Abstract
The ferrite-pearlite microstructure was cold-rolled to form dual phase (DP) steels, the percentage reduction of which varied. To do so, the steels were annealed in two steps and then the workpiece underwent water quenching. Accordingly, a decrease was observed in the average size of the ferrite grains, from above 15 µm to below 2 µm, subsequent to the thermomechanical processing. By an increase in the reduction percentage, the volume fraction of martensite grew. The balance between strength and elongation also improved nearly 3 times, equivalent to approximately 37,297 MPa% in DP in comparison to 11,501 MPa% in the ferrite-pearlite microstructure, even after 50% cold-rolling. Based on Hollomon and differential Crussard-Jaoul (DC–J) analyses, the DP steels under investigation deformed in two and three stages, respectively. The modified C–J (MC–J) analysis, however, revealed that the deformation process took place in four stages. The rate of strain hardening at the onset of the deformation process was rather high in all DP steels. The given rate increased once the size of the ferrite grains reduced; an increase in the volume fraction of martensite due to larger percentage of reduction also contributed to the higher rate of strain hardening. The observation of the fractured surfaces of the tensile specimens indicated ductile fracture of the studied DP steels.
Highlights
Dual phase (DP) steels, which are made up of hard martensite and soft ferrite grains, are formed by the common low alloy steels
The lattice geometrically necessary dislocations (GNDs) distribution maps was performed using curvatures which reflect from the local changes in the lattice orientation can be considered to calculate
The yielding behavior gradually changed to a usual continuous pattern subsequent to the intercritical annealing, which is typical of DP steels
Summary
Dual phase (DP) steels, which are made up of hard martensite and soft ferrite grains, are formed by the common low alloy steels. The nucleation sites of the ferrite are enhanced during cooling by the use of controlled rolling, in which the size of the grain reduces and the strain of the austenite lowers. In this technique, dislocations accumulate, annihilate and rearrange themselves and the grains recrystallize and grow, thereby evolving the microstructure of the sheet. Hot rolling and large deformation can effectively contribute to the formation of UFG in the DP microstructures, show two major limitations This structure entails the application of costly machinery having huge capacity for loading.
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