  Based on the assumption of the production of deconfined quark matter, we use
a quark combination model to systematically investigate hadron yields in heavy
ion collisions from RHIC $\sqrt{s_{NN}}=200, 130, 62.4$ GeV to SPS
$E_{beam}=158, 80, 40, 30, 20 $ AGeV. We find that as the collision energy is
greater than or equal to 30 AGeV the yields of various hadrons, their
correlations, in particular, the observables $A=\frac{\bar{\Lambda} k^{-}
p}{\Lambda k^{+} \bar{p}}$ and $B=\frac{\Lambda
k^{-}\bar{\Xi}^{+}}{\bar{\Lambda} k^{+}\Xi^{-}}$, are all reproduced; however,
as the collision energy drops to 20 AGeV quark combination fails. This
indicates that the constituent quark degrees of freedom represent a decisive
factor in thermal hadron production above 30 AGeV and seem to be invalid at 20
AGeV. In addition, hadron yields as well as particle ratios at midrapidity in
the most central Pb+Pb collisions at $\sqrt{s_{NN}}=5.5$ TeV are predicted.
