Alpha+beta titanium alloy ingot for hot working

Abstract

The α+β titanium alloy ingot for hot working has the following chemical composition: 2.5 to 8.0% of Al, 0.5 to 3.0% of Fe, and O in an amount satisfying 0.02% ≤ [O] ≤ ([Al] - [Fe] - 0.5×[Mo] - 0.5×[Nb] + 1.0) / 100 ([X] represents the content of element X when the unit is mass%), the balance being Ti and impurities, the ratio L / S of the circumference L (mm) of the cross section perpendicular to the length direction, i.e., the cross section, to the area S (mm 2 ) of the cross section is 0.010 or more, the average grain diameter D of the casting structure at a position 10 mm from the surface toward the center axis of the α+β titanium alloy ingot in the length direction satisfies D ≤ 10 mm and D ≤ L / 100 in a portion of 20 to 80% of the total length from one of the two end surfaces in the length direction of the α+β titanium alloy ingot toward the other end surface and relative to the length direction, and the thickness of the α+β titanium alloy ingot is 80 mm or more.

Description

Technical Field

[0001] This invention relates to α+β type titanium alloy ingots for hot working. Background Technology

[0002] The manufacturing process of α+β type titanium alloy bars generally includes: a process of melting and casting titanium alloy ingots using sponge titanium, master alloys, titanium scrap, etc. as molten raw materials; a billet-making process of heating the titanium alloy ingots to the β domain for forging or rolling; and a rolling process of heating to the β domain or α+β domain to form bars. On the other hand, to reduce costs, the billet-making process can be omitted, and the titanium alloy ingots can be directly rolled to manufacture bars.

[0003] For example, Patent Document 1 discloses a method for manufacturing titanium ingots, comprising: a compression forming step, wherein one or more selected from sponge titanium and titanium waste are compressed and formed with a by-product containing necessary elements for adjusting the chemical composition to obtain a titanium block; and a melting step, wherein an electron beam is irradiated onto the surface of the titanium block under a reduced pressure of 1 Pa or less, causing the titanium block to completely melt and form a titanium ingot. In the titanium ingot manufacturing method described in Patent Document 1, the melting step comprises: irradiating an electron beam onto any surface of the titanium block to melt a portion of it from its surface in the thickness direction; and irradiating an electron beam onto any other surface to melt at least the unmelted titanium block. In the titanium ingot manufacturing method described in Patent Document 1, a plate-shaped titanium ingot with a thickness of 7 to 80 mm is manufactured, or a cylindrical shape having a circular cross-section perpendicular to the length direction with a diameter of 10 to 80 mm, or a pentagonal or larger polygonal shape with an equivalent circle diameter of 10 to 80 mm is manufactured.

[0004] In addition, titanium alloy powder, made from titanium alloy rods, is used as a material for creating shapes using 3D printers.

[0005] Patent document 2 exemplifies a technology that uses metal powder as a material and employs a 3D printer for shaping.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2019 / 26251

[0009] Patent Document 2: Japanese Patent Application Publication No. 2017-222899 Summary of the Invention

[0010] The problem the invention aims to solve

[0011] However, Patent Document 1 discloses a preferred manufacturing method for thin titanium ingots with a thickness of 80 mm or less. The technology described in Patent Document 1 involves irradiating the surface of a titanium block with an electron beam to melt the titanium block, followed by solidification of the molten titanium block to manufacture the ingot. When manufacturing large titanium ingots by irradiating the surface of a titanium block with an electron beam, it is necessary to increase the melting depth of the titanium block produced by the electron beam. However, increasing the melting depth of the titanium block produced by the electron beam results in a slower cooling rate after melting, leading to coarser grains. When the grains are coarse, cracks or voids may sometimes occur when rolling the titanium alloy ingot into bars, resulting in deep defects on the surface of the bars. Therefore, the technology described in Patent Document 1 has room for improvement.

[0012] When the billet-making process is omitted and the ingot is directly hot-rolled to produce bars, a problem arises where the coarse casting structure leads to poor surface properties and reduced yield. In α+β type titanium alloys, especially those containing Al and O as α-stabilizing elements and Fe as a β-stabilizing element, due to the rapid diffusion rate of Fe and its low solid solubility limit in the α phase, a film-like α phase sometimes precipitates at the β grain boundaries during the phase transformation from the β phase to the α+β phase. This film-like α phase precipitated at the β grain boundaries can sometimes become the initiation point for cracks in the α+β type titanium alloy bars, resulting in a reduced yield. Therefore, to omit the billet-making process, it is necessary to manufacture ingots similar to those obtained by rolling titanium alloy ingots, resulting in a reduced depth of surface defects. It should be noted that the film-like α phase precipitated at the β grain boundaries will sometimes be referred to as the grain boundary α phase.

[0013] Furthermore, when titanium alloy rods have deep defects, in addition to the reduced yield and increased manufacturing costs due to defect removal, the chemical composition of titanium alloy powder made from the defective rods can sometimes deviate. Titanium alloy powder for 3D printers can be manufactured by melting titanium alloy rods and using gas atomization, for example. However, when the titanium alloy rod has significant defects, oxide scale or foreign matter may adhere to the defective portion. When oxide scale or foreign matter adheres to the defective portion, the titanium alloy powder produced by melting the defective portion will have a different chemical composition than the titanium alloy powder produced from the defect-free portion. In parts modeled using titanium alloy powder with different chemical compositions in a 3D printer, problems such as strength variations compared to other parts may occur. Therefore, titanium alloy powder for 3D printers preferably has a uniform chemical composition. Therefore, titanium alloy rods used as raw materials for 3D printer titanium alloy powder preferably have fewer large defects.

[0014] Furthermore, examples of suppressing surface defects in titanium alloy bars with diameters of 80 mm or more by skipping the initial forging of titanium alloy ingots and directly hot rolling them are still unknown.

[0015] In view of the above, the present invention aims to provide an ingot of an α+β type titanium alloy containing Al, Fe and O, which can be used to manufacture bars with good surface properties (shallow surface defect depth) in an industrially inexpensive and stable manner, even when the forging process is omitted and the bars are directly hot rolled to manufacture bars.

[0016] Solution for solving the problem

[0017] The inventors have conducted in-depth research on a method for manufacturing bars with good surface properties by directly hot rolling α+β type titanium alloy ingots without forging, thereby improving the properties of α+β type titanium alloy ingots.

[0018] To improve the surface properties of the bars, refining the casting microstructure of the α+β titanium alloy ingot can be considered first. For refining the casting microstructure of the α+β titanium alloy ingot, electron beam melting (EBM) or plasma melting (PAM) offers greater freedom in mold shape and allows for faster solidification compared to consumable electrode vacuum arc melting (VAR), making them preferred methods.

[0019] The grain diameter (hereinafter sometimes simply referred to as grain size) of α+β titanium alloy ingots manufactured by electron beam melting or plasma melting is affected by the size of the α+β titanium alloy ingot. The grain size of the cast microstructure of an α+β titanium alloy ingot with a diameter of 200 mm is typically around 10–20 mm. Our research has shown that, when skipping the initial forging and directly hot rolling, refining the grain size of the α+β titanium alloy ingot to less than 10 mm and less than 1 / 100 of the ingot's circumference is effective in suppressing surface defects in the bar stock.

[0020] However, the above-mentioned measures alone are not sufficient to reduce the depth of surface defects in bars to below a certain benchmark.

[0021] According to the results of the inventors' research, when a titanium alloy ingot is heated to the β region and rolled to a section reduction rate of 74% and the cross-section is observed, recrystallized β grains with a grain size of about 5 mm are formed on the surface. A film-like α phase further forms at the grain boundaries of these recrystallized β grains. Furthermore, it is known that cracks are generated along the film-like α phase formed at the grain boundaries of the recrystallized β grains, and these cracks extend to the surface. Moreover, it is known that the cracks caused by the film-like α phase formed at the grain boundaries of these recrystallized β grains reach a relatively deep location, thus becoming the main reason for the formation of deeper defects in the bar stock and a significant reduction in yield.

[0022] Further research by the inventors revealed that controlling the content of Al, Fe, and O within a certain range is effective in controlling the formation of the α phase at grain boundaries in titanium alloy ingots and reducing the surface defect depth of the bars.

[0023] This invention is based on further research into the above insights, and its main points are as follows.

[0024] (1) According to a certain aspect of the present invention, a hot-working α+β type titanium alloy ingot is provided, characterized in that it has the following chemical composition: by mass%, it contains Al: 2.5-8.0%, Fe: 0.5-3.0%, Sn: 0-3.0%, Zr: 0-3.0%, Mo: 0-3.0%, Si: 0-3.00%, Cu: 0-3.0%, Nb: 0-3.0%, and O in an amount satisfying the following formula (1), with the balance being Ti and impurities, and the perimeter L (mm) of the cross section perpendicular to the length direction of the above α+β type titanium alloy ingot relative to the area S (mm) of the above cross section. 2 The ratio L / S is 0.010 or more, and in the portion from one of the two end faces of the α+β type titanium alloy ingot along its length direction toward the other end face and relative to the total length of the α+β type titanium alloy ingot, the grain size D of the casting structure at a position 10 mm from the surface of the α+β type titanium alloy ingot toward the central axis along its length direction satisfies D≤10mm and D≤L / 100, and the thickness of the α+β type titanium alloy ingot is 80mm or more.

[0025] 0.02%≤[O]≤([Al]-[Fe]-0.5×[Mo]-0.5×[Nb]+1.0) / 100…(1) Formula

[0026] Where [X] represents the content of element X when the unit is set to mass %

[0027] (2) The α+β type titanium alloy ingot for hot working described in (1) above may also contain one or more of Sn, Zr, Mo, Cu and Nb, each less than 3.0%, to replace part of the above Ti, and contain less than 3.00% Si.

[0028] The effects of the invention

[0029] According to the above-described manner of the present invention, even when the initial forging is omitted and the ingot of the α+β type titanium alloy containing Al, Fe and O is directly hot rolled, it is possible to manufacture bars with good surface properties in an industrially inexpensive and stable manner. Attached Figure Description

[0030] Figure 1 This is a diagram illustrating a method for measuring the average grain diameter D of an α+β type titanium alloy ingot for hot working according to an embodiment of the present invention.

[0031] Figure 2 It is a diagram used to illustrate a method for controlling the irradiation ratio of an electron beam or plasma, and it is a diagram schematically showing the molten metal inside the mold as seen from the irradiation direction of the electron beam or plasma.

[0032] Figure 3 It is a diagram used to illustrate a method for controlling the irradiation ratio of an electron beam or plasma, and it is a diagram schematically showing the molten metal inside the mold as seen from the irradiation direction of the electron beam or plasma. Detailed Implementation

[0033] The following is a detailed description of an α+β type titanium alloy ingot for hot working according to one embodiment of the present invention. α+β type titanium alloy refers to a titanium alloy in which the α phase and β phase are the main phases at room temperature (25°C).

[0034] First, the chemical composition of the α+β type titanium alloy ingot for hot working in this embodiment will be explained. Hereinafter, "%" refers to "mass %".

[0035] [Al: 2.5-8.0%]

[0036] Al is an α-stabilizing element and is included to increase the proportion of the α phase. In the α+β type titanium alloy ingot for hot working in this embodiment, if the Al content is too low, surface oxidation easily occurs during heating, increasing the thickness of the oxide scale and oxide-hardened layer, resulting in deterioration of the surface properties when the ingot is made into a bar. Therefore, the Al content is set to 2.5% or more. If the Al content is too high, ductility and toughness decrease, resulting in deterioration of the surface properties when the ingot is made into a bar; therefore, the Al content is set to 8.0% or less. The Al content is preferably 2.6% or more, more preferably 2.7% or more, and even more preferably 3.5% or more. Furthermore, the Al content is preferably 7.5% or less, more preferably 7.0% or less, and even more preferably 6.5% or less.

[0037] [Fe: 0.5–3.0%]

[0038] Fe is an element that stabilizes the β phase, and its inclusion in titanium alloy ingots increases strength. However, if the Fe content is below 0.5%, the grain growth inhibition effect of the β phase decreases, leading to grain coarsening and deeper defects, resulting in deteriorated surface properties when the material is made into bars. Therefore, the Fe content is set to 0.5% or more. On the other hand, if the Fe content exceeds 3.0%, ductility and toughness decrease, resulting in deteriorated surface properties when the material is made into bars; therefore, the Fe content is set to 3.0% or less. The Fe content is preferably 0.6% or more, more preferably 0.7% or more. Furthermore, the Fe content is preferably 2.9% or less, more preferably 2.8% or less, and even more preferably 2.5% or less.

[0039] [Sn: 0 to 3.0%, Zr: 0 to 3.0%, Mo: 0 to 3.0%, Si: 0 to 3.00%, Cu: 0 to 3.0%, and Nb: 0 to 3.0%]

[0040] The titanium alloy ingot of the present invention may also contain one or more of Sn, Zr, Mo, Cu and Nb in the range of less than 3.0% to replace a portion of the above-mentioned Ti, and contain Si in the range of less than 3.00%.

[0041] Sn, Zr, Si, and Cu are effective elements for solid solution strengthening of the α and β phases; by including them in titanium alloy ingots, strength can be improved. Therefore, the ingot may contain 3.0% or less of each of Sn, Zr, and Cu, and 3.00% or less of Si. The Sn content may be 2.5% or less, 1.5% or less, or 0.5% or less. Similarly, the Cu content may be 2.0% or less, or 1.5% or less. The Zr content may be 2.8% or less, or 2.5% or less. Since Sn, Zr, Si, and Cu may be omitted, the Sn, Zr, Si, and Cu contents may each be 0% or more. The Si content may be 1.50% or less, or 0.50% or less.

[0042] Mo and Nb are β-stabilizing elements, which, in addition to improving strength through solid solution strengthening, also enhance ductility and hot workability. Therefore, titanium alloy ingots can contain up to 3.0% Mo or Nb each. The Mo content can be 2.9% or less, or 2.8% or less. Similarly, the Nb content can be 2.5% or less, or 2.0% or less. Since Mo and Nb can be absent, the Mo and Nb contents can both be 0% or more. The Mo content can also be 0.5% or more. Furthermore, the Nb content can be 1.0% or more.

[0043] [O: 0.02~0.08%]

[0044] O is an α-stabilizing element, which increases the fraction of the α-phase and improves strength. However, if the O content is less than 0.02%, refining costs increase significantly, making cost reduction impossible. Therefore, the O content is 0.02% or more. The O content can be 0.03% or more, or 0.04% or more. On the other hand, if the O content is excessive, grain boundary α-phase is easily formed. Furthermore, when the hot-working α+β type titanium alloy ingot of this embodiment is used as a starting material for metal powder used in 3D printers, in order to ensure the strength of the model formed using the 3D printer, the O content of the hot-working α+β type titanium alloy ingot of this embodiment is set to be slightly lower than the O content required to improve strength. When titanium alloy rods manufactured from titanium alloy ingots are made into material powder for 3D printers, the surface area to volume ratio increases, thus the influence of the surface oxide film becomes greater, and the O content becomes higher. The O content of this titanium alloy powder reaches the O content of the model manufactured using the 3D printer. Furthermore, the strength of the model manufactured using a 3D printer reaches a level corresponding to the oxygen content of the model. Therefore, in this embodiment, the oxygen content of the α+β type titanium alloy ingot for hot working is set to be slightly lower than the oxygen content required to improve strength. Therefore, the oxygen content is set to 0.08% or less. The oxygen content can also be 0.07% or less.

[0045] [0.02%≤[0]≤([Al]-[Fe]-0.5×[Mo]-0.5×[Nb]+1.0) / 100]

[0046] However, in α+β type titanium alloys containing Al and O as α-stabilizing elements and Fe as a β-stabilizing element, due to the rapid diffusion rate of Fe and its low solid solubility limit in the α phase, the grain boundary α phase precipitates at the β grain boundaries during the phase transformation from the β phase to the α+β domain, becoming the initiation point of cracks. Furthermore, O also promotes the formation of the grain boundary α phase, becoming the initiation point of cracks. Mo and Nb, like Fe, are elements that promote the precipitation of the grain boundary α phase. Therefore, in the hot-working α+β type titanium alloy ingot of this embodiment, the O content is controlled from the viewpoint of suppressing surface defects. Through the research of the inventors, it has been found that there exists a range of O contents for suppressing surface defects, depending on the Al, Fe, Mo, and Nb contents. When the unit is set as mass %, by using the contents of Al [Al], Mo [Mo], and Nb [Nb] as α-stabilizing elements, and the contents of Fe [Fe] as β-stabilizing element, and ensuring that the O content [O] satisfies 0.02% ≤ [O] ≤ ([Al] - [Fe] - 0.5 × [Mo] - 0.5 × [Nb] + 1.0) / 100, surface defects of the bar can be suppressed. Therefore, in the α+β type titanium alloy ingot for hot working in this embodiment, the contents of Fe, Al, Mo, Nb, and O need to further satisfy the following equation (1).

[0047] 0.02%≤[O]≤([Al]-[Fe]-0.5×[Mo]-0.5×[Nb]+1.0) / 100…(1) Formula

[0048] Where [X] represents the content (mass%) of element X. It should be noted that when the Mo content is 0, 0 is substituted into [Mo], and when the Nb content is 0, 0 is substituted into [Nb].

[0049] The chemical composition balance consists of Ti and impurities. Specifically, impurities include V, C, N, Y, Cr, Ni, Mn, B, and H. As long as they do not hinder the effectiveness of this application, the impurity content is acceptable as long as it is below 0.2% by mass for each element and below 0.5% in total. B may form large precipitates within the ingot. Therefore, even when present as an impurity, it is preferable to suppress the B content as much as possible. In the titanium alloy plate of this embodiment, the B content is preferably set to 0.01% or less.

[0050] The chemical composition of the titanium alloy ingots was analyzed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). O content was determined using the inert gas melting-thermal conductivity method. The Al, Fe, Sn, Zr, Mo, Si, Cu, Nb, V, Y, Cr, Ni, and Mn contents of the titanium alloy ingots were determined according to "JIS H 1632-2:2014 Titanium - ICP emission spectroscopic analysis method - Part 2: Quantitative methods for palladium, manganese, iron, magnesium, silicon, aluminum, vanadium, nickel, chromium, tin, copper, molybdenum, zirconium, niobium, tantalum, cobalt, and yttrium". The O content of the titanium alloy ingots was determined according to "JIS H 1620:1995 Quantitative methods for oxygen in titanium and titanium alloys". The N content of the titanium alloy ingots was determined according to "JIS H1612:1993 Quantitative methods for nitrogen in titanium and titanium alloys". The carbon (C) content in titanium and titanium alloys was determined according to "JIS H 1617:1995 Method for quantifying carbon in titanium and titanium alloys". The hydrogen (H) content in titanium alloy ingots was determined according to "JIS H 1619:2012 Method for quantifying hydrogen in titanium and titanium alloys". The boron (B) content in titanium alloy ingots was determined according to "JIS H 1632-3:2014 Method for quantifying boron by ICP emission spectroscopy of titanium - Part 3: Method for quantifying boron".

[0051] Next, the perimeter L (mm) and cross-sectional area S (mm²) of the α+β type titanium alloy ingot for hot working in this embodiment will be discussed. 2 Explain the relationship between them.

[0052] The perimeter L (mm) of the cross-section relative to the area S (mm²) 2 The ratio of L / S is greater than 0.010.

[0053] In this embodiment, the perimeter L (mm) of the cross-section of the α+β type titanium alloy ingot for hot working is relative to the area S (mm²) of the cross-section. 2 The ratio of L / S is 0.010 or higher.

[0054] Refining the casting microstructure of titanium alloy ingots is effective in improving the surface properties of bars. During solidification, if the titanium alloy ingot is large, the solidification process is lengthy, resulting in a larger grain size in the casting microstructure. Larger grain sizes hinder recrystallization, leading to increased strain mismatch at grain boundaries during hot rolling of the titanium alloy ingot into bars. This increases the likelihood of cracks and voids, and the surface is prone to wrinkling and deep defects during hot rolling.

[0055] A larger surface area per unit volume of a titanium alloy ingot results in greater heat dissipation, thus shortening the time until solidification and allowing for a smaller microstructure. In the α+β type titanium alloy ingot for hot working in this embodiment, the perimeter L (mm) of the cross-section (the section perpendicular to the length direction of the ingot) is used relative to the area S (mm²) of the cross-section. 2 The heat dissipation magnitude is evaluated by the ratio L / S, with L / S set to 0.010 or higher.

[0056] From a heat dissipation perspective, a higher L / S ratio is better, so there is no specific upper limit. For typical shapes, L / S is below 0.050 or around 0.030.

[0057] [In the portion of the α+β titanium alloy ingot extending from one end face towards the other along its length, and relative to the total length of the α+β titanium alloy ingot (20-80%), the average grain diameter D of the casting microstructure at a position 10 mm from the surface of the α+β titanium alloy ingot towards the central axis along its length is: D ≤ 10 mm and D ≤ L / 100]

[0058] For the α+β type titanium alloy ingot for hot working in this embodiment, in a portion of 20-80% of the total length of the α+β type titanium alloy ingot from one end face to the other end face along its length, at a position 10 mm from the surface of the α+β type titanium alloy ingot towards the central axis in the length direction, the average grain diameter D of the casting microstructure is 10 mm or less, and the average grain diameter D and the perimeter L of the cross-section satisfy D ≤ L / 100. By making the microstructure of the α+β type titanium alloy ingot for hot working in this embodiment such that when compressive strain acts on the circumference of the ingot during rolling, the strain at each grain boundary is mitigated, and the generation of cracks and voids when the titanium alloy ingot is hot rolled into a bar is suppressed. As a result, the formation of deep defects on the surface of the bar can be suppressed. It should be noted that, in the following text, the portion of the α+β type titanium alloy ingot with one end face facing the other and relative to the total length of the α+β type titanium alloy ingot along its length direction, which is 20% to 80% of the total length of the ingot, is sometimes referred to as the central portion along the length direction.

[0059] The grains constituting the casting structure are equiaxed and columnar grains. However, in this embodiment, the cross-section of the central portion of the hot-working α+β type titanium alloy ingot along the length direction is almost entirely composed of columnar grains at a position 10 mm from the surface towards the center of the cross-section, resulting in minimal deviation in the casting structure along the length direction. On the other hand, in the cross-sections of the two ends other than the central portion along the length direction, coarse equiaxed grains sometimes form at a position 10 mm from the surface towards the center of the cross-section. In cases where coarse equiaxed grains are formed, the average grain diameter D of the casting structure at a position 10 mm from the surface towards the center of the cross-section does not satisfy D≤10 mm and D≤L / 100. In such abnormal portions, in addition to issues caused by the casting structure, there are, for example, cases where surface defects arise due to a significant drop in temperature leading to a decrease in ductility and toughness, or due to collision with the rolling equipment. Therefore, improving the surface properties of the central portion is more effective in improving the yield by achieving good surface properties. Therefore, the average grain diameter D at the cross-section of the central portion along the length direction of the titanium alloy ingot is measured.

[0060] The cross section used to calculate the average grain diameter D is prepared using the following method. First, the titanium alloy ingot is cut at its center along a direction perpendicular to the length direction, and the cut surface is ground using diamond sandpaper and polishing. Next, the ground cut surface is etched using nitric acid (10% by mass) and hydrofluoric acid (5% by mass). This etched cut surface is used as the cross section for calculating the average grain diameter D. It should be noted that during the manufacturing process of the titanium alloy ingot, approximately 5 mm of the ingot surface is ground (or cut away). However, the average grain diameter D calculated from a position 10 mm from the surface of the ingot before grinding towards the central axis along the length direction is approximately the same as the average grain diameter D calculated from a position 10 mm from the surface of the ingot after grinding (or 15 mm from the surface of the ingot before grinding towards the central axis along the length direction). Therefore, the average grain diameter D can be calculated either from the position 10 mm from the central axis of the surface before grinding towards the length direction, or from the position 10 mm from the central axis of the surface after grinding towards the length direction.

[0061] Among them, reference Figure 1 The method for determining the average grain diameter D is explained. Figure 1 This is a diagram illustrating a method for measuring the average grain diameter D of an α+β type titanium alloy ingot for hot working according to an embodiment of the present invention. Figure 1The image shows one of the semicircles obtained by dividing a titanium alloy ingot with a circular cross-section by a straight line passing through the center of the cross-section.

[0062] The average grain diameter D is calculated from the cross-section at the center of the length direction of the titanium alloy ingot. Specifically, in the cross-section at the center of the length direction of the titanium alloy ingot, a line I corresponding to the shape of the cross-section is drawn at a position 10 mm from the surface of the titanium alloy ingot toward the central axis in the length direction, and the number of grain boundaries intersecting with this line I is calculated. Figure 1 In the diagram, dashed lines represent lines I that define the shape corresponding to the cross-sectional shape. For example, in... Figure 1 In this calculation, since the cross-section A is circular, the line I used to calculate the number of grain boundaries is a line that defines the circle. Furthermore, the value obtained by dividing the length of this line I by the number of grain boundaries is taken as the average grain diameter D.

[0063] [Thickness 80mm and above]

[0064] The thickness of the α+β type titanium alloy ingot for hot working in this embodiment is 80 mm or more.

[0065] Details will be described later. In this embodiment, the α+β type titanium alloy ingot for hot working is manufactured by irradiating the surface of molten titanium flowing into the mold with an electron beam or plasma. The technology described in Patent Document 1 involves irradiating the surface of solid titanium materials such as titanium blocks with an electron beam or plasma. In the technology described in Patent Document 1, the melting depth is limited because the electron beam or plasma is irradiated onto the surface of the solid titanium material. If the power of the electron beam is increased to increase the melting depth, the cooling rate after melting becomes slower, and the grains become coarse. However, if the electron beam or plasma is appropriately controlled and the surface of the molten titanium is irradiated, the formation of coarse grains can be suppressed. Therefore, by irradiating the surface of molten titanium with an electron beam or plasma, it is possible to manufacture titanium alloy ingots of larger size that can suppress surface defects when hot-working them into bars without forging. Therefore, the thickness of the α+β type titanium alloy ingot for hot working in this embodiment is set to 80 mm or more. The thickness of the α+β type titanium alloy ingot for hot working in this embodiment is preferably 130 mm or more, and more preferably 200 mm or more. On the other hand, there is no particular limitation on the upper limit of the thickness of the α+β type titanium alloy ingot for hot working in this embodiment. For example, the thickness of the α+β type titanium alloy ingot for hot working in this embodiment can be 400 mm or less, or 350 mm or less.

[0066] Regarding the thickness mentioned, in the case of a rectangular cross-section, it refers to the length of the shorter side of the cross-section; in the case of a circular cross-section, it refers to the diameter of the cross-section. Furthermore, in the case of a polygon with a cross-section that is pentagonal or larger, it is defined as the equivalent circle diameter of the cross-section. The equivalent circle diameter refers to the diameter of a circle whose area is equivalent to that of the cross-section. Additionally, the cross-section of a polygon also includes cross-sections formed by cutting away at least one of the four corners of a rectangle or other quadrilateral. Therefore, although the cross-section of a polygon may have various shapes, in such cases, the diameter of the cross-section is also defined as the equivalent circle diameter of the cross-section.

[0067] It should be noted that the width of the α+β type titanium alloy ingot for hot working in this embodiment is determined by the size of the mold and does not require special specification. However, its lower limit can be greater than or equal to the thickness, and its upper limit can be less than 2m. It should be noted that the width mentioned herein refers to the length of the longer side of the cross-section when the cross-section is rectangular. Furthermore, when the cross-section is a polygon with a pentagonal or larger shape, the width of the titanium alloy ingot is equal to its thickness.

[0068] Furthermore, since the α+β type titanium alloy ingot for hot working in this embodiment is manufactured by casting, no lower or upper limit is specified for its length. Considering the equipment, the length of the α+β type titanium alloy ingot for hot working in this embodiment can be either 2m or more, or 7m or less.

[0069] This concludes the description of the α+β type titanium alloy ingot for hot working according to this embodiment. The bar stock manufactured using the α+β type titanium alloy ingot for hot working according to this embodiment can be used as a material for 3D-printed objects.

[0070] Next, a preferred manufacturing method for obtaining the α+β type titanium alloy ingot for hot working according to this embodiment will be described. The manufacturing method of the α+β type titanium alloy ingot described below is merely an example and is not limited to the manufacturing method described below.

[0071] In this embodiment, the α+β type titanium alloy ingot for hot working is manufactured using sponge titanium, master alloy, or titanium scrap as molten raw materials through electron beam melting or plasma melting. Both electron beam melting and plasma melting utilize a cold hearth furnace, hence the term "cold hearth furnace melting."

[0072] Electron beam melting and plasma melting are methods of manufacturing ingots by injecting raw materials molten by electron beam or plasma irradiation into a cold hearth furnace, or by melting the raw materials on a cold hearth furnace, and then letting them flow into a mold, and pulling the solidified ingot out from the bottom of the mold.

[0073] The cold hearth furnace melting method has the following advantages: the molten pool (the portion where the raw material melts within the mold) is shallow, which reduces the grain size of the casting structure; furthermore, it allows for greater freedom in mold dimensions. Therefore, the cold hearth furnace melting method is used in the manufacturing method of the α+β type titanium alloy ingot for hot working in this embodiment.

[0074] The solidification morphology of titanium alloy ingots is affected by factors such as heat dissipation from the ingot to the mold sidewall, heat output to the surface of the molten metal using electron beams or plasma, and the temperature of the molten metal injected into the mold. By appropriately controlling these factors, the solidification morphology can be controlled.

[0075] In the manufacturing method of the α+β type titanium alloy ingot for hot working in this embodiment, the pouring temperature when the molten titanium alloy is injected into the mold is set to 200°C or less above the melting point of the titanium alloy; the pouring speed when the molten titanium alloy is injected into the mold is set to 0.05 tons / hour or more and 2.00 tons / hour or less; and the average irradiation density of the electron beam or plasma within 20 mm of the mold surface is set to 50-80% of the average irradiation density of the electron beam or plasma further inward than 20 mm of the mold surface. By controlling the pouring temperature, pouring speed, and average irradiation density as described above, the temperature of the molten metal in the mold can be reduced, and the coarsening of the grain size of the titanium alloy ingot can be suppressed. The ratio of the average irradiation density of the electron beam or plasma within 20 mm of the mold surface to the average irradiation density of the electron beam or plasma further inward than 20 mm of the mold surface is defined as the irradiation ratio.

[0076] Among them, reference Figure 2 and Figure 3 The method for controlling the irradiation ratio of electron beam or plasma is explained. Figure 2 This is a diagram illustrating the first control method for the irradiation ratio of electron beams or plasma. Figure 3 This is a diagram illustrating a second method for controlling the irradiation ratio of electron beams or plasma. Figure 2 and Figure 3 The image schematically shows molten metal inside a mold as seen from the direction of electron beam or plasma irradiation. Figure 2 and Figure 3 The area shown is 2, which represents the molten metal within 20 mm of the interface 1 where the molten metal meets the inner surface of the mold. The area shown is 3, which represents the molten metal further inside than 20 mm from the surface of the mold.

[0077] Solidification primarily occurs due to heat dissipation to the mold sidewalls. To prevent solidification caused by heat dissipation, the surface of the molten metal inside the mold is heated by electron beam or plasma. For example... Figure 2 As shown, the irradiation area B of the electron beam or plasma is narrower than the mold area. Therefore, by making the electron beam or plasma at, for example... Figure 2The solid arrows indicate the scanning method, thereby heating the entire surface of the molten metal.

[0078] The electron beam or plasma irradiating the molten metal can be one or more beams. The following explains the methods for controlling the irradiation ratio when there is one electron beam or plasma and when there are two beams.

[0079] First, the method for controlling the irradiation ratio when only one electron beam or plasma is irradiated onto the molten metal, namely the first control method, will be explained. When only one electron beam or plasma is irradiated onto the molten metal, the scanning speed of the electron beam or plasma irradiating the molten metal located within 20 mm of the inner surface of the mold is set to 1.25 to 2.00 times, relative to the scanning speed of the electron beam or plasma irradiating the molten metal located more than 20 mm from the mold surface. This allows the irradiation ratio of the electron beam or plasma to reach 50% to 80%.

[0080] Next, the method for controlling the irradiation ratio when there are two electron beams or plasmas irradiating the molten metal, namely the second control method, will be described. When there are two electron beams or plasmas irradiating the molten metal, one electron beam or plasma scans the entire area of ​​the molten metal surface, while the other electron beam or plasma scans the surface of the molten metal located more inward than 20 mm from the mold surface. Figure 3 In the diagram, solid arrows indicate the scanning paths of the electron beam or plasma scanning the entire surface of the molten metal, while dashed arrows indicate the scanning paths of the electron beam or plasma scanning the surface of the molten metal located 20 mm inside the mold surface. The number of scans for each of the two electron beams or plasmas can be varied depending on the irradiation ratio. For example, with an irradiation ratio of 80%, the electron beam or plasma scanning the entire surface of the molten metal will perform 80 cycles, while the electron beam or plasma scanning the surface of the molten metal located 20 mm inside the mold surface will perform 20 cycles. One cycle here refers to scanning the entire desired area with the electron beam or plasma. It should be noted that the scanning speeds of the two electron beams or plasmas can be equal. Furthermore, the power of the two electron beams or plasmas can be equal. This allows for an electron beam or plasma irradiation ratio of 50% to 80%.

[0081] Titanium alloys with the above-mentioned composition are obtained by controlling the solidification conditions in this way, thereby controlling the formation of the grain boundary α phase in the titanium alloy ingot and reducing the surface defect depth of the bar obtained by rolling the titanium alloy ingot.

[0082] As explained above, according to the present invention, ingots of α+β type titanium alloys containing Al, Fe, and O can be directly hot-rolled without forging, producing bars with good surface properties. Furthermore, when using the hot-working α+β type titanium alloy ingot of this embodiment to manufacture bars, forging can be omitted, thus enabling inexpensive industrial manufacturing of bars. Moreover, it will be explained below that, according to the hot-working α+β type titanium alloy ingot of this embodiment, surface defects in the bars are suppressed, thus enabling stable manufacturing of bars.

[0083] It should be noted that the effectiveness of this invention can be confirmed by measuring the depth of surface defects in titanium alloy bars obtained by rolling titanium alloy ingots. The depth of surface defects is evaluated using the following method.

[0084] The bar stock is cut perpendicular to the axial direction, and the cut surface is ground to achieve a mirror finish. Then, using an optical microscope at 15x magnification, the depth of surface defects is measured. Based on the depth of the surface defect with the largest depth in the field of view and the converted diameter of the bar stock, the defect depth ratio (%) is calculated as: (maximum surface defect depth) / (converted diameter of the bar stock) × 100, serving as an indicator of surface characteristics. The defect depth is determined by investigating five locations on the bar surface at the central portion along the length direction where defects are visually confirmed. Specifically, at the central portion of the bar stock along the length direction, cross-sections perpendicular to the bar's axial direction are obtained at each of the five locations where defects are visually confirmed. These five cross-sections are observed using an optical microscope at 15x magnification, and the arithmetic mean of the maximum depths obtained from each cross-section is taken as the maximum surface defect depth in the above formula. If fewer than five defects are visually identified in the central portion of the bar along its length, the maximum surface defect depth is calculated based on the visually identified defect locations. Conversely, if six or more defects are visually identified in the central portion of the bar along its length, five defects are measured, starting with the largest defect. It should be noted that if defects do not fall within a single field of view, the field of view can be moved and the defect depth calculated. Furthermore, as mentioned above, in abnormal portions of the bar along its length, such as the two ends (excluding the central portion), in addition to defects caused by casting structures, there may be surface defects caused by: a significant drop in temperature leading to a decrease in ductility and toughness; or surface defects caused by collision with the rolling equipment. To eliminate the influence of such surface defects and to evaluate the occurrence of surface defects caused by grain size, the maximum surface defect depth calculated based on the depth of defects identified in the central portion of the bar along its length is used when calculating the defect depth ratio, which is an indicator of surface properties. The central portion of the bar in the length direction refers to the portion that extends from one end face of the α+β type titanium alloy bar in the length direction toward the other end and is 20% to 80% of the total length of the α+β type titanium alloy bar in the length direction.

[0085] The following embodiments illustrate the results of confirming the effectiveness of the present invention using the depth of surface defects. It should be noted that the embodiments shown below are merely examples of the present invention, and the present invention is not limited to these examples.

[0086] Example

[0087] [Example 1]

[0088] (Manufacturing of titanium alloy ingots and bars)

[0089] For Nos. 1 to 29 shown in Table 1, titanium alloy ingots were manufactured using electron beam melting (EBM). The pouring temperature of the molten titanium alloy, injected from the furnace bed into molds of the shapes shown in Table 1, was set to below the melting point +200°C (only No. 1 exceeded the melting point +200°C); the pouring speed was set to 0.05–2.0 tons / hour; and the average irradiation density of the electron beam, located 20 mm from the center of the mold surface, was set to 0.05–0.10 kW / cm². 2 The average irradiation density of the electron beam within 20 mm of the mold surface is set to 50-100% of the average irradiation density of the electron beam on the central side, which is 20 mm from the mold surface. This irradiation ratio is 50-100%, thereby manufacturing a titanium alloy ingot with the chemical composition shown in Table 2 and the cross-sectional perimeter L, cross-sectional area S, and average grain diameter D shown in Tables 3A and 3B. It should be noted that in Table 1, the average irradiation density of the electron beam on the central side, which is 20 mm from the mold surface, is denoted as the "central average irradiation density," and the average irradiation density of the electron beam within 20 mm of the mold surface is denoted as the "surface average irradiation density." With the electron beam gun power constant, the scanning speed of the electron beam irradiating the molten metal located on the inner side, which is 20 mm from the mold surface, and the scanning speed of the electron beam irradiating the molten metal located within 20 mm of the inner surface of the mold are changed to control the average irradiation density.

[0090] Subsequently, the titanium alloy ingot is heated to 1200℃ and rolled with a section reduction rate of 41%–94% until it reaches φ100mm. For titanium alloy ingots No. 1–21 with a die cross-section (die shape) of 310mm × 440mm perpendicular to the casting direction of the titanium alloy ingot, a total of 17 reverse rolling passes are performed before the cross-section reaches 180mm × 195mm. The section reduction rate of the titanium alloy ingot after 17 reverse rolling passes is 74%. After 17 reverse rolling passes, 8 pass die rolling passes are performed until φ100mm is reached, with the section reduction rate relative to the titanium alloy ingot before reverse rolling set at 94%, to obtain the bar stock. For titanium alloy ingots No. 22 to 28 with mold shapes of φ200mm to φ500mm, reverse rolling and die rolling are performed in the same manner as described above. For titanium alloy ingots No. 29 with mold shapes of φ130mm, only the above-described die rolling is performed.

[0091] It should be noted that No. 30 and No. 31 shown in Table 1 are reference examples of manufacturing titanium alloy ingots according to existing technology and then producing bars through forging and hot rolling. In example No. 30, a φ750mm titanium alloy ingot is manufactured by consumable electrode vacuum arc melting (VAR), and this ingot is forged to produce a small square billet of 310×440mm. In example No. 31, a titanium alloy ingot is manufactured using the same method as No. 30, and a φ300mm round bar is produced by forging. Moreover, similar to No. 1 to 29, No. 30 and 31 are also rolled to φ100mm after being heated to 1200°C. It should be noted that the titanium alloy ingots of No. 30 and 31 are manufactured by VAR; therefore, the average irradiation density of the central portion, the average irradiation density of the surface side, and the irradiation ratio of No. 30 and 31 in Table 1 are not recorded.

[0092] [Table 1]

[0093]

[0094] (Chemical composition of titanium alloy ingots)

[0095] The Al, Fe, Mo, and Nb contents of each titanium alloy ingot were determined according to "JIS H 1632-2:2014 Titanium - ICP emission spectroscopic analysis method - Part 2: Quantitative method for palladium, manganese, iron, magnesium, silicon, aluminum, vanadium, nickel, chromium, tin, copper, molybdenum, zirconium, niobium, tantalum, cobalt and yttrium". The O content of the titanium alloy ingot was determined according to "JIS H 1620:1995 Quantitative method for oxygen in titanium and titanium alloys". In Example 1, apart from Al, Fe, and O, there were no elements whose content in the titanium alloy ingot was intentionally controlled; therefore, Ti and the elements shown in Table 2 were considered impurities. Furthermore, the Mo and Nb contents were within the range that could be considered impurities, and therefore their contents were considered to be 0%.

[0096] Table 2 shows the chemical composition of the titanium alloy ingot and whether it satisfies equation (1): 0.02% ≤ [O] ≤ ([Al] - [Fe] - 0.5 × [Mo] - 0.5 × [Nb] + 1.0) / 100. In the "Whether it satisfies equation (1)" item in Table 2, the case that satisfies equation (1) is marked as Good, and the case that does not satisfy it is marked as Bad.

[0097] [Table 2]

[0098]

[0099] (The perimeter L and the area S of the cross-section)

[0100] The shrinkage during solidification of each titanium alloy ingot has almost no impact on the evaluation of the ratio L / S, which is the perimeter L of the cross-section to the area S of the cross-section. Therefore, the area S and perimeter L of the cross-section of each titanium alloy ingot are calculated based on the cross-sectional shape of the mold.

[0101] (Determination of average grain diameter D)

[0102] The average grain diameter D was determined using the following method. First, the ingot was cut along a direction perpendicular to the length direction at a point 50% of its total length relative to the length of the ingot, from one end face towards the other. The cut surface was then ground using diamond sandpaper and polishing. Next, the ground cut surface was etched using nitric acid with a concentration of 10% by mass and hydrofluoric acid with a concentration of 5% by mass. The number of grain boundaries was measured at a point 5 mm from the short side towards the center of the etched cut surface.

[0103] In the “D≤L / 100” item in Tables 3A and 3B, the case where the average grain diameter D satisfies D≤L / 100 is marked as Good, and the case where the average grain diameter D does not satisfy D≤L / 100 is marked as Bad.

[0104] (Surface properties of bars)

[0105] For the five locations on the surface of the obtained bar where defects were visually observed, each location was cut perpendicular to the bar's axial direction. The cut surfaces were then ground to a mirror finish, and the depth of the surface defects was measured using an optical microscope. Specifically, sections perpendicular to the bar's axial direction were obtained at the five locations on the bar's surface where defects were visually confirmed along the bar's length. The five sections were observed using an optical microscope at 15x magnification, and the average of the maximum depths obtained from each section was taken as the maximum surface defect depth in the following formula. If fewer than five locations on the central section of the bar's length were visually confirmed as defects, the maximum surface defect depth was calculated based on the locations of the visually confirmed defects. Conversely, if six or more locations on the central section of the bar's length were visually confirmed as defects, five locations were measured starting with the largest defect. If defects did not fall within a single field of view, the field of view was moved, and the defect depth was calculated.

[0106] Based on the depth of the surface defect with the largest surface defect depth in the field of view and the converted diameter of the bar, the defect depth ratio (%) is calculated as follows: (maximum surface defect depth) / (converted diameter of the bar) × 100. A defect depth ratio below 3.0% is judged as having good surface properties, while a defect depth ratio exceeding 3.0% is judged as having poor surface properties. The results are shown in Tables 3A and 3B.

[0107] [Table 3A]

[0108]

[0109] [Table 3B]

[0110]

[0111] Even when the initial forging and hot rolling are omitted, the α+β type titanium alloy ingot of the present invention can be confirmed to have the same surface properties as the conventional examples (No. 30, No. 31).

[0112] It should be noted that by observing the microstructure, it is easy to distinguish the microstructure of a casting state, such as the α+β type titanium alloy ingot of the present invention, from the microstructure obtained by forging, such as No. 30 and No. 31.

[0113] Examples No. 1, 5, 6, 11, 13, 14, 15, 17, 19 to 23, 26, 28, and 29 are comparative examples.

[0114] No.1 Because the casting temperature exceeds the melting point +200°C, the irradiation rate is as high as 100%, and the relationship between the average grain diameter D and the perimeter L of the cross section is not satisfied (D≤L / 100), the average grain diameter D is outside the scope of this invention, resulting in poor surface properties after rolling.

[0115] Nos. 5, 6, 11, 17, and 19 do not satisfy equation (1), resulting in poor surface properties after rolling.

[0116] Nos. 15 and 26 have poor surface properties after rolling because the irradiation ratio is outside the preferred range and the particle size is outside the range of the present invention.

[0117] No. 22. Because the irradiation ratio is outside the preferred range, and the ratio of the perimeter L of the cross section to the area S of the cross section, L / S, is outside the scope of the present invention, and the particle size is outside the scope of the present invention, the resulting surface properties after rolling are poor.

[0118] No. 29 does not satisfy the relationship between the average grain diameter D and the perimeter L of the cross section, D≤L / 100, resulting in poor surface properties after rolling.

[0119] The titanium alloy ingot No. 13 had a high Fe content, resulting in poor surface properties after rolling.

[0120] The titanium alloy ingot No. 14 has a high Al content, resulting in poor surface properties after rolling.

[0121] The titanium alloy ingot No. 20 had a low Fe content, resulting in poor surface properties after rolling.

[0122] The titanium alloy ingot No. 21 has a low Al content, resulting in poor surface properties after rolling.

[0123] The ratio of the perimeter L of the cross section to the area S of the cross section in No. 23 is less than 0.010, resulting in poor surface properties after rolling.

[0124] No. 28 Because the relationship between the average grain diameter D and the perimeter L of the cross section is not satisfied (D≤L / 100), as a result, when the compressive strain acts on the circumference of the ingot during rolling, the strain at each grain boundary is not mitigated, resulting in voids and a deterioration in the surface properties after rolling.

[0125] [Example 2]

[0126] (Manufacturing of titanium alloy ingots and bars)

[0127] Using a mold with a cross-section of 310mm × 440mm perpendicular to the casting direction, the pouring temperature was set to below the melting point +200℃, the pouring speed to 0.5 tons / hour, and the average irradiation density of the electron beam, located 20mm from the mold surface and closer to the center, was set to 0.10 kW / cm². 2 The electron beam irradiation density within 20 mm of the mold surface was set to 50-80% of that in the central part, i.e., an irradiation ratio of 70%, thereby manufacturing titanium alloy ingots with the chemical composition and average grain diameter D shown in Table 4. The length of the manufactured titanium alloy ingots was 4200 mm. Then, similarly to Example 1, each titanium alloy ingot was heated to 1200°C, and then rolled with a section reduction rate of 94% until φ100 mm by reverse rolling and die rolling to obtain bars.

[0128] (Chemical composition of titanium alloy ingots)

[0129] The chemical composition of the titanium alloy ingots was determined by the following methods. The Al, Fe, Mo, and Nb contents of each titanium alloy ingot were determined in the same manner as in Example 1. The O content of the titanium alloy ingots was determined according to "JIS H 1620:1995 Quantitative Method for Oxygen in Titanium and Titanium Alloys". The Si, Zr, Sn, and Cu contents were determined according to "JIS H1632-2:2014 Titanium - ICP Emission Spectroscopy Analysis Method - Part 2: Quantitative Method for Palladium, Manganese, Iron, Magnesium, Silicon, Aluminum, Vanadium, Nickel, Chromium, Tin, Copper, Molybdenum, Zirconium, Niobium, Tantalum, Cobalt and Yttrium".

[0130] Table 4 shows the chemical composition of the titanium alloy ingot and whether it satisfies equation (1): 0.02% ≤ [O] ≤ ([Al] - [Fe] - 0.5 × [Mo] - 0.5 × [Nb] + 1.0) / 100. In the "Whether it satisfies equation (1)" item in Table 4, the case that satisfies equation (1) is marked as Good, and the case that does not satisfy it is marked as Bad. In addition, "-" in Table 4 indicates that no intentional addition was made.

[0131] [Table 4]

[0132]

[0133] (Determination of average grain diameter D)

[0134] The average grain diameter D was determined using the same method as in Example 1. In Table 5, under the "D≤L / 100" item, cases where the average grain diameter D satisfies D≤L / 100 are marked as Good, and cases where the average grain diameter D does not satisfy D≤L / 100 are marked as Bad.

[0135] (Surface properties of bars)

[0136] Five defects were observed on the surface of the obtained bar by visual inspection. Each defect was cut in a direction perpendicular to the axis of the bar, and the cut surfaces were ground to achieve a mirror finish. The depth of the surface defects was then measured using an optical microscope.

[0137] Based on the depth of the surface defect with the largest surface defect depth in the field of view and the converted diameter of the bar, the defect depth ratio (%) was calculated as follows: (maximum surface defect depth) / (converted diameter of the bar) × 100. A defect depth ratio below 3.0% was judged as having good surface properties, while a defect depth ratio exceeding 3.0% was judged as having poor surface properties. The results are shown in Table 5.

[0138] [Table 5]

[0139]

[0140] Even when the initial forging is omitted and hot rolling is performed, the α+β type titanium alloy ingot of this invention can be confirmed to have good surface properties. No. 2, 3, 6, 8, and 13 do not satisfy equation (1), resulting in poor surface properties.

[0141] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to these examples. Those skilled in the art will readily conceive of various modifications or alterations within the scope of the technical concept described in the claims, and these modifications should be understood as clearly falling within the technical scope of the present invention.

Claims

1. A type α+β titanium alloy ingot for hot working, characterized in that, It has the following chemical composition: By mass%, it contains Al：2.5～8.0％、 Fe: 0.5–3.0% Sn: 0-3.0% Zr：0～3.0％、 Mo: 0–3.0% Si: 0-3.00% Cu: 0–3.0% Nb: 0–3.0%, and The amount of O that satisfies the following equation (1) is equal to the amount of Ti and impurities. The perimeter L (mm) of the cross-section perpendicular to the length direction of the α+β type titanium alloy ingot is relative to the area S (mm²) of the cross-section. 2 The ratio of L / S is 0.010 or higher. In the portion of the α+β titanium alloy ingot extending from one end face to the other along its length, and relative to the total length of the α+β titanium alloy ingot, where the total length is 20-80%, the average grain diameter D of the casting microstructure at a position 10 mm from the surface of the α+β titanium alloy ingot towards the central axis along its length satisfies D ≤ 10 mm and D ≤ L / 100. The thickness of the α+β type titanium alloy ingot is 80 mm or more. 0.02%≤[O]≤([Al]-[Fe]-0.5×[Mo]-0.5×[Nb]+1.0) / 100…(1) Formula Where [X] represents the content of element X when the unit is set to mass % 2. The α+β type titanium alloy ingot for hot working according to claim 1, wherein it contains at least one of Sn, Zr, Mo, Cu and Nb, each at less than 3.0%, to replace a portion of the Ti, and contains at least 3.00% Si.

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