WO2025219739A1

WO2025219739A1 - A steelmaking method

Info

Publication number: WO2025219739A1
Application number: PCT/IB2024/053670
Authority: WO
Inventors: Sigrid Jacobs; Bart GOMMERS
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2024-04-15
Filing date: 2024-04-15
Publication date: 2025-10-23
Anticipated expiration: 2026-10-15

Abstract

The invention relates to a steelmaking method comprising the following successive steps: − producing hot metal in a blast furnace, the content in sulfur of said hot metal being below 500 ppm in weight, − tapping said hot metal in a vessel and submitting it to a first desulfurization step by adding desulfurization agents and creating a slag layer to which part of said sulfur is transferred to reach a content of sulfur below 50 ppm in weight in the hot metal, − removing said slag and transferring the hot metal into a BOF and submitting it to a first decarburization step, − tapping the resulting liquid steel into a ladle and submitting it to a second decarburization step, − performing an alloying step, − performing a second desulfurization step, − proceeding to an optional final alloying step using ferro-alloys containing less than 300 ppm in weight of titanium, 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, − casting said liquid steel.

Description

A steelmaking method

[001 ] 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.

[002] Steel can be currently produced through different manufacturing routes. Nowadays, the 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 ores consisting in iron oxides and then transform such hot metal into steel in a converter process or Basic Oxygen furnace (BOF)reducing down the level of carbon of the hot metal.

[003] However, iron ores may contain significant amounts of so-called residual elements, like sulfur or titanium, that may adversely impact the final properties of the steel. Moreover, there is a trend to replace part of the iron ores by recycled steel scraps to reduce the environmental impact of steelmaking. Such steel scraps may also include variable contents of residual elements. In addition, some specific grades like IF steels, or nonoriented electrical steels require also a control of the maximum content of carbon to low levels.

[004] The aim of the present invention is therefore to provide an improved steelmaking method allowing to limit the amounts of carbon, sulfur and titanium contained in such steel.

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

- producing hot metal in a blast furnace, the content in sulfur of said hot metal being below 500 ppm in weight, - tapping said hot metal in a vessel and submitting it to a first desulfurization step by adding desulfurization agents and creating a slag layer to which part of said sulfur is transferred to reach a content of sulfur below 50 ppm in weight in the hot metal,

- removing said slag and transferring the hot metal into a BOF and submitting it to a first decarburization step,

- tapping the resulting liquid steel into a ladle and submitting it to a second decarburization step,

- performing an alloying step,

- performing a second desulfurization step,

- proceeding to an optional final alloying step using ferro-alloys containing less than 300 ppm in weight of titanium, 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,

- casting said liquid steel.

[006] The method according to the invention may also have the optional features, considered individually or in combination and listed in claims 2 to 6.

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

[008] 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.

[009] The steelmaking method according to the invention comprises several successive steps. The first one consists in producing hot metal in a blast furnace, the content in sulfur of said pig iron being below 500 ppm in weight.

[010] 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 the colas used to reduce such 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, such hot metal comprises a maximum of 200 ppm, and even better of 150 ppm in weight of sulfur. In replacement of part of the iron ore, some steel scraps or direct reduced iron can be loaded as well in the blast furnace.

[011 ] Scraps are an optional element of the load of the blast furnace but are preferably added in an amount of 10 to 30 or 20 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 ppm in weight of sulfur. This maximum amount of sulfur contributes to limiting the global content in sulfur of the load. Such scraps can be recovered from the steel plant past productions or from external sources and classified as types E6, E8 or others. In another embodiment, such scraps can be added in the BOF at a later stage of the process.

[012] A reducing material like coke for example is also added in a conventional way to the blast furnace to convert iron oxide into iron.

[013] The second step of the steelmaking method according to the invention begins by tapping said hot metal in a vessel. The blast furnace slag is being removed from the hot metal at that stage.

[014] The vessel for tapping can be a torpedo car or a ladle that will be charged on a ladle transfer car, for example.

[015] The hot metal is then submitted to a first desulfurization step by adding desulfurization agents and creating a slag layer to which part of said sulfur is transferred to reach a content of sulfur below 50 ppm in weight in the hot metal. The desulfurization agents can, for example be lime and/or calcium aluminate. It can also be lime (CaO) and fluorite (CaF2) or any other fluidizer. 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.

[016] In another preferred embodiment, the desulfurization agents can be calcium carbide (CaC2), lime (CaO), and/or magnesium powder. [017] Whenever the hot metal was transferred to a ladle, the desulfurization process can be performed in a so-called Kanbara reactor which is well known to the man skilled in the art. It can also be done by injecting desulfurization agents in said ladle. If performed in a torpedo car, the hot metal is further transferred to a ladle for the rest of the steelmaking method.

[018] After the first desulfurization step, the slag layer is removed and the hot metal is then transferred to a BOF and submitted to a first decarburization step. A fresh slag layer is being generated by adding appropriate agents. Such decarburization step usually consists in blowing oxygen in the hot metal, which leads to the formation of carbon monoxide through the reaction of carbon and oxygen.

[019] After such first decarburization step, the liquid steel obtained is tapped in a ladle and can be submitted to a second decarburization step including a further treatment, preferably under vacuum, for example in a RH or VTDA/OD (Vacuum tank Degassing I Vacuum Oxygen Decarburization) device. 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.

[020] At the end of the decarburization steps, 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 blast furnace or in the BOF, 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.

[021 ] 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.

[022] 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.

[023] 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.

[024] 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.

[025] Then, the liquid steel can be submitted to a deoxidation step that can consist in adding elements that will combine with the oxygen in excess present in the steel, in a conventional manner. An alloying consisting in adding ferro-alloys to the liquid steel to set its composition to the product target is performed. 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 metals or ferro-alloys used in such step contain a maximum of 300 ppm, or 200 ppm, or 150 ppm in weight of titanium.

[026] The second 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.

[027] 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 0.1 or 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. [028] 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.

[029] Then, the next step 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.

[030] If needed to lower down the sulfur content to very low levels, it is possible to add intermediate additional desulfurization between the second desulfurization and the optional final alloying step.

[031 ] In a preferred embodiment, the alloying steps consist in adding metals or ferro alloys of aluminium Al and manganese FeMn and ferro-silicon FeSi containing a maximum of 300 ppm, or 200 ppm, or 150 ppm in weight of titanium.

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

[033] 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:

C : 0.0001 - 0.005 %

Mn : 0.08 - 0.7 %

Si : 2.0 - 3.6 %, preferably 2.5 - 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 %, preferably 0 - 0.025 %

S : 0 - 0.003 %

N : 0 - 0.09 %, preferably 0 - 0.009%

Sn : 0 - 0.2 %

Sb : 0 - 0.2 %

[034] Examples

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.

[035] Some hot metal was produced in a conventional blast furnace, with a sulfur content of 300 ppm in weight.

[036] After the first desulfurization step performed by adding desulfurization agents, the slag is removed and a sample of hot metal is taken and its composition is analyzed, the corresponding result being gathered in Table 1.

[037] The hot metal is transferred in a BOF and submitted to a first decarburization step by oxygen blowing. A sample was taken and analyzed, the corresponding result being gathered in Table 2.

[038] The liquid steel is then transferred to a ladle and submitted to a second decarburization step by vacuum. The steel is then submitted to a deoxidation step performed through appropriate aluminium addition in the frame of the first alloying step.

[039] The second 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. [040] 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.

[041] Finally, at the end of the steelmaking method, a sample is taken just before casting and analyzed, the corresponding result being gathered in

Table 3.

[042] Table 1 - Composition of the hot metal after first desulfurizing step, in wt. ppm, the balance being iron

[043] Table 2 - Composition of the hot metal after the first decarburization step, in wt. ppm, the balance being iron

[044] Table 3 - Composition of the liquid steel before casting in wt. ppm, the balance being iron

[045] 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 maximizing productivity and maximizing the overall yield of aluminium additions.

Claims

1 ) A steelmaking method comprising the following successive steps:

- producing hot metal in a blast furnace, the content in sulfur of said hot metal being below 500 ppm in weight,

- tapping said hot metal in a vessel and submitting it to a first desulfurization step by adding desulfurization agents and creating a slag layer to which part of said sulfur is transferred to reach a content of sulfur below 50 ppm in weight in the hot metal,

- performing an alloying step,

- performing a second desulfurization step,

- casting said liquid steel.

2) A steelmaking method according to claim 1 wherein said vessel is a torpedo car.

3) A steelmaking method according to claim 1 or 2 wherein said first and/or second desulfurization step is performed by bubbling an inert gas into said vessel or said ladle.

4) A steelmaking method according to claim 3 wherein said first and/or second desulfurization step includes generating a slag layer by adding agents comprising CaO and optionally fluorspar or AI2O3.

5) A steelmaking method according to any one of the preceding claims wherein said second 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. 6) 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 : 0.015 %

P: 0-0.15%

S: 0 - 0.003 % N: 0-0.09%

Sn : 0 - 0.2 %

Sb : 0 - 0.2 %