Looking through this thread, I didn't see any mention of three events that relate to all this:
- the Panic of 1837
- the Crimean War (1853-56)
- the Panic of 1857
In the 1830s, there was a boom in the US, particularly in iron manufacturing. The older technology was the type of furnace operation found in what became The Wilderness in Virginia; the newer tech was found in places like Pittsburgh (and cost about 10 times more). Hard times started in Britain; British manufacturers started dumping excess production in the US, driving down prices; the Panic of 1837 followed and spread from Britain to the US. US iron manufacturers started dropping like flies, particularly those with the old tech (less efficient, many newcomers who were overextended/underfunded.etc.) Bankruptcies followed. The Virginia iron industry that had prospered from about the 1720s west of Fredericksburg died out (furnaces and farms and wood-cutting operations abandoned) to become The Wilderness by 1860 -- similar problems hit in PA and NJ and elsewhere. US manufacturers going broke developed a grudge against the British that would be long-lasting.
The Crimean War period saw British production largely devoted to the war. It also saw the British and French merchant marine cut off from the Russian trade, so the Americans rushed in. You'll see a large increase in the US import of Pig Iron about then (the Russians were the low-cost/low quality pig iron producer, but could not sell to the British and French, so they sold to the US, which took over the Russian carrying trade while the war continued). Once the Crimean War ends, the British and French shipping interests moved in on trade with Russia (cutting out the Americans) and British iron/steel interests started dumping excess capacity to the US (the British were the world's high quality/high efficiency producer)
All this leads into the Tariff of 1857 and the Panic of 1857. With excess capacity after the War ended, the British started aggressively exporting to the US again (or dumping excess capacity). At the same time, the US shipping industry hit a bust: excess capacity because of the California Gold Rush ship-building boom, sail-vs-steam tech, and the general downturn in the US business. Also a financial crisis exacerbated by the bad-timing of the low Tariff of 1857, the Buchanan administrations' spending and opposition tothe government having cash, and the general financial downturn following years of expansion. The iron-and-steel interests, particularly in PA and parts of NJ, see the British as taking advantage of this and want protective tariffs to protect themselves-- remembering full well their grudge from 1837.
Those events play a large part in the pattern of imports of iron and steel before the American Civil War. They also largely explain why the country (particularly PA-NJ) wanted higher protective tariffs in 1860-61 (bailing out the government and protecting the iron-steel manufacturers)