How do you keep a constant velocity of an object Unity 5.5

I have a game object that the player hits around and what I want to do is have a constant speed of that game object even if its hit by the player.

Basically I start the object off with a directional translation which gives the magnitude of the velocity a 0. As soon as the object is hit by the player the magnitude is usually around 15-25. So after a hit by a player its not being moved with this line of code:

          transform.Translate(Vector2.down * speed * Time.deltaTime);

What I was trying to accomplish was the bullet moving in a direction with a certain speed and when the player hits it, it stops relying on that translation and only on the velocity given by the player hitting it but it seems that I can detect the change in magnitude its just not changing the speed. I want the object to stay at a certain speed after being hit and not to go over or under that.

     public float speed;
     private float vel;
     public Rigidbody2D rb;
 
     void Start()
     {
         rb = GetComponent<Rigidbody2D>();
     }
 
     void Update()
     {
         
         vel = rb.velocity.magnitude;
         if (vel == 0)
         {
             transform.Translate(Vector2.down * speed * Time.deltaTime);
             Debug.Log("Moving Via Translate");
         }else{
             Debug.Log("Moving Via player hits");
         }

         Debug.Log("Start velocity:" + vel);
 
         //attempt to slow down gameobject or atleast change the speed.
         if (vel > 25)
         {
             transform.Translate(Vector2.up * speed * Time.deltaTime);
         }
 
     }

Notes: When the magnitude of the object gets above 25 it doesn't seem to change any speeds As well the player is moved with the same .translation(Vector2.down * speed * Time.deltaTime);

I'm someone new to unity so if you could also explain that would be nice!