JP5875965B2 - Titanium plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a titanium plate made of industrially pure titanium, and more particularly to a titanium plate that has been subjected to a forming process and is used as a plate for a plate heat exchanger and a method for manufacturing the same.

  In general, titanium plates are excellent in specific strength and corrosion resistance, so heat exchanger members such as chemical, electric power and food production plants, consumer products such as camera bodies and kitchen equipment, transport equipment members such as motorcycles and automobiles, Used in exterior materials such as home appliances. Titanium plates are used in plate-type heat exchangers, which are being applied in recent years, among them, because high heat exchange efficiency is required. ing. Therefore, a titanium plate for a heat exchanger is required to have excellent formability in order to have a deep wave.

  Pure titanium plates frequently used for various applications are defined in the JIS H4600 standard. There are grades of JIS type 1, type 2, type 3, etc., depending on the concentration and strength of impurities such as Fe and O, etc. The minimum strength is high, and they are used properly according to the application. Conventionally, a JIS type 1 pure titanium plate having a low concentration of Fe or O has been used for a member requiring high formability because of its high ductility although it is inferior in strength. However, in recent years, in addition to improving the efficiency of heat exchangers, there has been an increasing demand for higher strength and lighter weight. In order to meet these requirements, it is necessary to apply JIS type 2 (yield strength 215 MPa or higher) or JIS type 3 with a higher strength level. However, when the strength level of these pure titanium plates is reached, heat exchange is not possible. It is difficult to apply to a vessel. In general, titanium materials can be increased in strength by increasing the concentration of impurities such as Fe and O, or by refinement of crystal grains. However, these methods greatly reduce moldability.

  In order to form a metal material, it is necessary to be plastically deformed. For that purpose, slip deformation or twin deformation of dislocations is required. Pure titanium is mainly composed of an α-phase crystal grain structure composed of a dense hexagonal crystal (hcp structure). A slip system that easily acts in the α phase of titanium is a column surface slip {10-10} <11-20>, and a bottom surface slip {0001} <11-20> and a weight surface slip. In addition, {11-22} <11-23> twins can be active during deformation during press molding. However, pure titanium has a smaller number of active slip systems than bcc-structured steel materials and fcc-structure aluminum, and it is difficult for a plurality of slip systems to easily operate, and plastic deformation is difficult. Therefore, it is considered important to activate a plurality of slip systems / twin systems in order to improve the formability.

  Thus, the following titanium plate technology with improved formability has been proposed. For example, Patent Document 1 proposes a titanium plate in which the texture (C-axis angle) and the crystal grain size after final annealing are defined (30 μm or more), and the balance between strength and formability is improved. Further, in Patent Document 2, the crystal grain size is set to a predetermined range by atmospheric annealing after cold rolling, and pickling and rolling under a light reduction (skin pass) with a reduction rate of 0.2 to 1.0% are performed. Titanium plates with improved press formability have been proposed. Furthermore, in Patent Document 3, by performing skin pass rolling with a rolling reduction of 0.7 to 5% after the final annealing, adjusting the texture (shift angle of the C axis) to obtain a specified accumulated strain amount, Titanium plates with improved press formability have been proposed.

Japanese Patent No. 4088183 Japanese Patent No. 4584341 JP 2011-026649 A

  However, there is room for improvement in the prior art described above in order to be applied to a heat exchanger. Since Patent Document 1 does not describe performing skin pass rolling, characteristics change when tempered by rolling. On the other hand, in patent document 2, the rolling reduction in skin pass rolling is low, and the strength is insufficient. In Patent Document 3, when Fe and O are relatively high in concentration, ear cracks are likely to occur during cold rolling, and productivity may be reduced.

  The present invention has been made in view of the above-mentioned problems, and provides a titanium plate that has both high proof strength equivalent to JIS type 2 or higher and high formability applicable to a heat exchanger, and a method for producing the same. Is an issue.

  As a result of diligent research, the present inventors have found that the existence ratio of twin grain boundaries existing between orientation differences of 60 ° to 70 ° satisfying a specific orientation relationship in the grain structure of titanium material is a balance between strength and formability. It was found that it contributes to the increase of Furthermore, the present inventors have experimentally found that twin grain boundaries having a misorientation between 82 ° and 87 ° reduce the effect of improving the balance between strength and formability.

That is, the titanium plate according to the present invention contains an α-phase grain structure, is made of industrial pure titanium, and has an orientation difference in the range of 60 ° to 70 ° in the orientation difference distribution of the α-phase grain boundaries . The maximum peak at 0.5 ° increments is a ratio of 0.0114 or more and 0.040 or less. Furthermore, in the titanium plate according to the present invention, in the orientation difference distribution of the α-phase grain boundaries, the maximum peak in 0.5 ° increments in the range of orientation difference of 82 ° to 87 ° is less than 0.010. It is preferable.

  The titanium plate having such a structure includes a large amount of specific orientation difference in the α phase crystal grain boundary, thereby improving the formability while having sufficient strength, and further restricting another specific orientation difference. The balance between strength and formability is not impaired.

Furthermore, the titanium plate according to the present invention preferably has an average crystal grain size of the α phase of 10 μm or more and 120 μm or less.
With this configuration, the titanium plate can easily obtain the orientation difference distribution of the α-phase grain boundaries.

Moreover, it is preferable that the titanium plate which concerns on this invention contains Fe: 0.020-0.120 mass%, O: 0.030-0.160 mass%, and remainder consists of titanium and an unavoidable impurity.
With this configuration, the strength of the titanium plate is further improved.

The titanium plate according to the present invention is molded and used as a plate heat exchanger plate.
With this configuration, the titanium plate can have a deep corrugation, increase the surface area, and form a heat exchanger plate with excellent heat exchange efficiency.

The titanium plate manufacturing method according to the present invention is such that the final cold rolling is performed at a rolling reduction of 50% or more and 88% or less, and then the α-phase crystal grains are in the range of 10 μm to 120 μm in average grain size. ° C. in annealing, and final annealing step of cooling to below 200 ° C. at 40 ° C. / s or more, after the final annealing step, light rolling at 5% one-pass reduction rate of 0.5% or more and the total rolling reduction of and the reduction rolling process, characterized by performing, further wherein the soft reduction rolling step, it is preferable to roll parallel to the rolling direction in the final cold rolling.

  By this procedure, the crystal grains are appropriately sized in the final annealing, so that twin deformation is likely to occur during subsequent cooling, etc., and strain is sufficiently introduced by further rapid cooling, so that α-phase crystals A titanium plate containing many specific orientation differences at the grain boundaries can be obtained. Furthermore, a pre-strain is imparted by rolling in a predetermined rolling direction and at an appropriate reduction rate, thereby obtaining a titanium plate having further improved strength and formability.

  According to the titanium plate according to the present invention, it is possible to provide high formability capable of press-working a plate for a heat exchanger plate while having a high proof strength equivalent to JIS type 2 or higher. Moreover, according to the manufacturing method of the titanium plate which concerns on this invention, the titanium plate which has the said effect can be obtained stably.

It is a conceptual diagram for demonstrating the degree of (0001) orientation of Ti hexagonal crystal in a titanium plate. It is a graph showing the orientation difference distribution between the crystal grains of the Example which concerns on this invention, and a comparative example, (a) is test material No.2. 2 and 10 in the orientation difference of 60 ° to 70 °, (b) is the test material No. The distribution is an orientation difference of 2, 15 at 82 ° to 87 °. The press molding die used in order to evaluate press formability in an example is shown, (a) is a top view and (b) is an EE line sectional view of (a).

Hereinafter, embodiments of the present invention will be described in detail.
[Titanium plate]
The titanium plate according to the present invention can be used for exterior materials such as heat exchanger members, transport equipment members, home appliances, etc. to which general industrial pure titanium is applied, and particularly requires high formability as well as strength. It is suitable for a plate type heat exchanger plate.

  The titanium plate according to the present invention contains an α phase crystal grain structure, is made of, for example, industrial pure titanium such as one type of pure titanium specified in JIS H 4600, and has an orientation difference between α phase crystal grain boundaries. In the distribution, the maximum peak in the orientation difference range of 60 ° to 70 ° is a ratio of 0.010 to 0.040.

(Orientation difference distribution of α phase grain boundaries)
In a conventional titanium product manufactured by a rolling process, as shown in FIG. 1, the C axis ((0001) axis) is oriented in a direction (ND) perpendicular to the rolling surface, and further from the rolling surface normal to the rolling width direction. They are distributed at positions inclined at about 30 ° to 40 ° on both sides to (TD) and tend to accumulate in that direction. Therefore, since the titanium material is a collection of crystal grain structures whose orientations are aligned in the same direction, the orientation difference between crystal grains is small (within 15 °), and there are many small-angle grain boundaries. There is a relatively large proportion of crystal grains that are misorientated corresponding to the fact that the C-axis is accumulated at positions inclined on both sides in the rolling width direction from the rolling surface normal line. Between crystal grains that are composed of a structure and have other orientation differences, the existence ratio is almost uniformly less than 0.01 (1%). The small-angle grain boundary is unlikely to become resistance to dislocation movement during plastic deformation, and thus hardly contributes to an increase in yield strength. The existence ratio of the low-angle grain boundaries does not change greatly with an increase in O (oxygen) content or refinement of crystal grains. Titanium materials are considered to become less likely to cause slip-type activity when the O content increases, and the yield strength increases due to increased resistance to dislocation movement itself. Deteriorates. Further, when the crystal grain of the titanium material is refined, the yield strength increases due to the resistance of dislocation movement due to the large-angle grain boundary other than the small-angle grain boundary. On the other hand, since such a titanium material has a small particle size, the distance traveled until the dislocation moves within the grain and is stopped at the grain boundary is small, and the amount of dislocation deposited at the grain boundary is reduced, so that they are adjacent to each other. Since the secondary slip system of crystal grains is not activated efficiently and the uniform secondary slip system is difficult to work, it is considered that uniform plastic deformation hardly occurs. Furthermore, since a normal large-angle grain boundary has high grain boundary energy and low grain boundary strength, it is assumed that it tends to be a starting point of crack generation and fracture when strain is concentrated, and deteriorates the formability of the titanium material.

  In the titanium plate according to the present invention, among the crystal grain boundaries that are resistance to dislocation movement, the twin grain boundaries having a relationship in the range of 60 ° to 70 ° with respect to the grain boundaries of other orientation differences. It is assumed that the frequency is high. This provides resistance to dislocations during plastic deformation, and efficiently activates the secondary slip system of adjacent crystal grains, making the secondary slip system easier to work uniformly, making the titanium plate more susceptible to plastic deformation. It is considered to be. Furthermore, twin grain boundaries have low grain boundary energy and high grain boundary strength, making it difficult for grain boundaries to break at such grain boundaries. It is thought that the balance of moldability is improved. However, if the frequency of twin grain boundaries is too high, the crystal grain size becomes smaller, and the phenomenon that uniform plastic deformation hardly occurs, which is recognized in fine grains, becomes prominent, and on the contrary, the formability is deteriorated. Therefore, the frequency of twin grain boundaries is set to the following range.

  In the orientation difference distribution of the α phase grain boundaries, when the maximum peak in the orientation difference range of 60 ° to 70 ° is less than 0.010, the frequency of twin grain boundaries is insufficient and the effect of improving the strength is obtained. Therefore, it is set to 0.010 or more and preferably 0.012 or more. On the other hand, when the maximum peak in the above range exceeds 0.040, the strength becomes excessive and the ductility is lowered, and the crystal grains become finer and uniform plastic deformation hardly occurs. Since a balance falls, it is set to 0.040 or less, and 0.025 or less is preferable. As will be described later, such an α-phase structure can be obtained by controlling the cooling rate at the time of final annealing or by applying a pre-strain after the final annealing in the production of a titanium plate.

  On the other hand, even if it is a large tilt grain boundary, a twin grain boundary having a relationship in the range of 82 ° to 87 ° is a twin crystal having a relationship in the range of the above orientation difference of 60 ° to 70 °. Unlike grain boundaries, it reduces strength. The reason for this is not sufficiently clear, but the degree of activation of the secondary slip system has changed due to the difference in orientation between adjacent grains depending on the type of twin, or It is considered that fine graining due to the increase in frequency tends to be significantly affected. Specifically, in the orientation difference distribution of the α phase grain boundaries, when the maximum peak in the orientation difference range of 82 to 87 ° is 0.010 or more, the effect of improving the balance between strength and formability is sufficient. Since it cannot be obtained, it is preferably less than 0.010, and more preferably 0.008 or less. Such an α-phase structure can be obtained by controlling the direction of prestraining after the final annealing in the production of a titanium plate, as will be described later.

  The orientation difference distribution of the α phase grain boundaries is measured by an electron backscatter diffraction (EBSD) method while scanning an electron beam with a scanning electron microscope (SEM) on a plane parallel to the rolling surface of the titanium plate. It is obtained by measuring and analyzing the EBSD pattern.

  The titanium plate according to the present invention preferably has an average crystal grain size of α phase of 10 μm or more and 120 μm or less. Moreover, it is preferable that the titanium plate which concerns on this invention contains Fe: 0.020-0.120 mass%, O: 0.030-0.160 mass%, and remainder consists of titanium and an unavoidable impurity.

(Average crystal grain size of α phase: 10 μm or more and 120 μm or less)
If the average grain size of the titanium plate is less than 10 μm, twin deformation hardly occurs when strain is introduced, and the maximum peak in the range of 60 to 70 ° in orientation cannot be obtained sufficiently. Therefore, the titanium plate according to the present invention preferably has an α-phase average crystal grain size of 10 μm or more, more preferably 20 μm or more, and further preferably 30 μm or more. On the other hand, roughening of the titanium plate is likely to occur when the α-phase crystal grain size increases. Therefore, the titanium plate according to the present invention preferably has an α-phase average crystal grain size of 120 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less. The crystal grain size of the α phase is an equivalent circle diameter, and the average is calculated by measuring with a known means such as SEM. The crystal grain size of the α phase can be obtained by adjusting the Fe content of the titanium plate or by controlling the final annealing conditions in the production, as will be described later.

(Fe: 0.020-0.120 mass%, O: 0.030-0.160 mass%)
The strength of the titanium plate decreases when the content of Fe and O is small. If an attempt is made to compensate for the lack of strength due to Fe or O deficiency, the amount of strain to be introduced increases, resulting in a decrease in formability. Therefore, the Fe content is preferably 0.020% by mass or more, more preferably 0.025% by mass or more, and further preferably 0.030% by mass or more. Moreover, 0.030 mass% or more is preferable, as for O content, 0.050 mass% or more is more preferable, and 0.070 mass% or more is further more preferable. On the other hand, when the Fe content increases, the segregation of the ingot increases and the productivity decreases. Further, since the crystal grains are refined by increasing the precipitation amount of the β phase, the maximum peak in the orientation difference range of 60 ° to 70 ° is insufficient, and the formability is deteriorated. Therefore, the Fe content is preferably 0.120% by mass or less, more preferably 0.080% by mass or less, and further preferably 0.070% by mass or less. Moreover, when O content increases, a titanium plate will become weak and it will become easy to produce the crack at the time of cold rolling, productivity will fall, and a moldability will fall. Therefore, the O content is preferably 0.160% by mass or less, more preferably 0.140% by mass or less, and further preferably 0.125% by mass or less.

  The titanium plate according to the present invention may contain C, H, N, Si, Cr, Ni, and the like as unavoidable impurities in addition to Fe, O, and Ti (titanium). C: 0.015% by mass or less, N: 0.02% by mass or less, H: 0.005% by mass or less, other elements: each 0.1% by mass or less inhibits the effects of the present invention. Is not acceptable.

[Production method of titanium plate]
The titanium plate according to the present invention is produced by performing ingot rolling on an ingot and performing hot rolling, intermediate annealing, cold rolling, and final annealing in the same manner as a conventional titanium plate. In the cold rolling step, appropriate rolling reduction and annealing conditions are selected according to the cold rolling properties of the material (ease of occurrence of ear cracks, deformation load, etc.), and cold rolling and intermediate annealing are repeated. The final annealing step is performed in order to recrystallize a material (titanium) that has been hardened by cold rolling to impart formability. Therefore, the cold rolling (final cold rolling) performed immediately before the final annealing step is performed at a processing rate necessary for recrystallization, specifically, a reduction rate of 50% or more. In addition, when the scale adheres to the titanium plate surface after annealing (intermediate annealing, final annealing), as a scale removal step, for example, salt heat treatment, acid treatment, etc., before the next step (subsequent cold rolling in the case of intermediate annealing) Perform washing treatment.

  The titanium plate according to the present invention can have the α phase crystal grain structure defined above by performing the final cold rolling step and the final annealing step under predetermined conditions. Hereinafter, the final cold rolling step and the final annealing step in the titanium plate manufacturing method according to the present invention will be described.

(Final cold rolling)
The titanium plate according to the present invention preferably controls the average crystal grain size of the α phase by adjusting the cold rolling rate in cold rolling (final cold rolling) performed immediately before the final annealing step. The rolling reduction of the final cold rolling is preferably 50% or more and 88% or less. When the rolling reduction is less than 50%, even if the conditions are adjusted in the subsequent final annealing, it is difficult to recrystallize as described above, and the recrystallized grain size is likely to become coarse after the final annealing, and the crystal grain size of the α phase. It is difficult to control the thickness to 120 μm or less. On the other hand, when the rolling reduction of the final cold rolling exceeds 88%, an ear crack at the time of cold rolling is likely to occur, and industrial problems such as a decrease in yield occur. In addition, the recrystallized grain size is likely to be refined after the final annealing, and the α phase crystal grain size is less than 10 μm.

(Final annealing)
The titanium plate according to the present invention preferably controls the average crystal grain size of the α phase by adjusting the temperature and time in the final annealing. Therefore, annealing temperature shall be 600-890 degreeC. If the temperature is less than 600 ° C., recrystallization does not proceed and the crystal grain size is small, so that twins are not sufficiently introduced. On the other hand, when the temperature exceeds 890 ° C., the fraction of β phase increases, the growth of α phase grains is inhibited, and the crystal grain size becomes finer. Furthermore, when the temperature exceeds the β transformation point temperature, the α phase grains become a needle-like structure after cooling, a remarkably fine structure, and press formability is hindered. As an example, the holding time is preferably within 15 minutes in the case of a continuous annealing furnace, and is further set according to the annealing temperature. That is, the higher the annealing temperature, the faster the recrystallization proceeds and the crystal grain size increases rapidly, so the time is short. Further, as described above, when the Fe content of the titanium plate is large, the crystal grains tend to become finer. Therefore, the final annealing is preferably performed at a high temperature for a long time.

  Furthermore, the titanium plate according to the present invention increases the cooling rate after the final annealing. Thereby, in the titanium plate, twins are introduced due to strain at the time of cooling, and the orientation difference distribution of the α-phase grain boundaries becomes within the specified range. Strain introduced by normal plastic working (tensile, rolling, etc.) has anisotropy, so twins generated in the strain mode also have anisotropy and are likely to be non-uniform in the grain distribution. . On the other hand, the strain at the time of cooling tends to be generated in an in-plane isotropic manner, so that anisotropy is reduced, and the introduced twins are likely to be uniform in the crystal grain distribution. Therefore, the effect of improving the balance between strength and formability can be remarkably obtained by introducing strain at the time of cooling the titanium plate. Specifically, after the final annealing under the above conditions, cooling is preferably performed at 40 ° C./s or more, more preferably 60 ° C./s or more, and further preferably 100 ° C./s or more. The cooling rate is up to 200 ° C. or less, and the rate for further cooling is not particularly specified. In order to perform such rapid cooling, the final annealing is preferably performed in a continuous furnace.

  Other conditions for the final annealing are not particularly defined, and can be performed by a known method. For example, the atmosphere may be air, vacuum, an inert gas such as Ar, or a reducing gas. In particular, when annealing (atmospheric annealing) is performed in an air atmosphere, a scale removal process such as pickling is performed as described above.

(Skin pass rolling)
Furthermore, the titanium plate according to the present invention, like the conventional titanium plate, is not only subjected to tension (stretching) or rolling after final annealing to increase flatness, but also imparts strain to provide strength and formability. May be improved. Since there is an upper limit to the amount of strain introduced during cooling, it is preferable to apply strain by performing stretching or rolling after the final annealing, whereby the titanium plate has the effect of further improving the balance between strength and formability. can get.

  Here, the orientation difference is in the range of 60 ° to 70 ° by making the direction to give strain due to tension or rolling after the final annealing parallel to the rolling direction in the rolling immediately before the final annealing, that is, the final cold rolling. The frequency of only the twin grain boundaries having the relationship increases. On the other hand, if the direction of applying the strain is perpendicular to the rolling direction in the final cold rolling (rolling width direction), the frequency of twin grain boundaries having a relationship in the range of 82 ° to 87 ° in orientation difference also increases. . Therefore, when pre-strain is applied, the application direction is preferably the same as the rolling direction in the final cold rolling.

  The total strain applied after the final annealing is preferably 0.5% or more in order to improve strength and formability. On the other hand, if the amount of strain exceeds 5%, the strength becomes excessive and the formability deteriorates. As an example of the strain application, light rolling (skin pass rolling) can be mentioned. In this case, the total rolling reduction is set to 5% or less. Further, if the amount of strain per one-time (one pass) of the skin pass rolling is small, it is difficult to obtain the crystal structure of the orientation difference distribution defined by the present invention up to the center of the thickness of the rolled plate. Therefore, the rolling reduction of one pass in skin pass rolling is preferably 0.5% or more, and more preferably 0.8% or more.

  As mentioned above, although the form for implementing this invention has been described, the Example which confirmed the effect of this invention is demonstrated concretely compared with the comparative example which does not satisfy | fill the requirements of this invention below. It should be noted that the present invention is not limited by this embodiment, and can be implemented with modifications within the scope shown in the claims, all of which are included in the technical scope of the present invention.

[Test specimen preparation]
After performing normal cold rolling and intermediate annealing on pure titanium (JIS H4600) hot-rolled sheet (sheet thickness: 4.0 mm) having the Fe and O composition shown in Table 1, the final cold rolling is performed at the rolling reduction shown in Table 1. Then, a cold-rolled sheet having a thickness of 0.5 mm was obtained by the final annealing under the conditions shown in Table 1 in an air atmosphere and the pickling process. This cold-rolled sheet was further subjected to light reduction by rolling in the direction shown in Table 1 (pre-strain direction) and a reduction ratio to obtain a test material. In addition, the prestraining direction (rolling direction after final annealing) shown in Table 1 is expressed as a direction with respect to the rolling direction in final cold rolling, and indicates RD: rolling direction and TD: rolling width direction. In Table 1, the specimens with a pre-strain reduction ratio of 0 are not rolled after the final annealing.

(Measurement of average crystal grain size of α phase)
The surface (plate surface) of the test material is polished, and 0.5 mm square (0. The region of 5 mm) was subjected to tissue observation by EBSD. For the EBSD measurement, an FE-SEM (manufactured by Carl-Zeiss, ULTRA55) and an EBSD detector (manufactured by Oxford Instruments, Nordlys II) were used. The measurement data was analyzed using the program of the EBSD data analysis software Channel 5: Tango. In Tango, select Grain Boundaries as the Boundary Component, set the boundary with an orientation difference of 5 ° or more as the crystal grain boundary, perform the Grain Area Determination operation, and calculate the equivalent circle diameter of each crystal grain and its average did. The average value obtained is shown in Table 1.

(Measurement of orientation difference distribution of α phase grain boundary)
From the EBSD measurement result in the measurement of the average crystal grain size, the distribution of orientation differences between adjacent crystal grains was analyzed, and the maximum frequency in each range of orientation differences of 60 ° to 70 ° and 82 ° to 87 ° was determined. Specifically, at Tango, Grain Boundaries was selected as Boundary Component, and the frequency was determined in increments of 0.5 ° from an azimuth difference of 0 ° to 94.5 ° using the Legend function. Table 1 shows the frequency in each range of the azimuth difference of 60 ° to 70 ° and 82 ° to 87 °. In addition, when a peak is observed in these ranges (corresponding to the existence of twin grain boundaries), the peak intensity is shown, and when no peak is observed (no twin grain boundaries are present), the maximum value in the above range. Was measured. In addition, test material No. The distribution in the orientation difference between 60 ° and 70 ° between 2, 10 crystal grains is shown in FIG. FIG. 2 (b) shows the distributions of orientation differences between 82 ° and 87 ° between 2,15 crystal grains.

[Evaluation]
(Strength)
A No. 13 test piece defined in JISZ2201 was taken from the test material in the direction in which the rolling direction of the test material coincided with the load axis, and a tensile test was performed based on JIS H4600 at room temperature to obtain a 0.2% proof stress (YS ) Was measured and shown in Table 1. The acceptance criterion was 0.2% proof stress of 215 MPa or more.

(Formability)
A heat exchange (plate) portion of a plate heat exchanger was simulated with respect to the test material, and press molding was performed with an 80-t press using a molding die having the shape shown in FIG. 3, and the moldability was evaluated. The molding die has four ridge line portions with a maximum height of 6.5 mm at a pitch of 17 mm in a molded portion of 100 mm × 100 mm, and the ridge line portions are formed in an R shape with R = 2.5 mm at the apex. . In press molding, a polysheet having a thickness of 0.05 mm is laid on both sides of the test material, and the test material is placed on the lower mold so that the rolling direction coincides with the vertical direction in FIG. The flange portion was restrained with a plate press, and the mold was pushed in at a press speed of 1 mm / second. Indentation was performed in increments of 0.1 mm, and the maximum indentation depth (mm) Y at which no cracks occurred in the test material was determined.

Based on the 0.2% proof stress (MPa) YS in the rolling direction measured in the tensile test and the indentation depth Y, the formability index (mm) F represented by the formula (1) is a positive value (F > 0) was accepted. The indentation depth and the formability index F are shown in Table 1.
F = Y− (9.844−0.016YS) (1)

  As shown in Table 1, the test material No. As for 1-9,15,16, the orientation difference distribution of the crystal grain boundary of (alpha) phase is in the range of this invention, and as shown to Fig.2 (a), the presence rate protruded in orientation difference 60 degrees-70 degrees. Since there existed inter-grain boundaries, that is, twin grain boundaries, it had sufficient strength and formability. In particular, specimen No. In Nos. 1 to 9, since the distribution of the orientation difference of 82 ° to 87 ° was suppressed, the balance between strength and formability was sufficiently obtained by the twin grain boundaries having the orientation difference of 60 ° to 70 °. Furthermore, specimen No. Since Nos. 1-3 and 5-9 were pre-strained by rolling in the same direction as the rolling direction in the final cold rolling after the final annealing, the strength and formability were further improved. The test material No. In No. 9, since the final annealing time was long and the crystal grains became large, the surface was roughened.

  On the other hand, the specimen No. Nos. 15 and 16 indicate the direction of applying pre-strain by rolling after the final annealing. While 2 and 8 are the same direction as the rolling direction in the final cold rolling, they are orthogonal directions (rolling width direction). As a result, a distribution with an azimuth difference of 60 ° to 70 ° indicates a specimen No. 2 and 8, but at the same time, as shown in FIG. Compared with 2 and 8, the moldability decreased.

  On the other hand, specimen No. Nos. 10 to 13 had a small distribution of orientation difference of 60 ° to 70 ° and lacked twin grain boundaries, so that the strength was insufficient and the balance with moldability was insufficient. Specimen No. Nos. 10 and 11 were not sufficiently strained during cooling because the cooling after the final annealing was slow at a general cooling rate. No. 10 was not rolled after that, and as shown in FIG. Less than 2. In addition, the specimen No. No. 12 has a low final annealing temperature. In No. 13, since the holding time with respect to the final annealing temperature was insufficient, recrystallization did not proceed sufficiently, the crystal grains were small, and twins were not sufficiently introduced even by rapid cooling or light pressure thereafter. . Furthermore, specimen No. No. 12 was particularly deficient in strength. On the other hand, the specimen No. No. 14, the reduction after the final annealing was excessive, the distribution of orientation difference 60 ° to 70 ° was excessive, the strength was excessive due to excessive twin grain boundaries, the ductility was lowered, and the formability was lowered.

Claims (7)

A titanium plate containing an α-phase grain structure and made of industrial pure titanium,
In the orientation difference distribution of the α-phase grain boundaries, the maximum peak in 0.5 ° increments in the orientation difference range of 60 ° to 70 ° is a ratio of 0.0114 to 0.040. Board. 2. The orientation difference distribution of the α phase crystal grain boundaries has a ratio of the maximum peak in 0.5 ° increments in the range of orientation difference of 82 ° to 87 ° being less than 0.010. 3. Titanium plate.   The titanium plate according to claim 1 or 2, wherein an average crystal grain size of the α phase is 10 µm or more and 120 µm or less.   4. Any one of claims 1 to 3, wherein Fe: 0.020 to 0.120 mass%, O: 0.030 to 0.160 mass% is contained, and the balance consists of titanium and inevitable impurities. The titanium plate according to claim 1.   The titanium plate according to any one of claims 1 to 4, wherein the titanium plate is used as a plate heat exchanger plate after being molded. A method for producing a titanium plate according to any one of claims 1 to 5,
Final cold rolling is performed at a reduction rate of 50% to 88%,
After the final cold rolling, as α-phase crystal grains is in the range of 120μm or more average particle size of 10 [mu] m, and a final annealing step of cooling at six hundred to eight hundred ninety ° C. annealing, to 200 ° C. or less at 40 ° C. / s or higher ,
A light reduction rolling process in which rolling is performed at a rolling reduction rate of 0.5% or more and a total rolling reduction rate of 5% or less in one pass after the final annealing step . The titanium plate manufacturing method according to claim 6 , wherein in the light rolling process , rolling is performed in parallel with a rolling direction in the final cold rolling.

Images