Method for casting semi-finished steel products with high titanium content

Abstract

A method for casting a steel blank from liquid steel, the steel blank having a target composition of at least 3.5 wt. % titanium.

Description

[Technical field]

[0001] The present invention relates to the casting of steel semi-finished products having a high titanium content. [Background technology]

[0002] FeTiB 2 Steels have been attracting a lot of attention due to their excellent high modulus of elasticity E, low density and high tensile strength, which are very promising for the automotive industry, where the weight reduction and safety of vehicles are always of utmost concern. However, these steels are difficult to manufacture due to limitations related to the precipitates that are formed. For this reason, different solutions have been developed to manufacture such steels, especially to solve the castability problem.

[0003] US20130174942 discloses FeTiB containing between 2.5 and 7.2% Ti. 2 A steel is disclosed which is cast at a casting temperature not exceeding 40° C. above the liquidus temperature of the steel, which allows a fine microstructure to be obtained.

[0004] EP 3612657 discloses a specific composition of steel, the content of which in the steel is at least 0.95% free Ti, and due to this content of free Ti the structure of the steel remains mainly ferritic at any temperature below the liquidus temperature. As a result, the hardness of the steel at high temperatures is significantly reduced compared to prior art steels, which results in increased castability. Casting is preferably carried out in the form of thin slabs.

[0005] However, regardless of the type of casting method, it is necessary that the liquid steel arrives at the tapping station with the right composition, temperature and viscosity. For these particular grades, this step is one of the trickiest. During steelmaking, some components of the slag may precipitate, depending on the slag composition and temperature. For grades with high titanium content, the crystallization rate of the slag increases dramatically, as titanium tends to partition and migrate towards the slag and titanium oxides tend to precipitate at the casting temperature. A good crystallization rate for steelmakers is when such slag still covers the molten metal to avoid contact with air, while at the same time allowing samples of the molten metal to be taken. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US Patent Application Publication No. 2013 / 0174942 [Patent Document 2] European Patent Application Publication No. 3612657 Summary of the Invention [Problem to be solved by the invention]

[0007] There is a need for a casting process that makes it possible to cast steel semi-finished products with a high titanium content, ie, greater than 3.5% by weight. [Means for solving the problem]

[0008] This problem is solved by the method according to the invention, in which the following steps are carried out: A / adding aluminum to the liquid steel so that the liquid steel contains at least 0.1% by weight of aluminum; B / Liquid steel with aluminum and / or calcium and optionally magnesium and CaF 2 Adding mineral compounds containing CaO vs. Al 2 O 3 (CaO / Al 2 O3 ) ratio is between 0.7 and 2, and the slag contains a maximum of 25% by weight of CaF 2 A process comprising: C / Adding titanium to liquid steel to achieve the target composition; D / The process of casting this steel in the form of semi-finished products.

[0009] The method of the invention may also comprise any of the following features, considered separately or according to all possible technical combinations: the amount of aluminum added is such that the liquid steel contains more than 0.2% by weight of aluminum, preferably more than 0.4% by weight, - the semi-finished steel product contains a minimum percentage by weight of boron satisfying the formula %B ≥ 0.45 × %Ti - 1.35%; - Between steps A and B, a step of heating the liquid steel is carried out; - after step C, a step of adding boron is carried out, - during step B boron is added, - Spar CaF during process B 2 Addition of CaF 2 A step of achieving a composition containing 6 to 15% by weight of - adding magnesia during step B to achieve a composition of the slag containing 5-15% by weight of MgO; - Mineral compounds are added during step B to reduce the CaO vs. Al 2 O 3 (CaO / Al 2 O 3 ) is between 0.9 and 1.3; - Mineral compounds are added to reduce CaO vs. Al 2 O 3 (CaO / Al 2 O 3 ) ratio is between 1.4 and 2, and the slag is between 6 and 12 wt.% CaF 2 achieving a slag composition further comprising: - the semi-finished metal product has a target composition of at least 5.8% by weight of titanium; - the mineral compound is selected from lime, spar and magnesia; - the semi-finished steel product has the following components, expressed as percentages by weight: 0.01%≦C≦0.2% 3.5%≦Ti≦10% (0.45×Ti)-1.35%≦B≦(0.45×Ti)+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 TiB 2 and optionally Fe 2 The remainder is Fe and unavoidable impurities resulting from refining. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The invention also relates to a steel slag having the following composition, expressed in contents by weight: 35%≦CaO≦55%, 15% or less Al 2 O 3 %≦55%, On the other hand, 0.7≦CaO 2 / Al 2 O 3 ≦2, 0%≦MgO≦15%, TiOx<20%, Compound B 2 O 3 , SiO 2 , CrOx, MnO, NiO, FeOx, and S are each less than 1%; 0%≦CaF 2 ≦25%; The remainder are oxides resulting from impurities present in the molten metal.

[0011] In one embodiment of the method according to the invention, liquid steel (also called molten metal), which can be obtained from an electric arc furnace or from any steel making plant such as a basic smelter or converter, is subjected to a deoxidation step. At that stage, the liquid steel usually has a temperature of about 1650°C. To carry out the deoxidation, aluminum is added to the molten metal, usually during ladle tapping, in order to promote a uniform reaction of the deoxidation. According to the invention, aluminum is added so that its amount in the molten metal is equal to or greater than 0.1% by weight, which is higher than the usual amount required for the deoxidation of liquid steel. In a preferred embodiment, aluminum is added so that its amount in the molten metal is equal to or greater than 0.2% by weight. In a preferred embodiment, its weight in the molten metal is equal to or greater than 0.4%. The oxides thus formed migrate towards the top of the molten metal and increase the amount of slag. The amount of aluminum added depends on the amount of titanium oxides allowed in the slag to limit the crystallization of the slag, and on the parameters controlling the titanium distribution, such as the molten metal composition, the slag composition and the temperature. Among these parameters, the main ones are the titanium content in the molten metal, the alloying elements in the molten metal such as boron, manganese, chromium that can change the slag / metal equilibrium, the respective temperatures of the molten metal and slag, the slag weight / molten metal weight ratio, and the slag composition. 2 , B 2 O 3 Other slag oxides that are reducible by titanium, such as , must be avoided because they limit titanium distribution. Calculations of the slag / metal thermodynamic equilibrium can be performed if a thermodynamic model and a thermodynamic database are available.

[0012] Based on the final TiOx target in the slag, the molten metal composition and the temperatures during the refining process, thermodynamic calculations are performed to optimize both the amount of aluminum added and the slag composition. For each step of the refining process of the grade, the aluminum content in the molten metal and slag composition is optimized to limit titanium distribution, ensure the TiOx target and limit slag crystallization.

[0013] To define the optimum conditions, the occurrence and crystallization fraction of TiOx in the slag is calculated as a function of the temperature and the variations in the molten metal composition and the slag composition. All these calculations are well known to those skilled in the art of steelmaking. If models and databases are not available, laboratory or pilot scale slag / metal equilibrations can be performed to simulate industrial conditions.

[0014] The addition of aluminum not only allows the deoxidation of the molten metal, but also reduces the content of TiOx in the slag. Part of the crystallization of the slag occurs through the precipitation of titanates, which are compounds of titanium oxide combined with other oxides such as aluminum oxide. By lowering the TiOx content in the slag, the precipitation of titanates is limited, resulting in a lower percentage of crystals. The nature of the titanates and the rate of crystallization can be somewhat complex depending on the slag composition.

[0015] According to the method of the present invention, mineral 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 spar CaF 2 In accordance with the present invention, these additions are based on the CaO to Al ratio. 2 O 3 The adjustment is made so as to achieve and maintain a slag composition in which the ratio of sulfur to sulfur (C / A) is comprised between 0.7 and 2. This composition makes it possible to limit the crystallization rate of the titanium oxide present in the slag by maximizing the sulfur content of the slag.

[0016] The limitation of TiOx by the addition of aluminum must indeed be linked to the optimization of the slag composition in order to optimize the nature and amount of titanates and to limit their precipitation in the slag to promote low slag crystallization at the casting temperature.

[0017] In a preferred embodiment, the ratio is comprised between 1.4 and 2, the slag further comprises between 6 and 25% by weight of spar CaF 2 and more preferentially between 6 and 12% by weight of spar CaF 2 Methods for calculating and controlling this ratio are well known to those skilled in the steelmaking art. In another embodiment, the ratio C / A is comprised between 0.9 and 1.3 and the slag comprises 5-15% by weight of magnesia. In a third embodiment, this ratio is comprised between 1.4 and 2 and the slag comprises between 6-12% by weight of spar CaF 2 and between 5 and 15% by weight of magnesia MgO. Magnesia makes it possible to reduce the liquidus temperature of the slag. This last composition makes it possible to further limit the crystallization rate of titanium oxide. Magnesia can be added to the melt and / or can come directly from the refractories surrounding the molten metal in the steelmaking vessel. The skilled person knows from experience what amount of magnesia must be dissolved from the refractories and what amount must be added to reach the required content.

[0018] By virtue of this control of slag composition and the addition of aluminum, the slag contains strictly less than 20% by weight titanium dioxide. In all embodiments, the remaining slag composition contains less than 1% by weight B. 2 O 3 , less than 1% SiO 2 , less than 1%w CrOx, less than 1%w MnO, less than 1%w NiO, less than 1%w FeOx. In the method according to the invention, no slag removal is required before proceeding to the next step, which reduces the refining time of the steel.

[0019] After the mineral addition step, titanium is added to the melt in an amount to achieve a target composition of the final semi-finished product, which is at least 3.5% by weight. This is the nominal composition. This titanium can be added in the form of titanium sponge or ferrotitanium pieces such as Fe-70%Ti or Fe-35%Ti, or pure Ti or ferrotitanium wire.

[0020] After the titanium addition, the slag has the following composition: 35%≦CaO≦55%, 15% or less Al 2 O 3 ≦55%, On the other hand, 0,7≦CaO 2 / Al 2 O 3 ≦2, 0%≦MgO≦15%, TiOx<20%, Compound B 2 O 3 , SiO 2 , CrOx, MnO, NiO, FeOx, and S are each less than 1%; 0%≦CaF2≦25%, The remainder is oxides resulting from impurities present in the molten metal.

[0021] The liquid steel thus formed is then sent to the casting station to be cast in the form of semi-finished products. The casting temperature is T liquidus +40℃ or less, T liquidus denotes the liquidus temperature of the steel, which in this case is, for example, about 1330° C. By semi-finished product is meant steel slabs, thick strip or thin slabs or any other product produced by continuous casting, vertical casting, horizontal casting, roll casting, thin slab casting, bed casting, strip casting.

[0022] In another embodiment of the method according to the invention, the cast semi-finished product contains at least 2% boron added after the mineral addition step by injection of Fe-18%B or ferro-boron pieces such as ferro-boron wire. In the most preferred embodiment, this addition is made during the mineral addition step.

[0023] In a preferred embodiment, the steel semi-finished product has the following composition, expressed in contents by weight: 0.01%≦C≦0.2% 3.5%≦Ti≦10% (0.45×Ti)-1.35%≦B≦(0.45×Ti)+0.70% S≦0.03% P≦0.04% N≦0.05% O≦0.05% and optionally including: Si≦1.5% Mn≦3% Al≦1.5% Ni≦1% Mo≦1% Cr≦3% Cu≦1% Nb≦0.1% V≦0.5% and TiB 2 and optionally Fe 2 The composition contains precipitates of B, the balance being Fe and unavoidable impurities resulting from refining. This preferred composition allows the steel to remain predominantly ferritic at any temperature below the liquidus temperature, thus reducing castability problems.

[0024] One way for a steelmaker to carry out the method according to the invention is to first define the target for titanium in the final semi-finished product and the casting temperature of this final semi-finished product. Then, one of skill in the art defines what slag composition he wants to have within the given framework of the invention, i.e., to remain within a given range of C / A ratios, potentially add spar, and add the amount of MgO provided by the refractory, depending on the crystallization volume that can be tolerated during casting. Finally, using known models, calculate the amount of aluminum and other mineral additions required to achieve this defined slag composition.

[0025] For all the embodiments mentioned above, the different processes carried out on the liquid steel can be carried out indiscriminately in the same vessel or in different vessels, depending on the configuration of the plant. No special equipment is required other than those classically used in steel mills. EXAMPLES

[0026] The following trials presented herein should be considered merely for illustrative purposes, not limiting in nature, and they illustrate advantageous features of the present invention.

[0027] <Calculation> The calculations were carried out using the thermodynamic model mentioned above and known to those skilled in the art. The temperature of the slag considered was 1350°C.

[0028] The parameters considered were the titanium content (%Ti) in the cast semi-finished product, the %CaO and %Al in the slag. 2 O 3 The ratio C / A using CaF in the slag 2 The amount of slag (or spar) and the target maximum amount of TiOx in the slag are shown in Table 1. The calculations were carried out considering 15 kg of slag per tonne of hot metal.

[0029] The content of magnesia (MgO) in the slag was always considered to be 10%w.

[0030] The calculations were carried out taking into account 15 kg of slag per tonne of hot metal.

[0031] Taking all this into account, the crystallized percentage of the slag was calculated, which expresses the percentage of solid volume / total slag volume. Thermodynamic calculations give the nature and amount of oxides precipitated in the liquid slag, and knowing the volume of the liquid slag, it is possible to determine the volume percentage of the crystallized slag.

[0032] <Aluminum addition> In this series of examples, the target titanium in the final product was varied between 2.5 and 10%. The C / A ratio was 1.1, and CaF 2It was not added.

[0033] All parameters and results are summarized in Table 1 below. The trial numbers with an asterisk * are not according to the present invention.

[0034]

Table 1

[0035] From this series of trials, when the target of titanium in the final product is 3.5% or more, in order to reduce the amount of TiOx in the slag and thus avoid the crystallization of the slag, the addition of aluminum is necessary.

[0036] <C / A ratio> In this series of examples, the target of titanium in the final product was equal to 8 or 10. The C / A ratio was varied from 0.5 to 2.3, and CaF 2 It was not added.

[0037] The addition amount of aluminum is 0.4%.

[0038] All parameters and results are summarized in Table 2 below. The trial numbers with an asterisk * are not according to the present invention. As explained before, the allowable amount of crystallization rate depends on the method, but the allowable crystallization rate for steel manufacturers is when the slag covers the molten metal surface while still allowing sampling in the molten metal.

[0039]

Table 2

[0040] From this series of trials, the addition of mineral compounds to obtain a slag composition within the C / A range of the present invention makes it possible to reduce the crystallization rate of the slag.

[0041] <Effect of CaF on aluminum addition 2 > In this set of examples, the target for titanium in the final product was set at 8%.

[0042] The C / A ratio is 1.1, and the spar (CaF 2 ) content was varied from 0 to 20%w.

[0043] The aluminum content was calculated to achieve a titanium oxide content in the slag equal to 5%.

[0044] All parameters and results are summarized in Table 3 below.

[0045] [Table 3]

[0046] CaF 2 The addition of makes it possible to reduce the amount of TiOx in the slag and thus the amount of aluminum required to limit crystallization.

[0047] <Crystallization rate vs. CaF 2 Impact of > In this set of examples, the titanium target in the final product was fixed at 8%. The C / A ratio was varied from 1.1 to 2, and the spar (CaF2) content was either 0 or 12%w. The aluminum content was fixed at 0.4%.

[0048] All parameters and results are summarized in Table 4 below.

[0049] [Table 4]

[0050] Spar CaF 2 The addition of makes it possible to further limit the crystallization rate of the slag.

[0051] <Pilot Trial> Trials were carried out in a pilot to reproduce the behavior of steel on a small scale. The molten metal, the initial composition of which is given in Table 5, was poured into a magnesia crucible placed in a furnace at the given temperature and atmospheric conditions. The presence of argon, and therefore of a non-oxidizing atmosphere, is relevant for the pilot composition and is not necessary in industrial conditions. A ball of slag, the composition of which is given in Table 5, was then added to the top of the molten metal. The results regarding crystallization are also given in Table 5.

[0052] Trial A608 was carried out in a manner not in accordance with the present invention, whereas the other five trials are in accordance with the present invention. For this trial, the standard aluminum value for deoxidation was added to the steel without exceeding it as in the method of the present invention.

[0053] Trial A608 is the only one that exhibits slag crystallization at the casting temperature, which therefore prevents further casting of the steel. The method according to the invention therefore makes it possible to avoid slag crystallization at the required casting temperature.

[0054] Additionally, in some methods, the distribution of titanium is hindered, thus reducing the amount of titanium added required to achieve the target composition.

[0055] [Table 5]

Claims

1. 1. A method for casting a steel semi-finished product having a target composition of at least 3.5% by weight titanium from liquid steel, comprising the steps of: A / adding aluminum to the liquid steel so that the liquid steel contains at least 0.2% by weight of aluminum; B / The liquid steel is added with CaF ２ and adding mineral compounds containing aluminum and calcium to balance CaO vs. Al. ２ O ３ (CaO / Al ２ O ３ % CaF ２ A process comprising: C / adding titanium to the liquid steel to achieve the target composition; D / Casting said steel in the form of a semi-finished product.

2. The method of claim 1, wherein the mineral compounds added to the liquid steel further comprise magnesium.

3. 3. The method of claim 1 or 2, wherein the amount of aluminum added is such that the liquid steel contains more than 0.4% by weight aluminum.

4. The mineral compounds are lime, spar CaF ２ The method according to any one of claims 1 to 3, wherein the oxidized silicate is selected from among magnesia and magnesia.

5. The steel semi-finished product has a composition satisfying the following formula: %B≧0.45×%Ti−1.35% 5. The method of claim 1, further comprising the step of:

6. The method according to any one of claims 1 to 5, wherein between steps A and B a step of heating the liquid steel is carried out.

7. 7. The method according to any one of claims 1 to 6, wherein the step of adding boron is carried out after step C.

8. 7. The method according to any one of claims 1 to 6, wherein the addition of boron is carried out during step B.

9. During step B, CaF contained in the mixture is 6 to 15% by weight. ２ To achieve the composition of CaF ２ The method according to any one of claims 1 to 8, wherein

10. 10. The method according to any one of the preceding claims, wherein during step B, magnesia is added to achieve a composition of MgO comprised between 5 and 15% by weight.

11. During step B, CaO vs. Al ２ O ３ (CaO / Al ２ O ３ 11. The method according to claim 1, wherein mineral compounds are added to achieve a slag composition in which the ratio of 0.9 to 1.3 is comprised between 0.9 and 1.

3.

12. During step B, CaO vs. Al ２ O ３ (CaO / Al ２ O ３ ) is comprised between 1.4 and 2, and the slag contains between 6 and 12 wt.% CaF ２ The method of any one of claims 1 to 8, wherein mineral compounds are added to achieve a slag composition further comprising:

13. A method according to any one of the preceding claims, wherein the steel semi-finished product has a target composition of at least 5.8 wt% titanium.

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