Vertical continuous annealing furnace and its operating method

A vertical continuous annealing furnace comprising a plurality of unit furnaces arranged in series, each unit furnace comprising:

• a furnace proper having a furnace mouth opening upward in the top thereof, for admitting a steel strip into the furnace, the strip forming a catenary-like loop therein;
• a first furnace cover horizontally mounted on the top of the furnace proper so as to close said furnace mouth, the first furnace cover being laterally slidable with respect to the furnace proper so as to open and close the furnace mouth and having long narrow inlet and outlet openings laterally extending from one side to the other thereof and longitudinal spaced so that the descending and ascending portions of the strip can pass therethrough;
• and
• a second furnace cover horizontally spaced below said first furnace cover so as to divide the furnace proper into an upper pre-chamber and a lower heating chamber, the second furnace cover being laterally slidable so as to permit the two chambers to be cut off and connected as required, having two long narrow openings laterally extending from one side to the other thereof and longitudinally spaced so that the descending and ascending portions of the strip can pass therethrough;
• the second furnace cover including a partition extending from the undersurface thereof to near the bottom of the furnace to vertically divide the heating chamber into a heating zone and a soaking zone.

A method of operating a vertical continuous annealing furnace, having a first furnace cover laterally slidable to open and close the furnace mouth in the top of the furnace proper and a second furnace cover spaced below the first furnace cover to divide the furnace proper into an upper pre-chamber and a lower heating chamber, the second furnace cover being laterally slidable and having a vertically hanging partition to divide the heating chamber into an upstream and a downstream zone, which heats and anneals a strip of steel charged and discharged through slit-like charging and discharging ports in the top and forming a free loop inside the furnace, comprising:

• providing detecting means on the side walls of said pre-chamber, the detecting means comprising a light emitter and a light receiver;
• lowering the strip after moving the first furnace cover out of the path line, with the heating chamber closed with the second furnace cover;
• charging the strip in the heating chamber after confirming that the lower end of the strip loop exists within the pre-chamber by use of said detecting means and returning the first furnace cover onto the path line;
• and
• discharging the strip outside the furnace by reversing said steps.