The  structure  evolution  of a  Nb-microalloyed  extra-low  carbon  steel  0.03C- 1.05Mn-0.6Cr-0.08Nb  during 1200  ℃  heating

then  cooling  to  850  ℃  and  reducing  50%

holding  at  850  ℃  for  20~1000  s  fast  cooling  to  450  ℃

air  cooling has been simulated and tested by Thermecmastor -Z thermal simulator. The test results showed that with increasing  holding time after reduction  50%

the amount of acicular ferrite and bainite in  steel and the dislocation density in  structure  decreased

where  the  size  of  martensite-austerite(M-A)island  increased;  As  the  holding  time  after  reduction  50%  was  more  than  50  s

  the  micro  hardness(Hv10)value  quick  decreased  but  that  more  than  500  s  the  Hv10  hardness  value  decreased  slowly.