Composite roll

Abstract: The present invention relates to a roll, comprising cemented carbide and cast iron. The roll may be used for hot or cold rolling. The cast iron has the following composition in addition to Fe, in weight %, Si 1.9 - 2.8, Mg 0.02 - 0.10, Ni 0.5 - 4, Mn 0.1 - 1 and with C eqv 2.5 - 6.

Description: The present invention relates to a composite roll including at least one cemented carbide ring and nodular cast iron casing. The composite roll can be available as a complete roll or as a roll ring to be attached to a driving spindle. Composite rolls with cemented carbide for hot or cold rolling comprise one or more cemented carbide rings with a casing of cast iron attached to a (driving) spindle by various couplings and locking devices. One problem with such rolls is that during cooling from the casting temperature the casing shrinks more than the cemented carbide ring. As a result 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 microcracks generated in the roll surface during rolling. Under the influence of these tensile stresses the microcracks propagate in depth, which may cause roll breakage or need for excessive amount of dressing, limiting the total rolling capacity of the roll. One solution to this problem is described in U.S. patent 5,044,056, according to which one or more cemented carbide rings are cast into a casing of an essentially graphitic cast iron, which after the casting contains the structure elements bainite and residual austenite, preferably 15-20 wt-%, which at subsequent one or more heat treatment steps totally or partly is transformed to bainite. In this way a favourable stress state is obtained. However, this heat treatment is a costly and time consuming operation that it would be favourable to eliminate. U.S. patents No. 5,248,289 and 5,359,772 disclose that even a complete roll can be made in the same way with maintained good bond between the cast iron and the cemented carbide. However, heat treatment of complete rolls with a length of up to 2500 mm requires furnaces with large dimensions and long cycle heat treatments increase the door to door time. The resulting structure with a mix of bainite and residual austenite is also very difficult to machine. According to the present invention it has been found that by using an alloy giving an as-cast material with a structure of pearlite and ferrite, the desired state of stress is provided in combination with a good metallurgical bond without the subsequent heat treatment of the roll. The cast iron is easily machined in an as-cast condition and has a hardness-toughness-strength which is well balanced during use of the roll. Figure 1 shows a composite roll consisting of a cemented carbide ring (2) and cast iron casing (1) to be mounted on a spindle. Figure 2 shows a complete roll including roll core and journals (1) with one cast-in cemented carbide ring (2). Figure 3 shows the microstructure of the nodular cast iron according to the invention in which 3 - graphite nodules 4 - pearlite 5 - ferrite According to the present invention the cemented carbide is cast into an essentially graphitic cast iron with a composition adjusted so that the carbon equivalent, C eqv = %C + 0.3· (%Si + %P) is 2.5 - 6, preferably 3.5 - 5. Ferro-silicium-magnesium and/or nickel-magnesium is/are added to the cast alloy to a magnesium content of 0.02-0.10 wt-%, preferably 0.04-0.07 wt-%. By inoculation with ferro-silicium the cast iron obtains a silicon content of 1.9-2.8 wt-%, preferably 2.1-2.5 wt-%. Thereby a nodular cast iron is obtained having dispersed, spheroidal graphite. Further, the iron shall be alloyed with elements delaying the diffusion of carbon, preferably nickel in an amount of 0.5-4, preferably 1-2 wt-%, and manganese in an amount of 0.1-1.0, preferably 0.6-0.7 wt-%, resulting in a structure of pearlite and ferrite with not less than 40 % by volume pearlite and some amount <5% by volume of residual austenite. Ni and/or Cu may partly be replaced by up to 1 wt-% Mo. For a weight of the cast iron portion of the roll in excess of 1000 kg an addition of <2 wt-%, preferably 0.01-1 wt-% Cu, is suitable. In as cast condition the cast iron in the roll has a Brinell hardness of 190-250 for a weight between 200 and 1000 kg of the cast iron portion of the roll. In one embodiment the roll is a complete roll including roll core and journals with at least one cemented carbide ring. Roll core and journals may be made of another cast alloy. For instance, Figure 1 shows a composite roll consisting of a cemented carbide ring 2 and cast iron casing 1 to be mounted on a spindle whereas Figure 2 shows a complete roll including roll core and journals 11 with one cast-in cemented carbide ring 2. As shown in Figure 3, the microstructure of the nodular cast iron of the roll includes graphite nodules 3, pearlite 4 and ferrite 5. In another alternative embodiment the roll comprises a cemented carbide ring (or rings) cast into a ring-shaped casing only which rings are attached to a (driving) spindle by various couplings and locking devices. According to the invention there is also provided a method for manufacture of a complete roll including roll core and journals or cemented carbide ring(s) cast into a ring-shaped casing only which ring(s) are attached to a spindle for hot or cold rolling. According to the method at least one sintered cemented carbide ring is placed in a mould with the inner surface of the ring and its side surfaces free to establish contact with the cast iron. The mould is filled with molten cast iron with the composition according to above and suitable temperature. After cooling to room temperature the roll is cleaned and machined to final shape and dimension. In one embodiment the casting is made by the static method. In a preferred embodiment the roll is cast by static casting in a mould where the inlet is directed in a tangential direction to the inner surface of the cemented carbide roll ring. In another embodiment the casting is made by centrifugal casting. The mould is rotated and when a suitable speed, about 400 rev/min, is achieved the molten cast iron is poured into the rotating mould. The rotating speed is continuously decreased during the pouring time which lasts about 1 min. As a result the molten iron is slung against the wall of the mould and solidifies under pressure. Alternatively the 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. In order to achieve optimum metallurgical bond between cemented carbide and cast iron it is necessary to use an over-temperature of 200-300 °C of the iron in the cradle, combined with amount controlled filling of the mould and in the case of centrifugal casting 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. A transition zone between cemented carbide and cast iron of 1 - 5 mm width has been found satisfactory. The present composite roll comprises, after machining to final shape and dimension, a complete roll or roll ring. The difficulties with existing cast in rolls is getting a heat treatment furnace with the necessary dimensions, and the costs and loss of time that this heat treatment generate is eliminated by using the cast alloy according to the invention. Example A sintered cemented carbide ring with the composition 70 % WC, 13 % Co, 15 % Ni, 2 % Cr, all wt-%, was molded in foundry sand. The dimensions of the cemented carbide ring were: Outer diameter: 340 mm Inner diameter: 260 mm Width: 100 mm After the molding the inner 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 roll was cast by static casting in a mould where the inlet was directed in a tangential direction to the inner surface of the cemented carbide roll ring. A cast iron melt, with the composition 3.5% C, 2.2 % Si, 0.6 % Mn, 1.65 % Ni, 0.05 % Mg and the balance Fe, all wt-%, at a temperature of 1540 °C was poured into the mould. The duration of the pouring was about 1 minute. After the cooling the composite roll was cleaned and checked by an ultrasonic method. The quality of the metallurgical bond was good. The roll dimensions were: Barrel: ⊘310 mm (cemented carbide ⊘340 mm) x 500 mm. Journals: ⊘220 x 300 mm + ⊘220 x 520 mm.

Claims: Composite roll, preferably for hot or cold rolling, comprising at least one cemented carbide ring cast into a casting alloy comprising an essentially graphitic cast iron, characterised in that the cast iron contains >40 vol-% pearlite and with the following composition in addition to Fe (in wt-%) Si 1.9 - 2.8 Mg 0.02 - 0.10 Ni 0.5 - 4 Mn 0.1 - 1 Cu <2 Mo ≤1 and with C eqv 2.5 - 6 where C eqv = %C + 0.3·(%Si + %P). Roll according to the preceding claim characterised in that the cast iron has the following composition in addition to Fe (in wt-%) Si 2.1 - 2.5 Mg 0.04 - 0.07 Ni 1 - 2 Mn 0.6 - 0.7 Cu 0.01 - 1 and with C eqv 3.5 - 5

Inventors: KARLSSON JAN-ERIK, MADERUD KARL-JOHAN, KARLSSON, JAN-ERIK, MADERUD, KARL-JOHAN
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1996
Main classification: C22C37
IPC codes: B21B27, C22C37, B22F7