KR20230066610A - Converter refining method - Google Patents

Abstract

A converter refining method capable of suppressing sloping in the initial stage of decarburization blow tempering and stably reducing P in molten steel after decarburization blow tempering is disclosed. In the converter refining method of the present disclosure, a first step of charging molten iron into a converter, and a second step of dephosphorizing molten pig iron while using a first flux with respect to the molten pig iron in the converter after the first step, After the second step, a third step of excluding at least a part of the slag in the converter outside the converter, and a fourth step of decarburizing after adding a second flux into the converter after the third step, , the second flux includes a CaO source and a SiO ₂ source, the charged CaO/SiO ₂ defined by a predetermined formula is 3.0 or more and 4.5 or less, and the charged CaO defined by a predetermined formula is 30.0 kg/ton-steel or less am.

Description

Converter refining method

The present application discloses a converter refining method.

A process for refining molten pig iron using a converter, wherein molten pig iron is dephosphorized in a first converter, and then the molten pig iron tapped from the first converter is charged into a second converter and decarburization is performed in the second converter (I); A process (II) has been developed in which dephosphorization is performed in one converter, slag produced by dephosphorization is removed (intermediate removal), and then decarburization is continued in the same converter. Process (I) has a high refining capacity, but since it requires two converters, the equipment cost is high, and furthermore, the dissipation heat loss increases, and the ability to melt iron ore and scrap is lowered. Compared with process (I), process (II) can shorten the overall blow time, can reduce the amount of flux required for dephosphorization, and can reduce heat loss at the time of scouring. However, in process (II), it is difficult to stably control the amount of intermediate waste, and it may be difficult to extremely reduce the P concentration in molten steel after completion of refining, for example.

Patent Literature 1 discloses a converter refining method in which 1.0 to 4.0 kg of flux containing 60 to 99% of SiO ₂ is added per ton of produced molten steel as SiO ₂ minutes before decarburization in Process (II). Patent Literature 2 discloses a method of using desiliconization slag in decarburization treatment after dephosphorization treatment.

Japanese Patent No. 3194212 Japanese Patent No. 6223249

According to what the present inventors newly discovered, the prior art related to process (II) has the following problems. That is, in the prior art, slag in the converter is reduced in basicity at the time of decarburization blow tempering, slag overflows from the inside of the converter in the initial stage of decarburization blow tempering (sloping), and there is a possibility that stable operation becomes difficult. Or, at the time of decarburization blow tempering, the slag in the converter becomes highly basic, and the flux introduced for decarburization is not sufficiently slag, making it impossible to sufficiently secure a sufficient amount of slag with high dephosphorization ability. It may become difficult to lower. Or, the amount of slag in decarburization blow tempering becomes large, and there is a possibility that sloping may occur in the initial stage of decarburization blow tempering.

This inventor performed actual measurement of the molten slag at the time of intermediate|middle loading with a "weighing machine". As a result, it was found that the intermediate removal rate fluctuated greatly from 50 to 95%, more than conventional assumptions. That is, in the prior art, since the intermediate rejection ratio rapidly decreased, a discrepancy between calculated basicity and actual basicity occurred, and this was found to be one of the causes of low basicity or high basicity of the slag in the converter during decarburization blasting. . Then, the present inventor invented a method of accurately specifying an intermediate material removal rate at least once by "weighing machine or the like" and operating based on the accurately specified intermediate material removal rate. Specifically, it is as follows.

As one of the means for solving the above problems,

The 1st process of charging molten iron into the inside of a converter,

A second step of dephosphorizing the molten pig iron in the converter after the first step, using a first flux;

A third step of discharging at least a part of the slag in the converter outside the converter after the second step;

A fourth step of performing decarburization after adding a second flux in the converter after the third step,

equipped,

The second flux includes a CaO source and a SiO ₂ source,

The following formulas (1) and (2) are satisfied,

Converter refining method

Initiate.

C2: CaO equivalent in the first flux (kg/ton-steel)

C4: CaO equivalent in the second flux (kg/ton-steel)

S2: SiO ₂ equivalent in the first flux (kg/ton-steel)

S4: SiO ₂ equivalent in the second flux (kg/ton-steel)

α3: Intermediate exclusion rate (%) in the third step

The converter refining method of the present disclosure,

A fifth step of performing steel tapping after the fourth step, while leaving the slag generated in the fourth step in the converter, and

After the fifth step, based on at least one of an estimated P ₂ O ₅ component amount of the slag in the converter and a target value of the P component of the steel of the next heat, the entire amount of the slag in the converter is left in the converter; or a sixth step of selecting and executing any of the treatment of removing the others while leaving a part of the slag in the converter in the converter.

can be provided,

After the sixth step, the first step of the next heat may be performed with the slag remaining in the converter.

According to the converter refining method of this indication, it is easy to suppress the sloping in the initial stage of decarburization blow tempering. Moreover, according to the converter refining method of this indication, it is easy to stably reduce P in molten steel after decarburization blow tempering.

1 is a schematic diagram for explaining an example of a flow of a converter refining method.

As shown in Fig. 1 (A) to (F), the converter refining method of the present disclosure is a first step (Fig. 1 (A)) of charging molten iron 10 into the converter 100. , After the 1st process, the molten pig iron 10 in the converter 100 is dephosphorized while using the 1st flux 21, the 2nd process (FIG. 1(B)), after the 2nd process, the converter After the third step (FIG. 1(C)) of removing at least a part of the slag 31 in the converter 100 out of the converter 100, and the third step, the second flux 22 is added into the converter 100. 4th process ((D) and (E) of FIG. 1) which performs decarburization after addition is provided. Here, in the converter refining method of the present disclosure, the second flux 22 contains a CaO source and a SiO ₂ source. In addition, in the converter refining method of the present disclosure, the following formulas (1) and (2) are satisfied.

C2: CaO conversion amount in the first flux (21) (kg/ton-steel)

C4: CaO conversion amount in the second flux (22) (kg/ton-steel)

S2: SiO ₂ conversion amount in the first flux 21 (kg/ton-steel)

S4: SiO ₂ conversion amount in the second flux 22 (kg/ton-steel)

α3: Intermediate exclusion rate in the third step (%)

1. First process

As shown in FIG. 1(A), in the first step, the molten iron 10 is charged into the converter 100. The conditions in the 1st process are not specifically limited.

As the converter 100, a conventional one may be used. In the converter refining method of the present disclosure, the converter 100 may be any of a top-blow converter, a low-blow converter, and a bottom-blow converter. In the top-blowing converter, since there is no agitation from the bottom, iron oxide produced by oxidation of iron by top-blowing oxygen is difficult to reduce, and the amount of slag tends to increase excessively. Also, slag rich in iron oxide tends to slag CaO. On the other hand, in the upper and lower blow converters and the lower blow converters, the iron oxide concentration should not be so high, and the slag of CaO tends to be less likely to proceed compared to the case in the top blow converter. From the viewpoint of obtaining a higher effect from this point and the technique of the present disclosure, the converter 100 may be either a low-blow converter or a top-bottom blow converter, or may be a top-bottom blow converter. 1(A) to (F) show a top-bottom blowing converter as an example of the converter 100. The top and bottom blow converter 100 may be equipped with a plurality of flow paths 101 for supplying low blow gas into the furnace at its bottom. Moreover, the top-bottom blow converter 100 may equip its side part with the tapping port 102 for tapping the molten steel 12.

As the molten iron 10 charged into the inside of the converter 100, any general blast furnace molten iron can be employed, for example. The molten iron 10 may contain Si as an impurity other than containing P and C. When molten iron|metal 10 contains Si, the desiliconization reaction of Si in molten iron|metal 10 advances by oxidative refining by oxygen gas, and then a dephosphorization reaction advances. In other words, desiliconization of molten pig iron may be performed after the 1st process and before molten pig iron dephosphorization in the 2nd process. Further, after desiliconization, the desiliconization slag in the converter 100 may be removed, or dephosphorization may be performed with the desiliconization slag remaining in the converter 100. In the latter case, desiliconization slag may be used as the first flux 21 . As shown in FIG. 1(A) , the molten iron 10 may be a mixture of molten iron 10a (for example, blast furnace molten iron) and additive materials such as scrap 10b.

The method of charging the molten iron 10 into the converter 100 is not particularly limited either, and examples thereof include a method of introducing the molten iron 10 into the converter 100 using a known container such as a molten iron pot.

2. Second process

As shown in FIG.1(B), in a 2nd process, molten pig iron dephosphorization is performed, using the 1st flux 21 after a 1st process. The conditions for dephosphorization in the second step are not particularly limited.

The 1st flux 21 may be what was put into the converter 100 before dephosphorization in the 2nd process, and the one using the component derived from molten iron|metal, such as desiliconization slag produced|generated by the desiliconization reaction as mentioned above Alternatively, the decarburization slag 32 after decarburization refining of the previous heat may be left in the converter 100 and used as a flux. The composition and amount of the first flux 21 are not particularly limited, as long as the composition and amount allow for the target dephosphorization. For example, the first flux 21 may contain a CaO source. Examples of the CaO source include quicklime, limestone, dolomite, and decarburized slag 32 of previous heat. In addition, the first flux 21 may contain a SiO ₂ source. Examples of the SiO ₂ source include desiliconization slag, former heat decarburized slag 32, silica stone, and olivine. The basicity CaO/SiO ₂ of the first flux 21 may be 0.9 or more and 1.4 or less. In addition, the amount of the first flux 21 may be 5 kg/ton-steel or more and 25 kg/ton-steel or less in terms of CaO. In addition, the amount of the first flux 21 may be 0 kg/ton-steel or more and 5 kg/ton-steel or less in terms of SiO ₂ . In addition, in this application, "kg/ton-steel" is the mass per 1 ton of molten steel finally obtained.

As shown in FIG. 1(B) , in the second step, for example, by blowing oxygen into the molten iron 10 from the top blowing lance 200, even if oxidative refining is advanced while stirring the molten iron 10 On the other hand, stirring of the molten pig iron 10 during refining may be enhanced by continuously or intermittently blowing in low-blowing gas from the bottom of the converter 100.

A part of P contained in molten iron|metal 10 is removed by dephosphorization in a 2nd process, and dephosphorization molten iron|metal 11 is obtained. The P concentration in the dephosphorized molten iron 11 is not particularly limited. For example, the dephosphorization hot metal 11 may contain 0.02 mass % or more or 0.03 mass % or more of P, and may contain 0.08 mass % or less or 0.06 mass % or less of P.

3. Third process

As shown in FIG. 1(C), in a 3rd process, at least a part of the slag 31 in the converter 100 is excluded from the converter 100 after a 2nd process. For example, as shown in FIG. 1(C), the slag 31 may be discharged out of the system by tilting the converter 100. Moreover, in a 3rd process WHEREIN: You may form the slag 31 by continuously blowing in low-blowing gas from the bottom part of the converter 100. Thereby, the exhaustion of the slag 31 becomes easier.

The material removal rate (intermediate material removal rate) of the slag 31 in the third step is not particularly limited, and may be, for example, 40% or more and 70% or less. The composition or production amount of the slag 31 may be arbitrarily changed depending on the dephosphorization conditions in the second process.

4. Fourth process

As shown in (D) and (E) of FIG. 1, in a 4th process, after the 3rd process, after adding the 2nd flux 22 in the converter 100, decarburization is performed. The fourth step is characterized in that the second flux 22 contains a CaO source and a SiO ₂ source, and that the above formulas (1) and (2) are satisfied. Other decarburization conditions are not particularly limited.

The second flux 22 is introduced into the converter 100 before decarburization in the fourth step. In the fourth process, together with the second flux 22, a part of the slag 31 remaining in the converter 100 without being eliminated in the third process can also be used as the flux 22x. The composition or amount of the second flux 22 is not particularly limited as long as the above formulas (1) and (2) are satisfied. The second flux 22 includes a CaO source and a SiO ₂ source. In the fourth step, the CaO source and the SiO ₂ source may be added simultaneously or separately in the converter 100 . Specific examples of the CaO source and the SiO ₂ source are as described above. In addition, the basicity CaO/SiO ₂ of the second flux 22 may be 3.2 or more or 4.2 or less. From the viewpoint of further increasing the amount of dephosphorization in the fourth step, the amount of the second flux 22 may be 8 kg/ton-steel or more and 25 kg/ton-steel or less in terms of CaO. In addition, the amount of the second flux 22 may be greater than 0 kg/ton-steel or less than 8 kg/ton-steel in terms of SiO ₂ .

In the converter refining method of the present disclosure, as in the above formula (1), charged CaO/ It is important that _SiO2 is 3.0 or more and 4.5 or less. Charged CaO/SiO ₂ may 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 findings of the present inventor, when the charge CaO/SiO ₂ is too small, CaO is insufficient and dephosphorization becomes difficult to proceed, and it becomes difficult to reduce the P concentration in the molten steel 12 finally obtained. Moreover, there exists a possibility that sloping may arise in the initial stage of decarburization blow tempering accompanying the low basicity of slag. On the other hand, in the past, it was common to add a large amount of CaO source as a flux in order to sufficiently advance dephosphorization, but according to the knowledge of the present inventors, even if a large amount of CaO source is added, not all of them contribute to dephosphorization. . According to the knowledge of the present inventors, when the charged CaO/SiO ₂ exceeds 4.5 due to a large amount of CaO source being added as a flux during the converter refining operation, the amount of SiO ₂ relative to CaO is insufficient, and during decarburization The slag is excessively high in alkalinity. When the slag becomes excessively basic during decarburization blow tempering, the flux injected into the converter is not sufficiently slag, and it becomes impossible to sufficiently secure the amount of slag with high dephosphorization ability, so that the P concentration in the molten steel 12 after decarburization blow tempering is stable. There is a risk that it will be difficult to lower it to .

In the converter refining method of the present disclosure, it is important that the charged CaO, defined by C2×(100−α3)/100+C4, is 30.0 kg/ton-steel or less, as in the above formula (2). In other words, the amount of slag in decarburization is below a certain level in terms of CaO. Charged CaO may 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. The lower limit of charged CaO is not particularly limited, and naturally exceeds 0 kg/ton-steel from the relationship of the above formula (1). The lower limit of charged CaO may be, for example, 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 findings of the present inventors, when the amount of slag in decarburization blasting is too large, there is a possibility that sloping may occur in the initial stage of decarburization blasting.

In the converter refining method of the present disclosure, the charged SiO ₂ defined by S2×(100−α3)/100+S4 is not particularly limited as long as the above formulas (1) and (2) are satisfied. The employable range of the charged SiO ₂ is naturally specified from the above formulas (1) and (2).

In the above formulas (1) and (2), C2 is the CaO equivalent amount (kg/ton-steel) in the first flux 21, and C4 is the CaO equivalent amount in the second flux 22 (kg/ton-steel). ton-steel), S2 is the SiO ₂ equivalent in the first flux 21 (kg/ton-steel), and S4 is the SiO ₂ equivalent in the second flux 22 (kg/ton-steel) am. That is, Ca contained in each flux is converted into CaO, Si is converted into SiO ₂ , and the amount thereof is specified. The amount of CaO or SiO ₂ in terms of each flux can be obtained from the composition of the flux before being charged into the converter and the amount of the charged flux. Alternatively, you may specify CaO conversion amount or SiO ₂ conversion amount based on the component etc. contained in the slag after removal. In addition, as described above, when desiliconization slag is used as the first flux 21, it is assumed that 100% of the silicon contained in the molten iron before desiliconization has changed to SiO ₂ , and in the first flux 21 You may specify the SiO ₂ conversion amount.

In the above formulas (1) and (2), α3 is the intermediate rejection ratio (%) in the third step. The intermediate rejection ratio can be determined from the amount of flux added to the converter 100 or the amount of flux that existed in the converter 100 and the amount of slag (metal removed) removed from the converter 100. The intermediate exclusion ratio may be empirically estimated from past operations or the like, or may be obtained from online or offline measurement values during operation. In particular, it is preferable to actually measure the intermediate exclusion ratio at least once with a weighing machine or the like. By measuring the intermediate material removal rate at least once, even if the use of the weighing machine is stopped due to a failure or trouble of the weighing machine, etc., using the measured value in the past, empirically based on operating conditions, etc. Intermediate exclusion rates can be estimated with high accuracy. In addition, as a weighing method other than the method using a weighing machine, as disclosed in Japanese Unexamined Patent Publication No. 2018-119195, a method of obtaining a waste removal rate based on the volume of discharged slag, and the like are exemplified.

Thus, charging CaO/SiO ₂ can be controlled with high precision within the target range by specifying the intermediate rejection ratio α3 in the third step. That is, in the converter refining method of the present disclosure, based on the step of specifying the intermediate rejection ratio α3 in the third step and the specified intermediate rejection ratio α3 and the CaO equivalent amount and SiO ₂ equivalent amount in the first flux, It may be provided with a step of determining the added amount of the second flux, the CaO equivalent amount of the second flux, and/or the SiO ₂ equivalent amount of the second flux so that formulas (1) and (2) are satisfied. More specifically, based on the process of specifying the waste removal amount of the slag 31 in the 3rd process, for example after the 3rd process, and the specified waste removal amount, the intermediate waste removal of the said formula (1) and (2), for example The step of specifying the ratio α3, the addition of the second flux so that the above equations (1) and (2) are satisfied, based on the specified intermediate exclusion ratio α3 and the amount of CaO equivalent and SiO ₂ equivalent in the first flux It may be provided with a step of determining the amount, the CaO equivalent amount of the second flux, and/or the SiO ₂ equivalent amount of the second flux. For example, by measuring the weight of the slag removed and subtracting the weight of the metal contained in the slag here, the slag removal amount and intermediate removal rate can be specified with high accuracy. The current weight contained in slag may be empirically estimated from past operations or the like, or may be actually measured online or offline during operation.

As described above, according to the converter refining method of the present disclosure, sloping at the initial stage of decarburization blow refining can be prevented by satisfying Expressions (1) and (2). In addition, at the time of decarburization, it is possible to perform effective dephosphorization securing the minimum amount of slag required for dephosphorization. Therefore, the P concentration of the finally obtained molten steel 12 can be reduced.

5. Supplements

In the converter refining method of the present disclosure, as shown in FIG. 1(F), the molten steel 12 in the converter 100 may be tapped out of the converter 100 after the fourth step. For example, the converter 100 may be tilted, and the molten steel 12 may flow out of the outlet 102 on the side of the converter 100.

Further, in the converter refining method of the present disclosure, as shown in FIG. 1 (F), after the fourth step, the slag 32 generated in the fourth step is left in the converter 100, You may be equipped with the 5th process. Then, after the fifth step, based on at least one of the estimated P ₂ O ₅ component amount of the slag 32 in the converter 100 and the target value for the steel P component of the next heat, the slag 32 in the converter 100 A sixth step of selecting and executing either a treatment for leaving the entire amount in the converter 100 or a treatment for leaving a part of the slag 32 in the converter 100 while excluding the others while remaining in the converter 100 You may have it. In this case, after the 6th process, you may perform the 1st process of the next heat, with the slag 32 remaining in the converter 100. In this way, by performing the first step of the next heat with the slag 32 remaining in the converter 100 after decarburization, the slag 32 can be reused as a flux for the next heat.

Example

Hereinafter, effects and the like of the technology of the present disclosure will be described in more detail while exemplifying examples, but the technology of the present disclosure is not limited to the following examples.

1. Example 1

1.1 First process

Into a 300t top-bottom blowing converter in which decarburization slag of all heats remained, molten iron and scrap were charged so as to be 300t.

Table 1 below shows the temperature and composition of the molten iron in the furnace in the first step. Table 2 below shows the residual amount of decarburized slag of all heats.

1.2 Second process

After the 1st process, the flux containing CaO was newly injected|thrown-in into a furnace, and dephosphorization blow tempering was performed. In addition, the 1st flux used for dephosphorization blow tempering corresponds to what combined the flux containing CaO newly injected|thrown-in into a furnace, the decarburized slag of the previous heat, and the desiliconization slag generated by desiliconization.

In Table 1 below, the molten iron temperature, molten iron composition, and slag composition at the time of completion of the second process are shown. In addition, Table 2 below shows the amount of CaO newly introduced 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. Table 3 below shows the CaO equivalent and SiO ₂ equivalent in the first flux.

1.3 The third process

After the second step, the intermediate removal of the slag in the furnace was performed by tilting the converter. At this time, the intermediate loading amount was measured by a weighing machine. The intermediate exclusion ratio was specified from the measured intermediate exclusion ratio. Specifically, the intermediate removal rate in the third step was obtained by dividing the weighing value measured by the weighing machine corrected for the ground powder by the amount of slag obtained in advance from the charge in the second step.

In Tables 1 and 3 below, the intermediate rejection ratio in the third step is shown.

1.4 4th process

After the 3rd process, the 2nd flux was injected|thrown-in into the furnace, and decarburization blow tempering was performed. Here, the charged amount of the CaO source and the silica source was adjusted according to the intermediate loading ratio so that the charged CaO/SiO ₂ represented by the following (1) became a predetermined value. In addition, the charged amount of the CaO source was adjusted according to the intermediate loading ratio so that the charged CaO represented by the following (2) became a predetermined value.

In Table 1 below, the molten steel temperature, molten steel composition, and slag composition at the time of completion of the fourth process are shown. In addition, in Table 2 below, the amount of CaO newly introduced for the fourth process, the amount of SiO ₂ and the amount of slag generated in the fourth process are shown. In addition, in Table 3 below, the amount of CaO converted and SiO ₂ in the second flux, the value of charged CaO/SiO ₂ calculated from the following formula (I), and the charged CaO calculated from the following formula (II) represents a value.

C2: CaO conversion amount in the first flux (kg/ton-steel)

C4: CaO conversion amount in the second flux (kg/ton-steel)

S2: SiO ₂ equivalent in the first flux (kg/ton-steel)

S4: SiO ₂ conversion amount in the second flux (kg/ton-steel)

α3: Intermediate exclusion rate in the third step (%)

2. Examples 2 to 6, Comparative Examples 1 to 5

The 1st process - the 4th process were performed on the conditions shown in Tables 1-3.

[Table 1]

[Table 2]

[Table 3]

3. Evaluation results

In Table 4, for each of Examples 1 to 6 and Comparative Examples 1 to 5, charged CaO/SiO ₂ for formula (I), charged CaO for formula (II), actual basicity of decarburized slag, decarburization The amount of CaO that has not been slaged in the process (determined from the difference between the actual basicity of charged CaO/SiO ₂ and decarburized slag), the presence or absence of sloping at the initial stage of decarburization, the P concentration in the finally obtained molten steel, and the system by passing through a series of steps The total amount of slag discharged to the outside (the total amount of slag discharged outside the system) and the amount of new CaO (the sum of the amount of CaO newly injected in the second process and the fourth process) are respectively shown. In addition, the "total amount of slag discharged outside the system" is the furnace when the weight of the slag discharged outside the furnace by the intermediate removal in the third step and the amount of slag in the furnace after the end of the fourth step are adjusted to the amount shown in Table 2 above. It was set as the integration value with the weight of the slag excluded outside.

[Table 4]

From the conditions shown in Tables 1 to 3 and the results shown in Table 4, the following can be seen.

(1) As is clear from the results of Example 1 and Comparative Example 1, even if the total amount of slag discharged out of the system was about the same, the charged CaO/SiO ₂ was higher and the unreacted CaO was higher than in Comparative Example 1. , In Example 1 in which the charged CaO/SiO ₂ was lowered and the SiO ₂ source was appropriately added, the P concentration in the molten steel could be lowered.

(2) As is clear from the results of Example 2 and Comparative Example 2, in Comparative Example 2 in which SiO ₂ source was excessively added and the charged CaO/SiO ₂ was less than 3.0, the P concentration in molten steel could not be reduced. On the other hand, in Example 2, although the addition of the SiO ₂ source was larger than in Example 1, since the charged CaO/SiO ₂ was about 3.8 or more than 3.0, the P concentration in the molten steel could be lowered.

(3) As is clear from the results of Comparative Example 2, when the charged CaO/SiO ₂ is less than 3.0, the slag during decarburization is excessively low in basicity, and a large amount of slag with high viscosity and easy to bubble is generated. Sloping, in which slag overflows from the inside of the converter, occurred in the initial stage of decarburization.

(4) As is clear from the results of Examples 3 and 4, when decarburization blow tempering was performed at a charge CaO/SiO ₂ around 4.5, the P concentration in molten steel could be sufficiently reduced. In particular, in Example 4 in which the CaO source and the SiO ₂ source were added so as to increase the amount of slag, the P concentration in the molten steel could be lowered, and the melting of the extremely low tensile steel was possible.

(5) As is clear from the results of Examples 3 and 4 and Comparative Examples 3 and 4, in Comparative Example 3 in which the loading CaO/SiO ₂ greatly exceeds 4.5, although the medium exclusion ratio was the same as in Examples 3 and 4. , there was a large amount of unreacted CaO, and slag was not sufficiently generated, so the P concentration in molten steel could not be lowered. In Comparative Example 4, although the SiO ₂ source was added, the amount of addition was not sufficient and the charge CaO/SiO ₂ still exceeded 4.5, so the P concentration in the molten steel could not be sufficiently lowered.

(6) As is clear from the results of Comparative Example 5, when the charged CaO exceeds 30.0 kg/t-steel, the P concentration in molten steel does not decrease, and sloping occurs at the initial stage of decarburization blow. On the other hand, in Example 5 in which the charged CaO was 30.0 kg/t-steel or less, dephosphorization could be satisfactorily performed without sloping.

(7) From the comparison between Examples 1 to 5 and Example 6, it can be seen that the same effect is exhibited both when decarburized slag of all heats is used as the first flux and when it is not used.

As described above, it was found that the converter refining methods according to Examples 1 to 6 are easy to suppress sloping in the initial stage of decarburization blow tempering, and it is possible to stably lower P in molten steel after decarburization blow tempering.

10: dragon ship
11: dephosphorized chartered ship
12: molten steel
21: first flux
22: second flux
31: slag
32: slag
100: converter

Claims (2)

The 1st process of charging molten iron into the inside of a converter,
A second step of dephosphorizing the molten pig iron in the converter after the first step, using a first flux;
A third step of discharging at least a part of the slag in the converter outside the converter after the second step;
A fourth step of performing decarburization after adding a second flux in the converter after the third step,
equipped,
The second flux includes a CaO source and a SiO ₂ source,
The following formulas (1) and (2) are satisfied,
converter refining method.

C2: CaO equivalent in the first flux (kg/ton-steel)
C4: CaO equivalent in the second flux (kg/ton-steel)
S2: SiO ₂ equivalent in the first flux (kg/ton-steel)
S4: SiO ₂ equivalent in the second flux (kg/ton-steel)
α3: Intermediate rejection rate (%) in the third step According to claim 1,
A fifth step of performing steel tapping after the fourth step, while leaving the slag generated in the fourth step in the converter, and
After the fifth step, based on at least one of an estimated P ₂ O ₅ component amount of the slag in the converter and a target value of the P component of the steel of the next heat, the entire amount of the slag in the converter is left in the converter; or a sixth step of selecting and executing any of the treatment of removing the others while leaving a part of the slag in the converter in the converter.
equipped,
After the sixth step, the first step of the next heat is performed with the slag remaining in the converter,
converter refining method.

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