Method for continously annealing steel strip having a curie point and continous annealing facility of the same

Abstract

A continuous annealing facility comprised of a heating zone, soaking zone, and cooling zone is used for continuous annealing of a steel strip having a Curie point (Tc) by an annealing temperature over Tc during which the heat treatment in the heating zone is divided into three regions, in a first heating zone, a radiant heating means using gas heating and/or a radiant heating means using electric heaters is used to heat the steel strip to less than Tc−50° C., in a next second heating zone, the heated steel strip is heated by a solenoid coil type high frequency induction heating means to a region of Tc−30° C. to Tc−5° C., and, in a final third heating zone, the heated steel strip is heated by a radiant heating means using gas heating and/or a radiant heating means using electric heaters to a treatment target temperature over Tc to thereby anneal a steel strip having a Curie point uniformly in the longitudinal direction.

1 . A method of continuously annealing a steel strip having a Curie point by an annealing temperature exceeding the Curie point at a continuous annealing facility comprising a heating zone, soaking zone, and cooling zone, or a heating zone, soaking zone, nitriding zone, and cooling zone, which method of continuously annealing a steel strip having a Curie point characterized by dividing heat treatment at the heating zone into a first to third, that is, three regions,
 in a first heating zone, using radiant heating means using indirect gas heating or direct gas heating and/or radiant heating means using electric heaters to heat the steel strip from 500° C. to less than the Curie point Tc (° C.)−50° C.,   in a following second heating zone, heating the heated steel strip by solenoid coil type high frequency induction heating means at a heating rate of 50° C./sec or more to a temperature region of Tc−30° C. to Tc−5° C., and   in a final third heating zone, heating the heated steel strip by radiant heating means using indirect gas heating or direct gas heating and/or radiant heating means using electric heaters to the treatment target temperature exceeding the Curie point.   
   
   
       2 . A method of continuously annealing a steel strip having a Curie point as set forth in  claim 1 , characterized by, in the second heating zone, heating the heated steel strip by a plurality of solenoid coil type high frequency induction heating means. 
   
   
       3 . A method of continuously annealing a steel strip having a Curie point as set forth in  claim 2 , further comprising simultaneously controlling the plurality of solenoid coil type high frequency induction heating means so that an output current value of an induction heating means of a downstream most control region becomes a target value and controlling them by detecting an actual output power value of the induction heating means of the downstream most control region, computing a difference between the detected actual output power value and the target output power value, and using the computed difference of power values to correct set output power values of the induction heating devices arranged upstream of the downstream most control region so that the actual output power values of the induction heating devices arranged upstream of the downstream most control region become the corrected set output power values and thereby make a rate of temperature elevation of the steel strip near the Curie point constant. 
   
   
       4 . A method of continuously annealing a steel strip having a Curie point as set forth in  claim 1 , characterized in that the steel strip having the Curie point is cold rolled grain-oriented electrical steel sheet containing Si≦4.5 mass %. 
   
   
       5 . A method of continuously annealing a steel strip having a Curie point as set forth in  claim 1 , wherein the steel strip having the Curie point is a cold rolled ferrite type stainless steel sheet or martensite type stainless steel sheet containing Cr≦18 mass %. 
   
   
       6 . A continuous annealing facility for continuously annealing a steel strip having a Curie point by an annealing temperature exceeding the Curie point comprising a heating zone, soaking zone, and cooling zone, or a heating zone, soaking zone, nitriding zone, and cooling zone, said continuous annealing facility for a steel strip having a Curie point characterized by dividing the heating zone into a first to third, that is, three regions,
 a first heating zone being provided with radiant heating means using indirect gas heating or direct gas heating and/or radiant heating means using electric heaters for heating the steel strip from 500° C. to less than Tc−50° C.,   a second heating zone being provided with solenoid coil type high frequency induction heating means for heating the steel strip heated at the first heating zone to a temperature region of Tc−30° C. to Tc−5° C., and   a third heating zone being provided with radiant heating means using indirect gas heating or direct gas heating and/or radiant heating means using electric heaters for heating the steel strip heated at the second heating zone to a treatment target temperature exceeding the Curie point.   
   
   
       7 . A continuous annealing facility for a steel strip having a Curie point as set forth in  claim 6 , wherein the second heating zone is provided with a plurality of solenoid coil type high frequency induction heating means. 
   
   
       8 . A continuous annealing facility for a steel strip having a Curie point as set forth in  claim 7 , further provided with control means for controlling the plurality of solenoid coil type high frequency induction heating means so that an output current value of an induction heating means of a downstream most control region among the plurality of solenoid coil type high frequency induction heating means becomes a target value, detecting means for detecting an actual output power value of the induction heating means of the downstream most control region, processing means for computing a difference between the detected actual output power value and the target output power value and using the computed difference of power values to correct set output power values of the induction heating devices arranged upstream of the downstream most control region, and control means for control so that the actual output power values of the induction heating devices arranged upstream of the downstream most control region become the corrected set output power values and thereby making a rate of temperature elevation of the steel strip near the Curie point constant.