This article describes in detail the composition and manufacturing process of cold rolling rolls.
Key words: cold rolling roll; composition; manufacturing process
Cold rolling rolls: The rolls used for cold rolling are generally flat rolls and are used for rolling plates and strips. In a broad sense, cold rolling rolls include work rolls that are in contact with the rolled piece and back-up rolls that support them. Because the back-up roll is self-contained, the cold roll is often narrowly referred to as the cold-rolled work roll. Cold rolling rolls must have sufficient strength, uniform high hardness and surface quality to withstand extremely high rolling forces, ensure sufficient wear resistance, and meet the precision requirements of rolled products. Among the forged steel rolls, those that can be quenched to high hardness are used as cold rolls.
Carbon is the main strengthening element. Part of the carbon is solid-dissolved in the matrix phase, that is, the carbon contained in martensite and retained austenite; the other part exists in the undissolved carbide in a combined state, and the undissolved carbide is dispersed in a fine form. Figure 2 Cold-drawn steel pipe The granular form of the production process is distributed in the matrix tissue. High carbon is beneficial to wear resistance, but it increases the brittleness of the material; at the same time, problems such as carbide strips, networks, and liquid precipitation are also aggravated. Therefore, rolling mills with a high accident rate should choose rolls with lower carbon content, while small-load rolling mills can choose rolls with higher carbon content. Silicon and manganese are generally not considered as alloying elements. Some of them are brought in by raw materials, and some are added for the purpose of deoxidation and desulfurization. Chromium is the most important alloying element. As a strong carbide forming element, chromium increases the amount and hardness of undissolved carbides, which is beneficial to improve wear resistance; chromium is also solid-soluble in the matrix phase, which plays a strengthening role and increases the hardenability of steel. When the amount of conventional chromium is contained, the maximum hardening depth of the roll body is 15mm. Cold rolled rolls containing 3% or more chromium (semi-high-speed steel) have come out, some for improving toughness and wear resistance, and some for deep hardened layers. The effect of molybdenum and chromium is similar. When the rolled piece is deformed in the roll gap, it is accompanied by high deformation heat, and molybdenum also plays the role of high temperature strengthening of the roll. Vanadium is used to refine the grains. Its dosage depends on the quenching temperature. Occasionally other alloying elements are used, such as nickel, silicon, tungsten, etc.
Materials and properties
It mainly refers to the chemical composition, metallographic structure, hardness and strength of the roll.
The steel for forged steel cold roll is mainly high-carbon chromium steel, the typical composition (%) is: C0.7～1.0, Si0.25～0.5, Mn0.25～0.5, Cr1.5～5.5, Mo0.2～0.6, V0.1～0.2.
The metallographic structure of the hardened layer of the cold roll body is: a certain amount of quenched undissolved carbides are distributed on the martensite matrix containing a small amount of retained austenite; under the optical microscope, the martensite is cryptocrystalline or fine needle-like ;Due to the very low tempering temperature, there is no obvious tempering phenomenon of martensite; under the electron microscope, both martensite twins and martensite laths can be seen, but twinned martensite is the most Main; a small amount of lath martensite can only appear at a lower quenching temperature or a high amount of chromium.
The strain energy accompanying the martensitic transformation has a decisive influence on the kinetics of the martensitic transformation, and a part of the parent phase (austenite) remains to form retained austenite. Retained austenite, as an unstable phase and plastic phase, will partially undergo transformation or work hardening during service, thereby increasing internal stress and impairing performance. Retained austenite that does not undergo these changes is still conducive to the toughness of the hardened layer. The content of retained austenite is generally controlled at 5% to 10%.
The carbide particles in cold rolled steel are (Fe, Cr) 3C, and part of (Fe, Cr) 7C3 can appear when the chromium is high. The size of carbide particles is about 1-3 μm, and the number is about 5%-10%.
Hardness is an important parameter to determine the wear behavior of metal, and it is regarded as the primary performance index of the working layer of the roll body in engineering. Only the working layer of the roll body is hardened, if not specified, it refers to the hardness of the roll body. Roll hardness is usually measured with a Shore durometer. The hardness of the work roll of the cold strip mill is about 90-95HS; the hardness of the flat roll is about 95-100HS. The hardness of the foil roll is about 61-64HRC. The hardness of the roll is affected by the residual compressive stress of the roll body, the higher the residual stress, the higher the hardness value. Therefore, the roll hardness of the rolling mill with small load is higher, and the roll with lower hardness should be selected for the rolling mill with high accident rate. The hardness of the roll neck is usually only 30-50HS, but with the change of the bearing structure, some cold rolls have required the roll neck to be hardened, and the quenched hardness of the roll neck is generally below 80HS.
The parts that are not in direct contact with the rolled piece, such as the core of the roll body and the roll neck, play the role of transmitting the rolling force, and its primary performance is strength. If not specified, the strength of the roll is the neck strength of the lower roll. The strength of cold rolled rolls depends on the pretreatment before quenching. The strength of only spheroidizing pretreatment is about 600MPa; the strength of normalizing pretreatment is about 950MPa; the strength of quenching and tempering pretreatment is about 1050MPa.
The main links in the cold roll manufacturing process are smelting, forging, heat treatment, processing and inspection.
The steel for cold rolling rolls must be smelted according to the requirements for cleanliness. Commonly used smelting equipment includes alkaline electric furnace, acid open hearth furnace, and vacuum ingot casting equipment. Secondary metallurgical methods can be used to improve the cleanliness of molten steel, such as electroslag remelting or vacuum melting of consumable electrodes. Depending on the equipment conditions of the production plant, other refining methods outside the furnace can also be used for degassing, desulfurization, removal of inclusions and fine adjustment of composition of molten steel. The steel ingot should adopt the ingot shape with polygonal wave surface and big head upward.
After the steel ingot is demolded, it should be hot sent to the forging workshop to be forged into a billet. Forging not only obtains the basic shape of the roll, but also breaks the coarse as-cast structure, especially the coarse network carbide, and greatly reduces the segregation and void defects in the steel ingot, such as shrinkage cavity, porosity, and microcracks. The benefits of microstructure refinement, homogenization and densification are not only the improvement of mechanical properties (mainly strength and toughness), but also the preparation of microstructure conditions for subsequent severe quenching. Forging generally adopts hydraulic press free forging process. In view of the quality advantages of small ingot forgings, it is advisable to forge one roll with one ingot. The main control parameters in the process are temperature, anvil shape and forging ratio. When the forging temperature is high, it is easy to crack; when the temperature is low, the diffusion effect is poor, and the density after forging is low. In order to forge through, sometimes the method of compacting the center after cooling the surface is used. Considering the structure of the working layer of the roll body, the last fire should give the roll body a certain amount of deformation, that is, the final forging temperature of the roll body should be taken into account. The anvil shape should be conducive to the axial flow of metal, so the V-shaped anvil is commonly used, especially the lower anvil. The size of the anvil affects the elongation and thus the quality of the forging: small elongation makes it difficult to forge through; large elongation makes it easy to forge cracks.
Forged cold rolls must have a sufficient forging ratio, generally greater than 3. For this reason, after cutting off the head and tail, pressure handle and chamfering of the steel ingot, it should be thickened first, and then drawn out. When elongating, in addition to controlling the amount of reduction to avoid forging impenetrability or forging cracks, the coaxiality between the center line of the ingot and the finished product should always be maintained. The forging process should be formulated and optimized according to the composition of the incoming material (steel ingot), the ingot shape and the size of the forging to optimize the structure and performance of the forging. Very small rolls can be manufactured directly from slabs.
After forging, the billet should be annealed directly in the furnace. After slow cooling, the matrix structure is flaky pearlite, while the carbides are still precipitated in the form of a network. If the forged billet is not shipped as a commodity, it can also be cooled by normalizing to break the carbide network, and the granular pearlite matrix can be obtained after isothermal annealing. If necessary, the final stage of annealing also allows sufficient hydrogen diffusion. In order to eliminate the network carbide of the forged blank and make the matrix spheroidize, normalize first, and then spheroidize, sometimes it can be simplified as normalizing and high temperature tempering, if the carbide of the forged blank is intermittent, then Spheroidizing annealing can be performed directly. When normalizing large rolls, special cooling measures should be taken.
The rough-machined rolls must be quenched and tempered. The purpose of quenching and tempering is to firstly treat the roll core and roll neck to the required strength and hardness, and make organizational preparations for the final quenching of the roll body. If the strength requirement is not high, it can also be directly quenched without quenching and tempering. Use oil cooling or spray cooling when tempering. Spray cooling can make the roll tempered deeply.
Quenching is the most important heat treatment operation for cold rolled rolls. The early use is the overall heating and quenching method after the roll neck is protected by a jacket. With the development of heating technology, medium frequency or power frequency induction heating is mostly used. This induction heating can be single-frequency or dual-frequency, integral or mobile. Rapid heating can also be achieved with a flame. High-pressure water is used for quenching and cooling, and cooling must be continued for a certain period of time after quenching. After quenching, low-temperature tempering should be carried out in time. For rolls with very low residual austenite content, a cold treatment process must be added before tempering. When the cold roll with a center hole is quenched, water should also pass through the hole to accelerate the heat dissipation of the core.
After the roll billet is annealed, rough machining is carried out first. To reduce the risk of quenching, cold rolls are often designed with a center hole. At this time, attention should be paid to the surface processing quality of the center hole to avoid damage caused by center hole fatigue during use. If the amount of hydrogen and residual stress are properly controlled, the central hole may not be left. The machining allowance should be considered in this way; no decarburization layer can be left on the final product; no defects that are not conducive to quenching can be left; too much machining allowance should not be left for finishing; the amount of grinding after quenching should be as small as possible . Finishing and grinding are carried out before and after quenching respectively. Cold rolls have high requirements for dimensional accuracy, concentricity and roughness. In addition, the quenched roll has high residual stress, and the grinding wheel and grinding parameters should be carefully selected during grinding to avoid grinding cracks. Roll processing should use special machine tools, and CNC machine tools should be used as much as possible.
In addition to routine material inspections, cold rolled rolls are subject to a series of product inspection items. The inspection of machining accuracy is carried out according to the conventional method. Roll hardness measurement includes three indicators, namely hardness (average hardness), hardness uniformity and distribution of hardness with depth. Routine testing refers to the first two items. When using the Shore hardness tester to measure the hardness of the roll, it is necessary to follow the specified testing conditions, especially the calculation method of the number of measuring points, the number of measurements per point, the average value and the uniformity. When measuring with other types of hardness testers, the hardness conversion table used must be specified in advance.
Non-destructive testing of rolls is usually carried out with ultrasonic flaw detectors. In the cold roll flaw detection standard, the roll body is usually divided into the surface area, the central area and the middle area according to the force condition, and then the critical size of the defects allowed to exist in each area is stipulated respectively according to the principle of fracture mechanics. Flaw detection workers should have the knowledge of roll manufacturing in order to determine the nature of defects, and also have the knowledge of roll use in order to estimate whether these defects may cause service damage under specific use conditions.
The residual stress of the cold roll is caused by the uncoordinated deformation during the quenching and cooling process. During the quenching and cooling process, the temperature changes drastically, and the outer layer turns into an elastic state in a short time, so the elastic contraction of the core is hindered in the later stage of cooling, causing the outer layer to be compressed and the core to be pulled. There is also phase transformation stress in the quenching process, that is, the outer layer of martensitic phase transformation, the specific volume increase is large, and the inner layer of pearlite phase transformation, the specific volume increase is small, so that the outer layer compressive stress increases. The residual compressive stress of the outer layer is the source of additional hardness of the roll; while the tensile stress of the core increases the risk of roll damage. The influence of residual stress on cold rolling rolls is multifaceted, involving hardness, wear resistance, contact fatigue resistance, strength (including core tensile strength and surface indentation resistance), fatigue strength, thermal crack resistance and even after shot peening. surface roughness. Determination of residual stress is not a routine inspection. Surface residual stress was measured by X-ray method. When measuring the residual stress distribution in the body, the roll needs to be destroyed, and the residual stress is gradually released in the process of measuring with a strain gauge or X-ray method.
Maintenance includes intermediate grinding, roughening and re-quenching of the rolls.
During the intermediate grinding of cold rolls, the surface cracks left after the last round of service should be removed. The new eddy current flaw detection equipment can be installed on the grinding and cooling bed to monitor whether the residual cracks are worn out at any time, which can not only control the amount of grinding at the optimal level, but also greatly avoid the accident of roll spalling.
Some strip steel products need to be deep drawn, painted or annealed in coils, and rough surface rolls are used in the finished rolling mill and skin pass mill during rolling. The traditional method of roughening is shot blasting. The surface roughness after peening depends on the shot quality, the speed parameters of the shot and roll, and the number of peening passes. (See roll surface roughening) Electric pulse roughening and laser roughening are relatively new roughening methods. Compared with shot peening, electric and laser pulse methods can control the roughness more accurately.
Cold rolled rolls can be re-quenched when the hardened layer is used up but the minimum roll diameter is not yet reached. Tempering is required before re-quenching to relieve stress and fully temper the semi-martensitic zone. The structure and mechanical properties of re-quenched rolls are generally not as good as new rolls, so when determining the quenching parameters, the quenching stress should be kept low.
Other types of cold rolls
Centrifugally cast high chromium cast steel rolls (see high chromium rolls) have been used cautiously for cold strip rolling. The advantages of casting rolls are that the manufacturing process is simple, no heavy quenching is required, and the rolling volume is high. Casting rolls are suitable for use on rolling mills with few accidents and stable rolling.
The material used for the work rolls of the temper mill, the intermediate rolls and the partial rolls of the six-high and eight-high mills is the same type as the work rolls of the four-high mills, that is, forged and quenched high-carbon chromium steel. This grade is also used for straightening rolls and other auxiliary rolls.
The work rolls of the Sendzimir mill are made of high-carbon, high-chromium cold-work die steel or high-speed steel; the middle rolls are made of medium-alloy cold-work die steel.
Cemented carbide rolls are mainly used in the field of cold rolling to flatten steel wires and produce flat steel wires or hairsprings. In view of the high hardness and wear resistance of cemented carbide, cemented carbide rolls can roll out rolled products with high precision and high surface quality, so they are also suitable for rolling foil.