JPH0559509A - Method for heat-treating (alpha+beta) type titanium alloy - Google Patents

Abstract

PURPOSE:To provide an (alpha+beta) type titanium alloy excellent in strength, tough ness and ductility by executing a heat treatment in specified conditions. CONSTITUTION:When the (alpha+beta) type titanium alloy having a compsn. satisfying the conditions of following inequalities (1) to (6) and the balance Ti with inevitable impurities is applied to a solution annealing aging treatment, it is heated to a soln. annealing aging treatment temp. of >= the beta transformation point (Tbeta) and to less than the beta transformation point (Tbeta)+100 deg.C at >=2 deg.C/s heating rate, is held for <=5min and is thereafter cooled at >=2 deg.C/s cooling rate; where it contains 3.0<=Al<=5.0 (1), 2.1<=V<=3.7 (2), 0.85<=Fe<=3.15 (3), 0.85<=Mo<=3.15 (4) and 0.06<=O<=0.20 (5) as well as 80<=20x (Fe)+13x (V)+10x (Mo)<=130 (6).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for an (α + β) type titanium alloy for obtaining excellent strength, toughness and ductility.

[0002]

2. Description of the Related Art Generally, there are (α + β) region heat treatment and β region heat treatment as heat treatment methods for (α + β) type titanium alloys. It is well known that the characteristic of the (α + β) heat treatment is that it is composed of equiaxed α and β transformation structures and is excellent in balance of strength and ductility and fatigue characteristics.

[0003] In addition, solution heat treatment aging treatment is used as a heat treatment for increasing the strength. This is because the solution aging treatment, which is the first heat treatment, is heated and maintained in the (α + β) region below the β transformation point, and then cooled to room temperature at a high speed such as cold water.
The aging treatment, which is the second stage heat treatment, is performed at around 450 ° C to 600 ° C. For example, a heat treatment method for Ti-6Al-4V, which is a typical (α + β) type titanium alloy, is specified in American Standards AMS4911E and 4965E. According to this, after keeping it at 720 ± 15 ° C. for 20 minutes, it is cooled by air, or solution treatment as solution aging treatment is 955 ± 1
Hold at 5 ° C for 1 to 2 hours and then cool with water, and then perform aging treatment at 485 to 6
After holding at 20 ° C. for 4 to 8 hours, it is a method of air cooling.

On the other hand, it is known that the heat treatment in the β region is characterized by having needle-like α and β transformation structures and excellent in fracture toughness and creep characteristics.

However, the β-particle size becomes coarse during heating, and needle-like α is precipitated at the grain boundary, resulting in a large decrease in ductility.

As a method for solving this problem, Japanese Patent Laid-Open No.
In Japanese Patent Laid-Open No. 3-223155, Ti-6Al-4V and Ti are used.
The -6Al-4V-2Sn alloy is heated to the β transformation point or higher and the β transformation point + 150 ° C, and then extruded at an extrusion ratio of 10 or higher, and subsequently cooled at 5 ° C / s or higher and annealed at a temperature of 700 to 850 ° C. Therefore, it is proposed that the structure has fine β grains and that the strength, toughness, and ductility be improved.

[0007]

However, in the above-mentioned technique, a ductile structure cannot be obtained unless the plastic working is performed in the β region, and it can be applied even to the case where the plastic working region such as forging becomes non-uniform. It's not like that.

The present invention is intended to solve the above-mentioned problems, and high strength, high toughness and high ductility can be obtained by heat treatment in the β region without plastic working (α +).
An object of the present invention is to provide a heat treatment method for β) type titanium alloy.

[0009]

In order to achieve the above object, the present inventors have examined the heating rate, temperature, time and cooling rate of liquefaction and the aging temperature, and as a result, the above characteristics can be achieved. I found it.

That is, the heat treatment method according to the present invention is characterized in that the alloy composition having an equiaxed structure as a pre-structure satisfies the following conditions, and when the solution aging treatment is performed on the α + β type titanium alloy, the alloy composition ( (Wt%) is the following (1) to (6)
When solution-aging aging treatment is performed on an (α + β) type titanium alloy having a composition that satisfies the conditions of the formula and the balance consists of Ti and unavoidable impurities, the solution treatment temperature is not less than β transformation point (Tβ) and not less than β transformation point (Tβ). ) It is a heat treatment method for an (α + β) type titanium alloy in which it is heated to less than + 100 ° C. at a heating rate of 2 ° C./s or more, held for 5 minutes or more, and cooled at 2 ° C./s or more.

3.0 ≦ Al ≦ 5.0 (1) 2.1 ≦ V ≦ 3.7 (2) 0.85 ≦ Fe ≦ 3.15 (3) 0.85 ≦ Mo ≦ 3.15 (4) ) 0.06 ≦ O ≦ 0.20 (5) and 80 ≦ 20x (Fe) + 13x (V) + 10x (Mo) ≦ 130 (6) Further, the heat treated (α + β) type titanium alloy is further added. The aging treatment is performed at 400 to 750 ° C. for the heat treatment of the (α + β) type titanium alloy.

First, the heating temperature will be described. Compared with Ti-6Al-4V, the present alloy composition has more β-stabilizing elements, and therefore, these β-stabilizing elements are concentrated in β before liquefaction, but are hardly contained in α. Therefore, it was found that the β transformation point of pro-eutectoid α itself is 100 ° C. or more higher than the equilibrium β transformation point of this alloy. Therefore, the present alloy is converted from β transformation point to β transformation point +100
Even if heated to ℃, the entire structure does not immediately become β structure, and α phase is transformed by diffusion from the grain boundary.
The particle size does not become coarse.

Next, the reason for limiting the heating rate will be described. If heating is performed at less than 2 ° C./s before reaching the above heating temperature, diffusion proceeds during heating and the α phase becomes small, and when the temperature is at or above the β transformation point, β is easily formed and grain coarsening occurs and ductility decreases. To do.

With respect to the heating time and the cooling rate, the β particle size is coarsened similarly to the case where the holding is carried out for 5 minutes or more or the cooling is carried out at 2 ° C./s or less.

Next, the reason for limiting the aging temperature will be described.
If the aging temperature is 400 ° C. or lower, the temperature is too low and the strength does not increase, and if it exceeds 750 ° C., the strength is immediately softened even if the strength increases. For this reason, the optimum aging temperature and 400 ° C or more and 500 ° C or less are set.

Finally, the reasons for limiting the alloy components in the present invention will be explained. Al: Al is a basic component that increases the strength of the α + β type titanium alloy, and if it is less than 3%, sufficient strength cannot be obtained. The range is 5%.

The β-stabilizing elements V, Fe and Mo have a large effect of lowering the β-transformation point, and are present in large amounts in the β-transformation structure. Therefore, the addition of these causes a large difference between the overall β transformation point and the β transformation point of only the α phase before heat treatment, and has an effect of suppressing coarsening of the β grain size in a certain temperature range.

V: When V is less than 2.1%, it becomes difficult to form an α + β type titanium alloy, while when 3.7% or more, it forms a solid solution in the α phase and the difference between the initial α phase and the overall β transformation point. On the contrary, it becomes smaller and the heating temperature range is narrowed.

Fe: Fe is one of the elements having the largest β stability, and if it is less than 0.85%, the contribution of the β stabilizing element to the β phase is small, and the difference between the pro-eutectoid α and the overall β transformation point is small. Is small and the heating temperature range is narrow. Further, since the diffusion coefficient is very large, if it is added in an amount of 3.15% or more, the β particle size becomes coarse during liquefaction, and the ductility is lowered.

Mo: Mo has a slow diffusion rate, and if it is less than 0.85%, coarsening of β grains occurs at the time of solution treatment and ductility decreases. Further, if it exceeds 3.15%, since Mo is a heavy element, the alloy density is increased.

O: O has a great effect on the strength and is 0.0
If it is less than 6%, the strength level is low. On the contrary, if it is 0.2% or more, the strength is large but the ductility is lowered.

80 ≦ 20x (Fe) + 13x (V) + 10x (Mo) ≦ 130 (6) Equation (6) shows the stability of the β phase and is closely related to the strength and the β transformation point. When this value is less than 80, the β-phase β-stability is small, and the difference between the pro-eutectoid α and the overall β-transformation point is small,
The heating temperature range becomes smaller. Further, when it exceeds 130, the β-stabilizing element becomes a solid solution also in the pro-eutectoid α, and the heating temperature range is reduced.

[0023]

【Example】

(Example 1) In Table 1, Ti-4.5Al-2Fe-Mo-0.10 (X produced in the α + β region and having an equiaxial two-phase structure)
= 99) Alloy φ15 mm round bar, notched tensile strength (NTS) and notched tensile strength are smoothed for tensile properties and notch tensile properties when heating rate, temperature, holding time and cooling rate are changed. It is considered that the toughness is high when the NTS / 0.2% PS is 1.0 or more, which is expressed by the value obtained by dividing the piece by 0.2% PS and the notch yield ratio (NTS / 0.2% PS). Notched tensile strength (NTS) is a test piece with a non-notched portion diameter of 8.0 mm, a notched bottom portion diameter of 5.66 mm, a notched shape of 60 ° V, a notched tip R of 0.1 mm, and a stress concentration factor Kt = 5.42. To use.

As a result, when the heating rate, temperature, holding time and cooling rate do not satisfy the conditions of the present invention, the elongation is 8.
If it is less than 0% and the notch yield ratio is less than 1.0, it becomes small. In addition, when aging is performed, the strength decreases, but the elongation and notch yield ratio remarkably increase.

[0025]

[Table 1]

Example 2 A φ15 mm round bar manufactured by heating and rolling the α + β type titanium alloy shown in Table 2 into the α + β type titanium alloy was heated at a heating rate of 2 ° C./s, a heating temperature of 1000 ° C., and held. The tensile properties and the notch tensile properties when heat treatment was performed for 30 seconds at a cooling rate of 2 ° C./s were investigated.

As a result, Al is less than 3% and O is 0.06.
If it is less than%, the tensile strength tends to be low. 20x
It can be seen that when (Fe) + 13x (V) + 10x (Mo) is less than 80 or greater than 130, the strength is increased, but the elongation and the notch yield ratio are reduced.

[0028]

[Table 2]

[0029]

As described above, according to the present invention, high strength, high toughness and high ductility can be obtained by the β region heat treatment without plastic working.

Claims (2)

[Claims] 1. The alloy composition (% by weight) below (1)
When the solution hardening aging treatment is performed on the (α + β) type titanium alloy having the composition of the formulas (6) and the balance being Ti and unavoidable impurities, the solution hardening aging treatment temperature is set to the β transformation point (Tβ) or more. Below β transformation point (Tβ) + 100 ° C, 2
After heating for 5 minutes or more at a heating rate of ℃ / s or more, 2 ℃
A method for heat treatment of an (α + β) type titanium alloy, characterized in that it is cooled at / s or more. 3.0 ≦ Al ≦ 5.0 (1) 2.1 ≦ V ≦ 3.7 (2) 0.85 ≦ Fe ≦ 3.15 (3) 0.85 ≦ Mo ≦ 3.15 (4) 06 ≦ O ≦ 0.20 (5) and 80 ≦ 20x (Fe) + 13x (V) + 10x (Mo) ≦ 130 (6) 2. The heat treatment according to claim 1 is performed (α + β
(Type) titanium alloy, and further aging treatment is performed at 400 to 750 ° C., a heat treatment method for (α + β) type alloy.