JP6432328B2 - High strength titanium plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a Ti—Fe—O-based high-strength titanium plate exhibiting excellent ductility and a method for producing the same.

  High-strength α + β-type titanium alloys represented by Ti-6Al-4V have various characteristics such as weldability, superplasticity, and diffusion bondability in addition to light weight, high strength, and high corrosion resistance. Has been widely used in the center.

  In recent years, in order to make further use of these characteristics, it has come to be widely used in sports equipment such as golf equipment. Furthermore, automobile engine parts, civil engineering materials, various tools, deep sea and energy. Expansion of application to the so-called consumer products field such as development applications is also being considered.

  However, the remarkably high production cost of α + β type titanium alloy has hindered the expansion of its application, and in order to expand the application to the field of consumer products, development of an inexpensive titanium alloy has been required.

  The reason for the high production cost of these high strength α + β type titanium alloys is that (1) an expensive β phase stabilizing element such as V is used, and (2) the α phase stabilizing element and the solid solution strengthening element. Since Al which is used remarkably increases the deformation resistance in the hot state, it becomes difficult to work and the hot workability is deteriorated, so that defects such as cracks are likely to occur.

  On the other hand, in order to achieve both high strength and excellent ductility of titanium alloy, Patent Documents 1 and 2 propose a titanium alloy in which room temperature strength is increased without increasing hot deformation resistance by adding oxygen or Fe. Has been. Patent Document 3 proposes a titanium alloy that contains oxygen or Fe but has a reduced oxygen content.

Japanese Patent No. 3481428 Japanese Patent No. 3749589 JP 2008-240026 A

  In recent years, there has been a demand for a high-strength titanium alloy that achieves cost reduction and has higher ductility.

  However, in Patent Documents 1 and 2, although various production conditions have been studied in order to reduce the in-plane anisotropy of high strength and high ductility, it is not possible to increase the characteristic values themselves of high strength and high ductility. Absent. For this reason, a structure capable of achieving both high strength and high ductility at a high level has not been obtained. According to the examination results described in these documents, the ductility is insufficient and further improvement is necessary to meet the recent demand.

  In the invention described in Patent Document 3, the ductility of the titanium alloy material due to softening of the α phase can be increased by reducing the content of oxygen which is an α stabilizing element. Although the document describes that annealing is performed at least once before the finish annealing, the strength is less than 500 MPa because the oxygen content is small.

  Patent Documents 1 and 2 disclose a titanium alloy containing 0.2% by mass or more of oxygen in order to increase the strength. However, Patent Document 3 discloses an oxygen concentration of 0.1% by mass in order to increase ductility. % Or less is disclosed. In other words, it is also difficult to combine these technologies that have conflicting technical ideas.

Thus, with the conventional technology, a high-strength titanium plate having high ductility that satisfies recent requirements cannot be obtained, and further studies are required.
Therefore, an object of the present invention is to provide a high-strength titanium plate having excellent ductility.

  When Al is contained and the strength is increased as in a conventional titanium alloy, the ductility is lowered and the toughness is impaired. Oxygen and Al, which are α-phase stabilizing elements, are effective for solid solution strengthening, but slip deformation and twin deformation are unlikely to occur due to their high strengthening ability. Furthermore, since the hot deformation resistance is high, cracking is likely to occur during forging or rolling. As described above, the inventors of the present invention focus on the Fe content after incorporating oxygen that can be reduced in cost and increased in strength to some extent, and intensively studying the relationship between these elements and the structure. Went. As a result, the following knowledge was obtained.

  (1) Oxygen has the effect of suppressing slip deformation and twin deformation as in the case of Al. However, if the content is 0.25 to 0.4% by mass, the ductility is not greatly impaired and the hot deformation resistance is also large. Therefore, cracks are difficult to occur during forging. Furthermore, when the Fe content is 0.8 to 1.5 mass%, the crystal grains can be refined and sized, and good ductility can be obtained.

  (2) A good ductility and formability can be obtained by finely dispersing the β phase in an equiaxed structure obtained by recrystallizing the α phase.

  (3) The reason why the α phase is recrystallized is as follows. With an equiaxed structure, the deformation of crystal grains is uniform and work hardening is easy, and good ductility and strength are obtained. On the other hand, in the needle-like structure, the shape of the crystal grains has an influence, and nonuniform deformation occurs and work hardening is difficult. Therefore, good ductility and strength cannot be obtained.

(4) The reason why a large amount of β phase is dispersed is as follows. The α phase and the β phase have a difference in hardness. When plastic deformation occurs, stress concentrates at the grain interface between the α phase and β phase, and when the plastic instability occurs, voids are generated at this interface. Since this can be a starting point for cracks in the forming process, it is necessary to disperse the stress in the material by increasing the interface between the α phase and the β phase as much as possible, that is, it is important to disperse the β phase in a minute amount. Therefore, in order to obtain excellent strength and ductility, it is necessary to control the particle size and number of β phases.
The inventors have repeatedly studied to control the particle size and number of β phases, and obtained the following knowledge.

  (5) In order to obtain a structure in which a large amount of β-phase is dispersed, the content of Fe, which is a β-phase stabilizing element, is adjusted to the above range, and the hot-rolled coil is 750 ° C to 850 ° C before finish annealing. In this temperature range, the intermediate annealing for 30 to 300 seconds must be performed twice or more. This is because the β phase having a sufficient solid solution amount of Fe remains by annealing and a large amount is dispersed in the titanium material.

(6) In order to cause adequate recrystallization after intermediate annealing and obtain sufficient ductility while maintaining the amount of Fe solid solution in the β phase, the final annealing temperature is 720 to 850 ° C., and the annealing time is 30 to 200 sec. To do.
As described above, the β-phase amount, β-phase particle size, and β-phase number are affected by the Fe content (mass%), the annealing temperature T (° C.), and the annealing time t (seconds). The relationship was unclear. It is very important that this relationship be clarified in order to control the β-phase distribution to provide an excellent balance between strength and ductility. Therefore, the present inventors conducted the following investigation. The amount of β phase strongly depends on the Fe content and annealing temperature, and the particle size and number strongly depend on the Fe content, annealing temperature and annealing time. Further, the number varies with the particle diameter. Therefore, we focused on the relationship between particle size and influencing factors. And the present inventors also obtained the following knowledge, as a result of conducting earnest regression analysis.

  (7) The β phase and the influencing factor are controlled to satisfy −0.20 ≦ A ≦ 0.57 and −0.20 ≦ B ≦ 0.55 in the relational expressions represented by the expressions (1) and (2). It is desirable to do. Thereby, the knowledge that high ductility was obtained even at high strength was obtained. That is, when a large number of β phases are finely dispersed, stress to the α / β phase grain boundaries is dispersed, voids are less likely to be generated, and high ductility is achieved.

A = 0.98 × [Fe] −1264 ÷ (273 + T ₁ ) + 0.05 × t ₁ ^0.25
(1)
B = 0.98 × [Fe] −1264 ÷ (273 + T ₂ ) + 0.05 × t ₂ ^0.25
(2)

In formulas (1) and (2), [Fe] is the Fe concentration of the high-strength titanium plate, T ₁ and T ₂ are the intermediate annealing temperature (° C.) and the finish annealing temperature (° C.), respectively, t ₁ and _{t 2} are each intermediate annealing time (s) and finish annealing time (s).

The present invention completed based on the above findings is as follows.
(1) In mass%, Fe: 0.8 to 1.5%, O: 0.25 to 0.40%, the remainder having a chemical composition consisting of Ti and impurities, and recrystallized equiaxed It has a two-phase structure consisting of an α phase and a β phase, the area ratio of the equiaxed α phase is 70 to 85%, the average crystal grain size of the β phase is 3 μm or less, and the number of β phases is in the rolling width direction A high-strength titanium plate having 0.02 or more on average in an area of 1 μm ² in cross section.

  (2) After hot rolling the titanium material, cold rolling and intermediate annealing are performed twice or more in this order, and finish cold working and finishing annealing are performed in this order after the last intermediate annealing ( 1) The method for producing a high-strength titanium plate according to 1), wherein the annealing temperature range of the intermediate annealing is set to 750 to 850 ° C., the annealing time of the intermediate annealing is set to 30 to 300 seconds, and the annealing temperature of the finish annealing is set to 720-850 degreeC, The annealing time of the said finish annealing shall be 30-200 second, The manufacturing method of the high strength titanium plate characterized by the above-mentioned.

  (3) As for the condition of the said intermediate annealing, the value of A in Formula (1) satisfy | fills -0.20-0.57, and the condition of the said finish annealing has the value of B in Formula (2) being -0.20. The method for producing a high-strength titanium plate according to the above (2), which satisfies -0.55.

A = 0.98 × [Fe] −1264 ÷ (273 + T ₁ ) + 0.05 × t ₁ ^0.25
(1)
B = 0.98 × [Fe] −1264 ÷ (273 + T ₂ ) + 0.05 × t ₂ ^0.25
(2)

In Formula (1) and Formula (2), [Fe] is the Fe content (% by mass) of the high-strength titanium plate, and T ₁ and T ₂ are the annealing temperature (° C.) of the intermediate annealing and the above It is the annealing temperature (° C.) of finish annealing, and t ₁ and t ₂ are the annealing time (s) of the intermediate annealing and the annealing time (s) of the finishing annealing, respectively.

  According to the present invention, a high-strength titanium plate having excellent ductility and a method for producing the same can be provided.

In the following description, “%” represents “mass%” unless otherwise specified.
1. High-strength titanium plate 1.1 Chemical composition (1) Fe: 0.8-1.5%
Fe is contained in the titanium material in an amount of 0.8 to 1.5%. In titanium materials, Fe is a β-phase stabilizing element, and some of them are known to dissolve in the β phase, although some of them dissolve in the α phase. That is, as the amount of Fe increases, the amount of β phase increases, and accordingly, the growth of α phase grains can be suppressed and a fine grain structure can be obtained. This is because sufficient strength cannot be obtained when the Fe content is less than 0.8%. Preferably it is 0.9% or more. When the Fe content exceeds 1.5%, the stability of the β phase increases, and even after cooling to room temperature, it remains as the β phase without being transformed into the α phase, and passes through heating processes such as hot rolling and annealing. In addition, the residual β phase may be coarsened. Moreover, there exists a possibility that corrosion resistance may fall. Preferably it is 1.4% or less. Furthermore, Fe tends to segregate during dissolution and solidification. From the viewpoint of homogeneity in the coil, 1.2% or less is more preferable.

(2) O: 0.25 to 0.40%
Oxygen is contained in the titanium material by 0.25 to 0.40%. Oxygen is an effective element for increasing the strength of all titanium materials. If the oxygen content is less than 0.25%, it may be difficult to impart sufficient strength to a product produced using a titanium plate. Preferably it is 0.28% or more, more preferably 0.32% or more. This is because if the oxygen content exceeds 0.40%, the strength becomes too high and the titanium plate has a low ductility. Preferably it is 0.39% or less, More preferably, it is 0.38% or less.
The balance is Ti and impurities.

1.2 Titanium metal structure (1) Two-phase structure composed of recrystallized equiaxed α-phase and β-phase The titanium plate of the present invention is composed of two phases of equiaxed α-phase and β-phase. The acicular α phase reduces the ductility because the crystal grains are difficult to rotate and are not easily deformed. The aspect ratio (major axis / minor axis) of the equiaxed α phase is preferably 1 to 2 from the viewpoint of enhancing ductility. The area ratio of the equiaxed α phase having an aspect ratio of 1 to 2 is desirably 70 to 85% in order to obtain sufficient ductility. More preferably, it is 72 to 80%. Further, since a good ductility and formability can be obtained by finely dispersing the β phase in an equiaxed structure obtained by recrystallizing the α phase, the ratio of the area ratio of the equiaxed α phase to the β phase is 7 : It is preferable that it is 3-17: 3.

(2) β-phase average crystal grain size: 3 μm or less, β-phase number: 0.02 or more on average within an area of 1 μm ² in the cross section in the rolling width direction It is composed of β phase. For this reason, stress concentrates at the interface between the α phase and the β phase in plastic deformation. The β-phase average crystal grain size is 3 μm or less, and the number of β-phases is 0.02 or more on average within an area of 1 μm ² in the cross section in the rolling width direction. When the average cross section in the direction cross section is less than 0.02 within an area of 1 μm ² , the interface between the α phase and the β phase is reduced, stress concentration is more likely to occur, and ductility is lowered. Preferably, the β phase particle size is 2.8 μm or less, the average number of β phases is 0.03 or more, more preferably the β phase particle size is 2.7 μm or less, and the β phase number is 0.04 or more on average. . Note that the “average” is a value obtained by observing the number of β phases at three locations in a field of view of 400 μm × 400 μm and dividing the total number by the total number of fields.

2. 2. Manufacturing Method of High Strength Titanium Plate 2.1 Hot Rolling of Pure Titanium Ingot In the present invention, the steps up to hot working can be manufactured by a general manufacturing method of a titanium plate. For example, a titanium ingot was manufactured from sponge titanium or the like, the ingot was forged into a slab shape, and this was processed into a hot-rolled sheet by hot rolling. Thereafter, descaling is performed on the hot-rolled sheet by shot blasting and pickling to produce a titanium material used for cold rolling. These conditions are not particularly limited, and may be general conditions.

2.2 Intermediate annealing In order to obtain the structure defined by the high-strength titanium plate of the present invention, intermediate annealing is performed twice or more in which the titanium material is held in the temperature range of 750 ° C. to 850 ° C. for 30 to 300 seconds until the final annealing. There must be. This is because the β phase generated by annealing remains and is dispersed in a large amount in the titanium material.

  When the number of intermediate annealing is less than 2, since the β phase is small, the stress concentration cannot be dispersed.

  When the intermediate annealing temperature is less than 750 ° C. and the holding time is less than 30 seconds, the amount of β phase generated is small. When the intermediate annealing temperature exceeds 850 ° C. and the holding time exceeds 300 seconds, the β phase grows and the interface between the α phase and the β phase decreases, and the amount of the β phase itself increases and the Fe phase in the β phase increases. This is because the amount of solid solution decreases and the β phase cannot remain and transforms into a needle-like α phase. Furthermore, the cooling rate after annealing is preferably 0.1 ° C./second or more. This is because the β phase generated by the intermediate annealing remains at room temperature. If the cooling rate is slower than this, the β phase is transformed into the α phase, and the amount of the β phase is reduced. This cooling rate is the same in finish annealing described later.

  Moreover, “intermediate annealing is performed twice or more” means that cold rolling and intermediate annealing are performed twice or more in this order on the titanium material after hot rolling. That is, in this invention, it is necessary to perform cold rolling and intermediate annealing twice or more each. The pressing ratio of cold rolling performed before the intermediate annealing is not particularly specified, but may be 50% or more for each.

2.3 Finish annealing In the present invention, in order to obtain a recrystallized equiaxed α-phase, the finish annealing temperature is set to 720 to 850 ° C., and the annealing time is set to 30 to 200 seconds. When the final annealing temperature is less than 720 ° C., recrystallization hardly occurs in a short time of several minutes, and sufficient ductility cannot be obtained. In addition, even if it is less than 720 degreeC, if it anneals for a long time, it will recrystallize, but will reach an equilibrium state. In the equilibrium state, the lower the temperature, the fewer β phases. Therefore, even if a large amount of β phases remain in the intermediate annealing, the number of β phases after finish annealing decreases. Therefore, the finish annealing temperature needs to be 720 ° C. or higher regardless of the annealing time. Preferably it is 725 ° C or more. On the other hand, when the temperature exceeds 850 ° C., the β phase generated by annealing has a small amount of Fe solid solution, so that it does not remain when cooled to room temperature and transforms into a needle-like α phase that reduces ductility.

  When the annealing time is set to 30 to 200 seconds, recrystallization hardly occurs in a time shorter than 30 seconds, and sufficient ductility cannot be obtained. If the time is longer than 200 seconds, the β phase grows and coarsens, the number of β phases decreases, and the stress at the interface between the α phase and the β phase does not disperse, resulting in a decrease in ductility.

2.4 Relationship with Fe Content, Annealing Temperature and Annealing Time In the relational expressions shown by the formulas (1) and (2), −0.20 ≦ A ≦ 0.57, −0.20 ≦ B ≦ 0. It is desirable to control to be 55. This is because high ductility can be obtained even at high strength. When a large number of β phases are finely dispersed, the stress concentration at the α / β phase grain boundaries is dispersed, voids are less likely to be generated, and high ductility is achieved.

  When A and B are smaller than this range, the Fe content is small and the β phase particle size is small, but the β phase amount is small and the β phase cannot be dispersed in a large amount. For this reason, stress concentrates on the α / β phase grain boundary and voids are easily generated, and good ductility cannot be obtained. On the other hand, if A and B are larger than this range, the β phase particle size becomes too large and the number of β phases is so small that the β phase cannot be dispersed in a large amount, so that good ductility cannot be obtained as described above.

  From the above, by substituting the Fe content, annealing temperature, and annealing time into the formulas (1) and (2), respectively, so that A and B are in the above ranges, the β phase particle size and the number of β phases are A large amount can be finely dispersed, and good strength and ductility can be obtained. The range of A is preferably -0.20 to 0.45, more preferably -0.20 to 0.40, and the range of B is preferably -0.20 to 0.50, more preferably- 0.20 to 0.45. A preferable range of combinations is that A is -0.36 to 0.57 and B is -0.38 to 0.55, more preferably A is -0.30 to 0.45, and B is -0.30. It is -0.50, More preferably, A is -0.20-0.40 and B is -0.20-0.45.

Intermediate annealing:
A = 0.98 × [Fe] −1264 ÷ (273 + T ₁ ) + 0.05 × t ₁ ^0.25
(1)
Finish annealing:
B = 0.98 × [Fe] −1264 ÷ (273 + T ₂ ) + 0.05 × t ₂ ^0.25
(2)

In formulas (1) and (2), [Fe] is the Fe concentration of the high-strength titanium plate, T ₁ and T ₂ are the intermediate annealing temperature (° C.) and the finish annealing temperature (° C.), respectively, t ₁ and _{t 2} are each intermediate annealing time (s) and finish annealing time (s).
Thereafter, the scale is completely removed with nitric hydrofluoric acid or the like and cooled to obtain a titanium material.

  As shown in Tables 3 and 4, by vacuum arc melting, various titanium ingots having different oxygen contents and Fe contents are manufactured, forged, hot-rolled and surface-cut, and subjected to intermediate annealing and cold rolling. This was performed three times, and a titanium plate having a thickness of 0.9 mm was obtained by finish annealing, cold rolling, and pickling. The obtained titanium plate was measured for Fe content, oxygen content, equiaxed α-phase area ratio, β-phase area ratio, β-phase number, β-phase particle size, tensile strength, and elongation.

  Specific cold rolling conditions, finish annealing, and pickling conditions are shown in Table 1. Table 2 shows specific evaluation methods. In addition, the cooling rate after each annealing is 0.1-20 degree-C / sec. Moreover, the rolling reduction of the cold rolled steel sheet performed before each annealing was 50% or more.

  In this example, the tensile strength was 900 MPa or more, and the tensile strength × elongation ≧ 23,000 MPa ·% was regarded as acceptable. An arbitrary range of 1 mm × 1 mm on the test piece TD surface was measured by a crystal orientation analysis method using a backscattered electron diffraction image EBSD (Electron Backscatter Diffraction Pattern) at a pitch of 3 μm, and the following was obtained using the attached analysis software. In addition, the particle size and the number of β-phases at three locations were observed with an optical microscope in a field of view of 400 μm × 400 μm. For the number, the value obtained by dividing the total number of three places by the total area of the field of view was taken as the average number. In addition, the aspect ratio (major axis / minor axis) of the α phase observed at the same location was determined, and the α phase having an aspect ratio of 1 to 2 was regarded as the recrystallized equiaxed α phase, and the area ratio was investigated. The area ratio of the β phase was determined by subtracting the area ratio of the α phase from 100, assuming that the area excluding the α phase (equal axis and needle-like) is occupied by the β phase.

  These results are shown in Tables 3 and 4. In Tables 3 and 4, “Expression (1) -1” is the value of A in Expression (1) under the first intermediate annealing (intermediate annealing 1) conditions. “Formula (1) -2” and “Formula (1) -3” are the values of A in Formula (1) under the second and third intermediate annealing (intermediate annealing 2, intermediate annealing 3) conditions, respectively.

  In all the inventive examples satisfying the conditions of the present invention, the tensile strength was 900 MPa or more, and the tensile strength × elongation was 23000 (MPa ·%) or more. In addition, No. Except for 11-14, 57 and 62, it was confirmed that the area ratio of the equiaxed α phase was 70 to 85%, and the fine β phase was dispersed.

On the other hand, No. which is a comparative example. Nos. 1 to 8 were inferior in tensile strength because of low Fe content.
No. 9 and 10 were inferior in tensile strength due to low oxygen content.

  No. Since No. 11-14 had a low finish annealing temperature, the area ratio of the equiaxed α phase was as low as less than 70%, the elongation was low, and the tensile strength × elongation was inferior.

  No. In 15, 21, and 26, the intermediate annealing temperature was low, so β phases were not sufficiently generated, the number of β phases was small, and tensile strength × elongation was inferior.

  No. 19, 20, 24, 25, 30, 34, 38, 42, 46, 50 and 70 have a high intermediate annealing temperature, so the β phase is transformed into an acicular α phase, the number of β phases is small, the tensile strength × The growth was inferior.

  No. In 35 to 37 and 39 to 41, since the finish annealing temperature was high, the β phase was transformed into an acicular α phase, the number of β phases was small, and the tensile strength × elongation was inferior.

  No. Nos. 43 to 45 reached an equilibrium state because the annealing temperature was low and annealed for a long time, the number of β phases was small, and the tensile strength × elongation was inferior.

  No. In Nos. 47 to 49, since the intermediate annealing was performed only once, the β phase did not remain, the number of β phases was small, and the tensile strength × elongation was inferior.

  No. No. 51 had a short intermediate annealing time, so the production amount of β phase was small, the number of β phases was small, and the tensile strength × elongation was inferior.

  No. In 55 and 56, since the intermediate annealing time was long, the β phase was transformed into an acicular α phase, the β number was small, and the tensile strength × elongation was inferior.

  No. 57 and 62 had a short finish annealing time, so the area ratio of the equiaxed α phase was lower than 70%, the elongation was low, and the tensile strength × elongation was inferior.

  No. In Nos. 60, 61, 65 and 66, since the finish annealing time was long, the β phase was coarsened, the number of β phases was small, and the tensile strength × elongation was inferior.

No. 71 and 72 were inferior in tensile strength × elongation due to high oxygen content.
No. 75 and 76 were inferior in tensile strength × elongation due to high Fe content.

Claims (2)

In mass%, Fe: 0.8 to 1.5%, O: 0.25 to 0.40% is contained, the balance has a chemical composition consisting of Ti and impurities,
Recrystallized equiaxed α phase with an aspect ratio of 1 to 2 and a β phase with a two-phase structure, the area ratio of the equiaxed α phase is 70 to 85%, and the average crystal grain size of the β phase Is 3 μm or less, the number of β phases is 0.02 or more on average in the area of 1 μm ² in the cross section in the rolling width direction , the tensile strength is 900 MPa or more, and tensile strength × elongation ≧ 23,000 MPa ·%. in it, a high strength titanium plate, characterized in that. The hot rolling is performed on the titanium material, and then the cold rolling and the intermediate annealing are performed twice or more in this order, and the finish cold working and the finishing annealing are performed in this order after the final intermediate annealing. A method for producing a high-strength titanium plate,
The annealing temperature range of the intermediate annealing is set to 750 to 850 ° C., the annealing time of the intermediate annealing is set to 30 to 300 seconds, and the condition of the intermediate annealing is such that the value of A in the formula (1) is −0.20 to 0 .57, an annealing temperature of the finish annealing is set to 720 to 850 ° C., an annealing time of the finish annealing is set to 30 to 200 seconds , and the condition of the finish annealing is such that the value of B in the formula (2) is − The manufacturing method of the high intensity | strength titanium plate characterized by satisfying 0.20-0.55 .
A = 0.98 × [Fe] −1264 ÷ (273 + T ₁ ) + 0.05 × t ₁ ^0.25
(1)
B = 0.98 × [Fe] −1264 ÷ (273 + T ₂ ) + 0.05 × t ₂ ^0.25
(2)
In Formula (1) and Formula (2), [Fe] is the Fe content (% by mass) of the high-strength titanium plate, and T ₁ and T ₂ are the annealing temperature (° C.) of the intermediate annealing and the above It is the annealing temperature (° C.) of finish annealing, and t ₁ and t ₂ are the annealing time (s) of the intermediate annealing and the annealing time (s) of the finishing annealing, respectively.