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

Abstract

Provided is a hot rolling method by which a hot-rolled coil having a small number of defects can be obtained. In the hot rolling method, a steel slab 1 is heated in a heating furnace and then hot-rolled. In a temperature range T in which the temperature of the steel slab 1 in the heating furnace is between 950 °C and 1,150 °C (both inclusive), when heating the steel slab 1 having a component composition in which the γ phase rate at 1,050 °C, which is the median value of the temperature range T, is 20 mol% or less, when using a heating furnace in which the distance between the plurality of skids 2 supporting the steel slab 1 exceeds 1.1 m, the average oxygen concentration in the heating furnace in the temperature range T is 5.0 vol% or less.

Description

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

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

[0002] Grain-oriented electrical steel sheets are typically produced by using a precipitate called an inhibitor to induce secondary recrystallization of Goss-oriented ({110}<001>) grains during final annealing. For example, Patent Document 1 discloses a method using AlN as an inhibitor, and Patent Document 2 discloses a method using MnS or MnSe as an inhibitor, both of which have been put into industrial use.

[0003] The method using the inhibitor is useful for stably developing secondary recrystallized grains, but the precipitates must be finely dispersed, which requires that the steel slab for grain-oriented electrical steel sheet (hereinafter simply referred to as "steel slab") be heated to a high temperature of 1300°C or higher before hot rolling.

[0004] However, high temperature heating of steel slabs not only increases the equipment costs, but also increases the amount of scale generated during hot rolling, thereby reducing yield and making equipment maintenance more complicated.

[0005] On the other hand, a manufacturing technique (inhibitor-less method) that does not use the inhibitors described above has also been proposed. Patent Document 3 discloses a technique in which a steel slab that is made higher in purity without containing inhibitor components is used as the steel slab, and secondary recrystallization is caused by texture (control of texture).

[0006] Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent Laid-Open No. 2000-129356

[0007] Steel slabs containing almost no inhibitor-forming elements do not need to be heated at temperatures exceeding 1300° C., so there is no need to use a special furnace when heating the steel slab. Therefore, hot rolling can be carried out using slab heating equipment such as a gas furnace that is used in general steel production, making it possible to produce grain-oriented electrical steel sheets at low cost.

[0008] However, in some products, surface defects occur at specific positions on the hot-rolled coil (hereinafter also referred to as "hot-rolled coil") obtained after hot rolling. As a result, these defects affect meandering of the hot-rolled coil in the subsequent annealing process of the hot-rolled sheet (hereinafter also referred to as "hot-rolled sheet") and breakage of the sheet in the cold-rolling process, which has been one of the factors hindering industrial-scale production.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to propose a hot rolling method that can obtain hot rolled coils with fewer surface defects.

[0010] The inventors conducted a detailed investigation into the characteristics of defect occurrence in hot-rolled coils in which surface defects actually occurred, and have come to the following findings: 1) The location where the surface defects occur generally corresponds to the position corresponding to the position of the skid that supported the steel slab when the material temperature was thought to be around 1050°C during heating of the steel slab (i.e., the position on the steel slab that the skid was in contact with). 2) The gamma phase fraction of the steel slab at around 1050°C is approximately 20 mol% or less. 3) Regarding the air-fuel ratio (ratio of the mass of air to the mass of fuel gas) during heating, the defect occurrence rate is high when the mass ratio of air is high. Note that the gamma phase fraction in 2) above was calculated using thermodynamic software Thermo-calc ver. 2019b (database TCFE7) manufactured by Thermo-Calc Software AB.

[0011] Figure 1 shows a schematic diagram of an example of a walking beam type slab heating furnace (hereinafter simply referred to as a "heating furnace"). In a walking beam type heating furnace, a steel slab 1 is generally supported and transported by a plurality of skids 2 extending approximately parallel to one another. Typically, the skids 2 are arranged alternately with fixed skids and movable skids, and the movable skids move up and down to lift the steel slab 1 and transport it little by little from the furnace charging side to the furnace unloading side.

[0012] In the above structure, if the position of the skid 2 used to support and transport the steel slab 1 is always the same relative to the steel slab 1 (i.e., if the same skid 2 continues to support the same position on the underside of the steel slab 1), it is difficult to heat the underside of the steel slab 1 directly above the skid 2. For this reason, one or more mechanisms called shift skids 3 are often provided inside the heating furnace, and the position of the skid 2 supporting the steel slab 1 is changed before and after the shift skid 3.

[0013] The inventors confirmed the corresponding position on the steel slab 1 with respect to the position where the surface defect occurred on the hot-rolled coil, taking into consideration the length of the steel slab 1 before rolling. As a result, it was found that the position almost coincided with the position of the skid 2 that supported the steel slab 1 when the temperature of the steel slab 1 was around 1050°C during its stay in the heating furnace (i.e., the position where the bottom surface of the steel slab 1 was in contact with the skid 2). However, in reality, the steel slab 1 meandered slightly during transport within the heating furnace, resulting in an error (deviation) of 0.15 m from the corresponding position on the steel slab 1.

[0014] Creep deformation occurring at high temperatures is characterized by being more likely to occur in the α (ferrite) phase and slower in the γ (austenite) phase, but the frequency of occurrence differs depending on the γ phase ratio of the steel slab 1, so creep behavior may be one of the factors. Furthermore, the occurrence rate differs depending on the air-fuel ratio during heating, so there is a possibility that the atmosphere, particularly the oxygen concentration, may have an effect.

[0015] Therefore, the inventors conducted the following laboratory experiment. First, a steel slab 1 was cast, containing, by mass%, C: 0.04%, Si: 3.0%, Mn: 0.10%, Al: 0.007%, N, O, and S + 0.405 × Se each suppressed to less than 0.0060%, with the balance being Fe and unavoidable impurities. Next, test pieces measuring 4 mm square and 40 mm long were taken from the surface of the cast steel slab 1, and a creep test was performed by applying stress to the test pieces so as to bend them three-point while soaking them in a heating furnace at a temperature of 900°C to 1200°C. At this time, the oxygen concentration in the heating furnace was 0% by volume (100% by volume N ２ ) to 20% by volume (80% by volume N ２As a result, it became clear that under conditions of high oxygen concentration and in a specific temperature range, the deformation of the test specimen did not stop there, but progressed to cracking.

[0016] Based on the findings obtained as described above, the inventors have deduced the mechanism by which surface defects are formed as follows. That is, when the oxygen concentration is high on the surface of the steel slab 1 where tensile stress occurs directly above the skid 2, grain boundary embrittlement occurs due to oxidation, which develops into cracks during hot working. However, in skids 2 where the distance between two adjacent skids 2 is small, the stress exerted by each skid 2 on the steel slab 1 is small and deformation is suppressed, so the defect does not develop into a crack.

[0017] Furthermore, because grain boundary embrittlement is promoted by grain boundary creep, it is more likely to occur in α (ferrite) single-phase steel, which is prone to creep as the steel slab 1, and the higher the γ (austenite) phase ratio, the less likely it is to develop into cracks. Normally, when considering creep deformation alone, the higher the temperature, the greater the deformation amount and deformation rate. However, one of the factors behind this phenomenon is the grain boundary segregation of elements contained as impurities. At high temperatures, these impurity elements are homogenized and no longer segregate, so cracks develop only within a specific temperature range.

[0018] Based on the above-mentioned assumed mechanism, the inventors investigated ways to reduce surface defects in hot-rolled coils. Electrical steel sheets 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 during high-temperature heating. Another element that significantly affects the γ phase ratio is C. Since C has the effect of improving the hot-rolled structure and the texture during primary recrystallization, there is an optimum amount of C from the perspective of improving the final magnetic properties. Therefore, it is difficult to adopt a method of increasing the γ phase ratio at a given temperature by significantly changing the composition of electrical steel sheets that have already been manufactured in a process.

[0019] Furthermore, since the temperature of the steel slab 1 gradually increases in the heating furnace, it is difficult to completely avoid the temperature range, even if it is possible to shorten the residence time in a specific temperature range. Furthermore, since the steel slab 1 is supported by the skid 2, it is difficult to prevent stress from being applied to the steel slab 1. Therefore, the inventors came up with a method for appropriately controlling the oxygen concentration in the heating furnace in a specific temperature range, and completed the present invention.

[0020] That is, the present invention that solves the above-mentioned problems is as follows: [1] A hot rolling method for heating a steel slab in a heating furnace and then hot rolling the same, wherein, in a temperature range T in which the temperature of the steel slab in the heating furnace is 950°C or higher and 1150°C or lower, when a heating furnace in which a distance between multiple skids supporting the steel slab exceeds 1.1 m is used to heat a steel slab having a chemical composition such that the γ-phase ratio is 20 mol% or lower at 1050°C, which is the median value of the temperature range T, the hot rolling method is characterized in that the average oxygen concentration in the heating furnace in the temperature range T is 5.0 vol% or lower.

[0021] [2] The hot rolling method according to [1], wherein the average oxygen concentration in the heating furnace in the temperature range T is 3.0% by volume or less.

[0022] [3] The hot rolling method according to [1] or [2], wherein the hot rolling includes two consecutive passes of hot rolling, each pass being carried out in a temperature range of 1030°C or higher and 1150°C or lower, under conditions of a reduction rate of 50% or lower and a strain rate of 15 / sec or higher, and the time between the two passes being 15 seconds or longer.

[0023] [4] A method for producing a grain-oriented electrical steel sheet, comprising hot-rolling a steel slab by the hot rolling method according to any one of [1] to [3] above, annealing the obtained hot-rolled coil, and then cold-rolling the resulting hot-rolled coil once or at least twice with intermediate annealing therebetween, and then optionally performing decarburization annealing, followed by final annealing to obtain a grain-oriented electrical steel sheet.

[0024] [5] A hot-rolled coil for a grain-oriented electrical steel sheet obtained by hot-rolling a steel slab by the hot rolling method according to any one of [1] to [3], wherein the steel slab has a position on the skid when heated, and a front-rear L in the rolling direction of the hot rolling. ０ A hot rolled coil for grain-oriented electrical steel sheet, characterized in that the number of surface defects in the range of L (m) is 0.3 or less on average. ０ (m) is the width of the skid X and the thickness of the steel slab Y １ (m), the thickness of the hot rolled coil for grain-oriented electrical steel sheet is Y ２ (m) is given by the following formula (1): ０ (m)={0.15(m)+X(m)}×Y １ (mm) / Y ２ (mm) (1)

[0025] According to the present invention, a hot rolled coil with few surface defects can be obtained.

[0026] 1 is a schematic diagram of an example of a walking beam type slab heating furnace, and FIG. 2 is a diagram showing details of a skid in a heating furnace used in Examples.

[0027] (Hot Rolling Method) Hereinafter, an embodiment of the present invention will be described. The following description exemplifies an embodiment of the present invention, and the present invention is not limited to the following embodiment. A hot rolling method according to the present invention is a hot rolling method in which a steel slab is heated in a heating furnace and then hot-rolled. Here, in a temperature range T in which the temperature of the steel slab in the heating furnace is 950°C or higher and 1150°C or lower, when a steel slab having a chemical composition in which the γ-phase fraction is 20 mol% or less at 1050°C, which is the median value of the temperature range T, is heated, the method is characterized in that, when a heating furnace in which the distance between multiple skids supporting the steel slab exceeds 1.1 m is used, the average oxygen concentration in the heating furnace in the temperature range T is set to 5.0 volume% or less.

[0028] [Steel Slab] In the present invention, a steel slab 1 for grain-oriented electrical steel sheet is used as a starting material. First, a suitable chemical composition of the steel slab 1 will be described. In the following description of the chemical composition, "%" represents "mass %" and "ppm" represents "mass ppm" unless otherwise specified.

[0029] The steel slab 1 preferably has a chemical composition containing C, Si and Mn in the ranges described below, with the balance being Fe and unavoidable impurities.

[0030] C: 0.03% or more and 0.08% or less. If the C content exceeds 0.08%, even after decarburization annealing, it becomes difficult to reduce the C content in the steel to 50 ppm or less, at which magnetic aging does not occur. Therefore, the C content is preferably 0.08% or less. Furthermore, in the present invention, the sulfides and selenides present in the central layer of the steel slab 1 can be reduced to a size that would be problematic during cold rolling by performing two hot rolling steps under appropriate conditions in a specific temperature range (1030°C or more and 1150°C or less). While the hot rolling temperature range typically results in the formation of a single ferrite phase in Si steel, the target temperature coincides with the temperature range in which austenite phase is formed, albeit to a small volume fraction, suggesting that this contributes to the fragmentation and destruction of sulfides and selenides. In fact, steel with a C content of 0.02% does not achieve the effect of fragmenting and destroying sulfides and selenides. Therefore, the C content is preferably 0.03% or more.

[0031] Si: 2.0% or more and 8.0% or less Si is a useful element that improves iron loss by increasing electrical resistance. In order to obtain good magnetic properties, the Si content is preferably 2.0% or more. On the other hand, Si is also an element that increases the brittleness of steel. If the Si content exceeds 8.0%, the risk of fracture during sheet threading through equipment increases and cold rolling properties also deteriorate significantly. Therefore, the Si content is preferably 8.0% or less. Since this can further reduce the risk during sheet threading through equipment, the Si content is more preferably 2.8% or more and 4.5% or less.

[0032] Mn: 0.005% or more and 3.0% or less Mn is an element that has the effect of improving hot workability during manufacturing. If the Mn content is less than 0.005%, the effect is poor in terms of both improving hot workability and controlling oxide film formation. Therefore, the Mn content is preferably 0.005% or more. On the other hand, if the Mn content exceeds 3.0%, the primary recrystallization texture deteriorates, leading to deterioration of magnetic properties. Therefore, the Mn content is preferably 3.0% or less. The Mn content is more preferably 0.010% or more and 0.5% or less.

[0033] In the present invention, it is preferable to reduce the contents of Al, N, S, and Se, which are components that form inhibitors, as much as possible. In this case, secondary recrystallization of the Goss orientation can be achieved by the texture inhibition effect. Therefore, it is preferable to reduce the contents of Al, N, S, and Se in the composition of the steel slab to the following ranges.

[0034] Al: Less than 0.010% If the Al content is 0.010% or more, it becomes difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition. Therefore, the Al content is preferably less than 0.010%. On the other hand, from the viewpoint of the texture inhibition effect, the lower the Al content, the better, and it may be 0%.

[0035] O: 0.006% or less O also forms oxides, which deteriorates the magnetic properties of the final product sheet. Therefore, the O content is preferably 0.006% or less, and more preferably 0.003% or less. On the other hand, from the viewpoint of the texture inhibition effect, the lower the O content, the better, and it may be 0%.

[0036] N: 0.006% or less N forms silicon nitrides after purification annealing. In order to prevent the formation of the silicon nitrides, the N content is preferably 0.006% or less. On the other hand, from the viewpoint of texture inhibition effect, the lower the N content, the better, and it may be 0%.

[0037] S + 0.405 × Se: 0.0060% or less In order to stably achieve secondary recrystallization, it is preferable to set S + 0.405 × Se to 0.0060% or less. On the other hand, from the viewpoint of texture inhibition effect, the lower S + 0.405 × Se is the better, and it may be 0%, but excessive reduction leads to increased manufacturing costs. Therefore, it is preferable to set S + 0.405 × Se to 0.0010% or more.

[0038] In the present invention, in addition to the above elements, one or more elements selected from the group consisting of the following may be contained: 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, B: 0.0005% or more and 0.0200% or less, Bi: 0.0005% or more and 0.0200% or less, Nb: 0.0005% or more and 0.0200% or less, Ti: 0.0005% or more and 0.0200% or less, and Te: 0.0005% or more and 0.0200% or less.

[0039] Ni: 0.005% or more and 1.50% or less Ni is a useful element that improves the hot-rolled sheet structure and enhances magnetic properties. However, if the Ni content is less than 0.005%, the effect of improving magnetic properties is small. Therefore, the Ni content is preferably 0.005% or more. On the other hand, if the Ni content exceeds 1.50%, secondary recrystallization becomes unstable and magnetic properties deteriorate. Therefore, the Ni content is preferably 1.50% or less.

[0040] 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, B: 0.0005% or more and 0.0200% or less, Bi: 0.0005% or more and 0.0200% or less. The use of grain boundary segregation elements Sn, Sb, Cu, Mo, P, Cr, B, and Bi can further improve magnetic properties. If the contents of these elements are below the lower limits of the above ranges, the effect of improving magnetic properties is small, while if the contents of these elements exceed the upper limits, the development of secondary recrystallized grains is suppressed. Therefore, the contents of Sn, Sb, Cu, Mo, P, Cr, B, and Bi are preferably set within the above ranges.

[0041] Nb: 0.0005% or more and 0.0200% or less Ti: 0.0005% or more and 0.0200% or less Te: 0.0005% or more and 0.0200% or less Nb, Ti, and Te are precipitate-forming elements. Although not necessarily required in manufacturing methods that do not use inhibitors, adding trace amounts within the range in which they form a solid solution during slab heating at relatively low temperatures can improve magnetic properties. Therefore, the contents of Nb, Ti, and Te are preferably equal to or greater than the lower limit values ​​described above. On the other hand, if the contents of Nb, Ti, and Te exceed the upper limit values ​​of the above ranges, secondary recrystallization becomes unstable. Therefore, the contents of Nb, Ti, and Te are preferably equal to or less than the upper limit values ​​described above.

[0042] The molten steel adjusted to the above-mentioned preferred composition is refined by a known method using a converter, an electric furnace, or the like, and if necessary, subjected to vacuum treatment or the like, and then a steel slab 1 is produced by a normal ingot-making method or a continuous casting method. Alternatively, a thin cast piece having a thickness of 100 mm or less may be directly produced by a direct casting method.

[0043] Next, the steel slab 1 having the above-described composition is hot-rolled to form a hot-rolled sheet. The steel slab 1 can be heated in a heating furnace to a temperature of, for example, 1050°C or higher but lower than 1300°C, and then hot-rolled. The steel slab 1 of the present invention does not need to be subjected to high-temperature treatment of 1300°C or higher, particularly for complete solid solution of precipitates, when the inhibitor component is preferably suppressed. If the steel slab 1 is heated to a temperature of 1300°C or higher, the crystal structure becomes too large, making it difficult to control the texture. Therefore, the maximum temperature during heating is preferably less than 1300°C. On the other hand, from the viewpoint of smooth rolling of the steel slab 1, it is preferable to heat the steel slab 1 to a temperature of 1050°C or higher. Here, the temperature of the steel slab 1 refers to the surface temperature of the steel slab 1.

[0044] Here, the inventors calculated based on the chemical composition of the steel slab 1 that, when heating a steel slab 1 in a temperature range T in which the temperature of the steel slab 1 in the heating furnace is 950°C or higher and 1150°C or lower, and the gamma phase fraction is 20 mol% or less at the median value of the temperature range T, 1050°C, if a heating furnace is used in which the distance between multiple skids 2 supporting the steel slab 1 (hereinafter also referred to as "skid spacing") exceeds 1.1 m, it is essential to set the average oxygen concentration in the heating furnace in the above temperature range T to 5.0 volume% or less.

[0045] Even within the same heating furnace, the skid spacing is not necessarily equal. As a result of investigations by the inventors, when focusing on a certain skid 2 among multiple skids 2 and the distance to two adjacent skids 2 on both sides exceeds 1.1 m, defects occur at a position on the hot-rolled coil corresponding to the contact position between that skid 2 and the steel slab 1. Therefore, focusing on a certain skid 2, when the distance to two adjacent skids 2 on both sides exceeds 1.1 m, the average oxygen concentration in the heating furnace is set to 5.0 volume % or less.

[0046] Note that the "skid spacing" does not refer to the distance between the centers of two skids 2, but rather refers to the distance of the space (gap) between the centers of two skids 2. Here, the skid spacing is not taken into consideration for the skids 2 at both ends. Also, the average oxygen concentration in the heating furnace refers to the time average of the oxygen concentration in the heating furnace. Furthermore, when the heating temperature of the steel slab 1 is in the range of 950°C to 1150°C, for example, 1050°C, the average oxygen concentration in the heating furnace is set to 5.0 volume % or less in the temperature range of 950°C to 1050°C.

[0047] From the viewpoint of suppressing hot cracking, the lower the oxygen concentration in the heating furnace, the better, but in the case of a heating furnace that uses a mixture of air and fuel gas, or a heating furnace that may draw in air when loading and unloading the steel slab 1, the effect of suppressing the formation of surface defects can be greatly exhibited by setting the average oxygen concentration to 5.0% by volume or less. The average oxygen concentration is more preferably 3.0% by volume or less, as this can further enhance the effect of suppressing the formation of surface defects, and even more preferably 0.5% by volume or less, as this can almost completely prevent the formation of surface defects.

[0048] The temperature range T of 950°C to 1150°C often corresponds to the middle of the heating process in slab heating. Therefore, in the temperature range T, the temperature of the steel slab 1 is rarely measured directly, or the atmospheric gas at the target location in the heating furnace is rarely continuously sampled to measure the oxygen concentration. On the other hand, the atmosphere in the furnace changes from moment to moment due to changes in the air-fuel ratio when the furnace temperature is changed, or the air being drawn in when the steel slab 1 is loaded or unloaded. Therefore, in order to control the oxygen concentration as described above, it is preferable to directly measure the slab temperature or to determine the target temperature range by calculation, and to use a heating furnace having a mechanism for continuously measuring the atmosphere at at least one location in the heating furnace to continuously determine the gas atmosphere in the target temperature range, and to increase the amount of inert gas introduced to reduce the oxygen concentration in response to fluctuations in the atmosphere.

[0049] Next, the steel slab 1 is hot-rolled. In order to improve the shape of the hot-rolled coil and prevent minor surface defects from becoming apparent, it is preferable to apply the following rolling conditions. That is, it is preferable that at least two consecutive passes of rolling from the stage of the steel slab 1 to the stage of the sheet bar are performed in a temperature range of 1030°C to 1150°C, with each pass having a reduction of 50% or less, a strain rate of 15 / sec or more, and the time between two passes being 5 seconds or more. It is more preferable that the time between two passes be 15 seconds or more.

[0050] When a steel slab 1 containing 0.03% or more C as its chemical composition and having a γ-phase fraction of 20 mol% or less at temperatures of 950°C to 1150°C in a heating furnace is used, the γ-phase fraction reaches its maximum at temperatures near 1030°C to 1150°C. Generally, the austenite phase has higher deformation resistance than the ferrite phase and is less likely to deform when rolled down. Therefore, it is preferable to limit the reduction rate of each pass to 50% or less. If the reduction rate is excessively large, even minor surface defects before rolling will be significantly expanded by friction with the rolls, making them more likely to become apparent. Furthermore, by setting the time between two passes to 15 seconds or more, dislocations formed by deformation will recover or disappear through recrystallization, allowing rolling without excessively increasing deformation resistance. This also works advantageously in terms of friction suppression. Furthermore, it is preferable to set the strain rate to 15 / s or more. If the strain rate is low, i.e., the rolling speed is slow, the temperature gradually decreases from when the material is caught in the roll bite until the end of rolling, making it difficult to perform rolling properly and potentially leading to shape deterioration.

[0051] The strain rate was calculated using the following Ekeland equation (2). where dε / dt is the strain rate ( / sec), v Ｒ is the roll peripheral speed (mm / sec), R' is the roll radius (mm), h １ is the roll entry side thickness (mm), and r is the rolling reduction (%). By applying such a rolling schedule, it is possible to improve the shape of the hot rolled coil while suppressing the manifestation of minor surface defects.

[0052] The presence or absence of surface defects in a hot-rolled coil can be evaluated visually. Alternatively, the presence or absence of surface defects can be more easily evaluated by subjecting the hot-rolled coil to a treatment that exposes surface defects. For example, a sample is cut from a portion of the hot-rolled coil that corresponds to the position of a skid 2 that supports a steel slab 1 in a heating furnace, and the sample is then pickled to remove surface scale, dried at 180°C for 1 minute, and then left to stand for several days. This causes localized rusting at the locations where defects exist, making it easier to evaluate the presence or absence of surface defects.

[0053] However, in actual operation, cutting out the hot-rolled coil leads to a significant deterioration in yield, so it is preferable to use a defect detector using a general defect evaluation device such as an eddy current sensor or an optical camera.

[0054] Here, a method for determining a position on a hot-rolled coil corresponding to the position of a skid 2 supporting a steel slab 1 in a heating furnace (i.e., the contact position between the steel slab 1 and the skid 2) will be described. For example, when rolling a steel slab 1 having a thickness of 200 mm to 2 mm, a skid 2 of interest (i.e., the skid spacing is more than 1.1 m) is located 3 m from the longitudinal end of the steel slab 1, and the width of the skid 2 supporting the steel slab 1 itself is 50 mm. In this case, the position 3 m from the longitudinal end of the steel slab 1 corresponds to a position on the hot-rolled coil that is 3 m × (200 mm / 2 mm) = 300 m from the longitudinal end of the hot-rolled coil. Furthermore, the width of the skid 2 in contact with the steel slab 1 is 50 mm, but the 50 mm-wide region in the steel slab 1 expands to a 35 m-wide region in the hot-rolled coil, taking into account the above-mentioned 0.15 m error and the 50 mm width of the skid 2. Therefore, a region of 17.5 m before and after the position 300 m from the longitudinal end of the hot-rolled coil corresponds to a position 3 m from the longitudinal end of the steel slab 1.

[0055] However, if the width of the steel slab 1 increases due to rolling (for example, if a 1-m-wide steel slab 1 becomes 1.1 m wide), the amount of elongation in the longitudinal direction is reduced to take into account the increase in width. In the above example, an area extending 17.5 m forward and backward from a position 300 m from the end is the target for surface defect evaluation, and the number of surface defects within that area is evaluated. By using the method of the present invention, it is possible to suppress the occurrence of surface defects in hot-rolled coils. When the number of surface defects is added up for multiple hot-rolled coils or for multiple target skids for a single hot-rolled coil and averaged as the number of defects per skid (for example, for 20 or more coils), it is possible to reduce the number to 0.3 or less.

[0056] In the above explanation, the evaluation method for hot-rolled coils has been described. However, even in the case of coils after cold rolling, which will be performed later, surface defects can be evaluated by setting similar target locations and using similar methods, although the target plate thickness is different.

[0057] (Method for manufacturing grain-oriented electrical steel sheet) Next, a method for manufacturing a grain-oriented electrical steel sheet according to the present invention will be described. The method for manufacturing a grain-oriented electrical steel sheet according to the present invention is characterized in that a steel slab 1 is hot-rolled by the hot rolling method according to the present invention described above, the resulting hot-rolled coil is subjected to hot-rolled sheet annealing, and then subjected to cold rolling once or two or more times with intermediate annealing in between, and then optionally subjected to decarburization annealing and final annealing to obtain a grain-oriented electrical steel sheet.

[0058] After the above-mentioned hot rolling, the hot-rolled sheet is annealed and cold-rolled. In the hot-rolled coil in which surface defects are suppressed, breakage during the cold rolling process can be suppressed.

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

[0060] After the above hot-rolled sheet annealing, the sheet is subjected to one or more cold rolling steps with intermediate annealing as required, and then decarburization annealing is carried out to reduce the C content to 50 ppm or less, preferably 30 ppm or less, at which magnetic aging does not occur.

[0061] In cold rolling, it is effective in terms of developing a Goss structure to perform aging treatment once or multiple times at a rolling temperature of 80° C. or higher and 150° C. or lower, and at an inter-pass temperature of 100° C. or higher and 300° C. or lower.

[0062] The decarburization annealing after the final cold rolling is also intended to decarburize the cold rolled sheet having a rolling structure and to adjust the grain size of the primary recrystallized steel to an optimum size for secondary recrystallization. ２ The dew point is introduced in a mixed atmosphere, and the annealing is carried out at 750°C to 900°C. When the temperature is increased during annealing, the temperature increase rate from 550°C to 680°C is set to 200°C / sec or more, which further enhances the texture improvement effect. Furthermore, after decarburization annealing, a technique for increasing the Si content by siliconizing or a technique for increasing the N content by nitriding may be used in combination.

[0063] Thereafter, a secondary recrystallization structure is developed by performing final annealing. At this time, a forsterite film may be formed using an annealing separator mainly composed of MgO. In this case, the formation of the forsterite film can be further promoted by adding an appropriate amount of Ti oxide, Sr compound, etc. to the separator. In particular, the addition of an auxiliary agent that promotes uniform forsterite film formation is also advantageous for improving the release properties. In addition, Al ２ O ３ Any annealing separator may be used to suppress film formation.

[0064] The final annealing is preferably carried out 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, so it can be carried out under any conditions. ２ , Ar, H ２Alternatively, any of these mixed gases can be used. To more effectively carry out secondary recrystallization, the temperature can be maintained isothermally near the secondary recrystallization temperature. However, this can also be achieved by slowing the heating rate, so isothermal maintenance is not necessarily required. Since the precipitation of trace elements in the final product leads to deterioration of magnetic properties, the maximum annealing temperature is preferably 1100°C or higher to purify the elements.

[0065] After the above-mentioned final annealing, an insulating coating can be further applied to the surface of the steel sheet and baked. The type of such insulating coating is not particularly limited, and any conventionally known insulating coating is suitable. For example, a method in which a coating liquid containing phosphate, chromate, and colloidal silica, as described in JP-A-50-79442 and JP-A-48-39338, is applied to the steel sheet and baked at about 800°C, is suitable.

[0066] Furthermore, the shape of the steel sheet can be adjusted by flattening annealing, and further, this flattening annealing can also be used in combination with the baking treatment of the insulating coating.

[0067] (Hot-rolled coil for grain-oriented electrical steel sheet) The hot-rolled coil for grain-oriented electrical steel sheet according to the present invention is a hot-rolled coil for grain-oriented electrical steel sheet obtained by hot-rolling a steel slab 1 by the hot-rolling method according to the present invention described above, and has a width L in the rolling direction of hot rolling relative to a position corresponding to the position on the skid 2 when the steel slab 1 is heated. ０ The number of surface defects in the range of (m) is an average of 0.3 or less. ０ (m) is the width of the skid 1, X is the thickness of the steel slab 1, Y is the １ (m), the thickness of the hot rolled coil for grain-oriented electrical steel sheet is Y ２ (m) is given by the following formula (1): ０ (m)={0.15(m)+X(m)}×Y １ (mm) / Y ２ (mm) (1)

[0068] As described above, the hot rolling method according to the present invention is characterized in that, in the temperature range T in which the temperature of the steel slab 1 in the heating furnace is 950°C or higher and 1150°C or lower, the average oxygen concentration in the heating furnace in the temperature range T is set to 5.0 volume % or lower. This makes it possible to reduce surface defects at a position on the hot rolled coil corresponding to the position on the steel slab 1 supported by the target skid 2. Specifically, the surface defects are reduced at a distance X (m) x Y from the end of the hot rolled coil. １ (mm) / Y ２ (mm) Position before and after L ０ The area (m) corresponds to the position where the steel slab 1 is supported by the target skid 2, and the occurrence of surface defects in the area can be suppressed to an average of 0.3 or less. The average number of surface defects can be, for example, an average for 20 or more coils.

[0069] Example 1 A steel slab 1 having a thickness of 200 mm and containing, in mass %, C: 0.035% to 0.055%, Si: 3.0% to 3.4%, Mn: 0.07%, Al: 0.005% to 0.008%, N, O, S + 0.405 × Se: each less than 0.0060%, and the balance being Fe and unavoidable impurities and no inhibitor component was heated in a heating furnace equipped with a plurality of skids 2 having a skid width of 50 mm and skid spacing as shown in Figure 2. The sum of the numerical values ​​in Figure 2 indicates the distance from the center of skid D or the center of skid C to the longitudinal end of the steel slab 1.

[0070] In the heating furnace, either of the following heating patterns was adopted: Pattern A, in which the steel slab 1 was heated to 1150°C upstream of the shift skid 3 (hereinafter also referred to as "before the shift") and then heated to 1250°C downstream of the shift skid 3 (hereinafter referred to as "after the shift"), or Pattern B, in which the steel slab 1 was heated to 950°C before the shift and then heated to 1200°C after the shift. In addition, a heat-resistant gas suction pipe was installed in the heating furnace, and a mechanism for continuously measuring the oxygen concentration was provided, and N gas supplied to the positions before and after the shift was measured. ２By controlling the amount of gas, fluctuations in oxygen concentration were controlled in real time when the heating furnace was opened and closed and the burner combustion efficiency was changed. Furthermore, under some conditions (No. 13 in Table 1), experiments were also conducted under conditions in which the oxygen concentration in the heating furnace momentarily increased due to the influence of air entering the furnace when the heating furnace was opened and closed. In Table 1, the values ​​in parentheses for the average oxygen concentration in the furnace before and after the shift in No. 13 indicate the values ​​after the heating furnace was momentarily opened and closed. The temperature of the steel slab 1 was calculated by numerically calculating the furnace temperature after evaluating the heat input to the steel slab 1 with a thermocouple attached in the heating furnace. The experiments were conducted based on the calculated slab temperature. After extraction from the heating furnace, the third and fourth passes of a four-pass rough rolling process were performed under the conditions shown in Table 1. Subsequently, the slab was subjected to multiple passes of finish hot rolling in a temperature range of 850°C to 950°C to finish it to a thickness of 2.0 mm. In the finish hot rolling, the steel sheet was given an appropriate tension between the rolls to minimize width expansion. Twenty hot-rolled coils were produced under the same manufacturing conditions.

[0071] Using the longitudinal end as a reference point, samples were cut out of the obtained hot-rolled coil from locations corresponding to the positions on the steel slab 1 supported by the target skid 2 (contact positions between the steel slab 1 and the target skid 2). These samples were pickled in 5% HCl at 80°C for 120 seconds to remove surface scale, then heat-treated at 180°C for 1 minute to dry the surface, and left for 7 days, after which the number of locations where localized rust had occurred was confirmed. The results are shown in Table 2.

[0072] In Table 2, the skids 2 targeted by the present invention are those designated "c" before the shift and "C and D" after the shift. The comparative example had a high number of surface defects, at 2.5 or more per skid. In contrast, the inventive example had a low number of surface defects, at 0.3 or less per skid. When the oxygen concentration was 3.0% by volume or less, the number of surface defects was 0.1 or less per skid, meaning that surface defects were virtually eliminated. Thus, it can be seen that the present invention can significantly reduce the number of surface defects.

[0073]

[0074]

[0075] Example 2 A steel slab 1 containing 0.04% C, 3.3% Si, and 0.05% Mn, as well as the other components shown in Table 3, and having a calculated γ-phase ratio of 20 mol% or less over the entire temperature range, was heated using a heating furnace having the skid arrangement shown in FIG. 2 in the same manner as in Example 1, with a heating pattern of heating to 1150°C before shifting and to 1250°C after shifting, and hot-rolled under the conditions shown in Table 3. Some hot-rolled coils (20 coils each) were produced under the same conditions, and portions of 10 of the coils were cut out at positions corresponding to the positions of the steel slab 1 supported by the skid 2 marked with the letter "c." The amount of surface defects was evaluated in the same manner as in Example 1. The hot-rolled coils (10 coils each) from which samples were not taken were subjected to hot-rolled sheet annealing at an ultimate temperature of 1020°C. Each material was then divided into two types: a two-pass rolling method, in which the final thickness was reached in two passes, and a single-pass rolling method, in which the final thickness was reached in one pass. For the two-pass rolling material, a primary cold rolling was performed in a reverse mill at 100 ° C to 1.7 mm. After the target thickness was reached, intermediate annealing was performed at 900 ° C for 1 minute, and then a secondary reverse cold rolling was performed again, with coiling aging treatment at 200 ° C during the process, resulting in a thickness of 0.22 mm. The material allocated to the single-pass rolling method was rolled to a thickness of 0.26 mm using a tandem mill. Each material (10 coils) was also evaluated for fracture during passage through the rolling line. The cold-rolled sheet having the final thickness was subjected to primary recrystallization annealing with a heating rate of 300°C / sec. at 550°C to 680°C, a soaking temperature of 840°C, and a soaking time of 60 seconds, and then subjected to primary recrystallization annealing with a temperature of 95% MgO, 95% TiO ２ A water slurry of an annealing separator containing 5% of phosphate, chromate, and colloidal silica was applied to the surface of the steel sheet, and the steel sheet was subjected to secondary recrystallization annealing. A coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:3 was applied to the surface of the thus-obtained finish-annealed sheet, and the steel sheet was baked at 800°C. The magnetic properties of the widthwise center of the obtained product sheet coil were also confirmed. In Table 3, when "other components" were not included, B ８If the magnetic flux density at a magnetizing force of 800 A / m is 1.910 or more, it can be judged that the magnetic properties are good, and if "other components" are included, B ８ If the value is 1.915 or more, it can be determined that the magnetic properties are good.

[0076]

[0077] From Table 3, it can be seen that the inventive examples have improved manufacturing stability and also provide good magnetic properties.

[0078] According to the present invention, a hot rolled coil with few surface defects can be obtained.

[0079] 1. Steel slab 2. Skid 3. Shift skid

Claims

1. A hot rolling method for heating a steel slab in a heating furnace and then hot rolling the slab, comprising: In a temperature range T in which the temperature of the steel slab in the heating furnace is 950°C or higher and 1150°C or lower, When heating a steel slab having a component composition in which the γ phase ratio is 20 mol% or less at 1050°C, which is the median value of the temperature range T, a heating furnace in which the distance between multiple skids supporting the steel slab exceeds 1.1 m is used, A hot rolling method characterized in that the average oxygen concentration in the heating furnace in the temperature range T is 5.0 volume % or less.

2. 2. The hot rolling method according to claim 1, wherein the average oxygen concentration in the heating furnace in the temperature range T is set to 3.0% by volume or less.

3. 3. The hot rolling method according to claim 1 or 2, wherein the hot rolling includes two successive passes of hot rolling, each pass being carried out in a temperature range of 1030°C or higher and 1150°C or lower, under conditions of a reduction rate of 50% or lower and a strain rate of 15 / sec or higher, and the time between the two passes is 15 seconds or higher.

4. 3. A method for producing a grain-oriented electrical steel sheet, comprising hot rolling a steel slab by the hot rolling method according to claim 1 or 2, and subjecting the resulting hot-rolled coil to hot-rolled sheet annealing, followed by cold rolling once or at least two times with intermediate annealing in between, and then optionally subjecting the resulting hot-rolled coil to decarburization annealing, followed by final annealing to obtain a grain-oriented electrical steel sheet.

5. 3. A hot-rolled coil for grain-oriented electrical steel sheet obtained by hot-rolling a steel slab by the hot rolling method according to claim 1 or 2, wherein the hot-rolled coil has a width L in the rolling direction of the hot rolling relative to a position on the skid during heating of the steel slab. ０ 1. A hot-rolled coil for grain-oriented electrical steel sheet, characterized in that the number of surface defects in the range of (m) is an average of 0.3 or less. However, L ０ (m) is the width of the skid X and the thickness of the steel slab Y １ (m), the thickness of the hot rolled coil for grain-oriented electrical steel sheet is Y ２ (m) is given by the following equation (1). L ０ (m)={0.15(m)+X(m)}×Y １ (mm) / Y ２ (mm) (1)

6. 4. A method for producing a grain-oriented electrical steel sheet, comprising hot rolling a steel slab by the hot rolling method according to claim 3, annealing the obtained hot-rolled coil, cold rolling it once or two or more times with intermediate annealing in between, optionally decarburizing it, and then final annealing it to obtain a grain-oriented electrical steel sheet.

7. A hot-rolled coil for grain-oriented electrical steel sheet obtained by hot-rolling a steel slab by the hot rolling method according to claim 3, wherein the steel slab has a front-rear L in the rolling direction of the hot rolling with respect to a position on the skid during heating of the steel slab. ０ 1. A hot-rolled coil for grain-oriented electrical steel sheet, characterized in that the number of surface defects in the range of (m) is an average of 0.3 or less. However, L ０ (m) is the width of the skid X and the thickness of the steel slab Y １ (m), the thickness of the hot rolled coil for grain-oriented electrical steel sheet is Y ２ (m) is given by the following equation (1). L ０ (m)={0.15(m)+X(m)}×Y １ (mm) / Y ２ (mm) (1)

8. 5. A hot-rolled coil for grain-oriented electrical steel sheet obtained by hot-rolling a steel slab by the hot rolling method according to claim 4, wherein the steel slab has a front-rear distance L in the rolling direction of the hot rolling relative to a position on the skid when the steel slab is heated. ０ 1. A hot-rolled coil for grain-oriented electrical steel sheet, characterized in that the number of surface defects in the range of (m) is an average of 0.3 or less. However, L ０ (m) is the width of the skid X and the thickness of the steel slab Y １ (m), the thickness of the hot rolled coil for grain-oriented electrical steel sheet is Y ２ (m) is given by the following equation (1). L ０ (m)={0.15(m)+X(m)}×Y １ (mm) / Y ２ (mm) (1)

9. 7. A hot-rolled coil for grain-oriented electrical steel sheet obtained by hot-rolling a steel slab by the hot rolling method according to claim 6, wherein the steel slab has a front-rear distance L in the rolling direction of the hot rolling relative to a position on the skid when the steel slab is heated. ０ 1. A hot-rolled coil for grain-oriented electrical steel sheet, characterized in that the number of surface defects in the range of (m) is an average of 0.3 or less. However, L ０ (m) is the width of the skid X and the thickness of the steel slab Y １ (m), the thickness of the hot rolled coil for grain-oriented electrical steel sheet is Y ２ (m) is given by the following equation (1). L ０ (m)={0.15(m)+X(m)}×Y １ (mm) / Y ２ (mm) (1)