Patent Grant
12036591
This application is a 371 of PCT/IB2021/055952, filed Jul. 2, 2021, which claims the benefit of Italian Patent Application No.102020000016120, filed Jul. 3, 2020.
The present invention relates to a plant and a process for the continuous production of hot-rolled ultra-thin steel strips down to a thickness of 0.3 mm and with a limited amount of scale, so as to make them suitable to be directly coated against corrosion without undergoing specific preliminary surface conditioning treatments.
It is well known that in the steel industry, in view of both the increases in the costs of raw materials and energy used and the increased competitiveness required by the global market, as well as increasingly restrictive regulations in terms of pollution, there is a particular need for a method of manufacturing high-quality hot-rolled steel strips which requires lower investment and production costs, resulting in ever thinner strip thicknesses. As a result, also the final product processing industry can become more competitive with lower energy consumption, so that the negative impact on the environment is also minimised.
The state of the art is essentially as described in previous patents by the same inventor such as EP 1558408, EP 1868748 and EP 1909979 to which reference is made for further details. In practice, the so-called ESP (Endless Strip Production) technology is used, which is based on “cast rolling” that combines the continuous casting of a thin slab with liquid core reduction (LCR=Liquid Core Reduction) with a first roughing phase through a roughing mill (HRM=High Reduction Mill) that produces an intermediate product, the so-called “transfer bar”. Casting is carried out from an ingot mould system based on patents EP 0946316, EP 1011896 and EP 3154726, also by the same inventor, to which reference is made for further details, which concern the geometric profile of both the horizontal and vertical sections of the ingot mould, as well as the particular geometry of the nozzle designed for a high mass flow of material up to 7-8 tons/min.
The above-mentioned patent EP 1558408 also envisages the possibility of extracting rough sheets after the first roughing phase as an emergency system in case of problems in the portion of the plant downstream of the roughing mill, in order to avoid the interruption of the continuous casting and consequently the production of the line, and not for a programmed production of sheets given the absence in the first portion of the plant of a controlled cooling system necessary for the production of high-quality sheets.
The transfer bar, after a phase of heating in an induction furnace and subsequent descaling, is further processed in a second phase of finishing rolling to transform it into a strip by controlling its temperature so that at the exit of the finishing rolling mill it still has a temperature above approximately 820-850° C., which corresponds to the lower end of the austenitic temperature range for most steels.
However, the results so far, although optimal in terms of steel strip quality, have proved to be improvable in terms of plant compactness, energy savings and the current minimum strip thickness value of 0.6 mm. In addition, although reduced oxide (scale) formation on the strip surface is achieved due to the minimum dwell time of the material at temperature, through the aforementioned induction heating of the transfer bar between the roughing and finishing stages, this reduced formation has not proved sufficient to avoid the pickling stage before the anti-corrosion coating is applied.
In order to ensure the desired final rolling in the austenitic field with greater production flexibility and to further reduce the formation of scale, a plant of the type described above is known from U.S. Pat. No. 9,108,234, which also includes a second induction furnace between the descaler and the finishing mill, the heating in said second furnace taking place in a protective atmosphere that prevents oxidation of the transfer bar being substantially composed of inert gas (nitrogen) with a minimum presence of oxygen (about 5% or less). Other examples of induction heating in a protective atmosphere prior to final rolling are found in U.S. Pat. No. 8,479,550 which, however, provides for only one induction furnace after the descaler, US 2012/043049 which also provides for a reducing atmosphere using hydrogen but no roughing, and DE 19936010 which, however, does not include an induction furnace after the descaler and which for the protective atmosphere teaches the use of combustion gas produced in the plant itself instead of an inert gas in order to reduce costs, this gas being able to be distributed also in various parts of the plant before and after the induction furnace (e.g. inductive edge heater, descaler, finishing mill, exit roller conveyor, winder).
None of these prior art documents, however, envisages obtaining a strip thickness below the current limit of 0.6 mm nor considers the particular problems arising when going below this limit. In fact, none of the plants described in these documents is suitable for this purpose because of the conflicting requirements of maintaining a high temperature of the transfer bar at the entrance to the finishing mill to ensure a completely austenitic rolling of such a thin strip and therefore subject to greater cooling, and the need to limit the formation of scale despite the strong heating both in terms of time and temperature.
The purpose of the present invention is therefore to provide a solution for the continuous production of hot-rolled strips with a thickness down to 0.3 mm and a maximum width of at least 2100 mm, or whatever is the provided maximum width of the ingot mould, starting from the casting of a slab with a thickness between 40 and 150 mm without passing through intermediate plants for pickling, cold rolling and annealing, and with a limited amount of scale such that these strips are suitable to be directly coated against corrosion (particularly in galvanising lines) without undergoing specific preliminary surface conditioning treatments, particularly in pickling lines.
This result is obtained with the use of continuous production technology (so-called endless), which minimises production time and consumption and consequently reduces production costs, in particular by adopting the following measures to control the temperature of the material and limit its reduction while avoiding excessive surface oxidation of the material:
a) in order to clean the slab from the scale before entering the roughing mill (HRM) and allow a number of roughing passes from a minimum of three up to a maximum of five, at the exit of the continuous casting (caster) there is an initial thermal conditioning and descaling section comprising in sequence, in the direction of slab advancement, an induction edge heater, an induction heater for the rest of the slab surface and a water descaler;
b) in order to prevent jets of water and steam from the descaler from damaging the induction coils of the surface heater, the descaler is provided at the inlet with transversely movable shutters that rest directly on the edges of the slab, while closure on the upper and lower faces of the slab is provided by a small drive stand, a so-called pinch roll, placed adjacent to said shutters on the inlet side of the descaler facing the surface heater;
c) given the low speed of the slab at the exit of the caster, lower than 10 m/min, in order to minimise the time taken for the slab to pass from the caster to the entrance of the roughing mill, so as to minimise scale formation and temperature drops, said initial section must be as compact as possible, so that said edge heater, surface heater and descaler, the latter including pinch roll and screening shutters, occupy a space in the order of 3-5 metres in length;
d) the edge heater is equipped with a handling system which allows the efficiency of the heating system to be kept constant as the width of the slab varies, to set the optimum width of the area of the edges to be heated and to remove/lift the induction coils in the event of “waves” on the slab due to cobbles in the roughing mill;
e) the edge heater is able to heat differently the right and the left edge of the slab to ensure an optimal and homogeneous profile of the slab entering the roughing mill, even if the slab exiting the caster presents temperature inhomogeneity between the two edges;
f) the descaler is designed to have a diameter of the cooling water nozzles and a delivery pressure such that the temperature drop at the outlet of the descaler is limited to less than 10° C. between when the descaler is active and when it is inactive.
Other advantageous arrangements preferably adopted in the present invention to improve the present plant and process are:
g) constructing the second water descaler, located between the two induction furnaces before the finishing mill, with a structure similar to the first descaler mentioned above and including pinch rolls at both inlet and outlet so as to protect said two induction furnaces from water and steam jets;
h) mounting the nozzles for feeding the protective atmosphere in the finishing mill on the mobile structure of the so-called “looper” arranged between the rolling stands, i.e. a roller equipped with a strip tension sensor that can move vertically and allows the material to be arranged with a suitable loop between the stands in such a way that the speed control system varies the reciprocal speed of the stands so as to maintain constant tension on the strip
i) providing a mechanical scale-breaking device, situated immediately before the second water descaler, consisting of at least three rollers arranged alternately above and below the feed line of the transfer bar and at a height sufficient to cause a plastic stretching of the surface thereof which causes a breakage of the rigid layer of scale and facilitates its removal in the subsequent water descaler;
j) in order to allow high temperatures for the winding of ultra-thin strips, up to 750° C. and in any case higher than the transformation points, providing also winding coilers close to the last rolling stand, above (“up-coilers”) or below (“down-coilers”) the surface of the exit roller conveyor, and preceded by a short cooling line and a high speed shear (in addition to the similar final coilers traditionally provided after a normal cooling line and the relative shear);
k) providing a first and a second mechanical descaler, located respectively between the cooling line and the shear of the close coilers and the final coilers, using counter-rotating abrasive brushes or abrasive slurry jets;
l) providing a corrosion protection coating line directly after the final coilers so that it is possible to apply said coating without the steel strip having to be previously wound onto a coiler to form coils;
m) providing a cooling tank in which the coils removed from the coilers can be immersed in water or in a slightly oxidising aqueous solution.
Further advantages and features of the plant and process according to the present invention will be apparent to those skilled in the art from the following detailed and non-limiting description of some of its embodiments with reference to the appended drawings in which:
Referring to
After HRM 2, an emergency system is arranged for the production and removal of rough sheets in case of problems in the portion of the plant downstream of the HRM, such system comprising a pendulum shear 15, a stacker 16 for the extraction of sheets, a rotary shear 17 and a loop-former 18, the latter two devices having the purpose of freeing the line from the material between the pendulum shear 15 and the subsequent first induction furnace 6.1 in the initial cobble phase.
Said first induction furnace 6.1 is the first component of the central thermal conditioning and descaling section 6 further comprising in sequence, in the direction of advancement of the transfer bar, a mechanical device 7 (optional) for breaking the scale of the type described above and formed in this case by five rollers, a water descaler 8 and a second induction furnace 6.2. In this way, the transfer bar undergoes a further heating before entering the adjacent finishing mill 3, which in the illustrated example is formed by seven stands 3.1-3.7 but could also be five or six. Finally, the strip is cooled in a controlled manner by a cooling roller conveyor 12 followed by a final winding station comprising a flying shear 10 and at least one pair of single coilers 11.
In order to allow high winding temperatures for ultra-thin strips, as mentioned above, the plant preferably also comprises close winding coilers, i.e. preceding the aforementioned elements 10-12, in the form of a pair of “carousel” coilers 9, arranged in proximity to the last rolling stand 3.7 and preceded by a short cooling roller conveyor 12′ and a high speed shear 10′ analogous to said elements 10, 12, although the roller conveyor 12′ may preferably be made to perform ultra-rapid cooling in order to obtain a scale that is more easily removable in the subsequent processes of applying the protective coating.
Between each pair of elements 10, 12 and 10′, 12′ there is also preferably arranged a respective mechanical descaler 14, 14′ of a known type, and therefore not further described, which uses counter-rotating brushes or jets of abrasive slurry for a final surface treatment of the strip before it is coiled onto coilers 9 or 11.
As mentioned above, the plant depicted in
A first innovative aspect of the present invention, as mentioned above, is the presence of an initial thermal conditioning and descaling section 4 arranged between the outlet of caster 1 and HRM 2, and designed so as to have a length of only slightly more than three metres to minimise the passage time between said two components. Said section 4 comprises an induction edge heater 4.1, an induction heater 4.2 and a water descaler 5 better illustrated in detail in
More specifically, the edge heater 4.1 is preferably designed to operate with transverse flux using side coils 4.1a in a “channel” configuration with flux concentrators, with the dual purpose of increasing the efficiency of the heating system and concentrating the magnetic flux on the chosen area of the slab to be heated. Furthermore, it is able to heat differently the right and the left edge of the slab thanks to the presence of two frequency converters, one for each coil 4.1a, instead of only one converter for the whole device as it is usually provided. From the experimental tests carried out by the applicant, it results that the width of the band to be heated should preferably reach up to 150 mm from the edge and that the optimum temperature rise in said band is up to 120° C. to avoid melting of the scale.
The edge heater 4.1 is provided with a handling system which performs a transversal movement to adapt the device to the slab width, to set the width of the area of the edges to be heated and to move away (and, if necessary, to lift by rotation) coils 4.1a from the edges of the slab in case there are “waves” on the slab due to cobbles in the roughing mill. Such a handling system can be realized, for example, by placing each coil 4.1a on a slide mobile along a transversal guide under the action of an actuator such as an electric motor driving a screw jack.
The induction heater 4.2 comprises a surface heating coil, designed to integrate with the edge heater 4.1, which can be controlled in such a way that the temperature increase of the slab reaches values of up to a maximum of 150° C., thus preventing melting of the slab.
The subsequent descaler 5 consists of the pinch roll 5.1, on the side towards the induction heater 4.2, and the actual descaler 5.2 on the side towards the HRM 2. As shown in
More specifically, in the embodiment illustrated in
The water descaling is carried out by means of a row 23 of upper nozzles and a row 24 of lower nozzles arranged transversely to the slab and with the nozzles inclined to deliver a jet in the opposite direction to the direction of movement of the slab. An upper scroll 25 and a lower scroll 26, arranged specularly upstream of the nozzles and with their openings facing the nozzles, collect most of the water through a lip in contact with the slab and convey it to their ends where it is discharged.
In addition, a row 27 of upper nozzles and a row 28 of lower nozzles arranged transversely to the slab upstream of the scrolls and with the nozzles inclined to deliver a jet of air in the direction of movement of the slab eliminate residual water. The combination of components 5.1, 20, 25, 26, 27 and 28 ensures that the induction coils of heater 4.2 are not damaged by the water used in descaler 5.
As mentioned above, descaler 5.2 is designed to limit the temperature drop to less than 10° C. between when it is active and when it is inactive, and to this end the cooling water pressure is less than 150 bar and the diameter of the nozzles is less than 3 mm. Note that the rows 23, 24 of the water nozzles shown in
The second water descaler 8, illustrated in
Furthermore, since the second descaler 8 is followed by the second induction furnace 6.2 which significantly increases the temperature of the transfer bar before the final rolling, the descaling can be stronger even at the expense of a greater temperature reduction. Therefore, there is provided a first row 33 of upper nozzles with a corresponding row 34 of lower nozzles, also arranged transversely to the transfer bar and with the nozzles inclined to deliver a jet in a direction opposite to the direction of movement of the bar, as well as an identical second row 33′ of upper nozzles with a corresponding row 34′ of lower nozzles. Preferably, the second rows 33′, 34′ are transversely staggered by half pitch, where the pitch is the spacing between two nozzles of a row, with respect to the first rows 33, 34 so that the two successive rows 33, 33′ and 34, 34′ completely cover the upper and lower surface of the bar, respectively, so as to increase the efficiency of the hydraulic descaling process by eliminating inefficiencies manifested in the overlapping bands of adjacent nozzles of each row.
The two rows 33, 33′ of upper nozzles are similarly preceded by an upper scroll 35, 35′ which, however, in this case is separated from the lip 32, 32′ which contacts the upper surface of the transfer bar and is movable between a rest position, illustrated in
Since descaler 8 is not required to be as compact in length as descaler 5, the transfer bar can be supported below by ordinary transport rollers 36, 36′ which perform a closing function on the lower side similar to that of the lower scroll 26. For this reason, descaler 8 does not comprise lower components corresponding to the upper components 32, 32′, 37, 37′ but only the lower water nozzles 34, 34′. Nevertheless, the combination of components 8.1, 8.1′, 32, 32′, 35, 35′, 36, 36′, 37 and 37′ ensures that the induction coils of furnaces 6.1 and 6.2 are not damaged by the water used in descaler 8. As mentioned above, since descaler 8 is designed for stronger descaling, the cooling water pressure can be up to 380 bar, again with nozzles of less than 3 mm in diameter, even though this can result in a reduction of up to 150-200° C. in the temperature of the transfer bar. Obviously, even in descaler 8 the rows 33, 34 and 33′, 34′ of the water nozzles are sized for the maximum width of the bar, with the nozzles outside the bar being processed that are closed with plugs or with jets that “cancel out” by colliding, and in this case the upper and lower nozzles must be vertically aligned and have the same angle of inclination (e.g. 5°).
Referring now to
This protective atmosphere can be of various types as long as it has a very low or zero oxygen content so as to limit or prevent surface oxidation of the material. Typically, the oxygen is reduced by continuously delivering nitrogen from nozzles 43 until a low-oxidising atmosphere with a maximum of 3% vol. oxygen content is obtained. Other possibilities are the use of an atmosphere composed entirely of inert gas (nitrogen, argon, etc.), or the addition of hydrogen to the inert gas up to a maximum content of 5% vol. to obtain a slightly reducing atmosphere.
As mentioned above, a similar solution can be envisaged for obtaining chambers between the stands of the finishing mill 3 by mounting the nozzles on the structure of the looper arranged in the space between two stands. A first embodiment of this solution is illustrated in
This system comprises on each side of the strip a pair of vertical feed ducts 52, 52′ mounted on the structure of looper 51, respectively on the upstream and downstream side thereof, and from each of said ducts 52, 52′ branch out two rows of substantially horizontal nozzles arranged longitudinally above and below the strip and parallel to its edges. More specifically, each of the two rows 53, 53′ of upper nozzles extends towards both stands 3.1, 3.2 almost up to the plane of section A-A passing through the centre of looper 51, while each of the two rows 54, 54′ of lower nozzles extends only towards the adjacent stand 3.1, 3.2 respectively. Moreover, as shown in the detail of
To limit the dispersion of the protective atmosphere, the rows of nozzles are preferably enclosed within a chamber formed by a pair of upper flaps 55, 55′ and a pair of lower flaps 56, 56′ which are obviously shaped to allow the strip to pass through the chamber. More specifically, each of the flaps is pivoted at one of its external ends to allow the opening of the containment chamber by means of a rotation of 90°, as indicated in
A second embodiment of the system analogous to the previous one is illustrated in
Finally, in
As mentioned above, the plant described above can be integrated with a line 13 for the application of a protective coating, typically a galvanising line, connected directly downstream of the final coilers 11 as shown in
Another possible alternative is to perform a liquid cooling of the coil wound on coilers 9 or 11 in a tank (not shown) containing water or a slightly oxidizing aqueous solution. This allows to obtain a scale which is more easily removable in the subsequent processes of applying the protective coating.
Furthermore, thermal scanners, not shown in the figure, are preferably positioned at the exit of caster 1, HRM 2, the first induction furnace 6.1, descaler 8, the second induction furnace 6.2, the finishing mill 3 and the cooling roller conveyors 12, 12′.
These thermal scanners are operatively connected to a temperature control and management system which, thanks also to thermocouples (not shown) inserted in the copper plates of the ingot mould, influences the temperature distribution of the steel in the mould by means of an electromagnetic brake (EMBR) inserted in the mould, also not shown. In fact, the thermal scanners and thermocouples provide an image of the temperature distribution in the slab, giving the control system the ability to take corrective action on the operating parameters of the EMBR and of the slab cooling system. This control system obviously also acts on all the other components that actively influence the temperature of the material being processed, both during heating (4.1, 4.2, 6.1, 6.2) and cooling (5.2, 7, 8.2, 12, 12′, 14, 14′).
By way of example, the following table represents a possible rolling sheet for the production of an ultra-thin strip of thickness 0.4 mm with a winding temperature on the final coilers of 680° C.:
A corresponding production process using the above-described plant in its most complete embodiment therefore comprises the following sequence of steps:
(a) continuous casting of thin or medium slabs (1);
(b) induction heating (4.1) of the slab edges;
(c) induction heating (4.2) of the rest of the slab surface
(d) first water descaling (5.2);
(e) rough rolling in 3-5 passes to obtain a transfer bar;
(f) first induction heating (6.1) of the transfer bar
(g) mechanical breaking (7) of the scale;
(h) second water descaling (8.2);
(i) second induction heating of the transfer bar;
(j) finishing rolling in 5-7 passes to obtain the strip;
(k) controlled cooling (12; 12′) of the strip;
(l) mechanical descaling (14; 14′);
(m) cutting of the strip (10; 10′) and winding on a coiler (9; 11); or
(n) direct passage of the strip to a step (13) of application of a protective coating with final winding;
wherein at least phases (i) and (j), at least up to the third pass, and preferably also phases (k) and (m), in the winding part, are carried out in a protective atmosphere that is slightly oxidizing, inert or slightly reducing as described above.
It is clear that the embodiments of the plant and process according to the invention described and illustrated above are only examples which are susceptible to numerous variations. For example, although all the rows of nozzles described above and shown in
Furthermore, it is clear that for reasons of space and/or cost the system could be without the containment chambers shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102020000016120 | Jul 2020 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/055952 | 7/2/2021 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2022/003641 | 1/6/2022 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 8479550 | Eckerstorfer | Jul 2013 | B2 |
| 9108234 | Eckerstorfer | Aug 2015 | B2 |
| 20090288798 | Ondrovic | Nov 2009 | A1 |
| 20120043049 | Klinkenberg | Feb 2012 | A1 |
| Number | Date | Country |
|---|---|---|
| 19936010 | Feb 2001 | DE |
| 102008003222 | Mar 2009 | DE |
| 0946316 | Jan 2002 | EP |
| 1011896 | Jan 2002 | EP |
| 1558408 | Aug 2006 | EP |
| 1868748 | Oct 2008 | EP |
| 1909979 | Oct 2010 | EP |
| 3154726 | Aug 2018 | EP |
| S63104724 | May 1988 | JP |
| H0466203 | Mar 1992 | JP |
| 2018189652 | Oct 2018 | WO |
| WO-2018189652 | Oct 2018 | WO |
| Entry |
|---|
| Translation of DE-102008003222 (Year: 2009). |
| International Search Report and Written Opinion for Corresponding International Application No. PCT/IB2021/055952, 14 pages, Nov. 4, 2021. |
| Number | Date | Country | |
|---|---|---|---|
| 20230082080 A1 | Mar 2023 | US |
