JP7640604B2 - Grain-oriented electrical steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to grain-oriented electrical steel sheets and manufacturing methods thereof, and more specifically to grain-oriented electrical steel sheets and manufacturing methods thereof that have improved magnetic properties by including multiple cold rolling and decarburization annealing processes.

Grain-oriented electrical steel sheet is a soft magnetic material having excellent magnetic properties in the rolling direction, the crystal orientation of which is {110}<001>, also known as Goss orientation, and composed of crystal grains.
Such grain-oriented electrical steel sheets are manufactured by rolling the slab to a final thickness through hot rolling, hot-rolling, annealing, and cold rolling after slab heating, and then undergoing primary recrystallization annealing and high-temperature annealing for forming secondary recrystallization.
Generally, the secondary recrystallization annealing process of grain-oriented electrical steel sheet requires a low heating rate and long-term purification annealing at high temperature, which is a process that consumes a lot of energy. In order to manufacture grain-oriented electrical steel sheet with excellent magnetic properties by forming secondary recrystallization through such an extreme process, the following process difficulties arise.

First, a temperature difference occurs between the outer and inner coils due to heat treatment in a coiled state, and the same heat treatment pattern cannot be applied to each part, resulting in a magnetic difference between the outer and inner coils. Second, various surface defects occur in the process of coating the surface with MgO after decarburization annealing and forming a base coating during high-temperature annealing, thereby reducing the yield. Third, the production process is divided into three steps, so that the yield is reduced because the decarburized sheet after decarburization annealing is wound into a coil, annealed at high temperature, and then flattened and coated with insulation.
In order to overcome such process limitations, a technique has been proposed that utilizes normal crystal growth without utilizing secondary recrystallization by controlling decarburization annealing and cold rolling reduction. However, in continuous annealing, the grain growth of the final grains is limited due to the short heat treatment time of moisture, and grains cannot grow to have an optimal grain size, so there is a limit to the improvement of iron loss.

The objective of the present invention is to provide a grain-oriented electrical steel sheet with improved magnetic properties by including multiple cold rolling and decarburization annealing processes, and a manufacturing method thereof.

The method for manufacturing grain-oriented electrical steel sheet according to the present invention is characterized by including the steps of heating a slab, hot rolling the slab to manufacture a hot-rolled steel sheet, hot-rolling the hot-rolled steel sheet, first cold rolling the hot-rolled steel sheet that has been annealed, decarburizing annealing the first cold-rolled steel sheet, second cold rolling the steel sheet that has been decarburizing annealed, continuous annealing the steel sheet that has been second cold-rolled, and batch annealing the continuously annealed steel sheet.

The slab contains, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the balance being Fe and unavoidable impurities;
It is particularly characterized by further containing Mn: 0.1 wt % or less and S: 0.005 wt % or less.

The step of hot-rolling the slab includes a decarburization process,
The hot-rolled sheet annealing step is performed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.
The step of decarburizing the first cold-rolled steel sheet is characterized in that the steel sheet is annealed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C.

The step of decarburizing the first cold-rolled steel sheet includes annealing the steel sheet in an austenite single phase region or in a region where a composite phase of ferrite and austenite exists,
After the first cold-rolled steel sheet is decarburized and annealed, the average grain diameter is 150 to 250 μm.
The steps of subjecting the first cold-rolled steel sheet to decarburization annealing and subjecting the steel sheet to the second cold-rolling after the decarburization annealing are repeated two or more times.

The continuous annealing step is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C. for 1 to 5 minutes;
The batch annealing step includes annealing at a temperature of 1000° C. to 1200° C. and a dew point temperature of −20° C. or less for 1 to 8 hours;
After the batch annealing step, the volume fraction of crystal grains making an angle of 15° or less from {110}<001> is 40% or more;
The crystal grains have a diameter of 1000 μm to 5000 μm, and the surface area ratio of the crystal grains is 20 to 70%.

The grain-oriented electrical steel sheet according to the present invention is characterized in that the surface area fraction of crystal grains with diameters of 1000 μm to 5000 μm among all crystal grains is 20 to 70%.

The electrical steel sheet contains, by weight percent, Si: 1.0% to 4.0%, C: 0.005% or less (excluding 0%), with the balance being Fe and unavoidable impurities;
Further containing Mn: 0.1 wt.% or less and S: 0.005 wt.% or less;
The volume fraction of crystal grains making an angle of 15° or less from {110}<001> is 40% or more;
The Goss crystal grains having a ratio (D2/D1) of the diameter (D1) of the circumscribing circle to the diameter (D2) of the inscribing circle of 0.5 or more account for 95% or more by area of all the Goss crystal grains.

The grain-oriented electrical steel sheet according to the present invention has excellent magnetic properties by stably forming large-diameter Goss grains while utilizing normal crystal growth,
Furthermore, since AlN and MnS are not used as grain growth inhibitors, there is no need to heat the slab at high temperatures of 1300° C. or higher.
In addition, it is no longer necessary to remove precipitates such as N and S, and the purification annealing time can be relatively short, thereby improving productivity.
It is also possible to provide a grain-oriented electrical steel sheet having magnetic properties in which cracks are formed in the width direction.

1 is a photograph of the surface of a grain-oriented electrical steel sheet manufactured using Inventive Material 2, observed with a scanning electron microscope. 1 is a photograph of the surface of a grain-oriented electrical steel sheet produced using comparative material 2, observed with a scanning electron microscope.

Terms such as first, second and third are used to describe various parts, components, regions, layers and/or sections, but are not limited thereto. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Thus, 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.
The terminology used herein is merely for the purpose of referring to particular embodiments and is not intended to limit the present invention. As used herein, the singular form includes the plural form unless the text clearly indicates otherwise. The term "comprising" as used in the specification embodies certain features, regions, integers, steps, operations, elements and/or components and does not exclude the presence or addition of other features, regions, integers, steps, operations, elements and/or components.
When a part is referred to as being "on" another part, it may be "directly on" the other part, or there may be other parts interposed between them. In contrast, when a part is referred to as being "directly on" another part, there are no other parts interposed between them.
Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted to have a meaning consistent with the relevant technical literature and the currently disclosed content, and are not interpreted in an ideal or very formal sense unless defined.
Moreover, unless otherwise specified, % means % by weight, and 1 ppm is 0.0001% by weight.
In an embodiment of the present invention, the inclusion of an additional element means that the additional element is included in place of the remaining iron (Fe) by an amount corresponding to the additional element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described in detail with reference to exemplary embodiments thereof, so that those skilled in the art will be able to easily practice the present invention. However, the present invention may be embodied in many different forms and is not limited to the embodiments set forth herein.
A method for manufacturing a grain-oriented electrical steel sheet according to the present invention includes the steps of heating a slab, hot rolling the slab to manufacture a hot-rolled steel sheet, hot-rolling the hot-rolled steel sheet, first cold rolling the hot-rolled steel sheet that has been annealed, decarburization annealing the first cold-rolled steel sheet, second cold rolling the steel sheet that has been subjected to decarburization annealing, continuous annealing the steel sheet that has been subjected to the second cold rolling, and batch annealing the continuously annealed steel sheet.

Each step will be explained in detail below.
First, the slab is heated.
The slab contains, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the balance being Fe and unavoidable impurities.
The reasons for limiting the composition are as follows.
Silicon (Si) reduces the magnetic anisotropy of electrical steel sheet and increases resistivity to improve core loss. If the Si content is less than 1.0 wt%, core loss becomes poor, and if it exceeds 4.0 wt%, brittleness increases. Therefore, the Si content in the grain-oriented electrical steel sheet after the slab and final annealing steps may be 1.0 to 4.0 wt%. More specifically, the Si content is 1.5 to 3.5 wt%.

Regarding carbon (C), since the Goss crystal grains in the surface layer diffuse to the center during intermediate decarburization annealing and final decarburization annealing, a process is required in which C in the center escapes to the surface layer, so the C content in the slab is 0.1 to 0.4 wt%. More specifically, the C content in the slab is 0.15 to 0.3 wt%. In addition, after the final annealing stage in which decarburization is completed, the carbon content in the final grain-oriented electrical steel sheet is 0.0050 wt% or less, more specifically 0.002 wt% or less.
The slab further contains Mn: 0.1 wt. % or less and S: 0.005 wt. % or less.

Mn and S form MnS precipitates and impede the growth of Goss grains that diffuse to the center during the decarburization process. Therefore, it is preferable not to add Mn and S. However, taking into consideration the amount that is inevitably mixed in during the steelmaking process, the Mn and S contents in the grain-oriented electrical steel sheet after the slab and final annealing stages are controlled to Mn: 0.1 wt% or less and S: 0.005 wt% or less, respectively.
The balance is composed of Fe and inevitable impurities. The inevitable impurities are impurities that are mixed in during the steelmaking stage and the manufacturing process of the grain-oriented electrical steel sheet, and are widely known in the art, so a detailed description will be omitted. In particular, components such as Al, N, Ti, Mg, and Ca react with oxygen in the steel to form oxides, and therefore need to be strongly suppressed, and therefore each component can be controlled to 0.005 wt% or less. In one embodiment of the present invention, the addition of elements other than the alloy components described above is not excluded, and various elements can be included within a range that does not harm the technical idea of the present invention. When an additional element is further included, it is included in place of the balance Fe.

More specifically, the slab contains, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the balance being Fe and unavoidable impurities.
The slab heating temperature is 1100°C to 1350°C, which is higher than the usual heating temperature. If the temperature is high during slab heating, the hot-rolled texture becomes coarse, which can have a negative effect on magnetic properties. However, in the method for producing grain-oriented electrical steel sheet according to the present invention, the carbon content of the slab is relatively high, so that the hot-rolled texture does not become coarse even if the slab reheating temperature is high, and since the slab is reheated at a higher temperature than usual, it is more advantageous during hot rolling.

The slab is then hot rolled to produce a hot rolled steel sheet.
The hot rolling is carried out to produce a hot rolled sheet having a thickness of 1.5 to 4.0 mm so that a proper rolling ratio can be applied in the final cold rolling stage to produce the final product thickness.
The hot rolling temperature and the cooling temperature are not particularly limited, but an example in which the steel sheet has excellent magnetic properties is one in which the hot rolling end temperature is 950°C or less, and the steel sheet is rapidly cooled with water and coiled at 600°C or less.
Next, the hot-rolled steel sheet is annealed. At this time, the hot-rolled steel sheet annealing includes a decarburization process. Specifically, the hot-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.
After the above-mentioned annealing, additional annealing is performed at a temperature of 1000 to 1200° C. and a dew point temperature of 0° C. or less. After the hot-rolled sheet annealing is performed, pickling is performed.

Next, a first cold rolling is carried out to produce a cold-rolled steel sheet.
In the manufacturing process of a conventional grain-oriented electrical steel sheet, it is effective to perform cold rolling once at a high reduction rate of nearly 90%. This is because it creates an environment favorable for grain growth of only the Goss grains in the primary recrystallized grains. However, the manufacturing method of the grain-oriented electrical steel sheet according to the present invention does not utilize the abnormal grain growth of the Goss orientation grains, but rather causes the Goss grains in the surface layer generated by the decarburization annealing and cold rolling to diffuse internally, so it is advantageous to form a large number of Goss orientation grains distributed in the surface layer.
Therefore, when cold rolling is performed at a rolling reduction of 50% to 70%, more specifically, 55% to 65%, a large amount of Goss texture is formed in the surface layer.
Next, the cold rolled steel sheet is decarburized and annealed. At this time, the decarburization annealing step is performed in an austenite single phase region or a region where a composite phase of ferrite and austenite exists. Specifically, annealing is performed at a temperature of 850°C to 1000°C and a dew point temperature of 50°C to 70°C. The atmosphere is a mixed gas atmosphere of hydrogen and nitrogen. The amount of decarburization during decarburization annealing is 0.0300 wt% to 0.0600 wt%. After the above annealing, additional annealing is performed at a temperature of 1000°C to 1200°C and a dew point temperature of 0°C or less.
During this decarburization annealing process, the crystal grains on the surface of the electrical steel sheet grow coarse, but the crystal grains inside the electrical steel sheet remain as fine structures. After this decarburization annealing, the average diameter of the crystal grains is 150 μm to 250 μm. At this time, the crystal grains are surface ferrite crystal grains. The diameter of the crystal grains refers to the diameter of an imaginary circle having the same area as the crystal grain.

Next, the steel sheet that has been subjected to the decarburization annealing is subjected to a second cold rolling. The second cold rolling is the same as the first cold rolling, and therefore a detailed description thereof will be omitted.
The steps of decarburization annealing the cold-rolled steel sheet and second cold rolling the steel sheet after the decarburization annealing may be repeated two or more times. By repeating the steps two or more times, a Goss texture is formed in a large amount in the surface layer.
Next, the steel sheet which has completed the secondary cold rolling is subjected to continuous annealing.
The continuous annealing step is performed 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 the continuous annealing is in a state where the carbon content is 40% to 60% of the carbon weight of the slab due to the decarburization annealing. Therefore, in the continuous annealing step, the crystal grains formed in the surface layer diffuse inward as the carbon is released. In the continuous annealing step, decarburization can be performed so that the carbon content in the steel sheet is 0.005 wt% or less.

The continuous annealing step is performed for 1 to 5 minutes. The purpose of the continuous annealing step is to grow the grains to a certain size or more after decarburizing the carbon in the steel. This is because the fraction of Goss grains increases continuously through the process of decarburization and the grain growth immediately thereafter. This is because the Goss grains grow while eating away at the surrounding non-Goss grains. However, since the annealing time is limited to a few minutes in consideration of the productivity of the continuous annealing, it can be said that the grain growth is restricted. In the present invention, it is claimed that the additional batch annealing induces the grain growth, which is effective in reducing the iron loss. At this time, the Goss fraction does not increase, but the iron loss is reduced due to the effect of the increase in the grain size.
After the continuous annealing step, an annealing separator is applied. Annealing separators are widely known in the art, so a detailed description will be omitted. For example, an annealing separator mainly composed of MgO can be used.

Next, the continuously annealed steel sheet is subjected to batch annealing. Batch annealing means that the steel sheet is wound in a coil and annealed.
In the batch annealing step, a texture having the Goss orientation diffused in the continuous annealing step grows. In the method for producing a grain-oriented electrical steel sheet according to the present invention, the Goss texture has a grain diameter of 5 mm or less, unlike the case where grains grow by conventional abnormal grain growth. Specifically, the grain fraction having a diameter of 1000 um to 5000 um increases. Therefore, compared to grain-oriented electrical steel sheets produced by conventional abnormal grain growth, there are many Goss grains having a small grain size, but the grain size is appropriately controlled to minimize iron loss. More specifically, the area fraction of grains having a diameter of 1000 μm to 5000 μm in the total grains is 20 to 70%. At this time, the area fraction of the grains is measured in a plane parallel to the rolled surface (ND surface) of the steel sheet. More specifically, the area fraction of grains having a diameter of 1000 μm to 5000 μm in the total grains is 20 to 60%. More specifically, the area ratio of crystal grains having a diameter of 1000 μm to 5000 μm to the total crystal grains is 20 to 50%.
The batch annealing step may be performed at a temperature of 1000° C. to 1200° C. and a dew point temperature of −20° C. or less.
The batch annealing is performed for 1 to 8 hours, more specifically, for 2 to 5 hours.
In addition, the method for producing grain-oriented electrical steel sheet according to the present invention improves magnetic properties due to a high Goss fraction, specifically, the volume fraction of crystal grains that form an angle of 15° or less with the {110}<001> is 40% or more, more specifically 40% to 75%, and even more specifically 45% to 60%.

In the grain-oriented electrical steel sheet according to the present invention, the area fraction of crystal grains having a diameter of 1000 μm to 5000 μm among all crystal grains is 20 to 70%.
The area distribution of crystal grains has been described in detail in relation to the manufacturing method of grain-oriented electrical steel sheet, so a duplicated description will be omitted.
The electrical steel sheet contains, by weight, 1.0% to 4.0% Si, 0.005% or less (excluding 0%) C, and the balance being Fe and unavoidable impurities.
The electrical steel sheet further contains Mn: 0.1% by weight or less and S: 0.005% by weight or less.
Except for C, the compositional limitations are the same as those for slabs, so duplicate explanations will be omitted.
The volume fraction of crystal grains making an angle of 15° or less from {110}<001> is 40% or more.
The ratio (D2/D1) of the diameter of the circumscribing circle (D1) to the diameter of the inscribing circle (D2) of 0.5 or more accounts for 95% or more of the area of the entire Goss crystal grains. The crystal grains of the above-mentioned shape are formed by the manufacturing process unique to the present invention.

The grain-oriented electrical steel sheet according to the present invention has a high Goss fraction, which improves magnetic properties. Specifically, the iron loss (W _17/50 ) is 1.3 W/kg or less, more specifically, the iron loss (W _17/50 ) is 1 to 1.3 W/kg, and even more specifically, the iron loss (W 17/50 ) is 1.1 to 1.25 W/kg. The iron loss W _17/50 is the magnitude of the iron loss (W/kg) induced under the conditions of 1.7 Tesla and 50 Hz.
Hereinafter, specific examples of the present invention will be described. However, the following examples are merely specific examples of the present invention, and the present invention is not limited to the following examples.

Example 1
The slab, containing, by weight, 2.32% Si, 0.195% C, and the balance Fe and unavoidable impurities, was heated at a temperature of 1250° C. and then hot-rolled, and then hot-rolled at an annealing temperature of 950° C. and a dew point temperature of 60° C. The steel sheet was then cooled and pickled, and cold-rolled at a rolling reduction of 65% to produce a cold-rolled sheet having a thickness of 0.8 mm.
The cold-rolled sheet was again decarburized in a wet mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60° C.) at a temperature of 950° C. for 80 seconds, and then cold-rolled again at a reduction rate of 65% to produce a cold-rolled sheet having a thickness of 0.28 mm.
Thereafter, in the 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, and then heat treatment was continuously performed in a mixed gas atmosphere of hydrogen and nitrogen (dew point temperature 60°C) at 1100°C as shown in Table 1, or heat treatment was performed in a coiled state in a mixed gas atmosphere of hydrogen and nitrogen at 1200°C for the time shown in Table 1 below.
Table 1 shows the Goss fraction of grains, the area fraction of grains between 1 mm and 5 mm, and iron loss of grains after high temperature annealing according to the embodiment. The Goss fraction was measured as the volume fraction of grains that make an angle of 15° or less from {110}<001>. The finally obtained steel sheet was surface-washed, and the iron loss was measured at 1.7 Tesla and 50 Hz using a single sheet magnetic measurement method.

表１に示すように、バッチ焼鈍を適切な時間の間に行った発明材１～発明材４は、直径が１～５ｍｍである結晶粒の面積分率が高いことを確認できる。ゴス分率が比較材に比べて比較的低くても鉄損がむしろ優れていることを確認できる。
図１および図２では、発明材２および比較材２で製造した方向性電磁鋼板の表面を走査電子顕微鏡で観察した写真を示す。
図１および図２で確認できるように、発明材２で製造した方向性電磁鋼板の結晶粒が比較的大きく形成されたことを確認できる。

As shown in Table 1, it can be confirmed that inventive materials 1 to 4, which were batch annealed for an appropriate time, the surface fraction of crystal grains having a diameter of 1 to 5 mm is high. It can be confirmed that the iron loss is rather superior even though the Goss fraction is relatively low compared to the comparative materials.
1 and 2 show photographs of the surfaces of grain-oriented electrical steel sheets manufactured from Inventive Material 2 and Comparative Material 2, observed with a scanning electron microscope.
As can be seen from Figs. 1 and 2, it can be seen that the grains of the grain-oriented electrical steel sheet manufactured using Inventive Material 2 were formed relatively large.

The present invention is not limited to the above-described embodiments and/or examples, but can be manufactured in a variety of different forms, and a person having ordinary knowledge in the technical field to which the present invention pertains should be able to understand that the present invention can be embodied in other specific forms without changing the technical concept or essential features of the present invention. Therefore, it should be understood that the above-described embodiments and/or examples are illustrative in all respects and not limiting.

Claims (6)

Heating a slab containing, by weight, 1.0% to 4.0% Si, 0.1% to 0.4% C, and the balance being Fe and unavoidable impurities;
hot rolling the slab to produce a hot rolled steel sheet;
annealing the hot-rolled steel sheet;
performing a first cold rolling on the hot-rolled steel sheet that has been annealed;
decarburization annealing the first cold rolled steel sheet;
a step of subjecting the steel sheet having been subjected to the decarburization annealing to a second cold rolling;
The method includes the steps of: continuously annealing the steel sheet after the second cold rolling; and batch annealing the continuously annealed steel sheet,
The hot rolling step is performed after heating the slab at a temperature of 1100° C. to 1350° C.,
The step of manufacturing the hot-rolled steel sheet includes setting the hot-rolling end temperature to 950° C. or less and coiling at 600° C. or less,
The thickness of the hot-rolled steel plate is 1.5 to 4.0 mm,
The hot-rolled sheet annealing step is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C.,
The reduction ratios of the first cold rolling step and the second cold rolling step are each 50% to 70%;
The step of decarburizing the first cold-rolled steel sheet is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C.
The continuous annealing step is performed at a temperature of 850° C. to 1000° C. and a dew point temperature of 50° C. to 70° C.,
The batch annealing step comprises annealing at a temperature of 1000° C. to 1200° C. and a dew point temperature of −20° C. or less for 1 to 8 hours. The method for manufacturing grain-oriented electrical steel sheet according to claim 1, characterized in that the hot-rolled sheet annealing step includes a decarburization process. 3. The method of claim 1 , wherein the step of decarburizing the first cold-rolled steel sheet comprises annealing the steel sheet in an austenite single phase region or in a region where a composite phase of ferrite and austenite exists. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3 , wherein the average diameter of crystal grains is 150 to 250 μm after the step of decarburization annealing the primarily cold-rolled steel sheet. 5. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the step of decarburization annealing the first cold-rolled steel sheet and the step of second cold-rolling the steel sheet after the decarburization annealing are repeated two or more times. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 5 , wherein the continuous annealing step comprises annealing for 1 to 5 minutes.