  A quasi-linear reduced transport model is developed from a database of
high-$\beta$ electromagnetic nonlinear gyrokinetic simulations performed with
Spherical Tokamak for Energy Production (STEP) relevant parameters. The
quasi-linear model is fully electromagnetic and accounts for the effect of
equilibrium flow shear using a novel approach. Its flux predictions are shown
to agree quantitatively with predictions from local nonlinear gyrokinetic
simulations across a broad range of STEP-relevant local equilibria. This
reduced transport model is implemented in the T3D transport solver that is used
to perform the first flux-driven simulations for STEP to account for transport
from hybrid-KBM turbulence, which dominates over a wide region of the core
plasma. Nonlinear gyrokinetic simulations of the final transport steady state
from T3D return turbulent fluxes that are consistent with the reduced model,
indicating that the quasi-linear model may also be appropriate for describing
the transport steady state. Within the assumption considered here, our
simulations support the existence of a transport steady state in STEP with a
fusion power comparable to that in the burning flat-top of the conceptual
design, but do not demonstrate how this state can be accessed.
