This article presents the results of a study on the effect of reinforcement on the microstructure of high-carbon steel castings. Plates with thicknesses of 0.5, 1.0, 1.5, and 3.0 mm are used as reinforcing elements. The implantation of the inserts into the castings is carried out by placing them into a preformed cavity in a polystyrene-foam model, which is then set into a mould and poured with molten high-carbon steel. Examination of the ‘insert−matrix’ interfaces revealed both defect-free and defective contact areas, the latter appearing as gaps with maximum widths exceeding 100 μm. A correlation is established between the heating temperature of the inserts and the length of the gap-containing interface. A quantitative relationship is determined between the insert thickness and the extent of the gap in the ‘reinforcing element−matrix’ contact zone, and a mechanism of this effect is proposed. The influence of insert thickness on the minimum, maximum, and average pearlite-grain area, as well as on its distribution within the matrix of the castings, is analysed. Analytical models are developed to describe the effect of insert thickness and gap length on the average pearlite-grain area in the microstructure of the casting near the insert surface and at a distance of 20 mm from it. As found, the minimum average pearlite-grain area near the insert surface reaches 170.4 μm² at an insert thickness of 2.2 mm and 148.2 μm² at a distance of 20 mm for a 2.0 mm thick insert, which is 6 and 14 times lower, respectively, than in castings without inserts. The most pronounced grain-refinement effect is observed for inserts with a thickness of 2.0−2.2 mm and a gap length at the ‘insert−matrix’ interface in the range of 62−72%.