EP4575005A1 - Method for producing a steel melt - Google Patents

Abstract

The invention relates to a method for producing a steel melt according to claim 1.

Description

• Erzeugen einer eisenbasierten Schmelze in einem elektrischen Einschmelzer, in welchem Eisenträger, optional Kohlenstoffträger und/oder optional Schlackenbildner eingebracht und im Schmelzbetrieb zu einer Roheisenschmelze und einer auf der Roheisenschmelze aufschwimmenden Flüssigschlacke erschmolzen werden;
• Behandeln der im elektrischen Einschmelzer erzeugten Roheisenschmelze in einem Konverter zur Erzeugung einer Stahlschmelze, wobei vor und/oder während der Konverterbehandlung zusätzliche Eisenträger eingebracht werden.

The invention relates to a method for producing a steel melt, comprising the steps:

• Producing an iron-based melt in an electric melter, in which iron carriers, optionally carbon carriers and/or optional slag formers are introduced and melted in the melting operation to form a pig iron melt and a liquid slag floating on the pig iron melt;
• Treating the pig iron melt produced in the electric melter in a converter to produce a steel melt, whereby additional iron carriers are introduced before and/or during the converter treatment.

A generic procedure is known from the publication EP 3 954 786 A1 It would be desirable to be able to reduce operating resources during converter operation and/or increase yield while maintaining the same converter performance.

Furthermore, the Richardson-Ellingham diagram is well known in expert circles, in which the oxygen potential of metal oxides is plotted as a function of temperature. In the Richardson-Ellingham diagram, curves for the more noble metals lie above those for the less noble ones, with the less noble metals correspondingly having a higher oxygen affinity and thus, in principle, being suitable as reducing agents, while they themselves are sometimes difficult to reduce. It can be seen from the Richardson-Ellingham diagram that a phase change (from solid to liquid metal) has a significant influence on the temperature dependence of the oxygen potential (phase changes lead to a change in the slope of the line, since the entropy increases from solid to liquid to gaseous). It must be noted, however, that the Richardson-Ellingham diagram assumes ideal conditions, so that, for example, a different activity of the metal and the oxide, due to the formation of mixed phases, for example, leads to a deviation from the described relationships. A further disadvantage is that only statements about the theoretically achievable equilibrium states are possible, while the path and time to reach equilibrium, i.e. the crucial area of reaction kinetics, remain unconsidered, cf. https://application.wiley-vch.de/books/sample/ 3527315802 c01.pdf page 37 ff.

The object of the present invention is to further develop this process in such a way that a higher thermal energy can be stored in the pig iron melt.

This object is achieved by a method having the features of claim 1. Further embodiments are described in the subclaims.

According to the invention, a silicon content of at least 0.40 to 5.0 wt.% and a titanium content of at least 0.040 to 0.90 wt.% are set and/or maintained in the pig iron melt during the melting operation and, by means of the converter treatment, a silicon content which is at least 10% lower and a titanium content which is at least 10% lower in the steel melt compared to the pig iron melt is achieved.

The elements silicon and titanium present in the pig iron melt are preferably in free or dissolved form.

By deliberately adjusting the concentrations of silicon and titanium in the pig iron melt during melting operations at the aforementioned levels, a higher chemical energy can be adjusted and/or maintained in the pig iron melt compared to the previously known state of the art. This energy, in turn, is released as thermal energy during converter operation and favors converter treatment by requiring fewer resources, such as oxygen, a specific amount of pig iron per ton of crude steel, electrical current, etc., to treat the pig iron melt. In particular, this allows a higher proportion of additional iron carriers, such as scrap, to be added and melted compared to the state of the art, while maintaining the same converter output. This can increase the yield or production volume relative to the available amount of pig iron.

The aforementioned contents of silicon and titanium in the pig iron melt serve as chemical storage components for energy. The pig iron melt can therefore have a silicon content of in particular at least 0.60 wt.%, preferably at least 0.80 wt.%, preferably at least 1.0 wt.%, particularly preferably at least 1.20 wt.%, further preferably at least 1.40 wt.%, and in particular at most 4.50 wt.%, preferably at most 4.0 wt.%, preferably at most 3.50 wt.%, particularly preferably at most 3.0 wt.%. Furthermore, the pig iron melt can have a titanium content of in particular at least 0.070 wt.%, preferably at least 0.10 wt.%, preferably at least 0.150 wt.%, particularly preferably at least 0.20 wt.%, further preferably at least 0.250 wt.%, and in particular at most 0.80 wt.%, preferably at most 0.70 wt.%, preferably at most 0.60 wt.%, particularly preferably at most 0.50 wt.%.

The converter treatment can achieve a silicon content that is at least 10% lower and a titanium content that is at least 10% lower in the steel melt compared to the pig iron melt, whereby the chemical energy can be released in the form of heat through oxidation of the storage components during the further course of the converter treatment and can preferably be used to melt additional iron carriers. The silicon content in the steel melt can be reduced compared to the pig iron melt, in particular by at least 15%, 20%, preferably by at least 25%, 30%, more preferably by at least 35%, 40%, particularly preferably by at least 50%, 60%, further preferably by at least 70%, 80%. The silicon content in the steel melt can be reduced to at least 0.10 wt.%. The titanium content in the steel melt can be reduced by at least 15%, 20%, preferably by at least 25%, 30%, more preferably by at least 35%, 40%, particularly preferably by at least 50%, 60%, further preferably by at least 70%, 80%, in comparison to the pig iron melt. The titanium content in the steel melt can be reduced to at least 0.0050 wt.%. Particularly when very high degrees of purity are required for certain steel grades, it is important, on the one hand, to remove slag-forming ingredients from the steel melt by agglomerating them with the liquid slag or slag. On the other hand, elements such as Ti in the steel product can significantly influence the production properties.

Furthermore, before and/or during the converter treatment, additional iron carriers are introduced, which are provided comprising or consisting of sponge iron pieces and/or sponge iron pellets and/or sponge iron briquettes, scrap, blast furnace pig iron, whereby the proportion of the additional iron carriers is at least 3 wt.% and at most 30 wt.% of the total mass of the steel melt produced or to be produced.

The proportion of additionally introduced iron carriers can in particular be at least 4, 5, 6 wt.%, preferably at least 7, 8, 9 wt.%, preferably at least 10, 11, 12 wt.%, particularly preferably at least 13, 14, 15 wt.% and in particular at most 28 wt.%, preferably at most 25 wt.%.

During converter treatment, oxidation processes take place at high temperatures. Due to the Si and Ti-containing pig iron melt to be conditioned, conventional oxygen blowing produces oxidised solid components, including SiO ₂ and TiO ₂ , which are transferred into the (converter) slag, as well as gaseous compounds, such as CO and CO ₂ , etc. The oxidation reaction of Si and Ti each generates an exothermic energy which is higher than the energy required to convert solid iron into a liquid phase, so that the kinetics occurring during converter treatment are suitable for being able to melt additional iron carriers, in particular, without having to incur additional effort and/or costs, and thus to increase the yield or production quantity in relation to the existing amount of pig iron in the converter.

Thus, according to a preferred embodiment, reduced iron ore carriers in the form of sponge iron pieces and/or sponge iron pellets and/or sponge iron briquettes with a carbon content between 0 and 4.8 wt.%, in particular > 0 wt.%, and a degree of metallization of at least 75% are preferably used as iron carriers (in the smelter). The degree of metallization reflects the ratio of the metallic iron content relative to the total iron content in the sponge iron. It can in particular be at least 80%, preferably at least 85%, preferably at least 90%, and ideally up to 100%, in particular up to 99%.

The preferred use of sponge iron as an iron carrier also brings with it slag-forming components which are naturally present in the iron ore carrier and cannot be expelled in a preceding reduction process, and are referred to as gangue. If the gangue provided via the sponge iron is insufficient, further slag-forming agents can be introduced as additives if required in order to produce a liquid slag which can be further processed. Thus, according to one embodiment, slag-forming agents can be added in order to be able to adjust a basicity B3 in the liquid slag between 0.9 and 1.8. B3 can in particular be at least 1.0, preferably at least 1.1 and in particular a maximum of 1.7, preferably a maximum of 1.6. The basicity B4 corresponds to the ratio of CaO+MgO to _SiO2 + _Al2O3 , whereby the determination of the characteristic quantities in the slag in the solid state is within the skill of the art _. are familiar. The adjustment of the desired basicity by appropriate mixing/addition is familiar to the expert. The slag former comprises at least one or more of the elements from the group (CaO, MgO, SiO ₂ , Al ₂ O ₃ ).

If the pig iron melt has a defined carbon content, which can be between 2.5, in particular 3.0, preferably 3.3, more preferably 3.7, particularly preferably 3.9 and 4.8 wt. %, and cannot be provided via the iron carrier alone, carbon carriers can be introduced according to one embodiment, in particular in an amount that allows the desired carbon content in the iron melt to be achieved. In principle, all materials in gaseous, liquid and/or solid form with reducible free carbon that can be introduced into the electric smelter are suitable as carbon carriers. In solid form, for example, coke dust, coke slaked coal, coke breeze or coal particles are suitable. In liquid form, for example, ethanol, methanol and other hydrocarbons are suitable. In gaseous form, carbon-containing gases, for example carbon dioxide, methane (natural gas), carbon monoxide, propane and butane are suitable. Preferably, the provided carbon carrier may comprise or consist of coal, biochar, biomass, biogenic and/or non-biogenic plastics.

Scrap can preferably be introduced as an additional iron carrier, particularly to increase the recycling rate. The scrap can introduce undesirable accompanying elements such as chromium (Cr), copper (Cu), molybdenum (Mo), tin (Sn), and/or nickel (Ni). Depending on the steel grade to be produced, these elements can affect both metallurgical processes and material product properties, with some of the aforementioned elements also representing alloying elements for certain steel grades. From a metallurgical perspective, this can, on the one hand, necessitate changes to the process control; on the other hand, it can also lead to negative impacts or even problems in the further processing of the semi-finished products produced from the molten steel.

To melt the iron and carbon carriers, the electric melter has several electrodes that can be charged with an electric current, thus providing the necessary energy to convert the solids into a liquid phase, comprising molten iron and liquid slag. Depending on the size/dimensions of the electric melter, three, four, five, six, or more than six electrodes can be used. The energy required for melting is preferably provided by renewable energy. Energy (solar, wind, water, biomass, geothermal) is provided. This allows the electric smelter to operate in a climate-friendly manner.

The electric melter can preferably be an OSBF (Open Slag Bath Furnace) furnace. These include submerged electric arc furnaces, especially SAF (Submerged Electric Arc Furnaces), which are melting furnaces with arc resistance heating that form arcs between the electrode and the charge material and/or the liquid phase, or which heat the charge material and/or the liquid phase using the Joule effect. In SAF, the electrodes are immersed in the charge material and/or the liquid phase, in particular the liquid slag. Depending on the functional principle/mode of operation, the submerged electric arc furnaces can be designed as alternating current submerged arc furnaces (SAFac) or direct current submerged arc furnaces (SAFdc). Alternatively, melting furnaces with direct arc action, which deviate from the functional principle/mode of operation described above, so-called EAF (Electric Arc Furnaces), which form arcs between the electrode and the liquid phase, can also be used. This includes the alternating current arc melting furnace (EAFac), the direct current arc melting furnace (EAFdc) and the ladle furnace LF (Ladle Furnace).

The advantage of using submerged arc furnaces (SAF) is that they operate in a reducing atmosphere, whereas direct arc furnaces (EAF) operate in an oxidizing atmosphere.

$$\begin{array}{c}1\mathrm{kg}\mathrm{Ti}\left(\mathrm{l},1550°\mathrm{C}\right)+\mathrm{0,67}\mathrm{kg}{\mathrm{O}}_2\left(\mathrm{g},25°\mathrm{C}\right)=\mathrm{1,67}\mathrm{kg}{\mathrm{TiO}}_2\left(\mathrm{l},1550°\mathrm{C}\right)\mathrm{bei}1550°\mathrm{C}\\ =-\mathrm{17785,34}\mathrm{kJ}\end{array}$$

$$1\mathrm{kg}\mathrm{Fe}\left(\mathrm{s},25°\mathrm{C}\right)=1\mathrm{kg}\mathrm{Fe}\left(\mathrm{l},1550°\mathrm{C}\right)=\mathrm{1307,66}\mathrm{kJ}$$

mit s=fest, l=flüssig und g=gasförmig.Using FactSage version 8.3, the energies for the oxidation reaction of the elements Si and Ti in molten pig iron at a temperature of 1550 °C in combination with oxygen at a temperature of 25 °C, and for the melting of Fe at an applied temperature of 25 °C to a temperature of 1550 °C, were determined and modeled. The functionality of FactSage is familiar to those skilled in the art. Only the respective raw data were considered in the calculation, not mixing enthalpies, etc. Thus, the following result was generated: $$\begin{array}{c}1\mathrm{kg}\mathrm{Si}\left(\mathrm{l},1550°\mathrm{C}\right)+\mathrm{1,14}\mathrm{kg}{\mathrm{O}}_2\left(\mathrm{g},25°\mathrm{C}\right)=\mathrm{2,14}\mathrm{kg}{\mathrm{SiO}}_2\left(\mathrm{l},1550°\mathrm{C}\right)\mathrm{bei}1550°\mathrm{C}\\ =-\mathrm{31467,81}\mathrm{kJ}\end{array}$$

$$\begin{array}{c}1\mathrm{kg}\mathrm{Ti}\left(\mathrm{l},1550°\mathrm{C}\right)+\mathrm{0,67}\mathrm{kg}{\mathrm{O}}_2\left(\mathrm{g},25°\mathrm{C}\right)=\mathrm{1,67}\mathrm{kg}{\mathrm{TiO}}_2\left(\mathrm{l},1550°\mathrm{C}\right)\mathrm{bei}1550°\mathrm{C}\\ =-\mathrm{17785,34}\mathrm{kJ}\end{array}$$

$$1\mathrm{kg}\mathrm{Fe}\left(\mathrm{s},25°\mathrm{C}\right)=1\mathrm{kg}\mathrm{Fe}\left(\mathrm{l},1550°\mathrm{C}\right)=\mathrm{1307,66}\mathrm{kJ}$$

with s=solid, l=liquid and g=gaseous.

Thus, in the above-mentioned model, the energy exothermically generated during the production of SiO ₂ would enable the melting of approximately 24 times the amount of iron without the input of additional energy. Similarly, the energy exothermically generated during the production of TiO ₂ would enable the melting of approximately 13 times the amount of iron without the input of additional energy.

The modeling shows that the aforementioned contents of silicon and titanium in the pig iron melt serve as chemical storage components for (additional thermal) energy.

As a result, additional iron carriers, preferably iron-containing scrap, can be fed into the converter in the existing process in addition to the previously known use of iron carriers, so that the energy alone in the pig iron melt by setting or maintaining silicon with a content between 0.40 and 5.0 wt.% and titanium with a content between 0.040 to 0.90 wt.% is sufficient for an additional iron yield, in particular by using additional scrap and/or sponge iron pieces and/or sponge iron pellets and/or sponge iron briquettes with a proportion of the additional iron carriers of at least 3 wt.% and a maximum of 30 wt.% based on the total mass of the steel melt produced or to be produced.

The liquid slag tapped from both the melter and the converter is preferably fed into a granulation process in order to provide slag, particularly for the construction industry.

Claims (6)

A method for producing a steel melt, comprising the following steps: - producing an iron-based melt in an electric melter, in which iron carriers, optionally carbon carriers and/or optional slag formers are introduced and melted in the melting operation to form a pig iron melt and a liquid slag floating on the pig iron melt; - treating the pig iron melt produced in the electric melter in a converter to produce a steel melt, with additional iron carriers being introduced before and/or during the converter treatment; characterized in that a silicon content of at least 0.40 to 5.0 wt.% and a titanium content of at least 0.040 to 0.90 wt.% is set and/or maintained in the pig iron melt during the melting operation and a silicon content of at least 10% lower and a titanium content of at least 10% lower in the steel melt compared to the pig iron melt is achieved by the converter treatment, wherein the iron carriers additionally introduced before and/or during the converter treatment consist of or comprise sponge iron pieces and/or sponge iron pellets and/or sponge iron briquettes, scrap, blast furnace pig iron, wherein the proportion of the additional iron carriers can be at least 3 wt.% and at most 30 wt.% of the total mass of the steel melt to be produced. Process according to claim 1, wherein reduced iron ore carriers in the form of sponge iron pieces and/or sponge iron pellets with a carbon content between 0 and 4.8 wt.% and a degree of metallization of at least 75% are used as iron carriers. Process according to one of the preceding claims, wherein slag formers are optionally introduced which comprise at least one or more of the elements from the group (CaO, MgO, SiO ₂ , Al ₂ O ₃ ) in order to be able to set a basicity B4 in the liquid slag between 0.9 and 1.8. Process according to one of the preceding claims, wherein carbon carriers are optionally introduced, which can be provided in gaseous, liquid and/or solid form with reducible free carbon in order to be able to adjust the carbon content in the pig iron melt between 2.50 and 4.80 wt.%. Process according to one of the preceding claims, wherein scrap is introduced as an additional iron carrier, which may comprise at least one or more of the elements from the group (Cr, Cu, Mo, Zn, Ni) up to 0.20 wt.% in total. Method according to one of the preceding claims, wherein the energy required for melting is provided from renewable energy.