The present invention relates to casting by the centrifugal casting method one or several cemented carbide rings into cast alloys based on iron, prefer¬ ably cast iron. The resulting product is a composite roll, made in one piece only, with metallurgical bond between cemented carbide and cast iron.

Composite rolls with cemented carbide for hot or cold rolling comprise one or several cemented carbide rings attached to a (driving) spindle by various couplings and locking devices. One method of trans- mitting the torque from the driving spindle to the cemented carbide roll is to use keyways or other driving grooves or driving lugs, made integral with the cemented carbide ring. However, cemented carbide is a brittle material with limited tensile strength and special notch sensitivity such as in inner cor¬ ners in above mentioned devices for the torque trans¬ mission. Methods based on such conventional joints have worked unsatisfactorily. Another method for the torque transmission is by means of frictional forces at the bore surface of the cemented carbide ring.

However, the radial force on this surface gives rise to tangential tensile stresses in the cemented carbide ring with a maximum at its inner diameter. These tensile stresses are added to other tensile stresses, generated when the roll is in use.

By casting a casing of an iron alloy onto a cemented carbide ring a composite roll for hot or cold rolling is obtained, in which required devices for torque transmission may be located to the cast iron, see for example the Swedish patent No. 7100170-5, publication

number 371114. However, this is not an entirely simple problem. Due to the fact that during cooling the casing shrinks more than the cemented carbide ring, inwardly directed forces on the cemented carbide ring are produced, giving rise to axially directed tensile stresses on the outer surface of the cemented carbide ring, which are acting perpendi¬ cularly to micro cracks generated in the roll surface during rolling. Under the influence of these tensile stresses the micro cracks propagate in depth, which may cause roll breakage or need for excessive dress¬ ing amount, limiting the total rolling capacity of the roll.

A solution to this problem is described in the

Swedish patent application No. 8804503-4, according to which one or several cemented carbide rings are cast into a casing of an iron alloy, at which the cast alloy comprises an essentially graphitic cast iron, which after the casting contains the structure element residual austenite, preferably 15-20 percent by weight, which at subsequent one or several heat treatment steps totally or partly is transformed under volume increase to mainly bainite, with the aim of reducing or totally eliminating the differential shrinkage between the cast iron and the cemented carbide as a result from cooling after the casting.

The method according to the mentioned patent applica- tion permits manufacture of merely roll rings, which then by couplings and locking devices on a driving spindle are assembled to a complete roll. Even if the necessary couplings for the torque transmission are made in the cast iron part there are dimensional limitations and thereby a limit for the transmittable torque. Further, the wear resistant surface (the

barrel) of the rolls is di ensionally restricted. According to the mentioned method or other existing systems of cemented carbide rolls, operating by way of some kind of torque transmitting couplings, one is not totally free to place cemented carbide rings on the barrel because of space requiring couplings and locking devices. Thereby, the surface to be used for rolling becomes substantially restricted.

Centrifugal casting is a well known technique for manufacture of usually rotary symmetric bodies. The technique utilizes the centrifugal force, accomp¬ lished by a rotating cylindrical mould, to sling the molten metal against the walls of the mould, where it solidifies under pressure to desired shape. The technique is commonly used for manufacture of tubes, linings, bushes etc., but is also used for manufac¬ ture or rolls. For example, by the Swedish patent application No. 8603987-2 a composite roll for hot or cold rolling mills is known, having a centrifugal cast outer wear resistant layer or casing of a high alloyed cast steel and a core of a vermicular cast iron, ductile iron or simple cast steel.

According to the invention a composite roll is now provided made in one piece only by means of centri¬ fugal casting with metallurgical bond between cemen¬ ted carbide and cast iron, where the barrel may be equipped with one or several cemented carbide rings according to actual rolling conditions and require¬ ments. This freedom in design is possible to achieve only by the present invention.

Figure 1, 2 and 3 show some alternative embodiments.

Figure 1 shows a roll of ductile iron (1) in which a cemented carbide ring (2) is cast in.

Figure 2 shows a roll of ductile iron (1) in which four cemented carbide rings (2) are cast in.

Figure 3 shows a roll with four cemented carbide rings (2) cast in ductile iron (1) and with core and journals in another material (3) .

The moulding of the cemented carbide ring or rings in the foundry sand must be made in such a way that the molten cast iron is prevented from reaching the outer surface of the cemented carbide ring. This is proven to be possible by the invention.

The outer surface of the cemented carbide ring (or rings) forms the barrel of the manufactured composite roll. Experience from previous manufacture of cast-in roll rings, according to the patent application No. 8804503-4, shows that a small amount of molten cast iron, having penetrated the outer surface of the cemented carbide ring, results in deteriorated sur¬ face quality of the cemented carbide under the pene- tration surface. By machining one or a few milli¬ meters of the outer surface of the cemented carbide ring this surface deterioration can be removed. For successful manufacture of a composite roll according to the present invention such surface penetrations must be avoided which, according to above, is possi¬ ble.

In order to achieve optimum metallurgical bond bet¬ ween cemented carbide and cast iron it is necessary to use high over-temperature of the iron in the cradle, combined with amount controlled filling of

the mould and a predetermined speed of rotation, to get a balanced heating and melting of a surface layer of the part of the cemented carbide ring which is not moulded in the foundry sand, i.e. the part that shall be metallurgically bonded to the cast iron.

The present composite roll comprises, after necessary heat treatment and machining to final shape and dimension, a complete roll or roll ring. The diffi- culties with existing cemented carbide rolls to con¬ fidently transmit the torque from the driving spindle to the cemented carbide part, as previously de¬ scribed, has been eliminated with this composite roll.

According to the invention the cemented carbide is cast into an essentially graphitic cast iron with a composition adjusted to the carbon equivalent, Ceqv. , in a way described in the Swedish patent No. 8601289- 7, publication number 399911. The composition of the cast iron is also chosen with regard to optimum metallurgical bond to the cemented carbide, to its strength, toughness and hardness, all necessary for the transmission of the torque and to its machin- ability. By addition of ferro-silicium-magnesium and/or nickel-magnesium the cast alloy gets a magne¬ sium content of 0,02-0,10, preferably 0,04-0,07 percent by weight. By inoculation with ferro-silicium the cast iron gets a silicon content of 1,9-2,8, preferably 2,1-2,5 percent by weight. Thereby a ductile iron is obtained having dispersed, spheroidal graphite. This ductile iron has a hardness-toughness- strength which is well balanced to the application. In heat treated condition the Brinell hardness is 250-300. Further, the iron shall be alloyed with austenite generating alloying elements, preferably

nickel in amount of 3-10, preferably 4-8 percent by weight, resulting in a certain amount of residual austenite viz. 5-30, preferably 10-25 or rather 15-20 percent by weight after the casting. By heat treat- ment in one or several steps a suitable amount of residual austenite can under volume increase be transformed to other structure elements for example bainite. This volume increase can be so adjusted that the differential shrinkage, taking place in the composite roll during cooling from the casting tem¬ perature, can be totally or partly eliminated. The method for this heat treatment is adjusted to cemen¬ ted carbide grade, composition of the cast iron and to roll application. The heat treatment includes heating to and holding at a temperature of 800-1000 °C, cooling to and holding at a temperature of 400- 550 °C, and cooling to room temperature.

In an alternative embodiment the roll comprises a cemented carbide ring (or rings) and a casing of mentioned cast iron only, whereas roll core and journals are cast of another casting alloy either by centrifugal or static casting.

In another alternative embodiment the roll comprises a cemented carbide ring (or rings) cast into a ring- shaped casing only.

According to the invention there is also provided a method for manufacture of a complete cemented carbide equipped roll for hot or cold rolling. According to the method at least one sintered cemented carbide ring is placed in a mould in a centrifugal casting equipment with the bore surface of the ring and its side surfaces free to establish contact between cemented carbide and cast iron. The mould is rotated

and when proper speed is achieved a molten cast iron with above described composition and suitable tempe¬ rature is tapped down into the rotating mould, in doing which the molten iron is slung against the wall of the mould and solidifies under pressure. After cooling to room temperature the roll is cleaned and is then heat treated according to above.

According to an alternative method a cemented carbide ring (or rings) is cast into a casing of mentioned cast iron only, after which core and journals are cast of another cast alloy either by centrifugal or static casting.

According to another alternativ embodiment a cemented carbide ring (or rings) is cast into a ring-shaped casing only, by centrifugal casting.

Example

A sintered cemented carbide ring with the composition 70 % tungsten carbide (WC) , 13 % cobalt (Co) , 15 % nickel (Ni) , 2 % chromium (Cr) , all percent by weight, was molded in foundry sand in a mould in a vertical centrifugal casting machine. The dimensions of the cemented carbide ring were:

- Outer diameter 340 mm

- Inner diameter 260 mm

- Width 100 mm.

After the molding the bore surface of the cemented carbide ring and its side surfaces between the inner diameter and a diameter of 310 mm were free in order to there create a metallurgical bond between the cemented carbide and the cast iron.

The mould was put to rotate at 400 revolutions per

minute. A cast iron melt, with the composition 3,7 % carbon (C) , 2,3 % silicon (Si), 0,3 % manganese (Mn) , 5,4 % nickel (Ni) , 0,2 % molybdenum (Mo), 0,05 % magnesium (Mg) and the balance iron (Fe) , all percent by weight, at a temperature of 1540 °C was tapped into the rotating mould. The duration of pouring was about l minute. During that time the rotation gra¬ dually ceased.

After the cooling the composite roll was cleaned and checked by an ultrasonic method. The quality of the metallurgical bond was good.

The roll was heat treated, with the aim of trans- forming the residual austenite to bainite, by heating to 900 °C, holding time 6 hours, then cooling to 450 °C, holding time 4 hours, followed by cooling to rcom temperature. The freely exposed surfaces of the cemented carbide ring were covered with an anti- oxidizing agent, as the heat treatment was performed in open air. The roll dimensions were:

Barrel: 0310 mm (cemented carbide 0340 mm) x 500 mm. Journals: 0220 x 300 mm + 220 x 520 mm.