Roller is an important part on the rolling mill of the rolling mill, which uses the pressure generated when a pair or group of rolls roll to roll the steel. It mainly bears the effects of dynamic and static loads, wear and temperature changes during rolling.
There are two types of rolls: hot rolls and cold rolls.
The materials of the work rolls in common cold rolls are 9cr, 9cr2, 9crv, 8crmov, etc. The cold rolls require surface quenching and the hardness is hs45~105.
The commonly used materials for hot rolls are 55mn2, 55cr, 60crmnmo, 60simnmo, etc. Hot rolls are used in the processing of billets, thick plates, and sections. It withstands strong rolling force, severe wear and thermal fatigue effects, and the hot rolls work at high temperatures and allow diameter wear within a unit workload, so surface hardness is not required, only high strength and toughness are required. and heat resistance. The hot roll is only normalized or quenched as a whole, and the surface hardness is required to be hb190~270.
The application of high-speed steel rolls is an important part of the development of modern steel rolling production. It is another major change to the development of the iron and steel industry after microalloying, controlled rolling and controlled cooling technology. High-speed steel rolls have been widely used in steel rolling production in the fields of plate, section steel, bar and wire, not only for work rolls, but also for backup rolls. The good wear resistance of high-speed steel rolls is inseparable from its characteristics that are significantly different from traditional rolls.
Rolls are tools for plastically deforming (rolled) metals, and are important consumable parts that determine the efficiency of rolling mills and the quality of rolling materials. The types of rolls can be divided into casting rolls and forging rolls according to the forming method; according to the process methods, they can be divided into integral rolls, metallurgical composite rolls and combined rolls. Integral rolls are divided into integral casting and integral forging rolls. Metallurgical composite casting rolls mainly include semi-flushing composite casting, overflow (full flushing) composite casting, and centrifugal composite casting. Types of composite rolls manufactured by special composite methods such as HIP-Hot Isostatically Pressed, electroslag welding, etc. The combined roll is mainly a set of combined rolls.
Compared with the composite roll, the integral roll is cast or forged with a single material for the outer layer, the core and the neck of the integral roll. The outer layer and the neck have different structures and properties through casting or forging. And the heat treatment process to control and adjust. Both forged rolls and static cast rolls are integral rolls.
Rolls are mainly divided into three categories according to manufacturing materials: cast steel series rolls, cast iron series rolls and forging series rolls.
Casting rolls refer to the types of rolls manufactured by direct casting of smelted molten steel or smelted molten iron. Casting rolls can be divided into two categories: cast steel rolls and cast iron rolls according to materials; according to manufacturing methods, they can be divided into two categories: integral casting rolls and composite casting rolls.
Blooming mills, continuous billet rolling mills, large section steel and rail beam rolling mills, hot-rolled sheet and strip mills, scale breaking and edge rolling machines, section steel universal rolling mills, and small section steel, wire and bar rolling mills Roughing stands and other rolling mills The rolls used are mostly produced by the integral casting method. These rolls use thick layers and deep hole patterns. In addition, the two-high roughing roll of the hot-rolled strip mill is also suitable for integral casting production. The process method of the integral casting roll is relatively simple, and the manufacturing cost is low.
Composite casting roll refers to the outer layer of the roll body, the core and the roll neck are made of two or more materials. performance. Composite casting methods include semi-flush composite casting, centrifugal composite casting and overflow composite casting. Composite casting rolls require special process equipment, the process is relatively complex, the control is difficult, and the manufacturing cost is high.
Composite casting is suitable for the production of rolls with high workload and high quality requirements. The performance requirements of the roll body and the roll neck of such rolls are very different, the surface hardness of the roll body is high, and the roll neck requires high strength and toughness. For example, work rolls and backup rolls of hot strip mills; work rolls of medium and heavy plate mills; work rolls and backup rolls of skin-pass mills; roll rings of section steel universal mills; Steel pipe rolling mill, continuous rolling of pipe rolls and reduction roll rings, etc. In recent years, centrifugal composite high-chromium cast iron small vertical rolls have been used more and more in hot strip rolling mills at home and abroad, showing excellent wear resistance and significantly extending the roll changing cycle.
Rolls cast from iron-based materials with a carbon content of less than 2.2% are collectively referred to as cast steel rolls. Among them, the cast steel with the total amount of alloying elements not more than 0.8% is called carbon steel; the cast steel with the total amount of alloy elements greater than 0.8% is called alloy cast steel; the alloy cast steel with carbon content not less than 1.3% called semi-steel.
The main varieties of cast steel rolls are 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, etc.
Carbon steel roll is a hypoeutectoid steel composition roll with low alloy content (not more than 0.8%), such as Z-U70, ZU80, ZU70Mn etc. A cast steel roll used in early my country. Its matrix structure is pearlite, but there are a lot of block or strip ferrite in the matrix structure, which reduces the impact toughness of the matrix and low wear resistance. At present, carbon steel rolls are still used in domestic cross-section steel rolling mills, double-duplex wire rod rolling mills and individual billet continuous rolling mills, and they have tended to be eliminated.
Alloy cast steel roll is a cast steel roll whose composition is in the range of eutectoid steel and hypereutectoid steel and whose alloy content is greater than 0.8%. Its carbon content is generally between 0.4% and 1.3%, and it contains some chromium, nickel and molybdenum. , vanadium and other alloying elements, the matrix structure is generally controlled to sorbite and tempered sorbite through the normalizing + tempering heat treatment process. Typical materials are: 60CrMnMo, 65CrNiMo, 70Mn2Mo, 75CrMo, 75CrNiMo, etc. The matrix structure of the alloy cast steel roll is mainly pearlite type, containing a small amount of carbides, and the hardness range is HS 35-50; because the matrix contains a small amount of secondary carbide, the wear resistance of the carbon steel roll is greatly improved. At the same time, the impact toughness of the roll is improved due to the elimination of the massive strip ferrite in the matrix. The performance characteristics of alloy cast steel rolls are high strength, good toughness, good bite, thermal crack resistance and impact resistance. The material of alloy cast steel rolls tends to reduce the carbon content and improve the degree of alloying. Various micro-alloying technologies are used, and at the same time, the metallurgical quality is further improved, so that the alloy cast steel rolls can better meet the needs of different rolling mills.
Alloy cast steel rolls are used in square/slab blooming mills, large section steel blooming and rail and beam rolling mills, strip steel two-high roughing mills and large vertical rolls in scale breaking stands, and section steel, bar and wire rod rolling mills, etc. . The backup rolls of integral casting and composite casting are all alloy cast steel rolls in the strict sense. The carbon content is controlled according to the hardness requirements and different alloying elements are added. The alloy cast steel materials for the backup rolls contain high chromium, molybdenum and vanadium. and other alloying elements, with the increase of alloy content, the matrix structure is transformed from sorbite to bainite or martensite, and contains secondary carbides with high hardness. The hardness range is HS 55-70, which has excellent wear resistance. resistance and peeling resistance.
Most alloy cast steel rolls are integral cast rolls. The traditional production process adopts the roll body cold sanding, bottom injection and normalizing and tempering heat treatment. It is mainly used for the production of carbon steel rolls and ordinary alloy cast steel rolls used in ordinary 3-high section steel rolling mills and double bar and wire rod rolling mills. In order to strengthen the roll body performance of the alloy cast steel roll and obtain a good casting structure of the working layer of the roll body, most of them are changed from cold-form hanging sand to cold-form spray coating, and the riser feeding adopts the electric heating process. With the improvement of the degree, the heat treatment of the roll increases the process means such as high temperature diffusion and spheroidization. There are two production processes for alloy cast steel rolls: one is smelting-casting-heat treatment-processing-performance, flaw detection and other testing-finished products; the second is smelting-casting-roughing-heat treatment-finishing-performance, flaw detection and other testing-finished products . In 1986, the German GP (Gontennnnn-Peipers Gmbh) roll manufacturing technology was introduced in China, and large-scale square/slab alloy cast steel blooming rolls were produced by the flowless casting process. The roll was successfully applied to Baosteel’s Φ1300mm l blooming mill. In 1997, Maanshan Iron and Steel and Laiwu Steel respectively introduced universal rolling mills for section steel with an annual output of 600,000 tons and 500,000 tons from Germany and Japan. According to the structure and special conditions of use of the alloy cast steel BD (Break Down) roll of the section steel universal rolling mill, the flowless casting process and the electric heating riser are still used in static casting in China; Sequential solidification is achieved in the as-cast state, and a dense core structure is obtained in the as-cast state, thereby developing and improving the manufacturing technology of alloy cast steel rolls.
Commonly used heat treatment types for rolls include stress relief annealing, isothermal spheroidizing annealing, diffusion annealing, normalizing, tempering, quenching, and cryogenic treatment.
Due to the presence of dendrites during the solidification of the cast steel roll, the intragranular segregation of the alloy components will occur. This segregation is caused by the inability to achieve sequential solidification of the upper and lower rolls, the inner and outer layers during the solidification of the roll. The larger the diameter of the roll body and the longer the length of the roll body, the greater the solidification temperature gradient, and the more serious the alloy solidification segregation caused. As the alloy content of the alloy cast steel roll increases, the segregation index generated during solidification increases, resulting in a greater degree of inhomogeneity of the composition. Therefore, in order to fully diffuse the atoms of the alloying elements and obtain the austenite structure with uniform composition, the high-alloy cast steel roll needs to be heated to above Ac3 at 150-250 °C for a certain period of time. This heat treatment method is called high temperature diffusion treatment.
The spheroidizing annealing heat treatment of the alloy cast steel roll is mainly to improve the cutting performance of the roll, improve the cutting efficiency and prepare the corresponding structure for the subsequent quenching treatment. The spheroidizing annealing treatment changes the carbides in the structure from flakes to spherical, that is, the spherical or granular carbides are uniformly distributed on the ferrite matrix, and the spherical pearlite structure is formed. The spheroidization temperature is selected in production, which is generally slightly higher than the critical point of Ac1. If the temperature is too low, the required spheroidization time is too long, and if the temperature is too high, granular pearlite with different particle size and thickness will appear.
The length of the rolling mill rolls generally considers two aspects: first, the stiffness of the roll body. The roll body is too long to affect the roll rigidity, which is detrimental to the dimensional accuracy of the rolled piece. Second, with rollers. The length of the roll can be matched with more grooves, which is beneficial to reduce the consumption of steel per ton of the roll and save the roll.
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