Method for detecting fluctuation of solidified layer and method for operating blast furnace

A method for detecting a fluctuation of a solidified layer, the method comprising:

• detecting a fluctuation of a solidified layer in a lower part of a blast furnace using an amount of heat supplied to pig iron in the lower part of the blast furnace represented by TQ and an amount of heat in the pig iron tapped in a predetermined period represented by Tpig;
• wherein:
• TQ is a furnace heat index (MJ/t-pig) that is an index of an amount of heat supplied to the lower part of the blast furnace, and Tpig is a temperature (° C.) of the pig iron tapped;
• determining that the solidified layer has decreased when expression (1) below is satisfied:
• α×TQ>a×Tpig+b (1);
• and
• determining that the solidified layer has grown when expression (2) is satisfied:
• α×TQ<a×Tpig+b (2),
• where in the expressions (1) and (2) above:
• a is a ratio of (an amount of heat (MJ/t-pig) of the pig iron at pig-iron temperature Tpig multiplied by an amount of pig iron tapped from the blast furnace)/(To multiplied by an amount of pig iron produced in the predetermined time period) in which heat supplied to the lower part of the blast furnace in a steady state where the solidified layer does not increase or decrease is transferred to the pig iron, a and b are each a constant determined by a component concentration of the pig iron tapped.