The present invention is directed to a grain-oriented electrical steel sheet and a method of manufacturing the same. More specifically, the present invention relates to a grain-oriented electrical steel sheet containing B, Ba, and Y in a predetermined amount to be segregated in grain boundaries, and a method for manufacturing the same.
The grain-oriented electrical steel sheet is a soft magnetic material having excellent magnetic properties in the rolling direction, composed of grains having a crystal orientation of {110}<001>, so-called Goss orientation.
In general, magnetic properties can be expressed by magnetic flux density and iron loss, and high magnetic flux density can be obtained by precisely aligning the orientation of the grains to the {110}<001> orientation. The electrical steel sheet having a high magnetic flux density not only makes it possible to reduce the size of the iron core material of the electric equipment, but also reduces the hysteresis loss, thereby making it possible to miniaturize the electric equipment and increase the efficiency at the same time. The iron loss is a power loss consumed as heat energy when an arbitrary alternating magnetic field is applied to the steel sheet, and varies greatly depending on the magnetic flux density and plate thickness of the steel sheet, the amount of impurities in the steel sheet, the specific resistance and the size of the secondary recrystallization grain. The higher the magnetic flux density and the specific resistance and the lower the plate thickness and the amount of impurities in the steel sheet, the lower the iron loss, thereby increasing the efficiency of the electrical equipment.
In order to cope with global warming by reducing CO2 emission worldwide, there is a tendency toward energy saving and high-efficiency commercialization. Further, as the demand for widening and spreading of highly efficient electric devices using less electric energy is increased, the social demand for the development of a grain-oriented electrical steel sheet having a low iron loss property is increasing.
Generally, a grain-oriented electrical steel sheet having excellent magnetic properties is required to strongly develop a Goss texture in a {110} <001> orientation in the rolling direction of a steel sheet. In order to form such a texture, grains in the Goss orientation should form an abnormal grain growth called the second recrystallization. This abnormal grain growth occurs when normal grain growth inhibits the movement of grain boundaries normally grown by precipitates, inclusions, or elements dissolved or segregated in the grain boundaries, unlike ordinary grain growth. As described in the above, precipitates and inclusions that inhibit grain growth are specifically referred to as a grain growth inhibitor. Studies on the production of grain-oriented electrical steel sheets by secondary recrystallization in the {110}<001> orientation has been focused on securing good magnetic properties by using a grain growth inhibitor to form secondary recrystallization with high degree of integration in the {110} <001> orientation.
In the conventional grain-oriented electrical steel sheet technology, precipitates such as AlN and MnS[Se] are mainly used as a grain growth inhibitor. For example, decarburization is carried out after one time of the strong cold-rolling. And then nitrogen is supplied to the inside of the steel sheet through a separate nitriding process using ammonia gas to produce secondary recrystallization by the Al-based nitride which exhibits a strong grain growth inhibiting effect.
However, in the process of high temperature annealing, the instability of the precipitates due to the denitrification or the re-nitrification based on the furnace atmosphere and the necessity of the stress relief annealing for a long time for 30 hours or more at a high temperature causes the complications and cost burden.
For this reason, recently, a method of manufacturing a grain-oriented electrical steel sheet without using a precipitates such as AlN or MnS as a grain growth inhibitor has been proposed. For example, there is a manufacturing method using grain boundary segregation elements such as barium (Ba) and yttrium (Y).
Ba and Y are excellent in the effect of inhibiting the growth of grains enough to form secondary recrystallization and are not affected by the atmosphere in the furnace during the high-temperature annealing process. However, they have a disadvantage in weakening the bonding strength of the grain boundaries. Therefore, there is a problem in that a large number of grain boundary cracks occur in the cold-rolling process in which the high pressure is required, so that the productivity decrease cannot be avoided.
In one embodiment of the present invention, a grain-oriented electrical steel sheet and a method of manufacturing the same are provided.
The invention is described in the claims.
The grain-oriented electrical steel sheet according to an embodiment of the present invention is excellent in magnetic properties by stably forming Goss grain.
In addition, since AlN and MnS are not used as a grain growth inhibitor, it is not necessary to heat the slab at a high temperature of 1300°C or more.
In addition, due to the grain boundary strengthening effect, generation of grain boundary cracks is reduced even under a strong cold-rolling. Thus, the productivity is increased and manufacturing cost is reduced.
Fig. 1 is a photograph of a cold-rolled steel sheet in the process of manufacturing the inventive material, which is a sample No. 2.FIG. 2 is a photograph of a cold-rolled steel sheet in the process of manufacturing the comparative material, which is a sample No. 1.
The terms first, second, third, and the like are used to describe various portions, components, regions, layers and/or sections, but are not limited thereto. These terms are only used to distinguish one portion, component, region, layer or section from another portion, component, region, layer or section. Thus, a first portion, component, region, layer or section described below may be referred to as a second portion, component, region, layer or section without departing from the scope of the present invention.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms as used herein include plural forms as long as the phrases do not specifically state the opposite meaning thereof. The "comprises" means that a particular characteristic, region, integer, step, motion, element and/or component is specified and that does not exclude the presence or addition of other characteristics, regions, integers, steps, motions, elements, and/or components.
When referring to a part as being "on" or "above" another part, it may be positioned directly on or above another part, or another part may be interposed therebetween. In contrast, when referring to a part being "directly above" another part, no other part is interposed therebetween.
Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms defined in the commonly used dictionary are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.
Unless otherwise stated, % means % by weight, and 1 ppm is 0.0001% by weight.
Hereinafter, embodiments of the present invention will be described in detail so that a person of ordinary skill in the art could easily carry out the present invention. The present invention may, however, be embodied in various forms and should not be construed as limited to the embodiments set forth herein.
In the conventional grain-oriented electrical steel sheet technology, precipitates such as AlN and MnS were used as the grain growth inhibitors. All the processes were strictly controlling the distribution of the precipitates and the process conditions were severely constrained by the conditions for removing precipitates remaining in the secondary recrystallized steel sheet.
On the other hand, in one embodiment of the present invention, precipitates such as AlN and MnS are not used as a grain growth inhibitor.
The present invention uses B and Ba or Y as a grain growth inhibitor, thus it is possible to increase the grain fraction of Goss and obtain an electrical steel sheet excellent in magnetic properties.
The grain-oriented electrical steel sheet of the present invention includes by weight, Si: 1.0 to 7.0%, Mn: 0.01 to 0.5%, B: 0.001 to 0.1%, and Ba and Y individually or in a total amount of 0.005 to 0.5%, and the remainder including Fe and other unavoidable impurities.
Hereinafter, each component will be described in detail.
Barium (Ba) and yttrium (Y) act as a grain growth inhibitor, during secondary recrystallization annealing, to suppress the growth of grains in a orientation other than the Goss grains, thereby improving the magnetic properties of the electrical steel sheet. Ba and Y may be added individually or in combination. Ba and Y are included individually or in a total amount of 0.005 to 0.5% by weight. That is, when Ba or Y is added individually, the content of Ba or Y is 0.005 wt% to 0.5 wt%, respectively. When Ba and Y are simultaneously added, the sum of the contents (i.e., the total amount) of Ba and Y is 0.005 wt% to 0.5 wt%. If the amount of Ba or Y or the total amount thereof is too small, it is difficult to exert a sufficient restraining force. If the amount of Ba or Y or the total amount thereof is too large, the brittleness of the steel sheet increases and cracks may occur during rolling.
Boron (B) is segregated at the grain boundaries to strengthen the grain boundary bonding force, thereby reducing generation of cracks and rolling times during rolling. In addition, it reacts with nitrogen in the steel to partially form BN precipitates. BN is excellent in high temperature stability and can act as an auxiliary inhibitor which suppresses grain growth together with Ba and Y described in the above. The content of B is 0.001 to 0.1% by weight. If B is included too little, it may be insufficient to alleviate the grain boundary brittleness due to Ba and Y. If B is included too much, grain boundary segregation of Ba and Y is suppressed, and a large number of inclusions are formed in the high-temperature annealing process, so that the magnetic properties may be deteriorated.
B satisfies the following Formula 1 in relation to Ba and Y
When the value of the Formula 1 is less than 0.5, grain boundary segregation of Ba and Y is suppressed. Further, a large number of inclusions are formed in the high-temperature annealing process, so that the magnetic properties may be deteriorated. When the value of the Formula 1 is more than 3, it may be insufficient to alleviate the grain boundary brittleness due to Ba and Y.
Silicon (Si) acts to lower the iron loss by increasing the specific resistance of the material. If the Si content in the slab and the electrical steel sheet is less than 1.0% by weight, the specific resistance may decrease and the iron loss property may be deteriorated. On the contrary, when the Si content exceeds 7% by weight in the grain-oriented electrical steel sheet, the Si content in the grain-oriented electrical steel sheet can be 7% by weight or less since the processing is difficult in manufacturing the transformer.
Carbon (C), as an austenite stabilizing element, is added to the slab in an amount of 0.001 wt% or more to refine the coarse columnar structure that occurs during the performance process and to suppress the slab center segregation of S. It is also possible to accelerate work hardening of the steel sheet during cold-rolling, thereby promoting generation of secondary recrystallization nuclei in the {110} <001> orientation in the steel sheet. However, if the content exceeds 0.1%, it may cause edge-cracks in hot-rolled steel. However, the decarburization annealing is performed during the production of the electrical steel sheet, and the C content in the final electrical steel sheet after decarburization annealing is 0.005 wt% or less. Preferably, it may be 0.003% by weight or less.
In one embodiment of the present invention, the precipitates, such as AlN and MnS, are not used as a grain growth inhibitor. Therefore, the elements which are essentially used in normal grain-oriented electrical steel sheets, such as aluminum (Al), nitrogen (N), sulfur (S), are regulated within the range of impurities. That is, when Al, N, and S are inevitably further included, it further includes 0.005 wt% or less of Al, 0.0055 wt% or less of S, and 0.0055 wt% or less of N.
In one embodiment of the present invention, since AlN is not used as a grain growth inhibitor, aluminum (Al) content can be positively suppressed.
Therefore, in one embodiment of the present invention, Al may not be added to the grain-oriented electrical steel sheet or is controlled to 0.005 wt% or less. In addition, in the slab, since Al can be removed during the manufacturing process, Al can be contained in an amount of 0.01 wt% or less. Since nitrogen (N) forms precipitates such as AlN, (Al,Mn)N, (Al, Si,Mn)N, Si3N4, and BN, in the embodiment of the present invention, N may not be added or is controlled to 0.0055 wt% or less.
Preferably, it may be 0.0030% by weight or less. In one embodiment of the present invention, the nitriding process can be omitted, so that the N content in the slab and the N content in the final electrical steel sheet can be substantially the same. The sulfur (S) is an element having a high dissolving temperature and a high segregation during hot-rolling, and thus, in one embodiment of the present invention, it may not be added or is controlled to 0.0055 wt% or less. Preferably, it may be 0.0035% by weight or less.
In one embodiment of the present invention, since MnS is not used as a grain growth inhibitor, manganese (Mn) may not be added. However, since Mn is a non-resistive element and has an effect of improving magnetic properties, it may be further included as an optional component in slabs and electrical steel sheets. When Mn is further included, the content of Mn is 0.01 wt% or more. However, if it exceeds 0.5% by weight, phase transformation may occur after the secondary recrystallization, and the magnetic property may be deteriorated. In the embodiment of the present invention, when additional elements are further included, it is understood that it is added replacing iron (Fe) which is the remainder.
In addition, as other unavoidable impurities, components such as Ti, Mg, and Ca react with oxygen in the steel to form oxides, which may interfere with the magnetic migration of the final product as an inclusion and cause magnetic deterioration. Thus, it is necessary to strongly suppress the unavoidable impurities. Therefore, when they are inevitably contained, they are controlled to 0.005% by weight or less for each component.
The grain-oriented electrical steel sheet has 10 mm or more of an average particle diameter of grains having 2 mm or more of the particle diameter. If the average particle diameter of the grains having a particle diameter of 2 mm or more is less than 10 mm, the grains may not grow sufficiently and thus the magnetic properties may be deteriorated. In one embodiment of the present invention, the particle diameter of grains means the diameter length of the grains of the circular form.
The grain-oriented electrical steel sheet according to an embodiment of the present invention is excellent in magnetic properties by stably forming Goss grain. Specifically, the grain-oriented electrical steel sheet according to an embodiment of the present invention may have a magnetic flux density B8 of 1.88T or more measured at a magnetic field of 800 A/m.
The method for manufacturing a grain-oriented electrical steel sheet according to the present invention includes a step of heating the slab containing, by weight, Si: 1.0 to 7.0%, B: 0.001 to 0.1%, and Ba and Y individually or in a total amount of 0.005 to 0.5%, and the remainder including Fe and other unavoidable impurities; a step of hot-rolling the slab to produce a hot-rolled sheet; a step of cold-rolling the hot-rolled sheet to produce a cold-rolled sheet; a step of the primary recrystallization annealing the cold-rolled sheet; and a step of the second recrystallization annealing the cold-rolled sheet after the primary recrystallization annealing is completed.
Hereinafter, a manufacturing method of the grain-oriented electrical steel sheet will be described in detail for each step.
First, the slab is heated.
Since the composition of the slab has been described in detail with respect to the composition of the electrical steel sheet, a duplicate explanation will be omitted.
The heating temperature of the slab is limited. If the slab is heated to a temperature of 1280°C or less, it may prevent the columnar structure of the slab from becoming coarse, thereby preventing cracks in the plate during the hot-rolling process. Thus, the heating temperature of the slab may be between 1000°C and 1280°C. In particular, in one embodiment of the present invention, since AlN and MnS are not used as a grain growth inhibitor, it is not necessary to heat the slab at a high temperature of 1300°C or more.
Next, the slab is hot-rolled to produce a hot-rolled sheet. The hot-rolling temperature is not limited, and in one embodiment, hot-rolling may be terminated at 950°C or lower. Thereafter, it is water-cooled and can be wound at 600°C or less.
Next, the hot-rolled sheet can be subject to a hot-rolled sheet annealing, if necessary. In the case of annealing the hot-rolled sheet, the hot-rolled steel sheet can be heated to a temperature of 900°C or more, cracked, and cooled to make the texture of the hot-rolled steel sheet uniform.
Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet. The cold-rolling can be carried out by a cold-rolling method using a reverse rolling mill or a tandem rolling mill through one cold-rolling, a plurality of cold-rolling, a plurality of cold-rolling including an intermediate annealing to produce a cold-rolled sheet having a thickness of 0.1 mm to 0.5 mm. Further, warm-rolling in which the temperature of the steel sheet is maintained at 100°C or higher during the cold-rolling can be performed.
In addition, the final reduction roll through cold-rolling can be 80% or more.
In the present invention, as described in the above, by containing a specific amount of B in the slab component, the grain boundary is segregated to strengthen the grain boundary's bonding force. As a result, cracking and rolling times can be reduced during rolling and the final reduction roll can be increased.
Next, the cold-rolled sheet is subject to the primary recrystallization annealing. The primary recrystallization occurs in which the core of the Goss grain nuclei is generated in the primary recrystallization annealing step. The decarburization of the cold-rolled sheet can be performed in the primary recrystallization annealing step. It is annealed at a temperature of 800°C to 900°C for decarburization. Further, the atmosphere is a mixed gas atmosphere of hydrogen and nitrogen. When the decarburization is completed, the carbon content in the cold-rolled steel sheet may be 0.005 wt% or less. In one embodiment of the present invention, since the AlN grain growth inhibitor is not used, the nitriding process can be omitted.
Next, the cold-rolled sheet having undergone the primary recrystallization annealing is subject to a secondary recrystallization annealing. At this time, after the annealing separator is applied to the cold-rolled sheet having undergone the primary recrystallization annealing, secondary recrystallization annealing can be performed. At this time, the annealing separator is not particularly limited, and an annealing separator containing MgO as a main component can be used.
The step of secondary recrystallization annealing includes a temperature elevating step and a soaking step. The temperature elevating step is a step of raising the temperature of the cold-rolled sheet, of which the primary recrystallization annealing is completed, to the temperature of the soaking step. The temperature of the soaking step is 900°C to 1250°C. If the temperature is less than 900°C, the Goss grains may not sufficiently grow and the magnetic properties may be deteriorated. When the temperature exceeds 1250°C, the grains may grow so large that the characteristics of the electrical steel sheet may be deteriorated. The temperature elevating step may be performed in a mixed gas atmosphere of hydrogen and nitrogen, and the soaking step may be performed in a hydrogen atmosphere.
In the method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, since the AlN and MnS are not used as a grain growth inhibitor, the stress relief annealing step can be omitted after the secondary recrystallization annealing is completed. In the conventional method of manufacturing a grain-oriented electrical steel sheet using MnS and AlN as a grain growth inhibitor, high-temperature stress relief annealing to remove precipitates, such as AlN and MnS, is required. However, in the method of manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention, the stress relief annealing process may not be necessary.
Thereafter, an insulating film may be formed on the surface of the grain-oriented electrical steel sheet or a magnetic domain refining treatment may be carried out, if necessary. In one embodiment of the present invention, the alloy component of the grain-oriented electrical steel sheet refers to a base steel sheet excluding a coating layer such as an insulating film.
Hereinafter, the present invention will be described in more detail with reference to examples. However, the embodiments are only for illustrating the present invention, and the present invention is not limited thereto.
A slab containing, by weight, Si: 3.2%, C: 0.05%, Mn: 0.06%, S: 0.0048%, N: 0.0032%, and Al: 0.005%, and barium (Ba), yttrium (Y), and boron (B) as shown in Table 1 below, and the remainder Fe and other inevitably incorporated impurities, was prepared.
The slab was heated at a temperature of 1150°C for 90 minutes, and hot-rolled to obtain a hot-rolled sheet having a thickness of 2.6 mm. The hot-rolled sheet was heated to a temperature of 1050°C or higher, held at 910°C for 90 seconds, cooled with water, and pickled. And then, the sheet was cold-rolled to a thickness of 0.30 mm through a total of seven passes using a reverse mill. The reduction roll per pass was the same for each test condition. The cold-rolled steel sheet was heated in a furnace, and then held in a mixed gas atmosphere of 50 vol% of hydrogen and 50 vol% of nitrogen and annealing temperature of 850°C for 120 seconds to carry out the primary recrystallization annealing along with the decarburization was performed until carbon content reaches 0.002 wt.%. Thereafter, MgO was applied and then wound into a coil, followed by the secondary recrystallization annealing. The secondary recrystallization annealing was carried out in a mixed gas atmosphere of 25 vol% of nitrogen and 75 vol% of hydrogen to elevate the temperature to 1200°C. After reaching 1200°C, the sheet was held in 100 vol% of hydrogen gas atmosphere for 20 hours, followed by cooling in the furnace. After the surface of the final steel sheet was cleaned, the magnetic flux density was measured at a magnetic field strength of 800 A/m using a single sheet measurement method.
As can be seen from Table 1, when the content of B was controlled within the range of the present invention depending on the contents of Ba and Y, the inventive material had no rolling cracks and excellent magnetic properties were obtained compared to the comparative material.
In addition, in
A slab containing, by weight, Si: 3.2%, C: 0.048%, Mn: 0.11%, S: 0.0051%, N: 0.0028%, and Al: 0.008%, and barium (Ba), yttrium (Y), and boron (B) as shown in Table 2 below, and the remainder Fe and other inevitably incorporated impurities, was prepared.
The slab was heated at a temperature of 1150°C for 90 minutes, and hot-rolled to obtain a hot-rolled sheet having a thickness of 2.6 mm. The hot-rolled sheet was heated to a temperature of 1050°C or higher, held at 910°C for 90 seconds, cooled with water, and pickled. And then, the sheet was cold-rolled to a thickness of 0.30 mm through a total of seven passes using a reverse mill. The reduction roll per pass was the same for each test condition. The cold-rolled steel sheet was heated in a furnace, and then held in a mixed gas atmosphere of 50 vol% of hydrogen and 50 vol% of nitrogen and annealing temperature of 850°C for 120 seconds to carry out the primary recrystallization annealing along with the decarburization was performed until carbon content reaches 0.003 wt.%. Thereafter, MgO was applied and then wound into a coil, followed by the secondary recrystallization annealing. The secondary recrystallization annealing was carried out in a mixed gas atmosphere of 25 vol% of nitrogen and 75 vol% of hydrogen to elevate the temperature to 1200°C. After reaching 1200°C, the sheet was held in 100 vol% of hydrogen gas atmosphere for 20 hours, followed by cooling in the furnace. After the surface of the final steel sheet was cleaned, the magnetic flux density was measured at a magnetic field strength of 800 A/m using a single sheet measurement method. In addition, the particle diameter of the grains was calculated as the average value based on the area after removing the coating layer on the surface by immersing into a hydrochloric acid heated to 60°C for 5 minutes.
Referring to Table 2, the average particle diameter of the grains having 2 mm or more of particle diameter in the electrical steel sheet according to an embodiment of the present invention was found to be 10 mm or more, and the magnetic properties were excellent.
It will be understood by those of ordinary skill in the art that various changes in form and details may be made herein without departing from the scope of the present invention as defined by the following claims. It will be understood that the invention may be practiced. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive.