MANUFACTURING METHOD AND MANUFACTURING FACILITY FOR ORIENTED ELECTROMAGNETIC STEEL SHEET

Abstract

 Provided is a method of producing a grain-oriented electrical steel sheet that can greatly improve manufacturability, and a production line that can realize this method. In the method of producing a grain-oriented electrical steel sheet, a steel slab having a defined chemical composition is hot-rolled, followed by hot-rolled sheet annealing, followed by cold rolling once or twice or more from a thickness of an obtained hot-rolled sheet to a thickness of a product for a total rolling reduction of 80 % or more, followed by primary recrystallization annealing, followed by application of an annealing separator to the steel sheet surface, followed by final annealing and flattening annealing for flattening. In this method, an occupancy rate of carbides relative to grain boundaries of recrystallized grains in the hot-rolled sheet after the hot-rolled sheet annealing, before cold rolling, is 80 % or more, and an initial rolling reduction in the cold rolling has a strain rate of 200/s or less, a rolling reduction of 30 % or less, and a steel sheet temperature at roll bite of 90 °C or lower.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of producing a grain-oriented electrical steel sheet and a production line.

BACKGROUND

[0002] A grain-oriented electrical steel sheet is a steel sheet that has excellent magnetic properties and a crystalline microstructure in which the <001> orientation, which is the easy magnetization axis of iron (Goss orientation), is highly concentrated in the rolling direction of the steel sheet. One method proposed for improving the magnetic properties of grain-oriented electrical steel sheets is to control the form of C in the steel by controlling a cooling process after annealing, before final cold rolling.

[0003] For example, in Patent Literature (PTL) 1, a technique is proposed for precipitating fine carbides having a particle size of 100 angstrom to 500 angstrom by subjecting an annealed steel sheet to rapid cooling and aging treatment under specific conditions. Further, in PTL 2, a technique is proposed for increasing solute C by cooling the annealed steel sheet at a cooling rate of 150 °C/min or more in a temperature range from 600 °C to 300 °C.

[0004] The techniques proposed in PTL 1 and 2 control carbon in steel as very fine carbides or solute C, and when dislocations are introduced during cold rolling, the solute C or the like is pinned to the dislocations to form a Cottrell atmosphere, thereby promoting non-uniform deformation during cold rolling, modifying the cold-rolled texture, and improving the texture after primary recrystallization.

[0005] This effect is also known as a method for increasing the {110} strength in the texture after recrystallization in annealing after cold rolling in typical steel. In grain-oriented electrical steel sheets, the {110}<001> orientation is ultimately accumulated using a metallurgical phenomenon known as secondary recrystallization, and the {110} microstructure can act as a good nucleus for secondary recrystallization. Therefore, in grain-oriented electrical steel sheets, a technique of forming carbides within crystal grains is extremely common.

CITATION LIST

Patent Literature

[0006] 

PTL 1: JP S58-157917 A

PTL 2: JP S52-094825 A

SUMMARY

(Technical Problem)

[0007] In recent years, due to the need for energy conservation, there has been a demand for electrical steel sheets that have lower iron loss. Products that meet these needs are actually being produced by thinning steel sheets and magnetic domain refining.

[0008] It is known that Si added to steel has an effect of increasing the electrical resistance of the steel, and decreases Joule heat generated during use of an electrical steel sheet, thereby greatly contributing to decreasing iron loss. As a result, an electrical steel sheet containing a large amount of Si in the steel can achieve decreased iron loss. However, Si is also known as an element that embrittles steel, and it is typically very difficult to work steel that has a Si content of more than 4.0 mass% by rolling.

[0009] Typically, steel material that has a large amount of alloy added has high strength and is often difficult to roll. Further, as mentioned above, Si promotes embrittlement of the material, and therefore even a content of some mass% can cause fracture problems. For this reason, various techniques are often employed in the production of electrical steel sheets, such as using rolling mills that have high rigidity, reverse mills instead of tandem mills, or warm rolling at high temperatures to soften the material.

[0010] On the other hand, in order to provide flexibility in production, it is desirable to be able to produce using not only rolling mills suitable for producing electrical steel sheets, but also various other rolling mills.

[0011] Accordingly, the inventors have investigated decreasing the rolling reduction per pass from the viewpoint of decreasing loads imposed on the rolling mill and rollers during rolling. As a result, an unexpected problem was faced: fracture was more likely to occur in a range of relatively slow rolling speeds.

[0012] The present disclosure is made in view of the above-mentioned problems, and it would be helpful to provide a method of producing a grain-oriented electrical steel sheet that can greatly improve manufacturability, and a production line for a grain-oriented electrical steel sheet that can realize the method.

(Solution to Problem)

[0013] In order to solve the above-mentioned problem, the inventors first conducted a study by investigating materials that fractured after the first pass of hot rolling. As a result, it was confirmed that a plurality of deformation twins were formed in the fractured material, and that the twins propagated through grain boundaries, affecting adjacent crystals.

[0014] Twins are known to cause material embrittlement by intersecting with other twins or by interacting with dislocations. Therefore, the inventors considered that suppressing the formation of such twins would be particularly effective in suppressing fracture.

[0015] Twins are normally formed so that the atoms that make up the material are mirror images of each other across a twin boundary, so are unlikely to form when dislocations are introduced into the crystal and there is a lot of strain in the crystal lattice. Under the conditions under which fracture occurred, a low rolling reduction was used to decrease the rolling load, and at the same time, the rolling speed was relatively slow to improve stability during sheet passing through the rolling mill. The inventors presumed that because the rolling was carried out under such conditions, a large number of deformation twins may have been formed before plastic deformation due to dislocations occurred. However, these rolling conditions were set so that rolling can be carried out not only with rolling mills designed for producing electrical steel sheets, but also with many typical rolling mills. Therefore, methods such as increasing the rolling reduction, which increases the load on the rolling mill, and methods such as using a high rolling speed are not preferable countermeasures.

[0016] Accordingly, the inventors recognized that completely suppressing the formation of twins is difficult, and decided to carry out research and develop a policy to decrease the amount of twins generated, by not allowing twins to propagate to other adjacent crystals, even when twins are generated. As a result, since the propagation of twins proceeds across grain boundaries, the inventors came up with the idea of precipitating carbides at grain boundaries as a way to give grain boundaries the function of barriers to propagation of twins. Experiments that led to the above discoveries are now described.

<Experiment 1>

[0017] A steel slab for a grain-oriented electrical steel sheet (hereinafter also referred to simply as "steel slab") was prepared, having a chemical composition of C: 0.03 mass%, Si: 4.2 mass%, Mn: 0.1 mass%, sol.Al: 0.02 mass%, S: 50 ppm, Se: 100 ppm, and N: 60 ppm, with the balance being Fe and other elements each decreased to less than 60 ppm. The prepared steel slab was heated at 1380 °C and then hot rolled to obtain a hot-rolled coil having a thickness of 2.5 mm (hereinafter also referred to as "hot-rolled coil"). Test pieces were cut out from the obtained hot-rolled coil, and hot-rolled sheet annealing was carried out on the test pieces in a laboratory experiment furnace to an end-point temperature of 980 °C, and an experiment was conducted in which cooling after hot-rolled sheet annealing was controlled.

[0018] First, with reference to Non-Patent Literature (NPL) 1 (New Edition of Steels and Alloying Elements, p. 395), conditions considered to be capable of precipitating carbide (Fe₃C) at grain boundaries were determined to be: a cooling rate of 1.5 °C/s in a temperature range from 700 °C to 600 °C, and a residence time in the temperature range of 1 minute or longer.

[0019] A portion was cut out from the obtained hot-rolled and annealed sheet so that a cross-section in the direction orthogonal to the rolling direction could be observed. The cutout was etched with nital, and then a mid-thickness part was continuously observed with a scanning electron microscope (SEM) over a range of 500 µm in the thickness direction and 1 mm in the direction orthogonal to the rolling direction (sheet transverse direction). As a result, it was found that carbides were precipitated in 85 % of the grain boundaries within the observed field of view. When etching is carried out with nital, the steel portion is etched, but the carbides remain unetched. Therefore, when carbides are formed on grain boundaries, the film-like carbides are observed to have a different contrast from the steel matrix. In addition, by analyzing the same region using a high-resolution electron probe microanalyzer (EPMA), it was found that it was possible to grasp the state of carbon concentration on the grain boundaries and to quantify how much of the total grain boundary length in the observation field was covered by carbides. Further, in the cooling after the hot-rolled sheet annealing, almost no carbides precipitate in a temperature range from the hot-rolled sheet annealing temperature to 700 °C. Therefore, after cooling at an arbitrary cooling rate (for example, 20 °C/s) in the temperature range from the hot-rolled sheet annealing temperature to 700 °C, the cooling conditions were changed so that a constant cooling rate was maintained in the temperature range from 700 °C to 600 °C, and the residence time in the temperature range from 700 °C to 600 °C was changed. The cooling rate in a temperature range of 600 °C or less was set to 50 °C/s. The steel sheets thus obtained were evaluated for Fe₃C, which is a carbide precipitated on grain boundaries, and the samples indicated in Table 1 were obtained. Table 1 indicates a relationship between the residence time in the temperature range from 700 °C to 600 °C and the grain boundary occupancy rate of carbides.

[0020] [Table 1]

Table 1

Residence time in temperature range from 700 °C to 600 °C (s)	Grain boundary occupancy rate of carbides (%)
2	2
3	4
5	5
10	10
50	70
67	85
80	100

<Experiment 2>

[0021] A steel slab was prepared having a chemical composition of C: 0.02 mass%, Si: 4.8 mass%, Mn: 0.3 mass%, and sol.Al: 0.005 mass%, with the balance being Fe and impurity elements such as S, N, Se, and O, each of which was decreased to 50 ppm or less. The prepared steel slab was heated at 1150 °C and then hot rolled to obtain a hot-rolled coil having a thickness of 2.5 mm. Test pieces were cut out from the obtained hot-rolled coil and subjected to hot-rolled sheet annealing in a laboratory experiment furnace, with an end-point temperature of the test pieces being 990 °C. Rapid cooling was carried out at 50 °C/s in the temperature range from 700 °C to 600 °C. Hot-rolled sheet annealing was then carried out, controlling to change the cooling rate in the temperature range from 600 °C to 500 °C after the hot-rolled sheet annealing, thereby changing the residence time in the temperature range from 600 °C to 500 °C. After the hot-rolled sheet annealing, since carbides do not precipitate at grain boundaries in the temperature range of 700 °C or higher, any cooling pattern may be used, but cooling was carried out at 30 °C/s, and after the residence time in the temperature range from 600 °C to 500 °C, rapid cooling was carried out at 60 °C/s. The grain boundary occupancy rate of carbides after annealing was quantitatively evaluated by the SEM observation mentioned above. Table 2 indicates a relationship between the residence time in the temperature range from 600 °C to 500 °C and the grain boundary occupancy rate of carbides. Comparing the results in Table 1 and Table 2, it can be seen that, as described in NPL 1, precipitation of grain boundary carbides is more likely to proceed during retention in the temperature range from 600 °C to 500 °C. Carbide precipitation also progresses at temperatures exceeding 600 °C. However, considering that it is not realistic to cool for more than one minute during actual production, it can be said that controlling the residence time in the temperature range from 600 °C to 500 °C, in particular, is extremely important.

[0022] [Table 2]

Table 2

Residence time in temperature range from 600 °C to 500 °C (s)	Grain boundary occupancy rate of carbides (%)
4	20
7	50
10	80
50	90

[0023] The obtained samples were each subjected to cold rolling with a first pass rolling reduction of 20 % and a strain rate of 150/s, and then to rolling by a plurality of passes to a sheet thickness of 1.0 mm. At this time, it was found that there was a certain probability of crack defects occurring, such as partial cracks occurring in the sheet, even when the sheet did not fracture completely. Here, the number of samples that were actually rolled for the experimental conditions was used as the denominator, and the number of samples in which crack defects occurred was used to calculate a crack occurrence probability, and the results illustrated in FIG. 1 were obtained.

[0024] As illustrated in FIG. 1 , the inventors have found that, in order to prevent fracture trouble that occurs during cold rolling under specific rolling conditions of low rolling reduction and low strain rate, it is effective to set the occupancy rate of carbides to 80 % or more of the grain boundaries of recrystallized grains in a steel sheet before cold rolling, regardless of the cooling pattern. Further, when the materials that had cracked under relatively low pressure were observed, it was confirmed that many deformation twins had been formed. However, once the rolling reduction had progressed to a certain extent, the worked microstructure became complex and intricate due to dislocations, making it difficult to interpret this as a change in twin density.

[0025] The results obtained are considered to be extremely useful discoveries from the viewpoint of improving manufacturability. On the other hand, in the production of grain-oriented electrical steel sheets, carbide control for texture control is also important. When the material is held at a high temperature for a long time, where the diffusion rate is high, and grain boundary precipitation proceeds, the carbon concentration present in crystal grains of course decreases greatly.

[0026] Therefore, the inventors have conducted intensive studies on how to increase the carbide occupancy rate at grain boundaries while retaining as much carbon as possible in the crystal grains, by maintaining the temperature at which carbides are formed at the grain boundaries for only the time required for nucleation, and carrying out a stage in which the precipitates grow at a low temperature at which the diffusion rate is as slow as possible, and have completed the present disclosure.

[0027] In order to solve the technical problems described above, the following are provided:

[1] A method of producing a grain-oriented electrical steel sheet, the method comprising: hot rolling a steel slab containing, in mass%, C: 0.01 % or more and 0.10 % or less, Si: 2.0 % or more and 6.5 % or less, and Mn: 0.01 % or more and 0.5 % or less; followed by hot-rolled sheet annealing; then cold rolling once or twice or more times from a thickness of an obtained hot-rolled sheet to a thickness of a product for a total rolling reduction of 80 % or more; then primary recrystallization annealing; then applying an annealing separator to a steel sheet surface; then final annealing; then flattening annealing for flattening, wherein,
after the hot-rolled sheet annealing an occupancy rate of carbides relative to grain boundaries of recrystallized grains in the hot-rolled sheet before the cold rolling is 80 % or more, and an initial rolling reduction in the cold rolling has a strain rate of 200/s or less, a rolling reduction of 30 % or less, and a steel sheet temperature at roll bite of 90 °C or lower.

[2] The method of producing a grain-oriented electrical steel sheet according to [1], wherein intermediate annealing is carried out one or more times between the cold rolling when cold rolling the twice or more times.

[3] The method of producing a grain-oriented electrical steel sheet according to [1] or [2], wherein during cooling after the hot-rolled sheet annealing, a residence time in a temperature range from 600 °C to 500 °C is 10 s or longer.

[4] The method of producing a grain-oriented electrical steel sheet according to [1] or [2], wherein, during cooling after the hot-rolled sheet annealing, a residence time in a temperature range from 600 °C to 500 °C is 3 s or longer and shorter than 10 s, an average cooling rate in a temperature range from 500 °C to 200 °C is 10 °C/s or less, and a cooling rate until coiling is 15 °C/s or more.

[5] The method of producing a grain-oriented electrical steel sheet according to any one of [1] to [4], wherein a heating rate in the primary recrystallization annealing is 200 °C/s or more in a temperature range from 550 °C to 680 °C.

[6] The method of producing a grain-oriented electrical steel sheet according to any one of [1] to [5], wherein the steel slab further contains, in mass%, sol.Al: 0.010 % or more and 0.050 % or less, N: 0.004 % or more and 0.015 % or less, and S+0.4Se: 0.010 % or more and 0.050 % or less.

[7] The method of producing a grain-oriented electrical steel sheet according to any one of [1] to [5], wherein the steel slab further contains sol.Al: less than 0.010 mass%, and each of the elements S, N, and O: 60 ppm or less.

[8] The method of producing a grain-oriented electrical steel sheet according to any one of [1] to [7], wherein the steel slab further contains, in mass%, one or more selected from the group consisting of Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, Co: 0.0001 % or more and 0.0100 % or less, Ga: 0.0001 % or more and 0.0100 % or less, Zn: 0.0001 % or more and 0.500 % or less, Bi: 0.0005 % or more and 0.0200 % or less, Pb: 0.001 % or more and 0.3 % or less, Ge: 0.001 % or more and 0.3 % or less, As: 0.001 % or more and 0.3 % or less, and Ag: 0.001 % or more and 0.3 % or less.

[9] A production line for a grain-oriented electrical steel sheet, used in producing a grain-oriented electrical steel sheet, the production line comprising: a plurality of cooling sections configured to cool a steel strip that has been annealed to a temperature of 700 °C or less in a plurality of stages; a thermometer provided to each of the cooling sections at least at a position partway through or on a delivery side, configured to measure a temperature of the steel strip; a controller configured to control a cooling rate in each of the cooling sections by using the temperatures measured by the thermometers for feedback control; and at least one coolant remover provided between the cooling sections configured to remove coolant from the steel strip, wherein the production line is capable of setting a coiling temperature to 100 °C or less.

[10] The production line for a grain-oriented electrical steel sheet according to [9], wherein the cooling sections include a first cooling section configured to control a residence time of the steel strip in a temperature range from 600 °C to 500 °C to 3 s or longer and shorter than 10 s, and a second cooling section configured to control a residence time of the steel strip in a temperature range from 500 °C to 200 °C to 30 s or longer.

(Advantageous Effect)

[0028] According to the present disclosure, it is possible to provide a method of producing a grain-oriented electrical steel sheet that can greatly improve manufacturability, and a production line that can realize this method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In the accompanying drawings:

FIG. 1 is a diagram illustrating a relationship between a grain boundary occupancy rate of carbides and a crack occurrence probability; and

FIG. 2 is a schematic diagram of an example of a production line for a grain-oriented electrical steel sheet according to the present disclosure.

DETAILED DESCRIPTION

(Method of producing a grain-oriented electrical steel sheet)

[0030] The following describes embodiments of the present disclosure. The method of producing a grain-oriented electrical steel sheet according to the present disclosure is a method comprising: hot rolling a steel slab containing, in mass%, C: 0.01 % or more and 0.10 % or less, Si: 2.0 % or more and 6.5 % or less, and Mn: 0.01 % or more and 0.5 % or less; followed by hot-rolled sheet annealing; then cold rolling once or twice or more times from a thickness of an obtained hot-rolled sheet to a thickness of a product for a total rolling reduction of 80 % or more; then primary recrystallization annealing; then applying an annealing separator to a steel sheet surface; then final annealing; then flattening annealing for flattening. Here, after the hot-rolled sheet annealing, before the cold rolling, an occupancy rate of carbides relative to grain boundaries of recrystallized grains in the hot-rolled sheet is 80 % or more, and an initial rolling reduction in the cold rolling has a strain rate of 200/s or less, a rolling reduction of 30 % or less, and a steel sheet temperature at roll bite of 90 °C or lower.

[Steel slab]

[0031] According to the present disclosure, a steel slab for a grain-oriented electrical steel sheet is used as a starting material. First, a chemical composition of the steel slab is explained. In the following explanation of the chemical composition, "%" represents "mass%" and "ppm" represents "mass ppm" unless otherwise specified.

[0032] The present disclosure is advantageous in improving manufacturability when rolling at low pressure and low speed is required to utilize a rolling mill that does not have mill rigidity, load capacity, or apparatus for carrying out warm rolling, or when rolling at low pressure and low speed is required due to processing requirements. Therefore, many other components and production processes can be adopted that are similar to those used in the production of typical grain-oriented electrical steel sheets. However, the contents of C, Si, and Mn are limited for the following reasons.

C: 0.01 % or more and 0.10 % or less

[0033] C is an essential element for precipitating carbides on grain boundaries and improving texture. However, when C content exceeds 0.10 %, decarburization in final decarburization annealing becomes difficult, and carbon remaining in the product becomes a cause of iron loss deterioration known as magnetic aging. The C content is therefore 0.10 % or less. Further, when the C content is less than 0.01 %, various precipitation controls cannot be carried out sufficiently. The C content is therefore 0.01 % or more. From the viewpoints of manufacturability and magnetic properties, the C content is preferably 0.02 % or more. The C content is preferably 0.06 % or less.

Si: 2.0 % or more and 6.5 % or less

[0034] Si is a useful element that decreases iron loss by increasing electrical resistance. In order to obtain good magnetic properties, the Si content needs to be 2.0 % or more. On the other hand, Si is also an element that increases the brittleness of steel. When the Si content exceeds 4.5 %, the risk of fracture during sheet passing through a production line increases and cold rolling manufacturability degrades greatly. However, according to the present disclosure, Si can provide an effect of suppressing cracking, and therefore the Si content can be made greater than normal. However, magnetostriction properties required along with iron loss saturate at 6.5 %, and therefore adding more than this amount does not produce a significant improvement in the magnetic properties. The Si content is therefore 6.5 % or less. Even when a risk reduction effect during passing through a production line is taken into consideration, the risk does not become zero, and therefore the Si content is preferably 2.8 % or more. The Si content is preferably 4.5 % or less.

Mn: 0.01 % or more and 0.5 % or less

[0035] Mn is a useful element from the viewpoint of controlling formation of an oxide coating during primary recrystallization, but when content is less than 0.01 %, no effect can be obtained from the viewpoint of controlling the formation of the oxide coating. Mn content is therefore 0.01 % or more. On the other hand, Mn also has an effect of improving hot workability during production, but when the Mn content exceeds 0.5 %, the primary recrystallized texture deteriorates, leading to degradation of magnetic properties. The Mn content is therefore 0.5 % or less.

[0036] Other typical compositions are as follows: According to the present disclosure, it suffices that the steel slab has a chemical composition that allows a grain-oriented electrical steel sheet to be obtained by sequentially carrying out known processes, namely, hot rolling, hot-rolled sheet annealing, cold rolling to a final sheet thickness in one pass, decarburization annealing (which also serves as primary recrystallization annealing), and final annealing (which also serves as secondary recrystallization annealing and purification annealing). Therefore, it is possible to use a composition that utilizes an inhibitor component to develop secondary recrystallized grains, or, as indicated in PTL 3 and the like, it is also possible to develop secondary recrystallized grains without using a precipitation inhibitor (AlN, MnS, MnSe, or the like). Depending on the type of inhibitor component, preferred content is as follows.

<When using an inhibitor component>

[0037] 

sol.Al: 0.010 % or more and 0.050 % or less

N: 0.004 % or more and 0.015 % or less

S+0.4Se: 0.010 % or more and 0.050 % or less

When sol.Al content is less than 0.010 %, the magnetic flux density of a produced grain-oriented electrical steel sheet decreases. On the other hand, when the sol.Al content exceeds 0.050 %, secondary recrystallization becomes unstable. The sol.Al content is therefore preferably 0.010 % or more. The sol.Al content is therefore preferably 0.050 % or less.

[0038] When N content is less than 0.004 %, AlN does not precipitate appropriately during processing, making it difficult to control grain size. Further, when the N content exceeds 0.015 %, it causes frequent occurrence of surface defects called blisters. The N content is therefore preferably 0.004 % or more. The N content is therefore preferably 0.015 % or less. The N content can be changed as needed by applying a nitriding process during production, and in many cases, sufficient precipitates can be formed with a N content of 0.010 % or less.

S+0.4Se: 0.010 % or more and 0.050 % or less

[0039] The absolute amounts of Se and S as inhibitor components are insufficient when S content + 0.4 × Se content is less than 0.010 %. On the other hand, when S+0.4Se exceeds 0.050 %, purification of the steel during final annealing becomes difficult. S+0.4Se is therefore preferably 0.010 % or more. S+0.4Se is therefore preferably 0.050 % or less. S and Se can be used as inhibitors in the form of MnSe and MnS, respectively, or as a compound thereof, Mn(S,Se). Further, AlN inhibitor and the MnSe and/or MnS inhibitor can coexist, thereby providing a synergistic effect.

<When no precipitation inhibitor component is contained>

[0040] 

sol.Al: less than 0.010 %

S: 60 ppm or less

N: 60 ppm or less

O: 60 ppm or less

When no precipitation inhibitor component is contained, the content of sol.Al, S, and O, which are elements that form precipitation inhibitors, is limited to extremely low levels. Specifically, sol.Al is limited to less than 0.010 %, S is limited to 60 ppm or less, and O is limited to 60 ppm or less. When these amounts are exceeded, it becomes difficult to obtain the secondary recrystallized microstructure due to an effect of texture inhibition.

[0041] It is desirable that the N content be 60 ppm or less in order to help prevent the formation of silicon nitrides after purification annealing. Further, it is preferable to decrease the content of each of the nitride-forming elements Ti, Nb, B, Ta, and V to 0.050 % or less. This is so that the action of texture inhibition is not interfered with, thereby helping prevent greater iron loss.

[0042] The content of inhibitor components is as described above, but by adding grain boundary segregation type elements to an inhibitor component, the magnetic properties can be improved. The elements may be contained in ranges of Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, Co: 0.0001 % or more and 0.0100 % or less, Ga: 0.0001 % or more and 0.0100 % or less, Zn: 0.0001 % or more and 0.500 % or less, and Bi: 0.0005 % or more and 0.0200 % or less.

[0043] Further, Pb, Ge, As, Ag, and the like may each be contained in a range of 0.001 % or more and 0.3 % or less. These elements can be used alone or in combination, thereby decreasing iron loss.

[0044] The steel slab that is the starting material having the chemical composition described above is heated at an appropriate temperature according to the chemical composition, and then subjected to hot rolling including rough rolling and finish rolling to produce a hot-rolled sheet. When a precipitation inhibitor component is contained, the steel slab is heated to a temperature range of 1350 °C to 1450 °C in order to completely dissolve Al, Se, S, and the like in solid solution. On the other hand, in the case of a chemical composition that does not contain a precipitation inhibitor component, when the heating temperature of the steel slab is too high, the inhibitor-forming components that have dissolved during heating will precipitate unevenly as fine grains during hot rolling, which will locally suppress grain boundary migration, resulting in an extremely uneven grain size distribution and inhibiting the development of secondary recrystallized grains in the Goss orientation. Therefore, a relatively low heating temperature is preferably adopted, for example, 1250 °C or lower. The hot rolling conditions are not particularly limited, and may be conditions normally employed for producing grain-oriented electrical steel sheets.

[0045] The hot-rolled sheet obtained as described above is subjected to hot-rolled sheet annealing. In the hot-rolled sheet annealing, it is preferable to carry out soaking treatment at a temperature of 800 °C or higher and 1150 °C or lower for 20 s or longer in order to homogenize the hot-rolled microstructure. The effect of the present disclosure in suppressing cracking was greater when the maximum annealing temperature was lower than 1000 °C. Although the mechanism is not clear, when hot-rolled sheet annealing is carried out at a low temperature of lower than 1000 °C, recrystallization nuclei are not formed, and relatively coarse structures resulting from hot rolling remain at mid-thickness, which may make the sheet more susceptible to developing cracks, and it is believed that this is why the crack suppression effect of the present disclosure is exerted.

[0046] Next, in the cooling carried out after the hot-rolled sheet annealing, one of the following controls is required from the viewpoint of carbide control.

1) The residence time in the temperature range from 600 °C to 500 °C is 10 s or longer.
At higher temperatures, the driving force for carbide precipitation decreases, and therefore longer holding times may be required. By holding the temperature in the above temperature range for 10 s or longer, carbides can be formed on grain boundaries. However, the material is maintained at a high temperature, and therefore most of the carbon present in the crystal grains diffuses to grain boundaries, resulting in a low carbon concentration in the crystal grains. As described above, under specific rolling conditions, the crack occurrence probability can be decreased by increasing the grain boundary occupancy rate of carbides, thereby achieving the effect of improving manufacturability. On the other hand, the utilization of carbides in crystal grains is limited and therefore, when these conditions are used, the texture obtained after primary recrystallization annealing may be inferior, and the magnetic properties of the product may be slightly inferior. However, these conditions allow productivity to be increased relatively easily, and therefore can be applied when a higher level of magnetic properties is not required. Although there is no particular upper limit to the residence time, the effect of suppressing cracking saturates even when the residence time is long, and a short residence time is desirable from the viewpoint of magnetic properties. The residence time is therefore preferably 15 s or shorter.

2) The residence time in the temperature range from 600 °C to 500 °C is 3 s or longer and shorter than 10 s, and an average cooling rate in the temperature range from 500 °C to 200 °C or lower is 10 °C/s or less (that is, the residence time is 30 s or longer), and a cooling rate until coiling is 15 °C/s or more.
By setting the residence time in the temperature range from 600 °C to 500 °C to 3 s or longer, carbide nuclei can be formed on grain boundaries. When the residence time in the above temperature range is less than 3 s, grain boundary precipitation does not proceed sufficiently. On the other hand, when the residence time in the above temperature range is 10 s or more, precipitation at grain boundaries may proceed too much, and even when the cooling rate at 500 °C and lower is controlled, an appropriate carbide morphology may not be possible to obtain. That is, there is a risk that the texture improving effect will not be obtained due to a decrease in the required amount of carbon in grains. The residence time in the above temperature range is therefore preferably 3 s or longer. The residence time in the above temperature range is therefore preferably shorter than 10 s.

[0047] The cooling rate is decreased in the temperature range from 500 °C to 200 °C, where the diffusion rate of carbon is extremely slow compared to the temperature range from 600 °C to 500 °C, and the residence time is 30 s or more. By using such a heat pattern, it is possible to promote the diffusion of carbon in crystal grains to grain boundaries to some extent, while at the same time retaining a certain amount of carbon in the crystal grains. After the grain boundary precipitation has been appropriately carried out, the cooling rate is increased again to suppress the diffusion of carbon in the crystal grains and so that a precipitation state of carbides in the crystal grains is also appropriate. After at least a retention treatment for grain boundary precipitation has been carried out, the cooling rate needs to be increased again to a cooling rate of 15 °C/s or more. This makes it possible to improve manufacturability while minimizing the effect on the texture. As a result, after cooling, the steel strip has carbides formed on grain boundaries, and at the same time, a thickness of a carbide depleted zone (depleted layer) formed in the vicinity of the grain boundaries can be decreased to 20 % or less of the grain size.

[0048] In particular, to realize the heat pattern indicated under 2), cooling sections following a continuous annealing furnace in a grain-oriented electrical steel sheet production line require the following mechanisms, as illustrated in the schematic diagram in FIG. 2.

1. A plurality of cooling sections (two cooling sections 1 and 2 in the example of FIG. 2) is provided for cooling the steel strip in a plurality of stages in a temperature range where the temperature of the steel strip is 600 °C or lower. In a temperature range from the hot-rolled sheet annealing temperature to 700 °C, the carbide Fe₃C hardly precipitates, and in a temperature range of 600 °C or higher, although precipitation progresses, the precipitation rate is very slow. Therefore, although cooling to 600 °C after annealing is not particularly limited from the viewpoint of carbide control, when the cooling rate is unnecessarily slowed, the required line length will increase. Therefore, it is sufficient to achieve a typical cooling rate, for example, 5 °C/s to 40 °C/s.

2. In the cooling section that cools the steel strip in the temperature range from 600 °C to 500 °C (cooling section 1 in the example of FIG. 2), a thermometer (thermometer 1 in the example of FIG. 2) capable of measuring the steel strip temperature is provided partway through the cooling section or on the delivery side.

3. A mechanism (controller) is provided that measures a steel strip temperature using the thermometers described under 2, above, feedback-controls the steel strip temperature, controls the cooling rate in the temperature range from 600 °C to 500 °C, and controls the residence time in the temperature range to 3 s or longer and shorter than 10 s.

4. In the temperature range from 500 °C to 200 °C, the temperature gradually decreases even when the strip is allowed to naturally cool. Therefore, at the delivery side of the cooling section responsible for cooling in the temperature range from 600 °C to 500 °C, coolant or the like remains on the steel strip and to prevent excessive cooling, at least one mechanism (coolant remover) is provided between the cooling sections to easily wipe and remove the coolant. The coolant remover is preferably provided before the cooling section where the cooling rate is greatly decreased. Further, coolant removers may be provided between all cooling sections.

5. A slow cooling section is provided that has a temperature holding function that secures a residence time of 30 s or longer in the temperature range from 500 °C to 200 °C, or a natural cooling section is provided without an active cooling function.

6. A cooling section that is after slow cooling or natural cooling and before a delivery coiling mechanism is provided (cooling section 2 in the example of FIG. 2), which is responsible for cooling the steel strip in a temperature range of 200 °C or less, and a thermometer (thermometer 2 in the example of FIG. 2) capable of measuring the steel strip temperature is provided partway through the cooling section or on the delivery side.

7. The steel strip temperature is measured using the thermometer described under 6, above, and the steel strip temperature is feedback-controlled to provide a mechanism for achieving a stable cooling rate of 15 °C/s or more in the cooling section. The steel strip in the cooling section is at a relatively low temperature, and therefore a plurality of functions may be provided, such as pickling after cooling.

8. When the coil is finally coiled at production line delivery, a function of being cooled to 100 °C or lower is provided.

[0049] Subsequently, the obtained hot-rolled and annealed sheet is subjected to cold rolling. At this time, the total rolling reduction from the thickness of the hot-rolled sheet to the thickness of the steel sheet as a finished product is 80 % or more. Such high reduction rolling puts a heavy burden on the rolling mill, and therefore the rolling conditions for the first pass by the rolling mill are a strain rate of 200/s or less and a rolling reduction of 30 % or less. The above rolling conditions are applied due to production and apparatus constraints, texture control, and other reasons, and are not necessarily recommended as optimal rolling conditions for producing grain-oriented electrical steel sheets. For example, apparatus constraints include a case where, although it is normally desired to achieve a final sheet thickness in one rolling pass without intermediate annealing, there is a limit to the rolling reduction per pass due to issues with the rolling load. Another example is a case where, when producing using a production method in which rolling is carried out twice or more times with intermediate annealing once or more, the rolling reduction per rolling pass is low, but there is no rolling mill that can adjust the number of passes like a reverse rolling mill, so a tandem mill has to be used, and the number of stands is determined by the specifications of the rolling mill, so the rolling speed in the first pass is inevitably slow.

[0050] When rolling is not carried out under such special conditions, it is not necessary to occupy the grain boundaries with carbides as specified in the present disclosure. Similarly, when the steel sheet temperature exceeds 90 °C on work roll bite during reduction in the first pass, when the steel sheet temperature is high, deformation due to dislocations is likely to occur, and twinning deformation is suppressed under these conditions, so there is no need to apply the present disclosure. There are no particular limitations on the cold rolling as long as the final sheet thickness is obtainable. If possible, as in many conventional techniques, the texture can be improved by carrying out warm rolling after the first pass, which utilizes the heat generated by working to increase the steel sheet temperature.

[0051] Subsequently, the final cold-rolled sheet is subjected to primary recrystallization annealing. The purpose of the primary recrystallization annealing is primary recrystallization of the cold-rolled sheet that has a rolled microstructure to adjust to an optimum primary recrystallized grain size for secondary recrystallization, and to decarburize carbon contained in the steel by making the annealing atmosphere wet hydrogen-nitrogen or wet hydrogen-argon atmosphere to form an oxide coating on the surface in the oxidizing atmosphere. Accordingly, the primary recrystallization annealing is carried out in a H₂ mixed atmosphere with a controlled dew point, at 750 °C or higher and 900 °C or lower. During the heating in the primary recrystallization annealing, the texture improving effect can be further enhanced by setting a heating rate in a temperature range from 550 °C to 680 °C to 200 °C/s or more.

[0052] An annealing separator is applied to the primary recrystallization annealed steel sheet surface. In order to form a forsterite film on the surface of the steel sheet after secondary recrystallization annealing, magnesia (MgO) is used as the main component of the annealing separator. The formation of the forsterite film can be further favored by adding an appropriate amount of Ti oxide, Sr compound, or the like to the separator. In particular, the addition of auxiliaries that promote uniform forsterite film formation is also advantageous for improving peeling properties.

[0053] This is followed by final annealing for secondary recrystallization and forsterite film formation. The annealing atmosphere may be any of N₂, Ar, H₂, or a mixture thereof. To more effectively carry out secondary recrystallization, the temperature can be maintained isothermally near the secondary recrystallization temperature. However, maintaining isothermally is not necessarily required, as a slow heating rate is also effective. Precipitation of trace components in the final product leads to degradation of magnetic properties, and therefore, for component purification, the maximum annealing temperature is preferably 1100 °C or higher.

[0054] After the final annealing, an insulating coating may be further applied to the steel sheet surface and baked. Such an insulating coating is not limited to a particular type, and any conventionally known insulating coating is applicable. For example, a preferred method is described in JP S50-79442 A and JP S48-39338 A, where a coating solution containing phosphate-chromate-colloidal silica is applied on a steel sheet and then baked at a temperature of around 800 °C.

[0055] Further, flattening annealing may be carried out to correct the shape of the steel sheet. This flattening annealing may also serve as an insulating coating baking treatment.

EXAMPLES

(Example 1)

[0056] A steel slab was prepared having a chemical composition of C: 0.05 mass%, Si: 3.2 mass%, Mn: 0.04 mass%, sol.Al: 0.0200 mass%, Se: 100 ppm, N: 100 ppm, S: 60 ppm, O: less than 50 ppm, with the balance being Fe and inevitable impurity. The prepared steel slab was heated to 1350 °C and then hot-rolled to produce a hot-rolled sheet having a thickness of 2.0 mm. Next, using a production line according to the present disclosure, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 990 °C for 30 s, and then cooled under a cooling condition listed in Table 3. A sample was cut out from the longitudinal end of an obtained coil at a transverse central position so that a cross-section orthogonal to the rolling direction could be observed. Subsequently, the cut-out sample was etched with nital, and then a mid-thickness part was subjected to SEM observation over a continuous area of 500 µm in the sheet thickness direction and 1 mm in the direction orthogonal to the rolling direction (sheet transverse direction). The obtained SEM images were subjected to image interpretation to determine the occupancy rate of carbides relative to the total length of grain boundaries within the field of view. Subsequently, the steel sheets were cold rolled either once or twice to the final sheet thickness, and some of the steel sheets were subjected to intermediate annealing at 1030 °C for 20 s. The final sheet thickness was 0.22 mm to 0.35 mm. Thereafter, primary recrystallization annealing was carried out in a temperature range from 550 °C to 680 °C at a heating rate of 250 °C/s, a soaking temperature of 800 °C, and a soaking time of 30 s. When an in-line fracture occurred during the process, the number of fractures was counted as one, and the in-line fracture rate was calculated using the number of coils passed during a one-week period as the denominator. After the primary recrystallization, an annealing separator containing 95 % MgO and 5 % TiO₂ was applied to the steel sheet surface as a water slurry, and the steel sheet was subjected to final annealing. A coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3 was applied to the surface of the final annealed sheet obtained in this way, and baked at 800 °C. In this way, a product sheet coil was obtained as a grain-oriented electrical steel sheet.

[Table 3]

[0057] 

Table 3

No.	Hot-rolled sheet annealing cooling conditions				Grain boundary occupancy rate of carbides (%)
	Residence time in temperature range from 600 °C to 500 °C (s)	Cooling rate	Soaking and natural cooling temperature range at 10 °C/s or less	Secondary cooling rate after natural cooling
1	5	from 500 °C to 150 °C: 50 °C/s	from 150 °C to 80 °C: 45 s	no	21
2	5	from 500 °C to 150 °C: 50 °C/s	from 150 °C to 80 °C: 48 s	no	21
3	12	from 500 °C to 150 °C: 50 °C/s	from 150 °C to 80 °C: 55 s	no	86
4	5	from 500 °C to 400 °C: 50 °C/s	from 400 °C to 300 °C: 35 s	from 300 °C to 100 °C: 20 °C/s	84
5	2	from 500 °C to 400 °C: 50 °C/s	from 400 °C to 300 °C: 28 s	from 300 °C to 100 °C: 20 °C/s	65
6	7	from 500 °C to 300 °C: 50 °C/s	from 300 °C to 200 °C: 60 s	from 200 °C to 100 °C: 15 °C/s	89
7	4	from 500 °C to 450 °C: 50 °C/s	from 450 °C to 200 °C: 28 s	from 200 °C to 100 °C: 10 °C/s	48

Note: underlining indicates a value outside the scope of the present disclosure.

[0058] The magnetic properties of a transverse central portion of the product sheet coil obtained as described above were investigated. To determine the magnetic properties, after stress relief annealing at 800 °C for 3 h, test pieces of 30 mm × 280 mm having a total mass of 500 g or more were cut out from a position corresponding to the outer winding of the coil during final annealing, and B₈ (magnetic flux density at a magnetizing force of 800 A/m) (T) was measured using the Epstein test specified in JIS C2550. The results are listed in Table 4.

[Table 4]

[0059] 

Table 4

No	Number of rolling passes	Final sheet thickness (mm)	First pass cold rolling conditions after hot-rolled sheet annealing			Intermediate annealing	Line fracture rate (%)	Magnetic properties B8(T)	Remarks
			Roll bite temperature (°C)	Strain rate (/s)	Rolling reduction (%)
1	1	0.30	110	250	32	no	4	1.91	Comparative Example
2	2	0.22	50	150	18	yes	11	1.91	Comparative Example
3	2	0.22	80	100	28	yes	3	1.90	Example
4	2	0.22	50	150	18	yes	3	1.91	Example
5	2	0.22	80	100	28	yes	9	1.90	Comparative Example
6	1	0.35	50	190	28	no	2	1.91	Example
7	1	0.35	50	190	28	no	10	1.88	Comparative Example

Note: underlining indicates a value outside the scope of the present disclosure.

[0060] As is clear from Table 4, it was confirmed that according to the present disclosure, the incidence of in-line fractures can be suppressed even when rolling conditions that are prone to cracking are applied, and that the magnetic properties of the grain-oriented electrical steel sheets are also maintained at a good level.

(Example 2)

[0061] A steel slab was prepared containing, in mass%, C: 0.04 %, Si: 3.3 %, Mn: 0.05 %, and other components listed in Table 5. The prepared steel slab was heated to 1200 °C and then hot-rolled to obtain a hot-rolled sheet. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing in an annealing furnace according to the present disclosure at 980 °C for 60 s, then cooled at 30 °C/s in a temperature range from 950 °C to 400 °C, held in a temperature range from 400 °C to 250 °C for 3 s or longer and 150 s or shorter, and finally cooled at 30 °C/s in a temperature range from 250 °C to 100 °C. Table 5 lists the holding (retention) times in the temperature range from 400 °C to 250 °C and the temperature range from 500 °C to 200 °C. A sample was cut out from the longitudinal end of the obtained coil at a transverse central portion so that the direction orthogonal to the rolling direction could be observed. Subsequently, the cut-out sample was etched with nital, and then a mid-thickness part was subjected to SEM observation over a continuous area of 500 µm in the sheet thickness direction and 1 mm in the direction orthogonal to the rolling direction (sheet transverse direction). The obtained SEM images were subjected to image interpretation to determine the occupancy rate of carbides relative to the total length of grain boundaries within the field of view. Next, rolling was carried out using a tandem mill consisting of 6 std that is not normally used for producing electrical steel sheets, with the rolling mill entry temperature set to 40 °C. The rolling reduction and strain rate of the first pass and the final sheet thickness after rolling are listed in Table 5. Twenty coils of the steel sheets were rolled under the same conditions, and evaluated as "×" when two or more coils fractured (rolling fracture rate: 10 %), and as "O" when one or less coils fractured. Further, for coils that fractured in the first half of rolling, a cross-section was cut out along the rolling direction from the fractured portion, and a length per unit area of grain boundaries having a twin orientation relationship (60 degrees around the <111> axis with respect to the matrix orientation: tolerance 15 degrees) was evaluated using EBSD. Coils that fractured in the later stage of rolling were not evaluated because it was difficult to determine whether twins were present. Thereafter, primary recrystallization annealing was carried out in a temperature range from 400 °C to 700 °C at a heating rate of 200 °C/s, a soaking temperature of 850 °C, and a soaking time of 40 s. Next, an annealing separator containing MgO as a main component was applied to the steel sheet, and the steel sheet was subjected to final annealing. A coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the final annealed sheet obtained as described above, and flattening annealing was carried out at 850 °C for 30 s. In this way, a product sheet coil was obtained as a grain-oriented electrical steel sheet.

[0062] The magnetic properties of a transverse central portion of the product sheet coil obtained as described above were investigated. During final annealing, test pieces of 30 mm × 280 mm having a total mass of 500 g or more were cut out from a position corresponding to the outer winding of the coil, and B₈ (T) was measured by the Epstein test specified in JIS C2550. Table 5 indicates a relationship between the obtained magnetic flux density and each experimental condition.

[Table 5]

[0063] 

Table 5

No	Added components (mass%)							Residence time in temperature range from 400 °C to 250 °C (s)	Residence time in temperature range from 500 °C to 200 °C (s)	Carbide occupancy rate (%)	Cold rolling conditions					B8(T)	Remarks
	C	Si	Mn	sol.Al	N	S	Other components				First pass rolling reduction (%)	First pass strain rate (/s)	Final sheet thickness (mm)	Fracture evaluation	Twin boundary
8	0.04	3.4	0.05	0.006	0.004	0.002		3	8	20	18	80	0.35	×	1.1 mm/mm2	1.905	Comparative Example
								10	15	55	18	80	0.35	×	not observable	1.902	Comparative Example
								30	35	80	18	80	0.35	O	0.4 mm/mm2	1.907	Example
								100	105	88	18	80	0.35	O	-	1.906	Example
								150	155	96	18	80	0.35	O	-	1.906	Example
9	0.04	3.4	0.05	0.005	0.003	0.001	Sb:0.02, P:0.03, Cr:0.05, Nb:0.001, Ti:0.00	20	25	69	28	150	0.26	×	not observable	1.916	Comparative Example
								100	105	89	28	150	0.26	O	-	1.917	Example
10	0.04	3.4	0.05	0.004	0.002	0.001	Ni:0.02, Sn:0.01, Cu:0.05, P:0.08, Cr:0.05	3	8	19	15	60	0.35	×	0.9 mm/mm2	1.916	Comparative Example
								100	105	88	15	60	0.35	O	-	1.914	Example
11	0.04	3.4	0.05	0.008	0.005	0.001	Ni:0.02, Sn:0.01, Mo:0.01, Ge:0.001	100	105	88	15	60	0.35	O	-	1.915	Example
12	0.04	3.4	0.05	0.005	0.003	0.001	Te:0.001, In:0.001, Pb:0.001	3	8	20	35	250	0.22	×	-	difficult to roll	Comparative Example
								3	8	20	15	60	0.35	×	-	1.917	Comparative Example
								150	155	96	15	60	0.35	O	-	1.919	Example
13	0.04	3.4	0.05	0.008	0.005	0.001	Zn:0.001, Ag:0.005	80	85	93	25	100	0.22	O	-	1.918	Example
14	0.04	3.4	0.05	0.003	0.002	0.001	Sb:0.02, Mo:0.01, Bi:0.001, As:0.001	80	85	92	25	100	0.22	O	-	1.918	Example

Note: underlining indicates a value outside the scope of the present disclosure.

[0064] As is clear from Table 5, the Examples were able to maintain good magnetic properties while suppressing the in-line fracture rate.

INDUSTRIAL APPLICABILITY

[0065] According to the present disclosure, it is possible to provide a method of producing a grain-oriented electrical steel sheet that can greatly improve manufacturability, and a production line that can realize this method.

Claims

1. A method of producing a grain-oriented electrical steel sheet, the method comprising: hot rolling a steel slab containing, in mass%, C: 0.01 % or more and 0.10 % or less, Si: 2.0 % or more and 6.5 % or less, and Mn: 0.01 % or more and 0.5 % or less; followed by hot-rolled sheet annealing; then cold rolling once or twice or more times from a thickness of an obtained hot-rolled sheet to a thickness of a product for a total rolling reduction of 80 % or more; then primary recrystallization annealing; then applying an annealing separator to a steel sheet surface; then final annealing; then flattening annealing for flattening, wherein,
after the hot-rolled sheet annealing, an occupancy rate of carbides relative to grain boundaries of recrystallized grains in the hot-rolled sheet before the cold rolling is 80 % or more, and an initial rolling reduction in the cold rolling has a strain rate of 200/s or less, a rolling reduction of 30 % or less, and a steel sheet temperature at roll bite of 90 °C or lower.

2. The method of producing a grain-oriented electrical steel sheet according to claim 1, wherein intermediate annealing is carried out one or more times between the cold rolling when cold rolling the twice or more times.

3. The method of producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein during cooling after the hot-rolled sheet annealing, a residence time in a temperature range from 600 °C to 500 °C is 10 s or longer.

4. The method of producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein, during cooling after the hot-rolled sheet annealing, a residence time in a temperature range from 600 °C to 500 °C is 3 s or longer and shorter than 10 s, an average cooling rate in a temperature range from 500 °C to 200 °C is 10 °C/s or less, and a cooling rate until coiling is 15 °C/s or more.

5. The method of producing a grain-oriented electrical steel sheet according to any one of claims 1 to 4, wherein a heating rate in the primary recrystallization annealing is 200 °C/s or more in a temperature range from 550 °C to 680 °C.

6. The method of producing a grain-oriented electrical steel sheet according to any one of claims 1 to 5, wherein the steel slab further contains, in mass%, sol.Al: 0.010 % or more and 0.050 % or less, N: 0.004 % or more and 0.015 % or less, and S+0.4Se: 0.010 % or more and 0.050 % or less.

7. The method of producing a grain-oriented electrical steel sheet according to any one of claims 1 to 5, wherein the steel slab further contains sol.Al: less than 0.010 mass%, and each of the elements S, N, and O: 60 ppm or less.

8. The method of producing a grain-oriented electrical steel sheet according to any one of claims 1 to 7, wherein the steel slab further contains, in mass%, one or more selected from the group consisting of Ni: 0.005 % or more and 1.50 % or less, Sn: 0.01 % or more and 0.50 % or less, Sb: 0.005 % or more and 0.50 % or less, Cu: 0.01 % or more and 0.50 % or less, Mo: 0.01 % or more and 0.50 % or less, P: 0.0050 % or more and 0.50 % or less, Cr: 0.01 % or more and 1.50 % or less, Nb: 0.0005 % or more and 0.0200 % or less, B: 0.0005 % or more and 0.0200 % or less, Te: 0.0005 % or more and 0.0200 % or less, Co: 0.0001 % or more and 0.0100 % or less, Ga: 0.0001 % or more and 0.0100 % or less, Zn: 0.0001 % or more and 0.500 % or less, Bi: 0.0005 % or more and 0.0200 % or less, Pb: 0.001 % or more and 0.3 % or less, Ge: 0.001 % or more and 0.3 % or less, As: 0.001 % or more and 0.3 % or less, and Ag: 0.001 % or more and 0.3 % or less.

9. A production line for a grain-oriented electrical steel sheet, used in producing a grain-oriented electrical steel sheet, the production line comprising: a plurality of cooling sections configured to cool a steel strip that has been annealed to a temperature of 700 °C or less in a plurality of stages; a thermometer provided to each of the cooling sections at least at a position partway through or on a delivery side, configured to measure a temperature of the steel strip; a controller configured to control a cooling rate in each of the cooling sections by using the temperatures measured by the thermometers for feedback control; and at least one coolant remover provided between the cooling sections configured to remove coolant from the steel strip, wherein the production line is capable of setting a coiling temperature to 100 °C or less.

10. The production line for a grain-oriented electrical steel sheet according to claim 9, wherein the cooling sections include a first cooling section configured to control a residence time of the steel strip in a temperature range from 600 °C to 500 °C to 3 s or longer and shorter than 10 s, and a second cooling section configured to control a residence time of the steel strip in a temperature range from 500 °C to 200 °C to 30 s or longer.

Drawing