JP2013011013A - Pure titanium sheet having excellent balance between press formability and strength and excellent corrosion resistance, and process for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pure titanium sheet having characteristics excellent in balance between strength and press formability, and excellent corrosion resistance (crevice corrosion resistance), and to provide a process for manufacturing the pure titanium sheet.SOLUTION: The pure titanium sheet contains 0.02-0.10% of Fe, 0.04-0.20% of O and the balance consisting of titanium and inevitable impurities, the contents of the Fe and O satisfying the following expression (1): [O content (mass%)]+0.12[Fe content (mass%)]≥0.050. In the pure titanium sheet, the area fraction of regions that exhibit Schmid factors of {11-22}<11-23> twin of 0.45 or more is at least 43%, each Schmid factor being determined with the axis set in the direction of rolling at a position corresponding to one-fourth of the sheet thickness. Further, the pure titanium sheet has a volume fraction of β phase of 0.3% or less.

Description

  The present invention relates to a pure titanium plate excellent in the balance between press formability and strength at room temperature and corrosion resistance, and a method for producing the same, and in particular, has a strength (tensile strength) of 343 MPa or more even at room temperature, and is excellent. The present invention relates to a pure titanium plate having both press formability and crevice corrosion resistance, and a method for producing the same.

  Titanium uses its excellent corrosion resistance, press formability, specific strength, lightness, etc., for example, aircraft parts, chemical plant members, coastal structural materials (especially, seawater comes into contact and corrosion is accelerated). Widely used in a wide range of fields and applications, such as (Wangan structural materials), automobiles and building materials.

  In recent years, environmental load reduction and energy saving have been demanded in various fields, and in order to meet the needs for further lightening and thinning of titanium, which is light and strong, higher strength is desired. In addition, it is used in camera casings, exterior products for home appliances, members for transportation equipment, and the like, and further improvement in moldability is required.

  Pure titanium plates frequently used for these are defined in the JIS H4600 standard, and have grades such as JIS 1, 2 and 3 depending on the amount and strength of impurities such as Fe and O. The higher the grade, the higher the minimum strength and the proper use of them according to the application.

  When the concentration of Fe or O is low as in JIS type 1, the strength is low but the ductility is high. Therefore, conventionally, a JIS type 1 pure titanium plate has been used for members that require high formability.

  For example, a plate type heat exchanger (PHE) is one of the most frequently used applications among pure titanium plates. The pure titanium plate used in this application has a heat exchange rate. In general, in order to increase the effective heat exchange area, press molding (anisotropy bulging) into a complex corrugated plate shape is performed cold, which is extremely severe for the material. Exposed to the press environment. As a pure titanium plate used for such severe press molding conditions, soft JIS type 1 pure titanium that is most easily molded according to the standard is used.

  However, the improvement of the heat exchange rate of the heat exchanger is not limited to the above-described one that depends on the shape of the heat exchanger itself, and can be achieved, for example, by increasing the flow rate of the heat medium (or refrigerant). Therefore, the pure titanium plate is required to have higher strength and excellent formability.

  As the pure titanium plate having high strength, JIS type 2 or type 3 having high Fe and O concentrations are used. However, the higher the Fe and O concentrations, the higher the strength, but the moldability deteriorates, and the conventional techniques cannot achieve both high strength and high moldability. Further, depending on the manufacturing history, crevice corrosion of titanium products may be a problem in a high temperature and high humidity environment where the usage environment is likely to corrode.

  As means for improving the press formability of a pure titanium plate, for example, it has been proposed to control the structure of titanium (Patent Document 1) or to alloy titanium (Patent Document 2). However, the titanium targeted by these technologies is intended to improve the formability of a pure titanium plate having a strength (yield strength) level equivalent to that of JIS Class 1. However, when the strength level is increased, the formability is improved. Absent.

  As a means for improving the press formability of a pure titanium plate having a high strength level, a technique for adjusting the content of Fe and O and controlling the crystal grain size of titanium has been proposed (Patent Document 3). However, it is difficult to achieve a balance between press formability and strength only by controlling the Fe and O contents and the crystal grain size of titanium.

  In addition, as described above, in recent years, the application field of pure titanium plates has been widespread, but when used in a corrosive environment, further improvement in crevice corrosion resistance has been demanded.

  As a titanium plate having improved crevice corrosion resistance, Patent Document 4 discloses that a platinum group element is attached to the surface of a pure titanium plate. However, platinum group elements are expensive, and there is a problem that the manufacturing cost is high, and it is difficult to use them in general.

  As a technique for improving the balance between strength and press formability, Patent Document 5 discloses a titanium alloy plate to which Cu, Ni, and Mo are added. However, when alloy components are added, the productivity is lowered and it is not suitable for industrial scale production. There is no specific disclosure about corrosion resistance.

  Thus, a pure titanium plate that has an excellent balance between press formability and strength at room temperature and also has excellent crevice corrosion resistance has not yet been provided.

JP 2004-285457 A JP 2002-317234 A JP 2009-228092 A Japanese Patent Publication No.49-31610 JP 2010-236067 A

  The present invention has been made in view of the above-mentioned conventional problems, and its purpose is a pure titanium plate having characteristics excellent in balance between press formability and strength, and corrosion resistance (crevice corrosion resistance), and production thereof. Is to provide a method. Specifically, it has a tensile strength of 343 MPa or more at room temperature, is excellent in press formability (isotropic stretch forming and ductility of isotropic stretch forming), and further is crevice corrosion resistance. It is providing the pure titanium plate which has the characteristic excellent also in the property, and its manufacturing method.

The present invention that has solved the above problems contains Fe: 0.02 to 0.10% (meaning “mass%”, the same applies to chemical components), and O: 0.04 to 0.20%. The balance is made of titanium and inevitable impurities, the contents of Fe and O satisfy the following formula (1), and {11-22} <11 with the rolling direction at the 1/4 position of the thickness as the axis. -23> Press formability having a gist in that the area ratio of the region where the twin Schmid factor is 0.45 or more is 43% or more, and the volume ratio of the β phase is 0.3% or less. It is a pure titanium plate with excellent strength balance and corrosion resistance.
[O content (% by mass)] + 0.12 × [Fe content (% by mass)] ≧ 0.050 (1)

  As a method for producing a pure titanium plate excellent in the balance between press formability and strength, and corrosion resistance, in a method for producing a pure titanium plate produced through hot rolling, intermediate annealing, cold rolling, and final annealing steps. After the intermediate annealing, after performing the scale removal treatment on the surface of the pure titanium plate, cold rolling with a rolling reduction of 70% or more is performed, and then the condition that H defined by the following formula (2) becomes a positive value It is recommended to perform final annealing at

However,
T: heating temperature during final annealing (° C.), t: annealing time (seconds)
X ₀ : Fe concentration in titanium (mass%)
A = 891, B = 0.428, n = 0.135

  Further, in the method for producing a pure titanium plate manufactured through the steps of hot rolling, cold rolling and final annealing, the pure titanium plate is scale-removed on the surface of the pure titanium plate without performing intermediate annealing after the hot rolling. By performing the cold rolling with a rolling reduction of 20% or more after the treatment, and then performing the final annealing under the condition that H defined by the following formula (2) becomes a positive value, Can also be obtained.

However,
T: heating temperature during final annealing (° C.), t: annealing time (seconds)
X ₀ : Fe concentration in titanium (mass%)
A = 891, B = 0.428, n = 0.135

  According to the present invention, the chemical component composition is strictly defined in consideration of the balance of each element, and the specific twin system Schmid factor and β phase of the crystal grains of the pure titanium plate are specified to a specific ratio. Thus, it is possible to obtain a pure titanium plate not only having high tensile strength even at room temperature but also excellent in press formability and crevice corrosion resistance. These pure titanium plates are used in a wide range of fields such as mobile phones, mobile PCs, camera bodies, eyeglass frames, plate heat exchanger components, fuel cell separators, etc., when high strength at room temperature is required. In addition to being able to exhibit good moldability, the corrosion resistance is further improved.

  The present invention also provides a production method for producing the pure titanium plate at a low cost.

It is a perspective view of the titanium plate used for a plate type heat exchanger (PHE). FIG. 2A is a plan view of a press mold for explaining a method for evaluating press formability, and FIG. 2B is a schematic cross-sectional view taken along line FF in FIG. 2A. .

  As for the press formability of a pure titanium plate, it is known that twin deformation is greatly contributed to deformation of a material in addition to slip deformation, so that the twin form deformation is more likely to occur and the press formability is excellent. It is known that pure titanium has a tendency to cause twin deformation as it is softer and has better press formability. However, when the strength level is increased, twin deformation is less likely to occur and the press formability is poor.

  In order to provide a pure titanium plate having improved press formability while maintaining a high strength level (tensile strength of 343 MPa or more) and having excellent crevice corrosion resistance, the present inventors have various angles with respect to the metal structure. The following findings were obtained.

  That is, when a cold rolling is performed in one direction, strength anisotropy occurs in the pure titanium sheet, and the strength in the rolling direction with high ductility is lower than the width direction with low ductility. Deformation at will proceed with priority. Therefore, in order to improve the press formability, the structure control that promotes the activation of twin deformation that contributes to deformation in the rolling direction which is the main deformation direction is effective, and the tensile strength level is set to 343 MPa or more. However, it is important to form a crystal orientation that easily causes twin deformation.

  Therefore, in the present invention, as a result of examining the Schmitt factor that affects the likelihood of twinning deformation, twinning deformation in the case where a tensile load is applied in the rolling direction on a 1/4 × thickness rolled surface of a pure titanium plate. When the Schmid factor value and its distribution ratio (area ratio) are in a specific range, it was found that press formability can be improved while maintaining a high strength level, and the present invention has been achieved.

  Specifically, the crystal grains of the main phase of the pure titanium plate have a hexagonal crystal structure, and the twin system mainly active during the stretch forming is {11-22} <11-23>. . Therefore, in order to improve the formability mainly when deformation occurs in the rolling direction, it is effective to activate {11-22} <11-23> twinning activities when a load is generated in the rolling direction. Is. For this purpose, it is effective to increase the {11-22} <11-23> twinned Schmid factor.

  As a result of repeated studies by the present inventors, in order to facilitate the twin deformation activity, it is necessary that the value of the above-mentioned twin-type Schmitt factor is at least 0.45 or more.

  However, if the ratio of the crystal grains having a Schmid factor greater than a specific value is too small, even if the twin deformation activity can easily occur, the overall material, that is, the press formability is greatly improved. Does not contribute. Therefore, the area ratio of the region where the Schmid factor is 0.45 or more needs to be 43% or more, preferably 45% or more of the whole. If the area ratio is 43% or more, a pure titanium plate having remarkably excellent press formability can be obtained while maintaining a strength level of 343 MPa or more. The upper limit of the area ratio is not particularly limited.

  In addition, in the Miller index indicating a crystal plane, a notation method in which a bar is added above a number when the index is negative is common. However, when the index becomes negative, in this specification, it is expressed as a negative number for convenience. Therefore, -2 in the {11-22} indicates that the index is negative.

  Thus, in this invention, the Schmid factor is employ | adopted as a parameter | index for improving press moldability. In general, the magnitude of critical decomposition shear stress (τ) required to move dislocations along a specific crystal plane depends on the crystal plane and the crystal axis direction, and [τ = σ cos φ cos λ] (where σ is the tensile axis direction). It is known that φ is given by the angle between the normal to the sliding surface and the tensile axis, and λ represents the angle between the sliding direction and the tensile axis.

  [Cos φ cos λ] in the above formula is called a Schmitt factor, and indicates the ratio of the force used to move the dislocation out of the external force applied to the tensile axis. When an external force is applied to the titanium plate, the slip system / twin system with a high Schmid factor value operates with a smaller external force, and as a result, the metal plate is easily plastically deformed. A pure titanium plate that satisfies the above Schmid factor requirements has a press formability (especially anisotropic overhanging) in which twin deformation is promoted when a tensile load is applied in the rolling direction and the principal strain direction is the rolling direction. Formability) is greatly improved.

  In addition, it is known that a pure titanium plate usually has higher strength as the content of impurity elements is larger, and twin deformation is less likely to occur, so that press formability is poor. However, in the present invention, the chemical composition is strictly defined as described later, and the Schmitt factor is controlled so that twin deformation is likely to occur. Therefore, even if the impurity element content is high and the tensile strength is high, Crystal deformation can be activated, and as a result, press formability is improved.

  In addition, since the present inventors are prone to crevice corrosion resistance, particularly in high-temperature and high-humidity environments, in order to increase crevice corrosion resistance in such an environment, the precipitation of β phase should be suppressed. Found that is important. That is, it is desirable that the titanium of the present invention has a metal structure mainly composed of an α phase (hexagonal crystal). Since the crevice corrosion resistance deteriorates when the volume ratio of the β phase (body-centered cubic crystal) is increased, the upper limit of the volume ratio of the β phase is 0.3% or less, preferably 0.2% or less, more preferably 0%. It is.

  Furthermore, in the pure titanium plate of the present invention, it is important to appropriately adjust Fe and O, which are elements that affect press formability and tensile strength. The reasons for setting the ranges of these components in the pure titanium plate of the present invention are as follows.

Fe: 0.02-0.10%
Fe is an element that contributes to improving the strength of the pure titanium plate. In order to exert such effects, the Fe content is 0.02% or more, preferably 0.03% or more. However, if the Fe content is excessively large, the β phase generated increases even if the annealing conditions are optimized, and the crevice corrosion resistance deteriorates. Therefore, the Fe content is 0.10% or less, preferably 0.08% or less, more preferably 0.07% or less.

O: 0.04 to 0.20%
O is an element effective for ensuring the strength of a pure titanium plate, and in order to exert such an effect, the content of O is 0.04% or more, preferably 0.045% or more, more preferably It is 0.050% or more, more preferably 0.07% or more. However, if the O content is too large, the strength becomes too high, and press formability may be deteriorated, or edge cracking or breakage may occur during cold rolling. Therefore, the O content is 0.20% or less, preferably 0.18% or less, more preferably 0.15% or less.

  Furthermore, in the present invention, from the viewpoint of improving the crevice corrosion resistance by suppressing the precipitation of the β phase while balancing the improvement in strength and press formability by adding Fe and O, the relationship between the contents of Fe and O is described. It is important to specify. Specifically, the content of Fe and O is within the above range so that [O content (% by mass)] + 0.12 × [Fe content (% by mass)] ≧ 0.050 (formula (1)). It is necessary to adjust with. When the contents of Fe and O derived from the above formula (1) are less than 0.050, the strength may be insufficient. In order to further enhance the effect of adding Fe and O, the value of the formula (1) is preferably adjusted to 0.055 or more, more preferably 0.060 or more, and further preferably 0.080 or more. Is desirable.

  The pure titanium plate of the present invention is composed of titanium and inevitable impurities in addition to the above components. “Inevitable impurities” refers to impurity elements (typically N, C, H, Si, Cr, Ni, etc.) that are inevitably contained in the raw sponge titanium, or can be incorporated into products in the manufacturing process. A characteristic element (for example, H) is included.

  Among these inevitable impurities, for example, N is also an element that contributes to improving the strength of the pure titanium plate. However, if the content is increased, the cold rolling property is impaired. Therefore, the N content is preferably 0.02% or less, More preferably, the content is 0.01% or less. On the other hand, the N content may be 0%, but excessive reduction leads to an increase in cost. Therefore, it is preferably 0.001% or more, more preferably 0.002% or more.

  C is an element that contributes to strength improvement. However, if the content increases, the cold-rolling property is impaired, so the C content is preferably less than 0.015%, more preferably 0.012% or less. On the other hand, the lower limit of the C content is not particularly limited, and may be 0%. However, excessive reduction causes an increase in cost, so 0.002% or more, more preferably 0.003% or more. is there.

  The pure titanium plate of the present invention has a tensile strength of 343 MPa or more at room temperature (25 ° C.). As described above, pure titanium plates are used in various fields, and high strength is required even at room temperature. The tensile strength of the pure titanium plate of the present invention is preferably 370 MPa or more. However, if the tensile strength is too high, the ductility is lowered and the press formability is deteriorated.

  As described above, the strength and ductility of the pure titanium plate are contradictory properties, but the present invention has a tensile strength of 343 MPa or more and excellent press formability as a pure titanium plate that can satisfy both of these properties. In addition, the present invention provides a pure titanium plate having excellent crevice corrosion resistance.

  Next, the manufacturing conditions will be described using the pure titanium plate as an example. For example, in the case of a pure titanium plate, it is generally manufactured by the following process. Since the physical properties of titanium vary depending on the chemical composition of the titanium material used and the setting conditions for each process, it should be determined by selecting conditions comprehensively as a series of manufacturing processes. The conditions are strictly determined for each individual process. Setting is not always appropriate.

  The pure titanium plate is generally obtained by casting a titanium ingot adjusted to a specific component composition. Thereafter, [I. Partial forging / rolling] → [II. Hot rolling] → [III. Intermediate annealing] → [IV. Cold rolling] → [V. It is manufactured through each step of final annealing. Between each process, you may perform a scale removal process by performing blasting, a pickling process, etc. as needed.

  [I. [Branch forging / rolling] is a process performed as necessary, and is performed to break a coarse cast structure. For example, forging by soaking at about 800 to 1100 ° C. for productivity (ease of processing) and the like. Or rolling may be started.

  [II. For example, the hot rolling may be performed at a temperature of about 700 to 950 ° C. and then hot rolling so as to obtain a desired plate thickness.

  [III. Intermediate annealing] is a process performed as necessary, and may be air-cooled after soaking at about 700 to 850 ° C., for example.

  When [intermediate annealing] is performed, scale removal treatment is performed after the intermediate annealing, and then [IV. Cold rolling] is performed. Performed before final annealing [IV. Cold rolling] is a rolling reduction of 70% or more, preferably 80% or more. By performing cold rolling at a rolling reduction of 70% or more, the subsequent [V. In the final annealing], the tensile strength of the pure titanium plate is maintained at 343 MPa or more, and the properties excellent in press formability can be obtained.

  That is, in manufacturing the pure titanium plate of the present invention, [IV. If the rolling reduction of cold rolling] is 70% or more, the area ratio of crystal grains having a {11-22} <11-23> twin-type Schmid factor of 0.45 or more can be 43% or more. . This is the result of [IV. The rolling reduction in cold rolling] is set to 70% or more, and then [V. By performing the final annealing, the desired crystal orientation can be obtained, so that good press formability can be obtained and the tensile strength can be maintained at 343 MPa or more.

  As the rolling reduction increases, press formability improves. However, if the rolling reduction is increased too much, edge cracking occurs at the end of the pure titanium plate, or the pure titanium plate breaks during cold rolling. In order to inhibit the properties, the rolling reduction is preferably 95% or less, more preferably 90% or less.

  On the other hand, [III. When not performing [intermediate annealing], [II. [Hot rolling], and after removing the scale, [IV. Cold rolling] is performed. And in this case, it is performed before the final annealing [IV. Cold rolling] is a rolling reduction of 20% or more, preferably 25% or more. Even when the intermediate annealing is omitted, the tensile strength of the pure titanium plate can be increased to 343 MPa or more by appropriately performing cold rolling and final annealing, and also has excellent press formability. When cold rolling without intermediate annealing, the reason why the above excellent characteristics can be obtained even if the rolling reduction is low is that the deformation structure formed in the material during hot rolling remains in the material without being canceled by the intermediate annealing. Because. Since the properties excellent in press formability can be obtained even at a low rolling reduction without performing intermediate annealing, for example, when the thickness of the pure titanium plate is thick and a high rolling reduction cannot be obtained, It can also be applied when the plate thickness is sufficiently thin.

  [V. In the final annealing, after annealing under the conditions of the heating temperature and annealing time satisfying the following formula (2), it is sufficient to air-cool to room temperature. By setting such conditions, crevice corrosion resistance is affected. Precipitation of β phase can be suppressed. The pure titanium plate of the present invention contains a small amount of Fe as described above, but the heating temperature and holding time at which the β phase precipitates are determined depending on the Fe content, and therefore the following formula (2) is satisfied. Final annealing is performed under the conditions. Equation (2) means that

In the formula, T: heating temperature during final annealing (° C.), t: annealing time (seconds), X ₀ : Fe concentration (mass%) in titanium, A = 891, B = 0.428, n = 0.135.

  Equation (2) is obtained based on the following experiment. That is, as described above, the precipitation rate of the β phase depends on the Fe concentration in titanium and the holding temperature during heat treatment, and increases as the Fe concentration increases and the temperature increases. Therefore, using the precipitation rate equation calculated based on the diffusion and concentration distribution of atoms, a relational equation (Equation 2) as a basis of the precipitation rate is obtained, and the coefficients A and B in Equation (2) are obtained from the experimental data. Decided on the basis.

  In the formula, T is a heating (annealing) temperature (° C.) at the time of final annealing, and it is desirable that 550 ° C. ≦ T (° C.) ≦ 890 ° C. When the annealing temperature T is lower than 550 ° C., reductivity does not occur and ductility may be significantly deteriorated. The lower limit temperature required for recrystallization varies depending on the holding time. For example, the shorter the holding time, the higher the temperature is required. The heating temperature T is preferably 600 ° C. or higher, more preferably 700 ° C. or higher. In addition, when annealing is performed by heating to the β transformation point or higher, it becomes a needle-like structure after cooling, and press formability is hindered. Therefore, it is preferable to perform the final annealing at a β transformation temperature or lower. The heating temperature T is preferably 870 ° C. or lower, more preferably 850 ° C. or lower.

  In addition, general conditions can be employ | adopted for the other conditions in cold rolling and final annealing, and the conditions of other processes.

  In the formula, t is the annealing time (seconds) and is the holding time at the heating temperature T (° C.). If the temperature is within the above temperature range, the heating temperature T (° C.) ± 5 It means that the temperature may be maintained within a range of ° C. If the annealing time t is too short, it is difficult to control the annealing process, and if the annealing time t is too long, the productivity is deteriorated. Therefore, it is desirable that the annealing time t (second) is in the range of 10 seconds ≦ t (seconds) ≦ 120,000 (seconds).

  The thickness of the pure titanium plate of the present invention is not particularly limited as long as it is set in consideration of the required strength and the like.

  Since the pure titanium plate according to the present invention has high strength and excellent ductility, and also has excellent crevice corrosion resistance in a high-temperature and high-humidity environment, the mobile phone, mobile personal computer, It can be widely applied to applications that require high formability and mechanical strength in a wide range of fields such as camera bodies, eyeglass frames, plate heat exchanger components, and fuel cell separators.

  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. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  A titanium ingot was obtained by casting using CCIM (Cold Crucible Induction Melting Method), which contained Fe and O shown in “Components” in Table 1, and the remainder of titanium and inevitable impurities. This ingot was divided and rolled into a block shape having a width of 130 mm and a thickness of 45 mm, and further heated to 750 ° C. and hot-rolled to obtain a hot rolled sheet having a thickness of about 4 mm (No. 3 to 10). In addition, in consideration of the final plate thickness (0.5 mm) and the cold rolling rate shown in Table 1, some of the test materials were subjected to face milling of the plate thickness of the hot-rolled plate and adjusted appropriately. Some hot-rolled sheets shown in Table 1 were subjected to intermediate annealing at 700 ° C. for 5 minutes, then immersed in a salt furnace, and then pickled and descaled. The other part was descaled without intermediate annealing. After descaling, the plate thickness was adjusted by chamfering and pickling, cold rolling was performed at the “reduction ratio” shown in Table 1, and then final annealing was performed under “final annealing conditions” shown in Table 1. . After the final annealing, it was immersed in a hydrofluoric acid solution and descaled to obtain a pure titanium plate having a thickness of 0.5 mm as a test material. The H value of each pure titanium plate was calculated based on the above formula (2) and listed in the “value H of formula (2)” column of Table 1.

  The following evaluation was performed with respect to each obtained pure titanium plate.

(Measuring method of Schmid factor)
Field orientation scanning electron microscope (FESEM) (manufactured by JEOL Ltd., JSM5410) equipped with a backscattered electron diffraction (Electron Backscatter Diffraction Pattern: EBSP) system for arbitrary orientation analysis The texture on the surface of the pure titanium plate was evaluated.

  Specifically, after polishing the rolled surface of a pure titanium plate to ¼ of the plate thickness, it is set in an SEM lens barrel, and EBSP projected on a screen by irradiating an electron beam with a high-sensitivity camera. The image was taken and captured as an image into a computer, and the crystal orientation was determined by analyzing the image. At this time, the {11-22} <11-23> twinned Schmid factor when the tensile load direction was made to coincide with the rolling direction was evaluated for each measurement point, and orientation mapping data was collected. In addition, this measurement visual field range was 1 mm x 1 mm, and the measurement pitch was 1 micrometer. Here, the ones whose orientation difference between adjacent measurement points is within ± 15 ° belong to the same crystal plane.

(Area ratio of crystal grains (α phase) having {11-22} <11-23> twin system Schmid factor of 0.45 or more)
After the measurement of the Schmid factor, the area ratio of the crystal grains having the Schmitt factor of 0.45 or more is obtained by dividing the sum of the measurement points having the Schmitt factor of 0.45 or more by the total number of measurement points and multiplying by 100. In Table 1, “texture (%)” column is shown.

(Volume ratio of β phase)
The surface of the rolled surface of the pure titanium plate was mechanically polished to a 1/4 position of the plate thickness, finished to a mirror surface by buffing and chemical polishing, and then reflected electron image (BSE) observation was performed. An arbitrary region of 270 μm × 230 μm was observed for each sample at a magnification of 10,000 times, and the average of the volume fraction of β phase was determined and described in the “β phase volume fraction (%)” column of Table 1. In this example, if the volume fraction of the β phase was 0.3% or less, it was determined to be acceptable.

(Tensile strength evaluation)
A No. 13 test piece (direction in which the rolling direction coincides with the load axis (L direction)) defined in JISZ2201 was taken from a pure titanium plate, a tensile test was carried out at room temperature based on JISH4600, and the tensile strength was measured. The results were entered in the “Tensile strength” column of Table 1. The case where the tensile strength was 343 MPa or more was evaluated as acceptable.

(Press formability (score))
FIGS. 2A and 2B are explanatory diagrams of a method for evaluating press formability (anisotropy stretch formability). Press die simulating the heat exchange part of a plate heat exchanger using a pure titanium plate produced as the test material [size: 160 mm × 160 mm (evaluation part: 100 mm × 100 mm), pitch: 10 mm, maximum height : 4 mm, curvature radius R: 0.4, 0.6, 0.8, 1.0, 1.4, 1.8 (mm) herringbone type having 6 types of ridge lines, and curvature radius R of the mountain by the ridge lines Were used, and the press formability of the titanium plate was evaluated with an 80 ton hydraulic press. The pressing conditions are as follows. Press oil (kinematic viscosity: 34 mm ² / s (temperature: 40 ° C.)) is applied to both surfaces of a mold as a test piece, and the rolling direction of each test material coincides with the vertical direction in FIG. In this way, after placing on the lower mold and restraining the flange portion with a plate press, it was carried out under the conditions of a press speed of 1 mm / second and an indentation depth of 3.4 mm.

  As shown in FIG. 2 [FIG. 2 (a) is a plan view, FIG. 2 (b) is a cross-sectional view], the crack measurement position of the test material is a ridge and a broken line (5 places on the mountain side and 1 place on the valley side). There are 36 intersections.

  For the A line (mountain side), C line (peak side), C 'line (valley side), and E line (peak side), which are the base points of cracking, 2 if there is no crack (healthy) when judged visually. Point, if there is a tendency to necking (partially thinned and constricted), 1 point if necking has occurred, 0 point if there is a crack, B line (peak side), D line (peak side) For no cracking (sound), it was 2 points, 0.5 points if necking occurred, and 0 points if cracking occurred.

The formability score was calculated by the following formula (3) and used as an index for evaluating the press formability in the present invention. Specifically, the number of points was multiplied by the reciprocal of the radius of curvature R to quantify the cracking state, and the total was obtained. After normalizing this total value as 100 when cracking and necking are not recognized, a function F (T, μ, which depends on temperature (T), lubricating oil viscosity (μ), and test piece plate thickness (t) is used. t) and the function G (α, p) depending on the angle (α) and pitch (p) of the twill lines of the mold were multiplied to calculate the formability score. In this example, the temperature (T), the lubricating oil viscosity (μ), the thickness of the test piece plate (t), the angle (α) of the twill line of the mold, and the pitch (p) were constant, so that F × G For convenience, the score was calculated as 1 and listed in the “formability score” column of Table 1.
Formability score = (F × G) × [ΣE (ij) / R (j)] / [(Σ _{A, C, C ′, E} 2 / R (j)) + (Σ _{B, D} 1 / R ( j))] × 100 (3)
Where
For A, C, C ′, E, E (ij) = 1.0 × (no cracking: 2, constriction: 1, cracking: 0),
B and D were calculated as E (ij) = 0.5 × (no cracking: 2, constriction: 1, cracking: 0).
A moldability score of 70 or more was evaluated as being excellent in moldability.

(Crevice corrosion test)
A 30 mm x 50 mm test piece was cut out from the manufactured pure titanium plate, a hole of φ7 mm was drilled in the center, and Teflon (registered trademark) multiclevis (ASTM G16-71) was attached to both sides through the hole. It was set as the test piece for property evaluation. This multi-clevis forms 12 gaps, but since the multi-clevis was attached to both sides of the test piece in this test, there are a total of 24 gaps. This test piece was immersed in a boiled 10% NaCl aqueous solution for 360 hours, and then the presence or absence of crevice corrosion was visually confirmed, and the number of occurrences of crevice corrosion was measured. When the number of crevice corrosion occurrences was 5 or less, the crevice corrosion resistance was evaluated as good. The results are shown in the “corrosion resistant corrosion rate” column of Table 1.

(Comprehensive evaluation)
As mechanical properties at room temperature, a case where the tensile strength at room temperature is 343 MPa or more, the score of press formability is 70 points or more, and the number of crevice corrosion occurrences is 5 places or less is passed, respectively. It was evaluated that the balance of formability was excellent and the crevice corrosion resistance was also excellent. The results are shown in Table 1.

  From Table 1, it can be considered as follows. That is, no. 2 to 6, 9, 10, and 12 to 16 are pure titanium plates that satisfy the requirements defined in the present invention, and not only have a good balance between strength at room temperature and press formability, but also have excellent crevice corrosion resistance. I understand that.

  In contrast, no. Since 1, 7, 8, and 11 did not satisfy the requirements defined in the present invention, the strength at room temperature could not be secured, and problems such as poor press formability and crevice corrosion resistance at room temperature occurred.

  No. 1 and No. 11 is an example in which the rolling reduction in cold rolling before final annealing is low. In this example, the area ratio of the crystal grains having a Schmid factor of 0.45 or more (in the table, “texture (%)”) was below 43%, and the press formability was poor.

  No. No. 7 is an example in which the amount of O contained in the pure titanium plate is small, and the value of the formula (1) is less than that of the present invention, and the tensile strength is less than 343 MPa.

  No. 8 is an example (the value of “H” is negative in the table) in which the value of the formula (2) is less than that of the present invention. In this example, the area ratio of the grains having a Schmid factor of 0.45 or more (in the table, “texture (%)”) is less than 43%, and the volume ratio of the β phase exceeds the specified value. Formability and crevice corrosion resistance were poor.

Claims (3)

Fe: 0.02 to 0.10% (meaning “mass%”, the chemical components are the same hereinafter),
O: 0.04 to 0.20%
And the balance consists of titanium and inevitable impurities,
While the contents of Fe and O satisfy the following formula (1),
The area ratio of the region where the {11-22} <11-23> twinned Schmitt factor is 0.45 or more with the rolling direction at the ¼ thickness position as the axis is 43% or more,
Furthermore, a pure titanium plate excellent in the balance between press formability and strength and corrosion resistance, wherein the volume fraction of β phase is 0.3% or less.
[O content (% by mass)] + 0.12 × [Fe content (% by mass)] ≧ 0.050 (1) In the method of manufacturing a pure titanium plate manufactured through the steps of hot rolling, intermediate annealing, cold rolling, and final annealing, after the intermediate annealing, the scale removal treatment is performed on the surface of the pure titanium plate, and then the reduction rate is 70. %, And then the final annealing is performed under the condition that H defined by the following formula (2) is a positive value, and excellent in balance between press formability and strength, and corrosion resistance. The manufacturing method of the pure titanium plate of Claim 1.
However,
T: heating temperature during final annealing (° C.), t: annealing time (seconds)
X ₀ : Fe concentration in titanium (mass%)
A = 891, B = 0.428, n = 0.135 In the manufacturing method of a pure titanium plate manufactured through the steps of hot rolling, cold rolling, and final annealing, after performing the descaling treatment on the surface of the pure titanium plate without performing intermediate annealing after the hot rolling, the rolling down Cold-rolling with a rate of 20% or more, and then performing final annealing under the condition that H defined by the following formula (2) is a positive value, and a balance between press formability and strength, and corrosion resistance The manufacturing method of the pure titanium plate of Claim 1 excellent in.

However,
T: heating temperature during final annealing (° C.), t: annealing time (seconds)
X ₀ : Fe concentration in titanium (mass%)
A = 891, B = 0.428, n = 0.135