As our company adjusted its product structure and expanded into international markets, we embarked on the development of Russian-series railway freight cars, taking on the research and production of cast steel components for bolsters and side frames. Due to Russia’s unique geographical and climatic conditions, the technical standards for railway freight cars have distinct characteristics, imposing specific requirements on the material properties of steel castings. Beyond meeting standard strength and plasticity indices, the specifications particularly demand that the bolsters and side frames exhibit a low-temperature impact toughness of no less than 20 J/cm² at -60°C in the normalized state, i.e., KV ≥ 20 J/cm² at -60°C. This requirement far exceeds the capabilities of China’s Ministry of Railways B+ grade steel for bolsters and side frames, necessitating dedicated material research and development.
Railway freight car bogies are devices that support the car body, with bolsters and side frames serving as critical structural components. These are medium-sized steel castings with complex geometries, operating year-round in exposed environments. They reliably support the car body, bear and transmit various forces (such as vertical and vibrational loads), and facilitate smooth vehicle movement and steering, playing a vital role in railway transportation safety. Consequently, the quality of these steel castings significantly impacts the operational safety of freight cars.
Rare earth elements are increasingly utilized as alloy additives in steel. Incorporating rare earths into steel enables purification of the molten steel, improves the morphology and distribution of non-metallic inclusions, refines the microstructure, enhances impact toughness, and improves low-temperature performance. The addition of rare earth elements to alloy and carbon steel castings leverages their unique physical and chemical properties to significantly enhance mechanical properties, particularly low-temperature impact toughness.
Applying rare earth micro-alloying technology to the material of Russian railway freight car bolsters and side frames exploits the purifying, modifying, and alloying effects of rare earth elements to substantially improve low-temperature impact toughness. This led to the successful development of a specialized steel for Russian-series railway vehicle bolsters and side frames, designated as EB steel, which has been implemented in cast components and supplied in bulk quantities.
This developed material is a low-temperature-resistant cast steel. Steel often exhibits brittleness at low temperatures below -40°C, characterized by a significant drop in toughness and plasticity. Therefore, maintaining sufficient toughness under low-temperature conditions is crucial to prevent fracture due to vibration and other factors during operation, making impact toughness a paramount property for the material.
Project Research Objectives
The overall approach was based on the specific service conditions and operating temperatures of Russian railway vehicle bolsters and side frames. The primary research focus was enhancing the low-temperature impact toughness of the material while ensuring that strength and plasticity indices remained unchanged under low-temperature conditions.
To achieve this objective, we leveraged the electric arc furnace – LF furnace duplex smelting process to reduce harmful elements such as N, O, S, and P, capitalizing on the advantages of clean steel production. Additionally, rare earth micro-alloying treatment of the molten steel was employed, utilizing the unique physical and chemical characteristics of rare earth elements to modify the nature, morphology, and distribution of non-metallic inclusions in the steel, thereby improving the toughness and low-temperature impact performance of the steel casting.
The technical goal of the project was to develop EB steel with excellent low-temperature properties. The specific mechanical property requirements are summarized in Table 1.
| Grade | Tensile Strength /MPa | Yield Strength /MPa | Elongation (%) | Reduction of Area (%) | Impact Toughness at -60°C /J·cm⁻² |
|---|---|---|---|---|---|
| EB Steel | ≥ 500 | ≥ 300 | ≥ 20 | ≥ 35 | ≥ 20 |
Research Process
Chemical Composition Requirements
The Russian railway freight car technical standard GOST 32400 specifies the chemical composition requirements for bolster and side frame cast steel materials, as shown in Table 2.
| C | Si | Mn | S | P | Cu | Ni | Cr |
|---|---|---|---|---|---|---|---|
| 0.17~0.25 | 0.30~0.50 | 1.10~1.40 | ≤ 0.020 | ≤ 0.020 | ≤ 0.60 | ≤ 0.30 | ≤ 0.30 |
This standard primarily specifies the “C, Si, Mn, S, P” components, while alloying elements such as Cu, Ni, Cr, and Mo are considered residuals. However, producing steel castings based solely on these requirements resulted in mechanical properties that failed to meet the technical standards, particularly the -60°C low-temperature impact toughness value.
Building upon the Russian railway freight car technical standard, we initiated the development of a new material. The fundamental concept for developing the EB steel for Russian railway vehicle bolsters and side frames was to enhance the low-temperature impact toughness index while maintaining the material’s strength, plasticity, and toughness indices, ensuring sufficient plasticity and toughness reserves under low-temperature conditions.
Consequently, the chemical composition was optimized and adjusted from the base material specified in GOST 32400.
Determination of Rare Earth Element Addition
To optimize and adjust the chemical composition, rare earth micro-alloying treatment was applied during steelmaking. The primary roles of rare earth elements in steel are as follows:
1. Purification of Molten Steel: Rare earth elements are powerful deoxidizers, even stronger than aluminum. Adding rare earth mischmetal to molten steel significantly reduces oxygen content. They also possess stronger desulfurization capability than manganese, lowering sulfur content. Furthermore, they mitigate the embrittling effects of low-melting-point impurities like Pb, Bi, Sb, and Sn.
2. Modification of Non-Metallic Inclusions: Without rare earth addition, inclusions typically exist as blocky or irregularly distributed common sulfides. Adding a certain amount of rare earth transforms these inclusions into spherical or elliptical, finely dispersed rare earth oxides, oxysulfides, and sulfides. Rare earths interact with oxygen and sulfur in the steel, and these inclusions agglomerate, distributing within the grains rather than at the grain boundaries. The modification effect can be conceptually represented by the transformation: $$ \text{Inclusion}_{\text{original}} + RE \rightarrow \text{Inclusion}_{\text{RE-modified}} $$ where the RE-modified inclusions are more globular and uniformly distributed.
3. Microstructure Refinement: Rare earth elements contribute to grain refinement in steel and can eliminate columnar crystals and Widmanstätten structures. The grain size refinement can be described by the Hall-Petch relationship: $$ \sigma_y = \sigma_0 + k_y d^{-1/2} $$ where $d$ is the average grain diameter, and refinement improves toughness.
4. Enhanced Toughness: Adding a specific amount of rare earth improves the impact toughness of steel, particularly its low-temperature impact toughness. The improvement in impact energy can be related to the reduction in inclusion size and grain refinement.
Based on the structural characteristics and wall thickness of the railway freight car bolster and side frame castings, theoretical analysis, and decades of accumulated research results on rare earth application within our organization, the rare earth addition parameter for the newly developed EB steel was determined. The addition was controlled such that the rare earth content in the molten steel is ≤ 0.20%.
EB Steel Chemical Composition Design and Experimentation
Production Equipment and Process
The steel casting process utilized a 20-ton eccentric bottom tapping electric arc furnace coupled with a 25-ton LF ladle refining furnace. The process flow consisted of charging, melting, oxidation, EAF tapping, refining, LF tapping, and molten steel pouring.
The rare earth addition method employed was the投入法 (addition method), where rare earth is added after final deoxidation during the refining process, followed by a minimum of 3 minutes of bottom argon stirring.
The rare earth material used was rare earth ferrosilicon alloy, containing lanthanide series, light rare earth elements.
EB Steel Chemical Composition Design
The researched and designed chemical composition for EB steel is presented in Table 3.
| C | Si | Mn | S | P | Cu | Ni | Cr | Mo | RE (Added) |
|---|---|---|---|---|---|---|---|---|---|
| 0.17~0.25 | 0.30~0.50 | 1.10~1.40 | ≤ 0.015 | ≤ 0.015 | ≤ 0.30 | 0.20~0.30 | 0.20~0.30 | ≤ 0.12 | ≤ 0.20 |
GOST 32400 specifies Ni content not exceeding 0.30% and Cr content not exceeding 0.30% for the bolster and side frame cast steel material.
In designing the EB steel composition, to ensure plasticity and low-temperature impact toughness indices, Ni content was controlled between 0.20%~0.30%, and residual Mo content was limited to ≤ 0.12%. To guarantee strength indices, Cr content was maintained between 0.20%~0.30%. Simultaneously, S and P contents were reduced, controlling them to ≤ 0.015%, and micro-alloying elements were incorporated.
From the perspective of ensuring strength-related properties, controlling Ni and Cr towards the upper limits of the composition range, reducing S and P contents, and adding micro-alloying elements are beneficial. Utilizing secondary refining reduces N, O gases and non-metallic inclusions, purifying the molten steel. Applying rare earth micro-alloying modifies inclusion morphology and distribution, refines grain size, and effectively enhances the low-temperature impact toughness index, ensuring adequate plasticity and toughness reserves under low-temperature conditions for the steel casting.
Process trials were conducted according to the design, involving three heats designated as 1#, 2#, and 3#. The chemical compositions of these trial heats are listed in Table 4.
| Heat No. | C | Si | Mn | S | P | Cu | Ni | Cr | Mo | RE |
|---|---|---|---|---|---|---|---|---|---|---|
| 1# | 0.22 | 0.41 | 1.26 | 0.012 | 0.010 | 0.10 | 0.26 | 0.24 | 0.04 | 0.010 |
| 2# | 0.22 | 0.38 | 1.21 | 0.006 | 0.007 | 0.095 | 0.22 | 0.25 | 0.06 | 0.012 |
| 3# | 0.21 | 0.42 | 1.18 | 0.009 | 0.005 | 0.094 | 0.27 | 0.23 | 0.08 | 0.013 |
EB Steel Gas Content Detection
The gas content in the EB steel was measured, and the results are shown in Table 5. It is evident that the oxygen and nitrogen gas contents in the molten steel were low, indicating effective control during the steel casting process.
| Heat No. | Steel Grade | [O] | [N] |
|---|---|---|---|
| 1# | EB | 0.026 | 0.055 |
| 2# | EB | 0.029 | 0.058 |
| 3# | EB | 0.027 | 0.060 |
EB Steel Casting Heat Treatment
The heat treatment parameters for EB steel castings are detailed in Table 6. The mechanical properties, metallographic structure, non-metallic inclusions, and grain size of the castings after heat treatment are presented in Table 7 and Table 8.
| Process Step | Charging Temp. /°C | Holding Temp. /°C | Heating Time /min | Holding Time /min | Cooling Medium | Cooling Time /min |
|---|---|---|---|---|---|---|
| Normalizing | ≤ 540 | 900~920 | 180~720 | 210~240 | Air | ≥ 120 |
| Tempering | ≤ 500 | 600~650 | 150~300 | 210~270 | Air | ≥ 120 |
| Heat No. | Tensile Strength /MPa | Yield Strength /MPa | Elongation (%) | Reduction of Area (%) | Impact Toughness at -60°C /J·cm⁻² |
|---|---|---|---|---|---|
| 1# | 571 | 350 | 29.5 | 66 | 33, 38 |
| 2# | 555 | 345 | 30 | 65 | 36, 34 |
| 3# | 560 | 347 | 31 | 69 | 37, 35 |
| Heat No. | Metallographic Structure | Non-Metallic Inclusion | Grain Size |
|---|---|---|---|
| 1# | Normalized Grade 4 | Type III Fine Series 0.5 Grade | 8.5 |
| 2# | Normalized Grade 4 | Type III Fine Series 0.5 Grade | 8.5 |
| 3# | Normalized Grade 4 | Type III Fine Series 0.5 Grade | 8.5 |
The results indicate that all mechanical property indices of the heat-treated castings met the specified EB steel technical standards and the requirements of the Russian technical standard GOST 32400. The consistent fine grain size and controlled inclusion level across heats demonstrate the effectiveness of the process for producing high-quality steel castings.
Conclusions
1. The chemical composition design for EB steel is rational, and the smelting and heat treatment processes are feasible. All mechanical property indices achieved the targeted EB steel technical standards.
2. The application of rare earth micro-alloying treatment to EB steel enhanced the mechanical properties of the castings, successfully meeting the required low-temperature impact toughness value.
This material has been successfully applied to the production of bolster and side frame steel castings for railway freight cars operating in Russia, Kazakhstan, and Mongolia. A total of 400 sets have been manufactured, with all product properties satisfying the technical standard requirements, demonstrating the robustness of this steel casting material for demanding low-temperature applications.