Grain-oriented electrical steel sheet and manufacturing method thereof
technical field
[One]
It relates to a grain-oriented electrical steel sheet and a method for manufacturing the same. Specifically, by including a plurality of cold rolling and decarburization annealing process, it relates to a grain-oriented electrical steel sheet having improved magnetism and a method of manufacturing the same.
background
[2]
The grain-oriented electrical steel sheet is a soft magnetic material with excellent magnetic properties in the rolling direction, which is made up of grains having a so-called Goss orientation in which the crystal orientation of the steel sheet is {110}<001>.
[3]
This grain-oriented electrical steel sheet is manufactured by heating the slab and then rolling to the final thickness through hot rolling, hot-rolled sheet annealing, and cold rolling, and then subjected to primary recrystallization annealing and high temperature annealing to form secondary recrystallization.
[4]
In general, the secondary recrystallization annealing process of grain-oriented electrical steel sheet requires a low temperature increase rate and long-time purification annealing at high temperature, so it can be said that it is a process that consumes a lot of energy. Since the grain-oriented electrical steel sheet having excellent magnetic properties is manufactured by forming secondary recrystallization through this extreme process, the following difficulties occur in the process.
[5]
First, due to the heat treatment in the coil state, the temperature deviation between the outer and inner windings of the coil occurs, so that the same heat treatment pattern cannot be applied to each part, resulting in magnetic deviation between the outer and inner windings. Second, since various surface defects occur in the process of coating MgO on the surface after decarburization annealing and forming the base coating during high-temperature annealing, the real rate is lowered. Third, after the decarburization annealing is finished, the decarburization annealing is wound in a coil shape, and after high-temperature annealing, it is again subjected to planarization annealing to apply insulation coating, so that the production process is divided into three steps, which causes a problem in that the error rate is lowered.
[6]
In order to overcome this process limitation, a technique using normal crystal growth without using the secondary recrystallization phenomenon by controlling decarburization annealing and cold rolling reduction has been proposed. However, through continuous annealing, there is a limit to the growth of final crystal grains due to a short heat treatment time of moisture, and there is a limit in iron loss improvement because it cannot grow into crystal grains having an optimal grain size.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[7]
Provided are a grain-oriented electrical steel sheet and a method for manufacturing the same. Specifically, by including a plurality of cold rolling and decarburization annealing processes, to provide a grain-oriented electrical steel sheet with improved magnetism and a method for manufacturing the same.
means of solving the problem
[8]
A method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises the steps of heating a slab; manufacturing a hot-rolled steel sheet by hot-rolling the slab; annealing the hot-rolled steel sheet; First cold rolling the hot-rolled sheet annealed hot-rolled sheet; decarburizing annealing the first cold-rolled steel sheet; Secondary cold rolling of the decarburization annealing is completed; Continuous annealing of the steel sheet on which the secondary cold rolling has been completed and the step of batch annealing the continuously annealed steel sheet.
[9]
The slab is in wt%, Si: 1.0% to 4.0%, C: 0.1% to 0.4%, and the balance may include Fe and unavoidable impurities.
[10]
The slab may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.
[11]
It may include a decarburization process in the step of annealing the hot-rolled sheet.
[12]
The annealing of the hot-rolled sheet may be annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.
[13]
The decarburization annealing of the primary cold-rolled steel sheet may be annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.
[14]
The decarburization annealing of the primary cold-rolled steel sheet may be annealed in an austenite single-phase region or a region in which a composite phase of ferrite and austenite exists.
[15]
After the decarburization annealing of the primary cold-rolled steel sheet, the average diameter of the grains may be 150 to 250㎛.
[16]
The decarburization annealing of the primary cold-rolled steel sheet and the secondary cold rolling of the decarburization annealing completed steel sheet may be repeated two or more times.
[17]
Continuous annealing may be annealed at a temperature of 850 °C to 1000 °C and a dew point temperature of 50 °C to 70 °C.
[18]
Continuous annealing may be annealed for 1 to 5 minutes.
[19]
The batch annealing may be annealed at a temperature of 1000°C to 1200°C and a dew point temperature of -20°C or less.
[20]
The batch annealing may be annealed for 1 to 8 hours.
[21]
After the batch annealing step, the volume fraction of the crystal grains forming an angle of 15 ° or less from {110} <001> may be 40% or more.
[22]
After the batch annealing step, the area fraction of the crystal grains having a diameter of 1000 μm to 5000 μm among the total crystal grains may be 20 to 70%.
[23]
In the grain-oriented electrical steel sheet according to an embodiment of the present invention, a grain area fraction having a diameter of 1000 µm to 5000 µm among all grains may be 20 to 70%.
[24]
Electrical steel sheet is by weight, Si: 1.0% to 4.0%, C: 0.005% or less (excluding 0%) and the remainder may include Fe and unavoidable impurities.
[25]
The electrical steel sheet may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.
[26]
The volume fraction of the crystal grains forming an angle of 15° or less from {110}<001> may be 40% or more.
[27]
The Goss crystal grains having a ratio (D2/D1) of 0.5 or more of the diameter D1 of the circumscribed circle and the diameter D2 of the inscribed circle may be 95 area% or more of the total Goss crystal grains.
Effects of the Invention
[28]
The grain-oriented electrical steel sheet according to an embodiment of the present invention has excellent magnetic properties by stably forming Goss crystal grains with a large diameter while using normal crystal growth.
[29]
In addition, since AlN and MnS are not used as grain growth inhibitors, there is no need to heat the slab to a high temperature of 1300° C. or higher.
[30]
In addition, since it is not necessary to remove the precipitates N and S, the purification annealing time may be relatively shortened, and productivity may be improved.
[31]
In addition, it is possible to provide a grain-oriented electrical steel sheet having magnetic properties cracked in the width direction.
Brief description of the drawing
[32]
1 is a photograph of the surface of a grain-oriented electrical steel sheet prepared in Inventive Material 2 observed with a scanning electron microscope.
[33]
2 is a photograph of the surface of the grain-oriented electrical steel sheet prepared in Comparative Material 2 observed with a scanning electron microscope.
Modes for carrying out the invention
[34]
Terms such as first, second and third are used to describe, but are not limited to, various parts, components, regions, layers and/or sections. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.
[35]
The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. The meaning of "comprising," as used herein, specifies a particular characteristic, region, integer, step, operation, element and/or component, and includes the presence or absence of another characteristic, region, integer, step, operation, element and/or component. It does not exclude additions.
[36]
When a part is referred to as being “on” or “on” another part, it may be directly on or on the other part, or the other part may be involved in between. In contrast, when a part refers to being “directly above” another part, the other part is not interposed therebetween.
[37]
Although not defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, they are not interpreted in an ideal or very formal meaning.
[38]
In addition, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.
[39]
In an embodiment of the present invention, the meaning of further including the additional element means that the remaining iron (Fe) is included by replacing the additional amount of the additional element.
[40]
Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.
[41]
A method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises the steps of heating a slab; manufacturing a hot-rolled steel sheet by hot-rolling the slab; annealing the hot-rolled steel sheet; First cold rolling the hot-rolled sheet annealed hot-rolled sheet; decarburizing annealing the first cold-rolled steel sheet; Secondary cold rolling of the decarburization annealing is completed; Continuous annealing of the steel sheet on which the secondary cold rolling has been completed and the step of batch annealing the continuously annealed steel sheet.
[42]
Hereinafter, each step will be described in detail.
[43]
First, the slab is heated.
[44]
The slab is in wt%, Si: 1.0% to 4.0%, C: 0.1% to 0.4%, and the balance may include Fe and unavoidable impurities.
[45]
The reason for limiting the composition is as follows.
[46]
Silicon (Si) improves the iron loss by lowering the magnetic anisotropy of the electrical steel sheet and increasing the specific resistance. If the Si content is less than 1.0% by weight, iron loss is inferior, and when it is more than 4.0% by weight, brittleness increases. Therefore, the content of Si in the grain-oriented electrical steel sheet after the slab and final annealing step may be 1.0 to 4.0 wt%. More specifically, the content of Si may be 1.5 to 3.5 wt%.
[47]
Since carbon (C) requires a process in which C in the center escapes to the surface layer in order for the Goss grains of the surface layer to diffuse to the center during the intermediate decarburization annealing and final decarburization annealing, the content of C in the slab may be 0.1 to 0.4% by weight. . More specifically, the content of C in the slab may be 0.15 to 0.3 wt%. In addition, the amount of carbon in the final grain-oriented electrical steel sheet after the final annealing step in which decarburization is completed may be 0.0050 wt% or less. More specifically, it may be 0.002 wt% or less.
[48]
The slab may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.
[49]
Mn and S form MnS precipitates and prevent the growth of Goss grains that diffuse to the center during the decarburization process. Therefore, it is preferable that Mn and S are not added. However, in consideration of the amount unavoidably incorporated during the steelmaking process, Mn and S in the grain-oriented electrical steel sheet after the slab and final annealing step can be controlled to Mn: 0.1 wt% or less, S: 0.005 wt% or less.
[50]
The balance contains Fe and unavoidable impurities. The unavoidable impurities are impurities that are mixed in the steelmaking step and the manufacturing process of the grain-oriented electrical steel sheet, which are widely known in the art, and thus a detailed description thereof will be omitted. Specifically, since components such as Al, N, Ti, Mg, and Ca react with oxygen in steel to form oxides, strong suppression is required, so each component can be managed at 0.005 wt% or less. In one embodiment of the present invention, addition of elements other than the above-described alloy components is not excluded, and may be included in various ways within a range that does not impair the technical spirit of the present invention. When additional elements are included, they are included by replacing the remainder of Fe.
[51]
More specifically, the slab is in weight %, Si: 1.0% to 4.0%, C: 0.1% to 0.4%, and the balance may be made of Fe and unavoidable impurities.
[52]
Slab heating temperature may be 1100 ℃ to 1350 ℃ higher than the normal heating temperature. When the slab is heated when the temperature is high, the hot-rolled structure is coarsened, which has a problem in that it adversely affects the magnetism. However, in the method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, the carbon content of the slab is relatively high, so even if the slab reheating temperature is high, the hot-rolled structure is not coarsened. more advantageous
[53]
Next, the slab is hot-rolled to manufacture a hot-rolled steel sheet.
[54]
Hot rolling can be manufactured into a hot-rolled sheet having a thickness of 1.5 to 4.0 mm by hot rolling so that it can be manufactured to a final product thickness by applying an appropriate rolling rate in the final cold rolling step.
[55]
The hot-rolling temperature or cooling temperature is not particularly limited, but as an example of excellent magnetic properties, the hot-rolling end temperature is 950° C. or less, and the cooling is quenched by water to be wound at 600° C. or less.
[56]
Next, the hot-rolled steel sheet is annealed. In this case, the hot-rolled sheet annealing may include a decarburization process. Specifically, hot-rolled sheet annealing may be annealed at a temperature of 850 °C to 1000 °C and a dew point temperature of 50 °C to 70 °C. After the annealing described above, it may be further annealed at a temperature of 1000 to 1200 ℃ and a dew point temperature of 0 ℃ or less. It can be pickled after hot-rolled sheet annealing.
[57]
Next, a cold-rolled steel sheet is manufactured by performing primary cold rolling.
[58]
It is known that it is effective to perform cold rolling once at a high-pressure reduction ratio close to 90% in a normal grain-oriented electrical steel sheet manufacturing process. This is because only Goss grains among the primary recrystallization grains create a favorable environment for grain growth. However, the method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention is to internally diffuse Goss grains in the surface layer generated by decarburization annealing and cold rolling without using abnormal grain growth of the Goss orientation grains. It is advantageous to form it so that it distribute|distributes many.
[59]
Therefore, when cold rolling is performed at a reduction ratio of 50% to 70% during cold rolling, a plurality of Goss textures may be formed in the surface layer portion. More specifically, it may be 55% to 65%.
[60]
Next, the cold-rolled steel sheet is decarburized and annealed. At this time, the step of decarburization annealing may be performed in an austenite single phase region or a region in which a composite phase of ferrite and austenite exists. Specifically, it may be annealed at a temperature of 850 °C to 1000 °C and a dew point temperature of 50 °C to 70 °C. In addition, the atmosphere may be a mixed gas atmosphere of hydrogen and nitrogen. In addition, the amount of decarburization during decarburization annealing may be 0.0300 wt% to 0.0600 wt%. After the annealing described above, it may be further annealed at a temperature of 1000 to 1200 ℃ and a dew point temperature of 0 ℃ or less.
[61]
In this decarburization annealing process, the size of the crystal grains on the surface of the electrical steel sheet grows coarsely, but the crystal grains inside the electrical steel sheet remain as a fine structure. After such decarburization annealing, the average diameter of the grains may be 150 μm to 250 μm. At this time, the crystal grains are surface ferrite grains. In addition, the diameter of the crystal grain means a diameter of the circle assuming an imaginary circle having the same area as the crystal grain.
[62]
Next, the secondary cold rolling of the decarburization annealed steel sheet is completed. Since the secondary cold rolling is the same as that of the primary cold rolling, a detailed description thereof will be omitted.
[63]
The decarburization annealing of the above-described cold-rolled steel sheet and the secondary cold rolling of the decarburization annealing completed steel sheet may be repeated two or more times. By repeating it two or more times, a Goss texture can be formed in large numbers in the surface layer part.
[64]
Next, the steel sheet on which the secondary cold rolling has been completed is continuously annealed.
[65]
Continuous annealing is annealing at a temperature of 850 °C to 1000 °C and a dew point temperature of 50 °C to 70 °C. The cold-rolled sheet before continuous annealing has been subjected to decarburization annealing, so that 40% to 60% of carbon remains relative to the carbon weight of the slab. Therefore, in the continuous annealing step, the crystal grains formed in the surface layer are diffused inside as carbon escapes. In the continuous annealing step, decarburization may be performed so that the amount of carbon in the steel sheet is 0.005 wt% or less.
[66]
Continuous annealing may be annealed for 1 to 5 minutes. The purpose of the continuous annealing step is to decarburize the carbon in the steel and then grow the crystal grains to a certain size or more. The reason is that the fraction of Goss grains continuously increases through the process of decarburization and grain growth immediately after it. This is because Goss grains grow while encroaching on the surrounding Non-Goss grains. However, considering the productivity of continuous annealing, crystal growth is limited because the annealing time is limited within a few minutes. In an embodiment of the present invention, it is intended to assert that it is effective in reducing iron loss by inducing crystal growth through additional batch annealing. At this time, the Goss fraction does not increase, but the iron loss decreases due to the effect of increasing the grain size.
[67]
After the continuous annealing step, an annealing separator may be applied. Since the annealing separator is widely known in the art, a detailed description thereof will be omitted. For example, an annealing separator containing MgO as a main component may be used.
[68]
Next, the continuously annealed steel sheet is batch annealed. Batch annealing refers to annealing by winding a steel sheet into a coil shape.
[69]
In the batch annealing step, the texture with the Goss orientation diffused in the continuous annealing step grows. In the method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, the Goss texture may have a diameter of 5 mm or less, unlike a case in which crystal grains are grown by conventional abnormal grain growth. Specifically, the fraction of grains having a diameter of 1000 μm to 5000 μm increases. Therefore, there are a plurality of goth grains having a smaller grain size than in the conventional grain-oriented electrical steel sheet manufactured by abnormal grain growth, but the grain size can be adjusted to an appropriate size to minimize iron loss. More specifically, the area fraction of the crystal grains having a diameter of 1000 μm to 5000 μm among all crystal grains may be 20 to 70%. At this time, the grain area fraction is measured in a plane parallel to the rolling surface (ND surface) of the steel sheet. More specifically, the area fraction of the crystal grains having a diameter of 1000 μm to 5000 μm among the total crystal grains may be 20 to 60%. More specifically, the area fraction of the crystal grains having a diameter of 1000 μm to 5000 μm among all crystal grains may be 20 to 50%.
[70]
The batch annealing may be annealed at a temperature of 1000°C to 1200°C and a dew point temperature of -20°C or less.
[71]
In addition, the batch annealing may be annealed for 1 to 8 hours. More specifically, it may be annealed for 2 to 5 hours.
[72]
In addition, in the method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, the Goss fraction is high, and the magnetism is improved. Specifically, the volume fraction of the crystal grains forming an angle of 15° or less from {110}<001> may be 40% or more. More specifically, it may be 40% to 75%. More specifically, it may be 45 to 60%.
[73]
In the grain-oriented electrical steel sheet according to an embodiment of the present invention, a grain area fraction having a diameter of 1000 µm to 5000 µm among all grains is 20 to 70%.
[74]
Since the area distribution of the grains has been described in detail in relation to the method of manufacturing the grain-oriented electrical steel sheet, the overlapping description will be omitted.
[75]
Electrical steel sheet is by weight, Si: 1.0% to 4.0%, C: 0.005% or less (excluding 0%) and the remainder may include Fe and unavoidable impurities.
[76]
The electrical steel sheet may further include Mn: 0.1 wt% or less and S: 0.005 wt% or less.
[77]
Except for C, since it is the same as the content of the component limitation of the slab, the overlapping description is omitted.
[78]
The volume fraction of the crystal grains forming an angle of 15° or less from {110}<001> may be 40% or more.
[79]
The Goss crystal grains having a ratio (D2/D1) of 0.5 or more of the diameter D1 of the circumscribed circle and the diameter D2 of the inscribed circle may be 95 area% or more of the total Goss crystal grains. In an embodiment of the present invention, the crystal grains of the above-described shape are formed due to a unique manufacturing process.
[80]
The grain-oriented electrical steel sheet according to an embodiment of the present invention has a high Goss fraction, and the magnetism is improved. Specifically, the iron loss (W 17/50 ) may be 1.3 W/kg or less. More specifically, the iron loss (W 17/50 ) may be 1 to 1.3 W/kg. More specifically, it may be 1.1 to 1.25 W/kg. The iron loss W 17/50 is a size (W / kg) of the iron loss is derived from 1.7Tesla and 50Hz conditions.
[81]
Hereinafter, specific examples of the present invention will be described. However, the following examples are only specific examples of the present invention, and the present invention is not limited thereto.
[82]
Example 1
[83]
A slab containing Si: 2.32%, C: 0.195% by weight and the remainder Fe and unavoidable impurities as weight % was heated at a temperature of 1250 ° C., and then hot rolled, followed by an annealing temperature of 950 ° C., a hot-rolled sheet at a dew point temperature of 60 ° C. Annealed was done. After cooling the steel sheet, pickling was performed, and cold rolling was performed at a reduction ratio of 65% to produce a cold-rolled sheet having a thickness of 0.8 mm.
[84]
The cold-rolled sheet is again subjected to decarburization annealing for 80 seconds in a wet mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60°C) at a temperature of 950 ° C. produced.
[85]
Thereafter, during final annealing, decarburization annealing was performed for 2 minutes in a wet mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60° C.) at a temperature of 950 ° C. As summarized in Table 1, hydrogen and nitrogen at 1100 ° C. The heat treatment was carried out in a mixed gas atmosphere (dew point temperature of 60° C.), or in a coiled state, in a hydrogen and nitrogen mixed gas atmosphere at 1200° C. for the time shown in Table 1 below.
[86]
Table 1 is a table showing the Goss fraction of grains of grain-oriented electrical steel sheet after high-temperature annealing according to Examples, the area fraction of grains of 1 mm or more and 5 mm or less, and iron loss. The Goss fraction was determined by measuring the volume fraction of grains forming an angle of 15 ° or less from {110} <001>. After surface cleaning of the finally obtained steel sheet, iron loss was measured under conditions of 1.7 Tesla and 50 Hz using a single sheet measurement method.
[87]
[Table 1]
[88]
As shown in Table 1, it can be seen that Inventive Materials 1 to 4, which were subjected to batch annealing for an appropriate time, have a high area fraction of crystal grains having a diameter of 1 to 5 mm. Although the goss fraction is relatively low compared to the comparative material, it can be confirmed that the iron loss is rather excellent.
[89]
1 and 2 show photographs observed with a scanning electron microscope of the surface of the grain-oriented electrical steel sheet prepared in Inventive Material 2 and Comparative Material 2 in FIG.
[90]
As can be seen in FIGS. 1 and 2 , it can be confirmed that the crystal grains of the grain-oriented electrical steel sheet prepared in Inventive Material 2 are relatively large.
[91]
The present invention is not limited to the above embodiments and/or embodiments, but may be manufactured in various different forms, and those skilled in the art to which the present invention pertains may change the technical spirit or essential features of the present invention It will be understood that the present invention may be implemented in other specific forms without the above. Therefore, it should be understood that the embodiments and/or embodiments described above are illustrative in all respects and not restrictive.
Claims
[Claim 1]
By weight%, Si: 1.0% to 4.0%, C: 0.005% or less (excluding 0%), Mn: 0.1% by weight or less (excluding 0%), S: 0.005% by weight or less (excluding 0%) ) and the balance contains Fe and unavoidable impurities, and grain-oriented electrical steel sheet having a diameter of 1000 μm to 5000 μm among the total crystal grains and a grain area fraction of 20 to 70%.
[Claim 2]
The grain-oriented electrical steel sheet according to claim 1, wherein the volume fraction of grains forming an angle of 15° or less from {110}<001> is 40% or more.
[Claim 3]
The grain-oriented electrical steel sheet according to claim 1, wherein the ratio (D2/D1) of the diameter (D1) of the circumscribed circle to the diameter (D2) of the inscribed circle is 0.5 or more of Goss grains of 95 area% or more of the total Goss grains.
[Claim 4]
heating the slab; manufacturing a hot rolled steel sheet by hot rolling the slab; annealing the hot-rolled steel sheet; first cold-rolling the hot-rolled steel sheet annealed with the hot-rolled sheet; decarburizing annealing the first cold-rolled steel sheet; Secondary cold rolling of the decarburization annealing is completed; Method of manufacturing a grain-oriented electrical steel sheet comprising the steps of continuously annealing the steel sheet on which the secondary cold rolling has been completed, and batch annealing the continuously annealed steel sheet.
[Claim 5]
The method of claim 4, wherein the annealing of the hot-rolled sheet includes a decarburization process.
[Claim 6]
The method of claim 4, wherein the annealing of the hot-rolled sheet is annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.
[Claim 7]
The method of claim 4, wherein the decarburizing annealing of the primary cold-rolled steel sheet is annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.
[Claim 8]
[Claim 5] The method of claim 4, wherein the decarburizing annealing of the primary cold-rolled steel sheet is annealed in an austenite single-phase region or a region in which a composite phase of ferrite and austenite exists.
[Claim 9]
5. The method of claim 4, wherein after the decarburization annealing of the first cold-rolled steel sheet, the average diameter of grains is 150 to 250 μm.
[Claim 10]
The method of claim 4, wherein the decarburization annealing of the primary cold-rolled steel sheet and the secondary cold-rolling of the decarburization annealing finished steel sheet are repeated two or more times.
[Claim 11]
The method of claim 4, wherein the continuous annealing is annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.
[Claim 12]
The method of claim 4, wherein the continuous annealing is annealing for 1 to 5 minutes.
[Claim 13]
The method of claim 4, wherein the batch annealing is annealed at a temperature of 1000°C to 1200°C and a dew point temperature of -20°C or less.
[Claim 14]
The method of claim 4, wherein the batch annealing is annealing for 1 to 8 hours.