Structure and Strength of IF Steel After Large Strain Deformation

Abstract

Interstitial free (IF) steel with an ultrafine microstructure has been produced by three different routes: (i) cold rolling, (ii) accumulative roll-bonding (ARB) and (iii) martensitic transformation followed by cold rolling. The microstructure refines with increasing strain without saturation to a value of about 100 nm at an equivalent strain (epsilon(VM)) of 8, which is the maximum strain investigated. At all strains a microscopic analysis by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) shows that the microstructure is subdivided by dislocation boundaries and high-angle boundaries. For both cold rolled samples and ARB samples the flow stress increases as the boundary spacing decreases. For the finest structures this leads to a flow stress at room temperature in the range 900-1000 MPa. Finally structure-property relationships are discussed especially the effect of post-processing treatments by annealing and by low strain deformation.