Converter refining process

Abstract

Disclosed is a converter refining method capable of suppressing slag spitting at the initial stage of decarburization blowing, capable of stably reducing P in molten steel after decarburization blowing, and capable of reducing the amount of molten slag discharged to the outside of the system. The converter refining method of the present invention includes: a first step of charging molten iron into a converter; a second step of performing first-stage molten iron dephosphorization using a first flux; a third step of discharging at least a part of molten slag in the converter to the outside of the converter after the second step; a fourth step of adding a second flux to the converter after the third step, and then performing second-stage molten iron dephosphorization; a fifth step of discharging at least a part of molten slag in the converter to the outside of the converter after the fourth step; and a sixth step of adding a third flux to the converter after the fifth step, and then performing decarburization, the third flux containing a CaO source and a SiO2 source, and a given charged CaO / SiO2 being 3.0 or greater and 4.5 or less.

Description

TECHNICAL FIELD

[0001] The present application discloses a converter refining method. BACKGROUND

[0002] As a process for refining molten iron using a converter, a process (I) in which dephosphorization of molten iron is performed in a first converter, and then the molten iron tapped from the first converter is charged into a second converter, and decarburization is performed in the second converter; and a process (II) in which after dephosphorization is performed in one converter, the molten slag generated by the dephosphorization is tapped (intermediate tapping), and then decarburization is continued in the same converter have been developed. The refining capacity of the process (I) is high, but on the other hand, two converters are required, so the equipment cost is high, and the heat generation and heat dissipation loss increases, and the melting capacity of iron ore and scrap iron decreases. In comparison with the process (I), the process (II) can shorten the overall blowing time, can reduce the amount of flux required for dephosphorization, and can reduce the heat loss during refining. However, in the process (II), it is difficult to stably control the intermediate tapping amount, and for example, it is sometimes difficult to reduce the P concentration in the molten steel after refining to an extremely low level.

[0003] In Patent Literature 1, a converter refining method is disclosed in which, before decarburization in the process (II), a flux containing 60 to 99% of SiO2 is added as a SiO2 component at an average of 1.0 to 4.0 kg per 1 ton of molten steel generated. In Patent Literature 2, a converter refining method is disclosed in which, after dephosphorization of molten iron in a first stage, intermediate tapping is performed, and then additional flux is added, dephosphorization of molten iron in a second stage is performed, and then the molten slag is removed.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

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

[0007] Patent Literature 2: Japanese Patent No. 5671801 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] In the method disclosed in Patent Literature 1, it is difficult to stably control the intermediate tapping amount. According to the new insight of the present inventor, when SiO2 is further added in a state in which the intermediate tapping amount is low, that is, in a state in which a large amount of molten slag remains in the converter before decarburization blowing, a large amount of molten slag having low basicity, high viscosity, and being easily foamed is generated, and at the initial stage of decarburization blowing, the molten slag overflows from the converter (slag spitting), and it is sometimes difficult to perform stable operation.

[0010] On the other hand, in the method disclosed in Patent Literature 2, the intermediate slag discharge amount is increased, that is, the molten slag amount in the converter is decreased. According to the new insight of the present inventor, in the prior art, if the flux is added at the time of decarburization blow in a state where the molten slag amount in the furnace is decreased, sometimes, dephosphorization is not good. For example, according to the new insight of the present inventor, in the case where the flux is excessively added for decarburization, the added flux is not sufficiently slagged, the molten slag amount with high dephosphorization capacity cannot be sufficiently ensured, and it is difficult to stably decrease the P concentration in the molten steel after decarburization blow.

[0011] Further, in the prior art, the molten slag amount discharged to the outside of the system is not sufficiently studied to be decreased as much as possible.

[0012] Method for solving the problem

[0013] The present inventor actually measured the molten slag at the time of intermediate slag discharge by a "weighing instrument". As a result, it was found that the intermediate slag discharge rate has a large deviation of 50 to 95% larger than the past expectation. That is, it was found that in the prior art, since the intermediate slag discharge rate is sharply increased and decreased, the deviation of the calculated basicity from the actual basicity occurs. Therefore, the present inventor invented a method for accurately determining the intermediate slag discharge rate at least once by a "weighing instrument or the like" and performing the operation based on the accurately determined intermediate slag discharge rate. Details are described below.

[0014] The present application discloses a converter refining method as one of methods for solving the above problem, the converter refining method comprising:

[0015] Step 1, charging molten iron into the inside of a converter;

[0016] Step 2, after the above Step 1, performing first-stage molten iron dephosphorization on the above molten iron in the above converter using a first flux;

[0017] Step 3, after the above Step 2, discharging at least a part of molten slag in the above converter to the outside of the above converter;

[0018] Step 4, after the above Step 3, adding a second flux to the inside of the above converter, and then performing second-stage molten iron dephosphorization;

[0019] Step 5, after the above Step 4, discharging at least a part of molten slag in the above converter to the outside of the above converter; and

[0020] Step 6, after the above Step 5, adding a third flux to the inside of the above converter, and then performing decarburization,

[0021] The above third flux contains a CaO source and a SiO2 source,

[0022] a CaO / SiO2of 3.0 or more and 4.5 or less defined by the following formula (1),

[0023] [Math. 1]

[0024]

[0025] C2: CaO conversion amount (kg) of the above-mentioned first flux

[0026] C4: CaO conversion amount (kg) of the above-mentioned second flux

[0027] C6: CaO conversion amount (kg) of the above-mentioned third flux

[0028] S2: SiO2conversion amount (kg) of the above-mentioned first flux

[0029] S4: SiO2conversion amount (kg) of the above-mentioned second flux

[0030] S6: SiO2conversion amount (kg) of the above-mentioned third flux

[0031] α3: intermediate slagging rate (%) of the above-mentioned third process

[0032] α5: intermediate slagging rate (%) of the above-mentioned fifth process.

[0033] In the converter refining method of the present application, the amount of the above-mentioned third flux added to the above-mentioned converter in the above-mentioned sixth process can be 8 kg / ton-steel or more and 25 kg / ton-steel or less in terms of CaO conversion.

[0034] The converter refining method of the present application can be provided with:

[0035] a seventh process of performing tapping in a state where the molten slag generated in the above-mentioned sixth process remains in the above-mentioned converter after the above-mentioned sixth process; and

[0036] an eighth process of selecting and performing, after the above-mentioned seventh process, any of the following treatments based on at least one of the estimated P2O5 component amount of the above-mentioned molten slag in the above-mentioned converter and the P component target value of the steel of the next heat treatment, the treatments being a treatment of leaving all of the above-mentioned molten slag in the above-mentioned converter or a treatment of leaving a part of the above-mentioned molten slag in the above-mentioned converter and performing slagging of the other,

[0037] After the above-mentioned eighth process, the first process of the next heat treatment can be performed in a state where the above-mentioned molten slag remains in the above-mentioned converter.

[0038] Effects of the Invention

[0039] According to the converter refining method of the present invention, slag spraying during the initial stage of decarburization blowing is easily suppressed. Furthermore, the converter refining method of the present invention can stably reduce phosphorus (P) in the molten steel after decarburization blowing. In addition, the converter refining method of the present invention easily reduces the amount of slag discharged from the system. Attached Figure Description

[0040] Figure 1 This is a schematic diagram illustrating an example of a converter refining process.

[0041] Symbol Explanation

[0042] 10 Molten Iron

[0043] 11. Stage 1 Dephosphorized Iron

[0044] 12. Stage 2 Dephosphorized Iron

[0045] 13 Molten Steel

[0046] 21 First flux

[0047] 22. Second flux

[0048] 23 Third flux

[0049] 31 Slag

[0050] 32 Slag

[0051] 33 Slag

[0052] 100 converter Detailed Implementation

[0053] like Figure 1 As shown in (A)~(H), 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 the first stage of dephosphorization is carried out; 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; the fourth process ( Figure 1 (D) and (E)), after the third process, the second flux 22 is added to the converter 100, and then the second stage of molten iron dephosphorization is carried out; the fifth process ( Figure 1 (F)), after the fourth process, at least a portion of the slag 32 inside the converter 100 is discharged outside the converter 100; and the sixth process ( Figure 1(G) and (H)), after the fifth step, a third flux 23 is added to the converter 100, and then decarburization is performed. Here, in the converter refining method of the present invention, the third flux 23 includes a CaO source and a SiO2 source. In addition, in the converter refining method of the present invention, the CaO / SiO2 ratio, as defined by the following formula (1), is 3.0 or more and 4.5 or less.

[0054] [Mathematical Expression 2]

[0055]

[0056] C2: Equivalent CaO content (kg) of flux 21 (first flux).

[0057] C4: CaO conversion amount (kg) of flux 22

[0058] C6: CaO conversion amount (kg) of flux 23 (3rd flux)

[0059] S2: Equivalent SiO2 content (kg) of flux 21 (first flux).

[0060] S4: Equivalent SiO2 content (kg) of flux 22

[0061] S6: Estimated SiO2 content (kg) of flux 23 (3rd flux)

[0062] α3: Intermediate slag discharge rate (%) of the third process

[0063] α5: Intermediate slag discharge rate (%) of the fifth process

[0064] 1. First process

[0065] 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.

[0066] 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)~(H) 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 13.

[0067] 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.

[0068] 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.

[0069] 2. Second process

[0070] like Figure 1 As shown in (B), in the second process, after the first process, the first flux 21 is used to perform the first stage of dephosphorization of the molten iron. The dephosphorization conditions in the second process are not particularly limited.

[0071] As the first flux 21, a component from molten iron such as a desiliconized slag generated by the desiliconization reaction as described above can be charged into the converter 100 before the dephosphorization in the second process, and a decarburized slag 33 after decarburization refining in the previous heat treatment can also be left in the converter 100 as a flux. The composition and amount of the first flux 21 are not particularly limited, and can be any composition and amount that can achieve the targeted dephosphorization. For example, the first flux 21 can include a CaO source. As the CaO source, quicklime, limestone, dolomite, the decarburized slag 33 after the previous heat treatment, and the like can be given. In addition, the first flux 21 can include a SiO2source. As the SiO2source, a desiliconized slag, the decarburized slag 33 after the previous heat treatment, silica, olivine, and the like can be given. The basicity CaO / SiO2of the first flux 21 can be 0.9 or greater, and can be 1.4 or less. In addition, the amount of the first flux 21 can be 5 kg / ton-steel or greater, and can be 25 kg / ton-steel or less, in terms of CaO. In addition, the amount of the first flux 21 can be 0 kg / ton-steel or greater, and can be 5 kg / ton-steel or less, in terms of SiO2. Note that, in the present application, "kg / ton-steel" means the mass corresponding to an average of 1 ton of the final molten steel.

[0072] As shown in Figure 1 In the second process, as shown in (B), for example, by blowing oxygen into the molten iron 10 from the top lance 200, oxidation refining can be performed while stirring the molten iron 10. On the other hand, by continuously or intermittently blowing a bottom gas from the bottom of the converter 100, the stirring of the molten iron 10 in the refining can be enhanced.

[0073] By the dephosphorization in the second process, a part of the P contained in the molten iron 10 is removed, and a first-stage dephosphorized molten iron 11 is obtained. The P concentration in the first-stage dephosphorized molten iron 11 is not particularly limited. For example, the first-stage dephosphorized molten iron 11 can include 0.03 mass% or greater and 0.08 mass% or less of P.

[0074] 3. Third Process

[0075] As shown in Figure 1 In the third process, as shown in (C), at least a part of the slag 31 in the converter 100 is discharged to the outside of the converter 100 after the second process. For example, as shown in Figure 1 As shown in (C), the slag 31 can be caused to flow out to the outside of the system by tilting the converter 100. In addition, in the third process, the slag 31 can be foamed by continuously blowing a bottom gas from the bottom of the converter 100. Thereby, the slag 31 becomes easier to discharge.

[0076] 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.

[0077] 4. Fourth process

[0078] like Figure 1 As shown in (D) and (E), in the fourth step, after the third step, a second flux 22 is added to the converter 100, and then the second stage of molten iron dephosphorization is carried out. The dephosphorization conditions in the fourth step are not particularly limited.

[0079] The second flux 22 is added to the converter 100 before dephosphorization in the fourth step. In the fourth step, a portion of the slag 31 remaining in the converter 100 that was not removed in the third step can be used together with the second flux 22 as flux 22x. There are no particular limitations on the composition and amount of the second flux 22, as long as it achieves the desired dephosphorization. For example, the second flux 22 can contain either a CaO source or a SiO2 source. Specific examples of CaO and SiO2 sources are as described above. Furthermore, the basicity of the second flux 22 (CaO / SiO2) can be 1.0 or higher, or 1.8 or lower. Additionally, the amount of the second flux 22, converted to CaO, can be 0 kg / ton-steel or higher, 2 kg / ton-steel or higher, or 5 kg / ton-steel or lower. Furthermore, the amount of the second flux 22, converted to SiO2, can be above 0 kg / ton-steel or below 5 kg / ton-steel. It should be noted that the amount of flux 22x used in the fourth process is less than the amount of the first flux 21 used in the second process.

[0080] like Figure 1 As shown in (E), in the fourth step, similar to the second step, for example, oxygen can be blown into the first-stage dephosphorized molten iron 11 from the top-blown lance 200 to stir the first-stage dephosphorized molten iron 11 while oxidative refining. On the other hand, bottom-blown gas can be continuously or intermittently blown into the bottom of the converter 100 to enhance the stirring of the first-stage dephosphorized molten iron 11 during refining.

[0081] In the fourth step of dephosphorization, a portion of the phosphorus (P) contained in the dephosphorized molten iron 11 of the first stage is further removed, resulting in dephosphorized molten iron 12 of the second stage. The concentration of P in the dephosphorized molten iron 12 of the second stage is not particularly limited. For example, the dephosphorized molten iron 12 of the second stage may contain more than 0.02% by mass and less than 0.06% by mass of P.

[0082] 5. Step 5

[0083] As Figure 1 (F) shows, in the 5th process, at least a part of the molten slag 32 in the converter 100 is discharged to the outside of the converter 100 after the 4th process. For example, as Figure 1 (F) shows, the molten slag 32 can be caused to flow out to the system by tilting the converter 100. In addition, in the 5th process, the molten slag 32 can be caused to foam by continuously blowing the bottom blowing gas from the bottom of the converter 100. Thus, the discharge of the molten slag 32 becomes easier.

[0084] The discharge rate of the molten slag 32 in the 5th process (intermediate discharge rate) is not particularly limited, and can be, for example, 40% or more and 70% or less. The composition and the amount of generation of the molten slag 32 can be arbitrarily changed depending on the dephosphorization conditions in the 4th process. Note that, as described above, the amount of the flux 22x used in the 4th process is less than the amount of the first flux 21 used in the 2nd process, that is, the amount of the molten slag 32 generated after the 4th process is also less. Even in the case where the amount of the molten slag 32 is less than that of the molten slag 31, by performing the blowing, the molten slag 32 can be caused to foam to the same degree as the molten slag 31, and the discharge of the molten slag 32 can be performed. Compared with the molten slag 31, the molten slag 32 has a tendency to easily sink while foaming, but a sufficient amount of discharge can be achieved. Note that, the discharge rate of the 5th process can be less than that of the 3rd process. As described above, the amount of the flux 22x used in the 4th process is less than the amount of the first flux 21 used in the 2nd process, and thus, even in the case where the discharge rate of the 5th process is less than that of the 3rd process, the amount of the molten slag remaining in the furnace after the 5th process is likely to become less than that after the 3rd process. In this way, by performing the 4th process and the 5th process in addition to the 3rd process, the amount of the molten slag in the converter 100 before the decarburization blowing in the 6th process is reduced, and thus, the spitting of the molten slag at the initial stage of the decarburization blowing can be suppressed, and a more stable operation can be achieved.

[0085] 6. 6th process

[0086] As Figure 1 (G) and (H) show, in the 6th process, after the 5th process, the third flux 23 is added to the converter 100, and then decarburization is performed. In the 6th process, it is characterized in that the third flux 23 contains a CaO source and a SiO2 source, and satisfies the above formula (1). The decarburization conditions other than this are not particularly limited.

[0087] The third flux 23 is charged into the converter 100 before the decarburization in the sixth process. In the sixth process, a part of the molten slag 32 remaining in the converter 100 without being discharged in the fifth process can be used as the flux 23x together with the third flux 23. The composition and the amount of the third flux 23 are not particularly limited as long as the above-described formula (1) is satisfied. The third flux 23 contains a CaO source and a SiO2source. In the sixth process, the CaO source and the SiO2source can be added to the converter 100 at the same time or separately. Specific examples of the CaO source and the SiO2source are as described above. In addition, the basicity CaO / SiO2of the third flux 23 can be 3.2 or more and can be 4.2 or less. In addition, from the viewpoint of further increasing the dephosphorization amount in the sixth process, the amount of the third flux 23 can be 8 kg / ton-steel or more and can be 25 kg / ton-steel or less in terms of CaO. In addition, the amount of the third flux 23 can exceed 0 kg / ton-steel and can be 8 kg / ton-steel or less in terms of SiO2.

[0088] In the converter refining method of the present application, it is important that the charged CaO / SiO2defined by the above-described formula (1) is 3.0 or more and 4.5 or less. The charged CaO / SiO2may 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 present inventors' insight, when the charged CaO / SiO2is too small, CaO is insufficient, dephosphorization is difficult to perform, and it is difficult to reduce the P concentration in the finally obtained molten steel 13. On the other hand, at present, in order to sufficiently perform dephosphorization, a CaO source is generally added in a large amount as a flux, but according to the present inventors' insight, even if a CaO source is added in a large amount, not all of the CaO source contributes to dephosphorization. According to the present inventors' insight, in a case where the charged CaO / SiO2exceeds 4.5 by adding a CaO source in a large amount as a flux or the like at the time of the operation of the converter refining, the amount of SiO2is insufficient with respect to CaO, and the molten slag excessively becomes high in basicity at the time of decarburization. In a case where the molten slag excessively becomes high in basicity at the time of decarburization blowing, the flux charged into the converter is not sufficiently slagged, the amount of the molten slag having a high dephosphorization capacity cannot be sufficiently ensured, and there is a risk that it is difficult to stably reduce the P concentration in the molten steel 13 after decarburization blowing.

[0089] In the above formula (1), C2 is the CaO conversion amount (kg) of the first flux 21, C4 is the CaO conversion amount (kg) of the second flux 22, C6 is the CaO conversion amount (kg) of the third flux 23, S2 is the SiO2 conversion amount (kg) of the first flux 21, S4 is the SiO2 conversion amount (kg) of the second flux 22, and S6 is the SiO2 conversion amount (kg) of the third flux 23. That is, the amount of Ca and Si contained in each flux is determined by converting Ca into CaO and Si into SiO2. Note that the CaO conversion amount and the SiO2 conversion amount of each flux can be found based on the composition of the flux before being charged into the converter and the amount of the flux charged. Alternatively, the CaO conversion amount and the SiO2 conversion amount can be determined based on the components contained in the molten slag after the slag has been discharged. Note that, as described above, in the case where the desiliconized molten slag is used 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.

[0090] In the above formula (1), a3 is the intermediate slagging rate (%) of the third process, and a5 is the intermediate slagging rate (%) of the fifth process. The intermediate slagging rate can be determined based on the amount of the flux added to the converter 100 or the amount of the flux present in the converter 100, and the amount of the molten slag (excluding base metal) discharged from the converter 100. The intermediate slagging rate can be empirically estimated based on past operations, or can be found based on the measured value in an on-line or off-line manner during the operation. It is particularly preferable to measure the intermediate slagging rate at least once using a weighing instrument or the like. By measuring the intermediate slagging rate at least once in advance, even in the case where the use of the weighing instrument or the like is interrupted due to a failure or a malfunction of the weighing instrument or the like, the past measured value can be used, and the intermediate slagging rate can be empirically estimated with good accuracy based on the operation conditions or the like. Note that, as a method of weighing other than the method using a weighing instrument, a method of finding the slagging rate based on the volume of the discharged molten slag, as disclosed in Japanese Patent Application Publication No. 2018-119195, or the like can be given.

[0091] By determining the intermediate slag discharge rates α3 and α5 for the third and fifth processes in this way, the charge CaO / SiO2 ratio 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 for the third process; a process of determining the intermediate slag discharge rate α5 for the fifth process; and a process of determining the amount of the third flux to be added, the amount of CaO to be added, and / or the amount of SiO2 to be added based on the determined intermediate slag discharge rates α3 and α5, and the CaO and SiO2 conversion amounts of the first and second fluxes, so that the charge CaO / SiO2 ratio of the above formula (1) reaches 3.0 or more and 4.5 or less. More specifically, for example, it may include: a process after the third process to determine the amount of slag 31 discharged in the third process; a process after the fifth process to determine the amount of slag 32 discharged in the fifth process; a process based on the determined amount of slag discharged to determine intermediate slag discharge rates α3 and α5; and a process based on the determined intermediate slag discharge rates α3 and α5, and the CaO and SiO2 conversion amounts of the first and second fluxes, to determine the amount of additional third flux, the CaO conversion amount of the third flux, and / or the SiO2 conversion amount of the third flux, such that the CaO / SiO2 ratio of the above formula (1) reaches 3.0 or more and 4.5 or less. 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 estimated empirically based on past operations, or it can be measured online or offline during operation.

[0092] As described above, in the converter refining method according to the present invention, since two intermediate slag discharges are performed, the amount of residual slag in the converter 100 can be reduced before decarburization. As a result, slag spraying during the initial stage of decarburization blowing can be prevented. In addition, by setting the CaO / SiO2 charge as defined by the above formula (1) within a given range, effective dephosphorization can be carried out during decarburization with the minimum amount of slag required to ensure dephosphorization. Therefore, the P concentration of the final molten steel 13 can be reduced. Furthermore, in the converter refining method according to the present invention, low-phosphorus steel can be smelted with a minimum amount of slag, and the amount of slag discharged outside the system can also be reduced.

[0093] 7. Supplement

[0094] In the converter refining method of the present invention, such as Figure 1 As shown in (I), the molten steel 13 inside the converter 100 can be discharged outside the converter 100 after the sixth process. For example, the converter 100 can be tilted so that the molten steel 13 flows out from the tapping port 102 on the side of the converter 100.

[0095] Furthermore, in the converter refining method of the present invention, such as​ (I) shown in FIG. 1, a seventh process of tapping the molten steel while the slag 33 generated in the sixth process remains in the converter 100 can be provided after the sixth process. Then, an eighth process of selecting and performing any of the following treatments based on at least one of the estimated P2O5 component amount of the slag 33 in the converter 100 and the P component target value of the steel for the next heat treatment, the treatment of leaving all of the slag 33 in the converter 100, or the treatment of leaving a part of the slag 33 in the converter 100 and performing deslagging of the other, can be provided after the seventh process. In this case, the first process of performing the next heat treatment while the slag 33 remains in the converter 100 can be provided after the eighth process. Thus, by performing the first process of the next heat treatment while the slag 33 after decarburization remains in the converter 100, the slag 33 can be reused as a flux for the next heat treatment.

[0096] Embodiment

[0097] Hereinafter, an embodiment will be shown to explain the effects and the like brought by the technology of the present application in more detail, but the technology of the present application is not limited to the following embodiment.

[0098] 1. Embodiment 1

[0099] 1.1 First Process

[0100] A 300-t top and bottom combined blown converter in which the decarburization slag of the previous heat treatment remained was charged with molten iron and scrap iron so as to reach 300 t.

[0101] The temperature and the composition of the molten iron in the converter at the end of the second process are shown in Table 1 below. In addition, the amount of the CaO newly charged for the second process, the basicity of the first slag used in the second process, and the amount of the slag generated in the second process are shown in Table 2 below. Furthermore, the CaO conversion amount and the SiO2 conversion amount of the first slag are shown in Table 3 below.

[0102] 1.2 Second Process

[0103] After the first process, a first dephosphorization blow was performed by newly charging a flux containing CaO into the converter. Note that the first flux used for the first dephosphorization blow corresponds to a flux composed of the flux containing CaO newly charged into the converter, the decarburization slag of the previous heat treatment, and the desiliconization slag generated by desiliconization.

[0104] The temperature and the composition of the molten iron in the converter at the end of the second process are shown in Table 1 below. In addition, the amount of the CaO newly charged for the second process, the basicity of the first slag used in the second process, and the amount of the slag generated in the second process are shown in Table 2 below. Furthermore, the CaO conversion amount and the SiO2 conversion amount of the first slag are shown in Table 3 below.

[0105] 1.3 3rd process

[0106] After the 2nd process, intermediate slagging of the molten slag in the furnace was performed by tilting the converter. At this time, the intermediate slagging amount was measured by the weighing instrument. The intermediate slagging rate was determined from the measured intermediate slagging amount. Specifically, the intermediate slagging rate of the 3rd process was calculated by dividing the measured value by the molten slag amount calculated from the charge in the 2nd process, using the weighing instrument corrected for the base metal components.

[0107] The intermediate slagging rate of the 3rd process is shown in Table 1 and 3 below.

[0108] 1.4 4th process

[0109] After the 3rd process, a dephosphorization blow was performed by charging a CaO-containing flux as a 2nd flux into the furnace.

[0110] The molten iron temperature, molten iron composition, and molten slag composition at the end of the 4th process are shown in Table 1 below. In addition, the amount of CaO newly charged for the 4th process is shown in Table 2 below. Further, the CaO conversion amount and Si02 conversion amount of the 2nd flux are shown in Table 3 below. In the 4th process, since Si02 source was not charged, the Si02 conversion amount was 0 kg.

[0111] 1.5 5th process

[0112] After the 4th process, intermediate slagging of the molten slag in the furnace was performed again by tilting the converter. At this time, the intermediate slagging amount was measured by the weighing instrument. The intermediate slagging rate was determined from the measured intermediate slagging amount. The intermediate slagging rate of the 5th process was determined by dividing the intermediate slagging amount of the 5th process measured by the weighing instrument corrected for the base metal components by the molten slag amount of the 5th process. Here, the molten slag amount remaining in the furnace after the 3rd process, i.e., the molten slag amount calculated from the charge after the 2nd process, was subtracted from the intermediate slagging amount measured by the weighing instrument corrected for the base metal components, and the resulting value was added to the flux amount charged in the 4th process, and the thus obtained value was used as the molten slag amount of the 5th process.

[0113] The intermediate slagging rate of the 5th process is shown in Table 1 and 3 below.

[0114] 1.6 6th process

[0115] After the 5th process, a decarburization blow was performed by charging a 3rd flux into the furnace. Here, the amounts of CaO source and Si02 source were adjusted in correspondence with the intermediate slagging rate so that the CaO / Si02 indicated by the following formula (1) reached a given value.

[0116] The molten steel temperature, molten steel composition, and molten slag composition at the time of the end of the 6th process are shown in Table 1 below. In addition, the amount of CaO, the amount of Si02, and the amount of molten slag generated in the 6th process, which were newly charged for the 6th process, are shown in Table 2 below. Further, the CaO conversion amount and the Si02conversion amount of the 3rd flux, and the value of the CaO / Si02charged, which is calculated by the following formula (1), are shown in Table 3 below.

[0117] [mathematical formula 3]

[0118]

[0119] C2: CaO conversion amount (kg) of the 1st flux

[0120] C4: CaO conversion amount (kg) of the 2nd flux

[0121] C6: CaO conversion amount (kg) of the 3rd flux

[0122] S2: Si02conversion amount (kg) of the 1st flux

[0123] S4: Si02conversion amount (kg) of the 2nd flux

[0124] S6: Si02conversion amount (kg) of the 3rd flux

[0125] a3: intermediate slagging rate (%) of the 3rd process

[0126] a5: intermediate slagging rate (%) of the 5th process

[0127] 2. Examples 2 to 5, Comparative Examples 1 and 4

[0128] The 1st process to the 6th process were performed under the conditions shown in Tables 1 to 3.

[0129] 3. Comparative Examples 2 and 3

[0130] The 1st process, the 2nd process, the 3rd process, and the 6th process were performed under the conditions shown in Tables 1 to 3. The 4th process and the 5th process were omitted.

[0131]

[0132]

[0133]

[0134] 4. Evaluation results

[0135] In Table 4 below, for each of Examples 1 to 5 and Comparative Examples 1 to 4, whether or not the 4th process and the 5th process were performed, the actual basicity of the charged CaO / SiO2, the decarburization slag, the amount of CaO that was not slagged during decarburization (determined from the difference between the actual basicity of the charged CaO / SiO2 and the decarburization slag), whether or not there was spattering of the decarburization initial stage, the P concentration in the final molten steel, the total amount of slag discharged to the outside of the system (total amount of slag discharged to the outside of the system), and the total amount of newly charged CaO in the series of processes are shown. Note that the "total amount of slag discharged to the outside of the system" is the cumulative value of the weight of the slag discharged to the outside of the furnace by the intermediate slagging in the 3rd process and the 5th process, and the weight of the slag discharged to the outside of the furnace when the amount of the molten slag in the furnace was adjusted to the amount in Table 2 after the end of the 6th process.

[0136]

[0137] From the conditions shown in Tables 1 to 3 and the results shown in Table 4, the following can be known.

[0138] (1) First, from the comparison between Examples 1 and 2 and Comparative Examples 1 and 4, the following can be known. That is, even if the charged CaO / SiO2 exceeds 4.5 as calculated by formula (1), the actual basicity (CaO / SiO2 (mass ratio)) as calculated from the analysis value of the slag by the 6th process does not exceed 4.5. This is because, if the charged CaO / SiO2 exceeds 4.5, the amount of SiO2 is relatively insufficient with respect to the amount of CaO, and therefore, the charged CaO is difficult to slag. As a result, in Comparative Examples 1 and 4, only a part of the CaO after slagging contributes to the dephosphorization reaction, and the amount of dephosphorization does not increase. In Comparative Example 1, the slagging of CaO is suppressed, and as a result, the amount of dephosphorization decreases. In Comparative Example 4, the total amount of slag discharged to the outside of the system and the amount of new CaO increase by an amount corresponding to the amount of suppression of the slagging of CaO. On the other hand, in Examples 1 and 2, by charging the SiO2 source in the 6th process and setting the charged CaO / SiO2 to 4.5 or less, it is possible to increase the CaO after slagging, and it is possible to increase the amount of effective slag that contributes to the dephosphorization reaction. As a result, it is possible to reduce the [%P] compared to Comparative Example 1. In addition, it is possible to reduce the total amount of slag discharged to the outside of the system and the amount of new CaO compared to Comparative Example 4.

[0139] (2) From the comparison between Examples 1 and 2 and Comparative Example 2, the following can be known. That is, in the case where the same degree of [%P] (≈0.015) is melted, compared to Comparative Example 2 in which dephosphorization and slagging are performed in only one stage, in Examples 1 and 2 in which dephosphorization and slagging are performed in two stages, it is possible to reduce the amount of slag discharged to the outside of the system and the amount of necessary CaO by appropriately adjusting the amount of necessary CaO and the SiO source.

[0140] (3) The following was known from the comparison between Example 3, 4 and Comparative Example 3. That is, even in the case where dephosphorization and slagging were performed in two stages, the amount of slag discharged outside the system and the amount of CaO necessary could be reduced when the melting load was high [%P] (≈0.010).

[0141] (4) In Comparative Example 3 where dephosphorization and slagging were performed in only one stage and the amount of slag was increased in order to reduce [%P], a large amount of slag remained in the converter at the 6th step, and slag spitting was observed at the early stage of the 6th step. On the other hand, in Examples 1 to 4, no slag spitting was observed.

[0142] (5) From the comparison between Examples 1 to 4 and Example 5, it was known that the same effect was exerted whether the decarburization slag of the previous heat treatment was used as the 1st flux or not.

[0143] As described above, the converter refining method of Examples 1 to 5 was easy to suppress slag spitting at the early stage of decarburization blowing, could stably reduce P in the molten steel after decarburization blowing, and was easy to reduce the amount of slag discharged outside the system.

Claims

1. A converter refining method, the method comprising: a first step of charging molten iron into a converter; a second step of performing first-stage molten iron dephosphorization on the molten iron in the converter using a first flux having a basicity CaO / SiO2 of 0.9 or more and 1.4 or less after the first step; a third step of discharging 40% or more and 70% or less of molten slag in the converter to the outside of the converter after the second step; a fourth step of adding a second flux to the converter and then performing second-stage molten iron dephosphorization after the third step; a fifth step of discharging 40% or more and 70% or less of molten slag in the converter to the outside of the converter after the fourth step; and a sixth step of adding a third flux to the converter and then performing decarburization after the fifth step, the third flux containing a CaO source and a SiO2 source, a charging CaO / SiO2 of 3.0 or more and 4.5 or less, C2: a CaO conversion amount of the first flux in kg C4: a CaO conversion amount of the second flux in kg C6: a CaO conversion amount of the third flux in kg S2: a SiO2 conversion amount of the first flux in kg S4: a SiO2 conversion amount of the second flux in kg S6: a SiO2 conversion amount of the third flux in kg a3: an intermediate slag discharge rate of the third step in % a5: an intermediate slag discharge rate of the fifth step in %.

2. The converter refining method according to claim 1, wherein the intermediate slag discharge rate of the third step or the fifth step is measured at least once by a weighing instrument or based on the volume of the discharged molten slag.

3. The converter refining method according to claim 1 or 2, wherein in the sixth step, the amount of the third flux added to the converter is 8 kg / ton-steel liquid or more and 25 kg / ton-steel liquid or less in terms of CaO.

4. The converter refining method according to claim 1 or 2, the method comprising: a seventh step of performing tapping with the molten slag generated in the sixth step remaining in the converter after the sixth step; and an eighth step of selecting and performing any of the following treatments based on at least one of the estimated P2O5 component amount of the molten slag in the converter and the P component target value of the steel of the next heat treatment, 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 eighth step, a first step of the next heat treatment is performed with the molten slag remaining in the converter.

5. The converter refining method according to claim 1 or 2, wherein the first flux, the second flux, and the third flux are each a flux containing a CaO source and a SiO2 source. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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