US20190226073A1

US20190226073A1 - Titanium sheet and method for producing the same

Info

Publication number: US20190226073A1
Application number: US16/314,323
Authority: US
Inventors: Hidenori Takebe; Reita CHIDA; Satoshi Matsumoto
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-06-30
Filing date: 2016-06-30
Publication date: 2019-07-25
Also published as: JPWO2018003098A1; JP6156597B1; CN109477168A; WO2018003098A1

Abstract

A titanium sheet having a sheet thickness of 0.2 mm or less and including a hardened layer on a surface, the titanium sheet including a chemical composition containing, in mass percent: Fe: 0.001 to 0.08%; and O: 0.03 to 0.08%, wherein a grain size satisfies following Formulas (1) to (3), a thickness of the hardened layer is 0.1 to 2.0 μm. The titanium sheet has both a sufficient strength and an excellent workability. Formulas (1) to (3) are dave≥2.5 (1), t/dave≥3.0 (2), and t/dmax—1.5 (3), where, in Formulas (1) to (3), t denotes the sheet thickness (μm), dave denotes an average grain size (μm), and dmax denotes a maximum grain size (μm).

Description

TECHNICAL FIELD

The present invention relates to a titanium sheet and a method for producing the titanium sheet. The titanium sheet refers to a titanium sheet having, for example, a sheet thickness of 0.2 mm or less.

BACKGROUND ART

Titanium materials have high specific strengths and excellent corrosion resistances and are widely used as a wide variety of starting materials for industrial use such as chemical plants and building materials, and as starting materials for consumer products such as camera bodies, timepieces, and sports equipment. Sheets such as foil having thicknesses of 0.2 mm or less are used as audio components (speaker diaphragms, etc.), anti-corrosion films/sheets, and the like.
In general, there is a trend for metal materials to be required to have high strengths as well as to have workabilities. Titanium materials are no exception. However, in general, when titanium materials are made to have high strengths, their workabilities drop. Thus, attempts for titanium products have been made to optimize a balance between strength and workability by controlling an amount of oxygen, an amount of iron, a grain size, and the like. Increase in an amount of oxygen causes solid-solution strengthening, increasing the strength. Increase in an amount of iron, which is a β phase stabilizing element, inhibits grain growth in a phase crystal grain boundaries to thereby refine grains, and the strength increases as well. In both cases, ductility is compromised with the increase in the strength, with which formability deteriorates.
With regard to a titanium foil of 25 μm thickness, JP2616181B (Patent Document 1) describes that a good Ericksen value can be ensured by rolling a titanium foil under predetermined rolling conditions to control a grain size into ASTM No. 12 to 14.
In contrast, good shape retention properties are required for titanium foils of 0.2 mm or less in thickness after shape working. In general, enhancing a strength of a material ensures a good shape retention property. However, as described above, a problem of decreased formability arises, and a good workability cannot be achieved.
WO 2014/027657 (Patent Document 2) discloses a titanium sheet having a sheet thickness of 0.2 mm or more, wherein bulk Fe is contained at 0.1% or less by mass, O (oxygen) is contained at 0.1% or less by mass, sheet thickness / particle size 3 is established, particle size 2.5 μm is satisfied, a hardened layer is included on its surface, and a region of the hardened layer is at a depth of 200 nm or more and 2 μm or less from the surface, so that the titanium sheet is made to be excellent in shape retention property and workability.
Documents mentioning a maximum grain size in a titanium plate include the following documents. JP2002-012931A (Patent Document 3) describes a hot-rolled titanium plate for a surface member of an electrodeposition drum, for which generation of coarse grains are avoided to improve grindability but is not intended for sheets having sheet thicknesses of 0.2 mm or less and makes no description about a relation between coarse grains and workability. JP2013-095964A (Patent Document 4) rather describes defining a lower limit of a number of coarse grains. JP2005-105387A (Patent Document 5) defines an upper limit of an abundance ratio of regions having grain sizes not less than 1.25 times a minimum average grain size, but the definition is for reducing macro patterns on a surface to improve a surface texture, and the document does not mention improvement of workability.

LIST OF PRIOR ART DOCUMENTS

Patent Document

Patent Document 1: JP2616181B
Patent Document 2: WO 2014/027657
Patent Literature 3: JP2002-012931A
Patent Literature 4: JP2013-095964A
Patent Literature 5: JP2005-105387A

SUMMARY OF INVENTION

Technical Problem

The invention described in Patent Document 2 enables titanium sheets having sheet thicknesses of 0.2 mm or less to be produced as titanium sheets having excellent workabilities and high strengths. Meanwhile, it has been found that conventional titanium sheets vary in value of elongation, which is an index of workability, degrading a balance between elongation and strength, with a result that a value of elongation may be insufficient when a predetermined strength is obtained. When elongation is insufficient, a titanium sheet cannot be processed into an intended shape, and in order to produce processed goods, it is necessary to bring at least an average value of elongation to a level that allows the processing. However, when there is great variation, the average value of elongation may fall below the level that allows the processing. Therefore, in a case of great variation, a yield of the processed goods drops as compared with a case of small variation.
The present invention has an objective to provide a titanium sheet that has both a sufficient strength and an excellent workability, and a method for producing the titanium sheet.

Solution to Problem

The gist of the present invention is as follows.
(1) A titanium sheet having a sheet thickness of 0.2 mm or less and including a hardened layer on a surface, the titanium sheet having a chemical composition containing, in mass percent:
Fe: 0.001 to 0.08%; and
O: 0.03 to 0.08%, wherein
a grain size satisfies following Formulas (1) to (3),
a thickness of the hardened layer is 0.1 to 2.0 μm, and
a maximum height Rz is 3.0 μm or less:
d_ave≥2.5 (1)
t/d _ave≥3.0 (2)
t/d _max≥1.5 (3)
where, in Formulas (1) to (3), t denotes the sheet thickness (μm), d_avedenotes an average grain size (μm), and d_maxdenotes a maximum grain size (μm).
(2) The titanium sheet according to claim 1, wherein a ratio of grains each having a grain size of t/2 or more is 15% or less in terms of number ratio.
(3) A method for producing the titanium sheet according to the above (1) or (2) having steps of cold rolling and annealing repeated on a titanium product a plurality of times, wherein the titanium product having an average grain size adjusted to 2.0 μm or less is finish cold rolled with a rolling ratio of 50 to 80% and thereafter finish annealed in an inert atmosphere at 570 to 750° C.

Advantageous Effects of Invention

According to the present invention, it is possible to reduce variation in elongation of a titanium sheet. As a result, a titanium sheet that has both a sufficient strength and an excellent workability can be obtained, and the yield of processed goods can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating a relation between sheet thickness t/average grain size d_aveand uniform elongation.

FIG. 2 is diagrams each illustrating metal micro-structures in a cross section of a titanium sheet, where FIG. 2(a) illustrates a case where coarse grains are present, and FIG. 2(b) illustrates a case where coarse grains are absent.

FIG. 3 is a graph illustrating a relation between sheet thickness t/maximum grain size d_maxand decrease of uniform elongation.

FIG. 4 is a graph of comparison of grain size distribution between materials with/without decrease in uniform elongation.

FIG. 5 is a graph illustrating a relation between coarse grain ratio and decrease of uniform elongation.

FIG. 6 is a graph illustrating a relation between rolling ratio of previous cold rolling and maximum grain size d_max/average grain size d_ave.

DESCRIPTION OF EMBODIMENTS

The present invention is intended for a pure-titanium sheet having a sheet thickness of 0.2 mm or less. A reason for limiting the sheet thickness to 0.2 mm or less is that attaining a balance between strength and workability for a sheet thickness of 0.2 mm or less is affected by grain size, to which application of the present invention is highly beneficial.
Hereafter, in the present invention, strength of a titanium sheet is evaluated in terms of 0.2% yield stress, and a workability of the titanium sheet is evaluated in terms of uniform elongation. As to pure titanium, a value of the uniform elongation decreases as the strength (0.2% yield stress) becomes higher. Also in the present invention, a target lower-limit value of the uniform elongation is to be changed according to a level of the 0.2% yield stress. Specifically, the target lower-limit value of the uniform elongation is defined as a function of the 0.2% yield stress by the following Formula [4].
0.2% yield stress≤185 MPa
Uniform elongation (%)−0.4×0.2% yield stress (MPa)+85
0.2% yield stress>185 MPa
Uniform elongation (%)−0.03×0.2% yield stress (MPa)+16.5 [4]
As to pure titanium, the 0.2% yield stress increases as oxygen concentration becomes higher or as grain size becomes smaller. The grain size becomes larger mainly with an increase in annealing temperature or an increase in annealing duration. In contrast, the grain size becomes smaller as a content of iron increases. Therefore, to bring the 0.2% yield stress to a target value, it is important to manage an oxygen concentration and an iron concentration in titanium, as well as to manage conditions for intermediate annealing and finish annealing. The 0.2% yield stress can be increased by controlling average grain size whether a grain size distribution of crystals is uniform or nonuniform.
Even a titanium sheet having a sheet thickness of 0.2 mm or less decreases in the elongation by refining grains, as with common findings. Meanwhile, as is clear from Patent Document 2, excessive coarsening grains also decrease the elongation. In particular, when t/d_avemade by a sheet thickness t (μm) and an average grain size d_ave(m) is less than 3.0, decrease in the elongation occurs. Therefore, by coarsening grains according to a sheet thickness t of a product within a range where t/d_ave≥3.0, it is possible to exploit an utmost workability of a titanium sheet having a sheet thickness of 0.2 mm or less.
In contrast, when the average grain size falls below 2.5 μm, non-recrystallized structures are likely to occur, which makes stable production difficult, and therefore the average grain size dave (μm) is set at 2.5 μm or more.
When pure titanium sheets having sheet thicknesses of 0.2 mm or less were produced with an oxygen concentration, an iron concentration, and a grain size adjusted, it has been found that there were variations in value of elongation as described above, resulting in a poor balance between elongation and strength, and caused values of elongation in case of predetermined strength were in some cases insufficient.
Hence, materials having substantially the same grain size condition were studied in detail, and as a result of observation of cross sections of titanium plates, there was a tendency for titanium plates having good values of elongation to have no coarse grains developed, while there was a tendency for titanium plates having poor values of elongation to have coarse grains having large grain sizes intermixed. Next, a tension test was conducted on JIS Class 1 pure-titanium sheets having sheet thicknesses of 0.1 mm and 0.03 mm subjected to cold rolling and annealing, to evaluate uniform elongation. FIG. 1 illustrates a graph of organized relations between uniform elongation (%) and t/dave. Note that dmax means maximum grain size (μm) of crystals present in a titanium plate, and t means sheet thickness (μm). Of titanium plates having t/d_maxof 1.5 or more (small d_max), those having sheet thicknesses of 0.1 mm are marked as ◯, and those having sheet thicknesses of 0.03 mm are marked as ●, and of titanium plates having t/d_maxless than 1.5 (large d_max), those having sheet thicknesses of 0.1 mm are marked as Δ, and those having sheet thicknesses of 0.03 mm are marked as ▴.
First, attention will be paid to marks ◯ and ● indicating small maximum grain sizes dmax. In FIG. 1, the marks ◯ and ● are on lines in which uniform elongation decreases with an increase in t/d_ave(a decrease in d_ave) where t/d_aveof a horizontal axis is three or more. This is because the strength increases with decrease in the average grain size d_ave, and the elongation decreases accordingly. Where t/d_ave<3.0, a phenomenon of decreasing elongation is seen, and therefore t/d_ave≥3.0 is specified also in the present invention.
In the same FIG. 1, it is understood that marks Δ and ▴ indicating large maximum grain sizes d_maxshow decreased elongation as compared with the marks ◯ and ● indicating small maximum grain sizes d_max, even when compared at the same average grain size d_ave. That is, it has been found that, for titanium sheets having sheet thicknesses of 0.2 mm or less, a cause of the decrease in elongation due to variations in grain size is presence of a coarse grain having a maximum grain size d_maxgiving t/d_max<1.5. FIG. 2 includes pictures each illustrating metal micro-structures in a cross section of a titanium plate having a sheet thickness of 0.03 mm, where FIG. 2(a) illustrates a case where coarse grains are present, and FIG. 2(b) illustrates a case where coarse grains are absent. In addition, in FIG. 1, traces of plots of the marks ◯ and ● are illustrated as solid lines. Next, for items of data on the marks Δ and ▴, differences between the items and the solid lines are evaluated at the same values of t/d_ave, and the differences are defined as “reduced amounts of uniform elongation (%)”. FIG. 3 illustrates the marks, with its horizontal axis representing t/d_maxand its vertical axis representing reduced amount of uniform elongation. As illustrated in FIG. 3, it has been found that decrease in elongation is not seen where t/d_maxis 1.5 or more, whereas the uniform elongation is likely to decrease with decrease in t/d_maxwhere t/d_max<1.5. That is, it is found that the elongation does not decrease if t/d_max≥1.5.
This is because, in a case where a number of grains in a plate-thickness direction is small, contribution of each grain to deformation becomes substantial, and the elongation is influenced by deformation of one grain. Titanium is highly anisotropic and exhibits different properties depending on directions of undergoing deformation, and deformation is likely to concentrate on coarse grains having poor properties. Such a relation between sheet thickness and grain size is a phenomenon that occurs irrespective of sheet thickness. For example, in a case where the sheet thickness is 0.5 mm, cases giving t/d_ave<3.0 are those where d_aveis more than 160 μm, but under normal producing conditions, dave is about 100 μm or less, and such coarse metal micro-structures never appear. However, in a case where, for example, the sheet thickness is 0.2 mm, d_avemore than about 65 μm makes t/d_ave<3.0, bringing about degradation of a property. As described above, under the normal producing conditions, d_avemay become about 100 μm, and it is understood that t/dave has to be managed in a case where the sheet thickness is 0.2 mm or less. The same is true for t/d_max. Therefore, the degradation of a property occurs in the sheet even in a case where the sheet has the same grain size distribution as that of a typical sheet.
Hence, in the present invention, it is determined that the average grain size d_aveis managed to be 2.5 or more and brought within a range that satisfies t/d_ave≥3.0 and t/d_max≥1.5 by inhibiting coarse grains from being developed in a titanium plate. As a result, it is possible to reduce variation in elongation of titanium plate and achieve a good workability.
In the present invention, by specifying t/d_max≥1.5, it is possible to achieve good elongation as described above. In addition, grain size distributions were studied in detail. For titanium plates having a sheet thickness of 0.03 mm, two grain size distributions were evaluated, which are illustrated in FIG. 4 as marks ● and ◯. Both titanium plates satisfied t/d_max≥1.5. As to the marks ◯, their values of uniform elongation were on the solid line of FIG. 1, not showing decrease in elongation, whereas the marks ● showed a slight decrease in elongation. A difference between both grain size distributions is that the marks ● occur with a high frequency where the grain size is 15 μm or more (t/2 or more). Hence, a ratio of grains having a size of t/2 or more is defined as coarse grain ratio (%), and a relation between the coarse grain ratio and the “decrease of uniform elongation” is illustrated in FIG. 5, where a horizontal axis represents the coarse grain ratio and a vertical axis represents the decrease of uniform elongation. Items of data illustrated in FIG. 5 all satisfy t/d_max≥1.5. As is clear from FIG. 5, it has been found that good elongations can be achieved more stably by setting the coarse grain ratio at 15% or less. Thus, in the present invention, the coarse grain ratio is preferably set at 15% or less.
In the present invention, a grain size distribution is determined by observing an L cross section under an optical microscope at a maximum magnification that allows an entire sheet thickness to be checked. The observation is conducted in randomly selected ten visual fields, and in each visual field, areas of grains are calculated in a region of (sheet thickness)×(length not less than ten times sheet thickness) using image analysis, and diameters of the grains are calculated by approximation using square. Using the diameters, the average grain size d_aveand the maximum grain size d_maxare calculated. In a determined grain size distribution, the coarse grain ratio is determined as a number ratio of grains having grain sizes that are not less than sheet thickness (t)/2.
On a surface after annealing, there is carbon derived from lubricant used in the rolling, and the like, which form a hardened layer. The formation of the hardened layer depends on amounts of elements adhered to the surface, and therefore, to remove the hardened layer totally, there is no choice but to remove a surface layer. However, the removal leads to a significant decrease in yield because of a small sheet thickness, and thus it is rather desirable to utilize this hardened layer. By forming the hardened layer on the surface layer within a range that does not cause deterioration of the workability, it is possible to provide scratch resistance, shape retention property, or the like. To prevent the workability from deteriorating, a thickness of the hardened layer needs to be 2.0 μm or less, and to provide the effect of scratch resistance or the like, the thickness needs to be 100 nm or more.
In the present invention, a maximum height Rz (JIS B 0601:2001) needs to be 3.0 μm or less. A maximum height Rz more than 3.0 μm fails to prevent fine cracks from developing on the surface, which degrades the balance between 0.2% yield stress and uniform elongation.
As the titanium sheet according to the present invention, use can be made of pure titanium of JIS Class 1 or Class 2. Specifically, the titanium sheet according to the present invention has the following chemical composition.
For pure titanium, a required strength and an excellent ductility are achieved typically by adjusting a content of oxygen and a content of iron. Industrially considered, oxygen is contained at 0.03% or more by mass as a lower-limit value, and 0.08% by mass is set as an upper limit. Industrially considered, iron is contained at 0.001% by mass as a lower-limit value, and 0.08% by mass is set as an upper limit. With the content of oxygen and iron set at these ranges, by adjusting the content of oxygen, the content of iron, and the average grain size according to a required strength level, it is possible to produce a titanium sheet that is excellent in workability while having a required strength. In addition to oxygen and iron, titanium and unavoidable impurities are contained.
As impurities, pure titanium contains nitrogen and carbon. When nitrogen and carbon fall within ranges of nitrogen: 0.001 to 0.08% by mass and carbon: 0.001 to 0.05% by mass, respectively, which are levels of unavoidable impurities that are normally contained, nitrogen and carbon do not have adverse effects on a quality of the titanium sheet according to the present invention.
Next, a method for producing a titanium sheet according to the present invention will be described.
In a typical process of producing a titanium sheet, a titanium product is subjected to cold rolling and annealing a plurality of times. In particular, annealing performed during the cold rolling is called “intermediate annealing”, cold rolling performed the last is called “finish cold rolling” and annealing performed after the finish cold rolling is called finish annealing. The intermediate annealing is a step of subjecting a titanium product to the recrystallization after the cold rolling. Preferable conditions for each step will be described below.
Rolling ratio of finish cold rolling: 50% to 80% It is known that, as a rolling ratio of the finish cold rolling increases, the average grain size after annealing decreases and a grain size distribution can be brought closer to uniform one. Therefore, the finish cold rolling is typically performed at least with a rolling ratio of 50% or more. However, when the finish cold rolling with a rolling ratio of 50% or more is performed on a sheet of 0.2 mm or less in thickness, the grain size distribution may become nonuniform. This is because, as described above, a number of grains in the sheet thickness direction greatly differs even in the same grain size distribution. When the rolling ratio is increased to obtain a more uniform grain distribution, a fine crack develops on a surface. In a case of a large sheet thickness, the fine crack is very small relative to the thickness, which will not degrade the property. However, in a case of a small sheet thickness, an influence of the fine crack cannot be ignored. Therefore, a rolling ratio of a cold rolling cannot be increased. In addition, carbon derived from rolling oil or the like used in the cold rolling is adhered, making the surface layer hard and susceptible to crack through the annealing, which makes it impossible to reduce a maximum height Rz to 3.0 μm or less, and thus it is necessary to set a rolling ratio of a finish cold rolling at 80% or less. However, only setting the rolling ratio of the finish cold rolling at 50 to 80% is insufficient, and it is necessary to prepare for obtaining more uniform metal micro-structure before the finish cold rolling.
Rolling ratio of cold rolling before the finish cold rolling (previous cold rolling): 30% to 80%
As described above, in a case of a titanium sheet of 0.2 mm or less in thickness, an influence of a fine crack cannot be ignored. Therefore, forming fine-grain metal micro-structures in a titanium product before the finish cold rolling makes it easy to place strain uniformly during the finish cold rolling. This is because, in titanium including coarse metal micro-structures, strain placed by rolling is maintained through twin deformation, which makes it difficult to form a dislocation cell, serving as a nucleus for recrystallization. In addition, deformation occurs on a grain basis, and therefore a nonuniform distribution of strain is less likely to occur when a deformation unit is small, which makes it easy to form uniform recrystallization nuclei.
To confirm the above, an experiment in which pure titanium of JIS Class 1 was repeatedly subjected to cold rolling and annealing to be produced into a titanium sheet was conducted. At this point, the rolling ratio of the finish cold rolling was set at 50%, and the rolling ratio of the previous cold rolling was changed to various rolling ratios. In addition, the finish annealing and intermediate annealing before the finish cold rolling (previous annealing) were performed in an Ar gas at a temperature of 670° C. for 10 mins. FIG. 6 illustrates a relation between the rolling ratio of the previous cold rolling and the maximum grain size dmax / the average grain size d_ave. A value of dmax/dave indicates a uniformity in strain placed during the finish cold rolling. In general, the maximum grain size is likely to be large in a portion where a placed strain is slight, and thus smaller d_max/d_aveindicates a strain placed more uniformly.
As illustrates in FIG. 6, the higher the rolling ratio of the previous cold rolling, the smaller d_max/d_avebecomes, and thus a strain is placed uniformly during the finish cold rolling, allowing development of coarse grains to be inhibited. From this result, the rolling ratio of the previous cold rolling is set at 30% or more, more desirably 40% or more, still more desirably 50% or more. Note that the rolling ratio of the previous cold rolling is set at 80% or less so as to prevent a crack from developing on a surface. This enables the maximum height Rz to be set at 3.0 μm or less.
Metal micro-structure before the finish cold rolling: Average grain size is set at 2.0 μm or less
As described above, controlling the rolling ratio of the cold rolling makes it easy to obtain a uniform grain size distribution. However, only controlling the rolling ratio of the cold rolling may fail to obtain a stable workability. Hence, it is effective to make metal micro-structures before the finish cold rolling fine grains, specifically, to make the metal micro-structures have an average grain size of 2.0 μm or less because, as to strain placed by cold working, fine grains allow many dislocations to be introduced by a small amount of work. Metal micro-structures having average grain sizes of 2.0 μm or less are mixed grain structures including recrystallized grains and non-recrystallized grains, or non-recrystallized structures. The non-recrystallized structures are in a phase before recrystallization and can be considered to be smaller than recrystallization nuclei. The recrystallization nuclei are of course smaller than recrystallized grains. Therefore, in a case of mixed structures including recrystallized grains and non-recrystallized grains, when an average grain size of the recrystallized grains is 2 μm or less, the non-recrystallized structures are naturally smaller than the recrystallized grains. Also in a case where all of the structures are non-recrystallized structures, recrystallization nuclei and recrystallized grains grown from the non-recrystallized structures are 2 μm or less in production within the scope of the present invention, and therefore the non-recrystallized structures can be considered to have sizes smaller than the recrystallization nuclei and the recrystallized grains. From this assumption, the finish cold rolling is enabled, and many dislocations (strain) can be provided even at a limited rolling ratio, and by the finish annealing, it is possible to obtain grains with high uniformity. As a result, a stable workability can be given to the titanium sheet.
The sheet requires, as described above, the limitation on the rolling ratio and the uniformity in grain size distribution, but the uniformity to this degree is not required for normal sheets, and surface cracks to some extent raise no problem. Therefore, the rolling ratio of the finish cold rolling can be increased to be high, and in particular, for the purpose of reducing producing steps, the rolling ratio of the finish cold rolling is typically increased by performing recrystallization sufficiently.
Temperature of intermediate annealing: 500 to 800° C.
As with the previous annealing, the intermediate annealing is preferably performed at a low temperature, at which microstructures are easily obtained. The metal micro-structures are not necessarily refined in this phase, but the intermediate annealing is desirably performed at 500 to 700° C. to obtain microstructures stably before the finish cold rolling. However, the intermediate annealing may be performed at a temperature higher than such a temperature, and in this case, the intermediate annealing needs to be performed in less than one minute. It is more desirable to perform the inteanediate annealing in less than 30 seconds, and in this manner, performing the intermediate annealing at 700 to 800° C. raises no problem.
Temperature of annealing before the finish cold rolling (previous annealing): 400 to 700° C.
A temperature of the previous annealing differs according to a difference in annealing method. In continuous annealing, it is desirable to set the temperature at 500 to 600° C. to obtain non-recrystallized structures. To obtain microstructures, the temperature may be 600 to 700° C. However, a temperature more than these temperatures causes coarse metal micro-structures to be formed by recrystallization and growth, and thus the annealing is performed at 500 to 700° C. A shorter retention duration allows microstructures to be obtained, but an excessively short retention duration makes reduction of strain accumulated in the cold rolling insufficient, failing to obtain a sufficient ductility, and thus it is preferable that the annealing is performed for about one minute as a guideline, and the retention duration is adjusted in consideration of a time taken for temperature rise and a stability of the temperature. In batch annealing, a temperature distribution develops in a coil, and nonuniformity is likely to occur, and thus a batch annealing at a low temperature for a long time is required. Therefore, the annealing may be performed at a temperature of 400 to 550° C. for about one hour, as a guideline. An excessively low temperature fails to recover the ductility sufficiently, and an excessively high temperature causes coarsening and nonuniformity.
Temperature of finish annealing: 500 to 750° C.
The average grain size d_aveis influenced mainly by the temperature and duration of the finish annealing, as well as a concentration of iron and a concentration of oxygen in a titanium product. Since the present invention specifies t/d _ave3, the upper limit of d_avediffers according to the sheet thickness, and an upper limit of the temperature of the finish annealing also differs to set t/d _ave3. When the finish annealing is to be performed in an inert atmosphere at 750° C. or less, it is possible to prevent grains from being coarsened excessively.
This annealing results in different productivities according to annealing methods. The continuous annealing enables the annealing to be performed over an entire length of a coil with stability. In addition, use can be made of carbon derived from rolling oil adhered to a surface to form a hardened layer, and if the hardened layer is insufficient because of a small amount of adhered carbon on the surface, the hardened layer can be formed by introducing nitrogen in an atmosphere or using a mixed gas of the air and an Ar gas. However, performing the annealing in the atmosphere or a nitrogen atmosphere makes discoloration or excessive formation of the hardened layer likely to occur, and for stable production, a hardened layer formed by dispersing carbon derived from rolling oil adhered to the surface. An annealing duration differs depending on the temperature or a targeted grain size, and for example, annealing at 570° C. for 5 mins causes recrystallization. To further improve productivity, it is desirable to perform the annealing at an annealing temperature of 600 to 750° C. In this case, performing the annealing for about 1 min can cause recrystallization.
The batch annealing is difficult to perform the annealing over an entire length of a coil uniformly, and it is necessary to perform the annealing at a temperature as low as 500 to 570° C. for a long time, as well as to set a rate of temperature increase and a cooling rate as low as possible, which results in a low productivity. A high temperature fails to make metal micro-structures uniform in the coil, and an excessively low temperature requires a still longer time for recrystallization and may fail to cause the recrystallization. In a case of batch finish annealing, a process thereof involves a temperature rise to 500° C. for 10 hours or more, retention for 10 hours or more, and thereafter cooling for 15 hours or more. Furthermore, there is a concern that even a process at a low temperature for a long time may fail to control metal micro-structures into predetermined metal micro-structures in a part of the coil, significantly losing yield. In addition, the annealing at a low temperature for a long time forms a thick hardened layer, the retention duration may be adjusted according to facilities, referred to 15 h or less. Therefore, for a high productivity, it is desirable to use the continuous annealing.

EXAMPLE

Pure titanium products of JIS Class 1 were repeatedly subjected to a cold rolling and an annealing, to be produced into titanium sheets. Table 1 shows chemical compositions and producing conditions of the titanium sheets.
In the table, a phrase “PREVIOUS COLD ROLLING” means cold rolling performed before the finish cold rolling, and a phrase “INITIAL COLD ROLLING” means cold rolling performed before the “previous cold rolling”. A phrase “PREVIOUS ANNEALING” means annealing performed before the finish cold rolling, and a phrase “INITIAL ANNEALING” means intermediate annealing performed before the “previous annealing”. Examples of a case where annealing durations are 1 min are examples simulating the continuous annealing, and examples of a case where annealing durations are 1 h or more are examples simulating the batch annealing. As an annealing atmosphere, an Ar gas was used except for Nos. 23 to 26 (comparative examples 10 and 11, examples 14 and 15), Nos. 24 and 25 were made in a nitrogen gas, and Nos. 23 and 26 were made in the atmosphere. Sheet thicknesses of the starting materials were adjusted by cutting or grinding, according to sheet thicknesses after the finish cold rolling and the annealing.
The grain size distribution after the final annealing was determined by observing an L cross section under an optical microscope at a maximum magnification that allowed an entire sheet thickness to be checked. The observation is conducted in randomly selected ten visual fields, and in each visual field, areas of grains are calculated in a region of (sheet thickness) x (length not less than ten times sheet thickness) using image analysis, and diameters of the grains are calculated by approximation using square. Using the diameters, the average grain size d_aveand the maximum grain size d_maxwere calculated. In a determined grain size distribution, the coarse grain ratio was determined as a number ratio of grains having grain sizes that were not less than sheet thickness (t)/2. In observation of metal micro-structures after the previous annealing, the EBSD was used to measure an average grain size of recrystallized grains, with an orientation difference of 5° or more assumed to be a grain boundary. The measurement was performed on randomly selected five visual fields each having a region of sheet thickness×length of 100 to 200 μm and separated by 0.2 μm, with a magnification of 500× or more that allows an entire sheet thickness to be checked in a visual field.
For the uniform elongation, a tension test was conduct on an ASTM 1/2 tensile test specimen taken in an L direction, at a strain rate of 12%/min until rupture, and an amount of strain reaching a maximum load point on an obtained nominal stress—nominal strain curve was evaluated as the uniform elongation.
For a thickness of the hardened layer, GDS was used to perfoun an analysis of oxygen, nitrogen, carbon, titanium, and iron in a depth direction in a region on a surface of a sample having a diameter of 4 mm by the Ar ion-sputtering, and a thickness within which a total concentration of oxygen, nitrogen, and carbon is 0.5% or more by mass is determined as the thickness of the hardened layer. For the determination at this point, zinc oxide (containing oxygen at 19.8% by mass) was used for oxygen, austenitic stainless steel (containing nitrogen at 0.3% by mass) was used for nitrogen, titanium alloy (containing carbon at 0.12% by mass) was used for carbon, and the depth was in terms of pure titanium of JIS Class 1. Results of the above are shown in Table 2. Numeric values falling out of the ranges according to the present invention are underlined. When a relation between the 0.2% yield stress and the uniform elongation did not meet Formula [4], an acceptance judgement was determined to be ×.

TABLE 1

						PRODUCING METHOD

ROLLING

		CHEMICAL	RATIO OF	INITIAL
		COMPOSITION	INITIAL	ANNEALING

(mass %)

COLD

TEMPER-

No.	CATEGORY	Fe	O	C	N	ROLLING	ATURE	DURATION

1	COMPARATIVE EXAMPLE 1	0.03	0.06	<0.01	<0.01	50%	620° C.	1 min
2	COMPARATIVE EXAMPLE 2	0.03	0.06	<0.01	<0.01	50%	620° C.	1 min
3	COMPARATIVE EXAMPLE 3	0.03	0.06	<0.01	<0.01	50%	650° C.	1 min
4	COMPARATIVE EXAMPLE 4	0.03	0.06	<0.01	<0.01	50%	500° C.	1 min
5	EXAMPLE EMBODIMENT OF	0.03	0.06	<0.01	<0.01	50%	500° C.	1 min
	PRESENT INVENTION 1
6	EXAMPLE EMBODIMENT OF	0.03	0.06	<0.01	<0.01	50%	500° C.	1 min
	PRESENT INVENTION 2
7	EXAMPLE EMBODIMENT OF	0.03	0.06	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 3
8	EXAMPLE EMBODIMENT OF	0.03	0.06	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 4
9	COMPARATIVE EXAMPLE 5	0.05	0.05	<0.01	<0.01	50%	700° C.	1 min
10	COMPARATIVE EXAMPLE 6	0.05	0.05	<0.01	<0.01	50%	700° C.	1 min
11	COMPARATIVE EXAMPLE 7	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
12	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 5
13	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 6
14	COMPARATIVE EXAMPLE 8	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
15	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 7
16	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 8
17	COMPARATIVE EXAMPLE 9	0.05	0.05	<0.01	<0.01	50%	500° C.	1 min
18	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 9
19	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 10
20	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	500° C.	1 min
	PRESENT INVENTION 11
21	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	500° C.	1 min
	PRESENT INVENTION 12
22	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 13
23	COMPARATIVE EXAMPLE 10	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
24	COMPARATIVE EXAMPLE 11	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
25	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 14
26	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 15
27	COMPARATIVE EXAMPLE 12	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
28	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 16
29	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 17
30	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 18
31	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 19
32	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
	PRESENT INVENTION 20
33	COMPARATIVE EXAMPLE 13	0.05	0.05	<0.01	<0.01	50%	550° C.	1 min
33	COMPARATIVE EXAMPLE 14	0.03	0.05	<0.01	<0.01	50%	650° C.	1 min
34	COMPARATIVE EXAMPLE 15	0.03	0.05	<0.01	<0.01	50%	650° C.	1 min
35	COMPARATIVE EXAMPLE 16	0.04	0.05	<0.01	<0.01	50%	650° C.	1 min
36	EXAMPLE EMBODIMENT OF	0.05	0.05	<0.01	<0.01	50%	650° C.	1 min
	PRESENT INVENTION 21

PRODUCING METHOD

ROLLING

METAL MICRO-

ROLLING

		RATIO OF	PREVIOUS	STRUCTURE	RATIO OF
		PREVIOUS	ANNEALING	IMMEDIATELY	FINISH	FINISH ANNEALING

		COLD	TEMPER-		BEFORE FINISH	COLD	TEMPER-
	No.	ROLLING	ATURE	DURATION	COLD ROLLING	ROLLING	ATURE	DURATION

	1	50%	600° C.	1 min	1.6	90%	670° C.	1 min
	2	90%	600° C.	1 min	1.8	70%	700° C.	1 min
	3	20%	500° C.	1 min	8.0	50%	660° C.	1 min
	4	15%	600° C.	1 min	22.0	50%	640° C.	1 min
	5	50%	520° C.	1 min	NOT	50%	680° C.	1 min
					RECRYSTALLIZED
	6	40%	520° C.	1 min	NOT	50%	670° C.	1 min
					RECRYSTALLIZED
	7	50%	520° C.	1 min	NOT	50%	650° C.	1 min
					RECRYSTALLIZED
	8	50%	500° C.	1 min	NOT	50%	630° C.	1 min
					RECRYSTALLIZED
	9	15%	600° C.	1 min	32.0	50%	800° C.	1 min
	10	50%	600° C.	1 min	22.0	50%	760° C.	1 min
	11	50%	500° C.	1 min	NOT	50%	760° C.	1 min
					RECRYSTALLIZED
	12	70%	500° C.	1 min	NOT	50%	740° C.	1 min
					RECRYSTALLIZED
	13	70%	500° C.	1 min	NOT	50%	730° C.	1 min
					RECRYSTALLIZED
	14	70%	650° C.	1 min	6.1	50%	720° C.	1 min
	15	70%	500° C.	1 min	NOT	50%	700° C.	1 min
					RECRYSTALLIZED
	16	70%	500° C.	1 min	NOT	50%	680° C.	1 min
					RECRYSTALLIZED
	17	70%	650° C.	1 min	9.0	30%	720° C.	1 min
	18	50%	500° C.	1 min	NOT	70%	750° C.	1 min
					RECRYSTALLIZED
	19	50%	500° C.	1 min	NOT	70%	740° C.	1 min
					RECRYSTALLIZED
	20	50%	600° C.	1 min	1.2	70%	730° C.	1 min
	21	50%	600° C.	1 min	1.3	80%	700° C.	1 min
	22	50%	600° C.	1 min	1.3	80%	680° C.	1 min
	23	50%	600° C.	1 min	1.4	80%	650° C.	1 min
	24	50%	600° C.	1 min	1.4	80%	630° C.	1 min
	25	50%	600° C.	1 min	1.4	80%	600° C.	1 min
	26	50%	600° C.	1 min	1.4	80%	620° C.	1 min
	27	50%	600° C.	1 min	1.4	80%	580° C.	20 h
	28	50%	600° C.	1 min	1.4	80%	580° C.	4 h
	29	50%	600° C.	1 min	1.4	80%	600° C.	1 h
	30	50%	500° C.	2 h	NOT	80%	600° C.	1 h
					RECRYSTALLIZED
	31	50%	400° C.	8 h	NOT	80%	580° C.	8 h
					RECRYSTALLIZED
	32	50%	400° C.	8 h	NOT	80%	520° C.	15 h
					RECRYSTALLIZED
	33	50%	400° C.	8 h	NOT	80%	480° C.	20 h
					RECRYSTALLIZED
	33	50%	650° C.	1 min	33.0	30%	770° C.	1 min
	34	50%	650° C.	1 min	33.0	30%	770° C.	1 min
	35	15%	600° C.	1 min	25.0	50%	770° C.	1 min
	36	70%	500° C.	1 min	NOT	50%	700° C.	2.5 h
					RECRYSTALLIZED

“METAL MICRO-STRUCTURE IMMEDIATELY BEFORE FINISH COLD ROLLING” means crystal micro structure immediately before the finish cold rolling, and its numerical values mean an average grain size (μm) of recrystallized grains

TABLE 2

	SHEET	HARDENED	COARSE

THICKNESS/

GRAIN SIZE/μm

t/d

LAYER

Rz/

GRAIN

0.2% PS/

UNIFORM

FORMULA [4]

No.

Claims

1. A titanium sheet having a sheet thickness of 0.2 mm or less and including a hardened layer on a surface, the titanium sheet having a chemical composition containing, in mass percent:

Fe: 0.001 to 0.08%; and

O: 0.03 to 0.08%, wherein

a grain size satisfies following Formulas (1) to (3),

a thickness of the hardened layer is 0.1 to 2.0 μm, and

a maximum height Rz is 3.0 μm or less:

d_ave≥2.5 (1)

t/d_ave≥3.0 (2)

t/d_max≥1.5 (3)

where, in Formulas (1) to (3), t denotes the sheet thickness (μm), d_avedenotes an average grain size (μm), and dmax denotes a maximum grain size (μm).

2. The titanium sheet according to claim 1, wherein a number ratio of grains each having a grain size of t/2 or more is 15% or less.

3. A method for producing the titanium sheet according to claim 1 having steps of cold rolling and annealing repeated on a titanium product a plurality of times, wherein the titanium product having an average grain size adjusted to 2.0 μm or less is finish cold rolled with a rolling ratio of 50 to 80% and thereafter finish annealed in an inert atmosphere at 570 to 750° C.

4. A method for producing the titanium sheet according to claim 2 having steps of cold rolling and annealing repeated on a titanium product a plurality of times, wherein the titanium product having an average grain size adjusted to 2.0 μm or less is finish cold rolled with a rolling ratio of 50 to 80% and thereafter finish annealed in an inert atmosphere at 570 to 750° C.