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

다국어 초록 (Multilingual Abstract)

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 beforeannealing was beneficial to improve the thermal stability and strength but harmful to plasticity. The grain boundaries movementwas 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 thefiner ferrite grains and coarse Fe3Cparticles. However, the thermal stability and strength of the Nb-UFG steel were alsoenhanced without any elongation sacrifice when Nb was partially dissolved in matrix before annealing. The superior balancebetween strength and ductility is ascribed to the precipitation strengthening of fine Nb(C, N) particles, appropriate ferritegrain size and finer Fe3Cparticles. In addition, although the thermal stability of the steel with completely solute Nb beforeannealing was better than that with partially solute Nb, the strength and elongation of the former decreased due to the inferiorwork hardening ability caused by finer ferrite grains and larger Fe3Cparticles.