Note: Descriptions are shown in the official language in which they were submitted.
W093/~02 PCT/~2/00113
211 7~)83 :
A PR~CESS AND AN APPARATUS FOR TH~ MANl!FACT'~JRE OF BILLETS AND BLOOMS FROM
A CONTINUOUSLY CAST STEEL SHOWING HIGH OR EXCELLENT QUALITY
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The present invention relates to a process for the di-
rect ~anufacture of billets and blooms from a conti-
nuously cast steel showing high or excellent quality,
as well as an apparatus for the realization of such
process.
It is known that the continuous casting is ever more
utilized for the manufacture of steel, because of the
clear and well known advantages therein involved, with
respect to other kinds of casting. It is clear however
that the thus obtained product, as it appears at the
extracting machine at the end of the curvilinear ca-
sting route, shows the features of a typical castingpiece, with all the qualitative drawbacks typical of
these semifinished products. It is in fact possible to
see, by a metallographic analysis, that the grain size
and the structure isotropy are unsatisfactory and that
the carbon percentage is not homogeneous but prevailin-
gly concentrated in the central zone of the product,
with a~(consequent) kish (segregation~l which makes the
product, coming from the continuous casting, not direc-
tly exploitable for the rolling, when it is desirable
to obtain end products consisting of high and excellent
quality steel.
In the case of a high quality steel, the continuous ca-
sting can be exploited for obtaining blooms, which, a-
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211~0~3 2 -
fter transit through an oven, are conveyed to a size-
breaking rolling mill, in order to be converted into
billets, which in their turn, optionally heated in a
further oven, are eventually conveyed to a finishing
rolling mill. In the case of an excellent quality
steel, the casting isieven occurring in the form of an
ingot, and not in a continuous way, and in each ingot
the cooling is adjusted by means of a predetermined cy-
cle; therefore, the ingots, or the big size blooms, are
conveyed to the size-breaking and subsequently to the
pre-rolling (cogging) and to the finishing rolling,
wherein there are usually provided, between two subse-
quent operations, intermediate heatings in an oven,
thus performing an extremely long and expensive opera-
tive cycle.
It is thus a purpose of the instant invention to supllya process allowing the direct manufacture, by means of
a continuous casting, of billets or blooms showing such
- features as to be easily trasferred, in a subse~uent
time and without any further operation, to the fini-
shing rolling. Another object of the instant invention
is residing in an apparatus for the realization of said
process.
An advantage coming from the process according to the
invention, described hereinbelow, is residing in that
the consequent inner grain of the casting product is
showing those features of fineness, homogeneity and i-
sotropy, as well as of absolute absence of kish (segre-
gation), which are usually obser~ed in a product ready
for the finishing rolling, thus omitting the steps con-
cerning size-breaking and pre-rolling, including the
WOg3/~80~ PCT/~2/00113
3 2117083 ~
respective intermediate heatings, and thus reaching a
considerable power saving.
The process according to the instant invention is cha~
racterized by a liquid core distortion of the casting
product, obtained by means of a continuous casting,
which causes a reduction of the product cross-section,
the perimeter of the same cross- section being unchan-
ged, in the route between the lowermost geometrical
point on the casting axis, where it is still possible
to find superheated liquid, and the end point of the
metallurgical length, where the product is completely
solidified.
According to a preferred embodiment of the instant in-
vention, said distortion is made to occur in a zone
where the concentration of the solid grains is from 10
to 80%.
The distortion hereinabove, from a practical point of
view, can involve the conversion of the shape from
round to square (billet) or to rectangular (bloom), or
from billet t bloom or also, starting from this latter,
to a more flattened cross-section.
The apparatus for carrying out the process according to
the invention is essentially comprising at least one
of the sectors of the roller train along the curvili-
near`route, which is given the possibility of movementwith respect to the opposite sector, and means fit for
bringing said mobile sector near the opposite one on
at least one plane containing the casting axis.
The purposes, the advantages and the features of the
process and of the apparatus according to the instant
invention will be clearer from the following detailed
-- 2 1 1 7 0 8 3
- 4 -
description and drawings, which are supplied for illu-
strating purposes but do not limit in any way the scope
of the same invention.
As to the drawings: ~-
FIGURE 1 is showing an extremely schematic view of the
sole casting product along the curvilinear route, in
order to point out the basic parameters of the process
according to the invention;
FIGURE 2 is showing an equally schematic view of a con-
tinuous casting apparatus, modified as to realize the
process according to the invention; and
FIGURE 3 is showing a section view along line III-III
of Fig. 2, in the case of the distortion of a rod.
Referring now to Fig^ 1, there is represented, in an
extremely schema~ic way and in a section containing the
casting axls X-X, a product made from steel during a
continuous casting. By lo there is indicated the free
surface of the liquid in the ingot mold (ladle) and by
S the lowermost geometrical point, on the casting axis, `
where it is still possible to find superheated liquid~
In other words, below S the temperature reaches the
"liquidus" value, typical of each peculiar steel, whe-
reas above the two lines s1 and s2 the temperature va-
lue is higher and the liquid, in such zone, is super-
heated and void of solid grains.
The position of point S can be determined, for each ca-
sting mill, depending on the temperature value of the
st~ l contained in the ~ above the "li~uidus" va-
lue, according to the kind of the steel, and can be ex-
pressed by means of time terms (t), corresponding to acertain speed. From the determination of the time t it
SUBSl`ITU~ SH~ET
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2117083 -~
is possible to extrapolate the position of S depending
on the different possible casting velocities.
It is usually supposed that point S be lying below the
ladle, and precisely in the first leg of the roller
train, commonly defined "segment zero" (extractable
section). L is representing the point where the casting
product is completely solidified and the distance l
observed between such point and point S, is commonly
defined "metallurgical length".
It is believed that the liquid lying below the two li~
nes sl and s2 and comprised between such lines and the
inner walls of the already solidified portion P
`("skin") be already containing solid grains with a con-
centration increasing towards the lower zone, until the
complete solidification in L. In this viscous or semi-
solid mass (M) are in fact in equilibrium either the
true actual liquid or the solid suspended grains On
the X-X axis, the concentration of the solid grains in
the mass (M) is e~ual to zero in s and to l00% in L,
linearly increasing along with the metallurgical
length.
The progression speed of the solid portion P is clearly
equal in every point, whereas the speed of the mass (M~
has to fulfill a different condition, namely to feed
the solidification process as to avoid the formation of
empty zones before point L, which would involve the
presence of cracks in the end product. It has to be
furtherly remarked that the metallurgical length lm has
to be such as to allow point L to be upstream of the
3o extraction device 20, lying at the end of the curvili-
near casting route (see Fig. 2).
W093/~02 ~CT/~2/00113
3 6
It was now surprisingly found that by compressing ac-
cording to the invention the walls P of the "liquid co-
re" casting product, namely before point L, thus redu-
cing the volume of the same product by distorting its
cross-section, there is obtained a bloom or a billet
showing all the desired features hereinabove fit for
obtaining a steel product having a high or excellent
quality.
The mechanism allowing such a transformation is not
lo completely clear, but it is believed that by bringing
the solid walls P near each other, there occurs, inside
the fluid mass M, which contains already solidified
grains and which can be defined semisolid or "viscous",
a speed gradient or acceleration, causing the breaking
of the dendritic ~ranchings which tend to form inside
the same mass. The thus crushed grains get reduced in
their size and are oriented in a way as random (casual)
as possible, thus acquiring particular isotropy and ho-
mogeneity features, while avoiding segregations, namely
increasing concentrations of carbon ~kish) towards the
inner portion of the product.
An essential condition, in order to make such results
to occur, is however residing in that the liquid core
distortion, carried out between points S and L, be re-
ducing the volume of the casting product, the lengthbeing equal (unchanged), whereas its lateral surface is
maintained constant; in this case in fact there are
neither stretching nor rolling action, in a true actual
sense, involving a creep (slippage) of solid material,
until we are in the presence of a viscous mass M inside
the product itself, namely along the whole length l
W093/~02 PCT'~ d ~ 3
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In terms of cross-section (the length in fact doesn't
have any impact, as it is kept constant), the area of
the cross-section has to be reduced, whereas its peri-
meter is approximately kept constant.
The following distortions will therefore be possible:
- from round product to square product (billets) or to
rectangular product (blooms); or
-from square billets to rectangular blooms; or, at last
- from a certain bloom to a more flattened product
(bloom or slab), namely a product showing a higher ra-
tio between the different sides.
From a theoretical point of view, there would be possi-
ble even a distortion involving the conversion of a ca-
sting cross-section having n sides to a distorted
cross-section having, at the end of the metallurgical
length, n-1 sides, even if such hypothesis has poor
chances to be reduced to practice.
It has to be underlined that should the distortion oc-
cur without the fulfilment of such a condition, the vo-
lume would increase or would at the most remain at aconstant leve~; there would therefore fail the presup-
position for obtaining the desired features hereinabo-
ve, allowing the product to be directly conveyed to the
finishing rolling, without any intermediate operative
step, as it is already endowed with all the features of
the desired semifinished product.
As it was already said, the distortion according to the
instant invention can occur all along the metallurgical
length, starting from point S, but preferably inside a
limited zone thereof; this latter can be defined as the
zone corresponding to the 10-80% of the percentage con~
W093/0~02 PCT/~92/00113
'~117083
centration x of solid grains in the mass M, which can
be easily determined if we take into account that xs=O
and that xL=lO0. Therefore if lS is the distance of S
from loJ the xl concentration in a whatsoever point far ^
l from lo is:
1 - lS
x = --____
lm
It can be useless in fact to carry out the distortion
with volume reduction in a too high zone of the ca-
sting, where the concentration of the solid grains is
least and the grains themselves couldn't be affected,
because of their dispersion in the liquid mass, by the
mechanical action exerted by the solidified walls P. .
From the other side, it can even be a drawback to
perform the distortion in the lowermost zone, in the
proximity of point L, where the walls P are already so
near as to easily build up a few weldings and conse- ~.
quently a few pockets containing liquid material which,
by solidification and consequent volume shrinking,
would give rise to cavities inside the product, which
drawback has preferably to be avoided.
Referring to Fig. 2, there is represented, in an ex~
tremely schematic way, an apparatus fit for carrying
out the process according to the invention, as it is
shortly decribed hereinbelow. The product l, contained
in the ingot mold lO, descends, through the so called
"foot rolls" ll along the roller train 13, defined by
pairs of opposed sectors of roller cages 12, 14, 12',
14' and so on. The first sector, immediately after the
foot rolls ll, where it is believed sufficient, in cer-
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- 21170~3 ;;
tain cases, to limit the volume reducing distortion ac~
cordîng to the present invention, is usually called
Se~e~rS
"segment zero". ~e~o~s 12, 12', 12'' and so on are all
o~ t~
lying on the ~ of the curvilinear~s, namely
Se~
they have a greater bending radius, whereas ~r~ 14,
~e~
14' and so on are defined ~s~ade~ segments. The rolls
i~4~r
of ,~c~t~r 12 are represented by the re~erence number
se~ t
12a and the ones of ~e~ 14 by the reference number
14a and so on.
According to the instant invention and to the hypothe-
. sis above, according to which the distortion according
d~ ~r
to the invention is determined by said first ~ ~r,
the t~oJ portion 14 is made to be mobile with re~
spect to the ~L-l~$ 12 in whatsoever known way, there
being provided means 16 for drawing cage 14 near the
opposite cage 12, which remalns motionless. As it is
reported on Fig. 2, the cage or bearing structure 14,
comprising rolls 14a, is pivoted in 15 at one of its
ends, preferably the superior one, and a hydraulic pi-
ston 16 pushes the opposite end of the Z~GQ~ struc-
~ ~J~
ture towards the motionless ~etra~eg 12. Of course,
there can be provided whatsoever other solution of the
problems, known to the skilled in the art; it is thus
possible, for instance, to let the 14
slide along a skid device and to provide one or more
hydraulic pistons for the thrust along the same skid
device. In any case, an apparatus of this kind can be
used when a square product or billet has to be conver-
ted into a bloom or when an already rectangular pro-
duct, like a bloom, has to be converted into an always
rectangular however more flattened shape, for instance
~S~ 5~EE~
1 o - 1 7 0 8 3
a (thick) slab.
Should on the contrary a round product be converted, by
means of distortion, into a product showing a square or
rectangular cross-section, it is no more sufficient to
work in the plane of Fig. 2, but a corresponding action
has to be contemporaneously carried out in a plane nor-
mal to the plane above, always containing the casting
axis X-X, as it is recorded on the section view of Fig.
3. In this case the distortion of a round product 1 is
contemporaneously caused by two pairs of rolls, lying
in diametrically opposed positions of the perimeter of
the same round product, namely rolls 12a and l~a of the
opposite ~r~ and -~nt~9~ cages 12 and 14, recor-
ded on Fig. 2, as well as rolls 22a and 24a, respecti-
vely belonging to roll cages not reported in Fig. 2,having the opposite rolls oriented normally to the
rolls 12a and 14a of cages 12 and 14.
Prefera~ly the subsequent rolls, downstream of the ones
arranged in the sector undergoing distortion, are sup-
plied with pistons fit for exerting a thrust towardsthe inside of the casting, no more for distortion pur-
poses but for opposing the ferrostatic pressure and the
possible consequent swelling, which can occur between
the contact with a roll and the subsequent one, thus
fitting to the size reached in the preceeding distor-
tion phase.
EXAMPLE
From a continuous casting -ladle, having a round cross-
section, it was cast, with a progression speed
3o V-2m/min., a round product (rod) having a diameter of
130 mm. Xn a zone between 28% and 76% of the concentra-
5 ~ S ~ r~T
W093/~2 PCT/~2/00113
11- 2117083
tion of the solid grains and by a metallurgical length
equal (in this case) to 8 m, there was caused a distor-
tion leading to a substantially square billet having a
100 mm side.
Subsequently the same rod was converted in another bil-
let, having a similar size, however increasing the
speed up to 3 m/min., and the distortion, according to
the description above, was made to occur in a zone bet-
ween the concentration values x equal to 14~ and 46%,
whereas the metallurgical length was 12 m.
In both the cases there were sampled specimens of the
cast product, once solidification was over, thus fin-
ding, by macrographic analysis, the following results:
a) fine structure, without dendritic evidence;
b) structure isotropy, without any main orientation of
the grains;
c3 absence of (kish) segregations, with homogeneity, in
terms of chemical analysis, all along the cross-
section; ~
20 d) isotropy of the mechanical features (tensile ;
strength, yield point, break elongation, impact
strength);
e) better mechanical features, with respect to the pro-
duct coming from a traditional casting, such as to al-
low the reaching of the same end features of the end
product with lower percentages of reduction in the xol-
ling step.
From the above it is clear that the apparatus according
to the instant invention allows the product, coming
from the continuous casting and handled according to
the process above, to be directly conveyed to a fini-
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- 12 -
shing rolling mill, by merely interposing a heati~g o-
ven, optionally an induction oven, for adjusting the
temperature according to the rolling values.
Optional additions and changes can be carried out by
the skilled in the art, as to the process according to
the instant invention hereinabove, as well as modifi-
cations of the described and illustrated apparatus for
carrying out such process, without exceeding the scope
of the same invention.
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