CN111902550A - Titanium alloy and method for producing same - Google Patents

Abstract

A titanium alloy, characterized in that, in mass %, C: 0.10-0.30%, N: 0.001-0.03%, S: 0.001-0.03%, P: 0.001-0.03%, Si: 0.001-0.10%, Fe : 0.01 to 0.3%, H: 0.015% or less, O: 0.25% or less, the balance is Ti and inevitable impurities, and the surface layer of the titanium alloy is α single phase.

Description

Titanium alloy and method of making the same

technical field

The present invention relates to a titanium alloy and a manufacturing method thereof.

Background technique

Industrial pure titanium also exhibits excellent corrosion resistance in seawater where general-purpose stainless steel such as SUS304 corrodes. Taking advantage of this high corrosion resistance, it is used in seawater desalination equipment and the like.

On the other hand, materials for chemical equipment are sometimes used in environments such as hydrochloric acid that are more corrosive than seawater. In such an environment, industrially pure titanium will also corrode significantly.

In order to be used in such a highly corrosive environment, a corrosion-resistant titanium alloy has been developed whose corrosion resistance in a highly corrosive environment is superior to that of industrial pure titanium.

Patent Document 1 discloses an alloy to which platinum group elements such as Pd are added. Patent Document 2 and Non-Patent Document 1 disclose alloys in which intermetallic compounds are precipitated in addition to platinum group elements.

Since these titanium alloys use rare elements such as Pd, the billet cost increases. Therefore, there is a technical problem of improving the corrosion resistance of titanium without using expensive rare elements. Therefore, various proposals have been made regarding titanium alloys using general-purpose elements without using rare elements.

On the other hand, Patent Document 3 discloses a technique of improving the corrosion resistance and strength of Ti by using C. However, as shown in FIG. 4 , the titanium alloy described in Patent Document 3 has a technical problem in terms of workability by precipitation of TiC, and has a problem in practical application to heat exchangers and equipment members.

prior art literature

Patent Literature

Patent Document 1: International Publication No. 2007/077645

Patent Document 2: Japanese Patent Application Laid-Open No. 6-25779

Patent Document 3: Japanese Patent Publication No. 2009-509038

Non-patent literature

Non-Patent Document 1: "Iron and Steel", vol. 80, No. 4 (1994), P353-358

SUMMARY OF THE INVENTION

Invention to solve problem

The technical problem of the present invention is to provide a titanium alloy which maintains high workability and improves corrosion resistance by adding C in place of rare elements.

solution to the problem

As a result of investigations conducted by the present inventors, it was found that the surface structure can be made into an α single phase by subjecting a titanium alloy to which 0.10 to 0.30% of C is added by heat treatment at 750 to 820° C. and cooling at a rate of 0.001° C./sec or more. , can maintain excellent workability, and can also improve corrosion resistance.

The gist of the present invention is as follows.

(1) A titanium alloy comprising, in mass %, C: 0.10-0.30%, N: 0.001-0.03%, S: 0.001-0.03%, P: 0.001-0.03%, Si: 0.001-0.10%, Fe : 0.01 to 0.3%, H: 0.015% or less, O: 0.25% or less, the balance is Ti and inevitable impurities, and the surface structure of the titanium alloy is α single phase.

(2) A method for producing a titanium alloy, comprising subjecting a titanium alloy to a final heat treatment at 750 to 820° C. and cooling it at a rate of 0.001° C./sec or more, wherein the titanium alloy is C: 0.10 to 0.30% in mass % , N: 0.001 to 0.03%, S: 0.001 to 0.03%, P: 0.001 to 0.03%, Si: 0.001 to 0.10%, Fe: 0.01 to 0.3%, H: 0.015% or less, O: 0.25% or less, the remainder for Ti and inevitable impurities.

effect of invention

According to the present invention, it is possible to provide a titanium alloy that maintains high workability and has good corrosion resistance. Specifically, when a titanium alloy in the composition range of the present invention is produced by the production method of the present invention, the surface structure becomes an α single phase, and both workability and corrosion resistance are improved.

Description of drawings

FIG. 1 is a graph showing the relationship between the corrosion rate and the amount of C added in the hydrochloric acid immersion test.

FIG. 2 is a graph showing the relationship between the corrosion rate and the heat treatment temperature in the hydrochloric acid immersion test.

FIG. 3 is an example of a photograph of the metallographic structure of the titanium alloy produced by the production method of the present invention.

FIG. 4 is an example of a metal photograph of a titanium alloy produced by a conventional production method.

Detailed ways

(Ingredient composition)

The titanium alloy of the present invention is C: 0.10-0.30%, N: 0.001-0.03%, S: 0.001-0.03%, P: 0.001-0.03%, Si: 0.001-0.10%, Fe: 0.01-0.3%, H: 0.015% or less (including 0%), O: 0.25% or less (including 0%), and the balance is Ti and inevitable impurities. In addition, in the following description, the content shown by "%" all means "mass %".

<C: 0.10 to 0.30%>

C plays an important role in improving corrosion resistance in the present invention. As the C content increases, the corrosion rate decreases and the corrosion resistance improves (Fig. 1). The effect of improving the corrosion resistance by the inclusion of C is prominent at 0.10% or more. On the other hand, when an α single-phase structure is formed and C exists in the α phase as an interstitial solid solution element, the effect of improving the corrosion resistance by adding C becomes the most significant, as will be described later. Furthermore, adding a large amount of C promotes the precipitation of TiC which adversely affects workability, which is not preferable. Adding a large amount of C not only adversely affects workability, but also prevents the effect of improving corrosion resistance from being sufficiently exhibited. Therefore, the content of C is set to 0.10 to 0.30%. In addition, the lower limit of the content of solid solution C is more preferably 0.12%, and the upper limit of the content of solid solution C is more preferably 0.28%. The α phase in which C is a solid solution element as an interstitial solid solution element is the α phase of the surface structure described later.

<N: 0.001 to 0.03%>

N is an essential element effective for increasing strength, but as its content increases, ductility and toughness deteriorate. In addition, N is an interstitial solid solution element like C which plays an important role in improving corrosion resistance in the present invention. Therefore, as the N content increases, the solid solution content of C may decrease. Therefore, the content of N is set to 0.001 to 0.03%. More preferably, the upper limit of the content of N is 0.025%.

<S: 0.001 to 0.03%>

S is an essential element effective for increasing strength, but as its content increases, ductility and toughness deteriorate. In addition, S is an interstitial solid solution element like C which plays an important role in improving corrosion resistance in the present invention. Therefore, as the S content increases, the solid solution content of C may decrease. Therefore, the content of S is set to 0.001 to 0.03%. More preferably, the upper limit of the content of S is 0.025%.

<P: 0.001 to 0.03%>

P is an essential element effective for increasing strength, but as its content increases, ductility and toughness deteriorate. In addition, P is an interstitial solid solution element like C which plays an important role in improving corrosion resistance in the present invention. Therefore, as the P content increases, the solid solution content of C may decrease. Therefore, the content of P is set to 0.001 to 0.03%. More preferably, the upper limit of the content of P is 0.025%.

<Si: 0.001 to 0.10%>

Si is a relatively inexpensive element and is effective for improving heat resistance (oxidation resistance, high temperature strength), but a large amount of Si promotes compound precipitation and deteriorates ductility and toughness. Therefore, the content of Si is set to 0.001 to 0.10%. The lower limit of the content of Si is more preferably 0.003%, and the upper limit of the content of Si is more preferably 0.08%.

<Fe: 0.01 to 0.3%>

Fe is an element effective for increasing strength, but as its content increases, ductility and toughness deteriorate. In addition, Fe is a strong β-stabilizing element among the elements contained in the titanium alloy of the present invention, and if it is added in a large amount, it becomes difficult to obtain an α single-phase structure described later. Therefore, the content of Fe is set to 0.01 to 0.30%. The lower limit of the content of Fe is more preferably 0.03%, and the upper limit of the content of Fe is more preferably 0.25%.

<H: 0.015% or less>

H is an element that forms titanium hydride and degrades the ductility and toughness of the billet. Therefore, the lower the content, the better, but the increase of H is unavoidable in the production process. In addition, H is an interstitial solid solution element like C which plays an important role in improving corrosion resistance in the present invention. Therefore, as the H content increases, the solid solution content of C may decrease. Therefore, the content of H is limited to 0.015% or less. In addition, in order to obtain such a low-H titanium alloy, it is sufficient to use high-purity titanium sponge, but excessive use of high-purity titanium sponge will increase the cost. In the present invention, H is an impurity element and may be 0%, but from the viewpoint of cost, H is preferably 0.001% or more. More preferably, the upper limit of the H content is 0.005%.

<O: 0.25% or less>

O is an essential element effective for increasing strength, but as its content increases, ductility and toughness deteriorate. In addition, O is an interstitial solid solution element like C which plays an important role in improving corrosion resistance in the present invention. Therefore, as the O content increases, the solid solution content of C may decrease. Therefore, the content of O is made 0.25% or less. In addition, in order to obtain such a low-O titanium alloy, it is sufficient to use high-purity titanium sponge, but excessive use of high-purity titanium sponge will increase the cost. In the present invention, O is an impurity element and may be 0%, but from the viewpoint of cost, O is preferably 0.01% or more. More preferably, the upper limit of the content of O is 0.20%.

<The surface layer is α single phase>

The surface layer being an α single phase means that the structure of the surface layer is an α phase, and the intensity of the X-ray diffraction peak of TiC is 10% or less of the intensity of the background. Here, the surface layer refers to a range from the surface to a depth of 5 μm. The α phase does not include the α' phase and the needle-like α phase. Fig. 3 shows the state of the surface of the titanium alloy produced by the production method of the present invention.

The α phase is composed of a close-packed hexagonal structure, and is different from the α' phase and the needle-like α phase formed from the β phase transformation in crystal structure and grain boundary distribution. C atoms that are solid-dissolved in the α-phase easily exist among Ti atoms as interstitial solid-solution elements, and by acting on the electronic state existing around Ti nuclei, the anodic reaction can be suppressed, and corrosion resistance can be improved. Anodic reaction refers to the reaction of metal corrosion and ionization. It is necessary to deviate electrons from Ti nuclei during metal ionization, and by making C solid in the α phase, it becomes difficult to deviate from electrons, and corrosion resistance is improved. Since the α' phase does not have a close-packed structure, the needle-like α phase is greatly affected by grain boundary segregation, and a sufficient effect of improving corrosion resistance cannot be obtained compared with the α phase.

TiC is a hard compound and significantly deteriorates the workability of the billet. However, in the surface layer of the titanium alloy of the present invention, carbon is almost dissolved in a solid solution, and TiC hardly precipitates, so the workability is not deteriorated.

<Heat treatment temperature>

Even in a billet that satisfies the above-mentioned composition, the structure of the surface layer changes depending on the heat treatment temperature. As a result, the performance exerted also changes. As shown in FIG. 2 , the corrosion rate of the titanium alloy produced by the heat treatment at about 800° C. was the most suppressed. Therefore, in the present invention, the heat treatment temperature is set to 750 to 820°C. The holding time in this temperature range is not particularly limited, and may be held for 1 second or longer, preferably 30 seconds or longer.

The reason why the corrosion rate of the titanium alloy is suppressed at 750 to 820°C is that if heat treatment is performed outside this temperature range, TiC is precipitated, or the structure changes to an α' phase or an acicular α phase. For example, FIG. 4 shows the state of the surface layer of the titanium alloy produced by the conventional method in which the heat treatment is performed outside this temperature range. Island-like TiC precipitates were produced in the surface layer (Fig. 4). TiC is a hard compound and significantly deteriorates the workability of the billet. Therefore, the workability of the titanium alloy produced by the conventional method is deteriorated.

＜Cooling rate＞

Even if the heat treatment temperature is within the above range, if the cooling rate is slow, TiC is precipitated during the cooling process, so that the surface layer does not become α. The cooling rate of the present invention is preferably 0.001°C/sec or more, preferably 1°C/sec or more. In addition, the precipitation of TiC can be suppressed as the cooling rate is faster, but an excessive cooling rate will adversely affect the shape maintenance of the titanium plate, so the upper limit is made 2000°C/sec.

<Manufacturing method>

Next, the manufacturing method of the titanium alloy of this invention is demonstrated. Like conventional industrial pure titanium, the titanium alloy of the present invention is subjected to shot peening and pickling at any time during the respective steps of casting→blanking (or hot forging)→hot rolling→annealing (→cold rolling→finish annealing). processing, etc., especially can be manufactured without the use of special methods. In the description of the above-mentioned steps, the steps in parentheses (→cold rolling→finish annealing) are not essential, but can be appropriately implemented according to the thickness, shape, size, etc. of the titanium to be produced.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. The present invention is not limited to the following examples.

Using the molten raw material containing sponge titanium and predetermined|prescribed additive elements, the titanium ingot of each component composition shown in Table 1 was cast by the vacuum arc melting furnace. Among the additive elements, Fe was added as electrolytic iron, and C was added as TiC powder.

It should be noted that Al, V, Cr, Ru, Pd, Ni, and Co in the table are not intentionally added elements, and the values in the table indicate that the contents of the above-mentioned various elements are impurity levels.

[Table 1]

Using the cast titanium ingot, forging and hot rolling were performed at a heating temperature of 800 to 1000° C. to prepare a hot-rolled sheet with a thickness of 4.0 mm, and a test piece for corrosion resistance evaluation was prepared by pickling and machining. Then, vacuum annealing was performed at the temperatures shown in Table 2, respectively, and the corrosion resistance was evaluated.

The identification of the surface structure was carried out by XRD (X-ray diffraction) and microstructure observation. The conditions of X-ray diffraction were to use CoKα rays as characteristic X-rays, and to set a voltage of 30 kV and a current of 100 mA. The range of X-ray diffraction was 10°≤2θ≤110°, the step width was 0.04°, the accumulation time was 2s, and the X-ray incident angle was 0.3°. The presence or absence of α phase, β phase, α' phase, and TiC was investigated from the positions of the X-ray diffraction peaks of the test piece (length 20 mm, width 20 mm), and the microstructure observation including the presence or absence of needle-like α was performed. to investigate the surface tissue. When the detected X-ray diffraction peak intensity exceeds 10% of the background, the formation of β phase, α' phase, and TiC is confirmed, and in other cases, it is determined to be α single phase.

The corrosion resistance was evaluated based on the magnitude of the calculated corrosion rate by immersing the test piece in a 3 mass % hydrochloric acid aqueous solution at 90° C. for 168 hours, comparing the weights before and after the immersion. The case where the corrosion rate was 2 mm/year or less was regarded as pass. Table 2 shows the results of the corrosion resistance evaluation test. The workability was subjected to a tensile test by the method described in JIS Z 2241, and was evaluated by its elongation. The measurement of the elongation was performed with an elongation meter, and the case where the total elongation was 40% or more was regarded as the pass.

[Table 2]

In Nos. 1 to 9, which all satisfy the billet composition, heat treatment temperature, and surface structure specified in the present invention, the corrosion rate is remarkably low, the corrosion resistance is improved, and sufficient elongation is exhibited, so it can be confirmed that both the corrosion resistance and the Processability.

In Nos. 10 to 16, ingot components such as carbon are within the scope of the present invention, but the heat treatment temperature and cooling rate are outside the scope of the present invention, so the surface structure does not become α single-phase, the corrosion rate is high, and no satisfactory results are shown. elongation. No. 14, 16, 18, and 20 had a slow cooling rate, so TiC precipitated during the cooling process.

In Nos. 17 to 24, elements that reduce the solid solution limit of C, such as S, P, Si, etc., which are beyond the scope of the present invention, are added. Corrosivity was not improved, and TiC was precipitated, so the elongation was low.

In contrast to No. 1 and 5, discoloration and the like were hardly observed in an outdoor environment, while No. 23 and 24 turned brown in an outdoor environment.

Claims (2)

1. A titanium alloy characterized by comprising, in mass%

C：0.10～0.30％、

N：0.001～0.03％、

S：0.001～0.03％、

P：0.001～0.03％、

Si：0.001～0.10％、

Fe：0.01～0.3％、

H: less than 0.015%,

O: the content of the active ingredients is less than 0.25%,

the balance being Ti and unavoidable impurities,

the surface layer of the titanium alloy is alpha single phase.

2. A method for producing a titanium alloy, characterized by subjecting a titanium alloy to a final heat treatment at 750 to 820 ℃ and cooling the titanium alloy at a rate of 0.001 ℃/sec or more, wherein the titanium alloy is produced by mass%

C：0.10～0.30％、

N：0.001～0.03％、

S：0.001～0.03％、

P：0.001～0.03％、

Si：0.001～0.10％、

Fe：0.01～0.3％、

H: less than 0.015%,

O: less than 0.25 percent,

The balance being Ti and unavoidable impurities.

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