Metal treatment – Stock – Magnetic
Reexamination Certificate
2000-11-06
2002-12-03
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C148S111000
Reexamination Certificate
active
06488784
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers to a process for controlling and guiding the secondary recrystallization in the production of grain oriented electrical steel strips and, more precisely, to a process in which during a continuous treatment after primary recrystallization it is possible to complete, or at least to start, the oriented secondary recrystallization.
STATE OF THE ART
It is known that, in the grain oriented electrical steel strips, the desired final magnetic characteristics are obtained through a complex series of interdependent transformations of the strip structure, which occur during a final treatment of secondary recrystallization. This step, here understood as the one in which the grains having Miller index <001> (110) develop with higher velocity, was up to now obtained during an extremely long annealing treatment at high temperature in static annealing furnaces (box annealing) in which tightly wound cold coils of the strip having the desired final thickness are introduced, having a weight typically comprised between 6 and 18 tons, which coils are annealed, cooled and then discharged. This static annealing also eliminates from the strip elements which would impair its final quality and forms on the strip surface a coating, called “glass film”, useful to electrically insulate the strip and to act as a substrate for further necessary coatings.
This box annealing, however, has some major disadvantages, among which the long duration of the treatment, requiring some days, and the fact that a single batch comprises a plurality of coils. Those coils, due to the high treatment temperatures and times, are deformed under their own weight, which makes it necessary to eliminate the deformed zones through a slitting operation. More scrap is produced due to sticking of adjacent coil spires, which occurs even if oxide powder annealing separators are utilised. Still more scrap is due to quality problems (deriving both from damages attributed to the handling of coils during loading and unloading of box annealing furnaces and from different treatment conditions experienced by the most external and the most internal spires of the coils during the slow annealing process) which call for the elimination of the initial and final spires of the coils. Moreover, the process imparts to the strips the form due to coiling, which strips will have to be further treated to bring them back to a flat shape, necessary for the manufacture of the final products, usually transformers cores.
Further disadvantages, deriving from the box annealing utilised for the metallurgical final treatment of the grain oriented strips, relates to the process control.
In fact, while on one hand the high temperature purification of the strip, substantially obtained through solid phase extraction of elements such as sulphur and nitrogen by interaction with the annealing atmosphere, is not critically influenced by the atmosphere and temperature differences along the coiled strip (longitudinal and transversal gradients), on the other hand the grain growth and oriented secondary recrystallization are greatly influenced by such differences. In fact, due to the microscopic scale of such metallurgical processes and to the peculiarities of the oriented secondary recrystallization, the course of the process is critically controlled by the physical and chemical “micro-environment” in which the different parts of the strip are.
To better clarify the importance of the process control during the final metallurgical annealing as well as the relevant difficulties linked to a static thermal treatment, some details will hereinafter be exposed with reference to the state of the art and to the physical and chemical phenomena occurring during the treatment. The final result of the oriented secondary recrystallization is a polycrystalline structure iso-oriented along the crystallographic direction of easier magnetisation (<100> according to the convention of the Miller indexes), with an angular dispersion, for a good industrial product, lesser than 100. This is obtained through a delicate process which selects for the growth only crystals already having the above orientation, such crystals representing, before the final annealing, a very small fraction of the starting microstructure. In this process, a dimensional change occurs in the product structure which varies from some micrometers before the annealing to some millimeters after.
The desired result of this process, difficult to obtain at industrial scale, strongly depends on the treatment conditions preceding the final annealing and determining geometry, the superficial state and the microstructure of the strip. As already mentioned, this result is obtained during the final metallurgical annealing in a way critically controlled by the evolution kinetics of the dimensions of some. particles such as sulphides and nitrides present in the metallic matrix and by the diffusion of relevant composing elements between the same particles as well as towards the strip surface, and through the latter towards both the exterior and the interior of the metal matrix. The last two phenomena are controlled by interaction with the annealing atmosphere (micro-environment).
Even small variations in the kinetics of said processes (as well as of the temperatures at which the same are activated and developed) in different zones of the strip depending on different micro-environments produced during the box annealing, bring to differences in the development of the grain growth, which in the best case mean final grain dimensions and orientation different from zone to zone, entailing variations of the magnetic characteristics along the strip and in the transverse direction.
In more critical cases, which however are not so rare in the industrial practice, such differences lead to a loss of control in the oriented secondary recrystallization, with totally inadequate magnetic characteristics in part of the final product, which must thus be further conditioned at the end of the production cycle, or downgraded or scrapped.
For analogous reasons the chemical reactions at the surface depend on the micro-environment: for instance, the superficial oxidation layer evolution with time and during the thermal treatment strongly influences the exchange reaction between the metal matrix and the annealing atmosphere, further complicating the already delicate aspects of the metallurgical process control.
The differences between the different superficial reactions induced by the different micro-environments depending on the coil geometry (head and tail of the strip, external layers and core of the coil, and so on) more directly lead to differences in morphology and composition of the superficial layer of the strips.
The superficial characteristics are another important aspect of the grain oriented strips, in that they directly or indirectly influence the magnetic and insulation characteristics thereof. Thus, variations of the superficial quality along the strip constitute an industrial problem of product quality and hence of process control. It is clear now that the box annealing of grain oriented electrical steel strips having the final thickness, utilised to start and develop the oriented secondary recrystallization, as well as to modify surface structure and morphology and to purify the matrix of some elements not desired in the final product, is a treatment technique for some aspects inconvenient and expensive, in that requires a large number of plants to sustain an adequate production capacity, has a low productivity, physical yields difficult to control, and above all do not allows to perform a process control absolutely necessary for such a complicated production and which is present in all the other production steps, form the steel shop production to the primary recrystallization.
As already said, the secondary recrystallization process consists, in this kind of products, in the selective growth of some grains having a specific orientation with respect to the rolling direction and the strip
Abbruzzese Giuseppe
Cicale′ Stefano
Fortunati Stefano
Abelman ,Frayne & Schwab
Acciai Speciali Terni S.p.A.
Sheehan John
LandOfFree
Process for the production of grain oriented electrical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the production of grain oriented electrical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the production of grain oriented electrical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2956313