Effect of (/% ：) 0.023Al

sol

0.035Al

sol

、

0.018~0.019Al

sol

-0.012~0.014Ti and 0.019~0.020Al

sol

-0.031 ~0.032Nb microalloying on mixed grain size of Φ120 mm case hardened steel ZF7 (% ： 0.18~0.20C

0.22~0.25Si

1.22~1.30Mn

0.008~0.016P

0.019~0.029S

1.19~1.25Cr

0.0018~0.0022B) produced by 60 t BOF-LF-VD-300 mm x 360 mm CC-continuous rolling process and heated at 930 °C has been studied. Results show that with heat treatment condition at 930 °C for 7.5 h

as Al

sol

content in steel is 0.035%

there is no mixed grain size occurred in steel ZF7

but in steel ZF7 microalloying with (/% ：) 0.023

Alsol

0.018 〜0.019Al

sol

-0.012~0.014Ti or 0.019~0.020Al

sol

-0.031 ~0.032Nb the phenomena of mixed grain size occurs in steel

it is indicated that the steel has sufficient Al

sol

content is key factor to avoid mixed grain size occurring in steel ZF7.