JP7653022B2 - Electric furnace and electric furnace steelmaking process - Google Patents

Description

The present invention relates to electric furnaces and electric furnace steelmaking methods.

In electric furnace steelmaking, which uses an electric furnace, carburization is required to supply a carbon source to the molten iron to melt it and for the purposes of melting the raw materials, degassing, and reduction treatment. In particular, when producing low-nitrogen high-grade steel, it is necessary to maintain a high carbon concentration in the molten iron when it is tapped from the electric furnace by carburization in order to promote denitrification through decarburization in the subsequent process.

It is known that carbon sources for carburizing molten iron include carbonaceous materials such as coke and graphite as in Patent Document 1, high-carbon alloys such as ferrochrome, ferromanganese, and pig iron as in Patent Document 2, and recarburizers containing these as in Patent Document 3. Patent Document 1 describes a method in which powdered coke that 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 for molten steel and as a reducing agent for oxides. Patent Document 2 describes a molten steel melting method in which the carbon concentration in molten steel is reduced in a vacuum degassing facility, and then a carbon-containing substance is added to the molten steel to adjust the carbon concentration of the molten steel to 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 electric furnace refining is added to the molten iron.

In this invention, the rate at which the carbon concentration in molten iron increases after carbon material is added to the molten iron being refining in an electric furnace is called the "carburization rate."

When using ordinary carbonaceous materials such as coke as a carbon source to be added to molten iron, the carburization speed 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.

Conventionally, known methods of supplying carbonaceous material to be added to the surface of the molten material in an electric furnace for carburization include, for example, a method of blowing the carbonaceous material using a feed 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 area.

Japanese Unexamined Patent Publication No. 174813/1983 JP 2003-27128 A JP 2017-186607 A JP 2020-94247 A

Regardless of which of the above conventional charging methods is used, the carburizing speed is determined by the type of carbon material and the temperature of the molten steel, so when using inexpensive carbon materials, it is currently not possible to improve the carburizing speed or shorten the cycle time.

The present invention aims to provide an electric furnace and an electric furnace steelmaking method that can improve the carburization speed while using inexpensive carbonaceous materials and realize carburization of molten iron without extending the cycle time.

That is, the gist of the present invention is as follows.
[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, 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 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], characterized in that 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 electric furnace according to any one of [1] to [3], characterized in that the oxygen lance that supplies oxygen into the furnace is installed such that the axial center of the oxygen lance faces a position at least twice the electrode radius away from the center point of the electrode .

[ 5 ] An electric furnace steelmaking method in which molten iron is carburized in an electric furnace using a graphite electrode (excluding hollow electrodes) , wherein an intersection point between a vertical line passing through the center of gravity of the graphite electrode at the bottom end as viewed vertically from below and a stationary surface of the molten iron is called an "electrode center point", the electric furnace has a carbonaceous material adding device, the carbonaceous material is supplied from the electric furnace to a range within a radius equal to the electrode radius by the carbonaceous material adding device, and when an oxygen lance for supplying oxygen into the furnace is provided, oxygen is not supplied to a position within twice the electrode radius from the electrode center point.
[ 6 ] The electric furnace steelmaking method according to [ 5 ], characterized in that the carbonaceous material adding device is a blowing lance or an injection chute.
[ 7 ] The electric furnace steelmaking method according to [ 5 ] or [ 6 ], characterized in that the carbonaceous material adding device is capable of changing the adding direction of the carbonaceous material.
[ 8 ] The electric furnace steelmaking method according to any one of [ 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 .

By utilizing the high temperature of the arc spot, it is possible to increase the carburizing speed while using inexpensive carbon material, and to carburize molten iron without extending the cycle time.

FIG. 1 is a diagram showing an example of an electric furnace of the present invention. FIG. 2 is a schematic diagram showing the arrangement of a carbonaceous material spraying lance and an oxygen lance and the relationship between the carbonaceous material arrival point and the oxygen arrival point.

The inventor came up with the idea of utilizing the high temperature field of the arc spot in an electric furnace to achieve high-speed carburization of molten iron using inexpensive carbonaceous material.

ここでｔ：時間、Ａ：反応界面積、Ｖ：溶鉄体積、ｋ_ｔ：総括反応速度定数、[Ｃ]_ｓ：飽和炭素濃度、[Ｃ]_ｂ：溶鉄の炭素濃度である。 It has been reported that the dissolution of a carbonaceous material added to molten iron into the molten iron is expressed as a first-order reaction equation of the carbon concentration, as shown in formula (1).

Here, t is time, A is the reaction interfacial area, V is the volume of molten iron, _kt is the overall reaction rate constant, [C] _s is the saturated carbon concentration, and [C] _b is the carbon concentration of the molten iron.

The overall reaction rate constant _kt is expressed as in formula (2) using the dissolution reaction rate constant _kr at the interface between the carbonaceous material and molten iron and the mass transfer coefficient _km of carbon atoms in the molten iron. It has been reported that the dissolution of carbonaceous materials is a mixed rate-determining factor of dissolution and mass transfer at the interface.

ここでＡ：原子の衝突頻度に関する係数である。式（３）より温度Ｔを高くすることでｋ_ｒが増大する。また、ｋ_ｍも高温ほど大きくなることが知られているため、温度Ｔを高くすることによりｋ_ｔを増大せしめる。 Therefore, if either or both of _kr and _km can be increased, the overall reaction rate constant _kt also increases, making it possible to increase the reaction rate of formula (1). Of these, the dissolution reaction rate constant _kr is expressed as formula (3) using the reaction activation energy _Ea and temperature T.

Here, A is a coefficient related to the frequency of atomic collisions. From formula (3), _kr increases by increasing the temperature T. It is also known that _km increases with increasing temperature, so _kt increases by increasing the temperature T.

Based on the above, the inventor came up with the idea of utilizing the arc spot generated just below the electrode of an electric furnace as a means for increasing the temperature of the molten iron at the position where the carbonaceous material is melted. Normally, the temperature of the 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 just below the electrode is 5000°C or higher, 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 is possible to melt the carbonaceous material at a higher temperature than when it is supplied to the molten iron other than the arc spot. In this case, for example, in the case of coke with an activation energy _Ea of about 300 to 480 kJ/mol, when the molten iron temperature is 2000°C compared to 1700°C, the melting reaction rate constant _kr becomes 11 to 47 times larger, resulting in a significant increase in the carburization rate.

The present invention will be described below with reference to FIGS.
A carbonaceous material adding device 3 can be used as a method for supplying carbonaceous material to the molten iron 20 in the electric furnace 1. As the carbonaceous material adding device 3, a method of blowing the carbonaceous material together with the blowing gas using a blowing lance 4 is considered. In addition, other supplying methods such as addition by a charging chute 7 can also be used. The direction of addition of the carbonaceous material in the carbonaceous material adding device 3 can be fixed or movable (changeable). When the graphite electrode is a hollow electrode, the carbonaceous material can be added via the hollow part of the hollow electrode instead of or together with the carbonaceous material adding device. In either supplying method, the high-temperature arc spot can be utilized by designing the supply position of the carbonaceous material to be the arc spot at the bottom of the electrode. In this embodiment, the electric furnace is equipped with one electrode, but even in an electric furnace equipped with multiple electrodes, it is possible to supply the carbonaceous material to the arc spot directly under one of the electrodes to utilize the high-temperature arc spot.

In the following, in an electric furnace using graphite electrodes, the intersection point between the vertical line passing through the center of gravity of the figure of the lower end of the electrode as viewed vertically from below and the molten iron stationary surface 21 is called the "electrode center point 22" (see FIG. 2(A)). Also, the arrival position of the carbonaceous material from the carbonaceous material adding device 3 on the molten iron stationary surface 21 is called the "carbonaceous material arrival point 23". The position of the carbonaceous material arrival point 23 can be determined by adding the carbonaceous material in advance using the carbonaceous material adding device 3. Alternatively, it can be determined as the arrival position of the carbonaceous material on the molten iron stationary surface 21 based on the trajectory 25 of the carbonaceous material calculated from the speed of the carbonaceous material at the discharge part of the carbonaceous material adding device 3 (see FIG. 2(A)). In order to supply the carbonaceous material to the arc spot directly below the electrode, it is sufficient that the position and angle of the carbonaceous material adding device 3 are set so that the distance between the carbonaceous material arrival point 23 and the electrode center point 22 is within the range of the electrode radius r.

In an electric furnace, oxygen gas may be supplied to the 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, it is necessary to supply the carbonaceous material to be carburized and the oxygen gas to a position away from each other. In the present invention, when the electric furnace has an oxygen lance 8 that supplies oxygen into the furnace, in other words, even when the electric furnace has an oxygen lance 8 that supplies oxygen into the furnace, the axial center 26 of the oxygen lance 8 is not directed to a position within twice the electrode radius r from the electrode center point 22 (see FIG. 2 (B)). When the oxygen lance 8 that supplies oxygen into the furnace has an oxygen lance 8, the axial center 26 of the oxygen lance 8 is installed to face a position more than 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 to a position away from each other. Hereinafter, the direction in which the axial center 26 of the oxygen lance 8 is facing on the molten iron stationary surface 21 is also referred to as the "oxygen arrival point 24."

The following table shows an example of the present invention implemented in an electric furnace 1 with an inner diameter of 6.5 m, a tapping capacity of 105 t, and 15 t of seed molten metal left in the furnace when 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 increases or decreases within this range as the raw materials are melted and the steel is tapped. The graphite electrode 2 is one graphite electrode with a diameter of 24 inches, i.e., 609.6 mm (radius r is 304.8 mm).

The electric furnace 1 has, as the carbonaceous material adding device 3, a charging chute 7 for charging only carbonaceous material, three spray lances 4 (fixed lances 5) fixed to the wall (fixed lance A (5A), fixed lance B (5B), fixed lance C (5C)) that can supply carbonaceous material into the furnace, 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.

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

The manipulator 11 has the movable spray 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 independently into the furnace, respectively.

The feeding chute 7 can change the feeding direction, and the distance between the carbonaceous material arrival point 23 of the carbonaceous material fed from the feeding 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 spraying) and the movable oxygen lance 10.

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 centre point 22 (the intersection of the molten iron stationary surface 21 and the vertical line passing through the centre of gravity of the figure of the lower end of the electrode when viewed vertically from below) 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), 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 centre 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 250 mm from the electrode center point 22 for fixed oxygen lance A (9A), and 785 mm from the electrode center point 22 for fixed oxygen lance B (9B) and fixed oxygen lance C (9C) on the molten iron stationary surface 21. Therefore, when blowing carbonaceous material and oxygen 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.

In both the present invention and comparative examples, scrap was used as the raw material, and anthracite was used as the carbonaceous material. In addition, in both cases where 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. In addition, when oxygen was supplied from the oxygen lance 8, the linear flow velocity of the oxygen jet was set to 500 m/s.

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

Electric furnace steelmaking was carried out using 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 "carbonaceous material-electrode distance," and the distance between the electrode center point 22 and the oxygen arrival point 24 is shown in "oxygen-electrode distance." The results are shown for bath depths of 1100 mm and 1250 mm. Table 1 shows the results. Note that 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, "*" is added to the right end of the values in Table 1. In Table 1, values outside the range of the present invention are underlined.

Examples 1 to 7 in Table 1 are examples of the present invention.
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) in all cases, satisfying the conditions of the invention and resulting in good carburization. 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.

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) in all cases, thereby failing to satisfy the conditions of the present invention and resulting in poor carburization.
Comparative Examples 4 to 7 are cases where oxygen was blown, 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 carburizing 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 carburizing state was poor.

Reference Signs List 1 Electric furnace 2 Graphite electrode 3 Carbon material adding device 4 Spray lance 5 Fixed lance 5A Fixed lance A
5B Fixed Lance B
5C Fixed Lance C
6 Movable lance 7 Input chute 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 arrival point 24 Oxygen arrival point 25 Carbon material trajectory 26 Shaft center r Electrode radius

Claims (8)

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. An electric furnace according to claim 1, characterized in that the carbonaceous material addition device is a spray lance or an injection chute. An electric furnace according to claim 1 or 2, characterized in that the carbonaceous material addition device is capable of changing the direction in which the carbonaceous material is added. 4. The electric furnace according to claim 1, further comprising an oxygen lance for supplying oxygen into the furnace, the oxygen lance being installed with its axial center facing a position at a distance of at least twice the electrode radius from the center point of the electrode. 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. The electric furnace steelmaking method according to claim 5 , wherein the carbonaceous material adding device is a blowing lance or an injection chute. 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. 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.

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