TITANIUM THIN SHEET

Abstract

A titanium thin sheet of 0.2 mm or less in thickness, containing: Fe of 0.1 mass % or less and O (oxygen) of 0.1 mass % or less in a bulk, wherein a sheet thickness (mm)/a grain size (mm) ≧3, and the grain size ≧2.5 μm are satisfied, and a hardened layer is included at a surface, and a region of the hardened layer is a depth of 200 nm or more and 2 μm or less from the surface. The titanium thin sheet is supplied with excellent workability and high surface hardness, and is able to be suitably used for various purposes such as, for example, acoustic components (a speaker vibration plate and so on).

Description

TECHNICAL FIELD

The present invention relates to a titanium thin sheet, in more detail, to a high-strength titanium thin sheet having excellent workability and high surface hardness, and excellent in workability capable of suitably being used for a speaker vibration plate and so on. This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2012-179861, filed on Aug. 14, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

A titanium material has high specific strength and excellent corrosion resistance, and is used for various purposes as industrial raw materials for a chemical plant, for architecture, and for many others, or as materials of consumer products such as camera bodies, clocks, sports equipments. A thin sheet such as a foil of 0.2 mm or less in thickness is used for purposes making use of characteristics thereof such as acoustic components (a speaker vibration plate and so on), an anticorrosive film, sheet.

In general, there is a tendency in which high-strength is required for metal materials, and in addition, workability is also required. The titanium material is no exception, and it is often the case in which the high-strength is also required in addition to the excellent workability. However, in general, the workability is lowered when it is highly strengthened, and therefore, in the titanium material, an attempt to optimize a balance between strength and workability has been done by controlling an oxygen amount, an iron amount, a crystal grain size, and so on.

For example, in Patent Document 1, a titanium sheet in which strength is improved while suppressing lowering of ductility of the titanium sheet by increasing an Fe content (Fe: 0.1 mass % to 0.6 mass %) while setting an O (oxygen) content in the titanium material at a predetermined value, and formability is improved by setting an average grain size to be 10 μm or less is disclosed.

In Patent Document 2, a Ti sheet material having fine forming workability whose nitrogen amount and hydrogen amount are limited in addition to an iron amount and an oxygen amount such that the Fe content is 300 ppm or more and a [Fe+O+N+H] amount is 1500 ppm or less is disclosed.

Besides, in Patent Document 3, a manufacturing method of a pure titanium sheet in which an iron amount, an oxygen amount, further nickel and chromium amounts are specified into a predetermined range and an average grain size is set to be 20 μm to 80 μm to keep fine formability even when a cheap raw material whose purity is low is used is disclosed.

However, all of the arts described in these Patent Documents are arts whose target is a general titanium material whose of 0.3 mm to 1 mm in thickness.

On the other hand, a thin sheet and a foil of 0.2 mm or less in thickness used for the speaker vibration plate and so on is thinner than a material for general purposes, and it is inferior in workability. Accordingly, there is a problem in which working failure occurs even if the arts described in the above-stated Patent Documents 1 to 3 are applied.

As for the workability of the titanium thin sheet of 0.2 mm or less in thickness, a manufacturing method of a titanium foil excellent in formability is disclosed in Patent Document 4. According to this art, a titanium foil of 25 μm in thickness is rolled under a predetermined rolling condition, and a crystal grain size is controlled to be ASTM No. 12 to 14, and thereby, the fine Erichsen value is secured.

However, in the titanium foil of 0.2 mm in thickness or less, fine shape retentivity after the forming work is required. In general, strength of a material is improved, and thereby, the fine shape retentivity is secured, but at the same time, there is a problem in which fine workability cannot be obtained. Besides, a part where a large work is performed improves in strength by work hardening and the fine shape retentivity can be obtained, but a part where working ratio is low is inferior in the shape retentinity.

For example, in Patent Document 5, an art in which a layer containing a carbide and/or nitride of titanium is formed as an inner surface layer by a bright annealing or a vacuum annealing, and thereafter, an electrolytic acid pickling is performed is disclosed. This art is one in which a contact between a soft titanium base material and a die is suppressed, to thereby prevent an adhesion of the titanium base material to the die, and at the same time, to form an oxide layer excellent in lubricity at a press time at a surface of titanium. According to this art, it is possible to avoid that the carbide and/or nitride of titanium is in contact with the die, and wear of the die is prevented.

However, it is a rare case in which a severe work as disclosed in the Patent Document 5 is performed for the titanium foil of 0.2 mm or less in thickness. For example, in the work of the speaker vibration plate and so on, it is often the case in which an internal pressure is applied to form into a dome state, and a possibility of the contact with the die during the work is small compared to a general forming by a thin-sheet press, and a surface lubricity of the material in itself is not such a problem. Accordingly, an workability improvement effect owing to a lubricating effect of the oxide is not exhibited even if the art described in the Patent Document 5 is applied. Further, in the art, the electrolytic acid pickling is performed, and therefore, lowering of yield when the art is applied for the titanium foil material of 0.2 mm or less in thickness cannot be overlooked. In addition, there is a case when shipment as a product becomes impossible caused by unevenness of the sheet thickness.

PRIOR ART DOCUMENT

Patent Document

Patent Document 1: Japanese Patent Publication No. 4605514

Patent Document 2: Japanese Laid-open Patent Publication No. S63-103043

Patent Document 3: Japanese Patent Publication No. 3228134

Patent Document 4: Japanese Patent Publication No. 2616181

Patent Document 5: Japanese Laid-open Patent Publication No. 2009-97060

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The present invention is made in consideration of circumstances as stated above, and an object thereof is to provide a titanium thin sheet of 0.2 mm or less in thickness, and excellent in shape retentivity and workability.

Means for Solving the Problems

To solve the above-stated problems, the present inventors focus attention on surface hardness of the titanium foil, and think that it is possible to enable both the shape retentivity and the workability if the surface is hard and an inner side is soft compared to the surface, and study about a method to improve the workability and the surface hardness of the titanium thin sheet.

As an effective method to improve the workability of the titanium thin sheet, at first, it is conceivable to reduce elements such as iron and oxygen. These elements are elements inevitably introduced at manufacturing time, but as described in the Patent Documents 1 to 3, it is necessary to limit to a predetermined amount or less.

Next, it is conceivable to make crystal grains coarse. It is possible to make a twinning deformation which is important for the workability of the titanium material easily occur by making the grain coarse, and the workability is improved. A crystal grain size is controlled at a finish annealing process at the last, and therefore, it is easily controlled by changing annealing conditions.

A tensile test is performed by using a titanium thin sheet with a sheet thickness of 0.2 mm or less to investigate elongation. As a result, the elongation is lowered by refining the crystal grain as same as a general knowledge also in the sheet thickness of 0.2 mm or less. However, it turns out that there is a case when the elongation is lowered if the crystal grain becomes too coarse in the titanium thin sheet of 0.2 mm or less in thickness. Besides, whether or not this phenomenon occurs is determined by a ratio between the sheet thickness and the grain size, and it turns out that this phenomenon occurs when the sheet thickness/the grain size <3. Note that in case of the thin sheet of approximately 0.3 mm to 1 mm in thickness, the phenomenon in which the elongation is lowered by the coarseness of the crystal grain does not occur because the grain size is approximately within a range of 10 μm to 60 μm.

From this investigation result, the crystal grain is made coarse within a range in which the sheet thickness/the grain size ≧3 in accordance with the product sheet thickness, and thereby, it becomes possible to exploit the workability of the titanium thin sheet of 0.2 mm or less in thickness to the maximum.

In a process further advancing the investigation, there is a case when cracks frequently occur at a press working time, and a cause thereof is investigated, then it turns out that a carbon amount and a nitrogen amount in a vicinity of a material surface are high at a part where the cracks occur. Normally, when the thin sheet of 0.2 mm or less in thickness is manufactured, a bright annealing (BA) to give formability and workability by softening is performed after a cold-rolling. However, when removal of rolling oil at a cleaning line before the annealing is insufficient, a lot of rolling oil remains at the material surface, and an entering amount of carbon in the vicinity of the material surface becomes large. Nitrogen is nitrogen gas remained at a gas exchange time of an annealing furnace, and when the exchange is insufficient, a lot of nitrogen remains, and an entering amount of nitrogen becomes large.

The entered carbon, nitrogen form TiC, TiN, incur solid-solution strengthening, and therefore, the surface hardness becomes high, and the shape retentivity becomes good also in an ultrathin shape titanium thin sheet of 0.2 mm or less in thickness. However, when they enter too deep, the elongation of the material is remarkably lowered. It is necessary to set entering depths of carbon, nitrogen, oxygen to be within a range of 200 nm to 2 μm from a surface to enable the above-stated both characteristics (namely, the improvement in the surface hardness and the suppression of the elongation lowering). Namely, it is necessary that a region of a hardened layer formed by the entering of carbon, nitrogen, oxygen is to be within the range of 200 nm to 2 μm from the surface.

The present invention is made based on the studied information, and a content thereof is a high-strength titanium thin sheet excellent in workability described below.

Namely, it is a titanium thin sheet of 0.2 mm or less in thickness, which contains Fe of 0.1 mass % or less and O (oxygen) of 0.1 mass % or less in a bulk, satisfies a sheet thickness (mm)/a grain size (mm) ≧3, and a grain size ≧2.5 μm, includes a hardened layer at a 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.

After a cold-rolling, it is desirable if a finish annealing (bright annealing) is performed for the titanium thin sheet of the present invention at 500° C. or more and 850° C. or less by a BAF (batch heat treatment) or a continuous annealing because stable workability is thereby secured.

The “titanium thin sheet” described here means industrial pure titanium defined by JISH4600, and a thin sheet or a foil of 0.2 mm or less in thickness.

The “grain size” means an average grain size found by a quadrature defined by JISH0501. There is a case when it is described as an “average grain size” with an emphasis on the above.

Besides, the “hardened layer” is a concentrated layer of oxygen, nitrogen, carbon formed at an annealing time by carbon, nitrogen and oxygen comes from rolling oil remaining at a surface, and nitrogen and oxygen gas contained in a gas atmosphere of an annealing furnace.

Effect of the Invention

A titanium thin sheet of the present invention is a titanium thin sheet of 0.2 mm or less in thickness, where excellent workability and high surface hardness are given, and is a titanium thin sheet (foil) capable of suitably being used for various purposes such as, for example, acoustic components (speaker vibration plate, and so on).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view exemplifying a relationship between a crystal grain size and elongation in a tensile test of a titanium thin sheet.

FIG. 2 is a view exemplifying a relationship between a stress and a strain in the tensile test of a titanium thin sheet (foil) with a thickness of 25 μm.

FIG. 3 is a view exemplifying a relationship between a sheet thickness/a grain size and elongation in the tensile test of the titanium thin sheet.

FIG. 4 is a view illustrating a relationship between a hardened layer thickness and a surface hardness at the titanium thin sheet.

FIG. 5 is a view exemplifying a relationship between a hardened layer thickness and elongation at the titanium thin sheet of 100 μm in thickness, and a sheet thickness/a grain size ≧3.

MODE FOR CARRYING OUT THE INVENTION

A titanium thin sheet of the present invention is a titanium thin sheet of 0.2 mm or less in thickness, which contains Fe of 0.1 mass % or less and O (oxygen) of 0.1 mass % or less in a bulk, satisfies a sheet thickness (mm)/a grain size (mm) ≧3, and a grain size ≧2.5 μm, includes a hardened layer at a 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.

In the present invention, a reason why the titanium thin sheet of 0.2 mm or less in thickness is intended for is to provide a high-strength titanium thin sheet excellent in workability capable of suitably being used also for, for example, the speaker vibration plate and so on.

In the titanium thin sheet of the present invention, a reason why the bulk Fe is defined to be 0.1 mass % or less is as described below. Namely, Fe is an element stabilizing β-phase, and when the β-phase exists, a growth of a crystal grain is disturbed by the β-phase during an annealing. When a content exceeds 0.1 mass %, a function thereof becomes remarkable, and therefore, the content of Fe is set to be 0.1 mass % or less. A lower limit is not particularly limited, but mixture of Fe is inevitable when it is industrially manufactured, and 0.01 mass % or more is contained, and therefore, a desirable lower limit is set to be 0.01 mass %.

Besides, a reason why the bulk O (oxygen) is defined to be 0.1 mass % or less is to suppress lowering of workability. O is added, and thereby, the titanium thin sheet is highly strengthened, but the workability is lowered, and when a content exceeds 0.1 mass %, a tendency thereof becomes remarkable, and therefore, the content of O is set to be 0.1 mass % or less. A lower limit is not particularly limited, but mixture of O is inevitable when it is industrially manufactured as same as Fe, and therefore, a desirable lower limit is set to be 0.01 mass %.

Note that the bulk means an inside of the titanium thin sheet except the hardened layer formed at a surface of the titanium thin sheet. In the present invention, an Fe concentration is 0.1 mass or less, and an O concentration is 0.1 mass % or less in the bulk.

In the titanium thin sheet of the present invention, a reason why the grain size ≧2.5 μm is to be satisfied is that when the grain size is less than 2.5 μm, the elongation is largely lowered, and it is inferior in the workability as illustrated in FIG. 1.

FIG. 1 is a view exemplifying a relationship between the crystal grain size and the elongation in a tensile test of the titanium thin sheet. As illustrated in the drawing, when the crystal grain size is less than 2.5 μm, it becomes too high-strengthened even if a non-recrystallized grain does not exist, and therefore, the elongation is largely lowered.

In the titanium thin sheet of the present invention, a reason why the sheet thickness (mm)/the grain size (mm) ≧3 (hereinafter, “the sheet thickness (mm)/the grain size (mm)” is just referred to as “the sheet thickness/the grain size”) is to be satisfied is as described below.

FIG. 2 is a view exemplifying a relationship between a stress and a strain in the tensile test of a titanium thin sheet (foil) of 25 μm in thickness. In the drawing, “the grain size: 5.3 μm” and “the grain size: 12.3 μm” are measurement results as for test pieces whose average grain sizes are respectively 5.3 μm and 12.3 μm.

As illustrated in FIG. 2, in any of the test pieces, after passing through a uniform elongation state, it starts local deformation, and reaches a fracture. A local deformation amount is small, and a uniform deformation amount, namely, the uniform elongation is an index of the workability, and when the uniform elongation is lowered, it means the lowering of the workability.

In a deformation of a polycrystalline material, when one grain deforms, relaxation of deformation occurs by crystal grains at a periphery thereof. However, when the number of crystal grains are small relative to a sheet thickness direction, a contribution to the deformation of one crystal grain becomes large, the deformation progresses at a specific crystal grain, and therefore, the local deformation early starts. FIG. 2 illustrates this state.

Accordingly, by the number of crystal grains existing in the sheet thickness direction, namely, by the ratio of the sheet thickness/the grain size, an upper limit of a range of an average crystal grain size capable of improving the workability by coarsening is determined.

FIG. 3 is a view exemplifying a relationship between the sheet thickness/the grain size and the elongation in the tensile test of the titanium thin sheet. As illustrated in FIG. 3, the elongation is remarkably lowered at around the sheet thickness/the grain size=3 in any of the titanium thin sheets of 25 μm to 150 μm in thickness, and it can be seen that it is necessary to satisfy the sheet thickness (mm)/the grain size (mm) ≧3.

Further, in the titanium thin sheet of the present invention, it is necessary to have the hardened layer at the region at the depth of 200 nm or more and 2 μm or less from the surface. In other words, it is necessary to have the hardened layer of 200 nm to 2 μm in thickness in the vicinity of the surface.

The hardened layer is the concentrated layer of oxygen, nitrogen and carbon formed at the annealing time by carbon, nitrogen and oxygen comes from the rolling oil remaining at the surface, nitrogen and oxygen gas contained in the gas atmosphere of the annealing furnace, and is a region containing oxygen of 0.5 mass % or more, a region containing nitrogen of 0.5 mass % or more, a region containing carbon of 0.5 mass % or more, or a region containing oxygen, nitrogen, and carbon of 0.5 mass % or more as a total. Note that the thickness of the hardened layer is able to be measured by a GDS (Glow discharge optical emission spectrometer).

FIG. 4 is a view illustrating a relationship between the hardened layer thickness and the surface hardness in the titanium thin sheet. As illustrated in FIG. 4, the thicker the thickness of the hardened layer is, the higher the surface hardness becomes. When the thickness of the hardened layer is thinner than 200 nm, it is the same degree as a material hardness (illustrated in FIG. 4) measured at a cross section of the material, and an increase of the hardness is not recognized. Besides, when the increase of the surface hardness is insufficient, it is inferior in the shape retentivity. Accordingly, the thickness of the hardened layer is set to be 200 nm or more.

FIG. 5 is a view exemplifying a relationship between the hardened layer thickness and the elongation in the titanium thin sheet of 100 μm in thickness, and the sheet thickness/the grain size ≧3. As illustrated in FIG. 5, even when the sheet thickness/the grain size ≧3, the elongation is lowered if the hardened layer thickness is too thick to lead to the lowering of the workability, and therefore, the hardened layer thickness is set to be 2000 nm (2 μm) or less.

The titanium thin sheet of the present invention is desirable if the finish annealing is performed at 500° C. or more and 850° C. or less by the BAF or the continuous annealing after the cold-rolling, because stable workability is secured.

When an annealing temperature is low, the non-recrystallized grain remains, and the workability is lowered. A recrystallization temperature of the titanium thin sheet of the present invention is 500° C., and therefore, the finish annealing is performed at 500° C. or more. Besides, the finish annealing is performed at 850° C. or less to obtain an equiaxed structure in which a balance between excellent strength and ductility (elongation) is easy to obtain. An operation in accordance with an object of an annealing process is performed also in a normal operation, but the workability is stably secured by performing the finish annealing under the above-stated desirable temperature condition.

The thickness of the hardened layer is able to be set to an objected thickness by, for example, changing a remaining amount of the rolling oil at the cleaning process normally performed after the cold rolling, and changing the remaining nitrogen and the oxygen amount of the bright annealing furnace.

EXAMPLE

To verify effects of the present invention, the following tests are performed.

At first, cold-rolled sheets of 25 μm to 150 μm in thickness are manufactured as for one kind of pure titanium (thickness of 0.5 mm) defined by JISH4600 by passing through the cold rolling and an intermediate annealing. Subsequently, the finish annealing is performed while changing conditions in an Ar atmosphere (dew point≦−40° C.) to thereby change crystal grain sizes variously. Besides, the hardened layer is formed at a surface of the sheet by concentrating any of oxygen, nitrogen, carbon by the rolling oil remained at the surface of the sheet and the gas atmosphere of the annealing furnace. The thickness (depth) of the hardened layer is adjusted by changing the remaining amount of the rolling oil and the nitrogen amount and the oxygen amount in the atmosphere at the bright annealing time.

Each of these cold-rolled sheets (each test piece) after the finish annealing is processed into a test piece with a parallel part of 6.25 mm in width, a parallel part of 50 mm in length, and thereafter, a tensile test is performed. Besides, a sheet thickness, a crystal grain size, surface hardness, and a thickness of the hardened layer are measured as for each test piece. Fe concentration (bulk mass %), O concentration (bulk mass %) of each test piece used for the example, and each measurement result are illustrated together in Table 1.

	TABLE 1
					SHEET	HARDENED
			SHEET	CRYSTAL	THICK-	LAYER	SURFACE	0.2%
			THICK-	GRAIN	NESS/	THICK-	HARD-	PROOF	TENSILE	ELONGA-
	Fe	O	NESS	SIZE	GRAIN	NESS	NESS	STRESS	STRENGTH	TION
	(mass %)	(mass %)	(μm)	(μm)	SIZE	(nm)	(HV0.025)	(MPa)	(MPa)	(%)
COMPARATIVE	0.04	0.05	25	NON-	—		202	329	420	5.9
EXAMPLE 1				RECRYSTAL-
				LIZED
				GRAIN
EXAMPLE 1	0.04	0.05	25	2.6	9.6	320	152	284	406	12.7
EXAMPLE 2	0.04	0.05	25	3.1	8.1	480	151	285	387	13.6
EXAMPLE 3	0.04	0.05	25	3.7	6.8	450	156	248	365	15.5
EXAMPLE 4	0.04	0.05	25	7	3.6	400	157	191	318	13.2
EXAMPLE 5	0.04	0.05	25	8.2	3	300	155	206	336	15
COMPARATIVE	0.04	0.05	25	12.3	2	460	156	142	261	11.4
EXAMPLE 2
COMPARATIVE	0.04	0.05	25	20.1	1.2	320	149	137	258	7.3
EXAMPLE 3
COMPARATIVE	0.04	0.05	50	NON-	—		190	362	459	13.9
EXAMPLE 4				RECRYSTAL-
				LIZED
				GRAIN
EXAMPLE 6	0.04	0.05	50	2.8	17.9	470	145	281	432	22.4
EXAMPLE 7	0.04	0.05	50	3.4	14.7	490	150	291	406	24.6
EXAMPLE 8	0.04	0.05	50	5.3	9.4	510	154	238	362	25
EXAMPLE 9	0.04	0.05	50	9.3	5.4	500	158	191	329	24.1
EXAMPLE 10	0.04	0.05	50	12.4	4	500	152	168	299	22.6
COMPARATIVE	0.04	0.05	50	19.9	2.5	510	147	151	271	18.4
EXAMPLE 5
COMPARATIVE	0.04	0.05	50	23.3	2.2	520	153	157	264	14.9
EXAMPLE 6
COMPARATIVE	0.03	0.03	100	2.1	47.6	480	151	272	372	25.1
EXAMPLE 7
EXAMPLE 11	0.03	0.03	100	3	33.3	500	156	250	353	30.4
EXAMPLE 12	0.03	0.03	100	3.2	31.3	530	155	223	354	30.2
EXAMPLE 13	0.03	0.03	100	4.7	21.2	450	151	187	341	33.7
EXAMPLE 14	0.03	0.03	100	8.2	12.2	630	158	151	315	33
EXAMPLE 15	0.03	0.03	100	11.6	8.6	1080	198	147	302	34.4
COMPARATIVE	0.03	0.03	100	15.6	6.4	2510	262	216	306	26.3
EXAMPLE 8
EXAMPLE 16	0.03	0.03	100	17.9	5.6	1160	184	145	303	35.2
EXAMPLE 17	0.03	0.03	100	31.3	3.2	1760	199	151	302	33
COMPARATIVE	0.03	0.03	100	30.2	3.3	180	135	90	288	36.1
EXAMPLE 9
COMPARATIVE	0.03	0.03	100	33.1	3	2300	242	182	298	27.7
EXAMPLE 10
COMPARATIVE	0.03	0.03	100	38.7	2.6	1820	227	147	282	29.3
EXAMPLE 11
COMPARATIVE	0.03	0.03	100	56.1	1.8	2430	254	167	278	25.2
EXAMPLE 12
COMPARATIVE	0.03	0.03	150	1.9	78.9	410	171	279	388	30.1
EXAMPLE 13
EXAMPLE 18	0.03	0.03	150	2.6	57.7	500	162	250	372	33.4
EXAMPLE 19	0.03	0.03	150	5.3	28.3	480	159	186	330	36.2
EXAMPLE 20	0.03	0.03	150	8.1	18.4	420	160	158	311	35
EXAMPLE 21	0.03	0.03	150	12.5	12	410	155	148	302	40.6
EXAMPLE 22	0.03	0.03	150	28.8	5.2	1810	246	150	289	39.5
COMPARATIVE	0.03	0.03	150	31.5	4.8	190	138	93	279	41.2
EXAMPLE 14
EXAMPLE 23	0.03	0.03	150	45	3.3	1930	254	141	281	37.6
COMPARATIVE	0.03	0.03	150	44.6	3.4	160	136	85	274	41.5
EXAMPLE 15
COMPARATIVE	0.03	0.03	150	52	2.9	2120	253	145	257	28.7
EXAMPLE 16
COMPARATIVE	0.03	0.03	150	68.3	2.2	600	168	91	252	28.3
EXAMPLE 17

The tensile test is performed in a direction (L direction) in parallel to a rolling direction, under conditions of a strain rate of 0.5%/min up to 0.2% proof stress, and thereafter, 20%/min up to fracture, under a room temperature.

The crystal grain size is found by using the quadrature and square approximation as for a region of 40,000 μm² or more of a sample surface.

As for the surface hardness, a Vickers hardness meter is used, a Vickers indenter is pressed onto the sample surface with a load of 0.245 N (25 gf), and it is evaluated by an average value of 10 points.

The thickness of the hardened layer is set to be a thickness in which a depth direction analysis of each of oxygen, nitrogen, carbon, titanium, and iron is performed by an Ar ion sputtering by using the GDS, at a region of 4 mm in diameter of the sample surface, and any of concentrations of oxygen, nitrogen, and carbon, or a total concentration of these becomes 0.5 mass % or more. As for quantification, each measurement value is calibrated by using each of zinc oxide (oxygen: 19.8 mass %) as for oxygen, austenitic stainless steel (nitrogen content: 0.3 mass %) as for nitrogen, titanium alloy (carbon content: 0.12 mass %) as for carbon, to be corresponded to a measurement portion (depth) of pure titanium (JIS one kind) to thereby perform the depth direction analysis of each element.

In Table 1, characteristic values of the titanium material change depending on the sheet thickness, contents (bulk concentration) of Fe, O, and therefore, they are each compared under approximately the same condition. Besides, even when the sheet thickness, the contents of Fe, O are the same, they are affected by the grain size, and therefore, the comparison is performed in consideration of the grain size. Note that it is a problem in the shape retentivity in which the shape deforms by deformation of a part where a processing amount is small, and therefore, the shape retentivity is able to be evaluated at a value of 0.2% proof stress at each sheet thickness.

A comparative example 1 and a comparative example 4 are both cases when non-recrystallized grains remain, and the elongations are remarkably low.

Each of comparative examples 2, 3, 5, 6, 11, 12, 16, 17 is a case when (the sheet thickness/the grain size) <3, and the elongation is remarkably low. In particular, the elongation of the comparative example 17 is lower than examples 18 to 23.

A comparative example 7 and a comparative example 13 are both cases when the crystal grain sizes are too fine (less than 2.5 μm), and the elongations are low.

Each of comparative examples 8, 10, 12 is a case when the thickness of the hardened layer is larger than the thickness (200 nm or more and 2 μm or less) defined in the present invention, and the elongation is low. In particular, in the comparative example 12, the sheet thickness/the grain size is less than 3, and the hardened layer is thick, and therefore, the elongation is lower than examples 11 to 17. In a comparative example 16, the sheet thickness/the grain size is less than 3, and the hardened layer is also thick, and therefore, the elongation is lower than examples 18 to 23.

In each of comparative examples 9, 14, 15, the thickness of the hardened layer is thin (less than 200 nm), 0.2% proof stress is low, and the shape retentivity is not good. In particular, in the comparative example 14, the proof stress is remarkably low compared to the example 22 having approximately the same grain size. In the comparative example 15, the proof stress is remarkably low compared to the example 23 having approximately the same grain size.

When they are summarized by the same sheet thickness, results are as follows.

“As for 25 μm material”

The comparative example 1 is a non-recrystallized structure, and therefore, the elongation is low.

In each of the comparative examples 2, 3, the sheet thickness/the grain size is less than 3, and the elongation, the proof stress, and the tensile strength are low compared to the examples 1 to 5.

“As for 50 μm material”

The comparative example 4 is the non-recrystallized structure, and therefore, the elongation is low.

In each of the comparative examples 5, 6, the sheet thickness/the grain size is less than 3, and the elongation, the proof stress, and the tensile strength are low compared to the examples 6 to 10.

“As for 100 μm material”

The comparative example 7 is too grain-refined, and therefore, the elongation is low.

In the comparative example 8, the sheet thickness/the grain size ≧3 is satisfied, but the hardened layer is thick, and the elongation is low.

In the comparative example 9, the hardened layer is thin, and the proof stress is remarkably low compared to the example 17 having approximately the same grain size.

In the comparative example 10, the hardened layer is thick, and the elongation is low compared to the examples 11 to 17.

In the comparative example 11, the sheet thickness/the grain size is less than 3, and the elongation is low compared to the examples 11 to 17.

In the comparative example 12, the sheet thickness/the grain size is less than 3, the hardened layer is also thick, and therefore, the elongation is low compared to the examples 11 to 17.

“As for 150 μm material”

The comparative example 13 is too grain-refined, and therefore, the elongation is low.

In the comparative example 14, the hardened layer is thin, and the proof stress is remarkably low compared to the example 22 having approximately the same grain size.

In the comparative example 15, the hardened layer is thin, and the proof stress is remarkably low compared to the example 23 having approximately the same grain size.

In the comparative example 16, the sheet thickness/the grain size is less than 3, the hardened layer is also thick, and therefore, the elongation is low compared to the examples 18 to 23.

In the comparative example 17, the sheet thickness/the grain size is less than 3, and the elongation is low compared to the examples 18 to 23.

On the other hand, each of the examples 1 to 23 is a case when the conditions defined in the present invention are satisfied, and exhibits high elongation and surface hardness.

INDUSTRIAL APPLICABILITY

The titanium thin sheet of the present invention includes excellent workability and high surface hardness, and is able to be used for wide purposes as materials of consumer products and for industries such as, for example, a speaker vibration plate.

Claims

1. A titanium thin sheet of 0.2 mm or less in thickness, containing:

Fe of 0.1 mass % or less and O (oxygen) of 0.1 mass % or less in a bulk,

wherein a sheet thickness (mm)/a grain size (mm) ≧3, and the grain size ≧2.5 μm are satisfied, and

a hardened layer is included at a surface, and a region of the hardened layer is a depth of 200 nm or more and 2 μm or less from the surface.

2. The titanium shin sheet according to claim 1,

wherein after a cold-rolling, a finish annealing is performed at 500° C. or more and 850° C. or less by a BAF or a continuous annealing.