Influence of Batch Annealing Soak Temperature on the Structure and Properties of Fully Stabilized and High Strength, Fully Stabilized Steels

摘要

In this study, laboratory batch annealing (BA) simulations using both a conventional BA cycle (1350℉ soak) and higher temperature BA cycles (1425 and 1500℉ soaks) were performed on both fully stabilized (FS) and high-strength, fully stabilized steels (HSFS) in an effort to determine if the formability of the steels could be improved with higher temperature batch annealing. The results of the investigation reveal the following: 1) Microstructural analysis revealed homogeneous, equiaxed grain structures for all grades undergoing the 1350℉ cycle with grain sizes in general agreement with commercial experience. For samples undergoing the 1425℉ cycle, slightly larger equiaxed grain structures were observed for FS grades whereas the HSFS steel grain sizes remained similar as for the 1350℉ cycle. For the 1500℉ cycle, the grain structures of the FS steels remained equiaxed and homogeneous albeit larger in size. However, two of the HSFS grades, steels 3 and 4, underwent abnormal grain growth when put through this cycle, making the practical application of this cycle to these steels unfeasible. These differences in annealing and grain coarsening behavior between the FS and HSFS grades no doubt have to do with the influence of alloying additions such as P, B, and Mn on annealing response and grain boundary mobility. 2) Mechanical testing revealed n-value improvements upon going from the conventional cycle (1350℉ soak) to the higher temperature cycle (1500℉ soak) to be nominal in a practical sense for all grades except for steel 5. For this grade, an HSFS steel with a high degree of P, B, and Mn alloying, the n-value jumped from 0.225 to 0.248. 3) The results of r-value (longitudinal) testing revealed little r-value improvement for the majority of the grades. For steel 1, the Ti-stabilized FS steel, however, an r_m-value of 2.8 was achieved, significantly higher than the value of about 2.0 expected from conventional processing.