Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-11-13
2004-10-19
Wyszowmierski, George (Department: 1742)
Metal treatment
Stock
Ferrous
C420S109000, C420S124000, C492S058000
Reexamination Certificate
active
06805757
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for the production of alloyed casting material, in particular of material for the working area of indefinite chill rolls, containing the elements carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, and optionally also other elements of group 5 of the periodic system, aluminum, and the remainder iron and impurities related to the manufacturing process.
The invention further relates to a casting material containing the elements listed above.
Finally, the invention includes a composite indefinite chill roll, especially for work rolls for forming flat steel, comprising a work or sleeve part made of a casting alloy with little tendency to adhere or weld to the rolling stock and of a high-strength core part made of low-alloy cast iron, especially of ductile iron.
2. Discussion of Background Information
Tools or machine parts that are exposed to numerous stresses of different types require a special set of properties. For this reason, the most appropriate materials and manufacturing process must be selected for each case with regard to feasibility and economical manufacture as well as the service life of the parts in practical use.
Parts which are exposed to varying temperatures above room temperature, especially for hot forming of workpieces, are made of casting materials in many applications. As a result of such a choice of material, distortion due to local differences in temperature can advantageously be minimized to a great extent, the manufacture of the parts can be made economical, and the material properties can be largely matched to the stresses.
Work rolls used for hot rolling of steel, for example, rolls in wide-strip hot rolling mills, especially in Steckel stands and in the finishing stands, on the one hand, are exposed to high mechanical and thermal stresses and, on the other hand, must exhibit the least possible tendency toward sticking and welding to the rolling stock. Specifically, the thinner the hot strip is rolled, the higher the specific contact pressures become between the work roll and rolling stock, which significantly increases a tendency toward adhesion of the strip to the roll surface, enhanced by the low final roll temperatures of the last stands. This sticking or welding of the roll surface to the strip can lead to tearing of material out of the strip, which in and of itself and through its adhesion to the roll can lead to further rolling defects, which often perforce leads to a reduced quality in the hot rolled strip.
It is known to use an indefinite chill cast material in the working area of the rolls in the last stands of a hot strip rolling mill in order to meet the requirements regarding a reduction of friction in the roll gap and reduction of a tendency of the strip to stick to the roll surface, and also to increase the resistance to stripping and thermal shock damage to the material.
The indefinite chill quality comprises three essentially different structural constituents that are formed as cast, specifically graphite, carbides, and a steel-like matrix. Only the matrix can be significantly altered by heat treatment. The indefinite chill roll quality and/or alloy forms a large amount of carbide and little graphite in the structure when solidification is rapid, and at lower solidification rates the relationship is reversed, i.e., less carbide and more graphite is formed. This has the consequence that rapidly solidified material is harder and slowly solidified material is softer. In an indefinite chill roll, this has the effect that the carbide content drops as the distance from the cast surface increases, the graphite content rises, and the hardness likewise decreases. Since no defined jump in hardness can be observed in this case, this quality has been named “indefinite.”
However, graphite precipitation can degrade the hardness and especially the wear properties of the material, so the microstructure should also contain hard carbides to minimize this disadvantage.
It is known to one skilled in the art to create a cast structure with graphite particles and carbides using alloying methods, where the content of elements that promote graphite formation, primarily nickel and silicon, and the content of carbide formers, primarily small amounts of chromium and molybdenum, as well as the carbon content in the melt must all be coordinated with one another and where their interactions during solidification must also be taken into account.
According to the prior art, indefinite chill rolls have a composition in wt-% of2.6 to 3.6% carbon, 0.6 to 1.1% silicon, 0.6 to 1.0% manganese, 1.5 to 2.1% chromium, 4.1 to 4.6% nickel, 0.3 to 0.5% molybdenum, with the remainder being iron, accompanying elements, and impurities. The structure of the working body or the jacket of a composite roll is primarily composed of a bainitic and/or martensitic matrix with constituents of 28 to 40% of eutectic carbides and 1.3 to 2.2% graphite by volume, with 5 to 20 graphite particles being present per mm
2
of polished surface.
In order to improve the performance characteristics of indefinite chill rolls, especially to increase their wear resistance in the working area, attempts have already been made (PCT/GB 93/02380) to introduce preferable surface-layered carbide particles of greater hardness into the melt provided for this purpose. It is known to practitioners of the art that small amounts of high-hardness carbides increase wear resistance of the material more than the increase produced by typical-quality carbides of lower hardness. If the roll or the roll jacket is made from a melt of this type by means of a centrifugal casting process, undesirable segregation and inhomogeneities can be produced as a result of the centrifugal force and the differences in specific gravity between the carbide particles and the melt. Furthermore, the formation of the necessary graphite can be disrupted by the change in the melt.
The proposal was made according to PCT/US 96/09181 to add 0.3 to 6.0 wt-% Nb to a melt having a balanced composition for indefinite chill rolls and to correspondingly increase the carbon content stoichiometrically with respect to the niobium carbide to be formed. While this process does increase the carbide content and the wear resistance of the material, higher niobium contents can lead to primary formation of carbides which can result in coarsening of the carbide grains and the graphite particles.
During the centrifugal casting of the working area of an indefinite chill roll, the alloy in the mold is subjected to a high centrifugal acceleration during solidification, for example, in the range of 80 to 108 g. Since the monocarbides of vanadium that are primarily formed in the melt have a lower density than the liquid metal, and those of niobium have a higher density, segregation and/or demixing can occur. A proposal has already been made (U.S. Pat. No. 5,738,734) to prevent segregation by alloying the melt equally with vanadium and niobium in such a manner that the monocarbides formed during solidification are mixed carbides (VNb)C and have essentially the same density as the melt. As a result of the highest possible content for the monocarbide-forming elements of 17 wt-% per the above U.S. Patent, the carbon concentration must also be adjusted according to the known relationship. However, such an alloy can exhibit an undesirable solidification structure with localized demixing and large graphite particles, which can produce impaired surface quality of the roll after even a short operating time, and also increases the tendency to stick.
SUMMARY OF THE INVENTION
Based on the prior art, the object of the invention is to specify a new, improved process by means of which the material of the working area of indefinite chill rolls has a significantly reduced tendency to stick or weld to the rolling stock and a consistently high abrasion resistance over the thickness of the area used.
The invention has the additional goal of producing a casting material that has finely disperse
Feistritzer Bernhard
Schröder Karl-Heinrich
Windhager Michael
Ziehenberger Karl-Heinz
Eisenwerk Sulzau-Werfen R. & E. Weinberger AG
Greenblum & Bernstein P.L.C.
Morillo Janelle
Wyszowmierski George
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