The effect of ultra-fast cooling(UFC)and conventional accelerated cooling(AcC)on the mechanical properties and microstructure of controlled rolled AH32 grade steel plates on industrial scale were compared using tensile test,Charpy impact test,welding thermal simulation,and microscopic analysis.The results show that the properties of the plate produced by UFC are improved considerably comparing to that by AcC.The yield strength is increased with 54 MPa without deterioration in the ductility and the impact energy is improved to more than 260 J at-60℃ with much lower ductile-to-brittle transition temperature(DBTT).The ferrite grain size is refined to ASTM No.11.5 in the UFC steel with uniform microstructure throughout the thickness direction,while that of the AcC steel is ASTM No.9.5.The analysis of nucleation kinetics of-ferrite indicates that the microstructure is refined due to the increased nucleation rate of-ferrite by much lower γ→αtransition temperature through the UFC process.The Hall-Petch effect is quantified for the improvement of the strength and toughness of the UFC steel attributed to the grain refinement.