Rolling method for strip and corresponding rolling line

Abstract

Rolling method for the production of flat products with low productivity, which includes a continuous casting step at a speed between 3.5 m/min and 6 m/min of a thin slab with a thickness between 25 and 50 mm. It also includes a roughing step to reduce the thickness in at least one roughing stand to a value between 6 mm and 40 mm, and suitable for winding, a rapid heating step by means of induction in order to at least restore the temperature lost in the segment downstream of casting and in the roughing step, a winding/unwinding step in a winding/unwinding device with two mandrels. The method also includes a rolling step in a rolling unit that consists of a single reversing stand of the Steckel type to roll the product unwound by the winding/unwinding device.

Claims

1. A rolling method for the production of flat products with low productivity comprising: a) continuously casting a slab having a thickness between 25 mm and 50 mm at a speed between 3.5 m/min and 6 m/min, b) roughing the slab in a single roughing stand to reduce the slab's thickness to between 10 mm and 30 mm, c) rapidly heating the slab downstream of the casting and roughing step, d) winding and unwinding the slab in a winding/unwinding device having at least two mandrels after the step of rapidly heating, e) rolling the slab in a rolling unit having a single reversing stand, wherein the rolling step further comprises no more than three rolling passes or two inversions to produce a final slab having a thickness between 4 mm and 10 mm.

2. The rolling method of claim 1 further comprising cooling the slab.

3. The rolling method of claim 1 further comprising winding the slab.

4. The rolling method of claim 1 wherein the roughing step further comprises roughing the slab in the single roughing stand to reduce the slab's thickness to between 6 mm and 20 mm.

5. The rolling method of claim 1 wherein the rolling step further comprises two rolling passes or two inversions to produce the final slab having a thickness between 1.4 mm and 4 mm.

6. A rolling line for the production of flat products with low productivity comprising: a continuous casting machine configured to continuously cast a slab at a speed between 3.5 m/min and 6 m/min, a single roughing stand connected to an exit of the continuous casting machine and upstream of a heating unit, the heating unit having an induction furnace to recover the temperature lost passage through the roughing stand, a winding/unwinding device having at least two mandrels and configured to selectively and alternatively wind the slab arriving from the continuous casting machine and unwind the slab to feed a reversing stand of a rolling unit, wherein the winding/unwinding device is downstream of the heating unit, the rolling unit comprising a single reversing stand configured to perform a rolling operation, wherein the rolling operation comprises no more than three rolling passes or two inversions to produce a final slab having a thickness between 4 mm and 10 mm.

7. The rolling line of claim 6 wherein each of the single roughing stand is configured to allow a reduction in thickness of between 20% and 60%.

8. The rolling line of claim 6 wherein the single roughing stand is configured to perform a reduction in thickness of the slab between 6 mm and 40 mm.

9. The rolling line of claim 6 wherein the final slab has a thickness between 4 mm and 8 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

(2) FIG. 1 schematically shows a first form of embodiment of the rolling line according to the present invention;

(3) FIG. 2 schematically shows a second form of embodiment of the rolling line according to the present invention.

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT

(4) FIGS. 1 and 2 respectively show two forms of embodiment of a rolling line 10 according to the present invention for the production of flat rolled products for example strip 111.

(5) The rolling line 10 comprises a machine 12 for continuous casting, which produces, in this case, a thin slab 11. The machine 12 conventionally provides a ladle 13, a tundish 15 and a crystallizer 17.

(6) In some forms of embodiment, in the curved path shown in the drawings at exit from the crystallizer 17, the thin slab 11 may be subjected to a dynamic soft reduction, in order to obtain a better metallurgic structure. According to the invention, the cast thickness, after soft-reduction, is comprised between 25 mm and 50 mm.

(7) In some forms of embodiment, the thin slab which is cast has a width of 800-1600 mm.

(8) The rolling line 10 of the present invention is configured overall to produce coils with a thickness from about 1.4-1.6 mm to about 8-10 mm and roll weight of 25 tons.

(9) Since the rolling line 10 is of the low productivity type, the rolling process according to the present invention provides a casting speed of the slab 11 comprised between 3.5 and 6 m/min.

(10) According to the present invention, at least a roughing stand 20, 20a, 20b is provided downstream of casting. Typically, the at least one roughing stand 20, 20a, 20b is of the four-high type.

(11) In particular, according to a first form of embodiment of the present invention, shown in FIG. 1, immediately downstream of the casting machine 12 a single roughing stand 20 is provided.

(12) In accordance with a second form of embodiment, shown in FIG. 2, two roughing stands 20a, 20b are provided, located in series.

(13) According to the present invention, the working diameter of the rolls of the roughing stand 20, or of each of the roughing stands 20a, 20b, is comprised between 550 mm and 650 mm, preferably between 575 mm and 625 mm, for example about 600 mm. The length of the rolls is about 1500-1800 mm, for example about 1750 when the diameter is 600 mm.

(14) Moreover, in some forms of embodiment the separation force of the roughing stand 20, or of each of the two roughing stands 20a, 20b, is about 3000 tons (30000 kN).

(15) Furthermore, in some forms of embodiment the nominal power of the motor of the roughing stand 20, or of each of the two roughing stands 20a, 20b is 1500 kW.

(16) The single roughing stand 20, or the pair of two roughing stands 20a, 20b, has the function of reducing the thickness of the thin slab 11 immediately exiting the casting machine 12. According to the present invention each stand allows to obtain reductions of less than about 60%, for example comprised between about 20% and about 60%, advantageously between about 35% and about 55%, of the initial thickness.

(17) In the first form of embodiment in FIG. 1, the roughing stand 20 reduces the thickness of the thin slab 11 up to about 10 mm and 30 mm, preferably between 10 mm and 20 mm.

(18) In the second form of embodiment in FIG. 2, the two roughing stands 20a, 20b reduce the thickness of the thin slab 11 up to about 6 mm and 20 mm, preferably between 6 mm and 15 mm.

(19) In both forms of embodiment, the roughing stand 20, or the pair of roughing stands 20, 20b, is disposed immediately downstream of the casting machine 12 with which it is in direct contact without a break in continuity.

(20) The main advantage of this disposition of the roughing stand 20, or the pair of roughing stands 20, 20b, is that the reduction in thickness is performed when the slab 11 is still with a very hot core, which requires a smaller stand and therefore less power installed with subsequent saving of energy.

(21) Immediately downstream of the single roughing stand 20 or the pair of roughing stands 20, 20b, a first shearing unit 14 is present by means of which the shearing to size of the bar 11 is performed.

(22) The shearing unit 14 is of the known type and in some forms of embodiment, can comprises a pendulum shear, while in other forms of embodiment it can comprise a rotary shear or “crank shear”.

(23) During the production cycle, the first shearing unit 14 shears the bar 11 into segments or pieces of a desired length, correlated to the desired weight of the coil or roll of final strip.

(24) In particular, the length of the segments of bar is such as to obtain a coil of a desired weight, for example 25 tons, so that the rolling process is carried out in the so-called coil-to-coil mode.

(25) Upstream of the roughing stand 20 or the pair of roughing stands 20, 20b, a de-scaler 16 may be provided. In some forms of embodiment, the de-scaler 16 is preferably of the type having rotary nozzles and performs a careful removal of the scale from the surface of the cast product, using the minimum delivery of water possible, with a modest drop in temperature of the cast product.

(26) According to the present invention, downstream of the roughing stand 20 or the pair of roughing stands 20, 20b, and downstream of the first shearing unit 14, along the rolling line 10 a rapid heating unit is disposed, in this case an inductor furnace 18 to perform a step of rapid heating and configured to at least recover the losses of temperature coming from the pass in the at least one roughing stand 20, advantageously with the function of homogenizing and heating the cast product.

(27) The rolling line 10 provides, downstream of the inductor furnace 18, a winding/unwinding device 34 with at least two mandrels 34a, 34b, to carry out a winding/unwinding step subsequent to the rapid heating step. The at least two mandrels 34a, 34b are able to selectively and alternatively perform the function of winding the bar coming from the at least one roughing stand 20 and to unwind it and feed it to a subsequent rolling train with a single reversing roughing stand 22 of the Steckel type which will be described more fully hereafter in the description. For example the winding/unwinding device 34 can be made as in the international application PCT/EP2010/070857 in the name of the Applicant, entirely incorporated here for reference.

(28) In some forms of embodiment, the winding/unwinding device 34 is the heated type, to function as a furnace to at least maintain the temperature, so that during the winding/unwinding steps the bar remains at a suitable temperature for subsequent rolling in the rolling train, also reducing costs and bulk.

(29) If the rolling mill is stopped, the winding/unwinding device 34 allows to accumulate at most two segments of bar inside it without stopping the casting machine 12, hence functioning as a store, and then introduces them again into the rolling line 10 when the rolling train 22 starts up again. In this way it is possible to operate, for example, in some functioning modes of the rolling line 10, in the event of a stoppage of the rolling train 22 in an emergency (for example blockage), or programmed stoppage (for example roll change). Advantageously, the time for winding the bar onto one or more mandrels 34a, 34b of the winding/unwinding device 34 is consistent with the time of the roll change in the stands of the rolling train.

(30) Immediately downstream of the winding/unwinding device 34 there is an emergency shear, or crop shear 30, of a known type.

(31) The rolling train according to the present invention is the reversing Steckel type, and according to the present invention consists of a single Steckel reversing stand 22 which cooperates with winding/unwinding reels 25a, 25b, in some forms of embodiment heated reels, also known as furnace reels. The winding/unwinding reels 25a and 25b cooperate with respective drawing units 27a, 27b.

(32) In the solution shown, immediately upstream of the only reversing stand 22 of the rolling train there are respective de-scaling devices, indicated by 28a and 28b respectively, which perform the function of removing the scale before and/or after each rolling pass, preventing the scale from being impressed on the surface of the strip by the action of the rolling rolls.

(33) The working diameter of the rolls of the single Steckel stand 22 is comprised between about 500 mm and 600 mm, with a length of about 2050 mm.

(34) The working diameter of the rolls of each winding/unwinding reel 25a, 25b is about 1350 mm, with a length of 2050 mm.

(35) The rolling method according to the present invention provides at most five double passes through the reversing stand 22, which determine the desired reductions in thickness.

(36) For example, in the configuration in FIG. 1 having a single roughing stand 20, to produce, starting from a thin slab of 35 mm in thickness, a strip of low carbon steel having a width of about 1,300 mm, with a final thickness from 8-10 mm down to 4 mm, three rolling passes (two inversions) are sufficient, while for final thicknesses under 4 mm and down to 1.4 mm, five rolling passes (four inversions) are sufficient in the reversing stand 22 of the rolling train.

(37) In particular, in this first form of embodiment the reduction in thickness in the single roughing stand 20 is 60% and a bar of 14 mm in thickness is obtained which in the rolling train is reduced to the final thickness, for example of 2 mm, in the following way: a first rolling pass through the reversing stand 22 (first reduction in thickness of the first rolling pass is about 40%) and winding onto the winding/unwinding reel 25b; a first inversion (second reduction in thickness of the second rolling pass is about 38%), with unwinding of the strip by the winding/unwinding reel 25b and winding onto the winding/unwinding reel 25a; a second inversion (third reduction in thickness of the third rolling pass is about 33%), with unwinding of the strip by the winding/unwinding reel 25a and winding onto the winding/unwinding reel 25b; a third inversion (fourth reduction in thickness of the fourth rolling pass is about 28%), with unwinding of the strip by the winding/unwinding reel 25b and winding onto the winding/unwinding reel 25a; a fourth inversion (fifth reduction in thickness of the fifth rolling pass is about 22%), with unwinding of the strip by the winding/unwinding reel 25a.

(38) Instead, in the configuration in FIG. 2 having two roughing stands 20a, 20b to produce a strip of low carbon steel having a width of about 1,300 mm and a final thickness from 8-10 mm down to 2 mm, starting from a thin slab of 35 mm in thickness, three rolling passes (two inversions) in the rolling train are sufficient, while for final thicknesses of less than 2 mm down to 1.4 mm only five rolling passes (four inversions) are sufficient.

(39) In particular, in this second from of embodiment the reduction in the first roughing stand is 60%, while in the second roughing stand it is 50% and a 7 mm thick bar is obtained which in the rolling train is reduced to the final thickness, 2 mm for example, in the following way: a first rolling pass through the reversing stand 22 (first reduction in thickness of the first rolling pass is about 41%) and winding onto the winding/unwinding reel 25b; a first inversion (second reduction in thickness of the second rolling pass is about 34%), with unwinding of the strip by the winding/unwinding reel 25b and winding onto the winding/unwinding reel 25a; a second inversion (third and final reduction in thickness of the third and final rolling pass is about 26%), with unwinding of the strip by the winding/unwinding reel 25a;

(40) In one form of embodiment of the present invention, with two roughing stands and for thicknesses of the final strip of more than 5-6 mm, rolling in the single reversing Steckel stand 22 occurs advantageously with a single pass and therefore without inversions and without using the winding reels, therefore the exposure time of the product to the air and hence the formation of scale are both drastically reduced.

(41) Finally, the rolling line 10 includes, after the reversing rolling stand 22 of the rolling train, a run-out table for the strip 111, at a speed of about 1.5-12 m/s, and a cooling unit 24. For example, the cooling unit 24 is of the laminar cooling type with showers.

(42) Downstream of the cooling unit 24 the rolling line 10 comprises a winding unit 26, for example formed by a winding reel (down-coiler), of the strip 11 to produce the coils of strip.