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

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 (N2) and hydrogen (H2) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH2O/pH2 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 (N2) and hydrogen (H2) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH2O/pH2 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 (N2) and hydrogen (H2) and acts on the cold-rolled strip, and which has a water vapor/hydrogen partial pressure ratio pH2O/pH2 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.