You offer a very good description of coating failure when the real-world conditions have exceeded the ability of the coating system to withstand bending of the substrate. This property is referred to as resistance to flexibility, or simply, ‘flexibility.’
Each powder coating has built-in property limits, a result of its original design and composition. The use of the coating in conditions beyond these limitations produces failures such as you describe. The powder coating data sheet should describe flexibility as a minimum diameter bend easily survived at a specific film thickness.
In addition to the given powder coating properties, there are other factors in the coating system that can reduce coating performance when bent or formed.
Film thickness – The coating flexibility will change inversely to film thickness. In other words, flexibility decreases as film thickness increases, just as you have seen on your part. Keep film thickness under tight control, especially on parts destined for further forming. From your description, it seems this is an area to focus on.
Adhesion – Here the relationship is direct: poor adhesion leads to poor flexibility. The quality of the substrate surface during coating application is very important to realizing the physical properties potential from the powder coating. An adhesion weakness can appear as an easily peeled coating in the area of the bend pinholes or cracks. A small blade or even a finger nail can reveal quite a bit about poor adhesion at a bend failure area. You describe a five-stage wash system and you do not mention peeling, so you may be OK here, though hot-rolled steel can be a cleaning challenge.
Cure – Flexibility is definitely reduced when the powder has not been cured properly. Powder coatings tolerate well the typical variations in line cure conditions, but when cured below or beyond the proper range, trouble soon begins. When compared to proper cure, undercure results from shortened oven time, reduced temperature or heavy parts that heat up more slowly. Overcure happens with extended oven time, higher temperature or lightweight parts that heat up readily. Undercure is not suspect here as its failure mode would be open cracks, or even shattering, when bent. Consider overcure, especially if the oven is typically set for heavier parts.
Looking again to the powder coating, it must be capable of surviving post-forming under the overall conditions of your coating system. Further details on your system controls and conditions, the coated parts, the bend radius and on the powder coating properties would be necessary to fully assess this capability.
Should it be that the system controls are proper, you may simply have a case of demanding more from the powder coating than it can provide. If so, the powder supplier can recommend modifications or alternatives that will work within the system parameters.
The move to greater physical performance can involve compromise in other properties. If the coating is reduced gloss or textured, a change in appearance may be necessary for greater flexibility. Changes in specific gravity or basic coating chemistry may also occur when improving flexibility.
Consider what can be done on the coating line to improve results, and consider what aspects of the powder coating may be negotiable if a material change is expected.
Testing flexibility – The common test for flexibility on bend diameters from 1/8 inch to 1.5 inches is ASTM D 522: Standard Test Methods for Mandrel Bend Test of Attached Organic Coatings.
For even tighter bends, the T-bend method is found in ASTM D 4145: Standard Test Method for Coating Flexibility of Prepainted Sheet.