Abstract
Rolling deformation results in the transformation of a lath martensite structure to a lamellar structure characteristic to that of IF steel cold-rolled to medium and high strains. The structural transition takes place from low to medium strain, and electron backscatter diffraction analysis shows that the frequency of medium angle boundaries with misorientation angles of 5-10° decreases with increasing strain, while the frequencies of boundaries with angles in the ranges of 1-5° and 10-25° increase, resulting in the evolution of a bimodal misorientation angle distribution. The microstructural evolution and the strength are characterized for lath martensite rolled to a thickness reduction of 30%, showing that large changes in the misorientation take place, while the strain hardening rate is low.
Highlights
Martensitic transformation (MT) is an efficient grain refinement process, which introduces high densities of high angle boundaries, low angle boundaries and dislocations
3.1 Un-deformed martensite Here we use an inverse pole figure (IPF) coloring in the electron backscatter diffraction (EBSD) maps to show the microstructure of the initial sample without deformation, as shown in figure 2
The percentage of misorientations between 5-10o is about 24 %, which indicates the presence of a high number of sub-block boundaries in the lath martensite (LM), in agreement with previous observations [2,3,7]
Summary
Martensitic transformation (MT) is an efficient grain refinement process, which introduces high densities of high angle boundaries, low angle boundaries and dislocations. In low carbon (below 0.4 wt % C) steels, the martensitic structure is divided at decreasing scale into packets, blocks, sub-blocks and laths, as shown in figure 1. A combination of MT and PD can further enhance the grain refinement, the hardening behavior of LM is different from that of the ferrite structure, especially at low strains [7]. The present study follows previous research [7], with an aim of characterizing the deformed LM martensite at low strains (up to 30% rolling reduction) using electron backscatter diffraction (EBSD). The structure evolution is analyzed based on the changes of misorientation angles, as obtained from the EBSD data, and the evolution of mechanical properties with deformation is discussed. To allow reliable statistics data for more than 1000 misorientation angles were collected in different areas of the EBSD maps for each sample
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