Method for inserting reduced iron into electric furnace

Abstract

To suppress an increase of a power input amount, when producing molten steel with an arc type electric furnace.SOLUTION: After a charging period in which iron sources are charged into a furnace body without electric current flowing to the electrodes, during the melting period where the iron sources in the furnace body are melted by applying current to the electrodes, reduced iron is charged into a boring hole formed within the iron sources in the furnace body, surrounded by melted and unmelted iron sources, while current is flowing to the electrodes. The charging of reduced iron is initiated when the following condition (1) is satisfied during the melting period: E / (A+1.56×B)≤190 (1). A is the amount (ton) of iron sources other than reduced iron loaded into the furnace body before the start of reduced iron charging during the melting period. B is the amount (ton) of reduced iron contained in the iron sources loaded into the furnace body before the start of reduced iron charging during the melting period. E is the amount of electric energy (kWh) delivered to the aforementioned arc furnace before the start of reduced iron charging during the melting period.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a method for charging reduced iron into an arc-type electric furnace when producing molten steel using an iron source containing reduced iron by means of an arc-type electric furnace.

Background Art

[0002] Conventionally, a method of charging an iron source such as scrap into an arc-type electric furnace to produce molten steel has been known. In the electric furnace, electrodes are installed at least above the furnace. By generating an arc between the lower end of the electrode and the iron source in the furnace, the iron source in the furnace is melted to produce molten steel.

[0003] In the furnace, there are a position called a hot spot, which is close to the electrode and where heat is easily transferred, and a position called a cold spot, which is far from the electrode and where heat is hardly transferred. Since the iron source located at the cold spot remains undissolved, the iron source at the cold spot is melted by, for example, extending the energization time of the electrode. However, this causes adverse effects in operation, such as an increase in heat loss and an increase in the amount of power input to the electric furnace due to the extension of the energization time.

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

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The cold spot of an electric furnace is generally described as a position far from the electrode in the horizontal direction when looking at the furnace from above, but it is not limited to this. When an arc irradiates near the lower end of the electrode, the iron source in the furnace melts from near the lower end of the electrode, and the bulk of the iron source near the lower end of the electrode decreases. Accordingly, as the electrode is lowered, the upper part of the furnace far from the arc irradiation site near the lower end of the electrode can also become a cold spot.

[0007] The method described in Patent Document 1 is a method of melting the iron source of a cold spot far from the electrode in the horizontal direction when looking at the furnace from above, rather than a cold spot in the upper part of the furnace. In order to melt the iron source located at the cold spot in the upper part of the furnace, it is still necessary to lengthen the energization time of the electrode, so the power input increases.

[0008] Also, in Patent Document 1, scrap or the like is used as the iron source (see

[0002] of Patent Document 1), but in recent years, the use of reduced iron as the iron source to be melted in an electric furnace has been considered. However, from the findings of the inventors of the present application, it has been found that reduced iron exhibits different behavior from scrap at the cold spots in the furnace. In addition, reduced iron has poor heat conduction and poor heat absorption efficiency compared to scrap. Therefore, when using reduced iron as the iron source to be melted in an electric furnace, it is considered that a unique operation different from the case of using scrap or the like as the iron source is required.

[0009] An object of the present invention is to provide a method capable of enhancing the thermal efficiency and suppressing an increase in power input when producing molten steel using an iron source containing reduced iron in an arc-type electric furnace.

Means for Solving the Problems

[0010] In the present invention, an iron source containing reduced iron is used as the iron source to be melted in an arc-type electric furnace. The inventors of the present application obtained the following findings regarding reduced iron.

[0011] Since the cold spots in the furnace are far from the heat source, the reduced iron located at the cold spots is difficult to melt. In addition, since the reduced iron contains iron oxide and also contains more gangue components than scrap, the reduced iron and the molten slag that has gradually melted adhere to each other or to the refractory and grow into coarse undissolved matter. The grown undissolved matter becomes even more difficult to melt. When using an iron source containing reduced iron as described above, it is extremely important how to prevent the reduced iron from being positioned at the cold spot.

[0012] In addition, since the reduced iron contains more gangue components and voids than scrap, its thermal conductivity is worse than that of scrap. Due to the poor thermal conductivity of the reduced iron, the heat absorbed by the reduced iron does not reach the inside of the reduced iron, and only the surface where heat is supplied (in the furnace, the part where heat is transmitted by the arc) becomes high temperature. The greater the temperature difference between the heat source (arc) and the reduced iron (the surface where heat is supplied), the easier it is for the reduced iron to absorb heat. However, since the reduced iron (the surface where heat is supplied) is at a high temperature, it is difficult for the reduced iron (the surface where heat is supplied) to absorb heat. Therefore, the reduced iron, combined with its poor thermal conductivity, has poor heat absorption efficiency.

[0013] Thus, compared with scrap, the reduced iron has poor thermal conductivity and poor heat absorption efficiency, so it is more difficult to melt than scrap. Therefore, when using reduced iron as the iron source to be melted in an arc-type electric furnace, it is considered that a unique operation different from the case of using scrap or the like is required.

[0014] Based on the above findings, the inventors of the present application have found the following method different from the conventional one.

[0015] The method for charging reduced iron into an electric furnace disclosed in this specification is a method for charging reduced iron into an arc-type electric furnace when producing molten steel using an iron source containing reduced iron. The arc-type electric furnace has a furnace body and at least an electrode installed above the furnace body. After the charging period in which the iron source is charged into the furnace body when the electrode is not energized, in the melting period in which the iron source in the furnace body is melted by energizing the electrode, while energizing the electrode, the reduced iron is charged into the boiling holes formed in the iron source in the furnace body and surrounded by the molten iron source and the undissolved iron source. In the melting period, the charging of the reduced iron is started when the following formula (1) is satisfied. E / (A + 1.56×B) ≤ 190 ···(1) Here, A is the amount (ton) of the iron source other than reduced iron among the iron sources charged into the furnace body before the start of charging the reduced iron in the melting period. B is the amount (ton) of the reduced iron contained in the iron source charged into the furnace body before the start of charging the reduced iron in the melting period. E is the amount of electric power (kWh) input into the arc-type electric furnace before the start of charging the reduced iron in the melting period.

[0016] According to the method described above, by charging the reduced iron into the boiling holes in the melting period, it is difficult for the reduced iron to exist in the hot spot at the upper part of the furnace. In addition, by starting the charging of the reduced iron at the above timing in the melting period, both the poor heat conduction and the poor heat absorption efficiency of the reduced iron can be compensated, heat loss can be reduced, and the heat efficiency can be increased. As a result, the melting of the reduced iron proceeds. Furthermore, the melting of the iron source around the boiling holes also proceeds. Since the increase in the energization time can be suppressed as described above, the increase in the power input amount can be suppressed.

Advantages of the Invention

[0017] When producing molten steel using an iron source containing reduced iron by an arc-type electric furnace, the heat efficiency can be increased and the increase in the power input amount can be suppressed.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0019] Hereinafter, preferred embodiments of the present invention will be described. The embodiments described below are an example of embodying the present invention and do not limit the present invention.

[0020] In the present embodiment, an arc-type electric furnace is used. FIG. 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 an electrode 13.

[0021] The furnace body 11 can accommodate an iron source 30. The upper end of the furnace body 11 is open. FIG. 1 shows a state in which the opening at the upper end of the furnace body 11 is closed by the furnace lid 12.

[0022] The furnace lid 12 is provided with a chute 21, an exhaust duct 22, etc. The chute 21 is used, for example, when charging a flux for component adjustment. With the furnace body 11 closed by the furnace lid 12, a flux or the like can be charged into the furnace body 11 from the chute 21. The exhaust duct 22 can discharge the gas, heat, etc. inside the furnace body 11.

[0023] The furnace lid 12 is movable in the vertical and horizontal directions by a moving mechanism (not shown). The furnace lid 12 may be rotatable.

[0024] FIG. 1 shows a case where the electrode 13 is installed above the furnace body 11. The electrode 13 penetrates the furnace lid 12. The lower end portion of the electrode 13 is disposed inside the furnace body 11. The electrode 13 is movable up and down by a lifting device (not shown). In addition, electrodes other than the electrode 13 may be further installed in the arc - type electric furnace 1.

[0025] By lowering the electrode 13, the lower end portion of the electrode 13 is brought closer to the iron source 30. In the arc - type electric furnace 1 shown in FIG. 1, when the electrode 13 is energized, an arc is generated between the lower end portion of the electrode 13 and the iron source 30, and the iron source 30 is melted by the heat of the arc. While the electrode 13 is energized, the opening at the upper end of the furnace body 11 is closed by the furnace lid 12.

[0026] In this embodiment, as the iron source 30 to be melted in the arc - type electric furnace 1, an iron source containing reduced iron is used. The iron source 30 is solid. The iron source 30 is charged into the furnace body 11.

[0027] Here, the "reduced iron" in this specification is obtained using the direct reduction method (which may also be referred to as the "direct ironmaking method" or "direct reduced 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, electric furnaces, etc. "Reduced iron" has undergone more reduction 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.

[0028] Table 1 shows the composition of HBI as an example of the composition of reduced iron (the sources of Table 1 are "Hidehiko Tanaka, Kobe Steel R&D Vol.64 (2014), No.1, p2-7." and "Hidehiko Tanaka et al., Iron and Steel 92 (2006), p1022-1028."). Note that Table 1 is an example, and the composition of reduced iron is not limited to the composition shown in Table 1.

Table 1

[0029] The iron source 30 melted in the arc-type electric furnace 1 may be only reduced iron, or may include reduced iron and an iron source other than reduced iron. The iron source other than reduced iron charged into the electric furnace generally has a higher thermal conductivity than reduced iron and contains an iron component. Examples of those having a higher thermal conductivity than reduced iron and containing an iron component include scrap, pig iron, ingots, and iron carbide. As the iron source other than reduced iron, for example, a combination of one or more of scrap, pig iron, ingots, and iron carbide may be used. The combinations of multiple types of iron sources other than reduced iron are not particularly limited. Note that the iron source other than reduced iron that can be charged into the electric furnace is not limited to the above examples. As the iron source other than reduced iron, for example, an iron source reduced by a method other than the direct reduction method may be used. The iron source 30 including 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.

[0030] The method according to this embodiment differs from the conventional method in the timing of charging part or all of the reduced iron. First, the conventional method will be described below, and the method according to this embodiment will be described while comparing it with the conventional method.

[0031] [Conventional method] Fig. 2 schematically shows a conventional method for producing molten steel (conventional method (1)).

[0032] First, with the charging port 11A, which is the opening at the upper end of the furnace body 11, open, the iron source 30 is charged into the furnace body 11 from the charging port 11A. The iron source 30 is charged from above the furnace body 11 by means of a clam shell X or the like.

[0033] In this specification, the period when the iron source 30 is charged into the furnace body 11 from 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 (charging port 11A) at the upper end of the furnace body 11 is not closed by the furnace lid 12 shown in Fig. 1. During the "charging period", the lower end of the electrode 13 shown in Fig. 1 is not disposed inside the furnace body 11. During the "charging period", no current is passed through the electrode 13. Even if there are other electrodes (not shown) other than the electrode 13 installed in the arc electric furnace 1 (for example, inside the furnace), no current is passed through all the electrodes during the "charging period".

[0034] In the conventional method, all of the iron source 30 to be melted in the arc electric furnace 1 during the "charging period" is charged into the furnace body 11. After all of the iron source 30 has been charged, the bulk of the iron source 30 inside the furnace body 11 is large, and the iron source 30 exists in the upper part of the furnace body 11.

[0035] Next, 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, the lower end of the electrode 13 is brought close to the iron source 30 inside the furnace as shown in Fig. 1, and current is passed through the electrode 13. The iron source 30 melts from the vicinity of the lower end of the electrode 13, and the bulk of the iron source 30 in the vicinity of the lower end of the electrode 13 decreases. As shown in the "melting period" of Fig. 2, as the bulk of the iron source 30 in the vicinity of the lower end of the electrode 13 decreases, the electrode 13 is lowered.

[0036] In this specification, the period when the iron source 30 in the furnace body 11 is melted by energizing the electrode 13 is referred to as the "melting period". During the "melting period", the opening (charging port 11A) at the upper end of the furnace body 11 is closed by the furnace lid 12. During the "melting period", the lower end of the electrode 13 is disposed in the furnace body 11. During the "melting period", the electrode 13 is energized. Note that during the "melting period", the energization may stop for a short time when attempting arc control or the like, but such an unintended short-time energization stop is included in the "melting period". However, when the electrode 13 is not intentionally energized, it is not the "melting period". During the "melting period", a melt (molten iron source) (M) and an undissolved solid iron source (S) exist in the furnace body 11.

[0037] All of the iron source 30 in the furnace body 11 is melted to become molten steel (or molten iron). Thereafter, the energization of the electrode 13 is continued to heat up the molten steel.

[0038] In this specification, the period when the iron source 30 in the furnace body 11 is all melted and the electrode 13 is energized is referred to as the "heating period". The "heating period" is a state in which the molten steel surface in the furnace is substantially flat (hereinafter referred to as the "flat bath state").

[0039] During the "heating period", slag is generated by charging fluxes such as lime, and oxidative refining is carried out by supplying oxygen gas as necessary. Thereafter, the molten steel in the furnace body 11 is tapped into a ladle (not shown).

[0040] In the conventional method as described above, all of the iron source 30 to be melted in the arc-type electric furnace 1 is charged into the furnace during the "charging period".

[0041] Note that in the above, all of the iron source 30 to be melted in the arc-type electric furnace 1 is charged into the furnace body 11 during one "charging period". However, as described below, the iron source 30 may be charged in multiple "charging periods". For example, when there are restrictions on the internal space of the furnace body 11 and all of the iron source 30 cannot be charged into the furnace during one "charging period", the iron source 30 is charged in multiple "charging periods". Hereinafter, a method of charging the iron source 30 in multiple "charging periods" (conventional method (2)) will be described.

[0042] Figure 3 schematically shows a conventional method for producing molten steel (conventional method (2)).

[0043] In the conventional method (2), a part of the iron source 30 is charged into the furnace body 11 in the first "charging period". Then, after a part of the iron source 30 in the furnace is melted in the "melting period", the iron source 30 is charged into the furnace in the next "charging period". Until all the iron source 30 is charged into the furnace, as shown in "f" of Figure 3, the "charging period" → "melting period" → "charging period" → ···, the "charging period" and the "melting period" are repeated.

[0044] In addition, in "f" of Figure 3, during the "melting period", as a part of the iron source 30 melts, the bulk of the iron source 30 decreases, creating a space above the melt M. In the next "charging period", the iron source 30 is charged onto the space above the melt M and the solid iron source S around it.

[0045] Even when the iron source 30 is charged into the furnace body 11 in multiple "charging periods", after all the iron source 30 is charged into the furnace body 11, as shown in the "charging period" of Figure 3, the bulk of the iron source 30 in the furnace body 11 is large, and the iron source 30 exists above the furnace body 11.

[0046] After all the iron source 30 is charged into the furnace body 11, all the iron source 30 in the furnace body 11 is melted in the "melting period" (see the "melting period" below "f" in Figure 3). Then, similar to the conventional method (1), after going through the "heating period", refining, etc., tapping is carried out.

[0047] Thus, in the conventional methods (1) and (2), all the iron source 30 is charged into the furnace body 11 in the "charging period". After all the iron source 30 is charged, the iron source 30 in the furnace exists in the upper part of the furnace, which becomes a cold spot. When the iron source 30 contains reduced iron, the reduced iron exists in the upper part of the furnace. The reduced iron in the upper part of the furnace is difficult to melt. Also, the reduced iron and the gradually melted molten slag adhere to each other or to the refractory, growing into coarse undissolved substances. The grown undissolved substances become even more difficult to melt. To melt these, it is necessary to increase the energization time of the electrode 13, but the power input increases.

[0048] In the method according to this embodiment, different from the conventional method, not all of the iron source 30 melted in the arc-type electric furnace 1 is charged during the "charging period". A part or all of the reduced iron among the iron source 30 melted in the arc-type electric furnace 1 is charged into the furnace at a timing different from the conventional method. Hereinafter, with reference to FIGS. 4 and 5, the method according to this embodiment will be described. FIG. 4 shows the case where the "charging period" is only once (this embodiment (1)), and FIG. 5 shows the case where there are multiple "charging periods" (this embodiment (2)).

[0049] [Method according to this embodiment] In the case where the "charging period" is only once (this embodiment (1)), as shown in FIG. 4, a part of the iron source 30 is charged into the furnace body 11 during the "charging period". The iron source 30 charged into the furnace during the "charging period" is the iron source 30 excluding a part or all (R) of the reduced iron among the iron source 30 melted in the arc-type electric furnace 1. When the iron source 30 melted in the arc-type electric furnace 1 is only reduced iron, a part of the reduced iron is charged into the furnace during the "charging period". When the iron source 30 melted in the arc-type electric furnace 1 includes reduced iron and an iron source other than reduced iron, during the "charging period", a part of the reduced iron and the iron source other than reduced iron, or only the iron source other than reduced iron is charged into the furnace. When the iron source 30 melted in the arc-type electric furnace 1 includes reduced iron and an iron source other than reduced iron, reduced iron may or may not be charged during the "charging period".

[0050] After the "charging period", the iron source 30 in the furnace body 11 is melted during the "melting period". When the iron source 30 in the furnace body 11 starts to melt from the vicinity of the lower end of the electrode 13 during the "melting period", the bulk of the iron source 30 in the vicinity of the lower end of the electrode 13 decreases. Thereby, the boring hole 40 is formed. As the electrode 13 is lowered, the boring hole 40 is formed around the electrode 13.

[0051] As used herein, the "boring hole 40" is a hole formed in the iron source 30 within the furnace body 11, and is a hole surrounded by the melt (molten iron source) M and the undissolved solid iron source S. The bottom of the boring hole 40 is mainly surrounded by the melt (molten iron source) M. The side of the boring hole 40 is mainly surrounded by the undissolved solid iron source S. Note that a part of the iron source at the bottom of the boring hole 40 may be undissolved. Also, a part of the iron source on the side of the boring hole 40 may be dissolved.

[0052] In this embodiment, as shown in FIG. 4, reduced iron R is charged into the boring hole 40 during the "melting period". The reduced iron R is the iron source 30 that was not charged into the furnace during the "charging period". During the "melting period", with the opening at the upper end of the furnace body 11 closed by the furnace lid 12, while energizing the electrode 13, the reduced iron R is charged into the boring hole 40.

[0053] Note that the charging rate of the reduced iron into the boring hole 40 during the "melting period" is assumed to be approximately corresponding to the power supply capacity of the electric furnace, and the charging rate may be set according to the specifications of the electric furnace. Also, the method of charging the reduced iron R into the boring hole 40 during the "melting period" is not particularly limited. For example, as shown in FIG. 3, the reduced iron R may be charged from the chute 21. When the iron source 30 melted in the arc-type electric furnace 1 includes reduced iron and other iron sources, whether the reduced iron R charged during the "melting period" is part or all of the reduced iron in the iron source 30 melted in the arc-type electric furnace 1 is not particularly limited, and may be determined, for example, from the cold spot corresponding to the furnace and the height of the iron source 30 charged during the "charging period". When the iron source 30 melted in the arc-type electric furnace 1 is only reduced iron, the amount of the reduced iron R charged during the "melting period" is not particularly limited, and may be determined, for example, from the cold spot corresponding to the furnace and the height of the iron source 30 charged during the "charging period". Also, when charging the reduced iron R during the "melting period", the reduced iron R may be charged continuously or intermittently.

[0054] The reduced iron R charged into the boring hole 40 is immediately melted by the arc heat. Therefore, the melting state of the iron source in the furnace hardly changes between the start and end of charging the reduced iron R into the boring hole 40.

[0055] All of the iron source 30 in the furnace is melted, and the "heating-up period" begins. After the "heating-up period", it is the same as the conventional method.

[0056] Note that in the above description, the "charging period" is once, but in this embodiment, there may be a plurality of "charging periods". Hereinafter, the case where there are a plurality of "charging periods" (this embodiment (2)) will be described with reference to FIG. 5.

[0057] In the method of this embodiment (2), a part of the iron source 30 is charged into the furnace body 11 during the first "charging period". After that, after a part of the iron source in the furnace is melted during the "melting period", the iron source 30 is charged into the furnace during the next "charging period". As shown by "F" in FIG. 5, the "charging period" → "melting period" → "charging period" → ···, and the "charging period" and the "melting period" are repeated. The iron source 30 charged into the furnace during the "charging period" is the iron source 30 excluding a part or all (R) of the reduced iron among the iron sources 30 melted in the arc-type electric furnace 1, similar to this embodiment (1).

[0058] After all the "charging periods" are completed, all of the iron source 30 in the furnace body 11 is melted during the "melting period" (refer to the "melting period" below "F" in FIG. 5). All of the iron source 30 in the furnace is melted, and the "heating-up period" begins. After the "heating-up period", it is the same as the conventional method.

[0059] When there are a plurality of "charging periods", there are a plurality of "melting periods". The plurality of "melting periods" are the "melting period" between the "charging periods" (the "melting period" of "F" in FIG. 5) and the "melting period" after all the "charging periods" are completed (the "melting period" below "F" in FIG. 5). In other words, when there are a plurality of "charging periods", that is, when there are a plurality of "melting periods", reduced iron R is charged into the boring hole 40 during one or more of the plurality of "melting periods". Reduced iron R may be charged into the boring hole 40 during any "melting period". The number of times of charging reduced iron R into the boring hole 40 during the "melting period" is not limited.

[0060] For example, as shown in FIG. 5, reduced iron R may be charged into the boring holes 40 during the "melting period" between the "charging periods" (the "melting period" of "F" in FIG. 5) and the "melting period" after all the "charging periods" are completed (the "melting period" below "F" in FIG. 5). Also, reduced iron R may be charged into the boring holes 40 only during the "melting period" between the "charging periods" (the "melting period" of "F" in FIG. 5). Reduced iron R may be charged into the boring holes 40 only during the "melting period" after all the "charging periods" are completed (the "melting period" below "F" in FIG. 5).

[0061] Regardless of which "melting period" reduced iron R is charged into the boring holes 40, the reduced iron R charged into the boring holes 40 will immediately melt due to the arc heat. The melting state of the iron source in the furnace hardly changes between the start and end of charging reduced iron R into the boring holes 40.

[0062] Thus, in the present embodiments (1) and (2), not all of the iron source 30 is charged into the furnace during the "charging period". During the "charging period", the iron source 30 excluding a part or all of the reduced iron (R) among the iron sources 30 melted in the arc furnace 1 is charged into the furnace. Therefore, after charging the iron source 30 during the "charging period", it is less likely to exist in the upper part of the furnace where the iron source 30 in the furnace becomes a cold spot.

[0063] Also, the reduced iron R not charged into the furnace during the "charging period" is charged into the boring holes 40 while energizing the electrode 13 during the "melting period". The reduced iron R melts due to the arc heat at the boring holes 40. While the reduced iron R is melting at the boring holes 40, the undissolved solid iron source S in the furnace melts, and the bulk of the iron source 30 decreases.

[0064] From the above, according to the methods of the present embodiments (1) and (2), it is less likely for reduced iron to exist in the upper part of the furnace where a cold spot occurs. Also, the above-mentioned coarse undissolved substances generated by the reduced iron are less likely to occur. Therefore, it is not necessary to extend the energization time. As a result, the power input can be reduced compared to the conventional method.

[0065] Furthermore, according to the present Embodiments (1) and (2), the following effects can be obtained. FIG. 6 shows an example of the "melting period" according to the present embodiment.

[0066] Reduced iron has poor heat conduction and poor heat absorption efficiency compared to scrap. Therefore, reduced iron is less likely to melt compared to scrap.

[0067] As shown at "just before the start of charging reduced iron" in FIG. 6, at the bottom of the boring hole 40, there is a melt (molten iron source) M, there is no gap between solid iron sources that reduce heat conduction, and there is some convection (natural convection caused by the temperature difference within the melt) in the melt (molten iron source) M. When reduced iron R is charged here, the poor heat conduction of the reduced iron R is compensated for.

[0068] Also, since the reduced iron R contains C and iron oxides such as FeO, when the reduced iron R is charged into the boring hole and the reduced iron R becomes high temperature due to arc heat, in the boring hole 40, C in the reduced iron reacts with iron oxides such as FeO, and CO gas is generated. An example of the reaction formula is as follows. Reaction formula: C + FeO = Fe + CO(g) Due to the secondary combustion heat of the CO(g) generated in the boring hole 40, the iron source around the boring hole 40 is heated up, promoting the melting of the surrounding solid iron source S. Also, in the boring hole 40, since the arc is surrounded by the iron source 30, the arc heat is less likely to escape to the furnace wall, furnace lid, and exhaust gas port, and since the heat loss is small, the thermal efficiency is high (see "during charging of reduced iron" in FIG. 6).

[0069] That is, by charging the reduced iron R into the boring hole 40 during the "melting period", both the poor heat conduction of the reduced iron R and the poor heat absorption efficiency of the reduced iron are compensated for, and the melting of the reduced iron R charged into the boring hole 40 proceeds.

[0070] Furthermore, since the melt M exists at the bottom of the boring hole 40 and the heat conduction of the melt M is high, when the reduced iron R is charged into the boring hole 40, the following cycle proceeds efficiently: the reduced iron R absorbs heat → a part of the reduced iron R melts → the heat absorbed is conducted to the surrounding reduced iron R and the solid iron source S → the arc heat is absorbed by the undissolved reduced iron → further melting proceeds.

[0071] Moreover, the heat of the high-temperature CO gas generated in the boring hole 40 transfers to the solid iron source S around the boring hole 40, and the reaction heat from the reaction (secondary combustion) in which CO2 gas is generated by the reaction of the CO gas and the oxygen in the furnace is also utilized for heating the solid iron source S. This is also one way to improve the thermal efficiency.

[0072] As described above, by charging the reduced iron R into the boring hole 40 during the "melting period", the thermal efficiency can be improved. As a result, the melting of the reduced iron R charged into the boring hole 40 and the solid iron source S around the boring hole 40 is promoted. This leads to a reduction in the power input.

[0073] Of course, it is a prerequisite that the charging rate of the reduced iron into the boring hole 40 during the "melting period" is approximately corresponding to the power supply capacity of the electric furnace. Since the energy by energization is consumed for heating and melting the charged reduced iron R during the charging of the reduced iron R into the boring hole 40, the situation of the iron source 30 in the furnace (the melting state of the iron source 30 in the furnace) at the start point of the reduced iron charging continues until the end point of the reduced iron charging. For example, as shown in FIG. 6, the situation of the iron source 30 in the furnace (the melting state of the iron source 30 in the furnace) from immediately before the start of the reduced iron charging to immediately after the end of the reduced iron charging is almost the same. The charging rate of the reduced iron R may be set according to the specifications of the electric furnace.

[0074] As described above, it has been found that it is good to charge reduced iron into the tapping hole during the "melting period". However, from the research of the inventors of the present application, even when charging reduced iron during the "melting period", there is an appropriate range for the timing. If the timing of charging the reduced iron is too late, as shown in Fig. 7, the amount of the melt M at the bottom of the tapping hole 40 in the furnace increases, and at the local position where the reduced iron reaches, the surface of the melt (molten metal) in which the iron source has melted is almost flat. Hereinafter, the fact that the surface of the melt (molten metal) is almost flat is referred to as "flat bath".

[0075] Fig. 7 shows the state when the melt (molten metal) M at the bottom of the tapping hole 40 is close to the flat bath during the "melting period". Fig. 7 shows the case of charging reduced iron in a state close to the flat bath.

[0076] As shown in "just before the start of charging reduced iron" in Fig. 7, when the melt (molten metal) M at the bottom of the tapping hole 40 is close to the flat bath, the bulk of the undissolved solid iron source S covering the periphery of the electrode 13 becomes small. Due to this influence, the arc heat escapes to the exhaust gas system (dust collection side) (for example, "exhaust duct 22" in Fig. 1), and heat loss occurs. Even if reduced iron R is charged here, the heat absorption efficiency of the reduced iron R is poor.

[0077] Also, while the density of the molten steel is about 7 t / m 3 whereas the reduced iron is about 5 - 5.5 t / m 3 Therefore, the reduced iron R charged into the tapping hole 40 floats on the melt (molten metal). As the reduced iron R rises in temperature while floating, C in the reduced iron R reacts with iron oxides such as FeO, and CO gas is generated. The CO gas generated on the surface of the melt (molten metal) M takes away the heat of the molten steel and escapes to the exhaust gas system (dust collector side) (for example, "exhaust duct 22" in Fig. 1) (see "during charging of reduced iron" in Fig. 7). Also, by charging the reduced iron R, the surface of the melt (molten metal) M vibrates, and the area of the molten steel in contact with the atmosphere increases, thereby inducing heat loss. Due to heat extraction of CO gas generated by chemical reactions and heat extraction from the molten steel caused by physical vibration of the molten steel surface in this way, the heat acquisition efficiency of the charged reduced iron R is poor.

[0078] Moreover, since CO gas is generated in a situation where there is almost no undissolved solid iron source S around the reduced iron R, the heat of the CO gas itself or the combustion heat (secondary combustion heat) of the CO gas is not utilized for preheating the undissolved solid iron source S.

[0079] When the reduced iron R is charged in a state close to a flat bath in this way, the amount of heat loss increases, and the effect of increasing the thermal efficiency cannot be enjoyed. The charged reduced iron R and the solid iron source S are difficult to melt, leading to an increase in the power input. Therefore, it is desirable to start charging the reduced iron R before the melt (molten metal) M in the tapping hole 40 reaches a state close to a flat bath.

[0080] As described above, an important control factor for increasing the thermal efficiency in the furnace is the "starting timing of charging the reduced iron".

[0081] Here, regarding the situation of the iron source in the furnace (molten state of the iron source in the furnace) at the start of charging the reduced iron, it can be inferred from the type and amount of the iron source charged into the furnace during the "charging period" and the amount of charging power until the start of charging the reduced iron during the "melting period". For example, when the amount of the iron source charged during the "charging period" is small, if the amount of power until the start of charging the reduced iron during the "melting period" is not also reduced, the melt (molten metal) at the bottom of the tapping hole will approach a state similar to a flat bath.

[0082] From these, the "starting timing of charging the reduced iron" is determined by the type and amount of the iron source charged into the furnace during the "charging period" and the amount of charging power until the start of charging the reduced iron during the "melting period".

[0083] By the way, different from scrap, the reduced iron contains iron oxide and consumes energy for the reduction reaction to obtain Fe from the iron oxide. Also, the reduced iron inevitably has voids from its manufacturing process, and these voids cause deterioration of the thermal efficiency. From these facts, it is considered that reduced iron requires more melting energy compared to iron sources other than reduced iron such as scrap.

[0084] Therefore, first, in examining the appropriate timing for starting the charging of reduced iron, in order to investigate the deterioration of the melting energy of reduced iron with respect to other iron sources such as scrap, that is, how much more melting energy of reduced iron is required compared to the melting energy of iron sources other than reduced iron, the following experiment (Experiment 1) was conducted. In Experiment 1, scrap was used as the iron source other than reduced iron.

[0085] (Experiment 1) Molten steel was produced using an arc-type electric furnace with the specifications shown in Table 2. In this experiment, all iron sources were charged into the furnace during the "charging period".

Table 2

[0086] As iron sources, scrap and reduced iron were used. In this experiment, the blending ratio of reduced iron with respect to the iron source was changed. As the reduced iron, HBI (apparent density: 5 - 5.5 ton / m 3 ) was used. The same HBI as shown in Table 1 was also used in this experiment.

[0087] During the "charging period", with the electrodes not energized, using a clam shell type basket, the iron sources were charged into the furnace body without a furnace lid from the top of the furnace. After the "melting period" and after transitioning to the "heating period", flux was added in the usual manner of those skilled in the art, oxidation refining, etc. were carried out, and then tapping was performed.

[0088] Table 3 shows other operating conditions (the blending amounts and ratios of scrap and HBI of the iron source, tapping amount), and also shows the index of the power unit in Table 3.

[0089]

Table 3

[0090] The "index of power consumption per unit" shown in Table 3 was obtained by the following method. The power consumption per unit (kWh / ton of molten steel tapped) for Experiment Nos. 1 to 8 was calculated. This "power consumption per unit" is the amount of electric power consumed to tap 1 ton of molten steel from the electric furnace. In Experiment No. 8 where only scrap was used as the iron source, the average value of the power consumption per unit (kWh / ton of molten steel tapped) was obtained from the power consumption of each charge for 1269 charges of normal operation carried out in the usual manner by those skilled in the art, and this average value was taken as the power consumption per unit for Experiment No. 8. Taking the power consumption per unit of Experiment No. 8 as 1, the values obtained by exponentiating the power consumption per unit of Experiment Nos. 1 to 7 were taken as the "index of power consumption per unit". The larger the "index of power consumption per unit" is than 1, the greater the melting energy is compared to the melting energy of scrap.

[0091] The relationship between the "index of power consumption per unit (kWh / ton of molten steel tapped)" and the "HBI blending ratio" for Experiment Nos. 1 to 8 is shown in Fig. 8. Using the "index of power consumption per unit (kWh / ton of molten steel tapped)" and the "HBI blending ratio" of Experiment Nos. 1 to 8 as explanatory variables, the following quadratic approximation formula was derived. y = 0.18x 2 + 0.38x + 1.00 ···(a) Here, x: HBI blending ratio (of the iron source) y: Index of power consumption per unit (kWh / ton of molten steel tapped) That is. The line of the above quadratic approximation formula ((formula (a))) is shown in Fig. 8.

[0092] From the above quadratic approximation formula ((formula (a))), the index of power consumption per unit when the HBI blending ratio of the iron source is changed can be calculated. The index of power consumption per unit when the HBI blending ratio of the iron source is changed by 10% from 0% was calculated from the above quadratic approximation formula ((formula (a))) and is shown in Table 4.

Table 4

[0093] From Table 4, it can be inferred that when the HBI blending ratio is 100%, the index of the power unit is 1.56. From this, it can be said that 1.56 times the melting energy is required for the melting of HBI compared to scrap.

[0094] In the above experiment, scrap was used as the iron source other than reduced iron. However, iron sources other than reduced iron charged into the arc furnace (for example, those having a higher thermal conductivity than reduced iron and containing iron, such as pig iron, ingots, and iron carbide) are as easy to melt as scrap. Therefore, it is presumed that the same results as the above experimental results can be obtained even when an iron source other than scrap charged into the arc furnace is used instead of scrap. In the above experiment, HBI was used as the reduced iron. However, reduced iron other than HBI is as easy to melt as HBI. Therefore, it is presumed that the same results as the above experimental results can be obtained even when reduced iron other than HBI is used instead of HBI.

[0095] From the above, it can be said that 1.56 times the melting energy is required for the melting of reduced iron compared to the iron source other than the reduced iron charged into the arc furnace.

[0096] As described above, regarding the situation of the iron source in the furnace (the melting state of the iron source in the furnace) at the start of charging reduced iron, it can be inferred from the type and amount of the iron source charged into the furnace during the "charging period" and the amount of electric power input up to the start of charging reduced iron during the "melting period". Also, 1.56 times the melting energy is required for the melting of reduced iron compared to the iron source other than the reduced iron charged into the arc furnace. Based on these, the following formula (b) was found as the formula representing the situation of the iron source in the furnace (the melting state of the iron source in the furnace) at the start of charging reduced iron during the "melting period". E / (A + 1.56×B) ··· (b) Here, A is the amount (ton) of the iron source other than the reduced iron among the iron sources charged into the furnace body before the start of charging reduced iron during the "melting period". B is the amount (ton) of reduced iron contained in the iron source charged into the furnace body before the start of charging reduced iron in the "melting period". E is the amount of electric power (kWh) input to the arc furnace before the start of charging reduced iron in the "melting period".

[0097] In addition, with the quadratic approximation formula (a) when the total amount of the iron source is charged in the "charging period" as the reference line, the exponent of the power consumption per unit derived by substituting the HBI blending ratio into the quadratic approximation formula (a) is positioned as the "exponent of the reference power consumption per unit". When the total amount of HBI is charged in the "charging period", it should be the same exponent of the power consumption per unit as the "exponent of the reference power consumption per unit". However, when HBI is charged into the tapping hole at an appropriate timing in the "melting period", it is considered that the exponent of the power consumption per unit becomes lower than the "exponent of the reference power consumption per unit".

[0098] In order to examine the appropriate start time of charging reduced iron using the above formula (b) and the "exponent of the reference power consumption per unit", the following experiment (Experiment 2) was conducted.

[0099] (Experiment 2) Scrap and HBI were used as the iron source. In the "charging period", with the electrodes not energized, a part of the iron source (a part of HBI and scrap) was charged into the furnace from the furnace top using a clam shell basket into the furnace body without a furnace lid. After all the "charging periods" were completed, the remaining HBI was charged into the tapping hole in the "melting period". After the charging of HBI, the furnace lid was opened to check the inside of the furnace, and it was confirmed that the charged HBI had entered the tapping hole or the melt (molten metal) in the tapping hole. Then, the power supply was started. The blending amount of the iron source, the HBI blending ratio, the charging time of HBI, etc. were changed. For HBI, the same HBI as shown in Table 1 above (apparent density: 5 - 5.5 ton / m 3 ) was used.

[0100] In addition, the charging rate of HBI during the "melting period" was set to a rate approximately corresponding to the power supply capacity of the arc-type electric furnace. During the "melting period", since energy by power supply is consumed for melting the charged HBI during the charging period from the start to the end of HBI charging, the state inside the furnace at the start of HBI charging continues until immediately after the end of charging, and the state inside the furnace at the start of charging and immediately after the end of charging is similar.

[0101] After the "melting period" and after transitioning to the "heating period", fluxes were introduced and oxidation refining etc. were carried out in the usual manner of those skilled in the art, and tapping was performed.

[0102] Table 5 shows the components of the molten steel after tapping.

Table 5

[0103] Table 6 shows other implementation conditions (iron source blending amount, HBI blending ratio, HBI charging timing during energization, etc.). Also, Table 6 shows the value of the following formula (b) and the exponent of the power unit etc. The value of formula (b) represents the state of the iron source inside the furnace (molten state of the iron source inside the furnace) at the start of charging reduced iron during the "melting period" as described above. E / (A + 1.56×B) ··· (b)

[0104]

Table 6

[0105] The "power unit exponent (P)" and the "reference power unit exponent (Q)" shown in Table 6 were obtained by the following method.

[0106] (Power unit exponent (P)) The power units (kWh / tap ton) of experiment numbers 11 to 14 shown in Table 6 were calculated. This "power unit" is the amount of electric power consumed to tap 1 ton of molten steel from the electric furnace. In Experiment 1, taking the power unit of Experiment No. 8 that used only scrap as the iron source as 1, and indexing the power units of Experiment Nos. 11 to 14, the result was defined as the "index of power unit (P)". The "index of power unit (P)" of Experiment Nos. 11 to 14 is greater than 1. This is because scrap and reduced iron were used as the iron sources in Experiment Nos. 11 to 14.

[0107] (Reference index of power unit (Q)) The value obtained by substituting the "HBI blending ratio" in Table 6 into x in the following quadratic approximation formula (a) obtained in Experiment 1 was defined as the "reference index of power unit (Q)". y = 0.18x 2 + 0.38x + 1.00 ···(a) The "reference index of power unit" (Q) is the index of the power unit of the conventional method of charging all the iron sources (scrap and HBI) into the furnace during the "charging period".

[0108] The difference (P - Q) between the "index of actual power unit" (P) and the "reference index of power unit" (Q) was calculated and shown in Table 7. When "P - Q" is less than 1, the power input is less than that of the conventional method of charging all the iron sources (scrap and HBI) into the furnace during the "charging period". When "P - Q" is less than 1, the judgment result was marked as "○", and when "P - Q" is 1 or more, the judgment result was marked as "×". These results are shown in Table 7.

[0109] Figure 9 shows the relationship between the "index of power unit (P)" and the "HBI blending ratio" of Experiment Nos. 11 to 14 using the judgment results ("○", "×") shown in Table 6. The reference line represented by the above quadratic approximation formula (a) is also shown in Figure 9. The reference line is the index of the reference power unit of the conventional method of charging all the iron sources into the furnace during the "charging period". The "○" below the reference line is considered to have improved thermal efficiency because the power input is less than that of the conventional method. The "○" corresponds to Experiment Nos. 11 - 13. The "×" above the reference line is considered not to have improved thermal efficiency because the power input is more than that of the conventional method. The "×" corresponds to Experiment No. 14.

[0110] To investigate the HBI charging start timing that improves the thermal efficiency compared to the conventional method, the relationship between the value of formula (b) and "P - Q" for Experiment Nos. 11 to 14 was examined using the judgment results ("○", "×") shown in Table 6. These relationships are shown in Fig. 10. The value of formula (b) represents the situation of the iron source in the furnace (molten state of the iron source in the furnace) at the start of charging the reduced iron.

[0111] From Fig. 10, when the value of formula (b) is 190 or less, it is considered that "P - Q" becomes less than 1. From this, when starting the charging of reduced iron when the value of formula (b) representing the situation of the iron source in the furnace (molten state of the iron source in the furnace) at the start of charging the reduced iron satisfies 190 or less, it is considered that the thermal efficiency is improved compared to the conventional method and the amount of power input can be reduced.

[0112] From the above, when charging reduced iron into the tapping hole while energizing the electrode during the melting period, start charging the reduced iron when the following formula (1) is satisfied. E / (A + 1.56×B) ≦ 190 ···(1) Here, A is the amount (ton) of the iron source other than the reduced iron among the iron sources charged into the furnace body before the start of charging the reduced iron during the melting period. B is the amount (ton) of the reduced iron contained in the iron source charged into the furnace body before the start of charging the reduced iron during the melting period. E is the amount of power (kWh) input to the arc - type electric furnace before the start of charging the reduced iron during the melting period. When charging reduced iron into the tapping hole in a plurality of melting periods, start charging the reduced iron when the above formula (1) is satisfied in each melting period.

[0113] When the above formula (1) is satisfied, the melt (molten metal) at the bottom of the tapping hole is not in a state close to a flat bath. By starting the charging of reduced iron at the timing when the above formula (1) is satisfied, both the poor heat conduction and the poor heat absorption efficiency of the reduced iron are compensated, heat loss is reduced, and the heat efficiency can be increased. As a result, the melting of the reduced iron charged into the tapping hole proceeds. Also, the melting of the iron source around the tapping hole proceeds. Therefore, an increase in the power input amount can be suppressed.

[0114] Figs. 11 and 12 schematically show examples of the situation of the iron source in the furnace (molten state of the iron source in the furnace). In Figs. 11 and 12, the upper figure is a schematic longitudinal sectional view of the furnace, and the lower figure is a schematic view of the furnace as seen from above.

[0115] "Just before the start of the melting period" shown in Fig. 11 means that there is an undissolved solid iron source S in the furnace. The electrode 13 is not energized. The tapping hole is not formed.

[0116] "Melting period (1)" shown in Fig. 11 is when time has elapsed since the start of the melting period, and is an example when the above formula (1) is satisfied. There is a melt (molten iron source) M at the bottom of the tapping hole 40. A part of the iron source at the bottom of the tapping hole 40 is melted, and the electrode 13 is surrounded by the undissolved solid iron source S on the side of the tapping hole 40. By starting the charging of reduced iron during the melting period (1), the above-described effects can be obtained.

[0117] "Melting period (2)" shown in Fig. 12 is when time has elapsed since the melting period (1), and is an example when the above formula (1) is satisfied. There is a melt (molten iron source) M at the bottom of the tapping hole 40. The iron source at the bottom of the tapping hole 40 is melted, but it is not in a state close to a flat bath. The electrode 13 is surrounded by the undissolved solid iron source S on the side of the tapping hole 40. By starting the charging of reduced iron during the melting period (2), the above-described effects can be obtained.

[0118] The "melting period (3)" shown in FIG. 12 is an example when time has passed since the melting period (2) and the above formula (1) is not satisfied. The boring hole 40 exists, but the molten steel (molten material M) at the bottom of the boring hole 40 is in a state close to the flat bath. The bulk of the undissolved solid iron source S is small, and the electrode 13 is hardly surrounded by the undissolved solid iron source S.

[0119] As described above, the embodiments of the present invention have been described with reference to the drawings. However, the specific configuration should be considered not to be limited to these embodiments. The scope of the present invention is shown not by the above description but by the claims, and includes all changes within the meaning and scope equivalent to the claims. Also, the above experiments do not limit the present invention, and all changes and implementations within the scope not departing from the spirit of this specification are included in the technical scope of the present invention.

[0120] For example, in Experiment 2 above, as shown in Table 6, a part of the reduced iron was charged during the "charging period", and the remaining reduced iron was charged into the boring hole during the "melting period". However, when the iron source charged into the arc-type electric furnace includes a reduced iron and an iron source other than the reduced iron, only the iron source other than the reduced iron may be charged during the "charging period", and the reduced iron may be charged into the boring hole during the "melting period". Even in such a case, the effects of the present invention can be obtained.

[0121] Also, in the "melting period" of FIG. 3, one boring hole is formed in the iron source 30 in the furnace. However, when charging the iron source 30 into the boring hole during the "melting period", two or more boring holes may be formed in the iron source 30 in the furnace.

[0122] Also, the operating conditions and operating conditions of the arc-type electric furnace are not particularly limited, and may be carried out as normally practiced by those skilled in the art.

Explanation of symbols

[0123] 1 Arc-type electric furnace 11 Furnace body 12 Furnace lid 13 Electrode 30 Iron source 21 Shoot 22 Exhaust Duct M Melt (molten iron source) S Undissolved solid iron source R Reduced iron

Claims

【Claim 1】 A method of charging reduced iron into an arc-type electric furnace when producing molten steel using an iron source containing reduced iron, the method comprising: The arc-type electric furnace has a furnace body and at least an electrode installed above the furnace body; After the charging period of charging the iron source into the furnace body when the electrode is not energized, in the melting period of melting the iron source in the furnace body by energizing the electrode, While energizing the electrode, charging reduced iron into a boiling hole formed in the iron source in the furnace body and surrounded by molten iron source and unmolten iron source; In the melting period, starting to charge reduced iron when the following formula (1) is satisfied: E / (A + 1.56 × B) ≦ 190... (1) A method for charging reduced iron into an electric furnace, characterized in that. Here, A is the amount (ton) of the iron source other than reduced iron among the iron sources charged into the furnace body before the start of charging reduced iron in the melting period; B is the amount (ton) of reduced iron contained in the iron source charged into the furnace body before the start of charging reduced iron in the melting period; E is the amount of electric power (kWh) input to the arc-type electric furnace before the start of charging reduced iron in the melting period

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