Lukens expanded rapidly in the late nineteenth and early twentieth centuries. In 1891 it installed its first two open hearth furnaces, then quickly upgraded them and added four new thirty-five-ton furnaces for a total of six, and on February 25, 1892, it poured its first heat of steel. In 1899 a forty-eight-inch Universal mill was added, able to roll plates from eight inches to forty-eight inches wide and one hundred feet long, and Open Hearth #2 was built with six fifty-ton furnaces for a total of twelve. In 1903 another rolling mill, larger than any of its predecessors, began operation with rolls one hundred forty inches long, producing the widest plates of any mill in the country, along with four furnaces and the ability to shear plates two feet thick. In 1904 the one hundred thirty-four-inch mill was reduced to the one hundred twelve-inch mill with rolls thirty-six inches in diameter, allowing plates one hundred eight inches wide to be rolled. About 1904-1905, four more fifty-ton furnaces were added to Open Hearth #2 for a total of sixteen. In 1916 Lukens built its two hundred four-inch rolling mill and enlarged it to two hundred six inches in 1918, giving it the distinction for many years of having the world's largest plate mill. This four-high rolling mill used two working rolls thirty-four inches in diameter that directly contacted the plate, reinforced by two backup rolls fifty inches in diameter, and had eight gas-fired furnaces to keep plates hot during rolling; it remains in operation today. In 1918 Open Hearth Shop #3 was built between the #2 shop and Main St., containing eight one-hundred-ton open hearth furnaces for a plant total of twenty-four.
