Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/008—Using a protective surface layer
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/02—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working in inert or controlled atmosphere or vacuum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
  • C22F1/18—High-melting or refractory metals or alloys based thereon
  • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B2001/221—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length by cold-rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
  • B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
  • B21B2001/228—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length skin pass rolling or temper rolling

Definitions

• the present disclosure relates to a titanium product and a production method of the same.
• 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.
• 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.
• titanium product 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 TiO 2 film that has good lubricity.
• Patent Document 1 JP2020-183551A
• the titanium product when a TiO 2 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.
• an objective of the present disclosure is to provide a titanium product that is capable of maintaining an inherent metallic color while increasing lubricity.
• 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.
• the inventors have earnestly studied to improve lubricity while maintaining a metallic color inherent in a titanium product and obtained findings as described below.
• 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.
• 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 TiO 2 that is effective for improving the lubricity.
• 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.
• the Vickers hardness HVs of surface measured at a load of 25 gf satisfies the following formula (i): HVs ⁇ 200
• 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.
• FIG. 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.
• GDS glow discharge emission spectrometry
• 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.
• 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.
• 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.
• 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.
• 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.
• Fine unevenness can be removed by skin passing and the like, as described later.
• 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.
• the fine unevenness is evaluated by using a glossiness Gs measured at an angle of incidence of 20°.
• 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 ⁇ 250 ⁇ 0.25 Gs
• 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.
• the thickness of the film is considered on the order of about 5 to 20 nm.
• the glossiness is defined in JIS Z 8741: 1997 and can be measured by a glossmeter.
• 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°.
• 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.
• the color difference ⁇ E*ab between the surface and the resultant surface after removal of 10 to 20 ⁇ m from the surface by pickling with fluonitric acid satisfies the following formula (iii): ⁇ E * ab ⁇ 3
• the color difference ⁇ E*ab is an indicator that indicates the difference in color. That is, the larger the color difference ⁇ E*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 ⁇ E*ab because the interference effect of light occurs.
• the color difference ⁇ E*ab between the surface and a region of 10 to 20 ⁇ m 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 ⁇ E*ab is less than 3.
• the lower limit of the color difference is not particularly limited and is most preferably 0.
• the color difference ⁇ E*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.
• 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 ⁇ m from the surface toward the substrate side.
• fluonitric acid 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 ⁇ m from the surface of the titanium product toward the substrate side.
• the measurements of L*, a*, and b* may be taken, and ⁇ L*, ⁇ a*, and ⁇ b* and then the color difference ⁇ E*ab may be calculated.
• 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.
• 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.
• 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 ⁇ titanium alloys, ⁇ + ⁇ titanium alloys, or ⁇ titanium alloys.
• the ⁇ 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.
• Ti-0.5Cu refers to a titanium alloy containing 0.5 mass% Cu
• Ti-1.0Cu-0.5Nb refers to a titanium alloy containing 1.0 mass% Cu and 0.5 mass% Nb.
• the terminologies of titanium alloy generally have "Ti-" followed by a contained element and the content of the element.
• the ⁇ + ⁇ titanium alloys include, for example, Ti-3Al-2.5V, Ti-5Al-1Fe, Ti-6Al-4V, and the like.
• the ⁇ 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.
• 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.
• measurements of the average coefficient of friction were taken by performing a pin-on-disc friction and wear test.
• 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 ⁇ 3.5 mm and Ra 0.12.
• the titanium product of the embodiment can be stably produced by, for example, a production method described below.
• 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.
• the hot rolled material is subjected to cold rolling.
• a cold rolling oil that contains C is used during the cold rolling.
• 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.
• a C-concentrated layer, which is formed on the outer layer is carburized during annealing so that the outer layer can be hardened.
• any cold rolling oil available on the market may be used.
• the cold rolling is generally performed by a sendzimir mill.
• the rolling takes place by passing the titanium product between a pair of work rolls back and forth multiple times.
• passing the titanium product between work rolls of a rolling mill is referred to as a "pass".
• 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.
• the average rolling reduction per pass excluding the last two passes in the cold rolling is more than 10%.
• 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.
• the range of the average rolling reduction is as described above from the viewpoint of promoting carburizing described later and hardening the outer layer.
• 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.
• Rolling reduction % h 1 ⁇ h 2 / h 1 ⁇ 100 where, each symbol in the above formula (a) is defined as follows:
• the vacuum annealing refers to annealing taking place with the furnace being kept under vacuum.
• 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.
• the annealing temperature is within the range of 580 to 850°C
• the annealing time is within the range of 0.5 minutes to 24 hours.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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).
• the formula (ii) is less likely to be satisfied, leading to the degradation of lubricity.
• 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.
• S/D is preferably 3.0 ⁇ 10 -5 or less.
• 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 ⁇ 10 ⁇ 5 ⁇ S / D ⁇ 3.0 ⁇ 10 ⁇ 5
• the amount of reduction refers to the amount of reduced thickness (mm) of the titanium product.
• 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.
• 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.
• the industrial pure titanium is simply indicated as pure titanium in Table 1.
• 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.
• mirror polish indicates that the surface of the titanium was polished with colloidal silica.
• brush polish 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.
• titanium sheet 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 ⁇ E*ab, and the average coefficient of friction were taken or calculated according to the procedure below.
• 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°.
• the glossmeter the gloss meter GM-1 from Suga Test Instruments Co., Ltd. was used.
• 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 ⁇ 3.5 mm and Ra 0.12 was used. The results are collectively shown in Table 3 below.
• Comparative 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.
• 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.
• 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 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 ⁇ E*ab, and the average coefficient of friction were taken. The results are collectively shown in Table 5 below.

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