smooth rotation with external command

Hi, i want manage a vassel but with an aexternal commant wich send some numbers, for example : 4 --> ToTheRight , 5 --> ToTheLeft etc... so, i use this code : Vector3 movement = new Vector3(-1.0f, 0.0f, 1.0f);

         _rigidbody.velocity = movement * _speed;
         _rigidbody.rotation = Quaternion.Euler(0.0f, 0.0f, _rigidbody.velocity.x * -tilt);

but the movement is too rough,

Have you got an idea for a smooth rotation, i want a same movement that i have with this code : float moveHorizontal = Input.GetAxis("Horizontal"); float moveVertical = Input.GetAxis("Vertical");

         Vector3 movement = new Vector3(moveHorizontal, moveVertical, 1.0f);
         _rigidbody.velocity = movement * _speed;
 
         //rotation du vaisseau
         _angleHorizontal = _speed * moveHorizontal;
         _angleVertical = _speed * -moveVertical;
         _rigidbody.rotation = Quaternion.Euler( _angleVertical,  _angleHorizontal, _rigidbody.velocity.x * - tilt);

but this time i don't use keyboard.

Thanks for your answers, and sorry for my english...