| More emphasis has being laid on the improvement of technical content instead of quantitative increase in the modern steel industry. As for new generation steel, reduced rolling technology has already successfully applied to its research and development, where the most effective method utilized is fine-grained strengthening. By taking this strengthening method into account, basing on the advance researches in the 863 project of The Advanced Industrial Manufacture Technology of 500MPa Carbon Steel, a new way was explored to better control of ultra-fine microstructures in combination with ultra-refinement of austenite grains and reasonable control of transformation behaviors during the subsequent cooling stage.Being sponsored by autonomic-project fund of State Key Lab. of Rolling & Automation, researches about the processes for ultrafining austenite grains in Nb-containing steel and Nb-V-Ti steel, growth kinetics of ultra-fine austenite grains, hot-deformation of ultra-fine grained austenite and subsequentγ→αtransformation behaviors etc. were systematically undertaken in the current paper, including some tentative rolling experiments in laboratory. The main works involved as follows:(1) Ultra-fine austenite grains in size of 1～3μm can be prepared with different initial microstructures in the process of repetitive reheating and quenching, while the most effective initial microstructure used is warm-rolled ferrite + pearlite. Moreover, ultra-fine austenite grains obtained in Nb-V-Ti steel are much smaller than those in Nb-containing steel.(2) All the influencing factors are related to each other and should be controlled well in the thermomechanial treatment which is a coupling process, in order to minimize the obtained austenite grain size.(3) Submicron crystallization of austenite grains can be obtained through recrystallization of ferrite and deformation promoted austenite transformation during the reheating stage.(4) Isothermal growth behavior of ultra-fine austenite grains with average grain size 1～3μm in Nb-V-Ti steel was systematically discussed and corresponding model of growth kinetics was also established. True stress-true strain curve is inclined to softening type as austenite grain size decreases at 900～950℃, which means grain boundary behaviors gradually participate in coordinating the deformation process. Austenite grains (or grain clusters) become more sensitive to the distribution of carbon concentration, and then, such 'quasi two phase' (hardening and softening phases) in austenite is formed which further influence the hot-deformation behaviors of ultra-fine grained austenite and subsequent dynamic ferrite transformation significantly.(5) Deformation-induced ferrite transformation and dynamic austenite grain growth simultaneously exist in the early stage of deformation at temperature range of Ad3～Ar3 around A3.(6) Ferrite grain size remarkably decreases through refinement of austenite grain size. Also, lowering the deformation temperature and increasing the strain rate are both favorable for the refinement of final microstructures. Ferrite grains in size of 100～300nm can be obtained when samples are deformed at relatively low temperatures in two-phase region with high strain rate (10s-1), while ferrite grains become a little larger (～500nm) if the strain rate is slowed down to 0.1s-1.(7) Ultra-fine grained C-Mn steel and Nb-V-Ti steel with ferrite grains size about 1μm and 100～300nm respectively were successfully prepared under laboratory conditions, and low strain hardening capacities were entirely found in those ambient tensile tests, which are partially attributed to initial grain boundary sliding/grain rotation besides the weak piling up of dislocations in ultra-fine grains.
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