Process for adjusting the secondary cooling of a machine for continuous casting of metal products

Process for secondary cooling of a metal product continuously cast on a machine in which secondary cooling is divided into n staggered independent zones, within which the flow of cooling fluid varies according to the speed of the product. An undesired change in the surface temperature of the product at a point HD of the metallurgical length of the machine, such as the straightening point, due to a foreseen or foreseeable variation starting at the time t.sub.vo, in the casting speed, is compensated for by anticipation. A determination is made, by means of the casting speed, of the time t.sub.o at which commences, at the upper part of the ingot mold, the portion of product which, at the time tvo, will reach the point HD; a further determination is made of the times t.sub.1, . . . , t.sub.1, . . . , t.sub.n at which the portion commencing at t.sub.o will emerge from the zones 1, . . . . i. . . . ,n of the secondary cooling; and, from the time t.sub.vo, the method of cooling conventionally used is recommenced, varying according to the actual speed.

FIELD OF THE INVENTION 
The invention relates to the adjustment of the secondary cooling of a 
machine for continuous casting of metal, particularly steel, products, 
such as slabs, blooms or billets. 
More precisely, the invention relates to the adjustment processes of this 
type in which the future speed of advance of the product in the machine is 
taken into account in determining the intensities of cooling in the 
various zones of the machine. 
BACKGROUND OF THE INVENTION 
In a machine for continuous casting of metallurgical products, secondary 
cooling of the product is conventionally provided by ramps of jets which 
spray a cooling liquid, generally water optionally mixed with air, onto 
the product. Spraying of the product commences immediately below the ingot 
mold and can be continued until the product reaches the bending and 
extraction zone. However, most often, spraying is interrupted before the 
straightening zone. 
Today, it is known that the final quality of the cast product is greatly 
influenced by the manner in which its secondary cooling has been conducted 
Good adjustment of the latter makes it possible, in particular 
to ensure complete solidification of the product before its straightening 
or its oxygen cutting; 
to ensure good mechanical behavior of the solidified skin along the machine 
and, in particular, to avoid the problems of bulging due to too high a 
surface temperature which can generate internal cracks and considerable 
central segregation; 
to ensure a certain uniformity in the cooling of the product and to avoid 
sudden reheating or cooling capable of creating cracks at the 
solidification front (internal cracks) or surface cracks; 
to maintain the surface temperature on straightening in the zone of good 
forgeability of the metal and to thus avoid the formation of transverse 
cracks on the undersurface. 
Secondary cooling is conventionally divided into various successive 
spraying zones along the cast product. Within each of these zones, the 
flow of water can be adjusted independently of the other zones The 
production of a good-quality product is connected to a correct definition 
of the flows of water in the various zones, particularly in relation to 
the speed of casting, i.e., the speed of extraction of the product from 
the machine. 
When the speed of casting is constant, the definition of a suitable 
secondary cooling method poses no problems. In the case of a small 
variation in the speed of casting, even if sudden, the cooling of the 
product deviates only relatively slightly from the ideal program defined 
for a continuous operation and the quality of the product is scarcely 
affected thereby 
This does not apply when the advance of the product passes through a major 
transition, corresponding to an increase or, above all, to a sudden and 
considerable reduction in the speed of casting, or even to stoppage of 
extraction. 
When such a transition occurs, the product present in the machine has its 
cooling disrupted relative to the ideal foreseen program. This disruption 
particularly affects the portion of the product which, during this 
transition, is passing through the zone of the machine located between the 
end of secondary cooling and the straightening point. In this zone, the 
product cools naturally, particularly by means of radiation, without being 
sprayed. The transition in speed has the effect of modifying the residence 
time of the product in this zone of natural cooling. As the operators are 
no longer able to control the speed of cooling of this portion of the 
product, this portion reaches the straightening point at a temperature 
which is substantially different from that which it would have had if the 
speed of casting had remained normal. This phenomenon is particularly 
damaging when the speed of casting becomes low or zero during the 
transition. In fact, under these conditions, cooling of the product is 
accentuated and the latter reaches the straightening point at a 
temperature which risks being too low because it is situated outside the 
zone of good forgeability of the metal. 
Such transitional phases occur unexpectedly when there are incidents 
connected with the operation of the machine. However, most frequently (in 
approximately 90% of the cases), they are connected with conventional and 
foreseeable operations, such as the completion of pouring, or a change of 
distributor. 
DESCRIPTION OF THE PRIOR ART 
Applicants' European Patent EP. 0116496, in the name of the Applicant, 
describes a process of anticipatory secondary cooling. For spraying the 
product in the various zones of secondary cooling, this process takes into 
account not only the present and past speeds of advance of the product, as 
performed to date, but also its future speed of advance when it is 
possible to predict at which moment a transition will commence, what its 
duration will be and what will then be the speed of advance 
This is achieved by introducing temporarily into the adjustment system, in 
place of the actual extraction speed, a "decoy" speed which is between the 
actual speed and the future speed. It is thus possible to tend to 
compensate by anticipation for the supplementary cooling which will be 
obtained by a slowing down or stoppage of the extraction, by reducing the 
cooling of the product even before the variation in the speed of casting 
has taken place. A similar argument may be followed if a sudden increase 
in speed of casting is foreseen: the intensity of cooling must then be 
increased in advance and the fictitious speed must be greater than the 
actual speed and lower than the future speed. 
This process is well suited to those cases in which the variation in speed 
is not too great or occurs progressively. However, in the case of a marked 
transition, such as a sudden stoppage of extraction, action in respect of 
secondary cooling may not be quick enough to sufficiently limit the fall 
in temperature of the product In fact, it is undesirable to impose, when 
beginning the operation of anticipation, a very low fictitious speed which 
would be well suited to the sections which will be subject to the 
transitional operation, but which would excessively disrupt the cooling of 
the portions of the product which are currently being cast and which will 
not be affected by this transition. 
SUMMARY OF THE INVENTION 
The invention aims to propose a method for adjusting the secondary cooling 
which also operates by anticipating those events which will lead to 
changes in the speed of casting, but which would be more suitable than 
existing methods in the case of transitional operations leading to sudden 
and considerable variations in this speed. 
To this end, the subject of the invention is a process for secondary 
cooling of a metal, particularly steel, product, such as a slab, a bloom 
or a billet, which is continuously cast on a machine whose secondary 
cooling is divided into n staggered independent zones, within which a flow 
of cooling fluid, varying according to the speed of casting of the 
product, is sprayed on the said product, in which process an undesired 
change in the surface temperature of the product at a point HD of the 
metallurgical length of the machine, such as the straightening point, 
beyond which it is no longer desired to control the temperature of the 
product, is compensated for by anticipation, this change in temperature 
being due to a foreseen or foreseeable variation in the speed of casting 
commencing at the time t.sub.vo, in which process: 
a determination is made, by means of the speed of casting, of the time 
t.sub.o at which commences, at the upper part of the ingot mold, the 
portion of product which, at the time t.sub.vo, will reach the point HD, 
a determination is made of the times t.sub.1,...t.sub.1,...t.sub.n at which 
the portion of product commencing at t.sub.o will emerge from the zones 
1,...i,...n of the secondary cooling, 
from the time t.sub.1, a flow of cooling fluid adapted to the compensation 
for the said change in temperature is imposed in the zone i, 
and, from the time t.sub.vo, the cooling method conventionally used on the 
machine is recommenced, varying according to the actual speed of casting. 
As will be understood, the invention consists in applying to the portions 
of product affected by the transitional operation a specific cooling which 
is independent of the present speed of casting and which is intended to 
compensate for the increase or the lack of cooling of these portions which 
would otherwise result from the transition. This specific cooling is not 
applied immediately throughout the machine, but it is implemented 
successively in the various zones of secondary cooling. This makes it 
possible to adapt the method of cooling of a given portion of product more 
precisely to the history of its path than do the prior art methods.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention will be better understood on reading the following 
description which is given with reference to the appended single figure. 
The single figure is composed of two diagrams having an abscissa axis in 
common. The top diagram shows the evolution, during the time t, of the 
speed of casting V. In the example described, this speed assumes a 
constant and non-zero value V1 up to a time t.sub.vo, when it becomes zero 
following an event such as a change of distributor. It remains zero up to 
a time t.sub.V1, when it reassumes the preceding value V1. 
The curves A, B, C, D, E in the bottom diagram represent the path, in the 
machine during the time t, of the extremely thin product portions, called 
sections, which commence at various times, t.sub.o, t.sub.B, t.sub.C, 
t.sub.D, t.sub.V1 at the upper part of the ingot mold. The ordinate of a 
point of one of these curves represents the point in the machine H at 
which the corresponding section is located at the time plotted on the 
abscissa. The length of the machine is divided into several zones through 
which the product passes successively; 
the ingot mold, denoted L on the diagram, extending from the point 0 to the 
point H1, 
the first zone of secondary cooling, denoted Z1, extending from the point 
H1 to the point H2, 
the second zone of secondary cooling, denoted Z2, extending from the point 
H2 to the point HR, 
a zone in which the product is not sprayed and is cooled naturally by 
radiation, denoted R, extending from the point HR to the point HD, 
the straightening zone, denoted D, which commences at the point HD, 
referred to as the "straightening point". 
It is considered that, in the zone D, the method of cooling of the product 
no longer has an influence on the quality of the product and it is no 
longer attempted to control it. 
The anticipatory secondary cooling operation is achieved in the following 
manner. At the time (t.sub.Vo -t.sub.ANT), the operator responsible for 
the functioning of the machine is alerted to the fact that, at the time 
t.sub.Vo which is still in the future, some event will oblige him to 
interrupt the extraction of the product which will not recommence until 
the time t.sub.V1. The operator (or computer which preferably manages the 
secondary cooling) then determines the time t.sub.o corresponding to the 
commencement, at the upper part of the ingot mold (that is to say at the 
point 0), of the extremely thin section of product which, at the time 
t.sub.Vo, will be located at the point HD, that is to say at the bending 
point. The sections commencing after t.sub.o will thus not yet have 
arrived at the straightening point when stoppage of extraction commences 
and it is these which will be subject to a modified secondary cooling. 
To this end, the time t.sub.1 at which the section commencing at t will 
emerge from the zone Z1 is determined. From the time t.sub.1, a 
predetermined minimum flow of water is applied in the zone Z1. This 
minimum flow can be zero flow, the technological minimum attainable on 
this zone 1, or a previously defined minimum flow which is different from 
the two preceding flows. This flow is maintained constant throughout the 
anticipation phase extending from t.sub.1 to t.sub.Vo. The choice of the 
minimum flow is determined before casting. It must comply with the safety 
constraints of the continuous casting machine and be adapted to the 
compensation for the change in temperature of the product at the 
straightening point which would be caused by the stoppage of extraction. 
Generally, the times t.sub.1 are determined, i being an integer less than 
or equal to the number n of zones of secondary cooling, at which times the 
section commencing at t will emerge from the various zones Zi. After 
t.sub.1, a predetermined minimum flow of water, as has just been defined, 
is applied in Zi. This minimum flow may be different in each zone. In the 
case illustrated, the number n of zones of secondary cooling equals 2, but 
it may, of course, assume any greater value. At the time t.sub.Vo, this 
procedure is interrupted and use of the cooling method conventionally 
employed on the machine in the case of stoppage of extraction is 
recommenced, then the latter is restarted. 
Two cases may be envisaged: 
it is foreseen at (t.sub.Vo -t.sub.ANT) that the extraction will be stopped 
at t.sub.Vo, and that the time t.sub.o, deduced from this forecast, is 
still to come. In this case, the procedure of secondary cooling by 
anticipation will affect all the sections commencing between t.sub.o and 
t.sub.Vo, as has just been described. 
at the time (t.sub.Vo -t.sub.ANT), stoppage of extraction at t.sub.Vo is 
foreseen, whereas the time t deduced from this forecast is already past. 
The procedure of secondary cooling in advance is then immediately set in 
motion. If the time t.sub.j is the last of the times t to have been past, 
the very first sections of product already cast will have been subject to 
normal cooling in at least one part of the zone j and of the preceding 
zones and not to a cooling according to the invention. They thus risk 
being, at the straightening point, at a temperature located outside the 
desired range. Nevertheless, even late application of the procedure of 
secondary cooling by anticipation will have made it possible to cast a 
greater length of the product under satisfactory conditions as compared 
with the case in which no specific action had taken place in respect of 
cooling 
The single figure illustrates the case in which it has been possible to 
forecast stoppage of extraction before the time t.sub.o. The various 
portions of the curves A, B, C, D, E are plotted in solid lines for the 
time intervals in which the section which they represent has been sprayed 
according to conventional procedures (corresponding to V=V1 before 
t.sub.Vo and after t.sub.V1, and at V=0 between t.sub.Vo and t.sub.V1), 
and in dotted lines for the time intervals in which they have been 
subjected to minimum spraying, following the anticipation procedure. It 
should be noted that, in this example, cooling of the portion of product 
present in the ingot mold is not modified during the anticipation 
procedure 
The section commencing at t (curve A) is subject, like the previous ones, 
to cooling of the conventional type over its entire path The section 
commencing at t.sub.B (curve B) is subject to minimum spraying as it 
finishes crossing the zone Z1 and as it finishes crossing the zone Z2. The 
section commencing at t.sub.C (curve C) is subject to minimum spraying 
throughout its passage through the secondary cooling zone. The section 
commencing at t.sub.D (curve D) is subject to minimum spraying between its 
entry into the zone Z1 and the time t.sub.Vo when it is stationary within 
Z1. The section commencing at t.sub.Vo (curve E) remains at the point 0 
throughout the entire duration of the stoppage of extraction and is the 
first section, since that commencing at t.sub.o, to be subject over its 
entire path to cooling according to conventional procedures, firstly for 
V=0 and then for V=V1. 
A similar argument could apply in the case where the instantaneous 
modification of the speed of casting would not be a stoppage of 
extraction, but a simple slowing down. 
A significant aspect in the implementation of a model of the type according 
to the invention is the forecasting, with a sufficient degree of 
certainty, of the time t.sub.Vo at which the transitional operation will 
commence. If this time in fact occurs substantially later than had 
initially been foreseen, considerable portions of the product will have 
meanwhile been subject to too little cooling. This risks bringing these 
portions of product to the straightening point in a state of 
insufficiently advanced solidification, which can give rise to the 
formation of defects during bending. 
The consequences of this difficulty can be limited if the operator 
allocates a degree of certainty "CERT" to the forecasting of the time 
t.sub.Vo, CERT is firstly assumed equal to 0 when forecasting is still 
uncertain, and to 1 when t.sub.Vo can be determined with certainty. 
According to this alternative embodiment of the model, while CERT equals 0, 
only the end zone or zones of secondary cooling (for example the zones 5 
and 6 if it has 6 zones) will be subject to the minimum of flow throughout 
the anticipatory cooling procedure. These zones are, in fact, those in 
which urgent action is most required, since the portions of product 
located therein will be the first to enter into the zone R in which no 
further action will be possible If, finally, the transition does not take 
place, or does not commence at t.sub.Vo but at a later time, these 
portions will have been subjected to unsuitable cooling only in the final 
zone or zones, and the effects on the quality of the product will be less 
than if cooling had been unsuitable in all the zones. 
When the operator becomes certain that the transition will indeed commence 
at t.sub.vo, he imposes CERT=1 on the model. Then, all secondary cooling 
can take place according to the procedure described above. 
If, on the other hand, the operator learns at the time (t'.sub.Vo 
-t'.sub.ANT) after (t.sub.Vo -t.sub.ANT), that the transition will 
commence at the time t'.sub.Vo, which is different from t.sub.Vo, with a 
degree of certainty CERT', the anticipation procedure which was in force 
is immediately interrupted. It is immediately replaced by a procedure 
based on the new data which reached the operator at the time (t'.sub.Vo 
-t'.sub.ANT). 
The process according to the invention can also be applied to straight 
continuous casting in which the product does not need to be unbent. In the 
above argument, the straightening point will then replaced by the point at 
which the product is cut, or more generally by any point beyond which it 
is estimated that the method of cooling the product no longer has an 
influence on its quality.