Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-02-26
2001-08-14
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S112000, C148S230000, C148S541000, C164S476000, C164S477000, C164S478000
Reexamination Certificate
active
06273964
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers to a process for the production of grain oriented electrical steel strip starting from thin slabs, and more precisely refers to a simplified process for the production of grain oriented electrical steel which provides a consistent and superior quality product.
STATE OF THE ART
Grain oriented electrical silicon steel is generically classified into two main categories, essentially differing in relevant induction value measured under the effect of an 800 As/m magnetic field, called B800 value; the conventional grain oriented product has a B800 lower than about 1890 mT, while the high-permeability product has a B800 higher than 1900 mT. Further subdivisions are made considering the core losses value, expressed in W/kg at given induction and frequency.
Said products have essentially the same field of application, mainly for the production of transformer cores. The high-permeability, oriented grain steel finds application in those fields in which the advantage of high permeability and low core losses can compensate for its higher cost relative to conventional products.
The grain orientation of electrical steel strips is obtained by finely precipitated second phases in a one of the last production steps which is known as secondary recrystallization. During secondary recrystallization, the growth of the grains or crystals of iron (body centered cube) are inhibited up to a certain temperature, beyond which, in a complex process, the crystals having an edge parallel to the rolling direction and a diagonal plane parallel to the strip surface (Goss structure), selectively grow.
The second phases, i.e. non-metallic precipitates within the solidified steel matrix, which are utilized to obtain the growth inhibition are mainly sulfide, and/or selenides, particularly of manganese, for the conventional oriented grain steels and nitrides, particularly containing aluminum, for the high-permeability oriented grain steels.
The intrinsic complexity of the oriented grain electrical steel production process is essentially attributable to the fact that said second phases precipitate in coarse form, during the relatively slow cooling of the continuously cast slab. The coarse form is useless for providing the desired effect in oriented grain steel product. For the desired effect, the coarse grains must be dissolved and reprecipitated in the proper form which must be maintained up to the moment during the final secondary recrystallization step when the grain is obtained which has the desired dimensions and orientation.
From the above, the following idea can be derived, that a quicker cooling during the continuous casting should improve the inclusional state of the slabs, thus rendering less complex the control of the various steps of the slab transformation process into strips. However, it was found that the thin slab continuous casting though having a cooling rate quite higher than the one obtainable in the conventional continuous casting, is not sufficient per se to allow obtaining the necessary quality.
This applicant has extensively studied the possibility of utilizing the technologies of thin slab or continuous strip casting, which has been utilized extensively for carbon steels and for more sophisticated steels such as silicon electrical steels. In this field, good results were obtained with conventional oriented grain and in one of the high magnetic characteristics of oriented grain steels.
DESCRIPTION OF THE INVENTION
The present invention aims to improve the conventional grain oriented electrical steel production, utilizing in an innovative way the thin slab continuous casting technology and introducing specific modifications of the transformation process.
In particular, the continuous casting process is carried out in such a way that a particular equiaxic to columnar grains ratio is obtained, as well as specific equiaxic grains dimensions and precipitates of limited dimensions.
The present invention refers to a silicon steel strip production process of the kind above identified as conventional, in which a silicon steel is continuously cast, high-temperature annealed, hot rolled, cold rolled in a single step or in a plurality of steps with intermediate annealings, the cold rolled strip so obtained is annealed to perform primary annealing and decarburization, coated with annealing separator and box annealed for the final secondary recrystallization treatment, said process being characterized by the combination in cooperation relationship of:
(i) continuously casting a thin slab of the following composition: 2 to 5.5 wt % Si, 0.05 to 0.4 wt % Mn, <250 ppm (S+5.04 Se), 30 to 130 ppm N, 0.05 to 0.35 wt % Cu, 15 to 300 ppm C, and 200 to 400 ppm Al, remaining being iron and minor impurities, and having a thickness of between 40 and 70 mm, preferably of between 50 and 60 mm, with a casting speed of 3 to 5 m/min, a steel overheating at the casting lesser than 30° C., preferably lesser than 20° C., such a cooling speed as to obtain a complete solidification between 30 to 100 s, preferably between 30 and 60 s, a mould oscillation amplitude of between 1 and 10 mm, and an oscillation frequency of between 200 ans 400 cycles per minute;
(ii) equalizing the thus obtained slabs and hot rolling them, after which the strip cooling is delayed for at least 5 seconds after the strip leaves the last rolling stand;
(iii) directly sending the strip to the cold rolling, avoiding the usual annealing step;
(iv) cold rolling in a single step or in a plurality of steps if necessary with intermediate annealing, with a reduction ratio in the last step of at least 80%, and maintaining a rolling temperature of at least 200° C. in at least two rolling passes during the last step;
(v) continuously annealing the cold rolled strip for a total time of 100 to 350 s, at a temperature comprised between 850 and 1050° C. in a wet nitrogen/hydrogen atmosphere, with a pH
2
O/pH
2
comprised between 0.3 and 0.7;
(vi) coating the strip with annealing separator, coiling it and box annealing the coils in an atmosphere having the following compositions during the heating-up: hydrogen mixed with at least 30% vol nitrogen up to 900° C., hydrogen mixed with at least 40% vol nitrogen up to 1100-1200° C., then maintaining the coils at this temperature in pure hydrogen.
In the hot rolling, the slabs are treated with a rolling starting temperature of 1000 to 1200° C. and a finishing temperature of 850 to 1050° C.
The steel composition can be different from the conventional one, in that very low carbon contents can be contemplated, between 15 and 100 ppm.
There can also be a copper content of between 800 and 2000 ppm. During the continuous casting, the casting parameters are chosen to obtain an equiaxic to columnar grain ratio of between 35 and 75%, equiaxic grain dimensions less than 1.5 mm, and mean second phase dimensions not greater than 0.06 micrometers.
Such an intermediate product is of paramount importance for a trouble-free development of the remaining of the process and for the final product quality.
If during the decarburization annealing the temperature is maintained below 950° C., the nitrogen content in the atmosphere of the subsequent box-annealing can be so controlled as to allow a nitrogen quantity lesser than 50 ppm to diffuse into the strip.
Such nitrogen absorption can also be obtained in the continuous furnace, after the decarburization annealing, maintaining the strip at a temperature comprised between 900 and 1050° C., preferably over 1000° C., in a nitriding atmosphere, e.g. containing NH
3
up to 10% volume. In this case water vapour must be present in a quantity comprised between 0.5 and 100 g/m
3
.
The above steps of the process can be interpreted as follows:
The steel treatments after the slab formation as well as the results obtainable with such treatments strongly depend on the way in which the steel solidifies, the type and dimensions of the steel grains as well as distribution and dimensions of non-metallic precipitates. For instance, very slow cooling rates enh
Abbruzzese Giuseppe
Cicale' Stefano
Fortunati Stefano
Acciali Speciali Terni S.p.A.
Hedman & Costigan ,P.C.
Yee Deborah
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