Final answer:
J.P. Morgan renamed Carnegie Steel to U.S. Steel after his purchase of the company in 1901, creating the first billion-dollar corporation in the United States.
Step-by-step explanation:
After purchasing Carnegie Steel, J.P. Morgan changed its name to U.S. Steel. This monumental transaction in 1901 led to the creation of the U.S. Steel Corporation after Morgan consolidated Carnegie's holdings with several smaller steel firms. With a capitalization of $1.4 billion, it became the country's first billion-dollar corporation, navigating an era where the domestic steel industry was crucial to the nation's expansion and industrialization. This consolidation allowed Morgan to control every aspect of steel production and distribution, making U.S. Steel a dominant force in the industry.