  We calculate the polarization of $\Lambda$ hyperons in Au-Au collisions at
RHIC Beam Energy Scan range $\sqrt{s_{\rm NN}}=7.7, \dots, 200$ GeV in a
state-of-the-art 3+1 dimensional cascade + viscous hydro model vHLLE+UrQMD. We
find that the polarization of $\Lambda$ in the out-of-plane direction decreases
substantially with collision energy. We explore the connection between the
polarization signal and thermal vorticity and discuss the feed-down and
hadronic rescattering effects on the mean polarization of all produced
$\Lambda$ hyperons.
