Electric furnace and electric furnace steelmaking process

Abstract

To provide an electric furnace and an electric furnace steel making method capable of improving a carburization speed while using an inexpensive carbonaceous material and realizing carburization to molten iron without elongating a cycle time.SOLUTION: An electric furnace 1 has a graphite electrode 2 and a carbonaceous material addition device 3. Regarding an intersection point (electrode center point 22) between a vertical line passing through a figure center of gravity of an electrode lower end viewed from below in a vertical direction and a molten iron stationary face, and an arrival position (carbonaceous material arrival point 23) from the carbonaceous material addition device 3 to the molten iron stationary face of a carbonaceous material, a position and an angle of the carbonaceous material addition device are set so that a distance between the carbonaceous material arrival point 23 and the electrode center point 22 falls within a range within an electrode radius r, and further, even when the electric furnace has an oxygen lance 8 feeding oxygen into the furnace, an oxygen arrival point 24 of the oxygen lance 8 is not positioned at a position within the double of the electrode radius r from the electrode center position 22.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to electric furnaces and electric furnace steelmaking processes. [Background technology]

[0002] In electric furnace steelmaking, a carbon source is supplied to the molten iron for melting, degassing, and reduction. This process is called carburization. When producing low-nitrogen high-grade steel, the carbon concentration in the molten iron must be kept high during tapping from the electric furnace, in order to promote decarburization in subsequent processes.

[0003] Known carbon sources used in carburizing molten iron include carbonaceous materials such as coke and graphite (see Patent Document 1), high-carbon alloys such as ferrochromium, ferromanganese, and pig iron (see Patent Document 2), and recarburizers containing these (see Patent Document 3). Patent Document 1 describes a method in which powdered coke, which has been pumped together with oxygen and compressed air, is ejected from a nozzle at the tip of an auxiliary burner, and the unburned powdered coke is used as a recarburizer and oxide reducer for molten steel. Patent Document 2 describes a molten steel production method in which the carbon concentration of molten steel is reduced using a vacuum degassing system, and then a carbon-containing substance is added to the molten steel to adjust the carbon concentration of the molten steel within the composition range. Patent Document 3 describes an electric furnace refining method in which a recarburizer with an apparent density higher than the calculated density of the slag in the electric furnace refining is added to the molten iron.

[0004] In the present invention, the rate at which the carbon concentration in molten iron increases after a carbon material is added to the molten iron during refining in an electric furnace is called the "carburization rate."

[0005] When ordinary carbonaceous materials such as coke are used as a carbon source to be added to molten iron, the carburization rate is slow, which leads to an extension of cycle time and reduced productivity. In addition, high-carbon alloys are expensive, which leads to increased costs.

[0006] Conventionally, known methods for feeding carbonaceous material onto the surface of the molten material in an electric furnace for carburization include a method of blowing the carbonaceous material using a feeding chute or a lance as described in Patent Document 4. In Patent Document 4, the carbonaceous material is added in a direction away from the arc heating region. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 174813 / 1983 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-27128 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-186607 [Patent Document 4] Japanese Patent Publication No. 2020-94247 Summary of the Invention [Problem to be solved by the invention]

[0008] Regardless of which of the above conventional charging methods is used, the carburization rate is determined by the type of carbonaceous material and the temperature of the molten steel, so at present, when using inexpensive carbonaceous materials, it has not yet been possible to improve the carburization rate and shorten the cycle time.

[0009] An object of the present invention is to provide an electric furnace and an electric furnace steelmaking method that can improve the carburization rate while using inexpensive carbonaceous materials and realize carburization of molten iron without extending the cycle time. [Means for solving the problem]

[0010] That is, the gist of the present invention is as follows. [1] Graphite electrodes (However, hollow electrodes are excluded.)the electric furnace using the graphite electrode, wherein the intersection of a vertical line passing through the center of gravity of the figure of the lower end of the graphite electrode as viewed from vertically below and the molten iron stationary surface is called the "electrode center point", the electric furnace has a carbonaceous material adding device, and the position at which the carbonaceous material from the carbonaceous material adding device reaches the molten iron stationary surface is called the "carbonaceous material arrival point", the position and angle of the carbonaceous material adding device are set so that the distance between the carbonaceous material arrival point and the electrode center point is within the range of the electrode radius, and further, if the electric furnace has an oxygen lance for supplying oxygen into the furnace, the axial center of the oxygen lance is not directed to a position within twice the electrode radius from the electrode center point. [2] The electric furnace according to [1], wherein the carbonaceous material adding device is a spray lance or an injection chute. [3] The electric furnace according to [1] or [2], characterized in that the carbonaceous material adding device is capable of changing the adding direction of the carbonaceous material. [ 4 ] The oxygen lance supplies oxygen into the furnace, and is installed so that the axial center of the oxygen lance faces a position at least twice the radius of the electrode from the center point of the electrode [1] to [ 3 ] An electric furnace according to any one of the above.

[0011] [ 5 ]Graphite electrode (However, hollow electrodes are excluded.) an electric furnace steelmaking method for adding carbon to molten iron in an electric furnace using a graphite electrode, wherein the intersection of a vertical line passing through the center of gravity of the figure of the lower end of the graphite electrode as viewed from vertically below and the surface of the molten iron at rest is called the "electrode center point", the electric furnace has a carbonaceous material adding device, which supplies carbonaceous material to a range within a radius equal to the electrode radius from the electrode center point, and if the electric furnace has an oxygen lance for supplying oxygen into the furnace, the electric furnace steelmaking method is characterized in that oxygen is not supplied to a position within twice the electrode radius from the electrode center point. [ 6 ]The carbonaceous material adding device is a spray lance or an injection chute. 5 ] An electric furnace steelmaking method as described above. [ 7 ] The carbonaceous material adding device is characterized in that the adding direction of the carbonaceous material can be changed. 5 ]or[ 6 ] An electric furnace steelmaking method as described above. [ 8 ] The oxygen lance is used to supply oxygen into the furnace together with the addition of carbonaceous material, and oxygen is supplied to a position at least twice the radius of the electrode from the center point of the electrode. 5 ]~[ 7 ] The electric furnace steelmaking method according to any one of the above. [Effects of the Invention]

[0012] By utilizing the high temperature of the arc spot, it is possible to increase the carburizing speed while using inexpensive carbon materials, and to carburize molten iron without extending the cycle time. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing an example of an electric furnace of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the arrangement of a carbonaceous material blasting lance and an oxygen lance and the relationship between the carbonaceous material arrival point and the oxygen arrival point. DETAILED DESCRIPTION OF THE INVENTION

[0014] The inventors came up with the idea of ​​utilizing the high temperature field of the arc spot in an electric furnace in order to realize high-speed carburization of molten iron using inexpensive carbonaceous materials.

[0015] It has been reported that the dissolution of carbonaceous material added to molten iron into the molten iron can be expressed as a first-order reaction equation of carbon concentration, as shown in Equation (1).

number

[0016] Overall reaction rate constant k t is the dissolution reaction rate constant k at the interface between the carbonaceous material and the molten iron rand the mass transfer coefficient k of carbon atoms in molten iron m It has been reported that the dissolution of carbonaceous materials is controlled by a mixture of dissolution and mass transfer at the interface.

number

[0017] Therefore, k r Or k m If we can increase either or both of the overall reaction rate constant k t The dissolution reaction rate constant k r is the activation energy of the reaction, E a and temperature T, it is expressed as equation (3).

number

[0018] Based on the above, the inventor came up with the idea of ​​utilizing the arc spot generated directly below the electrode of an electric furnace as a means of raising the temperature of molten iron at the position where the carbonaceous material is melted. Normally, the temperature of molten iron in an electric furnace is at most about 1700°C, but the temperature inside the arc generated from the electrode to the surface of the molten iron directly below the electrode is over 5000°C, and even the arc spot on the surface of the molten iron is about 2000°C. Therefore, if the carbonaceous material is supplied to the arc spot during carburization, it can be melted at a higher temperature than if it is added to the molten iron other than the arc spot. In this case, for example, the activation energy E a In coke with a melting point of 300-480 kJ / mol, the melting reaction rate constant k ris 11 to 47 times larger, resulting in a significant increase in the carburization rate.

[0019] The present invention will be described below with reference to FIGS. A carbonaceous material addition device 3 can be used to supply the carbonaceous material to the molten iron 20 in the electric furnace 1. The carbonaceous material addition device 3 can be a device that uses a blowing lance 4 to blow the carbonaceous material together with the blowing gas. Other supply methods, such as addition via a charging chute 7, can also be used. The direction of carbonaceous material addition in the carbonaceous material addition device 3 can be fixed or movable (changeable). If the graphite electrode is a hollow electrode, the carbonaceous material can be added via the hollow portion of the hollow electrode instead of or in addition to the carbonaceous material addition device. In either supply method, the carbonaceous material can be supplied to the arc spot below the electrode, thereby utilizing the high-temperature arc spot. While the electric furnace in this embodiment is equipped with one electrode, even in an electric furnace equipped with multiple electrodes, the high-temperature arc spot can be utilized by supplying the carbonaceous material to the arc spot directly below one of the electrodes.

[0020] In the electric furnace using graphite electrodes, the intersection of a vertical line passing through the center of gravity of the lower end of the electrode as viewed from vertically below and the molten iron stationary surface 21 is referred to as the "electrode center point 22" (see FIG. 2(A)). The position where the carbonaceous material reaches the molten iron stationary surface 21 from the carbonaceous material addition device 3 is referred to as the "carbonaceous material arrival point 23." The carbonaceous material arrival point 23 can be determined by adding the carbonaceous material in advance using the carbonaceous material addition device 3. Alternatively, the carbonaceous material arrival point 23 can be determined as the position where the carbonaceous material reaches the molten iron stationary surface 21 based on the carbonaceous material trajectory 25 calculated from the speed of the carbonaceous material at the discharge section of the carbonaceous material addition device 3 (see FIG. 2(A)). To supply the carbonaceous material to the arc spot directly below the electrode, the position and angle of the carbonaceous material addition device 3 need only be set so that the distance between the carbonaceous material arrival point 23 and the electrode center point 22 is within the electrode radius r.

[0021] In electric furnaces, oxygen gas is sometimes supplied to molten iron 20 in the furnace using an oxygen lance 8 for the purpose of oxidation refining, such as desiliconization and dephosphorization. If the supplied oxygen gas reacts with undissolved carbonaceous material, it will hinder carburization. Therefore, the carbonaceous material to be carburized and the oxygen gas must be supplied at positions separated from each other. In the present invention, even if an electric furnace has an oxygen lance 8 for supplying oxygen into the furnace, the axial center 26 of the oxygen lance 8 is not oriented within twice the electrode radius r from the electrode center point 22 (see FIG. 2(B)). When an oxygen lance 8 for supplying oxygen into the furnace is provided, the axial center 26 of the oxygen lance 8 is installed at a position at least twice the electrode radius r from the electrode center point 22. This allows the carbonaceous material to be carburized and the oxygen gas to be supplied at positions separated from each other. Hereinafter, the direction in which the axial center 26 of the oxygen lance 8 is directed on the molten iron stationary surface 21 is also referred to as the "oxygen arrival point 24." [Example]

[0022] The following table shows an example of the present invention implemented in an electric furnace 1 having an inner diameter of 6.5 m, a tapping capacity of 105 t, and 15 t of seed molten metal left in the furnace during tapping, along with a comparative example. The bath depth of molten steel in the electric furnace 1 is 1250 mm when the amount of molten steel in the furnace is 120 t, and 400 mm when the amount of molten steel in the furnace is 15 t, and it fluctuates within this range as the raw materials are melted and the steel is tapped. The graphite electrode 2 is provided as a single graphite electrode having a diameter of 24 inches, i.e., 609.6 mm (radius r is 304.8 mm).

[0023] The electric furnace 1 has, as the carbonaceous material addition device 3, a feeding chute 7 for feeding only the carbonaceous material, three spray lances 4 (fixed lances 5) fixed to the wall that can supply the carbonaceous material into the furnace (fixed lance A (5A), fixed lance B (5B), fixed lance C (5C)), and a manipulator 11 equipped with a movable spray lance 4 (movable lance 6). All of the spray lances 4 use argon gas as the carrier gas when supplying the carbonaceous material. Of these, the three spray lances 4 (fixed lances 5) fixed to the wall are designed so that their position and angle cannot be changed.

[0024] The blowing lance 4 (fixed lance 5) fixed to the wall has a nozzle for supplying carbonaceous material and a nozzle for supplying oxygen, and the two nozzles are installed with their axial centers spaced 51 mm apart. The nozzle portion for supplying carbonaceous material functions as three blowing lances 4 (fixed lances 5) fixed to the wall (fixed lance A, fixed lance B, fixed lance C). In addition, the nozzle portion for supplying oxygen functions as three oxygen lances 8 (fixed oxygen lance 9) (fixed oxygen lance A, fixed oxygen lance B, fixed oxygen lance C) that supply oxygen into the furnace.

[0025] The manipulator 11 has the movable blast lance 4 (movable lance 6 that supplies carbonaceous material) as well as an oxygen lance 8 (movable oxygen lance 10). The movable lance 6 and the movable oxygen lance 10 can supply carbonaceous material and oxygen into the furnace independently, respectively.

[0026] The charging chute 7 can change the charging direction, and the distance between the carbonaceous material arrival point 23 of the carbonaceous material charged from the charging chute 7 and the electrode center point 22 can be changed. The manipulator 11 can also change the direction of the movable lance 6 (carbonaceous material blowing) and the movable oxygen lance 10.

[0027] Of the fixed spray lances 4, one (fixed lance A (5A)) is installed at a position and angle that allows carbonaceous material to be supplied so that the carbonaceous material arrival point 23 (the position where the carbonaceous material from the carbonaceous material addition device 3 reaches the molten iron stationary surface 21) is 100 mm horizontally from the electrode center point 22 (the intersection of the vertical line passing through the center of gravity of the figure of the lower end of the electrode when viewed from vertically below and the molten iron stationary surface 21) on the molten iron stationary surface 21 when the bath depth of the molten iron is 1250 mm, and the other two (fixed lance B (5B) and fixed lance C (5C)) are installed at a position and angle that allows carbonaceous material to be supplied so that the carbonaceous material arrival point 23 is 635 mm from the electrode center point 22. As described above, the fixed lance 5 (for blowing carbonaceous material) and the fixed oxygen lance 9 are installed with their axial centers spaced 51 mm apart, and the direction in which the axial center 26 of the fixed oxygen lance 9 faces (oxygen arrival point 24) is such that, on the molten iron stationary surface 21, fixed oxygen lance A (9A) is 250 mm from the electrode center point 22, and fixed oxygen lance B (9B) and fixed oxygen lance C (9C) are 785 mm from the electrode center point 22. Therefore, when carbonaceous material and oxygen are blown simultaneously from the fixed lance 5 and fixed oxygen lance 9 at the same position, the carbonaceous material arrival point 23 and the oxygen arrival point 24 (oxygen supply position) are 150 mm apart.

[0028] In both the present invention and comparative examples, scrap was used as the raw material, and anthracite was used as the carbonaceous material. When the blowing lance 4 (fixed lance 5, movable lance 6) was used to supply the carbonaceous material, the initial velocity of the carbonaceous material at the nozzle outlet was set to an average of 70 m / s. When oxygen was supplied from the oxygen lance 8, the linear flow velocity of the oxygen jet was set to 500 m / s.

[0029] In the test, the scrap was first melted by applying current. Then, while continuing the current, carbonaceous material was added from the blast lance 4 at a total rate of 40 kg / min for 15 minutes, or carbonaceous material was added from the addition chute 7. The carbon concentration of metal samples taken before and after the carbonaceous material addition was evaluated. Sampling to analyze the [C] concentration in the molten steel before and after the carbonaceous material addition was performed 1 minute before the carbonaceous material addition began and 1 minute after the carbonaceous material addition was completed. Note that for all examples shown in Table 1, the carbonaceous material addition began when the bath depth of the molten iron reached 1100 mm. Therefore, the molten iron surface rose 150 mm during the carbonaceous material addition, and if the carbonaceous material addition position was not changed, the distance from the center of the electrode to the carbonaceous material arrival point also changed. Five minutes after the end of the carbonaceous material addition, the molten steel was tapped from the electric furnace. Assuming that all of the carbonaceous material added here was carbon, a total of 600 kg of carbon was added to the molten iron. In this test, the molten iron was evaluated as being well carburized when 50% or more of the supplied carbon was carburized into the molten iron, i.e., when the [C] concentration in the molten iron increased by 0.25% or more by mass after the supply of carbon material.

[0030] Electric furnace steelmaking was carried out under the carbonaceous material supply conditions and oxygen blowing conditions shown in Table 1. The distance between the electrode center point 22 and the carbonaceous material arrival point 23 is shown in the "carbonaceous material-electrode distance," and the distance between the electrode center point 22 and the oxygen arrival point 24 is shown in the "oxygen-electrode distance." The results are shown for bath depths of 1100 mm and 1250 mm. Table 1 shows the results. When the carbonaceous material arrival point 23 and the blowing lance 4, and the oxygen arrival point 24 and the oxygen lance 8 are located on different sides of the electrode center point 22, an "*" is added to the right end of the value in Table 1. In Table 1, values ​​outside the range of the present invention are underlined.

[0031] [Table 1]

[0032] Inventive Examples 1 to 7 in Table 1 are inventive examples. In Invention Examples 1 to 4, oxygen was not blown, and the carbonaceous material-electrode distance was within the graphite electrode radius r (304.8 mm), satisfying the conditions of the present invention, and the carburization state was good. In Invention Examples 3 and 4, a manipulator 11 was used to add the carbonaceous material, and of these, in Invention Example 3 the supply position was fixed, while in Invention Example 4 the movable lance 6 of the manipulator 11 was operated so that the carbonaceous material was supplied to the electrode center point 22 in accordance with the rise of the molten iron level during operation, regardless of the bath depth of the molten iron (*2 in Table 1). In Examples 5 to 8 of the present invention, oxygen was blown, and the carbonaceous material-electrode distance was within the graphite electrode radius r (304.8 mm) in all cases, and the oxygen-electrode distance exceeded twice the graphite electrode radius r (609.6 mm), satisfying the conditions of the present invention and resulting in good carburization.

[0033] Comparative Examples 1 to 7 in Table 1 are comparative examples. In Comparative Examples 1 to 3, oxygen was not blown and the carbonaceous material-electrode distance exceeded the graphite electrode radius r (304.8 mm), which did not satisfy the conditions of the present invention, and the carburization state was poor. Comparative Examples 4 to 7 were cases with oxygen blowing, and although the carbonaceous material-electrode distance in Comparative Examples 4 to 6 was within the graphite electrode radius r (304.8 mm), the oxygen-electrode distance was within twice the graphite electrode radius r (609.6 mm), which did not satisfy the conditions of the present invention and the carburization state was poor. In Comparative Example 7, neither the carbonaceous material-electrode distance nor the oxygen-electrode distance satisfied the conditions of the present invention, and the carburization state was poor. [Explanation of symbols]

[0034] 1 electric furnace 2. Graphite electrodes 3 Carbon material addition device 4 Spray Lance 5 Fixed Lance 5A Fixed Lance A 5B Fixed Lance B 5C Fixed Lance C 6 Movable Lance 7. Throw-in shot 8 Oxygen Lance 9 Fixed Oxygen Lance 9A Fixed oxygen lance A 9B Fixed oxygen lance B 9C Fixed Oxygen Lance C 10 Movable oxygen lance 11 Manipulator 20 Molten Iron 21 Molten iron stationary surface 22 Electrode center point 23 Carbon material reaching point 24 Oxygen Reach Point 25 The trajectory of carbon 26 axis center r electrode radius

Claims

1. In an electric furnace using graphite electrodes (excluding hollow electrodes), the intersection point between a vertical line passing through the center of gravity of the figure of the lower end of the graphite electrode when viewed from vertically below and the molten iron stationary surface is called the "electrode center point", the electric furnace has a carbonaceous material adding device, and the position at which the carbonaceous material reaches the molten iron stationary surface from the carbonaceous material adding device is called the "carbonaceous material arrival point", the position and angle of the carbonaceous material adding device are set so that the distance between the carbonaceous material arrival point and the electrode center point is within the electrode radius, and further, if the electric furnace has an oxygen lance for supplying oxygen into the furnace, the axial center of the oxygen lance is not directed to a position within twice the electrode radius from the electrode center point.

2. 2. The electric furnace according to claim 1, wherein the carbonaceous material adding device is a blowing lance or an input chute.

3. 3. The electric furnace according to claim 1, wherein the carbonaceous material adding device is capable of changing the direction in which the carbonaceous material is added.

4. 4. The electric furnace according to claim 1, further comprising an oxygen lance for supplying oxygen into the furnace, the oxygen lance being installed such that its axial center faces a position at a distance of at least twice the electrode radius from the center point of the electrode.

5. An electric furnace steelmaking method in which molten iron is carburized in an electric furnace using graphite electrodes (excluding hollow electrodes), wherein the intersection of a vertical line passing through the center of gravity of the graphite electrode at the bottom end as viewed vertically from below and the molten iron stationary surface is called the "electrode center point", and the electric furnace has a carbonaceous material adding device which supplies carbonaceous material to a range within a radius equal to the electrode radius from the electrode center point, and when the electric furnace has an oxygen lance which supplies oxygen into the furnace, the electric furnace does not supply oxygen to a position within twice the electrode radius from the electrode center point.

6. 6. The electric furnace steelmaking method according to claim 5, wherein the carbonaceous material adding device is a blowing lance or an injection chute.

7. 7. The electric furnace steelmaking method according to claim 5, wherein the carbonaceous material adding device is capable of changing the direction in which the carbonaceous material is added.

8. The electric furnace steelmaking method according to any one of claims 5 to 7, characterized in that oxygen is supplied into the furnace using the oxygen lance together with the addition of carbonaceous material, and oxygen is supplied to a position at least twice the electrode radius away from the center point of the electrode.

Images