U.S. shipments of shredded scrap to foreign steel mills and foundries dropped by nearly 14 percent in September from the previous month as China and Mexico trimmed their intake and Thailand abandoned the U.S. market altogether.

Exports sank to 758,113 tonnes, down a little more than 120,000 tonnes from 878,138 tonnes in August, which stands as the high water mark for the year thus far, according to the latest figures from the U.S. Commerce Department.

Shipments to market leader China sagged to 161,180 tonnes in September, down 54.9 percent from 357,309 tonnes a year earlier, but the year-to-date total...