Roll for hot rolling

Abstract

A roll for hot-rolling includes a body, wherein at least a part of an envelope surface of the body is made of a high speed steel that with reference to its chemical composition consists of the following elements, in weight %: 1-3 Carbon (C), 3-6 Chromium (Cr), 4.5-7 Molybdenum (Mo), 6-15 Tungsten (W), 3-14 Vanadium (V), 0-10 Cobalt (Co), 0-3 Niobium (Nb), 0-0.5 Nitrogen (N), 0.4-1 Yttrium (Y), eventualy distributed in the powder, and remainder iron (Fe) and unavoidable impurities, wherein contents of molybdenum (Mo) and tungsten (W) satisfy the formula Mo+0.5W=2.0-10.0 weight %.

Claims

1. A roll for hot-rolling comprising a body, wherein at least a part of an envelope surface of said body is made of a high speed steel by consolidation of a powder that with reference to its chemical composition consists of the following elements, in weight %: 1.0-3.0 Carbon (C); 3.0-6.0 Chromium (Cr); 4.5-7.0 Molybdenum (Mo); 6.0-15.0 Tungsten (W); 3.0-14.0 Vanadium (V); 0-10.0 Cobalt (Co); 0-3.0 Niobium (Nb); 0-0.5 Nitrogen (N); 0.4-1.0 Yttrium (Y), evenly distributed in the powder; and remainder iron (Fe) and unavoidable impurities, wherein contents of molybdenum (Mo) and tungsten (W) satisfy the formula Mo+0.5W=2.0-10.0 weight %, wherein the Yttrium (Y) evenly distributed in the powder comprises an oxide scale adhering to the high speed steel.

2. A roll for hot-rolling according to claim 1, wherein said body includes an axially extending core, and an axially extending sleeve arranged radially outside said core.

3. A roll for hot-rolling according to claim 2, wherein said sleeve is made of said high speed steel.

4. A roll for hot-rolling according to claim 2, wherein said sleeve is made of a consolidation of a powder of said high speed steel, which powder is subjected to elevated heat and elevated pressure causing consolidation.

5. A roll for hot-rolling according to claim 2, wherein said core is made of cast steel or cast iron or forged steel.

6. A roll for hot-rolling according to claims 2, wherein a material of said sleeve has carbide particles that have a mean carbide particle size<3.0 m.

7. A roll for hot-rolling according to claim 2, wherein the sleeve has an isotropic microstructure.

8. A roll for hot-rolling according to claim 2, wherein said sleeve is shrink fit on said core.

9. A roll for hot-rolling according to claim 1, wherein the yttrium (Y) content of said high speed steel is less than 0.6 weight %.

10. A roll for hot-rolling according to claim 1, wherein the yttrium (Y) content of said high speed steel is in the range 0.45-0.60 weight %.

11. A roll for hot-rolling according to claim 1, wherein contents of moludbenum (Mo) and tungsten (W) are based on weight % and satisfy formula Mo+0.5W=5.0-8.5 weight %.

12. A roll for hot-rolling according to claim 1, wherein the carbon (C) content of said high speed steel is in the range of from 1.1-1.4 weight %.

13. A roll for hot-rolling according to claim 1, wherein the chromium (Cr) content of said high speed steel is in the range of from 4.0-5.0 weight %.

14. A roll for hot-rolling according to claim 1, wherein the Molybdenum (Mo) content of said high speed steel is in the range of from 4.5-5.5 weight %.

15. A roll for hot-rolling according to claim 1, wherein the tungsten (W) content of said high speed steel is in the range of from 6.0-7.0 weight %.

16. A roll for hot-rolling according to claim 1, wherein the Vanadium (V) content of said high speed steel is in the range of from 3.0-5.0 weight %.

17. The roll for hot-rolling according to claim 1, wherein the oxide scale adhering to the high speed steel is at a saturation thickness.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

(1) The inventive concept will now be further explained using reference figures in connection with attached drawings and graphs, in which

(2) FIG. 1 is a perspective view of a compound roll,

(3) FIG. 2 is a schematic figure of a pin on disc test equipment,

(4) FIG. 3 shows a cross section of a typical groove obtained from a pin on disc evaluation, perpendicular to the longitudinal direction,

(5) FIG. 4 is a diagram showing the groove depth at room temperature and 650 C. for the alloys A, B and C in the pin on disc experiment,

(6) FIG. 5 is a diagram showing the volume loss per meter at 650 C. for the alloys A, B and C in the pin on disc experiment, and

(7) FIG. 6 shows the hardness in HRC for alloy A, B and C.

DETAILED DESCRIPTION

(8) The industrial production of semi-finished products, components and cutting tools based on powder metallurgical high speed steel started 35 years ago. The first powder metallurgical production of high speed steel was based on hot isostatic pressing (HIP) and consolidation of atomized powders. The HIP step was normally followed by hot forging of the HIP'ed billets. This method of production is still the dominating powder metallurgical method to produce high speed steel.

(9) The original objective for research and development on powder metallurgical processing of high speed steel was to improve the functional properties and performance of high speed steel in demanding applications. The main advantages from the powder metallurgical manufacturing process are no segregation with a uniform and isotropic microstructure. The well known problems with coarse and severe carbide segregation in conventional cast steel and forged steel are thus avoided in powder metallurgical high speed steel.

(10) Thus, the powder metallurgical manufacturing method of a high speed steel with sufficient amount of carbon and carbide forming elements results in a dispersed distribution of carbides that to a large extent solves the problem of low strength and toughness associated with conventionally produced high speed steel.

(11) FIG. 1 shows a composite roll 101 for hot-rolling. The roll 101 comprises an axially extending core 102 with an envelope surface 104 formed by an axially extending sleeve 103 arranged radially outside said core 102.

(12) The core 102 is manufactured of a material with good mechanical properties and good heat conductive properties, examples of such materials are ductile iron or steel. The core 102 is a cylindrical journal that comprises at a first end and at a second end means for support bearings. The support bearings allow the working roll to be mounted in the hot rolling mill. Between said first end and said second end is provided a longitudinal region arranged for shrink fitting of the sleeve 103 onto said core 102.

(13) The sleeve 103 is a cylindrical sleeve with an inner diameter that is dimensioned for shrink fitting the sleeve 103 onto said core 102. The wall thickness of the sleeve 103 is dimensioned with respect to heat transfer and work roll lifetime as well as geometrical constraints. In a preferred embodiment of the invention the thickness of the sleeve is 40 millimetres.

(14) According to the present invention, the sleeve 103 is made of a high speed steel that with reference to its chemical composition consists of the following elements: 1-3 wt-% Carbon (C), 3-6 wt-% Chromium (Cr), 0-7 wt-% Molybdenum (Mo), 0-15 wt-% Tungsten (W), 3-14 wt-% Vanadium (V), 0-10 wt-% Cobalt (Co), 0-3 wt-% Niobium (Nb), 0-0.5 wt-% Nitrogen (N), 0.2-1 wt-% Yttrium (Y), and remainder iron (Fe) and unavoidable impurities. It should be pointed out that the elements having a lower limit of 0% are optional and can thus be omitted. The manufacturing of the sleeve 103 comprises of a powder of said high speed steel to form a body from said powder. This forming may for example comprise pouring said powder into a capsule in the form of the sleeve 103; the capsule is then evacuated and sealed. In order to consolidate the powder, the capsule is subjected to heat and pressure in a so called hot isostatic processing (HIP) step.

(15) In an embodiment of the invention, the provision of the powder mixture comprises the step of argon gas-atomisation of molten metal comprising said elements into said powder. In an embodiment of the invention, the argon gas-atomisation of the molten high speed steel causes high speed steel particles of a maximum size of 160 m to be formed.

(16) After the provision of the powder, the sleeve is formed from said powder. This forming may for example comprise pouring said powder into a capsule; the capsule is then evacuated, e.g. by being subjected to a pressure of below 0.004 mbar for 24 hours in order to evacuate said capsule. The capsule is then sealed in order to maintain said pressure in the capsule. The consolidation of the powder is achieved by subjecting the capsule to an elevated temperature, e.g. about 1150 C., and an elevated pressure, e.g. about 1000 bar, for a long period of time, e.g. two hours. This last consolidation step is called hot isostatic pressing, HIP.

(17) A soft annealing step follows the HIP step, preferably the soft annealing step is performed at 900 C. followed by a temperature decrease to 700 C. at a cooling rate of 10 C./hour, from thereon the sleeve is allowed to naturally cool down to room temperature.

(18) After soft annealing the sleeve may be subjected to machining and preferably a hardening (austenizing) step at 1100 C. and three subsequent annealing steps at 560 C. for 60 minutes each, with natural cooling to room temperature there between.

(19) The resulting sleeve from these subsequent steps exhibits a very good uniformity without the aforementioned segregations and coarse carbide structure, and the most important effect is that the yttrium element is evenly distributed in the base-matrix of the high speed steel.

(20) TABLE-US-00001 TABLE 1 Car- bon Chromium Molybdenum Vanadium Tungsten Yttrium (C) (Cr) (Mo) (V) (W) (Y) Alloy wt-% wt-% wt-% wt-% wt-% wt-% A 1.28 4.2 5 3.1 6.4 0.0 B 1.18 4.2 5 3.1 6.4 0.5 C 1.19 4.2 5 3.1 6.4 1 D 1.55 4 0.0 3.5 12 0.5 E 1.05 4 4.5 3.5 0.0 0.5

(21) In order to demonstrate the superior properties of the material of the sleeve 103, a high speed steel was designed without the optional elements, see table 1. The exclusion of the optional elements causes a clear and concise demonstration of the improved high-temperature wear due to the method. A simple evaluation method pin-on-disc for high-temperature wear is described below.

(22) Table 1 shows the elements of the high speed steel used in the experiment. Smelts were produced with the elements in table 1, and from these smelts, powders were produced be means of gas atomization using argon. The powders of alloy B and C in table 1 have a particle size of <160 m, the powder of alloy A has a particle size of <500 m.

(23) In the following description, in order to further illustrate the present invention, a performed non-limiting experiment will be described in detail.

(24) The preparation of samples began with filling of the capsules with powder, with said capsules made from spiral welded tubes with a diameter of 73 mm. The capsules were then exposed to a pressure below 0.004 mbar for 24 hours. The capsules were then sealed in order to maintain said pressure.

(25) In order to consolidate the powder in the capsules a hot isostatic pressing operation was performed at 1150 C. and 1000 bar for 2 hours. The samples were then subjected to a soft annealing step at 900 C. followed by a temperature decrease to 700 C. at a cooling rate of 10 C./hour, from thereon the samples were allowed to naturally cool down to room temperature.

(26) The samples were then machined and heat treated with a hardening (austenizing) step at 1100 C. and three subsequent annealing steps at 560 C. for 60 minutes each, with natural cooling to room temperature there between.

(27) The final preparation step comprised of stepwise grinding and polishing of the samples in an automatic grinder/polisher. During the final polishing step a 1 m diamond suspension was used.

(28) FIG. 2 shows a simplified test set-up used for the tribological testing; this set-up is in the art called pin on disc. The principle for the pin on disc tribological testing is as follows; a sample 1 is rotated around an axis 5 with a speed for a number of revolutions. Simultaneously with the rotation of the sample 1, a force F is applied to a pin 2 that in turn applies the same force F to a ball 3. The ball 3 is made of Al.sub.2O.sub.3 and has a diameter of 6 mm. The rotation of the sample 1 and the force F on the ball 3 causes a groove 6 to be formed in the sample 1.

(29) In order to evaluate the wear behaviour at elevated temperatures the lower part of the pin on disc set-up is accommodated in a furnace 4. Thus, the furnace 4 can heat the sample 1, the ball 3 and the lower part of the pin 2 to the desired operating temperature.

(30) FIG. 3 shows a cross section of the groove 6 perpendicular to the longitudinal direction of the groove 6. The depth d measured from the polished surface of the sample to the bottom of the groove 6 is used as a measure of the wear resistance of the sample. Another figure of the wear resistance is the cross-sectional area 7, which is defined as the cross-sectional area of the groove 6 below the polished surface of the sample 1 perpendicular to the longitudinal direction of the groove 6. The profile and depth d of the groove 6 was estimated using a Veeco Wyko NT9100 white light interferometer.

(31) A series of samples according to the description above were produced and tested according to the pin on disc procedure outlined above. The pin on disc result is presented in FIG. 3. The linear speed in this test was 20 cm/s, the applied force F was 5N and 20N, respectively, and the samples were rotated 20000 revolutions.

(32) As can be seen in FIG. 4, the addition of yttrium caused the depth of the groove to decrease at 650 C.; see alloy A with a groove depth d equal to 5.7 m, alloy B with a groove depth d equal to 1.9 m and alloy C with a groove depth d equal to 3.7 m. This indicates the anticipated increased wear resistance at elevated temperatures for alloys produced by the inventive method. The addition of 0.5% yttrium to the high speed steel (Alloy B) caused a reduction of the groove depth d of roughly three times compared to the high speed steel without yttrium (Alloy A). Also the addition of 1% yttrium to the high speed steel (Alloy C) caused a reduction of the groove depth d at 650 C.

(33) A more representative measure of the wear resistance is the volume loss per meter (mm.sup.3/m). The calculation of the volume loss per meter is performed by integrating the cross sectional area 7 over the longitudinal direction of the track and divide by the circumference of the groove. In FIG. 5, the volume loss per meter is presented; volume loss for alloy A is 4.610.sup.5 mm.sup.3/m, volume loss for alloy B is 1.810.sup.5 mm.sup.3/m and finally the volume loss for alloy C is 410.sup.5 mm.sup.3/m. The relationship between the yttrium content of the high speed steel and the volume loss per meter thereof is illustrated in FIG. 5. From FIG. 5 one can conclude that the yttrium content of 0.5% clearly results in the lowest volume loss per meter. A higher yttrium content than 1% also has a beneficial effect on the volume loss per meter. This relationship implies that the yttrium content of 0.5% gives a superior increase in the implied wear resistance of the high speed steel. It should be noted that examples D and E, though not represented in the figures, also show corresponding positive effects due to the addition of yttrium thereto.

(34) According to the invention, the yttrium content of the high speed steel is within the range 0.2 to 1 weight %. It is preferred that the yttrium content of the high speed steel is more than 0.4 weight %, and less than 0.7 weight, more preferably 0.4 to 0.6 weight %, such as 0.4 to 0.5 weight %, such as 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49 and 0.5.

(35) In FIG. 6, the hardness of the samples is presented. The hardness is 63 HRC for alloy A, the hardness is 57 HRC for alloy B and the hardness is 56 HRC for alloy C. The conclusion from FIG. 6 is that the hardness is reduced with the addition of yttrium. Without wishing to be bound to any specific theory, one possible explanation for this reduction is that less carbon is available in the alloys that contain yttrium, thereby reducing the hardness. This illustrates the theory that the wear rate of the high speed steel, in FIG. 4, at room temperature is primarily dominated by the hardness of the high speed steel. At room temperature the wear rate increases with decreasing hardness. However, at elevated temperatures, other mechanisms are dominating the wear, such as the growth kinetics and the mechanical properties of the oxide scale.