Effect of copper on microstructure, recrystallization and precipitation kinetics in strip cast low carbon steel

Strip cast samples with different copper content have been developed by simulated direct strip casting. The as-cast microstructures were characterized by optical microscopy and electron microscopy. The microstructures mainly consisted of polygonal ferrite and Widmanstatten ferrite in as cast condition. X-ray diffraction showed that the copper was in solid solution in the as-cast condition. It was found that the equivalent tensile yield and maximum strengths, both increased with an increase in copper content. The strip cast samples were cold rolled to 70% reduction and then annealed isothermally at 600, 650 and 700 degrees C for various times to study the recrystallization and precipitation kinetics. After annealing, the hardness and strength were found to be decreased due to recovery and recrystallization. It was found that an increase in copper content retarded the recrystallization kinetics. During annealing, precipitation of copper was observed. The copper precipitates were bimodal with respect to particle size and distribution. The size and spacing of the particles were measured and the Zener pinning effect of the particles was calculated. High pinning forces of more than 2 MPa were predicted, and this correlated well with the observed decrease in recrystallization kinetics.