Formation mechanism of NbC precipitates in micro-alloyed Nb high-strength steel

Microalloying and heat treatment are the most important ways to improve the steel properties . In this study, the precipitation behavior of NbC precipitates with Nb content of 210 × 10^–6, 430 × 10^–6, and 690 × 10^–6 and heat treatment temperature of 1000, 1100, 1200, and 1300 ℃ were investigated. DH980 slab was melted in a silicon–molybdenum heating furnace with different Nb additions. The water-quenched steel samples were heated at different temperature in a furnace. The morphology and chemical composition of inclusions in steel samples were determined using an inclusion analysis system. The main inclusions in the Nb micro-alloyed high-Al high-strength steel were Al₂O₃, MnS, and NbC. The measured diameter of NbC precipitates ranged from 0.7 to 6.0 μm, which mainly concentrated on 1.0–2.0 μm. The precipitation temperature and amount of NbC were calculated using the thermodynamic calculation software Factsage. The initial precipitation temperature of NbC precipitates gradually increased to 1125, 1200, and 1260 ℃ as the Nb content increased from 210 × 10^–6 to 690 × 10^–6, respectively, and the NbC precipitation rate (the ratio of NbC mass to the mass of all inclusions) increased to 0.023%, 0.047%, and 0.076%, respectively. The precipitation temperature of MnS was 1450 ℃, which changed little with the content of Nb. Al₂O₃ was present at the melting temperature of steel. The amount of the precipitated NbC in steel increased with an increase in Nb content and heat treatment temperature. The NbC was dissolved in steel when the heat treatment temperature was 1300 ℃, resulting in a decrease in the precipitation of NbC. The size of the NbC precipitates was mainly influenced by the Nb content, heat treatment temperature, and heating time. With the increase in the initial Nb content, the difference in Nb content between the steel matrix and reaction boundary became larger, the diffusion driving force increased, and thus the size of NbC precipitates increased. The diffusion coefficient of Nb varied with the heat treatment temperature, which was hardly influenced by the Nb content. The diffusion coefficient increased with the increase in temperature, which promoted the diffusion of Nb. Consequently, the size of NbC increased with the temperature increased. The diffusivity of Nb increased with an increase in heating time, which also increased the size of NbC. Therefore, the size of NbC precipitates increased as the Nb content, heat treatment temperature, and heating time increased. A kinetic model of NbC precipitation in high-Al high-strength steel was developed to predict the effects of Nb content, heat treatment temperature, and heating time on the size of NbC precipitates.