TW201943862A - Titanium alloy and production method thereof capable of improving anti-corrosion property while maintaining high processing property - Google Patents

Abstract

Provided is a titanium alloy, which includes, 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 remainder being Ti and unavoidable impurities, wherein the surface layer of the titanium alloy is an <alpha> single phase. The invention also provides a production method of titanium alloy, which is characterized in performing a finish heat treatment at 750 to 820 DEG C and a cooling at a rate of 0.001 DEG C/sec or more on a titanium alloy including, 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 remainder being Ti and unavoidable impurities.

Description

Titanium alloy and manufacturing method thereof

FIELD OF THE INVENTION The present invention relates to a titanium alloy and a method for manufacturing the same.

BACKGROUND OF THE INVENTION As far as industrial pure titanium is concerned, in general stainless steel such as SUS304, it shows excellent corrosion resistance even in seawater where corrosion occurs. Utilizing this high corrosion resistance, it is used in seawater desalination plants.

On the other hand, in materials used in chemical plants, they are sometimes used in environments that are more corrosive than seawater, such as hydrochloric acid. In such an environment, even pure titanium for industrial use can significantly corrode.

In order to use it in such a highly corrosive environment, a corrosion-resistant titanium alloy having a corrosion resistance superior to 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 Literature 2 and Non-Patent Literature 1 disclose alloys in which intermetallic compounds are precipitated in addition to platinum group elements.

These titanium alloys increase the cost of materials because they use rare elements such as Pd. Therefore, there is a problem that the corrosion resistance of titanium is improved without using rare elements of high price. Therefore, various proposals have been made for titanium alloys that use common elements without using rare elements.

Among them, Patent Document 3 discloses an invention using C to improve the corrosion resistance and strength of Ti. However, as shown in FIG. 4, in the titanium alloy described in Patent Document 3, TiC precipitates and there is a problem in terms of workability, which poses a problem when it is actually applied to a heat exchanger or a plant component.

Prior Art Literature Patent Literature Patent Literature 1: International Publication No. 2007/077645 Patent Literature 2: Japanese Patent Application Laid-Open No. 6-25779 Patent Literature 3: Japanese Patent Application No. 2009-509038

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

SUMMARY OF THE INVENTION Problems to be Solved by the Invention The object of the present invention is to provide a titanium alloy in which C is added instead of a rare element, thereby maintaining high processability and improving corrosion resistance.

Means to Solve the Problem After research by the inventors of the present case, it was found that a titanium alloy to which 0.10 to 0.30% C was added was heat-treated at 750 to 820 ° C, and cooled at a speed of 0.001 ° C / sec or more Thereby, 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 this invention is as follows. (1) A titanium alloy based on 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, the rest is Ti and unavoidable impurities; the surface structure is α single phase.

(2) A method for manufacturing a titanium alloy, in which the following titanium alloy is subjected to a finishing heat treatment at 750 to 820 ° C and cooled at a rate of 0.001 ° C / sec or more; the titanium alloy is based on 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, The remainder is Ti and unavoidable impurities.

ADVANTAGE OF THE INVENTION According to this invention, the titanium alloy which can maintain a high workability and is excellent in corrosion resistance can be provided. Specifically, when the titanium alloy of the composition range of the present invention is manufactured by the manufacturing method of the present invention, the surface structure becomes an α single phase, and both workability and corrosion resistance are improved.

The form (composition) for implementing the invention The titanium alloy of the present invention is 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 (including 0%), O: 0.25% or less (including 0%), and the remainder is Ti and unavoidable impurities. In addition, all contents shown as "%" in the following description mean "mass%."

<C: 0.10 to 0.30%> C plays an important role in improving the corrosion resistance in the present invention. As the C content increases, the corrosion rate decreases and the corrosion resistance improves (Figure 1). The effect of improving corrosion resistance caused by the presence of C is clearly exhibited in the case of 0.10% or more. On the other hand, as will be described later, the effect of increasing the corrosion resistance by adding C is most significant when an α single-phase structure is formed and C exists in the α phase as an intrusive solid solution element. In addition, a large amount of C is not suitable because it promotes the precipitation of TiC, which adversely affects the processability. Adding a large amount of C not only adversely affects workability, but also does not exhibit a sufficient effect of improving corrosion resistance. Therefore, the C content is set to 0.10 to 0.30%. In addition, the lower limit of the preferred solid solution C content is 0.12%, and the upper limit of the preferred solid solution C content is 0.28%. The α phase in which C is a solid solution that is an intrusive 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 for effectively improving strength, as its content increases, ductility and toughness deteriorate. In addition, N 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 an increase in the N content. Therefore, the N content is set to 0.001 to 0.03%. The upper limit of the preferred N content is 0.025%.

<S: 0.001 to 0.03%> Although S is an essential element for effectively improving strength, as its content increases, ductility and toughness deteriorate. In addition, S is an intrusive solid solution element like C, which plays an important role in improving corrosion resistance in the present invention. Therefore, there is a fear that the solid solution content of C may decrease due to an increase in the S content. Therefore, the S content is set to 0.001 to 0.03%. The upper limit of the preferable S content is 0.025%.

<P: 0.001 to 0.03%> Although P is an essential element for effectively improving strength, as its content increases, ductility and toughness deteriorate. In addition, P is an intrusive solid solution element like C, which plays an important role in improving corrosion resistance in the present invention. Therefore, there is a fear that the solid solution content of C may decrease due to an increase in the P content. Therefore, the P content is set to 0.001 to 0.03%. The upper limit of the preferable P content is 0.025%.

<Si: 0.001 ~ 0.10%> Although Si is a cheaper element and an element that effectively improves heat resistance (oxidation resistance, high temperature strength), a large amount of addition will promote the precipitation of compounds, which will deteriorate the ductility and toughness. . Therefore, the Si content is set to 0.001 to 0.10%. The lower limit of the preferred Si content is 0.003%, and the upper limit of the preferred Si content is 0.08%.

<Fe: 0.01 to 0.3%> Although Fe is an essential element for effectively improving strength, as its content increases, ductility and toughness deteriorate. Furthermore, Fe is the strongest and most powerful β-stabilizing element among the elements contained in the titanium alloy of the present invention. Once added in large amounts, it becomes difficult to obtain an α single-phase structure as described later. Therefore, the Fe content is set to 0.01 to 0.30%. The lower limit of the preferred Fe content is 0.03%, and the upper limit of the preferred 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, the smaller the content, the better, but an increase in H cannot be avoided in the manufacturing steps. In addition, H is an intrusive solid solution element like C, which plays an important role in improving corrosion resistance in the present invention. Therefore, there is a fear that the solid solution content of C may decrease due to an increase in the H content. Therefore, the H content is limited to 0.015% or less. Further, in order to obtain such a low-H titanium alloy, although high-purity sponge titanium can be used, the cost will increase if the high-purity sponge titanium is excessively used. 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. The upper limit of the preferred H content is 0.005%.

<O: 0.25% or less> Although O is an essential element for effectively improving strength, as its content increases, ductility and toughness deteriorate. In addition, 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 O content is set to 0.25% or less. Further, in order to obtain such a low-O titanium alloy, although high-purity sponge titanium can be used, the cost will increase if excessively high-purity sponge titanium is used. In the present invention, O is an impurity element and may also be 0%; from a cost perspective, O is preferably 0.01% or more. The upper limit of the preferred O content is 0.20%.

<The surface layer is an alpha single phase> The so-called surface layer is an alpha single phase, which means that the surface layer structure is an alpha phase, and the intensity of the X-ray diffraction peak of TiC is 10% or less compared with the background intensity. In this case, the so-called surface layer ranges from the surface to a depth of 5 μm. The α phase does not include: an α 'phase or a needle-like α phase. FIG. 3 shows the appearance of a titanium alloy manufactured by the manufacturing method of the present invention.

The α phase is composed of the hexagonal closest-packed structure, and its crystal structure and grain boundary distribution are different from the α 'phase or needle-shaped α phase formed from the β phase metamorphosis. C atoms, which are solid-dissolved in the α phase, easily exist between Ti atoms as intrusive solid-solution elements, and suppress the anode reaction by acting on the electronic states existing around the Ti atomic nucleus, thereby improving the corrosion resistance. The so-called anodic reaction is a reaction in which metal is corroded and ionized. When the metal is ionized, the electrons must be detached from the Ti nucleus, and by dissolving C in the α phase, it is difficult for the electrons to detach and the corrosion resistance is improved. The α 'phase is not the densest structure, and the acicular α phase is greatly affected by grain boundary segregation. For these two reasons, compared with the α phase, the effect of improving corrosion resistance cannot be fully obtained.

TiC is a hard compound that significantly deteriorates the processability of the material. However, in the surface layer of the titanium alloy of the present invention, carbon is substantially solid-dissolved, and TiC is also substantially not precipitated, so there is no deterioration in processability.

<Heat treatment temperature> Even if it is a material satisfying the above-mentioned component composition, the structure of the surface layer changes depending on the heat treatment temperature. As a result, the performance will also change. As shown in Fig. 2, the corrosion rate of the titanium alloy produced by heat treatment at about 800 ° C is most suppressed. Therefore, in the present invention, the heat treatment temperature is 750-820 ° C. The holding time in this temperature range is not particularly limited, 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 once heat treatment is performed outside this temperature range, TiC will precipitate and the structure will become an α 'phase or a needle-like α phase. For example, FIG. 4 shows the surface appearance of a titanium alloy manufactured by a conventional method of performing heat treatment outside this temperature range. On the surface layer, island-like TiC precipitates were formed (Fig. 4). TiC is a hard compound and significantly deteriorates the processability of the material. Therefore, the titanium alloy produced by the conventional method has deteriorated workability.

<Cooling rate> Even if the heat treatment temperature is in the above range, when the cooling rate is slow, TiC will precipitate during the cooling process, so the surface layer cannot be changed to 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 faster cooling rate can suppress the precipitation of TiC, but an excessively fast cooling rate will adversely affect the shape maintenance of the titanium plate, so the upper limit is set to 2000 ° C / sec.

<Manufacturing method> Next, the manufacturing method of the titanium alloy of this invention is demonstrated. The titanium alloy of the present invention is inserted into a blast (blast) at any time during the steps of casting → block rolling (or hot forging) → hot rolling → annealing (→ cold rolling → final annealing), as is the case of pure industrial titanium. ), Pickling treatment, etc., whereby manufacturing can be performed without using a special method in particular. In addition, in the description of the above steps, the steps indicated by parentheses (→ cold rolling → final annealing) are not necessarily necessary, and can be appropriately implemented in accordance with the thickness, shape, size, and the like of titanium to be manufactured.

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

Titanium ingots having the composition shown in Table 1 were casted through a vacuum arc melting furnace using a melting material containing sponge titanium and a predetermined additive element. Among the added elements, Fe and C are added electrolytic iron and TiC powder, respectively.

In addition, Al, V, Cr, Ru, Pd, Ni, and Co in the table are unintentionally added elements, and the values in the table indicate the degree to which the content of each of the above elements is impure.

[Table 1]

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

The identification of the surface structure is performed by XRD (X-ray diffraction) and observation of the microstructure; the conditions for X-ray diffraction are to use CoKα rays as characteristic X-rays, the voltage is set to 30kV, and the current is set to 100mA. The range of X-ray diffraction is set to 10 ° ≦ 2θ ≦ 110 °, the step is set to 0.04 °, the accumulated time is set to 2s, and the X-ray incident angle is set to 0.3 °. Investigate the presence of α-phase, β-phase, α'-phase, and TiC from the X-ray diffraction peak position of the test piece (20mm in length and 20mm in width); comprehensively investigate the surface structure by observing the microstructure, including needle-like α Whether it is. When the X-ray diffraction peak intensity is detected to be greater than 10% of the background, it is considered to form β phase, α 'phase, and TiC; in other cases, it is determined to be α single phase.

The corrosion resistance was evaluated by immersing a test piece in a 90%, 3 mass% hydrochloric acid aqueous solution for 168 hours, calculating the corrosion rate by comparing the weights before and after immersion, and evaluating the calculated corrosion rate. A corrosion rate of 2 mm / year or less is considered acceptable. The results of the corrosion resistance evaluation test are shown in Table 2. The workability is evaluated by a tensile test according to the method described in JIS Z 2241 and the elongation. The measurement of the elongation is carried out by means of an extensometer; those with a total elongation of 40% or more are regarded as acceptable. [Table 2]

For Nos. 1 to 9 which all satisfy the ingredients, heat treatment temperature, and surface structure of the present invention, the corrosion rate is significantly lower, the corrosion resistance is improved, and a sufficient elongation is displayed, which confirms that it has both Corrosion resistance and processability.

As far as Nos. 10 to 16, the material components such as carbon are within the scope of the present invention, but the heat treatment temperature or cooling rate is outside the scope of the present invention, so the surface structure does not form an alpha single phase, and the corrosion rate is fast and Not showing sufficient elongation. For No. 14, 16, 18, and 20, TiC was precipitated during the cooling process because the cooling rate was slow. In terms of Nos. 17 to 24, elements such as S, P, and Si that decrease the solid solution limit of C are added above the scope of the present invention. Even if the temperature and cooling rate of the present invention are satisfied, an α single phase will not be formed. The corrosion resistance was not improved, and TiC was precipitated, so the elongation was low. In Nos. 1 and 5, almost no discoloration or the like was observed in the outdoor environment. In contrast, Nos. 23 and 24 became brown in the outdoor environment.

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 a relationship between a corrosion rate and a heat treatment temperature in a hydrochloric acid immersion test. FIG. 3 is an example of a metal structure photograph of a titanium alloy manufactured by the manufacturing method of the present invention. FIG. 4 is an example of a metal photograph of a titanium alloy manufactured by a conventional manufacturing method.

Claims (2)

A titanium alloy characterized by: based on 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 remaining portion is Ti and unavoidable impurities; and the surface layer is an alpha single phase. A method for manufacturing a titanium alloy, characterized in that: the following titanium alloy is subjected to a finishing heat treatment at 750 to 820 ° C and cooled at a rate of 0.001 ° C / sec or more; the titanium alloy is based on 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, The remainder is Ti and unavoidable impurities.