Modern armor against First World War guns

Can modern metals make armor that period large-caliber guns could not penetrate?

In the proposed counterfactual, modern armor could resist a first hit from period guns below 18 inches; a second hit in the same place or modern gun technology would change the result.

The claim concerns modern alloy knowledge taken back to battleships built around 1910, with their opponents still using period guns. Under that assumption, armor is judged capable of resisting a first hit from guns below 18 inches. A second hit in the same place remains dangerous because the first transfers heat and energy and alters the plate's condition. Modern gunmaking would also change the contest: a modern 15-inch gun could achieve greater velocity than its early twentieth-century counterpart. Sabot ammunition and smaller penetrators are offered as further hypothetical ways to challenge the improved protection. This is an argument about unequal rates of technological change, without a tested armor specification or universal immunity claim.