Textures of dual phase steels
The term crystallographic texture refers to the entity of all orientations and alls associated orientational aspects of the crystal lattice in polycrystalline aggregates.
In DP steels it is an important measure to design properties and microstructure features during processing.
In this project we conducted a systematic through-process analysis of the crystallographic texture evolution in DP steel. By using SEM/EBSD the substructure and texture evolution in dual phase
steels in the first steps of the process chain, i.e. hot rolling, cold rolling, and following annealing were characterized.
The starting material was hot rolled steel with a chemical composition consisting of 0.147 wt. % C, 1.9 wt. % Mn, 0.4 wt. % Si.
In order to obtain dual phase steels with high ductility and high tensile strength an industrial process was reproduced by cold rolling of industrially hot rolled steel sheets of a thickness of 3.75 mm with ferrite and pearlite morphology down to a thickness of 1.75 mm and finally annealing at different temperatures. Such technique allows a compilation of ferrite and martensite morphology typical for dual phase steels. Due to the competition between recovery, recrystallization and phase transformation during annealing a variety of ferrite martensite morphologies was produced by promoting one of the mechanisms through the variation of technological parameters such as heating rate, intercritical annealing temperature, annealing time, cooling rate and the final annealing temperature. Annealing induced changes of the mechanical properties were determined by hardness measurements and are discussed on the basis of the results of the substructure investigations.
We found that in hot rolled sheets, ferrite and pearlite showed a band structure in the center and a heterogeneous distribution at the surface of the sheet. Texture analysis yielded a through-thickness texture inhomogeneity and a maximum plane-strain texture in the center and a maximum shear texture close to the surface of the sheet. Due to cold rolling the in-grain orientation gradients also increased, which is related to an increase of dislocation density and thereby an increase of the driving force for recrystallization. The through-thickness texture inhomogeneity was reduced by cold rolling.
Recovery and recrystallization were found to be dominant for annealing at ferritic temperatures and at low intercritical temperatures up to 695 °C. There is an overlap of recrystallization and phase transformation only at intercritical annealing temperatures exceeding 740 °C. A correlation between
the volume fraction of martensite and hardness was observed at an intercritical annealing temperature of 740 °C. Increasing the annealing time leads to an increasing martensite fractions and therefore to an increasing hardness.