JP2006002175A - Method for preliminarily deoxidizing molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preliminarily deoxidizing method which keeps an oxygen content in a molten steel low, without adding such a deoxidizer as to form an oxide-based inclusion, in a top and bottom combined blown converter in which blowing has been finished. <P>SOLUTION: The method for preliminarily deoxidizing the molten steel comprises changing a stirring gas to be blown from a furnace bottom, from carbon dioxide gas to a gas mainly containing hydrocarbon, and rinsing the molten steel with the gas, simultaneously with stopping the top blowing of oxygen, in a method for refining dephosphorized molten pig iron in the top and bottom combined blown converter. The gas mainly containing hydrocarbon may be blown from the furnace bottom when the top blowing of oxygen at a final stage of the blowing is still continued. Thereby, the method can reduce an amount of the deoxidizer to be added in tapping, and produce high cleanliness steel. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for predeoxidizing molten steel by bottom blowing a gas mainly composed of hydrocarbons in a rinsing period after the end of blowing in the refining of steel by an upper bottom blowing converter.

  When steel is produced in a top-bottom blow converter using blast furnace iron as the main raw material, the carbon in the molten steel is oxidized with top-blown oxygen and decarburized so that the carbon content is slightly lower than the target content of the product steel. . In this case, the lower the target content of carbon ([C]) in the molten steel, the higher the amount of oxygen blown for decarburization, and the higher the oxygen ([O]) in the molten steel. For example, when the target [C] is 0.05%, [O] is about 500 ppm or more. [C] and [O] mean carbon and oxygen in the molten steel, respectively.

  If the above molten steel is used for continuous casting as it is, the concentration product of carbon and oxygen in the molten steel, that is, [C] × [O] decreases as the temperature of the molten steel decreases. Rimming action occurs due to the generation of CO by reaction, and casting cannot be performed.

[C] + [O] = CO (1)
Therefore, deoxidation is usually performed by adding a deoxidizer such as Al, Fe-Si, Si-Mn to the molten steel. Thus, continuous casting is made possible by killing the molten steel.

When the deoxidizer is added to the molten steel as described above, the rimming action is suppressed, but oxide inclusions such as Al ₂ O ₃ and SiO ₂ —MnO are generated in the molten steel. Since these inclusions deteriorate the mechanical properties of the steel, it is necessary to reduce them as much as possible, that is, to clean the molten steel.

  What is important for melting clean steel is that the carbon in the molten steel, that is, [C], is not reduced too much below the target value in the converter refining. In order to reduce [C], the amount of oxygen blown must be increased. Therefore, if [C] is excessively decreased, oxygen in the molten steel, that is, [O] increases, and the amount of the deoxidizer added is increased. Therefore, oxide-based inclusions are also increased.

  For example, if the target [C] is 0.05% and if it is mistakenly blown down to [C] = 0.03%, under the atmospheric pressure, the thermodynamic relationship [% C] × [% O] = 0.024, [O] rises to at least about 800 ppm. When the stirring is weak, [O] may further increase due to deviation from equilibrium. Thus, when [O] becomes high, not only a large amount of expensive deoxidizing agent such as Al is required, but also the amount of oxide-based inclusions increases as described above.

  In recent years, dephosphorization is often performed in the hot metal stage, and high-speed decarburization is often performed with a small amount of slag of about 20 kg / t (t is molten steel ton) in a converter. In this case, since the amount of slag is small, the decarbonation efficiency is high and the decarburization speed is fast. Therefore, when the stop of the oxygen blowing is delayed, [C] rapidly becomes lower than the target value, and conversely, the [O] concentration becomes high.

  Even when the oxygen blowing is stopped properly, for example, when steel with a low [C] of 0.03 to 0.05% is produced, a deoxidizer such as Al is used rather than the production of high [C] steel. Therefore, the problem of increased cost of deoxidizer and increased inclusions remains. Therefore, if pre-deoxidation can be performed without using strong deoxidizers such as Al or reducing the amount used to produce oxide inclusions as much as possible, the amount of deoxidizers such as Al can be reduced. In addition, it is possible to produce clean steel with few inclusions.

  In addition, preliminary deoxidation is deoxidation performed preliminarily in a converter before full-scale deoxidation after steel output.

  Conventionally, as a method for preventing the end point [C] from being set too lower than the target value, a method of increasing the end point ratio in the converter is fundamental. However, even if technology is used to obtain information on the composition and temperature of molten steel at the end of the smelting process using sublances and control the blowing conditions, [C] is excessively lower than the target [C]. It is inevitable to do this.

  The ideal method for reducing the [O] of the molten steel is to reduce the CO partial pressure by vacuum treatment and promote the reaction of the formula (1), as can be seen from the formula (1). However, the implementation of a vacuum processing technique such as the RH method increases the production cost of steel. Therefore, development of a technique for reducing [O] at the end of converter refining without using a high-cost vacuum deoxidation method has been directed. The following two methods are representative of such techniques.

  The first method is a method called K-BOP disclosed in Non-Patent Document 1. This is a method in which about 30% of the total oxygen is blown to the bottom, which is said to be advantageous for decarburization. When a large amount of oxygen is blown from the bottom of the furnace, decarburization reaction is active in the high carbon region, and as shown in the following formula (2), the bottom blown oxygen is 2 in moles compared to the reaction in formula (1). Double CO is produced, and the molten steel is vigorously stirred. However, near the end point of blowing, the decarbonation efficiency gradually decreases, and the amount of CO gas generated by equation (2) decreases. As a result, the molten steel stirring force due to bottom blown oxygen decreases, and the reaction of formula (1) is delayed and [O] increases.

2 [C] + O ₂ = 2CO (2)
Therefore, in this method, when [C] decreases to about 0.15%, an inert gas such as Ar is mixed with the bottom blowing gas (oxygen), or the bottom blowing gas is switched to the inert gas, and [C The reaction of formula (1) is promoted by compensating for the decrease in stirring force after the decrease in However, this method requires a large amount of inert gas and increases cost. Although it is conceivable to use nitrogen instead of the inert gas for cost reduction, this causes another problem that nitrogen [N] in the molten steel rises.

The second method is a method disclosed in Patent Document 1. In the method, when the decarbonation efficiency starts to decrease at the [C] concentration of 0.3 to 0.5% during the blowing in the top bottom blowing converter, for example, instead of the stirring gas such as CO ₂ from the furnace bottom. Inject hydrocarbons. For example, when propane gas (C ₃ H ₈ ) is blown, stirring of the molten steel is promoted by a large amount of generated hydrogen and the CO partial pressure is reduced by the reaction represented by the following formula (3) ( The reaction of formula (1) proceeds and the [O] of the molten steel decreases.

C ₃ H ₈ = 3C + 4H ₂ (3)
The amount of hydrocarbons blown is the usual amount of stirring gas (CO ₂ ), that is, about 0.2 to 0.3 Nm ³ / min · t. For example, in the case of propane, when 1 mol of propane is used, Since H ₂ decomposes and forms, this corresponds to blowing a stirring gas of 0.8 to 1.2 Nm ³ / min · t.

  However, this method is a reaction in a state in which refining is performed by blowing up blown oxygen, and is a method of promoting decarburization while supplying oxygen, so [O] is not reduced so much. That is, although stirring is sufficiently performed, it is insufficient in terms of reducing the CO partial pressure. Oxygen oxidizes [C] in the molten steel as well as C generated by the decomposition of the blown hydrocarbon, so that CO is in a state where a large amount of soot is generated and the CO partial pressure does not drop so much. Therefore, there is a limit in reducing [O] at the target [C] concentration. However, since there is an effect of increasing the amount of stirring gas as described above, an increase in [O] (deviation from equilibrium) due to reaction delay can be avoided.

Tsutomu Nozaki "Bottom Blowing Converter", Japan Iron and Steel Institute, issued February 29, 2000, pp. 121-123 JP-A-5-78726

  The purpose of the present invention is to keep oxygen in the molten steel low in the converter before adding a deoxidizer that generates inclusions such as Al after the completion of blowing in the top-bottom blowing converter. It is an object of the present invention to provide a preliminary deoxidation method capable of

  The present inventors conducted research with the goal of reducing oxygen [O] in molten steel more efficiently than before by using a converter. As a result, it is unreasonable to reduce [O] using hydrocarbons while blowing up oxygen, and after stopping the blowing of oxygen, using hydrocarbons that are reducing agents, I found the deoxidation method advantageous.

When the supply of top-blown oxygen is stopped, oxygen is only [O] in the steel and FeO in the slag, and when there is no slag, it is only [O] in the steel. In addition, the component in slag is represented with () like (FeO). When hydrocarbons are blown to the bottom in this state, C produced by the decomposition reacts with [O] to produce CO gas, but on the other hand, the CO partial pressure is lowered by H _{2 also} produced by the decomposition of hydrocarbons. As a result, the reaction of the formula (1) further proceeds by this amount, thereby reducing [O]. The effect of reducing the CO partial pressure is the same as the effect of the decompression process.

  When hydrocarbons were blown to the bottom in a state where the blowing of oxygen was stopped, C generated by the decomposition of the hydrocarbons was picked up by the molten steel, and there was a concern that [C] would rise. However, as a result of the experiment, the CO partial pressure is reduced by hydrogen, so that the reaction of the formula (1), that is, the reaction between [O] of molten steel and [C] and C generated by decomposition of hydrocarbons is promoted. As a result, when [C] was about 0.03%, it was confirmed that deoxidation proceeded with little carburizing even when the deoxidation amount (Δ [O]) was 500 ppm. That is, in the case of such a high [O], C generated by the decomposition of the hydrocarbon is consumed by the deoxidation reaction.

  The gist of the present invention based on the above knowledge is the following (1) to (4) pre-deoxidation method for molten steel.

  (1) In the method of refining the dephosphorized hot metal in the top bottom blowing converter, the top blowing of oxygen is stopped, and the stirring gas blown from the bottom of the furnace is changed to a gas mainly composed of hydrocarbons and rinsed. A preliminary deoxidation method for molten steel (hereinafter referred to as “first method”).

  (2) In the method of refining the dephosphorized hot metal in the top bottom blowing converter, the stirring gas blown from the bottom of the furnace is mainly composed of hydrocarbons while the oxygen top blowing is continued at the end of blowing and after the oxygen top blowing is stopped. A pre-deoxidation method for molten steel (hereinafter referred to as “second method”) characterized by rinsing instead of the gas.

  The above “gas mainly composed of hydrocarbon” refers to a gas containing 50% by volume or more of hydrocarbon.

  (3) Preliminary removal of molten steel according to (1) or (2) above, wherein one kind of gas or mixed gas of two or more kinds of methane, ethane, propane, butane and LPG is used as the hydrocarbon. Acid method.

  (4) After stopping the top blowing of oxygen, add one or more selected from CaO, MgO, and dolomite, and bottom blowing a gas mainly composed of hydrocarbons with the slag solidified (1), (2) or (3) A method for preliminary deoxidation of molten steel.

  In carrying out the above-described method of the present invention, it is desirable to blow off a gas containing hydrocarbon as a main component after blowing without adding flux. Moreover, the flux which has MgO type as a main component can also be used as a flux.

  First, the 1st method, ie, the preliminary | backup deoxidation method of the molten steel of said (1), is demonstrated along a process.

(i) Using an upper bottom blowing converter, decarburize to the target [C] with top blowing oxygen and bottom blowing CO ₂ or the like. The main raw materials charged into the converter are dephosphorized hot metal and scrap added as necessary.

  (ii) Stop the blowing of top-blown oxygen, change the bottom-blown stirring gas to a gas mainly composed of hydrocarbons, and proceed with preliminary deoxidation.

  (iii) After preliminary deoxidation, a deoxidizer (such as Al) is added at the time of steel output.

  Hydrocarbons include methane, ethane, propane, butane, LPG, and the like. Mixtures of two or more of these can also be used. For example, the deoxidation reaction with propane is as shown in the following formula (4).

C ₃ H ₈ +3 [O] = 3CO + 4H ₂ (4)
In the case of propane, CO partial pressure is reduced to 3/7 due to H ₂ generated by the reaction of the above formula (4), so that it is more effective than deoxidation only by oxidation of C. As a result, at the same [C] concentration, [O] can be reduced to 3/7 by propane rinsing. For example, when the end point [C] is 0.03%, the end point [O] is decreased from 800 ppm to about 340 ppm. As the hydrocarbon, the smaller the ratio of C to H, that is, C / H, is more effective because the CO partial pressure can be greatly reduced.

The object of the method of the present invention is a process in which only decarburization is performed in a converter using dephosphorized hot metal. In the case of converter blowing of hot metal that has not been dephosphorized, dephosphorization is required, so it is necessary to blow with a flux to produce slag containing P ₂ O _5. This is because rinsing with hydrogen reduces P ₂ O _{5 and} causes recovery.

  Moreover, if molten slag exists, oxygen is supplied from (FeO), and [O] in the molten steel is not efficiently deoxidized until the slag is reduced. Therefore, for efficient preliminary deoxidation, there is no slag when oxygen is blown up, or as little slag as possible (for example, 5 kg of slag per ton of molten steel) as long as spitting and fume loss are not a problem. It is preferable to decarburize.

  If there is molten slag, it is important to add one or more of MgO, CaO or dolomite to solidify before blowing hydrocarbons. Solidification means a state in which slag loses reactivity, and means that (T.Fe) is reduced, that is, Δ (T.Fe) is 3% or less.

  As described above, since dephosphorization is used in the method of the present invention, dephosphorization in the converter is no longer necessary. Therefore, in order to prevent melting of the converter refractory, it is also effective to replace the conventional CaO slag with the same MgO slag as the furnace refractory.

  The slag produced by the method of the present invention can be reused at the time of converter refining or dephosphorization of the hot metal, or it can be charged in a blast furnace and recycled.

  Hydrocarbon is blown into the molten steel from the stirring gas blowing tuyere installed at the furnace bottom. The tuyere can be either a single tube, a double tube, or a triple tube.

  Table 1 exemplifies bottom blowing gas blowing during and after stopping oxygen blowing for each of the above three types of tuyere. However, what can be used is not limited to the gases shown in Table 1.

The ratio of the amount of hydrocarbons to the total amount of gas blown when rinsing with hydrocarbons is 50% or more by volume. 70% or more is desirable for effective deoxidation. However, if the molar amount of hydrocarbons to be injected is larger than the molar amount of oxygen supplied from CO ₂ or the like, deoxidation proceeds, and there is no damage or clogging of tuyere, 50% That's all.

  Next, the second method, that is, the preliminary deoxidation method for molten steel (2) will be described.

  In the first method, hydrocarbons are blown only at the time of rinsing, that is, after the oxygen top blowing is stopped. However, oxygen is blown up in the top-bottom blowing converter, and hydrocarbons are blown in from the furnace bottom tuyere while continuing to blow up oxygen at the end of blowing (state where [C] is less than about 0.5%). After stirring and stopping the top blowing of oxygen, a gas containing hydrocarbon as a main component can be blown at the bottom. The example shown in Table 1 in which hydrocarbons are used during continued blowing of oxygen is an example of this second method.

Although there is no restriction | limiting in particular in the amount of blowing in of a hydrocarbon, Considering that it is preferable to be able to use with the conventional tuyere for bottom blowing stirring gas blowing, about 0.03-0.6Nm < ³ > / min * t is suitable. The blowing time is determined by [O] of the molten steel, the target [O] after preliminary deoxidation, and oxygen supplied from the slag. For example, in the case where there is no slag, when the propane bottom blowing rate is 0.2 Nm ³ / min · t, about 1 minute is blown to reduce the deoxidation amount “Δ [O]” to about 500 ppm. is required.

[Conventional example 1]
250 t of “dephosphorating hot metal A” having a composition shown in Table 2 dephosphorized in a converter type dephosphorizing furnace was charged into an upper bottom blowing converter together with 20 t of scrap and refined without adding flux. Refining uses an upper blowing lance with a four-hole Laval nozzle, and while blowing oxygen at ³ Nm ³ / min · t of oxygen, CO ₂ is 0.2 Nm ³ / min · from 4 single pipe tuyere attached to the furnace bottom. When the amount of [C] reached 0.4%, the stirring gas blown from the bottom of the furnace was changed to propane 0.2 Nm ³ / min · t and blown for 15 minutes. Thereafter, when [C] reached 0.03%, the top blowing of oxygen was stopped, the stirring gas was switched to Ar, and steel was immediately output. At this time, Al was added. The molten steel temperature after steel was 1620 ° C. Table 3 shows changes in the contents of C and O in the molten steel.

[Example 1]
250 t of “dephosphorating hot metal B” having the composition shown in Table 2 dephosphorized in a converter type dephosphorizing furnace was charged into an upper bottom blowing converter together with 20 t of scrap and refined without adding flux. In refining, an upper blowing lance with a four-hole Laval nozzle is used, and while blowing oxygen at ³ Nm ³ / min · t of oxygen, CO ₂ is 0.2 Nm ³ / min · from four single tube tuyere attached to the furnace bottom. tBlowed and blown for 14.5 minutes. When [C] reached 0.03%, the top blowing of oxygen was stopped, and the bottom stirring gas was changed to propane 0.2 Nm ³ / min · t and rinsed for 90 seconds. Thereafter, the stirring gas was switched to Ar, and steel was produced immediately. At this time, Al was added. The molten steel temperature after steel was 1620 ° C. Table 3 shows the transition of the contents of C and O in the molten steel in comparison with the conventional example 1.

  As is apparent from Table 3, according to the method of the present invention, preliminary deoxidation can be performed to a low oxygen level that could not be achieved by the conventional method. And since a small amount of Al may be added at the time of steel production, the basic unit is low, and the T.O. [O] (total oxygen) is also low. That is, there are few oxide inclusions, and the cleanliness of steel is high.

[Conventional example 2]
250 t of “Dephosphorating hot metal C” having the composition shown in Table 4 dephosphorized in a converter type dephosphorizing furnace was charged into a top bottom blowing converter together with 19 t of scrap, and 5 kg / t of MgO was added to form 4 holes. Using a top blow lance with a Laval nozzle, oxygen was blown at a rate of ³ Nm ³ / min · t, and CO ₂ was 0.25 Nm ³ / min · t (provided from four double pipe tuyere attached to the furnace bottom). Blowing was performed by blowing 0.05 Nm ³ / min · t from the outer pipe and 0.2 Nm ³ / min · t from the inner pipe.

When [C] reached 0.3%, the oxygen top blowing continued, the gas blown from the furnace bottom tuyere inner tube was propane 0.2 Nm ³ / min · t, and the gas blown from the tuyere outer tube was Ar 0.05 Nm ³ Switched to / min · t. Thereafter, when [C] reached 0.03%, the top blowing of oxygen was stopped, the inner tube blowing gas was switched to Ar, and steel was immediately produced. At this time, Al was added. The molten steel temperature after steel was 1610 ° C. The results are shown in Table 5.

[Example 2]
250t of "Dephosphorized hot metal D" having the composition shown in Table 4 dephosphorized in a converter type dephosphorization furnace is charged into a top bottom blown converter together with 19t of scrap, and MgO is added at 5kg / t. Using a top blowing lance with a Laval nozzle, oxygen is blown with ³ Nm ³ / min · t of oxygen, and CO ₂ is 0.25 Nm ³ / min · t (provided from four double pipe tuyere attached to the furnace bottom). 0.05 Nm ³ / min · t from the outer pipe and 0.2 Nm ³ / min · t from the inner pipe) and blown for 14.5 minutes.

At the same time as the top blowing of oxygen was stopped, the gas was blown in from the furnace bottom tuyeres, but the gas was blown in for 90 seconds while switching the propane gas to 0.2 Nm ³ / min · t and Ar 0.05 Nm ³ / min · t. After that, the inner pipe gas was switched to Ar and the steel was produced immediately. At this time, Al was added. The molten steel temperature after steel was 1610 ° C. The results are shown in Table 5 in comparison with the results of the conventional method 2 described above.

  As is apparent from Table 5, in the case of the method of the present invention, since the oxygen in the molten steel after rinsing is lower than in the conventional method, the Al basic unit added at the time of steel output can be reduced. Therefore, the T.O. [O] is low and cleanliness is high.

[Conventional example 3]
250 t of “dephosphorating hot metal E” having the composition shown in Table 6 dephosphorized in a converter type dephosphorizing furnace was charged into an upper bottom blown converter together with 18 t of scrap, and 8 kg / t of CaO was added thereto. Using a top blowing lance with a Laval nozzle, oxygen was blown at ³ Nm ³ / min · t of oxygen, and CO ₂ was 0.25 Nm ³ / min · t (provided from four double pipe tuyere attached to the furnace bottom) Blowing was started by blowing 0.05 Nm ³ / min · t from the outer pipe and 0.2 Nm ³ / min · t from the inner pipe.

When [C] reached 0.4%, the inner pipe gas was changed to propane 0.2 Nm ³ / min · t, and the top blowing oxygen was lowered to 1.5 Nm ³ / min · t. Thereafter, [C] was blown down to 0.025%, and then the top blowing oxygen was stopped and steel was immediately produced. At this time, Al was added. The molten steel temperature after steel was 1610 ° C. The results are shown in Table 7.

[Example 3]
250t of "Dephosphorized hot metal F" having the composition shown in Table 6 dephosphorized in a converter type dephosphorization furnace is charged into a top bottom blowing converter together with 18t of scrap, and 8kg / t of CaO is added to 4 holes. Using a top blowing lance with a Laval nozzle, oxygen is blown with ³ Nm ³ / min · t of oxygen, and CO ₂ is 0.25 Nm ³ / min · t (provided from four double pipe tuyere attached to the furnace bottom). Blowing was started by blowing 0.05 Nm ³ / min · t from the outer pipe and 0.2 Nm ³ / min · t from the inner pipe.

When [C] reached 0.4%, the CO ₂ of the inner pipe gas was changed to propane 0.2 Nm ³ / min · t, and the top blown oxygen was lowered to 1.5 Nm ³ / min · t. Then, after blowing down [C] to 0.03%, the top-blown oxygen was stopped, the outer tube gas was changed to Ar 0.05 Nm ³ / min · t, the inner tube gas remained as propane, and CaO was 3 kg / After adding t and solidifying the slag, rinsing was performed for 80 seconds. Thereafter, the inner pipe gas was also changed to Ar, and steel was immediately produced. At this time, Al was added. The molten steel temperature after steel was 1610 ° C. The results are shown in Table 7 in comparison with the above conventional example 3.

  As shown in Table 7, in the case of the method of the present invention, since the oxygen in the molten steel after rinsing is lower than in the conventional method, a small amount of Al may be added at the time of steel output. Therefore, the Al basic unit is low, the T. [O] of the molten steel is low, and the cleanliness of the steel is high.

In the method of the present invention, an existing top-bottom blowing converter can be used, and piping for bottom-blown stirring gas can be used as it is. Therefore, no special capital investment is required. In addition, sufficient preliminary deoxidation is possible by simply injecting hydrocarbons, the basic unit of strong deoxidizers such as Al and Fe-Si can be reduced, and the cost reduction effect of steelmaking can be obtained. Steel obtained by the method of the present invention has high cleanliness and excellent mechanical properties.

Claims (6)

  In the method of refining the dephosphorized hot metal in the top bottom blowing converter, after stopping the top blowing of oxygen, the molten steel is characterized in that the stirring gas blown from the bottom of the furnace is changed to a gas mainly composed of hydrocarbons and rinsed. Pre-deoxidation method.   In the method of refining the dephosphorized hot metal in the top bottom blowing converter, the stirring gas blown from the bottom of the furnace is the main component of the hydrocarbon gas during the continuous blowing of oxygen at the end of blowing and after the top blowing of oxygen is stopped. A preliminary deoxidation method for molten steel, characterized by rinsing instead of   The method for pre-deoxidizing molten steel according to claim 1 or 2, wherein one kind of gas or a mixed gas of two or more kinds of methane, ethane, propane, butane and LPG is used as the hydrocarbon. .   After stopping the top blowing of oxygen, one or more selected from CaO, MgO and dolomite are added, and the bottom gas is blown with hydrocarbon-based gas in the state of solidified slag. The preliminary deoxidation method for molten steel according to claim 1, claim 2, or claim 3.   The preliminary deoxidation of molten steel according to any one of claims 1 to 4, wherein the gas is mainly blown with hydrocarbons after blowing without adding flux in an upper bottom blowing converter. Method. 5. The gas according to any one of claims 1 to 4, wherein a gas mainly composed of hydrocarbons is bottom-blown after blowing using a flux mainly composed of MgO in an upper-bottom blowing converter. A preliminary deoxidation method for molten steel.