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

Abstract

With 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 addition to iron (Fe) and unavoidable impurities, contains Si: 2.00-4.00 wt %, C: 0.025-0.100 wt %, Mn: 0.060-0.500 wt %, Cu: 0.200-0.550 wt %, Al sl : 0.010-0.030 wt %, S: <100 ppm, N: 80-120 ppm, and one or more elements from the group comprising Cr, V, Ni and Mo, each <0.100 wt %, b) continuously casting the smelt by thin slab continuous casting to form a strand having a thickness of 50-120 mm, and dividing the strand into thin slabs, c) heating up the thin slabs, preferably in a linear furnace, to a temperature above 1050° C. and subjecting the slabs to homogenization annealing at a maximum temperature of 1250° C., d) immediately prior to the first hot rolling pass of a subsequent hot rolling process, passing the slabs through an induction heating device, in particular, a high frequency induction heating device, and heating the thin slabs to a maximum temperature of 1350° C., which is above the respective homogenization temperature of process step c), e) continuously hot rolling the thin slabs 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) pickling the hot-rolled strip after reeling and prior to a subsequent cold rolling step, h) cold rolling the hot strip in a first cold rolling stage to an (intermediate) thickness of 0.50 mm-0.80 mm, i) subjecting the resulting cold-rolled strip to recrystallization and decarburization annealing at a strip temperature ranging from 820° C.-890° C. for a period of 300-600 seconds in a gaseous annealing atmosphere which acts on the cold-rolled strip and contains nitrogen (N 2 ) and hydrogen (H 2 ), and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2 of 0.30 to 0.60, j) in a second cold rolling stage, cold rolling the cold strip which has been subjected to recrystallization and decarburization annealing to its (final) thickness or its nominal usable thickness of 0.15 mm-0.40 mm, k) applying an annealing separator (non-stick layer) containing MgO to the strip surface of the cold-rolled strip which has been rolled to its final thickness or usable thickness, l) 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 >1150° C., forming a finished steel strip having a pronounced Goss texture, and m) 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, an improved process for producing grain-oriented electrical steel strip by means of thin slab continuous casting is provided, by which it is possible to introduce an inhibitor into the steel strip, which controls secondary grain growth during secondary recrystallization annealing in a high-temperature bell-type annealing furnace.

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: 2.00-4.00 wt %,   C: 0.025-0.100 wt %,   Mn: 0.060-0.500 wt %,   Cu: 0.200-0.550 wt %,   Alsl: 0.010-0.030 wt %   S: <100 ppm,   N: 80-120 ppm, and one or more elements from the group comprising Cr, V, Ni and Mo, each <0.100 wt %,   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) heating the thin slabs, preferably in a linear furnace, to a temperature above 1050° C. and subjecting the slabs to homogenization annealing at a maximum of 1250° C., preferably at a maximum of 1200° C., in particular, at a maximum of 1150° C.,   d) immediately prior to the first hot rolling pass in a subsequent hot rolling process, passing the slabs through an induction heating device, in particular, a high-frequency induction heating device, and heating the thin slabs to a temperature of at least 1250° C. and maximum of 1350° C., which is above the respective 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) pickling the hot-rolled strip, after reeling and prior to a subsequent cold rolling step,   h) cold rolling the hot strip in a first cold rolling stage to an (intermediate) thickness of 0.50 mm-0.80 mm,   i) subjecting the resulting cold-rolled strip to recrystallization and decarburization annealing at a strip temperature ranging from 820° C.-890° C. for a period of 300-600 seconds in a particularly moist, gaseous annealing atmosphere which acts on the cold-rolled strip and contains nitrogen (N 2 ) and hydrogen (H 2 ), and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of 0.30 to 0.60,   j) in a second cold rolling stage, cold rolling the cold-rolled strip which has been subjected to recrystallization and decarburization annealing to its (final) thickness or its nominal usable thickness of 0.15 mm-0.40 mm,   k) applying an annealing separator (non-stick layer), in particular containing primarily MgO, to the strip surface of the cold-rolled strip which has been rolled to its final thickness or usable thickness,   l) 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 >1150° C., forming a finished steel strip having a pronounced Goss texture, and   m) 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.   
     
     
         2 . The process according to  claim 1 , characterized in that the hot rolling in process step e) is carried out at an initial rolling temperature in the first rolling pass that is greater than 1150° C., preferably greater than 1200° C., a final rolling temperature ranging from 850° C. to 980° C., and a final rolling speed of less than 12 m/s, preferably less than 10 m/s. 
     
     
         3 . The process according to  claim 1 , characterized in that a hot-rolled strip having a maximum relative thickness crown of less than 2%, preferably less than 1%, in particular, less than 0.7%, is rolled during hot rolling in process step e) 
     
     
         4 . The process according to  claim 1 , characterized in that a hot-rolled strip having a maximum relative thickness taper of less than 2% is rolled during hot rolling in process step e). 
     
     
         5 . The process according to  claim 1 , characterized in that, after the first cold rolling stage, at the start of the recrystallization and decarburization annealing in process step i), the cold-rolled strip is heated at a heating rate of more than 100 K/s. 
     
     
         6 . The process according to  claim 1 , characterized in that the casting in process step b) is carried out at a superheat temperature of the smelt during casting of less than 40 K, preferably less than 20 K, in particular, less than 12 K. 
     
     
         7 . The process according to  claim 1 , characterized in that during casting in process step b), a 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. 
     
     
         8 . The process according to  claim 1 , characterized in that, as part of the recrystallization and decarburization annealing in process step i), the cold-rolled strip is also nitridation annealed, wherein in this nitridation annealing phase the total nitrogen content of the annealing atmosphere is increased by at least 50% of its initial value in process step i) by adding ammonia (NH 3 ) to the annealing gas, wherein the ammonia (NH 3 ) is preferably added separately and in particular is blown onto the two opposing large-area strip surfaces of the cold-rolled strip. 
     
     
         9 . The process according to  claim 1 , characterized in that after process step m), a process step that effects a magnetic domain refinement of the coated finished steel strip is carried out, particularly optionally. 
     
     
         10 . 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: 2.00-4.00 wt %,   C: 0.025-0.100 wt %,   Mn: 0.060-0.500 wt %,   Cu: 0.200-0.550 wt %,   Alsl: 0.010-0.030 wt %   S: <100 ppm,   N: 80-120 ppm,   
       and one or more elements from the group comprising Cr, V, Ni and Mo, each <0.100 wt %,
 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) heating the thin slabs, preferably in a linear furnace, to a temperature above 1050° C. and subjecting the slabs to homogenization annealing at a maximum of 1250° C., preferably at a maximum of 1200° C., in particular, at a maximum of 1150° C., 
 d) immediately prior to the first hot rolling pass in a subsequent hot rolling process, passing the slabs through an induction heating device, in particular, a high-frequency induction heating device, and heating the thin slabs to a temperature of at least 1250° C. and maximum of 1350° C., which is above the respective 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) pickling the hot-rolled strip, after reeling and prior to a subsequent cold rolling step, 
 h) cold rolling the hot strip in a first cold rolling stage to an (intermediate) thickness of 0.50 mm-0.80 mm, 
 i) subjecting the resulting cold-rolled strip to recrystallization and decarburization annealing at a strip temperature ranging from 820° C.-890° C. for a period of 300-600 seconds in a particularly moist, gaseous annealing atmosphere which acts on the cold-rolled strip and contains nitrogen (N 2 ) and hydrogen (H 2 ), and which has a water vapor/hydrogen partial pressure ratio pH 2 O/pH 2  of 0.30 to 0.60, 
 j) in a second cold rolling stage, cold rolling the cold-rolled strip which has been subjected to recrystallization and decarburization annealing to its (final) thickness or its nominal usable thickness of 0.15 mm-0.40 mm, 
 k) applying an annealing separator (non-stick layer), in particular containing primarily MgO, to the strip surface of the cold-rolled strip which has been rolled to its final thickness or usable thickness, 
 l) 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 >1150° C., forming a finished steel strip having a pronounced Goss texture, and 
 m) 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.