PROCESS FOR PRODUCING CLEAN STEEL WITH A LOW NITROGEN CONTENT USING AN ELECTRIC ARC FURNACE AND A DEGASSING SYSTEM

Abstract

A process for producing clean steel products with low nitrogen content, below 35 ppm, in a steelmaking plant comprising a direct reduced iron (DRI) source, which may be a direct reduction plant or a DRI storage facility, an electric arc furnace (EAF), a vacuum degassing system (DS), and a continuous casting system (CC) is disclosed. The process comprises a first stage of melting and refining a metallic iron charge, a second stage of tapping molten steel from the electric arc furnace (EAF) into a ladle, a third stage of exposing molten steel to a pressure below the atmospheric pressure and a fourth stage of casting molten steel to clean steel products. Optionally, the molten steel tapped from the EAF is treated in a ladle furnace (LF) prior to being treated in the degassing system (DS). The metallic iron charge fed to the EAF comprises more than 70% by weight of DRI in the form of pellets or briquettes having a carbon content above 2.5 weight %. Preferably, the metallic iron charge is fed to the EAF at a temperature of 400 C. or higher. The low nitrogen level in the steel products made according to the Application is achieved by forming a first foamy slag in said first process stage and is maintained in a foamy state by controlling the feed of fluxes, oxygen, and carbonaceous materials to the EAF and by forming a second slag, after molten steel is tapped from the EAF, having a predetermined composition capable of continuing the desulfurization and providing a thermal and chemical insulation to prevent nitrogen pickup and promote nitrogen removal of molten steel. The process also comprises carrying out one or more of the following actions: (a) controlling the concentration of nitrogen and sulfur in the raw materials at each process stage, (b) promoting nitrogen removal from steel, (c) decreasing the time spent by the molten steel at each process stage and between each and subsequent process stages, and (d) preventing nitrogen pickup by the molten steel all along said process stages. Steel products made according to the Application comprise the following elements expressed in weight %: C0.05%, Si4.5%, Al2.0%; Mn2.0%; P0.20%; Ni0.200%, Cu0.200%; N0.0030%, Ni0.200%, S0.0035%.

Claims

1. A process for producing clean steel products with low nitrogen content below 35 ppm, in a steelmaking plant comprising a direct reduced iron (DRI) source, an electric arc furnace (EAF), a degassing system (DS), and a continuous casting system (CC), wherein said process comprises a first stage of melting, and refining in said electric arc furnace (EAF) a metallic iron charge comprising steel scrap and/or one or more of direct reduced iron (DRI), hot briquetted iron (HBI), pig iron, or other metallic iron-bearing materials, a second stage of tapping molten steel from the electric arc furnace (EAF) into a ladle, a third stage of exposing molten steel to a pressure below the atmospheric pressure in said degassing system (DS) and a fourth stage of casting molten steel to a steel product in said continuous casting (CC), said process being characterized by said hot metallic iron charge having a maximum of 60 ppm of sulfur and 35 ppm, preferably below 30 ppm of nitrogen, and a carbon content above 2.5%, forming a first foamy slag having a predetermined composition during said first process stage, maintaining said first foamy slag in a foamy state during said first process stage by controlling the feed of fluxes, oxygen, and carbon to said electric arc furnace (EAF), and forming a second desulfurizing slag during said second process stage, having a predetermined composition capable of continuing the desulfurization and providing thermal insulation and chemical insulation of the molten steel during the subsequent process stages.

2. A process for producing clean steel products with low nitrogen content below 35 ppm, according to claim 1, wherein the molten steel tapped from the electric arc furnace (EAF) is treated in a ladle furnace (LF) prior to being treated in the vacuum degassing system (DS).

3. A process for producing clean steel products with low nitrogen content according to claim 1, wherein in said first foamy slag the amount of each of the following oxides are comprised within the following ranges where the sum thereof represents 100% by weight: FeO from 24% to 40%, MgO from 7% to 15%, Al.sub.2O.sub.3: from 4% to 11%, CaO: from 16% to 40%, SiO.sub.2: from 8% to 24%.

4. A process for producing clean steel products with low nitrogen content according to claim 1, wherein in said second desulfurizing slag the amount of each of the following oxides are comprised within the following ranges where the sum thereof represents 100% by weight: MgO from 5% to 9%, Al.sub.2O.sub.3 from 30% to 35%, CaO from 50% to 58%, SiO.sub.2 from 3% to 8%.

5. A process for producing clean steel with low nitrogen content according to claim 1, wherein the Basicity B3, defined as B3=CaO/(Al.sub.2O.sub.3+SiO.sub.2), of said first foamy slag is within the range of 0.7 to 1.5.

6. A process for producing clean steel with low nitrogen content according to claim 1, wherein the metallic iron charge fed to the electric arc furnace (EAF) contains a maximum of 60 ppm of sulfur and 35 ppm of nitrogen, preferably below 30 ppm of nitrogen.

7. A process for producing clean steel with low nitrogen content according to claim 1, wherein the metallic iron charge fed to the electric arc furnace (EAF) comprises more than 70 weight % of direct reduced iron (DRI).

8. A process for producing clean steel with low nitrogen content according to claim 1, wherein said direct reduction plant produces direct reduced iron (DRI) in the form of pellets or briquettes having a carbon content greater than 2.5% by weight.

9. A process for producing clean steel with low nitrogen content according to claim 1, wherein said direct reduced iron (DRI) in the form of pellets or briquettes has a carbon content between 3% and 4% by weight.

10. A process for producing clean steel with low nitrogen content according to claim 1, wherein said direct reduced iron (DRI) in the form of pellets or briquettes has a carbon content between 4% and 6% by weight.

11. A process for producing clean steel with low nitrogen content according to 1, wherein said direct reduced iron (DRI) in the form of pellets or briquettes is charged to the electric arc furnace (EAF) at a temperature above 400 C.

12. A process for producing clean steel with low nitrogen content according to claim 1, wherein said direct reduced iron (DRI) source is a silo or bin storing direct reduced iron (DRI) in the form of pellets or briquettes.

13. A process for producing clean steel with low nitrogen content according to claim 1, wherein said MgO is added in the electric arc furnace (EAF) in the form of dolomite.

14. A process for producing clean steel with low nitrogen content according to claim 1, wherein argon is injected into the electric arc furnace (EAF) as transport gas to feed carbonaceous materials and to keep an inert atmosphere inside the electric arc furnace (EAF).

15. A process for producing clean steel with low nitrogen content according to claim 1, wherein said degassing system (DS) is a vacuum treatment system.

16. A process for producing clean steel with low nitrogen content according to claim 1, wherein said degassing system is a Ruhrstale-Heraeus furnace (RH) system.

17. A process for producing clean steel with low nitrogen content according to claim 1, wherein a stirring system is installed in the electric arc furnace (EAF).

18. A process for producing clean steel with low nitrogen content according to claim 17, wherein said stirring system is electromagnetic.

19. A process for producing clean steel with low nitrogen content according to claim 17, wherein said stirring system is based on a gas injection.

20. A process for producing clean steel with low nitrogen content according to claim 19, wherein said gas is argon.

21. A process for producing clean steel with low nitrogen content according to claim 1, wherein the composition of the steel after tapping from the electric arc furnace (EAF) includes: Carbon:<0.05 weight %; Oxygen:>0.04 weight %; Nitrogen:<0.002 weight %; Sulfur:<0.020 weight %;

22. A process for producing clean steel with low nitrogen content according to claim 1, wherein the composition of the steel product after continuous casting (CC) includes: Nitrogen:<0.0035 weight % and Sulfur:<0.015 weight %.

23. A process for producing clean steel with low nitrogen content according to claim 1, being further characterized by said process comprising feeding continuously to the electric arc furnace (EAF) one or more of steel scrap, direct reduced iron (DRI) (pellets or briquettes), and hot briquetted iron (HBI), pig iron or hot metal.

24. A process for producing clean steel with low nitrogen content according to claim 1, further comprising: (a) controlling the concentration of nitrogen and sulfur in the raw materials at each process stage, (b) promoting nitrogen removal from steel, (c) decreasing the time spent by the molten steel at each process stage and between each and subsequent process stages, and (d) preventing nitrogen pickup by the molten steel all along said process stages.

25. A process for producing clean steel with low nitrogen content according to claim 24, being further characterized by said process comprising limiting the sulfur input to the electric arc furnace (EAF) by controlling the sulfur content of carbon materials, direct reduced iron (DRI) (pellets or briquettes), hot briquetted iron (HBI), pig iron, hot metal, alloys, fluxes, which are sources of sulfur.

26. A process for producing clean steel with low nitrogen content according to claim 1, where the treating time of molten steel at said third stage is at least 15 minutes.

27. A process for producing clean steel with low nitrogen content according to claim 1, where the molten steel is exposed to a pressure level of less than 1 millibar.

28. A clean steel product with low nitrogen content made according to claim 1, comprising the following elements in weight %: C0.05%; Si4.5%; Al2.0%; Mn2.0%; P0.20%; Ni0.200%; Cu0.200%; N0.0030%; S0.0035%.

29. A clean steel product with low-nitrogen content made according to claim 1, further comprising one or more of the following elements within the respective limits expressed in weight %: Sb0.20%; Sn:0.05%; Cr:0.20%; Mo:0.20%; Ti:0.10%; Bi:0.010%; Pb:0.010%; V:0.100%; B:0.0050%; Rare Earths Elements:0.020%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 shows a schematic diagram of the steelmaking route and its main process stages of an embodiment not including a ladle furnace (LF) treatment stage.

[0028] FIG. 2 shows a schematic diagram of the steelmaking route and its main process stages of an embodiment including a ladle furnace (LF) treatment stage.

[0029] FIG. 3 shows a graph comparing the nitrogen content of molten steel after each stage of the process, according to an embodiment not including a ladle furnace (LF), without applying the process of the application and applying the process of the application.

[0030] FIG. 4 shows a graph comparing the nitrogen content of molten steel after each stage of the process, according to an embodiment including a ladle furnace (LF), without applying the process of the application and applying the process of the application.

[0031] FIG. 5 shows a graph showing the preferred relationship between the content of MgO and the content of FeO in the slag formed in the electric arc furnace (EAF) to assure that the first slag is maintained foamy enough to remove nitrogen from steel.

[0032] FIG. 6 shows a ternary diagram of the oxides CaO, Al.sub.2O.sub.3 and SiO.sub.2 showing the preferred relationship of these oxides in a second slag formed during tapping of molten steel from the EAF to assure that said second slag is capable of continuing desulfurization and thermal insulation and chemical insulation of molten steel.

[0033] FIG. 7 shows a ternary diagram of the same oxides of FIG. 4 showing the preferred relationship of these oxides in the second slag so that it remains liquid to continue the desulfurization and thermal insulation and chemical insulation of molten steel.

DETAILED DESCRIPTION

[0034] This description is intended to be exemplary only and there is no intention of limiting the applications or uses of the present inventions which is defined in the claims. To facilitate the reading of the description of the embodiments shown here, the same reference numerals are used to designate the same or equivalent elements throughout all the Figures.

[0035] Referring to FIG. 1, numeral 10 generally designates a process route for producing steel with low nitrogen content according to an embodiment. Route 10 comprises a direct reduced iron source (DRI) 11, an electric arc furnace (EAF) 12, a vacuum degassing system (DS) 24 and a continuous casting system (CC) 28.

[0036] A charge of metallic iron materials such as DRI and/or HBI 14 and steel scrap 13 are fed to the EAF 12 to form a molten steel bath to be processed adjusting the concentration of C, N, S, P, Mn, among other elements to meet the specifications of the desired steel product. Electrical energy is applied to one or more electrodes that heat and melt the iron charge from an electric arc between the electrodes and said metallic charge. Fluxes (for example, dolomite and lime) additives 15 and carbonaceous materials 16 are fed to the EAF to form a layer of a first foamy slag.

[0037] The metallic iron charge may be one or more of the following iron-bearing materials: steel scrap 13, direct reduced iron (DRI) (pellet or briquet) and/or hot briquetted iron (HBI) 14, solid or liquid pig iron, or hot metal. According to some embodiments, these materials have a carbon content above 2.5% by weight, preferably above 3% by weight or between 3% and 4% by weight and in some embodiments between 4% and 6% by weight because said carbon reacts with oxygen in the molten steel forming CO which evolves in small bubbles through the molten steel and then the nitrogen is withdrawn from the electric arc furnace (EAF). It is theorized that nitrogen is transferred from the liquid steel into the CO bubbles and is then withdrawn from the EAF along with other gases 18 that evolve during the melting, and refining stage of the EAF operation. The amount of CO bubbles generated from carbon in the metallic charge and therefore the amount of nitrogen removed, is directly proportional to the amount of carbon in the charge. Preferably, most of the carbon in the DRI is in the form of iron carbide Fe.sub.3C.

[0038] Additional carbonaceous materials 16 with low nitrogen content may be injected into the molten steel through suitable lances and carrier gases. The carbonaceous materials may be coke, coal, graphite or similar carbon carriers. According to some embodiments, the carbonaceous materials may also be derived from biomass or any biofuel. Carbonaceous materials 16 are injected into the electric arc furnace (EAF) 12 using argon 17 as transport gas.

[0039] The electric arc furnace (EAF) is a vessel that is not easy to be isolated from the environmental air and therefore, the molten steel therein may be exposed to air, and it may pick up nitrogen, especially in the zone of the electric arc due to the ionizing effect of the electric arc on the air. To decrease the entrance of air into the EAF that might contact and transfer nitrogen to the steel, the steel scrap 13, DRI (pellet or briquet), HBI or any other solid iron-containing charge 14 are fed to the EAF as continuously as possible so as to form and maintain a stable foamy slag layer, the slag door 20 of the EAF 12 is maintained closed to minimize air entrance, and argon 17 is used as transport gas to feed the carbonaceous materials 16 and also to maintain an inert and positive pressure atmosphere inside the EAF 12.

[0040] In some embodiments, an electromagnetic stirring system 19 and/or other stirring systems are installed in the EAF 12. This stirring of the liquid steel contributes to nitrogen removal because the extra stirring increases the residence time of the bubbles of CO, resulting in a larger travel trajectory of said bubbles and therefore, they have more time to catch and remove nitrogen. Next, in the interface between steel and slag layer, the bubbles of CO in the slag provide an increased area transfer; so, nitrogen elimination is favored (mass transfer controlling step); and as a result, the nitrogen gets into the bubbles and then is released to the air.

[0041] Stirring during tapping by means of argon injection 23 and additions of fluxes to produce CO (for example, CaCO.sub.3), contributes to remove nitrogen. During this stage it is important to maintain argon flowing through porous plugs which, added to the oxygen concentration and the generation of gases from various fluxes and slag formers, favor the removal of nitrogen and avoid its pickup by steel.

[0042] This effect may be explained by some reactions producing CO and CO.sub.2, for example CaCO.sub.3, that produce CO.sub.2 which in turn decreases the partial pressure of nitrogen in the gas phase, which inhibits the nitrogen transfer back to the molten steel.

[00001]$\mathrm{CaCO}3=CaO+{\mathrm{CO}}_2$$P_T=\underset{i}{\overset{n}{.Math.}}{p}_i=p_{\left[\mathrm{Ar}\right]}+p_{\left[\mathrm{CO}\right]}+p_{\left[O\right]}+p_{\left[N\right]}$

[0043] Where [0044] P.sub.T=total pressure of the system 1 atm [0045] p.sub.[Ar]=partial pressure of Argon [0046] p.sub.[CO]=partial pressure of CO [0047] p.sub.[O]=partial pressure of oxygen [0048] p.sub.[N]=partial pressure of nitrogen

[0049] When the partial pressure of other gases in the steel increases, the nitrogen partial pressure decreases.

[0050] After the full charge is melted and the temperature of the steel is adequate for continuing its processing (above 1600 C.), the molten steel 25 is tapped from the electric arc furnace (EAF) to a ladle. The ladle is then transferred to a vacuum degassing system (DS) 24.

[0051] The degassing system may be a conventional vacuum degasser (DS) or a Ruhrstale-Heraeus furnace (RH) or similar vacuum system where the pressure is reduced below atmospheric pressure to promote extraction of some elements in steel as carbon, hydrogen and nitrogen, which is extracted therefrom along with other gases 40.

[0052] After treatment in the vacuum degassing system (DS) 24, the molten steel 26 is transferred to a continuous casting installation (CC) 28 to be cast as sheet or in the form of other shaped product 30. To avoid nitrogen pickup at this fourth process stage, the molten steel contained in the ladle, tundish and mold is protected from contact with nitrogen of air by a cover of argon atmosphere and with casting powder, as well as with the use of other devices.

[0053] In some embodiments, as described with reference to FIG. 1, there is no need of processing the steel in a ladle furnace (LF), thus shortening the processing time and also decreasing the capital and operation costs of the steelmaking facilities. The shortened process route, without an LF treatment stage, also provides the benefit of decreasing the possible contact of steel with atmospheric air, thus maintaining the low content of nitrogen up to the final process stage.

[0054] Referring to FIG. 2, the process route 50, for producing steel with low nitrogen content according to another embodiment, comprises a direct reduced iron (DRI) source 11, an electric arc furnace (EAF) 12, a vacuum degassing system (DS) 24, a continuous casting system (CC) 28, and a ladle furnace (LF) 19 adding a fifth process stage carried out between the second process stage in the said EAF 12 and the second process stage in said vacuum degassing system 24. The LF treatment may be useful in some embodiments to carry out refining operations such as deoxidation, desulfurization, addition of alloying elements or inclusion modification.

[0055] In this embodiment, the molten steel 25 tapped from the electric arc furnace (EAF) 12 is transferred to a ladle furnace (LF) process stage 29 to be further refined in a ladle metallurgy stage. Thereafter, the molten steel 27 is treated in a vacuum degassing system (DS) 24 and thereafter transferred to a continuous casting system (CC) 28 in a similar manner as described with reference to the process route of FIG. 1.

[0056] FIG. 3 shows the results obtained in tests of the steelmaking process indicating the average nitrogen content of the steel after the first process stage at the EAF, the third process stage at the DS and the fourth process stage at the CC, according to an embodiment following process route 10 without an LF treatment stage. The content of nitrogen in the steel after each process stage is lower when practicing embodiments of the present application than the content of nitrogen not using the present embodiments and meets the desired nitrogen content in the final steel product after CC.

[0057] FIG. 4 shows the results obtained in tests of the steelmaking process indicating the average nitrogen content of the steel after the first process stage at the EAF, the fifth process stage LF treatment, the third process stage DS at the degassing system and the fourth process stage at the CC, according to an embodiment following process route 50 comprising an LF treatment stage. The content of nitrogen in the steel after each process stage is lower when practicing embodiments of the present application than the content of nitrogen not using the present embodiments and meets the desired nitrogen content in the final steel product after CC.

[0058] In another aspect, the low nitrogen content of the steel is achieved following three general strategies for nitrogen removal or nitrogen pickup prevention: [0059] (a) controlling the concentration of nitrogen and sulfur in the raw materials at each process stage, [0060] (b) promoting nitrogen removal from steel, [0061] (c) decreasing the time spent by the molten steel at each process stage and between each and subsequent process stages, and [0062] (d) preventing nitrogen pickup by the molten steel all along said process stages.

[0063] The strategy (a) includes the following actions: [0064] Feeding to the electric arc furnace (EAF) one or more of premium steel scrap, direct reduced iron (DRI) (pellet or briquet), hot briquetted iron (HBI), briquet iron, pig iron or hot metal constituting at least 70% by weight of the metallic iron charge, having a carbon content above 2.5% by weight. [0065] Feeding the direct reduced iron (DRI) (pellet or briquet) or hot briquetted iron (HBI) continuously to the electric arc furnace (EAF) and maintaining the slag door closed. [0066] Argon is used as transport gas to feed the carbonaceous material and to keep an inert atmosphere inside the electric arc furnace (EAF). [0067] Limiting the sulfur input to the electric arc furnace (EAF) by controlling the sulfur content of carbon materials, direct reduced iron (DRI) (pellet or briquet), hot briquetted iron (HBI), pig iron, hot metal, alloys, fluxes, which are sources of sulfur.-Using argon as carbon transport gas instead of compressed air.

[0068] The strategies (b) and (c) include the following actions: [0069] Preferably, feeding the direct reduced iron (DRI) (pellet or briquet) at a high temperature of 400 C. or higher. [0070] Forming a foamy slag layer in the electric arc furnace (EAF) for protection of the electric arc radiation and for improving the electrical energy transfer to the metallic charge, whereby the power-on time can be reduced and consequently the exposure of the metallic bath to air and therefore the nitrogen pickup is also reduced. [0071] Stirring system in the electric arc furnace (EAF) (electromagnetic stirring systems, argon injection through porous plugs systems, etc). [0072] Formulation and forming a second slag during tapping the steel from the electric arc furnace (EAF). [0073] Use only hot (more than 3 heats) and cleaned ladles (not skulls or slag are allowed) [0074] Coordinating the ladle moving operations of the steelmaking facilities so that the time that steel spends between each process stage is minimized. [0075] Shortening the decarburization time at the degassing system in order to extend the time of nitrogen removal after deoxidation.

[0076] The strategy (d) includes the following actions: [0077] Design and formulation of slag during tapping for the following purposes: [0078] carry over furnace slag should be maximum 4 kg/t [0079] formula of liquid slag capable to avoid reoxidation and nitrogen pickup. [0080] providing a thermal isolation for steel during the transfer through all process stages. [0081] able to keep the sulfur in the slag and to continue desulfurization in the subsequent process stages and helps to remove nitrogen. [0082] All lift gas nozzles in degasser must be available during the tests to allow required lift gas flow [0083] Vessel and snorkel in degasser must be well pre-heated and maximum 1 hour after the last treatment [0084] in degasser, at least 3 heats must be treated prior the tests with the same kind of grades which will be subjected to the test [0085] In degasser, vessel and snorkel must be clean and free of skull [0086] Purging system of tundish before and during casting [0087] Draining any hot heel remaining at the electric arc furnace (EAF) and changing the steel tap hole.

[0088] The above strategies define a steelmaking practice that the applicant has tested at industrial scale, provides an effective and efficient production process of clean steel with low nitrogen content. Embodiments of the present Application provide a process where the average nitrogen content in the steel is maintained at 35 ppm or less.

[0089] The heat produced by the electric arc is transferred to the metallic charge being melted and to the molten steel, but it may also overheat the refractory lining or in the wall and roof of the EAF causing damages that require repair or re-lining of the furnace or replacement of water-cooled panels. The foamy slag is formed by CO produced by the reaction of carbon in the metallic charge and oxygen from oxides in said charge and oxygen injected for decarburization protects the walls and roof of the EAF.

[0090] The composition of this first foamy slag comprises FeO, CaO, MgO, Al.sub.2O.sub.3, SiO.sub.2 among its main elements. The foamy characteristics and the basicity of the first slag produced in the EAF 12 are formulated so that the foamy state of said slag is maintained in the EAF 12. In some embodiments, the composition of the first slag is formulated on the basis of diagrams of thermal iso-activity of elements to promote nitrogen removal and prevent nitrogen pickup during this process stage.

[0091] With reference to FIG. 5, the first foamy slag formed at the electric arc furnace (EAF) 12 has a composition where the ratio of MgO to FeO lies above the line 38 in the phase diagram and within the zone 39 in said phase diagram. This line 38 indicates that the slag has the foamy properties to achieve the intended purposes to provide: thermal isolation and a physical barrier protecting the steel from contact with atmospheric air as well as an improved arc cover reducing the power-on time and as a result also the steel processing time is reduced.

[0092] Referring to FIG. 6, in some embodiments, the design and formulation of the second slag formed after tapping from the electric arc furnace (EAF) 12 provides to this second slag a high capacity of desulfurization that avoids the need of a ladle furnace (LF) treatment to remove sulfur and therefore advantageously the overall process time of the process route 10 of steel is shortened.

[0093] A second slag is formulated and formed after tapping the molten steel 25 from the electric arc furnace (EAF) 12 with a composition having a high capacity of desulfurization and providing thermal insulation and chemical insulation of the molten steel. The composition of the second slag, when the MgO content is between 5% and 8% by weight in said slag, comprises Al.sub.2O.sub.3, CaO and SiO.sub.2 in the following proportions in weight % of the sum of these compounds: [0094] Al.sub.2O.sub.3: 30%-35% [0095] CaO: 50%-58% [0096] SiO.sub.2: 3%-8%

[0097] These proportions provide the capability of slag to remove sulfur from steel with values of Ls=1000, Ls=500 and Ls=200 where Ls=(S)/[S] and where(S) is the concentration of sulfur in slag, and [S] is the concentration of sulfur in steel.

[0098] The second slag according to some embodiments is designed to form and remain as a liquid layer over the steel in order to avoid nitrogen pickup and continuing desulfurization after tapping from the electric arc furnace (EAF) 12 as well as to provide improved process conditions to remove the nitrogen content of steel during vacuum degassing treatment (DS).

[0099] Referring to FIG. 7, the composition of this second slag is therefore controlled so that on the basis of a constant content of MgO the amount of each of the relevant oxides are comprised within the following ranges where the sum thereof represents 100% by weight: MgO from 5% to 9%, Al.sub.2O.sub.3 from 30% to 35%, CaO from 50% to 58%, SiO.sub.2 from 3% to 8%. These proportions are indicated by the area 45 in the diagram.

[0100] In some embodiments, the Basicity B3 of said first foamy slag, defined as B3=CaO/(Al.sub.2O.sub.3+SiO.sub.2), is within the range of 0.7 to 1.5.

[0101] In some embodiments, the hot metallic charge of direct reduced iron (DRI) (pellet or briquet) and/or hot briquetted iron (HBI) and/or is fed to the electric arc furnace (EAF) at a temperature above 400 C.

[0102] In some embodiments, the MgO is added in the EAF in the form of dolomite.

[0103] In some embodiments, argon is injected into the EAF as transport gas to feed carbonaceous materials and to keep an inert atmosphere inside the EAF.

[0104] In some embodiments, the degassing system (DS) is a vacuum treatment system.

[0105] In some embodiments, the degassing system (DS) is a Ruhrstale-Heraeus furnace (RH) system.

[0106] In some embodiments, the residence time of steel at the degassing stage is at least 18 minutes, where it is exposed to a deep vacuum level of less than 1 milibar under reducing conditions. These conditions favor sulfur and nitrogen removal.

[0107] In some embodiments, the composition of the steel after tapping from the EAF includes: [0108] Carbon:<0.05%, [0109] Oxygen:>400 ppm [0110] Nitrogen:<20 ppm [0111] Sulfur:<0.020%

[0112] In some embodiments, the composition of the steel product after continuous casting includes Nitrogen:<25 ppm and Sulfur:<0.015%.

[0113] In some embodiments, one or more of steel scrap, DRI (pellet or briquet), HBI, pig iron or hot metal are continuously fed to the EAF.

[0114] In some embodiments, the sulfur content of carbon materials, premium steel scrap, DRI (pellet or briquet), HBI, pig iron, hot metal, alloys, fluxes, which are sources of sulfur, is limited within predetermined amounts.

EXAMPLES

[0115] Several heats in a steel plant were carried out according to some embodiments of the Application and the results are here described.

[0116] The heats were carried out in a direct current electric arc furnace (EAF) with conventional electrodes. The metallic iron charge was 100% direct reduced iron (DRI) having 3 w % of carbon and a hot heel of 55 tons. Average power was 103 MW. The average duration of heats was 56 minutes, and the tap temperature was 1630 C. A summary of these heats is shown in TABLE 1:

TABLE-US-00001 TABLE 1 N at EAF Degassing tapping T Oxygen Dolo lime Lime time HEAT [ppm] [ C.] [ppm] [kg/t] [kg/t] [min] 1 17 1632 895 84.0 2.7 20 2 10 1650 1056 77.4 3.3 20 3 11 1628 992 71.6 2.3 20 4 11 1635 1066 74.8 2.2 20 5 15 1610 983 68.8 2.3 20 6 20 1626 887 73.1 2.5 20 Average 14 1630 979.8 74.9 2.5 20

[0117] After tapping from the electric arc furnace (EAF) the nitrogen content was 21 ppm, at the beginning of the degassing stage 28 ppm, at the end of degassing 22 ppm and at the end of continuous casting (CC) 33 ppm.

[0118] The composition of the slag at the end of stages 1 and 2 are shown in TABLE 2:

TABLE-US-00002 TABLE 2 Stage 1 - EAF Stage 2 - Tapping EAF CaO 25.00 55.7 SiO.sub.2 15.97 4.9 Al.sub.2O.sub.3 7.32 30.9 MgO 14.11 8.4 FeO 37.60

[0119] Another aspect relates to the manufacture of clean steels where the necessary alloying elements to fulfill the specifications for particular applications formed as steel sheet.

[0120] The present Application provides a process to produce clean steel with reduced contents of carbon, nitrogen and sulfur by a combination of refining techniques through the use of specific slag compositions, vacuum degassing and inert gas purging, as well as preventing the steel from picking up nitrogen or other impurities during the various process stages from raw materials melting and refining up to continuous casting of said steel.

[0121] In some embodiments, clean steel products with low nitrogen content can be produced, comprising the following elements in weight %: [0122] C0.05%; [0123] Si4.5% [0124] Al2.0%; [0125] Mn2.0%; [0126] P0.20%; [0127] Ni0.200% [0128] Cu0.200%; [0129] N0.0030% [0130] S0.0035%.

[0131] In some embodiments, a clean steel product with low-nitrogen content made can be produced, further comprising one or more of the following elements within the respective limits expressed in weight %: [0132] Sb0.20%; [0133] Sn:0.05%; [0134] Cr:0.20%; [0135] Mo:0.20%; [0136] Ti:0.10% [0137] Bi:0.010%; [0138] Pb:0.010%; [0139] V:0.100% [0140] B:0.0050%. [0141] Rare Earths Elements:0.020%.

[0142] The clean steel products made according to some embodiments comply with the requirements set by the markets related to the automotive industry, particularly for applications where a high-quality exposed surface is demanded.

[0143] It is of course to be understood that the embodiments described here are only included for a better understanding of the scope and spirit of the inventions and not to limit in anyway its scope, which is defined in the appended claims.