<P> Slag, a byproduct of iron and steel - making composed primarily of highly impure glass, would normally be a waste product . However, it is in demand as an aggregate in concrete, asphalt paving, and construction fill . In 2014, the industry produced and marketed about 16.0 mt of slag, worth an estimated $270 million . </P> <P> The United States has been a major importer of steel and steel mill products since the 1960s . In 2014, the US exported 11 million tons of steel products, and imported 39 million tons . Net imports were 17 percent of consumption . As of 2012, the largest sources of net steel imports to the US were, in descending order, the European Union, Brazil, South Korea, Japan, and Russia . </P> <P> The US iron and steel industry has paralleled the industry in other countries in technological developments . In the 1800s, the US switched from charcoal to coke in ore smelting, adopted the Bessemer process, and saw the rise of very large integrated steel mills . In the 20th century, the US industry successively adopted the open hearth process, then the basic oxygen furnace . Since the American industry peaked in the 1940s and 1950s, the US industry has shifted to small mini-mills and specialty mills, using iron and steel scrap as feedstock, rather than iron ore . </P>

List of steel mills in the united states