JP7477762B2 - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Description

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

Grain-oriented electrical steel sheets contain 2% to 5% by mass of Si, and the orientation of the crystal grains of the steel sheet is highly concentrated in the {110}<001> orientation, which is called the Goss orientation. Grain-oriented electrical steel sheets have excellent magnetic properties and are used, for example, as iron core materials for stationary inductors such as transformers. Various developments have been made to improve the magnetic properties of electrical steel sheets. In particular, with the recent demand for energy saving, there is a demand for further reduction in iron loss in grain-oriented electrical steel sheets. To reduce the iron loss of grain-oriented electrical steel sheets, it is effective to increase the concentration of the grains of the steel sheet in the Goss orientation to improve the magnetic flux density and reduce hysteresis loss. In the manufacture of grain-oriented electrical steel sheets, the crystal orientation is controlled by utilizing a catastrophic grain growth phenomenon called secondary recrystallization. In order to properly control the crystal orientation through secondary recrystallization, it is important to ensure that fine precipitates in the steel, called inhibitors, precipitate uniformly in the steel and have thermal stability.

Various techniques have been proposed to manufacture grain-oriented electrical steel sheets with low iron loss by appropriately controlling secondary recrystallization. For example, Patent Document 1 describes a technique for manufacturing grain-oriented electrical steel sheets with low iron loss over the entire length of the coil by controlling the heat pattern during the temperature increase process of primary recrystallization annealing. In addition, Patent Document 2 describes a technique for reducing the iron loss value of grain-oriented electrical steel sheets by strictly controlling the average grain size of the crystal grains after secondary recrystallization and the deviation angle from the ideal orientation.

International Publication No. 2014/049770 Japanese Patent Application Laid-Open No. 7-268567

The precipitation state and thermal stability of the inhibitor, which determine the success or failure of secondary recrystallization in grain-oriented electrical steel sheets, are greatly affected by the α/γ phase ratio of the steel sheet. Grain-oriented electrical steel sheets contain Si to improve magnetic properties (particularly iron loss), but increasing the Si concentration improves the magnetic properties, but upsets the balance of the α/γ phase ratio. Therefore, simply increasing the Si concentration inhibits secondary recrystallization, and therefore leads to a deterioration in magnetic properties.

By increasing the Cu concentration as well as the Si concentration in the steel sheet, the imbalance of the α/γ phase ratio caused by the high Si concentration can be improved. However, according to the study by the inventors, when the Cu concentration of the steel sheet is increased, Cu may dissolve from the steel sheet into the pickling solution during pickling using the pickling solution and further precipitate on the steel sheet surface. Cu precipitated on the steel sheet surface inhibits the penetration of atmospheric gas into the steel sheet during decarburization annealing, causing the decarburization to deteriorate. In particular, when Cu precipitates in a film form on the steel sheet surface, the deterioration of decarburization is remarkable. Since insufficient decarburization leads to magnetic aging of the oriented electrical steel sheet, control of decarburization is important in order to manufacture oriented electrical steel sheets with excellent magnetic properties. For example, in the case of a thin material (e.g., a sheet thickness of less than 0.23 mm), decarburization is likely to proceed due to a relatively large surface area, and even if Cu precipitates, the deterioration of decarburization can be avoided by controlling the decarburization annealing process, etc. On the other hand, with thick materials, it was difficult to ensure the desired decarburization even by controlling the decarburization annealing process.

The present invention aims to solve the above problems and provide a method for manufacturing grain-oriented electrical steel sheets that can achieve good decarburization even in thick materials that contain high concentrations of Si and Cu, and can produce grain-oriented electrical steel sheets with excellent magnetic properties.

The present invention includes the following aspects.
[1] A hot rolling process in which a slab having a slab composition containing, in mass%, C: 0.02% to 0.10%, Si: 2.5% to 4.5%, Cu: [(Si%)/30-0.10]% to [(Si%)/30+0.10]%, Mn: 0.01% to 0.30%, S and Se: 0.001% to 0.050% in total, acid-soluble Al: 0.01% to 0.05%, N: 0.002% to 0.020%, P: 0.0400% or less, the balance Fe and impurities, is heated and hot-rolled to obtain a hot-rolled steel sheet;
A pickling process for obtaining a pickled sheet by immersing the hot-rolled steel sheet in a pickling solution, or by subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet and then immersing the hot-rolled annealed sheet in a pickling solution;
A cold rolling process of cold rolling the pickled steel sheet to obtain a cold rolled steel sheet;
A primary recrystallization annealing step of subjecting the cold-rolled steel sheet to primary recrystallization annealing to obtain a primary recrystallization annealed sheet;
A finish annealing process in which an annealing separator containing MgO is applied to the surface of the primary recrystallization annealed sheet, and then finish annealing is performed to obtain a finish annealed sheet;
A flattening annealing process for applying an insulating coating to the finish annealed sheet and then flattening annealing the sheet.
The pickling solution contains 0.5 to 20.0 vol. % of a Cu complexing agent;
The pH of the pickling solution is −1.5 or more and less than 7.0, and the solution temperature is 15° C. or more and 100° C. or less,
The method for producing a grain-oriented electrical steel sheet, wherein the immersion is carried out for 5 seconds or more and 200 seconds or less.
[2] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein the Cu complex-forming agent contains a nitrogen-containing ligand.
[3] The method for producing a grain-oriented electrical steel sheet according to the above [2], wherein the nitrogen-containing ligand is one or more selected from the group consisting of diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA), ethylenediaminetetraacetic acid (EDTA) and cyclohexanediaminetetraacetic acid (CyDTA).
[4] The method for producing a grain-oriented electrical steel sheet according to any one of the above [1] to [3], wherein in the primary recrystallization annealing step, a dew point in a temperature range of 30°C to 800°C is -50°C to 0°C.
[5] The method for producing a grain-oriented electrical steel sheet according to any one of the above [1] to [4], wherein the primary recrystallization annealing step includes a heating step and a decarburization annealing step, and the average heating rate in the temperature range of 750°C to 800°C in the heating step is 500°C/sec to 2000°C/sec.
[6] The method for producing a grain-oriented electrical steel sheet according to any one of the above [1] to [5], wherein the primary recrystallization annealing step includes a heating step and a decarburization annealing step, and the average heating rate in the temperature range of 500°C to 750°C in the heating step is 100°C/sec to 3000°C/sec.
[7] The method for producing a grain _- oriented electrical steel sheet according to the above [5] or [6], wherein the decarburization annealing step includes a soaking treatment carried out at a temperature of 750°C to 900°C in an atmosphere with an oxygen potential (P H2O /P _H2 ) of 0.2 to 0.6.
[8] The method for producing a grain-oriented electrical steel _sheet according to any one of the above [5] to [7], wherein the decarburization annealing step includes a first soaking treatment carried out in an atmosphere having a temperature of 750°C to _{900°C and an oxygen potential (P H2O /} P _H2 ) of 0.2 to 0.6, and, after the first soaking treatment, a second soaking treatment carried out in an atmosphere having a temperature of 900°C to 1000°C and an oxygen potential (P H2O /P H2 ) of less than 0.2.
[9] The method for producing a grain-oriented electrical steel sheet according to any one of the above [1] to [8], wherein a nitriding treatment is carried out after the cold rolling step and before the finish annealing step.
[10] The slab composition contains, in mass%, a part of the Fe replaced by
Sn: 0.500% or less,
Cr: 0.500% or less,
Bi: 0.0200% or less,
Sb: 0.500% or less,
The method for producing a grain-oriented electrical steel sheet according to any one of the above [1] to [9], comprising one or more selected from the group consisting of Mo: 0.500% or less, and Ni: 0.500% or less.

According to one aspect of the present invention, a method for manufacturing grain-oriented electrical steel sheets is provided that can produce grain-oriented electrical steel sheets with excellent magnetic properties by achieving good decarburization even in thick materials containing high concentrations of Si and Cu.

The following describes exemplary embodiments of the present invention, but the present invention is not limited to the following embodiments. Unless otherwise specified, the notation "A to B" for numerical values A and B means "greater than or equal to A and less than or equal to B."

The inventors have discovered that in grain-oriented electrical steel sheets with high concentrations of Si and Cu, suppressing Cu precipitation on the steel sheet surface caused by pickling allows decarburization to proceed smoothly and good magnetic properties can be obtained, and further discovered that adding a Cu complex-forming agent to the pickling solution is useful for suppressing Cu precipitation.

One aspect of the present invention is
a hot rolling process of heating a slab having a slab composition containing, by mass%, C: 0.02% or more and 0.10% or less, Si: 2.5% or more and 4.5% or less, Cu: [(Si%)/30-0.10]% or more and [(Si%)/30+0.10]% or less, Mn: 0.01% or more and 0.30% or less, one or two of S and Se: 0.001% or more and 0.050% or less in total, acid-soluble Al: 0.01% or more and 0.05% or less, N: 0.002% or more and 0.020% or less, P: 0.0400% or less, with the balance being Fe and impurities, and performing hot rolling to obtain a hot-rolled steel sheet;
A pickling process for obtaining a pickled sheet by immersing the hot-rolled steel sheet in a pickling solution, or by subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet and then immersing the hot-rolled annealed sheet in a pickling solution;
A cold rolling process of cold rolling the pickled steel sheet to obtain a cold rolled steel sheet;
A primary recrystallization annealing step of subjecting the cold-rolled steel sheet to primary recrystallization annealing to obtain a primary recrystallization annealed sheet;
A finish annealing process in which an annealing separator containing MgO is applied to the surface of the primary recrystallization annealed sheet, and then finish annealing is performed to obtain a finish annealed sheet;
A flattening annealing process for applying an insulating coating to the finish annealed sheet and then flattening annealing the sheet.
The pickling solution contains 0.5 to 20.0 vol. % of a Cu complexing agent;
The pH of the pickling solution is −1.5 or more and less than 7.0, and the solution temperature is 15° C. or more and 100° C. or less,
The present invention provides a method for producing a grain-oriented electrical steel sheet, characterized in that the immersion is carried out for 5 seconds or more and 200 seconds or less.

In the manufacturing method of the grain-oriented electrical steel sheet of this embodiment, the pickling conditions are specified in the pickling after hot rolling, thereby suppressing the precipitation of Cu from the pickling solution onto the steel sheet surface. This allows decarburization to proceed well in the subsequent primary recrystallization annealing process. Although not wishing to be bound by theory, it is believed that if Cu precipitates from the pickling solution onto the steel sheet surface during pickling, Cu is unevenly distributed near the steel sheet surface, and this Cu inhibits the penetration of atmospheric gas into the steel sheet in the primary recrystallization annealing process, thereby worsening the decarburization property. The inclusion of a Cu complex-forming agent in the pickling solution is extremely effective as a means of suppressing Cu precipitation onto the steel sheet surface during pickling. That is, the Cu complex-forming agent captures Cu in the pickling solution to form a stable Cu complex, thereby suppressing the precipitation of Cu onto the steel sheet surface. The Cu complex-forming agent is also advantageous in that its presence in the pickling solution does not cause any substantial inconvenience to the pickling itself.

The manufacturing method of the grain-oriented electrical steel sheet of this embodiment is particularly useful for thick materials. In one aspect, the thickness of the cold-rolled steel sheet (i.e., the base steel sheet after the cold rolling process) may be 0.23 mm or more, and may be selected depending on the application of the grain-oriented electrical steel sheet. In one aspect, the thickness of the cold-rolled steel sheet may be 0.35 mm or less, or 0.30 mm or less, or 0.27 mm or less.

The manufacturing method for the grain-oriented electrical steel sheet according to this embodiment will be described in detail below.

[Slab composition]
First, the composition of the slab used in the grain-oriented electrical steel sheet according to this embodiment will be described. In the following, unless otherwise specified, the notation "%" represents "mass %". The remainder of the slab other than the elements described below is Fe and impurities.

The C (carbon) content is 0.02% or more and 0.10% or less. C has various roles, but if the C content is less than 0.02%, the grain size becomes excessively large when the slab is heated, which increases the iron loss value of the final grain-oriented electrical steel sheet, which is not preferable. If the C content exceeds 0.10%, the decarburization time becomes long during decarburization after cold rolling, which is not preferable, and the manufacturing cost increases. Also, if the C content exceeds 0.10%, decarburization is likely to be incomplete, which is not preferable because it may cause magnetic aging in the final grain-oriented electrical steel sheet. Therefore, the C content is 0.02% or more and 0.10% or less, preferably 0.04% or more and 0.09% or less, and more preferably 0.05% or more and 0.09% or less.

The Si (silicon) content is 2.5% or more and 4.5% or less. Si increases the electrical resistance of the steel sheet, thereby reducing eddy current loss, which is one of the causes of iron loss. If the Si content is less than 2.5%, it is not preferable because it becomes difficult to sufficiently suppress the eddy current loss of the final grain-oriented electrical steel sheet. If the Si content exceeds 4.5%, it is not preferable because the workability of the grain-oriented electrical steel sheet decreases. Therefore, the Si content is 2.5% or more and 4.5% or less, preferably 2.7% or more and 4.0% or less, and more preferably 3.2% or more and 3.7% or less.

The Cu (copper) content is [(Si%)/30-0.10]% or more and [(Si%)/30+0.10]% or less. Cu contributes to avoiding the inhibition of secondary recrystallization caused by the imbalance of the α/γ phase ratio when the Si concentration in the steel is increased. If the Cu content is less than [(Si%)/30-0.10]%, the effect of improving the balance of the α/γ phase ratio is not sufficiently obtained, which is not preferable. If the Cu content exceeds [(Si%)/30+0.10]%, decarburization tends to be insufficient during decarburization annealing, which is not preferable. Therefore, the Cu content is from [(Si%)/30-0.10]% to [(Si%)/30+0.10]%, preferably from [(Si%)/30-0.05]% to [(Si%)/30+0.05]%, and more preferably from [(Si%)/30-0.03]% to [(Si%)/30+0.03]%.

The content of Mn (manganese) is 0.01% or more and 0.30% or less. Mn forms MnS, MnSe, etc., which are inhibitors that affect secondary recrystallization. If the content of Mn is less than 0.01%, the absolute amount of MnS and MnSe that cause secondary recrystallization is insufficient, which is not preferable. If the content of Mn is more than 0.30%, it is not preferable because it becomes difficult for Mn to be dissolved in solid solution during slab heating. In addition, if the content of Mn is more than 0.30%, it is not preferable because the precipitate size of MnS and MnSe, which are inhibitors, tends to become coarse, and the optimal size distribution as an inhibitor is impaired. Therefore, the content of Mn is 0.01% or more and 0.30% or less, preferably 0.03% or more and 0.20% or less, and more preferably 0.05% or more and 0.15% or less.

The total content of S (sulfur) and Se (selenium) is 0.001% or more and 0.050% or less. S and Se form inhibitors together with the above-mentioned Mn. Both S and Se may be contained in the slab, but at least one of them should be contained in the slab. If the total content of S and Se is outside the above range, it is not preferable because a sufficient inhibitor effect cannot be obtained. Therefore, the total content of S and Se is 0.001% or more and 0.050% or less, preferably 0.001% or more and 0.040% or less, and more preferably 0.005% or more and 0.030% or less.

In one embodiment, the content of acid-soluble Al (acid-soluble aluminum) is 0.01% or more and 0.05% or less. Acid-soluble Al constitutes an inhibitor necessary for manufacturing grain-oriented electrical steel sheets with high magnetic flux density. If the content of acid-soluble Al exceeds 0.05%, the AlN precipitated as an inhibitor becomes coarse, which is not preferable because it reduces the inhibitor strength. Also, if the content of acid-soluble Al is less than 0.01%, the inhibitor strength is low and is not preferable. The content of acid-soluble Al is preferably 0.01% or more and 0.04% or less, more preferably 0.01% or more and 0.03% or less.

The N (nitrogen) content is 0.002% or more and 0.020% or less. N forms AlN, an inhibitor, together with the acid-soluble Al described above. If the N content is outside the above range, it is not preferable because a sufficient inhibitor effect cannot be obtained. Therefore, the N content is 0.002% or more and 0.020% or less, preferably 0.004% or more and 0.015% or less. More preferably, it is 0.005% or more and 0.010% or less.

The P (phosphorus) content is 0.0400% or less. The lower limit includes 0%, but since the detection limit is 0.0001%, the actual lower limit is 0.0001%. P makes the texture after primary recrystallization annealing favorable for magnetic flux density. In other words, it is an element that improves magnetic properties. If the content is less than 0.0001%, the effect of adding P is not exerted. On the other hand, if it is added in excess of 0.0400%, the risk of breakage during cold rolling increases and the sheet passing property is significantly deteriorated. The P content is preferably 0.0030% or more and 0.0300% or less, and more preferably 0.0060% or more and 0.0200% or less.

Furthermore, in addition to the elements described above, the slab used in manufacturing the grain-oriented electrical steel sheet according to this embodiment may contain, in order to improve the magnetic properties, one or more elements selected from the group consisting of, by mass%, Sn: 0.500% or less, Cr: 0.500% or less, Bi: 0.0200% or less, Sb: 0.500% or less, Mo: 0.500% or less, and Ni: 0.500% or less, in place of a portion of the remaining Fe.
In one embodiment, the Sn content may be preferably 0.020% or more and 0.400% or less, and more preferably 0.040% or more and 0.200% or less.
In one embodiment, the Cr content may be preferably 0.020% or more and 0.400% or less, and more preferably 0.040% or more and 0.200% or less.
In one embodiment, the Bi content may be 0.0005% or more, preferably 0.0005% or more and 0.0150% or less, and more preferably 0.0010% or more and 0.0100% or less.
In one embodiment, the Sb content may be 0.005% or more, preferably 0.005% or more and 0.300% or less, and more preferably 0.005% or more and 0.200% or less.
In one embodiment, the Mo content may be 0.005% or more, preferably 0.005% or more and 0.400% or less, and more preferably 0.005% or more and 0.300% or less.
In one embodiment, the Ni content may be preferably 0.010% or more and 0.200% or less, and more preferably 0.020% or more and 0.100% or less.

A slab is formed by casting molten steel adjusted to the composition described above. The method for casting the slab is not particularly limited. In research and development, even if a steel ingot is formed in a vacuum melting furnace or the like, the same effects as when a slab is formed can be confirmed for the above components. Below, the preferred embodiments of each process for manufacturing grain-oriented electrical steel sheet from a slab are further explained.

[Hot rolling process]
In this process, the slab is heated and hot-rolled to obtain a hot-rolled steel sheet. In one embodiment, the heating temperature of the slab may be preferably 1280°C or higher, or 1300°C or higher, from the viewpoint of dissolving the inhibitor components (e.g., MnS, MnSe, AlN, etc.) in the slab to obtain a good inhibitor effect. In this case, the upper limit of the heating temperature of the slab is not particularly specified, but 1450°C is preferable from the viewpoint of equipment protection. Alternatively, in one embodiment, the heating temperature of the slab may be preferably less than 1280°C, or 1250°C or lower, from the viewpoint of reducing the burden on the heating furnace during hot rolling, reducing the amount of scale generation, and downstream processing of inhibitor control.

The heated slab is hot-rolled to produce hot-rolled steel sheet. The thickness of the hot-rolled steel sheet after processing is preferably 1.8 mm or more, since the temperature of the steel sheet is unlikely to decrease, and the precipitation of inhibitors in the steel can be stably controlled. It is also preferably 3.5 mm or less, since the rolling load in the cold rolling process can be reduced.

[Pickling process]
In this step, the hot-rolled steel sheet is immersed in a pickling solution, or the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet, and then the hot-rolled annealed sheet is immersed in a pickling solution to obtain a pickled sheet. Pickling is performed at least once after hot rolling and before primary recrystallization annealing. In one embodiment, pickling is performed before the cold rolling step in order to reduce roll wear in cold rolling.

The pickling solution contains a Cu complexing agent, which is a molecule or ion capable of forming a Cu complex. The Cu complexing agent may be a molecule or ion that is understood by those skilled in the art as a ligand of a Cu complex (e.g., a terminal ligand, a bridging ligand, a chelating ligand, a macrocyclic ligand, etc.). The Cu complexing agent may be linear, branched, or cyclic, and specific examples include anionic ligands such as carboxylate, oxalate, and oxo; amine-based ligands such as ammine, diamine, triamine, and tetramine; imine-based ligands such as pyridine, diimine, triimine, and porphyrin; Schiff bases; phosphine-based ligands having 1 to 3 phosphine sites; crown ethers; and the like. The Cu complexing agent may be a monodentate ligand or a multidentate ligand. From the viewpoint of the efficiency of capturing Cu in the pickling solution, a multidentate ligand is preferred. In a preferred embodiment, the Cu complexing agent contains a nitrogen-containing ligand. Suitable examples of nitrogen-containing ligands include ethyleneamine compounds (i.e., compounds having one or more ethylene groups and one or more amino groups) such as ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA), aminocarboxylic acids such as ethylenediaminetetraacetic acid (EDTA), and cyclohexanediaminetetraacetic acid (CyDTA), and iminocarboxylic acids such as iminodiacetic acid, and particularly preferred examples include one or more selected from the group consisting of diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA), ethylenediaminetetraacetic acid (EDTA), and cyclohexanediaminetetraacetic acid (CyDTA).

The stability of the Cu complex formed by the Cu complex forming agent can be evaluated by the stability constant (LogKd) of the metal complex. The stability constant of the Cu complex formed by the Cu complex forming agent of this embodiment is preferably 10 to 35, more preferably 10 to 30, and even more preferably 20 to 27. The stability constant of the Cu complex is preferably large, but may be 35 or less in one aspect from the viewpoint of the availability of the Cu complex forming agent. The stability constant in this disclosure means the total stability constant when multiple ligands form a Cu complex. In one aspect, the stability constant is a value based on the literature (specifically, the IUPAC Stability Constants Database (SC-Database), or Table 3 of Patent Document 1, or TABLE IV of Ueno Keihei, Japan Analyst, Vol. 14 (1965), pp. 962-968). The larger the stability constant of the Cu complex formed by the Cu complex forming agent, the easier it is for the Cu complex forming agent to capture Cu and the harder it is for the Cu complex forming agent to release it, and this is preferable. A Cu complex forming agent with high basicity tends to form a Cu complex with a large stability constant. The relationship between the type of Cu complex forming agent and the stability constant of the Cu complex formed by the Cu complex forming agent can be exemplified as follows (the stability constant is in parentheses): ammonia (NH ₃ ) (4.2); iminodiacetic acid (IDA) (10.3); ethylenediaminetetraacetic acid (EDTA) (18.3); ethylenediamine (EDA) (10.7); diethylenetriamine (DETA) (16.1); triethylenetetramine (TETA) (20.4); tetraethylenepentamine (TEPA) (23.1); pentaethylenehexamine (PEHA) (26.2).

The amount of Cu in the pickling solution, the sum of complexed and uncomplexed Cu, may be, for example, 0.01 g/L or more, or 0.05 g/L or more, or 0.08 g/L or more, and may be, for example, 3.00 g/L or less, or 2.00 g/L or less, or 1.50 g/L or less. The Cu in the pickling solution is typically a leachate of Cu in the slab. The amount of Cu in the pickling solution can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

The amount of Cu complex former to be contained in the pickling solution may be selected according to the composition ratio of Cu:Cu complex former corresponding to the structure of the target Cu complex. In one embodiment, when a complex with a composition ratio of Cu:Cu complex former of 1:n (n is an integer of 1 or more) is to be formed, the amount of Cu complex former in the pickling solution is preferably 1.1 or more, 1.2 or more, 1.5 or more, or 2.0 or more in terms of the Cu capture efficiency, as a molar ratio to the value obtained by multiplying the amount of Cu in the pickling solution by the above n, and is preferably 5.0 or less, or 3.0 or less in terms of avoiding an increase in costs due to the use of a Cu complex former in excess of the required amount.

In one embodiment, the amount of Cu complexing agent contained in the pickling solution is 0.5 vol.% or more and 20.0 vol.% or less, preferably 1.0 vol.% or more and 15.0 vol.% or less, and more preferably 3.0 vol.% or more and 10.0 vol.% or less, from the viewpoint of effectively capturing Cu in the pickling solution.

In one embodiment, the pH of the pickling solution may be -1.5 or more and less than 7.0. If it is 7.0 or more, the scale removal effect will be insufficient, which is not preferable. On the other hand, solutions that can be actually prepared have a pH of -1.5 or more. The pH is preferably less than 2.0, and more preferably less than 1.0. Examples of acid components contained in the pickling solution include sulfuric acid, hydrochloric acid, and nitric acid.

In one embodiment, the temperature of the pickling solution is 15°C or higher and 100°C or lower. If the temperature of the pickling solution is lower than 15°C, the effect of removing scale by pickling becomes insufficient, which is not preferable. If the temperature of the pickling solution exceeds 100°C, the handling of the pickling solution becomes difficult, which is not preferable. A pickling solution temperature of 15°C or higher and 100°C or lower is also advantageous in that it allows the Cu complex forming agent to exhibit its Cu capture ability well. The temperature of the pickling solution is preferably 50°C or higher and 90°C or lower, more preferably 60°C or higher and 90°C or lower.

In one embodiment, the time for which the steel sheet is immersed in the pickling solution is 5 seconds or more and 200 seconds or less. If the time for which the steel sheet is immersed in the pickling solution is less than 5 seconds, the effect of removing scale by pickling becomes insufficient, which is not preferable. If the time for which the steel sheet is immersed in the pickling solution exceeds 200 seconds, the equipment becomes long and large, which is not preferable. The immersion time is preferably 10 seconds or more and 150 seconds or less, more preferably 20 seconds or more and 150 seconds or less.

When free (i.e., uncomplexed) Cu is present in the pickling solution, it precipitates on the steel sheet surface as a segregated layer. During decarburization annealing, the Cu segregated layer acts as a barrier and inhibits decarburization. In the method of this embodiment, a Cu complex-forming agent is included in the pickling solution to capture Cu, thereby suppressing the deterioration of decarburization caused by the Cu segregated layer on the steel sheet surface.

[Cold rolling process]
In this process, the pickled sheet is cold-rolled once or twice or more times, or intermediate annealing is performed between the cold-rolled passes to obtain a cold-rolled steel sheet. For example, when cold rolling is performed by reverse rolling such as a Sendzimir mill, the number of passes in cold rolling is not particularly limited, but is preferably 9 or less from the viewpoint of manufacturing costs. Between passes of cold rolling, between rolling roll stands, or during rolling, the steel sheet may be heat-treated at about 300 ° C. or less. Such heating is preferable in that the magnetic properties of the final grain-oriented electrical steel sheet can be improved.

Between the multiple passes, intermediate annealing may be performed once or more. The temperature of the intermediate annealing may be 900° C. or more and 1200° C. or less. The holding time of the intermediate annealing is not particularly limited, but is preferably 200 seconds or less from the viewpoint of manufacturing costs. It is preferable to perform pickling after the intermediate annealing.
The cumulative reduction ratio (%) of the steel sheet in the cold rolling process may be appropriately designed so as to obtain a cold-rolled steel sheet of a desired thickness, and may be, for example, 80% to 95%. The cumulative reduction ratio (%) of the steel sheet in the cold rolling process is defined as [(hot-rolled sheet thickness-steel sheet thickness after final cold rolling pass)/hot-rolled sheet thickness]×100 when the intermediate annealing is not included, and is defined as [(steel sheet thickness after n-th intermediate annealing-steel sheet thickness after final cold rolling pass)/steel sheet thickness after n-th intermediate annealing]×100 when intermediate annealing is performed n times (n≧1).

[Primary recrystallization annealing process]
Next, in this process, the cold-rolled steel sheet is subjected to primary recrystallization annealing. In a typical embodiment, the primary recrystallization annealing process includes a temperature-raising process and a decarburization annealing process. After the temperature-raising process, the cold-rolled steel sheet is decarburized annealed in the decarburization annealing process. It is preferable that the temperature-raising process and the decarburization annealing process are performed continuously. When the temperature-raising process is performed at a rapid temperature rise, it is possible to increase the amount of Goss-oriented grains in the cold-rolled steel sheet before the finish annealing, and thus, in the finish annealing, secondary recrystallization of grains oriented close to the Goss orientation can be favorably formed.

(Heating process)
In the temperature increasing step, the cold-rolled steel sheet is heated to a desired decarburization annealing temperature. It is desirable to appropriately control the temperature increasing rate depending on the temperature range. Hereinafter, an example of the temperature increasing rate will be described.

In the heating process, the average heating rate between 30°C and 400°C is preferably 50°C/sec or more and 700°C/sec or less. This can prevent oxidation of Cu in the steel. The average heating rate between 30°C and 400°C may be, for example, 50°C/sec or more, or 100°C/sec or more, or 150°C/sec or more, and may be, for example, 700°C/sec or less, or 600°C/sec or less.

In the heating process, the average heating rate between 400°C and 500°C does not require any special control, but may be the same as the average heating rate between 30°C and 400°C exemplified above. For example, after dwelling at 400±50°C for 1 second or more, the material may be subsequently heated to 500°C or higher. For example, dwelling at 400±50°C for 1 second or more may be preferable from the viewpoint of magnetic properties.

In the heating process, it is preferable that the average heating rate between 500°C and 750°C is 100°C/sec or more and 3000°C/sec or less. This allows the amount of Goss-oriented grains before the finish annealing of the cold-rolled steel sheet to be increased, and the magnetic flux density of the final grain-oriented electrical steel sheet to be improved. By setting the average heating rate in the above range at least in the range of 500°C to 750°C, the recovery of dislocations in the steel sheet (i.e., the reduction in dislocation density in the steel sheet) does not progress excessively, and the start of primary recrystallization of orientation grains other than Goss-oriented grains can be avoided, and the primary recrystallization of other orientation grains can be avoided from being completed before the primary recrystallization of the Goss-oriented grains is completed. The average heating rate between 500°C and 750°C is preferably 100°C or more, or 400°C/sec or more, or 500°C/sec or more, or 600°C/sec or more, or 700°C/sec or more, from the viewpoint of increasing the amount of Goss-oriented grains before the final annealing and further improving the magnetic flux density of the grain-oriented electrical steel sheet. The upper limit of the average heating rate between 500°C and 750°C is not particularly limited, but from the viewpoint of equipment costs and manufacturing costs, it may be, for example, 3000°C/sec or less, 2000°C/sec or less, 1500°C/sec or less, or 1000°C/sec or less.

In the temperature increasing step, the average heating rate between 750 ° C. and 800 ° C. is preferably 500 ° C./sec or more, or 700 ° C./sec or more, or 1000 ° C./sec or more, from the viewpoint of suppressing the generation of SiO ₂ that inhibits decarburization during decarburization annealing. The upper limit of the average heating rate between 750 ° C. and 800 ° C. is not particularly limited, but from the viewpoint of equipment costs and manufacturing costs, it may be, for example, 2000 ° C./sec or less, 1800 ° C./sec or less, or 1600 ° C./sec or less. Such rapid heating can be performed, for example, by using a current heating method or an induction heating method.

The heating process may be performed using multiple devices.

The heating rate can be measured by measuring the steel plate temperature using a radiation thermometer or the like. There are no particular limitations on the method for measuring the steel plate temperature. However, if it is difficult to measure the steel plate temperature and it is difficult to accurately estimate the heating start point and heating end point at which the heating rate should be controlled, these temperatures may be estimated by analogy with the heat patterns of heating and cooling, respectively. Furthermore, the entry and exit temperatures of the steel plate into the heating device in the heating process may be taken as the heating start and end points.

(Decarburization annealing process)
After the temperature rise step, a decarburization annealing step is performed. In a typical embodiment, the decarburization annealing step includes a soaking treatment. The soaking treatment may be performed in a wet atmosphere containing hydrogen and/or nitrogen, for example, at 900° C. or less, or at 750° C. to 900° C. The decarburization annealing temperature may be the same as the end temperature of the temperature rise step described above, or may be higher or lower than the end temperature. If the end temperature of the temperature rise step is lower than the decarburization annealing temperature, the steel sheet may be further heated before the decarburization annealing. On the other hand, if the end temperature of the temperature rise step is higher than the decarburization annealing temperature, the steel sheet may be cooled by a heat dissipation treatment, a gas cooling treatment, or the like before the decarburization annealing. Furthermore, after the temperature rise step, the steel sheet may be cooled to a temperature lower than the decarburization annealing temperature, and then reheated in the decarburization annealing step.

The soaking may be performed once or twice or more. For example, when the soaking is performed twice, that is, the first soaking and the second soaking, the steel sheet may be cooled (for example, to room temperature) after the first soaking and then reheated, or the second soaking may be performed without cooling. From the viewpoint of improving the coating adhesion of the grain-oriented electrical steel sheet, it is desirable that a forsterite coating (Mg ₂ SiO ₄ ) is well formed on the steel sheet surface after the finish annealing. However, in the soaking, Fe ₂ Si ₄ O, which inhibits the formation of the forsterite coating, may be formed. When the first soaking and the second soaking are performed, even if Fe ₂ Si ₄ O is generated in the first soaking, it is reduced in the second soaking to form SiO ₂ , so that the forsterite coating can be well formed in the subsequent finish annealing process, and the coating adhesion can be improved. The improvement of the coating adhesion is advantageous for improving the coating tension and therefore the magnetic properties. Even if SiO ₂ is generated in the second soaking treatment, the decarburization has already progressed sufficiently in the first soaking treatment, so that the SiO ₂ does not adversely affect the decarburization properties.

The oxygen potential of the soaking atmosphere, i.e., the ratio of the water vapor partial pressure P _H2O to the hydrogen partial pressure P _H2 in the atmosphere (P _H2O /P _H2 ratio) is preferably 0.2 or more, or 0.3 or more, or 0.4 or more from the viewpoint of favorably progressing the internal oxidation and forming a uniform oxide film on the steel sheet surface, and is preferably 0.6 or less, or 0.55 or less from the viewpoint of obtaining good magnetic properties. In addition, for example, when the first soaking and the second soaking are performed, it is preferable that the P _H2O /P _H2 ratio of the first soaking is in the above range, and the P _H2O /P _H2 ratio of the second soaking is, for example, less than 0.2, or 0.1 or less, or 0.08 or less. In this case, the lower limit of the P _H2O /P _H2 ratio of the second soaking is not particularly limited, but may be, for example, 0.01 or more, or 0.02 or more from the viewpoint of ease of process control.

In one embodiment, the decarburization annealing step may include a first soaking treatment performed at 750° C. to 900° C. and a second soaking treatment performed at 900° C. to 1000° C. In one embodiment, the decarburization annealing step may include a first soaking treatment performed at a temperature of 750° C. to 900° C. in an atmosphere with an oxygen potential (P _H2O /P _H2 ) of 0.2 to 0.6, and a second soaking treatment performed after the first soaking treatment in an atmosphere with a temperature of 900° C. to 1000° C. and an oxygen potential (P _H2O /P _H2 ) of less than 0.2.

In the primary recrystallization annealing step, the dew point temperature of the atmosphere between 30°C and 800°C is preferably -50°C or higher and 0°C or lower. From the viewpoint of suppressing oxidation of the metal in the steel sheet and suppressing the generation _{of SiO2} due to external oxidation to smoothly proceed with decarburization, the dew point temperature is preferably 0°C or lower, or -5°C or lower, or -10°C or lower. From the viewpoint of ease of process control, the dew point temperature may be, for example, -50°C or higher, or -40°C or higher.

[Nitriding treatment]
After the cold rolling step, nitriding may be further performed before the finish annealing for the purpose of strengthening the inhibitor. In one embodiment, the nitriding may be performed after the soaking and before the finish annealing, for example, in the order of soaking-nitriding-finish annealing, first soaking-second soaking-nitriding-finish annealing, or first soaking-nitriding-second soaking-finish annealing. The nitriding may be performed in a nitriding gas (e.g., ammonia-containing gas) atmosphere.

[Finish annealing process]
Next, for the purpose of forming a primary coating and secondary recrystallization, the steel sheet after the primary recrystallization annealing step (primarily recrystallized annealed sheet) is subjected to finish annealing. In a typical embodiment, an annealing separator containing MgO as a main component is applied to the primary recrystallization annealed sheet before the finish annealing for the purpose of preventing seizure between steel sheets, forming a primary coating, controlling secondary recrystallization behavior, etc. The annealing separator is generally applied to the steel sheet surface in the form of a water slurry and dried, but an electrostatic application method or the like may also be used.

Finish annealing may be performed by, for example, heat treating the coiled steel sheet using a batch-type heating furnace or the like. Furthermore, in order to further reduce the iron loss of the finally obtained grain-oriented electrical steel sheet, the coiled steel sheet may be subjected to a purification treatment in which the temperature is raised to about 1200°C and then held at that temperature. Finish annealing is generally performed by raising the temperature from about room temperature, and the heating rate for finish annealing varies. The conditions for finish annealing are not particularly limited. For example, from the viewpoint of productivity and general equipment constraints, the heating rate may be 5°C/h to 100°C/h, but other known heat patterns may also be adopted. The cooling process pattern is also not particularly limited.

The atmosphere gas composition in the final annealing is not particularly limited. In the secondary recrystallization process, a mixed gas of nitrogen and hydrogen may be used. The atmosphere may be a dry atmosphere or a wet atmosphere. The atmosphere in the purification annealing may be, for example, dry hydrogen gas.

[Planarization annealing process]
After the finish annealing, an insulating coating (e.g., an insulating coating mainly composed of aluminum phosphate or colloidal silica) may be applied to the surface of the steel sheet in order to impart insulation and tension to the steel sheet. The components of the insulating coating may be appropriately selected so that the steel sheet is imparted with the desired insulation and tension. Next, planarization annealing may be performed in order to bake the insulating coating and planarize the shape of the steel sheet by the finish annealing.

Depending on the application of the resulting grain-oriented electrical steel sheet, further magnetic domain control processing may be performed.

By using the process exemplified above, it is possible to manufacture grain-oriented electrical steel sheets with excellent magnetic properties. The grain-oriented electrical steel sheets that can be manufactured by the method of this embodiment can be processed into wound cores or stacked cores during transformer manufacturing and can be used for the desired applications.

[Magnetic properties of grain-oriented electrical steel sheets]
The grain-oriented electrical steel sheet manufactured by the method of this embodiment has excellent decarburization, i.e., excellent iron loss. Here, the magnetic flux density B8 value and the iron loss W17/50, which are evaluation items of the grain-oriented electrical steel sheet manufactured by the method of this embodiment, will be described. The magnetic flux density is an index of the degree of integration of the Goss orientation. Here, the magnetic flux density B8 value is the magnetic flux density when a magnetic field of 800 A/m at 50 Hz is applied to the grain-oriented electrical steel sheet. B8 is an index of the degree of orientation integration of the Goss orientation, and if B8 is low, good iron loss cannot be obtained. When B8 is 1.88 T or more, good iron loss can be obtained, which is preferable. In addition, the iron loss W17/50 (W/kg) refers to the iron loss of a sample when the frequency is 50 Hz and the maximum magnetic flux density is 1.7 T. The magnetic flux density B8 value and W17/50 are values obtained in accordance with the single sheet magnetic property test method (Single Sheet Tester: SST) specified in JIS C2556. In research and development, when steel ingots are formed in a vacuum melting furnace or the like, it is difficult to obtain test pieces of the same size as those produced by actual manufacturing. In this case, for example, a test piece with a width of 60 mm and a length of 300 mm is obtained, and measurements are performed in accordance with the above-mentioned single sheet magnetic property test method. At this time, a correction factor may be applied to the obtained results so that the measured values are equivalent to those obtained by the method based on the Epstein test specified in JIS C2550.

The following provides further explanation of exemplary aspects of the present invention with reference to examples, but the present invention is not limited to these examples.

<Manufacturing of grain-oriented electrical steel sheets>
[Example 1]
A slab having the slab composition shown in Table 1 was obtained. The slab was heated at 1100 to 1350°C, and then hot-rolled to obtain a hot-rolled steel strip having a thickness of 2.3 to 2.6 mm. The hot-rolled steel strip was then heated to 1120°C for recrystallization, and then annealed at 900°C to obtain a hot-rolled annealed steel strip. The hot-rolled annealed steel strip was pickled under the pickling conditions shown in Table 2, and then cold-rolled to a final product thickness of 0.220 mm.

Next, primary recrystallization annealing was performed at 800-850°C for approximately 100-200 seconds. The primary recrystallization annealing consisted of a heating process and a decarburization annealing process, and the annealing atmosphere was a mixed atmosphere of hydrogen and nitrogen. In the heating process, the dew point was set to -10 to -30°C in the temperature range of 30°C to 800°C. Meanwhile, in the decarburization annealing process, the oxygen potential was controlled to 0.4 to 0.6. The heating rate in the heating process was controlled to 600°C to 800°C/sec in the range of 500-750°C and 900-1100°C/sec in the range of 750-800°C. After the decarburization annealing, the second soaking treatment, i.e., the second soaking treatment, was performed for approximately 10-50 seconds at 900-1000°C for slabs No. 1, 4 to 6. The annealing atmosphere for the second soaking treatment was also a mixed atmosphere of hydrogen and nitrogen, and the oxygen potential was controlled to 0.1 or less. Slabs No. 2 and 3 were not subjected to the second soaking treatment, but instead were subjected to nitriding treatment.

Next, final annealing was performed. Specifically, an annealing separator mainly composed of magnesium oxide (MgO) was applied to the surface of the steel sheet after the primary recrystallization annealing.

Then, the primary recrystallized annealed steel sheet coated with the annealing separator was heated to 1200°C to produce a finish annealed steel sheet. The annealing atmosphere for the finish annealing was a mixed atmosphere of hydrogen and nitrogen.

Next, an insulating film forming process was carried out on the finish annealed steel sheet. Specifically, an insulating film forming liquid mainly composed of colloidal silica and phosphate was applied to the surface of the steel sheet after the finish annealing, and the film was baked. From the grain-oriented electrical steel sheet produced by the above process, an evaluation sample with a width of 60 mm and a length of 300 mm was taken. The length direction of the sample was parallel to the rolling direction. Next, the evaluation sample was held at 150°C for 100 hours in a nitrogen atmosphere. By carrying out such a magnetic aging treatment, the effect of iron loss deterioration due to residual carbon can be accelerated.

The samples thus obtained were evaluated for B8 and W17/50 in accordance with JIS C2556.
For samples with a W17/50 value of 0.950 W/kg or more, it was determined that iron loss deterioration due to residual carbon occurred, and the magnetic properties were rated as NG. On the other hand, the magnetic properties of samples with a W17/50 value of 0.900 or more and less than 0.950 W/kg were rated as F (Fine), the magnetic properties of samples with a W17/50 value of 0.850 W/kg or more and less than 0.900 W/kg were rated as G (Good), and the magnetic properties of samples with a W17/50 value less than 0.850 W/kg were rated as VG (Very Good).

As shown in Table 2, in each example, by controlling the pickling conditions, decarburization proceeded well even in steel sheets with high Si and Cu contents, such as 3.40-3.65 mass% Si and 0.09-0.19 mass% Cu in the slab. On the other hand, in the comparative example that did not contain a complexing agent, a Cu segregation layer was formed during pickling, so the iron loss after magnetic aging treatment was inferior and the evaluation was NG.

In Examples 2 to 4, the stability constant and volume concentration of the complexing agent, and the temperature and immersion time of the acid solution were controlled within the more preferred ranges, so the results were particularly good. In Examples 1 and 5, the stability constant of the complexing agent was outside the more preferred ranges, so the evaluation remained at "G." In Examples 6 to 9, the stability constant and volume concentration of the complexing agent were both outside the more preferred ranges, so the evaluation remained at "F."

The grain-oriented electrical steel sheet obtained by the method disclosed herein can be suitably applied to various applications that require good magnetic properties.

Claims (10)

a hot rolling process of heating a slab having a slab composition containing, by mass%, C: 0.02% or more and 0.10% or less, Si: 2.5% or more and 4.5% or less, Cu: [(Si%)/30-0.10]% or more and [(Si%)/30+0.10]% or less, Mn: 0.01% or more and 0.30% or less, one or two of S and Se: 0.001% or more and 0.050% or less in total, acid-soluble Al: 0.01% or more and 0.05% or less, N: 0.002% or more and 0.020% or less, P: 0.0400% or less, with the balance being Fe and impurities, and performing hot rolling to obtain a hot-rolled steel sheet;
A pickling process for obtaining a pickled sheet by immersing the hot-rolled steel sheet in a pickling solution, or by subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled annealed sheet and then immersing the hot-rolled annealed sheet in a pickling solution;
A cold rolling process of cold rolling the pickled steel sheet to obtain a cold rolled steel sheet;
A primary recrystallization annealing step of subjecting the cold-rolled steel sheet to primary recrystallization annealing to obtain a primary recrystallization annealed sheet;
A finish annealing process in which an annealing separator containing MgO is applied to the surface of the primary recrystallization annealed sheet, and then finish annealing is performed to obtain a finish annealed sheet;
A flattening annealing process for applying an insulating coating to the finish annealed sheet and then flattening annealing the sheet.
The pickling solution contains 0.5 to 20.0 vol.% of a Cu complexing agent;
The pH of the pickling solution is −1.5 or more and less than 7.0, and the solution temperature is 15° C. or more and 100° C. or less,
The method for producing a grain-oriented electrical steel sheet, wherein the immersion is carried out for 5 seconds or more and 200 seconds or less. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein the Cu complex-forming agent contains a nitrogen-containing ligand. The method for producing grain-oriented electrical steel sheet according to claim 2, wherein the nitrogen-containing ligand is one or more selected from the group consisting of diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA), ethylenediaminetetraacetic acid (EDTA) and cyclohexanediaminetetraacetic acid (CyDTA). The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the dew point in the temperature range of 30°C to 800°C in the primary recrystallization annealing process is -50°C to 0°C. The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 4, wherein the primary recrystallization annealing process includes a heating process and a decarburization annealing process, and the average heating rate in the temperature range of 750°C to 800°C in the heating process is 500°C/sec to 2000°C/sec. The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 5, wherein the primary recrystallization annealing process includes a heating process and a decarburization annealing process, and the average heating rate in the temperature range of 500°C to 750°C in the heating process is 100°C/sec to 3000°C/sec. 7. The method for producing a grain-oriented electrical steel sheet according to claim 5, wherein the decarburization annealing _step includes a soaking treatment carried out in an atmosphere having a temperature of 750° C. to 900° C. and an oxygen potential (P _H2O /P H2 ) of 0.2 to 0.6. The method for producing a grain-oriented electrical steel _sheet according to any one of claims 5 to 7, wherein the decarburization annealing step includes a first soaking treatment carried out in an atmosphere having a temperature of 750°C to 900°C and an _oxygen potential (P _H2O /P _H2 ) of 0.2 to 0.6, and, after the first soaking treatment, a second soaking treatment carried out in an atmosphere having a temperature of 900°C to 1000°C and an oxygen potential (P H2O /P H2 ) of less than 0.2. The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 8, in which a nitriding treatment is carried out after the cold rolling process and before the finish annealing process. The slab composition is, in mass %, replacing a part of the Fe,
Sn: 0.500% or less,
Cr: 0.500% or less,
Bi: 0.0200% or less,
Sb: 0.500% or less,
The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 9, comprising one or more selected from the group consisting of Mo: 0.500% or less, and Ni: 0.500% or less.