Method for melting and refining iron source

Abstract

This method for melting and refining an iron source is characterized by: comprising, in melting and refining an iron source using an electric furnace, a first step for heating and melting the iron source charged into the electric furnace; a second step for discharging part or all of slag generated in the first step; a third step for adjusting the temperature and composition; and a fourth step for removing and tapping the slag, the first step, the second step, the third step, and the fourth step being performed in this order; at the end of the first step, specifying a B value by equation (3) on the basis of a CaO concentration (C), an SiO2 concentration (S), and an Al2O3 concentration (A) in the slag; and setting an MgO concentration (M) in the slag within a range of formula (1), and an FeO concentration (F) in the slag within a range of formula (2).　(1): 8.0(B+1.0)-1 ≤ M ≤ 60.0(B+1.0)-1, (2): 2.1(B+0.1)-1+3.0B-2.0 ≤ F ≤ 7.0(B+0.1)-1+10.0B+24.0, (3): B = C / (S+0.9A)

Description

Method for melting and refining iron sources

[0001] The present invention relates to a method for melting and refining an iron source using an electric furnace.

[0002] Currently, high-grade steel suitable for the production of automotive steel sheets is mainly produced using the blast furnace-converter method, with iron ore as the raw material. In the blast furnace method, coke is used as a reducing agent, and CO ２ In contrast, the electric furnace steelmaking process uses scrap as the main raw material, and CO ２ To achieve zero-carbon steel, it is necessary to expand the application of electric furnace steelmaking to the production of high-grade steel.

[0003] Patent Document 1 describes a method for producing low-phosphorus molten iron using a steelmaking electric furnace, focusing on the fact that when refining molten iron at a temperature of 1500°C or higher, the solid fraction and viscosity of the slag produced in the steelmaking electric furnace change significantly depending on the slag composition at the molten iron temperature. The method covers a process in which a solid iron source and optionally a molten iron source are charged during refining, the raw materials are melted using electrical energy, some or all of the slag produced during melting is discharged, and then a dephosphorization flux is added to perform dephosphorization. According to the document, in this process, the mass ratio of the slag composition to be discharged, CaO / (SiO ２ +Al ２ O ３ ) to the range of 0.25 to 0.70, which is claimed to reduce the lime consumption required for reducing phosphorus in molten iron and to efficiently produce low-phosphorus molten iron in a steelmaking electric furnace.

[0004] Scrap, the main raw material for electric furnaces, contains tramp elements such as Cu. These tramp elements are not removed during melting and refining in electric furnaces and remain in the steel, making it unsuitable for the production of automotive steel sheets. High-quality steel production in electric furnaces requires a high-quality iron source with a low content of these tramp elements. However, high-quality scrap is a limited resource. Therefore, it is necessary to reduce the amount of scrap used by utilizing reduced iron and pig iron (molten pig iron and molten pig iron) as iron sources. Furthermore, with the recent spread of electric vehicles, there is an increasing need to use scrap derived from electrical steel used in motors.

[0005] International Publication No. WO2022 / 054555

[0006] When using an iron source such as electromagnetic steel scrap in the melting and refining of iron sources using an electric furnace, the iron source contains a large amount of Si and Al compared to general scrap, and these are oxidized to produce SiO ２ and Al ２ O ３ Furthermore, when reduced iron is used as the iron source, the amount of slag increases due to the formation of CaO, SiO, etc. ２ and Al ２ O ３ When the iron source is heated and melted mainly by arc, SiO ２ When the amount of oxides generated is large, the large amount of slag can easily cause wear to the refractory. Therefore, when electromagnetic steel scrap or reduced iron is used as the iron source, the large amount of slag can cause chemical wear to the refractory used in the furnace walls, etc., if the conventional operating method is used. Furthermore, when the input power is increased to improve the productivity of melting and refining in an electric furnace, the refractory is melted by the radiant heat of the arc, and this causes the problem of thermal wear of the refractory as the arc power is increased. On the other hand, reducing the arc power requires more time for melting, resulting in a decrease in productivity.

[0007] The present invention relates to a method for reducing SiO2 derived from an iron source during melting and refining of the iron source using an electric furnace. ２ and Al ２ O ３A first object of the present invention is to provide a method for melting and refining an iron source, which can suppress the melting damage of a refractory material even if the amount of slag increases due to the generation of iron ore, etc., and can also suppress the melting damage of a refractory material even if the arc output is increased. A second object of the present invention is to perform good deP refining in the melting and refining of an iron source using an electric furnace.

[0008] That is, the gist of the present invention is as follows: [1] A method for melting and refining an iron source using an electric furnace, comprising: a first step of heating and melting the iron source charged into the electric furnace; a second step of discharging part or all of the slag generated in the first step; a third step of adjusting the temperature and composition; and a fourth step of discharging the slag and tapping the molten metal, wherein the first, second, third, and fourth steps are carried out in this order, and at the end of the first step, the B value is defined by the following formula (3) based on the slag composition, the MgO concentration (M) in the slag satisfies the following formula (1), and the FeO concentration (F) in the slag satisfies the following formula (2): 8.0(B+1.0) －１ ≦M≦60.0(B+1.0) －１ …(1) 2.1(B+0.1) －１ +3.0B-2.0≦F≦7.0(B+0.1) －１ +10.0B+24.0 ... (2) B = C / (S+0.9A) ... (3) In formulas (1) to (3), C, S, A, and M are the CaO concentration (C), SiO ２ Concentration (S), Al ２ O ３ The values ​​shown are the concentration (A) and MgO concentration (M). The FeO concentration (by mass) (F) was calculated assuming that all iron oxide in the slag was FeO. [2] The method for melting and refining an iron source according to [1], characterized in that, at the end of the third step, the B value is defined by the formula (3) based on the slag composition, and the MgO concentration (M) in the slag satisfies the formula (1), and the FeO concentration in the slag satisfies the formula (2). [3] The method for melting and refining an iron source according to [1] or [2], characterized in that, at the end of the third step, the slag composition satisfies the following formula (4): C / (S+0.9A)≧0.8 (4) In formula (4), C, S, and A are the same as those in formula (3).

[0009] In the present invention, in the melting and refining of an iron source using an electric furnace, at the end point of a first step in which an iron source charged into the electric furnace is heated and melted, the MgO concentration (M) in the slag satisfies the above formula (1) and the FeO concentration (F) in the slag satisfies the above formula (2), thereby making it possible to suppress the melting loss of a refractory.

[0010] The melting and refining of an iron source using an electric furnace according to the present invention includes a first step of heating and melting the iron source charged into the electric furnace, a second step of discharging part or all of the slag generated in the first step, a third step of adjusting the temperature and composition, and a fourth step of discharging the slag and tapping the molten metal. The first step, second step, third step, and fourth step are carried out in this order.

[0011] In the first process, the iron source placed in the electric furnace is heated and melted. Scrap, reduced iron, and pig iron (mold pig iron and molten pig iron) are charged into the electric furnace as the iron source, and the solid iron source is melted by arc heating. Iron scrap contains Si, Mn, Cr, Al, and other alloying elements that are easily oxidized. In particular, electrical steel contains a high concentration of Si. Reduced iron contains CaO, SiO, and other elements. ２ and Al ２ O ３ A portion of the Fe in the iron source and metal components other than Fe that are easily oxidized are oxidized by oxygen supplied into the electric furnace during melting of the solid iron source, and form slag together with the oxide components in the iron source.

[0012] Once the melting of the iron source is complete in the first step, in the next second step, some or all of the slag produced in the first step is discharged. This facilitates the formation of slag suitable for refining when adjusting the temperature and composition of the molten iron in the subsequent third step. In the third step, refining is carried out so that the temperature and composition of the molten iron reach the target values, and then in the fourth step, the slag is discharged and the molten iron is tapped.

[0013] Among the first to fourth steps, the first step takes the longest time and requires the highest arc output to melt the solid iron source, so it is in the first step that the erosion of the electric furnace refractories progresses most.

[0014] In the first step, CaO and SiO ２ , Al ２ O ３ The amount of CaO and SiO produced varies depending on the blending ratio of the iron source, but is 3 kg / t or more in total. In particular, when electrical steel scrap, reduced iron, and pig iron are used as the iron source at a high ratio, the amount of CaO and SiO produced from the iron source increases. ２ , Al ２ O ３ The amount of produced increases.

[0015] The analysis method for the slag composition is explained below. Iron in the slag is analyzed using metallic iron, FeO, Fe ２ O ３ The total iron concentration (% T.Fe) was determined by ICP atomic emission spectrometry. The metallic iron concentration (% M.Fe) was determined by the bromine-methanol method. The concentration of iron present as iron oxide was determined from the difference between these values, and converted to iron oxide concentration assuming that all iron oxide was present as FeO. The concentrations of other components were determined by ICP atomic emission spectrometry. The above analytical methods are common to those skilled in the art.

[0016] Other components contained in the slag include MnO, P ２ O ５ , Cr ２ O ３ , TiO ２ , Nb ２ O ３ , Na ２ O, CaS, CaF ２ The total amount thereof is usually 15% by mass or less.

[0017] <<Preferable Range of MgO Concentration in Slag>> A method of increasing the MgO content in slag by adding an MgO source to an electric furnace is known for the purpose of reducing the corrosion of refractories in an electric furnace, particularly refractories containing MgO. However, in the melting and refining of an iron source through the first to fourth steps as in the present invention, it was not known what the appropriate MgO concentration in the slag should be in each of the first and third steps.

[0018] First, we attempted to estimate the maximum MgO concentration at which MgO dissolves (hereinafter referred to as "solubility limit MgO concentration") in various slag compositions at 1600°C and 30% (FeO) using solgasmix (a multi-component equilibrium analysis calculation program). The solubility limit MgO concentration is determined by the amount of CaO, SiO in the slag. ２ , Al ２ O ３ It varies depending on the concentration of Al. ２ O ３ We searched for parameters of the slag composition that would keep the solubility limit of MgO constant even if the concentration fluctuated. ２ The concentration is S, Al ２ O ３ The concentration is indicated as A and the MgO concentration is indicated as M.

[0019] As a result, if the value of basicity B = C / (S + 0.9A) (3) is constant, Al ２ O ３ It was found that even if the concentration (A) fluctuates within the range of 0 to 20%, the solubility limit MgO concentration for the basicity B remains constant. Therefore, we decided to proceed with the following investigation using the basicity B in the above formula (3) as an index.

[0020] When the relationship between basicity B and solubility limit MgO concentration was investigated using solgasmix, it was found that the solubility limit MgO concentration value increases inversely proportional to "basicity B + 1.0". Therefore, next, P = (B + 1.0) －１ Using the above formula (5) as an index, the minimum MgO concentration (M) in the slag that can suppress refractory corrosion was evaluated for each value of parameter P calculated from the slag composition in the first step in actual electric furnace operation. As a result, it was found that refractory corrosion can be suppressed by ensuring an MgO concentration (M) in the slag that satisfies the formula 8.0 × P≦M (6).

[0021] In smelting processes using electric furnaces, particularly in processes such as the first process, where the arc heating output is high and the processing time is long, it is effective to cover the arc with slag foaming in order to reduce thermal refractory damage caused by the arc. However, it has been found that an excessively high MgO concentration in the slag suppresses slag foaming, exposing the arc and promoting thermal refractory damage. Therefore, the maximum MgO concentration in the slag (M) that can prevent slag foaming and prevent arc exposure was evaluated for each of the above parameters P. As a result, it was found that maintaining a MgO concentration in the slag (M) that satisfies the following formula can prevent foaming and suppress refractory damage.

[0022] Therefore, by substituting equation (5) into equations (6) and (7) above and combining them into a single inequality, we were able to derive the following equation (1) using B from equation (3): 8.0(B+1.0) －１ ≦M≦60.0(B+1.0) －１ ... (1) B = C / (S + 0.9A) ... (3) In formulas (1) and (3), C, S, A, and M are the CaO concentration (C) and SiO ２ Concentration (S), Al ２ O ３ The concentration (A) and MgO concentration (M) are shown.

[0023] <Preferable range of the coefficient in equation (1)> Although the coefficient on the left side of equation (1) is set to 8.0, it is more preferable that the coefficient be 10.0. It is even more preferable that the coefficient be 12.0. Furthermore, although the coefficient on the right side of equation (1) is set to 60.0, it is more preferable that the coefficient be 52.0. It is even more preferable that the coefficient be 45.0.

[0024] <Preferred range of FeO concentration in slag> In the first step, in which the iron source charged into the electric furnace is heated and melted, arc heating is performed and oxygen is supplied into the electric furnace. Oxygen gas is supplied to oxidize and remove impurities in the molten iron and to burn carbonaceous materials for the purpose of foaming the slag. The supplied oxygen gas oxidizes the Fe content in the molten iron in the electric furnace, forming iron oxide in the slag. The iron oxide in the slag contains FeO and Fe ２ O ３ However, in this case, all of the Fe constituting the iron oxide in the slag is converted to FeO and treated as the FeO concentration (mass basis) (F).

[0025] Therefore, first, we attempted to estimate the minimum FeO concentration (hereinafter referred to as the "critical FeO concentration") at 1600°C for various slag compositions, at which the slag becomes entirely liquid in equilibrium, using solgasmix (a multi-component equilibrium analysis calculation program). The critical FeO concentration is calculated by taking into account the amounts of CaO, SiO in the slag. ２ , Al ２ O ３ It varies depending on the concentration of Al. ２ O ３ We searched for parameters of the slag composition that would keep the limit FeO concentration constant even if the concentration fluctuated. ２ The concentration is S, Al ２ O ３ The concentration is indicated as A and the FeO concentration as F.

[0026] As a result, if the value of basicity B = C / (S + 0.9A) (3) is constant, Al ２ O ３ It was found that even if the concentration (A) fluctuates within the range of 0 to 20%, the change in the limit FeO concentration relative to the basicity B is the same. Therefore, hereinafter, the basicity B in the above formula (3) will be used as an index for further investigation.

[0027] When the relationship between basicity B and the critical FeO concentration was investigated using solgasmix, it was found that the value of the critical FeO concentration is minimum (=0) when the basicity B is between 0.7 and 1.2, and increases as the basicity B decreases when the basicity B is less than 0.7, and increases as the basicity B increases when the basicity B is more than 1.2. Next, a function showing a similar trend to the above was searched for using basicity B as an argument, and it was found that the function of the following formula (8) in which X is a constant coincides with the relationship between B and the critical FeO concentration when X = 3. From this, it was found that it is appropriate to use the following formula (8). P = X × ((B + 0.1) －１ +0.7 －１ ×B+5.3)-13 (8)

[0028] Next, using P in formula (8) as an index, the maximum value of the FeO concentration (F) in the slag capable of suppressing refractory corrosion was evaluated for each value of parameter P calculated from the slag composition in the first step in actual electric furnace operation. As a result, it was revealed that if the FeO concentration (F) in the slag satisfies the following formula (9), which is obtained by substituting X = 7.0 for X in formula (8), refractory corrosion can be suppressed at various values ​​of B. F ≦ 7.0 × ((B + 0.1) －１ +0.7 －１ ×B+5.3)-13 =7.0×(B+0.1) －１ +10.0 × B + 24.0 (9)

[0029] In smelting processes using electric furnaces, particularly in processes such as the first process, where the arc heating output is high and the processing time is long, it is effective to form slag to cover the arc with slag in order to reduce thermal refractory damage caused by the arc. However, it has been found that if the FeO concentration in the slag is too low, slag foaming is reduced, exposing the arc and promoting thermal refractory damage. Therefore, for each of the above parameters P, the minimum FeO concentration in slag (F) that can prevent arc exposure without reducing slag foaming was evaluated. As a result, it was found that an FeO concentration (F) that satisfies the following formula (10), obtained by substituting X = 2.1 for X in formula (8), can suppress the reduction in foaming, cover the arc, and suppress refractory damage at various values ​​of B. F ≥ 2.1 × ((B + 0.1) －１ +0.7 －１ ×B+5.3)-13 =2.1×(B+0.1) －１ +3.0 x B-2.0 (10)

[0030] Therefore, we were able to combine the above equations (9) and (10) into one inequality and derive the following equation (2) using B from the above equation (3): 2.1(B+0.1) －１ +3.0B-2.0≦F≦7.0(B+0.1) －１ +10.0B+24.0 ... (2) B = C / (S+0.9A) ... (3) In formulas (2) and (3), C, S, and A are the CaO concentration (C) and SiO concentration (SiO) in the slag by mass, respectively. ２ Concentration (S), Al ２ O ３ The FeO concentration (mass basis) (F) was calculated assuming that all the iron oxide in the slag was FeO.

[0031] <Preferable range of the coefficient in equation (2)> Although the coefficient on the left side of equation (2) is set to X = 2.1, it is more preferable that the coefficient is X = 2.5. It is even more preferable that the coefficient is X = 3.0. Furthermore, although the coefficient on the right side of equation (2) is set to X = 7.0, it is more preferable that the coefficient is X = 6.5. It is even more preferable that the coefficient is X = 6.0.

[0032] <<First Step Overall>> In the first step, the value of B = C / (S + 0.9A) in the slag composition defined by the formula (3) is not limited. However, it is preferable that the value of B is within a range of 0.3 to 3.0, since this satisfies the formulas (1) and (2), suppression of foaming is prevented, exposure of the arc is prevented, and refractory melting damage can be sufficiently suppressed.

[0033] According to the present invention, since the arc is covered with slag in the first step, it is possible to increase the arc output, and high-speed arc melting can be performed, thereby shortening the first step. This makes it possible to secure sufficient time for refining in the third step, and also to sufficiently promote deP refining in the third step.

[0034] The iron source may be continuously charged in the first step, which makes it easier to control the slag composition.

[0035] <<Second Step and After>> In the method for melting and refining an iron source according to the present invention, after heating and melting the iron source in the first step, the slag is discharged in the second step, and the temperature and composition of the molten iron are adjusted in the third step. The third step often requires a shorter processing time than the first step, and because it does not involve melting a solid iron source, the amount of heat generated by the arc is also less than in the first step. Therefore, although the attack on the refractories of the electric furnace is less severe in the third step than in the first step, taking measures to prevent refractory melting is still effective in the third step. Similarly to the first step, in the third step, favorable results can be obtained by defining the B value based on the slag composition at the end of the third step using the formula (3) above, and by ensuring that the MgO concentration (M) in the slag satisfies the formula (1) above and that the FeO concentration (F) in the slag satisfies the formula (2) above. The degree of wear of the refractory can be reduced by making the MgO concentration (M) in the slag at the end of the third step greater than the value given by the left side of equation (1) and making the FeO concentration (F) smaller than the value given by the right side of equation (2).Furthermore, by making the MgO concentration (M) in the slag at the end of the third step smaller than the value given by the right side of equation (1) and making the FeO concentration (F) larger than the value given by the left side of equation (2), the slag can be appropriately foamed, preventing exposure of the arc and reducing the degree of wear of the refractory.

[0036] In the third step, too, it is more preferable that the coefficient on the left side of equation (1) is 10.0. It is even more preferable that the coefficient is 12.0. It is also more preferable that the coefficient on the right side of equation (1) is 52.0. It is even more preferable that the coefficient is 45.0. It is also more preferable that the coefficient on the left side of equation (2) is X = 2.5. It is even more preferable that the coefficient is X = 3.0. It is also more preferable that the coefficient on the right side of equation (2) is X = 6.5. It is even more preferable that the coefficient is X = 6.0.

[0037] The compositional adjustment of the molten iron performed in the third step may involve simply adding alloying elements, but may also involve the dephosphorization of the molten iron. The slag in the third step is given dephosphorization capability, and oxidation refining is performed to reduce the P concentration in the molten iron. In the present invention, using the basicity B = C / (S + 0.9A) defined in the above formula (3) as an index, the slag composition at the end of the third step satisfies the following formula: C / (S + 0.9A) ≥ 0.8 (4). In formula (4), C, S, and A are as defined above. It is more preferable that B ≥ 1.0. While there is no particular upper limit for basicity B, a value of B ≤ 4.5 is preferred, as it reduces the amount of CaO that becomes supersaturated.

[0038] Any method can be used for removing the slag from the furnace as long as it allows the slag to be removed from the furnace. For example, it is possible to select whether or not to use arc heating or tilting the furnace body during the slag removal process. In addition, the slag may be discharged in parallel with heating and melting in the first step. In addition, the slag may be discharged in parallel with adjusting the temperature and composition in the third step.

[0039] The method for melting and refining an iron source according to the present invention was carried out using an electric arc furnace with a molten iron capacity of 175 tons. Of the 175 tons of molten iron produced, 110 tons were tapped, and 65 tons were left in the electric furnace to be used as a seed molten iron for the next heat. In addition to the seed molten iron, scrap and reduced iron were used as a solid iron source. The proportion of reduced iron in the solid iron source was 50%. The CaO concentration in the reduced iron was 1 mass %, SiO ２ The concentration is 2 mass%, Al ２ O ３The concentration was about 0.5% by mass.

[0040] The following steps were carried out in this order: a first step in which the iron source charged into the electric furnace was melted by arc heating; a second step in which some or all of the slag generated in the first step was discharged; a third step in which the temperature and components were adjusted; and a fourth step in which the slag was discharged and the molten metal was tapped.

[0041] Among the slag components generated in the first step, quicklime is added as a CaO source in addition to the CaO source in the reduced iron, and SiO ２ The source is SiO in reduced iron. ２ , SiO produced by oxidation of Si in the scrap ２ In addition to the source, silica or FeSi alloy is added as needed. ２ O ３ Al in reduced iron as a source ２ O ３ , Al generated by oxidation of Al in the scrap ２ O ３ In addition to the source of MgO, aluminum dross, alumina brick waste, and bauxite were added as needed. Light-burned magnesia was added as the MgO source.

[0042] After the melting in the first step was completed, 30 to 70% of the slag produced in the first step was discharged in the second step.

[0043] In the subsequent third step, deP refining was carried out by oxidation refining, adding quicklime as a CaO source and light-burned magnesia as an MgO source as needed in addition to the slag remaining in the electric furnace. After deP refining was completed, in the fourth step, 70-90% of the slag generated in the third step was discharged, and 110 tons of the 175 tons of molten iron were tapped. The 65 tons of molten iron remaining in the furnace was used as seed molten iron for the next heat.

[0044] Slag samples were collected from the electric furnace at the end of the first and third steps, and the slag components were analyzed. The CaO concentration (C) and SiO ２ Concentration (S), Al ２ O ３The concentrations (A) are designated as C, S, and A, respectively, and the results are shown in the "C / (S+0.9A)" and "MgO" columns of Table 1. Table 1 also lists the values ​​of the left and right sides of equation (1) calculated using the above analytical values. The FeO concentration (mass basis) (F) is also shown, calculated assuming that all iron oxide in the slag is FeO. Table 1 also lists the values ​​of the left and right sides of equation (2) calculated using the above analytical values.

[0045] The slag compositions and evaluation results for each example are shown in Table 1. Values ​​outside the range of the present invention are underlined.

[0046]

[0047] The operating conditions shown in Table 1 were each used for 100 consecutive heats.

[0048] For the refractory wear rate index, the remaining dimension of the refractory was measured at the height of the molten metal surface at the end of melting, and the difference between the remaining dimension of the refractory after 100 heats and the remaining dimension of the refractory before the heats was divided by the number of heats, and the value was graded according to the following criteria: A: Less than 0.10 mm / heat on average B: 0.10 mm / heat or more on average, less than 0.20 mm / heat C: 0.20 mm / heat or more on average, less than 0.35 mm / heat D: 0.35 mm / heat or more on average, less than 0.50 mm / heat E: 0.50 mm / heat or more on average

[0049] The deP index was evaluated as follows: W_P_in = mass of P in the seed molten metal + mass of P brought in the iron source W_P_out = mass of P in the molten iron at the end of refining Then, the deP index (%) was calculated as follows: DeP index = (W_P_in - W_P_out) / W_P_in x 100 The average value of the deP index for 100 heats was graded according to the following criteria: A: 70% or more B: 30% or more but less than 70% C: Less than 30%

[0050] The overall score was evaluated according to the following criteria. Overall scores A to D were considered to be pass. A: Refractory wear rate index A and de-P index A B: Refractory wear rate index B and de-P index A or Refractory wear rate index A and de-P index B C: Refractory wear rate index C and de-P index A D: Refractory wear rate index D and de-P index A E: Refractory wear rate index E

[0051] In Table 1, invention examples 1 to 13 are invention examples, and comparative examples 1 to 3 are comparative examples.

[0052] In all of Inventive Examples 1 to 3 and 8 to 13, the MgO and FeO contents of the slag in the first step were within the ranges of the present invention, and the MgO and FeO contents of the slag in the third step were within the preferred ranges of the present invention, and the refractory wear rate index was A to C, which was at an acceptable level. In the explanation of formula (1) showing the MgO concentration range in the slag and the explanation of formula (2) showing the FeO concentration range, the <preferred range for the coefficient of formula (1)> and <preferred range for the coefficient of formula (2)> describe numerical values ​​that define more preferred ranges and numerical values ​​that define even more preferred ranges for the coefficients on the left and right sides of formula (1) and formula (2). In Inventive Examples 8 to 9, the MgO and FeO contents in the first step were within the ranges of the formula employing the numerical values ​​described as preferred ranges in the above <Preferred ranges for the coefficients of formula (1)> and <Preferred ranges for the coefficients of formula (2)>, and the chemical corrosion of the refractory was small, the forming state was good, and the refractory wear rate index was good, being B. Inventive Examples 10 to 13, the MgO and FeO contents in the first step were within the ranges of the formula employing the numerical values ​​described as further preferred ranges in the above <Preferred ranges for the coefficients of formula (1)> and <Preferred ranges for the coefficients of formula (2)>, and the chemical corrosion of the refractory was particularly small, the forming state was particularly good, and the refractory wear rate index was particularly good, being A.

[0053] In Examples 4 to 7 of the present invention, the MgO content and FeO concentration in the slag in the third step were outside the preferred ranges of the present invention, and the refractory wear rate index was D.

[0054] In Example 13 of the present invention, the slag "C / (S+0.9A)" in the third step did not satisfy the preferred range of the present invention, and therefore the deP ability in the third step was insufficient, and the deP index was B.

[0055] In Comparative Examples 1 to 3, the MgO content of the slag in the first step was outside the upper or lower limit of formula (1), and the FeO content of the slag in the first step was outside the upper or lower limit of formula (2), and the refractory wear rate index was E in all cases.

Claims

1. A method for melting and refining an iron source using an electric furnace, comprising: a first step of heating and melting the iron source charged into the electric furnace; a second step of discharging some or all of the slag produced in the first step; a third step of adjusting the temperature and composition; and a fourth step of discharging the slag and tapping the molten metal, wherein the first, second, third, and fourth steps are carried out in this order, and at the end of the first step, the B value is defined by the following formula (3) based on the slag composition, the MgO concentration (M) in the slag satisfies the following formula (1), and the FeO concentration (F) in the slag satisfies the following formula (2): 8.0(B+1.0) －１ ≦M≦60.0(B+1.0) －１ …(1) 2.1(B+0.1) －１ +3.0B-2.0≦F≦7.0(B+0.1) －１ +10.0B+24.0 ... (2) B = C / (S+0.9A) ... (3) In formulas (1) to (3), C, S, A, and M are the CaO concentration (C), SiO ２ Concentration (S), Al ２ O ３ The iron oxide in the slag was converted to FeO as the FeO concentration (mass basis) (F).

2. The method for melting and refining an iron source according to claim 1, characterized in that, at the end point of the third step, the B value is defined by the formula (3) based on the slag composition, the MgO concentration (M) in the slag satisfies the formula (1), and the FeO concentration in the slag satisfies the formula (2).

3. The method for melting and refining an iron source according to claim 1 or 2, characterized in that, at the end of the third step, the slag composition satisfies the following formula (4): C / (S+0.9A)≧0.8 (4) In formula (4), C, S, and A are the same as those in formula (3).

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