EP0469803A1

EP0469803A1 - Water-cooled rolls

Info

Publication number: EP0469803A1
Application number: EP91306868A
Authority: EP
Inventors: Hideo C/O Union Carbide Services K.K. Nitta; Mamoru C/O Union Carbide Services K.K. Hisada
Original assignee: Union Carbide Coatings Service Technology Corp; Praxair ST Technology Inc; Union Carbide Coatings Service Corp
Current assignee: Praxair ST Technology Inc
Priority date: 1990-07-27
Filing date: 1991-07-26
Publication date: 1992-02-05
Also published as: JPH0437256U; KR920002242A; KR960006015B1; CA2047519A1; CA2047519C

Abstract

A water-cooled roll for cooling steel sheets is spray-coated on the surface that comes in contact with the steel sheets with a cermet composed of a metal oxide having a higher hardness and lower thermal conductivity than metals and a metal matrix of Ni- or Co-base heat-resisting alloy.

Description

This invention relates to a technique for improving the surface quality of water-cooled rolls to be used for cooling steel sheets in heat-treating furnaces.
Continuous annealing furnaces for steel sheets are provided with a quenching zone to help produce well-aged cold-rolled sheets or the like. One method of cooling in that zone uses water-cooled rolls.
The concept of the roll cooling is illustrated by Figure 1 of the accompanying drawings, the Figure 1 being a schematic view of a roll type cooling arrangement. An array of internally water-cooled metal rolls 1 cools a steel sheet 2 as the latter passes in direct contact with the rolls, under control such that the work is cooled down to a given finish temperature.
The water-cooled rolls hitherto used have been metal rolls. The metal rolls however have not been fully satisfactory. As one example, they have questionable durability to cope with the heat cycles involving contact with high-temperature steel sheets and internal water cooling. As another example, they are not quite resistant to the surface wear due to friction with the steel sheets usually conveyed under tension ranging from about 0.5 to about 3 kg/cm².
In view of these problems, it has already been proposed to reinforce the water-cooled rolls with metal carbide coatings (Japanese Utility Model Application Publication No.19317/1988) or metal oxide coatings (Japanese Patent Application Public Disclosure No.136634/1986).
However, metal carbide coatings have high thermal conductivity values, and the non-uniformity of surface roughness has a material bearing upon the irregularity of thermal conductivity on contact in the width direction of the rolls. This effect can result in an uneven rate of cooling in the width direction of steel sheets.
It was to eliminate this disadvantage that spray coating with metal oxides was proposed. The metal oxides are low enough in thermal conductivity to prevent the non-uniformity of surface roughness from influencing the evenness of the cooling rate in the width direction of steel sheets. The metal oxide coatings, however, exhibit such poor peeling resistance under service conditions that they require a double-layer bonding coat about 200µm thick. If desirable effects are to be achieved, the sprayed metal oxide coating itself must have a thickness of at least 200µm.
With the view to overcoming the problems of the prior art, the present invention provides the application of a cermet sprayed coating of a metal oxide and a heat-resisting metal or heat-resisting alloy matrix to water-cooled rolls of the character described above.
According to the present invention there is provided a water-cooled roll for cooling steel sheets wherein the roll surface that comes in contact with the steel sheets is spray-coated with a cermet composed of a metal oxide having a higher hardness and lower thermal conductivity than metals and a metal matrix of Ni- or Co-base heat-resisting alloy. The metal oxide may be chosen from among Al₂0₃, Cr₂0₃, Si0₂, and Zr0₂, and the metal matrix may comprise an MCrAlY (M = Co or Ni). More preferably, the metal oxide is alumina and the metal matrix, CoCrAlYTa.
The metal roll may be built of any known material usually used for the purposes, e.g., carbon steel or heat-resisting cast steel.
The sprayed coating can easily be formed by any known spraying technique, which involves spraying the materials, a metal oxide powder and a heat-resisting metal or alloy powder, onto a metal roll surface.
Useful metal oxides for use in the present invention include alumina, chromia, zirconia, and silica. Alumina is preferred because of its superior resistance to heat and wear.
Among the metal matrix materials which are used in the present invention are Ni- and Co-base heat-resisting alloys. The high heat resistance and good binding properties with respect to the substrate make CoCrYTa and CoCrAlYTa particularly suitable.
The ratio of the metal oxide to the matrix ranges from 10:90 to 70:30, preferably from 30:70 to 60:40. A ratio chosen from this range permits the formation of a coating with an appropriately selected thermal conductivity and excellent exfoliation resistance. Thus, the uniformity of the thermal conductivity in the width direction of the roll can be secured.
Table 1 lists desirable examples of spray material compositions according to the present invention.
The sprayed coating formed in conformity with the invention is so adherent to the substrate that a bonding coat is not always necessary. Where necessary, a single-layer coat as thin as 30 µm or less in thickness is satisfactory.
The present invention will now be further described with reference to and as illustrated in the accompanying drawings, wherein

Fig.1 is a schematic view of a roll type cooling arrangement (and has been described hereinabove);
Fig.2 is a diagrammatic view of a cooling roll embodying the present invention; and
Fig.3 is a fragmentary sectional view, on an enlarged scale, of the roll shown in Fig.2.

Referring to the accompanying drawings, Figs. 2 and 3 illustrate the construction of a water-cooled roll 10 according to the present invention. As shown, a conventional metal roll 11 which is cooled inside with water has a sprayed cermet coating 12 on the surface. The sprayed coating 12, as shown in Fig.3, comprises metal oxide particles 13 dispersed in a matrix 14 of a heat-resisting metal or alloy.
The invention will now be further illustrated by the following examples.

EXAMPLES

Coating materials of the compositions shown in Table 1 were prepared and applied to steel rolls by spray coating to form coatings about 50 µm thick.
These specimens were tested for their resistance to thermal shock. The thermal shock resistance was evaluated in terms of the number of thermal shock cycles, (each of which consisting of holding each test specimen at 900°C for 20 minutes and then placing it into water at 20°C), that the specimen withstood until its coating was peeled off.
The results are summarized in Table 2.
As can be seen from Table 2, the use of a metal matrix markedly improves the adhesion of the resulting coating to the substrate over the coating of the metal oxide alone, making the coating more stable against thermal shocks.
Next, thermal conductivity values of Specimens 1, 2, 3 of Table 1, CoCrYTa + Al₂O₃ (Specimen 6), and, for comparison purposes, Cr₃C₂ (65%) + Ni-Cr (35%), hard chromium plating, sprayed alumina coating (Specimen 5), and NiCoCrAlY + Cr₂O₃ 10% were determined, in cal/cm.sec°C. Table 3 gives the results.
As Table 3 indicates, the cermet type sprayed coatings have considerably low thermal conductivity values compared with the metal types and attain a soaking effect. Consequently, the former can accomplish uniform cooling regardless of variations in the coarseness of roller surface and in the thickness of coating.
Specimens 1 and 6 were further tested for wear resistance. The test was carried out by subjecting each specimen to 200 cycles of sliding runs at 1070°C and then measuring the abrasion quantity. By way of comparison, the sprayed coating of Cr₃C₂ + Ni-Cr, a dispersed system rather than an oxide system, was likewise tested. Table 4 shows the results.
As is clear from Table 4, the abrasion quantities indicate that the coatings composed of an oxide and a metal matrix were outstandingly resistant to wear.
As will be understood from the foregoing, the adjustment of the metal oxide content in a metal matrix makes it possible to choose a proper thermal conductivity and secure the uniformity of thermal conductivity in the width direction of the roll.
The spray coating has only to form a single layer rather than two over a water-cooled roll, and the sprayed coating may be as thin as 30 µm thick, thus making for a reduction of the spraying cost.
The sprayed cermet coatings according to the invention are superior in high-temperature wear resistance to the metal carbide systems and exhibit greater thermal shock resistance than metal oxide coatings.

Claims

A water-cooled roll for cooling a steel sheet which comprises a water-cooled roll body and a sprayed cermet coating formed on the roll surface that comes in contact with the steel sheet, the coating being composed of a metal oxide having a higher hardness and lower thermal conductivity than metals and a metal matrix of Ni- or Co-base heat-resisting alloy.
A roll according to claim 1, wherein the metal oxide is chosen from Al₂0₃, Cr₂0₃, Si0₂ and ZrO₂, and the metal matrix comprises an MCrXY, wherein M is cobalt and/or nickel, and wherein X is aluminium or tantalum.
A roll according to claim 2, wherein the metal oxide is alumina and the metal matrix comprises CoCrAlYTa or NiCoCrAlY or CoCrYTa.
A roll according to any of claims 1 to 3, wherein the ratio of metal oxide to matrix is from 10:90 to 70:30.
A roll according to claim 4, wherein the ratio of metal oxide to matrix is from 30:70 to 60:40.