DC ELECTRIC FURNACE

Abstract

Provided is a DC electric furnace using a DC electrode, the DC electric furnace including: a body that has a bottom surface on which a tapping hole is formed and has an inner space to which scraps are charged; a lower electrode that is mounted on the bottom surface of the body; and a bottom-blowing means that is provided on the bottom surface that is not interfered by the lower electrode and blows gas to the inner space of the body.

Description

TECHNICAL FIELD

The present invention relates to a DC electric furnace in which an interference by a lower electrode does not occur, and a bottom-blowing means is spaced apart from refractories of a body so that a stirring force for molten steel is ensured and stability problems are solved.

BACKGROUND ART

In general, an electric furnace has a problem in that a stirring force for molten steel is weak as compared with a converter furnace. Due to the weak stirring force, a [C]*[O] equilibrium value is high, and thus, an end-point oxygen content is high. Further, a T-Fe content in slag is high, and thus, a yield ratio is reduced.

The high end point oxygen content and the high T.Fe content in the slag cause an increase in a consumption rate of Al that is injected as a deoxidizer, and act as main factors that cause an increase in a quality defect, which results from Al₂O₃ inclusions that are generated during a deoxidation process, and an increase in production costs.

Further, there is no separate stirring method except for stirring by falling of molten steel during a tapping operation in the electric furnace. As a result, nitrogen is picked up due to an excessive strong stirring process in a ladle furnace, and thus a quality of products is negatively influenced and productivity deteriorates.

In case of an AC electric furnace among electric furnaces, considering characteristics of an electric furnace using a scheme of heating an upper portion thereof in a situation in which there is no stirring force for molten steel except for heat convection, stirring by the heat convection is also weak. Thus, in recent years, introduction and operation of bottom-blowing stirring equipment are attempted. However, in case of a DC electric furnace, because a stirring force is relatively high by electromagnetic and arc permeability or the like as compared with the AC electric furnace and a problem may occur in stability of equipment due to a large lower electrode that is installed below the DC electric furnace, operation of the bottom-blowing stirring equipment was not attempted.

The matters that are described as the above background art are merely for promoting understanding of the background of the present invention, and it should not be accepted that the matters correspond to the related art that has been already known to those skilled in the art.

DISCLOSURE

Technical Problem

An aspect of the present invention is to provide a DC electric furnace in which an interference by a lower electrode does not occur, and a bottom-blowing means is spaced apart from refractories of a body so that a stirring force for molten steel is ensured and stability problems are solved.

Technical Solution

To achieve the above aspects, a DC electric furnace according to the present invention may include: a body that has a bottom surface on which a tapping hole is formed and has an inner space to which scraps are charged; a lower electrode that is mounted on the bottom surface of the body; and bottom-blowing means that is provided on the bottom surface that is not interfered by the lower electrode and blows gas to the inner space of the body.

A plurality of bottom-blowing means may be provided and may be arranged at locations that are spaced from a center of the lower electrode by a predetermined distance.

When R refers to a distance from the center of the lower electrode to an inner wall of the body, which is closest to the center of the lower electrode, the bottom-blowing means may be arranged at locations of (⅝)*R˜(⅞)*R from the center of the lower electrode.

The bottom surface may be divided into a tapping area in which the tapping hole is formed and a coke side area that is opposite to the tapping area with respect to the center of the lower electrode, and at least one or more bottom-blowing means may be arranged at locations where a first virtual line that serves as a reference of the tapping area and the coke side area is placed.

At least one or more bottom-blowing means may be arranged at locations that are spaced apart from a second virtual line that pass through the center of the lower electrode and the tapping hole by the same distance, on opposite sides of the second virtual line.

The bottom surface of the body may be inclined to become deeper as it goes from the inner wall to the lower electrode at a center of the bottom surface, and a gas blowing direction of the bottom-blowing means may be perpendicular to the inclined portion.

A flow rate of gas that is supplied by the bottom-blowing means may be changed according to operating times.

Advantageous Effects

According to the above-described DC electric furnace of the present invention, as a stirring force for molten steel is increased due to gas that is blown by a bottom-blowing means, an electric power consumption rate may be reduced. Further, a tapping yield is increased due to a decrease in an end-point oxygen content.

An input amount of Al may be reduced, and cleanliness of molten steel is improved so that high-grade steel may be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a DC electric furnace according to an embodiment of the present invention.

FIG. 2 is a view illustrating a virtual model of a DC electric furnace for experiment.

FIG. 3 is a view illustrating a virtual model of the DC electric furnace for the experiment.

FIG. 4A is a graph depicting Maximum UDSs and Minimum UDSs according to a time in respective cases for the experiment.

FIG. 4B is a graph depicting Delta UDSs that is a difference between a maximum value and a minimum value and Mix UDSs according to a time in the respective cases for the experiment.

FIG. 5 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 6 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 7 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 8 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 9 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 10 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 11 is a graph depicting absolute values and relative values according to a time in the respective cases for the experiment.

FIG. 12 is a graph depicting Minimum UDSs according to a time in the respective cases for the experiment.

FIG. 13 is a view illustrating a state that is obtained by cutting the DC electric furnace in perpendicular to a first virtual line with reference to a bottom-blowing means of the DC electric furnace according to the embodiment of the present invention.

[Descriptions of reference numerals]
10: First virtual line 20: Second virtual line
100: Body              110: Tapping hole
200: Lower electrode   300: Bottom-blowing means

BEST MODE FOR THE INVENTION

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

As illustrated in FIG. 1, an electric furnace using a direct-current electrode according to the present invention has a bottom surface on which a tapping hole 110 is formed, and includes: a body 100 in which an inner space to which scraps are charged is provided; a lower electrode 200 that is mounted on a bottom surface; and a bottom-blowing means 300 that is provided on the bottom surface that is not interfered by the lower electrode 200 to blow gas to the inner space of the body 100.

The body 100 constitutes a DC electric furnace and has an inner wall and a bottom surface. The scraps are charged to the inner space that is defined by the inner wall and the bottom surface. Refractories are constructed on the inner wall to protect the body 100 from high temperature. Meanwhile, the tapping hole 110 is formed on the bottom surface. Thus, according to operating steps, the scraps are melted, heated and then tapped to a ladle through the tapping hole 110.

Further, the lower electrode 200 is mounted on a center of the bottom surface. The lower electrode 200 forms arc heat together with an electrode rod that is mounted on an upper portion of the DC electric furnace with the scraps interposed therebetween, to melt the scrap. As mentioned above, in case of the DC electric furnace, due to existence of the lower electrode 200, there has been no attempt to operate bottom-blowing stirring equipment in relation to a safety problem, in the past.

For bottom-blowing stirring, the bottom-blowing means 300 is provided at a location of the bottom surface, which is not interfered by the lower electrode 200, so that gas is blown from below to the inner space of the body 100. A direct porous plug may be used as the bottom-blowing means 300 that blows gas in this way. Further, argon gas (Ar) or nitrogen gas (N₂) may be used as the gas that is blown by the bottom-blowing means 300.

A comparison between an effect when bottom blowing is performed through the bottom-blowing means 300 and an effect when the bottom blowing is not applied may be identified in Table 1.

	TABLE 1
			Al
	Electric power	One point	consumption	Tapping
	consumption rate	oxygen	rate	yield
	(kWh/ton-steel)	(ppm)	(kg/ton-steel)	(%)
Bottom-blowing	478	827	3.7	89.1
is not applied
(320 times)
Bottom-blowing	454	711	3.3	89.6
is applied(380
times)

As identified in Table 1, even when the bottom-blowing is performed in a state in which the bottom-blowing means 300 is mounted at a location according to the present invention, operating may be performed about 380 times without a failure of the equipment. Further, because used electric power is small as compared with a case where the bottom-blowing is not applied, about 24 kWh of electric power per ton may be saved. Further, one point oxygen is lower by 116 ppm, and also less Al is used so that 0.4 kg of Al per ton may be saved.

A tapping yield may be increased by 0.5% as compared with a case where bottom-blowing is not applied.

Preferably, in case of the DC electric furnace according to the present invention, a plurality of bottom-blowing means 300 are provided, and may be spaced apart from the lower electrode 200 by a predetermined distance.

First, optimum locations at which the bottom-blowing means 300 are spaced apart from the lower electrode 200 by a predetermined distance and are thus not interfered by the lower electrode 200 are deduced, and the bottom-blowing means 300 are arranged at the corresponding locations so that stability of the equipment is ensured. When the stability is ensured, the plurality of bottom-blowing means 300 are arranged at the optimum locations that are deduced to maximally ensure a stirring force for molten steel. This is because if the stability is ensured, it is more effective in increasing the stirring force for the molten steel as the bottom-blowing means 300 are arranged as many as possible.

More preferably, when R refers to a distance from a center of the lower electrode 200 to the inner wall of the body 100, which is closest to the center of the lower electrode 200, the bottom-blowing means 300 may be arranged at locations that are spaced apart from the center of the lower electrode 200 by (⅝)*R˜(⅞)*R.

Because the bottom-blowing means 300 should not be interfered by the lower electrode 200 on the bottom surface of the body 100, which is a limited area, it is reasonable that the bottom-blowing means 300 are spaced apart from the lower electrode 200. Further, because the refractories may be damaged by gas that is sprayed by the bottom-blowing means 300 as the bottom-blowing means 300 are closer to the refractories that are constructed on the inner wall of the body 100, it is reasonable that the bottom-blowing means 300 may be spaced apart from the refractories by a predetermined distance.

Thus, the bottom-blowing means 300 are arranged at the locations that are spaced apart from the center of the lower electrode 200 by of (⅝)*R˜(⅞)*R such that R refers to a distance from the center of the lower electrode 200 to the inner wall. When the bottom-blowing means 300 are arranged at locations corresponding to distances that are smaller than (⅝)*R from the center of the lower electrode 200, the bottom-blowing means 300 become closer to the lower electrode 200, and thus the lower electrode 200 may be damaged.

On the other hand, when the bottom-blowing means 300 are arranged at locations corresponding to distances that exceed (⅞)*R from the center of the lower electrode 200, the bottom-blowing means 300 become closer to the refractories that are constructed on the inner wall, and thus the refractories may be damaged. Thus, it is reasonable that the bottom-blowing means 300 are arranged at the locations that are spaced apart from the center of the lower electrode 200 by (⅝)*R˜(⅞)*R.

In the DC electric furnace according to the present invention, the bottom surface is divided into a tapping area in which the tapping hole 110 is formed and a coke side area that is opposite thereto, with reference to the center of the lower electrode 200, and at least one or more bottom-blowing means 300 may be arranged at locations at which the first virtual line 10 that serves as a reference between the tapping area and the coke side area that are illustrated in FIG. 1 is placed.

Oxygen is blown to the coke side area. When the bottom-blowing means 300 are also arranged and thus gas are blown thereto together, the refractories are rapidly damaged, and thus, serious problems in durability may be caused. Thus, the bottom-blowing means 300 are spaced apart from the center of the lower electrode 200 by a predetermined distance, and are arranged as close to the tapping area where the tapping hole 110 exists as possible.

The bottom-blowing means 300 may be arranged at locations which are spaced apart from the lower electrode 200 by a predetermined distance and through which the first virtual line 10 by which the tapping area and the coke side area are divided passes. Two locations as described above are formed on the bottom of the body 100. The bottom-blowing means 300 may be arranged at least one or more locations among the two locations.

The reason why the bottom-blowing means 300 are arranged at locations which are spaced apart from the lower electrode 200 by a predetermined distance and through which the first virtual line 10 passes will be described below. In performing the operating through an electric furnace, a result of a computer simulation shows that cold zones are formed at these location. A probability that a non-molten ingot may be formed at locations where the cold zones are formed, due to insufficient stirring, is high, and as a result, a quality of molten steel deteriorates. Further, because the location corresponds to a location where subsidiary raw materials such as quicklime fall down, it is required to ensure a stirring force such that the subsidiary raw materials are sufficiently mixed with the molten steel.

Thus, the bottom-blowing means 300 are arranged at the locations which are spaced apart from the lower electrode 200 by a predetermined distance and through which the first virtual line 10 passes, so that the stirring force is ensured, the cold zones are suppressed from being formed, the non-molten ingot is suppressed from being generated, and thus, the quality of the molten steel may be improved.

Further, in the DC electric furnace according to the present invention, at least one or more bottom-blowing means 300 may be arranged at locations that are spaced apart from a second virtual line 20 that passes through the center of the lower electrode 200 and the tapping hole 110 by the same distance, on opposite sides of the second virtual line 20.

In determining the locations where the bottom-blowing means 300 are arranged, the bottom-blowing means 300 are arranged around the tapping hole 110 such that at least one or more bottom-blowing means 300 may be arranged on opposite sides of the second virtual line 20 with respect to the second virtual line 20 that passes through the center of the lower electrode 200 and the tapping hole 110, as illustrated in FIG. 1.

The reason why the bottom-blowing means 300 are arranged at the locations that are spaced apart from the second virtual line 20 by the same distance on the opposite sides of the second virtual line 20 will be described below. As above, the result of computer simulation shows that cold zones are formed at the locations. A probability that non-molten ingots are formed at the locations where the cold zones are formed due to insufficient stirring is high, and as a result, the quality of molten steel deteriorates. Further, there is a danger that slag is leaked through the tapping hole 110 during tapping of the molten steel, and the slag may be prevented from being leaked together with the molten steel by blowing gas through the bottom-blowing means 300 that are arranged adjacent to the tapping hole 110, at a predetermined flow rate even during the tapping.

Thus, the bottom-blowing means 300 are arranged at the locations that are spaced apart from the second virtual line 20 by the same distance on opposite sides of the second virtual line 20, so that the quality of molten steel may be improved and the slag may be prevented from being leaked.

An effect of ensuring a stirring force due to bottom blowing at a specific location will be identified below through an experimental result.

As illustrated in FIGS. 2 and 3, a virtual model of the DC electric furnace is generated, and a time for which stirring is performed is identified by measuring a time for which diffusion is performed through a concentration of a user defined scholar (UDS) volume according to a time.

In detail, an internal temperature of the virtual model is set to be about 1750K, a heat flux that is a heat transfer rate per unit area is set to be about 9.27 MW/m², a voltage of an upper electrode is set to be OV, and a voltage of the lower electrode 200 is set to be 700V. A diffusion time of the UDS volume that has a diffusion rate of 0.004 kg/ms under the above environment is measured.

An experiment is performed while the arrangement of the bottom-blowing means 300 that blow argon (Ar) gas is changed to a state in which locations where the bottom-blowing means 300 are arranged include the above-mentioned locations A and B that pass through the first virtual line 10 and locations C and D that are spaced apart from the second virtual line 20 by the same distance on opposite sides of the second virtual line 20, as represented in Table 2.

	TABLE 2
	Bottom-blowing flow rate	Bottom-blowing location
Case 1	No bottom-blowing	None
Case 2	Bottom blowing (80 NL/min)	C
Case 3	Bottom blowing (80 NL/min)	A, B, C
Case 4	Bottom blowing (80 NL/min)	A, C
Case 5	Bottom blowing (80 NL/min)	C, D
Case 6	Bottom blowing (80 NL/min)	B, C
Case 7	Bottom blowing (80 NL/min)	B, C, D
Case 8	Bottom blowing (150 NL/min)	C, D
Case 9	Bottom blowing (150 NL/min)	B, C, D
Case 10	Bottom blowing (150 NL/min)	A, B, C
Case 11	Bottom blowing (300 NL/min)	C, D
Case 12	Bottom blowing (300 NL/min)	B, C, D
Case 13	Bottom blowing (300 NL/min)	A, B, D

A numerical reference value that indicates a concentration is set to be 3977. When a numerical value that indicates the entire concentration in the virtual model arrives at 3977, it is considered that the UDS volume is completely diffused.

First, in FIG. 4A, Case 1, Case 2, Case 3, Case 4, Case 5, Case 6 and Case 7 are compared with each other. The first graph is a graph depicting times that are consumed until minimum values of numerical values that indicate concentrations in the virtual model arrive at 3977, and the second graph is a graph depicting times that are consumed until maximum values of numerical values that indicate concentrations in the virtual model arrive at 3977. In FIG. 4B, Case 1, Case 2, Case 3, Case 4, Case 5, Case 6 and Case 7 are compared with each other. The first graph is a graph depicting times consumed until differences between the maximum values and the minimum values of the numerical values that indicate the concentrations arrive at 0, and the second graph is a graph depicting times consumed until a mixing coefficient that is defined by Equation (1) arrives at zero.

$\begin{array}{cc}C=\sqrt{\frac{1}{N}\sum _{i=1}^N{\left(I_i-I_{\mathrm{mean}}\right)}^2}& \left(1\right)\end{array}$

(I_i: Numerical value that indicates a specific concentration, I_mean: Numerical value that indicates a mean concentration)

It can be identified through the graphs that the most excellent result value is provided in Case 7.

FIGS. 5 and 6 are a graph in which absolute values of the respective cases are compared with each other and a graph in which relative values of the respective cases are compared with each other, when a bottom-blowing flow rate is 80 NL/min. As a time consumed until an absolute value arrives at zero becomes smaller, the diffusion rate becomes higher, and as a relative value becomes farther from 1 according to elapsing of time with respect to Case 1, the diffusion rate becomes higher.

Likewise, FIG. 7 is a graph depicting absolute values and relative values in the respective cases when the bottom-blowing flow rate is 150 NL/min, and FIG. 8 is a graph depicting absolute values and relative values in the respective cases when the bottom-blowing flow rate is 300 NL/min.

Meanwhile, FIG. 9 is graphs in which absolutes values and relative values of Case 5, Case 8 and Case 11 in which the bottom-blowing means 300 are arranged at the locations C and D but the bottom-blowing flow rates thereof are different from each other and the absolute value and the relative value of Case 1 are compared with each other, FIG. 10 is graphs in which absolutes values and relative values of Case 7, Case 9 and Case 12 in which the bottom-blowing means 300 are arranged at the locations B, C and D but the bottom-blowing flow rates thereof are different from each other and the absolute value and the relative value of Case 1 are compared with each other, and FIG. 11 is graphs in which absolutes values and relative values of Case 7, Case 9 and Case 12 in which the bottom-blowing means 300 are arranged at the locations A, B and C but the bottom-blowing flow rates thereof are different from each other and the absolute value and the relative value of Case 1 are compared with each other.

FIG. 12 is a graph in which times for which stirring is performed under respective conditions with each other by comparing times for which the minimum values of the numerical values that indicate the concentrations arrive at 95% (3778.2) of 3977 that is the reference value with each other. The times for which the minimum values in the respective cases arrive at 95% of the reference value are represented in Table 3.

TABLE 3
Time for which minimum value arrives at 95%
of average
Case 1  448
Case 2  446
Case 3  334
Case 4  334
Case 5  332
Case 6  384
Case 7  330
Case 8  328
Case 9  272
Case 10 248
Case 11 324
Case 12 270
Case 13 246

As identified in Table 3, it is identified that an increase in the stirring effect is slight when the bottom-blowing flow rate is increased from 150 NL/min to 300 NL/min. Further, it can be identified that when the number of the bottom-blowing means 300 is not two but three, the stirring effect is high. When Case 4, Case 5 and Case 6 are compared with each other, it may be identified that when the bottom-blowing means 300 are arranged at two locations, the stirring is most effective if the bottom-blowing means 300 are arranged at the locations of C and D.

When the bottom-blowing means 300 are arranged at three locations, if the bottom-blowing flow rate is NL/min, the stirring effect of a case where the bottom-blowing means 300 are arranged at the locations B, C and D is higher than that of a case where the bottom-blowing means 300 are arranged at the locations A, B and C. However, if the bottom-blowing flow rate is 150 NL/min or 300 NL/min, the stirring effect of the case where the bottom-blowing means 300 are arranged at the locations A, B and C is higher.

According to the embodiment of the present invention, as illustrated in FIG. 13, the bottom surface of the body 100 is inclined to become deeper as it goes from the inner wall to the lower electrode 200 at the center of the bottom surface, and a gas blowing direction of the bottom-blowing means 300 may be perpendicular to the inclined portion.

As the gas blowing direction of the bottom-blowing means 300 is perpendicular to the inclined portion, gas may be sprayed to a center of the inner space of the body 100, so that the entire stirring effect of the molten steel may be expected. Further, because the gas blowing direction is a direction that becomes further away from the inner wall of the body 100, a probability that the gas is directly sprayed to the refractories becomes lower, so that the refractories may be prevented from being damaged by the bottom blowing.

In the DC electric furnace according to the present invention, a flow rate of the gas that is supplied by the bottom-blowing means 300 may be changed according to an operating time.

Operating of an electric furnace includes a primary melting period in which steel sources such as scraps are melted, a secondary melting period, a temperature rising period in which the temperature of molten steel is raised, a tapping period in which the molten steel is tapped and an operating standby period. As represented in Table 4, proper bottom-blowing flow rates are set according to operating times, and the gas is supplied through the bottom-blowing means 300.

	TABLE 4
		Flow
	Time point	rate (NL/min)
First melting period	~Input electric power	40
	5000 kWh
	~Input electric power	40
	10000 kWh
	~Input electric power	40
	30000 kWh
Second melting period	~Input electric power	40
	5000 kWh
	~Input electric power	60
	10000 kWh
	~Input electric power	80
	70000 kWh
Temperature rising period	~Input electric power	100
	3000 kWh
	~Input electric power	150
	20000 kWh
	~Input electric power	150
	30000 kWh
Tapping period	~2 minutes	40
	~Tapping is completed	60
	~After tapping is completed	40
Operating standby period	~1 hour	40
	~2 hours	60
	~	40

As above, the bottom-blowing flow rate is changed according to each time point, so that a productivity may be reduced even while the stirring force is ensured through effective bottom blowing.

Although specific embodiments have been illustrated and described in the present invention, it would be obvious to those skilled in the art that the present invention may be variously modified and changed without departing from the technical spirit that is provided by the following appended claims.

Claims

1. A DC electric furnace using a DC electrode, the DC electric furnace comprising:

a body that has a bottom surface on which a tapping hole is formed and has an inner space to which scraps are charged;

a lower electrode that is mounted on the bottom surface of the body; and

a bottom-blowing means that is provided on the bottom surface that is not interfered by the lower electrode and blows gas to the inner space of the body.

2. The DC electric furnace of claim 1, wherein a plurality of bottom-blowing means are provided and are arranged at locations that are spaced from a center of the lower electrode by a predetermined distance.

3. The DC electric furnace of claim 2, wherein when R refers to a distance from a center of the lower electrode to an inner wall of the body, which is closest to the center of the lower electrode, the bottom-blowing means are arranged at locations that are spaced apart from the center of the lower electrode by (⅝)*R˜(⅞)*R.

4. The DC electric furnace of claim 2, wherein the bottom surface is divided into a tapping area in which the tapping hole is formed and a coke side area that is opposite to the tapping area with respect to the center of the lower electrode, and

wherein at least one or more bottom-blowing means are arranged at locations where a first virtual line that serves as a reference of the tapping area and the coke side area is placed.

5. The DC electric furnace of claim 2, wherein at least one or more bottom-blowing means are arranged at locations that are spaced apart from a second virtual line that passes through the center of the lower electrode and the tapping hole by the same distance, on opposite sides of the second virtual line.

6. The DC electric furnace of claim 1, wherein the bottom surface of the body is inclined to become deeper as it goes from the inner wall to the lower electrode at a center of the bottom surface, and

wherein a gas blowing direction of the bottom-blowing means is perpendicular to the inclined portion.

7. The DC electric furnace of claim 1, wherein a flow rate of gas that is supplied by the bottom-blowing means is changed according to operating times.