in which business did andrew carnegie create a monopoly?

Andrew Carnegie created a monopoly in the steel industry. He founded the Carnegie Steel Company, which became one of the largest and most profitable steel companies in the United States during the late 19th century. Carnegie utilized vertical integration by controlling every aspect of production from raw materials to transportation, which lowered costs and improved efficiency. His company dominated the steel market through both vertical and horizontal integration strategies, although it did not eliminate all competition. In 1901, the company was sold and became part of United States Steel Corporation, which marked a more monopolistic position in the industry.