Since its foundation in 1910 and until 1970, The Steel Company of Canada acquired roughly 41 companies. These subsidiaries and affiliated companies served several business needs, such as increasing operational efficiencies and diversification, developing new products and services and, expanding into new geographical markets. Stelco, with its subsidiaries, became one type of conglomerate that comprises multiple different businesses. They did not necessarily operate in the same location, nor were they all in the same line of business. Instead, they played a complementary business role. Subsidiaries also had their own sub-companies, a line of succession forms or a corporate group with varying degrees of ownership. Several subsidiaries were founded at the beginning of 20th century and some of their oldest records are available in this series.
Subsidiaries had their own boards and directors'. However, approvals and decision-making pertaining to investment, expenditures or accounting issues were managed in cooperation with Stelco. Transaction and decisions between Stelco and its subsidiaries produced a large amount of records including photographs. They consists of:
- corporate records such as Charters, By-laws, letters patent, minutes of meetings and agreements;
- financial records: several types of ledgers, reports on sales, shares or stock books, appraisals and accounts;
- correspondence and reports with Stelco's executives;
- catalogues, advertisings and clippings.
Stelco's subsidiaries were founded at the beginning of 20th century and some of their records are available in this series. Among the oldest are: The Superior Rolling Mill, The Frost Wire Fence Company, The Canadian Drawn Steel Company, The Manitoba Frost Wire Fence Company, The Page-Hersey Iron and Tube Company, The Stave Manufacturing Company, The Laidlaw Bale Tie and Wire Company, The Stelco Coal Company of Pennsylvania, The Brantford Navigation Company, The Mohawk Conduit Company, The Hamilton By-Product Coke Ovens, The Ontario Forging Company, and others.
The series contains photographs and drawings showing the production of steel in farming (fences and tractors), in construction of buildings (Maple Leaf Gardens and 1st phase Jackson Square), boats (corvette boat and cargos) and pipelines (TransCanada, gas and oil, and plumbing). There are also some portraits showing employees and managers.