Influence of cooling paths on microstructural characteristics and precipitation behaviors in a low carbon V–Ti microalloyed steel

Based on ultra fast cooling, the microstructural characteristics, precipitation behaviors and mechanical properties of a low-carbon V–Ti microalloyed steel were investigated in details using optical microscope, electron back-scattered diffraction and transmission electron microscope. The results show that the ferrite grains can be slightly refined, the sheet spacings of interphase precipitation can be also slightly reduced and the number fraction of ferrite grains with higher precipitation hardening can be significantly enhanced by increasing cooling rate (by comparisons of air cooling and furnace cooling), and a ferritic steel precipitation-strengthened by nanometer-sized carbides was developed to produce hot rolled high strength steel with the tensile strength of ~810 MPa, elongation of ~24% and yield ratio of ~0.82. While for furnace cooling after ultra fast cooling, its tensile strength, elongation and yield ratio is only ~750 MPa, ~22% and ~0.84, respectively. The interphase precipitation in V–Ti microalloyed steel was observed, and these nanometer-sized carbides were detected as (V, Ti)C using energy dispersive X-ray spectroscopy spectra. In addition, the precipitation hardening was estimated as ~313 MPa and ~293 MPa for air cooling and furnace cooling after ultra fast cooling, respectively.