Converter refining method

Abstract

To provide a new converter refining method capable of stably producing a low phosphorous steel.SOLUTION: A molten iron is charged inside a converter, while executing dephosphorization blowing while using a first flux, thereafter, at least a part of slag in the converter is exhausted to the outside of the converter, and a second flux is added inside the converter to execute decarburization blowing. In this case, a third flux is blown into the molten iron together with a top-blown gas upon the decarburization blowing. The blowing of the third flux is started from the time of 70% or more of the total oxygen blowing amount of the decarburization blowing. Further, upon the decarburization blowing, the basicity of the flux in the converter after the addition of the second flux and before the third flux is blown is controlled to 2.5 or more, and the rise amount of the basicity by the blowing of the third flux is controlled to 0.8 or more to 2.0 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] The following processes have been developed for refining hot metal using converters: Process (I) in which dephosphorization and decarburization are performed consecutively in one converter without slag removal (intermediate slag removal), Process (II) in which hot metal is dephosphorized in the first converter, then the hot metal tapped from the first converter is charged into a second converter where it is decarburized, and Process (III) in which dephosphorization is performed in one converter, the slag generated by dephosphorization is removed (intermediate slag removal), and then decarburization is performed in the same converter. Process (I) has low heat loss during refining and high productivity because there is no intermediate slag removal, but it requires high slag basicity throughout the entire blowing process, which requires a large amount of CaO source as an auxiliary material, which is disadvantageous in terms of cost. Furthermore, while a large amount of slag is required to produce low-phosphorus steel, it is difficult to produce the desired low-phosphorus steel due to factors such as reduced slag volume caused by slopping. Process (II) has a high refining capacity and is easier to produce low-phosphorus steel than process (I). However, because it requires two converters, it requires higher capital costs and increases heat loss due to dissipation, which can reduce the melting capacity of iron ore and scrap. Process (III) can shorten the overall blowing time, reduce the amount of flux required for dephosphorization, and reduce heat loss during refining compared to process (II). However, process (III) makes it difficult to stably control the amount of intermediate slag discharge. For example, it can be difficult to significantly reduce the P concentration in the molten steel after refining is complete.

[0003] Patent Document 1 discloses a method of adding quicklime to the CaO source during 90-100% of the entire initial blowing period in order to reduce phosphorus by promoting the dissolution of the CaO source in process (I). This method requires a very large amount of CaO source, which is disadvantageous in terms of cost. Furthermore, since the timing for adding the CaO source is limited, it is difficult to produce low-phosphorus steel with a P concentration of 0.01% or less.

[0004] Patent Document 2 discloses a method of injecting a CaO source at the beginning and end of blowing to promote dephosphorization in process (I). This method also requires a very large amount of CaO source, which is disadvantageous in terms of cost. Furthermore, there is a risk of rephosphorization from the middle to end of blowing, making it difficult to produce low-phosphorus steel with a P concentration of 0.01% or less.

[0005] Patent Document 3 discloses a method for the decarburization blowing of processes (I) and (II), in which the dynamic pressure of the gas jet ejected from the lance nozzle on the molten iron surface is controlled and then all or part of the CaO source is injected through the lance nozzle into the molten iron. This method is advantageous for improving the yield and promoting dissolution of the CaO source injected through the lance nozzle, but since the P concentration after decarburization blowing varies depending on the amount of slag carried over after pretreatment, careful investigation is required for the stable production of low-phosphorus steel.

[0006] Patent Document 4 discloses a method of injecting a CaO source only in the early stage of decarburization blowing in process (III). This method can achieve the effect of promoting dephosphorization while suppressing nitrogen pickup, even when nitrogen is used as the carrier gas for the CaO source. However, there is a risk of rephosphorization from the middle to the end of blowing, making it difficult to produce low-phosphorus steel with a P concentration of 0.01% or less. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6379976 [Patent Document 2] Patent No. 5999157 [Patent Document 3] Patent No. 5574060 [Patent Document 4] Japanese Patent Application Publication No. 2020-105562 Summary of the Invention [Problem to be solved by the invention]

[0008] A new converter refining method that can stably produce low-phosphorus steel is needed. [Means for solving the problem]

[0009] As one of the means for solving the above problems, the present application provides: The first step is to charge molten iron into the 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; and a fourth step in which a second flux is added to the converter after the third step and then decarburization blowing is performed; Equipped with the fourth step includes blowing a third flux onto the molten iron together with a top-blown gas; the blowing of the third flux is started when 70% or more of the total amount of oxygen blown in the decarburization blowing has been blown; After the second flux is added and before the third flux is sprayed, the slag basicity in the converter is 2.5 or more. 3.7 or less and The increase in the slag basicity due to the spraying of the third flux is 0.8 or more and 2.0 or less. Converter refining method Disclose.

[0010] In the converter refining method of the present disclosure, the following formula (1) may be satisfied.

[0011]

number

[0012] In the converter refining method of the present disclosure, the top-blown gas may contain one or more of Ar, N2, CO2, and O2 as a carrier gas.

[0013] The converter refining method of the present disclosure comprises: A fifth step, after the fourth step, of tapping steel while leaving the slag generated in the fourth step in the converter; and a sixth step, after the fifth step, selecting and executing either a step of leaving all of the slag in the converter in the converter, or a step of leaving a portion of the slag in the converter in the converter while discharging the rest, based on at least one of an estimated P2O5 component amount of the slag in the converter and a target value of the P component of the steel for the next heat; and After the sixth step, the first step of the next heat may be carried out while the slag remains in the converter. [Effects of the Invention]

[0014] According to the converter refining method of the present disclosure, the third flux is sprayed onto the molten pig iron from a predetermined point in the latter half of the decarburization blowing, and the slag basicity during the decarburization blowing is controlled by the amounts of the first to third fluxes and the slag removal rate, thereby making it possible to stably produce low-phosphorus steel. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram for explaining an example of the flow of a converter refining method. DETAILED DESCRIPTION OF THE INVENTION

[0016] 1(A) to 1(G), a converter refining method according to one embodiment includes the following steps: a first step (FIG. 1(A)) of charging molten pig iron 10 into a converter 100; a second step (FIG. 1(B)) of performing dephosphorization blowing on the molten pig 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 portion 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) to 1(F)) of performing decarburization blowing after the third step by adding a second flux 22 into the converter 100. The fourth step includes blowing a third flux 23 onto the molten pig iron 11 (dephosphorized molten pig iron 11) together with a top-blown gas 42. The blowing of the third flux 23 starts when 70% or more of the total amount of oxygen 41 blown in the decarburization blowing has been blown. After the addition of the second flux 22 and before the blowing of the third flux 23, the slag basicity in the converter 100 is 2.5 or more, and the increase in the slag basicity due to the blowing of the third flux 23 is 0.8 or more and 2.0 or less. By performing the first to fourth steps, molten steel 12 with a low P concentration can be produced (FIG. 1(G)).

[0017] 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.

[0018] 1.1 Converter The converter 100 may be the same as a conventional converter. In the converter refining method of the present disclosure, the converter 100 may be either a top-blown converter or a top-blown converter. In a top-blown converter, the absence of bottom-blown stirring makes it difficult to reduce iron oxide formed when top-blown oxygen oxidizes iron, and the amount of slag tends to increase excessively. Furthermore, slag containing a large amount of iron oxide is prone to convert CaO into slag. On the other hand, in a top-blown converter, the iron oxide concentration is not so high, and the conversion of CaO into slag tends to be less likely than in a top-blown converter. Figures 1(A) to 1(G) show a top-blown converter as an example of a converter 100. The top-blown converter 100 may have multiple flow paths 101 at its bottom for supplying bottom-blown gas into the furnace. The top-blown converter 100 may also have a tapping hole 102 at its side for tapping molten steel 12.

[0019] 1.2 Molten iron The molten pig iron 10 charged into the converter 100 can be, for example, any ordinary blast furnace molten pig iron. The molten pig iron 10 contains impurities such as P and C, and may also contain Si. When the molten pig iron 10 contains Si, the desiliconization reaction of the Si in the molten pig iron 10 proceeds through oxidative refining with oxygen gas, followed by the dephosphorization reaction. In other words, the molten pig iron may be desiliconized after the first step and before the dephosphorization of the molten pig iron in the second step. After desiliconization, the desiliconization slag in the converter 100 may be drained, or the dephosphorization may be performed while the desiliconization slag is left 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 pig iron 10 may be a mixture of molten iron 10a (e.g., blast furnace molten pig iron) and additional materials such as scrap 10b.

[0020] The method for charging the molten iron 10 into the converter 100 is not particularly limited, and examples thereof include a method in which the molten iron 10 is poured into the converter 100 using a known container such as a molten iron ladle.

[0021] 2.Second process As shown in Fig. 1(B), in the second step, after the first step, dephosphorization blowing is performed using a first flux 21. The conditions for dephosphorization in the second step are not particularly limited.

[0022] 2.1 First Flux The first flux 21 may be a flux introduced into the converter 100 before the dephosphorization in the second step. Alternatively, it may utilize components derived from the molten iron, such as desiliconization slag produced by the desiliconization reaction described above. Alternatively, it may utilize the decarburization slag 32 remaining in the converter 100 after the decarburization and refining of a previous heat. The composition and amount of the first flux 21 are not particularly limited, as long as they are capable of achieving the desired dephosphorization. For example, the first flux 21 may contain a CaO source. Examples of CaO sources include quicklime, limestone, dolomite, and the decarburization slag 32 from the previous heat. The first flux 21 may also contain a SiO2 source. Examples of SiO2 sources include desiliconization slag, the decarburization slag 32 from the previous heat, silica, and olivine. The basicity (CaO / SiO2) of the first flux 21 may be, for example, 0.9 or more or 1.4 or less. The amount of the first flux 21 may be, for example, 5.0 kg / ton-steel or more and 25.0 kg / ton-steel or less in terms of CaO. The amount of the first flux 21 may be, for example, 0 kg / ton-steel or more and 18.0 kg / ton-steel or less in terms of SiO. In this application, "kg / ton-steel" refers to the mass per ton of molten steel finally obtained.

[0023] 2.2 Dephosphorized hot metal As shown in FIG. 1(B), in the second step, oxygen may be blown into the molten pig iron 10 from a top-blown lance 200 to stir the molten pig iron 10 while refining proceeds. Alternatively, bottom-blown gas may be continuously or intermittently blown from the bottom of the converter 100 to enhance stirring of the molten pig iron 10 during refining. The dephosphorization blowing in the second step removes a portion of the P contained in the molten pig iron 10, thereby obtaining dephosphorized molten pig iron 11. The P concentration in the dephosphorized molten pig iron 11 is not particularly limited. For example, the dephosphorized molten pig iron 11 may contain 0.02% by mass or more, 0.03% by mass or more, or 0.08% by mass or less, or 0.06% by mass or less of P.

[0024] 3. 3rd process As shown in FIG. 1(C), in the third step, after the second step, at least a portion 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 into the bottom of the converter 100. This makes it easier to discharge the slag 31.

[0025] 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 80% or less. The composition and amount of the slag 31 produced can be changed arbitrarily depending on the dephosphorization conditions in the second step.

[0026] 4. 4th step 1(D) to 1(F), in the fourth step, after the third step, a second flux 22 is added to the converter 100 and then decarburization blowing is performed. The decarburization conditions in the fourth step are not particularly limited as long as the requirements related to basicity, which will be described later, are satisfied.

[0027] 4.1 Second Flux As shown in FIG. 1(D), the second flux 22 is added to the converter 100 before the decarburization blowing in the fourth step. In the fourth step, a portion 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 basicity requirement described below is satisfied. The second flux 22 includes, for example, a CaO source and an SiO2 source. In the fourth step, the CaO source and the SiO2 source may be added to the converter 100 simultaneously or separately. Specific examples of the CaO source and the SiO2 source are as described above. From the viewpoint of further increasing the dephosphorization amount in the fourth step, the amount of the second flux 22 may be, for example, 3.0 kg / ton-steel or more and 25.0 kg / ton-steel or less in terms of CaO. The amount of the second flux 22 may be, for example, 0 kg / ton-steel or more, or 4.0 kg / ton-steel or less, in terms of SiO2.

[0028] 4.2 Decarburization blowing As shown in FIG. 1(E), in the fourth step, oxygen 41 may be injected from a top-blown lance 200 into the hot metal (dephosphorized hot metal) 11 to stir the hot metal 11 while refining. Alternatively, bottom-blown gas may be continuously or intermittently injected from the bottom of the converter 100 to enhance stirring of the hot metal 11 during refining. Also, as shown in FIG. 1(F), in the fourth step, a third flux 23 is blown onto the hot metal 11 together with a top-blown gas 42 from a certain point in the decarburization blowing. The top-blown gas 42 may be, for example, a mixture of oxygen 41 and a carrier gas. That is, the third flux 23 is transported to the top-blown lance 200 by the carrier gas, merges with the top-blown oxygen 41, and is blown upward from the tip of the lance. The top-blown gas 42 may contain, for example, one or more of Ar, N, CO, and O as a carrier gas. Among these, from the viewpoints of safety, cost, etc., it is preferable to use N2 as the carrier gas. Thus, in the fourth step, the third flux 23 is not blown from the top in the early stage of decarburization, and dephosphorization and decarburization proceed in the presence of the flux 22x. Thereafter, the flux 22x and the top-blown third flux 23 mix to form the flux 23x, and further dephosphorization and decarburization proceed in the presence of the flux 23x. Thus, in the fourth step, dephosphorization proceeds along with decarburization, and thereby a low-phosphorus steel having a desired P concentration can be finally produced.

[0029] 4.3 Third Flux The composition and amount of the top-blown third flux 23 are not particularly limited as long as the requirements for basicity described below are satisfied. Furthermore, the third flux 23 may have a shape (e.g., powder) that allows for top-blown sintering. The third flux 23 may contain, for example, a CaO source. Specific examples of the CaO source are as described above. To further increase the amount of dephosphorization in the fourth step, the amount of the third flux 23 may be, for example, 2.0 kg / ton-steel or more and 6.0 kg / ton-steel or less, calculated as CaO. The third flux 23 does not substantially contain a SiO2 source. That is, the amount of the third flux 23 may be less than 0.1 kg / ton-steel and 0.01 kg / ton-steel or less, calculated as SiO2. As described above, the third flux 23 does not contain any SiO source, or if it does contain any, it is in a very small amount. Therefore, for example, in the equations (1), (3) to (6) described later, it is not necessary to take into account the SiO equivalent amount in the third flux.

[0030] 4.4 Oxygen supply completion rate at the start of spraying the third flux In the converter refining method of the present disclosure, the spraying of the third flux 23 must be started when 70% or more of the total amount of oxygen sprayed in the decarburization blow has been sprayed. Starting the spraying of the third flux 23 too early can cause the following problems. Spraying the third flux 23 from the early stage of the decarburization blow to the end of the decarburization blow excessively increases the amount of flux required, which is disadvantageous in terms of cost. Furthermore, when nitrogen is used as the carrier gas, nitrogen is sprayed onto the molten pig iron 11 for a long period of time, which increases the N concentration in the final molten steel due to nitrogen pickup. On the other hand, if the spraying of the third flux 23 is terminated early in order to reduce costs and avoid nitrogen pickup, rephosphorization occurs in the latter half of the decarburization blow, making it impossible to produce the desired low-phosphorus steel. The above-mentioned problems are easily avoided by starting the blowing of the third flux 23 when 70% or more of the total amount of oxygen blown in the decarburization blowing has been blown. The blowing of the third flux 23 may start when 95% or less, 90% or less, or 85% or less of the total amount of oxygen blown in the decarburization blowing has been blown. The blowing of the third flux 23 is not particularly limited as long as the requirements related to basicity, which will be described later, are satisfied. For example, the blowing of the third flux 23 may be stopped when more than 95%, 97% or more, or 99% or more of the total amount of oxygen blown in the decarburization blowing has been blown.

[0031] 4.5 Basicity before spraying the third flux In the converter refining method of the present disclosure, it is also important that the slag basicity in the converter 100 after the second flux 22 is added and before the third flux 23 is sprayed is 2.5 or more. In other words, the basicity of the flux 22x before the third flux 23 is sprayed is 2.5 or more. If the basicity of the flux 22x before the third flux 23 is sprayed is too low, the dephosphorization effect (dephosphorization effect by the flux 22x) in the first half of the decarburization blowing cannot be obtained, and even if the third flux 23 is sprayed in excess to increase the basicity, it is difficult to sufficiently reduce the P concentration in the finally obtained molten steel.

[0032] On the other hand, the upper limit of the slag basicity in the converter 100 after the addition of the second flux 22 and before the blowing of the third flux 23 is not particularly limited, as long as the increase in basicity described below can be achieved. According to the findings of the present inventors, the slag basicity (actual basicity) in the decarburization blowing saturates at about 4.5 and does not increase beyond that, even when an excessive amount of a CaO source or the like is added as a flux. Considering this, in order to achieve the lower limit (0.8 or more) of the increase in basicity described below in the converter refining method of the present disclosure, the slag basicity in the converter 100 after the addition of the second flux 22 and before the blowing of the third flux 23 naturally needs to be 3.7 or less. If the basicity of the flux 22x before spraying the third flux 23 is too high, even if an excessive amount of the third flux 23 is sprayed, the increase in basicity described below cannot be achieved, and the transitional effect due to the spraying of the third flux 23 is difficult to obtain, making it difficult to produce the desired low-phosphorus steel.

[0033] The basicity of the flux 22x before spraying the third flux 23 may be measured directly, for example, by collecting a portion of the flux 22x and analyzing its components, or may be calculated from the components and amount of the flux charged into the furnace or the intermediate slag removal rate. For example, in the converter refining method of the present disclosure, when the following formula (2) is satisfied, it can be said that the slag basicity in the converter 100 after adding the second flux 22 and before spraying the third flux 23 is 2.5 or more.

[0034]

number

[0035] 4.6 Increase in basicity due to spraying of the third flux In the converter refining method of the present disclosure, it is also important that the increase in slag basicity caused by spraying the third flux 23 is 0.8 to 2.0, comparing the slag before and after spraying with the third flux 23. That is, the flux 23x has a basicity that is 0.8 to 2.0 higher than that of the flux 22x. If the increase in slag basicity is too small, it becomes difficult to obtain the dephosphorization effect caused by spraying the third flux 23, making it difficult to produce the desired low-phosphorus steel. On the other hand, if the increase in slag basicity is too large, the slag basicity before spraying becomes relatively too low, making it difficult to obtain the dephosphorization effect in the first half of the decarburization blowing, and making it difficult to sufficiently reduce the P concentration of the final molten steel.

[0036] The increase in slag basicity due to spraying the third flux 23 may be measured directly by, for example, collecting a portion of the flux 23x and analyzing its components, or may be calculated using a formula based on the components and amount of the flux charged into the furnace and the intermediate slag removal rate. For example, in the converter refining method of the present disclosure, when the following formulas (3) and (4) are satisfied, or when the following formulas (5) and (6) are satisfied, the increase in slag basicity can be said to be 0.8 or more and 2.0 or less.

[0037]

number

[0038] 4.7 Supplementary Information In the converter refining method of the present disclosure, for example, the following formula (1) may be satisfied.

[0039]

number

[0040] In other words, the "charged CaO / SiO2" ratio, defined by the formula [C1 × (100 − α3) / 100 + C2 + C3] / [S1 × (100 − α3) / 100 + S2], may be 3.3 or more and 4.5 or less. The charged CaO / SiO2 ratio may be 3.4 or more, 3.6 or more, 3.8 or more, 4.0 or more, 4.2 or more, or 4.4 or more. According to the inventors' findings, if the charged CaO / SiO2 ratio is too small, the CaO content becomes insufficient, making it difficult to proceed with dephosphorization and to reduce the P concentration in the final molten steel 12. Furthermore, as the slag becomes less basic, slopping may occur at the early stage of decarburization blowing. Furthermore, according to the inventors' findings, even if a large amount of CaO source is added as a flux to sufficiently proceed with dephosphorization, not all of it contributes to dephosphorization, and the actual basicity of the slag saturates at about 4.5. On the contrary, if the charged CaO / SiO2 ratio is too large, the flux charged into the converter will not be sufficiently slag-formed, and a sufficient amount of slag with high dephosphorization ability will not be secured, which may make it difficult to stably reduce the P concentration in the molten steel 12 after decarburization blowing.

[0041] 5.Effects As described above, in the converter refining method of the present disclosure, the third flux 23 is sprayed onto the molten pig iron 11 from a certain point in the decarburization blow, and the slag basicity during the decarburization blow is controlled by the amounts of the first to third fluxes 21 to 23 and the slag removal rate. This allows for a high transient effect due to the spraying of the third flux in addition to the effects of the first and second fluxes. For example, the P concentration of the final molten steel 12 can be reduced to 0.01% by mass or less. Furthermore, the converter refining method of the present disclosure only requires spraying a small amount of the third flux at the end of the blow, and can suppress nitrogen pickup even when nitrogen gas is used as the carrier gas. For example, the nitrogen concentration of the final molten steel 12 can be reduced to 40 ppm or less. Furthermore, the converter refining method of the present disclosure is economically advantageous over the conventional processes (I) and (II) because it can perform everything from dephosphorization to decarburization blowing using a single converter.

[0042] 6. Supplementary Information In the converter refining method of the present disclosure, as shown in Fig. 1(G), 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.

[0043] Furthermore, as shown in FIG. 1(G), the converter refining method of the present disclosure may include a fifth step after the fourth step, in which steel is tapped while the slag 32 generated in the fourth step remains in the converter 100. Then, after the fifth step, a sixth step may be included, in which, based on at least one of the estimated P2O5 content of the slag 32 in the converter 100 and the target P content of the steel for the next heat, either the entire amount of the slag 32 in the converter 100 remains in the converter 100 or a portion of the slag 32 in the converter 100 remains in the converter 100 while the rest is discharged. In this case, after the sixth step, the first step of the next heat may be performed while the slag 32 remains in the converter 100. In this way, by performing the first step of the next heat while leaving the decarburized slag 32 in the converter 100, the slag 32 can be reused as flux for the next heat. [Example]

[0044] The effects of the technology of the present disclosure will be described in more detail below with reference to examples, but the technology of the present disclosure is not limited to the following examples.

[0045] 1. Examples 1 to 6 and Comparative Examples 3 to 6 1.1 First step Molten iron and scrap were charged to a total weight of 350 tons into a 350-ton top-and-bottom blown converter containing residual decarburization slag from the previous heat. The average composition of the decarburization slag was 40% CaO and 10% SiO2 by mass. The temperature of the molten iron in the furnace during the first step was 1210–1250°C.

[0046] Table 1 below shows the C concentration, Si concentration, and P concentration of the molten iron in the furnace in the first step.

[0047] 1.2 Second process After the first process, new flux was added to the furnace, and dephosphorization blowing was performed while blowing oxygen from the top lance. At the end of the second process, the amount of slag in the furnace was 37-44 (kg / t-pig iron), and the hot metal temperature was 1320-1350°C. The first flux used in the dephosphorization blowing corresponds to the combination of the newly added flux, the decarburization slag from the previous heat, and the desiliconization slag produced by desiliconization.

[0048] The CaO equivalent amount and SiO2 equivalent amount in the first flux, as well as the basicity of the first flux, are shown in Table 1 below. The C concentration, Si concentration, and P concentration in the hot metal after dephosphorization blowing are also shown.

[0049] 1.3 Third step After the second process, the converter was tilted to remove the slag from the furnace. At this time, the amount of slag was measured using a weighing scale. The slag removal rate was determined from the measured amount of slag. Specifically, the weight value measured using the weighing scale, corrected for the metal content, was divided by the amount of slag previously determined from the charge in the second process to determine the slag removal rate in the third process.

[0050] Table 1 below shows the slag removal rate in the third step.

[0051] 1.4 4th step After the third step, the second flux was introduced into the furnace and decarburization blowing was performed while blowing oxygen from the top lance. Furthermore, once a predetermined amount of oxygen had been blown in the decarburization blowing (when a predetermined oxygen supply rate had been reached), the third flux was blown from above along with the top-blowing gas. The third flux was carried to the top lance by nitrogen as a carrier gas and merged with the top-blowing oxygen within the lance. The molten iron temperature at the end of the decarburization blowing was 1660–1680°C, and the slag volume was 10.0–11.1 tons.

[0052] Table 1 below shows the CaO equivalent amount and SiO2 equivalent amount of the slag remaining in the furnace before the fourth step, the CaO equivalent amount and SiO2 equivalent amount of the second flux, the CaO equivalent amount of the third flux, the slag basicity before spraying the third flux, the slag basicity after spraying the third flux, the increase in slag basicity due to spraying the third flux, the oxygen supply completion rate at the start of spraying the third flux, the oxygen supply completion rate at the end of spraying the third flux, and the C concentration, P concentration, and N concentration of the finally obtained molten steel.

[0053] 2. Comparative Examples 1 and 2 In the fourth step, the third flux was not sprayed, and the first to fourth steps were carried out under the conditions shown in the following Table 1. In each step, the conditions common to Examples 1 to 6 and Comparative Examples 3 to 6 are as described above.

[0054] In all of the above Examples 1 to 6 and Comparative Examples 1 to 6, the CaO source for the first and second fluxes was quicklime, limestone, dolomite, and / or decarburization slag from a previous heat, and the SiO2 source was decarburization slag, desiliconization slag, silica, and / or olivine. Furthermore, quicklime powder with a purity of 95% or higher was used as the third flux. In other words, the third flux did not substantially contain any SiO2 source, and the SiO2 equivalent amount in the third flux was also substantially zero.

[0055] [Table 1]

[0056] 3. Evaluation Results In Comparative Example 1, the third flux was not sprayed in the fourth step, and the slag basicity in the entire decarburization blowing was low at 3.2. In Comparative Example 1, the third flux was not sprayed, so the high transitory effect of the third flux was not obtained, and the P concentration in the finally obtained molten steel could not be sufficiently reduced.

[0057] In Comparative Example 2, a sufficient amount of CaO source was added as the second flux to the furnace to increase the slag basicity throughout the entire decarburization blow, as compared to Comparative Example 1. In this case, dephosphorization progressed more than in Comparative Example 1, but the high transitory effect of the third flux was not obtained because the third flux was not blown, and the P concentration in the final molten steel could not be reduced sufficiently.

[0058] In Comparative Example 3, although the third flux was sprayed in the fourth step, the slag basicity was high before the third flux was sprayed, and the increase in the slag basicity due to the spraying of the third flux was small. In this case, the P concentration in the finally obtained molten steel could be reduced more than in Comparative Examples 1 and 2, but it could not be said that the P concentration was sufficiently reduced.

[0059] Comparative Example 4 is an example in which the third flux was sprayed from the first half of the decarburization blowing in the fourth step and the spraying of the third flux was completed at an early stage of the decarburization blowing. In this case, rephosphorization occurred in the latter half of the decarburization blowing, and the P concentration in the finally obtained molten steel could not be reduced sufficiently.

[0060] In Comparative Example 5, although the third flux was sprayed in the fourth step, the amount of the second flux charged before the third flux was sprayed was small, and the slag basicity before the third flux was sprayed was low. In this case, a sufficient dephosphorization effect was not obtained in the first half of the decarburization blowing, and the P concentration in the finally obtained molten steel could not be reduced sufficiently.

[0061] Comparative Example 6 is an example in which the third flux was sprayed from the early to final stages of the decarburization blowing in the fourth step. In this case, a large amount of the third flux was sprayed, which significantly increased the basicity and reduced the P concentration in the final molten steel to some extent. However, the effect of reducing the P concentration was small compared to the amount of the flux sprayed. Furthermore, as a result of blowing nitrogen as a carrier gas onto the molten iron for a long period of time, nitrogen pickup occurred, and the decarburization blowing was completed without the nitrogen being removed, resulting in a significant increase in the N concentration in the final molten steel.

[0062] In comparison with the above Comparative Examples 1 to 6, Examples 1 to 6 are (1) In the fourth step, the third flux was blown together with the top-blowing gas. (2) The third flux was started to be blown when 70% or more of the total amount of oxygen blown in the decarburization blowing was blown. (3) The amount of the first and second fluxes and the slag removal rate were adjusted to control the slag basicity to 2.5 or more before the third flux was sprayed. (4) By adjusting the amount of third flux sprayed, the increase in slag basicity due to spraying of the third flux was controlled to 0.8 or more and 2.0 or less. As a result, the P concentration in the finally obtained molten steel could be significantly reduced to 0.01 mass % or less, and an increase in the N concentration could also be suppressed.

[0063] The results shown in Table 1 indicate that the nitrogen pickup rate increases with increasing third flux injection period and tends to increase the later the third flux is injected into the decarburization blow. Regarding the carrier gas, nitrogen is preferred from the viewpoint of equipment safety and cost, but Ar, CO2, or O2 may also be used depending on the upper limit of the nitrogen concentration of the steel being produced. When nitrogen is used as the carrier gas, it is best to limit the timing of injection of the third flux to the final stage of the decarburization blow. Furthermore, after the fourth step, a fifth step may be performed in which the slag generated in the fourth step is left in the converter while the steel is tapped. After the fifth step, either all of the slag in the converter may be left in the converter, or some of the slag may be left in the converter and the rest may be removed, based on at least one of the estimated P2O5 content of the slag in the converter and the target P content of the steel for the next heat. As described above, this allows the decarburization slag left in the converter to be reused as the first flux for the next heat.

[0064] From the above results, it can be said that a converter refining method that satisfies the following (A) to (D) can stably produce low-phosphorus steel with a P concentration of 0.01 mass % or less.

[0065] (A) The method comprises: (a) a first step of charging molten pig iron into a converter; (b) a second step of, after the first step, performing dephosphorization blowing on the molten pig iron in the converter using a first flux; (c) a third step of, after the second step, discharging at least a portion of the slag in the converter to the outside of the converter; and (d) a fourth step of, after the third step, adding a second flux into the converter and then performing decarburization blowing. (B) The fourth step includes blowing a third flux onto the molten pig iron together with top-blown gas, and the blowing of the third flux starts when 70% or more of the total amount of oxygen blown in the decarburization blowing has been blown. (C) The basicity of the slag in the converter after the second flux is added and before the third flux is sprayed is 2.5 or more. (D) The increase in the slag basicity due to the spraying of the third flux is 0.8 or more and 2.0 or less. [Explanation of symbols]

[0066] 10 Molten iron 11 Dephosphorized hot metal 12 Molten Steel 21 First Flux 22 Second Flux 23 Third Flux 41 Oxygen (top-blown oxygen) 42 Top-blown gas (top-blown oxygen and carrier gas) 31 Slag 32 Slag 100 converter 200 Top-blowing lance

Claims

1. A first step is to charge 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; and a fourth step of adding a second flux into the converter after the third step and then performing decarburization blowing; Equipped with the fourth step includes blowing a third flux onto the molten iron together with a top-blown gas; the blowing of the third flux is started when 70% or more of the total amount of oxygen blown in the decarburization blowing has been blown; a slag basicity in the converter after the second flux is added and before the third flux is sprayed is 2.5 or more and 3.7 or less; an increase in the slag basicity due to spraying of the third flux of 0.8 or more and 2.0 or less; Converter refining method.

2. The following formula (1) is satisfied: The converter refining method according to claim 1. [Equation 1] C1: CaO equivalent amount in the first flux (kg / ton-steel) C2: CaO equivalent amount in the second flux (kg / ton-steel) C3: CaO equivalent amount in the third flux (kg / ton-steel) S1: SiO in the first flux ２ Conversion amount (kg / ton-steel) S2: SiO in the second flux ２ Conversion amount (kg / ton-steel) α3: Intermediate slag removal rate (%) in the third step

3. The top blown gas is Ar, N ２ , CO ２ and O ２ as a carrier gas, The converter refining method according to claim 1 or 2.

4. a fifth step of tapping steel after the fourth step while leaving the slag generated in the fourth step in the converter; and After the fifth step, the estimated P of the slag in the converter ２ O ５ a sixth step of selecting and executing either a step of leaving all of the slag in the converter in the converter, or a step of leaving a portion of the slag in the converter in the converter while discharging the rest, based on at least one of the amounts of the components and the target value of the P component of the steel of the next heat; 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 any one of claims 1 to 3.

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