Field of the invention
5 The present invention relates to the field of rolling long products - in particular to the
field of casting and rolling products which have a polygonal section, in particular a
square, rectangular (with ratio between wide side and narrow side less than 4) or
octagonal section, or having oval or round section - for producing metallurgical
products, in particular metallurgical products of the merchant type.
10 Background art and background of the invention
In known rolling plants, the products in the merchant categories (the set of profiles
including, for example flat bars, section bars, angle bars of various types, for
example U, T, V, L, H, C, UPN angle bars, beams, in general of over 20 mm in
section size) are produced in a billet-to-billet mode.
15 This means that a plurality of individual billets are typically cast by multi-line casting
machines, all cut to a standard measure (for example 6 m, 13 m, etc.) and sent to a
storage and possibly cooling bed. When they are to be rolled, said individual billets
are sent one by one to a heating furnace to then be rolled. It is also normal procedure
to let them cool to then be rolled later, the rolling depending on the production needs.
20 Rolling is to be performed at high speed to allow the product to remain above about
800°C during the entire rolling stage. In particular, said temperature is to be
maintained at least up to the last rolling stand of the finishing unit. Indeed, surface
defects are avoided above this temperature.
Therefore, according to the prior art, the rolling of merchant products is entirely
25 unassociated from the casting and as mentioned, is performed in a billet-to-billet
mode at high speeds (also a few tens of m/min).
The classical layout for rolling merchant products is diagrammatically shown in Fig.
1A.
Fig. 1A shows, in particular, the continuous casting machine 121 and a rolling mill
30 comprising three rolling stand groups 131, 133, 134. There are cutting means 190
between the continuous casting machine 121 and the rolling mill, in particular
3
adapted to perform oxygen cutting to cut the casting product to work in a billet-tobillet mode.
During rolling in a billet-to-billet mode, each billet is cut by the cutting means 191
and/or the cutting means 192 arranged between the roughing stand group 131 and
5 the intermediate stand group 133 and between the intermediate stand group 133
and the finishing stand group 134, respectively.
The classical layout for rolling merchant products indeed provides a first loading
area of the previously cast (and typically cold) billets, a furnace for heating them to
the optimal temperature for rolling, and a successive rolling line (also referred to as
10 a rolling mill) consisting of various rolling stands (for example, twenty stands) which
in sequence bring about the deformation required to shape the desired finished
products.
It should be noted that in the rolling plants for merchant products belonging to the
prior art, the heating furnace upstream of the rolling line is the only active source for
15 bringing heat to the product, whereby it is essential for the rolling speed in known
plants to be high in order not to have excessive drops in temperature which may
affect the quality of the product.
Usually, however, excessive lowering of temperature has been encountered in the
heads (and often also in the tails) thereof during the crossing of the stands belonging
20 to the rolling mill since the rolling is performed for individual billets (this cooling also
changes from product to product as a function of the rolling speed, geometry of the
product in the various steps and the steel grade). Such cold heads may result in the
risk of cobbling in addition to objective difficulties in rolling since they are harder to
roll, whereby they are to be removed by suitable shears arranged along the rolling
25 mill, for example at one third and about two thirds of the length of the rolling mill.
Occasionally, also some tails are to be removed and often the cut is not very simple
to perform accurately since the merchant products have particular and in some
cases even non-symmetrical shapes and sections.
It is easy to understand how disadvantageously these required cropping operations
30 become a significant loss of product, especially the first cropping, since the section
of the product being rolled is high. Moreover, the cropping operations make
managing the lengths of finished product contained in each billet critical: a 6.5 m
4
billet may give rise to the exiting of a long angular product, also longer than 130 m,
from the twentieth stand. Supposing that the finished product to be sold is to be
shaped into 12 m bars, there would be ten bars 12 m long, with a short leftover bar
of 10 m. If some other cropping operations were required prior to the exiting from
5 the rolling mill, the calculation of the finished bars and short bars would need to be
revised and decreased.
Therefore, optimizing the finished products which may be produced from each billet
is complex in the rolling mills for merchant products belonging to the prior art, given
that the cropping operations may continuously change the possibilities.
10 Short bars are a strongly felt problem because they are to be disposed of if they are
too short, and they end up on a secondary market if they are non-standard.
Moreover, there is a need for dedicated equipment to manage and move them.
The phenomenon of short bars occurs when the billets cannot be perfectly divided
into multiples of commercial lengths (for example, due to intermediate cropping
15 operations, incorrect drawing between stands, wear of the channels bringing
increased influx of material in rolling steps which would require less thereof, etc.)
due to the multiple conditions to which they are subjected during the rolling stage.
Thus, bars are to be at least partly discarded since they would not be manageable
in the cooling bed and would result in uneven cooling of the contiguous bars in the
20 cooling bed. The classical merchant layout then includes a thermal product
treatment system, a system for measuring the tolerance of the exiting profile, and
shears for hot cutting the product at high speed, at the outlet of the rolling mill. Said
shears start to operate when the rolled billet exiting from the rolling mill has an
excessive length with respect to the cooling bed downstream.
25 Indeed returning to the previous example, supposing that the cooling bed has a
maximum length of 120 m, while the billet exiting from the rolling mill is 130 m long;
there is a need for the shears to make an intermediate cut, for example into two 65
m bars, so that the bars may be conveniently accommodated on the cooling bed
and cooled. Cooling shortens the bar by a few centimeters, in addition to twisting it
30 sometimes, since it may not be performed in the most suitable manner due to air
currents or the different sizes between adjacent bars which undergo uneven cooling
due to the spaces therebetween and the often different lengths thereof.
5
Therefore, a bar straightening and cold cutting unit, for example by means of
trimmers, is provided downstream of the cooling bed. This last unit is adapted to
obtain lengths of final product as a function of the various desired types according
to the production sheet. It indeed attempts to make cuts so as to minimize the losses
5 of material due to parts which cannot be straightened and short bars. Clearly, the
portion of product cropped in this passage is also lost.
The bars cut to size are then sent to a series of beds downstream, each intended to
accommodate products having specific lengths, so as to form bundles and tie and
strap them.
10 It therefore may be understood how the rolling process and division of the billets in
known rolling mills of the merchant type involves increased losses of material in the
various steps of the process, thus lowering the output of the plant and increasing
production costs.