EP4164823A1

EP4164823A1 - Plant to produce steel, and corresponding method

Info

Publication number: EP4164823A1
Application number: EP21737191.3A
Authority: EP
Inventors: Gianpietro Benedetti
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2020-06-16
Filing date: 2021-06-15
Publication date: 2023-04-19
Also published as: CN115715238A; US20230234123A1; IT202000014383A1; WO2021255767A1

Abstract

Steel production plant to obtain long products such as rods, bars or sections, with overall productivity comprised between 0.7 - 3.0 Mton/year, preferably between 1.0 and 3.0 Mton/year, comprising at least two co-rolling lines (11a, 1 lb).

Description

“PLANT TO PRODUCE STEEL, AND CORRESPONDING METHOD”

FIELD OF THE INVENTION

Embodiments described here concern a plant to produce steel, and a corresponding method, which use a multiline co-rolling apparatus to simultaneously produce at least two finished products, in particular, but not only, long products, for example bars, wire rod, beams or even other profiles, starting from respective metal materials.

The present invention is applied for high plant productivity, comprised between 0.7-3.0 Mton/year, preferably between 1.0-3.0 Mton/year.

BACKGROUND OF THE INVENTION

Steel producers throughout the world are increasingly committed to operating in a sustainable way, reducing energy consumption per ton and using new technologies available (Green Steel) to contain greenhouse gas emissions (GHG, Greenhouse Gases) and making production socially sustainable toward the community and the environment. Furthermore, maintaining a high level of steel consumption in the world, and the interest in further developing production in some geographical areas with renewed development prospects and where it can be produced more economically, keep producers’ interest high in investing in large and integrated plants, to support the engineering, automotive and infrastructure sectors.

There is therefore a need to make available technologies that allow to:

- improve the productivity of the plants and with it the per-capita added value;

- reduce GHG emissions per ton produced by applying technological solutions with a low environmental impact;

- speed up production processes, reducing time and costs and optimizing production efficiency by aggregating different processing steps with continuous operational solutions.

In the light of this, the aim is to have plants that function in direct, endless or semi-endless or billet-to-billet mode, thus avoiding cooling of the semi-finished product and therefore the corresponding loss of energy which would then have to be restored by heating the semi-finished product again, for example by means of combustion heating furnaces. With regard to the production of long products (bars, wire rod, sections), providing a single production line over 0.7 Mton/year sets limits for the maximum speed obtainable for the production of wire rod and small diameters for the production of bars, since the starting casting section has to be greater than 200mm. Therefore, for productivity higher than 0.7 Mton/year and up to 3.0 Mton/year, solutions are known that use two or more independent casting and rolling lines (hereafter, co-rolling) with the following two possible configurations:

1. Each line is served upstream by its own dedicated melting furnace, by a respective ladle and by a secondary metallurgy station. Each casting line is equipped with its own tundish and a ladle-turning turret to discharge the liquid steel from the ladle, coming from the secondary metallurgy station, to the tundish. Therefore, in this configuration there are two lines in which all the components, from the melting of the steel to the final product, are doubled.

2. The two co-rolling lines have in common the feed of liquid steel, which means having a single tundish in common, a single ladle-turning turret, a single ladle, a single secondary metallurgy station and a single melting furnace.

In the first case, having two furnaces and two secondary metallurgy stations, it is possible to cast very different types of steel (steel grades) on the two lines: for example, a first line can produce steel for reinforced concrete, while the second line can produce quality steels. This is possible because the two furnaces can be loaded separately with the appropriate charge mix that serves to obtain the desired chemistry according to the desired steel. On the other hand, the Capex and Opex of this solution are very high because the plants upstream of the lines are double.

In the second case, the Capex and Opex are reduced because the plant upstream of the lines is single, but the flexibility to produce significantly different steel grades on the two lines is greatly reduced: in fact, even if it is possible to modify the chemistry directly in the tundish, for example when its internal geometry provides suitable sectioning, this can only be done within certain limits, by introducing the alloy elements into the portion of the tundish that feeds the corresponding line. It is in fact possible that alloy elements intended for a certain finished product on one line contaminate the other product on the other line. Furthermore, a single supply of liquid steel by means of a common tundish does not allow to manage significantly different productivities on the two lines, because strong turbulence would be created in the tundish due to the very different steel flows exiting from it.

As can be seen, both cases have advantages and disadvantages which, however, do not satisfy the need to find an effective compromise between production flexibility, investment costs (Capex) and transformation costs (Opex).

The state of the art proposes US 2004/025320, in which two separate ladles are used to feed two casting lines that converge in a single rolling train. This solution therefore has both the limit of using two separate furnaces, and also that of limiting the productivity of the plant to that of the single rolling train shared between the two casting lines. GB 07811 only provides the possibility of filling two ladles with the material from the same furnace, but says nothing about how the individual ladles are then used to feed one or more co-rolling lines.

There is therefore a need to perfect a steel production plant to be used in multiline co-rolling apparatuses, which can overcome at least one of the disadvantages of the state of the art.

In particular, one purpose of the present invention is to allow two or more finished products to be processed simultaneously even when said finished products require different chemical composition specifications, such as for example different contents of alloy elements, or require different productivities between them.

Another purpose of the present invention is to considerably increase the productivity of known production plants, allowing to reduce Capex and Opex for the production and supply of steel even when it is necessary to produce different finished products with different contents of alloy elements, as well as of different sizes.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

The steel production plant according to the invention provides to use a single high-capacity melting furnace, for example of the EAF (Electric Arc Furnace) type, to feed at least two co-rolling lines for long products, fed independently of each other, with overall productivity for example comprised between 0.7 - 3.0 Mton/year, preferably between 1.0 - 3.0 Mton/year.

The single high-capacity furnace as above is suitable to distribute the quantity of liquid steel produced into at least two ladles with a smaller capacity. The total useful capacity of the at least two ladles, defined in terms of the quantity of steel, expressed in tons, which the ladles receive at each tapping cycle of the furnace does not exceed the capacity of the furnace, so that for each melting cycle the at least two ladles are filled with the quantity of molten steel from the single furnace.

In accordance with the invention, the at least two ladles can have the same capacities and, as a whole, substantially equal to the capacity of the single EAF. In one variant, the at least two ladles have different capacities, for example, but not only, in relation to the absorption capacity of the two respective casting machines downstream, that is, their productivity, which in turn depends on the type of finished product (e.g. rod or bars).

By co-rolling line we mean a line that provides a casting machine which directly feeds a respective rolling train that is aligned with said casting machine.

Each co-rolling line has a single line (strand) to allow the direct feed of billets from the casting to the rolling mill in continuous mode (endless), or in semi- continuous mode (semi-endless or billet-to-billet), without fossil fuel furnaces for heating the semi-product, thus improving Opex, Capex, occupied spaces and emissions.

In accordance with the invention, the at least two co-rolling lines can produce commercial steels or steels of medium or high quality.

In relation to the finished products to be obtained, the productivity of each line can vary from 0.3 Mton/year to 1.5 Mton/year.

The at least two co-rolling lines can produce the same type of product, or the final product can be differentiated.

For example, a first co-rolling line can produce bar in plate or bar in coil or rods, while the at least second line can produce sections.

According to one variant, one of the lines can be without the rolling mill and produce a semi-finished product (billets) intended for sale.

According to another variant, by providing special diverter means, one line can send the rolled product to the intermediate train or to the finishing train of another line when one of the lines is idle for maintenance or other.

The advantages of the plant/method according to the invention are as follows:

• Reduction of initial investment costs. The presence of a single EAF melting unit, with a higher capacity than what a single continuous casting machine can absorb, tapping the steel of the EAF into at least two ladles, allows to reduce Capex with the same overall productivity;

• Reduction of transformation costs. The costs of melting scrap, scrap + E1BI + cast iron or mix with hot metal, decrease as the value in tons of liquid steel produced at each casting increases; in fact, with a single furnace, fixed costs (operating personnel, auxiliary devices, etc.), the costs of consumables (electrodes, refractories, etc.) and costs relating to energy consumption and water consumption are reduced. Therefore Opex are reduced.

• Maximum flexibility to produce even very different steel grades, even with a single EAF.

As mentioned above, a single EAF satisfies the production capacity of at least two casting machines with tapping of the liquid steel divided between at least two ladles, to take into account the productivity of each single casting machine: for this reason, the EAF is provided with suitable devices for tapping liquid steel into the at least two ladles, in succession or simultaneously.

According to a first variant, the EAF can be of the tiltable type with a device for the selective closing of the tapping hole (for example of the EBT type) located on the bottom of the furnace. The filling of the at least two ladles occurs in succession and, in the transit step between one ladle and the other, the outflow of the steel is interrupted by tilting the furnace on the opposite side. As a function of the sizes of the shell and of the ladles, another variant provides that the furnace can be provided with more than one tapping hole, for example two adjacent to each other at a suitable distance, each equipped with its own selective closing device. This solution allows to tap into two ladles simultaneously and shorten the filling times of the ladles. This also facilitates the parallel execution of the subsequent working steps to which the ladles are subjected. This solution can be chosen as a function of the diameter of the ladles and the sizes of the shell, which determine the distance between the tapping holes.

A buffer rod, or stopper, can also be associated with the tapping hole or holes, which advantageously allows to reduce the flow of steel through the tapping hole by adjusting the distance between the tip of the buffer rod and the tapping hole.

In another variant, the furnace can always be of the tiltable type and equipped with a tapping spout, or sprue, located on one side of the furnace. In this variant, the filling of the at least two ladles occurs in succession and, in the transit step between one ladle and the other, the outflow of the steel is interrupted by tilting the furnace on the opposite side.

In another variant, the tapping device is of the “siphon” type and consists of a chamber disposed perimetrically to the shell in which the tapping can be induced by tilting the shell or with the aid of pumping systems that create a depression or with the combination of tilting and depression.

In another variant, the furnace is not tiltable and the tapping device consists of two opposite ducts, made for example on the lateral wall of the shell in the lower zone. These ducts are sealed with sand to prevent the steel from escaping during the melting step. For the tapping, the operators pierce the sand, for example with thermal lances, and the steel comes out filling two ladles simultaneously.

In one solution of the invention, the liquid steel thus divided into the at least two ladles is then treated in two respective secondary metallurgy stations which provide a treatment in a ladle furnace (LF - one per line), and possible degassing devices (VD/VOD or other refining device), which will allow to refine and possibly diversify the basic steel grade of the steel supplied by the EAF, in order to independently feed the at least two castings with the required steel grade of the finished product that said lines have to produce.

For significant variations in steel grade between the at least two lines, alloy elements can be added directly into the ladle, using special hoppers, during the step of tapping from the EAF. The additions can be made in only one of the two ladles or in both, so as to differentiate, or not, the chemical composition of the steel. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a schematic representation of a steel production plant in accordance with some embodiments described here;

- fig. 2 is a schematic representation of a possible example multi-line co-rolling apparatus used in the plant of fig. 1 ;

- figs. 3-6 schematically show different modes for tapping the steel into two ladles, with corresponding different types of EAF, which can be implemented in the present invention;

- figs. 7a-7e show a tapping sequence in a first embodiment of the invention;

- figs. 8a-8e show a tapping sequence in a second embodiment of the invention.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can conveniently be incorporated into other embodiments without further clarifications.

DETAILED DESCRIPTION OF SOME EMBODIMENTS Some embodiments, described by way of example by means of figs. 1-2, concern a steel production plant 10 that uses a multiple co-rolling apparatus 50 configured to simultaneously produce, along respective co-rolling lines 11a, l ib, two steel products that are the same or different, having the same or a different steel grade. The solution shown by way of example provides two co-rolling lines 11a, l ib and uses two ladles 14 to feed them, as described below, although the teaching of the invention is applied in a similar manner to a larger number of co rolling lines and/or a larger number of ladles 14.

The product made on one line can be made of steel with a steel grade known as “GB/T 1499.2-2018 Grade HRB400/HRBF400”, poor in manganese and without vanadium, while the product made on the other line can be enriched with alloy elements and the steel grades are known as “GB/T 1499.2-2018 Grade HRB400E/HRBF400E”, “GB/T 1499.2-2018 Grade HRB500”, “GB/T 1499.2- 2018 Grade HRB500E/HRBF500E’, which have progressively higher carbon, manganese, silicon and vanadium contents.

The two co-rolling lines 11a, l ib can function in endless mode, but also in billet-to-billet or semi-endless mode, in any case at high casting speed, for example greater than 6.5 m/min.

In some embodiments, each casting line 11a, lib of the multi-line casting apparatus 50 can comprise, downstream of the tundish 17, a casting machine 18 (schematically shown in figs. 1 and 2), an extractor unit 21, a cutting unit 22, usable in the case of billet-to-billet mode, an induction furnace 20 and a rolling train comprising rolling stands 12.

The sequence of rolling stands 12 can define a roughing train 24, an intermediate train 25 and a finishing train 26.

As can be seen in the drawings, neither of the two co-rolling lines 11a, l ib provides the presence of a fossil fuel (typically natural gas) furnace for heating the billets.

The heating inductor 20 is positioned upstream of the rolling train and advantageously consists of modular units.

The heating power of the inductor 20 is between 2 and 6 MW (from 20 to 60 kWh/t depending on the maximum AT to be compensated) in order to manage temperature increases between 30°C and 200°C. For high productivities and large AT (e.g. billets of 130mm at 180 t/h with a thermal integration of 350°C) it is possible to have an inductor power of up to 12-14 MW. The two co-rolling lines 11a, lib can have a number of rolling stands 12 that depends on the final product.

To produce rod, between 26 and 30 total rolling steps can be considered, for bars between 20 and 24 total rolling steps, while for sections/profiles approximately 20 total stands.

As can be seen in fig. 1, the plant 10 provides a single high-capacity melting furnace 13 to feed the two co-rolling lines.

The high-capacity furnace 13 is suitable to distribute the quantity of liquid steel produced into two respective ladles 14 with a smaller capacity. The total useful capacity of the two ladles 14, defined in terms of quantity of steel, expressed in tons, which the ladles 14 receive at each tapping cycle of the furnace 13 does not exceed the capacity of the furnace 13, so that for each melting cycle the two ladles 14 are filled with the quantity of molten steel from the single furnace 13.

In accordance with the invention, the two ladles 14 can have the same capacities and, as a whole, substantially equal to the capacity of the single EAF 13. In one variant, the two ladles 14 have different capacities, for example, but not only, in relation to the absorption capacity of the two respective casting machines downstream, that is, their productivity, which in turn depends on the type of finished product. In fact, as can be seen in fig. 1, the two lines 11a, lib can simultaneously produce different types of product, for example bars 27 to be unloaded into plate 28, or rod 29 to be wound in reel 39.

The present plant 10 can be used effectively to manage significantly different productivities on the two lines 11a, l ib, for example 75 ton/h on line l ib to produce rod and 150 ton/h on line 11a to produce bars, using ladles 14 of different capacities.

The furnace 13 can be advantageously, although not necessarily, of the tiltable electric arc (EAF) type. In fig. 1, the furnace 13 is shown as non-tiltable and with a double lateral tapping duct 15, so that the two ladles 14 can be filled simultaneously and continue in parallel in the subsequent operating steps.

The tiltable furnace, during the melting step, is kept in a horizontal position; during the slagging step, that is, when the slag layer covering the steel is at least partly evacuated from the furnace, the furnace is tilted on one side (for example by 2°-3°), while during the tapping step it is tilted on the opposite side (for example from 5° to 12° between the beginning and end of the tapping).

The tapping device through which the outflow of liquid steel occurs can be of various types.

In particular, the furnace 13 can be of the type with a tapping hole 32 located on the bottom of the furnace 13 and associated with selective closing means, in this specific case of the movable slide 30 type (fig. 3), with which there can possibly be associated a buffer rod 41, equipped with a respective movement mean 40. The buffer rod 41 can be used not only to close the tapping hole 32 once the filling of the ladles 14 has been completed, but also to regulate the flow of steel through the respective tapping hole 32, closing it only in part by partly introducing its end. In fig. 3, the buffer rod 41 is shown in a partly lowered position ready to close the tapping hole 32. On the opposite side of the tapping hole 32 the furnace 13 has a slag door 42.

In an alternative solution, the furnace 13 can be of the type with a tapping spout 38 (fig. 4). Again, it can be of the siphon type, with a tapping chamber 35 connected to a passage channel 37 that communicates with the inside of the furnace 13 (fig. 5). In this case, associated with the tapping chamber 35 there can be pumping means 43 to create the depression that draws the steel into the tapping chamber 35.

Finally, the furnace 13 can be of the type with double tapping hole 32. In this variant, shown in fig. 6, the tapping device consists of two tapping holes 32 adjacent to each other, which can be selectively closed with suitable and respective closing means 30 of the movable slide type (shown in dashed lines). Each tapping hole 32 is suitable to cooperate with a respective ladle 14 and the center to center distance between the tapping holes 32 is advantageously such as to be able to cooperate with a substantially central position of the underlying ladles 14 (also shown in dashed lines). This solution allows to fill both ladles 14 with a single tapping cycle, by tilting the furnace 13 according to the modes described for the solution of fig. 3. Also in this case, with each tapping hole 32 there can be associated a corresponding buffer rod 41 for the possible regulation of the flow of steel at exit.

The types of furnace of figs. 5 and 3 have been shown in the example tapping sequence of figs. 7a-7e and 8a-8e respectively. It is obvious that these sequences can also be applied to the other types of furnace 13 shown in figs. 4 and 6, or also to other types not shown here.

Figs. 7a-7e show a siphon tapping device consisting of a tapping chamber 35, equipped with a tapping duct 36, in which the liquid steel 31 located on the bottom of the furnace 13 can be drawn through the channel 37. The suction of the steel through the siphon can be carried out in different ways: for example by tilting the furnace on the side of the tapping chamber 35, so as to use the principle of communicating tanks to induce the exit of the steel, or by means of the pumping means 43 that create a depression in the tapping chamber 35 so as to draw the liquid steel inside. According to one variant, the two modes described above (inclination and depression) can be used in combination. Once the first tapping cycle has been performed, for example with a first ladle 14 (fig. 7b), the inclination or suction can be interrupted and then resumed once the ladle change has been performed. As can be seen in fig. 7c, the furnace 13 can be taken into an inclined position in the opposite direction in which the remaining steel 31 remains below the channel 37, until the second ladle 14 is ready for the filling cycle to be repeated (fig. 7d).

Once the filling of the second ladle 14 is also completed, the furnace 13 returns to a horizontal position (fig. 7e), with the liquid steel pool 31 inside it, for a new melting cycle.

In the example shown in figs. 8a-8e the furnace 13 is provided on the bottom with a selective closing device (for example of the movable slide type 30) to pour the liquid steel produced into two ladles 14 in succession.

To proceed with filling the first ladle 14, the furnace 13 is tilted on the side where the opening/closing device is located (fig.8b), the tapping hole 32 is opened by moving the movable slide device 30 and the filling of the first ladle 14 begins.

Once the first ladle 14 has been filled, the furnace 13 is tilted in the opposite direction, taking the liquid steel 31 to a position outside the tapping hole 32 (fig. 8c) in order to stop the outflow through the hole. The first ladle 14 is then evacuated from the tapping area and the second ladle 14 to be filled is introduced (fig. 8d). The furnace 13 is again tilted for the tapping and the filling of the second ladle 14 begins.

Once the second ladle 14 has also been filled, possibly keeping a volume of molten steel inside the shell of the furnace 13 as a liquid pool, in order to facilitate subsequent melting, the tapping hole 32 is closed with the movable slide device 30 (fig. 8e) and the furnace 13 is then returned to a horizontal position in order to prepare a new melting cycle. In this step, an introducer device 33 can be driven to discharge inert material 34, for example sand, in order to fill and close the tapping hole 32, until the next tapping cycle starts.

A similar method is also adopted when the tapping device consists of a casting spout 38 or channel located on a lateral wall of the furnace, as in the solution shown in fig. 4.

The ladles 14, in a first solution, can both have a capacity exactly equal to half the capacity of the furnace 13, so that for each melting cycle of the furnace 13 the two ladles 14 are both filled with a same quantity of steel.

In an alternative solution, not shown, the two ladles 14 have different capacities, in particular if the absorption capacity of the two respective casting machines downstream is different, that is, their productivity, which in turn depends on the type of finished product (e.g. rod or bars) is different.

The differentiation of the steel grade of the steel between one ladle 14 and the other, or in any case of at least one ladle 14, can occur both in the step of tapping from the electric furnace, and also in the secondary metallurgy station 16 in the ladle furnace.

In the first case, in the tapping step, it is possible to use elements, for example hoppers not shown here, for introducing alloy elements to chemically differentiate the metal materials independently and autonomously between the two ladles 14.

In the ladle furnace, on the other hand, the steel can be subjected to enrichment treatments with precise dosages of the various alloy elements (ferroalloys) in order to obtain the desired metallurgical quality.

The ladle furnace consists of a liftable vault, through which, in the secondary metallurgy station 16, electrodes can be inserted which are necessary for maintaining the bath at temperature.

For producing special steels and stainless steels, the secondary metallurgy treatment can provide, downstream of the ladle furnace LF, an additional vacuum degassing treatment stage (VD or VOD) for the removal of unwanted gases, such as nitrogen and hydrogen, and for decarburization.

From the respective ladles 14 the steel can be poured into a respective tundish 17 which feeds the respective continuous casting machine 18. As mentioned, each of the co-rolling lines 11a, lib has its own tundish 17 which operates independently of the tundish 17 of the other line.

The two continuous casting machines 18 form part of the co-rolling lines 1 la, l ib.

The present plant 10 as described above is suitable for always exploiting the productivity of the steel plant (EAF) to the maximum, even when one line 1 la or l ib has a reduced productivity because of the product to be made, for example rod which dictates a maximum productivity of 75 ton/h.

With this plant configuration, the invention provides the possibility of transferring billets from line 11a to line lib, for example by means of a transverse transferer of a known type, if the two lines produce products with the same steel grade.

The transfer of billets between the two lines can be carried out in case of emergency, for example in the event a rolling train is blocked due to accidents or jamming.

The transfer of billets from one line to another can also be carried out when, for example, there is a desire to increase the productivity of one of the two lines for a certain period (for example 1 month), for example line 1 la. In this case, line 11a would function in billet-to-billet mode. This option can be provided in the design stage of the plant by adequately sizing the rolling train of line 11a. In the design stage it is also possible to provide special diverter means, shown only schematically in dashed lines in figs. 1 and 2, to send the rolled product at exit from the roughing train 24 or from the intermediate train 25 of one line, to the intermediate train 25 or to the finishing train 26 of the other line, when one of the lines is idle for maintenance or other.

As stated, the two co-rolling lines 11a and l ib can be complete lines, as shown in fig. 2, or one of the two can be without the rolling mill and produce a semi-finished product (billets) intended for sale.

It is clear that modifications and/or additions of parts or steps may be made to the plant 10 and to the method as described heretofore, without departing from the field and scope of the present invention as defined by the claims.

Claims

1. Steel production plant to obtain long products such as rods, bars or sections, with overall productivity comprised between 0.7 - 3.0 Mton/year, preferably between 1.0 and 3.0 Mton/year, comprising at least two co-rolling lines (11a, l ib), characterized in that it provides a single high-capacity melting furnace (13) and at least two ladles (14) to receive the liquid steel from said melting furnace (13) and feed said co-rolling lines (11a, lib), wherein the total useful capacity of the at least two ladles (14) expressed in tons does not exceed the capacity in tons of the liquid steel tapped from said melting furnace (13).

2. Steel production plant as in claim 1, wherein the at least two ladles (14) have the same useful capacity and substantially equal, together, to the capacity in tons of tapped steel of the single melting furnace (13).

3. Steel production plant as in claim 1, wherein the at least two ladles (14) have different useful capacities and substantially equal, together, to the capacity in tons of tapped steel of the single melting furnace (13).

4. Steel production plant as in any claim hereinbefore, comprising at least two secondary metallurgy stations (16) for executing refining and/or thermal or other treatments.

5. Steel production plant as in any claim hereinbefore, comprising hoppers to add alloy elements directly to the ladle (14) during the step of tapping from the melting furnace (13).

6. Steel production plant as in any claim hereinbefore, wherein the furnace (13) is of the tillable type.

7. Steel production plant as in claim 6, wherein the furnace (13) is of the type with pouring spout (38).

8. Steel production plant as in claim 6, wherein the furnace (13) is of the type with a tapping hole (32) which can be selectively closed by means of closing means (30).

9. Steel production plant as in any claim up to 5, wherein the furnace (13) is of the type with siphon tapping with tapping chamber (35).

10. Steel production plant as in any claim up to 5, wherein the furnace (13) is of the non-tiltable type and with double lateral tapping duct (15).

11. Steel production plant as in any claim up to 5, wherein the furnace (13) is of the tiltable type with double tapping hole (32).

12. Method to produce steel to obtain long products such as rods, bars or sections, with productivity comprised between 0.7 - 3.0 Mton/year, preferably between 1.0 and 3.0 Mton/year, comprising the following steps: - melting the steel in a single high-capacity melting furnace (13);

- pouring the molten steel from the single melting furnace (13) into at least two ladles (14) whose total useful capacity expressed in tons does not exceed the capacity in tons of liquid steel tapped from said single melting furnace (13);

- feeding with each of said at least two ladles (14) a respective co-rolling line (1 la, 1 lb) of a multiple co-rolling apparatus (50).

13. Method as in claim 12, wherein the liquid steel contained in said two ladles (14) is diversified by type.

14. Method as in claim 13, wherein the diversification of the type of liquid steel occurs in the tapping step.

15. Method as in claim 13, wherein the diversification of the type of liquid steel occurs inside the ladles (14) in at least two secondary metallurgy stations (16).

16. Method as in any claim 12-15, wherein the co-rolling lines are equal to two, and one or both co-rolling lines (11a, l ib) work in endless mode.

17. Method as in any claim 12-15, wherein the co-rolling lines are equal to two, and one or both co-rolling lines (11a, l ib) work in billet-to-billet or semi-endless mode.