Effect of pre-straining on the forming limit of interstitial free high strength steel: Insights on microstructure and crystallographic texture evolution

Abstract

In the present work the forming behaviour and microstructure evolution of Interstitial free high strength (IFHS) steel have been investigated through Marciniak test (in-plane deformation) and miniature LDH test (out of plane deformation). Initially, the as-received IFHS steel sheet of 0.7 mm thickness has been deformed up to 2 % and 4 % strain in plane strain and biaxial condition through Marciniak test. Afterward, the miniature uniaxial, plane strain and biaxial specimens are fabricated from the bottom of the dome of the Marciniak test. The obtained samples are further deformed up to necking through miniature LDH test to generate the FLD. Thereafter, evolved microstructure has been investigated on the formed miniature samples at different strain levels and strain paths using electron backscattered diffraction (EBSD) and x-ray diffraction (XRD) techniques. The improved formability is noted due to the impact of pre-strain (both plane strain and biaxial pre-straining) on forming limit diagrams. The 2 % plane strain pre-straining increased uniaxial, plane strain, and biaxial responses by 17.12 %, 64.73 %, and 131.28 %, respectively, while 4 % resulted in 13.36 %, 49.02 %, and 64.94 %. Similarly, 2 % biaxial pre-straining caused 6.18 %, 56.39 %, and 129.88 %, whereas 4 % led to 26.78 %, 67.81 %, and 83.05 % increased formability. The microstructural analysis revealed that after 4 % pre-straining, the development of misorientations (KAM, GAM) and GOS is nearly negligible compared to 2 % pre-straining. At 2 % pre-strain, the development of dense dislocation structures and sub-grain boundaries caused more localized deformation and misorientation formation, explaining the higher levels of misorientation. However, as the strain increased to 4 %, the dislocations began to rearrange, forming a more stable structure, which reduced the overall misorientation and its distribution. XRD bulk texture analysis revealed that an increase in the fraction of copper orientation resulted in lower misorientation values, as observed in the near-neck region of the 2 % pre-strained specimen in comparison to that of neck-region. Another notable observation from the plot is that when the material was deformed from as-received condition to pre-straining the increase in Brass and Goss orientations led to higher misorientation under both uniaxial and biaxial strain conditions.