JP5327621B2 - Plasma torch for heating molten steel in tundish - Google Patents

Description

  The present invention relates to a plasma torch used in a plasma heating device for molten steel in a tundish for continuous casting in which the molten steel in the tundish is heated by a plasma arc generated from the plasma torch.

  Plasma torches used in the plasma heating apparatus for tundish inner molten steel for continuous casting include a water-cooled DC cold cathode transfer plasma torch and a water-cooled DC hot cathode plasma torch. The water-cooled DC cold cathode transfer type plasma torch has a function of cooling the tip of the torch with a large amount of circulating cooling water, and has heat resistance against a large-capacity plasma arc of several megawatts level. On the other hand, the water-cooled DC hot cathode plasma torch uses a highly heat-resistant material at the torch tip to reduce the amount of cooling water as much as possible, thereby minimizing the energy loss due to heat removal from the circulating cooling water. Is excellent.

  In general, in a continuous casting process, a water-cooled direct current hot cathode plasma torch is often used, and a technique for reducing the deflection arc and extending the life of the torch electrode in the torch is disclosed in Patent Document 1, for example. It is disclosed.

  A water-cooled DC cold cathode transfer type plasma torch having heat resistance against a large-capacity plasma arc of several megawatt level may be adopted along with an increase in the size of a continuous casting facility. Deterioration of the torch life due to the unique structure is a problem.

FIG. 5 shows the structure of the tip of a general water-cooled DC cold cathode transfer type plasma torch. A ring-shaped nozzle part is provided which serves as a guide for blowing Ar gas from the bottom of the electrode and blowing Ar gas toward the molten steel surface side of the tundish with directivity. Ar gas blown to the molten steel is turned into plasma to form a high-temperature plasma arc, and the molten steel is heated by the radiant heat of the arc. The role of the nozzle part is to guide the blown Ar gas to reliably control the plasma arc to the position between the electrode and the molten steel, to prevent electrode melting and plasma misfire due to misarcing to the tundish lid, etc. Plays an important role.
FIG. 4 shows a nozzle structure of a general water-cooled DC cold cathode transfer type plasma torch. In the water-cooled DC cold cathode transfer type plasma torch, it is necessary to machine a complicated cooling water path in the nozzle portion. For this reason, the nozzle part is not integrally formed by casting, but is usually formed by a two-piece structure including an inner peripheral part 12 and an outer peripheral part 11, machined and then integrated by welding. The enlarged view of the vicinity of the welded portion is shown in detail in FIG. 4B, but the inner peripheral part 12 and the outer peripheral part 11 are pointed to perform overlay welding. As the material of the nozzle portion, generally, a forged product of phosphorous deoxidized copper having high thermal conductivity and high strength is used.

  However, the conventional water-cooled DC cold cathode transfer type plasma torch shown in FIG. 4 has a short torch life, and on average about 17 hours of use, the welded portion between the inner peripheral part 12 and the outer peripheral part 11 of the nozzle part, There was a problem that water leakage was likely to occur from the inner peripheral portion of the nozzle bottom surface close to the molten steel in the dish.

JP-A-11-291023

  An object of the present invention is to provide a water-cooled DC cold cathode transfer type plasma torch that solves the above-mentioned problems and prevents the leakage of water from the nozzle portion to extend the torch life.

The water-cooled direct current cold cathode transfer type plasma torch of the present invention made to solve the above-mentioned problems is characterized in that a nozzle portion arranged at the lower end of the outer periphery of the plasma torch tip electrode includes a ring-shaped outer peripheral part and the outer peripheral part. The inner peripheral part of the ring-shaped protrusion formed on the lower part of the outer peripheral part is welded and integrated with the lower outer peripheral part of the inner peripheral part. A water-cooled direct current cold cathode transfer type plasma torch in which a nozzle inner peripheral surface cooling water channel is formed between parts of the nozzle, and a nozzle lower surface cooling water channel connected to the nozzle inner peripheral surface cooling water channel is a circle formed between the parts. It consists of a nozzle bottom surface cooling water channel radially disposed on the ring-shaped projecting portion and having a R-process on the lower end portion of the inlet, and the welded portion is connected to the lower end portion of the inlet of the circular cavity portion and the nozzle bottom surface cooling water channel. It is characterized in that the joint was staggered over 8 mm.

The invention described in claim 2 is the water-cooled direct current cold cathode transfer type plasma torch according to claim 1 , characterized in that the bottom surface of the nozzle portion is subjected to a heat-resistant and wear-resistant surface treatment. is there.

  As a result of a detailed investigation of the nozzle part where water leakage occurred, a crack extended from the cooling water channel side at the welded part of the inner peripheral part 12 and outer peripheral part 11 applied to the surface of the nozzle part facing the molten steel, and the nozzle part 1 It reached the surface and finally found out that a water leak occurred. In examining the countermeasures against water leakage at the relevant part, the present inventor, based on the thermal stress analysis of the nozzle part of the plasma torch at the time of plasma application, the weld connection part of the outer peripheral part 11 having high rigidity and the inner peripheral part 12 having low rigidity. It was found that a large thermal stress was generated around. When plasma is applied, the thermal load on the inner peripheral part 12 is high and the temperature rises and the linear expansion increases. Therefore, a large thermal stress is applied to the base X of the inner peripheral part 12 having a low rigidity connected to the outer peripheral part 11 having a high rigidity. This is because of this. The thermal stress is locally generated in a very narrow range of the root portion X of the inner peripheral part 12 described above. From the thermal stress analysis, the thermal stress at 8 mm away from the root portion X in the inner peripheral direction is It has been found that it is significantly reduced to about 10%. Further, it is generally known that the structure of the welded portion is coarser due to the influence of heat input during welding, so that the strength is weaker than other forged parts. Based on these findings, the present inventor has devised a configuration in which the thermal stress concentration position is shifted from the welding position with low strength. The shifting distance is preferably 8 mm or more. The water-cooled direct current cold cathode transfer type plasma torch of the present invention based on the idea avoids the thermal stress concentration part where the thermal stress is intensively generated at the time of plasma heating at the connection part between the inner peripheral part 12 and the outer peripheral part 11. The structure provided at the location prevents water leakage from a crack generated in the welded portion between the inner peripheral part 12 and the outer peripheral part 11 and can extend the torch life. By extending the torch life, water leakage repair costs are reduced, plasma non-operation time due to water leakage is shortened, temperature fluctuation of tundish molten steel is reduced, productivity is improved by high casting speed stability, and quality deteriorates due to low temperature molten steel Each effect of prevention and prevention of nozzle clogging can be enjoyed to the maximum extent.

Further, according to the present invention, the R stress 11c is applied to the lower end of the inlet of the nozzle bottom surface cooling water channel which is the thermal stress concentration portion X where the thermal stress is generated intensively at the time of plasma heating. can do.

As a result of detailed investigation of the nozzle portion where water leakage occurred, it was found that water leakage occurred also from the inner peripheral portion of the nozzle bottom surface close to the molten steel in the tundish. While examining countermeasures against water leakage in the part, the present inventor blows away the water leakage in the part with the kinetic energy of Ar plasma gas sprayed on the molten steel in large quantities by the cross-sectional EPMA analysis of the inner peripheral part of the nozzle bottom surface. It was found that the cause was a thinning due to attack of the molten steel droplets in the tundish, and it was proposed to perform a surface treatment excellent in heat resistance and wear resistance on this portion. The invention described in claim 2 based on the idea is the water-cooled direct current cold cathode transfer type plasma torch according to claim 1 , wherein the bottom surface of the nozzle portion is subjected to a heat-resistant and wear-resistant surface treatment. In addition, water leakage is prevented from the inner peripheral part of the nozzle bottom close to the molten steel in the tundish and the torch life is extended.

FIG. 3 is an explanatory diagram of a nozzle unit according to the first embodiment. It is AA sectional drawing of FIG. It is explanatory drawing of the nozzle part of the reference form 2. It is explanatory drawing of the nozzle part structure of a general water-cooling type direct current cold cathode transfer type plasma torch. It is explanatory drawing showing the structure of the plasma torch and the nozzle part.

  Preferred embodiments of the present invention are shown below.

(Embodiment 1)
FIG. 1 shows an explanatory view of a nozzle portion arranged at the lower end of the outer periphery of the plasma torch tip electrode in the water-cooled DC cold cathode transfer type plasma torch according to the first embodiment, and FIG. 2 shows an AA cross section of FIG. The figure is shown.

  In order to enhance the directivity of the plasma arc, the nozzle portion 1 arranged at the lower outer periphery of the plasma torch tip electrode is composed of an outer peripheral part 11 and an inner peripheral part 12 extending along the inner periphery and the lower surface of the outer peripheral part. Is done. As the constituent material of each part, a forged product of phosphorous deoxidized copper having high thermal conductivity and high strength was used.

  The cooling water for cooling the nozzle portion 1 cools the inner periphery of the nozzle through a nozzle inner peripheral surface cooling water passage 13 a formed in a confronting space between the inner peripheral part 12 and the outer peripheral part 11.

  As shown in FIG. 2, a ring-shaped protrusion having a radial nozzle bottom surface cooling water channel 11 a is formed at the lower part of the outer peripheral part 11.

  In general, the position where the change in the rigidity of the base material is maximized during plasma heating is a thermal stress concentration portion where the greatest thermal stress is generated. In the present embodiment, the position where the change in the base material rigidity is maximized during the plasma heating is the lower end of the inlet of the nozzle bottom surface cooling water channel 11a where the shape of the cooling water channel changes. Therefore, it is preferable to perform R machining 11c on the lower end of the inlet of the nozzle bottom surface cooling water channel 11a for the purpose of stress relaxation at the thermal stress concentration site. As a result of thermal stress analysis, it has been found that thermal stress can be reduced by 50% by setting the radius of curvature of R processing to 3.5 mm.

  Generally, the structure of the welded part is coarser due to the effect of heat input during welding, so the strength is weaker than other forged parts. In this embodiment, the lower part of the inner peripheral part at the bottom of the nozzle part By providing the connection part between the outer peripheral part 12a and the outer peripheral part 11 at a place avoiding the thermal stress concentration part at the time of use, the damage generated from the welded part is effectively prevented.

  It is preferable to apply a heat-resistant / abrasion-resistant surface treatment to the bottom surface of the nozzle that is affected by the rebound of molten steel in the tundish. As the surface treatment means, for example, Cr plating or W-C 10% Ni spraying can be employed. Since the melting point of Cr is as high as 1903 ° C. and the hardness is high, improvement in heat resistance and wear resistance can be expected. Since the melting point of WC is as high as 2800 ° C., further improvement in heat resistance can be expected. Although the surface treatment is applied to the entire bottom surface of the nozzle portion 1, the effect can be obtained even by performing only the inner peripheral portion of the bottom surface where the base material is extremely thinned.

( Reference form 2)
FIG. 3 shows the water-cooled DC cold cathode transferred plasma torch reference embodiment 2, an illustration of a nozzle portion which is disposed on the outer peripheral lower end portion of the plasma torch tip electrode.

Also in this reference embodiment, the nozzle unit 1 is composed of an outer peripheral part 11 and an inner peripheral part 12.

However, in this reference embodiment, the inner peripheral part 12 is formed such that the lower outer peripheral portion covers the entire bottom surface of the outer peripheral part 11, and the nozzle bottom cooling water channel 13 b is formed between these parts, and the inner peripheral part 12 and the outer peripheral part 11 are formed. Are screwed together at the outer periphery. Specifically, as shown in detail in FIG. 3D, the screws are cut so as to be engaged with the connecting portions of the inner peripheral part 12 and the outer peripheral part 11, and the screws are connected and tightened.

  The cooling water for cooling the nozzle portion 1 cools the nozzle inner periphery through the nozzle inner peripheral surface cooling water passage 13 formed in the space between the inner peripheral part 12 and the outer peripheral part 11, and then the nozzle bottom surface cooling water passage 13b is discharged from the nozzle part.

Unlike the first embodiment, this reference embodiment does not have a portion where the base material rigidity changes greatly with the change of the cooling water channel shape, and does not have a low-strength weld. In this reference embodiment, this configuration effectively prevents damage that has occurred from the welded portion.

  Table 1 shows the results of examination of the torch life according to the combination of the constituent elements of each claim of the present invention.

  As shown in Example 1 of Table 1, the connection between the lower outer peripheral part of the inner peripheral part and the outer peripheral part on the bottom surface of the nozzle part is provided at a position shifted by 10 mm from the thermal stress concentration part during use. Compared to the example (conventional example in FIG. 4), the torch life can be extended three times or more. Further, as shown in Example 3 of Table 1, a configuration in which R processing with a radius of curvature of 3.5 mm is further added to the configuration of Example 1 to the lower end of the inlet of the nozzle bottom surface cooling water channel, so that a comparative example (FIG. 4). Compared to the conventional example, the torch life can be extended by 4 times or more. Further, as shown in Examples 4 to 5 in Table 1, a configuration in which heat treatment and wear resistance surface treatment is applied to the bottom surface of the nozzle portion in addition to the configuration of Example 3 is added to the comparative example (Fig. Compared to the conventional example 4), the torch life can be extended by 6 times or more.

As shown in Reference Example 2 in Table 1, the inner peripheral part is shaped so that the lower outer peripheral part covers the entire bottom surface of the outer peripheral part, a nozzle bottom surface cooling water channel is formed between these parts, and the inner peripheral part and the outer peripheral part are By screwing in the outer periphery, the torch life can be extended three times or more compared to the comparative example (conventional example in FIG. 4).

Further, as shown in Reference Examples 6 to 7 in Table 1, a configuration in which heat treatment and wear resistance surface treatment is applied to the bottom surface of the nozzle portion is further added to the configuration of the second embodiment. Compared to the conventional example 4), the torch life can be extended by 6 times or more.

  Furthermore, by extending the torch life, water leak repair costs are reduced, plasma non-operation time due to water leaks is shortened, tundish molten steel temperature variation is reduced, productivity is improved by high casting speed stability, and quality deterioration is prevented by low temperature molten steel Each effect of preventing nozzle clogging can be enjoyed to the maximum extent.

DESCRIPTION OF SYMBOLS 1 Nozzle part 11 Outer peripheral part 11a Nozzle bottom surface cooling water channel 11b Inner peripheral edge part 12 of nozzle bottom surface cooling water channel Inner peripheral part 12a Lower outer peripheral part 13a Nozzle inner peripheral surface cooling water channel 13b Nozzle bottom surface cooling water channel 14 Screw connection part X Thermal stress concentration Part

Claims (2)

The nozzle part arranged at the outer peripheral lower end of the plasma torch tip electrode is composed of a ring-shaped outer peripheral part and an inner peripheral part extending along the inner and lower surfaces of the outer peripheral part, and is formed at the lower part of the outer peripheral part. This is a water-cooled direct current cold cathode transfer plasma torch in which the inner peripheral edge of the ring-shaped protrusion is integrated by welding with the lower outer peripheral part of the inner peripheral part, and a nozzle inner peripheral cooling water channel is formed between these parts. And
The nozzle lower surface cooling water channel connected to the nozzle inner peripheral surface cooling water channel is a circular cavity formed between the parts, and a nozzle radially disposed on the ring-shaped protrusion and subjected to R processing at the lower end of the inlet It consists of a bottom cooling water channel,
The water-cooled direct current cold cathode transfer plasma torch characterized in that the welded portion is disposed 8 mm or more away from the joint portion between the circular cavity portion and the lower end of the inlet of the nozzle bottom surface cooling water channel . 2. The water-cooled direct current cold cathode transfer plasma torch according to claim 1, wherein the bottom surface of the nozzle portion is subjected to a heat-resistant and wear-resistant surface treatment.