CN116648524A

CN116648524A - Titanium alloy sheet, titanium alloy coil, method for producing titanium alloy sheet, and method for producing titanium alloy coil

Info

Publication number: CN116648524A
Application number: CN202180088452.8A
Authority: CN
Inventors: 国枝知德; 塚本元气; 小池良树; 奥井利行; 岳边秀德
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2021-01-28
Filing date: 2021-01-28
Publication date: 2023-08-25
Also published as: EP4286551A1; WO2022162816A1; TW202229572A; KR20230110601A; TWI796118B; JPWO2022162816A1; US20240018629A1; EP4286551A4

Abstract

The titanium alloy sheet contains a predetermined chemical component, the area ratio of an alpha phase is 80% or more, the area ratio of an alpha phase having a circular equivalent diameter of 1 [ mu ] m or more is more than 53%, and in a (0001) pole diagram based on a plate thickness direction, an angle formed by a direction of a peak representing an aggregation degree and the plate thickness direction calculated by texture analysis when an expansion coefficient is 16 and a Gaussian half width is 5 DEG is 65 DEG or less in an inverse pole diagram using a spherical harmonic method with respect to an electron back scattering diffraction method, and an average plate thickness is 2.5mm or less.

Description

Titanium alloy sheet, titanium alloy coil, method for producing titanium alloy sheet, and method for producing titanium alloy coil

Technical Field

The present disclosure relates to a titanium alloy sheet and a titanium alloy coil, and a method for manufacturing a titanium alloy sheet and a method for manufacturing a titanium alloy coil.

Background

Titanium is a lightweight, high-strength and excellent corrosion-resistant material, and is a material suitable for use in the field of aircraft from the viewpoints of weight reduction and improvement of combustion consumption rate. Accordingly, development of titanium alloys corresponding to the characteristics required for each structural member of an aircraft is actively underway.

For example, patent document 1 discloses an α+β type titanium alloy wire rod containing 1.4% or more and less than 2.1% of Fe, 4.4% or more and less than 5.5% of Al, and the balance of titanium and impurities.

Patent document 2 discloses an α+β type titanium alloy bar containing 0.5% or more and less than 1.4% of Fe, 4.4% or more and less than 5.5% of Al, and the balance titanium and impurities.

Patent document 3 discloses a method for producing a Ti-6Al-4V alloy sheet by rolling a laminated sheet, which is a method for producing a sheet in which one or more sheet-like core materials are covered with a spacer material and a covering material to form a laminated sheet, and the laminated sheet is rolled to reduce the thickness of the core materials, wherein the thickness of the covering material is set so that the ratio of the core materials to the laminated sheet is at least 0.25 for each initial sheet thickness.

Patent document 4 discloses a method for producing a Ti-6Al-4V alloy sheet by rolling a clad sheet, which is a method for producing a sheet in which one or more sheet-like core materials are covered with a spacer material and a covering material, and the core materials are reduced in thickness by rolling the composite materials, wherein the reduction ratio of the sheet thickness before and after the reduction of the composite materials is 3 or more and the rolling rate per 1 pass is 15% or more.

Patent document 5 discloses a method for producing a titanium alloy sheet, which is characterized in that a hot-rolled annealed sheet of a titanium alloy containing, in wt.% Al, is cold-rolled in the same direction as the hot-rolling direction at a total rolling reduction of 67% or more and then annealed at a temperature of 650 to 900): 2.5 to 3.5 percent, V:2.0 to 3.0 percent, and the balance of Ti and common impurities.

Patent document 6 discloses a method for producing an α+β titanium alloy sheet, which is characterized in that in a production process of an α+β titanium alloy cold-rolled sheet, the method comprises the step of annealing at an annealing temperature: [ beta transus-25 ℃ or higher and less than beta transus temperature range, annealing time: cooling rate after heating and maintaining for 0.5-4 hours: a temperature range of 0.5 to 5 ℃/sec for cooling at the cooling rate: intermediate annealing performed after cold rolling is performed under the condition of 300 ℃ or lower.

Patent document 7 discloses an α+β titanium alloy sheet, which includes: at least 1 of the completely solid-solution type β stabilization elements in an amount of 2.0 to 4.5 mass% in terms of Mo equivalent, at least 1 of the eutectoid type β stabilization elements in an amount of 0.3 to 2.0 mass% in terms of Fe equivalent, at least 1 of the α stabilization elements in an amount of more than 3.0 mass% and 5.5 mass% or less in terms of Al equivalent, and the balance being Ti and unavoidable impurities, wherein the average grain size of the α phase is 5.0 μm or less and the maximum grain size of the α phase is 10.0 μm or less, the average aspect ratio of the α phase is 2.0 or less and the maximum aspect ratio of the α phase is 5.0 or less.

Patent document 8 discloses an α+β type titanium alloy sheet excellent in cold rolling property and handling property at the time of cold working, which is characterized in that the α+β type titanium alloy sheet is an α+β type titanium alloy hot rolled sheet, (a) a normal direction (plate thickness direction) of the hot rolled sheet is ND, (b 2) a normal direction of a (0001) plane of an α phase is c-axis direction, an angle formed by the c-axis direction and ND is θ, an angle formed by the plane including the c-axis direction and ND and the plane including ND and TD is Φ, (b 1) a strongest intensity of (0002) reflected relative intensities of X-rays by crystal grains in which θ is 0 to 30 degrees and Φ falls in the entire circumference (-180 to 180 degrees) is XND, (b 2) a strongest intensity of (0002) reflected relative intensities of X-rays by crystal grains in which θ is 80 to less than 100 degrees and Φ±10 degrees is X-ray, and (xtc) is X/d is X5.

Patent document 9 discloses a high-strength α+β titanium alloy sheet excellent in handling properties of a coil (strip) during cold working, which is characterized by comprising, in mass%, fe:0.8 to 1.5 percent of Al:4.8 to 5.5 percent of N:0.030% or less, and contains O and N satisfying a range of Q (%) = [ O ] +2.77- [ N ] defined by Q (%) = 0.14 to 0.38 when [ O ] is contained and [ N ] is contained, and the balance contains Ti and unavoidable impurities, (a) a maximum intensity of X-ray (0002) reflection relative intensity caused by crystal grains in which [ theta ] is 0 to 30 degrees, and [ phi ] falls within a whole circumference (-180 to 180 degrees) is set as XND, (b 2) a maximum intensity of X-ray (0002) reflection relative intensity caused by crystal grains in which [ theta ] is 80 to 100 degrees, and + -10/X-ray (0002) reflection relative intensity caused by crystal grains in which [ theta ] is set to 80 degrees, and less than 100 degrees, is set as XND, and an angle formed by the c-axis orientation and ND is set as θ, and an angle formed by the c-axis orientation and ND is set as [ theta ], and an X-ray (0002) reflection relative intensity caused by crystal grains in which [ phi ] falls within a whole circumference (-180 to 10 degrees, is set as XND.

Patent document 10 discloses a method for producing an α+β -type titanium alloy sheet, which is characterized in that an α+β -type titanium alloy sheet produced by rolling or forging is subjected to cold rolling with a rolling reduction of 20% or more and then annealed at a temperature of 700 ℃ or more and a β transformation point or less, thereby obtaining a sheet having a fine equiaxed α structure.

Non-patent document 1 discloses an α+β titanium alloy sheet having anisotropy in strength in a rolling direction and a direction perpendicular to the rolling direction.

Non-patent document 2 discloses an α+β titanium alloy sheet which is hot rolled at a temperature higher than the β transformation point to reduce anisotropy of strength in the rolling direction and in the direction perpendicular to the rolling direction.

Prior art literature

Patent literature

Patent document 1 Japanese patent application laid-open No. 7-62474

Patent document 2 Japanese patent application laid-open No. 7-70676

Patent document 3 Japanese patent application laid-open No. 2001-300603

Patent document 4 Japanese patent application laid-open No. 2001-300604

Patent document 5 Japanese patent application laid-open No. 61-147864

Patent document 6 Japanese patent laid-open No. 1-127653

Patent document 7 Japanese patent application laid-open No. 2013-227618

Patent document 8 International publication No. 2012/115242

Patent document 9 International publication No. 2012/115243

Patent document 10 Japanese patent application laid-open No. 62-33750

Non-patent literature

Non-patent documents 1:KOBE STEEL ENGINEERING REPORTS/Vol.59, no.1 (2009), P.81 to 84

Non-patent documents 2:KOBE STEEL ENGINEERING REPORTS/Vol.60, no.2 (2010), P.50 to 54

Disclosure of Invention

Problems to be solved by the invention

In addition, although titanium materials used for members requiring higher strength among structural members of an aircraft contain a large amount of Al, there are cases where the allowable load of a rolling mill is exceeded when manufacturing a thin plate because deformation resistance in hot rolling or cold rolling is large. Therefore, it is difficult to manufacture a high-strength titanium alloy sheet by the conventional hot rolling method or cold rolling method.

The present disclosure has been made in view of the above-described problems, and an object of the present disclosure is to provide a titanium alloy sheet and a titanium alloy coil having high strength, and a method for manufacturing the titanium alloy sheet and a method for manufacturing the titanium alloy coil.

Solution for solving the problem

The present inventors have found that a titanium alloy sheet has high strength and excellent workability by containing a predetermined amount of Al and forming a texture in which peaks of the degree of aggregation of crystal grains in a (0001) pole figure based on the sheet thickness direction are present at a predetermined angle or less with respect to the width direction of final rolling. And, a method of manufacturing a titanium alloy sheet capable of simultaneously achieving such chemical composition and texture by cold rolling was found, and thus completed the present disclosure.

The main matters of the present disclosure completed based on the above knowledge are as follows.

(1) The titanium alloy sheet according to one embodiment of the present disclosure contains, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn:0% or more and less than 0.25%, the balance being Ti and impurities, wherein the area ratio of the alpha phase of the titanium alloy plate is 80% or more and the area ratio of the alpha phase of the round equivalent diameter is 1 [ mu ] m or more is more than 53%, and the angle between the direction of a peak representing the degree of aggregation and the plate thickness direction calculated by texture analysis when the expansion coefficient is 16 and the Gaussian half width is 5 DEG is 65 DEG or less in a (0001) pole diagram based on the plate thickness direction, the average plate thickness of the titanium alloy plate is 2.5mm or less.

(2) The titanium alloy sheet according to (1) above has a microstructure composed of an equiaxed structure having an aspect ratio of 3.0 or less and a band-like structure having an aspect ratio of more than 3.0 and extending in the longitudinal direction, wherein the average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band-like structure relative to the area of the microstructure may be 10.0% or less.

(3) The titanium alloy sheet according to the above (1) or (2), which contains Fe in mass%: 0.5% or more and 2.3% or less, or V: any one of 2.5% or more and 4.5% or less.

(4) The titanium alloy sheet according to any one of the above (1) to (3), which contains, in mass%, a metal selected from the group consisting of Ni: less than 0.15%, cr: less than 0.25% and Mn: less than 0.25% of 1 or more than 2 of the group consisting of said Fe or said V is substituted for a part of said Fe or said V.

(5) The titanium alloy sheet according to any one of the above (1) to (4), wherein the smaller of the 0.2% yield strength in the longitudinal direction at 25 ℃ or the 0.2% yield strength in the width direction at 25 ℃ may be 700MPa or more and 1200MPa or less.

(6) The titanium alloy sheet according to any one of the above (1) to (5), wherein, in the (0001) pole diagram in the sheet thickness direction, an angle between the direction of a peak representing the degree of aggregation and the width direction calculated from texture analysis when the expansion coefficient is 16 and the gaussian half width is 5 ° with respect to an inverse pole diagram using the spherical harmonic method by the electron back scattering diffraction method is 10 ° or less, and a ratio of 0.2% yield strength in the width direction to 0.2% yield strength in the length direction may be 1.05 or more and 1.18 or less.

(7) The titanium alloy sheet according to any one of the above (1) to (5), wherein, in the (0001) pole diagram in the sheet thickness direction, the angle between the direction of the peak representing the degree of aggregation and the sheet thickness direction calculated from texture analysis when the expansion coefficient is 16 and the gaussian half width is 5 ° with respect to the inverse pole diagram using the spherical harmonic method by the electron back scattering diffraction method is 35 ° or less, and the ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the length direction may be 0.85 or more and 1.10 or less.

(8) The titanium alloy sheet according to any one of the above (1) to (7), wherein the dimensional accuracy of the sheet thickness may be 5.0% or less with respect to the average sheet thickness.

(9) Another embodiment of the present disclosure relates to a titanium alloy coil material including, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn:0% or more and less than 0.25%, the balance being Ti and impurities, wherein the area ratio of the alpha phase of the titanium alloy coiled material is 80% or more and the area ratio of the alpha phase of the round equivalent diameter is 1 [ mu ] m or more is more than 53%, and the angle between the direction of a peak representing the degree of aggregation and the plate thickness direction calculated by texture analysis when the expansion coefficient is 16 and the Gaussian half width is 5 DEG is 65 DEG or less in a (0001) pole diagram based on the plate thickness direction by a spherical harmonic function method in an electron back scattering diffraction method, and the average plate thickness of the titanium alloy coiled material is 2.5mm or less.

(10) A method for producing a titanium alloy sheet according to still another aspect of the present disclosure is the method for producing a titanium alloy sheet according to any one of (1) to (8), comprising the steps of: a cold rolling step of performing a cold rolling pass for 1 or more times in the longitudinal direction of a titanium material containing, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%, and the balance being Ti and impurities; and a final annealing step of annealing the titanium material after the last cold rolling pass, wherein the average rolling rate per cold rolling pass in the cold rolling step is more than 30%, and the total rolling rate is 60% or more.

(11) The method for producing a titanium alloy sheet according to (10) above, wherein when the plurality of cold rolling passes are performed, an intermediate annealing step of annealing the titanium material is included between the plurality of cold rolling passes, and annealing conditions in the intermediate annealing step and the final annealing step are as follows: the annealing temperature is 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T DEG C and the holding time T (seconds) at the annealing temperature satisfy the following formula (1).

22000≤(T+273.15)×(Log ₁₀ (t) +20). Ltoreq.27000 … (1)

Wherein T is _β Is beta transformation point (. Degree. C.).

(12) A method for producing a titanium alloy sheet according to still another aspect of the present disclosure is the method for producing a titanium alloy sheet according to any one of (1) to (8), comprising the steps of: cold cross rolling, in which cold rolling passes are performed in the longitudinal direction and the width direction of a titanium material containing, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%, and the balance being Ti and impurities; and a final annealing step of annealing the titanium material after the cold cross rolling step, wherein the total rolling ratio in the cold cross rolling step is 60% or more, and the ratio of the rolling ratio in the longitudinal direction to the rolling ratio in the width direction, that is, the cross rolling ratio is 0.05 to 20.00.

(13) The method for producing a titanium alloy sheet according to (12) above, wherein when the cold rolling step or the cold cross rolling step performs a plurality of cold rolling passes, an intermediate annealing step of annealing the titanium material is included between the plurality of cold rolling passes, and annealing conditions in the intermediate annealing step and the final annealing step are as follows: the annealing temperature is 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T DEG C and the holding time T (seconds) at the annealing temperature satisfy the following formula (1).

22000≤(T+273.15)×(Log ₁₀ (t) +20). Ltoreq.27000 … (1)

Wherein T is _β Is beta transformation point (. Degree. C.).

(14) The method for producing a titanium alloy coil according to still another aspect of the present disclosure is the method for producing a titanium alloy coil according to (9) above, comprising the steps of: a cold rolling step of performing a cold rolling pass for 1 or more times in the longitudinal direction of a titanium material containing, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%, and the balance being Ti and impurities; and a final annealing step of annealing the titanium material after the last cold rolling pass, wherein the average rolling rate per cold rolling pass in the cold rolling step is more than 30%, and the total rolling rate is 60% or more.

ADVANTAGEOUS EFFECTS OF INVENTION

As described above, according to the present disclosure, a titanium alloy sheet and a titanium alloy coil having high strength, and a method for manufacturing the titanium alloy sheet and a method for manufacturing the titanium alloy coil can be provided.

Drawings

Fig. 1 is an example of a (0001) pole diagram of a titanium alloy plate according to an embodiment of the present disclosure, based on a plate thickness direction (ND).

Fig. 2 is a view for explaining an angle formed between a direction of a peak showing the degree of aggregation and a width direction.

Fig. 3 is a view showing an example of an optical micrograph of the titanium alloy plate according to this embodiment.

Fig. 4 is an optical micrograph showing an example of a band-like tissue.

Fig. 5 is a schematic diagram for explaining a measurement method of the average plate thickness.

Detailed Description

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following procedure is described.

1. Titanium alloy plate

2. Method for producing titanium alloy sheet

< 1. Titanium alloy plate >)

First, a titanium alloy sheet according to the present embodiment will be described with reference to fig. 1 to 5. Fig. 1 is an example of a (0001) pole diagram of a titanium alloy sheet according to the present embodiment in the sheet thickness direction (ND). Fig. 2 is a view for explaining an angle formed between a direction of a peak showing the degree of aggregation and a width direction. The (0001) pole diagram based on the plate thickness direction (ND) in fig. 2 is the same as that of fig. 1. Fig. 3 is a view showing an example of an optical micrograph of the titanium alloy plate according to the present embodiment. Fig. 4 is an optical micrograph showing an example of a band-like tissue. Fig. 5 is a schematic diagram for explaining a measurement method of the average plate thickness. As will be described in detail later, the titanium alloy sheet according to the present embodiment can be produced by a method including a cold rolling step.

(1.1. Chemical composition)

First, chemical components contained in the titanium alloy sheet of the present embodiment will be described. The titanium alloy sheet according to the present embodiment contains Al in mass%: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn:0% or more and less than 0.25%, the balance being Ti and impurities. In the following description of chemical components, "%" means "% by mass" unless otherwise specified.

Al is an alpha-phase stabilizing element and has a high solid solution strengthening ability. When the Al content increases, the tensile strength at room temperature becomes high. If the Al content is more than 4.0%, a high tensile strength can be obtained. Further, the hot-rolled sheet before cold rolling can maintain high cold-rolling property. The Al content is preferably 4.5% or more, more preferably 4.6% or more. On the other hand, when the Al content is more than 6.6%, the cold rollability of the hot rolled sheet before cold rolling is significantly reduced, and a region where Al is excessively solid-solved due to solidification segregation or the like is locally generated, and Al is regularized. By this Al-regulated region, the impact toughness of the titanium alloy sheet is reduced. Therefore, the Al content is 6.6% or less, preferably 6.5% or less, and more preferably 6.4% or less.

Fe is a beta-phase stabilizing element. Since Fe is an element having high solid solution strengthening ability, the tensile strength at room temperature increases when the Fe content increases. Further, since the β phase and the α phase have higher workability, the workability of the titanium alloy sheet is improved and the dimensional accuracy can be improved by increasing the Fe content. Fe is not essential in the titanium alloy sheet, and therefore the lower limit of the content thereof is 0%. However, in order to maintain a β phase having good workability at room temperature and to obtain a desired tensile strength, the Fe content is preferably 0.5% or more. The Fe content is more preferably 0.7% or more. On the other hand, since Fe is an element that is very liable to solidify and segregate, if the Fe content is too large, fe may segregate locally, and there may be a variation in characteristics between the portion where Fe segregates and the portion where Fe does not segregate. Further, if Fe is excessively contained in the titanium alloy sheet, the fatigue strength may be lowered. Therefore, the Fe content is preferably 2.3% or less. The Fe content is more preferably 2.1% or less, and still more preferably 2.0% or less. Fe is cheaper than β -phase stabilizing elements such as V and Si.

The Fe that may be contained in the titanium alloy sheet according to the present embodiment may be replaced with V. V is a β -phase stabilizing element that is completely solid-solution, and is an element having solid-solution strengthening ability. V is not essential in the titanium alloy sheet, and therefore the lower limit value of the content thereof is 0%. However, in order to obtain the same solid solution strengthening ability as the above Fe, the V content is preferably 2.5% or more. The V content is more preferably 3.0% or more. If V is used instead of Fe, the cost increases, but V is less likely to segregate than Fe, and therefore variation in characteristics due to segregation is suppressed. As a result, stable characteristics are easily obtained in the longitudinal direction and the width direction of the titanium alloy sheet. In order to suppress variation in characteristics due to V segregation, the V content is preferably 4.5% or less. As described above, V is less likely to segregate than Fe, and therefore, when a large ingot is produced, V is preferably contained in a titanium ingot.

Si is a β -phase stabilizing element, but is also solid-soluble in the α -phase, exhibiting high solid-solution strengthening ability. As described above, fe may be contained in an amount exceeding 2.3% in the titanium alloy sheet and may be segregated, so Si may be contained as needed to increase the strength of the titanium alloy sheet. Since Si has a segregation tendency opposite to that of O described below and O is difficult to solidify and segregate, it is expected that a titanium alloy sheet contains a proper amount of Si and O to achieve both high fatigue strength and tensile strength. On the other hand, if the Si content is large, an intermetallic compound of Si called silicide may be formed, and the fatigue strength of the titanium alloy sheet may be lowered. If the Si content is 0.60% or less, the formation of coarse silicide is suppressed, and the reduction of fatigue strength is suppressed. Therefore, the Si content is preferably 0.60% or less. The Si content is more preferably 0.50% or less, and still more preferably 0.40% or less. The lower limit of the Si content is 0% because Si is not essential in the titanium alloy sheet, but the Si content may be, for example, 0.10% or more.

If C is contained in a large amount in the titanium alloy sheet, ductility and workability of the titanium alloy sheet may be lowered. Therefore, the C content is preferably less than 0.080%. C is not essential in the titanium alloy sheet, and therefore the lower limit value of the content thereof is 0%. The content of C is inevitably mixed, and is usually 0.0001% or more. The C content is more preferably 0.060% or less.

Similarly to C, if N is contained in a large amount in the titanium alloy sheet, ductility and workability of the titanium alloy sheet may be lowered. Therefore, the upper limit of the N content is preferably 0.050%. N is not essential in the titanium alloy sheet, and therefore the lower limit of the content thereof is 0%. N is an unavoidable compound, and the substantial content thereof is usually 0.0001% or more. The N content is more preferably 0.04% or less.

Similarly to C, if O is contained in a large amount in the titanium alloy sheet, ductility and workability of the titanium alloy sheet may be lowered. Therefore, the upper limit of the O content is preferably 0.40%, more preferably 0.38%, and even more preferably 0.35%. O is not essential in the titanium alloy sheet, and therefore the lower limit of the content thereof is 0%. O is an unavoidable compound, and the substantial content thereof is usually 0.01% or more.

Like Fe or V, ni is an element that improves tensile strength and workability. However, if the Ni content is 0.15% or more, intermetallic compound Ti as a balance phase may be generated ₂ The fatigue strength and room temperature ductility of the Ni, titanium alloy sheet deteriorate. Therefore, the Ni content is preferably less than 0.15%. The Ni content is more preferably 0.14% or less and 0.12% or less, and still more preferably 0.11% or less. Since Ni is not essential in the titanium alloy sheet, the lower limit of the Ni content is 0%, but the Ni content may be, for example, 0.01% or more.

Like Fe or V, cr is an element that improves tensile strength and workability. However, if the Cr content is 0.25% or more, an intermetallic compound TiCr as a balance phase may be formed ₂ The fatigue strength and room temperature ductility of the titanium alloy sheet deteriorate. Therefore, the Cr content is preferably less than 0.25%. The Cr content is more preferably 0.24% or less and 0.21% or less. Cr is not essential in the titanium alloy sheet, and therefore its contentThe lower limit is 0%, but the Cr content may be, for example, 0.01% or more.

Like Fe or V, mn is an element that improves tensile strength and workability. However, if the Mn content is 0.25% or more, intermetallic compound TiMn as a balance phase may be generated, and fatigue strength and room temperature ductility of the titanium alloy sheet may deteriorate. Therefore, the Mn content is preferably less than 0.25%. The Mn content is more preferably 0.24% or less, and still more preferably 0.20% or less. The lower limit of the Mn content is 0% because Mn is not essential in the titanium alloy sheet, but the Mn content may be, for example, 0.01% or more.

When considering the effects of the above chemical components, the titanium alloy according to the present embodiment preferably contains Fe as an optional element: 0.5 to 2.3 percent or V:2.5 to 4.5 percent of any one of the following components: 0 to 0.60 percent, and C: less than 0.080%, N: less than 0.050% and O: less than 0.40%.

In addition, when considering the effects of the above chemical components, fe is contained in the titanium alloy sheet: 0.5 to 2.3 percent or V: in the case of any one of 2.5 to 4.5%, the titanium alloy sheet according to the present embodiment preferably contains Ni: less than 0.15%, cr: less than 0.25%, and Mn: less than 0.25% of 1 or more than 2 of the group consisting of Fe or V is substituted for a part of the alloy.

When the titanium alloy sheet according to the present embodiment contains Fe, the alloy sheet contains Fe selected from Ni: less than 0.15%, cr: less than 0.25%, and Mn: when less than 0.25% is 1 or 2 or more of the group consisting of Fe, ni, cr and Mn, the total amount is preferably 0.5% or more and 2.3% or less. When the total amount of Fe, ni, cr and Mn is 0.5% or more, high tensile strength can be obtained. When the total amount of Fe, ni, cr, and Mn is 0.5% or more, the workability of the titanium alloy sheet can be improved while maintaining a β phase that is excellent in workability at room temperature, and therefore, the dimensional accuracy can be improved. In addition, when the total amount of Fe, ni, cr, and Mn is 2.3% or less, segregation of these elements is suppressed, whereby variation in characteristics of the titanium alloy sheet can be suppressed.

In the case where the titanium alloy sheet according to the present embodiment contains V, when V is contained in a metal alloy selected from the group consisting of Ni: less than 0.15%, cr: less than 0.25%, and Mn: when less than 0.25% is 1 or 2 or more of the group consisting of V, ni, cr and Mn, the total amount is preferably 2.5% or more and 4.5% or less. When the total amount of V, ni, cr and Mn is 2.5% or more, high tensile strength can be obtained. In addition, when the total amount of V, ni, cr, and Mn is 2.5% or more, the workability of the titanium alloy sheet can be improved while maintaining a β phase that is good in workability at room temperature, and therefore the dimensional accuracy can be improved. In addition, when the total amount of Fe, ni, cr, and Mn is 4.5% or less, segregation of these elements is suppressed, whereby variation in characteristics of the titanium alloy sheet can be suppressed.

The balance of the chemical composition of the titanium alloy plate according to the present embodiment may be Ti and impurities. Examples of the impurities include H, cl, na, mg, ca, B mixed in the refining step and Zr, sn, mo, nb, ta, cu mixed in the scrap. The total amount of impurities is 0.5% or less, which is a level without problems. The H content is 150ppm or less. B may become coarse precipitates in the ingot. Therefore, even when the catalyst is contained as an impurity, the content of B is preferably suppressed as much as possible. In the titanium alloy sheet according to the present embodiment, the B content is preferably 0.01% or less.

When the titanium alloy sheet according to the present embodiment contains 0.5 to 2.3% of Fe, V contained in the titanium alloy sheet may contain only the amount of the impurity to be regarded as the impurity, and when the titanium alloy sheet according to the present embodiment contains 2.5 to 4.5% of V, fe contained in the titanium alloy sheet may contain only the amount of the impurity to be regarded as the impurity.

The titanium alloy sheet according to the present embodiment may contain various elements instead of Ti, as long as it has high strength and can obtain excellent dimensional accuracy. As for the elements exemplified as impurities, the titanium alloy plate may be contained in an amount equal to or larger than the amount regarded as impurities, as long as the titanium alloy plate has high strength and excellent dimensional accuracy.

As described above, the titanium alloy sheet according to the present embodiment may have the chemical composition described above. More specifically, the chemical composition of the titanium alloy plate according to the present embodiment may be, for example, ti-6Al-4V, ti-6Al-4V ELI or Ti-5Al-1Fe.

(1.2. Texture and microstructure)

Next, the texture and microstructure of the titanium alloy sheet according to the present embodiment will be described.

[ texture ]

The titanium alloy sheet according to the present embodiment has the following texture: among (0001) pole diagrams in the plate thickness direction, an inverse pole diagram using a spherical harmonic method with respect to an Electron Back Scattering Diffraction (EBSD) method is calculated by texture analysis at a coefficient of expansion of 16 and a gaussian half width of 5 ° and has an angle of 65 ° or less between the direction of a peak representing the degree of aggregation and the plate thickness direction. In general, if hot rolling is performed at a high speed in one direction at a temperature of β domain or α+β high temperature domain having a higher β ratio, when the phase is changed from β phase to α phase, a titanium alloy forms a texture (T-texture) in which the c-axis of a hexagonal closed-packed (hcp) structure is oriented in a width direction perpendicular to the longitudinal direction on a rolling surface according to a variable selection rule. In the texture in which the hcp c-axis is oriented in the width direction, large anisotropy occurs in the stretching characteristics in the width direction and the length direction. If the tensile characteristics in the width direction and the length direction have large anisotropy, defects may occur during processing. The direction of the peak representing the degree of aggregation calculated by texture analysis (expansion coefficient=16, gaussian half width=5°) of the antipodal map using the spherical harmonic method of the EBSD method corresponds to the direction in which the degree of orientation of the c-axis of hcp is highest. In the (0001) pole view in the plate thickness direction, the titanium alloy plate according to the present embodiment has an angle of 65 ° or less between the direction in which the c-axis of hcp is most oriented (the direction indicating the peak of the aggregation) and the plate thickness direction, and thus can reduce anisotropy, ensure high workability, and improve dimensional accuracy. In the (0001) pole view in the plate thickness direction, the angle between the direction in which the orientation degree of the c-axis of hcp is highest and the plate thickness direction is preferably 60 ° or less, more preferably 55 ° or less, and further preferably 35 ° or less. The lower limit value of the angle formed by the direction of highest degree of orientation of the c-axis of hcp and the plate thickness direction is not particularly limited, but is 0 ° or more. When a titanium alloy sheet is produced by unidirectional rolling, the lower limit value of the angle between the direction in which the orientation degree of the hcp c-axis is highest and the sheet thickness direction is 20 ° or more.

Further, if one-way cold rolling is performed at an angle between the direction of the peak representing the degree of aggregation and the plate thickness direction, a texture (Split-TD type texture) in which the c-axis width direction (TD) of the hcp axis is inclined may be formed. The Split-TD texture is excellent in moldability, particularly in bendability. Therefore, the angle between the direction of the peak showing the degree of aggregation and the plate thickness direction is preferably 20 ° or more and 65 ° or less, which belongs to the Split-TD type texture.

The pole figure is obtained by subjecting the observation surface of a titanium alloy plate sample to chemical polishing and subjecting the observation surface to crystal orientation analysis by EBSD. Specifically, a cross section (L-section) of a titanium alloy plate cut in the plate thickness direction at the widthwise (TD) central position and in the longitudinal direction is chemically polished, and a (0001) pole figure can be produced by performing an EBSD method-based crystal orientation analysis at about 2 to 10 points at 1 to 2 μm intervals in a region of (total plate thickness) ×2mm of the cross section. (0001) The peak positions of the specific orientation in the polar plot are those obtained by OIM Analysis by TSL Solutions ^TM Software (ver.8.1.0) calculated by texture analysis of the antipodal map using the spherical harmonic method. The highest position of the contour line at this time is the peak position of the aggregation degree, and the value having the largest aggregation degree among the peak positions is the maximum aggregation degree. The degree of aggregation of a specific orientation in the (0001) pole figure is represented by: the frequency of existence of the crystal grains having this orientation is a multiple of the structure (degree of aggregation 1) having a completely irregular orientation distribution. In the above, the L-section at the widthwise central position is taken as the observation surface, but the crystal orientation of the titanium alloy sheet is uniformly distributed in the widthwise direction, and therefore, the L-section at an arbitrary sheet widthwise position may be taken as the observation surface.

Fig. 1 shows an example of a (0001) pole diagram of a titanium alloy plate according to the present embodiment in the plate thickness direction (ND). In fig. 1, the poles of each detected crystal orientation are concentrated according to the slopes in the final Rolling Direction (RD) and the final rolling width direction (TD), and the contour line of the concentration is plotted in the (0001) pole figure. The highest position of the contour line in the figure is the peak P1 of the crystal grain. Therefore, in the present embodiment, the angle between the direction of the peak P1 representing the crystal grain and ND is 65 ° or less. In general, the maximum aggregation degree is the aggregation degree of the peak P1 of the crystal grains.

In the titanium alloy sheet according to the present embodiment, in the (0001) pole diagram in the sheet thickness direction, an angle between the direction of the peak representing the degree of aggregation and the width direction calculated by texture analysis when the expansion coefficient is 16 and the gaussian half width is 5 ° with respect to the inverse pole diagram using the spherical harmonic method by the electron back scattering diffraction method may be 10 ° or less. As shown in fig. 2, the angle formed between the direction of the peak showing the degree of aggregation and the width direction is an angle θ2 formed between the direction of the position of the peak showing the degree of aggregation from the center of the (0001) pole diagram in the plate thickness direction and the width direction (TD). The above angle is preferably 5 ° or less, more preferably 3 ° or less, from the viewpoint of manufacturing and observation methods of the tissue.

In the titanium alloy sheet according to the present embodiment, in the (0001) pole diagram in the sheet thickness direction, an angle between the direction of the peak indicating the degree of aggregation calculated by texture analysis when the expansion coefficient is 16 and the gaussian half width is 5 ° and the sheet thickness direction may be 35 ° or less with respect to the inverse pole diagram using the spherical harmonic method of the electron back scattering diffraction method.

[ microstructure ]

The area ratio of the α -phase of the titanium alloy sheet according to the present embodiment is 80% or more. The titanium alloy sheet according to the present embodiment contains a large amount of α -stabilizing elements for the purpose of enhancing the strength. Therefore, if the amount of β -stabilizing element added is further increased, the strength becomes too high to be produced by cold rolling. Therefore, the area ratio of the α -phase of the titanium alloy sheet according to the present embodiment is 80% or more. The area ratio of the α phase may be 82% or more, for example. The upper limit of the area ratio of the α phase is not particularly limited, and the area ratio of the α phase may be, for example, 100% or less or 98% or less. The titanium alloy sheet according to the present embodiment has a structure composed of an α phase and a balance of structure including a β phase, tiFe, and Ti ₃ Al and silicide.

In the titanium alloy sheet according to the present embodiment, the area ratio of the α -phase having a circular equivalent diameter of 1 μm or more is more than 53%. When the area ratio of 1 μm or less is high, the ductility at room temperature may be poor, and therefore the area ratio of the α -phase having a circular equivalent diameter of 1 μm or more is more than 53%. The area ratio of the α phase of 1 μm or more may be 55% or more, or 60% or more. The upper limit of the area ratio of the α -phase having a circular equivalent diameter of 1 μm or more is not particularly limited, and the area ratio of the α -phase having a circular equivalent diameter of 1 μm or more may be 98% or less, for example. The microstructure of the titanium alloy sheet according to the present embodiment is, for example, the microstructure shown in fig. 3. The upper limit of the equivalent diameter of the α phase is not particularly limited, and the equivalent diameter of the α phase is, for example, 20 μm or less.

The area ratio of the α phase and the area ratio of the α phase having a circular equivalent diameter of 1 μm or more were measured by the following methods. A cross section (L-section) of a titanium alloy plate cut in the plate thickness direction at the widthwise (TD) center position and in the longitudinal direction was chemically polished, and an EBSD-based crystal orientation analysis was performed for about 2 to 5 fields of view at a gradient of 1 to 5 μm in a region of (total plate thickness). Times.200 μm of the cross section. The alpha phase was determined by crystal orientation analysis of the EBSD. The area ratio of the alpha phase relative to the area of the region is referred to as the area ratio of the alpha phase. The circular equivalent diameter (area a=pi× (particle diameter D/2)) 2 of the α -phase observed in the field of view was calculated, and the total area of the α -phase having a circular equivalent diameter of 1 μm or more relative to the area of the region was regarded as the area ratio of the α -phase having a circular equivalent diameter of 1 μm or more. The crystal grains of the alpha phase having a circular equivalent diameter of 1 μm or more contain a band structure described later. In the above, the area ratio of the α -phase and the area ratio of the α -phase having a equivalent circle diameter of 1 μm or more are measured based on the L-section at the widthwise central position, but since the α -phase is uniformly distributed in the widthwise direction, the area ratio of the α -phase and the area ratio of the α -phase having a equivalent circle diameter of 1 μm or more may be measured based on the L-section at any plate widthwise position.

The titanium alloy sheet according to the present embodiment has a microstructure composed of an equiaxed structure having an aspect ratio of 3.0 or less and a band-like structure having an aspect ratio of more than 3.0 and extending in the longitudinal direction, and the average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band-like structure to the area of the microstructure is preferably 10.0% or less. Each structure will be described below.

When hot rolling is performed at a temperature of α+β or β domains, as shown in fig. 4, the titanium alloy may form a structure called "band structure". The band-like tissue is, for example, a tissue extending in the longitudinal direction as shown in fig. 4. Specifically, it means a crystal grain having an aspect ratio of more than 3.0 expressed by the major axis/minor axis of the crystal grain. The titanium alloy sheet according to the present embodiment may have a strip-like structure extending in the longitudinal direction. When a band-like structure is formed, anisotropy in strength and defects in molding may occur. Therefore, the band tissue is preferably as small as possible. The area ratio of the band-like tissue to the area of the microstructure is preferably 10.0% or less. The area ratio of the band-like tissue is more preferably 8.0% or less. On the other hand, since the band-like tissue is preferably not present, the lower limit is 0%.

The aspect ratio and the area ratio of the band structure can be calculated as follows. A cross section (L-section) of a titanium alloy plate cut in the plate thickness direction at the widthwise (TD) center position and in the longitudinal direction was chemically polished, and an EBSD-based crystal orientation analysis was performed for about 2 to 5 fields of view at a gradient of 1 to 5 μm in a region of (total plate thickness). Times.200 μm of the cross section. From the results of the crystal orientation analysis of the EBSD, the aspect ratio of each crystal grain was calculated. Then, the area ratio of the crystal grains having an aspect ratio of more than 3.0 was calculated. In the above, the aspect ratio and the area ratio of the band structure were calculated based on the L section at the widthwise central position, but since the band structure is uniformly distributed in the widthwise direction, the aspect ratio and the area ratio of the band structure may be calculated based on the L section at an arbitrary plate widthwise position.

The remainder other than the band-like structure of the microstructure is preferably an equiaxed structure formed by recrystallization. From the viewpoint of formability, the titanium alloy sheet preferably has an equiaxed structure, and in particular, since the titanium alloy sheet is sometimes formed using superplastic properties, fine particles are preferable. The average grain diameter of the equiaxed structure is preferably 20.0 μm or less from the viewpoints of moldability and superplasticity. The average crystal grain diameter of the equiaxed structure is more preferably 15.0 μm or less, still more preferably 10.0 μm or less, and still more preferably 8.0 μm or less. On the other hand, when the average grain diameter of the equiaxed structure is less than 0.5 μm, sometimes the effect of fine grains causes the strength to become excessively large and the ductility to be significantly reduced. As a result, in particular, workability at cold working (room temperature) may be lowered. Therefore, the average grain diameter of the equiaxed structure is preferably 0.5 μm or more. The average grain diameter of the equiaxed structure is more preferably 1.0 μm or more.

It should be noted that the equiaxed structure and the band structure have more than 80% of the alpha phase, and the beta phase exists between the alpha phase and the alpha phase.

By measuring the aspect ratio (ratio of major axis to minor axis) of the crystal grains, the presence or absence of recrystallization can be determined. If the aspect ratio is 3.0 or less, the crystal grains can be judged as recrystallized grains. The lower limit of the aspect ratio of the equiaxed structure is 1.0.

The average grain diameter of the equiaxed structure can be calculated as follows. From the area of the crystal grains of the equiaxed structure measured by EBSD, the equivalent circle diameter (area A=pi× (particle diameter D/2) ² ) The average value of the number references was defined as the average grain diameter of the equiaxed structure.

(1.3.0.2% yield strength)

The titanium alloy sheet according to the present embodiment preferably has a smaller one of a 0.2% yield strength in the longitudinal direction at 25 ℃ and a 0.2% yield strength in the width direction at 25 ℃ of 700MPa or more. Hereinafter, the smaller of the 0.2% yield strength in the longitudinal direction and the 0.2% yield strength in the width direction is simply referred to as 0.2% yield strength. In the field of aircrafts and the like, a tensile strength close to that of Ti-6Al-4V, which is a general-purpose α+β type titanium alloy, at 25 ℃ is generally required. If the 0.2% yield strength of the titanium alloy sheet at 25 ℃ is 700MPa or more, the sheet can be used for applications requiring high strength. The 0.2% yield strength of the titanium alloy sheet at 25℃is more preferably 730MPa or more. On the other hand, if the strength is too high, the strength of the hot-rolled sheet before cold rolling is also high, and thus it may be difficult to cold-roll the hot-rolled sheet, and the number of cold rolling passes increases, which increases the cost. In addition, if the strength is too high, the notch sensitivity becomes high, and there is a possibility that the plate may be broken. Therefore, the 0.2% yield strength of the titanium alloy sheet at 25℃is preferably 1200MPa or less. The 0.2% yield strength of the titanium alloy sheet at 25℃is more preferably 1150MPa or less. Further, if the 0.2% yield strength of the titanium alloy sheet at 25 ℃ is 1000MPa or less, cracking during rolling is further suppressed, and therefore the 0.2% yield strength of the titanium alloy sheet at 25 ℃ is more preferably 1100MPa or less. The 0.2% yield strength may be based on JIS Z2241: 2011. That is, a 0.2% yield strength in the longitudinal direction and a 0.2% yield strength in the width direction may be used based on JIS Z2241: 2011. The longitudinal direction here is the final rolling direction. Determination of the final rolling direction is easy for a person skilled in the art, and the final rolling direction is clear.

(1.4. Anisotropy)

The ratio of 0.2% yield strength σt in the width direction at 25 ℃ to 0.2% yield strength σl in the length direction at 25 ℃, i.e., the conditional yield strength ratio σt/σl of the titanium alloy plate according to the present embodiment is preferably 0.85 or more and 1.18 or less. As described above, the α+β type titanium has an hcp phase (α phase), and thus exhibits higher anisotropy in the hcp direction. As described above, when T-texture is formed, anisotropy becomes large, and thus, particularly in the field of aircrafts, it is sometimes desirable to reduce the anisotropy as much as possible. Therefore, the condition yield strength ratio σt/σl is better as it is closer to 1.00, but if the condition yield strength ratio σt/σl is 1.18 or less, more excellent formability can be obtained. The conditional yield strength ratio σt/σl is more preferably 1.16 or less, still more preferably 1.15 or less, still more preferably 1.14 or less. If cold cross rolling is performed by cold rolling in the longitudinal and width directions, the conditional yield strength ratio σt/σl may be 0.85 or more and 1.10 or less. The conditional yield strength ratio σt/σl of the titanium alloy sheet produced by cold cross rolling is preferably 0.90 or more, more preferably 0.95 or more. The ratio σt/σl of the conditional yield strength of the titanium alloy sheet produced by cold cross rolling is preferably 1.05 or less. In the case of unidirectional cold rolling in the longitudinal direction, the conditional yield strength ratio σt/σl is difficult to be less than 1.05, and may be 1.05 or more. Since a titanium alloy sheet having a conditional yield strength ratio σt/σl of more than 1.18 can be produced by unidirectional cold rolling, σt/σl may be more than 1.18.

(1.5. Average plate thickness)

The titanium alloy sheet according to the present embodiment has an average sheet thickness of 2.5mm or less. For example, by using a titanium material containing the above chemical components, the average plate thickness of the titanium alloy plate can be set to 2.5mm or less by a method for producing a titanium alloy plate described later. Since the deformation resistance of the titanium ingot having an Al content of more than 4.0% and not more than 6.6% is large, the allowable load of the rolling mill may be exceeded in the course of manufacturing a thin plate in a conventional rolling mill. Therefore, it is difficult to produce a titanium alloy sheet having a sheet thickness of 2.5mm or less, which contains the above-mentioned chemical components. In addition, when hot rolling is performed without using plate rolling, if the plate thickness becomes thin, the temperature is rapidly lowered, and the deformation resistance increases. Thus, when hot-rolling a high-strength material, the allowable load of the rolling mill may be exceeded, and it may be difficult to set the average plate thickness to 2.5mm or less. On the other hand, although the lower limit of the average plate thickness of the titanium alloy plate according to the present embodiment is not particularly limited, the average plate thickness of the titanium alloy plate having the above strength is actually 0.1mm or more. Therefore, the average plate thickness of the titanium alloy plate according to the present embodiment is preferably 0.1mm or more. The thickness of the titanium alloy sheet according to the present embodiment is preferably 2.0mm or less, and more preferably 1.5mm or less. The average plate thickness of the titanium alloy plate according to the present embodiment is more preferably 0.2mm or more.

A method for measuring the average plate thickness will be described with reference to fig. 5. For the center position in the width direction (TD) and the positions at a distance of 1/4 of the plate width from both ends in the width direction, the plate thicknesses at 5 or more positions at intervals of 1m or more in the longitudinal direction were measured using an X-ray, a micrometer or a vernier, and the average value of the measured plate thicknesses was taken as the average plate thickness.

(1.6. Plate thickness dimensional accuracy)

The dimensional accuracy of the thickness of the titanium alloy sheet according to the present embodiment (hereinafter, the dimensional accuracy of the thickness may be simply referred to as "thickness dimensional accuracy") is preferably 5.0% or less with respect to the average thickness. In the lamination rolling, a titanium alloy sheet is produced by hot rolling a titanium material sandwiched between a plurality of layers of laminated steel materials, but it is difficult to produce a sheet having a uniform sheet thickness because the temperature distribution greatly changes the deformation resistance of the titanium material laminated in a plurality of layers. However, the titanium alloy sheet according to the present embodiment is produced by cold rolling as described later, and therefore is a titanium alloy sheet excellent in sheet thickness dimensional accuracy. The dimensional accuracy of the titanium alloy sheet according to the present embodiment is more preferably 4.0% or less relative to the average sheet thickness, and still more preferably 2.0% or less relative to the average sheet thickness.

The plate thickness dimensional accuracy was measured by the following method. The plate thickness at each of 5 or more positions at intervals of 1m or more in the longitudinal direction was measured using an X-ray, a micrometer or a vernier for each of the center position in the width direction (TD) and the positions at a distance of 1/4 of the plate width from both ends in the width direction. Using the actually measured plate thickness d and the average plate thickness dave, the maximum value of a' calculated by the following equation (101) is used as the plate thickness dimension accuracy a.

a' = (d-dave)/dave× … type (101)

As described above, the titanium alloy sheet according to the present embodiment is described. The titanium alloy sheet according to the present embodiment has the above-described chemical components and metallurgical structure, and thus has high strength. The titanium alloy sheet according to the present embodiment described above may be produced by any method, for example, the method for producing a titanium alloy sheet according to the present embodiment described below may be also used.

< 2 > method for producing titanium alloy sheet >

The method for manufacturing a titanium alloy sheet according to the present embodiment includes: a slab manufacturing step of manufacturing a titanium alloy slab; a hot rolling step of hot rolling a titanium alloy slab; a cold rolling step of cold-rolling the titanium material after the hot rolling step; and a temper rolling/stretch straightening step of temper rolling or stretch straightening the titanium material after the cold rolling step, as necessary. Hereinafter, each step of the method for producing a titanium alloy sheet according to the present embodiment will be described. In the cold rolling step, the titanium material after the hot rolling step is subjected to unidirectional cold rolling in which cold rolling passes are performed only 1 or more times in the longitudinal direction, or cold cross rolling in which cold rolling passes are performed on the titanium material in the longitudinal direction and the width direction. Hereinafter, as a first manufacturing method, a case where the titanium material after the hot rolling step is subjected to unidirectional cold rolling in the cold rolling step will be described, and as a second manufacturing method, a case where the titanium material after the hot rolling step is subjected to cold cross rolling will be described.

[ first manufacturing method ]

(2.1. Slab manufacturing Process)

In the slab manufacturing process, a titanium alloy slab is manufactured. The method for producing the titanium alloy slab is not particularly limited, and for example, the slab can be produced in the following order. First, an ingot is produced from titanium sponge by various melting methods such as a vacuum arc melting method, an electron beam melting method, and a hearth melting method such as a plasma melting method. Next, the obtained ingot is hot forged at a temperature of an α -phase high temperature region or an α+β -phase region or a β -phase single-phase region, whereby a titanium alloy slab can be obtained. The titanium alloy slab may be subjected to pretreatment such as cleaning treatment and cutting treatment, if necessary. In addition, when a rectangular shape which can be hot rolled is produced by the hearth melting method, hot rolling can be performed without hot forging or the like. The produced titanium alloy slab contains Al in mass%: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%.

(2.2. Hot Rolling Process)

In the hot rolling step, the titanium alloy slab is heated and then hot rolled. For example, heating a titanium alloy slab to a beta transus T _β After the temperature is in the range of 80% or higher, rolling is performed so that the total rolling reduction is 80% or higher. When hot rolling is performed from a temperature not higher than the temperature range of the α+β phase, cracks are generated in the titanium alloy slab, or the above-mentioned metallographic structure cannot be obtained even without cracks. Therefore, in this step, hot rolling is performed from the β -phase temperature range. The final temperature, which is the temperature immediately after hot rolling, is set to the temperature range of the α+β phase, and is based on titaniumThe composition of the alloy slabs varies, but may be, for example, (T _β -250) DEG C or higher and (T) _β -50) DEG C or less, and hot rolling is performed so that the rolling reduction reaches the rolling reduction in 1 hot rolling, or hot rolling may be performed so that the rolling reduction reaches the rolling reduction in a plurality of hot rolling. The titanium material after the hot rolling step contains Al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%.

In the present specification, the "β transformation point" refers to a boundary temperature at which the formation of the α phase starts when the titanium alloy is cooled from the β phase single phase domain. The beta transus point can be obtained from the phase diagram. The phase diagram can be obtained, for example, by the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method. Specifically, a phase diagram of a titanium alloy can be obtained by the calhad method using a Thermo-Calc integrated thermodynamic calculation system of Thermo-Calc software AB company and a predetermined database (TI 3), and a β transformation point can be calculated.

In the hot rolling step, a titanium alloy slab may be continuously hot-rolled using a known continuous hot rolling facility. In the case of using a continuous hot rolling apparatus, a titanium alloy slab is coiled by a coiling machine after hot rolling to produce a titanium alloy hot rolled coil. Therefore, the titanium material after the hot rolling step includes a plate-shaped titanium material and a rolled titanium material longer than the plate-shaped titanium material.

The titanium material after the hot rolling step may be annealed by a known method, removed by pickling or cutting oxide scale, or subjected to a cleaning treatment, if necessary. For example, the titanium material after the hot rolling step is annealed at a temperature of 650 ℃ to 800 ℃ for 20 minutes to 90 minutes. This can cause the unrecrystallized grains of the hot rolled sheet to precipitate as fine recrystallized grains, and can make the crystals in the metallographic structure of the finally obtained titanium alloy sheet more uniform and finer. The annealing may be optionally performed in an atmospheric atmosphere, an inert atmosphere, or a vacuum atmosphere.

In the above-described method for producing a titanium alloy sheet, the titanium material after the hot rolling step corresponds to a titanium ingot according to the present disclosure.

(2.3. Cold Rolling Process)

In this step, the titanium material after the hot rolling step is subjected to cold rolling for 1 or more times in the longitudinal direction thereof. The average rolling rate of each cold rolling pass in the cold rolling process is more than 30%, and the total rolling rate is more than 60%. By this cold rolling step, the c-axis of hcp approaches ND. However, when the average rolling reduction per cold rolling pass and the total rolling reduction are too small, the crystal orientation hardly changes, and the angle between the direction of the peak indicating the degree of aggregation and the plate thickness direction is not 65 ° or less. In this case, the anisotropy of the titanium alloy sheet cannot be improved. The strip structure is formed by hot rolling, but if the average cold rolling rate per cold rolling pass and the total cold rolling rate are small in cold rolling after hot rolling, the strip structure remains in the titanium material without being damaged. Therefore, the average rolling reduction per cold rolling pass in the cold rolling step is more than 30%, and the total rolling reduction is 60% or more. The total rolling reduction is preferably 70% or more.

Here, 1 cold rolling pass refers to cold rolling performed continuously. Specifically, the cold rolling pass is cold rolling from after the hot rolling process until the titanium material becomes the final product thickness, or from after the hot rolling process until before the finishing process when the finishing process is performed after the hot rolling process. When the intermediate annealing treatment is performed in the cold rolling step, cold rolling after the hot rolling step up to the intermediate annealing treatment, cold rolling from the intermediate annealing treatment up to the titanium material to the final product thickness, or cold rolling up to the temper rolling step are referred to as cold rolling passes, respectively. In the case of performing the intermediate annealing treatment a plurality of times, cold rolling from the preceding intermediate annealing treatment to the following intermediate annealing treatment is also referred to as a cold rolling pass. The rolling ratios of the cold rolling mills may be any ratio as long as the average rolling ratio is greater than 30% each time.

In the present cold rolling step, the titanium material, which is a long hot-rolled sheet or a long hot-rolled coil in the rolling direction, is rolled, so that the manufacturing cost can be reduced.

The cold rolling temperature is preferably 500 ℃ or less. When the cold rolling temperature is 500 ℃ or less, high dimensional accuracy can be obtained, and grains are miniaturized during cold rolling, and superplastic characteristics are easily exhibited. The cold rolling temperature is more preferably 400 ℃ or lower. The lower limit of the cold rolling temperature is not particularly limited, and the cold rolling temperature may be, for example, room temperature or higher. The room temperature herein means 0℃or higher.

[ intermediate annealing Process ]

In the cold rolling step, when a plurality of cold rolling passes are performed, it is preferable to provide an intermediate annealing step of annealing the titanium material between the plurality of cold rolling passes. In the intermediate annealing step, the annealing temperature T is preferably 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T DEG C and the holding time T (seconds) at the annealing temperature T satisfy the following formula (102), and annealing the intermediate material in the cold rolling step. The formula (102) was (T+273.15) X (Log ₁₀ (t) +20) is the Larson Miller parameter.

22000≤(T+273.15)×(Log ₁₀ (t) +20). Ltoreq.27000 … (102)

Wherein T is _β Is beta transformation point (. Degree. C.).

[ final annealing Process ]

The final annealing step is a step of annealing the titanium material after the final cold rolling pass. The annealing conditions in the final annealing step are not particularly limited, but in order to improve the formability of the titanium alloy sheet, it is preferable that the annealing temperature T be 600 ℃ or higher and (T _β -50) DEG C or less, and the annealing temperature T DEG C and the holding time T (seconds) at the annealing temperature T satisfy the above formula (102).

By performing the intermediate annealing step and the final annealing step under the above conditions, the uncrystallized grains are recrystallized so that the c-axis of the α -phase approaches the ND direction. This can reduce the anisotropy of the titanium alloy sheet. In addition, by recrystallization, excessive band-like tissues in the microstructure disappear. On the other hand, the annealing temperature is beta transformation point T _β In the above case, the secondary βThe phase changes toward the alpha phase, and the alpha phase thus produced becomes needle-like structure. In addition, even if the annealing temperature is just lower than the beta transus point, the structure becomes a bimodal structure in which an equiaxed structure and a needle structure exist in a mixed manner. Needle-like structures and bimodal structures sometimes lead to internal cracks and end cracks during cold rolling. In addition, needle-like structures or bimodal structures tend to be coarse particles, and it is difficult to exhibit superplastic properties. In the intermediate annealing step and the final annealing step, the annealing temperature T is 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T and the annealing time T are determined so as to satisfy the above formula (102), whereby the anisotropy of the titanium alloy sheet can be further reduced and the band structure in the microstructure can be further reduced by bringing the c-axis of the alpha phase closer to the ND direction by recrystallization. In the intermediate annealing step and the final annealing step, the annealing temperature T is 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T and the annealing time T are determined so that the annealing temperature T and the annealing time T satisfy the above formula (102), whereby the fine equiaxed structure is increased, internal cracks and end cracks at the time of cold rolling are suppressed, and superplastic characteristics are easily exhibited.

(2.4. Surface finishing and stretching correction step)

The titanium alloy sheet is produced by the cold rolling step, but the titanium alloy sheet after the cold rolling step is preferably subjected to surface finish rolling for adjusting mechanical properties or stretching correction for correcting the shape, as required. The reduction in the surface rolling is preferably 10% or less, and the elongation in the stretching correction is preferably 5% or less. In addition, no finishing rolling or stretching correction may be performed if not required.

According to the first production method, in the cold rolling step of cold-rolling a hot-rolled sheet produced using the blank of the titanium alloy sheet 1 or more times in the longitudinal direction, the rolling reduction per time in the cold rolling is more than 30% and the total rolling reduction is 60% or more, whereby a titanium alloy sheet having an angle of 65 ° or less between the direction of the peak representing the aggregation degree and the sheet thickness direction, which is calculated from the texture analysis with respect to the inverse pole diagram using the spherical harmonic method by the EBSD method, by the expansion coefficient of 16 and the gaussian half width of 5 ° in the (0001) pole diagram in the sheet thickness direction, can be obtained. In addition, according to the first manufacturing method, the average plate thickness of the titanium alloy plate can be set to 2.5mm or less, and the dimensional accuracy of the plate thickness can be set to 5.0% or less relative to the average plate thickness.

In addition, according to the first production method, the metallographic structure of the titanium alloy sheet has a microstructure composed of an equiaxed structure having an aspect ratio of 3.0 or less and a band-like structure having an aspect ratio of more than 3.0 and extending in the longitudinal direction, and the average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band-like structure relative to the area of the microstructure is 10.0% or less. Thereby, the anisotropy of the titanium alloy sheet is further reduced.

In addition, according to the first manufacturing method, the ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the length direction may be 1.05 or more and 1.18 or less.

In addition, according to the first manufacturing method, the titanium alloy sheet is easily exhibited in superplastic characteristics by cold rolling to make crystal grains finer, and is excellent in workability in sheet forming.

According to the method for producing a titanium alloy sheet according to the present embodiment, since the unidirectional cold rolling step is included, a long titanium alloy sheet or a titanium alloy coil can be produced. Therefore, the above-described production method may also be referred to as a production method of the titanium alloy coil. Therefore, it is needless to say that the titanium alloy coiled material manufactured by the above manufacturing method has the same characteristics as the titanium alloy sheet of the present disclosure. Specifically, the titanium alloy coil stock of the present disclosure contains, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn:0% or more but less than 0.25%, the balance being Ti and impurities, the area ratio of the alpha phase being 80% or more and the area ratio of the alpha phase having a circular equivalent diameter of 1 μm or more being more than 53%, and in a (0001) pole figure based on the plate thickness direction, the average plate thickness is 2.5mm or less as calculated from texture analysis with a coefficient of expansion of 16 and a Gaussian half width of 5 DEG for an inverse pole figure of the electron back scattering diffraction method using the spherical harmonic method.

In the case of manufacturing a titanium alloy coil, the "longitudinal direction" corresponds to the longitudinal direction of the titanium alloy coil, and the "width direction" corresponds to a direction perpendicular to the longitudinal direction of the rolled surface of the titanium alloy coil.

The first manufacturing method is described so far.

[ second manufacturing method ]

Next, a second manufacturing method will be described. The cold rolling step of the second manufacturing method is different from the first manufacturing method, and the other steps are the same as the first manufacturing method. Therefore, the cold rolling process will be described in detail herein, and the description of other processes will be omitted.

The cold rolling step in the second production method is a cold cross rolling step in which the titanium material after the hot rolling step is subjected to cold rolling in the longitudinal direction and the width direction.

The total rolling rate including the rolling in the longitudinal direction and the rolling in the width direction in this step is 60% or more. The final rolling direction in this step is defined as the longitudinal direction, and the direction perpendicular to the longitudinal direction is defined as the width direction. If the total rolling reduction is 60% or more, the c-axis of hcp is oriented closer to ND, and a titanium alloy sheet having small anisotropy can be produced. The higher the rolling reduction, the closer the c-axis of the α -phase of the titanium alloy sheet is to the sheet thickness direction and the higher the degree of aggregation, so the upper limit of the rolling reduction is not limited.

The cross rolling ratio is not particularly limited, and is, for example, 0.05 to 20.00. The cross rolling ratio referred to herein means a rolling ratio in the longitudinal direction (longitudinal rolling ratio/width rolling ratio) with respect to a rolling ratio in the width direction, which is performed until the plate thickness is changed from 4mm to the target plate thickness. If the cross rolling ratio is 0.05 or more and 20.00 or less, the c-axis of hcp is oriented closer to ND, and a sheet having a small anisotropy can be produced. In addition, the band tissue that is overproduced can be reduced. The cross rolling ratio is more preferably 0.07 to 15.00.

If the total rolling reduction is 60% or more, the rolling reduction per cold rolling pass is not particularly limited. The 1 cold rolling pass herein refers to a longitudinal cold rolling or a transverse cold rolling performed continuously on a hot rolled sheet. Therefore, in the present cold cross rolling step, when the hot rolled sheet is subjected to the cold rolling in the longitudinal direction and the cold rolling in the width direction a plurality of times, the total number of times is the number of cold passes. For example, in the case of performing cold rolling in the longitudinal direction and cold rolling in the width direction 1 time on a hot-rolled sheet, the number of cold passes is 2 times. In the second manufacturing method, the rolling in the longitudinal direction or the rolling in the width direction may be performed a plurality of times. In addition, even if the plate thickness is 4mm or less, reheating and the like can be performed. Further, the hot rolling in the width direction may be performed every time the hot rolling in the longitudinal direction is performed 1 or more times.

In addition, rolling in the width direction may be performed at any time.

The average rolling rate per cold rolling pass is not particularly limited, and may be 5% or more, for example. The average rolling reduction per cold rolling pass is preferably 10% or more, more preferably 20% or more. The average rolling rate per cold rolling pass may be 80% or less, or 75% or less.

The rolling temperature in the cold cross rolling step is preferably 500 ℃ or lower. If the rolling temperature is 500 ℃ or less, high dimensional accuracy can be obtained, and grains are miniaturized at the time of rolling. The rolling temperature is more preferably 400 ℃ or lower. The lower limit of the cold rolling temperature is not particularly limited, and the cold rolling temperature may be, for example, room temperature or more. The room temperature herein means 0℃or higher.

According to the second manufacturing method, the following titanium alloy sheet can be obtained: among the (0001) pole diagrams in the plate thickness direction, the inverse pole diagram using the spherical harmonic method for the electron back scattering diffraction method has an angle of 35 ° or less between the direction of the peak representing the aggregation degree and the plate thickness direction calculated by texture analysis when the expansion coefficient is 16 and the gaussian half width is 5 °, and the ratio of 0.2% yield strength in the width direction to 0.2% yield strength in the length direction is 0.85 or more and 1.10 or less. By performing the longitudinal rolling pass and the width rolling pass a plurality of times, the ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the length direction can be made to approach 1.00.

In addition, when a titanium ingot contains a large amount of β -phase stabilizing elements such as V, when hot rolling is performed at a high speed in one direction at a temperature in the β domain or the α+β high temperature domain having a higher β phase ratio, T-texture is likely to be formed, and anisotropy of the titanium alloy sheet tends to be large. However, according to the second production method, since cold cross rolling is performed, even when the titanium ingot contains β -phase stabilizing elements such as V, the formation of T-texture can be suppressed. As a result, a titanium alloy sheet having small anisotropy can be produced.

In addition, according to the second production method, the metallographic structure of the titanium alloy sheet has a microstructure composed of an equiaxed structure having an aspect ratio of 3.0 or less and a band-like structure having an aspect ratio of more than 3.0 and extending in the longitudinal direction, and the average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band-like structure relative to the area of the microstructure is 10.0% or less. Thereby, the anisotropy of the titanium alloy sheet is further reduced.

Examples

Hereinafter, embodiments of the present disclosure will be specifically described while illustrating examples. The embodiment shown below is only one example of the present disclosure, and the present disclosure is not limited to the following example.

Example 1

1. Production of titanium alloy sheet

First, a titanium alloy ingot, which is a material of a titanium alloy plate having the chemical composition shown in Table 1, was produced by any one of vacuum arc melting (VAR: vacuum Arc Remelting), electron beam melting (EBR: electron Beam Remelting) and plasma melting (PAM: prasma Arc Melting), and then a titanium alloy slab having a thickness of 150mm by 800mm by width by 5000mm was produced by blooming or forging. These titanium alloy slabs were then subjected to hot rolling, hot rolled sheet annealing, shot blasting and pickling to prepare hot rolled sheets having a thickness of 4 mm. In the hot rolling, the temperature of the titanium alloy slab is heated to 1050-1100 ℃ to change the temperature to the beta transformation point T _β The hot rolling is started from the above temperature, and the setting is made800-950 ℃ so that the final temperature becomes beta transus T _β The following is given. The elements other than those shown in table 1 are Ti and impurities.

For the chemical composition of the hot rolled sheet, al, fe, si, ni, cr, mn, V was measured by ICP emission spectrometry. O and N were measured by inert gas fusion, thermal conductivity, and infrared absorption method using an oxygen/nitrogen simultaneous analyzer. For C, measurement was performed by an infrared absorption method using a carbon-sulfur simultaneous analysis device. The chemical composition of each of the hot rolled plates produced was equal to that of the titanium alloy slab shown in table 1. Further, regarding the titanium materials A to P shown in Table 1, a phase diagram of a titanium alloy was obtained by the CALPHAD method using a Thermo-Calc and a predetermined database (TI 3) which are integrated thermodynamic calculation systems of Thermo-Calc software AB, inc., and a beta transformation point T was calculated _β 。

TABLE 1

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Next, the obtained hot-rolled sheet was subjected to a cold rolling process under the conditions shown in table 2. Examples 1 to 18, 30 and comparative example 3 of Table 2 were obtained by repeating the cold rolling in which the average rolling reduction per cold rolling pass was 35 to 60% and the intermediate annealing under the conditions shown in Table 2, and the cold rolling was performed until the total rolling reduction reached 70 to 94%. Inventive example 19 was an example in which cold rolling was repeated with an average rolling reduction per cold rolling pass of 35% and intermediate annealing under the conditions shown in table 2 until the total rolling reduction reached 60%. The invention example 20 is an example of cold rolling at 300 ℃. The invention example 21 is an example in which cold rolling was repeated under conditions shown in table 2 and intermediate annealing was performed under conditions such that the average rolling reduction per cold rolling pass was 40%, and the total rolling reduction was 78%. The intermediate annealing step in invention example 21 is an example that does not satisfy the above formula (102). Examples 22 and 23 were obtained by performing cold rolling with rolling ratios of 75% and 60%, respectively, without intermediate annealing. Inventive examples 24 to 26 were obtained by performing cold rolling with a rolling reduction of 75% in the 1 st cold rolling pass, and then performing intermediate rolling under the conditions shown in Table 2Annealing, and then cold rolling was performed with the rolling reduction of the 2 nd cold rolling pass set to 50% to give a total rolling reduction of 88%. Examples 27 to 29 were obtained by performing cold rolling under the conditions shown in table 2, performing intermediate annealing 1 st pass, performing cold rolling under the conditions shown in table 2, performing intermediate annealing 2 nd pass, performing cold rolling under the conditions shown in table 2, and performing cold rolling under the conditions shown in table 2, performing cold rolling under the conditions shown in table 3, and performing total rolling under the conditions of 60% and 90%. The reference example is a hot rolled sheet in which the cold rolling process is not performed. Comparative example 1 was an example in which the average rolling reduction per time was 20% and the total rolling reduction was 59%. Comparative example 2 shows an example in which the total rolling reduction was 50%. In comparative example 4 using the titanium ingot O having a high Al content, surface cracks and severe end cracks were generated at the time of cold rolling after hot rolling. Therefore, in comparative example 4, the intermediate annealing and the final annealing were not performed. In Table 2, "T _β "beta transformation point" and "Larson Miller parameter" are (T+273.15) × (Log ₁₀ (t) +20). In table 2, "pattern a" represents a cold rolling pattern in which cold rolling was performed with the rolling reduction of the 1 st cold rolling pass being 75% and the rolling reduction of the 2 nd cold rolling pass being 50%. In table 2, "pattern B" represents a cold rolling pattern in which cold rolling was performed with a rolling reduction of 50% in the 1 st cold rolling pass, 50% in the 2 nd cold rolling pass, and 60% in the 3 rd cold rolling pass.

TABLE 2

2. Evaluation

The titanium alloy sheets according to each of the invention examples, the reference examples and the comparative examples were evaluated as follows.

2.1. Chemical composition

The chemical components of the titanium alloy plates according to each of the invention examples, the reference examples and the comparative examples were measured by the same method as the method for measuring the chemical components of the hot rolled plate.

2.2. Concentration peak position

The observation surfaces of the samples of the titanium alloy plates according to each of the invention examples, the reference examples, and the comparative examples were chemically polished, and crystal orientation analysis was performed by using an electron back scattering diffraction method, thereby obtaining (0001) pole figures. Specifically, an L-section was chemically polished at the center in the width direction (TD) of each sample, and in this section, a crystal orientation analysis by the EBSD method was performed at intervals of 1 to 2 μm for about 2 to 10 fields of view in a region of (total plate thickness) ×2mm, to prepare a (0001) pole figure. (0001) The data of the aggregation peak position of the specific orientation in the polar diagram was calculated by texture Analysis (expansion coefficient=16, gaussian half width=5°) of the inverse polar diagram using the spherical harmonic method using OIM Analysis software manufactured by TSL Solutions.

2.3. Area ratio of alpha phase and area ratio of alpha phase with circle equivalent diameter of 1 μm or more

The area ratio of the α phase and the area ratio of the α phase having a circular equivalent diameter of 1 μm or more were measured by the following methods. A cross section of a titanium alloy plate cut at a widthwise (TD) center position perpendicularly to the widthwise direction was chemically polished, and a crystal orientation analysis by the EBSD method was performed for about 2 to 5 fields of view with a gradient of 1 to 5 μm in a region of (total plate thickness). Times.200 μm of the cross section. The area ratio of the alpha phase relative to the area of the region is referred to as the area ratio of the alpha phase. In addition, the equivalent circle diameter (area a=pi× (particle diameter D/2) of the α -phase observed in the above-mentioned field of view was calculated ² ) The total area of the alpha phase having a circular equivalent diameter of 1 μm or more relative to the area of the region is defined as the area ratio of the alpha phase having a circular equivalent diameter of 1 μm or more. The crystal grains of the alpha phase having a circular equivalent diameter of 1 μm or more contain a band structure described later.

2.4. Aspect ratio and area ratio of band-like structure

A cross section of a titanium alloy plate cut at a widthwise (TD) center position perpendicularly to the widthwise direction was chemically polished, and a crystal orientation analysis by the EBSD method was performed for about 2 to 5 fields of view with a gradient of 1 to 5 μm in a region of (total plate thickness). Times.200 μm of the cross section. From the results of the crystal orientation analysis of the EBSD, the aspect ratio of each crystal grain was calculated. The area ratio of the crystal grains having an aspect ratio of more than 3.0 was calculated as the band structure area ratio.

2.5. Average grain diameter of equiaxed structure

For the average grain diameter of the equiaxed structure, the equivalent circle diameter (area a=pi× (particle diameter D/2) was obtained from the area of the grains measured by EBSD for the equiaxed structure ² ) The average value of the number references was defined as the average grain diameter of the equiaxed structure.

2.6.0.2% yield strength

According to JIS Z2241: 2011, the 0.2% yield strength at 25℃of each of the titanium alloy plates according to the examples of the invention, the reference example and the comparative example.

2.7. Average plate thickness dave

The average plate thickness dave of the titanium alloy plates according to each of the invention examples, the reference examples and the comparative examples was measured by the following method. For each of the titanium alloy plates to be produced, the thickness of each of 5 or more positions was measured at intervals of 1m or more in the longitudinal direction using an X-ray, a micrometer or a vernier, at a position at which the width direction center position and the distance from both ends in the width direction were each 1/4 of the plate width, and the average value of the measured thicknesses was taken as the average thickness dave.

2.8. Plate thickness dimensional accuracy a

The sheet thickness dimensional accuracy a of the titanium alloy sheets according to each of the invention examples, the reference examples, and the comparative examples was obtained by using the sheet thickness d actually measured by the method described above and the average sheet thickness dave described above, and the maximum value of a' calculated by the following formula (101) was used as the dimensional accuracy a.

a' = (d-dave)/dave× … type (101)

3. Results

The evaluation results are shown in Table 3. The "θ" shown in table 3 is as follows: of the (0001) pole diagrams in the plate thickness direction, the inverse pole diagram using the spherical harmonic method with respect to the electron back scattering diffraction method is calculated by texture analysis at a coefficient of expansion of 16 and a gaussian half width of 5 ° as an angle between the direction of the peak representing the concentration degree and the plate thickness direction. In addition, "θ2" shown in table 3 is as follows: in the (0001) pole diagram based on the plate thickness direction, an angle between the direction of the peak showing the degree of aggregation from the center of the pole diagram and the plate width direction was calculated by texture analysis at a spreading factor of 16 and a gaussian half width of 5 ° for an inverse pole diagram using the spherical harmonic method by the electron back scattering diffraction method.

TABLE 3

In any of examples 1 to 30, reference examples and comparative examples 1 to 4, the contents of Al, fe, si, ni, cr, mn, V, O, N and C in the produced titanium alloy sheet were equal to the contents of the above elements contained in the hot rolled sheet used in each.

Regarding the invention examples 1 to 20, the angle θ between the direction of the peak indicating the degree of aggregation and the plate thickness direction in the (0001) pole diagram was 65 ° or less, and the angle θ2 between the direction of the peak indicating the degree of aggregation and the width direction was 0 °. The average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure is 10% or less. The area ratio of the alpha phase is more than 80%, and the area ratio of the alpha phase with the circle equivalent diameter of more than 1 mu m is more than 53%. The average plate thickness is 1.0-1.2 mm, and the dimensional accuracy is 0.8-4.5%. The ratio of 0.2% yield strength in the longitudinal direction at 25 ℃ to 0.2% yield strength sigma T in the width direction at 25 ℃ to 0.2% yield strength sigma L in the longitudinal direction at 25 ℃, i.e., the conditional yield strength ratio sigma T/sigma L, is not less than 1.05 and not more than 1.18.

Regarding invention example 21, the angle θ formed between the direction of the peak indicating the degree of aggregation and the plate thickness direction in the (0001) pole figure was 49 °, and the angle θ2 formed between the direction of the peak indicating the degree of aggregation and the width direction was 0 °. The average grain diameter of the equiaxed structure was 1.8. Mu.m, and the area ratio of the band structure was 5.0%. The area ratio of the alpha phase is 88% or more, and the area ratio of the alpha phase having a circle equivalent diameter of 1 μm or more is 88%. The average plate thickness was 0.9mm, and the dimensional accuracy was 2.0%. The 0.2% yield strength at 25℃was 805MPa, and the conditional yield strength ratio σT/σL was 1.12.

Regarding examples 22 and 23, the angle θ formed by the direction of the peak indicating the degree of aggregation and the plate thickness direction in the (0001) pole diagram was 50 °, and the angle θ2 formed by the direction of the peak indicating the degree of aggregation and the width direction was 0 °. The average grain size of the equiaxed structure of invention example 22 was 3.5. Mu.m, and the average grain size of the equiaxed structure of invention example 23 was 10.5. Mu.m. The area ratio of the band-like tissue was 15.0% and 20.0%, respectively. The area ratio of the alpha phase is more than 80%, and the area ratio of the alpha phase with the circle equivalent diameter of more than 1 mu m is more than 53%. The average plate thickness was 1.0mm and 1.6mm, and the dimensional accuracy was 2.0% and 2.5%. The 0.2% yield strength at 25 ℃ is 700MPa or more, and the conditional yield strength ratio [ sigma ] T/sigma ] L is 1.11 and 1.15.

Regarding invention examples 24 to 26, the angle θ formed between the direction of the peak indicating the degree of aggregation and the plate thickness direction in the (0001) pole diagram was 65 ° or less, and the angle θ2 formed between the direction of the peak indicating the degree of aggregation and the width direction was 0 °. The average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure is 10% or less. The area ratio of the alpha phase is more than 80%, and the area ratio of the alpha phase with the circle equivalent diameter of more than 1 mu m is more than 53%. The average plate thickness was 0.5mm, and the dimensional accuracy was 1.0. The 0.2% proof stress in the longitudinal direction at 25 ℃ is 700MPa or more, and the conditional proof stress ratio [ sigma ] T/sigma L is 1.05 or more and 1.18 or less.

Regarding invention examples 27 to 29, the angle θ formed between the direction of the peak indicating the degree of aggregation and the plate thickness direction in the (0001) pole diagram was 65 ° or less, and the angle θ2 formed between the direction of the peak indicating the degree of aggregation and the width direction was 0 °. The average grain diameter of the equiaxed structure is 0.1 μm or more and 20.0 μm or less, and the area ratio of the band structure is 10% or less. The area ratio of the alpha phase is more than 80%, and the area ratio of the alpha phase with the circle equivalent diameter of more than 1 mu m is more than 53%. The average plate thickness was 0.4mm, and the dimensional accuracy was 1.0% or less. The 0.2% yield strength in the longitudinal direction is 700MPa or more, and the conditional yield strength ratio [ sigma ] T/sigma ] L is 1.05 or more and 1.18 or less.

Regarding invention example 30, the angle θ formed between the direction of the peak indicating the degree of aggregation and the plate thickness direction in the (0001) pole figure was 45 °, and the angle θ2 formed between the direction of the peak indicating the degree of aggregation and the width direction was 0 °. The average grain diameter of the equiaxed structure was 3.5. Mu.m, and the area ratio of the band structure was 5.0%. The area ratio of the alpha phase is 85% or more, and the area ratio of the alpha phase having a circle equivalent diameter of 1 μm or more is 80%. The average plate thickness was 1.0mm, and the dimensional accuracy was 1.5%. The 0.2% yield strength was 800MPa, and the conditional yield strength ratio σT/σL was 1.14.

Regarding the reference example, the angle θ formed by the direction of the peak representing the degree of aggregation in the (0001) pole figure and the plate thickness direction is larger than 65 °. Thus, a conditional yield strength ratio σT/σL of greater than 1.18 shows a strong anisotropy.

In comparative example 1, the average rolling reduction per time was as small as 20%, and the total rolling reduction was also as small as 59%. Therefore, the angle θ between the direction of the peak indicating the degree of aggregation in the (0001) pole figure and the plate thickness direction is larger than 65 °. Thus, a conditional yield strength ratio σT/σL of greater than 1.18 shows a strong anisotropy. In comparative example 2, the average rolling reduction per time was 50%, but intermediate annealing and cold rolling were not repeated, and the total rolling reduction was as small as 50%. Therefore, the angle θ between the direction of the peak indicating the degree of aggregation in the (0001) pole figure and the plate thickness direction is larger than 65 °. Thus, a conditional yield strength ratio σT/σL of greater than 1.18 shows a strong anisotropy. Comparative example 3 has a 0.2% yield strength as low as 598MPa because of the low Al content. Comparative example 4 as described above, surface cracks and severe end cracks were generated at the time of cold rolling.

Example 2

A hot rolled plate having a thickness of 4mm and having the chemical compositions shown in Table 1, A, B, C, E and M, was produced in the same manner as in example 1.

Next, the hot-rolled sheet thus obtained was subjected to a cold rolling step under the conditions shown in Table 4. Inventive examples 31 to 37 in table 2 were performed in a plurality of cold rolling passes so that the average rolling rate per cold rolling pass was 5% or more and the total rolling rate shown in table 4 was reached. Inventive examples 31 to 35 in table 4 were examples in which cold cross rolling was performed by repeating a plurality of cold rolling passes at a rolling temperature of 25 ℃ and intermediate annealing under the conditions shown in table 2 until the total rolling reduction reached 60 to 75%. The intermediate annealing is carried out at 680-900 ℃ for 60-28800 s, and the final annealing is carried out at 650-930 ℃ for 120-28800 s. The cross rolling ratio of examples 32 to 36 was 0.4 to 7.0. The invention example 36 was an example in which cold cross rolling was performed by repeating a plurality of cold rolling passes at a rolling temperature of 400 ℃ and intermediate annealing under the conditions shown in table 4 until the total rolling reduction reached 75%. The intermediate annealing was performed at 800℃for 120s and the final annealing was performed at 850℃for 120s. The cross-rolling ratio of inventive example 36 was 13.0. Inventive example 37 was an example in which cold cross rolling was performed by repeating a plurality of cold rolling passes at a rolling temperature of 25 ℃ and intermediate annealing under the conditions shown in table 4 until the total rolling reduction reached 62%. The intermediate annealing was performed at 800℃for 120s and the final annealing was performed at 850℃for 120s. The cross-rolling ratio of inventive example 37 was 0.17. In this case, in order to achieve a dimension capable of rolling, cutting is appropriately performed according to the width of the roll, and rolling in the cross direction is performed.

TABLE 4

The titanium alloy sheets according to the respective invention examples were evaluated in the same manner as in example 1. The evaluation results are shown in Table 5.

TABLE 5

Regarding invention examples 31 to 37, the angle θ between the direction of the peak showing the degree of aggregation and the plate thickness direction in the (0001) pole diagram was 35 ° or less. The average grain diameter of the equiaxed structure is 0.1 μm or more and 10.0 μm or less, and the area ratio of the band structure is 10% or less. The area ratio of the alpha phase is more than 80%, and the area ratio of the alpha phase with the circle equivalent diameter of more than 1 mu m is more than 53%. The average plate thickness is 1.0-1.8 mm, and the dimensional accuracy is 1.5-3.5%. The ratio of 0.2% yield strength at 25 ℃ to 0.2% yield strength σT in the width direction at 25 ℃ to 0.2% yield strength σL in the length direction at 25 ℃, that is, the conditional yield strength ratio σT/σL, is not less than 0.85 and not more than 1.10.

The preferred embodiments of the present disclosure have been described in detail above, but the present disclosure is not limited to the examples. It is obvious that a person having ordinary knowledge in the art to which the present disclosure pertains can think of various modifications and corrections within the scope of the technical idea described in the claims, and these examples naturally fall within the technical scope of the present disclosure.

Claims

1. A titanium alloy sheet comprising, in mass%

Al: more than 4.0% and less than 6.6%,

Fe:0% to 2.3%,

V:0% to 4.5%,

Si:0% to 0.60%,

C: more than 0% and less than 0.080%,

N:0% to 0.050% by weight,

O:0% to 0.40%,

Ni:0% or more and less than 0.15%,

Cr:0% or more and less than 0.25%, and

mn:0% or more and less than 0.25%,

the balance of Ti and impurities,

the area ratio of the alpha phase of the titanium alloy plate is more than 80 percent,

The area ratio of alpha phase with the equivalent circle diameter of more than 1 mu m is more than 53 percent,

of the (0001) pole diagrams in the plate thickness direction, the inverse pole diagram using the spherical harmonic method by the electron back scattering diffraction method has an angle of 65 DEG or less between the direction of the peak representing the aggregation degree and the plate thickness direction calculated by texture analysis when the expansion coefficient is 16 and the Gaussian half-width is 5 DEG,

the titanium alloy sheet has an average sheet thickness of 2.5mm or less.

2. The titanium alloy sheet according to claim 1, which has a microstructure composed of an equiaxed structure having an aspect ratio of 3.0 or less and a band-like structure having an aspect ratio of more than 3.0 and extending in a longitudinal direction,

The average grain diameter of the equiaxed structure is more than 0.1 mu m and less than 20.0 mu m,

the area ratio of the band-shaped tissue relative to the area of the microstructure is 10.0% or less.

3. The titanium alloy sheet according to claim 1 or 2, which contains, in mass%, fe:0.5% or more and 2.3% or less, or V: any one of 2.5% or more and 4.5% or less.

4. The titanium alloy sheet according to any one of claims 1 to 3, which contains a metal selected from the group consisting of Ni: less than 0.15%, cr: less than 0.25% and Mn: less than 0.25% of 1 or more than 2 of the group consisting of said Fe or said V is substituted for a part of said Fe or said V.

5. The titanium alloy sheet according to any one of claims 1 to 4, wherein the smaller of the 0.2% yield strength in the longitudinal direction at 25 ℃ or the 0.2% yield strength in the width direction at 25 ℃ is 700MPa or more and 1200MPa or less.

6. The titanium alloy sheet according to any one of claims 1 to 5, wherein,

of the (0001) pole diagrams in the plate thickness direction, the angle between the direction of the peak representing the degree of aggregation and the width direction calculated by texture analysis when the expansion coefficient is 16 and the Gaussian half-width is 5 DEG is 10 DEG or less for the antipole diagrams using the spherical harmonic method by the electron back scattering diffraction method,

The ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the length direction is 1.05 to 1.18.

7. The titanium alloy sheet according to any one of claims 1 to 5, wherein,

of the (0001) pole diagrams in the plate thickness direction, the angle between the direction of the peak representing the concentration degree calculated by texture analysis when the expansion coefficient is 16 and the Gaussian half-width is 5 DEG with respect to the antipole diagram using the spherical harmonic method by the electron back scattering diffraction method and the plate thickness direction is 35 DEG or less,

the ratio of the 0.2% yield strength in the width direction to the 0.2% yield strength in the length direction is 0.85 to 1.10.

8. The titanium alloy sheet according to any one of claims 1 to 7, wherein dimensional accuracy of a sheet thickness is 5.0% or less relative to the average sheet thickness.

9. A titanium alloy coil material comprising, in mass percent

Al: more than 4.0% and less than 6.6%,

Fe:0% to 2.3%,

V:0% to 4.5%,

Si:0% to 0.60%,

C: more than 0% and less than 0.080%,

N:0% to 0.050% by weight,

O:0% to 0.40%,

Ni:0% or more and less than 0.15%,

Cr:0% or more and less than 0.25%, and

Mn:0% or more and less than 0.25%,

the balance of Ti and impurities,

the area ratio of alpha phase of the titanium alloy coiled material is more than 80 percent,

the average plate thickness of the titanium alloy coiled material is below 2.5 mm.

10. A method for producing the titanium alloy sheet according to any one of claims 1 to 8, comprising the steps of:

a cold rolling step of performing a cold rolling pass for 1 or more times in the longitudinal direction of a titanium material containing, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%, and the balance being Ti and impurities; and

A final annealing step of annealing the titanium material after the final cold rolling pass,

the average rolling rate of each cold rolling pass in the cold rolling process is more than 30%, and the total rolling rate is more than 60%.

11. The method for producing a titanium alloy sheet according to claim 10, wherein,

when the cold rolling process is performed in a plurality of cold rolling passes, an intermediate annealing process for annealing the titanium blank is included between the plurality of cold rolling passes, and annealing conditions in the intermediate annealing process and the final annealing process are as follows: the annealing temperature is 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T DEG C and the holding time T (seconds) at the annealing temperature satisfy the following formula (1),

22000≤(T+273.15)×(Log ₁₀ (t) +20). Ltoreq.27000 … (1)

Wherein T is _β Is beta transformation point (. Degree. C.).

12. A method for producing the titanium alloy sheet according to any one of claims 1 to 8, comprising the steps of:

cold cross rolling, in which cold rolling passes are performed in the longitudinal direction and the width direction of a titanium material containing, in mass%, al: greater than 4.0% and less than 6.6%, fe:0% or more and 2.3% or less, V:0% or more and 4.5% or less, si:0% or more and 0.60% or less, C:0% or more and less than 0.080%, N:0% or more and 0.050% or less, O:0% or more and 0.40% or less, ni:0% or more and less than 0.15%, cr:0% or more and less than 0.25%, and Mn: more than 0% and less than 0.25%, and the balance being Ti and impurities; and

A final annealing step of annealing the titanium ingot after the cold cross rolling step,

the total rolling rate in the cold cross rolling process is more than 60 percent,

the ratio of the rolling reduction in the longitudinal direction to the rolling reduction in the width direction, that is, the cross rolling ratio is 0.05 to 20.00.

13. The method for producing a titanium alloy sheet according to claim 12, wherein,

when the cold cross rolling process is performed in a plurality of cold rolling passes, an intermediate annealing process for annealing the titanium material is included between the cold rolling passes, and annealing conditions in the intermediate annealing process and the final annealing process are as follows: the annealing temperature is 600 ℃ or higher and (T) _β -50) DEG C or less, and the annealing temperature T DEG C and the holding time T (seconds) at the annealing temperature satisfy the following formula (1),

22000≤(T+273.15)×(Log ₁₀ (t) +20). Ltoreq.27000 … (1)

Wherein T is _β Is beta transformation point (. Degree. C.).

14. A method of manufacturing the titanium alloy coil of claim 9, comprising the steps of: