JP6311466B2 - Method of dephosphorizing molten steel using vacuum degassing equipment - Google Patents

Description

  The present invention relates to a molten steel dephosphorization method using a vacuum degassing facility, and uses molten steel recirculation type vacuum degassing facility (RH) for refining molten steel that has been refined in a converter, electric furnace, or the like and delivered to a ladle. The present invention relates to a dephosphorization method.

  In the steelmaking method, in general, main raw materials such as hot metal and scrap and auxiliary raw materials such as quick lime are charged into a converter or electric furnace, and oxygen is blown, and oxidative refining such as decarburization and dephosphorization is performed to obtain molten steel and After that, the steel is taken out into a ladle, and deoxidation and component adjustment are performed by adding Al or alloyed iron to the molten steel at the time of steeling. Thereafter, in many cases, secondary refining such as degassing treatment, component adjustment, inclusion removal, etc. is performed using secondary refining equipment such as vacuum degassing equipment (RH).

  However, in recent years, the use of steel products has expanded, and the demand for higher quality has increased. On the other hand, in recent years, the concentration of P in hot metal has risen due to the use of low-grade iron ore. The process of refining to form molten steel has been widely performed. However, there are still cases where the P concentration in the molten steel after oxidative refining has not been reduced to a level sufficient to bring the P concentration of the final product below the specified concentration. Such a situation lies in the background of the demand for higher quality of steel products, so that oxidation refining using an electric furnace also exists as in the case of using a converter.

  Regardless of whether using a converter or an electric furnace, if the P concentration in the molten steel after oxidative refining has not been reduced to a sufficient level, dephosphorization is often performed again before steelmaking. In some cases, it is known that the P concentration in the molten steel exceeds the standard concentration of the final product after the steel is taken out to the ladle. In that case, it is necessary to take measures to disturb the normal operation such as switching the destination of the molten steel to a product that can meet the P concentration standard, or charging the molten steel again into the converter and performing the dephosphorization treatment again. Manufacturing costs worsen.

  Instead, if additional dephosphorization treatment can be efficiently performed using secondary refining equipment such as vacuum degassing equipment (RH) in the next process of converters and electric furnaces, the above-mentioned normal operation may be disturbed. No longer needed. Furthermore, the amount of auxiliary raw materials such as quick lime used excessively in converters and electric furnaces can be reduced so as not to require such measures.

For example, Patent Documents 1 and 2 disclose inventions in which dephosphorization is performed by blowing a dephosphorizing agent into undeoxidized molten steel (free oxygen> 100 ppm) in a vacuum degassing tank. However, as the dephosphorizing agent, an example using 4 kg / t of “65% by mass CaO · 35% by mass CaF ₂ ” and an example of “80% by mass CaO · 20% by mass CaF ₂ ” for 6 kg / t are used. Examples of the above and examples using 3 kg / t of “65 mass% CaO · 35 mass% CaF ₂ ” are merely disclosed as respective examples.

  In the invention disclosed in Patent Document 1, the disadvantage of the invention disclosed in Patent Document 2 is “a situation where the dephosphorization reaction and the decarburization reaction compete with each other for free oxygen in the molten steel and the dephosphorization reaction rate decreases”. As a means to avoid this, since the degree of vacuum in the tank is controlled according to the C concentration level in the molten steel, in any of these inventions, the technical idea of simultaneously supplying oxygen to promote the dephosphorization reaction is It is understood that it did not exist. In any of the inventions, the dephosphorizing agent is blown through the powder blowing tuyere provided in the lower part of the vacuum degassing tank. Therefore, special care is required to prevent wear of the blowing tuyere.

On the other hand, Patent Document 3 discloses a powder containing CaO as a lime-based flux as a dephosphorizing agent, particularly a powder of 70% by mass CaO and 30% by mass CaF ₂ on a steel bath in a vacuum degassing tank. A method for producing ultra-low carbon steel by blowing oxygen for phosphorus with oxygen gas or a powder containing solid oxide, particularly Fe ₂ O ₃ powder, through an upper blowing lance is disclosed. However, other than the description that 1 to 4 kg / t of the above lime-based flux is added for the dephosphorization, there is little description regarding the flux addition conditions.

  Patent Document 4 discloses a method of dephosphorizing molten steel at the time of steel removal from a converter to a ladle, and the conditions for adding a low melting point sintered ore having a composition of calcium ferrite as a dephosphorizing agent are disclosed. Has been.

JP-A-2-122013 JP 62-205221 A JP-T-2002-544376 JP 61-217519 A

The inventions disclosed in Patent Documents 1 to 3 all use general CaO as a dephosphorizing agent, but the melting point of CaO having a high melting point and a poor melting property (hatchability) is reduced. In order to secure it in the next refining equipment, fluorite (CaF ₂ ) is introduced at the same time. However, elution of fluorine from the slag produced by this becomes a problem from the viewpoint of protecting the global environment such as soil environmental standards, and when the slag contains fluorine (F) above the soil environmental standards, the slag is used for roadbed materials, etc. Can not be recycled. For this reason, the steelmaking method which does not use a fluorite is calculated | required.

  The invention disclosed in Patent Document 4 is a technique that uses stirring during steelmaking and further adds bottom blowing bubbling in a ladle to react the molten steel with a dephosphorizing agent on the molten steel, and vacuum degassing. The basic reaction treatment form is different from the method of auxiliary dephosphorization after the molten steel arrives at the apparatus.

The present invention has been made in view of such problems of the prior art, and does not add CaF ₂ and does not require new molten steel bubbling treatment or powder injection treatment. An object of the present invention is to provide a method for efficiently and simply performing an additional dephosphorization process using a vacuum degassing facility (RH) in the next step of the furnace.

  As a result of intensive studies in order to solve the above problems, the present inventors have obtained knowledge A to E listed below and completed the present invention.

(A) When dephosphorization reaction of molten steel is written in ionic formula,
[P] + (5/2) [O] + (3/2) O ²⁻ = PO ₄ ³⁻
Thus, the dephosphorization reaction requires a high slag basicity (ie, the presence of CaO) and an increase in oxygen potential (T.Fe) in the slag. Conventionally, dephosphorization has been performed by increasing the slag basicity by using fluorite (CaF ₂ ). It is difficult to increase the slag basicity without using fluorite.

  Therefore, the present inventors examined maintaining the oxygen potential in the slag at a high level. In order to keep the oxygen potential in the slag high, it is necessary to keep the oxygen activity in the molten steel high. Therefore, once deoxidation is performed, the dephosphorization reaction does not proceed. Therefore, in order to perform dephosphorization in the secondary refining, it is assumed that undeoxidized steel is performed. If the acid is blown into a low carbon region (C = 0.02 to 0.08 mass%) in a converter or electric furnace, the oxygen activity in the molten steel increases to about 300 to 500 ppm.

  If the present inventors put CaO in this state, slag T.P. Fe was also expected to rise, slag was melted due to the formation of CaO-FeO, and dephosphorization reaction proceeded. However, when an input test was actually performed, dephosphorization reaction did not proceed well. Furthermore, oxygen was blown from above with RH, and the oxygen activity in the molten steel was further increased to about 500 to 900 ppm, with the same result.

  This is because RH is a vacuum reaction vessel and is not a machine suitable for stirring between slag and molten steel, so the oxygen in the molten steel does not sufficiently transfer to slag, and slag melting due to the formation of CaO-FeO does not proceed. It is thought that it was because of.

(B) Therefore, as a dephosphorization flux, it is thought that it is necessary to use an oxygen source in advance and a melting point is low, and such a dephosphorization flux is calcium ferrite (CaO: 25 to 50% by mass, Fe ₂ O ₃ : 30 to 50% by mass). As a result of the test, it was found that if the proportion of calcium ferrite in CaO conversion is 35% or more in the total dephosphorization flux, the dephosphorization rate becomes stable and high without using fluorite.

  It is thought that calcium ferrite has a low melting point and can be melted quickly after being added to produce high oxygen and high basicity molten slag. For example, when a mixture of quicklime and scale (FeO) is used as the dephosphorization flux, not only the melting point of quicklime is high, but also the contact frequency with FeO is affected. .

  (C) When the oxygen activity (free oxygen concentration) in the molten steel at the time of introducing the dephosphorization flux is less than 400 ppm, the dephosphorization rate decreases. This is probably because if the oxygen activity in the molten steel is too low, the oxygen activity in the slag cannot be kept high.

(D) Furthermore, by using fluorite (CaF ₂ ) in combination with calcium ferrite, the dephosphorization rate can be maintained at a higher level. This method may be selected as long as the elution of fluorine from the slag does not cause a problem.

  (E) It is desirable to secure a reflux time of 2 minutes or more from the introduction of the dephosphorization flux to the introduction of the deoxidizer. This is to prevent dephosphorization since the slag in the tank is deoxidized before the RH slag is discharged onto the ladle. In other words, using the number of reflux times, it is desirable that the number of reflux times = {refluxing treatment time T (min)) / (total mass M (t) of the molten steel / Q amount of molten steel Q (t / min)) ≧ 2. .

  The present invention is listed below.

(1) A method of dephosphorizing a molten steel having a free oxygen concentration of 400 ppm or more discharged from a converter or an electric furnace to a ladle using a vacuum degassing facility (RH),
As a dephosphorizing agent, it consists of CaO and calcium ferrite , and 35% or more and 76% or less of the total CaO mass contained in the dephosphorizing agent is calcium ferrite (CaO: 25-50 mass%, Fe ₂ O ₃ : 30-30 50 mass%), and a method of spraying the molten steel contained in the vacuum degassing tank from the upper blowing lance toward the molten steel bath surface, and introducing the alloy iron into the molten steel in the vacuum degassing tank Addition during reflux of the molten steel by either or both of the methods of adding from the charging hole,
After the addition of the dephosphorizing agent, the molten steel is refluxed to cause a dephosphorization reaction, and then deoxidation is performed by adding Al to the molten steel to make the Al content concentration 0.003% by mass or more, for example. A dephosphorization processing method using a vacuum degassing facility (RH).

(2) As the dephosphorizing agent, a dephosphorizing agent containing 20 to 50% of CaF ₂ as an external number with respect to the total CaO mass contained in the dephosphorizing agent is used as the item (1). The dephosphorization processing method of description.

  (3) The dephosphorization method according to (1) or (2), wherein the molten steel has a reflux time of 2 minutes or more.

According to the present invention, CaF ₂ is not added, no major equipment modification such as introduction of powder injection equipment is required, and no new process such as molten steel bubbling treatment is required, thereby reducing processing time and heat. Using the vacuum degassing equipment (RH) in the next process of the converter and electric furnace while suppressing energy loss, the additional dephosphorization process can be performed efficiently and simply. Thus, the dephosphorization of molten steel can be performed more stably than in the prior art.

FIG. 1 is a graph showing the relationship between the CaO basic unit and the P removal rate. FIG. 2 is a graph (no fluorite) showing the relationship between the calcium ferrite CaO ratio and the CaO dephosphorization efficiency. FIG. 3 is a graph showing the relationship between the calcium ferrite CaO ratio and the graph (with fluorite) showing the relationship between the CaO dephosphorization efficiency.

  The present invention will be described with reference to the accompanying drawings. In the following description, “%” relating to concentration means “% by mass” unless otherwise specified.

1. Process Object In the present invention, basically, molten steel with a free oxygen concentration of 400 ppm or more discharged from a converter or electric furnace to a ladle is dephosphorized using a vacuum degassing facility (RH). By decarburizing to [C] ≦ 0.06% using a converter or an electric furnace, free oxygen in the molten steel becomes 400 ppm or more.

  In such a condition where the free oxygen concentration is high, the iron oxide concentration (T.Fe%) in the slag produced in the converter or electric furnace also becomes high, which is advantageous for promoting the dephosphorization reaction.

  However, since the present invention dephosphorizes the molten steel after arrival at the RH, the present invention is not substantially applicable to a steel type that is deoxidized by introducing Al when steel is discharged from a converter or an electric furnace to a ladle. As a steel type in which Al is not charged at the time of steel removal from a converter or an electric furnace to a ladle, there are steel types that are decarburized with RH, that is, low-carbon steels including ultra-low carbon steels.

  In the present invention, it is assumed that it is applied to such a steel type that is decarburized with RH, and it is free in molten steel in consideration of suppression of slag forming during steel production and addition of a small amount of alloy iron such as Mn. It is assumed that the oxygen concentration is 400 ppm or more.

2. Dephosphorizing agent Conventionally, a dephosphorizing agent described in Patent Documents 1 and 2, that is, a mixture containing quick lime as a main CaO source and ₂₀ to 35% of CaF ₂ was used. However, in order to spray from the top blowing lance, the maximum particle size is a powder having a maximum of 0.15 mm or less.

In contrast, in the present invention, 35% or more of the total CaO mass contained in the dephosphorizing agent is calcium ferrite (CaO: 25 to 50%, Fe ₂ O ₃ : 30 to 50%) as the dephosphorizing agent. Use what is included. Calcium ferrite has a low melting point, dissolves quickly after charging, and can produce molten slag with high oxygen and basicity, so that the dephosphorization rate is stably high without using fluorite.

By using this dephosphorizing agent, in addition to using free oxygen in the molten steel and oxygen in the slag on the molten steel in the ladle, the dephosphorizing agent supplied to the dephosphorization reaction site in the molten steel is dephosphorized. Since it has a CaO source (CaO) and an oxygen source (Fe ₂ O ₃ ) necessary for promoting the treatment, the dephosphorization reaction with RH can be promoted.

Furthermore, as the dephosphorizing agent, a dephosphorizing agent containing 20 to 50% of CaF ₂ as an external number with respect to the total CaO mass contained in the dephosphorizing agent may be used. If the elution of fluorine does not cause a problem, this method may be selected to maintain a high dephosphorization rate.

3. Method of adding a dephosphorizing agent In the inventions described in Patent Documents 1 and 2, the dephosphorizing agent was blown in using tuyere. However, the tuyere requires special care to prevent wear caused by blowing the dephosphorizing agent, and to prevent clogging while the dephosphorizing agent is not blown, and the maintenance cost increases.

  On the other hand, in the present invention, since the above-described dephosphorizing agent is used, the dephosphorization reaction can be promoted without blowing from the tuyere. In the present invention, in recent years, a large number of RHs are equipped with spraying from an upper blowing lance inserted into a vacuum degassing tank toward a bath surface of molten steel, and alloy iron on the molten steel in the vacuum degassing tank. The above-mentioned dephosphorizing agent is added during the reflux of the molten steel by either or both of the methods of adding from the charging hole.

4). Reflux treatment In RH, after the ladle containing the molten steel to be treated arrives, the dip tube at the bottom of the vacuum chamber is immersed in the molten steel, and the vacuum chamber is decompressed to start the molten steel reflux treatment.

  The present invention uses free oxygen in molten steel and oxygen in slag on molten steel in a ladle, and further contains a CaO source and an oxygen source necessary for promoting dephosphorization treatment in the dephosphorizing agent. The dephosphorization reaction is promoted by supplying the dephosphorization reaction site in the molten steel.

  Therefore, in this invention, implementation of this invention is started prior to the process which reduces the free oxygen concentration in molten steel, such as adding Al or adding alloy iron.

  In the present invention, after the above-described dephosphorizing agent is added to the molten steel, the molten steel is refluxed to cause a dephosphorization reaction. The reflux time of the molten steel is 2 minutes or more, in order to prevent dephosphorization from occurring because the slag in the tank is deoxidized before the RH slag is discharged onto the ladle. is there.

  After the dephosphorization reaction is performed with RH in this manner, deoxidation is performed, for example, by adding Al to the molten steel so that the Al content concentration is 0.003% or more.

In the present invention, since no CaF ₂ is added in this way, no major equipment modification such as introduction of powder injection equipment is required, and no additional process such as molten steel bubbling is required. An additional dephosphorization process for molten steel can be performed efficiently and simply using a vacuum degassing facility (RH) while shortening the time and suppressing thermal energy loss.

  For example, after oxidative refining with the goal of C = 0.05% and P ≦ 0.013% in a converter, 260 tons of molten steel is discharged into a ladle without adding Al or Si in the discharged steel. The contained components were analyzed and confirmed.

  As a dephosphorizing agent, a powder containing 35% or more of the total CaO mass contained in the dephosphorizing agent as calcium ferrite is sprayed from the upper blowing lance inserted in the RH tank toward the bath surface of the molten steel. In addition, it was added to the molten steel by any one of the addition methods of adding the alloy iron to the molten steel in the RH tank from the charging hole. In addition, the oxygen activity before treatment was adjusted by sending acid from the top blowing lance so that it became 400 ppm or more.

Thereafter, the RH dip tube was immersed in the molten steel, and refluxing Ar gas was allowed to flow at 1.6 to 2.5 Nm ³ / min to immediately start the molten steel reflux. This reflux gas flow rate corresponds to 130 to 149 t / min as the molten steel reflux amount Q. The molten steel reflux amount Q was calculated using a known formula (1) shown in “Tatsuro Kuwahara et al .: Iron and Steel, 73 (1987), S176”.
Q = 11.4 × D4 / 3 × G1 / 3 × (ln (P1 / P0)) 1/3 (1)
In the formula (1),
Q: Molten steel reflux [t / min]
D: Inner diameter of dip tube [m]
G: Flow rate of reflux gas [Nl / min]
P0: Pressure in vacuum degassing equipment [torr]
P1: Gas blowing point pressure [torr]
It is. Here, D = 0.65 m, P0 = 5 torr, and P1 = 760 torr were used.

  In this target steel type, the free oxygen concentration is basically rarely less than 400 ppm, but in the unlikely event that it is less than 400 ppm, the oxygen source should be What is necessary is just to add operation which raises free oxygen concentration by adding to molten steel.

  As an example, after 3 minutes from the start of reflux, a powder having a composition of CaO: 0 to 100%, calcium ferrite: 0 to 100%, CaO equivalent concentration: 30 to 100% as a dephosphorizing agent is placed in the RH tank. It sprayed on the molten steel surface in a tank for 4 to 10 minutes from the top blowing lance.

  After the spraying of the dephosphorizing agent is completed, the molten steel reflux treatment is continued for 3 minutes so as to satisfy the molten steel reflux time of 2 minutes or more. In addition to adjusting the Al concentration to 0.020 to 0.050%, alloy components necessary for product specifications were added.

  The removal P results in this example are summarized in Table 1 and graphically shown in FIGS.

  FIG. 1 is a graph showing the relationship between the CaO basic unit and the P removal rate, FIG. 2 is a graph showing the relationship between the calcium ferrite CaO ratio and the CaO dephosphorization efficiency (without fluorite), and FIG. It is a graph which shows the relationship between the graph (with fluorite) which shows the relationship between a calcium ferrite CaO ratio and CaO dephosphorization efficiency.

  Here, the P removal rate in FIGS. 1 to 3 = ([P] i− [P] f) / ([P] i) × 100, and the converted CaO input amount = (the amount of dephosphorization agent added) × ( Respective CaO concentrations), calcium ferrite CaO ratio = (calcium ferrite input amount × calcium ferrite CaO concentration) / (total converted CaO), and CaO de-P efficiency = (de-P rate) / (converted CaO input amount) It is. [P] i: [P] before processing, [P] f: [P] after processing. Moreover, in FIG.1, 3, it is CaO: CaF = 1: 0.2-0.5.

Comparative Examples 1 and 2 in Table 1 are examples in which powders of CaO and CaF ₂ are used as a dephosphorization agent, and both CaO dephosphorization efficiency and slag recyclability are unsatisfactory.

  Comparative Examples 3 and 4 are examples using CaO and calcium ferrite powders as a dephosphorizing agent, and although slag recyclability is excellent, 14.7% of the total CaO mass contained in the dephosphorizing agent is calcium. Since it is contained as ferrite, CaO poor melting and oxygen deficiency in slag occurred, and CaO dephosphorization efficiency was unsatisfactory.

  Comparative Examples 5 and 6 are examples using CaO and calcium ferrite powder as a dephosphorization agent, and although the slag recyclability is excellent, the free oxygen concentration of the molten steel produced is 300,350 ppm. The dephosphorization efficiency was unsatisfactory.

  On the other hand, all of Invention Examples 1 to 8 have good P removal efficiency. In particular, Invention Examples 5 and 6 have high P removal efficiency, but since fluorite is used, these conditions may be selected if the frequency and environment are satisfactory for recycling.

From the results of Invention Examples 1 to 8,
(I) The dephosphorizing agent in the present invention may be introduced by spraying from an upper blowing lance inserted into the RH tank toward the bath surface of the molten steel, or alloy iron is introduced into the molten steel in the RH tank. That it may be charged by adding from the charging hole,
(II) What is necessary is just to use what contains 35% or more as calcium ferrite among the total CaO mass contained in a dephosphorizing agent as a dephosphorizing agent,
(III) It is understood that the free oxygen concentration of the molten steel to be subjected to the dephosphorization treatment with RH may be 400 ppm or more.

  Furthermore, from the graph shown in FIG. 1, if calcium ferrite is used, a dephosphorization rate according to the amount of CaO input can be stably obtained without using fluorite, and fluorite and calcium ferrite can be obtained. It can be seen that a higher P removal rate can be obtained when used in combination. Based on this relationship, by determining the amount of calcium ferrite or fluorite input according to the value of [P] before treatment, it is possible to lower the target [P].

  2 and 3 that the necessary amount of calcium ferrite for obtaining a stable dephosphorization rate is 35% or more in terms of CaO.

Claims (3)

A method of dephosphorizing a molten steel having a free oxygen concentration of 400 ppm or more discharged from a converter or an electric furnace to a ladle using a vacuum degassing equipment,
As a dephosphorizing agent, a material composed of CaO and calcium ferrite and containing 35% or more and 76% or less as calcium ferrite out of the total CaO mass contained in the dephosphorizing agent was inserted into a vacuum degassing tank. According to any one or both of a method of spraying from the top blowing lance toward the bath surface of the molten steel and a method of adding from the introduction hole for introducing the alloy iron to the molten steel in the vacuum degassing tank, the molten steel A dephosphorization method using a vacuum degassing facility, wherein the dephosphorization is performed after the molten steel is refluxed and dephosphorized after the addition of the dephosphorizing agent is completed. _{2. The} dephosphorization agent according to claim 1, wherein a dephosphorization agent containing 20 to 50% of CaF ₂ as an external number with respect to the total CaO mass contained in the dephosphorization agent is used as the dephosphorization agent. Processing method.   The dephosphorization method according to claim 1 or 2, wherein the molten steel has a reflux time of 2 minutes or more.

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