Preliminary treatment method for molten iron

Abstract

To provide a preliminary treatment method for molten iron which promotes dephosphorization treatment by securing a heat quantity necessary for melting a lime-based solvent and a heat quantity necessary for melting a cold iron source in the dephosphorization treatment, and prevents the ejection of dephosphorization slag from a converter type vessel.SOLUTION: A preliminary treatment method for molten iron comprises repeatedly performing the following steps in order: a molten iron charging step in which a first cold iron source having molten iron and FeO is charged into a converter type vessel, a desiliconization step in which the molten iron is de-siliconized, a dephosphorization step in which a second cold iron source having FeO is charged into the converter type vessel and the molten iron is dephosphorized, and a tapping step in which the molten iron subjected to the dephosphorization step is tapped out of the converter type vessel. The dephosphorized slag produced in the previous preliminary treatment for molten iron is left in the converter type vessel, and the first cold iron source and the second cold iron source are charged into the converter type vessel so that the FeO concentration of the dephosphorized slag in the dephosphorization treatment step is 20 mass% or more and 30 mass% or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for pre-treating molten iron, in which the molten iron is dephosphorized in a converter-type vessel. [Background technology]

[0002] In the past, a steelmaking method was developed in which the molten pig iron tapped from a blast furnace is decarburized and refined in a converter to produce molten steel. In the decarburization process, a preliminary dephosphorization process is performed at the molten pig iron stage to remove some of the phosphorus in the molten pig iron, and then the decarburization process is performed in a converter. In this preliminary dephosphorization process, an oxygen source (cold iron source) such as oxygen gas or solid iron oxide is added together with a dephosphorization refining agent mainly made of lime (CaO). That is, the phosphorus in the molten pig iron is oxidized by the added oxygen source to generate phosphorus oxide, and the generated phosphorus oxide is absorbed and fixed by a CaO-based dephosphorization refining agent to advance the dephosphorization reaction. The added oxygen source reacts with the phosphorus in the molten pig iron, as well as with carbon and silicon, and the temperature of the molten pig iron rises as a result of this reaction. However, since the dephosphorization reaction thermodynamically advances at lower temperatures, a coolant is added to control the temperature of the molten pig iron after the process to around 1300°C.

[0003] Here, several methods have been proposed for performing dephosphorization by adding an oxygen source (cold iron source) during preliminary dephosphorization treatment using a converter-type refining vessel. Patent Document 1 discloses, as a method for preliminary treatment of molten pig iron, a desiliconization treatment step in which molten pig iron and a cold iron source are charged into a converter-type vessel and the molten pig iron is desiliconized, an intermediate slag removal step in which the desiliconized molten pig iron is left in the converter-type vessel and a part of the desiliconization slag generated in the desiliconization treatment step is discharged from the converter-type vessel, a dephosphorization treatment step in which the molten pig iron left in the converter-type vessel in the intermediate slag removal step is dephosphorized, and a pouring step in which the dephosphorized molten pig iron is poured from the converter-type vessel.

[0004] Patent Document 2 discloses a method for charging a cold iron source into a converter-type vessel in a molten iron pretreatment method, the method being based on the weight relationship with the molten iron charged together. Patent Document 3 discloses a method for preliminary dephosphorization of molten iron, in which a cold iron source is charged at any time between when 30% and 90% of the dephosphorization refining time has elapsed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-167015 A [Patent Document 2] JP 2013-189714 A [Patent Document 3] JP 2007-270238 A [Patent Document 4] JP 2014-159629 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the pretreatment of molten iron, the FeO concentration in the dephosphorization slag generated during the dephosphorization process is an important factor that affects the progress of the dephosphorization process. Therefore, it is necessary to properly control the FeO concentration in the dephosphorization slag during the dephosphorization process of molten iron.

[0007] However, although the techniques disclosed in Patent Documents 1 and 2 disclose the charging of a cold iron source in the desiliconization and dephosphorization treatments, they make no suggestion whatsoever regarding the control of the FeO concentration in the dephosphorization slag. Also, the technique disclosed in Patent Document 3 discloses the timing of charging a cold iron source in the dephosphorization treatment, but makes no suggestion whatsoever regarding the control of the FeO concentration in the dephosphorization slag.

[0008] In addition, in the conventional hot metal pretreatment, the hot metal and the cold iron source are charged at the same time before the desiliconization treatment. This makes it difficult to simultaneously secure the amount of heat required to melt the lime-based flux and the amount of heat required to melt the cold iron source during the dephosphorization treatment.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for pretreatment of molten iron in a dephosphorization process, which accelerates the dephosphorization process and prevents the slag from spouting out of a converter-type vessel during the dephosphorization process. [Means for solving the problem]

[0010] [1] A method for pretreatment of molten pig iron, which sequentially repeats the following steps: a molten pig iron charging step of charging molten pig iron and a first cold iron source containing FeO into a converter-type vessel; a desiliconization step of desiliconizing the molten pig iron; a dephosphorization step of charging a second cold iron source containing FeO into the converter-type vessel and dephosphorizing the molten pig iron; and a pouring step of pouring the dephosphorized molten pig iron from the converter-type vessel, wherein a post-dephosphorization slag generated in a previous molten pig iron pretreatment is left in the converter-type vessel, and the first cold iron source and the second cold iron source are charged into the converter-type vessel such that the FeO concentration of the slag during the dephosphorization step is 20% by mass or more and 30% by mass or less. [2] A method for pre-treatment of molten iron as described in [1], comprising carrying out an intermediate slag removal process prior to the dephosphorization process, in which the molten iron that has been desiliconized is left in the converter-type vessel and at least a portion of the post-desiliconization slag generated in the desiliconization process is discharged from the converter-type vessel. [3] The method for pretreatment of molten iron according to [1] or [2], wherein the charging amounts of the first cold iron source and the second cold iron source are determined based on the FeO concentration of the dephosphorization treatment slag, the FeO content of the first cold iron source, and the FeO content of the second cold iron source, such that a ratio of the charging amount of the second cold iron source to a total cold iron source charging amount, which is the total charging amount of the first cold iron source and the second cold iron source, is 5 mass% or more and 30 mass% or less. [4] The method for pretreatment of molten iron according to any one of [1] to [3], wherein the first cold iron source and the second cold iron source have an FeO content of 1 mass % or more. [5] The method for pre-treating molten iron according to any one of [1] to [4], wherein the first cold iron source and the second cold iron source have an FeO content of 1 mass% or more and are recovered products that are generated and recovered in a steelmaking process. Effect of the Invention

[0011] According to the present invention, in the dephosphorization process, it is possible to accelerate the dephosphorization and prevent the slag from spouting out of the converter-type vessel during the dephosphorization. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of an example of a converter-type vessel. [Diagram 2] FIG. 1 is a diagram showing a schematic configuration for explaining a method for pre-treating molten iron in the order of steps. [Diagram 3] FIG. 2 is a graph showing the relationship between the FeO concentration in dephosphorization slag and the phosphorus concentration in molten iron after dephosphorization treatment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 shows a schematic diagram of an example of a converter-type vessel used in a method for pre-treating molten iron. Fig. 2 shows a schematic diagram for explaining the process of the method for pre-treating molten iron in order of steps.

[0014] In the method for pre-treating molten iron according to the present invention, a converter-type vessel 1 capable of top and bottom blowing as shown in FIG. 1 is used as an example of a converter-type refining furnace. In the converter-type vessel 1, top blowing is performed by blowing oxygen gas 12 from the tip of a top-blowing lance 2 that can be raised and lowered toward the molten iron 9. Here, industrial pure oxygen is used as the oxygen gas 12. Bottom blowing is performed using a bottom-blowing tuyeres 3 provided at the bottom of the converter-type vessel 1. The bottom-blowing gas 13 is generally a gas containing oxygen gas or an inert gas such as argon gas or nitrogen gas. However, it may also be a gas that has the function of enhancing the stirring of the molten iron 9 by injecting it into the molten iron and promoting the melting of the cold iron source, or a gas that has the function of injecting a flux into the molten iron together with a carrier gas. The molten iron 9 after refining is tapped from a tapping hole 4.

[0015] In the present invention, two or more converter-type vessels 1 are used to refine the molten iron 9, at least one of which is used for the molten iron pretreatment method according to the present invention, and the remaining one is used for decarburization refining the molten iron that has been pretreated by the method of the present invention. For example, it is preferable to perform pretreatment in the converter-type vessel 1 for molten iron pretreatment, and then transfer the pretreated molten iron to a converter-type vessel for decarburization refining to perform the decarburization treatment.

[0016] The method for pretreatment of molten iron according to the present invention, which uses a converter-type vessel 1, performs the steps of molten iron charging (A), desiliconization treatment (B), dephosphorization treatment (C), and tapping (D) in that order, as shown in the flow chart of FIG. 2. By repeatedly performing these steps in the same converter-type vessel 1, efficient pretreatment of molten iron is possible.

[0017] Here, the amount of cold iron source charged into the converter-type vessel 1 for each execution of steps (A) to (D) (hereinafter referred to as the "total cold iron source charging amount") has conventionally been determined based on the results of predetermined calculations, etc., for the slag generated after the completion of dephosphorization treatment during the previous molten iron pretreatment (hereinafter simply referred to as "slag after dephosphorization treatment"), on the assumption that it is charged all at once at the stage of the molten iron charging step (A).

[0018] In addition, when the entire cold iron source based on the total cold iron source charging amount is charged at once before the desiliconization treatment step (B), both the amount of heat required to melt the lime-based flux 16 charged in the desiliconization treatment step (B) and the amount of heat required to melt the entire cold iron source are required, but it is difficult to secure the amount of heat, and a limit is set on the supply amount of the entire cold iron source. This leads to a shortage of cold iron source charging, and as a result, there is a problem that the phosphorus concentration of the molten pig iron 9 that has been subjected to the dephosphorization treatment step (C) also becomes high.

[0019] In contrast, in the present invention, the total cold iron source charging amount calculated based on the slag after dephosphorization is divided into the amount of the first cold iron source 11a charged into the converter-type vessel 1 in the molten iron charging step (A) and the amount of the second cold iron source 11b charged into the converter-type vessel 1 in the dephosphorization step (C), and the first cold iron source 11a and the second cold iron source 11b contain FeO. That is, in the present invention, of the steps (A) to (D) (see FIG. 2) that are repeatedly performed, the first cold iron source 11a containing FeO is charged in the molten iron charging step (A), and the second cold iron source 11b containing FeO is charged in the dephosphorization step (C).

[0020] In the present invention, the amount of the first cold iron source 11a and the amount of the second cold iron source 11b are calculated before the hot metal charging step (A) for the purpose of adjusting the FeO concentration of the slag during the dephosphorization treatment in the dephosphorization treatment step (C). Specifically, the amount of each of the first cold iron source 11a and the second cold iron source 11b is determined so that the FeO concentration of the slag during the dephosphorization treatment in the dephosphorization treatment step (C) is 20 mass% or more and 30 mass% or less. In addition, the "slag during the dephosphorization treatment" here means the slag at the end of the dephosphorization treatment step (C). In other words, the "FeO concentration of the slag during the dephosphorization treatment" means the FeO concentration of the slag at the end of the dephosphorization treatment step (C). The FeO concentration of the slag during the dephosphorization treatment may be calculated by a calculation formula that has been used conventionally (such as formula (6) in Patent Document 4).

[0021] In this case, the respective charging amounts of the first cold iron source 11a and the second cold iron source 11b may be determined based on the FeO concentration of the dephosphorization treatment slag 17, the FeO content of the first cold iron source 11a, and the FeO content of the second cold iron source 11b, such that the ratio of the charging amount of the second cold iron source 11b to the total charging amount of the first cold iron source 11a and the second cold iron source 11b is 5 mass% or more and 30 mass% or less.

[0022] The molten pig iron charging step (A) shown in Fig. 2 will be described. In the molten pig iron charging step (A), new molten pig iron 9 is charged from a charging ladle 14 while leaving dephosphorization slag 17 generated in the previous preliminary treatment of the molten pig iron in the converter-type vessel 1, or a first cold iron source 11a such as iron scrap is charged before the molten pig iron is charged, and then the molten pig iron 9 is charged. The first cold iron source 11a charged in advance may be iron scrap as specified in the "Unified Standard for Iron Scrap Inspection" of the Japan Iron and Steel Association, or may be directly reduced iron, cold pig iron, or other iron-based material.

[0023] The amount of the first cold iron source 11a charged in the molten iron charging step (A) (hereinafter referred to as the "first cold iron source charging amount") may be calculated before the molten iron charging step (A) based on a calculated value of the FeO concentration in the post-dephosphorization treatment slag 17 generated in the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, and the amount and FeO content of the second cold iron source 11b charged into the converter-type vessel 1 in the dephosphorization treatment step (C).

[0024] The desiliconization process (B) shown in Fig. 2 will be described. In the desiliconization process (B), the converter-type vessel 1 is placed upright, and oxygen gas 12 is supplied to the molten iron 9 via the top-blowing lance 2 to perform desiliconization. In this desiliconization process, a silicon source 15 contained in a hopper 5 and a lime-based flux 16 contained in a hopper 7 are charged into the converter-type vessel 1 via a chute 6 and a chute 8, respectively, and carbonaceous material serving as a heat source, silicon source, and iron oxide serving as an oxygen source are also charged in the same manner. From the viewpoint of dissolving a large amount of the first cold iron source 11a, it is preferable to use only oxygen gas 12 as the oxygen source for the desiliconization process, without using iron oxide, which has a large heat absorption capacity.

[0025] In this desiliconization treatment, silicon contained in the molten pig iron 9 or silicon contained in the silicon source 15 and the first cold iron source 11a that transfers to the molten pig iron through melting reacts with the oxygen source (Si+O2→SiO2) to be desiliconized, which helps to improve the reaction efficiency in the subsequent dephosphorization treatment. During this desiliconization treatment reaction, oxidation heat is generated, which increases the temperature of the molten pig iron and promotes the melting of the cold iron source 11 in the molten pig iron.

[0026] The dephosphorization process (C) shown in Fig. 2 will now be described. After the desiliconization process (B), a lime-based flux is added to the molten pig iron remaining in the converter-type vessel 1, and oxygen is blown in to provide an oxygen source, to dephosphorize the molten pig iron. The phosphorus in the molten pig iron is oxidized by the oxygen in the oxygen source supplied to form phosphorus oxides (P2O5), which are stably incorporated into the slag produced by the slag formation of the lime-based flux, thereby progressing the dephosphorization of the molten pig iron.

[0027] In the present invention, in the dephosphorization step (C), a second cold iron source 11b containing FeO is charged into the converter-type vessel 1. The amount of the second cold iron source 11b charged (hereinafter referred to as the "second cold iron source charging amount") may be calculated before the molten iron charging step (A) based on a calculated value of the FeO concentration in the post-dephosphorization slag 17 generated in the previous molten iron pretreatment, the amount and FeO content of the first cold iron source 11a charged into the converter-type vessel 1 in the molten iron charging step (A), and the FeO content of the second cold iron source 11b.

[0028] The relationship between the FeO concentration in the dephosphorization slag and the phosphorus concentration in the hot metal after dephosphorization is shown in Figure 3. As shown in Figure 3, when the FeO concentration in the dephosphorization slag is low, the dephosphorization of the hot metal does not proceed smoothly, and the phosphorus concentration in the hot metal after dephosphorization becomes high. On the other hand, when the FeO concentration in the dephosphorization slag is high, the slag becomes overoxidized, which causes the slag to erupt from the converter-type vessel and the associated shutdown of operations.

[0029] In the present invention, the FeO concentration of the dephosphorization slag 20 in the dephosphorization process (C) is adjusted to an appropriate value by charging the first cold iron source 11a in the molten iron charging step (A) and charging the second cold iron source 11b into the converter-type vessel 1 in the dephosphorization process (C). That is, the FeO concentration of the dephosphorization slag 20 in the dephosphorization process (C) is adjusted to 20% by mass or more and 30% by mass or less by charging the first cold iron source 11a and the second cold iron source 11b into the converter-type vessel 1. The charging amounts of the first cold iron source 11a and the second cold iron source 11b may be determined so that the ratio of the second cold iron source charging amount to the total cold iron source charging amount is 5% by mass or more and 30% by mass or less.

[0030] Specifically, the amount of the first cold iron source charged may be determined based on a calculated value of the FeO concentration in the dephosphorization treatment slag 17 generated during the previous hot metal pretreatment, the FeO content of the first cold iron source 11a, the FeO content of the second cold iron source 11b, and the amount of the second cold iron source 11b charged in the dephosphorization treatment step (C). In this case, the amount of the first cold iron source charged may be determined so that the ratio of the amount of the second cold iron source charged to the total amount of the cold iron sources charged is 5% by mass or more and 30% by mass or less.

[0031] In this case, the amount of the first cold iron source 11a charged may be determined by adjusting the amount of the first cold iron source 11a to an optimal value relative to the calculated FeO concentration in the post-dephosphorization slag 17 generated in the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, the FeO content of the second cold iron source 11b, and the amount of the second cold iron source 11b charged in the dephosphorization process (C), so that the FeO concentration of the dephosphorization slag 20 in the dephosphorization process (C) is 20% by mass or more and 30% by mass or less.

[0032] The amount of the second cold iron source charged may be determined based on the calculated FeO concentration in the dephosphorization slag 17 generated during the previous hot metal pretreatment, the FeO content of the first cold iron source 11a, the amount of the first cold iron source 11a charged in the hot metal charging step (A), and the FeO content of the second cold iron source 11b. In this case, the amount of the second cold iron source charged may be determined so that the ratio of the amount of the second cold iron source charged to the total amount of the cold iron sources charged is 5% by mass or more and 30% by mass or less.

[0033] In this case, the amount of the second cold iron source 11b charged may be determined by adjusting the amount of the second cold iron source 11b to an optimal value relative to the calculated FeO concentration in the post-dephosphorization slag 17 generated in the previous molten iron pretreatment, the FeO content of the first cold iron source 11a, the amount of the first cold iron source 11a charged in the molten iron charging step (A), and the FeO content of the second cold iron source 11b, so that the FeO concentration of the dephosphorization slag 20 in the dephosphorization step (C) is 20% by mass or more and 30% by mass or less.

[0034] After the first and second cold iron source charging amounts have been determined, the amount of oxygen gas 12 added may be adjusted in order to precisely adjust the FeO concentration of the dephosphorization slag 20 to 20% by mass or more and 30% by mass or less in the dephosphorization process (C). In this case, in the desiliconization process (B) and the dephosphorization process (C), oxygen gas 12 with an adjusted amount added may be added via the top blowing lance 2 together with a dephosphorization refining agent mainly composed of lime (CaO). At this time, the height of the top blowing lance 2 and the blowing speed of the oxygen gas 12 may be adjusted to further adjust the FeO concentration of the dephosphorization slag 20.

[0035] Therefore, in the dephosphorization process (C), the FeO concentration of the dephosphorization slag 20 for dephosphorization of the molten pig iron can be ensured, and the dephosphorization of the molten pig iron can be efficiently promoted. Furthermore, by adjusting the FeO concentration of the dephosphorization slag 20, it is possible to prevent the deterioration of the phosphorus concentration of the molten pig iron in the dephosphorization process (C) and to prevent the occurrence of problems such as the slag spouting out of the converter-type vessel during the dephosphorization process.

[0036] Furthermore, in order to accurately adjust the FeO concentration of the dephosphorization slag 20 in the dephosphorization process (C), the FeO concentration of the dephosphorization slag 20 may be measured during the dephosphorization process (C), and the amount of the third cold iron source 11c containing FeO to be additionally charged into the converter-type vessel 1 may be calculated based on the measured FeO concentration of the dephosphorization slag 20. Then, the third cold iron source 11c may be charged into the converter-type vessel 1 based on the calculated additional amount of the third cold iron source 11c (hereinafter referred to as the "third cold iron source charging amount").

[0037] The FeO content of the first cold iron source 11a, the second cold iron source 11b, and the third cold iron source 11c containing FeO is preferably 1 mass% or more. This is because the dephosphorization reaction of molten iron can be promoted by making the FeO content at least 1 mass%. Furthermore, by individually recovering by-products containing FeO from among by-products generated in the iron-making process and using them as the first cold iron source 11a, the second cold iron source 11b, and the third cold iron source 11c, it becomes possible to supply the first cold iron source 11a, the second cold iron source 11b, and the third cold iron source 11c at low cost.

[0038] The tapping step (D) shown in Fig. 2 will now be described. In the tapping step (D), when the phosphorus concentration in the molten iron has decreased to a predetermined value through the dephosphorization treatment step (C), the converter-type vessel 1 is tilted to the side where the tapping port 4 is installed, and the molten iron in the converter-type vessel 1 is tapped into a molten iron holding vessel (not shown).

[0039] As described above, the method for pretreatment of molten pig iron according to the present invention can accelerate the dephosphorization of molten pig iron in the dephosphorization step. In addition, since the FeO concentration of the slag during the dephosphorization step can be adjusted, it is possible to prevent the phosphorus concentration of the molten pig iron from decreasing in the dephosphorization step (C) and to prevent problems such as the slag spouting out of the converter-type vessel during the dephosphorization step.

[0040] The method for pre-treating molten iron according to the present invention can be applied to various smelting processes as long as a converter-type vessel is used for pre-treating molten iron. For example, the method for pre-treating molten iron shown in the present embodiment may include an intermediate slag removal step.

[0041] In this case, specifically, before the dephosphorization treatment step (C), an intermediate slag removal step may be carried out in which the molten iron 9 that has been desiliconized in the desiliconization treatment step (B) is left in the converter-type vessel 1 and at least a portion of the post-desiliconization slag generated in the desiliconization treatment step (B) is discharged from the converter-type vessel 1.

[0042] By carrying out the intermediate slag removal step after the desiliconization step (B), the desiliconization slag with low basicity containing a large amount of SiO2 generated during the desiliconization step is discharged from the converter-type vessel 1. The intermediate slag removal of the desiliconization slag is effective in reducing the amount of lime-based flux used to adjust the slag basicity to an appropriate level in the next dephosphorization step. In addition, in the case of a hot metal pretreatment method in which a large amount of the dephosphorization slag generated during the previous hot metal pretreatment is left in the furnace, the desiliconization is carried out to prevent rephosphorization from the slag to the hot metal, so that the phosphoric acid concentration in the desiliconization slag becomes higher than in the past. Therefore, by carrying out the intermediate slag removal step, the amount of phosphoric acid contained in the desiliconization slag can be reduced, and the dephosphorization in the next dephosphorization step (C) can be further promoted.

[0043] In addition, the cold iron source containing FeO and the by-products (recovered materials) recovered in the iron-making process may be previously separated based on the FeO content and applied to the molten iron pretreatment method according to the present invention. Examples of the by-products (recovered materials) recovered in the iron-making process and containing FeO include sintered powder.

[0044] In the molten iron pretreatment method according to the present invention, the respective charging amounts of the first cold iron source 11a and the second cold iron source 11b are preferably determined so that the ratio of the charging amount of the second cold iron source 11b to the total charging amount of the first cold iron source 11a and the second cold iron source 11b is 5 mass % or more and 30 mass % or less.

[0045] That is, by adjusting the charging amount of the first cold iron source 11a to be greater than the charging amount of the second cold iron source 11b, it becomes possible to smoothly charge these cold iron sources in both the molten iron charging step (A) and the dephosphorization treatment step (C). Specifically, a large amount of the first cold iron source 11a can be charged in the molten iron charging step (A) where the converter-type vessel 1 can be tilted, while an appropriate amount of the second cold iron source 11b can be charged taking into account charging only from above in the dephosphorization treatment step (C) where the converter-type vessel 1 is maintained in an upright position.

[0046] Furthermore, by charging a large amount of the first cold iron source 11a during the molten iron charging step (A), in the next desiliconization treatment step (B), the melting of the first cold iron source 11a can be promoted by supplying a large amount of oxygen gas 12 via the top blowing lance 2. Therefore, in the subsequent dephosphorization treatment step (C), a small amount of the second cold iron source 11b can be smoothly melted, and the dephosphorization treatment can be further promoted.

[0047] Therefore, by adjusting the charging amount of the first cold iron source 11a to be greater than the charging amount of the second cold iron source 11b, the charging of the cold iron source can be smoothly performed in both the molten iron charging step (A) and the dephosphorization treatment step (C), and the melting of all the cold iron sources (the first cold iron source 11a and the second cold iron source 11b) can be further promoted in the stage of the dephosphorization treatment step (C), thereby further promoting the dephosphorization treatment.

[0048] Here, the "cold iron source" in the present invention may be interpreted in a broad sense to include iron scrap, reduced iron, iron oxide, and the like. EXAMPLES

[0049] Next, the results of the method for pretreating molten iron according to the present invention, which was carried out by charging molten iron into a converter-type vessel, will be described. In the example, the ratio of the amount of the first cold iron source charged to the amount of the second cold iron source charged to the total amount of cold iron source charged, calculated based on the slag after dephosphorization treatment generated in the previous molten iron pretreatment, was changed in the method for pretreating molten iron. Cold iron sources with an FeO content of 1 mass% or more were used as the first cold iron source and the second cold iron source. The results of the example are shown in Table 1.

[0050] [Table 1]

[0051] As shown in Table 1, in the examples, the FeO concentration (mass%) of the slag during the dephosphorization process, the phosphorus concentration (mass%) of the molten iron after the dephosphorization process, and the incidence (%) of problems such as spouting from the converter-type vessel were confirmed.

[0052] Comparative Example 1 shows an example of the prior art where the cold iron source was charged at one time in the molten iron charging process. That is, in Comparative Example 1, the amount of cold iron source charged in the molten iron charging process (shown as "100" in Table 1) corresponds to the total amount of cold iron source charged. It also shows that the FeO concentration of the slag during the dephosphorization treatment was 15 mass%. The results of the following examples (Inventive Examples 1-2, Comparative Examples 2-5) were evaluated based on the results of Comparative Example 1.

[0053] In Comparative Example 2, the ratio of the second cold iron source charging amount to the total cold iron source charging amount was set to more than 30% by mass and not more than 40% by mass, so that the FeO concentration in the slag during the dephosphorization treatment was more than 30% by mass and not more than 40% by mass, and the phosphorus concentration in the molten pig iron after the dephosphorization treatment process could be reduced by 0.003% by mass. However, since the FeO concentration in the slag during the dephosphorization treatment was increased compared to the conventional example (Comparative Example 1), the occurrence rate (%) of troubles such as blowing out from the converter-type vessel was 30%.

[0054] In Comparative Example 3, the ratio of the second cold iron source charging amount to the total cold iron source charging amount was set to less than 5 mass%, so that the FeO concentration in the slag during the dephosphorization treatment was 18 mass%, which was a slight increase in the FeO concentration compared to the conventional example (Comparative Example 1). However, the phosphorus concentration in the molten pig iron after the dephosphorization treatment process was only slightly decreased by 0.001 mass%.

[0055] Comparative Examples 4 and 5 show the results of charging the cold iron source all at once in the molten iron charging process, as in the conventional example (Comparative Example 1). As shown in Table 1, the FeO concentration of the slag during the dephosphorization process was 5 mass% in Comparative Example 4 and 10 mass% in Comparative Example 5, which were lower than the conventional example (Comparative Example 1). As a result, the phosphorus concentration of the molten iron after the dephosphorization process increased by 0.015 mass% in Comparative Example 4 and 0.010 mass% in Comparative Example 5.

[0056] In Example 1, the ratio of the second cold iron source charge to the total cold iron source charge was set to 10% by mass or more and 30% by mass or less, so that the FeO concentration of the slag during the dephosphorization treatment could be adjusted to 20% by mass or more and 30% by mass or less. As a result, the phosphorus concentration of the molten pig iron after the dephosphorization treatment process could be reduced by 0.003% by mass, and the occurrence of problems during the dephosphorization treatment process could be prevented.

[0057] In Example 2, the ratio of the second cold iron source charge to the total cold iron source charge was set to 5% by mass or more and 20% by mass or less, and the FeO concentration of the slag during the dephosphorization treatment was adjusted to 25% by mass. As a result, the phosphorus concentration of the molten pig iron after the dephosphorization treatment process was reduced by 0.003% by mass, and the occurrence of problems during the dephosphorization treatment process was also prevented.

[0058] In Example 3, the ratio of the second cold iron source charge to the total cold iron source charge was set to 15% by mass or more and 30% by mass or less, and the FeO concentration of the slag during the dephosphorization treatment was adjusted to 22% by mass or more and 30% by mass or less. As a result, the phosphorus concentration of the molten pig iron after the dephosphorization treatment process was reduced by 0.003% by mass, and the occurrence of problems during the dephosphorization treatment process was also prevented. [Explanation of symbols]

[0059] 1 Converter-type vessel 2 Top blowing lance 3 Bottom-blown tuyere 4. Tap hole 5, 7 Hopper 6, 8 shots 9. Molten Iron 11a 1st cold iron source 11b 2nd cold iron source 11c Third cold iron source 12 Oxygen gas 13 Bottom blown gas 14 Charging pot 15 Silicon source 16 Lime-based media 17 Slag after dephosphorization 20 Slag during dephosphorization treatment

Claims

1. a molten iron charging step of charging molten iron and a first cold iron source containing FeO into a converter-type vessel; a desiliconization treatment step for desiliconizing the molten iron; a dephosphorization treatment step of charging a second cold iron source containing FeO into the converter-type vessel and performing a dephosphorization treatment on the molten iron; a tapping step of tapping the dephosphorized molten iron from the converter-type vessel; A method for pre-treating molten iron, comprising the steps of: The dephosphorization slag generated during the previous hot metal pretreatment is allowed to remain in the converter-type vessel, The first cold iron source and the second cold iron source are charged into the converter-type vessel so that the FeO concentration of the slag during the dephosphorization treatment in the dephosphorization treatment step is 20% by mass or more and 30% by mass or less. A method for pre-treating molten iron.

2. 2. The method for pre-treatment of molten iron as claimed in claim 1, further comprising the step of: before the dephosphorization treatment step, carrying out an intermediate slag removal step in which the molten iron that has been subjected to the desiliconization treatment is left in the converter-type vessel and at least a portion of the post-desiliconization slag generated in the desiliconization treatment step is discharged from the converter-type vessel.

3. 3. The method for pre-treating molten iron according to claim 1 or 2, wherein the charging amounts of the first cold iron source and the second cold iron source are determined based on an FeO concentration of the slag after the dephosphorization treatment, an FeO content of the first cold iron source, and an FeO content of the second cold iron source, such that a ratio of the charging amount of the second cold iron source to a total charging amount of the first cold iron source and the second cold iron source is 5 mass % or more and 30 mass % or less.

4. 3. The method for pre-treating molten iron according to claim 1, wherein the first cold iron source and the second cold iron source have an FeO content of 1 mass % or more.

5. The method for pre-treating molten iron according to claim 3 , wherein the first cold iron source and the second cold iron source have an FeO content of 1 mass % or more.

6. 3. The method for pre-treating molten iron according to claim 1 or 2, wherein the first cold iron source and the second cold iron source have an FeO content of 1 mass % or more and are recovered products that are generated and recovered in a steelmaking process.

7. The method for pre-treating molten iron according to claim 3 , wherein the first cold iron source and the second cold iron source are recovered materials having an FeO content of 1 mass % or more and generated and recovered in a steelmaking process.

8. The method for pre-treating molten iron according to claim 4, wherein the first cold iron source and the second cold iron source are recovered materials having an FeO content of 1 mass % or more and generated and recovered in a steelmaking process.

9. The method for pre-treating molten iron according to claim 5 , wherein the first cold iron source and the second cold iron source are recovered materials having an FeO content of 1 mass % or more and generated and recovered in a steelmaking process.

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