To provide a method for stably preventing a streaky defect occurring in a galvannealed steel sheet after having been plated and alloyed.
The method for manufacturing the base material of the steel sheet, which is obtained by continuously casting a steel containing 0.035% or more P and is used for ultimately manufacturing a hot-dip galvannealed steel sheet, includes such a continuously casting step and a soaking step as to satisfy the following conditions: W/(82)(Dc tc)+(Dh th), wherein W is maximum value of width of defect in which P is negatively segregated; Dc and Dh are diffusion coefficients (cm2/s) of P at respective temperatures Tc and Th, in which Dc=Do exp(-Q/(R(Tc+273))) and Dh=Do exp(-Q/(R(Th+273))); Do is constant (cm2/s) dependent on steel type; Q is activation energy (cal/mol) of P dependent on steel type; R is gas constant (1.987 cal/mol/K); Tc is average temperature (C) of cast piece at position of 1 to 4 mm below the surface, in continuous casting machine; Th is soaking temperature (C) of cast piece before being hot-rolled; tc is staying time (s) of cast piece in machine; th is soaking time (s) of cast piece before being hot-rolled; and suffixes c and h represent continuous casting and soaking furnace respectively.
TAKAHASHI DAISUKE
SUZUKI MAKOTO
