Key Takeaways & Executive Findings
- •• A novel mathematical model integrating microsegregation, solidification, and TiN precipitation was developed and validated with high-temperature experiments. • Positive segregation of Ti and N at the solidifying front leads to maximum TiN precipitation at the 1/4 width of the steel, while high surface cooling rates cause negative segregation. • Increasing cooling rate reduces TiN precipitation by diminishing solute segregation, whereas higher initial solute content enhances TiN formation. • The model provides critical insights for controlling TiN inclusions to improve mechanical properties of Ti-microalloyed steels.
Abstract
In order to investigate the segregation process and clarify its effect on the formation of TiN during the solidification of a micro-alloy steel containing titanium (Ti), a new mathematical model concerning solute transportation, solidification, as well as TiN precipitation was successfully established and verified. The transportation of solute elements was described using the Brody-Fleming microsegregation model, while the thermodynamic principles governing the precipitation of TiN were derived within the framework of the model. Additionally, the model accounts for variations in the diffusion coefficient due to phase transition and the influence of non-equilibrium solidification on solute distribution. High-temperature tests were conducted to validate the mathematical model. Results show that during solidification, due to selective crystallization, there is positive segregation of Ti and N in the solidifying front. What’s more, due to the high cooling rate near the surface of this steel, negative segregation is easier to be formed in the surface area. The highest concentration of TiN precipitation is found in the 1/4 width of this steel. High-temperature experiment shows that when the solidifying front reaches the 1/4 width of the specimen, the concentration product of Ti and N elements biased at the solidifying front reaches the thermodynamic conditions of TiN precipitation, and exists a higher concentration of TiN distributed in this region. To address this phenomenon, a comparative analysis of the effects of cooling rate and initial solute element content on TiN precipitation behavior was conducted. An increase in the surface cooling rate accelerates the progression of the solidification front and diminishes solute segregation near the front, thereby reducing TiN precipitation. However, with the increase of the initial solute element content, the concentration product of Ti and N elements rises, then the content of TiN precipitation increases. The results of this model provide important insight into the micro segregation and TiN precipitation mechanism of the micro-alloy steels bearing titanium.
1. Introduction
Micro-alloy steels containing Ti are characterized by excellent mechanical property and low cost, which are widely used in chemical equipment, aerospace plane, ship building, as well as other fields [1-3]. During solidification and cooling process of this kind of steels, the precipitation of numerous inclusions such as TiC and TiN greatly damages the mechanical properties of final steel products [4-8]. The reason for this phenomenon is that the morphology of TiN is cubic or triangular, leading to stress concentration in the steels, especially at sharp corners, which result in crack defects [7, 8]. Consequently, gaining a thorough understanding of TiN precipitation is critical for mitigating its detrimental effects on steel, thereby enhancing both the performance and quality of the final rolled product.
The mechanism of TiN precipitation, along with strategies for its control, has aroused wide concern in today’s steel plants. Due to limitations in experimental technology, many researchers have turned to a combination of numerical simulation and experimental validation to explore the TiN precipitation process. Gui et al. [9] integrated a microsegregation model with a thermodynamic model of inclusion precipitation, comprehensively assessing the impact of TiN precipitation on steel quality. Their findings indicated that, irrespective of the macro-distribution of TiN in steels, a critical titanium content of 0.053wt.% achieved the highest toughness without being overly influenced by TiN inclusions. Liu et al. [10] conducted experiments to observe the morphology and distribution of TiN inclusions in cast billets, complementing their observations with thermodynamic modeling to determine the onset of TiN precipitation. They discovered that TiN particles began to precipitate when the solidification fraction exceeded 0.82, with 93% of the precipitation occurring by the end of solidification. This model was based on ideal thermodynamic equilibrium assumptions. Gui et al. [11] utilized the Vollere-Beckermann [12] microsegregation model to calculate elemental segregation, proposing the use of an equilibrium partition coefficient in the form of a phase-change and temperature-dependent function. This approach elucidated the precipitation mechanism of typical precipitation phases in high-sulfur steels, though it did not consider the effect of cooling rate on solute distribution. In reality, the cooling rate significantly influences the solidification path and solute redistribution, which in turn affects TiN precipitation. Therefore, a comprehensive model that accounts for cooling rate and non-equilibrium effects is essential for accurate prediction.
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Han Wang, Tian-peng Qu, Tian Liang, Xiang-long Li, De-yong Wang, Lei Fan, Zhi-xiao Zhang, Zheng-hong Yang (2025). A new mathematical model for investigating solidification, solute transportation, and TiN precipitation in a micro-alloy steel containing Ti. China Foundry. https://doi.org/10.1007/s41230-025-4171-5
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Frequently Asked Questions
What is the main objective of the study?
The study aims to develop and validate a mathematical model that simulates solidification, solute transportation, and TiN precipitation in a Ti-containing micro-alloy steel, to understand the segregation process and its effect on TiN formation.
How was the mathematical model validated?
The model was validated through high-temperature experiments, which confirmed the predicted segregation behavior and TiN precipitation distribution, particularly at the 1/4 width of the specimen.
What are the key findings regarding TiN precipitation?
The highest concentration of TiN precipitation occurs at the 1/4 width of the steel due to positive segregation of Ti and N at the solidifying front. Increasing cooling rate reduces TiN precipitation, while higher initial solute content increases it.
Why is controlling TiN precipitation important?
TiN inclusions with cubic or triangular morphology cause stress concentration and crack defects, degrading the mechanical properties of steel. Controlling their precipitation is crucial for improving product quality.
What is the significance of the model for industrial applications?
The model provides insights into microsegregation and TiN precipitation mechanisms, enabling optimization of cooling rates and alloy composition to minimize harmful inclusions and enhance steel performance.
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