Enhancing strength and ductility in ultrahigh-strength steel via flash-heated IQP with lamellar heterostructure and chemical heterogeneity

• A novel processing route combining full austenitization, intercritical quenching, and flash heating Q&P was developed. • The microstructure shows ferrite-martensite lamellar structure with Mn-induced chemical heterogeneity in martensite. • The sample shows greater retained austenite fraction and stability than conventional quenching and partitioning steels. • With 0.24 wt% C and 2.27 wt% Mn, the sample achieves an outstanding product of strength and elongation of 35.8 GPa·%. • The sample enhances both strength and ductility beyond traditional Q&P steels, breaking the strength-ductility dilemma. To develop ultrahigh-strength steel with both excellent mechanical properties and cost efficiency, this study integrates complete austenitization (CA), intercritical quenching (IQ), and flash heating quenching and partitioning (FHP). The resulting microstructure features a heterogeneous lamellar structure (HLS) of ferrite and martensite, along with chemical heterogeneity (CH) of the manganese (Mn) element within the martensite. The CA + IQ + FHP treatment imparts the steel with an exceptional strength-ductility synergy (YS: 1176 MPa, TS: 1868 MPa, TE: 19.2 %), which can be primarily attributed to the high density of geometrically necessary dislocations (GNDs) and the alignment of dislocations in ferrite that is perpendicular to the interfaces between soft and hard phases, thereby inducing significant hetero-deformation induced (HDI) strengthening. Nanoindentation tests demonstrate that the CA + IQ + FHP sample exceeds the rule of mixtures (ROM) due to HDI strengthening, a finding further validated by loading–unloading-reloading (LUR) tensile tests. In comparison to conventional quenching and partitioning (Q&P) steels, the CA + IQ + FHP sample achieves a superior product of strength and elongation (PSE) of 35.8 GPa·% with low carbon (C) (0.24 wt%) and Mn contents (2.27 wt%). Although the transformation-induced plasticity (TRIP) effect is not activated, both the fraction and stability of retained austenite (RA) are greater than those observed in traditional Q&P steels.