JP7832544B2 - Titanium material - Google Patents

Description

This invention relates to titanium materials.

Titanium exhibits extremely high corrosion resistance in atmospheric environments, making it suitable for building materials such as roofs and exterior walls. However, titanium materials used over long periods can discolor. This discoloration can be a problem from an aesthetic standpoint. Therefore, titanium materials with suppressed discoloration and superior colorfastness have been proposed.

For example, Patent Document 1 discloses a titanium material or titanium alloy material with excellent discoloration resistance, characterized in that an oxide film with a thickness of 100 Å or less is present on the substrate surface, the amount of carbon in the surface oxide film is 30 atomic percent or less, and the amount of carbon in the substrate surface layer beneath the oxide film is 30 atomic percent or less.

Patent Document 2 discloses titanium that is less prone to discoloration in an atmospheric environment, characterized by having an average carbon concentration of 14 at% or less in a range of 100 nm depth from the surface, and having an oxide film with a thickness of 12 to 40 nm on the outermost surface.

Patent Document 3 discloses a titanium material that is less prone to discoloration, characterized by having a fluorine content of 7 at% or less in the oxide film on its surface.

Patent Document 4 discloses a method for manufacturing titanium sheets, which efficiently produces titanium or titanium alloy sheets that show little discoloration in environments exposed to light. This method involves cold-rolling a titanium sheet using a lubricant that results in a carbon concentration layer on the surface of the titanium sheet having a carbon content of 150 mg/ ^m² or less after cold rolling, followed by annealing in an oxidizing atmosphere, and then descaling by molten salt immersion treatment and pickling with a nitrohydrofluoric acid aqueous solution.

Patent Document 5 discloses titanium or a titanium alloy that is less prone to discoloration in an atmospheric environment, characterized in that the average carbon concentration in a range of 100 nm depth from the surface is 14 at% or less, the surface has an oxide film with a thickness of 12 nm to 30 nm, and the arithmetic mean height (Ra) of the titanium surface is 0.035 μm or less.

Japanese Patent Publication No. 2000-1729 Japanese Patent Publication No. 2002-12962 Japanese Patent Publication No. 2002-47589 Japanese Patent Publication No. 2002-60984 Japanese Patent Publication No. 2005-272870

Tsutomu Miyashita, "Surface Roughness: A Review," Journal of the Japan Society for Precision Engineering, Vol. 73, No. 2, 2007, pp. 201-205.

The technologies described in Patent Documents 1 to 5 state that the deterioration of the discoloration resistance of titanium materials is suppressed by reducing the carbon concentration on the surface. Furthermore, conventional evaluation of the discoloration resistance of titanium materials is based on the color difference after immersion in sulfuric acid at pH 3 at 60°C for 14 days, as described in Patent Document 5. However, in recent years, there has been a demand for titanium materials with even better discoloration resistance than conventional materials.

This invention has been made in view of the above problems, and aims to provide a titanium material with excellent discoloration resistance that can suppress discoloration over a longer period than conventional titanium materials. For example, this invention aims to provide a titanium material that does not discolor even when immersed for a longer period than the evaluations described in Patent Documents 1 to 5.

Based on the above findings, the gist of the present invention is as follows:
[1] A titanium material according to one aspect of the present invention is a titanium material having a titanium substrate and an oxide film disposed on the surface of the titanium substrate, wherein the titanium substrate has a composition in mass%, of Co: 0% or more and 1.0%, Cr: 0% or more and 0.5%, Ni: 0% or more and 1.00%, Ta: 0% or more and 6.00%, Al: 0% or more and 7.0%, V: 0% or more and 5.0%, S: 0% or more and 0.3%, Cu: 0% or more and 1.50%, Nb: 0% or more and 0% ．． 70% or less, Sn: 0% or more and 1.40% or less, Si: 0% or more and 0.55% or less, Mo: 0% or more and 0.5% or less, W: 0% or more and 0.5% or less, Pd: 0% or more and 0.25%, Ru: 0% or more and 0.15% or less, Rh: 0% or less. Upper 0.15% or less, Os: 0% or more and 0.15% or less, Ir: 0% or more and 0.15% or less, Pt: 0% or more and 0.15% or less, REM: 0% or more and 0.10% or less, C: 0% or more and 0.18% or less, H: 0% or more and 0.015% The following is a description of the composition of the titanium material: O: 0% to 0.40%, N: 0% to 0.05%, Fe: 0% to 2.50%, with the remainder being Ti and impurities; the average nitrogen concentration and average carbon concentration are 14.0 atomic% or less, each within the range from the surface of the titanium material to the position where the oxygen concentration measured in the thickness direction from the surface by glow discharge spectroscopy is 1/3 of the maximum value; the average hydrogen concentration is 30.0 atomic% or less; the difference between the lattice constant of the c axis of α-phase Ti determined by X-ray diffraction measurement using a parallel beam method with an incident angle of 0.3 degrees at the surface and the lattice constant of the c axis of α-phase Ti determined by X-ray diffraction measurement using a concentrated method at the center of the plate thickness is 0.015 Å or less; when analyzed by X-ray photoelectron spectroscopy, the maximum value of the nitrogen concentration derived from nitrides in the oxide film is 2.0 to 10.0 atomic%, and the position where the nitrogen concentration derived from nitrides in the oxide film shows the maximum value is SiO When converted to a sputtering rate of ₂ , the nitrogen concentration derived from the nitride, which is present in a range of 2 to 10 nm from the surface of the oxide film and in a range of 20 nm from the position where the oxygen concentration is half of the maximum value to the titanium substrate side, is less than the maximum value of the nitrogen concentration derived from the nitride in the oxide film and is 7 atomic percent or less, and the maximum value of the nitrogen concentration derived from the nitride in the oxide film is equal to or greater than the carbon concentration derived from the carbide at the position where the nitrogen concentration derived from the nitride in the oxide film is maximum .
[2] The titanium material described in [1] above may have an oxide film with a thickness of 30.0 nm or less .
[3 ] The titanium material described in [1] or [2] above comprises a titanium substrate in which, in the roughness curve in the direction in which the arithmetic mean roughness Ra is maximum, the ratio Ra/RSm, which is the ratio of the arithmetic mean roughness Ra to the element length RSm, is 0.006 to 0.015, and the root mean square slope RΔq is 0.150 to 0.280, and the kurtosis Rku of the titanium substrate is greater than 3, and the skewness Rsk of the titanium substrate is greater than -0.5.
[ 4 ] The titanium material described in [1] or [2] above may have a roughness curve in the direction in which the arithmetic mean roughness Ra of the titanium substrate is maximized, where the ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, is 0.006 to 0.015, and the root mean square slope RΔq is 0.150 to 0.280, the kurtosis Rku of the titanium substrate is greater than 3, and the skewness Rsk of the titanium substrate is greater than -0.5.

According to the present invention, it is possible to provide a titanium material with excellent discoloration resistance, which can suppress discoloration over a longer period than conventional titanium materials.

This is a schematic enlarged cross-sectional view showing the layer structure of a titanium material according to one embodiment of the present invention. This figure shows an example of the change in the depth direction of the spectrum obtained by X-ray photoelectron spectroscopy of a titanium material according to the same embodiment. This figure shows an example of the depth-dependent change in the spectrum of a typical titanium material obtained by X-ray photoelectron spectroscopy. This figure shows an example of the elemental concentration distribution in the depth direction of a titanium material according to the same embodiment and a general titanium material, as determined by X-ray photoelectron spectroscopy. This figure shows the relationship between the maximum concentration of nitrogen derived from titanium nitrides within the oxide film and the color difference ΔE ^* ab before and after the discoloration test. This figure shows the relationship between the arithmetic mean roughness Ra and the average length RSm of the contour curve elements (Ra/RSm), the root mean square slope RΔq of the roughness curve elements, and discoloration resistance. This is a schematic diagram illustrating the kurtosis Rku.

The following describes a titanium material according to one embodiment of the present invention, with reference to the drawings. Note that the dimensions and ratios of the components shown in the drawings do not represent the actual dimensions and ratios of the components.

Note that the numerical ranges indicated by "～" below include both a lower limit and an upper limit. Numbers indicated as "less than" or "greater than" are not included in the numerical range.

First, we will detail the new findings obtained through the inventors' research that led to the completion of this invention.

Titanium materials have a structure in which an oxide film is arranged on the surface of a titanium substrate, and discoloration of titanium materials is thought to be caused by an increase in the thickness of the oxide film due to acid rain, etc. Since the carbon concentration near the surface of the titanium material affects the increase in the thickness of the oxide film, the carbon concentration on the surface is limited in conventional titanium materials aimed at improving discoloration resistance. The inventors of this invention conducted research to obtain a titanium material with excellent discoloration resistance that suppresses discoloration for a longer period than conventional materials.

First, the inventors measured the oxygen, carbon, and nitrogen concentrations in the thickness direction from the surface of the titanium material (in other words, the surface of the oxide film) using glow discharge spectroscopy (GDS). The titanium material has an oxide film, and it was found that the location where the oxygen concentration measured by GDS was 1/3 of the maximum value was located near the interface between the oxide film and the titanium substrate, on the titanium substrate side. Hereinafter, the range from the surface of the titanium material to the location where the oxygen concentration measured by GDS in the thickness direction is 1/3 of the maximum value will be referred to as the surface layer of the titanium material.

Next, the inventors immersed titanium material in a sulfuric acid aqueous solution at pH 3 and 60°C for four weeks and evaluated its discoloration resistance based on the color difference before and after immersion. Comparing titanium material that clearly discolored with titanium material that hardly discolored before and after immersion, it was found that there was a difference in the carbon and nitrogen concentrations measured by GDS in the titanium material before immersion. Specifically, in the case of titanium material that clearly discolored after immersion in a sulfuric acid aqueous solution at pH 3 and 60°C for four weeks, it was found that nitrogen and carbon were present in the titanium material before immersion, both inside the oxide film and near the interface between the oxide film and the substrate on the substrate side. Conventionally, it was not thought that nitrogen present in the oxide film and near the interface between the oxide film and the substrate on the substrate side would affect the discoloration of titanium material. However, when titanium material is exposed to an acid rain environment for a long period of time, it is presumed that nitrogen present in the oxide film and its vicinity, like carbon, will act as a starting point for the growth of the oxide film.

Furthermore, our investigations revealed that the nitrogen concentration in and near the oxide film, like the carbon concentration, affects the discoloration resistance of the titanium material, and that limiting the nitrogen concentration improves discoloration resistance. We also found that when the average nitrogen concentration and average carbon concentration are 14.0 atomic percent or less in the range from the surface of the titanium material to the point where the oxygen concentration measured in the thickness direction by GDS is 1/3 of the maximum value, the discoloration resistance of the titanium material improves compared to conventional methods. The average carbon concentration in the surface layer of the titanium material can be reduced by increasing the annealing temperature or lengthening the annealing time. The average nitrogen concentration can be reduced by increasing the vacuum level during heat treatment.

Next, the inventors focused on the hydrogen concentration in the surface layer of the titanium material and investigated the effect of the hydrogen concentration in the surface layer of the titanium material on its discoloration resistance. As a result, it was found that discoloration resistance was further improved when the average hydrogen concentration in the surface layer of the titanium material was 30.0 atomic percent or less. Titanium hydrides are thermodynamically less stable than titanium oxide in an acid rain environment. An increase in the hydrogen concentration in the titanium material may promote the formation of titanium oxide, potentially reducing discoloration resistance. However, if the average hydrogen concentration in the surface layer of the titanium material is 30.0 atomic percent or less, titanium hydride does not change into titanium oxide, and the decrease in discoloration resistance is thought to be suppressed.

Next, the inventors focused on the changes in the crystal structure of Ti on the surface of the titanium material. They found that changes in the c-axis of the α-phase Ti, which is a close-packed hexagonal crystal, affect the discoloration resistance of the titanium material.

Through our investigations, we have found that if the difference between the c-axis lattice constant of Ti in the α phase, determined by X-ray diffraction measurement using a parallel beam method with an incident angle of 0.3 degrees on the surface of the titanium material, and the c-axis lattice constant of Ti in the α phase, determined by X-ray diffraction measurement using a concentrated method at the center of the plate thickness (also called the thickness center), is 0.015 Å or less, the discoloration resistance can be significantly improved. Although the penetration level of X-rays differs depending on the X-ray diffraction energy in the above-mentioned X-ray diffraction measurement, the c-axis lattice constant of Ti in the α phase can be measured by performing the above-mentioned X-ray diffraction measurement on the surface of the titanium material and at the center of the thickness of the titanium material. In this application, the increase in the c-axis lattice constant of Ti in the α phase of the surface layer is defined as follows. In other words, the difference between the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement using the parallel beam method with an incident angle of 0.3 degrees on the surface of the titanium material and the c-axis lattice constant of α-phase Ti obtained by X-ray diffraction measurement using the concentrated method at the center of the plate thickness is referred to as the increase in the c-axis lattice constant of α-phase Ti in the surface layer. The measurement depth in X-ray diffraction measurement using the parallel beam method with an incident angle of 0.3 degrees does not strictly coincide with the range in the thickness direction of the surface layer measured by GDS, but it is possible to measure approximately the increase in the c-axis lattice constant of α-phase Ti in the surface layer.
It is believed that oxygen is involved in the increase in the c-axis lattice constant of α-phase Ti in the surface layer of titanium material. When oxygen is dissolved in α-phase Ti in the surface layer of titanium material, the c-axis lattice constant increases. If the c-axis lattice constant of α-phase Ti present on the surface of titanium material is greater than that of α-phase Ti present in the center of the thickness, it is presumed that the action of acid rain will generate titanium oxide with a high defect concentration, making it easier for an oxide film to grow and degrading the resistance to discoloration. The crystal structure of α-phase Ti in the surface layer of titanium material is affected by the temperature, time, and vacuum level of the heat treatment. By increasing the vacuum level in the heat treatment atmosphere, the amount of oxygen dissolved in α-phase Ti in the surface layer of titanium material decreases, and the increase in the c-axis lattice constant of α-phase Ti in the surface layer of titanium material is suppressed. The new findings obtained through the inventors' studies have been explained up to this point.

Next, a titanium material according to one embodiment of the present invention will be described with reference to Figure 1. Figure 1 is a schematic enlarged cross-sectional view showing the layer structure of the titanium material according to this embodiment. In the titanium material according to this embodiment, the average nitrogen concentration and average carbon concentration in the range from the surface to the position where the oxygen concentration measured in the thickness direction from the surface by glow discharge spectroscopy is 1/3 of the maximum value are 14.0 atomic% or less, and the average hydrogen concentration is 30.0 atomic% or less. The difference between the lattice constant of the c axis of Ti in the α phase, determined by X-ray diffraction measurement using a parallel beam method with an incident angle of 0.3 degrees at the surface, and the lattice constant of the c axis of Ti in the α phase, determined by X-ray diffraction measurement using a concentrated method at the center of the plate thickness, is 0.015 Å or less. The titanium material according to this embodiment will be described in detail below.

(Titanium material 1)
The titanium material 1 according to this embodiment is a titanium material in which an oxide film 20 is formed on the surface of a titanium substrate 10, as shown in Figure 1. In other words, the titanium material 1 has a titanium substrate 10 and an oxide film 20 formed on the surface of the titanium substrate 10. The surface layer 30 is a region from the surface of the titanium material 1 (in other words, the surface of the oxide film 20) in the thickness direction to a position where the oxygen concentration measured by GDS is 1/3 of the maximum value, and includes a part of the titanium substrate 10.

(Titanium base material 10)
The titanium substrate 10 of the titanium material 1 is pure titanium, industrial pure titanium, or a titanium alloy. The titanium substrate 10 is, for example, pure titanium, industrial pure titanium, or a titanium alloy with a Ti content of 70% by mass or more. Hereinafter, these may be collectively referred to as "titanium." The crystal structure of pure titanium is a close-packed hexagonal α phase and does not contain a body-centered cubic β phase. Industrial pure titanium mainly consists of the α phase, and may contain a β phase depending on the chemical composition. The titanium alloy may be an α-type alloy consisting only of the α phase, or an α+β-type alloy containing a body-centered cubic β phase. The titanium substrate 10 may also be, for example, industrial titanium. Examples of industrial titanium used in the titanium substrate 10 include various industrial titanium plates and strips described in JIS H 4600:2012, and various industrial titanium rods described in JIS H 4650:2016. When processability is required, industrial pure titanium of JIS Grade 1 (for example, JIS H 4600:2012) with reduced impurities is preferred. Furthermore, when strength is required, industrial pure titanium of JIS Grades 2 to 4 can be applied to the titanium substrate 10. Examples of titanium alloys include JIS Grades 11 to 23, which contain trace amounts of noble metal elements, such as palladium, platinum, and ruthenium, to improve corrosion resistance, and JIS Grade 60, which contains a relatively large number of elements, such as Ti-6Al-4V alloys, Grades 60E, 61, and 61F. In buildings, industrial pure titanium specified in JIS Grade 1 or its equivalent, ASTM Gr. 1, or equivalent materials, is mainly used.

Examples of titanium alloys primarily composed of the α phase include highly corrosion-resistant alloys (titanium alloys specified in JIS standards 11-13, 17, 19-22, and ASTM standards Grades 7, 11, 13, 14, 17, 30, and 31, as well as titanium alloys containing small amounts of various other elements (such as Ti-Ru-Mm)), Ti-0.5Cu, Ti-1.0Cu, Ti-1.0Cu-0.5Nb, Ti-1.0Cu-1.0Sn-0.35Si-0.25Nb, etc. Mm represents mischmetal.

Examples of α+β type titanium alloys include Ti-3Al-2.5V, Ti-5Al-1Fe, and Ti-6Al-4V.

When the titanium substrate 10 contains aluminum, such as in Ti-6Al-4V alloys, corrosion resistance may deteriorate and discoloration resistance may be adversely affected. Therefore, when forming an oxide film 20 on the surface of a titanium alloy as the titanium substrate 10, it is recommended to investigate the influence of alloying elements on the application beforehand and to appropriately adjust the composition and thickness of each layer according to the titanium substrate 10.

The titanium substrate 10 is, for example, in mass%
Co: 0% or more and 1.0% or less,
Cr: 0% or more and 0.5% or less,
Ni: 0% or more and 1.00% or less,
Ta: 0% or more and 6.00% or less,
Al: 0% or more and 7.0% or less,
V: 0% or more and 5.0% or less,
S: 0% or more and 0.3% or less,
Cu: 0% or more and 1.50% or less,
Nb: 0% or more and 0.70% or less,
Sn: 0% or more and 1.40% or less,
Si: 0% or more and 0.55% or less,
Mo: 0% or more and 0.5% or less,
W: 0% or more and 0.5% or less,
Pd: 0% to 0.25%,
Ru: 0% or more and 0.15% or less,
Rh: 0% or more and 0.15% or less,
Os: 0% or more and 0.15% or less,
Ir: 0% or more and 0.15% or less,
Pt: 0% or more and 0.15% or less,
REM: 0% or more and 0.10% or less,
C: 0% or more and 0.18%% or less,
H: 0% or more and 0.015% or less,
O: 0% or more and 0.40% or less,
N: 0% to 0.05%, and Fe: 0% to 2.50%,
The remainder consists of Ti and impurities, making it industrial-grade pure titanium or titanium alloy.
Here, REM refers to rare earth elements, specifically one or more elements selected from the group consisting of Sc, Y, light rare earth elements (La, Ce, Pr, Nd, Pm, Sm, Eu), and heavy rare earth elements (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).

Furthermore, the titanium substrate 10 is, for example, in mass%
C: 0% or more and 0.10% or less,
H: 0% or more and 0.015% or less,
O: 0% or more and 0.40% or less,
N: 0% to 0.05%, and Fe: 0% to 0.50%,
This is industrial-grade pure titanium, with the remainder consisting of Ti and impurities.

Furthermore, the titanium substrate 10 is, for example, in mass%
Co: 0% or more and 0.80% or less,
Pd: 0% or more and 0.25% or less,
Cr: 0% or more and 0.2% or less,
Ru: 0% or more and 0.06% or less,
Ni: 0% or more and 0.60% or less,
Ta: 0% or more and 6.0% or less,
N: 0% or more and 0.05% or less,
C: 0% or more and 0.08% or less,
H: 0% or more and 0.015% or less,
O: 0% to 0.35%, and Fe: 0% to 0.30%,
It is a titanium alloy, with the remainder consisting of Ti and impurities.

Furthermore, the titanium substrate 10 is, for example, in mass%
Al: 2.0% or more and 7.0% or less,
V: 1.0% up to 5.0%,
S: 0% or more and 0.3% or less,
REM: 0% or more and 0.08% or less,
N: 0% or more and 0.05% or less,
C: 0% or more and 0.10% or less,
H: 0% or more and 0.015% or less,
O: 0% to 0.35%, and Fe: 0% to 2.5%,
It is a titanium alloy, with the remainder consisting of Ti and impurities.

Furthermore, the titanium substrate 10 is, for example, in mass%
Cu: 0.3% or more and 1.50% or less,
Nb: 0% or more and 0.70% or less,
Sn: 0% or more and 1.40% or less,
Si: 0% or more and 0.55% or less,
N: 0% or more and 0.05% or less,
C: 0% or more and 0.10% or less,
H: 0% or more and 0.015% or less,
O: 0% to 0.15%, and Fe: 0% to 0.10%,
It is a titanium alloy, with the remainder consisting of Ti and impurities.

Furthermore, the titanium substrate 10 is, for example, in mass%
V: 0% or more and 0.5% or less,
Ni: 0% or more and 1.00% or less,
Cr: 0% or more and 0.5% or less,
Co: 0% or more and 1.0% or less,
Mo: 0% or more and 0.5% or less,
W: 0% or more and 0.5% or less,
Pd: 0% or more and 0.15% or less,
Ru: 0% or more and 0.15% or less,
Rh: 0% or more and 0.15% or less,
Os: 0% or more and 0.15% or less,
Ir: 0% or more and 0.15% or less,
Pt: 0% or more and 0.15% or less,
REM: 0.001% or more and 0.10% or less,
N: 0% or more and 0.03% or less,
C: 0% or more and 0.18% or less,
H: 0% or more and 0.015% or less,
O: 0% or more and 0.35% or less,
Fe: 0% to 0.30%, and the total of Pd, Ru, Rh, Os, Ir, and Pt: 0.01% to 0.15%.
It is a titanium alloy, with the remainder consisting of Ti and impurities.

Impurities are components that are present in titanium regardless of the intention of their addition and are not inherently necessary in the resulting titanium material. The term "impurities" is a concept that includes impurities introduced from raw materials or the manufacturing environment during the industrial production of titanium. Examples of impurities include Cl, Na, Mg, Ca, and B. The content of each element of impurities is preferably 0.1% by mass or less, and the total amount is preferably 0.4% by mass or less.

The titanium substrate 10 is usually in the form of a plate, strip, tube, rod, or wire, or in a shape that has been appropriately processed from these. The titanium substrate 10 may also be in any shape, for example, spherical or rectangular.

(Oxide film 20)
An oxide film 20 is formed on the surface of the titanium substrate 10. The thickness of the oxide film 20 is not particularly limited, but if it exceeds 30.0 nm, it may affect the color development of the titanium material 1 due to light interference. For this reason, the thickness of the oxide film 20 is preferably 30.0 nm or less. From the viewpoint of suppressing color development due to light interference, the thickness of the oxide film 20 is more preferably 25.0 nm or less, and even more preferably 20.0 nm or less. The thickness of the oxide film 20 is greater than 0 nm, but may be, for example, 10.0 nm or more. Furthermore, the discoloration resistance of the titanium material is improved by ensuring the thickness of the oxide film 20. For this reason, from the viewpoint of improving discoloration resistance, the thickness of the oxide film 20 on the surface of the titanium material 1 is more preferably 12.0 nm or more.

The thickness of the oxide film 20 is measured by GDS. The GDS measurement is performed using the following method: The GDS measurement is performed using a JOBIN YVON GD-Profiler 2 manufactured by Horiba, Ltd., in constant power mode at 35W, with an argon gas pressure of 600 Pa and a discharge range of 4 mm in diameter. The measurement pitch in the GDS measurement is 0.5 nm. In the GDS measurement, O (oxygen), N (nitrogen), C (carbon), H (hydrogen), and Ti are analyzed from the surface of the titanium material 1. The concentration (atomic %) of each of the above elements is calculated assuming the sum of the above elements is 100 atomic percent. The thickness of the oxide film 20 is determined from the oxygen concentration measured by GDS. Specifically, the thickness of the oxide film 20 is the distance in the thickness direction from the surface to the position where the oxygen concentration is halved from the maximum value. The average nitrogen concentration, average carbon concentration, and average hydrogen concentration are the arithmetic mean values of the nitrogen, carbon, and hydrogen concentrations at each measurement point.

The nitrogen concentration derived from nitrides near the interface between the titanium substrate 10 and the oxide film 20 is preferably less than the maximum nitrogen concentration in the oxide film 20 measured by XPS and 7 atomic percent or less. Although nitrides are formed on the oxide film 20 by the manufacturing method of the titanium material 1 according to this embodiment, as described later, according to this method, there are no nitrides or only a very small amount of nitrides formed near the interface between the titanium substrate 10 and the oxide film 20. The nitrogen content of the base material, which is the titanium substrate 10, is about 0.05 to 0.07 mass%, and at most about 0.20 atomic percent, which is at the impurity level, as obtained by chemical analysis. Since this content is below the solid solubility limit of nitrogen in titanium, nitrides are not formed. Therefore, if nitrides exist near the interface between the titanium substrate 10 and the oxide film 20, these nitrides are formed when nitrogen diffuses from the surface into the interior during the annealing process in one example of the manufacturing method of the titanium material 1 according to this embodiment. Therefore, it is preferable that the nitrogen concentration derived from nitrides near the interface between the titanium substrate 10 and the oxide film 20 is less than the maximum nitrogen concentration in the oxide film 20 measured by XPS and 7 atomic percent or less. When the nitrogen concentration derived from nitrides near the interface between the titanium substrate 10 and the oxide film 20 is less than the maximum nitrogen concentration in the oxide film 20 measured by XPS and 7.0 atomic percent or less, the growth of the oxide film is suppressed and discoloration is suppressed when the titanium material 1 is exposed to an acid rain environment for a long period of time. More preferably, the nitrogen concentration derived from nitrides near the interface between the titanium substrate 10 and the oxide film 20 is less than the maximum nitrogen concentration in the oxide film 20 and 3.0 atomic percent or less. On the other hand, there is no lower limit to the nitrogen concentration derived from nitrides near the interface between the titanium substrate 10 and the oxide film 20. Therefore, according to one example of the manufacturing method for the titanium material 1 according to this embodiment, the lower limit of nitrogen concentration derived from nitrides near the interface with the oxide film 20 on the titanium substrate 10 is 0 atomic%, but when considering those resulting from XPS peak separation (which will not be zero), it may be around 0.5 atomic%.

The vicinity of the interface between the titanium substrate 10 and the oxide film 20 refers to the 20 nm range from the interface toward the titanium substrate when measured by XPS. In the XPS measurement figure described later (for example, Figure 4), the interface is defined as the position where the oxygen concentration measured by XPS is half of its maximum value. Therefore, the 20 nm range from the position where the oxygen concentration measured by XPS is half of its maximum value toward the titanium substrate is defined as the vicinity of the interface between the titanium substrate 10 and the oxide film 21. The vicinity of the interface between the titanium substrate 10 and the oxide film 20 is a different region from the surface layer 30.
The oxide film 20 can be identified by GDS or XPS, but the thickness of the oxide film obtained by each measurement method often does not strictly match due to the differences in the measurement methods. However, the definition of the oxide film is consistent across all measurement methods, in that the oxide film is defined as the position where the oxygen concentration is half of the maximum value. In this application, the oxide film is measured by XPS when measuring the nitrogen concentration derived from nitrides near the interface with the oxide film 20 on the titanium substrate 10.

The oxide film 20 preferably contains nitrogen derived from nitrides. Nitrogen derived from nitrides in the oxide film 20 is measured by X-ray photoelectron spectroscopy (XPS). Figure 2 shows an example of the depth direction change of the spectrum of the titanium material 1 according to this embodiment, measured by X-ray photoelectron spectroscopy. Figure 3 shows an example of the depth direction change of the spectrum of a general titanium material, measured by X-ray photoelectron spectroscopy. Figures 2(A) and 3(A) show the depth direction change of the N1s spectrum, Figures 2(B) and 3(B) show the depth direction change of the C1s spectrum, Figures 2(C) and 3(C) show the depth direction change of the O1s spectrum, and Figures 2(D) and 3(D) show the depth direction change of the Ti2p spectrum. As shown in Figure 2(C), in the titanium material 1 according to this embodiment, a clear peak derived from nitrides may be observed at a depth corresponding to the oxide film 20. On the other hand, as shown in Figure 3(C), in general titanium materials, the nitride-derived peak is extremely small. Thus, it is preferable that the titanium material 1 according to this embodiment contains a predetermined amount of nitride-derived nitrogen in the oxide film 20. The nitride-derived nitrogen content in the oxide film 20 will be described in detail below. In the titanium material 1 according to this embodiment, as the peak intensity of the nitride (nitride) in Figure 2(C) increases, the intensity of the peak involving TiN in Figure 2(A) also increases, so it is considered that the nitride in Figure 2(C) originates from titanium nitride.

The nitrogen content derived from nitrides in the oxide film 20 (maximum nitrogen concentration) is preferably 2.0 to 10.0 atomic percent. The nitrogen content derived from nitrides in the oxide film 20 refers to the maximum nitrogen concentration derived from nitrides in the oxide film 20 measured by XPS. In the case of uncolored materials, if the oxide film 20 contains 2.0 atomic percent or more of nitrogen derived from nitrides, the discoloration resistance is further improved. The reason for this is not entirely clear, but it is thought that the presence of nitrides in the oxide film 20 disrupts the atomic arrangement, changes the strain distribution within the oxide film 20, or changes the conductivity and thus the potential distribution at the nanometer level, thereby changing the function of the oxide film 20 in shielding against ion permeation (shielding function).
When the nitrogen content derived from nitrides in the oxide film 20 is 2.0 atomic% or more, the shielding function can be more reliably improved, and the effect of improving discoloration resistance can be more reliably obtained. Considering manufacturing stability, the nitrogen content derived from nitrides in the oxide film 20 is more preferably 4.0 atomic% or more. On the other hand, when the nitrogen content derived from nitrides in the oxide film 20 exceeds 10.0 atomic%, the shielding function decreases, and the effect of improving discoloration resistance may not be obtained. However, when the nitrogen content derived from nitrides in the oxide film 20 is 10.0 atomic% or less, the shielding function is maintained, and a higher effect of improving discoloration resistance can be obtained. Considering manufacturing stability, the nitrogen content derived from nitrides in the oxide film 20 is more preferably 8.0 atomic% or less.

Furthermore, the inventors conceived of improving discoloration resistance by controlling the distribution of nitrogen derived from nitrides in the oxide film 20. Figure 4 shows an example of the elemental concentration distribution in the depth direction by X-ray photoelectron spectroscopy for titanium material 1 (example of the present invention) and general titanium material (conventional example) according to this embodiment. The sputtering depth on the horizontal axis in Figure 4 is the depth converted by the sputtering rate of _SiO2 . The example of the present invention in Figure 4 is the elemental concentration distribution for the titanium material according to this embodiment shown in Figure 2. The conventional example in Figure 4 is the elemental concentration distribution for general titanium material (industrial pure titanium type 1) shown in Figure 3.

As shown in Figure 4, in the titanium material 1 according to this embodiment, the nitrogen concentration derived from nitrides is maximum at a sputtering depth of 2 to 10 nm when calculated using the sputtering rate of _SiO2 . On the other hand, in general titanium materials, the nitrogen concentration is extremely low. Therefore, the depth at which the nitrogen concentration derived from nitrides in the oxide film 20 is maximum is preferably 2 to 10 nm when calculated using the sputtering rate of _SiO2 . The upper limit of this sputtering depth is preferably 30 nm or more, including the range near the interface with the oxide film 20 as described above. It may also be changed according to the thickness of the oxide film 20, and it is preferable to set it to about three times the thickness of the oxide film 20 or more.

Furthermore, the inventors investigated the effect of nitrogen concentration at the depth where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum on discoloration resistance. Figure 5 shows the relationship between the maximum value of nitrogen concentration derived from nitrides in the oxide film 20 and the color difference ΔE ^* ab before and after the discoloration test.

The color difference ΔE ^* ab was determined by the following method: The titanium material was immersed in a pH 3 sulfuric acid aqueous solution at 60°C for 4 weeks, and the L ^* a ^* b ^* of the titanium material surface before and after immersion were measured. From the differences ΔL ^* , Δa ^* , and Δb ^* of the lightness L ^* and chromaticity a ^* and b ^* before and after immersion, respectively, determined in accordance with JIS Z 8730:2009,
ΔE ^* ab = [(ΔL ^* ) ² + (Δa ^* ) ² + (Δb ^* ) ² ] ^1/2
The calculation was performed according to the formula. A smaller color difference ΔE ^* ab indicates less discoloration before and after the test, and thus better colorfastness.

As shown in Figure 5, when the nitrogen concentration at the depth where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum is less than 2.0 atomic%, the shielding performance is not sufficiently high, and the color difference may exceed 8. On the other hand, when the nitrogen concentration derived from nitrides exceeds 10.0 atomic%, the shielding performance decreases, and the color difference may exceed 8. Furthermore, when the nitrogen concentration at the depth where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum exceeds 10.0 atomic%, the color may become golden or yellowish. When the color becomes golden or yellowish, the color changes, making it unsuitable for applications where the silver color of titanium itself is required. This change in color is presumed to be due to the manifestation of the material color of the titanium nitrides. Therefore, the nitrogen concentration at the depth where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum is between 2.0 and 10.0 atomic%.

Furthermore, as shown in Figure 5, when the nitrogen concentration at the depth where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum was less than the carbon concentration derived from carbides at the same depth, the color difference sometimes exceeded 8. This is thought to be due to the influence on the strain distribution within the oxide film 20, the change in conductivity, and the influence on the nanometer-level potential distribution, as mentioned above. Therefore, it is preferable that the nitrogen concentration at the depth where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum is equal to or greater than the carbon concentration derived from carbides at the position where the nitrogen concentration derived from nitrides in the oxide film 20 is maximum.

The concentrations of N, C, O, and Ti derived from nitrides, carbides, and oxides in the titanium substrate 10 and oxide film 20 can be calculated using X-ray photoelectron spectroscopy by sputtering the surface of the titanium material with Ar ions. Specifically, the analysis conditions are as follows: X-ray source: mono-AlKα (hν: 1486.6 eV), beam diameter: 200 μmΦ (≒analysis area), detection depth: several nm, acquisition angle: 45°, sputtering conditions: Ar ⁺ , sputtering rate: 4.3 nm/min. ( _SiO2 equivalent value). The _SiO2 equivalent value is the sputtering rate obtained using an _SiO2 film whose thickness was previously measured using an ellipsometer, under the same measurement conditions.
The peak appearing at a bond energy of approximately 393–408 eV is measured as the N1s peak, separating N from organic sources at approximately 399–401 eV and N from nitrides at approximately 397±1 eV. The peak appearing at a bond energy of approximately 280–395 eV is measured as the C1s peak, separating C from organic sources at approximately 284–289 eV and C from carbides at approximately 281.5±1 eV. The peak appearing at a bond energy of approximately 525–540 eV is measured as the O1s peak, separating O from organic sources at approximately 399–401 eV and O from metal oxides at approximately 529.5–530.5 eV. The peak appearing at a bond energy of 450–470 eV is measured as the Ti2p peak. The bond energies of the above substances are general values and may change depending on the charge of the sample being measured. One method for correcting the charge is to use the peak position of the C-C bond in organic carbon as the basis for correction.
A common analytical method using these peaks is to analyze elemental concentrations and concentrations by chemical state using the analysis software MultiPak. The general procedure is as follows: Background correction is performed based on the Shirley method. Next, peaks are fitted for each element according to its chemical state using the Gauss-Lorentz function for compounds and the Asymmetric function for metals. Then, the concentration (atomic %) of each chemical state is calculated by multiplying the area ratio of the peaks originating from each chemical state by the elemental concentration (atomic %). The nitrogen content from nitrides and the carbon content from carbides are determined using this procedure. Note that the elemental concentrations mentioned above are calculated by calculating the peak area (without separation) including all peaks related to each element detected by XPS, and then dividing this by the sensitivity coefficient for each element to obtain a percentage.
Up to this point, we have explained in detail the nitrogen content derived from nitrides in the oxide film 20.

(Surface layer part 30)
The discoloration resistance of the titanium material 1 is improved by reducing the average nitrogen concentration and average carbon concentration of the surface layer 30. From the viewpoint of discoloration resistance, the average nitrogen concentration and average carbon concentration of the surface layer 30, measured in the thickness direction from the surface of the titanium material 1 by GDS of the above method, are each 14.0 atomic% or less. The average nitrogen concentration of the surface layer 30 of the titanium material 1 is preferably 12.0 atomic% or less, more preferably 10.0 atomic% or less. The average carbon concentration of the surface layer 30 of the titanium material 1 is preferably 13.0 atomic% or less, more preferably 12.0 atomic% or less, and even more preferably 10.0 atomic% or less. The average nitrogen concentration of the surface layer 30 of the titanium material 1 may be 0 atomic% or 1.0 atomic% or more. The average carbon concentration of the surface layer 30 of the titanium material 1 may be 0 atomic% or 1.0 atomic% or more.

The discoloration resistance of the titanium material 1 is further improved by reducing the average hydrogen concentration of the surface layer 30. The hydrogen concentration of the surface layer 30 of the titanium material 1 is 30.0 atomic percent or less, preferably 25.0 atomic percent or less, and more preferably 20.0 atomic percent or less. Titanium is a metal with high affinity for hydrogen, and the hydrogen concentration of the surface layer 30 may be 10.0 atomic percent or more.

(Difference in the lattice constant of the c axis of Ti in the α phase in the surface layer 30)
The crystal structure of α-phase Ti in the surface layer 30 of the titanium material 1 affects its resistance to discoloration. Specifically, an increase in the c-axis lattice constant of α-phase Ti in the surface layer 30 of the titanium material 1 degrades its resistance to discoloration. The increase in the c-axis lattice constant of α-phase Ti in the surface layer 30 of the titanium material 1 is evaluated by the difference between the c-axis lattice constant of α-phase Ti determined by X-ray diffraction measurement using a parallel beam method with an incident angle of 0.3 degrees on the surface of the titanium material 1 and the c-axis lattice constant of α-phase Ti determined by X-ray diffraction measurement using a concentrated beam method at the center of the plate thickness. From the viewpoint of resistance to discoloration, the increase in the c-axis lattice constant of α-phase Ti in the surface layer 30 of the titanium material 1 is 0.015 Å or less, preferably 0.010 Å or less. The smaller the increase in the c-axis lattice constant of α-phase Ti on the surface of the titanium material 1, the better, and it may even be 0 Å. If this increase becomes a negative value, the cause is measurement error, and the increase is considered to be 0 Å.

The lattice constant of the c-axis of the α-phase Ti in the surface layer 30 of the titanium material 1 is determined by X-ray diffraction measurement using the parallel beam method on the surface of the titanium material. A Rigaku SmartLab X-ray diffractometer was used for the parallel beam method, and the X-ray source was Co-Kα (wavelength λ = 1.7902 Å). A W/Si multilayer mirror was used on the X-ray incidence side to remove Kβ rays. The X-ray source load power (tube voltage/tube current) was 5.4 kW (40 kV/135 mA), respectively. The incidence angle of the X-rays on the sample was 0.3 degrees, and the diffraction angle 2θ was scanned. For the measurement, a sample cut from the titanium material to dimensions of 25 mm (length) x 50 mm (width) was used. The beam was irradiated around 12.5 mm (length) x 25 mm (width) of the sample, and the measurement was performed on the surface of the sample. Furthermore, since the cut-out sample may have dirt adhering to the surface to be measured, it should be washed with acetone or ethanol.

The α-phase Ti crystal structure at the center of the titanium sheet thickness is measured by X-ray diffraction using the focusing method. The sample used for analyzing the α-phase Ti crystal structure at the center of the titanium sheet thickness is finished by mechanical polishing and electrolytic polishing so that the center of the titanium sheet thickness becomes the measurement surface for X-ray diffraction. For X-ray diffraction measurement using the focusing method, the same X-ray diffractometer used for X-ray diffraction measurement using the parallel beam method can be used, and the X-ray source, Kβ-ray removal filter, and X-ray source load power should be the same as those for the parallel beam method. Since the Ti crystal structure is uniform at the center of the sheet thickness, the sample can be prepared from any point in the sheet width or rolling direction. Here, a sample was prepared from approximately one-quarter of the sheet width towards the center, and the test was conducted.

The c-axis lattice constant of the α-phase Ti at the surface and center of the titanium plate thickness is calculated from the diffraction peak of the (0002) plane using software (Expert High Score Plus) from Spectris Corporation. Even when the titanium substrate is of the α+β type, the c-axis lattice constant of the α-phase Ti is calculated from the diffraction peak of the α-phase Ti.

(Ra/RSm: 0.006-0.015)
(RΔq: 0.150 to 0.280)
Furthermore, the inventors conducted a detailed study on the relationship between the surface properties of titanium materials and their discoloration resistance, and found that the discoloration resistance of titanium materials is influenced by the ratio of the arithmetic mean roughness Ra of the titanium substrate surface to the average length RSm of the contour curve elements, Ra/RSm, and the root mean square slope RΔq of the roughness curve elements.

The arithmetic mean roughness Ra, the average length RSm of the contour curve elements, and the root mean square slope RΔq of the roughness curve elements can be measured using methods compliant with JIS B 0601:2013. Furthermore, the kurtosis Rku and skewness Rsk, described later, can also be measured using methods compliant with JIS B 0601:2013.

The arithmetic mean roughness Ra in this embodiment is the arithmetic mean roughness Ra specified in JIS B 0601:2013, and is the average of the absolute values of the total coordinate values Zj in the reference length. The arithmetic mean roughness Ra is calculated from the following formula (1).
The roughness curve that forms the basis for calculating the arithmetic mean roughness Ra is obtained by applying a low-pass filter with a cutoff wavelength λc = 0.8 mm to the measured cross-sectional curve of the oxide film to acquire the cross-sectional curve, and then applying a high-pass filter with a cutoff wavelength λs = 2.667 μm to this stepped curve. The reference length of the roughness curve is set to be equal to the cutoff wavelength λc, i.e., 0.8 mm. λc is a filter that defines the boundary between the roughness component and the waviness component. λs is a filter that defines the boundary between the roughness component and shorter wavelength components.

In equation (1) above, n is the number of measurement points, and Zj is the height of the j-th measurement point on the roughness curve.

The average length RSm of the contour curve elements is calculated using the following formula (2).

In equation (2) above, m is the number of measurement points, and Xsi is the length of the contour curve element at the reference length.

The root mean square slope RΔq of the roughness curve element is calculated using the following equation (3).

In equation (3) above, N is the number of measurement points. (dZj/dXj) is the local slope at the j-th measurement point in the roughness curve, and is defined by equation (4) below.

In the above equation (4), ΔX is the measurement interval. In this embodiment, the measurement interval ΔX can be determined as follows. That is, the measurement interval ΔX is a value set by the surface roughness shape measuring machine, and when the measurement length L is measured, if N points of numerical data are acquired, the measurement interval ΔX will be L/(N-1) on average. For example, when measuring a measurement length of 5 mm using Tokyo Seimitsu's SURFCOM 1900DX and software TIMS Ver. 9.0.3, if 25,601 points of digital numerical data are acquired, ΔX will be 5 mm/25,600 points, which will be approximately 0.195 μm on average.

The root mean square slope RΔq of the roughness curve element is a parameter that defines the slope angle (local slope dZ/dX) of the minute range formed by surface irregularities with respect to the reference length X of the roughness curve.

The inventors fabricated titanium materials with modified Ra/RSm and RΔq, and investigated the effect of Ra/RSm and RΔq of the titanium substrate on discoloration resistance. Figure 6 shows the relationship between the arithmetic mean roughness Ra of the titanium substrate, the ratio of Ra/RSm to the average length RSm of the contour curve elements, the root mean square slope RΔq of the roughness curve elements, and discoloration resistance.

As described above, discoloration resistance can be evaluated by the color difference ΔE ^* ab and visual observation. However, in the measurement of the color tone L ^* a ^* b ^* to evaluate the color difference ΔE ^* ab, light is irradiated from a daylight source placed directly above the titanium plate. Therefore, it may differ from the actual appearance. In particular, for titanium plates with a large RΔq, even if the color difference ΔE ^* ab is small, discoloration may be visible when observed under sunlight. Therefore, visual observation under sunlight is also important for evaluating discoloration resistance.

In Figure 6, "○" indicates a condition where the color difference ΔE ^* ab was 5 or less and the percentage of people who perceived the discoloration as not noticeable in visual sensory evaluation was 80% or more, while "×" indicates a condition where the color difference ΔE ^* ab was 5 or less but the percentage of people who perceived the discoloration as not noticeable in visual sensory evaluation was less than 80%. Here, in this visual sensory evaluation, titanium material that had not been subjected to the previous discoloration acceleration test and titanium material that had been subjected to the discoloration acceleration test were placed side by side on a flat plate, and 10 evaluators compared them from various angles under sunlight to determine whether there was an angle from which the discoloration was noticeably visible. The percentage of people who perceived the discoloration as not noticeable was compared. Note that this visual observation was conducted under conditions simulating the roofs and walls of actual buildings, and the evaluation took into account that the color tone would change depending on the viewing angle.

As shown in Figure 6, it was found that the titanium material according to this embodiment exhibits superior discoloration resistance even under high temperature and acidic environments when the ratio of the arithmetic mean roughness Ra to the average length RSm of the contour curve elements, Ra/RSm, is 0.006 to 0.015, and the root mean square slope RΔq of the roughness curve elements is 0.150 to 0.280. Such a titanium material, which exhibits superior discoloration resistance even under high temperature and acidic environments, can further suppress discoloration over long periods of time.

When Ra/RSm is less than 0.006, the surface irregularities of the titanium substrate are small and the spacing between these irregularities is wide. When Ra/RSm is less than 0.006, the surface of the titanium material is relatively smooth, and the optical path difference between the light reflected from the surface of the oxide film and the light reflected from the surface of the titanium substrate may cause the color of the light to be perceived as being amplified according to this optical path difference. In other words, the titanium material may discolor. When Ra/RSm is between 0.006 and 0.015, the relatively large slope of the titanium material surface reduces the optical path difference between the light reflected from the surface of the oxide film and the light reflected from the surface of the titanium substrate, and since there is no light to be amplified in the visible light range, discoloration is thought to be suppressed. Considering this mechanism of discoloration suppression, there is no reason to limit the upper limit of Ra/RSm to 0.015, however, it is difficult to industrially manufacture deep, narrow valley-like irregularities greater than 0.015. Therefore, the upper limit of Ra/RSm is preferably 0.015, which clearly provides the effects of the present invention.

When RΔq is 0.150 or higher, the gradient of finer irregularities in the oxide film is greater, and this local gradient suppresses specular reflection of light irradiated onto the titanium substrate surface, resulting in diffuse reflection. Therefore, the intensity of reflected light from the titanium substrate surface that reflects in the direction of light reflected from the oxide film surface becomes smaller. As a result, the color of the strengthened light is difficult to perceive. When RΔq is less than 0.150, the above effect does not occur, and the titanium material may appear discolored. On the other hand, when RΔq is greater than 0.280, although the color difference is small (less than 5), there may be angles in which the discoloration is noticeable under sunlight. This is thought to be because when RΔq is greater than 0.280, when the titanium material is viewed from an oblique angle, a gradient exists that results in specular reflection, and the interference colors due to the increased thickness of the oxide film are strengthened, making them visible to the naked eye.

If Ra/RSm is 0.006 to 0.015 and the root mean square slope RΔq of the roughness curve elements is 0.150 to 0.280, the above effects are superimposed, further suppressing discoloration of the titanium material. Furthermore, even if an oxide film of several tens of nanometers grows on the surface of a titanium material having the above surface condition, changes in color, i.e., discoloration, are suppressed. Therefore, it is preferable that the titanium substrate has a ratio of Ra/RSm, which is the ratio of the arithmetic mean roughness Ra to the element length RSm, of 0.006 to 0.015 in the roughness curve in the direction in which the arithmetic mean roughness Ra is maximum, and a root mean square slope RΔq of 0.150 to 0.280. The lower limit of Ra/RSm is more preferably 0.007. Also, RΔq is more preferably 0.190 or higher. When RΔq is between 0.190 and 0.0280, the color difference in the discoloration acceleration test becomes 6 or less, resulting in an even higher effectiveness.

The arithmetic mean roughness Ra and the average length RSm of the contour curve elements are preferably Ra/RSm is 0.006 to 0.015 as described above, but it is more preferable that the arithmetic mean roughness Ra is 0.700 to 3.0 μm and the average length RSm of the contour curve elements is 60 to 300 μm. Achieving an arithmetic mean roughness Ra of 0.700 to 3.0 μm and an average length RSm of 60 to 300 μm can be relatively easily achieved industrially by the manufacturing method described later.

[Kurtosis Rku: 3+]
Kurtosis Rku is an index that represents the sharpness of the amplitude distribution curve. Figure 7 is a diagram illustrating Kurtosis Rku. Note that Figure 7 is a diagram published in Tsutomu Miyashita, "Surface Roughness to Review Again," Journal of the Japan Society for Precision Engineering, Vol. 73, No. 2, 2007, p. 205. Kurtosis Rku represents the fourth-power mean of Zj in a dimensionless reference length obtained by raising the root mean square height Rq to the fourth power.

Zj is the height of the j-th measurement point in the roughness curve. Rq is the root mean square height and is expressed by the following equation (6).

The kurtosis Rku is an index indicating the sharpness of the height distribution. When the kurtosis Rku is 3, the height distribution is a normal distribution, as shown in Figure 7. As the kurtosis Rku is less than 3 and the value decreases, the surface becomes flatter. As the kurtosis Sku exceeds 3 and the value increases, the surface of the titanium material becomes more pronounced, with many sharp peaks and valleys.

In the roughness curve in the direction where the arithmetic mean roughness Ra is maximum, it is preferable that the kurtosis Rku of the titanium substrate is greater than 3. When the kurtosis Rku is greater than 3, the surface irregularities of the titanium substrate are sharp, and on a surface with sharp irregularities, the specular reflection component that causes interference colors to become apparent in the light reflected from the surface of the titanium substrate is further suppressed. As a result, even if the oxide film thickness increases, the interference colors become less noticeable, and thus discoloration of the titanium material is further suppressed.

[Skewness Rsk: greater than -0.5]
Skewness Rsk, also known as strain, is an index that represents the sharpness of surface irregularities. Skewness Rsk represents the mean cube of Z(x) at a dimensionless reference length obtained by cube the root mean square height Rq, and is expressed by the following formula (7).

In equation (7) above, N is the number of measurement points, and Zj is the height of the j-th measurement point on the roughness curve.

In a roughness curve, if the trough length is greater than the peak length, the skewness Rsk is greater than 0. In other words, when the skewness Rsk is greater than 0, the proportion of concave parts is high in the mean line of the roughness curve. That is, the peaks (convex parts) in the roughness curve are sharply pointed, and the ends of the troughs (concave parts) are broad. The mean line of the roughness curve is the curve that represents the long-wavelength components that are blocked by the cutoff wavelength λc.
On the other hand, when the trough length is smaller than the peak length, the skewness Rsk becomes less than 0. In other words, when the skewness Rsk is less than 0, the proportion of concave areas is high in the average line of the roughness curve. That is, the tips of the peaks (convex parts) in the roughness curve become wider, and the ends of the troughs (concave parts) become sharply pointed.
When the skewness Rsk is 0, the shape of the irregularities in the roughness curve is symmetrical with respect to the average surface.

In the roughness curve in the direction where the arithmetic mean roughness Ra is maximum, it is preferable that the skewness Rsk of the titanium substrate is greater than -0.5. When the skewness Rsk is greater than -0.5, the tips of the peaks (convex parts) in the roughness curve become sharper, and the light reflected from the surface of the titanium substrate is scattered more easily closer to the light source, i.e., at the peaks (convex parts), thus further suppressing discoloration. It is presumed that the valleys (convex parts) further away from the light source are less affected than the peaks (convex parts) because the interference colors that cause discoloration are less visible due to the shadow effect of the peaks (convex parts).

Up to this point, the titanium material according to this embodiment has been described. The thickness of the titanium material according to this embodiment may be, for example, 0.2 mm or more, or 0.3 mm or more. Furthermore, the thickness of the titanium material according to this embodiment is not particularly limited, and may be, for example, 5.0 mm or less, 3.0 mm or less, or 2.0 mm or less.

In the titanium material according to this embodiment, the average nitrogen concentration and average carbon concentration in the range from the surface to the position where the oxygen concentration measured in the thickness direction from the surface by glow discharge spectroscopy is 1/3 of the maximum value are 14.0 atomic percent or less, and the average hydrogen concentration is 30.0 atomic percent or less. The difference between the lattice constant of the c axis of α-phase Ti determined by X-ray diffraction measurement using a parallel beam method with an incident angle of 0.3 degrees at the surface and the lattice constant of the c axis of α-phase Ti determined by X-ray diffraction measurement using a concentrated method at the center of the plate thickness is 0.015 Å or less. As a result, the titanium material according to this embodiment exhibits suppressed discoloration over a longer period than conventional titanium materials and has excellent discoloration resistance.

Furthermore, if the maximum nitrogen concentration derived from nitrides in the oxide film, as analyzed by X-ray photoelectron spectroscopy, is 2.0 to 10.0 atomic percent, and the position where the nitrogen concentration derived from nitrides in the oxide film is at its maximum is located within a range of 2 to 10 nm from the surface of the oxide film when converted using the sputtering rate of _SiO2 , and the nitrogen concentration derived from nitrides near the interface between the titanium substrate and the oxide film is less than the maximum nitrogen concentration derived from nitrides in the oxide film and 7.0 atomic percent or less, and the maximum nitrogen concentration derived from nitrides in the oxide film is greater than or equal to the carbon concentration derived from carbides at the position where the nitrogen concentration derived from nitrides in the oxide film is at its maximum, then the material exhibits excellent discoloration resistance even in high-temperature and acidic environments. And, titanium materials that exhibit excellent discoloration resistance even in high-temperature and acidic environments can further suppress discoloration over long periods of time.

(Method of manufacturing titanium material)
An example of a manufacturing method for titanium material according to this embodiment will be described below. However, the titanium material according to this embodiment is not limited to that manufactured by the manufacturing method described below. Also, since pure titanium and titanium alloys used in building materials such as exterior materials are often in sheet form, an example of a manufacturing method for sheet-shaped titanium material will be described below.

Titanium materials are manufactured by cold-rolling pure titanium or titanium alloy, followed by annealing and cooling treatments.

Titanium materials used for cold rolling may be those manufactured by known methods. For example, using sponge titanium or a master alloy for adding alloying elements as raw materials, ingots of pure titanium or titanium alloy having the above-mentioned components are produced by various melting methods such as vacuum arc melting, electron beam melting, or plasma melting. Next, the obtained ingots are divided into slabs as needed and hot forged. Then, the slabs are hot-rolled to produce hot-rolled coils of pure titanium or titanium alloy having the above-mentioned composition.
Furthermore, the slab may be pre-treated as needed, such as by cleaning or cutting. Also, if it is a rectangular shape that can be hot-rolled by the hearth melting method, it may be subjected to hot rolling without hot forging or other processes.

Next, the hot-rolled coil is subjected to cold rolling. Lubricating oil is used during cold rolling, but the lubricating oil used may increase the carbon concentration on the surface of the titanium material during annealing. Therefore, preferably, before annealing, any oil present on the surface of the titanium material is removed by alkaline degreasing, dull rolling using dull rolls, coil grinding, or polishing. Note that if lubricating oil is applied to the surface of the titanium material and then cold-rolled, carbon will be incorporated into the surface of the titanium material due to a mechanochemical reaction. The cold titanium material subjected to the final annealing treatment may be pickled or annealed as appropriate.

A final annealing treatment is performed on the titanium material after cold rolling or after oil removal treatment. The final annealing treatment is generally a process to reduce the strain introduced into the titanium material, which is either pure titanium or a titanium alloy, by cold rolling, and to soften the titanium material. In the manufacturing of the titanium material according to this embodiment, the final annealing treatment and the subsequent cooling treatment are processes aimed at controlling the average nitrogen concentration, average carbon concentration, average hydrogen concentration, lattice constant of the c-axis of the α-phase Ti in the surface layer of the titanium material, and the thickness of the oxide film. It is believed that increasing the temperature of the final annealing treatment causes nitrogen and carbon in the surface layer of the titanium material, which are due to impurities, to diffuse in the thickness direction, thereby decreasing the average nitrogen concentration and average carbon concentration in the surface layer of the titanium material. It is believed that the average hydrogen concentration and the lattice constant of the c-axis of the α-phase Ti in the surface layer of the titanium material will decrease by increasing the vacuum level during the final annealing treatment. Therefore, the final annealing treatment is performed in an inert gas atmosphere (excluding nitrogen gas) after creating a vacuum, or directly in a vacuum. The heating temperature and atmosphere for the final annealing treatment are described below. Here, inert gas refers to a gas that is inert to titanium, and includes argon, helium, and neon.

The heating temperature (annealing temperature) for the final annealing treatment is 630°C or higher, from the viewpoint of reducing the average nitrogen and average carbon concentrations in the surface layer of the titanium material. Preferably, the annealing temperature is 650°C or higher, from the viewpoint of reducing the average carbon concentration in the surface layer of the titanium material. While there is no specific upper limit for the annealing temperature, 750°C or lower is preferred from the viewpoint of manufacturing costs. The annealing temperature referred to here is the temperature inside the heating furnace used for the annealing treatment, and is measured using a thermocouple installed in the heating furnace. The annealing time is preferably 5 hours or more, from the viewpoint of reducing the average nitrogen and average carbon concentrations in the surface layer of the titanium material. More preferably, the annealing time is 10 hours or more. The annealing time may be 3 hours or more, as cold-rolled titanium material can be annealed. On the other hand, from the viewpoint of productivity, the annealing time is preferably 48 hours or less. Furthermore, if the annealing time exceeds 10 hours, the grain size may become too coarse, leading to a decrease in tensile strength and defects such as wrinkles caused by processing. Therefore, from the viewpoint of maintaining tensile strength, the annealing time is preferably 10 hours or less. Note that the annealing time referred to here is the time during which the temperature inside the heating furnace containing the titanium material is maintained at the annealing temperature.

The final annealing treatment is performed in a vacuum atmosphere, an inert gas atmosphere (excluding nitrogen gas), or an inert gas atmosphere obtained by introducing an inert gas excluding nitrogen after creating a vacuum atmosphere. The vacuum level of the vacuum atmosphere is, for example, 1.0 × ^10⁻² Pa or less. The inert gas atmosphere is preferably a noble gas atmosphere, and more preferably an Ar atmosphere. The vacuum level before creating the inert gas atmosphere in the final annealing treatment is preferably 1.0 × ^10⁻² Pa or less from the viewpoint of suppressing the increase in the c-axis lattice constant of Ti in the α phase of the surface layer of the titanium material and reducing the average hydrogen concentration and average nitrogen concentration of the surface layer of the titanium material. The vacuum level before creating the inert gas atmosphere in the final annealing treatment is more preferably 5.0 × ^10⁻³ Pa or less from the viewpoint of reducing the average hydrogen concentration of the surface layer of the titanium material. The inert gas atmosphere can be, for example, an atmosphere containing 99.99 volume% or more Ar inside the heating furnace. The inert atmosphere may also be an atmosphere containing 99.99 volume% or more He (helium). The heating furnace may be made into a vacuum atmosphere before the start of heating for the final annealing treatment, and then changed to an inert gas atmosphere. Alternatively, the heating furnace may be made into a vacuum atmosphere before the start of heating, and then changed from a vacuum atmosphere to an inert gas atmosphere between the start of heating and the start of cooling.

By performing the final annealing treatment in a vacuum atmosphere, or in an inert gas atmosphere (excluding nitrogen) created by introducing an inert gas other than nitrogen after creating a vacuum atmosphere, at the aforementioned annealing temperature and annealing time, a state with low levels of oxygen, nitrogen, carbon, and hydrogen is formed on the surface of the titanium material. By achieving such a highly clean surface state in the final annealing treatment, the titanium material can be exposed to air, a nitrogen gas atmosphere, or an inert gas atmosphere containing 10% or more nitrogen gas during the subsequent cooling treatment, and a predetermined amount of nitride can be generated within the oxide film by the nitrogen contained in these atmospheres. In titanium material that has been cooled in the final annealing treatment and exposed to air, even if it is heated again in a nitrogen-containing atmosphere, the predetermined amount of nitride cannot be generated within the oxide film.

The nitrogen concentration of the annealing atmosphere is 0.005% by volume or less, and it is an inert gas. In general industrial pure gases, nitrogen accounts for less than half of the impurities, so the purity is sufficient for the annealing atmosphere in this embodiment using the aforementioned inert gas.

The atmosphere for the final annealing treatment is maintained at a temperature below which temper color does not appear on the titanium material after annealing, for example, below 300°C. The annealing atmosphere may be maintained until it cools to room temperature, or the heating furnace may be opened to the atmosphere or other external source at a temperature below 300°C.

After the final annealing treatment, the titanium material is cooled. The cooling atmosphere can be, for example, the same as the annealing atmosphere at the start of cooling, and when the temperature inside the heating furnace is below 300°C, it can be an atmosphere composed of nitrogen gas, a mixed atmosphere of argon or helium containing 10% or more by volume of nitrogen, or air.

The cooling rate after the final annealing treatment is not particularly limited. However, below 300°C, when the material is opened, it is preferable to use the conditions described below in order to keep the amount of nitrogen derived from nitrides present in the oxide film within a predetermined range. Furthermore, in order to suppress deformation due to thermal shrinkage of the titanium material during cooling, the cooling rate until opening (below 300°C) is preferably 50°C/min or less, more preferably 30°C/min or less, and even more preferably 1°C/min when cooling large titanium materials weighing 1 ton or more.

During the cooling process after the final annealing treatment, it is preferable to introduce nitrogen gas into the heating furnace or open the furnace to the atmosphere when the temperature inside the furnace drops below 300°C, thereby creating a nitrogen atmosphere containing 10% or more nitrogen by volume. The amount of nitrogen derived from nitrides present in the oxide film varies depending on the temperature, atmosphere, and the cleanliness of the titanium surface. This is because trace amounts of carbon, oxygen, hydrogen, and nitrogen present on the surface of the titanium material compete to react on the surface, and the amount of reaction between nitrogen and titanium changes depending on which reaction occurs preferentially, which in turn changes the amount of nitrides produced. However, if the temperature when changing the atmosphere (opening the atmosphere) is below 300°C, the reaction between titanium and nitrogen occurs sufficiently, and the amount of nitrides produced does not become excessive. Therefore, by the cooling treatment following the annealing treatment, the maximum value of nitrogen concentration derived from nitrides present in the oxide film can be set to 2.0 to 10.0 atomic percent. On the other hand, if the temperature when changing the atmosphere is below 200°C, the nitrogen content derived from nitrides in the oxide film will be less than 2.0 atomic percent. This is because the reaction between nitrogen and titanium slows down at lower temperatures. The above temperature is preferably 250°C or higher, and more preferably 280°C or higher.

It is preferable to allow a cooling time of 1.5 hours or more, and more preferably 2.0 hours or more, to reach 200°C after creating a nitrogen atmosphere inside the heating furnace. By allowing a cooling time of 1.5 hours or more to reach 200°C after creating a nitrogen atmosphere inside the heating furnace, the maximum value of nitrogen concentration derived from nitrides in the oxide film, as analyzed by X-ray photoelectron spectroscopy, becomes 2.0 to 10.0 atomic percent, and the position where the nitrogen concentration derived from nitrides in the oxide film shows the maximum value is located in a range of 2 to 10 nm from the surface of the oxide film, when converted to the sputtering rate of _SiO2 .

Furthermore, even if the titanium material after cooling is subjected to cold rolling, including rolling using dull rolls at a reduction ratio of 5% or less, the characteristics of the titanium material according to this embodiment are maintained.

In the titanium material manufacturing method according to this embodiment, it is preferable to have a polishing step in which the surface of the titanium material is polished using abrasive fine powder having a particle size distribution of #320 or less in accordance with JIS R 6001-2:2017, and a dull rolling step in which the titanium material is reduced using a rolling roll with a surface roughness Ra of 0.5 μm or more so that the total reduction ratio is 0.10% or more. The polishing step is performed before the final annealing treatment, and the dull rolling step is performed after the final annealing treatment. As a result of the polishing step and the dull rolling step, the Ra/RSm of the titanium substrate surface becomes 0.006 to 0.015, and the root mean square slope RΔq of the roughness curve elements becomes 0.150 to 0.280. The polishing step and the dull rolling step will be described below.

[Polishing process]
In this process, the surface of the titanium material is polished using abrasive powder having a particle size distribution of #320 or less in accordance with JIS R 6001-2:2017. The means for polishing the surface of the titanium material are not particularly limited, and known means such as brush rolls or coil grinders can be used.

For example, when using a coil grinder, the surface of a sheet-coil-shaped titanium material is polished in the following manner: The titanium material is polished using a coil line polishing machine with abrasive belts of grit #320 or lower, such as #320, #240, #100, #80, etc. The abrasive powder used in the polishing belt is preferably of grit #100 or lower. To obtain a more uniform polished surface, multiple polishing passes may be performed using the same grit or with different grits.

The titanium material used in the polishing process may be manufactured by known methods. For example, using sponge titanium or a master alloy for adding alloying elements as raw materials, an ingot of pure titanium or titanium alloy having the above components can be produced by various melting methods such as vacuum arc melting, electron beam melting, or plasma melting. Next, the obtained ingot can be divided into slabs as needed and hot forged. Then, the slabs can be hot-rolled to obtain hot-rolled coils of pure titanium or titanium alloy having the above composition. These hot-rolled coils can be cold-rolled, and the titanium material after cold rolling can be polished. The cold-rolled titanium material used in the polishing process may be annealed as appropriate.
Furthermore, the slab may be pre-treated as needed, such as by polishing or cutting. Also, if it is a rectangular shape that can be hot-rolled by the hearth melting method, it may be subjected to hot rolling without splitting or hot forging.
There are no particular restrictions on the cold rolling conditions; they can be used under conditions that appropriately yield the desired thickness, properties, etc.

[Dull rolling process]
In this process, the titanium material is reduced using a rolling work roll (hereinafter referred to as a rolling roll) with an arithmetic mean surface roughness Ra of 0.5 μm or more. By reducing the titanium material using the above rolling roll so that the total reduction rate is 0.10% or more, more localized inclined irregularities are imparted to the surface of the titanium material. If the arithmetic mean surface roughness Ra of the rolling roll is too large, the shape of the irregularities previously imparted may change significantly during the polishing process. Therefore, the arithmetic mean surface roughness Ra of the rolling roll is preferably 2.0 μm or less.
The surface roughness of the rolling mill rolls can be adjusted by polishing or shot blasting.

The total reduction ratio is preferably 0.10% or more, and more preferably 0.2% or more, to create localized inclined irregularities, and more preferably 0.2% or more, for the stability of surface preparation along the entire length of the coil. On the other hand, it is preferable to keep it at 1.5% or less so that the surface roughness formed by the polishing in the previous step is not flattened by cold rolling and the necessary irregular shapes are not eliminated. Furthermore, while one pass of cold rolling is sufficient to obtain the surface characteristics of the present invention, considering the need to finish the entire length of a long coil with a surface as uniform as possible, cold rolling may be performed in two or more passes. The total reduction ratio is defined taking this into consideration, and in the case of multiple passes, the total reduction ratio is the reduction ratio obtained from the difference between the initial and finished sheet thickness.

(Example 1)
The titanium material shown in Table 1 was cold-rolled to a thickness of 0.4 mm, and then degreased using an alkali or organic solvent to remove oil from the surface of the titanium material. In this example, JIS Grade 1 pure titanium (equivalent to ASTM Gr. 1), JIS Grade 2 pure titanium (equivalent to ASTM Gr. 2), JIS Grade 3 pure titanium (equivalent to ASTM Gr. 3), JIS Grade 4 pure titanium (equivalent to ASTM Gr. 4), JIS Grade 11 titanium alloy (equivalent to ASTM Gr. 11, Ti-0.15Pd), JIS Grade 21 titanium alloy (equivalent to ASTM Gr. 13, Ti-0.5Ni-0.05Ru), JIS Grade 17 titanium alloy (equivalent to ASTM Gr. 7, Ti-0.05Pd), Ti-Ru-Mm, Ti-3Al-2.5V, Ti-5Al-1Fe, and JIS Grade 60 titanium alloy (equivalent to ASTM Gr. 5, Ti-6Al-4V) were used in accordance with JIS H 4600:2012. In Ti-Ru-Mm, Mm represents mischmetal.
Subsequently, in Examples 1 to 23 of the present invention, each titanium material was subjected to annealing treatment under the conditions shown in Table 1. The opening temperatures shown in Table 1 are the temperatures at which the furnace was opened during the cooling process after each annealing temperature and time had been maintained. These temperatures were measured using a thermocouple.
In Examples 1-13, 19-23, and Comparative Examples 1-3 of the present invention, heating was started under a vacuum atmosphere as shown in "Vacuum Level" in Table 1, and 99.99% or more of Ar gas was introduced into the annealing furnace before the start of cooling. During the cooling process after each titanium material was held in the furnace at each annealing temperature for each annealing time, each annealing atmosphere was maintained until the opening temperature shown in Table 1, and the furnace was opened when the temperature inside the furnace had dropped to the opening temperature.
In examples 14 and 15 of the present invention, heating was started in an Ar atmosphere and the Ar atmosphere was maintained until the furnace was opened.
In Examples 16 to 18 of the present invention, heating was started under a vacuum atmosphere as shown in "Vacuum Level" in Table 1, and this vacuum level was maintained until the furnace was opened.
In Comparative Examples 4 and 5, the titanium material shown in Table 1 was cold-rolled to a thickness of 0.4 mm, then degreased using an alkali or organic solvent, and finally pickled with nitrate and hydrofluoric acid without a final annealing treatment.

The average nitrogen, carbon, and hydrogen concentrations in the surface layer were determined by the following method. O, N, C, H, and Ti were analyzed by GDS. A JOBIN YVON GD-Profiler 2 manufactured by Horiba, Ltd. was used for the measurements. The measurement conditions were a constant power mode of 35W, an argon gas pressure of 600 Pa, and a discharge range of 4 mm in diameter. The measurement pitch in the GDS measurements was 0.5 nm.
The concentrations (atomic percent) of each element listed above were calculated assuming the sum of the elements equaled 100 atomic percent. The surface layer of the titanium material was defined as the area from the surface of the titanium material to the point where the oxygen concentration, measured in the thickness direction by GDS, was 1/3 of the maximum value. The average nitrogen concentration, average carbon concentration, and average hydrogen concentration were calculated as the arithmetic mean of the nitrogen, carbon, and hydrogen concentrations at each measurement point.

The c-axis lattice constant of α-phase Ti on the surface of the titanium material was determined by X-ray diffraction measurement using the parallel beam method. A Rigaku SmartLab X-ray diffractometer was used for the parallel beam X-ray diffraction measurement, with a Co-Kα X-ray source (wavelength λ = 1.7902 Å). A W/Si multilayer mirror was used on the X-ray incidence side to remove Kβ rays. The X-ray source load power (tube voltage/tube current) was 5.4 kW (40 kV/135 mA), respectively. The incidence angle of the X-rays on the sample was 0.3 degrees, and the diffraction angle 2θ was scanned. For the measurement sample, a 0.4 mm thick titanium plate was machined to a size of 25 mm (length) x 50 mm (width). The beam was irradiated around the center of the titanium material surface, in other words, at a position of 12.5 mm (length) x 25 mm (width) on the surface of the measurement sample, and the measurement was performed. Furthermore, since the cut-out samples may have dirt adhering to the surface to be measured, they were washed with acetone.

The c-axis lattice constant of α-phase Ti at the center of the titanium plate thickness was measured by X-ray diffraction using the focusing method. The sample used for analyzing the crystal structure of α-phase Ti at the center of the titanium plate thickness was finished by mechanical polishing and electrolytic polishing so that the center of the titanium plate thickness served as the measurement surface for X-ray diffraction. For the focusing method X-ray diffraction measurement, the same X-ray diffractometer used for the parallel beam method was employed, and the X-ray source, Kβ-ray removal filter, and X-ray source load power were the same as those used for the parallel beam method.

The c-axis lattice constant of the α-phase Ti at the surface and center of the titanium material thickness was calculated from the diffraction peaks of the (0002) plane.
The difference in the c-axis lattice constant of the α-phase Ti on the surface of the titanium material was determined from the difference between the lattice constant calculated on the surface of the titanium material and the lattice constant calculated at the center of the plate thickness.

The thickness of the oxide film was determined by the oxygen concentration measured by the above method using GDS. Specifically, the thickness of the oxide film was defined as the distance in the thickness direction from the surface to the point where the oxygen concentration was halved from its maximum value.

The nitrogen content derived from nitrides in the oxide film was measured by the following method. Specifically, the maximum value within the oxide film, obtained from the distribution of nitrogen concentration derived from nitrides in the depth direction measured by the following method, was defined as the nitrogen content derived from nitrides in the oxide film.

The depth-direction (film thickness direction) distribution of nitrogen concentration derived from nitrides and carbon concentration derived from carbides within the oxide film, as well as the nitrogen concentration derived from nitrides near the interface with the oxide film on the titanium substrate (20 nm from the interface toward the titanium substrate), were measured by the following method. Specifically, the concentration distribution in the depth direction was measured by Ar ion sputtering of the titanium material surface using XPS, and the elemental state at each peak position of N1s, C1s, O1s, and Ti2p was analyzed to calculate the concentrations of N, C, O, and Ti derived from nitrides, carbides, and oxides. The details were calculated using the procedure described above. The XPS analysis conditions were as follows.
Equipment: ULVAC-FI Quantera SXM
X-ray source: mono-AlKα (hν:1486.6eV)
Beam diameter: 200 μmΦ (approximately analysis area)
Detection depth: several nm
Capture angle: 45°
Sputtering conditions: Ar ⁺ , sputtering rate 4.3 nm/min ( _SiO2 equivalent value)
The _SiO2 equivalent value is the sputtering rate obtained using an _SiO2 film whose thickness was previously measured using an ellipsometer, under the same measurement conditions.

The surface roughness parameters of the manufactured titanium material (arithmetic mean roughness Ra, average length RSm of the contour curve elements, root mean square slope RΔq of the roughness curve elements, crustosis Rku, and skewness Rsk) were measured in accordance with JIS B 0601:2013 under the following conditions.
Equipment: Surface roughness and shape measuring machine (SURFCOM 1900DX, manufactured by Tokyo Seimitsu Co., Ltd.; Analysis software: TiMS Ver. 9.0.3)
Measuring probe: Shape measuring probe manufactured by Tokyo Seimitsu Co., Ltd. (Model: DT43801)
Parameter calculation standard: JIS-01 standard Measurement type: Roughness measurement Cutoff type: Gaussian Slope correction: Least squares linear correction Measurement distance: 5.0 mm
Cutoff wavelength λc: 0.8 mm
Measurement range: ±64.0 μm
Measurement speed: 0.3mm/sec
Movement/return speed: 0.6 mm/sec
Return setting: Normal measurement Pre-drive length: (Cutoff wavelength / 3) × 2
Measurement interval Δx: 0.195μm
λs cutoff ratio: 300
λs cutoff wavelength: 2.667 μm
Pickup type: Standard pickup Polarity: Forward rotation

The measurement position involved taking three measurements in the direction where Ra was maximized and calculating the average value. Here, for titanium material in sheet form, the direction parallel to the rolling direction was defined as 0°, and the roughness was measured in four directions: 22.5°, 45°, and 90° (perpendicular to the rolling direction). The direction where Ra was maximized was determined by measuring the roughness in these directions. When titanium material was rolled using rolling rolls, or when the sheet surface was polished by rotating a roll embedded with abrasive grains in the rolling direction, Ra was maximized in the 90° direction, which is perpendicular to the rolling direction.

Each manufactured titanium material was immersed in a sulfuric acid solution at pH 3 and 60°C for four weeks. The color difference before and after immersion was calculated, and the colorfastness was evaluated based on the color difference value. A color difference ΔE of 0 to 5 was considered to indicate excellent colorfastness (A), a color difference of 5 to 10 was considered to indicate good colorfastness (B), and a color difference of 10 was considered to indicate poor colorfastness (C). The color difference ΔE before and after the test was calculated using the formula ΔE = ((L*2 - L*1) ^² + (a*2 - a*1) ^² + (b*2 - b*1) ^² )¹/².
Here, L*1, a*1, and b*1 are the color measurement results before the color change test, and L*2, a*2, and b*2 are the color measurement results after the color change test, based on the L*a*b* color method specified in JIS Z 8729.
Furthermore, the color difference was measured using a Konica Minolta CR400 under conditions of a measurement area diameter of 8 mm and a D65 light source.

Furthermore, each manufactured titanium material was subjected to a visual sensory evaluation. For the visual evaluation, titanium material that had not been subjected to this discoloration acceleration test and titanium material that had been subjected to this discoloration acceleration test were placed side by side on a flat plate, and 10 evaluators compared them from various angles under sunlight to determine whether or not there was an angle from which the discoloration was noticeably visible. A rating of A+++ was given when 90% or more of the 10 evaluators judged that the discoloration was not noticeable, A++ was given when 80% to less than 90% of the 10 evaluators judged that the discoloration was not noticeable, A+ was given when 70% to less than 80% of the 10 evaluators judged that the discoloration was not noticeable, A0 was given when 50% to less than 70% of the 10 evaluators judged that the discoloration was not noticeable, B was given when 30% to less than 50% of the 10 evaluators judged that the discoloration was not noticeable, and C was given when less than 30% of the 10 evaluators judged that the discoloration was not noticeable. A rating of C was considered a failure. This visual observation was conducted under conditions simulating the roofs and walls of actual buildings, and the evaluation took into account that the color tone may change depending on the viewing angle.
The results are shown in Tables 2 and 3. Underlined text in Tables 2 and 3 indicates that the topic is outside the scope of the present invention.

Examples 1 to 23 of the present invention had average nitrogen concentration and average carbon concentration of 14.0 atomic percent or less in the surface layer, an average hydrogen concentration of 30.0 atomic percent or less, and the difference in lattice constant of the c axis of Ti in the α phase at the surface and center of the thickness of the titanium material was 0.015 Å or less. The color difference evaluation results were B or higher, and the visual sensory evaluation results were B or higher.
In Comparative Example 1, the vacuum level during the annealing process was low at 1.0 × ^10⁻¹ Pa, resulting in a high average nitrogen concentration in the surface layer. Consequently, both the color difference evaluation and the visual sensory evaluation results for the titanium material in Comparative Example 1 were unsatisfactory.
Comparative Example 2 had a low annealing temperature and a high average carbon concentration in the surface layer. As a result, both the color difference evaluation and the visual sensory evaluation results for the titanium material of Comparative Example 2 were unsatisfactory.
In Comparative Example 3, the vacuum level during the annealing process was low at 8.0 × ^10⁻² Pa, which is presumed to have resulted in a high oxygen concentration in the surface layer of the titanium material, leading to a larger lattice constant of the c axis of the α-phase Ti in the surface layer and a larger difference in lattice constants. As a result, both the color difference evaluation and the visual sensory evaluation results for the titanium material in Comparative Example 3 were unsatisfactory.
In Comparative Examples 4 and 5, the average hydrogen concentration in the surface layer was high, and both the color difference evaluation results and the visual sensory evaluation results for the titanium material in Comparative Example 3 were unsatisfactory.

As demonstrated above, the titanium material according to this embodiment exhibited excellent discoloration resistance for a long period of time even in a pH 3 sulfuric acid aqueous solution simulating severe acid rain. The present invention is particularly effective for outdoor applications such as roofs or wall panels, and its industrial value is extremely high.

(Example 2)
The titanium materials shown in Table 4 were cold-rolled to a thickness of 0.4 mm, and then degreased using an alkali or organic solvent to remove oil from the surface of the titanium material. Afterward, each titanium material was annealed under the conditions shown in Table 4. The opening temperatures shown in Table 4 are the temperatures at which the furnace was opened during the cooling process after each annealing temperature and time had been maintained. These temperatures were measured using a thermocouple.

Each manufactured titanium material was evaluated in the same manner as in Example 1. The results are shown in Tables 5 and 6.

As shown in Tables 5 and 6, in Examples 24 to 39 of the present invention, the maximum nitrogen concentration derived from nitrides, as analyzed by X-ray photoelectron spectroscopy in the oxide film, was 2.0 to 10.0 atomic percent. The position where the nitrogen concentration derived from nitrides in the oxide film is at its maximum value is located in the range of 2 to 10 nm from the surface of the oxide film, when converted using the sputtering rate of _SiO2. The nitrogen concentration derived from nitrides near the interface between the titanium substrate and the oxide film is less than the maximum nitrogen concentration derived from nitrides in the oxide film and 7 atomic percent or less. The maximum nitrogen concentration derived from nitrides in the oxide film is equal to or greater than the carbon concentration derived from carbides at the position where the nitrogen concentration derived from nitrides in the oxide film is at its maximum. The color difference evaluation results were A, and the visual sensory evaluation results were all A++, indicating excellent discoloration resistance.

(Example 3)
After cold-rolling the titanium material shown in Table 7 to a thickness of 0.4 mm, the polishing, cleaning, annealing, and dull-rolling processes were carried out in the order shown in Tables 7 and 8 under the conditions indicated. The "number of polishing passes" shown in Table 7 refers to the number of times the titanium material was passed through the line of a coil grinder consisting of three polishing stands equipped with polishing belts.
In Examples 40 to 72 of the present invention, each titanium material was subjected to the final annealing treatment under the conditions shown in Table 8. The opening temperatures shown in Table 8 are the temperatures at which the furnace was opened during the cooling process after each annealing temperature and time had been maintained. These temperatures were measured using a thermocouple.
In Examples 40-61 and 63-72 of the present invention, heating was started under a vacuum atmosphere as shown in "Vacuum Level" in Table 8, and 99.99% or more of Ar gas was introduced into the annealing furnace before the start of cooling. During the cooling process after each titanium material was held in the furnace at each annealing temperature for each annealing time, each annealing atmosphere was maintained until the opening temperature shown in Table 8, and the furnace was opened when the temperature inside the furnace had dropped to the opening temperature.
In Example 62 of the present invention, heating was started under a vacuum atmosphere as shown in "Vacuum Level" in Table 8, and this vacuum level was maintained until the furnace was opened.
In Comparative Example 6, the titanium material was cold-rolled to a thickness of 0.4 mm, then degreased using an alkali or organic solvent, and finally pickled with nitrate and hydrofluoric acid without performing a final annealing treatment.

Each manufactured titanium material was evaluated in the same manner as in Example 1. The results are shown in Tables 9 and 10. Underlined text in Table 9 indicates that the material falls outside the scope of the present invention.

As shown in Tables 7-10, by controlling the heat treatment temperature, time, heat treatment atmosphere, and cooling atmosphere, it was found that in the roughness curve in the direction where the arithmetic mean roughness Ra is maximized, the ratio of arithmetic mean roughness Ra to element length RSm, Ra/RSm, is 0.006 to 0.015, the root mean square slope RΔq is 0.150 to 0.280, the crustosis Rku of the titanium substrate is greater than 3, and the skewness Rsk of the titanium substrate is greater than -0.5, resulting in even better evaluation results. In particular, in Examples 59 to 61 of the present invention, the average nitrogen concentration and average carbon concentration in the surface layer were both 14.0 atomic percent or less, the average hydrogen concentration was 30.0 atomic percent or less, the difference in the lattice constant of the c axis of Ti in the α phase at the surface of the titanium material and in the center of the plate thickness was 0.015 Å or less, the maximum value of nitrogen concentration derived from nitrides when analyzed by X-ray photoelectron spectroscopy in the oxide film was 2.0 to 10.0 atomic percent, the position where the nitrogen concentration derived from nitrides in the oxide film shows the maximum value is located in the range of 2 to 10 nm from the surface of the oxide film when converted using the sputtering rate of _SiO2 , the nitrogen concentration derived from nitrides located near the interface between the titanium substrate and the oxide film was less than the maximum value of nitrogen concentration derived from nitrides in the oxide film and 7 atomic percent or less, and the maximum value of nitrogen concentration derived from nitrides in the oxide film was greater than or equal to the carbon concentration derived from carbides at the position where the nitrogen concentration derived from nitrides in the oxide film is maximum. As a result, the color difference evaluation result was A, and the visual sensory evaluation result was A+++, indicating extremely excellent colorfastness.

1 Titanium material 10 Titanium substrate 20 Oxide film 30 Surface layer

Claims (4)

Titanium substrate and
A titanium material having an oxide film disposed on the surface of the titanium substrate,
The aforementioned titanium substrate is, by mass%,
Co: 0% or more and 1.0% or less,
Cr: 0% or more and 0.5% or less,
Ni: 0% or more and 1.00% or less,
Ta: 0% or more and 6.00% or less,
Al: 0% or more and 7.0% or less,
V: 0% or more and 5.0% or less,
S: 0% or more and 0.3% or less,
Cu: 0% or more and 1.50% or less,
Nb: 0% or more and 0.70% or less,
Sn: 0% or more and 1.40% or less,
Si: 0% or more and 0.55% or less,
Mo: 0% or more and 0.5% or less,
W: 0% or more and 0.5% or less,
Pd: 0% to 0.25%,
Ru: 0% or more and 0.15% or less,
Rh: 0% or more and 0.15% or less,
Os: 0% or more and 0.15% or less,
Ir: 0% or more and 0.15% or less,
Pt: 0% or more and 0.15% or less,
REM: 0% or more and 0.10% or less,
C: 0% or more and 0.18%% or less,
H: 0% or more and 0.015% or less,
O: 0% or more and 0.40% or less,
N: 0% or more and 0.05% or less,
Fe: Includes 0% to 2.50%,
The remainder consists of Ti and impurities.
From the surface of the titanium material , the average nitrogen concentration and average carbon concentration in the range up to the point where the oxygen concentration measured in the thickness direction from the surface by glow discharge spectroscopy is 1/3 of the maximum value are 14.0 atomic percent or less, and the average hydrogen concentration is 30.0 atomic percent or less.
The difference between the c-axis lattice constant of Ti in the α phase, determined by X-ray diffraction measurement using a parallel beam method with an incident angle of 0.3 degrees on the aforementioned surface, and the c-axis lattice constant of Ti in the α phase, determined by X-ray diffraction measurement using a concentrated beam method at the center of the plate thickness , is 0.015 Å or less.
When analyzed by X-ray photoelectron spectroscopy,
The maximum nitrogen concentration derived from nitrides in the oxide film is 2.0 to 10.0 atomic percent.
The position in the oxide film where the nitrogen concentration derived from the nitride is at its maximum value is located in a range of 2 to 10 nm from the surface of the oxide film, when converted using the sputtering rate of _SiO₂ .
The nitrogen concentration derived from the nitride, present in a 20 nm range from the position where the oxygen concentration is half of its maximum value to the titanium substrate side, is less than the maximum value of the nitrogen concentration derived from the nitride in the oxide film and is 7 atomic percent or less.
The maximum value of the nitrogen concentration derived from the nitride in the oxide film is equal to or greater than the carbon concentration derived from the carbide at the position in the oxide film where the nitrogen concentration derived from the nitride is maximum.
Titanium material. It has an oxide film with a thickness of 30.0 nm or less.
The titanium material according to claim 1. In the roughness curve in the direction where the arithmetic mean roughness Ra is maximized, the ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, is 0.006 to 0.015, and the root mean square slope RΔq is 0.150 to 0.280, comprising a titanium substrate.
The kurtosis Rku of the aforementioned titanium substrate is greater than 3.
The skewness Rsk of the aforementioned titanium substrate is greater than -0.5.
The titanium material according to claim 1 or 2. The titanium substrate has a roughness curve in the direction in which the arithmetic mean roughness Ra is maximum, where the ratio of the arithmetic mean roughness Ra to the element length RSm, Ra/RSm, is 0.006 to 0.015, and the root mean square slope RΔq is 0.150 to 0.280.
The kurtosis Rku of the aforementioned titanium substrate is greater than 3.
The skewness Rsk of the aforementioned titanium substrate is greater than -0.5.
The titanium material according to claim 1 or 2 .