Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2002-05-10
2003-11-25
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S243000, C075S246000
Reexamination Certificate
active
06652618
ABSTRACT:
TECHNICAL FIELD
The present invention relates to iron-based mixed powders for powder metallurgy, and particularly to an iron-based mixed powder used for manufacturing high strength sintered parts for automobiles.
BACKGROUND ART
In powder metallurgy, a metal powder is compacted by pressing and then sintered to form a sintered body. Since mechanical parts having a complicated shape can be precisely manufactured, powder metallurgy is widely used for manufacturing automobile parts such as gears, which are required to have high dimensional precision. When an iron powder is used as a metal powder, the iron powder is mixed with a Cu powder, a graphite powder, and so on, and the mixture is compacted, and then sintered to form a sintered body having a density of about 5.0-7.2 g/cm
3
.
The automobile parts are required to have high strength. In order to improve the strength, a sintered body containing alloy elements is heat-treated, namely, quenched and tempered, to manufacture products in general.
For example, in Japanese Examined Patent Application Publication No. 58-10962, an alloy steel powder containing a reduced amount of C, N, Si, Al, and O, at least one prealloyed element selected from a group consisting of Mn, Cr, Mo, and V, and the balance being unavoidable impurities and iron is proposed as a source powder for high strength parts manufactured by powder metallurgy, wherein the alloy steel powder has excellent compressibility, compactibility, and heat-treating properties.
Furthermore, in Japanese Unexamined Patent Application Publication No. 1-215904, partly alloyed steel powder is proposed to manufacture high strength automobile parts, wherein the alloy powder is prepared by diffusing and adhering Cu, Ni, and Mo powders to the surface of the steel powder simultaneously and has a small variation of dimensional change caused by heat treatment.
Furthermore, in order to reduce manufacturing cost, low-temperature sintering performed in a weak-oxidizing atmosphere at low temperature or the elimination of heat treatment after sintering have recently been required. A source powder for making sintered parts having high strength is also required, wherein the sintered parts are manufactured by performing low-temperature sintering or by a combination of performing low-temperature sintering and eliminating heat treatment after sintering.
When a prealloyed steel powder prepared by prealloying iron in a molten state with easily-oxidizable alloy elements such as Cr Mn and so on is sintered in a weak-oxidizing atmosphere, there is a problem in that a sintered body having a desired strength is not obtained because of the oxidation of the prealloyed alloy elements. When a partially alloyed steel powder prepared by partially alloying iron with alloy elements such as Ni, Mo, Cu and so on is used, the problem that alloy elements are oxidized does not arise but the following problem arises: the sintered body does not have a tensile strength of 800 MPa or more because of the insufficient diffusion of the alloy elements when low temperature sintering is performed, since high temperature sintering is necessary for the partially alloyed steel powder to diffuse alloy elements into an iron powder deeply and heat treatment is also necessary for the sintered body to have a high strength.
With regard to the above problem, for example, in PCT Japanese Translation Patent Publication No. 6-510331, an iron-based powder composition substantially consists of 0.5-4.5 mass % Ni, 0.65-2.25 mass % Mo, and 0.35-0.65 mass % C (balance being Fe)is proposed, wherein the iron-based powder composition is used for manufacturing a sintered body having a small variation dimensional change. In the iron-based powder composition, preferably the iron powder is diffusion-alloyed with Ni and/or Mo or prealloyed with Mo to obtain a high-strength sintered product having excellent dimensional stability after sintering.
In Japanese Unexamined Patent Application Publication No. 9-87794, a method for manufacturing a sintered iron alloy is proposed, wherein a mixed powder containing 1-2 mass % Cu, 1-3 mass % Ni, and 0.2-0.7 mass % C after sintering is prepared by mixing Cu, Ni and graphite powders into an alloy steel powder containing 3-5 mass % Ni, and 0.4-0.7 mass % Mo, and the balance being iron, the mixed powder is compacted, and the compact is sintered in a non-oxidizing atmosphere and then cooled at 5-20° C./min. in a sintering furnace.
In a method disclosed in PCT Japanese Translation Patent Publication No. 6-510331, there is a problem in that sufficient strength is not obtained, since a martensitic structure is not formed as a result of the low-temperature sintering. In another method disclosed in Japanese Unexamined Patent Application Publication No.9-87794, there is a problem in that sufficient strength is not obtained, since the density is low because the alloy steel powder has a low compressibility due to the high Ni content.
DISCLOSURE OF INVENTION
According to the above circumstances, it is an object of the present invention to provide an iron-based mixed powder used for manufacturing a high-strength sintered part having a tensile strength of 800 MPa or more, wherein the sintered component is only sintered at a low temperature, and is preferably only sintered at low temperature in a weak oxidizing atmosphere.
In order to achieve the above object, the inventors have diligently researched kinds and a alloying method of alloy elements. As a result, the following finding has been obtained: a sintered body having a martensitic structure including an austenitic phase in which Ni is partly concentrated can be obtained by performing only low-temperature sintering in a weak oxidizing atmosphere without further heat treatment when Ni, Mo, and Cu that are hardly oxidized during sintering are used as alloy elements increasing the strength and the contents of the elements are optimized, wherein Ni is added by both mixing the powders and by prealloying, Mo is added by prealloying, and Cu and graphite are added by mixing the powders. According to the above method, a high-strength sintered component having a tensile strength of 800 MPa or more can be manufactured.
The present invention has been completed according to the above finding and further studies.
The present invention provides an iron-based mixed powder used for high-strength sintered parts and, prepared by mixing an Ni powder, a Cu powder, and a graphite powder into an alloy steel powder, wherein the iron-based mixed powder contains 1-5 mass % of the Ni powder, 0.5-3 mass % of the Cu powder, 0.2-0.9 mass % of the graphite powder to the total of the alloy steel powder, the Ni powder, the Cu powder, and the graphite powder, wherein the alloy steel powder contains 0.5-3 mass % of prealloyed Ni, more than 0.7 to 4 mass % of prealloyed Mo, the balance being Fe and unavoidable impurities. In the iron-based mixed powder of the present invention, the alloy steel powder may contain 0.5-3 mass % of prealloyed Ni, more than 0.7 to 4 mass % of prealloyed Mo, 0.2-0.7 mass % of prealloyed Cu, and the balance being Fe and unavoidable impurities.
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, Ni, Mo, and Cu are used as alloy elements for increasing the strength. These elements are not oxidized during sintering in a weak oxidizing atmosphere such as a generally-used low-cost RX gas (hydrocarbon conversion gas) atmosphere and the elements increase the strength effectively.
An iron-based mixed powder of the present invention is prepared by mixing an alloy steel powder with Ni, Cu, and graphite powders. In the present invention, Ni is added both by mixing powders and by prealloying in terms of the acceleration of sintering by the Ni powder, the formation of a retained austenite phase, and the martensitic transformation of the matrix. Mo is added by prealloying. Cu is added by mixing powders mainly in order to accelerate sintering by liquid-phase sintering of Cu, and may be additionally added by prealloying.
The alloy steel powder contains a prealloyed steel powder in which Ni and
Uenosono Satoshi
Unami Shigeru
Kawasaki Steel Corporation
Mai Ngoclan
Oliff & Berridge
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