Process for producing grain-oriented electrical steel strip and grain-oriented electrical steel strip obtained according to said process

Abstract

A process for producing grain-oriented electrical steel strip by means of thin slab continuous casting, comprising the following process steps: a) smelting a steel, b) continuously casting the smelt by thin slab continuous casting, c′) heating up the thin slabs and subjecting the slabs to homogenization annealing at a maximum temperature of 1,250° C., d) heating to a temperature between 1.350° C. and 1.380° C., e) continuously hot rolling the thin slabs to form a hot-rolled strip, f) cooling and reeling the hot-rolled strip to form a coil, g) annealing the hot-rolled strip after reeling and prior to a subsequent cold rolling step, h) cold rolling the hot-rolled strip to the nominal usable thickness, i) subjecting the cold-rolled strip to recrystallization, decarburization and nitridation annealing, j) applying an annealing separator (non-stick layer) to the strip surface of the cold-rolled strip, k) subjecting the cold-rolled strip to secondary recrystallization annealing, forming a finished steel strip having a pronounced Goss texture, and l) stress-free annealing the finished steel strip, which has been coated with an insulating layer, provides an improved process for producing grain-oriented electrical steel strip by means of thin slab continuous casting. This is achieved in that the recrystallization, decarburization and nitridation annealing of the cold-rolled strip in process step h) comprises a decarburization annealing phase and a subsequent nitridation annealing phase, with an intermediate reduction annealing phase being interposed between the decarburization annealing phase and the nitridation annealing phase, and carried out at a temperature ranging from 820° C.-890° C., for a maximum period of 40 seconds, with a dry, gaseous annealing atmosphere, which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2 of less than 0.10 and wherein a cold-rolled strip is obtained, which primary recrystallized grains have a circle equivalent mean size (diameter) between 22 μm and 25 μm.

What is claimed is: 
     
         1 . A process for producing grain-oriented electrical steel strip by means of thin slab continuous casting, comprising the following process steps:
 a) smelting a steel with a smelt which, in particular after secondary metallurgical treatment, contains, in addition to iron (Fe) and unavoidable impurities,   Si: 2.50-4.00 wt %,   C: 0.030-0.100 wt %,   Mn: 0.060-0.300 wt %,   Cu: 0.100-0.300 wt %,   Alsl: 0.020-0.040 wt %   Sn: 0.050-0.150 wt %   S: <100 ppm,   N: <100 ppm,   
       and one or more elements from the group comprising Cr, V, Ni, Mo and Nb,
 b) continuously casting the smelt by thin slab continuous casting, without exposure of the strand to inert gas, to form a strand having a thickness of 50-120 mm, and dividing the strand into thin slabs, 
 c) carrying out a homogenization annealing comprising the steps of 
 c′) heating the thin slabs, preferably in a linear furnace, to a temperature above 1,050° C. and subjecting the slabs to an annealing at a maximum of 1,250° C., preferably at a maximum of 1,200° C., in particular, at a maximum of 1,150° C., 
 
       and
 d) feeding the thin slabs to an induction heating device, in particular, a high-frequency induction heating device, in which the thin slabs particularly while passing are immediately prior to the first hot-rolling pass heated for several seconds up to a temperature of 1,350° C.-1,380° C., preferably of 1.355° C.-1.370° C., particularly of 1.36,0° C. that is above the previous homogenization temperature of process step c′), 
 e) continuously hot rolling the thin slabs in a preferably linear, multiple-stand hot-rolling train to form a hot strip having a thickness of 1.8 mm-3.0 mm, 
 f) cooling and reeling the hot-rolled strip at a reeling temperature of less than 650° C. to form a coil, 
 g) annealing the hot-rolled strip, after reeling and prior to a subsequent cold rolling step, at a temperature of between 920° C. and 1,150° C., 
 h) cold rolling the hot-rolled strip, preferably on a reversible stand, in a single process step in more than three passes, to cold-rolled strip having a final thickness of 0.15 mm-0.40 mm, 
 i) subjecting the resulting cold-rolled strip to recrystallization, decarburization and nitridation annealing, 
 j) applying an annealing separator (non-stick layer), in particular containing primarily MgO, to the strip surface of the cold-rolled strip which has been recrystallization, decarburization and nitridation annealed, 
 k) subjecting the cold-rolled strip which has been coated with the annealing separator to secondary recrystallization annealing by high-temperature annealing in a bell-type furnace at a temperature of >1,150° C., forming a finished steel strip having a pronounced Goss texture, 
 l) coating the finished steel strip which has undergone secondary recrystallization annealing with an electrically insulating layer, and then stress-free annealing or stress-relief annealing the coated finished steel strip, 
 wherein 
 
       the recrystallization, decarburization and nitridation annealing of the cold-rolled strip in process step i) comprises 
       a decarburization annealing phase, which is carried out at a strip temperature ranging from 820° C.-890° C. for a maximum period of 150 seconds, with a gaseous annealing atmosphere, in particular moist, which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of 0.30 to 0.60, 
       and comprises a subsequent nitridation annealing phase, which is carried out at a temperature ranging from 850° C.-920° C. for a maximum period of 50 seconds, with a gaseous annealing atmosphere which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of 0.03 to 0.07, and comprises an intermediate reduction annealing phase, which is carried out between the decarburization annealing phase and the nitridation annealing phase and is carried out at a temperature ranging from 820° C.-890° C. for a maximum period of 40 seconds, with a gaseous annealing atmosphere, in particular dry, which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of less than 0.10 and wherein a cold-rolled strip is obtained, which primary recrystallized grains have a circle equivalent mean size (diameter) between 22 μm and 25 μm. 
     
     
         2 . The process according to  claim 1 , characterized in that at least 2 wt % to a maximum of 12 wt % ammonia (NH 3 ), in particular, referred to the total gas flow rate, is added separately to the annealing atmosphere during the nitridation annealing phase in process step i), and the ammonia is blown onto the two opposing, large-area strip surfaces of the cold-rolled strip. 
     
     
         3 . The process according to  claim 1 , characterized in that, during the annealing in process step i), which comprises the decarburization annealing phase, the intermediate reduction annealing phase and the nitridation annealing phase, the cold-rolled strip is annealed such that after annealing, the cold-rolled strip has a total nitrogen content of at least 180 ppm, preferably at least 250 ppm, in particular, at least 300 ppm. 
     
     
         4 . The process according to  claim 1 , characterized in that, at the start of recrystallization annealing in process step i), the cold-rolled strip is heated at a heating rate of more than 100 K/s. 
     
     
         5 . The process according to  claim 1 , characterized in that in the smelt in process step a), the ratio of manganese (Mn) to sulfur (S) is greater than 6, preferably greater than 20, and the ratio of aluminum (Al) to nitrogen (N) is greater than 4, preferably greater than 10. 
     
     
         6 . The process according to  claim 1 , characterized in that during casting in process step b), the superheating temperature of the smelt during casting is less than 40 K, preferably less than 20 K, in particular, less than 12 K, and the reduction in the thickness of the strand is carried out according to the “liquid core reduction” method, just below the metal mold, while the interior of the strand has a liquid core. 
     
     
         7 . The process according to  claim 1 , characterized in that the hot rolling in process step e) is carried out at an initial rolling temperature during the first working pass of greater than 1,150° C., preferably greater than 1,200° C., a final rolling temperature ranging from 850° C.-980° C. and a final rolling speed of less than 12 m/s, preferably less than 10 m/s. 
     
     
         8 . The process according to  claim 1 , characterized in that, during annealing of the hot-rolled strip in process step g), the annealed hot-rolled strip is quenched after annealing at a cooling rate of more than 25 K/s, preferably more than 30 K/s, in particular, more than 40 K/s, particularly preferably a cooling rate ranging from 25 K/s-52 K/s. 
     
     
         9 . The process according to  claim 1 , characterized in that the working during the cold-rolling of the hot-rolled strip in process step h) is carried out such, that during at least one or more of the final three passes, the hot-rolled strip reaches a temperature of at least 180° C. to a maximum of 260° C., resulting from the processing heat produced during rolling, for a period of at least five minutes. 
     
     
         10 . The process according to  claim 1 , characterized in that the cold rolling in process step h) is carried out in two stages, wherein the hot-rolled strip is pickled in a pickling step prior to the first cold-rolling stage, and upon completion of the first cold-rolling stage, the hot-rolled steel is annealed according to process step g). 
     
     
         11 . The process according to  claim 10 , characterized in that in the second cold-rolling stage, the thickness of the hot-rolled strip is reduced by at least 85%. 
     
     
         12 . The process according to  claim 1 , characterized in that the secondary recrystallization annealing in process step k) is carried out such that, during the heating phase of the high-temperature annealing in a bell-type furnace, the quantity of nitrogen (N 2 ), in terms of atomic percent, in the gaseous annealing atmosphere is greater than the quantity of hydrogen (H 2 ), in terms of atomic percent. 
     
     
         13 . The process according to  claim 1 , characterized in that, following process step l), a process step that effects a magnetic domain refinement of the coated finished steel strip is carried out, particularly optionally. 
     
     
         14 . A grain-oriented electrical steel strip obtained by a process comprising the following steps:
 a) smelting a steel with a smelt which, in particular after secondary metallurgical treatment, contains, in addition to iron (Fe) and unavoidable impurities,
 Si: 2.50-4.00 wt %, 
 C: 0.030-0.100 wt %, 
 Mn: 0.060-0.300 wt %, 
 Cu: 0.100-0.300 wt %, 
 Alsl: 0.020-0.040 wt % 
 Sn: 0.050-0.150 wt % 
 S: <100 ppm, 
 N: <100 ppm, 
   
       and one or more elements from the group comprising Cr, V, Ni, Mo and Nb, 
       b) continuously casting the smelt by thin slab continuous casting, without exposure of the strand to inert gas, to form a strand having a thickness of 50-120 mm, and dividing the strand into thin slabs, 
       c) carrying out a homogenization annealing comprising the steps of
 c′) heating the thin slabs, preferably in a linear furnace, to a temperature above 1,050° C. and subjecting the slabs to an annealing at a maximum of 1,250° C., preferably at a maximum of 1,200° C., in particular, at a maximum of 1,150° C., 
 
       and
 d) feeding the thin slabs to an induction heating device, in particular, a high-frequency induction heating device, in which the thin slabs particularly while passing are immediately prior to the first hot-rolling pass heated for several seconds up to a temperature of 1,350° C.-1,380° C., preferably of 1.355° C.-1.370° C., particularly of 1.36,0° C. that is above the previous homogenization temperature of process step c′), 
 e) continuously hot rolling the thin slabs in a preferably linear, multiple-stand hot-rolling train to form a hot strip having a thickness of 1.8 mm-3.0 mm, 
 f) cooling and reeling the hot-rolled strip at a reeling temperature of less than 650° C. to form a coil, 
 g) annealing the hot-rolled strip, after reeling and prior to a subsequent cold rolling step, at a temperature of between 920° C. and 1,150° C., 
 h) cold rolling the hot-rolled strip, preferably on a reversible stand, in a single process step in more than three passes, to cold-rolled strip having a final thickness of 0.15 mm-0.40 mm, 
 i) subjecting the resulting cold-rolled strip to recrystallization, decarburization and nitridation annealing, 
 j) applying an annealing separator (non-stick layer), in particular containing primarily MgO, to the strip surface of the cold-rolled strip which has been recrystallization, decarburization and nitridation annealed, 
 k) subjecting the cold-rolled strip which has been coated with the annealing separator to secondary recrystallization annealing by high-temperature annealing in a bell-type furnace at a temperature of >1,150° C., forming a finished steel strip having a pronounced Goss texture, 
 l) coating the finished steel strip which has undergone secondary recrystallization annealing with an electrically insulating layer, and then stress-free annealing or stress-relief annealing the coated finished steel strip, 
 wherein 
 
       the recrystallization, decarburization and nitridation annealing of the cold-rolled strip in process step i) comprises 
       a decarburization annealing phase, which is carried out at a strip temperature ranging from 820° C.-890° C. for a maximum period of 150 seconds, with a gaseous annealing atmosphere, in particular moist, which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of 0.30 to 0.60, 
       and comprises a subsequent nitridation annealing phase, which is carried out at a temperature ranging from 850° C.-920° C. for a maximum period of 50 seconds, with a gaseous annealing atmosphere which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of 0.03 to 0.07, and comprises an intermediate reduction annealing phase, which is carried out between the decarburization annealing phase and the nitridation annealing phase and is carried out at a temperature ranging from 820° C.-890° C. for a maximum period of 40 seconds, with a gaseous annealing atmosphere, in particular dry, which contains nitrogen (N 2 ) and hydrogen (H 2 ) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of less than 0.10 and wherein a cold-rolled strip is obtained, which primary recrystallized grains have a circle equivalent mean size (diameter) between 22 μm and 25 μm.