Abstract: A 10 mm thick Nb microalloyed high strength steel with ultra-fine grains (1～5 μm) in surface layer is obtained by applying the accelerated cooling intermediate slab-rolling-post rolling accelerated cooling process, the thickness of ultra-fine grains layer lies between 0.5 and 2.0 mm. It has 640 MPa yield strength, 740 MPa tensile strength, 27% elongation and more than 130 J impact absorbing energy at –40 ℃. Microstructure investigation is done by using optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show the mechanism of ultra-fined ferrite is deformation induced undercooling austenite to ferrite transformation (DIT), and conjunction with dynamic recrystallization (DRX) of proeutectoid ferrite and DIF ferrite. The strengthening mechanism is fine grain strengthening, Nb precipitation strengthening, dislocation and substructure strengthening. Before accelerated cooling of intermediate slab , to obtain refined austenite grains through recrystallization region rolling or deformed ones through non-recrystallization region rolling, or to increase the rolling compression ratio after accelerated cooling of intermediate slab , and to decrease the final cooling temperature of post-rolling accelerated cooling , all help obtain ultra-fine grains in surface and increase the proportion of ultra-fine grains over the whole plate thickness .

Key words: Accelerate cooling, Ferrite dynamic recrystallization, Nb microalloyed low carbon steel, Strain induced transformation, Surface layer, Ultra-fine grain