Andrew Carnegie, a Scottish immigrant, advanced the cheap and efficient mass production of steel rails for railroad lines, by adopting the Bessemer process.  After an early career in railroads, Carnegie foresaw the potential for steel to amass vast profits.  He asked his cousin, George Lauder to join him in America from Scotland. Lauder was a leading mechanical engineer who had studied under Lord Kelvin. Lauder devised several new systems for the Carnegie Steel Company including the process for washing and coking dross from coal mines, which resulted in a significant increase in scale, profits, and enterprise value.
Lauder would go on to lead the development of the use of steel in armor and armaments for the Carnegie Steel Company, spending significant time at the Krupp factory in Germany in 1886 before returning to build the massive armor plate mill at the Homestead Steel Works that would revolutionize naval warfare.
Carnegies first mill was the Edgar Thomson Works in Braddock, PA, just outside of Pittsburgh.  In 1888, he bought the rival Homestead Steel Works, which included an extensive plant served by tributary coal and iron fields, a 425-mile (685 km) long railway, and a line of lake steamships.  He would also add the Duquesne Works to his empire.  These three mills on the Monongahela River would make Pittsburgh the steel capital of the world.  In the late 1880s, the Carnegie Steel Company was the largest manufacturer of pig iron, steel rails, and coke in the world, with a capacity to produce approximately 2,000 tons of pig iron per day. A consolidation of Carnegies assets and those of his associates occurred in 1892 with the launching of the Carnegie Steel Company.
Lauder would go on to lead the development of the use of steel in armor and armaments for the Carnegie Steel Company, spending significant time at the Krupp factory in Germany in 1886 before returning to build the massive armor plate mill at the Homestead Steel Works that would revolutionize naval warfare.
By 1889, the U.S. output of steel exceeded that of Britain, and Andrew Carnegie owned a large part of it. By 1900, the profits of Carnegie Bros. & Company alone stood at $480,000,000 with $225,000,000 being Carnegie's share.
Carnegie, through Keystone, supplied the steel for and owned shares in the landmark Eads Bridge project across the Mississippi River in St. Louis, Missouri (completed 1874). This project was an important proof-of-concept for steel technology which marked the opening of a new steel market.
The Homestead Strike was a violent labor dispute in 1892 that involved an attack by strikers against private security guards. The governor called in the National Guard. The strike failed and the union collapsed. The dispute took place at Carnegie's Homestead Steel Works between the Amalgamated Association of Iron and Steel Workers and the Carnegie Steel Company. The final result was a major defeat for the union and a setback for efforts to unionize steelworkers.
Carnegie sold all his steel holdings in 1901; they were merged into U.S. Steel and it was non-union until the late 1930s.