Final answer:
Carnegie and his business partners reinvested their increased profits mostly in expanding steel production facilities and operations, leading to the dominance in the steel industry.
Step-by-step explanation:
Andrew Carnegie, known for having risen from humble origins to become a leading steel producer, applied his thrifty nature and business acumen in his dealings. After his profits doubled from hiring business partners, he, along with his partners, reinvested much of the extra money into expanding their business operations. This included saving profits during prosperous times and utilizing them to buy out other steel companies during economic lows, insisting on modern machinery and equipment, and encouraging innovation to increase production and cut costs. Ultimately, Carnegie's strategic moves allowed him to control virtually the entire production process, which led to significant cost reduction and the establishment of a dominant position in the steel industry. Later in life, Carnegie turned towards philanthropy, using his vast wealth to support various charitable causes.