Titanium material, processed product, and product

Abstract

Provided is a titanium material, wherein: in the L*a*b* color system, a color difference ΔE*ab between a color (L*, a*, b*) of the surface of the titanium material and a reference color (L0 *, a0 *, b0 *) represented by [(L0 *, a0 *, b0 *)=(68.905, 0.935, 2.295)] is 6.14 or less; and when a measurement by X-ray photoelectron spectroscopy is performed in the depth direction from the surface and the position at which the nitrogen concentration is highest is defined as a peak depth, the nitrogen concentration Np (at%), the titanium concentration Tp (at%), and the carbon concentration Cp (at%) at the peak depth satisfy [0.48≤Np / (Tp-Cp)≤1.09].

Description

Titanium materials, processed products and products

[0001] The present invention relates to titanium materials and processed products thereof, as well as products containing them.

[0002] Press forming is a common method for forming metal materials by pressing a material against a die and applying pressure. When press forming is performed, adhesion between the material and the die can become a problem. When adhesion occurs, lubrication deteriorates and formability decreases. It can also cause scratches on the surface of the material and shorten the life of the die. For this reason, there is a need to improve adhesion.

[0003] Commercially pure titanium and titanium alloys (hereinafter simply referred to as "titanium material") are materials that are prone to adhesion, meaning they have poor lubricity. For this reason, when titanium material is press-formed, a chemical called a film-type solid lubricant is usually applied to the surface. This improves lubricity and suppresses adhesion. However, using a film-type solid lubricant requires additional steps to dry the surface after application and to clean the surface after press-forming. This results in increased manufacturing costs. Furthermore, lubricant residue tends to accumulate on the mold, reducing workability.

[0004] In light of this, titanium materials have been developed that have good lubricity and do not require the use of a film-type solid lubricant. ２ Patent Document 2 discloses a titanium material whose lubricity is improved by forming an oxide film or nitride film on the surface.

[0005] JP 2020-183551 A JP 2005-298930 A

[0006] On the other hand, titanium materials such as those disclosed in Patent Document 1 are effective in improving lubricity. ２When a coating such as this is formed, the titanium material takes on a color due to the interference of light caused by the coating. This results in the loss of the titanium material's inherent silver-white metallic color, which can be undesirable in terms of appearance. Furthermore, forming a coating to the extent that the metallic color is lost can raise concerns about changes in properties such as thermal conductivity, or can make visual inspection after molding more difficult. While the titanium material disclosed in Patent Document 2 considers whether or not to color it, there is still room for improvement in terms of maintaining the metallic color.

[0007] In light of the above, an object of the present invention is to provide a titanium material, processed product, and finished product that have enhanced lubricity while retaining their original metallic color.

[0008] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is the following titanium material, processed product, and product.

[0009] (1) Titanium material, L ＊ a ＊ b ＊ In the color system, the color of the surface of the titanium material (L ＊ , a ＊ , b ＊ ) and a reference color (L ０ ＊ , a ０ ＊ , b ０ ＊ ) and color difference ΔE ＊ A titanium material in which ab is 6.14 or less, and when measurement is performed from the surface in a depth direction by X-ray photoelectron spectroscopy and the position at which the nitrogen concentration is maximum is defined as the peak depth, the nitrogen concentration Np, titanium concentration Tp, and carbon concentration Cp at the peak depth satisfy the following formula (ii): (L ０ ＊ , a ０ ＊ , b ０ ＊ )=(68.905, 0.935, 2.295) (i) 0.48≦Np / (Tp−Cp)≦1.09 (ii) where Np, Tp, and Cp in the above formula (ii) are each expressed in atomic percent.

[0010] (2) The titanium material according to (1) above, which further satisfies the following formula (iii): 0.54≦Np / (Tp−Cp) (iii)

[0011] (3) The titanium material according to (1) or (2) above, which further satisfies the following formula (iv): Np / (Tp-Cp)≦0.95 (iv)

[0012] (4) A processed product of titanium material, L ＊ a ＊ b ＊ In the color system, the color (L ＊ , a ＊ , b ＊ ) and a reference color (L ０ ＊ , a ０ ＊ , b ０ ＊ ) and color difference ΔE ＊ ab is 6.14 or less, and when measurement is performed from the surface in the depth direction by X-ray photoelectron spectroscopy and the position where the nitrogen concentration is maximum is taken as the peak depth, the nitrogen concentration Np, titanium concentration Tp, and carbon concentration Cp at the peak depth satisfy the following formula (ii). (L ０ ＊ , a ０ ＊ , b ０ ＊ )=(68.905, 0.935, 2.295) (i) 0.48≦Np / (Tp−Cp)≦1.09 (ii) where Np, Tp, and Cp in the above formula (ii) are each expressed in atomic percent.

[0013] (5) A product comprising the titanium material according to any one of (1) to (3) above and / or the processed product according to (4) above.

[0014] According to the present invention, it is possible to obtain titanium materials, processed products, and finished products that have enhanced lubricity while retaining their original metallic color.

[0015] FIG. 1 is a graph showing the concentration distribution in the depth direction of an example of a titanium material according to this embodiment.

[0016] The present inventors have conducted extensive research to improve lubricity while maintaining the original metallic color of titanium material, and have obtained the following findings.

[0017] (a) Forming a coating on the surface of titanium material is an effective means of improving lubricity. However, to improve lubricity, a relatively thick coating must be formed. In this case, the original metallic color of the titanium material cannot be maintained. This is because a thick coating will exhibit a different color due to light interference.

[0018] (b) Therefore, the present inventors have investigated ways to improve lubricity with a thin film. Titanium materials usually form a very thin passive film in the atmosphere. The passive film is composed of TiO ２ However, it has been thought that it is difficult to improve lubricity because peeling and breakage occur during press molding.

[0019] (c) The destruction and peeling of the passive film is thought to occur through the following mechanism. During press forming, the base titanium material directly below the passive film is subjected to compressive deformation and shear deformation due to frictional forces. Because the passive film is very thin and has low ductility, it cannot follow this deformation, and cracks form in the passive film.

[0020] As the forming process progresses, the die and titanium material slide against each other, forming wear debris containing both the passive film and the titanium base material. This wear debris is transferred to the die, further increasing the frictional force. This accelerates shear deformation of the titanium base material directly below the passive film, increasing the density of cracks in the passive film and causing the film to break down and peel off. This ultimately exposes the titanium base material, which is prone to adhesion.

[0021] (d) Therefore, the inventors investigated ways to suppress shear deformation of the titanium base material in the area where the passive film is in direct contact with the mold by hardening the titanium base material directly below the passive film, thereby suppressing destruction and peeling of the passive film. They found that concentrating N (nitrogen) directly below the passive film is effective, and that performing primary annealing and secondary annealing under the conditions described below is effective for concentrating N.

[0022] An embodiment of the present invention has been made based on the above findings. Each requirement of the titanium material of this embodiment will be described in detail below.

[0023] 1. Structure of titanium material 1-1. Color difference The titanium material of this embodiment has a passive film on the surface. This passive film must be thick enough not to affect the original metallic color of the titanium material. For this reason, the titanium material of this embodiment has a L ＊ a ＊ b ＊ In the color system, the color of the titanium material surface (L ＊ , a ＊ , b ＊ ) and a reference color (L ０ ＊ , a ０ ＊ , b ０ ＊ ) and color difference ΔE ＊ ab is 6.14 or less. (L ０ ＊ , a ０ ＊ , b ０ ＊ ) = (68.905, 0.935, 2.295) ... (i)

[0024] L ＊ a ＊ b ＊ The color system is a color system used to express colors numerically, as defined in JIS Z 8781-4:2013. ＊ , a ＊ , b ＊Each color can be defined based on the numerical values ​​(lightness index and chromaticness index) corresponding to the above. Here, the reference color is the silvery white color of ordinary titanium material, and (L ＊ , a ＊ , b ＊ ) is (L ０ ＊ , a ０ ＊ , b ０ ＊ ), specifically, the three values ​​on the right side of the above formula (i) are (L ＊ , a ＊ , b ＊ ) is the specific color represented.

[0025] In addition, the color difference ΔE ＊ ab is an index showing the difference between two colors. In this application, the color difference ΔE ＊ The larger the ab, the greater the difference in color between the surface color of the titanium material and the reference color.

[0026] The titanium material of this embodiment includes a passivation film and a titanium base material, which is a substrate portion covered with the passivation film. The thicker the passivation film formed on the surface of the base material, the more light interference occurs, and the greater the color difference ΔE ＊ Therefore, in order to maintain the original metallic color of the titanium material, the color difference ΔE ＊ ab is 6.14 or less. ＊ If ab exceeds 6.14, the color of the surface of the titanium material and the reference color cannot be considered to be substantially the same in a visual sensory inspection. ＊ The color difference ΔE is preferably 5.50 or less, and more preferably 5.00 or less. ＊ The smaller ab is, the more preferable it is, so the lower limit is 0.

[0027] The color difference between the surface of the titanium material and the reference color is measured by the following procedure. First, the L ＊ , a ＊ , and b ＊The measurement can be performed using a color difference meter (Konica Minolta CR-400 color difference meter) under light source C. Next, the surface of the titanium material is measured to determine the L obtained. ＊ , a ＊ , and b ＊ and the reference color L ０ ＊ , a ０ ＊ , and b ０ ＊ The difference between ＊ , Δa ＊ , Δb ＊ is calculated and substituted into the following formula (a) to obtain the color difference ΔE ＊ Calculate ab. ΔE ＊ ab = √{(ΔL ＊ ) ２ + (Δa ＊ ) ２ + (Δb ＊ ) ２} ...(a)

[0028] 1-2. XPS Measurement In the present invention, the separation and destruction of the passive film are suppressed by concentrating N (nitrogen) directly under the passive film. As a result, the lubricity is improved. Here, the passive film is usually made of TiO ２ On the other hand, Ti (titanium) usually reacts more easily with O (oxygen) than with N, resulting in dense TiO ２ When a passive film of N is formed on the surface of the titanium substrate, the penetration of N into the titanium substrate is inhibited. For this reason, it is difficult to concentrate N directly below the passive film by conventional annealing or other methods. On the other hand, if the annealing temperature or atmosphere is such that Ti reacts more easily with N than with O, a gold-colored nitride film is formed, and the original metallic color of titanium is lost.

[0029] Therefore, as described below, the production of the titanium material of this embodiment involves two annealing steps: primary annealing and secondary annealing. In the primary annealing, a certain amount of C is dissolved near the surface. Next, in the secondary annealing, the reduction action of the passive film caused by the dissolved C is utilized to concentrate N. If too much C is dissolved, the reduction action of C in the secondary annealing becomes too strong, resulting in excessive N concentration, and the titanium material surface will not achieve the desired color. Therefore, in the primary annealing, it is effective to dissolve a certain amount of C to a degree that does not result in supersaturation of the C concentration. It is also effective to form a certain amount of the compound TiC to prevent the C concentration from becoming supersaturated. In other words, the interaction between N, C, and Ti is utilized to improve lubricity.

[0030] Therefore, when the titanium material of this embodiment is measured from the surface in the depth direction by X-ray photoelectron spectroscopy (XPS), and the position at which the nitrogen concentration is maximum is taken as the peak depth, the nitrogen concentration Np, titanium concentration Tp, and carbon concentration Cp at this peak depth satisfy the following formula (ii): 0.48≦Np / (Tp−Cp)≦1.09 (ii), where Np, Tp, and Cp in the formula (ii) are each expressed in at % (atomic).

[0031] FIG. 1 is a graph showing the concentration distribution in the depth direction of an example of a titanium material according to this embodiment. FIG. 1 shows the concentration distributions of O, Ti, N, and C. As shown in FIG. 1, when XPS measurements were performed from the surface having the passive film toward the titanium base material (hereinafter also referred to as the "depth direction"), the N concentration initially increased, peaked, and then decreased, with the highest concentration occurring at a position slightly deeper from the outermost surface toward the base material. This highest concentration is Np. The position where the nitrogen concentration becomes Np is the peak depth, and the position directly below the passive film is where N is most concentrated. The Ti concentration at this peak depth is Tp, and the C concentration is Cp. By calculating the ratio of the difference between Tp and Cp to Np, the degree to which N is concentrated relative to the Ti of the base material can be determined.

[0032] Therefore, in the above formula (ii), Np / (Tp-Cp) is 0.48 or more. This is because N, which improves lubricity, is sufficiently concentrated just below the passive film, and TiC, which is also effective in improving lubricity, is formed. Np / (Tp-Cp) is preferably 0.54 or more. In other words, it is preferable to satisfy the following formula (iii): 0.54≦Np / (Tp-Cp) (iii)

[0033] On the other hand, in the above formula (ii), Np / (Tp-Cp) is 1.09 or less. If Np / (Tp-Cp) exceeds 1.09 and the concentration of N is promoted too much, golden TiN or brownish nitrogen-based compounds (compounds containing Ti with N, C, O, etc.) are formed, and the color difference ΔE ＊ The reason why Np / (Tp-Cp) is set to 1.09 or less is because the range of ab exceeds 6.14. ＊ This is because ab is within the range of the titanium material of this embodiment. Also, this makes it easier to suppress a decrease in lubricity. Np / (Tp-Cp) is preferably 0.95 or less. In other words, it is preferable to satisfy the following formula (iv): Np / (Tp-Cp)≦0.95 (iv)

[0034] As mentioned above, the relationship between Np, Tp, and Cp is important, so the individual values ​​are not particularly limited. However, from the viewpoint of concentrating N and suppressing destruction and peeling of the passivation film, Np is preferably 10 at% or more, more preferably 12 at% or more, and even more preferably 14 at% or more. On the other hand, excessive concentration of N may result in an undesired color, so Np is preferably 55 at% or less, more preferably 51 at% or less, more preferably 49 at% or less, and even more preferably 35 at% or less. That is, Np is preferably 10 to 55 at%, more preferably 12 to 51 at%, more preferably 14 to 49 at%, and even more preferably 14 to 35 at%.

[0035] Furthermore, although the above-mentioned TiC does not have a reducing effect, it is effective in improving lubricity. The Cp value increases in proportion to the amount of TiC. When Cp is 3 at% or more, sufficient TiC is formed, so Cp is preferably 3 at% or more, more preferably 10 at% or more, more preferably 12 at% or more, and even more preferably 14 at% or more. On the other hand, when Cp is 26 at% or less, excessive concentration of N can be suppressed, so Cp is preferably 26 at% or less, more preferably 24 at% or less, and even more preferably 22 at% or less. That is, Cp is preferably 3 to 26 at%, preferably 10 to 26 at%, more preferably 12 to 24 at%, and even more preferably 14 to 22 at%.

[0036] Considering the balance with the preferred ranges of Np and Cp, Tp is preferably 36 at% or more, more preferably 38 at% or more, and even more preferably 40 at% or more. Also, Tp is preferably 54 at% or less, more preferably 52 at% or less, and even more preferably 50 at% or less. That is, Tp is preferably 36 to 54 at%, more preferably 38 to 52 at%, and even more preferably 40 to 50 at%.

[0037] There is no particular limitation on the peak depth at which the nitrogen concentration is at its maximum. However, the titanium material of this embodiment is manufactured by two-stage annealing, and when manufactured by this method, the peak depth is directly below the passive film, so the depth is 50 nm or less. More preferably, the depth is 30 nm or less. The lower limit of the depth is greater than 0 nm, preferably 1 nm or more.

[0038] In the titanium material of this embodiment, the thickness of the passive film is not particularly limited as long as it satisfies the above-mentioned color difference requirement, but for example, it is preferably 20 nm or less, more preferably 15 nm or less, and even more preferably 10 nm or less. Note that, since the passive film is inevitably formed, the thickness will be greater than 0 nm. Furthermore, in consideration of lubricity, the thickness of the passive film is preferably 3 nm or more, and more preferably 5 nm or more.

[0039] The Np, Tp, and Cp can be measured according to the following procedure. XPS is used for the measurement, and the element concentration distribution in the depth direction is measured by repeating sputtering with Ar ions from the surface and measuring the element concentration in a 1 mm diameter region of a test piece taken from a titanium material. ２ The concentrations of Ti, O, N, C, Fe, Cu, Sn, Si, Nb, Al, and V are measured at 1 nm intervals from a converted depth of 1 to 5 nm, and at 5 nm intervals in the deeper region on the titanium base material side. From the measurement results, the position where the N concentration is greatest is designated as the peak depth, the N concentration at the peak depth as Np, the C concentration at the peak depth as Cp, and the Ti concentration at the peak depth as Tp. Note that if there are two or more peak depths, the depth closest to the surface is designated as the peak depth.

[0040] The thickness of the passive film is determined by measuring the O concentration from the surface to the depth of the base material. Specifically, when the maximum O concentration is 100%, the minimum depth at which the O concentration is 50% or less is defined as the thickness of the passive film.

[0041] 1-3. Type and Shape of Titanium Material The type of titanium material used in this embodiment is not particularly limited. That is, titanium material, commercially pure titanium, and titanium alloys may be used. Note that commercially pure titanium is specified by JIS, ASTM, etc., and typically has a Ti content of 99 mass% or more.

[0042] Examples of common commercially pure titanium include JIS types 1 to 4 and ASTM / ASME Grades 1 to 4. Typical impurity elements in commercially pure titanium are C, H, O, N, and Fe. In the commercially pure titanium described above, the contents of these elements are as follows: 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.

[0043] Titanium alloys are generally alloys containing 70 mass% or more of Ti. Examples of titanium alloys include α-type titanium alloys, α+β-type titanium alloys, and β-type titanium alloys. Examples of α-type titanium alloys include highly corrosion-resistant alloys (titanium alloys specified in JIS standards 11 to 13, 17, and 19 to 22, and ASTM standards Grades 7, 11, 13, 14, 17, 30, and 31, or titanium alloys containing small amounts of various elements), Ti-0.5Cu, Ti-1Cu, Ti-1Cu-0.5Nb, and Ti-1Cu-1Sn-0.35Si-0.25Nb. For example, Ti-0.5Cu refers to a titanium alloy containing approximately 0.5 mass% Cu, and Ti-1Cu-0.5Nb refers to a titanium alloy containing approximately 1.0 mass% Cu and approximately 0.5 mass% Nb. Thus, the names of titanium alloys typically include the Ti- followed by the elements and their amounts (see, for example, https: / / www.nipponsteel.com / product / titan / pdf / index02.pdf).

[0044] Examples of α+β titanium alloys include Ti-3Al-2.5V, Ti-5Al-1Fe, Ti-5Al-2Fe-0.25Si, Ti-6Al-4V, etc. Examples of β titanium alloys include Ti-11.5Mo-6Zr-4.5Sn, Ti-3Al-8V-6Cr-4Mo-4Zr, Ti-13V-11Cr-3Al, Ti-15V-3Al-3Cr-3Sn, Ti-20V-4Al-1Sn, Ti-22V-4Al, etc.

[0045] The shape of the titanium material is not particularly limited. It may be a plate material, a bar material, or any other shape. That is, the titanium material of this embodiment includes a titanium plate, a titanium bar, etc.

[0046] 2. Lubricity When the titanium material of this embodiment satisfies the above-mentioned requirements, it can improve lubricity, and the average coefficient of friction measured on the surface is preferably less than 0.20 when using a liquid lubricant as described below. Even without lubrication, the average coefficient of friction is preferably less than 0.30, and more preferably less than 0.20.

[0047] The average coefficient of friction is measured by conducting a pin-on-disk friction and wear test according to the following procedure. In this application, a pin-on-disk friction and wear tester is used, sliding a pin on the surface of a titanium material to perform the friction and wear test. When measuring using a lubricant, the lubricant is Castor (S-803T, manufactured by Taiyu Co., Ltd.) diluted four times with water. When measuring without lubrication, the test is performed without using a lubricant. The average coefficient of friction is calculated under the conditions of a surface pressure of 1 MPa, a speed of 0.1 m / min, and a sliding distance of 20 mm. The pin used in the test is made of high-carbon chromium bearing steel SUJ2 specified in Japanese Industrial Standards JIS G4805:2019, and has a smooth surface with a diameter of 3.5 mm and an arithmetic mean roughness Ra of 0.12 μm.

[0048] 3. Processed Products and Products The titanium material of this embodiment has excellent lubricity and can suppress mold wear, so it can be used as a processed product after various processes. That is, the processed product is obtained by processing the titanium material described above. Examples of processing methods include press processing and shearing. Examples of processed titanium products include expanded metal, separators, bipolar plates, and titanium plates for plate heat exchangers. Furthermore, titanium material and / or processed titanium material products may be used to make products such as plate heat exchangers.

[0049] Even in the state of being processed, at least a part of the surface remains as it was before processing. That is, in the processed product manufactured from the titanium material of this embodiment, the color (L ＊ , a ＊ , b ＊ ) and a reference color (L ０ ＊ , a ０ ＊ , b ０ ＊ ) and color difference ΔE ＊ ab is 6.14 or less. (L ０ ＊ , a ０ ＊ , b ０ ＊ ) = (68.905, 0.935, 2.295) ... (i)

[0050] Furthermore, when X-ray photoelectron spectroscopy is performed in the depth direction from the surface, and the position where the nitrogen concentration is maximum is defined as the peak depth, the nitrogen concentration Np, titanium concentration Tp, and carbon concentration Cp at the peak depth satisfy the following formula (ii). The reasons for these limitations are the same as those for titanium materials, and therefore will not be explained further: 0.48≦Np / (Tp−Cp)≦1.09 (ii) where Np, Tp, and Cp in formula (ii) are each expressed in atomic percent (at%).

[0051] 4. Manufacturing Method The titanium material of this embodiment can be stably manufactured, for example, by the following manufacturing method.

[0052] 4-1. Hot Rolling First, a material for hot rolling is prepared. This material can be commercially pure titanium or a titanium alloy, and the type is not particularly limited. The material for hot rolling can be produced according to a conventional method. For example, an ingot of titanium material can be produced by arc melting or the like, and then hot forged to produce the material for hot rolling.

[0053] The above-mentioned hot rolling material is hot rolled to produce a hot rolled material. The conditions for hot rolling are not particularly limited. They may be appropriately adjusted according to the desired properties. The obtained hot rolled material may be appropriately heat treated.

[0054] 4-2. Cold Rolling The hot-rolled material is then cold-rolled to produce a cold-rolled material. During cold rolling, a cold-rolling oil containing C is used. For the titanium material of this embodiment, it is preferable to use, for example, commercially available mineral oil or soluble oil as the cold-rolling oil. Considering reactivity, mineral oil is preferable to soluble oil. By using such cold-rolling oil, a mechanochemical reaction occurs during cold rolling, in which the titanium material surface reacts with the C in the cold-rolling oil. As a result, C is concentrated in the surface layer. Note that no spalling, polishing, etc. is performed before primary annealing after cold rolling. This is because spalling or polishing after cold rolling would remove the surface layer where C is concentrated.

[0055] Cold rolling is typically performed using a Sendzimir rolling mill. In a Sendzimir rolling mill, a titanium material is rolled by passing it back and forth between a pair of work rolls multiple times. Here, passing a titanium material through the work rolls of the rolling mill is called a pass. Therefore, in cold rolling, the target thickness is typically controlled through multiple passes. In other words, cold rolling is a process that involves multiple passes.

[0056] In the production of the titanium material of this embodiment, the average reduction rate for all passes in cold rolling is not particularly limited, but is preferably in the range of, for example, 7 to 20%. This is because a mechanochemical reaction is likely to occur when the average reduction rate is 7% or more. Furthermore, surface defects are less likely to occur when the average reduction rate is 20% or less.

[0057] The above-mentioned rolling reduction can be calculated from the following formula (b): Rolling reduction (%) = (h1 - h2) / h1 x 100 (b) where the symbols in the formula (b) are defined as follows: h1 (mm): thickness of the titanium material before the pass h2 (mm): thickness of the titanium material after the pass

[0058] 4-3. Primary annealing Subsequently, the obtained cold-rolled material is subjected to primary annealing. In the primary annealing, C concentrated in the surface layer during cold rolling is converted into a certain amount of C solid-solved in the surface layer and TiC, thereby stabilizing the C. Furthermore, in the primary annealing, recrystallization may be promoted and the grain size may be adjusted. Note that such structural control may also be performed in the secondary annealing described below.

[0059] Solute C is necessary to exert a reducing action during the secondary annealing described below, to temporarily remove the passive film, and to concentrate N. TiC does not exhibit a reducing action, but its presence immediately below the passive film after secondary annealing enhances lubricity. Also, a certain amount of TiC is formed to achieve the desired amount of solute C.

[0060] Here, the annealing temperature and time in the primary annealing are set within a range such that the Larson-Miller parameter (LMP) shown in the following formula (c) is 8600 or more. For example, if the annealing temperature is 550°C for 3 hours or more, 650°C for 20 minutes or more, 750°C for 3.5 minutes or more, and 850°C for 20 seconds or more, the LMP will be 8600 or more. When the annealing temperature and time satisfy the above ranges, excessive reduction reaction of C is unlikely to occur in the secondary annealing, and the color difference ΔE ＊ This is because the side values ​​in the formulas ab and (ii) can be set within a desired range.

[0061] The upper limit of the LMP for controlling the annealing temperature and time is not particularly limited, but it is preferable to control the annealing temperature and time so that the LMP is 11,000 or less. If the C concentrated on the surface is diluted by inward diffusion, the effect cannot be obtained, but this requires an unrealistically long time, so there is no practical upper limit. On the other hand, if the crystal grains of the titanium material become too coarse, the formability decreases, so it is preferable that the upper limit of the LMP be 11,000 or less.

[0062] LMP = (T + 273.15) × (10 + Log(t)) (c) In the above formula (c), Log is a common logarithm with base 10, and each symbol is defined as follows. Hereinafter, unless otherwise specified, Log will always represent a common logarithm. T: Annealing temperature in primary annealing (°C) t: Annealing time in primary annealing (h)

[0063] Furthermore, after cold rolling, there is no lower limit to the rate of temperature increase up to the annealing temperature of the primary annealing, but from the viewpoint of LMP and productivity, the rate of temperature increase up to the annealing temperature of the primary annealing is preferably 20°C / s or more.

[0064] The annealing atmosphere in the primary annealing is a vacuum or Ar atmosphere. In the case of a vacuum atmosphere, the pressure is preferably less than 0.01 Pa. On the other hand, in the case of an Ar atmosphere, it is preferable to use Ar gas with a purity of 5N or more, with an oxygen concentration of 1 vol. ppm or less and a dew point of -50°C or less. By using such an annealing atmosphere, excessive growth of a passive film on the surface is suppressed, and the color difference ΔE ＊ This is because ab can be set within a desired range.

[0065] After being held at the annealing temperature, the steel sheet is cooled to 200°C or less and removed from the furnace. After being cooled to 200°C and removed from the furnace, secondary annealing, which will be described later, is performed within seven days. By performing secondary annealing within seven days, the formation of a dense and thick passive film can be suppressed, and N can be sufficiently concentrated by secondary annealing.

[0066] Even after the primary annealing, a passive film is temporarily formed, but if this passive film is dense and thick, it becomes difficult for N to be concentrated during the secondary annealing. For this reason, the conditions for the primary annealing are set within the above ranges so as to prevent the formation of such a passive film.

[0067] 4-4. Secondary Annealing Next, the cold-rolled material that has undergone primary annealing is subjected to secondary annealing. Secondary annealing is what is known as nitriding annealing, in which the carbon that has dissolved in the primary annealing exerts a reducing effect, primarily removing the passive film and concentrating nitrogen in the titanium base material. This concentrated nitrogen improves lubricity.

[0068] Here, the annealing temperature in the secondary annealing is 500 to 850°C. When the annealing temperature is 500°C or higher, N can be sufficiently concentrated, and the value in the formula (ii) can be set within the desired range. On the other hand, when the annealing temperature is 850°C or lower, the color difference ΔE ＊This is because ab can be set in a desired range. After cold rolling, the rate of temperature rise up to the annealing temperature of the secondary annealing is preferably 20°C / s or more. This is because the rate of temperature rise within the above range can suppress excessive growth of the passive film.

[0069] Similarly, the holding time required after reaching the annealing temperature differs for each nitriding temperature, and it is preferable that the annealing time relative to the annealing temperature in the secondary annealing be within a range that satisfies the following equation (d) experimentally: Log(t.c)≧−20.633×Log(T+273.15)+63.093 (d) where Log is a common logarithm with the base 10, and each symbol is defined as follows: T: annealing temperature in the secondary annealing (°C) t.c: annealing time in the secondary annealing (s)

[0070] Based on the above formula (d), for example, N can be concentrated for 870 seconds or more at 550°C, 82 seconds or more at 650°C, 10 seconds or more at 750°C, and 4 seconds or more at 800°C, and the value in formula (ii) can be set within the desired range.

[0071] Furthermore, it is more preferable that the annealing time satisfy the following formula (e) because the central value of formula (ii) can be set in a more preferable range if the annealing time satisfies the following formula (e):

[0072] Log(t.c)≧−29.108×Log(T+273.15)+89.181 (e) where Log in the above formula (e) is a common logarithm with base 10, and each symbol is defined as follows: T: Annealing temperature in secondary annealing (°C), t.c: Annealing time in secondary annealing (s).

[0073] Although there is no upper limit to the annealing time from the viewpoint of lubricity, an excessively long annealing time is undesirable because a passive film grows and the appearance turns golden. This is because if the annealing time is too long relative to the annealing temperature, the reducing effect of C is lost and a passive film grows. For this reason, the upper limit of the common logarithm of the annealing time is preferably less than the right-hand side of equation (f), i.e., the annealing time preferably satisfies equation (f).

[0074] Log(t.c)≦−23.617×Log(T+273.15)+74.663 (f) where Log in the above formula (f) is a common logarithm with base 10, and each symbol is defined as follows: T: Annealing temperature in secondary annealing (° C.) t.c: Annealing time in secondary annealing (s)

[0075] The above formulas (d) to (f) were obtained experimentally, and although the details are unknown, they are presumed to be based on the diffusion distance of nitrogen.

[0076] The annealing atmosphere in the secondary annealing is a nitrogen atmosphere. In this case, it is preferable to use nitrogen gas with a purity of 5N or higher, with an oxygen concentration of 1 vol. ppm or less and a dew point of -50°C or less. By using such an annealing atmosphere, excessive growth of a passive film on the surface is suppressed, and the color difference ΔE ＊ This is because ab can be set in a desired range. Also, the value of the side in equation (ii) can be set in a desired range. After the secondary annealing, the material is cooled to about room temperature (25°C) to obtain the titanium material of this embodiment.

[0077] The titanium material according to this embodiment will be described in more detail below with reference to examples, but this embodiment is not limited to these examples.

[0078] A titanium material with a thickness of 4 mm was prepared and had the chemical composition shown in Table 1. This titanium material was manufactured through processes such as hot rolling. For simplicity, commercially pure titanium is simply referred to as pure titanium in Table 1.

[0079]

[0080] This titanium material was cold-rolled under the conditions shown in Tables 2 and 4. Commercially available mineral oil or soluble oil was used in the cold rolling until the plate thickness reached 0.5 mm. The cold rolling was performed in multiple passes, and the average reduction rate during each pass was also calculated. Primary annealing and secondary annealing were then performed to obtain the titanium material. In the tables, "0.001 Pa" or "0.01 Pa" in the annealing atmosphere refers to the air pressure inside the annealing furnace.

[0081] As shown in Tables 2 to 5, in some examples, surface grinding or polishing was performed before or after primary annealing. In surface grinding, fluoronitric acid was used to reduce the plate thickness by 10 μm or more on one side (a total of 20 μm or more on both sides). In some examples, polishing was performed by switching the grit of the abrasive paper from coarse to fine, in the order of P150, P600, and P1000, and finally finishing with P2000, thereby reducing the plate thickness by a total of 100 μm. The final finishing grit is listed in the table. The polishing direction was changed by 90° when the grit was switched. For mirror polishing, the plate was polished with P1000 abrasive paper, followed by buffing with colloidal silica to obtain a mirror finish. After primary annealing, the plate was cooled to below 200°C. The temperature at which it was removed from the annealing furnace is listed in the table as the removal temperature. The nitrogen used in secondary annealing had a purity of 5N or higher. The heating rates for the primary annealing and secondary annealing were 30° C. / s. The minimum and maximum t.c values ​​in the table were calculated based on formulas (d) and (f).

[0082] For Example No. 30 of the present invention, a blank having a diameter of 108 mm was cut out from the obtained plate-shaped titanium material, and then subjected to deep drawing using a punch having a diameter of 40 mm so as to have a drawing depth of 20 mm, thereby obtaining a processed product.

[0083]

[0084]

[0085]

[0086]

[0087] Color difference ΔE of the obtained titanium material ＊ The values ​​ab, the concentration of each element (Np, Cp, and Tp), and the average coefficient of friction were measured by the following procedure. For Example No. 30 of the present invention, the measurement was carried out on the flange portion of the processed product.

[0088] (color difference ΔE ＊ ab) First, on the surface of the flange portion of the titanium material or processed product, L ＊ , a ＊ , and b ＊The measurement was carried out using a color difference meter (Konica Minolta CR-400 color difference meter) under light source C. Next, the L obtained by measuring the surface of the flange portion of the titanium material or processed product was ＊ , a ＊ , and b ＊ and the reference color L ０ ＊ , a ０ ＊ , and b ０ ＊ The difference between ＊ , Δa ＊ , Δb ＊ is calculated and substituted into the following formula (a) to obtain the color difference ΔE ＊ ab was calculated. ΔE ＊ ab = √{(ΔL ＊ ) ２ + (Δa ＊ ) ２ + (Δb ＊ ) ２} ...(a)

[0089] (Concentration of each element (Np, Cp, and Tp)) Using a JEOL XPS (model number: JPS-9200), a 1 mm diameter region of a test piece taken from the flange portion of a titanium material or a processed product was measured for element concentration distribution in the depth direction by repeatedly performing sputtering with Ar ions from the surface and measuring the element concentration. Prior to the measurement, acetone (ultrasonic) degreasing was performed as a pretreatment, and the line connecting the maximum and minimum values ​​of binding energy (BE) specified in the range shown below was used as the background, and the element concentration was determined for the BE after background processing (removal). The background processing was performed using JEOL SPECSURF software.

[0090] The BE range was calibrated so that the peak of C graphite was 285 eV in the outermost surface qualitative measurement, and then the range was set to 275 to 300 eV for C, 445 to 470 eV for Ti, 520 to 540 eV for O, 385 to 410 eV for N, and arbitrary values ​​within the BE width of 25 eV for other elements so that all peaks fell within the BE width of 25 eV. ２The concentrations of Ti, O, N, C, Fe, Cu, Sn, Si, Nb, Al, and V were measured at 1 nm pitches from 1 to 5 nm equivalent depth, and at 5 nm pitches in the deeper region on the titanium base material side.

[0091] From the measurement results, the position where the N concentration was greatest was designated as the peak depth, the N concentration at the peak depth as Np, the C concentration at the peak depth as Cp, and the Ti concentration at the peak depth as Tp. When there were two or more peak depths, the depth closest to the surface was designated as the peak depth. The thickness of the passive film was calculated using the O concentration at the outermost surface and the O concentration at each depth from the surface. Specifically, when the O concentration at the outermost surface was designated as 100%, the minimum depth at which the O concentration was 50% or less was designated as the thickness of the passive film, and is listed in the table.

[0092] (Average Friction Coefficient) Friction and wear tests were conducted using a pin-on-disk friction and wear tester, with a pin sliding across the surface of the flange portion of the titanium material or processed product. Measurements were conducted under two conditions: with and without a liquid lubricant. When measuring with a lubricant, Castol (S-803T) diluted four times with water was used as the lubricant. When measuring without lubrication, the test was conducted without a lubricant. The average friction coefficient was calculated under the conditions of a surface pressure of 1 MPa, a speed of 0.1 m / min, and a sliding distance of 20 mm. The pins used in the tests were made of high-carbon chromium bearing steel SUJ2 specified in Japanese Industrial Standard JIS G4805:2019, and had a smooth surface with a diameter of 3.5 mm and an arithmetic mean roughness Ra of 0.12 μm. The results are summarized in the table below.

[0093]

[0094]

[0095] Inventive Examples Nos. 1 to 5, 7, 12, 13, 15, 17, 20, 22, and 24 to 30 have a color difference ΔE ＊ The requirements ab were satisfied, and the formula (ii) was also satisfied, so the lubrication was also good.

[0096] Inventive Examples Nos. 6, 8 to 11, the primary annealing conditions were slightly worse than those of the above-mentioned inventive examples, resulting in a slightly excessive reduction reaction of C. As a result, the side value in equation (ii) was 0.96 or more, resulting in slightly inferior lubricity compared to the above-mentioned inventive examples. Similarly, inventive Examples Nos. 14, 16, 18, 19, 21, and 23, the secondary annealing conditions were slightly worse, resulting in a side value in equation (ii) of less than 0.54, resulting in slightly inferior lubricity compared to the above-mentioned inventive examples such as Nos. 1 to 5.

[0097] In Comparative Examples Nos. 1 to 4, the surface was removed before the primary annealing, which resulted in insufficient C concentration in the surface layer and insufficient reduction in the secondary annealing. As a result, oxygen in the nitrogen gas reacted preferentially in the secondary annealing, forming a thick passive film and increasing ΔE ＊ In Comparative Examples Nos. 5 to 7, the annealing atmosphere in the primary annealing was outside the range of preferable conditions, so a thick passivation film was formed on the surface, and ΔE ＊ ab exceeded 6.14.

[0098] In Comparative Examples Nos. 8 to 15, the primary annealing temperature or holding time was outside the range of the preferred conditions, so the reduction reaction of C occurred excessively, the value in formula (ii) exceeded 1.09, and compounds of Ti, C, O, and / or N were formed in excess, resulting in a ΔE ＊ In Comparative Example No. 16, the removal temperature after primary annealing was too high, resulting in the formation of a dense passive film, and as a result, the reduction effect of C was not sufficiently achieved in secondary annealing. As a result, the value in equation (ii) was less than 0.48, resulting in a decrease in lubricity.

[0099] In the comparative examples Nos. 17 and 18, after the primary annealing, surface grinding or finish polishing with a coarse grit was performed, resulting in a decrease in the C concentration on the surface and insufficient reduction during the secondary annealing. As a result, oxygen in the nitrogen gas reacted preferentially, forming a thick passive film, resulting in a ΔE ＊ ab exceeded 6.14.

[0100] In Comparative Examples Nos. 19 to 21, the period from primary annealing and cooling to secondary annealing was longer than 7 days, resulting in a dense passivation film after primary annealing. As a result, the reducing action of C was not sufficiently achieved in secondary annealing. Therefore, the central value of equation (ii) was less than 0.48, resulting in a decrease in lubricity. In Comparative Examples Nos. 22 and 23, the annealing atmosphere in secondary annealing was outside the range of preferred conditions, resulting in the formation of a thick passivation film on the surface, and ΔE ＊ ab exceeded 6.14.

[0101] In Comparative Examples 24 to 26 and 28 to 31, the secondary annealing temperature or holding time was outside the range of the preferred conditions, so N was not concentrated sufficiently, the central value of equation (ii) was less than 0.48, and lubricity was reduced. In Comparative Example 27, the holding time of the secondary annealing was too long, so a thick passive film was formed on the surface, and ΔE ＊ In Comparative Examples Nos. 32 and 33, primary annealing was not performed, and therefore the reduction reaction of C occurred excessively, resulting in the formation of excessive compounds of Ti, C, O, and N. As a result, ΔE ＊ In Comparative Example No. 34, secondary annealing was not performed, so N was not concentrated, and the value in equation (ii) was less than 0.48, resulting in reduced lubricity.

Claims

1. Titanium material, L ＊ a ＊ b ＊ In the color system, the color of the surface of the titanium material (L ＊ , a ＊ , b ＊ ) and a reference color (L ０ ＊ , a ０ ＊ , b ０ ＊ ) and color difference ΔE ＊ A titanium material in which ab is 6.14 or less, and when measurement is performed from the surface in a depth direction by X-ray photoelectron spectroscopy and the position at which the nitrogen concentration is maximum is defined as the peak depth, the nitrogen concentration Np, titanium concentration Tp, and carbon concentration Cp at the peak depth satisfy the following formula (ii): (L ０ ＊ , a ０ ＊ , b ０ ＊ )=(68.905, 0.935, 2.295) (i) 0.48≦Np / (Tp−Cp)≦1.09 (ii) where Np, Tp, and Cp in the above formula (ii) are each expressed in atomic percent.

2. The titanium material according to claim 1, further satisfying the following formula (iii): 0.54≦Np / (Tp−Cp) (iii) 3. The titanium material according to claim 1 or 2, which further satisfies the following formula (iv): Np / (Tp-Cp)≦0.95 (iv) 4. Titanium processed products, L ＊ a ＊ b ＊ In the color system, the color (L ＊ , a ＊ , b ＊ ) and a reference color (L ０ ＊ , a ０ ＊ , b ０ ＊ ) and color difference ΔE ＊ ab is 6.14 or less, and when measurement is performed from the surface in the depth direction by X-ray photoelectron spectroscopy and the position where the nitrogen concentration is maximum is taken as the peak depth, the nitrogen concentration Np, titanium concentration Tp, and carbon concentration Cp at the peak depth satisfy the following formula (ii). (L ０ ＊ , a ０ ＊ , b ０ ＊ )=(68.905, 0.935, 2.295) (i) 0.48≦Np / (Tp−Cp)≦1.09 (ii) where Np, Tp, and Cp in the above formula (ii) are each expressed in atomic percent.

5. A product comprising the titanium material according to any one of claims 1 to 3 and / or the processed product according to claim 4.

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