Metallurgical apparatus – Means treating a continuum of work – With heating means
Patent
1996-06-26
1998-12-01
Kastler, Scott
Metallurgical apparatus
Means treating a continuum of work
With heating means
266114, C21D 162
Patent
active
058433678
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a device for the accelerated cooling of a continuous substrate moving rapidly in a vertical plane, more particularly a coated metal strip, for example a hot-galvanized sheet.
The cooling device, which is the subject of the present invention, will be intentionally described for the purpose of making it easier to understand within the scope of its use when cooling a galvanized steel strip after a galvannealing heat treatment, but this description in no way precludes the device of the invention from being able to be applied in other circumstances requiring similar cooling conditions.
BACKGROUND OF THE INVENTION
The heat treatment called "galvannealing" consists in subjecting a hot-galvanized steel sheet successively to a heating operation, a temperature hold and then a cooling operation.
The purpose of this treatment is to ensure that iron in the sheet diffuses through the coating layer, generally a zinc or zinc-alloy coating layer, so as to obtain iron contents of between 7% and 13% in the aforementioned coating layer. This heat-treatment operation has the effect of producing a coating having better drawability and weldability.
The conventional galvannealing operation is carried out in a plant in which the sheet performs at least two vertical passes, namely an ascending pass and a descending pass.
Diagrammatically, the sheet may be said to leave the galvanizing bath vertically to start its ascending phase and, directly after the wiping device consisting of air knives, enters a heating oven which heats the sheet to a temperature of between 460.degree. C. and 600.degree. C.; next, the sheet travels through a temperature-hold oven. Lying above this oven is at least one cooling device intended to ensure that the coating on the sheet solidifies in such a way that this coating is not damaged on the guide rollers terminating the ascending phase and guiding the sheet into the descending phase.
After the galvannealing operation, the quality of the coating obtained has proved to be directly related to the diffusion of iron into the zinc or zinc-alloy layer and this quality is favourably influenced by a sufficiently long temperature-hold duration, for example greater than 10 seconds.
Now, the length of the ascending phase is limited by mechanical-construction problems and by problems of controlling the vibrations produced in the sheet at the air knives.
SUMMARY OF THE INVENTION
In order to use to the best advantage the plant length available for carrying out the heating, temperature-hold and cooling operations before the sheet passes over the guide rollers located at the top of the ascending phase, the Applicant has developed a process in which both the heating power and the cooling flux are increased so as to limit the space required for these two operations and thereby to maintain a length sufficient to obtain as long as possible a passage time in the temperature-hold oven with a view to obtaining optimum conditions for the diffusion of iron into the coating layer; the cycle thus produced approaches an ideal so-called "square" cycle.
The subject of the present invention is an accelerated cooling device of short length, which is particularly suitable to be used for rapidly cooling, by means of nozzles spraying a water/air fog, a coated substrate after the latter has exited from a temperature-hold oven and before it passes over guide rollers, especially within the framework of galvannealing treatment.
The device for the accelerated cooling of a continuous substrate moving rapidly in a vertical plane, which is the subject of the present invention, formed by a closed chamber, is essentially characterized in that it comprises, in succession in the direction of travel of the substrate, at least one so-called main cooling zone provided with coolant injection means, a so-called secondary cooling zone promoting contact between the coolant and the substrate, a coolant extraction zone and coolant retention means both at the inlet and outlet of the cooling device.
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Dormal Benoit
Simon Pierre
Wilmotte Stephan
Centre de Recherche Metallurgiques A.S.B.L.
Kastler Scott
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