A steelmaking method

Abstract

The invention relates to a steelmaking method comprising the following successive steps: a. feeding an electrical furnace with an initial load consisting of 0 to 50 wt.% of pig iron or hot metal, 0 to 50 wt.% of scraps, and at least 50 wt.% of reduced iron, the respective load content in sulfur and titanium being below 100 ppm in weight after melting down, b. tapping the melted load in a ladle and submitting it to a decarburization step without prior deoxidation, then to an alloying step and then to a desulfurization step, then to an optional final alloying step to obtain a liquid steel having a sulfur content below 30 ppm in weight, a titanium content below 100 ppm in weight and a carbon content below 50 ppm in weight, c. casting said liquid steel.

Description

[0001] A steelmaking method

[0002]

[0001] The present invention relates to a steelmaking method for the manufacturing of steels containing limited amounts of elements like carbon, titanium and sulfur. Though not limited to any specific type of steel, such method is particularly suited to the manufacturing of non-oriented electrical steel.

[0003]

[0002] Steel can be currently produced through two mains manufacturing routes. Nowadays, most commonly used production route named “BF-BOF route” consists in producing hot metal (also called pig iron once solidified) in a blast furnace, by use of a reducing agent, mainly coke, to reduce iron oxides and then transform hot metal into steel in a converter process or Basic Oxygen Furnace (BOF). This route, both in the production of coke from coal in a coking plant and in the production of the hot metal, releases significant quantities of CO2.

[0004]

[0003] The second main route involves named “EAF route” consists in producing liquid steel in an Electric Arc Furnace fed by scrap, hot metal, pig iron or reduced iron obtained by different reduction methods. Among them are methods according to the brands MIDREX®, FINMET®, ENERGIRON® / HYL, etc., in which sponge iron is produced in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron), or HBI (hot briquetted iron) from the direct reduction of iron oxide carriers. Sponge iron in the form of HDRI, CDRI, and HBI undergoes further processing in electric furnaces to produce steel.

[0005]

[0004] One of the main options chosen by steelmakers to reduce CO2 emissions is therefore to switch from the BF-BOF route towards the EAF route. However, use of DRI products in classical electrical furnaces together with ferrous scraps has some limitations. Indeed, scraps contain a lot of impurities like carbon, sulfur, nitrogen and titanium and the resulting liquid melt will need to be further processed to produce high quality steel grades. i

[0005] It is known from WO 2023 / 028619 to provide a steelmaking method wherein the steel is fully deoxidized, before being desulfurized and then decarburized in a RH vessel by oxygen blowing. Such method is however not practical from an industrial point of view, as the decarburization step requires to blow a big quantity of oxygen in the liquid steel to remove carbon, all oxygen present in the steel being removed prior to the desulfurization step. Such succession of steps induces a long processing time which reduces the productivity of the plant.

[0006]

[0006] The aim of the present invention is therefore to remedy the drawbacks of the methods of the prior art by providing an improved steelmaking method allowing to limit the amounts of carbon, sulfur and titanium contained in such steel.

[0007]

[0007] For this purpose, a first object of the present invention consists in a steelmaking method comprising the following successive steps:

[0008] - feeding an electrical furnace with an initial load consisting of 0 to 50 wt.% of pig iron or hot metal, 0 to 50 wt.% of scraps, and at least 50 wt.% of reduced iron, the respective load content in sulfur and titanium being below 100 ppm in weight after melting down,

[0009] - tapping the melted load in a ladle and submitting it to a decarburization step without prior deoxidation, then to an alloying step and then to a desulfurization step, then to an optional final alloying step to obtain a liquid steel having a sulfur content below 30 ppm in weight, a titanium content below 100 ppm in weight and a carbon content below 50 ppm in weight,

[0010] - casting said liquid steel.

[0011]

[0008] The method according to the invention may also have the optional features listed in claims 2 to 11 , considered individually or in combination.

[0012]

[0009] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0013]

[0010] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive.

[0011] The steelmaking method according to the invention comprises several successive steps. The first one consists in feeding an electrical furnace with an initial load consisting of 0 to 50 wt.% of pig iron or hot metal, 0 to 50 wt.% of scraps, and at least 50 wt.% of reduced iron, the respective load content in sulfur and titanium being below 100 ppm in weight after melting down or even better below 50 ppm in weight after melting down.

[0014]

[0012] Pig iron or hot metal is an optional element of the initial load but is preferably added in an amount of 10 to 30 wt.%. In another embodiment, the initial load does not contain any pig iron or hot metal. Pig iron or hot metal is obtained by smelting iron ore in a blast furnace and its sulfur and titanium contents depends on the sulfur and titanium contents of such iron ores and on the coals used to reduce the iron ores. Such sulfur and titanium contents can be controlled by selecting iron ores and coals with low sulfur / titanium or by removing sulfur from the hot metal through desulfurization methods known by the man skilled in the art. In a preferred embodiment, the initial load includes pig iron or hot metal comprising a maximum of 200 ppm, and even better of 150 or 100 or 50 ppm in weight of sulfur. This maximum amount of sulfur contributes to limiting the global content in sulfur of the load after melting down.

[0015]

[0013] Scraps are an optional element of the initial load but are preferably added in an amount of 15 to 30 wt.% for environmental reasons. Scraps are made of steel that has been previously manufactured and used, and which has then come to the end of its life in that form and can be recycled. In a preferred embodiment, the initial load includes scraps comprising a maximum of 200 ppm, and even better of 150, 100 or 50 ppm in weight of sulfur. This maximum amount of sulfur contributes to limiting the global content in sulfur of the load after melting down. Such scraps can be recovered from the steel plant past productions or from external sources and classified as types E6, E8 according to Ell-21 Steel scrap classification, or others.

[0016]

[0014] Reduced iron is a mandatory element of the initial load and represents at least 50 wt.% of such load, and preferably at least 60 or 70 wt.% of such load. Reduced Iron is the product of the reduction of iron ore in the solid state by carbon monoxide and / or hydrogen derived from natural gas, coal or from renewable sources. In a preferred embodiment, the initial load includes reduced iron chosen among HBI (Hot Briquetted Iron) or DRI (Direct Reduced Iron). The content in sulphur and titanium of the reduced iron depends on the sulfur and titanium contents of the iron ores used for the manufacturing of such reduced iron. Such sulfur and titanium contents can be controlled by selecting iron ores with low sulfur / titanium.

[0017]

[0015] An essential feature of the steelmaking method according to the invention is that its content in sulfur after melting down of the load is limited to 100 ppm in weight. As detailed above, the sulfur of the load can come from the pig iron or hot metal, from the scraps and / or from the reduced iron. The sulfur content of the melted load is determined by the amount of sulfur introduced in the electric arc furnace and the distribution of sulfur over the slag and molten steel.

[0018]

[0016] Another essential feature of the steelmaking method according to the invention is that its content in titanium after melting down of the load is also limited to 100 ppm in weight and preferably limited to 50 ppm in weight. As detailed above, at this stage of the steelmaking process, the titanium of the load can mainly come from the pig iron or hot metal, from the scraps and / or from the reduced iron. The titanium content of the melted load is determined by the amount of titanium introduced in the electric arc furnace and the amount of slag carry-over from the furnace to the ladle.

[0019]

[0017] In a preferred embodiment, the steelmaking method according to the invention provides for a setting of the carbon content of the initial load to a range of 2 to 3 wt.% by carbon addition before and / or during the melting down. Whenever needed, such carbon addition can be made through charging of coke and / or injection of carbon fine (such as graphite or petrol coke), for example. Such carbon charging is preferably performed during the melting of the load. The setting of the carbon content of the load is the preferred option as it allows ensuring a good foaming of the slag layer thereby improving the energy efficiency of this melting step and protecting the electric furnace sidewall and refractories. It is also helpful to control the nitrogen content of the steel.

[0018] The addition of carbon by adding graphite is a preferred embodiment, in particular when said injected carbon includes a maximum of 10000 ppm in weight of sulfur, or even better a maximum of 9000, 8000, 6000 or 2500 ppm in weight of sulfur

[0020]

[0019] In a preferred embodiment, the steelmaking method according to the invention provides for an addition of lime to the load of the electric furnace, such lime comprising a maximum of 500 ppm in weight of sulfur. The lime, which is mainly consisting in calcium oxides and / or calcium hydroxides with a possible presence of magnesium oxides when added under the form of dolomite lime, helps generating a slag layer that will be able to capture part of the sulfur contained in the load. Such lime can usually be added to the furnace before and / or during melting of said load.

[0021]

[0020] The next step of the steelmaking method according to the invention begins by tapping the melted load in a ladle that will usually be transported on a ladle car throughout the different processing steps. A slag layer can be generated by adding appropriate agents like lime for example. Some metals or ferro alloys can optionally be added at that stage.

[0022]

[0021] The melted load is then submitted to a decarburization step, without prior deoxidation. The decarburization step usually consists of a treatment under vacuum, for example in a RH orVTD / VOD (Vacuum tank Degassing / Vacuum Oxygen Decarburization) device. Decarburization may be assisted by oxygen blowing in these vessels. The final content of carbon of the liquid steel is set to a maximum value of 50 ppm in weight and preferably a maximum value of 30 ppm in weight.

[0023]

[0022] At the end of the decarburization step, the slag present on top of the liquid steel will have captured a significant part of the titanium present in such steel. Apart from the titanium present in the raw materials loaded in the electric arc furnace, additional sources of titanium can come from the transfer from the ladle refractories into the liquid steel. Indeed, such refractories are usually made of alumina containing TiO2 and the contact with the ladle slag is eroding the refractories, thereby releasing some titanium oxide.

[0024]

[0023] In a preferred embodiment, the internal part of the ladle refractories can be coated with a lining containing no TiO2 and consisting of, for example, AI2O3 or MgO. The use of such specific lining is efficient in suppressing the transfer of titanium coming from the erosion of the refractories.

[0025]

[0024] Another additional source of titanium comes from the transfer from the slag back into the liquid steel through a phenomenon called titanium reversion. Indeed, a significant part of the titanium is being retained in such slag under an oxidised form but could be transferred back to the melt after reduction to metallic titanium, notably during desulfurization.

[0026]

[0025] In a preferred embodiment, the maximum content of titanium oxide in the ladle slag is maintained below 1 wt.%, or below 0.8 wt.% or even better below 0.7 wt.% to minimize the reservoir of titanium that may be reduced and transferred to the liquid steel.

[0027]

[0026] In another preferred embodiment, the ladle slag is removed at the end of the decarburization step and new agents are added to generate a fresh layer of slag containing as little titanium oxide as possible.

[0028]

[0027] Then, the melted load is submitted to a first alloying and to a desulfurization step. The alloying consists in adding metal or ferro-alloys to the liquid steel to set its composition to the product target. In particular, FeSi, Al and FeMn can be added. Such metals or ferro-alloys can also deoxidize the steel, to obtain a so-called killed steel. In a preferred embodiment, the metal or ferro-alloys used in such step contain a maximum of 300 ppm, or 200 ppm, or 150 ppm in weight of titanium.

[0029]

[0028] The desulfurization step is then performed, for example by adding desulfurization agents, like lime and / or calcium aluminate. In a preferred embodiment, the desulfurization agents contain a maximum of 200 ppm in weight of titanium that is usually present under the form of titanium oxide.

[0030]

[0029] In a preferred embodiment, the desulfurization agents comprise CaO and AI2O3 in a ratio of 4.5 to 5.5, preferably of 4.6 to 5.1 , carbon in a range from 0 to 1.5 wt.%, preferably of 0 to 0.8 wt%, and less than 0.05 or 0.02 wt.% of TiO2. Preferably the amount of CaO is at least equal to 70 wt.%. The use of such agents, with an appropriate basicity value represented by the CaO / Al2O3 ratio, ensures an efficient transfer of sulfur while minimizing the enrichment of the slag in titanium, thereby contributing to the control of such elements in the final steel composition.

[0030] In addition or in replacement of the above desulfurization practices, this desulfurization step can also be performed by bubbling an inert gas like argon through porous plugs in the bottom of the ladle or through a lance. Such bubbling will gently stir the liquid steel and promote the transfer of sulfur to the slag, while limiting the amount of titanium that may be transferred back from the slag to the liquid steel. In another preferred embodiment, no bubbling will be performed to avoid any titanium transfer to the liquid steel.

[0031]

[0031] Then, the next part of the steelmaking method according to the invention is to submit the liquid steel to an optional final alloying step, by using additional ferro alloys as described above to fine tune the composition setting, to obtain a liquid steel notably having a sulfur content below 30 ppm in weight, a titanium content below 100 ppm in weight and a carbon content below 50 ppm in weight.

[0032]

[0032] The final step of the steelmaking method according to the invention consists in casting said liquid steel.

[0033]

[0033] In a preferred embodiment, the steel manufactured by the steelmaking method according to the invention is a non-oriented electrical steel, which can, for example have the following composition, expressed in wt. %, the balance being iron:

[0034] C : 0.0001 - 0.005 %

[0035] Mn : 0.08 - 0.7 %

[0036] Si : 2.0 - 3.6 %, preferably 2.5 - 3.6%

[0037] Al : 0.35 - 1.3 %

[0038] Ti : 0 - 0.010 %

[0039] Ni : 0 - 0.05 %

[0040] Cr : 0 - 0.05 %

[0041] Cu : 0 - 0.05 %

[0042] Mo : 0 - 0.05 %

[0043] Nb : 0 - 0.05 %

[0044] P : 0 - 0.15 %, preferably 0 - 0.025 %

[0045] S : 0 - 0.003 %

[0046] N : 0 - 0.09 %, preferably 0 - 0.009% or 0-005%

[0047] Sn : 0 - 0.2 % Sb : 0 - 0.2 %

[0048]

[0034] Examples

[0049] The following examples and tests presented hereunder are non-restricting in nature and must be considered for purposes of illustration only. They will illustrate the advantageous features of the present invention, the significance of the parameters chosen after extensive experiments and further establish the results that can be achieved by following the method according to the invention.

[0050]

[0035] Different types of loads were introduced in an electric arc furnace and melted down, as summarized in Table 1. Reduced iron was introduced in the form of HBI containing various amounts of iron and sulfur, as detailed in Table 2. Whenever used, the other elements introduced in the electric arc furnace have the sulfur content gathered in Table 3.

[0051]

[0036] After melting down of the load, some samples of steel are taken and their composition is analyzed, the corresponding results being gathered in Table 4.

[0052]

[0037] A decarburization step by vacuum with oxygen blowing, and then a deoxidation step were performed. The deoxidation was done through vacuum treatment, followed by an appropriate aluminium addition in the frame of the first alloying step.

[0053]

[0038] The desulfurization step was performed using briquettes containing : 77 wt.% of CaO, 16 wt.%Al2O3, 4 wt.% of CO2 representing 1.09 wt.% of carbon, and 0.05 wt.% of TiO2 the rest being SiC>2, MgO and Fe2Os.

[0054]

[0039] A final alloying was performed with ferro alloys containing less than 200 weight ppm of sulfur as can be seen through comparing the different tables.

[0055]

[0040] Finally, at the end of the steelmaking method, some samples are taken just before casting and analyzed, the corresponding results being gathered in Table 5.

[0041] Table 1 - Loads

[0056]

[0042] Table 2 - HBI composition, in wt.%

[0057]

[0043] Table 3 - Other load elements composition, in wt. ppm

[0044] Table 4 - Composition of the load after melting down and before tapping in wt. ppm, the balance being iron

[0058] : according to the invention

[0059]

[0045] Table 5 - Composition of the liquid steel before casting in wt. ppm, the balance being iron

[0060] * : according to the invention

[0061]

[0046] Thanks to the steelmaking method according to the invention, the liguid steel content in carbon, sulfur and titanium can be controlled to reach appropriate ranges for steel, and especially for non-grain-oriented electrical steels while limiting the carbon footprint of such method, maximizing productivity and maximizing the overall yield of aluminium additions.

Claims

CLAIMS1 . A steelmaking method comprising the following successive steps: a. feeding an electrical furnace with an initial load consisting of 0 to 50 wt.% of pig iron or hot metal, 0 to 50 wt.% of scraps, and at least 50 wt.% of reduced iron, the respective load content in sulfur and titanium being below 100 ppm in weight after melting down, b. tapping the melted load in a ladle and submitting it, without prior deoxidation, to a decarburization step, then to an alloying step and then to a desulfurization step, then to an optional final alloying step to obtain a liquid steel having a sulfur content below 30 ppm in weight, a titanium content below 100 ppm in weight and a carbon content below 50 ppm in weight, c. casting said liquid steel.

2. A steelmaking method according to claim 1 wherein said pig iron or hot metal includes a maximum of 200 ppm in weight of sulfur.

3. A steelmaking method according to any one of claims 1 or 2 wherein said scraps include a maximum of 200 ppm in weight of sulfur.

4. A steelmaking method according to any one of the preceding claims wherein said reduced iron is chosen among HBI or DRI.

5. A steelmaking method according to any one of the preceding claims wherein the carbon content of the initial load is set to a range of 2 to 3 wt.% by carbon addition before and / or during the melting down.

6. A steelmaking method according to claim 5 wherein the carbon addition is made by injecting graphite comprising a maximum of 10000 ppm in weight of sulfur.

7. A steelmaking method according to any one of the preceding claims wherein lime is added to said electric furnace, such lime comprising a maximum of 500 ppm in weight of sulfur.

8. A steelmaking method according to any one of the preceding claims wherein the desulfurization step is performed by bubbling an inert gas into said ladle.

9. A steelmaking method according to claim 8 wherein the desulfurization step includes generating a slag layer by adding agents comprising CaO and optionally fluorspar or AI2O3.

10. A steelmaking method according to any one of the preceding claims wherein said desulfurization step includes the addition of agents comprising CaO and AI2O3 in a ratio of 4.5 to 5.5, carbon in a range from 0 to 1 .5 wt.% and less than 0.1 wt.% of TiO2.11 . A steelmaking method according to any one of the preceding claims wherein the liquid steel final composition comprises, in wt.%, the balance being iron:C : 0.0001 - 0.005 %Mn : 0.08 - 0.7 %Si : 2.0 - 3.6 %Al : 0.35 - 1.3 %Ti : 0 - 0.010 %Ni : 0 - 0.05 %Cr : 0 - 0.05 %Cu : 0 - 0.05 %Mo : 0 - 0.05 %Nb : 0 - 0.05 %P : 0 - 0.15 %S : 0 - 0.003 %N : 0 - 0.09 %Sn : 0 - 0.2 %Sb : 0 - 0.2 %

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