Titanium material and method for manufacturing the same

Abstract

A method for manufacturing titanium material includes a step of pressing a first pressing body 51 into the surface of a titanium blank 1. The first pressing body 51 has an arc-shaped first pressing surface 51a extending along a predetermined direction. The radius of curvature (mm) of the first pressing surface 51 in a cross section orthogonal to the extension direction of the first pressing surface 51a is 2.5 mm or more and 17.5 mm or less. The manufacturing method satisfies the following equations (1) and (2). The resulting processed titanium material is less prone to surface defects during hot rolling. 0.5≤X1≤R1×(1-cosθ1)(1), 1.0≤Y1≤(-0.16R1 2 +4.4R1)×(0.25X1+0.037)(2). Wherein, in the above formula, θ1 is 50°, R1 is the radius of curvature (mm) of the first pressing surface in the first cross section, X1 is the pressing amount (mm) of the first pressing surface pressed into the titanium blank, and Y1 is the distance (mm) between adjacent pressing positions of the first pressing surface in a direction orthogonal to both the extension direction of the first pressing surface and the pressing direction of the first pressing body.

Description

Technical Field

[0001] This invention relates to the processing of titanium materials and methods for manufacturing them. Background Technology

[0002] The typical manufacturing method for hot-rolled titanium materials is as follows. First, titanium is melted and solidified using a consumable electrode arc remelting (VAR) or electron beam remelting (EBR) method to produce an ingot. Next, the ingot is subjected to initial rolling processes such as billeting, forging, and rolling to produce hot-rolled titanium materials such as slabs and billets. In recent years, a technique has been developed to manufacture rectangular ingots that can be directly hot-rolled using electron beam remelting, thus eliminating the aforementioned initial rolling process.

[0003] However, the solidification structure of large ingots used in industry contains coarse grains, some as large as tens of millimeters. If such ingots are directly hot-rolled without a preliminary rolling process, uneven deformation can sometimes occur due to the coarse grains, leading to obvious surface defects. Furthermore, even when a preliminary rolling process is performed, if the processing rate is low or the temperature is inappropriate, casting structures may remain, or the structure may become coarser, resulting in surface defects during hot rolling.

[0004] If surface defects are generated in this way, the yield in the subsequent descaling process will be very poor. Therefore, we are looking for hot-rolled titanium materials that are less prone to surface defects.

[0005] Patent document 1 proposes the following method: When directly hot-working a titanium ingot, in order to refine the grains near the surface layer, after applying strain to the surface layer, it is heated to above the recrystallization temperature to recrystallize to a depth of more than 2 mm from the surface, and then hot-working is performed.

[0006] Furthermore, Patent Documents 2 and 3 describe a hot-rolled titanium material that uses a steel tool with a front end having a radius of curvature of 3 to 30 mm or a steel ball with a radius of 3 to 30 mm to cause plastic deformation of the titanium material's surface, thereby imparting strain to the surface layer. According to Patent Documents 2 and 3, by hot-rolling this type of titanium material, the influence of coarse solidification structures can be eliminated, and surface defects can be reduced.

[0007] Prior art literature

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 1-156456

[0010] Patent Document 2: International Publication No. 2010 / 090352

[0011] Patent Document 3: Japanese Patent Application Publication No. 2018-1249 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] In Patent Document 1, forging, rolling, and shot peening are listed as methods for imparting strain. However, the diameter of typical shot peening particles is as small as 0.5 to 1 mm, resulting in a small amount of strain. Furthermore, in forging and rolling, so-called dead zone metal is generated, leading to less strain or strain being introduced further inward. Therefore, there are cases where the required thickness of the recrystallized layer cannot be ensured, and the fine-graining becomes insufficient.

[0014] In patent documents 2 and 3, strain is imparted by striking or pressing with steel tools, which can be inefficient as it can take a long time to consistently apply strain to the entire surface. Furthermore, in the case of high-strength materials, the impact energy may not always be conducted to the interior, making it impossible to ensure the required thickness of the fine-grained structure. Therefore, there is room for further improvement.

[0015] The present invention was made in view of the above circumstances, and its objective is to provide a processed titanium material and a method for manufacturing the same, which can reduce surface defects generated during hot rolling.

[0016] Solution for solving the problem

[0017] The gist of the present invention for solving the above-mentioned problems is as follows.

[0018] A method for manufacturing a processed titanium material, wherein the processed titanium material is a processed titanium material in which a plurality of first grooves are formed on the surface of a titanium billet.

[0019] The manufacturing method includes a first step of pressing a first pressing body into the surface of the aforementioned titanium blank, wherein the first pressing body has an arc-shaped first pressing surface extending along a predetermined direction.

[0020] In the first cross-section orthogonal to the extension direction of the aforementioned first pressing surface, the radius of curvature of the aforementioned first pressing surface is 2.5 mm or more and 17.5 mm or less.

[0021] The aforementioned first process satisfies the following equations (1) and (2).

[0022] 0.5≤X1≤R1×(1-cosθ1) (1)

[0023] 1.0≤Y1≤(-0.16R1 2 +4.4R1)×(0.25X1+0.037) (2)

[0024] In the above formula,

[0025] θ1 is 50°.

[0026] R1 is the radius of curvature (mm) of the first pressing surface in the aforementioned first cross section.

[0027] X1 represents the pressing amount (mm) of pressing the aforementioned titanium blank into the aforementioned first pressing surface.

[0028] Y1 is the distance (mm) between adjacent pressing positions of the first pressing surface in a direction orthogonal to both the extension direction of the first pressing surface and the pressing direction of the first pressing body.

[0029] The effects of the invention

[0030] According to the present invention, the generation of surface defects during hot rolling can be reduced.

[0031] Furthermore, according to the present invention, even titanium billets in the cast state after omitting the initial rolling process of ingots can reliably reduce surface defects generated during hot rolling, and can provide excellent hot-rolled and cold-rolled products. Attached Figure Description

[0032] Figure 1 This is a perspective view showing an example of the shape of the processed titanium material in an embodiment of the present invention.

[0033] Figure 2 This is a perspective view illustrating the shape of the grooves arranged on the processed titanium material in an embodiment of the present invention.

[0034] Figure 3 This is a schematic diagram showing a cross-section of the processed titanium material according to an embodiment of the present invention, orthogonal to the extension direction of the groove.

[0035] Figure 4 This is a schematic diagram illustrating the pressing body used in the manufacturing method of processed titanium material according to an embodiment of the present invention. (a) shows a perspective view, and (b) shows a schematic diagram illustrating the pressing state in a cross section orthogonal to the axial direction of the pressing body.

[0036] Figure 5 This is a schematic diagram illustrating the pressing body used in a method for manufacturing titanium material according to other embodiments of the present invention. (a) shows a perspective view, and (b) shows a schematic diagram illustrating the pressing state in a cross section orthogonal to the axial direction of the pressing body.

[0037] Figure 6This is a schematic diagram illustrating the pressing body used in a method for manufacturing titanium material according to other embodiments of the present invention. (a) shows a perspective view, and (b) shows a schematic diagram illustrating the pressing state in a cross section orthogonal to the axial direction of the pressing body.

[0038] Figure 7 This is a perspective view of the pressing body used in a method for manufacturing titanium material according to another embodiment of the present invention.

[0039] Figure 8 The figures illustrate a method for manufacturing a processed titanium material according to an embodiment of the present invention. (a) is a plan view, and (b) is a schematic diagram showing a cross section orthogonal to the axial direction of the pressing body.

[0040] Figure 9 The figures illustrate a method for manufacturing a processed titanium material according to an embodiment of the present invention. (a) is a plan view, and (b) is a schematic diagram showing a cross section orthogonal to the axial direction of the pressing body.

[0041] Figure 10 This is a plan view of a groove for processing titanium material obtained by the manufacturing method of processing titanium material according to an embodiment of the present invention.

[0042] Figure 11 This is a graph showing the log-normal distribution of the crystal grain size of the recrystallized layers of Examples No.2, 18, and 16. Detailed Implementation

[0043] The following description uses the accompanying drawings to illustrate embodiments of the present invention.

[0044] Based on the viewpoint of reducing surface defects caused by hot rolling, the inventors have conducted in-depth research on methods for eliminating the influence of coarse solidified structures in ingots with grains as large as tens of millimeters, and on processing titanium materials suitable for such methods. As a result, the following insights were obtained, and the present invention was completed.

[0045] To refine coarse solidified structures, or to eliminate the influence of residual solidified structures in certain areas, it is advisable to create grooves (recesses) on the surface to impart strain, followed by recrystallization through prescribed heat treatments such as heating during hot rolling.

[0046] This invention includes a process of pressing a pressing body into the surface of a titanium billet to form multiple first grooves on the surface of the titanium billet. This creates multiple grooves on the surface of the titanium billet, thereby imparting strain. The processed titanium material obtained by this method can significantly suppress surface defects during hot rolling. Furthermore, in this invention, by actually pressing the pressing surface of the pressing body, physical plastic deformation occurs, forming grooves, thus stably introducing strain regardless of the crystallization orientation. Moreover, by performing multiple pressing steps, and ensuring that the extension directions of the grooves formed in each step do not overlap, strain can be effectively and sufficiently introduced into the grooves and their surroundings. Subsequent heating during hot rolling causes fine recrystallization on the surface, thereby suppressing the formation of surface defects.

[0047] The following describes the processing of titanium materials and the manufacturing method thereof according to this embodiment.

[0048] The processed titanium material of this embodiment has multiple grooves formed on its surface. In the thickness direction of the processed titanium material, the difference ΔHV between the Vickers hardness at a position 3 mm from the bottom of the groove and the Vickers hardness at a position halfway through the thickness is 20 or more. When the processed titanium material with a difference ΔHV of 20 or more is heat-treated at 800°C for 4 hours, grains with a round equivalent average grain size of 1.00 mm or less are formed at least from the bottom of the groove to a depth of 3.0 mm, and the standard deviation of the round equivalent grain size with respect to the logarithmic transformation value is 1.00 or less. In other words, the processed titanium material of this embodiment can refine the surface microstructure through heating during hot rolling, thus suppressing surface defects generated during hot working. Therefore, it is suitable for hot-rolled titanium materials.

[0049] In this embodiment, the angle between the inner surface of the groove and the surface of the processed titanium material in a cross-section orthogonal to the extension direction of the groove is preferably 50° or less. The titanium billet used in the manufacturing method of the processed titanium material in this embodiment is preferably formed from industrial pure titanium or a titanium alloy. Examples of titanium billets used in the manufacturing method of the processed titanium material in this embodiment include ingots, slabs, large steel billets, or small steel billets.

[0050] Figure 1 An example of the processed titanium material according to this embodiment is shown. The processed titanium material of this embodiment can be as follows: Figure 1 As shown in (a), it is slab 1, or it can be as shown in (a). Figure 1 As shown in (b), it is a large steel billet 2, and it can also be as shown in (b). Figure 1 As shown in (c), it is a small steel billet 3 with a rectangular cross-section perpendicular to the length direction. Alternatively, it can be a small steel billet with a circular cross-section as described above. Furthermore, in Figure 1 (a) slab 1, Figure 1 (b) large steel billet 2 and Figure 1(c) Each of the small steel billets 3 has a plurality of straight grooves 1b, 2b, and 3b formed on its surfaces 1a, 2a, and 3a. It should be noted that the extension direction of these grooves 1b, 2b, and 3b is shown in the figure as the length direction of each of the slab 1, large steel billet 2, and small steel billet 3, but is not limited to this. For example, it can be the width direction of each of the slab 1, large steel billet 2, and small steel billet 3. Alternatively, it can be formed in a direction having a predetermined slope relative to the width direction of each of the slab 1, large steel billet 2, and small steel billet 3. In the following description, the example of grooves 1b, 2b, and 3b being formed along the length direction of each of the slab 1, large steel billet 2, and small steel billet 3 will be used for illustration.

[0051] In this embodiment, the titanium material is processed at a depth of 3 mm from the bottom of the groove. Figure 3 The Vickers hardness at the location of the line with the symbol S in the figure is at 1 / 2 depth of the thickness. Figure 3 The Vickers hardness difference ΔHV at the location of the line marked with the symbol M is greater than 20. It should be noted that... Figure 3 This is a schematic diagram showing a cross-section of the processed titanium material that is orthogonal to the extension direction of the groove.

[0052] It should be noted that, regarding the 1 / 2 depth position of the thickness, in Figure 1 (a) or Figure 1 In the slab or large billet shown in (b), the positions are respectively the positions of 1 / 2t of the slab thickness t or the large billet thickness t. Additionally, in Figure 1 In the small steel billet with a rectangular cross-section and an aspect ratio of about 1 shown in (c), the position becomes the centroid of the small steel billet cross-section.

[0053] To suppress surface defects during hot rolling, it is necessary to refine the crystalline structure of the processed titanium material. While refining the overall crystalline structure of the processed titanium material can also suppress surface defects, this requires applying significant strain to the entire billet. Furthermore, sometimes rolling is performed along the width direction before hot rolling as required. If the reduction in the width direction relative to the cast titanium billet is large, wrinkles may occur due to the coarse cast structure, resulting in surface defects after hot rolling.

[0054] In order to reliably suppress surface defects caused not only by casting structure but also by wrinkles during rolling that increase width, it is necessary to ensure that the surface layer has a recrystallized structure. The surface layer mentioned here refers to the area from the bottom of the groove in the titanium processing section to a depth of 3 mm. To ensure that the surface layer has a recrystallized structure during hot rolling, it is necessary to ensure that the area from the bottom of the groove (1b1, 2b1, 3b1) to a depth of at least 3 mm (…) Figure 3Strain is applied to the region up to the position of the line marked with the symbol S. Through various analyses, the inventors have determined that if the equivalent strain from the bottom of the groove (1b1, 2b1, 3b1) to a depth of 3 mm is 0.2 or higher, recrystallization occurs during hot rolling, forming a fine microstructure on the surface. Furthermore, it has been determined that this equivalent strain is related to Vickers hardness; if the Vickers hardness at a depth of 3 mm from the bottom of the groove (1b1, 2b1, 3b1) is 20 or higher than the Vickers hardness at a position where the processed titanium material is half its thickness, then an equivalent strain of 0.2 or higher can be achieved. The Vickers hardness at a position where the processed titanium material is half its thickness is approximately the same as the hardness in the cast state; therefore, ΔHV corresponds to the increase in surface hardness when an equivalent strain of 0.2 or higher is introduced into the surface layer. If the ΔHV of the processed titanium material is 20 or higher, sufficient strain is introduced into the surface layer, and through subsequent heating (hot rolling), fine recrystallization with uniform particle size can be formed. The resulting recrystallized layer, with a thickness of 3 mm or more, can suppress surface defects during hot rolling. A recrystallized layer thickness of 3 mm or more is sufficient, and there is no specific upper limit. To increase this thickness, the compressive load used to introduce strain needs to be increased. Therefore, from the viewpoint of limiting the load that the press can withstand, the practical upper limit for the recrystallized layer thickness is 25 mm.

[0055] In the Vickers hardness test, a cross-section (orthogonal to the direction of the groove) cut in a manner including the grooved surface of the processed titanium material is mirror-polished, and the hardness is measured using a Vickers hardness tester. Seven measurements are taken at two locations: a depth of 3 mm from the bottom of the groove and a location at half the thickness of the processed titanium material, with a load of 1 kg. The average of the five measurements, excluding the maximum and minimum hardness values, is calculated. Furthermore, the hardness difference (ΔHV) between the location 3 mm from the bottom of the groove and the location at half the thickness is determined.

[0056] In addition, regarding the processing of titanium materials in this embodiment, Figure 1 (a) slab 1, Figure 1 (b) large steel billet 2 and Figure 1 (c) Each of the small steel billets 3 has multiple straight grooves 1b, 2b, 3b arranged along its length. The angle (θ) between the inner surface of grooves 1b, 2b, 3b and surfaces 1a, 2a, 3a in the cross section orthogonal to the extension direction of the grooves is preferably 50° or less.

[0057] Even if strain is applied to the surface of the titanium billet as described above, if excessively large grooves (with sharp angles on the inner surface) are created, surface defects may occur during hot rolling due to the groove shape. Therefore, as Figure 2As shown, the angle θ between the inner surfaces of grooves 1b, 2b, and 3b and surfaces 1a, 2a, and 3a in a cross-section orthogonal to the extending directions of grooves 1b, 2b, and 3b is preferably set to 50° or less. This prevents the inner surfaces of the grooves from being sharp, thus preventing surface defects caused by the groove shape. Angle θ is more preferably 45° or less. It should be noted that the smaller the angle θ, the less likely surface defects, especially those caused by the groove shape, will occur. Therefore, there is no particular limitation on the lower limit of angle θ. However, if the angle θ is excessively reduced while applying sufficient strain to the surface of the blank, it means repeatedly increasing the number of pressing processes, significantly reducing manufacturing efficiency. Therefore, angle θ is preferably 10° or more, and more preferably 20° or more.

[0058] In this embodiment, the processed titanium material preferably exhibits a grain structure with a round equivalent average grain size of 1.00 mm or less, formed at least from the bottom of the groove to a depth of 3.0 mm, during heat treatment simulating hot rolling, for example, at a temperature of 800°C and a heating time of 4 hours. Furthermore, the standard deviation σ of the round equivalent grain size with respect to the logarithmic transformation value is preferably 1.00 or less. The grain size formed by the heat treatment simulating hot rolling is relatively uniform.

[0059] Regarding surface defects that may occur during hot rolling of titanium materials, larger grains are more prone to these defects. For example, in the case of a mixed-grain structure containing both fine and coarse grains, large grains are more likely to become the starting point for hot rolling defects. Therefore, by simulating hot rolling heating, a polycrystalline structure with smaller grain size and smaller grain size deviation can be formed. Thus, the titanium material processed in this embodiment, heated to 800°C for 4 hours, can form a grain structure with a standard deviation σ of the logarithmic transformation value of the round equivalent grain size of less than 1.00. When the grain size of a metallic material exhibits a near-log-normal distribution, the narrower the distribution range of the log-normal distribution, the more uniform the grain size, and the less likely surface defects will occur during hot rolling. That is, if the grains are fine enough and the standard deviation of the log-normal distribution is below a certain specified value, a uniform structure will be formed, and surface defects will be less likely to occur.

[0060] If the standard deviation σ of the distribution of the converted values ​​obtained by converting the round equivalent grain size D of each grain to its natural logarithm LnD is 1.00 or less, then the generation of surface defects will be suppressed when the average round equivalent grain size is 1.00 mm or less. The standard deviation σ is more preferably 0.80 or less. The narrower the distribution of the crystal grain size, i.e., the smaller the standard deviation σ, the more difficult it is to generate surface defects. Therefore, there is no particular limitation on the lower limit of the standard deviation.

[0061] Regarding the average grain size, it is preferable to make it finer than the cast microstructure with an average grain size of 10 mm or more. In this embodiment, the roundness-equivalent average grain size of the processed titanium material after heat treatment at 800°C for 4 hours, extending from the bottom of the tank to a depth of 3.0 mm, is preferably 1.00 mm or less, more preferably 0.80 mm or less, and even more preferably 0.70 mm or less. If it is larger than this, surface defects may sometimes occur during hot rolling, even within the aforementioned standard deviation σ. The smaller the roundness-equivalent average grain size, the less likely surface defects will occur; therefore, the lower limit of the roundness-equivalent average grain size is not particularly limited.

[0062] The grain size coarsens during hot rolling. Investigations clearly show that if the grain size after heat treatment at 800℃ for 4 hours is within the aforementioned range, surface defects can be sufficiently reduced even within the hot rolling temperature range of practical machinery. Therefore, the range of the spherical equivalent average grain size and standard deviation σ represents the values ​​after applying strain to the surface and then heat treating at 800℃ for 4 hours.

[0063] In the method for determining crystal grain size, the cross-section obtained by cutting, including the strained surface of the titanium material, is chemically ground, and electron backscatter diffraction (EBSD) is used. Approximately 2 to 10 fields of view are measured in a 5 mm × 5 mm region with a step size of 5–20 μm. Subsequently, the circumspherical equivalent grain size (area A = π × (grain size D / 2)) is calculated from the grain area measured using EBSD. 2 The standard deviation σ in the log-normal distribution is calculated based on the crystal grain size distribution.

[0064] Titanium billets are hot-rolled titanium slabs, such as ingots, slabs, large billets, and small billets as shown in (A) or (B) below. That is, titanium billets do not include titanium plates that have been rolled to a thickness less than a specified thickness by hot rolling or cold rolling. Therefore, in the case of cuboid or cubic titanium billets, samples with a thickness of, for example, 100 mm or more are considered, and in the case of cylindrical titanium billets, samples with a diameter of, for example, 90 mm or more are considered. Furthermore, titanium billet (B) is formed from a solidified structure obtained by melting and casting titanium, and has a structure containing coarse grains with a crystal size of 10 mm or more, exhibiting a cast state.

[0065] (A) Titanium is temporarily melted and then solidified using methods such as consumable electrode arc remelting (VAR) and electron beam remelting (EBR). The resulting ingot is then subjected to preliminary rolling through hot processing such as billeting, forging, and rolling to form titanium billets such as slabs and small billets.

[0066] (B) Titanium billets are obtained by temporarily melting titanium using electron beam melting or plasma arc melting and then solidifying it, producing rectangular or cylindrical ingots that can be directly hot rolled, omitting the initial rolling process of (A) above.

[0067] In electron beam melting, the irradiated electron beam can be focused through polarization, thus heat can be easily supplied even in the narrow area between the mold and the molten titanium, allowing for good control of the casting surface. Furthermore, the mold's cross-sectional shape offers a high degree of freedom. Therefore, rectangular or cylindrical ingots of the size described above (B), which can be directly supplied for hot rolling, are preferably melted using an electron beam melting furnace.

[0068] The titanium billet is preferably formed from industrial pure titanium or titanium alloy.

[0069] Industrial-grade pure titanium includes types 1 to 4 of the JIS H4600 standard, and corresponding grades 1 to 4 of the ASTM 265B standard, as well as grades I (WL3.7025), II (WL3.7035), and III (WL3.7055) of the DIN 17850 standard. Specifically, the industrial-grade pure titanium used in this invention, by mass%, consists of C: less than 0.1%, H: less than 0.015%, O: less than 0.4%, N: less than 0.07%, Fe: less than 0.5%, and the balance Ti. Hereinafter, "%" for each element content means "mass %".

[0070] On the other hand, low-alloy, α-type titanium alloys can be made by using appropriate alloys for the required applications. More preferably, the alloy composition is substantially less than 5% low-alloy. Examples include, for instance, high corrosion-resistant alloys with Pd < 0.15%, Ru < 0.10%, and further addition of rare earth elements < 0.02%; and heat-resistant alloys with a total addition of less than 5% Cu, Al, Si, Sn, Nb, and Fe.

[0071] More specifically, low-alloy types include, for example, high corrosion-resistant alloys (ASTM grades 7, 11, 16, 26, 13, 30, 33 or their corresponding JIS grades, and further, alloys containing small amounts of various elements), Ti-0.5Cu, Ti-1.0Cu, Ti-1.0Cu-0.5Nb, Ti-1.0Cu-1.0Sn-0.3Si-0.25Nb, Ti-0.5Al-0.45Si, Ti-0.9Al-0.35Si, etc. Additionally, α-type titanium alloys include, for example, Ti-5Al-2.5Sn, Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si, etc.

[0072] Examples of α+β type titanium alloys include Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-7V, Ti-3Al-2.5V, Ti-3Al-5V, Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-6Al-2Sn-4Zr-6Mo, and Ti-1 Fe-0.35O, Ti-1.5Fe-0.5O, Ti-5Al-1Fe, Ti-5Al-1Fe-0.3Si, Ti-5Al-2Fe, Ti-5Al-2Fe-0.3Si, Ti-5Al-2Fe-3Mo, Ti-4.5Al-2Fe-2V-3Mo, etc.

[0073] Furthermore, as β-type titanium alloys, there are examples such as Ti-11.5Mo-6Zr-4.5Sn, Ti-8V-3Al-6Cr-4Mo-4Zr, Ti-10V-2Fe-3Mo, Ti-13V-11Cr-3Al, Ti-15V-3Al-3Cr-3Sn, Ti-6.8Mo-4.5Fe-1.5Al, Ti-20V-4Al-1Sn, and Ti-22V-4Al.

[0074] The titanium alloy of the present invention contains more than 0% of one or more elements selected from, for example, O: 0-0.5%, N: 0-0.2%, C: 0-2.0%, Al: 0-8.0%, Sn: 0-10.0%, Zr: 0-20.0%, Mo: 0-25.0%, Ta: 0-5.0%, V: 0-30.0%, Nb: 0-40.0%, Si: 0-2.0%, Fe: 0-5.0%, Cr: 0-10.0%, Cu: 0-3.0%, Co: 0-3.0%, Ni: 0-2.0%, platinum group elements: 0-0.5%, rare earth elements: 0-0.5%, B: 0-5.0%, and Mn: 0-10.0%, thereby enabling the surface of the processed titanium material to be endowed with the desired function.

[0075] Elements other than those mentioned above that can be present in titanium are those that, based on common knowledge of metallic materials, can be expected to improve strength through solid solution strengthening, precipitation strengthening (in cases where solid solution does not occur and precipitates are formed). Examples of such elements include those from hydrogen (1) to astatine (85) in atomic number (excluding noble gas elements that are Group 18 elements), and a total content of about 5% is acceptable.

[0076] The remainder, excluding the aforementioned amounts, consists of Ti and impurities. Impurities can be present within a range that does not impair the target properties. Other impurities mainly include impurity elements introduced from raw materials and waste, and elements introduced during manufacturing. Examples of representative elements include C, N, O, Fe, and H. Other elements introduced from raw materials include Mg and Cl, while elements introduced during manufacturing include Si, Al, and S. As long as these elements are present in amounts of approximately 2% or less, it can be considered within the range that does not impair the target properties of this application.

[0077] Furthermore, the titanium alloy described in this invention may contain, for example, O: 0.01–0.5%, N: 0.01–0.2%, C: 0.01–2.0%, Al: 0.1–8.0%, Sn: 0.1–10.0%, Zr: 0.5–20.0%, Mo: 0.1–25.0%, Ta: 0.1–5.0%, V: 1.0–30.0%, and Nb: 0.1–40%. One or more of the following: 0%, Si: 0.1–2.0%, Fe: 0.01–5.0%, Cr: 0.1–10.0%, Cu: 0.3–3.0%, Co: 0.05–3.0%, Ni: 0.05–2.0%, platinum group elements: 0.01–0.5%, rare earth elements: 0.001–0.5%, B: 0.01–5.0%, and Mn: 0.1–10.0%.

[0078] The titanium alloy of the present invention more preferably contains an alloy selected from the following: O: 0.02-0.4%, N: 0.01-0.15%, C: 0.01-1.0%, Al: 0.2-6.0%, Sn: 0.15-5.0%, Zr: 0.5-10.0%, Mo: 0.2-20.0%, Ta: 0.1-3.0%, V: 2.0-25.0%, Nb: 0.1%. 5-5.0%, Si: 0.1-1.0%, Fe: 0.05-2.0%, Cr: 0.2-5.0%, Cu: 0.3-2.0%, Co: 0.05-2.0%, Ni: 0.1-1.0%, Platinum group elements: 0.02-0.4%, Rare earth elements: 0.001-0.3%, B: 0.1-5.0%, and Mn: 0.2-8.0%. More preferably, it contains one or more of the following: O: 0.03–0.3%, N: 0.01–0.1%, C: 0.01–0.5%, Al: 0.4–5.0%, Sn: 0.2–3.0%, Zr: 0.5–5.0%, Mo: 0.5–15.0%, Ta: 0.2–2.0%, V: 5.0–20.0%, and Nb: 0.2–2.0%. One or more of the following: Si: 0.15–0.8%, Fe: 0.1–1.0%, Cr: 0.2–3.0%, Cu: 0.3–1.5%, Co: 0.1–1.0%, Ni: 0.1–0.8%, platinum group elements: 0.03–0.2%, rare earth elements: 0.001–0.1%, B: 0.2–3.0%, and Mn: 0.2–5.0%.

[0079] Here, as platinum group elements, specifically Ru, Rh, Pd, Os, Ir, and Pt can be listed, and one or more of these can be included. When two or more platinum group elements are included, the content of the aforementioned platinum group elements refers to the total amount of platinum group elements. Additionally, as rare earth elements (REM), specifically Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu can be listed, and one or more of these can be included. When two or more rare earth elements are included, mixtures or compounds of rare earth elements, such as cerium-lanthanum alloys (Mm) and neodymium-praseodymium alloys, can be used. Furthermore, when two or more rare earth elements are included, the aforementioned rare earth element content refers to the total amount of rare earth elements.

[0080] Next, the manufacturing method of the processed titanium material according to this embodiment will be described.

[0081] In the manufacturing method of this embodiment, a step is included in which a pressing body is pressed into the surface of a titanium billet to form a plurality of first grooves on the surface of the titanium billet. Generally, when attempting to apply strain to an ingot using forging, large-diameter rollers, etc., no metal flow occurs at the part in contact with the mold, resulting in a region known as dead zone metal. The strain of this dead zone metal region is small, so if strain is applied using forging or large-diameter rollers, the strain is not introduced to the surface layer, but rather to the interior, making it impossible to form a fine-grained structure in the surface layer. On the other hand, when applying strain by means of impact energy from impacts using protrusions, as described in Patent Document 2, strain can be applied to the surface layer, thus enabling the surface layer to form a fine-grained structure. However, in this method, it sometimes takes a long time to stably apply strain to the entire surface. Furthermore, for high-strength materials, the impact energy does not conduct to the interior, and sometimes it is not possible to ensure the required thickness of the fine-grained structure.

[0082] Therefore, the inventors have studied a method to prevent the formation of coarse grains by effectively and uniformly applying strain to the surface of a titanium billet, thereby preventing the formation of dead zone metal, and have found that if the following method is used for treatment, strain can be effectively applied to the surface.

[0083] The manufacturing method of the processed titanium material according to this embodiment will be described in detail below.

[0084] The manufacturing method of this embodiment is as follows: Figure 4 As shown, this is a method for manufacturing a processed titanium material in which a plurality of first grooves are formed on the surface of a titanium blank 10. The method includes a step (first step) of pressing a first pressing body 51 into the surface of the titanium blank 10. The first pressing body 51 has an arc-shaped first pressing surface 51a extending along a predetermined direction. This embodiment illustrates an example using a round bar (a bar with a circular cross-section orthogonal to the extending direction of the first pressing surface 51a).

[0085] Regarding the pressing surface 51a of the first pressing body 51, in a cross-section orthogonal to the extending direction of the first pressing surface 51a, the radius of curvature (mm) of the aforementioned pressing surface is 2.5 mm or more and 17.5 mm or less. If the radius of curvature is too small, the equivalent strain at a depth of 3 mm will be small. In addition, the processing time will be longer. Therefore, the radius of curvature is set to 2.5 mm or more. Preferably, the lower limit is 5.0 mm. On the other hand, if the radius of curvature is too large, the dead zone metal portion will be large, and sufficient strain cannot be imparted to the surface layer of the titanium blank, resulting in a small equivalent strain at a depth of 3 mm. Therefore, the radius of curvature is set to 17.5 mm or less. Preferably, the upper limit is 15 mm.

[0086] Here, the pressing body that can be used as the first pressing body is not limited in cross-sectional shape, as long as it has an arc-shaped pressing surface at least in the portion that contacts the titanium blank 10. For example, except Figure 4 In addition to the cylindrical rod-shaped pressing body 51 with a circular cross-sectional shape shown in (a), for example, as the first pressing body, a material such as... Figure 5 As shown in (a), the lower part (the portion in contact with the titanium blank 10) has an arc-shaped first pressing surface 52a extending along a predetermined direction, and the upper part has a cubic (rectangular cross-sectional) rigid pressing body 52. ​​This shape of pressing body 52 is particularly useful for rods with small radii of curvature, long rods, etc. In other words, this is because: by increasing the size of the rectangular rigid body located at the upper part, the section modulus can be increased, thereby improving the rigidity of the rod. Furthermore, as the first pressing body, for example... Figure 6 As shown in (a), the pressing body 53 can be a pressing body 53 with multiple pressing surfaces 53a at the bottom. While this shape of pressing body 53 has the disadvantage of increased pressing load, it allows multiple grooves to be formed simultaneously on the surface of the titanium blank 10, thus improving production efficiency. It should be noted that by increasing the load-bearing capacity of the stamping press and further increasing the size of the upper rectangular rigid body, etc., the pressing body 53 can achieve the following... Figure 7 As shown, a pressing body 54 with more pressing surfaces 54a at the bottom can be used. Using a pressing body 54 can reduce the number of pressing operations and improve production efficiency.

[0087] Here, the first process needs to satisfy equations (1) and (2) below. The following mainly focuses on the use of... Figure 4 The following explanation uses the pressing body as an example.

[0088] 0.5≤X1≤R1×(1-cosθ1) (1)

[0089] 1.0≤Y1≤(-0.16R1 2 +4.4R1)×(0.25X1+0.037) (2)

[0090] In the above formula,

[0091] θ1 is 50°.

[0092] R1 is the radius of curvature (mm) of the first pressing surface in the aforementioned first cross section.

[0093] X1 represents the pressing amount (mm) of pressing the aforementioned titanium blank into the aforementioned first pressing surface.

[0094] Y1 is the distance (mm) between adjacent pressing positions of the first pressing surface in a direction orthogonal to both the extension direction of the first pressing surface and the pressing direction of the first pressing body.

[0095] The pressing amount X1 of pressing the first pressing surface 51a into the titanium blank 10 is Figure 4 The distance indicated by the symbol X in (b) is the distance between the surface of the processed titanium material and the bottom of the groove in the thickness direction of the titanium billet 10. If the pressing amount X1 is too small, sufficient strain cannot be applied to the surface, and the processing time becomes longer. Therefore, the pressing amount X1 is set to 0.5 mm or more. A lower limit of 1.0 mm is preferred. On the other hand, if the pressing amount X1 is too large, ... Figure 2 In this process, the angle θ between the inner surfaces of grooves 1b, 2b, and 3b and surfaces 1a, 2a, and 3a becomes too large, resulting in defects such as overlap defects. Therefore, the pressing amount X1 is set to be R1×(1-cosθ1) or less. The preferred upper limit is 0.29×R1.

[0096] Interval Y1 is Figure 4 The distance indicated by the symbol Y in (b) is the distance between adjacent pressing positions of the first pressing surface 51a in a direction orthogonal to both the extending direction of the first pressing surface 51a and the pressing direction of the first pressing body 51. In this respect, in a cross-section parallel to the first section of the manufactured processed titanium material 1, it is equivalent to the distance between the bottom of any first groove and the bottom of another first groove adjacent to the aforementioned arbitrary first groove. If the interval Y1 is too small, the processing time becomes longer; therefore, it is set to 1.0 mm or more. A lower limit of 5.0 mm is preferred. On the other hand, if the interval Y1 is too large, sufficient strain cannot be applied to the surface layer. Therefore, the interval Y1 is set to (-0.16R1). 2 Below +4.4R1)×(0.25X1+0.037).

[0097] When the titanium billet is slab 1 and large steel billet 2, such as Figure 1 As shown, the surfaces 1a and 2a with the largest area in the titanium billet become the rolling surfaces. Therefore, it is sufficient to press the pressing body 51 into these surfaces to form a groove. When the titanium billet is a small steel billet, the entire surface extending along its length can become the rolling surface. Therefore, in, for example... Figure 3 In the case of a small steel billet 3 with a rectangular cross-section, ideally, a groove should be formed on its entire surface to introduce strain into the entire surface.

[0098] The following section details the processing method using a round rod as the pressing body. It should be noted that in the following description, the round rod is used as the first pressing body or, further, as the second pressing body. Figure 1 The manufacturing method of slab 1 of (a) will be illustrated by example. Figure 8 These figures illustrate the first pressing step (first step) in the manufacturing method of the processed titanium material according to this embodiment. (a) is a plan view, and (b) is a side view. Additionally, Figure 9 The diagram illustrates the second pressing step (second step) in the manufacturing method of processed titanium material according to other embodiments. (a) is a plan view, and (b) is a side view. It should be noted that the second step is not a mandatory step.

[0099] As a method for forming groove-shaped indentations on the surface of a billet by pressing in a round bar, firstly, a first process is repeatedly performed to form multiple groove-shaped indentations 1c on the surface 1a of the billet 1. The first process is as follows: Figure 8 In this manner, a pressing body (round bar) 5 is placed on the slab 1, and a force F is used to press the round bar 5 from the surface of the slab 1 into the thickness direction. After the load is removed, the round bar 5 is moved along a specified direction. Figure 8 The process involves moving the slab 1 (within its length direction) and similarly using force F to press the round bar 5 from the surface of the slab 1 towards its thickness, thus removing the load. It should be noted that in this specification, this operation is sometimes referred to as "pressing while moving." By performing this operation, the desired strain can be applied to the surface of the titanium billet. There is no limit to the number of presses. For example, it is also possible to use... Figures 4-6 The pressing bodies 51, 52, and 53 shown repeatedly undergo the processes of pressing in, unloading, moving, and pressing in. Additionally, in Figure 8 The example shown illustrates moving the round bar 5 along a predetermined direction, but this configuration is not limited to this. If the round bar 5 is moved and pressed in along the predetermined direction, then moved and pressed in the opposite direction, multiple grooves are formed side-by-side on the surface of the titanium blank 10, and the direction of movement is not limited. Moving the round bar 5 along the predetermined direction results in good production efficiency. Furthermore, methods such as... Figure 7 As shown, the pressing body 54 is a planar pressing body with more pressing surfaces 54a at the bottom. If this planar pressing body 54 is used, the number of pressing times of the pressing body can be reduced (for example, once), which can improve production efficiency.

[0100] After performing the first process on the entire surface 1a, the second process is then repeated to form multiple grooves 1b. The second process is as follows: Figure 9 As shown, the round bar 5 is pressed into the groove formed in the first stage by force F from the surface of the slab 1 towards the thickness direction, and the load is removed, so that the round bar 5 is moved along the specified direction ( Figure 9 The blank 1 is moved along its width, and the pressing and unloading processes are performed using force F. In this embodiment, the pressing process is described as being performed twice, but it can be performed three or four times, or multiple times as long as the blank itself does not break. The more pressing cycles, the higher the equivalent strain, which allows for further refinement of the microstructure, and is therefore preferred.

[0101] The second pressing body has an arc-shaped pressing surface in the portion that contacts the surface of the titanium blank 10. In a second cross-section orthogonal to the axial direction, the radius of curvature (mm) of the aforementioned pressing surface is 2.5 mm or more and 17.5 mm or less. The reason for this is the same as the reason for limiting the radius of curvature of the first pressing body. Furthermore, as long as the pressing body used as the second pressing body has an arc-shaped pressing surface at least in the portion that contacts the titanium blank 10, its cross-sectional shape is not limited. This is the same as for the first pressing body.

[0102] Here, the second process must satisfy equations (3) and (4) below. The following mainly focuses on the use of... Figure 4 The following explanation uses the pressing body as an example.

[0103] 0.5≤X2≤R2×(1-cosθ2) (3)

[0104] 1.0≤Y2≤50.0 (4)

[0105] In the above formula,

[0106] θ2 is 50°.

[0107] R2 is the radius of curvature (mm) of the second pressing surface in the aforementioned second section.

[0108] X2 represents the pressing amount (mm) of pressing the aforementioned second pressing surface into the aforementioned titanium blank.

[0109] Y2 is the distance (mm) between adjacent pressing positions of the aforementioned second pressing surface in a direction orthogonal to both the extension direction of the aforementioned second pressing surface and the pressing direction of the aforementioned second pressing body.

[0110] The pressing amount X2 of pressing the second pressing surface 51a into the titanium blank 10 is the same as the pressing amount X1 of the first pressing body. Figure 4 The distance indicated by the symbol X in (b) is the distance between the surface of the processed titanium material 1 and the bottom of the groove in the thickness direction. For the same reason as the pressing amount X1 of the first pressing body, the pressing amount X2 can be set to 0.5 mm or more, preferably with a lower limit of 1.0 mm. On the other hand, for the same reason as the pressing amount X1 of the first pressing body, the pressing amount X2 is set to R2×(1-cosθ2) or less. Preferably, the upper limit is 0.29×R1.

[0111] Interval Y2 is Figure 4The distance indicated by the symbol Y in (b) is, like the interval Y1 of the first pressing body, the distance between adjacent pressing positions of the second pressing surface 51a in a direction orthogonal to both the extending direction of the second pressing surface 51a and the pressing direction of the second pressing body 51. In this respect, in a cross-section of the manufactured processed titanium material 1 parallel to the second cross-section, it is equivalent to the distance between the bottom of any second groove and the bottom of another second groove adjacent to the aforementioned arbitrary second groove. For the same reason as the interval Y1 of the first pressing body, the interval Y2 can be set to 1.0 mm or more, preferably with a lower limit of 5.0 mm. It is acceptable to perform the second process on a surface that has already undergone the first process, even if the interval Y1 is wider than that of the first process. However, in order to apply sufficient strain to the surface layer, the interval Y2 can be set to 50.0 mm or less. The interval Y2, like the interval Y1 of the first pressing body, is preferably set to (-0.16R1). 2 Below +4.4R1)×(0.25X1+0.037).

[0112] Here, in the second process, when multiple grooves (second grooves) are formed extending in the same direction as the groove (first groove) formed in the first process, the strain, especially near the surface, becomes very small, potentially preventing the formation of a fine microstructure during hot rolling. Therefore, if the second process is performed following the first process, the pressing process can be performed by forming multiple second grooves extending in a direction different from the extension direction of the first groove. That is, in Figure 8 In the first process shown, the round bar (first pressing body) 5 is pressed in by moving along the length direction of the slab 1 to form a groove-shaped indentation (groove) 1c extending along the width direction of the slab 1. Figure 9 In the second process shown, the round bar (second pressing body) 5 is moved along the width direction of the slab 1 in an orthogonal manner, and pressed in such a way that a groove 1b is formed extending along the length direction of the slab 1. By using this method to form the groove 1b, strain (equivalent strain) can be stably applied to the surface layer. Furthermore, by applying strain from different directions, the aggregate structure will not develop during hot rolling heating, and the generation of surface defects can be suppressed. It should be noted that the angle formed by the extension direction of the first groove and the extension directions of the plurality of second grooves is as follows: Figure 9 As shown, the angle can be 90°, and there is no particular limitation as long as it exceeds 0°. However, in order to stably apply sufficient strain to the surface layer, it is preferable to set the angle in the range of 30° to 90°.

[0113] The above explanation primarily focuses on the method of forming grooves using a pressing surface extending in a straight line. However, this form is not limited to any method that can stably apply strain (equivalent strain) to the surface. For example, Figure 10As shown, a pressing body whose pressing surface bends midway can also be used to form a surface groove 10b in the titanium blank 10. In this case, observe the cross-section orthogonal to the extension direction of the pressing surface (using... Figure 10 In the cross-section indicated by the arrow view, when the first groove satisfies equations (1) and (2) above, and the second process is performed, when the second groove satisfies equations (3) and (4) above, the effects of the present invention can be obtained. Furthermore, the multiple grooves formed by the first or second process are preferably arranged, but they do not necessarily need to be parallel. In particular, non-parallel portions may exist. In this case, when the first groove of the observed portion satisfies equations (1) and (2) above, or when the second process is performed, when the observed portion satisfies equations (3) and (4) above, the effects of the present invention can be obtained. Furthermore, a pressing body with pressing surfaces intersecting in an X-shape can also be used. In any case, the first and second grooves may not be formed on the entire surface of the processed titanium material.

[0114] Figure 11 The log-normal distribution of the crystal grain size of the recrystallized layers of No.2 (pressed once, large diameter round bar), No.18 (pressed once, small diameter round bar), and No.16 (pressed twice) in the embodiments described later is shown. Figure 11 The horizontal axis represents the crystal grain size (natural logarithm ln), and the vertical axis represents the probability of occurrence (%). From Figure 11 It is also clearly known that when the pressing process is performed once, if a large-diameter round bar (radius of curvature: 30mm) is used as the pressing body, the log-normal distribution has a wide distribution range (large standard deviation σ), resulting in uneven crystal grain size. On the other hand, it is also known that when the pressing process is performed once, if a small-diameter round bar (radius of curvature: 5mm) is used as the pressing body, the log-normal distribution has a narrow distribution range (small standard deviation σ), resulting in more uniform crystal grain size. Furthermore, it is known that when two pressing processes are performed, the log-normal distribution has an even narrower distribution range (smaller standard deviation σ), resulting in more uniform crystal grain size. That is, by using a pressing body with a small radius of curvature for the pressing process, and then performing two or more pressing processes, the strain near the surface becomes very small, achieving finer and more uniform surface structure. As a result, the generation of surface defects can be significantly reduced.

[0115] The pressing process can be performed by cold rolling without heating the titanium billet, or it can be performed after heating the titanium billet to a temperature range below 500°C. The above heating temperature is acceptable up to 650°C depending on the chemical composition.

[0116] In this embodiment, strain is applied to the surface of the titanium material being processed, known as the rolled surface, by cold rolling to warm rolling. To reduce surface defects generated during hot rolling, a recrystallized structure reaching a certain depth needs to be formed. Especially for high-hardness titanium billets, strain is difficult to penetrate into the interior of the billet, requiring a large load to form grooves in order to apply strain deep into the surface layer. However, recent findings indicate that applying strain reduces ductility near the surface, leading to surface cracks. Therefore, to stably apply strain deep into the surface and improve surface ductility, it is effective to increase the temperature to some extent to reduce the strength of the titanium billet itself. On the other hand, for low-strength titanium billets, concentrating strain at the surface layer allows for a finer surface microstructure; therefore, applying strain at room temperature is preferable.

[0117] On the other hand, if the pressing process is performed at temperatures exceeding 500°C, the strain applied during processing may disappear immediately, preventing recrystallization during subsequent heating. Furthermore, at temperatures exceeding 500°C, an oxide coating may sometimes form on the surface of the titanium billet. This oxide coating, when pressed during processing, can create surface defects, which may be exacerbated during subsequent hot rolling. Therefore, depending on the chemical composition, temperatures up to 650°C are acceptable, but 500°C is preferably set as the upper limit.

[0118] Furthermore, the temperature range at which the strength and ductility of titanium billets increase varies depending on the alloy type; therefore, it is not simply a matter of operating at higher temperatures. For example, in industrial pure titanium, twinning, a key mechanism for titanium deformation, actively occurs near room temperature. This twinning ceases at temperatures around 400–500°C, resulting in reduced ductility compared to room temperature and a higher susceptibility to cracking. On the other hand, in alloy systems containing a large amount of Al, twinning is almost nonexistent even near room temperature; therefore, heating to below 500°C ensures ductility. Additionally, setting the titanium billet to a high temperature drastically reduces material strength, leading to excessively large groove depths (groove undulations) during plastic deformation, potentially causing surface defects. Therefore, it is advisable to select a temperature range that prevents surface cracking after rolling and yields a suitable recrystallized structure and surface condition. The lower limit of the surface temperature of the titanium billet during the pressing process is preferably set to 0°C.

[0119] As described above, in the manufacturing method of this embodiment, a round bar is actually pressed into the surface of a titanium billet, causing it to undergo physical plastic deformation to form a groove. As a result, regardless of the crystal orientation, strain can be stably introduced into the surface layer of the billet, thus enabling the uniform dispersion of fine grains on the surface. Based on this, if the round bar pressing process is performed multiple times under specified conditions, strain can be effectively and sufficiently introduced into the bottom of the groove, and fine recrystallization can be formed on the surface layer by subsequent heating during hot rolling, thereby suppressing the generation of surface defects.

[0120] By applying the present invention to the processing of titanium materials, surface defects after hot rolling are significantly suppressed. By applying the present invention to cuboid or cylindrical ingots (solidified structure in the casting state), the following effects can be achieved without initial rolling processes: surface defects can be suppressed to a problem-free level when hot-rolled into plates, strips, coils, or bars.

[0121] In this way, the processed titanium material manufactured according to this embodiment is not only suitable for hot rolling, but the hot-rolled material manufactured by hot rolling will have the following effect: surface defects are significantly suppressed, and then solid products can be manufactured even if cold rolling is performed.

[0122] In summary, according to this embodiment, even if the initial rolling process of the ingot is omitted and the titanium billet is in a cast state, the surface defects generated during hot rolling can be slight, and excellent hot-rolled and cold-rolled products can be provided.

[0123] Furthermore, when this embodiment is applied to titanium billets that have undergone a preliminary rolling process, the surface defects generated during hot rolling are extremely minor. As a result, the descaling process of hot-rolled plates and bars, and the yield of the final products, can be further improved.

[0124] Example

[0125] The present invention will now be described in more detail through examples.

[0126] <Example 1>

[0127] Electron beam melting (EBR) or plasma arc melting (PAM) is used to cast slabs (titanium billets) with the chemical composition shown in Table 1 and exhibiting a length of 1050 mm wide × 250 mm thick × 6000 mm. The pressing process shown in Table 2 is then performed on the cast titanium billets. In the examples shown in Nos. 6, 9, 13, and 16, the following methods are used: Figure 5The pressing body shown is a round bar, as used in other examples. In each of the first to fourth steps, the following operation is repeated: the pressing body is pressed into the surface of the titanium billet and the load is removed; then, the pressing body is moved and pressed into the surface of the titanium billet at that position, thereby forming multiple grooves on the surface of the titanium billet.

[0128] In Table 2, "radius of curvature of pressing surface" refers to the radius of curvature of the pressing surface of the pressing body (mm), "pressing amount" refers to the pressing amount (mm) of the titanium billet pressed into the pressing surface, "interval" refers to the distance (mm) between adjacent pressing positions of the pressing surface in a direction orthogonal to the extension direction of the pressing surface and the pressing direction of the pressing body, and "direction" refers to the angle formed by the extension direction of the groove formed by the first process and the extension direction of the groove formed by each process.

[0129] [Table 1]

[0130] Table 1

[0131]

[0132] [Table 2]

[0133]

[0134] Next, plastic deformation is performed as described above, and the groove angle of the machined titanium material with grooves is measured. The Vickers hardness of the machined titanium material is measured according to the following steps, and the hardness difference ΔHV is calculated.

[0135] First, the cross-section obtained by cutting the titanium material, including the grooved surface, is mirror-polished. At seven locations—3 mm from the bottom of the groove and at half the thickness of the processed titanium material—a Vickers hardness tester is used with a 1 kg load. The average of the five points, excluding the maximum and minimum hardness values, is calculated. Furthermore, the hardness difference (ΔHV) between the locations 3 mm from the bottom of the groove and half the thickness of the processed titanium material is determined.

[0136] Next, the mean circular equivalent diameter and standard deviation of the recrystallized structure (recrystallized layer) in the area from the bottom of the tank to a depth of 3 mm after heating at 800°C for 4 hours were determined according to the following steps.

[0137] First, the pre-hot-rolled titanium material was heat-treated by heating at 800°C for 4 hours in an Ar atmosphere. Next, the cross-section of the heat-treated titanium material, including the grooved surface, was chemically ground, and electron backscatter diffraction (EBSD) was used to measure approximately 2-10 fields of view in a 5mm × 5mm region with a step size of 5-20 μm. Subsequently, regarding the grain size, the spherical equivalent grain size (area A = π × (grain size D / 2)) was calculated based on the grain area A measured using EBSD. 2 The standard deviation σ in the log-normal distribution is calculated based on the crystal grain size distribution.

[0138] In addition, the “thickness of recrystallized layer (mm)” in the table shall be measured as follows.

[0139] First, the thickness of the recrystallized layer was measured while observing the cross-section obtained by cutting the heat-treated titanium material, including the surface with grooves, using EBSD. At this time, the area near the surface of the processed titanium material with a crystal grain size that is finer than the average crystal grain size at half the thickness of the processed titanium material was defined as the "recrystallized layer", and the thickness of this layer was defined as the "thickness of the recrystallized layer" and measured.

[0140] Next, the aforementioned plastic deformation was performed, and the grooved processed titanium material was inserted into a furnace at 820°C and heated for approximately 240 minutes. A 5mm thick hot-rolled sheet was then manufactured using a continuous hot-rolling strip mill and coiled. The hot-rolled sheet was then shot-peened and subsequently passed through a continuous pickling line using nitric acid and hydrofluoric acid to perform flame surface cleaning on approximately 50μm of each side. Afterward, both rolled surfaces were visually inspected to evaluate the presence of surface defects.

[0141] The results are shown in Table 3. In Table 3, “groove angle” refers to the angle (°) between the inner surface of the groove and the surface of the processed titanium material in a cross section orthogonal to the extension direction of the groove, and “hardness difference” refers to the difference (ΔHv) between the Vickers hardness at a position 3 mm from the bottom of the groove and the Vickers hardness at a position 1 / 2 of the thickness.

[0142] Regarding the evaluation of surface defects, for the rolled surface of hot-rolled plates after passing through a continuous pickling line, the number of surface defects larger than 10mm per 1m is considered to be... 2 A number exceeding 0.3 is considered unacceptable (Evaluation D), while fewer than 0.3 are considered acceptable (Evaluation A to C). The number of surface defects per 1m... 2Cases with fewer than 0.05 are rated A, cases with more than 0.05 but less than 0.2 are rated B, and cases with more than 0.2 but less than 0.3 are rated C.

[0143] [Table 3]

[0144] Table 3

[0145]

[0146] As shown in Tables 1-3, No.1 has a small radius of curvature of 1.5 mm for the pressing surface. As a result, the groove angle between the inner surface of the groove and the surface of the processed titanium material becomes sharp, and large surface defects frequently occur on the surface of the hot-rolled plate after hot rolling and pickling.

[0147] No. 2 Because the radius of curvature of the pressing surface is as large as 30mm, a sufficient hardness difference cannot be obtained. As a result, the crystal grain size of the recrystallized layer is large, and the log-normal distribution has a wide range (large standard deviation σ), resulting in uneven crystal grain size (also refer to...). Figure 11 Therefore, surface defects occur frequently.

[0148] The radius of curvature and indentation of the pressing surface in No. 3 are appropriate, but the spacing is too large. As a result, the crystal grain size of the recrystallized layer is large. In addition, the log-normal distribution has a wide distribution range (large standard deviation σ), and the crystal grain size is not uniform. Therefore, surface defects occur frequently.

[0149] While the radius of curvature and spacing of the pressing surface in No. 4 are appropriate, the indentation is too small, failing to achieve sufficient hardness difference. As a result, the recrystallized layer has large grain size, and the log-normal distribution has a wide range (large standard deviation σ), leading to uneven grain size. Consequently, surface defects occur frequently.

[0150] On the other hand, in Nos. 5 to 27, at least in the first process, the radius of curvature, pressing amount, and spacing of the pressing surface are appropriate, the hardness difference ΔHV of the processed titanium material is sufficiently large, and the grain size of the recrystallized layer is sufficiently reduced and made uniform. As a result, in these examples, the surface properties of the hot-rolled and pickled hot-rolled plates are good.

[0151] <Example 2>

[0152] Electron beam melting (EBR) was used to cast a slab (titanium billet) with the chemical composition shown in Table 4 and a length of 1050 mm wide × 250 mm thick × 5500 mm. The pressing process shown in Table 5 was performed on the cast titanium billet. In any example, a round bar presser was used. In each of the first and second processes, the following operation was repeated: the presser was pressed into the surface of the titanium billet and the load was removed; subsequently, the presser was moved and pressed into the surface of the titanium billet at that position, thereby forming multiple grooves on the surface of the titanium billet. The meanings of the terms in Table 5 are the same as in Table 2.

[0153] [Table 4]

[0154] Table 4

[0155]

[0156] [Table 5]

[0157] Table 5

[0158]

[0159] The underlined part indicates that it is outside the scope of this invention.

[0160] The hardness difference ΔHV, the roundness equivalent average grain size, the standard deviation, and the evaluation of surface defects were performed in the same manner as in <Example 1>. The results are shown in Table 6.

[0161] [Table 6]

[0162] Table 6

[0163]

[0164] The underlined part indicates that it is outside the scope of this invention.

[0165] In examples No. 28–36, at least in the first process, the radius of curvature, indentation amount, and spacing of the pressing surface are appropriate, the hardness difference ΔHV of the processed titanium material is sufficiently large, and the grain size of the recrystallized layer is sufficiently reduced and made uniform. As a result, in these examples, the surface properties of the hot-rolled and pickled hot-rolled plates are good.

[0166] Explanation of reference numerals in the attached figures

[0167] 1, 2, 3… Processing titanium materials (slabs, large billets, small billets)

[0168] 10…Titanium billet

[0169] 1b, 2b, 3b... slots

[0170] 5… Pressing body (round rod)

[0171] 51, 52, 53, 54… Pressing body

[0172] 51a, 52a, 53a, 54a… Pressing surfaces

Claims

1. A method for manufacturing hot-rolled titanium material, wherein, The hot-rolled titanium material is a hot-rolled titanium material in which multiple first grooves are formed on the surface of a titanium billet. The manufacturing method includes a first step of pressing a first pressing body into the surface of the titanium blank, the first pressing body having an arc-shaped first pressing surface extending along a predetermined direction. The radius of curvature of the first pressing surface in the first cross-section orthogonal to the extension direction of the first pressing surface is 2.5 mm or more and 17.5 mm or less. The first process satisfies the following equations (1) and (2). 0.5≤X1≤R1×(1-cosθ1) (1) 1.0≤Y1≤(-0.16R1 2 +4.4R1)×(0.25X1+0.037) (2) In the above formula, θ1 is 50°. R1 is the radius of curvature of the first pressing surface in the first cross-section, and its unit is mm. X1 represents the amount of material pressed into the first pressing surface of the titanium blank, expressed in mm. Y1 is the distance between adjacent pressing positions of the first pressing surface in a direction orthogonal to both the extension direction of the first pressing surface and the pressing direction of the first pressing body, and its unit is mm.

2. The method for manufacturing hot-rolled titanium material according to claim 1, wherein, The first process repeatedly performs the following actions: pressing the first pressing body into the surface of the titanium blank, and then moving and pressing the first pressing body in such a way that the pressing position of the first pressing surface satisfies the formula (2).

3. The method for manufacturing hot-rolled titanium material according to claim 1 or 2, wherein, A plurality of second grooves are formed on the surface of the titanium blank having the plurality of first grooves, the second grooves extending in a direction different from the extending direction of the first grooves. The manufacturing method includes a second step of pressing a second pressing body into the surface of the titanium blank having the plurality of first grooves formed thereon, the second pressing body having an arc-shaped second pressing surface extending along a predetermined direction. In the second section orthogonal to the extension direction of the second pressing surface, the radius of curvature of the second pressing surface is 2.5 mm or more and 17.5 mm or less. The second process satisfies the following equations (3) and (4). 0.5≤X2≤R2×(1-cosθ2) (3) 1.0≤Y2≤50.0 （4） In the above formula, θ2 is 50°. R2 is the radius of curvature of the second pressing surface in the second cross section, and its unit is mm. X2 represents the amount of material pressed into the titanium blank onto the second pressing surface, expressed in mm. Y2 is the distance between adjacent pressing positions of the second pressing surface in a direction orthogonal to both the extension direction of the second pressing surface and the pressing direction of the second pressing body, and its unit is mm.

4. The method for manufacturing hot-rolled titanium material according to claim 3, wherein, The second process repeatedly performs the following actions: pressing the second pressing body into the surface of the titanium blank, and then moving and pressing the second pressing body in such a way that the pressing position of the second pressing surface satisfies the formula (4).

5. The method for manufacturing hot-rolled titanium material according to claim 3, wherein, The angle formed by the extension direction of the first groove and the extension direction of the second groove is greater than 0° and less than 90°.

6. The method for manufacturing hot-rolled titanium material according to claim 4, wherein, The angle formed by the extension direction of the first groove and the extension direction of the second groove is greater than 0° and less than 90°.

7. The method for manufacturing hot-rolled titanium material according to claim 3, wherein, The first pressing body and the second pressing body may be the same or different.

8. The method for manufacturing hot-rolled titanium material according to claim 4, wherein, The first pressing body and the second pressing body may be the same or different.

9. The method for manufacturing hot-rolled titanium material according to claim 5, wherein, The first pressing body and the second pressing body may be the same or different.

10. The method for manufacturing hot-rolled titanium material according to claim 6, wherein, The first pressing body and the second pressing body may be the same or different.

11. A method for manufacturing hot-rolled titanium material according to any one of claims 1, 2, 4 to 10, wherein, The first and / or second processes are performed at a temperature where the surface temperature of the titanium billet is above 0°C and below 500°C.

12. The method for manufacturing hot-rolled titanium material according to claim 3, wherein, The first and / or second processes are performed at a temperature where the surface temperature of the titanium billet is above 0°C and below 500°C.

13. A hot-rolled titanium material, manufactured by the manufacturing method according to any one of claims 1 to 12, In the thickness direction of the hot-rolled titanium material, the difference ΔHV between the Vickers hardness at a position 3 mm deep from the bottom of the first groove and the Vickers hardness at a position half the thickness is 20 or more.

14. The hot-rolled titanium material according to claim 13, wherein, When heat treatment is performed at 800°C for 4 hours, grains with a round equivalent average grain size of 1.00 mm or less are formed in the thickness direction of the hot-rolled titanium material, from the bottom of the first groove to a depth of 3.0 mm, and the standard deviation of the round equivalent grain size of the grains with respect to the logarithmic transformation value is 1.00 or less.

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