RU2060067C1 - Roll for cold and hot rolling - Google Patents

Abstract

FIELD: rolled stock production. SUBSTANCE: a builtup rolling roll includes carbide inserts and a cast iron barrel. The barrel of the roll includes one or more carbide inserts, had been centrifugally cast to a body of the roll from ferrum base casting alloys, mainly from graphitized cast iron, including after casting process residual austenite. Upon next one-time or multiple heat treatment the residual austenite partially or completely is being converted to beinite with respective increase of volume. Such phase conversion allows to lower or to substantially limit a differential shrinkage of cast iron and carbides, caused by cooling from a casting temperature. A central part of the roll may be made of other alloys. EFFECT: enhanced wear resistance of roll. 2 cl, 3 dwg

Description

 The invention relates to centrifugal casting processes for carbide inserts (one or more) for products from cast alloys based on iron, mainly from cast iron. The final product is a composite monolithic roll, in the structure of which there is a transition zone between the carbide insert and cast iron.

 Compound rolls for hot and cold rolling have one or more inserts of carbide-containing materials that are mounted on the drive shaft using various devices and locking devices. The first method of transmitting force from the shaft to the composite roll with carbide inserts involves the use of keyways or guide streams and tides associated with the carbide insert.

 The disadvantages of carbide inserts are increased fragility, low tensile strength and high sensitivity to notches, which can adversely affect the manufacture of internal ribs in this device from torque transfer devices. Standard methods for connecting nodes are not well developed.

 The second method of transmitting torque can be carried out due to friction forces on the surface of the bored hole of the carbide insert. The presence on this surface of the radial component of the friction force leads to an increase in the tangential component of tensile stresses in the carbide insert. This parameter reaches its maximum value at the inner diameter. Such tensile stresses are combined with tensile stresses arising during the operation of the roll.

 A composite roll for hot or cold rolling should have a protective coating based on one of the iron alloys on the surface of the carbide insert. To enable the transmission of torque in the body of the roll, the necessary devices are strengthened. However, the task is not so simple. Under the influence of thermal stresses during cooling, the coating cools faster than the carbide insert itself, as a result of which forces are generated on its surface directed towards the center of the roll. This circumstance leads to an increase in axial tensile stresses on the outer surface of the carbide insert. Mentioned stresses are perpendicular to microcracks that occur during rolling on the surface of the rolls. Under the influence of tensile stresses, microcracks deepen, which can even lead to premature failure of the rolls.

 A known method of solving this problem, according to which one or more carbide inserts are cast in a chill mold, where graphite is largely graphitized; at the end of the casting process, the structure of cast iron contains 15-20 wt. austenite. After a single or multiple heat treatment, austenite partially or completely turns into bainite. The process of decomposition of austenite is accompanied by an increase in volume, which contributes to a significant reduction or limitation of the difference in volumetric shrinkage of cast iron and carbide insert that occurs during cooling of the casting. The method allows to establish the production of composite rolling rolls, which are in the final form mounted on the transfer shaft using couplings and locking devices. Even if all the necessary devices for ensuring the torque transmission are made of cast iron, there are still dimensional restrictions, and therefore restrictions on the amount of torque transmitted by the rolls. The working layer of the roll (drum), subject to wear, also has size restrictions. According to the well-known principles of assembling composite rolls, the latter are driven by clutches of various designs, transmitting torque. It is almost impossible to place carbide liners on the drum, as clutches and locking devices require space to accommodate them. Due to this circumstance, the surface area of the roll becomes very limited.

 Centrifugal casting technology is well known, it is widely used for the manufacture of parts representing bodies of revolution. This technology is based on the use of centrifugal forces created by a rotating casting mold and aimed at transporting metal to the walls of the casting mold, where the melt crystallizes under the influence of pressure and a casting of a given configuration is formed. This technology is widely used for the manufacture of pipes, liners, bearings, bushings, etc. It is also used for the manufacture of rolling rolls.

 According to the invention, it is proposed to produce a composite roll only by centrifugal casting in the presence of an intermediate zone between the carbide liners and the cast iron core of the roll. Depending on the specific rolling conditions and the requirements, the drum may have one or more carbide liners. Using this invention, you can get the choice of constructive rolls.

 In FIG. 1 shows a roll of viscous iron 1, in which one carbide insert 2 is cast; in FIG. 2 roll 1 of viscous iron, in which there are four carbide inserts obtained by the casting method; in FIG. 3 roll with four carbide liners 2, cast into ductile iron 1, and the core and neck of the roll are made of another material 3.

 The mold from the moldable mixture for casting one or more carbide liners should be designed to prevent molten iron from entering the outer surface of the liners. This invention allows this to be achieved.

 The outer surface of the carbide liners forms the body of the composite rolling roll. In accordance with the practical experience of using the technology for producing cast inserts for rolling rolls, it is known that small amounts of molten iron falling on the surface of the rolling rolls noticeably reduce the quality of carbide inserts. The damaged surface of the inserts is removed by machining the outer surfaces to a depth of one or more millimeters. To ensure conditions for the successful production of composite rolls, the ingress of melt onto the surface of the liners should be excluded.

 To ensure the optimum quality of the transition zone structure between the carbide insert and cast iron, it is necessary to ensure a high temperature of overheating of cast iron in the ladle, and to introduce strict control over the filling of the casting mold and the given mold rotation speed. These measures are necessary to ensure a balance between heating and melting of the surface part of the carbide insert that is not immersed in the molding mixture, i.e., that part of the liner that must be diffusion bonded to cast iron.

 The considered rolling roll after heat treatment according to a given mode and machining to the final dimensions is either the entire rolling roll as a whole, or just an insert to it.

Carbide inserts are poured into cast iron, which has a significant proportion of the graphitized structure, and its composition corresponds to the carbon equivalent. The chemical composition of cast iron is selected taking into account the possibility of forming the optimal transition zone between cast iron and carbide inserts, ensuring the necessary strength of the material. All of these properties are necessary to ensure optimal conditions for the transmission of torque and guarantee the workability of the material. Due to alloying, iron-silicon-magnesium and or nickel-magnesium cast iron contains 0.02-0.10 wt. magnesium (the optimum concentration of magnesium is 0.04 0.07 wt.). Due to the introduction of ferrosilicon, cast iron contains 1.9-2.8 wt. silicon (optimal range is 2.1-2.5 wt.). Silicon alloying of ductile iron makes it possible to obtain a dispersed spherical shape of graphite precipitates. In a heat-treated state, the hardness of such cast iron is 250-300 units. according to Brinell. In the future, cast iron should be alloyed with elements that are austenitic stabilizers, mainly nickel in the amount of 3.0-10.0 wt. (optimal concentration of 4-8 wt.); in the presence of stabilizers, pig-iron castings contain 5-30% residual austenite (optimal content is 10-25 or even 15-20 wt.). Due to a single or multiple step heat treatment, the volumetric content of residual austenite can increase somewhat and go into another structural state, for example, bainite. The increase in casting volume can be adjusted. The differential shrinkage occurring in the composite roll during cooling from the casting temperature can be completely or partially eliminated. For the case of carbide inserts and cast iron castings used for the manufacture of rolling rolls, the heat treatment technology is somewhat modified. The process comprises heat treatment of heating and holding at a temperature of 800-1000 ^{° C,} cooling and holding at a temperature of ^about 400-500 C, followed by cooling to room temperature.

 In an alternative method for producing composite rolls, carbide inserts (or insert) and cast iron mold are used, while the core of the roll and spikes are cast from another cast alloy using centrifugal casting technology or a conventional method.

 PRI me R. Ceramic-metal inserts from a mixture of carbides, wt. tungsten 70; cobalt 13; nickel 15; chrome 2, was cast into an earthen mold on a vertical centrifugal casting machine. Carbide inserts had the following dimensions: outer diameter 340 mm, inner diameter 260 mm, width 50 mm.

At the end of the casting process, the surface of the hole of the carbide insert and its side surfaces remained free, which created the conditions for the formation of a transition zone with a heterogeneous microstructure between cast iron and carbides. Then the mold was installed on a centrifugal casting machine, the rotation speed of which was 475 rpm. Molten cast iron containing, by weight. carbon 3,7; silicon 2,3; manganese 0.3; nickel 5.4; molybdenum 0.2; magnesium 0.05; the rest iron, at a temperature of 1500 ^{° C} was poured into the rotating mold. Casting was continued until the mold was completely filled, the casting time was about one minute. At the end of the casting, the mold was stopped and the mold was cooled to room temperature. After cooling, the composite roll was cleaned and ultrasonic quality control of the castings was performed. The intermediate casting zone provided a sufficiently high quality of diffusion adhesion of dissimilar materials.

After casting, the roll was subjected to heat treatment, the purpose of which was to convert residual austenite to bainite. To this end parts were heated to 900 ^{° C,} kept at this temperature for 6 hours and then cooled to a temperature of 450 ^{° C,} kept at this temperature for 4 hours, after which the item was cooled to room temperature.

 The rolls had the following dimensions: drum: D310 mm (in the presence of carbide inserts D 340 mm) x500 mm; shaft spike: D 220x300 mm + 220x520 mm.

Claims (2)

 1. Roll for hot and cold rolling, containing a barrel having one or more carbide inserts cast into the body of the roll of cast alloys of predominantly graphitized cast iron with 15 to 20 wt. residual austenite, which turns into bainite during the subsequent heat treatment with an increase in volume, which contributes to a decrease in the differential of shrinkage between cast iron and carbide inserts upon cooling from the casting temperature, characterized in that the roll has a neck connected to the barrel and is molded entirely for fine mechanical processing.  2. The roll according to claim 1, characterized in that the carbide inserts (or inserts) are cast from one casting alloy, and the barrel and the neck of the roll from another casting alloy are centrifugally cast.

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