AU657296B2

AU657296B2 - A roll including a cemented carbide ring cast into a cast iron body

Info

Publication number: AU657296B2
Application number: AU10635/92A
Authority: AU
Inventors: Jan-Erik Carlsson; Gert Sundstedt
Original assignee: Sandvik AB
Current assignee: Sandvik AB
Priority date: 1991-02-11
Filing date: 1992-02-04
Publication date: 1995-03-09
Anticipated expiration: 2009-12-04
Also published as: AU1063592A

Description

657296

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION PATENT OF ADDITION Invention Title: A ROLL INCLUDING A CEMENTED CARBIDE RING CAST INTO A CAST IRON BODY.

The following statement is a full description of this invention, including the best method of performing it known to me: A ROLL INCLUDING A CEMENTED CARBIDE RING CAST INTO A CAST IRON BODY.

The present invention relates to casting by the centrifugal casting method one or several cemented carbide rings into cast alloys based on iron, preferably 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 transmitting the torque from the driving spindle to the ceinented carbide roll is to use keyways or other driving grooves or driving lugs, made integral with the cemented carbide ring. However, cemented carbide 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 fkict%-ional 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 Nv, 7!00170-5, publication 2 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 perpendicularly 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 dressing 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 25 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 application permits manufacture of merely roll rings, which then by couplings and locking devices on a driving S 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 dimensiv,, 41 limitations and thereby a limit for the transmittable torque. Further, the wear resistant surface (the barrel) of the rolls is dimensionally 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 t' hnique for manufacture of usually rotary symmetric bodies. The technique utilises the centrifugal force, accomplished 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 15 for manufacture of tubes, linings, bushes etc., but is also used for manufacture of rolls. For exanmple, 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 casting of a high alloyed e 20 cast steel and a core of a vermicular cast iron, ductile V '"iron or simple cast steel.

According to a first aspect of the present invention there is provided a roll, for hot or cold rolling, in which a barrel comprises at least one cemented carbide ring cast into a casting alloy, comprising an essentially graphitic cast iron, which after casting contains between 5 to percent by weight residual austenite which after one or several subsequent heat treatment steps totally or partly is transformed under volume increase to mainly bainite, for reducing or totally eliminating the differential shrinkage between the graphitic cast iron and the cemented carbide resulting from cooling after casting, characterised in that said roll including journals is cast in one piece, which can later be machined to final shape and dimension.

According to a second aspect of the present invention there is provided a method of manufacturing a roll, for hot or cold rolling, in which a barrel comprises at least one cemented carbide ring cast into a casting alloy, comprising an essentially graphitic cast iron, which after casting contains between 5 to 30 percent by weight residual austenite which after one or several subsequent heat treatment steps totally or partly is transformed under volume increase to mainly bainite, for reducing or totaLly eliminating the differential shrinkage between the graphitic cast iron and the cemented carbide resulting from cooling after casting, said method comprising the step of casting the roll including journals in one piece, which can later be machined to final shape and dimension.

oo 15 Figure i, 2 and 3 show some alternative embodiments.

0* *•o Figure I shows a roll of ductile iron in which a cemented carbide ring is cast in.

Figure 2 shows L. roll of ductile iron in which four cemented carbide rings are cast in.

Figure 3 shows a roll with four cemented carbide rings cast in ductile iron and with core and journals in another material 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 th-e manufactured composite roll. Experience from previous :,anufacture 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 surface quality of Vise cemented carbide under the penetration surface. By machining one or a few millimeters 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 possible.

~mtalu'cTbn bt In order to achieve opiu ealria odbt ween cemented carbide and cast iron it is necessary to use high over-temperature of the iron in the cradle, combined with amnount 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, af ter necessary heat treatme 'nt and machining to final shape and dimension, a complete roll or roll ring. The difficulties with existing cemented carbide rolls to confidently transmit the torque from the driving spindle to the cemented carbide part, as previously described, 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- 25 ability. By addition of ferro-silicor\-macjnesium and/or nickel-magnesium the cast alloy gets a magnesium content of 0,02-0,10, preferably 0,04-0,07 percent by weight. By inoculation with ferro-silicov" the cast iron gets a silicon content of 1,9-2,8, preferably 2,1-2,5 percent by weight. Therei~y a ductile iron is obtained having dispersed, spheroidal graphite. This ductile .iron has a hardness-toughnessstrength 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 amiount 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 treatment 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 temnperature, an be totally or partly eliminated. The method f or this heat treatment is adjusted to cemented 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 OC, cooling to and holding at a temperature of 400- 550 OC, 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 S a cemented carbide ring (or rings) cast into a ringshaped casing only.

According to the invention there is also provided a mtethod 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 0: equipment with the bore surface of the ring and its side surfaf.es 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 temperature 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 alt"fniti44embodiment a cemented carbide ring (or rings) is cast into a ring-shaped casing only, by centrifugal casting.

Examle A sintered c,7anented carbide ring with the composition 7 0 tungsten carbide (WC) 13 t cobalt (Co) 15 nickel 2 chromium all percent by weight, was molded in foundry sand irn a mould in a vertical centrifuaal 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 8 minute. A cast iron melt, with the composition 3,7 t carborn 2,3 silicon 0,3 mangjanese (Mn), 5,4 It nickel 0,2 molybdenum 0.05 magnesium (Mg) and the, balance iron all percent by weight, at a temperature of 1540 OC 'was tapped into the rotating mould. T~duration of pouring was about 1 minute. During that time the rotation gradually 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 transforming the residual austenite to bainite, by heating to 900 holding time 6 hours, then cooling to 450 OC, holding time 4 hours, followed by cooling to room temperature. The freely exposed surfaces of the cemented carbide ring were covered with an antioxidizing agent, as the heat treatment was performed in open air. The roll dimensions were: Barrel: 0310 mm (cemented carbide 0340 mm) x 500 Journals: 0i220 x 300 mm 220 x 520 mm.

6

Claims

1. A roll, for hot or cold rolling, in which a barrel comprises at least one cemented carbide ring cast into a casting alloy, comprising an essentially graphitic cast iron, which after casting contains between 5 to percent by weight residual austenite which after one or several subsequent heat treatment steps totally or partly is transformed under volume increase to mainly bainite, for reducing or totally eliminating the differential shrinkage between the graphitic cast iron and the cemented carbide resulting from cooling after casting, characterised in that said roll including journals is cast in one piece, which can later be machined to final shape and dimension.

2. The roll according to claim 1 wherein the graphitic cast iron after casting contains 15 to 20 percent by weight residual austenite.

3. The roll according to either claim 1 or 2 characterised in that the roll is made by centrifugal casting. C 20 4. The roll according to any one of the preceding claims characterised in that it comprises a casing of said graphitic cast iron, holding at least one of said cemented carbide rings, and a core and said journals cast of another casting alloy by centrifugal or static casting.

5. The roll according to claim 3 characterised in that the roll comprises at least one cemented earbide ring cast into a ring-shaped casing only.

6. A method of manufacturing a roll, for hot or cold rolling, in which a barrel comprises at least one cemented carbide ring cast into a casting alloy, comprising an r i essentially graphitic cast iron, which after casting contains between 5 to 30 percent by weight residual austenite which after one or several subsequent heat treatment steps totally or partly is transformed under volume increase to mainly bainite, for reducing or totally eliminating the differential shrinkage between the graphitic cast iron and the cemented carbide resulting from cooling after casting, said method comprising the step of casting the roll including journals in one piece, which can later be machined to final shape and dimension.

7. The method according to claim 6 wherein the graphitic cast iron after casting contains between 15 to percent by weight residual austenite.

8. The method according to either claim 6 or 7 characterised in that casting is performed by a centrifugal 15 casting technique.

9. The method according to claim 8 characterised in that by centrifugal casting the cemented carbide ring is cast into a casing of said casting alloy, after which roll core and journals are cast of another casting alloy either 20 by centrifugal or static casting. *9*

10. The method according to claim 8 characterised in that a ring-shaped casing only is cast by the centrifugal casting technique.

11. The method according to any one of claims 6 to characterised in that an outer surface of the barrel is substantially separated from said casting alloy during casting of the roll.

12. A roll for hot or cold rolling substantially as herein described with reference to and as illustrated in 1 0 any=ee== the accompanying drawings.

14. 11 13. A method of manufacturing a roll substantially as herein. described with reference to and as illustrated in QeY!e--- fiWWMBE the accompanying drawings. DATED THIS 4TH DAY OF AUGUST, 1994. SANDVIK AB By its Patent Attorneys: GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia