PROCESS AND PLANT FOR PRODUCING HOT ROLLED STEEL INTO STRIP

Description

DESCRIPTION of the industrial invention entitled: "PROCESS AND PLANT FOR PRODUCING ROLLED STEEL "HOT IN TAPE" of the Italian company INNSE INNOCENTI ENGINEERING SpA based in MILAN — O — OOOOOO—OO—O — OOO — 0-0 0 _O _O — OOOO — O — O—OO- The present invention relates to a process for producing hot-rolled steel strip, as well as to a plant for carrying out this method. Various types of processes are known for producing hot steel strip from continuous casting, which have been developed in subsequent times starting from the so-called "indirect process" which involves: - continuous casting of 150-250 mm slabs thick. MI94A / OO123 - Cutting of slabs to pre-set lengths (generally by oxy-fuel cutting). - Cold storage of the slabs with inspection of the same and any repair of any defects found. TO - Heating of the slabs to a suitable temperature to rolling (about 1250°C) using a transverse heating furnace of the pusher or walking bean type. Rolling of slabs into bars of the thickness of approximately 40-50 mm by means of a continuous or reversible roughing train. - Shearing of the head and tail of the bar. -, Rolling on the finishing train to obtain tape of the sizes required by the market. As an alternative to this traditional process, more recently the so-called "direct process" has been used, which differs from the previous one essentially in that the slab is extracted from the continuous casting and sent while still hot to the furnace for limited heating up to the rolling temperature and for the homogenization of this temperature, with obvious energy advantages. More recently, the so-called "thin slab process" has been developed, which derives from the previous one and is based on the idea of ​​casting slabs of a thickness (approximately 50 mm) equal to that of the bar rolled at the roughing mill. This process essentially consists of the following phases: - casting of thin slab (approximately 50 mm). - Heating and homogenization of the temperature of the thin slab in a longitudinal roller furnace, given the length of the casting product which prevents the use of a pusher or moving bar furnace. - Shearing of the head and tail of the slab. - Rolling of the strip at the finishing mill up to the dimensions required by the market. It's true that compared to the traditional process, even the direct one, the thin-slab process offers energy advantages, primarily due to the elimination of the roughing mill. However, it's also true that this process also presents some problems, not only due to the difficulty of designing a mold suitable for thin-slab casting, but also due to a deterioration in product characteristics. This deterioration in the surface quality of the strip occurs due to both the high speeds required to achieve adequate productivity from a thin-slab casting and the problems associated with the formation of particularly tough scale in the longitudinal furnace.Furthermore, casting of all steel grades is difficult, particularly high-carbon and stainless steels. The metallurgical characteristics of the strip produced are unsatisfactory for certain applications, given the low rolling elongation ratio, especially for high final thicknesses. It is well known that the higher the rolling reduction ratio, the higher the final mechanical characteristics. thickness. Furthermore, in traditional longitudinal roller furnaces, the piece is not heated at the bottom and on the edges, where the greatest heat losses occur, resulting in temperature non-uniformity across the thickness and width. Finally, the connection between the casting and rolling mill is inflexible, both in terms of product dimensional variation, especially width, and productivity due to the limited buffer provided by the longitudinal heating zone, given the considerable length of the slab (at least 50 m). This also makes connection between multiple casting lines difficult. In particular, the lack of adequate buffer reduces casting productivity during roll changeover operations on the rolling mill, further reducing the quality of the affected slabs. Conversely, during casting downtime, the rolling mill must also be stopped, further reducing line productivity. It is therefore an object of the present invention to provide a process which allows the above mentioned drawbacks and difficulties for the thin slab process to be overcome whilst maintaining at the same time the energy advantages inherent in this process compared to traditional processes. The essential characteristics of the process according to the invention are reported in claim 1, while the plant which carries out this process and also forms the object of the present invention is defined by the characteristics of claim 7. Further objects, advantages and features of the present invention will become more apparent from the following detailed description with reference to the attached drawings in which: Figure 1 shows a schematic plan view of a plant suitable for carrying out the process of the present invention; Figure 2 shows, in the same representation as Figure 1, a variant of this system; and Figures 3a, 3b and 3c show a schematic cross-section of the only part of the system comprising the reheating furnace and the part downstream of it, respectively in a direct connection (mini-pull) configuration, with connection via a "coil-box" and in a free bar connection with panels. The process according to the present invention consists of the following phases: the first operational phase of the process according to The present invention concerns the continuous casting of slabs with a thickness between 80 and 120 mm. Greater thickness values ​​would lead to problems of compactness of the plant, while thicknesses lower than 80 mm would present the drawbacks, especially regarding the quality of the slab, already indicated in * \ Priority for the thin slab process. The casting speed is between 2 and 5?.m per minute, so it is not so high as to compromise the product surface while still allowing for high productivity. The outlet temperature can vary between 850 and 1150°C. After cutting the slabs to the desired length, there is a phase of alignment of the pieces - themselves, on a single production line if there are multiple casting lines. At this point, the possible insertion into the heating zone of semi-hot slabs coming from a storage area rather than from continuous casting and then heating the slab to a temperature suitable for rolling (up to 1200°C) in a transverse heating zone which can act as a buffer between the casting and rolling phases. After descaling the slab with means hydraulic and / or mechanical to ensure the quality of the rolled product, the pre-finishing rolling takes place to thicknesses equal to or less than 50 mm and then the ends of the bar thus obtained are sheared, further descaled and finally the finishing rolling is carried out to the desired thickness. With reference to the drawings, a typical plant capable of implementing the process according to the invention, outlined above in its essential phases, is now described. Figures 1 and 2 refer to a plant with a double continuous casting machine, represented by the reference number 1, whose ingot mold has parallel flat faces for thicknesses from 80 to 120 mm. The ingot mold can be of variable width to ensure greater operational flexibility, particularly in combination with a turret with independent rotating arms for quick changeover of the ingot mold and plunger, in order to allow for maximum compactness of the plant. The ingot mold can also be equipped with an oscillator capable of varying the frequency and oscillation stroke during casting depending on the type of steel and the casting speed. With reference to point 2 of figures 1 and 2, the cutting area of ​​the slabs to the desired length is represented, in particular by means of ossita fast. The length will naturally be such that it can be contained longitudinally in a transverse-advance furnace (approximately -30 m). An intermediate zone 3 consisting of an insulated roller conveyor or normalization furnace is followed by a slab alignment zone, indicated overall by A, which, given the relatively limited length of the slab, may comprise a series of insulated transverse trolleys 14 or sections of roller furnace, as shown in Fig. 1, in order to ensure optimal maintenance of the slab temperature and consequent energy savings. As an alternative to this solution, with reference to Fig. 2, the lateral transfer of the slabs may be achieved with a system of transverse furnaces 4', in a number equal to the number of castings, in particular of the mobile bar type like the subsequent furnaces indicated by the reference number 4, positioned so as to feed each other to provide good buffer for feeding the slabs. Please note that in both cases, alignment zone A may also include the possibility of access for slabs not coming directly from continuous casting, but for example semi-hot slabs coming from an intermediate storage area. It should also be noted that zone 3, in the case of Fig. 1, where the alignment in A is achieved with trolleys 14, is at least as long as a slab, while this is not necessary in the case of Fig. 2. In both cases, zone A is followed by a transversely advancing heating zone, illustrated by the reference number 4 and consisting in particular of a walking beam furnace, preferably with the entrance and exit on opposite sides in a transverse direction, both on roller conveyors inside the furnace to optimize the compactness of the system and ensure the minimum permanence of the piece in free air, primarily for reasons of energy saving. Such furnace 4 will preferably be equipped internally with known piece transfer devices, such as to allow independent loading and unloading cycles and thus optimise the exploitation of the buffer between the casting and the rolling mill. In this way, a high buffer capacity of at least twenty minutes can be achieved between the casting and the rolling mill in this area, allowing the rolling mill's productivity to be maintained during casting equipment changes and absorbing the casting's full productivity in the event of a roll change or small change. accidental rolling mill stops. A furnace control system may be provided to heat the slabs differently along their length to ensure maximum uniformity and consistency of temperature of the piece being rolled. Furthermore, direct access to the furnace 4 for slabs not directly coming from the casting process may be provided as an alternative to their insertion, as mentioned above, in the slab alignment area A immediately upstream. Finally, still with regard to the transverse heating zone, while in figure 2 the furnace 4 is fed directly by furnaces of the same type 4' (with moving bars), in figure 1 a scarfing device 4a, for example a torch, is inserted between the trolleys 14 of the slab alignment zone A and the furnace 4. Immediately downstream of the transverse heating zone 4, a pre-finishing unit 5 is provided, consisting of one or more rolling stands, whose purpose is to reduce the thickness of the piece from the initial 80-120 mm to less than 50 mm. The number of stands and their type ("duo" or "quarto") will be chosen according to the size of the piece, and in particular: of the input and output thicknesses. This pre-finishing unit can be designed with unidirectional or reversible rolling, meaning it can reverse the motion of the piece to perform multiple rolling passes with the same stand, thus reducing the number of stands installed downstream. Given the thickness values ​​entering the pre-finishing unit, vertical stands can be used, which are useful for various reasons. One reason is the ability to calibrate the width of the piece, adjusting it using an automatic control system based on the desired final width. Another function of vertical cages is to mechanically break up the scale, facilitating its removal using hydraulic systems and thus ensuring a better surface quality. For this purpose, hydraulic descaling devices can be installed between the horizontal and vertical cages. Furthermore, the entry into the horizontal cage is made easier in the event of significant reductions in thickness and it is possible to shape the head of the piece in order to limit waste during shearing, also reducing the possibility of jamming in the entry guide of the finishing train. As regards the connection area between the pre-finishing unit 5 and the finishing rolling mill 9, this connection can be implemented in various ways depending on the different production needs. In particular, three connection methods can be envisaged, represented schematically in Figures 3a, 3b and 3c respectively. With reference to Figure 3a, a first direct connection method is available. In this case, the piece is rolled simultaneously with the pre-finisher and the finishing mill, and the integrity of the piece is maintained by a tension regulation system that acts on it. This solution guarantees maximum system compactness and minimum investment costs. It is represented in the layout examples in Figures 1 and 2, where between the pre-finisher group 5 and the finishing mill 9, in both cases, an induction heater 6 for the edges of the piece, a rotary shear 7 for cutting the ends of bars with a thickness of less than 50 mm, and a descaler 8 are provided. With reference to figure 3b, a second connection method is shown, using a "coil-box" 15. With this solution the piece is wound at the exit of the pre-finishing unit 5, overturned at the entrance to the finisher 9 and unwound to be rolled on it. Temperature maintenance is guaranteed by the low surface exposed by the wrapped piece. This well-known solution maintains considerable compactness and allows for two pieces to be rolled simultaneously, respectively at the pre-finisher and at the finisher. The decoupling between the two rolling groups also allows the piece to be rolled at optimal speeds for the various passes. Figure 3c shows the third type of connection, that is, with a free bar. In this case, the piece leaves the pre-finishing unit 5 before entering the first cage of the finishing unit 9. Temperature maintenance is ensured by insulating panels or by a roller oven 16. The advantages of the previous solution are still available, but the footprint is larger, as a longer layout is required. The main advantage of this solution is that it allows the use of a reversible pre-finishing unit, which reduces the number of finishing passes required to obtain a certain strip thickness. The last two solutions, relating to Fig. 3b and 3c, are the most easily adoptable in case of modernization of traditional rolling mills. Finally, it should be noted that the finishing mill 9 can be any of the currently known types, chosen based on the required production and with the aim of ensuring the best product quality. Downstream of the finisher, there is a cooling zone 10 for the produced strip and then the downcoilers 11. From the above, the advantages of the process according to the present invention are clear. In addition to ensuring a high "buffer" capacity between casting and rolling mill, as already highlighted above, it allows for the casting of all types of steel, given the thickness of the slab and the casting speed, with a high surface quality of the product, again thanks to said speed, as well as the reduced formation of scale adhering to the piece. There is also considerable flexibility in varying the width of the piece, as it is possible to install a variable-width ingot mold due to the slab thicknesses not being too small. The transverse furnace then leads to a better homogenization of the slab temperature and allows for a good ease of connection between multiple casting lines entering the rolling mill with a Greater flexibility in plant usage. Furthermore, the ability to feed the cross-furnace with semi-hot or cold slabs not directly from continuous casting is advantageous, a benefit that is even more significant for steel grades requiring surface inspection and the elimination of defects off-line. Furthermore, the sending of cast material to scrapyards solely to deplete the load in the event of a major incident at the rolling mill can be avoided by diverting it to the plant's designated buffers. In addition to being more compact, given the transverse heating zone, the layout can be made modular in view of the increasing productivity levels planned in subsequent stages, given the ease of connecting multiple lines. Flexibility can also extend to the possible insertion of the initial part of the process on existing rolling mills operating according to the traditional indirect process, with obvious energy advantages. Finally, certainly not the least advantage is that of an elongation ratio sufficient to guarantee excellent metallurgical characteristics even for high final thicknesses, with reference to any type of steel, even stainless steel and high-grade steel. of carbon. Any additions and / or modifications may be made by those skilled in the art to the embodiments described and illustrated above of the process according to the present invention and of the related manufacturing plant, without departing from the scope of the invention itself.

Claims

CLAIMS 1. Process for the production of hot-rolled steel strip from continuous casting, characterised by the fact that it comprises the following phases in succession: " a) continuously casting slabs with a thickness between 80 and 120 mm at a casting speed between 2 and 5 m / min and at an exit temperature between 850 and 1150°C; b) cut the slabs to the desired length c) heat the slabs to a temperature suitable for rolling, up to 1200°C, in a transverse heating zone designed to act as a "buffer" between casting and the subsequent rolling phase; d) reduce the thickness of the slabs to thicknesses less than or equal to 50 mm in a rolling stage pre-finishing; and e) further reducing the thickness to the desired value in a finishing rolling step.

2. Process according to claim 1, charac- furthermore, it foresees a slab alignment phase on a single production line if casting takes place on several parallel lines. lele.

3. Process according to claim 1, charac- characterized by the fact that it also includes a phase of insertion of slabs coming from a storage area rather than from continuous casting to subject them to said heating phase.

4. Process according to claims 2 and 3, characterised in that said insertion occurs in correspondence with said slab alignment phase on a single production line, upstream of the heating phase.

5. Process according to any of the preceding claims, characterised by the further step of descaling the slab between said steps c) and d) and / or between steps d) and e).

6. Process according to any of the preceding claims, characterized by an end shearing step between the pre-finishing and finishing rolling steps.

7. Plant for the production of hot-rolled steel in continuous casting strip comprising at least one continuous casting line (1) with ingots parallel flat face ra for thicknesses from 80 to 120 mm, a subsequent cutting device (2) of the slabs to the desired length and a roller conveyor insulated (3), characterised by the fact that it includes subsequently a transverse heating zone (4), consisting of a furnace of length at least equal to said slab length, a pre-finishing group (5) consisting of one or more rolling stands and a finishing rolling mill (9) 5 a connecting zone being provided between said two rolling zones (5, 9).

8. System according to claim 7, characterised by the fact that it comprises a bramine alignment zone (A) upstream of said heating zone (4) when there are several continuous casting lines (1).

9. Plant according to claim 8, charac- characterized by the fact that the slab alignment area (A) is made up of a system of insulated moving trolleys (14), possibly in the form of roller furnace sections.

10. Plant according to claim 8, characterised in that said slab alignment area (A) is made up of transverse furnaces (4') with mobile bars arranged so as to feed each other.

11. Plant according to claim 7, characterised by the fact that it also comprises an area for inserting semi-hot slabs coming from storage areas independent of the lines continuous casting (1).

12. System according to claims 8 and 11, characterised in that said bramine insertion area coincides with said alignment area (A).

13. Plant according to claim 7, characterised in that said transverse heating zone (4) is made up of at least one section of a moving bar furnace with lateral entrance and exit on a roller conveyor.

14. Plant according to claims 10 and 13" characterised in that said transverse heating zone (4) coincides with said series of transverse furnaces (4') which constitutes said bramine alignment zone (A).

15. Plant according to claim 7, characterised in that said connection area between the pre-finishing group (5) and the finishing rolling mill (9) It is directly connected, as it includes control means for the tension which acts on the piece being rolled simultaneously with the pre-finisher and the finisher.

16. Plant according to claim 7, characterised in that said connection area comprises a "coil-box" (15) with winding and subsequent unwinding thus allowing the simultaneous rolling of two distinct pieces respectively at the pre-finisher and at the finisher. 17- System according to claim 7, charat characterized by the fact that the connection area is of the free bar type, with the piece leaving the pre-finishing group (5) before entering the first cage of the finisher (9), with insulating panels or a roller oven (16) being provided to maintain the rolling temperature.

18. Installation according to any of the claims indications 7-17, characterised by the fact that it comprises one or more descalers (8) downstream of a rotary shear (7) for cutting the ends of the bar with a thickness of less than 50 mm in said connection area between the pre-finisher (5) and the finisher (9).

19. System according to claim 7, charat characterized by the fact that the continuous casting mould has a variable width.

20. System according to claim 7, terized by the fact that the said prefinitor group includes vertical rolling cages. — O — O — O — O _ O _ O _ O _ 0 — O _ O — o _ o — O _ O _ 0 _ O — O — 0 „ 0 _ O — O — o_o_o~ pp: INNSE INNOCENTI ENGINEERING Sp The Mandatory ^ur. Luciano AIMI (Italian Short Society 130 MI / 010923 / IN / AD / pc