Abstract: The effect of forced intermediate water cooling on austenite grains in plates was studied to find a method of controlling grain growth in intermediate slabs due to staying for cooling. Changes in grain size of the intermediate slabs of low alloy steel Q345B and Nb-Ti bearing steel during cooling processes were analyzed by thermal simulating, which includes deforming at 1 050℃ and then rapidly cooling to scheduled temperature from 1 050 to 950℃ for a certain time. Control methods of grain growth were suggested for plates during cooling processes and grain growth models were established for Q345B steel and Nb-Ti bearing steel during the intermediate cooling process. The stability of austenite grains is poor for Q345B steel during the intermediate cooling process, whereas austenite grains in Nb-Ti bearing steel have good stability due to the pinning effect of precipitates, which mainly consist of niobium. Austenite grain growth in the intermediate slabs can be controlled by forced water cooling. The average grain size of 63 mm-thick intermediate slabs after forced water cooling reduces by about 20 μm. In actual production the impact toughness of 16 mm-thick Q345B steel plates after forced intermediate water cooling increases by 25% to 70%.