1. A method for quantitatively measuring internal defects in an as-cast steel product, comprising: optically scanning at least a portion of a surface of the steel product with a scanning device to create a digital image thereof; thresholding the image in a thresholding engine to isolate internal defects within the image; analyzing the thresholded image to determine an internal defect area, wherein the internal defect area comprises an area of at least a portion of the thresholded image that is occupied by the internal defects; determining an overall area of the portion of the thresholded image; calculating a fraction of the internal defect area relative to the overall area; and calculating an equivalent Mannesmann scale rating of the internal defects in the steel product based on the fraction.