| Two types of small strain deformation behaviors of IF steels were studied. The role of solute phosphorus in affecting the small strain yielding behavior was investigated in two IF steels containing 20 ppm and 600 ppm bulk P levels. The role of crystallographic texture in affecting the small strain unloading or springback behavior was theoretically modeled to identify those textures which would impart minimum variability in springback of IF steel sheets.; The addition of solute P brings about solid solution strengthening, the magnitude of which was determined to be 2.38 MPa/0.01 wt % P. The segregation of solute P to ferrite grain boundaries brings about extensive grain boundary hardening (GBH), which results in a substantial increase in the Hall-Petch slope, ky. Based upon the results from Auger Electron Spectroscopy (AES), tensile testing and hardness measurements, the increase in the Hall-Petch slope due to phosphorus segregation is quantified to be ∼18.6 MPa.mum 1/2/at% P segregated. Grain boundary hardening is postulated to be caused by a combination of dislocation forest hardening and the pinning of these grain boundary dislocations by segregated solute P.; Additionally, the role of FeTiP and FeTiNbP precipitates in affecting the strength of IF steels has been studied in Ti-bearing and (Ti+Nb)-bearing IF steels. The presence of large volume fractions of grain boundary phosphides results in a decrease in the strength due to scavenging of solute P at the grain boundaries.; Theoretical modeling proved that gamma-fiber texture components exhibit isotropic springback behavior in the plane of the sheet on account of their high in-plane elastic isotropy, whereas, alpha and epsilon fiber components exhibit highly anisotropic springback behavior in plane strain bending applications. FEM modeling in draw-bend test applications shows that sheet steels with high elastic moduli and low yield strengths along the axis of the draw-bend test specimen exhibit minimum springback. Thus, the presence of high volume fractions of gamma fiber components in IF steel sheets provides excellent deep drawability as well as isotropic in-plane springback behavior. |
