JP2005076098A - HIGH-STRENGTH alpha-beta TITANIUM ALLOY - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength α-β titanium alloy which achieves high strength through ageing treatment and shows no deterioration of ductility, even when used at an elevated temperature. <P>SOLUTION: The high-strength α-β titanium alloy comprises 3.0-7.0% Al and 0.08-0.25% C as α stabilizing elements, 0.5-3.5% Cr and 0.3-1.0% Fe as β stabilizing elements, 2.0-3.5% (Mo+0.66V) and the balance being Ti and unavoidable impurities and, if required, may further contain at least one element chosen from the group consisting of ≤5% Sn, ≤5% Zr and ≤0.8% Si. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention exhibits high strength in a practical range, has low deformation resistance at high temperatures and is excellent in hot workability (hot rollability and hot forgeability), or is ductile even if the strength is increased by aging treatment. The present invention relates to a high-strength α-β type titanium alloy that does not deteriorate and does not deteriorate ductility even when used at high temperatures. In addition, this titanium alloy can be widely used in the fields of aircraft, automobiles, ships, etc., taking advantage of its characteristics.

  Since α-β type alloys represented by Ti-6Al-4V alloy are lightweight and have high strength and excellent corrosion resistance, steel materials are used in various fields including aircraft, automobiles, and ships. Practical use is being actively promoted as structural materials and skin materials that will be replaced by

  However, a high-strength titanium alloy has a large deformation resistance in the α-β temperature region, that is, a hot working temperature region, and has poor workability and secondary workability. . For this reason, it is the actual situation that hot working is performed with a sufficient surplus at the expense of product yield by increasing the number of times and the number of heating during hot working. Even if you do it, you are bound by the limit size of applicable press capacity. Also, even when hot working in a rod shape or wire shape, if high speed rolling is adopted, large processing heat is generated due to a large deformation resistance, resulting in a defective structure. It has become a major obstacle.

  On the other hand, in α-β type and β type titanium alloys, it is also adopted to increase the strength by performing an aging treatment, but even if such a treatment is performed, the ductility is not deteriorated. Characteristics will be required. In addition, titanium alloys are required to be used in a high-temperature environment (for example, a temperature range of about 500 ° C.) like an impeller using a titanium material, and ductility does not deteriorate even when used at a high temperature after aging treatment. It is hoped that.

  The present invention has been made under such circumstances, and its purpose is to exhibit high strength in a practical range and low deformation resistance at high temperatures, and hot workability such as hot rollability and hot forgeability. A high-strength α-β type titanium alloy that does not deteriorate ductility even when high strength is achieved by aging treatment, and does not decrease ductility even when used at high temperatures. It is in.

  The high-strength α-β type titanium alloy of the present invention capable of achieving the above-mentioned object is Al: 3.0 to 7.0% and C: 0.08 to 0.25%, β as an α stabilizing element. As stabilizing elements, Cr: 0.5 to 3.5% and Fe: 0.3 to 1.0%, respectively, and (Mo + 0.66V): 2.0 to 3.5%, the balance being It has a gist in that it consists of Ti and inevitable impurities.

  The titanium alloy of the present invention may contain one or more elements selected from the group consisting of Sn: 5% or less, Zr: 5% or less, and Si: 0.8% or less, if necessary. The addition of these elements contributes to improving the heat resistance of the titanium alloy.

  The present invention is configured as described above, and by including appropriate amounts of Al and C as α-stabilizing elements and Cr, Fe, Mo and V as β-stabilizing elements, it exhibits high strength in a practical range and at high temperatures. High deformation strength with low deformation resistance and excellent hot workability, or even when high strength is achieved by aging treatment, ductility does not decrease, and ductility does not decrease even when used at high temperatures An α-β type titanium alloy could be provided.

  In order to achieve the above-mentioned object, the present inventors have made researches focusing on the alloy composition. By the way, a titanium alloy having excellent hot workability has been developed while having a strength comparable to or exceeding that of Ti-6Al-4V alloy in a practical temperature range from room temperature to about 500 ° C. Since its technical significance was recognized, it was filed earlier by the same applicant (Japanese Patent Application No. 2002-257053).

  This titanium alloy contains 3 to 7% Al and 0.08 to 0.25% C as an α stabilizing element, 2.0 to 6.0% Cr as a β stabilizing element, and 0 Fe. Although this alloy is excellent in terms of high-temperature strength and hot workability, it may cause ductile deterioration when subjected to aging treatment or ductility deterioration during high-temperature use. There was a problem that there was.

  The inventors of the present invention have further studied to improve the aging characteristics and ductility of the alloy based on the previously developed titanium alloy. As a result, the present inventors have found that a titanium alloy suitable for the above purpose can be realized by reducing Cr as a β-stabilizing element in the chemical component composition as much as possible and containing appropriate amounts of Mo and V, thereby completing the present invention. .

  In the titanium alloy of the present invention, the reason why the range of the content of each element is determined is as follows. First, the Al content is set to a lower limit for securing the strength equivalent to Ti-6Al-4V, and the upper limit suppresses an increase in deformation resistance and a decrease in hot workability under hot working conditions. It was defined as the allowable limit. That is, when the Al content is less than 3.0%, strength equivalent to Ti-6Al-4V cannot be ensured, and when it exceeds 7.0%, hot workability deteriorates. Also, for the amount of C, a lower limit is defined to ensure the strength equivalent to Ti-6Al-4V, and the upper limit is an allowable value that does not deteriorate hot workability and fatigue characteristics due to a large amount of TiC precipitation. As a limit.

  On the other hand, Cr and Fe are β-stabilizing elements, and these β-stabilizing elements generally increase the strength and deformation resistance, but the transition elements Cr and Fe diffuse at high speed in Ti, so It does not contribute much to strengthening. Therefore, if the addition amount of these elements is appropriately controlled, high strength is ensured in a practical temperature range from room temperature to 500 ° C., and deformation resistance under high temperature processing conditions is small, and excellent hot workability. It is thought that gives. In other words, the lower limit of Cr and Fe is set in order to ensure the strength equivalent to Ti-6Al-4V, and the upper limit value does not increase the deformation resistance during hot working and the β transformation point. Is set as a requirement not to lower the level too much. However, as the Cr content increases, ductility deterioration after aging treatment and ductility deterioration at high temperature use are caused, so the upper limit was set to 3.5%.

  The titanium alloy of the present invention contains Mo and V as essential components. These elements are β-stabilizing elements like Cr, but unlike Cr, they contribute to the development of age-hardening phenomena, ensuring good ductility after age-hardening, and suppressing ductility deterioration during high-temperature use. In order to exert such an effect, it is necessary to contain 2.0% at (Mo + 0.66V), but if it is excessively contained, the transformation point is lowered, so it should be 3.5% or less. The numerical value of “0.66” should be called Mo equivalent, and is a coefficient for making the degree of action of Mo and V uniform.

  The basic chemical component composition in the titanium alloy of the present invention is as described above, and the balance is substantially made of Ti, but it is also effective to contain a predetermined amount of Sn, Zr, Si, etc. if necessary. is there. These elements are effective in further improving the heat resistance of the titanium alloy, but Sn and Zr are each 5 in order not to impede high strength and hot workability in a temperature range from room temperature to 500 ° C. It is better to keep it to about 0.8% or less and Si about 0.8% or less.

  The production conditions of the titanium alloy of the present invention are not particularly limited, but preferred conditions include a titanium alloy satisfying the preferred requirements of the above component composition, after melting in accordance with a conventional method, and then the cast ingot in a temperature range slightly lower than that of the conventional method. Roughing is preferably performed at a processing rate of about 70 to 80% after heating to Tβ to (Tβ + 200 ° C.), more preferably Tβ to (Tβ + 100 ° C.) based on the β transformation point (Tβ) of the titanium alloy. Then, it is a method in which finish rolling is preferably performed at a processing rate of 60% or more in a temperature range of about (Tβ-50 ° C.) to 800 ° C. After the finishing, it is also effective to anneal at about 700 to 800 ° C. for about 120 minutes if necessary. The titanium alloy of the present invention is assumed to be used after aging treatment. The aging treatment conditions may be performed under normal conditions, for example, at 450 to 600 ° C. for 2 to 10 hours. Degree.

  As for the transformation point of titanium alloy, a minimum heating temperature of 850 ° C or higher is secured as a characteristic for enabling stable hot working even when the workpiece temperature drops during hot working. In order to perform hot working, the β transformation point is preferably higher than 850 ° C., more preferably 900 ° C. or higher. By the way, when the β transformation point is 850 ° C or lower, the heating temperature must be relatively low at 850 ° C (β transformation point) or lower to obtain an equiaxed structure, and the workability is remarkably deteriorated in any structure. It becomes easy to cause a crack etc. at the time of processing.

   EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Titanium alloys having various chemical composition shown in Table 1 below were melted by an arc melting method to produce a rod-shaped ingot (120 g) having a diameter of 50 mm × length of 15 mm, and subjected to subsequent processing.

  Each of the above test pieces was heated to 1200 ° C. and then hot-rolled to a thickness of 10 mm. And it hot-rolled 60% at 900 degreeC, and it was set as the shape of 4x50x80 (mm). Next, after heating for 700 × 2 hours → annealing with air cooling, solution treatment is performed by heating at 880 ° C. for 1 hour followed by water quenching (WQ), followed by aging treatment of 500 ° C. for 8 hours → air cooling (AC). did.

  Using the various aging treatment materials, mechanical suitability (yield stress YS, tensile strength TS, elongation El, and drawing RA) at room temperature was measured in accordance with ASTM E8. The results are shown in Table 2 below. Of these, the influence of the [Mo + 0.66V] content in the titanium alloy on the tensile strength TS and elongation El of the titanium alloy is shown in FIG. As is clear from these results, it can be seen that by containing Mo and V at a predetermined ratio, a good elongation El after the aging treatment can be secured.

  In addition to the titanium alloy D shown in Table 1, the Ti-4.5Al-4Cr-0.5Fe-0.2C alloy (titanium alloy I) and the conventional Ti-6Al-4V alloy were the same as in Example 1. The tensile properties (based on ASTM E8) from room temperature to 500 ° C. were measured using the hot-rolled material obtained without aging treatment. The strain rate at this time is 0.2% / min. In addition, each forged material was heated to 700 to 1000 ° C. for 5 minutes in an air atmosphere, and immediately after that, using a high-speed tensile test (Fuji Electric Koki Co., Ltd .: trade name “Cermec Master Z”), strain rate: 20 A high speed tensile test was conducted at% / min to measure tensile properties.

  The result is shown in FIG. In addition, in FIG. 2, the result at the time of using pure titanium (JIS 2 types) as an example of a high-speed tension test was also shown.

  As is clear from this result, the Ti-6Al-4V alloy, which is a typical high-strength titanium, has high strength in a practical temperature range of room temperature to 500 ° C., but has a hot working temperature of 700- Even at a high temperature of 1000 ° C., a considerably high strength is maintained, and since the deformation resistance is large, the hot workability is lacking.

  On the other hand, the titanium alloy D of the present invention and the titanium alloy I developed earlier have higher strength than the conventional Ti-6Al-4V alloy in the practical temperature range from room temperature to 500 ° C., In addition, it can be seen that the deformation resistance in the high temperature range of 800 to 1000 ° C. where hot working is assumed is as low as that of pure titanium, which is easy to work, and is very excellent in hot workability.

  Using the titanium alloy D (material of the present invention) and the titanium alloy I in Example 2, a test piece subjected to hot rolling, annealing and solution treatment in the same manner as in Example 1 was obtained, and subsequently, 500 ° C. × 8 hours → air cooling (AC ) Was examined for tensile properties (yield stress YS, elongation El, drawing RA).

  The result is shown in FIG. As is clear from this result, in the titanium alloy I (Ti-4.5Al-4Cr-0.5Fe-0.2C alloy) containing no Mo or V, ductility (elongation El and drawing RA) is obtained by aging treatment at 500 ° C. However, the ductility of the titanium alloy D of the present invention is not lowered.

  Using titanium alloy D (material of the present invention) in Example 2, a test piece rolled, annealed and solution treated in the same manner as in Example 1 was obtained, and subsequently in the temperature range of 480 to 570 ° C. for 8 hours → air cooling (AC ) Was subjected to tensile properties (tensile strength TS, yield stress YS, elongation El, drawing RA).

  The result is shown in FIG. As is clear from this result, it can be seen that in the titanium alloy D of the present invention containing a predetermined amount of Mo or V, a good strength-ductility balance is obtained by the aging treatment.

It is the graph which showed the influence which [Mo + 0.66V] content in a titanium alloy has on the tensile strength TS and elongation El of a titanium alloy. It is a graph which shows the tensile characteristics in room temperature-high temperature in various titanium alloys. It is a graph which shows the influence which it has on the ductility by aging treatment about each titanium alloy. The aging treatment temperature is a graph showing the effect on tensile properties (tensile strength TS, yield stress YS, elongation El, drawing RA).

Claims (2)

  Al as the α-stabilizing element: 3.0 to 7.0% (in the case of chemical components, mass%, the same applies hereinafter) and C: 0.08 to 0.25%, Cr as the β-stabilizing element: 0.0. 5 to 3.5% and Fe: 0.3 to 1.0%, respectively, and (Mo + 0.66V): 2.0 to 3.5%, with the balance being Ti and inevitable impurities A high-strength α-β type titanium alloy characterized by being.   The high-strength α- according to claim 1, further comprising at least one element selected from the group consisting of Sn: 5% or less, Zr: 5% or less, and Si: 0.8% or less as another element. β-type titanium alloy.

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