Experimental and numerical investigation of the yield point phenomenon and strain partitioning behavior in a dual-phase steel with lamellar structure

Abstract

Severe rolling techniques show potential for enhancing the strength and ductility of metallic materials, but the resulting microstructural anisotropy affects the mechanical behavior. The study investigated the anisotropic yield point phenomenon (YPP) and strain partitioning behavior in a dual-phase steel with ultrafine lamellar structure, fabricated through severe cold- and warm-rolling procedures. Experimental results show that tensile specimens aligned parallel to the rolling direction exhibit YPP and higher ductility, while those oriented normal to the rolling direction show continuous yielding behavior and decreased ductility. Using Digital Image Correlation techniques, we studied the evolution of the strain distribution on the surface of the gauge segment, revealing that the strain within the Lüders band was almost constant during the Lüders band propagation. Strain-induced martensitic transformation (SIMT) was observed within the Lüders band and the volume fraction of martensite keeps almost constant during the Lüders band propagation. Numerically, we integrated the mechanisms of SIMT and YPP into a dislocation density-based J2 plasticity constitutive framework. Simulated tensile tests indicate a smaller Lüders strain resulted from the initial higher strain hardening rate. Additionally, SIMT suppresses local instability and therefore promotes the stable propagation of Lüders bands. Simulations verify that the lamellar orientation and SIMT significantly affect the strain hardening rate and void growth. Our findings provided a new perspective on the intrinsic mechanism of the anisotropic YPP and ductility in lamellar dual-phase steels.