  We have studied the rapidity-odd directed flow in $^{197}$Au+$^{197}$Au
collisions in the beam energy range from $\sqrt{s_{NN}}$ = 7.7 to 39 GeV within
the framework of an extended multiphase transport model with both partonic and
hadronic mean-field potentials incorporated. Effects of the partonic
scatterings, mean-field potentials, hadronization, and hadronic evolution on
the directed flow are investigated, and it is found that the final directed
flow is mostly sensitive to the partonic scatterings and the hadronization
mechanism. Our study shows that a negative slope of the proton directed flow
does not necessarily need the equation of state with a first-order phase
transition.
