Method for manufacturing steel strip

Abstract

The invention relates to a method for producing steel strip, wherein a steel melt is first produced, then formed into a continuous cast strand (3) in a continuous casting plant (2), and then the continuous cast strand (3) is conducted to a heating unit (4) and then to a rolling mill (5). In order to enable energy-efficient production of hot-rolled steel strip and further processing, the invention provides that a steel melt is first produced, wherein a) starting material is melted; b) solid starting material containing iron and carbon and air, oxygen and / or natural gas are supplied; c) the melt is conveyed to a vacuum unit (7); d) the melt is conveyed to the continuous casting plant (2); e) the melt is cast; f) the continuous cast strand (3) / slab is conveyed to the heating unit (4); g) the continuous cast strand (3) / slab is conveyed to the rolling mill (5) and rolled into the hot-rolled strip (1).

Description

Technical Field

[0001] The present invention relates to a method for producing steel strip, in particular hot-rolled strip, in the form of wound coils or paneled sheets. A steel melt is first produced and then formed into strands in a continuous casting plant. The strands are then fed, either undivided or divided into individual slabs, to a heating unit, and the heated strands or slabs are subsequently rolled into steel strip in a subsequent rolling mill. The production of the steel strip, in particular the hot-rolled strip, preferably takes place without intermediate cooling of the strands or slabs to ambient temperature (20° C.). The aim is to provide a steel strip that is intended for further processing into finished products with a visually demanding surface, such as visible automotive components, packaging sheets, household appliances, or non-grain-oriented electrical steel sheets. Background Art

[0002] For example, hot-rolled strip made of ULC / IF steel, used for the production of automotive exterior materials or similarly demanding surfaces, is produced as a prefabricated material through a process flow from a blast furnace (pig iron), a blown steel plant (BOF), optional vacuum treatment, further secondary metallurgical steps, and continuous casting to form slabs. The slabs are then rolled into hot-rolled strip on a wide-band hot rolling mill (Warmbreitbandstraße). To achieve the desired combination of strength and other advantageous processing properties, the melt to be produced and its chemical composition are highly regulated to adhere to maximum levels of steel-related impurities (e.g., copper, chromium, nickel, molybdenum) and to minimize undesirable sulfur, nitrogen, and hydrogen contents.

[0003] The conventional approach enables the desired chemical composition to be achieved by desulphurising the pig iron before charging it into the steel blowing plant, by limiting the nitrogen content to a high decarburisation rate determined by the process when blowing oxygen into the steel blowing plant, by reducing the carbon content (if necessary, by vacuum treatment to 20 ppm (i.e. 0.002 wt. %) and below), by fine-tuning the analysis in secondary metallurgical treatment steps and by casting the steel into slabs having a thickness exceeding 200 mm.

[0004] This approach typically requires that the cast slabs cool in a slab storage and then undergo surface inspection. This inspection can be performed completely or only partially on a representative slab from the melt. The inspected (and, if necessary, repaired) slabs are then returned to the rolling process and fed into the downstream heating train in a pre-planned sequence.

[0005] This allows the production processes for the slab (steel plant and casting operation) and the hot rolling mill to be separated from one another not only in terms of time but also in terms of location and thus also to be plannable.

[0006] Different previously known solutions are described in the documents EP 1 752 549 A1, WO 2004 / 108971 A2, US 2016 / 108494 A1, DE 692 27014 T2, DE 697 13 639 T2, EP 2 998 046 B1, CN 106148639 A, JP 2003064412 A, KR 1063666 B1, KR 2019076164 A, KR 1017511 B1 and KR 1412566 B1.

[0007] Disadvantages are that so-called integrated smelters (comprising a blast furnace, optionally a coking plant and sintering equipment, and a converter) require relatively much space, result in relatively high CO2 emissions and have high investment costs.

[0008] Another disadvantage of conventional integrated steel plants is that the individual production steps in steel plants and hot forming are largely separated in time and location. This results in the cast slabs generally cooling to room temperature before further processing.

[0009] Energy-saving direct use shortly after casting is not possible under these conditions or can only be achieved by including special measures, for example by transporting the slab under a heat shield.

[0010] The known solutions for producing the steel grade to which the present invention is applicable therefore have the disadvantage that they are not sufficiently energy efficient in the case of a relatively high CO 2 load on the environment.

[0011] Mainly electrically operated melting plants, such as electric arc furnaces (EAFs) and induction melting furnaces (IFs), require less space and, in principle, can now already produce high-quality steels with appropriate selection of the input materials. However, these technologies have been primarily used to produce high-alloy quality and tool steels, as well as high-quality special steels with a high internal purity. Deeply decarburized steel melts with less than 150 ppm carbon and / or less than 50 ppm nitrogen have not been possible in electric arc furnaces with batch weights exceeding 100 tons and melt times of less than 50 minutes. This results in an average mass flow rate of 2 tons / minute for further processing. This mass flow rate is insufficient for continuous casting with high slab discharge temperatures to ensure direct use. For this reason, mass flows of more than 4 tons / minute are typically required.

[0012] Since characteristic surface defects may occur on the reheated continuously cast slabs, in particular when the cast slabs are placed in the reheating unit upstream of the hot rolling mill when the surface temperature is in the so-called low-toughness range, the material must be cooled to a temperature below this range, which lies between 700°C and 950°C, depending on the steel composition.

[0013] The temperature range can be defined differently for each steel composition and can be read from the material's time-temperature-transformation diagram (ZTU) and / or calculated using metallurgical simulation methods (structure models). Currently commercially available simulation tools can be used with ThermoCalc / DICTRA, MatCalc, etc.

[0014] The lower toughness observed in this temperature range, and the associated tendency for the steel to crack along austenite grain boundaries during reheating, is related to density changes during the austenite-ferrite-austenite transformation. When the cooled steel reaches its effective, chemically determined transformation temperature A3, structural transformation begins by nucleation at the previously austenite grain boundaries. Due to its lower density, the ferrite fraction expands, but the harder austenite fraction is stressed, thus initiating creep. If this structural transformation is interrupted and the steel is reheated, the previously transformed ferrite volume fraction contracts, exerting tensile stresses. These tensile stresses, combined with the precipitation of nitrides and / or carbides in the transformed structural region, weaken the grain boundaries and, in unfavorable circumstances, lead to tearing. Depending on the steel grade, these grain boundary damages may be superficial or deep. Such damaged surfaces no longer heal during further processing, becoming visible as microcracks on the slab surface, leading to very fine surface damage to the hot-rolled strip and ultimately to a loss of value. Consequently, damaged hot-rolled strip is no longer suitable for high-quality surface finishes.

[0015] In order to minimize surface damage by reheating, it is helpful to determine either the temperature range that should not be used directly in the reheating furnace (also called hot carburizing) or the still tolerable proportion of converted ferrite.

[0016] In this case, direct use of the slab means that the volume fraction of the microstructure near the surface that is transformed is less than 10% by volume. Based on experience, metallurgists assume that the volume fraction of the microstructure is present as ferrite at temperatures more than 10 K below the start temperature of the austenite-ferrite transformation A3. Metallurgical simulation tools are now better able to predict the onset of transformation and should preferably be used to determine the limiting temperature.

[0017] In contrast, hot carburizing (hot carburizing) means converting the structure to at least 75% by volume and thus minimizing damage along the former austenite grain boundaries. This structural state is generally assumed to be achieved at a temperature of A1+20 K. In this case, the use of metallurgical simulation methods is also preferred when determining this temperature.

[0018] Although the direct use method can be used for various thin slab technologies, the problem here is the significantly higher ratio between casting surface and volume, which increases the probability of surface defects caused by steel equipment and / or casting. Summary of the Invention

[0019] The object of the present invention is to improve a method of the type mentioned at the outset so that in particular hot-rolled steel strip can be produced in a most energy-efficient manner. In particular, further processing to high-quality cold-rolled and optionally coated steel strip should be possible, as is required, for example, for automotive body shells and relatively demanding surfaces.

[0020] The object is achieved by the invention in that a steel melt is first produced which has the following chemical composition:

[0021] - a maximum of 0.02% by weight of carbon, preferably less than 0.01% by weight of carbon;

[0022] - 0.01 to 3.5% by weight of silicon, preferably less than 0.1% by weight of silicon;

[0023] - a maximum of 2.5% by weight of manganese, preferably less than 1.0% by weight of manganese;

[0024] - 0.01 to 0.20% by weight of copper, preferably less than 0.15% by weight of copper;

[0025] - a maximum of 0.40% by weight of chromium and nickel, preferably less than 0.20% by weight of chromium and nickel;

[0026] - less than 0.10% by weight of niobium, titanium, vanadium and boron, respectively, preferably less than 0.05% by weight of titanium, vanadium and boron;

[0027] - maximum 70 ppm nitrogen, preferably less than 50 ppm nitrogen;

[0028] - optional further non-ferrous elements with a proportion of less than 1.0% by weight, which are specifically alloyed elements or which enter the melt as unavoidable additions via the input material; and

[0029] -The rest of the content is iron,

[0030] The manufacturing of the steel melt comprises the following steps:

[0031] a) melting the solid iron-containing starting material in a preferably electrically operated melting unit (for example in the form of an electric arc furnace, an induction furnace or a SAF);

[0032] b) solid iron- and carbon-containing starting materials as well as air, oxygen and / or natural gas are continuously fed into the melting unit in order to achieve an intense boiling reaction in a shallow bath phase for a duration of between 2 and 30 minutes, preferably between 10 and 20 minutes (in order to avoid nitrogen absorption from the furnace atmosphere);

[0033] c) conveying the melt into a vacuum apparatus and decarburizing the melt in the vacuum apparatus at a maximum decarburization rate of 180 ppm / min carbon;

[0034] Among them, the following steps are then performed:

[0035] d) conveying the thus pretreated melt to a continuous casting device;

[0036] e) casting the melt in a continuously operating continuous casting apparatus;

[0037] f) conveying the continuous cast strand or the slab produced therefrom into the heating unit and setting the required rolling temperature, wherein the continuous cast strand or slab enters the heating unit directly at a temperature greater than A3-20 K, so that the volume fraction of ferrite in the region close to the surface of the continuous cast strand or slab up to a depth of at least 5 mm, preferably up to a depth of 10 mm, is less than 5% by volume;

[0038] g) conveying the continuous cast strand or slab into the rolling mill and rolling the continuous cast strand or slab into the steel strip.

[0039] Step a) is preferably carried out in this case in such a way that the proportion of solid starting materials corresponds to 10% to 70% by weight of the total batch.

[0040] However, step a) can also be carried out in such a way that the solid starting material is at least partially replaced by liquid input material.

[0041] Step b) is preferably carried out in such a way that at least 20 kg of carbon per minute, preferably between 30 kg and 150 kg of carbon per minute, are fed into the melt.

[0042] The material continuously supplied according to step b) preferably has an average carbon content of at least 0.5% by weight, particularly preferably between 1.0 and 3.5% by weight.

[0043] Furthermore, step b) is preferably carried out in such a way that the melt has a nitrogen content of 5 to 60 ppm, preferably less than 30 ppm, before being discharged from the melting unit.

[0044] Step c) is preferably carried out in such a way that an average decarburization rate of between 30 ppm / min and 60 ppm / min, preferably between 40 ppm / min and 50 ppm / min, is achieved.

[0045] The melt preferably has a carbon content of 0.0005% by weight to 0.01% by weight, preferably less than 0.0040% by weight, before step d) is carried out.

[0046] The continuous cast strand preferably has a surface temperature (T1) of at least A3-20 K, preferably above 800° C., after the last section of the continuous casting system during step e).

[0047] For finishing purposes, the slab can also be removed from the production line before step f), in particular for inspection, repair of surface defects, and for segmentation. After finishing, the removed slab is preferably fed to a heating unit and heated to the required rolling temperature. When the slab is introduced into the heating unit during step f), the volume fraction of ferrite in the region near the surface of the slab is preferably at least 75% by volume, measured to a depth of at least 5 mm, preferably up to a depth of 10 mm.

[0048] Preferably, the casting of the continuous strand, its passage through the heating unit, and its rolling are carried out in a continuous process. This can be carried out as a completely continuous process or as a semi-continuous process (in which only sections of the continuous casting are performed). Although a coupled casting and rolling plant (i.e., using the cast continuous strand directly) represents a preferred embodiment of the proposed method, this is not mandatory.

[0049] The structural fraction is preferably determined by a computational model using known metallurgical simulation methods for thermodynamics and the kinetics of structural changes and phase formation. The computational model, designed as an automation system, provides the necessary information for controlling and regulating the casting process, as well as the necessary decision criteria for controlling the slab's entry into the direct use process or its removal into the slab finishing process.

[0050] The conduction of the entire process is preferably controlled and / or regulated by a superordinate process control system.

[0051] Therefore, in the first step a), a molten pool is generated in the melting unit by charging solid starting materials in combination with liquid starting materials (e.g., liquid pig iron and / or a liquid pool remaining in the melting unit). This can be achieved by feeding solid input materials, such as scrap steel and iron carriers, known as "raw" materials (raw materials), such as direct reduced iron (DRI), hot briquetted iron (HBI), or solid pig iron (PI), into the melting unit in blocks or via other suitable devices, such as vibrating tanks, preheating wells, etc. The proportion of these input materials should be no more than 70% of the desired discharge weight.

[0052] According to the above step b), due to the further supply of iron-containing and carbon-containing materials, at least 65 kg of carbon are introduced into the melt per minute while consuming this amount of carbon. This carbon input together with the artificially provided oxygen and the oxygen dissolved in the molten bath leads to a sufficiently strong boiling reaction.

[0053] [C] 溶解 +[O] 溶解 ={CO} 气态

[0054] or

[0055] 2[C] 溶解 +{O2} 气态 =2{CO} 气态

[0056] This in turn counteracts the tendency for nitrogen absorption during the melting process in the electric arc furnace due to physical reasons.

[0057] The above-mentioned step b) is preferably carried out in such a way that the melt has a nitrogen content in the liquid steel of preferably less than 30 ppm when it is discharged from the melting train.

[0058] The continuously added iron- and carbon-containing material preferably consists of sponge iron (DRI - direct reduced iron) and / or HBI (hot briquetted iron) and liquid and / or solid desulfurized pig iron. The selected mixture of the mixed materials preferably has an average carbon content of at least 0.5% by weight, particularly preferably between 1.0 and 3.5% by weight.

[0059] The above-mentioned step b) is preferably controlled and / or regulated by a process control system of the single melting unit.

[0060] The above-mentioned step c) is preferably carried out to achieve an average decarburization rate of between 30 ppm / min and 60 ppm / min, preferably between 40 ppm / min and 50 ppm / min. In addition, this step is preferably carried out until the melt has a carbon content of a maximum of 0.0020 wt.-%.

[0061] From an energy point of view and taking into account factors that influence the surface quality, it has proven particularly advantageous if the continuous cast strand has a temperature of at least 800° C., preferably above its austenite-ferrite transformation temperature A3-20K, after the last section of the continuous casting plant (i.e. ultimately directly after the continuous casting plant in the conveying direction).

[0062] It is preferably provided that the continuously cast strand or the cut slab has an average temperature of between 1.050° C. and 1.280° C. at the outlet from the heating unit.

[0063] The proposed method allows the production of high-quality end products, wherein the direct use method is used without intermediate cooling to room temperature; this has significant energy advantages.

[0064] Accordingly, the proposed concept enables energy-efficient production of hot-rolled steel strip for further processing into high-quality, cold-rolled and optionally coated steel strip, as is required, for example, in the automotive, household goods and packaging industries.

[0065] To this end, the solid input material (scrap, DRI, HBI, pig iron) is primarily melted in an electric arc furnace (electric melting furnace). This is followed by secondary metallurgical treatment in a vacuum for decarburization (any type of secondary metallurgical treatment in a vacuum system is conceivable, in particular circulating degassing or pot degassing) and, if necessary, ladle treatment for desulfurization. Continuous casting then takes place in a continuous casting system. The resulting slab or continuously cast strand is used in the heating unit and heated to the temperature required for hot rolling, or its temperature profile is adjusted, and then rolled into hot strip in a rolling mill.

[0066] It is preferably provided that in continuous operation a total production rate (throughput) of at least 5 t / min is measured over at least 120 minutes as a function of the casting efficiency of the continuous casting installation.

[0067] The method allows the production of a chemical composition of the steel strip with a copper+chromium+nickel fraction of less than 0.2% by weight, a sulfur fraction of less than 120 ppm and a nitrogen fraction of less than 50 ppm, while simultaneously achieving the highest possible scrap utilization in the electric arc furnace of at least 25% by weight.

[0068] Therefore, the production of hot-rolled strips, for example made of ULC / IF steel for the production of automotive exterior materials or correspondingly demanding surfaces, as prefabricated materials can be carried out via the electrical steel route according to the invention, thereby achieving more favorable investment costs, reduced CO2 emissions, recycling possibilities and a higher degree of flexibility.

[0069] When using the slabs directly, fast production cycles and high surface quality are also advantageous.

[0070] The fact that the advantages of utilizing casting heat known from the thin slab casting and rolling method are thus advantageously combined with a steel production process based on solid input material, which, despite a high scrap content (greater than 15% by weight), allows the production of a steel quality that is suitable in terms of its surface quality for housing applications, for example, in the automotive industry, is also suitable.

[0071] At the same time, the method allows energy-saving reheating of the cast strand or slab to rolling temperature by using the slab, preferably at a temperature above 800° C., directly from the continuous casting plant into the heating unit.

[0072] Furthermore, this direct use of the slabs produced in this way makes it possible to avoid at least 90% of the surface defects that typically occur during the cooling of the slabs to temperatures between 800°C and 600°C and the necessary subsequent reheating to the necessary hot rolling temperature (e.g. microcracks in the edge region and on the slab surface).

[0073] This requires the production of crude steel from scrap and other solid input materials, the necessary vacuum treatment of the crude steel to achieve a low carbon value and a minimum nitrogen content, and the logistic and technical coupling of continuous casting with the requirement for high casting efficiency to ensure that the continuous cast strands or cut slabs are introduced into the downstream heating unit at a temperature above the onset of the austenite-ferrite transformation A3-20K, which is adapted to the respective steel composition.

[0074] For melting in the electric arc furnace, the solid input materials are combined and charged, thereby ensuring a high decarburization rate that is limited downwards in terms of its minimum size. The yield of this crude steel grade advantageously exceeds the value predetermined by the last processing stage (continuous casting) by at least 10%.

[0075] The crude steel thus produced is then reduced to the desired carbon content in a vacuum apparatus. This is preferably done in such a way that the decarburization rate is set so that the output of this process stage is 5% higher than the value required for the final processing stage (continuous casting).

[0076] The continuous casting equipment for continuous production is preferably set so that the outlet temperature of the continuously cast strand or slab from the last section is about 20 K higher than the low toughness temperature range required for the respective steel.

[0077] The use of DRI / HBI with a specially adjusted carbon content (e.g., a high content for reduction coal blowing or a low content for better process control via coal blowing) facilitates this process. The boiling reaction in the electric arc furnace can thus be specifically influenced within the scope of the above-mentioned step b).

[0078] Furthermore, it is advantageous to use DRI / HBI produced or reduced without CO 2 , for example by direct reduction with H 2 . Hot charging of DRI is also possible.

[0079] Whenever continuous casting or the use of a continuous casting system is mentioned within the scope of the method described here, this is to be understood as encompassing all conventional possibilities for producing continuously cast metal strands. In addition to the preferred continuous casting system, thin slab casting systems with a casting thickness of 30 mm to 90 mm or so-called twin-roll systems with a casting thickness of between 1 mm and 30 mm can also be used, in which the strand is conveyed from the mold and deflected in an arc from a vertical to a horizontal line. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] An exemplary embodiment of the invention is illustrated in the drawings. The single FIGURE schematically shows the preparation of a steel melt, the subsequent continuous casting plant with a connected heating unit, and a rolling mill.

[0081] The drawing schematically shows a production plant with which a hot-rolled strip 1 can be produced. DETAILED DESCRIPTION

[0082] The starting material is first melted in an electric arc furnace. The melt is then conveyed to a vacuum system 7, where it undergoes secondary metallurgical processing. The melt, ready for casting, then reaches the continuous casting system 2, where a continuous strand 3 (slab) is cast in a known manner. Directly after the continuous casting system 2 (i.e., after its final section), the continuous strand 3 has a temperature T1.

[0083] The slab then passes to a heating unit 4 , where it is heated to a temperature T2 , at which temperature it then passes to a rolling mill 5 and is rolled into a finished hot-rolled strip 1 .

[0084] The method described makes it possible to produce steel of the highest quality (e.g. body shell quality for the automotive industry) by continuous casting into continuous ingots 3 with a thickness of 90 to 310 mm using an electric arc furnace by selecting the input material, by optimizing the process control, by avoiding non-metallic inclusions and by synchronizing with subsequent process steps (especially in the form of vacuum decarburization, i.e. secondary metallurgical treatment).

[0085] The crude steel preferably has a maximum carbon content of 0.020% by weight upon discharge from the melting unit and is produced in an electric arc furnace, as described above. Preferably, solid input material is used, with the process management resulting in a crude steel with a low content of undesirable brown elements (copper, chromium, nickel) and a minimal gas content (nitrogen, hydrogen). The produced crude steel is decarburized in a vacuum system 7 and subsequently formed into continuous cast strands 3 in a continuously operating continuous casting system 2.

[0086] In particular, scrap, pig iron and sponge iron (DRI and / or HBI) are used as metallic input materials, which result in a small sulfur input.

[0087] Furthermore, the metallic input material has a low content of undesirable brown elements.

[0088] The addition of new input material can be carried out with regard to undesired steel concomitants and adapted to the steel marking to be produced.

[0089] The metallic input material is selected in this case such that a total carbon input of at least 1% by weight is possible.

[0090] The metallic input material is preferably fed in such a way that a vigorous boiling reaction takes place during the entire shallow bath phase, which is ensured by adding at least 65 kg of carbon per minute.

[0091] Melting and slag removal are preferably carried out in such a way that a nitrogen content of less than 30 ppm is obtained in the liquid steel before tapping.

[0092] The crude steel melt is decarburized in the vacuum system 7 at a maximum decarburization rate of 120 ppm / min carbon to a carbon content of less than 0.010% by weight before being discharged to the continuous casting system.

[0093] Furthermore, the decarburization of the crude steel melt is preferably carried out in the vacuum device 7 in such a way that an average decarburization rate of 40 to 50 ppm / min of carbon is achieved during the entire decarburization phase.

[0094] Secondary metallurgical treatments can also be provided for deoxidation of the steel melt decarburized in the vacuum device 7 and for setting a target composition and temperature homogeneity in the vacuum device or, if appropriate, also in a downstream atmospheric treatment device.

[0095] The melt is cast in a continuously operating continuous casting system 2 , wherein an outlet temperature (temperature T1 ) of preferably at least 800° C. prevails on the surface from the last section.

[0096] The production time of the continuous casting installation 2 preferably includes at least four melts cast continuously in succession.

[0097] Furthermore, the slab produced in this way is fed directly to the downstream heating unit 4 in order to set an average discharge temperature (temperature T2) of 1.050° C. to 1.280° C.

[0098] An automatic surface inspection of the slab can be carried out between the slab being discharged from the final section of the continuous casting installation 2 and being fed into the downstream heating unit 4 .

[0099] Slabs with surface defects can be automatically removed from the production line and repaired after cooling. The repaired slabs can then be fed back into the production process.

[0100] The basic concept of the proposed method is therefore that the steel melting process in the electric arc furnace, the vacuum treatment in the vacuum device 7 and the continuous casting of the slabs are preferably carried out with a thickness of more than 110 mm so that the slabs discharged from the continuous casting device 2 have a sufficiently high temperature so that they can be inserted into the heating unit 4 (preferably a lifting beam furnace) without the risk of surface defects.

[0101] In order to ensure the essential requirement of a high slab temperature when fed into the heating unit 4 , the entire process is optimized for high productivity beforehand.

[0102] Correspondingly, at high slab temperatures, high casting speeds result and, as a result, a rapid supply of melt from the vacuum device 7 , which in turn leads to short processing times in the electric arc furnace.

[0103] Short treatment times in an electric arc furnace require a high boiling reaction in the bath and a constant decarburization rate in the melting phase as described above, while limiting the nitrogen content in the steel. This is facilitated by the continuous supply of DRI and / or other iron- and carbon-containing input materials.

[0104] The required minimum decarburization rate facilitates rapid processing under vacuum with a simultaneous reduction of the carbon content to a minimum.

[0105] The proposed concept is therefore suitable for coupled processes with a plurality of units arranged one behind the other, the processes of which are logically connected to one another in such a way that the slab can ultimately be fed directly into the heating unit 4 without subsequent formation of surface defects.

[0106] The method according to the invention, from steel production to steel strip, can be controlled and / or regulated by a superordinate process control system.

[0107] List of reference numerals:

[0108] 1 Hot rolled strip

[0109] 2 Continuous casting equipment

[0110] 3Continuous casting billet (slab)

[0111] 4Heating unit (reheating unit)

[0112] 5 rolling mill

[0113] 6 Melting Unit (Electric Arc Furnace)

[0114] 7 Vacuum equipment

[0115] T1 is the temperature of the continuous casting strand downstream of the last section of the continuous casting equipment

[0116] T2 is the temperature of the continuous casting billet at the outlet of the heating unit

Claims

1. A method for producing a steel strip (1) in the form of a wound coil or in the form of a single sheet for panelling, wherein: First, a steel melt is produced, which is then formed into a continuous cast strand (3) in a continuous casting plant (2), and the continuous cast strand (3) is then fed to a heating unit (4) either undivided or divided into individual slabs, and the heated continuous cast strand (3) or heated slab is subsequently rolled into a steel strip (1) in a subsequent rolling mill (5). in, First, a steel melt with the following chemical composition is produced: - maximum 0.02% by weight of carbon, - 0.01 to 3.5% by weight of silicon, - maximum 2.5% by weight of manganese, - 0.01 to 0.20% by weight of copper, - a maximum of 0.40% by weight of chromium and nickel, - less than 0.10% by weight of niobium, titanium, vanadium and boron, respectively, - Maximum 70 ppm nitrogen, - optional further non-ferrous elements, with a proportion of less than 1.0% by weight, which enter the melt as unavoidable additions via the input materials, and -The rest of the content is iron, The manufacturing of the steel melt comprises the following steps: a) melting solid iron-containing starting material in a melting unit (6); b) continuously feeding solid iron- and carbon-containing starting materials and air, oxygen and / or natural gas into the melting unit (6) in order to achieve an intense boiling reaction in a shallow bath phase for a duration of between 2 and 30 minutes; c) conveying the melt into a vacuum device (7), and decarburizing the melt in the vacuum device (7) at a maximum decarburization rate of 180 ppm / min of carbon; Among them, the following steps are performed next: d) conveying the thus pretreated melt to a continuous casting device (2); e) casting the melt in a continuously operating continuous casting device (2); f) feeding the continuous cast strand (3) or the slab produced therefrom into the heating unit (4) and setting the required rolling temperature, wherein the continuous cast strand (3) or the slab enters the heating unit (4) directly at a temperature greater than A3-20K, so that the volume fraction of ferrite in the region close to the surface of the continuous cast strand or slab up to a depth of at least 5 mm is less than 5% by volume; g) feeding the continuous cast strand (3) or slab into the rolling mill (5) and rolling the continuous cast strand or slab into the steel strip (1).

2. The method according to claim 1, characterized in that Step a) is carried out in such a way that the proportion of solid starting materials corresponds to 10% to 70% by weight of the total batch.

3. The method according to claim 1 or 2, characterized in that Step a) is carried out in such a way that the solid starting material is at least partially replaced by the liquid input material.

4. The method according to claim 1 or 2, characterized in that Step b) is carried out such that at least 20 kg of carbon are introduced into the melt per minute.

5. The method according to claim 1 or 2, characterized in that The material continuously supplied according to step b) has an average carbon content of at least 0.5% by weight.

6. The method according to claim 1 or 2, characterized in that Step b) is carried out in such a way that the melt has a nitrogen content of 5 to 60 ppm before being discharged from the melting unit (6).

7. The method according to claim 1 or 2, characterized in that Step c) is carried out so as to achieve an average decarburization rate of between 30 ppm / min and 60 ppm / min.

8. The method according to claim 1 or 2, characterized in that Before step d), the melt has a carbon content of 0.0005% by weight to 0.01% by weight.

9. The method according to claim 1 or 2, characterized in that During step e), the continuous casting strand (3) has a surface temperature (T1) of at least A3-20K after the last section of the continuous casting device (2).

10. The method according to claim 1 or 2, characterized in that Before step f) is carried out, the slab is removed from the production line for finishing, in order to carry out inspection work, repair surface defects and for segmentation.

11. The method according to claim 10, characterized in that After finishing, the removed slab is fed to the heating unit (4) and heated to the required rolling temperature.

12. The method according to claim 1 or 2, characterized in that The casting of the continuous casting strand (3), its passage through the heating unit (4) and the rolling are carried out in a continuous process.

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