Adhesive bonding has always been an alternative to traditional fastening systems. It sounds like you need a structural high-strength adhesive. It is very important that you choose a system that will: give reliable bonding to the steel, cure quickly to give high strength for handling, and have the durability to maintain the weight of the panel during the lifetime of the panels.
You really need to know what loads the assemblies will carry. The necessity for high strength and load-bearing capability will preclude many adhesives. In your case, a high-performance, two-component epoxy adhesive will be an obvious candidate. These adhesives can be formulated to give highly crosslinked thermoset systems that will resist creep under load for long periods of time. In addition, their cure speed can be varied to suit your assembly process. For example, a system that may take 24-72 hrs. to cure fully at room temperature can often be spot-heated to fixture and hold the parts together in a minute or two.