Description：roll is a tool to produce plastic deformation of metal. It is an important consuming part to determine the efficiency and quality of rolling mill.
Roll types can be divided into casting roll and forging roll according to forming method: integral roll, metallurgical composite roll and composite roll according to process method. Integral roll is divided into integral casting roll and integral forging roll. Metallurgical composite casting rolls mainly include semi flushing composite casting, overflow (full flushing) composite casting and centrifugal composite casting. In addition, there are also composite rolls manufactured by special composite methods such as continuous pouring process for cladding, spray deposition, hip – hot isostatic pressing and electroslag fusion welding. The combined roll is mainly the insert sleeve combined roll.
What is an Integral Roll?
An integral roll is made from a single material by casting or forging. Its outer layer, center, and neck are controlled through casting or forging and heat treatment. Forged rolls and static casting rolls are integral rolls.
Main Categories of Rolls by Material
Rolls are categorized into three main types:
- Cast steel rolls
- Cast iron rolls
- Forging series rolls
What are Casting Rolls?
Casting rolls are made by casting molten steel or iron. They are divided by material:
- Cast steel rolls
- Cast iron rolls
And by manufacturing method:
- Integral casting rolls
- Composite casting rolls
Suitable Rolls for Integral Casting
Integral casting is used for rolls in blooming mills, billet mills, large section steel mills, rail beam mills, scale breaking mills, edging mills, and small section steel mills. These rolls have a thick layer and deep pass. The process is simple and cost-effective.
What is a Composite Casting Roll?
Composite casting rolls use two or more materials for the outer layer, core, and neck. They achieve desired properties through material composition and heat treatment. Methods include:
- Semi-flushing composite casting
- Centrifugal composite casting
- Overflow composite casting
These rolls require special equipment and are complex and costly to make.
Suitable Rolls for Compound Casting
Compound casting suits rolls needing high load capacity and quality. These rolls require high surface hardness and neck toughness. Examples include:
- Work rolls and support rolls for hot strip mills
- Rolls for medium and heavy plate mills
- Roll rings for universal mills
Recent trends show high chromium cast iron rolls excel in wear resistance and durability.
What is a Cast Steel Roll?
Cast steel rolls are iron-based with less than 2.2% carbon. They include:
- Carbon steel (alloy elements ≤ 0.8%)
- Alloy cast steel (alloy elements > 0.8%)
- Semi steel (carbon ≥ 1.3%)
Varieties include:
- Carbon cast steel rolls
- Alloy cast steel rolls
- Semi steel rolls
- Graphite steel rolls
- High chromium steel rolls
- Composite cast steel support rolls
- High-speed steel rolls
- Semi high-speed steel rolls
What is a Carbon Steel Roll?
Carbon steel rolls are hypoeutectoid steel with low alloy content. Examples are z-u70, zu80, and zu70mn. The matrix is pearlite with ferrite, reducing toughness and wear resistance. These rolls are becoming obsolete.
What is an Alloy Cast Steel Roll?
Alloy cast steel rolls are eutectoid or hypereutectoid steel with alloy content >0.8%. They contain carbon (0.4%-1.3%) and elements like chromium and nickel. The matrix is sorbite or tempered sorbite. Typical materials are 60CrMnMo, 65CrNiMo, and 75CrNiMo.
Alloy cast steel rolls have high strength, toughness, and crack resistance. They are trending towards lower carbon and higher alloy content.
Main Uses of Alloy Cast Steel Rolls
Used in blooming mills, large section steel mills, and roughing mills. The backup rolls for integral and composite casting are also alloy cast steel rolls. They contain high alloy elements for wear and peeling resistance.
Production Process of Alloy Cast Steel Rolls
Most alloy cast steel rolls are integrally cast. Traditional methods include cold sand hanging and bottom injection. Improved methods use cold mold coating and electric heating. High temperature diffusion and spheroidization enhance roll performance.
Two processes exist:
- Smelting, casting, heat treatment, performance, and flaw detection
- Smelting, casting, rough machining, heat treatment, finish machining, performance, and flaw detection
High Temperature Diffusion Treatment
High temperature diffusion reduces alloy segregation caused by dendritic crystallization during solidification. It heats the roll to 150-250°C above AC3 to achieve uniform austenite structure.
Spheroidizing Annealing Treatment
Spheroidizing annealing improves cutting performance and prepares for quenching. It transforms flake carbides to spherical, creating spherical pearlite. The temperature is slightly above the AC1 critical point.
Quenching Heat Treatment
Quenching heats the roll above the critical temperature, austenitizes it, and cools it rapidly. This creates a bainite structure, improving hardness and wear resistance.
