KR102340036B1 - Titanium alloy and manufacturing method thereof - Google Patents

Abstract

In % 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% Hereinafter, O: 0.25% or less, the balance is Ti and unavoidable impurities, and the surface layer is a titanium alloy, characterized in that the α single phase.

Description

Titanium alloy and manufacturing method thereof

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

Industrial pure titanium exhibits excellent corrosion resistance even in seawater that is corroded by general-purpose stainless steels such as SUS304. Utilizing this high corrosion resistance, it is used in seawater desalination plants, etc.

On the other hand, materials for chemical plants may be used in environments with high corrosiveness than seawater such as hydrochloric acid. Under these circumstances, even industrial grade titanium is remarkably corroded.

In order to be used in such a highly corrosive environment, a corrosion-resistant titanium alloy excellent in corrosion resistance under a highly corrosive environment 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 deposited in addition to addition of a platinum group element.

Since these titanium alloys use rare elements, such as Pd, the material cost is improved. Therefore, it has the subject of improving the corrosion resistance of titanium without using an expensive rare element. Therefore, various proposals have been made regarding a titanium alloy utilizing a general-purpose element without using a rare element.

Then, in patent document 3, the invention which improved the corrosion resistance and strength of Ti using C is disclosed. However, as shown in FIG. 4, TiC precipitates in the titanium alloy described in Patent Document 3, and there is a problem in workability, and it becomes a problem when actually applied to a heat exchanger or a plant member.

International Publication No. 2007/077645 Japanese Patent Laid-Open No. 6-25779 Japanese Patent Publication No. 2009-509038 Publication

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

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

As a result of the research conducted by the present inventors, the surface structure of the 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, so that the surface structure can be made into an α single phase. It has been found that corrosion resistance can also be improved while maintaining excellent workability.

The gist of the present invention is as follows.

(1) 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 unavoidable impurities, and the surface structure is a titanium alloy of α single phase.

(2) 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 an unavoidable impurity, the titanium alloy is subjected to a finish heat treatment at 750 to 820°C, and a method for producing a titanium alloy in which 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 a titanium alloy having a composition range of the present invention was manufactured by the manufacturing method of the present invention, the surface structure became α single phase, and both workability and corrosion resistance were improved.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship between the corrosion rate and C addition amount in a hydrochloric acid immersion test.
2 is a diagram showing the relationship between the corrosion rate and the heat treatment temperature in the hydrochloric acid immersion test.
3 is an example of a photograph of the metal structure of the titanium alloy manufactured by the manufacturing method of the present invention.
4 is an example of a metal photograph of a titanium alloy manufactured by a conventional manufacturing method.

(Ingredient composition)

Titanium alloy of the present invention, 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 being Ti and unavoidable impurities. In addition, all content represented by "%" in the following description shows "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 containing C is remarkably expressed when it is 0.10% or more. On the other hand, as will be described later, the effect of improving corrosion resistance by the addition of C becomes most remarkable when an α single-phase structure is formed and C exists as an interstitial solid solution element in the α phase. In addition, the addition of a large amount of C is not preferable because it promotes the precipitation of TiC which adversely affects the workability. In addition to adversely affecting workability, addition of a large amount of C does not fully exhibit the effect of improving corrosion resistance. Therefore, the content of C is set to 0.10 to 0.30%. Moreover, the lower limit of the content of the more preferable solid solution C is 0.12%, and the upper limit of the content of the more preferable solid solution C is 0.28%. The α phase in which C is dissolved as an interstitial solid solution element is the α phase of the surface structure described later.

<N: 0.001 to 0.03%>

Although N is an essential element effective for strength improvement, ductility and toughness deteriorate as the content increases. In addition, N is an interstitial solid solution element similarly to C which plays an important role in improving corrosion resistance in this invention. Therefore, there exists a possibility that the solid solution content of C may fall with an increase in N content. Therefore, the N content 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%>

Although S is an essential element effective for strength improvement, ductility and toughness deteriorate as the content increases. In addition, S is an interstitial solid solution element similarly to C which plays an important role in improving corrosion resistance in this invention. Therefore, there exists a possibility that the solid solution content of C may fall with the increase of S content. 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%>

Although P is an essential element effective for strength improvement, as its content increases, ductility and toughness deteriorate. In addition, P is an interstitial solid solution element similarly to C which plays an important role in improving corrosion resistance in this invention. Therefore, there exists a possibility that the solid solution content of C may fall with an increase in 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 to 0.10%>

Si is a relatively inexpensive element and is an element effective for improving heat resistance (oxidation resistance, high temperature strength), but adding a large amount of Si 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 more preferable upper limit of the Si content is 0.08%.

<Fe: 0.01 to 0.3%>

Although Fe is an element effective for strength improvement, 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 when it is 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 more preferable upper limit of the Fe content is 0.25%.

<H: 0.015% or less>

H is an element that forms titanium hydride and deteriorates the ductility and toughness of the material. Therefore, although the one with less content is preferable, the increase of H in a manufacturing process is unavoidable. In addition, H is an interstitial solid solution element similarly to C which plays an important role in improving corrosion resistance in this invention. Therefore, there exists a possibility that the solid solution content of C may fall with an increase in 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 sponge titanium may be used, but if too much high-purity sponge titanium is used, the cost increases. 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 content of H is 0.005%.

<O: 0.25% or less>

Although O is an essential element effective for strength improvement, ductility and toughness deteriorate as the content increases. In addition, O is an interstitial solid solution element similarly to C which plays an important role in improving corrosion resistance in this invention. Therefore, there exists a possibility that the solid solution content of C may fall with an increase in O content. Therefore, the content of O is made 0.25% or less. In addition, when obtaining such a low-O titanium alloy, high-purity sponge titanium may be used, but if too much high-purity sponge titanium is used, the cost increases. In the present invention, O is an impurity element and may be 0%, and 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%.

<Surface layer is α single phase>

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

The α phase is composed of a hexagonal fine packing structure, and the crystal structure and grain boundary distribution are different from the α′ phase and the needle-like α phase formed by transformation from the β phase. The C atoms dissolved in the α phase tend to exist as interstitial solid solution elements between Ti atoms, and the corrosion resistance can be improved by inhibiting the anode reaction by acting on the electronic state existing around the Ti atom nuclei. The anode reaction refers to a reaction in which a metal is ionized by corrosion. When the metal is ionized, it is necessary to separate electrons from the Ti atom nucleus, and by dissolving C in the α phase, it is difficult to separate electrons and the corrosion resistance is improved. The α' phase does not have a tightest structure, and the needle-like α phase has a large influence of grain boundary segregation.

TiC is a hard compound and significantly deteriorates the workability of the material. However, since carbon is substantially dissolved in the surface layer of the titanium alloy of the present invention and TiC is hardly precipitated, workability is not deteriorated.

<Heat treatment temperature>

Even if it is a material satisfying the above-mentioned composition, the structure of the surface layer changes depending on the heat treatment temperature. As a result, the performance exhibited also changes. As shown in Figure 2, the corrosion rate of the titanium alloy produced by heat treatment around 800 ℃ 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 it is sufficient if the holding time is 1 sec or more, preferably 30 sec or more.

The reason why the corrosion rate of the titanium alloy is suppressed at 750 to 820°C is that when heat treatment is performed outside this temperature range, TiC is precipitated or the structure becomes α′ phase or acicular α phase. For example, FIG. 4 shows the state of the surface layer of a titanium alloy manufactured by a conventional method in which heat treatment is performed outside of this temperature range. In the surface layer, island-shaped TiC precipitates are generated (FIG. 4). TiC is a hard compound and significantly deteriorates the workability of the material. 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, when the cooling rate is slow, since TiC is precipitated in the cooling process, 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, although a faster cooling rate can suppress precipitation of TiC, since an excessively fast cooling rate causes a bad influence on shape maintenance of a titanium plate, an upper limit is made into 2000 degreeC/sec.

<Production method>

Next, the manufacturing method of the titanium alloy of this invention is demonstrated. The titanium alloy of the present invention, like ordinary industrial pure titanium, is subjected to frequent blasting and pickling treatment between each process such as casting → ingot rolling (or hot forging) → hot rolling → annealing (→ cold rolling → final annealing). It can be manufactured without using a special method in particular. In addition, in the description of the above process, the step of (→ cold rolling → final annealing) of parentheses is not necessarily required, but is appropriately carried out depending on the thickness, shape, size, etc. of the titanium to be manufactured.

Example

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

It was used as a melting raw material containing sponge titanium and a predetermined|prescribed additive element, and the titanium ingot of each component composition shown in Table 1 was cast with the vacuum arc melting furnace. Among the additional elements, electrolytic iron was added to Fe and TiC powder was added to C.

In addition, 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 content of each element is an impurity level.

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

The surface structure was identified by XRD (X-ray diffraction) and microstructure observation, and the conditions of X-ray diffraction were that Co Kα rays were used as characteristic X-rays, the voltage was 30 kV, and the current was 100 mA. The range of X-ray diffraction was 10°≤2θ≤110°, the step was 0.04°, the integration time was 2s, and the X-ray incident angle was 0.3°. From the position of the X-ray diffraction peak of the test piece (length 20 mm, width 20 mm), the presence or absence of α phase, β phase, α' phase, and TiC was investigated, The surface texture was investigated. When the X-ray diffraction peak intensity was detected to exceed 10% of the background, the formation of the β phase, the α′ phase and TiC was confirmed, and in other cases, it was judged to be an α single phase.

Corrosion resistance was evaluated by the magnitude of the corrosion rate calculated by immersing a test piece in 90 degreeC and 3 mass% hydrochloric acid solution for 168h, and comparing the weight before and behind 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. Workability performed a tensile test by the method described in JIS Z 2241, and evaluated by the elongation. The measurement of elongation was performed with an extensometer, and the case where total elongation was 40 % or more was set as the pass.

In Nos. 1 to 9, which satisfy all of the material components, heat treatment temperature, and surface layer structure stipulated in the present invention, the corrosion rate is remarkably low, corrosion resistance is improved, and since sufficient elongation is exhibited, both corrosion resistance and workability can be confirmed. there was.

In Nos. 10 to 16, the material components such as carbon are within the scope of the present invention, but since the heat treatment temperature or cooling rate is outside the scope of the present invention, the surface structure does not become α single phase, and the elongation at which the corrosion rate is greatly satisfied. did not show In Nos. 14, 16, 18, and 20, since the cooling rate was slow, TiC was precipitated in the cooling process.

In Nos. 17 to 24, elements that lower the solid solution limit of C, such as S, P, Si, are added beyond the range of the present invention, and even if the temperature and cooling rate of the present invention are satisfied, α single phase does not occur, and corrosion resistance also did not improve, and since TiC was also precipitated, the elongation was low.

In No. 1 and 5, discoloration etc. were hardly observed in an outdoor environment, whereas in No. 23 and 24, the surface became brown in an outdoor environment.

Claims (2)

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
and the balance is Ti and unavoidable impurities, and the surface layer is an α single phase. 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
And, a titanium alloy with the balance of Ti and unavoidable impurities is subjected to a finish heat treatment at 750 to 820 ° C., followed by cooling at a rate of 0.001 ° C./sec or more.

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