In according with issue of central segregation of 250 mm x 2 000 mm slab of steel Q345 (/% : 0.14-0.18C

0.20-0.50Si

1.30-1.50Mn

≤ 0.025P

≤ 0.025S

0.015-0.060A)

a solidification heat transfer mathematical model is established

and verified and modified by nail shooting test to study the effect of secondary cooling intensity (light

middle and strong) and casting speed (0.80-1.10 m/min) on casting slab temperature field and shell thickness

and optimize the soft reduction process and correspondent casting parameters. Results show that with typical casting speed 0. 95 m/min and light cooling scheduling

the final solidification point of slab is 23.43 m

the length of mushy (liquid-solidified phase) zone is 7.22 m； with decreasing the secondary cooling intensity

the final solidifying point extends 0.56-0.67 m and the length of mushy zone increases by 0.25-0.29 m； with increasing casting speed by 0.15 m/min

the final. solidifying point extends 3. 45 ~3. 90 m while the length of mushy zone increases by 0.94-1.22 m； with optimized light cooling schedule

casting speed 0. 95 m/min

solidified fraction

0.4-0.9 at soft reduction zone

and total reduction amount 6mm

the qualified percentage for ≤ I ratting central segregation of slab of steel Q345 increases to 98.0% from original 83.1%.