Strong dependence of martensite morphology in low-carbon dual-phase steel on initial microstructure

• The alteration of martensite morphology generated different mechanical responses. • In the full-annealed steel, successive hard and soft domains were formed. • The initial, full-annealed, and normalized DP samples exhibited higher martensite. • The RSHR mechanism led to higher strength-ductility in the full-annealed sample. • Dominant void initiation mechanism was decohesion of α-martensite interfaces. This study investigates how heat treatments modify the initial microstructure of Fe–0.07C steel and how these changes affect martensite morphology and the resulting mechanical behavior of dual-phase (DP) steels after intercritical annealing (IA). For this purpose, full-annealed, normalized, martensitic, and spheroidized samples were produced and then intercritical-annealed at 830 °C for 15 min, followed by water quenching to produce ferrite-martensite DP microstructures. The full-annealed sample with the highest average ferrite grain size (D f ) of 8.66 µm before IA, yielded a banded ferrite-martensite structure (successive hard and soft domains) with superior performance after IA. A chain-like morphology developed in the initial and normalized specimens, while the martensitic sample exhibited a lath morphology after IA. Also, grain boundary martensite was formed in the spheroidized microstructure after IA. The hardness values for the DP initial, full-annealed, and normalized steel sheets with 0.34–0.37 martensite volume fraction (V m ) were almost 226 HV, while the lowest hardness for DP samples was observed in the DP spheroidized sample with almost 143 HV, and only 0.14 V m ; also, the DP martensitic sample showed a moderate hardness of ∼197 HV and 0.27 V m . All DP samples exhibited superior properties, accompanied by the disappearance of the yield point phenomenon after IA. The highest yield and tensile strength values were achieved for the DP full-annealed sample (413.4 MPa and 661.5 MPa, respectively); whereas the lowest values were observed in the DP spheroidized sample with 308.3 MPa and 499 MPa, respectively. The full-annealed sample after IA revealed the best combination of ductility and strength. The occurrence of recovery of the strain hardening rate (RSHR) mechanism led to higher strength and ductility in the full-annealed sample after IA compared to other DP samples. All DP samples exhibited a perfect ductile fracture. The dominant void initiation in the DP samples was ferrite/martensite decohesion.