The Effect of Intercritical Annealing on the Microstructure and Mechanical Properties of Ferritic-Martensitic Two-Phase Steels

摘要

Modified press-hardening processes are very attractive for manufacturing safety-relevant vehicle body parts from steels with martensitic-ferritic microstructures. In the process developed, the formation of the two-phase microstructure and the hot sheet forming simultaneously occur subsequently to an intercritical annealing. By contrast, previously used process chains do not integrate setting of a multi-phase microstructure within the forming step. In order to successfully combine the intercritical annealing with the actual forming, comprehensive knowledge of the microstructural evolution and the resulting mechanical properties is needed. Specifically, different heat-treating routes are used to obtain different microstructures of ferritic-martensitic dual-phase steels and partial martensitic steels. As a result of intercritical annealing in the temperature range of Ac-1-Ac-3, it is possible to vary the martensite volume fractions from 7 to 96 vol%. The data obtained can be employed for numerically describing the microstructural transformation and for designing the heat treatment process. It is demonstrated that this combined process allows for designing steels that feature properties that are similar to complex-phase steels.