TWI667350B - Pretreatment method of lyophilization and manufacturing method of extremely low phosphorus steel - Google Patents

Abstract

The present invention provides a pretreatment method for lyotropic milling by appropriately performing delining in the pretreatment of lysing without hindering the intermediate slagging step of the converter and improving the dephosphorization treatment efficiency. The pretreatment method for lysing according to the present invention is to put iron oxide, gaseous oxygen and a lime-based flux into lyophilization in a refining vessel to perform a dislocation treatment and a dephosphorization treatment, and the refractory pretreatment method is characterized in that When the iron oxide equivalent conversion oxidizing agent basic unit is 25 kg/t or more of iron oxide, when the above-mentioned input is performed, 60% or more of the iron oxide to be supplied is supplied from above the refining vessel, and the Si content of the melt milling is adjusted. It is 0.05 to 0.30% by mass, and the P content is adjusted to 0.040 to 0.085% by mass.

Description

Pretreatment method of lyophilization and manufacturing method of extremely low phosphorus steel

The present invention relates to a pretreatment method for effectively reducing the Si concentration and the P concentration of a smelting mill in a hybrid milling machine and a method for producing an extremely low phosphorus steel.

BACKGROUND OF THE INVENTION In the case of converting molten oxygen into a molten steel in a converter, a "solution milling pretreatment" in which strontium, phosphorus, sulfur, and the like are removed from the converter in a converter is usually performed to reduce the blowing load in the converter. And make the molten steel easy to adjust to the desired composition.

For example, during the milling operation from the blast furnace, the smelting of the flux, the oxidant and/or the soda ash flux is used as a refining agent during the out-of-milling, pouring, or mixing and milling machine, such as: Nitrogen, oxygen) is blown into the smelting mill, or directly added to the smelting from above to remove bismuth, phosphorus and sulfur, etc., to the slag for removal.

On the other hand, a process for converting a converter for lyophilization pretreatment (converter-type refractory milling pretreatment) is also under development. The converter type smelting and tempering pretreatment is generally carried out by a double furnace method using a dephosphorization converter and a decarburization converter. However, Patent Document 1 proposes a method of performing the following series of treatment steps in a single furnace, the treatment step being dephosphorization. After the treatment, the converter is tilted to slag the dephosphorization furnace slag (intermediate slag discharge), followed by decarburization treatment, and the decarburization slag after slag removal is used again as a dephosphorization agent in the next dephosphorization treatment.

The great advantage of the method of Patent Document 1 is that it is the size of the thermal tolerance brought about by the single furnace method and the heat recovery of the decarburization slag. However, in the process of dislocation, dephosphorization, intermediate slag removal, and decarburization, if excessive dislocation occurs, the concentration of the main slag source, ie, cerium (Si), may become scarce in lyophilization, and once it becomes so, it becomes difficult. Perform intermediate slagging. As described above, the intermediate slag discharge becomes difficult depending on the degree of detachment, and in order to perform the intermediate slag discharge, it is necessary to add Si or sinter SiO ₂ as a slag source in the pulverization after the detachment is performed, The case of non-economic steps such as increasing the enthalpy concentration of lyophilization again.

In order to melt the extremely low-phosphorus steel, the following treatment is necessary. The foregoing treatment is to reduce the P concentration of the pulverization by lyophilization pretreatment, or to temporarily discharge the molten steel after the dephosphorization treatment in the converter, and to remove the dephosphorization slag. The total amount is discharged outside the system, and decarburization and dephosphorization blowing are performed again in the same converter. Further, the so-called extremely low phosphorus steel is a steel having a phosphorus content of 0.01% or less in steel.

The molten steel after the dephosphorization treatment in the converter is temporarily tapped, and the total amount of the dephosphorization furnace slag is discharged outside the system, and the decarburization and dephosphorization blowing are performed again in the same converter. This series of processing steps is complicated and the treatment is performed. The time has been greatly extended. As described above, in the case of melting ultra-low phosphorus steel by the method of Patent Document 1, there are many problems in the steps.

Thermodynamically, the dephosphorization reaction is carried out after the end of the depurination reaction. Therefore, when the pulverization pretreatment is used to sufficiently reduce the P concentration of the pulverization as described above, it is necessary to reduce the Si concentration of the smelting to about zero at the time of refractory pretreatment.

Therefore, in the prior art, in order to melt extremely low phosphorus steel, it is necessary to reduce the P concentration of the pulverization to a specification concentration (P content: 0.01% or less) in the pulverization pretreatment stage. However, a large amount of slag is produced to dephosphorize to a very low phosphorus grade. When using the torpedo type mixing and milling machine to perform dephosphorization as described above, since the torpedo's free side (the height formed in the space on the smelting mill to the furnace mouth) is small, it is necessary to reduce the P concentration of the pulverizing milling to the vicinity of the specification concentration. Have difficulty.

In addition, in the prior art, when dephosphorization is performed in the karst pretreatment in a mixed-milling vehicle, the following technique is often employed: a lime-based flux is blown in order to form a slag basicity of 3.0 or more, and a slag for promoting CaO The amount of flux is reduced and fluorite (CaF ₂ ) is added. As long as this technique is utilized, the melting point of CaO is lowered, and the slag is easily changed.

Patent Document 2 discloses a method in which a flux and an oxygen source mainly containing CaO are simultaneously blown into the same position by the following conditions, thereby simultaneously performing degumming and dephosphorization from smelting. The method disclosed in Patent Document 2 is carried out using a flux containing fluorite. The total oxygen feed rate ^{_{V O2 (Nm 3 /min.T)≧2.25[%Si] 0}} -0.03 However, [% Si] ₀ = initial [Si] concentration

However, the addition of fluorite (CaF ₂ ) increases the fluorine (F) concentration of the formed slag. In recent years, it has been considered as a problem to dissolve fluorine into the environment from civil engineering materials and construction materials using slag as raw materials, and the Environment Agency has imposed restrictions on fluorine in slag. Further, the fluorine in the slag adversely affects the refractory of the container used in the pretreatment, so that fluorine is not preferable.

Patent Document 3 proposes a method of pretreating a pulverizing mill which does not use fluorite and which can perform deamination and dephosphorization. The above method is to melt the lime-based flux and the oxidizing agent into the refining vessel, and to dislocate. During the period until the rate becomes 90%, the main amount of the flux is added to the pulverization, so that the final alkalinity of the slag is 1.2 to 2.5.

However, in the method of Patent Document 3, since the method of adding iron oxide is an injection method, the concentration of T.Fe (Total Fe) of the top slag is hard to rise, and the de-purification and dephosphorization cannot be simultaneously performed, not only the pulverization after the treatment. The Si concentration becomes about zero, and the amount of slag also becomes large.

Further, Patent Document 4 discloses a method for manufacturing a low-phosphorus pulverization method in which a smelting mill stored in a sterilizing and hoisting storage container is provided with an iron oxide source from above a bath surface, and a CaO-based body is blown under the bath surface. A flux, whereby the solution is lysed as a dephosphorization process to produce a low phosphorus solution milling. The method disclosed in Patent Document 4 is characterized in that the above-mentioned iron oxide source is applied to the surface of the bath surface so that the flux is coated in an area ratio of 40% or more of the blowing area of the bath surface. Iron oxide source. The above-described method disclosed in Patent Document 4 can omit a flux of a fluorine-containing source such as fluorite.

Patent Document 4 and Patent Document 1 are in the same manner as the single-furnace type, and are accompanied by an intermediate slagging step. Patent Document 4 teaches that the generated slag is slag discharged after the enthalpy concentration of the pulverization is lowered to a predetermined level by the dislocation treatment in the smelting pot, but it is not specifically taught that the intermediate slag removal step can be solved. Means of technical issues.

CITATION LIST Patent Literature Patent Literature 1: Japanese Laid-Open Patent Publication No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. Bulletin No. 288507

SUMMARY OF THE INVENTION Problems to be Solved by the Invention In the case of performing the smelting pretreatment followed by the intermediate slag removal, especially when the dephosphorization method by the single furnace method disclosed in Patent Document 1 is carried out, if excessive desorption occurs, there is a enthalpy in the smelting. The technical problem that the concentration becomes scarce and the intermediate slag is difficult.

Further, in the case of the melt milling treatment method including the intermediate slag discharging step, the slag having a high phosphorus concentration generated by dephosphorization blowing cannot be completely removed by the intermediate slag removing step, and the slag is after the intermediate slag discharging step Remained. Therefore, in the above-described dephosphorization method using a single furnace method, it is difficult to melt extremely low phosphorus steel.

Therefore, in view of the current state of the art, it is an object of the present invention to provide a crucible concentration which, in the pretreatment of smelting, leaves the slag removal step in the smelting process before the smelting process is performed by the converter, The P concentration in the smelting is reduced, thereby improving the dephosphorization treatment of the smelting step and the pretreatment method of the dislocation treatment efficiency and the melting method of the extremely low phosphorus steel.

Further, an object of the present invention is to effectively reduce the P concentration and the Si concentration of the pulverization without using CaF ₂ in the pretreatment of the smelting.

Means for Solving the Problem In the pretreatment of karst milling, if the Si concentration and the P concentration of the pulverization are appropriately reduced by simultaneously performing the decarburization treatment and the dephosphorization treatment, the extremely low phosphorus steel is melted in the subsequent converter refining. The inventors of the present invention have made intensive studies on the methods for solving the above problems based on the above-mentioned ideas, and the efficiency of the entire refining step has been improved.

As a result, it was found that the pulverization was pretreated by setting the Si content to 0.05 to 0.30% by mass and the P content to be 0.040 to 0.085% by mass, and the refining by the intermediate slagging step of the combination converter or the like was further enhanced. The overall efficiency of the steps.

Further, the inventors of the present invention have also intensively studied the conditions at the time of performing the dislocation treatment and the dephosphorization treatment of the pulverization by using the lime and the iron oxide-based flux in the pretreatment of the smelting.

As a result, it has been found that in order to make the Si content and the P content of the pulverization after the pretreatment in the above range, it is preferable to set the change (ΔP/ΔSi) of the Si content and the P content before and after the pretreatment to be more than 0.1.

As described above, it has been found that in the pretreatment of the smelting in the refining vessel, the Si concentration and the P concentration of the refractory after the pretreatment can be adjusted to suit the appropriate amount of the iron oxide input and the iron oxide input method. The composition of the subsequent refining steps, such as the refining step by the converter.

The present invention has been made based on the above findings, and its gist is as follows.

(1) A refractory pretreatment method in which iron oxide, gaseous oxygen, and a lime-based flux are introduced into a smelting vessel in a refining vessel to perform a de-slipping treatment and a dephosphorization treatment, and the pre-treatment method of the smelting milling is characterized by: When iron oxide of 25 kg/t or more is used as the basic unit of the oxidizing agent in terms of iron oxide, at the same time as the above-mentioned input, 60% or more of the iron oxide to be supplied is supplied from above the refining vessel, and the Si content of the aforementioned smelting is added. The ratio was adjusted to 0.05 to 0.30% by mass, and the P content was adjusted to 0.040 to 0.085% by mass. (2) The pretreatment method of lysing according to (1), wherein 80 to 100% of the total amount of iron oxide to be put into the refining vessel is supplied from above the refining vessel. (3) The pretreatment method of refractory milling according to (1) or (2), wherein the P concentration before and after the pretreatment and the Si concentration before and after the pretreatment are satisfied with the following formula 1, ΔP / ΔSi > 0.1 (formula 1), ΔP: the difference between the P concentration before pretreatment and the P concentration after pretreatment, ΔSi: the difference between the Si concentration before pretreatment and the Si concentration after pretreatment. (4) The pretreatment method of lysing according to any one of (1) to (3), wherein the lime-based flux contains no CaF ₂ . (5) The pretreatment method of lysing according to any one of (1) to (4), wherein the refining container is a mixed milling machine. (6) A method for producing an extremely low-phosphorus steel, characterized in that after the pretreatment method according to any one of (1) to (5), the intermediate slag is discharged in the converter.

Advantageous Effects of Invention According to the present invention, pretreatment can be performed in a manner that does not hinder the intermediate slag discharging step in the converter, so that the pretreatment method of the present invention can be used to efficiently melt in a refining step by a converter. Very low phosphorus steel.

Further, according to the present invention, in the pretreatment of the smelting, the P concentration and the Si concentration of the pulverization can be efficiently reduced without using CaF ₂ .

EMBODIMENT OF THE INVENTION The pretreatment method of the smelting process of the smelting process of the present invention (hereinafter may be referred to as "the method of the present invention") is a method of pretreating smelting iron oxide, gaseous oxygen, and a lime-based flux into a refining vessel. The disintegration treatment and the dephosphorization treatment are characterized in that: (i) the iron oxide in an amount of 25 kg/t or more based on the basic unit of the oxidizing agent in terms of iron oxide is supplied, and (ii) the above-mentioned input is performed. When 60% or more of the iron oxide to be supplied is supplied from above the refining vessel, (iii) the Si content of the pulverization is adjusted to 0.05 to 0.30% by mass, and the P content is adjusted to 0.040 to 0.085 mass%.

Hereinafter, the method of the present invention will be described.

The lyophilization which is the object of the pretreatment method of the present invention is not limited to a specific component composition but may be a general component composition, as long as it is 0.80 mass% or less and P: 1.200 mass% or less. Solving milling from the blast furnace or lysing by electric furnace melting.

The pre-treatment of lyophilization is carried out by using a mixing and milling machine mainly used for conveying the smelting-to-refining step as a refining container. Therefore, the pretreatment of the smelting and milling using the mixing and milling machine as a refining container will be described, but the refining container is not limited to the mixing and milling machine. As long as it is a container for transporting the lysate to a subsequent refining step (such as a smelting mill tank, etc.), it is a container capable of pre-refining.

Figure 1 shows schematically an aspect of the pretreatment of a smelting mill used as a refining vessel. As shown in Fig. 1, the lance 3 is immersed in the blasting 4 from the opening 2 of the mixer car 1, and the lime-based flux 5 and/or the oxidant 6 (iron oxide) are carried by the carrier gas 7 (gas oxygen), and the like. The time required to blow from the spray gun 3 into the smelting mill 4.

Si and P in the smelting mill 4 are oxidized and transferred to the slag 8, and the pretreatment (disintegration treatment and dephosphorization treatment) of the blast milling 4 is performed. When the Si concentration of the smelting is as high as 0.05 to 0.30% by mass, the pretreatment is interrupted and the mixing and milling machine 1 is tilted, and the generated slag 8 is discharged out of the mixing and milling machine 1. Next, the pretreatment is started again with the lime-based flux 5 and the oxidizing agent 6.

In the method of the present invention, in the pretreatment (disintegration treatment and dephosphorization treatment) of smelting, when iron oxide is used as an oxidant to be pulverized, 60% or more of the iron oxide 6a to be put into the iron oxide is supplied from above the mixing and milling machine. That is, it is introduced into the smelting mill 4 from the chute 9 disposed in the opening 2.

The iron oxide source is, for example, a mill scale, sinter, iron ore, and sintered dust. The lime-based flux may be CaO alone or a mixture of calcium carbonate (CaCO ₃ ) or converter slag having a main component of CaO. However, as described above, it is preferable to use fluorite instead of the refractory material of the container used for the external environment and the pretreatment.

In the method of the present invention, iron oxide of 25 kg/t or more is introduced into the lyophilization in terms of the basic unit of the oxidizing agent in terms of iron oxide, and this point will be described in detail later. In addition, the amount of iron oxide input is the total mass of iron oxide invested in the smelting of the milled car. Further, the "basic unit of the oxidizing agent in terms of iron oxide" refers to the mass obtained by converting the total mass of oxygen supplied for refractory milling for 1 t in the refractory pretreatment step into FeO.

In order to simultaneously carry out the depurination reaction and the dephosphorization reaction, it is necessary to lower the Si activity of the smelting and to increase the oxygen potential in the reaction interface between the smelting and the slag. It is presumed that Fe can be maintained at a high concentration by introducing iron oxide from above, but the reaction efficiency of the above addition is lower than that of direct injection into the pulverization, so even if only the total amount of iron oxide is added from above, The reaction efficiency will only decrease, and both the depurination treatment and the dephosphorization treatment will become insufficient.

Therefore, in the pretreatment of lyophilization, in order to maximize the de- ing efficiency and the dephosphorization efficiency, it is necessary to determine how much iron oxide should be put in the basic unit of the oxidant, and also to what extent the ratio should be added from above. The iron oxide described above has not been studied or examined on the quantitative determination of iron oxide which has been put into smelting, and of course, the quantitative measure cannot be displayed.

The inventors of the present invention conducted intensive studies on the amount of iron oxide which is put into the smelting and milling, and the basic unit of the oxidizing agent in terms of the iron oxide converted to the iron oxide which is put into the smelting and milling, and the ratio of the iron oxide which should be input from the top of the lyotropic milling ( %).

Figure 2 shows the ratio of iron oxide input from above the mixer truck: R _FeO (%) and the amount of change in P concentration before and after pretreatment (% by mass) ΔP and the ratio of change in Si concentration (% by mass) ΔSi: ΔP /ΔSi relationship. Fig. 2 shows the ΔP/ΔSi change when 35 kg/t of iron oxide is supplied in the basic unit of oxidizing agent in terms of iron oxide in smelting, and 25 kg/t in the basic unit of oxidizing agent in terms of iron oxide in kiln milling. ΔP/ΔSi change in iron oxide.

The more simultaneous desorption and dephosphorization, the larger the ΔP/ΔSi becomes. In order to melt the extremely low phosphorus steel in a converter step, it is desirable to leave a certain degree of Si and remove P as much as possible, so that the larger the ΔP/ΔSi is, the more appropriate.

Among the aforementioned pulverization, in particular, lyophilization including C: 4.50 to 4.70% by mass, Si: 0.50 to 0.60% by mass, and P: 0.100 to 0.120 is assumed to be the object of the pretreatment method of the present invention. After pre-processing the above-mentioned pulverization and reducing the Si content of the pulverization to 0.2% by mass, the extremely low-phosphorus steel is melted in a converter step, and ΔP/ΔSi>0.1 is preferable. Therefore, in the method of the present invention, ΔP/ΔSi>0.1 is used as an evaluation criterion.

As R _FeO (%) increases, ΔP/ΔSi also increases. When the basic unit of oxidant is 35kg/t, if R _FeO = 60%, it will become ΔP/ΔSi>0.1, if R _FeO ≧70%, R _FeO increases, and ΔP/ΔSi is 0.14 to 0.18.

In this case, the necessary conditions for simultaneously performing the deoximation treatment and the dephosphorization treatment are R _FeO ≧ 60%, preferably R _FeO ≧ 70%. The upper limit of R _FeO is 100%. If it is more than 85%, it is preferable to ensure the required ΔP/ΔSi. However, the reaction efficiency η ₀ of the input oxygen defined by the following formula is lowered, so this point should be considered. R _{FeO is} set as appropriate _.

η ₀ ={(ΔP×80/62+ΔSi×32/28)×1/100}/(basic unit of iron oxide input amount×1/1000×C ₀ ) Here, the basic unit of iron oxide input is: iron oxide Input amount (kg) / melt milling amount (t); ΔP: P concentration before pretreatment - P concentration after pretreatment; ΔSi: Si concentration before pretreatment - Si concentration after pretreatment; C ₀ : Oxygen ratio in iron oxide ( Oxygen quality in iron oxide / total mass of iron oxide)

The aforementioned reaction efficiency η ₀ reflects the amount of oxygen that has reacted with Si and P among the input oxygen. The Si component and the P component in the converter slag are almost both phosphorus pentoxide (P ₂ O ₅ ) and cerium oxide (SiO ₂ ). The amount of oxygen that has reacted with Si and P in the input oxygen can be calculated from the change in the P concentration and the Si concentration before and after the pretreatment, and the amount of iron oxide used for the pretreatment. Therefore, the aforementioned reaction efficiency η ₀ can be defined by ΔP, ΔS, and the basic unit of the amount of iron oxide input.

When the basic unit of the oxidizing agent is 25 kg/t, ΔP/ΔSi is lowered, and even if R _FeO ≧ 60%, ΔP/ΔSi<0.1 may be present. It is presumed that this is that the basic unit of the oxidant is insufficient relative to the Si concentration of the smelting, and the concentration of T.Fe in the slag of the remaining oxygen source is low, and the dephosphorization treatment and the dislocation treatment are not performed simultaneously.

In the method of the present invention, in consideration of the results shown in Fig. 2, the iron oxide which is put into the smelting is preferably set to 30 kg/t or more in terms of the basic unit of the oxidizing agent in terms of iron oxide. More preferably, it is above 35 kg/t. Further, the upper limit of the iron oxide to be put into the smelting is not particularly limited as long as it is not limited to the scale of the apparatus for carrying out the pretreatment method of the present invention. In the case of a general torpedo type mixed-milling vehicle, the upper limit of the iron oxide put into the smelting milling can also be set to 80 kg/t.

As described above, in the pretreatment of lyophilization, the conditions for efficiently performing the decoupling treatment and the dephosphorization treatment at the same time are: the iron oxide charged into the smelting milling is calculated to be 25 kg/t or more in terms of the basic unit of the oxidizing agent in terms of iron oxide, 30kg / t or more preferred, more preferably at 35kg / t or more, and to more than 60%, preferably from the melting ratio R _FeO into iron oxide milling over 70% or more.

In particular, the ratio R _FeO iron oxide into the solution above the milling of 80 to 100% is preferable. It is also preferable to set the above-mentioned R _FeO to 80 to 100% from the viewpoint of saving labor for the apparatus for blowing the spray gun 3 into the smelting mill 4.

EXAMPLES Next, the examples of the present invention will be described, but the conditions in the examples are only a conditional example used to confirm the practicability and effects of the present invention, and the present invention is not limited by the conditions. . The present invention can adopt various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention. In addition, in the following examples, the pre-treatment step is to remove and dephosphorize the melt milling from the blast furnace, followed by lyophilization using the converter refining pretreatment step to further perform dephosphorization.

[Pretreatment step of smelting] The pulverization (Si: 0.54% by mass, P: 0.118% by mass, C: 4.6% by mass) from the blast furnace is loaded into a mixer and conveyed to a blower with a lime-based flux and an oxidizing agent. The pretreatment field of the apparatus is subjected to pretreatment (disintegration treatment and dephosphorization treatment) of the smelting milling under various conditions.

Five additions were carried out under separate conditions and the average was calculated. In one feeding, the comminution milling in the mixer truck is 260~280 tons. The procedures for pretreatment (depurination treatment and dephosphorization treatment) are as follows.

First, iron oxide as an oxidizing agent and quicklime as a lime-based flux are blown into a smelting mill from a spray gun with a carrier gas (oxygen) (refer to FIG. 1). The flow rate is 600~800Nm ³ /hour. At the same time as blowing in, the required amount of iron oxide is fed onto the smelting mill from above the mixer car (see Fig. 1). After the iron oxide is introduced from above the mixer car, the blowing of the iron oxide and the quicklime and the blowing of the oxygen (carrier gas) are continued, and the solution is stirred and pulverized, and the pretreatment is terminated after 20 to 30 minutes.

Table 1 shows the results of the pretreatment (disintegration treatment and dephosphorization treatment) of the lyophilization. In Table 1, the reaction efficiency ηo (refer to the above definition formula) which contributes to the oxygen removal treatment and the dephosphorization treatment is an index for evaluating the pretreatment efficiency of the pulverization milling.

[Table 1]

Inventive Examples 1 to 6 are examples in which pretreatment (disintegration treatment and dephosphorization treatment) of smelting is carried out under the conditions of the present invention. Inventive Examples 1 to 5 are examples in which iron oxide having an oxidizing agent basic unit of 35 kg/t in terms of iron oxide is introduced, and quicklime having a basic unit of quicklime of 10 to 20 kg/t is introduced. In Inventive Examples 1 to 5, the ratio of iron oxide charged from above the mixer car: R _FeO was 70 to 95%.

Inventive Examples 1 to 3 satisfied that ΔP/ΔSi>0.1, and the result of the pretreatment was good. The ΔP/ΔSi of Inventive Example 3 is higher than the ΔP/ΔSi of Inventive Examples 1 and 2, but contributes to the oxygen efficiency of devolatilization treatment and dephosphorization treatment: ηo is slightly lower. This is considered to be because the R _FeO of Inventive Example 3 is 95% high, so that the oxidizing agent blown into the pulverizing milling is insufficient. From this, it is understood that R _{FeO is} preferably from 60 to 85%.

Inventive Example 4 is an example in which the basic unit of quicklime is changed to 20 kg/t, and Inventive Example 5 is an example in which the basic unit of quicklime is changed to 10 kg/t. The alkalinity of the slag of the invention example 4 is higher than that of the invention examples 1 to 3. The alkalinity of the slag of the invention example 5 is lower than that of the invention examples 1 to 3. However, in the inventive example 4 and the invention example 5, ΔP/ΔSi was not observed. There are big changes. In addition, the alkalinity of the invention examples and the comparative examples is the amount of CaO (total (CaO)) used for the pulverization processing (pretreatment (CaO) in the case of Table 1), and the amount of the treatment and the treatment target used for the processing for smelting. The ratio of the SiO ₂ equivalent mass (total (SiO ₂ )) of the total amount of cerium contained in the smelting (that is, "total (CaO) / total (SiO ₂ )".

6 embodiment of the invention the oxidant is iron oxide in terms of the basic unit is 31kg / t, the basic unit of quicklime 15kg / t, and the ratio of iron oxide into the above: 65% Examples R _FeO is. ΔP/ΔSi=0.13, ΔP/ΔSi>0.1 was satisfied, and the result of the pretreatment was good.

Comparative Example 1 is an example in which pretreatment was carried out under the following conditions: R _FeO = 30%, basic unit of oxidizing agent was 43 kg/t, and basic unit of quicklime was 15 kg/t. The ΔP/Si is 0.05 and is low, and the desired composition of the melt milling component cannot be obtained. This is because R _{FeO is} lower than the range of the present invention and the Fe concentration of the slag is low, and although the deuteration reaction proceeds, the dephosphorization reaction hardly proceeds.

Comparative Example 2 is an example in which R _FeO = 65%, the basic unit of the oxidizing agent is 25 kg/t, and the basic unit of quicklime is 15 kg/t. ΔP/ΔSi was 0.09, although it was higher than Comparative Example 1, but it was lower than the inventive example. We believe that this is that the oxidant base unit is insufficient relative to the Si concentration of the smelting, and the T.Fe (total iron) concentration in the slag of the remaining oxygen source is at a low level, and the dephosphorization treatment and the devolatilization treatment are not simultaneously performed.

It can be confirmed that in the pretreatment of the smelting, the conditions for the desulfurization treatment and the dephosphorization treatment for efficiently performing the Si concentration and the P concentration in the smelting at the same time are the basic units of the oxidant in terms of iron oxide conversion. 25 kg/t or more, and the ratio of iron oxide charged from above: R _FeO is 60% or more. Further, it was confirmed that R _{FeO is} preferably 80 to 100%.

[Refining step using a converter] After the pretreatment described above, each of the lyotropic milling of the inventive examples 1 to 9 and the comparative examples 1 to 3 was refined under the conditions of Table 2 to attempt to produce an extremely low phosphorus steel.

First, in each of the pulverization of the inventive examples 1 to 9 and the comparative examples 1 to 3, the quicklime as the lime-based flux and the carrier gas (nitrogen gas) were simultaneously blown from the bottom blowing nozzle of the converter into the lyotropic milling in the converter under the conditions of Table 2. And blowing gaseous oxygen to the lysed liquid surface.

After the lime-based flux is introduced into the converter, the slag is discharged in the middle to discard the slag. The alkalinity of slag during dephosphorization and blowing before intermediate slag discharge is 1.5~2.0. Thereafter, the gaseous oxygen is further blown into the lyophilization, the lyotropic milling is performed, and the refining step using the converter is ended after 15 to 20 minutes. The alkalinity of the slag during decarburization and blowing after the intermediate slag is 3.0 to 3.5.

Table 2 shows the P concentration after dephosphorization in the refining step using a converter. Further, the amount of gaseous oxygen to be charged is 40 to 60 Nm ³ /t in terms of the unit of the gas oxygen.

[Table 2]

An example in which very low phosphorus steel can be produced is taken as a qualified example (an example of "○" or "△" is shown in the item "Evaluation" in Table 2). In particular, an example in which the above-mentioned acceptable embodiment does not require the addition of Si is indicated by "○". In Inventive Examples 1 to 9, lyo milling in which the P content was less than 0.01% was obtained.

However, in Inventive Example 8, since the Si concentration and the P concentration of the pulverization after the pretreatment were high, the P concentration after the converter treatment became higher as compared with Inventive Examples 1 to 7. Further, in Inventive Example 9, the Si concentration of the pulverized after pretreatment was insufficient to perform intermediate slag discharge in the refining step using the converter. Therefore, in Inventive Example 9, Si was added in a refining step using a converter to produce slag. However, since it is the minimum amount of slag generated for intermediate slag discharge, the P concentration after the converter treatment is higher than that of Invention Examples 1 to 7.

Comparative Examples 1 to 3 are examples in which pulverization with a P content of less than 0.01% cannot be obtained (an example of "x" is shown in the item "Evaluation" in Table 2). In Comparative Examples 1 to 3, the ΔP/ΔSi of the pulverization after the pretreatment was less than 0.1.

Further, in Comparative Examples 1 and 3, although the P concentration of the pulverization after the pretreatment was more than 0.090% by mass, the Si concentration of the pulverization after the pretreatment was insufficient to reduce the P concentration in the refining step using the converter. Therefore, in Comparative Examples 1 and 3, Si was added in a refining step using a converter to generate slag in which the pulverized P was contained. However, in Comparative Examples 1 and 3, since the P concentration of the pulverization after the pretreatment was too high, it was not possible to obtain a pulverization having a P content of less than 0.01% by the refining treatment by a converter.

In Comparative Example 2, since the concentration of Si in the pulverization after the pretreatment was very high, it was not necessary to add Si in the refining step using the converter. However, since the Si concentration and the P concentration of the pulverization after the pretreatment are high, the treatment for reducing the P concentration by slag formation in the refining step using the converter may be insufficient.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, in the pretreatment of karst milling, the P concentration and the Si concentration in pulverization can be appropriately reduced without using CaF ₂ to such an extent that the dephosphorization step after the pretreatment is not hindered. The crucible phosphorus steel can be efficiently melted in the refining step. Therefore, the present invention is highly available in the steel industry.

1‧‧‧Mixed milling car

2‧‧‧ openings

3‧‧‧ spray gun

4‧‧‧Solution milling

5‧‧‧Lime flux

6‧‧‧Oxidizer

6a‧‧‧ Iron Oxide

7‧‧‧ carrier gas

8‧‧‧ slag

9‧‧‧Chute

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view schematically showing an aspect of pretreatment of a pulverizing mill used as a refining container. Figure 2 is a graph showing the ratio of iron oxide input from above the mixer car: R _FeO (%) and the amount of change in P concentration before and after pretreatment (% by mass) ΔP and the ratio of change in Si concentration (% by mass) ΔSi: ΔP Diagram of the relationship of /ΔSi.

Claims (6)

A pretreatment method for karching is to put iron oxide, gaseous oxygen and a lime-based flux into a smelting mill in a refining vessel to perform a de-slipping treatment and a dephosphorization treatment, and the pre-treatment method of the smelting milling is characterized in that: When the iron oxide-converted oxidizing agent basic unit is iron oxide of 25 kg/t or more, 60% or more of the iron oxide to be supplied is supplied to the smelting mill from the upper side of the refining vessel through the chute, and the above-mentioned input is performed. The remaining portion of the iron oxide is blown into the pulverization by a carrier gas through a spray gun; the Si content of the pulverization is adjusted to 0.05 to 0.30% by mass, and the P content is adjusted to 0.040 to 0.085% by mass. A pretreatment method for lysing according to claim 1, wherein 80 to 100% of the total amount of iron oxide to be put into the refining vessel is charged from above the refining vessel. In the pretreatment method of the solution milling according to claim 1 or 2, the P concentration before and after the pretreatment and the Si concentration before and after the pretreatment are satisfied with the following formula 1, ΔP / ΔSi > 0.1 (formula 1), ΔP: The difference between the P concentration before pretreatment and the P concentration after pretreatment, ΔSi: the difference between the Si concentration before pretreatment and the Si concentration after pretreatment. A pretreatment method of lysing according to claim 1 or 2, wherein the lime-based flux contains no CaF ₂ . A pretreatment method for solution milling according to claim 1 or 2, wherein The aforementioned refining vessel is a mixed milling vehicle. A method for producing an extremely low-phosphorus steel, characterized in that after the pretreatment method according to any one of claims 1 to 5, the intermediate slag is discharged in the converter.