With grain refinement, yield stress of ferritic steels is enhanced depending on the Hall-Petch relation (??y[MPa]=100+600xd[??m]-1/2)1) and it has been already reported in commercial low carbon steels that a high yield strength above 600MPa can be obtained by the grain refinement to a few microns2). On the other hand, it is true that ultra fine grained steels have good ductility despite of its high yield strength. To realize uniform deformation, an adequate work hardening is required corresponding to the flow stress. In this paper, effect of ultra grain refinement on work hardening behavior will be discussed in conventional low carbon steel.
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