WO2000050189A1

WO2000050189A1 - In-line continuous cast-rolling process for thin slabs

Info

Publication number: WO2000050189A1
Application number: PCT/IT1999/000050
Authority: WO
Inventors: Giovanni Arvedi
Original assignee: Giovanni Arvedi
Priority date: 1999-02-26
Filing date: 1999-02-26
Publication date: 2000-08-31
Also published as: TW445181B; AR018976A1; AU3273399A

Abstract

A process to obtain in continuous casting and in the immediately subsequent in-line rolling stage a pre-strip with characteristics such as to make it possible, in the subsequent processes downstream, to manufacture plates and coils in high strength steels with gauges as low as 0.6 mm, with a low or very low percentage carbon content, these coils having, for certain applications, the characteristics of a cold rolled product, in which the thickness of the thin slab leaving the mould is first reduced with a liquid core (soft reduction) and then rolled in-line with a solid core. Particular ranges of value are indicated which characterise the thickness, speed and temperature parameters of the pre-strip leaving the casting machine and entering the rolling stage, as well as cooling characteristics. Some characteristics of the cooling plant which allow the above-mentioned process to be performed are also defined.

Description

"IN-LINE CONTINUOUS CAST-ROLLING PROCESS FOR THIN SLABS"

The present invention concerns an uninterrupted in-line continuous casting and rolling process to obtain pre-strips in gauges between 6 and 15 mm starting from thin steel slabs in gauges of less than 60 mm on leaving the continuous caster after liquid core reduction, with a width of between 800 and 3000 mm, suitable for rolling, with or without intermediate coiling, immediately downstream of the caster for the manufacture of ultrathin strips in gauges down to 0.6 mm, coiled so as to form either the so-called "coils", or plates in gauges down to 6 mm. The steel is the low and/or very low carbon content type (<0.04%) with characteristics such that, for particular uses, it can replace a cold rolled, high strength product.

It is known that high mechanical resistance steels, also called "high strength" steels, and those with a low or very low carbon content, show metallurgical characteristics which make the casting and subsequent rolling processes particularly delicate when quality coils are to be obtained with the above-mentioned characteristics. So-called "mini-mills" are adopted in many industrial plants for this purpose, which provide a shear immediately after continuous casting so as to separate the casting stage from the subsequent rolling stage. In this way investment costs for plants for the manufacture of coils can be reduced, as well as the relative transformation costs. However, it has been observed that the use of these mini-mills cannot guarantee the production of coils with the desired characteristics, considering that the thin slab, at times with a constant thickness of about 50 mm on leaving the mould, shows not optimal internal structural characteristics both as regards grain size and because of the presence of segregations, and also presenting surface defects due to an insufficient quantity of liquid slag, essential for acting as a lubricant between the copper plates and the steel. Other defects may be due to the mould used, which, when for example it is of the type with a central funnel and on exit shows a rectangular cross-section without residual funnel, according to patent EP 0149734, induces considerable stress into the solidifying slab skin. On the other hand, even in the case in which the mould is without a funnel, as is possible for obtaining not thin slabs with gauges of over 90 mm (medium size), surface quality is certainly better, but the slabs, suitably cut on leaving the continuous caster to lengths of about 30-50 m, do not lend themselves to the production of quality ultrathin coils. The reason is due mainly to the thickness, always over 70 mm, on leaving the continuous caster which makes it impossible in practice to obtain hot coils in gauges of less than 1 mm with characteristics such as for them to replace cold rolled products in some important applications, above all in terms of close tolerances, as well as owing to surface roughness due to the formation of secondary scale caused by the excessive time spent in subsequent rolling. An alternative solution to that of the mini-mills has been proposed with patent EP 0344095 concerning a casting and rolling process with the aim of allowing final production of the above-mentioned type of coils. However, it can easily be observed, and has been shown in practice, that the teachings of this patent are not industrially practicable both for the fact that the secondaiy cooling is achieved, according to the mentioned patent, exclusively by means of cooling inside the rolls of the casting machine, which evidently leads to a limited lowering of temperature. The consequent solid core rolling just below solidification temperature, in particular in the temperature range between 1200 and 1500°C, is impracticable above all because of problems of reoxidation due to the excessive temperature.

A further problem is better illustrated with reference to Fig. la relating to the prior art in which some internally cooled guide rolls 10 are shown immediately downstream of the mould, being the seat of primary cooling. The consequent temperature, still too high, of the casting shell 1 gives rise to a shell thickness (A) that is too thin and therefore to an excessive bulging thereof between the rolls 1 because of the ferrostatic push. This bulging has particularly negative consequences on operating safety, productivity and casting quality. Moreover, this secondary cooling due only to internally cooled rolls leads to a solidification time that is too long, a greater metallurgical length between the meniscus 2 and the apex of the solidification cone ("sumpf) 3a (see Fig. 1) at constant casting speed v, as well as to disturbances in the casting process. These disturbances are characterized by oscillations of the meniscus 2, which lead not only to malformations of the slag layer 2a (still with reference to Fig. 1) and the casting shell itself, but also negatively affecting the surface quality of the cast, considering the irregular thermal flow densities in the transversal and longitudinal directions of the mould during the primary cooling therein.

These problems are felt even more because of slab thickness reduction during solidification, which can for example be 38%, from 65 mm on leaving the mould to 40 mm in the first reduction section. When the strong mechanical effect due to the movement of the residual cast between the casting surfaces is also considered, a precise control of the local and instant temperature of the continuous casting process is needed as regards the secondary cooling of the casting shell along the roll guide. The shells must be prevented from touching each other during thickness reduction, since should there be a total solidification and therefore a compression stage of the casting shells as described in patent EP 0286862, operation disturbances would arise due to the casting proceeding in an irregular way.

It is therefore absolutely vain to suppose a process of the type described and claimed in EP 0344095, i.e. with a thin slab casting machine which adopts this type of secondary cooling, also considering that for the process to be economical, casting must occur at a speed of > 4 m/min.

From the above-mentioned patent EP 0286862, a process is known for the manufacture of steel strip in which the slab leaving the continuous caster, already reduced through compression by welding the previously solidified walls to each other, is transformed into a strip in a subsequent rolling stand. However, this process and the relative plant are not suitable for the manufacture of ultrathin quality, "high strength" coils because of technological problems due to the welding process of the previously solidified slab walls and because of the limited productivity which makes its realization antieconomical; indeed, no industrial applications of this kind are known. Numerous other processes and plants for the manufacture of coils are known which foresee casting and subsequent rolling stages directly in-line, for example according to patent EP 0415987, but it has been found that without a particular liquid core reduction and without adopting the devices specified in the claims of the present invention, basically in independent claim 1, it is impossible to obtain ultrathin coils of the type mentioned above.

An object of the present invention is therefore to provide a process for the manufacture of a pre-strip with characteristics such as to be able to obtain from it ultrathin high strength steel coils with a low and/or very low carbon content in gauges down to 0.6 mm with the characteristics, for certain aspects, of a cold rolled product. Experimental tests have shown that such a pre-strip must have the following characteristics:

- a particularly constant transversal and longitudinal temperature profile on both surfaces leaving the continuous caster, with temperature variations in the range of ±30°C so as to allow rolling stability, in particular without the so-called "strip slashes", even in the presence of considerable reductions, and to also ensure geometrical characteristics and tolerances equal to or better than the international standards required for coils;

- mechanical characteristics in accordance with those provided by international standards for coils; - gauges and speeds such as to make an "endless" process to be possible until the final downcoiler, in particular for the manufacture of coils with gauges of between 0.6 and 1 mm.

The above-mentioned object, through the obtaining of the characteristics listed above, is achieved through a process characterized by the operating steps of claim 1. Advantageous characteristics and preferred embodiments of the process itself are recited in the claims dependent on that claim.

A plant for putting into practice the process according to the present invention is the object of claim 8, possibly in combination with the claims dependent on it. These and other objects, advantages and characteristics of the process and relative plant according to the invention will result more clearly from the following detailed description of a preferred embodiment, shown by way of a non-limiting example with reference to the attached drawings in which: Figure 1 shows a schematic view in longitudinal cross-section of a general continuous casting plant, without indicating the type of secondary cooling adopted;

Figure la shows a section of casting roll guide cooled internally by secondary cooling, as provided by the prior art;

Figure lb shows a corresponding section of roll guide in which secondary cooling is obtained according to the present invention.

The first essential stage of the process in question was to develop a particular type of air/water secondaiy cooling, specially studied in combination with the liquid core reduction process. The aim of this was to achieve a temperature variation of ±30°C along the transversal and longitudinal profiles, on both the external surfaces in contact with the casting rolls, in order to obtain a temperature distribution as homogeneous as possible in three directions, i.e. between the surface and the liquid core of the slab, on the perimeter of each transversal cross-section with respect to the casting direction and on the perimeter of each longitudinal cross-section along the casting direction. At the same time a particular internal structure of the material cast will be obtained, characterized by fine grains and absence of segregations, thanks above all to reducing the bulging effect to a minimum, as well as high casting speeds (up to 8 m/min) at an exit temperature below 1200°C in order to prevent phenomena of enlargement of the austenitic grain with negative effects on product quality during rolling.

Cooling must therefore be controlled in the various passages in the casting machine through parameters of intensity and homogeneity in the three directions mentioned above. As regards intensity, suitable specific volumes of water must be ensured, quantifiable in 0.6-3 1/kg of product, while the cooling density (1/min per m²) must be greater in the upper part of the casting machine, where slab temperatures are higher, cooling water vaporization is stronger and the skin still relatively thin, which is why the transmission of heat with the liquid core is facilitated. "Air-mist"-type nozzles will preferably be used.

As regards the homogeneity of the temperature in the three directions indicated above, a lower cooling density, compared with the average value, in the end-of- solidification area or lower area of the casting machine, already foreseen as stated above for controlling the cooling density, will be desired in order not to subject the skin, a relatively poor heat conductor and by now of a certain thickness, to excessive thermal gradients.

Temperature homogeneity on the perimeter of each transversal cross-section may be obtained by suitably choosing the number of nozzles and their spray pattern in the space between each pair of opposite rolls. Selective control of the delivery of the nozzles between the front side and back side of the slab must also be provided, by increasing the back side delivery in order to compensate for the lack of stagnation phenomena in the concave area between the front side rolls and the slab. For the same purposes it will also be useful to carry out selective dynamic control on some of the nozzles in each area between successive rolls, while observing for example the upper and/or lower slab surface temperature on the transversal sections, for example by means of an infrared scanner. For temperature homogeneity in the third direction, i.e. in the longitudinal section, dynamic control of the total delivery and/or the distribution of the cooling density along the casting machine is carried out in order to keep the desired temperatures of the slab surface constant in one or more detection points along the casting machine. It is to be noted that the temperatures in this direction may be affected by numerous parameters such as casting speed, the liquid steel casting temperature, the entity of thermal exchanges in the mould and the chemical composition of the cast steel. The temperature may be read either by pyrometers, placed along only one half or the whole perimeter of one of the longitudinal cross-sections, or by the above-mentioned transversal reading scanners which, with suitable software, allow average temperature readings to be obtained on perimetral sections of the transversal cross-sections.

The expected slab surface temperatures are calculated with suitable solidification models which consider:

- steel chemical composition;

- sensitivity of the steel to internal deformation (bulging); - sensitivity of the steel to thermal gradients (possible internal or surface cracks in the transversal or longitudinal direction);

- geometrical characteristics of the casting machine;

- foreseen casting speeds;

- foreseen metallurgical lengths. The aim of the present invention is also to provide a secondary cooling plant with various nozzle areas controlled by area valves for water and/or air in the case of air-mist, which in the upper part of the casting machine may include nozzles both on the front side and the back side, while in the lower part they may be differentiated between front side and back side. These valves may control only some of the nozzles present in each of the spaces between the rolls so as to have more than one active control of cooling in the transversal direction.

With reference to Fig. 1 , the pre-strip or casting product C leaves the mould 4 fed by a submerged nozzle 14 and cooled (primary cooling) by water jackets 5, for example with a thickness of 65 mm at the mould exit 4a and a speed of e. g. 5 m/min, enters the first segment 21 (tong-type) of the casting roll guide 20, where thickness is reduced during its solidification and at the end of this segment reaches for example 40 mm. Contraiy to patents DE 37 23 543 and EP 0286862, where the casting shells are pressed onto each other in the thickness reduction stage with the problems deriving therefrom, in this way there is still a residual cast between the shells of the pre-strip at the end of thickness reduction, which can continue beyond segment 21 , into part 22 where there can still be "soft reduction" i.e. liquid core reduction at least up to point 3b (3a in the case of secondary cooling wholly within the rolls 10 as in Fig. la according to the prior art). In this way 0.5 mm can be achieved advancing 2 mm/m of the casting guide, with excellent results as regards the internal quality of the solid cast product.

With reference to Fig. lb the casting guide 21 with rolls 1 1 according to the invention is provided with a secondaiy spray cooling, with spray nozzles for the cooling liquid set opposite between pairs of rolls 1 1, more precisely in the zones 13. Minimum bulging of the casting surface is noted or in any case very little compared with that of Fig. la without cooling outside the rolls. In this way there is no noticeable disturbance to the casting process, while shell growth is greater and therefore its thickness B compared with thickness A of Fig. la. At the same time the length of solidification is shortened (3b instead of 3 a, as already stated) and the external casting surface temperature is reduced, which is determined for example with an optical measuring device at the end of the continuous casting machine, immediately before the roughing mill.

It is to be noted that the surface temperature of the cast C, to which corresponds a suitable degree of slab energy maintenance, must be chosen in such a way that not only is the continuos casting process safe for production purposes, but also that intermediate rolling leads to a good quality pre-strip. For this purpose the effect of spray cooling is controlled in the zones 13 between rolls 11 of the casting guide, which could themselves be cooled internally. Some preferred methods of direct spray cooling control have been described above with reference to the part relative to the process of the present invention.

Water flows and possibly air pressures are planned in a so-called "spray plan" which defines for each area the cooling intensity on the basis of a basic parameter which is the casting speed. These spray plans reflect for each steel grade what results from the solidification model above. The simplest way of varying the cooling intensity with the casting speed is with proportional law, but laws which differ from proportionality are not excluded. Other variations may be made on the basis of control dynamicity so as to satisfy all the above-mentioned criteria of homogeneity.

A hardware device will also be provided with suitable software to suitably couple the casting stage with the in-line rolling stage so as to ensure, on leaving the casting machine, a drawing that annuls possible differences in speed due to the different dynamic responses of the technological processes involved in the two stages in-line (liquid core reduction and rolling). This device may be composed for example of a mechanical position transmitter and an electro-optical transducer which detects the position of the transmitter, the whole being managed by a special regulation algorithm equipped inter alia with its own calibration system.

Another hardware device with relative software may be provided in order to obtain the geometrical tolerances required by this strip, so as to contain the thickness variations of the slab leaving the continuos caster within the range of values of ±1 mm, irrespective of roll gaps and wear. For this purpose an active position actuator/regulator and parallelism control combined with the first part of the casting machine may be provided. On the other hand it has been found that the mould will preferably have a geometry such that on leaving it the slab shows a not perfectly rectangular section, but with a central crown of a value preferably between 0.5 and 5 mm at each side. The subsequent pre- strip, after solid core rolling, will preferably still have a central crown of up to 0.4 mm at each side.

Being in the presence of high temperatures (up to 1200°C) and having to ensure minimum drawing between the stands, from 3 to 6 N/mm², whereby the space between the rolling stands must be at most 2.8 m, a traditional looper can find no application and therefore a particular type of hydraulic fluid tension device has been provided, able to measure pressure indirectly and consequently the drawing between one stand and another. Moreover, considering that the optimal distance between the stands must be equal to about 2.6 m and that the maximum number of stands must be such as to allow rolling exclusively in the austenitic field, the diameter of the work rolls must be lower than 600 mm in order to ensure an optimal peripherical speed of the rolls, avoiding problems of pyrocracks and excessive consumption.

It has also been found that in the casting start-up stage, it is preferable, for a time exceeding fifteen minutes, to use a high melting speed powder, for example >1.5 mg/s, to allow high casting speeds from the start, so that the pre-strip can reach sufficient speeds (up to 60 m/min) after the rolling stands in-line with the casting to allow an "endless" type of rolling downstream.

Finally, in order to limit temperature losses to a maximum of 60°C, the insertion of a descaler is foreseen, characterized by low pressure (<60 bar) and volumes with coherent jet nozzles equipped with an oscillation mechanism in order to cover the whole surface.

Claims

1. A process to obtain in continuous casting and in the immediately subsequent in-line rolling stage a pre-strip with characteristics such as to make possible in subsequent processes downstream the manufacture of low and/or very low carbon content and high strength steel plate and coils with ultrathin gauges down to 0.6 mm, these coils having, for certain applications, characteristics of cold rolled products, characterized by the fact that they include an initial liquid core thickness reduction step of the thin slab leaving the mould, immediately followed without interruption by a solid core in-line rolling step in which:

- the thickness of the pre-strip obtained is between 6 and 15 mm, its speed between 10 and 60 m/min, preferably between 20 and 50 m/min, and its temperature upstream of rolling is below 1200°C; a secondary cooling during the liquid core reduction step with the following characteristics being provided:

- specific water delivery between 0.6 and 3 1/kg;

- decreasing cooling density in the sense of advancement due to liquid core reduction and greater with respect to the average value in the initial part of this advancement;

- selective control of cooling fluid flow rates between the front side and the back side of the slab, with increase on the latter; selective dynamic control in at least some areas where cooling water is delivered, according to temperature directions on the slab surfaces in various transversal cross-sections; as well as dynamic control of the total flow rate or distribution of the cooling density in the longitudinal direction along the whole advancement path with liquid core reduction.

2. A process according to claim 1, characterized by also comprising an active position regulation step and parallelism control in the initial part of the liquid core reduction step downstream of the mould in order to obtain at the end of continuous casting and upstream of solid core rolling a constant thickness with a tolerance of ±1 mm.

3. A process according to claim 1 or 2, characterized by the fact that in the casting start-up stage a high melting speed type of powder is used, for example greater than 1.5 mg/s, and subsequently after a period of time > 15 mins a different type of powder with a lower melting speed is used.

4. A process according to claim 1 or 2, characterized by the fact that between the casting step with liquid core thickness reduction and the rolling step, heating of the slab or pre- strip edges is provided.

5. A process according to any of the preceding claims, characterized by the fact that the slab leaving the mould is of not perfectly rectangular cross-section but with a central crown of a value preferably between 0.5 and 5 mm at each side.

6. A process according to one or more of the preceding claims, characterized by the fact that the pre-strip, after solid core rolling, has a not perfectly rectangular cross-section, with a central crown of up to 0J mm at each side.

7. A process according to one or more of the preceding claims, characterized by the fact that the solid core rolling step occurs exclusively in the austenitic field.

8. A plant for the accomplishment of the process of claim 1 , including a mould for thin slabs (4), a casting machine (20) provided with opposing rolls (10, 11) for liquid core thickness reduction and subsequently, downstream thereof, without interruption, a rolling mill for solid core thickness reduction in order to obtain a pre-strip having a thickness between 6 and 15 mm and a final gauge down to 0.6 mm after a subsequent rolling stage, characterized by the fact of including a secondaiy spray cooling system by means of spray nozzles (12) in correspondence with the said casting machine having the characteristics of claim 1, said selective and/or dynamic controls being carried out on the nozzles (12) of the said cooling system.

9. A plant according to claim 8, characterized by comprising an active position actuator/regulator and parallelism control combined with the first upper part (21 ) of the casting machine in order to obtain on leaving the latter a constant thickness with a precision of ±1 mm.

10. A plant according to any one of claims 8 or 9, characterized by comprising between the casting machine (20) and the rolling mill, an edge heating device with coils made exclusively in copper and with operating voltage lower than 200 V.

1 1. A plant according to one or more of claims 8-10, characterized by comprising between the casting machine (20) and the rolling mill a descaler for limiting temperature losses to a maximum of 60°C.

12. A plant according to one or more of claims 8-1 1, characterized by comprising between the casting machine (20) and the rolling mill a device with relative algorithm which ensures zero drawing between the caster and the rolling mill.

13. A plant according to one or more of claims 8-12, characterized by comprising tensiometers between the rolling stands and relative regulation algorithms, for ensuring a value of drawing between the stands of between 3 and 6 N/mm².

14. A plant according to one or more of claims 8-13, characterized by the fact that said rolling mill in-line with the caster comprises a number of stands such that rolling occurs exclusively in the austenitic field with interaxis between the stands of less than 2.8 m and diameter of the rolls of less than 600 mm.