Classifications

• • 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
  • C21D8/1272—Final recrystallisation annealing
• • 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
  • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
  • C21D8/1233—Cold rolling
• • 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
• • 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
  • 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
  • C21D8/1277—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties involving a particular surface treatment
• • 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
  • C21D8/1277—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties involving a particular surface treatment
  • C21D8/1283—Application of a separating or insulating coating
• • 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
  • C21D8/1277—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties involving a particular surface treatment
  • C21D8/1288—Application of a tension-inducing coating
• • 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
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
  • H01F1/16—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 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
  • C21D2201/00—Treatment for obtaining particular effects
  • C21D2201/05—Grain orientation

Definitions

• Iron-silicon material suitable for medium frequency applications is Iron-silicon material suitable for medium frequency applications
• the present invention relates to a grain-oriented steel strip and to the use of such a strip in electric transformers, in electric motors or in other electric devices, preferably in devices in which magnetic flux has to be channeled or contained.
• sheet or “strip” are used in the present text synonymously to indicate a flat steel product which is obtained by a rolling process an which length and width is much greater than its thickness.
• sheet or “strip” are used in the present text synonymously to indicate a flat steel product which is obtained by a rolling process an which length and width is much greater than its thickness.
• Grain-oriented electrical steel is a soft magnetic material which typically exhibits high silicon contents. GOES has a high permeability to the magnetic field and can be magnetized and demagnetized easily.
• the "iron crystal axis" is defined as an axis of easy magnetization of the body-centered cubic iron crystal. In GEOS sheets or strips this axis is closely aligned to the rolling direction. This distinct orientation results in excellent magnetic properties of the GOES sheet in the rolling direction as well. Those grains of GEOS sheets or strips which axis is aligned in this way are called“Goss grains”. Goss grains provide a strongly anisotropic behavior and reduce the power loss.
• An exemplary production route includes in the following manufacturing steps: Producing a steel by using a blast furnace and basic oxygen converter or by using an electric arc furnace - metallurgy refining of the steel melt by using a vacuum degassing vessel - casting the steel melt into an intermediate product, i.e.
• a common slab, a thin slab or a cast strip - optionally reheating the intermediate product - hot roiling the intermediate product to a hot rolled steel strip - coiling the hot rolled into a coil - coil surface preparation - hot strip annealing and pickling of the hot rolled strip - cold rolling the hot rolled strip in one or more passes to obtain a cold rolled strip with a final thickness - decarburization annealing of the cold rolled strip - optionally surface nitriding of the cold rolled strip - applying a MgO coating to the surface of the cold rolled strip - high temperature box annealing of the MgO coated cold rolled strip to decarburize the cold rolled strip, the cold rolled strip being coiled to coils which for the box annealing are stacked in a hood type furnace - heat flattening and insulation coating of the annealed strip - optionally magnetic domain refining of the strip.
• the casting and the high temperature slab reheating is performed at temperatures of up to 1400 °C.
• Such high temperature casting and reheating results in a well-developed inhibition system which comprises particles of AIN, MnS and other compounds in the iron matrix even before the cold process. The presence of said particles promotes an abnormal grain growth in the steel structure, which has a positive effect on the magnetic properties of the GEOS sheet.
• the primary recrystallization (PRX) occurring during the decarburization annealing prepares and controls the secondary grain growth.
• PRX The primary recrystallization
• this process step is unstable due to the large number of metallurgical phenomena that compete with each other during the decarburization annealing. These phenomena are in particular carbon removal, formation of the oxide layer, primary grain growth.
• decarburization annealing is essential to obtain efficient nitriding, a high-quality insulating glass film, and a sufficient number of Goss nuclei in the matrix.
• a dense oxide layer which occurs during the beginning of decarburization annealing, can promote surface quality but can also act as a barrier to decarburization and nitriding.
• the steel strip runs through a high temperature annealing cycle either in a batch annealing furnace or a rotary batch annealing furnace.
• a high temperature annealing cycle either in a batch annealing furnace or a rotary batch annealing furnace.
• SRX recrystallization
• a significant reduction of the losses induced by eddy currents can be obtained by reducing the thickness and by increasing the electrical resistivity of the material.
• An increase of the electrical resistivity can be achieved by increasing the content of at least one of the alloying elements of or by adding additional alloying elements to the Fe-Si-material. For example, a 10%-decrease of the thickness of a GEOS strip results in a reduction of approximately 20 % of the Eddy current losses at identical 50 Hz induction levels. Likewise, an 0.5%- increase of the Silicon content results in a reduction of 12% of the Eddy current losses at the identical 50 Hz induction levels.
• Eddy current losses account for about 10 to 25 % of the total specific losses at 50 Hz. However, at medium frequencies, which usually are in the range of 400 Hz to typically 2 kHz, much higher losses occur caused by Eddy current. In practice, these eddy current losses amount to at least 30 % of the total specific losses. For example, at a magnetic flux density of 1 .5 T and a frequency of 1 kHz, the share eddy current losses have on the total specific losses is typically 50 %. Here too, a dependency exists between the material thickness, the frequency and the induction values.
• the invention solved this problem by means of a grain oriented electrical steel sheet with at least the features specified in claim 1.
• a grain-oriented electrical steel according to the invention thus comprises
• core layer consisting of Fe, Si and optionally further alloying elements, the core layer having two outer surfaces,
• interface layers one of which being present on each outer surface of the core layer, the interface layers being formed by reaction products of at least one of the alloying elements of the core layer, and
• the thickness of the core layer is at least 25 times greater than the sum of the thicknesses of the outer layers
• t il ⁇ (t ol / t core ) x (p / m 0 x m dif x p x f) 2 x 10 -9 with t il being the thickness of the interface layer present in the respective surface of the core layer, indicated in nm,
• t core being the thickness of the core layer, indicated in pm
• f being the frequency of the respective current in Hz.
• the grain-oriented electrical steel sheet according to the present invention shows particularly improved magnetic loss behavior at medium frequencies, i.e.
• the interface layer present on each outer surface of the core are formed by reaction products which are the result of a chemical reaction of the alloying elements contained in the steel material of the core layer, which are at least Fe and Si.
• the alloying elements contained in the steel material of the core layer which are at least Fe and Si.
• the reaction products forming the interface layer can be oxides, nitrides and/or carbo-nitrides. Most commonly mixed oxides of iron and silica (“Fayalite”) form the interface layer.
• the outer layers being applied on each of the interface layers constitute an electrical insulation layer and can be of mineral or organic nature. For example, they may contain silica and aluminum-phosphate chemicals assembled together. As in common applications the insulating outer layer is provided for separating the layers of an electromagnetic converter core or the like.
• the invention provides a grain-oriented electrical steel sheet comprising a core layer containing at least Fe and Si having two outer surfaces, at least one interface layer present on each outer surface of the core and at least one outer layer present on each interface layer, wherein the thickness of the core layer is at least 25 times higher than the sum of the thicknesses of the outer layers.
• the grain-oriented electrical steel sheet according to the present invention consists at least of iron (“Fe”) and silicon (“Si”), wherein the Fe, as in common GEOS materials, accounts for by far the largest share.
• Si 1 to 8 % by weight Si can be present in the steel of the core layer of the grain-oriented electrical steel sheet according the invention, Si contain of 1 to 5 % by weight Si being especially effective for practical applications.
• Si-contents of 2 to 4 % by weight, particularly 2.5 to 3.5% by weight, of the core layer of prove to be especially advantageous with regard to the magnetic properties of a grain-oriented steel sheet according to the invention.
• the core layer of a grain-oriented steel sheet according to the invention optionally may contain as further alloying elements at least one element of the group“C, Mn,
• the amount of Mn, if present in the grain-oriented electrical steel sheet may amount to 0.001 to 3.0% Mn, particularly preferably 0.01 to 0.3 % by weight Mn.
• the amount of Cu, if present in the grain-oriented electrical steel sheet can be 0.001 to 3.0 % by weight, particularly 0.01 to 0.3% Cu.
• Al can be optionally present as well in the grain-oriented electrical steel sheet according to the invention in contents of 0.001 to 2.0 % by weight, particularly 0.01 to 1.0 % by weight.
• the contents of Cr, Sn, Ti, and B which may also be optionally present in the core layer of the grain-oriented electrical steel sheet according to the invention are delimited such that the sum of the contents of these elements is less than 3 % by weight, preferably less than 1 % by weight.
• a steel alloy which is especially suited for the core layer of a grain oriented steel sheet according to the invention preferably consists of, in % by weight, 2 to 5% Si, 0.01 to 0.3% Mn, 0.01 to 0.3% Cu, 0.01 to 1 .0% Al, the reminder being Fe and unavoidable impurities, which content in sum is preferably restricted to less than 0.5 % by weight.
• the sum of the sulfur (S) and selenium contents of the core layer of a grain-oriented steel sheet according to the invention is preferably restricted to less than 0.010 % by weight.
• the Sulfur (“S”) content of the core layer of the grain-oriented steel sheet fulfills at least one of the following provisions:
• the S-content is restricted to less than 7 ppm by weight and/or the S-content in the core layer is less than 0.0007 % by weight related to the total amount of Fe- and Si-contents of the core layer.
• the content of magnesium in the interface layers is lower than 1 % by weight.
• the grain-oriented electrical steel sheet according to the present invention comprises a soft magnetic material.
• a grain-oriented electrical steel sheet according to the invention comprises at least a core layer, at least one interface layer present on each outer surface of the core layer and at least one outer layer present being respectively applied on each of the interface layers.
• no further coating or layer is present on one or both outer layers of the grain-oriented steel.
• the thickness of the core layer is at least 25 times higher than the sum of the thicknesses of the outer layers. Accordingly, the thickness t oore of the core layer and the sum ⁇ t ol of the thicknesses of the outer layers have to fulfil the following provision (2): t core / S t ol 3 25 ' (2)
• t core being the thickness of the core layer, indicated in pm
• the core layer of the grain-oriented steel according to the present invention has a thickness of 50 to 220 pm, wherein a thickness of the core layer of at least 100 mm turned out to be especially useful for practical applications.
• the interface layer according to the present embodiment mainly differentiates from the core layer by its magnetic characteristics like magnetic permeability.
• the thickness of the interface layers of a steel sheet according to the invention typically amounts to 1 to 500 ran, wherein in practice thicknesses of at least 10 nm are observed.
• a restriction of the interface layer to a maximum of 100 nm turned out to be especially advantageous with regard to the magnetic properties of the steel sheet according to the invention.
• the grain-oriented steel according to the present invention com prises at least one interface layer present above each outer surface of the core.
• the grain-oriented steel according to the present invention comprises a first interface layer present beneath the top outer surface and a second interface layer beneath the bottom outer surface.
• the grain-oriented electrical steel sheet according to the present invention further comprises at least one, preferably exactly one, outer layer present on each interface layer.
• the sum of the thicknesses of the outer layers is preferably at least 0.1 pm and less than 5 pm, more preferably 0.1 to 2 pm.
• the grain-oriented electrical steel sheet according to the present invention can be manufactured by performing a method which comprises at least the following working steps:
• step (B) cold rolling the hot strip of step (A) in at least one pass to obtain a cold strip;
• step (D) performing a secondary recrystallization annealing treatment by heating the strip obtained in step (C) to a temperature OTAG2 with a heating rate of at least 40 K/s to obtain a grain- oriented electrical steel sheet according to the invention, wherein the temperature OTAG2 is set according to the following condition (I):
• PGS being the Average Grain Size of the grains obtained by the PRX, indicated in pm
• DN being the Nitriding Degree, indicated in ppm, and calculated as
• %S sum of the S- and Se-content of the core layer, indicated in ppm, pHAGB: High Angle (> 15°) primary Grain Boundary average density, indicated in mm -1 .
• Step (A) of the process comprises providing a hot rolled steel strip which is made from a steel alloyed in accordance with the explanations and provisions given above.
• Step (A) of the process according to the invention comprises a common steelmaking step to produce a steel melt which afterwards is cast into an intermediate product such as slabs, thin slabs or cast strip.
• the intermediate product obtained in this way is hot rolled to a hot rolled strip which is coiled to a coil and optionally undergoes a hot strip annealing and pickling if appropriate before further manufacturing.
• the hot rolled strip provided in step (A) of the process according to the invention preferably has a thickness of 0.5 to 3.5 mm, more preferably 1.0 to 3.0 mm. Examples for known methods suited for the production of a hot rolled strip to be provided in working step (A) can be found in
• step (A) hot band strips having the above mentioned composition and thickness are obtained. These hot band strips are preferably directly introduced into step (B) of the process.
• step (B) of the process according to the invention the hot rolled strip is cold rolled in at least one pass to obtain a cold rolled strip.
• Method for cold rolling a grain oriented steel strip are generally known to the skilled expert as well and, for example, described in
• the thickness of the cold rolled strip is 0.05 to 2.00 mm, preferably at least 0.10 mm, after the first cold rolling step, wherein after the second cold rolling a maximum thickness of 0.55 mm, preferably of £ 0.35 mm at most or of 0.22 mm at most, are especially favorable.
• Apparatuses in which such cold rolling can be performed are generally known to the skilled expert and, for example, disclosed in WO 2007/014868 A1 and WO 99/19521 A1.
• step (B) of the process according to the invention is preferably performed in at least two cold rolling steps to obtain a steel strip of minimized thickness. It turned that exactly to cold rolling steps are especially appropriate for the purposes of the invention.
• Two step cold rolling allows the strip to be subjected to a decarburization annealing between the cold rolling steps.
• decarburization can also be performed according to methods known to the skilled expert.
• an intermediate annealing is performed in a temperature range of 700 to 950 °C, preferably 800 to 900 °C, under an atmosphere which dew point is set to 10 to 80 °C. Installations with which such annealing can be performed are generally known and disclosed, for example, in WO 2007/014868 A1 and WO 99/19521 A1.
• the decarburization annealing is preferably performed such that the carbon content of the steel strip is lowered to less than 30 ppm by weight. Accordingly, in a two-step cold rolling with intermediate decarburization annealing the carbon content of the cold rolled strip preferably is less than 30 ppm by weight before the second cold rolling step in working step (B) of the process according to the invention.
• an annealing of the cold strip obtained in step (B) is performed to primary recrystallize and optionally nitride treating the cold rolled strip.
• the nitriding annealing preferably carried out at temperatures in the range of 400 to 950 °C, e.g. 600 to 900 °C. If a nitriding treatment is to be performed the annealing can be carried out under an atmosphere which comprises N 2 or N-com prising compounds, for example NH 3 .
• Annealing and nitriding can be conducted in two separate steps one after the other with the annealing being performed at first. As an alternative simultaneously annealing and nitriding can be performed.
• nitriding degree is calculated as the difference between the nitrogen content of the steel strip before the second recrystal isation annealing (working step (D)) minus the nitrogen content before the primary recrystallization annealing (working step (C)).
• the nitrogen content can be determined by usual means, such as the 736 analyzer offered by Leco Corporation, St. Joseph, USA.
• the average grain size of the structure of the core layer of the strip obtained after step (C) of the process according the invention typically is 5 to 25mm, especially 5 to 20 pm.
• the average High Angle primary Grain Boundary density of the strip obtained after step (C) lies in the range of 0.005 to 0.1 mm -1 , especially of 0.01 to 0.09 mm -1 .
• the Average Primary Grain Size can be determined with methods known to the skilled expert, for example Grain size measured by Electron Backscatter Diffraction ("EBSD") for which the common software OIM Analyses can be used (s. https://en.wikipedia.org/wiki/Electron_backscatter_diffraction;
• a pickling step may be performed after the annealing and the optional nitriding in a manner well known to the skilled expert as well.
• pickling can be performed by using aqueous solutions of acids like phosphoric acid, sulfuric acid and/or hydrochloric acid.
• the pickling step should preferably be performed after step (C) and before step (D) of the method according to the invention.
• step (D) of the process according to the invention the cold rolled strip undergoes a secondary recrystallization annealing treatment by heating to a temperature OTAG2 with a heating rate of at least 40 K/s, preferably at least 50 K/s, to obtain the grain-oriented electrical steel sheet. Heating rate of at least 70 K/s, more preferably at least 100 K/s, is especially favorable.
• the rapid heating can be carried out by any method known to the skilled expert, for example by induction heating, by resistive heating or by conductive heating.
• the respective temperature OTAG2 is calculated in accordance with the provisions already mentioned above and is set to 1420 K at most.
• the upper limit of OTAG2 is 1415 K.
• the Heating Rate to Secondary Recrystallization T reatment is 20 to 800 K/s, more preferably 50 to 750 K/s.
• the Heating Rate to Secondary Recrystallization Treatment is acquired with methods known to the skilled expert, for example as described in EP 2 486 157.
• the dew point of the atmosphere during heating is preferably set to 223 to 273 K, more preferably 243 to 270 K.
• the atmosphere dew point can be determined with methods well known to the skilled expert. Instructions for such determination can be found in
• the high angle (> 15°) primary grain boundary average density of the GEOS sheet according to the invention is preferably 0.005 to 0.1 mm -1 , in particular 0.01 to 0.09 mm -1 .
• the High Angle (> 15°) primary grain boundary average density of the GEOS sheet according to the invention is preferably 0.005 to 0.1 mm -1 , in particular 0.01 to 0.09 mm -1 .
• primary Grain Boundary can be measured as primary grain boundary length per unit area by EBSD analysis (OIM Analysis software).
• the pHAGB is the average of the values corresponding to a misorientation higher than 15° (>15°).
• step (D) of the process according to the invention the Secondary Recrystallization takes place which ensures that the grain-oriented steel sheet processed in this way is prepared to reliably develop the optimized properties of a grain-oriented steel sheet according to the invention as outlined above.
• step (D) on the surface of the cold rolled strip that is introduced into step (D) no outer coating is applied. That means that preferably no annealing separator, especially no MgO based coating, is present on the sheet material which is processed according to working step (D). Rather, the outer coating, i.e. the insulation coating preferably containing MgO, should be applied only after working step (D) to contribute to an optimized result of the SRX.
• the sheet material obtained after working step (D) of the process according to the invention should run through those process steps which in the common production of grain-oriented steel sheets usually are performed after the SRX.
• the heating and soaking is preferably carried out under a protective gas atmosphere, which, for example, comprises H 2 .
• a protective gas atmosphere which, for example, comprises H 2 .
• the heating to and soaking at the respective soaking temperature is performed under an atmosphere which comprises 5 to 95 Vol.-% H 2 , the reminder being nitrogen or any inert gas or a mix gas, the dew point of the atmosphere being at least 10 °C.
• the soaking time, during which the high temperature soaking is carried out in this way, can be determined in a common manner which is well known to the expert. By the soaking performed in this way atoms of elements are removed, which would deteriorate the properties of the grain-oriented steel sheet. These elements are in particular N and S.
• the steel strip After the high temperature annealing the steel strip is cooled down in a common manner, e.g. by natural cooling, down to room temperature.
• the steel strip is cleaned, and optionally pickled.
• Methods with which the steel strip is pickled are known to the skilled expert.
• the steel strip can be treated with an aqueous acidic solution. Suitable acids are for example phosphoric acid, sulfuric acid and/or hydrochloric acid.
• the grain-oriented electrical steel is presenting a core layer and two interface layers being present on the outer surfaces of the core layer, the chemical composition of the interface layer resulting from the migration of various species from the core layer towards the outer surface and arising from the reaction of the outer surface of the core layer with the various atmospheres and conditions encountered during the successive treatment phases as for example the ones explained in the previous sections of this text.
• the reaction products which are the result of these reactions is a mixed oxide of iron and silica, also called "Fayalite").
• the reaction products being present on the outer surface of the core layer form interface layers between the core layers and the outer layers.
• the outer layers constitute an electrical insulation media which can be of mineral or organic nature (e.g. it may contain silica and aluminum- phosphate chemicals assembled together) adapted to the necessary separation between layers of an electromagnetic converter core.
• the grain-oriented electrical steel sheets can be prepared in any format, like steel strips that are provided as coils, or cut steel pieces that are provided by cutting these steel pieces from the steel strips. Methods to provide coils or cut steel pieces are known to the skilled expert.
• the grain-oriented electrical steel sheet according to the present invention shows improved magnetic loss at medium frequencies compared to grain-oriented electrical steel sheets according to the prior art. Accordingly, the product according to the invention is in particular useful for the manufacture of parts for electric transformers, for electric motors or for otherelectric devices. This is particularly true for electrical applications in which the magnetic flux has to be channeled or contained.
• the grain-oriented electrical steel sheet according to the present invention shows improved magnetic loss at medium frequencies, in particular frequencies of at least 400 Hz and, for example, 3000 Hz or 2000 Hz at most.
• the thickness t il of the interface layers being present on the surfaces of the core layer, the thicknesses t il of the interface layers being identical;
• the thickness t ol of the outer layers being present on the surfaces of the core layer, the thicknesses t ol of the outer layers being identical;

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