| CLAIMS
1. A process for the manufacturing of steel strips comprising a step of continuous casting of thin slabs, having thickness from 45 to 90 mm, with solution of continuity, being provided downstream of said continuous casting with a step of cutting and subsequent heating, followed by a rolling step in several stands, characterized in that said heating involves almost exclusively external surface layers of the slab, thereby at the inlet of the rolling the temperature of the product is substantially homogeneous in any cross section of the slab with temperature from the external surface to in correspondence to the core of the slab of about 1150°C.
2. A process according to claim 1, wherein said heating step upstream of the rolling step is obtained by electrical induction at frequencies from 230 to 700 Hz.
3. A plant for the manufacturing of steel strips from thin slabs from continuous casting (21) having thickness ranging between 45 and 90 mm, comprising at least one heating furnace (25) upstream of a finishing rolling mill (29) with a plurality of stands, wherein the casting product (22) enters with solution of continuity after being cut down into slabs (24) by means of a shear (3), being provided a descaler (8) between the furnace (25) and the rolling mill (29), characterized in that said furnace (25) at the inlet of the first rolling stand of said rolling mill train (29) is of the induction type and the temperature of the product is substantially homogeneous in any cross section of the slab with temperature from the external surface to the correspondence with the core of the slab equal to about 1150°C, the distance between the output from the continuous casting (21) and the inlet in the rolling mill (29) being not longer than 100 m.
4. A plant according to claim 3, wherein said induction furnace (25) works at frequencies between 230 and 700 Hz, giving rise to a penetration (δ) of the heating equal to about 30í35% of the thickness of the slab, the induction frequency being proportional to the inverse of the square of the (δ) itself.
5. A plant according to claim 4, wherein the working frequency of the induction furnace (25) is selected in order to be sufficiently high so as to limit the heating action to the external surface layers, thus bringing them substantially to the same temperature, of about 1150°C, existing in the central zone or core of the slab.
6. A plant according to claim 3, wherein there is provided upstream and/or downstream of the induction furnace (25) a tunnel (26) for maintaining the temperature, having a length such as to keep the total distance between the continuous casting (21) and the finishing rolling mill (29) not longer than 100 m. 7. A plant according to claim 6, wherein said tunnel (26) comprises roller tables provided with insulating panels.
8. A plant according to claim 6 or 7, wherein said tunnel (6) is provided with gas burners and/or electric resistors.
9. A plant according to claim 6 or 7, wherein said induction furnace (25) is placed immediately upstream of the descaler (8).
10. A plant according to claim 6 or 7, wherein said induction furnace (25) is placed immediately downstream of the shear (3). |
"PROCESS AND RELATED PLANT FOR PRODUCING STEEL STRIPS WITH
SOLUTION OF CONTINUITY"
The present invention relates to a process and the related plant for the manufacturing of steel strips.
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 and 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, Indiana (US).
With reference to said Figure 1, wherein the continuous casting machine is schematically represented as 1, a thin slab 2, having thickness from 45 to 90 mm and a typical speed of 5 m/min, is produced at the outlet therefrom. The slab is cut at a typical length of 40 m by means of a shear 3, anyway depending on its thickness, its width and on the weight of the desired final strip coil. The thin slab, which is cut down into pieces 4 in this way, enters a tunnel furnace 5, whose purpose is to homogenize the temperature, especially through the cross section from the external surface to the core, then passes in a descaler 8 before entering the finishing mill 9 comprising, in the example shown, six stands 9.1 - 9.6. After the rolling step, 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 so as to form the desired coil. 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 from 20
to 40 min in correspondence to a speed as indicated above for heating and equalizing the temperature of the slab.
At a speed of 7 m/min at the outlet from the continuous casting, the tunnel furnace should have a length of about 300 m if it is desired to maintain the same residence time of the slab in the above-mentioned furnace longer than 20 min. By further increasing the casting speed, still for the same duration of residence in the furnace, the latter should have an even longer length, hardly feasible from both a technical and an economical point of view.
Still with reference to Figure 1, it shows three slabs 4, 4.1 and 4.2 inside the furnace 5, of which the first one is still connected with 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 train 9 through the descaler 8. Further, the virtual profiles of two additional slabs 4.3 and 4.4 that could find a place inside the furnace 5 without stopping the continuous casting in the case of jammings in the rolling mill or of operations of replacement of the cylinders, if these problems can be solved in a time shorter than 20 min, are represented with a dotted line.
The temperature trend in a cross section of the slab, immediately upstream of the first rolling stand, is substantially homogeneous, from a minimum of about 990°C at the ends, corresponding to the surface temperature, to a maximum of 1010°C at the central zone, corresponding to the core of the slab, from which derives a value of about 1000°C for the average temperature.
In fact, according to the prior art relative to this type of technology, it has been so far believed that the product at the outlet from the continuous casting 2, having a substantially inhomogeneous temperature profile with a surface temperature of about HOO 0 C and of about 1250°C at the core, should undergo a process of complete temperature homogenization, particularly through all the cross section of the slab, before entering the finishing rolling mill. A difference in temperature between surface and core of the product lower than 20°C is a precondition that the manufacturers of hot- rolling mills for strips normally require in order to give dimensional and qualitative guarantees about the final product.
It has always been a constant technical prejudice that, in order to obtain this
complete homogenization of cross-section temperature, the slab should have a minimum residence time in a furnace of about 20 minutes.
From this derives the further technical prejudice that, in order to achieve this target, a gas furnace is necessary even if its length, as previously stated, can reach remarkable dimensions along with the relative heavy duties of investment and maintenance. Moreover, it should be taken into consideration that the reliability of the current automation systems and the use of high wear-resistant rolling cylinders made of HSS quality (High Speed Steel) do not justify now the need for having a buffer of slabs inside the furnace in order not to stop casting in case of jamming or maintenance, as for the replacement of rolling cylinders. It is thus nullified this additional advantage of having large values of furnace length.
On the other hand, alternative heating systems, such as the induction furnace, have never been considered to be suitable for this purpose.
The patent EP 0415987 in the name of the same applicant teaches, among others, how to use an induction furnace for heating and homogenizing a product which does not come directly from the continuous casting, but through a roughing rolling mill, wherein the slab from continuous casting undergoes a process of temperature homogenization through its cross section. This homogenization process is mainly due to the deformation process, which reaches the slab core and thus transfers the heat from the core to the surface, and to the additional heat, which is transferred from the surface to the core, due to the kinetic energy that develops during the rolling process. In this case the induction furnace acts on a slab that is already partially rolled, having a thickness smaller than 30 mm.
Therefore, according to the prior art, only a previously rolled product, which has already undergone a homogenization process of the transversal temperature, can be subjected to the final homogenization step by means of an induction furnace, so as to obtain at the inlet of the finishing mill a strip with a temperature difference between surface and core lower than 2O 0 C, but not a slab from continuous casting with a temperature difference between core and surface which may be equal or higher than 150 0 C.
On the other hand, as it is seen above, the use of the gas furnace does not allow to
provide plants with the high casting speeds which could be theoretically reached (up to values of 12 m/min thanks to the current technology development), and thus with very high productivity, because of the inadmissible length that the furnace should have.
It would be desirable to homogenize the temperature of the slab by means of furnaces having reduced length between the continuous casting and the rolling mill so as to obtain very high productivity plants with space and investment savings.
For this purpose, it was possible to prove, by means of a mathematical model suitably prepared and experimentally confirmed, how it is instead possible to use, in place of or in addition to the traditional gas furnace, an induction furnace between the continuous casting and the rolling mill, thus obtaining in notably reduced spaces the same temperature homogeneity, both transversally and longitudinally, as it would be obtained with the gas furnace, thus overcoming a common prejudice of the prior art. This will be evident from the following description with reference to Fig. 2.
An object of the present invention is therefore to provide a process for manufacturing steel strips with solution of continuity which allows, by means of an induction furnace having reduced length, to achieve an adequate temperature homogenization with reduced residence time in the furnace and thus very high productivity as a consequence of a high casting speed.
These and other objects are achieved by a process having the characteristics mentioned in claim 1 and a plant whose characteristics are stated in claim 3, whereas further advantages and characteristics of the present invention will be evident from the following detailed description of one preferred embodiment thereof, which is reported by way of example, but it is not limitative, with reference to the annexed drawings wherein: Figure 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;
Figure 2 shows a schematic view of a plant according to the present invention; and Fifiure 3 shows an example of schematic partition in layers of a cross section of a slab to be taken as a reference for making the above-mentioned mathematical model and
for finding a correspondence between the temperatures and the frequencies of the induction furnace.
With reference to Figure 2, an example of plant is schematically shown, performing the process according to the present invention starting from a thin slab 22 at the outlet from a zone of continuous casting, which is schematically depicted on the whole with 21. The thin slab 22 comes out of the continuous casting 21 with the same values of thickness and speed as already indicated for the slab 2 of the plant of Fig. 1 , which is related to the prior art, and therefore with a thickness from 45 to 90 mm, e.g. 60 mm, a speed of 5 m/min and a length equal to 1600 mm. The temperature trend through the cross section, at the inlet in the furnace 25, is the same as the one obtained in the plant shown in Fig. 1, with a surface temperature of about HOO 0 C and of about 1250 0 C at the core.
By means of the shear 3, the thin slab is further cut down, as it is known, into pieces typically having a length of 40 m, depending on the weight of the final coil to be obtained, and enters a tunnel for maintaining the temperature and for a possible heating 26, which has the function of limiting the heat losses, and then enters an induction furnace 25, which has the function of homogenizing the temperature, and, if necessary, of heating the thin slab 24. It should be noted that, still with reference to the example of Fig. 2, while a slab 24 entering the tunnel 26 is still connected to the continuous casting before being cut by the shear 3, the precedent slab 24.2 is already drawn by the finishing rolling mill 29, while passing through the descaler 8. After rolling, through the roller table 15, the slab is finally coiled by one or two reels 16, as already shown according to Fig. 1.
After cutting by means of the shear 3, the slab is accelerated until it reaches the input speed of the rolling mill 29 ranging from about 15 and 20 m/min.
It should be noted that in order to limit the power of the induction furnace 25, and therefore also its length, the furnace itself could also be placed before the tunnel 26 on the part of the roller table immediately after the shear 3, while the slab 24 is still connected to the casting and therefore has a constant speed equal to the one of the casting itself. Further, the tunnel 26, which involves the roller tables between the casting and the rolling mill upstream and downstream of the induction furnace 25, is provided
with insulating panels, which could be also equipped with gas burners and/or with electric resistors to further limit the heat losses.
With reference to Fig. 3, for the purposes of the above-mentioned mathematical model a section of the slab has been represented, being practically divided in three layers, two of which are external, symmetrical with respect to the median plane, both subjected to the induction heating, and a central layer or core of the slab, which is slightly or not at all interested by the induction heating, already having a higher temperature. Thanks to a simple mathematical model it was possible to select a suitable frequency range, and consequently a suitable depth of heating penetration, also depending on the total thickness of the slab, in such a way that the heating itself extends only to one part of the cross section, in particular the surface portion, which is characterized by a lower temperature.
It has been possible to ascertain that at the end of the heating a complete temperature homogenization is achieved, as all three layers reach an average temperature of about 1150°C.
In fact, the surface layers are interested by the heating, with a decreasing power, in exponential form, from the surface to the maximum value of the penetration and their temperature rises on average of about 50°C, while the core reaches almost the same value of temperature by transferring heat to the external surface layers. Still with reference to Fig. 3 it has been possible to obtain the following table, where the optimal values of frequency, for different values of the total thickness T of the slab, are reported, as calculated on the basis of the above-mentioned mathematical model, for the purpose of having the desired temperature uniformity equal to about 1150 0 C starting from the initial condition, as already indicated, of 1100 0 C at the surface and 125O 0 C at the core of the slab. By δ it was indicated, for each thickness T, the entity of penetration of the thermal energy provided by the induction furnace, that is the thickness of the surface layers involved in the heating, which is variable within the range of values indicated from time to time.
TABLE
From this table it is evident that the optimal range of frequency in the range of values from 60 to 90 mm herein taken into consideration, is 230í700 Hz, more precisely 450í700 Hz for thicknesses of the slab around the value of 60 mm, 360í700 Hz for thicknesses around the value of 70 mm, 300í700 Hz for thicknesses around the value of 80 mm and 230í450 Hz for thicknesses around the maximum value of 90 mm. In case the induction heating was instead applied according to the teachings of EP 0415987, on a thin slab from continuous casting that is not subject to preliminary roughing rolling, and therefore having a higher thickness, the adopted field of frequencies would be ranging from about 140 Hz for the higher thicknesses, with distance of penetration δ equal to about 40-45 mm for thicknesses of the slab of 80-90 mm, up to a maximum of about 300 Hz for the thinnest slabs (60 mm) with a δ of 30 mm, that is until the core of the slab itself is reached in any case. In general it is possible to affirm that the value of heating penetration δ is equal to about 30í35% of the slab thickness, because the induction frequency is proportional to the inverse of the square of the value itself of δ according to the relation:
where p is a constant. Finally, it should be noted that the present invention can also be used, in all its variants, to provide processes and related plants with two casting lines feeding the same rolling mill 29.