Effects of Cu and Nb Addition on Microstructure and Mechanical Properties of Ultra-low Carbon Steel under Different Annealing Temperatures

Abstract

This paper investigated the beneficial effect of copper (Cu) and niobium (Nb) on grain size and mechanical properties of the ultra-low carbon (ULC) steel annealed at temperatures between 700 and 900 °C. The steel samples were prepared by melting input materials in an induction furnace, followed by hot forging, cold rolling into a thin plate, and annealing under an argon atmosphere. Mechanical testing and microstructural observations showed that the addition of Cu and Nb had a positive effect on the steel’s properties. The grain sizes and the tensile strengths of the steel without Cu and Nb varied in 50-98 μm and 254-204 MPa, respectively, while those of the steel alloyed with Cu and Nb decreased to 40-83 μm and increased to 286-242 MPa at the same annealing temperatures. SEM analysis did not find Cu-enriched zones at the grain boundaries, consistent with the predicted Cu content in this steel. It can be concluded that Nb contributed to grain-size refinement and Cu hardened the ferrite matrix, leading to improved mechanical properties of ULC steel at the same annealing temperatures.