Method for manufacturing non-oriented electrical steel sheet having excellent magnetic properties, and non-oriented electrical steel sheet manufactured thereby

Abstract

Disclosed are a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties and a non-oriented electrical steel sheet manufactured thereby. In a specific embodiment, a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties comprises the steps of: manufacturing a hot-rolled steel sheet using a slab, the slab comprises 0.4 wt% to 3.5 wt% of silicon (Si), 0 wt% (excluding) to 0.05 wt% (inclusive) of aluminum (Al), 0.002 wt% to 3.5 wt% of an austenitic stabilizing element, and the balance of iron (Fe) and other unavoidable impurities; performing cold rolling on the hot-rolled steel plate to obtain a cold-rolled steel plate; and cold annealing the cold-rolled steel sheet, wherein the cold annealing comprises: a primary heat treatment step of heating the cold-rolled steel sheet to an austenite single-phase region temperature and maintaining the temperature; and a secondary heat treatment step of cooling the cold-rolled steel sheet subjected to the primary heat treatment to a ferrite and austenite dual-phase region temperature and maintaining the same at the temperature.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties and a non-oriented electrical steel sheet manufactured thereby. Background Art

[0002] Electrical steel sheets can be classified into grain-oriented electrical steel sheets for transformers and non-oriented electrical steel sheets for rotating machine cores. In recent years, the attention to environmentally friendly products has been increasing with global environmental issues. In particular, the mode in the automotive industry has shifted to reducing the production of internal combustion engine vehicles and increasing the production of electric vehicles. With the increasing demand for electric vehicles, electrical steel sheets, as key components, also require high-efficiency and high-yield products.

[0003] For electrical steel sheets, two characteristics are important: magnetic flux density and iron loss. Magnetic flux density represents the number of magnetic lines of force induced in a material under a specific magnetic field, and is usually evaluated by the B50 value induced under a magnetic field of 5,000 A / m, with the unit of tesla. Factors that can improve magnetic flux density include chemical composition, grain size, and texture. Iron loss is the energy loss that occurs in an electrical steel sheet material during the magnetization process, with the unit of W / kg. Iron loss is divided into hysteresis loss caused by the magnetization phenomenon itself and eddy current loss formed by eddy currents induced during the magnetization process. To reduce the iron loss of electrical steel sheets, methods such as adding main alloy elements such as Si, Mn, and Al to increase the resistivity or thinning the material are used. However, as the addition amount of alloy elements increases, the magnetic flux density decreases and the rolling performance decreases, making it difficult to thin the steel sheet. In addition, the main alloy elements easily combine with impurity elements such as C, S, N, and Ti to form precipitates, and the formed precipitates hinder the movement of magnetic domains formed by an externally applied magnetic field, thus becoming the main cause of deteriorating magnetic properties. To improve the performance of such electrical steel sheets, the optimization of alloy elements and impurity elements is very important, and the performance can also be improved by improving the microstructure and texture that can affect magnetic flux density and hysteresis loss.

[0004] Currently, electrical steel sheets are manufactured through the following processes: steelmaking - continuous casting - hot rolling - annealing and pickling line (APL) - cold rolling - annealing and coating line (ACL). However, with the currently known processes and compositions, the effect of improving performance by improving the microstructure and texture is not obvious. In particular, it is more difficult to significantly improve the texture only by adding elements or improving some process conditions, and the texture has a significant impact on magnetic properties.

[0005] The background art related to the present invention is disclosed in Korean Patent No. 10-2325011 (publication date: November 11, 2021, patent title: "Non-oriented Electrical Steel Sheet and Method for Manufacturing the Same"). Summary of the Invention

[0006] Technical problem

[0007] According to one embodiment of the present invention, there is provided a method for manufacturing a non-oriented electrical steel sheet, the steel sheet having an excellent effect of improving the microstructure and texture, minimizing iron loss, and having excellent magnetic properties.

[0008] According to another embodiment of the present invention, there is provided a method for manufacturing a non-oriented electrical steel sheet, the steel sheet having an excellent effect of forming a texture favorable for magnetism.

[0009] According to still another embodiment of the present invention, there is provided a method for manufacturing a non-oriented electrical steel sheet having excellent mechanical properties.

[0010] According to yet another embodiment of the present invention, there is provided a method for manufacturing a non-oriented electrical steel sheet having excellent productivity and economic efficiency.

[0011] According to another embodiment of the present invention, there is provided a non-oriented electrical steel sheet manufactured by a method for manufacturing a non-oriented electrical steel sheet.

[0012] Technical solution

[0013] One aspect of the present invention relates to a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties. In one embodiment, the method for manufacturing a non-oriented electrical steel sheet includes the following steps: manufacturing a hot-rolled steel sheet from a slab, the slab containing 0.4 wt% to 3.5 wt% of silicon (Si), more than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of an austenite stabilizing element, and the balance of iron (Fe) and other inevitable impurities; cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet, wherein the step of annealing the cold-rolled steel sheet includes: heating the cold-rolled steel sheet to the austenite single-phase region temperature and holding at this temperature to perform a primary heat treatment on the cold-rolled steel sheet; and cooling the cold-rolled steel sheet that has undergone the primary heat treatment to the ferrite / austenite two-phase region temperature and holding at this temperature to perform a secondary heat treatment on the cold-rolled steel sheet that has undergone the primary heat treatment.

[0014] In one embodiment, the slab may further contain, in total weight, at least one of the following elements: more than 0 wt% to 0.0050 wt% or less of carbon (C), more than 0 wt% to 0.0050 wt% or less of sulfur (S), more than 0 wt% to 0.0050 wt% or less of nitrogen (N), and more than 0 wt% to 0.0050 wt% or less of titanium (Ti).

[0015] In one embodiment, the hot-rolled steel sheet can be manufactured by the following steps: reheating a slab at a reheating temperature of 1,110°C to 1,180°C; hot-rolling the reheated slab at a finish rolling temperature of 800°C to 900°C; and coiling the hot-rolled slab at a coiling temperature of 500°C to 700°C.

[0016] In one embodiment, the method for manufacturing a non-oriented electrical steel sheet may further include: before the step of cold-rolling the hot-rolled steel sheet, annealing the hot-rolled steel sheet, wherein the step of annealing the hot-rolled steel sheet may include: heating the hot-rolled steel sheet to a temperature of 940°C to 1,110°C and holding it at that temperature; and cooling the hot-rolled steel sheet.

[0017] In one embodiment, the step of annealing the hot-rolled steel sheet may be carried out as follows: heating the hot-rolled steel sheet at a heating rate of 20°C / s or higher and cooling it at a cooling rate of 20°C / s or higher.

[0018] In one embodiment, the reduction ratio of cold rolling is 50% to 90%, and the thickness of the cold-rolled steel sheet may be 0.1 mm to 0.5 mm.

[0019] In one embodiment, the primary heat treatment may include the following steps: heating the cold-rolled steel sheet to a temperature of 1,000°C to 1,250°C and holding it at that temperature for 30 seconds to 300 seconds; the secondary heat treatment may include the following steps: cooling the cold-rolled steel sheet that has undergone the primary heat treatment to the ferrite / austenite dual-phase region temperature and holding it at that temperature for 5 seconds to 300 seconds.

[0020] In one embodiment, the secondary heat treatment may be carried out as follows: cooling the cold-rolled steel sheet that has undergone the primary heat treatment at a cooling rate of 10°C / second or higher.

[0021] Another aspect of the present invention relates to a non-oriented electrical steel sheet manufactured by the above method for manufacturing a non-oriented electrical steel sheet. In one embodiment, the non-oriented electrical steel sheet contains: 0.4 wt% to 3.5 wt% of silicon (Si), more than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of austenite stabilizing elements, and the balance of iron (Fe) and other inevitable impurities, and the microstructure of the non-oriented electrical steel sheet includes ferrite.

[0022] In one embodiment, the non-oriented electrical steel sheet may further contain, by total weight, at least one of the following elements: carbon (C) in an amount greater than 0 wt% to 0.0050 wt% or less, sulfur (S) in an amount greater than 0 wt% to 0.0050 wt% or less, nitrogen (N) in an amount greater than 0 wt% to 0.0050 wt% or less, and titanium (Ti) in an amount greater than 0 wt% to 0.0050 wt% or less.

[0023] In one embodiment, the non-oriented electrical steel sheet may have: a thickness of 0.1 mm to 0.5 mm, a magnetic flux density (B 50 ) of 1.65 T or higher, an iron loss (W 15 / 50 ) of 2.42 W / kg or lower, a yield strength of 200 MPa or higher, and a tensile strength of 300 MPa or higher.

[0024] In one embodiment, the non-oriented electrical steel sheet may contain 15 area% or more of <100> / / ND texture.

[0025] Advantageous Effects

[0026] The method for manufacturing a non-oriented electrical steel sheet according to the present invention and the non-oriented electrical steel sheet manufactured thereby have excellent effects of improving the microstructure and texture, minimizing the iron loss, having excellent magnetic properties, having excellent effects of forming a texture favorable for magnetism, having excellent mechanical properties such as yield strength and tensile strength, and may have excellent productivity and economic efficiency. Brief Description of the Drawings

[0027] Figure 1 Shows a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention.

[0028] Figure 2 Shows a graph showing the heat treatment process during annealing of cold-rolled steel sheets in examples and comparative examples. Detailed Description

[0029] Hereinafter, the present invention will be described in detail. In the following description, when a detailed description of related well-known technologies or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

[0030] Furthermore, the terms used in the following description are terms defined in consideration of the functions obtained according to the embodiments of the present invention, and may be changed according to the selection or general practice of the user or operator. Therefore, the definitions of the terms should be based on the content of the entire present specification.

[0031] Method for manufacturing non-oriented electrical steel sheet with excellent magnetic properties

[0032] One aspect of the present invention relates to a method for manufacturing a non-oriented electrical steel sheet having excellent magnetic properties. Figure 1 A method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention is shown. Refer to Figure 1 , the method for manufacturing a non-oriented electrical steel sheet includes a step (S10) of manufacturing a hot-rolled steel sheet; a step (S20) of cold rolling; and a step (S30) of annealing the cold-rolled steel sheet.

[0033] More specifically, the method for manufacturing a non-oriented electrical steel sheet includes: a step (S10) of manufacturing a hot-rolled steel sheet from a slab, the slab containing 0.4 wt% to 3.5 wt% of silicon (Si), more than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of an austenite stabilizing element, and the balance of iron (Fe) and other inevitable impurities; a step (S20) of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and a step (S30) of annealing the cold-rolled steel sheet.

[0034] Hereinafter, each step of the method for manufacturing a non-oriented electrical steel sheet according to the present invention will be described in detail.

[0035] (S10): Step of manufacturing a hot-rolled steel sheet

[0036] This step is a step of manufacturing a hot-rolled steel sheet from a slab, the slab containing 0.4 wt% to 3.5 wt% of silicon (Si), more than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of an austenite stabilizing element, and the balance of iron (Fe) and other inevitable impurities.

[0037] Hereinafter, the components of the slab will be described in detail.

[0038] Silicon (Si)

[0039] Silicon (Si) is added as a component to reduce iron loss (eddy current loss) by increasing the resistivity of the material.

[0040] In one embodiment, the content of silicon is 0.4 wt% to 3.5 wt% based on the total weight of the slab. If the silicon content is less than 0.4 wt%, the effect of reducing iron loss is insufficient; if the silicon content exceeds 3.5 wt%, the magnetic flux density may decrease due to the increase in alloy element components, and the brittleness may increase, which will reduce the cold rolling performance and lead to a decrease in productivity.

[0041] Aluminum (Al)

[0042] Aluminum (Al) is a main additive element, which, together with silicon, reduces iron loss (eddy current loss) by increasing the resistivity. Aluminum can also be used as a ferrite stabilizing element.

[0043] In one embodiment, based on the total weight of the slab, the content of aluminum (Al) is greater than 0 wt% to 0.05 wt% or less. If the content of aluminum exceeds 0.05 wt%, aluminum may reduce the phase transformation region and form secondary phases such as AlN, thereby reducing the magnetic properties.

[0044] Austenite stabilizing elements

[0045] Austenite stabilizing elements can be added to ensure the austenite transformation region at high temperatures. If about 1.7 wt% or more of silicon is added to reduce the iron loss of the steel sheet, there may be a problem that only the ferrite single-phase region exists, and thus the transformation to austenite cannot occur. Therefore, in the present invention, austenite stabilizing elements can be added to ensure the austenite transformation region at high temperatures.

[0046] In one embodiment, the content of austenite stabilizing elements is 0.002 wt% to 3.5 wt% based on the total weight of the slab. If the content of austenite stabilizing elements is less than 0.002 wt%, the austenite transformation region may not be ensured; if the content of austenite stabilizing elements exceeds 3.5 wt%, the production cost may increase excessively, and the magnetic properties may decrease rather than increase.

[0047] In one embodiment, the austenite stabilizing elements may include manganese (Mn), cobalt (Co), nickel (Ni), and chromium (Cr). These elements can be included individually, or two or more of them can be included in combination.

[0048] In one embodiment, the slab may further contain at least one of the following elements in total weight: carbon (C) greater than 0 wt% to 0.0050 wt% or less, sulfur (S) greater than 0 wt% to 0.0050 wt% or less, nitrogen (N) greater than 0 wt% to 0.0050 wt% or less, and titanium (Ti) greater than 0 wt% to 0.0050 wt% or less.

[0049] Carbon (C)

[0050] Carbon is an element that can ensure the austenite phase transformation region, but it increases the iron loss by forming carbides such as TiC and NbC. Therefore, the addition amount is preferably as small as possible.

[0051] In one embodiment, the content of carbon can be greater than 0 wt% to 0.0050 wt% or less based on the total weight of the slab. When the content of carbon is within the above content range, the austenite phase transformation region can be ensured while preventing an increase in iron loss. For example, the content of carbon can be greater than 0 wt% to 0.0030 wt% or less.

[0052] Sulfur (S)

[0053] The addition amount of sulfur (S) can be as small as possible because sulfur forms precipitates such as MnS and CuS, thereby increasing iron loss and inhibiting grain growth.

[0054] In one embodiment, the content of sulfur (S) can be greater than 0 wt% to 0.0050 wt% or less based on the total weight of the slab. When the content of sulfur is within the above content range, the formation of precipitates such as MnS and CuS can be prevented, thereby preventing an increase in iron loss and not inhibiting grain growth. For example, the content of sulfur can be greater than 0 wt% to 0.0030 wt% or less.

[0055] Nitrogen (N)

[0056] The addition amount of nitrogen (N) can be as small as possible because nitrogen forms precipitates such as AlN, TiN, and NbN, thereby increasing iron loss and inhibiting grain growth.

[0057] In one embodiment, the content of nitrogen (N) can be greater than 0 wt% to 0.0050 wt% or less based on the total weight of the slab. When the content of nitrogen is within the above content range, the formation of precipitates such as AlN, TiN, and NbN can be prevented, thereby preventing an increase in iron loss and not inhibiting grain growth. For example, the content of nitrogen can be greater than 0 wt% to 0.0030 wt% or less.

[0058] Titanium (Ti)

[0059] Titanium (Ti) forms fine precipitates such as TiC and TiN, thereby inhibiting grain growth. Since adding titanium causes a decrease in magnetic properties, titanium can be added in the smallest possible amount.

[0060] In one embodiment, the content of titanium (Ti) can be greater than 0 wt% to 0.0050 wt% or less based on the total weight of the slab. When the content of titanium is within the above content range, the formation of fine precipitates such as TiC and TiN can be prevented, thereby not inhibiting magnetic properties while not inhibiting grain growth. For example, the content of titanium (Ti) can be greater than 0 wt% to 0.0030 wt% or less.

[0061] In one embodiment, the hot-rolled steel sheet can be manufactured by the following steps: reheating the slab at a reheating temperature of 1,110 °C to 1,180 °C; hot-rolling the reheated slab at a finish rolling temperature of 800 °C to 900 °C; and coiling the hot-rolled slab at a coiling temperature of 500 °C to 700 °C.

[0062] When reheating the slab under the above reheating temperature conditions, it is possible to prevent the precipitates formed by carbon, sulfur, and nitrogen from redissolving in the slab, thereby preventing the formation of fine precipitates in subsequent rolling and annealing processes, reducing the rolling load, and at the same time, it is possible to easily ensure magnetism without inhibiting grain growth.

[0063] When performing hot rolling under the above finish rolling temperature conditions, the performance fluctuations of the electrical steel sheet can be reduced, and the electrical steel sheet can have excellent mechanical properties and magnetic properties.

[0064] When coiling under the above coiling temperature conditions, the electrical steel sheet can have excellent surface quality, mechanical properties, and magnetic properties.

[0065] In one embodiment, the thickness of the hot-rolled steel sheet can be 1.6 mm to 2.6 mm. Under this thickness condition, an increase in the reduction ratio during cold rolling can be prevented, thereby preventing the deterioration of the texture and improving the magnetic properties of the electrical steel sheet.

[0066] The step (S11) of annealing the hot-rolled steel sheet

[0067] In one embodiment, the method for manufacturing an non-oriented electrical steel sheet may further include: the step (S11) of annealing the hot-rolled steel sheet before the cold rolling step of the hot-rolled steel sheet.

[0068] The step of annealing the hot-rolled steel sheet may include: the step of heating the hot-rolled steel sheet to a temperature of 940 °C to 1110 °C and holding it at that temperature; and the step of cooling the hot-rolled steel sheet.

[0069] When the hot-rolled steel sheet is heated to the above annealing temperature, a uniform microstructure can be formed by removing the elongated casting structure.

[0070] However, if the annealing temperature is too low, the elongated casting structure remaining after hot rolling may remain, resulting in non-uniform microstructure and the formation of small grains, which may hinder cold rolling. On the other hand, if the annealing temperature is too high, it may cause texture imbalance in the final product, thereby possibly leading to anisotropy of properties. For example, the annealing of the hot-rolled steel sheet can be carried out as follows: heating the hot-rolled steel sheet to a temperature of 940 °C to 1110 °C and holding it at that temperature for 30 seconds to 120 seconds. During annealing, grains with appropriate sizes can be formed, so that the electrical steel sheet can have excellent magnetic properties.

[0071] In one embodiment, the annealing of the hot-rolled steel sheet may include heating the hot-rolled steel sheet at a heating rate of 20 °C / s or higher. Under this condition, grains with appropriate sizes can be formed, and the electrical steel sheet can have excellent magnetic properties.

[0072] In one embodiment, annealing of the hot-rolled steel sheet may include heating the hot-rolled steel sheet and maintaining it at that temperature, and then cooling it at a cooling rate of 20 °C / s or higher. Under the above conditions, grains of appropriate size can be formed, and the electrical steel sheet can have excellent magnetic properties. For example, the hot-rolled steel sheet can be cooled to room temperature under the above cooling rate conditions.

[0073] Cold rolling step (S20)

[0074] This step is a step of cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet. For example, the hot-rolled steel sheet can be pickled and then cold-rolled.

[0075] In one embodiment, the temperature of the hot-rolled steel sheet can be raised to 75 °C to 200 °C to facilitate cold rolling.

[0076] In one embodiment, the reduction ratio (final reduction ratio) for cold rolling can be 50% to 90%. Under these conditions, the electrical steel sheet can have excellent magnetic properties while reducing the equipment load.

[0077] In one embodiment, the thickness of the cold-rolled steel sheet can be 0.1 mm to 0.5 mm. Under these conditions, the electrical steel sheet can have excellent magnetic properties and mechanical properties.

[0078] Annealing step of the cold-rolled steel sheet (S30)

[0079] This step is a step of annealing the cold-rolled steel sheet.

[0080] In one embodiment, the step of annealing the cold-rolled steel sheet includes: a step (S31) of heating the cold-rolled steel sheet to the temperature of the austenite single-phase region and maintaining it at that temperature to perform primary heat treatment on the cold-rolled steel sheet; and a step (S32) of cooling the cold-rolled steel sheet that has undergone primary heat treatment to the temperature of the ferrite / austenite dual-phase region and maintaining it at that temperature to perform secondary heat treatment on the cold-rolled steel sheet that has undergone primary heat treatment.

[0081] In the conventional technology, in order to improve the magnetic properties of the non-oriented electrical steel sheet by improving the texture, a heat treatment method based on the transformation from the austenite phase (γ) to the ferrite phase (α) is selected to induce the formation of the {100} / / ND plane, but there are the following disadvantages: Since secondary rolling for surface modification is performed after primary rolling, and then secondary heat treatment is required to eliminate residual stress, the production time and cost increase.

[0082] In another conventional technique, a method is used in which, without undergoing a phase change, an non-oriented electrical steel sheet is manufactured by performing heat treatment in a ferrite (α) single-phase region, and in which, by controlling the sulfur (S) content to an extremely low level, a decomposition reaction of MnS precipitates is induced, and segregation of the decomposed S in the grains is induced, thereby enabling the growth of the {100} / / ND plane. However, in this case, there are the following disadvantages: since high-temperature conditions of 1200 °C or higher and long-time heat treatment of at least 12 hours are required, the production time and cost increase.

[0083] In one embodiment, the primary heat treatment may include the steps of heating the cold-rolled steel sheet to a temperature of 1,000 °C to 1,250 °C and holding it at this temperature for 30 seconds to 300 seconds. Under these conditions, the microstructure of the cold-rolled steel sheet can be easily transformed into an austenite-phase single-phase region during the primary heat treatment.

[0084] In one embodiment, the secondary heat treatment may include the steps of cooling the cold-rolled steel sheet that has undergone the primary heat treatment to the ferrite / austenite dual-phase region temperature and holding it at this temperature for 5 seconds to 300 seconds. For example, the secondary heat treatment can be carried out by cooling to 700 °C to 980 °C. Under these conditions, nucleation of the ferrite phase can easily occur during the secondary heat treatment, so that the transformation to an austenite and ferrite microstructure can easily occur, such that the {100} / / ND orientation fraction that is favorable for magnetism increases while the orientation fraction that is relatively unfavorable for magnetism decreases, thereby enabling the electrical steel sheet to have excellent magnetic properties. For example, the secondary heat treatment may include the steps of cooling the cold-rolled steel sheet to the ferrite / austenite dual-phase region temperature and holding it at this temperature for 10 seconds to 100 seconds.

[0085] For example, the secondary heat treatment can be carried out as follows: cooling the cold-rolled steel sheet that has undergone the primary heat treatment at a cooling rate of 10 °C / s or higher.

[0086] In one embodiment, after the secondary heat treatment, the cold-rolled steel sheet that has undergone the secondary heat treatment can be cooled at a cooling rate of 10 °C / s or higher.

[0087] In one embodiment, the method may further include the step of forming a coating on the surface of the annealed cold-rolled steel sheet. The coating can be formed for the purpose of ensuring insulation and improving stamping properties.

[0088] In the present invention, based on phase change heat treatment, the inventors attempted to maximize the effect of improving the texture by utilizing the fact that the orientation preferentially formed during the phase change to the ferrite phase (α) varies with different temperature conditions.

[0089] In addition, the composition of the non-oriented electrical steel sheet contains 0.4 wt% to 3.5 wt% of silicon (Si) as a main additive element. As the content of silicon, which is a ferrite phase (α) stabilizing element, increases, the austenite phase transformation region disappears. Therefore, the content of the austenite phase (γ) stabilizing element is set to 0.002 wt% to 3.5 wt% in proportion to the silicon (Si) content to enable transformation heat treatment. Since the present invention is based on the phase transformation from the austenite phase (γ) to the ferrite phase (α), the addition of the austenite phase (γ) stabilizing element is proportional to the increase in the Si addition amount, thereby enabling phase transformation.

[0090] Among the added elements, aluminum (Al) combines with nitrogen (N) to form AlN, which is disadvantageous to magnetism and thus inhibits magnetic properties. In addition, like silicon (Si), aluminum acts as a ferrite phase (α) stabilizing element, so it is set to be greater than 0 wt% to 0.05 wt% or less to minimize its addition amount.

[0091] Meanwhile, in previous studies, since it was not known which orientation was preferentially formed according to the temperature change during the phase transformation process and there was no purpose of applying that orientation, a method of heating and maintaining the temperature during the annealing of the cold-rolled steel sheet and then reducing the temperature by continuous cooling was selected.

[0092] However, in the present invention, after holding the cold-rolled steel sheet at the austenite phase (γ) temperature for a certain period of time during the annealing process of the cold-rolled steel sheet to form a single phase, the inventors attempted to confirm which orientation was preferentially formed at each temperature where the austenite phase (γ) and the ferrite phase (α) coexisted and utilize this advantage.

[0093] Non-oriented electrical steel sheet manufactured by the method for manufacturing non-oriented electrical steel sheet

[0094] Another aspect of the present invention relates to a non-oriented electrical steel sheet manufactured by the above method for manufacturing a non-oriented electrical steel sheet. In one embodiment, the non-oriented electrical steel sheet contains: 0.4 wt% to 3.5 wt% of silicon (Si), greater than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of an austenite stabilizing element, and the balance of iron (Fe) and other inevitable impurities, and the microstructure of the non-oriented electrical steel sheet includes ferrite.

[0095] In one embodiment, the austenite stabilizing element may include manganese (Mn), cobalt (Co), nickel (Ni), and chromium (Cr). These elements may be included alone or in combination of two or more.

[0096] The alloying elements and their contents of the non-oriented electrical steel sheet may be the same as those of the above slab.

[0097] In one embodiment, the non-oriented electrical steel sheet may further contain, by total weight, at least one of the following elements: carbon (C) greater than 0 wt% to 0.0050 wt% or less, sulfur (S) greater than 0 wt% to 0.0050 wt% or less, nitrogen (N) greater than 0 wt% to 0.0050 wt% or less, and titanium (Ti) greater than 0 wt% to 0.0050 wt% or less.

[0098] In one embodiment, the thickness of the non-oriented electrical steel sheet may be from 0.1 mm to 0.5 mm.

[0099] In one embodiment, the non-oriented electrical steel sheet may further include a coating formed on its surface. When the coating is formed, it may have excellent effects of ensuring insulation and improving punching properties.

[0100] In one embodiment, the non-oriented electrical steel sheet may have a magnetic flux density (B 50 ) of 1.65 T or higher and an iron loss (W 15 / 50 ) of 2.42 W / kg or lower. Under the above conditions, the magnetic properties will be excellent.

[0101] The magnetic flux density (B 50 ) can be measured by applying a magnetic field of 5,000 A / m to the electrical steel sheet.

[0102] The iron loss (W 15 / 50 ) can be measured by applying a magnetic flux density of 1.5 tesla at a frequency of 50 Hz to the electrical steel sheet.

[0103] For example, the non-oriented electrical steel sheet may have a magnetic flux density (B 50 ) of 1.65 to 1.70 T and an iron loss (W 15 / 50 ) of 2.28 to 2.42 W / kg.

[0104] In one embodiment, the non-oriented electrical steel sheet may have a yield strength of 200 MPa or higher and a tensile strength of 300 MPa or higher.

[0105] In one embodiment, the non-oriented electrical steel sheet may contain <100> / / ND texture of 15 area% or higher. For example, the non-oriented electrical steel sheet may contain <100> / / ND texture of 15 area% to 40 area%. In another embodiment, the non-oriented electrical steel sheet may contain <100> / / ND texture of 30 area% to 35 area%. Under these conditions, the magnetic strength will be excellent.

[0106] <100> / / The <100> texture refers to a texture in which the <100> plane is parallel to the normal direction (ND) with respect to the surface of the non-oriented electrical steel sheet. The surface of the non-oriented electrical steel sheet can be referenced to the xy plane, where the x-axis represents the rolling direction (RD direction) of the steel sheet and the y-axis represents the transverse direction (TD direction) of the steel sheet.

[0107] Electron backscatter diffraction (EBSD) can be used to analyze the texture of the non-oriented electrical steel sheet based on the crystal orientation of the ND direction under the rolling (R), transverse (T), and normal (N) conditions of the steel sheet surface, using the orientation distribution function (ODF) of the surface intensity in each orientation.

[0108] The method of the present invention

[0109] Hereinafter, the configuration and effects of the present invention will be described in more detail through preferred embodiments. However, these embodiments are presented only as preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention in any way.

[0110] Example 1

[0111] (1) Manufacture of hot-rolled steel sheet: A slab containing the following composition was prepared: 1.97 wt% silicon (Si), 1.0 wt% nickel (Ni), greater than 0 wt% to 0.05 wt% or less aluminum (Al), 0.0040 wt% (40 ppm) carbon (C), 0.0040 wt% (40 ppm) sulfur (S), 0.0015 wt% (15 ppm) nitrogen (N), and 0.0012 wt% (12 ppm) titanium (Ti), the balance being iron (Fe) and other inevitable impurities. The slab with the above alloy composition ensured a stable austenite phase (γ) region at a temperature of 1,100 °C. Then, the slab was reheated at a slab reheating temperature (SRT) of 1,170 °C for 2 hours, hot-rolled at a finish rolling temperature of 850 °C, and then cooled and coiled at a coiling temperature of 600 °C to manufacture a hot-rolled steel sheet with a thickness of 2.0 mm.

[0112] (2) Annealing and cold rolling of the hot-rolled steel sheet: Thereafter, the hot-rolled steel sheet was heated to 975 °C at a heating rate of 20 °C / s or higher, held at this temperature for 60 seconds, and then cooled to room temperature at a cooling rate of 20 °C / s or higher, thereby annealing the hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet was pickled and cold-rolled to manufacture a cold-rolled steel sheet with a thickness of 0.35 mm.

[0113] (3) Annealing the cold-rolled steel sheet: Subsequently, the cold-rolled steel sheet is heated to 1,100 °C at a heating rate of 20 °C / s or higher and held at this temperature for 60 seconds, thereby performing the primary heat treatment on the cold-rolled steel sheet. The cold-rolled steel sheet after the primary heat treatment is cooled to 950 °C at a cooling rate of 10 °C / s or higher and held at this temperature for 60 seconds, thereby performing the secondary heat treatment on the cold-rolled steel sheet after the primary heat treatment. Then, the cold-rolled steel sheet after the secondary heat treatment is cooled to room temperature at a cooling rate of 10 °C / s or higher, as Figure 2 shown, and an non-oriented electrical steel sheet is thus manufactured.

[0114] Example 2

[0115] An non-oriented electrical steel sheet is manufactured in the same manner as in Example 1, except that the cold-rolled steel sheet after the primary heat treatment is cooled to 800 °C at a cooling rate of 10 °C / s or higher and held at this temperature for 60 seconds, thereby performing the secondary heat treatment, as Figure 2 shown.

[0116] Comparative Example

[0117] An non-oriented electrical steel sheet is manufactured in the same manner as in Example 1, except that the cold-rolled steel sheet after the primary heat treatment is cooled to room temperature at a cooling rate of 10 °C / s or higher, as Figure 2 shown.

[0118] Experimental Example

[0119] (1) XRD analysis: In Examples 1 and 2, the XRD of each cold-rolled steel sheet was measured using an in-situ XRD analyzer at the cooling temperature (950 °C or 800 °C) during the secondary heat treatment and expressed as the XRD spectrum peak intensity ratio (I / Imax). In the case of the comparative example, the XRD of the random texture of the cold-rolled steel sheet was measured when cooled to room temperature and expressed as the XRD peak intensity ratio.

[0120] [Table 1]

[0121]

[0122] Referring to Table 1 above, through XRD analysis of Example 1 (in Example 1, the cold-rolled steel sheet was heated to the austenite phase (γ) temperature and held at this temperature during the primary heat treatment process of the cold-rolled steel sheet annealing process, and then cooled to 950 °C during the secondary heat treatment process), it can be confirmed that the {110} peak does not exist, and only the {200} peak and the {112} peak exist. In the case of Example 2 (which was carried out in the same manner as Example 1, except that the hot-rolled steel sheet was cooled to 800 °C during the secondary heat treatment process), it can be confirmed that the {110} peak, the {200} peak, and the {112} peak can all be observed. Compared with the random peaks of the comparative example, the fraction of the {110} orientation peak is relatively low, while the fractions of the {200} and {112} orientation peaks are relatively high).

[0123] Based on the results in Table 1 above, it has been experimentally confirmed that during the phase transformation from the austenite phase (γ) to the ferrite phase (α), the preferentially formed orientation varies with temperature conditions. In addition, it can be confirmed from the results in Table 2 below that the texture finally formed can be intentionally controlled according to the process conditions using the results in Table 1.

[0124] (2) Analysis of the steel sheet texture and measurement of magnetic properties: For each example and comparative example, the texture of the finally produced non-oriented electrical steel sheet was analyzed, and the magnetic flux density and iron loss were measured. The results are shown in Table 2 below. The magnetic flux density (B 50 ) was measured by applying a magnetic field of 5,000 A / m to the electrical steel sheet, and the iron loss (W 15 / 50 ) was measured by applying a magnetic flux density of 1.5 tesla with a frequency of 50 Hz to the electrical steel sheet.

[0125] [Table 2]

[0126]

[0127] Table 2 above shows the results of process design and experiments using the characteristics in Table 1. In the case of the comparative example, the cold-rolled steel sheet was heated to 1,100 °C, held at this temperature for 60 seconds, and then furnace-cooled. This can be said to be an ordinary heat treatment cycle rather than a heat treatment cycle for inducing growth under specific temperature conditions.

[0128] In the case of Example 1 (wherein, the cold-rolled steel sheet was held at 1,100 °C (at which temperature the austenite phase (γ) is stable) for 60 seconds for primary heat treatment, and then held at 950 °C (at which temperature only the {200} peak and the {112} peak of the ferrite phase (α) exist) for 60 seconds for secondary heat treatment to intentionally induce nucleation, and then furnace cooling was performed), by analyzing the texture formed after cooling to room temperature after the secondary heat treatment, it can be confirmed that the orientation <100> / / ND favorable for magnetism increased to 32.6 area%, while the orientation <112> / / ND unfavorable for magnetism decreased to 24.3 area%.

[0129] It can be confirmed that in Example 2 (wherein, the cold-rolled steel sheet was cooled to 800 °C for secondary heat treatment), the fraction of <100> / / ND favorable for magnetism increased compared to the comparative example, but was less than that of Example 1. Additionally, it can be confirmed that the fraction of the orientation <112> / / ND unfavorable for magnetism decreased compared to the comparative example, but increased compared to Example 1.

[0130] It can be confirmed that in the texture of the non-oriented electrical steel sheet of the comparative example, the fraction of the orientation <100> / / ND favorable for magnetism was 12.1 area%, while the fraction of the orientation <112> / / ND unfavorable for magnetism was as high as 56.9 area%.

[0131] Referring to the results in Table 2 above, it can be confirmed that in Example 1 and Example 2 and the comparative example having the same component composition, only by improving the texture, the magnetic flux density [T] increased from 1.64 (comparative example) to 1.65 (Example 2), and then increased to 1.68 (Example 1), and the iron loss [W / kg] decreased from 2.45 (comparative example) to 2.42 (Example 2), and then decreased to 2.36 (Example 1).

[0132] Based on the comprehensive results of Table 2, it can be confirmed that by designing a component composition that can undergo a phase change from the austenite phase (γ) to the ferrite phase (α) through heat treatment and setting process conditions that can form an orientation favorable for magnetism during the phase change process, the orientation favorable for magnetism can be intentionally increased and the orientation unfavorable for magnetism can be decreased, thereby ultimately improving the magnetic properties.

[0133] Based on the above, the present inventor designed a heat treatment process, which induces the nucleation of <100> / / ND orientation by holding the cold-rolled steel sheet at 950 °C (at this temperature, the development of <100> / / ND orientation beneficial to magnetism is optimal) for a certain period of time. Therefore, it can be confirmed that, compared with the existing continuous cooling conditions, the <100> / / ND orientation beneficial to magnetism increases by 20%, and the <112> / / ND orientation not beneficial to magnetism decreases by 32.6%. The core of the present invention can be said to be a non-oriented electrical steel sheet and a method for manufacturing a non-oriented electrical steel sheet, which induces nucleation and allows it to grow within a specific temperature range where the development of <100> / ND orientation beneficial to magnetism is optimal during the phase transformation from austenite phase (γ) to ferrite phase (α), thereby improving the texture based on the phase transformation.

[0134] Those skilled in the art can easily make simple modifications or variations to the present invention, and all such modifications or variations are considered to be included within the scope of the present invention.

Claims

1. A method for manufacturing a non-oriented electrical steel sheet, the method comprising the following steps: manufacturing a hot-rolled steel sheet from a slab, the slab comprising 0.4 wt% to 3.5 wt% of silicon (Si), more than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of austenite stabilizing elements, and the balance of iron (Fe) and other inevitable impurities; cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; and annealing the cold-rolled steel sheet, wherein the step of annealing the cold-rolled steel sheet comprises: heating the cold-rolled steel sheet to the austenite single-phase region temperature and holding it at this temperature to perform a primary heat treatment on the cold-rolled steel sheet; and cooling the cold-rolled steel sheet that has undergone the primary heat treatment to the ferrite / austenite dual-phase region temperature and holding it at this temperature to perform a secondary heat treatment on the cold-rolled steel sheet that has undergone the primary heat treatment.

2. The method according to claim 1, wherein, the slab further comprises at least one of the following elements in total weight: more than 0 wt% to 0.0050 wt% or less of carbon (C), more than 0 wt% to 0.0050 wt% or less of sulfur (S), more than 0 wt% to 0.0050 wt% or less of nitrogen (N), and more than 0 wt% to 0.0050 wt% or less of titanium (Ti).

3. The method according to claim 1, wherein, the hot-rolled steel sheet is manufactured by the following steps: reheating the slab at a reheating temperature of 1,110 °C to 1,180 °C; hot-rolling the reheated slab at a finish rolling temperature of 800 °C to 900 °C; and coiling the hot-rolled slab at a coiling temperature of 500 °C to 700 °C.

4. The method according to claim 1, the method further comprising: before the step of cold-rolling the hot-rolled steel sheet, annealing the hot-rolled steel sheet, wherein the step of annealing the hot-rolled steel sheet comprises: heating the hot-rolled steel sheet to a temperature of 940 °C to 1,110 °C and holding it at this temperature; and cooling the hot-rolled steel sheet.

5. The method according to claim 4, wherein, the step of annealing the hot-rolled steel sheet is carried out as follows: heating the hot-rolled steel sheet at a heating rate of 20 °C / s or higher, and cooling it at a cooling rate of 20 °C / s or higher.

6. The method according to claim 1, wherein, the reduction ratio of cold-rolling is 50% to 90%, and the thickness of the cold-rolled steel sheet is 0.1 mm to 0.5 mm.

7. The method according to claim 1, wherein, the primary heat treatment comprises the following steps: heating the cold-rolled steel sheet to a temperature of 1,000 °C to 1,250 °C and holding it at this temperature for 30 seconds to 300 seconds, the secondary heat treatment comprises the following steps: cooling the cold-rolled steel sheet that has undergone the primary heat treatment to the ferrite / austenite dual-phase region temperature and holding it at this temperature for 5 seconds to 300 seconds.

8. The method according to claim 7, wherein, The secondary heat treatment is carried out as follows: The cold-rolled steel sheet that has undergone the primary heat treatment is cooled at a cooling rate of 10 °C / s or higher.

9. An non-oriented electrical steel sheet comprising: 0.4 wt% to 3.5 wt% of silicon (Si), more than 0 wt% to 0.05 wt% or less of aluminum (Al), 0.002 wt% to 3.5 wt% of austenite stabilizing elements, and the balance of iron (Fe) and other inevitable impurities, wherein, the microstructure of the non-oriented electrical steel sheet includes ferrite.

10. The non-oriented electrical steel sheet according to claim 9, further comprising, based on the total weight, at least one of the following elements: more than 0 wt% to 0.0050 wt% or less of carbon (C), more than 0 wt% to 0.0050 wt% or less of sulfur (S), more than 0 wt% to 0.0050 wt% or less of nitrogen (N), and more than 0 wt% to 0.0050 wt% or less of titanium (Ti).

11. The non-oriented electrical steel sheet according to claim 9, wherein, The non-oriented electrical steel sheet has a thickness of 0.1 mm to 0.5 mm, a magnetic flux density (B 50 ) of 1.65 T or higher, an iron loss (W 15 / 50 ) of 2.42 W / kg or lower, a yield strength of 200 Mpa or higher, and a tensile strength of 300 MPa or higher.

12. The non-oriented electrical steel sheet according to claim 9, wherein, the non-oriented electrical steel sheet contains <100> / / ND texture of 15 area% or higher.

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