Method for manufacturing steel sheet and device for continuous annealing steel sheet

Abstract

The present method for manufacturing a high strength steel sheet having a tensile strength of 780 MPa or higher includes continuous annealing by heating a steel sheet having a predetermined chemical composition to 750° C. to 900° C. and holding the steel sheet in the temperature range for 0 seconds to 300 seconds, in which, during heating and holding, a hydrogen concentration in a furnace atmosphere is less than 10 volume %, when a temperature of the steel sheet is 700° C. or lower, a furnace body average value is higher than −3.01 and lower than −0.07, when the temperature is higher than 700° C. and 800° C. or lower, the value is higher than −1.36 and lower than −0.07, when the temperature is higher than 800° C., the value is higher than −3.01 and −0.53 or lower, and a dew point is lower than −10° C.

1 . A method for manufacturing a high strength steel sheet having a tensile strength of 780 MPa or higher, the method comprising:
 continuous annealing by heating a steel sheet containing as a chemical composition, by mass %, C: 0.050% to 0.40%, Si: 0.10% to 2.50%, Mn: 1.20% to 3.50%, Cr: 0% to 0.80%, Ni: 0% to 5.00%, Cu: 0% to 3.00%, Nb: 0% to 0.10%, Mg: 0% to 0.010%, Ti: 0% to 0.10%, B: 0% to 0.010%, and Mo: 0% to 0.5% and a remainder being Fe and impurities, in which P: limited to 0.100% or less, S: limited to 0.010% or less, Al: limited to 1.200% or less, and N: limited to 0.0100% or less, up to a temperature range of 750° C. to 900° C. and holding the steel sheet in the temperature range for 0 seconds to 300 seconds,   wherein, in the continuous annealing, during the heating up to the temperature range and the holding in the temperature range,   a hydrogen concentration in an atmosphere of a furnace is less than 10 volume %,   when a temperature of the steel sheet is 700° C. or lower, a furnace body average value of log (P H2O /P H2 ) which is a relationship between a water partial pressure P H2O  and a hydrogen partial pressure P H2  in the atmosphere of the furnace is in a range of Expression (i) below,   when the temperature of the steel sheet is higher than 700° C. and 800° C. or lower, the furnace body average value of log (P H2O /P H2 ) in the atmosphere of the furnace is in a range of Expression (ii) below, and,   when the temperature of the steel sheet is higher than 800° C., the furnace body average value of log (P H2O /P H2 ) in the atmosphere of the furnace is in a range of Expression (iii) below, and a dew point is lower than −10° C.,
   −3.01<log( P   H2O   /P   H2 )<−0.07  (i)
 
   −1.36<log( P   H2O   /P   H2 )<−0.07  (ii)
 
   −3.01<log( P   H2O   /P   H2 )<−0.53  (iii).
 
   
     
     
         2 . The method for manufacturing a steel sheet according to  claim 1 ,
 wherein the steel sheet contains, by mass %, one or more of   Cr: 0.01% to 0.80%,   Ni: 0.01% to 5.00%,   Cu: 0.01% to 3.00%,   Nb: 0.001% to 0.10%,   Mg: 0.0001% to 0.010%,   Ti: 0.001% to 0.10%,   B: 0.0001% to 0.010%, and   Mo: 0.01% to 0.5%.   
     
     
         3 . The method for manufacturing a steel sheet according to  claim 1 ,
 wherein, when the temperature of the steel sheet is higher than 700° C. and 800° C. or lower, the furnace body average value of log (P H2O /P H2 ) in the atmosphere of the furnace is in a range of Expression (vii) below,
   −1.00<log( P   H2O   /P   H2 )<−0.67  (vii).
 
   
     
     
         4 . A device for continuous annealing a steel sheet containing as a chemical composition, by mass %, C: 0.050% to 0.40%, Si: 0.10% to 2.50%, Mn: 1.20% to 3.50%, Cr: 0% to 0.80%, Ni: 0% to 5.00%, Cu: 0% to 3.00%, Nb: 0% to 0.10%, Mg: 0% to 0.010%, Ti: 0% to 0.10%, B: 0% to 0.010%, and Mo: 0% to 0.5% and a remainder being Fe and impurities, in which P: limited to 0.100% or less, S: limited to 0.010% or less, Al: limited to 1.200% or less, and N: limited to 0.0100% or less, the device comprising:
 an in-furnace atmosphere adjustment unit for   setting a hydrogen concentration in an atmosphere of a furnace to less than 10 volume %,   when a temperature of the steel sheet is 700° C. or lower, adjusting a furnace body average value of log (P H2O /P H2 ) which is a relationship between a water partial pressure P H2O  and a hydrogen partial pressure P H2  in the atmosphere of the furnace to be in a range of Expression (iv) below,   when the temperature of the steel sheet is higher than 700° C. and 800° C. or lower, adjusting the furnace body average value of log (P H2O /P H2 ) in the atmosphere of the furnace to be in a range of Expression (v) below,   when the temperature of the steel sheet is higher than 800° C., adjusting the furnace body average value of log (P H2O /P H2 ) in the atmosphere of the furnace to be in a range of Expression (vi) below, and   adjusting a dew point to be lower than −10° C.,
   −3.01<log( P   H2O   /P   H2 )<−0.07  (iv)
 
   −1.36<log( P   H2O   /P   H2 )<−0.07  (v)
 
   −3.01<log( P   H2O   /P   H2 )≤−0.53  (vi).
 
   
     
     
         5 . The method for manufacturing a steel sheet according to  claim 2 ,
 wherein, when the temperature of the steel sheet is higher than 700° C. and 800° C. or lower, the furnace body average value of log (P H2O /P H2 ) in the atmosphere of the furnace is in a range of Expression (vii) below,
   −1.00<log( P   H2O   /P   H2 )<−0.67  (vii).