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Forming rolls - the main tool’s performance of which have an impact on the performance of rolling mills and product quality. Consumption of rolls – an integral part of the finished product cost. Also the value of quality of the rolls. The urgency of the problem of improving the performance of the rolls and above all, their persistence in the conditions of introduction of the continuous rolling production mills and endless rolling is constantly increasing.
Based on the working conditions of the hot rolling mill rolls and requirements , especially high wear resistance can be distinguished by the length and depth of the working layer at high temperatures and pressures; the increase in the required capture angle of the rolled metal; static strength (resistance to breakage); the constancy of the working diameter; purity gauge surface; precision machining and others.
The rolls roughing stands are usually made of forged steel. Prefinished and finishing (with high abrasion resistance, for high precision rolled) - iron (alloyed) rolls.
Key performance properties of the rolls are achieved by appropriate adjustment of the mechanical properties of the roll material, the hardness, ductility, surface roughness, toughness, tensile strength, and others. Currently, for the hot rolling of ferrous and non-ferrous metal is used as an iron and steel mill rolls, and the share of cast-iron rolls account for 65% of the total volume of produced rolls in the country.
Therefore, the material rolls can be supporting factor.
effective interventions to improve wear resistance which is the most important feature of the operational quality of the rolls are increasing their surface hardness and increase in carbon content and chromium roller steels.
However it has been established that increasing the hardness of the rolls and increased content of carbon and chromium in rolled steel has negative effect.
The chemical composition of the material steel rolls uniquely affects their utility properties. Thus, with an increase in carbon content increases wear resistance of rolls. For example, an increase to 0.6 - 0.8% C increases the wear resistance of the metal, due to the reduction in its ferrite structure; further increasing the carbon content in the steel causes excessive formation of carbides which, apart from increasing wear resistance rolls, it also contribute to improving the quality of rolled surface. Manganese in an amount of 0.5 - 0.9%, being a good deoxidizer cleanses nonmetallic inclusions from steel and gives it a spherical shape. At the same time alloying ferrite increases the strength of steel. Increase up to 1.4 - 2.2% Mn is beneficial to heat treatment and super cooling steel rolls during the normalization process. Content of 0.25 to 0,60% Si contributes to deoxidization of steel, but if its content is increased to 0.8 - 1.2% there is alloying ferrite, which increases the strength of the metal. Alloying elements (Ni, Cr, Mo, etc.), promote modification , getting fine-grained and dispersed structure, strengthen of structural steel components and improvement of its thermal processing.
Wear of rolls largely depends on their structure and chemical composition. when using the steel rolls of the highest wear resistance of hypereutectoid roll; they wear 2 - 3 times slower than their peers in hardness eutectoid rolls. Wear resistance is higher than the eutectoid dispersion structure of the rolls and the greater the amount of excess carbides contained in hypereutectoid rolls.
The mechanism of fracture of the working surface of the rolls eutectoid and hypereutectic chemical composition is different.
Rolls eutectoid class have high ductility and toughness. Deterioration of their differing considerable unevenness occurs in an offset thinnest layers of the working surface calibers. As a result, a smooth surface roller is broken, it appears on a number of alternating recesses and protrusions gradually increasing in volume.
mechanism of wear in hypereutectoid roller in which the structure contains excess carbides is uniform shearing of the working surface of the smallest particles in the rolling process. Such rollers during rolling wear more evenly and retain a sufficiently smooth surface for the duration of the roll.
The nature of the destruction of the working surface of cast iron roll is somewhat different. Observations have shown that the fractured surface of the roller calibers semisolid cast iron can be noted two successive stages: stage generate point (after regrinding the rolls) when subjected to only separate chipping micro platform surface of the roll body, and a step of intensive destruction of entire working surface of the roller.
Spot the original production occurs in the places where the graphite on the surface of the roll and then develops around the pearlite field weakened graphite inclusions.
As the number of rolled metal number destroyed micro areas is continuously increasing. They are distributed on the working surface of the roll (the second stage of wear) and cover the entire sections, and then all the working field gauge; thereby deteriorate the quality of the roll surface and finished steel.
The factors that accelerate mechanical wear of the rolls, should include internal conversion in a metal lattice in the presence of weak areas, and various defects in some cases the joints crystals. in the process of deformation of these nucleus are microcracks and cracks which increase in volume over time. Once begun, the destruction continues if try to continue operating.
The sharp increase in the durability of rolls can be achieved by increasing the hardness of the working layer. The greater the hardness of the rolls and above their resistance, the greater the amount of metal can be rolled over the period between the transhipment. The wear resistance of steel rolls higher than metal based structurally less free ferrite and more excess carbides. The greater the total surface of the carbide inclusions, finer grain and carbide particles, the greater the hardness of the rolls and above their resistance to abrasion.
Durability cast iron roll depends on the number of nonmetallic inclusions in the working layer of rolls with perlite- graphite and perlite- cementite-graphite structure, the amount and form of graphite inclusions, the degree of dispersion of metal base and the excess amount of carbides.
High performance inherent in the rolls, in which graphite inclusions of spherical shape. High wear of the rolls is explained form of graphite which crumble during operation with minimal disruption to the metal framework. At the same time the very basis due to the high resistance crumble too less.
The wear resistance of cast iron with graphite spherical shape more than the steel with high surface hardness. When changing the plate-shaped spherical graphite resistance rolls of gray cast iron is increased by 30 - 40%, as decreases the height and wear gauge.
High service properties are characterized by the rolls, molded profiled form. The hardness of the rolls is high (380 - 440 HB at Burt), they have a high wear resistance (by 2 - 3 times the usual) not only at the surface of the barrel, but also in the depth of incision in the cylinder.
prospective application of cast iron rolls with a low phosphorus content, made of magnesium, especially low-alloy cast iron.
the rolls of low iron phosphide characterized by improved mechanical properties (strength, elongation, toughness, resistance) compared with conventional rolls of cast iron. They are 30 - 50% stronger than normal, and their resistance almost three times higher. Increased resistance against wear and micro-chipping achieved by reducing the phosphorus, the amount of which is equal to 0.06 - 0.10%. When reduced phosphorus content in the microstructure of the rolls are almost absent phosphide (fragile components roll iron structure), contains a large amount of ferrite in the gray zone.
The absence in the microstructure of the roll brittle rolls generated in the iron containing more than 0.10% phosphorus, contributes to the strength of the core, the viscosity increase and wear resistance of the bleached working layer.
The downside of low phosphorus cast iron rolls is reduced hardness of bleached and gray areas. Reduction of phosphorus (without special measures) to 0.1% results in a decrease of the working roll surface hardness of 8 - 10 Brinell.
Progressive means of increasing resistance of milled rolls against the wear and tear is an alloying metal. It is noticed that in the iron, positive influence of alloying elements on the wear often exceeds their influence on mechanical properties. Alloying elements contribute to grain refinement, change the form of graphite, the metal structure of the advanced means of increasing the resistance of the rolls against the wear and tear is an alloying metal. It is noticed that in the iron, positive influence of alloying elements on the wear often exceeds their influence on mechanical properties. Alloying elements contribute to grain refinement, change the form of graphite, the metal base structure, composition and structure of the carbides, increase the efficiency of heat treatment, according to the rolls increased strength, hardness and durability. increased surface hardness promotes alloying iron with chromium, vanadium, molybdenum, nickel and boron.