Method for melting reduced iron

Abstract

To further improve the productivity of producing molten steel using an electric furnace without suppressing the charging rate of reduced iron.SOLUTION: In a method for melting reduced iron, when the reduced iron charged into the furnace of an electric furnace is melted, the total volume of the reduced iron satisfying the relation expressed by expression 1 in the reduced iron charged into the furnace is set to 70% or more of the total volume of the total reduced iron charged.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for melting reduced iron. [Background technology]

[0002] In recent years, molten steel production technology using an electric furnace to melt reduced iron has been attracting attention. In the operation of an electric furnace, reduced iron is continuously or intermittently charged from above a molten steel pool present in the furnace and melted. In order to achieve high energy efficiency and reduction efficiency in such reduced iron melting using an electric furnace, various technologies have been proposed in recent years (for example, see Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2001-515138 [Non-patent literature]

[0004] [Non-Patent Document 1] Takeaki Murayama, Yoichi Ono, Yasuji Kawai, Iron and Steel, 63, 1099 pages, 1977 [Non-Patent Document 2] K. Mori and T. Sakuraya, Trans. ISIJ, 22, p. 984, 1977 [Non-Patent Document 3] Q. Jiao and NJ Themelis, Canadian Metallurgical Quarterly, 32, p. 75, 1993 [Non-Patent Document 4] T. Yagi and Y. Ono, Trans. ISIJ, 8, p. 377, 1968 Summary of the Invention [Problem to be solved by the invention]

[0005] The reduced iron added to the molten steel is lighter than the molten steel and therefore exists in a floating state near the surface of the molten steel. In addition, since the temperature of the reduced iron is lower than the melting point of iron, it is known that the molten steel surrounding the reduced iron solidifies when the reduced iron is added, forming a solidified shell around the reduced iron.

[0006] If a solidified shell is formed around the reduced iron, the surrounding solidified shell must first be melted in order to melt the reduced iron. Therefore, it takes more time to melt the charged reduced iron, which is undesirable from the viewpoint of productivity. In addition, if nearby solidified shells are bonded together, a large lump of reduced iron called an iceberg is formed, and it takes even more time to melt the reduced iron.

[0007] From the above viewpoints, in the operation of an electric furnace for producing molten steel by melting reduced iron, it is considered necessary to suppress the charging rate of reduced iron in order to prevent solidification of molten steel or iceberg formation at the charging position of the reduced iron, which is considered to be a problem in improving productivity.

[0008] From the above viewpoints, it is considered that there is a need for a technology that can further improve the productivity of molten steel production using an electric furnace without restricting the charging rate of reduced iron.

[0009] Therefore, the present invention has been made in consideration of the above problems, and an object of the present invention is to provide a method for melting reduced iron that can further improve the productivity of molten steel production using an electric furnace, without suppressing the charging rate of reduced iron. [Means for solving the problem]

[0010] In order to solve the above problems, the present inventors first focused on the process of formation of a solidified shell. In this regard, it is known from knowledge about a general solid charged into molten steel that the thickness of a solidified shell formed around the solid present in the molten steel decreases as the heat transfer from the molten steel increases. Based on this knowledge, the present inventors came up with the idea of ​​promoting heat transfer from the molten steel to the reduced iron charged into the molten steel.

[0011] In addition, the coefficient of heat transfer from molten steel to a solid surface generally increases as the flow of the surrounding molten steel becomes stronger. In the operation of an electric furnace, in order to uniformize the temperature of the molten steel, the molten steel is stirred by imparting a global flow to the molten steel by gas blowing, electromagnetic stirring, or the like. However, as a result of the studies by the present inventors, it has become clear that when a large amount of reduced iron gathers and floats near the liquid surface of the molten steel, even if the above-mentioned global flow is imparted, it is not possible to impart a strong flow to the surface of the reduced iron, and the formation of a solidified shell on each reduced iron cannot be sufficiently suppressed.

[0012] Based on the above findings, the inventors have been focusing on chemical reactions taking place in molten steel and have noticed that a reaction is taking place in molten steel in which carbon C present in reduced iron becomes CO gas. Based on this, the inventors have come up with the idea that CO gas generated from the surface of the reduced iron can be used to stir the molten steel, thereby increasing the heat transfer coefficient and suppressing the growth of the solidified shell. Based on this idea, the inventors have conducted further studies and have come up with the present invention, which will be described in detail below. The gist of the present invention, which was completed based on these findings, is as follows.

[0013] [1] A method for melting reduced iron, which melts reduced iron charged into an electric furnace, comprising operating the electric furnace so that the total volume of reduced iron that satisfies the relationship expressed by the following formula (1) among the reduced iron charged into the furnace is 70% or more of the total volume of all reduced iron charged: Here, in the following formula (1), θ b =(Tb -T m ) / (T m -T0): Dimensionless bulk temperature T m : Melting point of reduced iron [unit: °C] T0: Input temperature of reduced iron [unit: °C] T b :Temperature of molten steel bulk in the furnace [unit: °C] k CO,m : Melting point of reduced iron T m Reaction rate constant in a C : Mole fraction of C in reduced iron a FeO : Molar fraction of FeO in reduced iron r0: spherical equivalent radius of reduced iron ρ1: Density of reduced iron ρ2: Density of the solidified shell formed around the reduced iron after charging α1: Thermal diffusion coefficient of reduced iron α2: Thermal diffusion coefficient of the solidified shell formed around the reduced iron after charging λ2: Thermal conductivity of the solidified shell formed around the reduced iron after charging C p1 : Specific heat of reduced iron ΔH: Latent heat of dissolution of reduced iron ΔH R : Reaction heat of CO gas generation reaction in reduced iron C R : Molar concentration of the lesser component between C and FeO ε: Void fraction of reduced iron It is. [2] The method for melting reduced iron according to [1], wherein a total amount of the reduced iron charged into the furnace satisfies the relationship expressed by the formula (1). [3] The method for melting reduced iron according to [1], wherein hot briquetted iron (HBI) is used as the reduced iron, and the furnace is operated so that the total volume of the hot briquetted iron charged into the furnace that satisfies the relationship expressed by the following formula (2) is 70% or more of the total volume of all the hot briquetted iron charged: Here, the following formula (2) is obtained by converting the right side of the formula (1) into a constant based on the physical properties of the hot briquette iron. [4] The method for melting reduced iron according to [3], wherein a total amount of the hot briquette iron charged into the furnace satisfies the relationship expressed by the formula (2). [5] The method for melting reduced iron according to [1], wherein direct reduced iron (DRI) is used as the reduced iron, and the furnace is operated so that the total volume of the direct reduced iron that satisfies the relationship expressed by the following formula (3) among the direct reduced iron charged into the furnace is 70% or more of the total volume of all the direct reduced iron charged: Here, the following formula (3) is obtained by converting the right side of the formula (1) into constants based on the physical property values ​​of the direct reduced iron. [6] The method for melting reduced iron according to [5], wherein a total amount of the direct reduced iron charged into the furnace satisfies the relationship expressed by the formula (3). [7] When scrap iron is charged into the furnace together with the reduced iron, and the temperature drop corresponding to the energy consumed until the scrap iron melts is ΔT, the temperature T of the molten steel bulk is b Instead of (T b -ΔT) and θ b =(T b -T m ) / (T m The method for melting reduced iron according to any one of [1] to [6], further comprising calculating the amount of reduced iron (T−T0).

[0014]

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[0015] As described above, according to the present invention, it is possible to further improve the productivity of molten steel production using an electric furnace without suppressing the charging rate of reduced iron. [Brief description of the drawings]

[0016] [Figure 1]FIG. 2 is a graph showing a schematic relationship between a variable x and a function φ used in the method for melting reduced iron according to the embodiment of the present invention. [Diagram 2] FIG. 4 is a graph for explaining the method for melting reduced iron according to the embodiment. [Diagram 3] FIG. 4 is a graph for explaining the method for melting reduced iron according to the embodiment. [Figure 4] FIG. 4 is a graph for explaining the method for melting reduced iron according to the embodiment. [Diagram 5] FIG. 4 is a graph showing verification results of the method for melting reduced iron according to the embodiment. [Figure 6] FIG. 4 is a graph showing verification results of the method for melting reduced iron according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same functional configurations are designated by the same reference numerals, and duplicated explanations will be omitted.

[0018] (Findings by the inventors regarding CO gas agitation) Before describing the method for melting reduced iron according to the embodiment of the present invention, the findings regarding CO gas stirring that have been completed by the present inventors based on the above-mentioned concept will be described in detail below.

[0019] As briefly mentioned above, the inventors came up with the idea of ​​utilizing agitation by CO gas generated from the surface of the reduced iron being charged to increase the heat transfer coefficient and suppress the growth of a solidified shell. When reduced iron is actually charged into molten steel, a solidified shell is not formed on the entire surface of the reduced iron immediately after it is charged, and there will be a mixture of areas where a solidified shell is formed and areas where no solidified shell is formed. Therefore, the CO gas generated from the surface of the reduced iron is very useful for efficiently stirring the molten steel around the reduced iron locally and suppressing the formation of a solidified shell on the surface of the reduced iron.

[0020] Furthermore, the inventors have found a method for estimating the amount of CO gas generated from the surface of reduced iron, its heat transfer coefficient, and its effect on the thickness of the solidified shell, and have identified an operation method capable of suppressing the thickness of the solidified shell due to CO gas generation.

[0021] The findings obtained by the present inventors as described above will be explained in order below. These findings are composed of a combination of three findings: (a) how CO gas is generated from reduced iron charged into an electric furnace, (b) how the generated CO gas affects the heat transfer coefficient, and (c) a method for predicting the extent to which the solidified shell is affected by the heat transfer coefficient.

[0022] (a) Prediction of CO gas generation from reduced iron First, we will explain in detail how to predict how CO gas will be generated from reduced iron fed into an electric furnace. The main chemical reactions in reduced iron that generate CO gas are expressed by the following two chemical reaction formulas. In the following chemical reaction formulas, (s) represents the solid state, and (g) represents the gas state.

[0023] C(s) + CO2(g) → 2CO(g) Reaction equation (1) FeO(s) + CO(g) → Fe(s) + CO2(g) Reaction equation (2)

[0024] That is, CO gas is generated by the gasification reaction of C in the reduced iron (reaction formula (1)) and the reduction reaction of FeO (reaction formula (2)). From the above reaction formulas (1) and (2), the apparent chemical reaction is expressed as the following reaction formula (3).

[0025] FeO(s) + C(s) → Fe(s) + CO(g) Reaction (3)

[0026] Here, for the above reaction formula (1) and reaction formula (2), the reaction rate formulas are as follows (101) and (103), respectively: where R1 is the reaction rate for reaction formula (1), and R2 is the reaction rate for reaction formula (2).

[0027]

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[0028] Here, in the above formulas (101) and (103), k is the reaction rate constant, K is the equilibrium constant, and P is the partial pressure. The subscript numbers indicate whether they relate to reaction formula (1) or reaction formula (2), and the subscript element symbols indicate that they relate to each component, such as C or CO. Furthermore, in the above formulas (101) and (103), c , a FeO are the activities of C and FeO, respectively, and are approximated by mole fractions in this embodiment.

[0029] Here, the temperature of the molten steel is about 1300 to 1500°C, and it is known that the above reaction in the reduced iron reaches a peak at about 1000°C. In the above temperature range, the amount of CO is sufficiently large compared to the amount of CO2. Therefore, a pseudo-steady-state approximation (i.e., an approximation that the molar concentration of CO2 does not change with time) is valid for CO2.

[0030] Here, from the above reaction formula (1) and reaction formula (2) and the above approximation of the pseudo-steady state, the CO2 production rate RCO2 is expressed as the following formula (105). Here, by substituting the above formulas (101) and (103) into the following formula (105), P CO2 Summarizing, we can obtain the following equation (107).

[0031]

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[0032] In addition, from the above reaction formula (1), the CO production rate R CO is expressed as the following formula (109). By substituting the above formulas (101), (103), and (107) into formula (109) and summarizing them, the following formula (111) can be obtained. In the following formula, C is the molar concentration, R is the gas constant, and T is the temperature at the reaction interface. In obtaining formula (111), the relationship expressed by the following formula (113) is used. Also, k in formula (111) can be obtained as CO and K e,CO Specifically, it is as shown in the following equations (115) and (117).

[0033]

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[0034] In addition, as described above, since the temperature of the molten steel is very high, the driving force in formula (101) for the reduced iron charged into the molten steel can be considered to be sufficiently large, and the relationship expressed by the following formula (119) can be considered to hold. CO is approximated as shown in the following equation (121).

[0035] Using these relational expressions, the CO gas generation rate (more specifically, the volume generation rate) is formulated as in the following equation (123). Here, in the following equation (123), 4π ri 2 is the reaction interfacial area in sphere equivalent.

[0036]

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[0037] The reaction area immediately after the reduced iron is added to the molten steel is the surface area of ​​the reduced iron. If the radius of the reduced iron in terms of a sphere (the radius of a sphere that is equal to the volume of the reduced iron) is expressed as r0, the surface area of ​​the reduced iron is 4πr0 2 It is known that the solidification and melting reactions of iron scrap and reduced iron in an electric furnace proceed at the rate determined by heat and mass transfer, and the interface between the reduced iron and molten steel can be considered to be in a local equilibrium state. Therefore, the temperature at the interface between the reduced iron and molten steel is approximately equal to the melting point T of the reduced iron. m Using the above, the initial CO gas generation rate q CO,Ini can be estimated as shown in the following formula (125): The following formula (125) was first proposed by the present inventors.

[0038]

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[0039] (b) Prediction of heat transfer coefficient Next, a method for predicting the heat transfer coefficient due to stirring of molten steel will be described. According to the above-mentioned Non-Patent Document 2, when CO gas is generated from an iron source immersed in molten steel, the mass transfer coefficient of the generated CO gas is the CO gas generation rate per unit area J, which is expressed by the following formula (131): CO It has been reported that it is proportional to the 0.3 power of

[0040]

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[0041] Here, for the heat transfer coefficient h and mass transfer coefficient K under forced convection, empirical formulas expressed by the following formulas (133) and (135) are known. When using these empirical formulas, if the stirring is sufficiently strong, the relationship expressed by the following formula (137) is established. Here, in the following formulas (133) to (137), λ is the thermal conductivity. Re is the Reynolds number, which is expressed by the following formula (139). Moreover, Pr is the Prandtl number, which is expressed by the following formula (141). Furthermore, Sc is the Schmidt number, which is expressed by the following formula (143). Moreover, in the following formulas (139) to (143), μ is the viscosity coefficient, D is the diffusion coefficient of the component, and the subscript l means the liquid phase.

[0042]

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[0043] Here, as in the above Non-Patent Document 2, the mass transfer coefficient K is the CO gas generation rate J per unit area. CO In the case where the heat transfer coefficient h is proportional to the 0.3 power of J CO The inventors came up with the idea that the heat transfer coefficient h is proportional to the 0.3 power of J CO Based on the idea that the heat transfer coefficient h is proportional to the 0.3 power of , the proportionality constant is set to A, and the heat transfer coefficient h is formulated as shown in the following equation (145), and the proportionality constant A is estimated based on the reported data disclosed in Non-Patent Document 2. As a result, it was clarified that the heat transfer coefficient h due to the generation of CO gas can be expressed as shown in the following equation (147).

[0044] From the above, it has become clear that the heat transfer coefficient due to stirring of molten steel caused by the generation of CO gas can be predicted by using the calculation formula for the CO gas generation rate obtained by the inventors (i.e., formula (125)) and formula (147).

[0045]

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[0046] (c) Prediction of solidified shell thickness Next, a method for predicting the thickness of the solidified shell formed around the reduced iron will be described. According to Non-Patent Document 3, the thickness Δr of the solidified shell formed around a solid charged into molten steel is expressed by the following equation (151): Here, the function φ(x) in the following equation (151) is a function of the variable x expressed by the following equation (153).

[0047] In the following formula (153), α1 is the temperature diffusion coefficient of the solid parent phase, ρ1 is the density of the solid parent phase, α2 is the temperature diffusion coefficient of the solidified shell, and ρ2 is the density of the solidified shell. In the above Non-Patent Document 1, a "solid" that is charged into molten steel is considered, but when considering applying this formula to the phenomenon of "charging reduced iron into molten steel", α1 corresponds to the temperature diffusion coefficient of the reduced iron, and ρ1 corresponds to the density of the reduced iron.

[0048] In addition, in the following formula (153), Bi is the Biot number expressed by the following formula (155), and θ b is the dimensionless temperature expressed by the following equation (157). λ2 is the thermal conductivity of the solidified shell, and T b is the molten steel temperature (temperature of the molten steel bulk), T0 is the initial temperature of the reduced iron to be charged, and ΔH is the latent heat. Note that the specific formula of the function φ(x) is unknown, and Non-Patent Document 1 describes that it is a function of the form as shown in FIG.

[0049]

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[0050] Here, when the knowledge disclosed in the above Non-Patent Document 1 is applied to the phenomenon of interest in this embodiment, the above formula (151) does not take into account the reaction heat of the above reaction formulas (1) and (2). Here, the reaction of reaction formula (3), which is the apparent reaction formula, is an endothermic reaction.

[0051] Therefore, in applying the knowledge disclosed in the above Non-Patent Document 1 to the phenomenon of interest in this embodiment, the present inventors decided to use the following equation (161), which is proposed by the present inventors, instead of the above equation (151).

[0052]

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[0053] Here, in the square brackets [] on the right side of the above formula (161), the second numerator in the fractional notation in the curly brackets {} is a term that takes into account the reaction heat from the above reaction formulas (1) and (2), which was introduced by the present inventors. Also, in the above formula (161), ΔH R is the reaction heat of the CO gas generation reaction in reduced iron, and C R is the molar concentration of the component that determines the reaction amount, and ε is the void fraction (porosity) of the reduced iron. In the above reaction formula (3), since the coefficients of C and FeO are both 1 in the reaction formula, C and FeO are consumed at the same rate in the proceeding reaction. Therefore, the molar concentration C of the component that determines the reaction amount R is the molar concentration of the lesser component between C and FeO.

[0054] Now, rearranging the above equation (161) with respect to the function φ(x), we obtain the following equation (163), where the term in square brackets [ ] on the right-hand side of equation (161) corresponds to the denominator on the right-hand side of equation (163).

[0055] Here, the inventors calculated the value in square brackets [ ] on the right-hand side of equation (161) (the value of the denominator on the right-hand side of equation (163)) taking into consideration the physical properties of various types of reduced iron, such as direct reduced iron (DRI) and hot briquetted iron (HBI), that may be present in an electric furnace, and the temperature range in actual operation.

[0056] For example, taking into consideration the physical properties of general reduced iron and the temperature range in actual operation, ρ1 = 3500 [kg / m 3 ], ρ2 = 7800 [kg / m 3 ], C p1 =350[J / K kg], ΔH=267520[J / kg], ΔH R = 170,000 [J / mol], the C content in the reduced iron = 4.0 [mass%], the FeO content in the reduced iron = 5.0 [mass%], and ε = 0.55, it was found that the value in square brackets [ ] on the right-hand side of equation (161) (the value of the denominator on the right-hand side of equation (163)) was at least approximately 0.16.

[0057] Therefore, the range of φ(x) of interest in this embodiment is φ(x)≦(Δr / 0.16·r0) from the following formula (163). In particular, in the range where the thickness of the solidified shell is sufficiently smaller than the size of the matrix (for example, the range where Δr is 1% or less of r0), φ(x)≦0.0625. When formulating the function (x) shown in FIG. 1 in such a range, it is only necessary to consider the behavior near the origin in FIG. 1, and it is understood that the function φ(x) may be linearly approximated as a linear function of the variable x. As a result of the study by the present inventors, it has become clear that the function φ(x) can be formulated as the following formula (165) in the range of interest in this embodiment.

[0058]

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[0059] Here, in the above formula (165), the variable x is given by the above formula (153), and the Biot number Bi in formula (153) can be made more detailed by substituting the above formula (125) and formula (147) into the above formula (155). Then, by using these formulas and the above formula (161), the "range in which Δr is 1% or less of r0" can be formulated as in the following formula (167). The obtained formula (167) can be rearranged to obtain the following formula (171). As is clear from the above explanation, this formula (171) is a relational expression that was first formulated by the present inventors.

[0060]

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[0061] As described above, if the reduced iron is melted so as to satisfy the relationship expressed by Equation (171), the thickness Δr of the solidified shell that is formed becomes sufficiently small, that is, 1% or less of the spherical equivalent radius r of the reduced iron, and the formation of the solidified shell can be suppressed.

[0062] In addition, we focus on the case where hot briquetted iron (HBI) is used as the reduced iron to be charged into the electric furnace. The inventors calculated the value of the right side of the above formula (171) taking into consideration the typical physical properties of HBI and the temperature range in actual operation.

[0063] For example, taking into account the typical physical properties of HBI and the temperature range in actual operation, ρ1 = 5500 [kg / m 3 ], ρ2 = 7800 [kg / m 3 ], C p1 = 500 [J / K kg], C p2 =627[J / K kg], λ1=10[J / s / m / K], λ2=30[J / s / m / K], ΔH=267520[J / kg], ΔH R= 170,000 [J / mol], the C content in the reduced iron = 1.5 [mass%], the FeO content in the reduced iron = 7.0 [mass%], ε = 0.30, and T0 = 25 [°C], the right-hand side of equation (171) is approximately 0.002.

[0064] From the above calculation results, taking into consideration the typical physical properties of HBI and the temperature range in actual operation, the right-hand side of the above equation (171) is at most 2.0 × 10 -3 The inventors' verification revealed that the degree of

[0065] Therefore, when HBI is used as the reduced iron to be charged into the electric furnace, the following formula (173) can be used instead of the above formula (171), in which the right-hand side of the above formula (171) is constantized based on the physical properties of HBI. This reduces the amount of calculation required to determine whether the conditions are satisfied, making it possible to carry out operations more easily.

[0066]

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[0067] Furthermore, we will focus on the case where direct reduced iron (DRI) is used as the reduced iron fed into the electric furnace. The inventors calculated the value of the right-hand side of the above formula (171) taking into consideration typical physical property values ​​of DRI and the temperature range in actual operation.

[0068] For example, taking into account the typical physical properties of DRI and the temperature range in actual operation, ρ1 = 3500 [kg / m 3 ], ρ2 = 7800 [kg / m 3 ], C p1 = 350 [J / K kg], C p2 =627[J / K kg], λ1=5[J / s / m / K], λ2=30[J / s / m / K], ΔH=267520[J / kg], ΔH R= 170,000 [J / mol], the C content in the reduced iron = 4.0 [mass%], the FeO content in the reduced iron = 20.0 [mass%], ε = 0.48, and T0 = 25 [°C], the right-hand side of equation (171) is approximately 0.0015.

[0069] From the above calculation results, taking into consideration the typical physical properties of DRI and the temperature range in actual operation, the right-hand side of the above equation (171) is at most 1.5 × 10 -3 The inventors' verification revealed that the degree of

[0070] Therefore, when DRI is used as the reduced iron to be charged into the electric furnace, the following formula (174) can be used instead of the above formula (171), in which the right-hand side of the above formula (171) is constant based on the physical property value of DRI. This reduces the amount of calculation required to determine whether the conditions are satisfied, making it possible to carry out operations more easily.

[0071]

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[0072] The findings regarding CO gas stirring obtained by the present inventors have been described in detail above.

[0073] (Method of dissolving reduced iron) As described above, the present inventors have conducted further studies based on the above formula (171), formula (173), and formula (174) proposed for the first time by the present inventors, and have come up with a method for melting reduced iron according to an embodiment of the present invention, as described below.

[0074] As described above, by performing the operation so as to satisfy the above formula (171) (so as to satisfy the above formula (171) or formula (173) when HBI is used, or so as to satisfy the above formula (171) or formula (174) when DRI is used), the thickness Δr of the formed solidified shell becomes sufficiently small, that is, 1% or less of the spherical radius r0 of the reduced iron, and the formation of the solidified shell can be suppressed.

[0075] According to the percolation theory, when focusing on a situation in which "among particles filled in a certain space, some particles bond with the surrounding particles," it is known that if the proportion of particles that bond is about 30% or more of the total, the aggregate formed by the bonding will become very large. In this embodiment, the above knowledge is applied to the dissolution treatment of reduced iron.

[0076] In light of the above-mentioned findings, in the method for melting reduced iron according to this embodiment, when reduced iron is charged into an electric furnace (for example, various types of arc furnaces or induction furnaces), operation is performed so that the total volume of reduced iron that satisfies the relationship expressed by the above formula (171) among the reduced iron charged into the furnace is 70% or more of the total volume of all reduced iron charged.

[0077] Similarly, when HBI is used as reduced iron, operation is carried out so that the total volume of HBI that satisfies the relationship expressed by the above equation (171) or (173) among the HBI charged into the furnace is 70% or more of the total volume of all HBI charged.

[0078] Similarly, when DRI is used as reduced iron, operations are carried out so that the total volume of DRI charged into the furnace that satisfies the relationship expressed by the above equation (171) or equation (174) is 70% or more of the total volume of all DRI charged.

[0079] As a result, in the method for melting reduced iron according to the present embodiment, it is possible to suppress the formation of a solidified shell, and in turn, to suppress the formation of large icebergs, and therefore, in the method for melting reduced iron according to the present embodiment, it is possible to further improve the productivity of molten steel production using an electric furnace, without suppressing the charging rate of reduced iron.

[0080] Needless to say, the higher the ratio of the total volume of reduced iron (HBI or DRI) satisfying the above formula (171), (173), or (174) to the total volume of all reduced iron (HBI or DRI) charged into the furnace, the better, and it is more preferable that all of the reduced iron charged into the furnace satisfies the above formula (171), (173), or (174).

[0081] Here, electric furnaces have been used in the past, but most of them use scrap iron as the raw material, and there are very few examples of melting reduced iron, mainly direct reduced iron (DRI). Here, typical operating conditions using direct reduced iron (DRI) and the properties of DRI are, for example, as follows: Molten steel temperature: around 1500~1600℃ DRI input temperature: around 30℃ DRI radius: 2~10[mm] DRI C content: 1~4% by mass FeO content of DRI: 5-20% by mass DRI density: 3500 [kg / m 3 ] Specific heat: 350 [J / kg / K] Thermal conductivity: 2~5[J / s / m / K]

[0082] Figure 2 shows the relationship expressed by the above formula (171) based on the above-mentioned typical operating conditions and DRI properties. In Figure 2, the horizontal axis represents the ball conversion coefficient r0 [unit: m] of the reduced iron to be charged, and the vertical axis represents the molten steel temperature Tb [unit: °C]. The region that satisfies the obtained formula (171) is the region above the curve shown in the figure. In some operations, the reduced iron is preheated to a maximum of about 600 °C before being charged. When the relationship expressed by the above formula (171) is shown in the same manner as in Figure 2, it is shown in Figure 3.

[0083] 2 and 3, it can be seen that within the range of molten steel temperatures conventionally set during operation (approximately 1300 to 1600°C), there are situations where the relational expression expressed by the above formula (171) is satisfied and situations where it is not satisfied. It is considered that these situations are the reasons why excessive formation of solidified shells and formation of icebergs occurs in conventional reduced iron melting treatments.

[0084] Considering the region where the relationship expressed by equation (171) holds, as is clear from the shapes of the curves shown in Figs. 2 and 3, the larger the spherical equivalent radius r0 of the DRI being fed, the smaller the allowable molten steel temperature T b It can be seen that the range of molten steel temperature T b In particular, when operating at the molten steel temperature T b When the line representing the DRI intersects with the line representing the DRI, it can be seen that the larger the sphere-equivalent radius r0 of the DRI, the easier it is to satisfy the relationship expressed by equation (171).

[0085] Here, in the past, when considering heat transfer from molten steel to a solid charged into the molten steel, it was thought that it was more advantageous to charge the solid with a smaller particle size because the surface area of ​​the solid that comes into contact with the molten steel would be larger. However, as is clear from Figures 2 and 3, the relationship expressed by equation (171), which was first defined by the present inventors, suggests that, from the viewpoint of suppressing the formation of a solidified shell, it is better to charge the reduced iron into the molten steel with a larger particle size, which is an operational guideline that goes against the conventional trend.

[0086] As for operations using hot briquetted iron (HBI), there have been few reports and typical operating conditions are unknown. However, typical properties of HBI are, for example, as follows: HBI ball conversion radius: 30~35mm C content of HBI: 0.5 to 1.5 mass% FeO content of HBI: 5.0-10.0 mass% Density of HBI: 5500 [kg / m3 ] Specific heat: 500 [J / kg / K] Thermal conductivity: 10[J / s / m / K]

[0087] Based on the above conditions, the state of the relational expression expressed by the above formula (171) is illustrated in Figure 4, similarly to Figures 2 and 3. In the case of the HBI illustrated in Figure 4, as in the case of DRI, the larger the spherical equivalent radius r0 of the HBI to be charged, the smaller the allowable molten steel temperature T b It can be seen that the range of molten steel temperature T b In particular, when operating at the molten steel temperature T b When the line representing the sphere-equivalent radius r0 of the HBI intersects with the line representing the sphere-equivalent radius r0, it becomes easier to satisfy the relationship expressed by equation (171).

[0088] In this manner, in the method for melting reduced iron according to this embodiment, when operating the method so that the ratio of the total volume of reduced iron satisfying the relationship expressed by the above formula (171) to the total volume of all reduced iron charged is at least 70%, it is more preferable to make the particle size of the reduced iron charged as large as possible.

[0089] In controlling the properties of reduced iron and the operating conditions so as to satisfy the relationship expressed by the above formula (171), it is preferable to first control the particle size of the reduced iron to be charged as large as possible. Then, as necessary, properties other than the particle size of the reduced iron to be charged (e.g., composition and various physical properties) may be controlled, or the molten steel temperature or the temperature of the reduced iron at the time of charging may be increased. By carrying out such control, the operating conditions of the melting process can be optimized more easily.

[0090] As described above, the method for melting reduced iron according to this embodiment aims to suppress the formation of a solidified shell by utilizing CO gas generated from reduced iron by introducing reduced iron into molten steel. In order to realize an operation in which the formation of a solidified shell is suppressed, the method for melting reduced iron according to this embodiment is set so that the total volume of reduced iron satisfying the relationship expressed by formula (171) is at least 70% or more of the total volume of all reduced iron introduced.

[0091] (Verification of the method for melting reduced iron) The method for melting reduced iron according to the present embodiment as described above was verified using an electric furnace and reduced iron used in actual operation.

[0092] First, a small-scale (furnace diameter and bath depth of approximately 300 to 400 mm) high-frequency melting furnace, which is equipment capable of melting an iron source added while heating molten steel, similar to the electric furnace used in the steelmaking process, was used to conduct a melting experiment in which HBI or crushed HBI was added to the high-frequency melting furnace. Since the melting behavior of HBI in a high-frequency melting furnace is considered to be similar to the melting behavior of HBI in an electric furnace used in the steelmaking process, the knowledge gained from this verification is considered to be applicable to electric furnaces as well.

[0093] The HBI used in this verification was measured in advance and found to have a C content of 1.50 mass%, an FeO content of 7.16 mass%, and a density of 5500 [kg / m 3]. The radius of the non-crushed HBI was 35 mm, and the radius of the crushed HBI was 10 to 15 mm. Molten steel at about 1560°C or 1650°C was placed in the high-frequency furnace. The curves representing the boundary of whether the relationship expressed by the above formula (171) is satisfied, calculated from the above conditions, and the curves representing the boundary of whether the relationship expressed by the above formula (173) is satisfied, are shown in FIG. 5. The region above the curves shown in FIG. 5 is the region that satisfies the relationships expressed by the respective formulas. In the case of FIG. 5, the curves corresponding to the above two formulas were located very close to each other, so that only one curve appears in FIG. 5.

[0094] When multiple uncrushed HBI pieces were added, the HBI did not coagulate (i.e., no icebergs were formed due to the solidified shells bonding together) and completely dissolved over time. On the other hand, when crushed HBI was added in an amount equal to the amount of uncrushed HBI added, the HBI coagulated significantly and dissolution hardly progressed at all. In Figure 5, cases where coagulation occurred are indicated with an ×, and cases where coagulation did not occur are indicated with an ◯.

[0095] As is clear from the results shown in FIG. 5, when HBI satisfying the relationships expressed by the above formulas (171) and (173) was charged, the charged HBI completely melted without solidifying at any molten steel temperature, whereas when HBI not satisfying the relationships expressed by the above formulas (171) and (173) was charged, the charged HBI solidified and hardly melted at any molten steel temperature.

[0096] Next, a test was conducted in which one HBI or pure iron was immersed in a high-frequency melting furnace holding molten steel at approximately 1560°C inside the furnace, and then pulled out halfway through. Three types of HBI with different C contents (0.20 mass%, 0.60 mass%, and 1.50 mass%) were used for this test. The C content of the HBI was adjusted by reducing HBI with 1.50 mass% C and 7.16 mass% FeO in advance under high-temperature inert gas. The FeO content was consumed simultaneously with C in the prior reduction reaction, and was 0.95 mass% (C: 0.20 mass%), 2.86 mass% (C: 0.60 mass%), and 7.16 mass% (C: 1.50 mass%), respectively, and the density of each was approximately 5500 [kg / m 3 The ball-equivalent radius of the HBI and pure iron used was approximately 35 mm.

[0097] The masses of the HBI and pure iron used were measured before they were immersed in the molten steel. In the verification, the HBI or pure iron was pulled out about 5 seconds after immersion, and the masses of the HBI or pure iron were measured.

[0098] As a result, a decrease in mass was confirmed before and after immersion for HBI with a C content of 1.50 mass%, while an increase in mass was confirmed before and after immersion for HBI with a C content of 0.20 mass% and 0.60 mass%, and for pure iron. The fact that a decrease in mass was confirmed before and after immersion means that the HBI had dissolved without the formation of a solidified shell, and the fact that an increase in mass was confirmed before and after immersion means that a solidified shell had formed and dissolution had not progressed.

[0099] Here, the horizontal axis represents the C content of the HBI charged, and the vertical axis represents the molten steel temperature Tb. The curves representing the boundary of whether the relationship expressed by the above formula (171) is satisfied and the curves representing the boundary of whether the relationship expressed by the above formula (173) is satisfied are shown in FIG. 6. Here, when calculating the relationship expressed by the above formula (171) and formula (173) in accordance with the above measurement conditions, the calculation was performed so that the FeO molar concentration also decreases with the decrease in the C molar concentration. Specifically, the calculation was performed with the reference C content set to 1.50 mass%, the FeO content set to 7.16 mass%, and the FeO molar concentration changing at the same rate as the C molar concentration (i.e., FeO mass%=7.16×C mass% / 1.50). Note that in FIG. 6 as well, the curves corresponding to the above two formulas are located very close to each other. The above verification results are also shown in FIG. 6. In FIG. 6, the results where the mass has decreased are shown as ◯, and the results where the mass has increased are shown as ×.

[0100] As is clear from the results shown in Figure 6, the HBI confirmed to have a mass decrease is located in the region satisfying the relationships expressed by the above formulas (171) and (173), while the HBI or pure iron confirmed to have a mass increase is located in the region not satisfying the relationships expressed by the above formulas (171) and (173). From these results, it can be seen that the relationships expressed by the above formulas (171) and (173) are appropriate.

[0101] In addition, a similar test to that shown in Figure 5 was performed using two types of HBI with different particle sizes, but varying the blending ratio of each. The results are shown in Table 1 below. Conditions under which no binding of the HBI was observed were given a rating of A, conditions under which the HBI was partially bound but did not form large lumps, and the final dissolution time was almost the same as in the case of rating A were given a rating of B, and conditions under which the HBI was partially bound to form large lumps and the final dissolution rate was slower than in the case of rating A were given a rating of C.

[0102] [Table 1]

[0103] As is clear from the results shown in Table 1 above, so long as the content of HBI satisfying the above formula (171) is at least 70%, the dissolution rate does not deteriorate.

[0104] As described above, the verification results confirm that it is possible to further improve the productivity of molten steel production using an electric furnace by performing operation based on the above formula (171) or (173), without suppressing the charging rate of reduced iron.

[0105] (Usage example) In an actual melting process using an electric furnace, the method for melting reduced iron according to the present embodiment as described above can be used, for example, in the following manner.

[0106] First, for the reduced iron used in the operation, the physical properties of the reduced iron that can be specified in advance, such as the melting point, reaction rate coefficient, various mole fractions, density, temperature diffusion coefficient, specific heat, latent heat, reaction heat, density and temperature diffusion coefficient of the solidified shell, etc., are analyzed by using a known analysis method for each physical property, and a database is created. In this case, the physical property values ​​are statistically processed using the respective measured values ​​obtained, and a representative value is determined in consideration of the average value, variation, etc. In this case, it is preferable to carry out the method for determining the representative value multiple times and confirm that there is reproducibility. For example, it is preferable to use a method for determining the representative value that determines the same value three times and that causes the variation to be within ±5% of the average value of the three representative values. In this case, it is preferable to adopt the average value of the three representative values ​​as the representative value.

[0107] As for the analytical method, for example, the molar fraction of C in the reduced iron is measured by an infrared absorption method, and the molar fraction of FeO is measured by a volumetric method. The temperature of the reduced iron during measurement should be the temperature immediately before it is charged into a furnace in actual operation. However, when the temperature at which the reduced iron is charged into a furnace in actual operation is 500°C or lower, almost no reaction occurs in the reduced iron and the molar fractions of C and FeO do not change, so the measurement may be performed at any temperature below 500°C.

[0108] Also, the equilibrium constant K e,1 , K e,2 may be determined by using a known empirical formula such as that disclosed in Non-Patent Document 1. The reaction rate constants k1 and k2 may be determined by fitting using a reduction experiment and an unreacted core model as disclosed in Non-Patent Document 4. In this case, it is preferable that the temperature during the experiment is 800°C or higher at which the reaction of reduced iron proceeds sufficiently, and that the experiment is performed at least three times at different temperatures, for example, at 800°C, 1000°C, and 1200°C, and fitting using the unreacted core model is performed.

[0109] The radius of the reduced iron in spherical form is calculated from the volume of the reduced iron, V, by the formula r0 = (3V / 4π). 1 / 3 Calculated from.

[0110] Also, the temperature of the molten steel in the furnace, T b is measured, for example, by immersing a thermocouple in the molten steel near the position where the reduced iron is charged.

[0111] Based on the obtained results, the reduced iron to be used is roughly divided into a number of groups, each of which is stored in a different storage container.

[0112] Furthermore, the above-mentioned obtained database and actual operating conditions, etc. are stored as data in various storage units of an information processing device such as a process computer realized by a CPU, ROM, RAM, etc., which manages the supply line of reduced iron to the electric furnace and the operation line composed of the electric furnace, etc.

[0113] Furthermore, a blending ratio calculation unit that calculates the blending ratio of reduced iron is implemented as one of the functions of the information processing device such as the process computer. The blending ratio calculation unit may have a function of setting the blending ratios of reduced iron roughly classified as above as parameters and determining a combination of parameters expressing the blending ratios so as to satisfy the relationship of the above formula (171), (173), or (174).

[0114] In addition, a blending control unit for controlling the blending of reduced iron is implemented as one of the functions of the information processing device such as the process computer. The blending control unit has a function of controlling the opening and closing of valves and the like provided on the outlet side of each storage container based on the calculated blending ratio. In this way, each device provided on the operation line works in conjunction with the information processing device, making it possible to create a mixed state of reduced iron that satisfies the relationship expressed by the above formula (171), formula (173), or formula (174).

[0115] In addition, in such an operation, it is also possible to provide, as necessary, a granulation step at any timing before the reduced iron is charged into the furnace, in which the reduced iron to be charged is granulated until the particle size satisfies the above formula (171), formula (173), or formula (174).

[0116] The above has briefly described an example of use of the method for melting reduced iron according to the present embodiment.

[0117] (Modification) In the above-described method for melting reduced iron according to the present embodiment, the case where reduced iron such as DRI or HBI is used as the iron source has been described as an example, but the method for melting reduced iron according to the present embodiment can also be applied to an operation in which scrap iron is also used as the iron source. In this case, the scrap iron is charged into the furnace from a charging port different from the charging port for the reduced iron, or is charged into the furnace at a timing different from that of the charging of the reduced iron.

[0118] In this case, unlike reduced iron, scrap iron does not float near the surface of the molten steel after being charged into the electric furnace, but is immersed in the molten steel, so it does not affect the bonding with the reduced iron floating near the surface or the formation of icebergs. Therefore, the only effect of scrap iron is to consider the temperature drop corresponding to the energy consumed until the scrap iron melts (sensible heat + latent heat). Specifically, when the input mass of scrap iron is Ms [kg], the sensible heat to the melting point is QS [J / kg], the latent heat of melting is QL [J / kg], the mass of molten steel is Ml [kg], and the specific heat is Cpl [J / kg / K], the temperature drop ΔT [K] of the molten steel due to the input of scrap iron can be approximately calculated as ΔT = Ms (QL + QS) / Ml Cpl.

[0119] Therefore, when using scrap iron in the operation, the temperature T b Instead, use (Tb-ΔT) to find θ b =(T b -T m ) / (T m −T0) and apply the above formula (171), formula (173), or formula (174).

[0120] The modified example of the method for melting reduced iron according to the present embodiment has been briefly described above.

[0121] Although the preferred embodiment of the present invention has been described in detail above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person having ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

[0122] The embodiments disclosed herein are illustrative and not restrictive in all respects. The above embodiments may be omitted, substituted, or modified in various forms without departing from the scope of the appended claims, the technical scope of the present invention as described below, and the spirit thereof. For example, the components of the above embodiments can be arbitrarily combined within a range that does not impair the effects of the components. Furthermore, the arbitrary combination naturally provides the actions and effects of each of the components in the combination, and also provides other actions and effects that are obvious to a person skilled in the art from the description of this specification.

[0123] In addition, the effects described in this specification are merely explanatory or exemplary, and are not limiting. In other words, the technology according to the present invention may achieve other effects that are apparent to a person skilled in the art from the description of this specification, in addition to or in place of the above effects.

Claims

1. A method for melting reduced iron, comprising the steps of: a total volume of reduced iron that satisfies the relationship represented by the following formula (1), among the reduced iron charged into the furnace, is 70% or more of a total volume of all reduced iron charged. [0010] Here, in the above formula (1), θ ｂ =(T ｂ -T ｍ ) / (T ｍ -T ０ ): Dimensionless temperature T ｍ : Melting point of reduced iron [unit: °C] T ０ :Feeding temperature of reduced iron [unit: °C] T ｂ :Temperature of molten steel bulk in the furnace [unit: °C] k ＣＯ，ｍ : Melting point T of reduced iron ｍ Reaction rate constant in a Ｃ : mole fraction of C in reduced iron a ＦｅＯ : mole fraction of FeO in reduced iron r ０ : spherical equivalent radius of reduced iron ρ １ : Density of reduced iron ρ ２ : Density of the solidified shell formed around the reduced iron after charging α １ : Thermal diffusion coefficient of reduced iron α ２ : Thermal diffusion coefficient of the solidified shell formed around the reduced iron after charging λ ２ : Thermal conductivity of the solidified shell formed around the reduced iron after charging C ｐ１ : specific heat of reduced iron ΔH: Latent heat of dissolution of reduced iron ΔH Ｒ : Reaction heat of CO gas generation reaction in reduced iron C Ｒ : Molar concentration of the lesser component of C and FeO ε: Void fraction of reduced iron It is.

2. The method for melting reduced iron according to claim 1 , wherein a total amount of the reduced iron charged into the furnace satisfies the relationship expressed by the formula (1).

3. Hot briquetted iron (HBI) is used as the reduced iron, 2. The method for melting reduced iron according to claim 1, wherein the total volume of the hot briquette iron charged into the furnace that satisfies the relationship expressed by the following formula (2) is 70% or more of the total volume of all the hot briquette iron charged into the furnace. [0025] Here, the above formula (2) is obtained by converting the right side of the formula (1) into a constant based on the physical properties of the hot briquette iron.

4. The method for melting reduced iron according to claim 3 , wherein a total amount of the hot briquette iron charged into the furnace satisfies the relationship expressed by the formula (2).

5. As the reduced iron, direct reduced iron (DRI) is used, 2. The method for melting reduced iron according to claim 1, wherein operation is performed such that, of the direct reduced iron charged into the furnace, a total volume of direct reduced iron satisfying the relationship expressed by the following formula (3) is 70% or more of a total volume of all direct reduced iron charged into the furnace. [0030] Here, the above formula (3) is obtained by converting the right side of the formula (1) into constants based on the physical properties of the direct reduced iron.

6. The method for melting reduced iron according to claim 5 , wherein a total amount of the direct reduced iron charged into the furnace satisfies the relationship expressed by the formula (3).

7. Feed scrap iron into the furnace together with the reduced iron; When the temperature drop corresponding to the energy consumed until the iron scrap is melted is ΔT, the temperature T ｂ Instead of (T ｂ -ΔT) to obtain θ ｂ = (T ｂ -T ｍ ) / (T ｍ -T ０ The method for melting reduced iron according to any one of claims 1 to 6, further comprising calculating a melting point of reduced iron (Mn) from the melting point of reduced iron (Mn) and ...).

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