Converter refining method

Abstract

This invention discloses a converter refining method capable of suppressing slag spraying in the initial stage of decarburization blowing and stably reducing phosphorus (P) in the molten steel after decarburization blowing. The converter refining method of this invention comprises: a first step of charging molten iron into a converter; a second step of, after the first step, applying a first flux to the molten iron in the converter and performing dephosphorization; a third step of, after the second step, discharging at least a portion of the slag in the converter to the outside of the converter; and a fourth step of, after the third step, adding a second flux into the converter and then performing decarburization, wherein the second flux comprises a CaO source and a SiO2 source, and the CaO / SiO2 ratio, as defined by a given formula, is 3.0 or more and 4.5 or less, and the CaO content, as defined by a given formula, is 30.0 kg / ton-steel or less.

Description

Technical Field

[0001] This application discloses a converter refining method. Background Technology

[0002] As a process for refining molten iron using converters, two processes have been developed: Process (I) in which dephosphorization of molten iron is performed in the first converter, and then the molten iron tapped from the first converter is charged into the second converter for decarburization; and Process (II) in which dephosphorization is performed in one converter, and the slag generated by dephosphorization is discharged (intermediate slag discharge), and then decarburization continues in the same converter. Process (I) has a high refining capacity, but on the other hand, it requires two converters, resulting in high equipment costs, increased heat loss, and reduced melting capacity of iron ore and scrap iron. Compared with Process (I), Process (II) can shorten the overall blowing time, reduce the amount of flux required for dephosphorization, and reduce heat loss during refining. However, in Process (II), it is difficult to stably control the amount of intermediate slag discharge, for example, it is sometimes difficult to reduce the P concentration in the molten steel after refining to an extremely low level.

[0003] Patent Document 1 discloses a converter refining method in which a flux containing 60-99% SiO2 is added as the SiO2 component before decarburization in process (II), at an average rate of 1.0-4.0 kg per ton of molten steel produced. Patent Document 2 discloses a method for using desiliconized slag in the decarburization process after dephosphorization treatment.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 3194212

[0007] Patent Document 2: Japanese Patent No. 6223249 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] According to the inventor's new discovery, the prior art related to process (II) has the following problems. Specifically, in the prior art, during decarburization blowing, the slag in the converter becomes low-basicity, and in the early stages of decarburization blowing, slag overflows from the converter (slag spray), posing a risk of difficulty in stable operation. Alternatively, during decarburization blowing, the slag in the converter becomes high-basicity, and the flux added for decarburization is not sufficiently slag-formed, failing to adequately ensure a high amount of slag with high dephosphorization capacity, posing a risk of difficulty in stably reducing the P concentration in the molten steel after decarburization blowing. Or, the amount of slag in decarburization blowing increases, posing a risk of slag spray occurring in the early stages of decarburization blowing.

[0010] Problem Solving Methods

[0011] The inventors conducted actual measurements of the molten slag during intermediate slag discharge using a weighing instrument. The results showed that the intermediate slag discharge rate deviated significantly by 50% to 95% compared to previous expectations. That is, it was found that in the prior art, due to the drastic increase or decrease in the intermediate slag discharge rate, a deviation occurred between the calculated basicity and the actual basicity, which is one of the reasons for the low or high basicity of the molten slag in the converter during decarburization blowing. Therefore, the inventors invented a method for accurately determining the intermediate slag discharge rate at least once using a weighing instrument, and for operating based on the accurately determined intermediate slag discharge rate. The details are as follows.

[0012] This application discloses a converter refining method as one of the methods for solving the above-mentioned problems, the converter refining method comprising:

[0013] The first step is to fill the converter with molten iron;

[0014] The second step involves applying the first flux to the molten iron in the converter and performing dephosphorization on the molten iron, following the first step described above.

[0015] The third step, following the second step, involves discharging at least a portion of the molten slag from the converter to the outside of the converter; and

[0016] The fourth step, following the third step, involves adding a second flux to the converter, followed by decarburization.

[0017] The second flux mentioned above includes a CaO source and a SiO2 source.

[0018] The above converter refining method satisfies the following equations (1) and (2).

[0019] [Mathematical Expression 1]

[0020]

[0021]

[0022] C2: CaO conversion amount of the first flux mentioned above (kg / ton-steel)

[0023] C4: CaO conversion amount of the second flux mentioned above (kg / ton-steel)

[0024] S2: The SiO2 equivalent of the first flux mentioned above (kg / ton-steel)

[0025] S4: The SiO2 equivalent of the second flux mentioned above (kg / ton-steel)

[0026] α3: Intermediate slag discharge rate (%) of the third process mentioned above.

[0027] The converter refining method of the present invention can include:

[0028] The fifth step involves tapping steel after the fourth step, with the slag generated in the fourth step remaining in the converter; and

[0029] In the sixth step, following the fifth step, based on at least one of the estimated P2O5 content of the slag in the converter and the target P content value of the steel for the next heat treatment, any of the following treatments is selected and performed: either the treatment that leaves all of the slag in the converter remaining in the converter, or the treatment that leaves a portion of the slag in the converter remaining in the converter and otherwise removes it.

[0030] After the sixth step described above, the first step of the next heat treatment can be carried out while the molten slag remains in the converter.

[0031] The effects of the invention

[0032] According to the converter refining method of the present invention, slag spraying in the initial stage of decarburization blowing is easily suppressed. Furthermore, according to the converter refining method of the present invention, phosphorus (P) in the molten steel after decarburization blowing is easily and stably reduced. Attached Figure Description

[0033] Figure 1 This is a schematic diagram illustrating an example of a converter refining process.

[0034] Symbol Explanation

[0035] 10 molten iron

[0036] 11 Dephosphorized Iron

[0037] 12 molten steel

[0038] 21 First flux

[0039] 22 Second flux

[0040] 31 Slag

[0041] 32 molten slag

[0042] 100 converter Detailed Implementation

[0043] like Figure 1 As shown in (A) to (F), the converter refining method of the present invention includes: a first step ( Figure 1 (A)), molten iron 10 is loaded into the converter 100; the second process ( Figure 1(B)) After the first process, the molten iron 10 in the converter 100 is treated with the first flux 21 and dephosphorized; the third process ( Figure 1 (C)), after the second process, at least a portion of the slag 31 inside the converter 100 is discharged outside the converter 100; and the fourth process ( Figure 1 (D) and (E)), after the third step, a second flux 22 is added to the converter 100, and then decarburization is performed. Here, in the converter refining method of the present invention, the second flux 22 includes a CaO source and a SiO2 source. In addition, in the converter refining method of the present invention, the following formulas (1) and (2) are satisfied.

[0044] [Mathematical Expression 2]

[0045]

[0046]

[0047] C2: CaO conversion amount of flux 21 (kg / ton-steel)

[0048] C4: CaO conversion amount of flux 22 (kg / ton-steel)

[0049] S2: Equivalent SiO2 content of flux 21 (kg / ton-steel)

[0050] S4: SiO2 equivalent amount of flux 22 (kg / ton-steel)

[0051] α3: Intermediate slag discharge rate (%) in the third process

[0052] 1. First process

[0053] like Figure 1 As shown in (A), in the first step, molten iron 10 is loaded into the converter 100. The conditions in the first step are not particularly limited.

[0054] As converter 100, a conventional converter can be used. In the converter refining method of the present invention, converter 100 can be any converter among top-blown converters, bottom-blown converters, and top-and-bottom combined-blown converters. In a top-blown converter, there is no bottom stirring; therefore, the iron oxide generated by the oxidation of iron by top-blown oxygen is difficult to reduce, and there is a tendency for excessive slag accumulation. Furthermore, slag with high iron oxide content tends to slag CaO. On the other hand, in top-and-bottom combined-blown converters and bottom-blown converters, the iron oxide concentration does not become as high, and compared to the case of a top-blown converter, there is a tendency for CaO slag formation to be difficult. Regarding this, from the viewpoint that higher efficiency can be obtained through the technology of the present invention, converter 100 can be one of a bottom-blown converter and a top-and-bottom combined-blown converter, or a top-and-bottom combined-blown converter. Figure 1 (A) to (F) show an example of a top-and-bottom blowing converter as converter 100. The top-and-bottom blowing converter 100 may have multiple flow paths 101 at its bottom for supplying bottom-blown gas into the furnace. In addition, the top-and-bottom blowing converter 100 may have a tapping port 102 on its side for tapping molten steel 12.

[0055] The molten iron 10 charged into the converter 100 can be, for example, typical blast furnace molten iron. Besides P and C as impurities, the molten iron 10 may also contain Si. When the molten iron 10 contains Si, a desiliconization reaction is carried out using oxygen for oxidative refining, followed by a dephosphorization reaction. In other words, desiliconization of the molten iron can be performed after the first process and before dephosphorization in the second process. It should be noted that after desiliconization, the desiliconized slag in the converter 100 can be discharged, or dephosphorization can be performed while desiliconized slag remains in the converter 100. In the latter case, the desiliconized slag can be used as the first flux 21. Figure 1 As shown in (A), molten iron 10 can be a mixture of molten iron 10a (e.g., blast furnace molten iron) and additives such as scrap iron 10b.

[0056] There is no particular limitation on the method of loading molten iron 10 into converter 100. For example, a known method of using a ladle or other container to flow into converter 100 can be cited.

[0057] 2. Second process

[0058] like Figure 1 As shown in (B), in the second step, after the first step, the first flux 21 is used and the molten iron is dephosphorized. The dephosphorization conditions in the second step are not particularly limited.

[0059] For the first flux 21, it can be added to the converter 100 before dephosphorization in the second process, or it can utilize components from molten iron such as desiliconized slag generated by the desiliconization reaction as described above, or it can retain the decarburized slag 32 from the previous heat treatment in the converter 100 as a flux. There are no particular limitations on the composition and amount of the first flux 21, as long as it achieves the desired dephosphorization. For example, the first flux 21 can contain a CaO source. Examples of CaO sources include quicklime, limestone, dolomite, and the decarburized slag 32 from the previous heat treatment. Additionally, the first flux 21 can contain a SiO2 source. Examples of SiO2 sources include desiliconized slag, the decarburized slag 32 from the previous heat treatment, silica, and olivine. The basicity of the first flux 21 (CaO / SiO2) can be 0.9 or higher, or 1.4 or lower. Furthermore, the amount of the first flux 21, converted to CaO, can be 5 kg / ton-steel or more, or 25 kg / ton-steel or less. Additionally, the amount of the first flux 21, converted to SiO2, can be 0 kg / ton-steel or more, or 5 kg / ton-steel or less. It should be noted that, in this application, "kg / ton-steel" refers to the mass equivalent to an average of 1 ton of molten steel ultimately obtained.

[0060] like Figure 1 As shown in (B), in the second process, for example, oxygen is blown into the molten iron 10 from the top blow lance 200, so that the molten iron 10 can be stirred and oxidized while being refined. On the other hand, the stirring of the molten iron 10 during refining can be enhanced by continuously or intermittently blowing bottom-blown gas from the bottom of the converter 100.

[0061] In the second process, dephosphorization removes a portion of the phosphorus (P) contained in the molten iron 10, resulting in dephosphorized molten iron 11. The concentration of P in the dephosphorized molten iron 11 is not particularly limited. For example, the dephosphorized molten iron 11 may contain 0.02% by mass or more, or 0.03% by mass or more, or it may contain less than 0.08% by mass or less than 0.06% by mass of P.

[0062] 3. Third process

[0063] like Figure 1 As shown in (C), in the third step, after the second step, at least a portion of the slag 31 inside the converter 100 is discharged outside the converter 100. For example, as... Figure 1 As shown in (C), the molten slag 31 can be discharged outside the system by tilting the converter 100. Furthermore, in the third step, the molten slag 31 can be foamed by continuously blowing bottom-blowing gas into the converter 100. This makes slag removal from the molten slag 31 easier.

[0064] The slag discharge rate (intermediate slag discharge rate) of molten slag 31 in the third process is not particularly limited; for example, it can be above 40% and below 70%. The composition and amount of molten slag 31 can be arbitrarily varied according to the dephosphorization conditions in the second process.

[0065] 4. Fourth process

[0066] like Figure 1 As shown in (D) and (E), in the fourth step, a second flux 22 is added to the converter 100 after the third step, and then decarburization is performed. The fourth step is characterized in that the second flux 22 comprises a CaO source and a SiO2 source, and satisfies the above formulas (1) and (2). There are no particular limitations on other decarburization conditions.

[0067] The second flux 22 is added to the converter 100 before decarburization in the fourth step. In the fourth step, a portion of the slag 31 remaining in the converter 100 after the third step was not discharged can be used as flux 22x together with the second flux 22. The composition and amount of the second flux 22 are not particularly limited as long as they satisfy the above formulas (1) and (2). The second flux 22 contains a CaO source and a SiO2 source. In the fourth step, the CaO source and the SiO2 source can be added to the converter 100 simultaneously or separately. Specific examples of the CaO source and the SiO2 source are as described above. In addition, the basicity of the second flux 22, CaO / SiO2, can be 3.2 or more or 4.2 or less. In addition, from the viewpoint of further increasing the amount of dephosphorization in the fourth step, the amount of the second flux 22, converted to CaO, can be 8 kg / ton-steel or more or 25 kg / ton-steel or less. In addition, the amount of the second flux 22, calculated in terms of SiO2, can exceed 0 kg / ton-steel or be less than 8 kg / ton-steel.

[0068] In the converter refining method of the present invention, as shown in formula (1) above, it is important that the charge CaO / SiO2 ratio, defined by [C2×(100-α3) / 100+C4] / [S2×(100-α3) / 100+S4], is 3.0 or more and 4.5 or less. The charge CaO / SiO2 ratio can be 3.2 or more, 3.4 or more, 3.6 or more, 3.8 or more, 4.0 or more, 4.2 or more, or 4.4 or more. According to the inventors' understanding, if the charge CaO / SiO2 ratio is too small, there will be insufficient CaO, making dephosphorization difficult and reducing the P concentration in the final molten steel 12 difficult. Moreover, there is a risk of slag spraying during the initial stage of decarburization blowing, accompanied by the low basicity of the slag. On the other hand, currently, in order to ensure sufficient dephosphorization, a large amount of CaO source is usually added as a flux, but according to the inventors' understanding, even if a large amount of CaO source is added, not all CaO sources are conducive to dephosphorization. According to the inventors' understanding, when a large amount of CaO source is added as flux during converter refining, resulting in a CaO / SiO2 ratio exceeding 4.5, the amount of SiO2 is insufficient relative to CaO, leading to excessively high basicity of the slag during decarburization. When the slag becomes excessively basic during decarburization blowing, the flux added to the converter is not fully slag-treated, making it impossible to adequately ensure a high dephosphorization capacity of slag. This poses a risk of difficulty in consistently reducing the P concentration in the molten steel 12 after decarburization blowing.

[0069] In the converter refining method of the present invention, as shown in equation (2) above, it is important that the amount of CaO charged, defined by C2×(100-α3) / 100+C4, is 30.0 kg / ton-steel or less. In other words, the amount of slag in decarburization blowing is a certain amount or less when converted to CaO. The amount of CaO charged can be 25.0 kg / ton-steel or less, 22.0 kg / ton-steel or less, 19.0 kg / ton-steel or less, or 16.0 kg / ton-steel or less. There is no particular limitation on the lower limit of the amount of CaO charged, and it will naturally exceed 0 kg / ton-steel according to the relationship in equation (1) above. For example, the lower limit of the amount of CaO charged can be 2 kg / ton-steel or more, 4 kg / ton-steel or more, 6 kg / ton-steel or more, 8 kg / ton-steel or more, or 10 kg / ton-steel or more. According to the inventors, if the amount of slag in the decarburization blowing process is too large, there is a risk of slag spraying in the early stage of the decarburization blowing process.

[0070] In the converter refining method of the present invention, the SiO2 charge defined by S2×(100-α3) / 100+S4 is only required to satisfy the above formulas (1) and (2), and there is no particular limitation. The range of SiO2 that can be charged can naturally be determined according to the above formulas (1) and (2).

[0071] In equations (1) and (2) above, C2 is the equivalent CaO content (kg / ton-steel) of the first flux 21, C4 is the equivalent CaO content (kg / ton-steel) of the second flux 22, S2 is the equivalent SiO2 content (kg / ton-steel) of the first flux 21, and S4 is the equivalent SiO2 content (kg / ton-steel) of the second flux 22. That is, the Ca content in each flux is converted to CaO, and the Si content is converted to SiO2 to determine their amounts. It should be noted that the equivalent CaO and SiO2 content of each flux can be calculated based on the composition of the flux before it is fed into the converter and the amount of flux fed into the converter. Alternatively, the equivalent CaO and SiO2 content can be determined based on the composition of the slag after slag discharge. It should be noted that, as mentioned above, when using desiliconized slag as the first flux 21, the SiO2 conversion amount of the first flux 21 can be determined by considering that 100% of the silicon contained in the molten iron before desiliconization is converted into SiO2.

[0072] In equations (1) and (2) above, α3 is the intermediate slag discharge rate (%) of the third process. The intermediate slag discharge rate can be determined based on the amount of flux added to the converter 100 or the amount of flux present in the converter 100, and the amount of slag discharged from the converter 100 (excluding base metal). The intermediate slag discharge rate can be empirically estimated based on past operations, or it can be calculated based on measurements taken online or offline during operation. It is particularly preferable to measure the intermediate slag discharge rate at least once using a weighing instrument or the like. By measuring the intermediate slag discharge rate at least once in advance, even if the use of the weighing instrument or the like is subsequently interrupted due to malfunction or other issues, the intermediate slag discharge rate can be empirically estimated with good accuracy based on past measurements and operating conditions. It should be noted that, as a weighing method other than using a weighing instrument, examples include the method for calculating the slag discharge rate based on the volume of discharged slag disclosed in Japanese Patent Application Publication No. 2018-119195.

[0073] By determining the intermediate slag discharge rate α3 of the third process in this way, the CaO / SiO2 charge can be controlled within the target range with good accuracy. That is, the converter refining method of the present invention can include: a process of determining the intermediate slag discharge rate α3 of the third process; and a process of determining the amount of the second flux, the CaO conversion amount and / or the SiO2 conversion amount of the second flux based on the determined intermediate slag discharge rate α3, and the CaO conversion amount and SiO2 conversion amount of the first flux, so as to satisfy the above formulas (1) and (2). More specifically, for example, it may include: a process after the third process, determining the amount of slag 31 discharged in the third process; a process based on the determined amount of slag discharged to determine the intermediate slag discharge rate α3 of the above formulas (1) and (2); and a process based on the determined intermediate slag discharge rate α3, and the CaO conversion amount and SiO2 conversion amount of the first flux to determine the amount of the second flux added, the CaO conversion amount of the second flux, and / or the SiO2 conversion amount of the second flux, so as to satisfy the process of the above formulas (1) and (2). For example, by measuring the weight of the discharged slag and removing the weight of the base metal contained in the slag, the amount of slag discharged and the intermediate slag discharge rate can be determined with good accuracy. The weight of the base metal contained in the slag can be empirically estimated based on past operations, or it can be measured online or offline during operation.

[0074] As described above, according to the converter refining method of the present invention, slag spraying during the initial stage of decarburization blowing can be prevented by satisfying equations (1) and (2). Furthermore, effective dephosphorization can be performed during decarburization to ensure the minimum amount of slag required for dephosphorization. Therefore, the P concentration of the final molten steel 12 can be reduced.

[0075] 5. Supplement

[0076] In the converter refining method of the present invention, such as Figure 1 As shown in (F), the molten steel 12 inside the converter 100 can be tapped out of the converter 100 after the fourth process. For example, the converter 100 can be tilted so that the molten steel 12 flows out from the tapping port 102 on the side of the converter 100.

[0077] Furthermore, in the converter refining method of the present invention, such as Figure 1As shown in (F), a fifth step can be performed after the fourth step, wherein the steel is tapped while the slag 32 generated in the fourth step remains in the converter 100. Then, a sixth step can be performed after the fifth step, wherein the sixth step is, based on at least one of the estimated P2O5 content of the slag 32 in the converter 100 and the target P content value of the steel for the next heat treatment, any one of the following treatments is selected and performed: either the treatment leaves all the slag 32 in the converter 100, or the treatment leaves a portion of the slag 32 in the converter 100 and removes the rest. In this case, the first step for the next heat treatment can be performed after the sixth step while the slag 32 remains in the converter 100. Thus, by performing the first step for the next heat treatment while the decarburized slag 32 remains in the converter 100, the slag 32 can be reused as a flux for the next heat treatment.

[0078] Example

[0079] The following examples illustrate the effects of the technology of the present invention in more detail, but the technology of the present invention is not limited to the following examples.

[0080] 1. Example 1

[0081] 1.1 First Process

[0082] Molten iron and scrap iron are added to a 300t top and bottom combined blowing converter that still contains decarburized slag from the previous heat treatment, bringing the total capacity to 300t.

[0083] Table 1 below shows the temperature and composition of the molten iron in the furnace during the first process. Table 2 below shows the amount of decarburized slag residue from the previous heat treatment.

[0084] 1.2 Second Process

[0085] After the first process, a new flux containing CaO was added to the furnace for dephosphorization blowing. It should be noted that the first flux used in dephosphorization blowing is equivalent to a flux composed of the new CaO-containing flux added to the furnace, the decarburized slag from the previous heat treatment, and the desiliconized slag produced by desiliconization.

[0086] Table 1 below shows the molten iron temperature, molten iron composition, and slag composition at the end of the second process. Table 2 below shows the amount of new CaO added for the second process, the basicity of the first flux used in the second process, and the amount of slag generated in the second process. Furthermore, Table 3 below shows the equivalent amounts of CaO and SiO2 in the first flux.

[0087] 1.3 Third Process

[0088] After the second step, intermediate slag removal was performed by tilting the converter. At this time, the amount of intermediate slag removed was measured using a weighing instrument. The intermediate slag removal rate was determined based on the measured amount. Specifically, the intermediate slag removal rate for the third step was calculated by dividing the weighing value obtained using a weighing instrument calibrated for the base metal composition by the amount of slag pre-calculated based on the charge in the second step.

[0089] The intermediate slag discharge rate for the third process is shown in Tables 1 and 3 below.

[0090] 1.4 Fourth Process

[0091] After the third step, a second flux was added to the furnace for decarburization blowing. Here, the amounts of CaO and silica sources were adjusted according to the intermediate slag discharge rate so that the CaO / SiO2 ratio shown in equation (1) below reached a given value. In addition, the amount of CaO source was adjusted according to the intermediate slag discharge rate so that the CaO ratio shown in equation (2) below reached a given value.

[0092] Table 1 below shows the molten steel temperature, molten steel composition, and slag composition at the end of the fourth process. Table 2 below shows the amount of new CaO and SiO2 added for the fourth process, and the amount of slag generated in the fourth process. Furthermore, Table 3 below shows the equivalent amounts of CaO and SiO2 in the second flux, the CaO / SiO2 ratio calculated using formula (I), and the CaO ratio calculated using formula (II).

[0093] [Mathematical Expression 3]

[0094]

[0095]

[0096] C2: CaO conversion amount of the first flux (kg / ton-steel)

[0097] C4: CaO conversion amount of the second flux (kg / ton-steel)

[0098] S2: Equivalent SiO2 content of the first flux (kg / ton-steel)

[0099] S4: Equivalent SiO2 content of the second flux (kg / ton-steel)

[0100] α3: Intermediate slag discharge rate (%) in the third process

[0101] 2. Examples 2-6, Comparative Examples 1-5

[0102] Processes 1 through 4 were carried out under the conditions shown in Tables 1 to 3.

[0103]

[0104]

[0105]

[0106] 3. Evaluation Results

[0107] Table 4 below shows the following for Examples 1-6 and Comparative Examples 1-5: the CaO / SiO2 charge of Formula (I), the CaO charge of Formula (II), the actual basicity of the decarburized slag, the amount of unslagified CaO during decarburization (determined based on the difference between the charge CaO / SiO2 and the actual basicity of the decarburized slag), the presence or absence of slag spraying in the initial stage of decarburization, the P concentration in the final molten steel, the total amount of slag discharged from the system after a series of processes (total slag discharged from the system), and the amount of new CaO (the sum of the amounts of new CaO added in the second and fourth processes). It should be noted that "total slag discharged from the system" is defined as the cumulative value of the weight of slag discharged from the furnace through intermediate slag discharge in the third process and the weight of slag discharged from the furnace when the amount of slag in the furnace is adjusted to the amount in Table 2 above after the fourth process.

[0108]

[0109] Based on the conditions shown in Tables 1-3 and the results shown in Table 4, the following can be determined.

[0110] (1) Based on the results of Example 1 and Comparative Example 1, it is clear that even if the total amount of slag discharged from the system is the same, compared with Comparative Example 1, which has a high CaO / SiO2 content and a large amount of unslag-treated CaO, Example 1, which has a lower CaO / SiO2 content and appropriately added SiO2 source, is more able to reduce the P concentration in the molten steel.

[0111] (2) Based on the results of Example 2 and Comparative Example 2, it is clear that in Comparative Example 2, where an excessive amount of SiO2 source was added and the CaO / SiO2 ratio was less than 3.0, the P concentration in the molten steel could not be reduced. On the other hand, compared with Example 1, although more SiO2 source was added in Example 2, the CaO / SiO2 ratio was around 3.8, which was greater than 3.0, and therefore, the P concentration in the molten steel could be reduced.

[0112] (3) According to the results of Comparative Example 2, it is clear that if the CaO / SiO2 ratio is less than 3.0, the slag during decarburization will be excessively low in basicity, and a large amount of highly viscous and easily foaming slag will be generated. In the early stage of decarburization blowing, slag overflows from the converter and sprays out slag.

[0113] (4) Based on the results of Examples 3 and 4, it is clear that when decarburization blowing is carried out with a CaO / SiO2 ratio of around 4.5, the P concentration in the molten steel can be significantly reduced. In particular, Example 4, which increases the amount of slag by adding CaO and SiO2 sources, can further reduce the P concentration in the molten steel, thus enabling the smelting of extremely low phosphorus steel.

[0114] (5) Based on the results of Examples 3 and 4 and Comparative Examples 3 and 4, it is clear that in Comparative Example 3, which had the same intermediate slag discharge rate as Examples 3 and 4 but a CaO / SiO2 ratio significantly exceeding 4.5, there was a large amount of unslag-treated CaO, which could not adequately generate slag and thus could not reduce the P concentration in the molten steel. Furthermore, in Comparative Example 4, although a SiO2 source was added, the amount was insufficient, and the CaO / SiO2 ratio still exceeded 4.5; therefore, it could not adequately reduce the P concentration in the molten steel.

[0115] (6) Based on the results of Comparative Example 5, it is clear that if the amount of CaO added exceeds 30.0 kg / t-steel, the P concentration in the molten steel will not decrease, and slag spraying occurs in the early stage of decarburization blowing. On the other hand, in Example 5, where the amount of CaO added is less than 30.0 kg / t-steel, dephosphorization can be carried out well without slag spraying.

[0116] (7) According to the comparison between Examples 1 to 5 and Example 6, the same effect can be achieved whether the decarburized slag from the previous heat treatment is used as the first flux or not.

[0117] As can be seen from the above, the converter refining methods of Examples 1 to 6 are good at suppressing slag spraying in the early stage of decarburization blowing and can stably reduce P in the molten steel after decarburization blowing.

Claims

1. A converter refining method, the method comprising: a first step of charging molten iron into a converter; a second step of performing molten iron dephosphorization on the molten iron in the converter using a first flux after the first step; a third step of discharging at least a part of a molten slag in the converter to outside the converter after the second step; and a fourth step of adding a second flux to the converter and then performing decarburization after the third step, wherein an intermediate slag discharge rate of the third step is determined at least once, the second flux contains a CaO source and a SiO2 source, the converter refining method satisfies the following formula (1) and formula (2), C2: CaO conversion amount (kg / ton-steel) of the first flux C4: CaO conversion amount (kg / ton-steel) of the second flux S2: SiO2 conversion amount (kg / ton-steel) of the first flux S4: SiO2 conversion amount (kg / ton-steel) of the second flux α3: intermediate slag discharge rate (%) of the third step. C2 + C4 > S2 + S4 (1) 0 < α3 < 100 (2) 2. The converter refining method according to claim 1, wherein the intermediate slag discharge rate of the third step is measured at least once by a weighing instrument or based on a volume of the discharged molten slag.

3. The converter refining method according to claim 1 or 2, the method comprising: a fifth step of performing tapping with the molten slag generated in the fourth step remaining in the converter after the fourth step; and a sixth step of selecting and performing any of the following treatments based on at least one of a presumed P2O5 component amount of the molten slag in the converter and a P component target value of steel for a next heat treatment after the fifth step, the treatments being a treatment of leaving all of the molten slag in the converter in the converter or a treatment of leaving a part of the molten slag in the converter and discharging the other, after the sixth step, a first step of the next heat treatment is performed with the molten slag remaining in the converter. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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