Based on double sub-lattice model

a compound thermodynamic model of (Nb

x

Ti

1-x

) (C

y

N

1-y

) is established

an

d the mol fraction

atom site ratio and precipitation sequence of each ingredient in precipitated phase of Nb-Ti microalloying ultra-low carbon steel (/% ：0.02C

0.12Si

1.70Mn

0.012P

0.004S

0.101Nb

0.009Ti

0.010Als) at 1023-1623 K have been calculated. The effect of precipitates on hot ductility of 230 mm casting slab of steel is studied and the established thermodynamic model is verified with test by using Gleeble thermal simulator

transmission electron microscope and energy dispersive spectrometer. Results show that at 1523 K the solute mol fraction of Nb and Ti in steel are respectively 5.4 x 10

-4

and 3.87 x 10

-5

with temperature decreasing to 1023 K the solute content of Nb and Ti tends to zero. With decreasing temperature the Ti and N atom site ratio gradually decreases while the Nb and C atom site ratio gradually increases

the sequence of evolution of precipitates is Nb

0.315

Ti

0.685

C

0.02

N

0.98

(Nb

x

Ti

1-x

) (C

y

N

1-y

)

Nb

0.85

Ti

0.1

5C

0.71

N

0.29

the calculated values are coordinate with experimental results. With precipitates size less than 60nm and number of particles more than 5 per/μm

2

the hot ductility o£ casting slab decreases obviously； at 1241 K the tensile strength of steel is 63.8 MPa to easily form cracks； meanwhile it is found at fracture of Gleeble specimen that Al

Si

Mn

Nb and Ti concentrate at grain boundaries

and the caves caused by carbo-nitride lead to formation of cracks at action of stress. Therefore during casting process the straightening temperature of casting slab should be ≥ 1241 K.