The  dual  phase  steel  adding  Nb(0.19～0.21C

0.7～0.8Si

1.9~2. 1Mn

0.02～0.04Nb)and  no adding Nb(0. 17~0. 19C

0.4~0.6Si

1.7~1.9Mn)were melted by using a 25 kg vacuum induction furnace and hot rolled to 3.5 mm plate by two-high mill then cold rolled to1.0 mm and 1.36 mm sheet in laboratory. The cold sheet heated in salt bath furnace to740~820 ℃

slow cooled to 680 ℃ then fast cooled with cooling rate ≥50 ℃/s to 280 ℃ for 240 s

air cooling. Results showed that with increasing heating temperature

the martensite content in ferrite-martensite struc- ture increased;As heating temperature was 820 ℃

the tensile strength of C-Mn-Si dual phase steel no adding Nb was up to1050 MPa

and as adding Nb to furher fine grain of structure

the tensile strength of steel was up to 1200 MPa.