Rolling method for thin flat products and relative rolling line

Abstract: Rolling method for thin flat products, used in the production of flat rolled products with a final thickness in the range of 0.6÷1.5 mm or more, up to 2.0÷3.0 mm, in a plant suitable to work thicknesses of up to 25.4 mm, the method being applied to slabs with a thickness of between 50 and 90 mm if arriving directly from the continuous casting machine or on slabs with a greater thickness, of between 80 and 200÷250 mm, if fed from a furnace to accumulate and heat the slabs (22), the method comprising at least a first heat treatment, a roughing or pre-finishing pass, a temperature equalisation treatment and a finishing pass in a finishing train (19) comprising at least three reduction passes, the finishing pass being followed by a step of cooling and coiling the flat finished product, the product at the outlet of the roughing or pre-finishing pass being in the austenitic state γ, the finishing pass taking place in the rolling line (10) at least partly in the ferritic step or in the austenitic step, as desired. Rolling line adopting the method as above, wherein the finishing train (19) cooperates with a system (24) to condition and adjust the temperature of the slab.

Description: This invention concerns a rolling method for thin flat products, and the relative rolling line, as set forth in the respective main claims. The invention is applied in the field of flat rolled stock, such as strip or sheet, and more particularly to obtain thin final products, around 1.5÷3.0 mm, and very thin, less than 1 mm, to as little as 0.6÷0.7 mm. The state of the art includes rolling lines for flat rolled products, both of the traditional type, where the furnace to heat the slabs is separated from the casting machine and connected to the rolling train, and also the type where the rolling train is directly connected to the continuous casting machine. The standard configuration of these rolling lines generally comprises, downstream of the continuous casting machine and the shears, a temperature-maintaining and possibly a temperature-restoring system, a roughing train comprising a number of stands which is normally between one and three, a temperature maintaining and equalisation system and the finishing train, normally comprising between four and ten stands, followed by a cooling system and a coiler to form the coil of rolled product. Apart from these basic components of the line, normally there are also fast heating devices, for example induction furnaces, scaling devices at the outlet and/or inlet of the temperature restoring systems, devices to heat the edges, emergency shears, intermediate coilers, devices to measure the size and other operative and/or conditioning assemblies which are known to the state of the art and are widely known to those skilled in this field. In the field of flat rolled products, obtaining strip or sheet with a final thickness of around 1.0÷1.5 mm has been widely explored and the results obtained, in terms of the quality of the finished product, can be considered substantially satisfactory. It should be considered that at the inlet to the finishing train there are usually temperature equalisation and temperature restoration systems by means of which it is possible to determine extremely rigorous and precise conditions of the product; this guarantees that at least the first passes to reduce the thickness can be carried out in the best possible rolling conditions, for example with the product in the austenitic state γ and in heat conditions far removed from the allotropic transformation step from the austenitic state γ to the ferritic state α. This requirement derives from the need to maintain constant values of rolling force and rolling torque in order to have a constant behaviour of the rolling stands, so that the quality of the rolled product is constant. However, there is a tendency in the markets at present to demand ever thinner final products, less than 1.0 mm, to 0.6÷0.7 mm and even less, or for products of conventional thickness, for example in the range of 1.5÷3.0 mm, but with special mechanical and metallurgical characteristics. Rolling ultra-thin products has demonstrated problems and disadvantages which had never been thought of before, which in conventional rolling lines of the type described above have a considerable impact on the surface and internal dimensional quality (thickness, profile and planarity) of the finished product. To be more exact, it has been found that the final reducing passes of the finishing train, which obtain this kind of reduced thicknesses, are performed on a product which, in the final rolling stands, is in a condition of allotropic transformation from the austenitic state γ to the ferritic state α. The temperature at which this transformation takes place depends mainly on the composition of the steel being worked and particularly on the percentage of carbon therein, the speed of cooling and the sequence of reduction of the thickness. In the most frequent cases found in practice, that is, in steels with a low carbon content, the temperature at which the allotropic transformation begins is around 800÷880°C. It is highly inadvisable therefore to carry out rolling operations at such a temperature, when the product is in the process of changing from state γ to state α. This is because the lack of structural homogeneity of the product, also due to the fact that it is very thin, leads to a product being obtained which is inferior in both surface and internal quality, with insufficient planarity, longitudinally uneven, with cracks and fissures, especially on the edges of the strip, and still other problems. Until now, these problems have prevented flat rolled products being produced which are both thin and very thin and characterised by a high standard of quality. This is also due to the need to maintain the basic structure of the rolling line, for reasons of both economics and production, so that it is possible to obtain, on the same rolling line, a relatively wide range of thicknesses as the production cycles are varied. WO-A-97-01401 discloses a plant for the manufacture of steel strip wherein two rolling lines, parallel therebetween, are disposed downstream of a single casting machine. Each rolling line comprises a 3-stand roll train which can reduce the thickness of the slab from a thickness of less than 100 mm to 10-15 mm, associated shears and a furnace apparatus for heating the rolled slab up to a temperature in the austenitic region, e.g. 1080°C. The furnace apparatuses are in the form of enclosures and are provided with conditioning means for creating and preserving a non-oxidizing gaseous atmosphere therein. A coiling apparatus is disposed downstream of each furnace apparatus to coil the heated slab. A single rolling train is disposed downstream of the coiling apparatus. Due to sealing means disposed around the coiling apparatus, the slab can be rolled in the finishing rolling train in the austenitic range, or at least at such a temperature that only a small part converts to ferrite. A measuring and control apparatus may be incorporated in the rolling train after or between the mill stands. After exiting the rolling train, the hot rolling strip, which thickness is of 1.0 mm and less, passes through a cooling line in which the strip is cooled at a temperature in the ferritic range by means of a water cooling. Finally the strip is coiled again into a coil on a further coiling apparatus. Alternatively the furnace apparatuses can be used as cooling apparatus to cool the rolled slab at a temperature in the ferritic region, e.g. 780°C. This plant disclosed in this document has the disadvantages not to permit to treat slab having a starting thickness more than 100 mm, because only a pre-finishing 3-stand roll train per each rolling line is provided, and that the same furnace apparatuses can not be use alternatively to heat the rolled slab in the austenitic region or to cool the rolled slab in the ferritic region. EP-A-0 771 596 discloses a rolling method and a rolling line for hot strips, with a final thickness in the range of about 0,5 to 2,5 mm, starting from slabs with a thickness of about 50 mm arriving directly from the continuous casting machine, the method comprising the following steps: conducting at least a first heat treatment (2) to heat the slabs; conducting a roughing or pre-finishing pass comprising one or more passes (4, 6) to reduce the thickness of the slabs (5) to provide a reduced thickness product in the austenitic range, conducting a temperature equalisation treatment (8, 9) and then conducting a finishing pass in a finishing train (12) comprising at least three reduction passes (13); conducting a cooling process (18) after the finishing pass to form a finished flat product and winding (20) the finished flat product; with the further step of controlling the temperature of the reduced thickness product in a temperature conditioning and adjusting system (14) comprising means for both heating and cooling, at an intermediate position of the finishing train to keep the reduced thickness product in an austenitic state or to transform the reduced thickness product to the ferritic state. In order to overcome these shortcomings, the present applicants have designed and experimentally embodied this invention, adopting a practical solution without excessive installation and maintenance costs but which still guarantees to provide a rolling line able to produce both thin and very thin products either in phase γ or in phase α, and also those thicknesses which can normally be obtained in conventional rolling lines and which are also rolled either in phase γ or phase α, and also to obtain further advantages. The invention is set forth and characterised in the respective main claims, while the dependent claims describe variants of the idea of the main invention. The purpose of the invention is to achieve a rolling method, and the relative rolling line, to obtain thin (1.5÷3.0 mm) and very thin (about 0.6-0.7 mm and less) flat rolled products, characterised by a very high standard both of the surface and internal quality. Another purpose of the invention is to achieve a rolling line which is not specifically applied only to these thicknesses, but which will be able to roll alternately, according to the planned cycle of production, both products having the aforesaid thickness and also products with a more conventional thickness, such as for example those in the range of 2.0÷12.7 mm and up to 20÷25.4 mm. With the rolling line which achieves the method according to the invention, it is possible, in at least part of the finishing assembly, to roll both in the austenitic field (phase y) and in the ferritic field (phase α) depending on the final product. According to a first embodiment of the invention, after the heating and temperature-maintenance furnace, there are one or two roughing or pre-finishing stands. The slabs produced by the continuous casting machine, according to the invention, are supplied to the heating and temperature-maintenance furnace with a thickness of between 50 and 90 mm. When the slab is between 50 and 60 mm thick, according to the invention only one roughing or pre-finishing stand is used, while for thicknesses of between 60 and 90 mm normally two or more roughing or pre-finishing stands are used. Alternatively, there may be a pre-finishing train comprising at least a reversible stand, preceded and followed by hot winding reels, in order to carry out the desired number of pre-finishing passes. The heating and temperature-maintaining furnace according to the invention is as long as is necessary to contain all the product cast with one ladle, or a slab as long as a desired plurality of coils of the desired product. According to a variant, upstream of the heating and temperature-maintaining furnace there is an accumulation and heating furnace which makes it possible to feed a cold load, or to contain the slabs cast with one ladle, or to feed thick slabs or slabs with an intermediate thickness of between 80 and 200÷250 mm. According to a variant, there is a welding system between the slab which is being rolled in the pre-finishing stand and the following slab so as to achieve continuity in the processing. According to the invention, downstream of the roughing train there is a tunnel furnace to equalise the temperature. Downstream of the tunnel furnace, according to a first embodiment of the invention, there is a temperature conditioning and adjustment system. This temperature conditioning and adjustment system takes the bar, which leaves the roughing train with a thickness of between 10 and 50 mm depending on the thickness of the final product, to the desired temperature so that at least part of the finishing rolling takes place in the austenitic or ferritic field according to the chosen field wherein rolling has to proceed. According to the invention, a thickness of between 10 and 15 mm serves to obtain final thicknesses of between 0.6 and 1.0 mm, whereas a thickness of 15 to 20÷25 mm serves to obtain final thicknesses of between 1 to 3 mm. A thickness of between 20÷25 mm and 50 mm serves to obtain final thicknesses of between 3.0 and 25.4 mm. According to a first variant, all the finishing rolling (that is to say, rolling carried out with the finishing stands) takes place in the austenitic or ferritic field. The temperature conditioning and adjustment system comprises means for both heating and cooling the bar in a homogeneous manner; this is because it serves to heat the bar to obtain thin strip in the austenitic field, that is to say with the temperatures of the strip at the outlet of the rolling mill above the austenite-ferrite transformation point. In this way it is possible to produce thicknesses of 0.6 mm even in the austenitic field. On the contrary, if the bar is to be rolled in the ferritic field it has to be cooled. The invention includes these heating and/or cooling means, that is to say, means which control the temperature of the bar and are connected with heating-adjustment and/or, respectively, cooling-adjustment means. After the temperature conditioning and adjustment system, the invention includes from five to seven finishing stands and, downstream thereof, a cooling system, a winding system, and a system to discharge the coils. According to the invention, immediately downstream of the finishing assembly there is a coiler assembly, with a single mandrel or a double carousel-type mandrel. The coiler assembly, which includes by-pass and exclusion means, serves for thin strip, that is to say, up to 3 mm, while for greater thicknesses a traditional method is followed. The positioning of the coiler immediately after the finishing assembly serves to control the temperature of the strip so that it does not go below the desired values due to radiance. The coiler assembly is associated with a shears assembly immediately upstream thereof. Whatever the final thickness desired, the invention makes it possible to roll the product in a heat condition which is far removed from the allotropic transformation step, thus avoiding the aforesaid disadvantages and ensuring better rolling conditions. The attached Figures are given as a non-restrictive example and show some preferential embodiments of the invention as follows: Fig. 1 shows schematically a rolling line of a substantially conventional type for flat products; Fig. 2 shows the rolling line of a first embodiment of the invention; The attached Figures show respectively, in an extremely schematic and simplified form, a conventional rolling line 10 for flat products. In Fig. 1, the rolling line 110 is of the conventional type. The rolling line 10 according to the invention as shown in Fig. 2 is of the type where rolling is directly connected to the continuous casting machine which in this case is advantageously equipped with a soft-reduction system. The continuous casting machine 11 is equipped with sott-reduction devices 12 for the slab leaving the ingot mold 13, followed by a shears 14 and a heating and maintenance furnace 15. There may possibly be other systems, such as descaling devices or other, before the shears 14. Between the shears 14 and the furnace 15, and/or between the outlet of the casting machine and the shears 14, according to the invention a descaling system 25 is advantageously included. At the outlet of the heating and maintenance furnace 15 there is a descaling system 25 followed by a roughing or pre-finishing train 16 comprising, in this case, two stands 17, which achieves a first reduction in the thickness of the cast product. According to a variant, the roughing train 16 can be of the reversible type and cooperate upstream and downstream with hot winding/unwinding systems such as are known to the state of the art and which are not shown here. Downstream of the roughing train 16 there is a tunnel furnace system 18, to equalise the temperature, which serves to feed to the finishing train 19, which includes at least three reduction passes, a product with a uniform temperature, both lengthwise and also on the transverse section, and with the correct heat conditions. Upstream of the finishing train 19 and downstream of the tunnel furnace 18 there is at least a descaling system 25. In the embodiment shown in Fig. 2, the temperature equalisation system, or tunnel furnace 18, is arranged so that at least the first passes of the finishing train 19 are carried out with the product in its austenitic state γ, thus guaranteeing rolling conditions such as to ensure that a final product is obtained with a high quality, both metallurgical and dimensional. The finishing train 19 in this case comprises seven rolling stands 17 and is followed by a cooling system 20 and in this case a downcoiler 21 to form the coil. When rolling thin products, in the order of 0.6÷0.7 mm, the final rolling passes in a finishing train 119 of the type shown in Fig. 1 are carried out on a product which is in the area of allotropic transformation from state γ to state α, and this causes considerable problems in the quality of the product and often makes the final product obtained unacceptable. According to the invention, in the embodiments of Fig. 2 for thicknesses of less than 3.0 mm, immediately at the outlet of the finishing train 19 there is a coiler assembly 26 with the shears assembly 27 upstream; these two assemblies 26 and 27 can be excluded from the line when larger thicknesses are rolled. The shears assembly 27 and the coiler assembly 26 may be preceded by a fast cooling assembly 28 shown for the sake of simplicity in Fig. 2 only. When strip wound has a thickness of less than 1.0 mm, a coiling assembly 26 is used which is not of a conventional type, for example including a carousel with two winding reels operating alternately, in order to avoid the problems which might arise with a strip of such a reduced thickness entering a conventional downcoiler. In the embodiment shown in Fig. 2, the temperature conditioning and adjustment system 24 located immediately downstream of the tunnel furnace 18 includes heating and cooling means.

Claims: Rolling method for thin flat products, used in the production of flat rolled products, such as strip or sheet, with a final thickness in the range of 0.6-1.5 mm or more, up to 2.0-3.0 mm, in a plant suitable to work thicknesses of up to 25.4 mm, the method being applied to slabs with a thickness of between 50 mm and 90 mm if arriving directly from the continuous casting machine or on slabs with a greater thickness, of between 80 and 200-250 mm, if fed from a furnace to accumulate and heat the slabs (22), the method comprising the following steps: conducting at least a first heat treatment to heat the slabs; conducting a roughing or pre-finishing pass comprising one or more passes to reduce the thickness of the slabs to provide a reduced thickness product in the austenitic state (γ); conducting a temperature equalisation treatment and then conducting a finishing pass in a finishing train (19) comprising at least three reduction passes; conducting a cooling process after the finishing pass to form finished flat product and winding the finished flat product, the method being characterised by the further step of controlling the temperature of the reduced thickness product in a single temperature conditioning and adjusting system (24) comprising both means for heating and cooling immediately before conducting the finishing pass (19) to keep the reduced thickness product in an austenitic state (γ) or to transform the reduced thickness product to the ferritic state (α), wherein the step of controlling the temperature of the reduced thickness product is conducted upstream of the finishing train (19). Method as in claim 1, in which the step of controlling the temperature of the reduced thickness product includes at least a step of cooling the product to take the whole product, at the inlet to the second finishing step, substantially to a ferritic state α. Method as in claim 1 or 2, in which the step of controlling the temperature of the reduced thickness product includes at least a step of heating the product to return the whole product, at the inlet to the second finishing step, substantially to an austenitic state γ. Method as in any claim hereinbefore, in which, in the production of flat rolled products with a final thickness of around 0.6-1.0 mm, the product enters the finishing step with a thickness of around 10-15 mm and is previously subjected to a descaling process. Method as in any claim from 1 to 3 inclusive, in which in the production of flat products with a final thickness of above 1.0 mm and up to 3.0 mm, the product enters the finishing step with a thickness of around 15-25 mm and is previously subjected to a descaling process. Method as in any claim from 1 to 3. inclusive, in which in the production of flat products with a final thickness of from 3.0 to 25.4 mm, the product enters the finishing step with a thickness of around 25-50 mm and is previously subjected to a descaling process. Method as in any claim hereinbefore, in which the finishing step comprises from three to seven reducing passes. Method as in any claim hereinbefore, in which the finishing step after the temperature conditioning and adjustment system (24) comprises from one to six reducing passes. Method as in any claim hereinbefore, in which downstream of the finishing step the strip with a thickness of around 0.6÷3 mm is immediately wound into a coil after a fast cooling step. Method as in any claim hereinbefore, in which downstream of the second finishing step there is a fast cooling treatment. Rolling line for the production of thin, flat rolled products, such as strip or sheet, used to obtain flat rolled products with a final thickness in the range of 0.6-1.5 mm or more, up to 25.4 mm, the rolling line adopting the method as in any claim from 1 to 12 inclusive and rolling slabs with a thickness of between 50 and 90 mm if arriving directly from the continuous casting machine or with a greater thickness, of between 80 and 200-250 mm, if fed from a furnace to accumulate and heat the slabs (22), the rolling line comprising a maintenance and possibly heating system (15), a roughing or pre-finishing train (16) comprising one or more stands (17), a temperature equalisation system (18) and a finishing train (19) to obtain the final thickness, the finishing train (19) being followed by a cooling system (20) and by a downcoiler assembly (21), the rolling line being characterised in that the finishing train (19) cooperates with a single temperature conditioning and adjustment system (24) comprising both means for heating and cooling placed immediately upstream of the finishing train (19). Rolling line as in claim 11, in which there is a descaling assembly (25) upstream of the finishing train (19). Rolling line as in any claim from 11 to 12 inclusive, in which there is a heating and temperature-equalisation tunnel furnace (18) between the roughing or pre-finishing train (16) and the finishing train (19). Rolling line as in any claim from 11 to 13 inclusive, in which the roughing or pre-finishing train (16) comprises one or more stands in sequence. Rolling line as in any claim from 11 to 13 inclusive, in which the roughing or pre-finishing train (16) comprises a reversible stand with hot winding reels upstream and downstream. Rolling line as in any claim from 11 to 15 inclusive, in which the finishing train (19) comprises a first finishing block (19a) and the second finishing block (19b). Rolling line as in any claim from 11 to 16 inclusive, in which the second finishing block (19b) can be excluded at least temporally from the rolling line. Rolling line as in any claim from 11 to 16 inclusive, which cooperates with a furnace to accumulate and heat thick slabs (22) with a thickness of up to 250 mm. Rolling line as in any claim from 11 to 16 inclusive, in which immediately downstream of the finishing train (19) there is a coiler assembly (26) for thin products preceded by a shears (27) and a system of rapid cooling.

Inventors: BOBIG PAOLO, DI GIUSTO BRUNO, BOBIG, PAOLO, DI GIUSTO, BRUNO
Applicants: DANIELI OFF MECC, DANIELI & C. OFFICINE MECCANICHE S.P.A.
Application year: 1998
Main classification: B21B1
IPC codes: B21B1, B21B15, C21D8, B21B45