AccelStepper Library: How do I go to a position without decelerating?
AccelStepper, as hinted by the name, uses acceleration always. By setting the acceleration high, you reduce the amount of time spent accelerating to the set speed.

setAcceleration() sets the rate in steps per second per second. If your MaxSpeed is 200 steps per second, then setting the acceleration to 200*10 means all acceleration is over in 1/10 second. You can go higher, etc.