With rapid urbanization and industrialization, steel in-use stocks (SIUS) have experienced significant growth, playing an important role in urban mining and future renewable resources. Although previous studies have quantified SIUS at the provincial level, a comprehensive understanding of its spatial distribution remains limited. This study uses Henan Province as a case to assess SIUS and its spatial distribution at the provincial level. A spatio-temporal characterization framework is developed to systematically analyze SIUS dynamics, integrating the bottom-up model, the spatial autocorrelation model, the Tapio–LMDI model, and the stock-driven model. The findings show that total SIUS has been continuously increasing, reaching 499.35 Mt in 2023, with the buildings sector being the largest contributor, accounting for 67%. However, due to its large population, per capita SIUS was 5.09 t/cap in 2023, lower than that of China. Spatial analysis reveals significant autocorrelation in per capita SIUS, with notable spatial heterogeneity in its density. Moreover, the average annual growth rate of SIUS is projected to decline from 10% in 2023 to 5% in 2060, suggesting that SIUS in Henan is approaching a saturation phase, consistent with theoretical expectations.