  Due to a ferromagnetic in-chain coupling between Co$^{3+}$ ions at trigonal
sites, chains Co$_2$O$_6$ are considered as large rigid spin moments. The
antiferromagnetic Ising model on the triangular lattice is applied to describe
an interchain ordering. An evolution of metastable states in a sweeping
magnetic field is investigated by the single-flip technique. At the first
approximation two steps in the magnetization curve and a plateau at 1/3 of the
saturation magnetization are found. Four steps in magnetization are determined
in high-order approximations in agreement with experimental results.
