Titanium material and method for producing same

Abstract

A titanium material in which the Vickers hardness HVs of the surface as measured at a load of 25 gf satisfies formula (i), the relationship between the Vickers hardness HVs and the glossiness Gs as measured at an incidence angle of 20 DEG satisfies formula (ii), and the color difference [Delta] E * ab between the surface and the surface obtained by removing the surface by 10-20 [mu] m by pickling with fluorine nitric acid satisfies formula (iii): HVs > = 200 (i) HVs > = 250-0.25 Gs (ii) [Delta] E * ablt; 3 (iii).

Description

Technical Field

[0001] The present disclosure relates to titanium materials and manufacturing methods thereof. Background Art

[0002] Stamping is a method of forming by pressing a blank against a die and applying pressure, and is common as a forming method for metal blanks. In the case of performing stamping, adhesion generated between the blank and the die becomes a problem. If adhesion occurs, lubrication deteriorates and formability decreases. In addition, defects are generated on the surface of the blank, or the life of the die becomes short. Therefore, improvement of adhesion is required.

[0003] Industrial pure titanium or titanium alloy (hereinafter, also simply referred to as "titanium material") is a blank that is prone to adhesion, that is, has low lubricity. Therefore, when stamping a titanium material, a chemical agent called a film-type solid lubricant is usually applied to the surface. Thereby, lubricity is improved and adhesion is suppressed. However, if a film-type solid lubricant is used, processes for drying the surface after coating and cleaning the surface after stamping are additionally generated. As a result, the manufacturing cost increases. For this reason, titanium materials with good lubricity and low adhesion as disclosed in Patent Document 1 have been developed. The titanium material disclosed in Patent Document 1 has a film of TiO2 with good lubricity formed thereon.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2020-183551 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] On the other hand, if a film of TiO2 effective for improving lubricity is formed as in the titanium material disclosed in Patent Document 1, the titanium material has a color due to the interference effect of light caused by the film. Moreover, since the silver-white metallic color originally possessed by the titanium material is lost, it is sometimes not preferable in terms of appearance. Therefore, there is a problem that it is difficult to obtain a titanium material that can improve lubricity and maintain its original metallic color.

[0009] In view of the above circumstances, an object of the present disclosure is to provide a titanium material that can improve lubricity and maintain its original metallic color.

[0010] Means for Solving the Problems

[0011] The present disclosure has been made to solve the above problems, and its gist lies in the following titanium material and manufacturing method.

[0012] (1) A titanium material, wherein the Vickers hardness HVs of the surface measured under a load of 25 gf satisfies the following formula (i),

[0013] The relationship between the aforementioned Vickers hardness HVs and the glossiness Gs measured at an incident angle of 20° satisfies the following formula (ii).

[0014] The color difference ΔE*ab between the surface and the surface after removing 10 to 20 μm of the aforementioned surface by pickling with fluonitric acid satisfies the following formula (iii).

[0015] HVs≥200 (i)

[0016] HVs≥250 - 0.25Gs (ii)

[0017] ΔE*ab < 3 (iii)

[0018] (2) A method for manufacturing the titanium material described in the above (1), wherein the following processes are carried out in sequence:

[0019] A process of cold rolling through multiple passes using cold rolling oil containing C;

[0020] A process of vacuum annealing or bright annealing; and

[0021] A process of skin rolling using a roll polished with abrasive paper of P220 - 800,

[0022] The average reduction ratio in the aforementioned cold rolling exceeds 10%,

[0023] The aforementioned skin rolling is carried out in such a manner that the relationship between the reduction amount S (mm) per pass and the roll diameter D (mm) satisfies the following formula (iv).

[0024] 0.1×10 -5 < S / D < 5.0×10 -5 (iv)

[0025] (3) According to the method for manufacturing a titanium material described in the above (2), wherein the average reduction ratio in the aforementioned cold rolling is in the range of 12 - 20%,

[0026] The aforementioned skin rolling uses a roll polished with abrasive paper of P280 - 800 and is carried out more than 2 passes in such a manner that the relationship between the reduction amount S (mm) per pass and the roll diameter D (mm) satisfies the following formula (v).

[0027] 0.5×10 -5 ≤ S / D ≤ 3.0×10 -5 (v)

[0028] Effects of the Invention

[0029] According to the present disclosure, a titanium material capable of improving lubricity and maintaining its original metallic color can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a graph showing the relationship between the C concentration of the titanium material of the present embodiment and the depth from the surface.

[0031] Figure 2 It is a graph showing the relationship between the surface hardness and the glossiness of the titanium material. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The present inventors conducted in-depth research in order to maintain the original metallic color of the titanium material and improve the lubricity, and obtained the following insights.

[0033] (a) Forming a film made of TiO2 on the surface of the titanium material is an effective means for improving the lubricity. On the other hand, in order to improve the lubricity, it is necessary to form a relatively thick film. Therefore, it is impossible to maintain the original metallic color of the titanium material. This is because, if a thick film is formed, different colors will appear due to the interference of light.

[0034] (b) Therefore, the present inventors studied improving the lubricity using a thin film. When the titanium material is exposed to the atmosphere, it naturally oxidizes to form a very thin passive film with a thickness of about 10 nm. Such a passive film is made of TiO2 with high lubricity and is formed without special treatment. Therefore, it is preferable to effectively utilize the passive film to improve the lubricity. However, generally, even if such a passive film is formed, it is difficult to improve the lubricity. The reason is based on the following mechanism.

[0035] In stamping forming, the die and the titanium material slide in a state where minute convex portions are in contact with each other. Near the surface where sliding occurs (hereinafter, also simply referred to as the "sliding surface"), the softer titanium material compared to the die undergoes plastic deformation. Since the passive film is very thin, it lacks ductility and cannot follow such plastic deformation. As a result, the passive film is locally damaged, adhesion occurs, and the lubricity is reduced.

[0036] (c) Therefore, if the destruction of the passive film can be suppressed, the lubricity can be improved. Therefore, it is preferable to suppress the plastic deformation occurring on the sliding surface. In suppressing plastic deformation, hardening of the surface layer and removal of minute convex portions where deformation concentrates are effective. In order to harden the surface layer, it is preferable to carburize the surface with C. In addition, in order to remove minute convex portions, it is preferable to perform skin rolling under specified conditions to crush the convex portions on the surface. By such a method, local destruction of the passive film can be suppressed and the lubricity can be improved.

[0037] One embodiment of the present disclosure was completed based on the above insights. Hereinafter, each condition of the titanium material of the present embodiment will be described in detail.

[0038] 1. Composition of the Titanium Material

[0039] 1-1. Vickers hardness of the surface

[0040] For the titanium material of the present embodiment, in order to improve lubricity and maintain the original metallic color of titanium, the passive film formed by natural oxidation on the surface of the titanium material is effectively utilized. This passive film is made of TiO2 which is effective in improving lubricity. On the other hand, the passive film is about 10 nm thick and very thin. Therefore, the passive film is damaged during stamping, and usually the effect of improving lubricity is small. Therefore, the titanium material of the present embodiment suppresses the damage of the passive film during stamping and improves lubricity.

[0041] In order to suppress the damage of the passive film, it is effective to increase the surface hardness. Therefore, the Vickers hardness HVs of the surface of the titanium material of the present embodiment measured under a load of 25 gf satisfies the following formula (i).

[0042] HVs≥200 (i)

[0043] If the above-mentioned Vickers hardness HVs of the surface is less than 200, the hardening of the surface layer is insufficient, and the damage of the passive film during stamping cannot be suppressed. As a result, adhesion occurs and lubricity decreases. Therefore, the Vickers hardness HVs of the surface is 200 or more. The Vickers hardness HVs of the surface is preferably 210 or more, more preferably 225 or more. It should be noted that the upper limit of the Vickers hardness HVs of the surface is not particularly limited and is usually 350.

[0044] The surface hardness of the titanium material of the present embodiment is high because, as described later, cold rolling is performed using a cold rolling oil containing C. As a result, C is enriched on the surface and the surface hardness increases. Figure 1 is a graph showing the relationship between the C concentration of the titanium material of the present embodiment and the depth from the surface, and is an analysis result obtained by GDS (glow discharge optical emission spectrometry) showing the change in the C concentration near the surface. GDS is an analytical method capable of investigating the concentration distribution of specific elements near the surface. From Figure 1 it can be seen that the C concentration of the titanium material of the present embodiment is the highest near the surface, and the C concentration decreases as the depth increases from the surface in the depth direction.

[0045] The Vickers hardness of the surface can be measured according to the following steps. According to JIS Z 2244-1:2020 (Vickers hardness test), the micro-Vickers hardness is measured at 5 points from the surface at a load of 25 gf and a spacing of 1 mm. The average of the measured values of 3 points out of the 5 measured values excluding the maximum value and the minimum value is taken as the Vickers hardness HVs of the surface. Here, the load is set to 25 gf, which is lower than the usual Vickers hardness test, in order to fully evaluate the hardness of the surface layer (shallow layer close to the surface).

[0046] 1-2. Vickers hardness of the interior

[0047] In the titanium material of the present embodiment, for example, from the viewpoint of formability, the Vickers hardness HVb inside the titanium material measured under 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 under a load of 500 gf is different from that measured under a load of 25 gf and is the Vickers hardness inside the titanium material. It should be noted that the lower limit of the internal Vickers hardness HVb is not particularly limited and is usually preferably 125 or more.

[0048] The internal Vickers hardness HVb can be measured according to the following steps. According to JIS Z 2244-1:2020 (Vickers hardness test), measure the micro-Vickers hardness at 5 points from the surface at a load of 500 gf and a spacing of 1 mm. Take the average of the measured values of 3 points out of the 5 measured values excluding the maximum and minimum values as the internal Vickers hardness.

[0049] 1-3. Relationship between Vickers hardness and glossiness of the surface

[0050] In order to suppress the breakage of the passive film during stamping forming, it is effective to harden the surface layer and remove fine protrusions.

[0051] The fine irregularities can be removed by skin rolling described later. However, since the fine irregularities to be evaluated are in the submicron range, it is difficult to evaluate such irregularities by a normal contact roughness test or the like. Therefore, the glossiness Gs measured at an incident angle of 20° is used for the evaluation of the fine irregularities. In order to suppress the breakage of the passive film and improve the lubricity, the relationship between the Vickers hardness HVs of the surface and the glossiness Gs measured at an incident angle of 20° needs to satisfy the following formula (ii).

[0052] HVs ≥ 250 - 0.25Gs (ii)

[0053] Here, formula (ii) is an experimentally obtained formula. Figure 2 It is a graph showing the relationship between the surface hardness and glossiness of the titanium material. As Figure 2 shown, good lubricity is obtained when formula (ii) and formula (i) are satisfied.

[0054] When the above formula (ii) is not satisfied, the plastic deformation of the sliding surface becomes large, and the passive film lacking ductility cannot follow the plastic deformation and is broken, resulting in adhesion. As a result, the lubricity is reduced. If a thick oxide film is formed to suppress this problem, the original metallic color cannot be maintained. That is, when formula (ii) is satisfied, both the improvement of lubricity and good color can be achieved.

[0055] It should be noted that, as described above, the titanium material of this embodiment has a passive film, but it is considered that the thickness of the film is about 5 to 20 nm.

[0056] Here, the glossiness can be measured with a glossmeter in accordance with JIS Z 8741:1997. In the titanium material of this embodiment, two arbitrary points are measured at an incident angle of 20° parallel to the cold rolling direction, and the average value thereof is used as the glossiness Gs measured at an incident angle of 20°.

[0057] 1-4. Color difference

[0058] The titanium material of this embodiment has a passive film on its surface. The thickness of this passive film needs to be set within a range that does not affect the original metallic color of the titanium material. Therefore, the color difference ΔE*ab between the surface of the titanium material of this embodiment and the surface after removing 10 to 20 μm of the surface by pickling with fluonitric acid satisfies the following formula (iii).

[0059] ΔE*ab < 3 (iii)

[0060] Here, the color difference ΔE*ab is an index indicating the color difference. That is, the larger the color difference ΔE*ab, the greater the color difference.

[0061] The titanium material of this embodiment includes a passive film and a titanium substrate as the substrate portion covered by the passive film. The thicker the passive film formed on the surface of the substrate, the more the interference effect of light is generated, and the larger the color difference ΔE*ab. When the color difference ΔE*ab between the surface and the range 10 to 20 μm from the surface does not satisfy formula (iii) and is 3 or more, the original metallic color of the titanium material cannot be maintained, and the titanium material has a color. Therefore, the above color difference ΔE*ab is less than 3. It should be noted that the lower limit of the above color difference is not particularly limited, and it is most preferably 0.

[0062] The color difference ΔE*ab can be calculated by measuring L*, a*, and b* in the L*a*b* color system representing the hue on two surfaces for calculating the color difference. L*, a*, and b* can be measured using a color difference meter with a light source C. Specifically, in the "surface" and the "surface after removing 10 to 20 μm of the surface", L*, a*, and b* are measured respectively. When the differences are set as ΔL*, Δa*, and Δb*, ΔE*ab = √{(ΔL*) 2 +(Δa*) 2 +(Δb*) 2} can be calculated by this formula.

[0063] In addition, for the above color difference ΔE*ab, after first measuring L*, a*, and b* of the surface, pickling is performed using hydrofluoric acid, and flame cleaning is carried out on the range from the surface to the substrate side of 10 to 20 μm. In the flame cleaning, it is difficult to uniquely determine the removed range. Therefore, it is only necessary to adjust in such a way that the surface is removed in the range from the surface of the titanium material to the substrate side of 10 to 20 μm. Then, measure L*, a*, and b*, calculate ΔL*, Δa*, and Δb*, and calculate the color difference ΔE*ab.

[0064] 1-5. Types and Shapes of Titanium Materials

[0065] The type of the titanium material of this embodiment is not particularly limited. That is, it may be any titanium material, commercially pure titanium, or titanium alloy. It should be noted that commercially pure titanium is defined by JIS, ASTM, etc., and usually has a Ti content of 99% by mass or more.

[0066] As general commercially pure titanium, JIS grades 1 to 4, or ASTM / ASME Grade 1 to 4 can be exemplified. As impurity elements of commercially pure titanium, typical elements are C, H, O, N, and Fe. In the above commercially pure titanium, the contents of the above elements are C: 0.08% by mass or less, H: 0.015% by mass or less, O: 0.40% by mass or less, N: 0.05% by mass or less, and Fe: 0.50% by mass or less.

[0067] In addition, a titanium alloy is usually an alloy containing 70% by mass or more of Ti. As titanium alloys, α-type titanium alloys, α+β-type titanium alloys, or β-type titanium alloys can be listed. As α-type titanium alloys, for example, there are high corrosion resistance alloys (titanium alloys specified by JIS standards of grades 11 to 13, 17, 19 to 22, and ASTM standards of Grade 7, 11, 13, 14, 17, 30, 31, and titanium alloys containing various elements in a small amount), Ti-0.5Cu, Ti-1.0Cu, Ti-1.0Cu-0.5Nb, Ti-1.0Cu-1.0Sn-0.3Si-0.25Nb, etc. It should be noted that, for example, Ti-0.5Cu means a titanium alloy containing 0.5% by mass of Cu, and Ti-1.0Cu-0.5Nb means a titanium alloy containing 1.0% by mass of Cu and 0.5% by mass of Nb. In this way, in the name of a titanium alloy, the contained elements and their contents are usually recorded after Ti-.

[0068] As α+β type titanium alloys, there are, for example, Ti-3Al-2.5V, Ti-5Al-1Fe, Ti-6Al-4V, etc. As β type titanium alloys, there are, 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, etc.

[0069] The shape of the titanium material is not particularly limited either. It can be a plate, a bar, or other shapes.

[0070] 2. Average coefficient of friction

[0071] Under the condition that the titanium material of this embodiment satisfies the above composition conditions, the lubricity can be improved, and the average coefficient of friction measured on the surface is less than 0.20 or less than 0.15.

[0072] It should be noted that the average coefficient of friction is measured by conducting a pin-on-disk friction and wear test. In this application, a pin-on-disk friction and wear testing machine is used to slide the pin on the surface of the titanium material to conduct the friction and wear test. The lubricant used is Castoal (S-803T), and the test is conducted under the conditions of a surface pressure of 1 MPa, a speed of 0.1 m / minute, and a sliding distance of 10 mm. In addition, the pin used in the test is made of high-carbon chromium bearing steel material SUJ2 specified in Japanese Industrial Standard JIS G4805:2019, and has a smooth surface with φ3.5 mm and Ra0.12.

[0073] 3. Manufacturing method

[0074] The titanium material of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0075] 3-1. Hot rolling, etc.

[0076] First, prepare a billet for hot rolling. This billet can be industrial pure titanium and titanium alloys, and its type is not particularly limited. It should be noted that the billet for hot rolling can be manufactured by a conventional method. For example, an ingot of titanium billet can be manufactured by arc melting, etc., and then hot forging can be carried out to make the billet for hot rolling.

[0077] Hot roll the above billet for hot rolling to manufacture a hot-rolled material. The conditions during hot rolling are not particularly limited either. It can be appropriately adjusted according to the desired properties. The obtained hot-rolled material can be appropriately heat-treated.

[0078] 3-2. Cold rolling

[0079] Then, the above hot-rolled material is cold-rolled. During cold rolling, a cold-rolling oil containing C is used. In the titanium material of this embodiment, in order to harden the surface layer, for example, mineral oil is used as the cold-rolling oil. By using mineral oil, a mechanical chemical reaction occurs during cold rolling. As a result, by forming a C-enriched layer on the surface layer and carburizing during annealing, the surface layer can be hardened. It should be noted that commercially available products can be used as the cold-rolling oil.

[0080] In addition, cold rolling is usually carried out using a Sendzimir mill. In the Sendzimir mill, the titanium material is passed through a pair of working rolls multiple times in a reciprocating manner for rolling. Here, passing the titanium material through the working rolls of the rolling mill is called a pass. Therefore, during cold rolling, it is usually controlled to the target thickness through multiple passes. That is, cold rolling is a process that goes through multiple passes.

[0081] Moreover, in the manufacture of the titanium material of this embodiment, during cold rolling, the average reduction ratio in each pass except for the last two passes exceeds 10%. If the above average reduction ratio is 10% or less, the Vickers hardness of the surface layer is less than 200 and the lubricity decreases. Therefore, the above average reduction ratio exceeds 10%, preferably 12% or more, and more preferably 16% or more.

[0082] Generally, in applications that emphasize the appearance such as surface defects, the number of passes is usually increased and rolling is carried out at a low average reduction ratio of 10% or less. However, in this application, from the viewpoint of promoting the carburization described later and hardening the surface layer, the average reduction ratio is set within the above range.

[0083] On the other hand, if the above average reduction ratio exceeds 20%, excessive strain is introduced, and as a result, the surface defects are likely to increase and it is difficult to maintain the appearance quality of the titanium material. Therefore, the above average reduction ratio is preferably 25% or less, and more preferably 20% or less. It should be noted that the average reduction ratio does not include the last two passes because these two passes are carried out to improve the dimensional accuracy of the thickness.

[0084] It should be noted that the above reduction ratio can be calculated by the following formula (a).

[0085] Reduction ratio (%) = (h1 - h2) / h1 × 100 (a)

[0086] Among them, each symbol in the above formula (a) is defined as follows.

[0087] h1 (mm): The thickness of the titanium material before the pass

[0088] h2 (mm): The thickness of the titanium material after the pass

[0089] 3-3. Annealing

[0090] Next, vacuum annealing or bright annealing is carried out to obtain an annealed material. Vacuum annealing means annealing is carried out with the furnace interior in a vacuum state. In addition, bright annealing means annealing is carried out in a non-oxidizing atmosphere. The annealing method can be continuous annealing or batch annealing. In vacuum annealing and bright annealing, since surface oxidation is suppressed, a good silver-white metallic color can be formed. Annealing is carried out to promote recrystallization, and in addition, it is also carried out for the purpose of carburizing the surface layer. Specifically, C contained in the mineral oil enriched in the surface layer by mechanochemical reaction diffuses during annealing to cause carburization. Thus, the surface layer can be hardened.

[0091] The conditions for vacuum annealing and bright annealing are not particularly limited and can be according to conventional methods. For example, usually the annealing temperature is set in the range of 580 - 850 °C, and the annealing time is in the range of 0.5 minutes - 24 h. It should be noted that in the case of continuous annealing carried out at a higher temperature and for a shorter time, it is preferred to carry out the annealing at a temperature in the range of 750 - 850 °C for 5 minutes or less. The degree of vacuum in vacuum annealing can also be according to conventional methods. For example, it is preferably 1 Pa or less, and in the case of continuous annealing rather than batch annealing, it is preferably less than 0.01 Pa.

[0092] The annealing atmosphere in bright annealing only needs to be an inert atmosphere. Usually, it is preferably carried out in Ar with a purity of 4N or more (99.99% or more). In the case of continuous annealing, the purity of Ar is preferably 5N or more. For example, it is preferably such that the oxygen concentration is 1 vol.ppm or less and the dew point is -50 °C or less. It should be noted that when annealing is carried out in the atmosphere, the oxide film formed is relatively thick, so that the silver-white metallic color of the titanium material cannot be maintained.

[0093] 3 - 4. Skin rolling (conditioning rolling)

[0094] Next, the above-mentioned annealed material is subjected to skin rolling. Skin rolling means carrying out processing such as rolling or drawing with a low and light reduction ratio. Skin rolling is generally carried out for the purpose of surface polishing and strain correction. In this application, it is also carried out to crush fine protrusions. Skin rolling, like cold rolling, is a process through multiple passes. In skin rolling, the number of passes is preferably 2 or more, more preferably 3 or more, and further preferably 4 or more.

[0095] It should be noted that the total reduction ratio of all passes, that is, the total reduction ratio of skin rolling, is preferably in the range of 0.5 - 5.0%. This is because if the total reduction ratio of skin rolling is less than 0.5%, it is difficult to sufficiently crush the protrusions. On the other hand, if it exceeds 5.0%, excessive strain is generated and the formability is reduced.

[0096] For skin rolling, a roll polished with abrasive paper of P220 to 800 is used for skin rolling. When using a roll polished with abrasive paper coarser than P220 or performing brush polishing, the lubricity of the surface is likely to decrease. Therefore, the number of the abrasive paper for the polishing roll is P220 or more. More preferably, the number of the abrasive paper is P280 or more.

[0097] The number of the abrasive paper can exceed P800, and the surface unevenness of the roll polished with P800 abrasive paper is sufficiently smaller than the surface unevenness of titanium. Therefore, even if abrasive paper exceeding P800 is used, no further improvement effect of lubricity can be obtained. In addition, the larger the number of the abrasive paper, the higher the polishing cost. For these reasons, the number of the abrasive paper for the polishing roll is P800 or less. It should be noted that the number of the abrasive paper is as defined in JIS R 6010:2000.

[0098] Moreover, in the method for manufacturing a titanium material of the present embodiment, skin rolling is performed in such a manner that the relationship between the reduction S per pass and the roll diameter D satisfies the following formula (iv). When the conditions of skin rolling do not satisfy the following formula (iv), it is difficult to satisfy formula (ii), and the lubricity decreases.

[0099] 0.1×10 -5 <S / D<5.0×10 -5 (iv)

[0100] It should be noted that S / D in the above formula (iv) is preferably 0.5×10 -5 or more. This is because the average friction coefficient is made less than 0.15, and thus the lubricity is more easily improved. On the other hand, S / D is preferably 3.0×10 -5 or less. This is because in this case, it is also easy to make the average friction coefficient less than 0.15. That is, in order to make the average friction coefficient less than 0.15, it is preferable to satisfy the following formula (v).

[0101] 0.5×10 -5 ≤S / D≤3.0×10 -5 (v)

[0102] Here, S and D in the above formula are defined as follows.

[0103] S (mm): reduction per pass

[0104] D (mm): roll diameter

[0105] The reduction amount refers to the reduction amount (mm) of the thickness of the titanium material. As described above, in this application, since multiple passes are performed, the reduction amount per pass is the value obtained by dividing the total thickness reduction amount by the total number of passes in skin pass rolling. It should be noted that in the case where the total reduction rate in skin pass rolling is as small as 2% or less and it is difficult to measure the total thickness reduction amount (mm), the blank is considered to have a constant volume, the elongation rate in the width direction is assumed to be 0, and the total thickness reduction amount (mm) can be calculated from the length change in the rolling direction.

[0106] Hereinafter, the titanium material of the present disclosure will be described more specifically by way of examples, but the present embodiment is not limited to these examples.

[0107] Example 1

[0108] A titanium blank with a thickness of 4 mm having the chemical composition shown in Table 1 was prepared. This titanium blank was manufactured through hot rolling and the like. It should be noted that in Table 1, for simplicity, industrial pure titanium is simply recorded as pure titanium.

[0109] [Table 1]

[0110] Table 1

[0111]

[0112] After cold rolling this titanium blank under the conditions shown in Table 2, intermittent annealing and skin pass rolling were performed. It should be noted that cold rolling was performed using cold rolling oil containing C and was carried out in multiple passes. In addition, Comparative Example 13 shows that under the conditions of Table 2, atmospheric oxidation was performed after vacuum annealing. In Comparative Example 14, it shows that atmospheric annealing was performed under the conditions of Table 2 instead of vacuum annealing, and other examples show only vacuum annealing.

[0113] In addition, the description of mirror polishing in Table 2 means polishing the surface of titanium with colloidal silica. In addition, the brush polishing in Table 2 means that for the cold-rolled titanium thin sheet, after annealing at 650 °C for 5 h in the atmosphere, a salt with 80% NaOH was set at a temperature of 520 °C, and salt treatment (immersion) was performed for 60 s, and a grinding brush made of nylon was used to polish while spraying warm water at 60 °C.

[0114] [Table 2]

[0115] Table 2

[0116]

[0117] Underline: Indicates not meeting the preferred manufacturing conditions. Indicates not meeting the more preferred manufacturing conditions.

[0118] For the titanium material of the obtained sheet (hereinafter, only referred to as "titanium plate"), measure or calculate the Vickers hardness HVs of the surface layer, the Vickers hardness HVb inside, the glossiness Gs, the color difference ΔE*ab, and the average coefficient of friction according to the following steps.

[0119] (Vickers hardness HVs of the surface)

[0120] According to JIS Z 2244-1:2020 (Vickers hardness test), measure the micro-Vickers hardness at 5 points from the surface of the test piece with a load of 25 gf and a spacing of 1 mm. Take the average of the 3 points other than the maximum and minimum among the 5 measured points as the Vickers hardness HVs of the surface layer.

[0121] (Vickers hardness HVb inside)

[0122] According to JIS Z 2244-1:2020 (Vickers hardness test), measure the micro-Vickers hardness at 5 points from the surface with a load of 500 gf and a spacing of 1 mm. Take the average of the measured values of the 3 points other than the maximum and minimum among the measured values of the 5 measured points as the Vickers hardness HVb inside.

[0123] (Glossiness Gs)

[0124] Perform arbitrary 2-point measurements at an incident angle of 20° parallel to the cold rolling direction, and take the average as the glossiness Gs measured at an incident angle of 20°. Use a gloss meter GM-1 manufactured by Suga Test Instruments Co., Ltd.

[0125] (Color difference ΔE*ab)

[0126] First, measure L*, a*, and b* on the surface, then perform pickling with fluonitric acid, dissolve a range of 10 - 20 μm from the surface toward the substrate side, and perform flame cleaning. After flame cleaning, measure L*, a*, and b* again. Calculate ΔL*, Δa*, and Δb* from the measured values. It should be noted that the color difference ΔE*ab is measured using a KONICA MINOLTA color difference meter CR-400 with light source C.

[0127] The pin-on-disk friction and wear test was carried out to measure the average friction coefficient. Using a pin-on-disk friction and wear testing machine, the pin was made to slide on the surface of the titanium material to conduct the friction and wear test. As the lubricant, a lubricant obtained by diluting the stamping oil "Castoal (S-803T)" manufactured by TAIYUCO., LTD. with water by 4 times was used, and the test was carried out under the conditions of a surface pressure of 1 MPa, a speed of 0.1 m / minute, and a sliding distance of 10 mm. In addition, the pin used in the test was made of high-carbon chromium bearing steel material SUJ2 specified in Japanese Industrial Standard G4805:2019, and had a smooth surface with φ3.5 mm and Ra0.12. Hereinafter, the results are summarized in Table 3.

[0128] [Table 3]

[0129] Table 3

[0130]

[0131] Underline: Indicates a condition that does not satisfy the conditions of this embodiment or a target characteristic value.

[0132] Inventive Examples No. 1 to 32 satisfy the preferred manufacturing conditions and also satisfy the conditions of this embodiment. Therefore, they have good lubricity and the metallic color of the titanium material is also maintained. On the other hand, Comparative Examples No. 1 to 14 do not satisfy the manufacturing conditions of this embodiment and do not satisfy the conditions of this embodiment. Therefore, at least one of the lubricity reduction and the inability to maintain the metallic color has occurred.

[0133] In the inventive examples, the manufacturing conditions of No. 5, 6, 8, 10 to 12, 15 to 19, 21 to 23, 25 to 28, 30 to 32 satisfy the more preferred range. Therefore, the average friction coefficient is less than 0.15. The manufacturing conditions of No. 1, 2, 3, 4, 7, 9, 13, 14, 20, 24, 29 do not satisfy the more preferred range. Therefore, although the average friction coefficient is less than 0.20, it is higher than that of No. 5 etc.

[0134] In Comparative Examples No. 1 to 8, since the average reduction ratio in cold rolling is low, the Vickers hardness HVs on the surface is less than 200, and the average friction coefficient is 0.20 or more. It should be noted that, accordingly, Comparative Examples No. 1 to 5 also do not satisfy formula (ii). In Comparative Example No. 9, the C-rich layer was removed by surface grinding, the Vickers hardness HVs on the surface was less than 200, and the average friction coefficient was 0.20 or more.

[0135] Comparative Examples Nos. 10 to 12 do not satisfy formula (iv), and thus do not satisfy formula (ii), and the average coefficient of friction is 0.20 or more. In Comparative Example No. 13, since atmospheric oxidation was performed after vacuum annealing, formula (iii) is not satisfied, and the metallic color cannot be maintained. In Comparative Example No. 14, atmospheric annealing was performed and skin rolling was not carried out, and thus the average coefficient of friction is 0.20 or more.

[0136] Example 2

[0137] Using the titanium billet of JIS 1 type industrial pure titanium described in Table 1 of Example 1, after cold rolling under the conditions described in Table 4, continuous annealing was performed in an Ar atmosphere. At this time, Ar gas with a purity of 5N or more was used, the oxygen concentration was set to 1 vol.ppm or less, the dew point was set to -50°C or less, and the heating rate until the annealing temperature was set to 30°C / s. It should be noted that the descriptions in the table other than this are the same as those in Example 1.

[0138] [Table 4]

[0139] Table 4

[0140]

[0141] For the obtained titanium plate, in the same manner as in Example 1, the Vickers hardness HVs of the surface layer, the Vickers hardness HVb inside, the glossiness Gs, the color difference ΔE*ab, and the average coefficient of friction were measured. Hereinafter, the results are summarized in Table 5.

[0142] [Table 5]

[0143] Table 5

[0144]

[0145] Inventive Examples Nos. 33 to 37 that satisfy the conditions of the present embodiment satisfy the preferred manufacturing conditions and also satisfy the conditions of the present embodiment, and thus have good lubricity and the metallic color of the titanium material is also maintained.

Claims

1. A titanium material, the Vickers hardness HVs of its surface measured under a load of 25 gf satisfies the following formula (i), the relationship between the Vickers hardness HVs and the glossiness Gs measured at an incident angle of 20° satisfies the following formula (ii), the color difference ΔE*ab between the surface and the surface after removing 10 - 20 μm of the said surface by pickling with fluonitric acid satisfies the following formula (iii), HVs≥200 (i) HVs≥250 - 0.25Gs (ii) ΔE*ab<3 (iii).

2. The manufacturing method of the titanium material according to claim 1, wherein, The following processes are carried out in sequence: A process of cold rolling through multiple passes using a cold rolling oil containing C; A process of vacuum annealing or bright annealing; and A process of skin pass rolling using a roll ground with abrasive paper of P220 - 800, the average reduction ratio in the said cold rolling exceeds 10%, the skin pass rolling is carried out in such a way that the relationship between the reduction amount S per pass and the roll diameter D satisfies the following formula (iv), where the units of the reduction amount S and the roll diameter D are mm, 0.1×10 -5 <S / D < 5.0×10 -5 (iv).

3. The manufacturing method of the titanium material according to claim 2, wherein, the average reduction ratio in the said cold rolling is in the range of 12 - 20%, the skin pass rolling uses a roll ground with abrasive paper of P280 - 800 and is carried out more than 2 passes in such a way that the relationship between the reduction amount S per pass and the roll diameter D satisfies the following formula (v), where the units of the reduction amount S and the roll diameter D are mm, 0.5×10 -5 ≤S / D≤3.0×10 -5 (v).

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