Microstructures and mechanical properties of DP and TRIP steels after laboratory simulated strip casting
The laboratory simulation of strip casting of dual phase (DP) and transformation-induced plasticity (TRIP) steels was carried out using heat treatments of dip-cast samples in a dilatometer. The microstructure of DP steel (0.08C-0.81Si-1.47Mn-0.03Al wt. %) consisted of ~80 % ferrite and remaining martensite with little of bainite, whereas the microstructure of TRIP steel (0.17C-1.52Si-1.61 Mn-0.03Al wt. %) consisted of ~65 % ferrite, 4% retained austenite and remaining bainite and martensite. Small amount of Widmiinstatten ferrite was also observed in both steels. The uniaxial tensile tests were carried out on sub-sized specimens. DP steel had the ultimate tensile strength (UTS) of 446 MPa at uniform elongation (e11 ) of 0.17 while TRIP steel had the UTS of 495 MPa at e11 of 0.20, which are lower than those in hot rolled steels. This can be a consequence of cast microstructure (absence of deformation), large grain size, and presence of some Widmanstiitten ferrite. In both steels the predominant fracture mode was ductile, with presence of some isolated cleavage facets associated with the second phase. TRIP steel had a more sustainable increase in strain hardening exponent than DP steel. Crussard-Jaoul model analysis has shown very weak TRIP effect, probably because of low amount of retained austenite ( 4% ).
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