Interstitial Solution Carbon Concentration and Defects of Ti + Nb ULC-BH Steel by Internal Friction and Positron Annihilation Methods

The interstitial solution carbon concentration and defects in continuously annealed Ti+Nb bearing ULC-BH (ultra-low carbon bake hardening) steel samples are investigated by multi-functional internal friction apparatus, positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS). The relationship of internal friction peaks, interstitial solution carbon concentrations and movable dislocations in the samples under different conditions is analyzed. A correlation of lifetime component τ1 values with interstitial solution carbon concentrations in the samples for different continuous annealing processes is established, while a correlation of lifetime component τ2 values with multi-vacancies, vacancy clusters, microvoids and other types of defects for various continuous annealing processes is also demonstrated. Furthermore, the average lifetime results illustrate the overall defect densities for various continuous annealing processes. The CDBS analysis reflects the chemical surroundings of the defects at the annihilation sites and reveals that the peak heights of the ratio curves relate to the total number of defects such as interstitial carbon atoms, dislocations, vacancies and other types of defects. The results show that internal friction, PALS and CDBS are effective techniques to identify and characterize the interstitial solution carbon concentration, multi-vacancies, vacancy clusters, microvoids and other types of microscopic defects in annealed Ti+Nb bearing ULC-BH steel.