Grain-Oriented Electrical Steel Sheet and Method for Its Production

Abstract

A grain-oriented electrical steel sheet includes, in wt. %: Si: 2.0 to 4.0, Mn: 0.01 to 0.5, C: up to 0.005, Alsl: up to 0.0030, N: up to 0.005, S: up to 0.002, the sum of C and N is ≤0.0065 wt. %, at least one element selected from the group consisting of V, Nb, Ti, Mo with the following contents, in wt. %, V: 0.0005 to 0.0060, Nb: 0.0005 to 0.0060, Ti: 0.0005 to 0.0030, Mo: 0.0005 to 0.03; in case the content of Mo is >0.010 wt. %, the sum of the contents of V, Nb, Ti and Mo is ≤0.040 wt. % and Cu is present in an amount of 0.02 to 0.6 wt. % and in case the content of Mo is ≤0.010% the sum of the contents of V, Nb, Ti and Mo is ≤0.030 wt. % and Cu is optionally present in an amount of 0.002 to 0.6 wt. %.

1 ) A method of producing a grain-oriented electrical steel sheet comprising at least the following working steps:
 a) providing a thick steel slab having a thickness of 140 to 280 mm with a composition comprising, in wt. %,
 Si: 2.0 to 4.0, 
 C: 0.01 to 0.10, 
 Al sl : 0.01 to 0.065, 
 N: 0.003 to 0.015, 
 Mn: 0.01 to 0.5, 
 at least one element selected from the group consisting of V, Nb, Ti, Mo with the following contents, in wt. % 
 V: 0.0005 to 0.0060, 
 Nb: 0.0005 to 0.0060, 
 Ti: 0.0005 to 0.0030, 
 Mo: 0.0005 to 0.030, 
 S: 0.003 to 0.03, 
 P: 0.0003 to 0.03, 
 wherein in case the content of Mo is >0.010 wt. %, the sum of the contents of V, Nb, Ti and Mo is ≤0.040 wt. % and Cu is present in an amount of 0.02 to 0.6 wt. % and wherein in case the content of Mo is ≤0.010 wt. % the sum of the contents of V, Nb, Ti and Mo is ≤0.030 wt. % and Cu may optionally be present in an amount of 0.002 to 0.6 wt. %, 
 optionally one or more elements selected from the group consisting of Se, Sn, Sb, Bi, Ni, Co, wherein the individual content of each of these elements is 0.005 to 0.2 wt. %, 
 optionally Cr in an amount of 0.005 to 0.6 wt. %, 
 optionally one or more elements selected from the group consisting of As, B, Te, wherein the individual content of each of these elements is 0.0003 to 0.1 wt. %, 
 the remainder of the composition being Fe and unavoidable impurities; 
   b) heating of the thick slab in a furnace to a temperature ranging between 105° and 1320° C.;   c) rough rolling in one or more passes   d) hot rolling of the thick steel slab obtained in step c) into a hot strip having a thickness of 0.5 to 4.0 mm in a multi-stand hot rolling mill, wherein the first rolling pass is carried out at a temperature of 900 to 1200° C. with a degree of deformation of more than 40%, and wherein a final rolling temperature is from 750 to 1000° C.;   e) cooling the hot strip by vapor and/or liquid spray cooling;   f) coiling the hot strip, wherein the maximum difference between the recrystallization degree of a middle portion to an edge portion in the hot strip is ≤20% determined using Electron Backscatter Diffraction (EBSD) and calculating the Kernel Average Misorientation from the EBSD data as described in the description;   g) annealing the hot strip;   h) cold rolling the hot strip in one or more passes into a cold strip, wherein the cold rolling optionally comprises an intermediate annealing;   i) decarburization annealing of the cold strip;   j) optionally nitriding annealing during or after decarburization annealing;   k) applying an annealing separator onto at least one surface of the cold strip;   l) final annealing of the cold strip;   m) optionally coating of the annealed cold strip with an electric insulation coating and annealing of the insulation coated cold strip; and   n) optionally performing domain refinement of the coated cold strip.   
     
     
         2 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein the hot strip resulting after step f) has at least a recrystallization degree depending on the final rolling temperature (FRT) according to the following formula:
   recrystallization degree>(FRT−750)*(FRT/450) 2 )/50,
   wherein the minimum recrystallization degree is evaluated over strip thickness and over of 2000 μm length in the rolling direction by determining the recrystallization degree at the middle of the strip and at 25 mm from both strip edges before optional side trimming using Electron Backscatter Diffraction (EBSD) and calculating the Kernel Average Misorientation from the EBSD data as described in the description.   
     
     
         3 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein the hot strip resulting after step g) and prior to step h) has a bending number before the occurrence of a crack of ≥1.5 as average value calculated from measurements at the middle of the strip width and measured at both edges of the strip in strip width direction 25 mm from the respective edge, determined according to DIN EN ISO 7799 with a bending radius of 5 mm. 
     
     
         4 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein depending on the amount of Si, in wt. %, in the thick steel slab provided in step a) the number of cold-rolling breakages in the cold-rolled strip after step h) satisfies the following formulae: 
       
         
           
             
               
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         wherein CRB is the number of cold-rolling breakages per 1000 t cold-rolled strip. 
       
     
     
         5 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein the duration of the heating in step b) is between 150 min to 500 min. 
     
     
         6 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein the thick steel slab obtained in step b) is hot rolled in step c) in one or more passes to an intermediate slab thickness of 90 mm to 35 mm. 
     
     
         7 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein the coiling of the hot strip in step f) is carried out at a temperature from 500° C. to 780° C. 
     
     
         8 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein step g) is carried out in a single or multi-step annealing, wherein the maximum temperature is at 800 to 1200° C. and the hot strip is annealed in total for 30 seconds to 6 minutes. 
     
     
         9 ) The method of producing a grain-oriented electrical steel strip according to  claim 1 , wherein production step h) is carried out by rolling in multiple passes, wherein the temperature T CR  of the surface of the strip determined directly after third and second last pass before reaching the final steel thickness is between 175 and 480° C. 
     
     
         10 ) A grain-oriented electrical steel sheet, wherein the steel bulk of the grain oriented electrical steel comprises, in wt. %:
 Si: 2.0 to 4.0,   Mn: 0.01 to 0.5,   C: up to 0.005,   Al sl : up to 0.0030,   N: up to 0.005,   S: up to 0.002;   wherein the sum of C and N is ≤0.0065 wt. %;   at least one element selected from the group consisting of V, Nb, Ti, Mo with the following contents, in wt. %, V: 0.0005 to 0.0060, Nb: 0.0005 to 0.0060, Ti: 0.0005 to 0.0030, Mo: 0.0005 to 0.03;   wherein in case the content of Mo is >0.010 wt. %, the sum of the contents of V, Nb, Ti and Mo is ≤0.040 wt. % and Cu is present in an amount of 0.02 to 0.6 wt. % and wherein in case the content of Mo is ≤0.010% the sum of the contents of V, Nb, Ti and Mo is ≤0.030 wt. % and Cu is optionally present in an amount of 0.002 to 0.6 wt. %   optionally one or more elements selected from the group consisting of Se, Sn, Sb, Bi, Ni, Co, wherein the individual content of each of these elements is 0.005 to 0.2 wt. %,   optionally Cr in an amount of 0.005 to 0.60 wt. %,   optionally one or more elements selected from the group consisting of As, B, P, Te, wherein the individual content of each of these elements is 0.0003 to 0.1 wt. %, the remainder of the composition being Fe and unavoidable impurities.   
     
     
         11 ) The grain-oriented electrical steel sheet according to  claim 10 , wherein the grain-oriented electrical steel sheet is produced from a thick slab with a thickness of 140 to 280 mm, and having a polarization J 800 ≥1.89 T determined according to IEC 60404-3 and an iron loss P 1.7 ≤0.85 W/kg determined according to IEC 60404-3. 
     
     
         12 ) The grain-oriented electrical steel sheet according to  claim 10 , wherein the grain oriented electrical steel sheet has a coarse grain structure in which up to 30% of the grain area in relation to the total area of the grains in the steel is occupied by grains having a grain size, measured according to DIN EN ISO 643 in rolling direction, of 50 mm. 
     
     
         13 ) The grain-oriented electrical steel sheet according to  claim 10 , wherein the increase in iron loss P 1.7  is ≤1.0% after performing an aging test with Epstein samples of the grain-oriented electrical steel sheet for 24 h at a temperature of 225° C. at ambient atmosphere after stress relief annealing according to IEC 60404-2. 
     
     
         14 ) The grain-oriented electrical steel sheet according to  claim 10 , wherein the grain-oriented electrical steel sheet has a bending number of ≥30 before the occurrence of a crack, determined according to DIN EN ISO 7799 with a bending radius of 5 mm.