Effect of warm rolling reduction on ferrite substructure and tensile fracture behavior in dual-phase steel

Heterostructure design represents an effective strategy for enhancing the synergy between strength and ductility in low-carbon microalloyed (LCMA) steels. The key challenge lies in regulating the interface behavior between soft and hard phases to exploit the hetero-deformation-induced (HDI) hardening effect while suppressing premature damage caused by interfacial strain localization. To address this challenge, the study proposes a "soft-phase substructure regulation" strategy. Through warm rolling processes with different reductions (R = 0%, 45%, 65%, and 85%), the phase fraction, strength difference, and substructure (low-angle grain boundaries, LAGBs) distribution between the soft phase (polygonal ferrite, PF) and the hard phase (layered structure, LS) are controlled, thereby improving the coordinated deformation ability of the heterostructure. The results show that under a 45% reduction, a high-density LAGBs network (1.96 × 10 9 mm/m 2 ) forms in PF grains, which not only enhances the hardening of PF (2.24 GPa) but also induces a strong HDI hardening effect. Additionally, this LAGBs network acts as a dislocation source, promoting the formation of multi-level slip microbands, thereby achieving uniform strain distribution and multi-stage strain hardening. Ultimately, the yield strength of the sample increases by 37.8% (from 498 to 686 MPa) while maintaining an elongation of 24.6%. In contrast, although the sample with a high reduction (85%) shows a higher strength of 858 MPa, the excessive hardening of PF (2.77 GPa) leads to strain localization in the grain boundary region, and the elongation is reduced to 14.8%. The microscopic damage mechanism shifts from PF-dominated crack-tip blunting to interface delamination-dominated quasi-cleavage fracture.