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• The laminate HMDP steels are produced by a combination of warm rolling and intercritical annealing. • Warm rolling accelerates cementite spheroidization, which enhances the kinetics of austenite formation. • The laminate HMDP steels present superior strength-ductility synergy than the non-laminate counterpart. • The high ductility of laminate HMDP steels is owed to the heterostructure with laminate ferrite surrounded by martensite. The steels with excellent strength and ductility are expected to be achieved by tailoring the microstructural features. In this work, laminate dual-phase (DP) steels with high martensite content (laminate HMDP steels) were produced by a combination of warm rolling and intercritical annealing. Influence of rolling strain and annealing temperature on the microstructural evolution and mechanical properties of laminate HMDP steels were systematically studied. The strength of HMDP steels was significantly improved to ∼1.6 GPa associated with a high uniform elongation of 7%, as long as the laminate structure is maintained. The strengthening and ductilizing mechanisms of laminate HMDP steels are discussed based on the influence of laminate structure and the high martensite content, which promote the development of internal stresses and can be correlated to the Bauschinger effect as measured by the cyclic loading-unloading-reloading experiments. Detailed transmission electron microscopy (TEM) observation was applied to characterize the dislocation structure in the deformed ferrite. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties (strength, fatigue and formability) of dissimilar/similar weld joints between DP780 and DP980 steels were studied. The microstructure in fusion zone (FZ) was lath martensite (LM), and alloying elements in the FZ were uniformly distributed. The hardness in the FZ of dissimilar weld joint was similar to the average value (375 HV) of the two similar weld joints. The microstructural evolution in heat affected zone (HAZ) of dissimilar/similar weld joints was as follows: LM (coarse-grained HAZ) →finer LM (fine-grained HAZ) →M-A constituent and ferrite (intercritically HAZ) →tempered martensite (TM) and ferrite (sub-critical HAZ). Lower hardness in intercritically HAZ and sub-critical HAZ (softening zones) was observed compared to base metal (BM) in dissimilar/similar weld joints. The size of softening zone was 0.2–0.3 mm and reduction in hardness was ∼7.6%–12.7% of BM in all the weld joints, which did not influence the tensile properties of weld joints such that fracture location was in BM. Formability of dissimilar weld joints was inferior compared to similar weld joints because of the softening zone, non-uniform microstructure and hardness on the two sides of FZ. The effect of microstructure on fatigue life was not influenced due to the presence of welding concavity. [ABSTRACT FROM AUTHOR]

DP780 steel sheets consisting of ferrite and martensite were successfully friction stir spot welded (FSSW) at the rotation rates of 500 to 1500 r/min using a W-Re alloy tool. The effect of rotation rate on microstructure and mechanical properties of the FSSW DP780 was investigated. The peak temperatures in the welds at various rotation rates were identified to be above A 3 temperature. FSSW caused the dynamic recrystallization in the stir zone (SZ), thereby producing the fine equiaxed grain structures. At the higher rotation rates of ≥1000 r/min, a full martensitic structure was observed throughout the SZs, whereas at the lower rotation rate of 500 r/min, the SZ consisted of a fine dual phase structure of ferrite and martensite due to the action of deformation induced ferrite transformation. The maximum average failure load as high as 18.2 kN was obtained at the rotation rate of 1000 r/min and the fracture occurred at the thinned upper sheet. [ABSTRACT FROM AUTHOR]