JP5064356B2 - Titanium alloy plate having high strength and excellent formability, and method for producing titanium alloy plate - Google Patents

Description

  The present invention relates to a titanium alloy plate excellent in strength and formability and a method for producing the titanium alloy plate.

  High strength α + β type titanium alloys represented by Ti-6Al-4V have various processing characteristics such as weldability, superplasticity and diffusion bonding properties in addition to lightweight, high strength and high corrosion resistance. Has been heavily used in the center. In order to further utilize these characteristics, in recent years it has come to be used for sports equipment such as golf equipment, such as consumer products such as automobile parts, civil engineering materials, various tools, Application to deep seas and energy development applications is also expanding. However, the remarkably high production cost of α + β type titanium alloy has hindered its application expansion, and in order to promote further application expansion to the field of consumer products, etc., without impairing the above-mentioned characteristics and An inexpensive titanium alloy is to be developed, and its development is awaited.

  The following two points can be cited as the reasons why the production cost of these high-strength α + β-type titanium alloys increases. Expensive β-phase stabilizing elements such as V are used. Al used as an α-phase stabilizing element remarkably increases hot deformation resistance and impairs hot workability, so that it is difficult to work and easily causes defects such as cracks. These are the above two points.

  In particular, the addition of Al is a major factor that increases the manufacturing cost when manufacturing the main product alloy plate, and requires reheating during rolling, or cracks at the end of the alloy plate. This has been a cause of problems such as a decrease in material yield.

  Under such circumstances, various low-cost titanium alloys have been proposed in recent years. Among them, Ti-Fe-O-N-based high-strength titanium alloys adopt inexpensive Fe as the β-phase stabilizing element, and replace Al as the α-phase stabilizing element, which reduces hot workability. Because of the use of cheap oxygen (O) and nitrogen (N) without impairing hot workability, considerable cost reduction is expected compared to conventional α + β type titanium alloys. .

  However, when this Ti—Fe—O—N-based high-strength titanium alloy is produced by normal unidirectional rolling, extreme in-plane material anisotropy occurs, and the rolling direction of the plate, that is, the lengthwise direction, Although it has excellent characteristics, it also has the problem that the ductility in the width direction becomes extremely poor.

  As an improvement plan to solve this problem, rolling in the direction perpendicular to the rolling direction only once, and reducing the in-plane anisotropy, high strength and high ductility in both the length and width directions Patent Document 1 discloses that a —Fe—O—N high-strength titanium alloy can be obtained. However, applying such cross rolling to an actual machine causes an increase in cost, and is not a substantial improvement. Accordingly, there is a long-awaited development of a titanium alloy plate that is low in cost, low in-plane anisotropy, high strength and excellent formability even when applied to an actual machine.

Japanese Patent Laid-Open No. 11-61297

  The present invention has been made as a solution to the above-mentioned conventional problems, and it is an object of the present invention to provide a titanium alloy plate having high strength, excellent formability, and an inexpensive method for producing the titanium alloy plate. It is.

  The invention according to claim 1 is characterized in that the metal structure has a maximum crystal grain size of β phase: 15 μm or less, an area ratio of α phase: 80 to 97%, an average crystal grain size of α phase: 20 μm or less, and The titanium alloy plate having high strength and excellent formability, wherein the standard deviation of the crystal grain size of the α phase ÷ the average crystal grain size of the α phase × 100 is 30% or less.

  The invention described in claim 2 is the titanium alloy plate having high strength and excellent formability according to claim 1, characterized in that it contains 0.8-2.5 mass% of a β-stabilizing element.

  The invention according to claim 3 is the titanium alloy plate having high strength and excellent formability according to claim 2, wherein the β-stabilizing element is Fe.

  The invention according to claim 4 is a titanium alloy plate having a metallographic structure according to claim 1, in which a titanium alloy ingot is used to perform ingot rolling, hot rolling, intermediate annealing, cold rolling, and final annealing in order. In the manufacturing process, the final annealing is performed twice at a temperature equal to or lower than the β transformation point, and the final annealing is performed with the second temperature higher than the first temperature. This is a method for producing a titanium alloy plate excellent in formability.

  According to the present invention, it is possible to obtain a titanium alloy plate that is high in strength and excellent in formability and that is inexpensive. In addition to the excellent durability inherent in titanium alloys, in addition to high mechanical strength, it has excellent formability, so it is a component of plate heat exchangers, fuel cell separators, mobile phones, mobiles It can be widely applied to uses that require high formability, such as personal computers, camera bodies, and spectacle frames.

  In order to obtain a titanium alloy plate having high strength and excellent formability at a low cost, the present inventors have intensively experimented and researched. As a result of dispersing the β phase, the conventional JIS has been obtained. While it was possible to realize a titanium alloy plate having a strength higher than that of a class titanium alloy plate, it was confirmed that when a coarse β phase was formed, it became a starting point of fracture during forming and the formability deteriorated.

  As a result, after setting the maximum crystal grain size of the β phase to an appropriate size, the area ratio of the α phase and the average crystal grain size are regulated within an appropriate range, and further, the α phase crystal grains with respect to the average crystal grain size of the α phase By controlling the ratio of the standard deviation of the diameter to an appropriate ratio, it has been found that high strength and excellent formability can be secured, and the present invention has been completed.

  Hereinafter, the present invention will be described in detail based on embodiments.

(Maximum grain size of β phase)
When a coarse β phase is formed, it becomes a starting point for destruction during molding, and the moldability deteriorates. In order not to deteriorate the moldability, the maximum crystal grain size of the β phase needs to be 15 μm or less. Preferably it is 10 micrometers or less, More preferably, it is 7 micrometers or less. The lower limit of the maximum crystal grain size of the β phase is not particularly specified, but a preferable lower limit is 0.1 μm.

(Area area ratio of α phase)
The crystal structure of the α phase in the titanium alloy is a hexagonal close-packed structure (HCP), and the crystal structure of the β phase is a body-centered cubic structure (BCC). Therefore, as the α phase decreases, the anisotropy of elongation decreases and the moldability improves. However, since the average crystal grain size of the α phase increases as the α phase decreases, the strength decreases. When the area ratio of the α phase exceeds 97%, the anisotropy of elongation becomes too large and the formability deteriorates. On the other hand, if the area ratio of the α phase is less than 80%, the maximum crystal grain size of the β phase becomes too large and the moldability deteriorates. Therefore, the upper limit of the area ratio of the α phase is 97%, and the lower limit is 80%. A preferable upper limit is 96%, and a lower limit is 90%.

(Average crystal grain size of α phase)
The smaller the average crystal grain size of the α phase, the greater the strength due to the effect of crystal grain refinement. Therefore, the upper limit of the average crystal grain size of the α phase is set to 20 μm, which is a limit value that can ensure excellent strength. A preferred upper limit is 15 μm. However, a titanium alloy having an α-phase average crystal grain size of 1 μm or less is difficult to produce in the current mass production process.

(Standard deviation of α phase crystal grain size ÷ average crystal grain size of α phase × 100)
When the standard deviation of the α-phase crystal grain size is larger than 30% of the average crystal grain size of the α-phase, the number of large crystal grains becomes too large compared to the average value of the α-phase crystal grain size. , The moldability will deteriorate. Therefore, the ratio (percentage) of the standard deviation of the crystal grain size of the α phase to the average crystal grain size of the α phase is set to 30% or less. A preferable upper limit is 20%, and a more preferable upper limit is 15%. The lower limit is not particularly specified because it is preferably as small as possible, but in reality, the lower limit is considered to be about 5%.

(Component composition)
The content of Fe as a β-stabilizing element is preferably 0.8 to 2.5% by mass. When the Fe content is less than 0.8% by mass, the necessary minimum strength cannot be obtained. On the other hand, if the Fe content exceeds 2.5% by mass, a coarse β phase is formed and the formability deteriorates. The lower limit is more preferably 1.0% by mass, and still more preferably 1.2% by mass. The upper limit is more preferably 2.3% by mass, and still more preferably 2.1% by mass. In addition, although it is possible to employ | adopt (beta) stabilizing elements other than Fe, also in that case, it is preferable that those content is 0.8-2.5 mass%.

  In addition, it is preferable to add O as an α stabilizing element that hardens the α phase in the titanium alloy because it is inexpensive. O, which is an α-stabilizing element, contributes to an increase in the strength of the material. However, if the content of O is too large, the elongation becomes small and the moldability deteriorates. Therefore, the content of O is preferably 0.1% by mass or less (not including 0% by mass). More preferably, it is 0.08 mass% or less, More preferably, it is 0.06 mass% or less.

  In the present invention, the component composition of the titanium alloy is not particularly specified. However, if the added β-stabilizing element is insufficient, the necessary minimum strength may not be obtained. On the other hand, if the β-stabilizing element to be added is excessive, a coarse β-phase is formed and the moldability may be deteriorated. As a β-stabilizing element, one or more of Mo, V, Fe, Cr, Ta, Nb, Mn, Cu, Ni, Ca, Si, and H can be added. It is preferable to add it because it is inexpensive.

(Production conditions)
Next, the manufacturing method of the titanium alloy plate of this invention is demonstrated. Ordinary titanium alloy sheets are manufactured by performing blasting and pickling treatment at any time between each process such as lump rolling → hot rolling → intermediate annealing → cold rolling → final annealing. Depending on the component composition and the setting conditions of each process, the physical properties and structure obtained will change, so the conditions should be selected and determined comprehensively as a series of manufacturing processes, and the conditions are strictly set for each process It is not always appropriate to do.

  However, as a manufacturing condition for manufacturing the titanium alloy plate of the present invention, the present inventors diligently studied, and by adopting the following conditions, the high strength and excellent formability intended by the present invention were achieved. It was confirmed that the titanium alloy plate can be reliably manufactured.

  The conditions are as follows: 1) Final annealing after cold rolling is performed twice. 2) The temperature of the second annealing is set to a temperature equal to or lower than the β transformation point in both times, and the second annealing temperature is set to a temperature higher than the first annealing temperature. 3) After the first annealing, it may be cooled to room temperature, or may be annealed as it is without cooling to room temperature. 4) Hold for 5 minutes or less at each annealing temperature in both annealings. By appropriately combining the above conditions and performing the final annealing, a titanium alloy having high strength and excellent formability intended in the present invention can be reliably produced.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, and the present invention is implemented with appropriate modifications within a range that can meet the gist of the present invention. These are all included in the technical scope of the present invention.

  In this example, first, ingots made of titanium alloys having respective component compositions shown in Table 1 were cast by CCIM (cold crucible induction melting method). The size of the ingot is a cylindrical shape of φ100 mm and is 10 kg. This ingot was used for ingot rolling, and a titanium alloy plate having a thickness of 0.3 mm was manufactured through steps of hot rolling → intermediate annealing → cold rolling → final annealing.

  The final annealing is no. It was carried out twice except for 10, and after the first annealing, the second annealing was performed as it was without cooling to room temperature. In addition, both the first annealing and the second annealing were held for 2 minutes. Only 10 was held for 4 minutes. The respective annealing temperatures are as shown in Table 1. Except for the first annealing of No. 11, the annealing temperature is a temperature below the β transformation point. Note that the β transformation point of the titanium alloy used in this example is about 830 to 870 ° C.

  Observation and measurement of the metal structure of each manufactured titanium alloy plate, and evaluation of strength and formability were performed as follows.

<Maximum crystal grain size of β phase, area ratio of α phase, average crystal grain size of α phase, standard deviation of crystal grain size of α phase ÷ average crystal grain size of α phase × 100>
In this example, the measurement of each of the above parameters was performed on a field emission scanning microscope (FESEM) (JSM5410, manufactured by JEOL Ltd.) using a back-scattered electron diffraction image (Electron Back Scattered (Scattered) Pattern). : EBSP) system mounted crystal orientation analysis method. This measurement method was used because the EBSP method has higher resolution than other measurement methods and can perform measurement with high accuracy. First, the measurement principle will be described.

  In the EBSP method, an electron beam is irradiated onto a sample set in a lens barrel of FESEM to project EBSP on a screen. This is taken with a high-sensitivity camera, captured as an image into a computer, and the image is analyzed. Since this process is automatically performed for all measurement points, data of tens of thousands to hundreds of thousands of points can be obtained at the end of measurement.

  As described above, the EBSP method has a wider observation field than the X-ray diffraction method or the electron diffraction method using a transmission electron microscope, and the maximum crystal of the β phase with respect to a large number of crystal grains of several hundred or more. There is an advantage that information on the particle size, the area ratio of the α phase, the average crystal particle size of the α phase, the standard deviation of the crystal size of the α phase, and the like can be obtained within a few hours. In addition, since the specified region is scanned at a fixed interval instead of the measurement for each crystal grain, there is an advantage that each of the above-mentioned information regarding the above-described many measurement points covering the entire measurement region can be obtained. Details of the crystal orientation analysis method in which the EBSP system is mounted on these FESEMs are described in Kobe Steel Technical Report / Vol. 52 no. 2 (Sep. 2002) P66-70 and the like.

  The maximum crystal grain size of the β phase of the titanium alloy plate, the area ratio of the α phase, the average crystal grain size of the α phase, and the standard deviation of the crystal grain size of the α phase were obtained from this measurement. For these measurements, as described above, using the crystal orientation analysis method in which the EBSP system is mounted on the FESEM, the surface is parallel to the surface of the titanium alloy plate and is a set of 1/4 t portions in the plate thickness direction. This was done by measuring the tissue. Specifically, the surface of the rolled surface of the titanium alloy plate was mechanically polished, followed by buffing and then electrolytic polishing to prepare a sample whose surface was adjusted. Then, the measurement by EBSP was performed using FESEM (JEOL JSM 5410) by JEOL. The measurement area was an area of 300 μm × 300 μm, and the measurement step interval was 0.5 μm. As the EBSP measurement / analysis system, EBSP: OIM (Orientation Imaging Microscopy) manufactured by TSL was used.

By such a measuring means, the maximum crystal grain size of β phase within the measurement range, the area ratio of α phase, the average crystal grain size of α phase, and the standard deviation of the crystal grain size of α phase were determined. The equivalent crystal diameter was adopted as the maximum crystal grain size of the β phase. Further, the average crystal grain size of the α phase and the standard deviation of the average crystal grain size were obtained from the following formulas. That is, when the number of measured crystal grains is n and each measured crystal grain size is x, the average crystal grain size is expressed by the formula (Σx) / n, and the standard deviation of the average crystal grain size is [ {NΣx ² − (Σx) ² } / n / (n−1)] ^1/2 .

<Measurement of tensile strength>
A No. 13 test piece defined in JISZ2201 was produced from each of the obtained titanium alloy plates, and the tensile strength (TS) in the rolling direction was measured for this test piece. At this time, the test speed (strain speed in the tensile test) was 0.25 mm / min up to 0.2% proof stress, and 10 mm / min thereafter.

  When the tensile strength (TS) in the rolling direction obtained in this test was 500 MPa or more, it was evaluated as high strength.

<Measurement of formability (Ericsen value)>
In the test of this example, the Eriksen test was adopted for the evaluation of moldability. The No. 2 test piece prescribed | regulated to JISZ2247 was produced from each obtained titanium alloy board, the Eriksen test based on the prescription | regulation of JISZ2247 was implemented about this test piece, and the Eriksen value was measured. At this time, the test speed (press speed in the Eriksen test, that is, the displacement speed of the press tool) was set to 5 mm / min.

  Those having an Erichsen value of 8.0 or more obtained in this test were evaluated as having excellent moldability.

  The test results are shown in Table 1.

  No. No. 2 has an upper limit of 2.50% by mass as defined in claim 3, and No. 2 No. 3 is the lower limit of 0.80% by mass as defined in claim 3, and No. 3 No. 1 has an intermediate Fe content of 1.80% by mass, and the final annealing conditions are satisfied.

  In contrast, no. Nos. 4 to 7 have annealing temperatures approximately upper and lower limits. No. 4 has the first annealing temperature as a substantially upper limit. No. 5 has the first annealing temperature as a substantially lower limit. No. 6 has the second annealing temperature as a substantially upper limit, 7 has the second annealing temperature as a substantially lower limit.

  These No. 1 to 7 are invention examples of the present invention, and the maximum crystal grain size of β phase, the area ratio of α phase, the average crystal grain size of α phase, the standard deviation of the crystal grain size of α phase ÷ the average of α phase The crystal grain size x 100 satisfies all the requirements stipulated in the present invention. The tensile strength (TS) in the rolling direction obtained in the test is all 500 MPa or more, and the Erichsen values are all 8.0 or more. . That is, it can be seen that a titanium alloy plate that satisfies the requirements defined in the present invention has high strength and excellent formability.

  On the other hand, no. Nos. 8 to 11 are comparative examples. No. 8 has a Fe content of 0.70% by mass less than the lower limit defined in claim 3, No. 8; No. 9 is 2.60% by mass with the Fe content exceeding the upper limit defined in claim 3; No. 10 was the final annealing only once. No. 11 is the one where the first annealing temperature exceeds the β transformation point.

No. Among the requirements defined in the present invention, Nos. 9 to 11 indicate that the maximum crystal grain size of the β phase is too large. In No. 8 , the area ratio of the α phase is too large. As a result, the tensile strength (TS) in the rolling direction obtained in the test did not reach 500 MPa, or the Erichsen value did not reach 8.0. That is, it can be seen that a titanium alloy plate that deviates from the requirements defined in the present invention cannot be said to have high strength and excellent formability.

Claims (4)

The metal structure is
Maximum crystal grain size of β phase: 15 μm or less,
α phase area ratio: 80-97%,
Average crystal grain size of α phase: 20 μm or less,
Further, a titanium alloy plate having high strength and excellent formability, wherein standard deviation of α phase crystal grain size ÷ average crystal grain size of α phase × 100 is 30% or less.   The titanium alloy plate having high strength and excellent formability according to claim 1, wherein the β-stabilizing element is contained in an amount of 0.8 to 2.5 mass%.   3. The titanium alloy plate having high strength and excellent formability according to claim 2, wherein the β-stabilizing element is Fe. In producing a titanium alloy plate having a metallographic structure according to claim 1 by sequentially carrying out rolling, hot rolling, intermediate annealing, cold rolling, and final annealing using a titanium alloy ingot.
The final annealing is performed twice at a temperature equal to or lower than the β transformation point, and the final annealing is performed by setting the second temperature to be higher than the first temperature. A method for producing a titanium alloy plate.