Titanium material, parts, heat treatment simulation method, heat treatment simulation apparatus, program, and method for manufacturing titanium material.

Abstract

Disclosed are rod-shaped or wire-shaped titanium materials made of pure titanium or α-type titanium alloy, which exhibit small variations in tensile properties. [Solution] The titanium material of this disclosure is a rod-shaped or wire-shaped titanium material made of pure titanium or α-type titanium alloy, wherein the average value D of the equivalent circular diameter of the α phase in a cross section perpendicular to the longitudinal direction. av However, it is 100 μm or less, and the standard deviation D of the equivalent circular diameter of the α phase sd However, D sd ≤0.20 × D av The following conditions are met, and the average aspect ratio of the α phase is 4.0 or less.

Description

[Technical Field]

[0001] This application discloses titanium material, parts, a heat treatment simulation method, a heat treatment simulation apparatus, a program, and a method for manufacturing titanium material. [Background technology]

[0002] Titanium and titanium alloys are lightweight and high-strength materials used in aircraft engines and window frames, automobile connecting rods and mufflers, and consumer goods such as golf club faces and building materials. Furthermore, due to their biocompatibility, they are widely used in jewelry such as watches and eyeglass frames, and in medical applications such as implants.

[0003] This document describes an example of a manufacturing process for hot-working materials made from pure titanium or α-type titanium alloys, where the α phase is the room-temperature phase. First, ingots are produced using methods such as vacuum arc melting, electron beam melting, or plasma melting, using sponge titanium, a master alloy, or titanium scrap as melting raw materials. Next, to adjust the coarse-grained structure derived from casting, the ingots are heated to the β single-phase region, the α+β two-phase region, or the α single-phase region, and then broken down by forging or rolling to produce hot-working materials such as slabs or billets. These hot-working materials are then further hot-worked into the shape of plates or rods. These hot-worked materials, consisting of plates and rods, may be annealed and pickled to become products as is, or they may undergo further cold working or strain removal annealing depending on the required dimensions and properties to become products. In this application, these are referred to as "titanium materials." After hot working, oxide layers and surface defects may be removed by pickling or cutting.

[0004] In the ingots described above, especially large ingots, coarse crystal grains with particle sizes of several tens of millimeters are formed. In the bloc splitting process, the coarse crystal grains are refined by uniformly processing the entire ingot. This suppresses surface defects in subsequent hot working and homogenizes the structure of the entire hot-worked product. Therefore, by using materials that have undergone the bloc splitting process and uniformly controlling the structure, titanium materials with various properties have been developed.

[0005] On the other hand, a technique has been developed to directly hot work the above-mentioned ingot without performing the block splitting process. When hot working is performed directly on an ingot without the block splitting process, the coarse crystal grains of the α phase having an hcp structure cause deformation anisotropy between each crystal grain, resulting in surface irregularities in the early stages of hot working, ultimately leading to deep surface defects. Therefore, after hot working, it is necessary to remove the surface defects through processes such as descaling, which reduces the yield. To address this, when the block splitting process is omitted, methods have been proposed to modify the surface structure of the ingot by processing or heat treatment to reduce surface defects after hot working, thereby minimizing them as much as possible.

[0006] For example, Patent Document 1 discloses a technique for suppressing surface wrinkles by heating a titanium ingot to a temperature of β-transformation point + 50°C or higher, then cooling it to β-transformation point - 50°C for hot working.

[0007] Patent Document 2 discloses a technique for performing hot working on a casting of pure titanium or a titanium alloy by applying processing strain to the surface, then heating it above the recrystallization temperature to recrystallize the structure of the surface layer.

[0008] Patent Document 3 discloses a method for manufacturing a titanium material mainly composed of the α phase, in which the ingot surface is heated to a depth of more than 5 mm to a temperature of (β transformation point + 30)°C or higher and (transformation point temperature + 130)°C or lower, and then the ingot surface is cooled at a cooling rate of fan air cooling or higher, and the surface is treated to have an arithmetic mean roughness Ra of 1 to 20 μm before hot rolling.

[0009] Thus, when hot working is performed directly on an ingot without the stepping-up process, it is important to obtain stable and excellent properties after hot working. For example, from the viewpoint of formability, it is preferable to make the microstructure, such as grain size, and the standard deviation of the tensile properties of the titanium material as small as possible.

[0010] Patent Document 4 discloses a titanium alloy sheet with excellent formability, having an α-phase area ratio of 80-97%, an average α-phase grain size of 10.0 μm or less, and a standard deviation of the α-phase grain size of 2.5 μm or less.

[0011] Patent Document 5 discloses a titanium alloy sheet with high strength and excellent formability, wherein the maximum crystal grain size of the β phase is 15 μm or less, the area fraction of the α phase is 80 to 97%, the average crystal grain size of the α phase is 20 μm or less, and the standard deviation of the crystal grain size of the α phase ÷ average crystal grain size of the α phase × 100 is 30% or less, and a method for manufacturing the same.

[0012] Patent Document 6 discloses a titanium plate having α-phase grain structure, containing Fe: 0.020 to 1.000 mass%, O: 0.020 to 0.400 mass%, with the remainder being titanium and unavoidable impurities, wherein the α-phase grains have an aspect ratio of 2.0 or more on average and a standard deviation of 0.70 or more, and an equivalent circle diameter of 5 μm to 100 μm on average and 300 μm or less on maximum, and possessing both strength and formability. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Application Publication No. 8-060317 [Patent Document 2] Japanese Patent Application Publication No. 1-156456 [Patent Document 3] Patent No. 6372373 [Patent Document 4] Japanese Patent Publication No. 2009-068098 [Patent Document 5] Japanese Patent Publication No. 2010-121186 [Patent Document 6] Japanese Patent Publication No. 2016-023315 [Overview of the project] [Problems that the invention aims to solve]

[0014] As described above, when obtaining hot-worked materials of pure titanium or α-type titanium alloy, it is important that the ingot can be processed using a general hot-working apparatus, without the need for a block splitting process, and that stable and excellent properties can be obtained after processing. For example, it is preferable that the variation in the tensile properties of the rod or wire obtained after hot-working be as small as possible. (1) In the prior art, there is room for improvement in reducing the standard deviation of the tensile properties of pure titanium or α-type titanium alloy rods or wires. (2) It cannot be said that sufficient consideration has been given to hot working conditions that allow for efficient hot working of ingots of pure titanium or α-type titanium alloy using general equipment, without the need for block splitting, and that reduce the standard deviation of the tensile properties of the hot-worked material, thereby obtaining stable and excellent properties. (3) Furthermore, in the analysis of induction heating, which has a high heating rate and is therefore frequently used for reheating on manufacturing lines, magnetic field analysis is necessary in addition to FEM analysis. Since induction heating generally uses frequencies of 50 to 450 kHz, the time step for its analysis should be a maximum of 2.0 × 10⁻⁶ -9 The time step needs to be around a second. If such a time step is adopted, FEM analysis of manufacturing processes such as heat treatment and processing of ingots cannot be completed in a realistic time. In this regard, when manufacturing various metal materials such as pure titanium or α-type titanium alloys, a new analytical method is needed that can accurately match the heat transfer analysis simulating induction heating with the actual thermal history applied to the metal material. [Means for solving the problem]

[0015] This application discloses several embodiments as means for solving each of the problems (1) to (3) above. <Aspect 1> A rod-shaped or wire-shaped titanium material made of pure titanium or α-type titanium alloy, In a cross-section perpendicular to the longitudinal direction, The average value D of the equivalent circular diameter of the α phase. av However, it is less than 100 μm, Standard deviation D of the equivalent circular diameter of the α phase sd However, D sd ≤0.20 × D av Satisfying the conditions, and The average aspect ratio of the α phase is 4.0 or less. Titanium material. <Aspect 2> A titanium material according to embodiment 1, When a tensile test is performed with the longitudinal direction as the tensile direction, the standard deviation of the 0.2% yield strength is 30 MPa or less, and the standard deviation of elongation is 4.0% or less. Titanium material. <Aspect 3> A titanium material according to embodiment 1 or 2, In mass%, Fe: 0~0.50%, O: 0~0.40%, N: 0~0.05%, C: 0~0.08%, H: 0~0.013%, and Remainder: Ti and impurities Having a chemical composition consisting of, Titanium material. <Aspect 4> A titanium material according to embodiment 1 or 2, In mass%, Al: 4.0-6.0%, Sn: 2.0~3.0%, Fe: 0~0.05%, O: 0~0.20%, N: 0~0.05%, C: 0~0.10%, H: 0~0.020%, and Remainder: Ti and impurities Having a chemical composition consisting of, Titanium material. <Aspect 5> A component comprising a titanium material according to any of embodiments 1 to 4. <Aspect 6> A heat treatment simulation method including the following steps 1-3: Step 1: Using the magnetic field obtained by assuming the heating coil is a solenoid, calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil. Step 2: Calculate the temperature of each element of the titanium material based on the FEM model for heat transfer analysis of the metal material, and Step 3: Calculate the amount of heat generated by each element in the FEM model using the Joule heat distribution. <Aspect 7> The following steps I-III are included: Heat treatment simulation method of aspect 6: Step I: Perform plastic deformation analysis and heat transfer analysis on a titanium material consisting of pure titanium or α-type titanium alloy when it is heated or cooled at a predetermined temperature change rate and subjected to plastic deformation. Step II: Using 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the heat generated during processing, predict the temperature change rate in the heat transfer analysis, and Step III: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step I, or by repeatedly performing Step I and Step II to perform a convergence calculation of the temperature change rates of each phase for each time and temperature. <Aspect 8> Includes steps A to C below, Heat treatment simulation method according to embodiment 6 or 7: Step A: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step B: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of phase fractions over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Step C: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step A, or by repeatedly performing Step A and Step B to perform convergence calculations of the transformation rate of each phase and the temperature change rate for each time and temperature. <Pattern 9> A heat treatment simulation device, It has a first calculation unit, a second calculation unit, and a third calculation unit, The first calculation unit uses the magnetic field obtained by assuming the heating coil is a solenoid to calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil, The second calculation unit calculates the temperature of each element of the titanium material based on an FEM model for heat transfer analysis of the metal material, The third calculation unit calculates the amount of heat generated by each element in the FEM model using the Joule heat distribution. Heat treatment simulation device. <Aspect 10> A heat treatment simulation apparatus according to embodiment 9, It has an analysis unit, a prediction unit, and a coupled analysis calculation unit. The analysis unit performs plastic deformation analysis and heat transfer analysis of a titanium material made of pure titanium or α-type titanium alloy when the titanium material is heated or cooled at a preset temperature change rate and subjected to plastic deformation. The prediction unit uses 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the processing heat generation amount to predict the temperature change rate in the heat transfer analysis. The coupled analysis calculation unit performs the coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in the analysis unit, or by repeatedly performing the analysis in the analysis unit and the prediction in the prediction unit to calculate the convergence of the temperature change rate of each phase for each time and temperature. Heat treatment simulation device. <Aspect 11> A heat treatment simulation apparatus according to embodiment 9 or 10, It has a first prediction unit, a second prediction unit, and a coupled analysis calculation unit. The first prediction unit predicts the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material made of pure titanium or an α-type titanium alloy is heated or cooled at a preset temperature change rate, The second prediction unit predicts the latent heat of transformation associated with each phase for each time and temperature based on the transformation rate of each phase for each time and temperature predicted by the first prediction unit, and predicts the temperature change rate of the titanium material and the time change rate of the phase fraction of each phase by performing a heat transfer analysis using the predicted latent heat of transformation. The coupled analysis calculation unit performs the coupled analysis calculation by either introducing the temperature change rate predicted by the second prediction unit into the temperature change rate in the first prediction unit, or by repeatedly performing the predictions in the first prediction unit and the second prediction unit to perform convergence calculations for the transformation rate of each phase and the temperature change rate for each time and temperature. Heat treatment simulation device. <Aspect 12> A program that operates a computer as a heat treatment simulation device, To the aforementioned computer, Step 1: Using the magnetic field obtained by assuming the heating coil is a solenoid, calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil. Step 2: Calculate the temperature of each element of the titanium material based on the FEM model for heat transfer analysis of the metal material, and Step 3: Calculate the amount of heat generated by each element in the FEM model using the Joule heat distribution. A program that executes something. <Aspect 13> A program according to embodiment 12, To the aforementioned computer, Step I: Perform plastic deformation analysis and heat transfer analysis on a titanium material consisting of pure titanium or α-type titanium alloy when it is heated or cooled at a predetermined temperature change rate and subjected to plastic deformation. Step II: Using 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the heat generated during processing, predict the temperature change rate in the heat transfer analysis, and Step III: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step I, or by repeatedly performing Step I and Step II to perform convergence calculations for the temperature change rates of each phase for each time and temperature. A program that executes something. <Aspect 14> A program according to embodiment 12 or 13, To the aforementioned computer, Step A: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step B: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of phase fractions over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Step C: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step A, or by repeatedly performing Step A and Step B to perform convergence calculations of the transformation rate of each phase and the temperature change rate for each time and temperature. A program that executes something. <Aspect 15> A method for manufacturing rod-shaped or wire-shaped titanium material made of pure titanium or α-type titanium alloy, To obtain an ingot made of pure titanium or α-type titanium alloy, The ingot is heated in a heating furnace under the first heating conditions. Cooling the ingot after heating under the first heating condition under the first cooling condition, Performing first hot working on the ingot after cooling under the first cooling condition to obtain a first intermediate material, Performing second hot working on the first intermediate material to obtain a second intermediate material, and Performing cold working on the second intermediate material and then performing final annealing, or performing final annealing without performing cold working to obtain a titanium material, including The first heating condition is that the set temperature T1 of the heating furnace is T β +10 °C or higher and T β +130 °C or lower (where T β is the β transformation point), and the ingot is heated in the heating furnace for a residence time t1, so that at least the position of the surface layer 10 mm of the ingot is T β +10 °C or higher and T β +130 °C or lower, The first cooling condition is that the position of the surface layer 10 mm of the ingot is cooled at a cooling rate of 5 °C / s or higher from T β +10 °C to 300 °C or lower, The temperature of the first hot working is T β +30 °C or higher and T β +200 °C or lower, The total area reduction rate of the first hot working is 60% or higher, The second hot working includes performing intermediate heating using a heating furnace installed after the first hot working, The temperature of the second hot working is T β -200 °C or higher and T β -20 °C or lower, The total area reduction rate of the second hot working is 70% or higher, The total area reduction rate of the cold working is 30% or higher, The temperature of the final annealing is T β -350 °C or higher and T β -20 °C or lower, A method for manufacturing a titanium material. <Aspect 16> A method for manufacturing titanium material according to embodiment 15, This includes performing intermediate annealing before or during the cold working process, The temperature for the intermediate annealing is T β -350℃ or higher and T β It is below -20℃. A method for manufacturing titanium materials. <Aspect 17> A method for manufacturing titanium material according to embodiment 15 or 16, The heating furnace installed during the second hot working process is an induction heating furnace. The current value and frequency of the induction heating furnace are determined based on the results of a heat treatment simulation method according to any of embodiments 6 to 8. A method for manufacturing titanium materials. <Aspect 18> A method for manufacturing titanium material according to any of embodiments 15 to 17, The shape of the cross-section of the ingot perpendicular to its longitudinal direction is rectangular. A method for manufacturing titanium materials. <Aspect 19> A method for manufacturing titanium material according to any of embodiments 15 to 18, The area S (mm²) of the cross-section perpendicular to the longitudinal direction of the ingot. 2 The ratio L / S of the circumference L (mm) of the cross-section to ) is 0.005 or more. A method for manufacturing titanium materials. [Effects of the Invention]

[0016] The titanium material of this disclosure has a small standard deviation in its tensile properties. Furthermore, the manufacturing method of the titanium material of this disclosure allows for hot working of a pure titanium or α-type titanium alloy ingot using general equipment, without the need for splitting, and makes it possible to reduce the standard deviation in the tensile properties of the hot-worked material. Moreover, the heat treatment simulation method, apparatus, and program of this disclosure can improve the accuracy of agreement between heat transfer analysis simulating induction heating and the actual thermal history applied to the metal material, making it possible to accurately determine appropriate induction heating conditions in the manufacturing method of a titanium material made of pure titanium or α-type titanium alloy, and in the form of a rod or wire. [Brief explanation of the drawing]

[0017] [Figure 1] This shows an example of the microstructure of pure titanium or α-type titanium. [Figure 2] This shows the relationship between temperature, stress, and microstructure in heat treatment simulations. [Figure 3] The temperature dependence of density for carbon steel and pure titanium is shown. [Figure 4] This diagram schematically shows the coordinate system used during induction heating of a round billet. [Figure 5] This graph compares the analytical values ​​and experimental values ​​of the temperature transition during induction heating. [Figure 6] The analysis results for the distance from the billet surface and the maximum temperature reached are shown. [Figure 7] The results of the tissue observation in the C section of the billet related to the comparative example are shown. [Modes for carrying out the invention]

[0018] 1. Background leading to the present invention When hot-working ingots made of pure titanium or α-type titanium alloy, omitting the block splitting process, surface defects are formed in the early stages of hot working due to the deformation anisotropy of coarse grains in the α phase (hcp). To address this issue, measures are being considered to suppress surface irregularities by refining the surface structure through heat treatment and processing. When such measures are implemented, the surface structure becomes finer than the center in cross-sections perpendicular to the longitudinal direction, resulting in differences in properties. Furthermore, in subsequent hot-working of pure titanium or α-type titanium alloy, reheating is performed using heating equipment installed on the line before hot working and as needed. Induction heating, which has a high heating rate, is often used for reheating. Unlike furnace heating, the heating time is short during reheating, so a temperature difference occurs between the surface and the center in cross-sections perpendicular to the longitudinal direction. If this temperature difference is large, variations in structure and mechanical properties will occur. Moreover, if these heating temperatures are high, grain growth may occur, leading to coarser structure and a decrease in the balance between strength and ductility. On the other hand, if these heating temperatures are low, the thermal energy required for recrystallization may be insufficient, resulting in the retention of a large aspect ratio α-phase that remains stretched, potentially leading to a decrease in ductility. Furthermore, depending on the manufacturing conditions, there may be a large difference in microstructure between the surface and the center within a cross-section perpendicular to the longitudinal direction, causing variations in mechanical properties. Since heat is released from the surface layer exposed to the atmosphere during transport, it is possible to control the microstructure uniformly after hot working by heating the surface layer to a higher temperature than the interior, ensuring uniform temperature distribution during hot working. Therefore, in order to propose a rolling process that can be realized within equipment constraints, we investigated methods for predicting the temperature during induction heating and methods for predicting the microstructure using these methods.

[0019] As a result of diligent research, the inventors have discovered a method for easily predicting temperature changes during induction heating used in reheating during hot working processes. Furthermore, they have found that by using the manufacturing conditions set using this method, it is possible to obtain a titanium material made of pure titanium or α-type titanium alloy, in which the metal structure in the cross-section perpendicular to the longitudinal direction is fine, has little variation, and exhibits little variation in mechanical properties. The following describes a titanium material, its manufacturing method, parts, and a heat treatment simulation method, heat treatment simulation apparatus, and program according to one embodiment.

[0020] 2. Titanium material The titanium material of this disclosure is a rod-shaped or wire-shaped titanium material made of pure titanium or an α-type titanium alloy. The titanium material of this disclosure has an average value D of the equivalent circular diameter of the α phase in a cross section perpendicular to the longitudinal direction. av However, it is 100 μm or less, and the standard deviation D of the equivalent circular diameter of the α phase sd However, D sd ≤0.20 × D av The titanium material satisfies the following conditions, and the average value of the aspect ratio of the α phase is 4.0 or less. The titanium material of this disclosure has a small standard deviation of tensile properties because the average value of the equivalent diameter of the α phase, the standard deviation of the equivalent diameter of the α phase, and the average value of the aspect ratio of the α phase are below a predetermined level. For example, when a tensile test is performed with the longitudinal direction as the tensile direction, the titanium material of this disclosure may have a standard deviation of 30 MPa or less for the 0.2% yield strength and a standard deviation of 4.0% or less for the elongation.

[0021] 2.1 Chemical composition The chemical composition of the titanium material disclosed herein consists of pure titanium or α-type titanium alloy. These materials have an α-phase with high deformation anisotropy as the main constituent phase at room temperature. The chemical composition of the titanium material disclosed herein may be that which is described as pure titanium or α-type titanium alloy in JIS H4600 (plates and strips), JIS H4650 (bars), JIS H4670 (wires), ASTM B265 (Strip, Sheet, and Plate), and ASTM B348 (Bars and Billets), or it may be otherwise. For example, the titanium material may have a chemical composition consisting of, by mass%, Fe: 0-0.50%, O: 0-0.40%, N: 0-0.05%, C: 0-0.08%, H: 0-0.013%, and the remainder: Ti and impurities. Alternatively, the titanium material may have a chemical composition consisting of, by mass%, Al: 4.0-6.0%, Sn: 2.0-3.0%, Fe: 0-0.05%, O: 0-0.20%, N: 0-0.05%, C: 0-0.10%, H: 0-0.020%, and the remainder being Ti and impurities. As mentioned above, the remainder of the chemical composition of the titanium material according to this embodiment may be Ti and impurities. Examples of impurities include, but are not limited to, Cl, Na, Mg, Ca, B introduced during the refining process, etc., and Zr, Sn, Mo, Nb, Ta, V introduced from scrap, etc. It is acceptable if the content of each element is 0.1% or less and the total amount of impurities is 0.5% or less.

[0022] 2.2 Shape The shape of the titanium material in this disclosure is rod-shaped or linear. The rod-shaped or linear titanium material may have a length of, for example, 0.1m to 100m, 0.1m to 50m, or 0.1m to 10m, and the equivalent diameter of the cross-section perpendicular to its longitudinal direction may be 3mm to 50mm, 5mm to 40mm, or 8mm to 20mm. The shape of the cross-section perpendicular to the longitudinal direction of the rod-shaped or linear titanium material is not particularly limited and may be, for example, circular (including elliptical and oblong shapes), angular, or other shapes. In this application, the "longitudinal direction" of the titanium material means the direction that coincides with the hot rolling direction of the titanium material.

[0023] 2.3 Organization Figure 1 shows an example of the microstructure of pure titanium or α-type titanium alloy. When an ingot is subjected to hot and cold working with sufficient strain, it develops a microstructure consisting of the α phase and trace amounts of β phase at the grain boundaries of the α phase, as shown in Figure 1.

[0024] 2.3.1 α phase In Figure 1, the α phase consists of equiaxed grains with a small aspect ratio. However, when the strain is small, and / or when the final heat treatment temperature is low or the final treatment time is short, it takes on an elongated form with a large aspect ratio. Here, in a cross-section perpendicular to the longitudinal direction of the titanium material, the average value D of the circular equivalent grain size of the α phase is... av As the value increases, the balance between the strength and ductility of the titanium material tends to decrease. This is because dislocations during processing tend to accumulate at the grain boundaries of the α phase, and as the equivalent diameter of the circle increases, the internal stress generated at the grain boundaries increases, making it easier for voids, which are the starting points for tensile and fatigue fracture, to form. In this embodiment, the average value D of the equivalent grain size of the α phase av The diameter is 100 μm or less, preferably 90 μm or less, more preferably 80 μm or less, even more preferably 70 μm or less, and particularly preferably 60 μm or less. Average value D of the equivalent circle diameter of the α phase av The lower limit is not particularly limited, but considering mass production, it may be 5 μm or more, and may also be 10 μm or more, 15 μm or more, 20 μm or more, or 25 μm or more.

[0025] In a cross-section perpendicular to the longitudinal direction of the titanium material, the standard deviation D of the equivalent circular diameter of the α phase. sd As the standard deviation increases, stress concentration in coarse grains becomes more likely, and the macroscopic strength and ductility of titanium materials tend to decrease. sd This increases as the temperature variation during hot working increases. In this embodiment, the standard deviation D of the equivalent circular diameter of the α phase sd D sd ≤0.20 × D av Satisfying the conditions, preferably D sd ≤0.15 × D av Satisfying, more preferably D sd ≤0.10 × D av Satisfying the standard deviation D of the equivalent circular diameter of the α phase. sd The lower limit is not particularly limited; for example, 0.01 × D av ≤D sd or 0.05 × D av ≤D sd It may also satisfy the following: Standard deviation D of the equivalent circular diameter of the α phase. sd For example, it may be 3.0 μm or more, 4.0 μm or more, or 5.0 μm or more, and may also be 15.0 μm or less, 13.0 μm or less, or 11.0 μm or less.

[0026] In pure titanium or α-type titanium alloys, the α-phase elongates in the machining direction during processing in the β-single-phase region and / or the α-single-phase region, increasing its aspect ratio. At the grain boundaries of such α-phases with large aspect ratios, stress concentration occurs during processing, making voids likely to form. These voids propagate along the grain boundaries of the α-phase, easily becoming the starting point for fracture in tension and fatigue. In the titanium material of this disclosure, as described later, annealing after processing promotes recrystallization and reduces the aspect ratio of the α-phase. In this embodiment, the average value of the aspect ratio of the α-phase is 4.0 or less, preferably 3.5 or less, more preferably 3.0 or less, even more preferably 2.5 or less, and particularly preferably 2.0 or less. The lower limit of the average value of the aspect ratio of the α-phase is not particularly limited, and is 1.0 or more, but may be greater than 1.0, 1.1 or more, or 1.2 or more.

[0027] Furthermore, the average value, standard deviation, and aspect ratio of the α-phase's equivalent diameter are thought to interact with each other, resulting in complex properties. These properties are also affected by temperature and strain during manufacturing; therefore, detailed analysis of manufacturing conditions using calculations such as FEM is necessary for their control.

[0028] The average value of the α-phase equivalent particle size, the standard deviation of the α-phase equivalent particle size, and the aspect ratio of the α-phase are measured as follows. (1) Observe any nine locations on the cross section perpendicular to the longitudinal direction of the titanium material using an optical microscope. For example, if the equivalent diameter of the circle of the cross section perpendicular to the longitudinal direction of the titanium material is X mm, observe a total of nine locations on the optical microscope: four at a depth of X / 50, four at X / 4, and one at X / 2, starting from the outer edge of the cross section and moving inward. Each observation field of view should be 3 mm × 3 mm. The four observation locations at depth X / 50 are on two straight lines passing through the centroid of the cross section perpendicular to the longitudinal direction of the titanium material (the approximate center of the 3 mm × 3 mm field of view is on these straight lines), and these two straight lines are orthogonal to each other (i.e., the four observation locations can be in the cardinal directions). The same applies to the four observation locations at depth X / 4. If there are overlapping areas, such as with round bars with a diameter of φ15 mm or less, the entire cross section should be the subject of observation. (2) Identify the α-phase contained in the observation field and measure its major and minor axes, assuming it is an ellipse. Specifically, the largest diameter of the α-phase is considered as the major axis, and the longest line segment between one point on the grain boundary of the α-phase and another, which is perpendicular to the major axis, is considered as the minor axis, and an ellipse having these major and minor axes is assumed. The equivalent diameter of the circle is the geometric mean of the major and minor axes, and the aspect ratio = major axis / minor axis. For all nine fields of view described above, determine the equivalent diameter of the circle and the aspect ratio for at least 80 α-phases (i.e., obtain a total of at least 720 values). Note that when measuring the major and minor axes of the α-phase, there are methods such as observing and measuring the sample after mirror polishing and etching the grain boundary with an optical microscope, or measuring by SEM-EBSD by considering a region with an orientation difference of 3° or more from an adjacent region as an equiaxial α-phase, but in this application, the former method will be adopted. (3) For the 720 or more equivalent circle diameters obtained, calculate their arithmetic mean and define this as the "average equivalent circle diameter of the α phase," and calculate its standard deviation and define this as the "standard deviation of the equivalent circle diameter of the α phase." Also, for the 720 or more aspect ratios obtained, calculate their arithmetic mean and define this as the "average aspect ratio of the α phase."

[0029] 2.3.2 Other Organizations As described above, titanium materials consisting of pure titanium or α-type titanium alloys may have trace amounts of β-phase present at the grain boundaries of the α-phase. As stated above, in the titanium materials of this disclosure, variations in tensile properties are suppressed by controlling the equivalent circular diameter and aspect ratio of the α-phase, so the shape of other structures included in the grain boundaries is not particularly limited.

[0030] 2.4 Mechanical Properties As described above, the titanium material of this disclosure has good tensile properties, and in particular, the standard deviation of its tensile properties is small. For example, when a tensile test is performed on the titanium material of this disclosure with the longitudinal direction as the tensile direction, the standard deviation of the 0.2% proof stress may be, for example, 30 MPa or less. Also, the standard deviation of elongation may be, for example, 4.0% or less.

[0031] 2.5 Applications (Parts) The technology disclosed herein also has aspects of a component containing the titanium material described above. The type of such component is not particularly limited. For example, the component may be one of the following: aircraft frame or structural material, airframe components such as seat rails and window frames, engine components, automobile connecting rods, valves and mufflers, or decorative items such as watches and eyeglass frames. The titanium material disclosed herein can be suitably used in consumer goods such as eyeglass frames and watches, as well as in structural materials for aircraft and automobiles.

[0032] 3. Manufacturing method of titanium material The technology disclosed herein also has aspects as a method for manufacturing titanium materials. One embodiment of the method for manufacturing a rod-shaped or wire-shaped titanium material made of pure titanium or an α-type titanium alloy is: To obtain an ingot made of pure titanium or α-type titanium alloy, The ingot is heated in a heating furnace under the first heating conditions. The ingot, after being heated under the first heating conditions, is cooled under the first cooling conditions. A first hot working process is performed on the ingot after it has been cooled under the first cooling conditions to obtain a first intermediate material. The first intermediate material is subjected to a second hot working process to obtain a second intermediate material, and To obtain a titanium material, the second intermediate material is subjected to cold working followed by final annealing, or to be subjected to final annealing without cold working. This includes, The first heating condition is that the set temperature T1 of the heating furnace is T β +10℃ or higher and T β Below +130℃ (here, T β (where is the β transformation point), and as the ingot is heated in the heating furnace for a time t1, the position at least 10 mm from the surface of the ingot is T β +10℃ or higher and T β The conditions were set so that the temperature would be below +130℃. The first cooling condition is that the position of the surface layer 10 mm of the ingot is T β The conditions are set so that the temperature is cooled from +10°C to below 300°C at a cooling rate of 5°C / s or more. The temperature of the first hot working process is T β +30℃ or higher and T β Below +200℃, The total reduction ratio of the first hot working is 60% or more. The second hot working includes performing intermediate heating using a heating furnace installed after the first hot working, The temperature of the second hot working process is T β -200℃ or higher and T β Below -20℃, The total reduction ratio of the second hot working process is 70% or more. The total reduction ratio of the cold working is 30% or more. The temperature of the final annealing is T β -350℃ or higher and T β The temperature is below -20℃.

[0033] In the above manufacturing method, the heating conditions in the heating furnace and the hot working conditions must be set to the optimal conditions according to the chemical composition, shape (rectangular, cylindrical), and dimensions of the ingot and intermediate material. In this case, it is preferable to predict the optimal conditions using a heat treatment simulation with analysis, and then set the conditions for actual operation based on the prediction results. That is, in the manufacturing method disclosed herein, it is preferable that the above-mentioned various conditions be determined based on the results of a predetermined heat treatment simulation method. Details of the heat treatment simulation will be described later.

[0034] The manufacturing method disclosed herein includes, for example, the ingot (T β +10℃) or more, (T β The process may include heat treatment at a set temperature T1 of a heating furnace (+130℃) or lower for a predetermined time t1, followed by cooling at a cooling rate faster than air cooling. The top 10 mm of the ingot has a structure that undergoes α⇒β transformation after being heated to above the β transformation point during this heat treatment and then cooled.

[0035] 3.1 Ingot Ingots made of pure titanium or α-type titanium alloy may be obtained by conventionally known methods. The chemical composition of the ingot may be the same as that of the titanium material described above. Furthermore, the microstructure of the ingot may be the as-cast microstructure. That is, the microstructure of the ingot may consist of columnar or equiaxed prior β grains several tens of mm in length, with the interior composed of two or more α grains. Ingots obtained by electron beam melting or plasma arc melting are preferred over those obtained by conventional consumable electrode vacuum arc remelting. This is because consumable electrode vacuum arc remelting is limited to cylindrical shapes with L / S = 0.005, whereas electron beam melting and plasma arc melting offer greater freedom in mold shape and can melt ingots with an L / S of a predetermined value or higher. In addition, electron beam melting has a higher output than plasma arc melting and can be held at a high temperature after melting, resulting in a more uniform temperature during solidification, a reduction in the standard deviation of particle size, and the ability to remove impurities such as O, C, and N.

[0036] The shape of the ingot is not particularly limited. For example, the cross-sectional shape perpendicular to the longitudinal direction of the ingot may be round or rectangular. For example, if the cross-sectional shape perpendicular to the longitudinal direction of the ingot is rectangular, it can be hot-rolled as is to process the ingot into a plate, wire, square bar, round bar, etc. Also, if the cross-sectional shape perpendicular to the longitudinal direction of the ingot is circular, it can be hot-rolled as is to process the ingot into a plate, wire, square bar, round bar, etc. The ingot may have width, thickness, and length. In this case, the width of the ingot may be 50 mm or more and 1500 mm or less, the thickness may be 30 mm or more and 1500 mm or less, and the length may be 0.1 m or more and 100 m or less. If the cross-sectional shape of the ingot is circular, the width and thickness of the ingot will substantially coincide with the diameter of the circle.

[0037] In the above manufacturing method, the area S (mm²) of the cross-section (transverse plane) perpendicular to the longitudinal direction of the ingot is 2The ratio L / S of the circumference L (mm) of the cross-section to the surface is preferably 0.005 or higher. This is because a larger L / S for titanium material results in greater heat removal from the surface during cooling, thus reducing the difference in thermal history between the surface and the center of the ingot and reducing the difference in microstructure. Furthermore, the time until solidification and the cooling time after solidification are shortened, and the microstructure becomes finer, improving the mechanical properties. From the viewpoint of increasing heat removal, a larger L / S is better, so there is no particular upper limit. As mentioned above, L / S is preferably 0.005 or higher, more preferably 0.006 or higher, and even more preferably 0.008 or higher. In addition, L / S may be, for example, 0.030 or less, 0.025 or less, or 0.020 or less.

[0038] 3.2 First heating conditions In the manufacturing method of the present disclosure, the ingot is heated in a heating furnace under first heating conditions. The first heating conditions are that the set temperature T1 of the heating furnace is T β +10℃ or higher and T β Below +130℃ (here, T β (where is the β transformation point), and as the ingot is heated in the heating furnace for a time t1, the position at least 10 mm from the surface of the ingot is T β +10℃ or higher and T β The conditions were set so that the temperature would be below +130℃.

[0039] The surface layer of the ingot is 10 mm thick. β To heat to +10℃ or higher, the set temperature T1 of the heating furnace is T β The temperature must be above +10℃. To stably promote the α⇒β transformation, the set temperature T1 should be T β A temperature of +30°C or higher is preferable. On the other hand, if the set temperature T1 is T β When the temperature exceeds +130℃, the initial β particle size tends to exceed 10mm due to the β single-phase temperature range and grain growth during cooling, resulting in the initial β particle size becoming coarser than before heat treatment. Also, when the set temperature T1 is T β When the temperature exceeds +130℃, a thick, hard oxygen-enriched layer forms on the surface of the ingot due to a reaction with oxygen in the atmosphere. This can cause cracking and indentation during hot rolling, requiring additional treatment such as mechanical cutting or grinding.β +10℃ or higher and T β By keeping the temperature below +130℃, oxidation of the ingot's surface layer can be suppressed while simultaneously allowing the surface layer to be made finer.

[0040] The furnace time t1 in a heating furnace is the time the ingot is heated in a heating furnace at the above-mentioned set temperature T1, and is determined by the size, shape, and heating method of the ingot. Naturally, the heat applied to the surface diffuses into the interior. If the ingot is large, or if it is cylindrical with a small (surface area / volume) ratio, the heat diffusion from the surface to the interior is small, so the furnace time t1 will be longer.

[0041] 3.3 First Cooling Conditions In the manufacturing method of the present disclosure, the ingot heated under the above-described first heating conditions is cooled under first cooling conditions. The first cooling conditions are such that the position of the surface layer 10 mm of the ingot is T β The conditions are set so that the temperature is cooled from +10°C to below 300°C at a cooling rate of 5°C / s or more.

[0042] The surface granulation by heating described above is insufficient to adequately reduce surface defects after hot working. In the manufacturing method of this disclosure, after heating described above, the T at the 10 mm position of the surface layer β By cooling the material from +10°C to below 300°C at a rate of 5°C / s or more, cooling strain is introduced into the surface layer. When heating to the β single-phase region before subsequent hot working, this type of cooling causes an α⇒β transformation using the cooling strain as driving energy, further refining the existing β grain size. This suppresses deformation anisotropy after hot working and neutralizes surface defects after hot working. The method of rapid cooling is selected according to the dimensions and shape of the titanium material. For example, it may be at least one method selected from cooling with a fan, gas cooling with inert gases such as He, Ar, or N2, cooling with mist, and water cooling by immersion in water. In addition, when using water cooling by immersion, it is also possible to circulate the water with a pump or the like.

[0043] Furthermore, the presence or absence of cooling strain in the surface layer can be confirmed by the difference in Vickers hardness ΔHV between the 10mm position on the surface and the center of the cross-section. Vickers hardness is affected by the cooling rate and the prior β particle size, but if the Vickers hardness at 10mm on the surface layer is 20HV or more greater than that at the center of the cross-section, it can be said that the effect of cooling strain, in addition to finer grain development, is clearly evident, and the desired cooling strain has been introduced.

[0044] 3.4 Additional information about time The above heating and cooling can reduce surface defects after hot working, however, β +10℃ or more, T β If the furnace time t1 in the set temperature range of +130℃ or below is not set appropriately, the prior β grain size at the 10mm position of the surface layer will vary, leading to problems such as the occurrence of wrinkles on the surface during hot working. In particular, in the heat treatment of ingots with a rectangular cross-section, the longer side of the cross-section perpendicular to the longitudinal direction is less heated and less cooled than the shorter side. Therefore, if the furnace time t1 is short, the temperature at the 10mm position of either the long or short side of the surface layer will drop below the β transformation point before cooling, so T β The cooling rate from +10°C to below 300°C cannot be set to 5°C / s or higher, making it difficult to introduce cooling strain into the surface. As a result, the effect of refining the prior β grain size cannot be obtained during heating before subsequent hot working, and surface defects may occur after processing. Thus, when hot working is performed on an ingot having an edge with a larger prior β grain size and deformation anisotropy compared to other parts, if that edge is a side perpendicular to the roll contact surface, irregularities will occur due to deformation anisotropy, resulting in surface defects. Furthermore, when the first hot working is performed by hot rolling, if that edge is the roll contact surface, problems such as warping due to deformation anisotropy may occur, which may lead to the stopping of rolling, and variations in material properties after hot working may occur. In the manufacturing method of this disclosure, these problems can be more effectively suppressed by setting appropriate heating conditions based on the results of the heat treatment simulation described later.

[0045] Furthermore, if the time t2 between the completion of heating and the start of cooling is too long, the surface temperature will reach T before cooling begins. βThe temperature drops below the point where the β⇒α transformation is completed, making it difficult to obtain the effects of rapid cooling as described above. Time t2 may correspond to, for example, the transport time from the heating step to the cooling step. In the manufacturing method of this disclosure, it is preferable to set an appropriate time t2 until the start of cooling based on the results of the heat treatment simulation described later.

[0046] 3.5 Heat treatment simulation method related to the first embodiment As a method for appropriately selecting the set temperature T1 and furnace time t1 in a heating furnace, we investigated a method that couples the temperature and phase fraction of pure titanium or α-type titanium alloy. In considering the furnace time t1, it is important that the temperature calculation accuracy is high, as is the consideration of β / α transformation and the prediction accuracy of the prior β grain size. Conventional heat transfer analysis only analyzes heat conduction due to the temperature gradient, so in titanium, which generates a large latent heat of transformation with β / α transformation, the calculation accuracy near the transformation point is low, and because β / α transformation during heating and cooling cannot be determined, there is a problem in that the prediction accuracy of the prior β grain size is low due to these factors.

[0047] On the other hand, in the heat treatment analysis of steel, as shown in Figure 2, analyses that couple the temperature distribution, stress, deformation, and phase transformation of the heat-treated area have been devised and implemented. Phase transformation is considered as an isothermal transformation progressing over a very small time interval, from austenite to ferrite, pearlite, bainite, and martensite. The analysis is performed using an isothermal transformation diagram and the isothermal transformation rate, and the latent heat of transformation during the phase transformation is considered in the heat transfer analysis, while the transformation strain is considered in the elastoplastic analysis. Efforts have also been made to consider the stress dependence of the transformation rate. Therefore, we considered using these methods for the heat transfer analysis of pure titanium or α-type titanium alloy and designing a processing schedule using the obtained temperature history.

[0048] Figure 3 shows the temperature dependence of the density of carbon steel and pure titanium, respectively. In carbon steel, the density changes significantly at point A1 (approximately 727°C) due to α / γ transformation. Therefore, the strain caused by the transformation (transformation strain) is large, and the residual stress generated during quenching from the γ region may affect the durability of the product. On the other hand, in pure titanium and α-type titanium alloys, β / α transformation occurs at the β transformation point (approximately 880°C for pure titanium), but no significant change in density is observed. Therefore, the transformation strain is smaller than that of steel, and its effect on the stress state is considered to be small. In other words, the effect of volume change due to transformation is small during heating as well as during cooling, and does not need to be considered, but the latent heat associated with transformation needs to be considered. By considering the latent heat of transformation, the accuracy of temperature-microstructure coupled simulations can be improved, and it is considered possible to provide pure titanium or α-type titanium materials with stable microstructures that strongly affect material properties and manufacturing defects. Furthermore, by determining the timing of the β / α transformation and performing the cooling process at the optimal timing when the proportion of the β phase, which has superior processability compared to the α phase, is optimal (performing the process when it has suitable deformation resistance for processing), it is possible to provide pure titanium or α-type titanium with a stable structure.

[0049] The heat treatment simulation method according to the first embodiment is based on the above technical concept. Specifically, the heat treatment simulation method according to the first embodiment includes the following first step, second step, and coupled analysis step. Step 1: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or an α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step 2: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of the phase fraction of each phase over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Coupled analysis step: The coupled analysis calculation is performed by either introducing the predicted temperature change rate into the temperature change rate in the first step, or by repeatedly performing the first and second steps to calculate the convergence of the transformation rate of each phase and the temperature change rate for each time and temperature.

[0050] Here, in the first step, the transformation start time may be when the β transformation point temperature is reached, and the transformation rate of each phase may be the β / α transformation rate, that is, the transformation rate from the α phase to the β phase during heating (transformation rate of α⇒β transformation), and the transformation rate from the β phase to the α phase during cooling (transformation rate of β⇒α transformation). In addition, in the second step, the heat transfer analysis may be performed using a three-dimensional transient heat conduction equation. Furthermore, in the above heat treatment simulation method, the deformation resistance at each time may be predicted based on the temperature and phase fraction at each time obtained by the coupled analysis calculation. Furthermore, the prior β particle size at each time may be predicted based on the temperature and phase fraction at each time obtained by the coupled analysis calculation.

[0051] 3.5.1 Step 1 In the first step, the transformation start time and the transformation rate of each phase for each time and temperature are predicted for the β / α transformation of the pure titanium or α-type titanium alloy when cooled or heated at a predetermined temperature change rate. Below, the method for predicting the transformation start time and the method for predicting the transformation rate of each phase for each time and temperature will be explained sequentially. Note that the transformation rate of each phase may be predicted for all constituent phases that may be included in the pure titanium or α-type titanium alloy, but in the following explanation, as an example, the prediction of the transformation rate of the transformation from the β phase to the α phase (β⇒α transformation) during cooling will be explained. Note that the initial value of the predetermined temperature change rate may be obtained by heat transfer analysis from the temperature distribution at the time immediately preceding. Furthermore, the prediction method below can also be easily applied to the transformation from the α phase to the β phase (α⇒β transformation) during heating.

[0052] When pure titanium is cooled from the β single-phase region, plate-like α phase (α grains) precipitate (diffusion transformation) from the grain boundaries of the β phase toward the grains. Among the α grains, clumps that are aligned in the same orientation are called colonies. Since the β / α diffusion transformation begins quickly after reaching the β transformation point, the transformation start timing can be set to the time when the β transformation point is reached. Note that the β / α transformation of pure titanium type 1 is completed in a narrow temperature range of 875-880°C. As the number of added elements increases, the temperature range in which the α+β two-phase region is reached widens. In this range, the transformation start timing of the β / α diffusion transformation can be assumed to be the temperature at which the equilibrium phase fraction, for example, determined from thermodynamic calculations, changes. In other words, in the first step, the transformation start time may be the time when the equilibrium phase fraction changes in thermodynamic calculations, and the transformation rate of each phase may be the transformation rate of the β / α transformation.

[0053] To predict the rate of diffusion transformation from the β phase to the α phase (transformation rate), for example, the following equation, which is a modified version of the Johnson-Mehl equation, can be used.

[0054]

number

[0055] Here, f(T) and n(T) are material constants, which are functions that depend on the alloy type and temperature T. For JIS Grade 1 and JIS Grade 2 pure titanium, f(T) = 2.0 and n = 2.0 are appropriate.

[0056] 3.5.2 Step 2 In the second step, based on the transformation rates of each phase for each time and temperature predicted in the first step, the latent heat of transformation associated with the β / α transformation is predicted for each time and temperature. Then, using the predicted latent heat of transformation, a heat transfer analysis is performed to predict the temperature change rate of the pure titanium or α-type titanium alloy and the time change rate of the phase fraction of each phase. In the above heat treatment simulation, it is preferable to couple the analysis of the phase transformation in the first step and the temperature analysis according to the heat conduction equation in the second step. In the second step, it is preferable to perform the heat transfer analysis using a three-dimensional transient heat conduction equation, which is commonly used in finite element methods (FEM). An example of a three-dimensional transient heat conduction equation is shown below.

[0057]

number

[0058] Here, ρ: density, c p : constant-pressure specific heat, k: thermal conductivity, T: temperature, t: time, Q: heat generation. Note that the heat conduction equation used in the second step is not limited to equation (1), and depending on the shape of the simulation target, transient heat conduction equations for two-dimensional, one-dimensional, or axisymmetric bodies may be used. Furthermore, on the surface of the simulation target, heat transfer from different materials considering convection, radiation, and contact resistance may be taken into account.

[0059] The heat generated Q includes Joule heat due to induction heating, processing heat generated by plastic deformation work, and latent heat of transformation associated with phase transformation, as described by the following equation.

[0060]

number

[0061] Here, H βα : Latent heat of transformation from β phase to α phase, ξ i : These are the volume fractions of the β phase and the α phase. These volume fractions of the β phase and the α phase can be determined from the transformation rates for each temperature predicted in the first step.

[0062] Furthermore, in the heat treatment process, pure titanium and α-type titanium alloy are considered to be a mixture of these two phases, and the linear mixing law can be applied to physical quantities such as specific heat and thermal conductivity. That is, the physical quantity C used in the heat transfer analysis is given by the following equation. C in equation (3) i is, i These are physical quantities of the single-phase state (β phase, α phase). The physical quantity C given by equation (3) is the same as ρ and C in equation (1). p This includes k.

[0063]

number

[0064] Using equation (3), it is possible to predict not only physical quantities but also macroscopic mechanical properties, such as deformation resistance and fatigue characteristics.

[0065] 3.5.3 Coupled Analysis Step The above coupled analysis of temperature and phase transformation is applicable regardless of whether it is an explicit or implicit method. In the explicit method, the temperature change rate of pure titanium or α-type titanium alloy predicted in the second step is introduced into the temperature change rate in the first step to perform the coupled analysis calculation. In the implicit method, the coupled analysis calculation is performed by repeatedly performing the first and second steps to converge the transformation rate of each phase and the temperature change rate for each time and temperature. Either method can be used for the calculation in the above heat treatment simulation method. Furthermore, the above simulation method is applicable not only to heat treatment but also to simulations that simultaneously consider plastic deformation.

[0066] The above heat treatment simulation can be performed not only using custom programs written in languages ​​such as Fortran or C, but also using the finite element method (FEM) based on the Galerkin method, where the temperature T is defined as a linear sum of the temperature vector φ and weight function N at each junction.

[0067] This coupled analysis of temperature and phase fraction allows for the prediction of the temperature and phase fraction history during heat treatment with higher accuracy compared to conventional heat transfer analysis. Below, we will further explain an example of a method for predicting prior β particle size using the temperature and phase fraction predicted in this way.

[0068] In the α-single-phase and β-single-phase regions, α-grains and β-grains grow according to the following n-th power law.

[0069]

number

[0070] Here, d0 is the initial particle size, d is the particle size after holding at temperature T for t seconds, R is the gas constant, and C and Q are material constants.

[0071] Also, particle size d p From the parent phase to the particle size after transformation d s This can be predicted using the following prediction formula, which takes the grain boundary surface as the nucleation site.

[0072]

number

[0073] Here, Is: nucleation rate, and G: growth rate. In this manner, the prior β particle size at each time can be predicted based on the temperature and phase fraction at each time obtained by coupled analysis calculations.

[0074] As described above, in the manufacturing method of this disclosure, the first heating conditions, first cooling conditions, etc., can be determined based on the results of the heat treatment simulation method according to the first embodiment described above.

[0075] 3.6 First Hot Working In the manufacturing method of this disclosure, a first hot working is performed on the ingot after cooling under the above first cooling conditions to obtain a first intermediate material. The temperature of the first hot working is T β +30℃ or higher and T βIt is below +200°C, and the total area reduction rate of the first hot working is 60% or more. The first hot working may be, for example, hot rolling by a rolling mill. The first hot working is, for example, heating an ingot after heating and cooling under the above first heating conditions and first cooling conditions in a heating furnace, and the temperature of the surface and inside of the ingot is T β +30°C or more and T β +200°C or less, and then hot rolling with a total area reduction rate of 60% or more may be performed by one stand or two or more stands of rolling mills.

[0076] If the temperature of the first hot working is too low, the casting structure will not recrystallize sufficiently and will not be refined, resulting in problems such as coarse crystal grains remaining after subsequent hot working or a large standard deviation. Also, if the temperature of the first hot working is too high, there is a problem that the thick oxide layer formed on the surface is pushed in during hot working and becomes a surface defect. When the temperature of the first hot working is T β +30°C or more and T β + less than 200°C, these problems are solved. Also, if the area reduction rate in the first hot working is too low, the casting structure will not be strained sufficiently and will not be refined, resulting in problems such as coarse crystal grains remaining after subsequent hot working or a large variation in the grain size of the crystal grains due to different progress of recrystallization depending on the initial orientation. When the area reduction rate of the first hot working is 60% or more, such problems are solved.

[0077] 3.7 Second hot working In the manufacturing method of the present disclosure, the first intermediate material obtained by the above first hot working is subjected to a second hot working to obtain a second intermediate material. The second hot working includes performing intermediate heating using a heating furnace installed after the above first hot working. The temperature of the second hot working is T β -200°C or more and T β - less than 20°C, and the total area reduction rate of the second hot working is 70% or more. The second hot working may be, for example, hot rolling by a rolling mill. The second hot working is, for example, the temperature of the surface and inside of the first intermediate material obtained by the above first hot working is T β -300°C or more and T βThe process may involve performing hot rolling at a temperature below -20°C using two or more rolling mills to achieve a total reduction of 70% or more, and further adjusting the temperature to the above-mentioned temperature by performing intermediate heating using a heating furnace installed on the line between the two or more rolling mills. Alternatively, the second hot working process may involve, for example, performing intermediate heating on the first intermediate material obtained by the first hot working process using a heating furnace installed on the line transporting it from the first hot working equipment to the second hot working equipment, without cooling the first intermediate material, thereby adjusting the temperature of the first intermediate material to the above-mentioned temperature.

[0078] In the second hot working process, the first intermediate material is T β Heating to temperatures below T is done to process the material into a shape close to the product shape and to refine the microstructure obtained by splitting the casting. If the temperature in the second hot working stage is too low, the surface temperature will drop, especially due to heat dissipation through contact with the rolls, resulting in insufficient ductility and surface defects such as wrinkles and scratches, requiring additional treatment by mechanical cutting or grinding. On the other hand, if the temperature in the second hot working stage is too high, the temperature will exceed the β transformation point due to the heat generated during processing, resulting in a large difference in microstructure between the inside and outside. β -300℃ or higher and T β These problems are resolved when the temperature is below -20℃.

[0079] The heating furnaces installed on the transfer line from the first hot working to the second hot working and on the intermediate line during the second hot working may be atmospheric furnaces, may be atmosphere furnaces, may be induction heating furnaces, or may be combinations thereof. In particular, the intermediate heating in the second hot working is preferably performed by an induction heating furnace. The indicators for controlling the temperature of the first intermediate material in these heating furnaces are the furnace temperature and the residence time in the furnace for an atmospheric furnace or an atmosphere furnace, and the current value and its frequency for an induction heating furnace, and are determined from the transfer time, the pass schedule, the shape and size of the first intermediate material, and the alloy type (including emissivity, specific heat, thermal conductivity, and latent heat of transformation). Naturally, as the transfer time becomes longer, the amount of heat extraction from the surface that occurs before the hot working increases, so the amount of heat that needs to be applied to the first intermediate material increases. On the other hand, when the amount of heat generation is large due to a large processing amount, it is necessary to set the amount of heat such that the temperature of the entire billet does not exceed the β transformation point. Considering the above, the lower limit of the target temperature of the titanium material in reheating is T β -300°C. On the other hand, for the upper limit temperature during reheating, T β -20°C is set so as not to exceed the β transformation point due to the heat generation during the subsequent hot working.

[0080] In addition, if a casting structure remains on the surface of the ingot or the first intermediate material, unevenness will occur on the surface at the initial stage of hot working due to the deformation anisotropy within the grains and between the grains caused by the coarse crystal grains of the α phase, and finally deep surface defects will occur. In the manufacturing method of the present disclosure, since heating under the first heating conditions and cooling under the first cooling conditions are performed before the first hot rolling, such problems are unlikely to occur.

[0081] 3.8 Cold Working In the manufacturing method of this disclosure, cold working may or may not be performed after the second hot working described above. Specifically, the manufacturing method of this disclosure includes obtaining a titanium material by cold working the second intermediate material and then performing final annealing, or by performing final annealing without cold working. When cold working is performed, the total reduction in surface area is 30% or more. In cold working, it is preferable to apply sufficient strain to the α phase and promote recrystallization of the α phase in the subsequent final annealing. For this reason, the total reduction in surface area is 30% or more. There is no particular upper limit to the total reduction in surface area, but it may be difficult to perform processing with a reduction in surface area of ​​70% or more without intermediate annealing.

[0082] 3.9 Intermediate annealing The manufacturing method of the present disclosure may include performing intermediate annealing before or during the cold working described above. Here, the temperature of the intermediate annealing is T β -350℃ or higher and T β The temperature may be below -20°C. Performing intermediate annealing at a predetermined temperature before or during cold working can be expected to promote recrystallization of the work-hardened structure, thereby controlling the structure and softening it to allow for subsequent cold working. The technical significance of the temperature in intermediate annealing is the same as that of the temperature in final annealing, which will be described later. The atmosphere for intermediate annealing may be air, vacuum, or inert gas. However, if air annealing is performed, it is preferable to perform a scale removal process in the next step, such as salt heat treatment, pickling, or mechanical cutting with a grinder. The annealing furnace may be either a batch furnace (box-type furnace) or a continuous furnace, and the heating method may be any of air furnace, atmosphere furnace, or induction heating furnace. The conditions for these heating furnaces are the furnace temperature and furnace time for air furnaces and atmosphere furnaces, and the current value and frequency for induction heating furnaces, and are determined by the transport time, the shape and size of the titanium material, and the alloy type (including emissivity, specific heat, thermal conductivity, and latent heat of transformation).

[0083] 3.10 Final annealing In the final annealing stage, it is preferable to control the equivalent circular diameter and aspect ratio of the α-phase by adjusting the temperature and time. The temperature range in which recrystallization proceeds in the annealing of general pure titanium and α-type titanium alloys is T β -350℃ or higher and T β It is below °C. Therefore, the lower limit temperature for final annealing is T β The temperature should be above -300℃. Regarding the upper temperature limit, T β If the temperature is below °C, the β-phase fraction during annealing will be high, and the β-to-α transformation will not proceed during subsequent cooling, leaving the β-phase behind after cooling, potentially resulting in an α-phase area ratio below 80%. Therefore, the upper limit of the annealing temperature is T β A temperature below -20°C is preferred. The final annealing atmosphere may be air, vacuum, or inert gas. However, if air annealing is performed, it is preferable to carry out a scale removal process in the next step, such as salt heat treatment, pickling, or mechanical cutting with a grinder. The annealing furnace may be either a batch furnace (box-type furnace) or a continuous furnace, and the heating method may be any of air furnace, atmosphere furnace, or induction heating furnace. The conditions for these heating furnaces are the furnace temperature and furnace time for air furnaces and atmosphere furnaces, and the current value and frequency for induction heating furnaces, and are determined by the transport time, the shape and size of the titanium material, and the alloy type (including emissivity, specific heat, thermal conductivity, and latent heat of transformation).

[0084] In the manufacturing method disclosed herein, it is preferable that appropriate heating conditions for the first and second hot working processes, as well as heating conditions for intermediate and final annealing, are set based on the results of a heat treatment simulation. For example, the heating furnace installed during the second hot working process may be an induction heating furnace, and the current value and frequency of the induction heating furnace may be determined based on the results of the heat treatment simulation method described later.

[0085] 3.11 Heat treatment simulation method related to the second form In the reheating in the above-mentioned second hot rolling and the continuous final annealing, an induction heating furnace with a high heating rate is frequently used. In order to appropriately select the settings of the induction heating furnace, an analysis method that facilitates the implementation of a coupled analysis with FEM analysis was investigated. The temperature distribution in the radial direction of the intermediate material (billet) during induction heating was analyzed by the following procedure.

[0086] Fig. 4 shows the coordinate system during the induction heating of a round billet. In an infinitely long coil (solenoid), the magnetic field is generated only in the z direction, and the magnetic flux density B inside the solenoid (x < R, R: coil radius) in the state without a billet z is given by the following equation (7).

[0087]

Equation

[0088] Here, N: number of turns of the coil, L: coil length, μ0: magnetic permeability of vacuum, I = I0sin(ωt): current value, ω = 2πf is the angular frequency calculated from the frequency f, and t is time.

[0089] The current value I b、 R b (x) at an arbitrary point P (coordinates x; x < R a R is the billet radius) inside the billet is obtained.

[0090] First, the magnetic flux Φ passing through the inside of point P is given by the following equation (8), assuming the relative magnetic permeability of titanium as μ r (= 1.0001).

[0091]

Equation

[0092] The electromotive force V generated in the minute region dx at point P a is given by the following equation (9) as the time change of the magnetic flux passing through the inside of the billet at point P (Faraday's law of electromagnetic induction).

[0093]

number

[0094] The electrical resistance R per unit length dz of this minute region a Since the induced current I generated at point P is given by equation (10) using the electrical resistivity ρ of the billet, a (x) is given by equation (11).

[0095]

number

number

[0096] In the above, we assumed that the magnetic field generated in the solenoid is constant. In reality, the magnetic field H generated in a solenoid of finite length is different. z (x) increases as the distance x from the central axis of the coil increases. Also, if there is a billet inside the solenoid, the induced current generated in the billet creates a magnetic field inside the billet that cancels out the magnetic field generated by the current flowing through the solenoid.Therefore, in the following, we assume that the induced current actually generated on the billet surface is equal to the product of equation (12) above and a correction value M that takes these effects into account.

[0097] In general, in induction heating, the distribution I(x) of the induced current generated inside the billet is given by the following equation (13).

[0098]

number

number

[0099] δ is called the current penetration depth. Using equation (13) above, the Joule heat Q(x) generated at point P is given by the following equation (14).

[0100]

number

[0101] The Joule heat calculated above is applicable regardless of whether it is an explicit or implicit method. Furthermore, the shape of the intermediate material may be rod-shaped, linear, circular (including elliptical and oblong shapes), angular, or any other shape. Here, the billet radius R b The equivalent radius of the circle of these cross-sections may be used. FEM analysis is applicable to simulations that simultaneously consider plastic deformation. In this case, it is preferable to use 50% to 100% of the plastic working energy generated by plastic deformation as the amount of heat generated by the processing in the heat transfer analysis. Furthermore, in pure titanium and α-type titanium alloys, latent heat of transformation is generated with the phase transformation of the α / β phase. Therefore, in the heat transfer analysis, further improvement in accuracy can be expected by calculating and coupling the phase fraction and latent heat of transformation separately from the temperature. The method for calculating and coupling the phase fraction and latent heat of transformation is as explained in the heat treatment simulation related to the first form. Regarding the heat treatment simulation method related to the second form, it is possible to perform simulations using the finite element method (FEM) based on the Galerkin method, by setting the temperature T as a linear sum of the temperature vector φ and weight function N at each junction.

[0102] As described above, the manufacturing method of the present disclosure makes it possible to perform hot working directly on a casting of pure titanium or α-type titanium alloy using a general rolling mill without the need for a splitting process, and also makes it possible to reduce the standard deviation of the tensile properties of the titanium material (bar or wire) after hot working.

[0103] 4. Heat treatment simulation method The technology disclosed herein also has aspects as a heat treatment simulation method. A heat treatment simulation method according to one embodiment includes the following steps 1 to 3: Step 1: Using the magnetic field obtained by assuming the heating coil is a solenoid, calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil. Step 2: Calculate the temperature of each element of the titanium material based on the FEM model for heat transfer analysis of the metal material, and Step 3: Calculate the amount of heat generated by each element in the FEM model using the Joule heat distribution.

[0104] The heat treatment simulation method of this disclosure corresponds to the heat treatment simulation according to the second embodiment described above. This heat treatment simulation may be combined with one or both of the following steps I to III and steps A to C.

[0105] In other words, the heat treatment simulation method of this disclosure may include, in addition to steps 1 to 3 above, the following steps I to III: Step I: Perform plastic deformation analysis and heat transfer analysis on a titanium material consisting of pure titanium or α-type titanium alloy when it is heated or cooled at a predetermined temperature change rate and subjected to plastic deformation. Step II: Using 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the heat generated during processing, predict the temperature change rate in the heat transfer analysis, and Step III: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step I, or by repeatedly performing Step I and Step II to perform a convergence calculation of the temperature change rates of each phase for each time and temperature.

[0106] Furthermore, the heat treatment simulation method of this disclosure may include, in addition to steps 1 to 3 above, or in addition to steps 1 to 3 and steps I to III above, the following steps A to C: Step A: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step B: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of phase fractions over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Step C: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step A, or by repeatedly performing Step A and Step B to perform convergence calculations of the transformation rate of each phase and the temperature change rate for each time and temperature.

[0107] Steps I to III and Steps A to C described above correspond to the first step, second step, and coupled analysis step in the thermal simulation method according to the first embodiment described above. Details are as described above, and further explanation is omitted. In addition, in the heat treatment simulation method of this disclosure, the deformation resistance for each time and temperature may be predicted based on the phase fraction of each phase obtained by the coupled analysis calculation. For example, by predicting the deformation resistance for each time and temperature and incorporating this into the hot working conditions, more appropriate hot working conditions can be determined.

[0108] 5. Heat treatment simulation device Furthermore, the technology of this disclosure also has aspects as a heat treatment simulation device. That is, the heat treatment simulation device of this disclosure has a first calculation unit, a second calculation unit, and a third calculation unit. Here, the first calculation unit calculates the Joule heat distribution generated in a titanium material made of pure titanium or α-type titanium alloy inside the heating coil using a magnetic field obtained by assuming the heating coil is a solenoid, the second calculation unit calculates the temperature of each element of the titanium material based on an FEM model for heat transfer analysis of the metallic material, and the third calculation unit calculates the amount of heat generated by each element in the FEM model using the Joule heat distribution. That is, in the heat treatment simulation device of this disclosure, step 1 described above is performed in the first calculation unit. Step 2 described above is performed in the second calculation unit. Step 3 described above is performed in the third calculation unit.

[0109] As described above, the heat treatment simulation apparatus of this disclosure is an apparatus capable of performing the heat treatment simulation method according to the second embodiment described above. The heat treatment simulation apparatus may further have the following parts.

[0110] In other words, the heat treatment simulation apparatus of this disclosure may have an analysis unit, a prediction unit, and a coupled analysis calculation unit. The analysis unit performs plastic deformation analysis and heat transfer analysis of a titanium material made of pure titanium or α-type titanium alloy when the titanium material is heated or cooled at a preset temperature change rate and subjected to plastic deformation. The prediction unit uses 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the processing heat generation amount to predict the temperature change rate in the heat transfer analysis. The coupled analysis calculation unit may perform the coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in the analysis unit, or by repeatedly performing the analysis in the analysis unit and the prediction in the prediction unit to perform a convergence calculation of the temperature change rate of each phase for each time and temperature. That is, in the heat treatment simulation apparatus of this disclosure, step I described above may be performed in the analysis unit. Step II described above may be performed in the prediction unit. Furthermore, step III described above may be performed in the coupled analysis calculation unit.

[0111] Furthermore, the heat treatment simulation apparatus of this disclosure may also include a first prediction unit, a second prediction unit, and a coupled analysis calculation unit. The first prediction unit predicts the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material made of pure titanium or an α-type titanium alloy is heated or cooled at a preset temperature change rate, The second prediction unit predicts the latent heat of transformation associated with each phase for each time and temperature based on the transformation rate of each phase for each time and temperature predicted by the first prediction unit, and predicts the temperature change rate of the titanium material and the time change rate of the phase fraction of each phase by performing a heat transfer analysis using the predicted latent heat of transformation. The coupled analysis calculation unit may perform coupled analysis calculations by either introducing the temperature change rate predicted by the second prediction unit into the temperature change rate in the first prediction unit, or by repeatedly performing the predictions in the first prediction unit and the second prediction unit to perform convergence calculations for the transformation rate of each phase and the temperature change rate for each time and temperature. That is, in the heat treatment simulation apparatus of this disclosure, step A described above may be performed in the first prediction unit. Step B described above may be performed in the second prediction unit. Furthermore, step C described above may be performed in the coupled analysis calculation unit.

[0112] Furthermore, the heat treatment simulation apparatus of this disclosure may also have a third prediction unit. The third prediction unit may predict the deformation resistance for each time and temperature based on the phase fraction of each phase obtained by the coupled analysis calculation unit.

[0113] Furthermore, the heat treatment simulation apparatus of this disclosure preferably includes at least a central computing unit as hardware, and the various calculation units, analysis units, prediction units, coupled analysis calculation units, etc., described above are preferably implemented as functions of the central computing unit.

[0114] 6. Program Furthermore, the technology disclosed herein also has an aspect as a program that operates a computer as a heat treatment simulation device. That is, the program disclosed herein operates on the computer, Step 1: Using the magnetic field obtained by assuming the heating coil is a solenoid, calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil. Step 2: Calculate the temperature of each element of the titanium material based on the FEM model for heat transfer analysis of the metal material, and Step 3: Calculate the amount of heat generated by each element in the FEM model using the Joule heat distribution. This is what causes it to execute.

[0115] As described above, the program of this disclosure causes a computer to execute the heat treatment simulation method according to the second embodiment described above. The program may also cause the computer to execute the following steps.

[0116] In other words, the program of this disclosure is provided to the computer, Step I: Perform plastic deformation analysis and heat transfer analysis on a titanium material consisting of pure titanium or α-type titanium alloy when it is heated or cooled at a predetermined temperature change rate and subjected to plastic deformation. Step II: Using 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the heat generated during processing, predict the temperature change rate in the heat transfer analysis, and Step III: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step I, or by repeatedly performing Step I and Step II to perform convergence calculations for the temperature change rates of each phase for each time and temperature. It may also be something that causes the execution of [something].

[0117] Furthermore, the program of this disclosure is installed on the computer, Step A: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step B: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of phase fractions over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Step C: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step A, or by repeatedly performing Step A and Step B to perform convergence calculations of the transformation rate of each phase and the temperature change rate for each time and temperature. It may also be something that causes the execution of [something].

[0118] Furthermore, the program of this disclosure is installed on the computer, Based on the phase fraction of each phase obtained by the aforementioned coupled analysis calculation, the deformation resistance for each time and temperature is predicted. It may also be something that causes the execution of [something]. [Examples]

[0119] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention allows for the adoption of various conditions without departing from its essence and insofar as it achieves its objective.

[0120] 1. Examination of induction heating simulations To verify the accuracy of the induction heating simulation, the surface temperature of the billet during heating was measured, and the microstructure of the round bar cross-section after heating was investigated. A 3 mm hole was machined in the side of a JIS Class 1 pure titanium billet (φ200 × L450) processed in the α single-phase region, and a K-type thermocouple in a protective tube was inserted. This billet was heated to 600°C in an atmospheric furnace, then heated at a frequency of 60 Hz and a current of 5000 A, and the temperature was measured. After heating, the billet was allowed to cool, and the microstructure of the C-section of the billet was observed. Coupled analysis of this induction heating and heat transfer analysis was also performed. The correction value M = 0.86 was used.

[0121] Figure 5 compares the analyzed temperature changes during induction heating with the experimental values. As shown in Figure 5, the analyzed temperature changes during induction heating reproduced the experimental values ​​well. Figure 6 shows the analyzed maximum temperature and microstructure distribution. In the region where the maximum temperature reached was below the β transformation point, the material had microstructure a, as shown in Figure 1. On the other hand, in the region where the maximum temperature reached was above the β transformation point, the material had microstructure b, as shown in Figure 7, which was the microstructure after cooling from the β single-phase region. From the above, the results of the induction heating analysis reproduced the microstructure observation results well.

[0122] 2. Manufacturing and evaluation of titanium materials 2.1 Preparation of the ingot Ingots were obtained by melting pure titanium or α-type titanium alloy having the chemical composition shown in Table 1 below using an electron beam.

[0123] 2.2 Heating and Cooling The obtained ingots were subjected to heat treatment (heating and water cooling) or disassembly forging under the conditions shown in Table 1 below. In cases No. 1 to 7, the heat treatment conditions were analyzed using the heat treatment simulation method described above for the first embodiment.

[0124] [Table 1]

[0125] 2.3 First Hot Working After heating ingot No. 5 to 1200°C and ingots No. 1-4 and No. 6-8 to 1000°C, a first hot rolling process with a reduction ratio of 80% was performed using a single stand to obtain the first intermediate material.

[0126] 2.4 Second Hot Working The first intermediate material obtained by the first hot working process was subjected to the second hot working process shown in Table 2 below to obtain the second intermediate material. In the second hot working process, the first intermediate material was heated to 600°C in an atmospheric furnace, then heated in an induction heating furnace to within the minimum and maximum temperature ranges shown in Table 2 below, followed by a hot working process consisting of one stand, and then further heated using an induction heating furnace installed on the line to within the minimum and maximum temperature ranges shown in Table 2 below, followed by a hot working process consisting of 10 stands. The induction heating conditions in the second hot working process were selected using the heat treatment simulation method related to the second embodiment described above.

[0127] 2.5 Cold rolling and intermediate annealing (Examples 1-9, Comparative Examples 11-16) The second intermediate material obtained by the second hot working process was subjected to cold working with the reduction ratio shown in Table 2 below. Furthermore, intermediate annealing was performed during the cold working process at the temperatures shown in Table 2 below.

[0128] 2.6 Final annealing For Examples 1-9 and Comparative Examples 11-16, the second intermediate material after cold rolling was subjected to final annealing at the temperatures shown in Table 2 below. In Example 10, the second intermediate material was subjected to final annealing at the temperatures shown in Table 2 below without cold rolling or intermediate annealing.

[0129] 2.7 Observation of crystal structure The crystalline structure was analyzed by measuring the equivalent circular diameter and aspect ratio of the α phase in a cross section perpendicular to the longitudinal direction of the obtained φ20 mm titanium rod. This was done at a total of nine fields of view (3 mm × 3 mm) at depths of 3 mm and 5 mm from the surface (representing the east, west, north, and south directions) and at a depth of 10 mm. The mean and standard deviation were calculated for each field of view. Approximately 800 data points were measured. The results are shown in Table 2 below.

[0130] 2.8 Tensile Test Tensile tests were conducted on titanium round bars, with the longitudinal direction as the tensile direction. Measurements were taken five times at each depth (15 data points in total) at 3mm, 5mm, and 10mm from the surface, and the mean and standard deviation were calculated. In accordance with ASTM E8, tensile test specimens were taken from a parallel section of φ3mm × L12.5mm at a depth of 3mm from the surface, and from a parallel section of φ6mm × L25mm at depths of 5mm and 10mm from the surface. The 0.2% yield strength was tested at a strain rate of 0.015 / min, and the elongation at 0.08 / min. The results are shown in Table 2 below.

[0131] [Table 2]

[0132] 2.9 Discussion In Examples 1 to 10, titanium materials with a uniform structure were obtained, in a cross-section perpendicular to the longitudinal direction, where the average value of the equivalent circle diameter of the α phase was 100 μm or less, its standard deviation was 0.2 times the average value of the equivalent circle diameter or less, and its average aspect ratio was 4.0 or less. This is thought to be due to the appropriate heat treatment conditions of the ingot, the temperature and reduction ratio during the first hot working, the temperature and total reduction ratio during the second hot working, the total reduction ratio during cold working, and the intermediate and final annealing.

[0133] In Comparative Example 11, due to the high current value during induction heating, the temperature of a portion of the intermediate material during the second hot working stage was T βBecause the temperature exceeded ℃, the structure shown in Figure 7 was formed after the second hot working. During the subsequent cold working, the elongated α grains resulting from this structure remained in some parts of the cross-section, resulting in low elongation in some tensile properties and a large standard deviation.

[0134] In Comparative Example 12, the total reduction ratio during the second hot working was low, resulting in the retention of a coarse microstructure in some parts of the titanium material after the second hot working. As a result, the average equivalent diameter of the α phase exceeded 100 μm, and the standard deviation of elongation became large.

[0135] For Comparative Example 13, the surface temperature of the intermediate material in the second hot working was T β Because the temperature fell below -200℃, surface defects occurred during the second hot working stage, reducing the yield, and a coarse, elongated structure remained. As a result, the average equivalent diameter of the α phase exceeded 100 μm, its standard deviation exceeded the average equivalent diameter × 0.2, and its average aspect ratio exceeded 4.0, leading to large standard deviations for the 0.2% proof stress and elongation.

[0136] In Comparative Example 14, the total reduction in surface area during cold working was small, resulting in the retention of unrecrystallized, stretched α-phase. Consequently, the standard deviation of elongation increased.

[0137] For Comparative Example 15, the intermediate annealing and final annealing temperatures were T β Because the temperature fell below -350℃, a mixed structure of coarse and fine α-phases was formed. As a result, the variability in tensile tests increased, and the standard deviation of the 0.2% proof stress and elongation became larger.

[0138] For Comparative Example 16, the intermediate annealing and final annealing temperatures were T β Because the temperature exceeded -20°C, the resulting titanium rods developed the microstructure shown in Figure 7, and the average aspect ratio of the α phase exceeded 4.0. As a result, the standard deviation of elongation increased.

[0139] 3. Summary Based on the above results, it can be said that rod-shaped or wire-shaped titanium materials made of pure titanium or α-type titanium alloy that satisfy the following conditions (1) to (3) have suppressed variations in tensile properties. (1) In a cross section perpendicular to the longitudinal direction, the average value D of the equivalent circular diameter of the α phase. av However, it is less than 100 μm. (2) Standard deviation D of the equivalent circular diameter of the α phase in a cross section perpendicular to the longitudinal direction sd However, D sd ≤0.20 × D av It satisfies the condition. (3) In a cross section perpendicular to the longitudinal direction, the average aspect ratio of the α phase is 4.0 or less.

Claims

1. A rod-shaped or wire-shaped titanium material made of pure titanium or α-type titanium alloy, In a cross-section perpendicular to the longitudinal direction, Average value D of the equivalent circular diameter of the α phase ａｖ However, it is less than 100 μm. Standard deviation D of the equivalent circular diameter of the α phase ｓｄ However, D ｓｄ ≤0.20 × D ａｖ Satisfying the conditions, and The average aspect ratio of the α phase is 4.0 or less. Titanium material.

2. The titanium material according to claim 1, When a tensile test is performed with the longitudinal direction as the tensile direction, the standard deviation of the 0.2% proof stress is 30 MPa or less, and the standard deviation of elongation is 4.0% or less. Titanium material.

3. A titanium material according to claim 1 or 2, In mass percent, Fe: 0 to 0.50%, O: 0 to 0.40%, N: 0-0.05%, C: 0-0.08%, H: 0-0.013%, and Remainder: Ti and impurities Having a chemical composition consisting of, Titanium material.

4. A titanium material according to claim 1 or 2, In mass percent, Al: 4.0-6.0%, Sn: 2.0 to 3.0%, Fe: 0 to 0.05%, O: 0-0.20%, N: 0-0.05%, C: 0-0.10%, H: 0-0.020%, and Remainder: Ti and impurities Having a chemical composition consisting of, Titanium material.

5. A component comprising the titanium material described in claim 1 or 2.

6. A heat treatment simulation method including the following steps 1-3: Step 1: Using the magnetic field obtained by assuming the heating coil is a solenoid, calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil. Step 2: Calculate the temperature of each element of the titanium material based on a FEM model for heat transfer analysis of the metal material, and Step 3: Calculate the amount of heat generated by each element in the FEM model using the Joule heat distribution.

7. The following steps I to III are included: The heat treatment simulation method described in claim 6: Step I: Perform plastic deformation analysis and heat transfer analysis on a titanium material made of pure titanium or α-type titanium alloy when it is heated or cooled at a predetermined temperature change rate and subjected to plastic deformation. Step II: Using 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the heat generated during processing, predict the temperature change rate in the heat transfer analysis, and Step III: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step I, or by repeatedly performing Step I and Step II to perform a convergence calculation of the temperature change rates of each phase for each time and temperature.

8. Includes steps A to C below, The heat treatment simulation method according to claim 6 or 7: Step A: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step B: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of phase fractions over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Step C: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step A, or by repeatedly performing Step A and Step B to perform convergence calculations of the transformation rate of each phase and the temperature change rate for each time and temperature.

9. A heat treatment simulation device, It has a first calculation unit, a second calculation unit, and a third calculation unit, The first calculation unit uses the magnetic field obtained by assuming the heating coil is a solenoid to calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil, The second calculation unit calculates the temperature of each element of the titanium material based on an FEM model for heat transfer analysis of the metal material, The third calculation unit calculates the amount of heat generated by each element in the FEM model using the Joule heat distribution. Heat treatment simulation device.

10. A heat treatment simulation apparatus according to claim 9, It has an analysis unit, a prediction unit, and a coupled analysis calculation unit. The analysis unit performs plastic deformation analysis and heat transfer analysis of a titanium material made of pure titanium or α-type titanium alloy when the titanium material is heated or cooled at a preset temperature change rate and subjected to plastic deformation. The prediction unit uses 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the processing heat generation amount to predict the temperature change rate in the heat transfer analysis. The coupled analysis calculation unit performs the coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in the analysis unit, or by repeatedly performing the analysis in the analysis unit and the prediction in the prediction unit to calculate the convergence of the temperature change rates of each phase for each time and temperature. Heat treatment simulation device.

11. A heat treatment simulation apparatus according to claim 9 or 10, It has a first prediction unit, a second prediction unit, and a coupled analysis calculation unit. The first prediction unit predicts the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material made of pure titanium or an α-type titanium alloy is heated or cooled at a preset temperature change rate, The second prediction unit predicts the latent heat of transformation associated with each phase for each time and temperature based on the transformation rate of each phase for each time and temperature predicted by the first prediction unit, and predicts the temperature change rate of the titanium material and the time change rate of the phase fraction of each phase by performing a heat transfer analysis using the predicted latent heat of transformation. The coupled analysis calculation unit performs the coupled analysis calculation by either introducing the temperature change rate predicted by the second prediction unit into the temperature change rate in the first prediction unit, or by repeatedly performing the predictions in the first prediction unit and the second prediction unit to perform convergence calculations for the transformation rate of each phase and the temperature change rate for each time and temperature. Heat treatment simulation device.

12. A program that operates a computer as a heat treatment simulation device, To the aforementioned computer, Step 1: Using the magnetic field obtained by assuming the heating coil is a solenoid, calculate the Joule heat distribution generated in the titanium material made of pure titanium or α-type titanium alloy inside the heating coil. Step 2: Calculate the temperature of each element of the titanium material based on a FEM model for heat transfer analysis of the metal material, and Step 3: Calculate the amount of heat generated by each element in the FEM model using the Joule heat distribution. A program that executes something.

13. The program according to claim 12, To the aforementioned computer, Step I: Perform plastic deformation analysis and heat transfer analysis on a titanium material made of pure titanium or α-type titanium alloy when it is heated or cooled at a predetermined temperature change rate and subjected to plastic deformation. Step II: Using 50-100% of the plastic deformation energy predicted from the plastic deformation analysis as the heat generated during processing, predict the temperature change rate in the heat transfer analysis, and Step III: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step I, or by repeatedly performing Step I and Step II to calculate the convergence of the temperature change rates of each phase for each time and temperature. A program that executes something.

14. A program according to claim 12 or 13, To the aforementioned computer, Step A: Predict the transformation start time and the transformation rate of each phase for each time and temperature when a titanium material consisting of pure titanium or α-type titanium alloy is heated or cooled at a predetermined temperature change rate. Step B: Based on the predicted transformation rates of each phase for each time and temperature, predict the latent heat of transformation associated with the transformation for each time and temperature, and predict the rate of temperature change of the titanium material and the rate of change of phase fractions over time by performing a heat transfer analysis using the predicted latent heat of transformation, and Step C: Perform a coupled analysis calculation by either introducing the predicted temperature change rate into the temperature change rate in Step A, or by repeatedly performing Step A and Step B to perform convergence calculations of the transformation rate of each phase and the temperature change rate for each time and temperature. A program that executes something.

15. A method for manufacturing rod-shaped or wire-shaped titanium material made of pure titanium or α-type titanium alloy, To obtain an ingot made of pure titanium or α-type titanium alloy, The ingot is heated in a heating furnace under the first heating conditions. The ingot, after being heated under the first heating conditions, is cooled under the first cooling conditions. A first hot working process is performed on the ingot after it has been cooled under the first cooling conditions to obtain a first intermediate material. The first intermediate material is subjected to a second hot working process to obtain a second intermediate material, and To obtain a titanium material, the second intermediate material is subjected to cold working followed by final annealing, or to be subjected to final annealing without cold working. Includes, The first heating condition is the set temperature T of the heating furnace １ is T β +10°C or higher and T β +130°C or lower (where T β is the β transformation point), and the ingot is heated in the heating furnace for the in-furnace time t １ such that the position of at least the surface layer 10 mm of the ingot becomes T β +10°C or higher and T β +130°C or lower under the set conditions, The first cooling condition is that the position of the surface layer 10 mm of the ingot is T β The conditions are set so that the temperature is cooled from +10°C to below 300°C at a cooling rate of 5°C / s or more. The temperature of the first hot working process is T β +30℃ or higher and T β Below +200℃, The total reduction ratio of the first hot working is 60% or more. The second hot working includes performing intermediate heating using a heating furnace installed after the first hot working, The temperature of the second hot working process is T β -200℃ or higher and T β It is below -20℃, The total reduction ratio of the second hot working process is 70% or more. The total reduction ratio of the cold working is 30% or more. The temperature of the final annealing is T β -350℃ or higher and T β It is below -20℃. A method for manufacturing titanium materials.

16. A method for manufacturing a titanium material according to claim 15, This includes performing intermediate annealing before or during the cold working process, The temperature for the intermediate annealing is T β -350℃ or higher and T β It is below -20℃. A method for manufacturing titanium materials.

17. A method for manufacturing a titanium material according to claim 15 or 16, The heating furnace installed during the second hot working process is an induction heating furnace. The current value and frequency of the induction heating furnace are determined based on the results obtained by the heat treatment simulation method described in claim 6. A method for manufacturing titanium materials.

18. A method for manufacturing a titanium material according to claim 15 or 16, The shape of the cross-section of the ingot perpendicular to its longitudinal direction is rectangular. A method for manufacturing titanium materials.

19. A method for manufacturing a titanium material according to claim 15 or 16, The area S (mm²) of the cross-section perpendicular to the longitudinal direction of the ingot. ２ The ratio L / S of the circumference L (mm) of the cross-section to the given cross-section is 0.005 or more. A method for manufacturing titanium materials.

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