Abstract: The effect of grain refinement by TiN particles in the simulated coarse-grained heat-affected zone of a high-strength low-alloy steel was investigated by means of analytical characterization techniques such as in-situ microscopy and electron back scattered diffraction analysis. Abundant finely dispersed nanoscale TiN particles form in the specimen and effectively retard the grain boundary migration during simulated high heat input welding,which results in refined austenite grains. TiN precipitates on the surface of aluminum oxide,which is the effective nucleation site for acicular ferrite during the cooling process and induces the austenite transform to the fine-grained mixed microstructure of a small proportion of acicular ferrite embedded in predominantly bainite.