Cast Iron Roll: A key component in the rolling process

In the field of metal processing, the rolling process is an important forming method, and the Cast Iron Roll is an indispensable key component in this process. Cast iron roll play a crucial role in shaping metal materials, ensuring product quality and enhancing production efficiency. The quality of its performance directly affects the accuracy and surface quality of the rolled products, as well as the stability and economy of the production process. Therefore, a thorough understanding of the relevant knowledge of cast iron rolls is of great significance for optimizing the rolling process and enhancing the competitiveness of products. ​

Working conditions and challenges of cast iron rolls

(1) Harsh temperature environment

Cast iron rolls are often in a high-temperature environment during operation, with the general working temperature reaching 700-800 ° C. In some special cases, the temperature of the rolled material they come into contact with can even reach 1200°C. Continuous high temperatures not only test the thermal stability of the roll material, but also cause problems such as thermal expansion and thermal deformation, affecting the dimensional accuracy of the rolls and the quality of the rolled products. ​

(2) Strong mechanical stress

The rolls need to withstand the strong pressure from the rolled material. This pressure acts continuously during the rolling process and is prone to cause fatigue damage to the rolls. Meanwhile, during the rolling process, there is a strong frictional force between the surface of the rolls and the rolled material, which will accelerate the wear of the roll surface and reduce the service life of the rolls. ​

(3) The threat of thermal fatigue

Due to continuous heating by hot-rolled materials and cooling by cooling water, the rolls undergo significant temperature changes in a short period of time and are subjected to severe thermal fatigue. Thermal fatigue can cause cracks on the surface of the rolls. Over time, these cracks may expand, eventually leading to the spalling and failure of the rolls. ​

2. Main types of cast iron rolls

(1) Chilled cast iron rolls

Working principle: The working layer of the chilled cast iron roll forms a white cast structure (matrix + carbide) due to the rapid cooling effect of the metal mold. During the casting process, by controlling the cooling rate, the surface of the roller is rapidly cooled to form a white cast layer with high hardness and high wear resistance, while the core maintains a relatively soft gray cast or pitted structure to ensure that the roller has a certain degree of toughness. ​

Characteristics: It features extremely high surface hardness and excellent wear resistance, effectively resisting wear during the rolling process. However, due to the high brittleness of the white cast iron layer, the thermal cracking resistance of cold-hardened cast iron rolls is relatively poor, and cracks are prone to occur when subjected to large thermal stress. ​

Application fields: It is often used in rolling processes with high surface quality requirements and relatively low rolling pressure, such as the precision rolling of thin plates and steel strips. ​

(2) Infinitely cold-hardened cast iron rolls

Working principle: By appropriately increasing the carbon equivalent of molten iron, the roll acquires a notched structure (matrix + carbide + graphite). This structure ensures that the chilled layer of the roll has no distinct boundary at the fracture surface, and the transition from the hard surface to the soft core is gradual without a clear transition zone. ​

Characteristics: It combines high hardness and good toughness. The presence of graphite improves the thermal cracking resistance and anti-spalling performance of the rolls, enabling them to maintain a good working state even when subjected to significant thermal and mechanical stresses. Compared with cold-hardened cast iron rolls, infinitely cold-hardened cast iron rolls have a longer service life and are suitable for a wider range of rolling conditions. ​

Application fields: Widely used in rough rolling, medium rolling and other processes, such as rough rolling of steel billets, intermediate rolling of bars and wires, etc. Among these processes, the rolls need to withstand considerable rolling forces and thermal loads. The performance of the infinitely chilled cast iron rolls can well meet the requirements. ​

(3) Semi-cooled hard cast iron rolls

Working principle: Casting is carried out using a metal mold with sand coating. A 10-20mm layer of molding sand is coated inside the metal mold to reduce the cooling rate of the roller body and obtain a notched structure in the working layer of the roller body. This casting method makes the hardness distribution of the rolls relatively uniform, with a small hardness drop from the surface to the core. ​

Characteristics: Semi-chilled cast iron rolls have excellent resistance to hot cracking, high strength and toughness. The surface hardness of the roller body is generally HS35-55, which can effectively resist thermal fatigue and mechanical fatigue while ensuring certain wear resistance. Among them, semi-cooled hard ductile iron rolls have more superior performance due to their unique spherical graphite structure. ​

Application fields: Mainly applicable to the billet opening stands and roughing mill stands of medium and small-sized rolling mills. In these cases, the rolls need to have good comprehensive performance to cope with more complex rolling conditions. ​

(4) Ductile iron rolls

Working principle: Ductile iron rolls are made by pouring molten iron that has undergone spheroidizing treatment into the mold, causing the graphite in the roll structure to take on a spherical shape. The presence of spherical graphite eliminates the fragmentation effect of flake graphite on the matrix and greatly improves the mechanical properties of the rolls. ​

Characteristics: It features high strength, high toughness and excellent wear resistance. Its resistance to thermal cracking and spalling is also outstanding. The hardness range of ductile iron rolls is relatively wide and can be adjusted according to different application requirements, with a wide range of applications. ​

Application fields: It can be used in various types of rolling mills, including rough rolling, medium rolling and finish rolling processes. In some special rolling processes with high requirements for the performance of rolls, ductile iron rolls can also demonstrate excellent performance.

3. The influence of alloying elements on the performance of cast iron rolls

(1) Carbon (C)
Influence mechanism: Carbon is one of the important elements affecting the performance of cast iron rolls. On the one hand, a high carbon content will hinder the precipitation of cementite, and at the same time, due to the increase in the number of formed graphite cores, the graphite can be refined. On the other hand, if the carbon content is too high, it will cause graphite to float, affecting the performance of the rolls. At a certain cooling rate, if the carbon content is appropriately increased, the depth of the white cast layer will decrease and the amount of surface cementite will increase.
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Impact on performance: An appropriate amount of carbon can enhance the hardness and wear resistance of the rolls, but an excessively high carbon content will reduce the toughness of the rolls and increase the risk of crack formation. Therefore, during the production process, it is necessary to precisely control the carbon content to balance the various properties of the rolls. ​

(2) Silicon

Mechanism of influence: Silicon can reduce the solubility of carbon in austenite, not only increasing the eutectoid transformation temperature, but also broadening the eutectoid transformation temperature range and shortening the incubation period of pearlite and bainite. Within a certain range, as the silicon content increases, the diameter of the graphite balls will decrease, thereby improving the structure and performance of the rolls. ​

Impact on performance: Silicon can enhance the strength and hardness of the rolls, and at the same time help improve the rolls' resistance to thermal cracking. However, excessive silicon content may lead to a decrease in the toughness of the rolls, so its content needs to be reasonably controlled. ​

(3) Manganese (Mn)

Influencing mechanism: Manganese elements lower the eutectoid transformation temperature, playing a role in stabilizing and refining pearlite. It can enhance the strength and hardness of the rolls. However, when the manganese content is too high, severe segregation will occur, and network carbides will precipitate along the grain boundaries in the cast state, reducing the toughness of the rolls. ​

Impact on performance: An appropriate amount of manganese can help enhance the overall performance of the rolls, but its content must be strictly controlled to avoid adverse effects on the performance of the rolls due to segregation and the precipitation of network carbides. ​

(4) Chromium (Cr)

Influence mechanism: Chromium is the most effective element for increasing the depth of the white cast iron layer in cold-hardened cast iron rolls, which can significantly counteract the adverse effects of silicon and is conducive to the formation of pearlite structure. In alloy ductile iron, the appropriate addition of chromium can cause some free carbides to appear in the microstructure, which is helpful to improve hardness and wear resistance. ​

Impact on performance: The addition of chromium can effectively enhance the surface hardness and wear resistance of the rolls, and improve their resistance to thermal fatigue. However, excessive chromium may lead to a decrease in the toughness of the rolls. Therefore, the chromium content needs to be precisely controlled according to the specific usage requirements of the rolls. ​

(5) Molybdenum

Influence mechanism: Molybdenum, as an element that stabilizes pearlite, can refine the white cast layer structure in cold-hardened cast iron, enhance material strength, and improve the thermal strength of the rolls. In alloy ductile iron rolls, appropriately increasing the molybdenum content can promote the formation of pearlite structure and increase the dispersion of pearlite. Molybdenum can also inhibit the decomposition of austenite and is conducive to the formation of bainite structure. However, molybdenum is prone to segregation, so its content should not be too high. ​

Impact on performance: An appropriate amount of molybdenum can enhance the comprehensive performance of the rolls, especially their performance stability in high-temperature environments. However, due to the segregation tendency of molybdenum, its distribution in the rolls needs to be strictly controlled to ensure the uniformity of the roll performance. ​

4. Manufacturing process of cast iron rolls

(1) Casting process

Metal mold casting: Cold-hardened cast iron rolls and some infinitely cold-hardened cast iron rolls are often cast by metal mold casting. During the casting process, the rapid cooling effect of the metal mold causes the surface of the roll to cool rapidly, forming the required white cast or pitted structure. By controlling parameters such as the temperature of the metal mold, the thickness of the coating, and the pouring temperature and speed of the molten iron, the microstructure and properties of the working layer of the rolls can be precisely controlled.
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Sand casting: For some rolls that have relatively low requirements for surface hardness and need higher toughness, such as semi-chilled cast iron rolls, sand casting can be adopted. Adding an appropriate amount of molding sand and chill iron to the sand mold can adjust the cooling rate of different parts of the rolls, enabling the rolls to achieve a suitable hardness distribution and microstructure. ​

Compound casting: The compound casting process is used to manufacture composite cast iron rolls. By successively pouring molten iron with different compositions, the rolls have working layers and cores with different properties. For instance, first pour the core material, and then pour the working layer material with high hardness and wear resistance on its surface, so that the roll has both good toughness and surface properties.
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(2) Heat treatment process

Annealing treatment: Annealing treatment can eliminate the internal stress generated during the casting process of the rolls and improve the microstructure and properties of the rolls. By holding the roller at an appropriate temperature for a certain period of time, the internal structure is homogenized, the hardness is reduced, the toughness is improved, and preparations are made for subsequent processing and use. ​

Normalizing treatment: Normalizing treatment can refine the grains of the rolls, enhancing their strength and hardness. Heat the rolls above the critical temperature, hold them for a period of time, and then cool them in the air to obtain a uniform pearlite or bainite structure for the rolls, thereby improving their overall performance. ​

Quenching and tempering treatment: For some rolls that require higher hardness and wear resistance, quenching and tempering treatment can be carried out. Quenching endows the surface of the rolls with a martensitic structure, significantly enhancing hardness. However, the martensitic structure is relatively brittle, so tempering treatment is necessary to adjust the balance between hardness and toughness, eliminate quenching stress, and increase the service life of the rolls. ​

5. Maintenance and care of cast iron rolls
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(1) Daily inspection

Surface inspection: Regularly check the surface of the rolls for defects such as cracks, spalling, and wear. By visual inspection and the use of non-destructive testing equipment such as ultrasonic flaw detectors and magnetic particle flaw detectors, potential problems can be identified in a timely manner, and corresponding measures can be taken to repair or replace them. ​

Dimensional inspection: Measure the diameter, cylindricity and other dimensional parameters of the rolls to ensure they are within the specified tolerance range. Excessive dimensional deviations may affect the accuracy of rolled products. Therefore, once dimensional abnormalities are detected, timely adjustments or repairs should be made. ​

(2) Lubrication and cooling

Lubrication: During the rolling process, in order to reduce the friction between the rolls and the rolled material and minimize wear, appropriate lubricants need to be used. Select lubricants with good lubrication performance, extreme pressure resistance and oxidation resistance, and ensure that they are evenly distributed on the surface of the rolls. Regularly inspect the supply system of lubricants to ensure its normal operation. ​

Cooling: Effective cooling is crucial for reducing the temperature of the rolls and preventing thermal fatigue. Ensure the normal operation of the cooling system, promptly clean the dirt and impurities in the cooling water pipelines, and guarantee that the flow rate and temperature of the cooling water meet the requirements. Meanwhile, the spray Angle and position of the cooling water should be reasonably adjusted to ensure uniform cooling of the surface of the rolls. ​

(3) Storage and handling

Storage: Store the rolls in a dry and well-ventilated environment to prevent them from getting damp and rusting. For rolls that have not been used for a long time, anti-rust treatment should be carried out, such as applying anti-rust oil and wrapping with anti-rust paper. At the same time, attention should be paid to the storage method to avoid the rolls being squeezed or collided, which may cause damage. ​

Handling: When handling rolls, dedicated handling equipment such as cranes and forklifts should be used, and it is necessary to ensure that the load-bearing capacity of the equipment is sufficient. During the handling process, handle with care to avoid the rollers colliding with other objects, preventing surface damage and internal structural damage. ​

6. Conclusion

Cast iron rolls, as the core components in the rolling process, their performance is directly related to the quality of rolled products and production efficiency. By understanding the characteristics of different types of cast iron rolls, the influence of alloying elements on their performance, manufacturing processes and maintenance methods, it is possible to better select and use cast iron rolls, give full play to their advantages and improve the overall level of the rolling process. With the continuous advancement of technology, the performance and quality of cast iron rolls are also constantly improving. In the future, they are expected to be applied in a wider range of fields and make greater contributions to the development of the metal processing industry.