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

Abstract

The present invention provides a hot-rolling method capable of obtaining a hot-rolled coil having few surface defects. The hot-rolling method is a hot-rolling method in which a billet (1) is heated in a heating furnace and then hot-rolled, wherein, in the case where the distance between a plurality of skids 2 supporting the billet 1 exceeds 1.1 m in the heating furnace when heating the billet (1) having a composition such that the γ phase ratio at the median value of the temperature range T, i.e., 1050°C, is 20 mol% or less, the average oxygen concentration in the heating furnace in the temperature range T is set to 5.0 vol% or less.

Description

Technical Field

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

[0002] The manufacture of oriented electromagnetic steel sheets typically uses precipitates known as inhibitors to induce secondary recrystallization of Gaussian-oriented ({110}<001>) grains during the final annealing process. For example, Patent Document 1 discloses a method using AlN as an inhibitor, and Patent Document 2 discloses methods using MnS and MnSe as inhibitors, both of which have been industrially applied.

[0003] Using the aforementioned inhibitors is a useful method for stably promoting the growth of secondary recrystallized grains, but the precipitates must be finely dispersed. Therefore, it is necessary to heat the oriented electromagnetic steel sheet billet (hereinafter, also referred to as "the billet") at a high temperature above 1300°C before hot rolling.

[0004] However, high-temperature heating of steel billets not only increases equipment costs, but also increases the amount of oxide scale generated during hot rolling, resulting in problems such as reduced yield and more complicated equipment maintenance.

[0005] On the other hand, manufacturing techniques that do not use the aforementioned inhibitors (inhibitor-free methods) have also been proposed. Patent document 3 discloses a technique that uses a steel billet with higher purity and no inhibitor components as the billet, and achieves secondary recrystallization through texture (aggregate structure control).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Publication No. 40-15644

[0009] Patent Document 2: Japanese Patent Publication No. 51-13469

[0010] Patent Document 3: Japanese Patent Application Publication No. 2000-129356 Summary of the Invention

[0011] Since steel billets contain virtually no inhibitory elements, heating at temperatures exceeding 1300°C is unnecessary, eliminating the need for specialized furnaces. Therefore, hot rolling can be performed using slab heating equipment such as gas furnaces commonly used in steel manufacturing, enabling the production of oriented electromagnetic steel sheets at low cost.

[0012] However, in some products, surface defects occur at specific locations on the hot-rolled coils (hereinafter also referred to as "hot-rolled coils") obtained after hot rolling. As a result, this affects the deviation of the hot-rolled coils during the subsequent annealing process of the hot-rolled sheet (hereinafter also referred to as "hot-rolled sheet") and the breakage of the sheet during the cold rolling process, becoming a cause that hinders industrial-scale production.

[0013] The present invention was made in view of the above-mentioned problems, and its object is to provide a hot rolling method that can obtain hot-rolled coils with fewer surface defects.

[0014] The inventors conducted a detailed investigation into the characteristics of defect generation in actual hot-rolled coils with surface defects, and thus obtained the following insights.

[0015] 1) The location of the surface defect is roughly the same as the location of the slide rail supporting the billet when the billet is considered to be around 1050℃ (i.e., the location on the billet that has been in contact with the slide rail).

[0016] 2) The γ phase fraction of steel billets near 1050℃ is approximately below 20 mol%.

[0017] 3) Regarding the air-fuel ratio (the ratio of the mass of air to the mass of fuel gas) during heating, a higher air mass ratio also results in a higher defect rate.

[0018] It should be noted that the γ phase ratio in 2) above was calculated using Thermo-calc ver.2019b (database TCFE7), a thermodynamic software manufactured by Thermo-Calc Software AB.

[0019] Figure 1 A schematic diagram of an example of a walking beam slab heating furnace (hereinafter also simply referred to as "heating furnace") is shown. In a walking beam heating furnace, a plurality of slide rails 2 extending approximately in parallel typically support and transport the steel billet 1. Typically, the slide rails 2 have the following structure: fixed slide rails and movable slide rails are alternately arranged, with the movable slide rails moving up and down to lift the steel billet 1 and transport it little by little from the furnace loading side to the furnace extraction side.

[0020] In the above structure, when the position of the slide rail 2 used to support and transport the billet 1 is always the same relative to the billet 1 (i.e., when the same slide rail 2 continuously supports the same position on the lower surface of the billet 1), the lower surface of the billet 1 directly above the slide rail 2 is difficult to heat. Therefore, the heating furnace is equipped with one or more mechanisms called displacement slide rails 3, which are usually configured such that the position of the slide rail 2 supporting the billet 1 changes before and after the displacement slide rail 3.

[0021] The inventors determined the location of surface defects in the hot-rolled coil by considering the length of the billet 1 before rolling. The result was essentially consistent with the position of the slide rail 2 supporting the billet 1 when the billet 1's temperature reached approximately 1050°C during its stay in the heating furnace (i.e., the position where the lower surface of the billet 1 contacts the slide rail 2). However, in reality, due to slight deviation of the billet 1 during transport within the heating furnace, an error (offset) of 0.15m occurred at the corresponding position in the billet 1.

[0022] Although creep deformation occurring at high temperatures is characterized by its ease of occurrence in the α (ferrite) phase and its slower occurrence in the γ (austenite) phase, the frequency of occurrence varies depending on the γ phase fraction of billet 1, thus creep behavior may be one of the reasons. Furthermore, the occurrence rate also varies due to changes in the air-fuel ratio during heating, therefore the atmosphere, particularly the oxygen concentration, may also play a role.

[0023] Therefore, the inventors conducted the following laboratory experiments. First, a steel billet 1 was cast, containing C: 0.04% by mass, Si: 3.0%, Mn: 0.10%, Al: 0.007%, and N, O, S + 0.405 × Se suppressed to less than 0.0060%, with the remainder consisting of Fe and unavoidable impurities. Next, a 4mm square and 40mm long test piece was taken from the surface of the cast steel billet 1. The test piece was homogenized in a heating furnace at a temperature above 900°C and below 1200°C, while stress was applied to the test piece to induce three-point bending, and a creep test was conducted. During this time, the oxygen concentration in the heating furnace changed from 0% by volume (100% by volume N2) to 20% by volume (80% by volume N2). The results showed that under high oxygen concentration conditions, in a specific temperature range, the test piece not only deformed but also developed cracks.

[0024] Based on the insights obtained above, the inventors speculate on the mechanism of surface defect formation as follows: On the surface of the steel billet 1, which experiences tensile stress directly above the slide rail 2, high oxygen concentration leads to grain boundary embrittlement due to oxidation, eventually resulting in hot cracking. However, in slide rails 2 with small intervals between adjacent slide rails 2, the stress exerted on the steel billet 1 by each slide rail 2 is reduced, and deformation is suppressed, thus preventing cracking.

[0025] Furthermore, grain boundary embrittlement is promoted by grain boundary creep, which easily occurs in the creep-prone α (ferrite) single-phase steel used as billet 1, and the higher the γ (austenite) phase ratio, the less likely it is to develop into cracking. Generally, if only creep deformation is considered, the higher the temperature, the higher the deformation amount and deformation rate. However, for this phenomenon, grain boundary segregation of elements contained as impurities is one of the factors. At high temperatures, these impurity elements will also homogenize and no longer segregate, so cracking will only develop in a specific temperature range.

[0026] Based on the aforementioned hypothetical mechanism, the inventors conducted research on methods to reduce surface defects in hot-rolled coils. To improve the final magnetic properties of the electromagnetic steel sheet, a high concentration of Si was incorporated into the steel. Si stabilizes the α phase and reduces the γ phase ratio during high-temperature heating. Additionally, C is another element that significantly affects the γ phase ratio, but C also improves the texture during hot rolling and primary recrystallization; therefore, from the viewpoint of improving the final magnetic properties, there exists an appropriate amount. Therefore, it is difficult to increase the γ phase ratio at this temperature by drastically altering the composition of the electromagnetic steel sheet already manufactured in the process.

[0027] Furthermore, as the temperature of the steel billet 1 gradually increases within the heating furnace, although the residence time in a specific temperature range can be shortened, it is difficult to completely avoid this temperature range. Consequently, as long as the steel billet 1 is supported by the slide rail 2, it is difficult to achieve a state where no stress is applied to the steel billet 1. Therefore, the inventors devised a method to appropriately control the oxygen concentration within the heating furnace in a specific temperature range, thus completing this invention.

[0028] That is, the present invention that solves the above-mentioned problems is as follows.

[0029] [1] A hot rolling method, which is a hot rolling method in which a steel billet is heated in a heating furnace and then hot rolled, characterized in that,

[0030] The temperature of the steel billet in the aforementioned heating furnace is within the temperature range T of 950℃ to 1150℃.

[0031] When heating a steel billet with a γ phase fraction of less than 20 mol% at the median of the aforementioned temperature range T, i.e., 1050 °C, in the case of using a heating furnace where the distance between the multiple slide rails supporting the steel billet exceeds 1.1 m,

[0032] The average oxygen concentration in the above-mentioned heating furnace within the above temperature range T is set to below 5.0% by volume.

[0033] [2] According to the hot rolling method described in [1], the average oxygen concentration in the heating furnace within the temperature range T is set to 3.0% by volume or less.

[0034] [3] According to the hot rolling method described in [1] or [2], wherein the hot rolling includes two consecutive hot rolling passes, each pass being carried out in a temperature range of 1030°C to 1150°C, with a reduction rate of less than 50% and a strain rate of more than 15 / second, and the time between the two passes being more than 15 seconds.

[0035] [4] A method for manufacturing an orientation-oriented electromagnetic steel sheet, wherein a steel billet is hot-rolled by any one of the hot rolling methods described in [1] to [3], the hot-rolled coil is hot-rolled and then annealed, followed by one or more cold rolling processes with intermediate annealing, then decarburization annealing is performed arbitrarily, and finally finished product annealing is performed to obtain an orientation-oriented electromagnetic steel sheet.

[0036] [5] A hot-rolled coil for oriented electromagnetic steel sheet is obtained by hot rolling a steel billet by any one of the hot rolling methods described in [1] to [3], characterized in that, relative to the position on the slide rail when the steel billet is heated, the average number of surface defects in the range L0 (m) before and after the rolling direction of the hot rolling is 0.3 or less.

[0037] Wherein, L0 (m) is given by the following formula (1), where the width of the slide rail is set as X, the thickness of the billet is set as Y1 (m), and the thickness of the hot-rolled coil for the oriented electromagnetic steel plate is set as Y2 (m).

[0038] L0(m)={0.15(m)+X(m)}×Y1(mm) / Y2(mm) (1)

[0039] According to the present invention, hot-rolled coils with fewer surface defects can be obtained. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of an example of a slab heating furnace using a walking beam method.

[0041] Figure 2 This is a diagram showing the details of the slide rails in the heating furnace used in the embodiment. Detailed Implementation

[0042] (Hot rolling method)

[0043] Hereinafter, embodiments of the present invention will be described. The following description exemplifies embodiments of the present invention, and the present invention is not limited to the following embodiments. The hot rolling method of the present invention is a hot rolling method in which a steel billet is heated in a heating furnace and then hot rolled. Here, the characteristic is that, when heating a steel billet with a composition having a γ phase fraction of 20 mol% or less at the median of temperature range T (1050°C) within a temperature range T of 950°C to 1150°C, and using a heating furnace in which the distance between the multiple slide rails supporting the steel billet exceeds 1.1 m, the average oxygen concentration in the heating furnace within the temperature range T is set to 5.0 vol% or less.

[0044] [Steel billet]

[0045] In this invention, a steel billet 1 for oriented electromagnetic steel sheet is used as the starting material. First, the preferred composition of the steel billet 1 will be described. In the following description of the composition, unless otherwise stated, "%" means "mass %" and "ppm" means "mass ppm".

[0046] The steel billet 1 preferably has a composition containing C, Si and Mn within the range described below, with the remainder being Fe and unavoidable impurities.

[0047] C: 0.03%~0.08%

[0048] If the carbon content exceeds 0.08%, even with decarburization annealing, it is difficult to reduce the carbon content in the steel to below 50 ppm, which prevents magnetic aging. Therefore, the carbon content is preferably below 0.08%. Furthermore, this invention preferably reduces the sulfides and selenides present in the central layer of the billet 1 to a size smaller than that that becomes problematic during cold rolling by performing two hot rolling processes under appropriate conditions within a specific temperature range (1030°C to 1150°C). Typically, in Si steel, the majority of the ferrite single-phase phase forms in the hot rolling temperature range, but the target temperature coincides with the temperature range where the austenite phase forms. Although the volume fraction of the austenite phase is low, this suggests that it facilitates the segmentation and destruction of sulfides and selenides. In practice, the segmentation and destruction effect of sulfides and selenides cannot be obtained in steel with a carbon content of 0.02%. Therefore, the carbon content is preferably 0.03% or higher.

[0049] Si: 2.0%~8.0%

[0050] Si is a useful element that improves iron loss by increasing electrical resistance. To obtain good magnetic properties, the Si content is preferably 2.0% or more. 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 plate handling increases, and cold rollability also deteriorates significantly. Therefore, the Si content is preferably 8.0% or less. Since it can further reduce the risk during plate handling, the Si content is more preferably 2.8% to 4.5%.

[0051] Mn: 0.005%~3.0%

[0052] Mn is an element that improves hot workability during manufacturing. When the Mn content is less than 0.005%, the effect is poor both from the viewpoint of 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 a deterioration in magnetic properties. Therefore, the Mn content is preferably 3.0% or less. The Mn content is more preferably 0.010% to 0.5%.

[0053] In this invention, it is preferable to minimize the content of the inhibitory components, namely Al, N, S, and Se. In this case, the Gaussian orientation can be recrystallized secondaryally through the texture inhibition effect. Therefore, it is preferable to reduce the content of Al, N, S, and Se in the billet composition to the following ranges.

[0054] Al: Less than 0.010%

[0055] When the Al content is above 0.010%, it is difficult to obtain a secondary recrystallization structure based on texture inhibition. Therefore, the Al content is preferably less than 0.010%. On the other hand, from the viewpoint of texture inhibition effect, the lower the Al content, the better, and it can be 0%.

[0056] O: below 0.006%

[0057] O can also form oxides, which degrade the magnetic properties of the final product board. Therefore, the O content is preferably 0.006% or less, more preferably 0.003% or less. On the other hand, from the viewpoint of texture suppression, the lower the O content, the better, and it can be 0%.

[0058] N: below 0.006%

[0059] N forms purified annealed Si nitrides. To prevent the formation of these Si nitrides, the N content is preferably 0.006% or less. On the other hand, from the viewpoint of texture suppression, the lower the N content, the better, and it can be 0%.

[0060] S + 0.405 × Se: below 0.0060%

[0061] 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 suppression, the lower the S + 0.405 × Se, the better; it can be 0%, but excessive reduction will lead to increased manufacturing costs. Therefore, it is preferable to set S + 0.405 × Se to 0.0010% or more.

[0062] In addition, in addition to the elements described above, this invention may also contain one or more elements selected from the following:

[0063] Ni: 0.005%~1.50%

[0064] Sn: 0.01%~0.50%

[0065] Sb: 0.005%~0.50%

[0066] Cu: 0.01%~0.50%,

[0067] Mo: 0.01%~0.50%,

[0068] P: 0.0050%~0.50%

[0069] Cr: 0.01%~1.50%

[0070] B: 0.0005%~0.0200%

[0071] Bi: 0.0005%~0.0200%

[0072] Nb: 0.0005%~0.0200%

[0073] Ti: 0.0005%~0.0200%, and

[0074] Te: 0.0005%~0.0200%.

[0075] Ni: 0.005%~1.50%

[0076] Ni is a useful element for improving the microstructure and magnetic properties of hot-rolled steel sheets. However, the improvement in magnetic properties is minimal when the Ni content is less than 0.005%. Therefore, the Ni content is preferably 0.005% or higher. On the other hand, if the Ni content exceeds 1.50%, secondary recrystallization becomes unstable, and the magnetic properties deteriorate. Therefore, the Ni content is preferably 1.50% or lower.

[0077] Sn: 0.01%~0.50%

[0078] Sb: 0.005%~0.50%

[0079] Cu: 0.01%~0.50%,

[0080] Mo: 0.01%~0.50%,

[0081] P: 0.0050%~0.50%

[0082] Cr: 0.01%~1.50%

[0083] B: 0.0005%~0.0200%

[0084] Bi: 0.0005%~0.0200%.

[0085] Magnetic properties can be further improved by using Sn, Sb, Cu, Mo, P, Cr, B, and Bi, which are grain boundary segregation elements. If the content of these elements is below the lower limit of the aforementioned range, the improvement in magnetic properties is minimal; conversely, if the content of these elements exceeds the upper limit, 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 aforementioned range.

[0086] Nb: 0.0005%~0.0200%

[0087] Ti: 0.0005%~0.0200%

[0088] Te: 0.0005%~0.0200%

[0089] Nb, Ti, and Te are precipitate-forming elements. While not strictly necessary in manufacturing methods without inhibitors, their magnetic properties may improve when added in minute quantities within the range where solid solution occurs during slab heating at relatively low temperatures. Therefore, the content of Nb, Ti, and Te is preferably above the aforementioned lower limit. On the other hand, if the content of Nb, Ti, and Te exceeds the upper limit of the aforementioned range, secondary recrystallization will become unstable. Therefore, the content of Nb, Ti, and Te is preferably below the aforementioned upper limit.

[0090] Molten steel with the above-mentioned preferred composition is refined using known methods such as converters or electric furnaces, and vacuum treatment is performed when necessary. Then, steel billets 1 are manufactured using conventional ingot casting or continuous casting methods. Alternatively, thin castings with a thickness of 100 mm or less can be directly manufactured using direct casting methods.

[0091] Next, the steel billet 1 having the above-described composition is hot-rolled to produce a hot-rolled plate. The steel billet 1 can be hot-rolled in a heating furnace, for example, at a temperature of 1050°C or higher but less than 1300°C. In this invention, the steel billet 1 is preferably subjected to high-temperature treatment above 1300°C, where the inhibitory components are suppressed, and in particular, it is not necessary to achieve complete solidification of the precipitates. If the steel billet 1 is heated to a temperature above 1300°C, the crystalline structure becomes too large, making it difficult to control the texture; therefore, the maximum heating temperature is preferably less than 1300°C. On the other hand, from the perspective of smooth rolling of the steel billet 1, it is preferable to heat the steel billet 1 to a temperature of 1050°C or higher. Here, the temperature of the steel billet 1 refers to its surface temperature.

[0092] Here, the inventors calculated based on the composition of the billet 1. The results showed that, within a temperature range T of 950°C to 1150°C in the heating furnace, when heating a billet 1 with a γ phase fraction of 20 mol% or less at the median of 1050°C in this temperature range T, in the case of a heating furnace using a plurality of slide rails 2 supporting the billet 1 with a distance (hereinafter also referred to as "slide rail spacing") exceeding 1.1 m, the key is to set the average oxygen concentration in the heating furnace within the aforementioned temperature range T to 5.0 vol% or less.

[0093] Even within the same heating furnace, the spacing between slide rails is not necessarily equal. Based on the inventors' findings, focusing on a specific slide rail 2, when the distance to any two adjacent slide rails 2 exceeds 1.1m on both sides, a defect occurs at the location on the hot-rolled coil corresponding to the contact point between that slide rail 2 and the billet 1. Therefore, focusing on a specific slide rail 2, when the distance to any two adjacent slide rails 2 exceeds 1.1m on both sides, the average oxygen concentration in the heating furnace is set to 5.0% by volume or less.

[0094] It should be noted that "rail spacing" does not refer to the distance between the centers of the two rails 2, but rather to the spatial portion (gap) within the distance between the centers of the two rails 2. Here, the rail spacing is not considered for the rails 2 located at both ends. Furthermore, the average oxygen concentration in the heating furnace refers to the time average of the oxygen concentration in the heating furnace. Therefore, when the heating temperature of the billet 1 is within the range of 950℃ to 1150℃, for example, 1050℃, the average oxygen concentration in the heating furnace within this temperature range is set to be 5.0% by volume or less.

[0095] From the viewpoint of suppressing hot cracking, the lower the oxygen concentration in the furnace, the better. However, in the case of furnaces that use a mixture of air and fuel gas, or furnaces where atmospheric entrainment may occur during the loading and unloading of billet 1, setting the average oxygen concentration to 5.0% by volume or less can significantly reduce the formation of surface defects. Since the average oxygen concentration can further enhance the suppression of surface defect formation, it is more preferably 3.0% by volume or less, and since it can substantially prevent the formation of surface defects, it is even more preferably 0.5% by volume or less.

[0096] The aforementioned temperature range T of 950°C to 1150°C largely corresponds to the middle of the heating process in slab heating. Therefore, within this temperature range T, the temperature of the billet 1 is rarely directly measured, nor is the oxygen concentration measured by continuously collecting atmospheric gas samples from the target area within the heating furnace. On the other hand, the atmosphere within the furnace constantly changes due to variations in the air-fuel ratio when the furnace temperature is altered, or due to atmospheric entrainment during the loading and unloading of the billet 1. Therefore, to achieve the oxygen concentration control described above, it is preferable to use a heating furnace with the following mechanisms: directly measuring the slab temperature or calculating the target temperature range; continuously measuring the atmosphere at at least one location within the heating furnace to continuously monitor the gas atmosphere within the target temperature range; and increasing the introduction of inert gases to reduce oxygen concentration relative to its variations.

[0097] Next, the billet 1 is hot-rolled. However, in order to improve the shape of the hot-rolled coil and suppress the manifestation of minor surface defects, the following rolling conditions are preferred: at least two consecutive rolling passes from the billet 1 stage to the production of the thin steel sheet, preferably in the temperature range of 1030°C to 1150°C, with each pass having a reduction rate of 50% or less, a strain rate of 15 / s or more, and a time between two passes of 5 seconds or more. Furthermore, the time between two passes is more preferably 15 seconds or more.

[0098] When using a steel billet 1 containing more than 0.03% C in its composition and with a γ phase fraction of less than 20 mol% at a temperature of 950°C to 1150°C in a heating furnace, the γ phase fraction is at its maximum at a temperature around 1030°C to 1150°C. Generally, austenite has higher deformation resistance than ferrite, making it difficult to deform even when pressed. Therefore, it is preferable to limit the reduction rate of each pass to less than 50%. Here, if the reduction rate is too high, even if there are slight surface defects before pressing, the cracks will expand significantly due to friction with the rolls and become easily visible. In addition, regarding the time between two passes, by setting it to 15 seconds or more, the dislocations formed by one deformation will disappear through recovery or recrystallization, so rolling can be performed without excessively increasing the deformation resistance. This is also advantageous from the viewpoint of suppressing friction. Furthermore, the strain rate is preferably 15 / second or more. When the strain rate is low, i.e. the rolling speed is slow, the temperature gradually decreases from the time the rolls bite into the rolls until the end of rolling, making it difficult to roll properly and potentially leading to shape deterioration.

[0099] It should be noted that the above strain rate was calculated using Ekelund's equation (2) below.

[0100]

[0101] Here, dε / dt represents the strain rate ( / s), v RR' is the roll circumferential speed (mm / s), R' is the roll radius (mm), h1 is the roll entry plate thickness (mm), and r is the reduction rate (%). By applying such a rolling process, it is possible to achieve a good shape for the hot-rolled coil while suppressing the manifestation of minor surface defects.

[0102] The presence or absence of surface defects in hot-rolled coils can be evaluated visually. Furthermore, by subjecting the hot-rolled coils to a treatment that makes surface defects visible, the presence or absence of surface defects can be evaluated more easily. For example, a sample is cut from the hot-rolled coil at a position corresponding to the slide rail 2 supporting the billet 1 in the heating furnace. The sample is then pickled to remove the surface oxide scale, dried at 180°C for 1 minute, and then left to stand for several days. This causes localized rusting at the defective areas, thus making the presence or absence of surface defects easier to evaluate.

[0103] However, in actual machine operation, cutting out hot-rolled coils leads to a significant deterioration in the yield. Therefore, it is preferable to use eddy current sensors as general defect evaluation devices or defect detectors using optical cameras.

[0104] Here, the method for determining the position in the hot-rolled coil corresponding to the position of the slide rail 2 supporting the billet 1 in the heating furnace (i.e., the contact position between the billet 1 and the slide rail 2) will be explained. For example, when rolling a billet 1 with a thickness of 200 mm into a thickness of 2 mm, assume there is a slide rail 2 located 3 m from the end of the billet 1 in the longitudinal direction (i.e., the slide rail interval exceeds 1.1 m), and the width of the slide rail 2 supporting the billet 1 is 50 mm. In this case, the position 3 m from the end of the billet 1 in the longitudinal direction corresponds to a position in the hot-rolled coil that is 3 m × (200 mm / 2 mm) = 300 m from the end of the hot-rolled coil in the longitudinal direction. In addition, although the width of the slide rail 2 in contact with the billet 1 is 50 mm, the 50 mm wide area in the billet 1 is extended in the hot-rolled coil to a width of (50 mm + 0.15 m × 2) × (200 mm / 2 mm) = 35 m due to the aforementioned error of 0.15 m and the width of the slide rail 2 of 50 mm. Therefore, the area 17.5m before and after the point 300m from the end of the hot-rolled coil in the longitudinal direction is the part corresponding to the point 3m from the end of the billet 1 in the longitudinal direction.

[0105] However, when the width of the billet 1 expands due to rolling (e.g., when a billet 1 with a width of 1m becomes a billet with a width of 1.1m), the width expansion is considered, and the length extension is subtracted. In the above example, a region of 17.5m before and after the end point, centered at a position 300m from the end, is used to evaluate surface defects, and the number of surface defects within this region is evaluated. By using the method of the present invention, the generation of surface defects in hot-rolled coils can be suppressed. The number of surface defects is accumulated for multiple hot-rolled coils or multiple object rails in a single hot-rolled coil, and averaged as the defect generation amount for each rail (e.g., for more than 20 coils), which can be reduced to less than 0.3 defects.

[0106] It should be noted that the evaluation method for hot-rolled coils was described in the above description, but even for coils that are subsequently cold-rolled, although the thickness of the target plate is different, the surface defects can still be evaluated by setting the same target area and using the same method.

[0107] (Manufacturing method of oriented electromagnetic steel sheet)

[0108] Next, the manufacturing method of the oriented electromagnetic steel sheet of the present invention will be described. The manufacturing method of the oriented electromagnetic steel sheet of the present invention is characterized in that the steel billet 1 is hot rolled by the hot rolling method of the present invention described above, the obtained hot rolled coil is subjected to hot rolled plate annealing, and then subjected to cold rolling once or twice or more with intermediate annealing, followed by arbitrary decarburization annealing, and finally subjected to final product annealing to obtain the oriented electromagnetic steel sheet.

[0109] After the hot rolling process, the hot-rolled plate is annealed and then cold-rolled. However, in the hot-rolled coil where surface defects are suppressed, the breakage during the cold rolling process can be suppressed.

[0110] Annealing of hot-rolled steel sheets is preferably performed below 1150°C. If the annealing temperature exceeds 1150°C, the inhibitory components inevitably mixed in will undergo solid solution and recrystallize unevenly upon cooling, making it difficult to achieve a uniform primary recrystallization structure and hindering the development of secondary recrystallization. Furthermore, if the annealing temperature exceeds 1150°C, the grain size of the annealed hot-rolled steel sheet will be too coarse, which is also disadvantageous in achieving a suitable primary recrystallization structure. Therefore, annealing of hot-rolled steel sheets is preferably performed below 1150°C.

[0111] After the hot-rolled plate is annealed as described above, it is cold-rolled once or more with intermediate annealing as needed, and then decarburized annealed to reduce C to below 50 ppm, preferably below 30 ppm, so that magnetic aging does not occur.

[0112] It should be noted that, from the perspective of developing Gaussian structure, setting the rolling temperature to 80℃~150℃ and raising the temperature between rolling passes to 100℃~300℃ for one or more aging treatments during cold rolling is effective.

[0113] Furthermore, the purpose of the final decarburization annealing after cold rolling is also to simultaneously recrystallize the cold-rolled sheet with the rolled structure, adjusting it to the primary recrystallization grain size most suitable for secondary recrystallization. Therefore, decarburization annealing is carried out in a H2 mixed atmosphere with dew point introduced, at 750℃~900℃. If the heating rate is set to 200℃ / second or higher during annealing (550℃~680℃), the texture improvement effect can be further enhanced. Alternatively, techniques for increasing Si content via silicon diffusion after decarburization annealing and for increasing N content via nitriding can also be used in conjunction.

[0114] Then, by performing final annealing, the secondary recrystallization structure is developed. At this time, an annealing separating agent with MgO as the main component can be used to form a forsterite coating. Adding appropriate amounts of Ti oxides, Sr compounds, etc., to the separating agent can further promote the formation of the forsterite coating. In particular, adding aids that help to uniformly promote the formation of the forsterite coating is also beneficial for improving exfoliation properties. Alternatively, any annealing separating agent such as Al2O3 can be used to inhibit coating formation.

[0115] The final annealing is preferably performed at 800°C or higher to facilitate secondary recrystallization, but the heating rate up to 800°C has little effect on the magnetic properties, so it can be performed under any conditions. The annealing atmosphere can be any of N2, Ar, H2, or mixtures thereof. To more favorablely facilitate secondary recrystallization, isothermal holding can be performed near the secondary recrystallization temperature. However, this can also be achieved by slowing down the heating rate, so isothermal holding is not always necessary. If trace components precipitate in the final product, it can lead to deterioration of the magnetic properties; therefore, the maximum annealing temperature is preferably set to 1100°C or higher for component purification.

[0116] After the final annealing described above, an insulating film can be further coated and baked on the surface of the steel plate. There are no particular limitations on the type of insulating film; any previously known insulating film may be used. For example, a preferred method is to coat the steel plate with a coating solution containing phosphate-chromate-colloidal silica, as described in Japanese Patent Application Publication No. 50-79442 or Japanese Patent Application Publication No. 48-39338, and bake it at approximately 800°C.

[0117] In addition, the shape of the steel plate can be adjusted by planarization annealing, which can also take into account the planarization annealing and the baking treatment of the insulating film.

[0118] (Hot-rolled coils for oriented electromagnetic steel sheets)

[0119] The hot-rolled coil for oriented electromagnetic steel sheet of the present invention is obtained by hot rolling a steel billet 1 using the hot rolling method of the present invention described above. Its characteristic is that, relative to the portion on the slide rail 2 corresponding to the position when the steel billet 1 is heated, the average number of surface defects within a range L0 (m) before and after the hot rolling direction is 0.3 or less. Wherein, L0 (m) is given by the following formula (1), where the width of the slide rail 1 is set as X, the thickness of the steel billet 1 is set as Y1 (m), and the thickness of the hot-rolled coil for oriented electromagnetic steel sheet is set as Y2 (m).

[0120] L0(m)={0.15(m)+X(m)}×Y1(mm) / Y2(mm) (1)

[0121] As described above, in the hot rolling method of the present invention, the average oxygen concentration in the furnace within the temperature range T of 950°C to 1150°C for the billet 1 is set to 5.0% by volume or less. This reduces surface defects at the position in the hot-rolled coil corresponding to the position supported by the guide rail 2 in the billet 1. Specifically, in the region L0 (m) before and after the end of the hot-rolled coil at a distance of X (m) × Y1 (mm) / Y2 (mm), corresponding to the position where the billet 1 is supported by the guide rail 2, the occurrence of surface defects in the aforementioned region can be suppressed to an average of 0.3 or less. The average number of surface defects can be set, for example, relative to the average of 20 or more coils.

[0122] Example

[0123] (Example 1)

[0124] With a slide rail width of 50mm and having Figure 2 A steel billet 1 is heated in a furnace with multiple slide rails 2 arranged at the shown slide rail intervals. The steel billet 1, by mass%, contains: C: 0.035%–0.055%, Si: 3.0%–3.4%, Mn: 0.07%, Al: 0.005%–0.008%, N, O, S + 0.405 × Se: each less than 0.0060%, with the remainder consisting of Fe and unavoidable impurities, and contains no inhibitory components. The billet 1 has a thickness of 200 mm. It should be noted that… Figure 2 The sum of the values ​​in the table represents the distance from the center of slide rail D or the center of slide rail C to the end of billet 1 along its length.

[0125] Inside the heating furnace, either Mode A or Mode B was used as the heating mode. In Mode A, the billet 1 was heated to 1150°C upstream of the shifting slide rail 3 (hereinafter referred to as "before shifting") and to 1250°C downstream of the shifting slide rail 3 (hereinafter referred to as "after shifting"). In Mode B, the billet 1 was heated to 950°C before shifting and to 1200°C after shifting. Furthermore, a heat-resistant gas suction pipe was installed inside the heating furnace, and a mechanism for continuously measuring oxygen concentration was installed. By controlling the amount of N2 gas supplied to the positions before and after shifting, the oxygen concentration fluctuations during furnace opening and closing and burner combustion efficiency changes were controlled in real time. Additionally, under certain conditions (No. 13 in Table 1), experiments were conducted to investigate the instantaneous increase in oxygen concentration inside the heating furnace due to atmospheric intrusion during furnace opening and closing. It should be noted that in Table 1, the values ​​in parentheses for the average oxygen concentration in the furnace before and after the relocation of No. 13 represent the values ​​after the instantaneous opening and closing of the heating furnace. For the temperature of billet 1, billet 1 equipped with a thermocouple was heated in the heating furnace. Based on an evaluation of the heat input to billet 1, a state that could be calculated numerically using the furnace temperature was determined, and experiments were conducted using this calculated billet temperature as a reference. After being removed from the heating furnace, the third and fourth passes of the four-pass rough rolling were performed under the conditions shown in Table 1. Next, multiple passes of finish hot rolling were performed in the temperature range of 850℃ to 950℃ to achieve a thickness of 2.0 mm. Furthermore, during the finish hot rolling, width expansion was minimized by applying appropriate tension to the steel sheet between the rolls. Twenty hot-rolled coils manufactured under identical conditions were produced.

[0126] From the obtained hot-rolled coil, a sample was cut from the position corresponding to the location in the billet 1 supported by the slide rail 2 (the contact position between the billet 1 and the slide rail 2), using the end along the length direction as a reference point. This sample was pickled at 80°C in 5% HCl for 120 seconds to remove the surface oxide scale. After drying, it was heat-treated at 180°C for 1 minute. After 7 days, the number of locally rusted areas was determined. The results are shown in Table 2.

[0127] In Table 2, the slide rail 2, which is the subject of this invention, is slide rail 2 indicated by "c" before displacement and slide rail 2 indicated by "C and D" after displacement. In the comparative example, there are more than 2.5 surface defects per slide rail, which is relatively high. In contrast, for the inventive example, the number of surface defects is less than 0.3 per slide rail, which is relatively low. When the oxygen concentration is less than 3.0% by volume, the number of surface defects is less than 0.1 per slide rail, which can generally prevent surface defects. It can be seen that the number of surface defects can be significantly reduced according to the present invention.

[0128]

[0129]

[0130] (Example 2)

[0131] The same as in Example 1 Figure 2 A heating furnace with a sliding rail configuration was used to heat billet 1 to 1150°C before displacement and to 1250°C after displacement. The billet 1 was then hot-rolled under the conditions shown in Table 3. Billet 1 contained 0.04% C, 3.3% Si, and 0.05% Mn, and also contained the composition shown in Table 3, with a calculated γ phase fraction of less than 20 mol% across the entire temperature range. Twenty hot-rolled coils were produced under identical conditions from each of the several types. From ten of these coils, portions corresponding to the positions supported by the sliding rail 2 marked with the symbol "c" in billet 1 were cut out, and the amount of surface defects was evaluated using the same method as in Example 1. For the hot-rolled coils (ten coils each) for which no samples were collected, hot-rolled sheet annealing was performed at a temperature of 1020°C. Then, each billet was divided into a two-rolling method (where the final sheet thickness was achieved through two rolling processes) and a one-rolling method (where the final sheet thickness was achieved through one rolling process). For the two-rolled billets, a single cold rolling process was performed at 100°C on a reversible mill to a thickness of 1.7 mm. After reaching the target thickness, an intermediate annealing was carried out at 900°C for 1 minute, followed by a second reversible cold rolling process with a coiling aging treatment at 200°C during the process, resulting in a thickness of 0.22 mm. Additionally, billets from the single-rolled process were rolled to a thickness of 0.26 mm using a tandem mill. The occurrence of fractures in each billet (10 coils) during the rolling line was evaluated. For the cold-rolled plates reaching the final thickness, a recrystallization anneal was performed at a heating rate of 300°C / second from 550°C to 680°C, a soaking temperature of 840°C, and a soaking time of 60 seconds. An annealing separating agent of 95% MgO and 5% TiO2 was applied as an aqueous slurry to the steel plate surface for the second recrystallization annealing. The surface of the annealed finished sheet obtained in this way was coated with a coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3, and baked at 800°C. The magnetic properties of the central portion of the width of the obtained product sheet roll were also confirmed. It should be noted that in Table 3, without "other components", if B8 (magnetic flux density at a magnetization force of 800 A / m) is 1.910 or higher, the magnetic properties can be judged as good; with "other components", if B8 is 1.915 or higher, the magnetic properties can be judged as good.

[0132]

[0133] As shown in Table 3, the manufacturing stability of the invention is improved, and good magnetic properties are also obtained.

[0134] Industrial availability

[0135] According to the present invention, hot-rolled coils with fewer surface defects can be obtained.

[0136] Symbol Explanation

[0137] 1. Steel billet

[0138] 2 Slide rails

[0139] 3. Shifting slide rail

Claims

1. A hot rolling method, characterized in that a steel billet is heated in a heating furnace and then hot rolled, wherein... The temperature of the steel billet inside the heating furnace is within the temperature range T of 950℃ to 1150℃. When heating a steel billet with a γ phase fraction of less than 20 mol% at the median of the temperature range T, i.e., 1050 °C, in a heating furnace where the distance between the multiple slide rails supporting the steel billet exceeds 1.1 m, The average oxygen concentration in the heating furnace within the temperature range T is set to below 5.0% by volume.

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

3. The hot rolling method according to claim 1 or 2, wherein, The hot rolling includes two consecutive hot rolling passes, each pass being carried out in a temperature range of 1030℃ to 1150℃, with a reduction rate of less than 50% and a strain rate of more than 15 / second, and the time between the two passes being more than 15 seconds.

4. A method for manufacturing an orientation-oriented electromagnetic steel sheet, comprising hot rolling a steel billet by any one of the hot rolling methods described in claims 1 to 3, performing hot rolling plate annealing on the obtained hot-rolled coil, performing one or more cold rolling processes with intermediate annealing, followed by arbitrary decarburization annealing, and finally performing final product annealing to obtain an orientation-oriented electromagnetic steel sheet.

5. A hot-rolled coil for oriented electromagnetic steel sheet, characterized in that, It is obtained by hot rolling a steel billet using the hot rolling method described in any one of claims 1 to 3. Relative to the position on the slide rail during billet heating, the average number of surface defects within a range L0 before and after the hot rolling direction is less than 0.3, where L0 is in meters. Where L0 is defined as the width of the slide rail as X, the thickness of the steel billet as Y1, and the thickness of the hot-rolled coil of the oriented electromagnetic steel plate as Y2, derived from the following formula (1), where the units of Y1 and Y2 are meters. L0(m)={0.15(m)+X(m)}×Y1(mm) / Y2(mm) (1).

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