Abstract: Carbon extraction replica analysis is employed to measure the quantities and size distributions of particles in Ti-Nb microalloyed steel and its simulated CGHAZ (coarse grain heat affected zone) to reveal the response behavior of second phase particle to the welding thermal cycle. The microstructure and impact toughness of CGHAZs in Ti-Nb microalloyed steel and a kind of high strength low alloyed steel with similar chemical content and manufacture process are investigated with OM, TEM and series charpy impact test. Experiment results and analysis show that the particles in the steel are irregular in shape, and exhibited a slight size-dependence in that the relative content of Nb decrease with the increase of particle size. The Nb contents of particles in parent metal fell in a wide range from 25%～82%. After weld thermal cycle, the shape of remaining particles in simulated CGHAZ changes from irregular shape to cubicle one, the particle number per unit area decrease, and the mean particle size increases. During the weld thermal cycling, the particles prohibit the original austenite grain from coarsening, prevent the formation of coarse upper bainite, accelerate the formation of acicular ferrite, and promote the decomposition of M—A constituent into ferrite and cementite, thus improve the toughness of CGHAZ in high strength low alloyed steel remarkably. With time t (cooling time form 800 ℃ to 500 ℃) increasing from 16 s to 120 s, the toughness of CGHAZ in Ti microalloyed steel increases because that secondary microstructure of CGHAZ change from upper bainite to acicular ferrite and the austenite grain size coarsening is not serious.

Key words: Austenite grain size, Microalloyed steel, The second phase particle, Toughness