Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-08-23
2003-03-11
Ip, Sikyin (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S663000, C148S578000
Reexamination Certificate
active
06531007
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for the manufacture of a steel product wherein the steel is subjected to isothermal martensite formation and precipitation hardening in a martensitic structure subsequent to soft annealing and shaping. The invention also relates to a steel product obtained with such method and to the use of said steel product.
2. State of the Art
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not necessarily be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
In published international patent application WO93/07303, a method of manufacture of the above mentioned kind has been described wherein the transformation into the martensitic structure is accomplished by air cooling after annealing in the austenitic region or by cold working. Air cooling after annealing normally results in the so-called athermal kinetic mode of martensite transformation. The formation of air cooling induced martensite is suppressed by alloying elements like nickel, titanium and aluminum, which are used for precipitation of hardenable steel. It may be that at relatively high concentrations of such alloying elements the austenite is stabilized such that the martensitic transformation start temperature becomes impracticably low.
SUMMARY OF THE INVENTION
It is an object of the invention to offer a method for the manufacture of steel products, steel products so manufactured and the use of said steel products whereby a practical optimum is achieved between ductility, strength, wear and corrosion resistance, homogeneity of martensite distribution and a practical level of the martensite transformation temperature.
One aspect of the present invention is a method for the manufacture of a steel product comprising the steps of subjecting the steel to precipitation hardening in a martensitic structure subsequent to soft annealing. The steel is then shaped into the desired form followed by solution annealing between a temperature of 1200° C. and 1050° C. and for a time period of from 5 to 30 minutes. From the annealing temperature, the steel is quenched at a rate of at least 5° C. per second to a temperature below 500° C. The quenched steel being subjected to an isothermal martensitic transformation. Hardening of the steel is then accomplished at a temperature of between 450° C. and 550° C. for at least 3 minutes to cause precipitation of particles out of solution into the martensitic structure.
REFERENCES:
patent: 62180040 (1987-08-01), None
patent: 62-180040 (1987-08-01), None
patent: 93/07303 (1993-04-01), None
Holmquist, M., Nilsson, J.-O., and Stigenberg, A. Hultin, Isothermal Formation of Martensite in a 12Cr-9Ni-4Mo Maraging Stainless Steel, Scripta Metallurgica et Materialia, vol. 33, No. 9, pp. 1367-1373, 1995.
Zhuravlev, L.G.; Shteinberg, M.M.; Petsakhov, Yu. B., Stabilization of austerite in alloys with Isothermal and a thermal kinetics of martensitic transformation, IZV. Vyssh. Uchebn. Zaved, Chem. Metall. (1976), (4), 138-40 (abstract only).*
U.S. Patent Application No. 09/713,021, Anna Lena Nyström et al, Nov. 16, 2000.
Blaauw Hubert Sjoerd
Nilsson Jan-Olof
Olsson Sören
Van der Grijn Adriaan
Burns Doane Swecker & Mathis L.L.P.
Ip Sikyin
Sandvik AB
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