Method for producing molten steel with reduced carbon footprint and high-grade steel products produced thereby

Abstract

The present invention relates to a method for producing molten steel with a reduced carbon footprint for casting into cast products in a continuous casting machine.

Description

Technical Field

[0001] The present invention relates to a method for producing molten steel with a reduced carbon footprint for casting into a cast product in a continuous casting machine.

Background Art

[0002] Steel is the most important engineering and construction material in the world. Steel is used in all aspects of our lives, ranging from automobiles and construction products, refrigerators and washing machines, to cargo ships and surgical scalpels. Steel is completely recyclable, has excellent durability, and requires a relatively small amount of energy for production compared to other engineering materials. Innovative lightweight steels (e.g., those used in automobiles and buildings) contribute to energy and resource conservation. The steel industry has made enormous efforts over the past decades to suppress environmental pollution. Today, the energy required to produce one ton of steel has decreased to 40% of that in 1960. Dust emissions have been reduced even further.

[0003] {> However, as governments and industries around the world aim to reduce carbon emissions to achieve ambitious net-zero goals, steel has also come under the spotlight. Currently, approximately 75% of steel is still produced in blast furnaces (BF) fueled by coal, and these blast furnaces emit large amounts of carbon dioxide into the atmosphere. Overall, steel production accounts for approximately 8% of the world's CO2 emissions. As CO2 reduction measures, options such as carbon capture and reuse, carbon capture and storage (e.g., storage in depleted gas fields) have been intensively studied, but these options are rather temporary bridging solutions rather than ultimate solutions towards a carbon-free steelmaking process.

[0004] Therefore, it is clear that cleaning up one of the world's largest sources of CO2 emissions is a crucial key to addressing climate change. Governments and steel companies around the world are setting ambitious targets to achieve global climate goals / climate targets.

[0005] In pursuing these goals, many companies, including Tata Steel of the Netherlands, are beginning to move towards "green steel." Green steel means producing steel without using carbon-based fossil fuels, and one of the most promising technologies is to switch the ironmaking process from coal-based to hydrogen-based. This alternative ironmaking route includes, for example, a combination of producing pre-reduced pellets (DRI) using a gas-based direct reduction plant (DRP) and then producing hot metal (HM) by melting in a reducing electric furnace (REF). Unfortunately, green hydrogen (derived from renewable energy) is not yet readily available in sufficient quantities. Therefore, as an intermediate step, natural gas-based processes are being developed that can be relatively easily converted to hydrogen-based processes in the future.

[0006] However, even setting aside the challenge of procuring sufficient amounts of green hydrogen, the carbon-free ironmaking route itself still faces many challenges. For example, obtaining the same carbon content in molten iron (HM) from a carbon-free ironmaking process as in conventional blast furnace pig iron is a major challenge. Conventional blast furnace pig iron has a high carbon content, typically in the range of 3.8 to 4.7 mass%. The carbon in molten iron (HM) is important for subsequent processes in the LD steelmaking process (also known as the Linz-Donawitz process or Basic Oxygen Steelmaking (BOS)). It has been found that simply adding carbon in REF (or EAF) to raise the carbon content of molten iron (HM) to the level of conventional pig iron is not sufficiently efficient. This method can significantly reduce the dissolved carbon content in the molten iron (HM), potentially to around 2.5 to 3.5 mass%.

[0007] Traditionally, LD converters are charged with molten iron (HM) obtained from the ironmaking process, along with steel scrap or iron scrap (approximately 15-30%). The thermal energy required to melt the scrap and refine the resulting molten metal into steel is generated during the oxidation process. Subsequently, a water-cooled lance equipped with a nozzle is lowered into the furnace / converter, and oxygen is blown onto the molten metal. This oxidizes the carbon dissolved in the molten metal, raising the average temperature of the molten metal to approximately 1700°C. Therefore, oxygen blowing helps to melt the scrap and mix it into the molten metal, reducing the overall carbon content of the molten metal and helping to remove unwanted chemical elements such as nitrogen, silicon, and phosphorus.

[0008] The presence of carbon in molten iron (HM) not only lowers the liquidus temperature, allowing for operation at lower temperatures, but also generates CO bubbles, which are essential for removing dissolved gases such as nitrogen and hydrogen in the LD process. Molten iron (HM) obtained through alternative blast furnace processes is expected to have a lower carbon content, as carbon usage is significantly reduced, or (ultimately) zero or low, and therefore the efficiency of dissolved gas removal decreases. In other words, the ability to remove nitrogen (and / or hydrogen) from the molten metal during the LD process is negatively affected by the low carbon content. Furthermore, when using green hydrogen-based ironmaking processes, hydrogen reduction can cause hydrogen to diffuse into the DRI pellets, potentially increasing the hydrogen content in the molten iron (HM). Therefore, the initial amounts of dissolved nitrogen and hydrogen in the molten iron (HM) may be too high compared to conventional blast furnace pig iron.

[0009] Attempts to increase the carbon level of REF-HM to mimic conventional blast furnace pig iron have so far yielded disappointing results. [Overview of the project] [Problems that the invention aims to solve]

[0010] Purpose of the invention The objective of the present invention is to provide an alternative refining process for producing high-grade steel through a steelmaking route that significantly reduces the amount of carbon used.

[0011] Furthermore, an object of the present invention is to provide an alternative refining process for producing high-grade steel through a CO2-neutral steelmaking route.

[0012] Furthermore, an object of the present invention is to provide a refining process for producing steel via a CO2-neutral steelmaking route that can process molten iron having large variations in composition, particularly in carbon content. [Means for solving the problem]

[0013] Description of the Invention In the first embodiment, the present invention is A method for producing molten steel for casting into a cast product in a continuous casting machine, comprising the following sequential steps: A process of producing directly reduced iron (DRI) from iron ore using reducing gas in a direct reduction plant (DRP) or a fluidized bed reactor; A process of producing molten iron (HM) by melting the DRI in an electric smelting furnace (ESF); If necessary, a step of desulfurizing the HM in a desulfurizing vessel; A step of removing nitrogen and hydrogen from HM in a vacuum degassing vessel; (a) A step of reducing one or more of carbon, silicon, and phosphorus in the HM in an LD converter (LD-vessel); (b) A step of reducing one or more of carbon, silicon, and phosphorus in the HM in an oxidizing ladle furnace, and then, if necessary, adding a deoxidizing agent such as Al, Si, or Mn in the furnace for alloy composition adjustment and / or deoxidation of the HM; (c) A step of reducing one or more of carbon, silicon, and phosphorus in the HM in a vacuum refining furnace; A step of producing molten steel by performing deoxidation and / or alloy trimming on the composition of HM, in a secondary metallurgical facility, preferably selected from the group comprising a ladle furnace (LF), a stirring station, or CAS(-OB), as needed; The process of manufacturing cast products by continuously casting the molten steel. The method including It will be materialized in this way.

[0014] The method according to the present invention makes it possible to produce molten steel suitable for casting into cast products such as slabs, thin slabs, strips, blooms, billets, or ingots. The preceding steps allow the method to produce molten steel having the same or similar properties and composition as molten steel currently produced by the conventional blast furnace-BOS route. A REF or submerged arc furnace (SAF) may also be called an electric smelting furnace (ESF). In this specification, ESF encompasses certain furnaces, including reducing electric furnaces (REF) and submerged arc furnaces (SAF).

[0015] This method also fulfills the objective of the present invention to provide an alternative refining process for producing high-grade steel through a steelmaking route that significantly reduces the amount of carbon used or a CO2-neutral steelmaking route.

[0016] This method is based on the production of DRI, which may be in the form of hot briquetted iron (HBI) as needed. HBI is a form of DRI obtained by compressing at high temperatures (usually above 650°C) during compression, and is generally 5000 kg / m³. 3 It has a density exceeding [a certain value]. By using HBI, problems associated with the transport and handling of DRI can be overcome. This is because the compression process makes HBI less porous than DRI, and therefore less reactive, and less susceptible to the risk of self-heating associated with DRI. Other forms of DRI include pellets and lumps. DRI fines can be used as cold briquetted iron after coagulation.

[0017] In one embodiment, iron ore is supplied directly to the reduction plant in the form of coagulated iron ore lump, pellets, or a combination thereof. The advantage of this form is that it is readily available and easy to handle in many iron and steel plants.

[0018] The reduction of iron ore is carried out in a direct reduction plant (DRP). Currently, the majority of DRPs are gas-based gravity shaft furnace processes, which account for more than 75% of DRI production (as of 2019), gas-based fluidized bed processes, which account for less than 1% of DRI production, and coal-based rotary kiln furnaces, which account for the remainder of DRI production. Therefore, the direct reduction plant in the method according to the present invention is preferably a reducing gas-based (gravity) shaft furnace. Depending on the selected process, the feedstock may be fine iron ore, agglomerated iron (e.g., sintered ore), or pellets.

[0019] The typical carbon content of DRI and HBI is approximately 0.5 to 2.5 mass%, and the metallization rate can exceed 90 mass% Fe. DRI (not HBI) is usually manufactured in the form of small pellets. The typical average diameter of these DRI pellets ranges from 1 to 1.5 cm. It is important to note that DRI or HBI may still contain some unreduced iron oxide.

[0020] These pellets or briquettes are then introduced into an electric smelting furnace (preferably a REF) and melted. The reducing atmosphere in the electric furnace promotes further reduction of any unreduced iron oxide remaining in the DRI or HBI, thereby increasing the yield of metallic iron. The resulting molten iron (HM), composed mainly of metallic iron, is further refined in subsequent processes.

[0021] The reducing gas in the DRP is rich in hydrogen and may contain carbon monoxide. This is particularly applicable when the reducing gas is supplied based on natural gas (NG) consisting of hydrocarbons. When using NG, not only carbon monoxide and hydrogen are formed by decomposition, but NG can also be used, for example, for carburizing pre-reduced DRI in the carburizing (cold) zone of DRP. In one embodiment, an additional carbon-containing compound can be optionally added to the ESF. This may be in the form of coal, charcoal, graphite, biomass, etc. Thereby, the amount of carbon in the hot metal (HM) increases, which is beneficial for the method of the present invention.

[0022] After melting the DRI, the produced hot metal (HM) is desulphurized (de-S) in a desulphurization vessel if necessary. Also, since the main origin of sulfur in the hot metal (HM) is coal, it is expected that when the amount of coal used decreases, the need for desulphurization will correspondingly decrease. If the amount of sulfur in the hot metal (HM) is less than a predetermined value, this desulphurization step can be omitted. In one embodiment, if the amount of sulfur in the hot metal (HM) is less than 0.01% by mass, the hot metal (HM) is not desulphurized.

[0023] It should be noted that the following abbreviations are used in the context of the present invention: de-S, de-N, de-H, de-Si, de-P, de-C and de-O. These respectively mean desulphurisation, de-nitrogenization, de-hydrogenisation, de-siliconisation, de-phosphorisation, de-carburisation and de-oxidation.

[0024] The next steps are the de-nitrification (de-N) and dehydrogenation (de-H) steps. This step is preferably carried out in a vacuum degassing apparatus such as a Ruhrstahl Heraeus (RH) apparatus or a vacuum tank degasser. In the prior art process, this step also includes decarburization (de-C), but in the method according to the present invention, decarburization is carried out in the next step, i.e., in the LD converter, so decarburization in this step is not required. Therefore, unlike the prior art and the normal BF-LD route, in the method of the present invention, the degassing treatment is carried out "before" rather than "after" the molten metal is treated in the LD converter. Thereby, the advantage that the oxygen level (which is a surface-active solute that hinders the removal of dissolved gas) is low is obtained, and it is also possible to eliminate or reduce the necessity of CO bubbles for degassing in the subsequent converter. Instead of the RH apparatus, a vacuum tank degassing apparatus (VTD) (also known as a Vacuum Degasser (VD)) treatment may be used to reduce the oxygen level. In VTD or VD, de-gassing (de-N and de-H) and decarburization (de-C) can be carried out, and the best conditions for desulfurization (de-S) and inclusion removal are provided. During VTD or VD treatment, the liquid steel ladle is placed in a vacuum tank that can be stationary or movable according to the requirements of a specific equipment layout. To promote stirring, an inert gas is injected through a porous plug. VTD or VD can be equipped with an additional oxygen lance (VD-OB) to obtain further advantages such as forced decarburization, chemical heating (e.g., chemical heating by adding Al).

[0025] In the LD converter, de-siliconization (de-Si) and de-phosphorization (de-P) are also carried out. This is the same as the conventional LD route. The main difference is that in the method according to the present invention, since most of the nitrogen has already been removed in the previous vacuum degassing step, the necessity of de-nitrification (de-N) in the LD is reduced.

[0026] The final step before transferring the molten steel to the casting process is secondary metallurgy in a ladle furnace (LF) facility, where deoxidation (de-O) and final adjustment of the steel alloy composition are performed on the molten iron (HM). The LF can also heat the molten steel so that it reaches the casting process at the operating temperature for casting.

[0027] This route can accommodate fluctuations in the carbon content of molten iron (HM). The carbon in the molten iron (HM) may originate from the carburizing of NG in the carburizing (cold) zone of the DRP, or from carbon-containing compounds added to the electric smelting furnace such as REF. In either case, the route of the present invention, which includes vacuum degassing followed by refining in LD, OLF, or VRP, can process molten iron (HM) with fluctuating carbon content and produce molten steel with a desired chemical composition and temperature.

[0028] In a preferred embodiment, one or more of carbon, silicon, and phosphorus are removed in the LD converter. In this embodiment, a conventional BOS plant can still be used with a modified configuration.

[0029] In one embodiment, at least a portion of the reducing gas in a DRP is produced by chemically reforming a mixture of natural gas and exhaust gas from a reducing furnace (DRP and / or REF) to produce a reducing gas rich in hydrogen and carbon monoxide.

[0030] In one embodiment of the present invention, at least a portion of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas, hydrogen, and exhaust gas from a reducing furnace (DRP and / or REF) to produce a reducing gas rich in hydrogen and carbon monoxide.

[0031] Other alternative sources of reducing gas for DRP include hydrogen (H2)-rich coke oven gas, carbon monoxide (CO)-rich BOF gas, or synthesis gas produced by the gasification of solid fuels (such as coal, coke, municipal waste, and other hydrocarbons).

[0032] In one embodiment, at least a portion of the reducing gas in the DRP consists primarily of hydrogen. A small amount of other gases may be present that do not significantly affect the hydrogen reduction process. If no carbon-containing gases or compounds are used in the production of molten iron and molten steel in this process, no carbon is present in the reducing gas, and therefore no carburizing occurs in the DRP. If carbon is present in the DRI, in this embodiment, that carbon must be added to the DRP feedstock or to the ESF as described above. A low carbon content in the DRI tends to result in a low sulfur content. This is because carbon-containing compounds such as coal are the primary source of sulfur. In this embodiment, since no carbon is added to the ESF, the sulfur content remains low. This low sulfur level may be sufficiently low for many steel grades. Therefore, the desulfurization (de-S) process in the desulfurization vessel is not always necessary. The desulfurization (de-S) process is optional and not mandatory.

[0033] In one embodiment, the reducing gas contains at least 95 mol%, preferably at least 98 mol%, and more preferably at least 99 mol% hydrogen.

[0034] In one embodiment, one or more carbon-containing compounds are added to ESF, preferably REF, to increase the carbon content in the molten iron. This is an option in processes where increasing the carbon level in the molten iron (HM) is desirable. The carbon-containing compounds may be in the form of coal, charcoal, graphite, biomass, biocoke, etc.

[0035] In a preferred embodiment, molten steel for casting into a cast product in a continuous casting machine is produced through the following continuous steps: The process of producing DRI by reducing iron ore, where the reducing gas in DRP is mainly composed of hydrogen; A process of melting the DRI in the ESF to produce molten iron (HM); If necessary, a step of desulfurizing the HM in a desulfurization vessel; A step in which nitrogen and hydrogen are removed from the HM in a vacuum degassing vessel; (a) A process of removing silicon, phosphorus, and optionally carbon in an LD converter, (b) an oxidizing ladle furnace, or (c) a vacuum refining furnace; A process in which molten steel is produced by deoxidizing and adjusting the composition of the HM in LF; The process of manufacturing cast products by continuously casting the molten steel. It is manufactured by a method that includes [a specific process].

[0036] Denitrification (de-N) and dehydrogenation (de-H) are carried out in the vacuum degassing vessel described herein, and then decarburization (de-C), silicon removal (de-Si), and phosphorus removal (de-P) are carried out in one of the following modified smelting furnaces: a. LD converter, or b. Oxidizing ladle furnace (OLF), or c. Vacuum refining furnace (VRF).

[0037] The distinctive features of these three refining options in relation to the method according to the present invention are as follows:

[0038] LD converters (so-called converters in BOS plants) can flexibly respond to fluctuations in the carbon content of the incoming molten iron (HM). Oxygen blowing (O2 blowing) promotes decarburization (de-C), which can then aid in further denitrification (de-N). Oxygen blowing also enables rapid dephosphorization (de-P) and desiliconization (de-Si). When carbon is absent, care must be taken to avoid excess FeO. Oxygen blowing (top blowing and / or bottom blowing, depending on the smelting furnace used) must be carefully controlled to form the appropriate amount of FeO necessary for dephosphorization (de-P), and dilution of the oxygen (O2) flow, for example, dilution of the oxygen (O2) flow with argon gas, may be necessary. To prepare oxygen blowing, it is preferable that the carbon content in the molten iron (HM) is accurately known in advance. Dephosphorization (de-P) proceeds via a slag / metal reaction, and there is no gas-slag-metal emulsion with respect to dephosphorization (de-P).

[0039] An oxidizing ladle (OLF), as its name suggests, is a ladle furnace with an oxidizing atmosphere. OLFs are equipped with electrodes that can raise the temperature of molten iron (HM). This is particularly important for low-carbon (low-C) molten iron (HM) because the lower the carbon content in the molten iron (HM), the higher the liquidus temperature of the molten iron (HM). Bottom-blowing of oxygen is possible, which allows for more effective interaction between the slag and the metal, and is important for dephosphorization (de-P) (e.g., the Oxygen Bottom Max Huette process). Top-blowing or side-blowing of oxygen is also possible. Oxygen blowing also promotes decarburization (de-C) of incoming molten iron (HM) with fluctuating carbon content. This can also produce some denitrification (de-N) depending on the relative carbon and nitrogen levels in the bath. Sufficiently large free-board space is required for the slag produced by desiliconization (de-Si). Following the treatment in the OLF, if necessary, a deoxidizing agent such as Al, Si, or Mn (one or more combinations) may be added in the furnace to deoxidize and / or adjust the alloy composition, thereby deoxidizing the molten steel.

[0040] A vacuum refining furnace (VRF) performs the vacuum refining process. This furnace is similar to a vacuum oxygen decarburization unit (VOD). This furnace enables degassing (denitrification (de-N) and dehydrogenation (de-H)), followed by refining by oxygen blowing. Therefore, in the process route of the present invention, vacuum degassing equipment such as an RH (Ruhrstahl-Heraeus) vessel is not necessarily required, or can be omitted. After degassing, dephosphorization (de-P), desiliconization (de-Si), and decarburization (de-C) (depending on the amount of carbon in the molten iron (HM)) can be performed by controlled oxygen blowing. A sufficiently large freeboard space is required because slag is generated by desiliconization. The vacuum refining furnace may also be applicable to desulfurization (de-S). Since the vacuum refining furnace can also handle denitrification and dehydrogenation, a vacuum degassing vessel is not required.

[0041] Preferably, silicon and phosphorus, and optionally carbon, are removed in the LD converter. The advantage of this option is that many steelmaking plants can utilize these facilities.

[0042] Finally, the last step before transferring the molten steel to the casting process is processing in a conventional ladle furnace (LF), where deoxidation (de-O) and final alloy composition adjustment are performed. If this deoxidation and final alloy composition adjustment have already been performed in the preceding oxidizing ladle furnace (OLF), this final step is not necessarily required before transfer to the casting process, and the molten steel can be transferred to the casting process after the completion of processing in the oxidizing ladle furnace (OLF). The ladle furnace (LF) can also heat the molten steel, thereby ensuring that the molten steel reaches the casting process at an appropriate temperature suitable for casting. Furthermore, a stirring station can be used to homogenize the temperature and chemical composition of the molten steel throughout the ladle. The molten steel can be stirred by injecting argon, for example, through a refractory-lined lance or through a permeable refractory block located at the bottom of the ladle. Alternatively, stirring with an electromagnetic coil is also possible. The addition of alloying elements can be carried out at the stirring station.

[0043] This method for producing carbon-free molten steel is achieved when the reducing gas in the direct reduction plant (DRP) consists solely of hydrogen and no carbon-containing compounds are added to the electric smelting furnace (ESF). Nevertheless, if carbon still remains in the molten iron due to drag-in through the feedstock of the direct reduction plant (DRP), variations in the carbon content in the molten iron can be easily treated by an LD converter, an oxidizing ladle furnace (OLF), or a vacuum refining furnace (VRF).

[0044] In one embodiment, a method for producing molten steel for casting into a cast product in a continuous casting machine comprises the following series of steps: The process of producing DRI by reducing iron ore, where the reducing gas in DRP is mainly composed of hydrogen; A process of melting the DRI in the ESF to produce molten iron (HM); If necessary, a step of desulfurizing the HM in a desulfurization vessel; A step in which nitrogen, hydrogen, silicon, phosphorus, and optionally carbon are removed from the HM in a vacuum refining furnace; A process in which molten steel is produced by deoxidizing and adjusting the composition of the HM in LF; The process of manufacturing cast products by continuously casting the molten steel. Includes.

[0045] In this embodiment, a separate vacuum degassing step for removing nitrogen (de-N) and hydrogen (de-H) from molten iron (HM) is not required.

[0046] In one embodiment, if a vacuum refining furnace is present, a vacuum degassing step for removing nitrogen (de-N) and hydrogen (de-H) from molten iron (HM) is not provided.

[0047] A potential problem with DRI pellets and REF molten iron (HM) containing no carbon is that the liquidus temperature of the molten metal becomes very high, increasing the risk of solidification. The lower the carbon content in the molten iron (HM), the higher the melting point (see the iron-carbon binary phase diagram in Figure 4). This can limit the productivity of ESF. Therefore, the lower the carbon content of the molten iron (HM), the higher the temperature of the molten iron (HM) should be. The temperature can be increased by applying superheat in REF, vacuum degassing furnaces (RH or VTD), conventional ladle furnaces, or oxidizing ladle furnaces (OLF). For example, superheat can be applied by electrode heating or by adding fuels such as Fe-Si or Al.

[0048] To avoid excessive superheating, it may be practical to retain a certain amount of carbon in the process chain (natural gas-derived carbon in DRP or carbon source-derived carbon in ESF). Alternatively, renewable forms of carbon sources, such as bio-coke, can be added. The use of renewable green carbon sources is considered a carbon-negative contribution.

[0049] If, at any point in the method according to the present invention, the need to add carbon to molten iron (HM) results in the generation of CO and / or CO2, the CO and / or CO2 can be recovered, purified, and reused (e.g., as fuel or as a raw material in other chemical processes). Carbon Capture, Utilization, and Storage (CCUS) can be used to remove trace amounts of residual CO2 generated in ironmaking processes (DRP and ESF) and steelmaking processes (LD, OLF, or VRF).

[0050] In a second aspect, the present invention is also embodied in molten steel produced by the method of the present invention, and in steel products produced by casting and further processing this molten steel.

[0051] The present invention will be further illustrated by the following non-limiting embodiments. [Examples]

[0052] Average carbon content [C] in ref molten iron (HM) HM Assuming a carbon content of 3 mass%, based on "simple" thermodynamics, the predicted nitrogen content is approximately twice as high as that observed in typical blast furnace (BF) molten iron. The results are shown in Figures 5 and 6. The calculated nitrogen content of BF molten iron (approximately 90 ppm) is in good agreement with the value actually observed in BF-HM (60-80 ppm). On the other hand, if REF molten iron (HM) contains approximately 3 mass% carbon, the nitrogen content [N] is calculated to exceed 160 ppm under atmospheric pressure conditions of 1500-1550°C.

[0053] Such high initial nitrogen levels require higher denitrification rates or longer refining times in primary (BOF / EAF) or secondary (RH / VTD) steelmaking compared to current operating conditions. As shown in Figure 6, sensitivity analyses with temperature and nitrogen partial pressure as parameters have also been performed to show the variability in nitrogen levels in Fe-C molten metal. Possible sources of nitrogen pickup in REF molten metal are as follows:

[0054] 1.Feed material: • DRI / HBI (20-30 ppm nitrogen), scrap (20-220 ppm nitrogen) • Carrier gas (gas used to transfer high-temperature DRI to REF) • Intrusion from the atmosphere (the atmosphere contains 78 vol.% nitrogen) • Natural gas (containing 1-14 vol.% nitrogen) used in DRI carburizing. 2. Dissociation of N2 molecules may occur around the arc: • Extremely high temperatures can be the cause. • If a thick REF slag layer is present, there is a "possibility" that absorption will be suppressed. 3. Air entrainment into the tapping stream: A diffused stream provides a larger interfacial area for nitrogen absorption; A compact stream is preferable. 4. Absorption from the atmosphere during the following transfer operations: • Runner Torpedo • Molten iron pot (HM ladle)

[0055] The process parameters were varied as follows: ·DRP-REF: • A natural gas-based DRI with a carbon content of 4.5% by mass was used as the starting point. By changing the degree of reduction in REF, the amount of carbon in the molten iron (HM) changes. The carbon content varied within the range of 2.5 to 4.5 mass%. Other components were also reduced to varying degrees. By adding different amounts of carbon and flux, variations in the molten iron (HM) composition were obtained. The predicted nitrogen content based on the thermodynamic solubility model fluctuated within the range of 130–220 ppm.

[0056] ·RH: • Industrial standard operating conditions, including vacuum (minimum value 5 mbar) and argon flow rate, were used. The final nitrogen concentration in RH was found to vary within the range of 25–35 ppm. • When the vacuum pressure was set to 1 mbar, the nitrogen content decreased (a decrease of 5-8 ppm).

[0057] ·LD / BOF: Based on the carbon content in HM, the blowing time (blow time) and scrap ratio were varied. The refining of other components such as C, Mn, P, and Si was simulated. The final values ​​were similar to those of conventional LD ​​operations. • In the converter, some additional nitrogen removal occurred due to decarburization (de-C). Nitrogen removal in the converter was predicted based on a first-order reaction rate model. The final nitrogen amount remained well within the range of conventional LD ​​operations.

[0058] Variations in the temperature and chemical composition of the molten metal are shown in Figures 8A to 8E.

[0059] The present invention is illustrated by the following non-limiting drawings. [Brief explanation of the drawing]

[0060] [Figure 1]Figure 1 shows the layout of each step in the steelmaking process, with the converter (LD unit) and vacuum degassing unit (VTD / RH-OB) or ladle furnace treatment (LF) arranged in the conventional order. [Step I] DRI production using NG (reduction of iron ore, carburizing of DRI); [Step II] Melting of DRI in ESF (REF), carburizing of HM; [Step III] Desulfurization of HM in a desulfurization vessel; [Step IV] Steelmaking in the LD converter (de-C, de-Si, de-P, and de-N); [Step V] Ladle furnace (de-O and de-S) or VTD / RH-OB (de-C, de-N, and de-H); [Step VI] Casting [Figure 2] Figure 2 shows the layout of each step in the steelmaking process, with the converter (LD unit) and vacuum degassing unit (VTD / RH-OB) or ladle furnace treatment (LF) arranged in the order according to the present invention. [Step I] DRI production using NG and H2 (reduction of iron ore, carburizing of DRI); [Step II] Melting of DRI in ESF (REF), carburizing of HM; [Step III] Desulfurization of HM in a desulfurization vessel; [Step IV] Ladle furnace (de-O and de-S) or VTD / RH-OB (de-C, de-N and de-H); [Step V] Steelmaking in LD converter (de-C, de-Si, de-P and de-N); [Step VI] Ladle furnace (de-O, alloy composition adjustment); [Step VII] Casting [Figure 3]Figure 3 shows the layout of each step in the steelmaking process, with the converter (LD unit) and vacuum degassing unit (VTD / RH-OB) or ladle furnace treatment (LF) arranged in the order according to the present invention. Step III (de-S), which is an optional step, is unnecessary because the DRI process and REF are no longer based on coal addition, but if necessary, de-S can be performed in step IV. There are three refining options in the process, one of which is the converter (LD unit). In one embodiment, if a vacuum refining furnace (VRP) is selected as the refining option, the VTD / RH-OB can be omitted. The VRP can also remove N and H. [Process I] DRI production by H2 (reduction of iron ore); [Process II] Melting of DRI in ESF (REF), carburizing of HM; [Process IV] VTD / RH-OB (de-C, de-N, de-H, de-S if necessary); [Process V] Steelmaking by one of the following: (a) LD converter (de-C, de-Si, de-P), or (b) OLF (de-C, de-Si, de-P), or (c) VRP (de-C, de-Si, de-P, de-N, de-H); [Process VI] Ladle furnace (LF) (de-O, alloy composition adjustment); [Process VII] Casting [Figure 4] Figure 4 shows the Fe-C binary phase diagram. [Figure 5] Figure 5 shows the nitrogen solubility of REF molten iron (HM) and BF molten iron (HM) under conditions of 1500°C and PN2 = 0.79 atm. The black triangles represent experimental values ​​quoted from the paper "Thermodynamics of Nitrogen in Fe-Mn-Al-Si-C Alloy Melts" by Min-Kyu Paek, Saikat Chatterjee, Jong-Jin Pak, and In-Ho Jung, Met&Mat Trans B, Vol. 47 (2016), pp. 1243-1262. These experimental values ​​are in very good agreement with the calculated results of nitrogen content relative to carbon content in molten metal (black dotted line). [Figure 6]Figure 6 shows nitrogen solubility under various possible molten iron temperatures and atmospheric pressure (PN2 approximately 0.79 atm). The solid phase begins to form at 1450°C and in the low carbon content region. [Figure 7] Figure 7 shows the operating sequence of each apparatus used to obtain the results shown in Figure 8. 1: DRP, 2: REF, 3: Vacuum degassing (RH / VTD), 4: BOF (modified refining) [Figures 8A-8E] Figures 8A–8E show the variability of carbon, nitrogen, sulfur, and phosphorus in molten iron (HM). Different lines represent different REF molten iron (HM) (see Figure 9). These lines correspond to different input conditions and are intended to show the distribution range rather than specifying individual absolute values. [Figure 9] Figure 9 is a table showing the temperature and composition variations of REF molten iron (HM) after adding different amounts of carbon and flux, starting from the same NG-DRI.

[0061] Figures 8A-8E show that, starting with NG-DRI containing approximately 4.5 mass% C and approximately 5 mass% SiO2+Al2O3, variations in the temperature and chemical composition of the molten iron (HM) cause significant variations in nitrogen, phosphorus, and carbon in units 2 and 3. However, the final result in unit 4 shows very little variation, indicating that even with significant variations in the composition and temperature of the incoming HM, the unit sequence according to the present invention can provide consistent results similar to those obtained by conventional routes in the prior art. On the other hand, the prior art process relies on the extremely stable temperature and composition of BF-HM, thereby gaining advantages.

[0062] Abbreviation BF=Blast Furnace BOS = Basic Oxygen Steelmaking CAS-OB = Composition Adjustment by Sealed Argon Bubbling-Oxygen Blowing CCUS = Carbon Capture, Utilization and Storage DRI = Directly Reduced Iron DRP = Direct Reduction Plant DV=Desulfurizing Vessel EAF = Electric Arc Furnace ESF = Electric Smelting Furnace / Vessel HBI = Hot Briquetted Iron HM=Hot Metal REF = Reducing Electric Arc Furnace LD=Linz-Donawitz LF=Ladle Furnace NG = Natural Gas OLF=Oxidizing Ladle Furnace RH = Ruhrstahl Heraeus RH-OB = Ruhrstahl Heraeus - Oxygen Blowing SS = Stirring Station VTD = Vacuum Tank Degasser VD = Vacuum Degasser VOD = Vacuum Oxygen Decarburization VRF = Vacuum Refining Furnace

Claims

1. A method for producing molten steel for casting into a cast product in a continuous casting machine, comprising the following sequential steps: A process of producing directly reduced iron (DRI) from iron ore in a direct reduction plant (DRP) or fluidized bed reactor using reducing gas; A process of producing molten iron (HM) by melting the DRI in an electric smelting furnace (ESF); If necessary, a step of desulfurizing the HM in a desulfurization vessel; A step of removing nitrogen and hydrogen from the HM in a vacuum degassing vessel; (a) A step of reducing one or more of carbon, silicon, and phosphorus in the HM in an LD converter; (b) A step of reducing one or more of carbon, silicon, and phosphorus in the HM in an oxidizing ladle furnace, and then, if necessary, adding a deoxidizing agent such as Al, Si, or Mn in the furnace for alloy composition adjustment and / or deoxidation of the HM; (c) A step of reducing one or more of carbon, silicon, and phosphorus in the HM in a vacuum refining furnace; A step of producing molten steel by deoxidizing and / or adjusting the alloy composition of the HM, if necessary, in a secondary metallurgical facility, preferably selected from the group comprising a ladle furnace (LF), a stirring station, or CAS(-OB); The process of manufacturing cast products by continuously casting the molten steel. The method, including the method described above.

2. The method according to claim 1, wherein one or more of carbon, silicon, and phosphorus are reduced in the LD converter.

3. The method according to claim 1 or 2, wherein at least a portion of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas and exhaust gas from the ESF to produce a reducing gas rich in hydrogen and carbon monoxide.

4. The method according to any one of claims 1 to 3, wherein at least a portion of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas, hydrogen, and exhaust gas from the ESF to produce a reducing gas rich in hydrogen and carbon monoxide.

5. The method according to any one of claims 1 to 4, wherein one or more carbon-containing compounds are added to the ESF in order to increase the amount of carbon in the HM.

6. The method according to any one of claims 1 to 5, wherein the reducing gas in the DRP is mainly composed of hydrogen, and preferably the reducing gas contains at least 90% hydrogen.

7. A method according to claim 6 for producing molten steel for casting into a cast product in a continuous casting machine, comprising the following continuous steps: A process for producing DRI by reducing iron ore, wherein the reducing gas in the DRI is mainly composed of hydrogen; A process of melting the DRI in the ESF to produce molten iron (HM); If necessary, a step of desulfurizing the HM in a desulfurization vessel; A step of removing nitrogen and hydrogen from the HM in a vacuum degassing vessel; (a) a step of removing silicon and phosphorus and optionally carbon in an LD converter, (b) an oxidizing ladle furnace, or (c) a vacuum refining furnace; A process of producing molten steel by deoxidizing and adjusting the composition of the HM in LF; The process of manufacturing cast products by continuously casting the molten steel. The method, including the method described above.

8. A method according to claim 6 for producing molten steel for casting into a cast product in a continuous casting machine, comprising the following continuous steps: A process for producing DRI by reducing iron ore, wherein the reducing gas in the DRI is mainly composed of hydrogen; A process of melting the DRI in the ESF to produce molten iron (HM); If necessary, a step of desulfurizing the HM in a desulfurization vessel; A step in which nitrogen, hydrogen, silicon, phosphorus, and optionally carbon are removed from the HM in a vacuum refining furnace; A process of producing molten steel by deoxidizing and adjusting the composition of the HM in LF; The process of manufacturing cast products by continuously casting the molten steel. The method, including the method described above.

9. The method according to any one of claims 1 to 8, wherein the amount of sulfur in the HM is less than 0.01% by mass, the HM is not desulfurized.

10. In the aforementioned ESF (REF), the desulfurization of the HM is not performed after the DRI is melted. The method according to claim 9, wherein one or more of nitrogen, hydrogen, silicon, and phosphorus are removed from the HM in a vacuum refining furnace.

11. The method according to any one of claims 1 to 10, wherein the direct reduction plant is a reducing gas type shaft furnace.

12. The method according to any one of claims 1 to 11, wherein the iron ore is supplied to the direct reduction plant in the form of agglomerated iron ore chunks, in the form of pellets, or in the form of a combination thereof.

13. Molten steel produced by the method according to any one of claims 1 to 12.

14. A steel product with a reduced carbon footprint, manufactured from molten steel according to claim 13.

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