EP4656752A1

EP4656752A1 - Titanium material and method for manufacturing same

Info

Publication number: EP4656752A1
Application number: EP23918668.7A
Authority: EP
Inventors: Ryotaro MIYOSHI; Sosuke Nishiwaki; Kazuhiro Takahashi; Junichi Tamenari; Atsushi Kurita
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-01-23
Filing date: 2023-12-26
Publication date: 2025-12-03
Also published as: JP7518462B1; CN120303420A; JPWO2024157714A1; KR20250136362A

Abstract

A titanium product, in which a Vickers hardness HVs of surface measured at a load of 25 gf satisfies a following formula (i), a relationship between the Vickers hardness HVs and a glossiness Gs measured at an angle of incidence of 20° satisfies a following formula (ii), and a color difference ΔE*ab between a surface and a resultant surface after removal of 10 to 20 µm from the surface by pickling with fluonitric acid satisfies a following formula (iii):

HVs \geq 200

HVs \geq 250 - 0.25 Gs

ΔE * ab < 3

Description

TECHNICAL FIELD

The present disclosure relates to a titanium product and a production method of the same.

BACKGROUND ART

Press forming, which is a method of forming performed by pressing a blank against a die and applying a pressure thereto, is a common forming method for a metal blank. Performing press forming is associated with a problem of adhesion that occurs between a blank and a die. The adhesion, if it occurs, leads to poorer lubrication and degrades formability. In addition, the blank may suffer from surface defects, or otherwise the life of the die may be reduced. Accordingly, there has been a need for suppression of adhesion.
Industrial pure titanium or a titanium alloy (hereinafter also simply referred to as "titanium product") is prone to adhesion, that is, a low-lubricity material. Accordingly, it is a common practice to apply a chemical agent referred to as a solid film lubricant to the surface when a titanium product is subjected to press forming. This improves lubricity to suppress adhesion. However, using a solid film lubricant additionally requires a process of drying the surface after the application and a process of cleaning the surface after the press forming. As a result, production costs increase. In view of such a situation, a titanium product that has good lubricity and is less prone to adhesion has been developed as in Patent Document 1. The titanium product disclosed in Patent Document 1 forms a TiO₂ film that has good lubricity.

LIST OF PRIOR ART DOCUMENTS

PATENT DOCUMENT

Patent Document 1: JP2020-183551A

SUMMARY OF INVENTION

TECHNICAL PROBLEM

Meanwhile, when a TiO₂ film that is effective for improving lubricity is formed as in the titanium product disclosed in Patent Document 1, the titanium product may become colored due to the interference effect of light caused by the film. This may not be desirable in terms of appearance because of the loss of a silvery-white metallic color inherent in the titanium product. Accordingly, it is challenging to obtain a titanium product that is capable of maintaining an inherent metallic color while increasing lubricity.
Keeping the above in mind, an objective of the present disclosure is to provide a titanium product that is capable of maintaining an inherent metallic color while increasing lubricity.

SOLUTION TO PROBLEM

The gist of the present disclosure, which has been made to solve the above problem, is a titanium product and a production method of the same as described below.

(1) A titanium product, wherein
- a Vickers hardness HVs of surface measured at a load of 25 gf satisfies a following formula (i),
- a relationship between the Vickers hardness HVs and a glossiness Gs measured at an angle of incidence of 20° satisfies a following formula (ii), and
- a color difference ΔE*ab between a surface and a resultant surface after removal of 10 to 20 µm from the surface by pickling with fluonitric acid satisfies a following formula (iii): $HVs \geq 200$ $HVs \geq 250 - 0.25 Gs$ $ΔE * ab < 3$
(2) A production method of the titanium product according to the above (1), including performing the following steps in order:
- a step of performing cold rolling in a plurality of passes by using a cold rolling oil that contains C;
- a step of performing vacuum annealing or bright annealing; and
- a step of performing skin passing by using a roll polished by an abrasive or polishing paper of P220 to 800, wherein
- an average rolling reduction in the cold rolling is more than 10%, and
- the skin passing is performed such that a relationship between an amount of reduction S (mm) per one pass and a roll diameter D (mm) satisfies a following formula (iv): $0.1 \times 10^{- 5} < S / D < 5.0 \times 10^{- 5}$
(3) The production method of the titanium product according to the above (2), wherein
- an average rolling reduction in the cold rolling is in a range of 12 to 20%, and
- the skin passing is performed in two or more passes by using a roll polished by an abrasive or polishing paper of P280 to 800 such that a relationship between an amount of reduction S (mm) per one pass and a roll diameter D (mm) satisfies a following formula (v): $0.5 \times 10^{- 5} \leq S / D \leq 3.0 \times 10^{- 5}$

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present disclosure, it is possible to obtain a titanium product that is capable of maintaining an inherent metallic color while increasing lubricity.

BRIEF DESCRIPTION OF DRAWINGS

[Figure 1] Figure 1 is a diagram illustrating the relationship between the concentration of C and the depth from the surface for a titanium product of the embodiment.
[Figure 2] Figure 2 is a diagram illustrating the relationship between the surface hardness and the glossiness for the titanium product.

DESCRIPTION OF EMBODIMENTS

The inventors have earnestly studied to improve lubricity while maintaining a metallic color inherent in a titanium product and obtained findings as described below.

(a) Forming a film made of TiO₂ on the surface of the titanium product is an effective way of improving the lubricity. On the other hand, it is necessary to form a relatively thick film to improve the lubricity. This makes it unable to maintain the metallic color inherent in the titanium product. The reason is that the formed thick film takes on a different color due to the interference effect of light.
(b) Accordingly, the inventors have studied to improve the lubricity with a thin film. When exposed to the air atmosphere, the titanium product is subjected to natural oxidation, and an extremely thin passivation film of the thickness as thin as 10 nm is formed thereon. Such a passivation film is made of TiO₂ that is highly lubricous and is formed without any special treatment. Accordingly, it is desirable to utilize the passivation film to increase lubricity. However, even when such a passivation film is formed, it is generally difficult to improve the lubricity. The reason for this is due to a mechanism described below.

In the press forming, the die and the titanium product slide on each other while fine projections are in contact with each other. Then, in the vicinity of surfaces that are sliding (hereinafter also simply referred to as "sliding surface"), plastic deformation occurs in the titanium product, which is relatively softer than the die. Since it is extremely thin and is thus inferior in terms of ductility, the passivation film cannot follow such plastic deformation. As a result, the passivation film is locally fractured, leading to adhesion and degradation of lubricity.
(c) Accordingly, it is possible to improve the lubricity if fractures in the passivation film can be suppressed. To this end, it is preferable to suppress plastic deformation from occurring in the sliding surface. To suppress the plastic deformation, it is effective to harden the outer layer and remove fine projections at which the deformation concentrates. To harden the outer layer, it is preferable to allow C to dissolve into the surface. Furthermore, to remove fine projections, it is preferable to perform skin passing under predetermined conditions to crush projections on the surface. In this way, local fractures in the passivation film can be suppressed to improve the lubricity.
An embodiment of the present disclosure has been made based on the findings described above. The requirements for the titanium product of the embodiment will now be described in detail.

1. Constitution of Titanium Product

1-1. Vickers Hardness of Surface

The titanium product of the embodiment utilizes a passivation film formed on the surface of the titanium product through natural oxidation to maintain a metallic color inherent in titanium while increasing lubricity. The passivation film is made of TiO₂ that is effective for improving the lubricity. On the other hand, since the passivation film is extremely thin as being on the order of about 10 nm, the passivation film is fractured during press forming and generally produces a smaller effect of improving the lubricity. Accordingly, in the titanium product of the embodiment, fractures in the passivation film during press forming are suppressed to improve the lubricity.
To suppress fractures in the passivation film, it is effective to increase the surface hardness. To this end, in the titanium product of the embodiment, the Vickers hardness HVs of surface measured at a load of 25 gf satisfies the following formula (i): $HVs \geq 200$
When the Vickers hardness HVs of surface described above is less than 200, hardening of the outer layer is not sufficient, and thus fractures in the passivation film during press forming cannot be suppressed. As a result, adhesion occurs, and the lubricity degrades. Accordingly, the Vickers hardness HVs of surface is 200 or more. The Vickers hardness HVs of surface is preferably 210 or more and more preferably 225 or more. Note that the upper limit of the Vickers hardness HVs of surface is generally, but not particularly limited to, 350.
The surface hardness of the titanium product of the embodiment is high because cold rolling is performed with a cold rolling oil that contains C, as described later. As a result, C is concentrated on the surface, so that the surface hardness increases. Figure 1 is a diagram illustrating the relationship between the concentration of C and the depth from the surface for the titanium product of the embodiment, which is an analysis result from the glow discharge emission spectrometry (GDS) indicating a change in the concentration of C in the vicinity of the surface. The GDS is an analysis method with which the concentration distribution of a specific element in the vicinity of the surface can be investigated. It can be seen from Figure 1 that the titanium product of the embodiment has the highest concentration of C near the surface and the concentration of C decreases as the depth increases in the depth direction from the surface.
The Vickers hardness of surface may be measured according to the procedure below. In compliance with JIS Z 2244-1: 2020 (Vickers hardness test), measurements of micro-Vickers hardness are taken from the surface at 5 points at a load of 25 gf and at a 1 mm pitch. Of the measured values from the measured 5 points, the average of the measured values from 3 points excluding the maximum and minimum values is taken as the Vickers hardness HVs of surface. Here, a load of 25 gf that is lower than that for a common Vickers test is used so that the hardness of the outer layer (a superficial layer near the surface) can satisfactorily be evaluated.

1-2. Internal Vickers Hardness

In the titanium product of the embodiment, for example, from the viewpoint of formability, the internal Vickers hardness HVb of the titanium product measured at a load of 500 gf is preferably less than 200, more preferably 180 or less, and further preferably 160 or less. Here, the Vickers hardness measured at a load of 500 gf is different from that measured at a load of 25 gf and is an internal Vickers hardness of the titanium product. Preferably, the lower limit of internal Vickers hardness HVb is generally, but not particularly limited to, 125 or more.
The internal Vickers hardness HVb may be measured according to the procedure below. In compliance with JIS Z 2244-1: 2020 (Vickers hardness test), measurements of micro-Vickers hardness are taken from the surface at 5 points at a load of 500 gf and at a 1 mm pitch. Of measured values from the measured 5 points, the average of the measured values from 3 points excluding the maximum and minimum values is taken as the internal Vickers hardness.

1-3. Relationship between Vickers Hardness of Surface and Glossiness

To suppress fractures in the passivation film during press forming, it is effective to control surface roughness while hardening the outer layer.
Fine unevenness can be removed by skin passing and the like, as described later. However, since the fine unevenness under evaluation is on the order of submicron, it is difficult to use general contact roughness test or the like to evaluate the unevenness. Accordingly, the fine unevenness is evaluated by using a glossiness Gs measured at an angle of incidence of 20°. To suppress fractures in the passivation film and improve lubricity, it is necessary for the relationship between the Vickers hardness HVs of surface and the glossiness Gs measured at an angle of incidence of 20° to satisfy the following formula (ii): $HVs \geq 250 - 0.25 Gs$
Here, the formula (ii) is a formula determined experimentally. Figure 2 is a diagram illustrating the relationship between the surface hardness and the glossiness for the titanium product. As illustrated in Figure 2, good lubricity can be obtained when both the formulas (ii) and (i) are satisfied.
When the above formula (ii) is not satisfied, the plastic deformation in the sliding surface increases, so that the passivation film that is inferior in terms of ductility cannot follow the plastic deformation and is fractured, leading to adhesion. As a result, the lubricity degrades. A thick oxide film for suppressing this situation will, if it is formed, make it unable to maintain the inherent metallic color. That is, both improvement of lubricity and a good color can be achieved when the formula (ii) is satisfied.
While the titanium product of the embodiment includes a passivation film as described above, the thickness of the film is considered on the order of about 5 to 20 nm.
Here, the glossiness is defined in JIS Z 8741: 1997 and can be measured by a glossmeter. In the titanium product of the embodiment, measurements are taken at arbitrary two points in parallel to the cold rolling direction at the angle of incidence of 20°, and the average value of the measurements is taken as the glossiness Gs measured at the angle of incidence of 20°.

1-4. Color Difference

The titanium product of the embodiment includes a passivation film on the surface. The passivation film needs to have a thickness to the extent that the metallic color inherent in the titanium product is not affected. To this end, in the titanium product of the embodiment, the color difference ΔE*ab between the surface and the resultant surface after removal of 10 to 20 µm from the surface by pickling with fluonitric acid satisfies the following formula (iii): $ΔE * ab < 3$
Here, the color difference ΔE*ab is an indicator that indicates the difference in color. That is, the larger the color difference ΔE*ab, the larger the difference in color-becomes.
The titanium product of the embodiment includes the passivation film and a titanium substrate, which is a base metal portion covered with the passivation film. A thicker passivation film formed on the substrate surface leads to an increase in the color difference ΔE*ab because the interference effect of light occurs. Then, when the color difference ΔE*ab between the surface and a region of 10 to 20 µm from the surface does not satisfy the formula (iii) and is 3 or more, the metallic color inherent in the titanium product cannot be maintained and the titanium product may become colored. Accordingly, the color difference ΔE*ab is less than 3. Note that the lower limit of the color difference is not particularly limited and is most preferably 0.
The color difference ΔE*ab can be calculated by taking measurements of L*, a*, and b* of the L*a*b* color system, which represents hue, for each of two surfaces for which the color difference is calculated. The measurements of L*, a*, and b* may be taken with a light source C by using a color difference meter. Specifically, the measurements of L*, a*, and b* are taken for each of "the surface" and "the resultant surface after removal of 10 to 20 µm from the surface", and the color difference can be calculated from the formula ΔE*ab = √{(ΔL*)² + (Δa*)² + (Δb*)²}, where ΔL*, Δa*, and Δb* are the respective differences.
For the color difference ΔE*ab described above, first, the measurements of L*, a*, and b* of the surface are taken, followed by pickling with fluonitric acid to dissolve a region from 10 up to 20 µm from the surface toward the substrate side. In the acid dissolving, it is difficult to uniquely determine the region to be removed. Accordingly, it is sufficient to make an adjustment such that the surface is removed from 10 up to 20 µm from the surface of the titanium product toward the substrate side. Thereafter, the measurements of L*, a*, and b* may be taken, and ΔL*, Δa*, and Δb* and then the color difference ΔE*ab may be calculated.

1-5. Type and Shape of Titanium Product

The type of the titanium product of the embodiment is not particularly limited. That is, it may be a titanium product, industrial pure titanium, and a titanium alloy. Note that the industrial pure titanium is defined in JIS, ASTM, and the like, and the content of Ti is generally 99 mass% or more.
General industrial pure titanium is exemplified as JIS Grades 1 to 4 or ASTM/ASME Grades 1 to 4. Typical elements as impurity elements of the industrial pure titanium are C, H, O, N, and Fe. In the industrial pure titanium as described above, the contents of the above-described elements are C: 0.08 mass% or less, H: 0.015 mass% or less, O: 0.40 mass% or less, N: 0.05 mass% or less, and Fe: 0.50 mass% or less.
The titanium alloy is an alloy that generally contains Ti 70 mass% or more. The titanium alloy includes α titanium alloys, α+β titanium alloys, or β titanium alloys. The α titanium alloys include, for example, a highly corrosion-resistant alloy (a titanium alloy defined in JIS standards, Grades 11 to 13, Grade 17, and Grades 19 to 22, and in ASTM standards, Grades 7, 11, 13, 14, 17, 30, and 31, as well as a titanium alloy additionally containing a minute amount of various elements), Ti-0.5Cu, Ti-1.0Cu, Ti-1.0Cu-0.5Nb, Ti-1.0Cu-1.0Sn-0.3Si-0.25Nb, and the like. Note that, for example, Ti-0.5Cu refers to a titanium alloy containing 0.5 mass% Cu, and Ti-1.0Cu-0.5Nb refers to a titanium alloy containing 1.0 mass% Cu and 0.5 mass% Nb. As described above, the terminologies of titanium alloy generally have "Ti-" followed by a contained element and the content of the element.
The α+β titanium alloys include, for example, Ti-3Al-2.5V, Ti-5Al-1Fe, Ti-6Al-4V, and the like. The β titanium alloys include, for example, Ti-11.5Mo-6Zr-4.5Sn, Ti-8V-3Al-6Cr-4Mo-4Zr, Ti-13V-11Cr-3Al, Ti-15V-3Al-3Cr-3Sn, Ti-20V-4Al-1Sn, Ti-22V-4Al, and the like.
The shape of the titanium product is not particularly limited. It may be a sheet material, a bar material, or may have any other shape.

2. Average Coefficient of Friction

The titanium product of the embodiment can improve the lubricity when the constituent requirements described above are satisfied, and the average coefficient of friction measured in the surface is 0.20 or less than 0.15.
Note that measurements of the average coefficient of friction were taken by performing a pin-on-disc friction and wear test. In the present application, a pin-on-disc friction and wear tester is used to rub the surface of the titanium product with a pin for performing the friction and wear test. The test is performed by using Castoal (S-803T) for the lubricant under the conditions of: surface pressure 1 MPa, speed 0.1 m/min, and sliding distance 10 mm. The pin used in the test is made of a high carbon chromium bearing steel material SUJ2 defined in the Japanese Industrial Standards JIS G4805: 2019 and has a smooth surface of φ 3.5 mm and Ra 0.12.

3. Production Method

The titanium product of the embodiment can be stably produced by, for example, a production method described below.

3-1. Hot Rolling and more

First, a material for hot rolling is prepared. The material may be industrial pure titanium or a titanium alloy, and the type thereof is not particularly limited. The material for hot rolling may be produced according to a usual method. For example, an ingot of a titanium material may be produced by arc melting and the like, and the ingot may be subjected to hot forging to obtain a material for hot rolling.
The material for hot rolling is subjected to hot rolling to produce a hot rolled material. The conditions for hot rolling are not particularly limited as well. Adjustments may be made as appropriate for desired characteristics. The resultant hot rolled material may be subjected to heat treatment as appropriate.

3-2. Cold Rolling

Thereafter, the hot rolled material is subjected to cold rolling. A cold rolling oil that contains C is used during the cold rolling. To harden the outer layer, for example, a mineral oil is used as the cold rolling oil for the titanium product of the embodiment. Using a mineral oil causes a mechanochemical reaction to occur during the cold rolling. As a result, a C-concentrated layer, which is formed on the outer layer, is carburized during annealing so that the outer layer can be hardened. Note that any cold rolling oil available on the market may be used.
The cold rolling is generally performed by a sendzimir mill. In the sendzimir mill, the rolling takes place by passing the titanium product between a pair of work rolls back and forth multiple times. Here, passing the titanium product between work rolls of a rolling mill is referred to as a "pass". Accordingly, in the cold rolling, the titanium product is generally controlled to be a target thickness in a plurality of passes. That is, the cold rolling is a process through a plurality of passes.
For the production of the titanium product of the embodiment, the average rolling reduction per pass excluding the last two passes in the cold rolling is more than 10%. When the average rolling reduction is 10% or less, the Vickers hardness of the outer layer falls less than 200, leading to the degradation of lubricity. Accordingly, the average rolling reduction is more than 10%, preferably 12% or more, and more preferably 16% or more.
Generally, in applications in which the appearance is important, such as with no surface defect, it is a common practice to perform rolling with an increased number of passes at a low average rolling reduction of 10% or less. However, in the present application, the range of the average rolling reduction is as described above from the viewpoint of promoting carburizing described later and hardening the outer layer.
On the other hand, when the average rolling reduction exceeds 20%, surface defects are likely to increase as a result of an excessive introduction of distortion, which makes it difficult to maintain the appearance quality of the titanium product. Accordingly, the average rolling reduction is preferably 25% or less and more preferably 20% or less. Note that the last two passes are excluded in the average rolling reduction because those two passes are performed for improving the dimensional accuracy of thickness.
The rolling reduction described above can be calculated by the following formula (a): $Rolling reduction (%) = (h 1 - h 2) / h 1 \times 100$ where, each symbol in the above formula (a) is defined as follows:

h1 (mm) : Thickness of titanium product before passes
h2 (mm) : Thickness of titanium product after passes

3-3. Annealing

Subsequently, vacuum annealing or bright annealing is performed to obtain an annealed material. The vacuum annealing refers to annealing taking place with the furnace being kept under vacuum. Furthermore, the bright annealing refers to annealing taking place in a non-oxidizing atmosphere. The type of annealing may be either continuous annealing or batch annealing. The surface oxidation can be suppressed during the vacuum annealing and the bright annealing, which makes it possible to preserve a good silvery-white metallic color. The annealing is performed to promote recrystallization, or otherwise for the purpose of carburizing the outer layer. Specifically, C contained in the mineral oil that is concentrated on the outer layer through the mechanochemical reaction is diffused during the annealing, leading to carburizing. In this way, the outer layer can be hardened.
The conditions for the vacuum annealing and the bright annealing are not particularly limited and may be according to a usual method. For example, typically, the annealing temperature is within the range of 580 to 850°C, and the annealing time is within the range of 0.5 minutes to 24 hours. Note that in the case of the continuous annealing, which takes place at a higher temperature for shorter time, the preferable annealing is at temperature of within the range of 750 to 850°C for 5 minutes or less. The degree of vacuum for the vacuum annealing may also be according to a usual method, and for example, is preferably 1 Pa or less, and in the case of the continuous annealing instead of the batch annealing, it is preferably less than 0.01 Pa.
The annealing atmosphere during the bright annealing may be an inert atmosphere and generally, processing may preferably take place in Ar of the purity being 4N or more (99.99% or more). In the case of the continuous annealing, the purity of Ar is preferably 5N or more and, for example, preferably, the oxygen concentration is 1 vol.ppm or less and the dew point is -50°C or less. Note that when the annealing is performed in the air atmosphere, a thicker oxide film is formed so that the silvery-white metallic color of the titanium product cannot be maintained.

3-4. Skin Passing (Temper Rolling)

Subsequently, the annealed material is subjected to skin passing. The skin passing refers to processing a material such as through light rolling with a low rolling reduction or drawing. The skin passing is generally performed for the purpose of surface polishing and distortion correction, and in the present application, it is performed to crush fine projections. As in the cold rolling, the skin passing is also a process through a plurality of passes. In the skin passing, the number of passes is preferably 2 or more, more preferably 3 or more, and further preferably 4 or more.
Note that the total rolling reduction of all passes, namely, the total rolling reduction of skin passing is preferably in a range of 0.5 to 5.0%. The reason is that when the total rolling reduction of skin passing is less than 0.5%, it is difficult to sufficiently crush projections, while formability degrades due to an excessive introduction of distortion when exceeding 5.0%.
The skin passing is performed by using a roll polished by an abrasive or polishing paper of P220 to 800. When a roll polished by an abrasive paper that is rougher than P220 is used or brush polishing is performed, the lubricity of surface is likely to degrade. Accordingly, the grit size of abrasive paper for polishing the roll is P220 or more. The grit size of the abrasive paper described above is preferably P280 or more.
While the grit size of the polishing paper may be more than P800, the surface unevenness of a roll polished by a polishing paper of P800 is sufficiently small relative to the surface unevenness of titanium. Accordingly, using a polishing paper of more than P800 no longer produces additional effects of improving the lubricity. In addition, a larger grit size of the polishing paper leads to an increase in polishing costs. For these reasons, the grit size of the polishing paper for polishing the roll is P800 or less. Note that the grit size of the polishing paper is as defined in JIS R 6010: 2000.
In the production method of a titanium product of the embodiment, the skin passing is performed such that the relationship between the amount of reduction S per one pass and a roll diameter D satisfies the following formula (iv). When the conditions for skin passing do not satisfy the following formula (iv), the formula (ii) is less likely to be satisfied, leading to the degradation of lubricity. $0.1 \times 10^{- 5} < S / D < 5.0 \times 10^{- 5}$
Note that S/D of the above (iv) is preferably 0.5 × 10^-5 or more. The reason is that the average coefficient of friction falls less than 0.15 and further, improvement of lubricity is facilitated. On the other hand, S/D is preferably 3.0 × 10^-5 or less. In this case, the reason is also that it is facilitated to make the average coefficient of friction less than 0.15. That is, to keep the average coefficient of friction less than 0.15, it is preferable to satisfy the following formula (v): $0.5 \times 10^{- 5} \leq S / D \leq 3.0 \times 10^{- 5}$
Here, S and D in the above formula are defined as follows:

S (mm) : Amount of reduction per one pass
D (mm) : Roll diameter

The amount of reduction refers to the amount of reduced thickness (mm) of the titanium product. As described above, in the present application, since a plurality of passes takes place, the amount of reduction per one pass equals to a value obtained by dividing the total amount of reduced thickness by the total number of passes during skin passing. If it is difficult to measure the total amount of reduced thickness (mm) because the total rolling reduction in skin passing is as small as 2% or less, the total amount of reduced thickness (mm) may be calculated from changes in the length in the rolling direction, assuming that the material has a constant volume and elongation in the width direction is zero.
Hereunder, the titanium product according to the present disclosure will specifically be described with reference to Examples, while the embodiments are not limited to the examples.

Example 1

Titanium materials that each have a sheet thickness of 4 mm and have chemical compositions shown in Table 1 were prepared. The titanium materials were produced through hot rolling and the like. For simplicity, the industrial pure titanium is simply indicated as pure titanium in Table 1.

[Table 1]

Table 1

Material	Chemical composition (mass%, balance: Ti and impurities)
Material	O	N	C	Fe	Cu	Sn	Si	Nb	Al	V	Cr
JIS Grade 1 pure titanium	0.044	0.002	0.003	0.033	-	-	-	-	-	-	-
JIS Grade 2 pure titanium	0.104	0.002	0.002	0.045	-	-	-	-	-	-	-
10Cu	0.45	0.001	0.002	0.025	1.00	-	-	-	-	-	-
10CSSN	0.047	0.002	0.002	0.03	1.10	1.05	0.35	0.70	-	-	-

The titanium materials were subjected to cold rolling under conditions shown in Table 2, followed by batch annealing and skin passing. The cold rolling was performed by using a cold rolling oil that contained C through a plurality of passes. Comparative Example 13 indicates that an atmospheric oxidation was performed after vacuum annealing under conditions in Table 2. Comparative Example 14 indicates that atmospheric annealing was performed in place of the vacuum annealing under conditions in Table 2, and other examples indicate that only the vacuum annealing was performed.
The entry of "mirror polish" in Table 2 indicates that the surface of the titanium was polished with colloidal silica. Furthermore, "brush polish" in Table 2 indicates that a cold-rolled titanium thin sheet was subjected to annealing at 650°C for 5 hours in the air atmosphere, and thereafter, subjected to 60s salt treatment (dip) with a salt containing 80% NaOH which was raised to a temperature of 520°C, and to polishing with a polishing brush made of nylon while 60°C warm water was being sprayed.
For the titanium products as resultant sheet materials (hereinafter simply indicated as "titanium sheet"), measurements of the Vickers hardness HVs of the outer layer, the internal Vickers hardness HVb, the glossiness Gs, the color difference ΔE*ab, and the average coefficient of friction were taken or calculated according to the procedure below.

(Vickers Hardness HVs of Surface)

In compliance with JIS Z 2244-1: 2020 (Vickers hardness test), measurements of micro-Vickers hardness were taken at 5 points from the surface of the test specimen at a load of 25 gf and at a 1 mm pitch. Of the measured 5 points, the average value of 3 points excluding the maximum and minimum was taken as the Vickers hardness HVs of the outer layer.

(Internal Vickers Hardness HVb)

In compliance with JIS Z 2244-1: 2020 (Vickers hardness test), measurements of micro-Vickers hardness were taken at 5 points from the surface at a load of 500 gf and at a 1 mm pitch. Of measured values from the measured 5 points, the average of the measured values from 3 points excluding the maximum and minimum values was taken as the internal Vickers hardness HVb.

(Glossiness Gs)

Measurements were taken at arbitrary two points in parallel to the cold rolling direction at the angle of incidence of 20°, and the average value of the measurements was taken as the glossiness Gs measured at the angle of incidence of 20°. As the glossmeter, the gloss meter GM-1 from Suga Test Instruments Co., Ltd. was used.

(Color Difference ΔE*ab)

First, measurements of L*, a*, and b* of the surface were taken, followed by pickling with fluonitric acid to dissolve a region from 10 up to 20 µm from the surface toward the substrate side. After the acid dissolving, measurements of L*, a*, and b* were taken again. From the measured values, ΔL*, Δa*, and Δb* were calculated. For the color difference ΔE*ab, measurements were taken with a light source C by using the Konica Minolta CR-400 Chroma Meter.
A pin-on-disc friction and wear test was performed to measure the average coefficient of friction. A pin-on-disc friction and wear tester was used to rub the surface of the titanium product with a pin for performing the friction and wear test. The press oil "Castoal (S-803T)" from Taiyu Co., Ltd. diluted 4-fold with water was used as the lubricant, and the test was conducted under the conditions of: surface pressure 1 MPa, speed 0.1 m/min, and sliding distance 10 mm. As the pin used in the test, one made of a high carbon chromium bearing steel material SUJ2 defined in Japanese Industrial Standards G4805: 2019 and having a smooth surface of φ 3.5 mm and Ra 0.12 was used. The results are collectively shown in Table 3 below.

[Table 3]

Table 3

Example	Physical properties and characteristics
Example	HVs I	HVb	Gs	250-0.25Gs	ΔE*ab	µ
Inventive Example 1	200	120	200	200	0	0.18
Inventive Example 2	200	120	250	188	0	0.19
Inventive Example 3	205	125	300	175	0	0.17
Inventive Example 4	200	120	350	163	0	0.15
Inventive Example 5	210	125	200	200	0	0.13
Inventive Example 6	215	120	250	188	0	0.14
Inventive Example 7	210	120	300	175	0	0.19
Inventive Example 8	210	120	530	118	0	0.11
Inventive Example 9	215	125	200	200	0	0.18
Inventive Example 10	230	125	250	188	0	0.12
Inventive Example 11	225	120	300	175	0	0.11
Inventive Example 12	225	125	330	168	0	0.10
Inventive Example 13	250	125	35	241	0	0.19
Inventive Example 14	245	130	50	238	0	0.17
Inventive Example 15	250	135	80	230	0	0.12
Inventive Example 16	240	140	120	220	0	0.10
Inventive Example 17	250	130	110	223	0	0.11
Inventive Example 18	250	135	100	225	0	0.10
Inventive Example 19	250	130	90	228	0	0.10
Inventive Example 20	260	135	35	241	1	0.19
Inventive Example 21	260	130	50	238	1	0.11
Inventive Example 22	265	125	80	230	1	0.11
Inventive Example 23	255	130	120	220	1	0.09
Inventive Example 24	270	130	35	241	2	0.19
Inventive Example 25	270	125	50	238	2	0.11
Inventive Example 26	275	130	80	230	2	0.12
Inventive Example 27	265	130	120	220	2	0.09
Inventive Example 28	270	135	160	210	2	0.09
Inventive Example 29	330	130	30	243	2	0.17
Inventive Example 30	320	140	50	238	2	0.11
Inventive Example 31	340	135	80	230	2	0.10
Inventive Example 32	300	135	120	220	2	0.09
Comparative Example 1	130	110	240	190	0	0.31
Comparative Example 2	140	115	300	125	0	0.22
Comparative Example 3	170	115	300	175	0	0.20
Comparative Example 4	170	120	220	195	0	0.35
Comparative Example 5	145	115	350	163	0	0.22
Comparative Example 6	165	115	350	163	0	0.23
Comparative Example 7	155	120	530	118	0	0.20
Comparative Example 8	185	110	530	118	0	0.21
Comparative Example 9	130	115	1200	-50	0	0.58
Comparative Example 10	200	120	100	225	0	0.52
Comparative Example 11	230	125	35	241	1	0.51
Comparative Example 12	200	125	170	208	0	0.49
Comparative Example 13	165	115	240	190	12	0.10
Comparative Example 14	185	120	30	243	2	0.53

Underline indicates that the values fell out of the requirements of this embodiment or its targeted property.

Inventive Examples No. 1 to 32, which satisfied preferable production conditions and also satisfied the requirements of the embodiment, had good lubricity and also maintained the metallic color of the titanium product. On the other hand, Comparative Examples No. 1 to 14, which failed to satisfy the production conditions of the embodiment and failed to satisfy the requirements of the embodiment, resulted in at least one of degradation of lubricity and inability to maintain the metallic color.
Among Inventive Examples, No. 5, 6, 8, 10 to 12, 15 to 19, 21 to 23, 25 to 28, and 30 to 32 satisfied a more preferable range of production conditions, and therefore, the average coefficient of friction was less than 0.15. No. 1, 2, 3, 4, 7, 9, 13, 14, 20, 24, and 29 failed to satisfy the more preferable range of production conditions, and therefore, the average coefficient of friction, which was still less than 0.20, was higher than No. 5 and the like.
In Comparative Examples No. 1 to 8, since the average rolling reduction during the cold rolling was low, the Vickers hardness HVs of surface was less than 200 and the average coefficient of friction was 0.20 or more. Note that Comparative Examples No. 1 to 5 failed to satisfy the formula (ii) accordingly. In Comparative Example No. 9, since the C-concentrated layer was removed due to surface grinding, the Vickers hardness HVs of surface was less than 200, and the average coefficient of friction was 0.20 or more.
Comparative Examples No. 10 to 12, which failed to satisfy the formula (iv), failed to satisfy the formula (ii) and the average coefficient of friction was 0.20 or more. Comparative Example No. 13, which was subjected to the atmospheric oxidation after vacuum annealing, failed to satisfy the formula (iii) and failed to maintain the metallic color. Comparative Example No. 14 was subjected to the atmospheric annealing but not subjected to skin passing, and therefore, the average coefficient of friction was 0.20 or more.

Example 2

Using titanium materials from JIS Grade 1 industrial pure titanium shown in Table 1 of Example 1, cold rolling was performed under conditions shown in Table 4, followed by continuous annealing in an Ar atmosphere. At this time, Ar gas, the purity of which was 5N or more, was used, the oxygen concentration was 1 vol.ppm or less, the dew point was -50°C or less, and the heating rate up to the annealing temperature was 30°C/s. Other entries in the table are the same as those in Example 1.

[Table 4]

Table 4

Example	Material	Surface hardening			Surface roughness control
		Cold rolling	Annealing	Atmospheric oxidation (°C, min)	Skin passing
		Average rolling reduction (%)	Air, Vacuum annealing (°C, min)	Atmospheric oxidation (°C, min)	Polishing of roll	Roll diameter (mm)	Total rolling reduction in skin passing (%)	S/D	Number of passes	Sheet thickness (mm)
Inventive Example 33	JIS Grade 1 pure titanium	16	750, 1	-	P800	500	1.0	1.0×10^-5	2	1
Inventive Example 34	JIS Grade 1 pure titanium	16	775, 1	-	P800	500	1.0	1.0×10^-5	2	1
Inventive Example 35	JIS Grade 1 pure titanium	16	800, 1	-	P800	500	1.0	1.0×10^-5	2	1
Inventive Example 36	JIS Grade 1 pure titanium	16	825, 1	-	P800	500	1.0	1.0×10^-5	2	1
Inventive Example 37	JIS Grade 1 pure titanium	16	850, 1	-	P800	500	1.0	1.0×10^-5	2	1

For resultant titanium sheets, as in Example 1, measuremants of the Vickers hardness HVs of the outer layer, the internal Vickers hardness HVb, the glossiness Gs, the color difference ΔE*ab, and the average coefficient of friction were taken. The results are collectively shown in Table 5 below.

[Table 5]

Table 5

Example	Physical properties and characteristics
Example	HVs	HVb	Gs	250-0.25Gs	ΔE*ab	µ
Inventive Example 33	236	132	112	222	0	0.12
Inventive Example 34	227	132	103	224	0	0.09
Inventive Example 35	252	126	104	224	0	0.11
Inventive Example 36	268	132	102	225	0	0.10
Inventive Example 37	246	128	88	228	0	0.13

Inventive Examples No. 33 to 37, which satisfied the requirements of the embodiment, satisfied preferable production conditions and also satisfied the requirements of the embodiment, and therefore, had good lubricity and maintained the metallic color of the titanium product.

Claims

A titanium product, wherein
a Vickers hardness HVs of surface measured at a load of 25 gf satisfies a following formula (i),

a relationship between the Vickers hardness HVs and a glossiness Gs measured at an angle of incidence of 20° satisfies a following formula (ii), and

a color difference ΔE*ab between a surface and a resultant surface after removal of 10 to 20 µm from the surface by pickling with fluonitric acid satisfies a following formula (iii): $HVs \geq 200$ $HVs \geq 250 - 0.25 Gs$ $ΔE * ab < 3$
A production method of the titanium product according to claim 1, comprising performing the following steps in order:
a step of performing cold rolling in a plurality of passes by using a cold rolling oil that contains C;

a step of performing vacuum annealing or bright annealing; and

a step of performing skin passing by using a roll polished by an abrasive or polishing paper of P220 to 800, wherein

an average rolling reduction in the cold rolling is more than 10%, and

the skin passing is performed such that a relationship between an amount of reduction S (mm) per one pass and a roll diameter D (mm) satisfies a following formula (iv): $0.1 \times 10^{- 5} < S / D < 5.0 \times 10^{- 5}$
The production method of the titanium product according to claim 2, wherein
an average rolling reduction in the cold rolling is in a range of 12 to 20%, and

the skin passing is performed in two or more passes by using a roll polished by an abrasive or polishing paper of P280 to 800 such that a relationship between an amount of reduction S (mm) per one pass and a roll diameter D (mm) satisfies a following formula (v): $0.5 \times 10^{- 5} \leq S / D \leq 3.0 \times 10^{- 5}$