Mannesmann initially remained purely a processor of steel and was therefore dependent on supplies of input stock from other companies. At the start of the 20th Century therefore, the primary strategic goal was to develop the company’s ability to supply its own material requirements. The first step was taken in 1906 with the acquisition of Saarbrücker Gußstahlwerke AG, in order to supply the Bous plant with the requisite high-quality input stock. Further acquisitions included the Blechwalzwerk Grillo Funke, which had its own steel-making facilities, as well as collieries, ore mines and limestone quarries. Finally, in 1929, the company’s own iron and steel works was commissioned in the Duisburg suburb of Huckingen. Mannesmann was now a vertically structured coal and steel group, an arrangement which remained typical of German heavy industry until the 1970s.
For Mannesmann, too, the 1930s and 1940s were dominated by the armaments and self-sufficiency policies of the National Socialist government. To compensate for the lack of labor during World War II the company employed prisoners of war and forced labor to maintain production.
After the end of the War Mannesmannröhren-Werke was liquidated by order of the Allies and divided in 1952 into three independent companies, Mannesmann AG, Consolidation Bergbau AG and Stahlindustrie und Maschinenbau AG. By 1955, however, the business was reunited under the management of Mannesmann AG, which from then on became the head of the Group.
Between 1952 and 1955 Mannesmann established tube works in Brazil, Canada and Turkey. The operation in South America was combined with a smelting and steel-making plant.