Although silicon is very Important to prevent carbide precipitation during annealing of low‐alloyed TRIP‐steels and thus allows the austenite to be stabilized by carbon, it causes problems during processing. Therefore, other alloying elements having a similar effect as silicon have to be considered. Possible candidates to substitute or reduce silicon are aluminium, copper and/or phosphorus, which are supposed to be capable of suppressing carbide formation, too.
The influence of reduced silicon contents in combination with alloying elements being capable of substituting silicon completely or partially on the phase transformations occurring during heat treatment is studied. The results of the investigations are compared with a conventional low alloyed TRIP‐steel. The phase transformations are investigated by dilatometric measurements on cold rolled material. The influence of the cooling rate after the intercritical annealing on the transformation behaviour of the austenite upon cooling and isothermal holding in the bainitic range is studied. The interpretation of the phase transformations are supported by investigations of the microstructure via light microscopy and measuring the content of retained austenite using a magnetic volumetric method.
Despite the extensive research and development work done in the field of high strength thin sheet multiphase steel grades, in the past, there are still two main challenges to be tackled, particularly for steel grades with tensile strength levels above 780 MPa. First a challenging optimisation and improvement of the overall processing in the steel plant is still necessary. This includes modified and improved layouts of processing lines. Further, due to the limited application and lacking feedback an overall customer oriented development and optimization is still necessary. Therefore, in a first step, this work reports on the alloy concepts and phase transformations in the annealing lines required for the adjustment of the microstructure of DP and TRIP grades. Then, the most critical processing steps are mentioned and in particular the demanded processing parameters in the annealing lines and the resulting layouts are highlighted. In a further part, three selected items of a costumer oriented development and optimisation of DP and TRIP steel grades are discussed. These include the improvement of the flange‐ability and bendability and efforts oriented towards the reduction of the banded structure in DP and TRIP steel grades. Finally, the improvement of the fracture appearance of spot welded joints of TRIP steels is highlighted and alloy concepts with a balance between weldability and formability are introduced.
The microstructures and the textures of TRIP-assisted and Dual Phase steel in undeformed state and after 10 % strain applied parallel to the rolling direction of the steel sheet were studied by optical microscopy, EBSD, TEM and XRD.It was found that the strain-induced transformation of retained austenite to martensite leads to localized deformation of ferrite close to the ferrite/martensite phase boundaries and the formation of a composite skeleton of several phases (bainite, retained austenite and martensite), clasping the ferrite grains, which thereby decrease in size. Ferrite and retained austenite deform simultaneously to minimize the local stresses at the phase boundaries, until the strain-induced martensitic transformation takes place. The compositelike strengthening behaviour in a TRIP-aided steel might be expressed by the decreasing free path of dislocations in ferrite due to the enlarging and thickening of the multiphase skeleton as plastic deformation progresses, without changing significantly the main texture components in the material.
The evolution of the microstructure of cold-rolled dual-phase steels during annealing is investigated. For this purpose, a cold-rolled dualphase grade is annealed in the laboratory. Annealing cycles are applied to systematically investigate various cooling rates and isothermal holding temperatures and times relevant for continuous annealing lines exhibiting an overaging zone. In addition to the characterization of the microstructure by means of light optical metallography and transmission electron microscopy, mechanical properties are presented.
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