By the end of the 1960s already, it was clear that Siemag Siegener Maschinenbau AG could only continue to maintain its international competitiveness by merging with another major market player. Consequently, Siemag Siegener Maschinenbau GmbH and Schloemann AG concluded a cooperation agreement in 1972. Previously, both companies had handled joint orders as part of international consortia, and their product and service portfolios complemented each other. Based on this cooperation agreement, they merged to form Schloemann-Siemag AG in December 1973. GHH Aktienverein, which later became M.A.N., held 51 percent of the shares in the new company, and Siemag Weiss KG 49 percent. Both partners each held half of the voting capital. In 1974, Heinrich Weiss took over the helm of the amalgamated company.

The name "SMS", which was already being used as the short form for Schloemann-Siemag AG, officially became part of the company's name on November 1, 1980. From then on, the company was known as SMS Schloemann-Siemag AG.

The merger of SMS Schloemann-Siemag AG with Mannesmann Demag Metallurgie resulted in the establishment of SMS Demag AG on September 1, 1999. The new company combined the strengths of both companies and thus consolidated its position on the market. The year 2000 saw the spin-off of the tube and copper plants business unit to create SMS Meer GmbH. SMS Meer GmbH was responsible for the "Long Products" business unit, while SMS Demag AG represented the "Flat Products" business unit.

Gaining full control of SMS was a particular objective of the Weiss family. In 2003 and 2007, it acquired the shares in M.A.N. through the family holding company and, in doing so, became the sole owner of SMS group.

In March 2009, the traditional name of "Siemag" returned. Following its "back to the roots" motto, SMS Demag AG was renamed SMS Siemag AG. Mid-2015 was when SMS Siemag AG and SMS Meer GmbH were merged to form SMS group GmbH.