Method for producing hot-rolled steel sheet, method for predicting temperature history of hot-rolled steel sheet, and method for predicting hardened portion of hot-rolled steel sheet

Abstract

A method for producing a hot-rolled steel sheet which enables predicting a temperature history of unevenness of the end face of a coil includes: measuring a surface temperature of hot-rolled strip-shaped steel; calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step; actually coiling the strip-shaped steel after the measurement step; scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.

1 . A method for producing a hot-rolled steel sheet, the method comprising:
 a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel;   a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step;   a coiling step of actually coiling the strip-shaped steel after the measurement step;   a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and   a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.   
     
     
         2 . The method for producing a hot-rolled steel sheet according to  claim 1 , further comprising:
 a second calculation step of calculating a phase transformation rate using the temperature history predicted in the first prediction step; and   a second prediction step of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step.   
     
     
         3 . The method for producing a hot-rolled steel sheet according to  claim 1 , wherein in the derivation step, a size of the unevenness is determined by using as a criterion, a median value of measurement values by the displacement meter. 
     
     
         4 . The method for producing a hot-rolled steel sheet according to  claim 1 , wherein in the derivation step, a size of the unevenness is determined by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter. 
     
     
         5 . A method for predicting a temperature history of a hot-rolled steel sheet, the method comprising:
 a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel;   a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step;   a coiling step of actually coiling the strip-shaped steel after the measurement step;   a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil; and   a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and the size of the unevenness derived in the derivation step.   
     
     
         6 . A method for predicting a hardened portion of a hot-rolled steel sheet, the method comprising:
 a measurement step of measuring a surface temperature of hot-rolled strip-shaped steel;   a first calculation step of calculating a temperature history in a natural cooling state after coiling, assuming that the strip-shaped steel has been coiled without unevenness on an end face, based on the surface temperature measured in the measurement step;   a coiling step of actually coiling the strip-shaped steel after the measurement step;   a derivation step of scanning with a displacement meter, an end face of a coil formed in the coiling step, and deriving a size of the unevenness of the end face over a radius of the coil;   a first prediction step of predicting a temperature history of the unevenness in a natural cooling state using: the temperature history calculated in the first calculation step; and   the size of the unevenness derived in the derivation step;   a second calculation step of calculating a phase transformation rate using the temperature history predicted in the first prediction step; and   a second prediction step of predicting a hardened portion of the strip-shaped steel using the phase transformation rate calculated in the second calculation step.   
     
     
         7 . The method for producing a hot-rolled steel sheet according to  claim 2 , wherein in the derivation step, a size of the unevenness is determined by using as a criterion, a median value of measurement values by the displacement meter. 
     
     
         8 . The method for producing a hot-rolled steel sheet according to  claim 2 , wherein in the derivation step, a size of the unevenness is determined by using a two dimensional coordinate system defined by: a protruding direction of the unevenness; and a direction of scanning with the displacement meter.