TITANIUM SHEET AND METHOD FOR PRODUCING THE SAME
20190226073 ยท 2019-07-25
Assignee
Inventors
Cpc classification
C22F1/18
CHEMISTRY; METALLURGY
International classification
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 m. The titanium sheet has both a sufficient strength and an excellent workability. Formulas (1) to (3) are d.sub.ave2.5 (1), t/d.sub.ave3.0 (2), and t/d.sub.max1.5 (3), where, in Formulas (1) to (3), t denotes the sheet thickness (m), d.sub.ave denotes an average grain size (m), and d.sub.max denotes a maximum grain size (m).
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 m, and a maximum height Rz is 3.0 m or less: d.sub.ave2.5 (1) t/d.sub.ave3.0 (2) t/d.sub.max1.5 (3) where, in Formulas (1) to (3), t denotes the sheet thickness (m), d.sub.ave denotes an average grain size (m), and dmax denotes a maximum grain size (m).
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 m 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 m 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.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0026]
[0027]
[0028]
[0029]
[0030]
[0031]
DESCRIPTION OF EMBODIMENTS
[0032] 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.
[0033] 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 stress185 MPa
Uniform elongation (%)0.40.2% yield stress (MPa)+85
0.2% yield stress>185 MPa
Uniform elongation (%)0.030.2% yield stress (MPa)+16.5 [4]
[0034] 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.
[0035] 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.sub.ave made by a sheet thickness t (m) and an average grain size d.sub.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.sub.ave3.0, it is possible to exploit an utmost workability of a titanium sheet having a sheet thickness of 0.2 mm or less.
[0036] In contrast, when the average grain size falls below 2.5 m, non-recrystallized structures are likely to occur, which makes stable production difficult, and therefore the average grain size dave (m) is set at 2.5 m or more.
[0037] 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.
[0038] 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.
[0039] First, attention will be paid to marks and .circle-solid. indicating small maximum grain sizes dmax. In
[0040] In the same
[0041] 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.sub.ave<3.0 are those where d.sub.ave is more than 160 m, but under normal producing conditions, dave is about 100 m 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.sub.ave more than about 65 m makes t/d.sub.ave<3.0, bringing about degradation of a property. As described above, under the normal producing conditions, d.sub.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.sub.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.
[0042] Hence, in the present invention, it is determined that the average grain size d.sub.ave is managed to be 2.5 or more and brought within a range that satisfies t/d.sub.ave3.0 and t/d.sub.max1.5 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.
[0043] In the present invention, by specifying t/d.sub.max1.5, 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
[0044] 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.sub.ave and the maximum grain size d.sub.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.
[0045] 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 m or less, and to provide the effect of scratch resistance or the like, the thickness needs to be 100 nm or more.
[0046] In the present invention, 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] Next, a method for producing a titanium sheet according to the present invention will be described.
[0051] 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.
[0052] 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 m 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.
[0053] Rolling ratio of cold rolling before the finish cold rolling (previous cold rolling): 30% to 80%
[0054] 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.
[0055] 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.
[0056] As illustrates in
[0057] Metal micro-structure before the finish cold rolling: Average grain size is set at 2.0 m or less
[0058] 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 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. 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 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. 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.
[0059] 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.
[0060] Temperature of intermediate annealing: 500 to 800 C.
[0061] 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.
[0062] Temperature of annealing before the finish cold rolling (previous annealing): 400 to 700 C.
[0063] 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.
[0064] Temperature of finish annealing: 500 to 750 C.
[0065] The average grain size d.sub.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.sub.ave 3, the upper limit of d.sub.ave differs according to the sheet thickness, and an upper limit of the temperature of the finish annealing also differs to set t/d.sub.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.
[0066] 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.
[0067] 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
[0068] 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.
[0069] 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.
[0070] 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.sub.ave and the maximum grain size d.sub.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. 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 thicknesslength 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.
[0071] 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 stressnominal strain curve was evaluated as the uniform elongation.
[0072] 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-US-00001 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 (m) of recrystallized grains
TABLE-US-00002 TABLE 2 SHEET HARDENED COARSE THICKNESS/ GRAIN SIZE/m t/d LAYER Rz/ GRAIN 0.2% PS/ UNIFORM FORMULA [4] No. CATEGORY mm AVERAGE MAXIMUM AVERAGE MAXIMUM THICKNESS/nm m RATIO (%) MPa ELONGATION/% ACCEPTANCE 1 COMPARATIVE EXAMPLE 1 0.03 8.8 13.1 3.41 2.29 120 3.2 0 211 10.1 x 2 COMPARATIVE EXAMPLE 2 0.03 9.3 14.9 3.23 2.01 130 3.3 0 207 10.2 x 3 COMPARATIVE EXAMPLE 3 0.03 7.1 21.3 4.23 1.41 100 1.6 15.3 228 8.9 x 4 COMPARATIVE EXAMPLE 4 0.03 6.8 22.3 4.41 1.35 70 1.4 11.8 231 8.7 x 5 EXAMPLE EMBODIMENT OF THE 0.03 8.6 17.4 3.49 1.72 140 1.2 6.1 213 15.6 PRESENT INVENTION 1 6 EXAMPLE EMBODIMENT OF THE 0.03 7.0 15.6 4.29 1.92 140 1.6 1.2 229 14.2 PRESENT INVENTION 2 7 EXAMPLE EMBODIMENT OF THE 0.03 5.4 12.3 5.56 2.44 120 1.3 0 251 11.3 PRESENT INVENTION 3 8 EXAMPLE EMBODIMENT OF THE 0.03 3.7 8.2 8.11 3.66 130 1.2 0 290 11.2 PRESENT INVENTION 4 9 COMPARATIVE EXAMPLE 5 0.1 50.1 81.3 2.00 1.23 930 1.4 36.2 127 29.6 x 10 COMPARATIVE EXAMPLE 6 0.1 30.5 74.3 3.28 1.35 420 1.2 3.8 144 26.3 x 11 COMPARATIVE EXAMPLE 7 0.1 30.3 82.2 3.30 1.22 500 1.2 6.5 145 24.6 x 12 EXAMPLE EMBODIMENT OF THE 0.1 26.5 59.8 3.77 1.67 1620 1.3 3.1 150 37.7 PRESENT INVENTION 5 13 EXAMPLE EMBODIMENT OF THE 0.1 22.3 58.3 4.48 1.72 1200 1.2 1.6 158 33.6 PRESENT INVENTION 6 14 COMPARATIVE EXAMPLE 8 0.1 17.8 67.3 5.62 1.49 630 1.3 0 162 20.1 x 15 EXAMPLE EMBODIMENT OF THE 0.1 13.1 35.1 7.63 2.85 320 1.3 0 185 28.3 PRESENT INVENTION 7 16 EXAMPLE EMBODIMENT OF THE 0.1 10.7 30.5 9.35 3.28 300 1.4 0 198 29.6 PRESENT INVENTION 8 17 COMPARATIVE EXAMPLE 9 0.1 25.9 76.7 3.86 1.30 730 1.6 15.2 151 20.6 x 18 EXAMPLE EMBODIMENT OF THE 0.2 32.1 61.2 6.23 3.27 840 1.7 0 143 39.8 PRESENT INVENTION 9 19 EXAMPLE EMBODIMENT OF THE 0.2 23.5 52.2 8.51 3.83 700 1.8 0 155 32.3 PRESENT INVENTION 10 20 EXAMPLE EMBODIMENT OF THE 0.2 21.4 52.8 9.35 3.79 680 1.1 0 160 32.2 PRESENT INVENTION 11 21 EXAMPLE EMBODIMENT OF THE 0.2 12.8 30.6 15.63 6.54 500 1.5 0 187 27.1 PRESENT INVENTION 12 22 EXAMPLE EMBODINENT OF THE 0.2 10.2 28.1 19.61 7.12 300 1.2 0 201 24.7 PRESENT INVENTION 13 23 COMPARATIVE EXAMPLE 10 0.2 10.6 32.6 18.87 6.13 2520 1.3 0 193 10.5 x 24 COMPARATIVE EXAMPLE 11 0.2 8.8 21.6 22.73 9.26 2150 1.4 0 199 9.9 x 25 EXAMPLE EMBODIMENT OF THE 0.2 5.6 18.3 35.71 10.93 1720 1.1 0 240 14.2 PRESENT INVENTION 14 26 EXAMPLE EMBODIMENT OF THE 0.2 8.4 72.2 23.81 2.77 1930 1.1 0 215 21.9 PRESENT INVENTION 15 27 COMPARATIVE EXAMPLE 12 0.2 24.6 56.8 8.13 3.52 2310 1.4 0 148 24.6 x 28 EXAMPLE EMBODIMENT OF THE 0.2 16.1 31.3 12.42 6.39 1820 1.4 0 159 28.4 PRESENT INVENTION 16 29 EXAMPLE EMBODIMENT OF THE 0.2 20.3 44.6 9.85 4.48 1520 1.5 0 151 28.9 PRESENT INVENTION 17 30 EXAMPLE EMBODIMENT OF THE 0.2 20.2 39.6 9.90 5.05 1390 1.2 0 153 29.8 PRESENT INVENTION 18 31 EXAMPLE EMBODIMENT OF THE 0.2 18.4 28.1 10.87 7.12 1420 1.1 0 161 29.4 PRESENT INVENTION 19 32 EXAMPLE EMBODIMENT OF THE 0.2 4.6 8.1 43.48 24.69 1060 1.7 0 261 9.8 PRESENT INVENTION 20 33 COMPARATIVE EXAMPLE 13 0.2 2.4 4.9 83.33 40.82 1040 1.2 0 340 5.9 x 33 COMPARATIVE EXAMPLE 14 0.2 44.3 138.6 4.51 1.44 760 1.2 13.9 130 31.6 x 34 COMPARATIVE EXAMPLE 15 0.4 45.2 141.3 8.85 2.83 790 1.4 0 131 43.1 35 COMPARATIVE EXAMPLE 16 0.5 41.4 113.1 12.08 4.42 690 1.3 0 130 42.2 36 EXAMPLE EMBODIMENT OF THE 0.2 62.1 131.4 3.22 1.52 1920 1.3 16.2 145 38.1 PRESENT INVENTION 21
[0073] For both of example embodiments of the present invention and comparative examples, their 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. As 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.
[0074] As to all of 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.sub.ave 2.5 m, t/d.sub.ave3, t/d.sub.max1.5, and thickness of hardened layer: 0.1 to 2.0 m were satisfied. As a result, 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.
[0075] As to a comparative example 1, its rolling ratio of the finish cold rolling was as high as 90%, and while its index for the grain size distribution was satisfied, its maximum height Rz was more than 3.0 and Formula [4] was not satisfied due to a fine crack on its surface. As to a comparative example 2, its rolling ratio of a last intermediate rolling was 90%, and its maximum height Rz was more than 3.0 m, and Formula [4] was not satisfied due to a fine crack on its surface.
[0076] As to comparative examples 3 to 6, and 8, since their rolling ratios of the last intermediate rolling were small, or their grains were coarsened by a last intermediate annealing, their metal micro-structures before the finish cold rolling were coarse, and their coarse grain ratios did not satisfy t/d.sub.max1.5, failing to satisfy Formula [4]. In addition, the results of comparative examples 5 and 6 were also due to their high finish annealing temperatures, which easily coarsened grains.
[0077] As to a comparative example 7, its finish annealing temperature was high, which easily coarsened grains, failing to satisfy Formula [4]. As to a comparative example 9, its metal micro-structures were coarse before the finish cold rolling, and its rolling ratio of the finish cold rolling was low, which makes uniformity of grains insufficient, failing to satisfy Formula [4].
[0078] As to 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. As to a comparative example 13, 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. As to comparative examples 10 to 12, their hardened layers had thicknesses of 2m or more, and their elongations were poorer than those of example embodiments 14 and 15 of the present invention, failing to satisfy Formula [4].
[0079] As to a comparative example 12, its finish annealing duration was long, with the result that its hardened layer was formed thick. The annealing at this temperature needs to be performed in a shorter time. As to a comparative example 13, its finish annealing temperature was low, and its grain size was less than 2.5 m even when the annealing was performed for 20 hours.
[0080] As to comparative examples 14 and 15, their production was under the same conditions, but their sheet thicknesses were different from each other. As to the comparative example 14, its sheet thickness was 0.2 mm, and its producing method did not meet the ranges according to the present invention, failing to satisfy Formula [4]. However, as to the comparative example 15, its sheet thickness was 0.4 mm, and its property did not deteriorate even when its producing method did not meet the ranges according to the present invention. As to a comparative example 16, similarly, its sheet thickness is large, and its property did not deteriorate even when its producing method is out of the ranges according to the present invention.