Abstract: The application of digital silk feeding spheroidization stations in the production of QT800-5 bracket castings. By leveraging a digital operation platform that integrates feeding, thermal analysis, and molten iron transfer systems, we ensure high-quality raw materials and precise control of alloying elements. This approach guarantees high spheroidization rates, optimal alloy compositions, and refined matrix structures, thereby producing QT800-5 bracket castings that meet stringent technical requirements in terms of chemical composition, mechanical properties, and metallographic structure.
1. Introduction
Bracket castings, as crucial components in automotive suspension systems, must withstand significant forces and moments. The adoption of QT800-5, a high-strength and high-ductility material, has become essential to meet the increasing demands for lightweight and durable castings in modern vehicles. This study explores the application of digital silk feeding spheroidization stations in the production of QT800-5 bracket castings, highlighting the production process, parameter control, and the advantages of digitalization.
2. Technical Requirements of Bracket Castings
Bracket castings play a vital role in automotive safety, supporting the vehicle’s suspension system. They must endure longitudinal, lateral forces, and moments from the steering arm. As vehicles evolve towards energy efficiency and lightweighting, the demand for high-performance materials like QT800-5 has surged. QT800-5 offers superior strength and ductility, making it an ideal replacement for lower-grade materials like QT450-10 or QT500-7.
Table 1: Technical Specifications of QT800-5 Bracket Castings
| Specification | Value |
|---|---|
| Material | QT800-5 |
| Weight | 25 kg |
| Average Wall Thickness | 20 mm |
| Tensile Strength | ≥800 MPa |
| Yield Strength | ≥480 MPa |
| Elongation | ≥5% |
| Matrix Composition | Pearlite + Ferrite |
| Hardness | 245–335 HBW |
| Spheroidization Rate | ≥90% |
| Defects | No shrinkage, porosity, or slag inclusions |
3. Production Process and Control Measures
3.1 Equipment and Conditions
The production process utilizes a 2-ton medium-frequency induction furnace for melting, equipped with a dedicated wire-feeding ladle. A green sand molding line with KW static pressure and a novel digital wire-feeding spheroidization platform ensures automated and intelligent control. Equipment such as a German OBLF direct-reading spectrometer, a molten iron thermal analyzer, a micro-controlled electronic universal testing machine, and a metallographic microscope facilitate rigorous quality control.
3.2 Melting Process
3.2.1 Chemical Composition Requirements
The chemical composition of QT800-5 bracket castings is carefully controlled to achieve optimal mechanical properties. Carbon content influences the graphite structure and matrix, ranging from 3.6% to 3.8%. Silicon promotes graphitization, reducing white iron tendencies and increasing ferrite content, maintained at 2.4% to 2.6%. Manganese stabilizes pearlite, enhancing tensile strength but reducing elongation, controlled at 0.3% to 0.5%. Alloying elements like copper and tin control the pearlite and ferrite content, with copper at 0.4% to 0.6% and tin at 0.02% to 0.04%. Antimony increases graphite nodule count and fineness, maintained at 0.015% to 0.03%. Residual magnesium and rare earths are also carefully controlled.
Table 2: Chemical Composition of Bracket Castings (Mass Fraction, %)
| Element | Initial Iron | Final Iron |
|---|---|---|
| C | 3.7–3.9 | 3.6–3.8 |
| Si | 1.3–1.5 | 2.3–2.5 |
| Mn | <0.3 | 0.3–0.5 |
| Cu | <0.05 | 0.4–0.6 |
| Sn | <0.02 | 0.02–0.04 |
| Sb | – | 0.015–0.03 |
| P | <0.05 | <0.05 |
| S | <0.01 | <0.01 |
| Mg | – | 0.03–0.05 |
| RE | – | 0.005–0.015 |
3.2.2 Charge Ratio
To ensure high strength and ductility, high-purity pig iron, quality scrap steel, and a small amount of cleaned return scrap are used. The charge ratio is 40% high-purity pig iron, 40% scrap steel, 20% return scrap, with additions of carburizer and other alloys.
3.2.3 Wire Feeding Spheroidization and Inoculation
(a) Selection of Cored Wire and Feeding Amount
Yttrium-based heavy rare earth cored wire is chosen for its ability to purify molten iron, ensure consistent spheroidization, resist decay, and minimize white iron tendencies. The wire diameter is φ13 mm + 0.5 mm, with a calculated feeding rate of 24 m/t.
(b) Selection of Wire Feeding Package and Feeding Speed
To prevent spattering, a safety height of 400–500 mm is maintained, with a height-to-diameter ratio of 1.5–2.0. The feeding speed is crucial, set at 20 m/min based on package size, iron weight, and temperature.
(c) Inoculation Process
Three inoculation steps ensure optimal results. Primary inoculation uses 0.5%–0.6% YFY-8B inoculant. Secondary inoculation uses BXX-Y2 inoculating wire at 15 m/t and 17 m/min. Tertiary inoculation adds 0.1%–0.15% YFY-1B silicon-barium-calcium powder during pouring.
Table 3: Inoculant Composition Requirements (Mass Fraction, %)
| Product | Component | Remarks |
|---|---|---|
| YFY-8B | Si 70–72%, Ca 1–1.5%, Ba 2–3%, Al 0.5–1% | 3–8 mm granularity |
| YFY-1B | Ce 1.5–2.0%, Si 63–68%, Ca 1–1.5% | 0.2–0.8 mm granularity |
3.3 Digital Optimization of Wire Feeding Parameters
Data on chemical composition, temperature, weight, thermal analysis, wire feeding parameters, mechanical properties, and metallographic structure are collected and used to optimize feeding parameters. Optimized parameters include a wire feeding rate of 21 m/t at 25 m/min for spheroidization and 15 m/t at 20 m/min for inoculation.
4. Production Results and Analysis
4.1 Production Results
Spectral analysis and mechanical testing of bracket casting samples show consistent compliance with technical requirements. Tensile strength, yield strength, elongation, and hardness values meet or exceed specified limits.
Table 4: Mechanical Properties of Bracket Casting Samples
| Sample No. | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HBW) |
|---|---|---|---|---|
| 1 | 826 | 501 | 5.7 | 302 |
| 2 | 815 | 498 | 6.0 | 295 |
| 3 | 822 | 496 | 5.8 | 298 |
| 4 | 818 | 512 | 6.1 | 310 |
| 5 | 812 | 488 | 5.9 | 290 |
Table 5: Metallographic Analysis Results of Bracket Castings
| Casting Name | Spheroidization Rate (%) | Graphite Size | Ferrite Quantity (%) | Pearlite Quantity (%) |
| A | 93 | 6 | 18 | 82 |
| B | 91 | 6 | 22 | 78 |
The metallographic analysis results of the bracket castings, as detailed in Table 5, reveal significant insights into the microstructure and composition of the castings. The spheroidization rate, which is crucial for ensuring the mechanical properties of the QT800-5 material, is consistently high across both samples, with 93% for casting A and 91% for casting B. This indicates effective nodularization, which is essential for achieving the desired strength and ductility.
The graphite size is noted as 6 in both castings, suggesting a uniform and fine distribution of graphite nodules, which contributes to the material’s ability to withstand loads and deformation.
The ferrite and pearlite quantities provide further information on the microstructure. Casting A has 18% ferrite and 82% pearlite, while casting B has 22% ferrite and 78% pearlite. The balance between ferrite and pearlite is critical for achieving the required mechanical properties of QT800-5. Generally, a mixture of ferrite and pearlite, with the latter typically being the majority phase, is desired to optimize tensile strength and elongation.
Finally, the remarks column refers to the figures that depict the microstructures of the castings. Figure 4 corresponds to casting A, and Figure 5 corresponds to casting B, providing visual confirmation of the metallographic analysis results.
In conclusion, the metallographic analysis results confirm that the bracket castings meet the technical requirements for QT800-5 material, with high spheroidization rates, uniform and fine graphite nodules, and an optimal balance between ferrite and pearlite. These findings support the effectiveness of the digital wire feeding spheroidization station in producing high-quality QT800-5 bracket castings for automotive applications.