Method for refining molten iron

Abstract

To provide a technology for efficiently reducing the phosphorus concentration of molten iron when dephosphorizing molten iron in a method for refining the molten iron using an electric furnace.SOLUTION: A method for refining molten iron using an electric furnace according to the present disclosure includes performing a dephosphorization treatment on the molten iron while stirring the molten iron. In the dephosphorization treatment, a condition of ε2<ε1 (ε1 is the stirring power density in a first period of the dephosphorization treatment, the first period is from the start of the dephosphorization treatment to a time point TM in the middle of the dephosphorization treatment, ε2 is the stirring power density in a second period of the dephosphorization treatment, and the second period is from the time point TM to the completion of the dephosphorization treatment) is satisfied.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present application discloses a method for refining molten iron using an electric furnace. [Background technology]

[0002] Patent Documents 1 and 2 disclose a method for refining molten iron using an electric furnace. When refining molten iron in an electric furnace, an arc is irradiated from an electrode toward the molten iron, and slag is foamed to encase the arc and protect the furnace wall refractory. When dephosphorizing molten iron in an electric furnace, the foamed slag is used to promote the dephosphorization reaction of the molten iron. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 3-267685 [Patent Document 2] Japanese Patent Application Publication No. 63-4010 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the new findings of the present inventors, when dephosphorization of molten iron is performed in an electric furnace, rephosphorization from the foamed slag to the molten iron occurs, and the phosphorus concentration in the molten iron may not be sufficiently reduced. [Means for solving the problem]

[0005] The present application discloses the following aspects as one of the means for solving the above problems. <Aspect 1> A method for refining molten iron using an electric furnace, comprising the steps of: and performing a dephosphorization treatment on the molten iron while stirring the molten iron, In the dephosphorization treatment, the following conditions are met: ε 2 <ε1 ε 1 : Agitation power density during the first period of the dephosphorization treatment First period: from the start of the dephosphorization treatment to a point T M to ε 2 : Agitation power density during the second period of the dephosphorization treatment Second period: the time T M From the completion of the dephosphorization treatment is satisfied, A method of refining molten iron. <Aspect 2> A method for refining molten iron according to embodiment 1, comprising the steps of: In the dephosphorization treatment, ε 2 ≦0.7×ε 1 The conditions are met, A method of refining molten iron. <Aspect 3> The method for refining molten iron according to embodiment 1 or 2, Said time T M is the time point when 30% to 80% of the dephosphorization treatment is completed; A method of refining molten iron. <Aspect 4> A method for refining molten iron according to any one of aspects 1 to 3, Said time T M is determined based on the temperature of the molten iron; A method of refining molten iron. <Aspect 5> A method for refining molten iron according to embodiment 4, comprising the steps of: Said time T M is the time when the temperature of the molten iron becomes equal to or higher than the threshold value; A method of refining molten iron. Effect of the Invention

[0006] According to the method for refining molten iron disclosed herein, the phosphorus concentration in the molten iron can be reduced. [Brief description of the drawings]

[0007] [Figure 1]This shows an example of the change in stirring power density from the start to the completion of dephosphorization treatment. The vertical axis represents stirring power density, and the horizontal axis represents time (or the stage of dephosphorization treatment). [Diagram 2] 4 shows the relationship between the stirring power density ratio and the phosphorus concentration in refined steel. [Diagram 3] 4 shows the relationship between the stirring power density drop point and the phosphorus concentration in the refined steel. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The method for refining molten iron according to the present disclosure will be described with reference to the drawings. However, the method for refining molten iron according to the present disclosure is not limited to the embodiment shown in the drawings.

[0009] The method for refining molten iron using an electric furnace according to the embodiment includes performing a dephosphorization treatment on the molten iron while stirring the molten iron. As shown in Fig. 1, in the method for refining molten iron according to the embodiment, the following conditions are satisfied in the dephosphorization treatment.

[0010] ε 2 <ε 1 ε 1 : Agitation power density during the first period of the dephosphorization treatment First period: from the start of the dephosphorization treatment to a point T M to ε 2 : Agitation power density during the second period of the dephosphorization treatment Second period: the time T M From the completion of the dephosphorization treatment

[0011] 1. Molten iron The molten iron is obtained by generating an arc in an electric furnace to melt an iron source. The iron source may include at least one selected from scrap, reduced iron, pig iron, and granulated pig iron, or may be molten iron or molten steel produced in another melting furnace or refining furnace. In particular, when reduced iron is used as the iron source, the method of the present invention is expected to be more effective. This is because reduced iron may contain a high concentration of phosphorus, which tends to increase the amount of dephosphorization of the molten iron by the slag and the amount of rephosphorization from the slag to the molten iron. The molten iron may contain various elements other than iron. The composition of the elements other than iron depends on the type of iron source. For example, the molten iron immediately after melting (molten iron at the start of the dephosphorization process) may contain P in an amount of 0.0050 mass% to 0.100 mass%, or 0.020 mass% to 0.050 mass%. The molten iron immediately after melting (molten iron at the start of the dephosphorization treatment) may contain 0.02% by mass or more and 3.00% by mass or less of C. The molten iron immediately after melting (molten iron at the start of the dephosphorization treatment) may contain 0.005% by mass or more and 0.050% by mass or less of S. The molten iron immediately after melting (molten iron at the start of the dephosphorization treatment) may contain 0.005% by mass or more and 0.050% by mass or less of N. The temperature of the molten iron at the start of the dephosphorization treatment may be any temperature suitable for the dephosphorization treatment. The density of the molten iron at the start of the dephosphorization treatment is, for example, 6600 kg / m 3 More than 7000kg / m 3 It may be the following.

[0012] 2. Mixing In the method for refining molten iron according to the present embodiment, the molten iron is stirred at least from the start to the completion of the dephosphorization treatment. The stirring of the molten iron is performed by bottom or top blowing of gas. The bottom or top blowing of gas is usually performed before the start of the dephosphorization treatment (at the stage of melting the iron source). In other words, it can be said that the molten iron is in a state of being stirred at a predetermined stirring power density at the start of the dephosphorization treatment. The form of bottom or top blowing of gas is not particularly limited, and may be the same as a known form. For example, the molten iron can be stirred by blowing gas into the molten iron from various gas supply means provided in the electric furnace. More specifically, the molten iron can be stirred by bottom blowing gas into the molten iron from a gas supply means (e.g., a bottom blowing tuyere) provided at the bottom of the electric furnace. The molten iron can also be stirred by top blowing gas toward the surface of the molten iron from a gas supply means (e.g., a lance) provided above the surface of the molten iron in the electric furnace. The number of gas supply means provided in the electric furnace is not particularly limited, and is at least 1. When the gas supply means is a lance, the lance may be at least one of a lance inserted from the furnace roof of the electric furnace (so-called main lance), a wall lance provided on the furnace wall, and a variable lance positioned by a manipulator or the like.

[0013] The types of bottom and top blown gases may be any gas generally used in the dephosphorization of molten iron. For example, the bottom blown gas may be an inert gas. The top blown gas may be an oxygen-containing gas. The flow rates of the bottom and top blown gases may be determined so as to satisfy the conditions related to the stirring power density described below.

[0014] 3. Dephosphorization treatment In the dephosphorization process of molten iron using slag, the phosphorus distribution ratio between slag metals decreases as the temperature of the molten iron rises, and especially at temperatures above 1600°C, the equilibrium phosphorus concentration calculated from the phosphorus distribution ratio between slag metals becomes higher than the phosphorus concentration in the molten iron, making it easier for rephosphorization to occur from the slag to the molten iron. In addition, when refining molten iron using an electric furnace, slag foaming is required to encase the arc, and it is important to know how to suppress rephosphorization from such foamed slag to the molten iron.

[0015] The present inventors have found that when the stirring force (stirring power density) applied to the molten iron is reduced during the dephosphorization process, rephosphorization from the foamed slag to the molten iron can be suppressed and the phosphorus concentration of the molten iron can be reduced more efficiently than when the stirring force is not reduced. This is believed to be due to the following reasons 1 and 2. Reason 1: By reducing the stirring force on the molten iron during the dephosphorization process, the mass transfer rate on the molten iron side can be reduced. Reason 2: By reducing the stirring force on the molten iron during the dephosphorization process, vertical mixing of the foamed slag can be suppressed.

[0016] Based on the above findings, in the refining method of molten iron according to the present embodiment, ε 2 <ε 1 The dephosphorization of molten iron is carried out so that the following conditions are satisfied. That is, as shown in FIG. 1, from the start of the dephosphorization to a point T M The period up to that point is called the "first period" and the M If the period from the start of dephosphorization to the completion of the dephosphorization treatment is defined as the "second period", the stirring power density ε 2 is the stirring power density ε 1 Lower than.

[0017] In this application, the entire period of dephosphorization refers to the period from the time when the entire iron source to be dephosphorized in the electric furnace is melted (the time when it is determined that melting of the iron source is complete based on visual inspection, video inspection, sounds generated inside the furnace, fluctuations in the current output, etc.) to the time when the next step in the dephosphorization process is started. In other words, the time when "the start of dephosphorization process" refers to the time when melting of the iron source is completed, and the time when "the completion of dephosphorization process" refers to the start of the next step. For example, when tapping (tapping steel) is performed after dephosphorization process, the time when "the completion of dephosphorization process" refers to the start of tapping.

[0018] In this application, "ε 2 <ε 1 " (Stirring power density ε 2 is the stirring power density ε 1 (lower than) means, for example, that in the first period, the stirring power density ε 1 The molten iron is stirred at T M In the second period, the stirring power density is reduced to ε 1 A lower stirring power density ε 2 This includes stirring molten iron at ε 1 is the time average value in the first period, and the stirring power density ε 2 is the time average value in the second period.

[0019] 3.1 Mixing power density In the method for refining molten iron according to the present embodiment, a stirring power density ε 1 and the stirring power density ε 2In the present application, the "agitation power density" is taken into consideration only that resulting from top and bottom blowing. The agitation power density by top blowing and the agitation power density by bottom blowing can be calculated by referring to known documents such as "Kai et al.: Tetsu to Hagane, vol. 69 (1983), pp. 228-237" and "Mori et al.: Tetsu to Hagane, vol. 67 (1981), pp. 672-695", and adopting a linear sum value. In addition to the above gas supply, the agitation may be achieved by any other means such as electromagnetic stirring, but the agitation power density resulting from these optional means is not taken into consideration in the present application. This is because, according to the knowledge of the present inventor, sufficient effects can be obtained by controlling the agitation power density by top and bottom blowing. More specifically, in the present application, the agitation power density ε (W / t) by bottom and top blowing is calculated based on the following formula (1).

[0020] ε = ε bot +0.1·ε top (1)

[0021] ε bot =371 Q T l ·{ln(1+9.8·ρ·h / P)+0.06·(1-T n / T l )} / W Q: Bottom blown gas flow rate (Nm 3 / s) T l : Molten iron temperature (K) ρ: Molten iron density (kg / m 3 ) h: Gas injection depth from the molten iron surface (m) P: Atmospheric pressure (Pa) T n : Gas temperature (K) W: Molten iron weight (t)

[0022] ε top =0.0453 Q T d t ·u 0 2 cosθ / (W X) Q T : Top blown gas flow rate (Nm3 / min) d t : Nozzle throat diameter (m) u 0 : nozzle exit gas linear velocity (m / s), θ: Nozzle angle (rad) X: Lance height (m)

[0023] As described above, in the method for refining molten iron according to the present embodiment, ε 2 <ε 1 In the dephosphorization treatment, the following condition is satisfied: 2 ≦0.9×ε 1 , ε 2 ≦0.8×ε 1 , or ε 2 ≦0.7×ε 1 In particular, according to the findings of the present inventors, in the dephosphorization treatment, ε 2 ≦0.7×ε 1 When these conditions are satisfied, the effect of decreasing the mass transfer rate of the molten iron and the effect of suppressing the vertical mixing of the slag become more significant, and rephosphorization from the slag to the molten iron can be more significantly suppressed. 2 The lower limit of is not particularly limited. For example, 0<ε 2 or 0.1×ε 1 ≦ε 2 ε 2 The above upper and lower limits may be combined in any combination.

[0024] ε 1 and ε 2 The specific values ​​of each of the above can take various values ​​as long as dephosphorization proceeds. 1 (W / t) may be, for example, 30 or more and 120 or less, or 38 or more and 110 or less. 2 (W / t) may be, for example, 4 or more and 100 or less, or 30 or more and 85 or less.

[0025] In dephosphorization treatment, ε 2 <ε 1The case where the above condition is satisfied is, for example, the case where the following condition A or B is satisfied. Condition A: In both the first and second periods, bottom blowing is not performed and only top blowing is performed, and the top blown gas flow rate in the second period is made lower than the top blown gas flow rate in the first period. Condition B: Bottom blowing and top blowing are performed in a first period, and the bottom blown gas flow rate and / or the top blown gas flow rate in a second period are made lower than the bottom blown gas flow rate and / or the top blown gas flow rate in the first period.

[0026] 3.2 Time T M As shown in FIG. 1, in the method for refining molten iron according to the present embodiment, the dephosphorization process is started at time T M The stirring power density ε during the first period up to 1 Rather, at time T M The stirring power density ε during the second period from the start of phosphorus removal to the completion of dephosphorization 2 is lower. At the point T that separates the first and second periods M is a time point during the dephosphorization process. M may be during the dephosphorization treatment and at the time when the stirring power density is reduced.

[0027] In one embodiment of the method for refining molten iron, M may be the time point when 30% or more and 80% or less of the dephosphorization treatment is completed. For example, in the first period, the stirring power density ε 1 The molten iron is stirred at T, and the dephosphorization process is completed at any time T when 30% to 80% of the dephosphorization process is completed. M In the second period, the stirring power density is reduced to ε 1 A lower stirring power density ε 2The molten iron may be stirred at a temperature of 1000° C. Before the dephosphorization treatment is completed by 30% to 80%, dephosphorization from the molten iron to the slag is likely to proceed, whereas after the dephosphorization treatment is completed, rephosphorization from the slag to the molten iron is likely to occur. For example, the temperature of the molten iron is likely to be 1600° C. or higher when the dephosphorization treatment is completed by 30% to 80%. Therefore, the temperature of the molten iron is likely to be 1600° C. or higher at the time when the dephosphorization treatment is completed by 30% to 80%. M Based on the standard, the stirring power density ε 2 The previous stirring power density ε 1 By lowering the temperature to less than 1000° C., rephosphorization from the slag to the molten iron can be more significantly suppressed.

[0028] The term "the point at which 30% to 80% of the dephosphorization process is completed" refers to the point at which 30% to 80% of the total time from the start to the completion of the dephosphorization process is completed, with the total time from the start to the completion of the dephosphorization process being taken as 100%. For example, if the total time of the dephosphorization process is 15 minutes, the point at which 80% of the dephosphorization process is completed is the point at which 12 minutes have elapsed since the start of the dephosphorization process.

[0029] In one embodiment of the method for refining molten iron, M may be determined based on the temperature of the molten iron. M may be the time when the temperature of the molten iron reaches or exceeds the threshold value. As described above, when dephosphorization is performed using slag, the phosphorus distribution ratio between the slag and metal decreases as the temperature of the molten iron rises, and especially when the temperature reaches 1600°C or higher, the equilibrium phosphorus concentration calculated from the phosphorus distribution ratio between the slag and metal tends to be higher than the phosphorus concentration in the molten iron, and rephosphorization from the slag to the molten iron tends to occur. In this regard, the above-mentioned time T M The time T when the temperature of the molten iron becomes equal to or higher than the threshold value may be set to 1600° C. M Based on the standard, the subsequent stirring power density ε 2 The previous stirring power density ε 1 By lowering the temperature to less than 1000° C., rephosphorization from the slag to the molten iron can be more significantly suppressed.

[0030] 3.3 Other Terms In the method for refining molten iron according to the present embodiment, the stirring power density ε 1 The stirring power density ε 2 The dephosphorization treatment may be performed under the same conditions as in the past, except for reducing the temperature of the dephosphorization furnace. That is, the type, amount and supply conditions of the auxiliary material (slag) for dephosphorization, the foaming conditions of the slag, the conditions for supplying oxygen to the molten iron, the temperature of the molten iron and other conditions may be the same as in the past. In addition, other treatments such as desulfurization treatment and decarburization treatment may be performed together with the dephosphorization treatment or separately from the dephosphorization treatment. The conditions of the other treatments may also be the same as in the past.

[0031] 4. Electric furnace As described above, the electric furnace may be any one that can change the stirring power density during dephosphorization treatment. The basic configuration of the electric furnace is the same as that of a conventionally known one. For example, the electric furnace has a melting furnace that melts the iron source by an arc. The melting furnace is a part that can be defined by a furnace cover, an inner wall, and a furnace bottom. The shape of the melting furnace in a top view (plan view) may have, for example, a circular part. The melting furnace may have a constant bath depth or a constant furnace diameter. The bath depth and furnace diameter of the melting furnace are not particularly limited.

[0032] The electric furnace may be of an AC type in which an arc is generated using only the upper electrode, or of a DC type in which an arc is generated using the upper and lower electrodes. The upper electrode may be installed so as to be inserted into the furnace through the furnace cover. The lower electrode may be installed at the bottom of the furnace. In the case of a DC type, the number of the upper electrode and the lower electrode is at least one. The positions of the upper electrode and the lower electrode are not particularly limited. For example, when the shape of the molten metal surface in the melting furnace is substantially circular in a top view (plan view), the center position of the circle may coincide with the central axis of one upper electrode or one lower electrode. Alternatively, a plurality of upper electrodes or a plurality of lower electrodes may be arranged around the center position of the circle in a top view. In the electric furnace, power is supplied from a power supply unit to the upper electrode (and the lower electrode) to generate an arc from the upper electrode. The power supply unit may be a general one that supplies power to the upper electrode (and the lower electrode). The power supplied from the power supply unit to the electrodes is not particularly limited as long as it can generate an arc between the electrodes.

[0033] The electric furnace may include an iron source charging means for charging an iron source into the melting furnace. The electric furnace may also include a slag removal door for removing slag and the like formed on the surface of the molten iron. The electric furnace may also include a tapping port for tapping the molten iron or steel. Any of these may be of a known configuration.

[0034] In the method for refining molten iron according to the present embodiment, the stirring power density in the dephosphorization treatment may be manually controlled by the judgment of an operator or may be automatically controlled by a control unit provided in the electric furnace. When the stirring power density is automatically controlled by a control unit provided in the electric furnace, the control unit, for example, continuously or intermittently determines whether or not the temperature of the molten iron exceeds a threshold value in the dephosphorization treatment of the molten iron, and at a time T when it is determined that the threshold value is exceeded, MIn the above, the flow rate of the bottom blown gas and / or the flow rate of the top blown gas may be reduced to reduce the stirring power density. The control unit may have a known configuration for enabling such control to be performed. For example, the control unit may have a CPU, a RAM, a ROM, etc.

[0035] 5.Effects According to the method for refining molten iron according to the present embodiment, the phosphorus concentration in the molten iron can be reduced, and for example, molten steel with a reduced phosphorus concentration can be produced. The molten steel may contain, for example, 0 mass% to 0.050 mass% P, or 0.005 mass% to 0.020 mass% P. The difference between the P concentration in the molten iron at the start of the dephosphorization treatment and the P concentration in the molten steel after the dephosphorization treatment may be 0.007 mass% or more. The molten steel may contain, for example, 0.01 mass% to 3.00 mass% C. The molten steel may contain, for example, 0.001 mass% to 0.050 mass% S. The molten steel may contain, for example, 0.001 mass% to 0.050 mass% N. EXAMPLES

[0036] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention can adopt various conditions as long as it does not deviate from the gist of the invention and achieves its object.

[0037] 1. Test conditions A dephosphorization test was carried out in an electric furnace. The initial conditions for the dephosphorization test were as follows: the amount of molten iron from the seed molten iron and the iron source melting was 170-180t, the molten iron temperature was 1570-1580°C, and the phosphorus concentration [P] in the molten iron was 0.030-0.033 mass% by adding a phosphorus-containing alloy. The amount of slag was 100-110 kg per ton of molten iron, and the composition of the slag was (CaO) / (SiO 2 ) = 1.8-1.9, (FeO) = 25-28 mass%, (MgO) = 9-11 mass%.

[0038] The entire dephosphorization period was defined as the period from the melting of the iron source to the start of tapping, and this period was set to 15 minutes. In the dephosphorization, oxygen required for the dephosphorization reaction, which is an oxidation reaction, was added by pumping oxygen from the wall lance to the molten iron. The temperature of the molten iron after the dephosphorization treatment was set to 1640-1650°C. Samples were taken before and after the dephosphorization treatment, and subjected to chemical analysis to investigate the phosphorus concentration [P] in the molten iron.

[0039] Dephosphorization was performed in two cases: one in which the stirring power density of the molten iron in the dephosphorization treatment was kept constant from the start to the end of the dephosphorization treatment, and the other in which the stirring power density of the molten iron in the dephosphorization treatment was changed halfway through the dephosphorization treatment. When the stirring power density of the molten iron in the dephosphorization treatment was changed halfway through the dephosphorization treatment, the stirring power density was changed at the time T M The molten iron was stirred by oxygen blowing from the wall lance and by gas blowing from the bottom blowing tuyeres at the bottom of the furnace. Argon, an inert gas, was used as the bottom blown gas.

[0040] 2. Test Results The test results are shown in Figures 2 and 3 and in the following Table 1. Note that the "agitation power density reduction ratio" in Figure 2 is the ratio of the agitation power density ε 1 The changed stirring power density ε 2 The ratio of ε 2 / ε 1 In addition, the "time when the stirring power density changes" in FIG. 3 is the time T at which the stirring power density is changed from the start of the dephosphorization treatment with respect to the total time T of the dephosphorization treatment. M It is defined as the ratio T' / T of the time T' to ... 1 , ε 2 ) was calculated using the following formula (1), and the stirring power density was changed by changing the bottom blown gas flow rate and / or the top blown gas flow rate. The results in Figures 2 and 3 and the following Table 1 all show the stirring power density ε before the change. 1The results shown in Fig. 2 are for the case where the stirring power density change time is fixed at 0.82, and the results shown in Fig. 3 are for the case where the stirring power density ratio is fixed at 0.8.

[0041] ε = ε bot +0.1·ε top (1)

[0042] ε bot =371 Q T l ·{ln(1+9.8·ρ·h / P)+0.06·(1-T n / T l )} / W Q: Bottom blown gas flow rate (Nm 3 / s) T l : Molten iron temperature (K) ρ: Molten iron density (kg / m 3 ) h: Gas injection depth from the molten iron surface (m) P: Atmospheric pressure (Pa) T n : Gas temperature (K) W: Molten iron weight (t)

[0043] ε top =0.0453 Q T d t ·u 0 2 cosθ / (W X) Q T : Top blown gas flow rate (Nm 3 / min) d t : Nozzle throat diameter (m) u 0 : nozzle exit gas linear velocity (m / s), θ: Nozzle angle (rad) X: Lance height (m)

[0044] As shown in Figure 2, when the stirring power density was reduced during the dephosphorization process, the phosphorus concentration [P] after the dephosphorization process was reduced compared to when it was not reduced. It is believed that by weakening the stirring during the dephosphorization process, the mass transfer coefficient of the molten iron becomes smaller, thereby slowing down the rephosphorization reaction rate. In addition, in electric furnaces, the slag is constantly foamed to protect the refractory from the arc radiation. By weakening the stirring during the dephosphorization process, it becomes difficult for the slag to mix vertically in the foamed slag, and the P concentration near the interface between the slag and the molten iron becomes relatively high. 2 O 5 In particular, by setting the stirring power density ratio to 0.7 or less, the phosphorus concentration [P] of the molten iron after dephosphorization treatment could be significantly reduced.

[0045] As shown in FIG. 3, at the time T M However, when the dephosphorization process was completed at between 30% and 80%, the phosphorus concentration [P] of the molten iron after the dephosphorization process was significantly reduced. If the stirring power density is reduced before 30% of the dephosphorization process is completed, it is believed that the dephosphorization reaction is suppressed due to insufficient stirring. On the other hand, if the stirring power density is reduced after 80% of the dephosphorization process is completed, rephosphorization from the slag to the molten iron will have progressed to a certain extent before the stirring power density is reduced, and the rephosphorization suppression effect will be reduced.

[0046] [Table 1]

[0047] No. 7 in Table 1 shows the results when the stirring power density was constant from the start to the end of the dephosphorization treatment. In No. 7, the phosphorus concentration [P] in the molten iron before the dephosphorization treatment was 0.024 mass%, and the phosphorus concentration [P] in the molten iron after the dephosphorization treatment was 0.018 mass%, meaning that the phosphorus concentration [P] in the molten iron was reduced by 0.006 mass% by the dephosphorization treatment.

[0048] Nos. 8 to 10 in Table 1 are the results when the stirring power density was increased during the dephosphorization treatment. For Nos. 8 to 10, the change in the phosphorus concentration [P] in molten iron due to the dephosphorization treatment is essentially the same as for No. 7. In other words, even if the stirring power density is increased during the dephosphorization treatment, no beneficial effect is obtained in terms of dephosphorization efficiency.

[0049] Nos. 1 to 6 in Table 1 are the results when the stirring power density was reduced during the dephosphorization process. Nos. 1 to 6 have a larger change in phosphorus concentration [P] in molten iron due to the dephosphorization process than No. 7. In other words, it can be said that Nos. 1 to 6 have better dephosphorization efficiency than No. 7.

[0050] In Table 1, the stirring power density before the change ε 1 The dephosphorization test results are shown for a case where the stirring power density is 75 (W / t), but the stirring power density before the change is not limited to this value. As long as the dephosphorization reaction proceeds both before and after the reduction of the stirring power density, the stirring power density before the reduction ε 1 Whether it is smaller than 75 (W / t) or larger, the same results as those of Nos. 1 to 6 in Table 1 are obtained. That is, when the stirring power density is reduced during the dephosphorization treatment, the stirring power density ε 1 and the stirring power density ε after the decrease 2 Regardless of the size of ε 2 <ε 1 As long as these conditions are met, the desired effect can be achieved.

[0051] Table 2 below shows the stirring power density ε before and after the change. 1 The test results for a stirring power density of ε 1 and the stirring power density ε 2 Regardless of the size of ε 2 <ε 1 It can be seen that as long as the following conditions are met, the desired effect is achieved.

[0052] [Table 2]

[0053] From the above results, in the case of a method for refining molten iron using an electric furnace, when the molten iron is dephosphorized while being stirred, the ε 2 <ε 1 It can be said that the phosphorus concentration in molten iron can be reduced by satisfying the following relationship. 2 ≦0.7×ε 1 It can be said that a higher effect can be expected when the following conditions are satisfied. In addition, it can be said that a higher effect can be expected by lowering the stirring power density when 30% to 80% of the dephosphorization treatment is completed.

Claims

1. A method for refining molten iron using an electric furnace, comprising the steps of: and performing a dephosphorization treatment on the molten iron while stirring the molten iron, In the dephosphorization treatment, the following conditions are met: e ２ <e １ ε １ : Stirring power density in the first period of the dephosphorization treatment First period: a time T from the start of the dephosphorization treatment to a point in time during the dephosphorization treatment Ｍ to ε ２ : Stirring power density in the second period of the dephosphorization treatment Second period: the time T Ｍ From the completion of the dephosphorization treatment is satisfied, A method of refining molten iron.

2. The method for refining molten iron according to claim 1, In the dephosphorization treatment, ε ２ ≦0.7×ε １ The conditions are met, A method of refining molten iron.

3. The method for refining molten iron according to claim 1 or 2, Said time T Ｍ is the time point when 30% to 80% of the dephosphorization treatment is completed; A method of refining molten iron.

4. The method for refining molten iron according to claim 1 or 2, Said time T Ｍ is determined based on the temperature of the molten iron; A method of refining molten iron.

5. A method for refining molten iron according to claim 4, Said time T Ｍ is the time when the temperature of the molten iron becomes equal to or higher than the threshold value; A method of refining molten iron.

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