JP6866901B2 - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet, and more particularly to a method for manufacturing a grain-oriented electrical steel sheet which has a high magnetic flux density and can obtain a grain-oriented electrical steel sheet which can be suitably used as an iron core material of a transformer.

The grain-oriented electrical steel sheet is a soft magnetic material used as an iron core material for transformers and generators, and has a texture in which the <001> orientation, which is the easy axis of iron magnetization, is highly aligned in the rolling direction of the steel sheet. There is. Such textures are formed by performing purification annealing with secondary recrystallization during the manufacturing process of grain-oriented electrical steel sheets. Here, the secondary recrystallization refers to a phenomenon in which crystal grains in the {110} <001> orientation, which are so-called Goth orientations, are preferentially grown in a huge manner.

As a typical method for causing the above-mentioned secondary recrystallization, there is a method using a precipitate called an inhibitor. In this method, crystal grain growth during the annealing step is controlled by dispersing precipitates such as AlN, MnS, and MnSe in the steel, and finally, crystal grains having a Goss orientation are selectively grown.

The method using the above-mentioned inhibitor has been widely used in the production of grain-oriented electrical steel sheets because it is possible to stably develop secondary recrystallized grains. However, in order to finely disperse the inhibitor in the steel, it is necessary to perform slab heating at a high temperature of 1300 ° C. or higher in advance to dissolve the inhibitor component once. Further, since the inhibitor element remaining in the steel after the secondary recrystallization causes deterioration of the magnetic properties of the grain-oriented electrical steel sheet, purification annealing is performed in a high temperature of 1100 ° C. or higher and a controlled atmosphere. It is necessary to remove the inhibitor element from the ground iron.

Therefore, in order to solve the problems caused by the use of inhibitors as described above, a method of manufacturing grain-oriented electrical steel sheets without using inhibitors (inhibitorless method) has been proposed (for example, Patent Document 1). ..

The inhibitorless method is a technique for expressing secondary recrystallization by using more purified steel and controlling the texture. Specifically, by demonstrating the grain boundary orientation difference angle dependence of the grain boundary energy of the crystal grain boundary at the time of primary recrystallization, the crystal grains having the Goss orientation are secondarily recrystallized without using an inhibitor. It becomes possible to make it. Such an effect is called a texture inhibition effect.

Japanese Unexamined Patent Publication No. 2000-129356

In the inhibitorless method, since it is not necessary to finely disperse the inhibitor in the steel, it is not necessary to perform the high temperature slab heating which is indispensable when the inhibitor is used. Therefore, the inhibitorless method has a great advantage not only in terms of manufacturing cost but also in terms of maintenance of manufacturing equipment.

However, the inhibitorless method has a problem that it is difficult to stably produce a grain-oriented electrical steel sheet having good magnetic characteristics. Therefore, there is a need for a method for manufacturing grain-oriented electrical steel sheets, which is an inhibitorless method excellent in terms of cost and can manufacture grain-oriented electrical steel sheets having more excellent magnetic characteristics, specifically, high magnetic flux density. Has been done.

The present invention has been made in view of the above circumstances, is an inhibitorless method excellent in terms of cost, and is a method for manufacturing a grain-oriented electrical steel sheet capable of manufacturing a grain-oriented electrical steel sheet having a high magnetic flux density. The purpose is to provide.

As a result of repeated studies to solve the above problems, the inventors have obtained the following findings.

(1) In a material containing no inhibitor component, the more {111} oriented grains, the more stable the secondary recrystallization is expressed. However, if the {111} orientation grains are excessive, the crystal grains in the orientation deviating from the ideal Goss orientation are likely to be recrystallized secondarily, so that the magnetic characteristics are rather deteriorated.

(2) On the other hand, the {411} orientation grain has an effect of increasing the secondary recrystallization orientation selectivity of the ideal Goss orientation. However, if the {411} orientation grains are excessive, it becomes difficult for secondary recrystallization to occur, and the magnetic properties are rather deteriorated.

(3) Therefore, in order to obtain a product having good magnetic properties in the inhibitorless method, the balance between {111} oriented grains and {411} oriented grains in the texture of the steel sheet subjected to secondary recrystallization annealing should be adjusted. It is important to control.

(4) In order to control the texture of the steel sheet subjected to the secondary recrystallization annealing as described above, the microstructure of the steel sheet during cold rolling may be controlled. Specifically, by controlling the area of pearlite and the area of the non-diffusion transformation phase in the steel sheet before cold rolling, the {111} orientation grains and {411} in the texture of the steel sheet at the time after the primary recrystallization annealing. The balance of azimuth grains can be controlled.

Here, an example of an experiment that led to the above findings will be described. In this experiment, a plurality of grain-oriented electrical steel sheets were prepared under different manufacturing conditions based on the inhibitorless method, and the magnetic properties of the obtained grain-oriented electrical steel sheets were evaluated. The specific procedure will be described below. In the following experiments, grain-oriented electrical steel sheets were manufactured by a one-time method in which cold rolling was performed only once.

First, molten steel was prepared by vacuum melting to obtain a steel slab. As the steel slab, by mass%,
C: 0.050%,
Si: 3.6%,
Mn: 0.08%,
sol. Al: 0.0050%,
N: 0.0025%,
S: 0.0010%,
Se: Contains 0.0010%
A steel slab having a component composition in which the balance was composed of Fe and unavoidable impurities was used.

The steel slab was heated to 1250 ° C. and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 2.3 mm. Then, the hot-rolled plate was annealed under the condition of the maximum temperature reached: 1010 ° C. In the hot-rolled sheet annealing, a different cooling rate was adopted for each grain-oriented electrical steel sheet to be manufactured in order to change the value of fs described later.

Next, the hot-rolled steel sheet after annealing was subjected to cold rolling to obtain a cold-rolled steel sheet having a final plate thickness of 0.23 mm. Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing. The primary recrystallization _{annealing, 50% H 2 -50% N} 2, under a humid atmosphere with a dew point of 50 ° C., soaking temperature: 850 ° C., soaking time: was carried out at 100 seconds conditions.

An annealing separator mainly composed of MgO was applied to the surface of the cold-rolled steel sheet after the primary recrystallization annealing, and then purified annealing accompanied by secondary recrystallization was performed. In the purified annealing, it was retained at 1200 ° C. for 5 hours in a hydrogen atmosphere. After the purification annealing, further flattening annealing at 830 ° C. for 40 seconds was performed to obtain a grain-oriented electrical steel sheet.

In order to evaluate the magnetic properties of the obtained grain-oriented electrical steel sheet, the magnetic flux density when excited at 800 A / m: B ₈ (T) was measured by the method specified by JIS C 2550.

Further, in order to evaluate the influence of the microstructure of the steel sheet during cold rolling on the magnetic properties, the following equation (1) is used for the microstructure in the cross section perpendicular to the rolling direction of the steel sheet after the hot-rolled sheet is annealed. The area ratio fs defined in is measured. The fs value was measured by the method described in Examples described later using a secondary electron image obtained using a scanning electron microscope (SEM).
fs = [Area of non-diffusion transformation phase] / ([Area of non-diffusion transformation phase] + [Area of pearlite]) ... (1)

The values of the obtained area ratio fs (dimensionless) and magnetic flux density B ₈ (T) are shown in Table 1 and FIG. As can be seen from this result, in the production of grain-oriented electrical steel sheet by the inhibitorless method, the microstructure before cold rolling is finally controlled so that fs is 0.35 or more and 0.75 or less. The magnetic flux density of the obtained grain-oriented electrical steel sheet can be remarkably improved. The reason why the magnetic flux density is improved in this way is not completely clear, but it is considered as follows.

As described above, in the production of grain-oriented electrical steel sheets, annealing is performed before cold rolling. For example, in the case of the one-time method in which cold rolling is performed only once as in the above experiment, annealing is performed after hot rolling and before cold rolling, and this annealing is called hot-rolled plate annealing.

The microstructure of the annealed steel sheet performed before cold rolling is mainly composed of a ferrite phase (α phase) as the first phase, but also contains a carbon-containing phase as the second phase. I'm out. Then, depending on the production conditions, the second phase may contain a non-diffusion transformation phase, pearlite, and cementite (Fe ₃ C) precipitated at the grain boundaries. Then, the fs value is the ratio of the area of the non-diffusion transformation phase to the total area of the non-diffusion transformation phase and pearlite in the microstructure as defined by the above equation (1).

The non-diffusion transformation phase is formed when the cooling rate during cooling in the annealing step is sufficiently high. On the other hand, the pearlite phase is formed by the diffusion of C and the precipitation of cementite when the cooling in the annealing step proceeds relatively slowly. The non-diffusion transformation phase and the pearlite phase are considered to have the following characteristics, respectively.

The non-diffusion transformation phase has a higher hardness than the pearlite phase and is less likely to be deformed during cold rolling. Therefore, if a non-diffusion transformation phase is present, it is considered that the ferrite phase around the non-diffusion transformation phase is deformed. In the absence of the non-diffusion transformation phase, during cold rolling, the deformation proceeds so as to accumulate on the γ fiber according to the slip surface. Here, the γ fiber refers to an texture in which the {111} plane has an orientation parallel to the rolled plane. On the other hand, when a non-diffusion transformation phase is present, the non-diffusion transformation phase interferes with the slip deformation, so that it is considered that shear deformation rather than slip deformation is likely to occur around the non-diffusion transformation phase.

On the other hand, the pearlite phase is a structure in which ferrite and cementite are deposited in layers. Of these, cementite is less susceptible to deformation due to its high hardness, but since the pearlite phase as a whole has a layered structure, it deforms so that the layer slides in the direction parallel to the layer during cold rolling. Can be done. Therefore, it is considered that the pearlite phase does not hinder the slip deformation as much as the non-diffusion transformation phase.

Therefore, it is considered that when the pearlite phase increases, the {111} azimuth grains increase in the steel sheet after the primary recrystallization annealing, and when the non-diffusion transformation phase increases, the {411} azimuth grains increase in the steel sheet after the primary recrystallization annealing. ..

For example, as shown in Table 1, the reason why the magnetic flux density decreases when fs is less than 0.35 is that the ratio of the pearlite phase is too high and the {111} directional particles are excessive. Is considered to be. Further, it is considered that the reason why the magnetic flux density decreases when fs exceeds 0.75 is that the ratio of the non-diffusion transformation phase is too high and the {411} directional particles are excessive. On the other hand, when the ratio of the non-diffusion transformation phase and the pearlite phase is in an appropriate range, the shear deformation and the slip deformation are balanced, and the {411} orientation grains and the {111} orientation grains of the texture are balanced. It is considered that good magnetic characteristics were obtained because the balance was improved.

In the above experiment, since the grain-oriented electrical steel sheet was manufactured by using the one-time method in which cold rolling was performed only once, the microstructure of the steel sheet to be subjected to cold rolling was formed before the cold rolling. It is affected by the conditions of hot-rolled sheet annealing. However, when cold rolling is performed twice or more, it is affected by the conditions of intermediate annealing performed immediately before the final cold rolling (final cold rolling). Therefore, when the annealing performed immediately before the last cold rolling of one or more times of cold rolling is defined as the final pre-cold pre-annealing, the microstructure of the steel sheet after the final cold pre-rolling is defined. It is important to control fs to be 0.35 or more and 0.75 or less.

The present invention has been made based on the above findings, and its gist structure is as follows.

1. 1. By mass%
C: 0.020 to 0.10%,
Si: 2.0-6.5%,
Mn: 0.005 to 0.50%,
sol. Al: less than 0.010%,
N: less than 0.0050% S: less than 0.0050%, and Se: less than 0.0050%, including
A steel slab having a component composition in which the balance is composed of Fe and unavoidable impurities is heated to a slab heating temperature of 1280 ° C. or lower.
The heated steel slab is hot-rolled to form a hot-rolled steel sheet.
The hot-rolled steel sheet is annealed by hot-rolled steel sheet, and then the hot-rolled steel sheet is annealed.
The hot-rolled steel sheet after annealing is subjected to one cold-rolling or two or more cold-rolling sandwiching an intermediate annealing to obtain a cold-rolled steel sheet.
The cold-rolled steel sheet is subjected to primary recrystallization annealing that also serves as decarburization annealing.
An annealing separator is applied to the surface of the cold-rolled steel sheet after the primary recrystallization annealing.
The cold-rolled steel sheet was subjected to purification annealing accompanied by secondary recrystallization, and then subjected to purification annealing.
A method for producing a grain-oriented electrical steel sheet, in which the cold-rolled steel sheet after purification annealing is flattened and annealed.
When the annealing performed immediately before the last cold rolling of the one or more times of cold rolling is defined as the final cold pre-rolling annealing, the direction of the steel sheet after the final cold pre-rolling annealing is defined. A method for producing a directional electromagnetic steel sheet, wherein the area ratio fs defined by the following equation (1) in the microstructure in a vertical cross section is 0.35 or more and 0.75 or less.
fs = [Area of non-diffusion transformation phase] / ([Area of non-diffusion transformation phase] + [Area of pearlite]) ... (1)

2. In the final pre-cold annealing,
The maximum temperature reached is 1000 ° C or higher and 1150 ° C or lower, and
When the temperature at which the γ phase transforms into the α phase in the equilibrium state is Tα,
The average cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. is 50 ° C./s or more.
The average cooling rate in the temperature range between Tα-200 ° C. and 350 ° C. is 20 ° C./s or less.
The method for manufacturing a grain-oriented electrical steel sheet according to 1 above.

3. 3. In the primary recrystallization annealing that also serves as the decarburization annealing,
The method for manufacturing a grain-oriented electrical steel sheet according to 1 or 2 above, wherein the heating rate in the temperature rising section of 500 to 680 ° C. is 100 to 400 ° C./s.

4. When the component composition is mass%,
Cr: 0.01-0.50%,
Cu: 0.01-0.50%,
Ni: 0.01-0.50%,
Bi: 0.005 to 0.50%,
B: 0.0002 to 0.0025%,
Nb: 0.0010 to 0.0100%,
Sn: 0.010 to 0.400%,
Sb: 0.010 to 0.150%,
Mo: 0.010 to 0.200%, and P: 1 or 2 or more selected from the group consisting of 0.010 to 0.150%.
The method for manufacturing a grain-oriented electrical steel sheet according to any one of 1 to 3 above.

According to the present invention, a grain-oriented electrical steel sheet having a high magnetic flux density can be manufactured by an inhibitorless method that is excellent in terms of cost.

It is a graph which shows the relationship between the area ratio fs and the magnetic flux density B _{8 (T).}

Next, the method of carrying out the present invention will be specifically described.

[Steel slab]
In the present invention, the steel slab used for manufacturing the grain-oriented electrical steel sheet needs to have the above-mentioned component composition. Therefore, first, the reason for limiting the component composition in the present invention will be described. In addition, "%" regarding a component composition shall mean "mass%" unless otherwise specified.

C: 0.020 to 0.10%
C is an element necessary for controlling the microstructure of the steel sheet. If the C content is less than 0.020%, the amount of precipitation of the pearlite phase is insufficient, so that the magnetic flux density decreases. Therefore, the C content is 0.020% or more, preferably 0.025% or more. On the other hand, if C remains in the finally obtained grain-oriented electrical steel sheet, it causes deterioration of magnetic properties due to magnetic aging. Therefore, in the production of grain-oriented electrical steel sheet, primary recrystallization annealing that also serves as decarburization annealing is performed. The C content is reduced. However, if the C content of the steel slab is higher than 0.10%, the C content cannot be sufficiently reduced even by decarburization annealing. Therefore, the C content of the steel slab is 0.10% or less, preferably 0.050% or less. The C content in the finally obtained grain-oriented electrical steel sheet is not particularly limited, but is preferably 0.005% or less from the viewpoint of suppressing magnetic aging.

Si: 2.0-6.5%
Si is an element necessary for increasing the specific resistance of steel and improving iron loss. In order to obtain the above effect, the Si content is set to 2.0% or more, preferably 2.5% or more. On the other hand, if the Si content exceeds 6.5%, the workability of the steel deteriorates and rolling becomes difficult. Therefore, the Si content is 6.5% or less, preferably 4.5% or less.

Mn: 0.005 to 0.50%
Mn is an element required to obtain good hot workability. In order to obtain the above effect, the Mn content is 0.005% or more, preferably 0.03% or more. On the other hand, when the Mn content exceeds 0.50%, the magnetic flux density of the finally obtained grain-oriented electrical steel sheet decreases. Therefore, the Mn content is 0.50% or less, preferably 0.20% or less.

The present invention relates to the production of grain-oriented electrical steel sheets by the inhibitorless method. Therefore, it is necessary to reduce the amounts of the inhibitor-forming components Al, N, S, and Se in the steel slab as much as possible. Specifically, the content of each of the above elements is in the following range.

sol. The Al content is less than 0.010%, preferably less than 0.007%. The N content is less than 0.0050%, preferably less than 0.0040%. The S content is less than 0.0050%, preferably less than 0.0030%. The Se content is less than 0.0050%, preferably less than 0.0030%.

In the present invention, a steel slab containing each of the above elements and having a component composition in which the balance is composed of Fe and unavoidable impurities is used. In one embodiment of the present invention, a steel slab having a component composition consisting of only the above elements, the remaining Fe, and unavoidable impurities can also be used.

In addition, the above-mentioned component composition is added to each of the above-mentioned elements.
Cr: 0.01-0.50%,
Cu: 0.01-0.50%,
Ni: 0.01-0.50%,
Bi: 0.005 to 0.50%,
B: 0.0002 to 0.0025%,
Nb: 0.0010 to 0.0100%,
Sn: 0.010 to 0.400%,
Sb: 0.010 to 0.150%,
It may further contain 1 or 2 or more selected from the group consisting of Mo: 0.010 to 0.200% and P: 0.010 to 0.150%.

Each of the above elements has an action of improving the magnetic characteristics of the grain-oriented electrical steel sheet, but if the content is lower than the above lower limit value, the effect of improving the magnetic characteristics of the element cannot be sufficiently obtained. On the other hand, when the content exceeds the above upper limit value, the development of secondary recrystallized grains is suppressed, so that the magnetic characteristics are rather deteriorated.

As the steel slab, any steel slab can be used without particular limitation as long as it has the above-mentioned composition. The method for producing the steel slab is not particularly limited, and the steel slab can be produced by any method. For example, a steel slab can be produced by an ingot forming method or a continuous casting method using molten steel adjusted to a predetermined composition. Further, a thin slab having a thickness of 100 mm or less can be produced by using a direct casting method, and this can be used as a steel slab. Since it is difficult to add each of the above-mentioned elements in the intermediate process, it is preferable to add each element at the stage of molten steel.

[heating]
-Slab heating temperature: 1280 ° C. or lower Prior to hot rolling, the slab is heated to the slab heating temperature. When an inhibitor is used, it is necessary to raise the slab heating temperature in order to dissolve the inhibitor component in solid solution. However, since the inhibitor is not used in the present invention, it is not necessary to raise the slab heating temperature. Therefore, from the viewpoint of cost reduction, the slab heating temperature is set to 1280 ° C. or lower, preferably 1250 ° C. or lower.

[Hot rolling]
Next, the steel slab heated to the slab heating temperature is hot-rolled to obtain a hot-rolled steel sheet. The conditions for hot rolling are not particularly limited, and the hot rolling can be performed under any conditions.

[Annealed hot-rolled plate]
Next, the hot-rolled steel sheet is annealed. The conditions for annealing the hot-rolled plate are not particularly limited, and can be performed under any conditions. When the one-time method in which cold rolling is performed only once is adopted, it is preferable to control the conditions for hot-rolled sheet annealing from the viewpoint of controlling the fs value. Specific conditions will be described later.

[Cold rolling]
Next, the hot-rolled steel sheet after annealing is subjected to cold rolling to obtain a cold-rolled steel sheet. The cold rolling may be performed only once (one-time method), or may be performed twice or more. When cold rolling is performed twice or more, intermediate annealing is performed between each cold rolling step. For example, in the case of the two-time method in which cold rolling is performed twice, hot-rolled sheet annealing, first cold rolling, intermediate annealing, and second cold rolling may be performed in this order.

The conditions for cold rolling and intermediate annealing are not particularly limited, and can be performed under any conditions. When cold rolling is performed twice or more, the condition of intermediate annealing performed immediately before the last cold rolling of the two or more cold rollings is set from the viewpoint of controlling the fs value. It is preferable to control. Specific conditions will be described later.

From the viewpoint of further improving the magnetic characteristics, it is effective to perform at least one of the above-mentioned one or two or more cold rollings in a state where the temperature is raised to 100 ° C. to 300 ° C. Further, it is also effective to perform the aging treatment at a temperature of 100 to 300 ° C. once or twice or more during the cold rolling (between passes) from the viewpoint of further improving the magnetic characteristics.

[Primary recrystallization annealing]
Next, the obtained cold-rolled steel sheet is subjected to primary recrystallization annealing that also serves as decarburization. The conditions for the primary recrystallization annealing are not particularly limited, and can be performed under any conditions. From the viewpoint of improving the decarburization property, it is preferable that the annealing temperature in the primary recrystallization annealing is 800 ° C. or higher and 900 ° C. or lower. Similarly, from the viewpoint of improving the decarburization property, it is preferable to make the annealing atmosphere a moist atmosphere.

Further, in the temperature raising process of the primary recrystallization annealing, if the temperature rising rate in the temperature rising section of 500 to 680 ° C. is 100 ° C./s or more, the {110} orientation grains are further increased and the magnetic characteristics are further improved. Can be made to. Therefore, it is preferable that the temperature rising rate in the temperature rising section of 500 to 680 ° C. is 100 ° C./s or more. However, increasing the rate of temperature rise also leads to an increase in {411} orientation grains. Therefore, the rate of temperature rise in the temperature rise section of 500 to 680 ° C. is preferably 400 ° C./s or less. By setting the temperature rising rate to 400 ° C./s or less, the increase of {411} directional grains can be suppressed and secondary recrystallization can be generated more stably. From the viewpoint of further reducing the frequency of occurrence of defective materials, it is more preferable that the temperature rising rate is 350 ° C./s or less.

[Application of annealing separator]
Then, an annealing separator is applied to the surface of the cold-rolled steel sheet after primary recrystallization annealing. When forming a forsterite film with an emphasis on iron loss, it is possible to develop a secondary recrystallization structure and form a forsterite film by applying an annealing separator containing MgO and then performing purification annealing. Is. As the annealing separator containing MgO, it is preferable to use an annealing separator containing MgO as a main component. On the other hand, when the forsterite film is not required with an emphasis on punching workability, an annealing separator containing no MgO that forms the forsterite film may be used. As the annealing separator containing no MgO, for example, an annealing separator containing at least one of silica and alumina can be used. When applying these annealing separators, it is effective to perform electrostatic coating that does not bring in moisture. Further, a heat-resistant inorganic material sheet (silica, alumina, mica) may be used.

[Purified annealing]
Then, purification annealing with secondary recrystallization is performed. The purification annealing can be carried out under any conditions without particular limitation, but the annealing temperature is preferably 800 ° C. or higher for the expression of secondary recrystallization. Further, from the viewpoint of more reliably completing the secondary recrystallization, it is preferable to hold the annealing temperature (holding temperature) of 800 ° C. or higher for 20 hours or longer.

Generally, in the purification annealing step, a forsterite film is formed by the reaction between the oxide present on the surface of the steel sheet and the annealing separator. It is not necessary to form a forsterite film. When the forsterite film is not formed, the holding temperature in the purified annealing is preferably 850 to 950 ° C. In that case, the purification annealing can be completed only by holding at the holding temperature. On the other hand, when forming a forsterite film for improving iron loss and reducing transformer noise, it is preferable to raise the temperature to 1100 to 1300 ° C.

After the purification annealing, it is preferable to remove the annealing separator adhering to the surface of the steel sheet. The method of removal is not particularly limited, and for example, one or two or more selected from the group consisting of washing with water, brushing, and pickling can be used.

[Flatration annealing]
Next, the cold-rolled steel sheet after purification annealing is subjected to flattening annealing. By the flattening annealing, the shape of the steel sheet can be corrected and the iron loss can be reduced.

[Insulation coating]
Although not essential in the present invention, an insulating coating can be formed on the surface of the grain-oriented electrical steel sheet. By providing the insulating coating, it is possible to reduce the iron loss when the grain-oriented electrical steel sheets are laminated and used. The insulating coating can be formed, for example, before or after flattening annealing. The material of the insulating coating is not particularly limited, and a coating made of any insulating material can be used, and an inorganic coating is generally used.

The method for forming the insulating coating is not particularly limited, and for example, a method of applying a coating treatment liquid, a physical vapor deposition method, a chemical vapor deposition method, or the like can be used. When the method of applying the coating treatment liquid is used, the coating treatment liquid can be baked by applying the coating before the flattening annealing and then performing the flattening annealing. The composition of the coating treatment liquid is not particularly limited, but for example, a treatment liquid containing phosphate and silica can be used.

[Magnetic domain subdivision processing]
Further magnetic domain subdivision treatment can be performed in order to further reduce iron loss. The method of the magnetic domain subdivision treatment is not particularly limited, and any method can be used. Examples of the magnetic domain subdivision treatment method include a method of introducing at least one of thermal strain and impact strain by irradiating the surface of the obtained directional electromagnetic steel sheet with a laser, an electron beam, plasma, or the like, or in the middle of a manufacturing process. For example, a method of forming a groove on the surface of a steel sheet by mechanical processing or etching after cold rolling can be mentioned.

[Area ratio fs]
In the present invention, a directional electromagnetic steel sheet is manufactured by sequentially performing each step as described above, and at that time, as described above, a cross section perpendicular to the rolling direction of the steel sheet after final pre-rolling annealing. It is necessary to control the microstructure in. Specifically, the area ratio fs defined by the following equation (1) in the microstructure in the cross section perpendicular to the rolling direction of the steel sheet after final cold rolling pre-annealing is set to 0.35 or more and 0.75 or less.
fs = [Area of non-diffusion transformation phase] / ([Area of non-diffusion transformation phase] + [Area of pearlite]) ... (1)

Here, the "final cold rolling" refers to the last cold rolling of the one or more cold rollings. For example, in the case of the one-time method in which cold rolling is performed only once, the one-time cold rolling is the final cold rolling. In the case of the two-time method in which cold rolling is performed twice, the second cold rolling is the final cold rolling.

Further, here, the “annealing before final cold rolling” refers to the annealing performed immediately before the “final cold rolling” defined as described above. For example, in the case of the one-time method in which cold rolling is performed only once, the hot-rolled sheet annealing performed before the one cold rolling is the final cold-rolled pre-annealing. Further, in the case of the two-time method in which the cold rolling is performed twice, the intermediate annealing performed between the first cold rolling and the second cold rolling is the final cold pre-rolling annealing.

As already explained with reference to the experimental data shown in Table 1, cold rolling is performed by controlling the microstructure before cold rolling so that the area ratio fs is 0.35 or more and 0.75 or less. The balance between the {411} azimuth grains and the {111} azimuth grains in the later steel sheet can be improved, and as a result, the magnetic flux density of the finally obtained directional electromagnetic steel sheet can be remarkably improved. In both the case where fs is less than 0.35 and the case where fs exceeds 0.75, the magnetic flux density of the finally obtained grain-oriented electrical steel sheet decreases. From the viewpoint of further improving the magnetic flux density, fs is preferably 0.40 or more. From the same viewpoint, fs is preferably 0.72 or less. The fs value can be measured by the method described in Examples described later using a secondary electron image obtained by using a scanning electron microscope (SEM).

The method of controlling the area ratio fs within the above range is not particularly limited, and any method can be used. Below, as an example of the method, a method of controlling the conditions of final pre-cold annealing pre-annealing will be described.

-Maximum temperature reached: 1000 ° C or higher and 1150 ° C or lower The maximum reached temperature (annealing temperature) in the final pre-rolling annealing is 1000 ° C or higher, preferably 1020 ° C, in order to completely recrystallize the structure formed by hot rolling. That's all. On the other hand, if the maximum temperature reached exceeds 1150 ° C., the crystal grains after annealing become remarkably coarse, which is extremely disadvantageous in realizing the subsequent primary recrystallization structure of the granules. Therefore, the maximum temperature reached is 1150 ° C. or lower, preferably 1100 ° C. or lower.

In addition, in order to realize fs satisfying the above conditions by controlling the annealing conditions, it is necessary to strictly control the cooling rate in a specific temperature range in the cooling process of the final pre-cooling annealing. Specifically, when the temperature at which the γ (austenite) phase transforms into the α (ferrite) phase in the equilibrium state is Tα, the temperature range between Tα + 100 ° C. and Tα-200 ° C. and Tα-200 ° C. to 350 ° C. The cooling rate in the temperature range between the two must satisfy the following conditions.

-Average cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C .: 50 ° C./s or more The average cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. is 50 ° C./s. By cooling the temperature range at a high speed of 50 ° C./s or more, most of the phases can be made into non-diffusion transformation phases. The average cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. is preferably 70 ° C./s or higher.

-Average cooling rate in the temperature range between Tα-200 ° C. and 350 ° C.: 20 ° C./s or less The average cooling rate in the temperature range between Tα-200 ° C. and 350 ° C. is 20 ° C./s or less. By cooling the temperature range at a low speed of 20 ° C./s or less, the pearlite transformation proceeds in the non-diffusion transformation phase.

The cooling conditions of 350 ° C. or lower are not limited, but it is preferable to cool to 100 ° C. or lower at 10 ° C./s or higher.

The value of Tα can be calculated by thermodynamic equilibrium calculation based on the CALPHAD method. Specifically, as described in the examples, it can be obtained by using Thermo-Calc (Thermo-Calc Software), which is commercially available thermodynamic equilibrium calculation software. The cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. is controlled in consideration of the deviation from the equilibrium state in the actual manufacturing process. By controlling the cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. as described above, stable and good magnetic characteristics can be obtained.

When the cold rolling is performed twice or more, the intermediate annealing before the final cold rolling may be performed under the above conditions, and the hot rolling plate annealing can be performed under any conditions. For example, hot-rolled plate annealing can be performed under the conditions of a soaking temperature of 900 to 1150 ° C. and a soaking time of 10 seconds or more.

Next, the present invention will be described in more detail based on Examples. The following examples show a suitable example of the present invention, and the present invention is not limited to the above examples.

(Example 1)
A grain-oriented electrical steel sheet was manufactured by a one-time method according to the procedure described below, and its magnetic properties were evaluated.

-Steel slab A steel slab having the composition shown in Table 2 was produced by a continuous casting method. The value of Tα was determined using Thermo-Calc (Thermo-Calc Software), which is thermodynamic equilibrium calculation software.

-Heating to hot-rolled sheet annealing The obtained steel slab was heated to a slab heating temperature of 1280 ° C. and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 2.2 mm. Then, the hot-rolled steel sheet was annealed by hot-rolled steel sheet. In the hot-rolled plate annealing, the annealing temperature of 1050 ° C. was maintained for 20 seconds, and then the mixture was cooled under the conditions shown in Table 3. After 350 ° C., the temperature was cooled to 20 ° C. within 30 seconds.

-Measurement of fs Next, the microstructure of the steel sheet after hot-rolled sheet annealing was observed by the following procedure, and the fs value was measured. First, an observation sample was taken from the steel sheet so that the cross section perpendicular to the rolling direction of the steel sheet was the observation surface. After etching the observation sample with nital, it was observed with a scanning electron microscope (SEM) at a magnification of 2000 times, and 20 secondary electron images were taken. As for the visual field, the non-diffusion transformation phase and the pearlite phase occupy 30% or more of the visual field area. The obtained secondary electron images were image-analyzed, the total area of the non-diffusion transformation phase for 20 sheets and the total area of the pearlite phase were obtained, and the area ratio fs was calculated. The measurement results are also shown in Table 3.

- cold rolling - flattening annealing then finished to a plate thickness of 0.23mm at cold rolling, further 150 seconds at _{830 ℃, 60% H 2 -40} % N 2, under a humid atmosphere with a dew point of 55 ° C., Primary recrystallization annealing with decarburization was performed. Further, an annealing separator mainly containing MgO was applied, and purification annealing was carried out at 1200 ° C. for 10 hours with secondary recrystallization in which it was retained in a hydrogen atmosphere. Then, flattening annealing was performed at 800 ° C. for 40 seconds.

_{-Measurement of magnetic flux density For each of the obtained grain-oriented electrical steel sheets, B 8} , which is the magnetic flux density when excited at 800 A / m, was measured by the method specified in JIS C 2550. The measurement results are also shown in Table 3.

As is clear from the results shown in Table 3, the grain-oriented electrical steel sheet manufactured under the conditions satisfying the conditions of the present invention has a high magnetic flux density.

(Example 2)
A grain-oriented electrical steel sheet was manufactured by a two-step method according to the procedure described below, and its magnetic properties were evaluated. The conditions were the same as in Example 1 unless otherwise specified.

A steel slab made of the steel types shown in Table 4 was heated to a slab heating temperature of 1250 ° C. and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 2.0 mm. Then, the hot-rolled steel sheet was annealed at 950 ° C. for 20 seconds.

Then, the first cold rolling was performed to obtain a thickness of 1.3 mm, and then intermediate annealing was performed. In the intermediate annealing, the annealing temperature was kept at 1080 ° C. for 60 seconds, and then the mixture was cooled under the conditions shown in Table 4. The microstructure of the steel sheet after the intermediate annealing was observed in the same manner as in Example 1, and the fs value was measured. The measurement results are also shown in Table 4.

Then, the second cold rolling was performed to obtain a cold-rolled steel sheet having a final sheet thickness of 0.20 mm. Then subjected for 60 seconds at _{850 ℃, 55% H 2 -45} % N 2, the primary recrystallization annealing with the decarburization in a wet atmosphere having a dew point of 60 ° C.. Further, an annealing separator mainly containing MgO was applied, and purification annealing accompanied by secondary recrystallization annealing was carried out at 1225 ° C. for 10 hours in a hydrogen atmosphere. After the purification annealing, flattening annealing was performed at 800 ° C. for 40 seconds to obtain a grain-oriented electrical steel sheet. _{The magnetic flux density: B 8} was measured for each of the obtained grain-oriented electrical steel sheets by the same method as in Example 1. The measurement results are also shown in Table 4.

As is clear from the results shown in Table 4, the grain-oriented electrical steel sheet manufactured under the conditions satisfying the conditions of the present invention has a high magnetic flux density even in the double method.

(Example 3)
A grain-oriented electrical steel sheet was manufactured by a one-time method according to the procedure described below, and its magnetic properties were evaluated. The conditions were the same as in Example 1 unless otherwise specified.

The steel slab of steel type Y shown in Table 2 was heated to a slab heating temperature of 1250 ° C. and then hot-rolled to obtain a hot-rolled steel sheet having a thickness of 2.2 mm. Then, the hot-rolled steel sheet was annealed at 1040 ° C. for 20 seconds. At that time, the average cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. was controlled to 70 ° C./s, and the average cooling rate from Tα-200 ° C. or lower to 350 ° C. was set to 15 ° C./s. Next, the microstructure of the steel sheet after hot-rolled sheet annealing was observed in the same procedure as in Example 1, and the fs value was measured.

Then, finishing the plate thickness of 0.23mm at cold rolling, further 60 seconds at _{850 ℃, 55% H 2 -45} % N 2, primary recrystallization annealing with the decarburization in a wet atmosphere having a dew point of 60 ° C. Was given. At this time, the rate of temperature rise in the temperature rise section of the primary recrystallization annealing at 500 to 680 ° C. was set as shown in Table 5.

Further, an annealing separator mainly containing MgO was applied, and purification annealing was carried out at 1225 ° C. for 10 hours with secondary recrystallization in which it was retained in a hydrogen atmosphere. After the purification annealing, flattening annealing was performed at 800 ° C. for 40 seconds to obtain a grain-oriented electrical steel sheet. _{The magnetic flux density B 8} was measured for each of the obtained grain-oriented electrical steel sheets by the same method as in Example 1. The measurement results are also shown in Table 5.

As can be seen from the results shown in Table 5, in the primary recrystallization annealing, a higher magnetic flux density can be obtained by setting the temperature rise rate in the temperature rise section of 500 to 680 ° C to 100 to 400 ° C / s. ..

Claims (4)

By mass%
C: 0.020 to 0.10%,
Si: 2.0-6.5%,
Mn: 0.005 to 0.50%,
sol. Al: less than 0.010%,
N: less than 0.0050% S: less than 0.0050%, and Se: less than 0.0050%, including
A steel slab having a component composition in which the balance is composed of Fe and unavoidable impurities is heated to a slab heating temperature of 1280 ° C. or lower.
The heated steel slab is hot-rolled to form a hot-rolled steel sheet.
The hot-rolled steel sheet is annealed by hot-rolled steel sheet.
The hot-rolled steel sheet after annealing is subjected to one cold-rolling or two or more cold-rolling sandwiching an intermediate annealing to obtain a cold-rolled steel sheet.
The cold-rolled steel sheet is subjected to primary recrystallization annealing that also serves as decarburization annealing.
An annealing separator is applied to the surface of the cold-rolled steel sheet after the primary recrystallization annealing.
The cold-rolled steel sheet was subjected to purification annealing accompanied by secondary recrystallization, and then subjected to purification annealing.
A method for producing a grain-oriented electrical steel sheet, in which the cold-rolled steel sheet after purification annealing is flattened and annealed.
When the annealing performed immediately before the last cold rolling of the one or more times of cold rolling is defined as the final cold pre-rolling annealing, the direction of the steel sheet after the final cold pre-rolling annealing is defined. A method for producing a directional electromagnetic steel sheet, wherein the area ratio fs defined by the following equation (1) in the microstructure in a vertical cross section is 0.35 or more and 0.75 or less.
fs = [Area of non-diffusion transformation phase] / ([Area of non-diffusion transformation phase] + [Area of pearlite]) ... (1) In the final pre-cold annealing,
The maximum temperature reached is 1000 ° C or higher and 1150 ° C or lower, and
When the temperature at which the γ phase begins to transform into the α phase in the equilibrium state is Tα,
The average cooling rate in the temperature range between Tα + 100 ° C. and Tα-200 ° C. is 50 ° C./s or more.
The average cooling rate in the temperature range between Tα-200 ° C. and 350 ° C. is 20 ° C./s or less.
The method for manufacturing a grain-oriented electrical steel sheet according to claim 1. In the primary recrystallization annealing that also serves as the decarburization annealing,
The method for manufacturing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the heating rate in the temperature rising section of 500 to 680 ° C. is 100 to 400 ° C./s. When the component composition is mass%,
Cr: 0.01-0.50%,
Cu: 0.01-0.50%,
Ni: 0.01-0.50%,
Bi: 0.005 to 0.50%,
B: 0.0002 to 0.0025%,
Nb: 0.0010 to 0.0100%,
Sn: 0.010 to 0.400%,
Sb: 0.010 to 0.150%,
Mo: 0.010 to 0.200%, and P: 1 or 2 or more selected from the group consisting of 0.010 to 0.150%.
The method for manufacturing a grain-oriented electrical steel sheet according to any one of claims 1 to 3.

Images