According to Newton's first law of motion, an object will remain at a constant velocity (including a constant speed) unless acted upon by an external force. In the scenario you described, where a body has accelerated and then acquired a constant speed, there would indeed be no net force acting on the body once it reaches that constant velocity.
When the velocity of an object becomes constant, it means that the acceleration is zero. In the equation you mentioned, F = m(vf - vi) / ∆t, if the final velocity (vf) is equal to the initial velocity (vi), then the numerator becomes zero. As a result, the net force (F) acting on the object would be zero.
However, it's important to note that even though there is no net force, the body will continue to move with constant velocity due to its inertia. Inertia is the property of matter that resists changes in its state of motion. Once the body reaches the constant velocity, it will tend to maintain that velocity unless acted upon by an external force.
In summary, when a body reaches a constant speed and maintains that speed, there is no net force acting on it according to Newton's laws. The body continues to move due to its inertia, which keeps it going at a constant velocity.