Explosive welding of titanium to hypoeutectoid steel (0.09 mass%C) was carried out, and interfacial aspects of as-welded and heat treated states have been investigated with a focus on microstructures of the steel. In as-welded joint, plastic flow occurred by high velocity collision was observed in the vicinity of the interface. The steel in the joints retained equiaxed structure consisting of ferrite and pearlite even after prolonged heat treatment up to 1173 K. Columnar grains were generated in the steel near the interface by the heat treatment at 1223 K and above. Although the region of the columnar ferrite structure increased with increasing heating temperature and holding time, texture with specific crystal orientation was not confirmed. It was found that such a microstructural change in the steel was closely related to constituent phases formed at the bonding interface. The formation mechanism of the columnar structure was also discussed.