This work deals with the deconfinement phase transition from a hadronic gas (HG) phase consisting of massive pions, to a quark-gluon plasma QGP phase consisting of gluons, massless up and down quarks and massive strange quarks, in addition to their antiquarks. Based on the Bag and coexistence models, we study the variations of the pressure characterizing the HG and the QGP phases. For this latter, we calculate the partition function of the color-singlet QGP within the projection method, using a density of states containing curvature and area terms additionally to the volume term. We investigate the phase diagram of the strongly interacting matter, in the T-µ plane, in several cases: with 2 massless quarks then when adding massive strange quarks in the QGP phase.