A method for casting a semi-finished steel product with high titanium content

Abstract

A method of casting a semi-finished steel product from a molten steel, the semi-finished steel product having a target composition of titanium of at least 3.5% by weight.

Description

A method for casting a semi-finished steel product with high titanium content

[001] The invention relates to casting a steel semi-finished product with a high titanium content.

[002] FeTiB2 steels have attracted much attention due to their high elastic modulus E, low density and high tensile strength, which make them a desirable choice for the automotive industry. In this industry, vehicle lightness and safety are constant concerns. However, these steels are difficult to manufacture due to the limitations associated with the deposits formed. Various solutions have been proposed for the production of these steels, in particular to overcome the problem of castability.

[003] Document US20130174942 discloses a FeTiB2 steel containing between 2.5 and 7.2 wt% titanium, which is cast at a casting temperature of up to 40°C above the liquidus temperature of the steel. This allows for a fine microstructure.

[004] Document EP3612657 discloses a specific steel composition in which the free titanium content of the steel is at least 0.95% and due to this free titanium content, the structure of the steel remains predominantly ferritic at all temperatures below the liquidus temperature. As a result, the hot hardness of the steel is significantly reduced compared to steels in the prior art, thereby improving the castability. Casting is preferably carried out in thin slabs.

[005] However, regardless of the casting method, the steel needs to reach the slag-making station at the right composition, temperature and viscosity. For those specific grades, this is one of the most difficult parts. During steelmaking, depending on the composition and temperature of the slag, some slag components may precipitate. In the case of high titanium grades, titanium tends to segregate and migrate to the slag, and titanium oxides tend to precipitate at the casting temperature, thus greatly increasing slag crystallization rates. A suitable crystallization rate for steelmaking is when samples of molten steel can be removed while the slag still covers the molten metal to prevent contact with air.

[006] Therefore, there is a need for a casting method that allows casting of semi-finished steel products with a high titanium content, i.e., more than 3.5% by weight.

[007] This problem is solved by a method according to the invention, in which the following steps are performed: a) Adding aluminum to molten steel until the molten steel contains at least 0.1% aluminum by weight. b) adding mineral compounds to the molten steel containing aluminum and / or calcium and optionally magnesium and CaF2, to achieve and maintain a slag composition in which the ratio of CaO to Al2O3 (CaO / Al2O3) is between 0.7 and 2, and the slag contains up to 25% by weight of CaF2, c) Adding titanium to molten steel to achieve the target composition, D) Casting steel as a semi-finished product.

[008] The inventive method may also include the following optional features, which are considered individually or in accordance with all possible technical combinations: - The amount of aluminum added is such that the molten steel contains more than 0.2% by weight of aluminum, preferably more than 0.4% by weight, - The semi-finished steel product must contain at least one percent boron by weight, which satisfies the following equation: %B≥0.45x%Ti-1.35% - Between steps (a) and (b), a heating step of the molten steel is performed. - A boron addition step takes place after step (c). - A boron addition step takes place after step (b). - During step (b), fluorspar CaF2 is added to achieve a CaF2 composition of between 6 and 15% by weight. - During step (b), magnesia is added to achieve a composition of MgO in the slag between 5 and 15% by weight. - During step (b), adding mineral compounds to achieve a slag composition in which the ratio of CaO to Al2O3 (CaO / Al2O3) is between 0.9 and 1.3. - Addition of mineral compounds to achieve a slag composition in which the ratio of CaO to Al2O3 (CaO / Al2O3) is between 1.4 and 2, the slag additionally containing 6 and 12% by weight of CaF2. - The semi-finished steel product has a target titanium composition of at least 5.8% by weight. - Mineral compounds are selected from lime, fluorspar, and magnesia. - The semi-finished steel product has the following components, expressed in terms of weight content: 0.01% ≤ C ≤ 0.2% 3.5% ≤ Ti ≤ 10% (0,45 xTi) – 1,35% ≤ B ≤ (0,45 xTi) + 0,70% S ≤ 0.03% P ≤ 0.04% N ≤ 0.05% O ≤ 0.05% And optionally includes the following: Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% It also contains precipitates of TiB2 and optionally Fe2B, with the remainder consisting of iron and unavoidable impurities resulting from preparation.

[009] The invention also relates to a steelmaking slag having the following composition, expressed in terms of weight content: 35% ≤ CaO ≤ 55%, 15% ≤ Al203 ≤55%, Which fulfills the following relationships: 0.7 ≤ CaO2 / Al2O3 ≤ 2, 0% ≤ MgO ≤ 15%, TiOx<20% Less than 1% of each of the following ingredients: B2O3, SiO2, CrOx, MnO, NiO, FeOx, S, 0%≤CaF2 ≤ 25% The remaining oxides are caused by impurities present in the molten metal.

[010] In one embodiment of the method according to the invention, liquid steel (also called molten metal), which can be the product of an electric arc furnace or a steelmaking device such as an alkaline oxygen furnace or a converter, is subjected to a deoxidation step. In said step, the liquid steel typically has a temperature of about 1650°C. To effect deoxidation, aluminum is typically added to the molten metal during discharge from the furnace to enhance the integrated deoxidation reaction. According to the invention, aluminum is added in such a way that the amount in the molten metal is greater than or equal to 0.1% by weight, which is greater than the amount normally required for deoxidation of liquid steel. In a preferred embodiment, aluminum is added in such a way that its amount in the molten metal is greater than or equal to 0.2% by weight. In a preferred embodiment, the amount is greater than or equal to 0.4% by weight. The oxides thus formed migrate to the surface of the molten steel and increase the amount of slag.The amount of aluminum added depends on the acceptable amount of titanium oxides in the slag to limit crystallization and the factors controlling titanium partitioning such as the composition of the molten metal, the composition of the slag and the temperature. Among these factors, the main components are the titanium content in the molten metal, the alloying elements in the molten metal such as boron, manganese, chromium and the like that can disturb the slag / metal equilibrium, as well as the temperature of the molten metal and slag, the weight ratio between the weight of the slag and the weight of the molten metal and the composition of the slag. Other slag oxides that can be reduced by titanium such as SiO2 and B2O3 should be avoided to limit titanium partitioning. Calculations of the thermodynamic equilibrium of the slag / metal can be performed when thermodynamic models and thermodynamic data centers are available.

[011] Thermodynamic calculations are performed to optimize the amount of aluminum to be added and the slag composition, based on the final TiOx in the slag, the composition of the molten metal, and the temperature during the refining process. For each stage of the grade refining process, the aluminum content in the molten metal and the slag composition are optimized to ensure limited titanium partitioning and the target TiOx, as well as limited slag crystallization.

[012] To define the optimum conditions, the TiOx and crystallized fraction in the slag are calculated depending on the temperature variations and the molten metal and slag compositions. All these calculations are known to the person skilled in the steelmaking industry. If the model and data center are not available, the slag / metal equilibrium is performed in the laboratory or pilot scale to simulate industrial conditions.

[013] The addition of aluminum allows the carbon dioxide of the molten steel to be reduced but also reduces the TiOx content of the slag. Part of the slag crystallization occurs through the precipitation of titanates, which are compounds of a titanium oxide combined with other oxides, such as aluminum oxides. By reducing the TiOx content of the slag, the precipitation of titanates is limited, which results in a lower crystal fraction. The nature and rate of crystallization of titanates can be more or less complex depending on the composition of the slag.

[014] Then, according to the method of the invention, inorganic compounds containing aluminum and / or calcium and / or magnesium, such as lime Ca(OH)2 or magnesia MgO and up to 25% by weight of fluorspar CaF2, are added to the molten metal. According to the invention, these additives are made with the aim of achieving and maintaining a slag composition in which the ratio of CaO to Al2O3 (C / A) is between 0.7 and 2. This composition makes it possible to limit the crystallization rate of the titanium oxides present in the slag by maximizing the sulfur capacity thereof.

[015] The limitation of TiOx thanks to the addition of aluminum should in fact be linked to the optimization of the slag composition to optimize the nature and amount of titanates and to limit their precipitation in the slag to promote low crystallization of the slag at casting temperatures.

[016] In a preferred embodiment, when the ratio is between 1.4 and 2, the slag additionally contains between 6 and 25% by weight of fluorspar CaF2 and more preferably between 6 and 12% by weight of fluorspar CaF2. The method of calculating and controlling this ratio is known to the person skilled in the art of steelmaking. In another embodiment, the C / A ratio is between 0.9 and 1.3 and the slag contains between 5 and 15% by weight of magnesia. In a third embodiment, this ratio is between 1.4 and 2 and the slag contains between 6 and 12% by weight of fluorspar CaF2 and between 5 and 15% by weight of magnesia MgO. Magnesia makes it possible to reduce the liquidus temperature of the slag. This last combination makes it possible to further limit the crystallization rate of titanium oxides. Magnesia can be added to the melt and / or directly from the refractory surrounding the molten metal in the steelmaking chamber. A person skilled in the art, having regard to the record, will be able to determine the amount of magnesia that must be dissolved from the refractory and how much is required to be added to achieve the required content.

[017] Thanks to this controlled slag composition and the addition of aluminum, the slag contains less than 20% by weight of titanium oxides. In all compositions, the remaining slag composition comprises less than 1%w B2O3, less than 1%w SiO2, less than 1%w CrOX, less than 1%w MnO, less than 1%w NiO and less than 1%w FeOx. According to the method according to the invention, no deslagging is required before starting the next step, which reduces the steel preparation time.

[018] After the mineral addition step, titanium is added to the melt in an amount that achieves the target composition in the final semi-finished product, which must be at least greater than or equal to 3.5% by weight. This composition is nominal. This titanium can be added in the form of titanium sponge or ferro-titanium pieces such as Fe-70%Ti or Fe-35%Ti or pure titania or ferro-titanium wires.

[019] After adding titanium, the slag has the following composition: 35% ≤ CaO ≤ 55%, 15% ≤ Al203 ≤55%, Which fulfills the following relationships: 0.7 ≤ CaO2 / Al2O3 ≤ 2, 0% ≤ MgO ≤ 15%, TiOx<20% Less than 1% of each of the following components: B2O3, SiO2, CrOx, MnO, NiO, FeOx, S, 0%≤CaF2 ≤ 25% The remainder consists of oxides resulting from impurities present in the molten metal.

[020] Then, the molten steel thus formed is sent to the casting place to be cast into a semi-finished product. The casting temperature is less than or equal to T|iquidus+40°C, where T|iquidus represents the liquidus temperature of the steel. In the present case, it is, for example, about 1330°C. The semi-finished product is a steel slab, a thick strip or a thin plate or any other product produced by continuous casting, vertical casting, horizontal casting, roller casting, thin plate casting, batch casting or strip casting.

[021] In another embodiment of a method according to the invention, the semi-finished product to be cast contains at least 2% boron. Boron is added after the mineral addition step by injection of ferro-boron (iron boron) particles such as Fe-18%B or ferro-boron wires. In a most preferred embodiment, this addition takes place during the mineral addition step.

[022] In a preferred embodiment, the semi-finished steel product has the following composition by weight content: 0.01% ≤ C ≤ 0.2% 3.5% ≤ Ti ≤ 10% (0,45 xTi) – 1,35% ≤ B ≤ (0,45 xTi) + 0,70% S ≤ 0.03% P ≤ 0.04% N ≤ 0.05% O ≤ 0.05% And optionally includes: Si ≤ 1.5% Mn ≤ 3% Al ≤ 1.5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0.1% V ≤ 0.5% and contains precipitates of TiB2 and optionally Fe2B, the remainder consisting of iron and unavoidable impurities resulting from preparation. This preferred composition allows the steel to remain predominantly ferritic at any temperature below the liquidus temperature, thus reducing castability problems.

[023] One way for a steelmaker to carry out the method according to the invention is to first define the target titanium in the semi-finished product and the casting temperature of this semi-finished product. Then, to define what composition of the overheads the maker intends within the scope of the invention, i.e., with a view to remaining within the C / A ratio range, the following can be done: potentially adding fluorspar, considering the amount of MgO provided by the refractories, depending on what volume of crystallization during casting is acceptable to him. Finally, using known patterns, calculating the amount of aluminum and other mineral additives necessary to achieve this defined slag composition.

[024] For all the above-mentioned constructions, the various steps carried out on the molten steel can be carried out, depending on the configuration of the plant, without change, in the same vessels or in different vessels. No special equipment is required other than that normally used in steelworks. Examples The following experiments are non-limiting in nature and should be considered for illustrative purposes only. They are illustrative of the advantageous features of the present invention. Calculation The calculation was performed using thermodynamic models as described above, which are known to those skilled in the art. The target temperature for the slag was 1350°C. The parameters considered were the titanium content in the semi-finished product for casting (%Ti), the C / A ratio with %CaO and %Al2O3 in the slag, the CaF2 (or fluorspar) content in the slag and a target maximum TiOX content in the slag. The calculation was made considering 15 kg of slag per ton of hot metal. The MgO content in the slag was always considered to be 10%w. The calculation was made considering 15 kg of slag per ton of hot metal. Considering all these conditions, a percentage of slag crystallization was calculated, which represented the percentage by volume of the solid phase over the total slag volume. The thermodynamic calculation indicated the nature and amount of oxides precipitated into the molten slag. Thus, by knowing the volume of the molten slag, it was possible to determine the percentage by volume of crystallized slag. - Adding aluminum In these examples, the target titanium in the final product varied between 2.5 and 10%. A C / A ratio of 1.1 was assumed and no CaF2 was added. All components and results are summarized in Table 1 below, the numbers of the starred experiments do not correspond to the invention. Table 1 N° %Ti C / A %Al %TiOx 1* 2,5 1,1 0,14 5% 2* 2,5 1,1 0 14,8 3 3,5 1,1 0,23 5% 4* 3,5 1,1 0 20 5 4 1,1 0,27 5% 6* 4 1,1 0 21,7 7 5 1,1 0,34 5% 8* 5 1,1 0 26,9 9 8 1,1 0,61 5% 10* 8 1,1 0 43,8 11 10 1,1 0,85 5% 12* 10 1,1 0 50,6 In this group of experiments, when the target titanium in the final product is greater than or equal to 3.5%, the addition of aluminum is necessary to reduce the amount of TiOx into the slag and avoid slag crystallization. - C / A ratio In this group of experiments, the target titanium in the final product was equal to 8 or 10. The C / A ratio varied from 0.5 to 2.3, and no CaF2 was added. The amount of added aluminum was considered to be 0.4%. All components and results can be seen in Table 2 below, the numbers of the starred experiments are not in accordance with the invention. As previously explained, the rate of crystallization depends on the process, but an acceptable crystallization rate for a steelmaker is when the slag covers the surface of the molten metal and at the same time the possibility of sampling the molten metal is still maintained. Table 2 N° %Ti C / A %Al % Crystallization 20* 8 0.5 0.4 48.7 21 8 0.7 0.4 3.8 22 8 1.1 0.4 1.7 23 8 1.6 0.4 4.9 24 8 2 0.4 16.7 25* 8 2.3 0.4 24.0 26* 10 0.5 0.4 37.4 27 10 0.7 0.4 1.5 28 10 1.1 0.4 1.7 29 10 1.6 0.4 4.1 30 10 2 0.4 15.2 31* 10 2.3 0.4 23.2 In this group of experiments, adding mineral compounds to achieve a slag composition in the C / A range of the invention allowed for a reduction in the slag crystallization rate. - Effect of CaF2 on aluminum addition In this group of experiments, the target titanium in the final product was considered to be 8%. The C / A ratio was 1.1 and the fluorspar (CaF2) content varied between 0 and 20%w. The aluminum content was calculated to achieve a titanium oxide content in the slag of 5%. All components and results are available in Table 3 below. Table 3 N° %Ti C / A %CaF2 %Al %TiOx 40 8 1,1 0 0,61 5 41 8 1,1 5 0,55 5 42 8 1,1 10 0,48 5 43 8 1,1 15 0,4 5 44 8 1,1 20 0,33 5 The addition of CaF2 made it possible to reduce the amount of aluminum required to reduce the TiOx content in the slag and thus limit the crystallization curve. - Effect of CaF2 on crystallization rate In this group of experiments, the target titanium in the final product was stabilized at 8%. The C / A ratio varied between 1.1 and 2, and the fluorspar (CaF2) content was 0 or 12%w. The aluminum content was stabilized at 0.4%. All components and results are available in Table 4 below. Table 4 N° %Ti C / A %CaF2 %Al % Crystallization 52 8 1,1 0 0,4 1,7 53 8 1,1 12 0,4 0,0 54 8 1,6 0 0,4 4,9 55 8 1,6 12 0,4 0 56 8 2 0 0,4 16,4 57 8 2 12 0.4 8.7 The addition of fluorspar CaF2 allowed for further limiting of the crystallization rate of the slag. Pilot tests Experiments were carried out on a pilot plant to reproduce the behavior of steel on a small scale. Molten metal, the initial composition of which is given in Table 5, was poured into a magnesia crucible placed in a furnace under the same temperature and atmosphere as the sample. The presence of argon and thus a non-oxidizing atmosphere is relevant to the pilot plant conditions and is not essential for industrial conditions. Slag pellets, the composition of which is given in Table 5, were added to the molten metal. The results in terms of crystallinity are also given in Table 5. The A608 test was conducted using a method that is not in accordance with the invention, while the other five tests were in accordance with the invention. For this test, a standard aluminum indicator, not in excess of the method in accordance with the invention, was added to the steel for deoxidation. The A608 test is the only case showing a crystallization of slag at the casting temperature, thus preventing further casting of the steel. Therefore, the method according to the invention makes it possible to avoid crystallization of slag at the required casting temperature. Table 5 Test No. % Aluminum Added Composition Conditions Steel Slag C / A CaF2 MgO B 2 O 3 TiO2 Slag Crystallization B Ti Initial Final Initial Final Ti A608* 0.04% 1390°C Ar-1atm 1.6 - 3.4 - 29.5 Yes 1.95 2.05 5.83 3.49 -40.1% A598 0.4% 1425°C Ar-1atm 1 - 8.5 0.35 7.7 No 1.92 1.91 5.96 5.85 -1.8% A596 0.4% 1450°C Ar-1atm 1.1 - 7.3 0.26 4.65 No 1.76 1.75 5.86 5.78 -1.4% A602 0.4% 1360°C Ar-1atm 1.1 - 7.1 0.4 4.2 No 1.95 1.98 5.85 5.62 -3.9% A601 0.4% 1380°C Ar-1atm 0.7 7.4 23.4 0.9 4.9 No 1.95 2.01 5.83 5.52 -5.3% A610 0.6% 1530°C Ar-0.25atm 0.7 5-6.5 4.7-12.6 - 0-20 No 1.95 2.04 5.83 6.08 +4.3% Furthermore, with a method according to the invention, titanium fragmentation is prevented, so that a smaller amount of titanium is required to achieve the target composition.

Claims

CLAIMS 1) A method of casting a steel semi-product from a liquid steel, the steel semi-product having a targeted composition in titanium of at least 3,5% in weight, the method comprising the following steps: A / addition of aluminium to the liquid steel so that liquid steel contains at least 0,1% in weight of aluminium, B / addition to the liquid steel of mineral compounds containing CaF2 and aluminium and calcium and optionally magnesium, to reach and maintain a slag composition wherein the ratio of CaO versus Al2 O3 (CaO / Al2 O3 ) is comprised from 0,7 to 2 and the slag contains up to 25%in weight of CaF2, C / addition of titanium to the liquid steel to reach the targeted composition, D / casting the steel in the form of a semi-product.2) A method according to claim 1 wherein the amount of aluminium added is such that the liquid steel contains more than 0,2% in weight of aluminium.3) A method according to claim 2, wherein the amount of aluminium added is such that the liquid steel contains more than 0,4% in weight of aluminium.4) A method according to anyone of the previous claims wherein the mineral compounds are chosen among lime, spar CaF2 and magnesia.5) A method according to any of claims 1 to 4, wherein the steel semi-product contains boron in a minimum percentage in weight fulfilling following equation: %B≥0,45x%Ti-1,35%6) A method according to any of claims 1 to 5 wherein between steps A and B a heating step of the liquid steel is performed.7) A method according to any of claims 1 to 6 wherein a step of addition of boron is performed after step C.8) A method according to any of claims 1 to 6 wherein an addition of boron is performed during step B.9) A method according to anyone of the previous claims wherein during step B, spar CaF2 is added so as to reach a composition in CaF2 comprised from 6 to 15%in weight.10) A method according to anyone of the previous claims wherein during step B, magnesia is added so as to reach a composition in MgO comprised from 5 to 15% in weight.11) A method according to anyone of the previous claims, wherein during step B addition of mineral compounds is done to reach a slag composition wherein the ratio of CaO versus Al2O3 (CaO / Al2O3) is comprised between 0,9 and 1,3.12) A method according to claims 1 to 8, wherein during step B addition of mineral compounds is done to reach a slag composition wherein the ratio of CaO versus Al2O3 (CaO / Al2O3) is comprised between 1,4 and 2, the slag furthermore comprising between 6 and 12% in weight of CaF2 .13) A method according to anyone of the preceding claims wherein the steel semi-product has a targeted composition in titanium of at least 5,8% in weight.14) A method according to claims 1 to 12 wherein the steel semi-product has the following composition expressed in content by weight: 0,01% ≤ C ≤ 0,2% 3,5% ≤ Ti ≤ 10 % (0,45 xTi) – 1,35% ≤ B ≤ (0,45 xTi) + 0,70% S ≤ 0,03% P ≤ 0,04% N ≤ 0,05% O ≤ 0,05% and optionally containing: Si ≤ 1,5% Mn ≤ 3% Al ≤ 1,5% Ni ≤ 1% Mo ≤ 1% Cr ≤ 3% Cu ≤ 1% Nb ≤ 0,1% V ≤ 0,5% and comprising precipitates of TiB2 and optionally of Fe2 B, the balance being Fe and unavoidable impurities resulting from the elaboration.