  The properties of phase diagram of strange quark matter in equilibrium with
hadronic matter at finite temperature are studied, where the quark phase and
hadron phase are treated by baryon density-dependent quark mass model and
hadron resonance gas model with hard core repulsion factor, respectively. The
thermodynamic conditions for the formation of metastable strange quark droplets
("strangelets") in relativistic nuclear collisions are discussed. We obtained a
rich structure of the phase diagram at finite temperature, and study the
dynamical trajectories of an expanding strange fireball. Our results indicate
that the strangeness fraction fs, perturbation parameter C, and confinement
parameter D have strong influence on the properties of phase diagram and the
formation of strangelets. Consider the isentropic expansion process, we found
that the initial entropy per baryon is less than or equal to 5, which gives a
large probability for the formation of strangelets. Furthermore, a sufficiently
large strangeness fraction fs and one-gluon-exchange interaction and
sufficiently small confinement interaction create possibilities for the
formation of strangelets. On the contrary, the fireball will always complete
the hadronization process when fs=0 or C>=0 or D^{1/2}>=170 MeV.
