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 (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.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The invention is directed to a process for producing grain-oriented electrical steel strip by means of thin slab continuous casting, said process comprising the process steps of:a) smelting a steel with a smelt which, particularly 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 ofc′) heating up the thin slabs, preferably in a linear furnace, to a temperature above 1,050° C. a...

AI Technical Summary

Benefits of technology

This patent proposes a method for improving the process of nitriding a strip of material. The method includes a step of reducing the oxidative barrier layers that can interfere with the nitridation process by using an intermediate reduction annealing phase. This phase is carried out under a drier annealing atmosphere with lower potential for oxidation. The reduction annealing phase is followed by a nitriding step, which further stabilizes the microstructure of the material and prevents any parasitic grain growth processes. Overall, the method leads to a more homogeneous and reproducible nitridation treatment.

Problems solved by technology

However, the grain growth inhibiting effect of the MnS phase is limited, so that, assuming customary hot-rolled strip thicknesses of, e.g., 2.30 mm, at least two cold rolling stages are required to bring the steel strip to its nominal usable thickness, with an intermediate recrystallization annealing being performed between the individual cold rolling stages.

However, it carries with it the disadvantage that it can achieve magnetic values only at the level of CGO material, and not those of High permeability Grain Oriented or HGO material.

All of the aforementioned inherent inhibitors that are produced in the hot strip require very high slab reheating temperatures greater than 1,350° C. In addition to requiring substantial energy input and high industrial expenditure, these temperatures result in a high occurrence of liquid slag (>1%) due to a relatively low-melting Fe—Si eutectic mixture.

In addition to the resulting substantial losses in mass yield, industrial annealing systems are heavily stressed, further adding to cost.

However, in these processes the inhibitor phase cannot be formed in the hot-rolled strip because the substances used as inhibitor particles cannot be dissolved out sufficiently at these temperatures to allow them to be re-precipitated finely dispersed in the subsequent process.

Inherent inhibition can thereby be formed only to a limited extent, and alone would not be sufficient to produce satisfactory magnetic characteristics in the finished material / finished strip.

However, this disadvantage can be overcome by combining this process with nitridation treatment, because the resulting additional acquired inhibition is enough to achieve sufficient total inhibition.

Sulfides are not used as acquired inhibitor phases, because sulfur can penetrate into the matrix only via vacancy diffusion, which would be far too slow, even with thermal activation.

Due to the limited thermal resistance of the thin slabs and the need to transport them through a roller hearth furnace, the temperature that can be reached by heating is limited by the thickness of the slab.

Producing grain-oriented electrical steel strip by means of thin slab continuous casting followed by homogenization annealing and hot rolling in line, cold-rolling the strip to its nominal usable thickness, and later nitridation annealing the strip to introduce an acquired grain growth inhibitor phase still results in practice in fluctuations in the ultimate magnetic characteristics across the length and width of the finished strip, and as a consequence, a decrease in the quality of the finished strip.