WO2025220520A1 - Method for manufacturing oriented electromagnetic steel sheet - Google Patents

Abstract

Provided is a method for manufacturing an oriented electromagnetic steel sheet having excellent magnetic characteristics and capable of reducing the heating temperature of a steel slab to at most 1,300ºC. This method for manufacturing an oriented electromagnetic steel sheet comprises subjecting a steel slab to hot rolling, hot-rolled sheet annealing, cold rolling, primary recrystallization annealing, and secondary recrystallization annealing, wherein, in the hot-rolled sheet annealing, first-stage soaking holding in which the hot-rolled steel sheet is held at a soaking temperature of 350-1,000ºC for a soaking time of 3.5-120 s is performed at least once, and second-stage soaking holding in which the hot-rolled steel sheet is held at a soaking temperature of higher than 1,000ºC and at most 1,100ºC is performed.

Description

Manufacturing method of grain-oriented electrical steel sheet

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

Grain-oriented electrical steel sheet is a soft magnetic material in which the crystal orientation of Fe-Si polycrystals is concentrated in the {110}<001> orientation (hereinafter referred to as the "Goss orientation") using secondary recrystallization, aligning the axis of easy magnetization in the rolling direction. Grain-oriented electrical steel sheet has low iron loss at commercial frequencies and can achieve high magnetic flux density with a low excitation field, making it primarily used as the iron core material for electrical equipment such as transformers. The iron loss of grain-oriented electrical steel sheet is expressed as the sum of hysteresis loss, which depends on factors such as the crystal orientation and purity of the steel sheet, and eddy current loss, which depends on factors such as sheet thickness, resistivity, and magnetic domain size. One known method for reducing hysteresis loss is to increase the concentration of Goss orientation and improve magnetic flux density. Other known methods for reducing eddy current loss include increasing the content of elements such as Si, which increase electrical resistance, reducing the steel sheet thickness, and refining the magnetic domains.

Among these methods for reducing iron loss, a method that has been put into industrial use involves increasing the degree of concentration of grain-oriented electrical steel sheets in the Goss orientation by using precipitates known as inhibitors to differentiate the mobility of grain boundaries during final annealing. Patent Document 1 discloses a method that uses AlN as an inhibitor, while Patent Document 2 discloses a method that uses MnS or MnSe as inhibitors. These methods require the steel slab to be heated to high temperatures of over 1,300°C in order to completely dissolve the constituent elements that make up the inhibitor in the steel. This has posed issues in terms of the energy consumption required for high-temperature heating and the cost of equipment.

To address these issues, Patent Document 3 discloses a method for secondary recrystallization of Goss-oriented grains without using inhibitors, using high-purity material and trace amounts of nitrogen to reveal the grain boundary misorientation angle dependence of the grain boundary energy of crystal grain boundaries during primary recrystallization. Patent Document 4 also discloses a method for producing grain-oriented electrical steel sheet with excellent magnetic properties, using a composition system that actively avoids inhibitors by minimizing Al, S, N, and Se, elements that can form inhibitors, but which cannot be completely removed in industrial-scale production. This method is said to achieve excellent magnetic properties by performing hot-rolled sheet annealing, with an average heating rate of 50°C/s or more from room temperature to 400°C, a time of 100 seconds or less to reach 900°C from 400°C, and soaking at a temperature of 950°C or higher.

Special Publication No. 40-15644 Japanese Unexamined Patent Publication No. 49-61019 Japanese Patent Application Laid-Open No. 2000-129356 JP 2017-160489 A

J. Kunze, Pungun O, K. Friedrich, J. Mater. Sci. Lett., 5(1986) 815-818.

The manufacturing methods for grain-oriented electrical steel sheets disclosed in Patent Documents 3 and 4 either do not use inhibitors at all or do not actively use inhibitors, thereby successfully reducing the heating temperature of the steel slab to 1,300°C or below. However, because these grain-oriented electrical steel sheets do not actively use inhibitors, depending on the manufacturing conditions, the degree of concentration in the Goss orientation may not necessarily be sufficient compared to conventional grain-oriented electrical steel sheets that actively use inhibitors, leaving room for improvement.

The present invention was made in consideration of the above-mentioned problems, and aims to provide a method for manufacturing grain-oriented electrical steel sheet that has excellent Goss orientation concentration and magnetic properties, and that can reduce the heating temperature of the steel slab to 1,300°C or less.

The inventors, in relation to the method of manufacturing grain-oriented electrical steel sheet disclosed in Patent Document 4, have searched for new heat treatment conditions for the hot-rolled sheet annealing performed on hot-rolled steel sheet obtained by hot rolling, to further increase the concentration of Goss orientation and stably obtain excellent magnetic properties. As a result, they discovered that grain-oriented electrical steel sheet with excellent magnetic properties can be manufactured by performing one or more extremely short soaking periods in the temperature range of 350°C or higher and 1000°C or lower before the conventional soaking period, and have thus completed the present invention.

The gist of the present invention is as follows:

[1] In mass ratio,
C: 0.0020% or more, 0.100% or less,
Si: 2.0% or more, 6.5% or less,
Mn: 0.020% or more, 1.00% or less,
sol. Al: 10 ppm or more and less than 100 ppm,
A steel slab having a chemical composition containing N: 10 ppm or more and 50 ppm or less, and S: 10 ppm or more and 50 ppm or less, with the balance being Fe and unavoidable impurities is prepared;
The steel slab is heated to a temperature of 1300°C or less, and then hot-rolled to form a hot-rolled steel sheet;
The hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet.
The hot-rolled annealed sheet is subjected to cold rolling once or twice or more times with intermediate annealing interposed therebetween to obtain a cold-rolled steel sheet having a final sheet thickness;
The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallization annealed sheet;
A method for producing a grain-oriented electrical steel sheet, comprising applying an annealing separator to the surface of the primary recrystallization annealed sheet and then performing secondary recrystallization annealing,
In the hot-rolled sheet annealing,
The hot-rolled steel sheet is subjected to a first-stage soaking holding step at least once, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less;
a second-stage soaking step in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and not more than 1100°C.
[2] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein, in the hot-rolled sheet annealing, the average heating rate when heating the hot-rolled steel sheet from 50°C to 350°C is 50°C/s or more.
[3] The component composition further comprises, in mass ratio:
Sb: 0.01% or more, 0.50% or less,
Sn: 0.01% or more, 0.50% or less,
Ni: 0.005% or more, 1.5% or less,
Cu: 0.005% or more, 1.5% or less,
Cr: 0.005% or more, 0.10% or less,
P: 0.005% or more, 0.50% or less,
Mo: 0.005% or more, 0.50% or less,
Ti: 0.0005% or more, 0.10% or less,
Nb: 0.0005% or more, 0.10% or less,
Bi: 0.005% or more, 0.10% or less,
Se: 10 ppm or more, 50 ppm or less,
Ca: 0.0005% or more, 0.0050% or less,
B: 0.0001% or more, 0.0020% or less,
V: 0.0005% or more, 0.10% or less,
Pb: 0.0002% or more, 0.050% or less,
The method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], wherein the steel sheet contains one or more elements selected from the group consisting of As: 0.0005% or more and 0.010% or less, and Zn: 0.0005% or more and 0.010% or less.

The manufacturing method of the present invention allows the heating temperature of the steel slab to be reduced to 1,300°C or less by using a chemical composition that contains trace amounts of sol. Al, N, and S, which are elements that form inhibitors. Furthermore, by performing a two-stage soaking process during the annealing of hot-rolled steel sheet, it is possible to manufacture grain-oriented electrical steel sheet with excellent magnetic properties.

1 is a graph for explaining a temperature profile of hot-rolled sheet annealing. 10 is a graph showing the relationship between the first-stage soaking temperature and the magnetic flux density. 10 is a graph showing the relationship between the first-stage soaking time and magnetic flux density. 10 is a graph showing the relationship between the first-stage soaking temperature and soaking time and the magnetic flux density.

The following provides a detailed description of the embodiments of the present invention.

In one embodiment, the present invention provides a method for producing a medicament for the treatment of a pulmonary arthritis.
In mass ratio,
C: 0.0020% or more, 0.100% or less,
Si: 2.0% or more, 6.5% or less,
Mn: 0.020% or more, 1.00% or less,
sol. Al: 10 ppm or more and less than 100 ppm,
A steel slab having a chemical composition containing N: 10 ppm or more and 50 ppm or less, and S: 10 ppm or more and 50 ppm or less, with the balance being Fe and unavoidable impurities is prepared;
The steel slab is heated to a temperature of 1300°C or less, and then hot-rolled to form a hot-rolled steel sheet;
The hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet.
The hot-rolled annealed sheet is subjected to cold rolling once or twice or more times with intermediate annealing interposed therebetween to obtain a cold-rolled steel sheet having a final sheet thickness;
The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallization annealed sheet;
A method for producing a grain-oriented electrical steel sheet, comprising applying an annealing separator to the surface of the primary recrystallization annealed sheet and then performing secondary recrystallization annealing,
In the hot-rolled sheet annealing,
The hot-rolled steel sheet is subjected to a first-stage soaking holding step at least once, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less;
The invention is a method for producing a grain-oriented electrical steel sheet, characterized in that the hot-rolled steel sheet is subjected to a second soaking stage in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and not more than 1100°C.

<Component composition>
The contents of elements contained in the steel slab prepared in the above embodiment will be described. In this specification, the contents of elements are expressed as mass ratios. The symbol "%" indicates percentages by mass. The symbol "ppm" indicates parts per million by mass.

1. Essential Elements C: 0.0020% or More, 0.100% or Less C is an element necessary for preventing brittle fracture when steel is heated to high temperatures. If the C content is 0.0020% or more, embrittlement at high temperatures is suppressed, thereby preventing brittle fracture of steel slabs during casting and hot rolling. If the C content is 0.100% or less, the C content can be reduced to 0.005% or less by decarburization treatment. This prevents magnetic aging due to incomplete decarburization. For this reason, the C content is set to 0.0020% or more, 0.100% or less. The C content is preferably 0.020% or more.

Si: 2.0% or more, 6.5% or less Si is an element necessary for increasing the resistivity of steel and reducing iron loss. If the Si content is 2.0% or more, it is effective in reducing iron loss. If the Si content is 6.5% or less, hot rolling and cold rolling can be easily performed. Therefore, the Si content is set to 2.0% or more, 6.5% or less. The Si content is preferably 2.5% or more. The Si content is preferably 4.0% or less.

Mn: 0.020% or more, 1.00% or less Mn is an element necessary for improving the hot workability of steel. If the Mn content is 0.020% or more, the hot workability is improved. If the Mn content is 1.00% or less, the magnetic flux density of the grain-oriented electrical steel sheet does not decrease significantly. Therefore, the Mn content is set to 0.020% or more, 1.00% or less. The Mn content is preferably 0.040% or more. The Mn content is preferably 0.30% or less.

Sol. Al: 10 ppm or more, less than 100 ppm. Al is an important element in grain-oriented electrical steel sheets because it combines with N dissolved in the steel to form AlN and precipitate, functioning as an inhibitor that suppresses normal grain growth of primary recrystallization grains during primary recrystallization annealing. However, in order for AlN to function as an inhibitor, it is necessary to prevent Al segregation in the steel slab and dissolve Al evenly throughout the steel. In conventional techniques, the Al content was set to 100 ppm or more in order to actively utilize AlN as an inhibitor. In this case, the steel slab had to be heated to a high temperature of over 1300°C in order to dissolve Al in the steel.

In the present invention, AlN is also used as an inhibitor, but the content of acid-soluble Al in the steel slab is set to 10 ppm or more and less than 100 ppm, so the amount of AlN is less than that of conventional techniques. Therefore, a heating temperature of 1300°C or less is sufficient for dissolving Al. Al is classified as acid-soluble Al or acid-insoluble Al depending on the difficulty of dissolving it in acid. Acid-soluble Al is used to improve the properties of steel sheets by being contained in steel in the form of solid solution Al or _AlN . Acid-insoluble Al is contained in steel in the form of _Al2O3 , etc., but its small amount has little effect on the properties. For this reason, the content of acid-soluble Al is specified in the present invention.

If the acid-soluble Al (hereinafter referred to as "sol. Al") content is 10 ppm or more, the necessary amount of AlN precipitates as the above-mentioned inhibitor, improving the magnetic flux density of the steel sheet. If the sol. Al content is less than 100 ppm, Al can be dissolved in the steel slab by heating to 1300°C or less, as described above. Therefore, the sol. Al content is set to 10 ppm or more and less than 100 ppm. The sol. Al content is preferably 80 ppm or less. The sol. Al content in the steel slab can be measured using, for example, the method specified in Japanese Industrial Standard JIS G 1257-10-2 (2013) or other known methods.

N: 10 ppm or more, 50 ppm or less As described above, N combines with Al and precipitates to form AlN, which acts as an inhibitor. If the N content is 10 ppm or more, a necessary amount of AlN precipitates as the inhibitor, improving the magnetic flux density of the steel sheet. If the N content is 50 ppm or less, there is no risk of N contained in the steel slab separating as nitrogen gas during hot rolling and causing blistering. Therefore, the N content is set to 10 ppm or more, 50 ppm or less. The N content is preferably 25 ppm or less.

S: 10 ppm or more and 50 ppm or less S combines with Mn to form MnS. If the S content is 10 ppm or more, the formed MnS functions as an inhibitor, improving the magnetic flux density of the steel sheet. If the S content is 50 ppm or less, the deterioration of the function of the inhibitor due to Ostwald ripening can be prevented. Therefore, the S content is set to 10 ppm or more and 50 ppm or less. The S content is preferably 25 ppm or less.

In the above embodiment, the steel slab prepared has a chemical composition consisting of the above elements, with the remainder consisting of Fe and unavoidable impurities.

2. Additional Components In a preferred embodiment, the present invention relates to a composition of matter, wherein the composition further comprises, in mass ratio:
Sb: 0.01% or more, 0.50% or less,
Sn: 0.01% or more, 0.50% or less,
Ni: 0.005% or more, 1.5% or less,
Cu: 0.005% or more, 1.5% or less,
Cr: 0.005% or more, 0.10% or less,
P: 0.005% or more, 0.50% or less,
Mo: 0.005% or more, 0.50% or less,
Ti: 0.0005% or more, 0.10% or less,
Nb: 0.0005% or more, 0.10% or less,
Bi: 0.005% or more, 0.10% or less,
Se: 10 ppm or more, 50 ppm or less,
Ca: 0.0005% or more, 0.0050% or less,
B: 0.0001% or more, 0.0020% or less,
V: 0.0005% or more, 0.10% or less,
Pb: 0.0002% or more, 0.050% or less,
The present invention relates to a method for producing a grain-oriented electrical steel sheet, which contains one or more elements selected from the group consisting of As: 0.0005% or more and 0.010% or less, and Zn: 0.0005% or more and 0.010% or less.

All of these elements are useful for improving magnetic properties. When the respective contents are equal to or greater than the lower limit of the above range, the magnetic properties are improved. When the respective contents are equal to or less than the upper limit of the above range, the formation of texture due to secondary recrystallization is not hindered. Among the added elements, Se combines with Mn to form MnSe. When Se is 10 ppm or more, the formed MnSe functions as an inhibitor, improving the magnetic flux density of the steel sheet. When Se is 50 ppm or less, the degradation of the function of the inhibitor due to Ostwald ripening can be prevented. Therefore, the Se content is preferably 10 ppm or more and 50 ppm or less. The Se content is more preferably 25 ppm or less.

Next, we will explain the manufacturing conditions for the grain-oriented electrical steel sheet in the above embodiment.

<Steel slab>
In the method for producing a grain-oriented electrical steel sheet according to the present invention, first, a steel slab having the chemical composition described above is prepared. The steel slab can be prepared by a general production method. The steel slab can be produced by an ingot casting method in which molten steel having a predetermined chemical composition is poured into a mold and then cooled and solidified. Alternatively, the steel slab can be produced by a continuous casting method in which molten steel is first received in a tundish, then poured into a water-cooled mold to solidify, and the steel slab is continuously withdrawn from the bottom of the mold. The method for producing the steel slab prepared in the present invention may be any of the above methods.

Among the essential elements mentioned above, the contents of C, Si, and Mn can be adjusted by changing the compounding ratio of the raw materials used when producing molten steel in various steelmaking furnaces. If necessary, the contents of C, Si, and Mn can be further adjusted by adding additional additives to the molten steel once it has been received in the ladle from the steelmaking furnace. Since it is difficult to add the additive elements mentioned above during steelmaking, it is also preferable to add them to the molten steel once it has been received in the ladle.

On the other hand, of the essential elements mentioned above, the contents of sol. Al, N, and S are often already contained as unavoidable impurities in the raw materials used when producing molten steel. When the contents of sol. Al, N, and S originally contained in molten steel satisfy the numerical ranges of content specified in this invention, steel slabs can be produced without adjusting the contents of these essential elements. When the contents of sol. Al, N, or S are less than the lower limit values mentioned above, the elements can be adjusted so that the contents are equal to or greater than the lower limit values by adding additional additives to the molten steel received in the ladle.

Ferrosilicon and ferromanganese may be used as raw materials for the Si and Mn essential components mentioned above. If the ferrosilicon and ferromanganese contain a large amount of C and S, the amount of C and S contained in the steel slab may exceed the upper limit values mentioned above. In such cases, it is preferable to adjust the composition of the molten steel using high-purity ferrosilicon and ferromanganese with low C and S contents.

<Hot rolling>
In the method for producing a grain-oriented electrical steel sheet according to the present invention, the prepared steel slab is then heated to a temperature of 1300°C or less, and then hot-rolled to produce a hot-rolled steel sheet. As described above, in the method for producing a grain-oriented electrical steel sheet according to the present invention, the contents of sol. Al, N, and S, which are elements that form inhibitors, are kept low among the chemical compositions contained in the steel slab. Therefore, even if the heating temperature of the steel slab is 1300°C or less, these elements can be sufficiently dissolved in the steel, thereby reducing the cost of heating the steel slab. The heating temperature of the steel slab is preferably 1100°C or more. Known means such as a gas furnace, an induction heating furnace, or an electric furnace can be used to heat the steel slab.

When hot rolling a heated steel slab, it is preferable to first perform one or more passes of rough rolling at a temperature of 1100°C or higher and 1300°C or lower, and then two or more passes of finish rolling at a temperature of 800°C or higher and 1100°C or lower, from the perspective of controlling the structure of the hot-rolled steel sheet. The total reduction in finish rolling is preferably 80% or higher. By setting the total reduction in the temperature range of 1100°C or lower to 80% or higher, dislocations are introduced into the hot-rolled steel sheet at a high density. Dislocations serve as nucleation sites for precipitates, contributing to the formation of fine, high-density precipitates and improving magnetic properties. Note that the temperature in hot rolling is based on the temperature at the surface of the steel sheet.

The hot-rolled steel sheet obtained by hot rolling is preferably coiled into a coil shape for easier handling. The coiling temperature for the hot-rolled steel sheet is preferably 400°C or higher and 750°C or lower, from the standpoints of both controlling the structure of carbides in the hot-rolled steel sheet and preventing defects such as cracks. The coiling temperature is more preferably 500°C or higher. The coiling temperature is more preferably 700°C or lower. The coiling temperature for the hot-rolled steel sheet is based on the temperature of the steel sheet surface immediately before coiling.

As described above, in the method for producing grain-oriented electrical steel sheet according to the present invention, the content of sol. Al solid-solved in the steel slab is reduced to less than 100 ppm. Therefore, during the hot rolling process, the N solid-solved in the steel slab hardly combines with the sol. Al, but combines with the Si contained in the steel slab in large amounts to form silicon nitride (Si ₃ N _{4 )} . Non-Patent Document 1 describes the results of an investigation into the solubility of nitrogen in Fe—Si alloys. According to this, the maximum temperature at which Si ₃ N ₄ can stably exist in an Fe—Si alloy is expressed as a function of the Si content in the Fe—Si alloy. For example, when the Si content is 3%, Si ₃ N ₄ is thought to stably exist in a temperature range of approximately 900°C or lower.

<Hot-rolled sheet annealing>
In the method for producing a grain-oriented electrical steel sheet according to the present invention, the hot-rolled steel sheet is then subjected to hot-rolled annealing to produce an annealed hot-rolled steel sheet. The hot-rolled annealing in the present invention is performed to replace Si contained in _{Si3N4 precipitates} formed in the steel during hot rolling with sol. Al to form AlN. FIG. 1 is a graph illustrating the temperature profile of hot-rolled annealing. As shown in FIG. 1, the hot-rolled steel sheet is subjected to one or more first-stage soaking holds in which it is held at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 seconds or higher and 120 seconds or lower, followed by a second-stage soaking hold in which it is held at a soaking temperature of higher than 1000°C and lower than 1100°C. The heat treatment conditions for each temperature range are described in detail below with reference to FIG. 1 as appropriate.

1. Temperature Range from 50°C to 350°C In the method for producing a grain-oriented electrical steel sheet according to the present invention, preferably, the average heating rate when heating the hot-rolled steel sheet from 50°C to 350°C during hot-rolled sheet annealing is 50°C/s or more. In this specification, the "average heating rate" refers to the average heating rate within a certain temperature range, specifically, the value obtained by dividing the difference between the minimum and maximum temperatures within that temperature range by the time required for heating. The temperature range from 50°C to 350° _C corresponds to the stage immediately prior to the first-stage soaking described below. By quickly heating the hot-rolled steel sheet in this temperature range at an average heating rate of 50°C/s or more, coarsening of _Si3N4 is prevented, _{and the first-stage soaking can be started while maintaining a dense distribution of fine Si3N4 precipitates} in the hot-rolled steel sheet. This improves the magnetic flux density of the steel sheet. There is no particular upper limit to the average rate of temperature increase when the temperature is increased from 50° C. to 350° C., but the average rate of temperature increase can be 500° C./s or less.

In the present invention, the heating rate in the temperature range of the hot-rolled steel sheet below 50°C has almost no effect on the morphology of _{Si3N4 precipitates} . Therefore, even if the temperature rise is started from a temperature below 50°C, the lower limit of the temperature range for evaluating the average heating rate may be 50°C.

2. Temperature Range of 350°C or More and 1000°C or Less In the method for producing a grain-oriented electrical steel sheet according to the present invention, the hot-rolled steel sheet is then subjected to one or more first-stage soaking in the hot-rolled sheet annealing step, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less. As shown in FIG. 1 , the first-stage soaking is performed after the previous temperature-raising step from 50°C to 350°C is completed, in which the hot-rolled steel sheet is heated to a soaking temperature _T1 and held for a soaking time _t1 . If _T1 is less than 350°C, Al diffusion is difficult _to proceed, and AlN precipitation does not proceed, resulting in a decrease in magnetic flux density. If _T1 is more than 1000°C, _Si3N4 dissolution occurs before AlN precipitation, resulting in a decrease in magnetic flux density of the steel sheet. For this reason, the soaking temperature _T1 is set to 350°C or more and 1000°C or less. The soaking temperature _T1 is preferably 400° C. or higher, more preferably 500° C. or higher. The soaking temperature _T1 is preferably 900° C. or lower.

If the first-stage soaking is not performed at all or if the first-stage soaking time _t1 is less than 3.5 seconds, there is insufficient time for the substitution of Si contained in the _Si3N4 precipitates with _sol . Al to proceed, resulting in a decrease in the magnetic flux density of the steel sheet. If the first-stage soaking time _t1 exceeds 120 seconds, excessive precipitation and coarsening of AlN will occur, resulting in a decrease in the magnetic flux density of the steel sheet. For this reason, the first-stage soaking time _t1 is set to 3.5 seconds or more and 120 seconds or less. The soaking time _t1 is preferably 5.0 seconds or more. The soaking time _t1 is preferably 60 seconds or less, more preferably 30 seconds or less.

Although it is unclear why the magnetic flux density of steel sheets is improved when soaking is performed for a short period of time compared to when the temperature is increased at a constant average rate in the temperature range of 350°C to 1000°C, the inventors believe the following. With soaking, the substitution of sol. Al for _Si in _Si3N4 progresses until a certain time, and Ostwald ripening occurs when sol. Al reaches saturated precipitation. This results in a uniform AlN grain size and improved magnetic flux density. On the other hand, with heating, the solid solution of _Si in _Si3N4 is promoted with increasing temperature, so the substitution by sol. Al progresses earlier. In this case, the AlN grain size is nonuniform from the beginning, making it more difficult to homogenize the AlN grain size in the subsequent process than with soaking. As a result, the magnetic flux density decreases. Therefore, soaking rather than continuously increasing the temperature in the temperature range of 350°C to 1000°C is essential for improving magnetic flux density.

In this specification, "soaking" refers to maintaining the temperature of a hot-rolled steel sheet at a constant target temperature during hot-rolled sheet annealing. In this specification, if the temperature fluctuates within ±5.0°C of the target temperature, the time during which the steel sheet temperature is within that range is considered to be "soaking." The temperature of a hot-rolled steel sheet during hot-rolled sheet annealing can be measured using known methods.

In the method for manufacturing grain-oriented electrical steel sheet according to the present invention, the first-stage soaking may be performed once, or may be repeated two or more times. When the first-stage soaking is performed two or more times, a cooling period may be provided between the soaking holds. The temperature of the second and subsequent soaking holds may be the same as the temperature of the first soaking hold, or may be higher or lower than the temperature of the first soaking hold as long as it is within the temperature range of 350°C or higher and 1000°C or lower. When the first-stage soaking is performed multiple times, the soaking time for each hold is 3.5 seconds or longer and 120 seconds or shorter.

In the temperature range of hot-rolled sheet annealing, AlN is more stable than _{Si3N4 precipitates} . Therefore, the substitution reaction of Si contained in _Si3N4 precipitates with sol. Al occurs irreversibly, and once generated, AlN does not return to _{Si3N4 precipitates} . Therefore, the opportunity to generate fine AlN from _{Si3N4 precipitates} is limited to the hot _- rolled sheet annealing process.

3. Temperature Range of More Than 1000°C and Less Than 1100°C In the method for producing a grain-oriented electrical steel sheet according to the present invention, the hot-rolled steel sheet is subsequently subjected to a second-stage soaking in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and less than 1100° _C during the hot-rolled sheet annealing. As shown in FIG. 1 , the second-stage soaking is performed after the completion of the first-stage soaking, which is the previous step, by raising the temperature of the hot-rolled steel sheet to a soaking temperature T2 and holding it for a soaking time _t2 . This holding allows the AlN precipitate diameter to be optimized by Ostwald ripening. If the soaking temperature _T2 is 1000°C or less, the precipitation diameter is not sufficiently adjusted by Ostwald ripening, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. If the soaking temperature _T2 is more than 1100°C, AlN is excessively coarsened or re-dissolved, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. Therefore, the soaking temperature _T2 is set to be higher than 1000°C and equal to or lower than 1100°C. The soaking temperature _T2 is preferably equal to or lower than 1050°C. The second-stage soaking time _t2 is not particularly limited. The soaking time _t2 is preferably equal to or longer than 10 seconds. The soaking time _t2 is preferably equal to or shorter than 60 seconds.

4. Shape of hot-rolled steel sheet in hot-rolled sheet annealing As described above, in hot-rolled sheet annealing, the hot-rolled steel sheet is repeatedly subjected to rapid heating or cooling and soaking. When the hot-rolled steel sheet after hot rolling is coiled into a coil shape, the hot-rolled steel sheet can be rapidly heated or cooled by unwinding the coil to return it to its original shape and performing hot-rolled sheet annealing using a known continuous annealing furnace. Performing hot-rolled sheet annealing in the shape of a hot-rolled steel sheet is also preferable in terms of performing appropriate temperature control with a small temperature fluctuation range during soaking after rapid heating or cooling.

<Cold rolling>
In the method for producing a grain-oriented electrical steel sheet according to the present invention, the hot-rolled and annealed sheet is then cold-rolled once or twice or more times with intermediate annealing in between to produce a cold-rolled steel sheet having a final thickness. The final thickness of the cold-rolled steel sheet is preferably 0.30 mm or less. If the final thickness of the cold-rolled steel sheet is 0.30 mm or less, eddy current loss can be reduced. The final thickness of the cold-rolled steel sheet is more preferably 0.23 mm or less, and even more preferably 0.20 mm or less. There is no particular lower limit for the final thickness of the cold-rolled steel sheet, but the final thickness in cold rolling is technically limited to approximately 0.10 mm or more.

Cold rolling may be performed once or twice or more times. When cold rolling is performed twice or more times, intermediate annealing is performed between cold rolling passes. Intermediate annealing is preferably performed at an annealing temperature of 900°C or higher and 1200°C or lower. If the annealing temperature is 900°C or higher, the recrystallized grains do not become too fine, and the number of nuclei with Goss orientation in the primary recrystallized structure increases, improving the magnetic flux density of the steel sheet. If the annealing temperature is 1200°C or lower, the recrystallized grains do not become coarse, and a primary recrystallized structure with uniform grain size can be achieved, also improving the magnetic flux density of the steel sheet. In the final stage of cold rolling, it is preferable to warm-roll the steel sheet by heating it to a temperature of 100°C or higher and 300°C or lower, or to perform aging treatment one or more times at a temperature of 100°C or higher and 300°C or lower between passes, as this increases the concentration of the recrystallized texture and improves the magnetic flux density of the steel sheet.

In cold rolling, it is preferable to perform rolling at least once with a reduction of 80% or more. Cold rolling with a reduction of 80% or more is advantageous in that it increases the concentration of recrystallized texture and improves the magnetic flux density of the steel sheet.

<Primary recrystallization annealing>
In the method for producing a grain-oriented electrical steel sheet according to the present invention, the cold-rolled steel sheet is then subjected to primary recrystallization annealing to obtain a primarily recrystallization annealed sheet. The primary recrystallization annealing may also serve as decarburization annealing. The annealing temperature for the primary recrystallization annealing is preferably 800°C or higher and 900°C or lower, and the atmosphere is preferably a moist atmosphere, in order to perform decarburization. However, if the C content in the steel slab is 0.0050% or lower, there is no need to further reduce the C content, and therefore the atmosphere for the primary recrystallization annealing may be other than the above. In order to increase the magnetic flux density of the steel sheet, it is preferable that the average heating rate to the holding temperature in the primary recrystallization annealing be 50°C/s or higher and 400°C/s or lower.

<Secondary recrystallization annealing>
In the method for producing a grain-oriented electrical steel sheet according to the present invention, an annealing separator is then applied to the surface of the primarily recrystallized annealed sheet, followed by secondary recrystallization annealing. An annealing separator mainly composed of MgO is used as the annealing separator. The secondary recrystallization annealing allows secondary recrystallization grains having a Goss orientation to develop and allows a forsterite film to be formed on the surface of the steel sheet. It is preferable to perform the secondary recrystallization annealing at a temperature of 800°C or higher for 20 hours or more in order to induce and complete secondary recrystallization. To form a forsterite film, it is preferable to perform the secondary recrystallization annealing at a temperature of 1200°C or higher.

<Post-processing>
After secondary recrystallization annealing, the steel sheet is washed with water, brushed, or pickled to remove the annealing separator adhering to the surface of the steel sheet, and then further subjected to flattening annealing to correct the shape of the steel sheet, which is effective for reducing iron loss.

When steel sheets are used in a stack, it is effective to apply an insulating coating to the surface of the steel sheets before or after flattening annealing in order to improve iron loss. In this case, applying a coating that can impart tension to the steel sheet is preferable in terms of reducing iron loss. Coating methods that can impart tension to the steel sheet include, for example, a tension coating application method in which a binder is used in the coating, and a coating method in which an inorganic substance is deposited on the surface of the steel sheet by physical vapor deposition or chemical vapor deposition. These coatings are preferable as they have excellent adhesion and are effective in reducing iron loss.

To further reduce iron loss, it is preferable to perform magnetic domain refinement. A commonly used method for magnetic domain refinement, for example, can be used, such as applying distortion to the final product sheet using an electron beam or laser. The target for magnetic domain refinement is not only the final product sheet, but also intermediate products such as cold-rolled steel sheets that have reached their final thickness.

The following describes examples of the present invention. Please note that the embodiments of the present invention are not limited to the following examples, and can be modified as desired without departing from the spirit of the present invention.

Example 1
A steel slab having a composition containing, by mass, 0.055% C, 3.2% Si, 0.12% Mn, 80 ppm sol.Al, 35 ppm N, and 32 ppm S, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting, heated to 1200°C for 60 minutes, and then hot-rolled to form a hot-rolled steel sheet with a thickness of 2.3 mm. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing according to the temperature pattern shown in Figure 1. The average heating rate from 50°C to 350°C was 50°C/s. The first-stage soaking time _t1 was fixed at 30 s, and the soaking temperature _T1 was changed from 300°C to 1020°C. The second-stage soaking temperature _T2 was set to 1030°C, and the soaking time _t2 was set to 30 s. The hot-rolled sheet annealing atmosphere was a dry nitrogen atmosphere. After the hot-rolled sheet annealing, scale on the surface of the annealed hot-rolled sheet was removed by pickling, and then the sheet was cold-rolled to obtain a cold-rolled steel sheet having a final thickness of 0.23 mm.

Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 830°C for 150 seconds in a humid atmosphere of 50 vol% N ₂ -50 vol% H ₂ with a dew point of 50°C, to obtain a primary recrystallization annealed sheet. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primary recrystallization annealed sheet, and secondary recrystallization annealing was performed at 1200°C for 5 hours in a hydrogen atmosphere to obtain 17 types of grain-oriented electrical steel sheet samples with different first-stage soaking temperatures. Next, the magnetic flux density _B8 of the obtained samples was measured using the method specified in Japanese Industrial Standard JIS C 2500 when the magnetic field strength applied to the sample was 800 A/m. The relationship between the first-stage soaking temperature _T1 and magnetic flux density _B8 is shown in Figure 2.

According to FIG. 2, it can be seen that an excellent magnetic flux density _B8 of 1.925 T or more was obtained when the first-stage soaking temperature _T1 was in the range of 350° C. or more and 1000° C. or less, indicated by the broken line.

Example 2
A steel slab having a composition containing, by mass, 0.048% C, 3.3% Si, 0.12% Mn, 83 ppm sol.Al, 41 ppm N, and 30 ppm S, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting, heated to 1230°C for 60 minutes, and then hot-rolled to form a hot-rolled steel sheet with a thickness of 2.3 mm. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing according to the temperature pattern shown in Figure 1. The average heating rate from 50°C to 350°C was 50°C/s. The first-stage soaking temperature _T1 was fixed at 750°C, and the soaking time _t1 was varied from 0 s to 120 s. The second-stage soaking temperature _T2 was 1010°C, and the soaking time _t2 was 30 s. The hot-rolled sheet annealing atmosphere was a dry nitrogen atmosphere. After the hot-rolled sheet annealing, scale on the surface of the annealed hot-rolled sheet was removed by pickling, and then the sheet was cold-rolled to obtain a cold-rolled steel sheet having a final thickness of 0.23 mm.

Next, the obtained cold-rolled steel sheets were subjected to primary recrystallization annealing and secondary recrystallization annealing under the same conditions as in Example 1 to obtain 16 types of grain-oriented electrical steel sheet samples with different first-stage soaking holding times. Next, the magnetic flux density _B8 of the obtained samples was measured using the same method as in Example 1. The relationship between the first-stage soaking time _t1 and the magnetic flux density _B8 is shown in Figure 3. In Figure 3, the scale of the soaking time _t1 on the horizontal axis is a logarithmic scale.

According to FIG. 3, it can be seen that an excellent magnetic flux density _B8 of 1.925 T or more was obtained when the first-stage soaking time _t1 was in the range of 3.5 seconds or more and 120 seconds or less, indicated by the broken line.

Example 3
A steel slab having a composition containing, by mass, 0.035% C, 3.3% Si, 0.13% Mn, 75 ppm sol. Al, 42 ppm N, 11 ppm S, and 0.075% Sb, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to 1160°C for 60 minutes, and then hot-rolled to form a hot-rolled steel sheet with a thickness of 2.4 mm. The resulting hot-rolled steel sheet was subjected to hot-rolled sheet annealing using the temperature pattern shown in Figure 1. However, unlike Figure 1, the average heating rate from 50°C to 350°C was changed within a range from 20°C/s to 100°C/s. Table 1 shows the average heating rate from 50°C to 350°C, the first-stage soaking temperature _T1 and soaking time _t1 , and the second-stage soaking temperature _T2 and soaking time _t2 . The hot-rolled steel sheet was annealed in a dry nitrogen atmosphere. After the annealing, the surface scale of the annealed hot-rolled steel sheet was removed by pickling, and then the sheet was cold-rolled to a thickness of 1.6 mm. Next, intermediate annealing was performed in a humid atmosphere of 70 vol% N ₂ -30 vol% H ₂ with a dew point of 40°C, and then cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.20 mm.

Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 60 seconds in a humid atmosphere of 50 vol% N ₂ -50 vol% H ₂ with a dew point of 50°C, to obtain a primary recrystallization annealed sheet. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primary recrystallization annealed sheet, and secondary recrystallization annealing was performed at 1220°C for 5 hours in a hydrogen atmosphere to obtain 40 types of grain-oriented electrical steel sheet samples with different manufacturing conditions. Next, the magnetic flux density _B8 of the obtained samples was measured using the same method as in Example 1. The results are shown in Table 1.

Furthermore, the results shown in Table 1, excluding Samples No. 2, No. 29, and No. 35, are plotted on a graph with the horizontal axis representing the soaking time _t1 and the vertical axis representing the soaking temperature _T1, as shown in Figure 4. In Figure 4, the horizontal axis representing the soaking time _t1 is scaled logarithmically. In Figure 4, white squares represent invention examples in which the magnetic flux density _B8 is 1.925 T or more. Black squares represent comparative examples in which the magnetic flux density _B8 is less than 1.925 T. Furthermore, in Figure 4, the invention examples and comparative examples of Example 1 are plotted with white circles and black circles, and the invention examples and comparative examples of Example 2 are plotted with white triangles and black triangles, respectively.

According to FIG. 4, it can be seen that an excellent magnetic flux density B8 of 1.925 T or more was obtained when the first-stage soaking temperature _T1 was 350° C. or more and 1000° C. or less and the soaking time _t1 was 3.5 s or more and ₁₂₀ s or less, in the range indicated by the dashed line.

Example 4
A steel slab containing the essential and additional elements shown in Table 2 by mass ratio, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. The slab was heated to 1200°C for 60 minutes and then hot-rolled to form a hot-rolled steel sheet with a thickness of 2.5 mm. The resulting hot-rolled steel sheet was subjected to hot-rolled sheet annealing using the temperature pattern shown in Figure 1. The average heating rate from 50°C to 350°C was 50°C/s. The first-stage soaking temperature _T1 was 750°C, and the soaking time _t1 was 20 seconds. The second-stage soaking temperature _T2 was 1030°C, and the soaking time _t2 was 30 seconds. The hot-rolled sheet annealing atmosphere was a humid atmosphere of 80 vol% _N2 - 20 vol% _CO2 with a dew point of 30°C. After the hot-rolled sheet annealing, scale on the surface of the annealed hot-rolled sheet was removed by pickling, and then the sheet was subjected to warm rolling at 150°C and cold rolling to obtain a cold-rolled steel sheet with a final thickness of 0.27 mm.

Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 180 seconds in a humid atmosphere of 40 vol% N ₂ -60 vol% H ₂ with a dew point of 50°C, to obtain a primary recrystallization annealed sheet. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primary recrystallization annealed sheet, and secondary recrystallization annealing was performed at 1175°C for 15 hours in a hydrogen atmosphere to obtain 25 types of grain-oriented electrical steel sheet samples with different composition. Next, the magnetic flux density _B8 of the obtained samples was measured using the same method as in Example 1. The results are shown in Table 2.

Table 2 shows that the grain-oriented electrical steel sheet samples produced by the production method defined in the present invention using steel slabs having the chemical composition defined in the present invention or a preferred chemical composition had an excellent magnetic flux density _B8 of 1.925 T or more.

_T1 First stage soaking temperature _t1 First stage soaking time _T2 Second stage soaking temperature _t2 Second stage soaking time

Claims (3)

In mass ratio,
C: 0.0020% or more, 0.100% or less,
Si: 2.0% or more, 6.5% or less,
Mn: 0.020% or more, 1.00% or less,
sol. Al: 10 ppm or more and less than 100 ppm,
A steel slab having a chemical composition containing N: 10 ppm or more and 50 ppm or less, and S: 10 ppm or more and 50 ppm or less, with the balance being Fe and unavoidable impurities is prepared;
The steel slab is heated to a temperature of 1300°C or less, and then hot-rolled to form a hot-rolled steel sheet;
The hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet.
The hot-rolled annealed sheet is subjected to cold rolling once or twice or more times with intermediate annealing interposed therebetween to obtain a cold-rolled steel sheet having a final sheet thickness;
The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallization annealed sheet;
A method for producing a grain-oriented electrical steel sheet, comprising applying an annealing separator to the surface of the primary recrystallization annealed sheet and then performing secondary recrystallization annealing,
In the hot-rolled sheet annealing,
The hot-rolled steel sheet is subjected to a first-stage soaking holding step at least once, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less;
a second-stage soaking step in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and not more than 1100°C. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein, during the hot-rolled sheet annealing, the average heating rate when heating the hot-rolled steel sheet from 50°C to 350°C is 50°C/s or more. The component composition further comprises, in mass ratio:
Sb: 0.01% or more, 0.50% or less,
Sn: 0.01% or more, 0.50% or less,
Ni: 0.005% or more, 1.5% or less,
Cu: 0.005% or more, 1.5% or less,
Cr: 0.005% or more, 0.10% or less,
P: 0.005% or more, 0.50% or less,
Mo: 0.005% or more, 0.50% or less,
Ti: 0.0005% or more, 0.10% or less,
Nb: 0.0005% or more, 0.10% or less,
Bi: 0.005% or more, 0.10% or less,
Se: 10 ppm or more, 50 ppm or less,
Ca: 0.0005% or more, 0.0050% or less,
B: 0.0001% or more, 0.0020% or less,
V: 0.0005% or more, 0.10% or less,
Pb: 0.0002% or more, 0.050% or less,
The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, further comprising the step of: containing one or more elements selected from the group consisting of As: 0.0005% or more and 0.010% or less; and Zn: 0.0005% or more and 0.010% or less.