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 %, 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 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 (N2) and hydrogen (H2), and which has a water vapor / hydrogen partial pressure ratio pH2O / pH2 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.

Description

RELATED APPLICATIONS[0001]This application is a continuation-in-part application of U.S. application Ser. No. 14 / 514,575 filed Oct. 15, 2014.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]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, b) continuously casting the smelt by thin slab continuous casting, c) heating up the thin slabs, d) heating the thin slabs, e) subjecting the thin slabs to continuous hot rolling, f) cooling and reeling the hot-rolled strip to form a coil, g) pickling the hot-rolled strip after reeling and prior to subsequent cold rolling, h) cold rolling the hot-rolled strip in a first cold rolling stage to an (intermediate) thickness, i) subjecting the resulting cold-rolled strip to recrystallization and decarburization annealing, j) in a second cold rolling stage, cold rolling the cold strip that has...

AI Technical Summary

Benefits of technology

The invention proposes adding a high amount of copper to the smelted alloy, which helps to strengthen the inhibitor. This allows for slab reheating at lower temperatures, reducing the formation of liquid slag.

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.

This traditional “high heating method” has a number of economic disadvantages: For one, it requires a particularly large amount of energy.

But the greatest disadvantage is the formation of liquid slag at temperatures above 1350° C. due to a low-melting Fe—Si eutectic mixture.

This liquid slag results in material losses of >1%, and necessitates considerable expenditure on equipment needed to protect the annealing device.

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

For this reason, process routes that are based on thin slab technology, i.e., thin slab continuous casting, are all essentially low-heating methods, in which the slab through-heating temperature is substantially lower than 1300° C. However, at such temperatures an inherent inhibitor phase can no longer be formed during hot rolling because the precipitations that form these inhibitor phases cannot be dissolved prior to hot rolling.

However, at the low temperatures of the low-heating method, an inhibitor phase cannot be precipitated in the hot strip during the hot working process, i.e., during hot rolling.

In general, the problem with such / these processes is that, although the method of two-stage cold rolling with intermediate decarburization annealing is effective in preventing the formation of liquid slag during slab heating, the decrease in the slab heating temperature to levels that will enable the use of thin slab technology results in problems with respect to the formation of an inhibitor phase.