Ameristeel started in Tampa, Florida in 1937 as Florida Steel Products, a construction industry-oriented reinforcing steel fabricating company, and became Florida Steel Corporation through a merger of steel fabricating groups in 1956. In 1960, Florida Steel began growing beyond its local focus. Product diversity grew along with market diversity, and the Company expanded into rail products, collated nails, and wire mesh with its 1986 purchase of the assets of Lancaster, South Carolina-based Stafford Rail Products, and New Orleans, Louisiana-based Atlas Steel & Wire Company.

The Company's next major expansion was in 1987, acquiring Tennessee-based Knoxville Iron and Steel Service Companies. This acquisition added one more electric steel minimill to the four the Company already owned, and five strategically located reinforcing steel fabricating plants.

In 1996, Florida Steel changed its name to Ameristeel to better reflect its increasing market area and continuing focus on growth and diversification.

In 1999, Ameristeel entered into an agreement with York, PA-based Brocker Rebar Company, Inc., a leading supplier of fabricated reinforcing steel for the concrete construction market, to purchase virtually all Brocker's assets. Ameristeel also acquired Milton Rebar Coating, Inc., an associated reinforcing steel epoxy-coating facility in Milton, PA.

Later that year, international minimill steel manufacturer Gerdau S.A. acquired Ameristeel, becoming Ameristeel's majority shareholder. Headquartered in Brazil, Gerdau S.A. has more than 100 years of successful business operations in North and South America, and is Brazil's largest producer of long-rolled steel and a leader in laminated steel products. Other products include bars, billets, and wire rod. Gerdau S.A. operates in South America and Canada. The Ameristeel acquisition marked Gerdau's first expansion into the United States market.

In 2000, Ameristeel acquired American Bright Bar (ABB), located in Orrville, Ohio and renamed it Ameristeel Bright Bar. Built in 1999, ABB produces cold finished steel bars for steel service centers and other markets across the U.S. An additional steel mill was added in January 2002, when the Company acquired the Cartersville, Georgia Steel Mill from Birmingham Steel Corporation. Later that spring, another Bright Bar location, also located in Cartersville, was purchased from Republic Technologies International.

In August, 2002, Ameristeel announced a merger with Canadian steelmaker Co-Steel. Co-Steel's founder, Gerald Heffernan, began construction on Lake Ontario Steel Company (which became Co-Steel Lasco), with a vision of revolutionizing the steel industry by designing and building a steel plant around the emerging technology of continuous casting matched with electric arc furnaces.

In 1970, Co-Steel incorporated under the laws of Ontario to consolidate the major Canadian holdings of Lasco, North Star, and Ferrco. In 1980, Raritan Steel Company (which became Co-Steel Raritan) was built in Perth Amboy, NJ. Raritan became one of the largest wire rod producers in the U.S.

Gallatin Steel, located in Gallatin, KY, is a 50 percent joint venture with Dofasco. Gallatin Steel began producing flat-rolled products in 1995. That same year, Co-Steel Lasco rolled out its scrap procurement business, creating Co-Steel Recycling. Three years later, in 1998, Co-Steel acquired 100 percent of the outstanding shares of New Jersey Steel, one of the largest producers of reinforcing bar in the Northeastern United States, and renamed it Co-steel Sayreville.

A historical merger between Ameristeel, Co-Steel, Gerdau Courtice Steel and Gerdau MRM Steel took place in October, 2002. The merger added five steel minimills, downstream concrete products and scrap operations in Canada and the U.S., and effectively doubled the size of Ameristeel, making it the second-largest steel minimill organization in North America. After the successful merger, the new organization took the name Gerdau Ameristeel.

In November of 2004, Gerdau Ameristeel acquired North Star Steel from Cargill, Incorporated, adding a team that operates four steel minimills, three wire rod processing facilities and a grinding ball facility to company.

The company's most recent acquisition closed in September, 2007. Gerdau Ameristeel acquired all of the outstanding shares of Chaparral Steel Company of Midlothian, TX. This added a team of employees operating mills at Midlothian, TX and Petersburg, VA to Gerdau Ameristeel.