Abstract: Behavior of dissolution precipitation and response to simulated weld thermal cycles of fine second phase particles in two microalloying steels were studied using transmission electron microscope (TEM) by means of "speed" carbon extraction replicas technique. The sizes, numbers and distribution characteristics were determined as with different simulated weld thermal cycles. Effects of parameters of weld thermal cycle (i. e pulse temperature T_p, holding time at pulse temperature t_p and cooling rate from pulse temperature t_max/8) on the size and distribution of particles were analysized. An investigation into effects of growth and dissolution behavior of particles on austenite grain sizes at high temperature and action of added element Nb to microalloying Ti steels was conducted.