Continuously plastically deformed main working parts and tools on the rolling mill. The roller consists of a roller body, a roll neck and a shaft head. The roll body is the middle part of the roll that actually participates in rolling the metal. It has a smooth cylindrical or grooved surface. The roll neck is mounted in the bearing and the rolling force is transmitted to the frame via the bearing housing and the pressing device. The shaft end of the transmission end is connected with the gear seat through the connecting shaft, and transmits the rotational torque of the motor to the roller. The rolls can be arranged in two, three, four or more rolls in the roll stand.
1. A brief history of the development of the roller
The variety and manufacturing process of rolls have continued to evolve with the advancement of metallurgical technology and the evolution of rolling equipment. The use of low-strength gray cast iron rolls in the rolling of soft non-ferrous metals in the Middle Ages. In the middle of the 18th century, the United Kingdom mastered the production technology of chilled cast iron rolls for rolling steel plates. In the second half of the 19th century, advances in European steelmaking technology required the rolling of larger tonnage steel ingots, whether the strength of gray cast iron or chilled cast iron rolls could not meet the requirements. Carbon steel is 0.4% to 0.6% of ordinary cast steel rolls. The appearance of heavy-duty forging equipment has further enhanced the toughness of forged rolls of this composition. The introduction of alloying elements and the introduction of heat treatment in the early 20th century have significantly improved the wear resistance and toughness of cast and forged hot and cold rolls. The addition of molybdenum to the cast iron rolls used for hot-rolled strips improves the surface quality of the rolled strips.
The rinsing compound casting significantly increases the core strength of the casting roll.The heavy use of alloying elements in rolls is after World War II. This is a higher requirement for roll performance after rolling equipment has grown in size, continuousness, high speed, automated development, increased rolling material strength and increased deformation resistance. the result of. During this period, semi-steel rolls and ductile iron rolls appeared. After the 1960s, powder tungsten carbide rolls were successfully developed. Centrifugal casting technology and differential temperature heat treatment technology for rolling rolls widely promoted in Japan and Europe in the early 1970s have significantly improved the overall performance of strip rolls. Composite high chromium cast iron rolls have also been successfully used on hot strip mills. In the same period, forged white iron and semi-steel rolls were used in Japan. In the 1980s, Europe introduced cold-rolled rolls with high-chromium steel rolls and ultra-deep-hardened layers, and special alloy cast iron rolls for the finishing of small-sized steels and wire rods. The development of modern steel rolling technology has led to the development of higher performance rolls. The cores produced by the centrifugal casting method and new composite methods such as continuous casting compound method (CPC method), spray deposition method (Osprey method), electroslag welding method, and hot isostatic pressing method are strong toughness forged steel or ductile ink Cast iron, composite high-speed steel rollers and metal ceramic rollers have been applied on new generation profiles, wire rods and strip mills in Europe and Japan respectively.
2. Roll classification
There are various methods for classifying rolls, which are: (1) There are steel strip rolls, profile steel rolls, wire rolls, etc., according to the type of product; (2) There are roll blanks, rough rolls, and the like according to the position of the rolls in the rolling mill series. Finishing rolls, etc.; (3) According to the roll function, there are broken scale rolls, perforated rolls, leveling rolls, etc.; (4) Roll rolls are divided into steel rolls, cast iron rolls, hard alloy rolls, ceramic rolls, etc.; (5) Press Manufacturing methods include casting rolls, forging rolls, surfacing rolls, nested rolls, etc.; (6) Hot rolled rolls and cold rolled rolls are divided according to the state of rolled steel. Various classifications can be combined to give the roller a more definite meaning, such as centrifugal cast high chromium cast iron work rolls for hot strip.
3. Roller selection
The commonly used roll materials and uses are shown in the table. Roll performance and quality generally depend on its chemical composition and manufacturing method and can be evaluated by its organization, physical and mechanical properties, and the type of residual stresses present inside the roll (see Roll Inspection). The effect of the roll in the rolling mill depends not only on the material of the roll and its metallurgical quality, but also on the conditions of use, roll design, and operation and maintenance. There are great differences in the operating conditions of the rolls of different types of rolling mills.
The factors causing the differences are:
(1) Mill conditions. Such as the mill type, mill and roll design, hole design, water cooling conditions and bearing types, etc.;
(2) rolling conditions such as rolling stock varieties, specifications and deformation resistance, pressing system and temperature system, production requirements and operations, etc.;
(3) Requirements for product quality and surface quality.
Therefore, different types of rolling mills and rolling mills of the same type and using different conditions have different requirements for the performance of the used rolls. For example, billets and slab bloomer rolls must have good torsional and bending strength, toughness, and bite in, Hot crack resistance and thermal shock resistance and abrasion resistance; and tropical finishing stands require high hardness, resistance to indentation, wear resistance, spalling, and thermal cracking resistance on the roll surface.
Understanding the conditions of use of the rollers and the failure modes of the rollers used in the same type of mill, and understanding the current performance and manufacturing processes of the various roller materials, can correctly formulate the technical conditions of the roller for the mill and select a suitable and economical roller material.
The most commonly used methods to evaluate the roll performance in the rolling mill are:
(1) Roll weight (kg) consumed for rolling 1T rolling stock (referred to as roll consumption), expressed in kg/t;
(2) Diameter reduction per unit roll diameter The weight of the rolled material is expressed int/mm.
With the modernization of rolling mills, the in-depth study of failures in the use of rolls, and improvements in the material and manufacturing process of rolls, the average roll consumption of industrially-developed countries has been reduced to less than 1 kg/t.
4. Roll performance requirements
(1) Hot crack resistance
Usually, the rough roll is mainly required for strength and heat crack resistance; the work roll weight of the small 20-roll mill is only about 100 grams, and the weight of the backup roll for the wide-thick plate mill is more than 200 tons. When the roller is selected, firstly, according to the basic strength requirements of the roller to the roller, the main body materials (cast iron, cast steel or forged steel of various grades, etc.) of safe loading are selected.
The high speed of the finishing roller requires a certain surface quality for rolling the final product. The main requirements are hardness and wear resistance. Then consider the wear resistance of the roller when used. Because the wear mechanism of the roller is complex, including mechanical stress action, thermal action during rolling, cooling action, chemical action of the lubricating medium, and other effects, there is no unified indicator for comprehensive evaluation of roll wear resistance. Because the hardness is easy to measure and can reflect the wear resistance under certain conditions, the radial hardness curve is generally used to approximately describe the wear index of the roller.
(3) Shock proof
In addition, there are some special requirements for the roll, such as a large amount of reduction, the roll requires a strong biting ability, more resistant to shock;
(4) Smooth finish
When rolling thin gauge products, the rigidity of the roll, uniformity of the structure and properties, processing accuracy, and surface finish are stricter;
(5) Cutting performance
When rolling sections with complex sections, the machining properties of the working layer of the roll body must also be considered.When the roller is selected, some performance requirements for the roller are often opposed to each other. The purchase cost and maintenance cost of the roller are also very expensive. Therefore, the technical and economic advantages and disadvantages should be fully weighed to decide whether to use cast or forged, alloyed or non-alloyed. The single material is composite material.
5. Carbide roll
Carbide roll ring (also called tungsten carbide roll ring) refers to a roll made of powder metallurgy method using tungsten carbide and cobalt as materials. Cemented carbide rolls are available in both monobloc and combi-type rolls. Superior performance, stable quality, high product precision, good wear resistance and high impact resistance.
With the increasingly fierce competition in the steel product quality and price market, iron and steel enterprises are constantly updating their own equipment technologies to continuously increase the rolling speed of rolling mills; at the same time, how to reduce the number of shutdowns of rolling mills and further increase the effective operating rate of rolling mills Become an important topic for rolling steel engineers. The use of roll materials with a higher rolling life is one of the important means to achieve this goal.
Carbide rolls have been widely used in the production of rods, wire rods, rebars and seamless steel tubes because of their good wear resistance, high temperature red hardness, heat fatigue resistance and high strength, which greatly improves the mill Effective operating rate. According to the differences in the working environment of each rack roll, various grades of carbide roll rings have been developed.
6. History of Cemented Carbide Rolls
Ring It was born in 1909 after the birth of powder metallurgy technology with the development of the metal processing industry. Since the introduction of the carbide drawing dies in Germany in 1918, it has stimulated the study of hard alloys in various countries. Rolls for various applications have also appeared one after another. However, a large number of applications of cemented carbide rolls are after 1960. In 1964, Morgen’s first high-speed twistless wire mill was launched, which increased the wire finishing speed by a factor of four. Because the finishing rolling mill is working under high speed and high stress, the wear resistance of the cast iron roll and the tool steel roll is poor, the life of the rolling groove is short, the loading and unloading of the roll is very frequent, and the efficiency of the rolling mill is affected, and the finishing rolling production is not suitable. The requirements were replaced by the combined cemented carbide roll. There are more than 200 sets of Morgan-type rolling mills in the world, which consume hundreds of tons of cemented carbide rolls.
7. Carbide Roll Performance
Hard alloy roll has high hardness, and its hardness value changes little with temperature. The hardness value at 700°C is 4 times that of high-speed steel; elastic modulus, compressive strength, bending strength and thermal conductivity are also 1 times more than tool steel. Due to the high thermal conductivity of the hard alloy roll, the heat dissipation effect is good and the time for the surface of the roll to be at a high temperature is short, so that the high temperature reaction time of the roll with harmful impurities in the cooling water is short. Therefore, the hard alloy roll is more resistant to corrosion and cold and heat fatigue than the tool steel roll.
Cemented carbide rolls are developed on the basis of cemented carbide tools. They are based on refractory metal compounds (WC, TaC, TiC, NbC, etc.) and transition metals (Co, Fe, Ni). Bonding phase, a cermet tool material prepared by powder metallurgy. It has a series of excellent properties such as high hardness, high red hardness and high wear resistance. Sometimes to obtain corrosion resistance, add a certain amount of nickel, chromium, and other elements.
The performance of the cemented carbide roll is related to the content of the bonding phase metal and the matrix phase, the size of the tungsten carbide particles. The different binder content and the corresponding tungsten carbide particle size form different carbide grades. Serialized cemented carbide grades have been developed for different grades. Tungsten carbide accounts for approximately 70% to 90% of the total composition in cemented carbides, and its average particle size is 0.2 to 14 μm. If the content of the metal binder is increased or the particle size of the tungsten carbide is increased, the hardness of the cemented carbide decreases and the toughness increases. The bending strength of cemented carbide rolls can reach more than 2200 MPa, the impact toughness can reach (4-6)×106 J/m2, and the Rockwell hardness HRA is 78-90.
Cemented carbide rolls can be divided into two types: solid carbide roll and composite cemented carbide roll. The whole hard alloy roll has been widely used in pre-finishing and finishing stands for high-speed wire rod mills (including fixed-reducing frames and pinch roll stands). The composite cemented carbide roll is composed of cemented carbide and other materials, and can be further divided into a cemented carbide composite roll ring and a solid carbide composite roll. The hard alloy composite roller ring is mounted on the roller shaft; the solid carbide composite roller is used to cast the carbide roller ring directly into the roller shaft to form a whole, which is applied to a rolling mill with large rolling load.
8. Research and Application of Cemented Carbide Roll Materials
New Process for Manufacturing Composite Carbide Rolls
1. Casting compound hard alloy roll ring
In order to meet the requirements of modern rolling production, a new cemented carbide composite roll cast (CIC, CAST IN CARBIDE) composite carbide roll ring. The technique is to cast the carbide ring with a ductile cast iron inner sleeve. The roller ring and the roller shaft are keyed. In this connection, the hard alloy material with extremely high hardness and excellent wear resistance on the outer layer of the composite roll ring is subjected to the rolling force, and the torque is transferred from the ductile iron with excellent strength and toughness in the inner layer. Structural features of CIC composite rolls:
(1) The use of a composite layer enhances the strength and toughness of the roll ring and can withstand large rolling loads;
(2) The coupling between the roller ring and the roller shaft adopts the interference fit, which solves the problem that the cold-loaded structure is easy to break the key and makes the rolling process more stable;
(3) There is no gap between the contact surface of the roller ring and the roller shaft, which avoids the deformation of the roller ring due to the corrosion of the contact surface caused by the cooling water containing impurities.
The development of cast-in-place CIC composite roll ring technology is a new combination of powder metallurgy technology and casting technology. It is a major advancement in the application of composite wear resistant material technology on rolls.
2. Powder metallurgy compound WC roll ring
This technology combines the cemented carbide ring with a steel substrate with Ni and Cr powders, and combines them with powder metallurgy technology. The main point of the process is to first compact and sinter the cemented carbide powder into a ring, and then mold and sinter with a selected steel-based powder. There is a solid metallurgical connection between cemented carbide and steel base. The key to the process is to master the sintering temperature of 1100-1200 °C and the pressure conditions of 100-120 MPa, and the sintered blanks are subjected to roughing, stress relief, etc., and the final car is then ground and shaped.
By choosing suitable matrix materials, coupled with advanced processes and ratios, the residual stress between the cemented carbide and the steel substrate in the composite roll ring can be very low. This powder metallurgy technology has created a new era in the preparation of roll materials.
Application of hard alloy roll ring material
In the hot rolling process, the WC roll ring is subjected to high temperature, rolling stress, hot corrosion and impact load. Compared with the WC roll rings produced abroad, the purity of the raw materials used in the production of the roll rings in China, the processing technology, and the performance of the roll rings There is still a certain gap between the indicators and other aspects. The wear resistance of the roller during use is poor, and the roller ring is easily broken. On the basis of the common hard alloy roll ring material, a gradient material LGM roll ring was developed by using a lubricating and wear resistant gradient material Lubrication Gradient Material (LGM).
The technology is the addition of sulfur and oxygen to common cemented carbide materials to form stable graded metal oxides and metal sulfides (Co3O4 and CoS, respectively) on the surface of metal substrates. Co3O4 and CoS have good lubrication and wear resistance. Industrial tests of LGM roll rings have shown that the sulfides and oxides in the gradient material can reduce the friction coefficient during rolling, significantly improve the lubrication performance of the roll ring under high temperature and large rolling force conditions, and reduce the transverse cracks. The life of the roll ring is 1.5 times that of the ordinary hard alloy roll ring, and it can reduce the grinding amount and the number of roll changes, and has significant economic benefits.
Using the CIC technology, developed the world’s cemented carbide roll ring H6T with the least bonding phase, its bonding phase content is only 6 %, while the hardness and wear resistance is significantly higher than the common brand alloy, especially the wear resistance increased by 50% When used on the finished frame and finished product front frame, the roll life is 2 times that of the common brand of hard alloy; it can solve the problem of changing the roll together with the finished frame and the finished frame, which can significantly reduce the change of groove. , change the number of rollers, thereby increasing the effective operating rate of the mill.
CIC Composite Carbide Roll Ring has been used in wire rod mills (medium or pre-finish), bar mills (medium and fine), small profile mills (square steel, hexagonal brazed steel, flat steel, angle steel, etc.) and Three-roll mill system (such as KOCK bar, seamless steel pipe stretch reducer) application. When the composite hard alloy roll ring is used in the finishing stand of a high-speed wire rod mill or a small bar mill, its single-slot rolling amount is 10 times that of ordinary cast iron rolls, and each grinding amount is only cast iron. 1/3 to 1/2 of the roll, therefore, compared with the conventional cast iron roll, the total rolling amount of the composite roll is 20 to 30 times that of the ordinary roll. When used in a 3-roller seamless steel pipe tension reducer frame and pipe jacking frame, compared with the conventional cast iron tension reducer roller, when rolling a larger diameter pipe, the single groove rolling amount of the composite roll is ordinary 20 times that of the cast iron roll, and when rolling the smaller diameter steel tube, the single groove rolling amount of the composite roll is 40 times that of the ordinary cast iron roll, and the finished product quality and dimensional accuracy of the steel tube are remarkably improved.
In order to solve the problems existing in the alloy tool steel and hard alloy roll materials used in the production of threaded steel wires, a cemented carbide GW30 was developed between the alloy tool steel and the cemented carbide. After forging, machining and heat treatment, the “bridging” phenomenon of carbides in the alloy was weakened, and the flexural strength and impact toughness of the material reached 2672 MPa and 18.0 J/cm2, respectively, which could prevent early brittle failure of the rolls. At the same time, the wear resistance of the hard phase in the hard alloy is fully utilized.
Under the condition that the toughness of the roller is maintained, the surface of the roller is treated with boronization, so that the boronized layer is firmly bonded with the steel substrate, and the surface of the alloy is fixed. The microstructure and properties tend to be consistent, so that the wear resistance of the alloy is further improved. Industrial test results show that the service life of the roller is more than ten times that of the alloy tool steel, and the economic benefits are significant.
9. Problems that exist
In recent years, carbide roll has been widely used in steel production for its excellent performance. However, the following problems still exist in the production and use of hard alloy rolls:
(1) Research and development of a new type of cemented carbide composite roll shaft material. As the rolling industry constantly raises new and higher demands on the rolls, conventional ductile iron roll shaft materials will be unable to withstand greater rolling forces and transmit more torque. For this purpose, high-performance carbide composite rolls must be developed. Roller shaft material.
(2) In the manufacturing process of the composite roll, the residual thermal stress caused by thermal expansion mismatch between the inner layer metal and the outer layer cemented carbide must be reduced or eliminated as much as possible. The residual thermal stress of cemented carbide is a key factor influencing the service life of the composite roll. Therefore, the difference in thermal expansion coefficient between the inner metal and the outer cemented carbide should be as small as possible. At the same time, the residual heat of the roll ring should be considered. The possibility of heat treatment of stress.
(3) Because the rolling force, rolling moment, and thermal conductivity of different stands are different, different grades of cemented carbide rolls should be used. In the design process of hard alloy roll materials, it is necessary to ensure the reasonable matching of the strength, hardness and impact toughness of the rolls. A database of different properties of the alloy materials should be established to optimize the material design of the roll.
(4) In the rolling process, wear of the cemented carbide roll is not only affected by external conditions such as temperature, rolling pressure, and thermal shock load, but also due to internal factors of the hard phase WC and the bonding phase Co/Co-Ni-. There are quite complex physical and chemical reactions between Cr. This makes the wear situation more complicated. To this end, research on the mechanism of this aspect must be strengthened.
In wire and bar rolling, the use of cemented carbide roll rings to replace traditional cast iron and alloy steel rolls has shown many advantages. With the continuous development of roll manufacturing technology and use technology, the use of carbide roll will continue to expand. Its role in rolling processing will become more and more important, and its application prospects will also be very broad.