from the conferences organized by TANGER Ltd.
The ferritic-martensitic microstructure is typical for the dual-phase steels such as DP600. This microstructure can be obtained during the hot rolling process followed by interrupted water cooling with short isothermal pause in the intercritical region. In manufacturing of the DP steel by hot rolling process the final deformation takes place slightly above Ar3, where the material is composed of austenite alone. Upon completion of plastic deformation, the hot strip is rapidly cooled to the temperature between Ar3 and Ar1, where supercooled austenite undergoes phase transformation into ferrite. Since both the temperature and time spent in this temperature region are not sufficient for complete phase transformation, remaining austenite is quenched during the second step of cooling process, which leads to the formation of a martensite. By knowing the relation between the temperatures used in the process and resulting microstructure - especially martensite volume fraction, it is possible to design industrial process so that the requirements regarding mechanical properties of a material can be obtained. Two experiments were carried out to analyze the mentioned process parameters. First, the quenching experiment, where samples with two different initial microstructures, ferritic-pearlitic and ferritic-martensitic, were annealed in wide range of temperatures and then quenched. Second, the two stage cooling from austenite region, where samples were first annealed at different temperatures just above Ar3, then cooled and held for a few seconds at different intercritical temperatures and finally quenched. Resulting microstructures allowed for evaluation of phase composition and its morphology in relation to temperatures used in experiments.
Keywords: dual phase, annealing, quenching, microstructure.© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.