Precipitation strengthening of titanium microalloyed high-strength steel plates with isothermal treatment

Titanium microalloyed low carbon steels were rolled into plates by controlled rolling process, followed by two subsequent cooling routes, including air cooling (AC) directly and isothermal treatment (IT) at 600 °C for 1 h. Fine precipitates were studied by high-resolution transmission electron microscope with energy dispersive X-ray spectroscope, as well as X-ray diffraction. It is noted that a larger amount of nano-sized TiC precipitated in the matrix of IT steel, and their orientation relationships with respect to the ferrite matrix were (100)TiC∥(110)α−Fe and [011]TiC∥[1¯11]α−Fe for strain-induced precipitated TiC of 10-40 nm in size, and (100)TiC∥(100)α−Fe and [011]TiC∥[001]α−Fe for random or interphase precipitated TiC of 1-10 nm in size. Contributions to the yield strength associated with grain refinement, solid solution, dislocation and precipitation were analyzed by using a modified form of Hall-Petch, Taylor, and Ashby-Orowan relationship, respectively. It is revealed that isothermal treatment is much advantageous to precipitation and the major increase in yield stress is mainly attributed to the precipitation strengthening of nano-sized TiC.