WO2001092589A1

WO2001092589A1 - Titanium alloy excellent in ductility, fatigue strength and rigidity and method for producing the same

Info

Publication number: WO2001092589A1
Application number: PCT/JP2000/003461
Authority: WO
Inventors: Nozomu Ariyasu; Satoshi Matsumoto
Original assignee: Sumitomo Metal Industries, Ltd.; Honda Giken Kogyo Kabushiki Kaisha
Priority date: 2000-05-29
Filing date: 2000-05-29
Publication date: 2001-12-06
Also published as: EP1295955A4; US20030084970A1; EP1295955A1

Abstract

A titanium alloy having a metal boride homogeneously crystallizated or/and deposited in the matrix thereof, characterized in that the matrix has an equiaxial α structure in an amount of 40 vol % or more. Such titanium alloy can be produced by subjecting it to a hot finishing working at a temperature 10 °C or more lower than its β transus temperture. This titanium alloy has a high rigidity, excellent ductility and excellent fatigue strength, which are properties required to a structural member, and can be widely used for parts of an automobile engine, structural members for an air craft, parts for a rapid transit rail car and the like.

Description

Specification

Titanium alloy with excellent ductility, fatigue strength and rigidity and its manufacturing method

INDUSTRIAL APPLICABILITY The present invention is useful for structural parts that require light weight and excellent mechanical properties, such as automobile engine conrods, valves, camshafts, crankshafts, pushrods, aircraft, and high-speed railway vehicles. The present invention relates to a titanium alloy having high rigidity and excellent ductility and fatigue strength, and a method for producing the same. Background art

Titanium alloys are lightweight, have high strength, and are also excellent in corrosion resistance and heat resistance. Therefore, application to various mechanical parts for automobiles, aircraft, and high-speed railway vehicles is being studied. However, titanium alloys have a Young's modulus that is about half that of steel materials, so when applied to mechanical structural materials, the occurrence of buckling and bending must be considered. For example, when a titanium alloy is applied to a long mechanical part such as a shaft condensor, a design that increases the cross-sectional area of the part is required to secure a certain strength. Such component design makes it impossible to take advantage of the excellent properties of titanium alloy such as light weight and high strength.

Therefore, conventionally, as a means for improving the Young's modulus of titanium alloys, various studies have been conducted on composite materials in which fibers or particles having a high Young's modulus are mixed in titanium. For example, JP-A-5-5142 proposes a method for producing a titanium-based composite material in which a TiB solid solution is dispersed at a predetermined volume ratio in a matrix made of a titanium alloy. According to this manufacturing method, a composite material exhibiting excellent properties such as strength, rigidity, and abrasion resistance from room temperature to high temperature can be obtained. However, in the proposed manufacturing method, melting, sintering, or powder metallurgy was premised because of poor plastic workability, and application to large structural materials was difficult. In addition, knowledge on the matrix structure of the manufactured composite materials is not disclosed, and it is unclear whether ductility and fatigue strength required for structural materials can be secured.

Japanese Patent Application Laid-Open No. 10-1760 proposes a particle-reinforced titanium-based composite material containing TiB or TiC particles and using a 1-type titanium alloy as a matrix and controlling the structure to a needle-like phase structure. Have been. However, even in the proposed composite material, TiB or TiC is used as the ceramic particles for reinforcement, so the powder metallurgy method is premised on its production, and it is difficult to apply it to large structural materials. In addition, since the matrix structure becomes a needle-like structure, the Young's modulus is high, but sufficient ductility cannot be obtained. Disclosure of the invention

As described above, titanium alloys have excellent specific strength, but have a problem that their Young's modulus is significantly lower than that of steel materials. To solve this problem, various types of composite materials have been studied, but problems remain in hot workability and ductility. On the other hand, structural components are used in even more severe environments and are required to reduce manufacturing costs, and are required to have excellent hot workability and to have predetermined strength characteristics. Become so. For example, in order to withstand severe use conditions and satisfy cost reductions, for example, automotive condolution parts are required to have hot workability, high rigidity, excellent ductility and excellent fatigue strength. It has become. However, titanium alloys having these characteristics have not been developed yet.

The present invention has been made in view of a request for development of a titanium alloy used for such a mechanical component, and is a titanium alloy which is hot workable and has excellent ductility, fatigue strength and rigidity, and a method for producing the same. Intended to provide ing. A specific object of the present invention is to be able to perform hot forging or hot rolling, to have a high rigidity with a tensile strength of at least lOOMpa and a Young's modulus of at least 130 Gpa, and to have a predetermined ductility and fatigue strength. We are developing titanium alloys.

The present inventors conducted various studies on the components, finely dispersed particles, and matrix structure in order to develop the above-mentioned titanium alloy. As a result, the following findings (a) to (c) were obtained. I got it.

(a) To improve the Young's modulus of a titanium alloy, it is effective to disperse particles with a high Young's modulus in the matrix. Applicable dispersed particles include titanium carbide or titanium boride generated by crystallization and / or precipitation reaction in the matrix, but their Young's modulus as particles is 1.3 times or more larger than titanium carbide It is effective to use titanium boride.

(b) In titanium alloys, various matrix structures appear even with the same alloy composition, but these structures are basically roughly classified into equiaxed structures and acicular structures. To ensure ductility and fatigue strength, the matrix structure must be equiaxed at a certain ratio.

In order to form an equiaxed structure in the matrix, it is necessary to perform a heat treatment after applying a processing strain, but the heating temperature during hot working performed at that time must be lower than that of Transus. Further, in the subsequent solution treatment, it is desirable to perform the treatment at a temperature lower than that of y5 transus.

(c) Al, oxygen (0), C, H and N, which are α-phase stabilizing elements, are contained in appropriate amounts to improve the Young's modulus of the matrix. Neutral elements such as Sn, Zr, and Hf have a small effect of improving Young's modulus, but have an effect of improving high-temperature strength and creep resistance.

When aging treatment is performed on titanium alloys containing Hf with Al, oxygen, or Sn or I, these components exhibit age hardening that promotes the formation of intermetallic compounds (Ti ₃ Al). Fatigue strength can be greatly improved. ^ Among phase stabilizing elements, solid solution V and Mo significantly lower Young's modulus However, the eutectoid type Fe and Cr are not so remarkable as the solid solution type. The β-phase stabilizing element has an effect of lowering the Young's modulus, but has an effect of improving hot workability.

The present invention has been completed on the basis of the above findings, and has a method for producing a titanium alloy of the following (1), (3) and (4), and a titanium alloy of the following (2), (3) and (4): The main point is.

(1) In mass%, Β: 0.5 to 3.0%, a titanium alloy in which metal boride is uniformly crystallized and / or precipitated in the matrix, and the matrix structure is equiaxed α structure Is a titanium alloy having excellent ductility, fatigue strength and stiffness characterized by having a content of at least 40 vol%. This titanium alloy is of type or α + type.

(2) A method for producing a titanium alloy in which ％: 0.5 to 3.0% is contained by mass%, and a metal boride is uniformly crystallized or precipitated in the matrix. This is a method for producing a titanium alloy with excellent ductility, fatigue strength and rigidity, characterized in that the heating temperature of the alloy is set to be 10 ° C or lower than that of Transus.

In the above-mentioned production method, it is preferable that the solution treatment is performed in a temperature range of (? Transus-350 ° C) to (? Transzas-10 ° C), and if necessary, an aging treatment is performed.

(3) In addition to the above titanium alloys (1) and (2), by mass%, Al: 5.5 to 10%, oxygen: 0.07 to 0.25%, C: 0.1% or less, H: 0.05% or less, and N: It is desirable to include 0.1% or less.

(4) Similarly, in the titanium alloy of (3) above, one or more of Sn, Zr, and Hf in a total of 20% or less, or one or more of the stabilizing elements of nitrogen and / or It is desirable to include 10% or less in the V equivalent shown by the following equation (a). 1 15 15 15

-— Fe + ~ b + —Ni + — W (a) v equivalent ^{= v + Mo +} 4.0 36 9 25

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram showing various properties of a titanium alloy tested in Example 1 after solution treatment.

FIG. 2 is a view showing various properties of the titanium alloy tested in Example 1 after solution treatment or after aging treatment. BEST MODE FOR CARRYING OUT THE INVENTION

In the titanium alloy of the present invention, the metal boride is finely crystallized and / or precipitated and uniformly dispersed in the matrix, and if necessary, an appropriate amount of a phase stabilizing element such as Al or oxygen is contained. In addition to improving the Young's modulus, the matrix structure is controlled to have an equiaxial structure ratio (hereinafter also referred to as “equiaxial ratio”) of at least 40% in terms of area ratio (same as volume ratio), thereby improving ductility and It is characterized by ensuring fatigue strength.

Furthermore, in the titanium alloy of the present invention, Sn, Zr, and Hf, which are neutral elements, may be contained as necessary to improve high-temperature strength, improve creep resistance, or use a ^ phase stabilizing element as a ^ phase monolayer. The amount of addition is limited by the V equivalent to such an extent that it does not become too small, so that β transduction is reduced and hot workability is improved. Next, the configuration of the present invention will be described with respect to the microstructure, component composition, and manufacturing method for the reasons specified above.

1. Microstructure

Titanium alloys are classified into three types according to their microstructure at room temperature: type 5, +5 and '/ ?, and the present invention is directed to α and type + titanium alloys. .

In general, for cast alloys and cast iron alloys, ductility and fatigue strength are not considered. Thus, equiaxed tissue is superior to acicular tissue. Further, according to the study by the present inventors, it is not necessary to form an equiaxed structure in all of the alloy matrix, and a mixture with a needle-like structure generated by transformation from the ^ phase may be used. However, in order to ensure ductility and fatigue strength in the mixed structure, the ratio of the equiaxed structure, that is, the equiaxed ratio must be 40% or more in area ratio. Furthermore, to ensure stable ductility and fatigue strength, it is desirable that the equiaxed ratio is 50% or more.

The microstructure can be observed with an optical microscope after a sample is taken from the alloy matrix and polished and corroded. The area ratio of the equiaxed α-structure specified in the present invention, that is, the isometric ratio, is obtained by performing image processing on the matrix of a microscopically observed tissue photograph, classifying the matrix into equiaxed tissue and needle-shaped tissue, and determining the ratio. Measure. The reason why the equiaxial ratio is defined in the present invention is that the properties of the ductility and the fatigue strength of the titanium alloy largely depend on the area ratio of the equiaxial braided structure. 2. Ingredient composition

Β Ingredients:

In order to uniformly disperse the metal boride (Ti B) in the matrix of the titanium alloy, B is contained and crystallized and / or precipitated during solidification and cooling. As a result, the Young's modulus of the titanium alloy is very high relative to the titanium alloy, and the Young's modulus of the titanium alloy can be improved in accordance with the composite rule.

If the B content is less than 0.5%, the amount of Ti B crystallized and / or precipitated is small, and the Young's modulus of the titanium alloy is not sufficiently improved. On the other hand, if the B content exceeds 3.0%, the dispersion amount of Ti B becomes too large, and the Young's modulus of the matrix is improved, but the hot ductility and the cold ductility are significantly reduced. Therefore, when B is contained, the content is set to 0.5 to 3.0%.

α-phase stabilizing element: Al and oxygen are phase stabilizing elements, have a large solid solution hardening effect, and significantly improve the Young's modulus. However, if 1 is less than 5.5% and oxygen is less than 0.07%, the effect is not sufficiently exhibited, while if A1 exceeds 10% and oxygen exceeds 0.25%, workability and ductility decrease. I do. Therefore, the content of both should be A1: 5.5 to 10% and 0: 0.07 to 0.25%. Desirably, A1: 7 to 9% and oxygen: 0.07 to 0.15%.

Other phase stabilizing elements include C, H and N, all of which reduce the room-temperature ductility, so the upper limits are C: 0.1%, H: 0.05% and N: 0.1%. .

Neutral element

In the present invention, a neutral element or a Z and? Phase stabilizing element can be blended. In this case, any of the added elements dissolve in the matrix. However, most of the neutral elements Zr and Hf form solid solutions in the matrix, but crystallize and / or precipitate zirconium boride and hafnium borohydride as metal borides in trace amounts. Since these are very small, improvement in Young's modulus cannot be expected.

One or more of Sn, Zr, and Hf, which are neutral elements, can be contained. Sn, Zr, and Hf exert the effect of solid solution strengthening and have little effect on improving Young's modulus, but can increase high-temperature strength. However, if the total content of one or more types exceeds 20%, the hot and cold workability is reduced and the alloy cost is increased. Therefore, the upper limit is set to 20% in total. Desirably, the total is 5% or less.

Phase stabilizing element:

? Phase stabilizing elements include V, Mo, Cr, Fe, Nb, Ni and W. The contained / phase stabilizing element has the effect of lowering the transus and improving the hot workability. In addition, these elements form a solid solution in the titanium alloy matrix and suppress intermetallic compounds (Ti ₃ Al) generated more than necessary. This has the effect of increasing the content of A1. However, an excessive content greatly reduces the Young's modulus, so one or more types can be added in a range of 10% or less in V equivalent shown by the following formula (a). Desirably, the V equivalent is 5% or less.

One, _s 15 15 15 15 15 15 _t ,

V equivalent = V + — Mo + "^ Cr + ^ Fe + ^ r b + 7Ni + one W *

10 6.3 4.0 36 9 25

3. Manufacturing process

Titanium alloy ingots are produced by adding titanium sponge, which is a titanium raw material, to pure Al, electrolytic Sn, Zr sponge, pure Hf, A1-V master alloy, Al-Mo master alloy, and Mo, Cr, V, etc. A single substance is appropriately selected and blended in a predetermined amount to prepare a compacted molten raw material. A1 boride and Fe boride are used as the B source in the raw material to crystallize, precipitate and disperse Ti B in the titanium alloy. The oxygen content of Ingo' metropolitan, can be adjusted to some extent by the type of titanium sponge, if you need to adjust in large amounts, the Ti_〇 ₂ is used as the adjustment member. The adjusted raw material is arc-melted by melting a consumable electrode by a vacuum melting furnace or by non-consumable electrode melting by plasma arc melting to produce an alloy ingot.

The produced titanium alloy ingot is subjected to hot working by forging or rolling to obtain a predetermined microstructure, and is appropriately subjected to heat treatment to adjust mechanical properties. As mentioned above, in order for the equiaxed structure to appear in the matrix, it is necessary to go through an appropriate thermal history after applying processing strain.

The structure in the matrix varies greatly depending on the heating conditions in the vicinity of the transus. When hot working is performed at a temperature higher than the transus, needle-like α-structures are likely to appear, and the heat at a temperature lower than the transus In the case of cold working, equiaxed tissue is likely to appear. For this reason, in the manufacturing method of the present invention, it is necessary to control the heating temperature at the time of finishing hot working to be lower than the temperature of Transus. By the way, in the temperature range just below the β transus, the phase and the / phase are mixed. When cooled to room temperature, the acicular structure and the equiaxed structure mix. As mentioned above-In order to secure the prescribed ductility and fatigue strength, to secure the equiaxed α-structure at an area ratio of 40% or more, the heating temperature during finishing hot working must be at least 10 ° C lower than Transus. There is a need to. The lower limit of the heating temperature is not particularly limited, but may be any temperature higher than the lower limit of the hot working. In the production method of the present invention, the heating temperature at the time of finishing hot working is specified, and the heating temperature at the time of rough working before finish working may be a temperature exceeding ^ transus.

In other words, hot working of a titanium alloy ingot is necessary not only to work to approximate the part shape, but also to make the matrix a predetermined microstructure. As described above, in order for the equiaxed structure to appear in the matrix, it is necessary to apply a heat treatment after applying the processing strain. For example, once the microstructure becomes a needle-like structure, it cannot be made into an equiaxed structure no matter how much heat treatment is applied thereafter. To change the matrix from acicular to equiaxed, it is necessary to heat it again to a temperature lower than?

In order to change the matrix from the needle-like structure to the equiaxed structure reliably, it is effective to apply a sufficient working strain, and it is desirable to secure a working ratio of 50% or more for hot working. Also, if Ti B is coarsely crystallized and precipitated in the matrix, ductility and fatigue strength decrease. In order to prevent this, it is necessary to destroy coarse crystallization and precipitation by hot working. In this case, it is desirable that the processing rate is 70% or more.

In the case of a titanium alloy, when the processing temperature is lowered, hot workability is reduced, and work cracks and the like are likely to occur. Therefore, in order to secure the processing temperature, it is necessary to apply a heat insulating material to the ingot, raise the ambient temperature to a warm or hot region, or reheat the ingot to a temperature below /? Valid is there.

The hot-worked titanium alloy is subjected to a solution treatment or an aging heat treatment to adjust its mechanical properties. By making the temperature of the solution treatment 10 ° C or more lower than that of Transus, it is possible to secure the equiaxed tissue formed by hot working as it is. On the other hand, if the treatment temperature is too low, the effect of the solution treatment is lost, so that the temperature is set to (? Transus-350 ° C) or more. Therefore, in the present invention, the solution treatment is performed at (? Transus-350 ° C) to β Transus-10 ° C), and more preferably

The temperature range is from {Transus-200 ° C) to (? Transus-100 ° C). Furthermore, the aging treatment promotes the formation of intermetallic compounds (Ti ₃ Al), thereby further improving the fatigue strength of the titanium alloy. The conditions of the aging treatment vary depending on the alloy composition, but it is desirable that the treatment temperature is 500 to 600 ° C and the treatment time is 5 hours or more.

(Example)

The effect of the present invention will be described in detail based on the case where solution treatment is performed after hot forging (Example 1) and the case where aging treatment is further performed (Example 2).

(Example 1)

A titanium alloy having the composition shown in Table 1 was arc-melted using a vacuum melting furnace to produce an ingot with a diameter of 140. The T / A trans of the titanium alloy tested is 1070 ° C.

Table 1 Ingredient composition (in mass%)

The obtained alloy ingot was subjected to hot forging and solution treatment twice under the following conditions to obtain a test material.

1. Rough forging Forging dimension: Outer diameter 80mm (working ratio 68%, forging ratio 3)

Heating temperature: 1170 ° C (^ Transus + 100.C)

2. Finish forging

Forging and elongation dimensions: Outer diameter 25mm (Processing ratio 90%, Forging ratio 10)

Heating temperature: 1040 ° C to 1170 ° C (The individual heating temperature is shown in Fig. 1.)

3. Solution treatment

Heating temperature: 700 ° C ~: 1100 ° C (The individual heating temperature is shown in Fig. 1.)

Heating time: 2 hours

Room temperature tensile properties, room temperature fatigue strength, and Young's modulus were measured as various properties of the test titanium alloy after solution treatment. Microscopic observation of each test piece was performed, and the equiaxed ratio (vol%) of the matrix was converted. Figure 1 shows these results.

From the results in Fig. 1, it can be seen that the tensile strength of all the test materials is l lOO Mpa or higher, the Young's modulus is 130 Gpa or higher, and high rigidity is ensured. 3-6 have an equiaxed ratio of 40 vol% or more, and have excellent fatigue strength and ductility in addition to rigidity.

That is, if the heating temperature at the time of finishing hot working is lower than Transus by 10 ° C or more, if the ratio of the equiaxed structure specified in the present invention is secured, high ductility can be achieved without deteriorating high rigidity characteristics. In addition, high fatigue strength can be ensured.

(Example 2)

Using the alloy ingot obtained in Example 1, the conditions of hot forging were changed, and the effect of the aging treatment after the solution treatment was confirmed. The tested titanium alloys were subjected to the following processes A to D.

1. Process A (Comparative example)

1-1 Finish Forging '

Forging dimension: 25mm outside diameter (working rate 97%, forging ratio 30) Heating temperature: 1170 ° C (? Transas + 100 ° C)

1-2 Solution treatment

Processing conditions: 900 ° C x 2 hours

Process B (Comparative example)

2-1 Finish forging

Forging and elongation dimensions: 25 mm outside diameter (working ratio 97%, forging ratio 30) Heating temperature: 1170 ° C (β Transus + 100 ° C)

2-2 Solution treatment

Processing conditions: 900 ° C x 2 hours

2-3 Aging treatment

Processing conditions: 580 ° C for 8 hours

Process C (Invention example)

3- 1 rough forging

Forging / extension size: Outer diameter 80 dragon (working ratio 68%, forging ratio 3) Heating temperature: 1170 ° C (β Transus + 100 ° C)

3-2 Finish forging

Forging and stretching dimensions: 25 outside diameter (working rate 90%, forging ratio 10) Heating temperature: 1040 ° C (? Transus-30 ° C)

3-3 Solution treatment

Processing conditions: 900 ° C x 2 hours

Process D (Invention example)

4- 1 rough forging

Forging and elongation dimensions: Outer diameter 80mm (Working ratio 68%, Forging ratio 3) Heating temperature: 1170 ° C (/ 3 Transus + 100 ° C) 4- 2 Finish forging

Forging / stretching dimensions: Outer diameter 25mm (Processing ratio 90%, Forging ratio 10) Heating temperature: 1040 ° C {Transus — 30 ° C) 4-3 Solution treatment

Processing conditions: 900 ° C x 2 hours

4-3 Aging treatment

Processing conditions: 580 ° C for 8 hours

After the solution treatment or the aging treatment of the test titanium alloy, the room temperature tensile properties, room temperature fatigue strength, Young's modulus, and matrix equiaxed ratio (vol%) were measured. Figure 2 shows the results.

Processes A and B, which are comparative examples, have a tensile strength of llOOMpa or higher, a Young's modulus of 130 Gpa or higher, and high rigidity.However, the heating temperature of finish forging is inappropriate. The ductility and fatigue strength are not secured. In contrast, processes C and D, which are examples of the invention, exhibit excellent ductility and fatigue strength in addition to high rigidity. Further, in the process D, by performing the aging treatment, it becomes possible to increase the heat resistance and the tensile strength and further improve the fatigue strength. Industrial applicability

ADVANTAGE OF THE INVENTION According to the titanium alloy of this invention and its manufacturing method, since it has the characteristic which is high rigidity required as a structural material, and is excellent in ductility and fatigue strength, it provides a mechanical part which satisfies excellent mechanical properties with a light weight. it can. Therefore, the titanium alloy of the present invention can be widely used for automobile engine conrods, camshafts, crankshafts and pushrods, aircraft structural members, high-speed railway vehicle parts, and the like.

Claims

The scope of the claims

B: 0.5 to 3.0% by mass; a titanium alloy in which a metal boride is uniformly crystallized and / or precipitated in the matrix, and whose matrix structure is equiaxed. Titanium 'alloy with excellent ductility, fatigue strength and rigidity characterized by having a structure of 40 vol% or more.

2. The titanium alloy having excellent ductility, fatigue strength, and rigidity according to claim 1, wherein the titanium alloy is a cast or a +5 type.

3. In addition, it must contain Al: 5.5 ~; 0%, oxygen (0): 0.07 ~ 0.25%, C: 0.1% or less, H: 0.05% or less and N ': 0.1% or less by mass%. The titanium alloy according to claim 1, which is excellent in ductility, fatigue strength and rigidity. .

4. The titanium alloy according to claim 3, further comprising one or more of Sn, Zr, and Hf in a total amount of 20% or less, and / or one or two of phase stabilizing elements. The above is a titanium alloy excellent in ductility, fatigue strength and rigidity, characterized by containing 10% or less in V equivalent shown by the following equation (a).

V equivalent W · · · (a)

5. A method for producing a titanium alloy in which B: 0.5 to 3.0% is contained by mass%, and a metal boride is uniformly crystallized and / or precipitated in the matrix. A method for producing a titanium alloy having excellent ductility, fatigue strength and rigidity, characterized in that the heating temperature of the alloy is 10 ° C or lower than that of Transus.

6. The method for producing a titanium alloy according to claim 5, wherein the solution treatment is performed in a temperature range of (? Transus-1 350 ° C) to (^ Transus-1 10 ° C). A method for producing a titanium alloy having excellent strength and rigidity.

7. The method for producing a titanium alloy according to claim 6, further comprising aging treatment. 7) A method for producing a titanium alloy having excellent ductility, fatigue strength, and rigidity.

8. The above titanium alloy further contains, by mass%, Al: 5.5 to 10%, Oxygen (0): 0.07 to 0.25%, C: 0.1% or less, H: 0.05% or less, and N: 0.1% or less. The method for producing a titanium alloy according to any one of claims 5 to 7, which is excellent in ductility, fatigue strength, and rigidity.

9. The titanium alloy according to claim 8, further comprising one or more of Sn, Zr, and Hf in a total amount of 20% or less, and / or one or two of phase stabilizing elements. The above is a method for producing a titanium alloy having excellent ductility, fatigue strength and stiffness, characterized in that the V equivalent represented by the following equation (a) is 10% or less.

1 c I K 15 1 15

V equivalent = V + _Mo + —— Cr + — -Fe + —Nb + —Ni + —W · ·. (A)

10 6.3 4.0 36 9 25