JPH06147758A - Method of operating dc arc furnace and profile of furnace body - Google Patents

Abstract

PURPOSE:To prevent the direct impact of heat gas, produced by an arc, against refractories and reduce the affection of heat to the refractories and a lower electrode as much as possible by a method wherein an arc voltage and an arc current under melting period are selected so that the remaining depth of molten metal immediately below an upper electrode becomes deeper than the depth (no) of recess due to arc jet, which is obtained by a specified formula. CONSTITUTION:When a DC arc furnace, having a lower electrode on a furnace wall or a hearth, is operated, an arc voltage and an arc current under melting period are selected so that the remaining depth of molten metal immediately below an upper electrode 3 becomes deeper than the depth (no) of recess due to arc jet which is obtained by a formula I. In this case, (no) in the formula I is the depth of a recess, (h) is the length of arc from the surface of the molten metal, PL is the specific gravity of molten steel, (g) is gravity and T is the power of an arc jet, which is obtained by a formula II. On the other hand, Mo in the formula II is a permeability in vacuum, I is current, Re is the cathode radius of the upper electrode and Rk is the radius of an arc pillar. According to this method, the possibility of direct collision of heat gas, produced by the arc, against the refractories of a hearth and the possibility of damaging of the refractories are eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for operating a DC arc furnace having a lower electrode on a furnace wall or a furnace bottom, and a furnace body profile.

[0002]

2. Description of the Related Art Usually, the operation of an electric furnace is carried out as follows. That is, after charging the material into the furnace, an arc is generated from the upper electrode, the material is melted by the high heat from the arc, and the upper electrode is gradually lowered. Eventually, the arc reaches the hearth where the molten metal that has been melted by that time has accumulated. The period up to this point is called the bowling period, which is usually about two to three minutes.
18 of "Industrial Heating / Japan Industrial Furnace Association Issue 2 Volume 3"
According to the page, this period is generally less than the melting period that follows, both voltage and current, to protect the hearth refractory from arc spots.

However, when the voltage and the current are reduced in this way, the input power is reduced, which causes a problem that the melting time is increased. Further, the voltage and current values selected here are obtained from the operator's many years of experience and achievements, and are not versatile. Furthermore, the operation is performed at the maximum voltage and maximum current immediately after the boring period and the melting period, but at this point, the amount of molten metal stored in the hearth is not so large, so the hearth refractory Is vulnerable to direct damage by hot gas from the arc and is subject to wear. In the case of the DC arc furnace, the arc jet force generated by the arc is much larger than that in the AC arc furnace, and the thermal effect on the refractory material is remarkable. In particular, when the lower electrode has a lower electrode, the life of the lower electrode may be shortened due to a great influence of heat on the lower electrode.

[0004]

SUMMARY OF THE INVENTION Therefore, the present invention provides a quantitative operation method and furnace body profile for preventing direct impact of hot gas on a refractory by an arc and minimizing thermal influence on the refractory and the lower electrode. It is intended to be provided to.

In recent years, the operation of residual hot water in an electric furnace is becoming common, and the amount of molten metal immediately below the upper electrode is relatively large even in the boring period, but the normal amount of residual hot water is about 10% of the amount of hot water discharged, Very few from the perspective of refractory protection.
Further, this residual hot water operation is carried out in order to prevent the slag in the furnace from being discharged together with the molten metal when the hot water is discharged from the bottom of the furnace, and is not carried out to protect the refractory, but has a different purpose.

[0006]

In order to solve the above problems, the present invention provides (1) a method of operating a DC arc furnace having a lower electrode on a furnace wall or a furnace bottom, in which the residual molten metal immediately below the upper electrode The arc voltage and arc current in the melting period are selected so that the depth of the molten metal is equal to or more than the depth n ₀ of the depression due to the arc jet obtained by the following formula.

[Equation 4] It is characterized by Where n _{0 is} the depth of the depression, h is the arc length from the molten metal surface (h = Va / Ep; Va is the arc voltage, Ep is the potential gradient), ρ _{L is the} specific gravity of the molten steel, and g is the gravitational acceleration. Yes, T is the force of the arc jet determined by the following equation.

[0007]

[Equation 5] Here, μ ₀ : magnetic permeability in vacuum, I: current, Rc: cathode radius of upper electrode, Rk: radius of arc column.

(2) In a furnace profile of a DC arc furnace having a lower electrode on the wall or bottom of the furnace, the depth of molten metal remaining immediately below the upper electrode at the beginning of operation is determined by the arc voltage and arc current during the melting period. The furnace body profile is such that the depth of the arc jet is n ₀ or more determined by the following equation.

[Equation 6] It is characterized by that. The arc generated from the upper electrode becomes an arc jet and collides with the molten metal surface. The force removes the molten metal and creates a large depression just below the upper electrode.

Bowman pays attention to the fact that the arc jet is very similar to the nozzle jet, and applies the force of the arc jet to the equation of the depression depth obtained by the nozzle jet to present the following equation. is doing.

[Equation 7] Where n _{0 is} the depth of the depression, h is the arc length from the molten metal surface (h = Va / Ep; Va is the arc voltage, Ep is the potential gradient), ρ _{L is the} specific gravity of the molten steel, and g is the gravitational acceleration. Yes, T
Is the force of the arc jet obtained by the following formula.

[0010]

[Equation 8] Here, μ ₀ : magnetic permeability in vacuum, I: current, Rc: cathode radius of upper electrode, Rk: radius of arc column.

The present inventors have paid attention to the application of the above formula to the protection measure of the bottom electrode of the furnace and succeeded in deriving a new effect of extending the life of the bottom electrode of the electric furnace. That is, from this equation, the depth of the residual molten metal under the upper electrode is n
_{If the} arc voltage and arc current are selected to be ₀ or more, it is possible to prevent the hot gas from directly hitting the refractory by the arc.

Further, the depth n _{0 of the} recess is calculated based on the maximum arc voltage and the maximum arc current used for operation, and molten metal having a depth of the recess n ₀ or more is introduced into the furnace from the beginning of the boring period. With the furnace body profile that can be stored, there is no risk that the hot gas from the arc directly collides with the hearth refractory and damages the refractory. In any case, since guidelines for hearth refractory protection can be obtained quantitatively and with versatility, even beginners can obtain the optimum operating method or furnace body profile without relying on the experience and results of operators. You can

[0013]

EXAMPLES The present invention will be further described below based on examples. 1 and 2 are examples of a furnace body profile of a 100 ton DC arc furnace according to the present invention. FIG. 1 shows a furnace body side view and FIG. 2 shows a furnace body plan view.
The maximum voltage used in the melting period is 720 in a 100 ton furnace
Assuming that V and the maximum current are 90 kA, calculation is performed using the above formula for obtaining the depression depth n ₀ by the arc jet,
n ₀ is about 340 mm. In order to secure a deeper molten metal depth from the beginning of the boring period, the amount of residual hot water is required to be about 30 tons. This is 30% of the amount of tapped steel. At this time, the ratio H / D between the depth (H) of the molten metal and the surface diameter (D) of the molten metal was 0.3.

In FIG. 1, the solid line indicates the remaining hot water amount of about 30.
The molten metal surface level when the ton is secured is shown. The one-dot chain line shows the total molten metal surface level including the amount of residual hot water after the material is completely dissolved. In a normal furnace body profile with a shallow depth of molten metal, it is necessary to leave a large amount of molten metal in the furnace in order to secure the amount of residual molten metal equal to or greater than the depth of the depression. Therefore, for the furnace body profile, H / D
It is better to choose a deep furnace body profile of about 0.3.

FIG. 3 shows a comparison between the calculated value and the actual value of the depression depth by the arc jet. this is,
This is an example in which the present inventors photographed the arc behavior in an actual DC arc furnace with a high-speed video, actually measured the depth of the depression by the arc jet, and compared it with the calculated value. The calculated value and the actual value agree relatively well.

[0016]

As described above, by setting the depth of the residual molten metal immediately below the upper electrode to be not less than the depression depth n ₀ by the arc jet obtained from the arc voltage and the arc current during the melting period, the heat generated by the arc is reduced. There is no risk of the gas colliding directly with the hearth refractory and damaging it. Therefore, the life of the refractory is improved and the amount of the repair material used is reduced, so that the cost can be reduced. Further, since the operation can be performed at the maximum voltage and the maximum current from the beginning of the boring period, the melting time can be shortened. As a result, cost reduction effects such as increased production and fixed costs can be enjoyed.

[Brief description of drawings]

FIG. 1 is a side view of a DC arc furnace body.

FIG. 2 is a plan view of a furnace body of a DC arc furnace.

FIG. 3 is a diagram showing a comparison between a calculated value and a measured value of a depression depth by an arc jet.

[Explanation of symbols] 1 electric furnace body 2 refractory 3 upper electrode 4 arc H depth of molten metal D surface diameter of molten metal

Claims (2)

[Claims] 1. A method of operating a DC arc furnace having a lower electrode on the wall or bottom of a furnace, wherein the depth of molten metal remaining immediately below the upper electrode is a depth n _{0 of an} arc jet obtained by an arc jet obtained by the following equation. A method for operating a DC arc furnace, comprising selecting the arc voltage and arc current in the melting period as described above. [Equation 1] Here, n _{0 is} the depth of the depression, h is the arc length from the molten metal surface, ρ _{L is the} specific gravity of the molten steel, g is the gravitational acceleration, and T is the force of the arc jet obtained by the following formula. [Equation 2] Here, μ ₀ : magnetic permeability in vacuum, I: current, Rc: cathode radius of upper electrode, Rk: radius of arc column. 2. In a furnace body profile of a DC arc furnace having a lower electrode on the wall or bottom of the furnace, the depth of molten metal remaining immediately below the upper electrode at the beginning of operation is the maximum arc voltage and maximum arc current during the melting period. A furnace body profile of a DC arc furnace, characterized in that the depth of the depression by the arc jet obtained from the following formula is n ₀ or more. [Equation 3]