JP6927418B2 - Titanium alloy and its manufacturing method - Google Patents

Description

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

Pure industrial titanium exhibits excellent corrosion resistance even in seawater, which is corroded by general-purpose stainless steel such as SUS304. Taking advantage of this high corrosion resistance, it is used in seawater desalination plants and the like.

On the other hand, materials for chemical plants may be used in an environment that is more corrosive than seawater such as hydrochloric acid. In such an environment, pure industrial titanium also corrodes significantly.

With the intention of using it in such a highly corrosive environment, a corrosion-resistant titanium alloy having excellent corrosion resistance in an environment more corrosive than industrial pure titanium 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 an alloy in which an intermetallic compound is precipitated in addition to the addition of a platinum group element.

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

Therefore, Patent Document 3 discloses an invention in which C is used to improve the corrosion resistance and strength of Ti. However, as shown in FIG. 4, the titanium alloy described in Patent Document 3 has a problem in workability due to precipitation of TiC, which causes a problem when it is actually applied to a heat exchanger or a plant member.

International Publication No. 2007/077645 Japanese Unexamined Patent Publication No. 6-25779 Special Table No. 2009-509038

"Iron and Steel", vol. 80, No. 4 (1994), P353-358

An object of the present invention is to provide a titanium alloy having improved corrosion resistance while maintaining high processability by adding C instead of a rare element.

As a result of the research conducted by the present inventors, a titanium alloy containing 0.10 to 0.30% C is heat-treated at 750 to 820 ° C. and cooled at a rate of 0.001 ° C./sec or more. Therefore, it has been found that the surface structure can be made into an α single phase, and the corrosion resistance can be improved while maintaining excellent processability.

The gist of the present invention is as follows.
(1)
By 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 unavoidable impurities, and the surface structure Is an α single-phase titanium alloy.
(2)
The titanium alloy according to (1), wherein the intensity of the X-ray diffraction peaks of the β phase, α'phase, and TiC in the surface structure is 10% or less as compared with the intensity of the background.

(3)
By 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, and the balance is Ti and titanium alloy which is an unavoidable impurity. The method for producing a titanium alloy according to (1) or (2) , wherein the finish heat treatment is performed at 760 to 820 ° C. and the titanium alloy is cooled at a rate of 0.001 ° C./sec or more.

According to the present invention, it is possible to provide a titanium alloy having good corrosion resistance while maintaining high workability. Specifically, when the titanium alloy having the composition range of the present invention was produced by the production method of the present invention, the surface structure became an α single phase, and both processability and corrosion resistance were improved.

It is a figure which showed the relationship between the corrosion rate and the amount of C addition in a hydrochloric acid immersion test. It is a figure which showed the relationship between the corrosion rate and the heat treatment temperature in a hydrochloric acid immersion test. This is an example of a metallographic photograph of a titanium alloy produced by the production method of the present invention. This is an example of a metal photograph of a titanium alloy manufactured by a conventional manufacturing method.

(Ingredient composition)
The titanium alloy of the present invention has 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 (including 0%), O: 0.25% or less (including 0%) The balance is Ti and unavoidable impurities. In the following description, all the contents indicated by "%" indicate "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 due to the inclusion of C is remarkably exhibited when the content is 0.10% or more. On the other hand, as will be described later, the effect of improving the corrosion resistance by adding C is most remarkable when an α single-phase structure is formed and C is present in the α phase as an intrusive solid solution element. Further, the addition of a large amount of C is not preferable because it promotes the precipitation of TiC, which adversely affects the processability. In addition to adversely affecting workability, the addition of a large amount of C does not sufficiently exhibit the effect of improving corrosion resistance. Therefore, the content of C is set to 0.10 to 0.30%. The lower limit of the more preferable content of the solid solution C is 0.12%, and the upper limit of the more preferable content of the solid solution C is 0.28%. The α phase in which C dissolves as an intrusive 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 improving strength, but ductility and toughness deteriorate as its content increases. Further, N is an invading solid solution element like 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 N content. Therefore, the content of N is set to 0.001 to 0.03%. The upper limit of the more preferable N content is 0.025%.

<S: 0.001 to 0.03%>
S is an essential element effective for improving strength, but its ductility and toughness deteriorate as its content increases. Further, S is an intrusive solid solution element like 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 S content. Therefore, the content of S is set to 0.001 to 0.03%. The upper limit of the more preferable S content is 0.025%.

<P: 0.001 to 0.03%>
P is an essential element effective for improving strength, but ductility and toughness deteriorate as its content increases. Further, P is an invading solid solution element like 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 P content. Therefore, the content of P is set to 0.001 to 0.03%. The upper limit of the more preferable P content is 0.025%.

<Si: 0.001 to 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 compound precipitation and deteriorates ductility and toughness. Therefore, the Si content is set to 0.001 to 0.10%. The lower limit of the more preferable Si content is 0.003%, and the upper limit of the more preferable Si content is 0.08%.

<Fe: 0.01-0.3%>
Fe is an element effective for improving strength, but its ductility and toughness deteriorate as its content increases. Further, Fe is a strong β-stabilizing element among the elements contained in the titanium alloy of the present invention, and when added in a large amount, it becomes difficult to obtain an α single-phase structure described later. Therefore, the Fe content is set to 0.01 to 0.30%. The lower limit of the more preferable Fe content is 0.03%, and the upper limit of the more preferable Fe content is 0.25%.

<H: 0.015% or less>
H is an element that forms a titanium hydride and deteriorates the ductility and toughness of the material. Therefore, it is better that the content is small, but an increase in H is unavoidable in the manufacturing process. Further, H is an intrusive solid solution element like 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 H content. Therefore, the H content is limited to 0.015% or less. Further, in order to obtain such a low H titanium alloy, high-purity titanium sponge may be used, but if too much high-purity titanium sponge is used, the cost will increase. In the present invention, H is an impurity element and may be 0%, but H is preferably 0.001% or more from the viewpoint of cost. The upper limit of the more preferable H content is 0.005%.

<O: 0.25% or less>
O is an essential element effective for improving strength, but its ductility and toughness deteriorate as its content increases. Further, O is an intrusive solid solution element like 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 O content. Therefore, the content of O is set to 0.25% or less. Further, in order to obtain such a low O titanium alloy, high-purity titanium sponge may be used, but if too much high-purity titanium sponge is used, the cost will increase. In the present invention, O is an impurity element and may be 0%, and O is preferably 0.01% or more from the viewpoint of cost. The upper limit of the more preferable O content is 0.20%.

<Surface layer is α single phase>
When the surface layer is α single phase, it means that the structure of the surface layer is α phase, and the intensity of the X-ray diffraction peaks of β phase, α'phase, and TiC is 10% or less of the background intensity. do. Here, the surface layer is a range from the surface to a depth of 5 μm. The α phase does not include the α'phase or the acicular α phase. FIG. 3 shows the surface of the 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 different crystal structure and grain boundary distribution from the α'phase and acicular α phase formed by metamorphosis from the β phase. The C atom dissolved in the α phase tends to exist as an interpenetrating solid solution element between Ti atoms, and the corrosion resistance can be improved by suppressing the anodic reaction by acting on the electronic state existing around the Ti nucleus. The anodic reaction refers to a reaction in which a metal is corroded and ionized. When the metal is ionized, it is necessary to dissociate the electrons from the Ti nucleus, and by dissolving C in the α phase, it is difficult for the electrons to dissociate and the corrosion resistance is improved. Due to the fact that the α'phase does not have a close-packed structure and that 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 processability of the material. However, since carbon is hardly dissolved in the surface layer of the titanium alloy of the present invention and TiC is hardly precipitated, the workability is not deteriorated.

<Heat treatment temperature>
Even if the material satisfies the above-mentioned component composition, the structure of the surface layer changes depending on the heat treatment temperature. Therefore, the performance to be exhibited also changes. As shown in FIG. 2, the corrosion rate of the titanium alloy produced by the heat treatment at around 800 ° C. is most suppressed. Therefore, in the present invention, the heat treatment temperature is 750 to 820 ° C. There is no particular limitation on the holding time in this temperature range, and holding for 1 sec or more, preferably 30 sec or more is sufficient.

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 and the structure becomes α'phase or acicular α phase. .. For example, FIG. 4 shows the surface layer of a titanium alloy produced by a conventional method that has been heat-treated outside this temperature range. Island-shaped TiC precipitates are generated on the surface layer (Fig. 4). TiC is a hard compound and significantly deteriorates the processability of the material. Therefore, the workability of the titanium alloy produced by the conventional method is deteriorated.

<Cooling speed>
Even if the heat treatment temperature is in the above range, if the cooling rate is slow, TiC is precipitated in 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. Further, the faster the cooling rate, the more the precipitation of TiC can be suppressed, but the too fast cooling rate adversely affects the shape maintenance of the titanium plate, so the upper limit is set to 2000 ° C./sec.

<Manufacturing method>
Next, the method for producing the titanium alloy of the present invention will be described. The titanium alloy of the present invention can be used at any time during each process such as casting → slabbing rolling (or hot forging) → hot rolling → annealing (→ cold rolling → final annealing) in the same manner as ordinary industrial pure titanium. By adding blasting, pickling treatment, etc., it can be manufactured without using a special method. In the above description of the process, the process of parentheses (→ cold rolling → final annealing) is not always necessary, but it is appropriately carried out depending on the thickness, shape, size, etc. of the titanium to be manufactured.

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

A titanium ingot having each component composition shown in Table 1 was cast by a vacuum arc melting furnace using a melting raw material containing titanium sponge and a predetermined additive element. Among the added elements, Fe was electrolytic iron and C was TiC powder.

In addition, Al, V, Cr, Ru, Pd, Ni, and Co in the table are not elements to be added intentionally, and the values in the table indicate that the content of each of the above elements is an impurity level. It is a thing.

Using the cast titanium ingot, forging and hot rolling are performed at a heating temperature of 800 to 1000 ° C. to obtain a hot-rolled plate with a thickness of 4.0 mm, and a test piece for corrosion resistance evaluation is prepared by pickling and machining. bottom. Then, vacuum annealing was carried out at each temperature shown in Table 2 to evaluate the corrosion resistance.

The surface structure was identified by XRD (X-ray diffraction) and microstructure observation. The conditions for X-ray diffraction were CoKα rays as characteristic X-rays, a voltage of 30 kV, and a current of 100 mA. The range of X-ray diffraction was 10 ° ≤ 2θ ≤ 110 °, the step was 0.04 °, the integration time was 2 s, and the X-ray incident angle was 0.3 °. Investigate the presence or absence of α phase, β phase, α'phase, and TiC from the position of the X-ray diffraction peak of the test piece (length 20 mm, width 20 mm), and observe the microstructure to comprehensively surface the surface including the presence or absence of needle-shaped α. The organization was investigated. When the X-ray diffraction peak intensity was detected to exceed 10% of the background, formation of β phase, α'phase, and TiC was observed, and in other cases, it was judged to be α single phase.

The corrosion resistance was evaluated by immersing the test piece in a 3 mass% hydrochloric acid aqueous solution at 90 ° C. for 168 hours and comparing the weights before and after the immersion, based on the calculated corrosion rate. The case where the corrosion rate was 2 mm / year or less was regarded as acceptable. The results of the corrosion resistance evaluation test are shown in Table 2. Workability is JIS
A tensile test was performed by the method described in Z 2241 and evaluated by its elongation. The elongation was measured by an extensometer, and the case where the total elongation was 40% or more was regarded as acceptable.

In Nos. 1 to 9 which satisfy all of the material components, 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 that both corrosion resistance and workability are compatible. It could be confirmed.

No. In 10 to 16, the material components such as carbon are within the range of the present invention, but since the heat treatment temperature or the cooling rate is outside the range of the present invention, the surface structure does not become α single phase, and the corrosion rate is greatly satisfied. It did not show any growth. No. Since the cooling rates of 14, 16, 18 and 20 are slow, TiC was precipitated during the cooling process.
No. In 17 to 24, elements such as S, P, and Si that lower the solid solution limit of C are added beyond the range of the present invention, and even if the temperature and cooling rate of the present invention are satisfied, the α single phase is formed. However, the corrosion resistance was not improved, and the elongation was low because TiC was also precipitated.
In Nos. 1 and 5, discoloration and the like were hardly observed in the outdoor environment, whereas in Nos. 23 and 24, the surface became brown in the outdoor environment.

Claims (3)

By 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: 0.01-0.3%,
H: 0.015% or less,
O: A titanium alloy characterized by having an O: 0.25% or less, the balance being Ti and unavoidable impurities, and the surface layer being an α single phase. The titanium alloy according to claim 1, wherein the intensity of the X-ray diffraction peaks of the β phase, α'phase, and TiC in the surface structure is 10% or less as compared with the intensity of the background. By 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: 0.01-0.3%,
H: 0.015% or less,
O: A claim characterized by subjecting a titanium alloy having an O: 0.25% or less and the balance being Ti and an unavoidable impurity to a finish heat treatment at 760 to 820 ° C. and cooling at a rate of 0.001 ° C./sec or more. Item 2. The method for producing a titanium alloy according to Item 1 or 2.

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