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 BA simulations using both a conventional BA cycle (1,350 deg F soak) and higher temperature Bacycles (1,425 and 1,500 deg F soaks) were performed on both FS and HSFS to determine if the formability of the steels could be improved with higher temperature BA. The results of he investigation reveal the following:(1) Microstructural analysis revealed homogeneous, equiaxed grain structures for all grades undergoing the 1,350 deg F cycle, with grain sizes in general agreement with commercial experience. For samples undergoing the 1,425 deg F cycle, slightly larger equiaxed grain structures were observed for FS grades, whereas the HSFS steel grain sizes remained similar, such as those for the 1,350 deg F cycle. For the 1,500 deg F cycle, grain structures of the FS steels remained equiaxed and homogeneous, albeit larger in size. However, two of the HSFS grades, steel Nos. 3 and 4, underwent abnormal grain growth when subjected to 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 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 (1,350 deg F soak) to the higher temperature cycle (1,500 deg F soak) to be nominal for all grades except for steel No. 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 No. 1 (the Ti-stabilized FS steel), an r_m-value of 2.8 was achieved. This is significantly higher than the value of about 2, which is expected from conventional processing.