Which new manufacturing method made the production of steel both cheap and quick?

The Bessemer Process is your answer.

The Bessemer process was used to mass produce steel cheaply and quickly, and was used throughout the Industrial Revolution.

~

The entry of the United States was the turning point of the war, because it made the eventual defeat of Germany possible. It had been foreseen in 1916 that if the United States went to war, the Allies' military effort against Germany would be upheld by U.S. supplies and by enormous extensions of credit.