Improved casting powders and improved casting slags enable production of steels having high aluminum contents of greater than or equal to 1% by weight and, in some cases, high manganese content of greater than or equal to 15% by weight. In some examples, such steels may also or alternatively include greater than or equal to 0.2% by weight titanium. The casting slag may result from a casting powder that comprises CaO and Al2O3 components essentially in the form of prefused calcium aluminate. Methods for casting steel, including methods for continuously casting steel, are also disclosed based on the use of the disclosed casting powders or casting slags.