Iron ore shipments on the Great Lakes totaled 6.6 million tons in July, a year-over-year decline of 9 percent.

And shipments of the key steelmaking ingredient from U.S. ports fared even worse, declining by 14 percent compared to July 2014.

About 1.5 tons of iron ore must be burned in a blast furnace to make a ton of steel. The Lake Carriers Association said the steep drop-off in July reflected cheap steel imports that currently command a record 31 percent of the U.S. market.

Iron ore trade on the Great Lakes stands at 27.7 million tons so far this year, an increase of 4.5 percent over the same period in 2014 but 5.2 percent lower than the long-term average.

Another key steelmaking input performed better last month. Metallurgic coke cargoes on the Great Lakes rose 23 percent over last year at 657,000 metric tons, according to the Chamber of Marine Commerce.

Year-to-date cargo tonnage through the St. Lawrence Seaway was only 14.5 million metric tons, a 7 percent decrease. Grains have been a bright spot with a 63 percent increase in the corn that's sent to Canada and the soybeans that are shipped off to international markets.

"The Great Lakes-Seaway is a vital trade corridor for American grain exports which will continue to be relied on into autumn when the new harvests begin," Chamber of Marine Commerce President Stephen Brooks said. "The more buoyant U.S. economy is also creating demand for other products such as construction, manufacturing and steel-making materials."