1. Field of Invention
This invention relates to a heat shielding apparatus for a vertical continuous annealing furnace in which heat treatment is performed on a metal strip while the strip is continuously transported.
2. Description of Related Art
Recently, an annealing process for recrystallizing steel strip after being subjected to cold rolling and for imparting good workability to the steel strip has been primarily carried out by continuous annealing instead of batch annealing. As a continuous annealing furnace for carrying out the continuous annealing, there are known horizontal continuous annealing furnaces, in which annealing is performed on a strip traveling along a horizontal pass, and vertical continuous annealing furnaces, in which a plurality of rolls are arranged in upper and lower portions of the furnace and annealing is performed on a strip traveling along a vertical pass. Of these continuous annealing furnaces, the vertical furnace is more advantageous for a mass-production process that is realized by increasing the passing (threading) speed of the strip.
Also, at present, indirect heating using a radiant tube is prevalent as a heating source for the vertical continuous annealing furnace, and steel strip is mainly heated with radiant heat from the heating source.
In a vertical continuous annealing furnace wherein a plurality of rolls are arranged in upper and lower portions of the furnace and annealing is performed on a steel strip being transported in the vertical direction by the rolls, while changing a travel direction from upward to downward or vice versa as the strip turns around each roll, it is important to prevent the steel strip from snaking or mistracking and to ensure stable passage of the strip. Generally, as shown in FIG. 11, each roll 12 arranged in the furnace is designed to have a convex roll crown with both shoulders tapered toward the ends. This design is intended to make the steel strip pass the furnace so that the strip always travels in match with the roll center, by utilizing a centering force (arrow F) acting on the strip, which has ridden over a tapered portion, in a direction from the roll edge toward the roll center based on a self-centering motion of the strip wound on the tapered portion of the roll with angle.
As shown in FIG. 12, however, radiant heat from a heating source (e.g., a radiant tube) 14 provided in the furnace heats not only a steel strip 10, but also the roll 12 arranged in the furnace. Therefore, an actual crown of the roll arranged in the furnace is given by the sum of a crown initially imparted to the roll (called an initial crown) and a crown imparted by the radiant heat from the heating source (called a thermal crown). As a result, when the temperature of the steel strip is lower than the roll temperature and when the thermal crown is larger than the initial crown, the temperature of a roll central portion is relatively reduced and the roll crown is rendered concave as indicated by solid lines in FIG. 12. If the steel strip 10 travels over the roll 12 having such a concave crown, a force produced in the width direction of the steel strip acts from the roll center toward the roll edge. Accordingly, once the steel strip undergoes snaking or mistracking, the strip is forced to ride over the roll edge beyond it at a stroke, which causes the problem during the strip passage that the strip comes into contact with the furnace wall.
To cope with this problem, some devices are proposed to prevent the roll temperature from being higher than the strip temperature , so, a shield plate has previously been provided to intercept the heat radiated from the heating source 14 toward the roll 12, as disclosed in Japanese Unexamined Utility Model Application Publication No. 63-119661. Also, Japanese Unexamined Patent Application Publication No. 57-79123 discloses a shielding apparatus employing a heat-resistant tube through which air, nitrogen gas or the like, flows for cooling.
Further, in view of the finding that a shield plate alone is not sufficient to suppress the thermal crown, Japanese Unexamined Patent Application Publication No. 52-71318 discloses a technique for spraying cooling gas to the roll to control the thermal crown in a positive way. Moreover, for the same purpose, Japanese Unexamined Patent Application Publication No. 53-119208 discloses a technique for water-cooling a roll edge portion, or changing a thermal conductivity between the roll central portion and the roll edge portion. In addition, Japanese Unexamined Patent Application Publication No. 53-130210 and Japanese Examined Patent Publication No. 57-23733 disclose techniques for arranging, separately from the rolls, a cooling apparatus that forms a cooling flow path.
Among the above-mentioned examples of the related art, techniques for suppressing the thermal crown imparted to the roll in a positive way are effective in preventing snaking of the strip, but have the problem of requiring a very large amount of equipment investment. Another problem is that, because of an increase in size of the apparatus itself, heat capacity of the apparatus is necessarily increased, which deteriorates the fuel unit consumption in the heating zone.
This invention has been made with the view of overcoming the above-described problems of the related art. An object of this invention is to provide an inexpensive and more efficient apparatus on the basis of the radiant heat shielding apparatus employing a cooling tube, which is disclosed in the above-cited Japanese Unexamined Patent Application Publication No. 57-79123, for example.
To achieve the above object, this invention provides a radiant heat shielding apparatus for a vertical continuous annealing furnace, in which a plurality of rolls are arranged in upper and lower portions of the furnace and heat treatment is performed on metal strip continuously transported by the rolls. The strip is transported in the vertical direction by the rolls while changing the travel direction from upward to downward, or from downward to upward, as the metal strip turns around each of the rolls. The radiant heat shielding apparatus is disposed below the roll positioned in the upper portion of the furnace, and/or above the roll positioned in the lower portion of the furnace, for intercepting heat radiated from a heating source provided within the furnace. Preferably, the radiant heat shielding apparatus is positioned just below the roll in the upper portion of the furnace, and/or just above the roll in the lower portion of the furnace. The radiant heat shielding apparatus comprises a double-walled tube including an inner tube having an outside atmosphere suction port projected horizontally or downward to be exposed to an outside atmosphere, and an outer tube having an exhaust port projected upward to be exposed to the outside atmosphere.
In the radiant heat shielding apparatus, preferably, the outer diameter D of the outer tube of the double-walled tube is not less than about 60 mm, the level difference H between the outside atmosphere suction port and the exhaust port of the double-walled tube is not less than about 150 mm, and the outer diameter D (unit: m) of the outer tube of the double-walled tube and the level difference H (unit: m) satisfy the following relationship:
D2xc3x97{square root over ((H+L ))}xe2x89xa72.2xc3x9710xe2x88x923xe2x80x83xe2x80x83(1)
Further, according to this invention, some embodiments of the radiant heat shielding apparatus comprise a plurality of double-walled tubes as described above. The double-walled tubes are horizontally arranged just below the roll positioned in the upper portion of the furnace and/or just above the roll positioned in the lower portion of the furnace.
Alternatively, in some embodiments, the radiant heat shielding apparatus comprises one or more double-walled tubes as described above, and the double-walled tubes are used as support tubes and a shield plate is attached to the support tubes.