Andrew Carnegie made his fortune primarily through the steel industry, becoming one of the richest Americans in history by building the Carnegie Steel Corporation into the largest steel manufacturing company in the world

. His wealth originated from several key business activities and strategies:

• Early Investments and Business Ventures: By his 30s, Carnegie had business interests in iron works, railroads, oil wells, and steamers on the Great Lakes. He also invested in railroads, railroad sleeping cars, bridges, and oil derricks during the 1860s

• Steel Industry and Innovation: Carnegie's major fortune came from steel production. He adopted and adapted the Bessemer process, which allowed cheap and efficient mass production of steel, significantly lowering steel prices and boosting demand, especially for rails

• Vertical Integration: Carnegie controlled the entire supply chain for steel production, including raw materials like coal and iron mines, transportation via railroads and lake steamships, and steel manufacturing plants. This integration reduced costs and increased efficiency

• Strategic Acquisitions: He purchased rival companies such as the Homestead Steel Works and combined various assets to form Carnegie Steel Company in 1892, which became the largest producer of pig iron, steel rails, and coke worldwide

• Relationship with Railroads and Tariffs: Carnegie worked closely with railroad industries, which were major consumers and transporters of steel. He also lobbied for favorable trade tariffs on steel, which helped protect and grow his business

• Sale of Carnegie Steel: In 1901, Carnegie sold his steel company to financier J.P. Morgan for $303 million (equivalent to over $11 billion today), which formed the basis of U.S. Steel and made Carnegie the richest man of his time

Carnegie's rise was marked by hard work, strategic partnerships, and investments during a period of rapid industrial growth in the United States, particularly post-Civil War as the country expanded its railroad system and industrial base

. After accumulating his fortune, he devoted much of his life to philanthropy, giving away nearly 90% of his wealth to causes such as libraries, education, and peace initiatives

. In summary, Andrew Carnegie made his fortune through savvy investments and control of the steel industry, leveraging innovation, vertical integration, and strategic business relationships during America's industrial expansion