In Chongqing Iron & Steel Co. China, heavy and medium plates (4100, 2700 and 1780 mm in width) from hypo-peritectic steel slabs are major products. According to reported statistics , the longitudinal cracking ratio of hypo-peritectic steel slabs in the No. 1 steelmaking plant from late 2009 to November 2012 ranged from 1 to 30%, in the worst case reaching 1000 tons of steel scrap per month caused by longitudinal cracks. Based on the steel grades and production procedure, a series of ultrahigh-basicity mold fluxes (basicity 1.7—1.8) was developed by Chongqing University [7-9] and applied on three casters in the No. 1 steelmaking plant from November 2012. Two-year statistics from 2012 to the end of 2014 indicates that after starting to use ultrahigh-basicity mold fluxes, the longitudinal cracking ratio and especially the scrap ratio caused by large cracks decreased significantly. The subsurface crack ratio also decreased significantly (see Tables 2 and 3). The data indicate that compared with traditional fluxes, the ultrahigh-basicity mold fluxes have good performance in improving the surface quality of slabs and final products. Over a period of two-years (2013 and 2014), no exceptional conditions such as sticker breakouts occurred.