The plastic response of martensitic steels is characterized by a steep initial hardening followed by a more gradual hardening regime. Literature attributes this behavior to residual stresses and dislocations inherited from the martensitic transformation, or to microstructural heterogeneities causing strength differences among the grains. Here, we argue that orientation-dependent yielding of lath martensite due to inter-lath sliding, which induces a substructure boundary sliding mechanism, may also contribute significantly to the observed behavior. To demonstrate this, we first propose a homogenized two-scale laminate model dedicated to lamellar microstructures containing softer lamellae, which are sufficiently thin to be considered as discrete slip planes, embedded in a matrix representing the harder martensite laths. Accordingly, the model is constructed as an isotropic visco-plastic model which is enriched with an additional orientation-dependent planar plastic deformation mechanism. We then employ the proposed effective laminate model for the martensite packets in simulations of randomly generated martensitic microstructures. To include the effect of carbon content, different levels of lath strength are considered. As a consequence of the sequential yielding of the martensite packets, depending on their orientation with respect to the applied loading, the macro-scale response of the microstructures exhibits a low yield stress, followed by a significant degree of initial hardening which continues until the saturation stress level is approached. The apparent work hardening rate depends on the contrast between the in-habit plane and out-of-habit plane yield strength used in the model. Moreover, our simulations can qualitatively capture other observations reported in the literature, e.g. that the initial plastic yielding is independent of carbon content and thus of lath strength, and that the uniform elongation increases with the macroscopic strength.