The precipitated phase composition

mass percent and particle size distribution in microalloyed high strength steels 70A(0.05C

1.90Mn

0.01V

0. 13Ti

0.06Nb) and B2C (0.06C

2.34Mn

0.03V

0.03Ti

0.04Nb

0.0067 B) have been studied by chemical phase analysis and small-angle X- ray scattering method. It was found that there were nano-scale M

2

C and M

2

C

6

carbide in large amounts in microalloyed steels:the mass percent of M

3

C + M

2

C

6

carbide in diameter less than 36 nm in steel 70A was 0.0664% and the mass percent of MC carbide in same diameter was 0.0371%; and that of M

3

C + M

23

C

6

carbide in diameter less than 36 nm in steel B2C was 0. 1341% while that of the MC in same diameter was 0.0084%.