Cast iron rolls – Alloy indefinite cast iron rolls

Description: The roll is a tool for plastically deforming (rolled material) metal, and is an important large-scale consumable part that determines the efficiency of the rolling mill and the quality of the rolled material. Rolls are the largest consumables in the metallurgical industry, so improving the wear resistance of rolls is the primary task of improving economic efficiency and production.

Keyword: Cast iron rolls

Working conditions and materials of hot rolls 

• Working conditions: Hot rolls often work in a high-temperature environment of 700-800°C (sometimes the rolled material they contact is as high as 1200°C), contact with the hot billet, bear strong rolling force, and the surface is subjected to strong wear of the rolled material , repeatedly heated by hot-rolled materials and cooled by cooling water, and subjected to thermal fatigue with large temperature changes.
• Main failure modes of hot rolls: thermal cracking and spalling caused by thermal fatigue, roll body surface wear, roll fracture, over-tempering and creep, roll winding, and the failure surface covers almost the entire working surface.

Among them, the peeling and wear of the roll surface is the most common failure mode of the hot roll

• Material requirements: hot roll materials not only have good strength and toughness, but also have high wear resistance
• Hot roll materials include: forged steel, infinite chilled cast iron, ordinary chilled cast iron, low NiCrMo, medium NiCrMo, high NiCrMo cast iron, cast steel, nodular composite cast iron, semi-steel and high hardness special semi-steel, high chromium cast iron, semi- High-speed steel and high-speed steel, etc.

Classification of cast iron rolls:

Generally classified by manufacturing process:

• lThe roll with white structure (matrix + carbide) in the working layer due to the chilling effect of the metal mold is called chilled cast iron roll;
• lThe roll that obtains mottled structure (matrix + carbide + graphite) by appropriately increasing the carbon equivalent of molten iron is called infinitely chilled cast iron roll.
• lSemi-chilled cast iron rolls that use sand-lined metal type and continue to increase the carbon equivalent to obtain rough texture.
• lIn the structure of all the above varieties, if the graphite is spherical, it is called a nodular cast iron roll; for a compound cast roll, the word “composite” is added。

Alloying principle of infinitely chilled cast iron rolls

• C: The high carbon content hinders the precipitation of cementite on the one hand, and on the other hand, due to the increase in the number of graphite cores formed, graphite can be refined. However, if it is too high, graphite will float. Under the condition of a certain cooling rate, the carbon content will be increased within a certain range, the depth of the white layer will decrease, and the amount of cemented body on the surface will increase.
• Si: Si silicon can reduce the solubility of carbon in austenite, not only can increase the eutectoid transformation temperature, but also widen the temperature range of eutectoid transformation, and shorten the incubation period of pearlite and bainite. Within a certain range, with the increase of silicon content, the diameter of graphite balls becomes smaller.

The two elements of carbon and silicon directly affect the comprehensive performance of cast iron rolls. Carbon and silicon should be considered uniformly. The upper limit of carbon equivalent should be based on the principle of avoiding graphite floating as much as possible, and the lower limit should meet the performance requirements of the roll to ensure that there is an appropriate content of carbide.

• Mn: Mn Manganese element lowers the eutectoid transformation temperature and plays the role of stabilizing and refining pearlite. Manganese can improve the strength and hardness of the roll, but when the content is too high, the segregation is serious, and the as-cast state will precipitate network carbide along the grain boundary. reduce its toughness.
• Cr: Cr Chromium is the most effective element to increase the depth of the white layer of chilled cast iron rolls. It can significantly offset the adverse effects of silicon and is beneficial to the formation of pearlite structure. In alloy ductile iron, proper addition of chromium can make some free carbides appear in the structure, which is beneficial to the improvement of hardness and wear resistance.
• Mo: Mo As a stable pearlite element, molybdenum can refine the white layer structure in chilled cast iron, improve the material strength, and improve the thermal strength of the roll. In alloy ductile iron rolls, appropriately increasing molybdenum content can promote the formation of pearlite structure and increase the dispersion of pearlite. Molybdenum inhibits the decomposition of austenite and is beneficial to the formation of bainite structure. But molybdenum is easy to segregate, so its content should not be too high.

Laser alloying of cast iron rolls

Laser alloying is to coat a layer of alloying powder on the surface, or use a nozzle to feed the alloy powder when the laser is irradiated, so that the powder and the matrix are melted at the same time to form a mixed molten pool, and the composition of the alloy layer is mixed by the added alloy powder and the matrix It is a new alloy, and its structure belongs to the rapid solidification structure under the new ratio.

Laser alloying technology overcomes the difficult problem of laser quenching and strengthening, effectively improves wear resistance and prolongs the service life of rolls; for cast iron rolls, it can effectively prevent cracks and no soft and hard bands appear on the surface.