of the shift skid mechanism furthest on the slab extraction side of the heating furnace (final shift skid mechanism), an atmosphere control mechanism configured to control convection of atmospheric gas in the heating furnace, and a furnace temperature control mechanism configured to control the heat treatment mechanism and the atmosphere control mechanism so that, with respect to the final shift skid mechanism, a furnace inner temperature on an extraction side of the heating furnace (also referred to as "after the final shift skid mechanism") is 50 °C or more lower than a furnace inner temperature on a charging side of the heating furnace (also referred to as "before the final shift skid mechanism"). On the other hand, to prevent the slab from becoming excessively cold, the furnace temperature after the final shift skid mechanism is preferably prevented from becoming 130 °C or more lower than the furnace temperature before the final shift skid mechanism.

[0067] The heat treatment mechanism can be a heating device, such as a burner. For example, the heating furnace may be divided into a plurality of heating sections (such as the sections between shift skid mechanisms), and a different system of burners may be provided for each heating section.

[0068] The atmosphere control mechanism is a mechanism for controlling the convection of the atmospheric gas within the heating furnace and can be, for example, a mechanism with separate intake and exhaust systems for each location in the heating furnace (for example, for each of the aforementioned heating sections). The atmosphere control mechanism can also supply inert gas to a specific location in the heating furnace to keep the temperature in that location and its surroundings low. The atmosphere control mechanism also contributes to controlling the furnace temperature. The heating and cooling by controlling the convection of the atmospheric gas is performed by the atmosphere control mechanism.

[0069] 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 50 °C or more lower than the furnace temperature before the final shift skid mechanism. To lower the slab temperature of an already heated slab by 10 °C or more, a temperature difference of 50 °C or more is required in the furnace. Here, the temperature inside the furnace can be measured by installing a sensor such as a thermocouple in the heating furnace. For example, if the heating furnace is divided into a plurality of heating sections, a sensor can be installed in each heating section. The furnace temperatures before and after the final shift skid mechanism is measured by the sensors that are installed before and after the final shift skid mechanism and are closest to the final shift skid mechanism. In a case in which the heating furnace is divided into a plurality of heating sections, the furnace temperatures before and after the final shift skid mechanism are measured by sensors installed in the heating sections adjacent to the final shift skid mechanism. Instead of measuring with a sensor, control may be performed based on a calculated value of the slab temperature.

[0070] Modifying the skid itself is equivalent to installing a new furnace, and there are significant constraints on its realization. Installation of a furnace temperature control mechanism, a heat treatment mechanism, and an atmosphere control mechanism, however, can be achieved more easily.

[0071] The heating furnace may be provided with furnace walls between adjacent heating sections to reduce the effect of radiant heat from the adjacent heating sections. The heating furnace may include an auxiliary skid for supporting the slab.

[0072] Since the slab itself carries heat, it is necessary to control the heat treatment mechanism and atmosphere control mechanism in order 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 a desired temperature difference in the furnace is achieved across the final shift skid mechanism. This allows the maximum arrival temperature Ta (°C) of the slab before reaching the final shift skid mechanism and the maximum arrival temperature Tb (°C) of the slab after the shift to satisfy the following relationship. Tb + 80 ° C > Ta > Tb + 10 ° C

[0073] Depending on the skid arrangement, the overhang length of the slab end after undergoing the shift by the final shift skid mechanism may be 110 % or more at one slab end and less than 110 % at the other slab end, as compared to the overhang length of the slab end before the shift. In such a case, the temperature may be lowered to Tb only at one end where the overhang amount is 110 % or more, which allows for more efficient operation.

[0074] The slab heating equipment of the present disclosure is not limited to the production of grain-oriented electrical steel sheets but can also be used for slab heating of steel with a low γ-phase ratio.

[0075] The slab extracted after the above-described slab heating is homogenized steel and also achieves little sinking at the overhang positions at the ends of the slab.

[0076] Next, the slab is subjected to hot rolling, which is relatively easy because sinking and defectiveness of shape at the slab ends have been controlled.

[0077] During the hot rolling process in the method of the present disclosure, at least two consecutive passes of rolling from the slab stage to the sheet bar stage can be carried out in a temperature range of 1030 °C or higher to 1150 °C or lower.

[0078] To improve the shape of the longitudinal end portions of the hot-rolled coil, the time between two consecutive passes is preferably 15 s or more, the rolling reduction of each pass is preferably 50 % or less, and the strain rate is preferably 15 s -1< or more.

[0079] In particular, even if the inhibitor components S and Se are present in the slab and form sulfides or selenides in the central layer of the slab, two-pass rolling under an appropriate set of conditions in a specific temperature range (1030 °C or higher and 1150 °C or lower) is advantageous in that a smaller size than the size that would cause problems during cold rolling can be achieved. 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 fragmentation and destruction due to sulfides and selenides may occur. Hence, the aforementioned hot rolling process is preferable.

[0080] The method of the present disclosure uses a slab in which the y phase ratio is 25 mol% or less at Ta, which is the maximum arrival temperature in the heating furnace, and the y phase ratio reaches its maximum near the temperature range of 1030 °C or higher to 1150 °C or lower. Generally, austenite has a higher deformation resistance than ferrite and is less likely to deform even when pressed. Therefore, the rolling reduction for each pass is limited to 50 % or less. From the perspective of homogenizing the structure of the hot-rolled coil, the rolling reduction is preferably 15 % or more. The rolling reduction is more preferably 20 % or more.

[0081] Furthermore, by setting the time between passes to 15 s or longer, dislocations formed once by deformation are recovered or disappear by recrystallization. Rolling can therefore be performed without excessively increasing deformation resistance. The time between passes is preferably 120 s or shorter in order to suppress the formation of precipitates nucleated by dislocations generated during deformation.

[0082] Setting the strain rate to 15 s -1< or more facilitates improvement of the shape of the longitudinal ends of the hot-rolled coil. The strain rate may be 50 s -1< or less.

[0083] Here, the strain rate ε can be calculated using the following Ekelund expression. ε ≒ υ R R ′ h 1 2 2 − r ⋅ r

[0084] In the expression, v R is the roller peripheral speed (mm / s), R' is the roll radius (mm), h 1 is the roll entry side thickness (mm), and r is the rolling reduction (%).

[0085] Typically, the longitudinal ends of a hot-rolled coil have larger thickness variations (the maximum thickness minus the minimum thickness) than the steady-state portion including the central portion. However, according to the method of the present disclosure, thickness variations at the longitudinal ends of the hot-rolled coil can be suppressed. For example, the widthwise thickness variation width at the longitudinal ends of the hot-rolled coil can be controlled to 1.5 times or less the widthwise thickness variation width 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 edge (i.e., positions between 0 % and 3 % and between 97 % and 100 %), where the entire longitudinal length of the hot-rolled coil is 100 %. The evaluation of the steady-state portion is preferably carried out at a position 10 % to 90 % from one end in the longitudinal direction.

[0086] The present disclosure also relates to a method of producing a grain-oriented electrical steel sheet, the method including hot rolling a slab by the method of hot rolling according to the present disclosure, performing hot-rolled sheet annealing on the resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, next optionally performing decarburization annealing, and then performing primary recrystallization annealing and final annealing. The hot-rolled sheet has an improved shape at the longitudinal end, which can suppress meandering, fracture during the cold rolling process, and the like.

[0087] It is important for the hot-rolled sheet annealing to be performed at 1150 °C or lower. If the hot-rolled sheet annealing temperature exceeds 1150 °C, the inhibitor-forming elements that are inevitably mixed in will become solute and will reprecipitate unevenly during cooling, making it difficult to achieve a homogenous primary recrystallization microstructure and inhibiting the development of secondary recrystallization. Furthermore, if the hot-rolled sheet annealing temperature exceeds 1150 °C, the grain size after hot-rolled sheet annealing becomes too coarse, which is also disadvantageous in realizing an appropriate primary recrystallization microstructure. The hot-rolled sheet annealing is preferably performed at 900 °C or higher to promote recrystallization.

[0088] After the hot-rolled sheet annealing, cold rolling is performed once, or twice or more with intermediate annealing performed therebetween. In cold rolling, it is effective to perform aging treatment once, or twice or more, at a rolling temperature of 80 °C or higher and 150 °C or lower, with the temperature between rolling passes being increased to 100 °C or higher and 300 °C or lower, in order to develop a Goss texture.

[0089] Next, primary recrystallization annealing is performed. The purpose of this primary recrystallization annealing is to subject the cold rolled sheet with a rolled microstructure to primary recrystallization and to adjust to the optimum primary recrystallized grain size for secondary recrystallization, and also to decarburize the carbon contained in the steel by setting the annealing atmosphere to a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere and simultaneously form an oxide film on the surface by the aforementioned oxidizing atmosphere. Therefore, the primary recrystallization annealing is preferably performed at 750 °C or higher with the dew point induced in an H 2 mixed atmosphere. The primary recrystallization annealing is preferably performed at 900 °C or lower with the dew point induced in an H 2 mixed atmosphere. During the temperature increase in the primary recrystallization annealing, the heating rate is preferably set to 200 °C / s or higher between 550 °C and 680 °C, as doing so can further enhance the effect of texture improvement. At the same time, decarburization annealing is carried out to reduce the C content to 50 mass ppm or less, at which no magnetic aging occurs. The C content is preferably reduced to 30 mass ppm or less.

[0090] A technique of increasing the Si content by siliconizing after primary recrystallization annealing may also be used in combination.

[0091] Thereafter, a final annealing is performed to develop a secondary recrystallized microstructure. In this case, a forsterite film may be formed using an annealing separator containing MgO as the main component. The forsterite film can be better formed by the addition of appropriate amounts of Ti oxides, Sr compounds, and the like to the separating agent. In particular, the addition of additives that promote uniform forsterite film formation is also advantageous for improving separation properties. Any annealing separator, such as Al 2 O 3 , may also be used to suppress film formation.

[0092] The final annealing needs to be performed at 800 °C or higher to induce secondary recrystallization, but the heating rate up to 800 °C does not have a significant effect on the magnetic properties. Hence, any condition may be used. The annealing atmosphere may be any of N 2 , Ar, or H 2 , or a mixed gas containing two or more of these. To promote secondary recrystallization more effectively, isothermal holding near the secondary recrystallization temperature may be employed. However, since the same effect can be obtained by, for example, slowing down the heating rate, isothermal holding is not essential. Precipitation of trace elements in the final product leads to a deterioration in magnetic properties. The maximum annealing temperature is therefore set to 1100 °C or higher for element purification.

[0093] After the final annealing, an insulating coating may 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, preferred methods are described in JP S50-79442 A and JP S48-39338 A, where a coating liquid containing phosphate-chromate-colloidal silica is applied on a steel sheet and then baked at a temperature of around 800 °C.

[0094] Furthermore, the shape of the steel sheet can be adjusted by performing flattening annealing. The flattening annealing may also be combined with baking of the insulating coating.EXAMPLES

[0095] The present disclosure will be described in detail with reference to examples, but the present disclosure is not limited to these examples.[Example 1]

[0096] A steel slab (total length 12.2 m, width 1 m, thickness 180 mm) containing Si: 3.2 % to 3.4 %, C: 0.035 % to 0.070 %, Mn: 0.07 %, Al: 0.0050 % to 0.0090 %, and less than 0.0060 % each of N, O, and S + 0.405 × Se, with the balance being Fe and inevitable impurities, and containing no inhibitor components, was heated using a walking beam type heating furnace according to the heating pattern illustrated in Table 1, and the third and fourth passes of four-pass rough rolling were carried out under the set of conditions illustrated in Table 1. Subsequently, finish hot rolling with multiple passes in the temperature range of 850 °C to 950 °C was performed to finish to a thickness of 2.2 mm. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0097] The walking beam type heating furnace is provided with one shift skid mechanism, which corresponds to the shift skid mechanism furthest on the slab extraction side of the heating furnace (final shift skid mechanism). This shift skid mechanism consists of movable skids and fixed skids arranged alternately, and when the aforementioned steel slab is disposed, the skid arrangement is such that at one end, the overhang length of the slab is 112 % from before to after the shift.

[0098] The heating furnace has separate heating sections before and after the final shift skid mechanism, and each section is equipped with a different system of burners. An inert gas supply system is connected to the heating section after the final shift skid mechanism, and the burner and the inert gas supply system are connected to a control system. Each heating section is provided with a sensor for measuring the furnace inner temperature.

[0099] To evaluate the shape of the longitudinal end of the obtained hot-rolled coil (total length 1000 m, width 1 m, thickness 2.2 mm), samples were cut out from positions 10 m, 12 m, 14 m, 16 m, 18 m, and 20 m from the longitudinal end of the coil. To evaluate the shape of the steady-state portion, six samples were similarly cut out at 2 m intervals from a position 200 m or more away from the end. The thickness profile in the width direction was measured, and the difference between the maximum and minimum values was calculated. Evaluation was made as the ratio of the thickness difference between the longitudinal ends and the steady-state portion. As is clear from the table, the shape is improved under the conditions of the disclosure. Table 1No.Chemical composition (mass%)*Before shift skidAfter shift skidInside the heating furnaceRough rolling third passRough rolling fourth passTime between third and fourth passesDifference in thickness at longitudinal end / difference in thickness at steady state portionNotesSiCMnAlNS + 0.405 SeMaximum arrival temperature TaMaximum arrival temperature TbMaximum arrival temperaturey phase ratioTemperatureEach rolling reductionStrain rateTemperatureEach rolling reductionStrain rate1-13.20.0350.070.00600.00400.00121150 °C1200 °C1200 °C4 mol%1110 °C30 %25 s -1< 1080 °C30 %25 s -1< 45 s1.6Comp. Example1-21200 °C1190 °C1200 °C4 mol%1080 °C30 %25 s -1< 1050 °C30 %25 s -1< 43 s1.1Example1-31240 °C1235 °C1240 °C0 mol%1160 °C31 %25 s -1< 1120 °C42 %40 s -1< 38 s1.8Comp. Example1-41240 °C1190 °C1240 °C0 mol%1080 °C55 %45 s -1< 1050 °C30 %28 s -1< 50 s1.3Example2-13.20.0550.070.00800.00500.00381200 °C1170 °C1200 °C23 mol%1090 °C28 %25 s -1< 1070 °C18 %13 s -1< 90 s1.3Example2-21200 °C1170 °C1200 °C23 mol%1110 °C32 %30 s -1< 1090 °C44 %42 s -1< 14 s1.3Example2-31250 °C1190 °C1250 °C15 mol%1080 °C30 %25 s -1< 1050 °C30 %25 s -1< 38 s1.1Example2-41250 °C1190 °C1250 °C15 mol%1100 °C35 %30 s -1< 1060 °C35 %30 s -1< 20 s1.1Example3-13.40.0550.070.00500.00300.00241060 °C1100 °C1100 °C18 mol%1000 °C25 %25 s -1< 970 °C38 %40 s -1< 28 s1.5Comp. Example3-21200 °C1170 °C1200 °C15 mol%1100 °C18 %22 s -1< 1070 °C55 %46 s -1< 55 s1.3Example3-31200 °C1170 °C1200 °C15 mol%1100 °C25 %25 s -1< 1070 °C26 %21 s -1< 50 s1.1Example3-41200 °C1110 °C1200 °C15 mol%1000 °C25 %25 s -1< 970 °C38 %36 s -1< 28 s1.6Comp. Example3-51140 °C1200 °C1200 °C15 mol%1100 °C55 %45 s -1< 1080 °C18%13 s -1< 90 s1.8Comp. Example43.40.0700.070.00900.00400.00301180 °C1200 °C1200 °C26 mol%1080 °C35 %25 s -1< 1040 °C45 %40 s -1< 14 s1.4Comp. Example* The balance is Fe and incidental impurities. [Example 2]

[0100] A steel slab (total length 8.5 m, width 1 m, thickness 170 mm) containing the components illustrated in Table 2, with the balance being Fe and inevitable impurities, and having a calculated y phase ratio of 25 mol% or less at the maximum arrival temperature during slab heating, was hot rolled using a walking beam type heating furnace under the conditions similarly illustrated in Table 2. The resulting hot-rolled coil was 1 m wide and 1.6 mm thick. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0101] The walking beam type heating furnace used was the same as in Example 1.

[0102] For some hot-rolled coils, two coils were produced under the same set of conditions, and one of the coils was used to evaluate the shape of the longitudinal end portion in the same manner as in Example 1.

[0103] The hot-rolled coil from which no samples were taken was subjected to hot-rolled sheet annealing at an end-point temperature of 1020 °C, and it was confirmed whether the sheet moved transversely and meandered for 20 mm or more during sheet passing. Thereafter, the sheet was subjected to primary cold rolling at 100 °C in a reverse mill to a thickness of 1.7 mm, followed by intermediate annealing at 900 °C for 1 minute, and then to reverse secondary cold rolling again, with coiling aging treatment at 200 °C during the process, to a sheet thickness of 0.22 mm. Subsequently, primary recrystallization annealing was performed with a heating rate of 300 °C / s between 550 °C and 680 °C, a soaking temperature of 840 °C, and a soaking time of 60 s. An annealing separator composed of MgO: 95 % and TiO 2 : 5 % was then applied to the steel sheet surface as an aqueous slurry, and the steel sheet was subjected to secondary recrystallization annealing. A coating liquid containing phosphate-chromate-colloidal silica at a 3:1:3 weight ratio was applied to the surface of the final annealed sheet obtained in this way and was baked thereon at 800 °C. The magnetic properties of the widthwise center of the product sheet coil of the obtained grain-oriented electrical steel sheet were also confirmed. As the magnetic properties, the magnetic flux density (B 8 ) was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz.

[0104] As is clear from Table 2, under the conditions of the present disclosure, the production stability is improved and good magnetic properties are obtained. Furthermore, although a reverse mill is used in these Examples, meandering within the line in a continuous line such as a tandem mill also affects fracture during cold rolling. INDUSTRIAL APPLICABILITY

[0105] According to the method of hot rolling of the present disclosure, the shape of the slab ends before hot rolling is improved, thereby obtaining a good shape for the longitudinal ends of the obtained hot-rolled coil, and ultimately making it possible to pass the grain-oriented electrical steel sheet more stably in the production processes after the hot rolling process. A grain-oriented electrical steel sheet can therefore be produced much easier than before. According to the present disclosure, a method of producing a hot-rolled coil and a grain-oriented electrical steel sheet can be provided, where the method includes a process of hot rolling a slab by the above method of hot rolling. Slab heating equipment that can be used in the method of hot rolling can also be provided.REFERENCE SIGNS LIST

[0106] 1 Slab heating furnace 2 Skid 2a Fixed skid 2b Movable skid 3 Shift skid S Slab O 0 Overhang length before undergoing shift furthest on the slab extraction side of the heating furnace O 1 Overhang length after undergoing shift furthest on the slab extraction side of the heating furnace L 0 Distance between the skid located furthest on the slab end side and the skid located one skid closer to the center of the slab, before undergoing the shift by the final shift skid mechanism L 1 Distance between the skid located furthest on the slab end side and the skid located one skid closer to the center of the slab, after undergoing the shift by the final shift skid mechanism

Examples

example 1

[Example 1]

[0096]A steel slab (total length 12.2 m, width 1 m, thickness 180 mm) containing Si: 3.2 % to 3.4 %, C: 0.035 % to 0.070 %, Mn: 0.07 %, Al: 0.0050 % to 0.0090 %, and less than 0.0060 % each of N, O, and S + 0.405 × Se, with the balance being Fe and inevitable impurities, and containing no inhibitor components, was heated using a walking beam type heating furnace according to the heating pattern illustrated in Table 1, and the third and fourth passes of four-pass rough rolling were carried out under the set of conditions illustrated in Table 1. Subsequently, finish hot rolling with multiple passes in the temperature range of 850 °C to 950 °C was performed to finish to a thickness of 2.2 mm. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0097]The walking beam type heating furnace is provided with one shift skid mechanism, which corresponds to the shift skid mechanism furthest on the slab extraction side of the he...

example 2

[Example 2]

[0100]A steel slab (total length 8.5 m, width 1 m, thickness 170 mm) containing the components illustrated in Table 2, with the balance being Fe and inevitable impurities, and having a calculated y phase ratio of 25 mol% or less at the maximum arrival temperature during slab heating, was hot rolled using a walking beam type heating furnace under the conditions similarly illustrated in Table 2. The resulting hot-rolled coil was 1 m wide and 1.6 mm thick. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0101]The walking beam type heating furnace used was the same as in Example 1.

[0102]For some hot-rolled coils, two coils were produced under the same set of conditions, and one of the coils was used to evaluate the shape of the longitudinal end portion in the same manner as in Example 1.

[0103]The hot-rolled coil from which no samples were taken was subjected to hot-rolled sheet annealing at an end-point temperature o...

TECHNICAL FIELD

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

[0002] Grain-oriented electrical steel sheets are typically produced by using precipitates called inhibitors to induce secondary recrystallization of Goss orientation ({110}<001>) grains during final annealing. For example, Patent Literature (PTL) 1 discloses a method using AlN as an inhibitor, and Patent Literature (PTL) 2 discloses a method using MnS or MnSe as an inhibitor. Both of these methods have been put into industrial use. The use of these inhibitors is a useful method for stably developing secondary recrystallized grains, but because the precipitates must be finely dispersed, slab heating needs to be performed at a high temperature of 1300 °C or higher before hot rolling.

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

[0004] On the other hand, a production technique that does not use the inhibitors described above (an inhibitor-less method) has also been proposed. For example, a technique has been proposed in which secondary recrystallization is achieved through the control of texture, using a higher-purity steel without adding inhibitor-forming components to the slab (PTL 3).CITATION LISTPatent Literature

[0005] PTL 1: JP S40-15644 B PTL 2: JP S51-13469 B PTL 3: JP 2000-129356 A SUMMARY(Technical Problem)

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

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

[0008] To solve the above problems, we conducted a detailed investigation into the hot rolling conditions of hot-rolled coils that actually suffered shape defects. The targeted hot-rolled coils were subjected to slab heating in a walking beam type slab heating furnace before being hot rolled.

[0009] FIG. 1 is a schematic diagram of an example of a walking beam type slab heating furnace. In a walking beam type slab heating furnace 1, a slab S is generally supported and transported by parts called skids. In FIG. 1, the slab S is supported on both the charging side and the extraction side, but the present disclosure is not limited to such a supporting method. Typically, the skids are arranged alternately with fixed skids 2a and movable skids 2b, and the movable skids 2b move up and down to lift the slab S and transport the slab S little by little from the charging side to the extraction side. If the positions of the skids used to support and transport the slab S are always the same relative to the slab S, it will 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 inside the heating furnace, and the positions of the skids supporting the slab S change before and after the shift skid 3.

[0010] FIG. 2 illustrates a state in which the slab S is supported by three fixed skids 2a or movable skids 2b. When the three movable skids 2b have moved downwards, the slab S is supported by the three fixed skids 2a, and when the three movable skids 2b have moved upwards, the slab S is supported by the movable skids 2b. FIG. 2 illustrates the latter case. In this case, if the overhang length is defined as the length from the skid, among the supporting skids 2, that is located closest to the longitudinal end of the slab S (hereinafter also referred to as the "slab end") to the longitudinal end of the slab, then in the skid arrangement of the heating furnace 1 in FIG. 1, the shift skid mechanism 3 makes an overhang length O 1 after the skid is shifted longer than an overhang length O 0 before the skid is shifted.

[0011] We discovered that shape defects often occur at the longitudinal end of a hot-rolled coil in the following cases. (1) The maximum arrival temperature of the slab in the heating furnace before hot rolling is 1150 °C or higher, and the slab has a chemical composition such that the γ phase ratio at the maximum arrival temperature is 25 mol% or less. (2) The overhang length O 1 of the slab after undergoing the shift furthest on the slab extraction side of the heating furnace is 10 % or more longer than the overhang length O 0 before the shift.

[0012] Here, the maximum arrival temperature of the slab can be determined by separately passing a slab equipped with a thermocouple through a heating furnace and recording the temperature change of the slab at each position in the heating furnace in advance. For example, thermocouples can be attached at a total of six locations, at the center in the slab longitudinal direction and at both longitudinal ends, on the surface and in the center of the thickness direction. The average value of the six measurements at each position in the heating furnace can be used as the temperature of the slab at that position. If the slab temperature is actually measured in the heating furnace, that value may be used.

[0013] The y phase ratio can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) produced by Thermo-Calc Software AB.

[0014] It is generally known that creep deformation occurring at high temperatures is more likely to occur in the α (ferrite) phase, whereas the rate of deformation is slower in the y (austenite) phase. The fact that a characteristic of hot-rolled coils with poor shape is that they have a chemical composition with a low γ-phase ratio suggests that the poor shape may be caused by creep deformation in the heating furnace. In other words, it is estimated that the ends of the slabs have undergone creep deformation due to their own weight inside the heating furnace, causing the ends to sink. The slab end is supported by the skid in a cantilever-like manner. Hence, as the overhang length increases, the creep deformation rate increases. It is presumed that this mechanism causes a slab with sunken ends to be extracted from the heating furnace and hot rolled, resulting in poor shape at the longitudinal ends of the hot-rolled coil.

[0015] Based on the presumed mechanism, we investigated the possibility of improving the shape of the hot-rolled coil.

[0016] Here, electrical steel sheets usually contain a high concentration of Si in order to improve the final magnetic properties. Si stabilizes the α phase and reduces the γ phase ratio when heated at high temperatures. C is also an element that has a large effect on the y phase ratio. As C has the effect of improving the hot-rolled microstructure and the texture during primary recrystallization, an appropriate amount of C is added from the perspective of improving the final magnetic properties. Therefore, it is difficult to adopt a method of increasing the y phase ratio at high temperatures by significantly changing the composition of an electrical steel sheet that has already undergone the production process.

[0017] The maximum arrival temperature of the slab during slab heating is usually set in order to dissolve trace amounts of impurity elements and precipitate-forming elements and to homogenize the slab. If impurity elements and the like can be reduced sufficiently, a slab heating temperature lower than 1150 °C can be adopted. In this case, the occurrence of the aforementioned shape defects is suppressed. However, in some cases, trace elements are intentionally added to improve magnetic properties, and lowering the maximum arrival temperature is not always an option.

[0018] On the other hand, if the cause of the defective shape of the longitudinal end of the hot-rolled coil is creep deformation of the slab end, 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 arrival temperature in the heating furnace, it may be possible to suppress the amount of creep deformation if the overhang length can be controlled.

[0019] We completed the present disclosure based on the above findings, and the primary features of the present disclosure are as follows. [1] A method of hot rolling, comprising: hot rolling a slab after heating the slab in a heating furnace, wherein skids that support and transport the slab shift at least once in the heating furnace, and a maximum arrival temperature Ta in °C reached by the slab from when the slab is charged into the heating furnace until prior to undergoing a shift furthest on a slab extraction side of the heating furnace and a maximum arrival temperature Tb in °C reached by the slab after undergoing the shift furthest on the slab extraction side of the heating furnace and before being extracted from the heating furnace satisfy Tb + 80 ° C > Ta > Tb + 10 ° C , Ta is a maximum arrival temperature reached by the slab in the heating furnace and is 1150 °C or higher, and the slab has a chemical composition such that a y phase ratio at Ta is 25 mol% or less. [2] The method of hot rolling according to [1], wherein the hot rolling includes two consecutive passes, each pass is performed in a temperature range of 1030 °C or higher and 1150 °C or lower, with a rolling reduction of 50 % or less and a strain rate of 15 s -1< or more, and a time between passes of 15 s or longer. [3] The method of hot rolling according to [1] or [2], wherein the slab is a steel slab comprising a chemical composition containing, in mass%, C: 0.02 % 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 inevitable impurities. [4] The method of hot rolling according to [1] or [2], wherein the slab further comprises, in mass%, one or more selected from the group consisting of Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, Ti: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, and Bi: 0.0005 % or more and 0.0200 % or less. [5] A method of producing a hot-rolled coil, comprising hot rolling a slab by the method of hot rolling according to any one of [1] to [4] to obtain a hot-rolled coil. [6] The method of producing a hot-rolled coil according to [5], wherein a widthwise thickness variation width at a position 10 m to 20 m from a longitudinal end of the hot-rolled coil is 1.5 times or less a widthwise thickness variation width at a longitudinal center. [7] A method of producing a grain-oriented electrical steel sheet, the method comprising hot rolling a slab by the method of hot rolling according to any one of [1] to [4], performing hot-rolled sheet annealing on a resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, and performing primary recrystallization annealing and final annealing. [8] Slab heating equipment for performing slab heating before hot rolling, the slab heating equipment comprising: a walking beam type heating furnace comprising at least one each of a skid configured to support and transport a slab in a heating furnace and a shift skid mechanism configured to shift a position at which the skid supports the slab; a heat treatment mechanism capable of independently controlling temperature on both sides of a shift skid mechanism furthest on a slab extraction side of the heating furnace; an atmosphere control mechanism configured to control convection of atmospheric gas in the heating furnace; and a furnace temperature control mechanism configured to control the heat treatment mechanism and the atmosphere control mechanism so that, with respect to the shift skid mechanism furthest on the slab extraction side of the heating furnace, a furnace inner temperature on an extraction side of the heating furnace is 50 °C or more lower than a furnace inner temperature on a charging side of the heating furnace. (Advantageous Effect)

[0020] According to the method of hot rolling of the present disclosure, the shape of the slab ends before hot rolling is improved, thereby obtaining a good shape for the longitudinal ends of the obtained hot-rolled coil, and ultimately making it possible to pass the grain-oriented electrical steel sheet more stably in the production processes after the hot rolling process. As a result, grain-oriented electrical steel sheets can be produced much more easily than before.

[0021] According to the present disclosure, a method of producing a hot-rolled coil and a grain-oriented electrical steel sheet can be provided, where the method includes a process of hot rolling a slab by the above method of hot rolling. Slab heating equipment that can be used in the method of hot rolling can also be provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In the accompanying drawings: FIG. 1 is a schematic diagram of an example of a walking beam type slab heating furnace; FIG. 2 is a schematic diagram illustrating the state in which a slab is supported by a skid in the slab heating furnace of FIG. 1; FIG. 3 is a diagram illustrating the correspondence between transportation of a slab in a walking beam type heating furnace on the one hand and an example of a profile of slab temperature in the method of the present disclosure (solid line) and a typical profile of a conventional slab temperature (dashed line) on the other; and FIG. 4 is a schematic diagram illustrating the arrangement of skids before and after a shift in a slab heating furnace. DETAILED DESCRIPTION

[0023] The method of hot rolling of the present disclosure will now be described in detail.

[0024] The present disclosure is a method of hot rolling including hot rolling a slab after heating the slab in a heating furnace. The heating furnace includes skids for supporting and transporting the slab, and the skids shift at least once within the heating furnace. Shifting of the skids may be accomplished by a shift skid mechanism. The number of shift skid mechanisms may be one or more.

[0025] The method of the present disclosure is useful when using a heating furnace having a skid arrangement in which an overhang length O 1 after a shift by the shift skid mechanism furthest on the slab extraction side of the heating furnace (hereinafter also referred to as the "final shift skid mechanism") is 110 % or more of an overhang length O 0 before the shift. It is particularly advantageous if, at both ends of the slab, the overhang length O 1 after the shift by the final shift skid mechanism is 110 % or more of the overhang length O 0 before the shift. The overhang length O 1 can be set to, for example, 130 % or less of the overhang length O 0 . "Before the shift" and "after the shift" refer to before initiating and after completing a change in the position of the skids supporting the slab.

[0026] If the furnace has only one shift skid mechanism, this shift skid mechanism corresponds to the final shift skid mechanism. If the heating furnace has a plurality of shift skid mechanisms, "before undergoing a shift by the final shift skid mechanism" means after undergoing a shift by the second-to-final shift skid mechanism and before undergoing a shift by the final shift skid mechanism.

[0027] In the method of the present disclosure, the maximum arrival temperature reached by the slab from when the slab is charged into the heating furnace until prior to undergoing a shift furthest on the slab extraction side of the heating furnace is designated Ta (units: °C; hereinafter omitted), the maximum arrival temperature reached by the slab after undergoing the shift furthest on the slab extraction side of the heating furnace and before being extracted from the heating furnace is designated Tb (units: °C; hereinafter omitted), and Ta and Tb satisfy Tb + 80 °C > Ta > Tb + 10 °C.

[0028] Tb is the highest temperature the slab reaches after being shifted by the final shift skid mechanism and before being extracted from the heating furnace.

[0029] As described above, Ta and Tb can be determined by separately passing a slab equipped with a thermocouple through a heating furnace and recording the temperature change of the slab at each position in the heating furnace in advance.

[0030] When there is only one shift skid mechanism, the slab reaches the maximum arrival temperature Ta before being shifted by the shift skid mechanism. When there is a plurality of shift skid mechanisms, the timing at which the slab reaches its maximum arrival temperature Ta is not particularly limited as long as it is between the time the slab is charged into the heating furnace and before it is shifted by the final shift skid mechanism. For example, this timing may be between the time when the slab is shifted by the second-to-final shift skid mechanism and before the slab is shifted by the final shift skid mechanism. In this case, the timing at which the slab reaches the maximum arrival temperature Ta may be immediately after the shift by the second-to-final shift skid mechanism, immediately before reaching the final shift skid mechanism, or any time therebetween. After the slab is charged into the heating furnace, the slab may reach the maximum arrival temperature Ta before reaching the second-to-final shift skid mechanism.

[0031] In the method of the present disclosure, the maximum arrival temperature Ta is also the maximum arrival temperature reached by the slab in the heating furnace and is a temperature of 1150 °C or higher. The maximum arrival temperature Ta can be set to 1300 °C or less. If Ta is within this range, the slab can be sufficiently homogenized.

[0032] In the method of the present disclosure, the slab is heated so as to satisfy Tb + 80 ° C > Ta > Tb + 10 ° C .

[0033] When a high-temperature slab is cooled, there is a possibility that elements that have been homogenized as a solute due to the high temperature may be reprecipitated. An excessively low temperature reduces the homogenization effect in the slab and can cause deterioration of characteristics. The temperature is therefore preferably not lowered by more than 80 °C relative to Ta, which is the maximum arrival temperature reached by the slab in the heating furnace. On the other hand, in order to obtain the effect of reducing the shape defects at the longitudinal end portions of the hot-rolled coil by suppressing creep deformation, the temperature is preferably lowered by 10 °C or more relative to Ta.

[0034] Here, a conventional method of controlling slab heating is as follows.

[0035] Generally, one of the purposes of slab heating in general steel is to lower the deformation resistance of the steel by raising the temperature of the slab, thereby enabling hot rolling under a large reduction to obtain hot-rolled coils. To fulfill this role, it is necessary to control the temperature of the slabs extracted from the heating furnace to a temperature suitable for hot rolling. The object of control is the temperature of the slab at the time of extraction from the heating furnace, and 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, it is common to gradually increase the temperature of the slab in the heating furnace and control the temperature to reach the maximum arrival temperature at the time of extraction.

[0036] Furthermore, in the production of grain-oriented electrical steel sheets, it is also necessary to dissolve trace amounts of impurity elements and precipitate-forming elements to homogenize the interior of the slab. Once an element is solute, it may reprecipitate if the steel temperature drops. For this reason as well, the slab temperature at the time of extraction is commonly controlled to be the maximum arrival temperature.

[0037] In addition, since heating furnaces increase fuel utilization efficiency during heating by gradually heating the slab to the desired maximum arrival temperature, operations that bring large-volume slabs to their maximum arrival temperature at an early stage in the heating furnace are not usually carried out.

[0038] In the method of the present disclosure, a slab having a chemical composition such that the y phase ratio in Ta is 25 mol% or less is used. Even if the temperature of the slab after the shift furthest on the extraction side of the heating furnace is lower than the maximum arrival temperature Ta reached by the slab prior to undergoing the shift, the maximum arrival temperature Tb reached by the slab after the shift is higher than (Ta - 80 °C). Therefore, elements that have once become solute and homogenized remain in a supersaturated state, making it easy to suppress the progression of precipitation. On the other hand, since the maximum arrival temperature Tb is lower than (Ta - 10 °C), creep deformation at the slab ends can be easily suppressed.

[0039] FIG. 3 is a diagram illustrating the correspondence between transportation of a slab in a walking beam type heating furnace on the one hand and an example of a profile of slab temperature in the method of the present disclosure (solid line) and a typical profile of a conventional slab temperature (dashed line) on the other.

[0040] In the method of the present disclosure, Tb + 50 ° C > Ta > Tb + 15 ° C is preferably satisfied.

[0041] In the method of the present disclosure, a slab having a chemical composition such that the γ phase ratio at Ta is 25 mol% or less is used. By using such a slab, it is possible to prevent excessive formation of a pearlite phase derived from the γ phase in the hot-rolled sheet microstructure. Since the pearlite phase has a higher strength than the ferrite phase, cold rolling may be difficult in some cases, but the method of the present disclosure can easily avoid such a situation. A slab having a chemical composition such that the γ phase ratio is 20 mol% or less is preferred. No lower limit is placed on the γ phase ratio, which may be 0 mol%.

[0042] The chemical composition of the slab preferably satisfies the following. Hereinafter, "%" indications regarding chemical compositions are "mass%" unless otherwise specified.C: 0.02 % or more and 0.08 % or less

[0043] C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, and therefore, from the perspective of improving the final magnetic properties, the C content is preferably 0.02 % or more. The C content is more preferably 0.03 % or more. On the other hand, if the C content exceeds 0.08 %, even if decarburization annealing is performed it becomes difficult to reduce the C content to 50 ppm or less, the content at which magnetic aging does not occur. From this perspective, the C content is preferably limited to 0.08 % or less. The C content is more preferably 0.06 % or less.Si: 2.0 % or more and 8.0 % or less

[0044] Si is an element useful in reducing iron loss by increasing electrical resistance. In order to obtain good magnetic properties, the Si content is preferably 2.0 % or more. The Si content is more preferably 2.8 % or more. On the other hand, Si is also an element that increases the brittleness of steel. From the perspectives of reducing the risk of fracture during sheet passing and suppressing deterioration of cold rolling manufacturability, the Si content is preferably limited to 8.0 % or less. The Si content is more preferably 4.5 % or less.Mn: 0.005 % or more and 3.0 % or less

[0045] Mn is a useful element from the perspectives of improving hot workability and controlling the formation of an oxide film during primary recrystallization. From this perspective, the Mn content is preferably 0.005 % or more. The Mn content is more preferably 0.010 % or more. On the other hand, from the perspective of avoiding deterioration of the magnetic properties due to deterioration of the primary recrystallized texture, the Mn content is preferably limited to 3.0 % or less. The Mn content is more preferably 0.5 % or less.Al: Less than 0.0100 %, N: 0.0060 % or less, O: 0.0060 % or less, S + 0.405 × Se: 0.0060 % or less

[0046] If Al is excessive, it may be difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition. The Al content is therefore preferably limited to less than 0.0100 %. The Al content is more preferably 0.0080 % or less.

[0047] To prevent the formation of silicon nitrides after purification annealing in the production of the steel sheet after final annealing, the N content is preferably limited to 0.0060 % or less. The N content is more preferably 0.0030 % or less.

[0048] O forms oxides and inhibits deterioration of the magnetic properties of the steel sheet after final annealing. The O content is therefore preferably limited to 0.0060 % or less. The O content is more preferably 0.0030 % or less.

[0049] To stably obtain a secondary recrystallized microstructure, the total amount of S and Se multiplied by 0.405 is preferably limited to 0.0060 % or less. This total amount is more preferably 0.0040 % or less.

[0050] Al, N, O, S and Se are inhibitor components.

[0051] The essential components and the inhibitor components have been described above, but one or more optional components selected from the elements described below may also be included as appropriate. Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, Ti: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, and Bi: 0.0005 % or more and 0.0200 % or less.

[0052] Ni is a useful element in terms of improving the microstructure of a hot-rolled coil for better magnetic properties thereof. In order to fully obtain this effect, when Ni is included, the Ni content is preferably 0.005 % or more. On the other hand, if Ni is excessive, secondary recrystallization becomes unstable and magnetic properties deteriorate. The Ni content is therefore preferably 1.50 % or less.

[0053] Sn, Sb, Cu, Mo, P, Cr, B, and Bi are grain boundary segregation elements that can improve various properties, but if they are in excess, the development of secondary recrystallized grains can be inhibited. From these perspectives, when these elements are contained, the amounts are set to the above ranges.

[0054] Nb, Ti and Te are precipitate-forming elements that may improve various properties, but excessive amounts may make secondary recrystallization unstable. From these perspectives, when these elements are contained, the amounts are set to the above ranges.

[0055] The slab used in the method of the present disclosure preferably has a chemical composition containing the aforementioned essential components and optional components, with the balance being Fe and inevitable impurities. In this chemical composition, the precipitate components are sufficiently reduced, and the elements that have become solute and homogenized can maintain a supersaturated state, with the progression of precipitation being suppressed, even if the maximum arrival temperature Tb reached by the slab after undergoing the shift by the final shift mechanism is lower than the maximum arrival temperature Ta reached by the slab before undergoing the shift by the final shift mechanism.

[0056] The slabs used in the method of the present disclosure can be produced by refining molten steel adjusted to a desired chemical composition by a known method using a converter, electric furnace, or the like, and then subjecting the steel to vacuum treatment, if necessary, followed by a conventional method of ingot casting or method of continuous casting. Alternatively, a thin slab or thinner cast steel having a thickness of 100 mm or less may be directly produced by direct casting and used as a slab.

[0057] The slab used in the method of the present disclosure has a chemical composition such that the γ phase ratio is 25 mol% or less at Ta, which is the maximum arrival temperature reached by the slab in the heating furnace. The γ phase ratio can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7) produced by Thermo-Calc Software AB.

[0058] After adjustment of the chemical composition of the slab, the temperature range in which the y phase fraction is 25 mol% or less can be calculated using the thermodynamic software Thermo-calc ver. 2019b (database TCFE7), and Ta, which is the maximum arrival temperature reached by the slab in the heating furnace, can be set based on the calculated temperature range.

[0059] Alternatively, the maximum arrival temperature Ta reached by the slab in the heating furnace may be set, and the chemical composition may then be adjusted so that the y phase ratio is 25 mol% or less at this value of Ta.

[0060] The slab heating in the method of the present disclosure can be performed using slab heating equipment provided with a walking beam type heating furnace including skids configured to support and transport a slab in a heating furnace and at least one shift skid mechanism configured to shift a position at which the skids support the slab.

[0061] In the heating furnace, on one of the two slab end sides of the final shift skid mechanism, the distance between the skid located furthest on the slab end side and the skid located one skid closer to the center of the slab after the shift by the final shift skid mechanism is L 1 , and the distance between the skid located furthest on the slab end side and the skid one skid closer to the center of the slab before the shift by the final shift skid mechanism is L 0 . In this case, if the distance L 1 is greater than the distance L 0 , the overhang length O 1 after the shift by the final shift skid mechanism easily becomes 110 % or more of the overhang length O 0 before the shift, which could result in poor shape. The benefits of the hot rolling method of the present disclosure can thus be fully enjoyed. In a case in which the furnace has a plurality of shift skid mechanisms, the position of the skid "before the shift by the final shift skid mechanism" corresponds to the position of the skid of the second-to-final shift skid mechanism. FIG. 4 is a schematic diagram. In FIG. 4, at the slab end where L 0 and L 1 are indicated, the skid located furthest on the slab end side is a fixed skid 2a, and the skid located one skid closer to the center of the slab than that skid is a movable skid 2b.

[0062] As has been revealed for the first time by the present disclosure, the deformation of the slab when the overhang length is increased is an operational problem limited to steel types with a low y phase ratio at the target temperature. Normally, the y phase develops in iron at high temperatures. In this case, if the steel contains a large amount of y phase suppressing elements such as Si and Al and a small amount of y phase forming elements such as C, Ni and Cr, a similar problem is likely to occur. Generally, electrical steel sheets contain 2.0 % or more Si to reduce iron loss. Furthermore, since impurities can affect performance degradation, the upper limits of many elements are strictly regulated, and these elements may therefore become components that can lead to the problem of slab deformation.

[0063] On the other hand, most slab heating furnaces currently in operation are designed and produced for general steel materials. The furnace is designed to gradually heat the slab from the entry side to the exit side to a high temperature, so that the target temperature (generally the maximum temperature) is reached at the exit side. Naturally, the higher the temperature, the greater the concern about slab deformation. Hence, with the idea of supporting the slab over a wider area, the skid on the outermost side of the slab is often shifted outward, as illustrated in FIG. 2. The second outermost skid may shift in the same direction or may shift inward for overall balance. When the outermost skid shifts outward and the second outermost skid shifts inward in this way, the overhang length increases. In cases in which the shift direction (inward or outward) causes an imbalance in the locations supporting the slab, a new skid may be installed when the shift occurs.

[0064] Furthermore, the length and width of the steel material required by manufacturers are not fixed. For example, even when using a heating furnace with a skid arrangement as illustrated in FIG. 2, if the required length of the product steel material is short, for example, the length of the slab to be charged will also be short, and the slab may be supported by the five skids on the left out of the six skids. Therefore, to accommodate various required product lengths and slab lengths, a plurality of heating furnaces and heating furnaces with various patterns of skid arrangements and shift mechanisms are used in parallel.

[0065] When a slab that satisfies the set of component conditions identified in the process of completing the present disclosure is used, and there is a mismatch between the slab length required for the product and the skid arrangement characteristics of the heating furnace, application of the present disclosure can yield particularly significant improvements.

[0066] The present disclosure also relates to slab heating equipment used in the method of the present disclosure. The slab heating equipment includes a heat treatment mechanism capable of independently controlling temperature on both sides of the shift skid mechanism furthest on the slab extraction side of the heating furnace (final shift skid mechanism), an atmosphere control mechanism configured to control convection of atmospheric gas in the heating furnace, and a furnace temperature control mechanism configured to control the heat treatment mechanism and the atmosphere control mechanism so that, with respect to the final shift skid mechanism, a furnace inner temperature on an extraction side of the heating furnace (also referred to as "after the final shift skid mechanism") is 50 °C or more lower than a furnace inner temperature on a charging side of the heating furnace (also referred to as "before the final shift skid mechanism"). On the other hand, to prevent the slab from becoming excessively cold, the furnace temperature after the final shift skid mechanism is preferably prevented from becoming 130 °C or more lower than the furnace temperature before the final shift skid mechanism.

[0067] The heat treatment mechanism can be a heating device, such as a burner. For example, the heating furnace may be divided into a plurality of heating sections (such as the sections between shift skid mechanisms), and a different system of burners may be provided for each heating section.

[0068] The atmosphere control mechanism is a mechanism for controlling the convection of the atmospheric gas within the heating furnace and can be, for example, a mechanism with separate intake and exhaust systems for each location in the heating furnace (for example, for each of the aforementioned heating sections). The atmosphere control mechanism can also supply inert gas to a specific location in the heating furnace to keep the temperature in that location and its surroundings low. The atmosphere control mechanism also contributes to controlling the furnace temperature. The heating and cooling by controlling the convection of the atmospheric gas is performed by the atmosphere control mechanism.

[0069] 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 50 °C or more lower than the furnace temperature before the final shift skid mechanism. To lower the slab temperature of an already heated slab by 10 °C or more, a temperature difference of 50 °C or more is required in the furnace. Here, the temperature inside the furnace can be measured by installing a sensor such as a thermocouple in the heating furnace. For example, if the heating furnace is divided into a plurality of heating sections, a sensor can be installed in each heating section. The furnace temperatures before and after the final shift skid mechanism is measured by the sensors that are installed before and after the final shift skid mechanism and are closest to the final shift skid mechanism. In a case in which the heating furnace is divided into a plurality of heating sections, the furnace temperatures before and after the final shift skid mechanism are measured by sensors installed in the heating sections adjacent to the final shift skid mechanism. Instead of measuring with a sensor, control may be performed based on a calculated value of the slab temperature.

[0070] Modifying the skid itself is equivalent to installing a new furnace, and there are significant constraints on its realization. Installation of a furnace temperature control mechanism, a heat treatment mechanism, and an atmosphere control mechanism, however, can be achieved more easily.

[0071] The heating furnace may be provided with furnace walls between adjacent heating sections to reduce the effect of radiant heat from the adjacent heating sections. The heating furnace may include an auxiliary skid for supporting the slab.

[0072] Since the slab itself carries heat, it is necessary to control the heat treatment mechanism and atmosphere control mechanism in order 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 a desired temperature difference in the furnace is achieved across the final shift skid mechanism. This allows the maximum arrival temperature Ta (°C) of the slab before reaching the final shift skid mechanism and the maximum arrival temperature Tb (°C) of the slab after the shift to satisfy the following relationship. Tb + 80 ° C > Ta > Tb + 10 ° C

[0073] Depending on the skid arrangement, the overhang length of the slab end after undergoing the shift by the final shift skid mechanism may be 110 % or more at one slab end and less than 110 % at the other slab end, as compared to the overhang length of the slab end before the shift. In such a case, the temperature may be lowered to Tb only at one end where the overhang amount is 110 % or more, which allows for more efficient operation.

[0074] The slab heating equipment of the present disclosure is not limited to the production of grain-oriented electrical steel sheets but can also be used for slab heating of steel with a low γ-phase ratio.

[0075] The slab extracted after the above-described slab heating is homogenized steel and also achieves little sinking at the overhang positions at the ends of the slab.

[0076] Next, the slab is subjected to hot rolling, which is relatively easy because sinking and defectiveness of shape at the slab ends have been controlled.

[0077] During the hot rolling process in the method of the present disclosure, at least two consecutive passes of rolling from the slab stage to the sheet bar stage can be carried out in a temperature range of 1030 °C or higher to 1150 °C or lower.

[0078] To improve the shape of the longitudinal end portions of the hot-rolled coil, the time between two consecutive passes is preferably 15 s or more, the rolling reduction of each pass is preferably 50 % or less, and the strain rate is preferably 15 s -1< or more.

[0079] In particular, even if the inhibitor components S and Se are present in the slab and form sulfides or selenides in the central layer of the slab, two-pass rolling under an appropriate set of conditions in a specific temperature range (1030 °C or higher and 1150 °C or lower) is advantageous in that a smaller size than the size that would cause problems during cold rolling can be achieved. 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 fragmentation and destruction due to sulfides and selenides may occur. Hence, the aforementioned hot rolling process is preferable.

[0080] The method of the present disclosure uses a slab in which the y phase ratio is 25 mol% or less at Ta, which is the maximum arrival temperature in the heating furnace, and the y phase ratio reaches its maximum near the temperature range of 1030 °C or higher to 1150 °C or lower. Generally, austenite has a higher deformation resistance than ferrite and is less likely to deform even when pressed. Therefore, the rolling reduction for each pass is limited to 50 % or less. From the perspective of homogenizing the structure of the hot-rolled coil, the rolling reduction is preferably 15 % or more. The rolling reduction is more preferably 20 % or more.

[0081] Furthermore, by setting the time between passes to 15 s or longer, dislocations formed once by deformation are recovered or disappear by recrystallization. Rolling can therefore be performed without excessively increasing deformation resistance. The time between passes is preferably 120 s or shorter in order to suppress the formation of precipitates nucleated by dislocations generated during deformation.

[0082] Setting the strain rate to 15 s -1< or more facilitates improvement of the shape of the longitudinal ends of the hot-rolled coil. The strain rate may be 50 s -1< or less.

[0083] Here, the strain rate ε can be calculated using the following Ekelund expression. ε ≒ υ R R ′ h 1 2 2 − r ⋅ r

[0084] In the expression, v R is the roller peripheral speed (mm / s), R' is the roll radius (mm), h 1 is the roll entry side thickness (mm), and r is the rolling reduction (%).

[0085] Typically, the longitudinal ends of a hot-rolled coil have larger thickness variations (the maximum thickness minus the minimum thickness) than the steady-state portion including the central portion. However, according to the method of the present disclosure, thickness variations at the longitudinal ends of the hot-rolled coil can be suppressed. For example, the widthwise thickness variation width at the longitudinal ends of the hot-rolled coil can be controlled to 1.5 times or less the widthwise thickness variation width 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 edge (i.e., positions between 0 % and 3 % and between 97 % and 100 %), where the entire longitudinal length of the hot-rolled coil is 100 %. The evaluation of the steady-state portion is preferably carried out at a position 10 % to 90 % from one end in the longitudinal direction.

[0086] The present disclosure also relates to a method of producing a grain-oriented electrical steel sheet, the method including hot rolling a slab by the method of hot rolling according to the present disclosure, performing hot-rolled sheet annealing on the resulting hot-rolled sheet, subsequently performing cold rolling once or two or more times with intermediate annealing in between, next optionally performing decarburization annealing, and then performing primary recrystallization annealing and final annealing. The hot-rolled sheet has an improved shape at the longitudinal end, which can suppress meandering, fracture during the cold rolling process, and the like.

[0087] It is important for the hot-rolled sheet annealing to be performed at 1150 °C or lower. If the hot-rolled sheet annealing temperature exceeds 1150 °C, the inhibitor-forming elements that are inevitably mixed in will become solute and will reprecipitate unevenly during cooling, making it difficult to achieve a homogenous primary recrystallization microstructure and inhibiting the development of secondary recrystallization. Furthermore, if the hot-rolled sheet annealing temperature exceeds 1150 °C, the grain size after hot-rolled sheet annealing becomes too coarse, which is also disadvantageous in realizing an appropriate primary recrystallization microstructure. The hot-rolled sheet annealing is preferably performed at 900 °C or higher to promote recrystallization.

[0088] After the hot-rolled sheet annealing, cold rolling is performed once, or twice or more with intermediate annealing performed therebetween. In cold rolling, it is effective to perform aging treatment once, or twice or more, at a rolling temperature of 80 °C or higher and 150 °C or lower, with the temperature between rolling passes being increased to 100 °C or higher and 300 °C or lower, in order to develop a Goss texture.

[0089] Next, primary recrystallization annealing is performed. The purpose of this primary recrystallization annealing is to subject the cold rolled sheet with a rolled microstructure to primary recrystallization and to adjust to the optimum primary recrystallized grain size for secondary recrystallization, and also to decarburize the carbon contained in the steel by setting the annealing atmosphere to a wet hydrogen-nitrogen or wet hydrogen-argon atmosphere and simultaneously form an oxide film on the surface by the aforementioned oxidizing atmosphere. Therefore, the primary recrystallization annealing is preferably performed at 750 °C or higher with the dew point induced in an H 2 mixed atmosphere. The primary recrystallization annealing is preferably performed at 900 °C or lower with the dew point induced in an H 2 mixed atmosphere. During the temperature increase in the primary recrystallization annealing, the heating rate is preferably set to 200 °C / s or higher between 550 °C and 680 °C, as doing so can further enhance the effect of texture improvement. At the same time, decarburization annealing is carried out to reduce the C content to 50 mass ppm or less, at which no magnetic aging occurs. The C content is preferably reduced to 30 mass ppm or less.

[0090] A technique of increasing the Si content by siliconizing after primary recrystallization annealing may also be used in combination.

[0091] Thereafter, a final annealing is performed to develop a secondary recrystallized microstructure. In this case, a forsterite film may be formed using an annealing separator containing MgO as the main component. The forsterite film can be better formed by the addition of appropriate amounts of Ti oxides, Sr compounds, and the like to the separating agent. In particular, the addition of additives that promote uniform forsterite film formation is also advantageous for improving separation properties. Any annealing separator, such as Al 2 O 3 , may also be used to suppress film formation.

[0092] The final annealing needs to be performed at 800 °C or higher to induce secondary recrystallization, but the heating rate up to 800 °C does not have a significant effect on the magnetic properties. Hence, any condition may be used. The annealing atmosphere may be any of N 2 , Ar, or H 2 , or a mixed gas containing two or more of these. To promote secondary recrystallization more effectively, isothermal holding near the secondary recrystallization temperature may be employed. However, since the same effect can be obtained by, for example, slowing down the heating rate, isothermal holding is not essential. Precipitation of trace elements in the final product leads to a deterioration in magnetic properties. The maximum annealing temperature is therefore set to 1100 °C or higher for element purification.

[0093] After the final annealing, an insulating coating may 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, preferred methods are described in JP S50-79442 A and JP S48-39338 A, where a coating liquid containing phosphate-chromate-colloidal silica is applied on a steel sheet and then baked at a temperature of around 800 °C.

[0094] Furthermore, the shape of the steel sheet can be adjusted by performing flattening annealing. The flattening annealing may also be combined with baking of the insulating coating.EXAMPLES

[0095] The present disclosure will be described in detail with reference to examples, but the present disclosure is not limited to these examples.[Example 1]

[0096] A steel slab (total length 12.2 m, width 1 m, thickness 180 mm) containing Si: 3.2 % to 3.4 %, C: 0.035 % to 0.070 %, Mn: 0.07 %, Al: 0.0050 % to 0.0090 %, and less than 0.0060 % each of N, O, and S + 0.405 × Se, with the balance being Fe and inevitable impurities, and containing no inhibitor components, was heated using a walking beam type heating furnace according to the heating pattern illustrated in Table 1, and the third and fourth passes of four-pass rough rolling were carried out under the set of conditions illustrated in Table 1. Subsequently, finish hot rolling with multiple passes in the temperature range of 850 °C to 950 °C was performed to finish to a thickness of 2.2 mm. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0097] The walking beam type heating furnace is provided with one shift skid mechanism, which corresponds to the shift skid mechanism furthest on the slab extraction side of the heating furnace (final shift skid mechanism). This shift skid mechanism consists of movable skids and fixed skids arranged alternately, and when the aforementioned steel slab is disposed, the skid arrangement is such that at one end, the overhang length of the slab is 112 % from before to after the shift.

[0098] The heating furnace has separate heating sections before and after the final shift skid mechanism, and each section is equipped with a different system of burners. An inert gas supply system is connected to the heating section after the final shift skid mechanism, and the burner and the inert gas supply system are connected to a control system. Each heating section is provided with a sensor for measuring the furnace inner temperature.

[0099] To evaluate the shape of the longitudinal end of the obtained hot-rolled coil (total length 1000 m, width 1 m, thickness 2.2 mm), samples were cut out from positions 10 m, 12 m, 14 m, 16 m, 18 m, and 20 m from the longitudinal end of the coil. To evaluate the shape of the steady-state portion, six samples were similarly cut out at 2 m intervals from a position 200 m or more away from the end. The thickness profile in the width direction was measured, and the difference between the maximum and minimum values was calculated. Evaluation was made as the ratio of the thickness difference between the longitudinal ends and the steady-state portion. As is clear from the table, the shape is improved under the conditions of the disclosure. Table 1No.Chemical composition (mass%)*Before shift skidAfter shift skidInside the heating furnaceRough rolling third passRough rolling fourth passTime between third and fourth passesDifference in thickness at longitudinal end / difference in thickness at steady state portionNotesSiCMnAlNS + 0.405 SeMaximum arrival temperature TaMaximum arrival temperature TbMaximum arrival temperaturey phase ratioTemperatureEach rolling reductionStrain rateTemperatureEach rolling reductionStrain rate1-13.20.0350.070.00600.00400.00121150 °C1200 °C1200 °C4 mol%1110 °C30 %25 s -1< 1080 °C30 %25 s -1< 45 s1.6Comp. Example1-21200 °C1190 °C1200 °C4 mol%1080 °C30 %25 s -1< 1050 °C30 %25 s -1< 43 s1.1Example1-31240 °C1235 °C1240 °C0 mol%1160 °C31 %25 s -1< 1120 °C42 %40 s -1< 38 s1.8Comp. Example1-41240 °C1190 °C1240 °C0 mol%1080 °C55 %45 s -1< 1050 °C30 %28 s -1< 50 s1.3Example2-13.20.0550.070.00800.00500.00381200 °C1170 °C1200 °C23 mol%1090 °C28 %25 s -1< 1070 °C18 %13 s -1< 90 s1.3Example2-21200 °C1170 °C1200 °C23 mol%1110 °C32 %30 s -1< 1090 °C44 %42 s -1< 14 s1.3Example2-31250 °C1190 °C1250 °C15 mol%1080 °C30 %25 s -1< 1050 °C30 %25 s -1< 38 s1.1Example2-41250 °C1190 °C1250 °C15 mol%1100 °C35 %30 s -1< 1060 °C35 %30 s -1< 20 s1.1Example3-13.40.0550.070.00500.00300.00241060 °C1100 °C1100 °C18 mol%1000 °C25 %25 s -1< 970 °C38 %40 s -1< 28 s1.5Comp. Example3-21200 °C1170 °C1200 °C15 mol%1100 °C18 %22 s -1< 1070 °C55 %46 s -1< 55 s1.3Example3-31200 °C1170 °C1200 °C15 mol%1100 °C25 %25 s -1< 1070 °C26 %21 s -1< 50 s1.1Example3-41200 °C1110 °C1200 °C15 mol%1000 °C25 %25 s -1< 970 °C38 %36 s -1< 28 s1.6Comp. Example3-51140 °C1200 °C1200 °C15 mol%1100 °C55 %45 s -1< 1080 °C18%13 s -1< 90 s1.8Comp. Example43.40.0700.070.00900.00400.00301180 °C1200 °C1200 °C26 mol%1080 °C35 %25 s -1< 1040 °C45 %40 s -1< 14 s1.4Comp. Example* The balance is Fe and incidental impurities. [Example 2]

[0100] A steel slab (total length 8.5 m, width 1 m, thickness 170 mm) containing the components illustrated in Table 2, with the balance being Fe and inevitable impurities, and having a calculated y phase ratio of 25 mol% or less at the maximum arrival temperature during slab heating, was hot rolled using a walking beam type heating furnace under the conditions similarly illustrated in Table 2. The resulting hot-rolled coil was 1 m wide and 1.6 mm thick. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0101] The walking beam type heating furnace used was the same as in Example 1.

[0102] For some hot-rolled coils, two coils were produced under the same set of conditions, and one of the coils was used to evaluate the shape of the longitudinal end portion in the same manner as in Example 1.

[0103] The hot-rolled coil from which no samples were taken was subjected to hot-rolled sheet annealing at an end-point temperature of 1020 °C, and it was confirmed whether the sheet moved transversely and meandered for 20 mm or more during sheet passing. Thereafter, the sheet was subjected to primary cold rolling at 100 °C in a reverse mill to a thickness of 1.7 mm, followed by intermediate annealing at 900 °C for 1 minute, and then to reverse secondary cold rolling again, with coiling aging treatment at 200 °C during the process, to a sheet thickness of 0.22 mm. Subsequently, primary recrystallization annealing was performed with a heating rate of 300 °C / s between 550 °C and 680 °C, a soaking temperature of 840 °C, and a soaking time of 60 s. An annealing separator composed of MgO: 95 % and TiO 2 : 5 % was then applied to the steel sheet surface as an aqueous slurry, and the steel sheet was subjected to secondary recrystallization annealing. A coating liquid containing phosphate-chromate-colloidal silica at a 3:1:3 weight ratio was applied to the surface of the final annealed sheet obtained in this way and was baked thereon at 800 °C. The magnetic properties of the widthwise center of the product sheet coil of the obtained grain-oriented electrical steel sheet were also confirmed. As the magnetic properties, the magnetic flux density (B 8 ) was measured in accordance with JIS C2550-1:2011 at an excitation magnetic field of 800 A / m and an excitation frequency of 50 Hz.

[0104] As is clear from Table 2, under the conditions of the present disclosure, the production stability is improved and good magnetic properties are obtained. Furthermore, although a reverse mill is used in these Examples, meandering within the line in a continuous line such as a tandem mill also affects fracture during cold rolling. INDUSTRIAL APPLICABILITY

[0105] According to the method of hot rolling of the present disclosure, the shape of the slab ends before hot rolling is improved, thereby obtaining a good shape for the longitudinal ends of the obtained hot-rolled coil, and ultimately making it possible to pass the grain-oriented electrical steel sheet more stably in the production processes after the hot rolling process. A grain-oriented electrical steel sheet can therefore be produced much easier than before. According to the present disclosure, a method of producing a hot-rolled coil and a grain-oriented electrical steel sheet can be provided, where the method includes a process of hot rolling a slab by the above method of hot rolling. Slab heating equipment that can be used in the method of hot rolling can also be provided.REFERENCE SIGNS LIST

[0106] 1 Slab heating furnace 2 Skid 2a Fixed skid 2b Movable skid 3 Shift skid S Slab O 0 Overhang length before undergoing shift furthest on the slab extraction side of the heating furnace O 1 Overhang length after undergoing shift furthest on the slab extraction side of the heating furnace L 0 Distance between the skid located furthest on the slab end side and the skid located one skid closer to the center of the slab, before undergoing the shift by the final shift skid mechanism L 1 Distance between the skid located furthest on the slab end side and the skid located one skid closer to the center of the slab, after undergoing the shift by the final shift skid mechanism

Examples

example 1

[Example 1]

[0096]A steel slab (total length 12.2 m, width 1 m, thickness 180 mm) containing Si: 3.2 % to 3.4 %, C: 0.035 % to 0.070 %, Mn: 0.07 %, Al: 0.0050 % to 0.0090 %, and less than 0.0060 % each of N, O, and S + 0.405 × Se, with the balance being Fe and inevitable impurities, and containing no inhibitor components, was heated using a walking beam type heating furnace according to the heating pattern illustrated in Table 1, and the third and fourth passes of four-pass rough rolling were carried out under the set of conditions illustrated in Table 1. Subsequently, finish hot rolling with multiple passes in the temperature range of 850 °C to 950 °C was performed to finish to a thickness of 2.2 mm. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0097]The walking beam type heating furnace is provided with one shift skid mechanism, which corresponds to the shift skid mechanism furthest on the slab extraction side of the he...

example 2

[Example 2]

[0100]A steel slab (total length 8.5 m, width 1 m, thickness 170 mm) containing the components illustrated in Table 2, with the balance being Fe and inevitable impurities, and having a calculated y phase ratio of 25 mol% or less at the maximum arrival temperature during slab heating, was hot rolled using a walking beam type heating furnace under the conditions similarly illustrated in Table 2. The resulting hot-rolled coil was 1 m wide and 1.6 mm thick. In the slabs of the Examples before hot rolling, sinking of the ends was suppressed, and the shape was good.

[0101]The walking beam type heating furnace used was the same as in Example 1.

[0102]For some hot-rolled coils, two coils were produced under the same set of conditions, and one of the coils was used to evaluate the shape of the longitudinal end portion in the same manner as in Example 1.

[0103]The hot-rolled coil from which no samples were taken was subjected to hot-rolled sheet annealing at an end-point temperature o...

Claims

1. A method of hot rolling, comprising: hot rolling a slab after heating the slab in a heating furnace, wherein skids that support and transport the slab shift at least once in the heating furnace, and a maximum arrival temperature Ta in °C reached by the slab from when the slab is charged into the heating furnace until prior to undergoing a shift furthest on a slab extraction side of the heating furnace and a maximum arrival temperature Tb in °C reached by the slab after undergoing the shift furthest on the slab extraction side of the heating furnace and before being extracted from the heating furnace satisfy Tb + 80 ° C > Ta > Tb + 10 ° C , Ta is a maximum arrival temperature reached by the slab in the heating furnace and is 1150 °C or higher, and the slab has a chemical composition such that a γ phase ratio at Ta is 25 mol% or less.

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

3. The method of hot rolling according to claim 1 or 2, wherein the slab is a steel slab comprising a chemical composition containing, in mass%, C: 0.02 % 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 inevitable impurities.

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

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

6. The method of producing a hot-rolled coil according to claim 5, wherein a widthwise thickness variation width at a position 10 m to 20 m from a longitudinal end of the hot-rolled coil is 1.5 times or less a widthwise thickness variation width at a longitudinal center.

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

8. Slab heating equipment for performing slab heating before hot rolling, the slab heating equipment comprising: a walking beam type heating furnace comprising at least one each of a skid configured to support and transport a slab in a heating furnace and a shift skid mechanism configured to shift a position at which the skid supports the slab; a heat treatment mechanism capable of independently controlling temperature on both sides of a shift skid mechanism furthest on a slab extraction side of the heating furnace; an atmosphere control mechanism configured to control convection of atmospheric gas in the heating furnace; and a furnace temperature control mechanism configured to control the heat treatment mechanism and the atmosphere control mechanism so that, with respect to the shift skid mechanism furthest on the slab extraction side of the heating furnace, a furnace inner temperature on an extraction side of the heating furnace is 50 °C or more lower than a furnace inner temperature on a charging side of the heating furnace.

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