What is hot rolled pipe?

Answer 1

Theoretically, the only difference between hot rolled and cold rolled steels is that hot rolled steel is rolled to its final dimensions while hot enough to scale (over 1700 degrees F) while cold rolled steel is rolled to its final dimensions well below scaling temperatures.
So----- If you are making ½” square hot rolled steel, you have to estimate what the final size will be after the product cools, whereas you can finish the cold rolled steel to much closer tolerances right in the sizing rollers and that is what you get.

Answer 2

In addition to the previous person's answer, hot rolled will be more ductile.