  Flat-bands induced by destructive interference of hoppings in frustrated
lattices such as kagome metals, have been extensively studied in recent years
since they may lead to strongly correlated phenomena. However, in realistic
multiorbital $d$-electron materials, such flat-bands usually appear in small
portions of Brillouin zone and are away from Fermi level (dubbed as
``incipient"). Whether such incipient flat-band portions can induce very strong
electron correlations is an open question. Here, by density functional theory
plus dynamical mean-field theory calculations on the superconducting kagome
CsCr$_3$Sb$_5$ and triangular CrB$_2$, we show that the roles of such incipient
flat-bands in driving electron correlations can be significantly amplified by
Hund's coupling $J_{\text{H}}$ in a Hund's metal. As a result, even moderately
$d$-electron heavy-fermions could be induced by enhancing the orbital
differentiation and Kondo-like effect of Hund's metals. This provides a
flexible route for generating $d$-electron heavy-fermion, and for controllably
inducing electron correlations and magnetic fluctuations suitable for emergence
of unconventional superconductivity, in frustrated Hund's metals with portions
of incipient flat-bands.
