Patent Description:
In a graphite electrode of an electric furnace, a structure of an electrode connection portion in which breakage of a nipple is prevented is disclosed (for example, see Patent Literature <NUM>). In this structure of the electrode connection portion, by providing a taper degree difference between the nipple and a socket, bias of stress that has been conventionally concentrated on a maximum diameter portion of the nipple is relaxed.

Similarly, in a graphite electrode of an electric furnace, a structure of a connection portion of the graphite electrode that prevents breakage of a nipple is disclosed (for example, see Patent Literature <NUM>). In this structure of the connection portion, a spiral peripheral edge cut part having a cut width that gradually increases as it moves to a maximum diameter portion from a small diameter portion side is formed in the tapered nipple or a thread abutting side portion of an electrode socket. According to this, stress in the maximum diameter portion of the tapered nipple is relaxed, and breakage of the tapered nipple is prevented.

Further, in a graphite electrode of an electric furnace, a connection portion of the graphite electrode in which breakage of a nipple is prevented is disclosed (for example, see Patent Literature <NUM>). The connection portion has a structure in which mountain portions of a plurality of threads are cut so as to gradually decrease from a small diameter portion side to a maximum diameter portion. According to this, stress concentration in the maximum diameter portion of the tapered nipple is relaxed, and breakage of the tapered nipple is prevented. Besides, a joint for an artificial graphite electrode used in an electric steelmaking furnace and a method for forming the same is disclosed (for example, see Patent Literature <NUM>). Further, a connection part of an artificial graphite electrode used, for example, in an electric steel-making arc furnace is disclosed (for example, see Patent Literature <NUM>). Moreover, a structure in which the upper and lower electrodes are screwconnected is disclosed (for example, see Patent Literature <NUM>).

Defects of graphite electrodes include a defect that a part of a graphite electrode falls due to loosening of a screw between the nipple and the socket, in addition to breakage of a nipple due to stress concentration described above. Further, graphite electrodes are poor in processibility because graphite electrodes are formed of graphite which is a hard brittle material, and there is a problem that when the socket and the nipple are formed into special shapes, as in Patent Literatures <NUM> and <NUM>, a great deal of cost is required to accurately process the socket and the nipple into the shapes.

It is therefore an object of the present invention to provide a graphite electrode capable of reducing loosening of a screw between a nipple and a socket and also suppressing manufacturing cost.

The above-described problem is solved as set out in the appended set of claims.

According to the present invention, it is possible to provide the graphite electrode in which loosening of the screw between the nipple and the socket is reduced.

Hereinafter, an electric furnace will be described with reference to the drawings. The electric furnace can melt scrap of metal such as iron in a furnace by heat generated by discharge (arc) to produce molten steel.

With reference to <FIG>, an electric furnace <NUM> of the present invention will be described. The electric furnace <NUM> includes a furnace body <NUM>, a graphite electrode <NUM> that is suspended inside the furnace body <NUM>, and a holder <NUM> that suspends the graphite electrode <NUM>. The electric furnace <NUM> may be either an AC furnace or a DC furnace. When the electric furnace <NUM> is an AC furnace, the number of graphite electrodes <NUM> may be multiple.

The graphite electrode <NUM> can melt metal scrap charged into the furnace body <NUM> by high heat by discharging from a tip end toward a bottom part of the furnace body <NUM>.

As shown in <FIG> and <FIG>, the graphite electrode <NUM> has one or more cylindrical poles <NUM>, and nipples <NUM> interposed as joints between the poles <NUM>. Each of the pole <NUM> and the nipple <NUM> is formed of a solid composition containing a graphite as a main component.

Each of the poles <NUM> has a socket <NUM> recessed in a truncated cone shape at an end surface <NUM> thereof. An internal screw is formed on an inner peripheral surface of the socket <NUM>. The nipple <NUM> can be received inside the socket <NUM>.

The nipple <NUM> has a shape in which bottom surfaces of two cones each in a truncated cone shape are joined to each other. The nipple <NUM> has a first fastening portion <NUM> formed in a taper shape, a second fastening portion <NUM> provided on an opposite side to the first fastening portion <NUM> and formed in a taper shape, a maximum diameter portion <NUM> positioned in a boundary between the first fastening portion <NUM> and the second fastening portion <NUM>, and a pair of small diameter ends <NUM> provided at respective tip ends of the first fastening portion <NUM> and the second fastening portion <NUM>. A taper of the first fastening portion <NUM> and a taper of the second fastening portion <NUM> are formed in opposite directions. The respective taper of the first fastening portion <NUM> and taper of the second fastening portion <NUM> are formed so that diameter of the nipple <NUM> gradually decreases toward the small diameter ends <NUM> positioned at both ends from the maximum diameter portion <NUM> in a center. External screws are formed on outer peripheral surfaces of the first fastening portion <NUM> and the second fastening portion <NUM>. The first fastening portion <NUM> of the nipple <NUM> can be fastened to the socket <NUM> of the pole <NUM>. In a state where the first fastening portion <NUM> is fastened to the pole <NUM>, a second pole <NUM> different from the pole <NUM> can be fastened to the second fastening portion <NUM> of the nipple <NUM>. The second pole <NUM> has a second socket <NUM> on an end surface <NUM>, and can be connected to the second fastening portion <NUM> via the second socket <NUM>.

In the state where the pole <NUM> and the second pole <NUM> are fastened to the nipple <NUM> like this, predetermined gaps are formed respectively between the small diameter end <NUM> on a first fastening portion <NUM> side of the nipple <NUM> and a bottom portion 24A of the socket <NUM>, and between the small diameter end <NUM> on a second fastening portion <NUM> side of the nipple <NUM> and a bottom portion 32A of the second socket <NUM>.

The holder <NUM> has a ring-shaped holding tool 14A, and a support portion 14B capable of supporting the graphite electrode <NUM> via the holding tool 14A.

An "effective diameter of the nipple" means a diameter of a circle located in an intersection portion of a plane orthogonal to a nipple shaft in a position at a central portion of the nipple and a cone configuring a pitch line of a nipple screw thread, as defined in JIS R <NUM>. As shown in <FIG>, an "effective diameter on a small diameter end side of the nipple" d of the present invention differs from this definition, and means a diameter of a circle located in an intersection portion of a plane orthogonal to a nipple axis in a position of the small diameter end <NUM>, and a cone configuring the pitch line of the nipple screw thread.

An "effective diameter of a socket" means a diameter of a circle located in an intersection portion of a plane orthogonal to a socket axis, that is, a plane corresponding to a terminal end portion of the pole, and a cone configuring a pitch line of a socket screw thread as defined in JIS R <NUM>. Unlike this definition, as shown in <FIG>, an "effective diameter on a small diameter end side of a socket" D of the present invention means a diameter of a circle located in an intersection portion of a plane of the nipple <NUM> orthogonal to a socket axis in a position of the small diameter end <NUM>, and the cone configuring the pitch line of the socket screw thread. At this time, the maximum diameter portion <NUM> of the nipple <NUM> is in a boundary position between the pole <NUM> and the second pole <NUM> adjacent to the pole <NUM>.

An effective diameter difference in the small diameter end <NUM>, that is, a value obtained by subtracting an effective diameter on a small diameter end side of the nipple <NUM> from an effective diameter on a small diameter end side of the socket <NUM> is <NUM> to <NUM>, preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>. If the effective diameter difference in the small diameter end <NUM> is less than <NUM>, torque that is required when fastening the nipple <NUM> and the second pole <NUM> to the pole <NUM> tends to be excessively large. If the effective diameter difference in the small diameter end <NUM> exceeds <NUM>, loosening torque that is required when detaching the nipple <NUM> and the second pole <NUM> from the pole <NUM> decreases, and the nipple <NUM> tends to be loosened with respect to the pole <NUM>.

A taper angle refers to a total angle of a cone represented by a pitch line of a screw thread as defined in JIS R <NUM>. Accordingly, as shown in <FIG>, a taper angle α of the nipple <NUM> corresponds to a value twice a gradient α/<NUM> with respect to the nipple axis. A taper angle β of the socket <NUM> corresponds to a value twice a gradient β/<NUM> with respect to the socket axis.

A taper angle difference between the nipple <NUM> and the socket <NUM>, that is, a value obtained by subtracting the taper angle of the socket <NUM> from the taper angle of the nipple <NUM> is -<NUM> minutes to -<NUM> minutes <NUM> seconds.

A linear expansion coefficient difference in a diameter direction of the pole <NUM> and the nipple <NUM>, that is, a value obtained by subtracting a linear expansion coefficient of the socket <NUM> from a linear expansion coefficient of the pole <NUM> is preferably from -<NUM> to +<NUM> (<NUM>-<NUM>/°C), and more preferably from -<NUM> to +<NUM> (<NUM>-<NUM>/°C). When the linear expansion coefficient difference in the diameter direction of the pole <NUM> and the nipple <NUM> exceeds +<NUM> (<NUM>-<NUM>/°C), a possibility of causing cracking to the pole <NUM> is increased with thermal expansion of the pole <NUM> during use at high temperatures, and a possibility of also causing cracking to the nipple <NUM> by a fastening force of the pole <NUM> is increased. On the other hand, when the linear expansion coefficient difference in the diameter direction of the pole <NUM> and the nipple <NUM> is less than -<NUM> (<NUM>-<NUM>/°C), the nipple <NUM> is thermally expanded greatly with respect to the pole <NUM>, a possibility of causing cracking to the nipple <NUM> is increased, and a possibility of also causing cracking to the pole <NUM> is increased by expansion pressure of the nipple <NUM>.

The loosening/fastening torque ratio is a ratio of loosening torque that is maximum torque required to loosen the nipple in the state of being fastened to the socket with respect to fastening torque that is maximum torque required when fastening the nipple to the socket. The loosening/fastening torque ratio is favorably at least one or more, preferably at least <NUM> or more, and more preferably at least <NUM> or more.

A method for manufacturing the pole <NUM> and the nipple <NUM> will be described. Needle coke derived from petroleum and/or needle coke derived from coal are ground and mixed respectively, and are heated to a high temperature, and the heated needle coke is mixed with a binder pitch at a predetermined rate. When a thermal expansion coefficient of the needle coke that is used at this time is small, a linear expansion coefficient in the diameter direction of the pole <NUM> and the nipple <NUM> that is finally obtained becomes small. The binder pitch is obtained by distilling and thermally modifying coal tar obtained by dry distillation of coal. Paste that is cooled to a constant temperature is charged into an extrusion molding machine and is pressed at a constant speed. A molded body (raw electrode) is cooled after extruded for each size. When needle coke having good acicular properties is used, needle coke is more likely to be oriented to be parallel to an extrusion direction in the extrusion molding operation. When a raw electrode is manufactured by extrusion conditions having the high orientation, the linear expansion coefficient in the diameter direction of the pole <NUM> and the nipple <NUM> that are finally obtained is increased.

Subsequently, in a primary firing step, the binder pitch in the molded body is carbonized. The raw electrode is placed in a firing furnace, and is fired to approximately <NUM>. This forms a carbon skeleton (fired electrode) of the electrode.

Subsequently, a pitch infiltration step is performed, and the fired electrode is impregnated with a pitch derived from coal tar in an impregnation tank. This achieves densification of the fired electrodes. By the densification, strength, electric resistance characteristics and the like of the electrode are improved.

Subsequently, a secondary firing step of the fired electrode is performed again in the firing furnace, the temperature is increased to approximately <NUM>, and the impregnated pitch is carbonized.

Further, in a graphitization step, in an LWG furnace or an Acheson furnace, the fired electrode is heated to an ultra-high temperature of about <NUM> to <NUM> and heat-treated. This crystallizes carbon structure into graphite. This forms a graphite electrode material. The higher the temperature of this heating treatment, the larger the linear expansion coefficient in the diameter direction of the pole <NUM> and the nipple <NUM> that are finally obtained.

The pole <NUM> and the nipple <NUM> are produced by processing the electrode material. In the processing step, profile processing and threading processing are performed according to dimensional standards by a dedicated processing machine.

The processed products (the pole <NUM>, the nipple <NUM>) undergo visual inspection, screw precision inspection and the like. Further, by a <NUM>% automatic inspection machine, a length, weight, and various characteristic values of each electrode are measured. The electrodes for which inspection is finished are packed and shipped.

In shipping, one nipple <NUM> may be fastened in advance to the socket <NUM> that is provided on one end surface of the pole <NUM>, and thus the pole <NUM> and the nipple <NUM> may be shipped as a product in an integrated state.

With respect to the graphite electrode (product of respective dimensional standards) manufactured by the manufacturing method described above, the graphite electrodes were each manufactured by setting the effective diameter d on the small diameter end side of the nipple, the effective diameter D on the small diameter end side of the socket, the effective diameter difference in the small diameter end, the nipple side taper angle, the socket side taper angle, and the taper angle difference as in Table <NUM> and Table <NUM> described below. Respective numeric values of the effective diameter d on the small diameter end side of the nipple, the effective diameter D on the small diameter end side of the socket, the nipple side taper angle, and the socket side taper angle are actual measured values measured by using a gauge. Further, a point at which a maximum value or a minimum value of the effective diameter d on the small diameter end side of the nipple is taken, and a point at which a maximum value or a minimum value of the effective diameter D on the small diameter end side of the socket is taken do not usually match with each other. Therefore, a value obtained by subtracting the maximum value of the effective diameter d on the small diameter end side of the nipple from the maximum value of the effective diameter D on the small diameter end side of the socket is not a maximum value of the effective diameter difference in the small diameter end.

Explaining the dimensional standards by taking comparative example A1 (<NUM>×<NUM> - 24T4W) as an example, with the hyphen in-between, the numbers on the left side indicate the dimensions of the pole, and indicate <NUM> inches in diameter by <NUM> inches in length. With the hyphen in-between, number <NUM> on the right side indicates the size of the nipple, and indicates the nipple of the type corresponding to the pole of <NUM> inches in diameter, and the letters indicate a predetermined model number.

Example A1 is improved in effective diameter difference and taper angle difference with respect to comparative example A1, similarly hereinafter, examples A2 and A2' are improved in effective diameter difference and taper angle difference with respect to comparative example A2, examples A3 and A3' are improved in effective diameter difference and taper angle difference with respect to comparative example A3, examples A4 and A4' are improved in effective diameter difference and taper angle difference with respect to comparative example A4, and example A5 is improved in effective diameter difference and taper angle difference with respect to comparative example A5.

In each of comparative examples and examples in which the poles are connected to each other via the nipple, the case in which loosening, falling-off, breakage, rattling or the like occurs to the connection portion was determined as a "defect", and the ratio of the number of "defects" to the total number of measurements was calculated as a "defect rate". The results are shown in Table <NUM>.

When comparative example A1 was improved so as to have the effective diameter difference and the taper angle difference as in example A1, the defect rate decreased to <NUM>% from <NUM>%, and a defect reduction rate was <NUM>%. When comparative example A2 was improved to have the effective diameter difference and the taper angle difference as in example A2 or example A2', the defect rate decreased to <NUM>% from <NUM>%, and the defect reduction rate was <NUM>%. When comparative example A3 was improved to have the effective diameter difference and the taper angle difference as in example A3 or example A3', the defect rate decreased to <NUM>% from <NUM>%, and the defect reduction rate was <NUM>%. When comparative example A4 was improved to have the effective diameter difference and the taper angle difference as in example A4 or example A4', the defect rate decreased to <NUM>% from <NUM>%, and the defect reduction rate was <NUM>%. When comparative example A5 was improved to have the effective diameter difference and the taper angle difference as in example A5, the defect rate decreased to <NUM>% from <NUM>%, and the defect reduction rate was <NUM>%.

A diameter direction CTE (Coefficient of Thermal Expansion) difference, that is, a value obtained by subtracting a linear expansion coefficient of the nipple with respect to the diameter direction of the nipple from a linear expansion coefficient of the pole with respect to the diameter direction of the pole was set as follows. Note that it is known that the linear expansion coefficients of the pole and the nipple have a positive correlation with volume resistivities thereof. It is possible to measure the linear expansion coefficients of the pole and the nipple, by obtaining the linear expansion coefficient corresponding to the linear expansion coefficient in advance to create an experimental calibration line, and measuring the volume resistivities.

In other words, the diameter direction CTE differences of comparative examples B1 to B7 were all large regardless of positive or negative, and specifically, absolute values thereof exceeded <NUM>. Here, the diameter of the pole of comparative example B1 is <NUM> inches, the diameter of the pole of comparative example B2 is <NUM> inches, the diameter of the pole of comparative example B3 is <NUM> inches, the diameters of the poles of comparative examples B4 to B6 are <NUM> inches, and the diameter of the pole of comparative example B7 is <NUM> inches.

The diameter direction CTE differences of comparative examples B1 to B7 were changed as shown in <FIG>, by properly changing the manufacturing conditions of the pole and the nipple (the thermal expansion coefficient of the needle coke, acicular properties, and the heat treatment temperature of the graphitization treatment). In other words, the diameter direction CTE difference in example B1 was -<NUM> to <NUM> (<NUM>-<NUM>/°C), the diameter direction CTE difference in example B2 was -<NUM> to <NUM> (<NUM>-<NUM>/°C), and the diameter direction CTE difference of example B3 was -<NUM> to <NUM> (<NUM>-<NUM>/°C). Further, the diameter direction CTE difference of example B4 was -<NUM> to <NUM> (<NUM>-<NUM>/°C), the diameter direction CTE difference of example B5 was -<NUM> to <NUM> (<NUM>-<NUM>/°C), the diameter direction CTE difference of example B6 was -<NUM> to <NUM> (<NUM>-<NUM>/°C), and the diameter direction CTE difference of example B7 was -<NUM> to <NUM> (<NUM>-<NUM>/°C). Here, the diameter of the pole of example B1 is <NUM> inches, the diameter of the pole of example B2 is <NUM> inches, the diameter of the pole of example B3 is <NUM> inches, the diameters of the poles of examples B4 to B6 are <NUM> inches, and the diameter of the pole of example B7 is <NUM> inches.

As a result, the number of occurrences of defects such as loosening, falling-off, breakage and rattling in the connection portion became zero, and the defect reduction rate was <NUM>%.

Relationship between the effective diameter difference and taper angle difference, and the loosening/fastening torque ratio of the pole and the nipple of the dimensional standards <NUM>×<NUM> - 24T4W was evaluated. The electrode connecting machine made by CIS was used in fastening work of the nipple to the pole, the loosening work that loosens the nipple from the pole, and the measuring work of the loosening/fastening torque ratio.

The taper angle differences of examples C1 to C4 were indiscriminately set at -<NUM> minutes, and the influences of the effective diameter differences (effective diameter differences in the small diameter ends) were evaluated. The effective diameter differences (effective diameter differences in the small diameter ends) of examples C1 to C4 were respectively <NUM>, <NUM>, <NUM>, and <NUM>. The number of evaluations for each of examples C1 to C4 was <NUM> (N = <NUM>), and an average value thereof was adopted as the result of the loosening/fastening torque ratio. The evaluation results are shown in <FIG>. The loosening/fastening torque ratios of examples C1 to C4 were respectively <NUM>, <NUM>, <NUM>, and <NUM>. Accordingly, it is understood that as for the effective diameter difference, <NUM>,<NUM> and a value in the vicinity thereof are the most desirable in the viewpoint of being able to prevent the nipple from being loosened from the socket of the pole.

Next, the effective diameter differences of example C2 and comparative examples C1 to C3 were indiscriminately set at <NUM>, and influences of the taper angle differences were evaluated. The taper angle difference of example C2 was -<NUM> minutes, and the taper angle differences of comparative examples C1 to C3 were parallel (taper angle difference of <NUM>), -<NUM> minutes, and -<NUM> minutes respectively. The number of evaluations of each of example C2, and comparative examples C1 to C3 was <NUM> (N = <NUM>), and an average value thereof was adopted as the result of the loosening/fastening torque ratio. The evaluation results are shown in <FIG>. The loosening/fastening torque ratio of example C2 was <NUM>, and the loosening/fastening torque ratios of comparative examples C1 to C3 were respectively <NUM>, <NUM>, and <NUM>. Accordingly, it is understood that the taper angle difference is most desirably -<NUM> minutes of example C2 and a value in the vicinity thereof from a viewpoint of being able to prevent the nipple from being loosened from the socket of the pole. On the other hand, it is understood that when the taper angle difference is parallel (taper angle difference of <NUM>), or -<NUM> minutes or less, a variation occurs to the loosening/fastening torque ratio, and the value of the loosening /fastening torque ratio does not become a stable high value.

According to the above-described invention and the above-described examples, the following can be said. The graphite electrode <NUM> includes the pole <NUM> having the socket <NUM> in an internal screw shape at the end portion, and the nipple <NUM> in an external screw shape that can be fastened to the socket <NUM>, the value obtained by subtracting the effective diameter on the small diameter end <NUM> side of the nipple <NUM> from the effective diameter on the small diameter end <NUM> side of the socket <NUM> is <NUM> to <NUM>, and the value obtained by subtracting the taper angle of the socket <NUM> from the taper angle of the nipple <NUM> is -<NUM> minutes to -<NUM> minutes <NUM> seconds.

According to this configuration, it is possible to increase the loosening/fastening torque ratio, and it is possible to realize the graphite electrode in which the nipple <NUM> is less likely to be loosened with respect to the pole <NUM>. Accordingly, it is possible to decrease the defect rate. Further, special processing such as cutting to the screw portion is not particularly required, and it is possible to prevent the manufacturing cost of the graphite electrode from being extremely increased.

The graphite electrode <NUM> includes the pole <NUM> having the socket <NUM> in the internal screw shape at the end portion, and the nipple <NUM> in the external screw shape that can be fastened to the socket <NUM>, and the value obtained by subtracting the linear expansion coefficient of the nipple <NUM> from the linear expansion coefficient of the pole <NUM> is -<NUM> to +<NUM> (<NUM>-<NUM>/°C). According to the configuration, it is possible to realize the graphite electrode <NUM> in which the nipple <NUM> is less likely to be loosened with respect to the pole <NUM> and reduce a probability of causing a defect such as loosening.

In these cases, the loosening torque that is required to loosen the nipple <NUM> fastened to the socket <NUM> is at least <NUM> times greater than the fastening torque that is required to fasten the nipple <NUM> to the socket <NUM>. According to the configuration, it is possible to realize the graphite electrode <NUM> in which the nipple <NUM> is easily fastened to the pole <NUM>, the nipple <NUM> is less likely to be loosened with respect to the pole <NUM>, and thereby a defect is less likely to occur, by increasing a so-called loosening/fastening torque ratio.

Claim 1:
A graphite electrode (<NUM>) comprising:
a pole (<NUM>) including a socket (<NUM>) in an internal screw shape at an end portion; and
a nipple (<NUM>) in an external screw shape that can be fastened to the socket (<NUM>), wherein
a value obtained by subtracting an effective diameter on a small diameter end side of the nipple (<NUM>) from an effective diameter on a small diameter end side of the socket (<NUM>) is <NUM> to <NUM>, and
a value obtained by subtracting a taper angle of the socket (<NUM>) from a taper angle of the nipple (<NUM>) is -<NUM> minutes to -<NUM> minutes <NUM> seconds, wherein
the effective diameter on a small diameter end side of the nipple (<NUM>) is a diameter of a circle located in an intersection portion of a plane orthogonal to a nipple axis in a position of the small diameter end (<NUM>) and a cone configuring the pitch line of the nipple screw thread,
the effective diameter on a small diameter end side of a socket (<NUM>) is a diameter of a circle located in an intersection portion of a plane of the nipple (<NUM>) orthogonal to a socket axis in a position of the small diameter end (<NUM>) and the cone configuring the pitch line of the socket screw thread, wherein a maximum diameter portion (<NUM>) of the nipple <NUM> is in a boundary position between the pole (<NUM>) and a second pole (<NUM>) adjacent to the pole (<NUM>),
the taper angle refers to a total angle of a cone represented by a pitch line of a screw thread as defined in JIS R <NUM>, wherein a taper angle α of the nipple (<NUM>) corresponds to a value twice a gradient α/<NUM> with respect to the nipple axis and a taper angle β of the socket (<NUM>) corresponds to a value twice a gradient β/<NUM> with respect to the socket axis, and
the effective diameter and the taper angle are determined according to the methods indicated in the description.