Method for charging reduced iron into an electric furnace

Abstract

To suppress the decrease in yield while keeping the increase in power input in check. [Solution] Molten steel is produced using an arc-type electric furnace with an iron source containing reduced iron. During the melting phase, after the charging phase in which the iron source is charged into the furnace body 11 when the electrode 13 is not energized, reduced iron R is continuously charged into the borehole 40 while energizing the electrode 13. During the melting phase, the ratio of the theoretical melting energy of reduced iron to the electrical energy supplied to the electric furnace in the initial stage of continuous charging (energy ratio) is set to 0.70 or less. In the later stage of continuous charging, the energy ratio is set to 0.90 or more. In the middle stage of continuous charging, the energy ratio is set to be greater than or equal to the energy ratio in the initial stage of continuous charging and less than or equal to the energy ratio in the later stage of continuous charging.

Description

[[TECHNICAL FIELD]]

[0001] The present invention relates to a method for charging reduced iron into an arc electric furnace when producing molten steel using an iron source containing reduced iron in an arc electric furnace. [[BACKGROUND ART]]

[0002] Conventionally, there has been known a method for producing molten steel by charging an iron source such as scrap into an arc electric furnace. An electric furnace is provided with electrodes at least above the furnace. An arc is generated between the lower end of the electrode and the iron source inside the furnace to melt the iron source in the furnace, thereby producing molten steel.

[0003] Inside the furnace, there exist a position close to the electrode where heat is easily transferred, called a hot spot, and a position far from the electrode where heat is not easily transferred, called a cold spot. Since the iron source located in the cold spot remains undissolved, the iron source in the cold spot is melted by measures such as prolonging the energization time of the electrode. However, this brings adverse effects on operation, such as an increase in heat loss accompanying the extension of energization time and an increase in the amount of power input to the electric furnace.

[0004] To melt the metal located in the cold spot, for example, there is known a method of performing cutting by supplying LNG (liquefied natural gas) and oxygen from side burners installed facing the interior of the furnace. In addition, Patent Document 1 describes a method of bringing the cold spot closer to the electrode by rotating the furnace during melting of the iron source.

[0005] Here, the cold spot of an electric furnace is generally described as a position far from the electrode in the horizontal direction when viewing the interior of the furnace from above, but this is not the only case. Due to arc irradiation near the lower end of the electrode, the iron source in the furnace starts to melt from the vicinity of the lower end of the electrode, and the bulk of the iron source near the lower end of the electrode decreases. Since the electrode is lowered accordingly, the upper part of the furnace far from the arc irradiation area near the lower end of the electrode can also become a cold spot.

[0006] The method described in Patent Document 1 is a method for melting iron sources in cold spots far from the electrodes in the horizontal direction when viewed from above inside the furnace, rather than in cold spots at the top of the furnace. In order to melt iron sources located in cold spots at the top of the furnace, it is necessary to extend the energizing time of the electrodes, which increases the amount of power input.

[0007] Furthermore, while Patent Document 1 uses scrap as an iron source (see

[0002] of Patent Document 1), in recent years, the use of reduced iron as an iron source for melting in an electric furnace has been considered. However, the inventors of this invention have found that reduced iron behaves differently from scrap in the cold spots inside the furnace. Also, reduced iron has poorer thermal conductivity and heat transfer efficiency compared to scrap. Therefore, it is considered that using reduced iron as an iron source for melting in an electric furnace requires a unique operation different from that when using scrap as an iron source.

[0008] The applicant has studied a method for producing molten steel using an iron source containing reduced iron in an arc-type electric furnace and has filed Japanese Patent Applications No. 2023-221731 and No. 2023-221790. In the method described in these applications, when producing molten steel using an iron source containing reduced iron in an arc-type electric furnace, reduced iron is charged into boreholes formed in the iron source within the furnace body while current is being applied to the electrodes. This method can compensate for the poor heat conductivity and heat transfer efficiency characteristic of reduced iron, as described above, and can suppress an increase in the amount of power input. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2016-89227 [Overview of the project] [Problems that the invention aims to solve]

[0010] When producing molten steel using an iron source containing reduced iron, it is desirable to further reduce the increase in power input. On the other hand, if the increase in power input is reduced, there is a risk that reduced iron, which has poor heat conductivity and low heat transfer efficiency, will remain undissolved. This may lead to a decrease in yield. It is desirable to reduce the increase in power input while also suppressing the decrease in yield.

[0011] The present invention aims to provide a method for producing molten steel using an iron source containing reduced iron in an arc-type electric furnace, which can suppress an increase in power input while suppressing a decrease in yield. [Means for solving the problem]

[0012] Reduced iron contains iron oxide and gangue components, and in the cold spots within the furnace, the molten slag gradually melts and adheres to each other or to refractories, growing into coarse undissolved material. Since this grown undissolved material is even more difficult to melt, when using reduced iron as an iron source, it is extremely important to prevent the reduced iron from being located in the cold spots.

[0013] Furthermore, reduced iron contains more gangue components than scrap. Also, due to the manufacturing process of reduced iron, voids inevitably exist within the reduced iron. Consequently, the apparent density of reduced iron is (5~5.5 t / m³). 3 ) is the density of iron (7.9 t / m³). 3 ) and the density of molten steel (7.0 t / m³) 3 It is smaller than ). Due to the gangue components and internal voids mentioned above, reduced iron has poorer thermal conductivity compared to scrap.

[0014] The poor thermal conductivity of reduced iron means that the heat transferred to the reduced iron does not penetrate to its interior, and only the surface to which the heat is supplied (in the case of a furnace, the part to which heat is transferred by the arc) becomes hot. The greater the temperature difference between the heat source (arc) and the reduced iron (the surface to which heat is supplied), the easier it is for the reduced iron to heat up. However, for the reasons mentioned above, the reduced iron (the surface to which heat is supplied) is hot, making it difficult for the reduced iron to heat up. In other words, reduced iron has poor heat transfer efficiency due to its low thermal conductivity.

[0015] From the above, reduced iron has poorer thermal conductivity and lower heat transfer efficiency compared to scrap. Therefore, reduced iron is more difficult to melt than scrap. Thus, when using reduced iron as an iron source in an electric furnace, it is considered that a unique operational method different from that used when using scrap or other materials as an iron source is required.

[0016] As described above, the applicant has filed patent applications (Japanese Patent Applications No. 2023-221731 and 2023-221790) for a method of charging reduced iron into a borehole formed in the iron source within the furnace body while energizing the electrodes when producing molten steel using an iron source containing reduced iron in an arc-type electric furnace. The inventors of the present invention have further researched this method and have found the following method that can suppress the decrease in yield while suppressing the increase in the amount of power input.

[0017] The method for charging reduced iron into an electric furnace disclosed herein is a method for charging reduced iron into an arc-type electric furnace when producing molten steel using an iron source containing reduced iron in the arc-type electric furnace, the arc-type electric furnace having a furnace body and electrodes installed at least above the furnace body, and after a charging period in which the iron source is charged into the furnace body when the electrodes are not energized, during a melting period in which the iron source in the furnace body is melted by energizing the electrodes, reduced iron is continuously charged into a borehole formed in the iron source in the furnace body and surrounded by a molten iron source and an unmelted iron source while energizing the electrodes, and in the initial stage of continuous charging in which an amount of reduced iron up to 25% by mass of the total amount of reduced iron to be continuously charged is continuously charged into the borehole, the amount of reduced iron relative to the electrical energy supplied to the arc-type electric furnace The ratio of theoretical dissolution energy is set to 0.70 or less. In the later stages of continuous charging, when 50% or more by mass of reduced iron is continuously charged into the borehole relative to the total amount of reduced iron to be continuously charged, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace is set to 0.90 or more. In the middle stages of continuous charging, when more than 25% but less than 50% by mass of reduced iron is continuously charged into the borehole relative to the total amount of reduced iron to be continuously charged, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace is set to be equal to or greater than the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace in the initial stages of continuous charging, and equal to or less than the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace in the later stages of continuous charging. In the following, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the electric furnace may be referred to as the "energy ratio."

[0018] According to the method described above, by charging reduced iron into the borehole during the "melting phase," reduced iron is less likely to be present in the hot spots at the top of the furnace. Furthermore, both the poor heat conductivity and poor heat transfer efficiency of reduced iron are compensated for. Further, in the "melting period" where reduced iron is continuously charged, it is considered that the charged reduced iron is likely to remain undissolved in the "initial stage of continuous charging" when the borehole is small. By setting the low "energy ratio" as described above, reduced iron can be reliably melted in the "initial stage of continuous charging". Furthermore, deepening of the borehole progresses. Thereafter, in the "late stage of continuous charging" when the borehole has become deep, by setting the aforementioned "energy ratio" which is higher than the "energy ratio" in the "initial stage of continuous charging", heat loss can be reduced, and the thermal efficiency of reduced iron and iron sources can be improved. This promotes melting of the charged reduced iron and the undissolved iron source in the furnace body. By these effects, a decrease in yield can be suppressed while suppressing an increase in power input.

[0019] Further, in the above method, the ratio of the theoretical melting energy of reduced iron to the electrical energy supplied to the arc electric furnace (energy ratio) may be controlled by changing the charging speed of reduced iron.

[0020] The above effects can be obtained by the simple method of changing the charging speed of reduced iron. Further, although there are methods such as reducing power input as a method of controlling the "energy ratio", compared to such methods, the method of changing the charging speed of reduced iron can suppress prolongation of operating time. Effects of the Invention

[0021] When molten steel is produced using an iron source containing reduced iron in an arc electric furnace, a decrease in yield can be suppressed while suppressing an increase in power input. Brief Description of Drawings

[0022] [Figure 1] It is a sectional view of an example of an arc electric furnace. [Figure 2] It is a figure showing sequentially a method using method (1) according to the present embodiment. [Figure 3] It is a figure showing sequentially a method using method (2) according to the present embodiment. [Figure 4]This diagram shows, in sequence, an example of the final dissolution stage. [Figure 5] This figure shows the experimental results. [Modes for carrying out the invention]

[0023] Preferred embodiments of the present invention will be described below. The embodiments described below are merely examples of the present invention and do not limit it.

[0024] In this embodiment, an arc-type electric furnace is used. Figure 1 schematically shows an example of an arc-type electric furnace. The arc-type electric furnace 1 has a furnace body 11, a furnace lid 12, and electrodes 13.

[0025] The furnace body 11 is capable of housing the iron source 30. The upper end of the furnace body 11 is open. Figure 1 shows the state in which the opening at the upper end of the furnace body 11 is closed by the furnace cover 12.

[0026] The furnace lid 12 is equipped with a chute 21 and an exhaust duct 22. The chute 21 is used, for example, when introducing flux for component adjustment. With the furnace body 11 closed by the furnace lid 12, flux can be introduced into the furnace body 11 from the chute 21. Gas and heat from inside the furnace body 11 can be discharged from the exhaust duct 22.

[0027] The furnace lid 12 is movable vertically and horizontally by a moving mechanism (not shown). The furnace lid 12 may also be rotatable.

[0028] Figure 1 shows the case where the electrode 13 is installed above the furnace body 11. The electrode 13 is inserted through the furnace lid 12. The lower end of the electrode 13 is located inside the furnace body 11. The electrode 13 can be raised and lowered by a lifting device (not shown). In addition, other electrodes besides electrode 13 may be installed in the arc-type electric furnace 1.

[0029] By lowering the electrode 13, the lower end of the electrode 13 is brought closer to the iron source 30. In the arc-type electric furnace 1 shown in Figure 1, when current is applied to the electrode 13, an arc is generated between the lower end of the electrode 13 and the iron source 30, and the heat of the arc melts the iron source 30. While the electrode 13 is energized, the opening at the upper end of the furnace body 11 is closed by the furnace lid 12.

[0030] In this embodiment, an iron source containing reduced iron is used as the iron source 30 to be melted in the arc-type electric furnace 1. The iron source 30 is solid. The iron source 30 is charged into the furnace body 11.

[0031] Herein, "reduced iron" as used herein refers to iron obtained using a direct reduction method (sometimes called "direct ironmaking method" or "direct reduction ironmaking method"). The iron used as the raw material for "reduced iron" may be, for example, iron ore, or iron-based dust generated in blast furnaces, converters, and electric furnaces. "Reduced iron" is more reduced than the raw material iron (e.g., iron ore, iron-based dust). Examples of "reduced iron" include DRI (Direct Reduced Iron) and HBI (Hot Briquetted Iron). HBI is obtained by compression molding DRI obtained by the direct reduction method.

[0032] Table 1 shows the composition of HBI as an example of the composition of reduced iron (the sources for Table 1 are "Hidetoshi Tanaka, Kobe Steel R&D Vol.64 (2014), No.1, pp. 2-7." and "Hidetoshi Tanaka et al., Iron and Steel 92 (2006), pp. 1022-1028."). Note that Table 1 is just one example, and the composition of reduced iron is not limited to the composition shown in Table 1. [Table 1]

[0033] The iron source 30 melted in the arc-type electric furnace 1 may consist solely of reduced iron, or it may contain reduced iron and other iron sources. The iron sources other than reduced iron charged into the electric furnace generally have a higher thermal conductivity than reduced iron and contain iron components. Examples of materials with higher thermal conductivity than reduced iron and containing iron components include scrap, pig iron, ingots, and iron carbide. As iron sources other than reduced iron, one or more of the following may be used: scrap, pig iron, ingots, and iron carbide. The combination of multiple types of iron sources other than reduced iron is not particularly limited. Furthermore, the iron sources other than reduced iron that can be charged into the electric furnace are not limited to the examples above. As an iron source other than reduced iron, for example, an iron source reduced by a method other than direct reduction may be used. The iron source 30, which includes reduced iron and an iron source other than reduced iron, may be, for example, a combination of reduced iron and scrap, or a combination of reduced iron, scrap, and pig iron.

[0034] Next, the method according to this embodiment will be described.

[0035] [Method (1) according to this embodiment] Figure 2 schematically shows a method for producing molten steel using method (1) according to this embodiment.

[0036] First, with the loading port 11A, which is the opening at the top of the furnace body 11, open, a portion of the iron source 30 is loaded into the furnace body 11 through the loading port 11A. The iron source 30 is loaded from above the furnace body 11 by a clamshell X or the like.

[0037] In this specification, the period during which the iron source 30 is charged into the furnace body 11 through the charging port 11A with the charging port 11A of the furnace body 11 open is referred to as the "charging period". During the "charging period", the opening at the upper end of the furnace body 11 (charging port 11A) is not closed by the furnace cover 12 shown in Figure 1. During the "charging period", the lower end of the electrode 13 shown in Figure 1 is not positioned inside the furnace body 11. During the "charging period", the electrode 13 is not energized. Even if other electrodes (not shown) besides the electrode 13 are installed in the arc-type electric furnace 1 (for example, inside the furnace), not all electrodes are energized during the "charging period".

[0038] The iron source 30 charged into the furnace during the "charging period" is the iron source 30 that is melted in the arc-type electric furnace 1, excluding some or all of the reduced iron (R). If the iron source 30 that is melted in the arc-type electric furnace 1 is only reduced iron, some of the reduced iron is charged into the furnace during the "charging period". If the iron source 30 that is melted in the arc-type electric furnace 1 includes reduced iron and iron sources other than reduced iron, some of the reduced iron and iron sources other than reduced iron, or only iron sources other than reduced iron, are charged into the furnace during the "charging period". If the iron source 30 that is melted in the arc-type electric furnace 1 includes reduced iron and iron sources other than reduced iron, reduced iron may be charged during the "charging period", or it may not be charged.

[0039] After the "charging phase," the iron source 30 inside the furnace body 11 is melted. The charging port 11A of the furnace body 11 is closed with the furnace lid 12, and as shown in Figure 1, the lower end of the electrode 13 is brought close to the iron source 30 inside the furnace, and current is applied to the electrode 13. The iron source 30 melts from near the lower end of the electrode 13, and the volume of the iron source 30 near the lower end of the electrode 13 decreases. As shown in the "melting phase" of Figure 2, as the volume of the iron source 30 near the lower end of the electrode 13 decreases, the electrode 13 is lowered.

[0040] In this specification, the period in which the iron source 30 inside the furnace body 11 is melted by applying current to the electrode 13 is referred to as the "melting period". During the "melting period", the opening at the upper end of the furnace body 11 (inlet 11A) is closed by the furnace cover 12. During the "melting period", the lower end of the electrode 13 is positioned inside the furnace body 11. During the "melting period", current is applied to the electrode 13. Note that during the "melting period", the current may be stopped for a short time, such as when attempting to control the arc, but such unintentional short-term interruptions in current are included in the "melting period". However, when current is not intentionally applied to the electrode 13, it is not the "melting period". During the "melting period", molten material (molten iron source) (M) and unmelted solid iron source (unmelted iron source) (S) exist inside the furnace body 11.

[0041] During the "melting phase," as the iron source 30 inside the furnace body 11 melts from near the lower end of the electrode 13, the volume of the iron source 30 near the lower end of the electrode 13 decreases. This forms a borehole 40. As the electrode 13 is lowered, the borehole 40 is formed around the electrode 13.

[0042] In this specification, "boring hole 40" refers to a hole formed in the iron source 30 within the furnace body 11, and is surrounded by molten material (molten iron source) M and unmelted solid iron source S. The bottom of the boring hole 40 is mainly surrounded by molten material (molten iron source) M. The sides of the boring hole 40 are mainly surrounded by unmelted solid iron source S. A portion of the iron source at the bottom of the boring hole 40 may be unmelted. Furthermore, a portion of the iron source at the sides of the boring hole 40 may be molten.

[0043] In this embodiment, as shown in the "dissolution phase" of Figure 2, reduced iron R is continuously charged into the borehole 40 during the "dissolution phase". The reduced iron R is the iron source 30 that was not charged into the furnace during the "charging phase". During the "dissolution phase", with the opening at the upper end of the furnace body 11 closed by the furnace lid 12, the reduced iron R is continuously charged into the borehole 40 while energizing the electrode 13. The method of charging the reduced iron R during the "dissolution phase" will be described later.

[0044] Furthermore, the method of charging reduced iron R into the borehole 40 during the "melting phase" is not particularly limited. For example, as shown in Figure 2, reduced iron R may be charged from the chute 21. Also, there is no particular limitation on whether the reduced iron R charged during the "melting phase" is part of or all of the reduced iron melted in the arc-type electric furnace 1. This can be determined, for example, based on the cold spot appropriate for the furnace and the height of the iron source 30 charged during the "charging phase".

[0045] The iron source 30 inside the furnace body 11 melts completely, becoming molten steel (or molten iron). The current is then continued to flow to the electrodes 13, further increasing the heat of the molten steel.

[0046] In this specification, the period when the iron source 30 in the furnace body 11 is completely melted and current is applied to the electrodes 13 is referred to as the "heating-up period." The "heating-up period" is a state in which the molten steel surface in the furnace is almost flat (hereinafter referred to as the "flat bath state").

[0047] During the "heating phase," slag is generated by adding flux such as lime, and oxidative refining is carried out by supplying oxygen gas as needed. After that, the molten steel in the furnace body 11 is tapped into a ladle (not shown). Reduced iron R is not charged after the "heating phase."

[0048] In the above, the "charging period" is assumed to be one. However, if the iron source 30 cannot be charged into the furnace in one "charging period" due to limitations in the internal space of the furnace body 11, the iron source 30 is charged in multiple "charging periods". The method of charging the iron source 30 in multiple "charging periods" (method (2) according to this embodiment) is described below. Note that the iron source 30 charged in multiple "charging periods" is the iron source excluding the reduced iron charged in the "dissolution period". The same applies below.

[0049] [Method (2) according to this embodiment] Figure 3 schematically shows a method for producing molten steel using method (2) according to this embodiment.

[0050] In the first "charging phase," a portion of the iron source 30 is charged into the furnace body 11. Then, in the "melting phase," a portion of the iron source in the furnace is melted, and in the next "charging phase," the iron source 30 is charged into the furnace. As shown in "F" of Figure 3, the "charging phase" → "melting phase" → "charging phase" → ... cycle of "charging phase" and "melting phase" is repeated. The iron source 30 charged into the furnace during the "charging phase" is, as in (1) above, the iron source 30 that is melted in the arc-type electric furnace 1, excluding some or all of the reduced iron (R) (the iron source excluding the reduced iron charged during the "melting phase").

[0051] After all "charging phases" are completed, the "melting phase" occurs in which all of the iron source 30 in the furnace body 11 is melted (see "melting phase" below "F" in Figure 3). Once all of the iron source 30 in the furnace has melted, the "heating phase" begins. From the "heating phase" onward, the process is the same as in the conventional method. No reduced iron R is charged from the "heating phase" onward.

[0052] If there are multiple "charging periods," there are also multiple "dissolution periods." These multiple "dissolution periods" are the "dissolution periods" between "charging periods" (the "dissolution period" labeled "F" in Figure 3) and the "dissolution period" after all "charging periods" have finished (the "dissolution period" below "F" in Figure 3). If there are multiple "charging periods," reduced iron R is charged into the borehole 40 during one or more of these "dissolution periods."

[0053] In this embodiment, reduced iron R is continuously charged into the borehole 40 during at least one of the multiple "dissolution periods," specifically during the "dissolution period" after all "charging periods" have finished. As long as this condition is met, reduced iron R may also be charged during the "dissolution period" between "charging periods" (the "dissolution period" labeled "F" in Figure 3), and it is not necessary to charge reduced iron R during the "dissolution period" between "charging periods." Furthermore, when charging reduced iron R during the "dissolution period" between "charging periods," it may be charged during any of these "dissolution periods." Additionally, if there are multiple "dissolution periods" between "charging periods," reduced iron R may be charged into the borehole 40 during one or more of these "dissolution periods."

[0054] For example, as shown in Figure 3, reduced iron R may be charged into the borehole 40 during the "dissolution period" between the "charging periods," and then continuously charged into the borehole 40 during the "dissolution period" after all the "charging periods" have finished. Alternatively, reduced iron R may not be charged into the borehole 40 during the "dissolution period" between the "charging periods," and may only be continuously charged into the borehole 40 during the "dissolution period" after all the "charging periods" have finished. A method for continuously charging reduced iron R during the "dissolution period" after all the "charging periods" have finished will be described later.

[0055] Here, the "dissolution period" between the "charging periods" is a "dissolution period" in which the iron source 30 is dissolved in order to secure space for charging the iron source 30 inside the furnace body 11. Therefore, the "dissolution period" between the "charging periods" is short, and the amount of reduced iron R charged during this "dissolution period" is small. On the other hand, the "dissolution period" after all the "charging periods" are finished is a "dissolution period" aimed at dissolving all of the iron source 30 in the furnace body 11. The "dissolution period" after all the "charging periods" are finished is long, and the amount of reduced iron R charged into the furnace during this "dissolution period" is greater than the amount of reduced iron R charged during the "dissolution period" between the "charging periods".

[0056] As described above, in the method using (1) or (2) of this embodiment, not all of the iron source 30 is charged into the furnace during the "charging period". During the "charging period", some or all of the iron source 30 to be melted in the arc-type electric furnace 1, excluding the reduced iron (R), is charged into the furnace. Therefore, after the iron source 30 is charged during the "charging period", it is difficult for the iron source 30 in the furnace to reach the upper part of the furnace where the cold spot occurs.

[0057] Furthermore, reduced iron R that is not charged into the furnace during the "charging phase" is charged into the borehole 40 during the "melting phase" while current is applied to the electrode 13. The reduced iron R melts in the borehole 40 due to arc heat. As the reduced iron R melts in the borehole 40, the unmelted solid iron source S in the furnace melts, and the volume of the iron source 30 decreases.

[0058] Based on the above, reduced iron is less likely to be present in the upper part of the furnace, which is a cold spot. Furthermore, the aforementioned coarse undissolved material caused by reduced iron is less likely to form. Therefore, the energizing time does not need to be extended. This reduces the amount of power input.

[0059] Furthermore, reduced iron has poorer thermal conductivity and lower heat transfer efficiency compared to scrap. Therefore, reduced iron is more difficult to dissolve than scrap.

[0060] At the bottom of the borehole 40, there is molten material (molten iron source) M (see "Melting Period" in Figure 2 and "Melting Period" in Figure 3), there are no gaps between the solid iron sources that would reduce heat conduction, and there is some convection in the molten material (molten iron source) M (natural convection caused by temperature differences within the molten material). When reduced iron R is introduced here, the poor heat conductivity of reduced iron R is compensated for.

[0061] Furthermore, since reduced iron R contains carbon and iron oxides such as FeO, when reduced iron R is charged into the borehole 40 and heated to a high temperature by arc heat, the carbon in the reduced iron R reacts with iron oxides such as FeO in the borehole 40, generating CO gas. An example of this reaction equation is shown below. Reaction equation: C + FeO = Fe + CO(g) The secondary combustion heat of CO(g) generated in the borehole 40 causes the iron source 30 surrounding the borehole 40 to heat up, promoting the melting of the surrounding solid iron source S. Furthermore, because the arc is surrounded by the iron source 30 in the borehole 40, the arc heat does not easily escape to the furnace wall, furnace lid, or exhaust gas port, resulting in less heat loss and high thermal efficiency.

[0062] As described above, by charging reduced iron R into the borehole 40 during the "melting phase," both the poor thermal conductivity of reduced iron R and the poor heat transfer efficiency of reduced iron are compensated for, and the melting of the reduced iron R charged into the borehole 40 proceeds.

[0063] Furthermore, since molten material M is present at the bottom of the borehole 40 and the thermal conductivity of the molten material M is high, when reduced iron R is charged into the borehole 40, a cycle efficiently proceeds in which heat is transferred to the reduced iron R → a portion of the reduced iron R melts → the transferred heat is conducted to the surrounding reduced iron R and solid iron source S → arc heat is transferred to the unmelted reduced iron → further melting proceeds.

[0064] Furthermore, the heat from the high-temperature CO gas generated in the borehole 40 is transferred to the solid iron source S surrounding the borehole 40, and the heat of reaction from the reaction (secondary combustion) in which CO gas is produced by the reaction of CO gas with oxygen in the furnace to generate CO2 gas is also utilized to raise the temperature of the solid iron source S. This is another way to improve thermal efficiency.

[0065] As described above, by charging reduced iron R into the borehole 40 during the "melting phase," thermal efficiency can be increased. This promotes the melting of the reduced iron R charged into the borehole 40 and the solid iron source S surrounding the borehole 40. This leads to a reduction in the amount of electricity required.

[0066] Furthermore, research by the inventors of this invention has shown that the amount of electricity input can be further reduced by further improving the "dissolution phase." It has also been found that even with reduced electricity input, the reduced iron R introduced during the "dissolution phase" is less likely to remain undissolved, thus suppressing a decrease in yield. The "dissolution phase" of this embodiment will be described in detail below.

[0067] In the following, the "dissolution period" refers to the "dissolution period" after the "charging period" (the "dissolution period" in Figure 2) when using the "Method (1) according to this embodiment" shown in Figure 2, and the "dissolution period" after all "charging periods" have been completed (the "dissolution period" below "F" in Figure 3) when using the "Method (2) according to this embodiment" shown in Figure 3. Hereafter, these "dissolution periods" may be referred to as the "final dissolution period".

[0068] The "final dissolution period" is divided into three sections: "early continuous charging period," "mid-continuous charging period," and "late continuous charging period." The "initial continuous charging phase" is the period during which reduced iron R is charged into the borehole 40 in an amount up to 25% by mass relative to the total amount of reduced iron R to be continuously charged during the "final dissolution phase." The "intermediate continuous charging period" is the period during which an amount of reduced iron R, exceeding 25% but less than 50% by mass relative to the total amount of reduced iron R to be continuously charged during the "final dissolution period," is charged into the borehole 40. The "late stage of continuous charging" is the period during which at least 50% by mass of reduced iron R is charged into the borehole 40 relative to the total amount of reduced iron R to be continuously charged during the "final dissolution stage."

[0069] (A) In the initial stage of continuous charging, the ratio of the theoretical dissolution energy of reduced iron (Y) to the electrical energy supplied to the electric furnace (X) (the "energy ratio") shall be 0.70 or less. (B) In the "late stage of continuous charging," the ratio of the "theoretical dissolution energy of reduced iron" (Y) to the "electrical energy supplied to the electric furnace (X)" ("energy ratio") shall be 0.90 or higher. (C) During the "mid-stage of continuous charging," the ratio of the "theoretical dissolution energy of reduced iron" (Y) to the "electrical energy supplied to the electric furnace" (X) ("energy ratio") shall be greater than or equal to the "energy ratio" in the "initial stage of continuous charging," and less than or equal to the "energy ratio" in the "late stage of continuous charging."

[0070] The "electrical energy supplied to the electric furnace" (X) used in calculating the "energy ratio" (the "energy ratio" at a given point in time) is the electrical energy (power input) supplied to the electric furnace from the start of the "final melting stage" up to a certain point in time (unit: for example, "kWh").

[0071] The "theoretical dissolution energy of reduced iron" (Y), used in calculating the "energy ratio" (the "energy ratio" at a given point in time), is the energy (in units of, for example, "kWh") required to dissolve the reduced iron R continuously supplied to the borehole 40 from the start of the "last dissolution period" up to a certain point in time. An example of how to determine the "theoretical dissolution energy of reduced iron" (Y) is explained below. In this specification, "%" indicating the unit of concentration means "mass%".

[0072] The composition of reduced iron R is analyzed. This reduced iron R is the reduced iron that is charged into the borehole 40 during the "final dissolution stage". For example, analytical instruments can be used to measure the T-Fe concentration (%T-Fe), M-Fe concentration (%M-Fe), FeO concentration (%FeO), SiO2 concentration (%SiO2), CaO concentration (%CaO), Al2O3 concentration (%Al2O3), and MgO concentration (%MgO) (where "%" represents "mass%)" of the "reduced iron".

[0073] Note that "reduced iron" also contains Fe2O3, but it may not be possible to measure the concentration of Fe2O3 using analytical instruments. In such cases, the Fe concentration obtained from (%T-Fe)-(%M-Fe)-(%Fe in FeO) of "reduced iron" is converted to the Fe2O3 concentration (%Fe2O3) to calculate the Fe2O3 concentration (%Fe2O3) contained in "reduced iron".

[0074] The iron source 30 containing reduced iron, which is charged into the furnace body 11, melts in the electric furnace and rises to about 1600°C. It is thought that the iron oxide (FeO, Fe2O3) in reduced iron is first reduced to Fe before dissolving, causing the temperature to rise to around 1600°C. The SiO2, CaO, Al2O3, and MgO components other than iron are common to the main components found in slag. Therefore, we consider the SiO2, CaO, Al2O3, and MgO components other than iron to be part of the slag, and assume that the slag will dissolve and the temperature will rise to around 1600°C. Based on these factors, the energy required to raise the temperature of reduced iron from 0°C to 1600°C is calculated using the values ​​shown in Table 2. The calculated energy is divided by the weight of reduced iron to obtain the "theoretical dissolution energy per unit weight of reduced iron" [kWh / kg-reduced iron].

[0075] [Table 2]

[0076] The "theoretical dissolution energy of reduced iron" (the "theoretical dissolution energy of reduced iron" at a certain point in time (Y)) is calculated by multiplying the "theoretical dissolution energy per unit weight of reduced iron" [kWh / kg-reduced iron] by the continuous rate of reduced iron charged into the electric furnace per unit time from the start of the "final dissolution period" to a certain point in time (units are, for example, [kg / h], [kg / min]). The continuous rate of reduced iron charged into the electric furnace per unit time from the start of the "final dissolution period" to a certain point in time is also the rate at which reduced iron is charged from the start of the "final dissolution period" to a certain point in time.

[0077] The method for ensuring that the "energy ratio" for the "initial stage of continuous charging," the "late stage of continuous charging," and the "mid-stage of continuous charging" satisfies (A), (B), and (C) above, respectively, is not particularly limited. For example, if the electrical energy supplied to the electric furnace per unit time (power input per unit time) is constant, the "theoretical dissolution energy of reduced iron" (Y) is changed. To change the "theoretical dissolution energy of reduced iron" (Y), for example, the charging rate of reduced iron may be changed. Alternatively, the "electrical energy supplied to the electric furnace" (power input) (X) may be changed. Furthermore, the "energy ratio" (Y / X) may be changed by changing the "theoretical dissolution energy of reduced iron" (Y) by changing the charging rate of reduced iron, and by changing the "electrical energy supplied to the electric furnace" (power input) (X).

[0078] Here, as an example, Figure 4 shows how the "energy ratio" (Y / X) is adjusted to satisfy (A), (B), and (C) above by changing the charging rate of reduced iron. Figure 4 shows the "final dissolution stage". In the example shown in Figure 4, the electrical energy supplied to the electric furnace per unit time (power input per unit time) is constant.

[0079] As shown in Figure 4, the charging rate of reduced iron R is slowed during the "initial stage of continuous charging." The charging rate of reduced iron R is slower than the charging rate of reduced iron R during the "later stage of continuous charging," which will be described later. This keeps the "energy ratio" (Y / X) below 0.70.

[0080] During the "mid-stage of continuous charging," the charging rate of reduced iron R is set to be greater than or equal to the charging rate of reduced iron R during the "initial stage of continuous charging," and less than or equal to the charging rate of reduced iron R during the "late stage of continuous charging," as described later. This ensures that the "energy ratio" (Y / X) during the "mid-stage of continuous charging" is greater than or equal to the "energy ratio" (Y / X) during the "initial stage of continuous charging," and less than or equal to the "energy ratio" (Y / X) during the "late stage of continuous charging." Note that the "energy ratio" (Y / X) during the "mid-stage of continuous charging" may be 0.70 or less, or 0.90 or more.

[0081] In the "late stage of continuous charging," the charging rate of reduced iron R is made faster than the charging rate of reduced iron R in the "initial stage of continuous charging." This makes the "energy ratio" (Y / X) 0.90 or higher. Note that the charging rate of reduced iron in the "late stage of continuous charging" may be the same as the charging rate of reduced iron in the "mid-stage of continuous charging," or it may be higher than the charging rate of reduced iron in the "mid-stage of continuous charging."

[0082] The following provides a detailed explanation of the "initial stage of continuous loading," the "mid-stage of continuous loading," and the "late stage of continuous loading."

[0083] <Early stages of continuous loading> In the initial stages of continuous charging, as shown in Figure 4, the borehole 40 is shallow. Also, the area with high thermal efficiency (for example, "Area U" shown in Figure 4) is small in and around the borehole 40. In areas other than the area with high thermal efficiency (U), the heat transfer efficiency of reduced iron R is low, and the reduced iron R does not dissolve easily. Reduced iron R in such areas tends to remain undissolved. This reduces the yield.

[0084] By keeping the "energy ratio" (Y / X) below 0.70, the reduced iron R charged in the "initial stage of continuous charging" can be reliably dissolved. In addition, a low "energy ratio" (Y / X) of 0.70 or less allows for deeper drilling of the borehole 40, expanding the area with high thermal efficiency. The "energy ratio" (Y / X) can be reduced by slowing down the charging rate of reduced iron R or increasing the amount of power input.

[0085] Furthermore, there is no particular limit to the lower limit of the "energy ratio" (Y / X) in the "initial stage of continuous charging." A smaller "energy ratio" (Y / X) is preferable in terms of ensuring the melting of the reduced iron R charged in the "initial stage of continuous charging" and deepening the borehole 40. For example, from the viewpoint of suppressing excessive temperature rise of the iron source 30 charged during the charging period and the resulting refractory wear of the furnace body 11, the "energy ratio" (Y / X) may be set to 0.40 or higher.

[0086] <Continuous loading, mid-term> During the "mid-stage of continuous charging," it is preferable not to make the "energy ratio" (Y / X) smaller than that of the "initial stage of continuous charging," due to factors such as longer operating hours, increased heat loss, and the resulting increase in power input (X). On the other hand, if the "energy ratio" (Y / X) is too large immediately after the "initial stage of continuous charging," the size of the area with high thermal efficiency (U) and the depth of the borehole 40 may not be able to adequately accommodate it. As a result, there is a risk that reduced iron R will not dissolve completely. Therefore, it is preferable not to make the "energy ratio" (Y / X) larger than that of the "late stage of continuous charging."

[0087] Based on the above, the "energy ratio" (Y / X) during the "mid-stage of continuous charging" should be greater than or equal to the "energy ratio" (Y / X) during the "initial stage of continuous charging," and less than or equal to the "energy ratio" (Y / X) during the "late stage of continuous charging." Note that the "energy ratio" (Y / X) during the "initial stage of continuous charging" may be less than 0.70. Also, the "energy ratio" (Y / X) during the "late stage of continuous charging" may be greater than 0.90. Therefore, the "energy ratio" (Y / X) during the "mid-stage of continuous charging" may be less than 0.70, between 0.70 and 0.90, or greater than 0.90.

[0088] <Late-stage continuous loading> If reduced iron R is continued to be charged in the "later stages of continuous charging" at a low "energy ratio" (Y / X) like that in the "early stages of continuous charging," the melting of the charged reduced iron R and the iron source 30 in the furnace body 11 will proceed excessively, and the molten material (molten iron source) M at the bottom of the borehole 40 will become close to a flat bath state. In addition, the volume of unmelted solid iron source S surrounding the electrode 13 will decrease. A "flat bath" is a state in which all the iron source in the furnace body 11 has melted and the molten steel surface in the furnace has become almost flat, as shown in the "heating up stage" in Figure 2 and the "heating up stage" in Figure 3. In a state close to a flat bath, arc heat escapes to the exhaust gas system (dust collector side) (for example, the "exhaust duct 22" in Figure 4), resulting in heat loss. Even if reduced iron R is introduced in this state, the heat transfer efficiency of the reduced iron R is poor.

[0089] Furthermore, the molten steel density is approximately 7 t / m³. 3In contrast, reduced iron is 5-5.5 t / m³ 3 Because of this, the reduced iron R charged into the borehole 40 floats on the molten material (molten iron source) M. As the temperature rises while the reduced iron R is floating, the carbon in the reduced iron R reacts with iron oxides such as FeO, generating CO gas on the surface of the molten material (molten iron source) M. CO gas generated on the surface of the molten material (molten iron source) M absorbs heat from the molten steel and escapes into the exhaust gas system (dust collector side) (for example, the "exhaust duct 22" in Figure 4). In addition, the charging of reduced iron R causes the surface of the molten material (molten iron source) M to vibrate, increasing the surface area of ​​molten steel in contact with the atmosphere, thus inducing heat loss. The heat absorption efficiency of the charged reduced iron R is poor due to the heat dissipation caused by the CO gas generated by the chemical reaction, and the heat dissipation from the molten steel due to the physical vibration of the molten steel surface.

[0090] Furthermore, since CO gas is generated when there is almost no undissolved solid iron source S around the reduced iron R, the heat of the CO gas itself, or the heat of combustion of the CO gas (secondary combustion heat), is not utilized to preheat the undissolved solid iron source S.

[0091] Thus, if reduced iron R is continued to be charged at a low energy ratio (Y / X) during the later stages of continuous charging, the reduced iron R will be charged in a state close to a flat bath, increasing heat loss and preventing the enjoyment of improved thermal efficiency. The charged reduced iron R and the solid iron source S remaining in the furnace body 11 are difficult to melt, and it is necessary to extend the operation time to melt them. However, extending the operation time increases the amount of electricity input.

[0092] Therefore, in the "late stage of continuous charging," the "energy ratio" (Y / X) is set to 0.90 or higher. This allows reduced iron R to be charged into the borehole 40 before it reaches a state close to a flat bath in the "late stage of continuous charging," thus compensating for both the poor heat conductivity and poor heat transfer efficiency of reduced iron R, reducing heat loss, and increasing thermal efficiency. As the charged reduced iron R dissolves, the dissolution of the solid iron source S around the borehole 40 also progresses. The "energy ratio" (Y / X) can be increased by increasing the charging rate of reduced iron R or by decreasing the amount of power input. More preferably, the "energy ratio" (Y / X) is 0.93 or higher.

[0093] Furthermore, there is no particular upper limit to the "energy ratio" (Y / X) in the "later stages of continuous charging." A high "energy ratio" (Y / X) is preferable in that reduced iron R is charged before the furnace reaches a state close to a flat bath. For example, in the "later stages of continuous charging," the amount of molten steel in the furnace body 11 is larger and the amount of heat contained in the molten steel is greater compared to the "early stages of continuous charging," so it is acceptable for the "energy ratio" (Y / X) to exceed 1.0, and it may also be set to 1.2 or less.

[0094] Thus, by continuously supplying reduced iron R to the borehole 40 during the "final dissolution stage" and controlling the "energy ratio" (Y / X) in the three sections described above, the following effects can be obtained. In the "final dissolution stage," during the "initial continuous charging phase" when the borehole 40 is small, it is thought that the charged reduced iron R is likely to remain undissolved. However, by using the low "energy ratio" (Y / X) described above, the reduced iron R charged during the "initial continuous charging phase" can be reliably dissolved. Furthermore, the borehole 40 can be deepened. Subsequently, in the "later stages of continuous charging" when the borehole 40 becomes deeper, the above-mentioned "energy ratio" (Y / X) is set higher than that of the "initial stages of continuous charging," thereby reducing heat loss and increasing the thermal efficiency of the reduced iron and iron source. As a result, the melting of the charged reduced iron R progresses, and the melting of the solid iron source S in the furnace body 11 also progresses. These measures make it possible to suppress the decrease in yield while suppressing the increase in power input.

[0095] Furthermore, when continuously charging reduced iron R, the power supply may be briefly interrupted, for example, when attempting to perform arc control, and consequently, the charging of reduced iron R may be interrupted. If such an unintended interruption of charging occurs during continuous charging of reduced iron R, and the continuous charging of reduced iron R is resumed after the power supply is restored, this is included in "continuous charging of reduced iron R".

[0096] Furthermore, Figure 4 illustrates a method of controlling the "energy ratio" (Y / X) by changing the charging rate of reduced iron R. However, the "energy ratio" (Y / X) may also be controlled by other methods, such as changing the amount of electricity input to the electric furnace, or by changing both the amount of electricity input to the electric furnace and the charging rate of reduced iron. Similar effects can be obtained by controlling the "energy ratio" (Y / X) in such ways as described above.

[0097] Furthermore, in each of the "initial continuous charging," "mid-continuous charging," and "late continuous charging" periods, the "energy ratio" (Y / X) may be constant or may change. Also, in one or more of these periods, there may be periods when the "energy ratio" (Y / X) is constant and periods when it changes. When the "energy ratio" (Y / X) changes, the manner of change is not particularly limited. For example, the "energy ratio" (Y / X) may change in a stepwise manner or gradually.

[0098] To keep the "energy ratio" (Y / X) constant, for example, the charging rate of reduced iron R and the power input per unit time may be kept constant. Alternatively, to change the "energy ratio" (Y / X), for example, the charging rate of reduced iron R may be changed, the power input per unit time may be changed, or both may be changed.

[0099] Next, I will explain the experiment in which the above findings were obtained.

[0100] [experiment] During the "charging phase," a portion of the iron source was charged into the arc-type electric furnace, and during the "final melting phase," the remaining iron source (reduced iron) was continuously charged near the electrodes inside the furnace, thereby continuously supplying it to the borehole.

[0101] In the "final dissolution phase," the amount of electricity input per unit time was kept constant, and the "energy ratio" of Nos. 1 to 7 was changed by varying the continuous charging rate of reduced iron for Nos. 1 to 7. In Nos. 1 to 5, the continuous charging rate of reduced iron was kept constant from the beginning to the end of the "final dissolution phase." As a result, the "energy ratio" remained constant from the beginning to the end of the "final dissolution phase" for Nos. 1 to 5. In Nos. 6 and 7, the continuous charging rate of reduced iron was varied in the "initial continuous charging phase," "mid-continuous charging phase," and "late continuous charging phase," changing the "energy ratio" in each section.

[0102] The implementation conditions are described below.

[0103] Molten steel was produced using an arc-type electric furnace with the specifications shown in Table 3. [Table 3]

[0104] Scrap and reduced iron were used as the iron source. Table 4, described below, shows the proportions of scrap and reduced iron and the timing of their charging. Here, scrap weighing 800 kg or less was used.

[0105] Reduced iron is HBI (apparent density: 5-5.5 ton / m³). 3 The following was used: In this experiment, an HBI with a composition similar to that shown in Table 1 was used. The theoretical dissolution energy per unit weight of reduced iron in this experiment was 0.491 [kWh / kg-reduced iron].

[0106] After the "final melting stage," the process transitioned to the "heating stage," where flux was added according to the conventional methods of those skilled in the art, followed by oxidation refining and other processes before the steel was tapped.

[0107] Table 4 shows the mixing ratios of scrap and pig iron with reduced iron, as well as the charging timing and other implementation conditions. Figure 5 shows the relationship between the "progress of continuous charging of reduced iron" and the "energy ratio" during the "final melting stage." The "progress of continuous charging of reduced iron" is the amount of reduced iron charged (mass ratio) to the total amount of reduced iron charged into the furnace body during the "final melting stage."

[0108] [Table 4]

[0109] Table 4 explains the "power consumption per unit" and its "index," as well as the "yield" and its "index."

[0110] [Electricity intensity and its index] "Electricity consumption per unit" is the amount of electricity consumed to extract 1 ton of molten steel from an electric furnace. To minimize yield reduction, it is necessary to ensure that the iron source charged into the furnace (especially the reduced iron charged during the "final melting stage") does not remain undissolved. One way to achieve this is to increase the amount of electricity input. However, increasing the amount of electricity input leads to longer operating hours for the electric furnace, which directly results in a decrease in productivity. Therefore, it is preferable to use less electricity and shorten the operating hours. Therefore, among No. 1 to 5, where the "energy ratio" in the "final dissolution period" was kept constant, No. 5, which had the smallest power intensity, was used as the standard, and the power intensity of each of No. 1 to 7 relative to the power intensity of No. 5 was defined as the "power intensity index."

[0111] [Yield and its index] In this experiment, the "yield" was defined as the value obtained by dividing the tapped steel weight by the weight of Fe charged into the furnace body. Similar to the power consumption unit index, No. 5 was used as the baseline, and the yields of No. 1 to 7 relative to the yield of No. 5 were defined as the "yield index".

[0112] From Table 4 and Figure 5, the following was found:

[0113] When comparing experiments where the "energy ratio" was kept constant during the "final dissolution period" of No. 1 to 5, the yields of No. 1 to 4, which had a higher power consumption per unit than No. 5, were equivalent to or higher than the yield of No. 5. From this, it can be inferred that, when the "energy ratio" in the "final dissolution stage" is kept constant, increasing the power input tends to increase the yield. Conversely, decreasing the power input tends to decrease the yield.

[0114] On the other hand, in No. 6 and No. 7, the "energy ratio" was changed in each section of the "final dissolution period." Although the power consumption per unit of No. 6 and No. 7 was lower than that of No. 5, the yield of No. 6 and No. 7 was about 10% higher than that of No. 5.

[0115] From the above, as shown in No. 6 and No. 7, by making the "energy ratio" smaller in the "initial stage of continuous charging" and larger in the "later stage of continuous charging," it is thought that the decrease in yield can be suppressed even if the increase in power input is suppressed. From No. 6 and No. 7, it is thought that the "energy ratio" in the "initial stage of continuous charging" should be 0.70 or less, and the "energy ratio" in the "later stage of continuous charging" should be 0.90 or more. Furthermore, it is thought that the "energy ratio" in the "mid-stage of continuous charging" should not be smaller than the "energy ratio" in the "initial stage of continuous charging," nor larger than the "energy ratio" in the "later stage of continuous charging." In other words, it is thought that the "energy ratio" in the "mid-stage of continuous charging" should be greater than or equal to the "energy ratio" in the "initial stage of continuous charging," and less than or equal to the "energy ratio" in the "later stage of continuous charging."

[0116] Based on the above, during the "final dissolution phase," the "energy ratio" should be 0.70 or less during the "initial continuous charging phase," 0.90 or more during the "later continuous charging phase," and the "energy ratio" during the "mid-stage continuous charging phase" should be greater than or equal to the "energy ratio" during the "initial continuous charging phase" and less than or equal to the "energy ratio" during the "later continuous charging phase." This makes it possible to suppress the decrease in yield while suppressing the increase in power input.

[0117] Although embodiments of the present invention have been described above with reference to the drawings, it should be understood that the specific configurations are not limited to these embodiments. The scope of the present invention is indicated by the claims rather than the above description, and all modifications within the meaning and scope of equivalence to the claims are included. Furthermore, the above experiments do not limit the present invention, and any modifications that do not depart from the spirit of this specification are all included within the technical scope of the present invention.

[0118] For example, in the experiment described above, as shown in Table 4, a portion of the reduced iron was charged during the "charging phase," and the remaining reduced iron was continuously charged into the borehole during the "final melting phase." However, if the iron source charged into the arc-type electric furnace contains both reduced iron and other iron sources, only the other iron sources may be charged during the "charging phase." Alternatively, all of the reduced iron contained in the iron source charged into the arc-type electric furnace may be continuously charged into the borehole during the "final melting phase." The effects of the present invention can be obtained in such cases as well.

[0119] Furthermore, in the "melting phase" shown in Figure 3, one borehole is formed in the iron source 30 inside the furnace. However, when the iron source 30 is charged into the borehole during the "melting phase," two or more boreholes may already be formed in the iron source 30 inside the furnace.

[0120] Furthermore, the operating conditions and other operational conditions of the arc-type electric furnace are not particularly limited and should be carried out as those normally practiced by those skilled in the art. [Explanation of Symbols]

[0121] 1. Arc-type electric furnace 11 Furnace body 12 Hearth cover 13 electrodes 30 Iron Source 21 shots 22 Exhaust duct M Molten material (source of molten iron) S Solid iron source (undissolved iron source) R Reduced Iron

Claims

1. This is a method for charging reduced iron into an arc-type electric furnace when producing molten steel using an iron source containing reduced iron in the said arc-type electric furnace. The aforementioned arc-type electric furnace comprises a furnace body and electrodes installed at least above the furnace body. After the charging phase, in which an iron source is charged into the furnace body when the electrodes are not energized, during the melting phase, in which the iron source in the furnace body is melted by energizing the electrodes, While energizing the electrodes, reduced iron is continuously charged into a borehole formed in the iron source within the furnace body and surrounded by a molten iron source and an unmolten iron source. In the initial stage of continuous charging, when reducing iron up to 25% by mass relative to the total amount of reduced iron to be continuously charged is continuously charged into the borehole, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace is set to 0.70 or less. In the later stages of continuous charging, in which an amount of reduced iron equal to 50% or more by mass relative to the total amount of reduced iron to be continuously charged is continuously charged into the borehole, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace is set to 0.90 or more. In the middle stage of continuous charging, when an amount of reduced iron exceeding 25% by mass but less than 50% of the total amount of reduced iron to be continuously charged is continuously charged into the borehole, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace is set to be greater than or equal to the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace in the initial stage of continuous charging, and less than or equal to the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace in the later stage of continuous charging. A method for charging reduced iron into an electric furnace, characterized by the following features.

2. By changing the charging rate of reduced iron, the ratio of the theoretical dissolution energy of reduced iron to the electrical energy supplied to the arc-type electric furnace is controlled. The method for charging reduced iron into an electric furnace according to feature 1.

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