Abstract: The microstructure， mechanical properties and precipitation behaviors in a low carbon Nb-Ti micro-alloyed steel were investigated using the dilatometer， Vickers-hardness tester and transmission electron microscope. The results showed that the microstructure of the experimental steel treated by an isothermal quenching process under different temperatures all mainly consisted of ferrite and martensite phases. The volume fraction of ferrite phase increased with the decrease of the isothermal temperature. Both of the interphase precipitation and supersaturated precipitation of carbides appeared in the samples treated by the isothermal quenching process. Along with the decrease of isothermal temperature， the precipitation form gradually changed from the interphase precipitation to supersaturated precipitation. Furthermore， the interphase-precipitated carbides had a NaCl-type crystal structure with a lattice parameter of 0.432nm and obeyed the Baker-Nutting (B-N) orientation relationship with respect to the ferrite matrix which confirmed by HRTEM. The Vickers-hardness of the ferrite increased with the decrease of the isothermal temperature.

Key words: HSLA steel, interphase precipitation, supersaturated precipitation, orientation relationship, micro-hardness