Effect of Niobium on the Thermal Stability and Mechanical Properties of a Low-Carbon Ultrafine Grain Steel

The effects of niobium (Nb) on the thermal stability and mechanical properties of a low-carbon ultrafine grain steel (UFG) were investigated. Results indicate that compared to the Nb-free UFG steel, the dissolved Nb in the Nb-UFG steel before annealing was beneficial to improve the thermal stability and strength but harmful to plasticity. The grain boundaries movement was inhibited by numerous Nb(C, N) particles re-precipitated during annealing, leading to the refined ferrite grains, subsequently the higher thermal stability and strength of the Nb-UFG steel. The decreased elongation was attributed to the finer ferrite grains and coarse Fe3C particles. However, the thermal stability and strength of the Nb-UFG steel were also enhanced without any elongation sacrifice when Nb was partially dissolved in matrix before annealing. The superior balance between strength and ductility is ascribed to the precipitation strengthening of fine Nb(C, N) particles, appropriate ferrite grain size and finer Fe3C particles. In addition, although the thermal stability of the steel with completely solute Nb before annealing was better than that with partially solute Nb, the strength and elongation of the former decreased due to the inferior work hardening ability caused by finer ferrite grains and larger Fe3C particles.

Automated summary

The effects of niobium (Nb) on the thermal stability and mechanical properties of a low-carbon ultrafine grain steel (UFG) were studied. It was found that the dissolved Nb in the Nb-UFG steel improved the thermal stability and strength, but reduced the plasticity. The inhibitory effect of numerous Nb(C, N) particles re-precipitated during annealing led to finer ferrite grains and higher thermal stability and strength. However, the decreased elongation was attributed to the finer ferrite grains and coarse Fe3C particles.