You say Y-axis does this imply in a horizontal plane?
What kind of acceleration do you plan to have?
What are you accounting for in your calculation (friction, inertia, static load, damping ...)?
My calculations give 75 newtons as the axial force along the ballscrew with those dimensions and a .12Nm torque, neglecting any losses.
keep in mind the torque output of a stepper motor is not constant over the speed range.