US20190226073A1 - Titanium sheet and method for producing the same - Google Patents

Titanium sheet and method for producing the same Download PDF

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US20190226073A1
US20190226073A1 US16/314,323 US201616314323A US2019226073A1 US 20190226073 A1 US20190226073 A1 US 20190226073A1 US 201616314323 A US201616314323 A US 201616314323A US 2019226073 A1 US2019226073 A1 US 2019226073A1
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grain size
titanium
annealing
sheet
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Hidenori Takebe
Reita CHIDA
Satoshi Matsumoto
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
    • C22F1/18—High-melting or refractory metals or alloys based thereon
    • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C14/00—Alloys based on titanium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/02—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working in inert or controlled atmosphere or vacuum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
    • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
    • C22F1/18—High-melting or refractory metals or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
    • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working

Definitions

  • 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.
  • 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.
  • titanium materials 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 ⁇ 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.
  • 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.
  • 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 ⁇ m 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 ⁇ m or less from the surface, so that the titanium sheet is made to be excellent in shape retention property and workability.
  • Patent 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.
  • Patent Document 5 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.
  • 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
  • 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.
  • value of elongation which is an index of workability, degrading a balance between elongation and strength
  • 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.
  • the gist of the present invention is as follows.
  • 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:
  • a thickness of the hardened layer is 0.1 to 2.0 ⁇ m
  • a maximum height Rz is 3.0 ⁇ m or less:
  • t denotes the sheet thickness ( ⁇ m)
  • d ave denotes an average grain size ( ⁇ m)
  • d max denotes a maximum grain size ( ⁇ m).
  • 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.
  • FIG. 1 is a graph illustrating a relation between sheet thickness t/average grain size d ave and 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 max and 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 .
  • 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.
  • a target lower-limit value of the uniform elongation is to be changed according to a level of the 0.2% yield stress.
  • 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].
  • 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.
  • 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.
  • the average grain size dave ( ⁇ m) is set at 2.5 ⁇ m or more.
  • FIG. 1 illustrates a graph of organized relations between uniform elongation (%) and t/dave.
  • dmax means maximum grain size ( ⁇ m) of crystals present in a titanium plate
  • t means sheet thickness ( ⁇ m).
  • titanium plates having t/d max of 1.5 or more 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 max less than 1.5 (large 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 ⁇ .
  • marks ⁇ and ⁇ indicating small maximum grain sizes dmax.
  • 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 ave of 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.
  • 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.
  • 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.
  • FIG. 2( a ) illustrates a case where coarse grains are present
  • FIG. 2( b ) illustrates a case where coarse grains are absent
  • traces of plots of the marks ⁇ and ⁇ are illustrated as solid lines.
  • 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 max and its vertical axis representing reduced amount of uniform elongation. As illustrated in FIG.
  • d ave more than about 65 ⁇ m makes t/d ave ⁇ 3.0, bringing about degradation of a property.
  • d ave may become about 100 ⁇ m, 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.
  • the average grain size d ave is 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.
  • t/d ave ⁇ 3.0 and t/d max ⁇ 1.5
  • 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.
  • 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 ave and the maximum grain size d max are 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.
  • 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.
  • 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.
  • a thickness of the hardened layer needs to be 2.0 ⁇ m or less, and to provide the effect of scratch resistance or the like, the thickness needs to be 100 nm or more.
  • a maximum height Rz (JIS B 0601:2001) needs to be 3.0 ⁇ m or less.
  • a maximum height Rz more than 3.0 ⁇ m fails to prevent fine cracks from developing on the surface, which degrades the balance between 0.2% yield stress and uniform elongation.
  • 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.
  • a required strength and an excellent ductility are achieved typically by adjusting a content of oxygen and a content of iron.
  • 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.
  • iron is contained at 0.001% by mass as a lower-limit value, and 0.08% by mass is set as an upper limit.
  • pure titanium contains nitrogen and carbon.
  • 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.
  • a titanium product is subjected to cold rolling and annealing a plurality of times.
  • annealing performed during the cold rolling is called “intermediate annealing”
  • cold rolling performed the last is called “finish cold rolling”
  • finish 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.
  • the fine crack is very small relative to the thickness, which will not degrade the property.
  • an influence of the fine crack cannot be ignored. Therefore, a rolling ratio of a cold rolling cannot be increased.
  • 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 ⁇ m or less, and thus it is necessary to set a rolling ratio of a finish cold rolling at 80% or less.
  • 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.
  • 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 ave indicates a strain placed more uniformly.
  • 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.
  • 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 ⁇ m or less.
  • Average grain size is set at 2.0 ⁇ m or less
  • controlling the rolling ratio of the cold rolling makes it easy to obtain a uniform grain size distribution.
  • only controlling the rolling ratio of the cold rolling may fail to obtain a stable workability.
  • 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 ⁇ m 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 ⁇ m 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 ⁇ m or less, the non-recrystallized structures are naturally smaller than the recrystallized grains.
  • recrystallization nuclei and recrystallized grains grown from the non-recrystallized structures are 2 ⁇ m 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.
  • 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.
  • 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.
  • 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.
  • a temperature of the previous annealing differs according to a difference in annealing method.
  • the temperature may be 600 to 700° C.
  • 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.
  • 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 ave is 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 ave 3, the upper limit of d ave differs according to the sheet thickness, and an upper limit of the temperature of the finish annealing also differs to set t/d ave 3. 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • a phrase “PREVIOUS COLD ROLLING” means cold rolling performed before the finish cold rolling
  • 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
  • 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.
  • 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 ave and the maximum grain size d max were 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.
  • 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 ⁇ m and separated by 0.2 ⁇ m, with a magnification of 500 ⁇ or more that allows an entire sheet thickness to be checked in a visual field.
  • 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.
  • zinc oxide containing oxygen at 19.8% by mass
  • austenitic stainless steel containing nitrogen at 0.3% by mass
  • titanium alloy containing carbon at 0.12% by mass
  • 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 ⁇ .
  • ROLLING ATURE DURATION COLD ROLLING ROLLING ATURE DURATION 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.
  • finish annealing temperatures were changed within the range according to the present invention and the average grain sizes are changed to obtain various strengths as the 0.2% yield stress.
  • a finish annealing temperature was increased, an average grain size became larger, a 0.2% yield stress decreased, and a value of uniform elongation increased.
  • example embodiments 1 to 4 of the present invention (sheet thickness 0.03 mm), example embodiments 5 to 8 of the present invention (sheet thickness 0.1 mm), and the present inventions 9 to 20 (sheet thickness 0.2 mm)
  • their chemical compositions and producing conditions fell within the respective ranges specified in the present invention, and d ave 2.5 ⁇ m, t/d ave ⁇ 3, t/d max ⁇ 1.5, and thickness of hardened layer: 0.1 to 2.0 ⁇ m were satisfied.
  • both of their 0.2% yield stresses and uniform elongations satisfied Formula [4], and thus good uniform elongations according to strength levels were successfully obtained.
  • a comparative example 10 an example embodiment 15 of the present invention, a comparative example 11, and an example embodiment 14 of the present invention, their hardened layers are intentionally formed by performing the annealing in the atmosphere in the comparative example 10 and the example embodiment 15 of the present invention and in the nitrogen atmosphere in the comparative example 11 and the example embodiment 14 of the present invention.
  • the annealing was performed in vacuum for a long time to disperse carbon derived from rolling oil remaining on its surface, forming the hardened layer.
  • their hardened layers had thicknesses of 2 ⁇ m or more, and their elongations were poorer than those of example embodiments 14 and 15 of the present invention, failing to satisfy Formula [4].

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