A Thermodynamic Analysis of Strengthening Mechanisms and Process-Structure-Property Relationships in Ti-Nb-Mo High-Strength Ferritic Alloy

We elucidate here the significant impact of processing and coiling temperature on microstructure and mechanical properties of Ti-Nb-Mo high-strength ferritic steel through thermodynamic modeling and quantitative analysis of strengthening mechanisms. The study clearly demonstrated that the moderate coiling temperature (570 °C) exhibited superior mechanical properties (yield strength of 773 MPa and elongation of 13%). At this coiling temperature, high percentage (58.64%) of nano-size (4-10 nm) precipitates were obtained with the finest average ferrite grain size of 3.34 ± 0.28 µm. The quantitative analysis of strengthening effects suggested that contributions of precipitation hardening from (Ti, Mo, Nb)C particles and grain refinement were 36.7 and 37.7%, respectively, and were remarkable. A precipitation model was utilized to predict the average size and volume fraction of precipitates and mass fraction of solute elements in the steel matrix. The thermodynamic model predicted precipitate size of 6.81 nm, consistent with the experimentally observed size of 7.01 ± 0.51 nm, when the steel was coiled at 570 °C for 2 h.