EP4640874A1

EP4640874A1 - Non-oriented electrical steel sheet and method for manufacturing same

Info

Publication number: EP4640874A1
Application number: EP23907462.8A
Authority: EP
Inventors: Hun-Ju Lee; Sang-Woo Lee; Yun-Su Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-11-24
Publication date: 2025-10-29
Also published as: CN120265814A; EP4640874A4; KR20240098933A; MX2025005330A; JP2025539171A; WO2024136176A1

Abstract

The present invention relates to a non-oriented electrical steel sheet and a method for manufacturing same, and more specifically, to a non-oriented electrical steel sheet that may preferably be used as an iron core for a drive motor for an environmentally friendly automobile or the like, and a method for manufacturing same. One aspect of the present invention has the purpose of providing a non-oriented electrical steel sheet with excellent magnetism and fatigue limit, and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a non-oriented electrical steel sheet and a method for manufacturing the same, and more particularly, to a non-oriented electrical steel sheet which may be preferably used as an iron core of a driving motor for an eco-friendly vehicle, and a method for manufacturing the same.

Background Art

A non-oriented electrical steel sheet may be mainly used in a motor converting electrical energy into mechanical energy, and excellent magnetic properties may be required to achieve high efficiency in the process. In particular, as eco-friendly vehicles driven by motors, instead of internal combustion engines, have recently gained attention, demand for a non-oriented electrical steel sheet used as a driving motor core material has increased, and to this end, a non-oriented electrical steel sheet having excellent magnetic properties and strength may be necessary.
Magnetic properties of a non-oriented electrical steel sheet may be mainly evaluated by iron loss and magnetic flux density. Iron loss may refer to energy loss occurring at a specific magnetic flux density and frequency, and magnetic flux density may refer to the degree of magnetization obtained under a specific magnetic field. The lower the iron loss, the more energy-efficient the motor may be manufactured under the same conditions, and as magnetic flux density increases, the motor may have a reduced size or copper loss may be reduced. Therefore, a driving motor with excellent efficiency and torque may be manufactured using a non-oriented electrical steel sheet having low iron loss and high magnetic flux density, thereby improving a driving range and output of an eco-friendly vehicle.
The characteristics of the non-oriented electrical steel sheet to be considered may also vary depending on operating conditions of a motor. As a general criteria for evaluating the characteristics of a non-oriented electrical steel sheet used in a motor, W15/50, which is the iron loss when a 1.5T magnetic field is applied at a commercial frequency of 50Hz, has been widely used. However, in a non-oriented electrical steel sheet having a thickness of 0.35mm or less used in an eco-friendly vehicle driving motor, magnetic characteristics may often be important at low fields of 1.0T or less and high frequencies of 400Hz or higher, such that the characteristics of a non-oriented electrical steel sheet may be evaluated using W10/400 iron loss or B1 magnetic flux density.
A non-oriented electrical steel sheet for an eco-friendly vehicle driving motor may require excellent fatigue limit as well as magnetic properties. Since an eco-friendly vehicle driving motor may be driven for long periods of time at various rotation speeds depending on the driving conditions of a vehicle, the fatigue limit of an electrical steel sheet may be one of the factors determining a lifespan of the vehicle. In particular, an eco-friendly vehicle driving motor may be designed with permanent magnets inserted into a rotor, and since the inserted permanent magnets exert a force to separate by continuous centrifugal force during driving, an electrical steel sheet having a high fatigue limit may be required.
A method commonly used to simultaneously increase magnetic properties and fatigue properties of a non-oriented electrical steel sheet may be to add alloy elements such as Si, Al, and Mn. The addition of these alloy elements may increase resistivity of steel, which may reduce the eddy current loss and may thus lower the overall iron loss. Also, alloy elements may be dissolved as substitutional elements in iron, and may causing a strengthening effect, thereby enhancing fatigue strength. However, as the amount of alloy elements such as Si, Al, and Mn added increases, magnetic flux density may deteriorate and brittleness may increase, and when a predetermined amount or more is added, it may be impossible to perform cold rolling, such that commercial production may be impossible. In particular, as a thickness of an electrical steel sheet decreases, the highfrequency iron loss may become excellent, but the decrease in rollability due to brittleness may become a serious problem. The maximum value of the total content of Si, Al, and Mn which may be commercially produced may be known to be approximately 4.5%, and also, by optimizing the content of trace elements, a top-quality non-oriented electrical steel sheet having excellent magnetism and strength may be produced.
Depending on a design intention of a motor, an electrical steel sheet having higher fatigue strength may also be used even when magnetic properties are relatively low. Methods for manufacturing an electrical steel sheet as above may include a method of using precipitation of interstitial elements and a method of reducing a grain size. Mainly, when miniaturizing a motor to increase a rotation speed or to increase the effect of a permanent magnet inserted into a rotor, a rotor manufactured with electrical steel having significantly improved strength may be used even when magnetic properties of the electrical steel are deteriorated. In this case, when micro-precipitates including interstitial solid-solution elements such as C, N, and S are formed, the effect of strength improvement may be desirable, but iron loss may become worse rapidly, which may lower the efficiency of the motor. Also, the method of reducing a grain size may have the disadvantage of increasing unevenness of a steel sheet material due to the inclusion of unrecrystallized regions, which may increase the quality deviation of the product.
To address the above problems, cited document 1 attempted to create a non-oriented electrical steel sheet having excellent magnetism and strength by controlling a cooling rate during the final annealing process, but it may be difficult to be applied to the mass production process due to an increase in material unevenness caused by inclusion of unrecrystallized regions. Also, most of the existing techniques proposed to simultaneously improve magnetism and strength may not be used due to reasons such as increased manufacturing costs, decreased productivity and yield, and insufficient improvement effects.

[Prior art]

(Cited document 1) International Laid-Open Patent Publication No. 2009-128428

Detailed description of present disclosure

Technical problems to solve

An aspect in the present disclosure is to provide a non-oriented electrical steel sheet, and a method for manufacturing the same.
A preferable aspect in the present disclosure is to provide a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit, and a method for manufacturing the same.

Solution to Problem

According to an embodiment of the present disclosure, a non-oriented electrical steel sheet includes, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities, and a ratio (D0.9/D0.0) between an average grain diameter (D0.0) of a central portion and an average grain diameter (D0.9) of a surface portion is 0.55 to 0.85,
where the surface portion indicates a region from a surface of the steel sheet to 1/10t (t: thickness of steel material), and the central portion indicates a region other than the surface portion.
The non-oriented electrical steel sheet may further include one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.
The non-oriented electrical steel sheet may further include one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.
The non-oriented electrical steel sheet may further include 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.
The surface portion may have 0.02 to 0.20 / µm² carbides having a diameter of 20 to 200 nm.
The non-oriented electrical steel sheet may have an average grain diameter of 50 to 100 µm.
The non-oriented electrical steel sheet may have a thickness of 0.1 to 0.35 mm.
The non-oriented electrical steel sheet may have a fatigue limit of 310 MPa or higher at 90°C.
The non-oriented electrical steel sheet may have a magnetic flux density (B1) of 1.12T or higher, a magnetic flux density (B50) of 1.67T or higher, and an iron loss (W10/400) of 11.4W/Kg or lower.
According to another embodiment of the present disclosure, a method for manufacturing a non-oriented electrical steel sheet includes heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities, finishing hot-rolling the heated slab and obtaining a hot-rolled sheet; hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more; pickling the hot-rolled sheet annealed hot-rolled sheet, cold-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a cold-rolled sheet; and final annealing the cold-rolled sheet.
According to another embodiment of the present disclosure, a method for manufacturing a non-oriented electrical steel sheet includes heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities; finishing hot-rolling the heated slab and obtaining a hot-rolled sheet; hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more; pickling the hot-rolled sheet annealed hot-rolled sheet, warm-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a warm-rolled sheet; and final annealing the warm-rolled sheet.
The slab may further include one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.
The slab may further include one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.
The slab may further include 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.
The heating the slab may be performed at 1050 to 1200°C.
The finishing hot-rolling may be performed at 800 to 950°C.
The final annealing may be performed at 850°C or lower.
The final annealing may be performed in an atmosphere in which hydrogen (H₂) and nitrogen (N₂) gases are mixed.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a non-oriented electrical steel sheet and a method for manufacturing the same may be provided.
According to a preferable aspect of the present disclosure, a non-oriented electrical steel sheet having excellent magnetic properties and fatigue limit and a method for manufacturing the same.

Best Mode for Invention

Hereinafter, a non-oriented electrical steel sheet according to an embodiment of the present invention may be described. First, an alloy composition may be described. The content of the alloy composition described below may be indicated in weight% unless otherwise indicated.

Si: 3.3 to 3.8%

Si may reduce iron loss by increasing resistivity of a material, and may increase strength through solid-solution strengthening. When the content of Si is less than 3.3%, the effect of addressing iron loss and improving strength may be insufficient. When the content of Si exceeds 3.8%, brittleness of the material may increase, which may rapidly reduce rolling productivity, and surface portion oxide layers and oxides which may be harmful to magnetic properties may be formed. Accordingly, the content of the Si may range from 3.3 to 3.8% preferably.

Al: 0.4 to 1.5%

Al may reduce iron loss by increasing resistivity of the material, and may increase strength through solid-solution strengthening. When the content of Al is less than 0.4%, fine nitrides may be formed, such that it may be difficult to obtain the effect of improving magnetic properties. When the content of Mn exceeds 1.5%, nitrides may be excessively formed, which may deteriorate magnetic properties and may cause problems in the entirety of processes such as steelmaking and continuous casting, such that productivity may be significantly reduced. Accordingly, the content of Al may have a range of 0.4 to 1.5% preferably. A lower limit of the Al content may more preferably be 0.6%. An upper limit of the Al content may more preferably be 1.3%.

Mn: 0.2 to 1.5%

Mn may improve iron loss by increasing resistivity of the material and may form sulfides. When the content of Mn is less than 0.2%, sulfides may be finely formed, which may deteriorate magnetic properties. When the Mn content exceeds 1.5%, fine MnS may be excessively precipitated, formation of {111} texture, which is unfavorable for magnetic properties, may be promoted, such that magnetic flux density may decrease rapidly. Accordingly, the Mn content may range from 0.2 to 1.5% preferably. A lower limit of the Mn content may more preferably be 0.3%. An upper limit of the Mn content may more preferably be 1.3%.

C: 0.0025% or less (excluding 0%)

C may cause magnetic aging and may combine with other impurity elements and may form carbides, thereby hindering grain boundary or domain wall movement and deteriorating magnetic properties. When the C content exceeds 0.0025%, fine carbides may increase rapidly, which may seriously deteriorate magnetic properties. Accordingly, the content of C may range from 0.0025% or less (excluding 0%) preferably.

S: 0.005% or less (excluding 0%)

S may form fine precipitates, MnS, which may deteriorate magnetic properties and hot workability. When the content of S exceeds 0.005%, it may be difficult to ensure sufficient magnetic properties and hot workability. Accordingly, the content of S may preferably range from 0.005% or less (excluding 0%). The content of S may more preferably be 0.003% or less.

N: 0.005% or less (excluding 0%)

N may form fine AlN precipitates in a base material, and may also combine with other impurities and may form fine precipitates, such that grain growth and domain wall movement may be inhibited, and iron loss may worsen. When the N content exceeds 0.005%, fine nitrides may increase rapidly, such that iron loss may worsen. Accordingly, the N content may preferably have a range of 0.005% or less (excluding 0%). The N content may more preferably be 0.002% or less.

Ti: 0.003% or less (excluding 0%)

Ti may have a relatively strong tendency to form precipitates in steel, and may form fine carbides, nitrides, or sulfides in a base material, such that grain growth and domain wall movement may be inhibited and iron loss may thus worsen. When the content of Ti exceeds 0.003%, it may be difficult to obtain sufficient magnetic properties. Accordingly, the content of Ti may preferably have a range of 0.003% or less (excluding 0%). The content of Ti may more preferably be 0.0025% or less, and even more preferably 0.002% or less.

Nb: 0.003% or less (excluding 0%)

Nb may have a relatively strong tendency to form precipitates in steel, and may form fine carbides, nitrides, or sulfides in a base material, such that grain growth and domain wall movement may be inhibited and iron loss may thus worsen. When the content of Nb exceeds 0.003%, it may be difficult to obtain sufficient magnetic properties. Accordingly, the content of Nb may preferably have a range of 0.003% or less (excluding 0%). The content of Nb may more preferably be 0.0025% or less, and even more preferably 0.002% or less.

V: 0.003% or less (excluding 0%)

V may have a relatively strong tendency to form precipitates in steel, and may form fine carbides, nitrides, or sulfides in a base material, such that grain growth and domain wall movement may be inhibited and iron loss may thus worsen. When the V content exceeds 0.003%, it may be difficult to obtain sufficient magnetic properties. Accordingly, the V content may preferably have a range of 0.003% or less (excluding 0%). The V content may more preferably be 0.0025% or less, and even more preferably 0.002% or less.

W: 0.0005 to 0.0050%

W may affect the behavior of carbide in steel, and may promote grain size gradient by inducing differences in carbide formation behavior depending on a thickness layer at a temperature of 700°C or lower. When the W content is less than 0.0005%, it may be difficult to induce differences in carbide formation behaviors between the surface portion and the central portion. When the W content exceeds 0.0050%, carbide formation between the surface portion and the central portion may be promoted, which may seriously deteriorate magnetic properties. Accordingly, the W content may preferably have a range of 0.0005 to 0.0050%. A lower limit of the W content may more preferably be 0.001%. An upper limit of the W content may more preferably be 0.0040%.

Sn and Sb: 0.005 to 0.1% in total

Sn and Sb may segregate at the initial grain boundary during final recrystallization annealing and may suppress development of {111} orientation, which may deteriorate magnetic properties. When the total content of Sn and Sb is less than 0.005%, it may be difficult to suppress the development of {111} orientation. When the total content of Sn and Sb exceeds 0.1%, it may deteriorate surface quality, which may degrade product productivity. Accordingly, the total content of Sn and Sb may preferably range from 0.005 to 0.1%. A lower limit of the total content of Sn and Sb may more preferably be 0.015%. An upper limit of the total content of Sn and Sb may more preferably be 0.08%.
The non-oriented electrical steel sheet of the present invention may further include one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.

P: 0.1% or less

P may be a grain boundary segregation element, which may delay recrystallization and may deteriorate strength uniformity in the rolling direction and the vertical rolling direction, and thus, an upper limit may be limited to 0.1%. More specifically, the P content may be 0.0001 to 0.1%. More specifically, the P content may be 0.001 to 0.05%.

Cr: 0.01 to 0.5%

Cr may improve iron loss by increasing resistivity. When the Cr content is less than 0.01%, the effect of resistivity improvement may not be sufficient. When the Cr content exceeds 0.5%, magnetic flux density may decrease. More specifically, the Cr content may be 0.02 to 0.3%.

Ni: 0.05% or less

Ni may react with impurity elements and may form fine sulfides, carbides, and nitrides, which may detrimentally affect magnetic properties, and thus, an upper limit may be limited to 0.05%. More specifically, the content of Ni may be 0.0001 to 0.050%. More specifically, the content of Ni may be 0.001 to 0.030%.

Cu: 0.005 to 0.2%

Cu may form sulfides with Mn. When the content of Cu is less than 0.005%, (Cu·Mn)S may be finely precipitated and may deteriorate magnetic properties. When the content of Cu exceeds 0.2%, high-temperature embrittlement may occur, which may form cracks during casting or hot rolling. More specifically, the content of Cu may be 0.010 to 0.1%.

Zn: 0.01% or less

Zn may act as an impurity and may deteriorate magnetic properties, and thus, an upper limit may be limited to 0.01%. More specifically, the content of Zn may be 0.0001 to 0.01%. More specifically, the content of Zn may be 0.001 to 0.008%.
The non-oriented electrical steel sheet of the present invention may further include one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.

Mo: 0.03% or less

Since the Mo may inevitably react with C, S, N, or the like, which may be inevitably added, and may form fine carbides, nitrides, or sulfides, which may adversely affect magnetic properties. Thus, an upper limit thereof may be limited to 0.03%.

B: 0.002% or less

B may form inclusions in steel, and may deteriorate magnetic properties. When the content of B exceeds 0.002%, it may be difficult to ensure excellent magnetic properties. Accordingly, the content of B may preferably have a range of 0.002% or less (excluding 0%). The B content may more preferably be 0.0005% or less.

Mg: 0.005% or less

The Mg may form inclusions in steel, and may deteriorate magnetic properties. When the content of Mg exceeds 0.005%, it may be difficult to ensure excellent magnetic properties. Accordingly, the content of Mg may preferably have a range of 0.005% or less (excluding 0%). The Mg content may more preferably be 0.002% or less.

Ca: 0.005% or less

The Ca may react with C, S, N, or the like, which may be inevitably included, and may form fine carbides, nitrides, or sulfides, which may adversely affect magnetic properties, and thus, an upper limit may be limited to 0.005%.

Zr: 0.005% or less

The Zr may form inclusions in steel, and may deteriorate magnetic properties. When the content of Zr exceeds 0.005%, it may be difficult to ensure excellent magnetic properties. Accordingly, the content of Zr may preferably have a range of 0.005% or less (excluding 0%). The Zr content may more preferably be 0.002% or less.
The non-oriented electrical steel sheet of the present invention may further include 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.
When the aforementioned elements are additionally added, the elements may segregate at the grain boundary, may relieve stress concentration at the grain boundary during cold-rolling, and may suppress recrystallization of <111>//ND orientation grains in the subsequent recrystallization annealing process, thereby improving magnetic flux density. When they are added appropriately, the aforementioned effect may be additionally obtained, but when they are included excessively, a large amount of segregation may occur, which may suppress grain growth and may deteriorate magnetic flux density and iron loss. More specifically, 0.0001 to 0.20% of one or more of Bi, Pb, Ge and As may be included individually or in combination. More specifically, 0.001 to 0.10% of one or more of Bi, Pb, Ge and As may be included individually or in combination.
A remainder of the present disclosure is iron (Fe). However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded. A person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
As for the non-oriented electrical steel sheet of the present invention, a ratio (D0.9/D0.0) between an average grain diameter (D0.0) of a central portion and an average grain diameter (D0.9) of a surface portion may be 0.55 to 0.85 preferably. The surface portion may have 0.02 to 0.20/ µm² of carbides having a diameter of 20 to 200 nm. By appropriately controlling the grain fraction of the surface portion and the grain fraction of the surface portion with optimized carbide distribution, good magnetic properties may be assured by the central portion grain having excellent aggregate structure, and also excellent fatigue properties may be assured by the surface portion grain. In particular, when the fine carbide of the surface portion is appropriately distributed, fatigue properties at a temperature higher than room temperature may be further improved. When the ratio (D0.9/D0.0) between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion is less than 0.55, iron loss may be deteriorated due to an increase in hysteresis loss. When the ratio (D0.9/D0.0) between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion exceeds 0.85, stress may be concentrated at the grain boundary positioned at the surface portion and the fatigue limit may be lowered. A lower limit of the ratio (D0.9/D0.0) between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion may more preferably be 0.60. An upper limit between the ratio (D0.9/D0.0) of the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion may more preferably be 0.80. When the number density of carbides having a diameter of 20 to 200 nm is less than 0.02/ µm², the surface portion grains may grow to a similar size to that of the central portion, which may lower the fatigue limit. When the number density of carbides having a diameter of 20 to 200 nm exceeds 0.20/ µm², the surface portion grains may become excessively fine and may hinder domain wall movement, such that magnetic properties may be deteriorated. Meanwhile, the surface portion may indicate a region from the surface of the steel sheet to 1/10t (t: thickness of steel material), and the central portion may indicate a region other than the surface portion. In the present invention, the type of carbide may not be specifically limited, and as an example, a precipitate in which one or more elements of Ti, Nb, V, W are combined with carbon may be formed.
The non-oriented electrical steel sheet of the present invention may have an average grain diameter of 50 to 100 µm. When the average grain diameter is less than 50 µm, hysteresis loss may increase rapidly and iron loss may worsen. When the average grain diameter exceeds 100 µm, stress may be concentrated at the grain boundary and the fatigue limit may be lowered.
As described above, the non-oriented electrical steel sheet of the present invention described above may have a thickness of 0.1 to 0.35 mm. Also, the non-oriented electrical steel sheet of the present invention may have a fatigue limit of 310 MPa or higher at 90°C. Also, the non-oriented electrical steel sheet of the present invention may have a magnetic flux density (B1) of 1.12 T or higher, a magnetic flux density (B50) of 1.67 T or higher, and an iron loss (W10/400) of 11.4 W/Kg or lower. In the present invention, the higher values of fatigue limit, magnetic flux density (B1) and magnetic flux density (B50) at 90°C may be advantageous, and thus, there is no particular limitation on an upper limit thereof. A lower value of iron loss (W10/400) may be advantageous, and thus, there is no particular limitation on a lower limit thereof. Meanwhile, electrical characteristics may be based on the case in which the thickness of the non-oriented electrical steel sheet is 0.25 mm.
Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention may be described.
First, a slab may be heated. The heating the slab may be performed at 1050 to 1200°C. When the slab heating temperature is lower than 1050°C, the shape may be poor after finishing rolling. When the slab heating temperature exceeds 1200°C, precipitates such as AlN and MnS may be re-precipitated in a fine size after re-dissolution, which may significantly deteriorate magnetic properties. A lower limit of the slab heating temperature may more preferably be 1100°C. An upper limit of the slab heating temperature may more preferably be 1170°C.
Thereafter, the heated slab may be finishing hot-rolled and a hot-rolled sheet may be obtained. The finishing hot-rolling may be performed at 800 to 950°C. When the finishing hot-rolling temperature is lower than 800°C, the coil shape may become poor due to deterioration in processability caused by an increase in deformation resistance. When the finishing hot-rolling temperature exceeds 950°C, an oxide layer may be excessively formed on the coil surface, which may cause defects.
Thereafter, the hot-rolled sheet may be hot-rolled sheet annealed to have an average grain diameter of 250 µm or more. When the average grain diameter of the hot-rolled sheet annealed hot-rolled sheet is less than 250 µm, it may be difficult to assure a difference in grain sizes between the central portion and the surface portion after final annealing, such that it may be difficult to assure both magnetic properties and the fatigue limit. In the present invention, the specific configuration for controlling the average grain diameter of the hot-rolled sheet annealed hot-rolled sheet to 250 µm or more may not be particularly limited. However, as an example, a method of controlling the annealing temperature and annealing time, or the like, may be used.
Thereafter, the hot-rolled sheet annealed hot-rolled sheet may be pickled and cold-rolled such that a section in which the surface temperature of the steel sheet is 400°C or higher may be included, and a cold-rolled sheet may be obtained. By including a section in which the surface temperature of the steel sheet is 400°C or higher during cold-rolling, a temperature and deformation structure advantageous for forming carbides may be formed on the surface portion, and the deformation structure occurring during cold-rolling may be partially recovered in the central portion, thereby obtaining an effect of generating a difference in grain sizes between the surface portion and the central portion. In the present invention, the higher surface temperature of the steel sheet may be advantageous during cold-rolling, there is no particular limitation on an upper limit thereof. However, an upper limit of the surface temperature of the steel sheet during cold-rolling may be, for example, 550°C.
In the present invention, there is no particular limitation on a specific configuration in which a section in which the surface temperature of the steel sheet is 400°C or higher during cold-rolling is included. However, as an example, a method of charging into a box furnace provided between passes during cold-rolling, a method of heating the rolls to a high temperature during rolling, a method of heating through an induction heating device provided before and after the rolling, or the like, may be used.
Thereafter, the cold-rolled sheet may be final annealed. The final annealing may be performed at 850°C or lower. When the final annealing temperature is lower than 850°C, recrystallization may not occur sufficiently, and magnetic properties of the steel sheet may deteriorate significantly. The final annealing may be performed in an atmosphere in which hydrogen (H₂) and nitrogen (N₂) gases are mixed.
Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention may be described.
The method for manufacturing a non-oriented electrical steel sheet according to another embodiment of the present invention may satisfy most of the manufacturing conditions described above, but instead of the process of pickling the hot-rolled sheet annealed hot-rolled sheet, cold-rolling the steel sheet such that the section in which the surface temperature of the steel sheet is 400°C or higher is included, and obtaining a cold-rolled sheet, a process of pickling the hot-rolled sheet annealed hot-rolled sheet, warm-rolling the steel sheet such that the section in which the surface temperature of the steel sheet is 400°C or higher is included, and obtaining a warm-rolled sheet. By including the section in which the surface temperature of the steel sheet is 400°C or higher during the warm-rolling, the effect of simultaneously improving magnetic properties and fatigue limit by generating a difference in microstructures between the surface portion and the central portion of the steel sheet during the final recrystallization annealing process may be obtained. In the present invention, the higher surface temperature of the steel sheet may be advantageous during warm-rolling, and thus, there is no particular limitation on an upper limit thereof. However, an upper limit of the surface temperature of the steel sheet during warm-rolling may be, for example, 550°C.

Mode for Invention

Hereinafter, the present disclosure may be described more specifically through embodiments. However, it should be noted that the embodiments below are merely intended to describe the present disclosure in greater detail based on embodiments, and are not intended to limit the scope of the rights of the present disclosure. This may be because the scope of rights of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

(Embodiment)

A slab having an alloy composition described in Table 1 below was heated at 1150°C, and hot-rolled at a finishing hot-rolling temperature of 900°C, thereby manufacturing a hot-rolled sheet having a thickness of 2.0 mm. Thereafter, the hot-rolled sheet was hot-rolled sheet annealed (at 1150°C for 120 seconds) under the conditions described in Table 2 below to obtain an average grain diameter, and cold-rolled, thereby manufacturing a cold-rolled sheet having a thickness of 0.25 mm. In this case, during the cold-rolling, the steel sheet was put in a box furnace set to the conditions described in Table 2 below between passes, and maintained for 5 minutes. Thereafter, the cold-rolled sheet was finally annealed at 1000°C for 100 seconds in an atmosphere in which hydrogen (H₂) and nitrogen (N₂) gases are mixed, and a non-oriented electrical steel sheet was manufactured.
For the non-oriented electrical steel sheet manufactured as above, the average grain diameter of the hot-rolled sheet annealed hot-rolled sheet, the ratio between the average grain diameter (D0.0) of the central portion and the average grain diameter (D0.9) of the surface portion (D0.9/D0.0), the number density of carbides having a diameter of 20 to 200 nm in the surface portion, the average grain diameter, the fatigue limit at 90°C, and the electrical properties were measured, and the results are listed in Tables 2 and 3.
The average grain diameter of the hot-rolled sheet annealed hot-rolled sheet was measured using an optical microscope on a rolling vertical direction cross-section (TD plane) of the hot-rolled sheet after hot-rolled sheet annealing the hot-rolled sheet.
The average grain diameter (D0.0) of the central portion was measured on the surface (ND plane) of 1/2t (t: thickness of steel material) and 1/10t (t: thickness of steel material) of the non-oriented electrical steel sheet using an optical microscope.
The number density of carbides having a diameter of 20 to 200 nm in the surface portion was measured on the surface (ND plane) of 1/10t (t: thickness of steel material) of the non-oriented electrical steel sheet using a scanning electron microscope (TEM).
The average grain diameter was measured on the rolling vertical direction cross-section (TD plane) of the non-oriented electrical steel sheet using an optical microscope.
As for the cold-rolled sheet surface temperature during cold-rolling, the highest temperature measured at the time of ejection of the steel sheet during cold rolling and the temperature measured at the exit of the rolling roll was listed.
The fatigue limit at 90°C was obtained by drawing an SN diagram through a fatigue test at a temperature of 90°C with a stress ratio of 0.05 and a frequency of 50Hz.

Among the electrical properties, magnetic flux density (B1), magnetic flux density (B50), and iron loss (W10/400) were measured by collecting 5 samples measuring width of 60 mm and length of 60 mm from a non-oriented electrical steel sheet, measuring the rolling direction and the vertical rolling direction using a single sheet tester, and calculating the average value thereof. Magnetic flux density (B1) and magnetic flux density (B50) indicate magnetic flux density induced in a magnetic field of 100 A/m and 5000 A/m, respectively, and iron loss (W10/400) may indicate iron loss when a magnetic flux density of 1.0 T is induced at a frequency of 400 Hz.

[Table 1]

Steel type
Steel type	Si	Al	Mn	C	N	S	Ti	Nb	V	W	Sn+Sb
Compar ative steel 1	3.35	1.00	0.20	0.003 1	0.000 9	0.001 7	0.000 7	0.000 7	0.000 8	0.001 3	0.078
Compar ative steel 2	3.35	1.00	0.20	0.001 6	0.001 4	0.001 7	0.003 7	0.000 7	0.000 7	0.001 2	0.039
Compar ative steel 3	3.35	1.00	0.20	0.001 7	0.001 4	0.002 3	0.000 8	0.001	0.001 1	0.000 2	0.074
Compar ative steel 4	3.35	1.00	0.20	0.001 8	0.001 6	0.001 0	0.001 5	0.001 8	0.001 7	0.004 6	0.120
Invent ive steel 1	3.35	1.00	0.20	0.000 9	0.001 2	0.001 6	0.001 6	0.000 9	0.001 4	0.001 5	0.056
Invent ive steel 2	3.35	1.00	0.20	0.001 8	0.000 9	0.000 8	0.001 6	0.001 4	0.001 5	0.001 1	0.084
Invent ive steel 3	3.35	1.00	0.20	0.001 4	0.000 7	0.001 1	0.001 7	0.001 4	0.000 9	0.003 8	0.092
Invent ive steel 4	3.35	1.00	0.20	0.000 7	0.000 8	0.002 1	0.000 9	0.001 2	0.001 5	0.001 0	0.076
Compar ative steel 5	3.55	0.70	0.50	0.001 6	0.005 9	0.001 7	0.001 1	0.001 8	0.001 6	0.002 2	0.012
Compar ative steel 6	3.55	0.70	0.50	0.001 0	0.001 8	0.001 8	0.001 7	0.004 1	0.001 2	0.002 9	0.052
Compar ative steel 7	3.55	0.70	0.50	0.000 9	0.001 4	0.000 9	0.001 4	0.001 8	0.000 7	0.002 4	0.003
Invent ive steel 5	3.55	0.70	0.50	0.002 3	0.000 7	0.001 8	0.000 9	0.001 4	0.000 7	0.002 3	0.018
Invent ive steel 6	3.55	0.70	0.50	0.001 7	0.001 8	0.001 2	0.000 8	0.001 1	0.000 7	0.004 2	0.049
Invent ive steel 7	3.55	0.70	0.50	0.001 0	0.001 8	0.001 2	0.000 9	0.000 7	0.000 8	0.003 3	0.008
Invent ive steel 8	3.55	0.70	0.50	0.001 6	0.000 9	0.001 0	0.001 4	0.001 6	0.001 4	0.004 0	0.033
Invent ive steel 9	3.55	0.70	0.50	0.001 7	0.001 8	0.001 7	0.001 7	0.001 8	0.001 7	0.004 2	0.054
Compar ative steel 8	3.75	0.50	0.60	0.001 6	0.001 6	0.006 2	0.000 3	0.000 9	0.001 4	0.002 8	0.056
Compar ative steel 9	3.75	0.50	0.60	0.001 4	0.001 5	0.001 1	0.001 8	0.001 8	0.003 6	0.004 1	0.080
Compar ative steel 10	3.75	0.50	0.60	0.001 8	0.000 9	0.001 7	0.000 9	0.001 8	0.000 9	0.005 9	0.054
Invent ive steel 10	3.75	0.50	0.60	0.001 4	0.000 9	0.001 4	0.001 8	0.001 4	0.000 7	0.002 6	0.017
Invent ive steel 11	3.75	0.50	0.60	0.001 2	0.001 5	0.001 1	0.001 7	0.000 7	0.000 8	0.001 6	0.022
Invent ive steel 12	3.75	0.50	0.60	0.001 1	0.000 7	0.000 3	0.000 2	0.001 7	0.000 3	0.002 2	0.069
Invent ive steel 13	3.75	0.50	0.60	0.001 8	0.001 6	0.001 8	0.000 9	0.001 0	0.001 5	0.001 3	0.037
Invent ive steel 14	3.75	0.50	0.60	0.001 6	0.000 9	0.000 9	0.000 9	0.001 6	0.001 6	0.002 9	0.038

[Table 2]

Classif ication	Steel type	Average grain diameter of hot-rolled sheet annealed hot rolled sheet (µm)	Surface temperatu re of cold-rolled sheet surface temperatu re during cold-rolling (°C)	Average grain diameter (D0.0) of central portion	Average grain diameter (D0.9) of surface portion	Average grain diameter (D0.9) of surface portion/ average grain diameter (D0.0) of central portion
Compara tive example 1	Compara tive steel 1	298	438	65	33	0.51
Compara tive example 2	Compara tive steel 2	277	410	76	39	0.51
Compara tive example 3	Compara tive steel 3	284	450	107	94	0.88
Compara tive example 4	Compara tive steel 4	314	449	106	54	0.51
Inventi ve example 1	Inventi ve steel 1	266	431	95	66	0.69
Inventive example 2	Inventive steel 2	281	418	92	73	0.79
Inventi ve example 3	Inventi ve steel 3	311	414	70	54	0.77
Inventi ve example 4	Inventi ve steel 4	284	422	81	55	0.68
Compara tive example 5	Compara tive steel 5	273	440	68	31	0.46
Compara tive example 6	Compara tive steel 6	286	432	71	37	0.52
Compara tive example 7	Compara tive steel 7	284	417	86	43	0.50
Compara tive example 8	Inventi ve steel 5	222	440	94	88	0.94
Inventi ve example 5	Inventi ve steel 6	273	440	109	84	0.77
Inventi ve example 6	Inventi ve steel 7	300	442	77	63	0.82
Inventi ve example 7	Inventi ve steel 8	310	421	92	75	0.82
Inventi ve example 8	Inventi ve steel 9	314	448	86	52	0.60
Compara tive example 9	Compara tive steel 8	268	450	59	30	0.51
Compara tive example 10	Compara tive steel 9	307	441	110	57	0.52
Compara tive example 11	Compara tive steel 10	282	440	78	21	0.27
Compara tive example 12	Inventi ve steel 10	284	384	89	81	0.91
Inventi ve example 9	Inventi ve steel 11	311	429	91	73	0.80
Inventi ve example 10	Inventi ve steel 12	308	426	79	58	0.73
Inventi ve example 11	Inventi ve steel 13	272	411	88	66	0.75
Inventi ve example 12	Inventi ve steel 14	269	417	107	74	0.69

[Table 3]

Classif ication	Steel type	Number density of carbides having a diameter of 20 to 200 nm within surface portion (µm²)	Averag e grain size (µm)	Fatigue limit at 90°C (MPa)	Magneti c flux density (B1) (T)	Magneti c flux density (B50) (T)	Iron loss (W10/400) (W/Kg)
Compara tive example 1	Compar ative steel 1	0.35	44	298	0.93	1.64	12.5
Compara tive example 2	Compar ative steel 2	0.25	67	293	0.94	1.64	12.4
Compara tive example 3	Compar ative steel 3	0.01	98	285	0.93	1.64	12.5
Compara tive example 4	Compar ative steel 4	0.26	87	287	0.95	1.64	12.6
Inventi ve example 1	Invent ive steel 1	0.11	84	318	1.13	1.67	11.3
Inventi ve example 2	Invent ive steel 2	0.07	86	314	1.14	1.67	11.4
Inventi ve example 3	Invent ive steel 3	0.06	64	316	1.13	1.67	11.2
Inventi ve example 4	Invent ive steel 4	0.14	75	313	1.12	1.67	11.3
Compara tive example 5	Compar ative steel 5	0.27	45	297	0.94	1.64	12.5
Compara tive example 6	Compar ative steel 6	0.24	56	290	0.93	1.64	12.3
Compara tive example 7	Compar ative steel 7	0.19	66	287	0.94	1.64	12.5
Compara tive example 8	Invent ive steel 5	0.01	91	283	0.95	1.64	12.6
Inventi ve example 5	Invent ive steel 6	0.13	91	312	1.13	1.67	11.4
Inventi ve example 6	Invent ive steel 7	0.14	70	315	1.12	1.67	11.4
Inventi ve example 7	Invent ive steel 8	0.06	81	314	1.13	1.67	11.3
Inventi ve example 8	Invent ive steel 9	0.08	68	315	1.12	1.67	11.2
Compara tive example 9	Compar ative steel 8	0.37	41	299	0.95	1.64	12.6
Compara tive example 10	Compar ative steel 9	0.28	90	285	0.95	1.64	12.4
Compara tive example 11	Compar ative steel 10	0.44	49	306	0.94	1.64	12.5
Compara tive example 12	Invent ive steel 10	0.01	85	294	0.93	1.64	12.5
Inventi ve example 9	Invent ive steel 11	0.13	89	313	1.12	1.67	11.1
Inventi ve example 10	Invent ive steel 12	0.04	71	315	1.13	1.67	11.3
Inventi ve example 11	Invent ive steel 13	0.15	78	314	1.12	1.67	11.1
Inventi ve example 12	Invent ive steel 14	0.11	89	315	1.14	1.67	11.2

As indicated in Tables 1 to 3, in inventive examples 1 to 12 satisfying the alloy composition and manufacturing conditions proposed in the present invention, the microstructure and carbide conditions which the present invention aims to obtain were satisfied, such that excellent magnetic properties and fatigue limit were assured.
In comparative examples 1 to 7 and 9 to 11, the alloy composition proposed in the present invention was not satisfied, such that a microstructure or carbide conditions which the present invention aims to obtain were not satisfied, and accordingly, magnetic properties and fatigue limit were low.
In comparative examples 8 and 12, the microstructure or carbide conditions which the present invention aims to obtain were not satisfied, and accordingly, magnetic properties and fatigue limit were low.

Claims

A non-oriented electrical steel sheet, comprising:
by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities,

wherein a ratio (D0.9/D0.0) between an average grain diameter (D0.0) of a central portion and an average grain diameter (D0.9) of a surface portion is 0.55 to 0.85,

where the surface portion indicates a region from a surface of the steel sheet to 1/10t, where t is a thickness of steel material, and the central portion indicates a region other than the surface portion.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet further includes one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet further includes one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet further includes 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.
The non-oriented electrical steel sheet of claim 1, wherein the surface portion has 0.02 to 0.20 / µm² carbides having a diameter of 20 to 200 nm.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has an average grain diameter of 50 to 100 µm.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has a thickness of 0.1 to 0.35 mm.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has a fatigue limit of 310 MPa or higher at 90°C.
The non-oriented electrical steel sheet of claim 1, wherein the non-oriented electrical steel sheet has a magnetic flux density (B1) of 1.12T or higher, a magnetic flux density (B50) of 1.67T or higher, and an iron loss (W10/400) of 11.4W/Kg or lower.
A method for manufacturing a non-oriented electrical steel sheet, the method comprising:
heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities;

finishing hot-rolling the heated slab and obtaining a hot-rolled sheet;

hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more;

pickling the hot-rolled sheet annealed hot-rolled sheet, cold-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a cold-rolled sheet; and

final annealing the cold-rolled sheet.
A method for manufacturing a non-oriented electrical steel sheet, the method comprising:
heating a slab including, by weight%, Si: 3.3 to 3.8%, Al: 0.4 to 1.5%, Mn: 0.2 to 1.5%, C: 0.0025% or less (excluding 0%), S: 0.005% or less (excluding 0%), N: 0.005% or less (excluding 0%), Ti: 0.003% or less (excluding 0%), Nb: 0.003% or less (excluding 0%), V: 0.003% or less (excluding 0%), W: 0.0005 to 0.0050%, Sn and Sb: 0.005 to 0.1% in total, and a balance of Fe and inevitable impurities;

finishing hot-rolling the heated slab and obtaining a hot-rolled sheet;

hot-rolled sheet annealing the hot-rolled sheet such that the hot-rolled sheet has an average grain diameter of 250 µm or more;

pickling the hot-rolled sheet annealed hot-rolled sheet, warm-rolling the steel sheet such that a section in which a surface temperature of the steel sheet is 400°C or higher is included, and obtaining a warm-rolled sheet; and

final annealing the warm-rolled sheet.
The method of claim 10 or 11, wherein the slab further includes one or more of P: 0.1% or less, Cr: 0.01 to 0.5%, Ni: 0.05% or less, Cu: 0.005 to 0.2% and Zn: 0.01% or less.
The method of claim 10 or 11, wherein the slab further includes one or more of Mo: 0.03% or less, B: 0.002% or less, Mg: 0.005% or less, Ca: 0.005% or less and Zr: 0.005% or less.
The method of claim 10 or 11, wherein the slab further includes 0.20% or less (excluding 0%) of one or more of Bi, Pb, Ge and As individually or in combination.
The method of claim 10 or 11, wherein the heating the slab is performed at 1050 to 1200°C.
The method of claim 10 or 11, wherein the finishing hot-rolling is performed at 800 to 950°C.
The method of claim 10 or 11, wherein the final annealing is performed at 850°C or lower.
The method of claim 10 or 11, wherein the final annealing is performed in an atmosphere in which hydrogen (H₂) and nitrogen (N₂) gases are mixed.