RU2607877C2 - Method for off-furnace steel treatment - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to ferrous metallurgy, particularly to production of steel using off-furnace treatment methods. Method includes cutting off furnace slag, tapping metal into a ladle, heating of metal in ladle and creation of highly basic slag, desulphurisation of metal, creation of low-basic slag, evacuation, continuous casting of metal and continuous mixing of metal with argon. When tapping metal into ladle, 10–12 kg/t of steel slag-forming materials are added in form of lime, aluminium-corundum mixture and silicon carbide at a ratio of (1.0–1.5):(0.20–0.25):(0.10–0.15) respectively and aluminium ingots in amount of 1.3–1.4 kg/t steel. Invention guarantees to perform complex metal refining from sulphur to 0.002–0.005 % with further alloying to 0.020–0.035 %, hydrogen to 0.0002 % and oxide non-metallic inclusions in metal of alumina nature to 0.0030–0.0035 vol%, and also to reduce total duration of off-furnace treatment to a level not exceeding duration of continuous casting.

EFFECT: disclosed is a method for off-furnace steel treatment.

5 cl, 2 ex, 1 tbl

Description

The invention relates to ferrous metallurgy, in particular to the production of steel using methods of out-of-furnace steel processing.

There is a method of out-of-furnace steel processing, in which freshly calcined lime, quartz sand and fluorspar are planted in the bucket before the metal is released from the steelmaking furnace; deoxidize and alloy the metal with ferrosilicon and ferromanganese additives; the acid slag formed in the ladle at the end of the discharge is downloaded, then the main slag in the ladle furnace is added with lime and fluorspar additives, if necessary, the metal is heated, then it is vacuumized; after the end of evacuation, acidic slag with a basicity of 0.7-1.1 is induced, and after obtaining homogeneous and liquid-moving slag, the heating installation (ladle furnace) is turned off and the metal is sent to the continuous casting machine of the UNRS (continuous casting machine for continuous casting machines). Source of information - A.N. Parshikov, M.P. Gulyaev, E.V. Ivanov, E.I. Laneweber, “Processing of steel for steel cord with acidic slag,” Proceedings of the IV Congress of Steelmakers, Moscow, 1997, p. 264-265.

Disadvantages:

a) the increased complexity of the method due to the need for two to three times a change in the composition of the slag;

b) only the “classes” of induced slags are given — basic and acidic. The composition of the main slag is not given. In acid, only the ratio (CaO) / (SiO ₂ ) is given. The absence or insufficient information on the composition of the slag does not guarantee reproducible metal refining results with their participation.

There is a method of out-of-furnace steel processing, RF patent No. 2362811 of 23.10.2007, in which the following actions are performed: deoxidation of steel in the ladle with aluminum, evacuation of the metal before it is purged with argon and the introduction of calcium, measurement of oxygen activity in the metal before the introduction of calcium and bringing to a given level of oxygen activity in the melt. The amount of calcium introduced into the metal, calculated on the assimilated level, is determined taking into account the specified CaO content in the non-metallic inclusion, the sulfur content before calcium treatment, and the oxygen activity in the metal. The oxygen activity in the metal is adjusted to a predetermined level by introducing a deoxidizer, for example aluminum, one or more times. The amount of calcium introduced into the metal must satisfy the condition 0.5 ... 2.0⋅α O≤ [Ca] ≤2.5⋅ [S] with oxygen activity of 0.0005 ... 0.002% and 2.0 ... 10.0⋅ α О≤ [Ca] ≤2.5⋅ [S] with an oxygen activity of 0.0001 ... 0.0005%. Calcium is introduced into the metal in the form of a flux-cored wire, which includes calcium containing an alloying component.

Closest to the invention in technical essence and the achieved result is a production method, V.V. Anders, D.S. Yashchuk, M.P. Gulyaev, “Optimization of the technology of out-of-furnace treatment of high-carbon high-quality steel in order to reduce oxide non-metallic inclusions”, Proceedings of the VII Congress of Steelmakers, Moscow, 2003, p. 435-438.

The method consists in the fact that out-of-furnace treatment is started at the time of release by introducing up to 0.1% silicon into the ladle and is doped with manganese; then the metal is transferred to the ladle furnace and heated to 1590-1620 ° C, highly basic slag is added with CaO and CaF ₂ additives, metal desulfurization is carried out, low-basic slag is introduced before vacuuming in the ladle furnace, then the metal is evacuated, it is deoxidized with carbon when evacuated, and corrected composition according to the content of carbon and silicon and transferred again to the ladle furnace, adjusted for carbon and temperature, then poured into a continuous casting machine. Steel of higher purity is obtained in terms of the content of non-metallic inclusions, and due to this, wire breakage is reduced by a factor of 1.7 when twisting steel cord.

The disadvantages of this method are:

a) There is no data on the composition of the used slag, which will not allow to successfully use the method for refining metal from sulfur and the adverse composition of non-metallic inclusions. So, for successful desulfurization of metal by highly basic slags, it is necessary to have a high value of the sulfide capacity of the slag Cs, as well as a low slag viscosity, which depend on the composition of the slag. The possibility of deep metal desulfurization by this method is practically impossible and its results are not shown.

b) Bringing low-basic slag and heating it and metal with electric arcs in ladles with a main lining for acidic slags causes an increased rate of chemical erosion of the ladle lining in the slag belt due to the increased solubility of magnesium oxide in the slag with low basicity and elevated slag temperature, approximately 100 ° C compared with the temperature of the metal, when such a slag is heated by electric arcs.

c) An excessively long duration of out-of-furnace treatment with a total duration of 45-210 minutes. This reduces the productivity of the production line DSP-Furnace-Bucket-Vacuum -ator CCM, increases the consumption of expensive refractories of the lining of the bucket, especially in the slag belt, and electricity. The duration of out-of-furnace treatment should be equal to or slightly less than the duration of continuous casting, currently 45-60 minutes.

The technical problem to which the invention is directed is the guaranteed complex refining of metal from sulfur to 0.002-0.005%, followed by alloying with it to 0.020-0.035%, hydrogen up to 0.0002% and non-metallic oxide inclusions in an aluminous metal up to 0.0030 -0.0035% vol. Reducing the total duration of out-of-furnace treatment to a level not exceeding the duration of continuous casting.

This problem is solved in that the desulfurization of the metal in the ladle is carried out by the slag of the CaO-SiO ₂ -Al ₂ O _{3 system} by the reaction:

3 (CaO) +3 [S] +2 [Al] = 3 (CaS) + (Al ₂ O ₃ ).

As can be seen from the metal desulfurization equation, lime is a desulfurizer. The additive of lime, alumina-corundum mixture and silicon carbide at a ratio of 1: 0.20: 0.10 allows to obtain slag with a high desulfurizing ability, to obtain a high value of {(S) / [S]} = 115 when the slag contains: lime 55% and alumina 30%. In the metal at the end of processing in a ladle furnace with such slag, the sulfur content decreases from 0.045% to 0.004%.

The addition to lime of a slag-forming alumina-corundum mixture and silicon carbide in a ratio of 1.5: 0.25: 0.15 results in a liquid-slag with a higher desulfurizing ability {(S) / [S]} = 150 and a slag with a content of: lime : 65% and alumina 25%. In the metal at the end of its processing in the furnace, the sulfur content decreases to 0.002-0.003%.

A decrease in the proportion of lime less than 1.0 or its increase above 1.5 in both cases leads to a significant decrease in the {(S) / [S]} indicator (with the same aluminum content in the metal) and worsening of metal desulfurization.

Adding alumina-corundum mixture to lime in a fraction of less than 0.20 reduces the desulfurization properties of slag and the efficiency of metal desulfurization. Increasing the proportion of alumina-corundum mixture above 0.25 also impairs the desulfurization properties of the slag.

It is proposed to use silicon carbide in the composition of the mixture, during carbon oxidation of which there is an intensive release of CO and an increase in the mass transfer of sulfur in the slag due to mixing of the slag and acceleration of metal desulfurization, the process is limited by sulfur mass transfer in the slag phase. We experimentally established a rational content of it in the mixture in the range of 0.10-0.15 shares.

Oxidation of silicon by silicon carbide within these limits increases the content of SiO ₂ in the slag to a level in the range of 10-8%, which ensures low slag viscosity, high desulfurizing and assimilating properties with respect to aluminous non-metallic inclusions.

An important element is the additional input of pig aluminum with the addition of slag-forming materials when melting into the ladle of pig or briquetted aluminum in the amount of 1.3-1.4 kg / t. This leads to the melting of slag-forming during the release process and the deoxidation of the resulting slag due to the release of additional heat from the reaction (a significant part of the oxidizing slag with a high FeO content gets from the furnace):

3 (CaO) +2 [Al] +3 (FeO) = 3 (CaO⋅Al ₂ O ₃ ) +3 [Fe];

ΔH = 208630 kcal / kg-at or 3864 kcal / kg;

ΔН is the reaction enthalpy;

When using slag-forming materials in an amount of 10 kg / t (1500 kg), heat will be required for its melting:

Q = m (cΔt + 50) = 1500. (0,298.1600 + 50) = 790,200 kcal;

Q is the amount of heat;

m is the mass;

C is the specific heat;

Δt is the temperature change;

To compensate for heat loss during heating and melting of such an amount of slag-forming materials, the required amount of aluminum is:

G _Al = (790200/3864) / 150 = 1.36 kg / t (or in the range of 1.3-1.4 kg / t);

G _Al - specific consumption of aluminum;

During evacuation of deoxidized steel, hydrogen is removed from the surface of a metal not covered by slag. The hydrogen removal rate is determined by a first order equation, the integral form of which is the equation:

log ([Hτ] - [Hp]) / ([H ₀ ] - [Hp]) = - 2.3kT;

Hτ is the current hydrogen content in steel;

Hp is the equilibrium hydrogen content in steel;

H ₀ - the initial hydrogen content in the steel;

l is the Boltzmann constant, J / K;

T is the temperature, K;

Hence, obtaining the actual or current hydrogen content in steel [Hτ] at the level of its non-wall sensitivity equal to 0.0002% is provided at [Hp] ≤ [Hτ] ≤0.0002%.

Values of [Hp] depending on pressure at a temperature of 1600 ° C are:

Our industrial data show that to ensure [Hτ] ≤2⋅10 ^-4 % it is necessary to vacuum the metal for 10-15 minutes at a residual pressure of 0.13-0.067 kN / m ² (1.0-0.5 mm Hg. Art.) and specific consumption of argon 4.5-5.0 l / (t⋅min). Evacuation at pressures above 0.13 kN / m ² will lead to unstable, often higher than 2⋅10 ^-4 %, values in [Hτ]. Evacuation at a pressure of less than 0.067 kN / m ² does not give significantly lower current hydrogen contents in steel under industrial production conditions.

Further, generally the slag system CaO-Al ₂ O ₃ -SiO ₂ hydrogen solubility is 66-76 cm ^3/100 g In the actual process of hydrogen slag less (30-40 cm ³ / 100g), but it is well established that the distribution of hydrogen between the slag (H) and the metal [H] is (H) / [H] = 5-6.

During evacuation, hydrogen is removed from the metal, but at the same time passes into it from the slag.

When the metal is evacuated in a ladle in the presence of the main slag, upon inducing it with the participation of electric arcs in the ladle furnace, increased contents of hydrogen and nitrogen are obtained in it. With a decrease in pressure, this leads to the emission of these gases from the slag and a strong foaming of the slag (visually - as the effect of "boiling milk") and its leakage from the ladle. This forces to stop the purging of the metal with argon and a sharp decrease and even the actual termination of the removal of hydrogen during evacuation of the deoxidized metal.

Therefore, for successful degassing, the amount of highly basic slag in the ladle during evacuation should not be, or its amount must be minimized as much as possible and the slag is downloaded.

One of the most important reasons for sorting metal deoxidized by aluminum is the increased presence of non-metallic inclusions of alumina or their accumulations in it, especially in combination with an increased [N]. In these areas, complex nonmetallic inclusions of Al (aluminum) oxides are found, often with Ca oxides of different stoichiometry (depending on the content of aluminum and calcium in low alloy steels).

To further reduce the content of such inclusions in the metal, properties assimilating to nonmetallic inclusions of alumina are enhanced by means of specially induced low-base slag, technological methods are used that increase the refining efficiency of the method in the period after vacuum processing. To do this, perform the following techniques:

- Reduce the activity of the corresponding component, in this case, aluminum oxide, in the slag melt. Slag corresponding to the compositions at the anorthite-gelenite boundary of the CaO-Al ₂ O ₃ -SiO _{2 system} , especially in the triple eutectic region, which allows maintaining the necessary slag viscosity and increased solubility, have a higher assimilating ability to non-metallic inclusions of alumina and lower liquidus and melting temperatures. Al ₂ O ₃ with an increase in the content of Al ₂ O ₃ up to 30-40%.

- Slag with a reduced ratio of CaO / SiO ₂ and have an increased solubility of MgO in them. The use of such slag in buckets with a main (magnesite) lining leads to an increase in the consumption of refractories. Further, during the melting and heating of such slags by an electric arc, the slag melt overheats 100-150 ° C higher than usual. This further enhances the dissolution of the MgO lining in such a slag, worsening its properties. Therefore, we propose not to include electric arcs when melting slag-forming materials to induce low-base slag.

- During continuous casting, a part of the slag from the steel pouring ladle inevitably falls into the intermediate ladle, which changes the composition of the slag in the intermediate ladle, reduces its properties assimilating to inclusions and affects the quality of the metal. Therefore, after evacuation, before slag casting, slag-forming ones are planted in the ladle, which ensure rapid melting and formation of low-base low-melting slag with high assimilating alumina properties similar in composition to the slag used in the CCM ladle. To obtain the above technological properties, after evacuation, a mixture of inexpensive and stable in chemical composition ingredients is planted after vacuum casting: cement, syenite concentrate, foundry sand, serpentinite, fluorite concentrate and carbon at a ratio of components (%), 28: 15: 39: 10: 3: 5, respectively, in an amount of 1.5-2.0 kg / t. The mixture is seated strictly in the indicated proportions, which makes it possible to obtain slag reproducible in composition and physicochemical properties. Get the slag, the composition is identical to the slag induced in the intermediate ladle CCM. This slag during casting, inevitably falling into the pit, does not change its chemical composition and retains its refining properties.

- To reduce heat loss and maintain the required temperature and assimilating properties, powdered rice husk is placed on the surface of the alumina inclusions after the mixture is melted, forming a two-layer metal coating. The amount of husk varies depending on the carbon content in the slag-forming mixture.

- The process of assimilation of inclusions by slag requires considerable time. To significantly reduce the contact time of low-base slag with the magnesite lining of the ladle and to eliminate the manifestation of its aggressiveness to the main refractories of the steel casting ladle, the process of assimilation of non-metallic inclusions was postponed during the continuous casting period. This increases the time of assimilating slag processing of metal from non-metallic inclusions and increases the efficiency of metal purification from them. At this time, there is a continuous decrease in the level of metal in the ladle, which is accompanied by a continuous change in the contact point of its lining with the slag, which significantly reduces the time of such contact while maintaining the lining of the ladle and prevents an undesirable increase in the MgO content in the slag composition.

- Purging metal with argon during casting floats non-metallic inclusions to the surface of the slag and accelerates the process of their assimilation by slag. We experimentally established that the main refining of inclusions of alumina with slag at this time occurs during the casting of the first 50-60% of the metal. A decrease in volume below 50% reduces the results of removal of inclusions, an increase in volume above 60% does not give a significant improvement in the results of removal of inclusions.

- The continuation of the metal refining process from inclusions in the steel pouring ladle during the continuous casting period allows to increase the throughput of out-of-furnace processing equipment by reducing the total time of its debt.

The method is as follows.

Example 1

When releasing melting from an electric arc furnace, for example, with a capacity of 150 tons, slag-forming materials are added to the ladle with the main lining and in the ratio: lime, alumina-corundum mixture MMKN-75 and silicon carbide with a component ratio of 1.5: 0.25: 015 or, respectively in the amount of (kg) 1500: 375: 225; 1.36 kg / t of briquetted aluminum (200 kg) are planted; ferroalloys are also planted, based on the lower limit of the grade composition. For example, when smelting steel grade C45E ferrochrome is planted 61 kg, ferrosilicon 65% 111 kg, ferrosilicon manganese 262 kg;

After release, the ladle with melting is transferred to the “ladle-furnace” installation, electric arcs are turned on and the metal is heated, heated and further melted is the slag-forming and slag formed. The whole process of processing the smelting is carried out with continuous stirring with argon with a specific intensity of 0.0059 m ³ / (t⋅min). Get the slag composition,%:

The metal is heated and treated with slag in a ladle furnace at a metal temperature of 1600-1620 ° C until the sulfur content in the metal is not more than 0.004%. The average metal desulfurization rate is V _S = 0.001% / min. The desulfurization time at [S] _nach = 0.040% is 35-36 minutes.

Next, an aluminum wire is introduced into the metal by a tribing apparatus to obtain [Al] in the range of 0.020-0.025% and the highly basic desulfurizing slag is downloaded. The heat is transferred to a ladle-type vacuum unit. The pressure is gradually reduced and the metal is continuously stirred with argon with an intensity of (4.5-5.0) * 10 ^-3 m ³ / (t⋅min) vacuum, providing processing at a residual pressure of less than 0.13-0.067 kN / m ² ( 1.0-0.5 mm Hg or 1.33-0.66 mbar) for at least 10 minutes.

After evacuation in the same bucket (without transporting it to another place and without stopping mixing the metal with argon), a mixture of cement, syenite concentrate, foundry sand, serpentinite, fluorite concentrate and carbon is placed on its surface at a component ratio (%) of 28:15: 39: 10: 3: 5, respectively, in an amount of 2.0 kg / t and after its melting from the heat of the metal (without turning on electric arcs), taking into account the influence of the always-present residual slag after evacuation, a low-melting slag with increased properties assimilating to inclusions of alumina. The chemical composition of the slag, wt. %:

Further, this resulting slag is covered with 170 kg of heat-insulating rice husk. An aluminum wire is introduced at a content of [Al] = 0.025-0.030%, then a calcium-containing flux-cored wire in an amount of 52 kg and after 5 minutes stirring the metal with argon, a sulfur-containing flux-cored wire in an amount of 110 kg is introduced, increasing the sulfur content in the metal from 0.004% to 0.027%. After 3-4 minutes of stirring and measuring the temperature, the ladle with heat is transferred to the caster table. Pour the first 60% of the metal by slag re-induced with stirring under argon intensity 1⋅10 ^-3 m ³ / (t⋅min). Then stirring with argon is stopped and the metal is cast without stirring with argon until the end of the casting.

Example 2

Perform the same way as example 1, but characterized in that to induce highly basic slag, slag-forming substances are added: lime, alumina-corundum mixture MMKN-75 and silicon carbide with a ratio of components of 1.0: 0.20: 0.10, respectively, in an amount, (kg ): lime 1000; MMKN-75 200; silicon carbide 100;

During continuous casting, under the newly induced low-melting slag, the first 50% of the metal is poured with stirring with argon at an intensity of 0.8⋅10 ^{* 3} m ³ / (t⋅min).

Held in the VTZ electric steelmaking shop melts with out-of-furnace treatment according to the claimed method showed the table, the following:

1. The duration of out-of-furnace steel processing is substantially reduced;

2. Low hydrogen and non-metallic oxide inclusions with a high content of alumina phase are obtained, which are especially harmful to the quality of the metal when combined with their high contents.

I The content of non-metallic inclusions in the metal after release from the arc steel furnace;

II The content of non-metallic inclusions after treatment with highly basic slag in a ladle furnace;

III Content non-metal. inclusions in the metal after processing by fusible slag in a casting ladle;

* ⁾ This includes ~ 25 minutes: downloading desulphurizing highly basic slag from a ladle before evacuation, additive materials to produce new low-melting slag in a steel casting ladle before continuous casting, introduction of an aluminum wire rod and SiCa and S-containing flux-cored wires.

Claims (5)

1. A method of out-of-furnace steel processing, including cutting off furnace slag, discharging metal into a ladle, heating metal in a ladle furnace and inducing highly basic slag, desulphurizing metal, inducing low-basic slag, evacuating, continuously casting metal, and continuously stirring metal with argon, characterized in that when releasing metal, 10-12 kg / t of steel of slag-forming materials in the form of lime, alumina-corundum mixture and silicon carbide are planted in the bucket with their ratio (1.0-1.5) :( 0.20-0.25) :( 0, 10-0.15), respectively, and aluminum ingots per stake honors 1.3-1.4 kg / t steel. 2. The method according to p. 1, characterized in that after heating the metal in a ladle furnace, guidance of highly basic slag and desulfurization of highly basic slag is removed, then steel deoxidized by pig aluminum is evacuated at a residual pressure of 0.13-0.067 kN / m. 3. The method according to p. 1, characterized in that the low-base slag is induced after the end of evacuation before continuous casting without turning on electric heating by feeding 1.5-2.0 kg / t of steel slag-forming mixture consisting of cement 28%, syenite concentrate 15% , foundry sand 39%, serpentinite 10%, fluorite concentrate 3% and carbon 5%. 4. The method according to p. 3, characterized in that in order to reduce heat loss and maintain the required temperature after melting the slag-forming mixture, powdery rice husk is planted on its surface. 5. The method according to p. 3, characterized in that they cast 50-60% of the initial volume of the metal with stirring in a steel pouring ladle with argon.