Effect of CAL parameters; temperature, time and cooling rates, on Ferrite+ martensite phase formation in DP steel
Design and development of continuous Annealing process parameters in full hard CR DP Steel in obtaining requisite ferrite + martensite structure by using annealing Simulator
Dual-phase steels derive their perfect blend of properties via hard second phase, namely martensite or bainite in a softer ferrite matrix. Key to refine the mechanical properties of DP steels rests on optimizing and tailoring the hard second phase distribution and size in the ferrite matrix. There could be several combinations of processing routes depending on governing mechanisms such as recrystallisation, phase transformation and pearlite dissolution, which can affect the morphology and distribution of the martensite phase. All these mechanisms are invoked at various stages of annealing process cycle. In the present study experimental simulation of various annealing parameters were carried out on a cold rolled steel using a custom designed annealing simulator. The evolution of microstructure was studied by field emission scanning electron microscope. The evolving microstructures were correlated with governing mechanisms of recrystallisation, pearlite dissolution and phase transformation. Through these simulations it was possible to tailor the microstructure and consequently improve the tensile properties of dual phase steel.
The present study addresses the effect of various physical phenomena occurring during annealing of cold rolled steel on formation of dual phase microstructure and its impact on tensile properties.
- Heating rate and annealing temperature ranging below inter-critical annealing temperature (Ac1) to upper critical-annealing temperature (Ac3) seems to greatly influence the final microstructure of dual phase steel.
- Ferrite recrystallisation before austenite nucleation promoted ferrite “in-grain” martensite formation. Further, effect of high heating rates causing grain boundary martensite formation could be minimized by ferrite recrystallisation and pearlite dispersion.
- Continuous heating to peak temperature without isothermal annealing resulted in “in-grain” martensite formation with serrated interfaces. This type of martensite was observed to improve the tensile properties of dual phase microstructure.
The modified annealing process showed predominant formation of martensite within the ferrite grains with serrated lath martensite interfaces. This nature of the martensite was considered responsible for the observed improvement in the tensile properties and bake-hardening response. Furthermore, along with improved bake-hardening response negligible loss in tensile ductility was also noted. This behaviour was correlated with delayed micro-crack initiation at martensite interface due to serrated nature.
Fig.2: (a) Micro crack at the martensite-ferrite interface at conventional DP steel; (b) microvoids coalescence at the interface at modified DP steel; (c) Engineering stress-strain plots corresponding to both steels.