Relationships among microstructure, precipitation and mechanical properties in different depths of Ti-Mo low carbon low alloy steel plate

Ti-Mo low carbon low alloy steel plate with 25 mm in thickness was produced through hot rolling in laboratory condition. Relationships among microstructure, precipitation and mechanical properties in different depths were investigated and the results revealed that substructures, such as dislocations and low angle boundaries, could vary the process of precipitation and finally made the mechanical properties changed from different depths. Mainly influenced by the high cooling rate during γ-α phase transition process, surface region of the plate was characterized by quasi-polygonal ferrite and large (Ti,Mo)C carbides (>10 nm). However, due to the relatively low cooling rate, some polygonal ferrite formed in the 1/2 thickness region, leading to the promotion of cluster precipitation. Different with the large (Ti,Mo)C carbides heterogeneously nucleated in the substructures, small (Ti,Mo)C carbides (∼2 nm) embedded in the precipitation cluster could homogeneously nucleate in the matrix and exhibited a coherent or semi-coherent interface. Besides, the fact that Ti/Mo atomic ratio increased with the size of (Ti,Mo)C carbides further confirmed the promoting effect of Mo during the nucleation process. Different with the Orowan strengthening mechanism, the movement of dislocation could traverse precipitation clusters. The formation of TiN during smelting process is harmful to the mechanical properties, thus a better performance can be achieved through the reasonable control of it.