In the field of steel rolling, the demand for high-performance rolls has driven extensive research into advanced materials. This article details the development and practical implementation of large high-strength ductile iron rolls, which leverage the unique properties of ductile iron casting to overcome limitations in traditional roll materials. By optimizing medium-alloy ductile cast iron through specialized heat treatment processes, these rolls exhibit superior strength, wear resistance, and thermal crack resistance, making them suitable for a wide range of applications in section and bar mills. The work focuses on comprehensive control of casting processes, chemical composition, high-temperature heat treatment, microstructure, and tensile strength, ensuring consistent performance in demanding industrial environments.
Ductile iron, also known as ductile cast iron, is characterized by its spherical graphite microstructure, which imparts excellent mechanical properties. In this study, high-strength ductile iron rolls were produced using a high-temperature recrystallization heating, rapid cooling, and tempering process. This approach significantly enhances the strength of the ductile iron casting, allowing it to replace cast alloy steel and semi-steel rolls in most stands of section mills and large bar mills. The presence of eutectic carbides and spherical graphite in ductile cast iron contributes to inherent advantages such as high wear resistance, thermal crack resistance, and minimal hardness variation, thereby expanding the application scope of ductile iron rolls.
The key benefits of high-strength ductile iron rolls can be summarized as follows:
| Advantage | Description |
|---|---|
| High Strength | Tensile strength exceeding 600 MPa at the roll neck and over 700 MPa in the roll body, surpassing conventional semi-steel rolls. |
| Uniform Hardness | Minimal hardness drop across the roll profile, with variations less than 5 HSD over 100 mm depth. |
| Thermal Crack Resistance | Enhanced by spherical graphite, which improves thermal conductivity and lubrication. |
| High Wear Resistance | Achieved through eutectic carbides and a bainite-troostite matrix structure. |
The production of high-strength ductile iron rolls involves precise control over several parameters. The casting process utilizes static casting methods with either chilled metal molds or sand-lined metal molds, depending on the desired carbide content. For carbide contents below 5%, sand-lined molds are preferred to achieve lower surface hardness and more uniform microstructure. Chemical composition is tailored to specific mill stands and rolling conditions, as shown in the following table:
| Stand | Surface Hardness (HSD) | C (%) | Si (%) | Mn (%) | Cr (%) | Mo (%) | Ni (%) | Roll Body Strength (MPa) |
|---|---|---|---|---|---|---|---|---|
| BD1 | 45-51 | 3.10-3.40 | 1.70-2.10 | 0.50-0.70 | ≤0.30 | 0.40-0.70 | 2.50-3.50 | 800 |
| BD2 | 52-58 | 3.20-3.50 | 1.50-2.00 | 0.60-0.90 | 0.30-0.80 | 0.40-0.80 | 2.00-3.00 | 750 |
| E | 57-63 | 3.20-3.50 | 1.50-2.00 | 0.60-0.90 | 0.50-1.00 | 0.40-0.80 | 2.00-3.00 | 550 |
| H, V Stand Rings | 55-70 | 3.20-3.50 | 1.30-1.80 | 0.60-0.90 | 0.50-1.00 | 0.40-0.80 | 2.50-3.50 | 650 |
Heat treatment is critical for achieving the desired properties in ductile iron rolls. The process involves pre-machining the roll profile before heat treatment to ensure uniformity. The heat treatment cycle includes stages such as heating to T1 (e.g., 560°C) for h1 hours (e.g., 6-8 hours), followed by rapid cooling and tempering. The temperature and time parameters are optimized based on composition and hardness requirements. The general heat treatment curve can be represented by the following equations, where T denotes temperature and t denotes time:
$$ T(t) = T_1 \quad \text{for} \quad t \leq h_1 $$
$$ T(t) = T_2 \quad \text{for} \quad h_1 < t \leq h_2 $$
$$ T(t) = T_3 \quad \text{for} \quad h_2 < t \leq h_3 $$
$$ T(t) = T_4 \quad \text{for} \quad h_3 < t \leq h_4 $$
Here, T1, T2, T3, and T4 are specific temperatures, and h1, h2, h3, h4 are holding times. For instance, T2 is set above the recrystallization temperature (A1 + 160–220°C), and the holding time h2 is proportional to the roll diameter, typically diameter/40 hours. This controlled process refines the microstructure, enhancing strength and wear resistance in ductile iron casting.
Performance testing of high-strength ductile iron rolls involves extensive mechanical and microstructural analysis. Samples taken from the roll body are evaluated for composition, hardness, tensile strength, impact toughness, and microstructure. The following table summarizes the chemical composition of four representative samples:
| Sample | C (%) | Mn (%) | Si (%) | P (%) | S (%) | Cr (%) | Mo (%) | Ni (%) | Mg (%) |
|---|---|---|---|---|---|---|---|---|---|
| a | 3.31 | 0.67 | 1.78 | 0.051 | 0.008 | 0.15 | 0.51 | 2.92 | 0.052 |
| b | 3.29 | 0.70 | 1.80 | 0.030 | 0.003 | 0.52 | 0.46 | 2.45 | 0.04 |
| c | 3.32 | 0.78 | 1.55 | 0.044 | 0.008 | 0.78 | 0.41 | 2.23 | 0.071 |
| d | 3.28 | 0.72 | 1.45 | 0.032 | 0.005 | 0.98 | 0.55 | 2.63 | 0.068 |
Hardness measurements across the roll radial direction show minimal variation, as illustrated in the table below for sample b:
| Distance from Surface (mm) | 5 | 15 | 25 | 35 | 45 | 55 | 65 | 75 | 85 | 95 |
|---|---|---|---|---|---|---|---|---|---|---|
| Hardness (HSD) | 61.3 | 61.3 | 61.8 | 60.8 | 60.0 | 58.9 | 58.8 | 58.3 | 58.6 | 57.8 |
The tensile strength and impact toughness are critical indicators of roll performance. Tests on samples from different radial depths yield the following results:
| Sample Location | Tensile Strength (MPa) | Impact Toughness (J/cm²) |
|---|---|---|
| a (30 mm depth) | 730 | – |
| b (60 mm depth) | 760 | – |
| c (15 mm depth) | – | 2.9 |
| d (35 mm depth) | – | 3.0 |
| e (55 mm depth) | – | 3.3 |
| f (75 mm depth) | – | 3.4 |
Microstructural analysis reveals the presence of spherical graphite and carbides in the ductile cast iron. The graphite morphology shows well-formed spheroids, with average sizes increasing from 40 µm near the surface to 80 µm at greater depths. The base microstructure consists of bainite and troostite, with eutectic carbides transitioning from blocky to skeletal forms with depth. This variation is attributed to cooling rate differences during solidification and heat treatment. The relationship between cooling rate (CR) and carbide size (CS) can be approximated by:
$$ CS = k \cdot CR^{-n} $$
where k and n are material constants. This equation highlights how slower cooling rates lead to larger carbides, affecting wear resistance. The spherical graphite in ductile iron casting enhances thermal conductivity, reducing the risk of thermal cracks, which is crucial in high-stress rolling applications.
Application practices demonstrate the effectiveness of high-strength ductile iron rolls in various mill stands. For example, in a section mill producing 60E1 rails, rolls used in the Z1 stand achieved a rolling capacity of 8,240 tons per groove with uniform wear. In large bar mills, R1 stand rolls endured 30,000 tons of rolling without significant surface defects. Compared to traditional alloy cast iron rolls, which averaged 8,000 tons per campaign, high-strength ductile iron rolls extended service life to approximately 15,000 tons in high-speed rail BD2 stands. Additionally, in universal mill stands, ductile iron rings reduced sticking and increased rolling capacity by nearly 50%, outperforming high-carbon semi-steel alternatives.
The success of ductile iron rolls in these applications stems from their optimized composition and heat treatment. For instance, the addition of elements like vanadium further enhances hardness and wear resistance. The wear rate (W) can be modeled as a function of hardness (H) and carbide volume fraction (C_v):
$$ W = \alpha \cdot H^{-\beta} + \gamma \cdot C_v^{-\delta} $$
where α, β, γ, and δ are empirical coefficients. This formula underscores the importance of microstructure control in ductile iron casting for achieving longevity in rolling operations.
In conclusion, the development of high-strength ductile iron rolls represents a significant advancement in roll technology. Through tailored chemical composition and advanced heat treatment, these rolls exhibit exceptional mechanical properties, including high tensile strength, uniform hardness, and superior resistance to thermal cracking and wear. The use of ductile cast iron enables replacements for traditional materials in section and bar mills, improving rolling efficiency and product quality. Continuous optimization of alloy elements, such as chromium and molybdenum, further enhances performance, ensuring that ductile iron rolls meet the evolving demands of the steel industry. The widespread adoption of these rolls in domestic and international mills validates their reliability and effectiveness, paving the way for broader applications in heavy-duty rolling processes.