Method for producing crude steel and plant for its production

Abstract

The invention relates to a method for producing low-nitrogen crude steel. The method comprises the following method steps: - melting direct-reduced iron and / or scrap in an electric-arc-resistance-heated smelting furnace into a metal melt and a slag; - taking the metal melt from the smelting furnace and filling it into a converter; - refining the metal melt in the converter into liquid crude steel and discharging the liquid crude steel with a nitrogen content [N] of not more than 50 ppm, in particular not more than 30 ppm.

Description

Technical Field

[0001] In steel production, two distinct routes are currently used: the blast furnace converter route and the electric arc furnace (EAF) route. In the blast furnace converter route, iron ore is reduced and melted in the blast furnace with the addition of coke. The resulting molten metal is then oxidized (“refined”) with oxygen in an oxygen-blown converter. Here, oxygen-loving associated elements (e.g., carbon, silicon, manganese, phosphorus) in the molten metal are oxidized and discharged as gas or slag. In the EAF route, the starting material used is directly reduced iron (“sponge iron”), which is partly in block form and / or scrap. This starting material is melted in an electric arc furnace and, similarly, oxygen-loving components can be removed by blowing in oxygen, for example, see WO 2004 / 108971 A1. Background Technology

[0002] The disadvantage of the blast furnace converter route is the release of a very large amount of CO2 during reduction with coke in the blast furnace. In contrast, the electric arc furnace (EAF) steel route generally has lower efficiency in removing oxygen-loving elements and impurities introduced from scrap steel. In the EAF steel route, associated elements and impurities must be further removed through complex downstream secondary metallurgical methods. For this reason, the EAF steel route is mainly used for construction steel and long products, for which higher levels of associated elements are permissible.

[0003] Crude steel with low levels of accompanying elements, such as starting materials for ULC steels like IF steel and amorphous electrical steel, is produced almost entirely via the blast furnace-converter route. Therefore, appropriate systems exist in steel plants worldwide to produce the required quantities of suitable crude steel for further processing.

[0004] ULC (ultra-low carbon) steel refers to steel with a carbon content of no more than 150 ppm (0.015% by weight), especially no more than 100 ppm, preferably no more than 50 ppm, and especially no more than 30 ppm.

[0005] IF steel refers to ULC steel with a nitrogen content of up to 50 ppm (0.005% by weight), preferably not exceeding 30 ppm.

[0006] Amorphous grain-oriented electrical steel refers to IF steel with a silicon content of 1.0-5.0%, preferably 2.0-4.0%.

[0007] The elemental content of the steel grades mentioned is based on steel that has solidified after casting, for example, in a continuous casting machine.

[0008] Here, a particular focus is on the nitrogen content of the crude steel produced, as it can only be reduced with great difficulty through secondary metallurgical methods, especially when the oxygen content also exceeds a certain level, a point that will be explained in detail later. Summary of the Invention

[0009] Therefore, the object of the present invention is to provide a method for producing low-nitrogen crude steel.

[0010] In this approach, carbon dioxide emissions are reduced, and as much of the existing equipment as possible can be used to keep investment costs low during technology transition.

[0011] This method for producing low-nitrogen crude steel includes at least the following steps:

[0012] - Directly reduced iron and / or scrap steel are melted in a smelting furnace heated by a resistance arc, especially in a reducing atmosphere, to form molten metal and slag.

[0013] -The molten metal is removed from the smelting furnace and filled into the converter.

[0014] - The molten metal is refined into liquid crude steel in a converter, and the liquid crude steel with a nitrogen content of up to 70 ppm, especially up to 50 ppm, is discharged.

[0015] Optionally, after the molten metal is removed from the smelting furnace and before it is fed into the converter, intermediate treatment, especially desulfurization of the molten metal, may be performed.

[0016] Alternatively or additionally, intermediate processing may include deslagging and / or desiliconization.

[0017] This method has many technical and economic advantages, which will be explained in detail below.

[0018] The present invention also relates to an apparatus for performing this method. The apparatus includes a melting furnace with resistance arc heating for producing molten metal and a converter located downstream of the melting furnace for refining the molten metal into liquid crude steel. In one specific embodiment, a desulfurization unit is arranged directly downstream of the melting furnace, and the converter is arranged directly downstream of the desulfurization unit.

[0019] In the sense of this application, "direct downstream" and "direct upstream" mean that the respective devices are directly successive to each other. Only the transport and / or intermediate storage of materials occur between these directly successive devices. In particular, no cleaning, mixing with other substances, or any other refining of materials occurs between two such devices.

[0020] When explaining elemental content, the following conventions are used: Element symbols in square brackets (such as "[N]") indicate the content of that element (here, nitrogen) in the molten metal by weight percentage. Element symbols in parentheses (such as "(P)") indicate the content of that element (here, phosphorus) in the slag by weight percentage. Element symbols without parentheses (such as "C") indicate the content of that element (here, carbon) in the cast steel by weight percentage.

[0021] In this application, unless otherwise expressly stated, percentage values ​​(or ppm values) shall in principle be considered as weight percentages, i.e., %.

[0022] Electric heating equipment used for melting or heating molten metal is classified into the following types:

[0023] 1. A melting furnace with direct electric arc action (English: Electric Arc Furnace EAF), which forms an electric arc between the electrodes and the metal. This includes alternating current electric arc furnaces (EAFac), direct current electric arc furnaces (EAFdc), and ladle furnaces (English: Ladle Furnace LF).

[0024] 2. Melting furnaces employing resistance arc heating, where an electric arc is formed between the electrodes and the charge or slag, or the charge or slag is heated via the Joule effect. This includes submerged electric arc furnaces (SAFs), where the electrodes are immersed in the charge or slag, such as AC submerged electric arc furnaces (SAFac) and DC submerged electric arc furnaces (SAFdc). On the other hand, this also includes furnaces where the electrodes end precisely above the slag. In this type of furnace, at least in the electrode region, the slag is not obstructed by the charge. Therefore, the slag is open upwards, and the brush arc formed towards the slag can be seen from above. This type of furnace is also known as an open slag bath furnace (OBSF).

[0025] Melting furnaces with direct electric arc heating operate in an oxidizing atmosphere to burn off unwanted byproducts. In contrast, melting furnaces using resistance arc heating operate in a reducing atmosphere.

[0026] In the first step of this method, directly reduced iron and / or scrap are melted into molten metal in an arc resistance heated smelting furnace, while slag is formed simultaneously.

[0027] According to the present invention, after processing in a melting furnace heated by electric arc resistance, the material is charged into a converter, where the molten metal is refined into liquid crude steel. Particularly during refining, a retractable nozzle is used to blow technically pure oxygen onto the molten metal from above, wherein 30 to 80 Nm³ of oxygen per ton of molten metal is used.3 Oxygen of technical purity (Normkubimeter, standard cubic meters) is preferred, ideally 40 to 60 Nm³ per ton of molten metal. 3 Oxygen of technical-grade purity is blown onto the molten metal for a duration of 10 to 40 minutes. This duration is preferably at least 12 minutes, particularly preferably at least 15 minutes. Independently, this duration is preferably at most 35 minutes, particularly preferably at most 30 minutes.

[0028] As is well known, converters are used for the oxidative removal of accompanying elements. This particularly involves carbon, such that in the converter, the molten metal is converted into crude steel with a carbon content [C] of at most 600 ppm, preferably at most 500 ppm. The carbon content [C] of the crude steel is particularly at least 200 ppm, preferably at least 300 ppm. Here, the converter is particularly configured as an oxygen-blown converter.

[0029] In the subsequent secondary metallurgical treatment of the produced crude steel, which will be described in detail later, the carbon content [C] of the crude steel is further reduced to a maximum of 150 ppm, especially a maximum of 100 ppm, preferably a maximum of 50 ppm, and especially a maximum of 30 ppm, of ULC steel grade carbon content C.

[0030] Oxygen-blown converters, also known in technical terms as Linz-Donauwitz converters (LD converters), consist of tiltable converter vessels lined with refractory materials.

[0031] Molten metal taken from the smelting furnace is fed into the converter. Optionally, scrap can also be added to the converter as a coolant. Alternatively, pig iron from the blast furnace process can also be added. This is, for example, when retrofitting existing equipment.

[0032] The molten metal is refined in a converter. Here, oxygen is blown into the molten metal through retractable water-cooled nozzles. The subsequent intense oxidation of iron and associated elements reduces the associated elements to the desired level after 10 to 40 minutes of blowing, and any usable scrap has melted. The burning iron and associated elements escape as gases or are bound in the liquid slag by added lime.

[0033] In addition to reducing unwanted accompanying elements, the exothermic reaction between the melt and the blown-in oxygen ensures sufficient vortexing of the melt, thereby improving the refining process results and shortening processing time. To further enhance this mixing, inert gases, typically argon and nitrogen, can be blown in through nozzles inserted into the converter base. As explained below, according to the invention, the nitrogen content is also reduced during refining. Therefore, argon is preferably used as the inert gas for mixing. Alternatively, the nitrogen content in the inert gas is reduced during refining so that at the end of refining, there is no or only a small amount of nitrogen in the inert gas.

[0034] In addition to other processes, it will be explained later that CO bubbles are formed in the molten metal through the oxidation of carbon as an accompanying element. Because the partial pressure of nitrogen in the CO bubbles is very low, nitrogen [N] dissolved in the molten metal diffuses into the CO bubbles and leaves the melt along with the CO. This denitrification process continues as long as CO bubbles form, i.e., as long as there is sufficient carbon in the molten metal to be oxidized to CO. Therefore, for the denitrification process, it is advantageous when the carbon-to-nitrogen ratio [C] / [N] of the molten metal is at least 20, preferably at least 100, especially at least 200, particularly preferably at least 500, and especially at least 1000, directly before refining.

[0035] In one preferred embodiment, the carbon content [C] of the molten metal directly before refining is at least 1.0%, preferably at least 1.5%, and particularly preferably at least 2.0%. In another preferred embodiment, the carbon content [C] of the molten metal directly before refining is at most 5.0%, preferably at most 4.5%, and particularly preferably at most 4.0%.

[0036] With these carbon contents [C] and the high ratio of carbon content to nitrogen content [C] / [N], effective denitrification can be achieved even when the nitrogen content [N] of the molten metal before direct refining is up to 450 ppm, so that the nitrogen content [N] of the liquid crude steel discharged after refining is up to 50 ppm, preferably up to 40 ppm, especially up to 30 ppm, particularly preferably up to 25 ppm, and especially up to 20 ppm.

[0037] The converter is specifically designed to be as enclosed as possible to minimize the reintroduction of nitrogen from the ambient atmosphere, and in particular, to prevent it entirely. Furthermore, this is also facilitated by the formation of CO. The amount of CO is so large that ambient air is displaced at the melt surface, thereby inhibiting the intake of nitrogen from the ambient air.

[0038] Since nitrogen is contained to some extent during secondary metallurgical processing and / or crude steel casting, it is advantageous to reduce the nitrogen content during refining in a converter to a level higher than that actually required for the steel grade. For example, when producing IF steel with a nitrogen content of no more than 30 ppm, the nitrogen content [N] of the refined liquid crude steel is reduced to a maximum of 25 ppm, preferably a maximum of 20 ppm.

[0039] The method according to the invention can successfully reduce the nitrogen content of molten metal when it is above 50 ppm, and can maintain a low nitrogen content or even further reduce it when the nitrogen content is below 50 ppm. Therefore, the nitrogen content [N] of the refined liquid crude steel is always 50 ppm or less.

[0040] In a preferred embodiment, the carbon content [C] of the molten metal is increased in the smelting furnace and / or converter. Therefore, the carbon content is increased before refining in the converter. This helps ensure sufficient CO bubbles are formed during refining to achieve an effective denitrification process. The carbon content [C] of the molten metal is increased in particular to such an extent that, directly before refining, the carbon-to-nitrogen ratio [C] / [N] is at least 20, preferably at least 100, especially at least 200, particularly preferably at least 500, and especially at least 1000.

[0041] The carbon content [C] of the molten metal is achieved, in particular, by blowing coke or process gas / pulverized coal into the smelting furnace or converter.

[0042] In a preferred embodiment, the iron (Fe) content of the slag in the melting furnace is less than 30% by weight, preferably less than 20% by weight. This makes the method particularly efficient because the iron loss through the slag is exceptionally low. Such a low iron content can be achieved, in particular, by using a melting furnace heated by electric arc resistance. In melting furnaces with direct arc action operating under oxidizing conditions, the loss of FeO in the slag is higher due to the oxidizing atmosphere, meaning that the efficiency of this type of melting furnace is lower. Therefore, the combination of a melting furnace with electric arc resistance heating and a downstream converter is materially more efficient than a melting furnace with direct arc action that combines melting and oxidation in one step. In addition, a melting furnace with electric arc resistance heating is also more energy efficient because, in a melting furnace with direct arc action, energy loss can be high if the arc is not well shielded by the foaming slag.

[0043] In another preferred embodiment, the melting furnace with arc resistance heating is implemented as a closed system. This prevents heat loss and reduces the introduction of oxygen, thus maintaining a reducing furnace atmosphere and minimizing oxidation losses.

[0044] Another method to reduce nitrogen content is through vacuum treatment in secondary metallurgy (such as the Ruhrstahl-Heraeus method, RH method). However, this can only be achieved to a limited extent in the production of ULC steels. Through converter refining and downstream secondary metallurgical treatment (here, vacuum treatment), extremely low carbon content of 150 ppm, especially 50 ppm, and preferably 30 ppm, is achieved in ULC steels. However, this simultaneously leads to the enrichment of dissolved oxygen in the crude steel during refining. The oxygen content [O] in the crude steel after converter refining is between 300 and 2300 ppm. The oxygen content is particularly at least 400 ppm, preferably at least 600 ppm, and particularly preferably at least 800 ppm. The oxygen content is particularly maximum of 2100 ppm, preferably maximum of 2000 ppm, and particularly preferably maximum of 1800 ppm. However, this oxygen content makes denitrification, which aims to achieve a nitrogen content of 50 ppm or less by vacuum treatment, ineffective. Studies have shown that vacuum denitrification can only be carried out within an economically feasible timeframe at ultra-low oxygen levels of 100 ppm or less [O].

[0045] Vacuum denitrification in secondary metallurgy will also cause more problems. First, it will require additional investment in the corresponding complete sets of equipment. Second, any change in the secondary metallurgical method during steel production will necessitate a complete overhaul of the production process for the end customer. In contrast, the method for producing crude steel according to this invention has the advantage that the further refining in secondary metallurgy remains unchanged, and therefore does not require recertification.

[0046] Therefore, low-nitrogen crude steel with particularly low carbon content can be produced using the method according to the invention, and thus can be used as a starting material for the production of ULC steel grades. The carbon content of the crude steel is particularly below 600 ppm, preferably below 500 ppm, and the nitrogen content is below 50 ppm, preferably below 30 ppm.

[0047] Although oxygen is also introduced into conventional electric arc furnace (EAF) steelmaking processes using furnaces with direct electric arc action to achieve decarburization, the furnace structure limits oxygen input, thus restricting the reduction in carbon content. Due to the limited oxygen input during refining, effective denitrification via CO bubbles is also absent in conventional EAF steelmaking processes. Furthermore, these furnaces operate in an oxidizing atmosphere (i.e., ambient air), thus introducing nitrogen from the surrounding atmosphere. Additionally, the flatter structure of these furnaces compared to converters further facilitates nitrogen introduction.

[0048] Another advantage of the method according to the invention is the low silicon content of the molten crude steel after tapping from the converter. During refining in the converter, silicon is oxidized very effectively and subsequently carried away by the slag; therefore, the Si content [Si] prior to tapping is not significant. The Si content of the tapped molten crude steel is at most 300 ppm, preferably at most 200 ppm.

[0049] In the case of typical starting materials, the Si content [Si] of the molten metal during the transition from the filler to the converter is at most 1.5%.

[0050] Compared to conventional electric arc furnace steelmaking routes using melting furnaces with direct electric arc action, another advantage of the converter method according to the present invention lies in the slag content. A slag content of 100-120 kg / t can be achieved in a converter, but in the case of melting furnaces with direct electric arc action, the slag content is only about 50 kg / t. Furthermore, in converter refining, the larger standard volume flow leads to intense mixing of the slag and melt, forming an emulsion of melt droplets in the slag. This results in a larger reaction surface area between the melt and slag, which has a positive impact on dephosphorization. Moreover, the deposition of phosphorus as P2O5 in the slag is an equilibrium reaction. Therefore, achieving the highest possible slag content is advantageous if the goal is to maximize the deposition of phosphorus from the melt into the slag. When using a converter, a phosphorus distribution of (P) / [P] = 60-80 wt% / wt% is obtained, while in the case of melting furnaces with direct electric arc action, the same proportion is only 30-40 wt% / wt%. Furthermore, in the case of smelting furnaces with direct electric arc action, the slag composition is optimized for foaming in foam slag processes, rather than for dephosphorization. The phosphorus content [P] in the molten metal directly before refining is between 100 ppm and 1500 ppm. In contrast, the phosphorus content [P] in the molten steel after tapping is at most 400 ppm.

[0051] As previously mentioned, desulfurization can be selectively performed after the molten metal is removed from the smelting furnace and before it is loaded into the converter. For this purpose, calcium oxide and / or calcium carbide and / or magnesium are added to the molten metal. In this case, the iron sulfide (FeS) contained therein essentially reacts to form calcium sulfite (CaS) or magnesium sulfite (MgS). The formed CaS or MgS is then bound in the alkaline slag.

[0052] Directly before refining (and therefore after selective desulfurization), the sulfur content [S] of the molten metal is at most 1500 ppm. The sulfur content [S] of the molten crude steel exiting the furnace is also at most 1500 ppm.

[0053] Both the molten metal and the tapped molten steel may selectively contain manganese. In this case, the manganese content [Mn] in the molten metal directly before refining is at most 0.5%. In contrast, the manganese content [Mn] in the tapped molten steel is at most 0.4%.

[0054] The molten metal and / or the molten crude steel exiting the furnace may selectively contain other unavoidable impurities, the total content of which may be up to 2.0%.

[0055] The iron content [Fe] of the molten metal directly before refining is at least 90.0%. The iron content [Fe] of the molten crude steel exiting the furnace is at least 97.0%.

[0056] In a preferred variant, the metal melt directly before refining contains at least one, preferably multiple, elemental contents of all accompanying elements, which are derived from the following combinations:

[0057] Carbon [C]: at least 1.0%, especially at least 1.5%, maximum 5.0%, especially maximum 4.5%.

[0058] Nitrogen [N]: Maximum 450 ppm, especially more than 50 ppm.

[0059] Selective oxygen [O]: 0-50 ppm,

[0060] Selective phosphorus [P]: 100-1500 ppm,

[0061] Selective sulfur [S]: 0-1500 ppm,

[0062] Selective silicon [Si]: 0-1.5%,

[0063] Selective manganese [Mn]: 0-0.5%.

[0064] Directly before refining, the molten metal contains, in particular:

[0065] Carbon [C]: at least 1.0%, especially at least 1.5%, maximum 5.0%, especially maximum 4.5%.

[0066] Nitrogen [N]: Maximum 450 ppm, especially more than 50 ppm.

[0067] Selective oxygen [O]: 0-50 ppm,

[0068] Selective phosphorus [P]: 100-1500 ppm,

[0069] Selective sulfur [S]: 0-1500 ppm,

[0070] Selective silicon [Si]: 0-1.5%,

[0071] Selective manganese [Mn]: 0-0.5%,

[0072] The balance is for iron and unavoidable impurities, with the total amount of impurities not exceeding 2.0% by weight.

[0073] In a preferred variant, the molten crude steel exiting the furnace has at least one, preferably multiple, elemental contents of all accompanying elements, which are derived from the following combinations:

[0074] Carbon [C]: Maximum 600 ppm, especially maximum 500 ppm.

[0075] Nitrogen [N]: Maximum 50 ppm, especially maximum 30 ppm.

[0076] Oxygen [O]: at least 300 ppm, maximum 2300 ppm.

[0077] Selective phosphorus [P]: 0-400 ppm,

[0078] Selective sulfur [S]: 0-1500 ppm,

[0079] Selective silicon [Si]: 0-300 ppm,

[0080] Selective manganese [Mn]: 0-0.4%.

[0081] The molten crude steel coming out of the furnace contains, in particular:

[0082] Carbon [C]: Maximum 600 ppm, especially maximum 500 ppm.

[0083] Nitrogen [N]: Maximum 50 ppm, especially maximum 30 ppm.

[0084] Oxygen [O]: at least 300 ppm, not exceeding 2300 ppm.

[0085] Selective phosphorus [P]: 0-400 ppm,

[0086] Selective sulfur [S]: 0-1500 ppm,

[0087] Selective silicon [Si]: 0-300 ppm,

[0088] Selective manganese [Mn]: 0-0.4%,

[0089] The balance is for iron and unavoidable impurities, the total of which does not exceed 2.0% by weight.

[0090] The advantage of the method according to the invention with a converter is that the composition of the smelting slag can be freely adjusted, whereas the slag in a smelting furnace with direct electric arc action is usually optimized for foaming and therefore cannot be arbitrarily changed.

[0091] Therefore, the composition can be adjusted to resemble, for example, blast furnace slag. Thus, smelting slag can be further applied, for example, in the cement industry, similar to blast furnace slag.

[0092] In a preferred embodiment, the smelting furnace with arc resistance heating includes at least one embodiment of the Soderberg. Electrode of an electrode.

[0093] Soderberg The electrode consists of a metal plate jacket with ribs (so-called guide plates) arranged on its inner side. The metal plate jacket is continuously filled with electrode material, for example, in the form of clumps, blocks, or cylinders. Because the electrode wears down at the end facing the melt during operation, the electrode is continuously lowered and refilled with electrode material from the top. Furthermore, the metal plate jacket can be continuously expanded through welding.

[0094] In a preferred embodiment, the melting furnace with arc resistance heating comprises exactly three electrodes and operates on three-phase alternating current.

[0095] In a preferred embodiment, the method includes an upstream direct reduction process for producing directly reduced iron. In this case, the equipment is upstream of a smelting furnace heated by a resistance arc, preferably directly upstream, comprising a direct reduction unit. In this direct reduction process, a solid-state reaction occurs in which oxygen is removed from the iron ore. For this purpose, coal or natural gas has traditionally been used as a reducing agent. Recently, hydrogen has also been frequently proposed as a reducing agent. The reaction takes place below the melting point of the iron ore, so that the external form of the ore remains unchanged. Since the removal of oxygen results in a weight reduction of approximately 27-30%, a honeycomb microstructure (solid porous iron with many air-filled gaps) is produced in the reaction product. Therefore, directly reduced iron is often referred to as sponge iron.

[0096] In a preferred embodiment, the direct reduction apparatus includes a shaft furnace with a reduction zone through which iron ore passes against the reducing gas.

[0097] In a particular variant of this method, the reduction zone is positioned above the cooling zone of the shaft furnace. Iron ore is then passed vertically downwards from the top of the shaft furnace. Due to the chimney effect upon which this shaft furnace is based, both cooling and reducing gases flow efficiently through the iron ore. The reducing gas flows through the reduction zone, specifically against the direction of iron ore movement. Correspondingly, the cooling gas also flows through the cooling zone against the direction of sponge iron production. Thus, a counter-current method is employed in both the cooling and reduction zones to achieve efficient gas-solid reaction.

[0098] In particular, CO or H2, or a mixture of CO and H2, is used as the reducing gas. The reduction reaction here is as follows ("(s)" indicates a solid; curly braces indicate a gaseous substance):

[0099] 3Fe2O3(s)+{CO}=2Fe3O4(s)+{CO2}

[0100] Fe3O4(s) + {CO} = 3FeO(s) + {CO2}

[0101] FeO(s) + {CO} = Fe(s) + {CO2}

[0102] 3Fe2O3(s)+{H2}=2Fe3O4(s)+{H2O}

[0103] Fe3O4(s) + {H2} = 3FeO(s) + {H2O}

[0104] FeO(s) + {H2} = Fe(s) + {H2O}.

[0105] Reducing gases are typically produced from fossil hydrocarbons, such as natural gas or coke oven gas. The following example illustrates a reaction using methane as a starting material. Other hydrocarbons can also be used as starting materials.

[0106] In the first implementation variant, the reducing gas is generated from methane, CO2, and water vapor in a gas reformer. (Process).

[0107] CH4 + CO2 = 2CO + 2H2

[0108] CH4 + H2O = CO + 3H2

[0109] This results in a gas cycle in which fresh methane is mixed with the cleaned exhaust gas from the shaft furnace before the gas reformer. The exhaust gas from the shaft furnace contains carbon dioxide and water vapor as products of the reduction reaction. With the aid of a catalytic reaction in the gas reformer, a reducing gas containing H2 and CO is produced from methane, CO2, and water vapor. This reducing gas is fed back into the shaft furnace, where the iron ore is reduced according to the reaction equation described above. The resulting reaction products are CO2, water vapor, and sponge iron. The carbon dioxide and water vapor, along with the unconsumed reducing gas, are mixed with methane and returned to the gas reformer.

[0110] In alternative implementation variant schemes ( In the process, through catalytic reaction:

[0111] CH4 + H2O = CO + 3H2

[0112] The reducing gas is generated by mixing methane with water vapor, heating it, and passing it through a catalyst. For example, the catalyst could be nickel present in an iron-nickel tube that transports the gas into the shaft furnace. In a particular design of this method, hot sponge iron itself acts as a catalyst in the lower part of the reduction zone. Simultaneously, carbon deposits form on the sponge iron, increasing its carbon content.

[0113] Alternatively, hydrogen is used as a reducing gas, particularly produced via electrolysis through carbon neutralization. In this case, the method further includes the following steps:

[0114] - Iron is produced directly from iron ore by electrolysis using hydrogen generated in a shaft furnace.

[0115] Using hydrogen produced by electrolysis reduces carbon dioxide emissions and the consumption of fossil fuel carriers, thereby improving the carbon balance of the method.

[0116] This hydrogen can either completely replace natural gas as a starting material or be partially added to the methods described above to reduce natural gas consumption. As the proportion of hydrogen increases, the reduction reaction further shifts to the given reaction equation utilizing H2, thus moving away from the three reaction equations utilizing CO.

[0117] In a preferred extension of this method, the direct reduction method includes a carburizing step in which a carbon-containing gas is applied to the produced directly reduced iron, thereby depositing carbon onto the iron. Natural gas or CO2 can be used as the carbon-containing gas. Depending on the gas used, different chemical reaction mechanisms occur during the carburizing reaction. Preferably, the carbon-containing gas is introduced into the cooling zone of the shaft furnace to simultaneously cool and carburize the directly reduced iron produced. Furthermore, the hot directly reduced iron in the cooling zone can also act as a catalyst for the carburizing reaction. The carburizing step increases the carbon content of the directly reduced iron, and therefore also increases the carbon content of the molten metal in the downstream smelting furnace. This has two advantages: first, the melting point of the directly reduced iron decreases, thereby reducing the energy consumption of the smelting furnace. Second, as already explained, the higher carbon content is advantageous for the denitrification mechanism in the downstream converter.

[0118] The present invention further relates to a method for producing ULC steel, particularly IF steel, preferably non-grain-oriented electrical steel strip, comprising the following steps:

[0119] - Low-nitrogen crude steel can be produced using the above method.

[0120] - The crude steel produced undergoes secondary metallurgical treatment.

[0121] - Casting crude steel in a continuous casting facility.

[0122] This method has the same advantages as the method for producing low-nitrogen crude steel described above.

[0123] The secondary metallurgical treatment of the crude steel produced includes, in particular, vacuum treatment.

[0124] In vacuum treatment, the carbon content [C] in the produced crude steel, which is at most 600 ppm, is reduced to a maximum of no more than 150 ppm, preferably 100 ppm, more preferably 50 ppm, and especially 30 ppm, as desired for ULC steel grades. Vacuum treatment is carried out, particularly by means of the Ruhrstahl-Heraeus method. Alternatively, vacuum treatment can also be achieved by means of ladle degassing (Pfannenstandsentgasung).

[0125] The present invention also relates to a complete set of equipment for performing the above-described method. This complete set of equipment includes a melting furnace with arc resistance heating for producing molten metal, having a converter arranged downstream, preferably directly downstream, for refining the molten metal into liquid crude steel.

[0126] Here, the complete set of equipment has the advantages related to the method mentioned above.

[0127] In a preferred embodiment, the equipment includes an upstream direct reduction unit, preferably directly upstream of a melting furnace with electric resistance heating, and a downstream secondary metallurgical unit, preferably directly downstream of the furnace. The advantage of directly connecting the direct reduction unit to the melting furnace is that the produced direct reduced iron can be added to the melting furnace while still hot. This reduces the energy input during the melting operation. Direct connection of the secondary metallurgical unit to the converter is also advantageous, as the molten steel can be directly fed into further processing.

[0128] The present invention also relates to a complete set of equipment for the above-described process of producing ULC steel. This complete set of equipment includes a melting furnace with arc resistance heating for generating molten metal, a downstream converter for refining the molten metal into liquid crude steel, a secondary metallurgical unit downstream of the converter, and a continuous casting unit downstream of the secondary metallurgical unit. The secondary metallurgical unit is particularly implemented as a vacuum degassing unit, preferably an RH unit.

[0129] This invention also relates to the retrofitting of existing blast furnaces and downstream converters used for producing low-nitrogen crude steel by adding an arc resistance-heated melting furnace upstream, preferably directly upstream, of the existing converter, and discontinuing the existing blast furnace. Surprisingly, it has been found that replacing the existing blast furnace with an arc resistance-heated melting furnace upstream of the existing converter can produce low-nitrogen crude steel and significantly reduce carbon dioxide emissions. Until now, such a melting furnace has not been coupled with a separate converter to produce a specific steel grade. Until now, only separate converters combined with blast furnaces are known. According to the invention, it has been recognized that the blast furnace can be replaced by the simple arc resistance-heated melting furnace. This combination achieves the synergistic effects elucidated by the combination method. The effect is particularly evident in the exceptionally low nitrogen content of the produced crude steel. Furthermore, this retrofit can be implemented relatively inexpensively since the existing converter can still be used. Similarly, due to the low nitrogen content, the downstream secondary metallurgical equipment can continue to be used in the same manner. The advantage of this is that it eliminates the need for recertification of the steel production process for the end customer. Since the certification of the production process only relates to the steps connected to the converter, recertification can be avoided if these steps remain unchanged. The modification of this invention precisely allows these steps to be carried over from the blast furnace process unchanged.

[0130] This invention also relates to the retrofitting of an existing complete set of equipment for producing ULC steel, comprising a blast furnace, an existing converter located downstream of the blast furnace, and secondary metallurgical equipment downstream of the converter. The method includes adding a melting furnace with arc resistance heating upstream of the existing converter, preferably directly upstream, and decommissioning the existing blast furnace. Here, this method of retrofitting an existing complete set of equipment for producing ULC steel has the same advantages as the method described above for retrofitting an existing complete set of equipment for producing low-nitrogen crude steel, because low-nitrogen crude steel is used as the starting material for producing ULC steel.

[0131] In a preferred variant, both of the above modifications include adding a direct reduction unit upstream of the smelting furnace with arc resistance heating, preferably directly upstream. The advantage of directly connecting the direct reduction unit to the smelting furnace is that the directly reduced iron produced can be added to the furnace while still hot. This reduces energy consumption in the smelting operation. Attached Figure Description

[0132] The invention is illustrated in more detail with the aid of the accompanying drawings. In the drawings:

[0133] Figure 1 A flowchart of a method for producing crude steel according to the present invention is shown.

[0134] Figure 2 A schematic diagram of a melting furnace with arc resistance heating is shown.

[0135] Figure 3 A schematic diagram of the converter is shown.

[0136] Figure 4 A schematic diagram of the direct restoration device is shown. Detailed Implementation

[0137] Figure 1 A flowchart of a method for producing low-nitrogen crude steel according to the present invention is shown. In a first optional step, direct reduced iron is produced from iron ore in a shaft furnace. Alternatively, direct reduced iron can also be purchased. In a subsequent step, the direct reduced iron is introduced into a melting furnace with arc resistance heating. Alternatively, scrap can also be introduced into the melting furnace. In the melting furnace, the iron and / or scrap are melted into molten metal and slag. Subsequently, the molten metal is removed from the melting furnace and fed into a converter. In the converter, the molten metal is refined into liquid crude steel. The liquid crude steel is then discharged from the converter.

[0138] Figure 2A melting furnace 13 with resistance heating in the form of an electric arc reduction furnace (submerged electric arc furnace, SAF) is shown. The melting furnace 13 includes a furnace chamber 15 lined with refractory material 17. Three electrodes 21, operating on alternating current, extend into the interior space 19. Molten metal 23 is already present within the interior space 19. A layer of slag 25 is deposited on the molten metal 23. The three electrodes 21 extend into the slag 25. Thus, a current is formed between the electrodes 21, extending through the slag layer 25 and heating the slag layer 25 by resistance heating. This heating is transferred from the slag layer 25 to the molten metal 23. The interior space 19 is closed upwards by a cover 29 through which the three electrodes 21 extend. The electrodes 21 are designed in a Söderberg configuration. electrode.

[0139] Figure 3 A converter 31 is shown. The converter 31 includes a converter chamber 33 with a refractory lining 35. Molten metal 37 is contained within the converter chamber 33. A nozzle 39 extending from the top into the converter chamber 33 can be used to blow oxygen onto the surface of the molten metal 37. The converter 31 is closed upwards by a cover 38 through which the nozzle 39 is guided. A nozzle 43 is located at the bottom 41 of the converter, through which inert gas is blown into the converter 31. A discharge port 45 is located on the side of the converter 31, through which molten crude steel can be removed by tilting the converter chamber 33 after refining.

[0140] Figure 4 A schematic diagram of a direct reduction apparatus 51 is shown. The direct reduction apparatus 51 includes a shaft furnace 53. A reduction zone 55 and a cooling zone 57 are arranged within the shaft furnace 53. The reduction zone 55 is positioned above the cooling zone 57. Iron ore is charged into the shaft furnace 53 from the top. At the lower end of the shaft furnace 53, the directly reduced iron produced can be removed. Simultaneously, reducing gas enters the shaft furnace 53 through an inlet 59. The reducing gas then flows through the iron ore in the reduction zone 55. Unconsumed reducing gas, along with any gaseous reaction products, is discharged again at an outlet 61. Therefore, the reducing gas flows through the reduction zone 55 against the direction of iron ore movement. After leaving the reduction zone 55, the directly reduced iron enters the cooling zone 57. In the cooling zone 57, cooling gas flows through the sponge iron against the direction of iron movement. For this purpose, the cooling gas enters the shaft furnace 53 through an inlet 63. Unconsumed cooling gas, along with any gaseous reaction products, is discharged again at an outlet 65. Alternatively, a certain proportion of the cooling gas may enter the reduction zone 55. Similarly, a certain proportion of reducing gas can enter the cooling zone 57. The cooling gas is preferably carbon-containing so that the directly reduced iron produced will undergo carburization.

Claims

1. A method for producing low-nitrogen crude steel, the method comprising the following steps: - The directly reduced iron and / or scrap are melted into molten metal and slag (25) in a closed smelting furnace (13) with electric arc resistance heating in a reducing atmosphere. - The molten metal is removed from the smelting furnace (13) and filled into the converter (31). - The molten metal is refined into liquid crude steel in a converter (31), and the liquid crude steel with a nitrogen content [N] of 50 ppm or lower is discharged, wherein the nitrogen content [N] is reduced when the nitrogen content [N] of the molten metal is higher than 50 ppm, or the nitrogen content is maintained at a low level or further reduced when the nitrogen content [N] is lower than 50 ppm, wherein, Directly before refining, the ratio of carbon content to nitrogen content [C] / [N] in the molten metal is at least 20, wherein the carbon content [C] in the molten metal directly before refining is at least 1.5% and the nitrogen content [N] is at most 450 ppm.

2. The method according to claim 1, characterized in that, The carbon content [C] of the molten metal is increased in the smelting furnace (13) and / or converter (31).

3. The method according to claim 1 or 2, characterized in that, Before refining, the carbon-to-nitrogen ratio [C] / [N] of the molten metal must be at least 100.

4. The method according to claim 1, characterized in that, The iron content (Fe) of the slag (25) in the smelting furnace (13) is less than 30% by weight.

5. The method according to claim 1, characterized in that, The molten metal directly before refining has the following associated element content: Carbon [C]: at least 1.5%, maximum 5.0%, Selective oxygen [O]: 0-50 ppm, Selective phosphorus [P]: 100-1500 ppm, Selective sulfur [S]: 0-1500 ppm, Selective silicon [Si]: 0-1.5%, Selective manganese [Mn]: 0-0.5%.

6. The method according to claim 1, characterized in that, The discharged molten crude steel has the following accompanying element contents: Carbon [C]: Maximum 600 ppm, Nitrogen [N]: Maximum 50 ppm, Oxygen [O]: at least 300 ppm, maximum 2300 ppm. Selective phosphorus [P]: 0-400 ppm, Selective sulfur [S]: 0-1500 ppm, Selective silicon [Si]: 0-300 ppm, Selective manganese [Mn]: 0-0.4%.

7. The method according to claim 1, characterized in that, In the refining process, oxygen is blown into the molten metal through a retractable water-cooled nozzle for a period of at least 10 minutes, and argon is selectively blown in through a nozzle (43) in the bottom (41) of the converter.

8. The method according to claim 1, further comprising the following steps: - Iron is produced directly from iron ore in a shaft furnace (53) using hydrogen generated by electrolysis or by using natural gas or coke oven gas.

9. A method for producing ULC steel, the method comprising the following steps: - Producing low-nitrogen crude steel by the method according to claim 1, - The crude steel produced undergoes secondary metallurgical treatment. - Casting crude steel in a continuous casting facility.

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