Alloys or metallic compositions – Ferrous – Chromium containing – but less than 9 percent
Reexamination Certificate
2002-10-02
2004-08-10
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Chromium containing, but less than 9 percent
C148S334000, C148S579000
Reexamination Certificate
active
06773662
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of Austrian Patent Application No. 1565/2001, filed on Oct. 3, 2001, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a hot-working steel article, in particular, a tool for forming metals and alloys at elevated temperatures. Components, above all tools, that are stressed at elevated temperatures, e.g., extrusion die matrices, forging tools, die casting dies, extrusion dies, mandrels and the like, require materials which have mechanical properties commensurate with the intended stress at temperatures of, if necessary, 550° C. and higher and which retain these properties during an extended operating time.
2. Discussion of Background Information
By working according to the prior art, however, a high hardness and toughness of the material, a low plastic deformation under extreme stresses, high wear resistance, retention of hardness and good fatigue strength properties of a hot-working steel in the temperature range of above 550° C. cannot be achieved simultaneously to the desired extent by alloying technology methods. At given thermal and mechanical strains, the chemical composition and thermal treatment of an article (e.g., tool) should thus be selected such that the profile of the material properties achievable thereby comes as close as possible to meeting all requirements, although a shorter service life of the article often has to be accepted in this case.
For a long time materials science has been faced with the problem of improving the long-term use properties at elevated temperature of articles made of hot-working steel and of providing an alloy with which a high toughness of the material in combination with a high degree of hardness thereof can be achieved by a thermal treatment, so that the risk of fracture, even with shock like strain of a piece, and the plastic deformation and wear are minimized thereby. In that regard, the retention of hardness and the heat conductivity of the material also are to be taken into consideration.
One skilled in the art is aware that retention of hardness and unchanged maintenance of the mechanical properties at elevated temperature of a thermally treated steel article are accomplished by special carbides that can develop at carbon concentrations in the range of 0.5% by weight and with chromium contents of 3 to 5% by weight Cr of the alloy, with molybdenum-tungsten and vanadium contents further increasing the heat resistance thereof. The common hot-working steels essentially have contents, in % by weight, of 0.35 to 0.665 C, 2.0 to 7.0 Cr, 1.5 to 8.0 Mo and/or 1.5 to 18.0 W and 0.4 to 2.0 V, where vanadium can be replaced by higher molybdenum or, in particular, by higher tungsten concentrations.
In order to achieve a high hardenability of the hot-working steel with good retention of hardness and wear resistance even when thermally treating articles of large diameter, EP 0249855 suggests to use a steel composition of essentially, in % by weight, C=0.42 to 0.5, Mn=0.35 to 0.6, Si=0.8 to 1.2, Cr=5.8 to 6.2, Mo=1.85 to 1.95, V=0.7 to 0.9. An improvement in toughness, hardness, strength and wear resistance is achieved with the above alloy composition compared to a steel according to AISI type H 13. However, tempering to a hardness of more than 58 HRC causes a coarse grain formation of the microstructure and disadvantageous losses in toughness.
In order to improve the mechanical high-temperature properties, in particular under cyclical strain, the use of a cobalt-containing hot-working steel produced by powder metallurgy (U.S. Pat. No. 6,015,446) has also been proposed.
It is known from AT 403 058 to use a hot-working steel with increased aluminum contents for tools for the non-cutting heat-shaping of metals and alloys. This steel, although quite suitable for elevated working temperatures, exhibits an embrittlement tendency at hardness values above 58 HRC.
An alloy consisting essentially, in % by weight, of C=0.3 to 0.5, Si<0.9, Mn<1.0, Cr=2.0 to 4.0, Mo=3.5 to 7.0, 0.3 to 1.5 V and/or Ti and/or Nb, Al=0.005 to 0.1 is proposed in EP 0632139 as a material for hot-working tools that must have a heat conductivity of more than 35 W/m K, in order to thereby achieve a lower stress on the tool surface and a flatter temperature gradient within the tool to avoid thermoshock- and tension cracks.
EP 0939140 discloses a hot-working steel consisting essentially of, in % by weight, C=0.25 to 0.79, Cr=1.10 to 7.95, Mo=0.56 to 3.49, V=0.26 to 1.48, Fe=balance. In the above alloy the contaminants and accompanying elements are restricted to improve the properties of the formed material at high temperatures. High values for the high-temperature strength, the high-temperature toughness and the high-temperature wear resistance are achieved with this measure after a thermal treatment of the material to a hardness of lower than HRC=56, but a strong scattering of the values of the respective mechanical properties was detected at high temperature after thermal treatment to a hardness of the article of higher than 58 HRC.
A powder metallurgically produced hot-working steel that is characterized by a content of 1.5 to 2.5% by volume of carbides of the MC type is known from WO 00/26427. At hardness values above 58 to 59 HRC, at which tools are increasingly to be provided for cold forming, higher as well as lower MC contents than 2.5 to 1.5% by volume have a detrimental effect on the flexural impact strength.
The disclosures of all documents mentioned above are hereby expressly incorporated by reference herein in their entireties.
It would be desirable to eliminate the shortcomings in the above prior art and to provide a hot-working steel article that with high degrees of material hardness and similar strength properties simultaneously ensures toughness values at a considerably higher level and, in combination with good heat conductivity, provides an improved wear resistance at elevated temperatures and an effective prolongation of the service life of the part under intensified and, optionally, shock like stresses.
SUMMARY OF THE INVENTION
The present invention provides a hot-working steel article comprising a material which comprises, in % by weight, 0.451 to 0.598 carbon, 0.11 to 0.29 silicon, 0.11 to 0.39 manganese, 4.21 to 4.98 chromium, 2.81 to 3.29 molybdenum, 0.41 to 0.69 vanadium, with the balance being iron, contaminants and accompanying elements. (Unless stated otherwise, the weight percentages given herein are based on the total composition.)
In one aspect, the ratio C/V of the material is 0.82 to 1.38. In another aspect, the ratio (Cr+Mo+V)/C is 15.2 to 18.4. In yet another aspect, the material comprises less than 0.1% by weight of W. In a still further aspect of the article, the content of carbides which are formed upon solidification of a melt on which the material is based is less than 0.45 vol.-%.
Furthermore, the material may comprise not more than 0.005% by weight of sulfur and/or not more than 0.007% by weight of phosphorus and/or not more than a total of 0.010% by weight of (sulfur+phosphorus) and/or not more than 0.15% by weight of nickel and/or not more than 0.1% by weight of cobalt and/or not more than 0.1% by weight of copper and/or not more than a total of 0.25% by weight of (nickel+cobalt+copper) and/or not more than 0.02% by weight of aluminum and/or not more than 0.001% by weight of magnesium and/or not more than 0.001% by weight of calcium and/or not more than a total of 0.02% by weight of (aluminum+magnesium+calcium). As used herein and unless stated otherwise, the phrase “not more than” in combination with weight percentages includes 0% by weight, i.e., absence of the respective component. Also, it should be understood that the values of the weight percentages given herein are a
Fisher Kay
Schweiger Herbert
Böhler Edelstahl GmbH & Co KG
Greenblum & Bernstein P.L.C.
Yee Deborah
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