Few industrial mergers in recent years have captured the business world’s imagination quite like the combination of giants Arcelor and Mittal Steel. The two were brought together in June 2006 to form the world’s biggest steel company, ArcelorMittal. It now has 320,000 employees in more than 60 countries and is a global leader in all its major customer markets, including automotive, construction, household appliances, and packaging. In 2007, the company earned revenues of more than $105 billion, while its steel production accounts for roughly 10 percent of the world’s output.

Behind the headlines, the primary challenge of any industrial merger is to integrate the separate management teams, sales and product groups, operating assets, and procurement divisions. At ArcelorMittal, the responsibility for driving this effort fell to Bill Scotting and Jérôme Granboulan, two experienced steel men and veterans of earlier mergers at Mittal Steel and Arcelor, respectively. The London-based Scotting, an executive vice president responsible for strategy at the combined group, and Granboulan, CEO of ArcelorMittal’s tubular-products division headquartered in Rotterdam, recently met McKinsey associate principals Seraf De Smedt and Michel Van Hoey to analyze the lessons of the integration and to review the progress to date.

The Quarterly: In...