Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2001-09-07
2003-11-25
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S559000, C148S579000
Reexamination Certificate
active
06652681
ABSTRACT:
The present invention relates to a method of reheating metallurgical products, in which solid products, especially steel products, are reheated so as to bring them from a temperature substantially below 400° C. to a temperature of at least about 1000° C. by passing them through a furnace having an upstream zone in which the said products are preheated and a downstream zone in which the said products are brought to their final temperature on leaving the furnace, the downstream zone of the furnace being fitted with burners, at least some of which operate with an oxidizer which is air, the smoke (flue gases) generated by these burners flowing as a countercurrent to the products and preheating these products in the upstream preheating zone (the terms “smoke” or “flue gases” are hereafter used with the same meaning).
DESCRIPTION OF THE RELATED ART
Reheat furnaces are used in the steel industry to reheat steel products coming especially from continuous casting and to bring them to the rolling temperature which is around 1000to 1300° C.
Furnaces of this type usually consist of several successive zones. Starting from the charge end (in the direction in which the products run through the furnace), these successive zones are the upstream zone called the flue gases exhaust (or recovery) zone in which the thermal energy of the flue gases, which is produced downstream in the furnace and which flows as a countercurrent to the products to be reheated, is recovered in order to start to preheat these products.
This preheating zone is followed by one or more heating zones, the furnace terminating in an equalization (or soaking) zone which serves to ensure that the temperature of the product leaving the furnace is homogeneous. Burners may be preferably installed on each side of the product which travels from the preheating zone to the end of the heating zones. Such burners may also be placed in the roof of the furnace (radiant roof case) or else in recesses depending on the width of the furnace.
While the products are passing through the various successive zones of the reheat furnace, the temperature of the product at the surface and inside it progressively increases. Owing to the characteristic times for thermal conduction, especially in steel, there is a temperature difference between the top side of the product and the underside or else between the top side of the product and the core of the product. Controlling these thermal inhomogeneities is an important aspect of the invention.
This problem of obtaining temperature homogeneity of the product is all the greater the more limited the thermal power that can be injected into a reheat furnace. There may be several reasons for this limitation: the limited volume of smoke, temperature of one or more zones of the furnace at maximum, temperature at the inlet of the energy recuperator at maximum, etc. In all cases, the limitation in injected thermal power results in a limitation in the energy transferred to the product and therefore to thermal inhomogeneities throughout the mass of the product appearing or increasing. In order to provide a better explanation of the problem facing a person skilled in the art,
FIG. 1
shows the curve of how the temperature difference &Dgr;T (defined below) varies as the product is being reheated.
For a furnace in which the products rest on the hearth, the temperature difference &Dgr;T will be the difference between the temperature of the top side of the product exposed to the radiation of the furnace and the temperature of the underside of the product in contact with the hearth.
For a walking beam furnace, that is to say one in which the hot gases of the furnace circulate all around the product, the temperature difference &Dgr;T will be the difference between the surface temperature and the core temperature of the product.
In
FIG. 1
, the position of the product in the furnace has been plotted on the x-axis and the &Dgr;T value on the y-axis. The initial temperature difference (&Dgr;T
init
) may be zero, when the product is at room temperature at the charge end of the furnace, or non-zero in the case of products whose temperature has not yet become homogeneous again, for example in the case of the treatment of metallurgical products shortly after their production. In
FIG. 1
, X represents the position of the product in the furnace, 0 being the charge end where the products enter the furnace, while X
B
is the discharge end or exit of the furnace.
The curve (C) showing the variation of &Dgr;T as a function of X in
FIG. 1
has a point A where the parameter &Dgr;T reaches a maximum (&Dgr;T
max
), a point D where the parameter &Dgr;T has a value &Dgr;T
init
, which is the value of &Dgr;T of the product at the charge end and a point B where the parameter &Dgr;T has a value &Dgr;T
final
of the product at the exit (discharge end) of the furnace.
Somewhere in the middle of the furnace, at the point X
A
, the temperature difference &Dgr;T reaches its maximum (&Dgr;T
max
). This &Dgr;T
max
value must be as small as possible, since a large temperature difference is equivalent to deformations (bending) of the product which may result in the product being damaged or in the furnace not being able to be operated or in the product leaving the furnace not being able to be rolled. Thus, in certain furnaces the operators must limit the power of the furnace and/or its production in order to avoid the appearance of excessively large temperature differences &Dgr;T. This is a major drawback for an industrialist.
It is therefore a first object of the present invention to prevent the appearance of excessively large temperature differences in the product throughout the time it is passing through the furnace.
FIG. 2
illustrates the relationship between the temperature difference &Dgr;T and the sag, that is to say the vertical deformation, of the product during its passage through the furnace.
This
FIG. 2
shows the curve (C), as in
FIG. 1
, and a curve (F) which represents the vertical deformation of the product as a function of X. It may be seen that the maximum deformation corresponds approximately to the maximum &Dgr;T (&Dgr;T
max
for X=X
A
).
Moreover, it has been shown that another important parameter is the temperature difference &Dgr;T
final
at the exit of the furnace. Ideally, &Dgr;T
final
should be zero at the exit (discharge end) of the furnace. In practice, a certain temperature difference &Dgr;T
final
is tolerated, but it must not exceed about 100° C. in the case of billets and 200° C. in the case of slabs and blooms. This is because a large temperature difference causes rolling difficulties which may result in mechanical hitches in certain stands of the rolling mill. In addition, any temperature inequality is manifested by a reduction in quality of the finished product.
It is also an object of the present invention to reduce &Dgr;T
final
of a product exiting a reheat furnace without increasing the consumption of energy in the furnace.
The article entitled
“Efficient operation of continuous reheat furnaces through oxygen optimization of combustion system”
by G. Gitman, T. Wechler and B. Levinson, published in the journal
Industrial Heating,
describes various systems for reheating metallurgical products and suggests the use of oxy-fuel burners instead of the usual air-fuel burners, so as to increase the energy transfer to the said products and maintain or even increase the &Dgr;T
max
of these products, as illustrated in
FIG. 7
of that article.
SUMMARY OF THE INVENTION
Contrary to the method described in the above article, the method according to the invention consists of the use of burners whose oxidizer has an oxygen concentration greater than 21 vol % and less than or equal to 100 vol % (hereafter called “oxy-burner”), these burners being installed in the furnace so that they are the first burners “seen” by the products to be treated as they progress through the furnace, after the latter has been charged therewith. The preheating zone formed by these oxy-burners is therefore the first preheating zone of
Ammoury Fouad
Delabroy Olivier
Le Gouefflec Gérard
Tsiava Remi
Burns Doane Swecker & Mathis L.L.P.
L'Air Liquide Societe Anonyme a Directoire et Conseil de Su
Yee Deborah
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