WO2025243810A1 - Grain-oriented electrical steel sheet production method - Google Patents

Abstract

Provided is a grain-oriented electrical steel sheet production method with which it is possible to produce a grain-oriented electrical steel sheet having better magnetic properties than conventional products in a component system in which an inhibitor is not actively used. This grain-oriented electrical steel sheet production method involves subjecting a steel slab to hot rolling, hot-rolled sheet annealing, cold rolling, primary recrystallization annealing, and secondary recrystallization annealing. In the final pass of the rough rolling in the hot rolling, the rolling temperature is set to 950-1150°C, and the reduction ratio is set to 25% or higher. In a temperature raising process in the hot-rolled sheet annealing, the average temperature increase rate when heating a hot-rolled steel sheet from 50°C to 350°C is set to 40°C/s or more.

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

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 sheets that can produce grain-oriented electrical steel sheets with magnetic properties superior to conventional steel sheets, using a composition system that does not actively use inhibitors.

The inventors, among the methods for producing grain-oriented electrical steel sheet disclosed in Patent Document 4, searched for new conditions for hot rolling and hot-rolled sheet annealing that would further increase the degree of concentration in the Goss orientation and consistently achieve excellent magnetic properties. As a result, they discovered that grain-oriented electrical steel sheet with excellent magnetic properties can be produced by setting the rolling temperature and reduction rate in the final pass of rough rolling in hot rolling, and the average heating rate in hot-rolled sheet annealing, within specific ranges, and thus completed the present invention.

The gist of the present invention is as follows:

[1] In mass ratio,
C: 0.002% or more, 0.100% or less,
Si: 2.0% or more, 6.5% or less,
Mn: 0.02% 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 final pass of rough rolling of the hot rolling, the rolling temperature is 950°C or more and 1150°C or less, and the rolling reduction is 25% or more;
10. The method for producing a grain-oriented electrical steel sheet, wherein, in the hot-rolled sheet annealing, the average temperature rise rate when raising the temperature of the hot-rolled steel sheet from 50°C to 350°C is 40°C/s or more.

[2] The method for producing grain-oriented electrical steel sheet described in [1] above, wherein during the temperature increase process of the hot-rolled sheet annealing, the hot-rolled steel sheet is subjected to one or more soaking periods in which it is held at a soaking temperature of 350°C or higher and 950°C or lower for 3.0 seconds or higher and 100 seconds or lower.

[3] The component composition further comprises, in mass ratio:
Se: 0.001% or more, 0.005% or less,
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,
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 makes it possible to produce grain-oriented electrical steel sheets with magnetic properties superior to conventional steel sheets using a composition system that does not actively utilize inhibitors.

1 is a graph showing the relationship between the reduction rate and magnetic flux density in the final pass of rough rolling in hot rolling.

First, we will explain the experiment that led to the idea for this invention.

<Experiment>
A steel slab having a composition consisting of, by mass, 0.055% C, 3.2% Si, 0.12% Mn, 76 ppm sol.Al, 33 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 obtain a hot-rolled steel sheet with a thickness of 2.3 mm. At this time, the rolling temperature in the final pass of rough rolling of the hot rolling was set to 1000 ° C, and the rolling reduction was changed in the range from 22% to 43%. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 1030 ° C for 10 s. At this time, the average heating rate from 50 ° C to 350 ° C was set to two conditions: 20 ° C/s and 50 ° C/s. In addition, some samples were subjected to soaking in which the surface temperature of the hot-rolled steel sheet was held at 800°C for 30 seconds during the temperature increase. 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% _H2 + 50 vol% _N2 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 32 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 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 reduction rate of the final pass of rough rolling in hot rolling and magnetic flux density _B8 is shown in Figure 1.

1 shows that an excellent magnetic flux density B8 of 1.925 T or more was obtained when the rolling reduction was 25% or more and the average temperature rise rate from 50°C to 350°C during hot-rolled sheet annealing was ₅₀ °C/s. In particular, it was found that an even more excellent magnetic flux density _B8 was obtained in samples that were subjected to soaking during the temperature rise during hot-rolled sheet annealing.

Next, we will explain in detail the form for implementing 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: 0.002% or more, 0.100% or less,
Si: 2.0% or more, 6.5% or less,
Mn: 0.02% 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 final pass of rough rolling of the hot rolling, the rolling temperature is 950°C or more and 1150°C or less, and the rolling reduction is 25% or more;
The invention is a method for producing a grain-oriented electrical steel sheet, characterized in that, in the hot-rolled sheet annealing, the average temperature rise rate when raising the temperature of the hot-rolled steel sheet from 50°C to 350°C is 40°C/s or more.

<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.

(Essential ingredient)
C: 0.002% 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 0.002% 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 makes it possible to avoid magnetic aging due to incomplete decarburization. For this reason, the C content is set to 0.002% 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: 0.02% or more, 1.00% or less Mn is an element necessary for improving the hot workability of steel. If the Mn content is 0.02% 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 0.02% or more, 1.00% or less. The Mn content is preferably 0.04% 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. When the S content is 10 ppm or more, the formed MnS functions as an inhibitor, improving the magnetic flux density of the steel sheet. When 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 40 ppm or less, more preferably 35 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.

(Optional addition ingredient)
In a preferred embodiment, the present invention relates to a composition of matter, wherein the composition further comprises, in mass ratio:
Se: 0.001% or more, 0.005% or less,
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,
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,
As: 0.0005% or more and 0.010% or less; and Zn: 0.0005% or more and 0.010% or less.
This is an invention of a manufacturing method for grain-oriented electrical steel sheets.

All of these elements are useful for improving magnetic properties. When the content of each is equal to or greater than the lower limit of the above range, the magnetic properties are improved. When the content of each is equal to or less than the upper limit of the above range, the formation of texture by secondary recrystallization is not hindered.

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

<Steel slab>
The steel slab is not particularly limited as long as it has the above-mentioned component composition. The method for producing the steel slab is not particularly limited, and known methods using a converter, an electric furnace, or the like can be used. From the viewpoint of productivity and the like, it is preferable to produce a slab (steel material) by continuous casting after the slab is produced, but the slab may also be produced by known casting methods such as ingot making-blooming rolling or thin slab continuous casting.

<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.

From the perspective of structural control of hot-rolled steel sheets, rough hot rolling is performed in one or more passes. One of the major features of the present invention is that in the final pass of rough hot rolling, the rolling temperature is set to 950°C or higher and 1150°C or lower, and the reduction ratio is set to 25% or higher. The temperature range of 950°C or higher and 1150°C or lower corresponds to the temperature range in which the gamma phase coexists. By performing the final pass reduction in this temperature range, the introduction of strain into the steel is promoted, making it easier for recrystallization of the stable rolling orientation {100}<011>, which is resistant to secondary recrystallization, to occur. By performing the final pass reduction with a high reduction ratio of 25% or higher in this temperature range, the steel sheet structure at the end of rough rolling can be made to have a high recrystallization ratio and a fine structure. This promotes the introduction of dislocations into the steel in the subsequent finish rolling, resulting in uniform and dense introduction of dislocations into the hot-rolled steel sheet. The rolling temperature in the final pass of rough rolling in hot rolling is preferably 980°C or higher and 1080°C or lower. The reduction in the final pass is preferably 30% or higher and 60% or lower. The rolling temperature in rough rolling is based on the temperature of the steel sheet surface.

By performing rough rolling in hot rolling under the above conditions, the magnetic flux density of the resulting final product is improved. The reason for this is not entirely clear, but the inventors believe it to _be as follows: In inhibitor-less materials, precipitates mainly composed of MnS and _Si3N4 are present in the hot-rolled steel sheet after hot rolling. During the hot-rolled sheet annealing process following hot rolling, Si contained _{in Si3N4} is replaced by Al to form AlN, and the formed AlN functions as an inhibitor. To improve the magnetic flux density of the final product, it is important to increase the amount of AlN precipitated and ensure that AlN is uniformly and finely distributed in the hot-rolled sheet annealed. To achieve this, it is necessary to select the hot-rolling conditions and the hot-rolled sheet annealing conditions so _{that Si3N4} precipitates uniformly and finely.

As described above, by performing reduction at a low temperature and a high reduction rate in the final pass of rough rolling in hot rolling, dislocations are introduced uniformly and densely into the hot-rolled steel sheet after finish rolling. Dislocations serve as nucleation sites for _Si3N4 , _which precipitates uniformly _and finely during the cooling process after hot rolling. Furthermore, by increasing the heating rate in _the subsequent hot-rolled sheet annealing process to prevent N diffusion and re-dissolution, a uniform and fine distribution of _Si3N4 is achieved, _and AlN inhibitors are generated uniformly and finely while maintaining the uniform and fine precipitation state _{of Si3N4} in the annealed hot-rolled sheet. As a result, it is believed that the inhibitor's ability to suppress normal grain growth during secondary recrystallization can be optimized, resulting in a grain-oriented electrical steel sheet with excellent Goss orientation concentration and magnetic properties.

From the perspective of controlling the structure of hot-rolled steel sheet, it is preferable that the finish rolling following rough hot rolling be performed at a rolling temperature of 800°C or higher and 1100°C or lower, with at least two passes. The rolling temperature in finish rolling is based on the temperature of the steel sheet surface. 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.

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 lower limit of the coiling temperature is more preferably 500°C or higher. The upper limit of 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.

<Hot-rolled sheet annealing>
The method for producing a grain-oriented electrical steel sheet according to the present invention then involves hot-rolled annealing the hot-rolled steel sheet to produce an annealed hot-rolled steel sheet. The hot-rolled annealing in the present invention is carried out for the purpose of forming AlN by substituting sol. Al for _Si contained in _Si3N4 precipitates formed in the steel during the hot rolling process. In the hot-rolled annealing, the surface temperature of the hot-rolled steel sheet is raised from 50°C to 350°C at an average heating rate of 40°C/s or more, and then soaked at a temperature preferably exceeding 950°C and not exceeding 1100°C.

Average heating rate from 50°C to 350°C: 40°C/s or more The faster the average heating rate from 50°C to 350°C, the more N diffusion is suppressed and the denser _{the Si3N4} distribution. By performing rough rolling in the final pass of rough rolling in hot rolling at the temperature range and reduction rate described above, the precipitation _{of Si3N4} is further promoted. After controlling the hot rolling conditions in this way, by setting the heating rate of the hot-rolled sheet annealing to 40°C/s or more, _{the Si3N4} distribution is made dense, thereby achieving a large amount of precipitation and a dense inhibitor precipitation state, and improving the magnetic flux density. The heating method is not limited, but in order to achieve an average heating rate of 40°C/s or more, induction heating methods and electric heating methods may be adopted in addition to conventional heating methods using heaters or burners. The average heating rate is more preferably 50°C/s or more. 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.

Temperature Range of More Than 950°C and Less Than 1100°C In the method for producing a grain-oriented electrical steel sheet according to the present invention, hot-rolled sheet annealing is performed by soaking the surface temperature of the hot-rolled steel sheet, preferably at a soaking temperature of more than 950°C and less than 1100°C. The soaking is performed by raising the surface temperature of the hot-rolled steel sheet to the soaking temperature after the completion of the previous temperature increase process from 50°C to 350°C, and then holding the temperature for the soaking time. This holding process optimizes the AlN precipitate diameter through Ostwald ripening. If the soaking temperature is 950°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 is more than 1100°C, AlN becomes excessively coarse or re-dissolves, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. For this reason, the soaking temperature is preferably more than 950°C and less than 1100°C. The soaking temperature is more preferably higher than 1000° C., and more preferably not higher than 1050° C. The soaking time is preferably not shorter than 10 seconds. The soaking time is preferably not longer than 60 seconds.

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 surface temperature of a hot-rolled steel sheet during hot-rolled sheet annealing can be measured using known methods.

Temperature range of 350°C or higher and 950°C or lower In a preferred embodiment, the method for producing a grain-oriented electrical steel sheet according to the present invention performs one or more soaking steps in the temperature-raising process of hot-rolled sheet annealing, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or higher and 950°C or lower for 3.0 seconds or longer and 100 seconds or shorter. The soaking in this temperature range involves raising the temperature of the surface of the hot-rolled steel sheet to the soaking temperature after the previous temperature-raising step from 50°C to 350°C is completed, and holding the temperature for the soaking time.

If the soaking temperature is less than 350°C, it becomes difficult to achieve a fine and dense distribution _{of Si3N4} , making it difficult to obtain an appropriate inhibitory effect on normal grain growth by the inhibitor, and the magnetic flux density is likely to decrease. If the soaking temperature exceeds 950°C, coarsening or dissolution _{of Si3N4} occurs, or AlN is likely to precipitate excessively, making it difficult to obtain an appropriate inhibitory effect on normal grain growth, and the magnetic flux density is likely to decrease. For this reason, the soaking temperature is preferably 350°C or higher and 950°C or lower. The soaking temperature is more preferably 400°C or higher, and even more preferably 500°C or higher. The soaking temperature is preferably 900°C or lower.

If soaking is not performed at all in the temperature range of 350° _C or higher and 950°C or lower, or if the soaking time is less than 3.0 seconds, it becomes difficult for _Si3N4 to be finely and densely distributed, it becomes difficult to obtain an appropriate inhibitory effect on normal grain growth by the inhibitor _, and the magnetic flux density tends to decrease. If the soaking time exceeds 100 seconds, coarsening or dissolution of _Si3N4 occurs, or AlN tends to precipitate excessively, it becomes difficult to obtain an appropriate inhibitory effect on normal grain growth, and the magnetic flux density tends to decrease. For this reason, the soaking time is preferably 3.0 seconds or higher and 100 seconds or lower. The soaking time is more preferably 5.0 seconds or higher. The soaking time is more preferably 60 seconds or lower, and even more preferably 30 seconds or lower.

Although the reason why the magnetic flux density of a steel sheet 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 950°C is unclear, the inventors believe the following: With soaking, the substitution of sol. Al for _Si contained 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 contained in _Si3N4 is promoted as the temperature increases _, 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 950°C is preferable for improving magnetic flux density.

In the method for manufacturing grain-oriented electrical steel sheet according to the present invention, soaking in a temperature range of 350°C or higher and 950°C or lower may be performed once, or may be repeated two or more times. When soaking in this temperature range is performed two or more times, the soaking temperature from the second time onwards may be the same as the temperature of the first soaking, or may be higher or lower than the temperature of the first soaking, as long as it is within the temperature range of 350°C or higher and 950°C or lower. When soaking is performed multiple times, it is preferable that the soaking time for each time be 3.0 seconds or longer and 100 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.

<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 excessively coarse, allowing for a primary recrystallized structure with uniform grain size, also improving the magnetic flux density of the steel sheet. In the final cold rolling, it is preferable to increase the cold rolling temperature to 100°C or higher and 300°C or lower, and to perform aging treatment at a temperature of 100°C or higher and 300°C or lower once or multiple times during cold rolling, as these are effective in changing the recrystallized texture and improving the magnetic properties.

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 0.005% 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. Secondary recrystallization annealing allows secondary recrystallization grains having a Goss orientation to develop, and a forsterite film to be formed on the surface of the steel sheet. Secondary recrystallization annealing is preferably carried out at 800°C or higher to induce secondary recrystallization. Furthermore, annealing at a temperature of 800°C or higher for 20 hours or more is preferred to complete secondary recrystallization. To form a forsterite film, the temperature is preferably raised to approximately 1200°C.

<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 processing. The preferred processing method is the commonly used method of applying distortion to the iron crystal lattice of the final product sheet using an electron beam or laser. It is also possible to pre-cut grooves not only in the final product sheet, but also in intermediate products such as cold-rolled sheets that have reached the final finished 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
Fourteen types of steel slabs containing the essential elements shown in Table 1, with the balance consisting of Fe and unavoidable impurities, were produced by continuous casting. The slabs were heated to 1160°C and then hot-rolled to form hot-rolled steel sheets with a thickness of 2.4 mm. The rolling temperature and reduction in the final pass of rough rolling in the hot rolling process were changed as shown in Table 2. The obtained hot-rolled steel sheets were subjected to hot-rolled sheet annealing at 1030°C for 30 seconds. The average heating rate from 50°C to 350°C was changed as shown in Table 2. Except for some samples, the hot-rolled steel sheets were subjected to soaking during heating, in which the temperature was held at the soaking temperature shown in Table 2 for 30 seconds. The hot-rolled sheet annealing atmosphere was a humid atmosphere of 90 vol% N ₂ + 10 vol% CO ₂ with a dew point of 40°C. After the hot-rolled sheet annealing, the surface scale of the annealed hot-rolled sheet was removed by pickling, and the sheet was subjected to a first cold rolling to a thickness of 1.6 mm. Intermediate annealing was then performed at 1000°C for 110 seconds in an atmosphere of 70 vol% _N2 + 30 vol% _H2 with a dew point of 40°C, and then a second cold rolling was performed 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% _N2 + 50 vol% _H2 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 the sheet was held at 900°C for 40 hours in a _N2 atmosphere, followed by secondary recrystallization annealing at 1220°C for 5 hours in a hydrogen atmosphere, to obtain 32 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 the above-mentioned experiment. The measurement results of magnetic flux density _B8 are shown in Table 2.

 

According to Table 2, it can be seen that for steel types A to D, an excellent magnetic flux density B8 of 1.925 T or more was obtained when the rolling temperature in the final pass of rough rolling in hot rolling was 950°C or more and 1150°C or less, the reduction ratio was ₂₅ % or more, and in addition, the average heating rate from 50°C to 350°C in hot-rolled sheet annealing was 40°C/s or more.

Example 2
A steel slab containing the essential and optional components shown in Table 3, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting, heated to 1200°C, and then hot-rolled to a 2.5 mm thick hot-rolled steel sheet. The rolling temperature in the final pass of rough rolling was 980°C, and the reduction ratio was 30%. The resulting hot-rolled steel sheet was subjected to hot-rolled sheet annealing. The average heating rate from 50°C to 350°C was 80°C/s, and the hot-rolled sheet annealing was performed by soaking at 800°C for 30 seconds, then holding at 1030°C for 30 seconds, and cooling. 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 warm-rolled at 150°C to obtain a cold-rolled steel sheet having 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% _N2 + 60 vol% _H2 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 the sheet was held at 870°C for 40 hours in a _N2 atmosphere, and then subjected to secondary recrystallization annealing at 1175°C for 15 hours in a hydrogen atmosphere to obtain 24 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 the above-mentioned experiment. The measurement results of magnetic flux density _B8 are shown in Table 3.

 

Table 3 shows that even for grain-oriented electrical steel sheets containing one or two optional added elements, an excellent magnetic flux density _B8 of 1.925 T or more was obtained.

Claims (3)

In mass ratio,
C: 0.002% or more, 0.100% or less,
Si: 2.0% or more, 6.5% or less,
Mn: 0.02% 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 final pass of rough rolling of the hot rolling, the rolling temperature is 950°C or more and 1150°C or less, and the rolling reduction is 25% or more;
10. The method for producing a grain-oriented electrical steel sheet, wherein, in the hot-rolled sheet annealing, the average temperature rise rate when raising the temperature of the hot-rolled steel sheet from 50°C to 350°C is 40°C/s or more. The method for manufacturing grain-oriented electrical steel sheet according to claim 1, wherein during the temperature increase process of the hot-rolled sheet annealing, the hot-rolled steel sheet is subjected to one or more soaking periods in which it is held at a soaking temperature of 350°C or higher and 950°C or lower for 3.0 seconds or higher and 100 seconds or lower. The component composition further comprises, in mass ratio:
Se: 0.001% or more, 0.005% or less,
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,
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.