Carnegie did not want to marry during his mother's lifetime, instead choosing to take care of her in her illness towards the end of her life. After she died in 1886, the 51-year-old Carnegie married Louise Whitfield, who was 21 years his junior. In 1897, the couple had their only child, a daughter, whom they named after Carnegie's mother, Margaret.  Carnegie made his fortune in the steel industry, controlling the most extensive integrated iron and steel operations ever owned by an individual in the United States. One of his two great innovations was in the cheap and efficient mass production of steel by adopting and adapting the Bessemer process, which allowed the high carbon content of pig iron to be burnt away in a controlled and rapid way during steel production. Steel prices dropped as a result, and Bessemer steel was rapidly adopted for rails; however, it was not suitable for buildings and bridges.  The second was in his vertical integration of all suppliers of raw materials. In the late 1880s, Carnegie Steel 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 metal per day. In 1883, Carnegie bought the rival Homestead Steel Works, which included an extensive plant served by tributary coal and iron fields, a 425-mile (684 km) long railway, and a line of lake steamships. Carnegie combined his assets and those of his associates in 1892 with the launching of the Carnegie Steel Company.  By 1889, the U.S. output of steel exceeded that of the UK, and Carnegie owned a large part of it. Carnegie's empire grew to include the J. Edgar Thomson Steel Works in Braddock, (named for John Edgar Thomson, Carnegie's former boss and president of the Pennsylvania Railroad), Pittsburgh Bessemer Steel Works, the Lucy Furnaces, the Union Iron Mills, the Union Mill (Wilson, Walker & County), the Keystone Bridge Works, the Hartman Steel Works, the Frick Coke Company, and the Scotia ore mines. Carnegie, through Keystone, supplied the steel for and owned shares in the landmark Eads Bridge project across the Mississippi River at 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.

Answer this question "what was the Bessemer process?" by extracting the answer from the text above.
which allowed the high carbon content of pig iron to be burnt away in a controlled and rapid way during steel production. Steel prices dropped as a result,