Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-02-26
2001-10-02
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S112000, C148S307000, C148S230000, C164S476000, C164S477000, C164S478000
Reexamination Certificate
active
06296719
ABSTRACT:
This application is a 371 of PCT/EP97/03 921 filed Jul. 21, 1997.
FIELD OF THE INVENTION
The present invention refers to a process for the production of grain oriented electrical steel strip having high magnetic characteristics, starting from thin slabs, and more precisely refers to a process in which the casting conditions are controlled to obtain such microstructural characteristics in the thin slab (high ratio of equiaxic to columnar grains, equiaxic grains dimensions, reduced precipitates dimensions and specific distribution thereof) as to simplify the production process still permitting to obtain excellent magnetic characteristics.
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. The conventional oriented grain steel sheet was first produced in the '30 ties and still has an important range of utilization; the high-permeability oriented grain steel came in the '60 ties second half and also has many applications, mainly in those fields in which its advantages of high permeability and of lower core losses can compensate for the higher costs with reference to the conventional product.
In the high-permeability electrical sheets, the higher characteristics are obtained utilizing second phases (particularly AlN) which, duly precipitated, reduce the grain boundary mobility and permit the selective growth of those grains (body-centered cubic) having an edge parallel to the rolling direction and a diagonal plane parallel to the sheet surface (Goss structure), with a reduced disorientation with respect to said directions.
However, during the liquid steel solidification the AlN which is responsible for better results, precipitates in coarse form, which will not provide the desired results, and must be dissolved and reprecipitated in the right form which has to be maintained up to the moment when the grain structure is obtained having the desired dimensions and orientation. This is achieved during a final annealing stage, after cold rolling to the final thickness, at the end of a complex and costly transformation process. It was immediately recognized that the production problems, mainly referred to the difficulties in obtaining good yields and uniform quality, were mainly attributable to all of the precautions required to maintain AlN in the necessary form and distribution during the whole steel transformation process.
In this respect, a technology was developed, for instance described in U.S. Pat. No. 4,225,366 and in EP patent 339,474, in which the aluminum nitride apt to control the grain growth process is produced by means of strip nitriding, preferably after cold rolling.
In this technology, the aluminum nitride coarsely precipitated during the slow solidification of the steel is maintained in this state utilizing low slab heating temperatures (lower than 1280° C. preferably lower than 1250° C.) before hot rolling; the nitrogen introduced into the strip after its decarburization immediately reacts forming silicon and manganese/silicon nitrides, which have a relatively low solution temperature and are dissolved during the final box annealing; the thus obtained free nitrogen diffuses through the strip and reacts with aluminum, reprecipitating in fine and homogeneous form along the strip thickness as mixed aluminum/silicon nitride; this process requires maintaining the steel at 700-850° C. for at least four hours.
In the above patents it is stated that the nitriding temperature must be near to the decarburizing one (about 850° C.) and anyhow must not exceed 900° C., to avoid an uncontrolled grain growth, due to the lack of suitable inhibitors. In effect, the best nitriding temperature seems to be of 750° C., the temperature of 850° C. being an upper limit to avoid uncontrolled grain growth.
This process seems to comprise some advantages, such as the relatively low temperatures of slab heating before hot rolling, of decarburization and of nitriding, and the fact that the need to keep the strip at 700-850° C. for at least four hours in the box-annealing furnace (to obtain mixed aluminum/silicon nitrides necessary for the grain growth control) does not add to the over-all production costs, in that the heating of the box annealing furnace in any case requires similar time.
However, the above only seem to be advantages. in that: (i) the low slab heating temperature keeps the coarse form of the aluminum nitride precipitates, unable to control the grain growth process, hence all the subsequent heatings, particularly in the decarburization and nitriding processes, must take place at relatively low, carefully controlled temperatures, precisely to avoid uncontrolled grain growth; (ii) the treating times at such low temperatures must be consequently prolonged; (iii) it is impossible to introduce, in the final annealings, possible improvements to speed-up the heating time, for instance utilizing continuous furnaces instead of the discontinuous ones of box annealing.
DESCRIPTION OF THE INVENTION
The present invention is intended to obviate to the drawbacks of known production processes, opportunely utilizing the thin slab continuous casting process, to obtain thin silicon steel slabs having specific solidification and microstructural characteristics, permitting to obtain a transformation process free of a number of critical steps. In particular, the continuous casting process is conducted so as to obtain in the slabs a given ratio of equiaxic to columnar grains, specific dimensions of equiaxic grains and fine precipitates.
The present invention refers to a production process of high magnetic characteristics silicon steel strip, in which a steel containing, in weight percent, 2.5-5 Si, 0.002-0.075 C, 0.05-0.4 Mn, S (or S+0.504 Se)<0.015, 0.010-0.045 Al, 0.003-0.0130 N, up to 0.2 Sn, 0.040-0.3 Cu, remaining being iron and minor impurities, 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 having a thickness of between 20 and 80 mm, preferably of between 50 and 60 mm, with a casting speed of 3 to 5 m/min, a steel overheating at the casting of between 20 and 40° C., such a cooling speed as to obtain a complete solidification within 30 to 100 s, a mould oscillation amplitude of between 1 and 10 mm, and an oscillation frequency of between 200 and 400 cycles per minute;
(ii) equalizing the thus obtained slabs at a temperature comprised between 1150 and 1300° C.;
(iii) hot rolling the equalized slabs with a starting rolling temperature of between 1000 and 1200° C. and a finishing rolling temperature of between 850 and 1050° C.;
(iv) continuously annealing the hot rolled strips for 30 to 300 at a temperature of between 900 and 1170° C., cooling the same at a temperature no lesser than 850° C. and maintaining said temperature for 30 to 300 s, and then cooling them, possibly in boiling water;
(v) cold rolling the strip in a single step or in a plurality of steps with intermediate annealings, the last step being performed with a reduction ratio of at least 80%, maintaining a rolling temperature of at least 200° C. in at least two rolling passes during the last step;
(vi) 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, wi
Abbruzzese Giuseppe
Cicale' Stefano
Fortunati Stefano
Acciai Speciali Terni S.p.A.
Hedman & Costigan ,P.C.
Yee Deborah
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