JP6686744B2 - Titanium alloy plate and its manufacturing method. - Google Patents

Description

  The present invention relates to a titanium alloy plate having excellent corrosion resistance and a method for manufacturing the same.

Since titanium or titanium alloy has excellent diffusion bonding properties, when annealing in a batch type vacuum furnace at high temperature, the materials that form the coil are bonded together, and the coil cannot be expanded or surface defects occur when expanded. There is such a problem.
Therefore, the annealing is preferably a step of annealing in a continuous annealing line in an air atmosphere.

In the annealing using the continuous annealing line in the air atmosphere, although high temperature annealing is possible, the oxide formed on the surface becomes thick.
This oxide is removed by pickling with hydrofluoric nitric acid, but removal of a large amount of oxide leads to deterioration of production efficiency.

Various prior arts are known for the corrosion resistance of titanium alloys (see, for example, Patent Documents 1 to 4).
Further, there is some knowledge about the relationship between the annealing temperature and corrosion resistance of titanium alloys (see, for example, Patent Document 5).

Japanese Patent Publication No. 62-20269 Japanese Patent Publication No. 63-4892 Japanese Patent Publication No. 63-12932 Japanese Patent Publication No. 5-77735 Japanese Patent Publication No. 64-2662

However, Patent Documents 1 to 4 do not describe the annealing conditions and the metal structure formed.
Further, Patent Document 5 does not describe the relationship between the element distribution near the surface and the cold rolling rate.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a titanium alloy plate that suppresses the amount of oxides formed in the atmosphere and has excellent corrosion resistance, and a method for manufacturing the same.

To achieve the above object, the titanium alloy sheet of the present invention has a mass% of Ru : 0.005 to 2.0%, Ni: 0.1 to 1.0%, C: 0.001 to 0. A titanium alloy plate containing 1%, the balance being Ti and unavoidable impurities, the thickness of the oxide covering the surface of the titanium alloy plate is 30 nm or less, and the average amount of Ni immediately below the oxide is 0.1. .About.5.5 mass%, and Ti 2 Ni is contained in the metal structure immediately below the oxide.

Further, the method for producing a titanium alloy plate of the present invention contains, in mass%, Ru : 0.005 to 2.0% and Ni: 0.1 to 1.0%, and the balance from Ti and inevitable impurities. 0.001 to 0.1 mass% of C is introduced into the titanium alloy plate raw material by cold rolling the titanium alloy plate raw material using a C-containing lubricant at a cold rolling rate of 10% or more. In the first step, the titanium alloy plate material is annealed at an annealing temperature of 650 ° C. to 700 ° C. to form an oxide on the surface of the titanium alloy plate material, and the average Ni content immediately below the oxide is 10. The second step is to form a Ni-enriched layer containing Ti2Ni in the metal structure immediately below the oxide with 0 to 30.0 mass%, and the oxide is removed by pickling the titanium alloy plate material. Part of the Ni enriched layer Then, the third step of reducing the average Ni content of the Ni-enriched layer immediately below the surface of the titanium alloy plate material to 0.1 to 5.5 mass% and the oxide having a thickness of 30 nm or less on the titanium alloy plate material. The fourth step of forming and the fourth step are sequentially processed.

  According to the present invention, it is possible to provide a titanium alloy plate that suppresses the amount of oxides formed in the atmosphere and has excellent corrosion resistance, and a method for producing the same.

It is a figure which shows the relationship between the oxide thickness before pickling of the titanium alloy plate and the average amount of Ni just under that of the titanium alloy plate which concerns on the Example of this invention. It is a figure which shows the corrosion rate of the titanium alloy plate which concerns on the Example of this invention, and the relationship of the average Ni content just under the surface oxide after pickling.

Hereinafter, embodiments of the present invention will be described.
The inventors first investigated in detail about the deterioration of manufacturing efficiency due to the increase of oxides. Generally, oxides are formed in a larger amount as the annealing temperature is higher and the holding time is longer. However, when annealed at a temperature of 700 ° C. or lower, the oxide formation amount hardly increases even if the holding time is extended. This is because Ni is concentrated just below the oxide formed on the surface. The term “immediately below the oxide” means within a range of 1 μm toward the metal side, starting from the interface between the oxide and the metal.

  The cold rolling rate is important to obtain such an element distribution of Ni. The present inventors have found that when the cold rolling ratio is 10% or more, the concentration of Ni immediately below the oxide formed on the surface is remarkable.

  It is considered that this is due to recrystallization in the annealing process after cold rolling. In the annealing step in the air atmosphere, the recrystallization of the work structure occurs at the same time when the surface oxide is formed. The temperature rise of the material is fastest on the surface and slowest in the center of the plate thickness. When the temperature of the surface of the material reaches or exceeds the recrystallization start temperature in the initial stage of annealing, α grains are recrystallized with Ni atoms as nuclei.

  This is because Ni is an element that is less likely to form an oxide than Ti, which is most of the constituent elements of the material of the present invention, and Ti atoms on the surface react with oxygen in the atmosphere and are consumed in the formation of titanium oxide. Therefore, the Ni atom that is difficult to oxidize becomes the nucleus of recrystallization.

  However, since Ni is a β-stabilizing element, it is discharged from the recrystallized α-grains and concentrated at the interface between the oxide and the metal.

  By the mechanism described above, Ni concentration immediately under the surface oxide is achieved, the Ni-rich layer that is difficult to oxidize functions as a protective layer, and the oxide increase is suppressed.

  Furthermore, the inventors of the present invention have earnestly studied to improve the corrosion resistance. In general, in order to improve the corrosion resistance of titanium, it is effective to add a platinum group element having a small hydrogen overvoltage. Here, the platinum group element means Ru (ruthenium), Rh (rhodium), Pd (palladium), Os (osmium), Ir (iridium) and Pt (platinum). Platinum group elements are rare metals and are therefore expensive, but Ru, Pd, Rh and Ir are not so expensive as Pt and are therefore preferable as additional elements applied to the present invention.

  In the present embodiment, a platinum group element such as Ru and Pd is selected as an additive element in addition to Ni which has the effect of suppressing oxide formation.

  Alloys in which Ni and a platinum group element were added to Ti were annealed at various temperatures and times, and the relationship between the metal structure of the surface layer and the corrosion resistance was investigated. The surface layer means a range up to 1 μm inside the material starting from the metal surface after oxide removal.

The titanium alloy plate used in the present embodiment has six kinds of metals such as α + Ti ₂ Ni + TiC, α + β + Ti ₂ Ni + TiC, and α + β + TiC in addition to α + Ti ₂ Ni, α + β + Ti ₂ Ni, and α + β structure, depending on increase / decrease of alloy components and manufacturing conditions. Tissue may form. TiC is formed because the rolling oil reacts with Ti due to the heat generated during processing during cold rolling. Such TiC is formed by working heat when cold-rolled at a rolling speed of 10 mpm or more using a rolling oil having a carbon content of 50% or more as a lubricating oil.

It has been clarified that among these surface layer metal structures, α + Ti ₂ Ni, α + β + Ti ₂ Ni, α + Ti ₂ Ni + TiC, and α + β + Ti ₂ Ni + TiC structures are excellent in corrosion resistance.

Even with the components within the scope of the present invention, the effect of improving corrosion resistance is insufficient in the case of α + β or α + β + TiC structure. This is because there is an α / β interface and there is no Ti ₂ Ni precipitate. The α / β interface has a low concentration of Ni and Ru, which bring about the effect of improving the corrosion resistance, and is inferior in corrosion resistance to the interfaces of other tissues. Even if the α / β interface is present, there is no problem as long as there is a structure in which Ni atoms that bring about an effect of improving corrosion resistance are concentrated, such as Ti ₂ Ni.

  Based on the above findings, the present invention has been made regarding a titanium alloy that suppresses the amount of oxides formed in the atmosphere and has excellent corrosion resistance, and a method for producing the same.

<Alloy composition>
[Platinum group element: 0.005 to 2.0 mass%]
The platinum group element is an element having a low hydrogen overvoltage, and when added to titanium, it promotes passivation and improves corrosion resistance. In order to bring about this effect, addition of at least 0.005 mass% is necessary. Since the platinum group element is expensive, addition of a large amount causes an increase in material cost, which is not preferable. Therefore, the upper limit of the added amount is set to 2.0 mass%. A desirable lower limit is 0.008 mass% and a desirable upper limit is 1.6 mass%. The platinum group element added to the titanium alloy plate of the present invention may be a single element or plural kinds of elements.

[Ni: 0.1 to 1.0 mass%]
Ni is an element having a low hydrogen overvoltage, and when added to titanium, it promotes passivation and improves corrosion resistance. In addition, as described above, it concentrates immediately below the surface oxide to suppress oxide formation. In order to bring about these effects, addition of at least 0.1 mass% is necessary. Ni is cheaper than other platinum group elements, but addition of a large amount of Ni causes an increase in material cost, which is not preferable. Further, by adding a large amount, a large amount of intermetallic compound Ti ₂ Ni precipitates, which deteriorates the manufacturability in hot and cold. Therefore, the upper limit of the amount added is 1.0 mass%. A desirable lower limit is 0.15 mass% and a desirable upper limit is 0.90 mass%.

[Average Ni content immediately below the oxide scale before pickling: 10.0 to 30.0 mass%]
By the mechanism described above, Ni is concentrated immediately below the oxide during annealing and suppresses oxide formation. In order to fully bring out this effect, Ni concentration of at least 10.0 mass% is necessary. Further, if Ni is excessively concentrated just below the oxide, the amount of Ti ₂ Ni precipitation is reduced and the β phase ratio is increased, so that sufficient corrosion resistance is not exhibited. Therefore, the upper limit of Ni concentrated immediately below the oxide is set to 30.0 mass%. A desirable lower limit is 12.0 mass% and a desirable upper limit is 26.0 mass%.
[Average Ni content immediately below the oxide after pickling: 0.1 to 5.5 mass%]
As shown in FIG. 1, the surface oxide thickness before pickling becomes thinner as the amount of Ni immediately below the oxide increases. That is, Ni immediately below the oxide suppresses the growth of the surface oxide, improves the yield reduction due to pickling, and prevents the oxide residue after pickling.
Moreover, as shown in FIG. 2, the corrosion rate deteriorates rapidly when the average Ni content immediately below the oxide after pickling exceeds 5.5 mass%.

<Surface metallographic structure>
[Α + Ti ₂ Ni, α + β + Ti ₂ Ni, α + Ti ₂ Ni + TiC or α + β + Ti ₂ Ni + TiC texture]
Ni is an element having a small hydrogen overvoltage and contributes to the improvement of the corrosion resistance of titanium, but its effect is greater when Ti ₂ Ni is formed than when Ni is in a solid solution state. TiC formed by the reaction with rolling oil also has the effect of improving corrosion resistance. Desirable volume ratios of Ti ₂ Ni are 0.1 to 10.0% and TiC is 0.01 to 1.00%.

  Forging or slab rolling, hot rolling, descaling, intermediate annealing, and cold rolling were performed on titanium alloy ingots having the chemical composition shown in Table 1 to obtain cold rolled coils having a plate thickness of 0.3 to 1.2 mm. It was manufactured and subjected to finish annealing (hereinafter simply referred to as "annealing" in the present invention). The production process from ingot production to intermediate annealing may be the same as the production process for producing a general titanium-titanium alloy.

  When performing the annealing, the annealing temperature and the annealing time shown in Tables 2 and 3 were annealed in the atmosphere, and the oxide formation amount was investigated. The atmosphere at the time of annealing does not necessarily have to be atmospheric air, and may be annealed in Ar or a vacuum atmosphere. In the case of an Ar atmosphere, an Ar gas having a purity of 90% or more and in a vacuum atmosphere, a vacuum degree of 0.01 Torr or less causes no problem. There is no problem if the soaking and holding time during annealing is 1 min or more, the temperature rising rate is 2 ° C./sec or more, and the cooling rate is 1 ° C./sec or less. Annealing in an air atmosphere is more advantageous in terms of manufacturing cost than annealing in an Ar atmosphere or a vacuum atmosphere, and is preferable because it can enjoy the effect of suppressing oxide formation according to the present invention.

The oxide formed after annealing was removed by bringing it into contact with a pickling solution using hydrofluoric nitric acid.
In order to exhibit the characteristics of the present invention, the temperature of the pickling solution is 20 to 60 ° C., the dipping time is 20 to 300 seconds, the composition of the pickling solution is 3 to 10 mass% of fluorine and 1 to 5 mass% of nitric acid. It is desirable to do. An acid-washed titanium alloy plate is washed with water and then left in the atmosphere to form an oxide having a thickness of 30 nm or less. Since the oxide having a thickness of 30 nm or less is an extremely thin film, it does not adversely affect the quality of the titanium alloy plate.

  The amount of oxide formed was determined by calculating the oxidation rate from the weight change before and after annealing.

  For the average Ni content and oxide thickness immediately below the surface oxide, a test piece was cut out from the material before and after removing the oxide, and the element distribution in the depth direction was analyzed using GDOES (Glow Discharge Optical Emission Spectrometry). It was measured by doing. The analysis range was 4 mmφ.

Regarding the metal structure of the surface, a test piece was cut out from the material after the oxide was removed, and phase identification by X-ray diffraction was performed as a primary evaluation, and microstructure observation by an optical microscope was performed as a secondary evaluation. In X-ray diffraction, CoKα was used as the characteristic X-ray, the voltage was 30 kV, and the current was 100 mA. The measurement range was 10 ° ≦ 2θ ≦ 110 °, the step was 0.04 °, the integration time was 2 s, and the incident angle of the X-ray on the surface of the test piece was 0.3 °. In the chart obtained by X-ray diffraction, the signal intensity at an angle at which peaks derived from each phase appear is 1.5 times or more of the average value of the background signal intensity at 0.2 ° before and after the angle. It was judged that each phase was detected in. In the microstructure observation, after mirror-polishing, etching was performed with hydrofluoric nitric acid and then observed with an optical microscope at a magnification of 500 times, and a white product was evaluated as α, a black product was evaluated as β, TiC, and Ti ₂ Ni. The metal structure on the surface was identified by comprehensively judging the primary evaluation and the secondary evaluation.

To evaluate the corrosion resistance, a test piece was cut out from the material after the oxide was removed, immersed in a 5 mass% hydrochloric acid aqueous solution at 70 ° C. for 168 hours, and the corrosion rate was calculated from the weight change before and after the immersion.
The above results are summarized in Tables 2 and 3.

  No. 1-No. No. 14 satisfies the alloy components, the cold rolling rate, the annealing conditions, and the surface metallographic structure specified in the present invention, and as a result, the amount of increased oxidation during annealing is small and the corrosion resistance is excellent.

  No. 15-No. 22 and No. 22. In No. 27, the heat treatment condition is outside the scope of the present invention. As a result, the metal structure on the surface and the average amount of Ni immediately below the oxide are out of the range of the present invention, the amount of oxidation increase is larger than that of the present invention, and the corrosion resistance is also poor.

  No. 23-No. No. 26 has a cold rolling rate outside the range of the present invention. As a result, the metallographic structure of the surface is within the range of the present invention, but the average amount of Ni directly below the oxide is outside the range of the present invention, the amount of oxidation increase is large and corrosion resistance is poor as compared with the present invention. This is because the cold rolling rate is small and recrystallization is not sufficient as compared with the present invention.

Claims (2)

mass%, titanium containing Ru : 0.005 to 2.0%, Ni: 0.1 to 1.0%, C: 0.001 to 0.1%, with the balance being Ti and unavoidable impurities An alloy plate, the thickness of the oxide covering the surface of the titanium alloy plate is 30 nm or less, the average amount of Ni immediately below the oxide is 0.1 to 5.5 mass%, and the metal structure immediately below the oxide has Ti2Ni. A titanium alloy plate characterized by containing. In% by mass, Ru : 0.005 to 2.0%, Ni: 0.1 to 1.0%, the balance is a titanium alloy plate material containing Ti and unavoidable impurities, and a lubricant containing C. The first step of introducing 0.001 to 0.1 mass% of C into the titanium alloy plate material by cold rolling at a cold rolling rate of 10% or more using
The titanium alloy plate material is annealed at an annealing temperature of 650 ° C. to 700 ° C. to form an oxide on the surface of the titanium alloy plate material, and the average Ni content immediately below the oxide is 10.0 to 30.0 mass. %, And a second step of forming a Ni-enriched layer containing Ti2Ni in the metal structure immediately below the oxide,
By pickling the titanium alloy plate material, oxides are removed and a part of the Ni concentrated layer is removed, and the average Ni content of the Ni concentrated layer immediately below the surface of the titanium alloy plate material is 0.1 to 0.1%. A third step of reducing to 5.5 mass%,
A fourth step of forming an oxide having a thickness of 30 nm or less on the titanium alloy plate material;
A method for manufacturing a titanium alloy plate, which comprises sequentially processing

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