<p>I have a Flashforge Creator Dual.</p>

<p>One corner of my print bed is warped down.  I am thinking about having a steel print bed made so it would tend to stay flat. </p>

<p>Has anyone tried this?</p>

I would consider getting another aluminum build plate for the following reasons:
Lightweight
. Aluminum is a very lightweight metal, making it suitable for most machines that have injection molded platform arms. This reduces potential sagging of the arms and overall load on the -Z- axis stepper motor.
Conductivity
. Referring to
this
simple Google search for heat conductivity, aluminum is significantly more conductive than steel (205 vs 43 respectively) with copper at about double that of aluminum.
Availability
. Aluminum is already a widely used material for 3D printing, so finding one will be relatively easy and probably cheaper than having a steel build plate custom made.
In conclusion, I personally would not recommend using steel for your build plate as aluminum seems to have the most benefits. Yes, steel will be more rigid and durable, but I don't think that these should be variables that are significantly more beneficial over aluminum.