U.S. patent application number 13/218093 was filed with the patent office on 2011-12-22 for process and related plant for producing steel strips with solution of continuity.
Invention is credited to Giovanni ARVEDI.
Application Number | 20110308289 13/218093 |
Document ID | / |
Family ID | 36204762 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110308289 |
Kind Code |
A1 |
ARVEDI; Giovanni |
December 22, 2011 |
PROCESS AND RELATED PLANT FOR PRODUCING STEEL STRIPS WITH SOLUTION
OF CONTINUITY
Abstract
A process for manufacturing steel strips with solution of
continuity. The steps of the process include a continuous casting
step for thin slabs with a high "mass flow," a shearing step and
subsequent heating in furnace, followed by a multiple stand rolling
step with the average temperature of the product at the inlet of
the rolling mill being higher than the surface temperature, which
is equal to at least 1100.degree. C., lower than that measured in
the inner central area by about 100.degree. C. A plant is also
described for the accomplishment of such process where at the inlet
of a furnace, possibly of the induction type, combined with a
temperature maintaining tunnel a shear is provided for cutting into
pieces a slab coming from continuous casting with the distance
between the outlet of the continuous casting and the inlet into the
finishing rolling mill is not greater than 100 m.
Inventors: |
ARVEDI; Giovanni; (Cremona,
IT) |
Family ID: |
36204762 |
Appl. No.: |
13/218093 |
Filed: |
August 25, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12102493 |
Apr 14, 2008 |
8025092 |
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13218093 |
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PCT/IT2005/000754 |
Dec 22, 2005 |
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12102493 |
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Current U.S.
Class: |
72/40 |
Current CPC
Class: |
B21B 1/466 20130101;
B21B 15/005 20130101; B21B 15/0007 20130101 |
Class at
Publication: |
72/40 |
International
Class: |
B21C 43/00 20060101
B21C043/00 |
Claims
1. A plant for production of steel strips from thin slabs having
thickness comprised between 45 and 110 mm coming from continuous
casting, comprising: a shear cutting a casting product, which is
formed with a solution of continuity, into the slabs; a multiple
stand finishing rolling mill located downstream of the shear, a
distance between an outlet of the continuous casting and an inlet
to the rolling mill being not greater than 100 m; at least one
heating furnace located downstream of the shear and upstream of the
multiple stand finishing rolling mill; and a descaler arranged
between the at least one furnace and the rolling mill, wherein one
of the at least one furnace is an induction furnace having a
sufficiently low working frequency to heat a core of the slab and
to substantially maintain a same temperature difference between an
inside and an outside of the slab from an end of the induction
furnace to an inlet of a first rolling stand of the finishing
rolling mill, and whereby an average temperature of each slab is
higher than a surface temperature of each slab, which is equal to
or higher than 1100.degree. C. and the average temperature at the
core is at least 100.degree. C. higher than the surface temperature
of each slab.
2. The plant according to claim 1, further comprising a second
furnace, which is a tunnel furnace that is heated by gas.
3. The plant according to claim 1, wherein the plant has only one
furnace and the furnace is an induction type furnace.
4. The plant according to claim 1, further comprising a temperature
maintaining tunnel in combination with the induction furnace,
upstream and/or downstream thereof, having a length to keep a total
distance between the continuous casting and the finishing rolling
mill not greater than 100 m, suitable for limiting the thermal
losses.
5. The plant according to claim 4, wherein the tunnel is formed by
roller tables having insulating panels.
6. The plant according to claim 4, wherein the tunnel has gas
burners and/or electrical resistors.
7. The plant according to claim 4, wherein the induction furnace is
placed immediately upstream of the descaler.
8. The plant according to claim 4, wherein the induction furnace is
placed immediately downstream of the shear.
9. The plant according to claim 3, further comprising intermediate
cooling and/or heating means among the rolling mill stands.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 12/102,493, filed Apr. 14, 2008, which in turn
claims priority of PCT Application Serial No. PCT/IT2005/000754
filed Dec. 22, 2005. The disclosures of these applications are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a process and related plant
for the manufacturing of steel strips.
BACKGROUND OF THE INVENTION
[0003] In the steel industry it is known the need, being however
present in every industrial field, for using manufacturing methods
involving lower investment and production costs. It is known as
well that in the last years manufacturing methods based on the
so-called "thin slab" technologies have had a remarkable
development and success in this direction of cost reduction, above
all under the energetic aspect. Three fundamental types of
manufacturing processes and related plants, accomplishing such a
technology, can be distinguished, namely a first type which does
not provide for solution of continuity between the continuous
casting step and the rolling one, a second type wherein said two
steps are separated, thereby with a solution of continuity
providing for the use of a Steckel rolling mill, and finally a
third type, again with solution of continuity, as shown in FIG. 1,
which represents the closest prior art to the present invention, as
is accomplished, for example, in the so-called CSP plant of the
American Company Nucor Steel in Crawfordsville, Ind. (US).
[0004] With reference to said FIG. 1, wherein the continuous
casting machine is schematically represented as 1, a thin slab 2 is
produced at the outlet thereof having thickness from 45 to 110 mm
and a typical speed of 5 m/min. The slab is cut by means of a shear
3 at a typical length of 40 m, anyway depending on its thickness,
on its width and on the weight of the desired final strip coil. The
thin slab, so cut down into pieces 4, enters a tunnel furnace 5,
whose purpose is to homogenize the temperature especially
throughout the transverse cross-section, from the external surface
to the core, then passes through a descaler 8 before entering the
finishing rolling mill 9 comprising, in the example shown, six
stands 9.1-9.6. After the rolling, from which it comes out on a
cooling roller table 15, it goes to the final coiling by means of
one or two reels 16 in order to form the desired coil.
[0005] It should be noted that the tunnel furnace 5 is
characterized, as it is known, by a length of about 200 m and by a
typical residence time of the slab inside thereof comprised between
20 and 40 min at a speed as indicated above. Of course, a
continuous casting speed higher than 5 m/min requires a tunnel
furnace length even greater than 200 m in order to heat the slab
and make its temperature uniform. For example, with a speed of 7
m/min at the outlet of the continuous casting, the tunnel furnace
should have a length of about 300 m if maintaining a residence time
of the slab in the furnace greater than 40 min is not desired. By
further increasing the casting speed, still for the same residence
duration in the furnace, this should have an even greater length,
hardly feasible both from a technical and an economical point of
view.
[0006] Still with reference to FIG. 1, it shows three slabs 4, 4.1
and 4.2 inside the furnace 5, among which the first one is still
connected to the continuous casting before being cut by the shear
3, the second one is free inside the furnace, ready to be rolled
and the third one is already drawn by the finishing rolling mill 9
through the descaler 8. The virtual profiles of two additional
slabs 4.3 and 4.4 are further represented by a dotted line, which
might find a place inside the furnace 5 without having to stop the
continuous casting in case of jammings in the rolling mill or of
replacement operations of the rolls, if these problems can be
solved in a time lower than 20 min.
[0007] The transverse temperature profile of the slab, immediately
upstream of the first rolling stand, has been represented by the
detail marked by reference number 7. The diagram of FIG. 1a further
shows that a slab with a average temperature of 1000.degree. C. at
the inlet of the finishing rolling mill requires a pressure or
"flow stress" Kf on the material equal to 100 N/mm.sup.2, whereas a
temperature of 800.degree. C., in the case of low carbon steel,
involves a pressure Kf of about 150 N/mm.sup.2. As it can be noted
in detail 7, the temperature profile of the slab at the inlet of
the finishing rolling mill is substantially homogeneous, as shown
by the slightly convex curve representing it from a minimum of
about 990.degree. C. at the ends, corresponding to the surface
temperature, to a maximum of 1010.degree. C. at the center zone,
corresponding to the core of the slab, from which comes the
previously indicated value of about 1000.degree. C. for the average
temperature.
[0008] In fact, according to the related prior art of this type of
technology, it has been so far believed that the product at the
outlet of the continuous casting 2, having a temperature profile as
shown in the diagram of detail 6, relative to a slab cross-section
at the inlet of the furnace 5, i.e. with a surface temperature of
about 1100.degree. C. and of about 1250.degree. C. at the core
(i.e. the apex of the diagram), should undergo a process of
complete temperature homogenization. The trend has always been to
homogenize such temperature as much as possible, especially
throughout the cross-section of the slab, before entering the
finishing rolling mill. In fact, it has been always thought that by
making the temperature uniform between surface and core of the
product, the advantage of a homogeneous fiber elongation could be
obtained, in order to show the same strain resistance by
substantially having the same temperature. On the basis of such a
constant technical prejudice, it has been always tried to have a
temperature difference being lower than 20.degree. C. between
surface and core of the product, as above indicated with reference
to detail 7, in order to have a homogeneous fiber elongation, until
now considered necessary for the achievement of a good quality of
the final product.
[0009] On the other hand, as seen above, the temperature uniformity
characteristic of the slabs does not allow building plants with the
high casting speeds, which would be theoretically possible to
achieve (up to values of 12 m/min due to the present technology
development), and thereby with very high productivities, due to the
inadmissible length the furnace should have.
[0010] On the other hand it would be desirable to have furnaces of
reduced length between continuous casting and rolling mill in order
to obtain space saving and reduction of investments, resulting in a
higher average temperature of the product, involving a lower total
power of the stands for the same strip thickness, as highlighted in
the diagram of FIG. 1a already mentioned.
[0011] In fact, thus overcoming a widespread prejudice of the prior
art, it has been found that with a temperature in the middle of the
cross-section of the slab being higher than 100-200.degree. C. with
respect to the surface temperature, maintained at about
1100.degree. C., a lower rolling pressure Kf is required in order
to obtain the same final thickness of the strip, because the
average rolling temperature is increased, without otherwise
worsening the product quality.
[0012] It has been also found that such temperature conditions are
not prejudicial for the final rolling product quality, when the
following conditions are met: the cast product shows a sufficiently
high "mass flow" value (i.e. the amount of steel flowing in the
time unit at the outlet of the continuous casting), with an outlet
speed >5 m/min after having undergone a process of liquid core
reduction or "soft reduction," in particular according to the
teachings of EP 0603330 in the name of the same applicant, in order
to guarantee the so-called "central sanity" characteristic of the
cast slab and to have a higher temperature at the core, and thereby
also a higher average temperature in the rolling step.
SUMMARY OF THE INVENTION
[0013] It is therefore an object of the present invention to
provide a process for the manufacturing of steel strips with
solution of continuity allowing the maximum possible reduction with
the minimum separating strength and therefore requiring a reduced
total power of the rolling stands with a consequent energy saving
for a given strip thickness at the outlet of the rolling mill.
[0014] Another object of the present invention is to provide a
process of the above-mentioned type being able to achieve, with a
limited furnace length, very high productivities as a consequence
of a high casting speed.
[0015] Broadly, the present invention relates to a plant for
production of steel strips from thin slabs having thickness
comprised between 45 and 110 mm coming from continuous casting. The
plant comprises a shear cutting a casting product, which is formed
with a solution of continuity, into the slabs, a multiple stand
finishing rolling mill located downstream of the shear with a
distance between the outlet of the continuous casting and the inlet
to the rolling mill being not greater than 100 m, at least one
heating furnace located downstream of the shear and upstream of the
multiple stand finishing rolling mill, and a descaler arranged
between the at least one furnace and the rolling mill. One of the
at least one furnace is an induction furnace which has a
sufficiently low working frequency to heat a core of the slab and
to substantially maintain a same temperature difference between an
inside and an outside of the slab from an end of the induction
furnace to an inlet of a first rolling stand of the finishing
rolling mill. Also, an average temperature of each slab is higher
than a surface temperature of each slab, which is equal to or
higher than 1100.degree. C., and the average temperature at the
core is at least 100.degree. C. higher than the surface temperature
of each slab.
[0016] The plant can further comprise a second furnace, which is a
tunnel furnace that is heated by gas.
[0017] Alternatively, the plant can have only one furnace and the
furnace is an induction type furnace. Further, the plant can have
intermediate cooling and/or heating means among the rolling mill
stands.
[0018] The plant can further comprise a temperature maintaining
tunnel in combination with the induction furnace, upstream and/or
downstream thereof, which have a length to keep a total distance
between the continuous casting and the finishing rolling mill not
greater than 100 m, suitable for limiting the thermal losses. The
tunnel can be formed by roller tables that have insulating panels.
The tunnel can also have gas burners and/or electrical resistors.
Moreover, the induction furnace can be placed immediately upstream
of the descaler. Alternatively, the induction furnace can be placed
immediately downstream of the shear.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Characteristics of the present invention will become evident
from the following detailed description of a preferred embodiment
thereof, given by way of non-limiting example with reference to the
accompanying drawings, in which:
[0020] FIG. 1 schematically shows a plant for the manufacturing of
steel strips from continuous casting, with solution of continuity,
according to the prior art, as already described above;
[0021] FIG. 1a is a diagram showing the trend of the rolling
pressure required as a function of the average temperature of the
material to be rolled;
[0022] FIG. 2 shows a schematic view of a plant according to the
present invention, similar to that of FIG. 1; and
[0023] FIG. 3 shows a schematic view of a variant of plant
according to the present invention, comprising an induction
furnace.
DETAILED DESCRIPTION OF THE INVENTION
[0024] With reference to FIG. 2, an example of plant carrying out
the process according to the present invention is schematically
shown starting from a thin slab 22 at the outlet of a continuous
casting zone schematically represented in its whole as 21 and
comprising, as it is known, a mould, as well as possible suitable
means to accomplish a liquid core reduction or "soft reduction."
The thin slab 22 comes out from the continuous casting 21 with the
same thickness and speed values already indicated for the slab 2 of
the plant of FIG. 1 relating to the prior art, i.e. with a
thickness between 45 and 110 mm, e.g. 60 mm, a speed equal to 5
m/min and a width equal to 1600 mm, that is to say with a high
"mass flow" as defined above. The temperature profile in the zone
upstream of the furnace 25 (here not shown) is still the one shown
in detail 6 of FIG. 1, with a surface temperature of about
1100.degree. C. and of about 1250.degree. C. at the core (diagram
apex).
[0025] The slab is still cut down in pieces, typically having a
length of 40 m, by means of the shear 3, according to the weight of
the final coil desired, and enters a traditional tunnel furnace 25
(gas heated), but being of a limited length, having the purpose of
maintaining the thin slab 24 in temperature by heating the same.
Therefrom it passes, through the descaler 8, into a finishing
rolling mill 29 from which comes out, upon its rolling, on a roller
table 15 in order to be coiled by means of one or two reels 16, as
already seen according to FIG. 1.
[0026] Differing from the plant of FIG. 1, the tunnel furnace 25
here shows a length that must be as reduced as possible and anyway
not greater than 100 m, so that the residence time of the thin slab
inside thereof be as short as possible. This is for the purpose of
maintaining a profile with a "triangular" trend at the outlet
thereof, as indicated in detail 27, being characterized by a
surface temperature of about 1100.degree. C., a temperature at the
slab core of about 1200.degree. C. and a average temperature of
about 1150.degree. C. The resulting trend is thereby substantially
less homogeneous than the profile shown in detail 7 of FIG. 1, for
the same feeding speed.
[0027] Inside furnace 25 two slabs 24 and 24.2 are represented of
which the first one is still connected to the continuous casting
before being cut by shear 3 and the second one is already drawn by
the finishing rolling mill 29 through the descaler 8, and thereby
is already in the rolling step. The dotted line 24.1, intermediate
between the two slabs, instead represents the space available for a
further slab, serving as a "lung" in case of jamming of the rolling
mill, if the slab thickness at the outlet and the weight of the
coil desired allow to have slabs of length <30 m, given the
above-mentioned limits of overall furnace length. Each slab, after
the shear 3 cut, is accelerated and transferred to the central part
of the furnace until it reaches the entering speed of the finishing
rolling mill, equal to about 15-20 m/min, in order to reduce the
residence time in the furnace itself as much as possible, which
will be able to be even lower than 10 minutes instead of the 20-40
min foreseen for a plant according to the prior art shown in FIG.
1.
[0028] As previously stated, it should be noted that anyway the
distance between the outlet from the continuous casting 21 and the
finishing rolling mill 29 will not be greater than about 100 m,
with the further consequent advantage of having a more compact
plant requiring a reduced space also with high speeds at the outlet
of the continuous casting. In such a way the average temperature of
the product will be higher than the surface temperature, being
higher of at least 100.degree. C. at the core with respect to the
external surface. From the diagram of FIG. 1a it is clear that a Kf
value of about 70 N/mm.sup.2 corresponds to a average temperature
of 1150.degree. C., instead of 100 N/mm.sup.2 as it happens with
the average temperature of 1000.degree. C. resulting from the plant
of FIG. 1.
[0029] It should be noted that, by using the above-mentioned higher
temperature of the "mass flow," greater reductions can be achieved,
in particular in the first rolling stands, allowing to obtain
thinner thicknesses with the same or a lower number of stands with
respect to the prior art. In FIG. 2, for example, the rolling mill
stands 29 have been represented in a number of five against the six
ones of the rolling mill 9 of FIG. 1.
[0030] FIG. 3 shows another embodiment of the present invention,
wherein the tunnel furnace 25, typically gas heated, is
substantially replaced by an induction furnace 35. In the prior art
(see for example EP 0415987 in the name of the same applicant)
induction furnaces have been used in order to heat a thin slab,
previously rolled to a thickness of about 15 mm in a roughing
rolling mill, and make it suitable for the subsequent finishing
rolling step. As the slab core was anyway hotter than the surface,
the working frequency of the furnace was generally chosen
sufficiently high so that the depth of penetration of thermal
energy, inversely proportional to frequency, were such to mainly
heat the surface layer characterized by a lower temperature.
[0031] On the contrary, according to the present invention, the
induction furnace 35 of FIG. 3 is used with a sufficiently low
working frequency so that the heating action, being performed in a
nearly homogeneous way throughout the whole transverse
cross-section of the slab to the core, substantially maintains the
same trend as at the inlet thereof until the end, such trend being
shown by the diagram of detail 6 in FIG. 1. Thus, if at the inlet
of furnace 35 the slab 34, to be cut by means of shear 3 from slab
32 coming out from the continuous casting 31, has a surface
temperature of 1100.degree. C. and of 1250.degree. C. at the core,
at the outlet of said furnace it will be able to have also a
surface temperature of 1150.degree. C. or higher and of about
1250.degree. C. at the core, not only maintaining a sensible
temperature difference inside-outside, but also increasing the
average temperature of the slab under rolling, with all the
advantages previously shown with reference to FIG. 1a.
[0032] Before entering the induction furnace 35, the thin slab 32
coming from the continuous casting 31, passes anyway, after the
shear 3, into a temperature maintaining and possible heating tunnel
36, which limits thermal losses.
[0033] It should be noted that the induction furnace 35,
differently from what is shown in FIG. 3, could also be placed
before said tunnel 36, in such a way to increase the slab
temperature while this is still connected to the continuous
casting, for the purpose of limiting its power dimensioning. After
the cut by shear 3, the slab cut down piece 34 is accelerated, as
already said for slab 24 with reference to FIG. 2, in order to
reach the entering speed of the rolling mill 39, equal to about
15-20 m/min. The tunnel 36 comprising the roller tables between
continuous casting and rolling mill, upstream and/or downstream of
the furnace 35, is formed of insulating panels, which might be
provided with gas burners and/or resistors in order to further
reduce the heat losses. To sum up, given the lower length of an
induction furnace with respect to a traditional one, it can be said
that also in this case, taken into account tunnel 36, being of a
reduced length with respect to furnace 25 of FIG. 2, the total
distance between the outlet of the continuous casting and the
rolling mill inlet is again not greater than 100 m.
[0034] Cooling systems or possibly intermediate heating systems,
not shown in the drawing, can be provided for among the stands of
the finishing rolling mill 29 or 39, being inserted between one
stand and another according to the rolling speed and to the steel
type to be rolled.
[0035] Finally, the present invention can also be used in order to
carry out processes and related plants with two casting lines
supplying the same rolling mill 29 or 39.
* * * * *