Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-06-22
2002-10-29
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S111000, C148S112000
Reexamination Certificate
active
06471787
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the production of oriented-grain electrical steel sheet with high magnetic characteristics, and more precisely to a process in which the slab obtained from continuous casting is annealed at a temperature that enables dissolution of part of the sulphides and nitrides present, to be subsequently re-precipitated in a form that is suitable for controlling the grain size during decarburization annealing, and which enables a subsequent high-temperature continuous heat treatment phase during which, by nitrogen diffusion throughout the thickness of the strip, aluminium is directly precipitated as nitride, complementing the second-phases fraction necessary to control the grain orientation in the end product.
STATE OF PRIOR ART
Oriented-grain silicon steel for electrical applications is generically classified into two categories, basically differentiated by the value of magnetic induction measured under the action of a magnetic field of 800 amp-turn/m, designated with the code B800: the category of conventional oriented-grain silicon steel, with B800 less than 1890 mT, and that of high-permeability oriented-grain silicon steel, with B800 higher than 1900 mT. Further subdivisions exist according to the so-called core losses, which are expressed in W/kg.
Conventional oriented-grain silicon steel, introduced in the thirties, and the super-oriented grain silicon steel having better permeability, introduced industrially in the second half of the sixties, are basically used for the production of cores, for electric transformers, the advantages of the super-oriented grain product regarding the higher permeability, which makes possible smaller-sized cores and lower losses, with resultant energy saving.
In electrical strips, permeability is a function of the orientation of the body-centered cubic crystals (grains) of iron, which must have a corner parallel to the direction of rolling. By using certain appropriately precipitated precipitates (inhibitors), the so-called second phases, which reduce the mobility of the grain boundaries, selective growth is obtained only of the grains having the desired orientation. The higher the temperature of dissolution in the steel of these precipitates, the greater the uniformity of orientation, and the better the magnetic characteristics of the end product. In the oriented-grain steel, the inhibitor consists prevalently of manganese sulphides and/or selenides, whilst in the super-oriented grain steel the inhibitor consists primarily of aluminium containing nitride.
However, in the production of super-oriented electrical strips, -during solidification of the liquid steel and the subsequent cooling of the resultant solid, the sulphides and aluminium nitride are precipitated in a coarse form, unsuitable for the desired purposes. They must therefore be redissolved and re-precipitated in the right form, and kept in this state up to the moment when grains of the desired size and orientation are obtained, in a final annealing stage, after cold-rolling to the desired final thickness and decarburization annealing, at the end of a complex and costly process of transformation.
It is evident that the production problems, which basically regard the difficulty of obtaining good yields and a constant quality, are to a large extent due to the necessary precautions to be taken to keep the aluminium nitride in the required form and distribution during the entire process of steel transformation.
To reduce these problems, a technology has been developed in which the aluminum nitride suitable for controlling the growth of the grains is produced via nitriding of the strip, preferably after cold-rolling, as described in U.S. Pat. Nos. 4,255,366, 3,841,924 4,623,406 and in European Pat. No. EP0339 474.
In the latter patent, the aluminium nitride, which is coarsely precipitated during the slow solidification of the steel, is kept in this state by the low temperature adopted for heating the slabs (i.e., lower than 1280° C., preferably lower than 1250° C.) before hot-rolling. After decarburization annealing, nitrogen is introduced, which immediately reacts producing, mainly in the surface layers of the strip, silicon nitrides and manganese and silicon nitrides, which have a relatively low solubilization temperature and which are dissolved in the final box annealing. The nitrogen thus liberated diffuses throughout the strip and reacts with the aluminium, re-precipitating in a fine and homogeneous form throughout the thickness of strip as a mixed aluminium and silicon nitride. This process entails the need to keep the material at 700-800° C. for at least four hours. In the above patent, it is stated that the temperature of introduction of the nitrogen must be close to the decarburization temperature (approx. 850° C.), and at all events certainly not higher than 900° C., to prevent an uncontrolled growth of the grains, in view of the lack of suitable inhibitors. Actually, the optimal nitriding temperature appears to be 750° C., whereas 850°C. is an upper limit, in order to prevent such uncontrolled growth. EP Application 539.858 follows the general ideas of the above EP Patent, imposing some further limitations on slab heating temperatures, at or below 1200 ° C.
U.S. Pat. Nos. 3,841,924 and 4,623,406 refer to a more classic process, in which the inhibitor is formed at the stage of hot rolled strip and there is no nitriding before final secondary recrystallization.
This process seems to involve certain advantages, such as the relatively low temperatures of heating of the slab before hot rolling, of decarburization and of nitriding as well as the fact that the need to keep the strip during box-annealing at a temperature of between 700° C. and 800° C. for at least four hours (with the aim of obtaining the mixed nitrides of aluminium and silicon necessary for controlling grain growth) does not add to the production cost, in so far as the heating of the box-annealing furnaces requires similar lengths of time in any case.
However, along with the advantages referred to above, there are also a number of disadvantages, among which: (i) owing to the low temperature of heating of the slabs, the sheet is very poor in precipitates useful as inhibitors of grain growth; consequently, all the strip heating cycles, in particular in the decarburization and nitriding processes, must be carried out at relatively low and critically controlled temperatures, in that in such conditions, the grain boundaries are very mobile, which implies the risk of an uncontrolled grain growth; (ii) it is impossible to introduce, in the final annealings, any improvements that might accelerate the heating times; for example, by replacing box-annealing furnaces with other furnaces of a continuous type.
DESCRIPTION OF THE INVENTION
The present invention aims at overcoming the drawbacks of the known production systems by proposing a process in which a slab of silicon steel for electrical applications is heated evenly at a temperature that is decidedly higher than the one adopted in cited know processes involving strip nitriding, but lower than the temperature of the classic process of production of high-permeability steel sheet, and then hot-rolled. The strip thus obtained undergoes two-stage rapid annealing followed by quenching, and is then cold-rolled, if necessary with a number of rolling steps at a temperature of between 180° C. and 250° C. The cold-rolled sheet first undergoes decarburization annealing and then nitriding annealing at a high temperature in an atmosphere containing ammonia.
There follow the usual final treatments, among which the deposition of the annealing separator and the secondary-recrystallization final annealing.
The present invention refers to a process for producing steel sheet with high magnetic characteristics in which a silicon steel containing from 2.5% to 4.5% of silicon; from 150 to 750 ppm, preferably from 250 to 500 ppm, of C; from 300 to 4000 ppm, preferably from 500 to 2000 ppm, of Mn; less than 120 ppm, preferably from 50 t
Abbruzzese Giuseppe
Cicale' Stefano
Fortunati Stefano
Acciai Speciali Terni S.p.A.
Hedman & Costigan ,P.C.
Oltmans Andrew L.
Sheehan John
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