## 4 Discussion
During the BOF campaign, the worn 5-hole copper tip was replaced without altering its orientation, as the new tip was always welded in the same position, resulting in oxygen streams maintaining a constant direction toward the MgO-C refractory lining (Figure 7). Laser measurements of residual lining thickness and gunning mixture consumption data reveal localized MgO-C lining wear in trunnion zones (S3/S9). Slag coating proves ineffective here due to converter movement and slag spillage, restricting trunnion area repairs to gunning or slag splashing (slag splashing is not applied at this steelwork). In contrast, adjacent zones at trunnion height (4-5.2 m) remain accessible for slag coating. This repair limitation, coupled with disproportionate gunning mixture usage in S3/S9 zones (Figure 7), confirms the wear's localized nature. The combined thermal, chemical, and mechanical degradation model helps explain this localized wear phenomenon. Thermal degradation initiates the process through cyclic stresses generated by temperature gradients within the refractory material. When oxygen jets consistently impact specific locations, they create thermal cycles that induce differential expansion and contraction, leading to microcrack formation along grain boundaries of the MgO-C material. These microcracks then facilitate chemical degradation as slag components penetrate the refractory, reacting with both MgO and carbon to form complex compounds with lower melting points. Finally, mechanical degradation completes the cycle as the weakened material succumbs to erosion from high-velocity oxygen jets and impinging metal and slag droplets. In total, 2326.343 t of gunning mixtures were consumed during the BOF campaign. Of this total, 18.13% was utilized in the trunnion area, equating to 421.77 t of gunning mixtures. The ratio of gunning mixtures used between the S3 zone and the S9 zone is 1:12.5 in favor of the S9 zone. Consequently, zone S9 was subjected to a more intense attack by a jet of pure oxygen, a sprayed and spitted mixture of molten metal and slag, compared to zone S3, for the whole duration of the campaign. The magnitude of this ratio cannot be explained by geometric factors alone but requires consideration of the compounding effects of multiple degradation mechanisms operating simultaneously in preferentially exposed areas. When oxygen jets consistently impact the same location, they create conditions that enhance both chemical reactions between slag and refractory and the mechanical removal of partially degraded material, creating a positive feedback loop of accelerated wear. The proposed solution to the problem of non-homogeneous wear of MgO-C refractory lining in the BOF in the trunnion area involves rotating the copper tip of the oxygen lance during replacements. This solution is also associated with a reduction in the consumption of the gunning mixtures used in the areas of the trunnions. Based on the scientific understanding of refractory wear mechanisms and the principles of stress distribution in materials, a solution emerges that addresses the fundamental cause of localized wear. In the case of a 5-hole copper tip of the oxygen lance, the new tip should be rotated 36\({}^{\circ}\) relative of the original worn copper tip (Figure 9). During the campaign, the rotation will always occur in the same direction, which will not change during the campaign (e.g., clockwise). This rotation effectively transforms concentrated stress patterns into distributed ones, following fundamental principles of materials engineering where stress redistribution prevents premature localized failure. For different tip configurations, the rotation angles are determined by geometric considerations that optimize impact distribution: 45\({}^{\circ}\) for 4-hole tips (Figure 10a) and 30\({}^{\circ}\) for 6-hole tips (Figure 10b). These angles are not arbitrary but represent solutions that maximize the separation between consecutive impact positions while maintaining a systematic rotation pattern throughout the campaign (Table 5).