Metal treatment – Stock – Titanium – zirconium – or hafnium base
Patent
1997-02-07
2000-05-16
Sheehan, John
Metal treatment
Stock
Titanium, zirconium, or hafnium base
148669, 420417, 420421, C22C 1400
Patent
active
060632119
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a high strength, high ductility titanium alloy and a process for producing the same. The present invention relates, in more detail, to a high strength, high ductility titanium alloy containing no alloying elements which increase the production cost, such as Al, V and Mo, and having a tensile strength as high as at least 700 MPa, preferably at least 850 MPa, particularly preferably at least 900 MPa and an elongation as high as at least 15%, preferably at least 20%, and a process for producing the same.
BACKGROUND ART
(.alpha.+.beta.)-alloys and .beta.-alloys containing Al, V, Zr, Sn, Cr, Mo, and the like have heretofore been known as high strength titanium alloys. In general, these conventional alloys have a tensile strength of at least 900 MPa, and there are few titanium alloys having a strength level between that of pure titanium and that of the conventional alloys, namely from about 700 to 900 MPa.
For example, Ti--6Al--4V alloy is a typical alloy of the (.alpha.+.beta.)-alloys, and has a tensile strength of 850 to 1,000 MPa and an elongation of 10 to 15% in an annealed state. There is Ti--3Al--2.5V alloy which has a strength level lower than the alloy mentioned above, and which has a tensile strength of 700 to 800 MPa and is excellent in ductility.
However, since these alloys contain V which is a high cost alloying element, they have the disadvantage that their cost is high.
Accordingly, the alloys mentioned below have been proposed in which V, a high cost alloying element, is replaced with Fe, a low cost element: Ti--5Al--2.5Fe alloy ("Titanium Science and Technology," Deutche Gesellshaft fur Metallkunde E.V. p1335 (1984)), and Ti--6Al--1.7Fe--0.1Si alloy and Ti--6.5Al--1.3Fe alloy (Advanced Material & Processes, p43 (1993)).
However, the above alloys which have been proposed contain a large amount of Al, and have high strength and low ductility at high temperature. The alloys have, therefore, poor hot workability compared with pure Ti. These alloys have the problem that the hot working cost is still high though the raw material cost is lowered by replacing V with Fe.
Accordingly, an alloy has been proposed which contains neither Al nor V and which utilizes O (oxygen) and N (nitrogen) as interstitial strengthening elements. For example, Japanese Patent Kokai Publication No. 61-159563 discloses a process for producing a pure Ti forged material having a tensile strength at the level of 80 kgf/mm.sup.2 class and an elongation of at least 20% which process comprises rough forging at high temperature including upsetting forging, finish forging, and heat treating at temperature of 500 to 700.degree. C. for up to 60 minutes. The process, however, requires complicated forging such as upsetting forging and heavy deformation, and it cannot be adopted in general.
Japanese Patent Kokai Publication No. 1-252747 discloses a high strength titanium alloy excellent in ductility which requires no specific forming, and which can be formed into products having various shapes such as sheets and rods by conventional rolling. The titanium alloy disclosed herein contains O, N and Fe as strengthening elements. The contents of these strengthening elements are defined as follows: the Fe content is from 0.1 to 0.8% by weight, and the oxygen equivalent value Q, which is defined to be equal to [O]+2.77[N]+0.1[Fe], is from 0.35 to 1.0. The N content is defined to be practically at least 0.05% by weight as disclosed in examples, and the titanium alloy is made to have fine microstructure in the (.alpha.+.beta.) dual and equiaxed phase or lamellar layers. As a result, the titanium alloy has a tensile strength of at least 65 kgf/mm.sup.2.
The disclosed titanium alloy attains a tensile strength of at least 65 kgf/mm.sup.2 and an elongation of at least 20% by solid solution strengthening with O and N, and by microstructural grain refining effects obtained by utilizing an Fe content higher than that of pure titanium, and it attains a tensile strength of at least 85 kgf/mm.sup.2 particula
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Liu et al, "Effects of Oxygen and Heat Treatment on the Mechanical Properties of Alpha and Beta Titanium Alloys", Metallurgical Transactions A, vol. 19A, Mar. 1988, 527-542, XP 002041099.
Donachi, "Titanium Groupings", Titanium, Technical Guide, 1988, 28, 14, 157-172, XP002041100.
Fujii Hideki
Hanaki Michio
Okano Hiroyuki
Soeda Seiichi
Nippon Steel Corporation
Sheehan John
Toho Titanium Co., Ltd.
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