Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2002-07-02
2004-12-07
Mai, Ngoclan T. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S238000, C075S246000, C419S011000, C419S029000, C419S032000, C419S048000
Reexamination Certificate
active
06827755
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel ferritic steel having high strength and high toughness, and a method of producing the same.
The ferritic steel of the invention has high durability in corrosive or stress loading environments and is suited for use for the manufacture of power-generating turbine parts, nuclear fuel cladding pipes, automobile mufflers and so on.
2. Description of the Prior Art
Among the ferrous materials, ferritic steel has the advantage not found in austenitic steel that it is resistant to stress corrosion cracking and low in thermal expansion coefficient, so that it is widely used as a material of structural components.
In recent years, there has been an increasing rise of demand for higher performance and smaller weight of products, so that even higher strength of structural materials has been desired. The conventional techniques for strengthening structural materials such as quenching and tempering heat treatment, solid-solution strengthening by an addition of alloying elements and precipitation strengthening had the problem of their tendency to cause deterioration of toughness of the produced material, and low toughness of the material has been a serious restriction on product designing. Recently, the researchers have pursued studies in earnest on grain refinement strengthening known as a material strengthening technique which causes no deterioration of toughness, and now it is possible to obtain a steel material having ultrafine crystal grains with an average grain size of not greater than 1 &mgr;m.
The powder metallurgy method adopting a mechanical grinding process such as mechanical alloying has made it possible to make large scale components, allowed enlargement of the degree of freedom of shaping after consolidation, and enabled refining of crystal grains to the nanometer order by mechanical pulverization, making it possible to obtain a high strength ultrafine grain structure with a grain size of several hundred nanometers depending on the consolidation process.
In order to obtain an ultrafine grain structure, it has been proposed and practiced to introduce dispersed particles which suppress the growth of crystal grains during consolidation. Carbides or oxides are used as dispersed particles, and one example using carbides is disclosed in JP-A-2000-96193. Also, examples using oxides are described in JP-A-2000-104140, JP-A-2000-17370 and JP-A-2000-17405.
JP-A-2000-17405 discloses a method of producing a high strength ultrafine grain steel containing SiO
2
, MnO, TiO
2
, Al
2
O
3
, Cr
2
O
3
, CaO, TaO and Y
2
O
3
. The role of the oxide-forming alloying elements is substantially defined to the supply of dispersed particles, and their amount is limited as excess precipitation results in a deterioration of toughness.
JP-A-2000-17370 describes a method of producing a high strength ultrafine grain steel directly from iron ore or iron sand by powder metallurgy method applying the mechanical alloying technique, and it states that since SiO
2
, Al
2
O
3
, CaO, MgO and TiO
2
in the raw powder are refined by mechanical alloying or finely precipitated during consolidation, it is possible to control the growth of crystal grains while making harmless the otherwise adverse effect of the oxides on mechanical properties of the produced steel.
JP-A-2000-17370 teaches also that it is possible to improve properties by adding one or more elemental powders of Al, Cu, Cr, Hf, Mn, Mo, Nb, Ni, Ta, Ti, V, W and Zr during mechanical alloying, but it is silent on effective amounts of the powders to be added and the properties to be improved.
As the effect of grain refining on toughness, it is known that the ductile-brittle transition temperature (DBTT) is lowered by such refining, and it has been reported that DBTT could be made lower than the liquid nitrogen temperature in the steel material having its crystal grains refined by thermomechanical treatment employing rolling vis-à-vis the material produced by melting/casting. However, with the art of powder metallurgy, it is difficult to attain high toughness simply by refining of crystal grains due to the brittlement factors such as particle boundaries of a starting powder and dispersed particles. Herein, the term “starting powder” means the powder produced by mechanical alloying.
SUMMARY OF THE INVENTION
Object of the Invention
An object of the present invention is to produce a ferritic steel having high strength and high toughness by powder metallurgy method making use of mechanical alloying techniques and to provide a novel ferritic steel having high strength and high toughness.
Statement of the Invention
According to the present invention, at least one compound-forming element selected from the group consisting of Zr, Hf, Ti and V is added when producing a ferritic steel powder by mechanical alloying.
The compound-forming elements are combined with O, C and N originally contained in the ferritic steel powder or getting mixed therein from the atmosphere to form a carbide, an oxide and a nitride, respectively, in the course of consolidation of the ferritic steel powder produced by mechanical alloying. The formed compounds function as pinning particles for controlling the growth of crystal grains to improve toughness of the consolidated ferritic steel.
The invention ferritic steel contains, by weight, not more than 1% Si, not more than 1.25% Mn, 8 to 30% Cr, not more than 0.2% C, not more than 0.2% N, not more than 0.4% O, and a total amount of not more than 12% of at least one compound-forming element selected from the group consisting of Ti, Zr, Hf, V and Nb in amounts of not more than 3% Ti, not more than 6% Zr, not more than 10% Hf, not more than 1.0% V and not more than 2.0% Nb. It may optionally further contain not more than 3% Mo, not more than 4% W and not more than 6% Ni. The balance consists of Fe and unavoidable impurities. The invention ferritic steel has an average crystal grain size of not more than 1 &mgr;m after consolidation.
The compound-forming element contained in the invention ferritic steel is preferably at least one selected from Ti, Zr and Hf, and it is particularly preferable that at least one of Ti, Zr and Hf be contained in amounts of not more than 3% Ti, not more than 6% Zr and not more than 10% Hf for a total amount of not more than 12%.
These compound-forming elements exist in the form of carbide, nitride and oxide in the consolidated ferritic steel.
The total content of O, C and N in the consolidated ferritic steel is a key factor for obtaining a ferritic steel having high strength and high toughness. It is desirable that the total content of O, C and N is not more than 66% by weight of the total content of Zr, Hf and Ti. In the case where Zr and Hf are contained as the compound-forming elements, the total content of O, C and N is preferably not more than 66% by weight of the total content of Zr and Hf.
According to the present invention, there are provided ferritic steels containing any one of Zr, Hf and Ti respectively as the compound-forming element, a ferritic steel containing all of Zr, Hf and Ti, a ferritic steel containing Zr and Hf, and a ferritic steel containing all of Zr, Hf, Ti, V and Nb.
The invention ferritic steel can be produced by encapsulating the steel powder produced by mechanical alloying, and subjecting the encapsulated steel powder to plastic deformation working.
The plastic deformation working is preferably carried out at a temperature of 700° C. to 900° C. The plastic deformation working can be effected by such a method of extrusion or hydrostatic pressing. Extrusion is preferably conducted in an extrusion ratio of 2 to 8, and hydrostatic pressing is preferably performed under a hydrostatic pressure of 190 MPa or higher. Preferably, hydrostatic pressing is followed by forging.
It is also desirable to conduct, after plastic deformation, a heat treatment for heating the work at 600° C. to 900° C. under a hydrostatic pressure of 10 to 1,000 MPa as this treatment contributes to the further enhancement of toughness.
I
Aono Yasuhisa
Fujikura Masakuni
Ishibashi Ryo
Masumoto Hiroki
Sumitomo Hidehiko
Antonelli Terry Stout & Kraus LLP
Mai Ngoclan T.
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