See the Math Potential Energy and Force

The potential energy between two bound quarks grows linearly with the distance between the quarks:

potential energy between two quarks

V(x) is the potential energy, which is a function of the distance, x, between the quarks. The force between the quarks is proportional to the derivative (rate of change) of the potential:

force between the quarks is proportional the derivative (rate of change) of the potential

Therefore, the strong force between two quarks stays constant as the quarks are pulled apart.