In the 1950s and 1960s August Thyssen-Hütte AG expands to a steel group. In 1954/55 August Thyssen-Hütte AG initially concentrates on acquisitions in the vertically upstream mining and rock products industries to restore its raw materials base. Subsequent horizontal diversification is effected by acquiring majority interests in Niederrheinische Hütte AG (1956), Deutsche Edelstahlwerke AG (1957), Phoenix-Rheinrohr AG Vereinigte Hütten- und Röhrenwerke (1964) and Hüttenwerk Oberhausen AG (1968). The group's range takes in sectional and flat products in all grades through to high-alloy stainless steel; rationalization gains are made by mutually coordinating production operations. In parallel, the iron and steel mills, which had been regarded as optimally sized, were rapidly enlarged. By the mid 1960s, August Thyssen-Hütte AG is Europe's biggest producer of crude steel and ranks number five worldwide. In addition to horizontal diversification, from 1960 a trading organization, Handelsunion AG, is added which in 1969 is renamed Thyssen Handelsunion AG. In the decades that follow, Thyssen Handelsunion AG evolves from an exclusive steel trader into a versatile service provider, concentrating by the mid 1990s on its core businesses materials, industrial and facility services, and project management.
The final phase of horizontal diversification at August Thyssen-Hütte AG leads to specialization through cooperation. In 1969 Mannesmann AG and August Thyssen-Hütte AG agree to a division of responsibilities, summed up in the phrase "tubes for Mannesmann, rolled steel for Thyssen".
By the end of the 1960s August Thyssen-Hütte AG is a monostructured steel group. It starts to change direction in 1973 with the acquisition of Rheinstahl AG, whose operations focus on manufacturing. This takeover reduces August Thyssen-Hütte AG's dominance in the steel sector and it becomes a conglomerate. The broad-based activities of Rheinstahl AG are combined with the corresponding Thyssen businesses in the four newly formed divisions Capital Goods and Manufactured Products, Trading and Services, Stainless Steel and Steel. Consequently August Thyssen-Hütte AG changes its name in 1977 to Thyssen Aktiengesellschaft vorm. August Thyssen-Hütte. To document the fact that Rheinstahl AG now represents the manufacturing division of the Thyssen Group, it is renamed Thyssen Industrie AG in 1976.