WO2025095089A1 - Manufacturing method and manufacturing facility for oriented electromagnetic steel sheet - Google Patents

Abstract

Provided are: a method for manufacturing an oriented electromagnetic steel sheet, the method being capable of significantly improving manufacturability; and a facility capable of realizing the same. This method for manufacturing an oriented electromagnetic steel sheet involves a series of steps of: hot-rolling a steel slab having a predetermined component composition; then annealing the hot-rolled sheet; performing cold rolling once or at least twice so that the total rolling reduction from the thickness of the obtained hot-rolled sheet to the thickness of a product after the cold rolling is at least 80%; performing primary recrystallization annealing; subsequently, applying an annealing separation agent onto the surface of the steel sheet; and then performing final finish annealing and flattening annealing for flattening, wherein the occupancy rate of carbides at crystal grain boundaries of recrystallized grains in the hot-rolled sheet before the cold rolling and after the annealing of the hot-rolled sheet is set to at least 80%, and then the first reduction in the cold rolling is performed under the conditions in which the strain rate is at most 200/sec, the rolling reduction is at most 30%, and the steel sheet temperature when biting into a roll is at most 90ºC.

Description

Manufacturing method and equipment for grain-oriented electrical steel sheet

The present invention relates to a manufacturing method and manufacturing equipment for grain-oriented electrical steel sheets.

Grain-oriented electrical steel sheet has excellent magnetic properties and a crystal structure (Goss orientation) in which the <001> orientation, the easy axis of magnetization of iron, is highly concentrated in the rolling direction of the steel sheet. One method that has been proposed to improve the magnetic properties of grain-oriented electrical steel sheet is to control the form of C in the steel by controlling the cooling process after annealing before the final cold rolling.

For example, Patent Document 1 proposes a technique for precipitating fine carbides with a grain size of 100 Å to 500 Å by subjecting annealed steel sheet to rapid cooling and aging treatment under specific conditions. Patent Document 2 also proposes a technique for increasing solute C by cooling annealed steel sheet at a cooling rate of 150°C/min or more in the temperature range of 600 to 300°C.

The technologies proposed in Patent Documents 1 and 2 control the carbon in the steel as very fine carbides and dissolved C, and when dislocations are introduced during cold rolling, the dissolved C etc. adheres to the dislocations to form a Cotterell atmosphere, promoting non-uniform deformation during cold rolling and modifying the cold rolling texture, improving the texture after primary recrystallization.

Similar effects are also known in general steel as a method for increasing the {110} strength in the texture after recrystallization during annealing after cold rolling. In grain-oriented electrical steel sheets, the {110}<001> orientation is ultimately accumulated using a metallurgical phenomenon known as secondary recrystallization, and in this case the {110} texture can act as a good nucleus for secondary recrystallization. For this reason, technology for forming carbides within the crystal grains is extremely common in grain-oriented electrical steel sheets.

Japanese Patent Application Publication No. 58-157917 Japanese Unexamined Patent Publication No. 52-094825

In recent years, the need for energy conservation has led to a demand for electrical steel sheets with lower iron loss. In fact, products that meet this need are being manufactured by making steel sheets thinner and refining magnetic domains.

Si added to steel has the effect of increasing the electrical resistance of the steel, and is known to reduce Joule heat generated during use of electrical steel sheets, thereby greatly contributing to improving iron loss. As a result, electrical steel sheets that contain a lot of Si in the steel can achieve good iron loss. However, Si is also known as an element that embrittles steel, and it is generally very difficult to process steel with more than 4.0% by mass by rolling.

Generally, steel with a large amount of alloy added has high strength and is often difficult to roll. Also, as mentioned above, Si promotes embrittlement of the material, so even a content of a few mass percent can cause breakage problems. For this reason, various measures are often taken to manufacture electrical steel sheets, such as using rolling machines with high rigidity, reverse mills instead of tandem mills, or warm rolling at high temperatures to soften the material.

On the other hand, to provide manufacturing flexibility, it is desirable to be able to manufacture using a variety of rolling mills, not just those suitable for manufacturing electromagnetic steel sheets.

The inventors therefore investigated lowering the reduction rate per pass in order to reduce the load on the rolling mill and rolling rolls during rolling. As a result, they faced an unexpected problem in that breakage was more likely to occur in the range where the rolling speed was relatively slow.

The present invention has been made to solve the above problems, and its purpose is to provide a manufacturing method for grain-oriented electrical steel sheets that can greatly improve manufacturability, and manufacturing equipment for grain-oriented electrical steel sheets that can achieve this.

In order to solve the above problem, the inventors first investigated materials that had fractured after the first pass of hot rolling. As a result, they confirmed that multiple deformation twins had formed in the fractured material, and that the twins had an effect on adjacent crystals by propagating through grain boundaries.

Twins are known to be a cause of material embrittlement by cutting other twins or interacting with dislocations. Therefore, the inventors believed that suppressing the formation of such twins would be particularly effective in suppressing fracture.

Twins are usually formed so that the atoms that make up the material are mirror images of each other on the twin plane, so they are difficult to form when dislocations are introduced into the crystal and there is a lot of distortion in the crystal lattice. Under the conditions under which the fracture occurred, low reduction was used to reduce the rolling load, and at the same time, the rolling speed was relatively slow to increase stability when the sheet is passed through the rolling mill. The inventors estimated that because rolling was performed under these conditions, many deformation twins may have been formed before plastic deformation due to dislocations occurred. However, these rolling conditions were set so that rolling can be performed not only with rolling mills designed for the production of electrical steel sheets, but also with many general rolling mills. Therefore, methods such as increasing the reduction rate, which increases the burden on the rolling mill, and methods using high rolling speeds are not preferable as countermeasures.

The inventors therefore concluded that it is difficult to completely suppress the formation of twins, and decided to develop a policy to reduce the amount of twins that occur by preventing them from propagating to other adjacent crystals, even if twins do occur, and to carry out research into this. As a result, the inventors came up with the idea of precipitating carbides at grain boundaries as a way to give grain boundaries the function of a wall for the propagation of twins, since twins propagate across grain boundaries. The details of the experiments that led to the above findings are explained below.

<Experiment 1>
A slab for grain-oriented electrical steel sheet (hereinafter also simply referred to as "steel slab") having a composition containing, in mass%, 0.03% C, 4.2% Si, 0.1% Mn, 0.02% sol.Al, 50 ppm S, 100 ppm Se, 60 ppm N, and the balance containing Fe and other elements each reduced to less than 60 ppm was prepared. The prepared steel slab was heated at 1380 ° C., and then hot-rolled to obtain a hot-rolled coil (hereinafter also referred to as "hot-rolled coil") having a plate thickness of 2.5 mm by hot rolling. A test piece was cut out from the obtained hot-rolled coil, and in a laboratory experimental furnace, the test piece was subjected to hot-rolled sheet annealing with an attained temperature of 980 ° C., and an experiment was conducted to control the cooling after the hot-rolled sheet annealing.

First, referring to Non-Patent Document 1 (New Edition of Steel Materials and Alloying Elements, p. 395), the conditions considered to be capable of precipitating carbide ( _Fe3C ) at grain boundaries were a cooling rate of 1.5°C/sec in the temperature range of 700°C or lower and 600°C or higher, and a residence time in the above temperature range of 1 minute or longer.

A part of the obtained hot-rolled annealed sheet was cut out so that a cross section perpendicular to the rolling direction could be observed, and after etching with nital, the central part of the sheet thickness was continuously observed with a scanning electron microscope (SEM) at 500 μm in the sheet thickness direction and 1 mm in the direction perpendicular to the rolling direction (sheet width direction). As a result, it was found that carbides were precipitated in 85% of the grain boundaries in the observation field. When etching with nital, the steel part is etched, while the carbides are not etched and remain, so that when carbides are formed on the grain boundaries, the carbides spreading in a film shape are observed with a different contrast from the steel matrix. In addition, by analyzing the same area using an electron probe microanalyzer (EPMA) with high resolution, it was also found that the state in which C is concentrated on the grain boundaries can be grasped, and it was also found that it is possible to quantify how much of the total grain boundary length in the observation field is covered with carbides. In addition, in the cooling after the above-mentioned hot-rolled sheet annealing, carbides are hardly precipitated in the temperature range from the hot-rolled sheet annealing temperature to 700 ° C or more. Therefore, after cooling at an arbitrary cooling rate (for example, 20°C/sec) 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 of 700°C to 600°C, and the residence time in the temperature range of 700°C to 600°C was changed. The cooling rate in the temperature range of 600°C or less was set to 50°C/sec. The steel sheet thus obtained was evaluated for _Fe3C , which is carbide precipitated on the grain boundaries, and the samples shown in Table 1 were obtained. Table 1 shows the relationship between the residence time in the temperature range of 700°C to 600°C and the grain boundary occupancy rate of carbide.

<Experiment 2>
A steel slab having a composition containing, by mass%, 0.02% C, 4.8% Si, 0.3% Mn, 0.005% sol.Al, and the balance containing Fe and other elements with impurity elements such as S, N, Se, and O each reduced to 50 ppm or less was prepared. The prepared steel slab was heated at 1150 ° C., and then hot-rolled coils with a plate thickness of 2.5 mm were obtained by hot rolling. Test pieces were cut out from the obtained hot-rolled coil, and hot-rolled sheet annealing was performed in a laboratory experimental furnace with the test pieces reaching a temperature of 990 ° C., and in the temperature range of 700 ° C. to 600 ° C., the cooling rate in the temperature range of 600 ° C. to 500 ° C. after the hot-rolled sheet annealing was changed, and the residence time in the temperature range of 600 ° C. to 500 ° C. was changed. After the hot-rolled sheet annealing, since carbides do not precipitate at the grain boundaries in the temperature range of 700 ° C or more, any cooling pattern may be used, but cooling was performed at 30 ° C / sec, and after the residence time in the temperature range of 600 ° C or less and 500 ° C or more was completed, rapid cooling was performed at 60 ° C / sec. The grain boundary occupancy rate of carbides after annealing was quantitatively evaluated by the above-mentioned SEM observation. Table 2 shows the relationship between the residence time in the temperature range of 600 ° C or less and 500 ° C or more and the grain boundary occupancy rate of carbides. Comparing the results of Tables 1 and 2, it can be seen that, as described in Non-Patent Document 1, the precipitation of grain boundary carbides is more likely to proceed when they are retained in the temperature range of 600 ° C or less and 500 ° C or more. Although carbide precipitation proceeds even at temperatures exceeding 600 ° C, it can be said that it is extremely important to control the residence time, especially in the temperature range of 600 ° C or less and 500 ° C or more, considering that it is not practical to perform cooling for more than 1 minute during actual machine manufacturing.

The obtained samples were cold-rolled with a first pass reduction rate of 20% and a strain rate of 150/sec, and then rolled to a thickness of 1.0 mm using multiple passes. During this process, it was found that there was a certain probability of crack defects, such as partial cracks occurring, even if the plate did not break completely. Here, the number of samples that were actually rolled for the experimental conditions was used as the denominator, and the number of samples that developed crack defects was used to calculate the crack occurrence rate, obtaining the results shown in Figure 1.

As shown in Figure 1, the inventors have found that, in order to prevent fracture problems that occur during cold rolling under specific rolling conditions of low pressure and low strain rate, it is effective to set the carbide occupancy rate to 80% or more of the grain boundaries of recrystallized grains in the steel sheet before cold rolling, regardless of the cooling pattern. In addition, when the material in which cracks occurred under relatively low pressure was observed, it was confirmed that many deformation twins had been formed. However, once the reduction had progressed to a certain extent, the processed structure had become complicated and intricate due to dislocations, and it was not easy to grasp this as a change in twin density.

The results obtained are considered to be extremely useful knowledge from the perspective of improving manufacturability. Meanwhile, in the manufacture of grain-oriented electrical steel sheets, carbide control for texture control is also important. When the material is held at high temperatures, where the diffusion rate is high, for long periods of time to promote grain boundary precipitation, the carbon concentration within the crystal grains naturally drops significantly.

The inventors therefore conducted extensive research into a method for increasing the carbide occupancy rate at the grain boundaries while retaining as much carbon as possible within the crystal grains by maintaining the temperature at which carbides form at the grain boundaries for only the time required for nucleation, and then performing the growth stage of the precipitates at a low temperature where the diffusion rate is as slow as possible. This led to the completion of this invention.

That is, the present invention that solves the above problems is as follows.
[1] A method for producing a grain-oriented electrical steel sheet, which includes hot rolling a steel slab containing, by mass%, C: 0.01% to 0.10%, Si: 2.0% to 6.5%, and Mn: 0.01% to 0.5%, followed by hot rolling, annealing the hot-rolled sheet, cold rolling the resulting hot-rolled sheet to a thickness of the product, one or more cold rollings with a total reduction of 80% or more, primary recrystallization annealing, applying an annealing separator to the surface of the steel sheet, and then performing final annealing and flattening annealing for flattening,
a method for producing a grain-oriented electrical steel sheet, characterized in that after the hot-rolled sheet annealing, an occupancy rate of carbides relative to the grain boundaries of recrystallized grains in the hot-rolled sheet before the cold rolling is set to 80% or more, and the initial reduction in the cold rolling is set to a strain rate of 200/sec or less, a reduction rate of 30% or less, and a temperature of the steel sheet when biting into the rolls is 90°C or less.

[2] A method for producing the grain-oriented electrical steel sheet described in [1], in which one or more intermediate annealing steps are performed between the two or more cold rolling steps.

[3] The method for producing grain-oriented electrical steel sheet according to [1] or [2], in which the residence time in the temperature range of 600°C or less and 500°C or more during cooling after the hot-rolled sheet annealing is 10 seconds or more.

[4] A method for producing grain-oriented electrical steel sheet according to [1] or [2], in which, during cooling after the hot-rolled sheet annealing, the residence time in the temperature range of 600°C or less and 500°C or more is 3 seconds or more and less than 10 seconds, the average cooling rate in the temperature range of 500°C or less and 200°C or more is 10°C/sec or less, and cooling is performed at a cooling rate of 15°C/sec or more until coil winding.

[5] A method for producing a grain-oriented electrical steel sheet according to any one of [1] to [4] above, in which the heating rate in the temperature range of 550°C to 680°C during the primary recrystallization annealing is 200°C/sec or more.

[6] The method for producing 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% to 0.050%, N: 0.004% to 0.015%, and S+0.4Se: 0.010% to 0.050%, in addition to the above-mentioned composition.

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

[8] In addition to the above-mentioned chemical composition, the steel slab further contains, by mass%, 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 0.020 The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [7] above, containing at least one selected from the group consisting of 0% or less, Co: 0.0001% to 0.0100%, Ga: 0.0001% to 0.0100%, Zn: 0.0001% to 0.500%, Bi: 0.0005% to 0.0200%, Pb: 0.001% to 0.3%, Ge: 0.001% to 0.3%, As: 0.001% to 0.3%, and Ag: 0.001% to 0.3%.

[9] A facility used in manufacturing grain-oriented electrical steel sheets, the facility having multiple cooling sections for cooling a steel strip that has been annealed to a temperature of 700°C or less in multiple stages, a thermometer for measuring the temperature of the steel strip provided at least halfway through each cooling section or at the exit side, a control unit for controlling the cooling rate in each cooling section by using the temperature measured by the thermometer for feedback control, and at least one cooling water removal unit for removing the cooling water from the steel strip provided between the cooling sections, and capable of setting the coil winding temperature to 100°C or less.

[10] The manufacturing equipment for grain-oriented electrical steel sheet described in [9], wherein the multiple cooling sections include a first cooling section that controls the residence time of the steel strip in a temperature range of 600°C or less and 500°C or more to 3 seconds or more and less than 10 seconds, and a second cooling section that controls the residence time of the steel strip in a temperature range of 500°C or less and 200°C or more to 30 seconds or more.

The present invention provides a method for manufacturing grain-oriented electrical steel sheets that can greatly improve manufacturability, and equipment that can realize this method.

FIG. 1 is a diagram showing the relationship between the grain boundary occupancy rate of carbide and the probability of crack occurrence. FIG. 1 is a schematic diagram of an example of manufacturing equipment for grain-oriented electrical steel sheets according to the present invention.

(Method of manufacturing grain-oriented electrical steel sheet)
The method for producing grain-oriented electrical steel sheet according to the present invention is a method for producing grain-oriented electrical steel sheet in a series of steps, which includes hot rolling a steel slab containing, by mass%, C: 0.01% to 0.10%, Si: 2.0% to 6.5%, and Mn: 0.01% to 0.5%, followed by hot-rolling annealing, cold rolling from the thickness of the obtained hot-rolled sheet to the thickness of the product, one or more cold rollings with a total reduction of 80% or more, primary recrystallization annealing, applying an annealing separator to the surface of the steel sheet, final annealing, and flattening annealing for flattening. Here, after hot-rolled sheet annealing, the occupancy rate of carbides relative to the grain boundaries of recrystallized grains in the hot-rolled sheet before cold rolling is set to 80% or more, and the initial reduction in cold rolling is set to a strain rate of 200/sec or less, a reduction ratio of 30% or less, and the steel sheet temperature when biting into the rolls is set to 90°C or less.

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

The present invention is advantageous in improving manufacturability when rolling at low pressure and low speed is required to utilize a rolling mill that does not have the mill rigidity, load capacity, or equipment for performing warm rolling, or when rolling at low pressure and low speed is required due to process requirements. Therefore, many other components and manufacturing processes can be adopted that are similar to those used in the manufacture of general grain-oriented electrical steel sheets. However, C, Si, and Mn are limited for the following reasons.

C: 0.01% to 0.10% C is an essential element for precipitating carbides on grain boundaries and improving texture. However, if the C content exceeds 0.10%, it is difficult to decarburize in the final decarburization annealing, and it remains in the product, which causes iron loss deterioration called magnetic aging. Therefore, the C content is set to 0.10% or less. Also, if the C content is less than 0.01%, various precipitation controls cannot be performed sufficiently. Therefore, the C content is set to 0.01% or more. From the viewpoint of manufacturability and magnetic properties, the C content is preferably set to 0.02% to 0.06%.

Si: 2.0% to 6.5% Si is a useful element that improves 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. If the Si content exceeds 4.5%, the risk of fracture during equipment threading increases and cold rolling properties also deteriorate significantly. However, in the present invention, since Si can provide the effect of suppressing cracking, the Si content can be contained more than usual. However, since the magnetostriction property required together with the iron loss is saturated at 6.5%, adding more than that does not provide a significant improvement effect on the magnetic properties. Therefore, the Si content is set to 6.5% or less. Since the risk is not zero even when the risk reduction effect during equipment threading is taken into account, the Si content is preferably set to 2.8% to 4.5%.

Mn: 0.01% or more and 0.5% or less Mn is a useful element from the viewpoint of controlling the formation of an oxide film during primary recrystallization, but if the content is less than 0.01%, the effect of controlling the formation of an oxide film cannot be obtained. Therefore, the Mn content is set to 0.01% or more. On the other hand, Mn also has the effect of improving hot workability during manufacturing, but if the Mn content exceeds 0.5%, the primary recrystallization texture deteriorates, leading to deterioration of magnetic properties. Therefore, the Mn content is set to 0.5% or less.

Other typical compositions are as follows. In the present invention, the steel slab only needs to have a 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 the final sheet thickness in one go, decarburization annealing (which also serves as primary recrystallization annealing), and final finish 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 shown in Patent Document 3 and the like, to develop secondary recrystallized grains without using a precipitation-type inhibitor (AlN, MnS, MnSe, etc.). The preferred contents of the inhibitor components according to their types are as follows:

<When an inhibitor component is used>
Sol.Al: 0.010% to 0.050% N: 0.004% to 0.015% S+0.4Se: 0.010% to 0.050% If the Sol.Al content is less than 0.010%, the magnetic flux density of the produced grain-oriented electrical steel sheet decreases. On the other hand, if the Sol.Al content exceeds 0.050%, the secondary recrystallization becomes unstable. Therefore, the Sol.Al content is preferably 0.010% to 0.050%.

If the N content is less than 0.004%, AlN will not precipitate properly during the intermediate process, making it difficult to control the particle size. Furthermore, if the N content exceeds 0.015%, it will cause frequent surface defects known as blisters. For this reason, it is preferable for the N content to be 0.004% or more and 0.015% or less. The N content can be changed as necessary by applying a nitriding process during production, so in many cases it is possible to form sufficient precipitates with a content of 0.010% or less.

S+0.4Se: 0.010% or more and 0.050% or less When the S content + 0.4 × Se content is less than 0.010%, the absolute amount of Se and S as inhibitor components is insufficient. On the other hand, when S+0.4Se exceeds 0.050%, purification in the final annealing becomes difficult. Therefore, it is preferable that S+0.4Se is 0.010% or more and 0.050% or less. S and Se can be used as inhibitors as MnSe and MnS, respectively, or as a compound of these, Mn(S,Se). In addition, AlN-based inhibitors and MnSe and/or MnS-based inhibitors can coexist, and a synergistic effect can be obtained.

<When no precipitate-type inhibitor component is included>
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 contents of precipitation inhibitor forming elements Sol.Al, S and O are restricted to extremely low values. Specifically, they are restricted to Sol.Al: less than 0.010%, S: 60 ppm or less, and O: 60 ppm or less. If these amounts are exceeded, it becomes difficult to obtain a secondary recrystallized structure due to the effect of texture inhibition.

It is desirable to keep N at 60 ppm or less to prevent the formation of silicon nitrides after purification annealing. It is also preferable to reduce the nitride-forming elements Ti, Nb, B, Ta and V to 0.050% or less each. This is to prevent deterioration of iron loss by not interfering with the texture inhibition effect.

The contents of the inhibitor components are as described above, but by adding grain boundary segregation elements to these, the magnetic properties can be improved. The elements are Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Mo: 0.01% to 0.50%, P: 0.0050% to 0.50%, Cr: 0.01% to 1.50%, Nb: 0.0005% to 0.02%. 00% 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.

Furthermore, Pb, Ge, As, Ag, etc. can be contained in the range of 0.001% to 0.3%. These elements can be used alone or in combination, which can improve iron loss.

The steel slab, which is the starting material having the above-mentioned composition, is heated at an appropriate temperature according to the composition system, and then hot-rolled, including rough rolling and finish rolling, to produce a hot-rolled sheet. When the steel slab contains a precipitation-type inhibitor component, it is heated to a temperature range of 1350°C to 1450°C in order to completely dissolve Al, Se, S, etc. In contrast, in the case of a composition system that does not contain a precipitation-type inhibitor component, if the heating temperature of the steel slab is too high, the inhibitor-forming components that were dissolved during heating will precipitate unevenly and finely during hot rolling, which will locally suppress grain boundary migration, make the grain size distribution extremely uneven, and inhibit the development of secondary recrystallized grains in the Goss orientation. For this reason, it is advisable to adopt a relatively low heating temperature, for example, 1250°C or less. There are no particular restrictions on the hot-rolling conditions, and the conditions normally used for manufacturing grain-oriented electrical steel sheets may be used.

The hot-rolled sheet obtained as described above is subjected to hot-rolled sheet annealing. In hot-rolled sheet annealing, it is preferable to perform soaking treatment at a temperature of 800°C to 1150°C for 20 seconds or more in order to homogenize the hot-rolled structure. The effect of the present invention was greater in terms of crack suppression when the maximum annealing temperature was less than 1000°C. Although the mechanism is not clear, when hot-rolled sheet annealing is performed at a low temperature of less than 1000°C, recrystallization nuclei are not formed and relatively coarse structures caused by hot rolling remain in the center of the sheet thickness, which may be more likely to develop into cracks, and it is believed that this is why the crack suppression effect of the present invention is exerted.

Next, in order to control carbides, one of the following controls is required for cooling after annealing the hot-rolled sheet.

1) The residence time in the temperature range of 500° C. or higher and 600° C. or lower is 10 seconds or longer.
The higher the temperature is held, the lower the driving force for carbide precipitation, so that a long time may be required. By holding at a temperature in the above temperature range for 10 seconds or more, carbides can be formed on the grain boundaries. However, since the material is held at a high temperature, most of the carbon present in the grains diffuses to the grain boundaries, and the carbon concentration in the grains decreases. As described above, under certain rolling conditions, the grain boundary occupancy rate of carbides is increased, thereby reducing the crack occurrence rate and improving manufacturability. On the other hand, since the use of carbides in the grains is limited, when this condition is used, the texture obtained after the primary recrystallization annealing is inferior, and the magnetic properties of the product may be slightly inferior. However, since this condition allows productivity to be increased relatively easily, it can be applied when a higher level of magnetic properties is not required. There is no upper limit on the residence time, but the effect of crack suppression is saturated even if the residence time is long, and a short time is desired from the viewpoint of magnetic properties. Therefore, it is desirable to hold the material for 15 seconds or less.

2) The residence time in the temperature range of 500°C or higher and 600°C or lower is 3 seconds or longer and shorter than 10 seconds, and the average cooling rate in the temperature range of 200°C or higher and 500°C or lower is 10°C/second or shorter (i.e., residence time 30 seconds or longer), and cooling is performed at 15°C/second or longer before coil winding.
By setting the residence time in the temperature range of 600°C or less and 500°C or more to 3 seconds or more, carbide nuclei can be formed on the grain boundaries. If the residence time in the above temperature range is less than 3 seconds, grain boundary precipitation does not proceed sufficiently. On the other hand, if the residence time in the above temperature range is 10 seconds or more, precipitation to the grain boundaries proceeds too much, and even if the cooling rate below 500°C is controlled, there is a risk that an appropriate carbide state cannot be obtained. In other words, there is a risk that the necessary amount of carbon in the grains is reduced, and the texture improvement effect cannot be obtained. Therefore, it is preferable to set the residence time in the above temperature range to 3 seconds or more and less than 10 seconds.

In the temperature range of 500°C to 200°C, where the diffusion rate of carbon is extremely slow compared to the temperature range of 600°C to 500°C, the cooling rate is reduced and the residence time is set to 30 seconds or more. By using such a heat pattern, it is possible to promote the diffusion of carbon within the crystal grains to the grain boundaries to a certain extent while at the same time retaining a certain amount of carbon within the crystal grains. After the grain boundary precipitation is properly performed, the cooling rate is increased again to suppress the diffusion of carbon within the crystal grains and to make the precipitation state of carbides within the crystal grains proper. After at least the residence treatment for grain boundary precipitation is performed, the cooling rate must be increased again and cooling must be performed at a cooling rate of 15°C/second or more. This makes it possible to minimize the effect on the texture and increase only the manufacturability. As a result, the steel strip after cooling is in a state in which carbides are formed on the crystal grain boundaries, and at the same time, the thickness of the depleted zone of carbides occurring near the grain boundaries can be reduced to 20% or less of the crystal grain size.

In particular, in order to realize the heat pattern shown in 2), the cooling section following the continuous annealing furnace in the grain-oriented electrical steel sheet manufacturing facility needs the following mechanism as shown in the schematic diagram of FIG.
1. The steel strip has multiple cooling sections (two cooling sections 1 and 2 in the example of FIG. 2) for cooling the steel strip in multiple stages in a temperature range where the temperature of the steel strip is 600° C. or less. In a temperature range from the hot-rolled sheet annealing temperature to 700° C. or more, Fe ₃ C, which is a carbide, hardly precipitates, and in a temperature range of 600° C. or more, although precipitation proceeds, the precipitation rate is very slow. Therefore, cooling to 600° C. after annealing is not particularly limited from the viewpoint of carbide control, but if the cooling rate is unnecessarily low, the required line length increases. Therefore, it is sufficient to set the cooling rate to a general rate, for example, 5° C./sec to 40° C./sec.
2. In the cooling section (cooling section 1 in the example of Figure 2) where the steel strip is cooled in a temperature range of 600°C or less and 500°C or more, a thermometer (thermometer 1 in the example of Figure 2) capable of measuring the temperature of the steel strip is provided midway through the cooling section or on the exit side.
3. The present invention has a mechanism (control unit) for measuring the temperature of the steel strip using the thermometer described in 2 above, feedback-controlling the steel strip temperature, controlling the cooling rate in a temperature range of 600°C or lower and 500°C or higher, and controlling the residence time in the temperature range to 3 seconds or longer and less than 10 seconds.
4. In the temperature range of 200°C or higher and 500°C or lower, the temperature gradually drops even when the strip is left to cool naturally. Therefore, at the exit side of the cooling section responsible for cooling in the temperature range of 500°C or higher and 600°C or lower, cooling water or the like remains on the steel strip. To prevent excessive cooling, at least one mechanism (cooling water removal section) is provided between the cooling sections to easily wipe and remove the cooling water. The cooling water removal section is preferably provided before the cooling section that significantly reduces the cooling rate. Also, the cooling water removal section may be provided between all cooling sections.
5. The apparatus has a slow cooling section having a temperature maintaining function that ensures a residence time of 30 seconds or more in the temperature range of 200°C or more but not exceeding 500°C, or a natural cooling section without an active cooling function.
6. After slow cooling or natural cooling, a cooling section (cooling section 2 in the example of FIG. 2) is provided between the exit coil winding mechanism and 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 temperature of the steel strip is provided midway through the cooling section or on the exit side.
7. The steel strip temperature is measured using the thermometer described in 6 above, and the steel strip temperature is feedback-controlled to achieve a stable cooling rate of 15°C/sec or more in the target cooling section. Since the steel strip in the target cooling section is also at a relatively low temperature, multiple functions can be performed, such as pickling following cooling.
8. When the coil is finally wound at the exit of the facility, it has the function of being cooled to below 100°C.

Then, the resulting hot-rolled and annealed sheet is cold-rolled. At this time, the total reduction from the thickness of the hot-rolled sheet to the thickness of the final product sheet is set to 80% or more. Because such high reduction places a large burden on the rolling mill, a strain rate of 200/sec or less and a reduction rate of 30% or less are adopted as reduction conditions for the first pass by the rolling mill. The above rolling conditions are applied due to manufacturing and equipment constraints, texture control, and other reasons, and are not necessarily recommended as optimal rolling conditions for the production of grain-oriented electrical steel sheet. For example, equipment constraints include cases where, normally, it is desirable to achieve the final thickness in one rolling pass without intermediate annealing, but the reduction rate per pass is limited due to issues with the rolling load; or, when manufacturing using a manufacturing method in which rolling is performed two or more times with one or more intermediate annealing, the reduction rate 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 rolling mill must be used, and the number of stands is determined by the specifications of the rolling mill, so the rolling speed on the first pass is inevitably slow.

If rolling is not performed under such special conditions, there is no need to make the grain boundaries, as specified in this invention, occupied by carbides. Similarly, when the steel sheet temperature exceeds 90°C when it is bitten into the work roll during the first pass reduction, if the steel sheet temperature is high, deformation due to dislocations is likely to occur, and twin deformation is suppressed, so there is no need to apply this invention. There are no particular restrictions on cold rolling as long as the final sheet thickness can be obtained. If possible, as with many conventional techniques, the texture can be improved by performing warm rolling to increase the steel sheet temperature after the first pass using heat generated by processing.

Next, the final cold-rolled sheet is subjected to primary recrystallization annealing. The purpose of this primary recrystallization annealing is to perform primary recrystallization of the cold-rolled sheet having a rolling structure, adjust the primary recrystallized grain size to an optimal size for secondary recrystallization, and decarburize the carbon contained in the steel by making the annealing atmosphere a wet hydrogen nitrogen or wet hydrogen argon atmosphere, and at the same time, form an oxide film on the surface by the above-mentioned oxidizing atmosphere. For this reason, the primary recrystallization annealing is performed at 750 ° C. or more and 900 ° C. or less in a H ₂ mixed atmosphere with a dew point. During the temperature rise of the primary recrystallization annealing, the temperature rise rate in the temperature range of 550 ° C. or more and 680 ° C. or less is set to 200 ° C. / second or more, so that the texture improvement effect can be further enhanced.

An annealing separator is applied to the surface of the above-mentioned primary recrystallization annealed steel sheet. In order to form a forsterite film on the steel sheet surface after secondary recrystallization annealing, magnesia (MgO) is used as the main agent of the annealing separator. At this time, the formation of the forsterite film can be further favored by adding an appropriate amount of Ti oxide, Sr compound, etc. to the separator. In particular, the addition of an auxiliary agent that promotes uniform formation of the forsterite film is also advantageous for improving the peeling characteristics.

This is followed by final annealing for secondary recrystallization and forsterite film formation. The annealing atmosphere can be _N2 , Ar, _H2 or any mixture of these gases. In order to more effectively perform secondary recrystallization, the material can be isothermally held near the secondary recrystallization temperature. However, this is not necessarily required, since slowing down the rate of temperature rise can also be effective. If trace elements are precipitated in the final product, this will lead to deterioration of the magnetic properties, so the maximum annealing temperature is preferably 1100°C or higher to purify the elements.

After the above-mentioned final annealing, an insulating coating can be applied and baked on the surface of the steel sheet. There are no particular restrictions on the type of insulating coating, and any conventionally known insulating coating is suitable. For example, a suitable method is to apply a coating liquid containing phosphate, chromate, and colloidal silica, as described in JP-A-50-79442 and JP-A-48-39338, to the steel sheet and bake it at a temperature of about 800°C.

Furthermore, flattening annealing can be used to adjust the shape of the steel sheet, and this flattening annealing can also be used in conjunction with the baking process of the insulating coating.

Example 1
A steel slab was prepared having a composition of mass% containing C: 0.05%, Si: 3.2%, Mn: 0.04%, sol.Al: 0.0200%, Se: 100 ppm, N: 100 ppm, S: 60 ppm, O: less than 50 ppm, and the balance being Fe and unavoidable impurities. The prepared steel slab was heated to 1350 ° C, and then hot-rolled to produce a 2.0 mm thick hot-rolled sheet. Next, using the equipment according to the present invention, hot-rolled sheet annealing was performed at 990 ° C for 30 seconds, and then cooled under the cooling conditions described in Table 3. Samples were cut out from the longitudinal end and width center position of the obtained coil so that the cross section perpendicular to the rolling direction could be observed. Next, the cut-out sample was etched with nital, and then SEM observation was performed on the center of the sheet thickness continuously at 500 μm in the sheet thickness direction and 1 mm in the sheet width direction perpendicular to the rolling direction. The carbide occupancy rate relative to the total grain boundary length in the field of view was determined by image analysis of the obtained SEM image. Subsequently, the steel sheet was cold-rolled under either one or two rolling conditions until the final plate thickness was reached, and intermediate annealing at 1030°C for 20 seconds was used for some conditions. The final plate thickness was set to 0.22mm to 0.35mm. After that, primary recrystallization annealing was performed with a heating rate of 250°C/sec in a temperature range of 550°C to 680°C, a soaking temperature of 800°C, and a soaking time of 30 seconds. If a break occurred in the line during the process, the number of breaks was counted as one, and the in-line breakage rate was calculated using the number of coils passed through the sheet during a one-week period as the modulus. The steel sheet after primary recrystallization was coated on the steel sheet surface with an annealing separator of MgO: 95% and TiO ₂ : 5% as a water slurry, and subjected to final finish annealing. A coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:3 was applied to the surface of the finish annealed sheet thus obtained, and baked at 800° C. In this way, a product sheet coil was obtained as a grain-oriented electrical steel sheet.

The magnetic properties of the product sheet coil obtained as described above were investigated at the width center. After stress relief annealing at 800°C for 3 hours, a test piece of 30 mm x 280 mm was cut out from a position corresponding to the outer winding of the coil during final annealing so that the total mass was 500 g or more, and _B8 (magnetic flux density at a magnetizing force of 800 A/m) (T) was measured by the Epstein test specified in JIS C2550. The results are shown in Table 4.

As is clear from Table 4, it has been confirmed that the present invention can suppress the incidence of in-line breakage even when rolling conditions that tend to cause cracks are applied, and that the magnetic properties of the grain-oriented electrical steel sheet are also well maintained.

Example 2
A steel slab containing, by mass%, C: 0.04%, Si: 3.3%, Mn: 0.05%, and other components shown in Table 5 was prepared. The prepared steel slab was heated to 1200 ° C, and then hot-rolled to form a hot-rolled sheet. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 980 ° C for 60 seconds in an inventive annealing furnace, and then cooled at 30 ° C / sec in a temperature range of 950 ° C or less and 400 ° C or more, held for 3 seconds to 150 seconds in a temperature range of 400 ° C or less and 250 ° C or more, and cooled at 30 ° C / sec in a temperature range of 250 ° C or less and 100 ° C or more. Table 5 shows the holding (retention) times in the temperature range of 400 ° C or less and 250 ° C or more and the temperature range of 500 ° C or less and 200 ° C or more. A sample was cut out so that the direction perpendicular to the rolling could be observed from the longitudinal end and width center position of the obtained coil. Next, the cut sample was etched with nital, and then the central part of the plate thickness was continuously observed with an SEM at 500 μm in the plate thickness direction and 1 mm in the direction perpendicular to the rolling direction (plate width direction). The carbide occupancy rate relative to the total grain boundary length in the field of view was determined by image analysis of the obtained SEM image. Next, rolling was performed using a tandem rolling mill consisting of 6 std that is not normally used for manufacturing electrical steel sheets, with the rolling mill inlet temperature at 40 ° C. The reduction rate and strain rate of the first pass at this time, and the final plate thickness after rolling are shown in Table 5. 20 coils were rolled under the same conditions, and the breakage was evaluated as × when 2 coils (breakage rate during rolling 10%) or more were broken, and ◯ when 1 coil or less were broken. In addition, for coils that broke in the first half of rolling, a cross section along the rolling direction was cut from the broken part, and the length per unit area of the grain boundary having a twin orientation relationship (60 degrees around the <111> axis with respect to the matrix orientation: tolerance 15 degrees) was also evaluated using EBSD. For coils that broke in the later stage of rolling, the presence or absence of twins was difficult to determine, so they were not evaluated. Thereafter, primary recrystallization annealing was performed in a temperature range of 400°C to 700°C, with a heating rate of 200°C/sec, a soaking temperature of 850°C, and a soaking time of 40 seconds. Next, an annealing separator containing MgO as the main agent was applied to the steel sheet, and the steel sheet was subjected to final finish annealing. A coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the finish annealed sheet obtained as described above, and flattening annealing was performed at 850°C for 30 seconds. In this way, a product sheet coil was obtained as a grain-oriented electrical steel sheet.

The magnetic properties of the widthwise center of the product sheet coil obtained as described above were investigated. Test pieces of 30 mm x 280 mm with a total mass of 500 g or more were cut out from a position corresponding to the outer winding of the coil during final finish annealing, and _B8 (T) was measured by the Epstein test defined in JIS C2550. The relationship between the obtained magnetic flux density and each experimental condition is shown in Table 5.

As is clear from Table 5, the invention example is able to maintain good magnetic properties while suppressing the in-line breakage rate.

The present invention provides a method for manufacturing grain-oriented electrical steel sheets that can greatly improve manufacturability, and equipment that can realize this method.

Claims (10)

A method for producing a grain-oriented electrical steel sheet comprising a series of steps of hot rolling a steel slab containing, by mass%, C: 0.01% to 0.10%, Si: 2.0% to 6.5%, and Mn: 0.01% to 0.5%, followed by hot-rolling annealing, cold rolling the resulting hot-rolled sheet to a thickness of a product, one or more cold rolling steps with a total reduction of 80% or more, primary recrystallization annealing, coating an annealing separator on the steel sheet surface, final annealing, and flattening annealing for flattening,
a method for producing a grain-oriented electrical steel sheet, characterized in that 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 set to 80% or more, and an initial reduction in the cold rolling is set to a strain rate of 200/sec or less, a reduction ratio of 30% or less, and a temperature of the steel sheet when biting into the rolls of 90°C or less. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein one or more intermediate annealing steps are performed between the two or more cold rolling steps. The method for producing grain-oriented electrical steel sheet according to claim 1 or 2, wherein the residence time in the temperature range of 600°C or less and 500°C or more during cooling after the hot-rolled sheet annealing is 10 seconds or more. The method for producing grain-oriented electrical steel sheet according to claim 1 or 2, wherein, during cooling after the hot-rolled sheet annealing, the residence time in the temperature range of 600°C or less and 500°C or more is 3 seconds or more and less than 10 seconds, the average cooling rate in the temperature range of 500°C or less and 200°C or more is 10°C/sec or less, and cooling is performed at a cooling rate of 15°C/sec or more until coil winding. The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 4, wherein the heating rate in the temperature range of 550°C to 680°C during the primary recrystallization annealing is 200°C/sec or more. The method for producing 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% to 0.050%, N: 0.004% to 0.015%, and S+0.4Se: 0.010% to 0.050%, in addition to the above-mentioned composition. The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 5, wherein the steel slab further contains, in mass%, sol. Al: less than 0.010%, and each of the elements S, N, and O is 60 ppm or less, in addition to the above-mentioned composition. In addition to the above-mentioned chemical composition, the steel slab further contains, by mass%, Ni: 0.005% to 1.50%, Sn: 0.01% to 0.50%, Sb: 0.005% to 0.50%, Cu: 0.01% to 0.50%, Mo: 0.01% to 0.50%, P: 0.0050% to 0.50%, Cr: 0.01% to 1.50%, Nb: 0.0005% to 0.0200%, B: 0.0005% to 0.0200%, Te: 0.0005% to 0.020 The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 7, which contains at least one selected from the group consisting of 0% or less, Co: 0.0001% to 0.0100%, Ga: 0.0001% to 0.0100%, Zn: 0.0001% to 0.500%, Bi: 0.0005% to 0.0200%, Pb: 0.001% to 0.3%, Ge: 0.001% to 0.3%, As: 0.001% to 0.3%, and Ag: 0.001% to 0.3%. Equipment used in manufacturing grain-oriented electrical steel sheets, the equipment has multiple cooling sections that cool the steel strip, which has a temperature of 700°C or less after annealing, in multiple stages, a thermometer that measures the temperature of the steel strip, which is provided at least halfway through each cooling section or at the exit side, a control unit that controls the cooling rate in each cooling section by using the temperature measured by the thermometer for feedback control, and at least one cooling water removal unit that removes the cooling water from the steel strip, which is provided between the cooling sections, and is capable of keeping the coil winding temperature at 100°C or less. The manufacturing equipment for grain-oriented electrical steel sheets according to claim 9, wherein the plurality of cooling sections include a first cooling section that controls the residence time of the steel strip in a temperature range of 600°C or less and 500°C or more to 3 seconds or more and less than 10 seconds, and a second cooling section that controls the residence time of the steel strip in a temperature range of 500°C or less and 200°C or more to 30 seconds or more.

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