KR102189097B1 - Pre-treatment method of molten iron and manufacturing method of ultra-low-tough steel - Google Patents

Abstract

The present invention provides a method for pretreatment of molten iron which improves the efficiency of dephosphorization treatment without interfering with the intermediate exclusion process of the converter by appropriately performing desiliconization in the pretreatment of molten iron. In the pretreatment method of the molten iron of the present invention, iron oxide, gaseous oxygen, and a lime-based flux are added to the molten iron in a refining vessel, and in the pretreatment method of the molten iron, the desiliconization treatment and the dephosphorization treatment are performed. In addition to adding 25 kg/t or more of iron oxide, at the time of the above addition, 60% or more of the added iron oxide was added from the top of the refining vessel, the Si content of the molten iron was 0.05 to 0.30 mass%, and the P content was 0.040. It is characterized by adjusting to 0.085 mass %.

Description

Pre-treatment method of molten iron and manufacturing method of ultra-low-tough steel

The present invention relates to a pretreatment method for efficiently reducing the Si concentration and P concentration of molten iron in cross-talk difference, and to a method for producing extremely low phosphorus steel.

When making molten steel by blowing oxygen in a converter, in order to reduce the blowing load in the converter and to make it easy to adjust the molten steel to the desired composition, silicon, phosphorus, sulfur, etc. "Colder iron preliminary treatment" to remove is usually performed.

For example, while the chartered barge from the blast furnace is still present in the tapping gutter, race gutter, or cross-talk vehicle, as a refining agent, lime-based flux, oxidizing agent, and/or soda-based flux, etc. It is blown with a carrier gas (for example, nitrogen, oxygen), or it is added directly from above, and silicon, phosphorus, sulfur, etc. are transferred to and removed from the slag.

On the other hand, the process of using a converter for the preliminary treatment of hot-metal (converter-type pre-treatment) is also making progress. The pretreatment of the converter type hot metal is generally carried out in a two furnace method using a converter for dephosphorization and a converter for decarburization, but in Patent Document 1, the converter is tilted after the dephosphorization treatment to eliminate dephosphorization slag (intermediate exclusion), and then , A method has been proposed in which a series of treatment steps in which decarburization treatment is performed and the exhausted decarburized slag is reused as a dephosphorization agent in the next dephosphorization treatment is performed in one furnace.

The method of Patent Literature 1 is a single furnace system, and a high thermal margin due to hot recycling of decarburized slag is a great advantage. However, if excessively degassed in the process of degassing and dephosphorization → intermediate exclusion → decarburization, when the concentration of silicon (Si), which is a main source of slag, in the molten iron becomes insufficient, it becomes difficult to perform intermediate exclusion. In this way, depending on the degree of desiliconization, it becomes difficult to remove the intermediate, and in order to remove the intermediate, Si is added to the molten iron in which the desilization has progressed, or SiO ₂ is newly added as a slag source to increase the silicon concentration of the molten iron again. In some cases, an uneconomical process is required.

In order to solvent the ultra-low phosphorus steel, the P concentration of the molten iron is sufficiently reduced by the preliminary treatment of the molten iron, or the molten steel subjected to the dephosphorization treatment in a converter is once poured out, and the entire amount of the dephosphorized slag is discharged out of the system, and in the same converter, Again, a process of performing decarburization and dephosphorization blowing is required. In addition, the extremely low phosphorus steel is a steel having a phosphorus content of 0.01% or less in the steel.

A series of treatments in which the molten steel subjected to dephosphorization treatment in a converter is once poured out, the entire amount of dephosphorized slag is discharged to the outside of the system, and decarburization and dephosphorization blown are performed again in the same converter, the process is complicated, and the treatment time is significantly extended. It becomes. In this way, when the ultra-low-toughness steel is melted by the method of Patent Document 1, there are many problems in the process.

Thermodynamically, the dephosphorization reaction proceeds after completion of the desiliconization reaction. Therefore, as described above, when the P concentration of the molten iron is sufficiently reduced by the pretreatment of molten iron, it is necessary to reduce the Si concentration of the molten iron to approximately zero in the pretreatment of the molten iron.

Therefore, in the prior art, in order to melt the extremely low phosphorus steel, it is necessary to reduce the P concentration of the molten iron to the vicinity of the standard concentration (P content: 0.01% or less) in the step of the pretreatment of the molten iron. However, in order to dephosphorize to the level of extremely low tensile steel, a large amount of slag is generated. In the case of performing such dephosphorization at the topedo car, since the freeboard of the topedo car (the height of the space formed on the molten iron to the furnace mouth) is small, it is difficult to reduce the P concentration of the molten iron to the vicinity of the standard concentration.

In addition, in the prior art, in the preliminary treatment of molten iron in a cross-talk vehicle, when dephosphorization is performed, a lime-based flux is blown so that the basicity of the slag to be formed is 3.0 or more, and the amount of flux is accelerated to slag CaO. Because of the reduction, the technique of adding fluorspar (CaF ₂ ) has been widely used. According to this technique, the melting point of CaO decreases, and its slag formation becomes easy.

For example, Patent Literature 2 discloses a method of simultaneously advancing desilication and dephosphorization from molten iron by simultaneously blowing a flux mainly containing CaO and an oxygen source at the same position under the following conditions. The method disclosed in patent document 2 is implemented using a flux containing fluorspar.

Total oxygen supply rate V _O2 (Nm ³ /min.T)≥2.25[%Si] ₀ -0.03

However, [%Si] ₀ = initial [Si] concentration

However, the addition of fluorspar (CaF ₂ ) increases the concentration of fluorine (F) in the formed slag. In recent years, the elution of fluorine into the environment from civil engineering and construction materials using slag as a raw material has become a problem, and the Ministry of Environment has also established regulations on fluorine in slag. In addition, since fluorine in the slag adversely affects the refractory material of the container used for the pretreatment, it is preferably not present.

In Patent Document 3, as a preliminary treatment method for molten iron that does not use fluorspar and performs degassing and dephosphorization, a lime-based flux and an oxidizing agent are blown into the molten iron in a refining vessel until the rate of deregulation is 90%. In the meantime, a method has been proposed in which the main amount of flux is added to the molten iron, and the final basicity 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, it is difficult to increase the T.Fe (Total Fe) concentration of the tops lug, so that the siliconization and dephosphorization do not proceed simultaneously, and the Si concentration of the molten iron after treatment Is roughly zero, and the amount of slag becomes vast.

In addition, Patent Document 4 dephosphorylates the molten iron by adding an iron oxide source to the molten iron held in the molten iron holding container from the upper side of the bath surface and by blowing a maelytic agent mainly containing CaO under the bath surface, A method of manufacturing is disclosed. The method disclosed in Patent Document 4 is characterized in that the iron oxide source is added so that the injection area of the iron oxide source on the bath surface covers 40% or more of the ejection area of the bath surface of the maeyong agent at an area ratio. I am doing it. The method disclosed in Patent Document 4 can omit the maeyongje containing a fluorine source such as fluorspar.

Patent document 4 is a single furnace system like the method of patent document 1, and involves an intermediate exclusion process. Patent Literature 4 teaches that the silicon concentration in the molten iron is reduced to a predetermined level by the desiliconization treatment in the molten iron port, and then the produced slag is removed, but specifically describes a means for solving the technical problem of the intermediate removal process described above. Does not teach.

Japanese Patent Laid-Open Publication No. Hei 05-247511 Japanese Patent Laid-Open No. Hei 02-93011 Japanese Patent Application Publication No. 2001-152226 Japanese Patent Application Publication No. 2001-288507

In the case of carrying out preliminary treatment for hot-metal heating and then performing intermediate removal, in particular, in the case of performing the dephosphorization method according to the one-furnace method disclosed in Patent Document 1, if excessively degassed, the silicon concentration in the hot-water metal becomes insufficient, making intermediate removal difficult. There is.

In addition, in the case of the molten iron treatment method including the intermediate exclusion process, the slag having a high phosphorus concentration generated by the dephosphorization blowing process cannot be completely removed by the intermediate exclusion process, and the slag remains after the intermediate exclusion process. For this reason, it is difficult to solvent the extremely low tensile steel with the dephosphorization method by the single furnace method described above.

Therefore, the present invention is based on the current situation of the prior art, in the preliminary treatment of the molten iron, before the molten iron treatment by the converter, while leaving a silicon concentration sufficient to perform the intermediate exclusion process in the converter. It is an object of the present invention to provide a pretreatment method and a solvent method for ultra-low phosphorus steel to improve the efficiency of dephosphorization treatment and desiliconization treatment in a smelting process by reducing the P concentration in molten iron.

In addition, it is an object of the present invention to efficiently reduce the P concentration and Si concentration of the molten iron without using CaF ₂ in the pretreatment of molten iron.

In the preliminary treatment of the molten iron, the inventors of the present invention believe that if the Si concentration and P concentration of the molten iron can be appropriately reduced by simultaneously proceeding the desulfurization treatment and the dephosphorization treatment in the pretreatment of the molten iron, the solvent efficiency of the ultra-low phosphorus steel is improved in the subsequent converter refining, Based on the idea that the overall efficiency of the refining process is improved, a method of solving the above problem has been studied.

As a result, by pre-treating the molten iron so that the Si content is 0.05 to 0.30 mass% and the P content is 0.040 to 0.085 mass%, and by combining it with an intermediate exclusion step in a converter, the overall efficiency of the refining process is improved. I could figure it out.

In addition, in the pretreatment of the molten iron, the present inventors intensively studied the conditions under which the desilization treatment and the dephosphorization treatment of the molten iron are simultaneously performed with lime and iron oxide-based flux.

As a result, it was found that the change (ΔP/ΔSi) of the Si content and the P content of the molten iron before and after the pretreatment is preferably more than 0.1 in order to make the Si content and P content of the molten iron after the pretreatment into the above ranges.

As described above, in the pretreatment of the molten iron in the refining vessel, if the input amount of iron oxide and the method of introducing iron oxide are appropriate, the Si concentration and P concentration of the molten iron after the pretreatment are converted to a component composition suitable for the next refining process, such as a refining process by a converter. I found that I can adjust it.

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

(1) In the pretreatment method of molten iron in which iron oxide, gaseous oxygen, and lime-based flux are added to the molten iron in a refining vessel, and desiliconization treatment and dephosphorization treatment are performed,

With the addition of more than 25kg/t of iron oxide as an oxidizing agent in terms of iron oxide,

At the time of the above addition, 60% or more of the iron oxide to be added is added from the top of the refining container,

A method for pretreatment of molten iron, wherein the Si content of the molten iron is adjusted to 0.05 to 0.30 mass%, and the P content is adjusted to 0.040 to 0.085 mass%.

(2) The pretreatment method for molten iron according to (1), wherein 80 to 100% of the total amount of iron oxide to be charged into the refining vessel is introduced from above the refining vessel.

(3) The pretreatment method for molten iron according to (1) or (2), wherein the P concentration before and after the pretreatment and the Si concentration before and after the pretreatment satisfy the following formula (1).

ΔP/ΔSi>0.1... (Equation 1)

However, ΔP: difference between P concentration before pretreatment and P concentration after pretreatment, ΔSi: difference between Si concentration before pretreatment and Si concentration after pretreatment

(4) The pretreatment method for molten iron according to any one of (1) to (3), wherein the lime-based flux does not contain CaF ₂ .

(5) The pretreatment method for molten iron according to any one of (1) to (4), wherein the refining container is a cross-talk difference.

(6) After the preliminary treatment method according to any one of (1) to (5), intermediate exhaust removal from a converter is performed.

According to the present invention, since the pretreatment can be performed so as not to interfere with the intermediate exclusion step in the converter, by using the pretreatment method of the present invention, it is possible to efficiently solvent the ultra-low phosphorus steel in the refining step by the converter.

Further, according to the present invention, in the pretreatment of molten iron, it is possible to efficiently reduce the P concentration and Si concentration of the molten iron without using CaF ₂ .

1 is a diagram schematically showing an aspect of a preliminary treatment of molten iron using a cross-talk vehicle as a refining container.
Fig. 2 shows the ratio of iron oxide introduced from the upper side of the crosstalk difference: R _FeO (%) and the ratio of the change amount (mass%) ΔP of the P concentration before and after pretreatment (mass%) ΔSi to the change amount (mass%) ΔSi: ΔP/ It is a figure which shows the relationship of ΔSi.

The preliminary treatment method of the molten iron of the present invention (hereinafter sometimes referred to as ``the method of the present invention'') is in which iron oxide, gaseous oxygen, and lime-based flux are added to the molten iron in the refining vessel, and the desiliconization treatment and dephosphorization treatment are performed. In the pretreatment method of the chartered iron,

(i) With the addition of 25 kg/t or more of iron oxide as an oxidizing agent in terms of iron oxide,

(ii) At the time of the above addition, 60% or more of the iron oxide to be added is added from the top of the refining container,

(iii) The Si content of the molten iron is set to 0.05 to 0.30 mass%, and the P content is adjusted to 0.040 to 0.085 mass%.

It features.

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

The molten iron targeted by the pretreatment method of the present invention is not limited to a specific component composition as long as it is Si: 0.80 mass% or less and P: 1.200 mass% or less, and specifically, molten iron having a common component composition, for example, a blast furnace And molten iron melted in an electric furnace.

Since the pretreatment of the molten iron is carried out by using a cross-talk car mainly used for conveying the molten iron to the refining process as a refining container, the pre-treatment of the hot-metal using the cross-talk car as a refining container will be described, but the refining container is It is not limited to a car, and it is a container (for example, a molten iron pot etc.) for conveying the molten iron to the next refining process, and what is necessary is just the container which can perform preliminary refining.

In Fig. 1, an aspect of the preliminary treatment of molten iron using a cross-talk car as a refining container is schematically shown. As shown in Fig. 1, the lance 3 is immersed in the molten iron 4 from the opening 2 of the cross-talk vehicle 1, and the lime-based flux 5 and/or the oxidizing agent 6 (iron oxide) is transferred to the carrier. It is conveyed by gas 7 (gas oxygen), and it blows into the molten iron 4 from the lance 3 for the required time.

Si and P in the molten iron 4 are oxidized and transferred to the slag 8, and the preliminary treatment (desulfurization treatment and dephosphorization treatment) of the molten iron 4 proceeds. For example, when the Si concentration of the molten iron reaches 0.05 to 0.30 mass%, the pretreatment is stopped, the crosstalk vehicle 1 is tilted, and the generated slag 8 is discharged out of the crosstalk vehicle 1. Subsequently, the pretreatment with the lime-based flux 5 and the oxidizing agent 6 is resumed.

In the method of the present invention, when iron oxide is introduced into the molten iron as an oxidizing agent in the preliminary treatment of molten iron (desulfurization treatment and dephosphorization treatment), the iron oxide 6a of at least 60% of the iron oxide to be introduced is supplied from the upper side of the crosstalk vehicle, that is, the opening It is put into the molten iron 4 from the chute 9 arranged in (2).

The iron oxide source is, for example, mill scale, sintered ore, iron ore, and sintered dust. The lime-based flux may be CaO alone, or may be a mixture of calcium carbonate (CaCO ₃ ) and converter slag in which the main component is CaO. However, as described above, it is preferable not to use fluorspar in consideration of the adverse effect on the external environment and the refractory of the container used for the preliminary treatment.

In the method of the present invention, 25 kg/t or more of iron oxide as an oxidizing agent in terms of iron oxide is added to the molten iron, but this point will be described later. In addition, the input amount of iron oxide is the total mass of iron oxide injected into the molten iron charged in the cross-talk vehicle. In addition, "the basic unit of oxidizing agent in terms of iron oxide" is a mass obtained by converting the total mass of oxygen injected into 1t of molten iron in the pretreatment step of molten iron into FeO.

In order to advance the dephosphorization reaction at the same time as the desulfurization reaction, it is necessary to increase the oxygen potential at the reaction interface between the molten iron and the slag while lowering the Si activity of the molten iron. It is thought that by adding iron oxide from the top, it is possible to maintain a high Fe concentration in the slag. However, since the reaction efficiency is lower than that of direct suction into the molten iron, the reaction efficiency is reduced even if the entire amount of iron oxide is simply added upward. It only degrades, and neither the desilication treatment nor the dephosphorization treatment becomes insufficient.

Therefore, in the pretreatment of the molten iron, in order to maximize both the desiliconization efficiency and the dephosphorization efficiency, it is necessary to clarify how much iron oxide is to be added in the basic unit of the oxidizing agent, and at what ratio to be added upwards. As a matter of course, there is no quantitative review and consideration on the input of iron oxide into the molten iron, and of course, no quantitative guidelines are shown.

The inventors of the present invention conducted a quantitative examination with respect to the introduction of iron oxide into the molten iron, and found the basic unit of the oxidizing agent in terms of iron oxide of the iron oxide introduced into the molten iron and the ratio (%) of iron oxide to be introduced from the upper side of the molten iron.

In Fig. 2, the ratio of iron oxide introduced from the upper side of the crosstalk difference: R _FeO (%) and the ratio of the change amount (mass%) of the P concentration before and after the pre-treatment ΔP and the change amount (mass%) of the Si concentration (ΔSi): ΔP/ It shows the relationship of ΔSi. In FIG. 2, the change in ΔP/ΔSi when 35 kg/t is added as an oxidizer basic unit in terms of iron oxide and iron oxide in the molten iron and ΔP/ΔSi when 25 kg/t is added as an oxidizer in terms of iron oxide. Show.

As the dephosphorization proceeds simultaneously with the degasification, ΔP/ΔSi increases. In order to solvent the ultra-low tough steel in the converter process, since it is preferable to remove P as much as possible while leaving Si to some extent, the larger ΔP/ΔSi is preferable.

Among the above-described molten irons, it is assumed that molten iron containing C: 4.50 to 4.70% by mass, Si: 0.50 to 0.60% by mass, and P: 0.100 to 0.120 becomes an object of the pretreatment method of the present invention. After pre-treatment of such molten iron to reduce the Si content of the molten iron to 0.2% by mass, ΔP/ΔSi>0.1 is preferable in order to solvent the ultra-low tough steel in the converter process. Therefore, in the method of the present invention, ΔP/ΔSi>0.1 was used as the evaluation criterion.

With the increase of R _FeO (%), ΔP/ΔSi also increases, and when the oxidizer basic unit is 35 kg/t, R _FeO =60% and ΔP/ΔSi >0.1, and R _FeO ≥70%, R _FeO is increased, and ΔP/ΔSi is 0.14 to 0.18.

From this, the conditions necessary for simultaneously advancing the desiliconization treatment and the dephosphorization treatment are R _FeO ≥60%, preferably R _FeO ≥70%. The upper limit of R _FeO is 100%, but when it exceeds 85%, it is preferable from the viewpoint of securing the required ΔP/ΔSi, but since the reaction efficiency η ₀ of the charged oxygen defined by the following formula decreases, taking this point into account, R Set _FeO appropriately.

η ₀ ＝{(ΔP×80/62+ΔSi×32/28)×1/100}/(Intrinsic unit of iron oxide input×1/1000×C ₀ )

Here, iron oxide input amount basic unit: iron oxide input amount (kg) / molten dose (t);

ΔP: P concentration before pretreatment—P concentration after pretreatment;

?Si: Si concentration before pretreatment—Si concentration after pretreatment;

C ₀ : Ratio of oxygen in iron oxide (oxygen mass in iron oxide/total iron oxide mass)

The reaction efficiency η ₀ reflects the amount of oxygen reacted with Si and P in the input oxygen. Most of the Si component and the P component in the converter slag are diphosphorus pentoxide (P ₂ O ₅ ) and silica (SiO ₂ ). The amount of oxygen reacted with Si and P in the charged oxygen can be calculated from the change in P concentration and Si concentration before and after the pretreatment, and the amount of iron oxide used in the pretreatment. Therefore, the reaction efficiency η ₀ can be defined from ΔP, ΔS, and the basic unit of the iron oxide input amount.

When the oxidizing agent basic unit is 25 kg/t, ΔP/ΔSi decreases, and even if R _FeO ≥60%, ΔP/ΔSi <0.1 may be obtained. It is considered that this is a position where the concentration of T.Fe in the slag of the excess oxygen source is low because the oxidizing agent is insufficient compared to the Si concentration of the molten iron, and the dephosphorization treatment did not proceed at the same time as the desulfurization treatment.

In the method of the present invention, based on the results shown in Fig. 2, it is preferable that the iron oxide injected into the molten iron is 30 kg/t or more in terms of an oxidizing agent in terms of iron oxide. More preferably, it is 35 kg/t or more. In addition, the upper limit of the iron oxide to be introduced into the molten iron is not particularly limited unless it is limited to the scale of the facility for carrying out the pretreatment method of the present invention. If it is a general toffee car, the upper limit of iron oxide added to the molten iron may be 80 kg/t.

As described above, in the pretreatment of the molten iron, the condition for efficiently simultaneously proceeding the desiliconization treatment and the dephosphorization treatment is that the iron oxide injected into the molten iron is 25 kg/t or more, preferably 30 kg/t or more, as an oxidizing agent in terms of iron oxide. , More preferably 35 kg/t or more, and the ratio of iron oxide added from the top of the molten iron R _FeO It is 60% or more, preferably 70% or more.

In particular, it is preferable that the ratio R _FeO of iron oxide injected from above the molten iron is 80 to 100%. Also from the viewpoint of making the facility for blowing into the molten iron 4 from the lance 3, it is preferable to make the R _FeO 80 to 100%.

[Example]

Next, examples of the present invention will be described, but the conditions in the examples are an example of a condition employed to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example. . The present invention is capable of adopting various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention. In the following examples, the molten iron degassed from the blast furnace is degassed and dephosphorylated by a preliminary treatment step, and then the molten iron in the preliminary treatment step is refined using a converter and further dephosphorization is performed.

[Pre-treatment process of chartered iron]

The molten iron (Si: 0.54% by mass, P: 0.118% by mass, C: 4.6% by mass) discharged from the blast furnace is charged into the cross-talk vehicle, and returned to a pretreatment plant equipped with a lime-based flux and an oxidizing agent suction device, under various conditions. Below, the preliminary treatment (desulfurization treatment and dephosphorization treatment) of molten iron was performed.

Five charges were carried out under each condition, and the average value was calculated. At the first charge, chartered ships charged to cross-talk vehicles are 260 to 280 tons. The procedure of the preliminary treatment (desulfurization treatment and dephosphorization treatment) is as follows.

First, iron oxide as an oxidizing agent and quicklime as a lime-based flux are blown into a molten iron from a lance with a carrier gas (oxygen) (see Fig. 1). Its flow rate is 600 to 800 Nm ³ /hour. Simultaneously with suction, a required amount of iron oxide is introduced into the molten iron from above the cross-talk vehicle (see Fig. 1). After iron oxide is introduced from the upper side of the cross-talk vehicle, suction of iron oxide and quicklime and suction of oxygen (carrier gas) are continued, the molten iron is stirred, and the pretreatment is terminated after 20 to 30 minutes elapse.

Table 1 shows the results of the preliminary treatment (desulfurization treatment and dephosphorization treatment) of molten iron. In Table 1, the reaction efficiency η ₀ of oxygen that contributed to the desiliconization treatment and the dephosphorization treatment (the definition formula, reference) is one index for evaluating the efficiency of the pretreatment of molten iron.

Inventive Examples 1 to 6 are examples in which preliminary treatment (desulfurization treatment and dephosphorization treatment) of molten iron was performed under the conditions of the present invention. Inventive Examples 1 to 5 are examples in which 35 kg/t of iron oxide was added as an oxidizing agent in terms of iron oxide, and 10 to 20 kg/t of quicklime was added as a basic unit of quick lime. In Inventive Examples 1 to 5, the ratio of iron oxide added from above the crosstalk vehicle: R _FeO is 70 to 95%.

Inventive Examples 1 to 3 satisfy ΔP/ΔSi>0.1, and the result of the preliminary treatment was good. The ΔP/ΔSi of Inventive Example 3 is a position higher than the ΔP/ΔSi of Inventive Examples 1 and 2, but the oxygen efficiency contributing to the de-siliconization treatment and dephosphorization treatment: η ₀ is a position slightly lower. This is considered to be because the oxidizing agent blown into molten iron was insufficient because R _FeO in Inventive _Example 3 was at a high position of 95%. From this, it can be seen that R _FeO is preferably 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. In Inventive Example 4, the basicity of slag is higher than Inventive Examples 1 to 3, and In Inventive Example 5, the basicity of slag is lower than Inventive Examples 1 to 3, but in Inventive Examples 4 and 5, ΔP/ΔSi is large. There is no change. In addition, the basicity of Inventive Examples and Comparative Examples is the mass of the sum of the amount of silicon used for the treatment of molten iron (preliminary treatment in the case of Table 1) and the amount of silicon contained in the target molten iron in terms of SiO ₂ (total (SiO ₂ )) to the amount of CaO (total (CaO)) used for the treatment of molten iron (that is, “total (CaO)/total (SiO ₂ )”).

Inventive Example 6 is an example in which the basic unit of oxidizing agent in terms of iron oxide is 31 kg/t, the basic unit of quicklime is 15 kg/t, and the ratio of iron oxide injected from the top: R _FeO is 65%. ΔP/ΔSi=0.13, ΔP/ΔSi>0.1 is satisfied, and the result of the preliminary treatment is good.

Comparative Example 1 is an example in which preliminary treatment was performed under the conditions of R _FeO =30%, oxidizing agent basic unit 43 kg/t, and quicklime basic unit 15 kg/t. ΔP/Si was as low as 0.05, and a preferable composition of the molten iron was not 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 the desiliconization reaction proceeds, but the dephosphorization reaction hardly proceeds.

Comparative Example 2 is an example in which R _FeO =65%, oxidizing agent basic unit 25 kg/t, and quicklime basic unit 15 kg/t. ΔP/ΔSi is 0.09, a higher position than Comparative Example 1, but a lower position than the Inventive Example. This is a position where the concentration of T.Fe (electric wire) in the slag of the excess oxygen source is low due to the lack of an oxidizing agent compared to the Si concentration of the molten iron, and it is considered that the dephosphorization treatment did not proceed at the same time as the desulfurization treatment.

As described above, in the pretreatment of the molten iron, the conditions for simultaneously and efficiently proceeding the desiliconization treatment and the dephosphorization treatment to appropriately the Si concentration and P concentration in the molten iron are 25 kg/t or more of the oxidizing agent in terms of iron oxide, and , It was confirmed that the ratio of iron oxide introduced from above: R _FeO was 60% or more. Further, it was confirmed that 80 to 100% of R _FeO is preferable.

[Refining process using a converter]

After the above-described preliminary treatment, the molten iron of each of the molten irons of Inventive Examples 1 to 9 and Comparative Examples 1 to 3 was refined under the conditions of Table 2, and an attempt was made to produce an ultra-low tough steel.

First, in each of the molten irons of Inventive Examples 1 to 9 and Comparative Examples 1 to 3, under the conditions of Table 2, quicklime as a lime-based flux was blown from the storage nozzle of the converter together with a carrier gas (nitrogen gas) into the molten iron in the converter, and gas Oxygen was injected into the liquid level of the molten iron.

After the lime-based flux was introduced into a converter, the slag was discarded by performing intermediate exclusion. The basicity of the slag at the time of dephosphorization blowing before intermediate exclusion was 1.5 to 2.0. Then, further, gaseous oxygen was blown into the molten iron, the molten iron was stirred, and after 15 to 20 minutes passed, the refining process using the converter was terminated. The basicity of the slag at the time of decarbonization blowing after the intermediate exclusion was 3.0 to 3.5.

Table 2 shows the concentration of P after dephosphorization in the refining process using a converter. In addition, the amount of gaseous oxygen added was 40 to 60 Nm ³ /t in terms of gaseous oxygen.

An example in which the ultra-low toughness steel was manufactured was taken as a passing example (an example in which “○” or “△” appeared in the item “evaluation” in Table 2). Particularly, among the examples of the pass, examples in which Si addition is not necessary is indicated by "○". In Inventive Examples 1 to 9, molten iron having a P content of less than 0.01% was produced.

However, in Inventive Example 8, since the Si concentration and P concentration in the molten iron after the preliminary treatment were high, the P concentration after the converter treatment was higher than in Inventive Examples 1 to 7. In addition, in Inventive Example 9, the Si concentration of the molten iron after the preliminary treatment was insufficient for intermediate exclusion in the refining step using a converter. Therefore, in Inventive Example 9, in order to generate slag, Si was added in the refining process using a converter. However, since it was the minimum amount of slag generation for intermediate exclusion, the concentration of P increased after the converter treatment compared with Inventive Examples 1 to 7.

In Comparative Examples 1 to 3, molten iron having a P content of less than 0.01% was not produced (an example in which "x" appeared in the item "evaluation" in Table 2). In Comparative Examples 1 to 3, ΔP/ΔSi of the molten iron after the pretreatment was less than 0.1.

In Comparative Examples 1 and 3, the P concentration of the molten iron after the preliminary treatment was more than 0.090% by mass, but the Si concentration of the molten iron after the preliminary treatment was insufficient for reducing the P concentration in the refining step using a converter. Therefore, in Comparative Examples 1 and 3, Si was added in a refining step using a converter in order to generate slag containing P in molten iron. However, in Comparative Examples 1 and 3, since the P concentration of the molten iron after the preliminary treatment was too high, the molten iron having a P content of less than 0.01% was not produced by the refining treatment using a converter.

In Comparative Example 2, since the Si concentration in the molten iron after the pretreatment was sufficiently high, it was not necessary to add Si in the refining step using a converter. However, since the Si concentration and the P concentration of the molten iron after the preliminary treatment were high, the treatment for reducing the P concentration due to slag formation in the refining step using a converter became insufficient.

As described above, according to the present invention, in the pretreatment of the molten iron, CaF ₂ is not used, and the P concentration and Si concentration in the molten iron are appropriately reduced so as not to interfere with the dephosphorization process after the pretreatment, and refining In the process, it is possible to efficiently melt ultra-low tough steel. Therefore, the present invention has high applicability in the steel industry.

1: cross-talk
2: opening
3: Lance
4: charter
5: Lime-based flux
6: oxidizing agent
6a: iron oxide
7: carrier gas
8: slag
9: suit

Claims (6)

In the pretreatment method of the molten iron in which iron oxide, gaseous oxygen, and a lime-based flux are added to the molten iron in a refining vessel, and desiliconization treatment and dephosphorization treatment are performed,
With the addition of 30kg/t or more of iron oxide as an oxidizing agent in terms of iron oxide,
At the time of the above addition, 60% or more of the iron oxide to be added is added from the top of the refining container,
A method for pretreatment of molten iron, wherein the Si content of the molten iron is adjusted to 0.05 to 0.30 mass%, and the P content is adjusted to 0.040 to 0.085 mass%. The method of claim 1,
A method for preliminary treatment of molten iron, characterized in that 80 to 100% of the total amount of iron oxide introduced into the refining vessel is introduced from above the refining vessel. The method according to claim 1 or 2,
A pretreatment method for molten iron, wherein the P concentration before and after the pretreatment and the Si concentration before and after the pretreatment satisfy the following formula (1).
ΔP/ΔSi>0.1... (Equation 1)
However, ΔP: difference between P concentration before pretreatment and P concentration after pretreatment, ΔSi: difference between Si concentration before pretreatment and Si concentration after pretreatment. The method according to claim 1 or 2,
The lime-based flux does not contain CaF ₂ , wherein the method for pretreatment of molten iron. The method according to claim 1 or 2,
The preliminary treatment method for molten iron, wherein the refining container is a cross-talk vehicle. After the preliminary treatment method according to claim 1 or 2, intermediate exhaust from the converter is performed.

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