Continuous casting rolls and method of using

Abstract

A roll for use in a continuous casting machine includes a roll body that is constructed and arranged to be mounted for rotation in a guide segment area of a continuous casting machine, and a generally cylindrical outer surface on the roll body. Preferably, the outer surface is fabricated at least in part from an age-hardenable nickel-based alloy that includes, in weight percent, up to 0.3% carbon, about 5% to about 12% chromium, and about 10% to about 30% molybdenum. The roll exhibits superior high temperature strength and stability, low thermal expansion, and superior oxidation resistance at temperatures that are typical during operation in a continuous casting machine. A process of making a metal casting using such a roll is also disclosed.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the field of continuous casting, and in particular to continuous casting rolls that are used to guide a newly-formed cast strand after it emerges from a continuous casting mold.

[0004] 2. Description of the Related Technology

[0005] Metals such as steel are continuously cast into strands by pouring hot, molten metal into the upper end of a mold and continuously withdrawing a metal strand from the mold's bottom. As the molten metal passes through the mold, the surfaces of the metal that are adjacent to the mold walls are cooled, thus solidifying and hardening the metal to form a casting or shell of solidified metal around the molten metal in the strand. After leaving the bottom of the mold, the metal continues to cool and the casing or shell of solidified metal around the molten core thickens until the whole strand section is solidified.

[0006] The shell of solidified metal around the molten core as the continuous cast strand leaves the mold is relatively thin and fragile and requires support. Such support, in continuous casting of metals, is customarily provided by “guide rolls” or “caster rolls” which engage and support the opposite sides of the continuously cast strand. The supporting rolls immediately below the mold, where the shell of solidified metal is relatively thin, are usually of relatively small diameter and are longitudinally spaced closely together. To assist cooling of the slab and to prevent the rolls and bearings from overheating, these supporting rolls may be liquid cooled. Further away from the mold bottom, where the metal has cooled and the shell of solid metal has thickened, rolls of larger diameter, spaced at greater longitudinal distance, are usually employed. To control the casting speed, certain of the supporting and guiding rolls may be driven. Typically, of course, the supporting rolls are arranged about an arcuate path or apron that defines the path of the strand as it emerges vertically downwardly from the mold, then gradually bends about a 90 degree arc until it emerges as a fully solidified, horizontally oriented casting. Space is generally provided between the rolls for permitting introduction of spray water to cool the cast strand.

[0007] Caster rolls are required to operate in a severe hostile service environment which subjects the rolls to cyclic thermal shock, thermal and mechanical bending stresses and elevated temperature abrasion. The temperature of the solidifying slab at the first caster roll may exceed 1200 degrees C. Also, the solidifying slab is rapidly quenched as it moves between the caster rolls, producing rapid thermal excursions in the rolls and superheated steam. All of these actions are occurring while the roll is in a highly corrosive aqueous environment of the cooling water which contains acids and/or bases from the flux and water treatment chemicals, most notably fluorine and fluoride solutions. Moreover, while the roll is in contact with the solidifying slab, the roll is elongating at that contact position such that the rolls are continuously undergoing tension-compression cycles.

[0008] To extend caster roll life in this severe service environment, the working surface of caster roll bodies are generally weld surfaced or cladded with alloys having superior resistance to the harmful effects of the service environment as compared to the base alloy steel roll material. Thus, the roll is a composite or bimetallic structure. Due to the narrow tolerances required to control slab thickness and solidification, the roll diameter must not vary (due to wear or other factors) more than about 0.040 in. When wear, thermal shock or corrosion produces even a small change in the roll surface, the casting operation must shut down. The assembly of rolls (typically 8 to 16 rolls in a segment) is then replaced and the affected rolls are re-machined and/or re-weld surfaced.

[0009] Surfacing of caster rolls has been performed with nickel base, cobalt base, iron base and stainless steel type alloys. Most typically, the cladding is martensitic stainless steel. As will be appreciated by those skilled in the art, the microstructure of stainless steel alloys and other materials is adversely affected by the heat input and cooling rate associated with surface cladding processes. This results in microstructure change, segregation and zones of deposit non-uniformity that affect service performance.

[0010] A need exists for an improved continuous casting roll that exhibits high temperature strength, and stability, low thermal expansion, and superior oxidation resistance at temperatures that are typical during operation in a continuous casting machine. A further need exists for such a roll to exhibit superior resistance to degradation in high temperature fluorine and fluoride environments.

SUMMARY OF THE INVENTION

[0011] Accordingly, it is an object of the invention to provide an improved continuous casting roll that exhibits high temperature strength and stability, low thermal expansion, and superior oxidation resistance at temperatures that are typical during operation in a continuous casting machine.

[0012] It is further an object of the invention to provide an improved continuous casting roll that exhibits superior resistance to degradation in high temperature fluorine and fluoride environments.

[0013] In order to achieve the above and other objects of the invention, a roll for use in a continuous casting machine includes, according to a first aspect of the invention, a roll body that is constructed and arranged to be mounted for rotation; and a generally cylindrical outer surface on the roll body, the outer surface comprising an age-hardenable nickel-based alloy that includes, in weight percent, up to 0.3% carbon, about 5% to about 12% chromium, and about 10% to about 30% molybdenum.

[0014] According to a second aspect of the invention, a process of making a metal casting includes steps of (a) introducing molten metal into a first end of continuous casting mold; (b) withdrawing a newly-formed casting from a second end of the continuous casting mold; and (c) guiding the newly formed casting with a plurality of continuous casting rolls, and wherein at least one of the rolls has an outer surface comprising an age-hardenable nickel-based alloy that includes, in weight percent, up to 0.3% carbon, about 5% to about 12% chromium, and about 10% to about 30% molybdenum.

[0015] According to a third aspect of the invention, a roll for use in a continuous casting machine includes a roll body that is constructed and arranged to be mounted for rotation; and

[0016] a generally cylindrical outer surface on the roll body, the outer surface including an age-hardenable Ni—Mo—Cr alloy in which the weight percent of Mo is greater than the weight percent of Cr.

[0017] According to a fourth aspect of the invention, a process of reconditioning a roll for a continuous casting machine includes steps of: (a) removing a continuous casting roll from a continuous casting machine; (b) stripping degraded material from a surface of the roll; (c) applying a new outer surface to the roll, said outer surface comprising an age-hardenable Ni—Mo—Cr alloy in which the weight percent of Mo is greater than the weight percent of Cr; and (d) installing the roll into a continuous casting machine.

[0018] These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a diagrammatical view of a continuous casting machine according to a preferred embodiment of the invention;

[0020] FIG. 2 is a diagrammatical longitudinal cross-sectional depiction of a roll in a continuous casting machine as shown in FIG. 1; and

[0021] FIGS. 3A and 3B are diagrammatical depictions of a fabrication of a roll as is shown in FIG. 2 according to a preferred process of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0022] Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 1, a continuous casting machine 10 according to a preferred embodiment of the invention includes a continuous casting mold 12 and a segment support section 14, as is typical in continuous casting machines. As is depicted in FIG. 1, continuous casting mold 12 has a first end 16 into which molten metal 19 is introduced during operation, and a second end 18 through which the thin-walled, newly formed casting or strand 20 emerges as the continuous casting machine 10 operates.

[0023] As is further shown in FIG. 1, the segment support section 14 includes, as is typical, a plurality of support rolls 22. As is better shown in FIG. 2, at least one of the rolls 22, and preferably most or all of the rolls 22 include a roll body 23 that may have journals 24 or other bearing elements thereon to support the roll 22 for rotation within the segment support 14, and a sleeve 26, which forms a protective layer about the base of the roll body 23 and defines an outer surface 28, which, according to one particularly advantageous aspect of the invention, is fabricated from an age-hardenable nickel-based alloy comprising, in weight percent, up to 0.3% carbon, about 5% to about 12% chromium, and about 10% to about 30% molybdenum. In the embodiment shown in FIG. 2, the age-hardenable material is a sleeve, but it could alternatively be applied in one of many conventional cladding processes, including welding, as is shown schematically in FIGS. 3A and 3B. In this type of process, the material 27 is applied, usually helically, about the roll body 23 until the entire outer surface that is designed for contact with the hot casting is formed together with roll body 23 into a completed roll 30. Alternatively, the material 27 could be applied by other known deposition techniques, including, without limitation, thermal spray techniques.

[0024] It is anticipated that the material 27 will often be applied to roll body 23 in a reconditioning process wherein degraded material is first stripped from the roll body, and then one of the techniques discussed above will be used to re-apply a new outer surface to the roll.

[0025] It has been found that this material gives the roll 22 high temperature strength and stability, low thermal expansion, and superior oxidation resistance at temperatures that are typical during operation in a continuous casting machine.

[0026] The alloy has unique long-range ordering characteristics. It has excellent ordering characteristics after an aging time of only 24 hours. The alloy has low thermal expansion characteristics with high impact strength after long-term aging. The alloy is not notch sensitive in notched rupture tests. The alloy does not require a coating to resist long-term thermal damage, i.e., oxidation. The excellent engineering properties of the alloy are provided by the close control of composition and especially the critical molybdenum plus tungsten to chromium ratio. As indicated in Table 1, the ratio of Mo+W:Cr must be between 2:1 and 7:1, or preferably between 2:1 and 6:1. In this invention there is a minor addition of chromium to a nickel-molybdenum base.

TABLE 1
PREFERRED ALLOY
COMPOSITION	BROAD	NARROW
WEIGHT PERCENT	RANGE	RANGE	TYPICAL
C	UP TO .3	0.02-0.06	ABOUT 0.04
Cr	5-12	7-9	ABOUT 8
Mo	10-30	24-26	ABOUT 25
Mo + W	22-40	22-40	ABOUT 25
Al	1.0 MAX	0.5 MAX	ABOUT 0.2
B	TRACE TO .015	.002-.006	ABOUT .003
Fe	5 MAX	2.0 MAX	ABOUT 1.0
Mn	2 MAX	0.8 MAX	ABOUT 0.5
Si	1.2 MAX	0.8 MAX	ABOUT 0.4
Re	0.1 MAX	0.07 MAX	ABOUT 0.03
Ni	BALANCE	BALANCE	BALANCE
RATIO (Mo + W) Cr	2-7.0 to one	2-6 to one	ABOUT 3 to
			one

[0027] It is well known in the art that molybdenum and tungsten are interchangeable in many alloy systems. In the alloy of this invention, these elements may be interchanged. Because of the lower cost of molybdenum and the high weight and metal working characteristics of tungsten, molybdenum is preferred. Thus, molybdenum may be present in the alloy of this invention at not less than 10% for optimum economic and technical benefits. It is well known in the art that a composition adjustment must be made because of the difference in the atomic weights of these elements, defined as about Mo=½W. For example, to obtain the equivalent of 25% molybdenum, it is necessary to have 10% molybdenum and 30% tungsten. Because of the possible interchange, molybdenum plus tungsten may total 22 to 40% in the alloy of this invention.

[0028] Boron may be present in the alloy of this invention in a small, but effective trace content up to about 0.015% to obtain certain benefits as is known in the art. Other elements may be present in the alloy of this invention as adventitious impurities or deliberate additions for certain benefits known in the art.

[0029] Some of the “impurities” may be present as residual elements resulting from certain processing steps, or be adventitiously present in the charge materials; for example, calcium, magnesium, vanadium, zirconium and the like. In actual practice, certain impurity elements are kept within established limits with a maximum and/or minimum to obtain uniform products as is well-known in the art and skill of melting and processing these alloys. Sulfur, phosphorous and zinc must generally be kept at low levels. Thus, the alloys of this invention may contain these and other impurities within the limits usually associated with alloys of this class, and as recited in commercial specifications.

[0030] An alloy as described above is commercially available from Haynes International of Kokomo, Ind. as Haynes Alloy 242, and is described in U.S. Pat. No. 4,818,486, the disclosure of which is hereby incorporated by reference as if set forth fully herein.

[0031] In operation, molten metal 19 is introduced into the first end 16 of continuous casting mold 12, and, as a result of the cooling that is caused by the mold, a casting or strand 20 is formed. The strand is withdrawn from the second end 18 of the mold, and is then guided by the rolls 22 as it moves away from the mold 12.

[0032] It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

1. A roll for use in a continuous casting machine, comprising: a roll body that is constructed and arranged to be mounted for rotation; and a generally cylindrical outer surface on said roll body, said outer surface comprising an age-hardenable nickel-based alloy comprising, in weight percent, up to 0.3% carbon, about 5% to about 12% chromium, and about 10% to about 30% molybdenum.

2. A roll according to claim 1, wherein said nickel based alloy comprises about 7% to about 9% chromium.

3. A roll according to claim 1, wherein said nickel based alloy comprises about 10% to about 26% molybdenum.

4. A roll according to claim 1, wherein said nickel-based alloy consists essentially of up to 0.3% carbon, 5% to 12% chromium, 10% to 30% molybdenum, 22% to 40% molybdenum plus tungsten, 1% maximum aluminum, trace to 0.015% boron, 5% maximum iron, 2% maximum manganese, 1.2% maximum silicon, 0.1% maximum rare earth metals, balance nickel plus normal impurities wherein the ratio of Mo+W:Cr is between 2:1 and 7:1 to provide a favorable combination of properties and wherein the chromium plus molybdenum content exceeds 3:1 to obtain optimum ordering characteristics.

5. A roll according to claim 1, wherein said roll body comprises a sleeve, and said outer surface is defined on said sleeve.

6. A roll according to claim 1, wherein said nickel-based alloy is deposited on said outer surface by a cladding process.

7. A process of making a metal casting, comprising steps of: (a) introducing molten metal into a first end of continuous casting mold; (b) withdrawing a newly-formed casting from a second end of the continuous casting mold; and (c) guiding the newly formed casting with a plurality of continuous casting rolls, and wherein at least one of the rolls has an outer surface that includes an age-hardenable nickel-based alloy comprising, in weight percent, up to 0.3% carbon, about 5% to about 12% chromium, and about 10% to about 30% molybdenum.

8. A process according to claim 7, wherein step (c) is performed with a nickel based alloy that comprises about 7% to about 9% chromium.

9. A process according to claim 7, wherein step (c) is performed with a nickel based alloy that comprises about 10% to about 26% molybdenum.

10. A process according to claim 7, wherein step (c) is performed with roll with a nickel-based alloy that consists essentially of up to 0.3% carbon, 5% to 12% chromium, 10% to 30% molybdenum, 22% to 40% molybdenum plus tungsten, 1% maximum aluminum, trace to 0.015% boron, 5% maximum iron, 2% maximum manganese, 1.2% maximum silicon, 0.1% maximum rare earth metals, balance nickel plus normal impurities wherein the ratio of Mo+W:Cr is between 2:1 and 7:1 to provide a favorable combination of properties and wherein the chromium plus molybdenum content exceeds 3:1 to obtain optimum ordering characteristics.

11. A roll for use in a continuous casting machine, comprising: a roll body that is constructed and arranged to be mounted for rotation; and a generally cylindrical outer surface on said roll body, said outer surface comprising an age-hardenable Ni—Mo—Cr alloy in which the weight percent of Mo is greater than the weight percent of Cr.

12. A roll according to claim 11, wherein said roll body comprises a sleeve, and said outer surface is defined on said sleeve.

13. A roll according to claim 11, wherein said age-hardenable Ni—Mo—Cr alloy is deposited on said cuter surface by a cladding process.

14. A process of reconditioning a roll for a continuous casting machine, comprising steps of: (a) removing a continuous casting roll from a continuous casting machine; (b) stripping degraded material from a surface of the roll; (c) applying a new outer surface to the roll, said outer surface comprising an age-hardenable Ni—Mo—Cr alloy in which the weight percent of Mo is greater than the weight percent of Cr; and (d) installing the roll into a continuous casting machine.

15. A process according to claim 14, wherein step (c) comprises applying the alloy by cladding.

16. A process according to claim 14, wherein step (c) comprises applying the alloy by securing a sleeve to the roll.

17. A process according to claim 14, wherein step (c) comprises applying the alloy by thermal spray deposition.