Nucor Corp. posted a third-quarter net loss of $29.5 million on sales that fell 58.1 percent to $3.12 billion, citing continued weak steel demand, uncertainty in the U.S. economy and its continued effort to work down high-cost pig iron inventories at its sheet mills.

The Charlotte, N.C.-based company had recorded net income of $734.6 million on sales of $7.45 billion in the same quarter last year.

Nucor logged a pre-tax loss of $21.5 million in the quarter ended Oct. 3 and total steel shipments of slightly more than 4.3 million tons, yielding an operating loss of about $5 per ton shipped. In the same quarter a year earlier, it posted earnings before income taxes of about $1.2 billion and shipments of more...