JP5233383B2 - Method for refining molten steel - Google Patents

Description

  The present invention relates to a method for promoting dephosphorization reaction by appropriately forming slag when decarburizing and decarburizing dephosphorization in a converter.

  When decarburizing and dephosphorizing carbon-containing iron in a converter, quick lime is usually used. However, since this quick lime has a melting point of 2000 ° C. or higher, it is not easy to melt and hatch quick lime alone in decarburization / dephosphorization blowing in a converter with a maximum temperature of about 1700 ° C.

  So, conventionally, halides such as fluorite were added to promote the melting and hatching of quicklime, but recently, legal regulations related to fluorine have become stricter due to environmental problems from the viewpoint of making slag into a useful material. In addition, the amount of fluorine elution and concentration are regulated in steelmaking slag products. For this reason, it is necessary to reduce the fluorine concentration in the slag to the limit, and development of a dephosphorization technique that does not use fluorite is strongly desired.

Under such a background, for example, Patent Document 1 discloses a method of using Al ₂ O ₃ as an agent for promoting the melting and hatching of quicklime in hot metal pretreatment.
JP 11-21608 A

However, when the present inventor investigated the dephosphorization behavior and the refractory erosion behavior when an Al ₂ O ₃ source was added when decarburizing and decarburizing dephosphorization in a converter, the dephosphorization reaction was promoted. When the Al ₂ O ₃ content (mass%, hereinafter also referred to as “(% Al ₂ O ₃ )”) in the added slag is more than 3.5 mass%, the refractory erosion loss amount increases rapidly. It became clear that it would. On the other hand, when the content of slag (% Al ₂ O ₃ ) is 3.5% by mass or less, the effect of promoting dephosphorization reaction due to the addition of Al ₂ O ₃ is relatively small. Thus, as a result of the investigation by the present inventor, means for further improving the productivity (excluding the factor for shortening the furnace life) and improving the quality (reducing the phosphorus concentration of the steel) is required. It became clear that there was.

The present invention is intended to provide this means. Specifically, when dephosphorizing a dephosphorization slag without using fluorite in a converter, it is contained in slag (% Al). _{2 O 3)} and has an object to provide a refining method of molten steel which are capable of promoting the dephosphorizing reaction be not more than 3.5 mass%.

The present invention provided to solve the above problems is as follows.
(1) When molten iron dephosphorized in a refining vessel is charged into an upper bottom blowing converter, which is another refining vessel, and decarburized and blown, a premelt containing CaO, SiO ₂ and Al ₂ O _{3 is} contained. Flux and TiO ₂ source were added, and the composition of the slag after treatment was (% Al ₂ O ₃ ) = 1.0 to 3.5% by mass, (% TiO ₂ ) = 0.2 to 1.0% by mass. The method for refining molten steel, wherein the basicity according to the following formula (1) is 3.0 to 4.5:
Basicity = (CaO) / (SiO ₂ ) (1)
However, (CaO): Total CaO content in slag (mass%)
(SiO ₂ ): SiO ₂ content (% by mass) in the slag.

(2) A ladle refining method according to (1), wherein ladle slag is used as the Al ₂ O ₃ -containing premelt flux.

(3) When the hot metal dephosphorized in a refining vessel is charged into a top bottom blowing converter, which is another refining vessel, and decarburized and blown, a CaO source, a SiO ₂ source, Al ₂ O ₃ and TiO _The pre-melt flux containing ₂ was added, and the composition of the slag after the treatment was (% Al ₂ O ₃ ) = 1.0 to 3.5% by mass, (% TiO ₂ ) = 0.2 to 1.0. wt%, refining method of soluble steel you characterized that you basicity by the following formula (1) and 3.0 to 4.5.
Basicity = (CaO) / (SiO ₂ ) (1)
However, (CaO): Total CaO content in slag (mass%)
(SiO ₂ ): SiO ₂ content in slag (mass%)

  According to the present invention, in a method of decarburizing and blowing dephosphorized rice in an upper bottom blowing converter, the hatching of flux such as quick lime is promoted while suppressing refractory melting, and after processing [P] Can be reduced.

Hereinafter, the best mode of a method for refining molten steel according to the present invention will be described with reference to the drawings.
In the method for refining molten steel according to the present invention, when dephosphorization is decarburized and blown in a converter (typically a top-bottom blown converter), an Al ₂ O ₃ -containing premelt flux is added, and Although melting and hatching are promoted, the slag after decarburization blowing treatment (% Al ₂ O ₃ ) is made 3.5% by mass or less so that the refractory erosion amount does not increase rapidly. Further, a TiO ₂ source such as Ti ore is added to control the content of TiO _{2 in} the slag after decarburization blowing (mass%, hereinafter also referred to as “(% TiO ₂ )”), The basicity of the slag represented by the following formula (1) is controlled.
Basicity of slag = (CaO) / (SiO ₂ ) (1)
Here, (CaO) is the total CaO content in the slag after the decarburization blowing treatment (mass%), (SiO ₂₎ is SiO ₂ content in the slag after the decarburization blowing process (wt% ).

1. Al ₂ O ₃
The pre-melt flux is one in which the components constituting the flux have been previously melted and hatched. As the pre-melt flux containing Al ₂ O _3, CaO and Al ₂ O ₃ are mixed in appropriate amounts to form calcium aluminate. In addition to synthetic flux, ladle slag and the like are exemplified. Such an Al ₂ O ₃ -containing pre-melt flux has a melting point as low as about 1500 ° C., and therefore hatches from the early stage of decarburization blowing. For this reason, it becomes possible to promote the hatching of quicklime when it comes into contact with quicklime.

Here, as the composition of the ladle slag, the total CaO content is 30 to 60% by mass, the Al ₂ O ₃ content is 10 to 40% by mass, the total iron content is 15% by mass or less, and the SiO ₂ content Is preferably 15% by mass or less. Such a composition has a low melting point and is useful as a CaO source as well as an Al ₂ O ₃ source.

Incidentally, even with the same _Al 2 _{O 3} source, waste of _Al 2 _{O 3 Al} 2 _{O 3} refractory as _Al 2 _{O 3} source other than containing pre-melt flux _(Al 2 _{O 3} content: 95 wt%) of When used, the dephosphorization reaction was not promoted even when the flux was blended to achieve the above slag composition. This means that if a high melting point quick lime, a high melting point Al ₂ O ₃ source and a TiO ₂ source are simply added to the furnace, the flux hardly hatches until the second half of the decarburization blowing, so that the dephosphorization reaction is not caused. Indicates that it is not promoted.

The upper limit of the slag after decarburization blowing (% Al ₂ O ₃ ) is 3.5% by mass or less from the viewpoint of preventing an increase in the refractory melt loss as described above. On the other hand, the lower limit of slag (% Al ₂ O ₃ ) is 1% by mass or more, and in the case of less than 1% by mass, the promotion of dissolution and hatching of quick lime is limited even when a TiO ₂ source is added, The effect of improving the dephosphorization efficiency becomes difficult to obtain.

_{Al 2 O 3} Al ₂ O ₃ content of containing the pre-melt in the flux is preferably 10 to 70 wt%. If it is less than 10% by mass, the pre-melt flux basic unit increases, which is disadvantageous in cost. On the other hand, when it exceeds 70 mass%, the melting point of the premelt flux becomes too high, and the effect of promoting hatching of quick lime during decarburization blowing is reduced.

Further, the CaO content in the Al ₂ O ₃ -containing premelt flux is preferably 10 to 70% by mass. When CaO is added to a flux having a high Al ₂ O ₃ content, the melting point can be remarkably lowered, so it is desirable to add 10% by mass or more. However, adding 70 mass% or more of CaO is not preferable because the melting point of the flux becomes too high.

2. TiO ₂
Al ₂ O ₃ content premelt flux as Al ₂ O ₃ source as described above may be facilitated dephosphorization reaction as compared to other Al ₂ O ₃ source, the upper base blown in converter Not slag-based flux Is unevenly distributed in the vicinity of the furnace wall on the bath surface, and unless it is added to the furnace with a certain amount of Al ₂ O ₃ -containing pre-melt flux, it cannot be efficiently contacted with the unincubated flux such as quicklime. The effect of promoting hatching will be reduced.

As a result of intensive studies by the present inventors paying attention to this point, when a small amount of TiO ₂ source is added together with a small amount of Al ₂ O ₃ -containing premelt flux, molten slag is formed from the initial stage of decarburization blowing, and the molten slag Will form appropriately and the total iron content in the slag (mass%, hereinafter also referred to as “(% T.Fe)”) will increase, and as a result, new knowledge will be obtained that the dephosphorization reaction is promoted. It was.

  The formed slag covers the inside of the furnace above the bath surface, and the slag is always in contact with unincubated flux (quick lime etc.), and the forming slag and unincubated flux are sufficiently stirred and mixed.

  In addition, FeO generated at the fire point during decarburization blowing is taken into the forming slag and the amount of slag (% T. Fe) increases. Since the volume of the forming slag is increased by forming, the hot metal-slag interface area per slag unit volume is small. Accordingly, the iron component in the forming slag is less likely to come into contact with the carbon dissolved in the hot metal, and the slag (% T. Fe) is easily maintained at a high value. As a result, the oxygen potential of the slag is increased and the dephosphorization reaction is promoted. In the slag after decarburization refining by the production method according to the present invention (% T. Fe) is typically 5 to 20% by mass.

Thus, ilmenite ore and rutile ore are exemplified as the effective TiO ₂ source for promoting the dephosphorization reaction.
In the method for refining molten steel according to the present invention, the content of TiO _{2 in} the slag after decarburization blowing is also referred to as “mass%, hereinafter“ (% TiO ₂ ) ”. Is 0.2-1 mass%. When this (% TiO ₂ ) is less than 0.2% by mass, it is difficult to obtain the effect of adding the TiO ₂ source. On the other hand, when this (% TiO ₂ ) exceeds 1% by mass, the amount of refractory is damaged. Tends to increase rapidly.

Even when a premelt flux containing both Al ₂ O ₃ and TiO ₂ is used, the above dephosphorization reaction promoting effect can be obtained.
In that case, it is desirable that (% Al ₂ O ₃ ) in the pre-melt flux is 10 to 70% by mass, and (% TiO ₂ ) is 0.5 to 20% by mass.

If (% TiO ₂ ) in the pre-melt flux is less than 0.5% by mass, the basic unit of the pre-melt flux increases so that the cost increases. On the other hand, if (% TiO ₂ ) in the premelt flux exceeds 20% by mass, when adjusting (% Al ₂ O ₃ ) in the slag, the slag (% TiO ₂ ) is controlled within an appropriate range. It becomes difficult.

3. Basicity, etc. In the molten steel refining method according to the present invention, the basicity of the slag after decarburization refining treatment represented by the above formula (1) (hereinafter also referred to as “actual basicity”) is 3.0 to 4.5. And By setting it as this range, the dephosphorization reaction promoting effect is stably realized. When the actual basicity is lower than 3.0, it is difficult to sufficiently reduce the phosphorus concentration because the dephosphorization ability of slag is low. On the other hand, when the basicity exceeds 4.5, the melting point of the slag is increased, so that the slag is not easily hatched, and the dephosphorization reaction does not proceed easily. In order to obtain the dephosphorization reaction effect particularly stably, the actual basicity is preferably set to 3.0 to 4.0.

Examples of the CaO source of slag include quick lime, limestone, slaked lime, and dolomite, and examples of the SiO ₂ source include quartzite and peridotite. The slag may contain MgO in addition to the above components, and examples of the MgO source include dolomite, natural magnesia, peridotite, and the like.

4). Hot metal composition, blowing conditions, etc. In the method for refining molten steel according to the present invention, (% Al ₂ O ₃ ), (% TiO ₂ ), and basicity in the slag after decarburization blowing are set within a predetermined range. If it can, the chemical composition of the hot metal used for the decarburization blowing process is not particularly limited. However, when the phosphorus concentration in the hot metal before the decarburization blown treatment is excessively high, the decarburization blown treatment according to the present invention may not be able to sufficiently reduce the phosphorus concentration. The hot metal used for the treatment is dephosphorized as a preliminary treatment, and the phosphorus content (% by mass, hereinafter referred to as “[P]”) in the hot metal is preferably 0.05% by mass, 0.03 If it is below mass%, it is especially preferable. The refining vessel in which this dephosphorization process is performed is not particularly limited, and a torpedo car and a hot metal ladle are exemplified.

The typical content of other elements in the hot metal used in the decarburization blowing process is as follows:
[C] ≦ 4.0% by mass or less,
[Si] ≦ 0.10 mass%,
[Mn] ≦ 0.30 mass%,
[S] ≦ 0.03 mass%,
[Ti] ≦ 0.01 mass%.

  Here, [element] means the content (mass%) in the molten steel of the said element similarly to [P].

The treatment conditions for the decarburization blowing process are not particularly limited as long as the slag after the treatment is appropriately controlled, similarly to the chemical composition of the hot metal.
Typically, an upper bottom blowing converter is used, a gas containing oxygen is supplied from an upper blowing lance, an inert gas such as Ar is supplied from the bottom blowing tuyere for stirring, and the molten steel temperature after processing is 1600. A decarburization blowing process for about 10 to 15 minutes is performed while maintaining the temperature to ˜1700 ° C.

The CaO source, the SiO ₂ source, the Al ₂ O ₃ -containing premelt flux and the TiO ₂ source added in the decarburization blowing process are charged as a lump into the hot metal before oxygen blowing or during oxygen blowing.

  The composition of the molten steel after the decarburization blowing process is not particularly limited. Low carbon steel may be obtained with a carbon content of about 0.3 mass or less, or medium carbon steel may be obtained with about 0.3 to 0.7 mass%. Alternatively, an extremely low carbon steel may be obtained with a carbon content of less than 0.05% by mass.

  Table 1 shows a summary of the hot metal and molten steel, the composition of slag after decarburization and dephosphorization, and the blowing conditions (temperatures before and after blowing) in the method for refining molten steel according to the present invention.

  EXAMPLES Hereinafter, although this invention is further demonstrated using an Example, this invention is not limited to a following example.

Example 1
A test converter (using magnesia-carbon brick) was charged with 2 ton dephosphorization having the following chemical composition:
[C]: 3.4 to 4.0% by mass,
[Si] ≦ 0.05 mass%,
[Mn]: 0.18 to 0.22% by mass,
[P]: 0.02-0.03% by mass,
[S]: 0.005 mass%,
[Ti] ≦ 0.01 mass%.

Subsequently, quicklime 13-20 kg / t, silica stone 3.0-5.5 kg / t, MgO particles 2 kg / t, ladle slag (composition: CaO = 40 mass%, SiO ₂ = 10 mass%, Al ₂ O ₃ = 20 wt%, T.Fe = 5 wt%) 0.8~6.5kg / t, Ti ore (composition: _TiO 2 = 40 wt%, T.Fe = 32 weight%) 0.1 to 1. 1 kg / t was added.

Thereafter, oxygen gas was blown from the top blowing lance at 2.7 Nm ³ / min / t, and Ar gas was blown from the bottom blowing tuyere at 0.2 Nm ³ / min / t.
The molten steel temperature at the end of blowing was about 1650 ° C., and its composition (mass%) was as follows except for phosphorus:
[C]: 0.05 to 0.10% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.10 to 0.17% by mass,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

The refractory erosion amount during blowing is the mass of MgO eluted into the slag during blowing (hereinafter referred to as “MgO elution amount”), and the MgO content in the slag (mass%, hereinafter referred to as “(% MgO)”. And the slag composition after treatment has a basicity of 3.8, (% Al ₂ O ₃ ) = 2.4, and (% TiO ₂ ) = 0.5 (No. 7 in Table 1). ), And an index normalized with the MgO elution amount (hereinafter referred to as “refractory material erosion index”).

The refractory melting index was set to 1.5 or less.
[P] after the treatment was targeted to be 0.010% by mass or less.
The results are shown in Table 2.

Here, the evaluation criteria in the evaluation column in Table 2 are as follows:
◯: When both [P] ≦ 0.010 mass% after treatment and refractory erosion index ≦ 1.5 are satisfied ×: [P] after treatment ≦ 0.010 mass%, refractory erosion index ≦ 1. When either one of 5 is not satisfied Δ: Even when the amount of flux is increased to increase the actual basicity, [P] is hardly changed after treatment, which is not preferable in terms of cost.

(Example 2)
A test converter (using magnesia-carbon brick) was charged with 2 ton dephosphorization having the following chemical composition:
[C]: 3.5% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.20 mass%,
[P]: 0.025 mass%,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

Subsequently, quicklime 16.5 kg / t, quartzite 4.1 kg / t, pre-melt flux (composition: CaO = 30 mass%, SiO ₂ = 7 mass%, Al ₂ O ₃ = 18 mass%, T.Fe = 8 (Mass%, TiO ₂ = 3.4 mass%) was added in an amount of 4.1 kg / t.

Thereafter, oxygen gas was blown from the top blowing lance at 2.7 Nm ³ / min / t, and Ar gas was blown from the bottom blowing tuyere at 0.2 Nm ³ / min / t.
The molten steel temperature at the end of blowing was about 1650 ° C., and its composition (mass%) was as follows:
[C]: 0.08% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.15% by mass,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

The post-treatment slag composition is: real basicity = 3.8, (% Al ₂ O ₃ ) = 2.4 mass%, (% TiO ₂ ) = 0.5%, and [P] = 0.005 mass in molten steel %, Refractory erosion index = 1.0.

[P] decreased after the treatment from the case where Al ₂ O ₃ -containing ladle slag and Ti ore were added (No. 7 in Table 1).

(Comparative Example 1)
A test converter (using magnesia-carbon brick) was charged with 2 ton dephosphorization having the following chemical composition:
[C]: 3.7% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.21% by mass,
[P]: 0.025 mass%,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

Subsequently, 18 kg / t quicklime, 5.0 kg / t silica, and 2 kg / t MgO particles were added.
Thereafter, oxygen gas was blown from the top blowing lance at 2.7 Nm ³ / min / t, and Ar gas was blown from the bottom blowing tuyere at 0.2 Nm ³ / min / t.

The molten steel temperature at the end of blowing was about 1645 ° C. and its composition was as follows:
[C]: 0.09% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.14% by mass,
[P]: 0.018 mass%,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

In the post-treatment slag composition, the actual basicity was 3.0, (% Al ₂ O ₃ ) was 0.5% by mass, and (% TiO ₂ ) was 0.1% by mass. The refractory erosion index was 0.9.

(Comparative Example 2)
A test converter (using magnesia-carbon brick) was charged with 2 ton dephosphorization having the following chemical composition:
[C]: 3.4% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.22% by mass,
[P]: 0.026% by mass,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

Subsequently, quicklime 18 kg / t, silica stone 5.0 kg / t, MgO particles 2 kg / t, ladle slag (composition: CaO = 40 mass%, SiO ₂ = 10 mass%, Al ₂ O ₃ = 20 mass%, T . Fe = 5% by mass) 3.0 kg / t was added.

Thereafter, oxygen gas was blown from the top blowing lance at 2.7 Nm ³ / min / t, and Ar gas was blown from the bottom blowing tuyere at 0.2 Nm ³ / min / t.
The molten steel temperature at the end of blowing was about 1646 ° C. and its composition was as follows:
[C]: 0.06% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.12% by mass,
[P]: 0.011% by mass,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

In the slag composition after the treatment, the actual basicity was 3.3, (% Al ₂ O ₃ ) was 2% by mass, and (% TiO ₂ ) was 0.1% by mass. The refractory erosion index was 0.9.

(Comparative Example 3)
A test converter (using magnesia-carbon brick) was charged with 2 ton dephosphorization having the following chemical composition:
[C]: 3.5% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.21% by mass,
[P]: 0.025 mass%,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

Subsequently, quicklime 16.5 kg / t, quartzite 4.2 kg / t, Al ₂ O ₃ brick waste (composition: Al ₂ O ₃ = 100 mass%) 0.8 kg / t, Ti ore (composition: TiO ₂ = 40) Mass%, T.Fe = 32 mass%) was added at 0.5 kg / t.

Thereafter, oxygen gas was blown from the top blowing lance at 2.7 Nm ³ / min / t, and Ar gas was blown from the bottom blowing tuyere at 0.2 Nm ³ / min / t.
The molten steel temperature at the end of blowing was about 1650 ° C. and its composition was as follows:
[C]: 0.10% by mass,
[Si]: <0.01% by mass,
[Mn]: 0.23 mass%,
[P]: 0.012 mass%,
[S]: 0.005 mass%,
[Ti]: <0.01% by mass.

The post-treatment slag composition has an actual basicity of 3.7, (% Al ₂ O ₃ ) of 2.0% by mass, and (% TiO ₂ ) of 0.6% by mass. 0.9.
Compared with the case where Al ₂ O ₃ -containing ladle slag and Ti ore were added (No. 7 in Table 1), [P] did not decrease after treatment.

Below, each result by said Example is demonstrated.
(1) No. 1 in Example 1 Slag composition subsequent blowing the actual basicity for 1-6 = 3.0 and _4.5, as (% TiO 2) = 0.2 to 1.0 wt%, alter the (% _Al 2 _{O 3)} As a result, when the Al ₂ O ₃ source addition (% Al ₂ O ₃ ) in the slag is 1 mass% or more, melting and hatching of the flux is promoted to increase the amount of molten slag effective for dephosphorization, Later [P] decreased to the target value.

However, when the amount of slag (% Al ₂ O ₃ ) exceeds 3.5% by mass, the amount of refractory erosion was rapidly increased.
After decarburization blowing, where in the slag (% Al 2 _{O 3)} was visually observed slag exceeds 3.5 mass%, the fluidity of the slag was significantly enhanced. Therefore, it is considered that the refractory has been eroded by this highly fluid slag.

(2) No. 1 in Example 1 About 7-10 As the slag composition after blowing is an actual basicity of 3.0-4.5, (% Al ₂ O ₃ ) = 1-3.5 mass%, the slag is changed in (% TiO ₂ ) The dephosphorization behavior and refractory erosion behavior were investigated.

When the amount of slag (% TiO ₂ ) was 0.2% by mass or more, the slag formed during blowing, and [P] decreased to the target value after the treatment.
On the other hand, when the amount of slag (% TiO ₂ ) exceeds 1.0% by mass, the slag started to form violently from the early stage of blowing and the refractory erosion amount increased rapidly. When slag (% TiO ₂ ) exceeds 1.0% by mass, (% T.Fe) in the slag increased from the early stage of blowing, and the slag fluidity continued for a long time. It is thought that the amount of material damage increased.

(3) No. 1 in Example 1 About 11 to 15 Slag composition after blowing _{(% Al 2 O 3) =} 1~3.5 _wt%, even (% TiO 2) = 0.2~1.0 wt%, slag fruit When the basicity was less than 3.0, [P] after the treatment could not be reduced to the target value.

It is known that the dephosphorization ability of slag increases as the actual basicity increases, and the slag composition after blowing is (% Al ₂ O ₃ ) = 1 to 3.5% by mass, (% TiO ₂ ) = From 0.2 to 1.0 mass%, it became clear that the actual basicity cannot be reduced to the [P] target value after the treatment unless the basic basicity is 3.0 or more.

In addition, even if the amount of added flux was increased to increase the charging basicity, and the actual basicity was further increased beyond 4.5, [P] after the treatment hardly changed.
The composition of slag whose basicity is over 4.5 has a very high melting point, so that the flux hatches only after the end of blowing and the basicity exceeds 4.5. Become. However, in order for the dephosphorization reaction to proceed, it is necessary for the slag to hatch, so in such a high melting point slag, it hatched at the end of blowing and the actual basicity suddenly increased. However, since the time for which the dephosphorization reaction proceeds is short, it is presumed that [P] changed almost after the treatment.

Thus, when the actual basicity is excessively high, [P] hardly changes after the treatment, which is economically disadvantageous as much as the amount of flux is increased.
Therefore, the actual basicity is desirably 3.0 to 4.5.

(4) About Example 2 By using a pre-melt flux containing Al ₂ O ₃ and TiO ₂ , ladle slag was used as the Al ₂ O ₃ -containing pre-melt flux, and Ti ore was used as the TiO ₂ -containing flux. Compared to the case, since Al ₂ O ₃ and TiO ₂ coexist in the molten slag, the synergistic effect of the two (moderately formed to produce slag having an appropriate (% T. Fe), It is presumed that the flux such as quicklime is rapidly hatched into the slag) and [P] is further reduced after the treatment.

In addition, when the pre-melt flux containing Al ₂ O ₃ and TiO ₂ was used, the above synergistic effect was efficiently realized, that is, only the hatching of the flux was promoted, There was no change such as slag forming becoming extremely intense. Therefore, even when a premelt flux containing Al ₂ O ₃ and TiO ₂ was used, the amount of refractory melt was hardly changed.

(5) About Comparative Example 1 When no Al ₂ O ₃ source is added, slag does not form during blowing, so that quick lime hatching does not progress, and [P] after processing decreases to the target value. There wasn't.

(6) About Comparative Example 2 Even if only a small amount of Al ₂ O ₃ source is added (in the slag (% Al ₂ O ₃ ) ≦ 3.5), quick lime hatching does not progress, and [P] after treatment is the target It did not drop to the value.

(7) About Comparative Example 3 When the waste material of Al ₂ O ₃ refractory was used as the Al ₂ O ₃ source, slag did not form much during blowing and [P] did not reach the target after treatment.
As described above, the high melting point quick lime, the high melting point Al ₂ O ₃ source and the TiO ₂ source are simply added to the furnace, so that the flux hardly hatches until the end of decarburization blowing. This is probably because the dephosphorylation reaction did not progress.

Claims (3)

When hot metal dephosphorized in a refining vessel is charged into an upper bottom blowing converter, which is another refining vessel, and decarburized and blown, a CaO source, a SiO ₂ source, an Al ₂ O ₃ -containing premelt flux, and TiO ₂ sources are added, and the composition of the treated slag is (% Al ₂ O ₃ ) = 1.0 to 3.5% by mass, (% TiO ₂ ) = 0.2 to 1.0% by mass, the following formula A method for refining molten steel, wherein the basicity according to (1) is 3.0 to 4.5.
Basicity = (CaO) / (SiO ₂ ) (1)
However, (CaO): Total CaO content in slag (mass%)
(SiO ₂ ): SiO ₂ content in slag (mass%) The ladle refining method according to claim 1, wherein ladle slag is used as the Al ₂ O ₃ -containing premelt flux . When the hot metal dephosphorized in the refining vessel is charged into the top bottom blowing converter, which is another refining vessel, and decarburized and blown, it contains CaO source, SiO ₂ source, Al ₂ O ₃ and TiO ₂ The pre-melt flux is added, and the composition of the slag after the treatment is (% Al ₂ O ₃ ) = 1.0 to 3.5% by mass, (% TiO ₂ ) = 0.2 to 1.0% by mass, refining method of the following formula (1) you characterized that you 3.0 to 4.5 basicity due soluble steel.
Basicity = (CaO) / (SiO ₂ ) (1)
However, (CaO): Total CaO content in slag (mass%)
(SiO ₂ ): SiO ₂ content in slag (mass%)