Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
1999-06-22
2001-12-04
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S111000, C148S230000, C148S221000, C148S231000
Reexamination Certificate
active
06325866
ABSTRACT:
The present application is the national stage filing of and claims priority to International Application No. PCT/EP97/04005, filed Jul. 24, 1997 and Italian Application Serial No. RM96A000905, filed Dec. 24, 1996.
1. Field of Invention
The present invention refers to a process for the production of grain oriented silicon steel sheet, and more precisely refers to a process that enables optimization of the production of grain oriented silicon steel strips, of a conventional type, via an appropriate synergistic combination between the specific choice of the composition levels of some elements and appropriate treatments enabling to control presence and type of inhibitors, and hence the primary-recrystallization grain size, as well as the secondary recrystallization conditions.
2. Prior Art
Silicon steel sheets are used basically for the manufacture of electric transformer cores.
Silicon steel consists of many adjacent to one another, grains having a cubic body-centred lattice where the axes corresponding to the corners of the cube, crystallographically designated by [100], constitute directions of easy magnetization.
Given:
(i) the structure of transformer cores, consisting of stacks of magnetic laminations made from silicon steel strip cut parallel with respect to the length of the rolled strip and combined to form a torus, and
(ii) the working scheme of the transformers themselves, in which the passage of current in the primary winding induces a magnetic flux in the core, which propagates through the core itself, it is evident how the work necessary for the magnetic flux to propagate is a function of the resistance that it encounters, and hence it is evident how the axes [100] must be parallel to the rolling direction of the strip, and hence to its length. In addition, it is obvious that it is not possible to have all the grains oriented exactly in the optimal way described above, and hence that big efforts have had to be made to reduce the degree of disorientation of the grains.
Furthermore, it is necessary to maintain the number and size of such grains within certain limits, which are well-known to those expert in the field.
Only by respecting these general conditions is it possible to obtain a material having good magnetization characteristics, among which magnetic permeability, expressed as density of magnetic flux caused in the core by a magnetic field of a given value, and dissipation of energy during operation, usually referred to as core losses at given frequency and permeability, and expressed in W/kg.
Correct orientation of the grains in the end product is obtained during a thermal treatment called secondary-recrystallization annealing, in which the growth only of the crystals originally having the desired orientation is possible. The number and orientation of the final grains depend to a certain extent on the corresponding initial values.
The grain growth process is activated by heat and is due to the fact that certain crystals, which for kinetic or energetic reasons are more “energized” than others, start growing at the expense of the adjacent crystals, at a temperature lower than is the one at which the other crystals are activated, thus reaching earlier the critical size that enables them to predominate in the growth process.
However, as is well known, the production process of grain oriented silicon steel sheet involves numerous heating cycles at high temperatures, during some of which grain growth could start, which, if it were to occur in not appropriate ways or times, would not allow the desired end results to be achieved.
Secondary recrystallization is controlled by some compounds, such as manganese suiphide, manganese selenide, aluminium nitride, etc., which, when appropriately precipitated in the steel, inhibit grain growth until they are solubilized, thus enabling initiation of secondary recrystallization. The higher the solubilization temperature of these compounds (also called inhibitors), the better their capacity to control grain growth, and the better the quality of the end product. Oriented-grain silicon steel for electrical applications is generically classified into two categories, basically differentiated by the levels of the magnetic induction value, expressed in mT, measured under the action of a magnetic field having the value of 800 amp-turn/m, designated with the code B800: the category of conventional grain oriented silicon steel, the so-called OG, with B800 values of up to approximately 1880 mT, and that of super-oriented grain silicon steel, with B800 values of over 1900 mT.
Conventional grain oriented silicon steel, introduced in the thirties, uses as inhibitors essentially manganese sulphides and/or selenides, whereas super-oriented grain silicon steel uses essentially aluminium-based nitrides, containing also other elements, such as silicon. For the sake of simplicity of presentation, hereinafter we shall refer to these inhibitors as aluminium nitrides.
The use of aluminium nitrides has enabled the achievement of very high-quality results, but has also entailed certain production problems due, to a large extent, to the following requirements:
higher carbon content;
higher reduction rate in cold-rolling;
adopting necessary precautions to maintain, from the hot-rolling phase to the final secondary-recrystallization annealing phase, two types of inhibitors simultaneously, namely sulphides and aluminium nitrides, in the optimal size and distribution for achieving the desired results.
Also in the production of conventional grain oriented silicon steel, difficulties are encountered in controlling the size and distribution of the inhibitors, even though at less extreme levels than in the case of the higher-quality product.
However, the production of a good-quality grain oriented silicon steel sheet is complex and costly, and it is evident how it is necessary to apply in a particularly careful way all possible techniques to enable reduction of production costs.
Consequently, in the production of conventional grain oriented silicon steel sheet, aluminium is not used in that it is considered an element that adversely affects the magnetic characteristics of the product because it forms undesired oxide precipitates, and the complications that it introduces into the process raise the cost of the treatment to an absolutely unacceptable extent.
This Applicant, which is one of the leading producers in Europe of steels for electrical applications, since a long time has been studying solutions aimed at optimizing the production and quality of grain oriented silicon steels, both in the category of super-oriented grain steels and in that of conventional grain oriented steels. In particular, for the latter type of product, the applicant has studied methods for eliminating, or in any case reducing, the critical aspects of the production process.
In previous patent applications, processes have been proposed in which silicon steel undergoes continuous casting to form a thin flat bloom, typically having a thickness of 40-70 mm, to exploit the favourable solidification structure which presents a preponderance of the so-called monodirectional small-sized grains, and the fine and well-distributed structure of the second phases, i.e., of the precipitates that inhibit grain growth. In addition, a concept that had been expressed in numerous patents of Japanese origin has been adopted, according to which it is possible to ignore completely the necessity to obtain a fine and well-distributed precipitate starting from the initial phases of the process; on the contrary the precipitates obtained during steel solidification have to remain as coarse as possible, whilst the precipitates necessary to control the secondary-recrystallization process are advantageously obtained during the slow heating phase preceding the above-mentioned secondary recrystallization.
This Applicant, however, has noticed that, in this way, during most of the process it is necessary to proceed in a particularly controlled way to prevent an uncontrolled grain growth, due to the fa
Abbruzzese Giuseppe
Cicale′ Stefano
Fortunati Stefano
Acciai Speciali Terni S.p.A.
Coy Nicole
Hedman & Costigan ,P.C.
King Roy
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