JP6536317B2 - α + β-type titanium alloy sheet and method of manufacturing the same - Google Patents

Description

  The present invention relates to an α + β-type titanium alloy sheet and a method of manufacturing the same.

  The high strength α + β type titanium alloy represented by Ti-6Al-4V has various processing characteristics such as weldability, superplasticity and diffusion bondability in addition to light weight, high strength and high corrosion resistance, so the space and aircraft industry It has been widely used around the world. However, the extremely high manufacturing cost of α + β-type titanium alloys hinders the expansion of their applications, and development of inexpensive high-strength titanium alloys is required in order to expand their application to these applications and consumer products. .

The following two points can be mentioned as a reason for the high manufacturing cost of these high strength alpha + beta-type titanium alloys.
(1) The use of an expensive β-phase stabilizing element such as V.
(2) Al used as an α-phase stabilizing element and a solid solution strengthening element significantly increases the deformation resistance in hot, so it becomes difficult to process and impairs the hot workability. Therefore, processing requires hot processing at high temperature and high processing power, and requires high heat resistance and high stress resistance of processing equipment. Further, as a result of difficulty in processing, defects such as cracks are likely to occur during processing, and the point of lowering the product yield is also mentioned as a problem of high cost.

  In recent years, high strength titanium alloys having higher ductility while achieving cost reduction have been sought. As prior art which does not add alloy elements, such as Al and V, the technique like patent documents 1-4 is disclosed.

  In Patent Document 1, the oxygen equivalent value determined by the amount of iron, oxygen, and nitrogen is taken as a predetermined range, and the tensile strength of the plate in the length direction and the width direction is 700 MPa or more in both the length direction and the width direction. A Ti-Fe-O-N based high strength titanium alloy sheet is described which has a tensile elongation of at least 15%.

  In Patent Document 2, as in Patent Document 1, the oxygen equivalent value is in a predetermined range, and a mixed structure of an equiaxed α phase of 5 to 30% by volume ratio and a needle-like α phase and β phase in which the balance is fine Described is a hot-rolled strip, a hot-rolled sheet or a hot-rolled strip consisting of a Ti-Fe-O-N titanium alloy having a tensile strength in the longitudinal direction of 700 MPa or more and a tensile elongation in the sheet width direction of 10% or more. It is done.

  Patent Document 3 describes a high-strength low-alloy titanium alloy having a tensile strength of 750 MPa or more, in which the contents of oxygen, carbon, nitrogen, and iron are in a predetermined range.

  Patent Document 4 has an oxygen equivalent value determined by the amount of iron, oxygen, and nitrogen as a predetermined range, and is composed of an equiaxed α + β structure having a particle diameter of 20 μm or less. High strength suitable for superplastic forming Titanium alloys are described.

JP-A-11-61297 Japanese Patent Application Laid-Open No. 10-265876 Japanese Patent Application Publication No. 2004-269982 Japanese Patent Application Laid-Open No. 8-295969

However, in Patent Documents 1 and 2, although various manufacturing conditions are studied to reduce high strength and high ductility in-plane anisotropy, each of the strength and ductility characteristic values themselves is further improved. There is room. For this reason, a structure in which strength and ductility can be compatible at a high level has not been obtained. As a result of examining the inventions described in these documents, ductility is insufficient in order to meet recent severe demands, and further improvement is necessary.
In the invention described in Patent Document 3, although O, N, C, and Fe are adjusted, although high strength is obtained in the example, ductility is low and elongation is 20% when the strength is sufficiently increased. Has not reached.
In the invention described in Patent Document 4, although ductility in a temperature range of superplasticity is examined, ductility at normal temperature is not substantially examined.
As described above, in the prior art, while maintaining high strength, a high strength titanium plate having high ductility that satisfies the recent requirements can not be obtained, and further study is necessary.
Then, this invention makes it a subject to provide the high strength titanium plate which has the outstanding ductility, maintaining high strength.

That is, the place made into the summary of the present invention is as follows.
(1) Recrystallization was made by containing 0.8% to 1.5% of Fe, 0.28% to 0.4% of oxygen (O) by mass, and the balance consisting of titanium and unavoidable impurities The circle equivalent average crystal grain size of the equiaxed α phase is 2 to 10 μm, and the recrystallized equiaxed α phase has an area ratio of 15% or more, and the recrystallized β phase ratio of 0.5% or more and 10% or less, Furthermore, an α + β-type titanium alloy sheet having a processed structure of 10% or more .
(2) The α + β-type titanium alloy sheet according to (1), which further contains 0.05% or less of nitrogen (N) by mass.
(3) The method for producing an α + β-type titanium alloy sheet described in (1) or (2), wherein, in the final annealing, in the case of batch annealing, it is held at 500 to 600 ° C. for 10800 to 54000 seconds, and in the case of continuous annealing Is held at 700 to 850 ° C. for 30 to 150 seconds, and a method of producing an α + β-type titanium alloy sheet.

If Al is contained as an α-phase stabilizing element to cause solid solution strengthening as in a conventional titanium alloy, it is effective to increase the strength. However, due to the high strengthening ability, it is difficult for slip deformation and twin deformation to occur, so the ductility is reduced and the toughness is impaired. Furthermore, since the hot deformation resistance is high, cracking is likely to occur by forging or rolling. Therefore, in the present invention, Al is not added.
On the other hand, oxygen, which is an α-phase stabilizing element like Al, is effective for solid solution strengthening, and by optimizing the content, high strength can be realized without inhibiting ductility. On the basis of such findings, the present inventors have included oxygen (O) capable of achieving cost reduction and high strength to some extent, and also focused on Fe content and N content. The study was conducted in consideration of the relationship between the elements and the structure of As a result, the following findings were obtained.

(1) Similar to Al, oxygen has the effect of suppressing slip deformation and twin crystal deformation, but if the content is 0.28 to 0.4 mass%, the ductility is not largely lost and the hot deformation resistance is also large. It is hard to produce a crack by forging etc. Further, by setting the Fe content to 0.8 to 1.5% by mass, the crystal grains can be made finer and sized, and good ductility can be obtained.
(2) If nitrogen having a higher solid solution strengthening ability than oxygen is added at 0.05% by mass or less, the strength can be enhanced without losing the ductility. Moreover, since nitrogen does not increase the resistance to hot deformation like oxygen, it is difficult for cracking to occur in forging.
(3) A high strength and high ductility can be obtained by adjusting the existing proportions of recrystallized equiaxed α phase and β phase and processed structures other than this. The crystal grain size grows and increases when recrystallization is complete. Therefore, in order to obtain fine equiaxed crystal grains of 2 to 10 μm or less, existence of machined structures other than equiaxed α phase and β phase which are recrystallized Adjust the percentage to 10-85%. And, by adjusting the existing proportions of the recrystallized equiaxed α-phase, β-phase and other processed structures, a good balance of strength and ductility can be obtained. The reason for making the alpha phase recrystallized equiaxed is as follows. If it is an equiaxial structure, deformation of crystal grains is uniform and work hardening is easy, and good ductility and strength can be obtained. In the needle-like structure, the shape of the crystal grains affects to cause non-uniform deformation, so that it is difficult to work harden, so that good ductility and strength can not be obtained.

  According to the present invention, in the α + β-type titanium alloy, high strength and high ductility can be simultaneously provided only by the inexpensive additive elements such as O, Fe and N without adding expensive alloy component additive elements.

Conceptual diagram showing the measurement site of alloy structure

  The component compositions of the present invention will be described in more detail below. The following% contents are all represented by mass%.

Oxygen content: 0.28 to 0.4% by mass
Oxygen is contained 0.28 to 0.4% in the titanium material. Oxygen is an effective element to increase the strength of titanium materials in general. If the oxygen content is less than 0.28%, it is not possible to impart sufficient strength to a product manufactured using a titanium plate. More preferably, it is 0.3% or more, more preferably 0.32% or more. On the other hand, when the oxygen content exceeds 0.4%, the strength becomes too high, resulting in a titanium plate with low ductility, so the upper limit is defined in this way. Preferably it is 0.39% or less, more preferably 0.38% or less.

Fe content: 0.8 to 1.5% by mass
Fe is contained 0.8 to 1.5% in the titanium material. In a titanium material, it is known that Fe is a β-phase stabilizing element, and although a part thereof is in solid solution in the α phase, most is in solid solution in the β phase. That is, when the amount of Fe increases, the amount of β phase increases, and accordingly, the grain growth of the α phase can be suppressed, and a fine grain structure can be obtained. If the Fe content is less than 0.8%, sufficient strength can not be obtained. Preferably it is 0.9% or more. When the Fe content exceeds 1.5%, the stability of the β phase increases, and even if cooled to room temperature, it does not transform to the α phase and most remains as the β phase, and passes through heating processes such as hot rolling and annealing There is a risk that the residual β phase may become coarser with each other. In addition, the corrosion resistance may be reduced. Preferably it is 1.4% or less. Furthermore, Fe is likely to segregate during melting and solidification. From the viewpoint of homogeneity in the coil, more preferably 1.2% or less.

Nitrogen content: 0.05% or less by mass Nitrogen is inevitably contained by about 0.002%. Furthermore, by containing 0.05% or less of nitrogen in the titanium material, the strength can be further improved. Nitrogen, like oxygen, is an element effective to increase the strength of titanium materials in general, and its solid solution strengthening ability is higher than that of oxygen. However, if the nitrogen content exceeds 0.05%, the strength becomes too high and the titanium plate becomes low in ductility, so the upper limit is defined as 0.05%. Preferably it is 0.048% or less, More preferably, it is 0.046% or less. If the nitrogen content is less than 0.002%, it is not preferable because the cost of nitrogen removal is increased. The lower limit of the nitrogen content is preferably 0.004% or more.

  The significance of determining each numerical range regarding the alloy structure will be described in detail.

  The titanium plate formed according to the present invention has high strength and high ductility by forming the titanium plate so that the grain size of the recrystallized equiaxed α phase, which is a leader of plastic deformation, is 2 to 10 μm. Be When the average crystal grain size of the equiaxed α phase is less than 2 μm, many crystal grains in which the operation of dislocation is difficult exist, so that plastic deformation hardly occurs and the ductility is lowered. Preferably, the average crystal grain size of the equiaxed α phase is 3 μm or more, more preferably 4 μm or more. On the other hand, if the average crystal grain size of the equiaxed α phase exceeds 10 μm, the surface area of the grain boundary for pinning the operation of dislocations decreases, so non-uniform plastic deformation easily occurs, and the work hardening ability decreases. High strength can not be obtained. Preferably, the average crystal grain size of the equiaxed α phase is 9 μm or less, more preferably 8 μm or less.

  In the titanium plate formed according to the present invention, high strength and high ductility can be obtained by forming the titanium plate so that the recrystallized equiaxed α phase has an area ratio of 15% to 90%. If the area ratio of the recrystallized equiaxed α phase is less than 15%, the α phase which is the carrier of plastic deformation is insufficient and sufficient ductility can not be obtained. In addition, if the recrystallized equiaxed α phase is less than 15%, the average grain size of the recrystallized equiaxed α phase tends to be small, and as a result, the ductility is lowered as described above. Preferably it is 20% or more, more preferably 30% or more. On the other hand, when the area ratio of the recrystallized equiaxed α phase exceeds 90%, plastic deformation easily progresses and high strength can not be obtained. If it exceeds 90%, the average crystal grain size of the recrystallized equiaxed α-phase tends to be coarsened, and the processed structure relatively decreases, so high strength can not be obtained. Preferably it is 85% or less, more preferably 80% or less.

In the present invention, the recrystallized β phase is present at an area ratio of 0.5% or more and 10% or less in order to prevent coarsening of the grain size of the recrystallized equiaxed α phase. Thereby, high strength and high ductility can be obtained. If the β phase is less than 0.5%, the grain growth of the recrystallized equiaxed α phase by the β phase can not be suppressed, and the structure of the α phase becomes coarse. When the recrystallized equiaxed α-phase structure becomes coarse, the strength decreases as described above. On the other hand, if it exceeds 10%, the crystal grain size of the recrystallized equiaxed α phase becomes too small, and as described above, the high ductility decreases.
In addition, when the recrystallized β phase increases, the interface area between the recrystallized equiaxed α phase and the recrystallized β phase increases. At the time of plastic deformation, stress concentrates on the interface between the α phase and β phase different in deformability, and voids are easily generated. For this reason, ductility is reduced as the recrystallized β phase increases.

Proportion of processed tissue (A value): 10 to 85%
The titanium plate formed according to the present invention has a ratio of non-recrystallized processed structure (ratio of unrecrystallized portion) of 10 to 85%. Exists. The processed structure here is determined by a crystal orientation analysis method using a backscattered electron diffraction image EBSD (Electron Backscatter Diffraction Pattern) with a step size of 0.2 μm, and determined by the attached analysis software (TSL OIM Analysis), In the obtained information, a portion where the CI value (coincidence index) was less than 0.1 was regarded as a processed tissue. Here, the ratio of processed tissue is described as A value. When recrystallization is complete, the grains start to grow and become coarse. For this reason, in order to obtain a titanium material excellent in the balance between strength and ductility, it is preferable that the machined structure before recrystallization is completely completed remains in an appropriate manner. However, if there are many processed structures, there is a possibility that the ductility may be lowered, but as a result of intensive studies by the present inventors, it was found that high strength and high ductility can be obtained if the A value is 10 to 85%. .
If the value of A is less than 10%, the crystal grains partially start to be coarsened, which may make it difficult to impart sufficient strength to a product manufactured using a titanium plate. That is, if the A value is low, the strength is insufficient. Preferably, the A value is 15% or more, more preferably 20% or more. On the other hand, if the A value exceeds 85%, the number of recrystallized equiaxed α-phase grains responsible for plastic deformation decreases, and sufficient ductility can not be obtained. Preferably, the A value is 80% or less, more preferably 70% or less. The titanium plate of the present invention has high strength because of the presence of a processed structure. And since the upper limit is limited, it is possible to obtain a titanium alloy excellent in the balance of strength and ductility within the range specified in the present invention even if there is a machined structure.

  Next, the production method of the present invention will be described.

Final annealing conditions: In the case of batch annealing, hold at 500 to 600 ° C. for 10800 to 54000 seconds. In the case of continuous annealing, it hold | maintains for 30 to 150 seconds at 700-850 degreeC.
In the present invention, the characteristic production conditions are the final annealing conditions. The hot rolling and cold rolling conditions do not particularly affect the production of the titanium plate having the alloy structure of the present invention, and it is important to control the final annealing conditions.
Here, the final annealing conditions for obtaining recrystallized equiaxed α phase, β phase, and processed structure (unrecrystallized portion) will be described.
In the case of batch annealing, the annealing temperature is 500 to 600 ° C., and the holding time is 10800 to 54000 seconds. If the temperature is lower than 500 ° C., a large amount of unrecrystallized portion remains, which may lower the ductility. At temperatures higher than 600 ° C., it is difficult to obtain 2 to 10 μm recrystallized equiaxed α-phase crystal grains because the crystal grains become coarse. On the other hand, with respect to the holding time, if it is less than 10,800 seconds, a large amount of unrecrystallized portions remain, which may lower ductility. If it exceeds 54000 seconds, it is difficult to obtain 2 to 10 μm recrystallized equiaxed α-phase crystal grains because the crystal grains become coarse.
On the other hand, in the case of continuous annealing, the annealing temperature is 700 to 850 ° C., and the holding time is 30 to 150 seconds. If the temperature is lower than 700 ° C., a large amount of unrecrystallized portion remains, which may lower the ductility. When the final annealing temperature is a high temperature exceeding 850 ° C., the β phase becomes unstable because the concentration of Fe dissolved in the β phase at the time of annealing is low, and the β phase remains at the time of cooling after annealing at high temperature It transforms into the α phase without oxidization. Since the α phase generated by cooling after annealing at this high temperature precipitates like needles, an undesirable needle-like structure is formed if the final annealing temperature is a high temperature exceeding 850 ° C. On the other hand, with respect to the holding time, if it is less than 30 seconds, a large amount of unrecrystallized portions remain, which may lower the ductility. If it exceeds 150 seconds, it is difficult to obtain 2 to 10 μm recrystallized equiaxed α-phase crystal grains because the crystal grains are coarsened.
Using the titanium alloy having a composition containing 0.8% to 1.5% of Fe, 0.28% to 0.4% of oxygen (O) and the balance being titanium and unavoidable impurities, the above final By applying the annealing conditions, the circle equivalent average crystal grain size of the recrystallized equiaxed α phase is 2 to 10 μm, and the recrystallized equiaxed α phase is 15% to 90% by area ratio, recrystallized β It is possible to manufacture an α + β-type titanium alloy sheet having a phase ratio of 0.5% or more and 10% or less, and further having a processed structure of 10% or more and 85% or less.

  In the present invention, the steps up to hot working can be manufactured by a general method of manufacturing a titanium plate. For example, titanium ingot was manufactured from sponge titanium etc., this ingot was made into slab shape by forging, and this was processed into a hot-rolled sheet by hot rolling. Thereafter, the hot-rolled sheet is subjected to shot blasting and descaling by pickling to produce a titanium material to be subjected to cold rolling. These conditions are not particularly limited, and may be general conditions. Specifically, manufacturing conditions with a heating temperature range of hot rolling of 900 to 1000 ° C., a heating time of 1 to 4 hours, a rolling reduction of 70% or more, and a cold rolling reduction of 50% or more. Here, the rolling reduction immediately before final annealing in cold rolling is preferably 50% or more. If it is less than 50%, the strain is insufficient, the recrystallization driving force is small, and it becomes difficult to become equiaxed. On the other hand, if the rolling reduction is 50% or more, the driving force for recrystallization is sufficient, and it becomes easy to become equiaxed recrystallized grains.

  EXAMPLES The present invention will next be described in more detail by way of examples, which should not be construed as limiting the invention thereto.

(Test specimen preparation)
A titanium ingot having adjusted Fe, oxygen and nitrogen contents by arc melting was produced, and the ingot was heated to 1000 ° C. and then forged to produce a slab. Subsequently, the slab was hot-rolled to a thickness of 4 mm and then annealing was performed to remove surface scale by shot blasting and nitric acid fluoride washing. Cold rolling, annealing, and cold rolling were performed to produce a thin titanium plate having a thickness of 0.9 mm.
The titanium thin plate was subjected to batch annealing while held at 450 to 650 ° C. for 14400 to 72000 h in vacuum. Moreover, it hold | maintained at 650-900 degreeC in air | atmosphere for 15 to 120 seconds, and performed continuous annealing. After annealing in the air, it was immersed in a salt (alkali molten salt) to reform the scale, and it was pickled with nitric hydrofluoric acid to be descaled.

(Composition analysis)
The Fe content of the sample from which the surface scale after hot rolling was removed was measured according to JIS H 1614, the oxygen content was measured according to JIS H 1620, and the nitrogen content was measured according to JIS H 1612 .

(Tensile test)
Prepare tensile test pieces of 6.25 × 32 mm in parallel, 25 mm between gauge points, 15 mm in width for chuck, 100 mm in total length, and measure the yield strength at 0.5% / min between gauge points until 0.2% proof stress measurement Performed a tensile test at a tensile rate of 20% / min. Here, the tensile strength and the elongation in the rolling width direction (T direction) were evaluated. The measurement of elongation was carried out at room temperature, but the joints after fracture were made, the numerical value of elongation was determined, and it was described as the elongation at break. A tensile strength of 800 MPa or more was regarded as passing, and a tensile strength of 23% or more was regarded as passing.

(Area fraction of recrystallized equiaxed α phase and recrystallized β phase, average grain size and A value of recrystallized equiaxed α phase)
The average crystal grain size, α phase ratio, β phase ratio and A value of the recrystallized α phase are arbitrary (plate thickness) x 1 mm of the cross section (L cross section) seen from the direction parallel to the rolling width direction of the test piece The step size was measured by a crystal orientation analysis method using a backscattered electron diffraction image EBSD (Electron Backscatter Diffraction Pattern) with a step size of 0.2 μm, and determined using the attached analysis software (TSL OIM Analysis). The direction of the measured L cross section is schematically shown in FIG. In the obtained information, since the processing distortion at the time of cold rolling remains in the part where CI value (Coincidence index) is less than 0.1, a clear diffraction pattern can not be obtained. On the IPF map, the CI value was distinguished to be 0.1 or more and less than 0.1, and the part having a CI value of less than 0.1 was regarded as a processed tissue. The portion having a CI value of 0.1 or more was regarded as a recrystallized structure. Whether the α phase regarded as a recrystallized structure is equiaxed or not was judged not to be equiaxed when the aspect ratio is 5 or less and equiaxed or more than 5 in the attached analysis software. In the recrystallized equiaxed α-phase, a boundary with a misorientation of 15 ° or more was set as a grain boundary, a circle equivalent diameter was determined, and an average grain size was calculated. In addition, the α phase and the β phase distinguish between the α phase and the β phase in the “Phase” mode, and the area ratio of the recrystallized equiaxed α phase and the β phase was calculated.

  The results are shown in Table 1. In the examples and comparative examples, those having a composition of 0.002% nitrogen were obtained by measuring the nitrogen content contained unavoidably. No. Except for 10, all the recrystallized α structures were equiaxed.

  No. 1 to 11 are comparative examples. No. Since No. 1 and Fe content and oxygen content did not reach the specified lower limit value, tensile strength was low. No. In No. 2, the Fe content was less than the specified lower limit value, so the tensile strength was low. No. No. 3 had a low tensile strength because the oxygen content did not reach the specified lower limit.

  No. 4, the elongation was low because the oxygen content exceeded the specified upper limit. No. 5 was low in elongation because the nitrogen content exceeded the specified upper limit. No. In No. 6, the Fe content was above the specified upper limit, so the elongation was low.

  No. In No. 7, the tensile strength was low because the annealing time was long, and the grain size and area ratio of the recrystallized equiaxed α phase exceeded the specified upper limit.

  No. No. 8 had a low annealing temperature, and the ratio of unrecrystallized portion (processed structure) exceeded the specified upper limit, so the elongation was low. No. In No. 9, the elongation was low because the annealing time was short, the grain size of the recrystallized equiaxed α phase was below the lower limit, and the ratio of the non-recrystallized part exceeded the specified upper limit.

  No. In No. 10, since the annealing temperature was high, the needle-like structure was mixed, so the elongation was low.

  No. In No. 11, since the annealing temperature is low, the grain size and area ratio of the recrystallized equiaxed α phase are lower than defined, and the ratio of the non-recrystallized portion (processed structure) exceeds the defined upper limit. It was low.

  On the other hand, no. As for 12 to 23, as a result of optimizing the annealing temperature and time of the final annealing, the grain size and area ratio of recrystallized equiaxed α phase, area ratio of recrystallized β phase, non-recrystallized part (processed structure) The tensile strength was 800 MPa or more and the elongation was 23% or more.

  According to the present invention, it is possible to provide an α + β-type titanium alloy which combines high strength and high ductility only with inexpensive additive elements such as O, Fe and N without adding expensive alloy component additive elements. .

Claims (3)

Contained by mass, 0.8% to 1.5% of Fe, 0.28% to 0.4% of oxygen (O), and the balance is titanium and unavoidable impurities, and is equiaxed with recrystallized α The circle equivalent average crystal grain size of the phase is 2 to 10 μm, the recrystallized equiaxed α phase has an area ratio of 15% or more, and the recrystallized β phase ratio is 0.5% or more and 10% or less. Α + β-type titanium alloy sheet in which 10% or more exists .   The α + β-type titanium alloy sheet according to claim 1, further comprising 0.05% or less of nitrogen (N) by mass.   It is a manufacturing method of the alpha + beta-type titanium alloy plate indicated in Claim 1 or Claim 2, and in final annealing, in batch annealing, it holds 10800-54000 seconds at 500-600 ° C, and 700- in case of continuous annealing. A method of manufacturing an α + β-type titanium alloy sheet characterized by holding at 850 ° C. for 30 to 150 seconds.