Predictions of the proton-directed flow $v_1$ in semicentral $\mathrm{Au}+\mathrm{Au}$ collisions in the energy range between 4.5 and 7.7 GeV are done. The calculations are performed within the model of three-fluid dynamics with crossover equation of state, which well reproduces the proton $v_1$ both below 4.5 GeV and above 7.7 GeV, as well as bulk observables in the energy range of interest. It is predicted that the proton flow evolves nonmonotonously. At the energy of 7.2 GeV it exhibits antiflow [i.e., negative slope of $v_1\left(y\right)$] in the midrapidity. At 7.7 GeV, the flow returns to the normal pattern in accordance with the STAR data. The midrapidity $v_1$-slope excitation functions within the first-order phase and crossover transitions to quark-gluon phase (QGP) turn out to be qualitatively similar, but the amplitude of the wiggle in the crossover scenario is much smaller than that in the strong first-order phase transition. Therefore, the change of sign followed by minimum at 7.2 GeV in the $v_1$-slope excitation function indicates onset of (weak phase or crossover) transition to QGP. The second change of the sign around 10 GeV results from interplay between incomplete baryon stopping and transverse expansion of the system.

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• Collective flow
• Quark-gluon plasma
• Relativistic heavy-ion collisions
• Hydrodynamic models