Method for annealing hot-rolled steel strip

Abstract

A method for annealing a hot-rolled steel strip controls the annealing temperature with high accuracy to obtain excellent magnetic properties across the entire length of the steel strip. Specifically, the method used for an electrical steel sheet containing 1.6 to 5.0 mass % Si, using a continuous annealing line including heating, soaking and cooling zones in this order from the upstream side, the annealing line heating conditions are determined from slab temperature distribution information in the longitudinal direction during heating, or from information steel strip temperature distribution in the longitudinal direction during hot rolling. The heating conditions of the annealing line are determined by a rapid heating device on the upstream side of the soaking zone, arranging a thickness meter on the upstream side of the rapid heating device, and setting the heating temperature for the steel strip by the rapid heating device to achieve a target annealing temperature.

1 . A method for annealing a hot-rolled steel strip for an electrical steel sheet obtained by heating a slab containing 1.6 to 5.0 mass % Si and hot-rolling the slab, using a continuous annealing line including a heating zone, a soaking zone, and a cooling zone arranged in this order from an upstream side, comprising
 determining a target annealing temperature for the steel strip in a longitudinal direction in the annealing line from information on a temperature distribution in a longitudinal direction of the slab during the slab heating or information on a temperature distribution in the longitudinal direction of the steel strip during the hot rolling; and   
       setting a heating condition for the steel strip in the annealing line. 
     
     
         2 . The method for annealing a hot-rolled steel strip according to  claim 1 , wherein
 the heating condition for the steel strip in the annealing line is determined by arranging a rapid heating device on an upstream side of the soaking zone, arranging a thickness meter on an upstream side of the rapid heating device, and setting a heating temperature for the steel strip by the rapid heating device so as to achieve the target annealing temperature based on a value of LSD defined by a thickness of the steel strip measured by the thickness meter and a threading speed of the steel strip during annealing and represented by the following Expression (1):
     LSD=t×LS   (1),
 
   where t represents the thickness (mm) of the steel strip, and LS represents the threading speed (m/min) of the steel strip.   
     
     
         3 . The method for annealing a hot-rolled steel strip according to  claim 1 , wherein
 the target annealing temperature in the longitudinal direction of the steel strip in the annealing line is determined using positional information on a skid during the slab heating.   
     
     
         4 . The method for annealing a hot-rolled steel strip according to  claim 3 , wherein
 the target annealing temperature in the longitudinal direction of the steel strip during the annealing is set using an in-furnace time during the slab heating, in addition to the positional information on the skid when the slab is heated.   
     
     
         5 . The method for annealing a hot-rolled steel strip according to  claim 2 , wherein
 the heating temperature for the steel strip by the rapid heating device is set by feeding back information on a furnace temperature on a downstream side of the rapid heating device.   
     
     
         6 . The method for annealing a hot-rolled steel strip according to  claim 2 , wherein
 the target annealing temperature in the longitudinal direction of the steel strip in the annealing line is determined using positional information on a skid during the slab heating.   
     
     
         7 . The method for annealing a hot-rolled steel strip according to  claim 6 , wherein
 the target annealing temperature in the longitudinal direction of the steel strip during the annealing is set using an in-furnace time during the slab heating, in addition to the positional information on the skid when the slab is heated.   
     
     
         8 . The method for annealing a hot-rolled steel strip according to  claim 3 , wherein
 the heating temperature for the steel strip by the rapid heating device is set by feeding back information on a furnace temperature on a downstream side of the rapid heating device.   
     
     
         9 . The method for annealing a hot-rolled steel strip according to  claim 4 , wherein
 the heating temperature for the steel strip by the rapid heating device is set by feeding back information on a furnace temperature on a downstream side of the rapid heating device.   
     
     
         10 . The method for annealing a hot-rolled steel strip according to  claim 6 , wherein
 the heating temperature for the steel strip by the rapid heating device is set by feeding back information on a furnace temperature on a downstream side of the rapid heating device.   
     
     
         11 . The method for annealing a hot-rolled steel strip according to  claim 7 , wherein
 the heating temperature for the steel strip by the rapid heating device is set by feeding back information on a furnace temperature on a downstream side of the rapid heating device.