A ring A of mass $m$ is attached to a stretched spring of force constant $K$, which is fixed at $C$ on a smooth vertical circular track of radius $R$. Points $A$ and $C$ are diametrically opposite. When the ring slips from rest on the track to point $B$, making an angle of $30°$ with $AC$ ($\angle ACB = 30°$), the spring becomes unstretched. Find the velocity of the ring at $B$.