Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-06-17
2002-06-18
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S112000, C148S221000, C148S230000, C148S232000
Reexamination Certificate
active
06406557
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the treatment of silicon steel; in particular it relates to a process for transforming a sheet of grain oriented silicon steel, wherein an initial controlled amount of precipitates (sulfides and aluminum as nitride) is produced in the hot-rolled strip in a fine and uniformly distributed form, suitable for the control of the grain size during decarburization annealing; the control of the subsequent secondary recrystallisation is obtained by adding to the initial precipitates further aluminum as nitride, directly obtained in a continuous high-temperature treatment.
STATE OF THE ART
Grain oriented silicon steel for electrical applications is generally classified into two categories, basically differing in the level of induction, measured under the influence of a magnetic field of 800 As/m, this parameter being indicated as ‘B800’. Conventional grain oriented steels have B800 levels lower than 1890 mT; high-permeability grain oriented steels have B800 higher than 1900 mT. Further subdivisions have been made according to the so-called core losses, expressed in W/kg.
The conventional grain oriented steel, introduced in the thirties and the super-oriented grain steel, industrially introduced in the second half of the sixties, are essentially used for the production of cores of electric transformers, the advantages of the super-oriented grain product being its higher permeability, allowing cores of lower dimensions, and its lesser losses, allowing energy saving.
The permeability of electrical steel sheets is a function of the orientation of the cubic, body-centred iron crystals (grains); the best theoretical orientation is the one showing one corner of the cube parallel to the rolling direction.
Certain suitably precipitated products (inhibitors) called second phases, reduce the mobility of the grain boundary. Their use allows to obtain the selective growth of grains having the desired orientation; the higher the dissolution temperature in the steel of these precipitates, the higher the uniformity of orientation, the better the magnetic features of the end product. In the oriented grain, the inhibitor consists essentially of manganese sulfides and/or selenides, whereas in the super-oriented grain the inhibition is produced by a number of precipitates comprising said sulfides and aluminum as nitride, also in a mixture with other elements, from now on being referred to as aluminum nitride.
Nevertheless, in the production of the grain oriented and grain super-oriented steel, during solidification of the liquid steel and cooling of resulting solid, the inhibitors are precipitated in a coarse form, unsuitable for the desired purposes; therefore they must be dissolved and reprecipitated in the correct form, and so maintained until the grain having the desired dimensions and orientation is obtained at the stage of final annealing, after the cold rolling to the desired thickness and the decarburization annealing, i.e. at the end of a complex and costly transformation process.
Clearly the production problems, essentially due to the difficulty of obtaining good yields and constant quality, are mainly due to the measures to be taken for maintaining the inhibitors in the required form and distribution during the whole steel transformation process. In the case of the super-oriented product, a new technology has been developed in order to overcome these problems, as described e.g. in U.S. Pat. No. 4,225,366 and in EP 339474; these documents show the production of the aluminum nitride suitable for controlling the grain growth, by nitriding the strip preferably after the cold rolling step.
In the latter patent, aluminum nitride, precipitated in a coarse form during the slow solidification and the following cooling of the steel, is kept in this state by using low heating temperatures of the thick stab (lower than 1280° C., preferably lower than 1250° C.) before the hot rolling step; after the decarburization annealing, nitrogen is introduced in the sheet (essentially in proximity of its faces); it then reacts by producing silicon- and manganese-silicon nitrides having a relatively low solubilization temperature, which are dissolved during the heating phase in the final box-annealing. Nitrogen released in this manner can now deeply penetrate the sheet and react with aluminum, reprecipitating in a fine and homogeneous form along the whole thickness of the strip in the form of mixed alluminum and silicon nitride; this process requires the permanence of the material at 700-800° C. for at least four hours. In cited EP patent it is stated that the temperature of nitrogen introduction must be close to the decarburization temperature (about 850° C.), and in any case not higher than 900° C., in order to avoid an uncontrolled grain growth, given the absence of suitable inhibitors. In fact, the optimal nitriding temperature appears to be 750° C., whereas 850° C. represents the upper limit to avoid such uncontrolled growth.
This process seems to comprise certain advantages, such as the relatively low heating temperature of the slab before the hot rolling step, or the relatively low decarburization and nitriding temperatures; another advantage lies in the fact that there is no increase in production costs in maintaining the strip in the box-annealing furnace at 700-800° C. for at least four hours (with the purpose of obtaining the mixed aluminum and silicon nitrides necessary for a controlled grain growth), because the time required for heating the box annealing furnaces is approximately the same.
However the above cited advantages are associated to some disadvantages, among which: (i) the almost total lack of precipitates inhibiting the grain growth, due to the low heating temperature of the slab; as a consequence, any heating of the strip, i.e. during the decarburization and nitriding processes, has to be performed at relatively low and critically controlled temperatures, in order to prevent an uncontrolled grain growth under the above referred conditions; (ii) the impossibility of taking any measures during the final annealing step, in order to accelerate the heating time, e.g. by replacing the box-annealing furnaces with other furnaces operating in continuous.
DESCRIPTION OF THE INVENTION
The present invention aims at overcoming the disadvantages of the known production systems, by proposing a new process allowing the control within optimal limits of the size of the grain of primary crystallisation and, at the same time, allowing to perform a high-temperature nitriding reaction enabling the correction of the total useful inhibition content, up to the necessary values, directly during continuous annealing.
According to the invention, the continuously cast slab is heated at a temperature sufficient to dissolve a limited but significant amount of second phases like sulfides and nitrides, which are thereafter reprecipitated in a way suitable to control the grain growth up to the decarburization annealing, included. In the course of a further high-temperature treatment during the same continuous annealing, further aluminum-bonded nitrogen is precipitated, in order to adapt the total amount of second phases to the desired grain orientation during the secondary recrystallisation.
The present invention relates to a process for the production of an electrical steel sheet, wherein a silicon steel is continuously cast, hot-rolled and cold-rolled, and wherein the obtained cold strip is annealed in continuous in order to perform primary recrystallisation, decarburization, and thereafter (still under continuous conditions) nitriding, coated with an annealing separator, and box-annealed in order to perform a final secondary crystallisation treatment, said process being characterised by the combination in cooperation relationship of the following steps:
(i) producing a hot-rolled sheet in which the inhibition level (Iz) necessary to control the grain growth, calculated according to the empiric formula:
Iz=
1.91
Fv/r
(where Fv is the volumetric frac
Abbruzzese Giuseppe
Cicale′ Stefano
Fortunati Stefano
Matera Susanna
Acciai Speciali Terni S.p.A.
Coy Nicole
Hedman & Costigan ,P.C.
King Roy
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