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The presented paper reviews,based on literature and own experimental data,some basics for the following phenomena:(1)Yield strength increase in DP steels at ferrite grain refinement,although classical Hall-Petch equation,well describing those changes,does not make an original physical sense,when grain boundaries are no longer are barriers for transferring plastic deformation due to abundance of mobile dislocations;(2)Strain hardening increase at structure refinement of DP steels whereas in single-phase materials it decreases at diminishing grain size;(3)Enhancement of TRIP effect(austenite stability)at refinement of austenite whereas grain refinement usually decreases hardenability of austenite at cooling after full austeitization;(4)Critical influence of grain refinement on impact of precipitation hardening on ductile-brittle transition temperature(DBTT);(5)Impact of grain size on bainitetransformation kinetics etc.Discussing different ways of structure refinement,unique advantages of niobium microalloyingof all types of AHSS(DP,TRIP,Carbide Free Bainitic steels as well as medium Mn,martensitic steels and steels processed using Q&P cycle)will be convincingly illustrated by numerous examples.