Steelmaking process

Abstract

This invention relates to a steelmaking method comprising the following sequential steps: - producing molten iron in a blast furnace, the molten iron having a sulfur content of less than 500 ppm by weight; - tapping the molten iron into a container and subjecting it to a first desulfurization step by adding a desulfurizing agent and forming a slag layer, wherein a portion of the sulfur is transferred to the slag layer, so that the sulfur content in the molten iron reaches less than 50 ppm by weight; - removing the slag and transferring the molten iron to a BOF and subjecting it to a first decarburization step; - tapping the resulting molten steel into a ladle and subjecting it to a second decarburization step; - performing an alloying step; - performing a second desulfurization step; - optionally performing a final alloying step using a ferroalloy with a titanium content of less than 300 ppm by weight to obtain molten steel with a sulfur content of less than 30 ppm by weight, a titanium content of less than 100 ppm by weight, and a carbon content of less than 50 ppm by weight; - casting the molten steel.

Description

Technical Field

[0001] This invention relates to a steelmaking method for producing steel containing limited amounts of elements such as carbon, titanium, and sulfur. While not limited to any particular type of steel, such a method is particularly suitable for producing non-oriented electrical steel. Background Technology

[0002] Currently, steel can be produced through different manufacturing routes. The most commonly used route today, known as the "BF-BOF route," involves reducing iron ore, which consists of iron oxides, in a blast furnace to produce molten iron (also known as pig iron after solidification) by using a reducing agent (mainly coke). This molten iron is then converted into steel in a converter process or a basic oxygen converter (BOF), thereby reducing the carbon level of the molten iron.

[0003] However, iron ore may contain significant amounts of so-called residual elements, such as sulfur or titanium, which can adversely affect the final properties of the steel. Furthermore, there is a trend to replace some iron ore with recycled scrap steel to reduce the environmental impact of steelmaking. Such scrap steel may also contain varying amounts of residual elements. Additionally, certain steel grades, such as IF steel or non-oriented electrical steel, require that the maximum carbon content be controlled at low levels. Summary of the Invention

[0004] Therefore, the object of the present invention is to provide an improved steelmaking method that allows for limiting the amount of carbon, sulfur and titanium contained in such steel.

[0005] Therefore, the first object of the present invention is a steelmaking method comprising the following sequential steps:

[0006] - Producing molten iron in a blast furnace, wherein the sulfur content in the molten iron is less than 500 ppm by weight;

[0007] - The molten iron is tapped into a container and subjected to a first desulfurization step by adding a desulfurizing agent and forming a slag layer, wherein a portion of the sulfur is transferred to the slag layer so that the sulfur content in the molten iron is less than 50 ppm by weight.

[0008] - Remove the slag and transfer the molten iron to a BOF for a first decarburization step;

[0009] - The resulting molten steel is tapped into a ladle and subjected to a second decarburization step;

[0010] - Perform the alloying step;

[0011] - Perform the second desulfurization step;

[0012] - An optional final alloying step is performed using an iron alloy with a titanium content of less than 300 ppm by weight to obtain molten steel with a sulfur content of less than 30 ppm by weight, a titanium content of less than 100 ppm by weight, and a carbon content of less than 50 ppm by weight.

[0013] - Cast the molten steel.

[0014] The method according to the invention may also have optional features, individually or in combination, listed in claims 2 to 6.

[0015] Other features and advantages of the present invention will be described in more detail in the following description.

[0016] The invention will be better understood by reading the following description, which is provided purely for illustrative purposes and is not intended to be limiting.

[0017] The steelmaking method according to the present invention comprises several sequential steps. The first step comprises producing molten iron in a blast furnace, wherein the sulfur content in the molten iron is less than 500 ppm by weight.

[0018] Molten iron is obtained by smelting iron ore in a blast furnace, and its sulfur and titanium content depends on the sulfur and titanium content of such iron ore and the coal used to reduce it. Such sulfur and titanium content can be controlled by selecting iron ore and coal with low sulfur / titanium ratios, or by removing sulfur from the molten iron using desulfurization methods known to those skilled in the art. In a preferred embodiment, such molten iron contains up to 200 ppm, and even better, 150 ppm, of sulfur by weight. To replace some of the iron ore, some scrap steel or direct reduced iron can also be loaded into the blast furnace.

[0019] Scrap steel is an optional component of blast furnace charge, but for environmental reasons, it is preferred to be added in an amount of 10 to 30 or 20% by weight. The scrap steel is made from steel that has been previously manufactured and used, subsequently reached the end of its life in that form, and can be recycled. In a preferred embodiment, the initial charge includes scrap steel containing up to 200 ppm, and even better, 150 ppm, of sulfur by weight. This maximum sulfur content helps limit the overall sulfur content of the charge. Such scrap steel can be recycled from past production at the steel mill or from external sources and is classified as E6, E8, or other types. In another embodiment, such scrap steel can be added to the BOF at a later stage of the process.

[0020] For example, reducing materials such as coke are also added to the blast furnace in a conventional manner to convert iron oxides into iron.

[0021] The second step of the steelmaking method according to the invention begins with tapping the molten iron into a container. At this stage, blast furnace slag is being removed from the molten iron.

[0022] For example, the container used for tapping iron can be a torpedo car or a ladle that will be loaded onto a ladle transfer car.

[0023] The molten iron is then subjected to a first desulfurization step by adding a desulfurizing agent and forming a slag layer, wherein a portion of the sulfur is transferred to the slag layer to reduce the sulfur content in the molten iron to less than 50 ppm by weight. The desulfurizing agent can be, for example, lime and / or calcium aluminate. It can also be lime (CaO) and fluorite (CaF2) or any other flux. In a preferred embodiment, the desulfurizing agent contains up to 200 ppm by weight of titanium, typically in the form of titanium oxide.

[0024] In another preferred embodiment, the desulfurizing agent may be calcium carbide (CaC2), lime (CaO), and / or magnesium powder.

[0025] Whenever molten iron is transferred to a ladle, the desulfurization process can be performed in a so-called Kanbara reactor, as is known to those skilled in the art. This can also be accomplished by injecting a desulfurizing agent into the ladle. If performed in a torpedo car, the molten iron is further transferred to the ladle for the remaining steps of the steelmaking process.

[0026] After the first desulfurization step, the slag layer is removed, and the molten iron is then transferred to a BOF (Boiler Air Formation) for a first decarburization step. A fresh slag layer is generated by adding appropriate reagents. Such a decarburization step typically involves blowing oxygen into the molten iron, which leads to the formation of carbon monoxide through the reaction of carbon and oxygen.

[0027] Following this first decarburization step, the resulting molten steel is tapped into a ladle and can undergo a second decarburization step, which includes further treatment, preferably carried out under vacuum, for example in an RH or VTD / VOD (vacuum tank degassing / vacuum oxygen decarburization) apparatus. The final carbon content of the molten steel is set to a maximum of 50 ppm by weight, and preferably a maximum of 30 ppm by weight.

[0028] At the end of the decarburization process, the slag on top of the molten steel has already captured a significant portion of the titanium present in such steel. Besides the titanium present in the raw materials loaded in the blast furnace or BOF, another source of titanium can be the transfer from the ladle refractory material into the molten steel. In fact, such refractory materials are typically made of alumina containing TiO2, and contact with the ladle slag is eroding the refractory material, thereby releasing some titanium oxide.

[0029] In a preferred embodiment, the inner portion of the ladle refractory material can be coated with a lining that does not contain TiO2 and is composed of, for example, Al2O3 or MgO. Using such a specific lining effectively inhibits the transfer of titanium from the refractory material.

[0030] Another additional source of titanium is its transfer from slag back into the molten steel through a phenomenon known as titanium reversion. In fact, a large portion of titanium is retained in such slag in oxidized form, but may be transferred back into the melt after reduction to metallic titanium, especially during desulfurization.

[0031] In a preferred embodiment, the maximum content of titanium oxide in the ladle slag is kept below 1% by weight, or below 0.8% by weight, or even better below 0.7% by weight, so as to minimize the reserves of titanium that can be reduced and transferred to the molten steel.

[0032] In another preferred embodiment, the ladle slag is removed at the end of the decarburization step, and new reagents are added to generate a fresh slag layer containing as little titanium oxide as possible.

[0033] The molten steel can then undergo a deoxidation step, which may include the conventional addition of elements that will combine with excess oxygen present in the steel. Alloying is performed, including the addition of ferroalloys to the molten steel to set its composition to the product target. Specifically, FeSi, Al, and FeMn may be added. Such metals or ferroalloys can also deoxidize the steel to obtain so-called killed steel. In a preferred embodiment, the metal or ferroalloy used in such a step contains up to 300 ppm, 200 ppm, or 150 ppm of titanium by weight.

[0034] A second desulfurization step is then performed, for example, by adding a desulfurizing agent such as lime and / or calcium aluminate. In a preferred embodiment, the desulfurizing agent contains up to 200 ppm of titanium by weight, which is typically present in the form of titanium oxide.

[0035] In a preferred embodiment, the desulfurizing agent comprises 4.5 to 5.5, preferably 4.6 to 5.1, of CaO and Al2O3, ranging from 0 to 1.5 wt%, preferably 0 to 0.8 wt%, of carbon, and less than 0.1, 0.05, or 0.02 wt% of TiO2. Preferably, the amount of CaO is at least 70 wt%. Using such a reagent with an appropriate basicity value, expressed as a CaO / Al2O3 ratio, ensures effective sulfur transfer while minimizing titanium enrichment in the slag, thereby helping to control such elements in the final steel composition.

[0036] In addition to or as an alternative to the desulfurization practices described above, the desulfurization step can also be performed by bubbling an inert gas, such as argon, through permeable bricks at the bottom of the ladle or through a spray gun. Such bubbling gently agitates the molten steel and promotes the transfer of sulfur to the slag, while limiting the amount of titanium that can be transferred back to the molten steel from the slag. In another preferred embodiment, bubbling is not performed to avoid any transfer of titanium into the molten steel.

[0037] Then, the next step of the steelmaking method according to the invention is to optionally perform a final alloying step on the molten steel, by using additional ferroalloys as described above to fine-tune the composition settings to obtain molten steel having, in particular, a sulfur content of less than 30 ppm by weight, a titanium content of less than 100 ppm by weight, and a carbon content of less than 50 ppm by weight.

[0038] If it is necessary to reduce the sulfur content to a very low level, an intermediate additional desulfurization can be added between the second desulfurization step and the optional final alloying step.

[0039] In a preferred embodiment, the alloying step includes adding a metal or iron alloy of aluminum (Al) and manganese (FeMn) and ferrosilicon (FeSi), with a titanium content of up to 300 ppm, 200 ppm, or 150 ppm by weight.

[0040] The final step of the steelmaking method according to the present invention includes casting the molten steel.

[0041] In a preferred embodiment, the steel produced by the steelmaking method according to the invention is a non-oriented electrical steel, which may, for example, have the following composition: expressed in weight %

[0042] C: 0.0001 - 0.005%

[0043] Mn: 0.08 - 0.7%

[0044] Si: 2.0 - 3.6%, preferably 2.5 – 3.6%

[0045] Al: 0.35 - 1.3%

[0046] Ti: 0 - 0.010%

[0047] Ni: 0 - 0.05%

[0048] Cr: 0 - 0.05%

[0049] Cu: 0 - 0.05%

[0050] Mo: 0 - 0.05%

[0051] Nb: 0 - 0.05%

[0052] P: 0 - 0.15%, preferably 0 – 0.025%

[0053] S: 0 - 0.003%

[0054] N: 0 - 0.09%, preferably 0 – 0.009%

[0055] Sn: 0 - 0.2%

[0056] Sb: 0 - 0.2%,

[0057] The balance is iron. Detailed Implementation

[0058] Example

[0059] The embodiments and tests presented below are non-limiting in nature and should be considered for illustrative purposes only. They will illustrate the advantageous features of the invention, the significance of parameters selected after extensive experimentation, and further establish the results achievable by following the methods according to the invention.

[0060] Some molten iron was produced in a conventional blast furnace, with a sulfur content of 300 ppm by weight.

[0061] After performing the first desulfurization step by adding a desulfurizing agent, the slag was removed, and a sample of molten iron was taken and its composition was analyzed. The corresponding results are summarized in Table 1.

[0062] The molten iron was transferred to a BOF (Boiler-Off Fluid) and subjected to the first decarburization step by oxygen blowing. Samples were taken and analyzed, and the results are summarized in Table 2.

[0063] The molten steel is then transferred to a ladle and subjected to a second decarburization step under vacuum. The steel is then subjected to a deoxidation step performed by the appropriate addition of aluminum within the framework of the first alloying step.

[0064] The second desulfurization step is performed using briquettes containing 77 wt% CaO, 16 wt% Al2O3, 4 wt% CO2 (representing 1.09 wt% carbon), and 0.05 wt% TiO2, with the remainder being SiO2, MgO, and Fe2O3.

[0065] Final alloying was performed using ferroalloys with a sulfur content of less than 200 ppm by weight, as can be seen by comparing different tables.

[0066] Finally, at the end of the steelmaking process, just before casting, samples were taken and analyzed, and the results are summarized in Table 3.

[0067] Table 1 – Composition of molten iron after the first desulfurization step, in ppm by weight, with the balance being iron.

[0068]

[0069] Table 2 – Composition of molten iron after the first decarburization step, in ppm by weight, with the balance being iron.

[0070]

[0071] Table 3 – Composition of molten steel before casting, in ppm by weight, balance being iron.

[0072]

[0073] Thanks to the steelmaking method according to the present invention, the contents of carbon, sulfur and titanium in molten steel can be controlled to a suitable range for steel, especially amorphous oriented electrical steel, while maximizing productivity and the total yield of aluminum addition.

Claims

1. A steelmaking method, comprising the following sequential steps: - Producing molten iron in a blast furnace, wherein the sulfur content in the molten iron is less than 500 ppm by weight; - The molten iron is tapped into a container and subjected to a first desulfurization step by adding a desulfurizing agent and forming a slag layer, wherein a portion of the sulfur is transferred to the slag layer so that the sulfur content in the molten iron is less than 50 ppm by weight. - Remove the slag and transfer the molten iron to a BOF for a first decarburization step; - The resulting molten steel is tapped into a ladle and subjected to a second decarburization step; - Perform the alloying step; - Perform the second desulfurization step; - An optional final alloying step is performed using an iron alloy with a titanium content of less than 300 ppm by weight to obtain molten steel with a sulfur content of less than 30 ppm by weight, a titanium content of less than 100 ppm by weight, and a carbon content of less than 50 ppm by weight. - Cast the molten steel.

2. The steelmaking method according to claim 1, wherein, The container is a torpedo tanker.

3. The steelmaking method according to claim 1 or 2, wherein, The first and / or second desulfurization steps are performed by bubbling inert gas into the container or the ladle.

4. The steelmaking method according to claim 3, wherein, The first and / or second desulfurization steps involve generating a slag layer by adding a reagent containing CaO and optionally fluorite or Al2O3.

5. The steelmaking method according to any one of the preceding claims, wherein, The second desulfurization step includes adding a reagent comprising 4.5 to 5.5% CaO and Al2O3, 0 to 1.5% carbon by weight, and less than 0.1% TiO2 by weight.

6. The steelmaking method according to any one of the preceding claims, wherein, The final composition of the molten steel, expressed as a percentage by weight, comprises: 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% The balance is iron.

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