The CASTRIP facility at Nucor Steel's Crawfordsville, Indiana plant is the world's first commercial installation for production of Ultra-Thin Cast Strip (UCS), via twin-roll strip casting. The facility has been producing plain, low-carbon sheet steel since 2002, with a range of structural grades available. The paper outlines some of the main parameters that influence the final microstructure and resultant mechanical properties of plain, low carbon UCS steels that have been recently made by the CASTRIP process, and then describes new Nb microalloyed UCS products that have been made in the CASTRIP process, significantly expanding the range of thin high strength hot rolled strip products for structural applications. In hot rolled UCS steels, niobium was in solute form retarding austenite grain refinement during in-line hot rolling thereby enhancing hardenability and promoting microstructural hardening. In particular, the formation of bainitic/ acicular ferrite microstructures was provided over proeutectoid ferrite in these steels using the CASTRIP process. This enabled higher strength levels to be achieved than with USC plain low carbon steels. Age hardened Nb UCS steels were found to be producible using heat treatment furnaces of a conventional hot dip galvanising line achieving improved strength, while also improving ductility. Yield strength levels for the age hardened and galvanised Nb UCS steels achieved over 550 MPa in combination with good ductility.