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 1250° C., d) heating to a temperature between 1250° C. and 1350° 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.

Description

RELATED APPLICATIONS[0001]This application is a continuation-in-part application of U.S. application Ser. No. 14 / 638,338 filed Mar. 4, 2015, which is a continuation-in-part of application Ser. No. 14 / 514,620 filed Oct. 15, 2014.BACKGROUND OF THE INVENTION1. 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:[0003]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: 0.160-0.300 wt %, Cu: 0.100-0.300 wt %, Alsl: 0.020-0.040 wt %, Sn: 0.050-0.150 wt %, S: <100 ppm, preferably ≦50 ppm, N: <100 ppm, and one or more elements from the group comprising Cr, V, Ni, Mo and Nb,[0004]b) continuously casting the smelt by thin slab continuous casting without exposure of the strand to inert gas...

AI Technical Summary

Benefits of technology

This patent text describes a method for improving the process of nitridation, which involves heating and treating a metal strip with a nitrogen gas to create a protective layer that prevents corrosion. The problem is that previous steps in the process, such as decarburization annealing, can lead to the formation of oxidative barrier layers that interfere with the subsequent nitridation. To solve this, the method introduces an intermediate reduction annealing phase where a drier annealing atmosphere with lower potential for oxidation is used to correct any local superoxidation that formed during decarburization annealing. This step ensures a more uniform and reproducible nitridation treatment, leading to better quality and performance of the metal strip. The intermediate reduction annealing phase lasts a maximum of 40 seconds at a temperature range of 820-890°C. This method provides significant inhibition during hot-rolling, but further inhibition is still required, so nitridation annealing is also employed to provide further inhibition and stability of the metal strip's microstructure for subsequent processing steps.

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 1350° 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.