| The CCT curves of two kinds of ultra-high strength low carbon bainite/martensite(B/M) duplex phase steel are obtained. The standard V-notch charpy impact samples are treated by Gleeble simulating different cooling rates and deforming temperatures. Toughness is tested by multifunctional digital impact tester. The results show that ferrite appears in high silicon test steel under lower cooling rates,but it is not found in low silicon test steel. When the cooling rate is getting lower,more bainite is got,and the impact energy increases markedly because of higher crack propagation energy. Strain induced phase transformation of ferrite occurs during the deformation of high silicon test steel,and more ferrite precipitates along the grain boundaries under lower deforming temperature. The toughness gets worse because of the weakened grain boundaries. In low silicon test steel,effect of deforming temperature on microstructure and properties is not significant.
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