Method for producing a steel melt with a reduced carbon footprint and high-grade steel product produced thereby

Abstract

This invention relates to a method for producing a steel melt for casting into a cast product in a continuous casting machine with a reduced carbon footprint.

Description

[0001] METHOD FOR PRODUCING A STEEL MELT WITH A REDUCED CARBON FOOTPRINT AND HIGH-GRADE STEEL PRODUCT PRODUCED THEREBY

[0002] Field of the invention

[0003] This invention relates to a method for producing a steel melt for casting into a cast product in a continuous casting machine with a reduced carbon footprint.

[0004] Background of the invention

[0005] Steel is the world's most important engineering and construction material. It is used in every aspect of our lives; from cars and construction products, refrigerators and washing machines, cargo ships to surgical scalpels. Steel is fully recyclable, possesses great durability, and compared to other engineering materials, requires relatively low amounts of energy to produce. Innovative lightweight steels (such as those used in automobiles and buildings) help to save energy and resources. The steel industry has made immense efforts to limit environmental pollution in the last decades. Producing one tonne of steel today requires just 40% of the energy it did in 1960. Dust emissions have been reduced by even more.

[0006] However, as governments and industries around the world look to reduce carbon emissions to reach ambitious net-zero targets, also steel is in the spotlight. Around 75% of steel is still largely made in coal-fired blast furnaces (BF) which release considerable amounts of carbon dioxide into the atmosphere. Overall, steel production accounts for around 8% of global CO2 emissions. Mitigating options like carbon capture and re-use and carbon-capture and storage, e.g. in empty gas fields, have been subject of intensive studies but these options are stopgap solutions rather than definitive solutions towards a carbon-free steelmaking process.

[0007] It is evident that cleaning up one of the largest industrial sources of CO2 is therefore one of the keys to tackling climate change. Governments and steel companies set ambitious targets to meet the world's climate goals / objectives.

[0008] In pursuit of these ambitions many companies, including Tata Steel in the Netherlands, have embarked on a route towards "green steel". Green steel is the manufacturing of steel without the use of carbon-based fossil fuels, and one of the most promising technologies is to convert the ironmaking step in the process from a coalbased process to a hydrogen-based process. This alternative ironmaking route includes, for example, a combination of gas based Direct Reduction Plant (DRP) to produce prereduced pellets (DRI), followed by their smelting in a Reducing Electric Furnace (REF) to produce hot metal (HM). Unfortunately, green hydrogen (from renewable sources) is not yet available in the abundance required. As an intermediary step a process is being developed based on natural gas that can, in due course, be converted to a hydrogenbased process relatively easily.

[0009] Leaving aside the challenges of sourcing enough green hydrogen the carbon-free ironmaking route itself is also not without its challenges. It is, for example, a challenge to produce hot metal (HM) from a carbon-free ironmaking process with comparable carbon levels as the traditional blast furnace pig iron which, has a high carbon content, typically in a range of 3.8-4.7 wt.%. This carbon in the HM is important for the further processing in the LD-steelmaking process (Linz-Donawitz process, also known as basic oxygen steelmaking (BOS)). It was found that a straightforward carbon addition in the REF (or EAF) to top up the carbon content of the HM to a conventional pig-iron level is not very efficient. This can result in significantly reduced dissolved carbon levels of 2.5- 3.5 wt.% in HM.

[0010] Conventionally an LD-vessel (converter) is charged with HM resulting from the ironmaking step together with steel or iron scrap (about 15-30%). The required thermal energy to melt the scrap and refine the resulting melt into steel is produced during the oxidation process. A water-cooled lance with nozzles is subsequently lowered into the vessel / converter and oxygen blown onto the melt, thereby oxidising the carbon dissolved in the melt and causing the average temperature of the melt to rise to about 1700 °C. The oxygen blowing therefore causes the scrap to melt and mix in the melt, lowers the overall carbon content of the melt, and helps to remove unwanted chemical elements such as nitrogen, silicon, and phosphorus.

[0011] The presence of carbon in hot metal not only reduces the liquidus temperature and thus enable operations at a lower temperature, but it also generates CO bubbles in the LD-process that is essential for removal of dissolved gases like nitrogen and hydrogen. The HM resulting from an alternative ironmaking process, either with a significantly reduced usage or (eventually) with no I low usage of carbon, is expected to have a lower carbon content and therefore will be less efficient for removing dissolved gases. Hence, the capacity to remove nitrogen (and / or hydrogen) from the melt during the LD-process is adversely affected when its carbon content is low. Moreover, when using a green-hydrogen based iron-making process , the hydrogen-based reduction may lead to diffusion of hydrogen in the DRI pellets and may result in increased hydrogen in the HM. The initial dissolved nitrogen and hydrogen content in the HM may therefore be too high and higher than that of conventional blast furnace-based pig iron.

[0012] Attempts to increase the carbon levels in the REF-HM thereby mimicking the traditional blast-furnace HM have produced disappointing results so far.

[0013] Objectives of the invention

[0014] It is an object of this invention to provide an alternative refining process to produce high-grade steel via a steelmaking route with a significantly lower use of carbon.

[0015] It is also an object of this invention to provide an alternative refining process to produce high-grade steel via a CC -neutral steelmaking route.

[0016] It is also an object to provide a refining process to produce steel via a CC -neutral steelmaking route that can process hot metal with a large variation in composition and of carbon in particular. Description of the invention

[0017] In a first aspect the invention is embodied in a method for producing a steel melt for casting into a cast product in a continuous casting machine comprising the following consecutive process steps:

[0018] Producing directly reduced iron, DRI, from iron ore in a direct reduction plant, DRP or a fluidized bed reactor, using a reducing gas;

[0019] Melting the DRI in an electric smelting furnace, ESF, to produce a hot metal, HM; Optionally de-sulphurising of the HM in a desulphurising vessel;

[0020] Removing nitrogen and hydrogen from the HM in a vacuum degassing vessel;

[0021] Reducing one or more of carbon, silicon, and phosphorus from the HM in an a. in an LD-vessel, or b. in an oxidizing ladle furnace, optionally followed by alloy trimming in said furnace and / or by adding a deoxidizer such as Al, Si or Mn for deoxidation the HM, or c. in a vacuum refining furnace;

[0022] Optionally deoxidising and / or alloy trimming the HM composition in a secondary metallurgy unit preferably selected from the group comprising a ladle furnace, LF; stirring station or a CAS(-OB) to produce a steel melt;

[0023] Continuous casting the steel melt to produce a cast product.

[0024] The method according to the invention allows the production of a steel melt that is suitable for casting into a cast product, such as a slab, thin slab, strip, bloom, billet, or ingot. Due to the processing steps preceding the casting, the method can produce a steel melt with the same or similar properties and composition as the current steel melts produced via the current conventional blast furnace - BOS route. Note that the REF or submerged arc furnace is sometimes also referred to as Electric Smelting Furnace (ESF). In the context of this descriptions ESF encompasses specific furnaces such as reducing electric furnaces (REF) and submerged arc furnaces (SAF).

[0025] This method also meets the objective of providing an alternative refining process to produce high-grade steel via a steelmaking route with a significantly lower use of carbon or even via a CO2-neutral steelmaking route.

[0026] The method is based on the production of a DRI, optionally in the form of a Hot Briquetted Iron (HBI) which is a form of DRI that has been compacted at an elevated temperature (typically more than 650° C) at time of compaction and which typically has a density of more than 5000 kg / m3. HBI may be used to overcome the problems associated with shipping and handling of DRI - due to the process of compaction it is less porous and therefore less reactive than DRI and does not suffer from the risk of self-heating associated with DRI. Other forms of DRI are pellets and lumps. DRI fines can be used after coagulation as cold briquetted iron.

[0027] In en embodiment the iron ore is provided to the direct reduction plant in the form of coagulated iron ore lumps or in the form of pellets, or in the form of a combination thereof. The advantage of this form is that it is readily available to most iron- and steelmaking plants and that it is easy to handle.

[0028] The reduction of the iron ore takes place in a direct reduction plant (DRP). The majority of current DRP's are gas-based gravitational shaft furnace processes accounting for over 75% of DRI production (2019), gas-based fluidized bed processes, accounting for less than 1% of DRI production and coal based rotary kiln furnaces accounting for the remainder of DRI production. It is therefore preferable wherein the direct reduction plant in the method according to the invention is a reducing gas-based (gravitational) shaft furnace. Depending on the chosen process the feedstock may be fine iron, coagulated iron lumps (e.g. sinter) or pellets.

[0029] Typical carbon contents of DRI and HBI are from about 0.5 to about 2.5wt.% and metallisation may be over 90 wt.% Fe. DRI (not HBI) is usually produced in the form of small pellets. The typical average diameters of these DRI pellets range between 1-1.5 cm. It is important to note that the DRI or HBI may still contain some unreduced iron oxide.

[0030] These pellets or briquettes are subsequently introduced in an electric smelting furnace, preferably a REF, and melted. The reducing atmosphere in the electric furnace ensures that further reduction of the unreduced iron oxide in the DRI or HBI takes place, thereby increasing the metallic iron yield. The resulting hot metal (HM) consists mainly of metallic iron and is further refined in the subsequent steps.

[0031] The reducing gas in the DRP is rich in hydrogen and may contain carbon monoxide. This is particularly the case if the reducing gas is provided based on natural gas (NG) which consists of hydrocarbons. Usage of NG not only forms carbon monoxide and hydrogen by cracking, but it can also be used for carburizing the pre-reduced DRI, e.g. in the carburization (cold) zone of a DRP. In an embodiment additional carbon containing compounds may optionally be added to the ESF. This may be in the form of coal, charcoal, graphite, biomass or the like. This will result in an increase in carbon content of the hot metal, which is beneficial for the method according to the invention.

[0032] The HM produced after melting the DRI is subsequently optionally desulphurised (de-S) in a desulphurization vessel. It is also noted that, since coal is the major source of sulphur in HM, it is expected that when the use of coal decreases that the need to de-S decreases accordingly. If the sulphur content in the HM is below a certain value, this de-S step may be skipped. In an embodiment the HM is not de-sulphurised if the sulphur content of the HM is below 0.01 wt.%.

[0033] It should be noted that in the context of this invention the following abbreviations will be used: de-S, de-N, de-H, de-Si, de-P, de-C, and de-0 which means desulphurisation, de-nitrogenization, de-hydrogenisation, de-siliconisation, dephosphorisation, de-carburisation and de-oxidation, respectively.

[0034] The next step is the de-N and de-H step. This is preferably performed in a vacuum degassing apparatus, such as the Ruhrstahl Heraeus (RH) unit or a vacuum tank degasser. In the prior art process this step also involves the de-C step, but this is not needed in the method according to the invention because de-C will be performed in a next step, viz. in the LD-vessel. So compared to the prior art situation and the normal BF-LD-route the degassing takes place before the molten metal is treated in the LD- vessel in the method according to the invention, instead of thereafter. This way the reduced oxygen levels (which is a surface-active solute that hinders removal of dissolved gases) are advantageous and these may be reduced or no need for CO bubbles for degassing in the converter later. Instead of the RH-unit a Vacuum Tank Degasser (VTD) (also known as Vacuum Degasser (VD)) treatment could be used to reduce the oxygen level. A V(T)D allows to degas (de-N and de-H) and de-C the melt and provides the best conditions for de-S and inclusions removal. During the V(T)D treatment, the liquid steel ladle is placed in a vacuum tank that can be of stationary or movable design according to the specific layout needs. To promote stirring, inert gas is injected through porous plugs. V(T)D can be equipped with an additional oxygen lance (VD-OB) and further benefits, such as forced de-C or chemical heating, e.g. by Al addition, can be obtained.

[0035] In the LD vessel also de-Si and de-P will take place. This is similar to the conventional LD route. The main difference is that de-N requirement in LD is now reduced because most of the N has already been removed in the previous vacuum degassing step.

[0036] The last step before transferring the melt to the casting shop is secondary metallurgy in a facility, such as a ladle furnace (LF) facility, to de-0 the melt and to perform any last-minute steel alloy trimming. The LF may also heat the melt so that the steel melt arrives in the casting shop at the operational temperature for casting.

[0037] This route can cope with varying amounts of carbon in the hot melt. The HM carbon may result from NG carburization in the carburization (cold) zone of the DRP and / or from addition of carbon containing compounds to the electrical smelting furnace such as a REF, but in any case the route according to the invention comprising vacuum degassing followed by refining operations in an LD, OLF or VRP, is able to treat the HM with varying carbon contents and produce a steel melt with the desired chemistry and temperature.

[0038] In a preferable embodiment one or more of carbon, silicon and phosphorus is removed in an LD-vessel. In this embodiment the facilities of a conventional BOS-plant can still be used, albeit in a different configuration.

[0039] In an embodiment at least part of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas and off-gas from the reducing furnaces (DRP and / or REF) to produce reducing gases that are that rich in hydrogen and carbon monoxide.

[0040] In an embodiment at least part of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas, hydrogen, and off-gas from the reducing furnaces (DRP and / or REF) to produce reducing gases that are that rich in hydrogen and carbon monoxide. Other alternative sources for DRP reducing gas are coke oven gas, which is rich in H2, BOF-gas, which is rich in CO, or syngas produced from solid fuel gasification (coal, coke, municipal waste, other hydrocarbons, etc.).

[0041] In an embodiment at least part of the reducing gas in the DRP consists mainly of hydrogen. Only insignificant amounts of gasses may be present that do not significantly affect the reduction process by hydrogen. If in this process no carbon containing gasses or compounds are used in the production of the hot metal and the steel melt then no carburisation takes place in the DRP because there is no carbon present in the reducing gas. If there is carbon present in the DRI then in this embodiment it must have been added to the feed stock for the DRP or added to the ESF as described above. As a result of the low carbon content of the DRI, the sulphur content is also likely to be low as carbon containing compounds like coal are the main source of sulphur. In this embodiment no carbon is added to the ESF, so the S-content stays low. This low level of sulphur may be low enough for most steel grades. This means that the de-S step in a de-S vessel may not always be needed. Thus, de-S step is therefore optional and not essential.

[0042] In an embodiment the reducing gas comprises at least 95 mol.%, preferably at least 98 mol.%, more preferably at least 99 mol.% hydrogen.

[0043] In an embodiment one or more carbon containing compounds are added to the ESF, preferably the REF, to increase the carbon content of the HM. This is an option in processes where it is desirable to increase the carbon level of the hot metal. This may be in the form of coal, charcoal, graphite, biomass, biocoal or the like.

[0044] In a preferable embodiment a steel melt for casting into a cast product in a continuous casting machine is produced according to a method comprising the following consecutive process steps:

[0045] Producing DRI by reducing iron ore wherein the reducing gas in the DRP consists mainly of hydrogen;

[0046] Melting the DRI in an ESF, to produce a hot metal, HM;

[0047] Optionally de-sulphurising the HM in a desulphurising vessel;

[0048] Removing nitrogen and hydrogen from the HM melt in a vacuum degassing vessel;

[0049] Removing silicon and phosphorus and optionally carbon, a. in an LD-vessel, or b. in an oxidizing ladle furnace, or c. in a vacuum refining furnace;

[0050] Removing oxygen and trimming the HM composition in a LF to produce a steel melt;

[0051] Continuous casting the steel melt to produce a cast product.

[0052] The de-N and de-H takes place in the vacuum degassing vessel as described herein, followed by de-C, de-Si, and de-P in either of the following modified refining vessels: a. an LD-vessel, or b. an oxidizing ladle furnace (OLF), or c. a vacuum refining furnace (VRF).

[0053] Specific characteristics of these three refining options in relation to the method according to the invention are as follows.

[0054] An LD-vessel (aka converter in a BOS-plant) can flexibly deal with varying amounts of carbon in the incoming HM. O2 blowing facilitates de-C, which in turn, can aid further de-N. O2 blowing also allows fast de-P and de-Si. Care must be taken to avoid excessive formation of FeO in the absence of carbon. The oxygen blowing (top and / or bottom blowing depending on the refining vessel used) must be carefully controlled to form the right amount of FeO needed for de-P, and dilution of the O2 stream might be needed, e.g. with Argon gas. It is preferable that the C-content of the HM is accurately known beforehand to tailor the oxygen blowing. De-P takes place via slag / metal reactions and there is no gas-slag-metal emulsion for de-P.

[0055] An oxidising ladle furnace (OLF) is, as the phrase already indicates, a ladle furnace with an oxidising atmosphere. It is equipped with electrodes to enable to raise the temperature of the HM. This is especially important for low-C HM, because the lower the carbon content of the HM, the higher the liquidus temperature of the HM. Options for bottom blowing with O2 allows a more effective interaction between slag and metal which is important for de-P (like the Oxygen Bottom Max Hutte process). Top or side blowing of O2 may also be possible. O2 blowing also facilitates de-C of the incoming HM with variable carbon contents. This can also result in some de-N depending on the relative C and N levels of the bath. A sufficiently large free-board space is needed for slag that is generated because of the de-Si. The process in the OLF may be optionally followed by adding deoxidizers such as Al, Si or Mn (or a combination of two or more) for deoxidation and / or alloy trimming in said furnace, thereby deoxidising the steel melt.

[0056] A vacuum refining furnace (VRF) executes a vacuum refining process. It is similar to a vacuum oxygen decarburisation (VOD). This furnace can facilitate degassing (de-N and de-H) followed by refining by 02-blowing, meaning that a vacuum degassing facility like a RH-vessel (Ruhrstahl-Heraeus) may not be needed or may be skipped in the process route. After degassing, de-P, de-Si, and de-C (depending on HM carbon) can be performed by controlled 02-blowing. A sufficiently large free-board space is needed for slag that is generated because of the de-Si. A vacuum refining furnace may also be suitable for de-S. Since the vacuum refining furnace can also take care of the de-N and de-H, the vacuum degassing vessel is not required.

[0057] Preferably the silicon and phosphorus and optionally carbon is removed in an LD- vessel. The advantage of this choice is that many steelmaking plants have these facilities available.

[0058] Finally, the last step before transferring the melt to the casting shop is the conventional ladle furnace (LF) treatment to de-0 the melt and to perform any last- minute steel alloy trimming. If this de-0 of the melt and last-minute steel alloy trimming has already been performed in the preceding step in an OLF, then this last step may be considered optional before transferring the melt to the casting shop and the melt may be transferred to the casting shop after completion of the treatment in the OLF. The LF may also heat the melt so that the steel melt arrives in the cast shop at the right temperature for casting. A stirring station may be used to establish a uniform temperature and chemical composition throughout the steel in the ladle. The steel can be stirred by argon injection, for example through a refractory- lined lance or through a permeable refractory block in the bottom of the ladle, or it can be stirred by an electromagnetic coil. Additions of alloying elements can be made at the stirring station.

[0059] The carbon-free method of producing a steel melt is achieved if the reducing gas in the DRP is hydrogen only and if no carbon containing compounds are added to the ESF. If despite this there is still carbon in the hot melt resulting from drag-in via the feedstock of the DRP then the variation in carbon content of the hot melt can be easily dealt with by the LD-vessel or the oxidising ladle furnace (OLF) or the vacuum refining furnace (VRF).

[0060] In an embodiment the method for producing a steel melt for casting into a cast product in a continuous casting machine comprises the following consecutive process steps: wherein the DRI is produced by reducing iron ore wherein the reducing gases in the DRP consist mainly of hydrogen;

[0061] Melting the DRI in an ESF, to produce a hot metal, HM;

[0062] Optionally de-sulphurising the HM in a desulphurising vessel;

[0063] Removing nitrogen, hydrogen, silicon, and phosphorus, and optionally carbon, from the hot melt in a vacuum refining furnace;

[0064] Removing oxygen and trimming the HM composition in a LF to produce a steel melt;

[0065] Continuous casting the steel melt to produce a cast product.

[0066] In this embodiment, there is no need for a separate vacuum-degassing step for removing nitrogen (de-N) and hydrogen (de-H) from the HM.

[0067] In an embodiment there is no vacuum-degassing step for removing nitrogen (de- N) and hydrogen (de-H) from the HM if there is a vacuum refining furnace.

[0068] A potential issue with zero carbon in DRI pellets and REF HM is that the melt liquidus can become very high and there is a risk of solidification of the HM. The lower the carbon content, the higher the melting point (see the iron-carbon binary phase diagram of Figure 4). It can also limit the productivity of the ESF. So the lower the C- content of the HM, the higher the temperature of the HM should be. The temperature may be raised by providing superheats at the REF or the vacuum degassing furnace (RH or VTD) or the conventional ladle furnace or the oxidizing ladle furnace (OLF), e.g. by electrode heating to adding fuels like Fe-Si or Al. To avoid excessively high superheats it may be practical to have some carbon in the process chain (either coming from NG in DRP or carbon sources in the ESF). An alternative is to add a renewable form of carbon source (like bio-coal). Using a renewable green source is considered a carbon-negative contribution.

[0069] If the need to add carbon to the HM results in the development of CO and / or CO2 at any time during the process according to the invention then this CO and / or CO2 may be captured, cleaned, and re-used e.g. as fuel or as a raw material in other chemical processes. CCUS (Carbon Capture, Utilization and Storage) is available to remove the last bit of CO2 generated in the ironmaking (DRP and ESF) and steelmaking step (LD, OLF or VRF).

[0070] According to a second aspect, the invention is also embodied in a steel melt produced according to the method of the invention and to a steel product produced based on this melt after casting and further processing into a steel product.

[0071] The invention will now be further explained by means of the following, non-limiting examples.

[0072] Examples

[0073] Considering an average [C]HM of 3 wt.% in REF HM, the expected nitrogen levels are around twice higher compared to that found in typical BF hot metal based on 'plain' thermodynamics. The results are depicted in Figure 5 and 6. The calculated values of nitrogen levels for the BF hot metal (~90 ppm) are comparable to those observed in BF-HM (60-80 ppm). With ~3 wt.% C in REF HM, the [N] is calculated to be >160 ppm at temperatures of 1500-1550 °C under atmospheric conditions.

[0074] Such high initial levels of nitrogen would necessitate higher de-nitrogenization rates or longer refining times in primary steelmaking (BOF / EAF) or secondary steelmaking (RH / VTD) - compared to the current practices. As shown in Figure 6, some sensitivity studies using temperature and partial pressure of nitrogen as parameters are also carried out to illustrate the variation in nitrogen levels in Fe-C melts. The various possible sources of nitrogen pickup in REF Hot Metal are as follows:

[0075] 1. Feed material:

[0076] O DRI / HBI (20-30 ppm N), Scrap (20-220 ppm N) o Carrier gas (hot DRI to REF) o Ingress from atmosphere (having 78 vol.% N) o NG (having 1-14 vol.% of N), if used for carburization of the DRI

[0077] 2. Dissociation of N2 molecules around the arc is possible: o Due to Very high temperature o The presence of a Thick REF-slag layer 'may' prevent absorption

[0078] 3. Air entrainment into the tapping stream: o Diffused stream will provide more interfacial area for N absorption; compact stream better

[0079] 4. Absorption from atmosphere during transfer operations in the: o Runner o Torpedo o HM ladle

[0080] The process parameters that were varied were as follows:

[0081] ■ DRP-REF: o Natural Gas based DRI with 4.5 wt.% carbon was used as the starting point o Varying degrees of reduction in REF gives varying HM carbon contents.

[0082] C varied from 2.5 to 4.5 wt.% o Other components were reduced to varying degrees o The variation in the HM composition was obtained due to addition of different amount of carbon and fluxes o Nitrogen prediction from thermodynamic solubility model gives a variation from 130 to 220 ppm.

[0083] - RH: o Standard industrial operating conditions of vacuum (5 mbar minimum) and Argon flow rates were used. Final nitrogen in RH was found to vary from 25 to 35 ppm. o A vacuum pressure of 1 mbar further decreases N (by 5-8 ppm).

[0084] ■ LD / BOF: o Blowing time and scrap ratio changes based on the HM carbon content Refining of other components such as C, Mn, P, Si is simulated. The final values are similar to a conventional LD operation. o There is some additional nitrogen removal due to de-C in converter. Nitrogen removal in converter is predicted from a 1st order kinetic model. The final nitrogen values are well within the limits of conventional LD operations.

[0085] The variation in temperature and chemical composition of the melt are shown in Figure 8A-E.

[0086] Brief description of the drawings

[0087] The invention will be explained by means of the following, non-limiting figures.

[0088] Figure 1 shows the layout of the steelmaking process in various steps with the converter (LD-unit) and the vacuum-degassing unit (VTD / RH-OB) or ladle furnace treatment (LF) in their conventional order.

[0089] Step I - DRI production with NG (reduction of iron ore, carburisation of DRI)

[0090] Step II - melting of DRI in ESF (REF), carburation of HM

[0091] Step III - desulphurisation of HM in desulphurisation vessel

[0092] Step IV - Steelmaking in LD-converter (de-C, de-Si, de-P, and de-N) Step V - Ladle Furnace (de-0 and de-S) or VTD / RH-OB (de-C, de-N, and de-H) Step VI - Casting

[0093] Figure 2 shows the layout of the steelmaking process in various steps with the converter (LD-unit) and the vacuum-degassing unit (VTD / RH-OB) or ladle furnace treatment (LF) in the order according to the invention.

[0094] Step I - DRI production with NG and H2 (reduction of iron ore, carburisation of DRI) Step II - melting of DRI in ESF (REF), carburation of HM Step III - desulphurisation of HM in desulphurisation vessel

[0095] Step IV - Ladle Furnace (de-0 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 trimming) Step VII - Casting

[0096] Figure 3 shows the layout of the steelmaking process in various steps with the converter (LD-unit) and the vacuum-degassing unit (VTD / RH-OB) or ladle furnace treatment (LF) in the order according to the invention. The optional Step III (de-S) is no longer needed because the DRI-process and the REF is no longer based on coal additions, although de-S could be performed in step IV if needed. There are three refining options, one of which is the converter (LD-unit). In an embodiment the VTD / RH- OB could be skipped as well if as the refining option the VRP is chosen. A VRP is also able to remove N and H.

[0097] Step I - DRI production with H2 (reduction of iron ore) Step II - melting of DRI in ESF (REF), carburation of HM Step IV - VTD / RH-OB (de-C, de-N and de-H, optionally de-S) Step V - Steelmaking in: 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)

[0098] Step VI - Ladle Furnace (de-O, alloy trimming) Step VII - Casting

[0099] Figure 4 shows the Fe-C binary phase diagram.

[0100] Figure 5 shows the nitrogen solubility in REF and BF HM at 1500°C and PN2 = 0.79 atm. The black triangles are experimental points taken from Min-Kyu Paek, Saikat Chatterjee, Jong-Jin Pak & In-Ho Jung, Thermodynamics of Nitrogen in Fe-Mn-AI-Si-C Alloy Melts, Met & Mat Trans B, Vol. 47, 2016, p 1243-1262. They correspond very well with the results of the calculations of the nitrogen content (black dotted line) as a function of the carbon content in the hot melt.

[0101] Figure 6 shows the nitrogen solubilities at various possible hot metal temperatures and ambient pressure (PN2 ~ 0.79 atm). Solid phases start to form at 1450 °C and low [C] levels. Figure 7 shows the sequence of units operating to produce the results presented in Figure 8: 1 : DRP, 2: REF, 3: Vacuum Degassing (RH / VTD), 4: BOF (Modified refining).

[0102] Figure 8A-E shows the variations in carbon, nitrogen, sulphur, and phosphorus in the HM. Different lines signify different REF HM (see Figure 9). It is noted that the various lines correspond to the different input variants and that these images are intended to demonstrate spread rather than absolute individually identifiable results.

[0103] Figure 9 is a table showing the variation in the REF HM temperature and composition after adding different carbon and flux to the same starting NG-DRI. The figures 8A-E show that the variety in temperature and chemistry of the HM, starting from a NG-DRI with ~4.5% C and ~5% of SiC +AhOs, results in considerable variation of nitrogen, phosphorus and carbon in units 2 and 3, but the final result in unit 4 shows very little variations, demonstrating that the sequence of units according to the invention, despite the large fluctuations in incoming HM composition and temperature, is able to provide consistent results comparable to the conventional route of the prior art, whereas the prior art process relies and benefits from a very consistent BF-HM temperature and composition.

[0104] Abbreviations

[0105] BF = Blast Furnace

[0106] BOS = Basic Oxygen Steelmaking

[0107] 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 = Desulphurising Vessel EAF = Electric Arc Furnace ESF = Electric Smelting Vessel HBI = Hot Briquetted Iron HM = Hot metal

[0108] REF = Reducing Electric Arc Furnace LD = Linz-Donawitz LF = Ladle Furnace NG = Natural Gas

[0109] OLF = Oxidizing Ladle Furnace RH = Ruhrstahl Heraeus

[0110] RH-OB = Ruhrstahl Heraeus, Oxygen Blowing SS = Stirring Station VTD = Vacuum Tank Degasser VD = Vacuum Degasser

[0111] VOD = Vacuum Oxygen Decarburisation

[0112] VRF = Vacuum Refining Furnace

Claims

CLAIMS1. Method for producing a steel melt for casting into a cast product in a continuous casting machine comprising the following consecutive process steps:Producing directly reduced iron, DRI, from iron ore in a direct reduction plant, DRP or a fluidized bed reactor, using a reducing gas;Melting the DRI in an electric smelting furnace, ESF, to produce a hot metal, HM;Optionally de-sulphurising of the HM in a desulphurising vessel;Removing nitrogen and hydrogen from the HM in a vacuum degassing vessel;Reducing one or more of carbon, silicon, and phosphorus from the HM in an a. in an LD-vessel, or b. in an oxidizing ladle furnace, optionally followed by alloy trimming in said furnace and / or by adding a deoxidizer such as Al, Si or Mn for deoxidation the HM, or c. in a vacuum refining furnace;Optionally deoxidising and / or alloy trimming the HM composition in a secondary metallurgy unit preferably selected from the group comprising a ladle furnace, LF; stirring station or a CAS(-OB) to produce a steel melt;Continuous casting the steel melt to produce a cast product.

2. Method according to claim 1, wherein one or more of carbon, silicon and phosphorus is reduced in the LD-vessel.

3. Method according to claim 1 or 2, wherein at least part of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas and off-gas from the ESF to produce a reducing gas that is rich in hydrogen and carbon monoxide.

4. Method according to claim 1 to 3, wherein at least part of the reducing gas in the DRP is produced by chemically reforming a mixture of natural gas, hydrogen, and off-gas from the ESF to produce a reducing gas that is rich in hydrogen and carbon monoxide.

5. Method according to any one of claims 1 to 4, wherein one or more carbon containing compounds are added to the ESF to increase the carbon content of the HM.

6. Method according to claim 1 to 5 wherein the reducing gas in the DRP consists mainly of hydrogen, and wherein the reducing gas preferably contains at least 90% hydrogen.

7. Method according to claim 6 for producing a steel melt for casting into a cast product in a continuous casting machine comprising the following consecutive process steps:Producing DRI by reducing iron ore wherein the reducing gas in the DRP consists mainly of hydrogen;Melting the DRI in an ESF, to produce a hot metal, HM;Optionally de-sulphurising the HM in a desulphurising vessel;Removing nitrogen and hydrogen from the HM melt in a vacuum degassing vessel;Removing silicon and phosphorus and optionally carbon, a. in an LD-vessel, or b. in an oxidizing ladle furnace, or c. in a vacuum refining furnace;Removing oxygen and trimming the HM composition in a LF to produce a steel melt;Continuous casting the steel melt to produce a cast product.

8. Method according to claim 6 for producing a steel melt for casting into a cast product in a continuous casting machine comprising the following consecutive process steps: wherein the DRI is produced by reducing iron ore wherein the reducing gas in the DRP consist mainly of hydrogen;Melting the DRI in an ESF, to produce a hot metal, HM;Optionally de-sulphurising the HM in a desulphurising vessel;Removing nitrogen, hydrogen, silicon, and phosphorus, and optionally carbon, from the hot melt in a vacuum refining furnace.Removing oxygen and trimming the HM composition in a LF to produce a steel melt;Continuous casting the steel melt to produce a cast product.

9. Method according to any one of claims 1 to 8, wherein the HM is not desulphurised if the sulphur content of the HM is below 0.01 wt.%.

10. Method according to claim 9, wherein there is no de-sulphurising of the HM after melting the DRI in the ESF (REF) and wherein one or more of nitrogen, hydrogen, silicon, and phosphorus is removed from the HM in a vacuum refining furnace.

11. Method according to any one of claims 1 to 10, wherein the direct reduction plant is a reducing gas-based shaft furnace.

12. Method according to any one of claims 1 to 11, wherein the iron ore is provided to the direct reduction plant in the form of coagulated iron ore lumps or in the form of pellets, or in the form of a combination thereof.

13. Steel melt produced according to the method of any one of claims 1 to 12.

14. Steel product with a reduced carbon footprint produced from the steel melt according to claim 13.

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