JP2002246171A - Artificial graphite electrode for steel making with superior spalling resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an artificial graphite electrode for a steel making electric arc furnace having good spalling resistance and capable of greatly reducing generation of a tip cracks or a transverse cracks of a screw during use. SOLUTION: In this artificial graphite electrode, having a diameter of 24 inches or more and used for the electric steel making arc furnace, a sample piece sampled in the direction parallel to the electrode longitudinal direction and provided with a diameter of 20 mm and a length of 100 mm is provided with a volumetric specific gravity of 1.66 or more, a resistivity of 5.2 μΩ or less, an elastic modulus of 17,000 MPa or less, a three-point bending strength of 9.80 MPa or more, and a thermal expansion coefficient of 0.5×10-6/ deg.C with the quartz glass as the reference up to 100 deg.C, and an average shape factor of a clearance inside the electrode is 500-3,000.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an artificial graphite electrode having a diameter of 24 inches or more used in an electric steelmaking arc furnace.
The present invention relates to an artificial graphite electrode for steel making which is excellent in spalling resistance and in which the occurrence of chipping at the tip portion and occurrence of lateral cracks at the screw bottom during operation is suppressed.

[0002]

2. Description of the Related Art Artificial graphite electrodes used in electric steelmaking arc furnaces are manufactured by kneading a raw material coke with a binder pitch, extruding, primary firing, impregnation, refiring, graphitization, and processing to a predetermined size. Is done.

[0003] Since the electrode may be broken during operation, measures to improve the shape and design of the screw portion and the like have been taken as a method of preventing the breakage. Also,
It has also been proposed to adjust the blending of raw coke and binder pitch to improve the thermal expansion coefficient, elastic modulus, bending strength, and other properties (Japanese Patent Publication No. 1-21085,
Japanese Patent Publication No. 2-20591.

[0004] With the recent widespread use of large high-load electric steelmaking arc furnaces, electrodes are also required to have a diameter of, for example, 26 inches.
Large diameters such as 8 inches are becoming mainstream.
As the size of the electrode has increased, the quality performance required for the electrode has become stricter, and breakage prevention measures have been more strongly required than ever.

[0005] In order to prevent breakage of an electrode for an electric steel arc furnace having a large diameter, quality design is performed so that four characteristic values of bending strength, elastic modulus, specific resistance and thermal expansion coefficient are adjusted to specific ranges. The proposed graphite electrode has been proposed (Japanese Patent Laid-Open No. 9-27390).
It is not sufficient as an index for predicting the spalling resistance of an electrode in an electric steel arc furnace, and is not always reliable.

The inventor of the present invention has taken measures to prevent breakage of a large-sized artificial graphite electrode for a steel-making arc furnace having a diameter of at least 24 inches and further having a diameter of 26 to 28 inches or more. In particular, after conducting experiments and studies on the relationship between the morphology of the voids and the spalling resistance of the electrode, we found that the voids inside the electrode are an important factor for the spalling resistance. A method for evaluating the shape of the internal voids which influences was found.

[0007]

SUMMARY OF THE INVENTION The present invention has been made based on the above findings, and an object of the present invention is to provide an artificial graphite electrode having a diameter of 24 inches or more used in an electric steel arc furnace and having a resistance to dust. It is an object of the present invention to provide an artificial graphite electrode for steelmaking which has excellent poling properties and is free from chipping at the tip during operation and occurrence of lateral cracks at the screw bottom.

[0008]

According to a first aspect of the present invention, there is provided an artificial graphite electrode for steelmaking, which is used in an electric steelmaking arc furnace and has a diameter of 24 inches or more. A sample having a diameter of 20 mm and a length of 100 mm taken in a direction parallel to the length direction of the electrode has a bulk specific gravity of 1.66 or more, a specific resistance of 5.2 μΩm or less, an elastic modulus of 17000 MPa or less, and a three-point bending strength. Is 9.80MP
a, the coefficient of thermal expansion from 0 ° C. to 100 ° C. based on quartz glass is 0.5 × 10 ⁻⁶ / ° C. or less, and the average shape factor of voids inside the electrode is 500 to 3000. Features. However, the average shape factor of the void inside the electrode is such that the cross-section perpendicular to the length direction of the electrode is measured using image analysis, the measurement area is 2800 ± 300 mm ^2, and the scale is 1 mm.
Read to be 8 to 10 pixels per pixel, the read image is binarized into gaps and parts other than gaps due to the difference in the color tone of the gaps and parts other than the gaps, and the binarized binarized image is subjected to image analysis. The perimeter and area of each void were measured (voids in the coke in the electrode were omitted from the measurement), and 50 voids were selected from the measured voids in descending order of the perimeter,
The average value of the peripheral lengths of the voids excluding the voids having an area of 15 mm ² or more from the selected voids is determined. For the voids whose void lengths exceed the average value, the peripheral length (L) and the area (S) are determined. The shape factor is calculated by the following equation, and the average is calculated to be the average shape factor. Shape factor = [(perimeter L) ²
/ {4π × (area S)}] × 100

[0009] The artificial graphite electrode for steelmaking having excellent spalling resistance according to claim 2 is a peripheral length of a void excluding a void having an area of 15 mm ² or more from the 50 voids selected in the descending order of the peripheral length. And the average value of the area is 3 to 2 respectively.
0 mm and 1 to 5 mm ² , and the deletion rate is 60 to 90%. However, the elimination rate means that, when calculating the average shape factor of the voids inside the electrode, 50 voids are selected in ascending peripheral length, and the area around the voids excluding the voids having an area of 15 mm ² or more from the selected voids. When calculating the average value of the lengths, the ratio of the number of voids (n) whose peripheral length is less than the average value to the number of voids (N) excluding the voids having an area of 15 mm ² or more ((n / N) × 100%).

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is applied to an artificial graphite electrode having a diameter of 24 inches or more used in an electric steel arc furnace, and further to a large-diameter artificial graphite electrode having a diameter of, for example, 26 to 28 inches or 30 inches. .

In the electrode of the present invention, a diameter of 20 mm and a length of 100 mm sampled in a direction parallel to the length direction of the electrode.
Has a bulk specific gravity of 1.66 or more and a specific resistance of 5.2 μΩ.
m, an elastic modulus of 17000 MPa or less, a three-point bending strength of 9.80 MPa or more, and a thermal expansion coefficient of 0.5 × 10 ⁻⁶ / ° C. or less from 0 ° C. to 100 ° C. based on quartz glass. ing.

When the specific resistance exceeds 5.2 .mu..OMEGA.m, the Joule heat generation increases, the thermal stress increases, and the elastic modulus becomes 1
If it exceeds 7000 MPa, the absorbed energy decreases, making it difficult to absorb impact energy. Flexural strength of 9.80
If it is less than MPa, the breaking moment is not sufficient. If the coefficient of thermal expansion exceeds 0.5 × 10 ⁻⁶ / ° C., the heat stress resistance due to Joule heat decreases.

The most important quality characteristic in the present invention is to specify the average shape factor of the void inside the electrode in the range of 500 to 3000. The average shape factor of the void inside the electrode is determined as follows. That is, a cross section perpendicular to the length direction of the electrode is measured using image analysis to obtain a measurement area of 280
The image is read so as to be 0 ± 300 mm ² and the scale is 8 to 10 pixels per 1 mm, and the read image is divided into a gap and a part other than the gap according to a difference in color tone between the gap and the part other than the gap. This operation is called binarization. Next, the binarized image (binary image) is subjected to image analysis to measure the peripheral length and area of each gap. In this case, the void in the coke in the electrode is omitted from the measurement.

From the measured individual voids, 50 voids are selected in ascending order of the perimeter, and an area having an area of 1 from the selected voids.
The average value of the peripheral lengths of the voids excluding the voids of 5 mm ² or more is determined. For the voids whose peripheral length exceeds the average value, the peripheral length (L) and the area (S) are determined, and the shape is calculated by the following formula. The coefficient is calculated, and the average value is determined to be the average shape coefficient. In the case of a perfect circle, the shape factor is 100. Shape factor = [(perimeter L) ² / {4π × (area S)}] × 100

In obtaining the average shape factor of the voids inside the electrode, as described above, 50 voids are selected in ascending order of the perimeter, and the voids excluding the voids having an area of 15 mm ² or more are selected from the voids. When calculating the average value of the perimeter,
The ratio of the number of voids (n) whose peripheral length is equal to or less than the average value to the number of voids (N) excluding voids having an area of 15 mm ² or more (that is, having an area of less than 15 mm ² ) ((n /
N) × 100%) is called a deletion rate. In addition, the perimeter (L) of a gap whose gap perimeter exceeds this average value
And the average value of the area (S) are referred to as an average peripheral length and an average area, respectively.

The average shape factor of the voids inside the electrode is more preferably 700 or more. In addition, the preferable average peripheral length and average area are 3 to 20 mm and 1 respectively.
５5 mm ² , a preferred removal rate is 60-90%, and more preferred average perimeter, average area and removal rate are in the range of 5-20 mm, 1-3 mm ² and 65-90%, respectively.

The electrode having a preferred average shape factor of the present invention can be produced by, for example, adjusting the types of raw coke and binder pitch, the mixing ratio thereof, and the atmosphere during kneading in the production of artificial graphite electrodes for electric steel arc furnaces. Obtainable.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, various characteristics such as bulk specific gravity, specific resistance, elastic modulus, bending strength and thermal expansion coefficient are adjusted to specific ranges, and a shape factor is introduced as a method for evaluating voids inside an electrode. Then, by specifying the average shape factor, to obtain an electrode with excellent spalling resistance,
Hereinafter, examples will be described in comparison with comparative examples to demonstrate the effects of the present invention. These examples show one embodiment of the present invention, and the present invention is not limited thereto.

Examples 1 to 3 and Comparative Examples 1 to 3 A diameter of 20 mm taken in a direction parallel to the length direction of the electrode,
The bulk specific gravity of the test piece having a length of 100 mm is 1.66 to 1.75,
Specific resistance is 4.4 to 5.0 μΩm, elastic modulus is 10,000
-14000 MPa, three-point bending strength 9.80-14.
0 MPa, the coefficient of thermal expansion from 0 ° C. to 100 ° C. based on quartz glass is 0.1 × 10 ^{−6 to} 0.3 × 10 ⁻⁶ / ° C.
Then, an artificial graphite electrode having an average shape factor, an average area, an average peripheral length, and a deletion rate shown in Table 1 was manufactured. In addition,
The specific resistance, elastic modulus, and three-point bending strength are based on JIS R722.
Measured according to 2-1997.

Measurement method of specific resistance: 4 by voltage drop method
It was measured using the terminal method. A current terminal was applied to an end of the sample (diameter: 20 mm, length: 100 mm), a direct current of 5 A was applied, and a potential difference was measured by applying a voltage terminal to the surface of the cylindrical portion. The distance between the voltage terminals is 66 mm. The relationship between the cross-sectional area S of the sample, the potential difference V, the current I, the distance L between the terminals and the specific resistance R is expressed by the following equation. R = VS / (IL)

Method of measuring elastic modulus: Elastic modulus E is calculated by measuring resonance frequency f. E = 4.08ρf ² L ² × 10 ⁻¹⁰ (kgf / mm ² ) 9.81 (MPa) = 1 (kgf / mm ² ) where ρ: bulk density (g / cc), L: sample length ( mm), f: resonance frequency (Hz)

Measuring method of bending strength: The bending strength St was measured by three-point bending. The distance L between fulcrums is L = 4 × D = 8c
m, crosshead descent speed is loading speed 5.0 ±
It was set to about 3 (mm / min) so as to be 2 (kg / s). The load P was measured with a load cell, and the sample diameter D (cm) was measured with a micrometer. St = 8PL / (πD ³ ) (kg / cm ² ) 9.81 × 10 ⁴ (Pa) = 1 (kgf / cm ² )

The average shape factor, average area, average perimeter and elimination rate were measured by the following methods. That is,
The electrode is cut in a direction perpendicular to the length direction, the cut surface is polished, and the polished surface is used as an observation surface. In this case, it is important that the polishing is performed so that the voids in the electrode are not buried, and that the polishing surface does not significantly reflect light like a mirror surface.

A photograph of the observation surface is taken, the coke portion of the photograph is painted in white, and this is read into an image analyzer (Luzex FS, manufactured by Nireco Co., Ltd.). The reading is performed by converting the photograph into a black-and-white image of 256 gradations,
00 mm ² , and the scale is captured so that 1 mm becomes 9 pixels (Pt).

In the captured image, the void portion is shown in a darker color than the portion other than the void, and the coke portion is filled with white, so that the color becomes lighter than the void portion and the portion other than the void. ing.

In this image, a dark part is colored and a light part is colorless with a certain tone color as a boundary. This operation is called a binarization process, and in the binarization process, only the void portions are colored. Due to the binarization process, for the image only the void portion excluding the void in the coke,
The image analyzer measures the peripheral length and area of the void, determines the shape factor, and calculates the average shape factor, average area, average peripheral length, and deletion rate according to the method described above.

This electrode was used in an electric furnace for steelmaking. In this steelmaking electric furnace, the electrode breakage during operation was small, but the tip of the electrode dropped off, and the tip fell off due to the occurrence of a lateral crack in the screw bottom. The presence or absence (spalling resistance) of the electrode (dropping of the electrode due to a lateral crack) determines the quality of use.

The service life as a result of use is shown in Table 1. The service life is the number of batches in which a single artificial graphite electrode could be used in a steelmaking electric furnace operated in a batch system, and is a numerical value generally used as an index indicating the performance of the artificial graphite electrode for steelmaking. It is.

[0029]

[Table 1]

As shown in Table 1, bulk specific gravity, specific resistance,
The electrodes of Examples 1 to 3 whose elastic modulus, bending strength, and coefficient of thermal expansion are within the range of the present invention, and whose average shape factor is also within the range of the present invention, have the average shape factor outside the range of the present invention. As compared with the electrodes of Examples 1 to 3, all of them have excellent service life.

[0031]

According to the present invention, there is provided an artificial graphite electrode for an electric arc furnace for steelmaking, which is excellent in spalling resistance and in which occurrence of chipping of a tip and occurrence of a lateral crack in a screw during use is remarkably suppressed. You.

Claims (2)

[Claims] 1. The diameter used for an electric steelmaking arc furnace is 2
An artificial graphite electrode of 4 inches or more, having a diameter of 20 mm and a length of 100 m sampled in a direction parallel to the length direction of the electrode.
m has a bulk specific gravity of 1.66 or more and a specific resistance of 5.2 μm.
Ωm or less, elastic modulus of 17000 MPa or less, three-point bending strength of 9.80 MPa or more, 0 ° C. based on quartz glass
A coefficient of thermal expansion from 0.5 to 100 ° C. is 0.5 × 10 ⁻⁶ / ° C. or less, and an average shape factor of voids inside the electrode is 500
An artificial graphite electrode for steelmaking having an excellent spalling resistance of from 3,000 to 3,000. However, the average shape factor of the voids inside the electrode is read and read so that the cross-section perpendicular to the length direction of the electrode is measured using image analysis, the measurement area is 2800 ± 300 mm ^2, and the scale is 8 to 10 pixels per mm. The image is binarized into gaps and parts other than the gaps based on the difference in color tone between the gaps and the parts other than the gaps, and the binarized binarized image is image-analyzed to measure the perimeter and area of each gap. (The voids in the coke in the electrode are omitted from the measurement.) From the measured voids, 50 voids are selected in ascending order of the perimeter, and the voids except the voids having an area of 15 mm ² or more from the voids selected are selected. The average value of the lengths is determined, and for the voids having a void length exceeding the average value, the peripheral length (L) and the area (S) are determined, the shape coefficient is calculated by the following formula, and the average is determined. The shape factor is used. Shape factor = [(perimeter L) ² / {4π × (area S)}] × 100 2. An average value of the peripheral length and the area of the voids, excluding the voids having an area of 15 mm ² or more, from the 50 voids selected in the descending order of the peripheral length are 3 to ²⁰ mm and 1 to 5 mm ² , respectively. 2. The artificial graphite electrode for steelmaking having excellent spalling resistance according to claim 1, wherein a deletion rate is 60 to 90%. However, the elimination rate means that, when calculating the average shape factor of the voids inside the electrode, 50 voids are selected in ascending peripheral length, and the area around the voids excluding the voids having an area of 15 mm ² or more from the selected voids. When calculating the average value of the lengths, the ratio of the number of voids (n) whose peripheral length is less than the average value to the number of voids (N) excluding the voids having an area of 15 mm ² or more ((n / N) × 100%).