Why foundries bottleneck wartime shipbuilding

Where did American steel production bottleneck when shipbuilding demand surged?

At the specialized foundries needed for armor-grade and other high-quality naval steel, because they take longest to establish and are costly to retain.

Warships require carefully controlled armor and structural steels. Large quantities of railway or pig iron cannot meet those specifications. Foundries capable of reaching the necessary temperatures and alloy precision take a long time to construct, staff, and bring to reliable output. They are also expensive to keep running when demand falls, so peacetime closures remove exactly the capacity needed during rapid mobilization. The same bottleneck constrained American expansion and British rearmament: scaling shipbuilding depended on preserving specialized industrial processes before the orders arrived.