US20190112697A1 - Electrical steel sheet and method of producing the same - Google Patents

Electrical steel sheet and method of producing the same Download PDF

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US20190112697A1
US20190112697A1 US16/089,734 US201716089734A US2019112697A1 US 20190112697 A1 US20190112697 A1 US 20190112697A1 US 201716089734 A US201716089734 A US 201716089734A US 2019112697 A1 US2019112697 A1 US 2019112697A1
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steel sheet
concentration
thickness
electrical steel
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Tatsuhiko Hiratani
Yoshihiko Oda
Yoshiaki Zaizen
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JFE Steel Corp
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
    • C23C10/06—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases
    • C23C10/08—Solid state diffusion of only metal elements or silicon into metallic material surfaces using gases only one element being diffused
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest
    • C21D8/1255—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest with diffusion of elements, e.g. decarburising, nitriding
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C10/00—Solid state diffusion of only metal elements or silicon into metallic material surfaces
    • C23C10/60—After-treatment
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/147—Alloys characterised by their composition
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
    • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
    • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
    • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
    • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
    • H01F1/147—Alloys characterised by their composition
    • H01F1/14766—Fe-Si based alloys
    • H01F1/14775—Fe-Si based alloys in the form of sheets
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • This disclosure relates to an electrical steel sheet used to produce iron cores included in high-frequency transformers, reactors, motors and the like for power electronics and a method of producing the electrical steel sheet.
  • the iron loss of an electrical steel sheet consists of the hysteresis loss of the electrical steel sheet, which is strongly dependent on a precipitate included in the steel, the size of crystal grains of the steel, the texture of the steel and the like, and the eddy-current loss of the electrical steel sheet, which is strongly dependent on the thickness, specific resistance, magnetic domain structure and the like of the steel sheet.
  • the content of impurities in the steel is reduced to a minimum level to facilitate the growth of crystal grains and thereby reduce hysteresis loss.
  • the hysteresis loss of an electrical steel sheet accounts for a large part of the iron loss of the electrical steel sheet.
  • the eddy-current loss of the electrical steel sheet becomes dominant, since eddy-current loss increases in proportion to the square of frequency while hysteresis loss increases in proportion to frequency.
  • Switching devices having an operating frequency of a few kilohertz to 50 kilohertz have been used in a power source having a relatively large capacity not only in the fields of automobiles and air conditioners, but also in the field of new energy sources such as photovoltaic power generation. Accordingly, an iron core material having a further low iron-loss at high frequencies has been anticipated.
  • ultrathin electrical steel sheets having a thickness of 0.1 mm or less, high-Si electrical steel sheets, dust cores formed of a compact of iron powder and the like have been used.
  • Mn—Zn ferrite which has a specific resistance several orders of magnitude higher than the specific resistances of soft magnetic metal materials, has been used.
  • an ultrathin electrical steel sheet having a thickness of 0.1 mm does not always have a sufficiently low eddy-current loss. It is not easy to produce a high-Si electrical steel sheet having a Si concentration of more than 4% by mass because such a steel sheet is hard and brittle. Since a dust core has a significantly higher hysteresis loss than an electrical steel sheet, the iron loss of a dust core considerably increases at frequencies of a few kilohertz.
  • Mn—Zn ferrite While Mn—Zn ferrite has a markedly low eddy-current loss, the saturation magnetic flux density of Mn—Zn ferrite is 0.5 T at most, which is significantly lower than the saturation magnetic flux density (2.0 T) of common electrical steel sheets. Therefore, when a power source having a large capacity is prepared using Mn—Zn ferrite, the size of the core needs to be increased disadvantageously.
  • Japanese Examined Patent Application Publication No. 6-45881 discloses a method of reducing the iron loss of an electrical steel sheet at high frequencies.
  • a 6.5-mass % Si steel sheet is produced by a siliconizing process.
  • a 3-mass % Si steel sheet having a thickness of 0.05 to 0.3 mm is caused to react with a silicon tetrachloride gas at a high temperature to increase the Si concentration in the steel.
  • a 6.5-mass % Si steel sheet has a specific resistance about double the specific resistance of a 3-mass % Si steel sheet, which enables an effective reduction in eddy current loss, and is advantageously used in a high-frequency application.
  • the magnetostriction of the 6.5-mass % Si steel sheet is substantially zero, the level of noise generated by an iron core may be markedly reduced.
  • Japanese Examined Patent Application Publication No. 5-49744 discloses a steel sheet in which the Si concentration changes in the thickness direction, that is, a “Si-gradient steel sheet”, that can be produced by, in a siliconizing process, pausing uniform diffusion of Si upon the Si concentration in the surface layer reaching 6.5% by mass.
  • the Si-gradient steel sheet has a lower iron loss at high frequencies than a steel sheet having a uniform Si concentration.
  • an electrical steel sheet containing 3% by mass or more Si does not transform into the austenite phase ( ⁇ phase) even when heated to a high temperature and remains in the ferrite phase ( ⁇ phase) until a liquid phase is formed. Therefore, the above-described siliconizing process is entirely performed in the ⁇ phase.
  • Japanese Unexamined Patent Application Publication No. 2000-328226 discloses an electrical steel sheet for motors having high workability and excellent high-frequency properties and in which the average Si concentration over the entire thickness of the steel sheet is 0.5% to 4% by mass, which is at a low level.
  • the electrical steel sheet is produced by siliconizing only the surface layer of a steel sheet containing less than 3% by mass Si at 900° C. to 1000° C.
  • Japanese Patent No. 5533801 and Japanese Patent No. 5648335 disclose a technique in which excellent magnetic properties are achieved by diffusing a ferrite-formation element from the surface of a steel sheet toward the inner austenite phase to transform the austenite phase into the ferrite phase and form a microstructure strongly accumulated in a particular crystal plane.
  • Japanese Unexamined Patent Application Publication No. 2015-61941 discloses a technique in which excellent magnetic properties are achieved by creating a portion of a steel sheet having a composition capable of causing ⁇ - ⁇ transformation and at which an element other than Fe is concentrated, the portion extending partially in the thickness direction, and thereby reducing the residual stress generated in the surface of the steel sheet.
  • Japanese Patent No. 5655295 discloses that it is possible to markedly reduce eddy-current loss by siliconizing a low-carbon steel sheet in the temperature range of 1050° C. to 1250° C., which is the austenite phase region, and cooling the siliconized steel sheet while only the Si concentration in the surface layer is maintained to be high to produce a Si-gradient steel sheet.
  • Japanese Patent No. 5644680 discloses a technique in which a steel sheet containing 0.003% to 0.02% by mass C which can be transformed into the austenite phase when heated to a high temperature is siliconized to produce a clad electrical steel sheet having excellent magnetic properties.
  • iron loss is the sum of hysteresis loss and eddy-current loss. It is known that, the higher the excitation frequency, the higher the proportion of eddy-current loss in the total iron loss.
  • the higher the specific resistance of a material the higher the resistance to eddy current passing through the material. Therefore, a material having a high specific resistance is used to produce a core used at high frequencies.
  • Si is primarily added to an electrical steel sheet to increase the specific resistance of the electrical steel sheet.
  • Si concentration in the material exceeds 4% by mass, the material becomes significantly brittle, which makes it difficult to cold-roll the material. Accordingly, the maximum amount of Si added to a steel sheet is normally set to about 4% by mass.
  • 1% to 4% by mass of Al and Cr are further added to the steel sheet.
  • a material for cores used at high frequencies is commonly designed in consideration of a certain amount of direct-current component of the excitation current and magnetic saturation of the material caused by a high current that may instantaneously pass through the material. It is necessary to increase the size of the core to compensate for the reduction in the saturation magnetic flux density of the material.
  • the material that is, the 6.5-mass % Si steel sheet
  • the material that is, the 6.5-mass % Si steel sheet
  • the saturation magnetic flux density of the steel sheet is about 1.80 T, which is at a low level.
  • a steel sheet in which the Si concentration changes in the thickness direction that is, a Si-gradient steel sheet.
  • the Si-gradient steel sheet has more excellent high-frequency properties than a 6.5-mass % Si steel sheet. While the Si concentration in the surface layer of the Si-gradient steel sheet is about 6.5%, which is at a high level, the Si concentration in the sheet-thickness center layer about 3% to 4% by mass, which is at a low level, and the average Si concentration over the entire steel sheet is low. Therefore, the Si-gradient steel sheet has higher workability than a 6.5-mass % Si steel sheet and has a high saturation magnetic flux density of 1.85 to 1.90 T.
  • Si-gradient steel sheet is an ultrathin steel sheet
  • Si atoms may reach the center of the steel sheet in the thickness direction during the siliconizing process and, consequently, the Si concentration over the entire steel sheet may be increased.
  • a material containing less than 3% Si is used to produce a steel sheet in which the Si concentration changes in the thickness direction to reduce the average Si concentration over the entire steel sheet and produce a high-frequency low iron-loss material having good workability.
  • a material containing Si at a low concentration can be transformed into the austenite ( ⁇ ) phase at high temperatures.
  • the material is siliconized at a high temperature exceeding 1000° C., that is, in the ⁇ phase, cracking may occur at the ⁇ / ⁇ transformation interface in the surface layer. Accordingly, the siliconizing process is performed in the temperature range of 900° C. to 1000° C., in which the austenite phase is hardly formed.
  • the above siliconizing process is an extension of the known siliconizing process performed in the ⁇ phase, and a reduction effect in eddy-current loss which can be achieved by the above siliconizing process will also be within an expected range.
  • the soft magnetic properties of a steel sheet are enhanced by diffusing a ferrite-formation element from the surface of the steel sheet toward the inner austenite phase and forming a particular texture through the use of the ⁇ transformation.
  • the change in texture markedly affects hysteresis loss, which accounts for a part of iron loss, it does not markedly affect eddy-current loss. Therefore, it appears that changing the texture of a steel sheet is not an effective way to reduce eddy-current loss which accounts for a large part of iron loss at high frequencies.
  • developing a texture effective to reduce hysteresis loss may increase the width of magnetic domain and, consequently, increase abnormal eddy-current loss.
  • the soft magnetic properties of a steel sheet are enhanced by changing the concentration of an element other than Fe in the thickness direction of the steel sheet and limiting the residual stress generated in the surface of the steel sheet to a low level.
  • the method in which residual stress is reduced to limit an increase in the hysteresis loss of a soft magnetic material has been practiced for a long time.
  • the relationship between a reduction in residual stress and a reduction in eddy-current loss is not clear.
  • the magnetic flux density B8 that corresponds to a magnetizing force of 800 A/m in a magnetization curve is about 0.75 T at most.
  • the dimensions of a core material used in practice are determined in accordance with the magnetic flux density at which the differential permeability starts rapidly decreasing in the magnetization curve, that is, the height of the shoulder of the B-H curve.
  • the B8 value is likely to be used as an index of such a magnetic flux density. Therefore, a material having poor direct-current magnetic properties and a low B8 value is substantially not suitable to reduce the size of a core even if the saturation magnetic flux density of the material is high.
  • An electrical steel sheet including, with a symmetry plane being the center of the steel sheet in the thickness direction, a surface part in which the Si concentration in the steel sheet changes continuously from a high Si concentration to a low Si concentration in the thickness direction of the steel sheet from the surface of the steel sheet, a boundary part in which the Si concentration changes discontinuously, and an inner part in which the Si concentration does not change substantially in the thickness direction of the steel sheet, the inner part including the center of the steel sheet in the thickness direction, the electrical steel sheet having a stress distribution such that an in-plane tensile stress is generated in the surface part and an in-plane compressive stress is generated in the inner part, the average aspect ratio of crystal grains included in the surface part, that is, the ratio of the dimension of the crystal grains in a direction parallel to the surface of the steel sheet to the dimension of the crystal grains in a direction (depth direction) perpendicular to the surface of the steel sheet, being 0.7 or more and 4.0 or less,
  • the average aspect ratio is the average of aspect ratios of 50 or more crystal grains and, when a crystal grain included in the surface part extends to the inner part beyond the boundary part, the dimension of the crystal grain in the direction (depth direction) perpendicular to the surface of the steel sheet is determined taking a portion of the crystal grain which is included in the inner part into account.
  • a method of producing an electrical steel sheet including: heating a steel sheet to 1100° C. to 1250° C. in a non-oxidizing atmosphere to transform the steel sheet into the austenite phase, the steel sheet having a composition containing, by mass, C: 0.020% or less, Si: 0.15% to 2.0%, Mn: 0.05% to 2.00%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.01% or less, with the balance being Fe and inevitable impurities; subsequently causing Si to penetrate the surface of the steel sheet at 1100° C.
  • An electrical steel sheet having a high saturation magnetic flux density and a low iron-loss at high frequencies may be produced. It is possible to produce an electrical steel sheet having a high saturation magnetic flux density, a low iron-loss at high frequencies, and consistent properties. Consequently, an iron core material that enables a reduction in the size of high-frequency transformers and the like may be provided.
  • Our steel sheets can be suitably used to produce an iron core included in a high-frequency transformer, a reactor, a motor or the like for power electronics.
  • FIG. 1 is a diagram illustrating the basic structure of a Si-gradient steel sheet.
  • FIG. 2 is a diagram illustrating the relationship between the average aspect ratio, b/a, of crystal grains included in the surface part and iron loss.
  • FIG. 3 is a diagram illustrating the aspect ratio, b/a, of a crystal grain included in the surface part.
  • the Si-gradient steel sheet illustrated in FIG. 1 is an electrical steel sheet that includes, with a symmetry plane being the center of the steel sheet in the thickness direction, a surface part in which the Si concentration in the steel sheet changes continuously from a high Si concentration to a low Si concentration in the thickness direction from the surface, a boundary part in which the Si concentration changes discontinuously, and an inner part in which the Si concentration does not change substantially in the thickness direction, the inner part including the center of the steel sheet in the thickness direction.
  • the Si-gradient steel sheet has a specific stress distribution such that an in-plane tensile stress is generated in the surface part and an in-plane compressive stress is generated in the inner part, which reduces iron loss at high frequencies.
  • samples each of which included a surface part constituted of a different form of crystal grains were prepared and the properties of the samples were determined.
  • a cold-rolled steel sheet having a thickness of 0.2 mm which contained, by mass, C: 0.0024%, Si: 0.6%, Mn: 0.12%, P: 0.008%, S: 0.003% or less, Al: 0.003%, N: 0.003%, and the balance being Fe and inevitable impurities was prepared.
  • Specimens having a width of 50 mm and a length of 200 mm were taken from the cold-rolled steel sheet. The specimens were siliconized and subjected to a diffusion treatment.
  • the conditions under which the siliconizing process and the diffusion treatment were performed were adjusted such that the amount of silicon used, that is, the amount of silicon added to the steel sheet in the siliconizing process, was 2.4% ⁇ 0.2% or less and the ratio of the thickness, ds, of the surface part, that is, the Si-concentrated layer, to the thickness, d0, of the steel sheet was 30% ⁇ 3% or less.
  • the width of the samples was reduced to 30 mm by shearing both ends of the samples.
  • the samples were subsequently subjected to a magnetic measurement in accordance with a method (Epstein test method) conforming to JIS C2550 with a small single-sheet testing frame. After the magnetic measurement had been terminated, the samples were further sheared. Subsequently, the microstructure of a cross section of each of the samples was determined with an optical microscope, and the Si distribution in the sample in the thickness direction was determined by EPMA.
  • the form of crystal grains included in the surface part can be adjusted by changing the conditions under which the siliconizing process is performed. For example, the higher the temperature at which the siliconizing process is performed within the austenite temperature range of the material (steel sheet) or the lower the concentration of the silicon tetrachloride gas, the higher the likelihood of the size of crystal grains included in the surface layer increasing in the direction parallel to the surface of the steel sheet. In contrast, the lower the temperature at which the siliconizing process is performed within the austenite temperature range of (steel sheet) or the higher the concentration of the silicon tetrachloride gas, the higher the likelihood of the size of crystal grains included in the surface part increasing in the thickness direction of the steel sheet.
  • the dimension of a crystal grain included in the surface part which is measured in the direction parallel to the surface is denoted by b
  • the dimension of the crystal grain which is measured in the thickness direction of the steel sheet is denoted by a.
  • the above dimensions of each of 50 or more crystal grains included in the surface layer were measured, and the aspect ratio, b/a, of each of the crystal grains was calculated. The average thereof was considered to be the representative value (average aspect ratio, b/a, of crystal grains included in the surface part) of the sample.
  • FIG. 3 is a cross-sectional view of the steel sheet which is taken in the L-direction (rolling direction), schematically illustrating the aspect ratio, b/a, of a crystal grain included in the surface part.
  • a and b denote the maximum dimension of each of the crystal grains measured in the thickness direction and the maximum dimension of the crystal grain measured in the direction parallel to the surface, respectively.
  • FIG. 2 illustrates the relationship between the average aspect ratio, b/a, of crystal grains included in the surface part (in FIG. 2 , this ratio is abbreviated as “average aspect ratio in surface part b/a”) and iron loss.
  • the samples prepared in the test had a ratio, b/a, of 0.5 to 4.5.
  • the iron losses of the samples were generally above a certain level regardless of the average aspect ratio of crystal grains included in the surface part, and reduction effect in iron loss were not confirmed.
  • the electrical steel sheet is a Si-gradient steel sheet produced by heating a steel sheet containing Si at a low concentration to the high-temperature austenite phase, increasing the Si concentration in the surface layer by siliconizing process and diffusion treatment, transforming the surface layer into the ferrite phase, and cooling the steel sheet such that the austenite phase having a low Si concentration remains in the inner layer.
  • the electrical steel sheet includes, with a symmetry plane being the center of the steel sheet in the thickness direction, a surface part in which the Si concentration in the steel sheet changes continuously from a high Si concentration to a low Si concentration in the thickness direction from the surface, a boundary part in which the Si concentration changes discontinuously, and an inner part in which the Si concentration does not change substantially in the thickness direction, the inner part including the center of the steel sheet in the thickness direction.
  • the inner part in which the Si concentration does not change substantially in the thickness direction and that includes the center of the steel sheet in the thickness direction is a part of the steel sheet extending from the boundary part to the center of the steel sheet in the thickness direction and in which the difference between the maximum and minimum Si concentrations in the region between two boundary parts is less than ⁇ 0.1%.
  • the boundary part in which the Si concentration changes discontinuously is a part of the steel sheet in which the Si concentration changes by 0.2% or more in a region having a thickness of ⁇ 1 ⁇ m or less and the minimum Si concentration in the surface part and the maximum Si concentration in the inner part occur discontinuously.
  • the electrical steel sheet has a stress distribution such that an in-plane tensile stress is generated in the surface part and an in-plane compressive stress is generated in the inner part. It is possible to reduce eddy-current loss and iron loss at high frequencies through the use of the above stress distribution.
  • the electrical steel sheet has a Si concentration distribution with a symmetry plane being the center of the steel sheet in the thickness direction. If the distribution of Si concentration in the steel sheet extending from the front surface to the rear surface is asymmetrical, the steel sheet may become significantly warped and the shape of the steel sheet may become degraded. Furthermore, the stress distribution such that an in-plane tensile stress is generated in the surface part and an in-plane compressive stress is generated in the inner part, which is unique to the Si-gradient steel sheet, may become asymmetrical with respect to the center plane of the steel sheet in the thickness direction and, consequently, the reduction effect in eddy-current loss may be limited. In consideration of the shape of the steel sheet and the reduction in iron loss at high frequencies, the difference between the Si concentrations at the front and rear surfaces of the steel sheet is desirably minimized and is preferably 0.2% or less.
  • the electrical steel sheet that is, the Si-gradient steel sheet produced by performing siliconizing process in the austenite phase, includes a discontinuous Si-concentration distribution region formed as a result of ⁇ / ⁇ transformation, that is, the boundary part (Si concentration gap) in which the Si concentration changes discontinuously.
  • the boundary part is a part of the steel sheet in which the Si concentration changes 0.1% or more per 1 micrometer in the thickness direction of the steel sheet (concentration gradient of 0.1%/ ⁇ m or more), that is, in which the Si concentration changes by 0.2% or more in a region within ⁇ 1 ⁇ m or less in the thickness direction.
  • the Si concentration gap which exists in the boundary part interposed between the surface part and the inner part, enables magnetic flux to concentrate at the surface part and thereby suitably reduces eddy-current loss.
  • cracking is likely to occur at the interfaces upon an impact force similar to the force generated in shearing process being applied to the steel sheet.
  • the cracks do not lead to the fracture of the material because they do not propagate over the entire steel sheet and remain in a narrow region, the cracks cause variations in magnetic properties and, in particular, variations in iron loss.
  • the average aspect ratio of crystal grains included in the surface part that is, the ratio of the dimension of crystal grains measured in the parallel-to-surface direction to the dimension of the crystal grains measured in the perpendicular-to-surface direction (depth direction), is specified.
  • the average aspect ratio of crystal grains included in the surface part is limited to 0.7 or more and 4.0 or less. This reduces the degree of variations in iron loss and enables the specific stabilization to be achieved.
  • Ratio of Dimension of Crystal Grains in Parallel-to-Surface Direction to Dimension of Crystal Grains in Perpendicular-to-Surface Direction (Depth Direction) is 0.7 or More and 4.0 or Less
  • the average aspect ratio, b/a, of crystal grains included in the surface part is the factor significantly important to the Si-gradient steel sheet. If the ratio, b/a, is less than 0.7, cracking and chipping may occur at the boundaries of crystal grains included in the surface part upon the steel sheet being sheared and, consequently, the degree of variations in iron loss may be increased to a significant level. If the ratio, b/a, is more than 4.0, cracking is likely to occur at the boundary part interposed between the surface part and the inner part upon the steel sheet being sheared and, consequently, the degree of variations in iron loss may be increased to a significant level. When the ratio, b/a, is 0.7 or more and 4.0 or less, such cracking hardly occurs and it is possible to reduce the degree of variations in iron loss to a markedly low level.
  • the average aspect ratio is the average of the aspect ratios of 50 or more crystal grains.
  • depth direction the dimension of the crystal grain in the perpendicular-to-surface direction
  • the texture of the surface part and the texture of the inner part are not limited and may be a microstructure constituted of crystal grains randomly oriented or highly accumulated in a particular plane or orientation.
  • the electrical steel sheet in which the Si concentration distributions in the surface part and the inner part are clearly different from each other, is constituted of randomly oriented crystal grains, cracking is less likely to occur at the crystal grains included in the surface layer having a high Si concentration and at the boundary part in which the Si concentration changes discontinuously upon the steel sheet being, for example, sheared, because the dislocation movements of the crystal grains are averaged.
  • the crystal grains are preferably randomly oriented.
  • Thickness of Surface Part 10% to 40% of Thickness of Steel Sheet (Preferable Condition)
  • the surface part may become almost magnetically saturated, which results in a reduction in magnetic permeability, when the excitation magnetic flux density is low. As a result, the inner part also starts becoming magnetized, which limits the reduction effect in eddy-current loss.
  • the thickness of the surface part is more than 40% of the thickness of the steel sheet, a large part of the steel sheet extending from the surface to a region around the center of the steel sheet in the thickness direction becomes magnetized and, consequently, a magnetic flux distribution similar to that formed in a Si-uniform material is formed, which limits the reduction effect in eddy current.
  • the thickness of the surface part is preferably 10% or more and 40% or less and is more preferably 20% or more and 35% or less of the thickness of the steel sheet.
  • the average Si concentration in the surface part is less than 2.5%, the reduction effect in eddy current may fail to be achieved at a sufficient level. If the average Si concentration in the surface part exceeds 6.5%, the likelihood of cracking in the surface layer may rapidly increase. Accordingly, the average Si concentration in the surface part is preferably 2.5% to 6.5%.
  • the average Si concentration in the inner part exceeds 2.0%, the likelihood of a discontinuous Si concentration distribution (boundary part) being formed at the boundary between the surface part and the inner part is small and, consequently, the reduction effect in eddy-current loss may fail to be achieved at a sufficient level. Accordingly, the average Si concentration in the inner part is preferably 2.0% or less.
  • the average Si concentration in the inner part is less than 0.15%, crystal grains included in the surface part are likely to grow in a slender shape elongated in the thickness direction of the steel sheet, the average aspect ratio, b/a, of crystal grains included in the surface part is likely to be less than 0.7 even when the conditions under which the siliconizing process and the conditions under which the diffusion treatment are performed are adjusted and, consequently, cracking is likely to occur in the surface layer. Accordingly, the average Si concentration in the inner part is preferably 0.15% or more.
  • the Si concentration gap in a region of the boundary part which separates the surface part and the inner part from each other, the region extending ⁇ 1 ⁇ m or less in the thickness direction of the steel sheet, is 0.4% or more, the reduction effect in eddy-current loss increases by 10% or more compared to when the Si concentration distribution is made completely uniform. If the Si concentration gap in the boundary part is less than 0.4%, the accumulation of magnetic flux at the surface part may fail to be achieved at a sufficient level because also the inner part is likely to be magnetized and, consequently, the reduction effect in eddy-current loss may fail to be achieved at a sufficient level. Accordingly, the Si concentration gap in the boundary part is preferably 0.4% or more.
  • the minimum Si concentration in the boundary part corresponds to the Si concentration in the inner part.
  • the maximum Si concentration in the boundary part corresponds to a possible minimum Si concentration in the surface part (a phase) which may occur in the temperature range in which the siliconizing process and the diffusion treatment are performed.
  • a stress distribution such that a tensile stress is generated in the surface part and a compressive stress is generated in the inner part is created to reduce eddy-current loss. It is preferable to set the tensile stress generated in the surface part to be 50 MPa or more and the compressive stress generated in the inner part to be 50 MPa or more to reduce eddy-current loss at a significant level (by 10% or more) compared to a Si-uniform steel sheet having the same thickness and the same average Si concentration.
  • the tensile stress generated in the surface part exceeds 200 MPa and the compressive stress generated in the inner part exceeds 200 MPa, severe cracking may occur during shearing process, which increases the degree of variations in iron loss, even when the aspect ratio of crystal grains included in the surface part falls within the desired range.
  • the tensile stress generated in the surface part is preferably 50 to 200 MPa
  • the compressive stress generated in the inner part is preferably 50 to 200 MPa.
  • Thickness of Steel Sheet 0.03 to 0.5 mm (Preferable Condition)
  • the thickness of the steel sheet is preferably 0.03 to 0.5 mm.
  • the above-described electrical steel sheet can be produced by heating a steel sheet to 1100° C. to 1250° C. in a non-oxidizing atmosphere to transform the steel sheet into the austenite phase, the steel sheet having a composition containing, by mass, C: 0.020% or less, Si: 0.15% to 2.0%, Mn: 0.05% to 2.00%, P: 0.1% or less, S: 0.01% or less, Al: 0.1% or less, and N: 0.01% or less, with the balance being Fe and inevitable impurities; subsequently causing Si to penetrate the surface of the steel sheet at 1100° C. to 1250° C.
  • a non-oxidizing atmosphere containing 10 mol % or more and less than 45 mol % silicon tetrachloride to transform a surface layer of the steel sheet into the ferrite phase; subsequently holding the steel sheet for a predetermined amount of time at 1100° C. to 1250° C. in a non-oxidizing atmosphere that does not contain Si until the thickness of the surface part that is in the ferrite phase reaches 10% to 40% of the thickness of the steel sheet, while maintaining the austenite phase in the inner part; and subsequently cooling the steel sheet to 400° C. at an average cooling rate of 5 to 30° C./s.
  • the C concentration in the material is limited to 0.020% or less. While the lower limit for the C concentration is not specified, a steel having an excessively low solute C concentration is likely to undergo intergranular fracture similarly to ultralow-carbon steel. Accordingly, the C concentration is preferably 0.0005% to 0.020%.
  • the Si concentration in the material is less than 0.15%, slender crystal grains that are elongated in the thickness direction of the steel sheet and have an aspect ratio of less than 0.7 are likely to be formed in the surface layer during the siliconizing process and the diffusion treatment. This increases the likelihood of cracking occurring during shearing process and the degree of variations in iron loss.
  • the Si concentration in the material exceeds 2.0%, the likelihood of the discontinuous Si concentration distribution (boundary part) being created at the boundary between the surface part and the inner part is small and, consequently, the reduction effect in eddy-current loss may fail to be achieved at a sufficient level.
  • the Si concentration in the material is limited to 0.15% to 2.0%.
  • Mn is an element effective to improve the toughness of steel. In steel, Mn bonds to S and precipitates in the form of MnS. If the Mn concentration in the material is less than 0.05%, the intergranular segregation of S may occur, which increases the likelihood of intergranular fracture occurring in the crystal grains included in the surface part having a high Si concentration. Mn is also an element that stabilizes the austenite phase. If the Mn concentration in the material exceeds 2.00%, a large transformation strain is likely to remain in the inner part when the inner part is transformed from the austenite phase to the ferrite phase during the cooling process performed subsequent to the siliconizing process and the diffusion treatment. This transformation strain disturbs the stress distribution created in the Si-gradient steel sheet and thereby limits the reduction effect in eddy current. Accordingly, the Mn concentration in the material is limited to 0.05% to 2.00%.
  • P is an element effective to strengthen steel, but also an element that causes embrittlement. Moreover, Mn may segregate at the phase-transformation interfaces.
  • the P concentration is 0.1% or less, the occurrence of intergranular cracking in the surface part and the occurrence of cracking in the boundary part can be reduced substantially to an insignificant level. Accordingly, the P concentration in the material is limited to 0.1% or less.
  • S is an element that is likely to segregate at grain boundaries, it is preferable to minimize the S concentration to prevent embrittlement.
  • the S concentration is 0.01% or less, the occurrence of cracking is reduced to a substantially insignificant level. Accordingly, the Si concentration in the material is limited to 0.01% or less.
  • Al is an element that increases the specific resistance of steel.
  • Al is added to an electrical steel sheet in combination with Si.
  • Si is an element that reduces the lattice spacing of Fe crystals
  • Al is an element that increases the lattice spacing of Fe crystals.
  • adding Al to the Si-gradient steel sheet disadvantageously mitigates the stress distribution suitable for reducing eddy current, which is formed by the addition of Si.
  • the adverse effect is not produced when the Al concentration is 0.1% or less.
  • the Al concentration in the material is limited to 0.1% or less.
  • the lower limit for the Al concentration is not specified, limiting the Al concentration to be less than 0.002% increases formation of a microstructure including crystal grains having various sizes, which increases iron loss.
  • the upper limit for the Al concentration is also not limited, it is advantageous to limit the Al concentration to 0.01% or less in consideration of workability. Accordingly, the Al concentration is preferably 0.002% to 0.01%.
  • N is limited to 0.01% or less.
  • the balance includes Fe and inevitable impurities.
  • a slab having the above-described composition is heated, hot-rolled, and repeatedly cold-rolled to form a steel sheet having a predetermined thickness.
  • Intermediate annealing may be performed once or twice or more between the cold-rolling steps.
  • Finish annealing may optionally be performed.
  • the steel sheet is heated to 1100° C. to 1250° C. in a non-oxidizing atmosphere to be transformed into the austenite phase.
  • Si is caused to penetrate the surface of the steel sheet at 1100° C. to 1250° C.
  • the steel sheet is held for a predetermined amount of time at 1100° C. to 1250° C. in a non-oxidizing atmosphere that does not contain Si until the thickness of the surface layer that is in the ferrite phase reaches 10% to 40% of the thickness of the steel sheet, while the austenite phase is maintained in the inner part. Subsequently, the steel sheet is cooled to 400° C. at an average cooling rate of 5 to 30° C./s.
  • the high-temperature steel sheet that is in the austenite phase is subjected to the siliconizing process and the diffusion treatment to transform only the surface part into a high-Si ferrite phase while maintaining the inner part to be in the austenite phase and, subsequently, the steel sheet is cooled to room temperature to transform the inner part into the ferrite phase.
  • an electrical steel sheet including, with a symmetry plane being the center of the steel sheet in the thickness direction, a surface part in which the Si concentration in the steel sheet changes continuously from a high Si concentration to a low Si concentration in the thickness direction of the steel sheet from the surface of the steel sheet, a boundary part in which the Si concentration changes discontinuously, and an inner part in which the Si concentration does not change substantially in the thickness direction of the steel sheet, the inner part including the center of the steel sheet in the thickness direction can be produced.
  • the conditions under which the siliconizing process is performed are one of the elements important to produce the electrical steel sheet.
  • Examples of a method of causing Si to penetrate the steel sheet (for siliconizing the steel sheet) include publicly known methods such as a gas-phase siliconizing process, a liquid-phase siliconizing process, and a solid-phase siliconizing process.
  • a Si-containing gas used in the process is not limited and is preferably, for example, one or two or more gases selected from silicon tetrachloride, trichlorosilane, dichlorosilane, monosilane, and disilane.
  • a gas-phase siliconizing process in which the steel sheet is heated in a non-oxidizing atmosphere and a silicon tetrachloride gas is used is described.
  • the amount of Si added to the steel sheet and the Si concentration distribution in the steel sheet by adjusting the concentration of the silicon tetrachloride gas in the non-oxidizing atmosphere such as, nitrogen or argon, the temperature at which the reaction is performed in the non-oxidizing atmosphere, the amount of time for which the reaction is performed in the non-oxidizing atmosphere, the temperature at which the subsequent diffusion treatment is performed in a non-oxidizing atmosphere that does not contain a silicon tetrachloride gas, and the amount of time for which the diffusion treatment is performed.
  • the non-oxidizing atmosphere such as, nitrogen or argon
  • the steel sheet To add a predetermined amount of Si to the steel sheet in a short time, it is preferable to produce the steel sheet using a silicon tetrachloride gas at a high temperature and a high concentration. To adjust the amount of Si added to the steel sheet and the Si concentration distribution in the steel sheet with high accuracy, it is preferable to produce the steel sheet using a silicon tetrachloride gas at a low temperature and a low Si concentration.
  • the siliconizing process is performed in the high-temperature austenite phase
  • the concentration of silicon tetrachloride in the non-oxidizing atmosphere has been set to about 50 to 75 mol % in consideration of the efficiency of the siliconizing process.
  • the silicon tetrachloride concentration is increased to the above level, the siliconizing rate is increased and the crystal grains included in the surface layer which has been transformed into the ferrite phase are likely to grow in the thickness direction of the steel sheet and have a small aspect ratio b/a.
  • the silicon tetrachloride concentration is limited to 10 mol % or more and less than 45 mol % to adjust the aspect ratio b/a of crystal grains included in the surface layer of the Si-gradient steel sheet to be 0.7 or more and 4.0 or less and to thereby reduce the occurrence of defects in shearing process and the degree of variations in iron loss.
  • the siliconizing process is performed at less than 1100° C., the sufficient tensile strength may fail to be generated in the surface part and, consequently, the reduction effect in eddy current may be limited.
  • the siliconizing process is performed at more than 1250° C., a liquid phase may disadvantageously be formed in a portion of the surface part which has the highest Si concentration. This may lead to the rupture, wrinkling, and warpage of the steel sheet. Accordingly, the temperature at which the siliconizing process is performed is limited to 1100° C. to 1250° C.
  • a diffusion treatment in which the steel sheet is maintained for a predetermined amount of time at 1100° C. to 1250° C. in a non-oxidizing atmosphere that does not contain Si until the thickness of the surface part that is in the ferrite phase reaches a predetermined thickness is performed. Specifically, the diffusion treatment is performed until the thickness of the surface part that is in the ferrite phase reaches 10% to 40% of the thickness of the steel sheet.
  • the steel sheet is cooled to 400° C. at an average cooling rate of 5 to 30° C./s. If the average cooling rate is less than 5° C./s, relaxation of the internal stress may occur and, consequently, the reduction effect in eddy-current loss may fail to be achieved at a sufficient level. On the other hand, if rapid cooling is performed at a cooling rate of more than 30° C./s, the microstructure of the inner part of the steel sheet may become distorted in various directions. This significantly degrades soft magnetic properties. Accordingly, to achieve good direct-current magnetic properties, it is necessary to perform cooling at an average cooling rate of 5 to 30° C./s until the temperature reaches at least 400° C.
  • Ingots having the compositions described in Table 1 with the balance being Fe and inevitable impurities were heated to 1100° C., hot-rolled to a thickness of 2.3 mm, and then cold-rolled to a thickness of 0.2 mm.
  • Specimens that were to be siliconized and had a width of 50 mm and a length of 150 mm were taken from each of the resulting cold-rolled steel sheets.
  • the specimens were heated in an argon atmosphere from room temperature to the temperature range of 1100° C. to 1225° C., in which the austenite phase is formed, while the specimens were transported.
  • an argon gas containing silicon tetrachloride at a concentration of 8% to 66% by volume was charged into the furnace, and a siliconizing process was performed at the same temperature as above for 1 to 6 minutes.
  • the atmosphere in the furnace was replaced with a non-oxidizing atmosphere that was an argon gas that did not contain silicon tetrachloride, and a diffusion treatment was performed at 1100° C. to 1250° C. for 2 to 30 minutes.
  • the amount of silicon used that is, the amount of Si added to each of the steel sheets, was adjusted by changing the concentration of silicon tetrachloride in the atmosphere and the amount of time during which the treatment was performed.
  • the Si concentration distribution in a cross section of each of the steel sheets was determined by EPMA (electron beam microanalyzer). For each of Test Nos., 12 samples having the same shape were prepared under the same treatment conditions.
  • samples that had been subjected to the siliconizing process and the diffusion treatment were then transported in a nitrogen atmosphere to the room temperature region to be cooled to 400° C. or less at an average cooling rate of 15° C./s.
  • the samples were removed when the temperature reached 100° C. or less.
  • samples prepared under the same conditions contained the same amount of silicon by determining the change in the mass of each of the samples which occurred during the treatments.
  • One of the 12 samples taken from each of Test Nos. was again heated in an argon atmosphere and subjected to an additional heating treatment in the ferrite phase region of 900° C. until the Si distribution in the sample became uniform in the thickness direction of the steel sheet.
  • One of the surfaces of another one of the 12 samples was covered with an adhesive label, and a portion of the sample that extended from the other surface to the center of the sample in the thickness direction was removed by chemical polishing with hydrofluoric acid.
  • the results of observation of warpage of the sample confirmed that the sample had a stress distribution such that a tensile stress was generated in the surface part and a compressive stress was generated in the inner part.
  • Each of the other 10 samples was subjected to a precision shearing machine for thin sheets to cut both ends of the sample at positions 10 mm from the respective ends in the width direction with an appropriate blade clearance.
  • single-sheet samples for magnetic property evaluation which had a width of 30 mm were prepared.
  • a single-sheet testing frame with which a sample having a width of 30 mm and a length of 100 mm can be excited and the magnetic properties of the sample can be evaluated was used and the iron loss (W1/10 k) of each of the samples was measured in accordance with a method (Epstein test method) conforming to JIS C2550.
  • the samples were cut with a high-speed rotary cutter for microstructure testing.
  • the microstructure of each of the samples was determined with an optical microscope.
  • the Si concentration distribution in each of the samples in the thickness direction was determined by EPMA.
  • the average Si concentration in the inner part, the Si concentration at the surface of the steel sheet, the average Si concentration in the surface part, the ratio of the thickness of the surface part to the thickness of the steel sheet, the average aspect ratio of crystal grains included in the surface part, the Si concentration gap in the boundary part, saturation magnetic flux density Bs, the average m of iron losses at high frequencies W1/10 k of samples excited at a magnetic flux density of 0.1 T and 10 kHz, the standard deviation ⁇ thereof, and the coefficient of variation ⁇ /m were measured.
  • the ratio of, to the iron loss of the sample having a uniform Si concentration, the average iron loss of the other samples was 0.9 or less.
  • Ingots having the compositions described in Table 3 with the balance being Fe and inevitable impurities were heated to 1100° C., hot-rolled to a thickness of 2.3 mm, and then cold-rolled to a thickness of 0.5 to 0.08 mm.
  • Specimens that were to be siliconized and had a width of 50 mm and a length of 150 mm were taken from each of the resulting cold-rolled steel sheets.
  • the specimens were heated in an argon atmosphere from room temperature to the temperature range of 1200° C., in which the austenite phase is formed, while the specimens were transported.
  • an argon gas containing silicon tetrachloride at a concentration of 8% to 57% by volume was charged into the furnace, and a siliconizing process was performed at the same temperature as above for 1 to 10 minutes.
  • the atmosphere in the furnace was replaced with a non-oxidizing atmosphere that was an argon gas that did not contain silicon tetrachloride, and a diffusion treatment was performed at 1200° C. for 2 to 40 minutes.
  • the amount of silicon used that is, the amount of Si added to each of the steel sheets, was adjusted by changing the concentration of silicon tetrachloride in the atmosphere and the amount of time during which the treatment was performed.
  • the Si concentration distribution in a cross section of each of the steel sheets was determined by EPMA (electron beam microanalyzer). For each of Test Nos., 11 samples having the same shape were prepared.
  • samples that had been subjected to the above treatments were transported in a nitrogen atmosphere to the room temperature region to be cooled to 400° C. or less at a cooling rate of 15° C./s.
  • the samples were removed when the temperature was reduced to 100° C. or less.
  • samples prepared under the respective conditions contained the same amount of silicon by determining the change in the weight of each of the samples which occurred during the treatments.
  • One of the 11 samples taken from each of Test Nos. was covered with an adhesive label, and a portion of the sample that extended from the other surface to the center of the sample in the thickness direction was removed by chemical polishing with hydrofluoric acid.
  • the results of observation of warpage of the sample confirmed that the sample had a stress distribution such that a tensile stress was generated in the surface part and a compressive stress was generated in the inner part.
  • Each of the other 10 samples was subjected to a precision shearing machine for thin sheets to cut both ends of the sample at positions 10 mm from the respective ends in the width direction with an appropriate blade clearance.
  • single-sheet samples for magnetic property evaluation which had a width of 30 mm were prepared.
  • a single-sheet testing frame with which a sample having a width of 30 mm and a length of 100 mm can be excited and the magnetic properties of the sample can be evaluated was used and the iron loss (W1/10 k) of each of the samples was measured in accordance with a method (Epstein test method) conforming to JIS C2550.
  • the samples were cut with a high-speed rotary cutter for microstructure testing.
  • the microstructure of each of the samples was determined with an optical microscope.
  • the Si concentration distribution in each of the samples in the thickness direction was determined by EPMA.
  • the Si concentration at the surface of the steel sheet, the average Si concentration in the surface part, the ratio of the thickness of the surface part to the thickness of the steel sheet, the average aspect ratio of crystal grains included in the surface part, the Si concentration gap in the boundary part, the average m of iron losses at high frequencies W1/10 k of samples excited at a magnetic flux density of 0.1 T and 10 kHz, the standard deviation ⁇ thereof, and the coefficient of variation ⁇ /m were measured. Table 4 summarizes the results.
  • Ingots having the compositions described in Table 5 with the balance being Fe and inevitable impurities were heated to 1100° C., hot-rolled to a thickness of 2.3 mm, and then cold-rolled to a thickness of 0.2 mm.
  • Specimens that were to be siliconized and had a width of 50 mm and a length of 150 mm were taken from each of the resulting cold-rolled steel sheets.
  • the specimens were heated in an argon atmosphere from room temperature to the temperature range of 1100° C. to 1250° C., in which the austenite phase is formed, while the specimens were transported.
  • an argon gas containing silicon tetrachloride at a concentration of 10% to 30% by volume was charged into the furnace, and a siliconizing process was performed at the same temperature as above for 1 to 6 minutes.
  • the atmosphere in the furnace was replaced with a non-oxidizing atmosphere that was an argon gas that did not contain silicon tetrachloride, and a diffusion treatment was performed at 1100° C. to 1250° C. for 2 to 30 minutes.
  • the amount of silicon used that is, the amount of Si added to each of the steel sheets, was adjusted by changing the concentration of silicon tetrachloride in the atmosphere and the amount of time during which the treatment was performed.
  • the Si concentration distribution in a cross section of each of the steel sheets was determined by EPMA (electron beam microanalyzer). For each of Test Nos., 12 samples having the same shape were prepared.
  • samples that had been subjected to the above treatments were transported in a nitrogen atmosphere to the room temperature region to be cooled to 400° C. or less at a cooling rate of 15° C./s.
  • the samples were removed when the temperature was reduced to 100° C. or less.
  • samples prepared under the respective conditions contained the same amount of silicon by determining the change in the weight of each of the samples which occurred during the treatments.
  • One of the 12 samples taken from each of Test Nos. was again heated in an argon atmosphere and subjected to an additional heating treatment in the ferrite phase region of 900° C. until the Si distribution in the sample became uniform in the thickness direction of the steel sheet.
  • One of the surfaces of another one of the 12 samples was covered with an adhesive label, and a portion of the sample that extended from the other surface to the center of the sample in the thickness direction was removed by chemical polishing with hydrofluoric acid.
  • the results of observation of warpage of the sample confirmed that the sample had a stress distribution such that a tensile stress was generated in the surface part and a compressive stress was generated in the inner part.
  • Each of the other 10 samples was subjected to a precision shearing machine for thin sheets to cut both ends of the sample at positions 10 mm from the respective ends in the width direction with an appropriate blade clearance.
  • single-sheet samples for magnetic property evaluation which had a width of 30 mm were prepared.
  • a single-sheet testing frame with which a sample having a width of 30 mm and a length of 100 mm can be excited and the magnetic properties of the sample can be evaluated was used and the iron loss (W1/10 k) of each of the samples was measured in accordance with a method (Epstein test method) conforming to JIS C2550.
  • the samples were cut with a high-speed rotary cutter for microstructure testing.
  • the microstructure of each of the samples was determined with an optical microscope.
  • the Si concentration distribution in each of the samples in the thickness direction was determined by EPMA.
  • the samples were cut with a high-speed rotary cutter for microstructure testing.
  • the microstructure of each of the samples was determined with an optical microscope.
  • the Si concentration distribution in each of the samples in the thickness direction was determined by EPMA.
  • the average Si concentration in the inner part, the Si concentration at the surface of the steel sheet, the average Si concentration in the surface part, the ratio of the thickness of the surface part to the thickness of the steel sheet, the average aspect ratio of crystal grains included in the surface part, the Si concentration gap in the boundary part, saturation magnetic flux density Bs, the average m of iron losses at high frequencies W1/10 k of samples excited at a magnetic flux density of 0.1 T and 10 kHz, the standard deviation ⁇ thereof, and the coefficient of variation ⁇ /m were measured.
  • the reductions in iron loss were smaller than reductions in the iron loss of the samples having a ratio ds/d0 of 10% to 40%.
  • the ratio of the iron loss of the sample in which the Si concentration gap in the boundary part was 0.1% to the iron loss of the sample having a uniform Si concentration was close to 1. That is, the iron loss of the sample was not reduced substantially by the formation of the Si concentration distribution.
  • the Si concentration gap in the boundary part was 0.2% or more
  • the average aspect ratio of crystal grains included in the surface part was 0.7 or more and 4.0 or less
  • iron loss was reduced by 10% or more compared to when the Si concentration was made uniform.
  • the coefficient of variation was less than 10%. That is, the degree of variations in iron loss was reduced to a sufficiently low level.

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US16/089,734 2016-03-31 2017-03-29 Electrical steel sheet and method of producing the same Abandoned US20190112697A1 (en)

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US11335485B2 (en) * 2017-12-12 2022-05-17 Jfe Steel Corporation Multilayer electrical steel sheet
US11355271B2 (en) 2017-12-12 2022-06-07 Jfe Steel Corporation Multilayer electrical steel sheet
US11401589B2 (en) * 2017-12-12 2022-08-02 Jfe Steel Corporation Multilayer electrical steel sheet
US11551839B2 (en) 2018-05-14 2023-01-10 Jfe Steel Corporation Motor
US11866796B2 (en) 2019-06-17 2024-01-09 Jfe Steel Corporation Grain-oriented electrical steel sheet and production method therefor
US12018357B2 (en) 2019-10-03 2024-06-25 Jfe Steel Corporation Non-oriented electrical steel sheet and method of producing same

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JP7334673B2 (ja) * 2019-05-15 2023-08-29 Ｊｆｅスチール株式会社 無方向性電磁鋼板およびその製造方法
WO2021132378A1 (ja) * 2019-12-25 2021-07-01 Ｊｆｅスチール株式会社 方向性電磁鋼板およびその製造方法

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Publication number Priority date Publication date Assignee Title
US11335485B2 (en) * 2017-12-12 2022-05-17 Jfe Steel Corporation Multilayer electrical steel sheet
US11355271B2 (en) 2017-12-12 2022-06-07 Jfe Steel Corporation Multilayer electrical steel sheet
US11401589B2 (en) * 2017-12-12 2022-08-02 Jfe Steel Corporation Multilayer electrical steel sheet
US11551839B2 (en) 2018-05-14 2023-01-10 Jfe Steel Corporation Motor
US11866796B2 (en) 2019-06-17 2024-01-09 Jfe Steel Corporation Grain-oriented electrical steel sheet and production method therefor
US12018357B2 (en) 2019-10-03 2024-06-25 Jfe Steel Corporation Non-oriented electrical steel sheet and method of producing same

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JPWO2017170749A1 (ja) 2018-04-05
EP3438314B1 (en) 2020-12-30
EP3438314A4 (en) 2019-02-20
KR102129846B1 (ko) 2020-07-03
CN108884535A (zh) 2018-11-23
EP3438314A1 (en) 2019-02-06
JP6319522B2 (ja) 2018-05-09
KR20180120717A (ko) 2018-11-06
WO2017170749A1 (ja) 2017-10-05
CN108884535B (zh) 2020-08-18

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