Microstructure development of complex phase steels during thermomechanical processing

Population growth and continuous infrastructural development have brought a high demand for private automotive vehicles, which is expected to continue rising in the coming years. As a result, there will be a larger number of vehicles in transit and therefore the probabilities of crash collisions and CO2 emissions into the environment will also increase. For this reason, the technological development of steels for automotive applications is increasingly important. CO2 emissions can be reduced by improving fuel consumption lowering the weight of vehicles. Therefore, automakers are investing in research to develop lighter and stronger automotive components which meet the safety requirements, without increasing production costs.
Complex phase (CP) steels have been introduced as candidates to improve steel properties through thermomechanical controlled processing (TMP). This thesis focuses on investigating different TMP schedules in a CP steel composition to evaluate the microstructure evolution and mechanical properties. The potential of the alloying elements has been assessed to develop a stronger steel grade through TMP. The effect of deformation parameters and cooling conditions on the final product was also considered in the analysis.
Small specimens of CP steel underwent four-pass TMP routes, simulating deformations conditions during finishing rolling. This was done in a TMP machine under the module of plane strain compression. The TMP routes were designed by defining the critical processing parameters such as transformation temperatures and recrystallization-stop temperature. The TMP explores the influence of the conditioning of the austenite during the finishing rolling and the subsequent development of the microstructure in the run-out table. An extensive characterization was done for the as-received material and the post-processed specimens.
The deformation parameters and the cooling strategies led to the development of microstructures corresponding to certain mechanical properties and volume fraction of phases. The higher tensile strength and higher amount of bainite were achieved in the specimen deformed at lower last-pass temperature but higher holding temperature during the step cooling. This generated high dislocation density with deformation bands and therefore high Sv to facilitate transformation products.
The heavily deformed and non-recrystallized austenite structure is ideal to create fine-grained products which increase the tensile strength. This is also favourable to start ferrite transformation in a relatively short time. Alloying elements play a key role in the microstructure development through TMP, e.g. Nb addition delays austenite recrystallization, retaining a high Sv.
Most of the TMP schedules designed in this work developed the mechanical properties and microstructure to be considered as CP for automotive applications.