A new lightweight material that is stronger than steel

Many materials are stronger than "steel"[1], and a few of them are quite manufacturable in principle, but nearly all of them suffer from defects in other attributes.

Hardness, toughness, resistance to temperature extremes and degradation by environmental chemicals, ductility/workability, cumulative stress resistance, material cost, toxicity, costs and complexity of making joints, repairs, and other workings, and the ability to vary strength, ductility, hardness, toughness, corrosion resistance, and thermal properties: all of these and more matter greatly.

"Steel" (as a category of materials) stands out as an all-rounder. Any new material is doomed to a niche, probably a small niche, unless it can exceed steel in all of the above.

[1] There is not just one "steel".