JP7469716B2 - Converter refining method - Google Patents

Description

This application discloses a converter refining method.

As a process for refining molten iron using converters, two processes have been developed: process (I) in which molten iron is dephosphorized in a first converter, and then the molten iron tapped from the first converter is charged into a second converter and decarburized in the second converter, and process (II) in which dephosphorization is performed in one converter, the slag generated by the dephosphorization is discharged (intermediate slag discharge), and then decarburization is continued in the same converter. Process (I) has a high refining capacity, but requires two converters, which requires high equipment costs, and further increases the dissipation heat loss, reducing the melting capacity of iron ore and scrap. Process (II) can shorten the total blowing time, reduce the amount of flux required for dephosphorization, and reduce heat loss during refining compared to process (I). However, in process (II), it is difficult to stably control the amount of intermediate slag discharge, and for example, it may be difficult to drastically reduce the P concentration in the molten steel after refining is completed.

Patent Document 1 discloses a converter refining method in which a flux containing 60 to 99% _SiO2 is converted to _SiO2 and added in an amount of 1.0 to 4.0 kg per ton of molten steel produced before the decarburization in process (II). Patent Document 2 discloses a method in which desiliconization slag is used in the decarburization treatment after the dephosphorization treatment.

Patent No. 3194212 Japanese Patent No. 6223249

According to the new findings of the present inventors, the conventional technology relating to process (II) has the following problems. That is, in the conventional technology, the slag in the converter becomes low in basicity during decarburization blowing, and the slag overflows from the converter (slopping) in the early stage of decarburization blowing, which may make stable operation difficult. Alternatively, the slag in the converter becomes high in basicity during decarburization blowing, and the flux added for decarburization does not turn into slag sufficiently, making it impossible to secure a sufficient amount of slag with high dephosphorization ability, which may make it difficult to stably reduce the P concentration in the molten steel after decarburization blowing. Alternatively, the amount of slag in decarburization blowing becomes large, which may cause slopping in the early stage of decarburization blowing.

The inventors actually measured the molten slag during intermediate slag removal using a "weigher." As a result, they found that the intermediate slag removal rate varied more widely, from 50 to 95%, than previously assumed. In other words, in the conventional technology, the intermediate slag removal rate dropped sharply, causing a discrepancy between the calculated basicity and the actual basicity, and it was found that this was one of the causes of the low or high basicity of the slag in the converter during decarburization blowing. Therefore, the inventors invented a method to accurately determine the intermediate slag removal rate at least once using a "weigher, etc." and to operate the converter based on the accurately determined intermediate slag removal rate. Specifically, it is as follows.

As one of the means for solving the above problems, the present application provides:
A first step of charging molten iron into a converter;
a second step of dephosphorizing the molten iron in the converter using a first flux after the first step;
A third step of discharging at least a portion of the slag in the converter to the outside of the converter after the second step;
a fourth step of adding a second flux into the converter after the third step and then carrying out decarburization;
Equipped with
The second flux includes a CaO source and a _SiO2 source;
The following formulas (1) and (2) are satisfied:
A converter refining process is disclosed.

C2: CaO equivalent amount in the first flux (kg/ton-steel)
C4: CaO equivalent amount in the second flux (kg/ton-steel)
S2: _SiO2 equivalent amount in the first flux (kg/ton-steel)
S4: _SiO2 equivalent amount in the second flux (kg/ton-steel)
α3: Intermediate slag removal rate (%) in the third step

The converter refining method of the present disclosure comprises:
A fifth step, which is performed after the fourth step, while leaving the slag generated in the fourth step in the converter, and
A _sixth step, after the fifth step, based on at least one of an estimated _P2O5 content of the slag in the converter and a target value of the P content of the steel of the next heat, is selected and executed to either leave the entire amount of the slag in the converter in the converter or leave a part of the slag in the converter in the converter while discharging the rest;
and
After the sixth step, the first step of the next heat may be carried out while the slag remains in the converter.

According to the converter refining method disclosed herein, it is easy to suppress slopping in the early stage of decarburization blowing. In addition, according to the converter refining method disclosed herein, it is easy to stably reduce the P in the molten steel after decarburization blowing.

FIG. 2 is a schematic diagram for explaining an example of the flow of a converter refining method.

As shown in Figs. 1(A) to (F), the converter refining method of the present disclosure includes a first step (Fig. 1(A)) of charging molten iron 10 into a converter 100, a second step (Fig. 1(B)) of dephosphorizing the molten iron 10 in the converter 100 using a first flux 21 after the first step, a third step (Fig. 1(C)) of discharging at least a part of the slag 31 in the converter 100 to the outside of the converter 100 after the second step, and a fourth step (Figs. 1(D) and (E)) of adding a second flux 22 to the converter 100 and then performing decarburization after the third step. Here, in the converter refining method of the present disclosure, the second flux 22 includes 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 equivalent amount in the first flux 21 (kg/ton-steel)
C4: CaO equivalent amount in the second flux 22 (kg/ton-steel)
S2: _SiO2 equivalent amount in the first flux 21 (kg/ton-steel)
S4: _SiO2 equivalent amount in the second flux 22 (kg/ton-steel)
α3: Intermediate slag removal rate (%) in the third process

1. First Step As shown in Fig. 1(A), in the first step, molten iron 10 is charged into a converter 100. The conditions in the first step are not particularly limited.

The converter 100 may be the same as that used in the past. In the converter refining method of the present disclosure, the converter 100 may be any of a top-blown converter, a bottom-blown converter, and a top-and-bottom-blown converter. In a top-blown converter, since there is no stirring from the bottom, the iron oxide formed by the top-blown oxygen oxidizing the iron is difficult to reduce, and the amount of slag tends to increase excessively. In addition, slag with a large amount of iron oxide is easy to turn CaO into slag. On the other hand, in a top-and-bottom-blown converter or a bottom-blown converter, the iron oxide concentration does not become so high, and compared to the case of a top-blown converter, the turn into slag of CaO tends to progress less. In this respect, from the viewpoint of obtaining a higher effect by the technology of the present disclosure, the converter 100 may be one of a bottom-blown converter and a top-and-bottom-blown converter, or may be a top-and-bottom-blown converter. Figures 1(A) to (F) show a top-and-bottom-blown converter as an example of the converter 100. The top and bottom blown converter 100 may have a plurality of flow passages 101 at its bottom for supplying bottom blown gas into the furnace. The top and bottom blown converter 100 may also have a tapping port 102 at its side for tapping the molten steel 12.

As the molten iron 10 charged into the converter 100, for example, any general blast furnace molten iron can be used. The molten iron 10 may contain Si in addition to P and C as impurities. When the molten iron 10 contains Si, the desiliconization reaction of Si in the molten iron 10 proceeds by oxidation refining with oxygen gas, and then the dephosphorization reaction proceeds. In other words, after the first step, the molten iron may be desiliconized before the molten iron dephosphorization in the second step. After desiliconization, the desiliconization slag in the converter 100 may be drained, or the desiliconization may be performed while leaving the desiliconization slag in the converter 100. In the latter case, the 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 (e.g., 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, and examples include pouring the molten iron 10 into the converter 100 using a known container such as a molten iron ladle.

1(B) , in the second step, after the first step, the hot metal is dephosphorized using a first flux 21. The conditions for dephosphorization in the second step are not particularly limited.

The first flux 21 may be a flux introduced into the converter 100 before the dephosphorization in the second step, or may be a flux using components derived from the molten iron, such as desiliconization slag generated by the desiliconization reaction as described above, or may be a flux using the decarburization slag 32 left in the converter 100 after the decarburization refining of the previous heat. The composition and amount of the first flux 21 are not particularly limited, and may be any composition and amount that can achieve the desired dephosphorization. For example, the first flux 21 may contain a CaO source. Examples of the CaO source include quicklime, limestone, dolomite, and the decarburization slag 32 of the previous heat. The first flux 21 may also contain a SiO ₂ source. Examples of the SiO ₂ source include desiliconization slag, the decarburization slag 32 of the previous heat, silica, and olivine. The basicity CaO/SiO ₂ of the first flux 21 may be 0.9 or more, or 1.4 or less. The amount of the first flux 21 may be 5 kg/ton-steel or more, or 25 kg/ton-steel or less, calculated as CaO. The amount of the first flux 21 may be 0 kg/ton-steel or more, or 5 kg/ton-steel or less, calculated as _SiO2 . In this application, "kg/ton-steel" refers to the mass per ton of molten steel finally obtained.

As shown in FIG. 1(B), in the second step, for example, oxygen may be blown into the molten iron 10 from a top-blowing lance 200 to stir the molten iron 10 while oxidative refining is proceeding, while bottom-blown gas may be blown continuously or intermittently from the bottom of the converter 100 to enhance the stirring of the molten iron 10 during refining.

By the dephosphorization in the second step, a portion of the P contained in the molten iron 10 is removed, and dephosphorized molten iron 11 is obtained. The P concentration in the dephosphorized molten iron 11 is not particularly limited. For example, the dephosphorized molten iron 11 may contain P in an amount of 0.02 mass% or more, or 0.03 mass% or more, or 0.08 mass% or less, or 0.06 mass% or less.

3. Third Step As shown in Fig. 1(C), in the third step, after the second step, at least a part of the slag 31 in the converter 100 is discharged outside the converter 100. For example, as shown in Fig. 1(C), the converter 100 may be tilted to allow the slag 31 to flow out of the system. In addition, in the third step, the slag 31 may be foamed by continuously blowing bottom blown gas from the bottom of the converter 100. This makes it easier to discharge the slag 31.

The slag removal rate (intermediate slag 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 and amount of slag 31 produced may vary arbitrarily depending on the dephosphorization conditions in the second step.

1(D) and (E), in the fourth step, after the third step, the second flux 22 is added to the converter 100 and decarburization is performed. The fourth step is characterized in that the second flux 22 contains a CaO source and a _SiO2 source, and that the above formulas (1) and (2) are satisfied. Other decarburization conditions are not particularly limited.

The second flux 22 is charged into the converter 100 before the decarburization in the fourth step. In the fourth step, together with the second flux 22, a part of the slag 31 remaining in the converter 100 without being removed in the third step can also be used as the flux 22x. The composition and amount of the second flux 22 are 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 into the converter 100. Specific examples of the CaO source and the SiO ₂ source are as described above. 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. The amount of the second flux 22 may be more than 0 kg/ton-steel and 8 kg/ton-steel or less in terms of _SiO2 .

In the converter refining method of the present disclosure, it is important that the CaO/SiO 2 charge defined by [C2×(100−α3)/100+C4]/[S2×(100−α3)/100+S4] is 3.0 or more and 4.5 or less, as shown in the above formula (1). The CaO/SiO ₂ charge 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 inventors, if _{the CaO/SiO 2 charge} is too small, the amount of CaO is insufficient, making it difficult to proceed with dephosphorization, and it becomes difficult to reduce the P concentration in the finally obtained molten steel 12. In addition, there is a risk that slopping will occur at the beginning of decarburization blowing due to the low basicity of the slag. On the other hand, in the past, it was common to add a large amount of CaO source as flux in order to sufficiently advance dephosphorization, but according to the knowledge of the present inventor, even if a large amount of CaO source is added, not all of it contributes to dephosphorization. According to the knowledge of the present inventor, when the charged CaO/ _SiO2 exceeds 4.5 due to the addition of a large amount of CaO source as flux during converter refining operation, the amount of _SiO2 relative to CaO becomes insufficient, and the slag becomes excessively high basic during decarburization. When the slag becomes excessively high basic during decarburization blowing, the flux charged into the converter does not become slag sufficiently, and it becomes impossible to secure a sufficient amount of slag with high dephosphorization ability, which may make it difficult to stably reduce the P concentration in the molten steel 12 after decarburization blowing.

In the converter refining method disclosed herein, it is important that the charged CaO, defined as C2 x (100 - α3)/100 + C4, is 30.0 kg/ton-steel or less, as shown in formula (2) above. In other words, the amount of slag in the decarburization blowing is equal to or less than a certain amount, calculated as CaO. The 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. There is no particular lower limit for the charged CaO, and it will naturally exceed 0 kg/ton-steel due to the relationship in formula (1) above. The lower limit of the 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 knowledge of the present inventors, if the amount of slag in the decarburization blowing is too large, slopping may occur in the early stage of the decarburization blowing.

In the converter refining method of the present disclosure, the charged _SiO2 defined by S2×(100−α3)/100+S4 is not particularly limited as long as the above formulas (1) and (2) are satisfied. The range of the charged _SiO2 is automatically determined by 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, C4 is the CaO equivalent amount (kg/ton-steel) in the second flux 22, S2 is the SiO ₂ equivalent amount (kg/ton-steel) in the first flux 21, and S4 is the SiO ₂ equivalent amount (kg/ton-steel) in the second flux 22. That is, the Ca contained in each flux is converted to CaO, and the Si is converted to SiO ₂ , and the amounts are specified. The CaO equivalent amount and the SiO ₂ equivalent amount in each flux can be obtained from the composition of the flux before being charged into the converter and the amount of the flux charged. Alternatively, the CaO equivalent amount and the SiO ₂ equivalent amount may be specified based on the components contained in the slag after slag removal. As described above, when desiliconization slag is used as the first flux 21, the SiO2 equivalent amount in the first flux 21 may be specified assuming that 100% of the silicon contained in the molten iron before _{desiliconization} is converted to _SiO2 .

In the above formulas (1) and (2), α3 is the intermediate slag discharge rate (%) in the third step. The intermediate slag discharge rate can be determined from the amount of flux added to the converter 100 or the amount of flux present in the converter 100, and the amount of slag (excluding ingots) discharged from the converter 100. The intermediate slag discharge rate may be empirically estimated from past operations, etc., or may be obtained from online or offline measurements during operation. In particular, it is desirable to actually measure the intermediate slag discharge rate at least once using a weighing device, etc. By actually measuring the intermediate slag discharge rate at least once, even if the use of the weighing device, etc. is subsequently suspended due to a breakdown or trouble of the weighing device, etc., the intermediate slag discharge rate can be empirically estimated with high accuracy from the operating conditions, etc. using the past measured values. In addition, weighing methods other than the method using a weighing device include a method of determining the slag discharge rate based on the volume of the discharged slag, as disclosed in JP 2018-119195 A.

In this way, by specifying the intermediate slag ratio α3 in the third step, the charged CaO/SiO ₂ can be accurately controlled within a target range. That is, the converter refining method of the present disclosure may include a step of specifying the intermediate slag ratio α3 in the third step, and a step of determining the additional amount of the second flux, the CaO equivalent amount of the second flux, and/or the SiO ₂ equivalent amount of the second flux based on the specified intermediate slag ratio α3 and the CaO equivalent amount and the SiO ₂ equivalent amount of the first flux so that the above formulas (1) and (2) are satisfied. More specifically, for example, the method may include the steps of: determining the amount of slag 31 discharged in the third step after the third step; determining the intermediate slag discharge rate α3 of the above formulas (1) and (2) based on the determined amount of slag discharged; and determining the additional amount of the second flux, the CaO equivalent amount of the second flux, and/or the SiO ₂ equivalent amount of the second flux based on the determined intermediate slag discharge rate α3 and the CaO equivalent amount and the SiO ₂ equivalent amount of the first flux so that the above formulas (1) and (2) are satisfied. For example, the amount of slag discharged and the intermediate slag discharge rate of the slag can be determined with high accuracy by measuring the weight of the discharged slag and subtracting the weight of the metal contained in the slag from the weight of the discharged slag. The weight of the metal contained in the 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 disclosed herein, formulas (1) and (2) are satisfied, thereby making it possible to prevent slopping in the early stage of decarburization blowing. In addition, during decarburization, effective dephosphorization can be performed by ensuring the minimum amount of slag required for dephosphorization. As a result, the P concentration of the final molten steel 12 can be reduced.

5. Supplementary Note: In the converter refining method of the present disclosure, as shown in Fig. 1 (F), after the fourth step, the molten steel 12 in the converter 100 may be discharged out of the converter 100. For example, the converter 100 may be tilted to allow the molten steel 12 to flow out from a tapping hole 102 on the side of the converter 100.

In addition, the converter refining method of the present disclosure may include a fifth step of tapping steel while leaving the slag 32 generated in the fourth step in the converter 100, as shown in FIG. ₁ (F). Then, after the fifth step, a sixth step of selectively performing either a procedure of leaving the entire amount of the slag 32 in the converter 100 in the converter 100 or a procedure of leaving a part of the slag ₃₂ in the converter 100 in the converter 100 and discharging the rest, based on at least one of the estimated P 2 O 5 content of the slag 32 in the converter 100 and the target value of the P content of the steel of the next heat. In this case, after the sixth step, the first step of the next heat may be performed while leaving the slag 32 in the converter 100. In this way, by performing the first step of the next heat while leaving the slag 32 after decarburization in the converter 100, the slag 32 can be reused as flux for the next heat.

Below, we will explain in more detail the effects of the technology disclosed herein while showing examples, but the technology disclosed herein is not limited to the examples below.

1. Example 1
1.1 First Process Molten iron and scrap were charged to a total volume of 300 tons into a 300 ton top-bottom blown converter in which decarburization slag from the previous heat remained.

Table 1 below shows the temperature and composition of the molten iron in the furnace in the first process. Table 2 below shows the amount of decarburization slag remaining in the previous heat.

1.2 Second step After the first step, a flux containing CaO was newly charged into the furnace, and dephosphorization blowing was performed. The first flux used in the dephosphorization blowing corresponds to a combination of the newly charged flux containing CaO, the decarburization slag of the previous heat, and the desiliconization slag generated by the desiliconization.

The following Table 1 shows the hot metal temperature, hot metal composition, and slag composition at the end of the second step. Table 2 shows the amount of CaO newly added for the second step, the basicity of the first flux used in the second step, and the amount of slag generated in the second step. Table 3 shows the CaO equivalent amount and _SiO2 equivalent amount in the first flux.

1.3 Third Process After the second process, the converter was tilted to remove the intermediate slag from the furnace. At this time, the amount of intermediate slag was measured using a weighing scale. The intermediate slag removal rate was determined from the measured amount of intermediate slag. Specifically, the intermediate slag removal rate in the third process was determined by dividing the weight value measured using the weighing scale with the metal content corrected by the amount of slag previously determined from the charge in the second process.

Tables 1 and 3 below show the intermediate slag removal rate in the third process.

1.4 Fourth step After the third step, the second flux was charged into the furnace to perform decarburization blowing. Here, the amounts of the CaO source and the silica source charged were adjusted according to the intermediate slag fraction so that the charged CaO/ _SiO2 ratio shown in (1) below was a predetermined value. Also, the amount of the CaO source charged was adjusted according to the intermediate slag fraction so that the charged CaO ratio shown in (2) below was a predetermined value.

The following Table 1 shows the molten steel temperature, molten steel composition, and slag composition at the time when the fourth step was completed. Table 2 shows the amount of CaO and _SiO2 newly charged for the fourth step, and the amount of slag generated in the fourth step. Table 3 shows the CaO equivalent amount and _SiO2 equivalent amount in the second flux, the charged CaO/ _SiO2 value calculated from the following formula (I), and the charged CaO value calculated from the following formula (II).

C2: CaO equivalent amount in the first flux (kg/ton-steel)
C4: CaO equivalent amount in the second flux (kg/ton-steel)
S2: _SiO2 equivalent amount in the first flux (kg/ton-steel)
S4: _SiO2 equivalent in the second flux (kg/ton-steel)
α3: Intermediate slag removal rate (%) in the third process

2. Examples 2 to 6 and Comparative Examples 1 to 5
Steps 1 to 4 were carried out under the conditions shown in Tables 1 to 3.

3. Evaluation results Table 4 below shows the charged CaO/ _SiO2 according to formula (I), the charged CaO according to formula (II), the actual basicity of the decarburization slag, the amount of CaO that was not slag-formed during decarburization (specified from the difference between the charged CaO/ _SiO2 and the actual basicity of the decarburization slag), the presence or absence of slopping at the beginning of decarburization, the P concentration in the finally obtained molten steel, the total amount of slag discharged outside the system through a series of steps (total amount of slag discharged outside the system), and the amount of new CaO (the sum of the amount of CaO newly charged in the second and fourth steps) for each of Examples 1 to 6 and Comparative Examples 1 to 5. The "total amount of slag discharged outside the system" is the integrated value of the weight of the slag discharged outside the furnace by intermediate slag discharge in the third step and the weight of the slag discharged outside the furnace when the amount of slag in the furnace is adjusted to the amount shown in Table 2 after the end of the fourth step.

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 from the system was about the same, Example 1, in which the charged CaO/ _SiO2 was reduced and the _SiO2 source was appropriately added, was able to reduce the P concentration in molten steel, compared to Comparative Example 1, in which the charged CaO/ _SiO2 was high and the undissolved CaO was large.
(2) As is clear from the results of Example 2 and Comparative Example 2, in Comparative Example 2 in which the _SiO2 source was excessively added and the charged CaO/ _SiO2 was less than 3.0, the P concentration in the molten steel could not be reduced. On the other hand, in Example 2, although the amount of added _SiO2 source was greater than that in Example 1, the charged CaO/ _SiO2 was about 3.8, which was 3.0 or more, and therefore the P concentration in the molten steel could be reduced.
(3) As is clear from the results of Comparative Example 2, when the charged CaO/ _SiO2 was less than 3.0, the basicity of the slag during decarburization became excessively low, and a large amount of slag that was highly viscous and prone to foaming was generated, resulting in slopping, in which the slag overflowed from the converter, at the initial stage of decarburization blowing.
(4) As is clear from the results of Examples 3 and 4, when decarburization blowing was performed with the charged CaO/ _{SiO2 ratio} in the vicinity of 4.5, the P concentration in the molten steel could be sufficiently reduced. In particular, Example 4, in which the CaO source and the _SiO2 source were added so as to increase the amount of slag, was able to reduce the P concentration in the molten steel more effectively, and it was possible to produce ultra-low phosphorus steel.
(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 intermediate slag removal rate was similar to that of Examples 3 and 4 but the charged CaO/ _SiO2 ratio was significantly greater than 4.5, the amount of unreacted CaO was large, so that slag could not be generated sufficiently and the P concentration in the molten steel could not be reduced. In Comparative Example 4, although a _SiO2 source was added, the amount added was insufficient and the charged CaO/ _SiO2 ratio still exceeded 4.5, so that the P concentration in the molten steel could not be reduced sufficiently.
(6) As is clear from the results of Comparative Example 5, when the charged CaO exceeded 30.0 kg/t-steel, the P concentration in the molten steel did not decrease, and slopping occurred at the early stage of decarburization blowing. On the other hand, in Example 5, in which the charged CaO was 30.0 kg/t-steel or less, dephosphorization could be carried out satisfactorily without slopping.
(7) From a comparison between Examples 1 to 5 and Example 6, it can be seen that the same effect is achieved whether or not the decarburization slag of the previous heat is used as the first flux.

As described above, it was found that the converter refining methods of Examples 1 to 6 can easily suppress slopping in the early stages of decarburization blowing and can stably reduce the P in molten steel after decarburization blowing.

10 Molten iron 11 Dephosphorized molten iron 12 Molten steel 21 First flux 22 Second flux 31 Slag 32 Slag 100 Converter

Claims (3)

A first step of charging molten iron into a converter;
a second step of dephosphorizing the molten iron in the converter using a first flux after the first step;
A third step of discharging at least a portion of the slag in the converter to the outside of the converter after the second step;
a fourth step of adding a second flux into the converter after the third step and then carrying out decarburization;
Equipped with
The intermediate slag discharge rate in the third step is measured at least once by a weighing device or based on the volume of the discharged slag;
The second flux includes a CaO source and a _SiO2 source;
The following formulas (1) and (2) are satisfied:
Converter refining method.
C2: CaO equivalent amount in the first flux (kg/ton-steel)
C4: CaO equivalent amount in the second flux (kg/ton-steel)
S2: _SiO2 equivalent amount in the first flux (kg/ton-steel)
S4: _SiO2 equivalent amount in the second flux (kg/ton-steel)
α3: Intermediate slag removal rate (%) in the third step The P concentration in the molten steel after the fourth step is 0.015% or less by mass%,
The converter refining method according to claim 1. A fifth step, which is performed after the fourth step, while leaving the slag generated in the fourth step in the converter, and
A _sixth step, after the fifth step, based on at least one of an estimated _P2O5 content of the slag in the converter and a target value of the P content of the steel of the next heat, is selected and executed to either leave the entire amount of the slag in the converter in the converter or leave a part of the slag in the converter in the converter while discharging the rest;
Equipped with
After the sixth step, the first step of the next heat is carried out while the slag remains in the converter.
The converter refining method according to claim 1 or 2 .

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