• The prior austenite grain size of martensite in the martensite-ferrite dual-phase microstructures was adjusted. • The nanotwins were observed as a main substructure of the refined martensite. • The refined martensite in the dual-phase steel led to a higher total elongation with respect to the coarse one. • The IHAZ retained in the steel after DQ treatment was conducive to enhance the post-uniform logarithmic strain. A ferrite-austenite 12CrNi2 alloy steel additively manufactured by laser melting deposition (LMD) has been treated by various quenching methods, including direct quenching (DQ), inter-critical quenching (IQ) and step quenching (SQ), respectively, to form the martensite-ferrite dual-phase steel with a similar major volume fraction (62 vol.%) but different grain sizes and lath substructures of martensite. The results demonstrate that SQ, DQ and IQ help to refine the grains of martensite in order, correspondingly decreasing the yield strength, ultimate tensile strength but increasing in the uniform logarithmic strain of the alloy steel. The effect of not only the interface structures and substructures of martensite but also the quenching-route-dependent structures of ferrite on mechanical properties was interpreted in detail. Curiously, the directly quenched steel displays a higher post-uniform logarithmic strain as compared to the inter-critically quenched steel. The abnormal result occurs due to that DQ cannot completely homogenize the microstructural difference between the laser-scanned layers and the interlayer heat-affected zones of the as-deposited alloy steel.