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US20030223902A1 - Titanium alloy bar and method for manufacturing the same - Google Patents

Titanium alloy bar and method for manufacturing the same Download PDF

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Publication number
US20030223902A1
US20030223902A1 US10/418,252 US41825203A US2003223902A1 US 20030223902 A1 US20030223902 A1 US 20030223902A1 US 41825203 A US41825203 A US 41825203A US 2003223902 A1 US2003223902 A1 US 2003223902A1
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rolling
titanium alloy
type titanium
phase
sec
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US10/418,252
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Hideaki Fukai
Atsushi Ogawa
Kuninori Minakawa
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: FUKAI, HIDEAKI, MINAKAWA, KUNINORI, OGAWA, ATSUSHI
Publication of US20030223902A1 publication Critical patent/US20030223902A1/en
Priority to US10/968,521 priority Critical patent/US20050051245A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
    • C22F1/18—High-melting or refractory metals or alloys based thereon
    • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C14/00—Alloys based on titanium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals

Definitions

  • the present invention relates to a titanium alloy bar having excellent ductility, fatigue characteristics and formability, particularly to an ⁇ + ⁇ type titanium alloy bar, and to a method for manufacturing thereof.
  • titanium alloys are used as structural materials in the fields such as chemical plants, power generators, aircrafts and the like.
  • an ⁇ + ⁇ type titanium alloy occupies a large percentage of use because of its high strength and relatively good formability.
  • Products made of titanium alloys have various shapes such as sheet, plate, bar and so on.
  • the bar may be used as it is, or may be forged or formed in complex shapes such as a threaded fastener. Accordingly, the bar is requested to have excellent formability as well as superior ductility and fatigue characteristics.
  • FIG. 1 shows a typical manufacturing method of bar.
  • An ingot prepared by melting is forged to a billet as a base material for hot rolling.
  • the billet is hot rolled to a bar after reheated in a reheating furnace using a reverse rolling mill or tandem rolling mills. If necessary, the billet is intermediately reheated during hot rolling to compensate the temperature needed for subsequent hot rolling.
  • a titanium alloy bar particularly as for an ⁇ + ⁇ type titanium alloy bar
  • the temperature of billet increases during hot rolling owing to the adiabatic heat, which disturbs stable hot rolling and manufacturing of a titanium alloy bar having excellent ductility, fatigue characteristics and formability.
  • the finally hot rolled bar has ⁇ microstructure consisting mainly of acicular ⁇ phase, thus failing in attaining superior ductility and fatigue characteristics.
  • JP-A-59-82101 discloses a rolling method in which cross sectional area reduction rate of billet is specified to 40% or less per rolling pass in ⁇ region or in ⁇ + ⁇ region.
  • JP-A-58-25465 discloses a method in which billet is water cooled during hot rolling to suppress the temperature rise caused by the adiabatic heat.
  • Article 1 “Hot Bar Rolling of Ti-6Al-4V in a Continuous Mill (Titanium '92 Science and Technology)” describes that hot rolling speed is reduced to the lower limit of keeping performance of mill in order to suppress the adiabatic heat.
  • FIG. 3 shows a relationship between temperature and rolling time during hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy.
  • the heating temperature was 950° C. for the Ti-6Al-4V alloy, and 850° C. for the Ti-4.5Al-3V-2Fe-2Mo alloy.
  • the Ti-4.5Al-3V-2Fe-2Mo alloy has lower ⁇ transus than that of the Ti-6Al-4V alloy by 100° C. so that the heating temperature was reduced by the difference, thus selecting 850° C. as the heating temperature thereof.
  • the rolling was conducted using a reverse rolling mill and tandem rolling mills, while selecting the same conditions of rolling speed, reduction rate and pass schedule to both alloys.
  • the rolling speed of reverse rolling mill was 2.7 m/sec, and the rolling speed of tandem rolling mills was 2.25 m/sec at the final rolling pass where the rolling speed becomes the maximum for both alloys.
  • the rolling speeds are lower than the rolling speed of Article 1 (6 m/sec).
  • the cross sectional area reduction rate was selected to maximum 26% for both alloys.
  • the rolling was conducted at a sufficiently lower temperature than 1000° C. which is the ⁇ transus of the alloy, thus giving favorable structure.
  • the heating temperature was decreased by the magnitude of low ⁇ transus
  • the low temperature rolling resulted in increased deformation resistance and in increased adiabatic heat, so the temperature increased to a temperature region exceeding the ⁇ transus, thus failed to obtain favorable microstructure.
  • excellent ductility, fatigue characteristics and formability were not obtained.
  • rolling conditions such as rolling temperature, reduction rate and time between rolling passes shall be considered, as well as the rolling speed.
  • An object of the present invention is to provide a high strength titanium alloy bar having excellent ductility, fatigue characteristics and formability, and to provide a method of manufacturing thereof.
  • the object is attained by an ⁇ + ⁇ type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary ⁇ phase, 10 ⁇ m or less of average grain size of the primary ⁇ phase, and 4 or less of aspect ratio of the grain of the primary ⁇ phase on the cross sectional plane parallel in the rolling direction of the bar.
  • the ⁇ + ⁇ type titanium alloy bar can be manufactured by a method comprising the step of hot rolling an ⁇ + ⁇ type titanium alloy consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, while keeping the surface temperature thereof to ⁇ transus or below.
  • FIG. 1 shows a typical method for manufacturing a bar.
  • FIG. 2 shows a process for hot rolling a bar.
  • FIG. 3 shows a relationship between temperature and rolling time during hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy.
  • FIG. 4 shows a relationship between average grain size of primary ⁇ phase and total elongation measured by high temperature tensile test.
  • FIG. 5 shows a relationship between average grain size of primary ⁇ phase and fatigue strength after 10 8 cycles observed in fatigue test.
  • FIG. 6 shows temperature changes with time at surface and center.
  • FIG. 7 shows a relationship between cross sectional area and temperature difference between surface and center.
  • the inventors of the present invention studied the microstructure of ⁇ + ⁇ type titanium alloy bar to provide excellent ductility, fatigue characteristics and formability, and found the followings.
  • the ⁇ + ⁇ type titanium alloy consists of primary ⁇ phase and transformed ⁇ phase. If, however, the alloy contains very large volume fraction of ⁇ phase that has HCP structure having little sliding system, or contains very large volume fraction of transformed ⁇ phase containing acicular ⁇ phase, formability and ductility deteriorate. Consequently, the volume fraction of primary ⁇ phase is specified to a range of from 10 to 90%. If the volume fraction of ⁇ phase and of ⁇ phase is equal or close to each other at reheating stage before hot rolling, the formability becomes better, so the volume fraction of primary ⁇ phase is preferably between 50 and 80%.
  • FIG. 4 shows a relationship between average grain size of primary ⁇ phase and total elongation measured by high temperature tensile test.
  • FIG. 5 shows a relationship between average grain size of primary ⁇ phase and fatigue strength after 10 8 cycles observed in fatigue test.
  • Forging a bar induces rough surface on a free deforming plane not contacting with a mold due to the shape of grains, or due to the aspect ratio of the grains.
  • the grains of bar tend to be elongated in the rolling direction.
  • elongated grains appear on a side face of the bar that becomes a free deforming plane. Therefore, it is necessary to avoid excessive increase in the aspect ratio during forging, more concretely to regulate the aspect ratio not exceeding 4 for the grains of the primary ⁇ phase on a cross section parallel in the rolling direction of the bar in order to prevent rough surface on the bar after forged.
  • a high strength titanium alloy bar having excellent ductility, fatigue characteristics and formability is obtained when the volume fraction of the primary ⁇ phase is between 10 and 90%, preferably between 50 and 80%, the average grain size in the primary ⁇ phase is 10 ⁇ m or less, preferably 6 ⁇ m or less, and further the aspect ratio of grains in the primary ⁇ phase is 4 or less.
  • the ⁇ + ⁇ type titanium alloy bar having above-described microstructure should consist essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti. The reasons to limit the content of individual elements are described below.
  • Aluminum is an essential element to stabilize the ⁇ phase and to contribute to the strength increase. If the Al content is below 4%, high strength cannot fully be attained. If the Al content exceeds 5%, ductility degrades.
  • Vanadium is an element to stabilize the ⁇ phase and to contribute to the strength increase. If the V content is below 2.5%, high strength cannot fully be attained, and ⁇ phase becomes unstable. If the V content exceeds 3.5%, range of workable temperature becomes narrow caused by the lowered ⁇ transus, and cost increases.
  • Molybdenum is an element to stabilize the ⁇ phase and to contribute to the strength increase. If the Mo content is below 1.5%, high strength cannot fully be attained, and ⁇ phase becomes unstable. If the Mo content exceeds 2.5%, range of workable temperature becomes narrow-caused by the lowered ⁇ transus, and cost increases.
  • Iron is an element to stabilize the ⁇ phase and to contribute to the strength increase. Iron rapidly diffuses to improve formability. If, however, the Fe content is below 1.5%, high strength cannot fully be attained, and the ⁇ phase becomes unstable, which results in failing to attain excellent formability. If the Fe content exceeds 2.5%, range of workable temperature becomes narrow caused by the lowered ⁇ transus, and degradation in characteristics is induced by segregation.
  • the ⁇ + ⁇ type titanium alloy bar according to the present invention may be manufactured by hot rolling an ⁇ + ⁇ type titanium alloy having above-described composition while adjusting the conditions of heating temperature, rolling temperature range, reduction rate, rolling speed, time between passes, and other variables to suppress the temperature rise caused by the adiabatic g heat, namely to keep the surface temperature of the alloy not exceeding the ⁇ transus.
  • the method comprises the steps of: heating an ⁇ + ⁇ type titanium alloy having ⁇ transus of T ⁇ ° C.
  • the reason of heating the surface before hot rolling in the range of from (T ⁇ 150) to T ⁇ ° C. is the following. If the surface temperature before hot rolling is below (T ⁇ 150) ° C., the decrease in temperature during the final rolling stage becomes significant to increase crack susceptibility and deformation resistance. And, if the surface temperature before hot rolling exceeds T ⁇ ° C., the microstructure of the bar becomes ⁇ microstructure consisting mainly of acicular ⁇ phase, which deteriorates ductility and formability.
  • the reason of limiting the surface temperature during hot rolling to the range of from (T ⁇ 300) to (T ⁇ 50) ° C. is the following.
  • the hot formability deteriorates to induce problems such as cracking. And, if the surface temperature during hot rolling exceeds (T ⁇ 50) ° C., the temperature rise caused by the adiabatic heat induces coarse grains and formation of acicular phase.
  • the reason of limiting the finish surface temperature immediately after the final rolling pass to the range of from (T ⁇ 300) and (T ⁇ 100) ° C. is the following. If the finish temperature thereof is below (T ⁇ 300) ° C., the crack susceptibility and the deformation resistance increase. And, if the finish temperature thereof exceeds (T ⁇ 100) ° C., grains become coarse.
  • the hot rolling is conducted by plurality of rolling passes. To prevent temperature rise caused by the adiabatic heat, it is preferable to keep the reduction rate not more than 40% per rolling pass.
  • the hot rolling is conducted by a reverse rolling mill, it is preferable to limit the rolling speed not more than 6 m/sec to prevent the temperature rise caused by the adiabatic heat.
  • the hot rolling is conducted by tandem rolling mills, it is preferable to limit the rolling speed not more than 1.5 m/sec.
  • the surface of the alloy receives temperature drop to some extent before entering succeeding pass even if a temperature rise exists caused by the adiabatic heat.
  • the temperature drop at center section of the alloy is small so that a large temperature difference appears between the surface and the center of the alloy.
  • the alloy is subjected to succeeding rolling pass before lowering the temperature of the center, which further increases the temperature owing to the adiabatic heat. If the phenomenon sustains, the center is hot rolled at higher temperature than the initial temperature. Consequently, the center of alloy having large diameter is required to be cooled with sufficient time between rolling passes.
  • the inventors of the present invention made a detailed study on the temperature difference between the surface and the center, and derived the finding described below.
  • the temperature difference significantly increases at or above 3500 mm 2 of cross sectional area of alloy normal to the rolling direction thereof.
  • S mm 2 of the cross sectional area securing the time before entering succeeding rolling at 0.167 ⁇ S 1/2 sec or more can make the temperature difference small and is favorable in manufacturing a bar having homogeneous characteristics.
  • the hot rolling is carried out while keeping the surface temperature of the alloy to ⁇ transus or below, thus there is a possibility for the surface temperature to decrease to a lower than the required rolling temperature range during hot rolling depending on the time between rolling passes and on the diameter of alloy.
  • reheating the alloy may be given using a high frequency heating unit or the like.
  • Materials having 125 square mm size were prepared by cutting each of the base alloy A01 (having composition within the range of the present invention) and the base alloy A02 (having composition outside the range of the present invention), both of which are ⁇ + ⁇ type titanium alloy having respective chemical compositions given in Table 1.
  • the materials are hot rolled using a caliber rolling mill under respective conditions (B01 through B18) given in Table 2 to produce bars having 20 mm and 50 mm in diameter, respectively.
  • ⁇ denotes the time between rolling passes of 0.167 ⁇ S 1/2 or more for all the rolling passes under each rolling condition
  • X denotes the time between rolling passes of less than 0.167 ⁇ S 1/2 .
  • Table 3 through Table 20 give cross sectional area S of alloy, reduction rate, 0.167 ⁇ S 1/2 , time between rolling passes, surface temperature, and rolling speed on each rolling pass under each rolling condition.
  • R in the table signifies a reverse rolling mill, and T signifies tandem rolling mills.
  • optical microstructure examination was performed at the center of the bar and at the position of quarter of diameter (1 ⁇ 4 D) to determine grain size of primary ⁇ phase, volume fraction of the grains, and aspect ratio of the grains on a cross section parallel in the rolling direction.
  • bars produced using A02 having chemical composition outside the range of the present invention under the rolling conditions of B10 and B12 could not attain satisfactory ductility and fatigue characteristics because the grain size in the primary ⁇ phase exceeded 10 ⁇ m, though the adiabatic heat was suppressed because the rolling conditions were within the range of the present invention.
  • Cylindrical specimens having 8 mm in diameter and 12 mm in height were cut from the center section in radial direction of bars produced in Example 1 under the rolling conditions B01 through B18, respectively.
  • the specimens were heated to 800° C. and were compressed to 70%. After the compression, the occurrence of cracks and of rough surface on the surface of each specimen was inspected to give evaluation of hot forging property.

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  • Metallurgy (AREA)
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Abstract

The invention relates to an α+β type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar. The α+β type titanium alloy bar has excellent ductility, fatigue characteristics and formability.

Description

  • This application is a continuation application of International Application PCT/JP02/01710 (not published in English) filed Feb. 26, 2002. [0001]
    • BACKGROUND OF THE INVENTION
  • 1. Field of the Invention [0002]
  • The present invention relates to a titanium alloy bar having excellent ductility, fatigue characteristics and formability, particularly to an α+β type titanium alloy bar, and to a method for manufacturing thereof. [0003]
  • 2.Description of Related Arts [0004]
  • Owing to high strength, light weight and excellent corrosion resistance, titanium alloys are used as structural materials in the fields such as chemical plants, power generators, aircrafts and the like. Among them, an α+β type titanium alloy occupies a large percentage of use because of its high strength and relatively good formability. [0005]
  • Products made of titanium alloys have various shapes such as sheet, plate, bar and so on. The bar may be used as it is, or may be forged or formed in complex shapes such as a threaded fastener. Accordingly, the bar is requested to have excellent formability as well as superior ductility and fatigue characteristics. [0006]
  • FIG. 1 shows a typical manufacturing method of bar. [0007]
  • An ingot prepared by melting is forged to a billet as a base material for hot rolling. As shown in FIG. 2A and FIG. 2B, the billet is hot rolled to a bar after reheated in a reheating furnace using a reverse rolling mill or tandem rolling mills. If necessary, the billet is intermediately reheated during hot rolling to compensate the temperature needed for subsequent hot rolling. [0008]
  • As for a titanium alloy bar, particularly as for an α+β type titanium alloy bar, however, the temperature of billet increases during hot rolling owing to the adiabatic heat, which disturbs stable hot rolling and manufacturing of a titanium alloy bar having excellent ductility, fatigue characteristics and formability. For example, if the temperature of billet increases to β transus or above, the finally hot rolled bar has β microstructure consisting mainly of acicular α phase, thus failing in attaining superior ductility and fatigue characteristics. In addition, even as for a Ti-6Al-4V alloy having high β transus, the increase in temperature during hot rolling owing to the adiabatic heat enhances grain growth, although the temperature during hot rolling hardly exceeds β transus, thus failing in attaining excellent ductility, fatigue characteristics and formability. [0009]
  • To solve the problem of temperature increase during hot rolling caused by the adiabatic heat, JP-A-59-82101, (the term “JP-A” referred herein signifies the “unexamined Japanese patent publication”), discloses a rolling method in which cross sectional area reduction rate of billet is specified to 40% or less per rolling pass in α region or in α+β region. JP-A-58-25465 discloses a method in which billet is water cooled during hot rolling to suppress the temperature rise caused by the adiabatic heat. Furthermore, [0010] Article 1 “Hot Bar Rolling of Ti-6Al-4V in a Continuous Mill (Titanium '92 Science and Technology)” describes that hot rolling speed is reduced to the lower limit of keeping performance of mill in order to suppress the adiabatic heat.
  • The methods disclosed in JP-A-59-82101 and JP-A-58-25465, however, cannot produce a titanium alloy bar that simultaneously has excellent ductility, fatigue characteristics and formability. [0011]
  • Even if cross sectional area reduction rate per rolling is 40% or less according to the method of JP-A-59-82102, it is not sufficient to suppress the adiabatic heat for some kinds of titanium alloys. The method of JP-A-58-25465 also causes characteristics deterioration by hydrogen absorption caused by water cooling, and difficulty in accurate temperature control because of deformation resulted from rapid cooling. [0012]
  • The method described in [0013] Article 1 deals with a Ti-6Al-4V alloy. As described below, the method is not necessarily applicable to alloys which generate large adiabatic heat and therefor should be hot rolled in low temperature region, resulting in poor ductility, fatigue characteristics and formability.
  • FIG. 3 shows a relationship between temperature and rolling time during hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy. [0014]
  • The heating temperature was 950° C. for the Ti-6Al-4V alloy, and 850° C. for the Ti-4.5Al-3V-2Fe-2Mo alloy. The Ti-4.5Al-3V-2Fe-2Mo alloy has lower β transus than that of the Ti-6Al-4V alloy by 100° C. so that the heating temperature was reduced by the difference, thus selecting 850° C. as the heating temperature thereof. The rolling was conducted using a reverse rolling mill and tandem rolling mills, while selecting the same conditions of rolling speed, reduction rate and pass schedule to both alloys. The rolling speed of reverse rolling mill was 2.7 m/sec, and the rolling speed of tandem rolling mills was 2.25 m/sec at the final rolling pass where the rolling speed becomes the maximum for both alloys. The rolling speeds are lower than the rolling speed of Article 1 (6 m/sec). The cross sectional area reduction rate was selected to maximum 26% for both alloys. [0015]
  • For the case of the Ti-6Al-4V alloy, the rolling was conducted at a sufficiently lower temperature than 1000° C. which is the β transus of the alloy, thus giving favorable structure. For the case of the Ti-4.5Al-3V-2Fe-2Mo alloy, however, even if the heating temperature was decreased by the magnitude of low β transus, the low temperature rolling resulted in increased deformation resistance and in increased adiabatic heat, so the temperature increased to a temperature region exceeding the β transus, thus failed to obtain favorable microstructure. As a result, excellent ductility, fatigue characteristics and formability were not obtained. The result suggests that rolling conditions such as rolling temperature, reduction rate and time between rolling passes shall be considered, as well as the rolling speed. [0016]
    • SUMMARY OF THE INVENTION
  • An object of the present invention is to provide a high strength titanium alloy bar having excellent ductility, fatigue characteristics and formability, and to provide a method of manufacturing thereof. [0017]
  • The object is attained by an α+β type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar. [0018]
  • The α+β type titanium alloy bar can be manufactured by a method comprising the step of hot rolling an α+β type titanium alloy consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, while keeping the surface temperature thereof to β transus or below.[0019]
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIG. 1 shows a typical method for manufacturing a bar. [0020]
  • FIG. 2 shows a process for hot rolling a bar. [0021]
  • FIG. 3 shows a relationship between temperature and rolling time during hot rolling for Ti-6Al-4V alloy and Ti-4.5Al-3V-2Fe-2Mo alloy. [0022]
  • FIG. 4 shows a relationship between average grain size of primary α phase and total elongation measured by high temperature tensile test. [0023]
  • FIG. 5 shows a relationship between average grain size of primary α phase and fatigue strength after 10[0024] 8 cycles observed in fatigue test.
  • FIG. 6 shows temperature changes with time at surface and center. [0025]
  • FIG. 7 shows a relationship between cross sectional area and temperature difference between surface and center.[0026]
    • DETAILED DESCRIPTION OF THE INVENTION
  • The inventors of the present invention studied the microstructure of α+β type titanium alloy bar to provide excellent ductility, fatigue characteristics and formability, and found the followings. [0027]
  • The α+β type titanium alloy consists of primary α phase and transformed β phase. If, however, the alloy contains very large volume fraction of α phase that has HCP structure having little sliding system, or contains very large volume fraction of transformed β phase containing acicular α phase, formability and ductility deteriorate. Consequently, the volume fraction of primary α phase is specified to a range of from 10 to 90%. If the volume fraction of α phase and of β phase is equal or close to each other at reheating stage before hot rolling, the formability becomes better, so the volume fraction of primary α phase is preferably between 50 and 80%. [0028]
  • FIG. 4 shows a relationship between average grain size of primary α phase and total elongation measured by high temperature tensile test. [0029]
  • When the average grain size of primary α phase exceeds 10 μm, the total elongation measured by high temperature tensile test rapidly decreases, and therefore the formability degrades. [0030]
  • FIG. 5 shows a relationship between average grain size of primary α phase and fatigue strength after 10[0031] 8 cycles observed in fatigue test.
  • If the average grain size of primary α phase exceeds 10 μm, the fatigue strength decreases. If the average grain size of primary α phase becomes less than 6 μm, higher fatigue strength is attained. [0032]
  • Forging a bar induces rough surface on a free deforming plane not contacting with a mold due to the shape of grains, or due to the aspect ratio of the grains. Generally, the grains of bar tend to be elongated in the rolling direction. Particularly for the case of upset forging, elongated grains appear on a side face of the bar that becomes a free deforming plane. Therefore, it is necessary to avoid excessive increase in the aspect ratio during forging, more concretely to regulate the aspect ratio not exceeding 4 for the grains of the primary α phase on a cross section parallel in the rolling direction of the bar in order to prevent rough surface on the bar after forged. [0033]
  • Based on the above-described findings, a high strength titanium alloy bar having excellent ductility, fatigue characteristics and formability is obtained when the volume fraction of the primary α phase is between 10 and 90%, preferably between 50 and 80%, the average grain size in the primary α phase is 10 μm or less, preferably 6 μm or less, and further the aspect ratio of grains in the primary α phase is 4 or less. [0034]
  • The α+β type titanium alloy bar having above-described microstructure should consist essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti. The reasons to limit the content of individual elements are described below. [0035]
  • Al [0036]
  • Aluminum is an essential element to stabilize the α phase and to contribute to the strength increase. If the Al content is below 4%, high strength cannot fully be attained. If the Al content exceeds 5%, ductility degrades. [0037]
  • V [0038]
  • Vanadium is an element to stabilize the β phase and to contribute to the strength increase. If the V content is below 2.5%, high strength cannot fully be attained, and β phase becomes unstable. If the V content exceeds 3.5%, range of workable temperature becomes narrow caused by the lowered β transus, and cost increases. [0039]
  • Mo [0040]
  • Molybdenum is an element to stabilize the β phase and to contribute to the strength increase. If the Mo content is below 1.5%, high strength cannot fully be attained, and β phase becomes unstable. If the Mo content exceeds 2.5%, range of workable temperature becomes narrow-caused by the lowered β transus, and cost increases. [0041]
  • Fe [0042]
  • Iron is an element to stabilize the β phase and to contribute to the strength increase. Iron rapidly diffuses to improve formability. If, however, the Fe content is below 1.5%, high strength cannot fully be attained, and the β phase becomes unstable, which results in failing to attain excellent formability. If the Fe content exceeds 2.5%, range of workable temperature becomes narrow caused by the lowered β transus, and degradation in characteristics is induced by segregation. [0043]
  • The α+β type titanium alloy bar according to the present invention may be manufactured by hot rolling an α+β type titanium alloy having above-described composition while adjusting the conditions of heating temperature, rolling temperature range, reduction rate, rolling speed, time between passes, and other variables to suppress the temperature rise caused by the adiabatic g heat, namely to keep the surface temperature of the alloy not exceeding the β transus. For example, the method comprises the steps of: heating an α+β type titanium alloy having β transus of Tβ ° C. so that the surface temperature ranges between (Tβ−150) and Tβ ° C.; and hot rolling the heated α+β type titanium alloy so that the surface temperature thereof during hot rolling is between (Tβ−300) and (Tβ−50) ° C., and so that the finish surface temperature thereof is between (Tβ−300) and (Tβ−100) ° C. [0044]
  • The reason of heating the surface before hot rolling in the range of from (Tβ−150) to Tβ ° C. is the following. If the surface temperature before hot rolling is below (Tβ−150) ° C., the decrease in temperature during the final rolling stage becomes significant to increase crack susceptibility and deformation resistance. And, if the surface temperature before hot rolling exceeds Tβ ° C., the microstructure of the bar becomes β microstructure consisting mainly of acicular α phase, which deteriorates ductility and formability. The reason of limiting the surface temperature during hot rolling to the range of from (Tβ−300) to (Tβ−50) ° C. is the following. If the surface temperature during hot rolling is below (Tβ−300) ° C., the hot formability deteriorates to induce problems such as cracking. And, if the surface temperature during hot rolling exceeds (Tβ−50) ° C., the temperature rise caused by the adiabatic heat induces coarse grains and formation of acicular phase. The reason of limiting the finish surface temperature immediately after the final rolling pass to the range of from (Tβ−300) and (Tβ−100) ° C. is the following. If the finish temperature thereof is below (Tβ−300) ° C., the crack susceptibility and the deformation resistance increase. And, if the finish temperature thereof exceeds (Tβ−100) ° C., grains become coarse. [0045]
  • The hot rolling is conducted by plurality of rolling passes. To prevent temperature rise caused by the adiabatic heat, it is preferable to keep the reduction rate not more than 40% per rolling pass. [0046]
  • When the hot rolling is conducted by a reverse rolling mill, it is preferable to limit the rolling speed not more than 6 m/sec to prevent the temperature rise caused by the adiabatic heat. When the hot rolling is conducted by tandem rolling mills, it is preferable to limit the rolling speed not more than 1.5 m/sec. [0047]
  • Since the alloy is cooled from surface after each rolling pass, the surface of the alloy receives temperature drop to some extent before entering succeeding pass even if a temperature rise exists caused by the adiabatic heat. As shown in FIG. 6, however, if the alloy has a large diameter (for the case of 106 mm in diameter), the temperature drop at center section of the alloy is small so that a large temperature difference appears between the surface and the center of the alloy. When the temperature drop at the center is small, the alloy is subjected to succeeding rolling pass before lowering the temperature of the center, which further increases the temperature owing to the adiabatic heat. If the phenomenon sustains, the center is hot rolled at higher temperature than the initial temperature. Consequently, the center of alloy having large diameter is required to be cooled with sufficient time between rolling passes. [0048]
  • To this point, the inventors of the present invention made a detailed study on the temperature difference between the surface and the center, and derived the finding described below. As shown in FIG. 7, the temperature difference significantly increases at or above 3500 mm[0049] 2 of cross sectional area of alloy normal to the rolling direction thereof. When an alloy having large cross sectional area is hot rolled to S mm2 of the cross sectional area, securing the time before entering succeeding rolling at 0.167×S1/2 sec or more can make the temperature difference small and is favorable in manufacturing a bar having homogeneous characteristics.
  • According to the manufacturing method of the present invention, the hot rolling is carried out while keeping the surface temperature of the alloy to β transus or below, thus there is a possibility for the surface temperature to decrease to a lower than the required rolling temperature range during hot rolling depending on the time between rolling passes and on the diameter of alloy. In that case, reheating the alloy may be given using a high frequency heating unit or the like. [0050]
    • EXAMPLE 1
  • Materials having 125 square mm size were prepared by cutting each of the base alloy A01 (having composition within the range of the present invention) and the base alloy A02 (having composition outside the range of the present invention), both of which are α+β type titanium alloy having respective chemical compositions given in Table 1. The materials are hot rolled using a caliber rolling mill under respective conditions (B01 through B18) given in Table 2 to produce bars having 20 mm and 50 mm in diameter, respectively. For the time between rolling passes given in Table 2, ◯ denotes the time between rolling passes of 0.167×S[0051] 1/2 or more for all the rolling passes under each rolling condition, and X denotes the time between rolling passes of less than 0.167×S1/2. Table 3 through Table 20 give cross sectional area S of alloy, reduction rate, 0.167×S1/2, time between rolling passes, surface temperature, and rolling speed on each rolling pass under each rolling condition. R in the table signifies a reverse rolling mill, and T signifies tandem rolling mills.
  • The produced bars were annealed at temperatures between 700 and 720° C. Tensile test was conducted to determine yield strength (0.2% PS), tensile strength (UTS), elongation (El), and reduction of area (RA). In addition, the smooth fatigue test (under the condition of Kt=1) and the notch fatigue test (under the condition of Kt=3) were given to determine fatigue strength. [0052]
  • Furthermore, optical microstructure examination was performed at the center of the bar and at the position of quarter of diameter (¼ D) to determine grain size of primary α phase, volume fraction of the grains, and aspect ratio of the grains on a cross section parallel in the rolling direction. [0053]
  • The results are given in Table 21. The columns of the microstructure in the table giving no grain size mean that the position consisted only of β microstructure consisting mainly of acicular α phase and that the equiaxed primary α phase could not be observed. [0054]
  • When the surface heating temperature is below (Tβ−150) ° C., the surface temperature of the alloy was excessively low, and the rolling load became excessive to fail in rolling. When the heating temperature exceeds Tβ ° C., the surface temperature of the alloy became too high even if the time between rolling passes was within the range of the present invention, which is seen under the rolling conditions of B02 and B11, so the surface temperature exceeded Tβ ° C. caused by the adiabatic heat to form β microstructure consisting mainly of acicular α phase at the center of the bar, thus deteriorated ductility and fatigue characteristics. [0055]
  • When the finish surface temperature was below (Tβ−300) ° C., the temperature of the alloy became too low, which deteriorated formability to generate cracks during hot rolling. When the finish surface temperature exceeded (Tβ−100) ° C., fine microstructure could not be attained, deteriorating ductility and fatigue characteristics as in the cases under the conditions of B04, B05, and B07. [0056]
  • When the surface temperature during hot rolling was below (Tβ−300) ° C., the surface temperature was too low, generating cracks. When the surface temperature exceeded (Tβ−50) ° C., the center and the ¼ D had β microstructure consisting mainly of acicular α phase after hot rolling, deteriorating ductility and fatigue characteristics. [0057]
  • When the reduction rate per rolling pass exceeded 40%, the adiabatic heat was enhanced, and the temperature of the alloy exceeded Tβ ° C., and fine microstructure could not be attained. [0058]
  • In the case of the rolling condition B14 which applied a reverse rolling mill and which selected the rolling speeds of higher than 6 m/sec, or in the case of rolling condition B15 which applied tandem rolling mills and which selected the rolling speeds of higher than 1.5 m/sec, the adiabatic heat became large, and the surface temperature exceeded Tβ ° C., thus failed to attain fine microstructure. [0059]
  • When the time between rolling passes was outside the range of the present invention, the surface temperature increase caused by the adiabatic heat overrode the temperature decrease caused by air cooling, thus the surface temperature exceeded Tβ ° C., and fine microstructure could not be attained. [0060]
  • With the bars using A01 which had the chemical composition within the range of the present invention and produced under the rolling conditions B01, B06, B08, B09, B16, B17, and B18, homogeneous microstructure of 10 μm or smaller grain size of primary α phase was observed, and they provided excellent ductility and fatigue characteristics. That is, further excellent ductility and fatigue characteristics could be attained giving 15% or larger elongation, 40% or larger reduction of area, 500 MPa or larger smooth fatigue strength, and 200 MPa of notch (Kt=3) fatigue strength. Furthermore, with the α+β type titanium alloy bars having 50 to 80% of volume fraction of primary α phase and 6 μm or less of average grain size of primary α phase, produced under the rolling conditions of B01, B06, B08, and B09, further excellent ductility and fatigue characteristics could be attained giving 20% or larger elongation, 50% or larger reduction of area, 550 MPa or larger smooth fatigue strength, and 200 MPa of notch (Kt=3) fatigue strength. [0061]
  • On the other hand, bars produced using A02 having chemical composition outside the range of the present invention under the rolling conditions of B10 and B12 could not attain satisfactory ductility and fatigue characteristics because the grain size in the primary α phase exceeded 10 μm, though the adiabatic heat was suppressed because the rolling conditions were within the range of the present invention. [0062]
    • EXAMPLE 2
  • Cylindrical specimens having 8 mm in diameter and 12 mm in height were cut from the center section in radial direction of bars produced in Example 1 under the rolling conditions B01 through B18, respectively. The specimens were heated to 800° C. and were compressed to 70%. After the compression, the occurrence of cracks and of rough surface on the surface of each specimen was inspected to give evaluation of hot forging property. [0063]
  • The results are shown in Table 21. [0064]
  • As for the bars produced under the rolling conditions of B01, B06, B08, B09, B16, B17, and B18 which were within the range of the present invention, no crack and rough surface appeared, and favorable hot forging property was obtained. [0065]
  • On the other hand, for the bars produced under the rolling conditions of B10 and B12 in which the grain size in the primary α phase exceeded 10 μm, rough surface appeared, though no crack was generated. As for the bars having only α phase at center and ¼ D produced under the rolling conditions of B02, B03, B04, B05, B07, B11, B14, and B15, both cracks and rough surface appeared. Furthermore, for the bars produced under the rolling condition B14 giving aspect ratios of more than 4 for the grains in a cross section parallel in the rolling direction, though giving the grain size in the primary α phase and the volume fraction within the range of the present invention, rough surface also appeared. [0066]
    TABLE 1
    β
    Alloy Al V Fe Mo O C N H transus
    A01 4.7 3.1 2.1 1.9 0.1 0.001 0.005 0.0017  900° C.
    A02 6.1 4.1 0.2 — 0.2 0.01 0.006 0.0016 1000° C.
  • [0067]
    TABLE 2
    Rolling speed Final rolling
    Maximum in speed in
    Rolling Total reduction rough rolling finish rolling
    Finish Reheat- temp. Finish Time number rate (Reverse (Tandem
    Rolling diameter ing temp. range temp. between of per rolling rolling mill) rolling mills)
    condition Alloy (mm) (° C.) (° C.) (° C.) passes passes pass (%) (m/sec) (m/sec) Remarks
    B01 A01 φ20 800 700-811 714 ◯ 17 25.8 2.7 1.125 E
    B02 A01 φ20 950 755-929 765 ◯ 17 25.8 2.7 1.125 C
    B03 A01 φ20 890 754-911 764 ◯ 17 25.8 2.7 1.125 C
    B04 A01 φ20 850 818-930 919 ◯ 8 42.4 2.7 1.125 C
    B05 A01 φ20 800 845-901 865 X 17 25.8 2.7 1.125 C
    B06 A01 φ50 800 711-804 731 ◯ 12 18.4 2.7 1.125 E
    B07 A01 φ50 830 864-909 874 X 12 18.4 2.7 1.125 C
    B08 A01 φ20 800 670-812 690 ◯ 17 25.8 2.7 1.125 E
    B09 A01 φ20 820 721-829 726 ◯ 17 25.8 2.7 1.125 E
    B10 A02 φ20 900 791-887 806 ◯ 17 25.8 2.7 1.125 C
    B11 A02 φ20 1050  815-1024 825 ◯ 17 25.8 2.7 1.125 C
    B12 A02 φ50 900 810-906 830 ◯ 12 18.4 2.7 1.125 C
    B13 A01 φ20 920 698-928 698 ◯ 17 25.8 2.7 1.125 C
    B14 A01 φ20 800 774-911 774 ◯ 17 25.8 10.8 1.125 C
    B15 A01 φ20 800 719-910 864 ◯ 17 25.8 2.7 2.250 C
    B16 A01 φ50 830 764-845 766 ◯ 12 18.4 2.7 1.125 E
    B17 A01 φ20 830 757-842 777 ◯ 17 25.8 2.7 1.125 E
    B18 A01 φ20 865 772-850 772 ◯ 17 25.8 2.7 1.125 E
  • [0068]
    TABLE 3
    Rolling condition: B01
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 790 R
    2 11000 15.4 17.5 25 2.7 796 R
    3 9500 13.6 16.3 25 2.7 801 R
    4 8000 15.8 14.9 25 2.7 803 R
    5 6500 18.8 13.5 25 2.7 811 R
    6 5200 20.0 12.0 25 2.7 801 R
    7 4150 20.2 10.8 25 2.7 779 R
    8 3300 20.5 9.6 25 2.7 761 R
    9 2450 25.8 8.3 25 2.7 738 R
    10 1850 24.5 7.2 25 2.7 719 R
    11 1450 21.6 6.4 5 0.350 721 T
    12 1150 20.7 5.7 5 0.466 732 T
    13 900 21.7 5.0 5 0.581 739 T
    14 700 22.2 4.4 5 0.733 745 T
    15 550 21.4 3.9 5 0.871 741 T
    16 420 23.6 3.4 5 0.982 730 T
    17 320 23.8 1.125 714 T
  • [0069]
    TABLE 4
    Rolling condition: B02
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 929 R
    2 11000 15.4 17.5 25 2.7 925 R
    3 9500 13.6 16.3 25 2.7 919 R
    4 8000 15.8 14.9 25 2.7 913 R
    5 6500 18.8 13.5 25 2.7 911 R
    6 5200 20.0 12.0 25 2.7 900 R
    7 4150 20.2 10.8 25 2.7 891 R
    8 3300 20.5 9.6 25 2.7 880 R
    9 2450 25.8 8.3 25 2.7 868 R
    10 1850 24.5 7.2 25 2.7 860 R
    11 1450 21.6 6.4 5 0.350 852 T
    12 1150 20.7 5.7 5 0.466 839 T
    13 900 21.7 5.0 5 0.581 829 T
    14 700 22.2 4.4 5 0.733 822 T
    15 550 21.4 3.9 5 0.871 803 T
    16 420 23.6 3.4 5 0.982 785 T
    17 320 23.8 1.125 765 T
  • [0070]
    TABLE 5
    Rolling condition: B03
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 890 R
    2 11000 15.4 17.5 25 2.7 894 R
    3 9500 13.6 16.3 25 2.7 899 R
    4 8000 15.8 14.9 25 2.7 906 R
    5 6500 18.8 13.5 25 2.7 911 R
    6 5200 20.0 12.0 25 2.7 902 R
    7 4150 20.2 10.8 25 2.7 889 R
    8 3300 20.5 9.6 25 2.7 881 R
    9 2450 25.8 8.3 25 2.7 867 R
    10 1850 24.5 7.2 25 2.7 860 R
    11 1450 21.6 6.4 5 0.350 852 T
    12 1150 20.7 5.7 5 0.466 839 T
    13 900 21.7 5.0 5 0.581 830 T
    14 700 22.2 4.4 5 0.733 820 T
    15 550 21.4 3.9 5 0.871 803 T
    16 420 23.6 3.4 5 0.982 784 T
    17 320 23.8 1.125 764 T
  • [0071]
    TABLE 6
    Rolling condition: B04
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 9300 40.5 19.0 25 2.7 849 R
    2 5500 40.9 17.5 25 2.7 865 R
    3 3300 40.0 16.3 25 2.7 879 R
    4 1900 42.4 14.9 25 2.7 896 R
    5 1100 42.1 13.5 25 2.7 912 R
    6 660 40.0 12.0 25 2.7 921 R
    7 400 39.4 10.8 25 2.7 930 R
    8 320 20.0 2.7 919 R
  • [0072]
    TABLE 7
    Rolling condition: B05
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 10 2.7 791 R
    2 11000 15.4 17.5 10 2.7 805 R
    3 9500 13.6 16.3 10 2.7 819 R
    4 8000 15.8 14.9 10 2.7 836 R
    5 6500 18.8 13.5 10 2.7 850 R
    6 5200 20.0 12.0 10 2.7 865 R
    7 4150 20.2 10.8 10 2.7 871 R
    8 3300 20.5 9.6 10 2.7 875 R
    9 2450 25.8 8.3 10 2.7 879 R
    10 1850 24.5 7.2 10 2.7 884 R
    11 1450 21.6 6.4 5 0.350 901 T
    12 1150 20.7 5.7 5 0.466 899 T
    13 900 21.7 5.0 5 0.581 895 T
    14 700 22.2 4.4 5 0.733 895 T
    15 550 21.4 3.9 5 0.871 883 T
    16 420 23.6 3.4 5 0.982 875 T
    17 320 23.8 1.125 860 T
  • [0073]
    TABLE 8
    Rolling condition: B06
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 791 R
    2 11000 15.4 17.5 25 2.7 796 R
    3 9500 13.6 16.3 25 2.7 801 R
    4 8000 15.8 14.9 25 2.7 804 R
    5 6700 16.3 13.7 25 2.7 806 R
    6 6000 10.5 12.9 25 2.7 784 R
    7 5200 13.3 12.0 25 2.7 764 R
    8 4650 10.6 11.4 25 2.7 746 R
    9 3800 18.3 10.3 25 2.7 733 R
    10 3100 18.4 9.3 5 0.622 733 T
    11 2600 16.1 8.5 5 0.837 734 T
    12 2210 15.0 1.125 731 T
  • [0074]
    TABLE 9
    Rolling condition: B07
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 10 2.7 819 R
    2 11000 15.4 17.5 10 2.7 836 R
    3 9500 13.6 16.3 10 2.7 849 R
    4 8000 15.8 14.9 10 2.7 873 R
    5 6700 16.3 13.7 10 2.7 879 R
    6 6000 10.5 12.9 10 2.7 896 R
    7 5200 13.3 12.0 10 2.7 901 R
    8 4650 10.6 11.4 10 2.7 904 R
    9 3800 18.3 10.3 5 2.7 909 R
    10 3100 18.4 9.3 5 0.622 902 T
    11 2600 16.1 8.5 5 0.837 883 T
    12 2210 15.0 1.125 874 T
  • [0075]
    TABLE 10
    Rolling condition: B08
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 790 R
    2 11000 15.4 17.5 25 2.7 795 R
    3 9500 13.6 16.3 25 2.7 799 R
    4 8000 15.8 14.9 25 2.7 804 R
    5 6500 18.8 13.5 25 2.7 812 R
    6 5200 20.0 12.0 25 2.7 800 R
    7 4150 20.2 10.8 25 2.7 780 R
    8 3300 20.5 9.6 25 2.7 759 R
    9 2450 25.8 8.3 25 2.7 741 R
    10 1850 24.5 7.2 25 2.7 720 R
    11 1450 21.6 6.4 10 0.350 719 T
    12 1150 20.7 5.7 10 0.466 724 T
    13 900 21.7 5.0 10 0.581 730 T
    14 700 22.2 4.4 10 0.733 729 T
    15 550 21.4 3.9 10 0.871 721 T
    16 420 23.6 3.4 10 0.982 705 T
    17 320 23.8 1.125 690 T
  • [0076]
    TABLE 11
    Rolling condition: B09
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 810 R
    2 11000 15.4 17.5 25 2.7 816 R
    3 9500 13.6 16.3 25 2.7 821 R
    4 8000 15.8 14.9 25 2.7 824 R
    5 6500 18.8 13.5 25 2.7 829 R
    6 5200 20.0 12.0 25 2.7 821 R
    7 4150 20.2 10.8 25 2.7 800 R
    8 3300 20.5 9.6 25 2.7 779 R
    9 2450 25.8 8.3 25 2.7 761 R
    10 1850 24.5 7.2 25 2.7 749 R
    11 1450 21.6 6.4 5 0.350 741 T
    12 1150 20.7 5.7 5 0.466 751 T
    13 900 21.7 5.0 5 0.581 760 T
    14 700 22.2 4.4 5 0.733 766 T
    15 550 21.4 3.9 5 0.871 761 T
    16 420 23.6 3.4 5 0.982 751 T
    17 320 23.8 1.125 726 T
  • [0077]
    TABLE 12
    Rolling condition: B10
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 886 R
    2 11000 15.4 17.5 25 2.7 884 R
    3 9500 13.6 16.3 25 2.7 884 R
    4 8000 15.8 14.9 25 2.7 887 R
    5 6500 18.8 13.5 25 2.7 885 R
    6 5200 20.0 12.0 25 2.7 859 R
    7 4150 20.2 10.8 25 2.7 841 R
    8 3300 20.5 9.6 25 2.7 820 R
    9 2450 25.8 8.3 25 2.7 800 R
    10 1850 24.5 7.2 25 2.7 791 R
    11 1450 21.6 6.4 5 0.350 801 T
    12 1150 20.7 5.7 5 0.466 810 T
    13 900 21.7 5.0 5 0.581 830 T
    14 700 22.2 4.4 5 0.733 836 T
    15 550 21.4 3.9 5 0.871 829 T
    16 420 23.6 3.4 5 0.982 821 T
    17 320 23.8 1.125 806 T
  • [0078]
    TABLE 13
    Rolling condition: B11
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 1024 R
    2 11000 15.4 17.5 25 2.7 1015 R
    3 9500 13.6 16.3 25 2.7 1003 R
    4 8000 15.8 14.9 25 2.7 996 R
    5 6500 18.8 13.5 25 2.7 985 R
    6 5200 20.0 12.0 25 2.7 969 R
    7 4150 20.2 10.8 25 2.7 961 R
    8 3300 20.5 9.6 25 2.7 949 R
    9 2450 25.8 8.3 25 2.7 930 R
    10 1850 24.5 7.2 25 2.7 921 R
    11 1450 21.6 6.4 5 0.350 911 T
    12 1150 20.7 5.7 5 0.466 901 T
    13 900 21.7 5.0 5 0.581 891 T
    14 700 22.2 4.4 5 0.733 881 T
    15 550 21.4 3.9 5 0.871 864 T
    16 420 23.6 3.4 5 0.982 845 T
    17 320 23.8 1.125 825 T
  • [0079]
    TABLE 14
    Rolling condition: B12
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 891 R
    2 11000 15.4 17.5 25 2.7 895 R
    3 9500 13.6 16.3 25 2.7 899 R
    4 8000 15.8 14.9 25 2.7 905 R
    5 6700 16.3 13.7 25 2.7 906 R
    6 6000 10.5 12.9 25 2.7 886 R
    7 5200 13.3 12.0 25 2.7 865 R
    8 4650 10.6 11.4 25 2.7 845 R
    9 3800 18.3 10.3 25 2.7 836 R
    10 3100 18.4 9.3 5 0.622 835 T
    11 2600 16.1 8.5 5 0.837 834 T
    12 2210 15.0 1.125 830 T
  • [0080]
    TABLE 15
    Rolling condition: B13
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 929 R
    2 11000 15.4 17.5 25 2.7 925 R
    3 9500 13.6 16.3 25 2.7 919 R
    4 8000 15.8 14.9 25 2.7 913 R
    5 6500 18.8 13.5 25 2.7 911 R
    6 5200 20.0 12.0 25 2.7 900 R
    7 4150 20.2 10.8 25 2.7 891 R
    8 3300 20.5 9.6 25 2.7 880 R
    9 2450 25.8 8.3 25 2.7 868 R
    10 1850 24.5 7.2 25 2.7 850 R
    11 1450 21.6 6.4 10 0.350 832 T
    12 1150 20.7 5.7 10 0.466 804 T
    13 900 21.7 5.0 10 0.581 777 T
    14 700 22.2 4.4 10 0.733 749 T
    15 550 21.4 3.9 10 0.871 728 T
    16 420 23.6 3.4 10 0.982 713 T
    17 320 23.8 1.125 698 T
  • [0081]
    TABLE 16
    Rolling condition: B14
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 10.8 810 R
    2 11000 15.4 17.5 25 10.8 836 R
    3 9500 13.6 16.3 25 10.8 861 R
    4 8000 15.8 14.9 25 10.8 883 R
    5 6500 18.8 13.5 25 10.8 911 R
    6 5200 20.0 12.0 25 10.8 901 R
    7 4150 20.2 10.8 25 10.8 869 R
    8 3300 20.5 9.6 25 1.8 841 R
    9 2450 25.8 8.3 25 10.8 808 R
    10 1850 24.5 7.2 25 10.8 779 R
    11 1450 21.6 6.4 10 0.350 781 T
    12 1150 20.7 5.7 10 0.466 792 T
    13 900 21.7 5.0 10 0.581 799 T
    14 700 22.2 4.4 10 0.733 805 T
    15 550 21.4 3.9 10 0.871 801 T
    16 420 23.6 3.4 10 0.982 790 T
    17 320 23.8 1.125 774 T
  • [0082]
    TABLE 17
    Rolling condition: B15
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 790 R
    2 11000 15.4 17.5 25 2.7 796 R
    3 9500 13.6 16.3 25 2.7 801 R
    4 8000 15.8 14.9 25 2.7 803 R
    5 6500 18.8 13.5 25 2.7 811 R
    6 5200 20.0 12.0 25 2.7 801 R
    7 4150 20.2 10.8 25 2.7 779 R
    8 3300 20.5 9.6 25 2.7 761 R
    9 2450 25.8 8.3 25 2.7 738 R
    10 1850 24.5 7.2 25 2.7 719 R
    11 1450 21.6 6.4 5 0.700 751 T
    12 1150 20.7 5.7 5 0.932 782 T
    13 900 21.7 5.0 5 1.162 829 T
    14 700 22.2 4.4 5 1.466 865 T
    15 550 21.4 3.9 5 1.742 891 T
    16 420 23.6 3.4 5 1.964 910 T
    17 320 23.8 1.500 864 T
  • [0083]
    TABLE 18
    Rolling condition: B16
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 821 R
    2 11000 15.4 17.5 25 2.7 817 R
    3 9500 13.6 16.3 25 2.7 834 R
    4 8000 15.8 14.9 25 2.7 838 R
    5 6700 16.3 13.7 25 2.7 845 R
    6 6000 10.5 12.9 25 2.7 824 R
    7 5200 13.3 12.0 25 2.7 794 R
    8 4650 10.6 11.4 25 2.7 776 R
    9 3800 18.3 10.3 25 2.7 767 R
    10 3100 18.4 9.3 5 0.622 764 T
    11 2600 16.1 8.5 5 0.837 769 T
    12 2210 15.0 1.125 766 T
  • [0084]
    TABLE 19
    Rolling condition: B17
    Number
    of Cross sectional Reduction 0.167{square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 822 R
    2 11000 15.4 17.5 25 2.7 825 R
    3 9500 13.6 16.3 25 2.7 833 R
    4 8000 15.8 14.9 25 2.7 834 R
    5 6500 18.8 13.5 25 2.7 842 R
    6 5200 20.0 12.0 25 2.7 830 R
    7 4150 20.2 10.8 25 2.7 809 R
    8 3300 20.5 9.6 25 2.7 790 R
    9 2450 25.8 8.3 25 2.7 765 R
    10 1850 24.5 7.2 25 2.7 757 R
    11 1450 21.6 6.4 5 0.350 759 T
    12 1150 20.7 5.7 5 0.466 772 T
    13 900 21.7 5.0 5 0.581 771 T
    14 700 22.2 4.4 5 0.733 774 T
    15 550 21.4 3.9 5 0.871 771 T
    16 420 23.6 3.4 5 0.982 779 T
    17 320 23.8 1.125 777 T
  • [0085]
    TABLE 18
    Rolling condition: B18
    Number
    of Cross sectional Reduction {square root}{square root over (S)} Time between Rolling speed Temp. Rolling
    passes area (mm2) rate (*) (sec) passes (sec) (m/sec) (° C.) mill
    15625
    1 13000 16.8 19.0 25 2.7 850 R
    2 11000 15.4 17.5 25 2.7 847 R
    3 9500 13.6 16.3 25 2.7 847 R
    4 8000 15.8 14.9 25 2.7 845 R
    5 6500 18.8 13.5 25 2.7 844 R
    6 5200 20.0 12.0 25 2.7 845 R
    7 4150 20.2 10.8 25 2.7 843 R
    8 3300 20.5 9.6 25 2.7 834 R
    9 2450 25.8 8.3 25 2.7 830 R
    10 1850 24.5 7.2 25 2.7 829 R
    11 1450 21.6 6.4 5 0.350 821 T
    12 1150 20.7 5.7 5 0.466 814 T
    13 900 21.7 5.0 5 0.581 803 T
    14 700 22.2 4.4 5 0.733 794 T
    15 550 21.4 3.9 5 0.871 790 T
    16 420 23.6 3.4 5 0.982 782 T
    17 320 23.8 1.125 772 T
  • [0086]
    TABLE 21
    Fatigue Microstructure (primary α) Forging
    strength 1/4D Center section characteristics
    Rolling 0.2% Smooth Notch Grain Volume Grain Volume Occur- Occurrence
    con- PS UTS E1 RA test test size fraction Aspect size fraction Aspect rence of rough
    dition (MPa) (MPa) (%) (%) (Kt = 1) (Kt = 3) (μm) (%) ratio (μm) (%) ratio of crack surface Remark
    B01 931 1030 20.4 51.9 565 230 2.5 66 1.5 2.7 66 1.8 Not Not E
    occurred occurred
    B02 885 1009 3.5 12.3 350 120 3.7 59 4.1 — — — Occurred Occurred C
    B03 879 1010 4.1 13.5 355 125 3.4 58 4.4 — — — Occurred Occurred C
    B04 881 1011 4.1 11.6 365 115 — — — — — — Occurred Occurred C
    B05 874 1014 3.8 11.1 360 100 3.8 29 4.2 — — — Occurred Occurred C
    B06 921 1020 20.0 50.8 560 225 5.4 60 2.1 5.8 68 2.2 Not Not E
    occurred occurred
    B07 887 1005 3.7 12.1 355 120 5.9 31 4.3 — — — Occurred Occurred C
    B08 930 1030 20.5 52.3 570 240 1.7 67 1.9 1.9 69 2.3 Not Not E
    occurred occurred
    B09 929 1027 20.1 50.1 550 210 4.1 62 1.7 4.9 64 2.1 Not Not E
    occurred occurred
    B10 911 1019 14.8 43.3 480 185 11.4 89 2.8 12.0 88 3.2 Not Occurred C
    occurred
    B11 863 1012 3.6 9.8 230 95 13.2 85 2.9 — — — Occurred Occurred C
    B12 902 1011 13.8 42.1 440 175 14.5 80 3.0 15.0 89 3.4 Not Occurred C
    occurred
    B13 899 987 12.1 38.2 395 155 5.5 85 4.2 5.8 87 4.5 Not Occurred C
    occurred
    B14 884 971 13.7 34.5 345 115 5.2 84 4.2 — — — Occurred Occurred C
    B15 894 955 11.9 33.3 340 120 5.3 81 4.3 — — — Occurred Occurred C
    B16 910 1014 17.4 40.1 505 205 6.2 63 2.5 6.4 60 2.7 Not Not E
    occurred occurred
    B17 914 1021 18.3 42.3 510 205 5.8 64 2.7 6.3 61 2.9 Not Not E
    occurred occurred
    B18 902 1008 15.6 40.1 500 200 6.5 60 3.1 6.6 60 3.3 Not Not E
    occurred occurred

Claims (9)

What is claimed:
1. An α+β type titanium alloy bar consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, and having 10 to 90% of volume fraction of primary α phase, 10 μm or less of average grain size of the primary α phase, and 4 or less of aspect ratio of the grain of the primary α phase on the cross sectional plane parallel in the rolling direction of the bar.
2. The α+β type titanium alloy bar of claim 1, wherein the volume fraction of primary α phase is 50 to 80%, and the average grain size of the primary α phase is 6 μm or less.
3. A method for manufacturing an α+β type titanium alloy bar comprising the step of hot rolling an α+β type titanium alloy consisting essentially of 4 to 5% Al, 2.5 to 3.5% V, 1.5 to 2.5% Fe, 1.5 to 2.5% Mo, by mass, and balance of Ti, while keeping the surface temperature thereof to β transus or below.
4. The method for manufacturing an α+β type titanium alloy bar of claim 3 comprising the steps of: heating an α+β type titanium alloy having a β transus of Tβ ° C. while keeping the surface temperature thereof between (Tβ−150) and Tβ ° C.; and hot rolling the heated α+β type titanium alloy while keeping the surface temperature thereof during hot rolling between (Tβ−300) and (Tβ−50) ° C. and keeping the finish surface temperature thereof, as the surface temperature immediately after the final rolling pass, between (Tβ−300) and (Tβ−100) ° C.
5. The method for manufacturing an α+β type titanium alloy bar of claim 4, wherein the α+β type titanium alloy is hot rolled at a reduction rate of 40% or less per rolling pass.
6. The method for manufacturing an α+β type titanium alloy bar of claim 4, wherein the rolling speed is selected to 6 m/sec or less when a reverse rolling mill is applied to hot rolling.
7. The method for manufacturing an α+β type titanium alloy bar of claim 4, wherein the rolling speed is selected to 1.5 m/sec or less when tandem rolling mills are applied to hot rolling.
8. The method for manufacturing an α+β type titanium alloy bar of claim 4, wherein when the α+β type titanium alloy having 3500 mm2 or larger cross sectional area in normal to the rolling direction is hot rolled to the cross sectional area of S mm2, a waiting time before starting succeeding rolling is 0.167×S1/2 or more sec.
9. The method for manufacturing an α+β type titanium alloy bar of claim 4, wherein the α+β type titanium alloy is reheated during hot rolling.
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