JP2024131663A - Molten steel manufacturing method and electric arc furnace - Google Patents

Abstract

To disclose a method for producing molten steel that can efficiently produce a target reaction when an oxygen jet is injected into molten iron in an arc furnace and an auxiliary raw material is supplied.SOLUTION: Disclosed is a method for producing molten steel by using an arc furnace provided with oxygen feeding means and auxiliary raw material supplying means. The method comprises injecting an oxygen jet from the oxygen feeding means to molten iron in the arc furnace and supplying an auxiliary raw material from the auxiliary raw material supplying means toward a position P on the surface of the molten iron. The position P is outside an impact surface between the oxygen jet and the molten iron.SELECTED DRAWING: Figure 3

Description

This application discloses a method for producing molten steel using an arc furnace and the arc furnace.

Patent Document 1 discloses a technique in which an immersion lance is immersed in molten iron in an arc furnace, carbonaceous material is injected from the immersion lance, and oxygen is injected into the slag formed on the surface of the molten iron. In Patent Document 1, the carbon concentration in the molten iron is increased in an arc furnace, and the molten iron is then tapped from the arc furnace and refined in a converter to produce molten steel. In the refining in the converter, for example, CO gas and the like are generated by a decarburization reaction using an oxygen jet, and the CO gas and the like promotes denitrification.

On the other hand, it is also possible to carry out the above-mentioned denitrification in an electric arc furnace. For example, it is thought that the above-mentioned denitrification is possible by simultaneously supplying oxygen and carbonaceous material to the molten iron in an electric arc furnace. However, at present, sufficient research has not been conducted on denitrification in an electric arc furnace. In addition, in electric arc furnaces, oxygen and carbonaceous material may be simultaneously injected, mainly for the purpose of promoting slag foaming. In this case, it is common to reliably generate CO gas in the slag by merging the oxygen jet with the carbonaceous material and supplying carbon and oxygen to one location of the molten iron, thereby stabilizing the foaming state of the slag.

It is also possible to simultaneously supply oxygen and auxiliary materials other than carbonaceous materials to the molten iron in the arc furnace. Examples of auxiliary materials other than carbonaceous materials include Ca-containing materials (lime, etc.), Si-containing materials (quartz sand, etc.), Al-containing materials (calcium aluminate, etc.), Mg-containing materials (magnesia, etc.), etc.

JP 2016-145393 A

According to the findings of the present inventors, when auxiliary materials are supplied to the molten iron in an arc furnace so as to be entrained together with an oxygen jet, the efficiency of the target reaction is likely to decrease. For example, when denitrifying molten iron in an arc furnace, if carbonaceous material is merged with the oxygen jet and carbon and oxygen are supplied intensively to one location of the molten iron, sufficient denitrification efficiency cannot be obtained.

ここで、
ｒは、前記酸素噴流の中心軸と前記溶鉄の表面との交点Ｏから、前記位置Ｐまでの距離（ｍ）であり、
Ｗ_ｍは、前記溶鉄の重量（ｔ）である。
＜態様４＞
溶鉄を処理するアーク炉であって、少なくとも１つの送酸手段と、少なくとも１つの副原料供給手段とを備え、
前記送酸手段は、前記アーク炉内の前記溶鉄の表面に向かって酸素噴流を噴射するように構成され、
前記副原料供給手段は、前記アーク炉内の前記溶鉄の表面の位置Ｐに向けて副原料を供給するように構成され、かつ、前記位置Ｐが前記酸素噴流と前記溶鉄との衝突面の外側の位置となるように構成される、
アーク炉。 The present application discloses the following aspects as one of the means for solving the above problems.
<Aspect 1>
A method for producing molten steel using an arc furnace equipped with an oxygen supplying means and an auxiliary raw material supplying means,
Injecting an oxygen jet from the oxygen supplying means onto the molten iron in the arc furnace, and supplying an auxiliary material from the auxiliary material supplying means toward a position P on the surface of the molten iron,
The position P is a position outside the collision surface between the oxygen jet and the molten iron.
A method for producing molten steel.
<Aspect 2>
The auxiliary material is a carbonaceous material.
2. A method for producing molten steel according to embodiment 1.
<Aspect 3>
The following relationship (1) is satisfied:
The method for producing molten steel according to aspect 1 or 2.

Where:
r is the distance (m) from an intersection O between the central axis of the oxygen jet and the surface of the molten iron to the position P,
_Wm is the weight of the molten iron (t).
<Aspect 4>
An electric arc furnace for treating molten iron, comprising at least one oxygen supplying means and at least one auxiliary raw material supplying means,
The oxygen supplying means is configured to inject an oxygen jet toward a surface of the molten iron in the arc furnace,
The auxiliary material supplying means is configured to supply the auxiliary material toward a position P on the surface of the molten iron in the arc furnace, and is configured so that the position P is located outside a collision surface between the oxygen jet and the molten iron.
Arc furnace.

According to the molten steel manufacturing method and arc furnace disclosed herein, when an oxygen jet is injected onto the molten iron in the arc furnace and an auxiliary raw material is supplied, the target reaction can be efficiently caused. For example, when an oxygen jet is injected onto the molten iron and a carbonaceous material is supplied as an auxiliary raw material, the molten iron can be efficiently denitrified.

2 shows a schematic diagram of an example of the positional relationship of each member when the arc furnace is viewed from above. 1 is a schematic diagram showing an example of the positional relationship between an oxygen supplying means and an auxiliary raw material supplying means when the arc furnace is viewed from the side. The upper electrode and the like are omitted. 1 shows an example of a positional relationship between an oxygen jet and a secondary raw material supply position.

1. Manufacturing method of molten steel The manufacturing method of molten steel of the present disclosure will be described with reference to the drawings. However, the manufacturing method of molten steel of the present disclosure is not limited to the form shown in the drawings. As shown in Figs. 1 to 3, the manufacturing method of molten steel according to one embodiment is a method of manufacturing molten steel using an arc furnace 100 equipped with an oxygen supplying means 20 and an auxiliary raw material supplying means 30, and includes injecting an oxygen jet 21 from the oxygen supplying means 20 to molten iron 10 in the arc furnace 100, and supplying an auxiliary raw material 31 from the auxiliary raw material supplying means 30 toward a position P on the surface of the molten iron 10. Here, the position P is a position outside the collision surface between the oxygen jet 21 and the molten iron 10.

1.1 Molten iron The molten iron 10 is obtained, for example, by generating an arc between the upper electrode 40 and the lower electrode 50 in an arc furnace 100 to melt an iron source. The iron source may include at least one solid iron source such as scrap, reduced iron, pig iron, and granulated pig iron, and may be molten iron or molten steel produced in another melting furnace or refining furnace. The molten iron 10 may include various elements other than iron. The composition of the elements other than iron depends on the type of iron source. For example, the molten iron 10 before the auxiliary raw material 31 is supplied may include C in an amount of 0.02% by mass to 3.0% by mass, N in an amount of 0.005% by mass to 0.030% by mass, and P in an amount of 0.003% by mass to 0.1% by mass. In particular, in the manufacturing method disclosed herein, when a carbonaceous material is supplied to the molten iron 10 as the auxiliary material 31 while the oxygen jet 21 is injected to cause a decarburization and denitrification reaction, a more remarkable effect is obtained in a low carbon region where the solute carbon movement in the molten iron 10 is the rate-limiting factor in the decarburization reaction. In the manufacturing method disclosed herein, when a carbonaceous material is supplied to the molten iron 10 as the auxiliary material 31, the molten iron 10 before the auxiliary material 31 is supplied may contain 0.02 mass% to 3.0 mass% C, 0.005 mass% to 0.030 mass% N, or 0.003 mass% to 0.1 mass% P. Alternatively, in the manufacturing method disclosed herein, when an auxiliary material other than a carbonaceous material is supplied to the molten iron 10 as the auxiliary material 31, it is preferable to avoid carrying out the method in an extremely low carbon concentration region where the decarburization reaction is difficult in terms of equilibrium, and for example, it is preferable that the molten iron 10 before the auxiliary material 31 is supplied contains 0.3 mass% or more C. More specifically, in the manufacturing method of the present disclosure, when a secondary raw material other than a carbon material is supplied to the molten iron 10 as the secondary raw material 31, the molten iron 10 before the secondary raw material 31 is supplied may contain 0.3 mass% or more and 3.0 mass% or less of C, 0.010 mass% or more and 0.030 mass% or less of N, and 0.003 mass% or more and 0.1 mass% or less of P. The density of the molten iron 10 may be, for example, 6600 kg/m3 ^or more and 7000 kg/m3 ^or less.

1.2 Oxygen supplying means The arc furnace 100 includes at least one oxygen supplying means 20. The oxygen supplying means 20 injects an oxygen jet 21 into the molten iron 10 in the arc furnace 100. The oxygen supplying means 20 may be a lance. The number of oxygen jets 21 injected from one lance is not particularly limited. For example, the lance may be a single-hole lance as shown in Figs. 2 and 3. The lance may also be straight, may have a Laval structure, or may include a coherent burner in which gaseous fuel and a combustion-supporting gas are injected so as to surround the oxygen jet. As shown in Fig. 1, the oxygen supplying means 20 may be at least one of a lance (so-called main lance) inserted from the furnace cover of the arc furnace 100, a wall lance provided on the furnace wall, and a variable lance positioned by a manipulator or the like. As shown in Figs. 2 and 3, in the arc furnace 100, oxygen is blown upward from the oxygen supplying means 20 toward the surface 10x of the molten iron 10. This allows various chemical reactions, such as decarburization, denitrification, and dephosphorization, to occur in the molten iron 10 depending on the type of auxiliary raw material 31 described below.

The shape of the oxygen jet 21 sprayed from the oxygen supply means 20 depends on the inclination of the oxygen supply means 20 and the shape of the injection hole of the oxygen supply means 20. The direction of the oxygen jet 21 sprayed from the oxygen supply means 20 may be inclined with respect to the vertical direction. That is, as shown in FIG. 3, the direction of the oxygen jet 21 sprayed from the oxygen supply means 20 may be inclined by an angle θ with respect to the vertical direction. The inclination angle θ may be specified as the angle between the central axis of the oxygen supply means 20 and a line parallel to the vertical direction. The inclination angle θ may be, for example, 5° or more and 30° or less. Also, as shown in FIG. 3, the oxygen jet 21 may be sprayed from the injection hole of the oxygen supply means 20 to the molten iron 10 with a certain spread angle α. The spread angle α depends on the shape of the injection hole of the oxygen supply means 20. The spread angle α may be, for example, 10° or more and 13° or less. Also, as shown in FIG. 3, a certain height h may be provided from the injection hole of the oxygen supplying means 20 to the surface 10x of the molten iron 10. When the oxygen supplying means 20 is tilted with respect to the vertical direction, the height h refers to the distance from the upper end of the injection hole of the oxygen supplying means 20 to the surface 10x of the molten iron 10. The height h may be, for example, 0.2 m or more and 0.8 m or less. Also, as shown in FIG. 3, the oxygen supplying means 20 may have an injection hole with a hole diameter d. The hole diameter d refers to the circle equivalent diameter of the injection hole. The hole diameter d may be, for example, 20 mm or more and 100 mm or less.

The flow rate of the oxygen jet 21 sprayed from one oxygen supplying means 20 is not particularly limited and may be, for example, 1000 ^Nm3 /h or more and 4000 ^Nm3 /h or less. In addition, the flow velocity of the oxygen jet 21 sprayed from the oxygen supplying means 20 (the flow velocity at the injection hole of the oxygen supplying means 20, that is, the flow velocity at the central axis) is not particularly limited and may be, for example, 10 m/s or more and 300 m/s or less.

1.3 Auxiliary material supply means The arc furnace 100 includes at least one auxiliary material supply means 30. The auxiliary material supply means 30 supplies the auxiliary material toward the position P of the surface of the molten iron 10 in the arc furnace 100. The auxiliary material can be supplied into the furnace, for example, through a supply port provided in the arc furnace 100. The supply port may be provided in any part of the furnace. For example, the supply port may be a hole provided in the furnace inner wall (side wall) or a hole provided in the furnace cover. As shown in FIG. 3, the supply port may be installed above the surface 10x of the molten iron 10. The number of supply ports may be one or more.

There is no particular limitation on the method of supplying the auxiliary material by the auxiliary material supplying means 30. For example, a method of supplying the auxiliary material 31 through a hole provided in the inner wall of the furnace as shown in FIG. 2(A), and a method of supplying the auxiliary material 31 through a hole provided in the furnace cover as shown in FIG. 2(B) can be mentioned. Depending on the positional relationship between the hole provided in the inner wall or the furnace cover and the supply position P of the auxiliary material 31 on the surface 10x of the molten iron 10, a lance, a charging chute, etc. may be appropriately adopted. As the charging chute, a known one may be adopted. When a lance is adopted as the auxiliary material supplying means 30, the auxiliary material 31 may be supplied to the molten iron 10 together with the top blown gas. In this case, the top blown gas from the auxiliary material supplying means 30 may be one that does not generate a hot spot on the surface of the molten iron 10, or one that generates a hot spot. For example, from the viewpoint of cost, it is preferable to use one or both of air and _N2 gas, and from the viewpoint of low nitrogen, it is preferable to use at least one selected from pure oxygen, Ar gas, and _CO2 gas, and when the auxiliary raw material is a carbon material or a deoxidizer, it is preferable to use one or both of Ar gas and _CO2 gas, which have lower reactivity, in order to avoid consumption in the air before reaching the iron bath. In this way, the top blown gas may be selected according to the production conditions, and may be a mixture of at least two or more types of gases in a predetermined ratio within the range of these operational constraints. The lance as the oxygen supplying means 20 and the lance as the auxiliary raw material supplying means 30 are separate from each other.

The auxiliary raw material 31 may be supplied vertically downward or obliquely downward to the molten iron 10. In either case, the auxiliary raw material supplying means 30 supplies the auxiliary raw material toward a predetermined position P. Here, "toward position P" means that the target position for supplying the auxiliary raw material 31 is position P, and a part of the auxiliary raw material 31 may be supplied to a part other than position P by diffusion or the like. For example, a part of the auxiliary raw material 31 may be supplied inside the collision surface between the oxygen jet 21 and the molten iron 10. In the manufacturing method disclosed herein, it is preferable that the majority of the auxiliary raw material 31, for example 50 mass% or more, 70 mass% or more, or 90 mass% or more, is supplied to position P outside the collision surface between the oxygen jet 21 and the molten iron 10.

There is no particular limit to the amount of auxiliary raw material 31 supplied from the auxiliary raw material supply means 30. For example, the amount of auxiliary raw material 31 supplied from one auxiliary raw material supply means 30 may be 10 kg/min or more and 100 kg/min or less.

There is no particular limit to the type of auxiliary material 31 supplied by the auxiliary material supply means 30. The auxiliary material 31 may be, for example, a carbonaceous material, or an auxiliary material other than carbonaceous material. The auxiliary material other than carbonaceous material may be at least one selected from Ca-containing materials (lime, etc.), Si-containing materials (quartz sand, etc.), Al-containing materials (calcium aluminate, etc.), Mg-containing materials (magnesia, etc.), etc. The shape of the auxiliary material 31 may be a shape that can be appropriately supplied from the auxiliary material supply means 30 to the molten iron 10, and may be various shapes such as powder, granules, and lumps. The auxiliary material 31 may also be a pressure-molded product. It may also be a mixture of multiple auxiliary materials 31. The auxiliary material 31 may have a particle size of, for example, 0.1 mm or more and 5 mm or less. If the auxiliary material 31 is large, it is easy to clog the powder transport system provided in general steelmaking equipment, and the specific surface area is reduced, which deteriorates heat transfer, and the time it remains unmelted on the molten iron 10 may be long. On the other hand, if the auxiliary raw material 31 is small, it will be highly likely to scatter in the furnace and be easily sucked into the exhaust gas system, which may result in a poor yield.

In the manufacturing method disclosed herein, the auxiliary raw material 31 is supplied toward a predetermined position P outside the fire point, thereby reducing the amount of auxiliary raw material 31 supplied to the fire point and suppressing a drop in temperature at the fire point. This allows the desired reaction to occur efficiently. In addition, in the manufacturing method disclosed herein, when the auxiliary raw material 31 is a carbonaceous material, a decrease in yield caused by the carbonaceous material burning in the oxygen jet 21 is suppressed, and further, the carbonization rate in the molten iron 10 is improved by supplying the carbonaceous material toward a predetermined position P outside the fire point, and denitrification efficiency can be significantly improved.

1.4 Position P
As shown in Figures 2 and 3, the position P is a position outside the collision surface between the oxygen jet 21 and the molten iron 10. Preferably, as shown in Figure 2, the auxiliary material 31 is supplied to the position P without crossing the oxygen jet 21 (in other words, it is preferable that the oxygen jet 21 does not exist between the auxiliary material 31 and the molten iron 10 from the auxiliary material supply means 30). The "collision surface between the oxygen jet 21 and the molten iron 10" can be geometrically specified from the above-mentioned inclination angle θ, spread angle α, height h, and hole diameter d. For example, as shown in Figure 3, the distance z from the intersection O between the central axis of the oxygen supply means 20 and the surface 10x of the molten iron 10 to the outer edge X of the collision surface can be specified as z = (tan (θ + α) - tan θ) h + d / (2 cos θ). 3, if the distance r from the intersection O between the central axis of the oxygen jet 21 and the surface 10x of the molten iron 10 to the position P is greater than the above-mentioned distance z, the position P can be outside the collision surface between the oxygen jet 21 and the molten iron 10. That is, in the manufacturing method disclosed herein, the distance r from the intersection O between the central axis of the oxygen jet 21 and the surface 10x of the molten iron 10 to the position P may satisfy the relationship r>(tan(θ+α)-tan θ)h+d/(2cos θ).

As described above, the position P may be any position outside the collision surface between the oxygen jet 21 and the molten iron 10, and there is no particular limit to the upper limit of the distance r. However, in the manufacturing method disclosed herein, by making the distance r shorter than the representative length calculated based on the weight of the molten iron 10, the movement of the auxiliary material 31 to the ignition point is less likely to be delayed, the auxiliary material concentration near the ignition point is improved, and the target reaction can be more efficiently caused. Consider the case where the auxiliary material 31 is a carbonaceous material. In this case, when the decarburization of the molten iron 10 progresses due to the oxygen jet 21 and the carbon concentration of the molten iron 10 falls below the critical carbon concentration, the consumption rate of carbon in the molten iron 10 by the decarburization reaction exceeds the supply rate of dissolved carbon to the reaction site, so that the ratio of oxygen supplied by the oxygen supply that contributes to decarburization (decarbonation oxygen efficiency) gradually decreases, and the CO generation rate at the ignition point decreases, which may be unfavorable for denitrification. In addition, there is a risk that the Fe in the molten iron 10 will be over-oxidized, resulting in a decrease in iron yield and an increase in the FeO concentration in the slag, which may damage the refractory material of the arc furnace 100. In response to this, by making the distance r shorter than the representative length calculated based on the weight of the molten iron 10 and increasing the dissolved carbon concentration near the fire point, the activity of dissolved [N] will increase, further improving the denitrification efficiency, and further suppressing the over-oxidation of Fe.

From the above viewpoint, the position P for supplying the auxiliary raw material 31 may be determined by referring to the representative length of the iron bath calculated from the weight of the molten iron 10 in each charge and the value of the stirring power density. For example, according to the well-known document "Asai et al.: Tetsu to Hagane, 68 (1982), vol. 3, pp. 426-434," the representative flow rate in the inertial region of the fluid is proportional to the 1/3 power of the stirring power density. Therefore, if the distance r from the intersection point O to the position P is sufficiently short compared to this representative flow rate, the delay in supplying the auxiliary raw material 31 to the fire point is alleviated. For example, if the auxiliary raw material 31 is a carbonaceous material, the rate of CO gas generation due to the decarburization reaction is maintained high, which is favorable for denitrification. That is, for example, for a furnace in which top-blowing and bottom-blowing stirring are dominant, the upper limit of the distance r can be determined so as to satisfy the following relationship:

ここで、Ｌ：代表長（ｍ）、ε_ａ：合計攪拌動力密度（Ｗ／ｔｏｎ）、Ｗ_ｍ：溶鉄の重量（ｔ）、ρ_ｌ：溶鉄の密度（ｔ／ｍ^３）、ε_Ｔ：上吹き攪拌動力密度（Ｗ／ｔｏｎ）、ε_Ｂ：底吹き攪拌動力密度（Ｗ／ｔｏｎ）である。

Here, L is characteristic length (m), ε _a is total stirring power density (W/ton), W _m is weight of molten iron (t), ρ _l is density of molten iron (t/m ³ ), ε _T is top blowing stirring power density (W/ton), and ε _B is bottom blowing stirring power density (W/ton).

The stirring power density can be determined by linear summing the values for top and bottom blowing, etc., by referring to known literature such as Kai et al.: Tetsu-to-Haganen, vol. 69 (1983), pp. 228-237 and Mori et al.: Tetsu-to-Haganen, vol. 67 (1981), pp. 672-695. The means of stirring is not limited to gas supply, and any means can be treated in the same way. The stirring power density of stirring means with different positions and principles does not have to be a linear sum, and each can be treated as having an independent contribution rate.

According to the inventor's findings, in the manufacturing method disclosed herein, by satisfying the following relationship (1), the movement of the auxiliary raw material 31 to the fire point is less likely to be delayed, the concentration of the auxiliary raw material near the fire point is improved, and the desired reaction is more likely to occur efficiently.

ここで、
ｒは、酸素噴流２１の中心軸と溶鉄１０の表面１０ｘとの交点Ｏから、上記の位置Ｐまでの距離（ｍ）であり、
Ｗ_ｍは、溶鉄１０の重量（ｔ）である。

Where:
r is the distance (m) from the intersection O between the central axis of the oxygen jet 21 and the surface 10x of the molten iron 10 to the position P,
W _m is the weight (t) of the molten iron 10.

1.5 Other configurations of the arc furnace As described above, the arc furnace 100 has a melting furnace for melting the iron source. The melting furnace is a portion that can be defined by a furnace cover, a furnace inner wall, and a furnace bottom. The planar shape of the melting furnace may have a circular portion as shown in FIG. 1. The melting furnace may have a constant bath depth and a constant furnace diameter. The bath depth and furnace diameter of the melting furnace are not particularly limited.

The arc furnace 100 may have an upper electrode 40 and a lower electrode 50. When the arc furnace 100 is of a direct current type, the upper electrode 40 may be a cathode and the lower electrode 50 may be an anode. Alternatively, the arc furnace 100 may be of an alternating current type. The upper electrode 40 is installed so as to be inserted into the furnace through the furnace cover. The lower electrode 50 is installed at the bottom of the furnace. The number of upper electrodes 40 and lower electrodes 50 is at least one each. The positions of the upper electrode 40 and the lower electrode 50 are not particularly limited. For example, when the shape of the molten metal surface in the melting furnace is approximately circular when viewed from above (planar view), the center position of the circle may coincide with the central axis of one upper electrode 40 or one lower electrode 50. Alternatively, when viewed from above, multiple upper electrodes 40 and multiple lower electrodes 50 may be arranged around the center position of the circle. In the arc furnace 100, for example, power is supplied from a power supply unit (not shown) to the upper electrode 40 and the lower electrode 50, generating an arc between the upper electrode 40 and the lower electrode 50. The power supply unit may be a general one that supplies power to the upper electrode 40 and the lower electrode 50. The power supplied from the power supply unit to the electrodes is not particularly limited as long as it can generate an arc between the electrodes.

As shown in FIG. 1, the arc furnace 100 may include an iron source charging means 60 for charging an iron source into the melting furnace. The arc furnace 100 may also include a slag removal door 70 for removing slag and the like that has formed on the surface of the molten iron 10. Furthermore, the arc furnace 100 may also include an eccentric hearth tap 80 for tapping the molten iron 10 or molten steel. All of these may be of a known configuration.

The arc furnace 100 may be equipped with various control units. The control unit may, for example, control the supply position P of the auxiliary material 31 supplied from the auxiliary material supply means 30 to the molten iron 10 according to the position of the oxygen jet 21 injected from the oxygen supply means 20 to the molten iron 10, or may control the position of the oxygen jet 21 injected from the oxygen supply means 20 to the molten iron 10 according to the supply position P of the auxiliary material 31 supplied from the auxiliary material supply means 30 to the molten iron 10, or may control both the position of the oxygen jet 21 injected from the oxygen supply means 20 to the molten iron 10 and the supply position P of the auxiliary material 31 supplied from the auxiliary material supply means 30 to the molten iron 10. The control unit may be capable of executing the above control, and may have a known configuration for enabling the control. For example, the control unit may be equipped with a CPU, RAM, ROM, etc.

1.6 Molten Steel The composition of the molten steel produced by the method of the present disclosure is not particularly limited. In the production method of the present disclosure, as described above, the oxygen jet 21 is sprayed onto the molten iron 10 and the auxiliary raw material 31 is supplied, and the molten iron 10 can be decarburized, denitrified, dephosphorized, etc. depending on the type of the auxiliary raw material 31. The molten steel produced by the method of the present disclosure may contain, for example, 0.01 mass% to 3.0 mass% C, 0.002 mass% to 0.030 mass% N, or 0.003 mass% to 0.1 mass% P. The molten steel in the arc furnace 100 may be tapped, for example, through the eccentric hearth tap hole 80 described above. The tapped molten steel may be further refined or may be directly subjected to continuous casting or the like.

2. Arc Furnace The technology disclosed herein has an aspect as an arc furnace in addition to the aspect as a manufacturing method of molten steel as described above. That is, as shown in Figs. 1 to 3, an arc furnace 100 according to one embodiment processes molten iron 10 and includes at least one oxygen supplying means 20 and at least one auxiliary material supplying means 30. Here, the oxygen supplying means 20 is configured to inject an oxygen jet 21 toward the surface 10x of the molten iron 10 in the arc furnace 100. The auxiliary material supplying means 30 is configured to supply an auxiliary material 31 toward a position P of the surface 10x of the molten iron 10 in the arc furnace 100, and is configured so that the position P is located outside the collision surface between the oxygen jet 21 and the molten iron 10. Details of the oxygen supplying means 20 and the auxiliary material supplying means 30 are as described above. The oxygen supplying means 20 and the auxiliary material supplying means 30 may be controlled, for example, by the above-mentioned control unit. That is, the arc furnace 100 according to one embodiment further includes a control unit, which may control one or both of the oxygen supplying means 20 and the auxiliary raw material supplying means so that the oxygen supplying means 20 injects an oxygen jet 21 into the molten iron 10 in the arc furnace 100, and the auxiliary raw material supplying means 30 supplies auxiliary raw materials 31 toward a predetermined position P on the surface of the molten iron 10.

3. Effects and Benefits The effects and benefits of the method for producing molten steel according to the present disclosure will be further supplemented below, focusing on the case where a carbonaceous material is used as the auxiliary raw material 31.

As described above, when carbonaceous materials are used as the auxiliary raw material, denitrification can occur along with oxygen-flow decarbonization. Here, the factors governing the rate of adsorption and denitrification during general oxygen-flow decarbonization include at least the following:
These factors are thought to include (1) the rate of CO generation at the fire point, (2) the temperature of the C-O reaction site at the fire point, and (3) the area of the gas-liquid reaction interface between the CO bubbles and the molten iron. Other factors include the air entrained by the oxygen jet (soft blowing increases the amount of air entrained to the fire point), and the concentration of surface active components in the molten iron (high [S] and [O] decrease the reaction rate).

Conventionally, oxygen jets and carbonaceous materials have been simultaneously injected into electric arc furnaces mainly for the purpose of promoting slag foaming, but in this case, both are injected into the same position on the molten iron surface. In other words, by merging the carbonaceous materials with the oxygen jets and intensively supplying carbon and oxygen to one location on the molten iron surface, CO gas is reliably generated in the slag and the foaming state of the slag is stabilized. However, when the carbonaceous materials and oxygen are injected into the same position on the molten iron surface, from the viewpoint of the adsorption and denitrification reaction,
(1) The CO reaction site is dispersed not only at the fire point but also in the slag or air, so the rate of CO generation at the fire point decreases.
(2) Since the carbonaceous material is close to room temperature and is directly charged to the fire point, the temperature of the C-O reaction site at the fire point also decreases.
(3) The C-O reaction sites are dispersed not only at the hot points but also in the slag or air, reducing the amount of decarburization from the molten iron and reducing the gas-liquid reaction interface area between the CO bubbles and the molten iron. As described above, the conventional method leads to a decrease in the denitrification reaction rate.

In contrast, according to the method for producing molten steel of the present disclosure, when the oxygen jet 21 is injected into the molten iron 10, the carbonaceous material as the auxiliary raw material 31 is supplied toward the position P outside the collision surface between the oxygen jet 21 and the molten iron 10 (i.e., outside the ignition point range).
(1) C-O reaction sites are concentrated at the fire point, increasing the rate of CO generation at the fire point;
(2) Carbonaceous material close to room temperature is added outside the range of the ignition point, suppressing the temperature drop of the C-O reaction site at the ignition point;
(3) The CO reaction sites are concentrated at the hot points, the amount of carbon removed from the molten iron increases, and the gas-liquid reaction interface area between the CO bubbles and the molten iron increases, which is thought to increase the denitrification reaction rate.

The effect of suppressing the drop in the ignition point temperature is exerted regardless of the type of auxiliary raw material 31. In other words, according to the manufacturing method disclosed herein, regardless of the type of auxiliary raw material 31, the amount of auxiliary raw material 31 supplied directly to the ignition point can be reduced, the temperature of the ignition point is less likely to drop, and the desired reaction can be efficiently caused to occur.

The present invention will be further explained below with reference to examples, but the present invention is not limited to the following examples. The present invention allows various conditions to be adopted as long as the purpose is achieved without departing from the gist of the invention.

In this example, molten steel was produced in an electric arc furnace having the following configurations (1) to (6) using scrap as a cold iron source.
(1) The diameter of the melting furnace shell is 7 m.
(2) The maximum amount of molten iron that can be processed at one time is 200 tons.
(3) It is a three-phase AC type that uses three upper graphite electrodes.
(4) Two wall lances for supplying oxygen and two wall lances for supplying powder are provided and fixed to the furnace wall.
(5) A manipulator (movable arm) is provided that can be inserted into the furnace from outside the furnace, and the manipulator is provided with a variable lance for supplying oxygen and a variable lance for supplying powder.
(6) The hearth is equipped with three bottom-blowing tuyeres.

In this embodiment, a conventional electric furnace operation method was adopted. The oxygen supply and auxiliary materials were each supplied from one location, with the respective supply amounts being 1,000 to 4,000 ^Nm3 /h and 0.01 to 0.10 t/min. The lances used, the supply positions of the auxiliary materials, and the ranges of the supply amounts were appropriately changed.

Specifically, in each charge, after visually confirming that all the charged cold iron source had melted through the slag removal port on the side of the furnace body, carbonaceous material was supplied as an auxiliary material from a fixed powder lance installed on the furnace inner wall, and oxygen gas was simultaneously supplied from a movable oxygen supply lance inserted into the furnace through the slag removal port. The range of the fire point was geometrically determined from the relationship shown in Figure 3, and the charging position and blowing angle of the movable oxygen supply lance were sequentially adjusted so that the difference Δr between the distance r from the intersection O of the central axis of the oxygen supply lance and the surface of the molten iron to the auxiliary material supply target position P and the distance z from the intersection O to the outer edge of the fire point range was a predetermined value. When Δr is positive, the supply target position P is outside the fire point range, and when it is negative, the supply target position P is inside the fire point range. In this embodiment, the smaller Δr is, the more it is necessary to lay the movable lance down (the angle θ is increased), and as a result, the distance z tends to be larger. In addition, the smaller the weight of the molten iron, the lower the molten iron surface, the larger the height h, and the larger the distance z. Sampling was carried out before and after the start of oxygen and coal feeding and was subjected to chemical analysis, and the superiority or inferiority of each level was evaluated based on the nitrogen concentration change Δ[N] within the same charge obtained by the analysis. The evaluation indexes are as follows.
◎:Δ[N]≦-30ppm
○: -30ppm<Δ[N]≦-20ppm
△: -20ppm<Δ[N]≦-10ppm
×: -10ppm<Δ[N]

The test conditions and evaluation results for Δ[N] for each of the examples and comparative examples are shown in Table 1. In Table 1, "distance A" is a value calculated by the following formula using the weight W _m (t) of molten iron.

In this embodiment and comparative example, the auxiliary materials were blown in from the fixed wall lance and oxygen was fed from the movable lance inserted from the slag removal port. However, even when oxygen was fed from the wall lance and the auxiliary materials were supplied from the movable lance for each level, no significant difference in Δ[N] was observed, and the results were similar to those shown in Table 1 above. In addition, the amount of molten iron was changed by adjusting the amount of cold iron source charged. In this case, the arrangement of the refractories was changed to adjust so that the geometric positional relationship between the fixed wall lance and the molten metal surface would not change.

At each level, the results of the first sampling were [C] = 0.48-0.52%, and the results of the second sampling were [C] = 0.04-0.06%, so the amount of decarburization derived from the iron bath was roughly the same. The results of the first sampling for [N] were [N] = 0.0070-0.0075%. When carbonaceous material was sprayed as an auxiliary material, the carbon source was continuously supplied from the outside, so the desired effect was achieved regardless of the amount of carbon derived from the iron bath. On the other hand, in Examples 1 and 2 and Comparative Examples 1 and 2, something other than carbonaceous material was used as an auxiliary material, so it is thought that only the decarburization reaction derived from the iron bath contributed to denitrification.

From the results shown in Table 1, in an arc furnace, when an oxygen jet is injected from an oxygen supplying means into the molten iron in the furnace and an auxiliary material is supplied from an auxiliary material supplying means toward position P on the surface of the molten iron, it can be said that molten steel with a lower nitrogen concentration can be produced when position P is outside the fire point range, i.e., outside the collision surface between the oxygen jet and the molten iron (Examples 1 to 16), than when it is inside (Comparative Examples 1 to 3). Also, from a comparison of Examples 1 and 2 with Examples 3 to 16, it can be said that molten steel with an even lower nitrogen concentration can be produced when the auxiliary material is carbonaceous material. Furthermore, from the results of Examples 3 to 16, it can be said that molten steel with an even lower nitrogen concentration can be produced when the following relationship (1) is satisfied.

ここで、
ｒは、酸素噴流の中心軸と溶鉄の表面との交点Ｏから、上記の位置Ｐまでの距離（ｍ）であり、
Ｗ_ｍは、溶鉄の重量（ｔ）である。

Where:
r is the distance (m) from the intersection O between the central axis of the oxygen jet and the surface of the molten iron to the position P,
W _m is the weight of molten iron (t).

100 Arc furnace 10 Molten iron 10x Surface of molten iron (melt surface)
20: Oxygen supply means 21: Oxygen jet 30: Auxiliary material supply means 31: Auxiliary material 40: Upper electrode 50: Lower electrode 60: Iron source charging means 70: Slag removal door 80: Eccentric furnace bottom tapping port

Claims (4)

A method for producing molten steel using an arc furnace equipped with an oxygen supplying means and an auxiliary raw material supplying means,
Injecting an oxygen jet from the oxygen supplying means onto the molten iron in the arc furnace, and supplying an auxiliary material from the auxiliary material supplying means toward a position P on the surface of the molten iron,
The position P is a position outside the collision surface between the oxygen jet and the molten iron.
A method for producing molten steel. The auxiliary material is a carbonaceous material.
The method for producing molten steel according to claim 1 . The following relationship (1) is satisfied:
The method for producing molten steel according to claim 1 or 2.

Where:
r is the distance (m) from an intersection O between the central axis of the oxygen jet and the surface of the molten iron to the position P,
_Wm is the weight of the molten iron (t). An electric arc furnace for treating molten iron, comprising at least one oxygen supplying means and at least one auxiliary raw material supplying means,
The oxygen supplying means is configured to inject an oxygen jet toward a surface of the molten iron in the arc furnace,
The auxiliary material supplying means is configured to supply the auxiliary material toward a position P on the surface of the molten iron in the arc furnace, and is configured so that the position P is located outside a collision surface between the oxygen jet and the molten iron.
Arc furnace.

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