ROLLING MILL FOR PRODUCING STEEL FOR TUBES AND THIN STRIP

Abstract

The invention relates to a rolling mill for producing metal strips, preferably steel for tubes and/or thin strip, in which the temperature control can be influenced between the finish stands Fi and Fi+1 using a rapid heating device (induction heater), characterized in that the distance between the stands Fi and Fi+1, between which a rapid heating device is arranged, is 5 to 25 m, and in addition to an induction heater, a roller flattening unit (for example, roller leveler 14), and/or shears 8, and/or a driving roller pair 12, and/or a descaling sprayer 15 are arranged as additional units between the two stands. The roll stand Fi and/or regions in front of the roll stand Fi further comprise actuators for influencing the strip warping and/or strip ski-up/-down at the strip head.

Description

The invention relates to a rolling mill, in particular, a CSP-installation for producing strips, in particular, steel for tubes and/or thin strips, including a caster for producing thin slabs and a rolling mill for rolling a thin slab to a strip or thin strip.

Production of steel strips or steel plates by hot rolling is adequately described in the state-of-the art. Corresponding disclosures are found, e.g., in a paper of P. Uranga et al., “Improvement of Micro-structural Homogeneity in Thermo-mechanically Processed Nb Steel by Thin Slab Casting,” 43^rd Mechanical Working and Steel Processing Conference, Charlotte, ISS, Vol. 39, pages 511-529; in a paper of Kinkenberg, et al., “Processing of Niobium Microaloy API Grade Steel on a Thin Slab Plant”, Material Science Forum, Vols. 500-501, 2005, pages 253-260; and in a paper of S. V. Subramanian, et al., “Process modeling of micro-alloyed steel for near net shane casting” Proc. Of the Int. Conf. on Thermomechanical Processing;” “Mechanics, Microstructure, ed. by E. J. Palmiere et al., The University of Shefield, Shefield 2003, pages 148-156.

CSP) (Compact Strip Production)—installations are casting and rolling installations in which two separate working steps for production of steel strip are closely connected with each other, namely, the casting of liquid steel in thin slabs in the caster and rolling of thin slabs in steel strip in the rolling installation. With this, usually the rolling of the previously cast strand is carried out directly with use of the casting heat or by adjusting the desired rolling temperature using a compensation furnace or a heating device between the caster and the rolling mill.

Conventional rolling mills of a thick slab installation have at least one roughing stand or heavy-plate stand and a finishing rolling train arranged after the roughing stand or heavy-plate stand at a distance therefrom. While in the roughing stand or heavy-plate stand, the thin slab is rolled, usually in a reversing operation, to an intermediate strip with a predetermined thickness, the rolling in the finishing rolling train takes place in tandem operation, with the finishing rolling train being formed as a continuous rolling train. While the distance between separate stands of such finishing train is usually constant and usually amounts to 5.5 m, the distance between the roughing stand or heavy-plate stand and the first stand of the finishing train is usually many times greater in order to insure the reversing operation in the roughing stand or heavy plate stand. In this connection, the distance between the roughing stand or heavy-plate stand and the finishing train of about 50 m or more is no rarity.

Conventional rolling mills have, because of a usually alignment arrangement of the thick slab—extraction roller table of the slab furnace, the roughing stand or heavy plate stand, and the finishing train, a large length, and require powerful stands, so that investment costs are high. Energetically, conventional rolling mills are inferior in comparison with CSP-installations. In particular, during production of thin strips, the entry temperature in the finishing train is very low, which makes rolling of thin strips difficult. Also, the production of steel for tubes in a conventional hot rolling mill, because of the necessary temperature control, is very time-consuming and reduces the production of a conventional rolling mill.

Accordingly, the object of the invention to provide a rolling mill of the type described above in which the above-described drawbacks can at least be reduced. Within the meaning of the invention, the object of the invention is achieved with a rolling mill having features of claim 1. Advantageous embodiments of the invention are defined by dependent claims.

The rolling mill according to the present invention, in particular, of a CSP-installation, advantageously consists of a compact rolling mill for producing metal strips, especially steel for tubes and/or thin strips, and in which the temperature between two, following one another finishing stands Fi and Fi+1 can be influenced by a rapid heating device, in particular, an induction heating device. In addition to the rapid heating device, advantageously a straightening unit, preferably, a roller straightening device, and/or shears, or/and, optionally, a descaling sprayer, or another strip cooling device can be arranged between the stands Fi and Fi+1. All of the devices are so compactly located that they fit in the spacing between the stands of 5-25 m. Further, the stand Fi is equipped upstream of the rapid heating device with actuators for influencing the strip warping and/or the ski-phenomenon on the strip head.

Together with the straightening unit, advantageously, the roller straightening device, and/or the shears and above-mentioned actuators, a reliable passage in a limited space through the rapid heating device, in particular, induction heating device, can be insured with a minimal clearance in the thickness direction. Dependent on the process, the rapid heating device, the shears, and/or the descaling sprayer is (are) used or are displaced sidewise, dependent on the to-be-rolled product, or there, alternatively, a roller table, with or without heat insulation, or a table can be arranged. Also for carrying out thin strip rolling, shears can be provided between the stands, in order to take care of as straight as possible heads and ends of the deformed strip.

As a result of additional devices between the stands, the normal spacing between the stands, e.g., ≧5.5, is increased. In order to eliminate a possible secondary scale formation, advantageously, a single descaling sprayer is provided, which is formed as a compact device and, advantageously, is arranged before the stand Fi+1 behind the rapid heating device. For rolling of strip, with high requirements to surface quality (e.g., for rolling of thin strips), the descaling sprayer in front of the stand Fi+1 can be activated. During production of tubes, the descaling sprayer can be optionally deactivated or removed from the rolling line.

In order to save space, optionally, a looper can be dispensed with and, instead of it, means for regulating tension, with or without a tension measuring roller, can be provided.

In comparison with conventional rolling mills with induction heating between the roughing and finishing trains, the inventive use of a rolling mill, preferably, a rolling mill of a CSP-installation with corresponding additional units, noticeably reduces the space occupied by the rolling mill. The use of additional units, which are selectively inserted in the finishing rolling mill, advantageously improves the rolling process as with regard to carrying out the process so with regard to obtaining of a corresponding structure. The additional units require, according to the invention, little space and are arranged between two finishing stands, the distance between which amounts to 5-25 m. On the other hand, the space requirement between two stands with additional units is much smaller than the length of the intermediate strip which is rolled there.

Advantageously, a thin strip, which is cast by a caster of CSP-installation, has a thickness, equal or less than 120 mm. Thereby, there is provided a CSP-installation that alone, without use of a reversing roughing stand, and with the use of a number of finishing stands forming a rolling mill, is in a condition to produce a desired production spectrum from a thick tube strip to a thin strip.

As an example and a preferred embodiment, a method and the production of a tube strip in a CSP-installation will be described. The TM-method (thermomechanical method) in a CSP-installation consists as a rule of a step of one or more deformations of an austenitic initial structure in recrystallization temperature range for producing a uniform, fine, recrystallized austenite structure, and a following step of one or several deformations of the recrystallized, austenitic structure for producing a high-dislocation, extended in an area as much as possible, non-recrystallized austenite structure (so-called pancake structure). Thus, these steps can be called conditioning of the austenite.

Finally, in a further step, cooling of austenitic structure which has been produced in the first step, is carried out for producing a fine-grain structure in the finished hot rolled strip or hot rolled sheet with a phase conversion. The structure of the finished hot rolled strip or hot rolled sheet consists of a combination of ferrite, perlite, bainite, and martensite, wherein the content of these four structure components can, respectively, amounts to between 0% and 100%.

As a result of the above-described TM-method, it is also possible to dispense with the above-mentioned step of deformation in the non-recrystallized temperature range of the austenite. In this case, the conditioning of the austenite takes place entirely in the recrystallization temperature region of the austenite.

The difficulty with the mechanical hot rolling consists, however, in that for producing the uniform, fine recrystallized austenite grain in the recrystallization range, as large as possible deformation should be undertaken. A fine recrystallised structure is distinguished by the fact that not only the former, non-uniform cast structure, but also individual coarse grains or structure regions have to be completely transformed into a uniform, fine recrystallised structure with small scatter around the mean grain size. This condition frequently is not fulfilled or is fulfilled incompletely and leads to an inadequately conditioned austenite structure.

If a step of hot rolling in the non-crystallization temperature range of the austenite follows the step of hot rolling in the recrystallisation temperature range of the austenite, often only little residual deformation for the succeeding actions in the thermomechanical treatment remains when there is a large ratio of the thickness of the finished hot strip or hot sheet and the thickness of the slab or the intermediate strip. Even when individual stands are taken out, this, on occasion, is not sufficient to transform possibly still present residues of the cast structure as well as individual coarse grains or structure regions into a uniform flat pancake structure of non-recrystallised austenite grains. An insufficiently conditioned austenite is also present in this case.

Inadequately conditioned austenite has the disadvantageous consequence in the finished hot strip or hot sheet consisting in the presence of individual coarser grains outside the normal distribution about the mean grain size and/or of structure regions having a substructure characterized by small-angle grain boundaries. However, structure regions of that kind lead to deterioration in the mechanical properties of the strip or sheet, particularly to a reduced toughness.

In correspondence with the description above of TM-method, the deformation in the recrystallization range of the austenite is of critical importance for the characteristics of the finished steel strip or sheet. The degree of the deformation required in first step of the thermomechanical treatment can, in fact, be replaced in part by an elevated entry temperature. However, this possibility is limited by the maximum furnace temperature as well as by the cooling at the time of contact with the roll and by thermal radiation between the stands when several stands participate in this step.

Particularly advantageously is a CSP-installation in which a heating device is arranged between two stands Fi and Fi+1 of the rolling mill, in particular between a first stand F1 and the second stand F2. If needed, the heating device can be removed from the rolling mill and again inserted therein. A particularly advantageous is an induction heating device, especially one with one-four induction elements. Altogether, with such heating devices, one strives for a construction as compact as possible with a high power density. The power density lies, advantageously in a range with at least 1500 mw per square meter, preferably, with about 4,000 mw per square meter, when measuring the power density that has been just applied to the strip or induced therein. As an inductor element, such constructional element is designated through which power is applied to the strip. One or several induction elements can provide induction heating.

At the end of the installation, the rolling mill, preferably a continuous rolling mill is used within which the above-described, in the example, deformation steps are used at high temperature and, if needed, with a support of a heating device between the finishing stands and, at an optional cooling of rolls at lower temperatures. Here, no roughing stand or heavy-plate stand, which is conventional for thick slab installations, is used.

Advantageously, the deformation steps are joined together, which means that all of the involved stands are simultaneously operated in accordance with entry of thin slabs or intermediate strips. On this occasion, the rolling stands operate in tandem, so that operationally, the thin slab or the intermediate strip simultaneously passes through common rolling stands. Separate stands can be displaced and, thus, would not take part in the deformation operation.

In a continuous operation, a method can be used in which the cast strand need not be cut in slabs, but rather is continuously, preferably, through a tunnel furnace, is fed to a hot rolling mill, is rolled in a hot rolled strip, is separated before a reel, and is wound in a coil. This process prevents accumulation of scrap as no head or foot ends of a strip are produced. In addition, even thinner strips with advantageously a thickness less than 1 mm can be produced because the danger of a rise (hills) during entry of a thinner strip in the last stand of the hot rolling mill at the start is reduced. During continuous rolling, the feeding speed of the first active stand is reduced to the casting speed which can lead to increased temperature losses before and/or during the hot rolling process. However, for rolling, high rolling temperatures and, thus, heating of the strip is necessary to avoid rolling temperatures in the ferrite phase and/or the two-phase area austenite plus ferrite.

One of such method and appropriate apparatuses permit in principle the production of different steels with a reduced austenite phase area, e.g., with silicon content of more than 1%. For this, for rolling, higher rolling temperatures are necessary to reliably avoid rolling temperatures in the ferrite phase and/or the intermediate phase austenite plus ferrite. Such steel goods can be produced, without further addition, on the inventive CSP-installation.

During the production of strips or thin strips, as discussed above, advantageously an induction heating is provided between the front stands or at least in the region between the front stands. Advantageously, the induction heating is provided between the first and second or/and the second and third stands of the finishing mill.

The induction heating, however, is characterized by a comparatively small penetration in the thickness direction and is a sensible component. During rolling in a finishing mill, in particular in the first stand, often, so-called skis, or other such strip cambers, or unflatness are present on the head, which endanger passage of tube strips or thin strips through the induction heating means or other units (shears, descaling sprayer) between two finishing stands. Besides, the above-mentioned ski-up or ski-down phenomenon makes the insertion of the pre-rolled intermediate strip in a following stand very difficult. In an unfavorable case, the intermediate strip can damage the units between the two finishing stands.

The reduction or elimination of skis can, e.g., be carried out with hold-down rollers or bending and straightening devices. However, it is most advantageous when in the inventive installation, the pre-rolled intermediate strip heads are cut off by a suitable cutting device. Optionally, with such a cutting device, both the head and the end of a pre-rolled intermediate strip can be cut off when thin finished strips are produced in a batch operation.

In a further advantageous embodiment of the inventive installation, dependent on the rolled strip material, descaling means can be arranged downstream, in the rolling direction, of the heating means for heating the intermediate strip, if necessary, after the strip passes through the heating device and the downstream located drive roller pair and before the intermediate strip enters in one or several finishing rolling stands. Thereby, it is insured that an almost impurities-free thin strip or a tube strip can be finish-rolled, without the scale damaging the surface of the strip or thin strip.

In order to insure a reliable passage of the intermediate strip through the devices between two finishing stands, there is provided, according to the invention, a number of possible control elements which can be used separately or in any arbitrary combination.

The rolling stand itself can have a twin drive and different speed settings on the upper and lower rolls dependent, e.g., on an entry thickness, thickness reduction, material or the temperature, and different diameters of the upper and lower working rolls.

For influencing the temperature distribution of the rolled product in thickness direction, the adjustment of slab and strip cooling before the rolling stand Fi can be carried out so that as symmetrical as possible temperature profile over the thickness of the rolled stock is provided, or by a well-aimed temperature trimming, the ski-phenomenon is influenced. Alternatively, to this end, trimming of the slab temperatures on upper and lower sides is possible in front of a stand with a heating device.

There is further provided for table height adjustment before the finishing sand Fi so that a predetermined slab entry position into the roll gap (e.g., the rolling stock middle=rolling gap middle) is reliably established.

The occurrence of a non-linear strip run (ski, amber, curvature) can be particularly easily prevented by using a straightening unit. This device for correction or prevention of skis or strip camber in form of a straightening unit can be, in addition to a roller straightening machine, also a hold-down roller, a hold-down plate, a hold-down strut, a bending and straightening machine, a strip head-straightening channel, a strip head pressing device.

As it has already been discussed above, a ski can be cut off the rolling stock with shears.

Finally, occurrence of unevenness in the intermediate strip, in particular before the entry into the heating device can be detected advantageously with suitable surface distance sensors. Suitable sensors such as mechanical, optical, or the like are known to one of ordinary skill in the art. Based on the alarm signal, control means which is connected with the sensors, can initiate appropriate measures for eliminating or reducing the detected non-linear strip run, or turn the intermediate strip or slab back.

Not only the heating device can protect the above-described devices from damage by strip curvatures, but also the damage, in particular produced by induction heating as a result of cooling water remaining on the strip or thin strip, can be minimized by using suitable blowers advantageously located between the rolling stands of the inventive finishing rolling mill or inserted therebetween.

The above-mentioned auxiliary units and measures for ski-elimination can be used preferably in a CSP-finishing mill for different application. However, they can also be inserted between the finishing stands of a conventional rolling mill.

Induction heating and auxiliary units can be fixedly arranged or displaced in and out from the rolling line. As a substitute for the movable units, a heat insulation hood is provided.

The invention will now be explained in detail below with reference to the drawings which show several embodiments of the inventive CSP-installation. The drawings show:

FIG. 1 a first embodiment of the invention in which induction heating means and a driving roller pair are arranged between two stands of a schematically shown strip hot rolling mill;

FIG. 2 a second embodiment of the invention with a device for correction/prevention of a ski-phenomenon on the intermediate strip head which is arranged upstream of the heating device in a strip rolling direction;

FIG. 3 a third embodiment of the inventive hot rolling mill in which a heating device is arranged between a device for correction/prevention of a ski-phenomenon on the intermediate strip head and a descaling sprayer, with all three being located between two rolling stands of a hot rolling mill;

FIG. 4 a fourth embodiment of an inventive hot rolling mill with arranged one after another, in the rolling direction shears, rapid heating means, driving rollers, and descaler; and

FIG. 5 a fifth embodiment of an inventive hot rolling mill.

FIG. 1 shows a portion of a hot rolling mill 2 according to the first embodiment in which a metal strip 1 runs through a first rolling stand designated as Fi and a second rolling stand designated as Fi+1. The distance between the stands Fi and Fi+1 amounts to from 5 m to 25 m. After exiting the finishing stand Fi, the strip 1 runs directly in induction heating means 8 and, finally, into the driving roller pair 12. This driving roller pair 12 can provide a strip tension (even before the strip head reaches the following stand) of the strip 1 and, the water which remains on the strip 1, can be easily squeezed. A minimal tension control can be carried out between the stands Fi and Fi+1. The driving roller pair 12 helps to precisely adjust the set speed in the stand Fi+1. Alternatively, a tension measuring roller or a looper (not shown) can be arranged between the stands in order to insure the mass flow rate. Finally, downstream of the rolling stand Fi+1 in the rolling direction, there is provided a cooling section 11 for cooling the strip 1, which was heated in the heating device 8 to a temperature above the recristallization temperature, to a temperature lying in a non-recristallization region. Both devices 8 and 12 can change their places. Also, arrangement of the driving roller pair 12 before or behind the heating device 8 can be contemplated.

FIG. 2 shows a further embodiment of the hot rolling mill 2 according to the invention in which again the finishing stand Fi and the finishing stand Fi+1 are shown. Again, a heating device 8 for a rapid heating of the metal strip 1 by induction heating is arranged between the rolling stands Fi and Fi+1.

In the rolling direction W, a device 14 for correction/prevention of a ski-phenomenon on the intermediate strip that have left the finishing stand Fi, is arranged between the finishing sand Fi and the heating device 8. Such a device 14 for correction/prevention a ski-up or -down can, e.g., be formed as a bending and straightening driver or as a hold-down roller. Such device 14 should correct a phenomenon of, e.g., elevated strip head after exiting a rolling stand (a so-called ski-up), i.e., to eliminate or to reduce it to an acceptable achievable minimum. First of all, this serves to prevent downtime resulting from imprecise or non-entry of the intermediate strip head in a following unit, e.g., the rapid heating device 8, other units (shears, descaling sprayer/or each further stand Fi+1). In order to prevent the ski-phenomenon, the hot rolling mill according to the invention has a least one rolling stand, preferably, the first rolling stand Fi, with separated and not connected with each other drives for the upper working roll Fia and the lower working roll Fib. Thereby, with a suitable control of the drives, the ski-up or also ski-down-phenomenon is reduced to a minimum value already during rolling.

FIG. 3 shows a third embodiment of the inventive hot rolling mill 2 in which between the first shown rolling stand Fi and the second shown rolling stand Fi+1, a rapid heating device 8 follows the device 14 for correction of the ski-phenomenon on the intermediate strip head. Between the rapid heating device 8 and the rolling stand Fi+1, a descaling sprayer 15 is arranged with which the scale that sticks to the surface of the strip 1, can again be reliably removed. Optionally, this embodiment of the invention can have a driving roller pair (not shown) and/or a cooling section (not shown).

FIG. 4 shows a fourth embodiment of the inventive hot rolling mill 2 in which between the first shown rolling stand Fi and the second shown rolling stand Fi+1, there are arranged, in the rolling direction W of the strip 1, shears 13, rapid heating device 8, driving roller pair 12, and descaling sprayer 15. While the rapid heating device 8, the driving roller pair 12, and the descaling sprayer 15 function in the same way as in the embodiments according to FIGS. 2-4, the shears 13 serves primarily for cutting off the strip head and, if necessary, the strip end on which rolled tongues or bends (so-called ski) can be formed. Thus, the shears 13 replaces, on one hand, devices shown in previous embodiments (not shown here) for correction/prevention of a ski-phenomenon and, in addition, the shears 13 advantageously support a further rolling process by cutting off the tongues which can adversely influence the running of the strip 1 and the working rolls of the following stands.

In FIG. 5, a fifth embodiment of the inventive hot rolling mill 2 is shown in which there are arranged, in the rolling direction of the strip 1, finishing stand Fi, device 14 for correction/prevention of a ski-up, shears 13, and rapid heating device 8. After exiting the rapid heating device 8, the metal strip 1 enters a driving roller pair 12 which squeeze the residual water that remained on the metal strip 1. After leaving the driving roller pair 12, the metal strip 1 enters the descaling sprayer 15 in which the residual scale that has formed on the surface of the strip 1, is reliably removed before entry of the strip 1 in the finishing stand Fi+1. Finally, dependent on application, the strip 1 that have been heated in the rapid heating device 8, can be cooled again in the cooling section 11 and be rolled to a finished strip thickness.

The rapid heating device 8 and the shears 13 can be arranged, viewing in the strip running direction, alternatively or in reverse.

In case the distance between the stands should be further reduced, the rapid heating device 8 and the descaling sprayer 15 can be located, viewing in the strip running direction, at the same location and, alternatively, be displaced sidewise in the rolling line so that either the descaling sprayer 15 or the rapid heating device 8 is used.

Claims

1. Rolling mill for producing metal strips, preferably steel for tubes and/or thin strip in which a rapid heating device (8) is arranged between two, following one another, finishing stands (Fi, Fi+1), characterized in that

distance between the stands (Fi, Fi+1) between which the rapid heating device (8) is arranged amounts to 5-25 m, and that in addition to the rapid heating device, at least one further auxiliary unit, in particular a straightening unit, preferably a roller straightening device (14) (62) is arranged.

2. Rolling mill according to claim 1, characterized in that the rolling stand (Fi) which is arranged before the rapid heating device (8), or/and regions before and after the rolling stand (1) are equipped with actuators for influencing strip curvature and/or strip-ski phenomenon on the strip head.

3. Rolling mill according to claim 1, characterized in that the rolling stands (Fi, Fi+1) are finishing stands of preferably compact continuous train.

4. Rolling mill according to claim 1, characterized in that it advantageously forms a rolling mill of CPS-installation for producing thin slabs with a thickness of ≦120 mm.

5. Rolling mill according to claim 1, characterized in that the heating device, in particular, an induction heating device, advantageously, with from 1 to 4 induction elements, is arranged between the first and second stands (F1, F2) or/and between second and third stands (F2, F3), or is insertable therebetween.

6. Rolling mill according to claim 1, characterized in that at least one driving roller pair is arranged between the stand (Fi) and the following stand (Fi+1) or is insertable therebetween.

7. Rolling mill according to claim 1, characterized in that a device for correction or prevention of ski-phenomenon in the intermediate strip is arranged between the stand (Fi) and the following stand (Fi+1), preferably, before the heating device, or is insertable therebetween.

8. Rolling mill according to claim 7, characterized in that the device for correction or prevention of the ski-phenomenon is a hold-down roller, a hold-down plate, a hold-down strut, a bending and straightening machine, a strip held-straightening channel or a strip head-pressing device.

9. Rolling mill according to claim 1, characterized in that at least one rolling stand (Fi) has separate drives for the upper and lower working rolls (Fia, Fib).

10. Rolling mill according to claim 1, characterized in that strip cooling or strip heating means is located before a rolling stand (Fi) with which temperature distribution over the slab or the strip thickness is adjusted for influencing the ski-phenomenon and strip curvature.

11. Rolling mill according to claim 1, characterized in that the rolling stand (Fi) has, at a run-in side thereof, table height-adjusting means with which the strip running position is adjusted.

12. Rolling mill according to claim 1, characterized in that shears are arranged between at least two rolling stands (Fi, Fi+1) or are insertable therebetween.

13. Rolling mill according to claim 1, characterized in that a descaling sprayer is arranged between at least two rolling stands (Fi Fi+1) or is insertable therebetween.

14. Rolling mill according to claim 1, characterized in that a distance between two, following one another stands (Fi, Fi+1) is smaller than the intermediate strip length rolled between the stands.

15. Rolling mill according to claim 1, characterized in that a heat insulation hood is arranged between at least two rolling stands (Fi, Fi+1), in particular, as a replacement for previously removed units, or is insertable therebetween.

16. Rolling mill according to claim 1, characterized in that the rolling mill is connected with a process model which controls and regulates different measures and adjustments for influencing the ski-phenomenon.

17. Rolling mill according to claim 1, characterized in that the rapid heating device is formed compact and a power density, which is inducted in the strip, amounts to at least 1,500 megawatt per square meter, preferably, to about 4,000 megawatt (+/−18%) per square meter.