In the field of metallurgy, the study of non-metallic inclusions in steel has always been a critical area of focus. Coarse inclusions severely degrade the performance of castings, and numerous researchers have conducted extensive studies on how to remove these inclusions. Recent research has shown that controlling the microstructure of castings through inclusions in steel is one of the most effective methods. Fine inclusions can mitigate or even eliminate their detrimental effects on steel properties, while also effectively refining grains and providing secondary phase strengthening. High manganese steel casting is widely used due to its high strength and excellent wear resistance. However, high manganese steel has low thermal conductivity and slow heat diffusion, leading to coarse solidification structures in castings and the segregation and growth of inclusions at grain boundaries, which reduces casting strength. The high manganese content in high manganese steel casting results in strong affinity between manganese and sulfur, making MnS inclusions one of the primary inclusions in such castings. Coarse inclusions cause defects like spalling and cracks during the service life of high manganese steel casting, significantly affecting its durability. In this study, we investigate the effect of microalloying with trace amounts of titanium and vanadium on the formation and distribution of MnS inclusions in thick-section high manganese steel casting, providing a technical approach for obtaining high-performance thick-section high manganese steel casting components.
Our research focuses on thick-section high manganese steel casting components, specifically combination frog point castings, with maximum dimensions of 2000 mm in length, 135 mm in thickness, and 190 mm in width. The castings were produced using water glass sand molds, with a pouring temperature of (1450 ± 10)°C. During the experiment, aluminum was used for final deoxidation, after which titanium (0.02%–0.1%) and vanadium (0.04%–0.1%) were added to the high manganese steel casting for microalloying. Two samples were cast for both non-alloyed and microalloyed conditions, with chemical compositions as shown in Table 1. Metallographic specimens of dimensions Φ20 mm × 20 mm were taken 750 mm from the root end of the casting, as illustrated in the sampling location. The specimens were ground, polished, etched with a 5% nitric acid alcohol solution, and scrubbed with a 4%–6% hydrochloric acid solution. Microstructural observation and analysis were performed using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).
The distribution of inclusions in non-alloyed and microalloyed thick-section high manganese steel casting combination frog point castings is shown in the micrographs. In non-alloyed castings, the inclusions are large, with sizes around 20 μm, whereas in microalloyed castings, the inclusions are significantly refined, with sizes approximately 5 μm, about one-third to one-fourth of those in non-alloyed castings, but the number of inclusions increases. This indicates that microalloying refines the inclusions, making them uniformly dispersed. The morphology of MnS inclusions, as observed through SEM and EDS analysis, includes both spherical and elongated shapes in both non-alloyed and microalloyed castings, but there are microstructural differences. In non-alloyed castings, MnS inclusions exhibit structures such as pure MnS and composites of MnS with Al2O3, where Al2O3 is embedded within the MnS. Additionally, aggregated inclusions of MnO, Al2O3, and MnS are present. In microalloyed castings, the microstructure includes composites of MnS with (Ti,V)N, where (Ti,V)N is partially surrounded by MnS, and sometimes ternary composites of MnS, (Ti,V)N, and Al2O3. The (Ti,V)N inclusions have distinct edges, and Al2O3 is located within the (Ti,V)N inclusions.
The formation mechanism of MnS inclusions in non-alloyed high manganese steel casting involves the initial solidification of coarse Fe dendrites. As solidification progresses, manganese and sulfur segregate and enrich in the interdendritic regions. When the supersaturation of manganese and sulfur reaches a high level, metastable MnS begins to nucleate. Due to poor wettability between liquid MnS and the solid-liquid interface of Fe dendrites, MnS is pushed into the interdendritic spaces, where it grows through collision and coalescence. A small amount of Al2O3 formed during crystallization provides nucleation sites for MnS inclusions, but in regions enriched with MnO, low-melting-point compounds like MnO·Al2O3 can form, which do not serve as effective nucleation sites for MnS and are brittle, reducing casting strength. The size of MnS depends on the cooling rate; at low cooling rates, MnS has sufficient time to grow and coalesce, leading to coarsening, whereas faster cooling rates help reduce MnS size.
In microalloyed high manganese steel casting, the formation mechanism differs. After deoxidation with Al, titanium and vanadium are added, leading to the formation of (Ti,V)N with minimal formation of (Ti,V)O inclusions. The precipitation temperatures of these inclusions are calculated as follows: Al2O3 precipitates at approximately 1572°C, (Ti,V)N at around 1475°C, and MnS at about 1375°C. Thus, the precipitation sequence is Al2O3 first, followed by (Ti,V)N, and finally MnS. The early precipitation of (Ti,V)N provides numerous nucleation sites for MnS, enabling heterogeneous nucleation of liquid MnS on (Ti,V)N particles. This refines the MnS inclusions and promotes a uniform distribution. Additionally, Al2O3 particles, which form at high temperatures, also act as nucleation sites for MnS, but the presence of abundant (Ti,V)N nuclei reduces the chance of MnS aggregation and growth. The (Ti,V)N inclusions, with their sharp edges, can cause stress concentration, so the amounts of titanium and vanadium added must be controlled.
The microalloying treatment in high manganese steel casting also refines the grain structure. In non-alloyed castings, the grain size is coarse, with a grain size rating of about 2.5, and uneven distribution. In microalloyed castings, the grain size rating improves to approximately 3.5, with finer and more uniform grains. This grain refinement contributes to enhanced mechanical properties. The yield strength (σ0.2) and tensile strength (σb) of non-alloyed castings are 335 MPa and 725 MPa, respectively, while microalloyed castings exhibit significantly improved properties, with σ0.2 of 390 MPa and σb of 840 MPa. The precipitation of (Ti,V)N particles during cooling provides nucleation sites for Fe dendrites, inhibits grain boundary movement, and suppresses grain growth, thereby improving the performance of high manganese steel casting.
To quantify the effects, we can use formulas to describe the nucleation and growth processes. The nucleation rate for MnS on (Ti,V)N particles can be expressed using classical nucleation theory: $$I = I_0 \exp\left(-\frac{\Delta G^*}{kT}\right)$$ where I is the nucleation rate, I_0 is a pre-exponential factor, ΔG* is the activation energy for nucleation, k is Boltzmann’s constant, and T is temperature. The solubility product for MnS formation is given by: $$K_{sp} = [Mn][S]$$ where [Mn] and [S] are the concentrations of manganese and sulfur, respectively. For (Ti,V)N precipitation, the solubility product is: $$K_{sp}^{(Ti,V)N} = [Ti][V][N]$$ which influences the number of nucleation sites available for MnS.
| Sample ID | C | Mn | Si | S | P | Ti | V | Al | O | N | Fe |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Non-alloyed 1 | 1.05 | 12.45 | 0.42 | 0.041 | 0.041 | 0 | 0 | 0.010 | 0.007 | 0.018 | Bal. |
| Non-alloyed 2 | 1.03 | 12.68 | 0.46 | 0.042 | 0.043 | 0 | 0 | 0.008 | 0.010 | 0.012 | Bal. |
| Microalloyed 1 | 1.10 | 12.36 | 0.41 | 0.042 | 0.044 | 0.053 | 0.089 | 0.009 | 0.006 | 0.014 | Bal. |
| Microalloyed 2 | 1.08 | 12.22 | 0.38 | 0.044 | 0.040 | 0.042 | 0.077 | 0.008 | 0.009 | 0.016 | Bal. |
| Condition | Average Inclusion Size (μm) | Grain Size Rating | Yield Strength σ0.2 (MPa) | Tensile Strength σb (MPa) |
|---|---|---|---|---|
| Non-alloyed | 20 | 2.5 | 335 | 725 |
| Microalloyed | 5 | 3.5 | 390 | 840 |
The refinement of inclusions in high manganese steel casting through microalloying can be further analyzed using kinetic models. The growth rate of MnS inclusions is governed by diffusion-controlled processes, described by the equation: $$\frac{dr}{dt} = \frac{D}{r} \left( C_b – C_i \right)$$ where r is the radius of the inclusion, t is time, D is the diffusion coefficient, C_b is the bulk concentration, and C_i is the interface concentration. With microalloying, the increased number of nucleation sites reduces the average diffusion distance, limiting growth. The effectiveness of (Ti,V)N as nucleation sites depends on their interfacial energy with MnS, which can be represented as: $$\Delta G_{hetero} = \Delta G_{hom} \cdot f(\theta)$$ where ΔG_hetero is the activation energy for heterogeneous nucleation, ΔG_hom is that for homogeneous nucleation, and f(θ) is a function of the contact angle θ between MnS and (Ti,V)N. A lower θ promotes easier nucleation.
In practical applications for high manganese steel casting, controlling the addition levels of titanium and vanadium is crucial. Excessive additions may lead to coarse (Ti,V)N inclusions with sharp edges, increasing stress concentration and potentially initiating cracks. Therefore, optimal ranges of 0.02%–0.1% for titanium and 0.04%–0.1% for vanadium are recommended for thick-section high manganese steel casting. Additionally, the cooling rate during solidification plays a role; although microalloying reduces inclusion size even at lower cooling rates, combining microalloying with accelerated cooling can further enhance refinement. The interplay between microalloying and solidification parameters in high manganese steel casting can be optimized using computational thermodynamics, such as CALPHAD-based simulations, to predict phase precipitation sequences.
In conclusion, microalloying with 0.02%–0.1% titanium and 0.04%–0.1% vanadium significantly refines MnS inclusions in thick-section high manganese steel casting, promoting a uniform distribution. The formation of (Ti,V)N particles provides heterogeneous nucleation sites for MnS, reducing their size and minimizing aggregation. This refinement, coupled with grain size improvement, enhances the mechanical properties of high manganese steel casting, offering a viable technical pathway for producing high-performance components. Future work could explore the combined effects of microalloying with other elements or advanced processing techniques to further optimize the properties of high manganese steel casting.