The continuous advancement of industries such as automotive, heavy machinery, and defense has placed increasingly stringent demands on the quality of steel castings. Modern components often require a paradoxical combination of high strength and excellent weldability. This typically translates to a need for high strength coupled with a low carbon equivalent (Ceq) to mitigate cracking risks during welding. Traditionally, many foundries have relied on quenching and tempering (quench & temper) heat treatments to achieve high strength from low-Ceq steel grades for steel castings. However, as casting technology pushes towards greater precision and complexity, controlling distortion during heat treatment has become a critical challenge. For large, slender, or thin-walled steel castings, the quenching stage of Q&T, while effective for mechanical properties, often leads to unacceptable levels of deformation and warpage.
This presents a significant bottleneck in the manufacturing of high-integrity steel castings. An alternative pathway is required—one that delivers the necessary strength through metallurgical means other than severe quenching, thereby allowing the use of a gentler heat treatment like normalizing. This is where the strategic application of rare earth (RE) elements becomes a powerful tool. The well-documented effects of RE addition—including grain refinement, modification of harmful inclusions, purification of the melt, and subtle microalloying—offer a route to enhance the strength of normalized steel castings without escalating the carbon equivalent. This research is centered on harnessing these effects to develop a novel grade of steel castings suitable for normalized condition.
The core objective was to design, produce, and characterize a new grade of steel castings. The goal was to achieve a carbon equivalent of Ceq ≤ 0.61% and meet the mechanical property requirements of high-strength cast structural steel (e.g., ASTM A148 grade 90-60) solely through a normalizing heat treatment. The base composition chosen for optimization was ZG25Mn2, a commonly used cast manganese steel. The strategy involved a dual approach: first, rationally reducing the content of key alloying elements to lower the Ceq; and second, introducing a RE-containing ferroalloy to compensate for the potential loss in strength and to improve the overall metallurgical quality of the steel castings.
1. Composition Design Philosophy for Advanced Steel Castings
The foundation of weldability assessment for steel castings often lies in the carbon equivalent formula. For this study, the International Institute of Welding (IIW) formula was employed:
$$Ceq = \omega(C) + \frac{\omega(Mn)}{6} + \frac{\omega(Cr) + \omega(Mo) + \omega(V)}{5} + \frac{\omega(Cu) + \omega(Ni)}{15}$$
Where $\omega(X)$ represents the weight percentage of element X. The target was to maintain Ceq at or below 0.61%. The standard ZG25Mn2 composition has a Ceq ranging from approximately 0.48% to 0.61%. After normalizing, its typical properties are: Yield Strength (YS) 345-440 MPa, Tensile Strength (TS) 590-685 MPa, with good ductility and toughness. Our aim was to preserve or exceed these strength levels while ensuring superior weldability.
The optimized composition design involved the following key adjustments, summarized in the design rationale table below:
| Element | Standard ZG25Mn2 Range (Typical) | Optimized Design Rationale | Target Range for New Steel Castings |
|---|---|---|---|
| Carbon (C) | ~0.22-0.32% | Primary driver of Ceq and strength. Content was carefully limited to the lower-mid range to control Ceq while retaining strength potential with RE addition. | 0.25% – 0.30% |
| Manganese (Mn) | ~1.50-1.80% | Significant contributor to Ceq (divided by 6 in IIW formula). Content was reduced to help meet the stringent Ceq target, accounting for residual elements. | 1.40% – 1.60% |
| Silicon (Si) | ~0.30-0.60% | Maintained for deoxidation and solid solution strengthening. Its contribution to Ceq in the IIW formula is minor, so it was kept at standard levels. | 0.35% – 0.45% |
| Sulfur (S) | <0.035% | Stringently controlled. RE addition is highly effective in forming stable, globular oxy-sulfides, mitigating the detrimental effect of S on toughness and weldability. | < 0.015% |
| Phosphorus (P) | <0.035% | Stringently controlled to prevent embrittlement, especially important for weldability of the final steel castings. | < 0.025% |
| Rare Earths (RE) | 0% | Key innovation. Added via Fe-Si-RE alloy (containing Ce, Ti, Si, B). Targets: grain refinement, inclusion modification, purification, microalloying. | 0.30% – 0.40% (of Fe-Si-RE alloy) |
The role of RE in steel castings is multifaceted and critical to this study’s success. The added Fe-Si-RE alloy introduces active elements like Cerium (Ce) and Titanium (Ti). Their functions can be described by the following mechanisms, which collectively enhance the performance of normalized steel castings:
- Grain Refinement: RE elements are surface-active. They lower the interfacial energy during solidification, increasing nucleation sites and hindering the growth of columnar grains. During the subsequent normalizing heat treatment, RE segregate at austenite grain boundaries, pinning them and preventing coarsening. The Hall-Petch relationship underscores the importance of this: $$ \sigma_y = \sigma_0 + k_y d^{-1/2} $$ where $\sigma_y$ is the yield strength, $\sigma_0$ and $k_y$ are material constants, and $d$ is the average grain diameter. Reducing $d$ directly increases $\sigma_y$.
- Inclusion Modification: RE have a stronger affinity for oxygen and sulfur than Mn or Fe. They form high-melting-point, globular complexes like RE-oxysulfides (e.g., Ce2O2S). This “modifies” harmful, elongated MnS or silicate stringers into harmless, spherical particles. This净化 (purification) improves ductility, toughness, and fatigue resistance of the steel castings.
- Microalloying: Elements like Ti form fine, stable carbides (TiC) or carbonitrides. These precipitates contribute to strength via precipitation hardening and also help inhibit grain growth.
2. Experimental Methodology for Steel Castings Production and Evaluation
To validate the composition design, experimental heats were produced. A 250 kg medium-frequency induction furnace with a neutral lining was used for melting. This scale is representative of many foundry operations for premium steel castings. The charge consisted of selected scrap steel, ferromanganese, and other necessary ferroalloys. Deoxidation was carried out using standard practices. The crucial RE addition was made via the ladle inoculation method: the calculated amount of Fe-Si-RE alloy was placed in the bottom of the pouring ladle, and the molten steel was tapped onto it, ensuring good mixing.
Two distinct experimental compositions were melted, differing primarily in the amount of RE addition. Their final chemical compositions and calculated carbon equivalents are presented below:
| Material ID | C (%) | Mn (%) | Si (%) | S (%) | P (%) | Ce (%) | Ti (%) | B (ppm) | Ceq (IIW) (%) |
|---|---|---|---|---|---|---|---|---|---|
| Q1 (0.3% RE) | 0.28 | 1.41 | 0.40 | 0.008 | 0.026 | 0.012 | 0.020 | 28 | 0.57 |
| Q2 (0.4% RE) | 0.27 | 1.48 | 0.39 | 0.006 | 0.022 | 0.022 | 0.031 | 41 | 0.56 |
As intended, both materials successfully met the carbon equivalent target of Ceq ≤ 0.61%. From each heat, standard keel blocks (foundry test coupons) were poured to produce specimens for mechanical testing and microstructural analysis.
The heat treatment variable investigated was the normalizing temperature. Specimens from both materials (Q1 and Q2) were subjected to normalizing at three different temperatures: 880°C, 900°C, and 920°C. All specimens were held at temperature for 4 hours to ensure complete austenitization, followed by air cooling. This simulates a practical and relatively low-distortion heat treatment cycle suitable for complex steel castings.
The evaluation suite consisted of:
- Mechanical Testing: Tensile tests were performed on a 1000 kN universal testing machine to determine Yield Strength (YS), Tensile Strength (TS), Elongation (El%), and Reduction of Area (RA%).
- Microstructural Analysis: Optical microscopy (OM) was used to examine the general microstructure (e.g., ferrite/pearlite morphology) and to determine the austenite grain size (ASTM grain size number). Scanning Electron Microscopy (SEM) was employed for higher-resolution imaging and for characterizing the morphology and distribution of non-metallic inclusions.
3. Results, Analysis, and Discussion on Steel Castings Performance
3.1. The Potent Influence of Rare Earths on Mechanical Properties
The tensile test results unequivocally demonstrate the benefit of RE addition for steel castings. The table below consolidates the mechanical properties for both materials across the different normalizing temperatures.
| Material | Normalizing Temp. (°C) | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) | Reduction of Area (%) |
|---|---|---|---|---|---|
| Q1 (0.3% RE) | 880 | 412 | 628 | 24 | 46 |
| 900 | 419 | 655 | 25 | 54 | |
| 920 | 416 | 647 | 24 | 48 | |
| Q2 (0.4% RE) | 880 | 421 | 640 | 28 | 59 |
| 900 | 437 | 642 | 29 | 62 | |
| 920 | 423 | 634 | 29 | 60 |
Comparing the baseline (unalloyed ZG25Mn2 with YS ~384 MPa after normalizing) to the RE-containing grades, a significant strength boost is evident. Both Q1 and Q2 show yield strength increases of over 30 MPa. This confirms that the RE addition successfully compensates for the reduced C and Mn content, maintaining the strength class of the steel castings.
Furthermore, comparing Q1 and Q2 reveals the effect of RE dosage. Increasing the RE addition from 0.3% to 0.4% (of the Fe-Si-RE alloy) provided a marginal further increase in yield strength but a very pronounced improvement in ductility and toughness, as reflected in the Elongation and Reduction of Area values. This suggests that while grain refinement (a strength mechanism) may saturate, benefits related to inclusion modification and purification—which greatly benefit ductility—continue to accrue with higher RE levels. However, an optimum window exists, as excessive RE can lead to the formation of large, clustered RE-containing inclusions that are detrimental.
3.2. Optimizing the Normalizing Process for Steel Castings
The data in the table above also reveals a clear trend regarding normalizing temperature. For both Q1 and Q2 materials, the optimal combination of strength and ductility was achieved at 900°C. The strength peaks at this temperature before declining at 920°C.
This can be explained by the interplay of several factors:
- Solid Solution Strengthening: At higher austenitizing temperatures, the solubility of Mn (and other elements like Ti) in austenite increases. Upon subsequent air cooling, these elements remain in solid solution in the ferrite or contribute to finer pearlite, enhancing strength. The increase from 880°C to 900°C captures this benefit.
- Austenite Grain Growth and Stability: As the temperature rises further (e.g., to 920°C), austenite grains begin to coarsen. According to the Hall-Petch equation, this leads to a decrease in strength. Concurrently, higher alloy element solubility can increase the hardenability, potentially leading to the retention of some austenite or the formation of non-equilibrium phases upon air cooling, which might slightly reduce strength and alter ductility.
The optimal 900°C temperature represents a balance: sufficient temperature for homogenization and solid solution effects, but not so high as to cause excessive grain growth in these RE-modified steel castings. The RE elements themselves, by pinning grain boundaries, help raise this optimal temperature window compared to RE-free steels.
3.3. Microstructural Evolution: The Root Cause of Enhanced Performance
Microscopic examination provides the fundamental explanation for the improved properties. The microstructure of the standard ZG25Mn2 after normalizing consisted of pearlite with pro-eutectoid ferrite, often in a somewhat blocky morphology, with an ASTM grain size of approximately 6.0.
In contrast, the RE-modified steel castings (Q2) exhibited a noticeably refined structure. The grain size was refined to ASTM 7.5. The ferrite distribution appeared more uniform. SEM analysis confirmed the powerful effect on inclusions. In the RE-free steel, manganese sulfide (MnS) stringers and silicate inclusions were observable. In the Q2 material, inclusions were predominantly small, globular oxy-sulfides. This morphological change from elongated to spherical dramatically reduces stress concentration sites, directly contributing to the enhanced ductility and toughness measured. The purification effect also cleans up the grain boundaries, leading to more efficient grain refinement and solid solution strengthening.
The strengthening contribution in these advanced steel castings can be conceptually modeled as a summation:
$$ \sigma_{total} = \sigma_{Fe} + \sigma_{ss}(Mn,Si) + \sigma_{gs}(d) + \sigma_{ppt}(TiC) + \sigma_{disl} + \Delta\sigma_{clean} $$
Where:
- $\sigma_{Fe}$: Lattice friction stress of pure iron.
- $\sigma_{ss}$: Solid solution strengthening from Mn, Si.
- $\sigma_{gs}$: Grain size strengthening (Hall-Petch term, greatly enhanced by RE).
- $\sigma_{ppt}$: Precipitation strengthening from microalloy carbides (Ti).
- $\sigma_{disl}$: Dislocation strengthening.
- $\Delta\sigma_{clean}$: A positive contribution from cleaner steel with modified inclusions (reducing defect sites).
The RE addition positively impacts $\sigma_{gs}$, $\sigma_{ppt}$ (indirectly via Ti), and $\Delta\sigma_{clean}$, making it a highly efficient multi-functional alloying strategy for normalized steel castings.
3.4. Production Validation: Castability and Weldability of the New Steel Castings
The transition from laboratory test bars to actual production steel castings is critical. A trial production run was conducted using a 3-ton basic electric arc furnace. The target component was a vehicle spring bracket, a part with varying sections (max 30 mm, min 15 mm) produced via resin sand molding. Over 5 tons of the Q2-composition steel was melted and used to pour multiple molds.
Castability: The pouring temperature was carefully controlled between 1595°C ± 5°C. All molds filled completely without any signs of misruns or cold shuts. This indicates that the RE addition, known to lower melt surface tension and improve fluidity, successfully maintained excellent casting characteristics—a vital property for manufacturing sound, complex steel castings.
Weldability: A key design goal was low Ceq for weldability. The IIW Ceq was ~0.56%. For a more conservative assessment of crack sensitivity, the Ito-Bessyo (Pcm) formula is often used for low-alloy steels:
$$ P_{cm} = \omega(C) + \frac{\omega(Si)}{30} + \frac{\omega(Mn) + \omega(Cu) + \omega(Cr)}{20} + \frac{\omega(Ni)}{60} + \frac{\omega(Mo)}{15} + \frac{\omega(V)}{10} + 5\omega(B) $$
For the Q2 composition, Pcm calculates to approximately 0.20-0.25%. This very low value confirms excellent inherent weldability. In practice, using low-hydrogen electrodes with a moderate preheat of 150-200°C resulted in crack-free, high-quality weld repairs and fabrications on trial steel castings, fulfilling a major industrial requirement.
4. Conclusions and Future Outlook for RE-Modified Steel Castings
This comprehensive study successfully demonstrates the feasibility and advantages of a novel RE-optimized, low-carbon-equivalent steel for castings requiring high strength in the normalized condition. The principal conclusions are:
- Effective Strength Enhancement: The strategic addition of 0.3-0.4% RE-containing ferroalloy to a leaner ZG25Mn2 base composition effectively increases the yield strength by over 30 MPa, compensating for the reduction in C and Mn needed to achieve Ceq ≤ 0.61%.
- Superior Combination of Properties: The optimized material (0.4% RE addition, normalized at 900°C) achieves a superior property profile: YS > 430 MPa, TS > 640 MPa, El > 28%, RA > 60%. This meets or exceeds the requirements for high-strength normalized steel castings while offering exceptional ductility.
- Microstructural Mastery: The property improvements are rooted in RE-induced metallurgical benefits: significant austenite grain refinement (from ASTM 6.0 to 7.5), transformation of harmful elongated inclusions into benign globular particles, and overall melt purification.
- Industrial Practicality: The new grade exhibits excellent castability, filling thin sections without defects, and possesses outstanding weldability due to its low Ceq and Pcm indices, validated in foundry trials.
The development pathway outlined here provides a robust template for designing the next generation of high-performance, fabricable, and dimensionally stable steel castings. Future work could explore:
- The fine-tuning of RE and microalloying (Ti, Nb) combinations for even higher strength-toughness balance.
- Investigating the impact on fatigue and wear resistance, crucial for many mechanical and automotive steel castings.
- Developing integrated computational materials engineering (ICME) models to predict microstructure and properties based on composition and normalizing parameters for these advanced steel castings.
This approach moves beyond reliance on distortion-prone quenching, aligning perfectly with the industry’s drive towards “high-precision, near-net-shape” manufacturing of critical steel castings.