JP5088876B2 - Titanium alloy plate with high strength and excellent formability and manufacturing method thereof - Google Patents

Description

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

  High-strength α + β-type titanium alloys represented by Ti-6Al-4V have various processing characteristics such as weldability, superplasticity, and diffusion bondability in addition to light weight, high strength, and high corrosion resistance. Widely used mainly in industry. In recent years, in order to make further use of these characteristics, so-called consumer products such as automobile parts, materials for civil engineering and construction, various tools, deep sea and energy development applications, as well as sports equipment such as golf equipment. Application expansion to the field is also progressing. However, there is a problem that the remarkably high production cost of the high strength α + β type titanium alloy hinders application expansion.

In this way, the reason for the high manufacturing cost of the high strength α + β type titanium alloy is as follows:
i) using an expensive β-phase stabilizing element such as V;
ii) Al used as an α-phase stabilizing element and a solid solution strengthening element remarkably enhances hot deformation resistance and impairs hot workability, so that it is difficult to work and is liable to cause defects such as cracks. Two points can be mentioned.

  In particular, the above ii) is a large high cost factor when manufacturing the main product, such as requiring reheating during rolling, and cracking at the edge of the plate, resulting in a decrease in material yield. was there.

  Under such circumstances, various low-cost titanium alloys have been proposed in recent years in order to promote application expansion to the consumer products field. Among these, Ti-Fe-O-N high strength titanium alloys employ inexpensive Fe as a β-phase stabilizing element, and are inexpensive and hot workable instead of Al, which reduces hot workability. Oxygen and nitrogen that do not impair the temperature are used as the α-phase stabilizing element, and therefore, a considerable cost reduction is expected compared to conventional α + β-type titanium alloys.

However, problems still remain in the practical application of this Ti—Fe—O—N high strength titanium alloy.
Ti-Fe-O-N-based titanium alloys have extreme in-plane material anisotropy when the plate is produced by ordinary unidirectional rolling, and the plate length direction characteristics are excellent, but the axial direction There is a problem that the characteristics, particularly ductility, becomes extremely poor. In addition, as an improvement measure, for example, in Patent Document 1, by performing cross rolling in which rolling is performed in a direction perpendicular to the rolling direction, the above anisotropy is reduced, and both the length direction and the width direction have high strength. -It has been shown that a high ductility Ti-Fe-ON-based high strength titanium alloy sheet is obtained. However, since an increase in cost is inevitable when performing the above cross rolling with an actual machine, establishment of a method for obtaining a high-strength titanium alloy sheet excellent in formability regardless of the rolling direction at low cost is eagerly desired.
Japanese Patent Laid-Open No. 11-61297

  The present invention has been made in view of such circumstances, and an object thereof is to produce a titanium alloy plate having high strength and excellent formability at low cost.

The titanium alloy plate having high strength and excellent formability according to the present invention is Fe: 0.8 to 2.5% (meaning mass%, the same shall apply hereinafter), O: 0.10% or less (including 0%) Not) and the balance: a titanium alloy plate made of Ti and inevitable impurities,
The metal structure is
The average value of the azimuth angle formed by the normal of the (0001) plane of the α phase and the normal of the rolled surface, which is a hexagonal close packed structure: 45 ° or less
It is characterized in that it satisfies the area ratio of the α phase: 80 to 97% and the average crystal grain size of the α phase: 7.0 μm or less.

  The present invention also defines a method for producing such a titanium alloy plate, and the method uses ingots satisfying the above component composition, and is used for ingot rolling, hot rolling, intermediate annealing, cold rolling. Further, when performing the final annealing sequentially, the hot rolling start temperature is set to 800 ° C. or higher, and the final annealing temperature is set to 700 ° C. to 850 ° C.

  According to the present invention, a titanium alloy plate having high strength and excellent formability can be produced at a low cost, so that the above-mentioned sports equipment, automobile parts, civil engineering and building materials, various tools, etc., the deep sea and energy The expansion of application of titanium alloy materials to development applications can be promoted.

  The present inventors have intensively studied to obtain a titanium alloy plate having high strength and excellent formability at low cost. As a result, in particular, in the metal structure, the average value of the azimuth angle formed by the normal line of the (0001) plane of the α phase and the normal line of the rolled surface, which is a hexagonal close-packed structure, and the average crystal grain size of the α phase It has been found that a high-strength titanium alloy sheet exhibiting excellent formability can be obtained regardless of the rolling direction by controlling together with the area ratio of the α phase.

  In addition, in order to obtain such a high strength and excellent formability of the titanium alloy sheet, the specific conditions are based on the idea that the conditions of hot rolling and final annealing should be controlled in the manufacturing process. I found. Hereinafter, the present invention will be described in detail.

<Average value of azimuth angle between normal of (0001) plane of α phase and normal of rolled surface: 45 ° or less>
In the titanium metal structure, the slip direction of the α phase, which is a hexagonal close-packed structure (HCP), is in the hexagonal plane [(0001) plane] of the crystal lattice of the hexagonal close-packed structure, When a load is applied in a direction perpendicular to this plane [the normal direction of the (0001) plane], slip deformation of the crystal does not occur and a larger deformation tends to occur, that is, excellent formability tends to be exhibited.

  Therefore, the present inventors investigated the relationship between the azimuth angle formed by the normal of the (0001) plane of the α phase and the normal of the rolled surface and the formability in order to reliably improve the formability. As a result, as shown in the examples described later, the azimuth angle formed by the normal of the (0001) plane of the α-phase crystal lattice and the normal of the rolled surface (measured in FIG. 1) was measured for all α-phase crystal lattices in the observation region. It has been clarified through experiments that if the average value of θ) is 45 ° or less, excellent moldability is exhibited and elongation anisotropy is also reduced. The average value of the azimuth is preferably 40 ° or less, and more preferably 30 ° or less.

  Thus, the smaller the average value of the azimuth angle, the better the moldability. However, from the viewpoint of manufacturing at a low cost in the current mass production process, the lower limit of the average value of the azimuth angle is about 20 °.

<Area ratio of α phase: 80 to 97%>
As described above, even when the average value of the azimuth angles formed by the normal line of the (0001) plane of the α phase and the normal line of the rolled surface satisfies 45 ° or less (that is, “the thickness perpendicular direction of the entire α phase” If the ratio of the α phase with a small tilt angle relative to the metal structure is small), if the ratio of the α phase itself in the metal structure is small, the absolute amount of the α phase with a small azimuth that contributes to the improvement of the formability is reduced. A level of formability cannot be obtained. Therefore, in the present invention, the area ratio of the α phase in the entire structure is set to 80% or more. Preferably it is 85% or more, more preferably 90% or more.

  However, when the area ratio of the α phase is close to 100%, the anisotropy of elongation due to the α phase increases, and the formability deteriorates. Therefore, in the present invention, the area ratio of the α phase in the entire structure is set to 97% or less. Preferably it is 95% or less, More preferably, it is 93% or less.

<Average crystal grain size of α phase: 7.0 μm or less>
The smaller the average crystal grain size of the α phase, the higher the strength due to the grain refinement effect. In the present invention, in order to exert such effects, the upper limit of the average crystal grain size of the α phase is set to 7.0 μm. Preferably it is 5 micrometers or less, More preferably, it is 3 micrometers or less. Thus, the smaller the average crystal grain size of the α phase, the better the characteristics. However, from the viewpoint of manufacturing at a low cost in the current mass production process, the lower limit of the average crystal grain size of the α phase is about 1 μm.

<About component composition>
The titanium alloy plate of the present invention satisfies Fe: 0.8 to 2.5% and O: 0.10% or less (excluding 0%).

  If the amount of Fe is less than 0.8%, the crystal grain size of the α phase becomes too large, so that the necessary minimum strength cannot be ensured and the moldability is also deteriorated. Preferably, the amount of Fe is 1.0% or more. On the other hand, if the amount of Fe exceeds 2.5%, the strength becomes unnecessarily high and the moldability deteriorates, which is not preferable. Preferably, the amount of Fe is 2.3% or less.

  O (oxygen) is an element mixed mainly as an impurity. The strength increases as the amount of O increases. However, if the amount of O exceeds 0.10%, the strength becomes unnecessarily high and the moldability deteriorates, which is not preferable. Therefore, in the present invention, the amount of O is made 0.10% or less. Preferably it is 0.08% or less.

  The remainder of the titanium alloy plate of the present invention is composed of Ti and inevitable impurities. As the inevitable impurities, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing facilities, etc. can be allowed. For example, the total amount of Ni and Cr is about 0.05% or less, the total amount of H (hydrogen), C (carbon), and N (nitrogen) is about several tens of ppm. It is about 05%.

<About manufacturing method>
Next, manufacturing conditions for the titanium alloy plate will be described. A titanium alloy plate is generally manufactured by the following steps. The physical properties and structural state of the titanium alloy plate vary depending on the chemical composition of the titanium alloy plate used and the setting conditions of each process, so it should be determined by selecting the conditions comprehensively as a series of manufacturing processes. It is not always appropriate to set the conditions strictly.
[Casting] → [Bundled rolling] → [Soaking / hot rolling] → [Intermediate annealing] → [Cold rolling] → [Final annealing] (Blasting or pickling treatment is performed at any time between the above processes. Called)

  However, the present inventors have examined the production conditions using the titanium alloy having the above-described component composition. In particular, if the following conditions (1) and (2) are employed, the present invention has high strength and excellent formability. It has been found that a titanium alloy plate can be obtained reliably and at low cost.

(1) Hot rolling start temperature: 800 ° C. or more In order to set the average azimuth angle between the normal line of the (0001) plane of the α phase and the normal line of the rolled surface to 45 ° or less, It is necessary to increase the starting temperature, that is, the hot rolling starting temperature is 800 ° C. or higher. Preferably it is 850 degreeC or more, More preferably, it is 870 degreeC or more.

  On the other hand, since the crystal grains become too large even if the hot rolling start temperature is too high, it is preferable that the temperature be equal to or lower than the β transformation point temperature.

  In addition, after hot rolling, by rapidly cooling to 300 ° C., a fine α phase that can be a nucleus of the recrystallized α phase is generated in the subsequent soaking step. By generating this fine α phase, the recrystallized α phase can be made fine. In order to perform such cooling, for example, the hot-rolled material obtained by hot rolling is cooled by applying a large amount of water as shown in the examples to be described later, and actively blowing or mist. The method of performing cooling is mentioned.

(2) Final annealing temperature: 700 ° C to 850 ° C
When the annealing temperature of the final annealing exceeds 850 ° C., the average crystal grain size of the α phase becomes too large and the strength is lowered. Therefore, in the present invention, final annealing is performed at an annealing temperature of 850 ° C. or lower. The annealing temperature is preferably 825 ° C. or lower, and more preferably 800 ° C. or lower. However, even if the annealing temperature of the final annealing is too low, unrecrystallized parts remain after annealing and formability deteriorates, which is not preferable. Therefore, the annealing temperature of the final annealing is set to 700 ° C. or higher (preferably 710 ° C. or higher, more preferably 720 ° C. or higher).

  In addition, the time of the last annealing is not specifically limited, What is necessary is just to perform for 0.1 to 10 minutes in the said temperature range.

  General conditions can be adopted as the other conditions in the hot rolling and final annealing and the conditions of other processes.

  The titanium alloy plate (thickness of about 0.2 to 1 mm) according to the present invention has excellent formability in addition to high mechanical strength as well as original excellent corrosion resistance. In addition to the above-mentioned components, it can be widely applied to applications that require high formability, such as fuel cell separators, mobile phones, mobile personal computers, camera bodies, and eyeglass frames.

  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, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  A titanium alloy was melted by CCIM (cold crucible induction heating method) melting to produce a cylindrical 10 kg ingot with a diameter of 100 mm. Table 1 below shows the results of component analysis of the ingot. Using this ingot, the forging was performed at 1050 ° C., and after the forging, the plate was allowed to cool to obtain a plate-shaped forging material having a thickness of 45 mm. Then, it hot-rolled on the conditions shown in Table 1, scale removal was performed, and the hot rolled sheet about 5 mm thick was obtained.

  Cooling to 300 ° C. after completion of the hot rolling was performed by the following methods. That is, No. 1 shown in Table 1. Nos. 1-6, 8-10, 12-15, 17, 18 are cooled by positively applying wind. In Nos. 7 and 11, it was cooled with a large amount of water. No. 16 was air-cooled (cooled) while being left standing.

  Next, as shown in Table 1, it was heated at 700 ° C. for 5 minutes in an atmospheric furnace, and then subjected to an annealing treatment (intermediate annealing) that was air-cooled, and then the scale was removed. Then, as shown in Table 1, after performing cold rolling at a cold rolling rate of 89%, an annealing process (final annealing) is performed in an atmospheric furnace, followed by heating at the annealing temperature shown in Table 1 for 3 minutes and then air cooling. After that, the scale was removed to obtain a titanium alloy plate having a thickness of 0.3 mm.

  Observation and measurement of the metal structure of the obtained titanium alloy plate, and evaluation of strength and formability were performed in the following manner, respectively.

<Area area ratio of α phase, average value of azimuth angle formed by normal of (0001) plane of α phase and normal of rolled surface, and average crystal grain size of α phase>
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 that was adjusted so that the 1/4 t portion of the rolling surface in the thickness t direction could be observed. Then, using this sample, a backscattered electron diffraction image [EBSP: Electron Back] mounted on a Schottky field emission scanning electron microscope (FESEM) (manufactured by JEOL Ltd., JSM-6500F). The crystal orientation and crystal grain size were measured by Scattering (Scattered) Pattern] (JEOL JSM 5410, manufactured by JEOL Ltd.). The measurement area was 100 μm × 100 μm, and the measurement step interval was 0.2 μm. As the EBSP measurement / analysis system, OED (Orientation Imaging Microscopy) (ver. 4) manufactured by EEDAX / TSL was used.

  Since the EBSP observation / measurement system mounted on the FESEM has a high resolution, the measurement of the metal structure parameters can be performed with extremely high accuracy. Hereinafter, 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 and captured as an image on a computer. In the computer, the orientation of the crystal is determined by analyzing this image and comparing it with a pattern obtained by simulation using a known crystal system. The calculated crystal orientation is recorded as a three-dimensional Euler angle together with position coordinates (x, y) and the like. Since this process is automatically performed for all measurement points, tens of thousands to hundreds of thousands of crystal orientation data can be obtained at the end of measurement.

  Thus, the EBSP method has a wider observation field of view than the electron beam diffraction method using an X-ray diffraction method or a transmission electron microscope, and the average crystal grain size and the average crystal for a large number of crystal grains of several hundred or more. There is an advantage that information such as a standard deviation of particle size or orientation analysis can be obtained within a few hours. In addition, since the measurement is performed by scanning a specified region at an arbitrary fixed interval instead of measurement for each crystal grain, there is also an advantage that each of the above-described information on the numerous measurement points covering the entire measurement region can be obtained. is there. 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) p. 66-70 and the like.

  Here, in the case of a normal titanium alloy, the β phase (BCC) has {111} orientation (specified by {111} <112>, {111} <110>), {001} <100> orientation, {011} A texture composed of <100> orientation, {112} <110> orientation, {554} <225> orientation, and the like is formed.

  In the present invention, basically, those whose orientation deviation is within ± 15 ° from each crystal orientation belong to the same crystal orientation. Further, the boundary between crystal grains in which the orientation difference between adjacent crystal grains is 5 ° or more was defined as a crystal grain boundary.

By such measurement means, the orientation of all crystal grains of the α phase and β phase within the measurement range is individually identified, and the orientation angle formed by the normal of the (0001) plane of the α phase and the normal of the rolled surface The average value and the average crystal grain size of the α phase (the total circle equivalent diameter of each crystal grain of the α phase measured / the number of crystal grains of the α phase measured) were determined. Further, the area ratio of the α phase was determined by the following formula (1).
Area ratio of α phase = [area of α phase / (area of α phase + area of β phase)] × 100 (1)

<Evaluation of tensile strength and elongation anisotropy>
From the above test material (0.3 mm titanium alloy plate), No. 13 test piece defined in JISZ2201 is prepared so that the longitudinal direction is the rolling direction (L), and this test piece is used to define in JISZ2241. A tensile test was performed by the method, and the tensile strength (TS) and total elongation in the rolling direction (L) were measured. At this time, the test speed (strain speed in the tensile test) was 0.5% / min up to 0.2% proof stress, and 10 mm / min thereafter. Further, from the above test material, No. 13 test piece defined in JISZ2201 is prepared so that the longitudinal direction is the direction (T) on the rolling surface orthogonal to the L direction, and using this test piece, A tensile test was performed in the same manner as described above, and the total elongation in the T direction was measured.

  And it evaluated that TS of 500 MPa or more of rolling direction (L) was high intensity | strength. Further, an elongation anisotropy of 7% or less determined from (T direction total elongation)-(L direction total elongation) was evaluated as having low elongation anisotropy.

<Measurement of formability (Ericsen value)>
The Eriksen test was conducted as follows. That is, the No. 2 test piece prescribed | regulated to JISZ2247 was produced from the test material, and the Eriksen test was implemented by the method prescribed | regulated to JISZ2247 using this test piece.

  At this time, the test speed (press speed in the Eriksen test, that is, the displacement speed of the press tool) was 5 mm / min. And the Erichsen value evaluated that it was excellent in a moldability 7.5 mm or more.

  These results are also shown in Table 1.

  From Table 1, it can be considered as follows. That is, no. 1-11 satisfy | fills the requirements prescribed | regulated by this invention, and it turns out that it is high intensity | strength and excellent in a moldability.

  In contrast, no. Since Nos. 12 to 18 did not satisfy the requirements defined in the present invention, problems such as failure to secure high strength and poor formability occurred.

  Specifically, no. No. 12 has a small amount of Fe, so that the crystal grain size of the α phase becomes large. As a result, high strength cannot be ensured and the moldability is inferior.

  No. In No. 13, since the amount of Fe is excessive, the strength is higher than necessary, and the moldability is inferior.

  No. No. 14 is inferior in moldability because the amount of O is excessive.

  No. No. 15, since the hot rolling start temperature is low, the average value of the azimuth angles formed by the normal of the (0001) plane of the α phase and the normal of the rolled surface exceeds 45 °, and the elongation anisotropy is large. Also, the formability is inferior.

  No. From FIG. 16, it can be seen that high strength can be achieved by quenching to 300 ° C. after hot rolling to suppress coarsening of α phase crystal grains.

  No. Since the final annealing temperature of No. 17 is too high, the crystal grain size of the α phase becomes remarkably large. As a result, high strength cannot be ensured and the moldability is inferior.

  No. In No. 18, since the annealing temperature of the final annealing was too low, an unrecrystallized part remained after annealing, resulting in poor formability.

It is a figure which shows the azimuth ((theta)) which the normal line of the (0001) surface of (alpha) phase and the normal line of a rolling surface make.

Claims (2)

  It is a titanium alloy plate that satisfies Fe: 0.8 to 2.5% (meaning of mass%, the same shall apply hereinafter), O: 0.10% or less (not including 0%), and the balance: Ti and inevitable impurities. Thus, the average value of the azimuth angle formed by the normal of the (0001) plane of the α phase and the normal of the rolled surface, which is a hexagonal close-packed structure of the metal structure: 45 ° or less, and the area ratio of the α phase: 80 to A titanium alloy plate having high strength and excellent formability characterized by satisfying 97% and an average crystal grain size of α phase: 7.0 μm or less.   When performing ingot rolling, hot rolling, intermediate annealing, cold rolling, and final annealing sequentially using an ingot that satisfies the component composition defined in claim 1, the hot rolling start temperature is 800 ° C. or higher. And the annealing temperature of the said last annealing shall be 700 to 850 degreeC, The manufacturing method of the titanium alloy plate excellent in the high intensity | strength and the moldability characterized by the above-mentioned.

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