Introduction
In the modern steel industry, milling rolls, as the core components of rolling mills, directly affect rolling efficiency, product quality, and production costs. Milling rolls can be classified into various types based on manufacturing processes, material compositions, and structural features. Among them, integral milling rolls and composite casting milling rolls play a key role in different rolling scenarios due to their distinct performance characteristics. This article will systematically elaborate on the classification system, material properties, manufacturing processes, and heat treatment technologies of milling rolls, and deeply explore the applicable scenarios and development trends of various types of milling rolls.
I. Basic Classification and Definition of Milling Rolls
1.1 Integral Milling Roll
Integral milling rolls refer to milling rolls made from a single material through casting or forging processes. Their outer layer, roller body, and roller neck are all made of the same material system through casting or forging and subsequent heat treatment processes to control performance. Integral milling rolls mainly include two categories: forged steel milling rolls and static casting milling rolls. The manufacturing process of this type of milling roll is relatively simple and the cost is low. It is suitable for some rolling environments with relatively small loads or not extremely high performance requirements.
1.2 Classification by Material Category
Milling rolls can be classified into three major series based on the different manufacturing materials:
Cast steel milling rolls: Iron-based material milling rolls with a carbon content of less than 2.2%.
Cast iron milling rolls: Milling rolls mainly made of cast iron.
Forged series milling rolls: Milling rolls formed through the forging process.
II. Detailed Classification and Technical Characteristics of Casting Milling Rolls2.1 Production Principle of
Casting Milling Rolls
Casting milling rolls are milling rolls made by pouring molten steel or molten iron into shape. They are classified into cast steel milling rolls and cast iron milling rolls according to different materials. According to the manufacturing method, they can be classified into integral casting milling rolls and composite casting milling rolls.
2.2 Applicable Scenarios for Integral Casting Milling Rolls
The integral casting process is particularly suitable for the milling rolls used in primary rolling mills, billet rolling mills, large section steel rolling mills, rail and beam rolling mills, scale breaking machines, vertical rolling machines, and small section steel rolling mills. This type of milling roll features a thick surface layer and deep holes and grooves. Its production process is simple and cost-effective, and it can meet most conventional rolling requirements.
2.3 Technical Characteristics of Composite Casting Milling Rolls
Composite casting milling rolls are made of two or more different materials for the outer layer, core, and roll neck respectively, and the ideal performance matching is achieved through material combination and heat treatment process. Its main manufacturing methods include:
Semi-rinse composite casting: The combination of different materials is achieved through a special pouring process
Centrifugal composite casting: Utilizing centrifugal force to distribute different materials in layers
Overflow composite casting: A composite material structure is achieved by controlling the flow of molten metal
Composite casting milling rolls require specialized equipment, have complex manufacturing processes and high costs, but they can achieve an ideal combination of high hardness on the outer layer and high toughness on the roll neck.
2.4 Application Fields of Composite Casting Milling Rolls
Composite casting is particularly suitable for rolling scenarios that require high load-bearing capacity and high-quality products. This type of milling roll needs to possess both high surface hardness and high toughness of the roll neck. Its main applications include:
The working rolls and support rolls of the hot-rolled strip steel rolling mill.
Milling rolls for medium and heavy plate rolling mills.
Roll rings of universal rolling mills.
In recent years, high-chromium cast iron milling rolls have performed particularly well in this field due to their excellent wear resistance and service life.
III. Material System and Performance Characteristics of Cast Steel Milling Rolls
3.1 Basic Concepts of Cast Steel Milling Rolls
Cast steel milling rolls refer to iron-based material milling rolls with a carbon content of less than 2.2%, mainly including the following types:
Carbon steel milling rolls: The total amount of alloying elements does not exceed 0.8%
Alloy cast steel milling rolls: The total amount of alloying elements is greater than 0.8%
Semi-steel milling rolls: Carbon content is greater than or equal to 1.3%
3.2 Characteristics and Limitations of Carbon Steel Milling Rolls
Carbon steel milling rolls belong to hypoeutectoid steel with low alloy content. Representative materials include Z-U70, ZU80, ZU70Mn, etc. Their matrix structure is pearlite and ferrite, which results in relatively low toughness and wear resistance. With the development of rolling technology, this type of milling roll is gradually being phased out.
3.3 Technical Advantages of Alloy Cast Steel Milling Rolls
Alloy cast steel milling rolls are made of eutectoid or hypereutectoid steel composition, with an alloy content greater than 0.8%. Their carbon content is usually between 0.4% and 1.3%, and they also contain alloying elements such as chromium and nickel. The matrix structure is sorbite or tempered sorbite, and typical materials include 60CrMnMo, 65CrNiMo, 75CrNiMo, etc.
Alloy cast steel milling rolls feature high strength, high toughness and high crack resistance. The current development trend is to reduce carbon cont ent and increase the proportion of alloying elements to further enhance performance.
3.4 Main Uses of Alloy Cast Steel Milling Rolls
Alloy cast steel milling rolls are widely used in primary rolling mills, large section steel rolling mills and rough rolling mills. The support rollers for integral casting and composite casting also mostly adopt alloy cast steel materials. The addition of high alloy elements enhances the wear resistance and anti-spalling performance of the milling rolls, thereby prolonging their service life.
IV. Production Process and Heat Treatment Technology of Alloy Cast Steel Milling Rolls
4.1 Production Process Flow
Most alloy cast steel milling rolls are produced by an integral casting process. The traditional production process employs cold-hung sand molds and bottom injection methods, while the improved process uses advanced technologies such as cold mold coating and electric heating. High-temperature diffusion and spheroidizing treatment further enhance the performance of the milling rolls.
At present, two main process routes are adopted:
Smelting → Casting → heat treatment → performance testing → flaw detection
Smelting → Casting → Rough machining → heat treatment → fine machining → performance testing → flaw detection
4.2 High-Temperature Diffusion Treatment Technology
High-temperature diffusion treatment aims to reduce the inhomogeneity of alloying elements caused by dendrite segregation during solidification. This process heats the milling rolls to 150-250°C above AC3, homogenizing the austenite structure and improving the consistency of material properties.
4.3 Spheroidizing Annealing Treatment Process
Spheroidizing annealing treatment aims to improve the machinability and prepare for quenching. By transforming lamellar carbides into spherical shapes, a spherical pearlite structure is formed. The processing temperature should be slightly higher than the critical point of AC1 to ensure the integrity of the tissue transformation.
4.4 Quenching Heat Treatment Process
The quenching process involves heating the milling rolls above the critical temperature, performing austenitizing, and then rapidly cooling them. This process generates bainite structure, significantly enhancing the hardness and wear resistance of the milling rolls, and is a key procedure for improving the performance of the milling rolls.
V. Development Trends and Future Prospects of Milling Roll Technology
Modern milling roll technology is developing towards high performance, long service life and low cost. The application of composite casting technology and high-alloy materials is constantly expanding, and heat treatment processes are constantly innovating. High-chromium cast iron, high-speed steel and semi-high-speed steel milling rolls perform well in specific fields and are gradually replacing traditional material milling rolls.
The advancement of milling roll manufacturing technology not only enhances rolling efficiency and quality but also reduces production costs by extending service life. In the future, the research and application of new materials and innovative processes will further enhance the performance of milling rolls, thereby strongly supporting the sustainable development of the steel industry.
Conclusion
As key components in the steel rolling process, the technological development level of milling rolls directly affects the production efficiency and product quality of the entire industry. From monolithic casting to composite casting, from carbon steel to high alloy steel, every advancement in milling roll technology has driven the development of rolling processes. A thorough understanding of the classification, materials, manufacturing processes, and application characteristics of milling rolls is of great significance for the rational selection and use of milling rolls and the improvement of economic benefits in rolling production. With the continuous advancement of technology, milling rolls will continue to develop towards high performance and specialization, providing more reliable equipment support for the steel industry.
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