CN111902550B - 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 for producing same

Technical Field

The present invention relates to a titanium alloy and a method for producing the same.

Background

Industrial pure titanium exhibits excellent corrosion resistance even in seawater in which general-purpose stainless steel such as SUS304 is corroded. The high corrosion resistance is utilized for seawater desalination equipment and the like.

On the other hand, materials for chemical plants are sometimes used in environments where hydrochloric acid or the like is more corrosive than seawater. Under such circumstances, commercially pure titanium also corrodes significantly.

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

Patent document 1 discloses an alloy to which a platinum group element such as Pd is added. Patent document 2 and non-patent document 1 disclose alloys in which an intermetallic compound is precipitated in addition to a platinum group element.

Since these titanium alloys use rare elements such as Pd, the cost of the material 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 on titanium alloys using general-purpose elements without using rare elements.

In contrast, patent document 3 discloses a technique for improving 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 workability due to precipitation of TiC, and has a problem when it is practically applied to a heat exchanger or a device member.

Documents of the prior art

Patent document

Patent document 1 International publication No. 2007/077645

Patent document 2 Japanese patent application laid-open No. 6-25779

Patent document 3 Japanese patent application laid-open No. 2009-509038

Non-patent document

Non-patent document 1, iron and Steel, vol.80, No.4(1994), P353-358

Disclosure of Invention

Problems to be solved by the invention

The present invention addresses the problem of providing a titanium alloy having improved corrosion resistance while maintaining high workability by adding C instead of rare elements.

Means for solving the problems

The present inventors have conducted studies and found that a titanium alloy containing 0.10 to 0.30% C is subjected to a heat treatment at 750 to 820 ℃ and cooled at a rate of 0.001 ℃/sec or more, whereby the surface structure can be made into an α -single phase, excellent workability can be maintained, and corrosion resistance can be improved.

The gist of the present invention is as follows.

(1) A titanium alloy, which comprises, 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: 0.015% or less, O: 0.25% or less, and the balance Ti and inevitable impurities, wherein the surface structure of the titanium alloy is an alpha single phase.

(2) A method for producing a titanium alloy, which comprises subjecting a titanium alloy to a final heat treatment at 750-820 ℃ and cooling the titanium alloy at a rate of 0.001 ℃/sec or more, wherein the titanium alloy comprises, 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: 0.015% or less, O: less than 0.25%, and the balance Ti and inevitable impurities.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a titanium alloy having excellent corrosion resistance while maintaining high workability can be provided. Specifically, when the titanium alloy having 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 the workability and the corrosion resistance are improved.

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 a photograph showing a metallographic structure of a titanium alloy produced by the production method of the present invention.

Fig. 4 is an example of a metal photograph of a titanium alloy manufactured by a conventional manufacturing method.

Detailed Description

(composition of ingredients)

The titanium alloy of the 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 Ti and inevitable impurities. In the following description, the content indicated by "%" represents "% by mass".

＜C：0.10～0.30％＞

C plays an important role in improving corrosion resistance in the present invention. As the content of C increases, the corrosion rate decreases, improving the corrosion resistance (fig. 1). The effect of improving the corrosion resistance by the inclusion of C is remarkable when the content is 0.10% or more. On the other hand, as described later, when an α single-phase structure is formed and C exists as an interstitial solid solution element in the α phase, the effect of improving corrosion resistance by adding C becomes most remarkable. Further, addition of a large amount of C is not preferable because precipitation of TiC adversely affecting workability is promoted. The addition of a large amount of C has an adverse effect on workability, and the effect of improving corrosion resistance cannot be sufficiently exhibited. Therefore, the content of C is set to 0.10 to 0.30%. 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 solid-dissolved as an interstitial solid-solution element is an α phase of a surface structure described later.

＜N：0.001～0.03％＞

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

＜S：0.001～0.03％＞

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

＜P：0.001～0.03％＞

P is an essential element effective for improving strength, but as the content thereof increases, ductility and toughness deteriorate. P is a interstitial solid solution element, similarly to C, which plays an important role in improving corrosion resistance in the present invention. Therefore, the solid solution content of C may decrease due to the increase in the P content. 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：0.001～0.10％＞

Si is a relatively inexpensive element and is an element effective for improving heat resistance (oxidation resistance, high-temperature strength), but addition of a large amount promotes precipitation of compounds, 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：0.01～0.3％＞

Fe is an element effective for improving strength, but as the content thereof 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 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 deteriorates the ductility and toughness of the ingot. Therefore, the smaller the content, the better, but the increase of H in the production process is inevitable. Further, H is a interstitial solid solution element, similarly to C which plays an important role in improving corrosion resistance in the present invention. Therefore, the solid solution content of C may decrease due to the increase of the H content. Therefore, the content of H is limited to 0.015% or less. In addition, in the case of obtaining such a low H titanium alloy, high purity titanium sponge may be used, but if high purity titanium sponge is excessively used, the cost increases. In the present invention, H is an impurity element and may be 0%, but H is preferably 0.001% or more in terms of cost. The upper limit of the content of H is more preferably 0.005%.

< O: 0.25% or less

O is an essential element effective for improving strength, but as the content thereof increases, ductility and toughness deteriorate. Further, O is a interstitial solid solution element, similarly to C, which plays an important role in improving corrosion resistance in the present invention. Therefore, the solid solution content of C may decrease due to the increase in the O content. Therefore, the content of O is set to 0.25% or less. In addition, in the case of obtaining such a low-O titanium alloy, high-purity titanium sponge may be used, but if high-purity titanium sponge is excessively used, the cost increases. In the present invention, O is an impurity element, and may be 0%, and in terms of cost, O is preferably 0.01% or more. The upper limit of the content of O is more preferably 0.20%.

< surface layer is alpha Single phase >

The surface layer is an α -phase, which 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 means a range from the surface to a depth of 5 μm. The alpha phase does not include alpha' phase, acicular alpha phase. FIG. 3 shows the state of the surface of a titanium alloy produced by the production method of the present invention.

The α phase is composed of a hexagonal close-packed structure, and has a crystal structure and a grain boundary distribution different from those of an α' phase and a needle-like α phase transformed from a β phase. The C atoms that are solid-dissolved in the α phase are likely to exist as interstitial solid-solution elements between Ti atoms, and act on the electron states existing around the Ti nuclei to suppress the anodic reaction, thereby improving the corrosion resistance. The anodic reaction refers to a reaction in which metal is ionized by corrosion. When ionization of a metal is performed, electrons need to be deviated from Ti nuclei, and C is dissolved in an α phase, so that it is difficult to deviate electrons, and corrosion resistance is improved. Since the α' phase is not a close-packed structure and the acicular α phase is greatly affected by grain boundary segregation, a sufficient effect of improving corrosion resistance cannot be obtained as 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 solid-dissolved, and TiC is hardly precipitated, so that the workability is not deteriorated.

< temperature of Heat treatment >

Even in the case of a billet satisfying the above composition, the structure of the surface layer changes depending on the heat treatment temperature. And therefore 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 ℃. Therefore, in the present invention, the heat treatment temperature is set to 750 to 820 ℃. The holding time in this temperature range is not particularly limited, and may be 1 second or more, preferably 30 seconds or more.

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

< Cooling Rate >

Even if the heat treatment temperature is within the above range, if the cooling rate is slow, TiC precipitates during cooling, and therefore the surface layer does not become α. The cooling rate of the present invention is 0.001 ℃/sec or more, preferably 1 ℃/sec or more. The precipitation of TiC can be suppressed as the cooling rate is higher, but an excessively high cooling rate adversely affects the shape maintenance of the titanium plate, and therefore the upper limit is 2000 ℃/sec.

< manufacturing method >

Next, a method for producing the titanium alloy of the present invention will be described. The titanium alloy of the present invention can be produced without using a special method, in particular, by adding shot blasting, pickling treatment, and the like as needed in each step of casting → blooming (or hot forging) → hot rolling → annealing (→ cold rolling → final annealing), similarly to conventional industrial pure titanium. In the above description of the steps, the bracketed (→ cold rolling → finish annealing) steps are not essential, but may be appropriately performed depending on the thickness, shape, size, and the like of the titanium to be produced.

Examples

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

Titanium ingots having the respective composition shown in table 1 were cast using a melting raw material containing titanium sponge and predetermined additive elements in a vacuum arc melting furnace. Fe as electrolytic iron and C as TiC powder are added to the additive elements respectively.

In the table, Al, V, Cr, Ru, Pd, Ni, and Co are not intentionally added elements, and the values in the table indicate the contents of the above-described elements as impurity levels.

[ Table 1]

A cast titanium ingot is forged and hot-rolled at a heating temperature of 800 to 1000 ℃ to form a hot-rolled sheet having a thickness of 4.0mm, and a test piece for corrosion resistance evaluation is produced by pickling and machining. Then, vacuum annealing was performed at the temperatures shown in table 2 to evaluate the corrosion resistance.

The surface structure was identified by XRD (X-ray diffraction) using CoK.alpha.rays as a characteristic X-ray, a voltage of 30kV and a current of 100mA, and observation of a microstructure. The range of X-ray diffraction is 10-2 theta-110 degrees, the step is 0.04 degrees, the accumulation time is 2s, and the X-ray incidence angle is 0.3 degrees. The presence or absence of the alpha phase, beta phase, alpha' phase and TiC was examined from the positions of X-ray diffraction peaks of a test piece (20 mm in length and 20mm in width), and the surface structure was examined comprehensively by observing the presence or absence of acicular alpha by microscopic structure. When the X-ray diffraction peak intensity is detected to exceed 10% of the background, it is confirmed that a β phase, an α' phase, and TiC are formed, and otherwise, it is determined as an α single phase.

The corrosion resistance was evaluated by immersing the test piece in a 3 mass% aqueous hydrochloric acid solution at 90 ℃ for 168 hours, comparing the weight before and after immersion, and calculating the corrosion rate. The corrosion rate was 2 mm/year or less. The results of the corrosion resistance evaluation test are shown in table 2. The workability was evaluated by a tensile test according to the method described in JIS Z2241 and the elongation. The elongation was measured by an elongation meter, and a case where the total elongation was 40% or more was regarded as pass.

[ Table 2]

In nos. 1 to 9, which all satisfied the composition of the material, the heat treatment temperature, and the surface structure defined in the present invention, corrosion rate was significantly low, corrosion resistance was improved, and sufficient elongation was exhibited, so that it was confirmed that corrosion resistance and workability were compatible.

In Nos. 10 to 16, the ingot components such as carbon were within the range of the present invention, but the heat treatment temperature and the cooling rate were outside the range of the present invention, and therefore the surface structure did not become an α single phase, the corrosion rate was high, and satisfactory elongation was not exhibited. Since Nos. 14, 16, 18 and 20 were slow in cooling rate, TiC precipitated during the cooling process.

In nos. 17 to 24, elements such as S, P, Si which lower the solid solubility limit of C were added beyond the range of the present invention, and even if the temperature and cooling rate of the present invention were satisfied, no α single phase was formed, the corrosion resistance was not improved, and TiC was precipitated, and therefore the elongation was low.

In contrast to nos. 1 and 5, which hardly observed discoloration or the like in the outdoor environment, nos. 23 and 24 had brown surfaces in the outdoor environment.

Claims (2)

1. A titanium alloy, characterized in that, in mass %, C: 0.10 to 0.30%, 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 balance is Ti and inevitable impurities, The surface layer of the titanium alloy is α single phase, and the intensity of the X-ray diffraction peak of TiC is less than 10% compared with the intensity of the background, The surface layer refers to a range from the surface to a depth of 5 μm. 2. A method for producing a titanium alloy, characterized in that a final heat treatment is performed on the titanium alloy at 750 to 820° C., and the titanium alloy is cooled at a rate of 0.001° C./sec or more, and the titanium alloy is calculated as mass %. C: 0.10 to 0.30%, 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 balance is Ti and inevitable impurities.

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