As shown in FIGS. 1 and 2, an annealing process of a strip sheet, which is manufactured by rolling mills in a hot rolling plant, a cold rolling plant or an electrical steel plant, is performed by releasing a strip sheet 100 from a pay off reel 120 mounted to an inlet of an annealing furnace 110 and winding the strip sheet 100, which has passed through the annealing furnace 110, around a tension reel 130 mounted to an outlet of the annealing furnace 110.
Hearth rolls 140 are provided to feed the strip sheet 100 having a constant tension in the annealing furnace 110 between the pay off reel and the tension reel.
For example, a grain oriented or non-oriented electrical steel sheet is produced by continuously feeding the strip sheet from the pay off reel to the tension reel and beat treating in the annealing furnace.
A reference numeral 112 in FIG. 1 is a wall of the annealing furnace, and a reference character C in FIG. 2 is an auxiliary heater coil which is arranged as shown in FIG. 2 on an inner surface of the annealing furnace wall (see FIG. 6) and heated by radiation.
FIG. 3 is a structure view showing a conventional hearth roll for the annealing furnace schematically depicted in FIGS. 1 and 2, and FIG. 4 is a partial sectional view showing a mounting state of a conventional hearth roll to the annealing furnace.
A conventional hearth roll 140 shown in FIG. 3 is disclosed in Korean Patent Laid-open Publication No. 2003-0054539. The hearth roll 140 is mounted in the annealing furnace 110 to feed the strip sheet 100 (see FIGS. 1 and 2).
The conventional hearth roll 140 includes a ceramic tube 142 having a good thermal resistance, a roll shaft 144 inserted through the ceramic tube 142, and tube fixing sleeves 146 coupled to both ends of the ceramic tube 142 to fix the roll shaft 144.
As shown in FIG. 3, the tube fixing sleeves 146 are fixed to the ceramic tube 142 by using first fixing pins 148a. 
And, the tube fixing sleeves 146 are fixed to the roll shaft 144 by using second fixing pins 148b. 
As shown in FIGS. 3 and 4, bearing blocks 148 are coupled to both end portions of the roll shaft 144 for rotation of the roll shaft.
As shown in FIG. 4, each of the bearing blocks 148 is mounted to a bracket fixed to an external steel frame 114 of the wall 112 of the annealing furnace, so that the roll shaft 144 can rotate by the bearing block.
A cover is coupled to an outer surface of the bearing block so as to surround the end of the roll shaft, and the cover is filled with grease G which is injected through an injecting pipe (not shown).
The grease G prevents external air from flowing into the annealing furnace in which hydrogen gas atmosphere is formed.
Such grease is also used in a hearth roll apparatus according to the present invention, which will be described later with reference to FIGS. 13 and 14.
A plurality of hearth rolls 140 pass through the wall 112 of the annealing furnace 110, and are driven individually. For example, as shown in FIG. 1, by a driving motor and a driving chain connected to the driving motor, the hearth rolls 140 rotate individually and feed the strip sheet 100.
As shown in FIGS. 1 and 3, a sprocket 150, which is engaged with the driving chain, is coupled to the end of the roll shaft of the hearth roll 140.
However, in the present, different from the structure of FIG. 1, driving motors are separately connected to the respective hearth rolls, so as to independently drive the hearth rolls.
For example, a coupling ring S (see FIG. 6, which will be described later), which is connected to a driving shaft (not shown) of the driving motor, may be connected to the roll shaft to independently drive the hearth rolls.
Because a temperature in the annealing furnace is kept to be high, about 1050° C., for the heat treatment of the strip sheet, the hearth roll 140 in the annealing furnace 110 is affected by heat.
In general, the roll shaft 144 of the hearth roll 140 is made from heat resistant steel (e.g., SCH 24), and the tube 142 is made from ceramic.
When the hearth roll is driven in the temperature of 1050° C. in the annealing furnace, a problem occurs because a coefficient of linear expansion of the roll shaft 144 made from heat resistant steel is different from that of the ceramic tube 142.
For example, the coefficient of linear expansion of the roll shaft 144 is about 30 times as large as that of the ceramic tube 142.
Accordingly, if comparing the roll shaft with the ceramic tube, because an elongation of the roll shaft is much larger than that of the ceramic tube, thermal deformation like creep is generated at the roll shaft 144 rather than the ceramic tube 142 in the high temperature.
The problem caused by the difference of the elongation of the ceramic tube 142 and the roll shaft 144 in the conventional hearth roll 140 is as follows.
For example, when the ceramic tube 142 has an outer diameter of 150 mm and an inner diameter of 75 mm and the roll shaft 144 made from heat resistant steel (e.g., SCH24) has an outer diameter of 74 mm, the elongation of the roll shaft 144 is measured to be about 5 times as large as that of the ceramic tube 142 in the temperature (about 1050° C.) of the annealing furnace.
In other words, the elongation of the roll shaft is larger than that of the tube made from ceramic which has a larger heat resistance than the heat resistant steel.
However, because of the difference of the elongation between the roll shaft and the ceramic tube in the conventional hearth roll 140, when the ceramic tube 142 and the tube fixing sleeves 146 are coupled to each other, gaps are generated at contacting portions (X and Y in FIG. 3), and coupling force between the components 142, 144 and 146 is loosened.
Especially, if heat in the annealing furnace is transferred to the roll shaft 144 through the gaps at the contacting portions (X and Y in FIG. 3) between the ceramic tube and the tube fixing sleeves, the roll shaft is subject to the thermal deformation (bending) like creep.
Thus, as illustrated by a dotted line at the roll shaft in FIG. 3, the center of the roll shaft 144, which is elongated more than the ceramic tube 142, hangs down.
If the center of the roll shaft hangs down, the load of the roll shaft is concentrated inward of an inner race of the bearing block 148 supporting the roll shaft 144. This hinders normal concentric rotation of the roll shaft 144.
Therefore, the conventional hearth roll 140 rotates eccentrically, and vibration is highly increased. When the conventional hearth roll is used for about 2 weeks to 2 months, the hearth roll malfunctions, and the strip sheet cannot be fed normally in the annealing furnace. Thus, the hearth roll should be replaced frequently.
Because the hearth roll 140 used in the annealing furnace is very expensive, when considering that about two hundred hearth rolls are necessary in the annealing furnace, the frequent replacement of the hearth roll 140 causes much costs for materials and labor.
Further, because the operation of the annealing furnace stops for a long time during the replacement of the hearth rolls, the normal production flow is cut, thereby deteriorating the productivity and the product quality.
On the other hand, in the real working field, in order to decrease the influence of the centrifugal force due to the deformation of the roll shaft on the strip sheet, for example, that the hung-down center of the roll shaft strikes the strip sheet when rotating and generates the defect on the surface of the strip sheet, the line speed in the annealing furnace is reduced.
For example, in case of the electrical steel sheet manufacturing process, even though the normal line speed in the annealing furnace is about 160 mpm, the line speed in real is reduced to about 120 mpm.
Accordingly, if a tensile force is forcedly applied to the roll shaft for stretching the roll shaft, the elongation of which is larger than that of the ceramic tube, because the hanging-down of the center of the roll shaft by the thermal effect is prevented, the operating performance of the hearth roll is improved, and the line speed of the strip sheet in the annealing furnace can be increased. Such an improvement of the hearth roll has been required.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a hearth roll apparatus for an annealing furnace which can improve an operating performance by restraining thermal deformation (creep) by which a roll shaft of the hearth roll apparatus is bent down in an annealing furnace of a high temperature.
It is another object of the present invention to provide a hearth roll apparatus for an annealing furnace which can increase a rotating speed of a hearth roll, thereby increasing a line speed of a strip sheet and improving productivity.