METHOD AND APPARATUS FOR MEASURING ON LINE PROGRESS OF RECOVERY-RECRYSTALLIZATION OF STEEL PLATE BEING ANNEALED AND METHOD FOR CONTINUOUS ANNEALING OF STEEL PLATE

PROBLEM TO BE SOLVED: To quickly measure on line a state in which a warpage of a steel plate is relieved and anneal under an optimum condition on the basis of a measurement result by measuring a magnetic characteristic of the steel plate at least at one point after a heating band of an annealing furnace. SOLUTION: An apparatus for measuring on line progress of recovery/ recrystallization of a steel plate being annealed is provided with, e.g. coils 1, 2 which face to a steel plate 10 to be measured and a steel plate 20 of a standard sample and to which a high frequency current is fed, and detection means 3, 4 detecting an intensity of a magnetic field based on an eddy current generated at the steel plates 10, 20. The coil 1 is preferably arranged at a temperature drop region after an exit of a heating band of an annealing furnace at a continuous annealing process 7. The steel plate 20 is preferably a perfect recrystallization of the same steel plate as the steel plate 10. Outputs of the detection means 3, 4 are compared by a comparison means 5, so that a temperature in the furnace, a feed speed of the steel plate, etc., are controlled on the basis of a comparison result. The above method is particularly effective for an ultralow carbon steel plate containing C by not larger than 0.0200 wt.%.